NATIONAL BUREAU OF STANDARDS REPORT

EQUIPMENT FOR THERMAL EMITTANCE MEASUREMENTS

ABOVE 1400°K

ANNUAL SUMMARY REPORT

January, 1964

Contract No. H-41986

04-84-3-75-0905-09-000-8400-2561-1

04-84-3-75-0905-11-000-8400-2561-1

80X0108 (63) R&D, MSFC, OBJECT 25

GEORGE C. MARSHALL SPACE FLIGHT CENTER

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION

Huntsville, Alabama

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

8175
U. S. DEPARTMENT OF COMMERCE

Luther H. Hodges, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director

THE NATIONAL BUREAU OF STANDARDS

Functions and Activities

The functions of the National Bureau of Standards include the development and maintenance of the national standards of measurement and the provision of means and methods for making measure - ments 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, including assistance to industry, business and con- sumers in the development and acceptance of commercial standards and simplified trade practice

recommendations. The work includes basic and applied research, development, engineering, instru- mentation, testing, evaluation, calibration services, and various consultation and information services. Research 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 listingof divisionsand sections on the inside

of the back cover.

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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 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, D.C., 20402.
NATIONAL BUREAU OF STANDARDS REPORT

MBS PROJECT

1009-11-10493

MBS REPORT

8175

EQUIPMENT FOR THERMAL EMITTANCE MEASUREMENTS

ABOVE 1400°K

ANNUAL SUMMARY REPORT

January, 1964

Contract No. H-41986

04-84-3- 75-0905-09-000-8400-2561-

1

04-84-3-75-0905-11-000-8400-2561-1

80X0108 (63) R&D, MSFC, OBJECT 25

George C. Marshall Space Flight Center

National Aeronautics and Space Administration

Huntsville, Alabama

Prepared for

IMPORTANT NOTICE NATIONAL BUREAU OF STAN|

for use within the Government. Bef and review. For this reason, the pu whole or in part, is not authorized Bureau of Standards, Washington, D

the Report has been specifically pre|

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

ccounting documents intended

jected to additional evaluation

ing of this Report, either in

fice of the Director, National

e Government agency for which

s tor its own use.

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

FOREWORD

A research program aimed at developing equipment and procedures suitable for reliable measurements of the total and spectral emittance of both

metals and nonmetals in the temperature range 1400 - 2500°K was initiated

on January 1, 1961 at the National Bureau of Standards under the sponsorship of the George C. Marshall Space Flight Center of the National Aeronautics and Space Administration. The contract was under the technical

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

The present report summarizes the progress that was made during the

calendar year 1963. The report was prepared by D. G. Moore of the National

Bureau of Standards staff (Div. 10, Sec. 9) who served as the group leader

during the course of the investigation. Staff personnel who were active in

the investigation included H. E. Clark, F. J. Kelly, J. L. Alderman,

J. C. Richmond and W. N. Harrison.
ABSTRACT

The furnaces used with the rotating cylinder equipment were redesigned

to give improved performance at the higher operating temperatures. At the

same time an automatic temperature control system was incorporated that

permitted the specimen and reference blackbody thermocouples to be maintained

at the same temperature to within ± 0.5 K.

Normal spectral emittances from 1 to 15p were measured for platinum at

1400°K and for 11 ceramic oxide specimens at 1200°K. Room-temperature re- flectance for six of these oxide materials was measured from 0.26 to 2.10(i

under conditions of near normal illumination and hemispherical viewing.

The normal spectral emittances computed from these reflectances were then

compared in the range 1.0 to 2.1 m. with emittances measured with the rotatingspecimen equipment.

the Gouffe expression as used in the shallow-hole method was tested ex- perimentally through use of a reflectance approach. The theoretical values

were in excellent agreement with the experimental values, thus indicating

that no significant error is introduced when the Gouffe equation is used for

converting flux ratios to emittances; the same study showed, however, that an error will occur if the specimen is formed from a material that is a poor

diffuser of radiant energy.

A few total normal emittance measurements on specimens of beryllium were

made in the temperature range 1000 - 1550°K.

Two manuscripts were prepared describing different phases of the program. Both papers were accepted for presentation at the March 1964 Symposium

on the Thermal Radiation of Solids.
I. OBJECTIVES

The primary objective of this project is to develop reliable and accur- ate 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 emittance (1 to 15n) in the range of about

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

emittance at temperatures 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 shme temperature .

Progress made in perfecting both types of measurement during the period

Jan. 1, 1963 to Jan. 1, 1964 is described in the present report.

II. ROTATING SPECIMEN METHOD FOR MEASUREMENTS TO 1800°K (2780 °F)

A. Description of Method

In the rotating specimen method 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. A theoretical analysis of the periodic heat flow in such a specimen, which was performed

as one phase of the investigation, has been published (Ref. 1). Although this

analysis was valuable in predicting the general pattern of temperature fluctuations in the specimen with increased speed, the expressions were not useful

for quantitative calculations because reliable thermal property data were not

available on the specimen materials. Instead a method was devised for determining the minimum permissible speeds experimentally. This was done by increasing the speed in increments until the apparent emittance as measured at

1..4p reached a constant value. Details of this experimental method are given

in Ref . 2

The 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

1
is largely eliminated by using a second blackbody furnace to obtain a calibration or "100% curve" prior to obtaining the specimen data. Figure 1 is a schematic showing the arrangement of the equipment and the optical

paths

.

