Salt weathering of sandstone during drying : effect of primary and secondary crystallisation

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A Temperature Analysis of High-power AlGaN/GaN HEMTs

Galliumnitride has become a strategic superior material for space, defense and civil applications, primarily for power amplification at RF and mm-wave frequencies. For AlGaN/GaN high electron mobility transistors (HEMT), an outstanding performance combined together with low cost and high flexibility can be obtained using a System-in-a-Package (SIP) approach. Since thermal management is extremely important for these high power applications, a hybrid integration of the HEMT onto an AlN carrier substrate is proposed. In this study we investigate the temperature performance for AlGaN/GaN HEMTs integrated onto AlN using flip-chip mounting. Therefore, we use thermal simulations in combination with experimental results using micro-Raman spectroscopy and electrical dc-analysis.Comment: Submitted on behalf of TIMA Editions (http://irevues.inist.fr/tima-editions

Hygroscopic behaviour of paper and books

This study presents experimental analysis and numerical modeling of hygroscopic moisture buffering by paper and books. First, a literature review of moisture transport properties of paper is presented. Experimental work on two paper types includes SEM analysis of the paper structure, determination of sorption isotherms and water vapor permeability measurements. A hysteretic model for paper is presented, which is based on the measurement of the main adsorption and desorption curves. It is shown that the water vapor permeability in a hysteretic model is dependent on the moisture content and not on the relative humidity. Books consist of several paper sheets with air layers between the sheets. To take the air layers into account, a parallel transport model is proposed to determine the effective moisture transport properties of books taking into account the air layers. The dynamic hygroscopic behavior of small book samples was measured. It is shown that, although the water vapor permeability of different paper types can be quite different, the effusivity of a book highly depends on the presence of the air layers and can therefore remain comparable for different paper type

Texture and mineralogy influence on durability: The Macigno sandstone

The behaviour of ornamental stones in response to environmental changes or interactions is crucial when dealing with the conservation of cultural heritage.Weathering factors affect each rock differently, depending on structure, mineralogy, and extraction and implementation techniques. This work focuses on the Macigno sandstone, a dimension stone often employed in Tuscany over the centuries. A thorough mineralogical (optical microscopy, scanning electron microscopy and X-ray powder diffraction) and petrophysical characterization (i.e. mercury intrusion porosimetry, X-ray computed tomography, hygroscopic adsorption behaviour, ultrasounds, image analysis and capillary uptake) was made of the sandstone type extracted in the area of Greve in Chianti. The lithotype shows mineralogical (i.e. presence of mixed-layer phyllosilicates) and microporosimetric features, leading to a high susceptibility to relative humidity variation. Moreover, the influence of swelling minerals is related to weathering due to saline solution. The joint application of petrographic and petrophysical techniques allows an understanding of the characteristic weathering pattern of exfoliation (i.e. detachment of multiple thin stone layers, centimetre scale, that are sub-parallel to the stone surface)

Vertical breakdown of GaN on Si due to V-pits

Gallium nitride on silicon (GaN/Si) is an important technological approach for power electronic devices exhibiting superior performance compared to devices based on a pure silicon technology. However, the material defect density in GaN/Si is high, and identification of critical defects limiting device reliability is still only partially accomplished because of experimental difficulties. In this work, atomic force microscopy, scanning electron microscopy, secondary ion mass spectrometry, and cathodoluminescence were employed to investigate commonly occurring epitaxial overgrown V-pits and inhomogeneous incorporation of oxygen and carbon across layer stacking in the vertical direction. These experiments identified V-pits as regions with higher n-type carrier concentrations and paths for vertical leakage through the buffer, as directly probed by conductive atomic force microscopy. The deleterious effect of V-pits on device performance is demonstrated by evaluating test devices fabricated on two wafers with significantly diverse density of buried V-pits induced by varying growth conditions of the aluminum nitride nucleation layer. A clear correlation between observed vertical breakdown and density of V-pits within the C-doped GaN layer below the device structures is obtained. Back-gating transient measurements also show that the dynamic device behavior is affected by the V-pit density in terms of the detrapping time constants.Gallium nitride on silicon (GaN/Si) is an important technological approach for power electronic devices exhibiting superior performance compared to devices based on a pure silicon technology. However, the material defect density in GaN/Si is high, and identification of critical defects limiting device reliability is still only partially accomplished because of experimental difficulties. In this work, atomic force microscopy, scanning electron microscopy, secondary ion mass spectrometry, and cathodoluminescence were employed to investigate commonly occurring epitaxial overgrown V-pits and inhomogeneous incorporation of oxygen and carbon across layer stacking in the vertical direction. These experiments identified V-pits as regions with higher n-type carrier concentrations and paths for vertical leakage through the buffer, as directly probed by conductive atomic force microscopy. The deleterious effect of V-pits on device performance is demonstrated by evaluating test devices fabricated on two wafers with s..

