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NBS TECHNICAL NOTE 514

ARPA-NBS Program

of Research on High

Temperature

Materials and Laser

Materials
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards ' was established by an act of Congress March 3, 1901 . Today,

in addition to serving as the Nation's central measurement laboratory, the Bureau is a principal

focal point in the Federal Government for assuring maximum application of the physical and

engineering sciences to the advancement of technology in industry and commerce. To this end

the Bureau conducts research and provides central national services in four broad program

areas. These are: (1) basic measurements and standards, (2) materials measurements and

standards, (3) technological measurements and standards, and (4) transfer of technology.

The Bureau comprises the Institute for Basic Standards, the Institute for Materials Research, the

Institute for Applied Technology, the Center for Radiation Research, the Center for Computer

Sciences and Technology, and the Office for Information Programs.

THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United

States of a complete and consistent system of physical measurement; coordinates that system with measurement systems of other nations; and furnishes essential services leading to accurate and

uniform physical measurements throughout the Nation's scientific community, industry, and commerce. The Institute consists of an Office of Measurement Services and the following technical

divisions:Applied Mathematics—Electricity—Metrology—Mechanics—Heat—Atomic and Molecular Physics—Radio Physics -—Radio Engineering -—Time and Frequency -—Astrophysics -—Cryogenics.2 THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research leading to im- proved methods of measurement standards, and data on the properties of well-characterized

materials needed by industry, commerce, educational institutions, and Government; develops,

produces, and distributes standard reference materials; relates the physical and chemical properties of materials to their behavior and their interaction with their environments; and provides

advisory and research services to other Government agencies. The Institute consists of an Office of Standard Reference Materials and the following divisions: Analytical Chemistry—Polymers—Metallurgy—Inorganic Materials—Physical Chemistry.

THE INSTITUTE FOR APPLIED TECHNOLOGY provides technical services to promote

the use of available technology and to facilitate technological innovation in industry and Government; cooperates with public and private organizations in the development of technological

standards, and test methodologies; and provides advisory and research services for Federal, state, and local government agencies. The Institute consists of the following technical divisions and

offices: Engineering Standards—Weights and Measures — Invention and Innovation — Vehicle Systems Research—Product Evaluation—Building Research—Instrument Shops—Measurement Engineering—Electronic Technology—Technical Analysis.

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provides technical services designed to aid Government agencies in the selection, acquisition, and effective use of automatic data processing equipment; and serves as the principal focus

for the development of Federal standards for automatic data processing equipment, techniques,

and computer languages. The Center consists of the following offices and divisions: Information Processing Standards—Computer Information — Computer Services — Sys- tems Development—Information Processing Technology.

THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and

accessibility of scientific information generated within NBS and other agencies of the Federal government; promotes the development of the National Standard Reference Data System and a system of information analysis centers dealing with the broader aspects of the National Measure- ment System, and provides appropriate services to ensure that the NBS staff has optimum accessibility to the scientific information of the world. The Office consists of the following

organizational units:

Office of Standard Reference Data—Clearinghouse for Federal Scientific and Technical Information ' —Office of Technical Information and Publications—Library—Office of Public Information—Office of International Relations.

1 Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D.C 20234.

- Located at Boulder. Colorado 803,02.

3 Located at 5285 Port Royal Road. Springfield. Virginia 22151.
UNITED STATES DEPARTMENT OF COMMERCE

Maurice H. Stans, Secretary

NATIONAL BUREAU OF STANDARDS • Lewis M. Branscomb, Director

NBS TECHNICAL NOTE 514

ISSUED JANUARY 1970

Nat. Bur. Stand. (U.S.), Tech. Note 514, 98 pages (Jan. 1970)

CODEN: NBTNA

ARPA-NBS Program of Research

on High Temperature Materials

and Laser Materials

Reporting Period

January 1 to June 30, 1969

Edited by A. D. Franklin and H. S. Bennett

Inorganic Materials Division

Institute for Materials Research

National Bureau of Standards

Washington, D.C. 20234

Supported by the

Advanced Research Projects Agency

of the Department of Defense

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the open literature.

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C, 20402

(Order by SD Catalog No. C 13.46:514). Price $1.00.

TABLE OP CONTENTS

Page

1. Introduction 1

2. Objectives 1

3. Project Summaries. 4

3.1. Diffusion in Oxides

3.1.1. Diffusion in Refractory Materials. . 4

3.1.2. Diffusion of Oxygen in Oxides. ... 8

3.1.3. Crystal Growth from Vapor 22

3-2. Properties of Refractory Borides

3.2.1. Electronic Structure of Refractory

Hard Metals 24

3-3. High Temperature Metals

3.3.1. Optical Constants of Titanium in the

Visible Region of the Spectrum ... 29

3-3.2. High Temperature Creep in Metals . . 31

3.4. Ceramics

3.4.1. Deformation and Fracture of Ionic

Crystals 38

3.5. Polymers

3-5.1- The Volatilization and Decomposition

of Materials • ... 40

3.5.2. Relaxations Near the Melting Point . 43

3.6. Laser Materials

3.6.1. Bulk Properties of Laser Materials . 44

3.6.2. Crystal Defect Studies on Laser

Materials 50

3.6.3. Examination of Ruby Laser Rods by

X-ray Diffraction Techniques .... 58

3.6.4. Laser Induced Damage Studies .... 62

3.6.5. Characterization Standards for Laser

Materials 67

3.6.6. Studies on Techniques to Detect

Platinum Inclusions and to Measure

Nd-Glass Quality by Light Scattering 73

3.6.7. Appendix - Trip Reports 75
ARPA-NBS PROGRAM OP RESEARCH ON

HIGH TEMPERATURE MATERIALS

AND

LASER MATERIALS

Work Performed at the National Bureau of Standards

Supported by the Advanced Research Projects Agency,

Department of Defense

Reporting Period January 1 - June 30, 1969

Edited by

A. D. Franklin and H. S. Bennett

Brief reviews are given of work performed during the

period January 1 to June 30, 1969, on a number of projects

concerned with High Temperature Materials and with Laser

Materials. Under the High Temperature Materials heading,

topics include diffusion of oxygen in oxides, growth of

Alp0_ crystals by chemical vapor deposition, the electronic

structure of transition metal borides and related compounds,

the optical constants of titanium, high temperature creep

in copper, fracture in glass, the mechanism of volatilization of polymers and long-chain compounds, and the interaction between mechanical relaxation and annealing in

polymers. Work on Laser Materials includes measurement of

bulk optical and elastic properties of laser materials, a study of the "orange" degradation of ruby, measurements of

sub-crystal misalignment in rubv , damage in glass induced

by high-energy laser pulses, chemical analyses for ruby and

Nd-doped laser glasses, and preliminary studies on detection of submicroscopic inhomogeneities in glass.

Key words: Band structure, chemical analysi's, copper,

creep, crystal defects, crystal growth,

diffusion, evaporation, fracture, glass, high

temperature materials, inhomogeneities in glass,

laser damage, laser glass, lasers, mass

transport, materials properties, mechanical

relaxation, metal borides, optical properties,

opto-elastic properties, oxides, polymers,

ruby, titanium, transition
ARPA-NBS PROGRAM OP

RESEARCH ON HIGH TEMPERATURE MATERIALS

Work Performed at the National Bureau of Standards

Supported by the Advanced Research Projects Agency,

Department of Defense

Edited by

A. D. Franklin

1. INTRODUCTION

The National Bureau of Standards, with support from

the Advanced Research Projects Agency of the Department of

Defense, is carrying out a program of research on High

Temperature Materials. In this program are included projects on their preparation, and others designed to explain

the basic phenomena limiting the use of materials at high

temperatures. A summary of the results achieved in the

period is given here.

2. OBJECTIVES

One of the crucial factors limiting the advance of

Defense technology is the failure of materials at elevated

temperatures. This fact has long been recognized, and the

Department of Defense has found it necessary to support a great deal of research to improve the high temperature

performance of a wide variety of materials.

The continued success of these efforts depends upon

having the right tools on hand when they are needed. In

the case of advanced R&D, the tools include the science and

practice of materials preparation and characterization, and

of the measurement of materials properties. It is the overall objective of the present program to identify some of

the more important of such key problems and to provide the

needed advances where NBS possesses the competence to do so These efforts will be directed mainly toward providing
techniques for the preparation and characterization of

research materials, providing techniques for the measurement of properties, obtaining reliable values of key pieces

of data, and improving the understanding of material properties particularly relevant to high-temperature performance,

In pursuit of this program, a continuing attempt is

being made to analyze technological problems in hightemperature materials use and to distill from this analysis

major scientific problems whose solution would advance the

technology. In addition, the problems chosen should fall

within areas in which NBS has front-rank competence available. As a final criterion, the number of problems included

in the program should be small enough so that each one can

be approached in a comprehensive fashion, ensuring adequate

attention to specimen preparation and characterization.

In line with these criteria, the program is continuing

to evolve. A further concentration of the work on fewer

problems, perhaps one or two, will undoubtedly take place.

One of these will be concerned with the diffusion of oxygen

in oxides at high temperature. We are also examining the

possibility of making an expanded study of the properties

of transition metal borides as the focus of the rest of the

program.

It is difficult to overemphasize the practical importance to DOD of improvements in the measurement of diffusion

of metals and non-metals, particularly oxygen, in oxides.

Diffusion is the process that controls the rate of attack

on oxidation-resistant metals at high temperature, of

corrosion (other than stress-corrosion) of many metals, of

sintering of ceramics, swelling of reactor fuels, operation

of ceramic fuel cells, and many other processes.

For instance, the corrosion and oxidation of structural metals at high temperatures is an outstanding problem
for DOD technology. The remedy lies in protection by some

sort of coating, either deliberately applied or else formed

during the early stages of corrosion. This coating must

provide a barrier to the passage of ions, since for corrosion to proceed either oxygen must penetrate to the metal

surface or metal ions must come out to meet the oxygen. The

ultimate solutions to problems of this class will require

understanding and control of the processes of diffusion in

the oxide coatings.

At present the diffusion of ions in oxides is not at

all well understood, nor subject to anything but very crude

control. Among the major reasons are lack of sufficiently

pure crystals upon which to make measurements; lack of

reliable techniques for measuring diffusion rates, and

therefore, of reliable diffusion data, particularly for

oxygen diffusion; and lack of well-characterized grainboundaries to assess the relative importance of bulk and

grain-boundary diffusion in polycrystalline oxide ceramics

and films.

Transition metal borides possess properties that promise considerable usefulness at very high temperatures.

Thus, the diborides early in the transition metal series

exhibit melting points in the neighborhood of 3000°C with

the high thermal conductivities more characteristic of

metals and with low thermal expansion. This combination

of properties produces an excellent resistance to thermal

shock. When combined with high chemical stability and

oxidation resistance, they suggest promising materials

where sudden extremes of temperature are encountered.

While considerable work has been done on a variety of

borides, there is a very real need for the development of

methods of preparing research-grade materials and for
characterizing them with respect to composition, purity,

and homogeneity, and also with respect to lattice structure

and perfection.

We are at present studying this problem area to determine if NBS can make a worthwhile contribution to the preparation of some of these borides at the research-grade level.

If it appears that we can, it is planned to mount a research

effort into the preparation techniques and such ancillary

problems as high temperature and pressure phase equilibria.

Specimens produced in this effort would form the basis for

development of an electron-microprobe light-elementcompound standard, and for continuation of our ongoing

work (by J. R. Cuthill, et.al.) on the electronic structure

of these materials. Studies on the chemical analysis of

the borides, for major constituents and also for minor but

important amounts of C, N, H, 0, etc., would also be

carried out. In the future, successful growth of good

crystals would make possible work on lattice defects and

their influence on mechanical properties.

3. PROJECT SUMMARIES

3.1 Diffusion in Oxides

3.1.1 Diffusion in Refractory Materials

A. L. Dragoo

Inorganic Materials Division

Institute for Materials Research

The objectives of this program are to measure and to

interpret diffusion of oxygen in single crystal oxides.

Experimentally these two objectives consist of (1) extending the sensitivity of present techniques so that the l"o

distribution in a single crystal sample can be measured,

(2) comparing the results of these concentration profile

measurements with measurements of the rate of exchange of
-| o between the gas phase and the oxide crystal, (3) determining the dependence of the diffusion rate on temperature,

oxygen partial pressure, impurities in the solid, etc., and

(4) obtaining reliable diffusion and exchange coefficients.

In regard to interpretation, this objective translates into determining the mechanisms for oxide ion diffusion in

the oxide studied. Currently, rutile (TiCO is the oxide

of interest. A mass spectrometer is being used to determine isotope ratios.

A major portion of the time during this period was

spent in designing, and installing equipment. The isotope

ratio mass spectrometer was finished by the Analytical

Mass Spectrometry Section and moved into our laboratory.

The installation of it is nearly completed. The graphitereduction system for liberation of the oxygen from the

oxide sample is undergoing modification to reduce the volume

of the CO- COp conversion system and to improve the control

of the conversion filaments. Construction of the manifold

for the inlet system to the mass spectrometer is in progress

In the previous report, mention was made of the solution of the diffusion equation for a problem involving

competition between two different solid host media for the

isotope in the vapor phase. Further details are given

here, plus an approximate solution which was developed in

order to have a convenient equation for the treatment of

experimental data. This equation was applied to experimental data involving competition between an A1 9 Q furnace

-i o ^ -> tube and a TiO

? sample for the ' isotope in the vapor

phase. We found that the experiment could be performed by

nearly saturating the walls of the alumina tube before

introducing the sample. The rate of depletion of from

the vapor phase was measured before and after introducing
the sample (a thin, single crystal, rutile wafer).

A mathematical description of the competition between

-l o two solid oxide diffusion media for in the vapor phase

was obtained by solving the diffusion equation,

3c/3t = D(3 2 c/3x2

) for two models. The first model involved only one semiinfinite solid slab (called the tube) with a general

1 Pi continuous distribution of whose surface was at all

times in isotopic equilibrium with the vapor. The vapor

was assumed to be finite and well stirred. The second

model included a second semi-infinite solid slab (called

the sample) along with the tube and vapor. The sample

was assumed to have a normal uniform isotopic composition

initially and its surface was assumed to be in equilibrium

with the vapor at all times.

The two solutions were used to obtain a time-rate of

1 8 change of in the vapor phase. By subtracting the rate

given by the first model from that given by the second

model, a differential rate was obtained which contained

both the independent effect of the sample and the interaction of the tube and sample.

Since the differential rate expression was very complicated, an approximation had to be found that was both convenient and accurate for the treatment of experimental data.

A zero-order approach consisting of the elimination of all

interaction terms agreed with simpler solutions of the

diffusion equation. A first order expression was obtained

by estimating the magnitude of each part of the interaction

term of the differential rate expression. Since this term

involved an integral containing an unspecified function

which described the initial isotopic distribution in the

wall of the tube, the contribution and form of this function
had to be considered. Noting that this function multiplied

functions in the integrands which were nearly 6-functions

for short times and that it could be assumed to be equal

to the initial mole fraction u

n of in the gas (at t = 0),

the integrations were easily performed by means of the

property of the 6-function

/

f(x)*<5(x)tfx = f (°) = u n 0- u

The first approximation to the differential rate equation is

9u - 9u (u -v )y r\/D n ll/(y/Trt)+l

at 1+2 dt

1 o o,2 2 2

- 2(1 + y/t) eerfc(y/t)].

The various parameters are given in the accompanying table 1

Table 1

u = Initial mole fraction of the isotope

in the gas

v n

-- Initial mole fraction of the isotope

o 2

' in the solid

Y = n Sd

1 + n

2

/D"

2

r\. = A.N. /V.N , where A. is the area, V.

i i i l g' i ' l

the volume, N the quantity of the

o

diffusing element (all isotopes) in

the gas and N. the quantity of the

element in the solid (the element is

assumed to have the same molecular

form in the solid phase as in the

gas phase)
The subscripts 1 and 2 refer to the tube and sample,

respectively. The special function eerfc (z) is defined

by the equation

p eerfc(z) = exp(z )

• erfc(z)

where erfc(z) is the complimentary error function.

The above equation was used by Mr. F. P. Knudsen, of

this laboratory, to treat data which he obtained for oxygen

diffusion in rutile at 1350°C; he calculated

D = 2.19 • 10" 11

cm

2 /s

for the diffusion coefficient of rutile which can be compared with Haul and Dumbgen's [1] values

D = 1.73 • 10" 11cm

2 /s, 13^7 C

D = 1.88 • 10" 11cm

2 /s, 1348°C.

In addition, he estimated that

D = 5 • 10~ 12

cm

2 /s

for the tube.

References

[1] Haul, R. and Diimbgen, G., J. Phys . Chem. Solids 26 , pp. 1-10 (1965).

