Superconductivity in NbGe2 and isostructural C-40 compounds

The superconductivity of NbGe₂ and isostructural C-40 compounds has been studied in a quantitative manner. The high transition temperatures T _c = 16 k exhibited by sputtered films of NbGe₂ are shown to be an intrinsic property of the compound and are attributed to the degree of order produced during the formation process. The roles of excess germanium and thermal effects are also discussed for NbGe₂. One of the major conclusions established in this study is that a new crystal structure type, namely the hexagonal C-40 system, is superconducting. Four binary compounds are shown to exhibit superconductivity; they are NbGe₂ T _c = 16 k, NbSi₂ T _c =2.9 k, TaSi₂ T _c = 4.4 k, and TaGe₂ T _c = 2.7 k. Even though superconductivity has been demonstrated on these compounds, the mechanism responsible for the superconductivity has not been established.This research supported in part by the National Science Foundation.     

Liquid phase epitaxy of hexagonal ferrites

Single crystal films of the hexagonal ferrite Zn₂Y, having the chemical formula Ba₂Zn₂Fe₁₂O₂₂, were grown by the isothermal dipping method of liquid phase epitaxy using a PbOBaOB₂O₃ flux. The substrates were flux-grown M-type hexagonal ferrite crystals having the chemical formula BaFe₁₂O₁₉. The films and substrates were distinguishable by the differences in their crystallographic and magnetic properties.     

Mechanochemical modification of the phase diagrams of hexagonal oxide ferrimagnets

Mechanical activation and fine comminution of hexagonal oxide ferrimagnets by milling to a nanocrystalline state (ultradisperse powder), followed by sintering, leads to significant changes in the phase diagram, the temperature of synthesis, and the magnetic properties as compared to those of the materials obtained by traditional ceramic technology. The final material structure is formed through “chemical assembly”—sequential consolidation of nanodimensional particles (structure-forming blocks)—rather than through the formation of low-temperature phases and their decomposition with increasing temperature.     

六韧带手性蜂窝结构的动力学响应特性研究

利用显式动力有限元ANSYS/LS-DYNA数值研究了六韧带手性蜂窝结构的面内冲击动力学特性。在保证圆环节点半径不变的前提下,通过改变韧带长度和胞元厚度,首先建立了六韧带手性蜂窝的有限元模型,具体讨论了冲击速度和胞元微结构参数对手性蜂窝材料的面内宏/微观变形行为、密实应变、动态平台应力和比能量吸收能力的影响。研究结果表明,随着冲击速度的增加,六韧带手性蜂窝结构表现为3种宏观变形模态：“> <”型模式、“过渡”模式和“I”型模式。在中、低速冲击载荷下,能够明显观察到拉胀材料在轴向压缩时独特的“颈缩”现象,其主要与韧带绕着圆环中心节点的旋转变形有关。通过引入无量纲“动态敏感因子”,还研究了六韧带手性蜂窝材料的面内动态冲击强化效应。     

Deformation texture and microtexture developments in a cold rolled single phase hexagonal Zircaloy 2

Abstract

A single phase hexagonal close packed Zircaloy 2 was cold deformed to different reductions by laboratory rolling. Systematic characterisations of the structural developments were carried out. Bulk texture developments were gradual, strongest developments being noticed at the highest strain. Although formation of well defined deformation fibre(s) could not be identified, overall developments in deformation texture were best captured through Taylor type models incorporating only prismatic slip. Strain localisations were observed as single or double walled dislocation structures at approximately 45 and 60° to the rolling direction. Such strain localisations were always associated with significant lattice reorientations or misorientation developments. Relative softening in lattice strain, observed at the higher reductions, can possibly be explained by the appearance of extensive strain localisations and the associated concurrent local dynamic recovery. The grains or orientations with dominant presence of strain localisations could be indirectly related to negative textural softening.     

Calculation of the band structure and superconductivity in the hexagonal close-packed phase of silicon

We report a calculation of the band structure and superconductivity of silicon in the hexagonal close-packed phase under pressure. The effect of pressure on the band structure is obtained by means of the linear muffin-tin orbital method. The superconducting transition temperatureT _c is calculated using the formulas of Allen and Dynes¹ as well as those of McMillan². It is found that the value ofT _c increases with pressure up to 74.3 GPa. The increase inT _c is attributed to the continuous s d electron transfer under pressure. The calculated values ofT _c are compared with the available experimental data. Further, Heine's fifth power law³ has been tested from thed-band widths obtained from the band-structure results.     

Liquid phase synthesis of ZnO microrods highly oriented on the hexagonal ZnO sheets

We have successfully synthesized zinc oxide microrods perpendicularly oriented on hexagonal ZnO sheets by a simple heat treatment approach using LDH (layered double hydroxide) precursor in an aqueous solution. The synthesized ZnO microrods have an average diameter of 500 nm and length of 2–3 μm, and form highly-oriented array. In this work, the effect of heating temperature and time on morphology and orientation of ZnO microrods was studied experimentally and the formation mechanism was discussed in detail. Transformation from hexagonal precursor to ZnO microrods can be attributed to dissolution and re-precipitation of the precursor, which should be caused by its thermal unstability under heating temperature.     

