1、Designation: E 423 71 (Reapproved 2008)Standard Test Method forNormal Spectral Emittance at Elevated Temperatures ofNonconducting Specimens1This standard is issued under the fixed designation E 423; the number immediately following the designation indicates the year oforiginal adoption or, in the ca
2、se of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.INTRODUCTIONThe general physical properties of ceramic materials combine to make thermal gradients a s
3、eriousproblem in the evaluation and use of thermal emittance data for such materials. Ceramic materials ingeneral tend to be somewhat translucent, and hence emit and absorb thermal radiant energy within asurface layer of appreciable thickness. Ceramic materials in general also tend to have low therm
4、alconductivity and high total emittance as compared to metals. These properties combine to producethermal gradients within a heated specimen unless careful precautions are taken to minimize suchgradients by minimizing heat flow in the specimen. The gradients tend to be normal to a surface thatis emi
5、tting or absorbing radiant energy. As a further complication, the gradients tend to be nonlinearnear such a surface.When a specimen is emitting from a surface layer of appreciable thickness with a thermal gradientnormal to the surface, it has no unique temperature, and it is difficult to define an e
6、ffective temperaturefor the emitting layer. Emittance is defined as the ratio of the flux emitted by a specimen to that emittedby a blackbody radiator at the same temperature and under the same conditions. It is thus necessaryto define an effective temperature for the nonisothermal specimen before i
7、ts emittance can beevaluated. If the effective temperature is defined as that of the surface, a specimen with a positivethermal gradient (surface cooler than interior) will emit at a greater rate than an isothermal specimenat the same temperature, and in some cases may have an emittance greater than
8、 1.0. If the thermalgradient is negative (surface hotter than interior) it will emit at a lesser rate. If the “effectivetemperature” is defined as that of an isothermal specimen that emits at the same rate as thenonisothermal specimen, we find that the effective temperature is difficult to evaluate,
9、 even if theextinction coefficient and thermal gradient are accurately known, which is seldom the case. If spectralemittance is desired, the extinction coefficient, and hence the thickness of the emitting layer, changeswith wavelength, and we have the awkward situation of a specimen whose effective
10、temperature is afunction of wavelength.There is no completely satisfactory solution to the problem posed by thermal gradients in ceramicspecimens. The most satisfactory solution is to measure the emittance of essentially isothermalspecimens, and then consider the effect of thermal gradients on the e
11、mitted radiant flux whenattempting to use such thermal emittance data in any real situation where thermal gradients normal tothe emitting surface are present.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.1. Scope1.1 This test metho
12、d describes an accurate technique formeasuring the normal spectral emittance of electrically non-conducting materials in the temperature range from 1000 to1800 K, and at wavelengths from 1 to 35 m. It is particularlysuitable for measuring the normal spectral emittance of mate-rials such as ceramic o
13、xides, which have relatively low thermalconductivity and are translucent to appreciable depths (severalmillimetres) below the surface, but which become essentiallyopaque at thicknesses of 10 mm or less.1.2 This test method requires expensive equipment andrather elaborate precautions, but produces da
14、ta that are accu-rate to within a few percent. It is particularly suitable forresearch laboratories, where the highest precision and accuracyare desired, and is not recommended for routine production oracceptance testing. Because of its high accuracy, this testmethod may be used as a reference metho
15、d to be applied toproduction and acceptance testing in case of dispute.1.3 This test method requires the use of a specific specimensize and configuration, and a specific heating and viewingtechnique. The design details of the critical specimen furnaceare presented in Ref (1),2and the use of a furnac
16、e of this designis necessary to comply with this test method. The transferoptics and spectrophotometer are discussed in general terms.1.4 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.1.5 This standard does not purport to addr
17、ess all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:3E 349 Terminol
18、ogy Relating to Space Simulation3. Terminology3.1 Definitions of Terms Specific to This Standard:3.1.1 spectral normal emittanceThe term spectral normalemittance (Note 4) as used in this specification follows thatadvocated by Jones (2), Worthing (3), and others, in that theword emittance is a proper
