β-Ga_(2)O_(3)晶体本征缺陷诱导的宽带超快光生载流子动力学

利用超快瞬态吸收光谱,针对氧化镓(β-Ga_(2)O_(3))晶体中本征缺陷诱导的载流子俘获和复合等动力学进行研究.实验发现,由本征缺陷诱导的宽带吸收光谱具有很强的偏振依赖性,特别是从不同探测偏振下的瞬态吸收光谱中可以提取出两个缺陷态吸收响应.该缺陷诱导的吸收响应归因于从价带到本征缺陷(镓空位)不同电荷态的光学跃迁,利用基于单缺陷的多能级载流子俘获模型拟合得到缺陷俘获空穴的速率远快于俘获电子,且缺陷态的吸收截面相较于自由载流子吸收截面大至少一个数量级.本文的研究结果不仅能明确本征缺陷与光生载流子动力学之间的关系,而且为β-Ga_(2)O_(3)在超快宽带光电子器件中的应用提供科学指导.

准晶数学弹性力学和缺陷力学

对准晶数学弹性理论的基本概念和基本框架作了介绍，在此基础上分别针对目前已经发现的几类一维准晶、二维准晶和三维准晶讨论了其数学弹性的理论体系．为了求解准晶弹性的边值问题或初值一边值问题，还必须发展相应的方法论．物理工作者在研究准晶位错弹性问题中发展了Ｇｒｅｅｎ函数方法．针对一维与二维准晶弹性中几类问题提出了分解与叠加程序，这一程序的使用，使极其复杂的准晶弹性问题得到简化，进而引进位移函数或应力函数，把数目。庞大的准晶弹性基本方程化成一个或少数几个高阶偏微分方程，进一步使求解步骤大为简化．对三维立方准晶弹性也采用了类似步骤使求解过程大为简化．在以上化简的基础上，发展了准晶弹性的边值问题或初值一边值问题的复交函数方法和 Ｆｏｕｒｉｅｒ分析方法，求得了一系列准晶位错问题和裂纹问题的分析解（古典解）．在研究准晶弹性的边值问题古典解的同时，也讨论了同这些边值问题相对应的变分问题和广义解（弱解）以及这种弱解的数值方法──有限元法．在物理学家工作基础上开展的这些工作可以看作对经典数学弹性理论和方法、经典Ｖｏｌｔｅｒｒａ位错理论、普通结构材料断裂力学和经典有限元的某些发展．此外。

熔焊填充+FSW修补搅拌摩擦焊缝匙孔型缺陷的接头组织性能研究

采用熔焊填充+搅拌摩擦补焊复合工艺成功进行了搅拌摩擦焊缝匙孔型缺陷的修补,焊后对补焊接头的微观组织和力学性能进行了分析。结果表明,焊核区呈细小的再结晶等轴晶粒,热机影响区发生粗化长大,逐渐丧失了原始母材轧制晶粒边界形貌,并残留熔焊柱状晶或树枝状晶粒。第二相析出物粗化长大,呈网状结构分布于热机影响区域。热影响区/热机影响区是整个补焊接头的薄弱环节,拉伸试样亦断裂在该区域。补焊接头断口呈现出韧性断裂与铸态组织脆性断裂的混合断口形貌。

电子束辐照对矿物可磨性的影响

颗粒材料破裂过程实质上是外力作用下力学缺陷和损伤形成和演化的过程。电子束辐射在颗粒体内部形成的大能量释放和电击穿 ,可在颗粒体内造成微观缺陷及显微裂隙 ,从而强化颗粒的磨碎过程。研究了电子束辐照对石英等几种颗粒材料磨碎细度的影响 ,试验结果显示 :电子束辐照可以大幅度提高细粒级的产率。辐照剂量 70 0kGy时 ,石英磨碎产品中 <4 5 μm粒级产率提高了 13.7个百分点 ;铁矿石则表现为辐照后粗粒级产率明显降低。

Atomistic simulation of defected magnesium hydroxide as flame retardants

The mechanical properties and the point defect energy of magnesium hydroxide（Mg（OH）2） were studied using the molecular dynamics. Moreover, the microelectronic structure of Mg（OH）2 with point defects in the bulk and on its surface were investigated using the first principles. The simulation results indicate that Mg（OH）2 was easily modified by other cations because of its strong, favorable interstitial and substitution defects via point defect energy calculation. Mg（OH）2 can provide high-efficiency flame retardancy because of the strong OH（OH Schottky defect） or H bond（H Frenkel defect and Schottky defect）. The potential model of Mg（OH）2 was established, and molecular dynamics simulation was used to investigate the relations between the crystal structure and the mechanical properties. Mg（OH）2 with special morphology such as nano-sheets was a prior consideration to maintain the composite mechanical properties. The detailed electronic structures of Mg（OH）2 with defects were determined. This work may provide theoretical guidance for choosing dopant element and reveal the element doping mechanism of Mg（OH）2.

A Kinetic Model of Dielectric Ageing

A kinetic model of dielectric ageing is presented. The central finding of this investigation is that there is a power-law relationship between the local electric field concentration and the rate of defect-tip initiated conducting crack growth. By applying such a power-law conducting crack growth rate expression to the evaluation of the life of solid dielectrics, the empirical classical ageing law of insulation materials can be derived theoretically as a lobical result. All the results are universal and agree with the experimental data of oxide films.

Atomistic simulation of thermal effects and defect structures during nanomachining of copper

Molecular dynamics （MD） simulations of monocrystalline copper （100） surface during nanomachining process were performed based on a new 3D simulation model. The material removal mechanism and system temperature distribution were discussed. The simulation results indicate that the system temperature distribution presents a roughly concentric shape, a steep temperature gradient is observed in diamond cutting tool, and the highest temperature is located in chip. Centrosymmetry parameter method was used to monitor defect structures. Dislocations and vacancies are the two principal types of defect structures. Residual defect structures impose a major change on the workpiece physical properties and machined surface quality. The defect structures in workpiece are temperature dependent. As the temperature increases, the dislocations are mainly mediated from the workpiece surface, while the others are dissociated into point defects. The relatively high cutting speed used in nanomachining results in less defect structures, beneficial to obtain highly machined surface quality.

Mechanical behavior and failure analysis of brittle sandstone specimens containing combined flaws under uniaxial compression

Based on the axial stress-axial strain curves,the effect of fissure angle on the strength and deformation behavior of sandstone specimens containing combined flaws is analyzed.The mechanical parameters of sandstone specimens containing combined flaws are all lower than that of intact specimen,but the reduction extent is distinctly related to the fissure angle.The results of sandstone specimens containing combined flaws are obtained by the acoustic emission,which can be used to monitor the crack initiation and propagation.The ultimate failure mode and crack coalescence behavior are evaluated for brittle sandstone specimens containing combined flaws.Nine different crack types are identified on the basis of their geometry and crack coalescence mechanism(tensile crack,hole collapse,far-field crack and surface spalling)for combined flaws.The photographic monitoring was also adopted for uniaxial compression test in order to confirm the sequence of crack coalescence in brittle sandstone specimens containing combined flaws,which recorded the real-time crack coalescence process during entire deformation.According to the monitored results,the effect of crack coalescence process on the strength and deformation behavior is investigated based on a detailed analysis for brittle sandstone specimens containing combined flaws by using digital photogrammetry.

Eliminating shrinkage defects and improving mechanical performance of large thin-walled ZL205A alloy castings by coupling travelling magnetic fields with sequential solidification

ZL205 A alloys with large thin-walled shape were continuously processed by coupling travelling magnetic fields(TMF)with sequential solidification,to eliminate the shrinkage defects and optimize the mechanical performance.Through experiments and simulations,the parameter optimization of TMF and the influence on feeding behavior,microstructure and properties were systematically studied.The results indicate that the magnetic force maximizes at the excitation current of 20 A and frequency of 200 Hz under the experimental conditions of this study,and increases from center to side-walls,which is more convenient to process thin-walled castings.TMF can break secondary dendritic arm and dendrites overlaps,widen feeding channels,prolong the feeding time,optimize the feeding paths,eliminate shrinkage defects and improve properties.Specifically,for as-cast state,TMF with excitation current of 20 A increases ultimate tensile strength,elongation and micro-hardness from 186 MPa,7.3%and 82.1 kg/mm^(2) to 221 MPa,11.7%and 100.5 kg/mm^(2),decreases porosity from 1.71%to 0.22%,and alters brittle fracture to ductile fracture.

Interface microstructure and mechanical properties of selective laser melted multilayer functionally graded materials

Functionally graded material(FGM)can tailor properties of components such as wear resistance,corrosion resistance,and functionality to enhance the overall performance.The selective laser melting(SLM)additive manufacturing highlights the capability in manufacturing FGMs with a high geometrical complexity and manufacture flexibility.In this work,the 316L/CuSn10/18Ni300/CoCr four-type materials FGMs were fabricated using SLM.The microstructure and properties of the FGMs were investigated to reveal the effects of SLM processing parameters on the defects.A large number of microcracks were found at the 316L/CuSn10 interface,which initiated from the fusion boundary of 316L region and extended along the building direction.The elastic modulus and nano-hardness in the 18Ni300/CoCr fusion zone decreased significantly,less than those in the 18Ni300 region or the CoCr region.The iron and copper elements were well diffused in the 316L/CuSn10 fusion zone,while elements in the CuSn10/18Ni300 and the 18Ni300/CoCr fusion zones showed significantly gradient transitions.Compared with other regions,the width of the CuSn10/18Ni300 interface and the CuSn10 region expand significantly.The mechanisms of materials fusion and crack generation at the 316L/CuSn10 interface were discussed.In addition,FGM structures without macro-crack were built by only altering the deposition subsequence of 316L and CuSn10,which provides a guide for the additive manufacturing of FGM structures.

