各向同性热解石墨在压痕试验中的尺寸效应

为促进各向同性热解石墨的精密/超精密加工技术的发展,采用压痕试验的方法研究了其在纳米/微米尺度上的尺寸效应.在确立压痕试验中使用载荷可行性域的基础上,对不同载荷作用条件下存在的尺寸效应进行了研究.结果表明,为提高尺寸效应分析的准确性,本试验条件下所施加的载荷应大于450 μN,且各向同性热解石墨压痕试验中硬度及弹性模量均存在一定的尺寸效应.同时,尺寸效应主要是由各向同性热解石墨表面附近球形单元内部的位错运动引起的.位错滑移阻碍较小时产生的尺寸效应较明显,位错滑移阻碍较大时产生的尺寸效应并不显著.     

c向蓝宝石单晶纳米压痕尺度效应研究

本文以c向蓝宝石单晶为对象采用纳米压痕方法测试了不同载荷下蓝宝石单晶的硬度与杨氏模量.分析表明,当载荷P=25±2 mN时出现了第一次位移突变现象,且位移突变量与最大压入载荷无关;试样的硬度与杨氏模量值随着压入深度的增大而减小,呈现出明显的压痕尺度效应;利用Nix-Gao尺度效应模型对测试数据进行了拟合,最终得出试样的硬度值与杨氏模量为30.956 GPa与382.316 GPa,与前人分析结果符合程度较高,证明Nix-Gao模型适用于蓝宝石单晶纳米压痕测试尺度效应的模拟与分析.     

不同晶面单晶铝纳米压痕测试及有限元模拟

采用纳米压入法对单晶铝(100)、(110)、(111)晶面进行力学性能分析,结合实验得到荷载-位移曲线,利用连续刚度技术获得单晶铝的弹性模量和硬度.结果表明:单晶铝具有良好的塑性性能,不同晶面的塑性能力从大到小依次为(110)晶面、(111)晶面、(100)晶面.材料弹性模量和硬度变化趋势一致,不同晶面弹性模量和硬度的大小关系为:(110)晶面＞(111)晶面＞(100)晶面.由于晶体变形时发生位错形核与发射,导致荷载-位移曲线在压入初期表现出位移爆发现象.同时运用ABAQUS有限元软件,使用单晶各向异性本构模型定义材料属性,对不同晶面的单晶铝进行仿真模拟,通过反复对比实验与模拟所得的荷载-位移曲线得到不同晶面各向异性的相关材料参数,并分析得出了最大压深时的正应力、剪应力及残余压痕分布的对称性.     

CdZnTe(211)面纳米力学特性分析

采用研磨-机械抛光-化学机械抛光工艺对CdZnTe(211)面进行加工,当表面粗糙度Ra达到0.94 nm时,采用纳米压痕仪对CdZnTe(211)面进行纳米力学性能测量,试验结果表明:弹性模量和硬度随着压力的增大而减小,存在明显的尺寸效应.当针尖离开晶体表面时,黏附现象特别明显.在不同晶向上,弹性模量和硬度存在显著的各向异性.     

基于纳米压痕测试的激光晶体光学加工性能研究

针对高功率固体激光器中使用的四类重要激光晶体(YAG、LBO、YCOB、KDP)元件,开展了晶体材料的光学加工性能研究.基于纳米压痕测试实验,研究了几种激光晶体材料在激光器实际使用晶向角度下的的表面力学特性差异,获得了载荷、弹性模量、压痕硬度等参数与压痕深度之间的变化规律;利用超景深显微镜及场发射扫描电镜观察了激光晶体表面的纳米压痕形貌,揭示了裂纹形貌随压力载荷的演变规律以及晶体材料力学各向异性现象.以熔石英材料为参照,从力学特性角度横向对比了几种激光晶体材料获得超光滑表面的光学加工难易程度.     

氟化钙晶体缺陷对应力双折射影响机制的研究

氟化钙(CaF2)晶体具有紫外透过率高(>99;@193 nm)、抗激光损伤阈值高等特点,在高功率紫外激光器、深紫外光刻机等领域具有广泛的应用.应力双折射是氟化钙晶体在实际应用中的关键性能指标,会导致通过晶体的光束发生形变,严重影响成像质量.采用坩埚下降法制备了尺寸为φ210 mm的氟化钙晶体,系统研究了CaF2晶体的位错和小角度晶界以及结晶质量对晶体应力双折射的影响,实验结果表明,位错密度的增高、小角度晶界的聚集、结晶质量的变差,会引起局部残余应力的集中,加剧应力双折射现象.     

