P(iA/y r) O -

NATIONAL BUREAU OF STANDARDS REPORT

7857

EQUIPMENT FOR THERMAL EMITTANCE MEASUREMENTS

ABOVE 1400°K

ANNUAL SUMMARY REPORT

January, 1963

Contract No. H-22727

3-84-2-41-1306-01-000-8400-2561-4

80X0108 (62) R&D, MSFC, OBJECT 25

GEORGE C. MARSHALL SPACE FLIGHT CENTER

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION

Huntsville, Alabama

NBS

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

Functions and Activities

The functions of the National Bureau of Standards are set forth in the Act of Congress,

March 3, 1901, as amended by Congress in Public Law 619, 1950. These include the development and maintenance of the national standards of measurement and the provision of means

and methods for making measurements consistent with these standards; the determination of

physical constants and properties of materials; the development of methods and instruments

for testing materials, devices, and structures; advisory services to government agencies on

scientific and technical problems; invention and development of devices to serve special needs

of the Government; and the development of standard practices, codes, and specifications. The

work includes basic and applied research, development, engineering, instrumentation, testing,

evaluation, calibration services, and various consultation and information services. Researcli

projects are also performed for other government agencies when the work relates to and supplements the basic program of the Bureau or when the Bureau's unique competence is required.

The scope of activities is suggested by the listing of divisions and sections on the inside of

the back cover.

Publications

The results of the Bureau’s research are published either in the Bureau's own series of

publications or in the journals of professional and scientific societies. The Bureau publishes

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published in four separate sections, presents complete scientific and technical papers; the Technical News Bulletin presents summary and preliminary reports on work in progress; and the

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Miscellaneous Publications, and Technical Notes.

A complete listing of the Bureau’s publications can be found in National Bureau of Standards Circular 460, Publications of the National Bureau of Standards, 1901 to June 1947 ($1,25),

and the Supplement to National Bureau of Standards Circular

0 460, July 1947 to June 1957

($1.50), and Miscellaneous Publication 240, July 1957 to June 1960 (includes Titles of Papers

Published in Outside Journals 1950 to 1959) ($2.25); available from the Superintendent of

Documents, Government Printing Office, Washington 2o, D.C.
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1009-11-10493 7857

EQUIPMENT FOR THERMAL EMITJANCE MEASUREMENTS

ABOVE 1400 K

ANNUAL SUMMARY REPORT

January. 1963

Contract No. H-22727

3-84-2-41-1306-01 -000-8400-2561-4

80X0108 (62) R&D . MSFC, OB JE CT 25

George C. Marshall Space Flight Center

National Aeronautics and Space Administration

Huntsville, Alabama

Prepared for

IMPORTANT NOTICE NATIONAL BUREAU OF STAI

for use within the Government. B* and review. For this reason, the | whole or in part, is not authorize Bureau of Standards, Washington

the Report has been specifically pi

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

accounting documents intended

ubjected to additional evaluation

listing of this Report, either in Office of the Director, National he Government agency for which

pies for its own use.

U. S. DEPARTMENT OF COMMERCE

NATIONAL OUREAU OF STANDARDS
i *

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foreword

able equipment r «*—«.• -«ltmetals and nonmetals in the t _ total and spectral emittance of both

on Jan. 1 1961 at the NnM

teTMparature ran8 e 1*00 - 2500°K was initiated

of the George C. Marshall Space Flight

^A

SP °nS °rShip

and Space Administration tv»« ~~ , t *le Natlona l Aeronautics

of Dr. Klaus Schocken, M-RP-T, Bldg 4488

3S

^ ^

technical supervision

The present report summarizes the Droere<?<; f-v,a«- ttqo , calendar vaar- i oto mi F § s that was made durins the

t r,ri

r

d - “• »°»* <* »«*«..

during the course of the invlltigatiTM S aff

?

8 the Sr°Up leader

the investigation included A. G. Eubanks, B. A.^a^H "e° Culk^fl

^

Crigler, J. C. Richmond, and W. N. Harrison.

’ k’

’ W ‘

ABSTRACT

A spectrometer was procured for use with the rotating cylinder equipment. The source optics were removed and replaced with transfer optics

that permitted focusing of beams from a blackbody furnace and from the

rotating specimen respectively on the two slits of the monochromator.

Preliminary results with platinum and sintered alumina at 1 to 15p indicated that the equipment was capable of satisfactory operation; however

more work is indicated to eliminate several potential sources of error.