3.1.2 Diffusion of Oxygen in Oxides

P. P. Knudsen

Inorganic Materials Division

Institute for Materials Research

Introduction

This project is not an ARPA-sponsored project. However,

the work is closely associated with, and virtually an

integral part of, the research being done under the project
described in the preceding section, which is an ARPA

sponsored project. For this reason, a report is submitted

herewith to ARPA. Because of the integral nature of the

two projects, this report will deal, in most instances,

with combined efforts of both projects.

Objectives

(1) To obtain sound, reliable, data on the diffusion and

exchange of oxygen in oxides, in such a manner that both

the data and the experimental techniques will be widely

accepted as standards and as a basis for comparison by

experimenters using other unproven or controversial

techniques

(2) To improve the sensitivity and precision of the

"sectioning method" of determining diffusion rates, so as

to extend its applicability from the present diffusion

-11 2 -1

coefficient limit of 10 cm /s to values as low as 10 J

-14 ? or 10 cm /s.

(3) To compare, by means of simultaneous experiments,

the widely used, indirect, "gas depletion method" of determining diffusion rates with the direct "sectioning method"

of determining diffusion rates.

(4) To define the pressure and temperature dependence

of the diffusion of oxygen in oxides and to attempt to

define the means by which oxygen diffusion occurs in the

oxides under study.

Materials

Rutile (TiOp) was selected as the first oxide to be

studied. - Single crystal disks of rutile were obtained

from a commercial supplier. The axis of each disk was

parallel to the crystallographic c axis. The disks had

been cut from boules specially grown by the supplier for

this study. NBS spectrographic analysis of samples of
the disks showed .001 percent B as the only measureable

trace element. Suspect traces of Al, Cu and Si were also

noted

.

Oxygen containing five percent of oxygen isotope 18

was used as tracer gas in the study. The isotope was

-i o obtained from a commercial supplier in the form of 0-

enriched water.

Summary of Previous Work

A rutile single crystal specimen was annealed for three

weeks at 1350°C in normal research grade oxygen at a pressure of 1/3 atm in a platinum diffusion chamber to

bring it into equilibrium with the anneal atmosphere with

respect to oxygen activity, the latter being both temperature and pressure dependent. During this annealing

treatment, platinum crystallites formed on the specimen.

The crystallites caused small local irregularities in the

contour of the specimen surfaces. These were removed by

repolishing the faces of the specimen.

At this point, a consultant to the project expressed

the opinion that the large thermal gradient in the diffusion chamber might cause some slight isotopic fractionation of any oxygen in the chamber. Gas samples taken

during subsequent trial heatings disclosed that no

measureable fractionation occurred under the test conditions.

However, during one of the trial heatings, a leak

developed in the protective gas-tight alumina tube enclosing the platinum diffusion chamber. The platinum

chamber was exposed to a compressive pressure differential of 2/3 atm at 1350°C. Although 0.15 mm thick,

the chamber wall collapsed and sintered to itself, thereby

ruining the chamber.

10
To continue the work while these difficulties (i.e. the

deposition of platinum crystallites and the likely failure

of another protective alumina tube) were being studied and

overcome, the platinum chamber was temporarily replaced

with one made of high-purity polycrystalline alumina. It

was ascertained, by means of trial heatings and spectrographs analysis, that no measureable amount of alumina is

adsorbed by rutile under the aforementioned experimental

conditions. The attempt to use a polycrystalline alumina

closed-end tube as a diffusion chamber met repeatedly with

difficulty. Three such tubes, helium-gas-tight at room

temperature, developed minute leaks at small flaws during

use at 1350°C. To minimize the extraneous effect of the

1 f) 1 8

alumina chamber exchanging for the O-enriched oxygen

during diffusion anneals, the alumina tubes were pre-

-i o annealed for 3 weeks at 1500°C in the O-enriched oxygen

1 8

at a pressure of 3/^ atm. From the depletion of from

the enriched oxygen during the satisfactory pre-anneal

of the fourth alumina tube at 1500°C, a diffusion co-

-11 2 efficient of 2 x 10 cm /s was calculated for the diffusion

of oxygen in this commercial alumina body having a nominal

density of 3.78 g/cm^ and a nominal purity of +99.7$. The

l 8 relative content of samples of the enriched oxygen

was determined by isotopic ratio mass spectrographic

analysis at the University of Iowa.

Thereafter, the equilibrated rutile specimen mentioned

earlier was diffusion-annealed in this fourth alumina

1 8

chamber at 1350°C at 1/3 atm pressure of the enriched

l 8

oxygen tracer gas. The depletion of from the enriched

oxygen by diffusion into the crystal had to be distin1 8

guished from the associated extraneous depletion of

by diffusion into the polycrystalline alumina diffusion

11
chamber. With this objective, the enriched oxygen initially

was permitted to diffuse into the alumina in the absence of

the rutile crystal and thereafter the crystal was periodically introduced into and withdrawn from the high temperature diffusion region of the alumina chamber. Contrary to

prior expectations, the precision of the experimental procedures and measurements proved to be sufficient to distinguish satisfactorily between the two rates of depletion

-i o of from the tracer gas. However, the two simultaneous

diffusion processes cannot be completely differentiated

even by this means because they are not simply additive

but interact with one another. In fact, slight evidence

of the existence of this interaction was noted in the data

of the periodic changes in the rate of depletion of

from the tracer gas. If the degree of interaction is

assumed to be negligible, one obtains from the data of the

— 11 2 above measurements, a value of (1.11 + .17) x 10 cm /s

for the coefficient of diffusion of oxygen parallel to the

c axis in single crystal rutile at 1350°C in oxygen at a pressure of 1/3 atm. In order to provide a means of evaluating the degree of interaction and the diffusion coefficient in a manner which would properly compensate for

the interaction, A. Dragoo derived a general equation for

the competitive coupled diffusion process. The equation

relates both diffusion coefficients of the coupled system

to the time dependence of the difference between the

observed two alternating rates of depletion of ambient

tracer gas during a cyclic diffusion anneal, ,such as the

one previously described. The diffusion coefficients can

be evaluated by means of an approximation to the general

equation. The general equation, its boundary conditions,

the approximation, and the inherent assumptions therein,

12
are presented in the appendix to this section.

At the outset of the study, the projects had access to

and part-time use of an isotope ratio mass spectrometer at

the nearby University of Maryland. Although this arrangement was often inconvenient, analytical results could be

obtained within twenty-four hours. Subsequently, the instrument was transferred to the University of Iowa where the

analysis of samples was continued by the University on a fee basis. However, the three to six weeks usually required for the receipt of analytical results by this

arrangement, forced the projects, in effect, to conduct

their experiments blindly without any means of discovering

the numerous possible daily errors or failures in established experimental procedures. Furthermore, as is obvious,

it made the development of essential new experimental procedures and techniques unacceptably lengthy. A survey of

nearby research facilities disclosed that there was no

available isotope ratio mass spectrometer in this area.

The need for such an instrument was clearly evident. In

as much as the National Bureau of Standards had the skill,

knowledge, facilities and experience to build such an

instrument and could do so at a cost much less than that

of a comparable commercial instrument, arrangements were

made to have such an instrument constructed here at the

National Bureau of Standards

Progress in the Present Report Period

By means of the aforementioned diffusion equation approximation, the previously reported coefficient of 1.11 + .17

_1 1 p x 10 cm /s for the self-diffusion of oxygen in rutile at

1350°C was recalculated without the use of the questionable presumption of the degree of interaction being negligible. The approximation and the evaluation of the

13
coefficient are presented in the appendix to the present

report. When the interaction was taken properly into

-11 2 account, by the above means, a value of 2.19 x 10 cm /s

was obtained. For a comparable determination, other investigators. Haul and Diimbgen [1] have reported a value of 1.88

-11 2 x 10 cm /s. Note, the degree of interaction was far from

negligible, even though the chamber had been pre-saturated

so thoroughly with the enriched oxygen that the relative

content of the ambient enriched oxygen decreased by

only three percent during the diffusion anneal. Even of

that slight decrease, only half was attributable to diffusion

of the enriched oxygen into the chamber wall: Yet the

interaction approximately doubled the value of the diffusion coefficient. This observation should prove to be of

particular concern to other investigators using chambers

capable of adsorbing or exchanging tracer gas.

Incidentally, the evaluation also yielded a value of

-12 2

5 x 10 cm /s as the diffusion coefficient of oxygen in

the polycrystalline chamber wall at 1350°C in oxygen at a

pressure of 1/3 atm. The determined values for the diffu-

-11 2 sion of oxygen in polycrystalline alumina, 2 x 10 cm /s

at 1500°C and 5 x 10" 12

cm

2 /s at 1350°C are several orders

of magnitude greater than the values reported by Oishi and

Kingery [2] for polycrystalline alumina. The difference

might be attributable to the fact that the nominal impurity

content of the polycrystalline alumina tube chamber was

approximately an order of magnitude greater than the

impurity content of Oishi and Kingery' s specimens.

The uncertainties and the more than twofold loss in

time and precision in dealing with a coupled system, together with the complexity and controversial nature of

the method of calculating the diffusion rates in such a

14
system, make it very undesirable to continue to use polycrystalline alumina as a diffusion chamber. The use of

such a chamber was intended to be a temporary expedient.

In the interim, means have been found which presumably will overcome the difficulties previously encountered

in the use of platinum as a diffusion chamber. The deposition of platinum crystals will be eliminated, or reduced

to a tolerable degree (1) by suspending the diffusion

specimen by a fine (i.e., approximately . 04 mm diameter)

single crystal sapphire filament instead of by fine platinum and platinum-rhodium thermocouple wires and (2) by

decreasing or eliminating the appreciable quantity of

1 o water vapor formerly present in the enriched oxygen

tracer gas. The possibility of an accidental collapse of

the platinum chamber will be eliminated by maintaining the

pressure outside the chamber always equal to that within

the chamber by means of a differential aneroid manostat.

The differential manostat and a suitable platinum

chamber diffusion furnace were designed and constructed

during the present report period while the projects were

awaiting the completion of the construction of the isotope

ratio mass spectrometer. The diffusion furnace consists

of a closed-end platinum tube within a protective gastight polycrystalline alumina closed-end tube which, in

turn, is within a molybdenum wire wound alumina heater core,

The platinum chamber is fashioned of 0.08 mm thick platinum,

It is 3.5 cm in diameter and 43 cm in length. The furnace

components are joined together by high vacuum "0" ring seal

flanges in such a manner that different gases can be

maintained in isolation and regulated in the different

sections of the furnace; that is, (1) within the platinum

chamber, (2) between the platinum tube and the protective

encompassing alumina tube and (3) within the section

15
external to the protective alumina tube (i.e., the section

containing the heater core and the insulation of the furnace). The differential aneroid manometer is essentially

comprised of a pair of sensitive three inch diameter bellows

actuated by pressure changes within the diffusion chamber

proper. The bellows directly actuate gas-tight seals

connecting vacuum and bleed lines to the system immediately

outside the platinum chamber.

In the simultaneous associate study of diffusion by

the "sectioning method", a change in method of recovering

the oxygen from the crystal sections was undertaken. The

proposed change created the need for an apparatus for

drying, encapsulating, and storing, ground sample sections,

all continuously under vacuum. Such an apparatus was

designed and constructed during the present report period.

Construction and installation of the isotope ratio

mass spectrometer was virtually completed during the present

report period and work is currently in progress on reassembling the diffusion system (i.e., diffusion furnace, gas

handling system, hydrolysis cell, gas storage flasks, etc.)

about the spectrometer and connecting the system thereto.

Objectives for the Next Report Period

To complete the reassembly and calibration of the

various components and to proceed with the determination

of the rate of oxygen self-diffusion in rutile as functions

of temperature and pressure of ambient oxygen by both the

"gas depletion method" and the "sectioning method".

References

[1] Haul, R. and Dumbgen, G., "Sauerstof f-Selbstdiffusion

in Rutilkristallen," J. Phys . Chem. Solids: Vol. 26_,

pp. 1-10 (1964).

[2] Oishi, Y. and Kingery, W. D. , "Self-Diffusion of Oxygen

in Single Crystal and Polycrystalline Aluminum Oxide,"

J. Chem. Phys.; Vol. 33, No. 2, pp. 480-86 (I960).

16
Appendix

A mathematical description of the competition between

1 Pi two solid oxide diffusion media for in the vapor phase

was obtained by solving the diffusion equation,

9c _ 8 2 c it

m D "IF

for two models. The first model involved only one semiinfinite solid slab (called the tube) with a general con-

-i o tinuous distribution of whose surface was at all times

in isotopic equilibrium with the vapor. The vapor was

assumed to be finite and well stirred. The second model

included a second semi-infinite solid slab (called the

sample) along with the tube and vapor. The sample was

assumed to have a normal uniform isotopic composition

initially and its surface was assumed to be in equilibrium with the vapor at all times.

The two solutions were used to obtain a time-rate of

change of in the vapor phase. By subtracting the rate

given by the first model from that given by the second

model, a differential rate was obtained which contained

both the independent effect of the sample and the interaction of the tube and sample.

Since the differential rate expression was very complicated, an approximation had to be found that was both

convenient and accurate for the treatment of experimental

data. A first order expression was obtained by estimating

the magnitude of each part of the interaction term of the

differential rate expression. Since this term involved an

integral containing an unspecified function f ( £) which

described the initial isotopic distribution in the wall of

the tube, the contribution and form of this function had to

17
n l n 2

1 1 g

A = the surface area of the interior of that

portion of the alumina diffusion chamber

2 approximately at the test temperature (cm )

n = the quantity of total oxygen, taken as 9 , within the walls per se of that portion of

the alumina diffusion chamber approximately

at the test temperature (mol)

n = the quantity of total oxygen, taken as

p

, in the gas within the entire diffusion

chamber (mol)

D. = the diffusion coefficient of oxygen in the

2 alumina (cm /s

)

V-, = the volume of solid of that portion of the

alumina diffusion chamber approximately at

the test temperature (cm ) n 2 A 2 n

2 /Dr

a V n

2 2 g

A 2 t the surface area of the crystal specimen

(cm )

n 2

= the quantity of total oxygen, taken as 0„,

in the crystal specimen (mol)

D 2 e the diffusion coefficient of the crystal

o specimen (cm /s

)

18
be considered. Noting that

f(0) = u

and that the terms multiplied by f(£) in the integral were

approximately <5 functions, the integral was replaced by

/. f(0 • 6(0^= f(0) = uq

The result is the following first order approximation,

, 3u>. / 3Us

t+c

Y (*) (u - v )

' a

2 ° °? I yvirt

1 -

2ey2t erfcCy/t)!- ("(2e y2t

)Y/t erfcCy/t)"]

j

wherein

(—

)

k 3t ; t+c

^3t ;

the rate of change in the mole fraction of

l ft in the gas in the diffusion chamber when

the crystal specimen is in the chamber.

the theoretical rate of change in the mole

1 o fraction of in the gas in the diffusion

chamber had the specimen not been introduced

into the chamber: the theoretical rate is

calculated by extrapolation of the rate

change observed prior to the introduction

of the specimen.

19
Vp = the volume of the crystal specimen (cm ) 2

1

a, 1

1

a

2

2

-l o u = the mole fraction of in the gas in the

o

to diffusion chamber at the time the crystal

specimen was introduced into the chamber

1 8

v = the mole fraction of in the crystal

o

2 specimen at the time it was introduced into

the diffusion chamber

t = the period of time subsequent to the introduction of the crystal specimen into the

diffusion chamber (seconds)

e = the Napierian base e

erfc = error function compliment

T = the test temperature ( °C

)

P = the pressure of the gas within the time

the crystal specimen was introduced into

the chamber (Torr)

a) =' the mass of the crystal specimen

Data

A, 200 cm (estimated value)

n

1

/V

1

.058417 mol/cm3

n .013893 mol

o

A

2

4.146 cm

2

20
n

2

.0084193 mol

V

2

.1637 cm 3

u .088762

o v .0040683

°2

T 1350 °C

p 316.35 Torr

w .6727 g

By evaluating some of the terms in the equation and simplifying, one obtains,

9t t+c dt

t

- 1.299756 y/D,

wherein,

—-— + 1 - (l+y/t)2eY2t erfc(y/t)

y/rrt"

Y = n

1

/D

The quantities,

,

, . , + 15.34646 /D~

(n 1 /D

1

) and D

2

were evaluated by successive "least squares" approximations

calculated to reduce the sum of the squares of their coefficients of variation to a minimum, where

LHS - RHS

coefficient of variation LHS

and

LHS = the value of the left hand side of the equation

RHS = the value of the right hand side of the equation

The coefficient of variation was used in preference to the

21
variance because the values of the left hand side of the

equation ranged over more than two orders of magnitude

and because it was deemed likely that deviations would be

proportional to the magnitude of LHS rather than constant,

The calculation of the "least squares" approximations encompassed values of (ri 1 /D 1

) ranging from 1x10 through

-6 -Q 1x10 and values of D

p ranging from 1x10 J through

-IS 1x10 ±3

.