TRANSFER OPTICS —:5 MONOCHROMATOR

A. Bkxkbody furnace No. I 3. SWackbody furnace No. 2

C. Rotating (pacinian furnace

0. Flat front eurfoce mirror

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

H 10 epe mirror - type chopper

K. NaCI prism

L. Movable mirror (for total normal measurements)

0. Vacuum -type thermocouple detector

Figure 1. Optical paths in rotating-cylinder equipment.

The spectrometer being used for the measurements is a modified Beckman

IR-5A. Before modification this instrument provided a linear response to

within 1%. However, because the modifications that were made to convert it

for emittance measurements could have disturbed this linearity, an independent test of linearity was made after the equipment was installed and ready

for operation.

To conduct this test, the two blackbodies were first adjusted to 1200

± 1°K. A sector-disc attenuator was then inserted into the beam from blackbody no. 2 (B in Fig. 1) and rotated at high speed (> 500 RPM) . Six different discs were used which had nominal transmit tances of 75.1, 50.0, 25.3,

12.7 and 5.1%. The resulting spectral curves, which were found to be reproducible to within 2%, showed that instrument response was lower than the

actual beam attenuation. This lowering was the same at all wavelengths.

Since no simple method could be devised to restore linearity to the instrument, a correction curve (Fig. 2) was prepared and used to correct all spectral emittance values that were obtained from the recorder charts.

2
Further details relative to the procedures used for making measurements

are given in ref, 2.

B. Modification of Blackbody Furnaces

The blackbody furnaces described in the preceding annual summary report

were found to have a short operating life at temperatures above 14Q0°K.

Hence, it was necessary to incorporate several new design features and re- build the furnaces so as to achieve the required trouble-free operation at

the higher test temperatures.

Fig. 3 is a schematic of the new design. The inner cavity is formed of

fused alumina bonded with 20% by weight of a calcium aluminate cement. This

mixture, which sets hydraulically, was mixed with water and vibrated into a greased plaster mold. The mold was made by using a Norton alumina core—/as a pattern; a brass mandrel was positioned at the center of the mold to form the

outline of the cavity. After allowing 20 hrs . for curing, the core was re- moved from the mold, dried in an oven for 24 hours, and then heated to 1925°K

for 1 hr. prior to the winding with 0.032 in. diam. platinum-40% rhodium re- sistance wire. As shown in Fig. 3, the spacing of the winding was varied to

minimize temperature gradients. No power taps were used on the windings.^-/

The alumina core material has a low emittance in the wavelength region

1 to 5|i. Inasmuch as the cavity emittance depends, in. part, on the emittance

of the walls, it was necessary to line the cavity with a coating that would

have high emittance at the short as well as the long wavelengths. The coating developed for this purpose consisted of black nickel oxide (Nia 03 ) bonded

with 7.5%, by weight of clay. This coating was prepared as a water suspension

(slip) and applied at a thickness of about 0.005 in. to the inner wall sur- faces of the cavity by first pouring the slip into the cavity, allowing it to

remain for a few seconds, and then pouring it out again. After drying, the core was heated to 1850°K for 1 hr. to bond the coating to the alumina. The

spectral emittance of a cylindrical specimen of the core material coated in

this way was found to be above 0.875 at all wavelengths from 1 to 15p; hence,

the lowest possible cavity emittance, at any wavelength from 1 to 15p, when

computed by the expression given by Gouffe (Ref. 3) was 0.990.

Temperature gradients in the two blackbody furnaces were measured at

each of the four test temperatures by means of a calibrated Pt vs Pt-10%

Rh thermocouple inserted into the furnace through the entrance slit. The

arrangement was such that the bead of the thermocouple could be positioned

within 1/8 in. of the core wall at any point along the length of the cavity.

1/ Norton RA139 core no. 10445 (Norton Co., Worcester, Mass.)

2/ Power taps were used in a preliminary design. Temperature gradients

could be minimized by increasing power to the ends; however, this procedure caused overloading of the end coils with a subsequent short operating life of the furnace at 1 800 °K.

3
Fig. 2. Calibration curve (solid line) for correcting for lack of instrument linearity. Dashed line

shows type of behavior desired of an instrument

(linear response).

ALUMINA DISC CAVITY LINED WITH

BRASS MULLITE WOOL INSUL. ^

ALUMINA REFRACTORY

I’ '.'I ALUMINA CEMENT

K/V ''/] ALUMINA BONDED WITH CALCIUM ALUMINATE

Fig. 3. Schematic of blackbody furnace.