纳米羟基磷灰石/聚酰胺66复合材料的研究及应用

背景:纳米羟基磷灰石在骨修复替代材料中有明显优势,但骨诱导活性低、力学性能差等缺陷限制了其临床应用。为克服弊端,国内外学者从仿生学等角度出发,以纳米羟基磷灰石为基础,掺杂、复合有机或无机材料,得到多种仿生复合材料。目的:综述纳米羟基磷灰石/聚酰胺66复合材料的研究及应用进展。方法:应用计算机检索1987年1月至2012年12月PubMed数据库相关文章,检索词为"nano,hydroxyapatite,polyamide 66"。同时,计算机检索1987年1月至2012年12月中国期刊网全文数据库相关文章,检索词为"纳米,羟基磷灰石,聚酰胺66"。共检索到文献93篇,最终纳入符合标准的文献56篇。结果与结论:纳米羟基磷灰石/聚酰胺66复合材料具有良好的热稳定性、生物力学性能及生物相容性。目前,纳米羟基磷灰石/聚酰胺66复合材料研究及引用主要集中于人工椎体、人工椎板及椎间融合器等,并取得了良好临床治疗效果,但仍有很多问题尚需要解决,如诱导成骨、降解情况都缺少长期而详尽的随访资料,而且目前主要是通过细胞学、组织学等方面来评价其生物安全性,尚未涉及到分子水平。

Energy Radiation of the Active Galactic Nuclei

In the Hellings Nordtvedt theory, we obtain some expressions of energy radiation and mass defect effect for a kind of the active galactic nuclei, which is meaningful to calculating the energy radiation in the procession of forming this kind of celestial bodies. This calculation can give some interpretation for energy source of the jet from the active galactic nuclei.

Quantum-mechanical study of effect of lattice defects on surface properties and copper activation of sphalerite surface

The electronic properties of sphalerite(110)surface bearing Fe,Mn and Cd impurities were calculated using density-functional theory,and the effects of impurities on the copper activation of sphalerite were investigated.Calculated results indicate that both Fe and Mn impurities narrow the band gap of sphalerite surface and lead to the Fermi level shifting to conduction band.Impurity levels composed of Fe 3d and Mn 3d orbital appearing in band gap are beneficial to electrons transfer from the valence band to the conduction band and promote the surface conductivity and the electrochemical activity.The results show that Fe and Mn impurities cannot be replaced by Cu atom,which reduces the exchange sites(Zn)for Cu atom,hence Fe-and Mn-bearing sphalerites are hard to be activated by copper.Cd impurity has little effect on electronic structure of sphalerite surface;however,Cd atom is easily replaced by Cu atom,and this is the reason why the Cd-bearing sphalerite can be easily floated.

Unveiling Defect-Mediated Carrier Dynamics in Few-Layer MoS2 Prepared by Ion Exchange Method via Ultrafast Vis-NIR-MIR Spectroscopy

Defect-mediated processes in two-dimensional transition metal dichalcogenides have a significant influence on their carrier dynamics and transport properties,however,the detailed mechanisms remain poorly understood.Here,we present a comprehensive ultrafast study on defect-mediated carrier dynamics in ion exchange prepared few-layer MoS2 by femtosecond time-resolved Vis-NIR-MIR spectroscopy.The broadband photobleaching feature observed in the near-infrared transient spectrum discloses that the mid-gap defect states are widely distributed in few-layer MoS2 nanosheets.The processes of fast trapping of carriers by defect states and the following nonradiative recombination of trapped carriers are clearly revealed,demonstrating the mid-gap defect states play a significant role in the photoinduced carrier dynamics.The positive to negative crossover of the signal observed in the mid-infrared transient spectrum further uncovers some occupied shallow defect states distributed at less than0.24 e V below the conduction band minimum.These defect states can act as effective carrier trap centers to assist the nonradiative recombination of photo-induced carriers in few-layer MoS2 on the picosecond time scale.

Analytical Bond-order Potential for hcp-Y

The lattice parameters, elastic constants, cohesive energy, structural energy differences, as well as the properties of point defects and planar defects of hexagonal closepacked yttrium （hcpY） have been studied with ab initio density functional theory for constructing an ex tensive database. Based on an analytical bondorder poial scheme, empirical manybody interatomic potential for hcpY has been developed. The model is fitted to some properties of Y, e.g., the lattice parameters, elastic constants, bulk modulus, cohesive energy, vacancy formation energy, and the structural energy differences. The present potential has ability to reproduce defect properties including the selfinterstitial atoms formation energies, vacancy formation energy, divacancy binding energy, as well as the bulk properties and the thermal dynamic properties.

