Use of solar furnaces—II thermophysical properties

Solar Energy Vol, 28, No. 4, pp. 273-280, 1982

0038-092X/821040273-0$$03.00/0 Pergamon Press Ltd.

Printed in Great Britain.

REVIEW PAPER

USE OF SOLAR FURNACES--II T H E R M O P H Y S I C A L PROPERTIES D. SURESHand W. W. S. CHARTERS Department of MechanicalEngineering,Universityof Melbourne, Parkville, Victoria 3052, Australia and P. K. ROHATGI Regional Research Laboratory,C.S.I.R., Bhopal 462003,India

(Received 10 July 1981; accepted 31 August 1981) 1. INTRODUCTION There is a requirement for special high temperature materials to be developed and studied for use in several new areas such as the generation of nuclear or M.H.D. power, space applications and chemical technologies. Severe materials problems arise from thermophysical and thermomechanical effects resulting from the space re-entry environment as well as from high temperature thermochemical effects encountered in modern reactors. However, data on the different thermophysical properties needed are not readily available in the literature. This is especially true in the case of thermally and electrically insulating materials. One of the reasons for this is the low efficiency of the heat sources used and several other complications arising out of their inherent drawbacks. Under these conditions it becomes imperative to make further studies on the production and use of extremely high temperatures with the aid of powerful sources. Also, it is necessary to develop research tools which are reliable for use in the required high temperature range. Solar furnaces are more advantageous than electron or induction heating which are normally used, because, any solid, irrespective of its electrical or magnetic properties, can be heated without any contamination to extremely high temperatures. In the present paper we have given a review of work done on the measurement of high temperature thermophysical properties of materials using solar furnaces. This includes the measurements of thermal expansion, thermal conductivity and diffusivity, thermal energy absorption, heat content, ablation characteristics, mechanical properties, optical characteristics and electrical properties of materials. 2. MEASUREMENTOF TIEgRMALEXPANSION

In several applications, data on thermal expansion at elevated temperatures are required for a suitable choice of structural materials and apparatus design, e.g. in the design of casting moulds and of glass-to-metal seals.

Measurements of thermal expansion can provide information on the disorder state of solid solutions, allotropic modifications, and the like. A precise knowledge of the thermal expansion of materials at elevated temperatures is necessary to analyse fatigue failures in anisotropic and two-phase systems, and to estimate the thermal-spalling resistance. The latter is especially needed in the case of refractory materials subject to severe thermal shock. However, one finds relatively little data on thermal expansion of refractory oxides at high temperatures. In addition, there is considerable disagreement on the data available when samples of different origin, and different measuring techniques have been used. The lack of agreement is also sometimes due to impurities and sintering which may cause appreciable changes in the expansion. In the case of many refractories, because of differences in porosity, many of these measurements are not in agreement[l]. Solar furnaces have been used to measure thermal expansion. Chalmin [2] was the first to work with a solar furnace to test zirconium silicate up to 1900°C for expansion. He utilised a twin microscope eomparator in conjunction with a cavity-type dilatometer, as shown in Fig. 1. This method introduced many practical difficulties. A cavity-type, push-rod dilatometer was described by Glaser[3]. The accuracy of results in such push-rod dilatometers will be affected by slipping and deformation of the specimen, and the inhomogeneous heat distribution. These systems could still be effectively modified by introducing differential dilatometers. A solar furnace, in which a specimen can be heated in an oxygen atmosphere, provides a favourable means to heat oxide crystals. When it is not easy to prepare single crystals of certain oxides which are of convenient size to be used in any dilatometer the X-ray method is used. Kamada et al.[4] reported the results of their work on the thermal expansion of CaP, up to 21500C, heated in a 10 m solar furnace equipped with a specially designed 273

