International audience ; In France, like in all developed countries, the amount of solid wastes generated per year has increased continuously since the 1960's. To hold back this trend, waste policies have been set up, as illustrated by current EU waste policy and its five main priorities: prevention, reuse, recycling, recovery and disposal. Composting can be defined as the process whereby aerobic micro-organisms convert organic substrates into compost: a hygienic, biostable product that can be beneficially applied to land (Haug, 1993; Mohee & Mudhoo, 2005). Therefore, it fits perfectly with the fourth priority, recovery, which fosters extraction of useful material or energy from wastes. In this context, interest towards composting has increased continuously during the last few years. During treatment, micro-organisms breakdown organic matter and produce carbon dioxide, water and heat. Heat generated by biological activity modifies moisture content and temperature conditions. These changes result in the appearance of a temperature peak in first days of treatment and most pathogens are killed by the high temperatures reached (around 70-80°C), turning waste into a hygienic product. Among the various physical parameters taking part in the composting process, thermal conductivity seems to be of major importance, and could be used as an indicator to follow heat transfers within the organic matrix. However, as all physical parameters involved in the process, the initial preparation of the substrate (adjustment of moisture content or C/N ratio, addition and mixing with bulking agent …) has an influence on the physical parameters involved in the process (such as bulk density, Free Air Space or air permeability) and thus, on thermal conductivity. Moreover, difficulties often occur in composting experiments because the effects of compaction on physical properties are ignored, or information about these effects is lacking. As soon as the pile of waste is built, the settlement of the composting matrix begins. This ...
International audience ; In France, like in all developed countries, the amount of solid wastes generated per year has increased continuously since the 1960's. To hold back this trend, waste policies have been set up, as illustrated by current EU waste policy and its five main priorities: prevention, reuse, recycling, recovery and disposal. Composting can be defined as the process whereby aerobic micro-organisms convert organic substrates into compost: a hygienic, biostable product that can be beneficially applied to land (Haug, 1993; Mohee & Mudhoo, 2005). Therefore, it fits perfectly with the fourth priority, recovery, which fosters extraction of useful material or energy from wastes. In this context, interest towards composting has increased continuously during the last few years. During treatment, micro-organisms breakdown organic matter and produce carbon dioxide, water and heat. Heat generated by biological activity modifies moisture content and temperature conditions. These changes result in the appearance of a temperature peak in first days of treatment and most pathogens are killed by the high temperatures reached (around 70-80°C), turning waste into a hygienic product. Among the various physical parameters taking part in the composting process, thermal conductivity seems to be of major importance, and could be used as an indicator to follow heat transfers within the organic matrix. However, as all physical parameters involved in the process, the initial preparation of the substrate (adjustment of moisture content or C/N ratio, addition and mixing with bulking agent …) has an influence on the physical parameters involved in the process (such as bulk density, Free Air Space or air permeability) and thus, on thermal conductivity. Moreover, difficulties often occur in composting experiments because the effects of compaction on physical properties are ignored, or information about these effects is lacking. As soon as the pile of waste is built, the settlement of the composting matrix begins. This ...
Bulk SrTiO3 is a quantum paraelectric in which an antiferrodistortive distortion below ≈105 K and quantum fluctuations at low temperature preclude the stabilization of a long-range ferroelectric state. However, biaxial mechanical stress, impurity doping, and Sr nonstoichiometry, among other mechanisms, are able to stabilize a ferroelectric or relaxor ferroelectric state at room temperature, which develops into a longer-range ferroelectric state below 250 K. In this paper, we show that epitaxial SrTiO3 thin films grown under tensile strain on DyScO3 exhibit a large reduction of thermal conductivity, of ≈60% at room temperature, with respect to identical strain-free or compressed films. The thermal conductivity shows a further reduction below 250 K, a temperature concurrent with the peak in the dielectric constant [J. H. Haeni et al., Nature (London) 430, 758 (2004)]. These results suggest that strain gradients in the relaxor and ferroelectric phase of SrTiO3 are very effective phonon scatterers, limiting the thermal transport in this material. ; F.R. acknowledges financial support of the Ministerio de Economía y Competitividad of Spain (Project No. MAT2016- 80762-R), and Xunta de Galicia (Centro Singular de Investigación de Galicia accreditation 2016-2019) and the European Union (European Regional Development Fund ERDF), and the European Commission through the project 734187 - SPICOLOST (H2020-MSCA-RISE-2016). J.A.M. would like to acknowledge the National Science Foundation, Division of Electrical, Communication and Cyber Systems, Award No. 1901972. This work was supported by the National Natural Science Foundation of China (Grant No. 51876186), Natural Science Foundation of Zhejiang Province (Grant No. LZ19E060002), the Fundamental Research Funds for the Central Universities (K20200145), and ZJUI with W.-L. Ong as one of the Principal Supervisors. ; Peer reviewed
