Raman spectroscopy of dawsonite NaAl(CO3)(OH)2

Raman spectroscopy both at 298 and 77 K complemented with infrared spectroscopy was used to study the structure of dawsonite. Previous crystallographic studies concluded that the structure of dawsonite was a simple one; however, both Raman and infrared spectroscopy show that this conclusion is incorrect. Multiple bands are observed in both the Raman and infrared spectra in the antisymmetric stretching and bending regions, showing that the symmetry of the carbonate anion is reduced and in all probability the carbonate anions are not equivalent in the dawsonite structure. Multiple OH deformation vibrations centred around 950 cm⁻¹ in both Raman and infrared spectra show that the OH units in the dawsonite structure are non‐equivalent. Calculations using the position of the Raman and infrared OH stretching vibrations enabled estimates of the hydrogen‐bond distances of 0.2735 and 0.27219 pm at 298 K, and 0.27315 and 0.2713 pm at 77 K to be made. This indicates strong hydrogen bonding of the OH units in the dawsonite structure. Copyright © 2007 John Wiley & Sons, Ltd.     

纳米Ni在77 K温度下压缩行为的研究

利用低温力学测试系统研究了电化学沉积纳米Ni在77 K温度下的压缩行为. 室温下纳米Ni 的屈服强度为 2.0 GPa, 77 K温度下的屈服强度为3.0 GPa, 压缩变形量则由室温的10%左右下降到5%. 借助应变速率敏感指数、激活体积、扫描电子显微和高分辨透射电子显微分析, 对纳米Ni的塑性变形机制进行了表征. 研究表明, 在77 K温度下的塑性变形主要是由晶界-位错协调变形主导, 晶界本征位错弓出后无阻碍地在晶粒内无位错区运动, 直至在相对晶界发生类似切割林位错行为. 同时分析了弓出位错的残留位错部分在协调塑性变形时起到的增加应变相容性和减小应力集中的作用. 利用晶界-位错协调机制和残留位错运动与温度及缺陷的相关性揭示了纳米Ni室温和77 K温度压缩性能差异的内在原因. 关键词： 塑性变形 强度 位错     

Effects of cold rolling deformation on microstructure,hardness, and creep behavior of high nitrogen austenitic stainless steel

Effects of cold rolling deformation on the microstructure, hardness, and creep behavior of high nitrogen austenitic stainless steel （HNASS） are investigated. Microstructure characterization shows that 70% cold rolling deformation results in significant refinement of the microstructure of this steel, with its average twin thickness reducing from 6.4 μm to 14 nm. Nanoindentation tests at different strain rates demonstrate that the hardness of the steel with nano-scale twins （nt-HNASS） is about 2 times as high as that of steel with micro-scale twins （mt-HNASS）. The hardness of nt-HNASS exhibits a pronounced strain rate dependence with a strain rate sensitivity （m value） of 0.0319, which is far higher than that of mt-HNASS （m = 0.0029）. nt-HNASS shows more significant load plateaus and a higher creep rate than mt-HNASS. Analysis reveals that higher hardness and larger m value of nt-HNASS arise from stronger strain hardening role, which is caused by the higher storage rate of dislocations and the interactions between dislocations and high density twins. The more significant load plateaus and higher creep rates of nt-HNASS are due to the rapid relaxation of the dislocation structures generated during loading.     

