The application of rare earths and rare earth oxides in ceramic materials is mainly used as additives to improve the sinterability, compactness, microstructure and crystal phase composition of ceramic materials, thereby greatly improving their mechanics and electrical properties. , Optical or thermal properties to meet the performance requirements of ceramic materials used in different occasions. This article briefly reviews the applications of rare earth oxides in structural ceramics and functional ceramics.
1 Mechanism of rare earth oxides in ceramic materials
2 Application of rare earth oxides in structural ceramic materials
Structural ceramics refer to a class of ceramic materials mainly composed of ionic and covalent bonds between grains, which have good mechanical properties, high temperature and biocompatibility. Structural ceramics are widely used in daily life, and they have been expanded to high-tech fields such as aerospace, energy environmental protection and large and medium-sized integrated circuits.
2.1 Oxide ceramics
Oxide ceramics refer to ceramics containing oxygen atoms in ceramics, or ceramics formed from various simple oxides having a melting point higher than the crystal melting point of silicon dioxide (SiO2: melting point 1730 ° C). Oxide ceramics have good physical and chemical properties, and their electrical conductivity is inversely proportional to temperature. Oxide ceramics are often used as heat-resistant, wear-resistant and corrosion-resistant ceramics, and are used in chemical, electronic and aerospace fields.
2.1.1 Alumina ceramics
Alumina ceramics are widely used in the manufacture of circuit boards, vacuum devices, and ceramic packages for semiconductor integrated circuits. In order to obtain a ceramic with good performance, it is necessary to refine the crystal grains and distribute them as equiaxed crystals, reduce the porosity of the ceramics, and increase the density. It is best to reach or approach the theoretical density. The high sintering temperature of alumina ceramics and the high price of high-purity alumina for firing raw materials limit its promotion and application in some fields. Studies have shown that the addition of rare earth oxides can form a liquid or solid solution with the matrix oxide, reduce the sintering temperature, and improve its mechanical properties. Commonly used rare earth oxide additives are Dy2O3, Y2O3, La2O3, CeO2, Sm2O3, Nd2O3, Tb4O7 and Eu2O3.
2.1.2 Zirconia ceramics
Zirconium oxide (ZrO2) has three crystal forms: monoclinic phase, tetragonal phase and cubic phase. At a certain temperature, when the crystal form of zirconia undergoes volume expansion and shear strain, the volume expansion may cause the product to crack. Zirconia has a high melting point, strong resistance to acid and alkali erosion, good chemical stability, high bending strength and fracture toughness. The mutual conversion of the three crystal forms will accompany the expansion or contraction of the volume, leading to unstable performance, and stabilization measures must be taken. Adding a rare earth oxide as a stabilizer to zirconia can form a stable cubic zirconia solid solution after high temperature treatment, and can also improve its toughness, strength and conductivity. Zirconia ceramics have been widely used in refractory materials for converter steelmaking, medical artificial teeth, various sensors and high-temperature heating elements.
2.2 Nitride ceramics
2.2.1 Silicon nitride ceramics
Silicon nitride has poor thermal conductivity, high thermal expansion coefficient, strong creep resistance at high temperature, and good chemical stability at high temperature. Based on excellent performance, it is highly practical in high-temperature ceramic bearings, radomes, support parts for nuclear reactors, and corrosion-resistant components in chemical processes. Pure silicon nitride is difficult to sinter, and due to covalent bonding, its diffusion coefficient is small and atom migration is difficult. Therefore, a sintering aid must be added during the sintering process to promote the sintering reaction of silicon nitride. With the rapid development of the electronics industry, the demand for electronic materials continues to increase, and improving the thermal conductivity of silicon nitride ceramics has become one of the research hotspots. The addition of rare earth oxide can effectively improve the disadvantages of low plastic toughness and poor stability of silicon nitride ceramics.
2.2.2 Aluminum nitride ceramics
Aluminum nitride (AlN) is a hexagonal wurtzite structure, which has good thermal shock resistance, insulator, low thermal expansion coefficient and mechanical properties. The theoretical thermal conductivity is 320W · m-1 · K-1, but its oxidation resistance Very poor. When the aluminum nitride crystal lattice is defect-free, the structure is dense, scattering of phonons is very weak, the average free path of phonons is large, and the thermal conductivity is high. Theoretically, if you want to obtain high thermal conductivity, you should reduce the additives as much as possible, but actual research has found that the addition of a small amount of rare earth oxides is not only helpful to stimulate the theoretical thermal conductivity, but also effectively promote aluminum nitride ceramic Sintering.
