ASTM E423-71(1996)e1
(Test Method)Standard Test Method for Normal Spectral Emittance at Elevated Temperatures of Nonconducting Specimens
Standard Test Method for Normal Spectral Emittance at Elevated Temperatures of Nonconducting Specimens
SCOPE
1.1 This test method describes an accurate technique for measuring the normal spectral emittance of electrically nonconducting materials in the temperature range from 1000 to 1800 K, and at wavelengths from 1 to 35 m. It is particularly suitable for measuring the normal spectral emittance of materials such as ceramic oxides, which have relatively low thermal conductivity and are translucent to appreciable depths (several millimetres) below the surface, but which become essentially opaque at thicknesses of 10 mm or less.
1.2 This test method requires expensive equipment and rather elaborate precautions, but produces data that are accurate to within a few percent. It is particularly suitable for research laboratories, where the highest precision and accuracy are desired, and is not recommended for routine production or acceptance testing. Because of its high accuracy, this test method may be used as a reference method to be applied to production and acceptance testing in case of dispute.
1.3 This test method requires the use of a specific specimen size and configuration, and a specific heating and viewing technique. The design details of the critical specimen furnace are presented in Ref (1), and the use of a furnace of this design is necessary to comply with this test method. The transfer optics and spectrophotometer are discussed in general terms.
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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e1
Designation: E 423 – 71 (Reapproved 1996)
Standard Test Method for
Normal Spectral Emittance at Elevated Temperatures of
Nonconducting Specimens
This standard is issued under the fixed designation E 423; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
e NOTE—Section 11 was added editorially in May 1996.
INTRODUCTION
The general physical properties of ceramic materials combine to make thermal gradients a serious
problem in the evaluation and use of thermal emittance data for such materials. Ceramic materials in
general tend to be somewhat translucent, and hence emit and absorb thermal radiant energy within a
surface layer of appreciable thickness. Ceramic materials in general also tend to have low thermal
conductivity and high total emittance as compared to metals. These properties combine to produce
thermal gradients within a heated specimen unless careful precautions are taken to minimize such
gradients by minimizing heat flow in the specimen. The gradients tend to be normal to a surface that
is emitting or absorbing radiant energy. As a further complication, the gradients tend to be nonlinear
near such a surface.
When a specimen is emitting from a surface layer of appreciable thickness with a thermal gradient
normal to the surface, it has no unique temperature, and it is difficult to define an effective temperature
for the emitting layer. Emittance is defined as the ratio of the flux emitted by a specimen to that emitted
by a blackbody radiator at the same temperature and under the same conditions. It is thus necessary
to define an effective temperature for the nonisothermal specimen before its emittance can be
evaluated. If the effective temperature is defined as that of the surface, a specimen with a positive
thermal gradient (surface cooler than interior) will emit at a greater rate than an isothermal specimen
at the same temperature, and in some cases may have an emittance greater than 1.0. If the thermal
gradient is negative (surface hotter than interior) it will emit at a lesser rate. If the “effective
temperature” is defined as that of an isothermal specimen that emits at the same rate as the
nonisothermal specimen, we find that the effective temperature is difficult to evaluate, even if the
extinction coefficient and thermal gradient are accurately known, which is seldom the case. If spectral
emittance is desired, the extinction coefficient, and hence the thickness of the emitting layer, changes
with wavelength, and we have the awkward situation of a specimen whose effective temperature is a
function of wavelength.
There is no completely satisfactory solution to the problem posed by thermal gradients in ceramic
specimens. The most satisfactory solution is to measure the emittance of essentially isothermal
specimens, and then consider the effect of thermal gradients on the emitted radiant flux when
attempting to use such thermal emittance data in any real situation where thermal gradients normal to
the emitting surface are present.
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E 423
1. Scope normal emittance refers to the radiant flux emitted by a
specimen within a narrow wavelength band and emitted into a
1.1 This test method describes an accurate technique for
small solid angle about a direction normal to the plane of an
measuring the normal spectral emittance of electrically non-
incremental area of a specimen’s surface. These restrictions in
conducting materials in the temperature range from 1000 to
angle occur usually by the method of measurement rather than
1800 K, and at wavelengths from 1 to 35 μm. It is particularly
by radiant flux emission properties.
suitable for measuring the normal spectral emittance of mate-
rials such as ceramic oxides, which have relatively low thermal
NOTE 1—All the terminology used in this test method has not been
conductivity and are translucent to appreciable depths (several standardized. Definitions E 349 contain some approved terms. When
agreement on other standard terms is reached, the definitions used herein
millimetres) below the surface, but which become essentially
will be revised as required.
opaque at thicknesses of 10 mm or less.