单晶金刚石刀具切削各向同性热解石墨的磨损机理

为了研究各向同性热解石墨切削过程中单晶金刚石刀具磨损及其对加工质量的影响,对各向同性热解石墨材料进行了切削试验.观测了刀具磨损形貌的演变过程,分析了刀具磨损机理以及刀具磨损对加工表面质量的影响.研究结果表明,在切削距离逐渐增加的情况下,单晶金刚石刀具后刀面磨损区域逐渐增大,磨损形貌分为平行沟槽和微细网状两种.切削过程中产生的颗粒状切屑和交变应力分别是导致磨粒磨损和解理磨损的主要原因.切削距离300 m内,试件加工表面粗糙度值Ra在0.2～0.4 μm之间波动,切削距离达到l000 m以后,试件加工表面粗糙度值明显增大,但是保持了相对稳定,表面粗糙度值Ra约为0.8μm.     

基于纳米压痕的氧化镓晶体断裂韧度检测方法研究

为了快捷、精准测量脆性易解理氧化镓晶体的断裂韧度,为氧化镓精密加工提供理论依据.采用G200纳米压痕仪对氧化镓晶体进行纳米压痕试验,通过扫描电子显微镜分析压痕形貌,分别采用纳米压痕法、能量法两种方法计算断裂韧度.纳米压痕法测得(010)面氧化镓晶体的断裂韧度为0.769 MPa·m1/2;能量法测得其断裂韧度为0.782 MPa·m1/2.与传统的断裂韧度检测方法相比,基于纳米压痕仪的压痕法和能量法能够微损伤快速检测脆性材料的断裂韧度.能量法是更精准、更便捷的纳米级断裂韧度检测方法.     

掺杂对GaN晶体力学性能影响的研究

对GaN单晶力学性能的研究有助于解决其在生长、加工和器件应用中的开裂问题。本文围绕掺杂对GaN单晶力学性能的影响，通过纳米压痕法测试了不同掺杂类型(非掺、Si掺和Fe掺)GaN单晶的弹性模量和硬度，测试结果表明掺杂对GaN单晶的硬度有重要影响。Si掺、Fe掺GaN较非掺样品硬度有所提升，用重掺杂的氨热GaN单晶作为对照，也证明了这一结论。通过高分辨X射线衍射分析和原子力显微镜表征实验发现，晶体结晶质量、接触面积等因素对GaN单晶硬度的影响较小。对GaN表面纳米压痕滑移带长度和晶体晶格常数进行测试，结果表明，掺杂影响GaN单晶硬度的主要原因是缺陷对GaN位错增殖、滑移的阻碍作用和掺杂引起的GaN晶格常数的变化。     

PCD刀具切削各向同性热解石墨时石墨转移膜的形成机理

为研究石墨转移膜在刀具表面形成的可能性和演变规律及形成机理,采用聚晶金刚石(PCD)刀具进行了各向同性热解石墨切削试验.对刀具磨损区域形貌、表面加工质量和切削力进行了分析.根据试验结果分析表明:切削过程中PCD刀具后刀面上形成了一层石墨转移膜.石墨转移膜的形成机理以物理吸附作用和摩擦挤压为主.石墨晶体结构特性、石墨晶体缺陷和环境氛围是影响石墨转移膜形成的三个主要因素.刀具磨损、表面加工质量和切削力的变化情况与石墨转移膜的动态变化过程具有较高的一致性,石墨转移膜的形成可以有效地降低刀具磨损速度,提高表面加工质量,降低切削力.     

Review: Indentation size effect,indentation cracks and microhardness measurement of brittle crystalline solids – some basic concepts and trends

Indentation size effect, indentation cracks and microhardness measurement of some brittle crystals are reviewed against the background of the existing concepts of indentation deformation of crystalline solids. Several approaches reported in the literature devoted to relationships between applied indentation test load P and indentation diagonal length d are applied to analyze the experimentally observed normal and reverse indentation size effect (ISE) in brittle compounds. Using typical examples of normal and reverse ISE it is shown that the indentation induced cracking model does not give load‐independent hardness and the final expression describing the experimental data for various compounds is essentially another form of the Meyer law. Analysis of experiment data on crack lengths and indentation diagonals for different indentation loads suggests that the origin of ISE is associated with the processes of formation of indentation cracks following the general concepts of fracture mechanics. The load‐independent hardness H₀ may be determined reliably from plots of P /d against d of the proportional resistance model or of H_V against 1/d as predicted by strain gradient plasticity theories. It was found that the load‐independent hardness of depends on crystal orientation and state of the indented surface. Finally, some comments on determination of fracture toughness and brittle index of crystals are made. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of Various Impurities on the Hardness of NaCl Crystals

