A method and apparatus is described for the measurement of spectral normal emittance in air of a variety of materials. The system permits measurements to be performed over a wavelength region of 1.0 through 15.0 microns and over a temperature range of 600F to 1,8000F with an accuracy of 5.0 percent. The advantages of this system are described. Results obtained by this system are compared with results reported by another observer using a different technique

Slemp, Wayne S.

Wade, William R.

English

NASA Technical Reports Server

I .  - .  . .  
L 
A MFPHOD FOR MMSURING TEE SPECTRAL NORMAL ENITTANCE IT3 AIR 
OF A VARIGM OF MATERIAIS HAVING SCABLE E3ECTANCE CHARAcrwISTICS 
By Wayne S. Slemp and W i l l i a m  R. Wade 
I?ASA Langley Research Center 
Langley Station, Bampton, Va. 
For Presentation a t  the Symposium on Measurement of 
Thermal Radiation Properties of Solids 
Dayton, Ohio 
September 5-7, 1962 
c 
https://ntrs.nasa.gov/search.jsp?R=19620002966 2020-03-11T16:43:00+00:00Z
c . .  
A W H O D  FOR l@JEWUNG THE SPECTRAL NORMAL ESrlITTANCE I N  A I R  
e 
OF A VARIETY OF MATERIALS HAVING STABLE l3MTEWNCE CHARACTERISTICS 
By Wayne S. Slemp" and W i l l i a m  R. Wade* 
ABSTRACT 
A method and apparatus i s  described for  the measurement of spectral 
normal emittance in  a i r  of a variety of materials. 
urements t o  be performed over a wavelength region of 1.0 through 15.0 microns 
and over a temperature range of 600° F t o  1,8000 F with an accuracy of 
5.0 percent. 
by this system are compared with results reported by another observer using 
a different technique. 
The system permits meas- 
The advantages of this system are described. Results obtained 
IITRODJCTION 
In order that the proper selection of materials for  the design of space 
radiators, heat rejection systems, and power plants for  use i n  space can be 
made, accurate heat-transfer calculatians must be performed. 
t r a l  emittance of these materials plays an extremely important part  i n  these 
calculations, a program has been init iated a t  the Langley Research Center of 
NASA f o r  the investigation of these properties. 
emittance program i s  the measurement of s p e c t r a l n o m l  emittance i n  a i r  of 
a variety of materials with stable emittance properties. 
include ceramics, cermets, stably oxidized metals, and coatings on metallic 
substrates. 
Since the spec- 
One phase of the spectral 
These materials 
L 
In effor ts  t o  provide an improved system for obtaining these measure- 
ments, a technique together with necessary apparatus was developed which has 
provedto be relatively simple, fas t ,  and reliable. 
i n  this system consisted of a modified commercially available recording spec- 
trophotometer and a blackbody furnace similar t o  that  used by McMahon i n  h is  
investigation of glasses i n  reference 1. T h i s  system permits measurements of 
spectral normal emittance t o  be made over a temperature range from 6000 F t o  
1,800° F. Since the primary use of the data obtained by th i s  system w i l l  be 
f o r  heat-transfer calculations, the near infrared region from 1.0 through 
15.0 microns was chosen. T h i s  region, according t o  Planck's spectral energy 
distribution l a w ,  contains a minirmun of 90 percent of the  energy in the 6000 F 
t o  1,800° F temperature range. 
The apparatus ut i l ized 
*Aerospace Technologist . 
. ,  
The description of this system and the procedure used t o  obtain these 
measurements i s  the subject of t h i s  paper. 
APPARATUS 
The apparatus under consideration can be divided into two main parts. 
A spectrophotometer with i t s  detector and recorder i s  one part. 
which contains the t e s t  specimen and a blackbody cavity, and an associated 
temperature monitoring system and power supply comprise the second part. 
These two parts of the apparatus are described i n  de ta i l  i n  the following 
sections. 
A furnace, 
Spectrophotometer and Associated Apparatus 
A commercially available spectrophotometer i s  used i n  t h i s  investigation. 
A minor modification was made t o  the optical system t o  allow the use of an 
external source. 
are shown schematically i n  figure 1. Mirrors MI and M2 as well as the 
internal source of the standard instrument, were removed. 
