What Are the Different Types of Raw Materials for Ceramics?

Ceramic materials refer to a class of inorganic non-metal materials made of natural or synthetic compounds through shaping and high-temperature sintering. It has the advantages of high melting point, high hardness, high wear resistance, and oxidation resistance. It can be used as a structural material and a cutter material. Since ceramic also has some special properties, it can also be used as a functional material.

Ceramic material

Mechanical properties

Ceramic materials are the materials with the best stiffness and highest hardness among engineering materials, and their hardness is mostly above 1500HV. Ceramics have higher compressive strength, but lower tensile strength, and poor plasticity and toughness.

Thermal characteristics

Ceramic materials generally have a high melting point (mostly above 2000 ° C), and have excellent chemical stability at high temperatures; the thermal conductivity of ceramics is lower than that of metal materials, and ceramics are also good thermal insulation materials. At the same time, the linear expansion coefficient of ceramics is lower than that of metals. When temperature changes, ceramics have good dimensional stability.

Electrical characteristics

Most ceramics have good electrical insulation, so they are widely used to make insulation devices of various voltages (1kV ~ 110kV). Ferroelectric ceramics (barium titanate BaTiO3) have a high dielectric constant and can be used to make capacitors. Ferroelectric ceramics can change the shape and convert electrical energy into mechanical energy (with the characteristics of piezoelectric materials) under the action of an external electric field. It can be used as loudspeaker, record player, ultrasonic instrument, sonar, medical spectrometer, etc. A few ceramics also have semiconductor characteristics and can be used as rectifiers.

Chemical properties

Ceramic materials are not easily oxidized at high temperatures, and have good corrosion resistance to acids, alkalis, and salts.

Optical characteristics

Ceramic materials also have unique optical properties. They can be used as solid laser materials, optical fiber materials, optical storage, etc. Transparent ceramics can be used for high-pressure sodium lamps. Magnetic ceramics (such as ferrites: MgFe2O4, CuFe2O4, Fe3O4) have a wide range of applications in audio tapes, records, transformer cores, and large-scale computer memory components.

Heat radiation

The basic ways of heat exchange are: conduction, convection and radiation. In order to effectively dissipate heat, people often achieve it by reducing the thermal resistance of the heat flow path and enhancing the convection coefficient, and often ignore the heat radiation. LED lamps generally use natural convection to dissipate heat. The heat sink quickly transfers the heat generated by the LED to the surface of the heat sink. Due to the low convection coefficient, the heat cannot be dissipated to the surrounding air in time, which causes the surface temperature to rise and the working environment of the LED to deteriorate. . Increasing the emissivity can effectively remove the heat from the surface of the radiator through thermal radiation. Generally, aluminum radiators improve the surface emissivity by anodizing, and the ceramic material itself can have high emissivity characteristics without the need for complicated subsequent processing. .

Radiation mechanism

The radiation mechanism of ceramic materials is produced by the two-phonon and multi-phonon with non-resonant effects of random vibration. High-radiation ceramic materials such as silicon carbide, metal oxides, and borides all have strong infrared-activated polar vibrations. These polar vibrations have strong anharmonic effects, and their absorption coefficients in the dual frequency and frequency regions are generally It has an order of magnitude of 100 ~ 100cm-1, which is equivalent to the lower reflectance of the remaining reflection band of the medium-intensity absorption region in this region. Therefore, it is beneficial to form a relatively flat and strong radiation band.

Generally speaking, the radiation band with high thermal radiation efficiency extends from the strong resonance wavelength to the short-wave entire two-phonon combination and frequency region, including some multi-phonon combination regions. This is a common feature of most high-radiation ceramic material radiation bands. It can be said that the strong radiation band is mainly derived from the two-phonon combined radiation in this band. With few exceptions, the radiation band of general radiating ceramics is concentrated in the two-phonon and three-phonon regions greater than 5m. Therefore, for infrared radiation ceramics, the radiation in the 1 ~ 5m band mainly comes from the in-band transition of free carriers or the direct transition of electrons from the impurity energy level to the conduction band. radiation.

Liu Weiliang and Luo Suming studied the infrared radiation of normal temperature ceramics. The infrared emissivity of the ceramic samples tested was about 0.82 ~ 0.94, and the far infrared ceramic glazes with different surface qualities were also tested. The emissivity was about 0.6 ~ 0.88. The SEM photos show that the radiation performance, glaze quality, color and cost are better when the far-infrared ceramic powder is added to the glaze at 10wt%, and its emissivity reaches 0.83, and other properties meet the national daily porcelain standards. Cui Wanqiu and Wu Chunyun tested the low-temperature far-infrared ceramic block samples, and the infrared emissivity was 0.78 ~ 0.94. Research by Li Hongtao and Liu Jianxue found that the emissivity of normal temperature far infrared ceramics can reach 0.85, and the highest emissivity of foreign Enecoat glaze coatings can reach 0.93 ~ 0.94. Numerous studies have shown that ceramic materials or glazes have high infrared emissivity, which is an important parameter to replace traditional aluminum radiators. ^[1]

The original ceramic refers to the general name of pottery and porcelain. That is, a shaped sintered body obtained by molding and high temperature sintering. Traditional ceramic materials mainly refer to aluminosilicates. At the beginning, people did not have high requirements for the choice of aluminosilicate, the purity was not large, the particle size was not uniform, and the molding pressure was not high. The ceramic obtained at this time is called traditional ceramic. Later, it developed to high purity, small and uniform particle size, high molding pressure, and the sintered body obtained by sintering was called fine ceramics.

In the next stage, people studied the foundation of the ceramic materials that make up ceramics, and the concept of ceramics changed a lot. The internal mechanical properties of ceramics are related to the chemical bond structure of the materials constituting the ceramics. Chemical substances that can form a relatively strong three-dimensional network structure when crystals are formed can be used as ceramic materials. This mainly includes ionic compounds with relatively strong ionic bonds, elements and compounds capable of forming atomic crystals, and substances that form metal crystals. They can all be used as ceramic materials. Secondly, people have developed fiber-reinforced composite materials by drawing on the characteristics of three-dimensional bonding. The range of ceramic materials has been further expanded. Therefore, ceramic materials have developed into a general term for materials that can be bonded in three dimensions.

The concept of ceramics has evolved into a sintered body that can be obtained by molding and high temperature sintering with the aid of three-dimensional bonding materials. (This concept also includes glass into the scope of ceramics)

The theory for studying the structure and properties of ceramics has also been unfolded: the influence of ceramic materials, internal microstructures (microcrystalline surface effect, porous multiphase distribution) on mechanical properties has been developed. The relationship between material (optical, electrical, thermal, magnetic) properties and forming, as well as the particle size distribution, and the bonding interface have also been developed. Ceramics should become the existence form of substances with certain properties. It should be related to disciplines such as quantum mechanics, nanotechnology, and surface chemistry. The ceramics discipline became a comprehensive discipline.

This development is related to polymer molding to some extent. They should influence each other.