Patent Application: US-58430103-A

Abstract:
the invention relates to an improved method of manufacturing a compound having a sub - micron primary particle size such as a metal compound such as metal oxides , metaloxy hydroxides metal hydroxides , metal carbides , metal nitrides , metal carbonitrides , metal borides , electroceramics and other such compound , said method comprising the steps of : introducing a solid reactor filling material in a reactor , introducing a metal - containing precursor , a semi - metal - containing precursor , a metal - containing oxide or a semi - metal - containing oxide in said reactor , introducing a reactant or a substitution source into the said reactor , and introducing a supercritical solvent into the said reactor . these steps result in the formation of said compound in the proximity of the said solid reactor filling material .

Description:
the invention , resulting in the production of nano - sized metal oxides , metaloxy hydroxides , or metal hydroxides , preferably makes use of a sol - gel process , in which a precursor of a metal alkoxide or a metal salt is used . in the case of producing tio 2 a precursor of a metal alkoxide may be e . g . titanium tetraisopropoxide , ti ( opr i ) 4 , titaniumbutoxide , ti ( obu ) 4 , titaniumethoxide , ti ( oet ) 4 , titaniummethoxide ti ( ome ) 4 , or precursor of a metal salts may be e . g . ticl 4 , ti ( so 4 ) 2 . the sol - gel process starts with the hydrolysis of the precursor , when it comes into contact with water . the hydrolysis continues simultaneously with the condensation of the hydrolyzed monomers leading to formation of nano - sized particles . the overall process can generally be expressed as follows [ livage et al ., 1988 ]: as an example , the total hydrolysis / condensation reaction can for the case of tio 2 formation be expressed as the process must be controlled to obtain a desired structure and size of the final product . the colloid solution starts out as a sol . if the sol is stable , the solution will remain unchanged . often , however , a gelation or precipitation of particulate material takes place . regardless of whether a sol , a gel , or a precipitate is formed , the product will , in the traditional sol - gel process , be dried and often calcined to obtain the final product . a schematic illustration of the development of the particle size as a function of the reaction time can be seen in fig1 . it is seen that in the traditional sol - gel process a final particle size of 1 - 10 μm is obtained [ soloviev , 2000 ]. utilizing a supercritical solvent ( e . g . co 2 ) can arrest the process shown in fig1 . the supercritical solvent makes it possible to control and stabilize the particles such that the particle growth is arrested before the steep part of the curve ( in fig1 ) is reached , consequently resulting in nano - sized particles . by producing the particles in a supercritical fluid at specified process parameters and including a reactor material acting as seed or catalyst according to the present invention , it is furthermore possible to obtain partially or wholly crystalline products at relatively low temperatures . a supercritical fluid is used as a solvent in this process . a supercritical fluid is defined as a fluid , a mixture or an element , in a state in which the pressure is above the critical pressure ( p c ) and the temperature is above the critical temperature ( t c ). the critical parameters for selected fluids are shown in table 1 . the characteristics of a supercritical fluid are often described as a combination of the characteristics of gasses and those of liquids . as such , the supercritical fluid has the viscosity of a gas and the density of a liquid . this makes them ideal as solvents in chemical reactions . a comparison of these physical characteristics is shown in table 2 . due to the high density and the low viscosity the supercritical fluids are ideal for obtaining high reaction rates as well as stabilizing and controlling the sol - gel process . this results in the possibility of arresting the sol - gel process in fig1 and stabilizing the particles at a size in the nano - regime of roughly 1 - 100 nm . to enable the production and collection of nano - sized particles , a solid reactor filling material is introduced in the production . these filling materials can act both as seed or catalyst as well as a reservoir for collecting the nano particles . examples of different filling material are polymers , ceramics , metal fibres , and natural materials . the filling materials can be coated and thereby have different surface properties such as hydrophilic or hydrophobic surfaces . it is believed that the reactor filling material is especially helpful in facilitating the formation of crystalline phases at low temperatures . a generalized sketch of the equipment used to obtain the sub - micron product is shown in fig2 . central to this equipment is the reactor in which the product is formed under supercritical conditions . the reactor is in general constructed such that both the temperature and the pressure can be controlled . both the metal containing precursor , the reactant / initiator / this like , the substitution source , co - solvent , the solvent and the reactor