2.3 Carbide ceramics
2.3.1 Silicon Carbide Ceramics
Silicon carbide has a small self-diffusion coefficient, it is difficult to sinter without adding a sintering aid, and it is difficult to sinter a dense structure even at high temperature and pressure. The addition of sintering additives can form a liquid phase, reduce the sintering temperature, promote the densification of the sintered body structure, and can improve the purity, particle size and phase composition of silicon carbide. For example, after Sc2O3 and AlN are added in combination, the SiC sintered silicon carbide grain boundary has no glass phase and is clean, but its strength and toughness are very low. The addition of Al2O3-Y2O3 can not only improve the density of silicon carbide ceramics, but also Improve the brittle toughness, strength and hardness of ceramics.
2.3.2 Boron Carbide Ceramics
Boron carbide (B4C) has a high melting point, a low coefficient of thermal expansion, and excellent thermal stability. It is widely used in the production of neutron absorbing materials (shielding plates, control rods, etc.), thermocouples and various nozzles. However, its brittleness, poor plasticity, small specific surface area, large grain boundary movement resistance, and high sintering temperature (above 2000 ° C) make it difficult to sinter and achieve densification of boron carbide ceramics. In order to obtain dense boron carbide ceramics, rare earth oxides and other sintering aids are often added in the sintering process to promote sintering and densification, and at the same time improve the strength, fracture toughness and oxidation resistance of the ceramic.
3 Application of rare earth oxides in functional ceramics
Functional ceramics refer to a class of ceramic materials with specific uses and functions. Its characteristics are mainly manifested in electricity, light, magnetism, heat, biology, etc., in microelectronic technology, fuel cells, military industry, nuclear energy industry, and biomedicine. The high-tech field has an irreplaceable status, mainly including electronic ceramics, porous ceramics, gradient ceramics, nano-ceramics and biomedical ceramics.
3.1 capacitor ceramics
The most widely used capacitor ceramics are ceramics based on barium titanate and lead titanate-based solid solutions, which have a wide temperature stability interval (-55 to 125 ° C). The dielectric constant is taken at the Curie temperature. The maximum value, but it does not have a linear relationship with temperature. Commonly used in various capacitors, sensors, ultrasonic transducers and other BST (Ba0.65Sr0.35TiO3) ceramics. The sintering temperature is higher than 1350 ° C. Adding a low melting point sintering aid can reduce the sintering temperature, increase the dielectric constant and reduce the dielectric constant. Electricity loss.
3.2 Piezoelectric ceramics
Piezoelectric ceramics are polycrystalline crystals formed by mixing several oxides and sintering at high temperatures. They have a piezoelectric effect and are accompanied by energy conversion. It is mainly based on lead zirconate titanate. Because it contains PbO components and is highly volatile, it poses serious harm to the human body and the environment. In order to reduce environmental pollution and protect human health, research on lead-free piezoelectric ceramics has been highly valued. Na0.5Bi0.5TiO3 (BNT) -based lead-free piezoelectric ceramics are considered as one of the ceramic materials with great potential for development, but BNT ceramics have high electrical conductivity, are not easily polarized, have a narrow sintering temperature range and are difficult to control. High temperatures It is volatile, etc., so it is difficult to achieve practical use of BNT alone. Doping an appropriate amount of rare earth oxide is beneficial to promote the growth of crystal grains and can effectively improve the ceramic ferroelectric and piezoelectric properties.
3.3 Varistor ceramics
Varistor ceramics refer to semiconductor ceramics that have non-linear volt-ampere characteristics under certain conditions and whose resistance is sensitive to voltage changes. Pressure-sensitive ceramics are used in silicon rectifiers, integrated circuits, and overvoltage protection devices. Zinc oxide semiconductor ceramics are the most widely used in medium and high voltage varistors, which have the advantages of small leakage current, absorption of noise, and inrush current. The main defect is interstitial zinc ions; the addition of rare earth oxides suppresses the growth of grains. Large, can significantly improve the non-linear coefficient.
3.4 Transparent ceramics
Transparent ceramic refers to a ceramic material with a certain transparency. Transparent light functional materials are mainly single crystal and glass, and transparent ceramics have excellent light transmittance (for example, the total light transmittance of Al2O3 ceramics reaches 95%). To improve the transparency of ceramics, the reflection loss, absorption loss and scattering loss of light should be reduced as much as possible. A dense structure should be obtained. Residual pores should be eliminated, the grain size should be controlled, and the grain boundaries should be controlled. Anisotropy, etc. Transparent ceramics have both good transparency and good dielectric properties, mechanical properties and thermal conductivity of ordinary ceramics. Adding additives, such as: La2O3, MgO, and ZrO2, etc., can obtain a completely dense structure and also improve its light transmission.