The calculated "least squares" approximations were,

(n 1

/D^) = 1.87 x 10" 3

D

2 = 2.19 x 10" 11

Accordingly, on the basis of the initial 48 hours of

-l o diffusion of into the rutile crystal, parallel to the

c axis

D 1350°C

= 2 ' 19 x 10 1:L cm2/s

3.1.3 Crystal Growth From Vapor

H. S. Parker and C. A. Harding

Inorganic Materials Division

Institute for Materials Research

The objectives of this program are, first, the growth

of aluminum oxide mono- and bicrystals of sufficient

physical perfection and chemical purity for use as research

materials; second, the reduction of both cation and anion

impurities in vapor grown aluminum oxide crystals to

sufficiently low levels to permit meaningful property

measurements at the intrinsic level; and third, the extension of the technique to other materials of interest.

The major portion of the effort during this reporting

22
period has been devoted to the growth of bicrystals containing symmetrical tilt boundaries of a size and quality

suitable for use as research specimens. Bicrystals of

15° and 30° misorientation have been grown, with either

[1120] or [1010] as the rotation axis. In addition, vapor

grown, self-seeded whiskers are being enlarged to a size

sufficient for use as vapor grown seeds for subsequent bicrystal growth. Research-sized specimens of both monoand bicrystals have been supplied to Drs. Heuer and Cooper

at Case Western Reserve University for use in the ARPA

program on Mass Transport in Oxides.

Various minor modifications to the growth equipment

have been made with the intention of eliminating or minimizing possible sources of crystal contamination. As

reported previously, activation analysis has indicated the

chlorine content of the vapor grown crystals to be of the

order of 70 parts per million or less, Spectrochemical

analysis of crystals grown using ruby seeds and sapphire

seeds shows that cation impurities are at quite low levels,

as shown in the table.

Spectrochemical Analysis of A

^0- Crystals

Material Impurities Found, % by Weight

Ruby Seed Plate Cu, Mg, Mn, Si - less than 0.001

Fe - questionable

Vapor Grown Crystal Cu, Mg, Mn, Si - less than 0.001

sapphire seeded Pe - questionable

Vapor Grown Crystal, Cu, Mg, Mn, Si - less than 0.001

ruby seeded Fe - questionable

The level of these impurities is at the lower limits

of detection for the general qualitative spectrochemical

technique utilized. Because of this, it is impossible to

state whether any real differences exist among the samples

23
and seed. Discussions with personnel of the Analytical

Chemistry Division of NBS indicate that by the use of a

more sophisticated, semi-quantitative technique it may be

possible to make definitive analyses at the 1-2 ppm level

for copper and magnesium and at the 25 ppm level for silicon. The possibility of activation analysis for these

impurities at these levels is currently under consideration.

A paper entitled "Vapor Growth of AlpCU Bicrystals"

will be presented at the forthcoming ACCG-NBS Conference

on Crystal Growth and subsequently submitted for publication.

Plans for the next period include an increased effort

on both anion and cation impurity reduction in addition to

supplying specimens for physical property measurements by

other investigators.

3.2 Properties of Refractory Borides

3-2.1 Electronic Structure of Refractory Hard Metals

J. R. Cuthill, L. H. Bennett, G. C. Carter

A. J. McAlister and I. D. Weisman

Metallurgy Division

Institute for Materials Research

The objective of this project is to study changes in

the density of states upon compound formation, especially

in high temperature borides. The information should contribute to the understanding of the basic principles leading to the design of high temperature alloys with prescribed

properties. The class of compounds being considered in

this study are those binary and pseudo-binary , compounds

characterized by one of the constituents having an incomplete d or f shell and the other constituent having no d

or f electrons. The transition metal borides, aluminides,

and beryllides are examples.

24
Information is being obtained in the present study from

soft x-ray spectroscopy, nuclear magnetic resonance, Mossbauer spectroscopy, magnetic susceptibility, and other probes

of the electronic density of states.

The efforts during this report period could be divided

into three categories: (1) Preparation and Characterization

of Transition Metal Borides , (2) Physical Property Measurements of Transition Metal Borides, (3) Obscure Instrumental Corrections Peculiar to Investigations Involving Boron.

1. Preparation and Characterization of Transition Metal

Borides

Initial screening has indicated that specimens of

CrB„, VB„ and TiB„ which we have in rod form and the following compounds which we have in powder form are more or less

satisfactory for the measurements being made: Fe p B, ScB„,

TiB

2

, VB

2

, CrB

2

, VB, CrB, NiB . The nuclear magnetic resonance spectra proved to be particularly useful as a characterization tool, quite apart from its value in revealing information on the electron density of states of

these materials. One such instance was the mixed boride,

VCrBj. having the CrB„ structure but with vanadium atoms

substituting for Cr atoms on half of the chromium sites.

The particular sample appeared from the x-ray diffraction

pattern and from the metallography to be such a single

phase structure. It was revealed by the NMR spectra (fig.

1) and independently confirmed by quantitative x-ray

microanalysis that this particular specimen was in actuality

merely a mixture of VB and CrB„ particles sintered but with

little diffusion having taken place.

The B nuclear magnetic resonance spectrum from this

specimen is shown in figure 1. The 2 pairs of peaks

labelled "a" and "b" correspond to the quadrupole coupling

25
CmO + T3 •> <D CO g C\JP CD 3 PQ M P M > O -H t3

-P aM c O CD (UH Cti CD .C m CD a-p p CM w C cti pq hO CD CO >

<D

C e O «H •H

m

O

C .H

O Cti

p

cti cti CD

H

C 0) QO'C O > co a c

CO CD 3 o

ID in cti ?h

a

M T3 CO • N Cm cti 0) >j O ffi O^H •H S Q" £-t CD -P M O >

U3 CD

O t-\

C

H P • P

hO o -=rc o

cti P cti CQ X) CD S cti m m = a

Cti

O CO MrH ,C > T3

CtirH o c P*

CD

C CD Cti rH O C\J,Q CM o U PQ = pq 3 o M O Cti M S .Q O P = O

CDM

hO

•H

26
constants in VBp and CrBp respectively. The main boron

resonance peak is at the same location and of the same

width as it is in CrB . Only one B resonance is seen

11 because the B ' resonance in VB

? is at the same location

SI and is much weaker, so it is masked. The V resonance in

this sample showed values for quadrupole constant and Knight

shift that coincide with those of pure VB ?

. In the case of a specimen that was supposedly NbB Q

11 the B resonance was typical of a single boride phase.

The Nb resonance showed one resonance at the position of

Nb metal and another one shifted from this to higher field

(smaller Knight shift). This again Indicated the presence

of two phases, probably Nb metal and one of the niobium

borides. In addition a hydrogen resonance was seen in

this sample.

A last case in point here is our MnB

? sample of which

11 the B resonance shows unexpected and unexplained oscillations when the field is swept. These oscillations have

been found to be independent of the spectrometer frequency.

Although this might be a true property of MnB~, comparison

with the results in other diborides suggest this experiment

must be redone with a sample prepared with more certainty

to be MnB„.

These results clearly indicate the importance of

specimen characterization by a variety of independent techniques. The literature contains a large body of physical

as well as mechanical property data obtained from observations on specimens which have been inadequately characterized. As we have shown above, characterization by only

one or even two techniques, such as x-ray diffraction and

metallography is not sufficient, and published properties

stemming from such investigations are open to considerable

doubt

27
2

.

Physical Property Measurements

Knight shift and quadrupole coupling constant measurements have been continued. In the diborides our values are

in substantial agreement with values found in the literature,

where such data are available. The diborides studied include ScB TiB

2

, VB

2

, CrB

2

, MnB

2

, NbB

2

. The boron resonances shown a central line with a Knight shift between

-0.01 and -0.05$ with respect to triethyl borate. The V^ 1

resonance shows a negative Knight-shift with a negligible

temperature dependence down to 77°K. The Sc resonance in

ScBp, observed for the first time, shows strong second order

quadrupole effects, having a quadrupole coupling constant

of near 6.2 MHz

.

Preliminary measurements on transition metal monoborides have been undertaken by NMR. An extensive literature search revealed no previous work on monoborides.

Samples we have studied include NiB, VB, CrB, NbB, with

shifts ranging from -0.04 to -0.16%. The resonance lines

show additional structure in the wings which in a few cases

can be interpreted as quadrupole satellites. Two other

samples studied are Cr B and Cr,-B n .

3

.

Obscure Instrumental Corrections Peculiar to Investi - gations Involving Boron

Boron, or boron compounds occur in various components

normally used in laboratory equipment and produces signals

which must either be removed or corrected for. Often the

boron signal due to the apparatus is not suspected and

causes complications and uncertainities in da*ta reduction.

In the course of our work we have seen structure in

the central resonance which was determined to be the signal

of the boron in the NMR probe. This was done by directly

integrating the signal of the probe and sample in the add

28
mode of an on-line computer and subtracting out an equivalent number of scans of the probe signal with the sample

removed. The B resonance in VB gave the most clear

example of this interference. Others also report similar

spurious central structure in some diborides but failed to

account for its origin.

Another source of spurious boron signal in the NMR

spectra was found to be the boron in the glass dewars being

used. Also, boron was detected in soft x-ray spectra of

materials which should not have contained boron. A boron

nitride insulator which was not in the electron beam but

was heated moderately by the thermal radiation was found to

be responsible for the boron emission.

Papers Published During This Report Period

1. Resonance Studies in Ferromagnetic Fe

? B and Fe? Zr, I. W.

Weisman, L. J. Swart zendruber, and L. H. Bennett, Phys

Rev., 177 , 465-^71, 10 January 1969.

2. Magnetic Behavior of Intermetallic Compounds of

Beryllium, . N. M. Wolcott , R. L. Falge, Jr., and

L. H. Bennett, J. Appl. Phys. 40, 1377-1378, 1 March

1969.

3.3 HIGH TEMPERATURE METALS

3.3.1 Optical Constants of Titanium in

The Visible Region of Spectrum

A. J. Melmed

Metallurgy Division

Institute for Materials Research

Efforts during the period of this report were directed

1) towards building an ultrahigh vacuum system and a tube

for making optical constants measurements on titanium characterized in situ as clean by low-energy-electron diffraction

29
techniques, and 2) towards establishing techniques for

meaningful measurements of optical constants using vapordeposited titanium films.

The ultrahigh vacuum system has been assembled and is

being tested. The experimental tube is about 90% completed.

Special care was taken in its design and construction to

ensure that the fused-quartz windows for entry and exit of

polarized light were strain-free and accurately aligned for

proper ellipsometric measurements. These important considerations are frequently neglected and can cause serious errors

in optical constants measurements, especially in vacuum.

The windows in the new tube were designed so that they could

be completely oven-annealed and were tested with our ellipsometer. No evidence of strain was detected.

We began a series of experiments with evaporated metal

films . Before working with titanium films we tried iron

films, since reliable modern measurements have been reported

for the optical constants of bulk iron cleaned and measured

in ultrahigh vacuum. Thus a good basis for comparison of

results was available. This was felt necessary in order

to establish that measurements made on evaporated films

gave optical constants for the bulk metal. We found that

iron films vapor-deposited on a previously cleaned singlecrystal (Oll)-oriented tungsten substrate exhibited optical

constants (in the visible spectral range) which changed with

film thickness and reached limiting values in good agreement

(M-l#) with the most recently published bulk iron optical

constants provided the films were briefly (^10 sec.) annealed

The thickness necessary for this agreement was about 300-

o 400A, determined by ellipsometry . The film formation process

was also followed by LEED on the same specimen and by

tungsten field-electron emitter located approximately at

30
the same distance from the metal vapor source. These techniques showed that the as-deposited film was very granular

o with grain sizes of the order of ^200A, and had very little

long-range order. After annealing (temperature <600°C) longrange crystallographic order was apparent and the film was

epitaxially oriented, with the (Oil) planes parallel to

the (Oil) planes of the substrate and with a measured lattice constant agreeing within ^1% with the accepted value.

We have therefore begun experiments of a similar type

with titanium. To date, the ellipsometry data looks normal,

but there appears to be a complication in interpreting the

LE'ED results. We are presently analyzing the data.

3.3.2 High Temperature Creep in Metals

A. A. 0. Rukwied, W. A. Willard, D. E. Harne

Metallurgy Division

Institute for Materials Research

The objectives of this investigation are to investigate

the microscopic mechanisms that control high temperature

creep, i.e., creep occurring at temperature T>T /2, where

T is the melting point. More specifically it is planned

to study: (a) the effect of temperature on the creep rate

and compare this to the temperature dependence of dislocation velocity, (b) the effect of stress and compare this

to measurement of dislocation density carried out by etch

pit techniques, (c) study the effect of stacking fault

energy through its variation in an alloy series and (d)

study the effect of grain boundaries by comparison of

studies in single crystals and polycrystals . For these

studies copper and copper-alloys have been selected to begin with especially since etch pit techniques for these

materials are already developed.

The work on polycrystalline high purity copper which

was the subject of previous progress reports has been

31
continued by obtaining and measuring high temperature creep

curves as a function of temperature at two more stress

levels. All the creep data obtained so far have been

evaluated and analyzed. They will be presented here in a

preliminary form.

The apparent activation energies derived from measurements of the second stage creep rates turned out to be

practically constant over the whole range of temperature

covered, and with the values U = ( 176+6 . 3 )kJ/mol , or

(42+1.5) kcal/mol, were about 10 to 20$ smaller than the

activation energy for self diffusion which may be assumed

to be somewhere between 197 kJ/mol, or 47.1 kcal/mol,

(Kuper et. al. Phys . Rev., 96 (1954), p. 1224) and the

latest measurement of 211 J/mol, or 50.5 kcal/mol (Rothman

et. al., APS Meeting, Chicago, March 1969).

This finding differs in two major points from the

measurements reported by Barrett and Sherby in the literature :(1) Within a temperature interval of 260°C the apparent

activation energy for high temperature creep was found to

be constant, demonstrating that one single rate controlling

mechanism operates over the entire temperature interval.

This interval includes the temperature of transition of the

apparent activation energy from a lower value (117 kJ/mol,

or 28 kcal/mol) to the one for self diffusion (201 kJ/mol,

or 48 kcal/mol), as reported by Barrett and Sherby.

(2) The value of the apparent activation energy agrees

neither with the lower, nor with the higher, value given by

Barrett and Sherby, but is closer to the higher value, the

one for self diffusion.

An explanation for this discrepancy necessitates a check of various factors, such as purity, test procedures,

etc., which might affect creep properties. A comparison

of some of these factors is given in table 1.

32
a

in O

hO

•H

-PO

<M

Cm<

COMOPO

cd

CmOCo

CO •HM

Cdgoo

CD rH

,Q

cd Eh

ShO

t3Paw

CD

Sh ph

>3M

oo

xsa

cd pp

CUmM

cd CQ

c\

ffi • M H

C

>i o "CO

O UP •• c H Cd -H ft-H C < g p O m -H « a M cd o 3 cd c\ S a c o

ap S a «h -h

>>

C ftH c <m P fe^ o g • -H »H

•H crs o a i-HO cd X5

m cr\

a P «H • 3C^ o O « C rH CD CO Pm • P c\J hOO O £h CD 1 c^ •»§ o co 3 >

•H 0\ Tl •» o P •H

ffi CD C\ •»H

M o CO

H

N

O 6 g cd cd 3 cd o cd >s a a-H h M M O C r-\ g a a m 3 P X3 K-H n! a r-i CO cd < s c a IC\ r-\ P rH £1 co O cd • • cd CD 3 o < C^rvj m o § o CO s

oo

M S *

•< cd ^.

a c\j

u

p lh as

CD P CTN ^-< a CD CT\CM g CM a^3 • ao o ctn •> a U >iCT\ hO g p < c\j a o -h c a

ffi

m cd

g «

Ph ^ ,c!

a cd o o ap ao c\j

cd •HM

CD p

cd s

bD

m

C

CD -H

rC a>H p cd cd •h a CD s c

TJ

M cd

CD CD X g CD

CO CD

m

•HO

rH

O Cm

O \ (D Ph CM ,Q o I -H C P

CD cd g M

•H cd o a a m

co

a

CM

tJ

cd

CD

O

ffi rH

Tj P

CD

C

•H cd <m p H CO M C 3 O

P-. o

to -d to

CD

g CD

N 3 M

•H 3 p

T3 o CO •h cd M > p O C

CD

m cd T3 O p

CO >j « C M CM O Q ffi O

CDM

CD x;

M

a

C

CO •H o -d g cd p o < Ml

oo -=r

CO

CD

CO

CO

CDMp

CO

cd

tO >H a toa oo o o o m o w OO

ooo-=r o o H

COaoo

LTi

O

d• c

r-\ Cd d o C o

Cd LO.

