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Low temperature radiative properties of materials used in cryogenics
Cryogenics 45 (2005) 529–536 www.elsevier.com/locate/cryogenics
Low temperature radiative properties of materials used in cryogenics V. Musilova *, P. Hanzelka, T. Kralik, A. Srnka Institute of Scientific Instruments, Kra´lovopolska´ 147, Brno CZ-612 64, Czech Republic Received 16 August 2004; received in revised form 18 August 2004; accepted 29 November 2004
Abstract Radiative heat transfer between two parallel surfaces, a sample surface and a black surface, was measured. One of the surfaces was cooled with liquid helium to about 5 K and the other one was step by step heated to temperatures ranging between 30 and 140 K. As a result, the total hemispherical absorptivity and emissivity of the sample surface were determined in dependence on the temperature of the heat radiation. Aluminium samples were made of Al sheet, Al foil and aluminized mylar. Further measurements were performed on sheets of aluminium alloy, Cu, zinc brass and stainless steel. The influence of different types of sample treatment such as chemical and mechanical surface finishing and material annealing on the radiative properties is presented. 2005 Elsevier Ltd. All rights reserved. Keywords: Metals (A); Structural materials (A); Radiant properties (C); Heat transfer (C); Cryostats (F)
1. Introduction Material data on radiative properties like total hemispherical absorptivity and emissivity of thermal radiation are important for the minimisation of parasitic heat flows in cryogenic devices. Dispersion and lack of published values of these radiative properties lies probably in their sensitivity to surface treatment and the difficulty of measurement, especially at low temperatures. Radiative properties could be predicted on the basis of a suitable theory on optical properties of a material in question. For example, theoretically predicted data on the total hemispherical emissivity of pure Cu, calculated by means of the anomalous skin effect theory, were presented by Domoto et al. [1], but experimental data for comparison at low temperatures were missing. For application in cryogenics, a knowledge of optical properties of materials at low temperatures and in the far infrared spectral region is required. A common method *
Corresponding author. Tel.: +420 5 415 14 271; fax: +420 5 415 14 402. E-mail address: [email protected] (V. Musilova). 0011-2275/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.cryogenics.2004.11.010
used for a study of optical properties is the measurement of normal spectral reflectivity. Tsujimoto et al. [2] measured the normal reflectivity spectra of rolled Al alloys and evaporated Al and Au. They obtained data for samples cooled down to 16 K in a wavelength interval from 0.4 to 20 lm and fitted a theoretical model to them. The spectra extrapolated from the model towards longer wavelengths enabled them to calculate the radiative properties also for low temperature thermal radiation. In the case of metals, it is difficult to acquire precise data on infrared optical properties from reflectivity measurement due to their low absorptivity. The errors of reflectivity measurements can in this case be avoided by a calorimetric detection of the absorptivity. Spectral absorptivities of very pure Al and Al alloys were measured at 4.2 K by Benbow and Lynch [3]. On the basis of a theoretical model, the authors separated the contribution of the mechanism dominating short wavelength absorption from contribution of the mechanism responsible for absorption in the far infrared region. The wavelengths in this experimental study were limited to 6 lm and extrapolation to wavelengths of several tens microns is necessary for the evaluation of low temperature
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radiative properties. Nevertheless, their results indicate that long wavelength absorptivity of commercially pure aluminium, with about 99.5% of Al, might be close to the absorptivity of high-purity Al at low sample temperatures. The above mentioned examples illustrate the problems associated with the prediction of low temperature radiative properties of metals from optical spectra. Another obstacle arises from the fact that optical studies are typically performed under stringent conditions of surface treatment and predictions based on them do not include effects connected with real surfaces as they are used in cryoengineering. Most direct measurements of radiative properties concentrate on one quantity, total hemispherical emissivity [4–7] or absorptivity [8–11]. Measurements of emissivities do not always reach very low temperatures region [4–6]. Furthermore, the values of absorptivity measured on samples cooled to liquid helium temperature are often obtained for individual temperatures of thermal radiation, usually 77 K [9,10] and 300 K [8– 10]. In addition, the materials or their surface treatment are not always fully specified [5,6,9–11]. And last but not least comparatively high values for low temperature emissivities are published in [7,9]. Such values are in discrepancy with our experience concerning the evaporation rates of cryoliquids in long-term operating cryostats which we designed and operated. To get reliable values of radiative properties at low temperatures, we resolved to perform our own measurements. In the first place, the dependence of total hemispherical absorptivity on the temperature of the thermal radiation was measured for samples kept at a temperature near to 5 K. This measurement was carried out on samples of several cryoengineering materials prepared with various surface treatments. For some samples the total hemispherical emissivity was measured in dependence on the sample temperature and compared with the measured absorptivity. The values resulting from our study can be used for calculations of thermal radiation heat transfer between surfaces cooled to temperatures ranging from about 4 to 140 K.
ment, bolometric detection of heat irradiated by the cold sample and the record of decay of the sample temperature were used in [7,4,6], respectively. The device (Fig. 1), described in [12], enables us to measure both absorptivity and emissivity. The principle consists in the measurement of heat transfer between two parallel surfaces, a sample surface and a black surface. Heat transferred by thermal radiation from the heated surface (radiator at the temperature TR) to the cold surface (absorber at the temperature TA) flows through a thin-walled steel support (thermal resistor) into a heat sink (LHe bath). The transferred heat is evaluated from the temperature gradient measured on the thermal resistor. Total hemispherical absorptivity a(TR) is measured, when the sample is cold and irradiated with heat emitted by the heated black surface. On the contrary, the sample is heated and the black surface is cold during total hemispherical emissivity e(TR) measurements. Both absorptivity and emissivity are evaluated as a ratio between the transferred heat and heat emitted by a black body surface heated to the temperature TR and having an area equal to that of the sample. An electrical heater attached to the radiator sets the temperature TR to values ranging from 30 K to about 140 K. When the radiator is heated, the absorber temperature TA varies from 5 to 15 K, in dependence on the heat flow transferred. In the case of metallic samples, it is possible to assume that the measured absorptivity is slightly influenced by this change of sample temperature (for example, as to Al and its alloys, see [2]).
2. Experimental 2.1. Method of measurement The described methods for the measurement of radiative properties of materials at low temperatures employ various measurement techniques for the absorbed or emitted radiative heat flux. For example, measurement of cryoliquid evaporation rate [9,10] and measurement of the temperature gradient over a thermal resistor conducting the absorbed heat to a heat sink [11] were applied in absorptivity studies. In emissivity measure-
Fig. 1. Schematic diagram of sample chamber in apparatus for measurement of total hemispherical emissivity and absorptivity.
