A Common Measure Of High Thermal Conductivity Insulators
The Efficient Thermal Management in Electronics is often complicated by the need to maintain a balance between electrical efficiency and effective heat dissipation. Materials that transfer heat rapidly have high thermal conductivity and are used as metals in heat sinks, while materials that transfer heat slowly have low thermal conductivity and are used as insulation. The amount of thermal energy that is transferred in a unit of time by a given material, regardless of the temperature difference between it and surrounding matter, is called its thermal conductivity (also known as displaystyle k or ldisplaystyle kappa) and is measured in watts per meter-Kelvin.
Thermal conductivity depends on the material's density, with higher denser materials having lower conductivity than lower density ones. It also varies with the pressure applied, with higher pressures increasing the conductivity due to closer packing of atoms and molecules. It is also affected by changes in the material's phase, with solids having higher conductivity than liquids and gases.
A common measure of the insulating effectiveness of a material is its R-value, which is equal to the material's thickness divided by its thermal conductivity. This value is a useful index of the insulating ability of any material, but does not take into account the effects of convection and radiation on the material's overall thermal properties.
Materials that transfer heat slowly act as good insulators, as they effectively block the passage of thermal energy. Many natural insulators, such as fur and feathers, do this by trapping air in pockets or voids. Expanded and extruded polystyrene, which is commonly referred to as Styrofoam, and silica aerogel are examples of synthetic insulators that function in the same way. The air in these insulators is disrupted into small gas cells or bubbles that prevent the normal flow of thermal energy by convection.
Another source of thermal resistance is the gaps that occur between adjacent materials. These can be reduced by using a thermal interface material, or TIM. For example, the gaps produced between bare copper and silicon are often filled with a polymer matrix composite, such as Polyimine.
Researchers at MIT have made an important step towards this goal by developing a technology that allows the thermal conductivity of a material to be varied on demand. This technique could lead to smart windows, walls and clothing that can be controlled to regulate the amount of insulation they provide.
Polymer based thermally conductive materials are widely used in applications that require both good thermal conductivity and insulation. These materials are usually based on polyimine, and filled with AlN, h-BN or carbon based fillers such as graphene nano fibers (GNSs) or reduced graphene oxide. These materials can also be incorporated into other materials, such as TIMs, to improve their thermal performance. The resulting polymer matrix composites are characterized by excellent insulation, flame resistance and tensile strength and are used in a variety of electronic components such as: