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IRTMENT OF MMERCS

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NBS TECHNICAL NOTE 792

U.S.

'ARTMENT

OF

OMMERCE

National

^

of OO ds

5153

<n3

Investigation of

Viscous Flow in Glass

During Phase Separation
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act of Congress March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology and facilitate their effective application for public benefit. To this end. the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau consists of the Institute for Basic Standards, the Institute for Materials Research, the Institute for Applied Technology,

the Institute for Computer Sciences and Technology, and the Office for Information Programs. THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United

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

2 Part of the Center for Radiation Research.

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* Part of the Center for Building Technology.
tial Bureau of Sta«ia#

Investigation of Viscous Flow in Glass

During Phase Separation

Joseph H. Simmons, Susan A. Mills,

Albert Napolitano, Douglas H. Blackburn, and

Wolfgang K. Haller

Inorganic Materials Division

Institute for Materials Research

•^/V National Bureau of Standards '" Washington, D.C. 20234

7eChnica/ noi-e- no *79o

Sponsored by:

The Office of Naval Research

Contract Naonr 12-73, NR 032-536 Department of the Navy

Code 471, Washington, D.C. 20360

S

U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director

Issued September 1973
National Bureau of Standards Technical Note 792

Nat. Bur. Stand. (U.S.), Tech. Note 792, 43 pages (Sept. 1973) CODEN: NBTNAE

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 (Order by SD Catalog No. C13.46:792). Price $0.65
TABLE OF CONTENTS

Page

1

.

Introduction 1

2

.

Experimental Section 3

2 .1 Microstructure Measurement 3

2 .

2

Viscos ity Measurement 9

3

.

Results and Discussion 13

3.1 Analysis of Microstructure Growth 13

3.2 Analysis of Viscosity Change 15

3.3 Formulation of Viscosity Change 26

4. Conclusions 33

5

.

References 36

Investigation of Viscous Flow in Glass During Phase Separation

Joseph H. Simmons, Susan A. Mills, Albert Napolitano

Douglas H. Blackburn and Wolfgang K. Haller

The isothermal viscosity of two borosilicate glasses, of

which one is a commercial glass widely used for chemical

glassware, shows a large increase (4 to 5 orders of magnitude)

with heat-treatment time (ranging up to 100,000 min) near the

annealing point. The two glasses have similar compositions,

but differ greatly in their phase separation characteristics.

Electron micrographs are used to analyze the development of

microstructure during the suspected phase separation. In both glasses, it is found that the structure development is

primarily responsible for the viscosity increase. An analysis

of the data, and a theoretical interpretation of the effect

are presented.

Key words: Glass; microstructure; phase separation; viscosity.

1. Introduction

The study of viscosity during liquid-liquid phase transitions in

glasses offers a promising means of investigating the effect of microstructure on viscous flow processes. While recent measurements of

viscosity and other properties [1,2]— have strongly indicated the

presence of microstructure in some non-phase separating glasses,

attempts to detect or measure this structure have generally yielded

negative or conflicting results [3-5]. Conversely, microstructure

arising from liquid-liquid phase transitions can generally be analyzed

by electron microscopy [6] and its size may readily be selected within

wide ranges by suitable heat-treatments. Further, analytical work con- ducted at this laboratory on results from ultrasonic relaxation measurements on oxide glass mixtures above the phase transition temperature [7]

1/ Figures in brackets indicate the literature references at the end

of this paper.
have suggested that the interaction of microstructure with viscous flow

processes depends only upon the activation energy differences between

the phases. Therefore, it is expected that results obtained on phase

separated glasses can be extended to glasses with microstructure of a different nature.

We present, here, results from an investigation of the change in

viscosity behavior of two oxide glasses, resulting from a developing

phase separation. These glasses were chosen with basically equivalent

compositions but different immiscibility behavior.

The first is a model glass for immiscibility studies with a com- position of SiO , BO and Na in the proportions: 70:23:7 by weight

(hence known as Type I). Its phase-transition temperature (755°C) occurs near the maximum of the immiscibility surface for the system

(Sample 42 in Ref. 8). Consequently, the viscosity is low (10 poises)

at the phase-transition temperature (see fig. 1) [9] and the glass

iv ' '

! 1 1 1 1 I0

16 A A Vl 0".°' -

10 l4 NA V: (T.CO) o % n;oj _ 10

' 2 • 7?I

(T,00) _

(A

0) \ H> v

(/> J io

l0 \ HA _ >

i- "\ . V\

£ 10 o

8 \ V \

o Tc X \ CO> IO 6

Tc ^^TYPE E

-

IO 4 TYPE I -

IO 21

I

I

1 I l^

600 800 1,000

TEMPERATURE (°C)

1,200 1,400

Fig. 1. Viscosity curves for glasses of Type I and II. n(T,o) is the viscosity of the

single phase glass (prior to immiscibility)

,

and n(T,°°) is the viscosity of the totally

phase separated glass.
readily displays very evident phase separation characteristics. Its microstructure develops quickly; it has a large chemical difference

between phases, with an overall poor chemical durability,

and its phases tend toward a higher volume fraction of the lower viscosity phase. Numerous studies have been conducted on its relaxation

properties and phase-separation characteristics [7,10,11,12],

In contrast, the second glass (Type II) undergoes a phase separation

which is not easily noticeable, and has apparently little effect on the

macroscopic properties. It is commonly used for chemical glassware with

a composition consisting of SiO , BO, Na and Al in the proportions:

81:12.6:3.9:2.4 by weight, as determined by chemical analysis. Its phase-transition temperature is low (649 °C) (this measurement is

described in Section 3.2.3) and occurs at a high viscosity (10 poises)

(see fig. 1). The microstructure is expected to be small with the

phases having little chemical difference.

The viscosity change in both these glasses during phase separation

will be investigated and will be related to the growing microstructure

by a model developed earlier [7] but modified here. Microstructure

measurements on the Type I glass have been reported [13] , and we will

present results from measurements conducted on Type II samples.

2. Experimental Section

2. 1 Microstructure Measurement 2.1.1 General

The samples were in the form of rods 0.1 to 0.2 cm in diameter

(which we will denote as fibers) for the Type I and Type II glasses and

rods 0.7 to 1.0 cm in diameter for the Type II glass only. They were

heat treated at elevated temperatures, then quenched to room temperature.

The fibers and rods were fractured perpendicularly to the axial direction,

and the freshly formed faces were prepared for examination by electron

microscopy. Only structure near the center of the fracture faces was measured in order to avoid surface effects. When fibers were' used, they were often also fractured along the axial direction to check for development of any anisotropy in the structure.
There are generally two significant problems encountered in the

examination of phase separated glasses by electron microscopy, and these

are particularly pronounced for samples of the Type II glass. First,

the two phases have similar compositions and second, the structures have

very small domain sizes.