The hardness of NaCl crystal in the presence of mono, divalent and polyvalent ions were measured. Measurements were made in the indentation load range from 5 x 10^‐3 to 20 x 10^‐3 N. The measured data showed that there is an indentation size effect. Classical Meyer's law was used for the characterization of crystal hardness f NaCl. The Meyer index was found to be smaller than 2 indicating brittle material characteristic. The PRS model was also used for the determination of the load‐independent microhardness value. It was found that the crystal hardness of NaCl is chancing depending on the type of impurity and the concentration.     

Microhardness studies of nanocrystalline calcium tungstate

Nanocrystals of calcium tungstate (CaWO₄) of three different grain sizes were synthesized through chemical precipitation technique and the grain sizes and crystal structure were determined using the broadening of X‐ray diffraction patterns and transmission electron microscopy. The microhardness of compacted pellets of nanocrystalline calcium tungstate (CaWO₄) with different grain sizes were measured using a Vickers microhardness tester for various applied loads ranging from 0.049 N to 1.96 N. The values of microhardness showed significant reverse indentation size effect at low indentation loads. The microhardness data obtained for samples of different grain sizes showed grain size dependent strengthening obeying normal Hall‐Petch relation. The dependence of compacting pressure and annealing temperature on microhardness of the nanostructured sample with grain size of 13 nm were also studied. The samples showed significant increase in microhardness as the compacting pressure and annealing time were increased. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

硬质合金表面去钴和脱碳对金刚石薄膜粘结性能的影响

金刚石涂层的粘结性能是影响CVD金刚石薄膜涂层刀具使用寿命的关键因素.本文分别对硬质合金(YG6)表面进行酸蚀去钴和原位脱碳两种不同的预处理后,在热丝CVD系统中沉积金刚石薄膜,运用压痕试验评价金刚石薄膜与硬质合金基体的粘结性能.结果表明,经过原位脱碳预处理的硬质合金表面上金刚石涂层的粘结性能比酸蚀去钴法提高了近一倍;同时分析了硬质合金表面酸蚀去钴对金刚石薄膜粘结性能的影响及其剥离机制,并讨论了硬质合金表面原位脱碳的化学机制.     

Mechanical properties of pentaerythritol tetranitrate(PETN) single crystals from nano‐indentation: Depth dependent response at the nano meter scale

The current work presents the results from an investigation of the mechanical behavior of single crystals of the energetic material pentaerythritol tetranitrate (PETN) using a nano‐indentation technique. The indentation tests have been performed on the maximum growth habit face (110) of the PETN, using a spherical, Berkovich and an anisometric (wedge‐shaped) tip. The load displacement curves along with analysis have been used to extract the mechanical properties and identify the anisotropic indentation modulus for the PETN. The indentation moduli of the PETN single crystal were found to decrease as indentation depth increases and become independent of indentation depth for depths greater than 200 nm for the spherical indenter and 300 nm for the anisometric wedge indenter and Berkovich tip. The indentation modulus obtained from spherical tip indentation is compared with the results calculated by using literature values of the elastic constants. The wedge indenter measurements and Berkovich indentation at various tip orientations are different due to the anisotropy of the PETN. The yield behaviors of the PETN single crystal were also explored using both spherical and wedge tip indentation and the differences are discussed.     

A review of measurement of thermophysical properties of silicon melt

Measurements of thermophysical properties of Si melt and supplementary study of X-ray scattering/diffraction by the authors' group were reviewed. The values obtained differed variously from those of literature. Density was 2–3% larger, surface tension 20–30% smaller, viscosity up to 40% larger, electrical conductivity 8% smaller, spectral emissivity more or less in good agreement with literature values, and thermal diffusivity a few percent larger. An anomalous density jump was found near the melting point. Surface tension and viscosity also showed anomaly. A strange time-dependent change of density was observed over 3 h after melting. X-ray analyses suggested a slight change in local atom ordering, but showed no sign of cluster formation. An addition of 0.1 at% gallium caused the density jump to disappear, while that of boron caused no change. An EXAFS study of the former melt indicated a strong interaction between Ga and Si atoms as if molecules of GaSi₃ existed. The implications of the measured properties are a possibility of soft-turbulence in an Si melt in a relatively large crucible, a more complicated manner of intake of oxygen depleted molten Si from the free surface region to underneath the growing crystal, and a relaxation of the melt after melting arising from trapped gas species.     

