1、Designation: E434 10Standard Test Method forCalorimetric Determination of Hemispherical Emittance andthe Ratio of Solar Absorptance to Hemispherical EmittanceUsing Solar Simulation1This standard is issued under the fixed designation E434; the number immediately following the designation indicates th
2、e year oforiginal adoption or, in the case 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.1. Scope1.1 This test method covers measurement techniques for
3、calorimetrically determining the ratio of solar absorptance tohemispherical emittance using a steady-state method, and forcalorimetrically determining the total hemispherical emittanceusing a transient technique.1.2 This standard does not purport to address all of thesafety concerns, if any, associa
4、ted 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:2E349 Terminology Relating to Space Simulation3. Summary of T
5、est Method3.1 In calorimetric measurements of the radiative propertiesof materials, the specimen under evaluation is placed in avacuum environment under simulated solar radiation with coldsurroundings. By observation of the thermal behavior of thespecimen the thermophysical properties may be determi
6、ned byan equation that relates heat balance considerations to measur-able test parameters.3.2 In a typical measurement, to determine a/ as defined inDefinitions E349, the side of the specimen in question isexposed to a simulated solar source, through a port havingsuitable transmittance over the sola
7、r spectrum. This port, orwindow, must be of sufficient diameter that the specimen andradiation monitor will be fully irradiated and must be ofsufficient thickness that it will maintain its strength withoutdeformation under vacuum conditions. The radiant energyabsorbed by the specimen from the solar
8、source and emitted bythe specimen to the surroundings cause the specimen to reachan equilibrium temperature that is dependent upon the a/ ratioof its surface.3.3 In the dynamic radiative method of measuring totalhemispherical emittance, the specimen is heated with a solarsimulation source and then a
9、llowed to cool by radiation to anevacuated space chamber with an inside effective emittance ofunity. From a knowledge of the specific heat of the specimen asa function of temperature, the area of the test specimen, itsmass, its cooling rate, and the temperature of the walls, its totalhemispherical e
10、mittance may be calculated as a function oftemperature.4. Apparatus4.1 The main elements of the apparatus include a vacuumsystem, a cold shroud within the vacuum chamber, instrumen-tation for temperature measurement, and a solar simulator.4.2 The area of the thermal shroud shall not be less than 100
11、times the specimen area (controlled by the specimen size). Theinner surfaces of the chamber shall have a high solar absorp-tance (not less than 0.96) and a total hemispherical emittanceof at least 0.88 (painted with a suitable black paint),3and shallbe diffuse. Suitable insulated standoffs shall be
12、provided forsuspending the specimen. Thermocouple wires shall be con-nected to a vacuumtight fitting where the temperature offeedthrough is uniform. Outside of the chamber, all thermo-couples shall connect with a fixed cold junction.4.3 The chamber shall be evacuated to a pressure of1 3 106torr (0.1
13、 mPa) or less at all times.4.4 The walls of the inner shroud shall be in contact withcoolant so that their temperature can be maintained uniform atall times.4.5 A shutter shall be provided in one end of the chamberwhich can be opened to admit a beam of radiant energy froma solar simulator. When open
14、, this shutter shall provide anaperture admitting the full simulator beam. When the shutter isclosed, all rays emitted by the specimen shall be intercepted by1This test method is under the jurisdiction of ASTM Committee E21 on SpaceSimulation and Applications of Space Technology and is the direct re
15、sponsibility ofSubcommittee E21.04 on Space Simulation Test Methods.Current edition approved Nov. 1, 2010. Published December 2010. Originallyapproved in 1971. Last previous edition approved in 2002 as E434 71 (2002).DOI: 10.1520/E0434-10.2Annual Book of ASTM Standards, Vol 15.03.3Nextel Brand Velve
16、t Coating 401-C10 Black, available from ReflectiveProducts Div., 3M Co., has been found to be satisfactory.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.a blackened surface at the coolant temperature (the shuttermust be at least co
17、nductively coupled to the shroud).4.6 The vacuum chamber shall be provided with a fusedsilica window large enough to admit the simulator beam anduniformly irradiate the entire specimen projected area. Thiswindow shall have high transmittance through the solar spec-trum wavelength region. The chamber
18、 shall be provided with avacuumtight sleeve for opening and closing the shutter andstandard vacuum fittings for gaging, bleeding, leak testing, andpumping. If low a/ specimens are to be measured, the solidangle subtended by the port from the specimen should be small(dependent upon desired accuracy).