Mechanical behavior of silicon carbide nanoparticles under uniaxial compression

The mechanical behavior of SiC nanoparticles under uniaxial compression was investigated using an atomic-level compression simulation technique. The results revealed that the mechanical deformation of SiC nanocrystals is highly dependent on compression orientation, particle size, and temperature. A structural transformation from the original zinc-blende to a rock-salt phase is identified for SiC nanoparticles compressed along the [001] direction at low temperature. However, the rock-salt phase is not observed for SiC nanoparticles compressed along the [110] and [111] directions irrespective of size and temperature. The high-pressure-generated rock-salt phase strongly affects the mechanical behavior of the nanoparticles, including their hardness and deformation process. The hardness of [001]-compressed nanoparticles decreases monotonically as their size increases, different from that of [110] and [111]-compressed nanoparticles, which reaches a maximal value at a critical size and then decreases. Additionally, a temperature-dependent mechanical response was observed for all simulated SiC nanoparticles regardless of compression orientation and size. Interestingly, the hardness of SiC nanocrystals with a diameter of 8 nm compressed in [001]-orientation undergoes a steep decrease at 0.1–200 K and then a gradual decline from 250 to 1500 K. This trend can be attributed to different deformation mechanisms related to phase transformation and dislocations. Our results will be useful for practical applications of SiC nanoparticles under high pressure.     

Hole localization effect on determination of charge accumulation in double barrier resonant tunneling structures

We have investigated photoluminescence of double-barrier diodes under various bias voltages and observed the saturation of the broadening of the photoluminescence lines. We have also studied the integrated photoluminescence intensity with increasing applied voltages near resonant voltages for high quality samples at low temperature (4.2K). The results of the 77K photoluminescence experiments confirm those at 4.2K. The saturation of the broadening is due to weak hole localization; however, the saturation of the integrated photoluminescence intensity is mainly due to reduced nonradiative recombination. Our results suggest that the hole localization may arise from interface roughness.     

Mössbauer study of aluminum-substituted hematites

A Mössbauer spectroscopic study is reported on a series of aluminum-substituted hematites, α(Fe_1-cAl_c)₂O₃, with c up to 0.32. These samples were prepared by heating aluminum-substituted goethites, αFeOOH, at 500°C. X-ray line broadening gives particle dimensions of ?200 Å to >1000 Å. Heating the samples to 900° improves crystallinity but reduces the maximum obtainable c to ?0.15. At 77 K for c?0.04 the magnetic structure is antiferromagnetic with the spins aligning close to the (111) axis. For c?0.08 and for all compositions at 298 K the spins are perpendicular to (111) and for this weakly ferromagnetic phase the supertransferred hyperfine field is (5±1) kOe per magnetic neighbor. Samples with 0.04?c<0.08 at 77 K have both magnetic phases present in varying proportions. The magnetic field at 298 K varies with aluminum content according to the 1/3-power law for the reduced sublattice magnetization. The asymmetric shape of the spectra for c?0.17 has been accounted for by a model based on the molecular field approximation with nearest neighbor superexchange interaction strength J_nn?15 K.     

Deformation-induced martensite and nanotwins by cryogenic laser shock peening of AISI 304 stainless steel and the effects on mechanical properties

Laser shock peening (LSP) of stainless steel 304 was carried out at room and cryogenic temperature (liquid nitrogen temperature). It was found that the deformation-induced martensite was generated by LSP only when the laser-generated plasma pressure is sufficiently high. Compared to room temperature laser shock peening (RT-LSP), cryogenic laser shock peening (CLSP) generates a higher volume fraction of martensite at the same laser intensity. This is due to the increase in the density of potential embryos (deformation bands) for martensite nucleation by deformation at cryogenic temperature. In addition, CLSP generates a high density of deformation twins and stacking faults. After CLSP, an innovative microstructure, characterised by networks of deformation twins, stacking faults and composite structure (martensite and austenite phases), contributes to material strength and microstructure stability improvement. The combined effect of higher surface hardness and a more stabilised microstructure results in greater fatigue performance improvement of the CLSP samples compared to that of the RT-LSP samples.     

Thermally stable coherent domain boundaries in complex-structured Cr2Nb intermetallics

The strength and hardness of nanostructured materials are significantly enhanced owing to the large amount of grain boundaries (GB) produced by a reduced grain size. The thermal stability of the GB is a key to maintaining the grain size and thus the strength/hardness in nanostructured materials at high temperatures. In this work, coherent domain boundaries (DB) were introduced by compressive processing to sub-divide a complex-structured intermetallic Cr₂Nb into nanograins of size down to 2 nm. These DB persisted after an annealing of 10 h at 1273 K. The coherent DB have been investigated by aberration-corrected high-resolution transmission electron microscopy and first-principles calculations. The high thermal stability is evidently a result of low formation energies of the DB.     

