Hebei Messi Biology Co., Ltd. stated that magnesium oxide is a fairly common alkaline oxide and is the main raw material for the production of magnesium hydroxide and magnesium metal. In the application of ceramic materials, magnesium oxide has a melting point of up to 2800℃ and has some special and excellent properties, so it is also very popular in the field of advanced ceramics. It can be directly sintered into magnesium oxide ceramics and can also be used as an additive.
Preparation of magnesium oxide powder
The preparation of magnesium oxide is divided into brine-ammonia precipitation method, dolomite carbonization method, thermal decomposition of magnesium salts and thermal decomposition of Lingmei ore. In terms of preparation, the raw materials come from minerals or seawater. Magnesium oxide is extracted from minerals or seawater. Most of them are first made into magnesium hydroxide or magnesium carbonate, and then calcined and decomposed into magnesium oxide. This magnesium oxide can be further chemically treated or heat treated to obtain high-purity magnesium oxide. If high-purity magnesium oxide ceramics are required, the method of adding additives cannot be used to promote sintering and grain growth. Instead, the method of activated sintering is used, that is, magnesium hydroxide is calcined at an appropriate temperature to obtain active magnesium oxide with many lattice defects, which is used to make sintered magnesium oxide ceramics.
Magnesium oxide ceramics
(1) Magnesium oxide transparent ceramics
Magnesium oxide transparent ceramics have good transparency and infrared transmittance. It is an optically isotropic body with good resistance to alkali metal vapor corrosion, high melting point, high thermal conductivity, small theoretical density, high insulation and high infrared transmittance. It has a wide range of applications in the field of visible light and infrared transparent materials, and its performance is better than that of alumina ceramics. It is a very promising material.
Typical applications: It is an important material for the preparation of infrared windows and fairings for spacecraft and rocket missiles, infrared windows for high-temperature furnaces, high-temperature infrared optical devices, optical filters and optical detectors, high-pressure sodium lamp light-emitting tubes and infrared detector covers.
(2) Magnesium oxide foam ceramics
As a new type of ceramic functional material, the development of foam ceramic materials began in the 1970s. Magnesium oxide foam ceramics have a unique three-dimensional mesh skeleton structure, which gives them an open porosity of 60%-90%. They can efficiently remove large debris and most of the tiny suspended inclusions in the molten metal. They have high refractoriness, high porosity, low thermal conductivity, low manufacturing cost, simple preparation process, and good mechanical properties. In China, Li Wenxia and others from Peking University used magnesium oxide as aggregate and added an appropriate amount of ZrO2 to prepare magnesium oxide partially stabilized zirconium oxide foam ceramic filters. They have achieved good results in the production of nickel-based alloy single crystals, filling the domestic technical gap.
Typical applications:
a. Filters. Filter cake effect, adsorption effect and rectification effect are used to filter impurities in magnesium alloy melt and improve filtration efficiency.
b. Energy-saving heat-insulating material. It can be used as kiln lining and can be used to insulate and protect aerospace equipment in the aerospace industry.
c. Sound-absorbing material. According to research, its noise reduction coefficient is close to that of glass wool and has good application prospects.
d. Catalyst carrier material. It has good adsorption capacity and can be widely used as a carrier for automobile exhaust purifiers.
e. Food and pharmaceutical industry. It is used for the concentration, separation and refining of physiologically active substances such as enzymes, vaccines and nucleic acids in the pharmaceutical industry to ensure food safety.
(3) Magnesium oxide microwave dielectric ceramics
With the update and iteration of mobile communication and satellite communication technology, people have higher and higher requirements for communication time and frequency bands, making low-dielectric high-Q ceramics a research hotspot. On the one hand, magnesium oxide ceramics themselves have excellent dielectric properties (εr=9.1, tanδ<1.6×10-6) and are an ideal microwave dielectric substrate material for 5G communication.
Magnesium metatitanate microwave dielectric ceramics have an ilmenite structure of space group R3. The material has been studied and applied to ceramic capacitors and resonators due to its low tanδ and high frequency thermal stability. In addition, magnesium metatitanate has the potential to be used in filters, communication antennas, radars, direct broadcast satellites and microwave frequency global positioning systems.
Magnesium aluminum spinel ceramics have a spinel structure, high hardness, high melting point, and high strength. Pure magnesium aluminum spinel ceramics are transparent and can be used in transparent armor, infrared windows, radar covers and other fields. Not only that, magnesium aluminum spinel ceramics also have the characteristics of low εr and can also be used as microwave dielectric ceramics. Magnesium aluminum spinel ceramics prepared by microwave sintering have excellent comprehensive performance and can meet the use requirements of various fields.
