Guide
Hexagonal boron nitride was born in the Berman laboratory in the 1840s. Its structure and properties are very similar to those of graphite. Because of its white color, it is known as "white graphite." As a new type of composite ceramic-based material, hexagonal boron nitride ceramics has the advantages of low density, high melting point, low hardness, thermal shock resistance and good machinability. It also has high temperature resistance, small thermal expansion coefficient and thermal conductivity. High performance, low dielectric constant, reliable electrical insulation and many other excellent properties are high temperature structural ceramic materials with great development potential.
Structure and properties of hexagonal boron nitride
1.1 Structure of hexagonal boron nitride
Boron Nitride (BN) is a new wide bandgap nanomaterial with excellent performance, great development potential and application prospects. It is a typical III-V compound consisting of a nitrogen atom and a boron atom. The nitrogen atom and the boron atom are combined with each other in different hybrid ways to form boron nitride of different phase structures: hexagonal boron nitride (h-BN), rhombohedral boron nitride (r-BN), cubic boron nitride. (c-BN) and wurtzite boron nitride (w-BN), orthogonal boron nitride (o-BN). Among them, hexagonal boron nitride is the only boron nitride phase existing in nature.
Structure of hexagonal boron nitride
Hexagonal boron nitride belongs to the hexagonal crystal system and has the same hexagonal crystal structure as graphene. Its lattice constant a=0.2504nm, c=0.6661nm, is stacked by multi-layer structure, and BNB is interposed by van der Waals. Force connection, easy to peel, light weight, non-conductive, wide band gap (5.1eV), high hardness (Mohs hardness 2), high melting point (>3000K), high anti-oxidation temperature 900°C, High temperature resistance 2000 ° C, low thermal expansion / shrinkage, and good thermal performance along the C axis, has a wide range of applications, and single or multi-layer hexagonal boron nitride can be curled into hexagonal boron nitride tube nano Material [1].
1.2 Performance of hexagonal boron nitride
The structural characteristics of hexagonal boron nitride have many excellent properties, such as high thermal conductivity, high heat resistance, lubricity, low friction coefficient, low thermal expansion coefficient, excellent dielectric properties, and strong oxidation resistance and resistance. Chemical properties such as strong corrosivity and chemical stability.
Basic properties of hexagonal boron nitride (h-BN)
The specific basic properties of h-BN ceramics are as follows [2]:
(1) High heat resistance
The h-BN ceramic is sublimed at 3000 ° C in 0.1 MPa nitrogen, twice as strong as room temperature at 1800 ° C, and has excellent thermal shock resistance. It does not break at 1500 ° C for several tens of times to room temperature.
(2) High thermal conductivity
The hot-pressed h-BN ceramic product has a thermal conductivity of about 33 W/m•k, which has a thermal conductivity similar to that of stainless steel, and is one of the materials with the highest thermal conductivity among ceramic materials.
(3) Low thermal expansion coefficient
The linear expansion coefficient of h-BN ceramics is (2.0~6.5)x10-6/°C, which is second only to quartz glass, which is the smallest among ceramics. In addition, it has high thermal conductivity, so the thermal shock resistance of h-BN ceramics Very good performance.
(4) Excellent electrical insulation properties
The high temperature insulation of h-BN ceramics is 1014 Ω•cm at 25 °C, 103 Ω•cm at 2000 °C, and the maximum volume resistivity of high purity h-BN ceramics is 1016~1018 Ω•cm, even at 1000. At °C high temperature, there is still 104~106Ω•cm, which is the best high temperature insulation material in ceramics.
(5) Good corrosion resistance
h-BN ceramics have good chemical stability and are not wetted by most molten metals, glass and salt, so they have high resistance to acid, alkali, molten metal and glass, and have good chemical inertness.
(6) Low coefficient of friction
The h-BN ceramic has excellent lubricating properties, the friction coefficient μ is 0.16, does not increase at high temperature, is higher than that of molybdenum disulfide and graphite, and the oxidizing atmosphere can be used up to 900 ° C, and the vacuum can be used up to 2000 ° C.
(7) Machinability
h-BN ceramics are easy to use for conventional metal cutting technology for finishing products with a turning accuracy of 0.05 mm. Therefore, h-BN blanks can be processed to obtain complex shaped products.
Preparation of hexagonal boron nitride ceramics
2.1 Preparation of hexagonal boron nitride powder
Hexagonal boron nitride is mainly prepared by synthesis and decomposition of boron-containing and nitrogen-containing compounds. Boron-containing compounds mainly include boron halides, oxides and boric acid, and nitrogen-containing compounds mainly include ammonia gas, ammonia salt and urea. And other organic ammonia [3].