Transmission electron microscopy is suitable for high magnification,

and high resolution analysis. However, the minimal differences in

electron absorption and lack of contrast due to small composition differences between the phases greatly reduce the utility of the method for

this study. Difficulty in thinning the glass without introducing sizeable damage [14] , specimen heating and confusion of the true morphology

by projection effects [15] further deter from using this method of

examination.

The use of shadowed replicas in electron microscopy requires the

development of a 3-dimensional surface which depends critically' upon the

feasibility of differentially etching the phases. Small composition

differences as found in the Type II samples make this approach difficult,

but with proper etching techniques, the problem can be overcome. Here,

care must be taken to dissolve the more soluble phase sufficiently to identify the phase boundaries when Pt is evaporated at a selected angle

to the surface (shadowing) , without dissolving the less soluble phase

and changing its size. Selective solutions are difficult to find, so that it is necessary to use solutions which dissolve both phases and to develop a suitable 3-dimensional structure by the proper selection of

strength and duration of the etch. The problem increases with reducing

microstructure size, and as will be seen below, structures smaller than

50 to 60 A are very difficult to shadow properly. Crystallization of

the evaporated Pt reduces the resolution and further complicates the

analysis of small structures.

Present-state scanning electron microscopy lacks the resolution

necessary to provide more than a cursory look at relatively large

structures. Therefore, the electron microscopy of Pt-shadowed carbon

replicas of etched surfaces offers the most suitable approach to
measuring micros tructure size and volume fraction, and estimating the

degree of interconnectivity of the phases.

2.1.2 Etch and Replication Technique

Samples of the Type I glass generally separated into phases with a marked chemical durability difference. Therefore, proper etching was readily accomplished with very weak acid solutions (0.5% HF in water) [13]

The commercial glass, Type II, besides having small microstructures

,

separated into phases with a less pronounced chemical durability difference. This required an extensive study of etching solutions and

durations

.

The criterion which we used to recognize a suitable etch was a surface with depressions caused by the removal of the more soluble phase

separated by flat areas representing the durable phase. Any rounding

of these areas indicated that the durable phase had been slightly dissolved and indicated uncertainty in the location of the phase boundaries

.

This is a very stringent requirement, which could not be met adequately

at times. However, all microstructure size measurements reported come from micrographs meeting the above criterion.

We used aqueous solutions with 0.5% to 10% HF, to 5% HC1 and

to 26% H^SO. . It was found that some HF was necessary in all our

2 4 solutions. The HC1 and H SO additions were effective in reducing the

etching times without an increase in HF concentrations. This was found

desirable, as long etch treatments inevitably resulted in a decrease in

the phase boundary definition. In general, the solutions selected

changed from a 4.5% HF + 26% H„S0 aqueous solution for samples with

short heat-treatments (4 to 8 hours) to a 1% HF + 5% HC1 aqueous

solution for the long heat treatments. (On the basis of normality, a sulfuric acid solution is approximately 3 times stronger than a hydrochloric solution with the same acid volume.) Stronger solutions were

required for samples heat-treated at higher temperatures. This follows

an expected trend since the chemical difference between phases is less

for higher temperatures , and therefore the soluble phase is more durable

than at lower temperatures. A comparison of the results of using
different solutions, where the etch is progressively strengthened on a sample heat-treated at 585°C for 96 hours is shown in figure 2.

,. -__ ---*.; • - ^.~- -»",£ *J-r*

---;'-" *c ' ' T^t'

'iV-.rV 'V.-'-^""^.'

r- * r

'""-'

' -v* ; »'/»-'** <

;.s. A- -V « »»; * '- .»-;. v 7 Vu

'

1

• t'j v\ '».' '.•. - ^v^--'

'

-%

.' ** si '»'-" >:":-?. • , -'-'

<- ~-

Fig. 2. Micrographs illustrating the variation of the etching solution on a sample treated at 585 °C for 96 hours.

From left to right

:

(a) 1% HF + 5% HC1

(b) 1% HF + 5% H SO

(c) 1% HF + 26% H SO

(d) 4.5% HF + 26% Hpo*

The bar measures 1/2 urn.

After the acid treatment, we found it necessary to wash the sample

in alcohol in order to eliminate the hexagonal SiF, crystals which

resulted from the etch. Once the freshly fractured faces were etched
and washed, the samples were transferred to a vacuum system (fig. 3)

Fig. 3. Bell Jar used for evaporation. The carbon

evaporator is at the left; the Pt evaporator

is at the right. On top the substrate holder

in the middle, and the N fill and exhaust

lines can be seen.

where the replica was evaporated. The films were evaporated with a background pressure of 2 x 10 torr or less. The Pt was evaporated

first, generally at an angle of 45°C to the specimen surface-normal.

Smaller angles were used for deep structures and larger ones for shallow

structures. The evaporation guns for the Pt and carbon operate by

resistance heating of two contacting carbon points (see fig. 4).

The lower carbon rod is blunt and held in a fixed position while the

pointed upper one is attached to a moveable weighted arm. Contact is

maintained during evaporation by gravity only. For the evaporation of

Pt, 3 mediums were tried: Pt-carbon pellets, Pt discs, and Pt wire
held between two carbon points as in figure 4 ; the latter being our

Fig. 4. Carbon resistance heating evaporator. The

pointed carbon rod is kept against the

lower blunt rod by gravity only. When Pt wire is used, it is wound around the point

of the upper rod.

choice. The pellets gave a coarser replica, and the discs required a high current to melt, causing carbon to be evaporated as_well.. This

produced either a good Pt film but a coarse replica, or a fine grain

replica with insufficient Pt. Fine Pt wire (,13mm dia. , 3 cm long)

wound tightly around the upper carbon rod gave the best results. In the

evaporation of Pt and carbon, we always tried to make the films as thin

as possible (<100 A)

.

The guns were designed to fit inside a shield with a small restricting aperture aligned with the etched glass substrate, in order to insure

a direct, controlled jet of carbon or Pt vapor at the sample. The

sample was in turn surrounded by a liquid nitrogen shield with two apertures. This reduced contamination of the substrate and the films

from the surrounding surfaces and reinforced the control over the vapor

8
from the carbon and Pt sources. The stringent restriction on the beam

of particles reaching the substrate enabled us to obtain clear and high

resolution micrographs showing well-defined phase boundaries. This is

reflected in our ability to replicate and measure microstructure as small as 60 A in size when 100 A was the previous limit. There are provisions in the system for cooling the specimen, however, we found

that this had no substantial effect on the replicas produced.

