Impact of Screw and Edge Dislocation on the Thermal Conductivity of Nanowires and Bulk GaN

We report on thermal transport properties of wurtzite GaN in the presence of dislocations, by using molecular dynamics simulations. A variety of isolated dislocations in a nanowire configuration were analyzed and found to reduce considerably the thermal conductivity while impacting its temperature dependence in a different manner. We demonstrate that 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$^{-1}$ variation in combination with an exponent factor which depends on the material's nature, the type and the structural characteristics of the dislocation's core. Furthermore, the impact of the dislocations density on the thermal conductivity of bulk GaN is examined. The variation and even the absolute values of the total thermal conductivity as a function of the dislocation density is similar for both types of dislocations. The thermal conductivity tensors along the parallel and perpendicular directions to the dislocation lines are analyzed. The discrepancy of the anisotropy of the thermal conductivity grows in increasing the density of dislocations and it is more pronounced for the systems with edge dislocations

Simulation à l échelle atomique des cœurs de dislocations dans le nitrure de gallium

CAEN-BU Sciences et STAPS (141182103) / SudocSudocFranceF

The atomic and electronic structure of dislocations in Ga based nitride semiconductors

International audienceThe atomic and electronic properties of dislocations in III-N semiconductor layers, especially GaN are presented. The atomic structure of the edge threading dislocation is now well established with three different cores (8 or full core, 5/7 or open core and 4 atom ring). The use of atomistic simulations has confirmed these atomic structures and has given a good understanding of the electronic structure of the screw dislocation. Partial dislocations which are mostly confined in the area close to the substrate are now also being investigated. It is becoming clear that the electrical activity of all these defects is dependent on the layer quality, which is governed by the growth conditions

The effect of ergocalciferol on the precipitation of calcium carbonate

International audienceWe have conducted the first study on the effect of ergocalciferol (vitamin D-2) on the polymorphism of calcium carbonate. CaCO3 particles are synthesised from aqueous solutions of sodium carbonate and calcium chloride, in the presence of vitamin D-2 with different concentrations, at temperatures ranging from 25 to 80 degrees C. The effect of the additive on the polymorphism and the morphology of the CaCO3 crystallites are characterised using X-ray quantitative phase analysis, scanning electron microcopy (SEM) and Fourier transform infrared spectroscopy (FT-IR). Our results show that the presence of vitamin D-2 has an impact on the morphology and composition of the CaCO3 crystallites. Vitamin D-2 was demonstrated to promote the formation of aragonite a 50 degrees C with a volume fraction that can reach 83%. Such effect can be attributed to the formation of D-2-Ca+2 ion solution complexes and their adsorption on the surfaces of the CaCO3 crystallites

The effect of ergocalciferol on the precipitation of calcium carbonate

We have conducted the first study on the effect of ergocalciferol (vitamin D-2) on the polymorphism of calcium carbonate. CaCO3 particles are synthesised from aqueous solutions of sodium carbonate and calcium chloride, in the presence of vitamin D-2 with different concentrations, at temperatures ranging from 25 to 80 degrees C. The effect of the additive on the polymorphism and the morphology of the CaCO3 crystallites are characterised using X-ray quantitative phase analysis, scanning electron microcopy (SEM) and Fourier transform infrared spectroscopy (FT-IR). Our results show that the presence of vitamin D-2 has an impact on the morphology and composition of the CaCO3 crystallites. Vitamin D-2 was demonstrated to promote the formation of aragonite a 50 degrees C with a volume fraction that can reach 83%. Such effect can be attributed to the formation of D-2-Ca+2 ion solution complexes and their adsorption on the surfaces of the CaCO3 crystallites

Impact of dislocations on the thermal conductivity of bulk GaN. Molecular dynamics study

International audienc

Structure and microstructure study of Charonia lampas lampas shell

International audienc

Screw threading dislocations in AlN: Structural and electronic properties of In and O doped material

Density functional theory calculations were performed on undoped AlN screw threading dislocations (TDs) as well as TDs doped by indium and oxygen, prompted by integrated experiments through transmission electron microscopy and spectroscopic techniques demonstrating enhanced In and O concentrations in screw dislocation cores. It is revealed that screw TDs act as conduction pathways to charge carriers, introducing multiple levels in the bandgap due to overstrained, dangling, and “wrong” bonds formed even in the undoped cores. The presence of impurities and especially metallic In elevates the metal-like electronic structure of the distorted material and promotes the conductivity along the dislocation line. Hence screw dislocations in AlN are established as highly prominent conductive nanowires in semiconducting thin films and prospects for novel, highly functional nano-device materials through exploitation of screw TDs are attested

Chitosan-multilayered graphene oxide hybrid beads for Zn 2 + and metoprolol adsorption

International audienc

Chitosan-multilayered graphene oxide hybrid beads for \protect \text{Zn}^{2+} and metoprolol adsorption

Chitosan (CS) hydrogel beads and hybrid beads made of a blending of CS hydrogels and Multilayer Graphene Oxide (MGO) were synthesized. The hybrid beads were prepared by gelation in NaOH solution of a 1 wt% CS acid solution with addition of MGO at either 1.5 wt% or 3 wt% loading rates. Prepared beads were characterized by infrared spectroscopy, thermogravimetric analysis (TGA), scanning electron cryo-microscopy and Brunauer–Emmett–Teller (BET) specific surface area measurements. $\text{Zn}^{2+}$ and Metoprolol (MTP) adsorption kinetics and isotherms were studied on the pristine and hybrid CS hydrogel beads. The adsorption kinetics of $\text{Zn}^{2+}$ and MTP in hybrid beads is limited by the diffusion to the MGO sites depending on their accessibility. While pure CS is not efficient for the MTP adsorption, the Langmuir-type isotherms of the 3 wt% MGO hydrogel beads (dose: 5 mg/100 mL) show 163 mg$\cdot$g$^{-1}$ maximum adsorption uptake. The MTP adsorption kinetics and isotherm suggest a MTP trapping on the MGO anionic sites (carboxylate groups) by electrostatic interactions. The $\text{Zn}^{2+}$ adsorption capacities are the highest for the 3 wt% MGO hydrogel beads (236 mg$\cdot$g$^{-1}$), and only of 40 mg$\cdot$g$^{-1}$ for the pure CS beads. The presence of $\text{Zn}^{2+}$ adsorption sites in the hybrid bead, such as MGO carboxylate groups giving electrostatic interactions, and CS amine groups leading to complexation, provides synergic adsorption effects. The competitive adsorption of $\text{Zn}^{2+}$ with respect to MTP in equimolar mixture was observed on hybrid beads (dose: 200 mg/100 mL) at 2 mmol$\cdot$L$^{-1}$ initial total concentration. At pollutant initial total concentration lower than 1.5 mmol$\cdot$L$^{-1}$, no competition occurs. The regeneration at pH 4 of the hybrid beads toward MTP or $\text{Zn}^{2+}$ adsorption was found to be 35–40% of the initial adsorption uptake for five adsorption/regeneration cycles