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2.2. Measurement accuracy Two silicon diode temperature sensors Lake Shore DT470 SD are used to measure the temperature difference TA TK on the thermal resistor with a sensitivity of 1 mK. The relation between the values of radiative heat power transferred to the absorber and the corresponding increments of temperature difference TA TK was obtained by a calibration process. During calibration, the radiative heating of the absorber was simulated using an electrical microWatt-heater which is incorporated into the absorber body (Fig. 1). The increments of temperature difference TA TK induced during calibration could be reproduced with 1 mK precision. Polynomial fitting was applied to the calibration data. The temperature TR of the radiator was measured by a further Lake Shore DT470 SD sensor with an absolute accuracy of 0.25 K. Relative error in the measured absorptivity or emissivity resulting from errors in the calibration and measurement of TR is about 10% at the temperature TR = 30 K and about 1–3% at temperatures TR > 50 K. In addition to the inaccuracy caused by the measurement of temperatures, the value of the measured radiative property is systematically reduced by two effects: Firstly, a part of the transferred radiative heat leaks out of the gap between the absorber and radiator, and, secondly, the absorptivity of the blackened surface does not reach 100%. The first effect was experimentally tested when absorptivity of an unfinished Al sample was measured in dependence on the radiator to the sample distance. The second effect was assessed on the basis of measurements of heat transfer between two blackened surfaces. Together, these two effects may reduce the measured absorptivity and emissivity by 5–10% of their value. Only in the case of the stainless steel surface, which has a higher emissivity and absorptivity than the other samples, the reduction is higher and may reach 10–15% of the measured value. 2.3. Samples 2.3.1. Material specification of samples prepared from metal sheets • Al 99.5: Aluminium produced according to the Czech standard CSN 42 4005.21, equivalent to EN 573-3 [AW-1050A]. Composition according to the standard: concentration of impurities is less than 0.5%; maximum contents of individual impurities are Cu 0.05%, Fe 0.4%, Si 0.3%, Ti 0.05% and Zn 0.07%. • Cu 99.5: Copper produced according to the Czech standard CSN 42 3005. Composition according to the standard: concentration of impurities is less than
531
0.5%; maximum contents of individual impurities are Fe 0.05%, Al 0.05%, Pb 0.1%, Sn 0.15%, As 0.1% and Sb 0.08%. • AlCu4Mn: Aluminium alloy produced according to the Czech Standard CSN 424201.60, equivalent to EN 573-3 [AW-2017A]. Composition according to the standard: Cu 4.3%, Mg 0.6%, Mn 0.6%, Si maximally 0.7%. • Copper–zinc brass with 63% of Cu: Ascertained by analysis. • Stainless steel: Produced according to DIN 1.4301 (equivalent to US type 304, Czech Standard CSN 17 240).
2.3.2. Foil samples • Al foils, 10 lm thick, bought as wrapping foils (assumed composition: 99.5% of Al). • Aluminized polyester foils 6 and 12 lm thick, one side aluminized (AM) and double side aluminized (DAM), provided by Austrian Aerospace.
2.3.3. Preparation of samples Samples prepared from metal sheets have a circular shape and a thickness of 1 mm. Different types of surface treatment of the sheets were applied, like chemical polishing, etching, mechanical polishing and fine turning on a lathe (Table 1). The Al sheet was chemically polished in a hot mix of acids (90–100 C), 800 ml H3PO4 + 120 ml HNO3 + 80 ml H2SO4 + 2 g Cu(NO3)2 for 30 s, cleaned with 20% HNO3, rinsed with water and dried. The Cu sheet was chemically polished in a H3PO4 + CH3COOH + HNO3 (10:8:5 in litres) bath, 1–5 min at room temperature and slightly neutralised with a weak alkali. Copper– zinc brass (63% Cu) was chemically polished with the same procedure as applied to the Cu samples. Properties of chemically polished Al and Cu were measured within 24 h after treatment and again after a 6 months exposure to air in the laboratory. Mechanically polished surfaces were lapped and then polished with gradually reduced grain until a mirror-like surface appeared. Mechanically finished surfaces were degreased with acetone. Foil samples were glued to a 1 mm thick copper disk of the same diameter. The copper support disks were covered with an absorbing layer, 0.1 mm in thickness, composed of epoxy and polyester fabric. The aluminized foils were glued to this layer by Loctite 480. The thickness of the absorbing layer, substantially higher than the glue thickness, should ensure reproducible properties of substrates in the case of transparency of aluminized foils. All samples of a specified material were made from the same piece of sheet or foil.
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Table 1 Specification of measured surfaces No.
Surfacea
Al 99.5 (equivalent to EN 573-3 [AW-1050A])-sheet 04/2 04/3 08/1 09/1 10/1 11/1 11/2 12/1 12/2
Chemically polished, 1 h exposed to air Sample 04/2 for half a year exposed to air in laboratory Unfinished Vacuum annealed at 350 C for 20 min, then chemically polished, 20 min exposed to air Abraded with stainless steel wool Finely turned on a lathe Sample 11/1 for half a year exposed to air in laboratory Mechanically polished Sample 12/1 for half a year exposed to air in laboratory
Cu 99.5 (Czech Standard CSN 42 3005)-sheet 14/1 14/2 15/1 15/2 15/3 22/1 22/2 27/1
Chemically polished Sample 14/1 for half a year exposed to air in laboratory Surface layer of 0.1 mm etched off in concentrated HNO3 Chemically polished sample 15/1 Sample 15/2 was again chemically polished and measured after half a year Finely turned on a lathe Sample 22/1 vacuum annealed at 650 C for 30 min Mechanically polished
Al foils 10 lm thick 07/1 20/1 24/1
Wrapping foil, shiny side Foil (AAE-Colli No. K61b), shiny side Wrapping foil, matt side
Copper–zinc brass sheets, 63% Cu 13/1
Chemically polished
AlCu4Mg (equivalent to EN 573-3 [AW-2017A])-sheet 16/1 Finely turned on a lathe Stainless steel (equivalent to US type 304)-sheet 25/1 26/1
Finely turned on a lathe, 14 days exposed to air in laboratory Unfinished
AAE (aluminized polyester foilsb) Nr. K73 Nr. K76 Nr. 2151
6 lm, DAMc CRYOGENIC Standard 12 lm, DAMc CRYOGENIC Standard 6 lm, AMd CRYOGENIC Standard, crinkled
a b c d
Samples were placed in vacuum within 24 h after surface treatment. Provided by Austrian Aerospace. Double aluminised mylar. Aluminised mylar.
3. Results The dependences of total hemispherical absorptivity a(TR) or emissivity e(TR) on the temperature TR of thermal radiation were obtained as a result of our measurement. The presented graphs include data for Al, Cu, their alloys and stainless steel, prepared with various surface treatments. The data of other authors are added in some cases. The ISI samples are labelled with their number corresponding to Table 1. 3.1. Al and Cu with highly reflecting surface. Comparison with published data To compare our results with previously published data, first measurements were performed on Al 99.5 and Cu 99.5 sheets with chemically polished surfaces.