Deformation and annealing behaviour of a low carbon high Mn TWIP steel microalloyed with Ti

ABSTRACT

A low carbon high Mn, Ti microalloyed dual phase TWIP steel has been processed through cold rolling and annealing. X-ray diffraction reveals the maximum austenite (≈92%) in HRACST sample whereas, the 50CD sample shows 29% ferrite. The microstructure of HRAC and HRACST samples reveal austenite grains with annealing twins and deformation induced ferrite (DIF). The higher amount of DIF along with deformation twins form during cold deformation. Annealing at 500°C shows recovery, whereas at 700°C shows partial recrystallisation and at 900°C reveals almost full recrystallisation. TEM microstructures of the 900°C for 30?min samples reveal annealing twins with TiC particle. Strong Brass {110}<112> and Goss{110}<001> texture components are observed in HRAC, HRACST and 50CD samples. Goss Twin (GT) {113}<332> and Copper Twin (Cu-T) {552}<115> components are observed in 50CD sample. Addition of Ti results in an average grain size of 20?μm. Maximum YS (1176?MPa) and UTS (1283?MPa) values with the lowest ductility of 11% have been obtained for the 50CD sample which is related to the formation of extensive deformation twin and a higher fraction of DIF. 700°C-30?min and 700°C-60?min samples show an increase in ductility (23% and 34%, respectively) with a marginal decrease in tensile strength (1054?MPa). Annealing at 900°C shows ductility restoration up to 60% with higher tensile strength compared to HRACST sample. Ductile fracture of HRAC and HRACST samples transform to brittle fracture in the 50CD sample. Annealing at 900°C for 30?min shows ductile fracture with some (Fe, Mn)S and TiC particles.     

Preparation and hydrogen storage of activated rayon-based carbon fibers with high specific surface area

Activated carbon fibers were prepared from rayon-based carbon fibers by two step activations with steam and KOH treatments. Hydrogen storage properties of the activated rayon-based carbon fibers with high specific surface area and micropore volume have been investigated. SEM, XRD and Brunauer-Emmett-Teller (BET) were used to characterize the samples. The adsorption performance and porous structure were investigated by nitrogen adsorption isotherm at 77 K on the base of BET and density functional theory (DFT). The BET specific surface area and micropore volume of the activated rayon-based carbon fibers were 3144 m²/g and 0.744 m³/g, respectively. Hydrogen storage properties of the samples were measured at 77 and 298 K with pressure-composition isotherm (PCT) measuring system based on the volumetric method. The capacities of hydrogen storage of the activated rayon-based carbon fibers were 7.01 and 1.46 wt% at 77 and 298 K at 4 MPa, respectively. Possible mechanisms for hydrogen storage in the activated rayon-based carbon fibers are discussed.     

Microstructure and its influence on CH4 adsorption behavior of deep coal

In this paper we investigate the influence of microstructure on the CH4 adsorption behavior of deep coal. The coal microstructure is characterized by N2 adsorption at 77 K, scanning electron microscopy （SEM）, Raman spectroscopy, and Fourier transform infrared spectroscopy （FT-IR）. The CH4 adsorptions are measured at 298 K at pressures up to 5.0 MPa by the the volumetric method and fitted by the Langmuir model. The results show that the Langmuir model fits well with the experimental data, and there is a positive correlation with surface area, pore volume, ID/IG, and CH4 adsorption capacity. The burial depth also affects the methane adsorption capacity of the samples.     