- 4 .
These measurements showed that at 1800°K the back wall of the cavity was 2°K

below the l\ in. long isothermal region near the center and the front of the

cavity (rear wall of the entrance slit) was 3 K below the isothermal region.

The cavity thermocouple (see Fig. 3) gave an emf corresponding to the temperature of the isothermal region of the cavity.

C. Modification of Specimen Furnace

Preliminary trials indicated that because of large heat losses, the temperature of the specimen furnace could not be raised above about 1600 K. The

furnace was therefore rebuilt with the following changes: (a) a more durable

wire (607c Pt;407o Rh) was used for the windings, (b) a new top insert with improved thermal insulation properties was added, (c) platinum radiation shields were placed at both the top and bottom of the furnace tube, and (d) gold wire

rather than copper was used for bringing the power leads through the glassmetal seals.

Fig. 4. Schematic

of rotating-specimen furnace.

5
Fig. 4 is a schematic of the modified design. Although this particular

version was completed late in the contract year, it has nevertheless been

operated at 1800 K on numerous occasions for periods of several hours without

difficulties developing. Power requirements at 1800°K are moderate and there

is no overheating of the shell. Also, the refractory pedestal has shown no evidence of distortion.

Adjustment of power to the booster coil is used to control vertical

gradients in the specimen furnace which would otherwise occur because of the

large heat losses through the viewing port. When the power to the booster coil

is adjusted at the proper level, no thermal gradient can be detected from top

to bottom on a rotating alumina specimen when temperatures are measured with a micro-optical pyrometer sighted on the specimen through the viewing port. The

pyrometer is capable of detecting differences of as small as 2°K.

D. Temperature Control System

The measurements given in the preceding annual summary report were obtained after adjustment of the temperature of the specimen furnace to that of

blackbody no. 1 by manual control of power input. This manual control was not

only time consuming but in those cases where the temperature of the specimen

furnace changed slightly during measurement, the data had to be discarded and

the test repeated. To eliminate these difficulties, a differential thermocouple control system was installed during the contract year. This system is

similar to the one described by Harrison, et al (Ref. 4). The thermocouples

in both furnaces were made from calibrated Pt;Pt-10% Rh wires taken from the same spools. The new controller meters the power into the specimen furnace

as required to attain a zero temperature difference between the two thermocouples. Temperature equality between the two furnaces is reached in as short

a time as 5 min.; the temperatures of the two furnaces can then be automatically maintained to better than 1°K until such time as the control point is

changed. The new system is expected to improve not only the precision of the

spectral measurements, but also it will appreciably reduce the time required

to complete a series of determinations.

E. Emittance Measurements

Because of the large amount of time spent in refining the equipment during the contract year it was not possible to obtain large quantities of emittance data. However, some preliminary measurements were made in air on speci- mens of ceramic oxides at 1200°K and also on platinum at 1400 K. The measurements are included in this summary report to indicate, in a general way, the

capabilities of the equipment; they should not be interpreted as representing

highly accurate data on well characterized specimens.

Table 1 identifies the specimens that were measured. All were in the

form of cylinders approximately 1 in. outside diameter with smooth surface

finishes. The wall thickness of the platinum specimen was approximately 1/16

in.; that of the sintered ceramic specimens approximately 1/8 in.

6
Table 1. Identification of platinum and sintered ceramic specimens.

No. of Porosity

Letter

Symbol Material n . a/ , DescriptionSpec

Tested

in % Por<

by Vol

P Platinum Commercially pure 1 0

AN Alumina Coarse grained furnace tube

refractory; > 95 A1 8 03

1 45

AD Alumina Fine grained, dense; 96 alumina 5 11

AL Alumina Fine grained; 99 + A18 03 3 36

CZ Calcium

Zirconate Fine grained body

CaO 31; ZrOB 68 1 30

M Magnesium

Oxide Fine grained magnesia

99.2 MgO 2 35

AS Mul 1 ite Fine grained body

A12 03 67; SiOg 30 1 32

S Silica Fine grained body—

SiOg 95 ; Alg03 4.0 1 8

MA Spinel Fine grained body

Al g 03 71 ; MgO 28 1 25

T Thoria Fine grained; 99+ Th02

1 36

ZS Zircon Fine grained body

Zr02 66 ; SiOg 32 2 26

Z Zirconia Fine grained, lime stabilized

Zr02 + HfOg 94; CaO 4.5 1 18

a/ Compositions are nominal for major constituents in percent by we

b/ Major crystalline phases - crystobalite and quartz.

7
COz

o

CEO

GO

O

XN8

Figure

5.

Spectral

emittance

curves

of

polished

platinum

and

three

types

of

sintered

alumina.
-

9

Figure

6.

Spectral

emittance

curves

for

sintered

ceramic

oxide

specimens.
10

Figure

7.

Spectral

emittance

curves

for

sintered

ceramic

oxide

specimens.
The spectral curves are shown in Figs. 5, 6 and 7. Fig. 5A is for a polished platinum specimen that had been annealed for one hour in air at

1525°K prior to testing. The agreement of the curve determined with the

rotating specimen method with that reported by Harrison, et al (Ref. 5) for

polished platinum as measured by a carefully calibrated heat-strip method

is good and these data, by themselves, indicate that the transfer optics

and monochromator are capable of satisfactory operation.