A large number of measurements of total normal emittance were made

with the induction heating equipment. Tests with specimens of steatite

(magnesium aluminum silicate) indicated that the measured emittances were

too low when the depth-to-radius ratios of the reference holes were greater than 2.0. On the other hand, total normal emittance measurements on platinum and tungsten specimens were as much as 307o higher than prevailing

literature values when the blackbody reference holes had a depth-to-radius

of 10 (3.5 mm depth by 0.35 mm radius). Reexamination of the optics used

with the induction heating equipment showed that aberrations were present

to an extent that would cause high emittance readings if the reference

hole was less than 0.35 mm in radius. This source of error can be eliminated either through use of larger holes or by substitution of an ellipsoidal mirror for the present 5" spherical mirror.

Typical results obtained with both the rotating cylinder and induction

heating equipment are included.
.
I. OBJECTIVES

The primary objective of this project is to develop reliable and accurate

techniques for the measurement of the thermal radiation properties of both

metals and nonmetals at temperatures up to 2500 K. A secondary objective is

to provide emittance data of known accuracy for materials of interest to the

space program.

Two measurement approaches are being used in the investigation. The

first is for normal spectral measurements (1 to 15p) in the range of about

1000° to 1800°K (rotating cylinder method) and the second for total normal

emittances at temperatures that may go as high as 2500°K (induction heating

method). In both approaches emphasis has been placed on devising measurement

procedures that will reduce temperature gradients in nonmetals and that will

permit the specimen and the reference blackbody to operate at very nearly the

same temperature.

Progress made in perfecting both types of measurement during the period

Jan. 1, 1962 to Jan. 1, 1963 is described in the present report.

II. ROTATING SPECIMEN METHOD FOR MEASUREMENTS TO 1800°K

A. A Brief Description of Method

The rotating specimen method has been described in detail in the first

summary report (ref. 1) which also includes a description of the furnace.

Briefly, the procedure is as follows: A hollow cylindrical specimen,

1" outside diameter with a 1/8" wall, is rotated in a platinum-wound furnace

equipped with a water-cooled viewing port. Because the specimen is rotating,

a freshly heated surface is arriving continuously at the port. The method

is based on the premise that at a sufficiently high speed of rotation, the

temperature change of any given area on the specimen surface while passing

the port will be too low to significantly affect the measurements and, also,

that temperature gradients from the surface inward will be largely eliminated.

The method by which these gradients were analyzed is given in the next section.

Emittance measurements are made by comparing the radiant flux density

from the rotating specimen with the flux density from a blackbody at the same

temperature. This is done by using suitable transfer optics to focus the two

beams onto the entrance slits of a double beam monochromator which continuously records the ratio of the flux densities in the two beams. Possible error

from uncompensated differences in optical conditions affecting the beams is

largely eliminated by using a second blackbody furnace to obtain a calibration

or

M 1007o curve" prior to obtaining the specimen data.

1
B. Analysis of Periodic Heat Flow in a Rotating Specimen

A theoretical analysis of periodic heat flow in a specimen rotating

before a viewing port was performed, and a report of this analysis has been

prepared for publication (ref. 2). This analysis is more rigorous than that

given in the first annual summary report. The equations that were developed

permitted computation of cyclic temperatures of a specimen as a function of

system geometry, rotational speed, thermal properties of the specimen material, and a derived average heat flux for the portion of the cycle during

which the specimen is being heated by the furnace.

A comparison was made between temperatures of a porous alumina specimen

computed theoretically and those obtained experimentally at low speeds of

rotation. The agreement in the two temperatures was within 6°K. This agreement seemed well within both the possible experimental error and the uncertainty as to the thermal properties of the specimen.

The theoretical analysis showed that at low rotational speeds the surface temperature of a specimen, at the center of the viewing port opening, is

substantially lower than the temperature of the thermocouple in the specimen

cavity. However, this difference decreases with increasing speed reaching in

the case of the porous alumina specimen a computed value of 4°K at about 450

RPM. In the actual measurements, if the temperature near the specimen surface at the viewing port is lower than cavity temperature, the measured emittance will be low. However, as the speed increases the measured emittance

will rise to some maximum value, after which a further increase in speed will

have no measurable effect. This predicted type of behavior is used to determine experimentally the optimum speed of rotation for each specimen at each

test temperature (See Sect.II-E).