filling material are introduced into the reaction chamber . the exact order of introduction and circumstances under which these are introduced may vary substantially . for example in one production route , which is considered to be an example of a pure batch route , the metal containing precursor , the co - solvent , and the reactor filling material may be introduced into the reaction chamber at room temperature and room pressure , albeit separated in some fashion so as to not start the hydrolysis . once the reaction chamber is closed , the temperature and the pressure can be raised to the supercritical level by either first raising the temperature , or raising the pressure or by some more complicated combination of the two . raising the pressure may for example be performed as a direct result of introducing the solvent , in sufficiently large quantities . in any combination of raising the temperature and the pressure , it is paramount that supercritical conditions are reached quickly . the solvent will transport the metal containing precursor and the co - solvent until they come into contact with each other , at which time hydrolysis will commence . after some time the chamber can be depressurized , cooled and opened such that the reactor - filling material and the product which is located in proximity to the reactor filling material can be removed from the reactor . in another example , which considered to be an example of a quasi - batch process , some of the components may be introduced into the reaction chamber at room temperature and room pressure . for example , the reactor filling material and the metal - containing precursor , may be introduced at room temperature and room pressure . in such a quasi - batch process , the temperature and the pressure may be raised in arbitrary order , or perhaps following any number of more complicated temperature pressure routes . as in the above batch process the rise in pressure may happen as a direct result of the introduction of the solvent or by any other means available in the prior art . to start the hydrolysis , it is necessary to introduce the co - solvent . this can be performed simultaneously with the introduction of the solvent , perhaps even mixing the solvent and co - solvent before introduction into the reaction chamber . alternatively , the introduction of the co - solvent can be performed well after the introduction of the solvent and well into the supercritical conditions . in this case the rate of hydrolysis can be controlled by the rate of co - solvent introduction into the reaction chamber . it is of course completely natural to rather consider the introduction of the reactor filling material and the co - solvent at room temperature and pressure and to consider the later introduction of the metal - containing precursor and the solvent . one may also as a further extension of the semi - batch process consider only the reactor filling material to be placed in the reactor chamber in room temperature and room pressure conditions , and for the solvent , co - solvent and metal - containing precursor to be added subsequently in preferably advantageous order and rates . after some time , the chamber can be depressurized , cooled and opened such that the reactor - filling material and the product can be removed from the reactor . finally , a continuous process is envisioned in which the reaction chamber is continuously ( or for very long times ) maintained at supercritical temperature and pressure . in such a system the introduction and extraction of reactor filling material may be continuous , or quasi - continuous as for example if a load lock system capable of introducing and removing the reactor filling material to and from the reaction chamber , while in supercritical conditions , was available . such a load lock system may function by introducing the reactor filling material into the load lock , closing the load lock , bringing the load lock area to conditions comparable to those in the reaction chamber , opening a valve between the reaction chamber and the load lock , introducing the reactor filling material into the reaction chamber , letting the reaction take place with the resulting product formed in proximity to reaction filling material , removing the reactor filling material from the reaction chamber into the load - lock , closing off the reaction chamber from the load lock , reducing pressure and temperature in the load lock , removing the reactor filling material ( and the thereby the product ) from the load - lock and subsequently taking steps to remove the product from the reactor filling materials by one or more of the means above . with two such load lock systems , production may be almost continuous by utilizing alternating load lock to introduce the reactor filling material . in the continuous process the introduction of respectively metal - containing precursor , co - solvent and solvent can take place in any number of imaginable combinations of rates and routes to ensure the desired product characteristics . in all of the above processing routes , one ends up with the product in proximity to the reactor filling material . in contrast to the prior art the process for separating the product from the fibre does not require a