JULY, 1939
INDUSTRIAL AND ENGINEERING CHEMISTRY
denses within them, they may deteriorate and lose their insulation value. Whatever is used as an insulant for cold storage purposes would have to be vapor-sealed by asphalt or some one of the other means known to the industry. Even so, very few if any of the sealing methods are so tight that they would not permit sufficient moisture to condense within the material over a period of years to reduce the insulation value considerably. Cork is usually preferred in such constructions, but even that is far from perfect. Practically all installations in cold storage warehouses are so made that the insulation material becomes part of the building, and to remove it is expensive. A possible solution is to install insulating materials in the form of panels, each one hermetically sealed as completely as possible, and then to set the panels against the walls, ceilings, and floors so that they can be removed and replaced with ease. If a panel becomes defective in any way, it can readily be replaced, and the insulating efficiency of the cold storage plant is maintained a t the highest point a t all times. Panels can also be made with vents on the cold side, which will tend to keep the inside dry, for moisture will evaporate into the cold room and condense on the brine coils.
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The question of the heat capacity of insulating materials has been receiving more thought within recent years. I n installations where rapid coolings are required-for example in loading and cooling a refrigerated railroad car-it is desirable to have walls of low heat capacity so that the cooling is not unnecessarily delayed. I n furnaces low-heat-capacity walls would permit rapid heating and cooling. To accomplish this, insulating materials of as low a heat capacity together with as high an insulation value as possible, should be selected. I n houses where air conditioning is to be installed, it is also desirable to reduce the heat capacity and maintain a high insulation value.
Literature Cited (1) Finck, J. L.,Bur. Standards J. Research, 5 (Nov.,1930). (2) Heilman, R. H., Trans. A m . SOC.Mech. Engrs., Fuels, Steam Power, 51, 257 (1929). (3) Rowley, F.B.,and Algren, A. B.,J. Am. SOC.Heating Ventilating Enurs., 35, 165 (1929). (4) Van Dusen, M. S.,.and Finck, J. L., Bur. Standards J . Research, 6 (March, 1931). (5) Wilkes, G. B.,and Peterson, C. M. F., Heating, Piping, Air Conditioning, 9,505-10 (1937).
Silica Aerogel
a
Effect of Variables on Its Thermal Conductivity J. F. WHITE The structure, preparation, and previous data on the conductivity of silica aerogel are briefly reviewed. The failure of aerogel to block radiant heat adequately a t high temperature and the need for an opacifying agent is discussed. An investigation showed that admixtures for increasing opacity fall into three classes: (I) red transparent admixtures, (11) opaque nonreflective substances, and (111) material exhibiting a metallic luster. Class I11 is superior, and of this class, silicon was found to be most favorable. Three fourths of a pound of fine ground silicon per cubic foot of aerogel is required for best results. Methods of addition of the silicon are discussed. The effect of the density of the aerogel on unopacified and opacified aerogel is shown. An increase i n density of the aerogel causes greater opacity. This increase in opacity with density causes a lowering of conductivity with unopacified aerogel. If opacified the decrease is not so marked, since radiation is already well blocked by the opacifier. Since the conductivity of aerogel is less than that of air, a correct gradation of particle size of the aerogel to give maximum packing results in a low conductivity. A comparison of the values for silica aerogel and still air is given. Tests of aerogel as insulation for refrigerators show that condensation of moisture is prevented presumably b y decreasing the diffusion rates in the insulating chamber.
Merrimac Division of the Monsanto Chemical Company, Boston, Mass.
T
H E term “aerogel” has been applied to a new class of materials that are prepared by drying a gel or gelatinous substance in such manner that the structure of the solid phase remains unchanged. They were discovered by Kistler and were defined in his patent (1) as a gel having an apparent specific gravity not in excess of 15 per cent of the true specific gravity; further, they are substantially free of liquids and consist essentially of the skeleton of the colloid as i t existed in the original undried gel. Although the method of preparation was described in the literature (1, S), it will be briefly reviewed since it has some bearing on the characteristics to be discussed. The scope of this discussion is limited to silica aerogel.