C. Blackbody Furnaces and Temperature Control

The blackbody furnaces used with the rotating- specimen equipment were

constructed as indicated in fig. 1. The alumina core was wound with

Pt-207, Rh resistance wire. Power taps (not shown in fig. 1) were positioned

lV from each end of the winding. These taps permitted adjustment of power

inputs to equalize temperatures from front to back. A thermocouple placed

near the center winding of one of the furnaces was connected to a controller

which maintained the temperature in the furnace cavity constant to within

±0.5°K. The second blackbody furnace and the rotating specimen furnace were

adjusted to this same temperature to within about ±1°K by manual control of

power input .i/

1J Final adjustment of the blackbody furnaces was based on the appearance of

the 1007, spectral curve when blackbody no. 1 was inserted in the reference

beam and blackbody no. 2 in the specimen beam of the spectrometer. If blackbody no. 2 was at a slightly higher temperature than no. 1, the curve from 1 to 2p rose above 1007o while if the temperature was slightly lower, the curve

fell below the 1007o line. When the two temperatures coincided the curve was

substantially flat over the entire range of 1 to 15p. This method of determining temperature equivalence has high sensitivity and it is more reliable

for precise work than depending entirely on thermocouple indications.

2
WATER

M'H

BRASS

ALUMINA REFRACTORY A*y.

MULLITE WOOL INSUL.

HIGH TEMP INSUL. BRICK

HIGH TEMP ALUMINA CEMENT

Figure 1. Schematic of blackbody furnace

D. Monochromator and Transfer Optics

A Beckman IR-5A spectrometer with NaCl prism is being used for the

spectral measurements. A flip mirror was inserted into this instrument so that the prism could be bypassed when total normal emittance measurements

were desired (see fig* 2).

The source optics of the IR-5 instrument were removed and replaced by

the transfer optics as shown schematically in figure 2. The off-axis

angle of the spherical mirrors is approximately 5°.

3
A. Blockbody furnac* No. 1 B. Blockbody furnace No. 2

C. Rotating specimen furnace

0. Flat front surface mirror

E. Movable flat mirror FandG. spherical mirrors, 3" diam., 18" focal length

H. 10 cps mirror - type chopper

K. NaCI prism

L. Movable mirror (for total normal measurements)

0. Vacuum -type thermocouple detector

Figure 2. Sketch of optical paths in rotating- specimen equipment

The monochromator is operated in double-beam mode. Energy from each

beam is chopped at 10 cps. Reference and specimen beams are alternately

directed through the prism to a Littrow mirror and the dispersed radiation

is then brought to a focus on a vacuum-type thermocouple detector. The

thermocouple converts the radiant energy to an electrical signal proportional to the difference in energy in the two beams. This signal passes

through a preamplifier and a signal amplifier to a pen- comb servo amplifier. The amplified signal then drives a servomotor to return a linear

reference comb to null position, equalizing the intensity of the specimen

and reference beams. A mechanical connection to the comb drives the pen

which in turn records the ratio of the energies in the two beams. A separate motor drives the wavelength and slit cams as well as the lateral

movement of the pen carriage.

4
E. Experimental Procedure for Determining Optimum Rotational Speed

The following procedure, which is based largely on the theoretical

analysis described in Section II-B, has been developed to determine ex- perimentally the optimum speed of rotation for any given specimen at any

given test temperature:

(1) The specimen is heated to the test temperature while rotating

at approximately 20 RPM.

(2) The monochromator is adjusted to a wavelength setting of 1.4 \i and the measured emittance at this wavelength is recorded—/.

(3) The specimen speed is next increased in increments until the

emittance at 1.4|~i shows no measurable increase with increasing RPM.

(4) At this rotational speed the temperature in the specimen cavity and the temperature of the specimen surface are very nearly

equivalent and hence this speed can be used as the optimum speed of

rotation for obtaining the 'spectral curve. The speed may be increased

somewhat beyond this optimum, but in no case are the measurements made

at slower speed.

F. Measurement Procedure

Once the rotational speed has been fixed by the procedure outlined in

the preceding section, the specimen beam is blocked and a "zero line" obtained. Next mirror E in the transfer optics (fig. 2) is moved into position to focus radiation from blackbody No. 2 on the specimen slit of the

monochromator. The wavelength drive is then started and a "1007, line" is

obtained, after which Mirror E is shifted to the rotating specimen position

and the specimen curve is determined. All three curves are recorded on the

same graph. The true emittance of the specimen is obtained for each wavelength by measuring the height of the recorded specimen curve above the

"zero curve", and dividing this distance by the height of the 1007, line

above the zero line. Due mostly to a lack of perfect optical equivalence in

the two beams neither the "100% line" nor the "zero line" is completely

flat; hence errors would result if a spectral correction of this type were

not made.

Total normal emittances are obtained in the same way as spectral emittances except that Mirror L is moved into position to bypass the prism in

the monochromator and the wavelength drive is not used. The slit openings,

while not critical, must be sufficiently narrow to prevent overloading the

detector

Fig. 3 is a photograph of the equipment.