Clinical Observation on Acupuncture Intervention for Children with Attention Deficit Hyperactivity Disorder

Objective： To observe the effect of acupuncture for children with attention deficit hyperactivity disorder （ADHD）. Methods： A total of 36 cases were recruited in this study. Before treatment, a Chinese medical constitution survey was conducted on these children. Based on the survey findings, the children were divided in different groups for acupuncture intervention. Additionally, the heart rate variability and attention were detected respectively before treatment, in the 12th and 24th week after treatment. Results： ADHD kids commonly manifest as a body constitution of yang predominance and heat exuberance. The kids＇ Body Mass Indexes （BMI） are moderately correlated with attention deficit disorder （ADD）. After the treatment of 24 weeks, the ADHD scores were reduced from （10.94±1.98） to （8.56±2.48）, showing a statistical significance （P〈0.05）; in addition, the low frequency/high frequency （LF/HF） and normalized low frequency （nLF） between the two groups were also statistically different （P〈0.05）. Conclusion： Most ADHD kids present with a body constitution of yang predominance and heat exuberance. A well-controlled BMI is helpful for ADHD treatment. Acupuncture can be a supplementary therapy to improve the ADHD symptoms.

A molecular dynamics-based comparison of defect production in collision cascades during the bombardment of iron with different ions

We present a computer simulation study on the influence of incident ions on the energy transferred to primary knock-on atoms(PKAs)and defects produced in the cascade collision of irons.Three types of ions(H,Fe,and Xe,which are frequently used in irradiation experiments)with an energy of 3 MeV were simulated.According to the calculation results of SRIM,the average energy transferred to PKAs by Fe ions was the highest among the three types.Then,cascade collisions induced by PKAs with different energies were simulated by the molecular dynamics method.The maximum number of defects produced during irra-diation increased,and the time taken by defect number peak formation was extended with the increased energy of PKAs.The difference in radial distribution function between pre-and post-irradiation irons showed that a higher energy of PKA transferred resulted in a flatter curve.Besides,the law of defects varying in temperature was also investigated.All the researches imply that heavy ions can substitute for neutrons in irradiation experiments which is a practicable way,but the influence of conditions must be taken into account.

Yielding behavior of copper nanowire in the presence of vacancies

In the present research,the measurement fluctuations of mechanical properties in nanowires (NWs) are investigated by using the molecular dynamics simulation.The large numbers of simulations are performed to study the yield behaviors of the NWs.The results have shown that the yield behavior of the smaller diameter NW is more sensitive to the presence of vacancies,and the dispersion of the measured mechanical properties for the small scale NW is larger than that for the large scale NW.Present results have also shown that vacancies escape from the bulk to the free surfaces as a result of high stress applied at the small scale systems similar to the dislocation starvation phenomenon observed in the compression test of nano-pillars,and dislocation nucleation induced by surface defect occurs after the vacancy reaches free surface leading to lower yield strength.Moreover,the strong surface vacancy interactions at the nanoscale level are also investigated.

Point defects in L1_0 FePt studied by molecular dynamics simulations based on an analytic bond-order potential

The point defects and their related physical properties in L10 FePt are investigated by molecular dynamics simulations based on an analytic bond-order potential. The calculated results agree well with the experimental value, indicating that the analytic bond-order potential is suitable to describe the structural properties and surface energies of the FePt alloy in the L10 phase. However, the calculated vacancy formation energy of an Fe atom is higher than that of a Pt atom, which disagrees with some other previously calculated results. This result indicates that the analytic bond-order potential is unable to describe the related point defect properties. The analytic bond-order potential needs to be modified in order to study these defect properties of an FePt alloy.

Early fluid loading for septic patients： Any safety limit needed？

Early adequate fluid loading was the corner stone of hemodynamic optimization for sepsis and septic shock. Meanwhile, recent recommended protocol for fluid resuscitation was increasingly debated on hemodynamic stability vs risk of overloading. In recent publications, it was found that a priority was often given to hemodynamic stability rather than organ function alternation in the early fluid resusci- tation of sepsis. However, no safety limits were used at all in most of these reports. In this article, the rationality and safety of early aggressive fluid loading for septic patients were discussed. It was concluded that early aggressive fluid loading improved hemodynamics transitorily, but was probably traded off with a follow-up organ function impairment, such as worsening oxygenation by reduction of lung aeration, in a part of septic patients at least. Thus, a safeguard is needed against unnecessary excessive fluids in early aggressive fluid loading for set）tic patients.