Large anisotropy of electrical properties in layer-structured In2Se3 nanowires

Layer-structured indium selenide (In 2Se 3) nanowires (NWs) have large anisotropy in both shape and bonding. In 2Se 3 NWs show two types of growth directions: [11-20] along the layers and [0001] perpendicular to the layers. We have developed a powerful technique combining high-resolution transmission electron microscopy (HRTEM) investigation with single NW electrical transport measurement, which allows us to correlate directly the electrical properties and structure of the same individual NWs. The NW devices were made directly on a 50 nm thick SiN x membrane TEM window for electrical measurements and HRTEM study. NWs with the [11-20] growth direction exhibit metallic behavior while the NWs grown along the [0001] direction show n-type semiconductive behavior. Excitingly, the conductivity anisotropy reaches 10 (3)-10 (6) at room temperature, which is 1-3 orders magnitude higher than the bulk ratio.     

Superplastic Behaviour of a Rapidly Solidified Al-17Si Alloy

A rapidly solidified Al-17Si alloy was pre-pared by the ultrasonic gas atomization andextrusion method.Its superplasticity was studiedunder constant cross-head speed tensile test condi-tion over the temperature range of 450℃ to 550℃and strain rate range of 1.67×10~(-4) to 6.17×10~(-2) s~(-1).The superplastic behaviour of theas-extruded and the thermomechanically treatedsamples was compared.It was found that thethermomechanical treatment was essential toachieving superplastic deformation.A maximumelongation of 445% was obtained at test tempera-ture of 550℃ and strain rate of 6.17×10~(-4) s~(-1).The microstructures before and after deformationwere studied using OM,SEM and TEM.Void for-mation on the primary Si phase interfaces wasfound to have detrimental effect on superplasticity.It was also noted that the primary Si phasecoarsened rapidly during superplastic deformation.The micromechanisms of superplasticity,phasecoarsening and void formation were discussed.     

Low-temperature in situ large strain plasticity of ceramic SiC nanowires and its atomic-scale mechanism

Large strain plasticity is phenomenologically defined as the ability of a material to exhibit an exceptionally large deformation rate during mechanical deformation. It is a property that is well established for metals and alloys but is rarely observed for ceramic materials especially at low temperature ( approximately 300 K). With the reduction in dimensionality, however, unusual mechanical properties are shown by ceramic nanomaterials. In this Letter, we demonstrated unusually large strain plasticity of ceramic SiC nanowires (NWs) at temperatures close to room temperature that was directly observed in situ by a novel high-resolution transmission electron microscopy technique. The continuous plasticity of the SiC NWs is accompanied by a process of increased dislocation density at an early stage, followed by an obvious lattice distortion, and finally reaches an entire structure amorphization at the most strained region of the NW. These unusual phenomena for the SiC NWs are fundamentally important for understanding the nanoscale fracture and strain-induced band structure variation for high-temperature semiconductors. Our result may also provide useful information for further studying of nanoscale elastic-plastic and brittle-ductile transitions of ceramic materials with superplasticity.     

A kink-bands reinforced titanium alloy showing 1.3 GPa compressive yield strength:Towards extra high-strength/strain-transformable Ti alloys

1.Introduction In structural metallic materials, the occurrence of particular deformation mechanisms such as dislocation slip, deformation twins (DTs) [1,2] deformation kink bands (KBs) [3,4] or stressinduced phase transformations (SIM) [5], are closely related to both their crystal structures [6-8] (e.g.FCC, BCC and HCP) and loading conditions (e.g.temperature and/or strain rate).Reaching an accurate control on the activation of the above competing deformation mechanisms is one of the critical challenges to achieve optimized strength/ductility trade-off.Regarding titanium alloys,huge efforts have been produced in that sense, in recent years, with the quick development of new strain-transformable materials displaying complex stress induced deformation mechanisms such as combined TRIP and TWIP effects (TWIP: twinning-induced plasticity;TRIP: transformation-induced plasticity).[1,9-17].Elementary deformation mechanisms have been extensively investigated in many different Ti systems.Several efficient pathways, such as solute solution strengthening or second phase precipitation (ex:ω phase [18] or α phase [19,20] have now been successively proposed for strengthening, while keeping a strain-transformable matrix.     

