Mechanical properties and hardness of boron and boron-rich solids

A brief review of the recent studies of the crystal and electronic structure and its stability under large strain-shear and tensile deformation of selected boron compounds, which have been predicted to be superhard on the basis of their high elastic moduli, is presented. It will be shown that in many cases, the materials undergo electronic instability and transformation to softer phases with a lower shear resistance than the original equilibrium structure. Therefore, high values of elastic moduli (“low compressibility”) do not guarantee high hardness. These results also challenge the recent “models of theoretical hardness of an ideal crystal”, which are based on the equilibrium electronic properties. It is shown that appropriately nanostructured materials open the way to the design of superhard materials.     

钴纳米线填充(12，0)硼氮纳米管的电子结构与磁性

采用基于密度泛函理论的第一性原理方法,研究了密排六方结构的Co₁₀纳米线填充（12,0）硼氮纳米管（BNNT）的结构稳定性、电子性质和磁性质。完全弛豫后的Co₁₀/BNNT（12,0）复合体系,外围的BNNT沿着Co₁₀纳米线的三重旋转对称轴变成了近似正三角形的结构。复合体系Co₁₀/BNNT（12,0）的形成是自发地放热过程。电荷由Co₁₀纳米线转移到BNNT上。Co₁₀纳米线最外层的Co原子与其近邻的N原子形成的Co-N键具有极性共价键特征。Co₁₀纳米线封装入BNNT后,其平均磁矩有所减小,但相对于纯BNNT,Co₁₀/BNNT（12,0）体系具有较大的磁矩值。同时复合体系Co₁₀/BNNT（12,0）表现出明显的半金属特征。因此这种稳定的纳米复合体系在高密度磁数据记录设备及自旋电子学器件等方面具有重要的应用价值。     

The mechanical properties of (SN)X single crystals

The compression stress-strain behaviour of single crystals of sulphur nitride, (SN)_x, a metallic polymer, was measured in the ambient environment. Measurements were made both parallel and perpendicular to the chain axis on crystals which were about 1 mm in size. The plastic stress-strain behaviour resembled that of a highly anisotropic metal. Deformation to large strains produced a fibrilated structure which graphically exhibits the polymeric nature of (SN)_x. Young's moduli parallel and perpendicular to the chain were 21 and 1.4 GPa respectively. An analysis of the possible slip systems in (SN)_x indicated that there is only one easy glide system, (1 0 0) [0 0 1].Supported by the National Science Foundation MRL Program under Grant No. DMR 76-80994.     

Influence of different boron precursors on superconducting and mechanical properties of MgB2

The superconducting, structural and mechanical properties of MgB₂ bulk samples have been studied as a function of precursor B powder particle size by means of AC susceptibility, XRD and microhardness measurements, respectively. The in situ processed MgB₂ samples have been prepared by means of conventional solid state reaction method with magnesium powder (99.8 %, 325 mesh) and four different types of boron powders (95.2, >95, 91.9 and 86.7 %) from two sources, Pavezyum and Sigma Aldrich. The XRD measurements showed that the diffraction peaks for our samples belong to the main phase of the MgB₂ diffraction patterns. The highest critical temperature T _c = 37.7 K was achieved for the MgB₂ sample which was fabricated by using >95 % purity amorphous boron. Microhardness measurements were performed to investigate the mechanical properties. Load independent hardness, Vickers microhardness, Young’s modulus, fracture toughness, and yield strength values were calculated separately for all samples. The results were analyzed by using the Meyer’s law, proportional sample resistance model, elastic–plastic deformation model, Hays Kendall approach, and indentation induced cracking (IIC) model. It was found that the IIC model is the most successful model to describe the mechanical properties of our samples.     

Effective mechanical properties of hexagonal boron nitride nanosheets

We propose an analytical formulation to extract from energy equivalence principles the equivalent thickness and in-plane mechanical properties (tensile and shear rigidity, and Poisson's ratio) of hexagonal boron nitride (h-BN) nanosheets. The model developed provides not only very good agreement with existing data available in the open literature from experimental, density functional theory (DFT) and molecular dynamics (MD) simulations, but also highlights the specific deformation mechanisms existing in boron nitride sheets, and their difference with carbon-based graphitic systems.     

Radial mechanical properties of single-walled boron nitride nanotubes

The radial mechanical properties of single-walled boron nitride nanotubes (SW-BNNTs) are investigated by atomic force microscopy. Nanomechanical measurements reveal the radial deformation of individual SW-BNNTs in both elastic and plastic regimes. The measured effective radial elastic moduli of SW-BNNTs are found to follow a decreasing trend with an increase in tube diameter, ranging from 40.78 to 1.85 GPa for tube diameters of 0.58 to 2.38 nm. The results show that SW-BNNTs have relatively lower effective radial elastic moduli than single-walled carbon nanotubes (SWCNTs). The axially strong, but radially supple characteristics suggest that SW-BNNTs may be superior to SWCNTs as reinforcing additives for nanocomposite applications.     

