Patent Application: US-201214005919-A

Abstract:
the present invention relates to thermal insulation materials made of hollow oxide particles . use of hollow oxide particles having an overall thermal conductivity of less than 0 . 026 w / is for example suitable for the building sector or other areas where thermal insulation is required .

Description:
three different methods for producing nanoporous hydrophobic materials have been investigated , i . e . membrane foaming , gas release and templating . in all cases , hydrolysis and condensation of silane precursors may be used to form the solid network . this is the same reaction that is used for production of aerogels and may be depicted as : in membrane foaming , the idea is to produce foams with nanoscale bubbles , followed by condensation and hydrolysis within the bubble walls to obtain a silica nanofoam . in the process , gas is pressed through a membrane to obtain bubbles with controlled size as depicted in fig3 . hydrolysis and condensation of precursors at the bubble - liquid interface should result in formation of gas capsules . this method was previously used to obtain nitrogen - containing capsules with titania - polypyrrole composite shells . their diameter was in the range 1 - 5 μm as shown in fig3 . initial experiments indicated that preparation of silica nanofoams might be difficult to accomplish . this was supported by theoretical considerations . the gas pressure must be very accurately adjusted ; if the pressure is too low , no bubbles will be formed and if it is too high , a continuous gas stream will result . the size of the bubbles may be decreased by decreasing the pore size of the membrane and adjusting its surface properties to obtain a high contact angle with the solvent , i . e . the solvent should be repelled from its surface . furthermore , the solvent density should be rather high and its surface tension low . it should in principle be possible to design a reaction system that fulfills these requirements , so that production of nanosized bubbles is viable . for the production of a solid nanofoam , the liquid foam must be stable long enough for the reactions in eqs . 4 - 6 to proceed . furthermore , if the foam is to be of interest as an insulator , its walls must be thin . otherwise , the solid contribution to the overall thermal conductivity will be too high . wall thicknesses of about 20 nm may be achieved if surfactant bilayers are used to stabilize the walls and the applied solvent has low viscosity and is rapidly drained from the wall interior . it is possible to achieve this in water - based systems . however , the reactions in eqs . 4 - 6 are generally performed in alcohol solutions like ethanol or isopropanol . no surfactant was found that could stabilize nanofoams long enough , thus this line of work was so far abandoned . the gas release method would require simultaneous formation of nanosized gas bubbles throughout the reaction system , followed by hydrolysis and condensation ( eqs . 4 - 6 ) to form a solid at the bubble perimeter . bubble formation could be achieved by either evaporation or decomposition of a component in the system . this method is similar to the process described by grader et al . [ 13 ], where crystals of alcl 3 ( pr i 2 o ) were heated to produce foams with closed cell structures . in this case , the crystals themselves decomposed . upon further heating , the remaining solid dissolved in the generated solvent . then a polymerization reaction occurred at the temperature of solvent evaporation . the solvent bubbles were trapped within the polymerizing gel , forming stable foam with pore sizes 50 - 300 μm after completion of the reaction . the gas release process entails several challenges . to obtain nanosized bubbles with a sufficiently narrow size distribution , the temperature must be the same throughout the liquid phase . in ordinary reaction conditions , this would be difficult to achieve . further , the reaction to form the solid shell must proceed very rapidly if the shell is to be formed before the bubbles grow too large . this would require very reactive chemicals , and their application would require strict control of humidity both in the working environment and in the solvents used . in the templating process , a nanoscale structure in the form of a nanoemulsion or polymer gel is prepared , followed by hydrolysis and condensation by eqs . 