Study On Mechanism And Modification Of Cordierite Synthesized From Different Kaolin Raw Materials

Cordierite?2MgO·2Al₂O₃·5SiO₂?is a kind of important ceramic material with low thermal expansion coefficient and low dielectric constant.With the development of society and the progress of science and technology,cordierite ceramic materials are constantly being excavated for new functions and new applications,which are used as refractories,structural ceramics,electronic packaging materials,catalyst carriers,foam ceramics,printed circuit boards and low temperature thermal radiation materials.Therefore,cordierite ceramics are widely used and have huge demand.However,few natural cordierite resources can be widely used.The synthetic and preparation of cordierite has been a hot scientific issue.Cordierite has poor stability at high temperature,so the preparation process of cordierite ceramics needs should be carefully controlled.Kaolin-talc-alumina system has become the mainstream of preparation of cordierite ceramics because of its wide source of raw materials,low synthesis temperature and high cordierite quality.Kaolin is an important factor affecting the quality of cordierite because of its various mineralization types,varieties and wide distribution.In order to study the mechanism of synthesizing cordierite ceramics from kaolin-talc-alumina system,and elucidate the effect of kaolin raw materiasl on the synthesis of cordierite and its crystal structure,cordierite ceramics are prepared by high temperature solid state reaction with four different types of kaolin as raw materials.Systematic research has been carried out:1)raw material characteristics of kaolin and crystal structure of kaolinite;2)preparation,structure and properties of cordierite ceramics;3)reaction mechanism of synthetic cordierite;4)the structure and mechanism of synthetic cordierite by solid state NMR;5)modification of cordierite ceramic powder.The main research contents and conclusions are as follows:?1?In order to study the raw material composition,microstructure and technological properties of Ordos coal series kaolin,Beihai kaolin,Maoming kaolin and Australian kaolin,and analyze the crystal structure of four kinds of kaolin,the chemical composition,microstructure,rheology and plasticity of kaolinite were analyzed,and the order/disorder of kaolinite crystal structure was characterized by XRD?based on different scanning rate?,FTIR and TG-DSC.It was found that the chemical compositions of the four kaolin raw materials were obviously different,such as SiO₂/Al₂O₃ ratio and the content of impurities.The SiO₂/Al₂O₃ ratios of Ordos kaolin,Beihai kaolin,Maoming kaolin and Australian kaolin were 1.36,1.59,1.45and 1.44 respectively,and the impurity contents were 2.11 wt%,5.25 wt%,2.14 wt%and 0.58 wt%.The structural ordering of the four kinds of kaolin was qualitatively analyzed by Hinckley Index,the ratio of infrared absorption peaks at 3695 cm^-1 and3620 cm^-1,and the symmetry of dehydroxylation absorption peaks.The order of structural ordering was Australia kaolin,Ordos kaolin,Maoming kaolin and Beihai kaolin from high to low.The chemical composition and structural ordering of the four kinds of kaolin were obviously different.It is universal and representative for the results of preparation of cordierite ceramic with excellent properties that four kinds of kaolin are used as raw material variables of kaolin-talc-alumina system.?2?In order to study the effect of different kaolin on the preparation,structure and properties of cordierite ceramics,the sintering process,microstructure,thermal expansion coefficient and electrical properties were studied by means of heating temperature microscope,SEM and optical dilatometer et al.The sintering process of cordierite ceramic by kaolin-talc-alumina system could be divided into four stages:1)the initial stage of sintering with a small amount of linear shrinkage?about 1000-1200^oC?;2)the re-expansion process?about 1200¹360 ^oC?;3)the densification stage?about 1360¹420 ^oC?;4)the rapid deformation stage?about 1420¹460 ^oC?.The temperature range of the sample during re-expansion was wide,making the sintering temperature range of cordierite ceramics become sharply narrow.The sintering temperature range of samples was determined by whether the cordierite body containing different kaolin materials could be sintered before 1200 ^oC.In kaolin raw materials,the content of K2O+Na2O determined the size and length-diameter ratios of cordierite grain,the content of liquid determines the evolution of pores in microstructure,and further determined the thermal expansion coefficient and electrical properties of cordierite ceramics.Cordierite ceramics prepared from low-grade kaolin had the advantages of low cost,wide firing temperature range,high bending strength and so on.Its comprehensive properties met the requirements of high-temperature electrical insulation materials.Cordierite ceramics prepared from high-grade kaolin materials had small thermal expansion coefficient and high porosity,so it is suitable for application in porous materials.?3?In order to study the effect