The heat capacity and thermal conductivity of multiferroics Bi1–xPrxFeO3 (0 ≤ x ≤ 0.50) has been studied in the temperature range of 130–800 K. A slight substitution of praseodymium for bismuth is found to lead to a noticeable shift of the antiferromagnetic phase transition temperature whilst the heat capacity increases. The temperature dependences of the heat capacity and thermal conductivity exhibit additional anomalies during phase transitions. The experimental results suggest that the excess heat capacity can be attributed to the Schottky effect for three-level states. The basic mechanisms of the heat transfer of phonons are highlighted and the dependence of the mean free path on temperature is determined. ; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union's Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART²
In France, like in all developed countries, the amount of solid wastes generated per year has increased continuously since the 1960's. To hold back this trend, waste policies have been set up, as illustrated by current EU waste policy and its five main priorities: prevention, reuse, recycling, recovery and disposal. Composting can be defined as the process whereby aerobic micro-organisms convert organic substrates into compost: a hygienic, biostable product that can be beneficially applied to land (Haug, 1993; Mohee & Mudhoo, 2005). Therefore, it fits perfectly with the fourth priority, recovery, which fosters extraction of useful material or energy from wastes. In this context, interest towards composting has increased continuously during the last few years. During treatment, micro-organisms breakdown organic matter and produce carbon dioxide, water and heat. Heat generated by biological activity modifies moisture content and temperature conditions. These changes result in the appearance of a temperature peak in first days of treatment and most pathogens are killed by the high temperatures reached (around 70-80°C), turning waste into a hygienic product. Among the various physical parameters taking part in the composting process, thermal conductivity seems to be of major importance, and could be used as an indicator to follow heat transfers within the organic matrix. However, as all physical parameters involved in the process, the initial preparation of the substrate (adjustment of moisture content or C/N ratio, addition and mixing with bulking agent …) has an influence on the physical parameters involved in the process (such as bulk density, Free Air Space or air permeability) and thus, on thermal conductivity. Moreover, difficulties often occur in composting experiments because the effects of compaction on physical properties are ignored, or information about these effects is lacking. As soon as the pile of waste is built, the settlement of the composting matrix begins. This settlement, called primary settlement or physical compressive settlement (Gourc et al., 2010; Yue et al., 2008) is related to the vertical load and leads to compaction. Despite its major importance, until now little has been written on thermal conductivity in composting, in particular about its link with compaction. That's why this study focus on it and aims to investigate how it evolves with (i) compaction or depth within the pile of waste, (ii) preparation parameters of the substrate and (iii) temperature. This investigation was carried out on mixtures of urban sludge and wooden palettes used as bulking agents. To understand how preparation parameters of the mixtures would affect thermal conductivity, two moisture content (50 and 65%), two types (fresh and recycled) and two meshes of bulking agent ( 20 mm) were tested. The influence of compaction (or depth) was evaluated in two steps. First, a "Schaub-Szabo" device (strongly inspired by the apparatus described in Schaub-Szabo and Leonard (Schaub-Szabo & Leonard, 1999)) was used to get depth-bulk density profiles in the different substrates. Then, these bulk densities were recreated in a modified air pycnometer where thermal conductivity was measured with a thermal probe directly embedded in the composting sample. Therefore, a link between thermal conductivity and compaction (or depth) could be established. On the other hand, the study of the impact of temperature on thermal conductivity was carried out in 10 liter cells where biological activity was prevented by a nitrogen atmosphere. The cells were filled with the same eight sludge-wooden palettes mixtures as before, and a thermal probe was put directly inside each sample. Then, they were put in different constant-temperature baths with target temperature from 5 to 75°C. In this study, thermal conductivity increased with depth and a statistical analysis highlighted the fact that it was only significantly impacted by moisture content (among the three preparation parameters cited above). Moreover, the impact of temperature on thermal conductivity was clear and a linear relationship between these two parameters could be established. Each correlation was specific to the substrate but with a similar slope. These results are interesting in two ways: first, until now little has been written on thermal conductivity in organic solid wastes, and in particular about its link with compaction. Besides this originality, the data obtained can now be used in numerical modeling to get a more thorough and accurate way to model heat transfers, an essential part of modeling composting systems.
