Stability and mechanical properties of various Hf-H phases: A density-functional theory study

We performe first-principles density functional theory calculations to investigate the stability and mechanical properties of various Hf Hx(0 ≤ x ≤ 1) phases. For pure Hf phases, the calculated results show that the HCP and FCC phases are mechanically stable, while the BCC phase is unstable at 0 K. Also, as for various Hf Hx phases, we find that H location and concentration could have a significant effect on their stability and mechanical properties. When 0 ≤ x ≤ 0.25, the HCP phases with H at(tetrahedral) T sites are energetically most stable among various phases. The FCC and BCC phases with H at T sites turn to be relatively more favorable than the HCP phase when H concentration is higher than 0.25. Furthermore, our calculated results indicate that the H solution in Hf can largely affect their mechanical properties such as the bulk moduli(B) and shear moduli(G).     

H13钢激光熔覆Co基涂层组织及热疲劳性能

针对热作模具用H13钢工况下易产生热疲劳失效的问题,采用Nd:YAG激光器在H13钢表面制备Co基合金涂层。利用光学显微镜、扫描电镜、能谱仪和X射线衍射仪,对涂层组织、合金元素分布及物相组成进行检测。利用显微硬度计和热震实验法,测试热疲劳对Co基合金涂层和淬火回火态H13钢硬度影响。结果表明,Co基合金涂层从底部到表层,依次为平面晶、胞状晶、树枝晶和等轴晶。Co基合金涂层物相主要由-Co和M23C6相组成,热疲劳后涂层表面形成M2O3和M3O4(M=Fe,Co,Cr)氧化物。Co基合金涂层硬度最高可达706HV0.2且呈梯度降低;热循环1000次后,Co基合金涂层表面硬度降低24.4%,H13钢表面硬度降低37.7%,Co基合金涂层硬度下降幅度低于H13钢。热循环1000次后,Co基合金涂层表面未发现明显热裂纹,H13钢表面形成大量网状热裂纹。Co基合金涂层中,Cr元素形成致密Cr2O3氧化膜使其热疲劳性能优于H13钢。     

Evolution of the commensurate and incommensurate magnetic phases of the S = 3/2 kagome staircase Co?V?O? in an applied field

Single crystal neutron diffraction studies have been performed on the S = 3/2 kagome staircase compound Co(3)V(2)O(8) with a magnetic field applied along the magnetization easy-axis ([Formula: see text]). Previous zero-field measurements (Chen Y et al 2006 Phys. Rev. B 74 014430) reported a rich variety of magnetic phases, with a ferromagnetic ground state as well as incommensurate, transversely polarized spin density wave (SDW) phases (with a propagation vector of [Formula: see text]) interspersed with multiple commensurate lock-in transitions. The magnetic phase diagram with [Formula: see text] adds further complexity. For small applied fields, μ(0)H ≈ 0.05 T, the commensurate lock-in phases are destabilized in favor of the incommensurate SDW ones, while slightly larger applied fields restore the commensurate lock-in phase with δ = 1/2 and yield a new commensurate phase with δ = 2/5. For measurements in an applied field, higher-order scattering is observed that corresponds to the second harmonic.     

Study of the spin reorientation phenomenon in Nd2(Fe, Si)14BHy

We have studied the influence of the Si for Fe substitution and of the H insertion on the spin reorientation phenomenon in the Nd₂Fe₁₄B phases. To determine the temperature of spin reorientation and the tilt angle between the c-axis and the easy magnetisation direction, we have measured the angular dependence of the components of the magnetisation vector. Our measurements are based on powder samples that have been previously aligned under an external magnetic field. The Si for Fe substitution induces a decrease of the spin reorientation temperature and of the tilt angle. Meanwhile, we have found that the tilt angles are almost the same for Nd₂Fe₁₃SiB and Nd₂Fe₁₂Si₂B. These features are analysed in terms of changes in the crystal electric field and the unit cell volume induced by the Si for Fe substitution. The insertion of hydrogen in the Si-containing samples leads to an additive decrease of the spin reorientation temperature. This decrease is not linked to a change in the tilt angle at 4 K but to a different thermal behaviour in the H-containing samples. The magnitude of the effects of H insertion on the crystal electric field in comparison with the Si for Fe substitution is discussed together with the role of the lattice expansion.     

