The opening of special refractory materials is closely related to the opening of high temperature skills, especially modern high-tech skills. Modern spatial skills, high speed flight vehicle (artificial earth satellite) eruption with open, especially the bulk of the gas turbine engine rotating blades, nozzle, before the cone (radome), tail cone commutator direct effect of high temperature, high speed air flow, such as refractory metals and heat resistant alloy rupture strength and creep at high temperature and oxidation resistance function to use the limit, It is necessary to find better specialty refractories. Advanced metallurgical skills, demand more high temperature resistance, corrosion resistance, thermal shock resistance of the function data. Training all kinds of new metals, special alloys and semiconductor data purity requirements are very high, but at the melting temperature and general fire resistance data reaction and general fire resistance data corrosion. Metallic containers are not suitable for melting, distilling, billowing, alloying or single crystal growth of these materials, as they may contaminate the training materials. There are rockets, missiles, electronics and other modern skills require high functional refractory materials. These special refractories have the following characteristics compared with traditional refractories:
(1) most of the special refractory material is now beyond the silicate scale, and high level, high purity, melting point is above 2000℃ (a single 1728℃);
(2) the molding process is not limited to semi-dry molding, in addition to many use of grouting method and plastic method molding, also choose isostatic pressing, vapor deposition, hot pressing, electromelting, and most of the use of micron (μm) level of fine powder;
(3) the product firing temperature is very high (1600 ~ 2000℃, or even higher), and in a variety of firing atmosphere or vacuum firing;
(4) It is not only made of bricks, rods, POTS and other solid products, but also made of tubes, plates, chips, crucible and other thin products, hollow spherical products, highly loose loose materials, but also can be made of transparent or translucent products, soft as silk fiber, a variety of gem-like single crystal and the hardness of super hard data second only to diamond;
(5) It in addition to fire resistance function, some also have better electrical, thermal, mechanical, chemical and other functions, so it in addition to high temperature industry, but also widely used in other parts. Almost every sector of the national economy;
A, special refractory classification
Special refractory materials are summarized into five aspects
(1) high melting point oxide;
(2) refractory compounds;
(3) cermet;
(4) High temperature inorganic coating;
(5) Fiber and its reinforcement data;
Although single data has its own advantages, it also has its own disadvantages. For example, the metal material has excellent ductility, mechanical strength and impact resistance, but the strength of this material decreases sharply at high temperature and is very simple oxidation; The function of organic materials is changeable, but it is easy to aging, low strength, not high temperature resistance; Although inorganic nonmetals have good function at high temperature, they have the disadvantage of being brittle and unable to withstand impact. Therefore, several kinds of data with certain methods compound together, so that the function of all kinds of data, and the composition of a general function of the new data. One of the key contents of special refractory materials is high temperature composite materials. It contains cermet, high temperature inorganic coating, fiber reinforced data, etc.
Metal ceramic both must have the same patience as metal, can be endured ceramic cannot be endured by the thermal shock and mechanical shock, has high mechanical strength, like ceramic can withstand the metal cannot be endured high temperature, thus not only improved the brittleness of ceramics, and improved the high temperature resistance of metals, metal and ceramic generalization of the function.
High temperature inorganic coating is a kind of inorganic protective layer or outer membrane coated on the surface of metal or other structures. It has the effect of changing the chemical composition and structure of the surface of the substrate, and then giving new or improving the function of the substrate. If a layer of high temperature resistant coating is added to the surface of the metal data, the heat insulation effect on the metal substrate is improved, so that the temperature of the metal is relatively higher; There are heat resistant alloys or graphite and other materials coated with a layer of oxidation resistance, chemical corrosion resistance coating. In recent years, there have been high temperature electrical insulation, high temperature wear resistance, corrosion resistance, temperature indication, temperature control, lubrication, particle radiation, spectral selection absorption or emission, infrared radiation and other effects of the coating.
With fiber (whisker) and metal, plastic or ceramic composite, can produce high temperature resistance, high strength, fatigue resistance and other excellent functions of various enhanced composite materials.
Two, the function of special refractory materials
All kinds of different special refractories, although the chemical composition and structure are different, their function also has certain differences, but from the special refractories all have many excellent functions than the general refractory materials.
(1) Thermal properties
(1) Thermal expansibility: thermal expansibility refers to the function of reversibly increasing and decreasing the linear and volume temperature rise and fall of data. Often indicated by linear expansion number or volume expansion coefficient. Most special refractory materials have relatively large linear expansion coefficient, only fused quartz, boron oxide, silicon oxide linear expansion coefficient is relatively small.
(2) Mechanical properties
The elastic modulus of special refractories is large. Most have high mechanical strength, but low impact strength due to brittleness compared to metal data. The vast majority of special refractory materials have higher hardness, so wear resistance, airflow resistance or dust scour is better. The high temperature creep of most special refractories is relatively small, with the largest being molybdenum disilicide. The size of creep is related to crystallization size, grain boundary material, porosity, etc.
(3) electrical properties
Most high melting point oxides belong to insulators, among which thorium oxide (ThO2) and stable zirconia (ZrO2) have conductivity at high temperature, as shown in Table 3. The resistance of carbide and boride is very small. Some nitrides are good conductors of electricity, while others are typical insulators. For example, TiN has a metal conductivity (ρ is 30×10-6 ω ·㎝), BN is an insulator (ρ is 1018 ω ·㎝). All silicides are good conductors of electricity.
(4) Nature of application
(1) Fire resistance: the melting point of special refractories is almost above 2000℃, and the highest hafnium carbide (HfC) and tantalum carbide (TaC) are 3887℃ and 3877℃. Refractoriness is also very high, in the oxidation atmosphere, the application temperature of oxide even close to the melting point. Nitrides, borides and carbides have higher operating temperatures than oxides in neutral or restorative atmospheres. For example, TaC can be applied to 3000℃ in N2 atmosphere, and BN can be applied to 2800℃ in Ar atmosphere. The function of high temperature resistance is in order: carbide > boride > nitride > oxide. And their high temperature oxidation resistance is: oxides > borides > nitrides > carbides.
(2) thermal shock resistance: in special refractories, because the thermal conductivity of beryllium oxide is low, the thermal conductivity of most borides is not high, and the linear expansion coefficient of fused quartz is particularly small, so the thermal shock resistance is very good. Some fiber products and fiber reinforced composite products have higher porosity and tensile strength, and these data have better thermal shock resistance. Silicon carbide, silicon nitride, boron nitride, molybdenum disilicate also have good thermal shock resistance.
Three, special refractory material arrangement structure
Special refractory material is a polycrystalline material. The microstructure of most special refractory materials is composed of crystal phase, and does not contain glass phase. A single special refractory mixed with trace impurities constitutes eutectic liquid phase at a certain temperature. The area where grains meet constitutes the distance between grains, referred to as grain boundaries. For polycrystals composed of small grains, the volume of grain boundaries accounts for almost more than half, which has an obvious influence on crystal properties. When the grain is fine, the material has a high mechanical strength, while the coarse grain simply forms cracks and defects, so that the mechanical strength of the material decreases. The structure of special refractories also contains certain porosity, which also affects the data function. Therefore, it is generally required that the arrangement structure of special refractories is uniform, the glass phase is less, and the grains are fine and uniform.
