Computer simulation of the growth of heterostructure systems

We have investigated the growth of heterostructures with high lattice mismatch and the growth of quantum wires on the top of ridges and through shadowing masks. Simulations are performed within a Monte Carlo scheme using tetrahedral lattice structure of semiconductor materials. It is shown that results of different simulations present similarities that we attribute to the primary role of kinetic effects as the driving force during epitaxial growth. The formation of 3D islands showing (111) facets, or (111) side walls when depositing through a shadowing mask, has been observed. The facets are of better quality than the top (001) surface because of the higher mobility of atoms on these facets.     

Physics of epitaxy and c-BN films optimized growth

The cubic zinc-blende phase of boron nitride (c-BN) affords a plethora of potential applications based on material hardness, low chemical reactivity, high thermal conductivity and also on its very large band gap. A modern growth experiments are currently carried out to elaborate monocrystalline c-BN films with a strong film-substrate adherence and up to now several substrates have been used. However, although the film concentration of c-BN is increasing, the growth optimization is still a question under debate. Optimum conditions must be found in energetic growth techniques (IBAD) as well as substrate conditions (temperature, polarization). In this field basic physics is at the very heart of any strategy which aims to elaborate high quality c-BN films. This involves the physics of heteroepitaxy, an aspect of the growth process which consists in taking advantage of complementary freedom degrees as substrate choice and introduction of buffer layers to optimize heterointerfaces. Although several substrates have been tested an optimized choice still remain to be found. On the other hand, the strategy of buffer layers to improve film quality has been less investigated. In this communication, we give the state of art of this field and we discuss the problem of the optimization of c-BN film quality and we demonstrate that the variation of c-BN content in BN films grown on different substrates can be understood on the basis of elasticity theory.     

A novel modified lattice Boltzmann method for simulation of gas flows in wide range of Knudsen number

To cover a wide range of the flow regimes, a new relaxation time formulation by considering the rarefaction effect and the effective dynamic viscosity has been obtained. By using the modified lattice Boltzmann method (LBM), pressure driven flow through micro and nano channels has been modeled for wide range of Knudsen number, Kn, covering the slip, transition and to some extent the free molecular regimes. The results agree very well with existing empirical and numerical data. The velocity profile was predicted as well as the volumetric flow rate and for the first time, the well known Knudsen minimum effect has been captured about Kn = 1.     

Multi-scale modelling of silicon nanocrystal synthesis by Low Pressure Chemical Vapor Deposition

A multi-scale model has been developed in order to represent the nucleation and growth phenomena taking place during silicon nanocrystal (NC) synthesis on SiO₂ substrates by Low Pressure Chemical Vapor Deposition from pure silane SiH₄. Intrinsic sticking coefficients and H₂ desorption kinetic parameters were established by ab initio modelling for the first three stages of silicon chemisorption on SiO₂ sites, i.e. silanol Si―OH bonds and siloxane Si―O―Si bridges. This ab initio study has revealed that silane cannot directly chemisorb on SiO₂ sites, the first silicon chemisorption proceeds from homogeneously born unsaturated species like silylene SiH₂. These kinetic data were implemented into the Computational Fluid Dynamics Fluent code at the industrial reactor scale, by activating its system of surface site control in transient conditions. NC area densities and radii deduced from Fluent calculations were validated by comparison with experimental data. Information about the deposition mechanisms was then obtained. In particular, hydrogen desorption has been identified as the main limiting step of NC nucleation and growth, and the NC growth rate highly increases with run duration due to the autocatalytic nature of deposition.     

Model algorithmic control (MAC); basic theoretical properties

A mathematical framework for the analysis of model algorithmic control is developed and the operations of the main components of the control structure are described. The single input-single output case is treated in detail and results pertaining to the stability and the robustness of such systems are derived, both for deterministic and stochastic (colored output additive noise) environment. The case of the nonminimum phase plant is also considered. The robustness problem is discussed and performance ‘measures’ of robustness are proposed. The results derived in this paper are more generally applicable to optimizing type of control laws. In particular, the paper establishes a close link between the stability and closed loop properties of optimizing type and feedback type of control laws.     

利用核函数法中的自适应带宽同时对一维空间两个储集层属性参数进行粗化

在多孔介质流体流动模拟中,应用网格粗化方法必须在保留重要储集层参数的空间分布的前提下,首先利用大网格粗化原始地质模型(尤其是孔隙型介质),并用数值方法求解。提出了根据属性参数的空间分布、以核函数的自适应带宽为基础的网格粗化新方法,该方法既减少了网格数量,同时也保留原始精细模型的主要非均质性特征。该方法的关键点在于可以同时对两种储集层属性参数进行网格粗化。首先计算每个储集层属性参数的带宽量或最优门槛值并得到粗化结果,然后根据最大带宽和最小带宽采用两种不同的方法对两个属性参数同时粗化。现在已经能够实现利用不同方法建立储集层两个属性参数的最终粗化模型,不同方法的网格单元数和网格位置都相同。最小带宽法的粗化误差小于最大带宽法。图6表1参29     

Effect of apple cider vinegar agent on the microstructure,phase evolution,and magnetic properties of CoFe2O4 magnetic nanoparticles

In the present study, spinel structure CoFe₂O₄ nanoparticles were successfully synthesized by the sol-gel auto-combustion technique. The effect of apple cider vinegar (ACV) addition as an organic biocompatible agent on the size, morphology, and magnetic properties of CoFe₂O₄ nanoparticles was investigated in detail. The phase evolution, particle size, and lattice parameter changes of the synthesized phase have been estimated by using Rietveld structure refinement analysis of X-ray powder diffraction data. Also, Fourier transform infrared spectra (FT-IR) of the samples verified the presence of two expected bands correspond to tetrahedral and octahedral metal-oxygen complexes within the spinel structure. Furthermore, microstructural observations revealed that ultrafine particles have a semi-spherical morphology. It was shown that the particles size decreased from ~45 to ~17 nm with an increase in the amount of ACV. Magnetic properties were carried out by vibrating sample magnetometer (VSM) at room temperature. Both the saturation magnetization (M_s) and coercivity (H_c) were found to be significantly dependent on the crystallite size and the amount of ACV.     

Magnetic behavior of nickel–bismuth ferrite synthesized by a combined sol–gel/thermal method

The bulk NiFe_2?xBi_xO₄ ferrites with various Bi³⁺ concentration (x=0, 0.1, 0.15) were synthesized via sol–gel procedure, starting from nickel, bismuth and iron nitrate powders, followed by the conventional thermal treatment. The structural and magnetic properties of the as-prepared ferrites were studied by means of X-ray diffraction, alternating gradient force magnetometry and Faraday balance. The anisotropy constant was determined by the law of approach to saturation (LAS) model. An increasing Bi³⁺ concentration in NiFe_2?xBi_xO₄ leads to a decrease in the saturation magnetization, Néel temperature and the anisotropy constant of the material.