CO

CD

t\l •H

toM

•H

cdu

C5

LH c

•H

cdM

hO

co

Ih

vo

rH\oooo

CXD to

CD

tsi •H

COC

•H

cdM

hO

N

•H

CO

•Hg

CH6o

oo o o O OOo o o o o • o

I>- OCX3

C -H

•H

g

cdM

O

hOVO hD \g o g go g o

-=3-

O

O • o • OHH

T3C

cd hDP

N c c •H

•H CO rH g

cd P C

cd •H

C cd M M M < P hD

>5co

to p

CO p

rH

cdgM

-CPo

to hO H C

•H

-d

rH i -H O >3

CO •H CO P d to C

cd P

hOH C cd •H g

<-<

M

O

>)£ O P Co

•HP

cdC

•HgMP

T3I O

COM

rH

cd>

CmOPgM

hO

cd

VD

33
If we discount the constant load type test as being

responsible for the observed discrepancy*, we have to

attribute responsibility to structural factors in our

material other than grain size and dislocation density.

Therefore an extensive study was undertaken using optical

and electron microscopy correlated with etch pit experiments, to better characterize the test material. As a

result, a cellular solidification substructure was found,

due to segregation of impurities during continuous casting

in a constitutionally supercooled melt.

On an electropolished and etched surface, the region

of the crystal to which segregation has occurred is distinguished by clusters and rows of etch pits, the result

of a preferred attack by the etchant. Within this region

cells of densely tangled dislocations have been observed.

After cold work, recrystallization and prolonged annealing

treatment at relatively high temperature (800°C/20 hrs),

the solidification substructure can still be revealed by

etch pitting. However, small particles in the segregated

region could not be revealed unambiguously, although at

least solid solubility measurements of carbon in copper

strongly suggest the formation of carbon particles during

solidification. Since carbon can be in solid solution in

equilibrium at the melting point of copper at a concentration of 7 ppm only, and the overall carbon concentration

in our material is 3.5 ppm, precipitation of carbon must

* One can make an estimate on the effect of testing under

constant load as compared with constant stress. Using

the relation e^o k * 8 it can be shown, that constant load

tests measure activation energies not more than 6.3

kJ/mol, or 1.5 kcal/mol, greater than constant stress

tests

.

3

4
occur due to the rise of carbon concentration in the

region in which segregation does occur. Similar considerations should hold for the other possible contaminants in

the test material, like oxides of Cu, Al, Mg, Si and

silicon carbide which are present in trace concentrations.

Thus, since bigger particles are not observed, their size

o must be smaller than 5 to 8 nm (50 to 80A) and the particles

are probably finely dispersed in the solidification substructure. The thermal stability and low mobility of the

solidification substructure up to the melting point of

copper explain the mechanical and thermal stability of the

solidification substructure.

In summary we can state the following points which are

relative to the copper used for the creep tests:

(1) After the annealing treatment the main fraction

of the impurities are still in the substructure arising

from cellular solidification.

(2) The impurities contained in the test material

have very low or no mobility at the creep test temperatures.

(3) We have to expect the existence of small second

phase particles in the growth substructure, but since they

could not be seen directly, the particle size probably is

o less than 5 to 8 nm, or 50 to 80A.

(4) The regions of the crystal bounded by the segregation substructure probably has a high degree of purity

and perfection.

(5) The dislocation density is low.

(6) So far - and this is to be taken as a very

preliminary statement - no interaction has been observed

to have taken place between the remainders of the cellular

solidification structure and the dislocation cells formed

during creep.

35
Returning to the observed apparent activation energy

for second stage creep, Barrett and Sherby argue that the

lower apparent activation energy level in copper is due

to dislocation-enhanced diffusion of vacancies. Following

Hart (Acta Met., 5 (1957), p. 597) they represent the macroscopically observed effective self diffusion coefficient as

a volume weighted sum of dislocation and lattice diffusivity

Deff " D

d

<*> + D

e

(1 -6>>

D, - dislocation diffusivity

D - lattice diffusivity

e

J g - lattice site fraction in

high diffusivity region

around dislocations

indicating that, depending upon the temperature, D , , D , or both can determine the effective diffusion coefficient,

and should therefore manifest themselves in the temperature dependence of the apparent activation energy for high

temperature creep.

If we want to explain the value of the apparent activation energy measured by us in such a model of enhanced

diffusivity, we have to find a diffusion path that is easier

than self diffusion, and faster than dislocation enhanced

diffusion within the temperature range in question. We

would like to suggest that such a diffusion path could be

provided by the segregation substructure, which interconnects the entire material by a regularly spaced channel

system, consisting of "columns" and walls distorted by a relatively high solute concentration, which might be of the

order of 1% , and very probably by the presence of second

phase particles of a size not resolvable by the electron

microscope, in the segregation region. In such a model a transition to another rate controlling process of steady

36
state creep characterized by the self-diffusion activation

energy may be expected at a higher temperature, and a

transition to a lower apparent activation energy due to

dislocation-enhanced diffusion at lower temperatures,

cannot be excluded. The low dislocation density, since it

enters the preexponential term of the diffusivity, could

probably help to shift downwards the temperature of transition to creep controlled by dislocation-enhanced diffusion

As a consequence of the assumption that the diffusivity in

the segregated region controls second stage creep, one

should expect the rate controlling creep process to take

place within the region containing the segregation substructure. We are working on that problem presently, but

have not yet obtained conclusive evidence.

In conclusion it should be emphasized, that in high

purity fee materials a very careful characterization with

respect to impurity content and distribution is imperative

for high temperature . creep tests. It is obvious that more

tests varying these parameters are needed to clarify their

roles in high temperature creep.

On the definitions of creep stages I and II : Data being obtained in creep tests indicate that a redefinition may be in order for steady state or minimum

creep rates. Consideration is being given to the best way

to redefine these stages.

SUMMARY : (1) High temperature creep tests have been carried

out on high purity copper in a high purity He atmosphere.

The test material contained a cellular solidification substructure . (2) The starting material for the creep tests is

being characterized using optical and electron microscopy.

37
(3) The apparent activation energy for steady state

creep has been measured from isothermal tests in the temperature range between 440 and 702°C. U has been found to be

constant over the entire temperature range and with a value

of 176 kJ/mol, or 42 kcal/mol, to be about 10 to 20$ smaller

than the activation energy for self diffusion. This value

is probably somewhat higher than a value one would derive

from creep tests carried out at constant stress.

(4) It has been suggested that this value of U is

due to lattice distortion-enhanced diffusion along the

cellular solidification substructure.

(5) Consideration is being given to a redefinition

of creep stages I and II.

The constant resolved shear stress creep testing unit

was finally accepted from SATEC Systems, Inc. No time

however has been available since to take it into operation

and to do research work, because the study on the copper

polycrystals had to be brought to a point from which a

publication of results obtained so far, is within reach.

3.4 CERAMICS

3.4.1 Deformation and Fracture of Ionic Crystals

S. M. Wiederhorn and L. H. Bolz

Inorganic Materials Division

Institute for Materials Research

Research on this project has been directed towards

understanding brittle failure of ceramic materials. Glass

has been chosen as an object of study because of its

commercial importance and because it serves as a model

material for failure of other high temperature ceramic

materials

.

Stress corrosion cracking data has been obtained to

elucidate the failure mechanisms in a variety of glasses.

38
In the last report it was stated that crack velocity data,

obtained in liquid water as a function of temperature,

satisfied the Charles-Hillig theory for stress corrosion.

Experimental data has been found to fit an equation of the

form v = v expf-(E*-bK T )/RT], where v is the velocity, v

is a pre-exponential constant, E* an activation energy,

b a constant and K

T

the stress intensity factor. During

this period the data has been analyzed and it is believed

that the rate limiting step in the corrosion reaction

involves a direct attack of the hydroxyl radicals on the

silicon oxygen network. This conclusion is based on the

following experimental evidence:

1. The pre-exponential of the above equation is

directly proportional to the [0H~~] of glasses

ground up in limited amounts of water.

2. The activation energy, after a correction for

the [OH ] temperature dependence, is approximately 92 J/mol, or 22 kcal/mol for all glasses

tested, and is close to that obtained for- known

OH attack on glass.

Some data is still being collected on the stress

corrosion process. When finished a paper will be submitted for publication in the Journal of the American

Ceramic Society.

Papers Published During This Report Period

1. S. M. Wiederhorn, "Fracture Surface Energy of Glass",

J. Am. Ceramic Society, 52 (2) 99-105 (1969).

2. S. M. Wiederhorn, "Fracture of Ceramics" from Mechanical & Thermal Properties of Ceramics, Editor, J. B.

Wachtman, Jr.; NBS Special Publication - 303, May 1969

39
3 . 5 POLYMERS

3.5.1 The Volatilization and Decomposition of Materials

L. Wall and S. Straus

Polymers Division

Institute for Materials Research

The rates of vaporization of five aromatic compounds

were measured as a function of temperature and their activation energies obtained. In terms of kinetic theory for

vaporization the activation energy E for vaporization is

related to the heat of vaporization at the temperature of

the measurement, by the relation,

E = AH„ -

3 RT V ^

Table 1 now includes all the phenylene ether type compounds

we have available for study. The fifth compound listed

which was previously reported had not behaved like a pure

compound.

This 7-unit meta linked phenylene ether compound,

m-bis[m-(m-phenoxy phenoxy )phenoxy ] benzene, was purified

by collecting the middle fraction on distillation. This

fraction, when heated in vacuum at 211, 220, 227, and 233°C,

vaporized at a constant rate independent of the amount

vaporized, indicating more clearly a volatilization of a pure compound. Previously the rate had decreased rather

than remaining constant as vaporization proceeded. The

activation energy, for the purified sample, was calculated

to be 134 J/mol, or 32 kcal/mol, similar to the previous

value for the extrapolated slopes.

The results in table 1 show that for the first 5 compounds, which are a series of meta-linked phenylene ethers,

the energy increment per unit decreases above the 5 unit

species with the larger compounds. This is evidence that

40
Table 1. Experimental Activation Energies for

Molecular Vaporization

Compound Formula

v M.W. kcal/mol

m-diphenoxy benzene H4C

6

H

4

0^

2

C

6

H

5

262.32 15

bis(m-phenoxy phenyl)ether H^-CgH^O-^CgH 354.41 20

m-bis(m-phenoxy phenoxy) H-^CgH^O^CgH 446.00 25

bis[m(m-phenoxy phenoxy)

phenyl Jether

o-Q

o

/V // N\

H4C

6

H

4

0^

5

C

6

H

5

538.60 29

4l
Table 1. Continued

Compound

m-bis[m(m-phenoxy phenoxy)

phenoxy] benzene

(pP

4-phenyl phenyl ether

(para)

2-phenyl phenyl ether

(ortho

)

/^

p-bis (o-phenoxy phenoxy)

benzene

m-(m-phenoxy phenoxy)

phenyl-p-phenoxy phenyl

ether

Formula

C

6

H

5

C

6

H

4

OC

6

H

5

C

6

H

5

C

6

H

4

OC

6

H

5

H4C

6

H

4

04

4

C

6

H

5

E.

v

M.W. kcal/mol

H4C

6

H

4

0^

6

C

6

H 630.70 32

246.31 23

246.31 16

HfC 6

H

4

C4

4

C

6

H 446.50 25

446.50 24

42
the larger species can evaporate with a dense conformation minimizing the energy requirement. Work is continuing with purification, preparation and activation

energy measurements of various linear and non-linear ester

structures

3.5.2 Relaxations Near the Melting Point

J. P. Colson and R. K. Eby

Polymers Division

Institute for Materials Research

If the dynamic mechanical properties of mats of

solution-grown crystals of polyethylene are measured with

a torsion pendulum, the strength of the a (higher temperature) relaxation depends upon the temperature at which

the crystals have been annealed. Furthermore, the magnitude of the relaxation correlates with the reciprocal of

the thickness of the crystals. This has been interpreted

as meaning that the origin of the relaxation is in the

crystal surface. However, we have previously shown that

the area of the basal (a, b) plane of the unit cell also

correlates with the strength of the a relaxation. We have

now found yet another correlation between the a relaxation

and one of the parameters necessary to describe completely

the crystal mats. This parameter is the small residual

solvent left in the mats from the preparation process. As

the crystals are annealed at higher temperatures, the solvent content decreases in a fashion which corresponds with

the decrease of the strength of the a relaxation. We are

not prepared at present to interpret the correlations of

the solvent content and the cell dimensions with the

strength of the a relaxation. However, they do show that

there is some question about the significance of the correlation with the reciprocal of the crystal thickness since

this correlation is not unique. Clearly there is a need

for more research to determine the mechanism which leads to

these correlations. 43
3.6. LASER MATERIALS

3.6.1. BULK OPTICAL PROPERTIES OP LASER MATERIALS

G. W. Cleek and R. M. Waxier

Inorganic Materials Division

Institute for Materials Research

Objectives

This project was established with the objective of evaluating the bulk optical properties of laser crystals and

glasses, particularly synthetic single crystal ruby and

neodymium-doped glass. These data are needed to calculate

the corrections for the distortion of the wave front in

light generated by lasers. The distortion is produced by

thermal effects when the rods are operated.

Specifically, determinations are being made of the

appropriate thermo-optic and elasto-optic properties required

for the above calculations, as well as other properties

needed to characterize these materials.

Technical Approach

Measurements are being made on commercially available

ruby and neodymium-doped glasses. The single crystal ruby

was doped with 0.05$ Cr. Five specimens of neodymium-doped

glasses have been received. These are identified as samples

A, B, C, D, and E. The properties that have been determined

during this reporting period on each of the specimens are

discussed below.

Property Measurements

Photoelastic Constants

(a) Ruby

Since the last semi-annual report, the two remaining

piezo-optic constants, q, j, and q^, have been measured for

single-crystal ruby. The entire set of eight q. . has now

been determined and the results are given in table 1. A

check of calculations showed that some values given in the

4'i|
Table 1. The photoelastic constants of synthetic

ruby doped with 0.05% Cr.

piezo-optic constants

q ll q 33 q 12 q l3 q l4 q 31 q 4l q 44

xlO 7 bar" 1

-0.52 -.41 .08 .13 -.07 .01 -.01 -.71

elasto-optic constants

p ll p 33 p12 p 13 Pl4 p 31 P4l p 44

-0.23 -.20 -.03 .02 .00 -.04 .01 -.10

last semi-annual report were in error; the correct values

are given in table 1.

Using the data of Wachtman [1] for values of the

elastic moduli, c the eight elasto-optic constants, P..,

have been evaluated according to the equation

6

p

ij =Z<Iik c kj (1)

k=l

and the results are also given in table 1.

The phenomenological theory of photoelasticity developed

by P. Pockels [2,3] assumes that the elastic deformations and

not the stresses are primarily responsible for the changes

in refractive index. It is interesting to note that for ruby

(table 1) P-,-,3 Poo> P-ip and Po-i a H show negative values. Of

all the crystals topaz and diamond and the partly covalent

crystal MgO, exhibit negative values for p., (h,k = 1,2,3).

In topaz it is found that p-.-. 9 p ?? and p^^ a^e all negative

in value. MgO has negative values for both p,., and p,„ 3

45
while diamond has a negative value for p, , only. All these

crystals are noted for having strong interatomic bonding.

Also, it may be seen in table 1 that p^ is of the same

order of magnitude as the other constants. This is typical

of covalent crystals where it is found that, in general,

Poo (^ = ^j5,6) are of the same order of magnitude as p.,

(h,k = 1,2,3). This is not the case in ionic crystals

where the p op are much smaller than the p , [31.

Mueller [4,3] has developed a physical theory to explain

the changes in the refractive index ellipsoid that take

place when a solid is stressed. In this theory, calculations are made of the changes in the density, the Coulomb

field, the Lorentz-Lorenz field, and the intrinsic polarizability of the scattering centers. The contribution of

density is always positive and in most crystals exceeds the

combined effect of the other three changes so that the p.,

are generally positive. The theory has been worked out for

glasses and cubic crystals, but not for crystals of lower

symmetry such as ruby (crystal class 3m) where the computations become extremely complicated. Although the individual

contributions can not be calculated out for ruby, it may be

noted that the combination of the latter three changes outweighs the contribution due to change in density and, with

the exception of p.-, the p of ruby have negative values.

