Controllable Synthesis Of Nano Calcium Fluoride And Their Applications

Calcium fluoride（CaF₂）has been widely used in the fields of fluorescent materials,drug carriers,laser ceramics,dental restorations and anti-reflection coatings owing to its high chemical resistance,biocompatibility,photostability and wide band gap.Nano CaF₂ possesses more excellent properties,but the controllable synthesis of morphology,size and dispersity are significantly important.At present,several methods have been developed for the preparation of nano CaF₂.However,there are still challenges in the synthesis process,such as large partice,non-uniform distribution of particle size,serious agglomeration,and complicated preparation process.In this work,high gravity reactive precipitation method,combined with surface modification were put forward,realizing the controllable preparation of doped/highly dispersed nano CaF₂;meanwhile,the surface coating and spray drying technology were adopted to create various high-performance CaF₂-based nano/micro-structured particles;the upconversion fluorescence non-contact temperature measurement,dental restoration and water vapor adsorption application properties of as-prepared nano CaF₂ were further studied.The main contents and innovations of are as follows:1.CaF₂ nanopowders were prepared in a rotating packed bed（RPB）by high gravity reactive precipitation method.The effects of reactant concentration,feed flow rate and rotational speed on particle preparation were mainly investigated,and the optimal process conditions were determined as:feed rate of 300 m L/min,rotational speed 2500 rpm,and the reactant concentration(Ca²⁺)of 0.25 mol/L;the average particle size of the obtained CaF₂ nanopowder was 32 nm,which was significantly smaller than that of traditional stirred tank reactor（STR）prepared products（84 nm）.Based on the above optimal process,the hydrothermal method was further combined to preapre Er³⁺doped and Yb³⁺/Er³⁺co-doped CaF₂upconversion nanophosphors.By optimizing the doping process,the uniform dispersions of Yb³⁺and Er³⁺in the nano CaF₂ matrix were achieved;under the near-infrared excitation at 980 nm,the up-conversion emission intensities of CaF₂:Er³⁺（2%）and CaF₂:Yb³⁺（10%）/Er³⁺（1%）prepared by RPB are 12 times and 2 times higher than those of STR-prepared products.The non-contact temperature measurement based on fluorescence intensity ratio（FIR）exhibitted that the absolute sensitivity（S_A）of CaF₂:Yb³⁺（10%）/Er³⁺（1%）prepared by RPB is 0.0049 K^-1,1 times higher than that of STR-product(0.00245 K^-1).2.Transparent CaF₂ nanodispersions were successfully prepared by reactive precipitation method without surface modification.The effects of reaction conditions on particle size,morphology,dispersity,and transparency of dispersions were mainly investigated.At the optimal process conditions（methanol as solvent,reaction temperature at 25°C and concentration of calcium source at 0.2 mol/L）,the as-prepared particles have an average particle size of 15 nm,which can be dispersed in water stably to form transparent nanodispersion,with zeta potentials up to+43m V.Based on this,the surface modification/phase transfer method was further adopted to prepare monodisperse CaF₂ nanoparticles（NPs）with the particle size of 8-9 nm which can be stably dispersed in cyclohexane.The optimal amount of sodium oleate as modifier and temperature during modification were 40 wt.%and 60°C,respectively;square and spherical shape of CaF₂ NPs can be achieved by tuning the excess of calcium source and fluorine source.3.Based on aqueous CaF₂ nanodispersion,the TEOS hydrolysis method was adopted to prepare CaF₂/SiO₂ core-shell structure nanoparticles（CaF₂/SiO₂ NPs）.The effects of the solid content of aqueous dispersion,p H value and molar ratio of water to alcohol on the SiO₂ coating effect were mainly explored,and the optimal process conditions were determined.CaF₂/SiO₂ NPs with different SiO₂ coating amounts could be obtained by changing the adding amount of TEOS,with the shell thickness of 3～6 nm and average pore size of about 1.8 nm.The resin composites（RCs）for dental restoration were further prepared using CaF₂/SiO₂ NPs as fillers,and the optimal mechanical properties were reached at the filling amount of 50 wt.%;compared with uncoated CaF₂ NPs,the core-shell NPs at coating amount of 50 wt.%（CaF₂/50SiO₂）significantly improve the mechanical properties of RCs,with the flexural strength,flexural modulus,compressive strength,and hardness increased by 111.5%,41.4%,61.7%and 66.8%,respectively.The fluoride ion release（F release）study displayed that the CaF₂/50SiO₂ NPs endowed the RCs with more stable and sustained F release,the corresponding F release ratio（5.22‰）and long-term F release rate（0.96±0.15μg/（h·cm²））were much lower than those of the CaF₂ filled RCs（12.1‰and 1.8±0.3μg/（h·cm²））.In addition,the ratio of the initial F release rate to long-term F release rate for CaF₂/50SiO₂ NPs filled RCs was 1.5,much lower than that of other resins reported traditional glass-ionomer materials for dental restoration.4.To further increase the filling amount of CaF₂/SiO₂ and improve the properties of RCs,CaF₂/SiO₂ nanoclusters（NCs）were constructed applying spray drying technology.The effects of spray drying parameters and raw material on the morphology and size of CaF₂/SiO₂ NCs were basically investigated.Under the conditions of feed flow rate of 7.4m L/min,solid content of 2 wt.%,using unmodified CaF₂/SiO₂ NPs for spray drying,CaF₂/SiO₂ NCs with average particle size of 2.4μm and the regular morphology can be obtained.And compact spherical CaF2/SiO2nanoclusters.The optimal filling amount of CaF₂/SiO₂ NCs is 55 wt.%,5wt.%（absolute value）higher than that of NPs.At the same filling amount,the mechanical properties of the RCs filled with NCs were better than The NPs filled RCs,exhibitting unique nano/micro-structure advantages.The ratio of initial F release and long-term F release rate revealed that CaF₂/50SiO₂ NCs（1.7）endowed RCs with more stable F release compared with corresponding NPs（1.9）;under the optimal filling amount of CaF₂/50SiO₂ NCs,the composite resin possessed good antibacterial properties with the antibacterial rate of streptococcus mutans reaching 93%.5.Based on the preparation process of CaF₂/SiO₂ NCs,pure CaF₂ NCs were prepared by spray drying technology using uncoated CaF₂nanodispersions as raw materials,and their water vapor adsorption properties were studied.The morphology of as-prepared CaF₂ NCs is regular,with an average particle size of 1.2μm,pore size of 6 nm,and the specific surface areas are larger than those of the CaF₂ NPs;the specific surface area of the CaF₂ NCs prepared at F excess was as high as 129.9m²/g,increased by 78.7%compared with the corresponding NPs（72.7m²/g）.Under the same humidity（10%,40%and 70%）,the water vapor adsorption capacity of NCs is higher than that of corresponding NPs,and the water vapor adsorption capacity of CaF₂ NCs prepared at Ca excess is the highest,which were 11%,12%,and 19%higher than the corresponding NPs,respectively;and the water vapor adsorption capacity of the NCs obtained at F excess was greatly increased by 87%,75%,and 68%compared with the corresponding NPs;Both CaF₂ NPs and NCs obey linear driving force adsorption kinetic model.