Thermal conductivity measurement
After assuming one-dimensional heat flow and an isotropic medium, Fourier's law is then used to calculate the measured thermal conductivity,
The transient hot wire method (THW) is a very popular, accurate and precise technique to measure the thermal conductivity of gases, liquids,[3] solids,[4] nanofluids[5] and refrigerants[6] in a wide temperature and pressure range.
The technique is based on recording the transient temperature rise of a thin vertical metal wire with infinite length when a step voltage is applied to it.
An ASTM standard is published for the measurements of engine coolants using a single-transient hot wire method.
It covers a thermal conductivity range of at least 0.01-500 W/m/K (in accordance with ISO 22007-2) and can be used for measuring various kinds of materials, such as solids, liquid, paste and thin films etc.
The Transient Plane Source technique typically employs two samples halves, in-between which the sensor is sandwiched.
The flat sensor consists of a continuous double spiral of electrically conducting nickel (Ni) metal, etched out of a thin foil.
During the measurement a constant electrical effect passes through the conducting spiral, increasing the sensor temperature.
The heat generated dissipates into the sample on both sides of the sensor, at a rate depending on the thermal transport properties of the material.
This modification provides a one-sided interfacial measurement in offering maximum flexibility in testing liquids, powders, pastes and solids.
The physical model behind this method is the infinite line source with constant power per unit length.
For large times, the exponential integral can be approximated by making use of the following relation where This leads to the following expression Note that the first two terms in the brackets on the RHS are constants.
Thus if the probe temperature is plotted versus the natural logarithm of time, the thermal conductivity can be determined from the slope given knowledge of Q.
[10] A variation on the Transient Line Source method is used for measuring the thermal conductivity of a large mass of the earth for Geothermal Heat Pump (GHP/GSHP) system design.
[11][12][13] Understanding the ground conductivity and thermal capacity is essential to proper GHP design, and using TRT to measure these properties was first presented in 1983 (Mogensen).
The laser flash method is used to measure thermal diffusivity of a thin disc in the thickness direction.
With a reference sample specific heat can be achieved and with known density the thermal conductivity results as follows where It is suitable for a multiplicity of different materials over a broad temperature range (−120 °C to 2800 °C).
The idea behind this technique is that once a material is heated up, the change in the reflectance of the surface can be utilized to derive the thermal properties.
The change in reflectivity is measured with respect to time, and the data received can be matched to a model which contain coefficients that correspond to thermal properties.
This temperature response is measured by logging the amplitude and phase delay of the AC voltage signal from the heater across a range of frequencies (generally accomplished using a lock-in-amplifier).
The main advantages of the 3ω-method are minimization of radiation effects and easier acquisition of the temperature dependence of the thermal conductivity than in the steady-state techniques.
Although some expertise in thin film patterning and microlithography is required, this technique is considered as the best pseudo-contact method available.
[16] (ch23) The transient hot wire method can be combined with the 3ω-method to accurately measure the thermal conductivity of solid and molten compounds from room temperature up to 800 °C.
[19] The freestanding sensor-based 3ω technique[20][21] is proposed and developed as a candidate for the conventional 3ω method for thermophysical properties measurement.
