Thermal ConductivityThermal conductivity(often denotedk,, or) is thepropertyof a material toconduct heat. It is evaluated primarily in terms ofFourier's Lawforheat conduction.Heat transfer occurs at a higher rate across materials of high thermal conductivity than across materials of low thermal conductivity. Correspondingly, materials of high thermal conductivity are widely used inheat sinkapplications and materials of low thermal conductivity are used asthermal insulation. The thermal conductivity of a material may depend on temperature. The reciprocal of thermal conductivity is called thermal resistivity.1) Comparative methodThe thermal conductivity of metals, alloys or composites with can be measured by comparative method with steady state longitudinal heat flow in a temperature range room temperature up to about 1000C. The comparative instrument measures heat flow based upon the known thermal properties of standard reference materials. The test specimen is sandwiched between two identical reference samples. This stack is placed between two heating elements controlled at different temperatures. A guard heater is placed around the test stack to ensure a constant heat flux through the stack and no lateral heat flow losses. As heat flows from the hot element to the cold element the temperature gradient (T/L) across the stack is measured with thermocouples. Once the specimen reaches a state of thermal equilibrium its thermal conductivity is calculated from the expression

where Q is the heating power of the heater.

Figure 1: Comparative method for measurement of thermal conductivity

The experimental error is in the range of approximately 10 %. The specimen geometry is cylindrical (diameter 25 or 50 mm, height approx. 10-40 mm). 2) Laser flash (Thermal Diffusivity) Thermal conductivity may be calculated using thermal diffusivity (Laser flash), Specific Heat (DSC) and density (Archimedes). The laser fires a pulse at the sample's front surface and the infrared detector measures the temperature rise of the sample's back surface. The software uses literature-based analysis routines to match a theoretical curve to the experimental temperature rise curve. The thermal diffusivity value is the diffusivity value associated with the selected theoretical curve. To determine specific heat, the infrared detector measures the actual temperature rise of the sample. The response of the infrared detector is calibrated with a reference sample of known specific heat. The instrument can measure thermal diffusivity and specific heat simultaneously. The software uses these values and the bulk density to calculate thermal conductivity from the equation:

Figure 2: Schematic setup of the laser flash methodThese measurements can be performed very rapidly with an accuracy of about 5 %. Within the last years, a great variety of very different materials like metals, ceramics, glasses, minerals etc. have been characterised with respect to thermophysical properties. Thermal conductivity can be calculated with high accuracy using thermal diffusivity. The specimen geometry is cylindrical (diameter ~12 mm, height 1-3 mm).

Reference:-1) G.Korb, E. Neubauer, Thermophysical Properties of Metal Matrix Composites MMC-Assess Consortium, July 2001