Fig. 5B, 5C and 5D show the curves obtained for three types of sintered

alumina. All three of these curves are in fair agreement with data for alumina specimens reported by Blau (Ref. 6) and by Slemp and Wade (Ref. 7) at

wavelengths beyond about 5p, but below 5p the present measurements give

values which are more in keeping with those reported by Folweiler (Ref. 8).

It should be pointed out, however, that cross comparison of emittance meas- urements are of only limited value unless the specimens are prepared from

the same material by the same processing and with the same surface finish.

Such factors as grain size, porosity, and the presence of impurities are known to have an appreciable effect on measured emittance.

The emittance peak at 2.9p for AD and AN alumina (Fig. 5C and 5D) was obtained for all specimens of this material that were tested. A similar peak

has been noted for flame-sprayed alumina (Ref. 9 and 10). It is probably

caused by chemically-held water in the structure. An observation made in the

present measurements that the intensity of the peak decreased with time of

heating at 1200°K tends to support this explanation. A similar maximum was observed for mullite (Fig. 7C) .

With the exception of the emittance peak at about 4 6p for zirconia

(Fig. 6A) and the low emittance band for silica at about 9. Op (Fig. 6D), the

other deviations are relatively minor and need confirmation by additional

measurements especially at higher test temperatures before they can be fully

accepted. The low emittance band for silica at 9. Op has been observed previously (Ref. 11).

Of the 11 ceramic materials measured, thorium oxide and magnesium oxide

have the longest wavelength region of low emittance. The emittance of thori- um oxide does not reach 0.9 until the wavelength is approximately 10p; the

wavelength at which this emittance is attained by magnesium oxide is about 9p.

This behavior should be reflected in the computed total normal emittances and

Table 2 shows that such is the case. These two oxides gave the lowest total

normal emittances at 1200 K of any of the oxides measured; the silica specimen

had the highest.

The selected ordinate method (Ref. 5) with 25 ordinates was used for com- puting the total normal emittances from the spectral data.

11
Table 2. Total normal emittances from

spectral

at 1200°K as data

.

computed from

Material

Ident

.

Symbol £ T N Material

Ident

.

Symb o

1

®T N

Aluminum

Oxide

AL 0.29 Silica S 0.60

Aluminum

Oxide

AD 0.52 Calcium

Zirconate

CZ 0.29

Aluminum

Oxide

AN 0.42 Zirconium

Silicate

(Zircon)

• ZS 0.39

Zirconium

Oxide

Z 0.34 Aluminum

Silicate

(Mul 1 ite)

AS 0.40

Thorium

Oxide

T 0.23

Magnesium

Oxide

M 0.20 Magnesium

Aluminat e (Spinel

)

MA 0.26

F. Room Temperature Reflectance Measurements

Because of the many possible sources of error in measuring spectral

emittance of ceramic materials, especially in the low wavelength region, it

seemed desirable to test the reliability of the preliminary emittance data

by means of spectral reflectance measurements. Spectral reflectance of an opaque material determined under conditions of normal illumination and hemispherical viewing is the complement of normal spectral emittance; hence,

reflectances determined in this way can be converted to emittances by subtracting them from unity. Since the spectral emittance varies only slightly

with temperature, such measurements should provide a reasonably good check

of the emittance data even though the spectral reflectances were obtained at room temperature.

A Cary Model 14M recording spectrophotometer equipped with an integrating sphere was used for these reflectance measurements. Data were obtained

over three different ranges (0.26 to 0.4p, 0.36 to 0.7p, and 0.6 to 2. Ip)

with different source-receiver combinations for each range. The specimens were flat discs, lk in. diam. by % in. thick, prepared of materials AL, M.

Z, ZS, CZ, and T (Table 1). The discs were prepared of the same materials

and with the same processing used for the 1 in. diam. cylinders.

12
The measurements were made relative to smoked magnesium oxide and cor- rected to "absolute" values using the data of Middleton and Sanders

(Ref. 12 and 13). The values used for conversion are listed in Table 3,

while the reflectance curves, corrected through use of these data, are shown

in Figs. 9, 10 and 11. The curves in each case were plotted from the average

values obtained from two specimens. These values were in good agreement over

the entire range except for the two magnesium oxide specimens in the region

0.26 to 0.60p where deviations in spectral reflectance of as great as 0.2 were observed. Because of this poor agreement, the average curve for magnesium oxide is not shown hpl nw 0.6u (Fie. 8Bl .

X - MICRONS

A

B

Fig. 8. Room-temperature spectral reflectance curves for

alumina and zirconia specimens.

