Thermal Conductivity - Different Methods for Measurement of Thermal Conductivity

By AZoM Editors

Table of Contents

Thermal Conductivity

Consider a sample of cross section A across which a thermal gradient exists. T₂ and T₁ are the temperatures measured over a length ÄL. Let Q be the quantity of heat flowing through A as shown in Fig. 1 below.

Figure 1

Now, thermal conductivity K is given by the ratio of the heat flux Q/A to the thermal gradient ÄT / ÄL.

Measurement of Heat Flux

The heat flux can be measured directly or indirectly. These methods are:

• The absolute method where the electrical power supplied to the heater is measured and
• The comparative method where a comparative measurement is made

The heat flux has to be uniaxial in all these methods and hence radial heat loss or gain must be minimized by methods such as insulation.

Thermal Conductivity and System Configuration

The length of a sample is influenced by the magnitude of the thermal conductivity. When the thermal conductivity of the sample is high, the amount of heat flowing is high and the heat lost from the sample’s lateral surface is small. As a high temperature gradient is established in this case, it is possible to measure it accurately.

On the other hand, samples with low thermal conductivity (and correspondingly low heat flux) are usually of a smaller thickness, which is sufficient to generate an accurately measurable thermal gradient. Smaller thicknesses also mean less lateral losses. Sometimes, self-guarding is provided for lateral surfaces by the use of additional pieces of the sample material.

For low temperatures, the sample is packed inside insulation to minimize heat losses or heat gains along the radial direction. Installation of a guard, which can be controlled to have a temperature gradient same as that across the sample, is often required at high temperatures.

At such high temperatures, heat losses are difficult to control. Therefore, the ratio of conductance of the sample to the conductance of lateral insulation becomes significant, as does the quality of guarding.

Methods for Conductivity Measurement

Here we discuss a few methods of measuring thermal conductivity of solid materials at temperatures ranging from sub ambient temperatures to 1500°C. These include:

• Axial flow methods
• Absolute axial heat flow method
• Comparative cut bar method
• Guarded/Unguarded Heat Flow Meter Method
• Guarded Hot Plate Method
• Hot Wire Method
• The Probe Method

Axial Flow Methods

Axial flow methods are especially deployed at cryogenic temperatures and produce consistent, accurate results. Radial heat losses are negligible at such low temperatures.

Absolute Axial Heat Flow Method

For sub ambient environments, the absolute axial heat flow method is suitable. In this method, the electrical power applied to the heater has to be precisely determined.

Comparative Cut Bar Method

The comparative cut bar method, which is widely used for determining axial thermal conductivity, is based on the principle of comparing thermal gradients. In this method, heat flux is passed through samples of known and unknown materials.

Figure 2

A sample of unknown material is sandwiched between two reference samples as shown in Figure 2. If K_R is the thermal conductivity of the reference samples and K_S that of the sample of unknown material, the heat flux through the unknown sample can be calculated from the expression:

Guarded/Unguarded Heat Flow Meter Method

The guarded/unguarded heat flow meter method, which is deployed in many testing instruments, is based on a flux gauge, which functions similar to the reference samples used in the above method. The thermal conductivity of the sample of unknown material is given by:

Thermal conductivities of building insulation materials are determined using this method.

Guarded Hot Plate Method

Another widely used method for measuring the conductivity of insulations is the guarded hot plate method. The samples used in this method are larger.

Three different measurement apparatus are required for carrying out thermal conductivity measurements at room temperatures, low temperatures (up to -180°C), and high temperatures (600°C or greater), respectively. The most common symmetric configuration in which two specimens sandwich the heater assembly is shown in Figure 3.

Figure 3

Hot Wire Method

Thermal conductivities of materials such as insulating bricks, powder or fibrous materials are measured using this method. Thermal properties of liquids and plastics can also be determined by this method, which requires isotropic samples.

The Probe Method

In this modified method, the thermal conductivity of a sample is determined by inserting a ‘hypodermic needle probe’ into the sample and measuring its response. A thermocouple and a heater is attached to the probe. This method is suitable for measurement of thermal conductivity in materials that are in semi rigid form, such as soils.

Anter Thermal Analysis Equipment

Anter Corporation manufactures thermal properties analyzers to measure:

• Thermal expansion and shrinkage
• Thermal conductivity and resistivity
• Thermal diffusivity and specific heat capacity of a wide range of materials.

This information has been sourced, reviewed and adapted from materials provided by Anter Corporation.

For more information on this source, please visit Anter Corporation.