二氧化硅氣凝膠的熱導率

1、Thermal Conductivity of Silica AerogelsINTRODUCTIONSilica aerogel is well known as a translucent thermal insulating material and is a material on which many studies are being carried out to investigate its potential for use as thermal insulating material in covers for solar collectors. We measured t

2、he thermal con ductivity of hydrophobic silica aerogels, prepared by Matsushita Electric Works, Ltd., using the thermal flow method un der con diti ons of con sta nt flow. Our results showed that the thermal conductivity of the silica aerogels consists of the extremely small thermal conductivity of

3、the solid, plus the conductivity of the micro pores of less than, or equal to half that of static air. Silica aerogel shows higher thermal insulating properties than static air, even at atmospheric pressure. This is due to the large porosity and the fine pore size peculiar to silica aerogel.SILICA A

4、EROGEL AS A TRANSLUCENT THERMAL INSULATING MATERIALSilica aerogel is a porous material obta ined by rem oving the solve nt from an alcogel, using a supercritical drying process and leav ing a three-dime nsional skelet on, the product of the in itial sol el reacti on .Its porosity can be con trolled

5、within a range of about 80 to 99% (Fricke, 1988; Tillotson, 1992). Due to this high porosity, it shows the highest thermal insulating capacity of all known as thermal insulating materials used at atmospheric pressure and at the same time high translucency, owing to its micro structure; this material

6、 has also bee n studied for applicati on to solar collectors, as a tran sluce nt thermal in sulati ng material (Hartmann, 1985; Wittwer, 1986; Fricke, 1989; Svendsen,1992). Figure 13-1 shows the thermal conductivity of conventional thermal insulating materials, excluding vacuum systems, and Figure 1

7、3-2 shows a photograph of silica aerogel prepared by Matsushita Electric Works, Ltd.To obtain a thermal insulating material that shows lower thermal conductivity than static air at atmospheric pressure, it is important to prepare a porous skeleton that has voids smaller than the“ mean free path ” of

8、10e aim(6Further廠by realizing a structurewhere heat tran sfer by solid con duct ion, convection and radiati on, other tha n thermal conduction by the air, can be minimized, thermal insulating materials with extremely high thermal in sulati ng performa nee can be obta in ed. On the other hand, in ord

9、er that a porous skeleton will exhibit translucency, it is important to obtain a structure where the size of the pores and the particles constituting the solid are smaller, by a factor of ten or more, than the wavelength of visible light (about 400-800 nm). Silica aerogel is able to act as a translu

10、cent thermal insulating material, with record low thermal conductivity, by satisfying all these con diti ons.MICROSCOPIC STRUCTURE AND THERMAL INSULATING PERFORMANCE入二Thermal conductivity (入 jof general thermal insulating materials (porous bodies) is described by the follow ing equati on:(13-1)Super

11、 insulationConv&nttanl instjiationparliite boartfparticle boardwooden Tiberinorganic Ober扌 Sms wo： 宮tyenE fosim .UreLhane foamSilica aerogelpowder insulation0.010 020.030 040.05Tberffral conductivity WAnKFigure 13-1. Thermal conductivity of thermal insulation materials.Figure 13-2. Photograph of 210

12、 X300 X10 mm silica aerogel plate (Matsushita Electric Works, Ltd.).where gCis thermal con duct ion through the air, sc is thermal Con duct ion through the solid,2c is heat tran sfer by conv ecti on, and r is heat transfer by radiati on.Each of these thermal conductions mechanism is shown in Figure

13、13-3. Here, the thermal conduction through the air can be regarded as a transport phenomenon with kinetic energy driven by the collision of gas molecules in the air under a temperature gradient. Therefore, the thermal conductivity of a gas depends on the“ mean free path ” of theand “ the mean free p

14、ath ”(of) ajgdosed in a narrow space can be given by equation (13-2), from the mean pore sizgLs) and the “ mean free path ” of the gas in fLee,space ( and this can be transformed as in equation (13-3) (Takahama, 1995; Takita, 1983):卜conduction 蘋A.甲$ Jby gas molecules:.卑K：Tharmal convectronttirovfl h

15、 tha tiOlid人；Thermal raclition w111=+ LfLsLgLfLg1Lg / Ls(13-2)(13-3)Figure 13-3. Factors contributing to the thermal conduction through porous materials.That is, it can be see n that, whe n making the mean pore sizel(s) smaller tha n the“ meanfree path ” of the La)s Lf will be smaller and the overal

