Structure and properties of highly crystallized graphite films based on polyimide Kapton

Highly crystallized graphite films were prepared by heat treatment of carbonized polyimide films (Kapton) at temperatures of 2700 and 3050° C. Interlayer spacing d ₀₀₂ at room temperature, and electrical resistivity, magnetoresistance and Hall coefficient at room and liquid nitrogen temperatures were measured. All of these data indicate high crystallinity of the graphitized Kapton films obtained. For the graphite films heat treated at 3050° C mean-square mobilities were estimated from the magnetoresistance data at 1 T to be 0.91 m² V^–1 sec^–1 at room temperature and 2.3 m² V^–1 sec^–1 at liquid nitrogen temperature; the value at liquid nitrogen temperature corresponds to that for a pyrolytic graphite heat treated at 3200° C for 1 h (PG 3200). Magnetic field dependence of Hall coefficient at liquid nitrogen temperature for this sample also agrees well with that for PG 3200. Scanning electron micrographs on the surfaces show that the present graphite films consist of grains of large crystallites, and grain size increases as the crystallinity of the material improves.     

Structure and properties of highly oriented polyoxymethylene/multi-walled carbon nanotube composites produced by hot stretching

Highly-oriented polyoxymethylene (POM)/multi-walled carbon nanotube (MWCNT) composites were fabricated through solid hot stretching technology. With the draw ratio as high as 900%, the oriented composites exhibited much improved thermal conductivity and mechanical properties along the stretching direction compared with that of the isotropic samples before drawing. The thermal conductivity of the composite with 11.6 vol.% MWCNTs can reach as high as 1.2 W/m K after drawing. Microstructure observation demonstrated that the POM matrix had an ordered fibrillar bundle structure and MWCNTs in the composite tended to align parallel to the stretching direction. Wide-angle X-ray diffraction results showed that the crystal axis of the POM matrix was preferentially oriented perpendicular to the draw direction, while MWCNTs were preferentially oriented parallel to the draw direction. The strong interaction between the POM matrix and the MWCNTs hindered the orientation movement of molecules of POM, but induced the orientation movement of MWCNTs.     

Unique photoluminescence properties of highly crystallized BaMoO4 film prepared by chemical reaction

Highly crystallized luminescent BaMoO₄ films have been prepared directly on molybdenum substrates at room temperature (25 °C) in barium hydroxide aqueous solution by a simple chemical reaction. In order to form the desired surface morphology and crystal structure, a mild oxidizing agent, NaClO, seems to be the key to the high crystallinity. In alkaline solution, the molybdenum substrate can gradually convert into MoO₄²⁻ ions, combined with Ba²⁺ ions which have a poor migration just depending on thermal motion to form a film. XRD and SEM results indicate that the films have single phase and uniform surfaces. FA results reveal that the films appear to have scintillating luminescent character when they are under different excitations at room temperature. Further, the excitation and emission wavelengths comply with a linear relationship which will be a potential technological application of BaMoO₄ in a variety of optical devices.     

Numerical modeling of steady state photoconductivity process in highly crystallized undoped μc-Si:H films

The steady state photoconductivity as a function of temperature and light intensity was measured on plasma-deposited highly crystalline undoped hydrogenated microcrystalline silicon (μc-Si:H) thin films possessing different microstructure and morphology. Different phototransport behaviors were observed in films having dissimilar microstructural attributes and we have shown this to be linked to different features of the density of states (DOS) in the gap. The phototransport properties of μc-Si:H films having small grains, high density of inter-grain and inter-columnar boundary regions containing disordered phase show light intensity exponent (γ) variation between 0.5 and 1, and thermal quenching effect in temperature-dependent photoconductivity. Based on our mutually corroborative simulation and experimental results, we have proposed the effective DOS maps for the μc-Si:H system, which differ for films having different microstructures.     

Structure and properties of a Pb-Ca-Sn battery alloy crystallized under nonequilibrium conditions

We have studied the properties of a Pb-Ca-Sn alloy for the positive current collector of lead-acid storage batteries, prepared by an existing industrial process and an experimental technique that involves melt jet quenching from the liquid state (QLS). The QLS process ensures good stability of the properties and structure of the alloy at elevated temperatures. Doping with trace levels of barium is shown to influence the properties of QLS ribbons.     

The tribological properties of highly oriented cobalt and Co-Cr ion platings

Ion-plated cobalt coatings have been found to have poor tribological properties owing to the high proportion of the f.c.c. phase present. Co-Cr alloy coatings deposited by triode ion plating consist only of h.c.p. phase and have strong basal plane orientation, fine columnar structures and good tribological properties. Measurements of the tribological properties are reported and related to the coating parameters.     