In Figs. 2 and 3, the temperature dependences of the total hemispherical emissivity and absorptivity obtained at our laboratory are plotted together with the data measured by other authors. Ramanthan et al. obtained emissivity and absorptivity on annealed and electro-polished Cu and Al with purity higher than 99.9% [8,4]. Also Giullietti et al. [5,6] presented emissivity measurements on high purity metals, but without surface specification. Ruccia et al. [13] studied the emissivity of evaporated Al and Cu with 99.9% purity. Tsujimoto et al. [2] predicted radiative properties of evaporated Al 99.99 from reflectivity spectra and Anderson [11] examined absorptivity of 13 lm thick Cu foil, material used for thermal shielding. Our measurements of radiative properties on Al and Cu are compatible with data of Ramanathan et al. [4,8] with the exception of emissivity of Cu [4].
V. Musilova et al. / Cryogenics 45 (2005) 529–536 2.0
2.0 ISI 04/3 Giu 1981 Ram 1977 Tsu 1982 Ruc 1967
1.8 1.6
ISI 04/3 ISI 09/1 Ram 1952 Tsu 1982
08/1 12/1 11/1 10/1
1.8 1.6 1.4
[%]
1.2 1.0
α
α , ε [%]
1.4
0.8
unfinished mech.pol. turned abraded
Chemically polished samples: 04/2 04/3 aged on air 09/1 annealed before ch.p.
1.2 1.0 0.8 0.6
0.6
0.4
0.4
Aluminium
0.2 0.0
533
0
50
100
150
200
250
300
Al 99.5
0.2 0.0
350
0
20
40
60
Fig. 2. Total hemispherical absorptivity (solid symbols) and emissivity (open symbols) of aluminium. Plotted data: ISI—see Table 1, other authors—[2,4,6,8,13]. Sample temperature during absorptivity measurement: ISI—from 5 to 15 K, [8]—4.2 K.
2.0 ISI 15/3 Ruc 1967 Ram 1977 Giu 1979 Giu 1981
1.8 1.6
[%]
1.2
α, ε
1.4
1.0
ISI 15/2 ISI 14/1 Ram 1952 And 1971 Dom 1969
0.8 ASE
0.6 0.4 0.2 0.0
Copper 0
50
100
150
200
80
100
120
140
TR [K]
TR [K]
250
300
350
TR [K] Fig. 3. Total hemispherical absorptivity (solid symbols) and emissivity (open symbols) of copper. Plotted data: ISI—see Table 1, other authors—[4,8,5,6,11,13], ASE—calculated according to anomalous skin-effect theory [1]. Sample temperature during absorptivity measurement: ISI—from 5 to 15 K, [8]—4.2 K, [11]—about 0.2 K.
Fig. 4. Total hemispherical absorptivity of Al 99.5 sheet with various surface treatments.
to that of an unfinished surface. On the other hand, an increase of absorptivity by about 30% was caused by abrasion of an unfinished surface with steel wool. The lowest values of absorptivity were achieved by chemical polishing. Vacuum annealing of Al sheet at 350 C for 20 min before chemical polishing did not influence absorptivity. Absorptivity of chemically polished samples was not changed by a half year exposure to the laboratory air. Slightly higher absorptivity and emissivity, in comparison with the chemically polished Al sheets, were measured on the matt side of a wrapping foil made of Al (Fig. 5). The shiny side of the same foil yielded a 10–20% higher absorptivity compared to the matt side. In Fig. 6, absorptivities and emissivities of aluminium samples with different surface treatments are plotted. The values of radiative properties of the finely turned surface are close to those of mechanically polished surfaces and both emissivities and absorptivities are from
0.8
3.2. Aluminium with various surface treatments The absorptivities of several types of surfaces prepared from Al 99.5 sheet are plotted in Fig. 4. Mechanical treatment like polishing and fine turning on a lathe decreased the absorptivity by about 30% in comparison
Al sheets and foils 0.6
α [%]
At TR = 170 K, where the both compared dependences overlap, we obtained an emissivity value which is lower by about 25% in comparison with the RamanathanÕs value [4]. On the other hand, values of total hemispherical emissivity of Cu 99.5 (Fig. 3) roughly agree with those calculated according to the theory of anomalous skin-effect (ASE) [1]. For total hemispherical absorptivity of Cu at lower temperatures, a similar agreement with ASE theory was observed in [11].
0.4
Foils:
0.2
0.0
24/1 matt side 07/1 shiny side 20/1 shiny side
0
20
40
Sheets: 09/1 annealed & chem.pol. 04/2 chemically polished 04/3 chem. polished & aged
60
80
100
120
140
TR [K] Fig. 5. Total hemispherical absorptivity of 10 lm Al foils and chemically polished Al 99.5 sheet. Random errors, typical for all measurements, are marked in case of sample 07/1 ISI.
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V. Musilova et al. / Cryogenics 45 (2005) 529–536 1.0
Al sheets and foils
[%]
0.6
α, ε
0.8
0.4 Mech. polished sheet 12/1 12/2
0.2
0.0
0
20
Turned sheet 11/1 11/2
40
Chem. polished sheet 04/3 04/3
60
80
Wrapping foil 24/1 tt
100
120
140
than the absorptivity of chemically polished Al 99.5 at radiation temperatures TR above 80 K. The absorptivity of a mechanically polished surface at TR = 140 K is 2.5 times higher than the absorptivity of a chemically polished one. Analogically, a factor of 2.4 results from [8] for electro-polished and mechanically polished electrolytically pure Cu measured at TR = 300 K. Fine turning of a copper sheet on a lathe resulted in relatively high absorptivity. It was reduced by subsequent vacuum annealing (650 C, 30 min) and approached the absorptivity of a sample with a mirror-like surface, which was obtained by mechanical polishing.
TR [K] Fig. 6. Total hemispherical absorptivity (solid symbols) and emissivity (open symbols) of matt side of Al wrapping foil in comparison with mechanically and chemically treated Al 99.5 sheet.
