Ab initio study of palladium and silicon carbide

Ab initio methods have been used to investigate the properties of Pd as impurity in bulk SiC at five charged states within the framework of density functional theory using the local spin density approximation. It was found that Pd interstitials and substitutionals have similar energy to their intrinsic counterparts. In addition, Pd substitutes for a vacancy, di-vacancy, and tri-vacancy with similar energies. Pd diffuses through SiC via an interstitial mechanism employing the tetrahedral sites and Pd can substitute for Si and C at positive charged states. Removing electrons (p-type doping) from SiC lowers the formation and migration energies of Pd defects in SiC for most configurations.     

Atomic-scale studies of native point defect and nonstoichiometry in silicon oxynitride

The native point defects and mechanism of accommodating deviations from stoichiometry of Si₂N₂O crystal have been investigated using atomistic simulation techniques. This work firstly provides a reliable classical interatomic potential model derived from density functional theory calculations. The force-field parameters well reproduce the crystal structure, elastic stiffness, and dielectric constants of Si₂N₂O. It is expected that the force-field parameters are useful in future investigations on Si₂N₂O by molecular dynamic simulation. The calculated formation energies for native defects suggest that intrinsic disorder in stoichiometric Si₂N₂O is dominated by antisites and a degree of oxygen Frenkel defect may also exist in this system. In nonstoichiometric Si₂N₂O, the calculated reaction energies indicate that excess SiO₂ or Si₃N₄ is most likely accommodated by the formation of antisite in the lattice. And we also find that SiO₂ excess is energetically more favorable than Si₃N₄ surplus in Si₂N₂O.     

Magnetism of single-walled silicon carbide nanotubes doped by boron,nitrogen and oxygen

We calculated, using spin polarized density functional theory, the electronic properties of zigzag (10,0) and armchair (6,6) semiconductor silicon carbide nanotubes (SiCNTs) doped once at the time with boron, nitrogen, and oxygen. We have looked at the two possible scenarios where the guest atom X (B, N, O), replaces the silicon X_Si, or the carbon atom X_C, in the unit cell. We found that in the case of one atom B @ SiCNT replacing a carbon atom position annotated by B_C exhibits a magnetic moment of 1 μ_B/cell in both zigzag and armchair nanotubes. Also, B replacing Si, (B_Si), induce a magnetic moment of 0.46 μ_B/cell in the zigzag (10,0) but no magnetic moment in armchair (6,6). For N substitution; (N_C) and (N_Si) each case induce a magnetic moment of 1 μ_B/cell in armchair (6,6), while N_Si give rise to 0.75 μ_B/cell in zigzag (10,0) and no magnetic moment for N_C. In contrast the case of O_C and O_Si did not produce any net magnetic moment in both zigzag and armchair geometries.     

Lithium absorption on single-walled boron nitride,aluminum nitride,silicon carbide and carbon nanotubes: A first-principles study

Using the DFT-B3LYP calculations we investigate the adsorption of Li atom on CNT, BNNT, AlNNT and SiCNT. We found that Li atom can be chemisorbed on zig-zag SiCNT with binding energy of −2.358 eV and charge transfer of 0.842 |e|, which are larger than the results of other nanotubes. The binding energy of Li on SiCNT is foun to be stronger than activation energy barrier indicating that Li metal could be well dispersed on SiCNTs. Furthermore, the average voltage caused by the lithium adsorption on SiCNT demonstrated that SiCNTs could exhibit as a stable anode similar to the lithium metal anode. The binding nature has been rationalized by analyzing the electronic structures. Our findings demonstrate that Li-BNNT, Li-SiCNT and Li-AlNNT systems exhibit spin polarized behaviors and can fascinating potential application in future spintronics. Also, Li-SiCNT system with rather small band gap might be a promising material for optical applications and active molecule in its environment.     