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M~icrpyrometer °Ptir°°l metertelescopes Fig. 1. Cavity-typedilatometer in a solar furnace. goniometer used for the high-temperature X-ray diffractometry. The expansion of CaO with temperature was found to be nearly linear below 1700°C, whilst it showed a slight curl upwards above 1800°C, which might be due to anharmonicity of lattice vibration at high temperatures. Sakurai and Takizawa[5] studied the anisotropic thermal expansion of SnO2 in the temperature range of 15-1625°C, using a 10m solar furnace. It was found that, even in air, free vacancies were possibly generated in SnO2 at high temperatures giving rise to an excess thermal expansion. Further study on SnO2 in an oxygen atmosphere, at high temperatures and pressures, will be necessary to explore the origin of this excess thermal expansion. The most difficult problem in the estimation of thermal expansion using a solar furnace is to measure the true temperature of the target. The other difficulty is due to the deformation of the sample. The latter could be overcome by improving the methods used in preparing sampies. A very important drawback with solar furnaces is that the concentrated images are too small to heat the plane surfaces of the samples uniformly and the decrease of the concentrated radiation owing to the interception with the X-ray apparatus. Hence only with large scale solar furnaces can such experiments be favoured. 3. THERMALCONDUCTIVITY Thermal conductivity is important in that it affects other properties such as the spalling resistance of refractories [6]. Reliable data on thermal conductivity of most of the engineering materials at high temperatures are not readily available. Also, correlations of thermal conductivity with melting point, thermal expansion and the like are quite unsatisfactory.

In the past the thermal conductivity of substances has been investigated by a radial heat flux method, by heating the samples either directly with high electric currents or with electron bombardment. It is true that the application of electron heating has made it possible to work on small samples of relatively simple geometry at any desired temperature[7]. However it is difficult to use such furnaces in the case of thermally and electrically insulating materials because of the low efficiency of these furnaces. Image furnaces such as solar furnaces have an advantage over conventional furnaces because transmission of heat to the test material is by radiation only from a far away heat source. Glaser[3] underlined the principle involved in analytically estimating the high temperature thermal conductivity of poor conductors. Annaev et al.[8] devised a method for determining the thermal conductivity of disc shaped specimens in a solar furnace. In this method, it was possible to study heat conduction in solids up to high temperatures, by gradually increasing the thermal flux. So far, no experimental values appear to have been reported. The inherent difficulty with a solar furnace is that it is impossible to heat the specimen uniformly unless flux redistributors are used[9]. A method therefore has to be developed so as to be adopted in the case of nonuniform radiant flux. 4. THERMALDIFFUSIVITY The thermal diffusivity of metals is normally based on studying the propagation of thermal waves in a plane specimen heated in an induction furnace or by electron bombardment, up to about 3000°C. A shortcoming of these heating methods is again their inability to be adapted for non-metals. The advantage of using solar furnaces is that the measurements can be carried out in vacuum or in a desired gaseous atmosphere, for materials with low as well as high thermal and electrical conductivities. The first reported experiment on the estimation of heat wave transmission through ceramics, in a solar furnace, was carried out at the Arizona State College[10]. The general principle underlying this method is to propagate and study thermal waves generated by special devices. Mavashev[l 1] used a 2 m solar furnace equipped with a modulating disc and a heat-flux regulator to measure thermal diffusivity of refractory and heat-resistant materials up to the melting point or destruction of the material tested. The experimental apparatus used was as shown in Fig. 2. Mavashev and Borukhov[12] determined the thermal diffusivities of ZrO2 and AI203 in vacua down to 0.01 Pa, in air and in an argon atmosphere by the method of planar temperature waves cited above. They also determined the diffusivities of NiO in an oxygen atmosphere at a pressure of about 10~Pa, and of boron carbonitride in a nitrogen atmosphere (pressure not quoted). In another paper[13] the results obtained with samples of ZrO2 and Al20~, when heated in a nitrogen atmosphere at the focus of a solar furnace, have been reported. Their results did not depend on the frequency of modulation of the thermal flux. Borukhov et a/.[14] employed two

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sorbtance values for AMG alloy and steel-20. They also investigated the temperature dependence of absorption coefficient and emissivity of these materials.