6-inch focal-length parabolic mirror, and M4, a plane-surface mirror, were 
positioned t o  transmit radiation from the external source and t o  focus t h i s  
radiation a t  the same point as the original instrument optics. A new cover 
for t h i s  porticm of the instrument was designed with an aperture and water- 
cooled shutter so that  the radiation from the external source could be pre- 
vented from passing into the instrument when so desired. 
These modifications together with the original optical path 
Mirrors M3, a 
Since th i s  spectrophotometer has several interchangeable prisms and 
detectors available for  use i n  various regions of the spectrum, the sodium 
chloride prism with the high-speed evacuated thermocouple detector with a 
potassiumbromide window were chosen t o  cover th i s  spectral region, from 1.0 
through 15.0 microns. 
Furnace and Associated Apparatus 
The furnace, shown i n  figure 2, i s  designed similarly t o  that  used by 
McMahon (ref. 1) i n  his investigations of glasses. 
t i a l l y  of three cubes. 
body cavity. 
1/16 inch thick and 6 inches on each side. 
silicon carbide which serves as  the furnace core. The silicon carbide i s  
wound with l7-gage (AWG) Nichrome V resistance wire that serves as the heating 
element. The core i s  w e l l  insulated by an outer cube of high-temperature 
refractory firebrick. 
i t a t e  specimen placement. 
tapered, water-cooled viewport, as well as provisions for rotating the t e s t  
specimen within 1/16 inch of the Inconel l iner  by an external 15-rpm motor. 
It i s  constructed essen- 
The inner cube, called the l iner ,  serves as the black- 
It i s  composed of heavily oxidized Inconel approximately 
This l iner  f i t s  inside a cube of 
A l l  these cubes are fabricated i n  two halves t o  faci l -  
The lower half of the furnace contains a blackened, 
2 
8 
i 
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J 
. 
\ 
4 
Since the furnace i s  divided into t w o  halves and the Nichrome V windings 
are separated, the power i s  supplied and controlled by two continuously vari- 
able autotransformers. 
a 
In operation 30-gage (AWG) chrome1 alumel thermocouples are spotwelded 
t o  the outer surface of the Inconel liner. 
monitored t o  adjust and hold the temperature of the cavity (and thus the t e m -  
perature of the t e s t  specimen) a t  the desired value. 
These thermocouples are constantly 
TES" PROCEDURES 
Specimens used for t h i s  investigation were cut from 1/8-inch stock i n  the 
form of a 4-inch-diameter semicircle. 
rotating shaft i n  the m a c e  normal t o  the axis of the viewport and passing 
within 1/16 inch of the viewport for each cycle of rotation. By manual con- 
t r o l  of the autotransformers, the  furnace cavity i s  brought t o  equilibrium a t  
the desired tezgperature, and emittance measurements are begun. 
These specimens were positioned on the 
e 
. 
An example of a typical recorder trace showing the measuring technique 
i s  shown i n  figure 3 for  a particular temperature and wavelength. 
any change i n  t h i s  setting. 
wavelength and the water-cooled shutter i s  opened. 
then recorded and the water-cooled shutter i s  closed. 
repeated at  0.5-micron intervals from 1.0 through 15.0 microns. The monochro- 
mator sl i ts  are held constant and the instrument amplifier gain i s  adjusted at  
each reading i n  order that  a maximum recorder deflection can be obtained. The 
large recorder deflection minimizes the error i n  calculating emittance values 
from recorder charts. 
i s  then obtained by the ratio: 
The 
1 recorder zero Vo i s  displaced up scale t o  allow the complete monitoring of 
The monochromator i s  then set  a t  the desired 
The radiant flux intensi- 
0 t ies from the blackbody reference VB and fromthe test specimen Vs are 
T h i s  procedure i s  
The calculation of the spectral n o m 1  emittance % 
CALIBRATION 
C 
Spectrophotometer 
8 
Since several prisms are interchangeable for this spectrophotometer, the 
wavelength indicator i s  divided i n  arbitrary units. 
instrument must, therefore, be calibrated through the wavelength region 
covered by this prism. 
i s  of interest  i n  t h i s  study, the sodium chloride prism was chosen. 
Each prism used i n  t h i s  
c 
Since the infrared region from 1.0 through 13.0 microns 
This prism 
was then calibrated through t h i s  region by the use of absorption and emission 
3 
. .  
spectra of known compounds and elements and a calibration curve of wavelength 
indicator number as  a function of wavelength was constructed. 