New phases of water ice predicted at megabar pressures

Based on density functional calculations we predict water ice to attain two new crystal structures with Pbca and Cmcm symmetry at 7.6 and 15.5?Mbar, respectively. The known high-pressure ice phases VII, VIII, X, and Pbcm as well as the Pbca phase are all insulating and composed of two interpenetrating hydrogen bonded networks, but the Cmcm structure is metallic and consists of corrugated sheets of H and O atoms. The H atoms are squeezed into octahedral positions between next-nearest O atoms while they occupy tetrahedral positions between nearest O atoms in the ice X, Pbcm, and Pbca phases.     

‘Guest–host’ effect in liquid crystal mixtures

The most important goal of our research is to show the influence of the ‘guest’ (bent-core mesogen, 1,3-phenyldicarboxylatebis{4-[(4-octylbenzoyl)sulphanyl]phenyl} [IFOS8], banana-shaped liquid crystal [BLC]) on the ‘host’ (calamitic liquid crystal [CLC], (S)-(+)-1-methylheptyloxybiphenyl-(4-n-octylphenyl)thiobenzoate [MHOBS8]), on the stability and the destabilization of the antiferroelectric B2 and the ferroelectric smectic C* (SmC*) phases, and change of the temperature ranges of other phases in the binary liquid crystal mixtures. This work is focused on polymorphism of three new binary liquid crystal mixtures, exhibiting a ‘guest–host’ (guest liquid crystal–host liquid crystal [GH-LC]) effect. MHOBS8 has, among others, a ferroelectric SmC* phase, and IFOS8 assumes the B2 phase with antiferroelectric properties. The observed properties of the mixtures, such as variation of the phase transition temperatures, spontaneous polarization, tilt angle and switching time, are characteristic of a ‘guest–host’ mixture. The influence of BLC on the character of the interactions within the CLC host is discussed, with particular attention paid to electro-optical properties of the GH-LC mixtures.     

Phase behavior of symmetric ternary block copolymer-homopolymer blends in thin films and on chemically patterned surfaces

The phase diagram of symmetric ternary blends of diblock copolymers and homopolymers in thin films was determined as a function of increasing volume fraction of homopolymer (phi(H)) and was similar to that for these materials in the bulk. Blends with compositions in the lamellar region of the diagram (phi(H)< or =0.4) could be directed to assemble into ordered lamellar arrays on chemically striped surfaces if the characteristic blend dimension (L(B)) and the period of the stripes (L(S)) were commensurate such that L(S)=L(B)+/-0.10L(B). Blends with compositions in the microemulsion region of the diagram (phi(H) approximately 0.6) assembled into defect-free lamellar phases on patterned surfaces with L(S)> or =L(B), but formed coexisting lamellar (with period L(S)) and homopolymer-rich phases when L(S)相似文献   

Metastable phases in HxLi1-xTaO3 waveguide layers and pure LiTaO3

The formation of high-temperature phases in low-doped H:LiTaO₃ waveguide layers in Z-cut LiTaO₃ has been observed both by refractive-index and IR-spectra measurements. This permits us to correlate the index jumps to the changes of the OH bonds in the crystal lattice. Reversible phase transitions were detected in the temperature interval T=50–200 °C over a wide range of hydrogen content including as-grown LiTaO₃. The high-temperature phases are metastable close to room temperature. This was demonstrated by tracing the time evolution of the refractive-index change. It was shown that the high-temperature phases are responsible for the long-term refractive-index instabilities in both H:LiTaO₃ waveguides and virgin LiTaO₃. Received: 7 May 2001 / Revised version: 10 August 2001 / Published online: 30 October 2001     

Search of hydrogen transition states on α-Fe: The monomer adapted to first principles calculations

In the context of the density functional theory (DFT), we use the ab-initio electronic structure code, SIESTA, coupled to an economic technique to find activated states (or migration barriers), in order to study atomic defects diffusion through the crystal lattice: The monomer method [V.P. Ramunni, M.A. Alurralde, R.C. Pasianot, Phys. Rev. B 74 (2006) 054113]. We test our technique calculating the activated transition states of interstitial hydrogen (H) between two sites of high symmetry, tetrahedral and octahedral, in the α and β phases of Fe. The results are discussed in the context of experimental data and other available models.     