Ab initio study of electronic and magnetic properties in TM-doped 2D silicon carbide

The magnetic properties of SiC monolayer with different TM atoms and substitutional sites are investigated using first-principles method. Magnetism is observed for all the TM dopants. The magnetic moments and binding energies are quite different between Si (TM_Si) and C (TM_C) sites. Dependent to the larger magnetic moments and binding energy, we also investigate the interaction between two Mn atoms in the TM_Si system. The results show that the ferromagnetic states are originated by the p–d hybridization mechanism between Mn and its neighboring C atoms. Moreover, the antiferromagnetic coupling is observed with increasing Mn-Mn distance, which can be explained by two-impurity Haldane-Anderson model using quantum Monte Carlo method.     

Mechanism of reversible photo-induced magnetization in prussian blue analogues

Photo-induced magnetic phase transitions observed in Co—Fe Prussian blue analogues are discussed from a theoretical point of view. In this material a magnetization is induced by visible light, and the induced magnetization can be completely diminished not only by thermal annealing but also by irradiation of infra red light. After a brief survey of relevant experiments, we present an overview of our theoretical approaches to clarify the underlying mechanism of this interesting phenomenon. The ligand field at the Co site is shown to be the most relevant variable controlling spin states. The presence of Fe vacancies and the resultant ligand substitution by water molecules turns to be an essential prerequisite for the photoinduced magnetization.     

Amorphous silicon hexaboride: a first-principles study

We report for the first time the atomic structure, electronic structure and mechanical properties of amorphous silicon hexaboride (a-SiB₆) based on first-principles molecular dynamics simulation. The a-SiB₆ model is generated from the melt and predominantly consists of pentagonal pyramid-like configurations and B₁₂ icosahedral molecules, similar to what has been observed in most boron-rich materials. The mean coordination number of B and Si atoms are 5.47 and 4.55, respectively. The model shows a semiconducting behaviour with a theoretical bandgap energy of 0.3?eV. The conduction tail states are found to be highly localised and hence the n-type doping is suggested to be more difficult than the p-type doping for a-SiB₆. The bulk modulus and Vickers hardness of a-SiB₆ are estimated to be about 118 and 13–17?GPa, respectively.     

Half-metallic magnetism of Co2CrX (X=As, Sb) Heusler compounds: An ab initio study

In this study, we present the electronic, magnetic, and structural properties of two novel half-metallic full-Heusler compounds, Co₂CrAs and Co₂CrSb, in cubic L2₁ geometry. The calculations are based on the density functional theory within plane-wave pseudopotential method and spin-polarized generalized gradient approximation of the exchange-correlation functional. The electronic band structures and density of states of the systems indicate half-metallic behavior with vanishing electronic density of states of minority spins at Fermi level, which yields perfect spin polarization. The calculated magnetic moments of both systems in L2₁ structure are 5.00 μ_B, which are largely localized on the chromium site. The energy gaps in minority spin states are restricted by the 3d-states of cobalt atoms on two different sublattices. The formation enthalpies for both structures are negative indicating stability of these systems against decomposition into stable solid compounds.     

反应烧结碳化硅平面反射镜的光学加工

介绍了100mm口径反应烧结碳化硅平面反射镜的光学加工工艺流程。按照流程依次介绍了在粗磨成形、细磨抛光和精磨抛光过程中使用的机床、磨具和磨料以及采用的工艺参数和检测方法。介绍了在光学加工各个步骤中应注意的问题。展示了加工后反应烧结碳化硅平面反射镜的实物照片。给出了面形精度和表面粗糙度的检测结果:面形精度(95%孔径)均方根值(RMS)为0.030λ(λ=632.8nm),表面粗糙度RMS值达到了1.14nm(测量区域大小为603 6μmⅹ448 4μm)。     

High-resolution transmission electron microscopy on silicon carbide whiskers

SiC whiskers were grown from the reaction of silicon monoxide (SiO) with activated carbon containing iron impurities. Growth proceeds through a VLS growth mechanism with SiO and CO as reacting gases. HRTEM combined with EDS shows that the SiC whisker is topped by a Fe₃Si catalyst droplet. The SiC whisker is found to be one-dimensionally disordered along the [111] growth direction of an fcc crystal structure. Although the catalyst droplet is usually larger than the top face of the whisker, we observed a number of situations where the diameter of the droplet was smaller. The study of the SiC-Fe₃Si interface showed that the growth is nucleated from the edges.     