The replicas were removed from the glass samples by stripping in a 5% HF aqueous solution. The floating replica was picked up with a copper grid disc and washed in distilled water. The replicas were

examined in an electron microscope, generally at a magnification of

22,000 to 65,000 times. Resulting micrographs are shown in figures 5-8

for the 620, 600, 585 and 565 °C isotherms.

2.1.3 Size Measurement

The size of the microstructure was obtained in two steps. First,

a straight line was drawn through a photograph of the replica and the

cumulative length of the portions associated with each phase was measured. This was done in several random directions and determined the

volume fraction of each phase. Then from the total number of intersections of each line with a phase boundary, the average distance

between phase changes was calculated. In general, we made 10 measurements of lines with 100 to 120 intersections. This yielded uncertainties

below 10% and usually around 5% of the numbers obtained, except when

the sizes were smaller than 150 A. In the latter case, our uncertainty

was around 20-25 A.

2.2 Viscosity Measurement

The shear viscosity was measured by the fiber elongation technique

described in detail elsewhere [16] (see fig. 9) , because it offers two essential features. The method lends itself well to a continuous

measurement of the viscosity as a function of time at a fixed temperature, and allows these measurements to be started immediately after

insertion of the fiber into the furnace. Analysis of the data obtained

demonstrates that the latter feature is necessary for a good
i*.~r.—.J,

"-. : £-':- '* -'-' '': yJ '? •!>

~-^~->

.

.""

' - . •' ~~^'~'~ '' -"/ ?'?•. ''•? ." -,

r -^ ' H

ft

Fig. 5. Micrographs showing

the 620 °C isotherm

for heat treatments

lasting:

(a) 8 hours

(b) 48 hours

(c) 192 hours

(d) 768 hours

The bar is 1/2 urn.

Fig. 6. Micrographs showing

the 600°C isotherm

'':'. ''_ .-

. ~— ' , /,'.' *;:. ', ".-

'- r •

T'ifSS/t y\ i/'i <M^**M

(a) 24 hours

(b) 48 hours

(c) 96 hours

(d) 192 hours

The bar is 1/2 um.

i^r>Cc^--^£^{fc/Sr^

10
'mm

Fig. 7. Micrographs showing

the 585 °C isotherm

c t

-!.*'

, ,

" "*'; r > " v" "" '- — - -

..-.-*-. \f-'f ^T2 s"^* f**-\

"'

*

'_ .'"*

- - V - *-« J *,' " _'<*

*>*V

.' * f~ ' / s-* /•--.'"": -

- **./ „ ~ - - :."./ --"/>*r.*

.• *&~>

. . <- '._-* ?

" <"'*

* . "—'*'.—' r *

,*"" t • ' *

f*'. * *- . ^,"; " ~,T i .,"",< /\'

:

"'- 1 '"'/.*"•

*5& ' J J,

' - - *-

* . *

'•* . »* '"'/.--" *." ' * *, * "^".'-

(a) 24 hours

(b) 48 hours

(c) 96 hours

(d) 192 hours

The bar is 1/2 um.

Fig. 8. Micrographs showing

the 565 °C isotherm

(a) 24 hours

(b) 48 hours

(c) 96 hours

(d) 192 hours

The bar is 1/2 um.

&y£^M&ffi-'Mt* Steffi

ii
TOP CIRCUIT

BOTTOM

CIRCUIT

SHELL

CONTROL THERMOCOUPLE

GLASS FIBER ^ TRANSITE EZ3 BRASS ^ SILVER CORE

REFRACTORY

SIL-O-CELL

WEIGHTS

Fig. 9. Fiber elongation apparatus reprinted

from reference 16

.

interpretation of the results [11]

.

The behavior of the isothermal viscosity as a function of heattreatment time was measured by continuous recording of the elongation

of the fiber. Special care was taken to begin the measurements within

the first minute after insertion of the fiber into the hot furnace.

It is estimated that the fibers reached the heat-treatment temperature

in less than 2 minutes. The measurements were then continued for times

lasting up to 150,000 minutes. Some fibers were loaded immediately,

and others were heat-treated for different time periods before the load was applied in order to determine the delayed elastic effects. Several

measurements were conducted at the same temperature, but with different

loads in order to detect stress-dependent behavior.

12
3. Results and Discussion

3.1 Analysis of Microstructure Growth

In both glasses, the structure appeared to grow in size during the

isothermal heat treatments. The volume fractions of each phase remained

constant throughout the measurements. The volume fractions of durable

to soluble phase are 50:50 and 55:45 for Type I and Type II glasses,

respectively. The change in etching solution necessary for the analysis

of the structure in the Type II samples indicated that the chemical

composition of the phases may have changed during the heat treatments

as a function of both temperature and treatment duration. This question

is deferred to another report [17] where the chemical durability of the

samples is measured as a function of heat-treatment time and temperature.

Further analysis of the micrographs indicated that the interconnectivity

remained high throughout the microstructure growth. These results, when

taken together, suggest that we are observing primarily the rearrangement stage of the phase separation process.

It is now necessary to ascribe a microstructure size to each micrograph and analyze the change of this quantity with heat-treatment time

and temperature. An easily measured quantity, as shown before, is the

distance between consecutive phase changes. However, in order to remain

consistent with out previous work on the subject [7,13], let us determine a correlation length to characterize the phases. This is done

through a correlation function, G(r) t which represents the increased

probability due to phase separation of going a distance r from any point

and remaining within the same phase. The distance between phase changes,

r, is therefore defined as:

= /rG(r)I47rr

2 'rG(r)I4irr'dr]/ drJ IGi /G(r)I4Trr

2 dr] (1)

Using a probability function previously shown to describe these systems

well [Ref. 7]:

G(r) = G

q exp [-r

2 /2A

2

] (2)

where G

q is a constant and A is the correlation length, we obtain the

13
following relationship:

A = 0.63 r (3)

The isothermal time dependence of the correlation length, A, may

be readily calculated from the measured dependence of r on heattreatment time and temperature. The data for the Type I glass were

reported in Ref. 13. Those for the Type II glass are shown in figure 10,

400

I I I I I I 1 1 i i i i i i i

I

200 —

a

A /

100 - — - o/

o 620 °C - A 600°C -

50

i i

1 1 1 1 i i

585°C

l 1 100 1,000 10,000 100,000

TIME AT 620° C CORRESPONDING TO THE

REDUCTION-. t*=te E 2/ RT/( |_T/Tc) (min)

Fig. 10. Plot of correlation length, A, against

reduced time:

t* = t eE

2 /RT /(l-T/Tc).

The line follows 1/3 power of time.

