Glass ceramics

Glass ceramics, also called glass-ceramics or ceramics, is a new type of inorganic crystalline material developed in the early 1950s. It is known for its high mechanical strength, hardness, and excellent chemical stability. Moreover, because this new type of inorganic material does not age or deteriorate, and is stored in human tissues for a long time without irritation and adverse reactions, in recent years, domestic and foreign scientists have conducted in-depth research on the application of glass ceramic materials in the medical field.

1. Classification of glass ceramics

After heat-treating the base glass of specific composition with (or without adding) a nucleating agent at a certain temperature, a composite material with uniform distribution of microcrystals and glass phase is obtained, which is called glass ceramic.

According to the basic glass composition, the classification of glass ceramics can be divided into five categories: silicate, aluminosilicate, borosilicate, borate and phosphate.

 

2. Production process of glass ceramics

The production process of glass ceramics varies with the types of products, and the specific process systems have their own characteristics. The basic process

The process is roughly as follows:

Batch preparation → glass melting → forming → processing → crystallization treatment → reprocessing

The key to the production of glass ceramics is the crystallization stage: the first stage is the nucleation stage, and the second stage is the growth stage. These two stages are closely related. It must be fully nucleated in the first stage, and control the growth of crystal nuclei in the second stage.

The crystallization process of glass ceramics is determined by three factors: ① the rate of crystal nucleation; ② the rate of crystal growth; ③ the viscosity of the glass. These three factors are all related to temperature.

The crystallization rate of glass ceramics should not be too small or too large, which is conducive to the control of the crystallization process.

In order to promote nucleation, a nucleating agent is generally added. One kind of nucleating agent is precious metal, such as gold, silver, platinum plasma, but the price is more expensive; the other is Zengtong's nucleating agent, including TiO2, ZrO2, P2O5, V2O5, Cr2O3, MoO3, oxide, fluoride , Sulfide. Of course, the choice of nucleating agent for bioceramics should also consider whether it is toxic.

3. The structure and performance of glass ceramics

Glass ceramics are composed of a crystalline phase and a glass phase, without pores, and are different from glass and ceramics. The crystalline phase is a polycrystalline structure, the crystals are fine, much smaller than general crystalline materials, generally less than 1~2μm, the content of the crystal phase is generally 50%~90%, and the content of the glass phase is generally 5%~50%. The distribution state of the crystalline phase and the glass phase in glass ceramics varies with their ratio. When the proportion of the glass phase is large, the glass phase is a continuous matrix, and the isolated crystal phases are evenly distributed in it; for example, when the number of glass phases is small, the glass phase is dispersed between the crystal grids, forming a continuous network. ; When the amount of the glass phase is small, it is distributed between the crystals in the state of a thin film.

The performance of glass ceramics is mainly determined by the types of precipitated crystals, the size of crystal grains, the number of crystal phases, and the types and quantities of remaining glass phases. The above factors depend on the composition of the glass and the heat treatment system. In addition, the proper use of nucleating agents plays a key role in the microcrystallization of glass.

Glass ceramics with different main crystalline phases will have great differences in their properties (such as chemical stability, expansion coefficient, etc.). For example, glass ceramics whose main crystal phase is cordierite (2MgO·2Ai2O3·5SiO2) has good high-frequency insulation properties; glass ceramics whose main crystal phase is β-quartz solid solution has low expansion coefficient and high transparency. To manufacture glass ceramics with special performance requirements, it is necessary to control the types of main crystal phases.

The grain size of the main crystalline phase has an impact on the mechanical properties and optical properties of glass ceramics. Within a certain size range, the strength of glass ceramics is also related to the grain size.

In addition, the content of the crystal phase in glass ceramics also affects the properties of the material, and the content of the crystal phase has a decisive effect on the properties. The nature and quantity of glass also have a certain influence on the properties of glass ceramics (such as mechanical properties and thermal properties). Chemical stability, thermal stability, etc. depend on the nature and quantity of the glass phase under certain conditions. The existence of the glass phase is a prerequisite for the quenching and strengthening of glass ceramics. When the expansion coefficient of the glass phase is greater than that of the crystal phase, and the amount exceeds 15% to 20%, the glass can be strengthened by quenching.

Glass ceramics have many excellent properties, such as high mechanical strength. The mechanical strength of glass ceramics is much higher than that of ordinary glass. The compressive strength is 0.59~1.02GPa, and the bending strength is 88.2~220.5MPa, the tensile strength is 49~137.2MPa, the special or reinforced glass ceramic has a bending strength up to 411. 6~548.6MPa.

Glass ceramics have high hardness and outstanding wear resistance. Its hardness is higher than that of high-carbon steel and granite, and is close to that of quenched tool steel. Its Vickers hardness value is 5.9~9.3GPa.

The elastic modulus of glass ceramics is generally 88~98GPa, and the Poisson's ratio is 0.25~0.29.

The resistance of glass ceramics to strong acids and alkalis is higher than that of ordinary glass. For example, glass ceramics with β-spodumene as the main crystalline phase will react with 15% HCI at 90°C. After 24 hours, the erosion amount will be 0.02%~0.03%.

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