Structure and properties of amorphous and nanocrystalline NiTi prepared by severe plastic deformation and annealing

The formation of homogeneous nanocrystalline structure by nanocrystallization of amorphous NiTi subjected to high pressure torsion is demonstrated. Structural evolution during annealing was investigated and homogeneous nanocrystalline structures with different grain sizes have been obtained by controlled annealing. Nanocrystallization results in the record value of room temperature strength for this material equal to 2650 MPa with an elongation to failure of about 5%. At elevated temperatures of (0.4…0.5)T_m nanocrystalline nitinol showed a high ultimate strength with sufficient elongation (up to 200%). The observation that the shape and the size of grains after deformation remain close to that of the initial state suggests that in nanocrystalline NiTi such mechanism as grain boundary sliding and grain rotation are active and the generation and motion of dislocations play the role of accommodation of stress concentration.     

Growth of highly oriented ZnO films by the two-step electrodeposition technique

Compact and transparent ZnO films were deposited on the ITO/glass substrates from zinc nitrate aqueous solution by the two-step electrodeposition technique. While the first potentiostatic step was used to produce ZnO seed layer, the ZnO film growth has been done galvanostatically. Effects of the potentiostatic parameters on the crystal structure, morphology and optical properties of ZnO films were investigated. Results show that ZnO films with highly c-axis preferred orientation can been obtained when the potentiostatic deposition at −1.2 V for 15 s has been applied. Such an observation might be attributed to the etching process of ITO substrate in the diluted HCl solution. The film exhibits smooth and compact morphology, high transmittance in the visible band (>80%) and sharp absorption edge (at ∼370 nm). The analysis on the growth mechanism indicates that the short potentiostatic process prior to the film growth can produce ZnO seed layer and substitute the initial nucleation process in the conventional one-step galvanostatic deposition, thus increasing the nucleation density and preventing the formation of loose structures.     

Preparation of highly transparent conductive Al-doped ZnO thin films and annealing effects on properties

Aluminum-doped ZnO (ZAO) thin films were deposited on fused quartz substrates by radio frequency sputtering in pure argon ambient at 450 °C. Effects of in situ annealing temperature and annealing atmosphere on microstructure, electrical and optical properties of ZAO films have been investigated. Results showed that as-grown film without annealing treatments attained lowest resistivity of 1.1 × 10⁻⁴Ω cm. And all films performed high average transmittance greater than 90% in visible region. X-Ray diffraction (XRD), photoluminescence (PL), X-ray photoelectron spectroscopy (XPS) were utilized to characterize the microstructure properties of films. XRD results indicated that as-grown film had higher crystalline quality and larger grain size than annealed films. Al atoms replaced Zn efficiently to provide electrons stable in all samples. PL spectra revealed that high annealing temperature and oxygen atmosphere would generate more Zn vacancy (V_Zn) and oxide antisite defect (O_Zn), respectively and composition content results from XPS provided supports to this.     

采用室温原电池电化学技术制备SrWO4多晶膜的研究

电化学技术可以分为电解型电化学技术和原电池型电化学技术两个大类.本文采用原电池型电化学路线,在含Sr2 的碱性溶液中,在室温环境下制备出了钨酸锶多晶膜,并且采用XRD、SEM、XPS和EDAX等技术对多晶膜进行了分析表征.研究发现,沉积在钨片上的材料是结晶良好的、具有单一白钨矿结构的钨酸锶多晶膜;利用原电池方法制备的SrWO4多晶膜大致呈层状或近层状生长,没有出现用恒电流电化学方法制备的SrWO4多晶膜中所出现的束状或花状枝蔓晶团簇.其原因可归因于晶体在微小电流条件下能够更加容易地呈层状生长或近层状生长.     

Constrained annealing of biaxially oriented SAN copolymer films

Biaxially oriented styrene-acrylonitrile (SAN) copolymer films with a draw ratio of 6.9 in the machine direction (MD) and 2.9 in the transverse direction (TD) were annealed under constraint in both directions. The films fractured during annealing developed cracks along the TD. The temperature dependence of the time to fracture showed an Arrhenius plot with an activation energy of 114 kcal mol^–1. When annealing was terminated before fracture, the work to fracture on tensile deformation of the annealed film increased with annealing time, with a maximum increase of about 53% in the MD and 40% in the TD. The annealed films showed fewer bands or more fine shear bands as compared with unannealed films. These observations suggest a toughening effect in oriented films when annealed under dimensional constraint.     

Mechanical anisotropy of highly oriented polymers

Recent progress on the understanding of the mechanical anisotropy of orientedpolymers is reviewed. The key issues that are discussed include theexperimental validation of theoretical predictions for the elastic constantsof fibres, quantitative interpretations of the mechanical anisotropy of chain-extended polyethylenes and liquid crystalline polymers using aggregate models,the analysis of anisotropic polymers and fibre/resin composites and compositemodels for specially oriented polyethylene sheets and high moduluspolyethylene fibres.     