Both glasses follow a 1/3 power dependence of structure size on time,

generally expressed as [18] :

A

3 (T,t)-A

Q

3 (T) = D(t-t

Q

) (4)

with the time and size at which this 1/3 power-law behavior begins

(A and t ) sufficiently smaller than the values reported that they may

14
be ignored. Near the transition temperature, the diffusion coefficient,

D, can be expressed in terms of a mobility term and the chemical potential with the following result:

A

3 (T,t) = D (1-T/T )t e"

E

2 /RI

(5) o c

where D is a constant; t is the time; T is the temperature; T is the

o c transition temperature, and E is the activation energy controlling the

diffusion process by which the phase rearrangement is occurring. The

parameters of the fit of the data to this equation are as follows:

Type I Glass

D = 1.19 x 10

30 A^/min

o E

2 = 97 kcal/mol (4.06 x 10

5 J/mol)

Type II Glass

D = 8.6 x 10

22

X

3 /min

oE = 70 kcal/mol (2.93 x 10

5 J/mol)

Since bulk diffusion through the fluid phase is likely to be the rate

controlling step, as indicated by the 1/3 power relationship of Eq. 5 between time and size, then we may assume that it is characterized by

an activation energy whose value is E .

3. 2 Analysis of Viscosity Change

3.2.1 General

The isothermal viscosity measured at this laboratory for the two glasses under consideration is shown in figures 11 and 12. It is

interesting to note in passing that despite the large viscosity differences between the two glasses above and at the transition temperature

(fig. 1), the range of temperatures for which the viscosity is suitably

measured by the fiber elongation method is the same, 540-660 °C.

Figures 11 and 12 show that the isothermal viscosity Increases by

as much as five orders of magnitude over the heat-treatment times

studied. Before correlating this increase to microstructure development,

we must identify the effect of the elastic response from application of

15
Fig. 11. Composite illustration of the isothermal viscosity

of samples of Type I glass from 620°C to 540°C.

The solid lines represent the fit of the data by

the relaxation model (Eqs. 12 and 13). The lower

right plot shows how, by the complexity of the

viscosity effect, there are heat-treatment times

between 10 and 100 minutes when the viscosity at 560°C is lower than at 600°C.

16
0.1 I 10 100 1,000 10,000 100,000 HEAT-TREATMENT TIME (mm)

10 100 1.000 10,000 100,000

HEAT-TREATMENT TIME (mm)

0.1 I 10 100 1,000 10,000 100,000 HEAT -TREATMENT TIME (min)

1 o

16 -

1 1 1

1 1 .-***-

I0 15

//

I0 14 - -

Q. •7

VISCOSITY

O

O

'/

560° C "

10" -

I0 10

1

1 1 10 100 1,000 10,000 100,000 HEAT -TREATMENT TIME (mm)

10 100 1,000 lOPOO 100,000 HEAT-TREATMENT TIME (mm)

i s*

o

15

W -

f' &^

~

o

13 -

600 "C/ '560 'C

-

o

12 rP J -

io" - -

in10 i i 1 1 i 10"' 10° 10' 102

IO3 TIME (mini

17
Fig. 12. Composite illustration of the isothermal viscosity

of samples of the Type II glass from 545 °C to 660 °C. The solid lines represent the fit of the

data by the relaxation model (Eqs. 12 and 13).

18
0.1 I 10 100 1,000 10,000 100,000 °- ! ' l0

_ l0° L000 l0.°° 100,000 HEAT TREATMENT TIME (min) HEAT-TREATMENT TIME (min)

0.1 I 10 100 1,000 10,000 100,000 HEAT-TREATMENT TIME (min)

I 10 10 1,000 10,000 100,000 HEAT TREATMENT TIME (min)

i r / io 12

R io 10 -

1 1 I 1 1

- 630 °C ....r^""^-.

* 634°C

- 637.6 °C .*£*%*•**" ~oa „0 =n 644.6 °C

.jKj» e a °°°" °° 660°C

' 652 °C

- -

-

!

1 1 1 1

-

I 10 100 1,000 10,000 100,000 HEAT TREATMENT TIME(min)

I 10 100 1,000 10,000 100,000 HEAT TREATMENT TIME (mm)

19
the load at the beginning of the viscosity measurement (delayed

elastic effect)

.

3.2.2 Effects Associated with Application of the Load

For the Type I glass samples, several sets of measurements were made at 600 °C. The unloaded fibers were inserted into the furnace and

then were loaded after different lengths of heat treatment to detect and

identify the delayed elastic effects (see fig. 13)

.

10 100 1,000

HEAT -TREATMENT TIME (min)

10,000 100,000

Fig. 13. Test of delayed elastic effects in

samples of Type I at 600 °C. W indicates

load application.

If the viscosity change observed is a delayed elastic effect and

not due to phase separation, then it will take the same time after

loading for the viscosity to reach a given value, whether the load was applied immediately after the fiber was inserted into the furnace, or after a long elapsed time at the measuring temperature. In other words,

the duration of the heat treatment prior to loading should have no

20
effect on the measured time-dependent viscosity after loading. This

test is shown in table 1. The first column of table 1 lists the

Table 1. Test for the Increase in Viscosity

Equilibrium viscosity Time at which Measured time Calculated time

when load was applied

Poises

load was

applied

Minutes

for relaxation

of delayed

elastic effect

Minutes

for relaxation

of delayed

elastic effect

Minutes

2 0.2 0.1

30 10 11.5

400 150 150

1580 240 300

6320 600 450

10

11

10

13

1.3 X 10

14

2.6 X 10

3.6 X 10

14

14

viscosity of samples loaded immediately after insertion into the furnace

but corresponding to an elapsed time listed in column 2. Other fibers were heat-treated for the same times before loading (delayed-loaded)

.

The third column then lists the time taken by the viscosity of the

delayed-loaded specimens to reach the viscosity value listed in column 1. If the viscosity change observed on fibers loaded immediately after

insertion into the furnace is a delayed elastic effect, then columns 2 and 3 should list equal times, i.e., the viscosity curves should be

identical regardless of the elapsed heat-treatment time prior to loading. It is evident from the table that column 2 lists times much

larger than column 3, particularly for the higher viscosities.

Therefore, it can be concluded that the viscosity change observed on the fibers loaded immediately upon insertion into the furnace is not a delayed elastic effect but rather a true viscosity change. In these

cases, due to the initial low viscosity, the expected delayed elastic

effect is hidden by the 2-minute warm-up period, as shown below.

21
A delayed elastic effect on the viscosity is, however, visible at 540°C for times less than 3 minutes, due to the high starting viscosity.

The delayed elastic effects can be further investigated by studying

the fibers which were loaded after different heat-treatment durations.