Preparation of Silica Aerogel When a solution of sodium silicate is added to a dilute acid solution, a silica sol is first formed which soon sets to a silica aquagel. This gel is firm, rigid, and transparent. The concentration can be varied between 3 and 15 per cent silica, and by this means the density of the final product can be regulated. If the aquagel is dried by heating a t normal pressure, a marked shrinkage takes place, and the final result is a hard glasslike product having about one fifth the volume of the
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original gel. A material so prepared is similar to the wellknown silica gel. I n the aerogel process the water present in the aquagel is replaced with alcohol by placing the gel in alcohol and allowing diffusion to take place. This alcogel is heated in an autoclave to the critical temperature of the alcohol while a pressure is maintained in excess of the critical pressure. When the critical temperature is reached, the pressure is reduced to
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Figure 2 is a sketch of the apparatus. Measurements could be completed in 2 hours after the hot plate had attained the desired temperature. Normally the heating-up period took about one hour so that a complete determination could be made in about 3 hours. It was not intended that this apparatus should give a true conductivity but rather that it should show the relative values of various mixtures. For this purpose it was very successful. The results were reproducible to within 5 per cent, a n accuracy sufficient to develop the best method of opacification and to explore other possible variables.
Selection of Opacifying Agent
300
400
I
FIGURE1. THERMAL CONDUCTIVITY OF SILICA AEROGEL
atmospheric. This technique prevents the shrinkage that was previously mentioned from taking place and results in a product of low density. If the original aquagel contained 8 per cent silica by weight, the final product will have a density of approximately 6.5 pounds per cubic foot. The material is made up of a system of extremely small pores of submicroscopic size. The pore volume, calculated from the known density of the silica and density of the aerogel, is estimated to be about 94 per cent. A material with such a high porosity obviously has possibilities as a heat insulator. Kistler investigated this application and came to the conclusion (2) that the conductivity a t about room temperature was 10 per cent lower than the data given in the International Critical Tables for still air a t that temperature. The theory was advanced that the diameter of the pores was of the same order of magnitude as the mean free path of the air molecule. This would cause a reduction of the mean free path of the enclosed air molecules so that a thermal conductivity less than that of still air would be expected. A material with a conductivity of this order is extremely interesting, and further investigation into this application has been encouraged. The aerogels had one outstanding oharacteristic that was a serious disadvantage for some purposes. When they were used a t temperatures substantially above room temperature, the conductivity increased rapidly since the mass was pervious to wave lengths in the infrared range and so offered small resistance to heat transfer byradiation. This is demonstrated by Figure 1, where the over-all conductivity is plotted against mean temperature. For contrast a similar curve is given for an insulating material that is opaque to radiant heat. Therefore, a study of admixtures to prevent this radiant transfer became necessary. At the beginning of this work an autoclave of 200-cc. capacity was the only apparatus available for carrying out the drying step a t the critical temperatures and pressures. This made it necessary to develop an apparatus for determining the conductivity on samples of this size. At the same time it was apparent that many measurements would have to be made, and so it was desirable that they be made quickly.
At the start of the investigation of means for opacifying the aerogel, 15 per cent of the opacifying material on the weight of the aerogel was used for comparison. The choice of this amount was fortunate since later work showed it to be about the optimum amount to use on the type of aerogel being investigated. A great many materials were tried in this search, and it was soon discovered that there were three distinct classes. Figure 3 shows the results obtained. Group I consists of the addition of other oxides to the aerogel in a manner to give a color and still retain the high degree of transparency characteristic of the aerogel. Only two mixtures of this type were made. I n neutralizing the sodium silicate when making the aquagel a solution of ferric sulfate was used to replace part of the acid ordinarily used. Thus, on precipitation of the gel, some Fe203.zH20 was occluded. When converted to an aerogel the product was red and highly transparent. Similarly, chromic sulfate was used in replacing part of the acid. The aerogel so prepared was a transparent green that turned to a dark transparent brown when subjected to the temperature employed in determining the conductivity. It is obvious that the occlusion of these materials has aided the transfer of radiant heat.
m
R
FIGURE2. CROSSSECTION OF APPARATUS FOR DETERMISING CONDUCTIVITY Electric hot plate Iron plate fitted over A ThermocouDles fitted i n B Iron ring E. Asbestos ring t o support G F. Sample G. Guard ring H. Calorimeter I. Dam built around K A. B. C. D.
J. K. L. M.
N. 0. P.
Water inlet to H Water outlet from H Water inlet t o G Water outlet from G Pinhole in I t o prevent air binding Asbestos between H and G Cover, space underneath filled with aerogel
In group I1 the majority of materials tried were oxides. Many trials of compounds of this class were made. The color of the oxides used ranged from dark manganese dioxide to the very white pigment-grade titanium dioxide. Several
JULY, 1939
FIWRE 3.