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V Fig. 2. Experimental setup used in a solar furnace for thermal diffusivity measurements. experimental variants--axial temperature waves in the case of metallic rods and plane waves in the case of non-metals in the shape of plates. In a different paper the same authors [15] reported the determination of thermal diffusivity for different materials in their two"universal solar furnaces". Values of this parameter were obtained (up to their melting points) for Armco iron, zirconium dioxide, aluminium dioxide, boron carbonitrate, nickel oxide and titanium carbide. This method was also suggested as a means of yielding thermal conductivity and specific heat values from a determination of the absolute values of temperature amplitude and thermal flux input. In all the "pulsed methods" mentioned above difficulties arise due to inhomogeneity of the heating and the finiteness of pulses, in addition to the effects of heat exchange. It should be noted here that lasers can be used in such applications but again the inhomogeneity of the light flux in the case of lasers could be a critical quantity in limiting the accuracy of the results obtained[16]. 5. THERMALENERGYABSORPTION The radiant energy absorption coefficient of metals, at high temperatures, is another important parameter often required in the study of phase changes of metals in gaseous atmospheres. The usual method of measuring this quantity is to use a plasma jet, which transmits energy to the metal specimen kept at a constant temperature. The absorbed energy is then calculated using calorimetric measurements and the absorption coefficient is given by the ratio of the absorbed energy to the incident energy. The main disadvantage of this method is the presence of convection. But in a solar furnace convectional heat losses are not present when the sample is heated in a vacuum enclosure and this increases the accuracy of the results. Struss[17] conducted experiments with a 1.5 m solar furnace at the Sandia Laboratories, on 6061-T6 aluminium, cold rolled molybdenum and naval brass. In the calculation of absorption values, inaccuracies were introduced due to the neglect of conduction and re-radiation losses. However, this method could still be effectively modified to give accurate results. Mavashev et al.[18] designed an apparatus to estimate the solar absorption coefficient of ceramic materials and coatings heated in vacua. Using this device, they obtained ab-

6. SPECIFICHEATMEASUREMENTS

A measure of heat content is extremely useful, e.g. in selecting a suitable ablating material to be used in heat shields of nose cones of re-entry vehicles or in combustion chambers. Relatively little data is available on this important property of materials at high temperatures. An apparatus was designed by Butler and Inn[3, 19] to determine the specific heat of materials up to 2500°C by using it in conjunction with a solar furnace. The diagram of this apparatus is given in Fig. 3. No experimental values have, however, been given in these references. 7. ABLATIONCHARACTERISTICS

The evaluation of material properties under the high temperature thermochemical effects of a re-entry environment is very important in heat-shield design. Because, kinetic factors such as thermal shock and time of exposure control materials performance on exposure to high temperature aerodynamic heating, the materials used under such operating conditions must have very low thermal conductivity and diffusivity with the highest heat of vaporization and specific heat[20,21]. They should also resist thermal or mechanical shock. Under these conditions, ablating materials are used as surface layers of heat-shield materials. These materials absorb heat by their fusion, often followed by evaporation and so protect the interior[22]. The usual facilities available for testing the heat-shield materials utilise plasma arcs. Under the simulated reentry environment, the specimen is heated by convection and radiation simultaneously, but regulating the relative magnitudes of the two components so as to correspond to actual conditions is difficult[20]. Hence, these two

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heating effects are studied separately. Convective heating effects are studied using electric arcs and wind tunnels, while solar furnaces are used to study the radiant heating effects on ablating materials. The first reported work in this direction seems to be that of the General Electric Missile and Ordnance Systems Department in Philadelphia[23]. Grunffest and Shenker[24] tested a number of selected plastics such as phenolic asbestos, epoxy glass, vulcanized cellulose fiber and phenolic nylon in a 1.5 m solar furnace under normal atmospheric conditions. The same tests were also run under vacuum conditions. Their work showed the possibility of using solar furnaces for ranking the plastics with respect to their resistance to thermal attrition. However, the work was only exploratory at this stage. Sheehan et aL[25] used a 1.5 m solar furnace as a tool in ablation tests. The experimental arrangement was as shown in Fig. 4. It was shown that the surface recession rate and heat of ablation could be measured as a function of incident thermal radiation. The only difficulty encountered in this method, was the lack of accuracy in measuring the absolute values of surface temperature and the re-radiated flux. Laszlo et a/.[26] studied the surface recession rates of two charring ablators as a function of time and incident flux. At high incident Light pipe FLux control

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fluxes, the recession rate of the denser material was appreciably lower than that of the other. A very low recession rate was observed for Teflon because of its high reflectivity. This result has emphasized the importance of considering the optical properties of materials selected as ablators. A suitable device was built and tested at the Odeilio Solar Energy Laboratory for studying these ablation phenomena [27]. 8. MECHANICALPROPERTIES AT ELEVATED TEMPERATURES