The times taken for their viscosities to reach the instantaneouslyloaded curve are listed in column 3 and represent the relaxation of the

applied stresses. At any time during the heat treatment, these values

are related to the viscosity by the expression:

t_ = C t = C n/G . (6) DE o o

OT

C relates the observed elastic effect relaxation time to the average

relaxation time t , and we shall assume that it remains unchanged with

phase separation. The shear modulus, G , of an immiscible system undergoes little change due to composition fluctuations, and therefore, will

also be assumed to be independent of phase separation. The observed

time for the relaxation of the delayed elastic effects is thus directly

proportional to the viscosity at the application of the load. By calculating the proportionality constant for one of the delayed loadings

14

(t = 150 min for n =1.3x10 poises), we can test the relationship

DE

for the other loadings. The times thus calculated from Eq. (6) are listed in column 4 of table 1. The good agreement between columns 3 and 4 indicates that the times listed in column 3 indeed represent the

relaxation of the delayed elastic effects by a shear flow process

characterized by the viscosity listed in column 1.

Fibers loaded at a viscosity of 10 poises exhibit delayed elastic

effects for only 0.1 minutes, therefore, the viscosity curves of fibers

loaded immediately after insertion into the furnace (fig. 11) represent

real changes in the viscosity as a function of time after heat-treatments

of 2 minutes (warm-up period)

.

Measurements were also made with loads varying up to

2 7 2 2,500 lb/in (1.72 x 10 N/m ). No stress-dependent behavior was

observed,, indicating that Newtonian viscous flow is occurring during

the measurements.

22
The same test was conducted on samples of the Type II glass at 565°C and 620°C (fig. 12). Here the analysis is more complicated since

the curves reveal two separate periods of viscosity increase. Examining

the results of loading the fibers after different treatment durations,

indicates that neither apparent viscosity increase is caused by the

effect of applying the load. As in the first glass, varying the applied

loads showed the viscosity to be stress independent, up to stress levels

of 2,500 psi (1.72 x 10

7 N/m2

)

.

3.2.2 Phase -Trans it ion Temperature Measurement

The phase-transition temperature of the Type I glass was measured

by the opalescence-clearing method which is described in detail in

ref. 8. However, this convenient technique could not be used on samples

of Type II because opalescence was very difficult to achieve. (We have

one sample of Type II which scatters light with the characteristic blue

color, and it was heat-treated for approximately two months at 590 °C.)

Therefore, we resorted to the viscosity measurements to determine the

transition temperature.

Measurements of the isothermal viscosity at 652 and 660°C

exhibited no time dependence after the first three minutes (see fig. 12,

lower right) . Since this period is not long enough to reach equilibrium

in a phase-separating system due to the high viscosity of the glass, it was concluded that these temperatures were above the phase-transition

temperature. (See the next section for a discussion of the early viscosity changes.) Below 652 °C, the isothermal viscosity increases with

time in an approximately linear fashion on a log-log plot. The slope of

this dependence was found to vary linearly with temperature and goes to zero at 649°C (see fig. 14). Since this slope is a measure of the

phase separation effect on the viscosity, the intercept at 649 °C is the

phase transition temperature.

23
0-5

0.4

uj 0-3

Q_O

<» 0.2

0.1

0.0

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

*>*•^v Tc = 649°C

v.

v^ _ v. \ ^««* •

_ \ \ ^ \

• "^

-N. ^

\

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 >< 1 . 1 1

1 •

620 630 640 TEMPERATURE (°C)

650 660

Fig. 14. Measurement of temperature for which

the time dependence of the viscosity

ceases

.

3.2.4 Effects Associated with the Micros tructure Growth

The analysis of microstructure development in the glasses investigated indicates that the increase in microstructure size is the

primary change occurring during the viscosity increase. The volume

fractions remain constant, and the differences in phase composition

between samples of Type II treated at different temperatures has been

discussed. Following Eq. 5, we may plot the isothermal viscosity as a function of microstructure size in figures 15 and 16. The curves demonstrate the same behavior. In the Type II glass samples, however,

the early increase in viscosity also occurs at temperatures above the

phase transition temperature: 652 and 660°C, where only supercritical

composition fluctuations may exist.

The effect is not simply due to heating of the fiber to the treatment temperature because it lasts longer than the 2-minute heating time

at the lower temperatures , and because simple heating would cause a decrease in viscosity. Our microstructure size measurements at later

times indicate that significant subcritical microstructure growth does

not occur during these short treatment times and, therefore, cannot

account for the effect.

24
10'

10*

I0'

:

S io 14

10'-

10"

ill -

IO

11

II 1 1 1 1 1 1

^ ^ 560°C 540 °C »°°°

• <P .' ° _ 580°C

. "^

o o % ~* 600°C

•• / .-?

- »"

o ° »° „«*••*

- .• <.o° #

620°C

- / » B

.<* <• f • y : : 00

- ** ** ; ° o#° 8° o ° „B •• o fl D

• „tf" f» ° 0=°° ..-<•

700 1500 ii i i i i i i 70 100 200 400 1000 2000

MICROSTRUCTURE SIZE A (angstroms)

Fig. 15.

Viscosity of

Type I samples

plotted as a function of microstructure size,

A.

Fig. 16.

Viscosity of

Type II samples

plotted as a function of microstructure size,

A.

ivj1 | I

I

1 1 | 1

jf 545 °C

IO

15 f y 565 °c -

•" * <pJ° / „• ..• 585 °C

IO 14 //

'to ^v^-\>V'^°600X

a>

"> ii o IO 13 a. . ,° ..-* 00°° .^620°C

• • O o •• >- . ° .•'* o°°

1- • • o o . *• r g IO

12

• o °° ..•*" -&**» 630 °C

CO>

IO 11

• * o° ° .•J..'

00 ^.— ' 6376 °C

00=°° .* ° rT- '„'*"" """ " ° 644.6 °C

.

° »" .

* °TM

.' ° <f o°

• °o* •

•

°o • °

IO 10 — o°° • * —

IO 9 1 1 1 1 1 I 1 1 50 100 200

MICROSTRUCTURE SIZE (Angstroms)

500

25
There are two possible processes which can result in this early

increase in viscosity. This is an expulsion of water from the glass in

heating, or the growth of supercritical composition fluctuations.

Vacuum infrared absorption measurements and a microscopic examination

of fibers heat-treated for very short times are needed for a further

analysis of the effect. These are both difficult tests and it appears

that the effect is not related to subcritical microstructure growth,

therefore, we will defer further discussion of the effect to another

report.

Herein we shall analyze the entire viscosity increase in the

Type I glass and the later increase in the Type II glass in terms of the

associated microstructure growth as shown in figures 15 and 16.

3.3 Formulation of Viscosity Change

3.3.1 Model

A model describing the effect of point-to-point variations in the

local molecular environment of inorganic glasses on the viscous flow

processes has been proposed [7]

.

