On the role of plastic deformation during the mild wear of alumina

Polycrystalline -alumina was worn against Mg-partially stabilized zirconia (Mg-PSZ), using water lubrication, a sliding speed of 0.24 m/s and a load of 10 N. Differential wear between grains (maximum 33 nm) and fine (0.3–1.9 μm diameter) abrasive grooves were found on the worn surface. TEM of back-thinned samples indicated widespread dislocation flow at the surface, heterogeneously distributed between grains, and largely associated with abrasive grooves. Those grains standing proud of the surface invariably contained extensive dislocation damage. The dominant slip system was pyramidal ( , , and ) although occasional basal slip was also found. No prism slip was observed. The pyramidal slip planes were concentrated at angles of 6–33° to the worn surface. Basal slip was frequently associated with basal twinning on planes at 72–73° to the worn surface. Dislocation pile-ups at grain boundaries often coincided with grain boundary cracking. The extent of damage from abrasive grooves varied from grain to grain and was dictated by crystallographic orientation more than the grain height. No evidence of mechanical damage was found in those grains that had suffered the highest wear, indicating that material removal had been controlled by tribochemical mechanisms. The origin of the differential wear between grains is considered and the implications of the experimental observations on the time-dependent transition to severe wear in aluminas are discussed.     

A model for predicting grain boundary cracking in polycrystalline viscoplastic materials including scale effects

A model is developed herein for predicting the mechanical response of inelastic crystalline solids. Particular emphasis is given to the development of microstructural damage along grain boundaries, and the interaction of this damage with intragranular inelasticity caused by dislocation dissipation mechanisms. The model is developed within the concepts of continuum mechanics, with special emphasis on the development of internal boundaries in the continuum by utilizing a cohesive zone model based on fracture mechanics. In addition, the crystalline grains are assumed to be characterized by nonlinear viscoplastic mechanical material behavior in order to account for dislocation generation and migration. Due to the nonlinearities introduced by the crack growth and viscoplastic constitution, a numerical algorithm is utilized to solve representative problems. Implementation of the model to a finite element computational algorithm is therefore briefly described. Finally, sample calculations are presented for a polycrystalline titanium alloy with particular focus on effects of scale on the predicted response. This revised version was published online in July 2006 with corrections to the Cover Date.     

Bismuth phosphates as intermediate temperature proton conductors: From polycrystalline powders to amorphous glasses

Proton conducting electrolyte materials operational in the intermediate temperature range of 200–400 °C are of special interest for applications in fuel cells and water electrolysers. Bismuth phosphates in forms of polycrystalline powders and amorphous glasses are synthesized and investigated by scanning electron microscopy, X-ray diffraction, FT-IR, thermogravimetric analysis and AC impedance. Under dry atmosphere the pure crystalline and amorphous phosphates exhibit an intrinsic conductivity of up to 10^?5 S cm^?1 at 250 °C. In the presence of atmospheric humidity the conductivity of both types of phosphates is significantly enhanced, reaching about 10^?2 S cm^?1 at a water vapor partial pressure above 0.5 atm. During a period of more than 100 h with four humidity cycles from zero to 0.58 atm of the water vapor partial pressure, the phosphates show good stability, suggesting the potential as an intermediate temperature electrolyte.     

A thermomechanical crystal plasticity constitutive model for ultrasonic consolidation

We present a micromechanics-based thermomechanical constitutive model to simulate the ultrasonic consolidation process. Model parameters are calibrated using an inverse modeling approach. A comparison of the simulated response and experimental results for uniaxial tests validate and verify the appropriateness of the proposed model. Moreover, simulation results of polycrystalline aluminum using the identified crystal plasticity based material parameters are compared qualitatively with the electron back scattering diffraction (EBSD) results reported in the literature. The validated constitutive model is then used to simulate the ultrasonic consolidation process at sub-micron scale where an effort is exerted to quantify the underlying micromechanisms involved during the ultrasonic consolidation process.     