20% to 50% higher than the corresponding values measured on the chemically polished surfaces. The relation a(TR) < e(TR) was observed for all these samples at temperatures above TR = 50–60 K. At lower temperatures, emissivity equalled absorptivity. 3.3. Copper with various surface treatments Absorptivities of the samples made of Cu 99.5 sheets, prepared with either chemical or mechanical surface treatment, are plotted in Fig. 7. Absorptivities of etched and chemically polished surfaces were nearly equal. Their values are 0.3–0.5% and they depend more slightly on the temperature TR of heat radiation than the absorptivity values of chemically polished aluminium. The exposure of chemically polished surface to the air in a laboratory for 6 months caused an increase in absorptivity by about 10–20%. The absorptivity of this aged surface still remained lower
3.4. Alloys The radiative properties of aluminium alloy AlCu4Mg prepared by fine turning on a lathe are plotted in Fig. 8. The absorptivity of the turned AlCu4Mg was by 30–80% higher in comparison to the turned Al 99.5 and it was lower than the absorptivity of Al 99.5 abraded by steel wool (Fig. 7). Copper–zinc brass, chemically polished, had an absorptivity of about 3 times higher than the chemically polished Cu. On the other hand, its absorptivity at TR > 50 K was lower than the absorptivity of turned Cu. Radiative properties of stainless steel finely turned and stainless steel with an unfinished surface are presented in Fig. 9. They do not differ by more than 15% in their values. In comparison, absorptivity of the turned surface is about 3–4 times higher than the absorptivity of turned AlCu4Mg alloy. The emissivities of both the turned stainless steel and turned AlCu4Mg were also measured. The emissivity was found to equal approximately the absorptivity for
2.0 1.8
2.0
1.4
α
[%]
1.4 1.2 1.0
1.2 1.0 0.8 0.6
0.8
16/1 AlCu4Mg, turned Tsu 1982: Al alloy, mech. pol. & annealed 13/1 copper-zink brass, chem. polished
0.4
0.6
0.2
0.4
0.0
Cu
0.2 0.0
[%]
1.6
14/1 chem. polished 14/2 chem. p. & aged 15/1 etched
α, ε
22/1 turned 22/2 turned & annealed 27/1 mech. polished
1.8
Alloys of Al and Cu
1.6
0
20
40
60
80
100
120
140
TR [K] Fig. 7. Total hemispherical absorptivity of Cu sheet with various surface treatments.
0
20
40
60
80
100
120
140
TR [K] Fig. 8. Total hemispherical absorptivity (solid symbols) and emissivity (open symbols) of aluminium alloy and copper–zinc brass (63% Cu). Composition of sample ‘‘Tsu 1982’’, i.e. sample 2024B in [2]: Al 94, Cu 4.5, Mg 1.2, Mn 0.52.
V. Musilova et al. / Cryogenics 45 (2005) 529–536
4. Conclusions
8.0 7.0
Stainless steel
[%] α, ε
6.0 5.0 4.0 3.0 26/1 unfinished 25/1 turned 25/1 turned
2.0 1.0 0.0
0
20
40
60
80
100
120
140
TR [K] Fig. 9. Total hemispherical absorptivity (solid symbols) and emissivity (open symbols) of stainless steel sheet (US type 304).
2.0
Aluminised polyester foils
1.8 1.6
[%] α, ε
1.4 1.2 1.0 Nr.2151, 6 ο m AM crin., S2 Nr.K76, 12 ο m DAM, S2
0.8 0.6 0.4
Nr.K73, 6ο m DAM χ , substrate S1 χ , substrate S2
0.2 0.0
535
0
20
40
60
80
100
120
140
TR [K] Fig. 10. Total hemispherical absorptivity (solid symbols) and emissivity (open symbols) of aluminised polyester foils. S1—foil was glued on clean Cu substrate, S2—foil was glued on 0.1 mm thick epoxy layer covering Cu substrate. Dash dot and dash lines plot total hemispherical emissivity and absorptivity of chemically polished Al 99.5 sheets.
both materials in the whole range of applied temperatures TR.
The influence of material treatment and its surface finish on total hemispherical absorptivity of some cryoengineering materials were studied. The temperature of samples was held near to 5 K and the temperature of absorbed radiation varied from 30 to 140 K. Beside the absorptivity, total hemispherical emissivity of several surfaces was measured in dependence on their temperature within the interval 30–140 K. The lowest values of absorptivity and emissivity were found for chemically polished sheets of Cu. Nearly equal values were measured on chemically polished Al sheets below a temperature of about 80 K. Above this temperature the measured absorptivity and especially emissivity of chemically polished Al are higher in comparison with those properties of chemically polished Cu. On the other hand, the polished Al surface was more stable when exposed to air in laboratory. Both Al and Cu with improper surface treatment show higher values of absorptivity than their alloys with appropriate treatment. Emissivity of all measured samples approximately equalled absorptivity for radiation temperature lower than 50 K. Above this temperature a difference between absorptivity and emissivity was observed only for surfaces of Al and Cu. In the case of alloys, the values of emissivity and absorptivity did not differ within the whole measured temperature interval. The values of radiative properties we have obtained on chemically polished surfaces of Al 99.5 and Cu 99.5 sheets are comparable to the data obtained by other authors on Al and Cu with purity 99.9% and higher.
Acknowledgment This work is supported by the Academy of Sciences of the Czech Republic, projects No. IBS 2065109 and No. KSK 2067107.
3.5. Aluminised polyester foils The absorptivities of aluminized polyester foils are plotted in Fig. 10. The measured absorptivity of foils may be influenced by the substrate in a case of a partly transparent foil. Measurement on 6 lm foil with an absorbing layer (Section 2.3) and without it was performed. The measured absorptivities of the foil on those different substrates are nearly equal. Other measurements were performed using the substrate with absorbing layer. Almost the same values of absorptivities were obtained for 12 and 6 lm thick double aluminized foils. About 20% higher absorptivities were measured for the aluminised side of the crinkled foil.
References [1] Domoto GA, Boehm RF, Tien CL. Prediction of the total emissivity of metals at cryogenic temperatures. Adv Cryog Eng 1969;14:230–9. [2] Tsujimoto S, Kanda M, Kunitomo T. Thermal radiative properties of some cryogenic materials. Cryogenics 1982;22:591–7. [3] Benbow RL, Lynch DW. Optical absorption in Al and dilute alloys of Mg and Li in Al at 4.2 K. Phys Rev B 1975;12(12): 5615–21. [4] Ramanathan KG, Yen SH, Estalote EA. Total hemispherical emissivities of copper, aluminium, and silver. Appl Opt 1977;16: 2810–77. [5] Giulietti D, Gozzini A, Lucchesi M, Stampacchia R. A calorimetric technique for measuring total emissivity of solid materials
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V. Musilova et al. / Cryogenics 45 (2005) 529–536 and coatings at low temperatures. J Phys D: Appl Phys 1979;12: 2027–36. Giulietti D, Lucchesi M. Emissivity and absorptivity measurements on some high-purity metals at low temperatures. J Phys D Appl Phys 1981;14:877–81. Hawks KH, Cottingham WB. Total normal emittances of some real surfaces at cryogenic temperatures. Adv Cryog Eng 1970;16: 467–74. Ramanathan KG. Infra-red absorption by metals at low temperatures. Proc Phys Soc London A 1952;65:532–40. Obert W. Emissivity measurements of metallic surfaces used in cryogenic applications. Adv Cryog Eng 1982;27:293–300.