of different kaolin raw materials on the synthesis of cordierite,the phase composition and content of cordierite at different sintering temperatures were analyzed by XRD,determining the phase evolution and reaction process.The thermodynamics and kinetics of cordierite synthesis were analyzed.The mechanism of cordierite synthesis was revealed from the point of view of chemical reaction process,which was compared with that of pure chemical reagents.It was found that cordierite synthesized from kaolin-talc-alumina system is mainly derived from the reaction of mullite,enstalite and cristobalite.In addition,a small amount of cordierite was synthesized from spinel and cristobalite.The ratio of the two cordierites was determined by the SiO₂/Al₂O₃ ratio of kaolin raw materials.The synthesis range of cordierite is narrow?1100¹200 ^oC?with lower starting temperature low and more fast synthesis rate.Four kinds of kaolin raw materials and pure oxides had no effect on the phase composition of the final synthetic cordierite,but kaolin raw materials had an effect on the temperature of the synthetic cordierite about 20 ^oC.The temperature range of cordierite synthesized by pure oxide system was wide.The synthesizing rate of cordierite of two systems mainly depended on the form of silica phase.The process of synthesizing cordierite in pure oxide system was controlled by the phase transformation of quartz and cristobalite.The presence of cristobalite remarkably increased the temperature of synthesizing cordierite.The synthesis of cordierite in kaolin-talc-alumina system is controlled by silica-rich glass phase and accompanied by liquid-phase sintering process.The liquid-phase sintering belongs to viscous flow.The rate of liquid phase sintering depended on the content of silica-rich glass phase,which was determined by the SiO₂/Al₂O₃ ratio of kaolin component and the content of impurities.Silica phase was the key factor affecting the rate and temperature of synthesizing cordierite.The transformation of quartz,cristobalite and silica-rich glass phase should be controlled in the process of synthesizing cordierite.?4?In order to study the crystal structure of cordierite synthesized from different kaolin materials,and reveal the synthesis mechanism of cordierite from the point of view of structural evolution,the Si/Al ordering in the crystal structure of synthetic cordierite had been studied by ²⁹Si NMR,and the structural evolution of synthetic cordierite has been studied by ²⁷Al NMR.It is found that ²⁹Si NMR was the most effective method for analyzing Si/Al ordering in cordierite crystal structure.The Pearson-VII function was used to fit the ²⁹Si NMR spectra,and the relative strength values of the polymerization states of Si_T1?nAl?and Si_T2?nAl?were calculated.A mathematical formula for calculating the Si/Al ordering of cordierite crystal structure by ²⁹Si NMR spectra was constructed.The structural formula of cordierite synthesized by kaolin-talc-alumina system was Mg₂Al_3-xSi_x[Al_1+xSi_5-xO₁₈]?x=0.7⁰.9?,and the structural formula of pure oxide synthesized cordierite was Mg₂Al_0.8696Si_2.1304[Al_3.1304Si_2.8696O₁₈].The number of Al-O-Al bonds in the crystal structure was calculated by ²⁹Si NMR spectroscopy,which could quantitatively judge the Si/Al ordering of cordierite structure prepared from different kaolin materials.The crystal structure gene of the intermediate product of synthesizing cordierite was inherited into the process of synthesizing cordierite,which determines the ordering of Si/Al.The smaller the SiO₂/Al₂O₃ ratio of kaolin was,the worse the order of cordierite Si/Al synthesis was.²⁷Al NMR spectra confirmed that[AlO₄]tetrahedron in cordierite structure was transformed from[AlO₆]hexahedron in mullite,and the addition of Mg accelerates the transformation from hexahedron to tetrahedron.Therefore,mullite was the key phase affecting the structural change of synthetic cordierite,and the changing the structure of cordierite needed to control the formation and content of mullite.?5?In order to obtain high quality cordierite composite powders,amorphous silica coated cordierite composite powders were prepared by treating cordierite ceramic powders with hydrofluoric acid/nitric acid mixed acid.The surface of the composite powders was characterized by SEM-EDS,XRD and FTIR.The morphology of cordierite ceramic powders transformed into amorphous state and the evolution of silicate structure were analyzed by SEM and ²⁹Si NMR.It was found that HF dissolved the glass phase on the surface of cordierite ceramic powder,but did not destroy the crystal structure of cordierite.HNO₃ destroyed the crystal structure of cordierite and forms amorphous silica by removing Mg²⁺and Al³⁺.In the cordierite crystal structure,the[AlO₄]of T1 was easier to react with HNO₃.After modification with mixed acid,the high temperature resistance of cordierite composite powder was improved,and the melting temperature was increased by 30 ^oC.Low grade cordierite ceramic powders were modified by HNO₃-HF mixed acid,which provided an effective solution for obtaining high quality cordierite composite powders.