Sol-Supercritical Fluid Technology: Preparation Of Ultrafine Silica Particles

Supercritical anti-solvent atomization(SAS-A) is a combination of particle formation from gas-saturated solution(PGSS) and solution enhanced dispersion by supercritical fluids(SEDS) processes.It can be applied to aqueous solutions to remove water to achieve fine solid powders at mild operation conditions(low temperature and low pressure);it has shown good applications in preparing various fine materials,such as lipids and polymers including proteins.This work extends the SAS-A process to prepare inorganic materials-nano-and micro-silica powders.Firstly,tetraethoxysilicane(TEOS) used as the precursor was studied with acetic acid as the catalyst to produce acid silica sol,and the sol was further treated by the SAS-A process to obtain silica powder.The effect of the sol flowrate,aging time,concentration of the sol,pre-expansion temperature,pre-expansion pressure,nozzle size,and different supercritical fluids(CO₂ and N₂) on the particle morphology,particle size and particle size distribution(PSD) was investigated.FT-IR,XRD and SEM were implemented to characterize the obtained particles,and the specific surface area of the particles was measured by BET method.Results show that the particles when using supercritical N₂ are amorphous,spherical,and uniformly distributed under all the investigated conditions with an average size of 0.2-0.4μm;particle size decreases and PSD narrows with the decrease of the sol concentration;particle size increases and PSD broadens with the increase of the sol flowrate;particle size firstly decreases and then increases and PSD narrows with the increase of the pre-expansion temperature.Results also show that the nozzle size has obvious effect on the particle morphology;the increase of the aging time or the pre-expansion pressure decreases the particle size and narrows PSD.The measured specific surface area suggests that the particles are nearly solid spherical particles.When using supercritical CO₂,particles produced at 8.0MPa are well dispersed and uniform with an average size of 0.5μm,but following the increase of the pressure,particles tend to conglomerate.Secondly,ammonia used as the catalyst(TEOS as the precursor) was studied to produce alkaline silica sol.The sol was further processed by the SAS-A technology. Similarly,the effect of the concentration of the sol,flowrate of the sol,pre-expansion pressure and different supercritical fluids on the particle morphology,particle size and PSD was investigated.In addition,conventional centrifugal separation method was employed to the sol to obtain silica particles.FT-IR,XRD and SEM were used to characterize the particles,and the particles' specific surface area was measured.Results show that amorphous,spherical,and uniformly distributed silica particles were produced under all the investigated conditions with an average particle size of 50-100nm and a specific surface area of about 62m²/g.The centrifugal separation method provided slightly larger silica particles with a specific surface area of about 27m²/g.The centrifugal separation method needs relatively complicated procedures and long time to obtain final silica powders, however,SAS-A process indicates its advantages in treatment of sol to obtain fine powders: continuous operation with high energy efficiency and high product yield.Finally,water glass used as the precursor was studied to produce silica sol.Before the preparation of the sol,the water glass was treated by a cation exchange resin column for obtaining acid sols with pH ranged from 2.52 to 3.20.The acid sol was then processed by the SAS-A process with different supercritical fluids(CO₂ and N₂) at different operating pressures.In addition,three alkaline sols with pH values of 9.04,9.85 and 10.04 obtained by adding ammonia were also studied by the SAS-A process.FT-IR,XRD and SEM were used to characterize the obtained particles,and the specific surface area of the particles was measured.Results show that spherical silica particles were obtained under all the conditions and for all the sols with an average size of 0.3-2μm.Among them,when using N₂,nano-silica particles were obtained at pH = 9.85;the measured specific surface area suggests that the obtained particles are nearly solid particles.