19、ty of a specimen; it is the ratio ofradiant flux emitted by a specimen per unit area (thermal-radiant exitance) to that emitted by a blackbody radiator at thesame temperature and under the same conditions. Emittancemust be further qualified in order to convey a more precisemeaning. Thermal-radiant e
20、xitance that occurs in all possibledirections is referred to as hemispherical thermal-radiant exi-tance. When limited directions of propagation or observationare involved, the term directional thermal-radiant exitance isused. Thus, normal thermal-radiant exitance is a special case ofdirectional ther
21、mal-radiant exitance, and means in a directionperpendicular (normal) to the surface. Therefore, spectralnormal emittance refers to the radiant flux emitted by aspecimen within a narrow wavelength band and emitted into asmall solid angle about a direction normal to the plane of anincremental area of
22、a specimens surface. These restrictions inangle occur usually by the method of measurement rather thanby radiant flux emission properties.NOTE 1All the terminology used in this test method has not beenstandardized. Terminology E 349 contain some approved terms. Whenagreement on other standard terms
23、is reached, the definitions used hereinwill be revised as required.4. Summary of Test Method4.1 The principle of the test method is direct comparison ofthe radiance of an isothermal specimen at a given temperatureto that of a blackbody radiator at the same temperature. Thedetails of the method are g
24、iven by Clark and Moore (1,4).NOTE 2With careful attention to detail, overall accuracy of 62 % canbe attained.4.2 The essential features of the test method are (1) the useof a cylindrical sample that rotates in an electrically heatedfurnace and attains essentially isothermal conditions, and (2)the u
25、se of electronic controls to maintain the host specimen andblackbody reference at the same temperature.4.3 Atheoretical analysis (5) was made of thermal gradientsin the rotating cylinder, supplemented by measurements of thetemperature and temperature changes indicated by a smallthermocouple imbedded
26、 0.025 mm below the surface of aspecimen of alumina, as the specimen rotated in front of awater-cooled viewing port. In brief, it was found that (1) thetemperature fluctuations at the surface of the specimen wereinversely related to the speed of rotation, and because negligi-bly small (2 K or less)
27、at speeds of rotation greater than 50r/min, and (2) the temperature indicated by a radiation shieldedthermocouple suspended in the center of the rotating specimenwas the same within 1 K as the average temperature indicatedby the embedded thermocouple at speeds of rotation greaterthan 10 r/min.NOTE 3
28、An electronic-null, ratio-recording spectrophotometer4is pre-ferred to an optical-null instrument for this use. Special precautions maybe necessary to obtain and maintain linearity of response of an optical-nullinstrument if the optical paths are not identical to those of the instrumentas manufactur
29、ed. Clark and Moore (1) describe linearity calibration of anoptical-null instrument.5. Significance and Use5.1 The significant features are typified by a discussion ofthe limitations of the technique. With the description andarrangement given in the following portions of this testmethod, the instrum
30、ent will record directly the normal spectral1This test method is under the jurisdiction of ASTM Committee E21 on SpaceSimulation and Applications of Space Technology and is the direct responsibility ofSubcommittee E21.04 on Space Simulation Test Methods.Current edition approved May 1, 2008. Publishe
31、d July 2008. Originally approvedin 1971. Last previous edition approved in 2002 as E 423-71(2002)2The boldface numbers in parentheses refer to the references listed at the end ofthis test method.3For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at
32、serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.4The Perkin-Elmer Model 13-U prism spectrophotometer is one of severalinstruments found suitable for this test method.E 423 71 (2008)2emittance of a specimen. However
33、, the following conditionsmust be met within acceptable tolerance, or corrections mustbe made for the specified conditions.5.1.1 The effective temperatures of the specimen and black-body must be within1Kofeach other. Practical limitationsarise, however, because the temperature uniformities are often
34、not better than a few kelvins.5.1.2 The optical path length in the two beams must beequal, or, preferably, the instrument should operate in anonabsorbing atmosphere, in order to eliminate the effects ofdifferential atmospheric absorption in the two beams. Measure-ments in air are in many cases impor