Even with p.-, the positive numerical value is very small

(p 13

= .01). . The photoelastic constants of calcite have been determined by Pockels [5 3 3], and it is interesting to compare

these data with the results on ruby because both crystals

belong to class 3m. In both crystals the oxygen atoms are

arranged in triangular groups perpendicular to the optic

axis. However calcite has a true layer lattice, whereas

46
AlpO- possesses an isosthenic lattice in which the oxygen

atoms are almost in hexagonal close packing. The elastic

compliances of calcite are much greater than those of ruby

[3]. This pronounced difference in the elastic compliances

manifests itself in the photoelastic constants where it is

found that the p, , of calcite are all positive, and the

p , of ruby are generally negative.

(b) Neodymium-doped laser glasses

The two piezo-optic constants, q, , and q-,p S and two

elasto-optic constants, p.,-, and P-,p, have been determined

for the five neodymium-doped glasses, and the results are

given in table 2. It can be seen in table 2 that the values

Table 2. The photoelastic constants of neodymium

doped laser glasses

7 7 7

GLASS (q 12 -q 11 )xl0' q^xlO' q 12 xl0' p 11 p 12

A

B

C

D

E

for the various glasses are largely the same, with the

exception of the sample E glass where the values are markedly different. This difference may be attributed to the

fact that the composition of the sample E glass differs

notably from the other glasses as shown in table 3. It may

be noted that the optical measurements on the samples B

, -1

bar . -1 bar . -1 bar

1.64 0.53 2.16 0,134 0.225

1.59 .58 2.18 .139 .222

1.58 .78 2.36 .153 .232

1.49 .54 2.04 .139 .218

1.09 .12 1.21 .105 .184

47
>10 >10 60.31 >10

0.1-1 0.1-1 1.91 1-10

1-10 1-10 3.02

1-10 1-10 17.1^

1-10 1-10 5.88

.93 1-10

0.1-1 1-10 0.73

1-10 1-10

1.82

1-10 1-10 5.81 1-10

0.01-.1 0.01-.1 1-10

Table 3- Major constituents in wt

.

% in Nd-doped glasses

as determined by chemical analysis

Oxide A B* C D E*

Si0

2

66.56

A1

2

1.80

BaO 5.46

K

2

10.06

Na

2

6.56

Li

2

1.03

Sb

2 3

0.73

PbO

ZnO 1.57

Nd

2 3

5.35

CaO

*Spectrochemical analysis only

and C glasses were practically identical. The differences

in the q. . and p. . which are reported arise from differences

in the elastic moduli which data are needed in the calculations of the photoelastic constants.

Thermo-Optic Data

An optical interference method has been used to determine the change in refractive index with temperature from

25°C to 300°C for three neodymium-doped glasses. These are

the samples B, C, and D. The results are shown in figures

1, 2, and 3 respectively. It can be seen in figures 1 and

2 that each of the samples B and C show a minimum in the

temperature-refractive index curve. In the case of sample

-4 ' B the greatest change is only about 1 x 10 and the refractive index at 300°C is almost the same as at room tempera-

-4

ture. With sample C, there is a change of almost 4 x 10

from the initial value to the minimum, and the value at

300°C is lower than the room temperature value by about 2.5

48
-4

x 10 . The sample D glass shows a continuous decrease

refractive index, the overall change being about 4 x 10

The shape of the curve indicates that there is probably a minimum somewhere above 300°C.

Thermal Expansion

The linear thermal expansion of samples B, C, and D has

been measured, and the results are shown in figure 4. The

curves for the samples B and C almost coincide, and, for the

temperature range 0°C to 300°C, a = 10.65 x 10" for either

glass. The sample D glass exhibits a somewhat steeper curve,

and, for the same temperature range, a = 11.88 x 10

Density

The densities of two glass samples were determined by

the buoyancy method using water as the immersion liquid.

The values found were:

Glass Sample Density, g/cm

D 2.6206

P 2.5^90

Elastic Constants

The elastic constants of two glass samples were determined by George Dickson, Dental Research Section, using a pulse echo technique. The values were:

Sample D Sample E

Young's Modulus, E, Newtons/m2 6.58 x 10 10 9-08 x 10 10

Shear Modulus, G " 2.66 x 10 10 3.62 x 10 10

Bulk Modulus, K " 4.16 x 10 10 6.14 x 10 10

Poisson's Ratio, y 0.236 0.255

Thermal Conductivity

Values for thermal conductivity of sample E glass at

and 100°C were given in our last report to the sponsor

covering the period July 1 - December 31, 1968. Since there

appeared to be a discrepancy between our results at 0°C and

those of others on this laser glass, a second set of

measurements was made. The results of both sets were:

4 9
Temperature, Thermal Conductivity

Set °C Wm- 1 °C- 1

1 5.91 1.21

6

1 104.78 1.35

2

2 5-95 l-20

3

2 45.47 l-28

3

The two measurements near 6°C differ by about 1.1$, well

within the expected error of about two percent. To obtain

smoothed values at 0, 50, and 100°C, the two points near

6°C were averaged, and a parabola was fitted to this point

and the two points near 45 and 105°C. The results for the

smoothed thermal conductivity values are:

Thermal Conductivity

Temperatures, °C Wm"* lo C1

1.19

7

50 1.29

100 1 - 34

8

References

[1] J. B. Wachtman, Jr., W. E. Tefft, D. G. Lam, Jr., and

R. P. Stinchfield, J. Res. NBS 64A (Phys. and Chem.

)

No. 3, 213 (I960).

[2] F. Pockels, Wied. Ann. _37, 151 (1889): Lehrbuch der

Kristalloptik (B. G. Teubner, Leipzig and Berlin,

1906).

[3] R. S. Krishnan, Progress in Crystal Physics, Volume I

(Interscience Publishers, New York, London, 1958).

[4] H. Mueller, Phys. Rev. 47_, 947 (1935).

[5] F. Pockels, Ann. Phys. (Leipzig), 11, 726 (1903).

3.6.2. CRYSTAL DEFECT STUDIES ON LASER MATERIALS

R. F. Blunt, T. Chang, and T. Tsang

Inorganic Materials Division

Institute for Materials Research

Objectives

This investigation is a study of the causes responsible

for degradation of performance of solid state lasers. The

50
effort has been redirected to consist of a study of "orange"

degradation of laser ruby, by EPR and optical means.

Technical Approach

Experimental measurements consist mainly of optical

absorption spectra and electron paramagnetic resonance on a variety of "ruby" samples. The following outline is being

used:

1. Detailed spectra taken on undamaged "laser quality"

ruby is to be compared with corresponding spectra on

"orange" degraded samples.

2. Spectra will be examined for evidence of trace

impurities, and, in the case of "orange" ruby, typical EPR

crystal defect spectra that correlates with the orange coloring bands. It is noted that Stickley et al [1] suggests

that the discoloration in laser rods may arise from color

centers created by the pumping light at defects initially

present in the crystals.

3. Special attention will be given to the detection of

Cr and/or Cr ions that may exist in the orange samples.

4. A number of "doped" sapphire crystals are grown and

studied. These include: rubies covering a wide range of

Cr concentration; crystals containing single metal impurities;

and several samples of "double doping" with Cr and one other

metal.

5. Most crystals are colored by radiation with 50 KV

x-rays and the resultant damage studied.

Experimental Results

Most of the samples employed were cut from Verneuil

crystals grown in this laboratory. Metallic dopant impurity

concentrations quoted are either nominal "starting material"

values, or, as in the case of Cr, determined optically by

the optical absorption method described by Dodd, Wood,

and Barnes [2].

51
Ruby samples of various Cr concentrations were prepared,

the optical absorption was measured, and then the crystals

were colored by 50 KV x-rays and re-measured. The added

coloration is seen to reach a maximum or saturation value

with increasing irradiation.

Figure 1 is a plot of a typical sample of 0.076 mol %

Cr 9 q ruby. The top curves are the well known polarized

spectra taken before irradiation. The bottom curves are

plots of the added absorption at saturation, all plotted vs.

wavelength. A simple reflection loss correction is made and

the ordinate plotted as absorbance per cm sample thickness.

On the bottom is the added x-ray coloration produced in a "pure" sapphire prepared from the same A£„0~ powder used

to grow the rubies.

A piece of commercial laser quality pink ruby was similarly examined and yielded similar data. The orange coloration produced by the irradiation consists of a broad band

peaking near 0.46um, and a very strong band (or bands) at

shorter wavelengths. Our principle attention has been

focused, so far, on the 0.46um band.

Data taken on a large number of rubies verify that the

coloration saturates with irradiation in a manner analogous

to the familiar color center growth curves. There is also

some evidence that the coloration exhibits a maximum when

plotted as a function of Cr. concentration. These results

have been reported by Novothy and Spurny [3] s while coloration bands shown in figure 1 have been observed by a number

of authors

.

A brief search was made for possible impurity involvement in the coloration, by doping "pure" sapphire with a

number of single metallic impurities. Included were V, Mn,

and Ti , all presumably substitutional as trivalent ions.

The absorption spectra were similar to that reported by

52
1 1 //

1 1 1 / / / /

/ /

/ /

/ / ^' / u, \

/

\ I // 1 // 1 \ \ '/ 1 \ \ / /

\ \ / /

\ \ / /

\ \ / / 1 — M~ / / t" \| f / '

' / !

fl u = ' // t \ // * \ i u y7 \ \ i ^^ S/ * \ ' UI W \ \

/ * — * *

\ \ /

i \

w N \ i »>^ \ "^ \ \ 1 NA \ >\^

— y _i / ^r^ IO 1 / ^•"^ o 1 / ^t^"^ ** 1 / ^-"^^*"^ < . l^^" /

1 . III/

E

c\i

to^ or

I O

• +>

>i tO 3 T3 U tO

fn t* Sh tH

(1)

E O

o o

•H

<M p

O D.U

C O

o to •H £1 P tO

too o. •H o

PE-t a

o

•a a>

<u u

o =S

T3 = cH CH

co c

•H O P -H q.p

u o. O Sh

to o o to

tO JD

to

(D T3

TJ) <V

tO TJ P

tO K

<U

0) 0) > > C o o to

t, B -H

11 on JJ -p -p C -P 0) moo o m -^

(sr^-fun ^aBa^-fqJV) ^fsuaa T^OTldO

53
McClure. [4] Irradiation produced added coloration, and

samples were found to damage more easily than the undoped

sapphire, but the absorption bands were observed only in

the ultraviolet.

Double doped samples were studied in which single

metals were added to ruby. One example, Mg + Cr, is of

2+ interest. Magnesium, presumably present as Mg , could

3+ 4+ possibly result in either oxidation of Cr to Cr or

oxygen vacancies or perhaps even both. These crystals were

orange and optically resemble irradiated ruby.

Three sapphire crystals were grown to study the effect

of Mg in greater detail. One crystal was doped with 0.025

mol % MgO and was colorless. The second crystal doped with

0.003 mol % Cr

?

0~ yielded the expected Cr spectra. The

third crystal was doped with both Mg and Cr in like amounts.

Figure 2 summarizes the optical measurements on the

second and third Crystals. The bottom curve is the Cr doped

crystal; the middle curve the double doped crystal as

initially grown; and the top curve is the double doped

crystal after irradiation.

The coloration resulting from Mg doping appears to be

similar to that produced by irradiation without the magnesium. The limited additional effect of irradiation in

4+ the double doped sample suggests a Cr model in which case

essentially all of the Cr was oxidized. An obvious difficulty with this is the apparent requirement of a very large

4+ oscillator strength for the presumed maximum possible Cr

concentration. Thus a trapped hole or electron, i.e., color

center model would be more desirable in this respect.

Electron paramagnetic resonance measurements have been

made on most of the ruby crystals that were studied optically. Comparison pieces cut from the same crystals were

used. Measurements were made before and after irradiation.

54
E

=1

a>u

o

<M

CD £1

>> £> M 3 £ U

. o 13 >> c 0) XI ft 3 O

Eh ^a

£1 1 0 3 bO

(D u s

fto %«. C

1 <D •H c

bO>H c o S o O -H B •H 4-> C« P CO O on ft-H o u 73 ^-^ CO O CO -P O co U X

•H £1 U CD •P * cd •H -p ft CD U > •* u c

O h CO <D •H

co 3 <U •P £1 o > <M co CO U CO rH B 3 •H

rH o o a CO CO .p c P

O .p ft CO 0) •H o O o

.p CQ En —

ft O

CM

CDU

3M

ft,

«n

(s^fun ^JBJ^Tqav) ^Tsusa TBOf^do

55
Figure 3. The EPR line of Cr

Large line -1/2

Small line -1/2

3+ in ruby

+ 3/2 transition

+ 1/2 forbidden transition

56
It was hoped that extra lines or obvious readily recog3+ nizable changes in normal Cr spectra would be found and

correlated with the accompanying optical data. However,

3+ only typical Cr spectra have been observed to date. A

.general systematic search for extra lines attributable to

foreign paramagnetic impurities or to trapped holes or

electrons present in paramagnetic configuration was made.

Special attention was paid to the region where Hoskins and

Soffer [5] report a line in orange ruby that they attribute

to Cr

The sensitivity of the apparatus as used on the magnesium doped ruby of figure 2 is illustrated in figure 3,

where two of the lines are seen. This sample could, of

4+

course, have Cr ions even if the coloration were due to

some other mechanism. The spectrometer operates at X-band

frequencies, and the sample was at 4.2K. The magnetic field

was approximately parallel to the C-axis of the ruby. From

the measured values of B, and the calculations of du Bois [6],

the large line is the -1/2 < >+3/2 transition near B + 0.3^

Tesla with a g ff + 1.9955 and a line width of about 2 x 10

Tesla. The small line is from the forbidden -1/2 <—> 1/2

transition near B + 0.39 Tesla, which is seen by virtue of a slight crystal misalignment. The curve shown is a copy of

the recorder chart paper and illustrates the signal to noise

ratio available during the search for new lines.

The apparent lack of obvious paramagnetic "defects" in

the orange rubies is rather surprising, as is the lack of

3+

any observable change in the Cr spectra after coloration.

However, efforts to observe either or both of these are continuing. Optical pumping on samples at low temperature will

be further explored.

A new optical EPR cavity is under construction that

will permit non-destructive resonance studies on laser rods

down to very low temperatures.

57
The optical studies on x-ray irradiated rubies having

special dopings will be continued. The evidence presented

by Stickley [1] that the optically pumped coloration is

different from x-ray coloration would make mandatory the

study of samples colored by optical pumping.

This will be done when rubies that are easily damaged

by "pumping" light become available.

References

LI] C. M. Stickley, H. Miller, E. E. Hoell, C. C. Gallagher

and R. A. Bradbury, J. Appl. Phys. 40, 1792 (1969).

[2] D. M. Dodd, D. L. Wood, and R. L. Barnes, J. Appl.

Phys. 35, 1183 (1964).

[3] J. Novothy and Zd. Spurny, Czechoslovak Jour, of Phys.

16B, 119 (1966).

[4] Donald S. McClure, J. Chem. Phys. 36, 2757 (1962).

[5] R. H. Hoskin and B. H. Soffer, Phys. Rev. 133, A490

(1964).

[6] E. 0. Schulz-duBois, Bell System Tech. J. 38_, 271

(1959).

3.6.3. EXAMINATION OF RUBY LASER RODS BY

X-RAY DIFFRACTION TECHNIQUES

E. N. Farabaugh and F. A. Mauer

Inorganic Materials Division

Institute for Materials Research

Objective

This study was undertaken to determine whether x-ray

diffraction techniques may be used to characterize ruby laser

rods. A correlation between crystal perfection and its performance in a laser was considered.

Approach and Results

Comparison of laser performance and crystal perfection

has been carried out on three ruby rods. Dr. John Hall of

the NBS Boulder Laboratories supplied the rods and evaluated

their laser performance. Their performance was described as

"Fairly Good" (rod A), "Medium to Poor" (rod B) , and "Lousy"

(rod C). The Laue back-reflection, Laue transmission and

58
pseudo-Kossel techniques were used in the x-ray examination

to detect small-angle grain boundaries and c-axis wander.

By translating the rod in an x-ray beam and taking

back-reflection films at points along the rod, it was determined that all rods had some c-axis wander. The magnitude

was of the order of 1/2 to 1 1/2° for each rod. Examination

of the individual diffraction spots showed some splitting in

the case of each of the rods. This splitting probably indicates that the beam was incident on a subgrain boundary, but

some of the observed splitting may be due to grinding done

in fabricating the rods.

Back-reflection Laue films were also taken across the

end faces of all three rods. Rod A yielded the best Laue

films and rod C yielded the worst Laue films. Those for

rod B were somewhere between those for rods A and C. The

diffraction spots from one end of rod C were badly split

and there was a variation in orientation of approximately

half of a degree across the face of the rod. The other two

rods had no misorientation across their faces. The structure

of the spots was best for rod A and slightly poorer for rod

B.