Buffer breakdown in GaN-on-Si HEMTs: A comprehensive study based on a sequential growth experiment

Abstract The aim of this work is to investigate the breakdown mechanisms of the layers constituting the vertical buffer of GaN-on-Si HEMTs; in addition, for the first time we demonstrate that the breakdown field of the AlN nucleation layer grown on a silicon substrate is equal to 3.2 MV/cm and evaluate its temperature dependence. To this aim, three samples, obtained by stopping the epitaxial growth of a GaN on Silicon stack at different steps, are studied and compared: Si/AlN, Si/AlN/AlGaN, full vertical stack up to the Carbon doped buffer layer. The current-voltage (IV) characterizations performed at both room temperature and high temperature show that: (i) the defectiveness of the AlN nucleation layer is the root cause of the leakage through an AlN/Silicon junction, and causes the vertical I-V characteristics to have a high device-to-device variability; (ii) the first AlGaN layer grown over the AlN, beside improving the breakdown voltage of the whole structure, causes the leakage current to be more stable and uniform across the sample area; (iii) a thick strain-relief stack and a carbon-doped GaN buffer enhance the breakdown voltage up to more than 750 V at 170 °C, and guarantee a remarkably low device-to-device variability. Furthermore, a set of constant voltage stress on the Si/AlN sample demonstrate that the aluminum nitride layer shows a time dependent breakdown, with Weibull-distributed failures and a shape factor greater than 1, in line with the percolation model

Thermal conductivity of gypsum plasterboard beyond dehydration and its correlation with the pore structure

Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.Gypsum plasterboard is a material used in the building industry for its low weight (porosity 50-65%) and its high resistance to fire due to the endothermic dehydration taking place between 150 and 200°C. Its thermal conductivity which is a decisive thermal property regarding reaction to fire drops by 50% of its initial value after dehydration due to the loss of water (20 mass %) but starts to rise again with rising temperature and reaches its initial value around 750°C. The present study shows that this rise is not due to the increasing radiative or conductive heat transfer but to changes in the bimodal pore structure which leaves the overall structural dimensions nearly unchanged (dilatation of around 2%). Different methods such as mercury intrusion porosimetry, scanning electron microscopy and in-situ X-ray diffraction up to 1000°C were carried out to investigate the correlation between pore structure and thermal conductivity of this material.dc201

Durability of the Indian Kandla Grey sandstone under Western European climatic conditions

An increasing amount of imported natural building stones are being used in Western Europe, often as a replacement of more traditional, local building stones. Unlike for these traditional stones, which have been used under the prevailing climatic conditions in Western Europe, the durability of these imported stones is largely unknown. Therefore, it is essential to study their behaviour under these climatic conditions in order to predict their weathering resistance. The chemical and structural properties of these new building materials need to be determined and their behaviour under changing environmental conditions needs to be studied. When these materials are being used in Western Europe, they have to resist to significant mechanical stresses due to the imbibition of de-icing salt solutions. These de-icing salts are very frequently used during winter in Western Europe, while temperature fluctuates between freezing and thaw conditions. In this research, focus has been laid on the multi-disciplinary characterization of the compact Kandla Grey layered sandstone. This stone is recently frequently imported from India to Belgium. Besides traditional techniques, (according to European Standars for natural stone testing) highly advanced research techniques such as µ-XRF and HRXCT were used to characterize and monitor the changes under different external conditions such as freezing, thawing and salt crystallization. The results of this study demonstrate that the structural properties of the laminations inside Kandla Grey have an influence on the resistance of the stone to frost and salt weathering. Based on these results, it can be concluded that Kandla Grey can be vulnerable to these types of weathering under the current climatic conditions in Western Europe

Examining the psychological wellbeing of refugee children and the role of friendship and bullying

BACKGROUND: Refugee children might have experienced violent and traumatic events before settling into a new country. In the United Kingdom, the number of refugee children is increasing; however, little is known about their psycho-social and physical well-being. AIM: This study aims to investigate the psychological well-being and behaviour of refugee children compared to British-born children on a number of psychological, social, behavioural, and health-related issues and to investigate the role of friendship as a protective factor. SAMPLES: This study utilized a sample of 149 refugee children recruited from two charities, 79 of which are children aged 6-10 years and 70 older refugee children aged 11-16 years. The study also included 120 non-refugee children recruited from primary schools aged 6-10 years. METHODS: This is a cross-sectional study that investigates the psycho-social well-being of refugee children compared to non-refugee British-born children. The study explored symptoms of posttraumatic stress disorder, emotional and behavioural problems (Strengths and Difficulties Questionnaire), self-esteem, friendships and popularity, bullying and victimization, physical health, and psychosomatic problems. RESULTS: Young refugee children reported more peer problems, functional impairment, physical health, and psychosomatic problems compared to the control children and older refugee children groups. On the other hand, older refugee children had lower self-esteem (academic and social self-peers) compared to the younger refugee children group. The differences between the groups were explained by friendship quality, number of friends, peer bullying/victimization, or sibling bullying/victimization except for physical health and psychosomatic problems. CONCLUSIONS: While refugee children were found to be at risk on various levels, the findings also point to the fact that social relationships including friendship quality and number of friends played an essential protective role. Conversely, bullying was a risk factor that explained many of the refugees' problems. These findings pave the way for future research to further probe into the well-being of refugee children in the United Kingdom while also targeting relevant intervention schemes specifically tailored to address their needs