Hebei Messi Biology Co., Ltd. stated that magnesium oxide (MgO) is a widely used chemical material with excellent chemical inertness, heat resistance, insulation and thermal conductivity. Among them, the most prominent ones are good high-temperature oxidation resistance, moderate alkalinity, and electrophilicity due to the presence of oxygen vacancies and single electrons. These properties provide important basic conditions for the application of magnesium oxide.
Generally, magnesium oxide is a flaky crystal, but studies have found that some magnesium oxides with special morphologies 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. It has very important and effective applications.
Types of thermal conductive fillers
The thermal conductivity of polymer materials is generally low, and the thermal conductivity of most common materials is around 0.3W/m·K. Therefore, in order to improve the thermal conductivity of polymers, it is necessary to fill thermal conductive fillers in polymer materials. The thermal conductive filler with high thermal conductivity is evenly dispersed into the polymer matrix by blending, and the fillers form a thermal conductive network chain that contacts each other, so that the thermal conductivity of the polymer meets the application requirements.
Thermal conductive fillers are mainly divided into three types: carbon-based materials, metal materials and non-metallic inorganic materials.
1. Carbon-based materials
The thermal conductivity of some carbon-based materials is significantly higher than that of metal materials and inorganic non-metallic materials. Carbon-based materials have a very unique microstructure and anisotropic thermal conductivity. Taking graphite as an example, graphite has a typical layered structure and works with the dual mechanism of electrons and phonons. Therefore, graphite has good thermal conductivity and anisotropic characteristics. At the same time, it is cheap and can be well mixed with the matrix. It is generally considered to be the preferred thermal conductive filler.
2. Metal materials
Metal materials are recognized as good conductors of heat. They are not only fillers for polymer materials, but also have relatively mature and wide applications in aerospace, machinery manufacturing and other fields. There are a large number of free electrons inside metal materials. Their thermal conductivity mainly depends on the free movement of a large number of electrons inside. Generally, metal materials have a high thermal conductivity. At the same time, because metal materials have good electrical conductivity, they can provide electrical conductivity in composite materials prepared as fillers.
However, metal materials have a large density and are difficult to mix evenly with polymer materials, which restricts their application in polymer material thermal conductive fillers.
3. Inorganic non-metallic materials
Inorganic non-metals mainly rely on phonons for thermal conductivity. Generally, their thermal conductivity is lower than that of carbon-based materials and metal materials, but they have good insulation. They are mainly divided into metal nitrides and metal oxides. Metal nitride fillers include: BN, AlN, etc.; metal oxide fillers include: MgO, Al2O3, etc.
Among them, nitrides exist in the form of crystals, with regular and dense structures. Phonons have less resistance to propagation in crystals, so heat can be transferred more effectively. However, the higher the purity of nitrides, the higher the price. Although metal oxides have low thermal conductivity, they are cheap and have a wide range of material sources, so they are widely used.
Among oxides, aluminum oxide and magnesium oxide are the most commonly used. Aluminum oxide has relatively low thermal conductivity, but the cost is not high, so it is widely used. Although the thermal conductivity of magnesium oxide is lower than that of boron nitride, it is higher than that of aluminum oxide, at 36W/m·K, and the cost is low, so it has attracted more and more attention in the application of thermal conductive fillers.
The influence of different filler morphologies on thermal conductivity
Generally, rod-shaped and sheet-shaped fillers with a certain aspect ratio are easier to form thermal conductive network chains when added to polymer materials, thereby improving the thermal conductivity of composite materials. However, such fillers will be oriented and distributed during processing, that is, the directions of the rod-shaped structures are inconsistent, which will cause the thermal conductivity of the composite materials to be anisotropic, and the thermal conductivity in the processing direction is much higher than that in the perpendicular processing direction.
Therefore, when designing and producing the shape of filler products, try to make the orientation direction of the fillers consistent, so as to improve the thermal conductivity efficiency of the composite materials.
In contrast, due to the isotropy of the spherical structure, spherical fillers have more advantages in improving the thermal conductivity of composite materials than rod-shaped or sheet-shaped structures. At the same time, the particle size of spherical powder is small and evenly distributed, the surface morphology is regular, and the packing density of the powder is significantly increased, which can greatly improve the fluidity and dispersibility of the powder, eliminate the influence of agglomeration to the maximum extent, and improve the defects inside the powder.
Current Development Status of Spherical Magnesium Oxide
For spherical magnesium oxide products, due to the application of high-performance chip technology, foreign countries are highly confidential about the spherical synthesis technology of magnesium oxide, and international procurement of products is relatively difficult, and it is impossible to obtain complete technical parameters and professional manufacturing equipment and other information of the product. According to relevant literature reports, only a few technologically developed countries such as Japan, the United States, and Israel in the world currently have the synthesis and manufacturing technology of this product.
Physical and chemical methods for sphericalization of powders
Currently, spherical magnesium oxide is mainly prepared by two methods:
1) Using magnesium salt as a raw material, first obtain a precursor for preparing spherical magnesium oxide, and heat-treat the precursor to obtain spherical magnesium oxide. Generally, the precursor is spherical basic magnesium carbonate or spherical magnesium hydroxide or spherical basic magnesium oxalate.
2) After mixing magnesium oxide powder with a solvent and a binder, spherical magnesium oxide is obtained by mechanical molding, and spherical magnesium oxide products are obtained by heat treatment.
However, in the exploration of industrialization, the sphericalization of magnesium oxide still relies more on the technical accumulation based on spherical alumina and spherical silicon powder. Globally, Japan is currently in the forefront, and domestic companies such as Hebei Messi Biology Co., Ltd. are currently laying out 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 thermal conductive filler” after spherical alumina, can also be mass-produced as soon as possible to achieve domestic substitution, thereby starting large-scale application and promotion.