19、 If flat specular specimensare to be measured, the port plane should be canted withrespect to the specimen plane to eliminate multiple reflectionsof the simulator beam. Multiple reflections could result in asmuch as a 7 % apparent increase in a/.4.7 The solar simulator should duplicate the extraterr
20、estrialsolar spectrum as closely as possible. A beam irradiance of atleast 7000 W/m2at the specimen plane shall be available fromthe solar simulator (;5 solar constants). This irradiance maybe required to raise the temperature of certain specimens to adesired level.5. Coating Requirements5.1 Any typ
21、e of coating may be tested by this test methodprovided its structure remains stable in vacuum over thetemperature range of interest.5.2 For high emittance specimens the accuracy of themeasurements is increased if only one surface of the substrateis coated with the specimen coating in question. The r
22、emainingarea of the substrate shall be coated with a low emittancematerial of known hemispherical emittance (such as evapo-rated aluminum or evaporated gold).5.3 The thickness and density of the coating shall bemeasured and its heat capacity calculated from existing refer-ences (see Refs (1) and (2)
23、.46. Specimen Preparation6.1 The substrates used for the measurements describedhere shall be of a material whose specific heat as a function oftemperature can be found in standard references (for example,OFHC copper or a common aluminum alloy such as 6061-T6)(Ref (1).6.2 The substrate shall be machi
24、ned from flat stock and to asize proportioned to the working area of the chamber.6.3 Each specimen shall be drilled with a set of holes, nearthe edge, through which suspension strings are to be inserted.6.4 Each substrate shall be drilled with two small shallowholes in the back for thermocouples.6.5
25、 Ideally the back and sides of the substrate shall bebuffed and polished and one uninsulated thermocouple insertedin the back of the specimen (one wire in each hole). One ofthese wires shall be peened into each hole.6.6 Alow-emittance coating shall be applied to the back andsides of the substrate an
26、d to the thermocouple wires for severalinches at the specimen end.6.7 The substrates shall be coated with the material inquestion. The coating shall be of sufficient thickness so as to beopaque. (This will avoid any substrate effects.)6.8 The specimens shall be suspended from the top of theshroud by
27、 means of thread or string. These strings shall be ofsmall diameter, low thermal conductivity, and low emittance inorder to minimize heat losses through the leads.6.9 An alternative method of specimen mounting (massdependent) shall be to suspend the specimens by their ownsmall wire thermocouple lead
28、s. In this case the thermocoupleholes shall be drilled as before but radially around the edge.The suspension holes may also be eliminated in this case.7. Procedure7.1 Suspend the test specimen in the chamber normal to theincident solar radiation, but geometrically removed from thecentral axis of the
29、 chamber so that radiation from the specimento the chamber walls is not specularly reflected back to thespecimen. Since the chamber walls are designed to be cold andhighly absorbing, first reflections from the walls are usually allthat need be considered.7.2 Determine the simulated solar irradiance
30、incident on thespecimen with a suitable radiometric device such as a com-mercial thermopile radiometer or a black monitor sample ofknown a/ which may be suspended similarly to the testspecimen within the incident beam of simulated solar radiation.Take care in the latter case that the irradiance and
31、spectraldistribution of the incident energy is the same for bothspecimen and monitor.7.3 Then close the system and start the evacuation andcooling of the shroud (see Ref (3) for a typical system).Maintain a pressure of 1 3 106torr (0.1 mPa) or less and thewalls of the chamber must be at coolant temp
32、erature. Recordthe specimen, monitor, and shroud temperatures.7.4 When the specimen has reached thermal equilibrium,that is, when the specimen temperature becomes constant withconstant surrounding conditions, shut off the solar simulator.When specimens of large thermal mass are used, carefullyevalua
33、te the DT/Dt = 0 conditions, that is, the Dt chosen shouldbe dependent on the specimen time constant.7.5 Close the moveable door in the shroud and allow thespecimens to cool to a desired temperature. Measure thespecimen temperature as a function of time and calculate therates of change of the temper
34、ature.8. Calculation8.1 Calculate the aes/ (T1) ratio from the following equa-tion:aes T1!5AtApEsST142 T0! T1!T04D(1)where:aes= Effective solar absorptance relative to the illumi-nating source, (T0) = hemispherical emittance of the specimen at Tem-perature T0, (TI) = hemispherical emittance of the s