The pseudo-Kossel technique was also used to study the

end faces of the rods. Analysis of the diffraction patterns

taken by this technique gave results which were in agreement

with those from the Laue back-reflection films. Both ends

of rod A yielded pseudo-Kossel patterns which were sharp and

had no discontinuities. One face of rod B gave a high

quality pattern, while the opposite face gave a pattern with

several discontinuities. One face of rod C gave a sharp

pattern while the opposite face gave a very poor one.

Transmission Laue films were taken at three points along

each rod. This technique samples a volume extending through

the rod rather than a surface layer. Pictures taken in this

59
Figure 1. A transmission Laue pattern of rod A. The

diffraction spots are only slightly split.

All the transmission patterns for rod A were

of good quality.

60
Figure 2. A transmission Laue pattern of rod C. The

diffraction spots are so badly split that it

is difficult to tell where one spot ends and

the next begins. A pattern like the above

arises when the x-ray beam passes through a volume of the crystal containing regions of

differing orientation. All films of rod C

were similar to the above.

61
manner revealed qualitative differences that were quite

striking:

1. Well formed spots with only slight splitting were

observed in the case of rod A (fig. 1).

2. Moderate splitting of spots was seen in one of the

three pictures from rod B.

3. Extreme splitting of spots occurred in all three

pictures from rod C (fig. 2).

Although the x-ray examination did reveal differences

between rods A, B, and C that correlated qualitatively with

laser performance, these techniques have not been applied

to the differentiation of high performance rods. Better

rods are being obtained, and an attempt will be made to

modify the techniques which are required to classify these

rods

.

3.6.4. LASER INDUCED DAMAGE STUDIES

N. N. Winogradoff, A. H. Neill, Jr., and J. Mitchell

Inorganic Materials Division

Institute for Materials Research

Objective

The objective of this work was to:

1. Devise techniques for comparing the energy and power

damage thresholds for various samples of laser glasses,

2. Develop an understanding of the damage mechanism,

and,

3. Relate the bulk physical properties of the glasses

to the above, with the aim of determining the optimum properties of glasses for very high power laser operation.

Approach

With the exception of consulting trips to other laboratories and relevant conferences, the work described below

was charged to the STO of ARPA, and included:

1. Technical literature survey of laser damage work,

62
2. Serving as consultants in advising on the nature of

the problem and in setting up a laser system capability for

experimental work on laser glass damage,

3. Preliminary experimental work on long pulse and Qswitched beam characterization of a ruby laser,

4. Preliminary experiments on the comparison of the

nature of the damage produced by focused long and Q-switched

ruby pulses in glass and plexi-glass samples.

The visits to other laboratories and relevant conferences, charged to the Materials Office of ARPA, included

The Conference on Lasers and Opto-Electronics , University of

Southampton, England*; The National Physical Laboratory,

England*; The American Optical Company, Massachusetts; and

the ASTM Laser Glass Damage Symposium in Boulder, Colorado.

Technical Status Review

All the work we came across was carried out with focused

laser beams. Both verbal consultations and the literature

survey showed the field to be very confusing except for the

fact that the (apparently intrinsic) laser induced damage can

exist in two basic forms; (a) circular disc-like cracks, the

distribution and orientation of which can be controlled by

the application of an external stress, and (b) thin lines

running parallel to the beam direction. The width of these

lines can be so fine as to defy resolution of any structure

within the line or wide enough to easily display a randon

"debris" type structure. In some cases, the damage exhibits

localized clusters of type (a) cracks which may or may not

be connected by type (b) lines.

A third type of damage related to the presence of absorbing inclusions results in a "star-burst" type of locali-

*We refer the reader to the trip report, which is in the

appendix, for details.

63
zed cracking. With improved, platinum free samples of glass

this type of damage is almost extinct and will not be considered in this report.

There appear to be at least three distinct mechanisms

responsible for the damage, (a) localized heating (or energy

absorption), (b) multiphoton ionization and subsequent

acceleration of the free electron to destructive avalanching

energies, and (c) electrostriction resulting in self-focusing

and acoustic effects.

By using two identical short pulses with a separation

of some lOOu sec, it was shown that preheating of the focused

region by the first pulse did not reduce the damage threshold

energy density for the second pulse, and it was concluded

that the damaging factor was the power rather than the energy

content of the beam [1]. In contrast with this experiment,

the same authors found that an increase in the over-all

temperature of the sample reduced the destruction threshold

[1].

They also found strong similarity in the damage produced

in sapphire by both neodymium and ruby laser pulses and also

found that the frequency doubled beams from these lasers

yielded damage thresholds which were constant over a temperature range from 77°K to 700°K, and that the damage due to

these higher photon energies was mainly in the form of very

thin long lines as distinct from the circular disc-type fracture observed with the Q-switched fundamental frequencies.

They also found a linear relationship between the

energy density (joules per square centimeter 'per pulse

duration) for damage and the pulse width (at constant power).

The extrapolation of this curve suggests that the damage

threshold would be reduced to zero at vanishingly small

pulse durations (pico second pulses). This experimental

evidence appears to be contrary to common belief.

64
Technical Report

Because of the relative ease of preparing polished

blocks of plexiglass, we compared the nature of the damage

produced with ruby laser beams focused by a wlO cm focal

length lens in glass and in methyl methacrylate blocks.

Visual and microscopic examination did not reveal any

difference in the nature of the damage in these two materials which always exhibited the disc-like cracks with long

pulse excitation and thin «1 mm x 5 cm long tracks with 30

n sec Q-switched pulses having energy contents of ^25 and

3 joules, respectively.

These experiments showed that preliminary setting-up,

optical alignment, and focusing of glass damage experiments

could be easily and economically established with plexiglass blocks in such a way that the scarce experimental

glass blocks can be conserved for damage studies when the

system had been optimized on the methyl methacrylate samples.

All our long pulse mode experiments yielded similar

disc-like cracks in the glass and methyl methacrylate blocks

and showed that this type of damage was limited to the

upstream side of the focus.

The Q-switched pulses invariably yielded the thin long

tracks, described above, which passed through the focus

without highlighting the focal point. This result contrasts

with the thin tracks observed by Edwards [2], only on the

upstream side of his focal point.

Both types of damage were luminescent during formation,

and it has been suggested that the emission could be due to

high temperature incandescence. Assuming black body

emission, temperatures of >10 °K have been calculated. However, it is difficult to correlate these temperatures with

the apparent temperature dependence of the damage threshold

where a few °K can apparently produce a marked difference.

65
Since the track-like damage is almost certainly a form

of self- focusing process, it would be expected to be dependent on the degree of initial focusing of the beam, essential

to produce damage, by "normal" laboratory laser systems.

For this reason, more significant quantitative measurements could be obtained by using collimated beams where the

required energy densities could be obtained by optical

reduction of the beam width. However, under these circumstances, surface damage having a lower threshold may set in

before the radiation could produce bulk damage so that an

optimum degree of convergence in thick samples will have to

be determined for the evaluation of a the bulk damage

threshold.

J. Mitchell is currently re-designing and constructing

a holder for the bi-planar phototube with improved impedence

matching which should result in response times of better

than 230 pico seconds for power measurements of the pulses

used in the damage studies, and A. H. Neill's calorimeter

is being recalibrated and evaluated for pulse energy measurements in Boulder.

References

[1] G. M. Zverev, T. N. Mikhailova, V. A. Pashkov, and

N. M. Solov'eve, Soviet Physics JETP 26, p. 1053

(1968). —

[2] N. N. Winogradoff, Trip report covering attendence at

the Laser and Opto-electronics Conference, Southampton

and the National Physical Laboratory, March 1969.

66
3.6.5. CHARACTERIZATION STANDARDS FOR LASER MATERIALS

J. L. Torgesen

Assisted by the following staff in the Analytical Chemistry

Division: B. A. Thompson, E. Miller, Virginia C. Stewart,

T. A. Rush, T. C. Rains, E. R. Deardorff

Inorganic Materials Division

Institute for Materials Research

Objectives

1. To develop precise methods for the chemical analyses of solid laser materials to include determinations of

major constituents and trace quantities of impurities.

2. To provide characterization of laser materials as

to physical defects and dislocations.

Approach and Results

Trace Quantities of Impurities in Ruby

Neutron activation analysis was used for the determination of trace elements in specimens cut from three different ruby boules. The three ruby boules were doped with

Cr

2

at nominal levels of 0.05$, 0.075$, and 0.10$, respectively. Two specimens, cut from opposite ends of each boule,

were analyzed.

Each sample (200-800mg) was sealed in polyethlyene and

irradiated for 30 minutes in pneumatic tube RT-3 of the NBS

13 -2 -1 Reactor at a power level of 5 MW ( 3 x 10 n • cm • sec ) A copper flux monitor was taped to the outside of each capsule. After irradiation the samples were allowed to decay

? Pi to 30 minutes to reduce the activity of 2.3-minute Al from

the Alp0_ matrix. Since the ruby crystals could not be

dissolved readily, all analyses were non-destructive. The

crystals, after irradiation, were rinsed in alcohol and in

1:1 HN0~ to remove surface contamination and were then

3 counted with a 47-cc Ge(Li) detector and a 2048-channel

analyzer. Several counts were made at different times to

permit observation of nuclides of short, intermediate, and

67
OAOo

COoo

VD

LH

ono oo r-\ oo ^r o o O on o\

CM o o o o O oo oo O H VD • • • o o • • • o o O • • o O o V V V o o

rH

Cti EH 6

a,

CO H

cd -p

CO>>o

Sh

CD

coa o

>5-H

X3 -P 3 ac

hO ^ »H O CO

CD co "d

•H

CO <H

>s cd rH -P

cd co

cd ^o o

•H

-P

cd>H

-Po

cdo

CO -p

rH

CO

CD K oo

OnmO

inO

OAoOOmo o

VO

oooo

ooo

oo

LPi

oo

oo in

H oo CM • o H o • V o O

rH LO,

m

O O^

O • • V o OO

Lno

OA o

LO. OO o o m

LO

CM o o oo -=T O o o CM 00 • • O o • o o • • o V V o o oo

oo c- o o CM CO vo o o O oo OA • • o o o o • • • V V o o o o

oo

CM OO o o oo r- o o o

oo o

ooo

rH OO

CDA LOOO

Ln

ino

CM

CM CM

oo

CMOO

CO

vo

H

oo o o o

co CM

O CM

O

O

O

CTNO c— CM t~- o

CO o\ On

O CM CM oo oo

O ^t rH

O

O o o o H o V o O o o oo

CMOo

oo

p o c CM

CD in B O

CD H ^ tss. cd

U 3 o W O-^ CQ CO o o 3 Cd £ rH cd < j 2 h crs 68
long half-life. The concentration of each element detected

was determined by comparison with a standard composed of a known amount of the element, irradiated and counted under

the same conditions as the samples.

The results obtained are shown in table 1. It can be

seen that a number of elements were observed at the ppb

level or above. We estimate, on the basis of previous

experience, that the standard deviation in each case,

except as noted below, is about + 5%

Although the aluminum activity produced by thermal

neutrons in the Al^O^ matrix has only a 2.3 minute halflife and thus does not interfere with the measurement of

nuclides of longer half-lide, two other isotopes are produced by the action of fast neutrons on the aluminum:

^Al (n,p) ^Mg

?7f ?4

Al (n,a) Na

27 The Mg, with a half-life of 9-5 minutes, does not consti24 tute a serious interference, but the Na has a half-life

of 15 hours and, thus, can interfere with the measurement

of all activities having half-lives shorter than 15 hours.

To minimize this interference we irradiated the samples in

the facility with the highest available ratio of thermal

to fast neutrons. Even here, the magnitude of the sodium24 "background" required large corrections for the Cr, Ga,

Sr, and the low Mn activities and, therefore, the uncertainty for these values is probably poorer than i 5%. This is

particularly true for copper because, in addition to the

24 high Compton continuum, or "background", the Na emits a small amount of 0.511 MeV radiation, which is the same

64

energy as that from Cu. A very large correction was thus

required for the copper values and, in fact, it would probably be safest to consider them as upper limits to the Cu

concentrations in the crystals.

69
Within the next month or two we should have access to

a terminal in the NBS Reactor with a much higher thermalto-fast neutron ratio and so we should be able to obtain much

better results for these shorter half-life nuclides. It is

possible that we could also determine K, Rb , Ca, and Zn at

the ppm level or below if this interference were removed.

Analyses of Nd-doped Glasses

Specimens of three commercially available Nd-doped

laser glasses were analyzed by a qualitative spectrochemical method. The glasses are identified as samples B, C,

and E. The results of this analysis are shown in table 2.

It is apparent that the samples B and C contain mixed alkalis, and evidently are based on similar compositions. The

sample E glass is of a much different composition and evidently is a lithium aluminum calcium silicate.

Data on the optical properties of the glasses, obtained on an associated project, bears out the fact that the

sample E glass is of a different composition.

Based on the results of the spectrochemical analyses,

precise determinations of the major constituents of the

glasses were then made. The results are given in table

3, along with the method used for each element.

Future Work

Due to personnel changes the project on Characterization Standards for Laser Materials has been discontinued

as of June 30, 1969. The work on chemical characterization

will be continued under Project 3130423, Physical and

Chemical Properties of Laser Materials.

Tasks that are underway or will be undertaken include:

(a) the determination of dn/dC for single crystal ruby.

Refractive index measurements are now being made on ruby

prisms doped at nominal levels of 0.03$, 0.05% and 0.064$

Cr^O-. Determinations of the absolute concentrations of

70
Element

Al

B

Ba

Ca

Cu

Pe

K

LI

Mg

Na

Nd

Pb

Rb

Sb

SI

Tl

Table 2. Spectrochemlcal analyses

of three laser glasses

Glass Sample

B c E

0.1-1 0.1-1 1-10

•<.001 <.001 <.001

1-10 1-10 .001-. 01

.Ol-.l .Ol-.l 1-10

<.001 <.001 - .001-. 01 .001-. 01 .001-. 01

1-10 1-10 <.001

<.001 <.001 1-10

<.001 <.001 <.001

1-10 1-10 .Ol-.l

1-10 1-10 1-10

1-10 1-10 - .001-. 01 .001-. 01 - .1-1 1-10 - >10 >10 >10

.1-1 .1-1 _

Note: >, greater than; <} less than,

-, not detected: values given in

percent composition by weight.

71
Table 3- Maj or

Nd-dop

constituents in three

>ed glasses

Oxide Composition

B C

- wt#

E

Analytic method

Si0

2

67.8 66.3 66.1 Gravimetric

Na

2

7.90 3.60 0.05 Flame Emission

Spectrometry

CaO 0.04 0.02 10.1 Flame Emission

Spectrometry

K

2

13.8 18.2 - Flame Emission

Spectrometry

Li

2 - - 14.5 Flame Emission

Spectrometry

PbO 1.10 1.75 - Atomic Absorption

Spectrometry

A1

2 0^ 0.12 0.06 4.38 Flame Emission

Spectrometry

Sb

2

1.26 3.57 - Atomic Absorption

Spectrometry

Nd

2

5.49 3.42 3-19 Solution Spectrometry

Fe

2

0.007 0.009 0.006 Solution Spectrometry

Ti0

2

0.19 0.39 - Solution Spectrometry

Cr in the specimens will be made and the data correlated with

refractive index.

(b) the determination of Cr absorption coefficients

in ruby and the correlation with absolute concentration.

(c) completion of the analysis of commercially-made

Nd-doped glasses.

72
3.6.6. STUDIES ON TECHNIQUES TO DETECT PLATINUM INCLUSIONS

AND TO MEASURE ND-GLASS QUALITY BY LIGHT SCATTERING

Herbert S. Bennett

Inorganic Materials Division

Institute for Materials Research

Objective

To determine whether dielectric relaxation, NMR,

and light scattering capabilities, for which NBS

has at present the equipment and the personnel may be

employed for the study of inclusions in Nd-glass and for

quality tests of Nd-glass.

Technical Information

Feasibility studies on two techniques by which one might

hope to measure the size, shape, and concentration of platinum inclusions in Nd-glass were considered. The dielectric

relaxation experiments, which are based upon the MaxwellWagner dispersion, measure the conductivities and the permittivities of a suspension (platinum in Nd-glass for example) as

a function of frequency. The suspension is viewed as electrically equivalent to a simple resistance-capacitance network. The dispersion frequency for Pt in Nd-glass is

1 ft estimated to be about 10 Hz and the change in resistance

and capacitance to be about one part in 10 . Such high frequencies and small changes in resistance make this mechod

unsuitable for Pt detection in Nd-glass.

The conventional NMR (nuclear magnetic resonance) techniques are estimated to be insensitive to the concentration

of Pt inclusions which are expected, even when methods to

increase the signal to noise ratio are employed, such as using

high magnetic fields, low temperature, and time averaging.

However, one should not conclude that all magnetic resonance

techniques will be unsuccessful in detecting Pt inclusions.