- 13 -
Table 3. Spectral diffuse reflectances of smoked magnesium oxide

X

R “/ \R

^

\

R

^

\

0 . 20p 0.955 0 . 80p 0.965 1.50 .945

.30 .970 .90 .960 1.60 .945

.40 .980 1.00 .960 1.70 .945

.50 .975 1.10 .955 1.80 .940

.60 .970 1.20 .955 1.90 .925

.70 .965 1.30 .950 2.00 .925

.80 .965 1.40 .950 2.10 .930

a/ Selected from data given by Middleton and Sanders (J. Am. Opt.Soc.,

41, 419, 1951).

b/ Taken from Fig. 2 of paper by Sanders and Middleton (J. Am. Opt.Soc.,

43, 58, 1953).

G. Comparison of Reflectance and Emittance Measurements

Between 1.0 and 2. Op it is possible to compare the spectral emittances

computed from the reflectance measurements with those obtained with the ro- tating cylinder equipment. Table 4 shows this comparison at 1.0, 1.5 and

2. Op.

Considering both the difference in temperature of the two measurements

(900 K) and the uncertainty in converting reflectances from relative to

"absolute" values, the agreement is surprisingly good for AL, CZ and M; reasonably good for T and ZS; but poor for Z.

In general, spectral emittance increases only very slowly with increasing temperature; hence this comparison at the two different test temperatures

is of interest. However, until such time as measurements are available at

1200, 1400, 1600 and 1800°K and temperature coefficients are established, it

is not possible to obtain a more valid comparison of the two sets of data.

The- data listed in Table 4 are nevertheless of value in obtaining an "order of magnitude" comparison.

14
Table 4. Spectral emittances of oxide specimens at

1.0, 1.5, and 2.0 microns.

I dent

.

WaveMaterial Symbol lengt

M Aluminum AL 1.0

Oxide 1.5

2.0

Calcium CZ 1.0

Zirconate 1.5

2.0

Magnesium M 1.0

Oxide 1.5

2.0

Thorium T 1.0

Oxide 1.5

2.0

Zirconium Z 1.0

Oxide 1.5

2.0

Zirconium ZS 1.0

Silicate 1.5

2.0

Normal Spectral Ernittance

Rot. Cyl. Method Comp

.

from

1200°K R, T. Refl.

0.08 0.08

0.07 0.06

0.06 0.06

0.21 0.11

0.16 0.13

0.13 0.11

0.09 0.10

0.07 0.07

0.06 0.04

0,11 0.07

0.11 0.09

0.11 0.06

0.19 0.09

0.18 0.08

0.16 0.03

0.18 0.11

0.16 0.21

0.09 0.02

H. Discussion of Errors in Spectral Ernittance

Measurements

.

The most serious sources of error in ernittance measurement of this

type are (1) lack of optical equivalence in the two beams, (2) reflected

flux entering the specimen beam, (3) temperature gradients in the specimen,

(4) lack of temperature equality between specimen and blackbody, and (5)

radiant flux from the specimen cavity being transmitted through the wall of

the specimen and into the specimen beam. Any sizeable error from the first

of these sources is avoided through use of the two blackbodies to establish

the "lOOT, line"; the error from the second source was minimized by maintain

ing the clearance between the viewing port and the specimen as small as pos

sible; and error from the third source was reduced to a low level by rotating the specimen at high speed.

- 15
Lack of temperature equality between specimen and blackbody, although potentially a large source of error, was minimized by reducing the temperature

difference, as indicated by the blackbody and specimen thermocouples, to less

than 1°K. Even if the temperature of the specimen surface had been as much as 3°K lower than that indicated by the thermocouple at the specimen center, the error in emittance at lp for T = 1200 K would be only ±37>. Since the precision with which the emittance could be obtained from the recorder chart was 0.005 units, this error would be detectable only when the emittance was above

0.17. Errors from this source would, of course, be smaller both at higher

temperatures and at longer wavelengths.

i.o -

0 8 -

0.6 - x

o

0.4 -

0.2 -

0 1-

0 2

r t ~ i i i r [i i—i

1—i

1—i

1—i r

MAGNESIUM

OXIDE

0.4 0.6

_J I l I I I I 0.8 1.0 1.2 1.4 X -M I CRONS

A

_l i L

i.6 i.e

_j i 20 2 2

X - MICRONS

B

Fig. 9. Room temperature spectral reflectance curves

for magnesia and thoria specimens.

16 -
The greatest uncertainty that remains in the measurement is the possibility of transmission of blackbody radiation from the specimen cavity through

the wall of the specimen. This radiation, if it were present, would reinforce

the emitted flux from the specimen and cause the measured emittance to be high

For example, a specimen with a true emittance of 0.15 in the near infrared

would show a measured emittance of 0.20 if as little as 5% of the radiation

from the specimen cavity was being transmitted through the wall. This amounts

to an error of 33%.

X - MICRONS

B

Fig. 10. Room-temperature spectral reflectance curves for

zirconium silicate and calcium zirconate specimens.

