Microcrystalline glass microstructure
The microstructure of the glass-ceramics is mainly determined by the composition and heat treatment process, and has great influence on the physical properties of the glass-ceramics such as mechanical strength, fracture toughness, light transmittance, and thermal shock resistance. The microstructure of glass-ceramics mainly includes dendritic structure, ultrafine particles, porous membrane, residual structure, building block structure, columnar interlocking structure, island structure, and flaky twin crystal.
The dendritic structure is caused by the accelerated growth of crystals in a certain lattice direction. The overall profile of the dendrites is similar to the usual crystal morphology, with a high proportion of residual glass phase remaining in the dendritic structure. The dendrites continuously penetrate in a three-dimensional direction to form a skeleton. Since hydrofluoric acid erodes lithium silicate faster than aluminosilicate glass, lithium silicate dendrites are easily nucleated by silver, and complex patterns can be transferred to the glass ceramics.
The grain size of the highly crystallized glass ceramics can be controlled within several tens of nanometers to obtain an ultrafine grain structure. In lithium aluminum silicon transparent glass ceramics, due to sufficient nuclear, a large amount of zirconium titanate crystal nucleus is formed in the base glass, and the β-quartz solid solution crystal phase is epitaxially grown on the crystal nucleus to form a uniform average grain size of about 60 nm. Fine particle structure. Since the grain size is much smaller than the visible light wavelength, and the birefringence of the β-quartz solid solution is low, the transmittance of the glass ceramic is high.
In many glass-ceramics, the residual glass phase can form a porous membrane structure. In the lithium aluminum silicon opaque glass ceramics with β-spodumene solid solution as the main crystal phase, the residual glass phase has a high SiO2 content and a high viscosity, which can hinder the aluminum ion film network. Therefore, lithium aluminum silicon glass ceramics have very good particle stability at high temperatures and can be used for a long time at a high temperature of 1200 °C.
The so-called residual structure means that the glass ceramics retain the original structure in the base glass as it is. The first step in nucleation of glass-ceramics is often liquid-liquid phase separation, which forms droplets. As in binary aluminosilicate glass, high aluminum droplets resembling mullite are separated from a high silicon matrix. During heat treatment, the high aluminum droplets crystallize into mullite microcrystals, and their shape inherits the spherical appearance of the parent droplets. Due to the small size of the microcrystals, only a few tens of nanometers, although the refractive index difference between mullite and siliceous glass is large, the scattering of visible light is small, and it is a transparent glass ceramic.
Crystallization of mica silicate minerals in two dimensions can produce an interlocking building block structure and is a typical microstructure of machinable glass-ceramics. Since the mica crystal phase is soft, and the cracks are passivated, deflected, and branched by the tip of the cutting tool to cause chipping, no catastrophic damage occurs, so even if the crystal phase volume fraction is only 40%, it has good machinability. In addition, the continuity of the mica phase also makes such glass-ceramics have a high electrical resistivity and dielectric strength.
Glass-ceramics having a columnar or needle-like interlocking microstructure have the highest mechanical strength and fracture toughness. Photomicrograph of microscopic glass of amphibolite glass-ceramic with potassium fluoroalkali amphibole as the main crystalline phase. The columnar interlocking microstructure has structural features similar to the random arrangement of whiskers in whisker-reinforced ceramics. The glass ceramic has a flexural strength of 150 MPa and a high fracture toughness (3.2 ± 0.2) Mpa·m. The fluorosilicite feldspar glass-ceramics with a chain silicate mineral fluoroalkali-calcium as the main crystalline phase and a higher degree of crystallization have a columnar interlocking microstructure with a bending strength close to 300 MPa and a fracture toughness of up to 5.0 MPa. ·m.
When the equilibrium phase is formed along the interface of various metastable phases, a typical island structure is created. The garnet crystal phase produced by the silicate glass-ceramics in the presence of the mullite crystals and the residual glass phase has an island microstructure.
The crystal phases of several glass-ceramics, such as enstatite, anorthite and leucite, undergo structural transformation during the cooling process to produce polymerized twins, producing a sheet-like twin structure capable of improving fracture toughness. The enstatite begins to form the original enstatite. When cooled to 1000 °C, the enstatite undergoes a martensitic transformation transition oblique enstatite, and the enstatite particles are highly crystallized. Since the microstructure of the germanium wafer allows the crack to deflect energy, the glass ceramic has the highest fracture toughness, on average about 5.0 Mpa·m, and has a high modulus of elasticity.
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