Thermal conductivity is the property of a material that expresses its capacity to transfer heat. It is defined as the heat quantity transferred through a unit length of a conductor of unit cross-section area due to differences in temperature between the two ends of the conductor. Conductivity can be expressed in watts per meter Kelvin and is determined through a number of methods.
Absolute Axial Flow Method
The absolute axial flow method of determining conductivity involves the use of an electric heating element such as a heater on one end of a cylindrically uniform rod of the material in question. The power through the heating element is known at hand and is equal to the quantity of heat causing the temperature gradient between the two ends. The length and the cross-section area are determined as well for use in the calculation of the conductivity value. This method is used to determine conductivity in sub-ambient settings and is greatly affected by heat losses to the surroundings, especially when the specimen temperature exceeds room temperature.
Comparative Bar Method
The comparative bar method or the comparative cut bar method involves the use of a material of already known conductivity to determine the conductivity of another. In this method, a rod of the material with unknown conductivity is sandwiched in between a material with known conductivity on both of its ends. A heating element is then placed at one end of the setup in a similar manner to the absolute axial flow method, and the segment lengths and cross-section area are determined for the calculations involved.
Guarded Hot Plate
The guarded hot plate method of measuring conductivity is mostly used to determine the conductivity of insulating materials. In this method, flat metering sections electrically heated and surrounded by guard heater sections on all of the lateral sides are used as the heat supply for the hot side of the specimens. The most common method involves sandwiching the heater assembly between two specimens, with the guarded hot plates arranged in a symmetrical fashion. However, other parameters, such as the metered region's unidirectional heat flux, temperature readings at both hot and cold ends of the specimen and its thickness are necessary for calculations using this method.
Hot Wire Method
The hot wire method is used to measure the conductivity of plastics, liquids and other materials with comparatively low conductivity values, such as powders and bricks. A more advanced version of this method is the probe method, involving hypodermic needles sensitive to heat being inserted into specimens. An example is determining the conductivity of soil placed in a furnace to provide the heat for an experiment. The probes also have electrically heated elements installed in them. A temperature against time curve is obtained from the setup from which the conductivity is calculated.
Tags: cross-section area, determine conductivity, heating element, absolute axial flow, area determined