Preparation And Properties Of Micro-nano Cerium-based Polishing Powders And Slurries

Chemical mechanical polishing（CMP）is an important technique for obtaining ultra-smooth surfaces,and is widely used in the processing of high-precision surfaces such as optical glass,liquid crystal panels,mobile phone covers,and integrated circuit chips.Cerium oxide-based micro-nano materials are the most widely used polishing or abrasive materials.In order to meet the requirements of diversified processing objects with ultra-high precision,it is necessary to develop polishing materials with higher efficiency and better surface quality.In this paper,cerium-based polishing particles were prepared by precipitation and hydrothermal method respectively.The effects of synthesis conditions on the surface properties of particles and the polishing performance for optical glass were studied.Cerium based colloid particles（CBCP）were synthesized by the precipitation reaction of Ce（NO₃）₃·6H₂O with NH₃·H₂O in the presence of H₂O₂ or air at room temperature,and the effects of the concentration and ratio of reactants on product composition and particle characteristics were studied by means of XRD,Raman spectra,SEM,TEM.It was found that synthesized CBCP are composed of Ce（OH）₃O·OH,Ce（OH）₄ and CeO₂,and the content of Ce（OH）₃O·OH is increased with the increase of H₂O₂concentration,meanwhile the size of CBCP decreases and the suspension stability became better.It was also demonstrated that when heat treating CBCP in water bath at90℃for 2 h,Ce（OH）₃O·OH in CBCP was decomposed and cerium based precipitation particles（CBPP）are formed,which are composed of Ce（OH）₄ and CeO₂ with increasing particle size and less volume of deposition layer.The filtered CBCP can be directly used to prepare polishing slurry and show good polishing performance for K9 glass.The material remove rate（MRR）values are increased with the increase of the dosage of H₂O₂ employed in the precipitation process.However,the MRR values by the slurry of filtered HT-CBCP decline greatly and can be recovered by adding H₂O₂into the slurry.However,the MRR values for HT-CBCP both with or without the addition of H₂O₂ all increase with the increase of the dosage of H₂O₂ employed in the precipitation process.The results of Raman,TG and XRD analysis proved that the formation of Ce（OH）₃O·OH and the resulting nano sized effect play a major role in polishing.Slurries containing 0.4 mol/L of H₂O₂ and p H=8 with different solid contents of HT-CBCP synthesized with molar ratio of Ce（III）:H₂O₂=1:1.5 were prepared and used to polish K9 glass.The results indicated that the MRR values increased greatly first and then decreased slightly.The highest MRR value of 307.18 nm/min with RMS 0.56 nm was achieved for slurry containing7%of HT-CBCP.Cerium oxide synthesized by calcining HT-CBCP precursor with low MRR at different temperatures are aggregated seriously.Therefore,the effects of calcination temperature and subsequent ball milling conditions on the characteristics of cerium oxide particles and polishing performance were studied.The optimal ball milling conditions were determined to be:milling time 4 h,speed 350 r/min,the ratio of material to water 1:2 and the ratio of ball to material 15:1.The highest MRR reaches261.28 nm/minis.When increasing calcination temperature to 1000℃,the MRR value increased slightly to 263.00 nm/min with RMS 0.86 nm.According to the ball grinding conditions,the HT-CBCP obtained by different molar ratios of Ce（III）:H₂O₂ was used for polishing experiments.It was found that with the decrease of Ce（III）:H₂O₂ molar ratio or the increase of H₂O₂ dosage,the MRR of K9 glass polishing increased slightly,but the MRR of soft glass ZF7 polishing increased significantly.The maximum MRR was 331.98 nm/min,and the polished surface was RMS 0.98 nm.The measurement results of XPS found that the Ce³⁺content on the product surface decreased with the increase of the amount of H₂O₂ in the synthesis process,and the polishing rate decreased,indicating that the increase of Ce³⁺content contributed to the improvement of the polishing effect.At the same time,hierarchical ceria octahedra（edge length～200 nm）formed by self-assembly of 5～8 nm spherical-like primary grains were prepared by a hydrothermal method with Ce（NO₃）₃·6H₂O as the cerium source and the assistance of surfactants at p H=3.8.Octahedral cerium oxide polishing powders with different microstructures and surface characteristics were prepared by calcination at 500～900℃,respectively.The results showed that the specific surface area,surface active oxygen and surface Ce³⁺content of the product decreased gradually with the increase of calcination temperature.When calcination at 500-700℃,the product still has a hierarchical structure,and the surface of the octahedron presents convex particles.The primary grain changed from5～8 nm spherical small particles to 8～10 nm octahedral particles.When calcination at800℃,significant sintering occurred between the primary grains,and the interface was not obvious;The surface of the octahedral aggregates became smooth.When calcination at 900℃,the hierarchical structure disappeared and single-crystal octahedral structure cerium oxide was obtained.The MRR values of products for polishing K9 glass increased firstly and then decreased with the increase of calcination temperature.The highest MRR of 187.9 nm/min was obtained for the product calcined at 700℃,the corresponding surface roughness was RMS 0.73 nm.The optimal temperature for obtaining polishing material by the calcination of precursor is 200℃lower than that of common precipitation method,which is related to the higher specific surface area and surface chemical activity of the hierarchical structure.This peak shape change relationship between polishing efficiency and calcination temperature is due to the polishing process being affected by multiple factors such as chemical,mechanical and electrical properties.The structural features of the precursor and its structural evolution and boundary surface reorganization during calcination are just at a transition point at 700℃.