Stress-induced wurtzite to hexagonal phase transformation in zinc oxide nanowires

The stress-induced wurtzite to hexagonal phase transformation in [0110] oriented zinc oxide nanowires were investigated using a molecular dynamics simulation and reactive force field potentials. The yield strength of the 2.13 x 1.93 nm wurtzite nanowires is 12 GPa at 50 K. The wurtzite to hexagonal phase transformation was successfully observed at stress plateaus (5-5.5 GPa at 50 K) located after the yield point of the wurtzite phase. The wurtzite to hexagonal phase transformation was a result of the propagation of {0111} twinning boundaries. During the phase transformation, the wurtzite and hexagonal phases were clearly separated by the {0111} twinning boundaries. To analyze the difference between ceramic and metallic systems, all the calculation data of wurtzite to hexagonal transformation were compared with stress-induced phase transformation in metallic nanowires such as CuZr and NiA1. As the result of the [0110] tensile loading of the ZnO nanowires, the hexagonal phase was obtained.     

Tetragonal → twinned hexagonal crystal phase transformation in polybutene-1

The spontaneous transformation of the metastable (Form II) tetragonal crystals into the stable twinned hexagonal (Form I) crystals in polybutene-1 was studied using several techniques. The mechanical properties of the heat moulded material undergo significant changes in the process and the crystalline melting point increases from 112 to 128‡ C. Results from an Avrami analysis suggest the nucleation of the stable crystalline phase occurs shortly after crystallization from the melt and the subsequent growth of the nuclei follows a rod-like geometry. Transmission electron microscopy of melt-grown thin films shows that nucleation occurs at random positions within a spherulite and growth propagates along the radially oriented fibrillar crystals. As a result of multiple nucleation, each tetragonal crystal generates several twinned hexagonal crystallites with different crystallographic orientations. Besides exhibiting multiple nucleation within each individual tetragonal crystal, solution-grown single crystals also reveal twisting of the crystal lattice about thec-axis. The information obtained shows that residual stresses present in the material do not appear essential for the nucleation of the stable phase. Current concepts of the transformation mechanism are examined.     

Cathodic arc co-deposition of highly oriented hexagonal Ti and Ti2AlC MAX phase thin films

Ti₂AlC belongs to a family of ternary nanolaminate alloys known as the MAX phases, which exhibit a unique combination of metallic and ceramic properties. Here we report pulsed cathodic arc deposition of c axis normal oriented Ti₂AlC thin films on α-Al₂O₃ (001) single crystal substrates heated to 900 °C, without an intentionally pre-deposited seed layer. Oriented hexagonal Ti is observed in some films and an in-plane epitaxial relationship between the α-Al₂O₃ (001) substrate, the hexagonal Ti and Ti₂AlC MAX phase is observed. We observe formation of the Ti₂AlC phase in all films despite variations in elemental composition. The electrical resistivity of our films was in the range 0.48-0.67 μΩ m, higher than other values found for Ti₂AlC in the literature.     

Investigation of interfacial thermal resistance of hybrid graphene/hexagonal boron nitride

International Journal of Mechanics and Materials in Design - Hybrid graphene/hexagonal boron-nitride (G/h-BN) has shown significant physical properties and has been fabricated recently. Structural...     

Influence of strontium on the cubic to ordered hexagonal phase transformation in barium magnesium niobate

Oxides of the type Ba_3-xSr_xMgNb₂O₉ were synthesized by the solid state route. Thex = 0 composition (Ba₃MgNb₂O₉) was found to crystallize in a disordered (cubic) perovskite structure when sintered at 1000C. For higher Sr doping (x ≥ 0.5), there was clearly the presence of an ordered hexagonal phase indicated by the growth of superstructure reflections in the powder X-ray diffraction patterns. In all the compositions there was the presence of a minor amount of Ba_5-xSr_xNb₄O₁₅ phase which increased with Sr substitution up tox = 1 and then it remained nearly constant at about 5%. Samples sintered at 1300C showed the hexagonally ordered phase for the entire range of composition (0 ≤x ≤ 3). The degree of ordering being considerably greater than in the 1000C heated samples as evidenced by several superstructure reflections     

Thermal oxidation behavior of hexagonal BC2N

Non-isothermal thermogravimetry and differential scanning calorimetry (DSC) kinetic analysis methods were used to study the thermal oxidation behavior of hexagonal BC₂N. The results show that the oxidation of hexagonal BC₂N begins at about 900 °C under a dry air atmosphere. The apparent activation energy E_a for the oxidation of hexagonal BC₂N is calculated to be 223.57 and 231.48 kJ/mol from the Kissinger and Ozawa methods, respectively. These values are higher than those (153.24 and 161.10 kJ/mol) for carbon, which means that hexagonal BC₂N has higher thermal stability than carbon at high temperature in an oxygen-containing environment.     

Liquid phase epitaxy of hexagonal ferrites and spinel ferrites on non-magnetic spinel substrates

Using a PbOBaOB₂O₃ fluxed melt which had been employed previously to grow hexagonal ferrite Zn₂Y films on hexagonal ferrite M substrates, isothermal dipping liquid phase epitaxy produced epitaxial films on non-magnetic spinel substrates. The substrates studied were single crystals of three different compositions: MgAl₂O₄, MgGa₂O₄ and Mg(In,Ga)₂O₄. Two different film phases were identified: a lightly Zn-substituted M-type hexagonal ferrite and a heavily Zn-substituted magnetite.