The Extended Pulse Method for the Measurement of the Thermal Diffusivity of Solids

Abstract

The paper presents a complete theory for a new method for the determination of the thermal diffusivity of a bulk solid in the form of a cylinder using a pulse of energy of finite duration delivered on one face and the subsequent temperature rise detected on a parallel face. It is an important feature of the method that the departure from equilibrium in the solid sample is small so that the temperature rise is no more than a few degrees Kelvin. The energy pulse may be of any temporal distribution and the detection of the temperature rise can be conducted at any point on the opposing face of the sample. The theory explicitly accounts for heat losses at all the surfaces of the sample and enables absolute measurement of the thermal diffusivity of the sample. A prototype instrument is described to realize this theory in which the heating pulse is generated by an array of light emitting diodes in a circular configuration which is then guided by a light pipe so that a uniform distribution is ensured across the flat face of the solid sample being tested. The instrument is designed for operation over the temperature range from ambient to 1300 K but, in the current proof of principle, measurements are conducted at room temperature on a sample of Pyroceram™ 9606.¹ In this case, the detection is performed with a micro-thermocouple at the center of the sample. Several different rectangular heating pulse durations are employed to show that the theory provides an appropriate description of the experiment. The potential for future applications of the technique is demonstrated.