Peelback of highly oriented cellulosic fibres

An original study of peelback of cellulosic fibres spun using the N-methylmorpholine N-oxide (NMMO) process was performed. 13 micrometre diameter fibres were peeled under a microscope. Due to the high orientation of the material, the rupture could propagate in the centre of the fibres. Fibres presenting very different Fibrillability Index (FI = 0 or FI = 5) and hydration (swollen in water or freezedried) were studied. The resulting surfaces were analysed using Scanning Electron Microscopy (SEM). They showed the original formation of regularly spaced ribs corresponding to tearing of a homogeneous oriented material for the non fibrillable fibres. The structure of fibrillable fibres appeared through the creation of fibrils. Measurements of the energy release rate were performed with a set of each kind of fibres. Low values of this energy were obtained, corresponding to the formation of small ribs, due to the high orientation of the cellulose molecules. The energy values can be increased by the occurence of fibrils in the case of fibrillable fibres.     

磁场退火制备FePt/Ag垂直磁化薄膜

采用磁控溅射法在自然氧化的单晶Si（100）基底上制备了（FePL／Ag）10多层薄膜,并在10kA／m磁场中进行了不同温度的真空热处理,研究了磁场作用下,不同热处理温度对FePt薄膜有序化转变及磁性能的影响。X射线衍射研究表明,在磁场作用下通过多层膜设计可以比较容易获得垂直生长的易磁化轴;选择适当的热处理温度、降低多层膜中每层膜厚可以制备出晶粒尺寸细小均匀的FePL／Ag垂直磁化薄膜,适用于高密度垂直磁记录介质材料。     

Effect of solvent ratio on the properties of highly oriented sprayed fluorine-doped tin oxide thin films

Transparent conducting thin films of F:SnO₂ have been deposited onto preheated glass substrates by a spray pyrolysis technique using pentahydrate stannic chloride (SnCl₄·5H₂O) and ammonium fluoride (NH₄F) as precursors and mixture of water and propane-2-ol as solvent. The concentration of SnCl₄·5H₂O and NH₄F is kept fixed and the ratio of water and propane-2-ol solvent in the spraying solution is varied. A fine spray of the source solution using air as a carrier gas has grown films of thickness up to 995 nm. Optical absorption, X-ray diffraction, Van der Pauw technique for measurement of a sheet resistance and Hall effect measurements at room temperature for determination of carrier density and conductivity have been used. The as-deposited films are of polycrystalline SnO₂ with a tetragonal crystal structure and are preferentially having orientation along the (200) direction with texture coefficient as high as 6.16. The average grain size for the as-deposited sample is found to be of the order of 44 nm. The films have moderate optical transmission (up to 70–85% at 550 nm). The figure of merit (ϕ) values vary from 1.95 · 10^− 3 to 35.68 · 10^− 3 Ω^− 1. The films are heavily doped, degenerate and exhibit n-type electrical conductivity. The lowest sheet resistance (R_s) for the optimized sample is 5.1 Ω. The films have a resistivity of 5.43 · 10^− 4 Ω cm and mobility around 7.38 cm² V^− 1 s^− 1.     

Anisotropic properties of unidirectionally crystallized calcium metasilicate

The anisotropy displayed by materials which contain aligned, fibrous crystals in a crystalline or glass matrix has been investigated. The materials were prepared by the unidirectional crystallization of calcium metasilicate from glasses and melts of approximately metasilicate compositions. Rectangular prism test specimens were fabricated such that the fibre axis ran parallel to the long axis, across the width or through the depth. This enabled the variation of properties with fibre orientation to be established. The high-temperature polymorph,-CaSiO₃, could be crystallized dendritically such that the primary arms were aligned, and the growth of secondary arms inhibited, by mutual impingement.-CaSiO₃, the low-temperature polymorph, has a chain silicate structure and crystallized as fibres by the spherulitic crystallization mechanism. These fibres grew in approximately parallel alignment due to transcrystallization. The glass remaining after crystallization of the primary phase was either stable or subsequently crystallized from nuclei on the primary crystals, giving materials with either a glass or crystalline matrix respectively. Thermal expansion data were obtained from an automatic recording dilatometer, and the moduli of rupture and shear strengths were found using three-point bend tests of appropriate span-to-depth ratios. The results of these tests showed that the unidirectionally crystallized materials behaved similarly to conventional brittle matrix / brittle fibre composites.     

Hot deformation and annealing of cube oriented aluminium single crystals

Abstract

Single crystals of the {001}〈100〉 orientation of an Al–0.05Si single phase alloy have been deformed in plane strain compression at temperatures of 300–500°C and strain rates of 0.5–50 s^-1, and annealed in a salt bath at temperatures of 300–450°C. Quantitative texture measurements by electron backscatter diffraction (EBSD)show that, in agreement with previous work, the cube orientation is stable at lower strain rates and higher temperatures (lower Zener–Hollomon parameter Z), whereas this orientation is unstable at higher values of Z. During annealing of the deformed crystals there is a competitive migration of subgrain boundaries of a wide range of orientations, and recrystallisation starts preferably at deformation bands of high orientation gradient. Measurement of subgrain growth has enabled the dependence of the mobility of low angle grain boundaries on misorientation to be determined. The results are in accord with those obtained for lower angle (<6°)boundaries in the same material.