2/ Difference in temperature between specimen and blackbody can be shown t have the greatest effect on emittance at the shorter wavelengths. The se- lected setting of 1.4|i is well below the energy peak of blackbody radiation

for all four of the preselected test temperatures of 1200, 1400, 1600, and

1800°K.

'
NORMAL

SPECTRAL

EMITTANCE

Figure 3. Photograph of rotat ing-specimen equipment

G. Preliminary Measurements

Figure 4 shows a curve obtained for a polished platinum specimen at

1400 K that had been annealed for one hour in air at 1525 K prior to test;

Fig. 4. Preliminary measurements of normal

spectral emittance of a platinum specimen.

6
WAVE LENGTH- MICRONS

Figure 5. Preliminary measurements of normal spectral

emittance of a sintered alumina specimen.

fig. 5 is a spectral curve for a sintered alumina specimen at 1425 K. Roth

of these curves must be considered preliminary because certain possible

sources of error had not been eliminated at the time of measurement. The

values of emittance in the wavelengths range 1 to 5p for the sintered

alumina are suspect because of their poor agreement with recently reported

data for a similar material (ref. 3 and 4). On the other hand, the agreement of the curve obtained for platinum with that reported by Richmond, et al

(ref. 5) is fairly good and these data, of themselves, indicate that the

equipment is capable of satisfactory operation. The total normal emittances

measured with the equipment at 1400 K were 0.13 for platinum and 0.21 for

alumina

H. Present Status of Measurements and Future Plans

Current work with the equipment is concerned with adjustment of the

blackbody furnaces and the specimen furnace to minimize temperature gradients. When this task has been completed, the transfer optics will be re- aligned. and response linearity will be investigated with sectored disc

attenuators at each of the four selected test temperatures (1200, 1400, 1600,

and 1800 K) . If the response is found to be sufficiently linear, an error

analysis will be performed to determine the overall accuracy of measurement.

In this analysis special attention will be given to the effect of temperature uncertainties and also to the effect of possible translucency of the

7
specimen walls over certain wavelength regions. Finally, the spectral^and

total normal emittance will be determined at 1200, 1400, 1600 and 1800 K

on three specimens each of the following materials: platinum, sintered alumina, magnesia, zirconia, thoria, silica, calcium zirconate, mullite,

magnesium aluminate spinel, zirconium silicate, and steatite (magnesium

aluminum silicate). Specimens of all of these materials have been procured

for the testing program. The nominal chemical analysis of each has been

obtained. All 11 materials are of commercial purity.

Fig. 6 Schematic

of induction

furnace

8
III. INDUCTION HEATING EQUIPMENT FOR

TOTAL NORMAL EMITTANCE MEASUREMENTS TO 2500°K

A. Brief Description of Equipment and Theory of Method

The design of the induction equipment and the reasons for its selection

were given in the first annual summary report (ref. 1), and also in a paper

to be published as a part of the Proceedings of the Symposium on the Measurement of Thermal Radiation Properties of Solids (ref. 6). Fig. 6 is a schematic of the furnace equipment while fig. 7 is a schematic of the optics.

Basically, the measurement consists of determining the ratio of the density

of radiant flux from the surface of a small specimen to the density of flux

from a cylindrical hole drilled into the specimen surface. Fig. 8 shows

the two types of specimen in current use.

AXIS)

Figure 7. Sketch showing both optical paths and electronic components.

SPECIMEN

Fig. 8 Two types

of specimen. Metal

specimen shown has

type of reference

hole on which best

results were obtained. Heavy line

on hole walls re- presents high emittance coating.

9
The principle modification introduced into the equipment during the

contract year was the introduction of a radiation trap at the top of the

bell jar to prevent back-reflected energy from the top of the bell jar and

from the NaCl or CaFs window from entering the specimen beam. This trap

is water cooled so as to eliminate overheating of the window "0" ring

which was one of the more serious problems encountered with the earlier

design

The equipment is used for both metals and nonmetals. For metals the

reference cavity is designed so as to radiate very nearly as a blackbody.

This is accomplished through the use of cavities in which the wall surfaces have been modified by roughening and/or application of a high emittance coating. Fortunately, temperature differences between surface and

reference cavity are not serious with most metals because of their high

heat transfer coefficients and their low emittances which means that

reasonably good temperature equality can be achieved even when sizeable

reference cavities are used. However, this is not the case with nonmetals and for this class of materials a small, shallow reference cavity

is imperative (fig. 8) so as to reduce as much as possible the differences

in temperature between the surface and the bottom of the cavity. An ex- pression derived by Gouffe (ref. 7) permits the emittance of a specimen

with a shallow reference hole to be computed from (a) the ratio of the

flux densities from surface and reference hole and (b) the hole dimensions.

