SEM and TEM characterization of the microstructure of post-compressed TiB2/2024Al composite

In the present work, 55 vol.% TiB(2)/2024Al composites were obtained by pressure infiltration method. Compressive properties of 55 vol.% TiB(2)/2024Al composite under the strain rates of 10(-3) and 1S(-1) at different temperature were measured and microstructure of post-compressed TiB(2)/2024Al composite was characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM). No trace of Al(3)Ti compound flake was found. TiB(2)-Al interface was smooth without significant reaction products, and orientation relationships ( [Formula: see text] and [Formula: see text] ) were revealed by HRTEM. Compressive strength of TiB(2)/2024Al composites decreased with temperature regardless of strain rates. The strain-rate-sensitivity of TiB(2)/2024Al composites increased with the increasing temperature. Fracture surface of specimens compressed at 25 and 250°C under 10(-3)S(-1) were characterized by furrow. Under 10(-3)S(-1), high density dislocations were formed in Al matrix when compressed at 25°C and dynamic recrystallization occurred at 250°C. Segregation of Mg and Cu on the subgrain boundary was also revealed at 550°C. Dislocations, whose density increased with temperature, were formed in TiB(2) particles under 1S(-1). Deformation of composites is affected by matrix, reinforcement and strain rate.     

Influence of Catalyses on the Preparation of YVO<Subscript>4</Subscript>:Eu<Superscript>3+</Superscript> Phosphors by the Sol–gel Methodology

YVO(4):Eu(3+) phosphors have been prepared by the hydrolytic sol-gel methodology, with and without alkaline catalyst. The solid powder was obtained by reaction between yttrium III chloride and vanadium alkoxides; the europium III chloride was used as structural probe. The powder was treated at 100, 400, 600, or 800 °C for 4 h. The samples were characterized by X-ray diffraction, thermal analysis, and photoluminescence. The XRD patterns revealed YVO(4) crystalline phase formation for the sample prepared without the catalyst and heat-treated at 600 °C and for the sample prepared in the presence of ammonium as catalyst and heat-treated at 100 °C. The average nanosized crystallites were estimated by the Scherrer equation. The sample which was produced via alkaline catalysis underwent weight loss in two stages, at 100 and 400 °C, whereas the sample obtained without catalyst presented four stages of weight loss, at 150, 250, 400, and 650 °C. The excitation spectra of the samples treated at different temperatures displayed the charge transfer band (CTB) at 320 nm. PL data of all the samples revealed the characteristic transition bands arising from the (5)D(0) → (5)F(J) (J = 0, 1, 2, 3, and 4) manifolds under maximum excitation at 320, 394, and 466 nm in all cases. The (5)D(0) → (7)F(2) transition often dominates the emission spectra, indicating that the Eu(3+) ion occupies a site without inversion center. The long lifetime suggests that the matrix can be applied as phosphors. In conclusion, the sol-gel methodology is a very efficient approach for the production of phosphors at low temperature.     

Effect of ZnAl2O4 coating on the compressive behaviors of aluminum borate whiskers-reinforced aluminum composites at elevated temperatures

Aluminum borate whiskers (ABOw) with or without ZnAl₂O₄ coating reinforced pure aluminum composites (ABOw/Al, ABOw/ZnAl₂O₄/Al) were fabricated by squeeze casting. The effects of ZnAl₂O₄ coating on the compressive behaviors, microstructures, and matrix textures of the composites were investigated at different temperatures and strain rates. The results indicate that the maximum compressive flow stress of the composites almost linearly decreases with the increase in temperature. The maximum compressive flow stress of ABOw/ZnAl₂O₄/Al composite is higher than that of ABOw/Al composite at the same temperature when the strain rate is larger, however, it is reversed when the strain rate is smaller. It is more serious for the fracture of whiskers in ABOw/ZnAl₂O₄/Al composite than that in ABOw/Al composite at the high compressive strain rate. However, the average length of whiskers in ABOw/ZnAl₂O₄/Al composite is larger than that in ABOw/Al composite at the low compressive strain rate. The strong matrix texture in ABOw/ZnAl₂O₄/Al composite appears at the high compressive strain rate, however, it is observed in ABOw/Al composite at the low compressive strain rate.     