temperature treatment . in most cases it requires a simple mechanical or dynamic manipulation to separate the product from the filling material . examples of such manipulations can be flushing in a liquid , rubbing , shaking , vibrating , jolting , sucking e . g . use of vacuum , ultrasonically agitating etc . it is a key feature of the invention and a prerequisite for obtaining reproducible results that the chemical sol - gel process takes place in a supercritical environment . it is assumed that the reaction in the supercritical environment together with the presence of reactor fill that enables the production of , for example , the meta - stable anatase phase tio 2 at low temperatures without the need for after treatment . by changing the process parameters it is possible to vary the characteristics of the product . in the following table various process parameters and their influence on the end product is listed . it is seen in table 3 that by changing the temperature , it is possible to vary the crystalline phases . the lowest possible process temperature would be the temperature required to obtain a supercritical state , which for co 2 as the supercritical fluid is , 31 . 1 ° c . temperature has a significant influence on which phase of for example tio 2 is produced . in fig3 the crystalline phases of tio 2 is shown as a function of temperature . it is seen that the commercially important phases of tio 2 ( anatase and rutile ) normally are obtained at temperatures of respectively 350 - 500 ° c . and over 900 ° c . [ stojanovic et al ., 2000 ]. the pressure can also be varied , as long as the pressure is kept above the critical pressure that for co 2 is 73 . 8 bar . by changing the pressure and temperature it is possible to change the characteristics of the solution , in terms of density . the solvent density can have a great influence on the stability of a colloidal suspension as well as on the solubility parameters for the materials in the solution . from fig5 it is seen that co 2 has a low density at normal conditions ( 20 ° c . and 1 bar ), where co 2 is a gas . furthermore , it is seen that a significant increase in density is obtained near the critical pressure . thus it is possible to fine - tune these parameters in order to obtain an optimal production environment . in addition to changing the process parameters , the product can also be subjected to supercritical drying after the normal production process has taken place . drying is done by opening valve v 2 while still supplying the supercritical solvent fluid through value v 1 at a given flow ( f 1 ) in a given time . the additional supercritical drying is expected to have an effect on the crystallinity as well as on the specific surface area . a solid can be considered as crystalline from a theoretical point of view if a bravais lattice can describe the structure of the solid . the crystallinity of the product produced by the present method is determined by x - ray powder diffraction patterns ( xrd ). the patterns can be recorded by any number of standard commercial diffractometers , but were in the present case recorded using a cukα radiation ( λ = 1 . 540 å ) from a stoe transmission diffractometer . the x - ray diffraction patterns are measured over a range of angles , which for the present case ranged from 2θ = 10 ° to 2θ = 50 ° for tio 2 samples and from 2θ = 10 ° to 2θ = 80 ° for alooh samples . the crystallinity , as used in this document , is defined with respect to a 100 % reference sample , caf 2 , and the crystallinity is defined as being the background subtracted area of the 100 % peak of the sample with unknown crystallinity divided by the background subtracted area of the 100 % peak of the 100 % crystalline caf 2 . the crystallinity ratio is compared to table values of the ratio between the respective peaks for a 100 % crystalline sample and caf 2 . the sample with unknown crystallinity and caf 2 are mixed with a weight ratio of 50 %. it is in the following shown how the crystallinity of a tio 2 sample is determined . the ratio between the background subtracted area of the 100 % peak for anatase ( 101 ) and corundum in a 50 % weight ratio is : and the ratio between the 100 % peak of caf 2 and corundum in a 50 % weight ratio is : this gives a ratio between 100 % crystalline anatase and caf 2 in a 50 % weight ratio is : this method can be demonstrated for degussa p25 from degussa gmbh , germany , which is a commercial tio 2 powder prepared by the flame oxidation synthesis and consists of both the anatase phase as well as the rutile phase . the ratio between rutile ( 110 ) and caf 2 is 0 . 85 . the sample is mixed in a weight ratio of 50 % with caf 2 . the diffraction pattern for the determination of the crystallinity of degussa p25 is shown in fig6 . as shown on fig6 degussa consists of both the anatase as well as the rutile phase of tio 2 . by analyzing the measured spectra from degussa p25 powder and calculating the area of the peaks gives a fraction of 71 % crystalline anatase phase and 27 % crystalline rutile phase while the remaining 2 % is an amorphous fraction . this is in agreement with [ pozzo et al ., 2002 ] who have measured the degussa p25 powder to consist of 75 % anatase and 25 % rutile and [ porter et al ., 1999 ] who got 76 . 