16、l thermal conduction can be less tha n that of the air. A rough relati on ship betwee n the mea n pore siz(L s) and the thermal conduction of the air is shown inFigure 13-4. It is clear that the thermal conductivity of the solid will become smaller as the skeleton is made thinner and the solid porti

17、on becomes less.Heat tran sfer by gas conv ecti on takes place whe n the pore size becomes larger and fluid movement overtakes molecular movement. However, when the pore size is several millimeters or less, gas convection can be suppressed and completely ignored in a porous material.Heat tran sfer b

18、y radiati on, since it in creases in proporti on to the fourth power of the absolute temperature, needs to be taken into account in high-temperature applications. It is known that this heat tran sfer depe nds on the properties of the comp onent substa nces rather than the structure of the thermal in

19、 sulati ng material, and can be suppressed by ble nding heat-absorbing and heat-reflecting materials into the thermal insulating composite.Pore diameter (nm)Figure 13-4. Thermal conductivity of the static air in micro pores.As shown below, we prepared various silica aerogel samples and measured thei

20、r thermal con ductivity. We also con firmed, from observati ons of microscopic structures, that silica aerogel is an excelle nt thermal in sulat ing material which shows all the above characteristics.PREPARATION OF SAMPLES AND MEASUREMENT OF THERMAL CONDUCTIVITYPreparation of Hydrophobic Silica Aero

21、gelsAs described in another chapter, since conventional silica aerogels absorb moisture from the air, which causes white turbidity and shri nkage, and have difficulty in main tai ning temporally stable properties, these authors and colleagues prepared hydrophobic silica aerogels that showed no tempo

22、ral degradati on, by appl ying a hydrophobic process to silica micro particles, which are the formi ng un its of the silica aerogel skelet on, by treatme nt with trimethylsilyl (TMS) groups (Yokogawa, 1995). More specifically, after preparing an alcogel by sol el reacti on, using methylsilicate olig

23、omer as the raw material, and the n appl ying orga nic modificati on by hexamethyldisilaza ne and carry ing out the supercritical drying process using a CO overall process diagram of this preparati on is show n iFigure 13-5. Further, in this preparation method, by adjusting the concentration of meth

24、ylsilicate in the raw solution, we prepared hydrophobic silica aerogel samples, approximately 50 mm in diameter and 10 mm in thickness, with densities varying between 45 and 450 kg/mMeasurement of Thermal ConductivityWe measured the thermal con ductivity of the hydrophobic silica aerogel, prepared a

25、s described above. We used the thermal flow method, un der con diti ons of con sta nt flow, in accorda nee with ASTM-C518, and used the thermal con ductivity measuri ng system show n in Figure 13-6. We carried out the measureme nts at 40C and 20 C, on the high and low temperature plate respectively,

26、 so that heat tran sfer by radiati on could be igno red. Further, by installing this measuring system in a vacuum chamber, we also carried outmeasurements under conditions where the pressure was reduced to 27 Pa (0.2 mmHg), in order to measure the apparent thermal conductivity of the solid skeleton

27、of the silica aerogel un der con diti ons where the in flue nee of the gas (air) is reduced.Methyl silicateGelationreaction)Aloog&l(CH3)3&-MH-Si(CH3)3Organic modificationSupercritical dryg (COs)Hydrophobic silica aerogelFigure 13-5. Process diagram for preparing hydrophobic silica aerogel samples.Fi

28、gure 13-6. Thermal conductivity measurement system by thermal flow method.0 0300.025 7G.020 r0.01 S -0.010 70 J in atmospheric pressure . o自就皿&/too 200300400500Density of siliod aerogels (kg/m3)Figure 13-7. Thermal conductivity of silica aerogels measured by thermal flow method.Figure 13-7 shows the

29、 measurementresults for thermal conductivity. The thermal con ductivity of the silica aerogel at atmospheric pressure showed values lower tha n the thermal conductivity of static air (approximately 0.026 W/mK) in the density range between 45 and 400 kg/m . The thermal conductivity showed its minimum

