Special refractory overview

With the development of science and technology, many new materials have appeared in a wide range of materials. Special refractory materials are a group of new inorganic materials developed on the basis of traditional ceramics and refractory materials, also known as high temperature ceramic materials.

The traditional ceramic production process is to make the raw materials into fine powder and reshape. The use of ceramics to make refractory materials is also a special process and should be described in Chapter 5, but due to the high purity of the chemical composition of the special refractory materials, super fire resistance, various special properties, complex product shapes, special conditions of use. Etc., can be a separate system, as a chapter is systematically described here.

The development of special refractory materials is closely related to the development of high temperature technology, especially modern high technology. Modern space technology, the development of jet propulsion equipment for high-speed aircraft (artificial earth satellites), especially the gas turbine rotating blades, nozzles, nose cones (radomes), and tail cone commutators of jet engines are directly affected by high temperature and high-speed airflow. The rupture strength, creep, oxidation resistance and other properties of refractory metals and heat-resistant alloys have reached the limit of use at high temperatures, and it is necessary to find better special refractories. Advanced metallurgical technology requires functional materials that are more resistant to high temperatures, corrosion, and thermal shock. The smelting of various new metals, special alloys and semiconductor materials requires high purity, but it is easy to react with ordinary refractory materials at the melting temperature to corrode the general refractory materials. Metallic containers are less suitable as a container for melting, distilling, pouring, alloying, or single crystal growth of these materials because they contaminate the smelted material. Modern technologies such as rockets, missiles, and electronics all require high-performance refractories. These special refractory materials have the following characteristics compared to conventional refractory materials:

(1) The material of most special refractory materials has exceeded the range of silicate, and the grade is high, the purity is high, and the melting point is above 2000 °C (individually 1728 ° C);
(2) The molding process is not limited to semi-dry molding. In addition to a large number of applications of grouting and plastic forming, isostatic pressing, vapor deposition, hot pressing, electrofusion, etc., and most of them are fine in micrometer (μm) grade. Powder
(3) The firing temperature of the product is very high (1600-2000 ° C, or even higher), and is fired in various firing atmospheres or vacuums;
(4) It is not only made into thick products such as bricks, rods, pots, etc., but also made into thin products such as tubes, plates, sheets, and enamels, hollow spherical products, highly dispersed bulk materials, and can be made transparent or translucent. Products, soft silky fibers, various jewel-like single crystals and superhard materials with hardness second only to diamonds;
(5) In addition to its fire resistance, some have better electrical, thermal, mechanical, chemical and other properties, so it is widely used in other sectors besides high temperature industry. Almost all parts of the national economy;

First, the classification of special refractory materials

Special refractory materials are summarized in five aspects

(1) a high melting point oxide;
(2) refractory compounds;
(3) cermet;
(4) high temperature inorganic coating;
(5) Fiber and its reinforcing materials;

Although a single material has its own advantages, there are also insurmountable shortcomings and weaknesses. For example, metal materials have good ductility, mechanical strength and impact toughness, but the strength of this material drops sharply at high temperatures and is extremely easy to oxidize; the properties of organic materials are ever-changing, but it is easy to age, low in strength, and not resistant to high temperatures. Inorganic non-metals, although high temperature performance, but the fatal weakness is brittle, can not withstand the impact. Therefore, several materials are combined in a certain way, so that the performance of various materials can complement each other to form a new material with comprehensive performance. One of the key elements of special refractory materials is high temperature composite materials. It includes cermets, high temperature inorganic coatings, fiber reinforced materials, and the like.

The cermet has a certain metal-like toughness, can withstand the thermal shock and mechanical shock that the ceramic cannot withstand, and has high-temperature mechanical strength like ceramics, and can withstand the high temperature that the metal cannot withstand, thus improving the ceramic. The brittleness improves the high temperature resistance of the metal and has the comprehensive properties of both metal and ceramic.

The high temperature inorganic coating is a general term for an inorganic protective layer or surface film which is applied to the surface of a metal or other structural substrate. It acts to alter the chemical composition and structure of the outer surface of the substrate, thereby imparting new or improved substrate properties. For example, a layer of high temperature resistant coating is applied on the surface of the metal material to insulate the metal substrate to increase the temperature of the metal. The heat resistant alloy or graphite is coated with a layer of anti-oxidation and chemical resistance. Corrosion coating. In recent years, there have been coatings of various functions such as high-temperature electrical insulation, high-temperature wear resistance, corrosion resistance, temperature display, temperature control, lubrication, particle radiation prevention, spectral selective absorption or emission, and infrared radiation.

The fiber (whisker) is compounded with metal, plastic or ceramic to produce a variety of reinforced composite materials with high temperature resistance, high strength and fatigue resistance.

Second, the performance of special refractory materials

Different kinds of special refractory materials, although their chemical composition and structure are different, their performances are also different, but from the general refractory materials in general, it has many excellent properties than ordinary refractory materials.

(1) Thermal properties

(1) Thermal expansion property: Thermal expansion property refers to a property in which the linearity and volume temperature of a material rise and fall reversibly. Often expressed as the number of linear expansion or volume expansion coefficient. Most of the special refractory materials have relatively large linear expansion coefficients, and only the fused silica, boron oxide, and silicon oxide have relatively small linear expansion coefficients.