14N NQR study of the structural phase transitions in NH4NO3

The temperature dependences of the¹⁴N nuclear quadrupole resonance frequencies have been measured in phases II, III, IV and V of ammonium nitrate with the help of the¹H–¹⁴H nuclear quadrupole double resonance technique. The experimental results are related to the proposed crystal structures and disorder in the various crystallographic phases.     

Predicting the solid state phase diagram for glass-forming alloys of copper and zirconium

The free energies of six crystal structures associated with Cu-Zr alloys-Cu (face centred cubic), Cu(2)Zr, Cu(10)Zr(7), CuZr, CuZr(2) and Zr (hexagonal close packed)-are calculated using the embedded atom potential of Mendelev et al (2009 Phil. Mag. 89 967). We find that the observed low temperature stability of the Cu(10)Zr(7) and CuZr(2) phases is not reproduced. Instead, the model predicts that the CuZr phase remains stable down to T = 0 K. This discrepancy is largely removed when the interaction potentials are cut off at a short distance, such as that used by Duan et al (2005 Phys. Rev. B 71 224208). We present evidence, however, that the cut-off distance necessary to achieve the change in phase stability results in pathological artefacts in the energetics of some crystal phases.     

Effect of temperature on the phase behavior and microstructure of aggregates in SDS/Triton X-100/aH2O systems

The phase behavior and microstructure of a system containing polyoxyethylene tert-octylphenyl (Triton X-100), sodium dodecyl sulfate (SDS) and water were investigated using polarized optical microscopy (POM), differential scanning calorimetry (DSC) and small-angle X-ray diffraction (XRD). Phase-behavior study revealed that lamellar and hexagonal phases occurred in different compositions, and that lyotropic liquid crystal phases tended to form an H phase with increased Triton X-100 content. The effects of temperature on the phase behavior and microstructure of lyotropic liquid crystals were also studied. Heating induced a change from hexagonal to lamellar phase and from gel to hexagonal phase. POM and small-angle X-ray diffraction (SXRD) revealed that the phase-transition temperature of LLCs increased with increased Triton X-100 content, as further supported by DSC data.     

高压下铌铁矿相TiO2直接相变的第一性原理研究

利用第一性原理的GGA+U方法对TiO2的金红石相、铌铁矿相、萤石相、黄铁矿相和四方P42/nmc相进行了计算。首先,优化了这几个相的晶体结构及相关参数,分析了焓曲线图,发现了在50.1GPa压强下,铌铁矿相突然直接转变成一个新的四方P42/nmc相。其次,通过GGA+U和HSE06两种方法计算讨论了四方P42/nmc相的能带结构,并拟合分析了三阶B-M状态方程和XRD图谱。最后,通过分析相变压强范围、体弹模量和XRD图谱,本文认为四方P42/nmc相与立方萤石相是共存的,能很好符合2004年立方TiO2的实验结果。     

ZnS晶体3C⇔2H结构转变的LAUE衍射图中“奇异点”的标定

在ZnS晶体3C⇔2H结构转变的X—射线Laue衍射分析中,为了解释一维无序造成的异常衍射现象,本文推导了Laue衍射图曲线晶带的“奇异点”计算公式。并应用此公式,计算了ZnSLaue衍射照片上数条晶带上的“奇异点”,计算值与实验值完全吻合。另外,本文还对“奇异点”的形成、特点、标定的意义以及计算公式适用范围进行了讨论。     

高压下铌铁矿相TiO2直接相变的第一性原理研究

利用第一性原理的GGA+U方法对TiO2的金红石相、铌铁矿相、萤石相、黄铁矿相和四方P42/nmc相进行了计算。首先,优化了这几个相的晶体结构及相关参数,分析了焓曲线图,发现了在50.1GPa压强下,铌铁矿相突然直接转变成一个新的四方P42/nmc相。其次,通过GGA+U和HSE06两种方法计算讨论了四方P42/nmc相的能带结构,并拟合分析了三阶B-M状态方程和XRD图谱。最后,通过分析相变压强范围、体弹模量和XRD图谱,本文认为四方P42/nmc相与立方萤石相是共存的,能很好符合2004年立方TiO2的实验结果。     

高压下铌铁矿相TiO_2直接相变的第一性原理研究

利用第一性原理的GGA+U方法对TiO_2的金红石相、铌铁矿相、萤石相、黄铁矿相和四方P42/nmc相进行了计算.首先,优化了这几个相的晶体结构及相关参数,分析了焓曲线图,发现了在50.1GPa压强下,铌铁矿相突然直接转变成一个新的四方P42/nmc相.其次,通过GGA+U和HSE06两种方法计算讨论了四方P42/nmc相的能带结构,并拟合分析了三阶B-M状态方程和XRD图谱.最后,通过分析相变压强范围、体弹模量和XRD图谱,本文认为四方P42/nmc相与立方萤石相是共存的,能很好符合2004年立方TiO_2的实验结果 .     

CeMgAl11O19-SrAl12O19固溶体和CeMgAl11O19-SrMgAl10O17固溶体在稀土和碱土复合六角铝酸盐体系中的光谱学性质研究

本文研究CeMSAl11O19-SrAl12O19和CeMeAl11O19-SrMgAl10O17体系固溶体的发光性质。发现在CeMgAl11O19-SrAl12O19体系中,能形成完全的固溶体,随着SrAl12O19含量的增加,晶胞常数a线性地减小,c保持不变,Ce3+的Nephelauxetic效应和晶场强度减弱,Ce3+最低5d激发带边能量呈指数形式变化,Ce3+的发射能量线性地高移。在CeMgAl11O19-SrMgAl10O17体系中,不能形成完全的固溶体。固溶体终端组成大约为0.3CeMgAl11O19-0.1SrMgAl10O17。在这种固溶体中,晶胞常数α和c则随SrMgAl10O17含量分别线性地减小和增大。样品制备中添加H3BO3,引起固溶体晶僻特性改变,大大提高Ce3+的发光强度。但当H3BO3量超过0.4(摩尔比)时,硼将进入晶格,反而减弱Ce3+的发光。     

Effect of rare earth on the microstructures and properties of a low expansion superalloy

A new Fe-Ni-Co-Nb-Ti-Si superalloy containing trace additions of selective rare earths and having good combination of very low thermal expansion coefficient, high-resistance to stress accelerated grain boundary oxygen embrittlement and fairly good notch-bar rupture strength has been successfully developed. The resistance to oxidation for long time exposure at high-temperatures and the stress rupture life has been improved significantly with trace yttrium addition. The microstructures of the alloys have been studied by means of analytical electron microscopy, chemical and X-ray analysis techniques. The results reveal that the trace yttrium segregates in the strengthening phase with platelet morphology, and helps in transforming A(3)B type epsilon phase into A(5)B type H. The morphology and crystal structures of the grain boundary phases also change with selective additions of rare earth elements. Compared with those in the conventional alloy, the platelet precipitates in the yttrium-containing alloy densely segregate within the grains and along the grain boundaries with smaller size. The segregation of the platelet precipitates within the grains is helpful in improving the strength of the alloy. In addition, its precipitation along the grain boundaries can improve the resistance to stress accelerated grain boundary oxidation and stress rupture property of the alloy and thereby contribute to its temperature stability.     

Thermotropic biaxial nematic liquid crystals

We have synthesized liquid crystal (LC) mesogens based on a nonlinear oxadiazole unit that exhibit nematic phases near 200 degrees C. Polarized microscopy and conoscopy indicate that these LCs are biaxial nematics. Unambiguous and quantitative evidence for biaxiality is achieved using 2H NMR spectroscopy. "2D powder" spectra, obtained by rotating 2H-labeled samples about an axis perpendicular to the magnetic field at approximately 200 Hz, yield phase biaxiality parameters of approximately 0.1 when coupled with rigorous and proven simulations.