Email us!

We look forward to hearing from you.

How to improve the moisture resistance of magnesium oxide \”heat-conducting filler\”?

Heat dissipation in electronic devices is becoming increasingly important as the packaging density of these devices continues to increase, resulting in the generation of large amounts of heat in a compact packaging space, and poor heat dissipation can significantly compromise the lifetime and performance stability of electronic products. To improve heat dissipation, electrical insulating materials with high thermal conductivity are required. A common approach to improving the heat dissipation capability of electronic products is to use polymer-based composites containing thermally conductive fillers.

The average particle size is about 20μm, with high fluidity and high filler magnesium oxide powder

Usually, silicon dioxide, aluminum oxide, aluminum nitride and boron nitride are widely used as insulating and thermally conductive fillers. Silicon dioxide is cost-effective, but its low thermal conductivity and heat dissipation capacity cannot cope with the increase in heat generation, alumina has higher thermal conductivity than silicon dioxide and thus has better heat dissipation, but the disadvantage is high hardness and easy wear of manufacturing equipment. Aluminum nitride, boron nitride and other nitride based fillers have excellent thermal conductivity, but are expensive and have limited applications. Magnesium oxide thermal conductivity is an order of magnitude higher than silicon dioxide, about twice as high as alumina [45-60W/(m.K), the hardness is lower than alumina (alumina Mohs hardness 9, magnesium oxide Mohs hardness 6), can reduce the wear and tear of manufacturing equipment, and its price is lower than the nitride series of thermal conductivity filler, is regarded as the \”thermal conductivity filler\” It is considered as the best candidate for \”thermal conductivity filler\”.

In Ref. 1, thermal conductivity of up to 3 W/(m.K) was achieved for an epoxy plastic sealer (EMC) using a 56% volume fraction MgO filler. The thermal conductivity of the 56 vol% filled EMC is approximately twice that of a conventional silica filled EMC with the same filler volume fraction and has equivalent electrical insulation, thermal expansion and water absorption properties.

However, one fatal injury of magnesium oxide is its higher hygroscopicity than silica and alumina, and when it hydrates with atmospheric moisture, volume expansion occurs which can lead to problems such as cracks and decreased thermal conductivity of the composite, so the resistance to hydrolysis of magnesium oxide needs to be improved to enhance its usefulness. In addition, when magnesium oxide is used as a thermally conductive filler, it is necessary to have high filler in resin compositions to obtain higher thermal performance, so high fluidity and good compatibility with the matrix magnesium oxide is also extremely important. The moisture resistance of magnesium oxide can be effectively improved by special surface treatment.

Hydrothermal reaction under CO2 pressurized condition was carried out to obtain core-shell structure powder of magnesium carbonate (MgCO3) coated magnesium oxide with excellent moisture resistance. Magnesium carbonate is a stable compound with low solubility in water, the core of magnesium oxide particles should be completely covered by the shell of magnesium carbonate reaction layer to avoid the reaction of magnesium oxide with water, however, the thermal conductivity of magnesium carbonate is 15W/(m-K), which is lower than MgO, and the production conditions should be controlled to make the shell (magnesium carbonate) as thin and dense enough to ensure that water will not pass through the shell.

Cross section of MgO particles with core-shell structure

The moisture resistant MgO powder is manufactured in a multi-step process, firstly, the MgO powder is fired. Afterwards, a silica film is formed on the magnesium oxide powder by spray plating, chemical deposition or spray bonding to coat the surface of the magnesium oxide powder. Alternatively, the surface of the magnesium oxide powder is coated with a silica film by mixing micronized silica in the magnesium oxide powder and firing. As such a solution, there are problems of increased manufacturing processes and the need for equipment for manufacturing.

A low hygroscopicity magnesium oxide powder was manufactured by a simple method and applied to a thermosetting resin composition. The electrical insulation layer made with this resin composition has excellent moisture resistance, processability, and thermal conductivity, and is suitable as an insulation layer for circuit boards such as printed circuit boards in which heat-generating parts are mounted. The inventors used magnesium oxide with a silica content of 1 to 6% by mass as a raw material, and fired it at 1650°C to 1800°C (a temperature near the melting point of silica).

By this operation, the silica exuded on the surface of magnesium oxide powder does not completely separate from the magnesium oxide powder, but is coated on the surface of the magnesium oxide powder, and a silica film is formed on the surface of the magnesium oxide powder. No special process is needed, only the original firing process of magnesium oxide powder can be done, which can simplify the manufacturing process, and the surface of the resulting magnesium oxide powder is coated with silica film, which can improve the moisture absorption of magnesium oxide powder. If the silica content is less than the total mass of 1%, the fused silica can not fully cover the surface of magnesium oxide powder, can not complete the improvement of reducing the hygroscopicity of magnesium oxide powder. In addition, if it is more than 6% of the total mass, the thermal conductivity of the resin molding object is reduced because the thickness of the silica film covering the surface of the magnesium oxide powder becomes thicker and the thermal conductivity of the magnesium oxide originally cannot be exerted.

In order to improve the filling rate of the powder to the resin, it is necessary to make the shape of the powder close to spherical, spherical covered magnesium oxide powder whose surface is covered by composite oxide, the composite oxide covering the surface of the magnesium oxide powder preferably contains one or more elements selected from aluminum, iron, silicon and titanium with magnesium, such as magnesium olivine (Mg2SiO4), spinel (Al2MgO4), magnesium ferrite ( Fe2MgO4), magnesium titanate (MgTiO3), etc. The first purpose of the composite oxide coating is to improve the moisture resistance of the magnesium oxide powder, and the second purpose is to make the spherification step of the magnesium oxide powder easy. The spherification is made easy by forming the composite oxide on the surface of the magnesium oxide powder with lower melting point than the flame temperature, which makes the surface of the magnesium oxide powder low melting point.

The melting point of the composite oxide is preferably 2773K or below, more preferably 2273K or below. In addition, the invention document mentions that the spherical magnesium oxide powder can be surface treated with silane-based coupling agent, titanate-based coupling agent, aluminate-based coupling agent as needed, which can further improve the filling property. Silane-based coupling agents: vinyltrichlorosilane, vinyltrialkoxysilane, epoxypropyltrialkoxysilane, methacryloxypropylmethyldioxysilane, etc. Titanate system coupling agent: isopropyl triisostearyl titanate, tetraoctyl bis(bis(tridecyl phosphate) titanate, bis(dioctyl pyrophosphate) oxyacetate titanate, etc.

A method of manufacturing phosphorus-containing coated magnesium oxide powder with good water resistance and a resin composition containing the powder. The composite oxide coated magnesium oxide has good water resistance, but still tends to have areas where the coating is incomplete. In order to improve the water resistance by filling the incompletely coated area of the compound oxide on the surface of magnesium oxide powder, the invention has obtained magnesium oxide powder with better water resistance by further forming a coating layer of magnesium phosphate compound on the coating layer formed by the compound oxide.

Leave a Comment

Your email address will not be published. Required fields are marked *

Scroll to Top