This model is an outgrowth of viscous relaxation measurements con- ducted in a series of soda-borosilicate glasses above their phaseseparation temperatures [7,10]. (One of the glasses in the series is

identical to the material investigated herein denoted as Type I.) Above

the phase-separation temperature, composition fluctuations occur in the

melt, and have well characterized temperature-dependent wavelengths and

lifetimes. These fluctuations, in effect, produce distributions of

local molecular environments which vary with temperature. The effect

of changes in these local environments on viscous flow was investigated

by shear ultrasonic spectroscopy. It was found that viscous flow was greatly affected by differences in local molecular environments around

flow species in the melt. The environmental relaxation model developed

in that investigation describes quantitatively the behavior of the

distribution of shear relaxation times as a function of temperature

for changing distributions of microstructure sizes.

26
The model is based upon the concept that the shear structural

relaxation processes which control the viscosity are highly dependent

upon the size and type of microstructure present in the melt. If one

has a system whose interconnected microstructure may be represented by

two phases of widely different viscosities there are three types of

viscosity behavior which will ensue, depending upon microstructure

size only. The latter which we will denote by A is considered relative

to a size, r , which represents the range of influence for viscous flow

of a molecule. A sphere of radius, r , is defined such that the com- position of a region contained within it has a direct effect upon the

shear structural relaxation properties of the molecule in the center,

while the region outside r has no effect. There are, therefore,

three viscosity regions which can be characterized by the ratio A/r

(see fig. 17)

.

Fig. 17. Graphical demonstration of the effect

of relative sizes, A/r , on the viscous

flow properties of molecules located

in each phase.

27
In the case of very small micros tructure, A<<r , space averaging

within r leads to similar compositional make-up around the various

molecules. Their relaxation times or activation energies are equivalent.

Therefore, distributions are narrow and normal viscosity behavior is

expected. As the microstructure grows, A^r , the neighborhoods differ

and lead to non-equivalent relaxation times or activation energies.

This produces a broadening distribution of relaxation times and leads

to an anomalous increase in viscosity. The broadening in the distribution and increase in viscosity are accentuated as A grows. However,

for very large microstructure sizes, A>>r , most neighborhoods are well

embedded within the fluctuations and further changes in A have relatively little effect on their relaxation properties. The distributions

are broad, but are no longer dependent on A. The anomalous viscosity

increase saturates.

The formulation of the model [7] leads to an expression for the

width of the distribution of activation energies in terms of the ratio

of microstructure size to range of influence, A/r .

<(6E)

2 > = RTAE f(A/r

Q

) = RTAE [ 1+(

* /A)

2]

3/2

(7a)

where AE is a constant representing the difference in activation energies between the totally mixed state E , and either phase after total

separation. Implicit in this expression is the formulation of the

activation energy spectrum in terms of a Gaussian distribution (i.e.

,

2 2 the definition of <(<5E) >) . In this case, E + 1/2 <(6E) > is the

activation energy of the viscous phase, and E - 1/2 <(6E) > is the

RT activation energy ascribed to the fluid phase. The function f(A/r ) represents the gradual broadening of the distribution resulting from

an increase in structure size, A.

A Gaussian distribution describes well the effect of supercritical

fluctuations, however, below T , the changes in composition are larger

and thus, the differences in activation energies between the phases are high and more suitably represented by two well separated values.

28
Another modification is necessary because the effect of microstructure on the activation entropy was neglected for simplicity in the

analysis of reference 7. Since this provided a good fit of the data,

the activation entropy contribution was left out of the model. As will

be seen in the subsequent analysis below, the data from the Type II glass

cannot be treated without an activation entropy contribution. Therefore,

we will introduce this term into equation (7a) before proceeding.

<(6H)

2

> = RT AH f(A/r

Q

) = RT AH [ x + (r /K) 2]

3/2

(7b)

where the activation enthalpy is a sum of the activation energy and

entropy: AH = AE - TAS.

We consider, below, the case where the volume fractions are not

equal, and the distribution is bimodal. The viscosity at any temperature or time is a measure of the average relaxation time, t, of the

system:

r

n(T,t) = G t = G t g (lnx)dlnT (8)

g(lnx) is the viscoelastic distribution of relaxation times and G is ° V ' CO the solid-like shear modulus . In terms of activation enthalpies the

viscosity is written as:

1^°

n(T,t) = G^A eH/RT g(H,t)dH (9)

A is the pre -exponential from the rate equation. The activation enthalpy

distribution g(H) may now be written as:

g(H,t) = v

1 6(H-H

1 )+v

2 6(H-H

2

) (10)

where v and v„ are the respective volume fractions, and the viscous

phase has H,lolo = H + AH n f(A/r ) and the fluid phase has H n = H -AH„ f(A/r ) I o 2 o

29
This has the following result on the viscosity:

^1 f(A/r ) ^2 f(A/r ) lnn(T,t) = ln[G A] + H /RT + ln[vn e RT ° + v,e RT °

] (11) 00 O 1 Z

where A contains the implicit time dependence. Unless v »v , or AH

is very small (<4 kcal/mol) the second term in the right-hand bracket

is always negligible at the temperatures considered since f(A/r ) only

varies between and 1.

The expression for the time-dependent viscosity may then be simplified to:

(AE - TAS -) ( 1 „\3/2 "W~ X

\ l+[r /A(t)] Z lnn(T.t) ) = lnn(T.o) + -RT [ l+[r„/A(t)] ) (12)

where the mixed system has the viscosity:

lnn(T.o) = Inn + E / . (13)

O O KL

The constant S /R is incorporated in Inn . The temperature dependence

is explicitly shown in the equation and only the value of the deviation

of the viscous phase activation enthalpy from the mixed system

(AH = AE - TAS ) affects the result. The time dependence and the

effect of the activation energy for flow in the fluid phase are introduced through the microstructure size, A, as in Eq. (5).

3.3.2 Analysis of Results

The model allows for a fit of the viscosity data and this is shown

by the solid line in figures 11 and 12, with the following parameters:

30
Type I Glass

r = 57 1

o Inn = -50.43

°

5 E = 128 kcal/mol (5 . 35x10 J/mol) ; AS =

E = 97 kcal/mol (4.06xl0

5 J/mol)

AE (620) = 18.9 kcal/mol AE (580) = 17.8 AE (540) = 15.6

AE (600) = 18.2 AE (560) = 17.8

Type II Glass

r = 58 i

o Inn = -20.20

°

5 E = 84 kcal/mol (3.51x10 J/mol)

5 E

2 = 70 kcal/mol (2.93x10 J/mol)

^S

± = 33.6

AE (620) = 65.1 kcal/mol AE (585) = 66.5 AE (545) = 64.3

AE

1 (600) = 66.0 AE1 (565) = 65.8

In general, the theoretical curves fit the data very well and

describe accurately the viscosity change as a function of time. The

mixed state viscosity, n(T,0) deviated slightly from the assumed

Arrhenian behavior (Eq. 12) in samples of the Type I glass (see fig. 18),

therefore, the value of Inn varied ±0.4 in order to fit the data.

o The small variation in AE from 620 °C to 560°C in the Type I samples

is probably experimental uncertainty since in all cases we have measurements where f(A) has approached its saturation region. At 540°C,

however, the lower value of 15.6 kcal/mol is probably due to a yet

incomplete phase separation. The activation energy for flow in the

viscous phase is 146 kcal/mol [E- = E +AE- ] . As in reference 7, the

1 o 1 activation entropy AS., is negligible.