5 % anatase and 23 . 5 % rutile . [ porter et al ., 1999 ] also report about an amorphous fraction in the degussa p25 powder . the x - ray powder diffraction patterns are also used to determine the crystallite size , τ , or primary particle size of the sample from scherer &# 39 ; s formula [ jenkins et al ., 1996 ]: β τ = width of the peak at half the maximum intensity subtracted from instrumental noise the crystallite size of degussa p25 for the ( 101 ) peak is 35 nm . the size of the primary particles , which can be different than the size of the crystallites determined above , can be determined by scanning electron microscopy ( sem ) and small - angle x - ray scattering ( saxs ). the saxs data can be obtained using any number of commercial or home - built systems , but in the present case was obtained using an adaptation of a brukers axs , nanostar saxs system , with a rotating anode x - ray generator , cross - coupled goebel mirrors and a bruker axs hi - star area detector . the scattering intensity , i , was measured in terms of the scattering vector modulus q = 4n sin ( θ ))/ λ , where λ = 1 . 54 å . the scattering intensity was measured from q = 0 . 0071 å − 1 to q = 0 . 334 å − 1 . the data was corrected for background and azimuthally averaged to obtain a spectrum of average intensity vs . q . the data was then analyzed by fitting to the beaucage model [ beaucage and schaefer , 1994 ]: b : pre - factor specific to the type of power - law scattering , specified by the regime in which the exponent p , falls the beaucage model gives information of the size of the primary particle through the radius of gyration . the radius of gyration is defined as the weight average radius of the particles . in difference from xrd data saxs can determine the size of primary particles of both crystalline as well as amorphous samples . a sorptomatic 1990 from thermoquest is used to determine the specific surface area of the produced powder . the apparatus measures the adsorption isotherm of nitrogen on the sample and calculates the surface area from this isotherm . in this example the production of nano - sized crystalline tio 2 by a batch process is described . the precursor in this example is a 97 % titaniumtetraisopropoxide , ti ( opr i ) 4 , from sigma aldrich . it will in the following be referred to as ttip . the ttip reacts with distilled water in a supercritical environment including reactor filling material acting as seeds or catalyst material . the supercritical fluid is in this example co 2 . the experimental set up is shown in fig2 and the batch process is generically described in the equipment and preparation section . the process equipment consists of a reactor where the supercritical sol - gel reaction takes place . the reactor in this example comprises reactor filling material in the form of fibres . the reactor is placed in an oven where the pressure and temperature can be controlled . the pressure can be changed from 1 - 680 bars depending on the desired product and is controlled by a pump ( p 1 ). the temperature can be changed from 25 - 250 ° c . and is controlled by a temperature regulator ( t 1 ). the setup is a spe - ed sfe - 2 from applied separation inc . in the batch experiment the supercritical reactor is first filled with reactor filling material . the ttip is than injected in the top of the reactor into the reactor filling material and the water is injected in the bottom of the reactor into the reactor filling material . the amount of reactor filling material is adjusted as to prevent the reaction to take place before the co2 is added to the reactor . the reactor is than placed in the preheated oven at 96 ° c . the co 2 is added immediately having an entering temperature of 1 . 3 ° c . and a pressure of 60 bar . the pressure is raised to the starting set point , 100 bar . the temperature in the reactor is reaching the set point in 30 minutes . as a result of the increasing temperature of the reactor , from room temperature to 96 ° c ., the pressure is increasing from 100 bar to approximately 170 - 200 bar in 30 minutes . the experimental parameters and the reactants amount for a standard experiment for tio 2 is shown in table 4 . the amount of ttip in a standard experiment is 2 . 10 ml and the amount of distilled water is 1 . 00 ml that gives a hydrolysis ratio on 7 . 87 . the filling material used is hydrophilic polypropylene polymer fibres ( pp ). the standard experiment with the above process parameters has , according to the present invention , enabled the production of a pure anatase phase tio 2 . this is shown in fig4 , where an x - ray diffraction spectrum of a powder produced using the above equipment and method is shown . in the figure , the spectrum of the product is compared to diffraction lines expected from pure anatase . it is seen that except for broadening , which is due to the small size of the crystalline particle the observed lines coincide with those expected from anatase . no other tio 2 phases are present . the crystallite size , τ , of this production run has been determined to be approximately 10 nm . the following table shows the characteristic results of materials produced by the above preparation method and process parameters . table 5 shows that the result obtained by the present invention . the crystallinity of the product is 40 ± 5 % over a series of 5 experiments . the remaining part is amorphous tio 2 . the average particle size estimated by the crystallite size is 10 . 