A pEek on the lab

The main characterization parameters in the materials department are: thermal conductivity, thermal diffusivity, specific heat, enthalpy of fusion and radiative properties. The Laboratoire National de Metrologie et d’Essais (LNE), which is a world leader in the characterisation of the thermophysical properties of materials (thermal diffusivity, thermal conductivity, specific heat, enthalpy of fusion and radiative properties) and studies caloric quantities within its “Materials Department” has good experience in the development of metrological reference facilities in order to address the needs of traceable measurements with uncertainties sufficiently low to meet the requirements of the existing and emerging industries.

The French National Metrology Institute (LNE) has modified a high temperature Laser Flash apparatus in order to perform the measurement of the specific heat of solids by drop calorimetry. A sample hanging on a thin wire held by a motorised gripper, is heated by the inductive furnace and dropped into a heat-flux Calvet calorimeter maintained at near ambient temperature. The calorimeter is equipped with an in-situ electrical calibration system in order to perform accurate and reliable measurements of energy directly traceable to the International System of Units (SI). The electrical calibration system has been designed for the calibration of the calorimeter by electrical substitution (Joule effect) and remains in-situ during the drop of the heated sample, keeping exactly the same experimental configuration during both steps of calibration and measurement.

The calorimeter is composed of two Calvet thermopiles plugged in a thermalized cylindrical massive block of aluminum. The block is stabilised at near room temperature (25 °C) using temperature regulated water circulation in a jacket around the block. Each thermopile delivers a voltage proportional to the heat flux from the cell inside the fluxmeter to the aluminum block. The thermopiles of the two fluxmeters are electrically connected in opposition in order to compensate the parasitical heat fluxes due to any temperature fluctuation of the block. An in-situ electrical calibration system enabling to perform the heat calibration of the calorimeter by Joule effect was developed in order to prevent any modification of the experimental conditions during measurement. This calibration system consists in a resistance heating wire of nickel–chromium alloy wound on a length of 60 mm around an alumina crucible of 30 mm external diameter and 90 mm long.