Thermal transport in chemically doped polyaniline materials

Simultaneous measurements of effective thermal conductivity (λ_e) and effective thermal diffusivity (χ_e) of four samples S1 (polyaniline mixed with 100% Ni), S2 (polyaniline mixed with 90% Ni and 10% Al), S3 (polyaniline mixed with 80% Ni and 20% Al) and S4 (polyaniline mixed with 70% Ni and 30% Al) of polyaniline mix with metal Ni and Al in different percentage have been made using transient plane source technique. In the temperature range from room temperature to 170 °C. Both λ_e and χ_e increase with increase in temperature. Addition of aluminium concentration at the cost of nickel in the composite increases the value of λ_e and χ_e over the entire range of temperature under investigation. It has been found that the values of λ_e and χ_e are maximum when 70% of Ni and 30% of Al are mixed in polyaniline matrix. This behaviour is also predicted by an empirical relation, which is obtained from polynomial fit of the experimental data and is explained on the basis of bond formation between metal (Ni and Al) and nitrogen of polyaniline matrix.     

Mechanical Properties of Ni-Coated Single Graphene Sheet and Their Embedded Aluminum Matrix Composites

The effects of Ni coating on the mechanical behaviors of single graphene sheet and their embedded Al matrix composites under axial tensionare investigated using molecular dynamics (MD) simulation method. Theresults show that the Young's moduli and tensile strength of grapheneobviously decrease after Ni coating. The results also show that the mechanical properties of Al matrix can be obviously increased by embedding asingle graphene sheet. From the simulation, we also find that the Young'smodulus and tensile strength of the Ni-coated graphene/Al composite isobviously larger than those of the uncoated graphene/Al composite. Theincreased magnitude of the Young's modulus and tensile strength ofgraphene/Al composite are 52.27 and 32.32 at 0.01 K, respectively,due to Ni coating. By exploring the effects of temperature on the mechanicalproperties of single graphene sheet and their embedded Al matrix composites, it is found that the higher temperature leads to the lower critical strain and tensile strength.     

In situ environmental transmission electron microscopy to determine transformation pathways in supported Ni nanoparticles

We have applied in situ environmental transmission electron microscopy (ETEM) to follow the dynamic phase transformations that take place in SiO(2) supported Ni nanoparticles during oxidation and reduction processes. The gas environments used for in situ ETEM studies were relevant to partial oxidation of methane (POM) reaction. In the presence of the CH(4)+O(2) gas mixture (in 2:1 ratio) at 400°C, Ni transforms to NiO due to the high O-chemisorption energy. NiO void structures were formed during the oxidation because of the Kirkendall type process where diffusion of Ni cations along NiO grain boundaries is eight orders of magnitude greater than the diffusion of O anions. Reduction was performed under a CO+H(2) mixture at 400°C (in 1:2 ratio) and also in the presence of CH(4) at 500°C. Particle reduction processes also takes place via the diffusion of Ni cations along the NiO grain boundaries leaving NiO on the surface of the nanoparticle. NiO is the phase that is present on the surface of the nanoparticle during the intermediate stage of reduction.     

Residual microstructure associated with impact crater in Ti-6Al-4V meshes reinforced 5A06Al alloy matrix composite

In this paper, TC4(m)/5A06Al composite was hypervelocity impacted by 2024 aluminium projectile with the diameter of 2mm and with the impact velocity of 3.5 km/s. The residual microstructure was observed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HREM). The TC4-Al interface before impact was composed of TiAl(3) phase and Ti(3)Al phase. Near the pithead, separation of TC4 fibers and Al matrix occurred along the impact direction. Around the middle of the crater, TC4 fibers were sheared into several sections. Near the bottom of crater, adiabatic shear band (ASB) occurred in TC4 fiber, while the angle between shear plane and cross section was 45°. The crack propagated along TC4-Ti(3)Al interface during impact and some Ti(3)Al phase at the TC4-Al interface transformed to amorphous with few nanocrystals after hypervelocity impact.     