INDUSTRIAL AND ENGINEERING CHEMISTRY
EFFECTOY VARIOUS A D M ~ X T U ~ E S
compounds such as ultramarine, cobalt blue, and other silicates were tried. Without exception all of these materials gave values in area I1 of Figure 3. Group I11 is mainly confined to the metals or materials exhibiting a metallic luster. Many of the metals are prohihitive in cost, and others will not resist corrosion satisfactorily a t high temperatures. Aluminum of the pigment form used in bronzes was tried with encouraging results. The results with graphite were noteworthy, but i t was feared that at high temperatures oxidation would be excessive. The literature indicated that high reflectivity would be obtainable with a metallic sulfide. Pyrites is easily available and was tried. A noticeable improvement was found; but, as was t o be expected, the pyrites quickly oxidized a t the high temperature and finally a value corresponding to that of group I1 was ohtained.
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Presumably materials in group 111 are superior to group I1 because they reflect the greater part of the radiant heat rather than absorb it. When radiation falls upon a particle of the materials used in group 11, it is absorbed; the ternperature is raised, and this causes further radiation or conduction into the mass. Such is not the case with the reflective class. Hence the radiation is reflected hack to the source without causing any temperature rise in the mass. From a consideration of these two groups i t is obvious that if low conductivities a t high temperature were to he obtained, a reflective material had to be used. Although no values for the emissivity for silicon could be found in the literature, i t does have a metallic luster and is resistant to corrosion; hence it was investigated. Silicon of 98 per cent purity was ohtained and reduced to 200 mesh by ball milling. Fifteen per cent of this on the weight of the aerogel gave a mixture having the lowest conducti7it.y so far ohtained. A comparison of silicon, alnminum, and graphite is given in Figure 4. Because of its great effectiveness in blocking transfer by radiation, silicon was selected as the opacifying agent to be used in future work.
Effect of Variables A series of investigations was next undertaken to determine the effectof other variables on the conductivity of silica aerogel--optmum amount of opacifier, method of addition of the opacifier, density of the aerogel, particle size of the opacifier, and'particle size of the aerogel. OFTIMUM AMOUNT OF AEROGEL. Up to this point 15 per cent of the opacifying agent had been used for purposes of
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comparison. A series was run to determine the optimum amount (Figure 5 ) . For this series an aerogel with an apparent specific gravity of 0.08 was used. The results show that 15 per cent silicon is about the proper amount for an aerogel of this density.
0
/00
200
300
400
is preferred since a siliceous coating is formed on the opacifier and so increases its resistance to corrosion. DENSITYOF AEROGEL.The density of the aerogel affects the conductivity in various ways. The density can be increased by partially drying the aquagel or alcogel to cause some shrinkage before placing it in the autoclave for the final drying. This shrinkage results in an increase of the solid phase per unit volume of the aerogel, and so conduction by this phase is increased. Aerogels so made are somewhat opaque to visible light and possibly are also opaque to the infrared range.. The results of a series of experiments made in this manner on unopacified aerogel are given in Figure 6. At 500 O F. mean temperature it is apparent that the amount of radiant heat blocked as a result of the increased opacity is greater than the increase by conduction through the solid; the net result is a lower coefficient with an increase in density. At 250" F. mean temperature these values appear to balance each other. I n Figure 6 a similar series on an opacified aerogel is given. The decrease in conductivity with an increase in density is less since in this series the major part of the opacification is due to the silicon. If the silicon were totally reflecting the radiation, an increase in conductivity with density would be expected as a result of the greater conduction through the solid phase.
FIGURE 4. COMPARISON OF REFLECTIVE ADMIXTURES 0 Aerogel Aerogel 15% aluminum A Aerogel -t 15% graphite 0 Aerogel 15% silicon
+
+
Since the function of an opacifier is to provide a surface for the reflection of the radiant heat, i t would be expected that the opacifier should be applied on a volume basis rather than a weight basis. This has been found to be true; an aerogel of 0.036 specific gravity requires about 30 per cent silicon for best opacification. Therefore the optimum amount of silicon for opacification is set a t 0.75 pound per cubic foot of insulation.
