Choosing High Thermal Conductivity Insulators
The ability of a material to absorb and disperse thermal energy is an important factor in its performance as insulation. High thermal conductivity materials can transmit heat quickly and can act as an efficient heat sink. Conversely, materials that transfer heat slowly are good insulators and can protect against unwanted thermal radiation.
Efficient Thermal Management in Electronics is a vital part of the design process for many electrical and electronic applications. Whether you are designing a chip, battery pack or system that requires high power density and/or low package profile, choosing the right thermal insulator is essential.
Often, the thermal characteristics of insulation are compared using the R-value. This is an easy way to determine the thermal resistance of a material. However, it only gives a rough indication of a material's thermal conductivity. The actual figure is determined by many factors including the density of the material, the temperature at which it is tested and the geometry of the test piece. These factors make the R-value of a material difficult to compare between different products.
In addition to the R-value, thermal conductivity and the thermal diffusivity of a material are also important. The higher the thermal diffusivity of a material, the more it spreads the heat evenly through the material and reduces the effect of hot spots.
The thermal conductivity of a material is the ease with which a material transfers thermal energy and can be measured in watts per metre-kelvin (W/m*K). This value increases as a material's temperature rises. A pure metal has a high thermal conductivity, while wood has a lower value. In most cases, a material's thermal conductivity will increase with its thickness.
Other factors that affect thermal conductivity are the specific heat capacity and the specific heat loss of a material. The specific heat capacity of a material is the amount of heat needed to raise the temperature of one kilogram of the material by 1K. This value is usually higher for denser materials.
Examples of high thermal conductivity insulation are polyurethane foams, which can be sprayed to provide a range of insulation levels. These foams typically use non-chlorofluorocarbon blowing agents, which helps to decrease the amount of damage to the ozone layer.
Other insulation options include ceramic fibre, flexible closed cell sheet insulation and a new class of materials called aerogels, which have the ability to conduct very little heat and can be used as thermal insulators with a very low thickness. These insulations have been designed for applications such as high power electronics and insulated metal substrates, where the requirement is to balance high-power performance with low package size.