High quality p-type Ag-doped ZnO thin films achieved under elevated growth temperatures

By correlating the effects of substrate temperature, oxygen pressure and laser energy on the electrical and microstructural properties of Ag-doped ZnO films grown on a sapphire (0001) substrate, p-type conductivity is achieved under various substrate temperatures in the wide range of 250-750?°C. All of the samples were deposited by pulsed-laser deposition under various designed conditions. Hall measurements indicate that the best conductivity is achieved in Ag-ZnO films under a substrate temperature of 500?°C, a partial oxygen pressure of 250-300?mTorr and laser energy between 330 and 345?mJ. The hole-carrier concentration is 2.29?×?10(18)?cm(-3), the resistivity is 0.9?Ω?cm and the mobility is 3.03?cm(2)?V(-1)?s(-1). Transmission-electron microscopy (TEM) studies on the p-type films reveal similar microstructural properties to one another, but different properties to that of the n-type films deposited at the same temperatures with different deposition parameters.     

Accurate method for measuring the thermal coefficient of group birefringence of polarization-maintaining fibers

We present a method to accurately measure the group birefringence variation with temperature in high-birefringence polarization-maintaining (PM) fibers using a distributed polarization analyzer. By analyzing polarization cross-talk peaks purposely induced at both ends of a PM fiber, the temperature coefficient of group birefringence can be accurately obtained. We confirm the theoretical prediction that the group birefringence of PANDA and TIGER PM fibers decrease linearly with temperature from -40 °C to 80 °C, and find that the temperature coefficients are -5.93 × 10(-7) °C(-1) and -5.29 × 10(-7) °C(-1) for two types of PANDA fibers, and -5.36 × 10(-7) °C(-1) for a TIGER fiber.     

原位自生20vol.％TiCP/LD7Al基复合材料蠕变的应力指数和门槛应力

在523 K,573 K和623 K恒应力压缩条件下研究了原位自生20vol%TiC_p/LD7Al基复合材料和LD7Al合金的高温蠕变行为.对蠕变速率与外加应力在双对数坐标中进行拟合,获得了复合材料和基体铝合金的应力指数;通过在幂率方程中引入有效应力（σ-σ₀）,对实验数据进行线性回归外推至零蠕变速率得到相应的门槛应力.实验结果显示,复合材料的应力指数和门槛应力均高于LD7Al合金.TiC颗粒的存在,明显改善了LD7Al合金的高温蠕变 关键词： p/LD7Al基复合材料')" href="#">TiC_p/LD7Al基复合材料 蠕变 应力指数 门槛应力     

Electron microscopic study on interfacial characterization of electroless Ni-W-P plating on aluminium alloy

The interface between electroless plating Ni-W-P deposit and aluminium alloy (Al) matrix at different temperature heated for 1 h was studied using transmission electron microscope. The results show that the interface between as-deposited Ni-W-P deposit and Al matrix is clear. There are no crack and cavity. The bonding of Ni-W-P deposit and Al matrix is in good condition. The Ni-W-P plating is nanocrystalline phase (5-6 nm) in diameter. After being heated at 200 °C for 1 h, the interface of Ni-W-P deposit and Al matrix is clear, without the appearance of the diffusion layer. There exist a diffusion layer and educts of intermetallic compounds of nickle and aluminium such as Al₃Ni, Al₃Ni₂, NiAl, Ni₅Al₃ and so on between Ni-W-P deposit and Al matrix after being heated at 400 °C for 1 h.     

Defect states in Lu 3 Al 5 O 12 :Ce crystals

Wavelength-resolved TSL measurements after x-ray irradiation at room temperature were performed on undoped, Ce- and (Ce, Zr)-doped Lu 3 Al 5 O 12 crystals. Several glow peaks were observed in the 20-450 v C temperature region, whose intensities and emission spectra are influenced by doping. Namely, the TSL spectra are governed by defect and trace impurity-related emissions in the undoped crystal, while only the Ce 3+ emission is detected in the doped crystals. Moreover, EPR measurements performed at 12 v K revealed typical spectra of Ce 3+ (4f 1 , S =1/2) occupying Lu 3+ substitutional positions. In situ light irradiation (250-330 v nm) of the crystals results in a step-like decrease of the Ce 3+ EPR intensity. Such a behaviour can reflect a creation of Ce 4+ ions under UV irradiation or an increase of the Ce 3+ population in the excited (5d) state.     