3.5 Gas-sensitive ceramics
Since the 1970s, many studies have been done on the role of rare earth oxides in the gas-sensitive ceramic materials such as ZnO, SnO2 and Fe2O3, and ABO3 and A2BO4 rare earth composite oxide materials have been prepared. Some research results show that the addition of rare earth oxides to ZnO can significantly increase its sensitivity to propylene; and the addition of CeO2 to SnO2 can obtain sintered components sensitive to ethanol.
3.6 Smart Ceramics
Smart ceramics refer to a class of functional ceramics with features such as self-diagnosis, self-adjustment, self-recovery, and self-conversion. For example, lead lanthanum zirconate titanate (PLZT) ceramics obtained by adding rare earth lanthanum to lead zirconate titanate (PZT) ceramics are not only an excellent electro-optical ceramic, but also because of their shape memory function, they exhibit shape self-recovery. The self-tuning mechanism is also an intelligent ceramic. The proposal of the concept of intelligent ceramic materials advocates a new concept of research and development of ceramic materials.
3.7 Bioceramics
Bioceramic is a kind of ceramic material with special properties and specific functions. It is mainly used to make up for a class of repairing materials that are caused by illness or injury. It can effectively treat human diseases and maintain human health. And prolong life. Clinical medicine has found that alloys can meet the strength requirements, but they are not biologically active. Materials such as hydroxyapatite (HA) and tricalcium phosphate have good biological activity, and contain a large number of elements of human bone composition. Low strength and high brittleness cannot meet the mechanical properties of living organisms. Therefore, the preparation of bio-coated alloy composite ceramic materials is very important for the development of medical materials.
3.8 Antibacterial ceramics
Antibacterial ceramics refer to a class of functional ceramics that have a bactericidal effect on the ceramic surface due to the inclusion of inorganic antibacterial agents. The purpose is to reduce or eliminate the harm to the human body, so that the number of bacteria is controlled within the scope of medical regulations. Antibacterial ceramics can effectively prevent or kill bacteria and reduce their spread. Photocatalytic antibacterial materials are attracting attention mainly as TiO2. TiO2 antibacterial ceramics refers to the surface of TiO2 coated with antibacterial agents, and catalytic reaction sterilization occurs under the condition of light. Jiang Li et al. Studied TiO2 photocatalytic antibacterial materials with different contents of La and Ho. The study found that the bactericidal rates of the highly active La-TiO2, Ho-TiO2 thin-film antibacterial materials against E.coli reached 92.21% and 88% after being irradiated with ordinary fluorescent lamps for 1.5 and 1 h, respectively.
3.9 Porous ceramics
Porous ceramics are ceramics fired at high temperature and containing a large number of interconnected or closed pore structures. The main structures are: porous, honeycomb, foam, corrugated, gradient and other ceramics. Porous ceramics have stable chemical properties, good mechanical properties, uniform distribution of pore channels, and light weight. Foam ceramic is made of foam plastic as a raw material, and is made of a ceramic material with a large amount of porosity and interconnected special functions. In the steel industry, foam ceramics are mostly used in the continuous casting stage. As a filter, it can effectively filter refractory inclusions in molten steel, which can significantly improve the cleanliness of molten steel and the structural properties of steel. In order to improve the shortcomings of low strength and poor thermal shock resistance of foamed ceramics, studies have shown that the addition of rare earth oxides as reinforcement and sintering agents can improve the performance of foamed ceramics.
3.10 Ceramic color glaze
Rare earth oxides are used in ceramic color glazes, which are bright, stable, high temperature resistance, strong hiding power and uniform color. The research on the application of rare earth materials in ceramics is also the earliest in the application of ceramic pigments. ZrO2 and rare earth element hafnium or rare earth element hafnium form hafnium zirconium yellow and hafnium blue glaze, which are pure and bright; hafnium yellow and hafnium zirconium blue can be combined into light green, soft color and good effect. The "high temperature neodymium color changing glaze" based on calcium, magnesium and zinc white glaze containing 5% neodymium oxide has a two-color effect.It shows different shades under the illumination of different light sources, and appears purple under natural light or incandescent lamps. It is sky blue under fluorescent light, which is a color glaze of great artistic value.
Summary
Adding rare earth oxides to the ceramic matrix can not only improve the sinterability of the ceramic material, optimize its structure, but also improve its mechanical properties and functional properties. It will definitely become one of the important research and development directions in the field of ceramic materials.