Different space vehicle components such as the nose cones, and nozzle blocks, turbine blades and combustion chamber of jet engines have protective oxidation resistant coatings which are, under the actual operating conditions, subjected to a unidirectional surface heating in an oxidizing atmosphere. This results in surface phase transformations and subsequent thermal fatigue and failure of the coatings which will, in turn, adversely affect the strength of base materials which are under highly stressed conditions. Therefore, mechanical properties such as tensile strengths must be known in order to assess the reliability of various coatings at very high temperatures. In the existing literature very little information is readily available. With a solar furnace, it is possible to test materials under situations similar to actual flight conditions. Solar furnaces offer the possibility of subjecting the specimens, kept under stress, to surface heating from a single direction only in any chemical environment. However, the inherent difficulty in controlling the temperature and the need to utilise small specimens of the size of the focal area restrict the effectiveness of solar units. Leon and Shank [28] suggested the use of furnaces with larger apertures or of special shapes in such applications. An analytical study was presented and two different heating schemes were proposed, as shown in Fig. 5. A number of solar furnaces with a generic designation of "SGU" were developed and utilized in studying several mechanical properties of materials at the Institute of Materials Science of the Ukranian Academy of Sciences[29]. The SGU-1 solar furnace of 1.5 m dia was used to study the oxidation-resistant coatings in any

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Fig. 6. Specimenloading device for mechanicaltesting. desired medium whose chemical composition and density could be varied. The SGU-2M installation had a compact specimen loading device as shown in Fig. 6. With this apparatus the mechanical characteristics of several heatresistant alloys and glass-reinforced plastics were investigated in air up to 3000K[30, 31]. The refractoriness of silicide and enamel coatings on a TSM-5 type Mo alloy was also evaluated. Coatings of mixed MoS6enamel, based on ZrOz, ZrC and SiO2 were reported to resist a temperature of about 2000K for 6 rain. However, it was observed that owing to the difference in the plasticity of the enamel and the metal, the coating was destroyed before the base. The SGU-3 solar unit had a 1.5 m furnace and a VRD-20 air-breathing jet engine. Rupture failures of protective coatings under the action of the working medium and different modes of heating were studied with this installation. Kuzovkov et al. [32] used the SGU4 solar furnace, in conjunction with a loading device, to determine the limiting displacements as well as loads under which normal operating conditions were disrupted for plastic specimens. In their apparatus pure bending was effected and the application of concentrated solar rays was in a direction perpendicular to the plane of bending. In a recent paper[33], the fracture characteristics of crystalline mica under intense radiant heating has been reported. Schreyer et a/.[34] employed the 30 kW White Sands Solar Facility to select favourable candidates of plasma-sprayed coatings such as ErB~2 for further study. 9. SPECTRALEMISSIVITY At high temperatures, the emittance of the material has to be known accurately for calculating the radiant heat exchange as well as to interpret temperatures as measured with an optical pyrometer. As with any other thermophysical property, very little data is available in the existing literature, on the spectral emissivity of different materials at elevated temperatures. Many attempts to predict the temperature dependence of emissivities have so far not been successful [35-37]. This is because the emissivity of a material is an intrinsic property, its value being affected by stoichiometry, phase changes, sintering and the like. Also, total emittance can vary depending on the texture of the sample.

When conventional furnaces are used for estimating emissivity, serious problems arise as a result of stray radiations from the furnace wall and other surrounding objects. Contamination is another problem which is avoided in the image furnaces as the sample is heated only by radiation from a far away source. When such a furnace is used it is also possible to eliminate the reflected radiation from the sample by the use of a suitable shutter mechanism as suggested by Conn et al. [38]. Glaser[3, 39] was the first to suggest the use of solar furnaces in measuring the spectral emissivities of construction materials such as metals, refractories and refractory-coated metals. In a typical study, with a 1.5 m solar furnace the wavelength range employed was varying from the near infra-red to 15/zm in the temperature range of 1000-3500K. The schematic diagram of the experimental arrangement was as shown in Fig. 7. Laszlo et a/.[26, 40] had calibrated a 1.5 m solar furnace and used that calibration for investigating spectral emissivities of alumina, magnesia and zirconia. They obtained interesting results in the case of zirconia for which a sudden change in emittance was observed at its melting point. But a large amount of error had been introduced in the measurements due to inaccuracies in the estimation of sample temperature. There is room for considerable further work in this area. Laszlo's method had been used elsewhere also for emittance measurements[15], but it had been found difficult to apply this technique to large solar furnaces. Therefore, Giutronich[41], used special equipment in conjunction with the solar furnace of the U.S. Army Natick Laboratories. Data on the spectral emittance of coarse granular 90 per cent zirconia were collected by him, over a temperature range from 1950 to 2200K. An error of at least 5 per cent was reported in emittance estimation caused by the coarse texture of the samples, but the error quoted in that paper could not be attributed to texture alone.