Residual microstructure associated with impact craters in TiB2/2024Al composite

Residual microstructures associated with hypervelocity impact craters in 55 vol.% TiB(2)/2024Al composite were investigated by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). TiB(2)-Al interface, TiB(2) particles and Al matrix before and after hypervelocity impact were compared to discuss the effect of hypervelocity impact. A new Al(x)O(1-x) phase with the fcc structure and the crystal parameter of 0.69 nm was formed at TiB(2)-Al interface. Stacking fault with width of 10-20 nm was formed along the (001) plane of TiB(2) particle. Formation of nanograins (≈ 100 nm) was observed within Al matrix, moreover, lamellar S' phase was transformed into lenticular or spherical S phase after hypervelocity impact.     

Image resolution and sensitivity in an environmental transmission electron microscope

An environmental transmission electron microscope provides unique means for the atomic-scale exploration of nanomaterials during the exposure to a reactive gas environment. Here we examine conditions to obtain such in situ observations in the high-resolution transmission electron microscopy (HRTEM) mode with an image resolution of 0.10nm. This HRTEM image resolution threshold is mapped out under different gas conditions, including gas types and pressures, and under different electron optical settings, including electron beam energies, doses and dose-rates. The 0.10nm resolution is retainable for H(2) at 1-10mbar. Even for N(2), the 0.10nm resolution threshold is reached up to at least 10mbar. The optimal imaging conditions are determined by the electron beam energy and the dose-rate as well as an image signal-to-noise (S/N) ratio that is consistent with Rose's criterion of S/N≥5. A discussion on the electron-gas interactions responsible for gas-induced resolution deterioration is given based on interplay with complementary electron diffraction (ED), scanning transmission electron microscopy (STEM) as well as electron energy loss spectroscopy (EELS) data.     

Effect of Al content on impact resistance behavior of Al-Ti-B4C composite fabricated under air atmosphere

Reaction behavior, mechanical property and impact resistance of TiC-TiB₂/Al composite reacted from Al-Ti-B₄C system with various Al content via combination method of combustion synthesis and hot pressed sintering under air was investigated. Al content was the key point to the variation of mechanical property and impact resistance. Increasing Al content could increase the density, strength and toughness of the composite. Due to exorbitant ceramic content, 10 wt.% and 20 wt.% Al-Ti-B₄C composites exhibited poor molding ability and machinability. Flexural strength, fracture toughness, compressive strength and impact toughness of 30–50 wt.% Al-Ti-B₄C composite were higher than those of Al matrix. The intergranular fracture dispersed and defused impact load and restricted crack extension, enhancing the impact resistance of the composite. The composite with 50 wt.% Al content owned highest mechanical properties and impact resistance. The results were useful for the application of TiC-TiB₂/Al composite in impact resistance field of ceramic reinforced Al matrix composite.     

Compressive strength and hot deformation mechanisms in as-cast Mg-4Al-2Ba-2Ca (ABaX422) alloy

The behaviour of an as-cast ABaX422 Mg alloy has been evaluated with regard to its compressive strength in the temperature range 25–250?°C and hot working characteristics in the range 260–500?°C. The microstructure of the as-cast alloy has intermetallic phases Mg₁₇Ba₂ and (Al, Mg)₂Ca at the grain boundaries and is fine grained. The alloy has compressive strength better than AZ31 with Ca and Zn, which was attributed to the finer grain size. A processing map developed to characterize its hot working behaviour revealed two dynamic recrystallization domains in the temperature and strain rate ranges of (1) 300–390?°C/0.0003–0.001?s^?1 and (2) 400–500?°C/0.0003–0.5?s^?1. In the first domain, basal?+?prismatic slip occurs along with recovery by climb while in the second domain, second-order pyramidal slip dominates and recovery occurs by cross-slip. The apparent activation energy estimated in Domains 1 and 2 are 169 and 263?kJ/mol respectively, both being higher than that for self-diffusion suggesting that the intermetallic particles in the matrix cause considerable back stress. Bulk metal working of this alloy may be done in Domain 2 which ensures high workability while finish working may be done in Domain 1 in order to achieve a fine grained component. The alloy exhibits flow instability regimes at higher strain rates, in both the lower and higher temperature regions of the processing map, the manifestation being adiabatic shear band formation and flow localization respectively.     

