Simultaneous measurement of temperature, thermal diffusivity, thermal conductivity and spectral emissivity by photothermal radiometry

The Laser Absorption Radiation Thermometer (LART) technique has been developed at the National Physical Laboratory (NPL) to measure temperature independently of target emissivity, surrounding background radiation or gaseous absorption. The technique is based on photothermal radiometry and utilises fibre optics in the latest instrument. Thermal properties can in principle be deduced from measurements of modulated (ac) and steady (dc) thermal radiations, in particular thermal diffusivity, thermal conductivity and spectral emissivity. The paper presents progress so far on “Multi LART” and initial measurements made of oxidised Inconel and platinum targets. The Multi LART instrument has great potential for meeting industrial needs for temperature and thermal properties measurements in challenging environments

Improvement of thermal nodal models with negative compensation capacitors

The objective of the present study is to improve the modelling of heat transfer by elementary cells, aiming to increase the quality of their representation in the Laplace space. From the twoport representation and its connections with the classical nodal method, we show that the systematic increase of the order leads to improve the simulation results in transients. But, we would like to find a better reduced topology of the equivalent elementary network of heat conduction, closer to the analytical solution and verifying its terms for higher orders. The wall representation can be performed by an impedance network with “Π” or “T” shaped cells. The approximation of these impedances leads to define a new cell topology, which introduces capacitances with a negative value called "compensation capacitors". The value of these new elements only depends on the model nodal thermal capacitances in a wall. We study the transfer functions of these various equivalent networks as twoports that we will then compare to the analytical solution of the heat transfer equation. Some interesting values of the negative compensation capacitors are then obtained from transfer function; however, the optimal value would only be given from simulation results. All the established results will be confirmed by transient response simulations, which show the high performances of these new structures. These results are also validated by a modal analysis of these systems. The study of the model's accuracy show that the importance of the reduction for equivalent maximum errors corresponds to the square of the number of elementary cells

A temperature-aware battery cycle life model for different battery chemistries

With the remarkable recent rise in the production of battery-powered devices, their reliability analysis cannot disregard the assessment of battery life. In the literature, there are several battery cycle life models that exhibit a generic trade-off between generality and accuracy. In this work we propose a compact cycle life model for batteries of different chemistries. Model parameters are obtained by fitting the curve based on information reported in datasheets, and can be adapted to the quantity and type of available data. Furthermore, we extend the basic model by including some derating factors when considering temperature and current rate as stress factors in cycle life. Applying the model to various commercial batteries yields an average estimation error, in terms of the number of cycles, generally smaller than 10%. This is consistent with the typical tolerance provided in the datasheets

On the behavior of the LixNiO2 system: an electrochemical and structural overview

Lithium nickel oxide exhibits a departure from stoichiometry (Li1-zNi1+zO2) consisting in the presence of extra-nickel ions within the lithium sites. Using optimized experimental synthesis conditions, compositions very close to the ideal stoichiometry (z = 0.02) can be obtained. By using the sensitivity of the lithium site isotropic temperature factor to the stoichiometry, the amount of extra-nickel ions can be determined in a very precise way. The loss of reversibility at the first cycle is mainly related to the change in the oxidation state of the extra-nickel ions, which induces a local collapse of the structure and makes difficult the lithium re-intercalation, A systematic structural study of LixNiO2 phases has been performed by extended X-ray absorption fine structure (EXAFS) as well as X-ray and electron diffraction. In the case of the starting Li0.98Ni1.02O2 phase, a local distortion of the NiO6 octahedra, resulting from a dynamic Jahn-Teller effect of low spin trivalent nickel ions has been evidenced from the EXAFS study. For the partially de-intercalated materials (0.50 < x < 0.75) which crystallize in the monoclinic system, the EXAFS study shows that the NiO6 octahedra are only slightly distorted due to the occurrence of a hopping phenomenon between Ni-IV and Ni-III. Electron diffraction experiments show the existence of a superstructure due to a peculiar lithium-ion ordering. Systematic electrochemical studies have shown that this ordering is strongly sensititve to the presence of extra-nickel ions