Dislocation modeling in bcc lithium: A comparison between continuum and atomistic predictions in the modified embedded atoms method

In this study, the modified embedded-atom method (MEAM) was applied to compare the predictions of dislocation core properties obtained by molecular statics with the continuum predictions obtained in the framework of the simplified 1D-Peierls–Nabarro model. To this end, a set of four fictive Li potentials in the MEAM framework was proposed with the condition that all four potentials reproduce the same elastic constants, the same transition energies between bcc and fcc crystal structures, and between bcc and hcp crystal structures, while the unstable stacking fault energy on the plane {110} in the direction <111> was varied around the value predicted by first-principles. Within these potentials, direct atomistic calculations were performed to evaluate dislocation core properties such as dislocation half width and Peierls stress and the results were compared with continuum predictions. We found that the trends predicted by the Peierls–Nabarro model, i.e. (i) a decrease of the dislocation half width with increasing unstable stacking fault energy, and (ii) an increase of the Peierls stress with increasing the magnitude of the unstable stacking fault energy, were recovered using atomic calculations in the MEAM framework. Moreover, the magnitude of the dislocation half width and the Peierls stress calculated in the MEAM framework are in good agreement with the Peierls–Nabarro predictions when the dislocation half width is determined using a generic strategy. Specifically, the dislocation half width is defined as the distance for which the disregistery is included between b/4 and 3b/4. It was, therefore, demonstrated herein that the set of fictive potentials could be parameterized in the MEAM framework to validate or to disprove the continuum theory using atomistic methods.     

Oxidation fracturing of the graphitic BN sheet

Graphitic BN sheets with well-defined structure are promising candidate materials for future applications in nanoelectronics and molecular devices. The local oxidation is regarded as an effective means to produce a regular nanostructure. However, the underlying fracturing mechanism of such system is unclear. Here we aim to resolve this issue by the ab initio method. we predict the equilibrium configuration and the oxidative cutting process by introducing an epoxy-like chain and an added oxygen atom placed nearby, respectively. The results show that the intermediate epoxy-like pair can be eventually broken up after a key structure formation of B₃O during the oxidative processes.     

Theoretical prediction of the structural,elastic, electronic and thermodynamic properties of V3M (M = Si,Ge and Sn) compounds

Density functional theory (DFT), is used in our calculations to study the V₃M (M = Si, Ge and Sn) compounds, we are found that V₃Sn compound is mechanically unstable because of a negative C₄₄ = −19.41 GPa. For each of these compounds considered, the lowest energy structure is found to have the lowest N(E_f) value. Also there is a strong interaction between V and V, the interaction between M (M = Si, Ge, Sn) and V (M and M) is negative, not including Si [Sn]. In phonon density of states PDOS, the element contributions varies from lighter (high frequency) to heaviest (low frequency).     

Ab initio study of some fundamental properties of the M3X (M=Cr,V; X=Si,Ge) compounds

M₃X (M=Cr, V; X=Si, Ge) compounds are studied using first-principles calculations based on the Density Functional Theory (DFT). It is found that the bulk of Cr₃X (X=Si, Ge) compounds are comparable to those of Al₂O₃, the nearest-neighbor distance D_M−M and D_M−X in these compounds increase and the bulk modulus decrease, there is a strong interaction between M and M (M=Cr the interaction is stronger). Also the interaction between M (M=Cr, V) and X (X=Ge) is negative, an anti-bonding-type interaction is dominant between these atoms.     

Volume and heat of solution of hydrogen in rare earths from proton screening charges

In this work we determine some fundamental microscopic and macroscopic properties of rare earth-hydrogen (RE-H) systems. The behaviour of the electronic density variation of RE-H systems is obtained, using a program based on the density functional formalism. This information allows us to calculate the volume of solution of hydrogen in rare earths, as well as their heat of solution, and to compare with experimental results.     