The. complicated double resonance techniques, the combination

of optical and microwave resonance techniques, and ENDOR

73
experiments might prove successful. But, each of the latter

will lead only to a research project and not to analytic

techniques which are applicable to many materials.

Light scattering experiments were performed on two Ndglass rods (sample F, 2.54 cm in diameter by 5-08 cm long).

One sample was rated excellent and the other was rated as

less than excellent but not poor either. The ratings are

based upon examining the white light from a projector which

is scattered at 90°. Seed-and-gas-bubble inclusions have

little change in brightness as the sample is rotated. Large

enough Pt inclusions exhibit an easily observed change in

brightness upon rotation of the sample. The ratings were

given by the supplier of sample F. A light scattering photometer was used for these experiments. The intensity of

light scattered from 30° to 150° in the azimuthal plane (the

plane which is perpendicular to the axis of the rod) was

measured for the 546 nm and the 436 nm lines of a mercury

arc lamp. The scattering intensity as a function of scattering angle for both vertical and horizontal polarizations

of the incident light showed no difference between the two

samples. The negative results may be due to the fact that

the light source (mercury arc) and detector (photo-multiplier tube) are not stable enough to distinguish passively

between excellent and good samples of Nd-glass. They also

suggest that if information on rod quality is available in

the light-scattering data it is probably contained in the

forward (0°) and backward (180°) scattering directions. One

should not conclude however that light scattering measurements are not sensitive enough. Perhaps, more sophisticated

equipment using He-Ne lasers and vacuum photo diodes might

provide the answer. The writer acknowledges and thanks

Dr. Lois Frolen and Mr. Given W. Cleek for performing the

above light scattering experiments and for the sample preparation . 74
3.6.7. APPENDIX

SUMMARY FOREIGN TRIP REPORT

Report by:

Nicholas Winogradoff, Research Physicist

Solid State Physics Section (313.07)

Inorganic Materials Division, IMR

National Bureau of Standards

Locations Visited:

24 March 1969 - 28 March 1969 University of Southampton

Southampton, England

31 March 1969 National Physical Lab

Teddington, Middlesex

England

Purpose of Trip:

(a) To attend a Joint Conference on Lasers and OptoElectronics sponsored by the Institution of Electronic and Radio Engineers, The Institution of

Electrical Engineers (Electronics Division), The

Institute of Physics and Physical Society, The

Institute of Electrical and Electronics Engineers,

and the University of Southampton, with the specific purpose of learning the latest state of the art

in laser glass damage problems and study procedures, and laser glasses in general.

(b) To establish contact with others working on

semi-conductor injection lasers, and other related

fields. Being international, and having 122 contributing authors drawn from countries including

France, Germany, India, Japan, Switzerland, U.K.

,

U.S.A., and U.S.S.R., this conference was particularly well suited for the above purposes.

Trip Summary

Although we were particularly interested in the damage

properties of neodymium-doped glass, the main paper relating

to laser glass damage was given by Mr. J. G. Edwards of the

75
National Physical Laboratory, and this work was related to

the elucidation of damage mechanisms rather than the comparison of various glasses.

I was interested in the French attitude to research in

general, and introduced myself to some of the French delegates attending the meeting, and was most impressed by their

friendliness. Two of them, Drs . Jean Michel Jego and Lain

Ternaud were active in laser glass damage problems and gave

me reprints of some of their work relating to studies of the

changes in the refractive index of glass samples subjected

to focused laser beams and the generation of giant pulses in

small samples of material by focused optical pumping respect2 ively. The latter produced 375MW/cm pulses having half

widths of the order of 1 nanosecond out of samples measuring

about 4 mm in length.

The refractive index studies were based on an extension

of Bell and Landt ' s work in water (reported by Dr. Bennet )

,

and included the use of direct and Schlieren photography, the

latter proved to be the more informative. Dr. J. M. Jego had

written a thesis on the study of laser damage in glasses —

this represented a comprehensive volume of work.

The rest of the summary given below will be restricted

to the actual proceedings of the conference.

While in Southampton, I also had most interesting and

instructive discussions on band tailing in gallium arsenide

with Dr. G. H. B. Thompson, whom I had met on a previous visit

to the Clarendon Laboratories, Oxford University. These

discussions will be continued by mail.

The Russian papers on semiconductor lasers were very

poorly presented and lacked clarity in objective and design.

76
Conference Proceedings:

(1) J. G. Edwards, National Physical Laboratories.

Laser-Amplifier Source used in Damage Studies

The laser source used, consisted of an auxiliary ocilla3+ tor feeding an unusual amplifier which consisted of a Nd

sample B glass block, measuring %10 x 10 x 1 cm sandwiched

between pumping flash lamps. Following initial amplification

in passing through a strip of this block, measuring 0.118 cm

in height, 1.0 cm in width, the beam was deflected by a tilted block of fused silica spinning at %4ll herz, (serving

as a Q switch) forced into the TEM mode by rectangular

oo J to diaphragms formed out of fused silica rods, and reflected

back through the spinning block of quartz into another strip

of the amplifying block, having the same dimensions as given

above, but displaced some 0.148 cms below the former.

This displacement was governed by the speed of rotation

of the block and the external optical path n °ngth. The beam

was then successively swept through different layers of the

block which thus served as a compact series of amplifier rods,

and after some 56 amplifying passes, the beam traversing the

lowest "strip" of the block was deflected out.

The system used enabled the pulse half widths to be

varied from 2.9 ns to 80 ns by placing suitable glass fiber

grids between the input face of the amplifier block and a mirror. An amplification coefficient of 0.07 cm" was

obtained and this resulted in a gain of 4 for each double

pass through the block. The energy density in the slab was

>e2

2 16.5 J/cm and was claimed to be well below the fracture

energy density of the glass 30-100 J/cm

(2) J. G. Edwards, National Physical Laboratory. Laser

Damage Mechanisms in Glasses

The above 1.06 ym diffraction limited Q-switched pulses

having pulse widths ranging from 3-60 ns with powers up to

77
100 MW were focused into the volume of various glass samples.

The peak powers were measured by means of biplanar photo

emissive diodes contained in special modified containers

where impedance matching for the output was obtained by

tapered co-axial shaping techniques. This procedure was

claimed to shift the response time for the wide area detector to ^250 pico seconds.

The energy in the beam was computed from the pulse shape

displayed on an oscilloscope and claimed to be known to 7%

absolute accuracy. (The writer has considerable misgivings

about this technique and its ability to produce energy measurements to this accuracy, but there does not seem to be any

better way of making such measurements at the present time).

Energies entering and leaving the sample were measured and

the absorbed energy deduced. A plot of absorbed energy

against input energy was made. The energy threshold for

damage was given by the value of input energy for which an

extrapolated portion of this curve intersects the input

energy axis.

Visually, most of the damage produced by the focused

beams was in the form of very thin "hair lines" about 1 ym

in width and several centimeters in length. A plot of the

2

square of the length, I , against the incident energy was

also a straight line whose intercept on the energy axes, also

gave a corresponding energy threshold for damage. These

linear plots- and the fine line structure of the damage

usually observed suggested that they were of intrinsic

nature

.

Occasional large deviations from the linear plots were

taken to indicate a different damage mechanism due to inclusions but even in large samples of laser glass B these were

rare. Typical energy threshold levels for the "intrinsic"

2 2 damage were 1.5«J/cm for 50 ns pulses 30 Mw/cm , and

100J/cm2 for 3 ns pulses «33 GW/cm2

. When surface damage was

78
observed, the threshold energies were a factor 3 lower.

The above pulse length dependence suggested that the plco

second pulse component often observed in Q-switched lasers

may not contribute to the damage threshold.

By screening a side viewing photo detector from the

scattered 1.06 ym radiation, Edwards showed that each damage

track emitted "white" light during its formation and suggested that the damage was due to plasma formation initiated

and propagated by multiphoton freeing of an electron and its

subsequent acceleration by the E field in the radiation.

Recent work by Russians showed that x-rays could be emitted

from such plasma in gases and these could be used to estimate temperatures in the plasma. Temperatures in excess of

c 10 °K have been calculated.

Basing his arguments on this model, Edwards suggested

that the energy threshold for damage should be increased

by the incorporation of electron traps in the glass. Such

an effect was apparently observed in ruby doped with TiOp,

in Germany, where, at high TiO

?

contents the energy threshold for damage was increased by a factor of 45.

(3) G. H. B. Thompson, Standard Telecommunication

Laboratories, Ltd., Harlow, Essex, Injection and Recombination of Carriers in GaAs Electro-Luminescent Diodes Below

Threshold

Examination of the spectral output and lifetimes of the

injected electrons in a p-n GaAs junction made by diffusing

Zn into an Si doped material indicated the presence of two

distinct lifetimes, one of which is quadratic and the other

linear with current. The lifetimes were said to depend on

the part of the junction examined, the latter being dominant

in the p-type side further away from the function.

(Writer's comment: Since Si is known to be amphoteric, it

was a pity that this dopant was used in the n-type side of

the diode, as the two lifetimes may be due to differences in

79
recombination cross sections for the two types of acceptors . )Thompson attributes a low mobility to the electrons

lying in the low energy states in the tail of the conduction

band. Prom our point of view, it is particularly interesting to note that he finds evidence of a distinct energy

step of the order of O.Olev associated with marked changes

in electron mobility — this adds further evidence to our

contention of the existence of a "discrete" energy level

lying close to the conduction band.

(4) H. R. Whittman, U. S. Army Missile Command, Alabama,

Temperature Effects in GaAs Lasers

This paper described the performance of GaAs p-n junctions formed by solution regrowth, over a temperature range

extending "below 50°K to above 100°K". The nature of the

diodes was not well defined and the interpretation of the

results in terms of conduction band tailing with radiative

recombination to a "discrete" state near the valence band

edge lacked direct experimental evidence. A comment by

Burrel of STL Ltd. indicated that the latter had evidence

for exponential tailing at both band edges. Whittman and

I have discussed the problem of band tailing at some length

and are now engaged in further discussions by mail. We

hope that mutual laboratory visits will take place shortly.

(5) A. R. Goodwin, Standard Telecommunication Laboratories, Ltd. Harlow, Essex, Cooled and Room Temperature GaAs

Lasers

Efficiencies up to 50$ for Zn diffused GaAs laser diodes

at 77°K and up to 15$ at 300°K have been reported. Although

efficiencies of 35$ were observed by using solution regrown

n-type material as a substrate for the Zn diffusion and the

switch on delay times observed with these lasers could be

reduced by suitable heat treatment, problems with short pulse

operation were still to be solved.

80
(6) J. R. Peters and C. E. E. Stewart, Standard Telecommunication Laboratories, Ltd. Harlow, Essex, Ga( AsP)

Visible Lamps and Arrays

The diodes were generally formed by Zn diffusion into

layers of GaAs p, produced by vapour phase epitaxy on to X _L TM" X

GaAs substrates using arsenic and phosphorous trichloride

predoped with Se or S. In some cases, mixed arsine and

phosphine were used for the formulation of the n-Ga As p, X X " X

layer.

A compact 5x7 alphanumeric array of Ga As P,_ diodes

(retaining direct band gap characteristics) was demonstrated,

(it was a pity to note that no attempt was made to improve

the efficiency for the power output by controlled compensation. ) Visit to Mr. J. G. Edward's Laboratory, National Physical

Laboratory, Middlesex:

This visit proved to be particularly fruitful, as I was

able to see the actual nature of the damaged glass. A

particularly interesting aspect of this was that all the

samples I saw indicated that using focused Q-switched Nd

laser flashes caused damage in the form of hair line tracks

originating at or near the focus and extending several

centimeters upstream . No bubbles could be resolved under

the microscope and there was no evidence of scattered debris

or damage areas as seen in the damage tracks we generated

by focusing Q-switched 30 ns ruby pulses in glass.

In a quantitative evaluation of glass it is, of course,

essential to correlate the damage and beam characteristics,

and the latter has to be specified with the best possible

precision. Since the Q-switched pulses can vary from shot

to shot, each individual pulse was measured by a special

beam splitter feeding a small fraction of the beam into a biplanar photo emissive detector. Edwards has done very

extensive work on beam measurement and characterization and

81
his methods may well be regarded as a standard for the

present state of the art.

Using special tapered coaxial connectors, drawings of

which were given to me, Edwards was able to decrease the

response time to some 250 pico seconds and demonstrated that

this was a considerable improvement on the standard housing.

A copy of Edward's beam splitter and detector system

is now being constructed and will be used in our damage

work.

TRIP REPORT

Report by:

Herbert S. Bennett

Physical Properties Section 313.05

Inorganic Materials Division,

National Bureau of Standards

Locations and Persons Visited:

I MR

7 June

9 June

10 June

11 June

Dr. E. C. Crittenden

Physics Department

Navy Postgraduate School

Monterey, California

Prof. S. Harris

Microwave Laboratory

Stanford University

Palo Alto, California

Dr. J. Swain

Lawrence Radiation Laboratory

Livermore, California

Dr. H. Winston and Dr. C. Giuliano

Hughes Aircraft Research Labs.

Malibu, California

12 June

16 June

Dr. A. Keig

Union Carbide (Linde)

San Diego, California

Dr. G. Gobeli and Dr. E

Reactor Building

Sandia Corporation

Albuquerque, New Mexico

Jones

82
18 June

19 & 20 June

17 June Dr. P. Avizonis and Capt. Abela

Air Force Weapons Laboratory

Kirtland Air Force Base

Albuquerque, New Mexico

Prof. D. Edwards

Physics Department

Colorado State University

Fort Collins, Colorado

ASTM - Subcommittee II on Lasers

and Laser Materials

National Bureau of Standards

Boulder, Colorado

Purpose of Trip:

(a) To visit representative laboratories associated

with universities, government installations, and industries

which have experienced damage or degradation in systems

containing lasers and laser materials.

(b) To establish contact with researchers at these laboratories in the fields of high power pulsed lasers, laser

materials, and non-linear optics.

(c) To learn the materials problems which the above

researchers have encountered and how they have overcome

them or how they have managed to accommodate them when

practical solutions were not possible. Also, to attempt to

learn the type of problems which are not discussed in the

published literature.

(d) To investigate possible areas of laser damage

research in which the National Bureau of Standards might

make a. unique and definitive contribution within the limitations of its present resources.

This report contains two sections. The first section

is a summary of the writer's views on current laser research

and on its problems in the United States. The second

section contains brief accounts of the technical information

obtained from each of the laboratories listed above.

83
Summary and Subjective Comments:

One of the current topics among laser researchers is

the comparison of United States, Russian, and French Ndglass lasers. This topic was discussed at most of the

laboratories. Much more is known about the French Nd-glass

than about the Soviet Nd-glass. The French Nd-glass is

selected by coring from large melts (one meter in diameter

by 0.31 m thick), while the U.S. Nd-glass is drawn from

much smaller melts. Most agree that the U.S. Nd-glass is

inferior to both the French and the Russian Nd-glass. The

reasons for this are in part financial. It is estimated

that the French Nd-glass laser research effort is funded at

twice the United States rate. But, money alone does not

solve problems. Based on the scant information the writer

could obtain, the organization of the French research

differs substantially from the organization in the U.S. The

French effort is directed and coordinated more strongly than

the U.S. It is certain that the total number of French

laser research laboratories is less than that for the U.S.

There is only one French manufacturer of high power pulsed

Nd-glass lasers, one French supplier of Nd-glass rods and

one major contract granting office. Such reasons as the

above are most speculative. However, the writer does know

that in one instance the French effort on materials research

is more sophisticated than that of the U.S. When damage

tracks occur in glass or when surface damage (pits) occur,

the defects emit light. The researchers (J. Ernest, et al.)

have studied the spectral content of such emissions with

time. Unfortunately, these experiments are not reported in

the open literature and the quantitative results of the

experiments are not available to the writer. Except for NBS

the writer has not been able to locate one group in the U.S.

which has or plans similar studies on either Nd-glass lasers

in the 30 nanosecond pulse range. This is one way in which

84
the changes in the electronic energy levels of either the

bulk or surface may be studied as a function of doping

(e.g., TiOp) or as a function of surface treatment (etching

and ion exchange). When electrons lie in deeper states (are

bound more tightly) larger electric feilds are necessary

before the electron can be raised to a conduction band or

free electron state. Conduction or free electrons may cause

damage more readily by avalanching than may tightly bound

electrons

As the writer travelled from one laboratory to the other

he began to realize that a lack of communication among laser

researchers in high power pulsed lasers exists. If one

assumes that the development of high power pulsed lasers is

a national undertaking, then open and frank discussions of

one anothers efforts and problems are most essential. The

common means of communication is the published and open

literature. This is a medium in which researchers are

understandably reluctant to state their unsolved problems.

But these must be made known to others if the device development is to proceed with a minimum of time and effort.

