Magnesium hydroxide [Mg(OH)2] is a flame retardant that we are all familiar with. It is often added to polymer materials to improve the safety factor of polymer materials that have problems such as flammability and large smoke. When encountering a fire, magnesium hydroxide will decompose and release bound water after being heated to absorb a large amount of latent heat and reduce the surface temperature of the filled synthetic material in the flame; at the same time, it can also suppress smoke and greatly reduce the smoke density during combustion , to reduce the risk of suffocation of those trapped in the fire due to dense smoke.
With such an excellent flame retardant, it is good to do your job well. But sometimes it can perform exceptionally well—for example, researchers have recently discovered that under certain conditions, when magnesium hydroxide is mixed with other thermally conductive fillers into polymer materials, it can also play a role of “synergistic heat conduction” at the same time.
Why study the thermal conductivity of magnesium hydroxide?
At present, high thermal conductivity insulating materials are widely used in the fields of aerospace and electrical equipment. In terms of electrical equipment, with the rapid growth of power demand, the capacity of power transmission equipment such as transformers and insulated cables is increasing, and the heat generated is getting higher and higher. Therefore, improving the thermal conductivity of insulating materials in power cables has important practical significance for increasing the current carrying capacity of the cable core.
For cable materials, insulation is important, as is thermal conductivity
At present, improving the thermal conductivity of composite materials at home and abroad is mainly to dope the thermally conductive filler with high thermal conductivity in the polymer matrix through a certain blending method. There are many kinds of thermally conductive fillers. At present, the research on thermally conductive and insulating composite materials with single fillers has been relatively complete. The prominent fillers include alumina (Al2O3), aluminum nitride (AlN), boron nitride (BN), etc.
Among them, AlN has a very high thermal conductivity, but is expensive. In order to obtain better heat conduction effect, manufacturers often add two or more heat conduction fillers to polymer materials in the form of “mix and match”. For the power cable insulation materials mentioned above, the selection of the second filler needs to consider the rigid demand of the cable – the limiting oxygen index (LOI) of the insulation materials commonly used in power cables is mostly below 21%, which means that these materials are in the air. It is extremely flammable, and adding a large amount of flame retardants to insulating materials can increase the oxygen index of the composite material, and can quickly absorb heat and eliminate smoke when the material is burned, improving its reliability and safety. Therefore, flame retardants can be used as the first The second filler, namely magnesium hydroxide [Mg(OH)2] was used in this study.
Influence of Incorporation of Magnesium Hydroxide
According to research, the incorporation of Mg(OH)2 mainly brings about the following effects:
(1) In multi-filler composites, the incorporation of Mg(OH)2 can improve the thermal conductivity of composites, and have a certain degree of synergy with BNNs in axial thermal conductivity, further improving the axial conductivity of composites. towards thermal conductivity.
Axial Thermal Conductivity of Composite Materials
(2) Under different doping content, the thickness will greatly affect the thermal conductivity of the material. Compared with the composite material with thicker thickness, the composite material with thin thickness is more likely to promote the radial arrangement of BNNs along the sample, so that in Macroscopically, the radial thermal conductivity of the composite material is improved, and the composite material shows stronger anisotropy in thermal conductivity.
Radial Thermal Conductivity of Composite Materials
(3) Under a certain thickness, when the filler content of Mg(OH)2 is similar to that of BNNs, Mg(OH)2 will further enhance the radial arrangement of BNNs and improve the radial thermal conductivity of the composite material. When Mg When the (OH)2 filler content is much higher than the BNNs filler content, Mg(OH)2 will inhibit the radial arrangement of BNNs and reduce the radial thermal conductivity of the composite material, while the incorporation of high filler content Mg(OH)2 It will also hinder the formation of large thermal conduction pathways in BNNs, which will also reduce the radial thermal conductivity of the composite.
Left: Cross-sectional topography of single-filler composite material, it can be seen that the radial arrangement of BNNs is very high;
Right: Cross-sectional morphology of multi-filler composites, the incorporation of a large amount of Mg(OH)2 will block the interconnection of BNNs.
(4) The incorporation of Mg(OH)2 and BNNs will increase the dielectric constant and dielectric loss factor of the composite material at power frequency, and the dielectric properties will increase with the increase of the content of the two fillers, resulting in The dielectric properties of the composite material decrease, but compared with the increase in thermal conductivity, the decrease in the dielectric properties of the composite material is small.
The above is the reason why magnesium hydroxide can conduct heat synergistically in the cable insulation material. Although the relevant research is still in the early stage, it also proves that similar “atypical heat-conducting materials” may play unexpected roles as long as they appear in the right place in the right posture.