Synthesis And Stabilization Of Cubic Leucite With Low Coefficient Of Thermal Expansion

Because leucite has a refractive index close to that of the feldspathic glass,relatively high strength and extraordinary high coefficient of thermal expansion,it has been used to improve the strength and the coefficient of thermal expansion of the porcelain fused to metals for a long time,so that to ensure a good thermal expansion match between the porcelain and the metal without reducing the strength and the transparency of the porcelain.Leucite stabilizes in a cubic phase at high temperature,however,when the temperature reaches about 625 ?,it undergoes a transformation from cubic to tetragonal,associated with a 1.2 % volume contraction,resulting in an especially high coefficient of thermal expansion?³0×10-6 /??.The newly developed dental restorations aimed to improve the biocompatibility and durability are based on titanium alloy and porcelains such as zirconia with a relatively lower coefficient of thermal expansion,so the leuctie with high coefficient of thermal expansion is detrimental to the thermal expansion match.In this paper,the methods to stabilize cubic leucite and decrease its coefficient of thermal expansion such as ion doping,increasing Si/Al ratio and adding CaF₂,etc.were studied emphatically in order to meet the need of new type of dental restorations to the leucite with low coefficient of thermal expansion.In the study,feldspar was used as the main raw material and TiO₂ as the nucleating agent,when 16 mol% Cs₂ O and 1 wt% CaO were added to the raw material and heated at 900 for ?3 h,the cubic leucite stabilized at room temperature was synthesized.The sample with cubic leucite had a coefficient of thermal expansion in the range of 30 ?⁷00 ? of?17.3 ± 0.2?×10-6 /?,which was obviously lower than that of the sample with tetragonal leucite prepared by us??24.1 ± 0.2?×10-6 /??.By adding Li₂CO₃,Na₂CO₃ and H₃BO₃ respectively to the cubic leucite fomulation to make the Li+,Na+ substitute for K+ and B3+ substitute for Al3+,the samples with different coefficient of thermal expansion were obtained.When the doping amount of Li+ and Na+ was x=0.03 and the doping amount of B3+ was x=0.2,the coefficients of thermal expansion of the cubic leucite samples were?13.6 ± 0.2?×10-6 /?,?14.4 ± 0.1?×10-6 /? and?14.5 ± 0.1?×10-6 /? in the range of 30 ?⁷00 ?,which indicated that doping Li+,Na+ and B3+ might reduce the coefficient of thermal expansion of cubic leucite obviously.When the Si/Al ratio of leucite was increased to 2.75,the main crystal phase of the sample was still cubic leucite,and the coefficient of thermal expansion of the sample in the range of 30 ?⁷00 ? was?13.2 ± 0.1?×10-6 /?,which was apparently lower than that of the cubic leucite sample prepared with Si/Al=2 in the same temperature range.B3+ doping on the basis of Si/Al=2.75 changed the cubic leucite morphology to small spherical particles.The addition of 2 wt% CaF₂ to the cubic leucite formulation promoted the crystallization of cube leucite but part of them would transform to tetragonal leucite.The effect of CaF₂ on the reduction of coefficient of thermal expansion of leucite was limited.