This expression was derived by assuming that (1) the emittance of the hole

walls is the same as the surface, (2) the specimen material is a perfectly

diffuse reflector, and (3) there is no temperature difference between hole

and surface. None of these conditions can be achieved in a real specimen

but they can be very closely approached by most nonmetallic specimens if

the reference hole is of small diameter and of shallow depth.

The Gouffe expression that is employed with the shallow-hole method

is

e = E (1 + R-Rn) ~R (1)

1 + R + E (R-R0 ) where

e = emittance of specimen

E = Ratio of radiant flux density from the sur- face to the flux density from the hole

R = 1

, (for a cylindrical cavity of

2(l+h/r) depth, h, and radius, r.)

R = l

1 +(h/r) 2

Note: The RQ given by Gouffe" is an approximation . It can be shown that

a more rigorous expression for R0 is 2 ( 1 - h A\/h2 +r2

) . However, no significant error is introduced through use of the approximation at h/r values

above about 2.5.

10
B. Measurements on Nonmetals

Data already presented in ref. 6 will not be repeated in this report

but instead attention will be focused on results obtained since preparation of the paper for the Symposium.

1. Steatite . Steatite was selected for an investigation of the effect of hole depth on the apparent emittance. Earlier tests with alumina

had indicated that specimens with reference holes of 0.25 mm radius and

having h/r ratios of 2.5 and greater gave lower emittance values than did

specimens with holes having h/r ratios of less than 2.0. This type of

result suggested that the temperature of the bottom of the deeper holes

was higher than that in the shallow holes, possibly because of less heat

loss by radiation when the hole is deep. Steatite, because of its easy

machineability , appeared to be an excellent material with which to investigate this possible effect.

Six specimens were machined from a rod of unfired steatite. The re- ference hole in each specimen was precision drilled with a flat bottom so that after firing it would have a radius of about 0.25 mm. The hole in

each specimen was drilled to a different depth so as to give a range of

h/r values from about 1 to 4. After firing at 1475 K for 1 hr., measureo y ments were made at 1360 K.

The results are plotted in fig. 9. The upper curve, A, shows the apparent total normal emittance when the hole is considered as a blackbody

(E in Equation 1); the middle curve, B, results when the relative flux

density ratios from surface and hole are reduced to emittance by Equation

1, while the lower curve, C, shows the corrected emittance values when the

exact solution for RQ is used. It should be noted that curves B and C are

practically superimposed for h/r values greater than 2.

Two tentative conclusions can be implied from these data. The first

concerns the use of the exact solution for R0. If the Gouffe expression is

sound, and if no significant temperature difference exists between hole and

surface, then the emittance as computed from the flux ratio, E, should be

independent of the h/r ratio. Fig. 9 shows that this independence is

achieved at h/r values between 1.0 and 2.0 only when the exact solution for

R

q

is used in the Gouffe^ equation (Curve C) . At h/r ratios greater than

2.0, the computed emittances decrease with increasing hole depth. This is

the type of behavior that would occur if the bottom of the hole was at a higher temperature than the specimen surface which is precisely the condition that would be expected for relatively deep reference holes. Hence a second implication from the data is that the h/r ratio of the reference

hole is that the h/r ratio of the reference hole in an oxide specimen such

as steatite should be less than 2.0 if significant errors from temperature

differences are to be avoided.

11
T

.80

0

1

LlJ o

z

<

I-K

UJ<

s

a

o

.70

< .60

h-o

UJ

cr<

Q.0.<

.50

Fig. 9 Effect of

depth- 1 o- radius

(h/r) ratio of the

reference hole on the total normal

emittance of steatite specimens:

(A) c considering

reference hole as

a blackbody; (B)e

computed from Equation 1; (C)e a com- puted from Equation

1 with exact solution for R

q

.

1.0

I L

2.0 3.0

h/r OF REFERENCE HOLE

4.0

2. Other nonmetals : A considerable number of measurements were made on such nonmetafs as alumina, zirconia, graphite, and silicon carbide. Some of

these data are given in ref. 6 and some have been presented in the monthly

letter reports. In most cases, the measurements were made on specimens with

reference holes, 0.25 mm radius, and depths of 0.25 to 0.50 mm (h/r ratios

of 1.0 to 2.0). Fig. 10 shows a recently obtained curve for a commercial

grade of sintered alumina while fig. 11 shows results for a dense recrystallized grade of silicon carbide (code 45 in ref. 8).

12
TOTAL

NORMAL

EMITTANCE

.70

Figure 10. Change in normal emittance with temperature for

a specimen of sintered alumina.