17 -
Since the transmittance of the specimen walls could not be measured

readily, an effort was made to obtain a qualitative Indication of wall transmission at room temperature. This was accomplished by placing each specimen

in the beam of a He-Ne laser (A. = 0.6328) with the assumption that if the

1/8 in. thick wall was opaque at 0.63p it would also be opaque in the range

of 1.0 to 2. Op. The relative flatness of the reflectance curves (Figs. 9,

10, 11) in the region 0.6 to 2. Ip suggests that this assumption may be reason- ably valid. The results of this qualitative type of test indicated that the

ZS, T and Z specimen walls were opaque; the AL walls were nearly opaque; the

AD, AN, M and CZ walls were slightly transmitting; and the S and MA walls

were appreciably transmitting.

Fig. 11. Schematic of optical paths and electronic components for

total normal emittance equipment.

I. Future Plans

The work for calendar year 1964 will include the following:

(1) Factors affecting the' accuracy of the rotating-cylinder method will

be analyzed and computations will be made to arrive at a realistic value for

the accuracy of the measurements.

(2) Spectral emittance data will be obtained for specimens of representative nonmetals at 1200, 1400, 1600 and 1800°K.

18
(3) The variation with temperature of the spectral emittance of the

tested materials will be evaluated and an attempt will be made to arrive at

the most probable room-temperature spectral emittance curve from a knowledge

of the temperature dependency in the 1200-1800 K range. If this extrapolation

proves to be feasible, comparisons will be made of these extrapolated room- temperature emittances with those computed from room-temperature spectral re- flectance curves measured on the same materials.

III. INDUCTION HEATING EQUIPMENT FOR TOTAL NORMAL EMITTANCE

MEASUREMENTS TO 2500°K (4040°F)

A. Equipment

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

selection have been given in an earlier report (Ref. 14). Basically, the

measurement consists of determining the ratio of the radiant flux density from

the surface of a small specimen heated to a high temperature to the flux density from a cylindrical hole drilled into the specimen surface.

The holes are of different dimensions for metals than for nonmetals as indicated in Fig. 11 and the method of arriving at the emittance from the flux

ratio is also different.

A schematic of the equipment is shown in F ig . 12. The only major change

that was incorporated during the contract year was the substitution of a 5 in.

diameter off-axis ellipsoidal mirror for the ordinary spherical mirror used

in the early design. This substitution resulted in an improved quality of the

image at the detector, which in turn resulted in less error from optical aberrations .

SPECIMEN

Fig. 12. Two types of specimen used in induction heating

equipment. Heavy line on hole walls of metal specimen re- presents high emittance coating.

19
TOTAL

NORMAL

EMITTANCE

B. Measurements on Metals

A number of total normal emittance measurements were made on beryllium

specimens submitted by NASA. The specimens were of the type shown in Fig.

12B. The beryllium had a reported purity of 98.5% beryllium metal with BeO

as the principal impurity.

Fig. 13. Total normal emittance of beryllium specimens before

treatment (Curves AB and DE) and after heat treatment of 15 sec.

in air at 1 atm. at 1500 K (Curves C and F). Specimen B-2 was polished and Be-4 was abraded with 400C paper. All measurements

were made in a vacuum of 2 x 15“^ of Hg.

20
Fig. 13 shows the results of measurements on two of the four specimens

that were submitted. Specimen Be-2 was polished (RMS 8) while Be-4 was sur- face ground to RMS 20—.

The difference in e T N between the two specimens is in keeping with what

might be expected from the variation in surface finish. The clean untarnished beryllium appears to have both a total normal emittance and a temperature coefficient comparable to those of tunbsten and several other mptals.

However, the tests showed that beryllium tarnishes ranidly at temperatures

above about 1150 K even in a vacuum as low as 2 x 10 ^ torr. This tarnish,

which has a dark gray appearance, increases the emittance. Other tests showed

that treatment in air at 1275°K for as short a time as 1 sec. could increase

the emittance by as much as 20%.

C. Measurements on Nonmetals

In the case of nonmetals with their relatively low thermal conductivity

and relatively high thermal emittance, a small, shallow reference hole is

needed so as to minimize the temperature difference between the surface and

the bottom of the hole (Ref. 14). A shallow hole of this type does not, of

course, radiate as a blackbody. Nevertheless, if the geometry of the hole is

known, and if the emittance of the hole walls is the same as that of the test

surface, then it is possible through use of an expression derived by Gouffe

(Ref. 3) to compute the emittance from the measured ratio of the flux from

the surface to that from the hole.

The Gouffe expression for the apparent emittance of a cavity may be written as follows:

P W

ri + (1 ' p w

) " f)]

( 1 ) *w (1 - f

)+ K

where: e c = cavity emittance

€ = emittance of cavity walls

w

f =

1

1 + (L/R)'

where L = cavity depth and R = cavity radius

a/A = ratio of area of cavity opening to total area of cavity,

opening included, =

2 [1 + (L/R)]

for a cylindrical cavity

3/ RMS finish refers to root-mean-square average of the peak- to-val ley

depth of surface irregularities as expressed in microinches.

