| Hard porcelain has drawn great attention due to its hard texture,good light transmittance,excellent thermal shock resistance,high glaze hardness and scratch resistance,but its mechanical strength is low.The alumina reinforced porcelain obtained by introducing corundum improves the mechanical strength of hard porcelain,but its transmittance is greatly reduced.In order to solve this problem,a new R2O-RO-Al2O3-SiO2quaternary system,breaking the traditional ternary system of K2O-Al2O3-SiO2,was innovatively used to prepare hard porcelain.The as-prepared porcelain contains only mullite crystal phase by introducing CaO and MgO instead of K2O as flux.Firstly,the effects of composition and sintering temperature of KAS system on crystal phase composition of porcelain body were systematically studied.Then,the effects of flux agents of CaO and MgO on the structure and properties of mullite porcelain were investigated,and the formation mechanism of cluster-and whisker-like mullite crystals in this system was clarified,and the formation mechanism of mullite whisker tip was analyzed.Furthermore,the regulation of in-situ mullite crystals in the system was preliminarily realized.The method of flux prefiring was proposed to solve the problem that the hard porcelain was difficult to achieve densification sintering in the system.The light transmittance was optimized by introducing high refractive index oxides such as La2O3and BaO,and the mullite hard porcelain with high strength,high transmittance and high whiteness was eventually prepared.Finally,high temperature transparent glaze suitable for the mullite hard porcelain was prepared to further improve the quality of hard porcelain.The results shows that:(1)In KAS ternary system,the more R2O content as flux,the less SiO2content is,and the higher sintering temperature are beneficial to the elimination of residual quartz.When the sintering temperature ranges from1410?to 1430?,no residual quartz in the samples is observed when the R2O content is higher than 3.5 wt%and SiO2content is lower than 62 wt%.Replacing K2O with CaO and MgO as flux can eliminate the corundum crystals and promote the formation of secondary mullite crystals in the porcelain body.Meanwhile a large number of closed pores with large size will be formed,which makes it difficult to achieve densification sintering.When the mass ration of CaO/MgO is 1.20,the content of mullite crystal,the bending strength,light transmittance,water absorption and porosity of the samples at 1410?for 1 h are 51.9%,107.3 MPa,12.6%(thickness of 1mm),0.20%and 15.45%,respectively.(2)The mullite crystals in the system show two morphology including cluster and whisker.The mullite clusters are in situ formed on the basis of?-Al2O3particles.With the increase of firing temperature,the Al2O3particles gradually become loose.When the firing temperature is 1300?,the alumina particles react with SiO2to form mullite cluster in situ.When the firing temperature is increased to 1400?,the outer edge of mullite clusters grows in one-dimensional direction and forms hedgehog.The mullite whisker shows single crystal structure,and its growth mechanism belongs to the axial screw dislocation growth mechanism.The whisker preferentially grows along[001]direction.Most mullite whiskers are formed during the cooling process,and the maximum yield temperature range is 1200??1300?.After 1410?quenching treatment,the end of mullite whisker shows flat shape.When the quenching temperature is lower than 1300?,the end of mullite whisker becomes‘tip'shape.Importantly,secondary growth of mullite whisker occurs by cooling to 1250?for a period of time,although this process can not promote the content of mullite whisker.The content of mullite whisker increases by holding at 1410?.With the extension of holding time at 1410?,the bending strength of the sample increase gradually.When the sample is obtained at 1410?for 3 h,the sample has the maximum bending strength of125.3 Mpa.(3)Compared with pure Al2O3power,the surface of Al2O3particles in hard porcelain adsorbs certain amount of K2O,CaO,MgO and SiO2,which inhibits the phase transition of?-Al2O3.This phenomenon can significantly increases the phase transition temperature of?-Al2O3to?-Al2O3from 1200?to 1300?,and then improves the reactivity of Al2O3to mullite.With the increase of pregrinding time of alumina,the clustered mullites gradually become small mullite whiskers,but the bulk density of the sample decreases,water absorption increases,and bending strength decreases.(4)Changing the molding method,holding time and heating rate can not promote densification sintering of the system,and the porosity of the prepared samples is still greater than 15%.The bending strength of the sample is highly consistent with the pore fractal dimension,that is,the smaller the pore fractal dimension is,the more regular the shape is,and the higher the bending strength of the sample is.Prefiring flux can significantly reduce the pore contents in the sample,greatly reduce the pore size and improve the bending strength of the sample.This is because the temperature of liquid phase produced in the system decreases from 1122?to 868?after prefiring flux above 1300?,so that the hard porcelain can realize liquid phase sintering at lower temperature.When the prefiring temperature of flux is 1500?,the high density mullite hard porcelain is obtained.Compared with the sample without prefiring flux,the bending strength increases from 107.3 MPa to 156.4 MPa,the porosity decreases from 15.45%to 4.57%,and the average pore size decreases from 38.1?m to 14.4?m.(5)When the contents of La2O3and BaO are 0.57%and 1.33%,respectively,the mullite hard porcelain under the optimal processing conditions possess the best comprehensive property,which have bending strength of 182.9 MPa,light transmittance of 13.9%(thickness of 1 mm),whiteness of 88.6%,thermal expansion coefficient of 4.65×10-6/?(600?),porosity of 2.78%and volume density of 2.554g/cm3.Compared with the traditional hard porcelain,the bending strength and whiteness of the obtained porcelain are 2.3 and 1.2 times of the traditional hard porcelain,respectively,while maintaining a higher transmittance.Compared with high alumina reinforced porcelain,the transmittance of mullite hard porcelain is greatly improved,about 2.5 times that of high alumina reinforced porcelain,and the bending strength is increased by 16.4%,which solves the contradiction between high strength and high transmittance of traditional hard porcelain.Mullite hard porcelain has low thermal expansion coefficient,which is conducive to improving its thermal stability.(6)In the glaze of CaO-MgO-Al2O3-SiO2system,the introduction of a small mount of K2O can significantly reduce cristobalite contents,thereby greatly reduces thermal expansion coefficient of the glaze.When K2O contents increases from 0.29%to 0.81%,the average thermal expansion coefficient at 27?200?decreases from 6.29×10-6/?to 3.10×10-6/?.With the increase of pregrinding time,the glaze glossiness is obviously improved,and the glaze surface roughness is gradually reduced.When the pregrinding time is 6 h,the glaze glossiness reaches 103%.The average roughness of the unpregrinding quzrtz glaze is 1.95 times of that of pregrinding 6 h.Pregrinding quartz has little effect on the hardness of the glaze.When pregrinding time is 6 h,the maximum hardness of the glaze is 6.85 GPa.When high-temperature transparent glaze is applied on the hard porcelain body,the bending strength of the sample increases from 182.9 MPa to 196.2 MPa.This is because on one hand,the thermal expansion coefficient of the green body and the glaze are 4.65×10-6/?and 3.28×10-6/?(600?).The thermal expansion coefficient of the two matches very well,so that the glaze bears compressive stress.In addition,the wedge structure is formed between them,which results good combination between the green body and the glaze. |
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