The preparation method of early boron nitride is generally a direct synthesis method, and the reaction is 2B+N2—2BN. Due to the high price of raw material boron, the manufacturing cost is high, which limits its development and application. After the 1950s, research on the synthesis of boron nitride powders has developed rapidly. The main synthetic methods are:
Boric anhydride nitridation method: B2O3+NH3—2BN+3H2O
Borax-ammonium chloride method: Na2B4O7+2NH4Cl+2NH3—4BN+2NaCl+7H2O
Borax-urea method: Na2B4O7+2(NH2)2CO-4BN+Na2O+4H2O+2CO2
As the research on boron nitride continues to deepen, the properties of some nanostructured boron nitride are gradually being discovered. On the one hand, nano-powder has higher specific surface energy and higher sintering activity, which can effectively promote the densification of h-BN ceramics. On the other hand, using nano-powder as raw material can reduce the sintering temperature and reduce the grain size of ceramic sintered body. Improve the toughness of ceramics and enhance the mechanical properties of h-BN ceramics, laying a foundation for the large-scale industrial application of h-BN ceramics [4].
At present, there are many preparation methods of nano boron nitride powders, which can be roughly divided into two categories according to their principles: one of them is a synthesis method, mainly including high temperature synthesis method, solvothermal synthesis method, template method and chemical vapor deposition method ( CVD), etc.; and the other is the stripping method, including liquid phase ultrasonic stripping method, laser etching stripping method, mechanical ball milling method, etc. [5].
Method for preparing hexagonal boron nitride powder
In recent years, with the deepening of research on hexagonal boron nitride materials, various new preparation methods have emerged. Among them, the precursor ceramic technology occupies an extremely important position in the preparation of BN ceramics and its composite materials with its unique advantages and characteristics.
Precursor ceramic technology is a ceramic material preparation technology that uses organic or inorganic compounds as precursors to transform into ceramics through inorganic processes such as cross-linking cracking or gas phase pyrolysis [6].
Process flow chart for preparing hexagonal boron nitride particles by precursor method
Compared with traditional ceramic preparation processes, precursor conversion has many advantages, including [7]:
(1) The designability of precursor molecules can be designed and optimized by the molecular design of the composition and structure of the precursors, thereby achieving the design and control of the composition, structure and properties of the final ceramic materials;
(2) The preparation temperature is low, and cracking ceramization can be realized at a lower temperature (1000 to 1400 ° C), thereby avoiding damage of the reinforcement by high temperature sintering;
(3) High-purity ceramic materials can be prepared without adding a sintering aid;
(4) Good processability.
In the future, this technology will focus on aerospace fields such as high-performance engines, high-efficiency thermal protection systems, and high-temperature permeable radomes.
2.2 Sintering of hexagonal boron nitride ceramics
Hexagonal boron nitride (h-BN) is a ceramic material that is difficult to densify due to its special lamellar structure and low self-diffusion coefficient. At present, the commonly used preparation methods for h-BN ceramics include pressureless sintering (PLS), hot press sintering (HP) and spark plasma sintering (SPS) [8].
Pressureless sintering
It refers to a method of directly heating a sample and sintering it under normal pressure and in a certain atmosphere. The pressureless sintering process is simple, low in cost and high in efficiency, and can prepare large-sized and complicated-shaped products in batches, but the disadvantage is that the prepared articles have low density and poor mechanical properties, and can only satisfy the non-load-bearing use.
Hot press sintering
It refers to filling a dry powder into a special graphite mold, uniaxially pressing the mold by two-way or one-way pressing, and heating at a certain temperature to make a sintering simultaneously with molding and sintering. Method [9].
Hot press sintering is generally considered to be an ideal sintering method for the preparation of h-BN ceramics, because the applied driving force can destroy the card support structure of the sheet-shaped h-BN and promote the rearrangement of h-BN grains, thereby obtaining high An h-BN ceramic sintered product excellent in density and mechanical properties.
Spark plasma sintering
It is an emerging high-efficiency sintering technology in which a powder is pre-packed in a special graphite mold and a pulse current is applied to the sintered powder to generate a plasma to rapidly sinter the powder while applying a uniaxial equiaxed pressure.
The SPS method is similar in principle to hot press sintering, but its heating method is different from hot pressing sintering, and its temperature rise and fall is faster, which can realize the sintering of ceramics in a short time, thus effectively suppressing the growth of crystal grains. . However, the high energy consumption of complex equipment and high cost of sintering limits its application to a certain extent.
Hot isostatic pressing
Hot isostatic pressing is a process in which a material (powder, a green body or a sintered body) is sintered and densified during heating by a combination of high temperature and uniform pressure of high pressure gas. This process can produce a material with uniform microstructure, finer grain and higher density at a lower sintering temperature, and can produce a product with a complicated shape. The disadvantage is that the blank is difficult to package, the cost of the device is high, and the operation is complicated, which hinders the promotion of the process.
Reaction sintering
Also known as activated sintering, it is a sintering technique that uses a variety of chemical reactions occurring between a solid phase, a gas phase, and a liquid phase at a certain temperature to form a specific component while performing a sintering densification process. In the process of reaction sintering, the whole system is in a state of high energy level to low energy level conversion, so the sintering activation energy is relatively low, which can lower the sintering temperature and inhibit grain growth. In addition, its reaction speed is fast, heat transfer and mass transfer throughout the sintering process, which is conducive to the increase of product density [10].
Application of hexagonal boron nitride ceramics
h-BN ceramics can be said to be a new industrial material developed with the development of aviation and electronics industry, and has broad application prospects in the fields of metallurgy, chemical industry, electronics and new energy.