35、tant, and will notnecessarily give the same results as in a vacuum, thus theequality of the optical paths for dual-beam instruments be-comes very critical.NOTE 4Very careful optical alignment of the spectrophotometer isrequired to minimize differences in absorptance along the two paths of theinstrum
36、ent, and careful adjustment of the chopper timing to reduce“cross-talk” (the overlap of the reference and sample signals) as well asprecautions to reduce stray radiation in the spectrophotometer are requiredto keep the zero line flat. With the best adjustment, the “100 % line” willbe flat to within
37、3 %.5.1.3 Front-surface mirror optics must be used throughout,except for the prism in prism monochromators, and it should beemphasized that equivalent optical elements must be used inthe two beams in order to reduce and balance attenuation of thebeams by absorption in the optical elements. It is rec
38、om-mended that optical surfaces be free of SiO2and SiO coatings:MgF2may be used to stabilize mirror surfaces for extendedperiods of time.The optical characteristics of these coatings arecritical, but can be relaxed if all optical paths are fixed duringmeasurements or the incident angles are not chan
39、ged betweenmodes of operation (during 0 % line, 100 % line, and samplemeasurements). It is recommended that all optical elements beadequately filled with energy.5.1.4 The source and field apertures of the two beams mustbe equal in order to ensure that radiant flux in the two beamscompared by the app
40、aratus will pertain to equal areas of thesources and equal solid angles of emission. In some cases itmay be desirable to define the solid angle of the source andsample when comparing alternative measurement techniques.5.1.5 The response of the detector-amplifier system mustvary linearly with the inc
41、ident radiant flux, or must becalibrated for linearity, and corrections made for observeddeviations from linearity.6. Apparatus6.1 SpectrophotometerThe spectrophotometer used forthe measurement of spectral normal emittance is equipped witha wavelength drive that provides automatic scanning of thespe
42、ctrum of radiant flux and a slit servomechanism thatautomatically opens and closes the slits to minimize thevariations of radiant flux in the comparison beam. For mostmaterials the wavelength band-pass of the instrument is gen-erally smaller than the width of any absorption or emissionband in the sp
43、ectrum to be measured. Operation of thespectrophotometer at a higher sensitivity level or in a single-beam mode can be used to evaluate band-pass effects. In aprism instrument, several prism compositions can be used tocover the complete wavelength range; however, a sodiumchloride prism is typically
44、used to cover the spectral rangefrom 1.0 to 15 m, and a cesium bromide prism to cover thespectral range from 15 to 35 m. As a detector, a vacuumthermocouple with a sodium chloride window is used in thespectral range from 1 to 15 m, and a vacuum thermocouplewith a cesium bromide window in the spectra
45、l range from 1 to35 m. A black polyethylene filter is used to limit strayradiation in the 15 to 35-m range.6.1.1 In order to reduce the effects of atmospheric absorp-tion by water vapor and carbon dioxide, especially in the 15 to35-m range, the entire length of both the specimen andreference optical
46、 paths in the instrument must be enclosed indry nonabsorbing gas5(dew point of less than 223 K) by anearly gastight enclosure maintained at a slightly positivepressure relative to the surrounding atmosphere.6.2 Specimen FurnaceFig. 1 is a schematic drawing ofthe specimen furnace used at the National
47、 Institute of Stan-dards and Technology. The high-temperature alumina coresurrounding the specimen is wound with 0.8-mm diameterplatinum-40 % rhodium wire. The winding is continuous to theedges of the rectangular opening that is cut into the core topermit entrance of the viewing port. A booster wind
48、ing of thesame wire positioned on the outer alumina core, as indicated inFig. 1, is used to compensate for the large heat losses at thecenter.6.2.1 The water-cooled viewing port is machined fromcopper, and its inner surface is curved to the same radius as thespecimen. A shield of platinum foil, 0.05
49、 mm thick, surroundsthe outer surfaces of the port, including the edges that face thespecimen. This helps to isolate the viewing port thermally fromthe furnace interior. The inner surfaces of the viewing port andthe portion of the platinum shield nearest the specimen areblackened to minimize the possibility of errors from reflectedradiation.The opening at the inner end of the port is 3 mm wideby 12.7 mm high.6.2.2 The alumina support tube (Fig. 1) is surface ground tothe same tolerance as given in 7.1 for the test specimen. Thespindle is driven by a18-hp motor that is