(4) Magnesium oxide ceramic core
Magnesium oxide has good high temperature performance. When using magnesium oxide-based ceramic core to cast stainless steel castings, even if the pouring temperature is as high as 1650℃, the core material will not react with the alloy, and the internal surface finish of the casting is high. In addition, it is a weakly alkaline refractory material and can be dissolved in organic acid solutions such as phosphoric acid and acetic acid. It is easy to remove the core and does not produce thermal cracking defects. Currently, there is little research on magnesium-based ceramic cores, and there is a lot of room for development.
Application of magnesium oxide as an additive
1. As a sintering agent for alumina ceramics
As a common sintering aid, magnesium oxide has the following effects on alumina ceramics:
a. Adding an appropriate amount of magnesium oxide can reduce the sintering temperature of alumina ceramics, inhibit grain growth, and improve density.
b. Adding magnesium oxide accelerates grain boundary diffusion, has a certain grain refinement effect, and has good density and mechanical properties.
c. An appropriate amount of magnesium oxide can inhibit the rapid movement of grain boundaries, making the pores more completely discharged, the ceramics more dense, and the transmittance higher.
2. As a sintering aid for high-performance ceramic heat dissipation substrates
With the development of high-power electronic devices in the fields of high-speed rail, aerospace and military industry towards high temperature, high frequency and high integration, efficient heat dissipation has become an urgent need. High-power devices achieve heat exchange with the outside world through ceramic copper-clad boards. At present, the mainstream ceramic substrates are silicon nitride, aluminum nitride and aluminum oxide, all of which require magnesium oxide as a sintering aid. Especially for silicon nitride ceramics with excellent comprehensive performance, in order to avoid the lattice defects caused by aluminum oxide as an additive to increase phonon scattering, magnesium oxide has become the preferred sintering aid for the preparation of high thermal conductivity silicon nitride ceramics, and its usage is about 3%.
3. As a sintering aid for zirconium oxide toughened alumina wear-resistant ceramics
Both aluminum oxide and zirconium dioxide have the characteristics of high temperature resistance, wear resistance and good biocompatibility. The preparation of ZTA nanocomposite ceramics with zirconium dioxide toughened alumina can make the best use of its strengths and avoid its weaknesses, giving full play to its integrated advantages, and has important applications in aerospace, engine wear-resistant parts and artificial femoral head. The densification and grain refinement mechanism of magnesium oxide in ZTA ceramics is similar to that in alumina, and its usage is about 2%.
4. As a ferroelectric ceramic additive
- (1) Added to barium titanate-based ceramics
Using a uniform precipitation method to uniformly coat magnesium oxide on the surface of barium titanate-based ceramic powder can effectively inhibit grain growth, thereby obtaining ceramics with uniform grains. This fine grain effect is due to the inhibitory effect of magnesium oxide in the grain boundary region; magnesium oxide helps to form “shell-core” structure grains, reduce and broaden the ε peak of barium titanate-based ceramics, and increase resistivity and breakdown voltage strength.
- (2) Added to barium strontium titanate ceramics
Barium strontium titanate (BST) ferroelectric ceramic materials have very good application prospects as phase shifters in phased arrays and adjustable devices at microwave frequencies due to their high adjustability and low dielectric loss.
Since various ferroelectric materials currently have certain deficiencies, improving their comprehensive performance through various means has become a key issue that must be solved for the large-scale application of barium strontium titanate materials.
In addition to using rare earth element ions for A-site doping substitution, adding compounds such as magnesium oxide, magnesium metatitanate, and magnesium silicate to BST ceramics and films can also reduce their dielectric constant and dielectric loss.
5. Doped in zinc oxide linear ceramics
Zinc oxide linear ceramic resistors have the advantages of a large resistivity variation range, large current density, low nonlinear coefficient, and small resistance temperature coefficient. They are widely used in power-electronics, transportation, communications, and household appliances. Traditional zinc oxide composite ceramics still have many problems, such as poor structural uniformity, low industrial production repetition rate, poor stability, insufficient theoretical research, etc.
The addition of magnesium oxide helps to improve the temperature resistance coefficient of zinc oxide ceramic resistors. An appropriate amount of magnesium oxide can promote sintering and increase the density of ceramics, but excessive addition will reduce the density of ceramics.