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The effect of the structural inhomogeneity and oxygen defects on the thermal conductivity of polycrystalline aluminum nitride (AlN) thin films deposited on single-crystal silicon substrates is experimentally and theoretically investigated. The influence of the evolution of crystal structure, grain size, and out-of plane disorientation along the cross plane of the films on their thermal conductivity is analyzed. The impact of oxygen-related defects on thermal conduction is studied in AlN/AlN multilayered samples. Microstructure, texture, and grain size of the films were characterized by X-ray diffraction and scanning and transmission electron microscopy. The measured thermal conductivity obtained with the 3-omega technique for a single and multiple layers of AlN is in fairly good agreement with the theoretical predictions of our model, which is developed by considering a serial assembly of grain distributions. An effective thermal conductivity of 5.92 W m(-1) K(-1) is measured for a 1107.5 nm-thick multilayer structure, which represents a reduction of 20% of the thermal conductivity of an AlN monolayer with approximately the same thickness, due to oxygen impurities at the interface of AlN layers. Our results show that the reduction of the thermal conductivity as the film thickness is scaled down, is strongly determined by the structural inhomogeneities inside the sputtered films. The origin of this non-homogeneity and the effect on phonon scattering are also discussed.

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Theoretical and experimental study of a thermal damper based on a CNT/PCM composite structure for transient electronic cooling. Energy Conversion and Management, Elsevier, 2017, 142, pp.257-271 AbstractThe present study focuses on a thermal damper that aims at smoothing the temperature peaks experienced by electronic components during transient solicitations. It consists of a silicon casing containing a densified or undensified carbon nanotube (CNT) array -linking directly both sides of the system -filled with phase change material (PCM). Theoretical consideration enables to define the concept of ideal thermal damper in order to study the foreseeable performance of this kind of system. Its thermal effectiveness can be predicted by means of two non-dimensional numbers, linked to the thermal capacity of the system and to the latent heat of the PCM. A numerical model shows that the behavior of a non-ideal thermal damper can differ from that of an ideal thermal damper: it is mostly affected by the thermal resistance at the interface between the silicon and the CNT and the temperature glide during the PCM phase change. To complete the study, prototypes of thermal dampers are experimentally characterized, in terms of heat storage and heat conduction performance. An estimation method of the total apparent thermal capacity of the tested sample is developed in order to quantify its latent heat storage capacity. The latent energy storage density is 1.6 J cm -2 for the best sample and is observed to be preserved after 850 thermal cycles. The total thermal resistance of the thermal damper is estimated by means of a laser flash test and a simple model of the sample. Sensitivity analyses show that the main thermal resistances are located at the interfaces between silicon and CNT.Keywords: vertically aligned carbon nanotubes, phase change material, thermal energy storage, interfacial thermal resistance, transient electronic cooling Highlights: Two non-dimensional parameters enabling to design an ideal thermal damper are identified  Latent energy storage capacity of embedded PCM is estimated and reaches 1.6 J cm -²