Furnace 
I n  order t o  determine whether any possible temperature different ia l  
exists between a test specimen and the cavity, an Inconel specimen with three 
thermocouples attached, as shown i n  figure 4(a), was placed i n  the flumace 
out of view of the port. The temperature of the cavity was measured by ther- 
mocouples attached a t  eight points on the Inconel cavity. 
these eight thermocouples i s  shown i n  figure 4(b). 
4 
The placement of 
The temperatures of the specimen and of the cavity were measured over the 
temperature range from 600° F t o  1,800° F and were found t o  be i n  close agree- 
ment as  shown i n  figure >. Although these temperatures are  averages of a l l  
thermoccmples on the specimen or cavity, the actual  differences are less  than 
1.0 percent. The specimen temperature w a s  lower than the cavity temperature 
over the entire temperature range, but the  maximum deviation was only 0.87 per- 
cent of the absolute temperature. These measurements indicate tha t  the  assump- 
t ion  of thermal equilibrium between specimen and cavity i s  valid for  the non- 
measuring portion of the specimen cycle. Thus, the measurement of cavity 
tance error of less  than 3.50 percent. 
4 
, 
temperature may be taken also as specimen temperature with a possible emit- t 
When the specimen i s  rotated i n  front of the viewing port it radiates t o  
the cooler surroundings and obviously must cool. 
determined by many factors, one of which i s  the emittance of the specimen. 
To determine the effects of t h i s  cooling on emittance measurements the fol-  
lowing procedure was used: A n  Inconel specimen was placed i n  the furnace, 
out of view of the port, and heated t o  1,800° F. A f t e r  the furnace tempera- 
ture  was stabilized, measurements of radiant flux intensity were obtained by 
use of a t o t a l  radiation bolometer. 
the bolometer was focused in to  the cavity, and a measurement of the blackbody 
flux intensity was recorded. 
and a measurement of the specimen flux intensity w a s  recorded. 
of obtaining measurements fo r  the blackbody and specimen alternately w a s  con- 
tinued for I 2  cycles, The resul ts  are shown i n  figure 6 where 
* 
The ra te  of cooling i s  
With the specimen out of view of the port 
The specimen was then rotated past the viewport, 
This  procedure 
radiometer output fo r  specimen, rms volts fS 
radiometer output fo r  blackbody, rms volts f B  
From a comparison of the r a t i o  of the radiant f lux intensi t ies  of specimen 
and blackbody, an indication of the cooling effect  i s  obtained. 
emittance measured for  the 1/2 cycle ( the f i r s t  viewing cycle) was 0.97, and 
tha t  measured for the 3/2 cycle was 0.94. 
cent, which indicates that  the specimen coolir?g from rotating past the view- 
port i s  negligible for  most materials. 
The specimen 
This difference i s  about 1.00 per- 
The reproducibility of t h i s  system as measured during these t e s t s  i s  
within 0.5 percent. 
4 
.. . .  
c 
. 
The t o t a l  error possible i n  considering a l l  these factors cannot exceed 
5.00 percent and is  probably much less. 
This system for the measurement of normal spectral emittance i n  a i r  of 
materials with stable emittance properties a t  moderately high temperatures 
has several dist inct  advantages over m a ~ y  other systems used in  these studies. 
One advantage is  the elimination of attached thermocouples on the surface 
of the specimen or use of other temperature measuring devices, such as optical  
pyrometers, t o  obtain the temperature of the t e s t  specimen. 
of the cavity i s  measured by thermocouples spotwelded t o  the outer surface of 
the cavity and the specimen temperature can be maintainedto within 1.00 per- 
cent of the cavity temperature, which makes the system practical for use with 
a large variety of materials. These include ceramics, cermets, thick coatings 
on metallic substrates, and other mter ia l s  t o  which it i s  d i f f i cu l t  or impos- 
sible t o  attach a thermocouple. 
The temperature 
Methods ut i l iz ing a dual-beam spectrophotometer for  these measurements 
are complicated by the cost and t i m e  involved i n  equalizing the path lengths 
and absorption of mirrors i n  each beam. 
body reference is used in that type of operation, a problem of maintaining 
them a t  equal temperatures is present. 
single-beam aperation and separated reference and specimen are confronted with 
the same problem of temperature control or the problem of absorbing gases i n  
the optical path. 