原位氮化法制备TiN纳米粉体

用溶胶凝胶法合成的纳米TiO2粉体作为原料,将该粉体在氨气中进行原位氮化制备了TiN纳米粉体.用XRD,TEM,化学分析等手段对合成的TiN纳米粉体的物相组成、形貌、成分进行了分析.实验分析表明:在1000℃和1100℃下分别氮化5h,可以制备粒径大约为40nm和80nm的TiN粉体,其TiN的含量分别为95.40;和98.37;;而在1000℃条件下氮化时间减少到2h时,TiN的含量仅为58.36;.氮化温度和氮化时间是合成纳米TiN的重要因素,提高合成温度和延长氮化时间均可形成纯度较高的TiN纳米粉体,但延长氮化时间更有利于获得粒径小的氮化钛粉体.     

Mesomorphism Dependence on the Position of Substitution of a Pair of Esters

A novel homologous series of ethylene derivatives of thermotropic liquid crystals has been synthesized. The methoxy to octyloxy derivatives are nematogenic, the decyloxy to tetradecyloxy derivatives are smectogenic, in addition to nematogenic, and the hexadecyloxy homologue is smectogenic only. All the members of the series are enantiotropically mesogenic. Thermotropic behavior was determined by an optical polarizing microscope equipped with a heating stage and Differential Scanning Calorimetry (DSC) study. Analytical and spectral data confirm the molecular structures of homologues (infrared, nuclear magnetic resonance, mass spectra, X-ray, and DSC data). Textures of the nematic phase are threaded or Schlieren and that of smectic phase are focal conic fan-shaped of smectic A or C. Transition curves of the phase diagram behave in a normal manner except one or two deviations from the normal trend. The mesophase range (Sm+N) varies from 3°C to 44°C. The average thermal stability for smectic is 93°C and that for nematic 117.4°C. The LC behavior of the novel series is compared with a structurally similar known series.     

On the influence of structural features of DNA on the possibility of metabolic transfer of electrons

Abstract

A fragment of a DNA molecule is considered as one of the channels of metabolic electron transfer. The heterogeneity of the complementary chains is effectively taken into account. This made it possible to find the speed of the electron injected into the DNA conduction band and the current density that it creates. Estimates of electron mobility in nucleic acid chains are made. They were an order of magnitude smaller than that of typical semiconductors. For the specific conductivity of nucleic acid chains, estimates provide a conductivity of one to two orders of magnitude lower than in graphite.     

Effect of alumina on enthalpy of mixing of mixed alkali silicate glasses

The enthalpy of mixing of mixed alkali (Na₂O and K₂O) silicate glasses containing various concentrations of alumina was determined using an ion-exchange equilibrium method. For glasses with a constant alkali concentration, the enthalpy of mixing was found to become less negative with alumina addition. Consistent with our previous results on the enthalpy of mixing of alumina-free mixed alkali silicate glasses, the magnitude of enthalpy of mixing exhibited a good correlation with the molar volume mismatch of the corresponding two single alkali glasses as well as with the extent of conductivity mixed alkali effect, e.g. excess activation energy of conductivity, ΔE. The reduction of the magnitude of the enthalpy of mixing with alumina addition can be attributed to the reduction of non-bridging oxygen and ionic field strength. Combining the present results with results obtained earlier, the magnitude of the enthalpy of mixing for all mixed alkali (Na₂O and K₂O) silicate glasses with and without alumina was expressed by a simple function of a modified Tobolsky parameter, which takes into account the alkali concentration and the difference in cation-to-effective anion distances. The enthalpy of mixing data of the mixed alkali glasses was then compared with reported experimental data on the conductivity of mixed alkali aluminosilicate glasses. What appears to be conflicting experimental data can be understood in terms of the magnitude of the enthalpy of mixing and we can conclude that the mixed alkali effect is closely correlated with the negative enthalpy of mixing.