高应变率下TC4及TC9钛合金的动态断裂

 报导了由短脉冲激光引起的应力波加载及电炮驱动Mylar膜平面飞片碰撞TC4、TC9钛合金靶的实验及数值计算研究结果。由回收靶样品的金相分析表明：在电炮驱动飞片碰撞及由激光引起的应力波加载所造成的应变率分别在10⁶ s^-1左右及10⁷ s^-1以上两种情形下，TC4、TC9钛合金的层裂特性都是以微孔洞的成核、增长、汇合为特征的韧性断裂。运用一维流体弹塑性动力学程序进行数值研究表明：成核增长NAG模型在一定程度上可以用来描述高应变率下TC4、TC9钛合金的损伤破坏特性。     

Initial study on the structure and photoluminescence properties of SiC films doped with Al

SiC films doped with aluminum (Al) were prepared by the rf-magnetron sputtering technique on p-Si substrates with a composite target of a single crystalline SiC containing several Al pieces on the surface. The as-deposited films were annealed in the temperature range of 400-800 °C under nitrogen ambient. The thin films have been characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The results show that the introduction of Al into films hinders crystalline formation process. And with the increase of annealing temperature, more Si particles are formed in the films, which strongly affect the optical absorption properties. The photoluminescence (PL) spectra of the samples show two peaks at 370 nm and 412 nm. The intensities of the PL peaks are evidently improved after Al doped. We attribute the origin of the two PL peaks to a kind of Si-related defect centres. The obtained results are expected to have important applications in modern optoelectronic devices.     

Structures of sulfur on TiO2(1 1 0) determined by scanning tunneling microscopy, X-ray photoelectron spectroscopy and low-energy electron diffraction

The temperature dependent adsorption of sulfur on TiO₂(1 1 0) has been studied with X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), and low-energy electron diffraction (LEED). Sulfur adsorbs dissociatively at room temperature and binds to fivefold coordinated Ti atoms. Upon heating to 120°C, 80% of the sulfur desorbs and the S 2p peak position changes from 164.3±0.1 to 162.5±0.1 eV. This peak shift corresponds to a change of the adsorption site to the position of the bridging oxygen atoms of TiO₂(1 1 0). Further heating causes little change in S coverage and XPS binding energies, up to a temperature of 430°C where most of the S desorbs and the S 2p peak shifts back to higher binding energy. Sulfur adsorption at 150°C, 200°C, and 300°C leads to a rich variety of structures and adsorption sites as observed with LEED and STM. At low coverages, sulfur occupies the position of the bridging oxygen atoms. At 200°C these S atoms arrange in a (3×1) superstructure. For adsorption between 300°C and 400°C a (3×3) and (4×1) LEED pattern is observed for intermediate and saturation coverage, respectively. Adsorption at elevated temperature reduces the substrate as indicated by a strong Ti³⁺ shoulder in the XPS Ti 2p_3/2 peak, with up to 15.6% of the total peak area for the (4×1) structure. STM of different coverages adsorbed at 400°C indicates structural features consisting of two single S atoms placed next to each other along the [0 0 1] direction at the position of the in-plane oxygen atoms. The (3×3) and the (4×1) structure are formed by different arrangements of these S pairs.     

生长温度对ZnO薄膜晶体质量和发光特性的影响

采用激光脉冲沉积法在Si(100)衬底上生长ZnO薄膜,衬底温度分别为室温,200℃,300℃,400℃和500℃.用X射线衍射仪、拉曼光谱、扫描电子显微镜对薄膜的微结构进行了测量,并测量了室温下薄膜的光致发光特性.结果表明,300℃时.ZnO具有最佳择优取向,随着衬底温度升高.衍射峰半峰全宽减小,薄膜晶粒尺寸增大,400℃时,薄膜具有各向等大的品粒尺寸.同时拉曼谱结果显示,薄膜内部的缺陷随衬底温度变化无明显差别,应力表现为张应力,400℃时应力最小,紫外发光峰在衬底温度为400℃时最强,而黄绿光带最弱.在减少薄膜缺陷,提高择优长向和晶粒尺寸的同时.使晶粒横向尺寸和纵向尺寸尽可能相同,可极大提高薄膜的发光特性.