Since a separated specimen and black- 
On the other hand, methods favoring 
The use of the blackbody-cavity method requires only one path, so that  
the problem of equalizing the beams, as i n  dual-beam operation i s  avoided. 
The blackbody reference and t e s t  specimen are kept a t  approximately equal 
temperatures, with differences of less than 1.00 percent, by control of the 
furnsce temperature alone; thus the need for elaborate temperature regulating 
devices i s  eliminated. 
2 seconds, so that  the effects of changes i n  composition and quantity of 
absorbing gases i n  the optical path are canceled out. 
The reference and t e s t  specimen are viewed within 
TYPICAL m m s  
Measurements of normal spectral emittance us ingth is  method have been 
performed on several high-temperature materials including boron nitride,  
Inconel, Inconel-X, alumina, and zirtonia as  w e l l  as several other refractory 
metals. 
on refractory metals. 
The apparatus i s  currently being usedto  study flame sprayed ceramics 
Figures 7, 8, and 9 present comparisons of the results obtained by t h i s  
system and those obtained by Henry H. Blau, Jr. and associates using a differ-  
ent technique, as reported i n  reference 2. 
were taken from reference 3. 
The IUASA data shown i n  figure 7 
5 
The measurements reported by M r .  Blau and associates were made on t e s t  
specimens heated i n  a i r  a t  temperatures from 600° C t o  1,0000 C and over a 
wavelength region from 2.0 t o  15.0 microns, with a reported accuracy of 
54 percent. 
i n  that temperatures of both specimen and blackbody were measured by embedded 
thermocouples and values of radiant intensity from the specimen and blackbody 
reference were measured independently. Although the specimen and blackbody 
heat source varied from those used in  this investigation, almost a l l  other 
apparatus and techniques were very similar t o  those reported i n  th i s  paper. 
The procedures used differed fromthose described in  this paper c 
, 
One main problem discussed i n  reference 2 i s  that  of determining the 
temperature lag of the thermocouples used t o  obtain the temperature of the 
blackbody and specimen. 
Another problem, although not discussed i n  reference 2, i s  the control 
of absorbing gases i n  the optical path due t o  the time interval necessary 
between measurements of specimen and blackbody radiation. 
and humidity of the room were closely controlled so that the errors caused by 
these absorbed gases would be extremely small. 
4 
The temperature 
A s  i l lustrated i n  figures 7, 8, and 9, agreement i n  the results obtained 
by the t w o  systems i s  extremely close, and the error i n  each system i s  approx- 
such other factors a s  the ease of specimen preparation and of operation of the 
apparatus. 
imately the same. Therefore, the choice between these two systems depends on I 
e 
CONCLUDING RpllARKs 
A system has been developed for  the measurement of spectral normal emit- 
tance i n  a i r  of materials with stable emittance properties. 
materials which can be tested, the simple method of specimen and blackbody 
temperature control, and the elimination of problems associated with absorbing 
gases i n  the optical path are some of the advantages of this system over 
others now i n  use. 
The variety of 
Both consideration of possible errors and comparison of results with 
those of a prior investigator indicate that the results are accurate and 
reliable. 
6 
. .  
REFERENCES 
1. McMahon, Howard 0.: Thermal Radiation Characteristics of Same Glasses. 
Journal American Ceramic Soc., vol. 34, no. 3, 1951, p. 91. 
r 
2. BLSu, Henry E., Jr., Marsh, John B., Martin, W i l l l a m  S.,  Jasperse, John R., 
and Chaffee, Eleanor: 
Materials. 
Infrared mectral  Emittance Properties of Solid 
AFCRL-TR-60-416, Geophys . Res. Directorate , Air Force Res. 
mv., oct. 1960. 
3.  Walker, G i l b e r t  E., and Casey, Francis W., Jr.: 
Normal Emittance of Boron nit r ide From 1,2000 
Spectral Emittance Data at 1,m0 F. NASA TN 
Measurement of Total 
F t o  1,9000 F With Normal 
D - H ,  1961. 
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A Method for Measuring the Spectral Normal Emittance in Air of A Variety of Materials Having Stable Emittance Characteristics

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