21
Now, if E is the measured ratio of normal radiant flux from surface and

hole, I s is the normal radiant flux per unit area from a plane (non-cavity)

surface of a specimen, I is the normal radiant flux per unit area from a cavity formed in the specimen material, Igg is the radiant flux from a blackbody radiator under the same conditions at the same temperature, c s is the

emittance of the specimen, and e c is the emittance of the cavity, then it is

possible to write the following:

E _ -

I

s /I

BB =

^_s ' ( 2 )

[ BB c V c1 !

Assuming that the emittance of the cavity wall, p is the same as that

of the specimen surface, Gouffe then combines equation (2) with equation

(1)

to obtain:

a a

p =

s E (1 +

a A - a

f) - A mK '

1 - K + E(K - f)

Equation (3) is the expression that is used in the shallow-hole method.

Obviously, it can be valid only if equation (1) is valid. Therefore, before

an accuracy can be ascribed to measurements made by the shallow-hole method,

it was first necessary to test equation (1) experimentally. This was done by

determining the spectral reflectance from 0.4 to 0.75p of an adjustable-depth,

paper-lined cylindrical brass cavity, and then converting the spectral re- flectances to emittances by subtracting from unity. Because the incident beam

of the spectrophotometer was near normal and the viewing was hemispherical,

the reflectance measurement was, in effect, the complement of the normal emittance, which is the property of interest in the shallow-hole method.

The results for a cavity lined with a coating that was a good diffuser

are shown in Fig. 14. The measured values fall on the curve predicted by the

Gouffe expression (equation 3) to within the precision of measurement. The

more important conclusions from this study of cavity emittances were:

(1) When the wall material was a good diffuse reflector, the Gouffe ex- pression predicted the emittance of shallow cylindrical holes to within 0.01.

(2) When the wall material was somewhat specular in nature (a poor diffuser) the experimental and theoretical emittances still agreed well for a hole with a depth-to-radius ratio (L/R) of 0.5.

(3) The Gouffe expression (equation 3) can be used for converting flux

ratios obtained by the shallow-cavity method to emittance without significant

error for all those materials that are good diffuse reflectors. Fortunately,

this includes most of the nonmetals.

A paper entitled "A Test of Analytical Expressions for the Thermal Emittance of Shallow Cylindrical Cavities" was prepared. This will be presented

at the March 1964 Symposium on Thermal Radiation of Solids.

- 22
Fig. 14. Room-temperature emittances

(circled points) determined by a reflectance

technique for cavities lined with a diffusely

reflecting coating. Solid lines were computed

from Gouffe expression.

D. Future Plans

The following work is planned for calendar year 1964:

(1) An analysis will be made of all potential sources of error in the

shallow-hole method and an attempt will be made to arrive at a figure for

its overall accuracy.

(2) Emittances will be measured on specimens at temperatures in the

range 1200 - 1800 K and compared to total normal emittance values for the same materials obtained with the rotating-specimen equipment.

(3) Methods of heating oxide specimens to high temperatures without the

aid of a metal crucible will be explored. This is needed to avoid contamination of the specimen by the crucible material, which has occurred in a number

of measurements at temperatures above 2000 K.

(4) Emittance-temperature curves will be determined for several representative metallic and nonmetallic specimens at temperatures up to 2500°K.

23
REFERENCES

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

rotating in a furnace with a viewing port. Measurement of Thermal Radiation Properties of Solids. NASA SP-31, p 553, 1963. (For sale by Supt

.

of Documents, U. S. Govt. Printing Office, Was., D. C. 20402. Price

$3.50.)

2. Clark, H. E. and Moore, D. G. Equipment for measuring thermal emittance

of ceramic oxides to 1800 K. (Preprint of paper to be presented at the

Symposium on Thermal Radiation of Solids, San Francisco, Calif., March

1964.)

3. Gouffe, A. Revue d'Opt. 24, 1 (1945). '

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

thermal emittance standards. Measurement of Thermal Radiation Properties

of Solids. NASA SP-31, p. 403, 1963.

5. Harrison, W. N., Richmond, J. C., Shorten, F. J. and Joseph, H. S.

Standardization of thermal emittance measurements. WADC TR 59-510, Pt IV

(Dec. 1962).

6. Blau, H. H., Marsh, J. B., Martin, W. S.,Jasperse, J. R. and Chaffee, E.

Infrared spectral emittance properties of materials. AFCRL-TR-60-416

(Oct. 1960).

7. Slemp, W. S. and Wade, W. R. Measurement of Thermal Radiation Properties

of Solids. NASA SP-31, p. 433, 1963.

8. Folweiler, R. C. Thermal radiation characteristics of transparent, semitransparent, and translucent materials under non-isothermal conditions.

ASD-TDR- 62- 719 (Aug. 1962).

9» Gravina, A. and Katz, M. Investigation of high emittance coatings.

WADD TR 60-102 (March 1961).

& 10. Edwards, D. K. and Roddick, R. D. Basic studies on the use and control of

solar energy. U. of Calif., Report No. 62-27.

11. Schatz, E. A. and McCandless,' L. C. Research on high and low emittance

coatings. ASD TR 62-443 (1962).