But the general rule was that of isolation. It is true

that pico-second, nano-second, and micro-second pulses

differ, but much knowledge in one time domain can be applied

to other time domains. For example, flash lamp designs for

the pico-second and nano-second time domains have many

common characteristics. Yet, the practical advantages and

disadvantages of helical lamps, axial lamps, (parallel to

the laser axis) and perpendicular lamps (perpendicular to

the laser axis) are not widely known among the laboratories.

Workers in one time domain tend to isolate themselves from

workers in other time domains. One result of this lack of

communication becomes apparent at meetings such as the ASTM

Symposium on Damage in Laser Glass, which was held on 20

June 1969 at the National Bureau of Standards (Boulder,

85
Colorado). Each invited paper was almost completely selfcontained and related little to the other papers presented

at the Symposium. Laboratories associated with universities tend to communicate more freely than those associated with industries and the Government. This same ASTM

symposium also illustrates the clinical nature of the

recent studies on damage and degradation. That is, the

results from laser "a" at laboratory A are frequently unique

and are most difficult to compare with the results from

laser "b" at laboratory B. The difficulty lies mostly in

obtaining the same reproducible pulses of known spatial and

temporal behavior from different lasers. However, improved

inter-laboratory communication could alleviate the situation.

One problem concerns the incomplete reporting of

results in the literature and the incomplete nature of the

research. Two striking examples exist. First, it has been

reported that etching the end surfaces of Nd-glass rods

with hydrofluoric acid increases the threshold for surface

damage by about a factor of three. But, the surfaces after

treatment are deteriorated and are not of good optical

quality for laser materials. Second, doping ruby rods with

Ti0

2

increases the internal damage threshold substantially.

Some reports indicate that an order of magnitude increase

obtains. Again, the rods after treatment are no longer

suitable for use as laser rods. Even though these effects

have been known for at least a year, no one in the United

States, to the writer's knowledge, is studying the possible

mechanisms, chemistry or physics, responsible for these

increased thresholds or how to increase the threshold without degrading the optical and laser qualities.

The empirical approach to laser materials research has

been adequate in the past. That is, the damage or degradation to laser materials has been circumvented by equipment

86
design, by using another material, or by simply asking for

lower laser power. When a material was found to be superior,

little effort was devoted to understanding the chemistry

and or physics of why it brought about an improvement. The

writer feels that now the empirical approach to the materials problem must be complemented by a fundamental materials

research program which is coordinated closely with the needs

of those who use specialized lasers.

Finally, the damage to Nd-glass laser rods remains a confusing area. It is rare that more than two researchers

from different laboratories agree with one another. Definitive experiments which permit reasonable and confident

interpretation of the damage results have not been performed

yet. Most damage studies use photographs exposed for times

much greater than the pulse width. More useful information

obtains if the damage tracks are photographed as a function

of time on a time scale comparable to the pulse width. One

then acquires information on the rate of growth of the track,

direction of growth of the track, and the time at which the

damage site first appears. These quantities are essential

when one attempts to suggest the dominant mechanism which

is responsible for the damage site or track. Present damage

studies do not include such details. The writer is aware of

only one case in which future plans include them. He feels

that only by knowing the spatial and temporal shape of the

laser pulse, by examining the spectral emission, with time

and by taking photographs as the damage develops can one

hope to reduce the present extent of confusion and lack of

consensus of opinion.

Technical Information:

Prof. E. C. Crittenden and his students have generated

acoustic waves at 500 MHz by the electrostrictive beating of

two coherent ruby laser pulses in dense lead glass. They

expect to obtain frequencies as high as 50 GHz. The gener87
ated acoustic waves had an amplitude greater than that

expected from quantum considerations; but still not great

enough to fracture the dense lead glass. Some of the

pulses fractured the dense lead glass. They say that the

fractures might have been due to inclusions and not due to

the absorption of acoustic waves. If they had better knowledge about the change of index of refraction with temperature and with stress they would have had fewer design

problems with their experiment. In addition, they also

observed surface damage pits on the ruby laser rod after a

few thousand pulses. Due to thermal drifting, the pulse

repetition rate never exceeded one pulse every twenty

minutes

.

Prof. S. Harris has five major projects. Two or more

graduate students work on each project. Four projects

develop non-linear optical devices. One of the projects

studies the materials problems encountered by the other four

projects. Dr. R. Byers heads the materials project. They

have encountered many damage and degradation problems with

the following non-linear materials; LiNbCU, BapNaNbj-O, ,-

,

KH

2

P0

2| , HICU, LiI0 3> CdS, CdSe, CsPbCl^, LiTaO^, and PbMoCK.

In many cases only a factor of ten exists between the

threshold energy density for the non-linear mechanism to

occur and the threshold energy density for surface burns and

internal tracks. One tries in designing devices to optimize the figure of merit (D /n ) and the second harmonic

generation (SHG) power against the burn or damage threshold.

The index of refraction is n and the non-linearity (nonlinear susceptibility) is D. The bulk optical properties

of the above materials (dn/doo, dn/dT, birefringence, and the

photo-elastic coefficients) have not been measured well in

any of the above materials. These quantities are needed for

designing non-linear optical devices. The researchers consequently must do these measurements themselves. They

commonly employ Q-switched pulses which are tens of nano-
seconds in length and which have power densities from 10 to

2 100 MW/cm . In addition, in many cases they have had to

grow the crystals themselves. They say and their output

shows that one cannot develop devices without high quality

optical materials. They experience considerable burn damage

in the complex dielectric and metallic coatings for antireflection elements and mirrors. They also find that damage

is less likely to occur when the laser mode is closely controlled and consistently reproducible. Prof. Pantell says

that materials are the main limitations in non-linear optic

studies. There is a need to gather data on testing techniques, to enumerate the data on non-linear materials and

to keep the data current on materials from the far infrared

to the ultra-violet.

Dr. J. Swain's group designs high power pulsed lasers

to study high density and high temperature plasmas and shock

waves. They seek to obtain as much energy as possible in

nanosecond and picosecond pulses. In order to circumvent

the damage problem in ruby, they previously brought the

beams from 12 ruby lasers to the surface of a sphere and

focused them to a central point with lenses (5 cm. focal

length). They now are building Nd-glass disc lasers. They

plan to have a 5 element Nd-glass disc (12.5 cm. diameter by

1.25 cm. thick) laser in operation by August 1969- A 15

element Nd-glass disc (12.5 cm. diameter by 1.25 cm. thick)

laser, which has the mirrors about 55m. from the nine-pass

amplifier, is being constructed. The feasibility of pumping

a 39 cm. diameter disc by a shock wave excited flash is

being examined. Conventional flash lamps are not efficient

optical pumps for this size disc. The 5 element disc laser

contains axial flash lamps and is expected to produce a 10

nanosecond pulse with an energy of about 10 joules. Power

levels of the order of 100 GW are expected by focusing the

_2 beam down to a 10 cm. spot. The 15 element disc laser

contains perpendicular flash lamps and is expected to

89
2 produce 1000 joules per 60 cm in a 3 nanosecond pulse.

They estimate that the 39 cm. disc laser will produce pulses

having 10,000 joules in a tens of nanosecond pulse, if they

can find an efficient way to transfer stored energy to

create a uniform population inversion. Studies to bring

about a population inversion by chemical energy (chemical

reactions) have also been conducted. The source of most

problems and limitations associated with high power and high

radiance laser rods is due to optical pumping through the

cylindrical surface. This Livermore group does not seek

high repetition rates and hence is not concerned with

excessive heating problems. The above disc lasers are face

pumped. The discs are centimeters apart and set at Brewster

angles with respect to one another. The optical absorption

is an asset here because it means a large energy storage

capability. Face pumping also leads to a more radially uniform population inversion. The internal damage due to self

focusing is lessened when discs of a thickness less than the

critical self- focusing distance are used. They use these

large aperture, disc systems to reduce thermal distortion,

improve pumping efficiency, and to have a more radially uniform population inversion. Their major damage problem

arises from surface damage pits. Because no one else has

examined the problem to their satisfaction they have undertaken an empirical surface damage study. Even though

treating the surface with hydrofluoric acid increases the

surface damage threshold by a factor of three, they find

that the optical quality becomes unsuitable for laser applications . Dr. Winston and Dr. C. Giuliano plan to cooperate and

to collaborate very closely with Dr. M. Stickley and Dr. E.

Bliss of the Air Force Cambridge Research Laboratory

(AFCRL). They will study damage to ruby and sapphire samples with very slightly converging beams (long focal length

90
compared to sample size). The Hughes group will examine

surface and internal damage thresholds with 3 to 100 nanosecond pulses and the APCRL group will examine the same with

0.1 and 100 nanosecond pulses. They will use single modes

and know the temporal properties of the beam. They plan to

examine the ruby damage tracks and cleavage faces as they

develop in time with a streak camera (image converter

camera). A possibility exists that they also will examine

the tracks and cleavage faces with an electron microscope.

A debate exists as to whether the threshold for surface

damage in ruby is greater or less than the internal damage

threshold in ruby. They believe that Czochralski crystals

may have more contaminants than the Verneuil crystals but

that the optical qualities of Czochralski crystals are

better than those of the Verneuil crystals. In order to

compare results at laboratory A with those at laboratory B,

they assert that one must use well-behaved, non-fluctuating

beams in both space and time.

Dr. A. Keig directs the characterization of crystals

grown at Union Carbide. The Electronics Division of Union

Carbide (San Diego) is an important supplier of Czochralski

grown ruby crystals in the United States. Dr. L. Rothrock

(Union Carbide) says that electrostrictive self- focusing has

been observed in white sapphire (Alp0~) but not in ruby.

The debate persists as to whether "yellow" ruby is still a problem with recent (less than one year) Union Carbide grown

crystals. One side says that the problem is most likely

related to the impurity content and the other says that it

is related to changes in the volume of chromium ions. On

the other hand, his group observes that bulk damage in ruby

is power-density-dependent and suggests that multiphoton

absorption occurs or that other impurities, which do not

play a role in the "yellow" ruby debate, absorb energy and

modify the energy levels. It is also their opinion that the

91
Schulz-Wei method is as good as the Laue-x-ray-back-scattering method to study the wander of the c-axis. The most

recently grown ruby crystals are so good now that the caxis orientations differ by less than a minute. New techniques are needed to test the most recent ruby crystals

because present techniques are not sensitive enough. The

Nd-YAG crystals have many more problems than ruby and Ndglass. They also have a program to improve the optical

quality of Nd-YAG. During the past year they have reduced

the average number of interference fringes across the face

of a rod in an interferometer from 2 for a .64cm x 5. Oftcm

rod to 1.5 for a 64cm x 7.62cm rod.

Dr. G. Gobeli and Dr. E. Jones have constructed a Ndglass laser which emits 75 joules in a one to two picosecond

pulse. The pulse length is expressed in terms of the two

photon fluorescence experiments. The interpretation of the

two photon fluorescence experiments is controversial and is

not the subject of their research. They built this laser as a tool with which to study solid state physics and do not

consider the continual development of high power lasers to

be their major endeavor. They hope to generate thermonuclear

neutrons by focusing the laser on a solid LiD surface. They

20 2 expect to obtain power densities of 10 watts/cm over an

area 10 cm . The spatial and temporal properties of the

picosecond pulses are of secondary interest to them. Their

major damage problem centers on the metallic and dielectric

coatings of the 100 percent mirrors. The laser rod damage

occurs much less frequently. Most rods last thousands of

pulses before replacement due to damage is necessary. But,

most mirrors last only 20 to 50 pulses before a "burn" spot

develops and the mirror must be replaced. The 100 percent

reflection coatings provided by all manufacturers are universally of a poor quality for their purposes. They have

found that rods from the same melt have widely different

92
damage characteristics. One rod will last perhaps only three

pulses: while another rod from the same melt will last 1000

pulses. Removing such differences from supposedly identical

laser rods is more important to them than debating about

which Nd-glass rod supplier produces the most superior product. Damage to mirrors and rods sometimes occurs in their

system when the (always present but hopefully small) diffuse

scattering from an optical surface causes prelasing in high

gain regions. A significant difference exists between a single picosecond pulse and a train of several picosecond

pulses. The latter often produces plate-like damage sites

which are very similar to the damage sites produced by Qswitched nanosecond pulses in plastics. They plan to study

damage in all of their laser components to enable them to

accomplish their primary mission (to make a tool for solid

state physics studies). They will attempt to understand the

propagation characteristics of high power laser beams in

transparent materials such as silica, sapphire, and glass.

The lack of sufficient controls on damage experiments to

date make inter-laboratory comparisons very hazardous undertakings. The experimental techniques and the spatial and

temporal properties of the laser pulses in a given laboratory differ markedly from those in other laboratories.

Capt. A. Abela is familiar with the Nd-glass damage

problems encountered by the Kirtland Air Force Weapons Laboratory researchers. Due to limitations on funds and personnel, other projects have a higher priority than the glass

damage studies. They recognize however the importance of

damage studies. They do plan some damage studies which will

be limited probably to visual observations and which will be

undertaken only to the extent necessary to accomplish their

mission. Their efforts are directed towards the application

of Q-switched pulses which are 30 to 80 nanoseconds in

length. They initially seek 50 joules per pulse, 5 pulses

per second, and 5 cm. diameter beam. Their studies will be

93
mainly with collimated beams. They would like a figure of

merit to be provided for each type of Nd-glass. That is,

an evaluation based upon lasing efficiency, laser threshold,

beam divergence, signal gain, and rod distortion during

pumping. They need specific glasses around which they will

design their systems. He also agrees with others that comparing data at one location with that from another location

is difficult and provides little additional understanding.

They have a slab (disc) laser designed with the discs very

close together. Only close examination reveals that the

laser "rod" is composed of many discs. The laser is pumped

through the cylindrical surface. They employ the disc system mainly to circumvent the cooling problem of a conventional laser rod. Their needs and applications are in the

realm of limited access information.

Prof. D. Edwards and his students are sponsored in part

by the Air Force Cambridge Research Laboratories to carry

out laser damage studies in transparent materials. His

group examines the details of the Raman spectra before damage occurs, near the damage sites after it occurs, and as a

function of uniaxial stress. They have studied most

thoroughly quartz, but also have made preliminary examinations on ruby, sapphire, salt (NaCl) and glass. They measure the line position, line width, and line intensity. The

latter two quantities relate to the anharmonic parts of the

lattice potential. The above quantities are used first to

characterize the undamaged or stress-free materials. After

characterization by the Raman spectra they have two

approaches. First, the material is damaged by a laser beam

and then examined a second time near the damage sites by

Raman spectroscopy. The amount by which the above quantities change may suggest possible mechanisms for the damage.

Second, a uniaxial stress is applied in the range of stress

which they estimate from a laser beam without specifying

9 h
the exact nature of the electromagnetic wave-lattice coupling. The above quantities are studied as a function of

the applied uniaxial stress. They expect to be able to

make some correlations between how the line width changes

with stress and the damage thresholds. He observes that the

Russians have a substantially greater effort on damage in

resin materials (plastics) by lasers. He also remarks that

past successes have been accomplished without a sound

materials effort; but now materials research is going to be

essential for further achievements.

Because some of the foregoing technical information

also was presented at the ASTM Nd-glass Symposium (20 June

1969, Boulder, Colorado) and because the Symposium proceedings are to be distributed, the writer feels that it is

redundant to include a summary of the ASTM Symposium in this

trip report.

95

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and industrial practice (including safety codes) de- veloped in cooperation with interested industries,

professional organizations, and regulatory bodies. Special Publications. Proceedings of NBS conferences, bibliographies, annual reports, wall charts,

pamphlets, etc. Monographs. Major contributions to the technical

literature on various subjects related to the Bureau's

scientific and technical activities. National Standard Reference Data Series.

NSRDS provides quantitive data on the physical

and chemical properties of materials, compiled from

the world's literature and critically evaluated. Product Standards. Provide requirements for sizes, types, quality and methods for testing various industrial products. These standards are developed cooperatively with interested Government and industry groups

and provide the basis for common understanding of product characteristics for both buyers and sellers. Their use is voluntary.

Technical Notes. This series consists of communications and reports (covering both other agency and

NBS-sponsored work) of limited or transitory interest. Federal Information Processing Standards Publications. This series is the official publication within

the Federal Government for information on standards

adopted and promulgated under the Public Law

89-306, and Bureau of the Budget Circular A-86

entitled, Standardization of Data Elements and Codes

in Data Systems.

CLEARINGHOUSE

The Clearinghouse for Federal Scientific and

Technical Information, operated by NBS, supplies

unclassified information related to Government-generated science and technology in defense, space, atomic energy, and other national programs. For

further information on Clearinghouse services, write: Clearinghouse

U.S. Department of Commerce

Springfield, Virginia 22151

Order NBS publications from: Superintendent of Documents

Government Printing Office

Washington, D.C. 20402
U.S. DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20230

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