Highly Efficient CaF₂ And MgF₂ Water-adsorbents

The corrosive electronic gas HF,as one of the key gases in the manufacture of micro-nano electronics,is mainly used for etching and surface cleaning of silicon wafers in the semiconductor industry.With the development of micro-nano electronics towards high integration,large wafer size,narrower line width,high uniformity and integrity,the purity of HF electronic gases is increasingly demanded.The requirement on the content of impurities in the electronic gas HF is also becoming more stringent,because the moisture impurity leads to defects in downstream products,releasing of secondary pollutants,and reducing of the purity of the electronic gas.Because MgF₂ and CaF₂ have not only a strong adsorption affinity to water molecules but also the resistance to the corrosion of HF containing water,they can be used as adsorbents for the removal of the trace moisture from gaseous HF.However,the CaF₂ materials reported in the literature have low water-adsorption capacities and few investigations on the adsorption of water vapor on MgF₂ have been carried out.Thereof,the purposes of the thesis are to develop the synthesis method to prepare MgF₂ and CaF₂ with high specific surface areas and high thermal stability and systematically investigate their water-adsorption properties.The main research results are summarized as follows:1 Using silica as a hard template,a co-precipitation method was used to prepare Mg-Si oxygen composites,and then the resultant composites were treated with hydrofluoric acid to prepare porous MgF₂.The factors,such as the type and dosage of Mg salt,silicon source,and metal dopant,on the textural properties of the resultant MgF₂ were investigated in detail.The results show that when magnesium acetate and fumed silica are used as starting chemicals,the molar ratio of silicon to magnesium is 1:6,the BET surface area of the synthesized MgF₂ is as high as 67 m²·g^-1.In addition,for MgF₂ doped with metal,the BET surface area of the synthesized sample first increases and then decreases with increasing the amount of metal dopant（Cr or Al）,and when MgF₂ is doped with a proper amount of Cr,the resultant sample has a BET specific surface area of 170 m²·g^-1.The results indicate that the doping of the metal can significantly increase the surface area of MgF₂.2 The hierarchically structured flower-like CaF₂ microspheres with nanopetals were prepared by a hydrothermal method.The effects of the type of chelating reagents,the pH in the synthesis solution,and the hydrothermal reaction time on the morphology and size of the prepared CaF₂ samples were investigated in detail. The results show that the prepared flower-like CaF₂ composed of multi-layered nanosheets can be synthesized with disodium ethylenediamine tetraacetate as a chelating reagent.The pH in the synthesis solution and the hydrothermal reaction time have a great influence on the diameter of the resultant CaF₂ microspheres.When the pH in the synthesis solution is 4.4 and the hydrothermal reaction time is 1 h,the synthesized CaF₂ microspheres have a diameter of ca.0.55μm,a BET specific surface area of 37 m²·g^-1,and the pore size distribution mainly concentrated at 7.2 nm.When the pH in the synthesis solution is adjusted to 3.8 and the hydrothermal reaction time is 6 h,the prepared CaF₂ microspheres have a diameter of ca.1.4μm,a BET specific surface area of 81 m²·g^-1,and the pore size distribution mainly concentrated at 3.3 nm.When the pH in the synthesis solution is adjusted to 3.9,the prepared CaF₂ microspheres have a diameter of ca.1.0μm,a BET specific surface area of 116 m²·g^-1,and the pore size distribution mainly concentrated at 2.8 nm.3 The adsorption of water vapor on the synthesuized MgF₂ and CaF₂ was investigated by means of a volumetric method.The results show that under the same conditions,MgF₂ doped with 10 mol%Cr（0.1 Cr-MgF₂）has the best water-adsorption properties,compared with other synthesized MgF₂ adsorbents.Among the synthesized flower-like CaF₂ samples,the one with a higher BET specific surface area,a higher thermal stability,and a diameter of 1.0μm has the best water-adsorption properties,also much better than other CaF₂ water-adsorbents reported in the literature.Finally,the application of these synthesized water-adsorbents in the removal of water vapor from gaseous HF gas to produce the electron gas HF is anticipated.