Atomistic study on shock behaviour of NiTi shape memory alloy

Abstract

The shock behaviour of NiTi shape memory alloy is investigated by using molecular dynamics simulation. The nano-pillar samples of the alloy are subjected to the impact of a piston with a velocity of 350 m/s at initial environment temperatures of 325 and 500 K. At 325 K, we observe two different pathways of the formation of BCO phase, the gradient twins, and the detwinning phenomena, strongly depending on the local stress and the deformation state. As the initial temperature increases to 500 K, the plasticity is dominated by the dislocation movements rather than the twinning at 325 K. The phase transformation and plasticity result in stress attenuation when the stress wave propagates through the nano-pillar. Furthermore, it is interesting to note that multiple stress peaks occur due to the formation of local complex atomic structures with various wave speeds, leading to the catch up and overlap of the stress waves.     

Electron tunneling through superconducting A1/monolayer/Pb junctions

The current-voltage characteristic of Al/adsorbed monolayer/Pb junctions was measured at 77, 4.2 and 1.8K at applied voltages from 1 to 3 mV. At 77K the current changes linearly with voltage whereas at 4.2 and 1.8 K the relationship becomes nonlinear. From the results at 1.8 K we obtain an approximate band gap for Pb equal to 2.6 meV. The observation of a nonlinear current-voltage characteristic at temperatures where Pb becomes superconducting is strong evidence that the observed current through the insulator is a tunneling current.     

Temperature-induced rearrangement of the dislocation pattern of Persistent Slip Bands in copper single crystals

Experimental investigations are reported on mechanisms by which dislocation arrangements of Persistent Slip Bands (PSBs) respond to changes of the deformation temperature. Copper single crystals orientated for single slip were cyclically deformed well into saturation at 300 K at an applied resolved plastic shear-strain amplitude, , such that the plastic strain became localized in PSBs. The spacings of the dislocation walls in these PSBs are about 1.4 m. After the temperature had been lowered to 77 K, cyclic deformation was continued with unchanged . A transformation of the dislocation pattern started. A certain fraction of the PSBs produced at 300 K finally showed a mean wall spacing of about 0.7 m which is typical for PSBs formed at 77 K. The remaining PSBs did not finish the transformation and became obviously inactive. In the state of cyclic saturation reattained at 77 K 50% of the PSBs, which had been formed at 300 K, show the dislocation pattern characteristic of 77 K. It is concluded that the amplitude of the resolved plastic shear strain localized in a PSB, , must be twice as large at 77 K as at 300 K. In an additional series of experiments crystals were cyclically deformed at constant temperatures of 430 K, 300 K, 190 K, and 77 K. In the temperature range covered by these experiments, the amplitude of the saturation flow stress, _S, appears to be proportional to the intrinsic amplitude of the PSBs, .     

强流脉冲电子束作用下金属锆的微观结构与应力状态

利用强流脉冲电子束 (HCPEB) 技术对金属纯锆进行表面处理, 采用X射线衍射, 扫描电子显微镜及透射电子显微镜详细分析了辐照诱发的表层微观结构和缺陷. X射线分析结果表明, HCPEB辐照后在材料表层诱发幅值为GPa量级的压应力, 并形成{0002}, {1012}, {1120}及{1013}织构. 表层微观结构观察表明, 与其他金属材料不同, HCPEB辐照在材料表层诱发的熔坑数量极少, 多次轰击甚至几乎没有表面熔坑的形成. 此外, 在快速的加热和冷却状态下, 在表面熔化层形成大量的超细晶粒结构, 同时诱发马氏体相变和强烈的塑性变形. 1次HCPEB辐照后表层内形成的变形微结构以位错为主, 孪晶数量较少; 5 次辐照样品的位错密度迅速增高, 孪晶数量也显著增加; 10次辐照后样品中的变形微结构以变形孪晶为主, 且出现二次孪晶现象. 表层晶粒内部变形的晶体学特征不仅决定了表层的织构演化行为, 而且还起到细化晶粒的作用, 为纯锆及锆合金表面强化提供了一条有效的途径. 关键词： 强流脉冲电子束 纯锆 微观结构 应力状态     

Effect of temperature on solid-state formation of bulk nanograined intermetallic Al3Ni during high-pressure torsion