[10] Amano T, Ohara A. An experimental study of thermal radiation at cryogenic temperatures. Heat Transfer-Jap Res 1991;20: 307–23. [11] Anderson AC, Folisbee JT, Johnson WL. Measurement and control of thermal radiation below 6 K. J Low Temp Phys 1971;5(5):591–605. [12] Kralik T, Hanzelka P, Musilova V, Srnka A. Device for measurement of thermal emissivity at cryogenic temperatures. In: Proceedings of the 8th Cryogenics 2004, IIR int conf, Prague, April 2004. p. 23–9. [13] Ruccia FE, Hinckley RB. The surface emittance of vacuummetalized polyester film. Adv Cryog Eng 1967;12:300–7.
Low temperature radiative properties of materials used in cryogenics V. Musilova *, P. Hanzelka, T. Kralik, A. Srnka Institute of Scientific Instruments, Kra´lovopolska´ 147, Brno CZ-612 64, Czech Republic Received 16 August 2004; received in revised form 18 August 2004; accepted 29 November 2004
Abstract Radiative heat transfer between two parallel surfaces, a sample surface and a black surface, was measured. One of the surfaces was cooled with liquid helium to about 5 K and the other one was step by step heated to temperatures ranging between 30 and 140 K. As a result, the total hemispherical absorptivity and emissivity of the sample surface were determined in dependence on the temperature of the heat radiation. Aluminium samples were made of Al sheet, Al foil and aluminized mylar. Further measurements were performed on sheets of aluminium alloy, Cu, zinc brass and stainless steel. The influence of different types of sample treatment such as chemical and mechanical surface finishing and material annealing on the radiative properties is presented. 2005 Elsevier Ltd. All rights reserved. Keywords: Metals (A); Structural materials (A); Radiant properties (C); Heat transfer (C); Cryostats (F)
1. Introduction Material data on radiative properties like total hemispherical absorptivity and emissivity of thermal radiation are important for the minimisation of parasitic heat flows in cryogenic devices. Dispersion and lack of published values of these radiative properties lies probably in their sensitivity to surface treatment and the difficulty of measurement, especially at low temperatures. Radiative properties could be predicted on the basis of a suitable theory on optical properties of a material in question. For example, theoretically predicted data on the total hemispherical emissivity of pure Cu, calculated by means of the anomalous skin effect theory, were presented by Domoto et al. [1], but experimental data for comparison at low temperatures were missing. For application in cryogenics, a knowledge of optical properties of materials at low temperatures and in the far infrared spectral region is required. A common method *
Corresponding author. Tel.: +420 5 415 14 271; fax: +420 5 415 14 402. E-mail address: [email protected] (V. Musilova). 0011-2275/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.cryogenics.2004.11.010
used for a study of optical properties is the measurement of normal spectral reflectivity. Tsujimoto et al. [2] measured the normal reflectivity spectra of rolled Al alloys and evaporated Al and Au. They obtained data for samples cooled down to 16 K in a wavelength interval from 0.4 to 20 lm and fitted a theoretical model to them. The spectra extrapolated from the model towards longer wavelengths enabled them to calculate the radiative properties also for low temperature thermal radiation. In the case of metals, it is difficult to acquire precise data on infrared optical properties from reflectivity measurement due to their low absorptivity. The errors of reflectivity measurements can in this case be avoided by a calorimetric detection of the absorptivity. Spectral absorptivities of very pure Al and Al alloys were measured at 4.2 K by Benbow and Lynch [3]. On the basis of a theoretical model, the authors separated the contribution of the mechanism dominating short wavelength absorption from contribution of the mechanism responsible for absorption in the far infrared region. The wavelengths in this experimental study were limited to 6 lm and extrapolation to wavelengths of several tens microns is necessary for the evaluation of low temperature
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radiative properties. Nevertheless, their results indicate that long wavelength absorptivity of commercially pure aluminium, with about 99.5% of Al, might be close to the absorptivity of high-purity Al at low sample temperatures. The above mentioned examples illustrate the problems associated with the prediction of low temperature radiative properties of metals from optical spectra. Another obstacle arises from the fact that optical studies are typically performed under stringent conditions of surface treatment and predictions based on them do not include effects connected with real surfaces as they are used in cryoengineering. Most direct measurements of radiative properties concentrate on one quantity, total hemispherical emissivity [4–7] or absorptivity [8–11]. Measurements of emissivities do not always reach very low temperatures region [4–6]. Furthermore, the values of absorptivity measured on samples cooled to liquid helium temperature are often obtained for individual temperatures of thermal radiation, usually 77 K [9,10] and 300 K [8– 10]. In addition, the materials or their surface treatment are not always fully specified [5,6,9–11]. And last but not least comparatively high values for low temperature emissivities are published in [7,9]. Such values are in discrepancy with our experience concerning the evaporation rates of cryoliquids in long-term operating cryostats which we designed and operated. To get reliable values of radiative properties at low temperatures, we resolved to perform our own measurements. In the first place, the dependence of total hemispherical absorptivity on the temperature of the thermal radiation was measured for samples kept at a temperature near to 5 K. This measurement was carried out on samples of several cryoengineering materials prepared with various surface treatments. For some samples the total hemispherical emissivity was measured in dependence on the sample temperature and compared with the measured absorptivity. The values resulting from our study can be used for calculations of thermal radiation heat transfer between surfaces cooled to temperatures ranging from about 4 to 140 K.
ment, bolometric detection of heat irradiated by the cold sample and the record of decay of the sample temperature were used in [7,4,6], respectively. The device (Fig. 1), described in [12], enables us to measure both absorptivity and emissivity. The principle consists in the measurement of heat transfer between two parallel surfaces, a sample surface and a black surface. Heat transferred by thermal radiation from the heated surface (radiator at the temperature TR) to the cold surface (absorber at the temperature TA) flows through a thin-walled steel support (thermal resistor) into a heat sink (LHe bath). The transferred heat is evaluated from the temperature gradient measured on the thermal resistor. Total hemispherical absorptivity a(TR) is measured, when the sample is cold and irradiated with heat emitted by the heated black surface. On the contrary, the sample is heated and the black surface is cold during total hemispherical emissivity e(TR) measurements. Both absorptivity and emissivity are evaluated as a ratio between the transferred heat and heat emitted by a black body surface heated to the temperature TR and having an area equal to that of the sample. An electrical heater attached to the radiator sets the temperature TR to values ranging from 30 K to about 140 K. When the radiator is heated, the absorber temperature TA varies from 5 to 15 K, in dependence on the heat flow transferred. In the case of metallic samples, it is possible to assume that the measured absorptivity is slightly influenced by this change of sample temperature (for example, as to Al and its alloys, see [2]).