新型铝奥氏体耐热合金的高温塑性变形行为和热力工性能

新型含铝奥氏体耐热合金(AFA)进行压缩热模拟试验,使用OM和EBSD等手段研究了这种合金在950~1150℃和0.01~5 s^-1条件下的微观组织演变、建立了基于动态材料模型热加工图、分析了变形参数对合金加工性能的影响并按照不同区域组织变形的特征构建了合金的热变形机理图。结果表明：新型AFA合金的高温流变应力受到变形温度和应变速率的显著影响。在变形温度为950~1150℃和应变速率为0.18~10 s^-1条件下,这种合金易发生流变失稳。在变形温度为1050~1120℃、应变速率0.01~0.1 s^-1和变形温度1120~1150℃、应变速率10^-0.5~10^-1.5 s^-1这两个区间,这种合金发生完全动态再结晶行为且其再结晶晶粒均匀细小,功率耗散因子η达到峰值45%。新型AFA合金的热加工艺,应该优先选择再结晶区域。     

Mechanical properties of ZnO nanowires under different loading modes

A systematic experimental and theoretical investigation of the elastic and failure properties of ZnO nanowires (NWs) under different loading modes has been carried out. In situ scanning electron microscopy (SEM) tension and buckling tests on single ZnO NWs along the polar direction [0001] were conducted. Both tensile modulus (from tension) and bending modulus (from buckling) were found to increase as the NW diameter decreased from 80 to 20 nm. The bending modulus increased more rapidly than the tensile modulus, which demonstrates that the elasticity size effects in ZnO NWs are mainly due to surface stiffening. Two models based on continuum mechanics were able to fit the experimental data very well. The tension experiments showed that fracture strain and strength of ZnO NWs increased as the NW diameter decreased. The excellent resilience of ZnO NWs is advantageous for their applications in nanoscale actuation, sensing, and energy conversion.      

Lateral self-aligned p-type In2O3 nanowire arrays epitaxially grown on Si substrates

Lateral orientated growth of In2O3 nanowire (NW) and nanorod (NR) arrays has been achieved by a vapor transport and condensation method on (001) and (111) surfaces of Si substrates. The single crystalline In2O3 NWs and NRs were grown along [211] in parallel to the Si +/-[110] and lying in the substrate plane. The electrical measurements show that the In2O3 NWs are p-type semiconductor. By N+ doping, the resistivity of the In2O3 NWs has been tuned. The lateral self-aligned In2O3 NW and NR arrays on Si can offer some unique advantages for fabricating parallel nanodevices that can be integrated directly with silicon technology.     

Growth of high-density titanium silicide nanowires in a single direction on a silicon surface

Understanding the growth mechanisms of nanowires is essential for their successful implementation in advanced devices applications. In situ ultrahigh-vacuum transmission electron microscopy has been applied to elucidate the interaction mechanisms of titanium disilicide nanowires (TiSi2 NWs) on Si(111) substrate. Two phenomena were observed: merging of the two NWs in the same direction, and collapse of one NW on a competing NW in a different direction when they meet at the ends. On the other hand, as one NW encounters the midsection of the other NW in a different direction, it recedes in favor of bulging of the other NW at the midsection. Since crystallographically the nanowires are favored to grow on Si(110) only in the [1 -1 0] direction, this crucial information has been fruitfully exploited to focus on the growth of a high density of long and high-aspect-ratio Ti silicide NWs parallel to the surface on Si(110) in a single direction. The achievement in growth of high-density NWs in a single direction represents a significant advance in realizing the vast potential for applications of silicide NWs in nanoelectronics devices.     

ZK60镁合金的热压缩变形行为

采用Gleeble-1500热模拟机在温度250～400℃、应变速率0.001～1s-1、最大变形程度105%的条件下对ZK60镁合金进行了高温压缩模拟实验研究。分析了实验合金在高温变形时的流变应力和应变速率及变形温度之间的关系,计算了变形激活能和应力指数,并观察了热压缩变形过程中组织的变化。结果表明,合金的峰值流变应力随应变速率的增大而增加,随温度的升高而减小;在给定的变形条件下,计算出合金的变形激活能为63～130kJ/mol,应力指数为2.78～3.79;降低变形温度和提高应变速率可使再结晶晶粒的平均尺寸减小。     

Electron-beam-induced elastic-plastic transition in Si nanowires

It is generally accepted that silicon nanowires (Si NWs) exhibit linear elastic behavior until fracture without any appreciable plastic deformation. However, the plasticity of Si NWs can be triggered under low strain rate inside the transmission electron microscope (TEM). In this report, two in situ TEM experiments were conducted to investigate the electron-beam (e-beam) effect on the plasticity of Si NWs. An e-beam illuminating with a low current intensity would result in the bond re-forming processes, achieving the plastic deformation with a bent strain over 40% in Si NWs near the room temperature. In addition, an effective method was proposed to shape the Si NWs, where an e-beam-induced elastic-plastic (E-P) transition took place.     

Phase Transformation and Deformation Mechanism of Ti_3Al-base Alloy

The phase transformation and deformationmechanism of the alloy based on composition Ti_3Alwith addition of Nb,V,Mo have been studied by useof transmission electron microscopy (TEM).It hasbeen shown that the orientation relationship ofα_2 phase transformed from β phase is:(0001)α_2//(l10)β,[1210]α_2//[111]β.The present dislocationslip systems in α_2 phase are (1100)[0001] and(1100)<1120>.There also exist α_2 twins whichhave new twin relationship and the twin plane is(2021).     