12. Middleton, W. E. K. .and Sanders, C. L. J. Opt. Soc. Am. 41, 419 (1951).

13. Sanders, C. L. and Middleton, W. E. K. J. Opt. Soc. Am. 43, 58 (1953).

14. Moore, D. G. Investigation of shallow reference cavities for hightemperature emittance measurements, NASA SP-31, p. 515, 1963.

24
THE NATIONAL BUREAU OF STANDARDS

The scope of activities of the National Bureau of Standards at its major laboratories in Washington, D.C.,and

Boulder, Colorado, is suggested in the following listing of the divisions and sections engaged in technical work.

In general, each section carries out specialized research, development, and engineering in the field ind icated by

its title. A brief description of the activities, and of the resultant publications, appears on the inside of the

front cover.

WASHINGTON, D. C.

Electricity. Resistance and Reactance. Electrochemistry. Electrical Instruments. Magnetic Measurements.

Dielectrics. High Voltage. Absolute Electrical Measurements.

Metrology. Photometry and Colorimetry. Refractometry. Photographic Research. Length. Engineering Metrology.

Mass and Volume.

Heat. Temperature Physics. Heat Measurements. Cryogenic Physics. Equation of State. Statistical Physics.

Radiation Physics. X-ray. Radioactivity. Radiation Theory. High Energy Radiation. Radiological Equipment.

Nucleonic Instrumentation. Neutron Physics.

Analytical and Inorganic Chemistry. Pure Substances. Spectrochemistry. Solution Chemistry. Standard Refer- ence Materials. Applied Analytical Research. Crystal Chemistry.

Mechanics. Sound. Pressure and Vacuum. Fluid Mechanics. Engineering Mechanics. Rheology. Combustion

Controls. Polymers. Macromolecules: Synthesis and Structure. Polymer Chemistry. Polymer Physics. Polymer Characterization. Polymer Evaluation and Testing. Applied Polymer Standards and Research. Dental Research.

Metallurgy. Engineering Metallurgy. Metal Reactions. Metal Physics. Electrolysis and Metal Deposition.

Inorganic Solids. Engineering Ceramics. Glass. Solid State Chemistry. Crystal Growth. Physical Properties.

Crystallography.

Building Research. Structural Engineering. Fire Research. Mechanical Systems. Organic Building Materials. Codes and Safety Standards. Heat Transfer. Inorganic Buildjng Materials. Metallic Building Materials. Applied Mathematics. Numerical Analysis. Computation. Statistical Engineering. Mathematical Physics. Operations Research. Data Processing Systems. Components and Techniques. Computer Technology. Measurements Automation.

Engineering Applications. Systems Analysis.

Atomic Physics. Spectroscopy. Infrared Spectroscopy. Far Ultraviolet Physics. Solid State Physics. Electron Physics. Atomic Physics. Plasma Spectroscopy.

Instrumentation. Engineering Electronics. Electron Devices. Electronic Instrumentation. Mechanical Instru- ments. Basic Instrumentation. Physical Chemistry. Thermochemistry. Surface Chemistry. Organic Chemistry. Molecular Spectroscopy. Ele- mentary Processes. Mass Spectrometry. Photochemistry and Radiation Chemistry.

Office of Weights and Measures. BOULDER, COLO.

Cryogenic Engineering Laboratory. Cryogenic Processes. Cryogenic Properties of Solids. Cryogenic Technical

Services. Properties of Cryogenic Fluids. CENTRAL RADIO PROPAGATION LABORATORY

Ionosphere Research and Propagation. Low Frequency and Very Low Frequency Research. Ionosphere Research. Prediction Services. Sun- Earth Relationships. Field Engineering. Radio Warning Services. Vertical Soundings Research. Troposphere and Space Telecommunications. Data Reduction Instrumentation. Radio Noise. Tropospheric Measurements. Tropospheric Analysis. Spectrum Utilization Research. Radio-Meteorology. Lower Atmosphere

Physics.

Radio Systems. Applied Electromagnetic Theory. High Frequency and Very High Frequency Research. Fre- quency Utilization. Modulation Research. Antenna Research. Radiodetermination.

Upper Atmosphere and Space Physics. Upper Atmosphere and Plasma Physics. High Latitude Ionosphere

Physics. Ionosphere and Exosphere Scatter. Airglow and Aurora. Ionospheric Radio Astronomy.

RADIO STANDARDS LABORATORY

Radio Standards Physics. Frequency and Time Disseminations. Radio and Microwave Materials. Atomic Fre- quency and Time-Interval Standards. Radio Plasma. Microwave Physics.

Radio Standards Engineering. High Frequency Electrical Standards. High Frequency Cal ibration Services. High

Frequency Impedance Standards. Microwave Calibration Services. Microwa v e Circuit Standards. Low Frequency

Calibration Services. NBS LABORATORY ASTROPHYSICS GROUP

(Joint Institute for Laboratory Astrophysics at Univ. of Colo.)

U S COMM-N BS-DC