(2) Mechanical properties

Special refractories have large elastic modulus. Most have high mechanical strength, but compared to metallic materials, the impact strength is very low due to brittleness. Most of the special refractory materials have high hardness, so wear resistance, airflow resistance or dust scouring is better. Most special refractory materials have relatively low temperature creep, the largest being molybdenum disilicide. The magnitude of the creep value is related to the crystal size, grain boundary material, porosity, and the like.

(3) Electrical properties

Most of the high melting point oxides are insulators, in which yttrium oxide (ThO2) and stabilized zirconia (ZrO2) have electrical conductivity at high temperatures, as shown in Table 3; carbides and borides have small electrical resistance; some nitrides are electrically Good conductors, while others are typical insulators. For example, TiN has a conductivity of metal (ρ is 30 × 10 -6 Ω·cm), and BN is an insulator (ρ is 1018 Ω·cm). All silicides are good conductors of electricity.

(4) Nature of use

(1) Fire resistance: The melting point of special refractories is almost above 2000 °C, and the highest cesium carbide (HfC) and tantalum carbide (TaC) are 3887 ° C and 3877 ° C. The degree of refractoriness is also high, and in an oxidizing atmosphere, the use temperature of the oxide is even close to the melting point. Nitrides, borides, and carbides have a higher use temperature than oxides in a neutral or reducing atmosphere. For example, TaC can be used up to 3000 ° C in an N 2 atmosphere, and BN can be used up to 2800 ° C in an Ar atmosphere. The high temperature resistance performance is: carbide > boride > nitride > oxide. Their high temperature oxidation resistance is: oxide > boride > nitride > carbide.

(2) Thermal shock resistance: In special refractory materials, due to the low thermal conductivity of cerium oxide, the thermal conductivity of most borides is not high, and the linear expansion coefficient of fused silica is particularly small, so the thermal shock resistance is very high. it is good. Some fiber products and fiber reinforced composite products have high porosity and tensile strength, and these materials have better thermal shock resistance. Silicon carbide, silicon nitride, boron nitride, molybdenum disilicide, etc. also have good thermal shock resistance.

Third, the structure of special refractory materials

A special refractory material is a polycrystalline material. The microstructure of most of the special refractory materials is composed of crystal phase, but no glass phase. Some special refractories are mixed with trace impurities, forming a eutectic liquid at a certain temperature. The grain boundary is formed at the place where the grain meets the grain, which is called the grain boundary. For the polycrystal composed of small grains, the volume of the grain boundary accounts for almost half or more, which has a significant influence on the properties of the crystal. When the crystal grains are fine, the material has high mechanical strength, and the coarse crystals are liable to cause cracks and defects, so that the mechanical strength of the material is lowered. The structure of special refractory materials also contains a certain amount of pores, which also affects the material properties. Therefore, it is generally required that the structure of the special refractory material is uniform, the glass phase is small, and the crystal grains are fine and uniform.

Fourth, the use of special refractory materials

(1) Special refractory materials have been widely used as structural materials and functional materials for high temperature engineering.

(2) In the metallurgical industry, it is widely used in parts resistant to high temperature, oxidation, reduction or chemical corrosion; smelting rare metals, precious metals, refractory metals, ultra-pure metals, special alloys, vessels, boats and other containers; Casting separation ring, filter device for molten metal, and conveying pipe.
(3) In the aerospace and flight technology, the head cover for the rocket missile, the combustion chamber lining, the tail pipe bushing, the turbine blade of the jet aircraft, the exhaust pipe, the fuselage, the structural components of the wing Wait.
(4) In the electronics industry, it is used as a container for melting high-purity semiconductor materials and single crystal materials, a semiconductor solid diffusion source; various high-temperature-resistant insulating heat-dissipating components in electronic equipment; an integrated circuit substrate, and an evaporation coating film Conductive boat etc.
(5) In the high temperature industry, it is used as a high-temperature heating element, furnace tube, furnace structural material and thermal insulation material for special electric furnaces, and internal and external protection sleeves for various thermocouples.
(6) Used in the machinery and defense industry as abrasives, abrasive tools, cutting tools, armored protective plates, etc.
(7) In the chemical and light industry departments, it is used as a submersible pump and chemical pump mechanical seal ring; glass wire drawing, flow trough and glass pool kiln brick.
(8) In the medical and agricultural sectors, it is used as artificial joints, artificial teeth, and uses its special properties such as biocompatibility.

V. Prospects for special refractory materials

From the perspective of the microstructure of the material, structural non-uniformities, such as microcracks, micro-defects, grain boundaries and interfacial properties, will be used to obtain some special properties. Translucent ceramics have a lot to do with the development of optical communication in the future. The use of ceramic solid electrolytes (βAl2O3, ZrO2, etc.) will be further expanded, and it will be promising in terms of energy, environment, and pollution. Superhard materials such as synthetic diamond and cubic boron nitride will enter the industrial production stage. The replacement of metals with special refractory materials, such as Si3N4 and Al-Si-NO replacement metals for bearing and turbine blades, will be greatly developed. Efficient composite materials are promising and will be greatly developed.

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