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The physicochemical properties of boron carbide

Release time:

21 Oct,2022

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It does not react with acidic or alkaline solutions, has high chemical stability, neutron absorption, wear resistance, and semiconductor conductivity. It is one of the substances stable against acids and remains stable in all concentrated or diluted acidic or alkaline aqueous solutions. After treatment with a mixed acid of sulfuric acid and hydrofluoric acid, it can be completely decomposed and form carbon trioxide and boron trioxide when calcined in air at 800°C for 21 hours.

It has special stability when some transition metals and their carbides coexist.

Boron carbide reacts strongly with transition metals from groups IV, V, and VI of the periodic table at temperatures between 1000 and 1100°C to form metal borides.

In the presence of nitric acid, boron carbide easily decomposes and forms a solution when melted with sodium hydroxide, potassium hydroxide, sodium carbonate, or potassium carbonate.

Its Mohs hardness is 9.3, making it the fifth known hardest material after boron nitride, diamond, fullerene compounds, and diamond-like carbon.

The properties of boron carbide.

Applications of boron carbide

The properties of boron carbide.

Applications of boron carbide

Oct 21,2022

Boron carbide has a high melting point (2450℃), high hardness (second only to diamond and boron nitride, ~29.1 GPa), high elastic modulus, low density (2.52 g/cm3), good thermal stability, and a high thermal neutron absorption cross-section (6×10−22 cm−2), making it an excellent special ceramic hard material.

The fundamental properties, uses, and chemical stability of boron carbide.

Oct 21,2022

Boron carbide crystals have a rhombohedral structure, and the lattice belongs to the D3d5-R3m space group. Its rhombohedral structure is shown in Figure 7 and can be described as a cubic unit cell lattice extending in the spatial diagonal direction, forming appropriately regular dodecahedra at each corner. Parallel to the spatial diagonal, it becomes the hexagonal c-axis, composed of three boron atoms connecting with adjacent dodecahedra to form a linear chain. Therefore, the unit cell contains 12 dodecahedral orientations, with three orientations located on the linear chain. If the B atoms are considered as positions caused by the dodecahedra, and the C atoms are seen as being on the linear chain, then the chemical formula for B12C3 is B4C. 1. Basic properties and uses of boron carbide 1) Low density B4C has a low density of 2.52 g/cm3. In the homogeneous region, the relationship between density and carbon content can be expressed by the formula (9): ρ = 2.4224 + 0.00489C% (9) Due to the low density of boron carbide, it can achieve high density while maintaining excellent properties such as high strength and hardness, making it suitable for use as lightweight armor to reduce the weight of tanks and other vehicles, thus saving energy consumption. 2) Hardness and wear resistance B4C possesses super hardness and extremely high wear resistance. In the homogeneous region, the Vickers hardness of B4C increases with the addition of carbon content. When the carbon content is 10.6%, the hardness is 29.1 GPa; when the carbon content is 20%, the hardness can reach 37.7 GPa. Its hardness remains very high (>30 GPa) at elevated temperatures. The variation of hardness with temperature can be expressed by the formula (10): H = H0 - exp(-aT) (10) Where: H0 is the hardness at room temperature; T is the temperature; a is a constant related to carbon content. This formula is applicable in the range of 20~1700°C. It should be noted that B4C is one of the hardest materials in the world, second only to diamond and cubic BN. The wear resistance of B4C increases with temperature. In the range of 20~1400°C, the friction coefficient decreases as the temperature rises, dropping to about 0.05 around 1400°C, and the wear rate also continuously decreases. Due to its super hardness and friction characteristics, B4C has been used as a sandblasting nozzle, diamond nozzles, and nozzles for hydraulic jet cutters, among other wear-resistant materials. It is widely used in military applications for armor materials in tanks, aircraft, etc. With advancements in technology and increasing demand for high-precision grinding, B4C has increasingly demonstrated its superiority, and its usage has been continuously increasing in recent years. Additionally, B4C can also be used for grinding hard alloys, ceramics, and hard gemstones, as well as for abrasive materials used in free abrasives or ultrasonic processing of these super hard materials. However, compared to Europe and the United States, the usage in China is still relatively low. 3) Coefficient of thermal expansion and specific heat capacity The melting point of boron carbide is 2450°C, and the boiling point is 3000°C. The coefficient of thermal expansion is 5.73×10-6/°C (28~1770°C), and the specific heat capacity can be calculated using the formula (11): C = 22.99 + 5.40×10-3T - 10.72×10^5T-2 (11) 2. Chemical stability Boron carbide is one of the most stable compounds, not easily undergoing oxidation reactions below 600°C; above 600°C, it forms a B2O3 film on the surface, preventing further oxidation of B4C. Therefore, B4C is currently used as an antioxidant in refractory materials. At room temperature, B4C generally does not react with chemical reagents; above 800°C, B4C reacts with Br to form tribromide compounds; at high temperatures, B4C reacts with metal oxides to generate metal borides and carbon monoxide, with the resulting FeB film exhibiting very high microhardness (HV=24 GPa) and wear resistance. Thus, B4C can be used for boronizing steel and alloys.

The fundamental properties, synthesis, and performance of boron carbide.