Superplasticity and Diffusion Bonding of Ti_3AI Intermetallic Compounds

The superplasticity of Ti_3Al intermetallic compounds has been investigated in this paper.TheTi-14Al-21Nb ternary alloy showed 477% elongation at the strain rate of 1.49×10~(-5) s~(-1) and950℃.The elongation of Ti-14Al-21 Nb-3Mo-1V quinary alloy approached to 573% at the strainrate of 4.52×10~(-5) s~(-1) and the same temperature,and it was found that the elongation value in-creased to 1096.4%as temperature was raised up to 980℃ at the same strain rate.Ti_3Al base al-loys were bonded by diffusion bonding technology and good joints were created,the simulatedspecimens were performed by SPF/DB process.     

Residual-strain-induced nanocoils of metallic nanowires

Metallic nanocoils are attractive nanowire (NW) structures, which are expected to have an application as small inductors, but have not been reported before. This study proposes a coating technique for permanently bending a straight, metallic NW into a helix. A physical vapor deposition is applied to oblique NWs standing on a substrate. The deposition produces a biased thickness of the coating on NWs, resulting in a mismatch in internal strains, namely thermal strain and intrinsic strain, between a NW and the coating. These residual strains are driving forces for the bending process of NWs. In particular, the intrinsic strain of the overlayer contributes the bending deformation. In addition, elastic anisotropy of NWs couples bending with a twist, contributing to the formation of helixes. We have demonstrated nanocoils, comprised of Cr-coated Cu NWs, with a coil diameter of about 300 to 500 nm.     

Deformation mechanism transition in Fe–17Mn–0.4C–0.06V TWIP steel with different strain rates

The dynamic tensile behaviour and deformation mechanism of the Fe–17Mn–0.4C–0.06V twinning-induced plasticity (TWIP) steel were investigated over a wide range of strain rates from 10^?4 to 10³ s^?1. With increasing strain rate, the stacking fault energy increased due to the increase of adiabatic heating temperature, ΔT. At 10^?4 to 10¹ s^?1, the transformation-induced plasticity (TRIP) effect coexisted with the TWIP effect and weakened with increasing strain rate. With the increase of strain rate in the range of 10^?1 to 10¹ s^?1, the TWIP effect strengthened gradually and intersected deformation twins were formed. When the strain rate was higher than 10¹ s^?1, the TRIP effect disappeared and the twinning was inhibited since the adiabatic heating effect elevated.     

Rate-limiting mechanisms in high-temperature growth of catalyst-free InAs nanowires with large thermal stability

We identify the entire growth parameter space and rate-limiting mechanisms in non-catalytic InAs nanowires (NWs) grown by molecular beam epitaxy. Surprisingly huge growth temperature ranges are found with maximum temperatures close to ~600°C upon dramatic increase of V/III ratio, exceeding by far the typical growth temperature range for catalyst-assisted InAs NWs. Based on quantitative in situ line-of-sight quadrupole mass spectrometry, we determine the rate-limiting factors in high-temperature InAs NW growth by directly monitoring the critical desorption and thermal decomposition processes of InAs NWs. Both under dynamic (growth) and static (no growth, ultra-high vacuum) conditions the (111)-oriented InAs NWs evidence excellent thermal stability at elevated temperatures even under negligible supersaturation. The rate-limiting factor for InAs NW growth is hence dominated by In desorption from the substrate surface. Closer investigation of the group-III and group-V flux dependences on growth rate reveals two apparent growth regimes, an As-rich and an In-rich regime defined by the effective As/In flux ratio, and maximum achievable growth rates of > 6 μm h(-1). The unique features of high-T growth and excellent thermal stability provide the opportunity for operation of InAs-based NW materials under caustic environment and further allow access to temperature regimes suitable for alloying non-catalytic InAs NWs with GaAs.     

Direct gravure printing of silicon nanowires using entropic attraction forces

The development of a method for large-scale printing of nanowire (NW) arrays onto a desired substrate is crucial for fabricating high-performance NW-based electronics. Here, the alignment of highly ordered and dense silicon (Si) NW arrays at anisotropically etched micro-engraved structures is demonstrated using a simple evaporation process. During evaporation, entropic attraction combined with the internal flow of the NW solution induced the alignment of NWs at the corners of pre-defined structures, and the assembly characteristics of the NWs were highly dependent on the polarity of the NW solutions. After complete evaporation, the aligned NW arrays are subsequently transferred onto a flexible substrate with 95% selectivity using a direct gravure printing technique. As a proof-of-concept, flexible back-gated NW field-effect transistors (FETs) are fabricated. The fabricated FETs have an effective hole mobility of 17.1 cm(2) ·V(-1) ·s(-1) and an on/off ratio of ~2.6 × 10(5) .     