Growth and mechanical properties of GeSi bulk crystals

Bulk crystals of Ge_1–xSi _x alloys were grown by the Czochralski technique. Full single crystals were obtained for the alloys of composition 0 < x < 0.15 and 0.9 < x < 1, while single crystal parts near the seeds of ingots provided alloys of intermediate composition. The dislocation velocity and mechanical strength of the GeSi alloys were investigated by the etch pit technique and compressive deformation tests, respectively. In the GeSi alloys of the composition range 0.004 < x < 0.080 the dislocation velocity decreases monotonically with increasing Si content in the temperature range 450–700°C and the stress range 3–24 MPa. In contrast, in the composition range 0.94 < x < 1 the dislocation velocity first increases and then decreases with decreasing Si content in the temperature range 750–850°C and the stress range 3–30 MPa. The velocity of dislocations was determined as functions of stress and temperature. The stress–strain behaviour in the yield region of the GeSi alloys of composition 0 < x < 0.4 is similar to that of Ge at temperatures lower than about 600°C. However, the yield stress becomes temperature-insensitive at high temperatures and increases with increasing Si content. The stress–strain curves of the GeSi alloys of composition 0.94 < x < 1 are similar to those of pure Si at temperatures of 800–1000°C and the yield stress increases with decreasing Si content down to x = 0.94. The yield stress of the GeSi alloys is dependent on the composition, being proportional to x(1 – x). The strengthening mechanism in alloy semiconductors is discussed.     

Growth-dependent properties of KTP crystals and PPKTP structures

Growth of optically uniform and single ferroelectric domain KTP crystals is of prime importance for frequency-conversion applications. In the course of KTP growth from pure self-fluxes the flux becomes enriched in potassium causing a gradual increase of potassium content in the crystal as well. We have shown that such an effect can be well characterized by a corresponding increase in the Curie temperature of the crystal. Establishment of the potassium concentration gradients is followed by charge separation and production of a built-in electric field, which can be enhanced or diminished also by the incorporation of charge-compensating residual impurities. The magnitude of the built-in electric field is directly proportional to the projection of the potassium concentration gradient on the crystals Z-axis, and it defines the domain direction in immersion seeded or different configurations of the top-seeded growth of KTP crystals. Detailed investigation of the domain formation mechanisms has allowed us to suggest a number of ways of growing single domain crystals, such as top-seeded growth with pulling in the Z-direction. Pulling in the X-direction is shown to yield predominantly bi-domain crystals. The formation of bi-domains and complex domain structures along the growth sector boundaries is explained in terms of edge-like and apex-like growth perturbations, respectively, which are due to temperature fluctuations at the growth interface. The knowledge of parameters influencing the domain formation mechanisms has allowed us to develop a technique for obtaining as-grown periodic domain structures necessary for large aperture (high-power) frequency conversion applications. © 2001 Kluwer Academic Publishers     

Microstructure and mechanical properties of high boron white cast iron

In this paper, high boron white cast iron, a new kind of wear-resistant white cast iron was developed, and its microstructure and mechanical properties were studied. The results indicate that the high boron white cast iron comprises a dendritic matrix and an interdendritic eutectic boride in as-cast condition. The distribution of eutectic boride with a chemical formula of M₂B (M represents Cr, Fe or Mn) and with a microhardness of HV2010 is much like that of carbide in high chromium white cast iron. The matrix includes martensite and a small amount of pearlite. After quenching in air, the matrix changes to martensite, but the morphology of boride remains almost unchanged. In the course of austenitizing, a secondary precipitation with the size of about 1 μm appears, but when tempered at different temperature, another secondary precipitation with the size of several tens of nanometers is found. Both secondary precipitations, which all forms by means of equilibrium segregation of boron, have a chemical formula of M₂₃(C,B)₆. Compared with high chromium white cast iron, the hardness of high boron white cast iron is almost similar, but the toughness is increased a lot, which attributes to the change of matrix from high carbon martensite in the high chromium white cast iron to low carbon martensite in the high boron white cast iron. Moreover, the high boron white cast iron has a good hardenability.     