35、pecimen at Tem-perature T1,4The boldface numbers in parentheses refer to the list of references appended tothis method.E434 102s = Stefan-Boltzmann constant,Ap= projected area of the specimen exposed to solarradiation,E = incident total irradiance,T1= specimen equilibrium temperature with simulateds
36、olar radiation,T0= chamber wall temperature with solar source off,andAT= total radiating area of the specimen.8.2 This equation is derived in the following manner: If aspecimen coated on all sides with the material in question, witha projected area as viewed in the direction of irradiation, Ap,atota
37、l area, AT, effective simulated solar absorptance, aes,emittance at T1, (T1), and specific heat cpis suspended in anevacuated high absorptance isothermal cold-walled chamberand exposed to a simulated solar irradiance, E, the rate oftemperature change can be determined by evaluating the heatbalance e
38、quation.The energy balance of an irradiated specimenemitting radiant energy in a vacuum is given by the followingequation (assuming parasitic heat losses can be ignored):mcpSdTdtD5 ApsEp1 Ep2 At T1! s T141 AtatrT0! s T04(2)where Ep= AsT24, the thermal radiation from the port. Todetermine the inciden
39、t thermal radiation, Ep, see Ref (3). Thelast term, Atatr(T0) s T04, is the amount of heat energyabsorbed by the sample from the chamber walls. Kirchoffs lawtells us that at a given temperature the infrared absorptance isequal the infrared emittance. This means that it will emit asmuch heat as it ab
40、sorbs from a black body at the sametemperature as the sample. Therefore, to know how muchenergy is absorbed by the sample from the shroud walls wemust know the infrared absorptance (and hence the emittance)of the sample at the temperature of the shroud wall. Theinfrared absorptance at T0atr(T0), by
41、Kirchoffs law is equal tothe infrared emittance of the sample at that temperature so wecan write:mcpSdTdtD5 ApsEp1 Ep2 At T1! s T141 At T0! s T04(3)If Epis eliminated from Eq 2 when an equilibrium temperatureis reached, mcp(dT/dt) = 0, and,From Eq 2, solving for the a/ ratio we obtainaes T1!5AtApEsS
42、T142 T0! T1!T04D(4)Eq 4 is used to calculate the aes/ (T1) ratio when theparameters AT, E, and Apare determined and the equilibriumtemperature is measured.8.3 If the source is blocked by the shutter and the specimenlooses energy only by radiation, the energy balance equationbecomes:mcSdTdtD5 At T1!
43、s T14 AtatraT0! s T041 Qll1 Qrg Qts(5)Where Qlland Qgrepresent the heat losses from the supportleads and the heat lost from the residual gasses in the vacuumchamber, respectively. The last term Qtsis any heat input fromthe temperature sensor. See Ref (7) and Ref (8) for a treatmentof the lead loss a
44、nd residual gas heat loss terms.8.4 If the term T04is neglected, and the parasitic heat lossesand gains can be ignored, the above equation can be integratedand expanded into: T1! 5mscs1 mccc!3sAtDtS1T1321T23 D(6)where:ms= mass of the substrate,mc= mass of the coating,cs= thermal capacitance of the s
45、ubstrate,cc= thermal capacitance of the coating,T = temperature of the specimen, andDt = change in time from T1to T2and magnitude such thatcsand ccmay be assumed constant over smalltemperature ranges.When the temperature decay is recorded with time, then thetotal hemispherical emittance of the sampl
46、e can be determinedwith Eq 5 or Eq 6. The use of Eq 6 is preferable since Eq 5involves the experimental determination of two quantities(dT/dt and T4), thereby introducing more possible errors than inEq 6.8.5 Data from specimens which are coated on one side onlyshall be reduced by use of the followin
47、g equation: T!c5mscs1 mccc!3sAcDtS1T1321T23 D2sAT2 Ac!Ac(7)where:s= total hemispherical emittance of substrate,Ac= area of coating, andc= total hemispherical emittance of coating.8.6 To obtain an a/ measurement or an effective solarabsorptance, a, for a specimen coated only on one side, onemust cons
48、ider the following expression:ATT5 Acc1 Ass(8)where:AT,Ac,As= total area, area of the coating, and uncoatedarea of the substrate, respectively, andT, c, s= total hemispherical emittance of the speci-men, coating, and substrate respectively.Rearrangement shows that:T5 Acc1 Ass!/AT(9)Multiplying the a
49、/ value obtained from Eq 4 by T(at thesame temperature of equilibrium) obtained from Eq 9 will givethe solar absorptance, a. In order to acquire the (a/) coating,divide the asvalue by c(already measured in a transient cooldown).9. Report9.1 The report should include the methods used for tem-perature and irradiance measurements, and the actual data usedfor the calculations.9.2 A complete characterization of the specimen shall begiven whenever possible. This shall include specimen dimen-sions, specimen composition, coati
