In the preparation and production of some advanced ceramics, magnesium oxide is a common additive. What changes it will bring to the ceramics during the sintering process? Let\’s understand it by several cases. Magnesium oxide on alumina ceramics what are the influencing factors?
1, magnesium oxide on alumina ceramic sintering temperature and densification of the impact
First of all, magnesium oxide as a common sintering additive, is able to effectively reduce the sintering temperature of alumina ceramics. The effect of magnesium oxide addition and sintering temperature on the densification process and microstructure of alumina ceramics was studied, and the diffusion and evolution of pores during the sintering process was analyzed. The results showed that the addition of appropriate amount of MgO can reduce the sintering temperature, inhibit the grain growth and improve the densification of alumina ceramics, and 0.25% (mass fraction) is the best addition amount of MgO.
2、The effect of magnesium oxide on the mechanical properties and grain growth of alumina ceramics
The alumina ceramics were prepared by using alumina powder as raw material, magnesium oxide as additive and two-step sintering process, and analyzed the influence of the change of additive on the densities, mechanical properties and microstructure of alumina ceramics. It was found that the relative density, flexural strength and hardness of the sintered alumina ceramics showed a tendency to increase and then slightly decrease with the increase of magnesium oxide addition. When the magnesium oxide content is lower than the solid solution limit, the magnesium accelerates the grain boundary diffusion, and has a certain refining effect on the grain, and the density and mechanical properties are better; when the magnesium oxide content exceeds the solid solution limit, although the refining effect on the grain is enhanced, but the formation of magnesium aluminum spinel at the grain boundary will prevent the pore discharge.
In terms of grain growth, with the increase of MgO addition, the grain size and homogeneity of the samples showed a trend of rising first and then slightly decreasing. When the magnesium oxide content in 0.25%, the average grain size is the smallest, grain distribution is also more concentrated, the best performance; when the magnesium oxide content in 0.5%, the grain is finer, grain distribution is more uniform, the largest grain size is only 2.2μm, but the worst denseness.
3, magnesium oxide on alumina transparent ceramic optical properties of the impact
Magnesium oxide on alumina transparent ceramic mechanical properties and denseness of the influence of the law similar to ordinary alumina ceramics, the appropriate amount of magnesium oxide added to its mechanical properties, denseness, inhibit grain growth has a positive effect. As for the optical properties of transparent alumina ceramics, when the doping amount of magnesium oxide is low, the light transmission rate of transparent ceramics is relatively high, which is due to the appropriate amount of magnesium oxide can inhibit the rapid movement of grain boundaries, making the pore discharge more complete, the ceramic is denser, and the transmission rate is higher. But with the increase of doping, magnesium oxide content more than the solid solution in Al2O3, will locally form the second phase, the formation of light scattering center, resulting in the decline of transparent ceramic transmittance.
4, magnesium oxide doping on zinc oxide linear ceramics
Industrial manufacturing of zinc oxide linear ceramic resistors has the advantages of large resistivity variation range, high flux density, low nonlinear coefficient, small resistance temperature coefficient, widely used in power – electronics, transportation, communications and household appliances. However, traditional ZnO composite ceramics still have many problems, such as poor structural uniformity, low industrial production repeatability, poor stability, and insufficient theoretical studies. The effects of magnesium oxide doping on the nonlinear coefficient, resistivity and temperature resistance coefficient of ZnO linear ceramics were investigated.
The ZnO linear ceramic resistors with the base formulation of ZnO, Al2O3, MgO, FeO, TiO2 and SiO2 were prepared by solid-phase sintering method. The effects of MgO doping on the microstructure, impedance, resistance-temperature characteristics and resistance-frequency characteristics of ZnO linear ceramic resistors were investigated. It was also concluded that the appropriate amount of magnesium oxide doping in ZnO linear ceramic resistors can effectively improve the resistivity and temperature resistance coefficient of the samples. The effect of magnesium oxide on ferroelectric ceramics
1, magnesium oxide on the structure and performance of barium strontium titanate ceramics
Strontium barium titanate (BST) ferroelectric ceramic material with its high tunability and low dielectric loss in the phased array as a phase shifter and microwave frequencies under the tunable devices have very good prospects for application. As the current variety of ferroelectric materials are deficient in certain aspects, improving their comprehensive performance by various means has become a key issue that must be solved for the realization of large-scale applications of barium strontium titanate materials. In addition to A-site doping substitution with rare earth element ions, compounds such as magnesium oxide, MgTiO3 and Mg2SiO4 added to BST ceramics and thin films can also reduce their dielectric constants and dielectric losses.
2, the effect of magnesium oxide on BaTiO3-based ceramic properties
Using uniform precipitation method to uniformly coat magnesium oxide on the surface of BaTiO3-based ceramic powder, the BaTiO3-based ceramic composite particles coated with magnesium oxide were prepared, and the effects of different coating amounts on the microstructure, microscopic morphology, dielectric properties, insulation properties and breakdown voltage of BaTiO3-based ceramics were studied. The experiments show that: the cladding agent MgO can effectively inhibit the grain growth, so as to obtain a homogeneous ceramic grain, and this fine crystal effect is due to the inhibiting effect of MgO in the grain boundary area; MgO helps to form a \”shell and core\” structure grain, reduce and broaden the ε-peak of BaTiO3-based ceramics, increase the resistivity and breakdown voltage strength.
3, magnesium oxide on the sintering performance of YAG transparent ceramics
YAG transparent ceramics have high melting point (1950 ℃), high strength, high thermal conductivity, stable physical and chemical properties, whether as a functional material, or as a structural material shows excellent application prospects, magnesium oxide is a common sintering aid in its preparation process. YAG transparent ceramics were prepared by vacuum solid-phase reaction sintering technique, and the effect of the addition of magnesium oxide as a sintering aid on the microstructure and optical transmittance of YAG transparent ceramics was investigated in depth. It was found that the high quality YAG transparent ceramics could be prepared by using trace amount of MgO as sintering aid and the vacuum solid-phase reaction sintering process. Magnesium oxide as a sintering aid is beneficial to control the diffusion of grain boundaries, grain growth and the exclusion of pores. But with the increase of magnesium oxide content, too much added will form the second phase in the ceramic, or cause the ceramic sintering shrinkage too fast to form pores and cannot exclude the grains, thus greatly reducing the YAG ceramic transmission rate.
4, magnesium oxide on the mechanical properties of Sialon ceramics
Sialon is a Si3N4-based solid solution, in the synthesis of Sialon, due to the low diffusion coefficient of Si3N4, generally in the arrival of Sialon ceramic sintering temperature (greater than 1800 ℃) accompanied by decomposition reaction, so low-temperature sintering is the development direction of the preparation of Sialon ceramics. It is found that MgO increases the density of β-sialon ceramics and decreases the sintering temperature, and the flexural strength and fracture toughness of β-sialon ceramics increase and then decrease with the increase of sintering temperature, and when the sintering temperature is 1600°C, the prepared β-sialon ceramics are tightly bonded, and the flexural strength and fracture toughness reach the maximum.