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The all-inorganic perovskite cesium lead bromide (CsPbBr3) has attracted considerable attention as a promising material for optoelectronics and high-energy radiation detectors. In order to obtain a bulk single crystal from a melt, it is crucial to understand the peculiarities of melting and crystallization processes. Here, the solid–liquid and liquid–solid phase transitions were studied by differential thermal analysis at different heating/cooling rates (0.1, 1, 3, 5, and 10 °C/min). A two-stage melting mechanism of CsPbBr3 perovskite was proposed. The critical maximal sample temperature (Tcritical) was determined for each heating rate. If the sample was heated to a temperature below Tcritical, the crystallization occurred at a temperature higher than the melting point. Contrarily, if the sample was heated to a temperature higher than this critical value, the melt crystallization occurred with supercooling. We believe that such crystallization features are closely related to the melt structure, which changes during the sample heating. The activation energies of melting and crystallization processes of CsPbBr3 were determined to be 1846 and 1940 kJ/mol, respectively. For the first time, this study demonstrates the impact of heating and cooling conditions on the melting and crystallization processes of the bulk CsPbBr3. It is significant for gaining a fundamental understanding of the crystal growth and fabrication of high-quality monocrystalline materials.

Recently, it has been shown that high density nanoconfined water was the reason of the important enhancement of the effective thermal conductivity up to a factor of 50% of a nanoporous silicon filled with water. In this work, using molecular dynamics simulations, we further investigate the role of the temperature T (from 285 to 360 K) on the thermal conductivity enhancement of nanohybrid porous silicon and water system. Furthermore, by studying and analysing several structural and dynamical parameters of the nanoconfined water, we give physical insights of the observed phenomena. Upon increasing the temperature of the system, the thermal conductivity of the hybrid system increases reaching a maximum for T = 300 K. With this article, we prove the existence of new heat flux channels between a solid matrix and a nanoconfined liquid, with clear signatures both in the radial distribution function, mean square displacements, water molecules orientation, hydrogen bond networks and phonon density of states.

International audience

International audience; We report the thermal transport properties of wurtzite GaN in the presence of dislocations using molecular dynamics simulations. A variety of isolated dislocations in a nanowire configuration are analyzed and found to considerably reduce the thermal conductivity while impacting its temperature dependence in a different manner. Isolated screw dislocations reduce the thermal conductivity by a factor of two, while the influence of edge dislocations is less pronounced. The relative reduction of thermal conductivity is correlated with the strain energy of each of the five studied types of dislocations and the nature of the bonds around the dislocation core. The temperature dependence of the thermal conductivity follows a physical law described by a T À1 variation in combination with an exponent factor that depends on the material's nature, type and the structural characteristics of the dislocation core. Furthermore, the impact of the dislocation density on the thermal conductivity of bulk GaN is examined. The variation and absolute values of the total thermal conductivity as a function of the dislocation density are similar for defected systems with both screw and edge dislocations. Nevertheless, we reveal that the thermal conductivity tensors along the parallel and perpendicular directions to the dislocation lines are different. The discrepancy of the anisotropy of the thermal conductivity grows with increasing density of dislocations and it is more pronounced for the systems with edge dislocations. Besides the fundamental insights of the presented results, these could also be used for the identification of the type of dislocations when one experimentally obtains the evolution of thermal conductivity with temperature since each type of dislocation has a different signature, or one could extract the density of dislocations with a simple measurement of thermal anisotropy.

Raman spectroscopy was applied to investigate laser-induced heating of silicon nanowires (SiNWs) formed by metal-assisted chemical etching of lightly boron-doped crystalline silicon (c-Si) wafers. Low-frequency shift of the Raman peak from 520.5 cm−1 to about 517 cm−1 for SiNWs with length of 40–65 µm under laser irradiation with wavelengths of 632.8 or 488 nm was observed, and it was explained by an increase in the average temperature of SiNWs on about 150 K for the laser intensity about 1 kW/cm2. The same photoinduced heating was confirmed by analyzing the ratio between the Stokes and anti-Stokes components of the Raman scattering. The obtained experimental data allow us to estimate the thermal conductivity coefficient of SiNW array ~0.1 W/(m K), which is three orders of magnitude smaller than that of c-Si. Furthermore, the Raman spectra of SiNWs under excitation with intensity above 0.2–0.5 kW/cm2 consisted of an additional low-frequency peak, which is related to an overheated subsystem of well spatially separated fine SiNWs up to 600–700 K. The observed strong photoinduced heating can be used for local laser-induced treatment of SiNWs and biomedical applications.

International audience; An approach for the size evaluation of fine morphological features of fractal-like porous materials is described. The method is based on the measurements of thermal conductivity of porous layers with empty and filled pore networks. Being associated to a heat transfer model, the experimentally obtained values of thermal conductivity allow the size evaluation of very small structural constrictions ensuring interconnections between the main particles constituting the porous material. The method is applied to determine the size values of the structural constrictions interconnecting silicon nanocrystals forming meso-porous silicon layers.

In this paper, thermal transport properties study of partially amorphous porous silicon obtained by irradiation with swift (110 MeV) uranium ions is reported. A photoacoustic technique allows thermal conductivity evaluation of the samples with enhanced amorphous fraction (>80%). In particular, 3-fold thermal conductivity decrease is shown to be achieved for the completely amorphous porous silicon layers in comparison with as-prepared non-irradiated porous samples.

Results of a study of the photoacoustic transformation in microporous nanocrystalline silicon are described. The amplitude-frequency and phase-frequency characteristics of the photoacoustic signal from microporous silicon samples on a monocrystalline substrate exposed to illumination at various wavelengths are experimentally determined. Informative response was measured by the gas-microphone and piezoelectric detection methods. In terms of the proposed mathematical model, it is shown that the difference in the parameters of the photoacoustic signal for different wavelengths of exciting radiation is attributed to a shift of the fundamental absorption edge in nanocrystalline silicon. It is pointed out that the piezoelectric detection method is more sensitive to changes in the thermophysical and optical parameters of the porous layer.

In this paper photothermal transformation in semiconductor structures with modified properties of subsurface layer under its irradiation by pulse laser (~10 ns) radiation was analyzed. It was show that the presence of this surface modified layer leads to increasing of surface temperature in comparison with homogeneous case. Moreover, this increasing could even compensate the temperature decreasing induced by thermal source redistribution caused by charge carrier diffusion.

International audience; In the paper the experimental study of heat transport across the interface “porous silicon/liquid” by photoacoustic technique is reported. Two cases with and without liquid covering of porous silicon surface were considered. Thermal perturbations were excited at the surface of porous silicon as a result of absorption of the light with modulated intensity. The resulting thermal-elastic stresses arising in the system were registered with piezoelectric transducer. The amplitude-frequency dependencies of the voltage on the piezoelectric electrodes were measured. The presence of the liquid film leads to decreasing of the amplitude of photoacoustic signal as a result of the thermal energy evacuation from the porous silicon into the liquid. The experimental dependencies were fitted with the results of simulation that takes into account heat fluxes separation at the porous silicon/liquid interface. With the presented method one can precisely measure heat fluxes transferred from the solid into contacting fluid. Moreover, the presented approach can be easily adopted for the thermal conductivity study of the different nanofluids as well as thermal resistance at the interface nanostructured solid/fluid.