30、 value at a density of approximately 110 to 150 kg/m . On the other hand, since the solid portion in creases in proporti on to the den sity of the silica aerogel, the thermal con ductivity at 27 Pa showed a steady in crease with in creas ing den sity of the silica aerogel. Here, assu ming that the d

31、ifferenee between the thermal conductivities at atmospheric pressure and at27 Pa reflects the thermal conductivity of air in the silica aerogel, we can see that the thermal con ductivity of the air sudde nly decreases at around 11 50 kg/m3. It the n con verges to about 0.08-0.05 W/mK.Pore diameter (

32、nm)Figure 13-8. Pore size distribution of silica aerogels, determined by nitrogen-adsorption isotherms.MICROSCOPIC STRUCTURE OF THE SILICA AEROGELThe results of the thermal con ductivity measureme nts clearly show that silica aerogel isa porous structure consisting of pores smaller than the“ mean fr

33、ee path ” of the air. wmeasured the pore radius more accurately, using the n itroge n-adsorpti on methocF igure13-8 shows the pore size distribution, calculated using the BJH method from then itroge n-adsorpti on measureme nt, of silica aerogels of three den sities: 190, 110 and 70kg/m . The calcula

34、ted peak pore diameters were about 20, 25 and 40 nmrespectively.These results show that silica aerogels have a pore size almost equivale nt to the mea n freepath of air in free space in the 70 kg/mversi on and less in the 190 and 110 kg/nversi ons;and the fact that the thermal conductivity of the ai

35、r of these silica aerogels is half or less of that of static air agrees with Formula (13-3).Figure 13-9 shows a SEM micrograph of the silica aerogel of den sity 110 kg/m. Fromthis, we can also confirm that the silica aerogel has a pore diameter of several nano meters.CONCLUSIONAs described above, si

36、lica aerogel shows a solid structure with low thermal con ductivity, con sisti ng of bon ded microscopic particles as the skelet on, and with uniformly distributed pores of a size equal to or smaller than the“ mean free path ” of air molecules. Owing tpeculiar microscopic structure, it dem on strate

37、s the behavior of a high-performa nee thermal insulating material with thermal conductivity below that of static air. The thermal conductivity of silica aerogel with the density adjusted around 110-50 kg/m shows the minimum value and this value shows highest thermal insulating property within all of

38、 the atmospheric thermal insulating materials.On the other hand, silica aerogel has also optical function, tran sluce nt or tran spare nt, as described in ano ther sect ion (Chapter 3). This excelle nt thermal in sulati ng material is mostideal for solar en ergy applicati on.APPLICATIONS AND FUTURE

39、CHALLENGESThe above account of thermal conductivity of silica aerogels is based on our measureme nt results at room temperature. Many applicati ons of silica aerogel as thermal insulating material have already been reported, as mentioned earlier. There have been many studies of the additi on of carb

40、 on or TiO 2 to suppress radiati on, for applicati ons at high temperatures, and on beaded packing to improve the handling of fragile materials (Wittwer, 1986; Lee, 1995; Wan g, 1995; Kwon, 2000). Figure 13-10 shows the external appearance of a double-glazed window with its internal space filled wit

41、h 110 kg/m silica aerogel beads prepared as described in this chapter. Although not completely tran spare nt, it has good tran sluce ncy.On the other hand, to avoid the prohibitively high cost of preparation using the supercritical drying process that would prevent its use as a general thermal insul

42、ating material, trials have beg un into a drying process un der atmospheric pressure (Smith, 1995) and the use of the sodium silicate as the raw material (Herrmann, 1995). From the standpoint of environmental protection, silica aerogel, with its superlative heat-insulating properties, has great pote

43、 ntial for use in gen eral in dustrial applicati ons. Cost reducti on and the improvement of its handling could be main subjects for the future of silica aerogels applicati on.Figure 13-10. Photograph of double glazed window filled with silica aerogel beads.ReferencesFricke J. Aerogel.“ Scientific A

44、merican” Japanese edition. 1988; 18(7): 80-Fricke J., Hummer E., Morper H.J., Scheuerpflug P. Thermal properties of silica aerogels. J. Physique 1989; 50: 487 -497Herrmann G., Iden R., Mielke M., Teich F., Ziegler B. On the way to commercial production of silica aerogel. J. Non-Cryst. Solids 1995; 186: 380-87Hartma nn J., Rub in M., Arasteh D. Thermal and solar-optical