1.2 This test method requires expensive equipment and
4. Summary of Test Method
rather elaborate precautions, but produces data that are accu-
4.1 The principle of the test method is direct comparison of
rate to within a few percent. It is particularly suitable for
the radiance of an isothermal specimen at a given temperature
research laboratories, where the highest precision and accuracy
to that of a blackbody radiator at the same temperature. The
are desired, and is not recommended for routine production or
details of the method are given by Clark and Moore (1,4).
acceptance testing. Because of its high accuracy, this test
method may be used as a reference method to be applied to
NOTE 2—With careful attention to detail, overall accuracy of 6 2 % can
be attained.
production and acceptance testing in case of dispute.
1.3 This test method requires the use of a specific specimen
4.2 The essential features of the test method are (1) the use
size and configuration, and a specific heating and viewing
of a cylindrical sample that rotates in an electrically heated
technique. The design details of the critical specimen furnace
furnace and attains essentially isothermal conditions, and (2)
are presented in Ref (1), and the use of a furnace ofthis design
the use of electronic controls to maintain the host specimen and
is necessary to comply with this test method. The transfer
blackbody reference at the same temperature.
optics and spectrophotometer are discussed in general terms.
4.3 A theoretical analysis (5) was made of thermal gradients
1.4 This standard does not purport to address all of the
in the rotating cylinder, supplemented by measurements of the
safety concerns, if any, associated with its use. It is the
temperature and temperature changes indicated by a small
responsibility of the user of this standard to establish appro-
thermocouple imbedded 0.025 mm below the surface of a
priate safety and health practices and determine the applica-
specimen of alumina, as the specimen rotated in front of a
bility of regulatory limitations prior to use.
water-cooled viewing port. In brief, it was found that (1) the
temperature fluctuations at the surface of the specimen were
2. Referenced Documents
inversely related to the speed of rotation, and because negligi-
2.1 ASTM Standards:
bly small (2 K or less) at speeds of rotation greater than 50
E 349 Terminology Relating to Space Simulation
r/min, and (2) the temperature indicated by a radiation shielded
thermocouple suspended in the center of the rotating specimen
3. Terminology
was the same within1Kasthe average temperature indicated
3.1 Definitions of Terms Specific to This Standard:
by the embedded thermocouple at speeds of rotation greater
3.1.1 spectral normal emittance—The term spectral normal
than 10 r/min.
emittance (Note 4) as used in this specification follows that
NOTE 3—An electronic-null, ratio-recording spectrophotometer is pre-
advocated by Jones (2), Worthing (3), and others, in that the
ferred to an optical-null instrument for this use. Special precautions may
word emittance is a property of a specimen; it is the ratio of
be necessary to obtain and maintain linearity of response of an optical-null
radiant flux emitted by a specimen per unit area (thermal-
instrument if the optical paths are not identical to those of the instrument
radiant exitance) to that emitted by a blackbody radiator at the
as manufactured. Clark and Moore (1) describe linearity calibration of an
optical-null instrument.
same temperature and under the same conditions. Emittance
must be further qualified in order to convey a more precise
5. Significance and Use
meaning. Thermal-radiant exitance that occurs in all possible
5.1 The significant features are typified by a discussion of
directions is referred to as hemispherical thermal-radiant exi-
the limitations of the technique. With the description and
tance. When limited directions of propagation or observation
arrangement given in the following portions of this test
are involved, the term directional thermal-radiant exitance is
method, the instrument will record directly the normal spectral
used. Thus, normal thermal-radiant exitance is a special case of
emittance of a specimen. However, the following conditions
directional thermal-radiant exitance, and means in a direction
must be met within acceptable tolerance, or corrections must
perpendicular (normal) to the surface. Therefore, spectral
be made for the specified conditions.
5.1.1 The effective temperatures of the specimen and black-
body must be within1Kof each other. Practical limitations
This test method is under the jurisdiction of ASTM Committee E-21 on Space
Simulation and Applications of Space Technology and is the direct responsibility of
arise, however, because the temperature uniformities are often
Subcommittee E21.04 on Space Simulation Test Methods.
not better than a few kelvins.
Current edition approved March 19, 1971.
The boldface numbers in parentheses refer to the references listed at the end of
this test method. The Perkin-Elmer Model 13-U prism spectrophotometer is one of several
Annual Book of ASTM Standards, Vol 15.03. instruments found suitable for this test method.