13
T T T i—:—

.95

W

o

< .90

j2

UJ<

| .85

O

z

<t

h-o

.80

.75

SILICON CARBIDE

SPECIMEN IN ARGON

0-0 -—

I J 1— ... I U

1000 1200 1400 1600 1800 2000 ' 2200

TEMPERATURE -°K

Figure 11. Change in normal emittance with temperature

for a specimen of recrystallized silicon carbide.

C. Measurement on Metals

The Gouffe^ equation does not apply to metals because materials of

this type, when the surfaces are smooth, are specular rather than diffuse

reflectors. Metals, however, have relatively high thermal conductivity

and low emittance; hence fairly deep reference holes can be used without

encountering a serious temperature difference between the bottom of the

hole and the surface. If such a hole can be made to approximate a blackbody closely, then the emittance can be determined directly from the ratio

of the relative flux density from the surface to that from the hole.

14 -
The first attempts at such measurements were made with platinum and

tungsten specimens into which flat-bottomed holes 0.25 mm radius by 5 mm

deep (h/r = 10) had been drilled. The walls of these holes were purposely made rough; also, a special tool was used to roughen the bottoms. The

purpose of the roughening was to produce a diffusely reflecting surface

that would raise the effective emittance of the hole to an emittance of

0.99 or better (ref. 9 and 10). Measurements made with these specimens

consistently gave total normal emittance values that were higher than

accepted literature values by as much as 30%.

An attempt was made to devise conditions that would bring the emittance

values for polished metals, as determined by this method, into line with the

accepted measurements reported in the literature. Different polishing

methods were tried. Also, methods of specimen heating and support were

changed so as to reduce as much as possible any temperature gradients that

might be present in the specimens. In addition hole geometry was modified

by making the bottom of the hole cone shaped rather than flat. None of

these modifications were successful in reducing the measured emittance. In

fact the only modification that gave results in keeping with best literature

values was to use a reference hole of the type shown in fig. 8.

Up to the present time this cavity has been used only in a single platinum specimen. The hole was 1.90 mm diam. by 2.05 mm deep (h/r = 2.16).

The hole was drilled and the test surface mechanically polished after which

the side walls and bottom of the hole were roughened by grit blasting with

a small dental unit. Finally, successive layers of a silicon carbide coating were applied to the walls of the hole. The coating consisted of 5%, by

weight 600 mesh SiC suspended in a 1% by weight sugar solution. A small

drop of this coating suspension was placed in the hole and worked around

the sides with a wire of small diameter. After drying, the specimen was

tested at 1100 K. Table 1 summarizes the results.

The specimen surface remained virtually the same in all five tests and

only the hole treatment was changed. With no coating in the hole, the apparent emittance of the platinum was 0.183; yet with an applied coating

layer only 0.02 mm thick the measured emittance dropped to 0.117. The coating thickness does not appear to be critical for layers thinner than about

0.2 mm. Coatings prepared in this way contain numerous pores that tend to

cause low heat conduction. At thicknesses of the order of 0.2 mm and

greater the temperature drop through the coating is apparently sufficiently

high to cause the surface of coating to operate at a significantly lower

temperature than the platinum, thus causing the measured emittance values

to be high.

Fig. 12 shows the results obtained when the emittance of this same

specimen (coating thickness in hole, 0.02 mm) was measured up to 1500 K.

Agreement is good with recent values reported for polished platinum by

Abbott. Alvares and Parker and also with the emittance computed from electrical resistivity measurements (ref. 11).

- 15 -
Table 1

Effect of thickness of a silicon carbide coating in the

reference hole on the measured total normal emittance of

a platinum specimen

Coating

Thicknessin mm

Apparent c of

TN Platinum Surface

0 .183

0.02 .117

0.06 .116

0.21 .120

0.30 .134

a/ Measured with a microscope at center of hole bottom.

16
The emittance of silicon carbide is about 0.90 (fig. 11) . Assuming the

applied coating to have this value, the emittance of the cavity as computed

by the Gouffe^ expression is 0.98. The results plotted in fig. 12 and listed

in Table 1 suggest that the coated-wall cavity closely approximates a blackbody when the coating is applied at a thickness as low. as 0.02 mm.

Fig. 12. Change

in total normal

emittance with

temperature for

a specimen of

polished platinum .

D. Investigation of Possible Sources of Error

A large difference in the total normal emittance of sintered alumina

from the same source was observed when measurements were made by^the rotating cylinder and the shallow hole methods (0.21 and 0.39 at 1400 K) . This

type of result required a careful review of both methods for possible sources

of error. The approaches being used with the rotating cylinder equipment are

discussed in Section TI-G; the present section is concerned with possible

sources of error that might still be present in the induction heating method.