31
10''

"i i i r

146 kcal/mole *Ss*

l50Kcol/mole

COo

u

to

10"

I0

C • i7n

(T,00) J L_ |

i i i 5-6 5.8 6.0 6.2 INVERSE TEMPERATURE (I04/RT)

Fig. 18. Plots of the mixed-state viscosity,

n(T,o), and the totally-separated

viscosity, ti(T,°°), as a function

of inverse temperature for both

glasses

.

In samples of the Type II glass, the mixed state viscosity ,n(T ,o)

,

is Arrhenian (see fig. 18). In this glass, however, there is a sizeable

activation entropy. Fitting the data without it required temperature

dependent values for AE , and values too small to be physically meaningful.

With an activation entropy, we have an excellent fit of the data, and

the value of AE at 66 kcal/mol yields an activation energy of 150 kcal/mol

for the more viscous (high SiO_) phase. This compares well to the

activation energy for viscous flow in pure SiO [20]

.

32
4. Conclusions

We have presented viscosity measurements conducted on two glasses

with similar compositions but different immiscibility behavior. Both

glasses are basically soda-borosilicates . The first (Type I) is a "model" composition for immiscibility studies. Its phase transition

occurs near the maximum of the immiscibility surface for the system.

Therefore, the transition is easily observed and phase separation

occurs quickly. The phases have equal volume fractions, and exhibit

a marked difference in chemical durability. The second glass (Type II)

is widely used for chemical glassware, and its transition is difficult

to detect. The glass contains more silica and less soda than Type I, and 2% alumina. The composition is located in the narrowing portion

of the immiscibility dome, and the transition temperature is approximately 100°C below that of Type I.

The viscosity measurements were made below the phase transition

temperature of each glass in order to investigate the effect of microstructure growth on the viscous flow processes. The viscosity increased

by 4 to 5 orders of magnitude as a function of heat-treatment time

(ranging to 100,000 min.) at a fixed temperature.

An analysis of the microstructure development with electron

microscopy of replicated, etched surfaces, showed that an increase in

microstructure size was primarily responsible for the viscosity increase.

This growth followed a 1/3 power of time suggesting the rearrangement

stage of phase separation. An analysis of the growth rate at different

temperatures yields an activation energy for the regarrangement process

which is probably occurring by bulk diffusion of silica through the

fluid phase. This was 97 kcal/mol for the Type I and 70 kcal/mol for

the Type II glass. The low activation energy for Type II was coupled

with a very small pre-exponential term indicating the effect of an activation entropy. The latter explains the observed slow microstructure

growth. The difference in growth rates between the glasses is as expected, since the fluid phase of Type I contains less silica. Also

expected is the value of 97 kcal/mol for the activation energy for

33
bulk diffusion in the fluid phase of Type I. The value of 70 kcal/mol

for Type II is surprisingly low. This is not due to a possible different

rate-controlling mechanism for rearrangement since they would all exhibit

a higher energy than bulk diffusion. It has been suggested that the

tie lines in this system run approximately parallel to the Na„0-SiO„

boundary [19], therefore, the fluid phases of Types I and II contain

approximately equal amounts of sodium oxide (see points 35 and 42 in

fig. 1 of Ref . 8 for comparison) . It appears that the change in BO

affects the pre -exponential term (possibly through the activation

entropy). It is interesting to note finally, that while the measurements were conducted in the same viscosity range for both glasses, the

structure of Type I developed faster than Type II because of the change

in the p re-exponential term.

A model relating a change in viscosity to a change in microstructure

size through the distribution of activation energies was modified to apply to subcritical microstructure growth. The resulting equation

was fitted to the data with excellent quantitative results. The parameters from the fit yield the mixed-state viscosity, the viscosity of

the silica-rich phase, and the range of molecular influence for viscous

flow in the liquid. The mixed-state viscosities for both glasses follow

well the data measured above the transition temperature (see fig. 1)

.

Curiously again, the Type II glass has a lower activation energy than

Type I despite their composition differences, indicating that the preexponential terms in the rate equation dominate in this temperature

range. The change in viscosity resulting from phase separation is

smaller in Type II as expected, since the change in composition is less

due to a narrowing of the immiscibility dome (see Ref. 8). This

narrowing also accounts for the small difference in chemical durability

observed in Type II when leaching during sample preparation for electron

microscopy. The activation energy for diffusion in the viscous phase of

Type I is 146 kcal/mol, which is within the range of values reported for

silica [20] . Type II shows 150 kcal/mol which is also close to silica,

and is a reasonable value since its viscous phase is primarily composed

34
of SiO and Al .

Finally, in comparing the value of r , the range of molecular

influence for viscous flow, between the two glasses, we obtain excellent

consistency with 57 and 58 A for Types I and II, respectively. This

also agrees well with a value obtained for the Type I glass (41-56A)

by ultrasonic relaxation spectroscopy far above the transition temperature (800-1200°C) . This parameter defines the extent of the local

environment around the flow species or molecules and the shape of the

viscosity curve in terms of microstructure size is sensitive to its

value. The consistency obtained here along with the excellent quantitative fits of the data supports the application of the model to this

process.

In closing, we find that there is a large effect of subcritical

microstructure growth on the viscosity of glass. As shown in figure llf,

this effect is complex and will strongly affect stress release in the

glass due to the time-dependent viscosity increase. The effect can be

quantitatively described in terms of a model presented here. Values

of the parameters used in the model appear to support the physical

significance assigned to them during the development of the model.

Application of the model only depends upon the activation enthalpy

differences and the size of the structure. This, therefore, facilitates

its use on microstructure development in materials, arising from causes other than immiscibility

.

* Chemical analysis of a similar glass phase-separated and leached at this laboratory indicated that all the A1„0 remained in the phase

rich in silica.

35
5. References

[I] J. Konnert, J. Karle and G. A. Ferguson, Bui. Am. Ceram. Soc.

5_1, 374 (1972).