Oct 21,2022

Boron carbide is a general term for compounds of carbon (C) and boron (B). Depending on the cooperative conditions, two compounds, B4C and B6C, can be generated, with B4C being the one commonly referred to as boron carbide. 1. Fundamental Properties of Boron Carbide B4C belongs to the trigonal crystal system, with 12 B atoms and 3 C atoms in the unit cell. The C atoms in the unit cell form a connecting spatial diagonal configuration, and C is in a dynamic state, which can be replaced by B atoms, forming a substitutional solid solution, and may leave the lattice to form high-boron compounds with defects. The molecular weight of B4C is 52.25, containing 21.74% C and 78.26% B. It generally appears in gray to black color, with a density of 2.519 g/cm3, a Mohs hardness of 9.36, and a microhardness of about 50 GPa, second only to diamond and cubic boron nitride. Therefore, B4C powder has a very high grinding ability, with its grinding efficiency reaching 60%-70% of that of diamond, higher than SiC by 50%, and 1-2 times that of corundum. The melting point of B4C is 2450°C (decomposes). The coefficient of thermal expansion between 1000°C is 4.5×10-6°C-1. The thermal conductivity at 100°C is 121.4 W/m·k, and at 700°C it is 62.79 W/m·k. B4C is mainly used as an abrasive material; hot-pressed B4C products can be used for wear-resistant and heat-resistant components. In the refractory materials industry, B4C is mainly used as an additive, such as being added to carbon-bonded refractory materials to act as an antioxidant, and added to amorphous materials to enhance the strength and corrosion resistance of the body. 2. Composition and Typical Functions of Boron Carbide The commonly used method for producing B4C powder industrially is by reducing boron oxide with excess carbon: 2B2O3 + 7C → B4 + 6CO↑ The synthesis reaction can be carried out in a resistance furnace or an electric arc furnace. In the resistance furnace, boron oxide B2O3 and carbon C are heated below the decomposition temperature of B4C, resulting in B4C containing very little free C (sometimes containing 1%-2% free boron), which is a better synthesis method. In the electric arc furnace, due to the high temperature of the arc, B4C decomposes into a carbon-rich phase and boron at around 2200°C, and boron will evaporate at high temperatures, resulting in the reaction products containing a large amount of free C (20%-30%), thus the quality of the obtained B4C is slightly inferior. When synthesizing B4C in an electric arc furnace, boric acid (content greater than 92%), artificial graphite (fixed carbon greater than 95%), and petroleum coke (fixed carbon greater than 85%) are generally selected as materials. The amount of boric acid added is about 2% higher than the theoretical amount calculated based on the reaction formula, with artificial graphite and petroleum coke each accounting for 50% of the total carbon input, and then being 3%-4% higher than the theoretical amount. The three prepared materials are mixed in a ball mill and then fed into the electric arc furnace for reduction and carbonization at 1700-2300°C to obtain B4C. Finally, the molten mass is selected and subjected to washing, crushing, grinding, acid washing, and sedimentation classification to obtain various particle sizes of B4C.

Boron carbide powder materials from Mudanjiang account for 40% of the international market.

Oct 21,2022

In recent years, Mudanjiang City has actively developed the new materials industry, which has now taken shape—there are 48 enterprises above designated size, accounting for about 7% of the industrial proportion. The main products include four major series: hard materials, special fibers and composite materials, new chemical raw materials, and new functional building materials. Mudanjiang has rich mineral resources, with 80 types of minerals discovered and proven reserves of 41 types, accounting for 47.1% of Heilongjiang Province and 17.5% of the country. It has abundant energy resources such as coal and oil shale, as well as metal resources like iron, copper, and gold, and non-metal resources including graphite, cordierite, wollastonite, calcite, quartz sand, refractory clay, basalt, granite, perlite, pumice, volcanic ash, and marble. Through recent development, the variety of new materials industry products in Mudanjiang has evolved from a single type of boron carbide to dozens of types including silicon carbide, boron carbide, industrial silicon, special ceramic materials, and finished products. Boron carbide powder materials account for 40% of the international market and 80% of the domestic market; 80% of industrial finished products are exported, accounting for 15% of the international market and 80% of the domestic market. Mudanjiang is the largest export base for international green silicon carbide powder materials, with an annual export volume of over 20,000 tons, accounting for 60% of the international market. The special materials industrial base in Mudanjiang has been recognized as a national high-tech industrialization base, becoming one of the three national high-tech industrialization bases in Heilongjiang Province. Mudanjiang is also an important production area for graphite in our province, with proven graphite ore reserves reaching 240 million tons. During the 13th Five-Year Plan period, there are plans to build three deep processing projects for graphite. Mudanjiang is also actively promoting the research and development of new materials and has become a national "863 Program" special materials industrial base designated by the Ministry of Science and Technology. The Jindiamond Boron Carbide Company has a postdoctoral research station and a provincial-level enterprise technology center. The Chenxi Boron Carbide Company has established a production, learning, and research alliance with the Shanghai Institute of Ceramics, Chinese Academy of Sciences. It has several research and development organizations, including the Provincial Paper Industry Research Institute and the Hard Alloy Research Institute. The Provincial Key Laboratory for New Carbon-Based Functional and Superhard Materials at Mudanjiang Normal University currently has 33 doctoral and master's degree holders, undertaking 25 provincial and ministerial-level research projects, and has obtained 3 national patents, mainly engaged in applied research on diamond films and related materials, low-dimensional materials, and electronic functional materials.

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