2. Experimental 2.1. Method of measurement The described methods for the measurement of radiative properties of materials at low temperatures employ various measurement techniques for the absorbed or emitted radiative heat flux. For example, measurement of cryoliquid evaporation rate [9,10] and measurement of the temperature gradient over a thermal resistor conducting the absorbed heat to a heat sink [11] were applied in absorptivity studies. In emissivity measure-
Fig. 1. Schematic diagram of sample chamber in apparatus for measurement of total hemispherical emissivity and absorptivity.
V. Musilova et al. / Cryogenics 45 (2005) 529–536
2.2. Measurement accuracy Two silicon diode temperature sensors Lake Shore DT470 SD are used to measure the temperature difference TA TK on the thermal resistor with a sensitivity of 1 mK. The relation between the values of radiative heat power transferred to the absorber and the corresponding increments of temperature difference TA TK was obtained by a calibration process. During calibration, the radiative heating of the absorber was simulated using an electrical microWatt-heater which is incorporated into the absorber body (Fig. 1). The increments of temperature difference TA TK induced during calibration could be reproduced with 1 mK precision. Polynomial fitting was applied to the calibration data. The temperature TR of the radiator was measured by a further Lake Shore DT470 SD sensor with an absolute accuracy of 0.25 K. Relative error in the measured absorptivity or emissivity resulting from errors in the calibration and measurement of TR is about 10% at the temperature TR = 30 K and about 1–3% at temperatures TR > 50 K. In addition to the inaccuracy caused by the measurement of temperatures, the value of the measured radiative property is systematically reduced by two effects: Firstly, a part of the transferred radiative heat leaks out of the gap between the absorber and radiator, and, secondly, the absorptivity of the blackened surface does not reach 100%. The first effect was experimentally tested when absorptivity of an unfinished Al sample was measured in dependence on the radiator to the sample distance. The second effect was assessed on the basis of measurements of heat transfer between two blackened surfaces. Together, these two effects may reduce the measured absorptivity and emissivity by 5–10% of their value. Only in the case of the stainless steel surface, which has a higher emissivity and absorptivity than the other samples, the reduction is higher and may reach 10–15% of the measured value. 2.3. Samples 2.3.1. Material specification of samples prepared from metal sheets • Al 99.5: Aluminium produced according to the Czech standard CSN 42 4005.21, equivalent to EN 573-3 [AW-1050A]. Composition according to the standard: concentration of impurities is less than 0.5%; maximum contents of individual impurities are Cu 0.05%, Fe 0.4%, Si 0.3%, Ti 0.05% and Zn 0.07%. • Cu 99.5: Copper produced according to the Czech standard CSN 42 3005. Composition according to the standard: concentration of impurities is less than
531
0.5%; maximum contents of individual impurities are Fe 0.05%, Al 0.05%, Pb 0.1%, Sn 0.15%, As 0.1% and Sb 0.08%. • AlCu4Mn: Aluminium alloy produced according to the Czech Standard CSN 424201.60, equivalent to EN 573-3 [AW-2017A]. Composition according to the standard: Cu 4.3%, Mg 0.6%, Mn 0.6%, Si maximally 0.7%. • Copper–zinc brass with 63% of Cu: Ascertained by analysis. • Stainless steel: Produced according to DIN 1.4301 (equivalent to US type 304, Czech Standard CSN 17 240).
2.3.2. Foil samples • Al foils, 10 lm thick, bought as wrapping foils (assumed composition: 99.5% of Al). • Aluminized polyester foils 6 and 12 lm thick, one side aluminized (AM) and double side aluminized (DAM), provided by Austrian Aerospace.
2.3.3. Preparation of samples Samples prepared from metal sheets have a circular shape and a thickness of 1 mm. Different types of surface treatment of the sheets were applied, like chemical polishing, etching, mechanical polishing and fine turning on a lathe (Table 1). The Al sheet was chemically polished in a hot mix of acids (90–100 C), 800 ml H3PO4 + 120 ml HNO3 + 80 ml H2SO4 + 2 g Cu(NO3)2 for 30 s, cleaned with 20% HNO3, rinsed with water and dried. The Cu sheet was chemically polished in a H3PO4 + CH3COOH + HNO3 (10:8:5 in litres) bath, 1–5 min at room temperature and slightly neutralised with a weak alkali. Copper– zinc brass (63% Cu) was chemically polished with the same procedure as applied to the Cu samples. Properties of chemically polished Al and Cu were measured within 24 h after treatment and again after a 6 months exposure to air in the laboratory. Mechanically polished surfaces were lapped and then polished with gradually reduced grain until a mirror-like surface appeared. Mechanically finished surfaces were degreased with acetone. Foil samples were glued to a 1 mm thick copper disk of the same diameter. The copper support disks were covered with an absorbing layer, 0.1 mm in thickness, composed of epoxy and polyester fabric. The aluminized foils were glued to this layer by Loctite 480. The thickness of the absorbing layer, substantially higher than the glue thickness, should ensure reproducible properties of substrates in the case of transparency of aluminized foils. All samples of a specified material were made from the same piece of sheet or foil.
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Table 1 Specification of measured surfaces No.
Surfacea
Al 99.5 (equivalent to EN 573-3 [AW-1050A])-sheet 04/2 04/3 08/1 09/1 10/1 11/1 11/2 12/1 12/2
Chemically polished, 1 h exposed to air Sample 04/2 for half a year exposed to air in laboratory Unfinished Vacuum annealed at 350 C for 20 min, then chemically polished, 20 min exposed to air Abraded with stainless steel wool Finely turned on a lathe Sample 11/1 for half a year exposed to air in laboratory Mechanically polished Sample 12/1 for half a year exposed to air in laboratory
Cu 99.5 (Czech Standard CSN 42 3005)-sheet 14/1 14/2 15/1 15/2 15/3 22/1 22/2 27/1
Chemically polished Sample 14/1 for half a year exposed to air in laboratory Surface layer of 0.1 mm etched off in concentrated HNO3 Chemically polished sample 15/1 Sample 15/2 was again chemically polished and measured after half a year Finely turned on a lathe Sample 22/1 vacuum annealed at 650 C for 30 min Mechanically polished
Al foils 10 lm thick 07/1 20/1 24/1
Wrapping foil, shiny side Foil (AAE-Colli No. K61b), shiny side Wrapping foil, matt side
Copper–zinc brass sheets, 63% Cu 13/1
Chemically polished
AlCu4Mg (equivalent to EN 573-3 [AW-2017A])-sheet 16/1 Finely turned on a lathe Stainless steel (equivalent to US type 304)-sheet 25/1 26/1
Finely turned on a lathe, 14 days exposed to air in laboratory Unfinished
AAE (aluminized polyester foilsb) Nr. K73 Nr. K76 Nr. 2151
6 lm, DAMc CRYOGENIC Standard 12 lm, DAMc CRYOGENIC Standard 6 lm, AMd CRYOGENIC Standard, crinkled
a b c d
Samples were placed in vacuum within 24 h after surface treatment. Provided by Austrian Aerospace. Double aluminised mylar. Aluminised mylar.
