Dongguan Ming Rui Ceramic Technology Co., Ltd
Boron nitride ceramics were first discovered in behrman's lab more than 100 years ago, and the material was developed on a large scale in the late 1950s.
Boron nitride (BN) ceramics are a new type of ceramic materials with excellent properties and great potential for development. They include five isomers, namely hexadecyclic boron nitride (h-bn), wurtzite boron nitride (w-bn), ternary boron nitride (r-bn), cubic boron nitride (c-bn) and rhombic boron nitride (o-bn).
Boron nitride ceramics used in machinery, metallurgy, chemical industry, electronics, nuclear energy and aerospace fields.
Boron nitride hexagonal, sphalerite and fibre-zinc crystals
Boron nitride ceramics are crystals composed of nitrogen and boron atoms, with the molecular formula BN and molecular weight 24.81.The chemical composition is 43.6% boron and 56.4% nitrogen with theoretical density of 2.27g/cm3.
Boron nitride ceramic powder has properties of looseness, lubrication, light weight, moisture absorption and white color.Boron nitride products are ivory white.At present, studies on boron nitride mainly focus on its hexagonal (h-bn) and cubic (c-bn) phases.
The properties of boron nitride can be divided into the following aspects:
✔ in the aspects of mechanical properties: has no abrasion, low wear, size safety, good lubricity, refractory and easy processing, etc.
✔ in the electric features: has good dielectric strength, low dielectric constant, high frequency and low loss, microwave penetration, good electrical insulation, etc.
✔ in the aspects of thermodynamic features: with high thermal conductivity, heat capacity, thermal expansion, thermal shock, high temperature lubricity and high temperature stability, etc.
✔ in the aspects of chemical properties: non-toxic, chemical stability, corrosion resistance, oxidation resistance, low moisture, biological stability and non-stick, etc.
Hexagonal boron nitride ceramics (h-bn) is the most commonly used form of boron nitride.H-bn is similar to graphite in structure, with hexagonal layered structure, lattice constant a=0.2504nm, c=0.6661nm, theoretical density 2.27g/cm3, melting point 3000℃, soft texture, strong processability and white color, commonly known as "white stone ink".
Graphite structure and hexagonal boron nitride ceramic structure
Boron nitride hexagonal (h-bn) has excellent electrical insulation, excellent chemical stability and excellent dielectric properties.
Heat properties of boron nitride: no obvious melting point, sublimation at 3000℃ in 0.1MPA nitrogen, melting point at 3000℃ in inert gas, heat resistance to 2000℃ in neutral reducing atmosphere, temperature up to 2800℃ in nitrogen and argon, poor stability in oxygen atmosphere, temperature below 1000℃.
Boron nitride is one of the materials with the highest thermal conductivity in ceramic materials. Its thermal conductivity is ten times that of quartz, and it has a high thermal conductivity of 60W/(m·K) in the direction perpendicular to the c-axis.
The low thermal expansion coefficient, which is equivalent to quartz, is the smallest among the ceramics. The thermal expansion coefficient in the c-axis direction is 41*10-6m/K, and in the d-axis direction is 2.3*10-6m/K, so the thermal shock resistance is very good.
Mechanical properties of hexagonal boron nitride ceramics: friction coefficient is low to 0.16, does not increase at high temperature, higher temperature resistance than molybdenum disulfide, graphite, oxidation atmosphere can be used to 900℃, can be used to 2000℃ in vacuum.At room temperature, the lubrication performance is poor, so it is often mixed with fluorine, graphite and molybdenum disulfide as high-temperature lubricant.Boron nitride is a soft material with mohs hardness of only 2.Good mechanical processing, can turn, milling, planing, drilling, grinding, cutting, and the processing accuracy is high, so the general machining method can be used to process parts and components with high precision.
Electrical properties of hexagonal boron nitride: hexagonal boron nitride is a good conductor of heat and a typical electrical insulator.Room temperature conductivity can be up to 10 ^ 16 ~ 10 ^ 18 Ω/cm, even at 1000 ℃, there are still 1014 ~ 106 Ω resistivity/cm.H -BN permittivity 3~5.The dielectric loss is (2~8) *10-4, and the breakdown strength is twice that of Al2O3, up to 30~40kV/mm. Therefore, it is an ideal high frequency insulation, high voltage insulation and high temperature insulation material.
Chemical properties of boron nitride: HBN has excellent chemical stability.It does not react with general metal, rare earth metal, precious metal, semiconductor material, glass, molten salt, inorganic acid and alkali.For most metal melts, such as steel, stainless steel, Al, Fe, Ge, Cu, Ni, Zn, neither wetting nor action.Therefore, it can be used as high temperature thermocouple protective sleeve, melting metal crucible, utensils, conveying liquid metal pipeline, pump parts, cast steel mold and high temperature electrical insulation materials.
Cubic boron nitride (c-bn) was first synthesized by general electric (GE) in the 1950s by artificial methods under high temperature and high pressure. Its hardness is only second to diamond, but much higher than other materials. Therefore, it and diamond are collectively referred to as superhard materials.
Superhard materials are widely used in sawing tools, grinding tools, drilling tools and cutting tools.Diamond is easy to oxidize at high temperature, especially has good affinity with iron elements, so it is not suitable for ferrous metal processing.Cubic boron nitride crystal structure similar to diamond, hardness is slightly lower than diamond, commonly used as abrasive and tool materials.
Boron nitride was first developed in 1957 by R.H. weintov of the United States, but no natural cubic boron nitride has been found.
Cubic boron nitride cell
The properties of cubic boron nitride mainly include high hardness and thermal stability.
✔ its thermal stability is better than that of artificial diamond, can still maintain a high enough at high temperature mechanical properties and hardness, good red hardness;
✔ stable structure, has the high oxidation resistance, good chemical stability, compared with diamond is particularly good, at up to 1100 ~ 1300 ℃ temperature also does not react with elements of iron group, therefore particularly suitable for processing ferrous metal materials;
✔ coefficient of thermal conductivity is smaller than the diamond, but than a hard high, has the good heat conductivity;
✔ high bending strength;
✔ as abrasive material, long service life, good abrasion resistance.
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