A bulk form of nanograined intermetallic Al₃Ni was produced by severe plastic deformation using high-pressure torsion (HPT). Powder mixtures of 75?mol% Al and 25?mol% Ni were processed by HPT at a selected temperature in the range of room temperature (RT) to 573?K under a pressure of 6?GPa. X-ray diffraction analysis revealed that the Al₃Ni intermetallic formed after processing for 50 revolutions at RT but, as the processing temperature increased, less revolutions (i.e. lower imposed strain) were required for the formation of Al₃Ni. Observations by transmission electron microscopy showed that the microstructure consists of ultrafine grains having a size of 300–2000?nm after 3 and 10 revolutions. Once the Al₃Ni formed after a higher number of revolutions, equiaxed nanograins with a size of ~30?nm prevailed with a significant increase in hardness. The increase in hardness was more significant when processed at higher temperatures because of increasing the fraction of Al₃Ni. It was shown that the solid-state formation of Al₃Ni occurred due to enhanced diffusion (i.e. decreased activation energy for diffusion) through the presence of high density of lattice defects.     

Temperature studies of Fe−Mn nodules

Sea nodules were extensively studied over a wide range of temperature ranging from 77K to 1175K using Mossbauer effect and ESR. The Mossbauer studies of the nodules at 77K and room temperature show a quadrupole doublet whereas at higher temperatures magnetic spectra were obtained starting at around 875K which ultimately gives a hyperfine field of around 390 KOe at 1175 K. The Mossbauer spectra recorded at 30K did not show any significant change in the room temperature spectra. The analysis of the spectra upto 775K showed two positions of Fe^R+, vie., octahedral and tetrahedral which were varified by ESR of the diluted samples.     

Quadrupole interaction in In0.95 Ag0.045 Ga0.005 alloy

The temperature dependence of the quadrupole interaction of¹¹¹Cd in the In_0.95 Ag_0.045 Ga_0.005 alloy is determined by the time differential perturbed angular correlation method from 77 to 422 K. The electric field gradients produced by the lattice are derived. The temperature dependence of the electric field gradient is found to follow theT ^3/2 law observed in In between 77 K and room temperature and to deviate from it at higher temperatures. The widths of the electric field gradient distribution are 0.1147(259), 0.0924(340), 0.4954(601) and 1.344(175) at 77, 298, 373 and 422 K, respectively. This illustrates that the alloy is well ordered at 77 and 298 K and that it becomes less ordered at 373 and 422 K.     

Localization of plastic deformation in calcium fluoride crystals at elevated temperatures

The strain distribution was experimentally studied in CaF₂ crystals subjected to compression tests along [110] and [112] at a constant strain rate at temperatures T = 373–1253 K. At T > 845 K, the plastic deformation in deformed samples is found to be strongly localized in narrow bands, where the shear strain reaches several hundred percent. The physical deformation conditions are determined under which the plastic flow loses its stability and, as a result, the deformation is localized. The temperature dependence of the critical stress of the transition to a localized flow is found. A scenario is proposed for the nucleation and development of large localized shears during high-temperature deformation of single crystals.     

新型溶胶-凝胶二氧化硅微孔增透膜的制备及性能研究

以正硅酸乙酯为前驱体,二甲基二乙氧基硅烷为造孔剂,采用溶胶凝胶技术在酸催化条件下制备了二氧化硅溶胶;采用提拉法在K9玻璃基片上双面镀膜,经500 ℃热处理,制备得到一种新型单层微孔二氧化硅增透膜。通过改变造孔剂加入量,膜层峰值透过率可达到99.7％,而硬度仍保持在2H以上,同时具有良好的耐摩擦性及粘附性。加速腐蚀实验表明,膜层的环境稳定性是常规膜层的10倍以上。由于该新型增透膜兼具高透过率、良好的机械性能以及很强的环境稳定性,因而在改善太阳能玻璃增透性能方面有极大的应用价值。