3. Results The dependences of total hemispherical absorptivity a(TR) or emissivity e(TR) on the temperature TR of thermal radiation were obtained as a result of our measurement. The presented graphs include data for Al, Cu, their alloys and stainless steel, prepared with various surface treatments. The data of other authors are added in some cases. The ISI samples are labelled with their number corresponding to Table 1. 3.1. Al and Cu with highly reflecting surface. Comparison with published data To compare our results with previously published data, first measurements were performed on Al 99.5 and Cu 99.5 sheets with chemically polished surfaces.
In Figs. 2 and 3, the temperature dependences of the total hemispherical emissivity and absorptivity obtained at our laboratory are plotted together with the data measured by other authors. Ramanthan et al. obtained emissivity and absorptivity on annealed and electro-polished Cu and Al with purity higher than 99.9% [8,4]. Also Giullietti et al. [5,6] presented emissivity measurements on high purity metals, but without surface specification. Ruccia et al. [13] studied the emissivity of evaporated Al and Cu with 99.9% purity. Tsujimoto et al. [2] predicted radiative properties of evaporated Al 99.99 from reflectivity spectra and Anderson [11] examined absorptivity of 13 lm thick Cu foil, material used for thermal shielding. Our measurements of radiative properties on Al and Cu are compatible with data of Ramanathan et al. [4,8] with the exception of emissivity of Cu [4].
V. Musilova et al. / Cryogenics 45 (2005) 529–536 2.0
2.0 ISI 04/3 Giu 1981 Ram 1977 Tsu 1982 Ruc 1967
1.8 1.6
ISI 04/3 ISI 09/1 Ram 1952 Tsu 1982
08/1 12/1 11/1 10/1
1.8 1.6 1.4
[%]
1.2 1.0
α
α , ε [%]
1.4
0.8
unfinished mech.pol. turned abraded
Chemically polished samples: 04/2 04/3 aged on air 09/1 annealed before ch.p.
1.2 1.0 0.8 0.6
0.6
0.4
0.4
Aluminium
0.2 0.0
533
0
50
100
150
200
250
300
Al 99.5
0.2 0.0
350
0
20
40
60
Fig. 2. Total hemispherical absorptivity (solid symbols) and emissivity (open symbols) of aluminium. Plotted data: ISI—see Table 1, other authors—[2,4,6,8,13]. Sample temperature during absorptivity measurement: ISI—from 5 to 15 K, [8]—4.2 K.
2.0 ISI 15/3 Ruc 1967 Ram 1977 Giu 1979 Giu 1981
1.8 1.6
[%]
1.2
α, ε
1.4
1.0
ISI 15/2 ISI 14/1 Ram 1952 And 1971 Dom 1969
0.8 ASE
0.6 0.4 0.2 0.0
Copper 0
50
100
150
200
80
100
120
140
TR [K]
TR [K]
250
300
350
TR [K] Fig. 3. Total hemispherical absorptivity (solid symbols) and emissivity (open symbols) of copper. Plotted data: ISI—see Table 1, other authors—[4,8,5,6,11,13], ASE—calculated according to anomalous skin-effect theory [1]. Sample temperature during absorptivity measurement: ISI—from 5 to 15 K, [8]—4.2 K, [11]—about 0.2 K.
Fig. 4. Total hemispherical absorptivity of Al 99.5 sheet with various surface treatments.
to that of an unfinished surface. On the other hand, an increase of absorptivity by about 30% was caused by abrasion of an unfinished surface with steel wool. The lowest values of absorptivity were achieved by chemical polishing. Vacuum annealing of Al sheet at 350 C for 20 min before chemical polishing did not influence absorptivity. Absorptivity of chemically polished samples was not changed by a half year exposure to the laboratory air. Slightly higher absorptivity and emissivity, in comparison with the chemically polished Al sheets, were measured on the matt side of a wrapping foil made of Al (Fig. 5). The shiny side of the same foil yielded a 10–20% higher absorptivity compared to the matt side. In Fig. 6, absorptivities and emissivities of aluminium samples with different surface treatments are plotted. The values of radiative properties of the finely turned surface are close to those of mechanically polished surfaces and both emissivities and absorptivities are from
0.8
3.2. Aluminium with various surface treatments The absorptivities of several types of surfaces prepared from Al 99.5 sheet are plotted in Fig. 4. Mechanical treatment like polishing and fine turning on a lathe decreased the absorptivity by about 30% in comparison
Al sheets and foils 0.6
α [%]
At TR = 170 K, where the both compared dependences overlap, we obtained an emissivity value which is lower by about 25% in comparison with the RamanathanÕs value [4]. On the other hand, values of total hemispherical emissivity of Cu 99.5 (Fig. 3) roughly agree with those calculated according to the theory of anomalous skin-effect (ASE) [1]. For total hemispherical absorptivity of Cu at lower temperatures, a similar agreement with ASE theory was observed in [11].
0.4
Foils:
0.2
0.0
24/1 matt side 07/1 shiny side 20/1 shiny side
0
20
40
Sheets: 09/1 annealed & chem.pol. 04/2 chemically polished 04/3 chem. polished & aged
60
80
100
120
140
TR [K] Fig. 5. Total hemispherical absorptivity of 10 lm Al foils and chemically polished Al 99.5 sheet. Random errors, typical for all measurements, are marked in case of sample 07/1 ISI.
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Al sheets and foils
[%]
0.6
α, ε
0.8
0.4 Mech. polished sheet 12/1 12/2
0.2
0.0
0
20
Turned sheet 11/1 11/2
40
Chem. polished sheet 04/3 04/3
60
80
Wrapping foil 24/1 tt
100
120
140
than the absorptivity of chemically polished Al 99.5 at radiation temperatures TR above 80 K. The absorptivity of a mechanically polished surface at TR = 140 K is 2.5 times higher than the absorptivity of a chemically polished one. Analogically, a factor of 2.4 results from [8] for electro-polished and mechanically polished electrolytically pure Cu measured at TR = 300 K. Fine turning of a copper sheet on a lathe resulted in relatively high absorptivity. It was reduced by subsequent vacuum annealing (650 C, 30 min) and approached the absorptivity of a sample with a mirror-like surface, which was obtained by mechanical polishing.