E 423
5.1.2 The optical path length in the two beams must be 35 μm. A black polyethylene filter is used to limit stray
equal, or, preferably, the instrument should operate in a radiation in the 15 to 35-μm range.
nonabsorbing atmosphere, in order to eliminate the effects of 6.1.1 In order to reduce the effects of atmospheric absorp-
differential atmospheric absorption in the two beams. Measure- tion by water vapor and carbon dioxide, especially in the 15 to
ments in air are in many cases important, and will not 35-μm range, the entire length of both the specimen and
necessarily give the same results as in a vacuum, thus the reference optical paths in the instrument must be enclosed in
equality of the optical paths for dual-beam instruments be- dry nonabsorbing gas (dew point of less than 223 K) by a
comes very critical. nearly gastight enclosure maintained at a slightly positive
pressure relative to the surrounding atmosphere.
NOTE 4—Very careful optical alignment of the spectrophotometer is
6.2 Specimen Furnace—Fig. 1 is a schematic drawing of
required to minimize differences in absorptance along the two paths of the
the specimen furnace used at the National Institute of Stan-
instrument, and careful adjustment of the chopper timing to reduce
“cross-talk” (the overlap of the reference and sample signals) as well as dards and Technology. The high-temperature alumina core
precautions to reduce stray radiation in the spectrophotometer are required
surrounding the specimen is wound with 0.8-mm diameter
to keep the zero line flat. With the best adjustment, the “100 % line” will
platinum-40 % rhodium wire. The winding is continuous to the
be flat to within 3 %.
edges of the rectangular opening that is cut into the core to
5.1.3 Front-surface mirror optics must be used throughout,
permit entrance of the viewing port. A booster winding of the
except for the prism in prism monochromators, and it should be same wire positioned on the outer alumina core, as indicated in
emphasized that equivalent optical elements must be used in
Fig. 1, is used to compensate for the large heat losses at the
the two beams in order to reduce and balance attenuation of the center.
beams by absorption in the optical elements. It is recom- 6.2.1 The water-cooled viewing port is machined from
mended that optical surfaces be free of SiO and SiO coatings:
copper, and its inner surface is curved to the same radius as the
MgF may be used to stabilize mirror surfaces for extended specimen. A shield of platinum foil, 0.05 mm thick, surrounds
periods of time. The optical characteristics of these coatings are
the outer surfaces of the port, including the edges that face the
critical, but can be relaxed if all optical paths are fixed during specimen. This helps to isolate the viewing port thermally from
measurements or the incident angles are not changed between
the furnace interior. The inner surfaces of the viewing port and
modes of operation (during 0 % line, 100 % line, and sample the portion of the platinum shield nearest the specimen are
measurements). It is recommended that all optical elements be
blackened to minimize the possibility of errors from reflected
adequately filled with energy. radiation. The opening at the inner end of the port is 3 mm wide
5.1.4 The source and field apertures of the two beams must
by 12.7 mm high.
be equal in order to ensure that radiant flux in the two beams 6.2.2 The alumina support tube (Fig. 1) is surface ground to
compared by the apparatus will pertain to equal areas of the
the same tolerance as given in 7.1 for the test specimen. The
sources and equal solid angles of emission. In some cases it spindle is driven by a ⁄8-hp motor that is coupled to a gear
may be desirable to define the solid angle of the source and
reducer. With the arrangement used, the rotation of the speci-
sample when comparing alternative measurement techniques. men can be adjusted to any speed in the range from 1 to 300
5.1.5 The response of the detector-amplifier system must
r/min.
vary linearly with the incident radiant flux, or must be 6.2.3 The design of the furnace shell is such that the furnace
calibrated for linearity, and corrections made for observed
can be operated in an inert atmosphere, as well as in air.
deviations from linearity. Glass-metal seals are used for power leads and rubber O-ring
seals are used for the shell ends, as well as for a sodium
6. Apparatus
chloride viewing window. The thrust bearings are designed to
6.1 Spectrophotometer—The spectrophotometer used for
provide a reasonably gastight seal at the point where the
the measurement of spectral normal emittance is equipped with
spindle shaft enters the shell.
a wavelength drive that provides automatic scanning of the
6.2.4 Axial temperature gradients in the specimens are
spectrum of radiant flux and a slit servomechanism that
reduced to low values by adjusting the power to the booster
automatically opens and closes the slits to minimize the
coil. The gradients are measured by sighting a micro-optical
varia
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