How to simulate a structural phase transition by the first-principles method?

We outline the procedure and approximations used to study the structural phase transition. The standard ab initio programs provide the Hellmann-Feynman forces, which allow, by the direct method, to describe the dispersion curves including the soft mode. The values of the anharmonic terms of the order parameter and order parameter-strain coupling terms, found from symmetry analysis, can be calculated within the ab initio total energy approach, by carring on the calculations for a variety of atom configurations. The method is used to describe the cubic-tetragonal phase transition in SrTiO₃, for which a soft mode and anharmonic coefficients have been found.     

基于4H-SiC的高能量分辨率α粒子探测器

为突破传统半导体核探测器耐高温与抗辐照性能不足的瓶颈,采用4H-SiC宽禁带半导体材料研制了4H-SiC探测器,并研究其构成的探测系统对α粒子的能量分辨率和能量线性度。所研制4H-SiC探测器漏电流低,当外加反向偏压为200V时,其漏电流仅14.92nA/cm2。采用具有5种主要能量α粒子的226 Ra源研究其构成的探测系统对α粒子的能量分辨率,获得4H-SiC探测系统对4.8~7.7 MeV能量范围内α粒子的能量分辨率为0.61%~0.90%,与国际上报道的高分辨4H-SiC探测系统能量分辨率一致。同时,实验结果表明:4H-SiC探测系统对该能量范围内α粒子的能量线性度十分优异,线性相关系数为0.999 99。     

基于4H-SiC的高能量分辨率α粒子探测器

为突破传统半导体核探测器耐高温与抗辐照性能不足的瓶颈,采用4H-SiC宽禁带半导体材料研制了4H-SiC探测器,并研究其构成的探测系统对粒子的能量分辨率和能量线性度。所研制4H-SiC探测器漏电流低,当外加反向偏压为200 V时,其漏电流仅14.92 nA/cm2。采用具有5种主要能量粒子的226Ra源研究其构成的探测系统对粒子的能量分辨率,获得4H-SiC探测系统对4.8~7.7 MeV能量范围内粒子的能量分辨率为0.61%~0.90%,与国际上报道的高分辨4H-SiC探测系统能量分辨率一致。同时,实验结果表明：4H-SiC探测系统对该能量范围内粒子的能量线性度十分优异,线性相关系数为0.999 99。     

EPR investigations of silicon carbide nanoparticles functionalized by acid doped polyaniline

Nanocomposites (SiC-PANI) based on silicon carbide nanoparticles (SiC) encapsulated in conducting polyaniline (PANI) are synthesized by direct polymerization of PANI on the nanoparticle surfaces. The conductivity of PANI and the nanocomposites was modulated by several doping levels of camphor sulfonic acid (CSA). Electron paramagnetic resonance (EPR) investigations were carried out on representative SiC-PANI samples over the temperature range [100–300 K]. The features of the EPR spectra were analyzed taking into account the paramagnetic species such as polarons with spin S=1/2 involved in two main environments realized in the composites as well as their thermal activation. A critical temperature range 200–225 K was revealed through crossover changes in the thermal behavior of the EPR spectral parameters. Insights on the electronic transport properties and their thermal evolutions were inferred from polarons species probed by EPR and the electrical conductivity in doped nanocomposites.     

Ab initio study of electronic structures of BaMoO4 crystals containing an interstitial oxygen atom

The electronic structures of the perfect BaMoO₄ and BaMoO₄ crystals containing an interstitial oxygen atom situated at an appropriate position with the total energy being the lowest are studied within the framework of the density functional theory with the lattice structure optimized. The calculated results reveal that the interstitial oxygen atom situated at two different interstitial sites would combine with formal lattice oxygen ions forming molecular ions in two different ways, and the interstitial oxygen atom would cause visible range absorption band peaked at about 320 nm.