17
(1) Lack of flat response' from detector : The thermocouple detector

used in the induction heating equipment is of a type commonly used in infrared monochromators. The sensing area of this detector is coated with

a special black paint that supposedly provides non-selective (flat) spectral response. A rapid method of determining whether >or not the detection

system does in fact react in this manner, is to determine whether the detector response follows the Stefan-Boltzmann relation, i.e., increases

linearly with the fourth power of the temperature of a blackbody source.

As the temperature increases the peak energy in the blackbody radiation

shifts to lower wavelengths in accordance with Wien's law. It follows

that if the detector gives a flat response, a straight line will result

when the detector response is plotted against the fourth power of the temperature. Likewise if the detector is selective some deviation from the

straight-line relation will occur. This test method involves the premise

that the detector responds linearly to radiant flux density at a given

temperature, which fact was established earlier (ref. 1)*

TEMPERATURE °K

Figure 13. Potentiometer response with temperature for a deep hole drilled in a specimen of graphite. Temperatures

were measured with a calibrated optical pyrometer.

The data plotted in fig. 13 were obtained with the detector focused on

a hole drilled in a graphite specimen. The emittance of this hole when computed by the Gouffe equation was 0.99. The measurements were made in vacuum with a sodium chloride window at the top of the radiation trap. The

plotted points all fall on a straight line which intercepts the temperature

axis at very close to room temperature (T

4 = 0.007 x 1012

). These results

show that the detector has a substantially flat response to the wavelengths

of interest, hence there is no measurable error from this source.

18
(2) Error from detector not being at exact focal point ; Alignment of

the optics was accomplished initially by determining visually the point of

sharpest focus of the specimen image. To determine this point more accurately, the detector was replaced with a camera with a focal plane shutter.

The lens was removed from the camera and a series of photographs taken of

a platinum specimen at distance increments of 1 mm along a 2 cm length near

the focal point.

An enlargement of these pictures showed that the image of the reference hole tended to be elliptical because of the slight astigmatism introduced by the off-axis optics except at one distance where the picture of

the hole was sharp and circular (see fig. 14). This position, which was

almost 1 cm from the focal point originally located by less precise means,

was about 7 mm farther when either a NaCl or CaFg window was inserted at

the top of the radiation trap; hence, the detector position was made adjustable so that it could be properly located for different test conditions. Numerous emittance determinations showed that earlier measurements

were approximately 10% too high because of the slightly off-focus positioning of the detector.

(3) Error from aberrations ; A densitometer

curve was made from the negative used for preparing fig. 14. The effect of this aberration

would be to cause a blurring of the hole image

which, -in the case of holes of small diameter,

would mean that the measured flux densities

would be too low. However, for holes that are

sufficiently large, no significant error from

this source would be expected.

The effect of this aberration on the relative flux densities from the hole and the surface was investigated with specially prepared

nickel specimens. The reference holes were

drilled completely through the specimens. A

narrow shoulder was machined near the top sur- face of each specimen so that the specimen could

be supported from the top of the field concentrator by means of a thin mica sheet into which

a hole was drilled so as to support the specimen

at the shoulder. A water-cooled blackened cone

was placed at the bottom of the concentrator

beneath the hole in the specimen. With this

arrangement, the detector when placed on the

image of the hole in a heated specimen would

"see" the cold blackened cone and, hence, the detector, at this position,

should indicate a zero response with respect to the hot specimen surface.

However, if optical aberrations were present some of the flux from the

specimen surface would be "smeared out" onto the hole image. This, in

turn, would be evidenced by hole readings that were greater than zero, as the edges of hole were - approached

Fig. 14. Photograph

taken at image plane of

a heated platinum specimen with a deep reference hole.

19
Figure 15. Change in detector response on scanning across

the image of a 1.6 mm diam. hole in a heated oxidized

nickel specimen. Hole was drilled through specimen so as to permit detector, when centered on hole image, to "see 1" water-cooled blackened surface at bottom of field concentrator. Resolution is determined by 0.5 mm diameter

aperture on detector.

Fig. 15 shows the results obtained for an oxidized nickel specimen

with a hole 1.6 mm diam. (image size 3.2 mm diam.). It is obvious from

this graph that there are aberrations in the optics, and furthermore that

an error from optical aberrations can be expected unless hole diameters

of greater than about 0.7 mm are used. In similar test made with a platinum specimen having a 0..5 mm diam. hole, the flux from the center of the

20
hole was about 10% of that from the surface. It follows that all of the

emittance measurements previously made on specimens with 0.5 mm diam.

(0.25 mm radius) holes are too high, because of aberration in the optics.