For the high-power (HP) electronic applications the existing Si-based devices have reached the performance limits governed by the material properties. Hence the device innovation itself is unable to enhance the overall performance. GaN, a semiconductor with wide bandgap, high critical breakdown field, and high electronic saturation velocity is regarded as an alternative of Si. The material properties of GaN make it very suitable for fast-switching HP electronic devices and contribute to the fast growing of GaN technology. The state-of-the-art GaN devices operating up to 650 V have recently become commercially available. Further goal is to reach higher breakdown voltage which can be done via device engineering and material growth optimization. AlxGa1−xN is an ultrawide-bandgap (UWBG) semiconductor which is considered as a natural choice for next generation in the development of GaN-based HP electronic devices. This material attracts particular interest due to the possibility for bandgap tuning from 3.4 eV to 6 eV which allows nonlinear increase of avalanche breakdown field. Furthermore, both n- and p-type conductivity can be achieved on this material permitting variety of device design with reduced energy losses during operation. β−Ga2O3 is also a promising material for HP electronics because of its ultra-wide bandgap (4.8 eV) and a huge value of Baliga's figure of merit (FOM) exceeding by far that of GaN. More interesting feature making this material attractive is the availability of low-cost natural substrates, and then the possibility to obtain high crystal quality of device structures. For the HP electronic devices thermal conductivity is one of the key parameters determining the device's performance. The initial studies have shown that the thermal conductivity of AlxGa1−xN and β−Ga2O3 is quite low comparing with that of GaN. This is one of the biggest challenges slowing the development of these materials for HP device applications. Nevertheless, AlxGa1−xN- and β−Ga2O3-based field-effect transistors and Schottky-barrier diodes have been demonstrated showing performances superior to that of GaN. To optimize and maintain good performance and reliability, heat generated in the device active regions has to be effectively dissipated. Therefore the thermal conductivity of the materials in the device structures needs to be systematically studied and accurately determined. This information is critically important for the thermal management of the devices. Transient thermoreflectance (TTR) is a contactless nondestructive method for measuring of the thermal conductivity of materials. TTR, which is based on a pump-probe technique, has shown its potential in evaluation of the thermal conductivity in bulk crystals as well as in thin layers in hetero-epitaxial structures. The method requires an analysis of experimental data based on the fit of thermoreflectance transients with the solution of the one-dimensional heat transport equations by a least-square minimization of the fitting parameters. Such a procedure allows to extract not only the thermal conductivity of the constituent materials in the structures, but also the thermal boundary resistance at different hetero-interfaces. The main research results of the graduate studies presented in this licentiate thesis are summarized in three scientific papers. Paper I. In this paper thermal conductivity of β−Ga2O3 and high Al-content AlxGa1−xN thin layers was studied. For β−Ga2O3 the the effects of Sn doping and phonon-bondary scattering on the reduction of thermal conductivity were discussed. For the AlxGa1−xN we studied the effect of Al-Ga alloying which gives rise to phonon-alloy scattering. It was found that this scattering process accounts for low thermal conductivity of this material. Finally, a comparison for the thermal conductivity of the two materials was made. Paper II. In this paper the effect of layer thickness on the thermal conductivity of AlxGa1−xN layers grown by HVPE were investigated. Due to Al alloying the thermal conductivity of this material is degraded and reduced by more than one order of magnitude. On top of that we also observed further reduction of thermal conductivity when the layer thickness goes thinner. The mechanism of this phenomenon has been revealed by studying the phonon transport properties in bulk crystal and thin layer. Paper III. This study emphasizes the role of defects in GaN and AlxGa1−xN to the thermal conductivity of these materials. The dislocations, impurities, free carries, and random alloying have been separately studied and discussed. Thermal conductivity of samples containing these defects with various concentrations was measured and the results were interpreted by a theoretical model based on relaxation time approximation (RTA). ; Additional funding agencies: the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University, Faculty Grant SFO Mat LiU No. 2009 − 00971
We analyzed the phonon spectrum of graphene. The flexural phonon contribution on thermal properties of graphene is discussed. The thermal conductivity has been calculated using the Boltzmann equation in the relaxation time approximation. The temperature dependence of the thermal conductivity and of the phonon specific heat has been calculated in the low temperatures domain.
