Magnesium oxide (MgO) is a very widely used chemical material, with excellent chemical inertness, heat resistance, insulation and thermal conductivity, of which the more prominent is good resistance to high temperature oxidation, moderate alkalinity, due to the presence of oxygen vacancies and single electrons and the production of electron affinity, etc., these properties provide important basic conditions for the application of magnesium oxide.
Generally, magnesium oxide is a flaky crystal, but some special morphological magnesium oxide has been found to have very effective applications in many aspects. For example, spherical magnesium oxide can be used as a stationary phase material in chromatography, as a material for adsorbing toxic substances, and added to plastics to improve thermal conductivity.
The type of thermal filler
The thermal conductivity of polymer materials is generally low, and the thermal conductivity of most common materials is about 0.3W/m·K, so in order to improve the thermal conductivity of polymers, it is necessary to fill the polymer material with thermal conductivity fillers. The thermal conductivity filler with high thermal conductivity is uniformly dispersed into the polymer matrix by the blending method, and the thermal conductivity network chain in contact with each other is formed between the fillers to make the thermal conductivity of the polymer meet the application requirements.
Thermal conductive filler is mainly divided into three kinds of carbon-based materials, metal materials and non-metallic inorganic materials, and the thermal conductivity of several common fillers is as follows:
1. Charcoal base material
The thermal conductivity of some carbon-based materials is significantly higher than that of metallic materials and inorganic non-metallic materials, and carbon-based materials have various heterogeneities due to their unique microstructure and thermal conductivity. Taking graphite as an example, graphite has a more typical layered structure, and at the same time plays a role in the dual mechanism of electronic and phonons, so the thermal conductivity of graphite is good, with the characteristics of various opposite sexes, and the price is cheap and can be well mixed with the matrix, which is generally considered to be the preferred thermal conductive filler.
2. Metal materials
Metal materials are recognized as good conductors of heat, not only in terms of fillers for polymer materials, but also in aerospace, machinery manufacturing and other aspects have more mature and wide applications. There are a large number of free electrons inside the metal material, and its thermal conductivity mainly depends on the free movement of a large number of electrons inside these, and the thermal conductivity of the general metal material is high. At the same time, because the electrical conductivity of metal materials is good, in composite materials prepared as fillers, their conductivity can be provided.
However, the density of metal materials is large, and it is difficult to mix uniformly with polymer polymeric materials, which restricts its application in thermal conductivity fillers of polymer materials.
3. Inorganic non-metallic materials
Inorganic non-metals mainly rely on phonon heat conductivity, and the general thermal conductivity is lower than that of carbon-based materials and metal materials, but has better insulation. Mainly divided into metal nitrides and metal oxides, metal nitride fillers include: BN, AlN, etc.; Metal oxide fillers include: MgO, Al203 and so on.
Among them, nitrides exist in the form of crystals, the structure is regular and dense, and the phonons have less resistance to propagation in the crystals, so heat can be transferred more efficiently. But the higher the purity of the nitride, the higher the price. Although the thermal conductivity of metal oxides is not high, they are cheap and have a wide range of material sources, so they are widely used.
The most commonly used in oxides are alumina and magnesium oxide, alumina thermal conductivity is relatively low, but the cost is not high, so the application is more extensive. Although the thermal conductivity of magnesium oxide is lower than that of boron nitride, it is higher than alumina, which is 36W/m·K, and the cost is low, so it is also receiving more and more attention in the application of thermal conductive fillers.
The influence of different filler morphologies on thermal conductivity
General rod-like and sheet structure fillers with a certain length-to-diameter ratio, it is easier to form a thermal network chain in the polymer material, thereby improving the thermal conductivity of composite materials, but such fillers will occur in the orientation distribution during processing, that is, the direction of the rod structure is not uniform, which will lead to anisotropy in the thermal conductivity of the composite materials, and the thermal conductivity of the processing direction is much higher than the thermal conductivity of the vertical processing direction.
Therefore, when designing and producing the shape of the filler product, try to make the orientation of the filler consistent, so as to improve the thermal conductivity efficiency of the composite material.
In contrast, due to the isotropic nature of the spherical structure, the spherical filler has an advantage over the rod or flake structure for improving the thermal conductivity of the composite material. At the same time, the particle size of the spherical powder is small and uniformly distributed, the surface morphology is regular, and the bulk density of the powder is significantly increased, which can greatly improve the fluidity and dispersion of the powder, eliminate the impact of agglomeration to the greatest extent, and improve the defects inside the powder.
Development status of spherical magnesium oxide
For spherical magnesium oxide products, due to the application of high-performance chip technology, foreign magnesium oxide spherical synthesis technology is highly confidential, the international procurement of products is more difficult, and it is impossible to obtain the complete technical parameters of the products and professional manufacturing equipment. According to relevant literature reports, at present, only a few scientific and technologically developed countries in the world, such as Japan, the United States and Israel, have mastered the synthesis and manufacturing technology of the product.
Physical and chemical methods of powder spheroidization
At present, spherical magnesium oxide is mainly prepared by two methods:
1) Using magnesium salt as raw material to first obtain a precursor to prepare spherical magnesium oxide, the precursor is heat treated to obtain spherical magnesium oxide, and the general precursor is spherical basic magnesium carbonate or spherical magnesium hydroxide or spherical basic magnesium oxalate.
2) After mixing the magnesium oxide powder with the solvent and the binder, the spherical magnesium oxide is obtained by mechanical molding, and the spherical magnesium oxide product is obtained by heat treatment.
However, in the exploration of industrialization, the sphericalization of magnesium oxide is more dependent on the accumulation of technology based on spherical alumina and spherical silicon powder, on a global scale, Japan has been at the forefront, and domestic Xingtai Magnesium Xi Environmental Protection Materials Co., Ltd. and other enterprises are laying out their production lines. It is believed that with the popularity of emerging markets such as 5G and new energy vehicles, spherical magnesium oxide, which is regarded as the "next generation of thermal conductive filler" for receiving ball-shaped alumina, can also be mass-produced as soon as possible to achieve localization substitution, thus starting large-scale application and promotion.