4 - 6 to form a solid . this procedure is used for preparing e . g . catalysts and membrane materials . the approach is to prepare hollow silica nanospheres , followed by condensation and sintering to form macroscale particles or objects . for thermal insulation materials , small pore size is preferred , combined with a small wall thickness . assuming a pore diameter of 100 nm and a wall thickness of 15 nm , the solid volume fraction of the particle would be 54 %. with cubic close packing ( ccp ) of monosized spheres , the solid fraction of a “ nanosphere compact ” would be 41 %. in practice , the sphere packing will always be less efficient than ccp , thus lower solid fractions are envisaged — as is also desired . several methods for nanosphere production are given in the literature . the current work is based on the work reported by du et al . [ 14 ], who used the method for preparing antireflection coatings . the method is described in more detail by wan and yu [ 15 ], and their schematic depiction of the synthesis process is shown in fig4 . synthesis starts with dissolving the polyelectrolyte polyacrylic acid ( paa ) in ammonia , followed by addition of ethanol to form an emulsion . the droplet size in the emulsion increases with increasing concentration of polyelectrolyte in the solution . the next step is gradual addition of tetraethoxysilane ( teos ), which reacts with water at the droplet surface and forms a solid silica shell . when the sample is washed with water , the polyelectrolyte diffuses through the shell , and after drying , hollow silica nanospheres are obtained . an initial templating experiment was conducted . first , 0 . 27 ml of paa ( polyacrylic acid , mw ˜ 5 000 , 50 % aqueous solution , polysciences ) was dissolved in 4 . 5 ml 25 % nh 4 oh solution ( merck ). an emulsion was formed by adding 90 ml of absolute ethanol ( kemetyl ) was added during magnetic stirring at a rate of 760 rpm . teos ( tetraethoxysilane , purum 98 . 0 %), fluka ) was added in five 0 . 45 ml portions , with at least one hour between additions . the total time of addition was 22 h . the sample was diluted with water to twice the volume and centrifuged at 10 000 rpm for 10 min . the supernatant was removed and the particles were redispersed in water . this cleaning procedure was performed four times . the particle size of the nanospheres in water was measured using a malvern zetasizer nano zs . the particles were dried and investigated by scanning electron microscopy ( sem ), using backscattered electrons ( bse ) in a helios nanolab ” instrument . to further determine whether the spheres were hollow , they were subjected to focused ion beam ( fib ) cutting . the produced spheres had a reasonably narrow size distribution , with an average particle size of 190 nm measured by the malvern zetasizer . sem images showed that most particles had diameters between 90 and 400 nm , with most around 200 nm ( fig5 ), which is well in agreement with the size measured in water . in fig5 the spheres show up as circles with dark centers and light edges . this is due to the atomic contrast in bse images . sio 2 is shown lighter than void areas . if the particles were dense sio 2 spheres , they would also have light centers . thus , this image is the first indication that most of the spheres are indeed hollow . images of separate particles before and after fib etching are shown in fig6 . for convenience , relatively large spheres were chosen for the fib experiments . it is clearly seen that the ion beam removes material from the sphere surface , showing an empty interior . eventually , the sphere is slightly deformed . if the walls of the sphere are thin , the sphere collapses after etching . the rectangular hole in front of the particle in fig6 ( middle and right photos ) is a result of ion beam etching of the sample holder . the thermal insulation material according to the invention comprises hollow particles . the inner diameter size of the hollow particles will typically be in the range 10 - 1000 nm , 20 - 400 nm , 20 - 300 nm , 20 - 200 nm or 20 - 100 nm . the dense or porous shell / wall will have a typical thickness of less than 50 nm . the aim is to produce hollow particles with as small wall thicknesses as possible , but avoid collapsing of the particles . for use as thermal insulation the overall thermal conductivity of the porous nano insulation materials should be less than that of normal air , e . g ., 0 . 026 w /( mk ), preferably less than 4 mw /( mk ). the particles are filled with gas , they are preferably filled with air . in one embodiment of the invention , the shell of the hollow particles consists essentially of inorganic oxide material . in other embodiments the shell consists essentially of a metal oxide or a semi - metal oxide . the shell may be a single phase material or a composite consisting of silica , titania , alumina , zinc oxide , iron oxide , manganese oxide , etc . any type of oxide that can be prepared from soluble alkoxy compounds can be used . the hollow particles may be spherical , cubic , elliptical , or tube - like . the hollow oxide particles used for thermal insulation can be used without any further treatment . in one embodiment of the invention hollow spherical particles of silica are prepared and used as heat insulation material . in order to make the nanospheres stick together to form a macroscale material with low solid content different methods should be further investigated . in one embodiment the particles are preferably made hydrophobic , by a hydrophobic surface treatment . 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