The boron nitride product is excellent in high temperature resistance and electrical insulation, and can be used as an electrical insulating material at a high temperature, and has excellent thermal shock resistance. With its high thermal conductivity and penetration into microwave radiation, it can be used as a transmission window for radars in the electronics industry. The h-BN product can be used as a high-temperature metal smelting crucible, a heat-resistant material, a heat sink and a heat-conducting material, because of its high melting point, small coefficient of thermal expansion, and stability to almost all molten metals. Using the excellent thermal stability of h-BN ceramics, it can be used under repeated rapid cooling conditions from 1500 °C to room temperature [11].
The use of h-BN ceramics for good corrosion resistance to acid, alkali and glass slag, as well as the ability to neither wet nor react to most molten metals, can be used as a bismuth for the smelting of non-ferrous metals, precious metals and rare metals. Vessels, pipes, pumps and other components. The use of h-BN ceramics is both a good conductor of heat and an electrical insulator, and can be used as an ultra-high temperature insulating material. The h-BN ceramic is transparent to microwaves and infrared rays, and can be used as a window for transmissive infrared and microwave, such as radar windows [12].
The h-BN ceramic has strong neutron absorption capacity and can be mixed with various plastics and graphite in the atomic energy industry as a shielding material for the atomic stack. The h-BN ceramic has high thermal stability, chemical stability and electrical insulation, and has the characteristics of high thermal conductivity, good dielectric property and easy processing of the product. It can be combined with TiB2 to prepare a conductive ceramic evaporation boat.
In addition, h-BN is stable under ultra-high pressure and can be used as a pressure transmitting material and container. The use of h-BN is the lightest ceramic material available for high temperature structural materials for aircraft and spacecraft. The luminescence of h-BN can be used as an electroluminescent material.
In general, different grades of hexagonal boron nitride particles (nanoscale, several micron, tens of micron particles) have different applications. Among them, the nano-sized particles have small particle size and good lubricity, and will be applied to lubricating oil additives, cosmetics and other industries; several micron particles have good heat resistance, low thermal expansion coefficient and good electrical insulation, and will be applied to high temperature resistant coatings and synthetic cubic nitrogen. Boron, preparation of special ceramics and other industries; tens of micron particles with good thermal conductivity and stable chemical properties, will be used in thermal conductive materials, aerospace materials, electrical materials and other fields.
Conclusion
Since the first synthesis of hexagonal boron nitride, scholars have made many explorations on its preparation methods, structural characteristics and properties, and made a series of important progress. In recent years, with the development of multiphase ceramic technology, ceramic materials have developed into a multi-directional direction and become a research direction of today's ceramic materials. In this process, hexagonal boron nitride-based composite ceramics have also been developed accordingly. Increasing the density of h-BN ceramics and preparing h-BN ceramics with high purity and excellent performance have been the focus of research at home and abroad. In the future, the development of a synthetic method with low cost, low energy consumption, no pollution and simple process is the main development direction in the future.
references:
[1] Wan Hongbing. Preparation and Thermal Conductivity of Hexagonal Boron Nitride-Graphene Composites [D]. Chongqing: Chongqing Normal University, 2016.
[2]翟凤瑞.Discharge plasma sintering and performance of hexagonal boron nitride ceramics [D]. Beijing: Beijing University of Science and Technology, 2017.
[3] Jing Jie. Preparation and properties of hexagonal boron nitride ceramics [D]. Shandong: Shandong University, 2018.
[4] Li Chen. Preparation, Functionalization and Application of Hexagonal Boron Nitride Nanosheets [D]. Shandong: Shandong University, 2016.
[5] Wang Xiaobing. Preparation and characterization of hexagonal boron nitride and boron carbonitride materials [D]. Beijing: Beijing University of Chemical Technology, 2016.
[6] He Dongqing, Liang Jiaming, Liang Bing. Research Progress in Preparation Methods of Hexagonal Boron Nitride Particles[J]. æ料导报,2015(05) .
[7] Ding Yang, Cao Feng, Chen Li. Research Progress in Preparation of Boron Nitride Ceramic Materials by Precursor Method[J]. æ料导报,2013(05) .
[8] Wang Zheng. Effect of grain size and crystal form of boron nitride ceramics on mechanics and sputtering resistance [D]. Harbin: Harbin Institute of Technology, 2015.
[9] Geng Fengrui, Shanke et al. Sintering and structure and properties of hexagonal boron nitride ceramics[J]. Journal of The Chinese Ceramic Society, 2018(06).
[10] Gao Xiaoju, Wang Hongjie, Zhang Dahai. Preparation of Hexagonal Boron Nitride Ceramics by Reaction Sintering[J]. Aerospace Materials Technology, 2009(01).
[11] Li Hongbo. Study on mechanism and process of combustion synthesis of hexagonal boron nitride based composite ceramics [D]. Harbin: Harbin Institute of Technology, 2010.
[12] Xue Yafang. Peeling modification and properties of hexagonal boron nitride and graphite [D]. Shanghai: Donghua University, 2013.
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