Direct Coagulation Casting Via High Valence Counter Ions (DCC-HVCI) From Chelate Complex By Tailoring Temperature And PH Value

Ceramics with high strength and reliability are highly demanded in engineering applications.Direct coagulation casting via controlled release of high valance counter ions(DCC-HVCI)are ability to solve the problems of ceramic colloidal processing,such as toxic gel system,low green strength and poor adaptability.We propose a new method using chelate complex as the coagulating agent for controlling release of high valence counter ions.Finally,ceramic the formating process and sintering properties are studied by the DCC-HVCI method of three kinds of chelating systems of sodium poly phosphate,ammonium phosphate and ammonium citrate.The following conclusions are obtained:A novel approach was adopted using sodium tripolyphosphate(STPP)chelate complex as coagulation agent for direct coagulation casting of alumina and zirconia via controlled release of high valence counter ions(DCC-HVCI).Sodium tripolyphosphate chelate complex suspension was prepared from sodium tripolyphosphate and calcium chloride.Calcium ions used as high valence counter ions were chelated by STPP.The average particle size of chelate complex is 0.11 ?m with a narrow particle size distribution,which is close to the size of alumina particles.Glycerol diacetate was used to tailor pH value of the suspension by hydrolysis,which produces acetic acid.The lowering of the pH value helps to decompose the chelate complex and release of calcium ions.It is indicated that the viscosity of suspension with the addition of STPP chelate complex suspension and glycerol diacetate increases to ca.20 Pa·s after heating at 40-70? for 1.5-7 h,which is high enough to coagulate the suspension.Suspension with addition of 3 vol%STPP chelate complex suspension and 3 vol%glycerol diacetate treated at 60? could coagulate completely within 2 h with compressive strength of 2.7±9 MPa.Alumina and zirconia have been obtained with relative density of above 93%and 99%,respectively.The existence of sodium inhibited the densification of alumina ceramics.Ammonium polyphosphate(APP)chelate complex as the coagulating agent was adopted to direct coagulation casting of alumina via controlled release of high valence counter ions.APP chelate complex suspension was prepared from APP and calcium chloride.Calcium used as high valence counter ions was chelated by APP.The average particle size of the chelate complex is 0.13?m with a narrow particle size distribution.It is indicated that the viscosity of the suspensions with the addition of APP chelate complex suspension and glycerol diacetate increases to approximately 20 Pa s after heating at 40?70? for 1.5-5 h,which is high enough to coagulate the suspension.55 vol%alumina suspension with a addition of 3 vol%APP chelate complex suspension and 5 vol%glycerol diacetate treated at 60? could coagulate completely within 2 h with a compressive strength of 2.1 MPa.Dense alumina with a relative density of above 97.9%and a flexural strength of 388±23 MPa can be prepared by this method from 55 vol%alumina suspensions without a burnout process.A new method using calcium citrate complex assisted by glycerol diacetate for controlling release of high valence counter ions have been proposed.Calcium citrate complex suspensions were prepared by mixing tri-ammonium citrate and calcium chloride in water.Effect of reaction time on the chelating properties of the prepared suspensions was investigated.Concentrated alumina suspensions with a solid loading of 50 vol%were prepared by mixing the calcium citrate complex suspensions and alumina powder at pH of 10.5.Then the suspensions were coagulated by adding 3?6 vol%glycerol diacetate at temperatures of 40?70 ? for 2?6 h.The compressive strength of the coagulated wet samples is in the range 1.1?2.4 MPa.Alumina ceramics sintered at 1550? shows homogeneous microstructures with flexural strength and Weibull modulus of 455±17 MPa and 30,respectively.