TR [K] Fig. 6. Total hemispherical absorptivity (solid symbols) and emissivity (open symbols) of matt side of Al wrapping foil in comparison with mechanically and chemically treated Al 99.5 sheet.
20% to 50% higher than the corresponding values measured on the chemically polished surfaces. The relation a(TR) < e(TR) was observed for all these samples at temperatures above TR = 50–60 K. At lower temperatures, emissivity equalled absorptivity. 3.3. Copper with various surface treatments Absorptivities of the samples made of Cu 99.5 sheets, prepared with either chemical or mechanical surface treatment, are plotted in Fig. 7. Absorptivities of etched and chemically polished surfaces were nearly equal. Their values are 0.3–0.5% and they depend more slightly on the temperature TR of heat radiation than the absorptivity values of chemically polished aluminium. The exposure of chemically polished surface to the air in a laboratory for 6 months caused an increase in absorptivity by about 10–20%. The absorptivity of this aged surface still remained lower
3.4. Alloys The radiative properties of aluminium alloy AlCu4Mg prepared by fine turning on a lathe are plotted in Fig. 8. The absorptivity of the turned AlCu4Mg was by 30–80% higher in comparison to the turned Al 99.5 and it was lower than the absorptivity of Al 99.5 abraded by steel wool (Fig. 7). Copper–zinc brass, chemically polished, had an absorptivity of about 3 times higher than the chemically polished Cu. On the other hand, its absorptivity at TR > 50 K was lower than the absorptivity of turned Cu. Radiative properties of stainless steel finely turned and stainless steel with an unfinished surface are presented in Fig. 9. They do not differ by more than 15% in their values. In comparison, absorptivity of the turned surface is about 3–4 times higher than the absorptivity of turned AlCu4Mg alloy. The emissivities of both the turned stainless steel and turned AlCu4Mg were also measured. The emissivity was found to equal approximately the absorptivity for
2.0 1.8
2.0
1.4
α
[%]
1.4 1.2 1.0
1.2 1.0 0.8 0.6
0.8
16/1 AlCu4Mg, turned Tsu 1982: Al alloy, mech. pol. & annealed 13/1 copper-zink brass, chem. polished
0.4
0.6
0.2
0.4
0.0
Cu
0.2 0.0
[%]
1.6
14/1 chem. polished 14/2 chem. p. & aged 15/1 etched
α, ε
22/1 turned 22/2 turned & annealed 27/1 mech. polished
1.8
Alloys of Al and Cu
1.6
0
20
40
60
80
100
120
140
TR [K] Fig. 7. Total hemispherical absorptivity of Cu sheet with various surface treatments.
0
20
40
60
80
100
120
140
TR [K] Fig. 8. Total hemispherical absorptivity (solid symbols) and emissivity (open symbols) of aluminium alloy and copper–zinc brass (63% Cu). Composition of sample ‘‘Tsu 1982’’, i.e. sample 2024B in [2]: Al 94, Cu 4.5, Mg 1.2, Mn 0.52.
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4. Conclusions
8.0 7.0
Stainless steel
[%] α, ε
6.0 5.0 4.0 3.0 26/1 unfinished 25/1 turned 25/1 turned
2.0 1.0 0.0
0
20
40
60
80
100
120
140
TR [K] Fig. 9. Total hemispherical absorptivity (solid symbols) and emissivity (open symbols) of stainless steel sheet (US type 304).
2.0
Aluminised polyester foils
1.8 1.6
[%] α, ε
1.4 1.2 1.0 Nr.2151, 6 ο m AM crin., S2 Nr.K76, 12 ο m DAM, S2
0.8 0.6 0.4
Nr.K73, 6ο m DAM χ , substrate S1 χ , substrate S2
0.2 0.0
535
0
20
40
60
80
100
120
140
TR [K] Fig. 10. Total hemispherical absorptivity (solid symbols) and emissivity (open symbols) of aluminised polyester foils. S1—foil was glued on clean Cu substrate, S2—foil was glued on 0.1 mm thick epoxy layer covering Cu substrate. Dash dot and dash lines plot total hemispherical emissivity and absorptivity of chemically polished Al 99.5 sheets.
both materials in the whole range of applied temperatures TR.
The influence of material treatment and its surface finish on total hemispherical absorptivity of some cryoengineering materials were studied. The temperature of samples was held near to 5 K and the temperature of absorbed radiation varied from 30 to 140 K. Beside the absorptivity, total hemispherical emissivity of several surfaces was measured in dependence on their temperature within the interval 30–140 K. The lowest values of absorptivity and emissivity were found for chemically polished sheets of Cu. Nearly equal values were measured on chemically polished Al sheets below a temperature of about 80 K. Above this temperature the measured absorptivity and especially emissivity of chemically polished Al are higher in comparison with those properties of chemically polished Cu. On the other hand, the polished Al surface was more stable when exposed to air in laboratory. Both Al and Cu with improper surface treatment show higher values of absorptivity than their alloys with appropriate treatment. Emissivity of all measured samples approximately equalled absorptivity for radiation temperature lower than 50 K. Above this temperature a difference between absorptivity and emissivity was observed only for surfaces of Al and Cu. In the case of alloys, the values of emissivity and absorptivity did not differ within the whole measured temperature interval. The values of radiative properties we have obtained on chemically polished surfaces of Al 99.5 and Cu 99.5 sheets are comparable to the data obtained by other authors on Al and Cu with purity 99.9% and higher.
Acknowledgment This work is supported by the Academy of Sciences of the Czech Republic, projects No. IBS 2065109 and No. KSK 2067107.
3.5. Aluminised polyester foils The absorptivities of aluminized polyester foils are plotted in Fig. 10. The measured absorptivity of foils may be influenced by the substrate in a case of a partly transparent foil. Measurement on 6 lm foil with an absorbing layer (Section 2.3) and without it was performed. The measured absorptivities of the foil on those different substrates are nearly equal. Other measurements were performed using the substrate with absorbing layer. Almost the same values of absorptivities were obtained for 12 and 6 lm thick double aluminized foils. About 20% higher absorptivities were measured for the aluminised side of the crinkled foil.
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[10] Amano T, Ohara A. An experimental study of thermal radiation at cryogenic temperatures. Heat Transfer-Jap Res 1991;20: 307–23. [11] Anderson AC, Folisbee JT, Johnson WL. Measurement and control of thermal radiation below 6 K. J Low Temp Phys 1971;5(5):591–605. [12] Kralik T, Hanzelka P, Musilova V, Srnka A. Device for measurement of thermal emissivity at cryogenic temperatures. In: Proceedings of the 8th Cryogenics 2004, IIR int conf, Prague, April 2004. p. 23–9. [13] Ruccia FE, Hinckley RB. The surface emittance of vacuummetalized polyester film. Adv Cryog Eng 1967;12:300–7.