M. Sheindlin

Thermal Conductivity of Uranium Dioxide up to 2900 K from Simultaneous Measurement of the Heat Capacity and Thermal Diffusivity.

The thermal diffusivity and heat capacity of uranium dioxide have been measured from 500 to 2900 K with an advanced laser-flash technique. These two quantities were determined simultaneously by means of an accurate numerical fitting of the experimental thermograms. At high temperatures the precision of the method used is much better than that associated with conventional laser-flash measurements. It was found that the heat capacity continues to increase even at temperatures above the expected lambda transition (2670 K). The inverse of the thermal diffusivity increases linearly with temperature up to 2600 K, whilst at higher temperatures the slope markedly decreases. A new expression for the thermal conductivity as a function of temperature is proposed, which is corroborated by some theoretical considerations on the underlying heat transport mechanisms.

New techniques for high-temperature melting measurements in volatile refractory materials via laser surface heating

An original technique for the measurement of high-temperature phase transitions was implemented based on a laser-heating method, enabling chemically unstable, refractory materials to be melted under controlled conditions. This technique includes two independent but correlated methods: In the first, fast multichannel pyrometry is employed to measure thermograms and spectral emissivity; in the second, a low-power probe laser beam is used for the detection of reflectivity changes induced by phase transitions on the sample surface. The experiments are carried out under medium ( approximately 10(2) kPa) or high ( approximately 10(2) MPa) inert-gas pressures in order to kinetically suppress evaporation in volatile or chemically instable samples. Two models for the simulation of the laser-heating pulses are as well introduced. Some results are presented about the successful application of this technique to the study of the melting behavior of oxides such as UO(2+x), ZrO(2), and their mixed oxides. The method can be extended to a broad class of refractory materials.

Melting point of MgO

Despite large commercial production of MgO-based ceramics for a wide gamut of applications, the melting point of magnesia remained uncertain for almost one century. This article shows that a number of problems must be solved to attain experimental conditions where the solid–liquid phase transition of magnesia can be unambiguously detected, and the temperature be measured with sufficient accuracy. The method adopted in the reported work is based on controlled laser pulse heating. The solidification point was measured by the thermal arrest occurring during cooldown from the melt. The measurement of temperature, a most delicate problem for pyrometry applications in semitransparent materials, was obtained by using combined brightness and spectral pyrometers. The experimental and analytical methods are described in some detail. The resulting melting point of MgO is 3250±20 K, which is approximately 150 K higher than the value currently recommended.

Pressure Dependence of UO2 Melting Measured by Double-Pulse Laser Heating

The melting point of uranium dioxide was measured as a function of isostatic pressure in an autoclave filled with helium up to 0.25 GPa. Containerless laser surface-heating was applied to measure the melting point by thermal arrest measurements during sample cooling. Precise control of the cooling rate, aimed at a precise determination of the freezing plateau, was obtained with a two-pulse method, where two Nd:YAG high power laser pulses are mixed within the same optical fiber and then focused on the sample surface. Calibration of the power of the two pulses permits the measurement of the melting point with very low uncertainty (0.5%). The measured melting slope of UO2 is 170 and 200 K⋅GPa−1, a value which is markedly higher than that predicted by the Clausius–Clapeyron equation, which expresses this slope in terms of the enthalpy of fusion and the melting expansion. Possible reasons for this discrepancy are discussed.

A universal laser-pulse apparatus for thermophysical measurements in refractory materials at very high temperatures

This paper presents a new experimental technique enabling thermophysical measurements to be carried out at very high temperatures in a very simple and small pressurized vessel in which the sample is heated by a continuous wave laser, and subsequently subjected to a short temperature pulse. The adopted method is essentially an extension of the “laser-flash” technique, widely used for thermal diffusivity measurements, whereby, in addition, the heat capacity and, hence, the thermal conductivity, λ, are simultaneously evaluated from the pulse analysis. Results are presented for the thermal diffusivity and heat capacity of graphite, zirconia, and uranium dioxide up to temperatures above 3000 K.

Experimental determination of the thermal conductivity of liquid UO2 near the melting point

The article gives an account of measurements of the thermal conductivity of liquid UO2. The sample was heated up to above the melting point by a laser pulse of a controlled shape, and the produced thermogram of temperature history was measured by a fast and accurate pyrometer with a time resolution of 10 μs. The experiment shows that the rate of temperature increase during the ascending part of the pulse changes moderately across the melting point. Due to the high power input, this effect cannot be explained in terms of the sole intake of latent heat of fusion. By solving the related heat transfer equation with a 2D-axisymmetric numerical model, it is demonstrated that this feature depends principally on heat conduction in the sample, and proves that the thermal conductivities of solid and liquid are not very different. A theoretical sensitivity study assessing the influence of the liquid thermal conductivity on the pulse temperature evolution showed that the conductivity of the liquid can be deduced from...