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It is difficult to estimate the true temperature of an unenclosed specimen heated in a solar furnace because of the high temperature gradient existing across the specimen. In such cases, the emissivity measurements could be accomplished by finding the reflection coefficient of the sample at the operating wavelength of the pyrometer[42]. Shcherbina[43] described a reflection method to measure the "coefficient of blackness" and to estimate the temperature of materials with low thermal conductivity. Yanulis and Mayauskas[44] developed a special device for determining the spectral reflectance of materials heated in a solar furnace. From the reflectance measurements they calculated, using Kirchhoff's equation, the spectral emissivities for certain coarse-grained, non-transparent materials such as ZrO2 and MgO. They also indicated the possibility of determining the true temperatues of samples heated in their device. It was also possible to investigate the variation of the reflectance of materials as a function of their physicochemical properties at high temperatures. Shcherbina et aL[45] also used a similar method and reported the reflectance values for aluminium oxide at different angles of incidence. In a recent paper, Mavashev et a1.[46] gave a description of their device to measure the spectral emissivity of heat-resistant materials at temperatures above 1000°C in a solar furnace of 1.5m aperture. Results for ZrO2 and A1203 were quoted. It has now become a well established technique to measure the target temperature of melting oxides in a solar furnace by knowing the emissivities[47-53]. However, some researchers [54] still feel that the existing methods of measuring temperature of melting targets in a solar furnace need refinement. Several interesting experiments are being carried out in different laboratories to study the optical properties of solar selective coatings. As an example, the work of Kirgizbaev et al. [55] can be quoted. They determined the emissive power of the ICrl8Ni9Ti stainless steel as a function of surface treatment, between 950 and 1400K. Their work revealed that the emissive power of the surface coated with black chromium was not very different from that for the sand-blasted surface. They also studied the emissive power of reactor graphite. In a more recent work, Matthews and Mulholland[56] reported the approximate surface emissivity values for different refractory materials.

gas ionisation currents, high contact resistances, combustion products and the surrounding electromagnetic field. Image furnaces such as solar furnaces are ideal for such studies because high temperatures are achieved without any of the above restrictions. As no electrical current is required to heat the specimen, properties such as thermionic emission from ceramic-like materials and Hall coefficients can be measured relatively easily in a solar furnace. The breakdown characteristics of insulators in a field-free region could also be studied as could the conducting electrode materials for use in M.H.D. applications[57]. In spite of all these advantages, in a solar furnace, the sample is heated only on one side encouraging a steep temperature gradient across the specimen. Laszlo et a1.[58] have given a method for measuring the electrical resistivity of materials at high temperatures in a solar furnace. They designed a sample holder (Fig. 8) in which the sample could be rotated at a sufficiently high speed perpendicular to the optical axis of the furnace. Afzal et aL[59] reported their results of electrical conductivity measurements on single crystals of magnesium oxide in a 3.7 m solar furnace. Measurements were taken in a temperature range of 1300-2050°C. With repeated runs it was found that the material became purified and hence measurements were made on crystals of different impurities. Incidentally, this conforms to the idea of purifying materials in a solar furnace as has been pointed out elsewhere [60]. Using a 10m solar furnace, Sakurai et a/.[61] measured the electrical conductivity of MgO crystals, at temperatures from 1800°C up to the melting point, and that of ot-Fe203 between 800 and 1500°C, by a microwave ellipsometric method. The major advantage of this method is that the electrical properties of oxides could be analysed in an oxidising atmosphere at elevated temperatures without the use of electrodes, which is not possible with any other method. However, the accuracy attainable in this method is comparatively less than in the four-probe method. To test the utility of several materials as RF transmission windows or as shields in the microwave bands, it is important to study their dielectric properties at high temperatures. The primary advantage in using solar furnaces for studying the microwave loss characteristics is that the heating environment is "clear" and the RF

