Translated Abstract
Due to the ever-increasing demands of dental prosthetic materials, lithium disilicate (Li2Si2O5) glass-ceramics has gained an important place in dental restoration because of its excellent mechanical properties, optical properties, corrosion resistance and biological properties, and have a broad application prospect. However, few studies on glass-ceramics of this series are performed in China, and there are still some problems such as unclarified crystallization process and poor mechanical performance. Therefore, it has important research significance and application value to analyze the crystallization process and optimize the material performance.
In this paper, lithium disilicate glass-ceramics in Li2O-SiO2-Al2O3-K2O-P2O5-ZrO2-CeO2 system is used as the research object. The nucleation and growth mechanism of the processes about precipitation of the lithium metasilicate crystal in glass matrix, convertion of lithium metasilicate into lithium disilicate, and precipitation of lithium disilicate crystal in glass matrix are analyzed. The effects of heat treatment temperature and time of two steps insulation processes on the parameters of the samples such as phase compositions, microstructure, flexural strength and fracture toughness are also studied.
Through DSC results, it is confirmed that the crystallization process of lithium disilicate glass-ceramics is divided into two processes: generation of lithium metasilicate (Li2SiO3) crystal at low temperature and generation of lithium disilicate crystal at high temperature. Through experimental study, we found out that the crystallization rate, size and shape of lithium disilicate crystal can be effectively adjusted by controlling the socking time if first step heating temperature is limited in 600~700°C and the second step heating temperature is limited in 800~880°C. The differences in macroscopic color of samples are also studied, and it is confirmed color of the samples with low light transmittance and small crystalline grain size is blue due to scattering, the samples with high light transmittance and big crystalline grain size is showed as yellow, and the samples situated between them is showed as white.
The research shows that in the lithium disilicate glass, the nucleation mode of lithium metasilicate crystal is heterogeneous, and the nucleation mode of lithium disilicate crystal can be heterogeneous or spontaneous depending on the precipitation situation of lithium metasilicate. When the content of lithium metasilicate is high (insulate the basic glass in 600°C for 10h), the volume of lithium disilicate will become really small because of the large amount of nuclei. Extended the socking time, part of silica-oxygen networks in lithium disilicate crystals or glass matrix decomposed, and the crystals in the system grow into a bimodal distribution structure, in which big rod-like crystals are surrounded by small rod-like crystals. When the content of lithium metasilicate is low (heating the basic glass in 870°C directly), there are so little lithium disilicate nuclei so that the crystals have adequate growth space, and eventually the crystals would grow into uniform rod-like structure. So lithium disilicate glass-ceramics with small crystals, uniform rod-like structure, or bimodal distribution structure can be prepared by controlling the content of lithium metasilicate mesophase and the temperature and soaking time of second step heat treatment.
It is found that the second step soaking temperature and time have the greatest influence on the mechanical properties of the samples through optimizing the parameters of heat treatment by orthogonal test. Through analyze the microstructure and mechanical properties of orthogonal test samples, it is found that samples with small crystals have high strength and low toughness, samples with bimodal distribution structure have low strength and high toughness, and samples with uniform rod-like structure have relatively high strength and toughness. When insulating the samples with adequate lithium metasilicate at 850°C, as the thermal soaking time extended from 1h to 8h, the grain size gradually increases, the microstructure changes from small crystals to bimodal distribution, and the flexural strength decrease at first, then increase and then decrease, with an increase of the fracture toughness at first and then decrease. When soak the samples for 5h, as the thermal soaking temperature increased from 800°C to 875°C, the microstructure changes from small crystals into bimodal distribution and then turn to uniform rod-like structure, and the grain size gradually increases, while both the flexural strength and fracture toughness increase at first and then decrease. Through analyze and summarize the experimental results, it is finally confirmed that the optimized mechanical properties with the flexural strength of 344.8MPa, and fracture toughness of 2.33MPa•m1/2 can be obtained when soaking the samples at 600°C for 10h followed at 850°C for 5h.
Translated Keyword
[Anode material, Biochar, Lithium ion battery, Multi-porous graphitized carbon]
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