an 0.40
FIGURE 6. EFFECTOF SPECIFICGRAVITY ON CONDUC-
0.30
TIVITY
0
A
Unopacified aerogel Opacified aerogel
0.20
FIGURE 5. EFFECTOF AMOUNTOF SILICA Aerogel 4- 7.5% silicon A Aerogel f 16 silioon
0
0 Aerogel
+ 30%
silicon
METHODOF ADDINGOPACIFIER. I n the majority of the experiments the opacifier was added to the finished aerogel and mixed by shaking or rotating several minutes. A uniform mix was quickly obtained. Another method was also used in which the opacifier was suspended in the aquasol before it set to a gel. Either method gives about the same effect on the conductivity of the mixture. The latter method
PARTICLE SIZE OF OPACIFIER, The effect of the particle size of the silicon used is not so great as might be expected. Increasing the mesh from 60 to 100 gives considerable improvement, but an increase from 100 to 400 results in no further improvement although the increase in surface is considerable. PARTICLE SIZEOF AEROGEL. The conductivity of aerogels of various particle size has been investigated. There seems to be no appreciable effect when the size is varied from through-4 on-10 mesh to through-100 on-200 mesh (2). However, noticeable differences can be obtained b y mixing small particles with larger particles. A mixture with the following screen analysis had an apparent density of 8.8 pounds per cubic foot although the density of any individual fraction had a density of 7.5 pounds per cubic foot:
JULY, 1939
INDUSTRIAL AND ENGINEERING CHEMISTRY
Mesh On-3 Through-3 on-6 Through4 on-10 Through-10 on-20
Per Cent 20.7 20.7 17.2 14.7
Mesh Through-20 on-40 Through-40 on-60 Through-60 on-SO Through-100
Per Cent 13.0 4.2 2.5 7.0
The conductivity of such a mixture is approximately 10 per cent lower than that of a through-20 on-40 mesh mixture. This is caused by replacing the air voids between the particles of the even grain mixture with small particles in the case of the mixture given in the table. Since the conductivity through aerogel particles is less than in air even in small void spaces, the result is a decrease in conductivity.
Conductivity Determinations Figure 7 gives the result of several conductivity determinations made on various opacified aerogel samples by Gordon B. Wilkes, of the Massachusetts Institute of Technology, in an apparatus capable of giving very accurate data. The data for still air from the International Critical Tables are included for comparison.
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was responsible, and a series of experiments to determine this is planned. The low conductivity of silica aerogel has suggested its use as an insulator where the available space is limited and high thermal efficiency must be had. The refrigeration field is an example. When considering aerogel for this use the question of its resistance to moisture arises, since usually a considerable amount of moisture condenses in or near insulation applied to cold surfaces. An experiment was made to determine the effect of these conditions on aerogel. Several cabinets of the type using solid carbon dioxide as the refrigerant were insulated with aerogel. After two-year service, during which the inner chamber was kept at a n average temperature of 0" F., the cabinets were opened and the aerogel was examined. There was no indication of moisture. Cabinets similarly constructed but using an insulation with much larger void spaces often contain considerable moisture after a similar operation period. The absence of moisture in the case of the aerogel-insulated cabinets is undoubtedly due to the fact that the aerogel packs so that there are no large air pockets. Convection currents are eliminated and so there is no circulation of fresh moistureladen air. During the experiment an appreciable drop in the operating costs of the aerogel-filled cabinets was noted. Because of the favorable results from this and other experiments, it is indicated that silica aerogel will prove to be of considerable value to the refrigeration field.
Conclusions
A , Still ai?: 0 opacified fterogel. Connected points indicate determinations on same sample.
It has been confirmed that the thermal conductivity of silica aerogel is less than the conductivity reported for still air. An effective method of opacifying the aerogel to radiant heat transfer has been found to be the addition of finely ground silicon. The method of addition of the opacifier, the density of the aerogel, and the particle size of the aerogel affect the conductivity. Tests carried out using aerogel as insulation in a refrigerator have indicated its value for this use.
The average conductivity for aerogel is well below that for still air. The conductivities of 0.144 a t 104" F. mean temperature and 0.148 a t 138" F. are reproducible. The data in the lowest curve were obtained on a sample prepared early in this investigation. It has not yet been possible to reproduce them. We suspect that the quality of the silicon used
(1) Kistler, S. S., U. 5. Patent 2,093,454(Sept. 21, 1937); Nature, 127, 741 (1931); J. Phys. Chem., 36, 52 (1932). (2) Kistler, S. S., and Caldwell, A. E., IND. ENQ.CHDM.,26, 658 (1934). (3) Weiser, H. B.,"Hydrous Oxides and Hydroxides", Vol. 2, p. 223, New York, McGraw-Hill Book Co., 1927.
FIGURE 7. COMPARISON OF AEROGEL AND STILL AIR
Literature Cited
GENERAL VIEW OF THE PILOT PLANTFOR PRODUCING SILICA AEROGEL