There should, however, be no measurable error from this source when the

reference holes have diameters appreciably larger than 0.5 mm. such as was the case for the platinum specimen used in obtaining the data plotted

in fig. 12.

Obviously the most desirable solution to the aberration error is to

employ optics that will give an image free of aberrations. Replacement of

the 5" spherical mirror with an off-axis ellipsoidal mirror should result in

a significant improvement. Use of larger holes is not a desirable solution

because of the larger temperature differences that the increased depth of

such holes would create in the nonmetallic specimens.

(4) Window error : The window error mentioned in the Symposium paper

(Ref. 6) has been completely eliminated through use of the radiation trap

at the top of the bell jar coupled with the repositioning of the detector

to the point of sharpest focus.

D. Summary of Present Status and Future Plans

Considerable effort during the latter part of the contract year was devoted to investigating possible sources of error. The measurements with

steatite specimens indicated that a modified R0 should be used for those

specimens with reference holes having h/r values below 2. Although the

measured emittances of these specimens are undoubtedly too high because of

focus and aberration errors, they should all be high in the same proportion;

hence, the conclusion on h/r ratio should be valid. Errors from deviation

of the detector position from that of sharpest focus were detected and these

will be eliminated from future measurements, as will be the error from the

presence of a viewing window.

The most serious error that still remains is caused by the aberration

which is present with the current optical system when hole diameters are below 0.7 mm. Since the use of larger, and hence deeper holes would introduce

troublesome temperature differences between the surface and the bottom of the

reference hole, the elimination of existing aberrations will be sought

through substitution of an off-axis ellipsoidal mirror for the present spherical mirror.

Even when all measurement errors have been reduced to negligible

amounts there will still be uncertainties caused by the failure of real specimens to fulfill the assumptions made in the derivation of the Gouffe ex- pression (Section III-A) . The magnitude of the error caused by these un- certainties will be evaluated during the coming year by comparing emittances

obtained by the shallow-hole method with those obtained with the rotating

cylinder equipment.

21
E. Restrictions on Measurements Above 2000°K

Many gf the measurements that have been attempted at temperatures

above 2000 K have encountered difficulties from two sources; (1) filming

of the viewing window, and (2) change in specimen emit»tance due to such

factors as thermal etching, specimen contamination from the crucible,

volatilization of impurities, and specimen shrinkage. For many but not

all materials these difficulties will present serious obstacles to the

achievement of reliable measurements up to the target temperature of

2500°K.

22
REFERENCES

1. Equipment for thermal emittance measurements above 1400°K. First annual

summary report to NASA on Contract No. H-3646. NBS report No. 7514,

Jan. 1962.

2. Peavy, B. A. and Eubanks, A. G. Periodic heat flow in a hollow cylinder

rotating in a furnace with a viewing port. Prepared for publication in

the Proceedings of Symposium on Measurement of Thermal Radiation Properties of Solids, Dayton, Ohio, Sept. 1962.

3.. Slemp, W. S. and Wade, W. R. A method for measuring the spectral normal

emittance in air of a variety of materials having stable emittance characteristics. Vol. 11, p. 75 of preprints of papers for Symposium on Measurement of Thermal Radiation Properties of Solids, Dayton, Ohio,

Sept. 1962.

4. Blau, H. H., Marsh, J. B., Martin, W. S. and Chaffee, E. Infrared spectral emittance properties of solid materials. Rept . No. AFCRL-TR-60-416,

Geophysical Res. Dir., AF Res. Div., Oct. 1960.

5. Richmond, J. C., Harrison, W. N., and Shorten, F. J. An approach to

thermal emittance standards. Vol. 11, p. 102 of Preprints of Papers for

Symposium on Measurement of Thermal Radiation Properties of Solids,

Dayton, Ohio, Sept. 1962.

6. Moore, D. G. Investigation of shallow reference cavities for hightemperature emittance measurements. Ibid. Vol. iii, p. 159, Sept. 1962.

7. Gouffe, A. Revue d’Opt. 24, 1 (1945).

8. Lang, S. M. Properties of high-temperature ceramics and cermets; elasticity and density at room temperature. NBS Monograph No. 6, 1960.

9. Vos, J. C. de. The emissivity of tungsten ribbon. Thesis Amsterdam

1953. Also, Physica 20, 690 (1954).

10. Buckley, H. Phil. Mag. 4, 753 (1927); 6, 447 (1928); 17, 576 (1934).

11. Abbott, G. L., Alvares, N. J. and Parker, W. J. Total normal and total

hemispherical emittance of polished metals. WADD TR 61-94, Nov. 1961.

23
.

.
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Luther H. Hodges, Secretary

NATIONAL BUREAU OF STANDARDS

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