Elastic and photoelastic properties of KY(WO4)2 single crystals

The complete elastic modulus matrix of a KY(WO₄)₂ single crystal has been determined for the first time using sound velocities measured in different directions of the crystal by a pulse-echo method. We have studied the entire isotropic part of the elasto-optic modulus matrix of KY(WO₄)₂ and identified high acousto-optic coupling efficiency directions. Calculated sound velocities in the three main crystallographic planes demonstrate that there are directions with considerable group velocity dispersion.     

Co、Mn共掺杂SnO_2超晶格的电子结构和光学性质

采用基于第一性原理的线性缀加平面波(FP-LAPW)方法,应用广义梯度近似来处理相关能,研究了Co、Mn掺杂SnO2超晶格的电子态密度、能带结构和光学性质。研究结果表明,Co、Mn掺杂使材料表现出金属性。共掺杂Co、Mn的3d电子和O的2p电子产生了强烈的杂化作用,杂化作用使Co和Mn原子的磁矩3.0μB。单掺杂Co时没有杂化作用,导致每个Co原子产生的磁矩为1.56μB。两种掺杂都在低能量区1.0 eV处形成新的介电峰,Co、Mn共掺杂与Co掺杂相比在1.5 eV处形成介电峰,吸收系数和反射系数也发生了相应的变化。     

Characterization of porous glass fiber-reinforced composite (FRC) implant structures: porosity and mechanical properties

The aim of this study was to characterize the microstructure and mechanical properties of porous fiber-reinforced composites (FRC). Implants made of the FRC structures are intended for cranial applications. The FRC specimens were prepared by impregnating E-glass fiber sheet with non-resorbable bifunctional bis-phenyl glycidyl dimethacrylate and triethylene glycol dimethacrylate resin matrix. Four groups of porous FRC specimens were prepared with a different amount of resin matrix. Control group contained specimens of fibers, which were bound together with sizing only. Microstructure of the specimens was analyzed using a micro computed tomography (micro-CT) based method. Mechanical properties of the specimens were measured with a tensile test. The amount of resin matrix in the specimens had an effect on the microstructure. Total porosity was 59.5 % (median) in the group with the lowest resin content and 11.2 % (median) in the group with the highest resin content. In control group, total porosity was 94.2 % (median). Correlations with resin content were obtained for all micro-CT based parameters except TbPf. The tensile strength of the composites was 21.3 MPa (median) in the group with the highest resin content and 43.4 MPa (median) in the group with the highest resin content. The tensile strength in control group was 18.9 MPa (median). There were strong correlations between the tensile strength of the specimens and most of the micro-CT based parameters. This experiment suggests that porous FRC structures may have the potential for use in implants for cranial bone reconstructions, provided further relevant in vitro and in vivo tests are performed.     

微量硼对Ti-Fe-Cu-Sn-Nb合金力学性能的影响

基于细晶强化和第二相强化原理,通过在一种近β钛合金中加入微量硼(B)元素,以强化该合金.首先设计不同含硼量的Ti85 Fe6 Cu5 Sn2 Nb2合金,并用真空非自耗电弧炉制备,随后对合金在800℃下进行多道次热轧及最终淬火.通过组织观察、拉伸力学性能测试、断口观察及透射电子显微分析,考察不同硼含量对Ti85 Fe6 Cu5 Sn2 Nb2合金组织及力学性能的影响.结果表明,微量硼元素可以使合金的晶粒细化,强度明显提高,但伴随着塑性下降.添加质量分数为0.15％硼可以使合金具有较好的综合力学性能(σ0.2=1105 MPa,δb=4.5％).随着硼含量的增加,合金的强度升高,最高可达1156 MPa.硼的加入在合金中形成正交结构的TiB相,分布于β钛基体中.变形过程中,TiB断裂、TiB割裂基体及其与基体脱粘,产生裂纹源,导致合金塑性下降.     

Electronic, thermal and mechanical properties of carbon nanotubes

A review of the electronic, thermal and mechanical properties of nanotubes is presented, with particular reference to properties that differ from those of the bulk counterparts and to potential applications that might result from the special structure and properties of nanotubes. Both experimental and theoretical aspects of these topics are reviewed.     

Photoelectrical properties of layered GaS single crystals and related structures

The photoconductivity of pure and Cu-doped layered gallium monosulfide (GaS) single crystals, and of Ni-GaS(Cu)-In and GaS(Cu)-ZnO structures, is investigated. The activation energies of surface states were found as 0.10 eV, 0.40 eV and 17 meV, ∼400 meV for GaS and GaS(Cu), respectively. Cu acceptor levels are localized at 0.44 eV and 0.52 eV above the valence band of GaS. ZnO-GaS(Cu) heterojunctions show remarkable photosensitivity in the wavelength range of 250-700 nm.