阴极蒸发物电子发射现象的研究

为研究钡钨阴极蒸发物的电子发射现象,采用一种新设计的测试装置,对沉积在多晶钨表面上阴极蒸发物的电子发射曲线进行了采集,利用电子发射显微镜和扫描电镜对蒸发物沉积层的电子发射像、表面形貌和成分进行了分析。结果表明,电子发射曲线分3段,即陡升段、快升段和缓升段。分析认为,发射曲线的3段依次对应着多晶钨表面的晶界及划痕发射、晶面发射和3维岛状发射。实践证明,在覆膜阴极表面构造均匀弥散分布的岛状晶体发射点,可大幅度提高阴极的电子发射性能。     

The structural evolution and lithiation behavior of vacuum-deposited Si film with high reversible capacity

The silicon film (6 μm) was prepared by vacuum deposition method on a surface modified copper foil as anode for lithium-ion batteries with high capacity and long cycle life. The modified copper foil has a rough and pyramid-like surface which helps the deposited Si film to connect toughly. The deposited hill-like Si is favorable to reduce the mechanical stress coming from the volume expansion and shrinkage of active materials during lithiation and de-lithiation. Moreover, the Si film exists mostly in an amorphous state. After cycling, partial amorphous phase transforms into the polycrystalline Si grains, forming a combination of amorphous and crystalline structure. Cyclic voltammograms (CVs) and electrochemical impedance spectra (EIS) research indicate that the vacuum-deposited thick Si film has a good reversibility of lithiation/de-lithiation. As a consequence, the thick Si film exhibits an excellent cycling performance with high reversible capacity.     

Deposition of polycrystalline Si thin films on glass substrates by direct negative Si ion beam deposition

Polycrystalline Si (Poly-Si) thin films were deposited on a glass substrate by direct negative Si (Si⁻) ion beam deposition. The glass substrate temperature was kept constant at 500 °C for all depositions. Prior to deposition, the ion energy spread and ion-to-atom arrival ratio were evaluated as a function of the ion beam energy.The Si⁻ ion energy spread was less than 10% regardless of the ion energy, while the ion-to-atom arrival ratio increased proportionally from 1.3 to 1.6 according to the ion beam energy.Atomic force microscopy images showed that a relatively rough surface was obtained at 50 eV of Si⁻ ion energy and it is also concluded that the Si⁻ ion beam irradiation at 50 eV is effective to deposit Si thin film with small grains as shown in Fig. 3.     

Development of low cost production technologies for polycrystalline silicon solar cells

To develop a technology of forming grooves for low cost cell production, a multi-blade wheel grinding method was investigated. The process time of groove formation on the surface of 10 × 10 cm² polycrystalline silicon substrate was reduced to 30 s by a newly developed high-speed groove formation machine. Simultaneous formation of junction and anti-reflection coating by atmospheric pressure chemical vapor deposition (APCVD) technique was also investigated. For electrodes formation process, single firing method for both side electrodes made possible to simplify the firing process and to speed up from a conventional speed of 400 mm/min to 5000 mm/min.     

Influence of the substrate's surface structure on the mechanism and kinetics of the electrochemical UPD formation of a copper monolayer on gold

The copper electrodeposition process was studied onto different gold substrates, single crystal (1 1 1) and polycrystalline, using electrochemical techniques. It was found, from the analysis of the experimental current density transients, that the potentiostatic formation of a full copper monolayer onto the gold electrode under UPD conditions follows the same mechanism, regardless of the crystallinity of the substrate. The mechanism involved the simultaneous presence of an adsorption process and of two 2D nucleation processes, progressive and instantaneous, respectively.     

Effect of polycrystalline mullite fibers on the properties of vitrified bond and vitrified CBN composites

The effect of polycrystalline mullite fibers (PMFs) on the properties of vitreous bonds and vitrified CBN composites was investigated. The results show that the addition of PMFs can increase the porosity of composites and reduce the fluidity of binders. The vitrified composites incorporating 6.4 wt% PMFs display excellent mechanical strength, which is enhanced by 21.2% compared with that of composites without PMFs sintered at the optimal sintering temperature. Meanwhile the thermal expansion coefficient of vitrified bond reduces from 6.256×10⁻⁶ °C⁻¹ to 4.805×10⁻⁶ °C⁻¹ with increasing fraction of PMFs. The improvement of mechanical strength is associated with the change of cracking mechanisms of the composites with fibrous crystals and the existence of several observed mechanisms, including fiber pull-out, fiber bridging and rupture.