[2] A. Napolitano and P. B. Macedo, J. Res. Nat. Bur. Stand. (U.S.)

72A , 425 (1968)

.

[3] J. Zarzycki and R. Mezard, Phys . Chem. Glasses 3_ (5), 163 (1962).

[4] E. A. Porai-Koshits , V. I. Averjanov, N. S. Andreev and

V. V. Golubkov, IX International Congress on Glass 1, 391 (1971).

[5] T. P. Seward, III and D. R. Uhlmann, Amorphous Materials,

R. W. Douglas and B. Ellis, Eds. (John Wiley & Sons, London,

1972) p. 327.

[6] W. Vogel, Silikattechn. 9_, 51 (1958); ibid. 1£, 241 (1959);

T. H. Elmer, M. E. Nordberg, G. B. Carrier and E. J. Korda,

J. Amer. Ceram. Soc. 53^, 171 (1970).

[7] J. H. Simmons and P. B. Macedo, J. Res. Nat. Bur. Stand. (U.S.)

75A , 175 (1971)

.

[8] W. Haller, D. H. Blackburn, F. E. Wagstaff and R. J. Charles,

J. Amer. Ceram. Soc. 53_, 34 (1970).

{9] J. H. Simmons, A. Napolitano and P. B. Macedo, J. Chem. Phys. 53 , 1165 (1970).

[10] J. H. Simmons and P. B. Macedo, J. Chem. Phys. 5_3, 2914 (1970);

ibid 54_, 1325 (1971) . [II] J. H. Simmons, P. B. Macedo, A. Napolitano and W. Haller,

Discussions of the Faraday Society 5_0, 155 (1970) . [12] W. Haller, J. Chem. Phys. 42_, 686 (1965).

[13] R. Mahoney, G. R. Srinivasan, P. B. Macedo, A. Napolitano and

J. H. Simmons, Phys. Chem. Glasses, submitted.

[14] H. Bach, J. Non-Crystalline Solids 3_, 1 (1970).

[15] P. F. James and P. W. MacMillan, Phys. Chem. Glasses 11, 59 (1970).

[16] A. Napolitano and E. G. Hawkins, J. Res. Nat. Bur. of Stand. (U.S.)

68A , 439 (1964).

[17] B. F. Howell, J. H. Simmons and W. Haller, "Chemical Durability

Deterioration of a Phase-Separated Glass," in preparation.

36
[18] J. Zarzycki and F. Naudin, J. Non-Crystalline Solids 1, 215 (1969)

[19] 0. V. Mazurin, V. P. Kluyev and G. P. Roskova, Phys. Chem.

Glasses 11, 192 (1970)

.

[20] G. Hofmaler and G. Urbain, Science of Ceramics 4, 25 (1968).

U. S. GOVERNMENT PRINTING OFFICE : 1 973—5^2-6^9/39

37

ormNBS-114A (1-71) U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS TN-7922. Gov t Accession No.3. Recipient's Accession No.

4. TITLE AND SUBTITLE Investigation of Viscous Flow in Glass During Phase Separation

5. Publication Date September 1973

6. Performing Organization Code

7. authorCs) J. H. Simmons, S. A. Mills, A. Napolitano

D. H. Blackburn, W. K. Haller

8. Performing Organization

9. PERFORMING ORGANIZATION NAME AND ADDRESC NATIONAL BUREAU OF STANDARDS DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 3130426

11. Contract/Grant No.

Naonr 12-73, NR 032-536

12. Sponsoring Organization Name and Address Office of Naval Research

Department of the Navy

Code 471

Washington, D.C. 20360

13. Type of Report & Period Covered 1st Annual Report

(11/72 - 6/73)

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant bibliography or literature survey, mention it here.) The isothermal viscosity of two borosilicate glasses, of which one is a commercial

glass widely used for chemical glassware, shows a large increase (4 to 5 orders of magnitude) with heat-treatment time (ranging up to 100,000 min) near the annealing

point. The two glasses have similar compositions, but differ greatly in their

phase separation characteristics. Electron micrographs are used to analyze the

development of microstructure during the suspected phase separation. In both

glasses, it is found that the structure development is primarily responsible for

the viscosity increase. An analysis of the data, and a theoretical interpretation

of the effect are presented.

17. KEY WORDS (Alphabetical order, separated by semicolons) Glass; microstructure; phase separation; viscosity.

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Studies and compilations designed mainly for the mathematician and theoretical physicist. Topics in mathematical statistics, theory of experiment design, numerical analysis, theoretical physics and chemistry,

logical design and programming of computers and

computer systems. Short numerical tables. Issued quarterly. Annual subscription: Domestic, $9.00; Foreign,

$11.25.TECHNICAL NEWS BULLETIN

The best single source of information concerning the Bureau's measurement, research, developmental, co- operative, and publication activities, this monthly

publication is designed for the industry-oriented

individual whose daily work involves intimate contact with science and technology

for engineers, chemists,

physicists, research managers, product-development

managers, and company executives. Includes listing of

all NBS papers as issued. Annual subscription: Domestic, $6.50; Foreign, $8.25.

H0HPER100ICALS Applied Mathematics Series. Mathematical tables, manuals, and studies. Building Science Series. Research results, test methods, and performance criteria of building materials, components, systems, and structures. Handbooks. Recommended codes of engineering 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 quantitative 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 co- operatively 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. Consumer Information Series. Practical information, based on NBS research and experience, cover- ing areas of interest to the consumer. Easily understandable language and illustrations provide useful background knowledge for shopping in today's tech- nological marketplace.

BIBLIOGRAPHIC SUBSCRIPTION SERVICES

The following current-awareness and literature-survey bibliographies are issued periodically by the Bureau : Cryogenic Data Center Current Awareness Service (Publications and Reports of Interest in Cryogenics). A literature survey issued weekly. Annual subscription : Domestic, $20.00; foreign, $25.00.

Liquefied Natural Gas. A literature survey issued quarterly. Annual subscription: $20.00. Superconducting Devices and Materials. A literature survey issued quarterly. Annual subscription : $20.00. Send subscription orders and remittances for the preceding bibliographic services to the U.S. Department

of Commerce, National Technical Information Service, Springfield, Va. 22151. Electromagnetic Metrology Current Awareness Service (Abstracts of Selected Articles on Measurement

Techniques and Standards of Electromagnetic Quantities from D-C to Millimeter- Wave Frequencies). Issued monthly. Annual subscription: $100.00 (Special rates for multi-subscriptions). Send subscription order and

remittance to the Electromagnetic Metrology Information Center, Electromagnetics Division, National Bureau

of Standards, Boulder, Colo. 80302. Order NBS publications (except Bibliographic Subscription Services) from: Superintendent of Documents, Government Printing Office, Washington, D.C. 20402.
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