10. ELECTRICALPROPERTIESAT ELEVATED TEMPERATURES

In the literature, the data existing on the electrical properties of pure ceramics and melts are very sparse and contradictory. As for all the other thermophysical property measurements of pure materials, it is imperative here to avoid contamination and to heat either in vacuum or in any inert atmosphere. Otherwise it is probable that a change in the chemical nature of the surface of the material under study may introduce an appreciable change in the electrical properties of high temperatures. With the conventional furnaces, there could be several difficulties in measuring the electrical properties because of the possibilities for the existence of surface currents,

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Use of solar furnaces attenuation measured is not partially due to the ionised gases. Borukhov et a1.[62] studied the dielectric properties of inorganic materials by a phase method using microwaves in the centimeter and millimeter wavebands. The advantage of this phase method is that it is not affected by diffraction. Bassett[63] employed the 1000 kW Odeillo solar furnace and obtained the X-band transmission data for several electromagnetic antenna window materials--ceramic samples such as slip cast fused silica, aluminium oxide, beryllium oxide and hotpressed boron nitride, and organic samples such as Avcoat 9080, Duroid 5650 and Teflon. Walton and Royere [64] also reported the preliminary results of their experiments with the Odeillo furnace on dielectric response and thermal shock reisitance of several ceramics including the reaction-sintered Si3N4. 11. SUMMARY AND CONCLUSIONS

In this review we have summarized the several applications of solar furnaces in measuring the high temperature thermophysical properties of materials. Distinct advantages and disadvantages of solar furnaces have been highlighted. We have also tried to identify the scope for further research in certain clearly defined areas. In general, thermal expansion experiments lack accuracy and by introducing homogeneous heating methods as well as precision dilatometers, this defect could be overcome. The abnormal thermal expansion behaviour of oxide crystals at high temperatures and pressures could be studied. The most severe difficulty as for any other solar furnace experiment is that due to problems in measuring the true temperature of the target. It could, therefore, be of utmost importance to develop an independent method to solve this problem. Further work could be done in the areas of thermal conductivity, diffusivity and absorption. Although a limited number of references are available, virtually no useful data has been presented in them. In setting up such experiments one has to bear in mind that a device has to be designed to evenly heat the specimen. It is also necessary to design solar furnaces, which give larger focal areas of uniform flux density. Alternately it could also be an interesting proposition to modify the pulsed method so as to eliminate the requirement for homogeneity of the energy flux. From the theoretical work of Leon and Shank [op. cit] it became apparent that deviation from the usual paraboloidal shape furnace would be necessary to eliminate the temperature gradient across large size specimens. It is worthwhile to design a suitable loading device to obviate complications arising out of elongation of the specimen during testing. To evaluate the performance of ablative materials under aerodynamic shear, precision in measuring the reradiated flux must be improved. Any researcher trying to calculate heat transfer from solar-heated specimens would require many emissivity values, which are unavailable in the current data books. Estimation of target temperature, in many cases, also depends on emissivity determination and vice versa. Inaccuracies in one parameter will introduce a large amount of error in the other. As mentioned earlier, it

could really be an area of further research to design an apparatus which determines the exact surface temperature of a heated specimen. Variation of emissivity due to porosity, sintering and the like could be studied. Further work may be possible in improving the ellipsometric method so as to accurately measure the electrical conductivity of oxides. It is apparent that at present only a very limited number of papers are available on high temperature thermophysics dealing with the topics which have been mentioned herein. The utilisation of solar furnaces for these applications form an area in which a lot of further research could be done in the near future.

Acknowledgements--We sincerely thank Dr. J. P. Coutures, CNRS, Odeillo for his critical comments on the original manuscript. Our thanks are due to Mr. A. J. Doss of the Indian Institute of Science, Bangalore for the neat illustrations. REFERENCES

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