Controlled Sn-doping in TiO2 nanowire photoanodes with enhanced photoelectrochemical conversion

We demonstrate for the first time the controlled Sn-doping in TiO(2) nanowire (NW) arrays for photoelectrochemical (PEC) water splitting. Because of the low lattice mismatch between SnO(2) and TiO(2), Sn dopants are incorporated into TiO(2) NWs by a one-pot hydrothermal synthesis with different ratios of SnCl(4) and tetrabutyl titanate, and a high acidity of the reactant solution is critical to control the SnCl(4) hydrolysis rate. The obtained Sn-doped TiO(2) (Sn/TiO(2)) NWs are single crystalline with a rutile structure, and the incorporation of Sn in TiO(2) NWs is well controlled at a low level, that is, 1-2% of Sn/Ti ratio, to avoid phase separation or interface scattering. PEC measurement on Sn/TiO(2) NW photoanodes with different Sn doping ratios shows that the photocurrent increases first with increased Sn doping level to >2.0 mA/cm(2) at 0 V vs Ag/AgCl under 100 mW/cm(2) simulated sunlight illumination up to ~100% enhancement compared to our best pristine TiO(2) NW photoanodes and then decreases at higher Sn doping levels. Subsequent annealing of Sn/TiO(2) NWs in H(2) further improves their photoactivity with an optimized photoconversion efficiency of ~1.2%. The incident-photon-to-current conversion efficiency shows that the photocurrent increase is mainly ascribed to the enhancement of photoactivity in the UV region, and the electrochemical impedance measurement reveals that the density of n-type charge carriers can be significantly increased by the Sn doping. These Sn/TiO(2) NW photoanodes are highly stable in PEC conversion and thus can serve as a potential candidate for pure TiO(2) materials in a variety of solar energy driven applications.     

Hot Deformation and Modeling of Flow Stress for a Ti-containing HSLA Steel

Single-stage and double-stage interrupted hot compression tests for simulating hot rolling have been carried out for a Ti-containing HSLA steel (10Ti). Physical simulation of hot rolling was in progress utilizing a Thermecmastor-Z simulator in 850~1150℃ and strain rate of 0.1~60 s-1.A model for residual strain ratio λ was designed, and a model of flow stress considering residual strain has been obtained. The hot deformation behaviour at various strain rates has been studied.     

Direct observation of enhanced cathodoluminescence emissions from ZnO nanocones compared with ZnO nanowire arrays

We report, for the first time, direct observation of enhanced cathodoluminescence (CL) emissions from ZnO nanocones (NCs) compared with ZnO nanowires (NWs). For direct and unambiguous comparison of CL emissions from NWs and nanocones, periodic arrays of ZnO NW were converted to nanocone arrays by our unique HCl [aq] etching technique, enabling us to compare the CL emissions from original NWs and final nanocones at the same location. CL measurements on NW and nanocone arrays reveal that emission intensity of the nanocone at ～ 387 nm is over two times larger than that of NW arrays. The enhancement of CL emission from nanocones has been confirmed by finite-difference time-domain simulation of enhanced light extraction from ZnO nanocones compared to ZnO NWs. The enhanced CL from nanocones is attributed to its sharp morphology, resulting in more chances of photons to be extracted at the interface between ZnO and air.     

Hot Deformation Behaviour of SiC/AA6061 Composites Prepared by Spark Plasma Sintering

In this study, Si C/AA6061 composites with different Si C volume fractions(5%, 10%, 15% and 20%) were fabricated by spark plasma sintering. The deformation behaviour of the composites was studied by uniaxial compression test at temperatures from 573 K to 773 K and strain rates between 0.001 s(-)~1.and 1 s(-)~1..Results indicate that the flow stress of Si C/AA6061 composites increases with the increase of Si C volume fraction, with the decrease of deformation temperature and with the decrease of strain rate. The main deformation mechanism of the composites is dynamic recrystallisation(DRX), and the DRX degree depends on the processing parameters of deformation. Higher Si C volume fraction, higher deformation temperature and lower deformation strain rate promote the occurrence of DRX. The strain rate sensitivity and deformation activation energy of Si C/AA6061 composites are calculated. Results show that with the increase in deformation temperature and the decrease in Si C volume fraction, the strain rate sensitivity of the composites increases. From 573 K to 773 K, the average deformation activation energy of 5vol.%Si C/AA6061, 10 vol.%Si C/AA6061, 15 vol.%Si C/AA6061 and 20 vol.%Si C/AA6061 are 207.91, 230.88, 237.7 and249.87 k J mol(-)~1., respectively. The optimum hot working zone of the Si C/AA6061 composites is in the temperature range of 723 K to 773 K at strain rates from 0.1 s(-)~1.to 1 s(-)~1.