7 ± 1 . 0 nm . both particle size and crystallinity were derived from spectra like the one shown in fig8 . the saxs measurement confirms that the powder consist of primary particle of 10 - 15 nm . the sem analysis also reveals that the samples are made out of nano - sized primary particles in a range from 15 - 25 nm . these primary particles are then agglomerated into larger aggregates . the bet measurement shows that the samples have a large surface area of 236 m 2 / g . in the following example the consequence of changing the process time is described . the experiment is a standard experiment as described in example 1 but the reaction time is changed . in the following table the influence of changing the process time is shown . by changing the reaction time the primary particle size changes slightly from 2 to 4 hours but does not change from 4 to 8 hours . the increase of the reaction time does not result in an increase of the crystallinity of the samples . the crystallinity is at all reaction times approximately 40 %. in this example a standard experiment is carried out as described in example 1 but the temperature is lowered to 43 ° c . the results from this experiment is shown in table 7 it is shown in table 7 that the powder is amorphous when produced at 43 ° c . the size of the primary particles is determined by saxs and is as low as 5 . 6 nm in diameter . in this example the influence of different reactor filling material is investigated . 5 different filling materials are examined and the influence on the product properties is determined . the reactor filling material is divided into 4 categories : polymers ( in form of fibres ), ceramics ( in form of small balls ), metals ( in form of steel wool ) and natural material ( a sheet of flax ). two polypropylene ( pp ) polymers with different surface properties are investigated . ten standard experiments , like those described in example 1 , were carried out . five different reactor filling materials were used and the amount was adjusted separately . for each experiment the amount was determined so the reactants did not react before the supercritical co 2 was added . in table 8 the results from these experiments are shown . the results are average values from the 2 experiments for each material . from table 8 it can be seen that the highest crystallinity comes from using the hydrophilic pp as reactor filling material . it gives 40 % crystalline tio 2 on anatase phase . the natural material is not so applicable for producing crystalline tio 2 , only 16 % anatase phase . in between is the hydrophobic pp , the ceramic and metal fibres . these 3 reactor filling materials gives all around 25 - 33 % crystalline tio 2 and because of uncertainties it is not possible to distinguish between these 3 reactor filling materials regarding crystallinity . it can also be seen that these 3 materials plus the hydrophilic pp gives the same crystal size of 12 . 4 to 13 . 4 nm . the natural material gives a larger crystal size of 18 . 5 nm . the larger crystallite size is due to bigger uncertainties in determining the peak parameters resulting from a smaller peak . from the results in table 8 it is shown that using these 5 materials all give crystalline tio 2 at anatase phase . in this example the production of nano - sized al 2 o 3 by a batch process is described . the precursor in this example is aluminium - sec - butoxide , al ( obu s ) 3 , from sigma aldrich . the hydrophilic polypropylene fibres are used as reactor filling material . the reactor filling material , al ( obu 5 ) 3 and water is placed in the reactor before inserting it in the oven and the experiment is carried out as in example 1 . in table 9 the process parameters and reactant amount are shown . the reactant amounts give a hydrolysis ratio of 6 . 8 . the produced material is nano - sized and weak crystalline . the particle properties are shown in table 10 . in this example al203 is produced at a higher temperature and hydrolysis ratio than example 3 . a batch process makes the production and the precursor in this example is aluminium - sec - butoxide , al ( obu 5 ) 3 , from sigma aldrich . the metal fibre is used as reactor filling material . the reactor filling material , al ( obu 5 ) 3 and water is placed in the reactor before inserting it in the oven and the experiment is carried out like example 1 . in table 11 the process parameters and reactant amounts are shown . the reactant amounts give a hydrolysis ratio of 29 . 9 . the produced material is nano - sized and consists of the crystalline aluminium oxide hydroxide phase boehmite . the characteristics of the produced powder are shown in table 12 and the diffraction spectrum is shown in fig9 . the powder consists of 94 % crystalline boehmite the main remaining part is amorphous powder but the powder also consists of a small fraction of aluminium transitions oxide / hydroxide phase . the crystals are 12 . 7 nm in dimensions determined by scherrers formula . 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