Compressive Surface Layer to Avoid Edge Cracking in Laminar Ceramic Composite

Si 1 − x Ge x ∕ Si 1 − y Ge y ∕ Si ( 100 ) heterostructures grown by molecular beam epitaxy were used in order to study B surface segregation during growth and B lattice diffusion. Ge concentration and stress effects were separated. Analysis of B segregation during growth shows that (i) for layers in epitaxy on (100)Si, B segregation decreases with increasing Ge concentration, i.e., with increased compressive stress; (ii) for unstressed layers, B segregation increases with Ge concentration; (iii) at constant Ge concentration, B segregation increases for layers in tension and decreases for layers in compression. The contrasting behaviors observed as a function of Ge concentration in compressively stressed and unstressed layers can be explained by an increase of the equilibrium segregation driving force induced by Ge additions and an increase of near-surface diffusion in compressively stressed layers. Analysis of lattice diffusion shows that (i) in unstressed layers, B lattice diffusion coefficient decreases with increasing Ge concentration; (ii) at constant Ge concentration, the diffusion coefficient of B decreases with compressive biaxial stress and increases with tensile biaxial stress; (iii) the volume of activation of B diffusion [ΔV=−kT(dlnD∕dP)] is positive for biaxial stress while it is negative in the case of hydrostatic pressure. This confirms that under a biaxial stress the activation volume is reduced to the relaxation volume.

Delayed fracture of alumina ceramics with compressive surface layers

Cracking and spalling of the oxide layer developed in high-burnup Zircaloy-4 cladding under normal operating conditions in a PWR

Role of the Magnum and Uterus in the Determination of Albumen Quality of Laid Eggs

Due to the lattice mismatch between the epilayer and the substrate in ZnSe/ZnO stuctures, it is cucial to know the limiting conditions leading to a pseudomorphic growth by the accommodation of the biaxial strains caused by difference in lattice constants and/or thermal expansion coefficients of the materials forming the heterostucture [1, 2]. Moreover, hydrostatic pressure decreases the lattice constant of the materials. The difference in the compressibility of ZnO from that of ZnSe generates under hydrostatic pressure a tensile strain which progressively compensates the lattice mismatch induced compressive strain. The valence band at the Γ point of the unstrained ZnSe crystals consists of a fourfold degenerate P3/ 2 valence band (J = 3/2, m = ±3/2, m = ±1/2). Under biaxial compressive or tensile stress this valence band splits into a light-hole (J = 3/2, mj = f 1/2) and a heavy-hole (j = 3/2, m = ±3/2) branches [3-5]. The trend in II-VI compounds is that the materials have a larger negative deformation potential as their lattice constants get smaller. It may be suspected that the deformation potential will get more negative as we apply pressure, since the average lattice constant gets smaller [3]. In addition, a situation similar to the pressure induced biaxial strain can occur for heterostructures where the thermal expansion coefficients for materials vary greatly. The lattice constants of constituent materials at each temperature must adjust each other. In this paper, we present for the first time a study of the exciton emission of the strained ZnSe/ZnO stuctures as a function of the biaxial compressive or tensile stress and temperature using photo and electroluminescence methods. These

Improving the fatigue life of electro-discharge-machined SDK11 tool steel via the suppression of surface cracks

Localized impact damage in stressed members

NDT of Coatings, Surface Modified Layers, and Adhesives

Effects of interlayer bonding conditions between semi-rigid base layer and asphalt layer on mechanical responses of asphalt pavement structure

Potential distribution across a membrane with surface charge layers: Effects of nonuniform charge distribution

AIN thin films were grown on c-plane sapphire by metalorganic chemical vapor deposition from tritertiarybutylaluminum and ammonia at 1050°C. These films exhibit a full width at half maximum of the 002 X-ray rocking curve below 200 arcsec indicating high epitaxial quality. By measuring asymmetric reflections, a structural disorder of the lattice mainly due to edge dislocations can be observed. For further investigations, atomic force microscopy and photothermal deflection spectroscopy were performed. In order to study the effect of increasing AIN layer thickness on the optical and structural properties of GaN in an AIN/GaN heterostructure, AIN thin films with increasing thickness ranging from 0.02 to 0.36 μm were used as sublayers for the deposition of 0.75 μm thick GaN layers. Photoluminescence, micro- Raman and X-ray diffraction measurements confirm the relaxation of biaxial compressive stress in the GaN layers due to different thermal expansion coefficients by increasing AIN layer thickness. The pressure dependence of the band gap shift was determined as 24 meV/GPa for biaxial compressive stress. Our results indicate that the growth of AIN with metal organic chemical vapor deposition from tritertiarybutylaluminum and ammonia is a promising method for obtaining high quality epitaxial films.

The thickness of noncrystalline surface layers of polyethylene single crystals

The high efficiency and power density of AlGaN/GaN high electron mobility transistors (HEMTs) can be limited by parasitic charge trapping at sites on the semiconductor surface. Trapped electrons at the surface deplete the two-dimensional electron gas (2DEG), resulting in effects known as current collapse, increased dynamic on-resistance, DC-RF dispersion, among others. A SiN layer is conventionally implemented as surface passivation for GaN HEMTs, however the properties of this SiN layer and the impact of this layer on current collapse depend drastically on the deposition conditions of this film. Although passivation of GaN HEMTs has been studied extensively, the specific details behind the SiN passivation deposition are not frequently provided in literature. In this work, we investigate several passivation techniques and study their performance under dynamic switching conditions, under off-state stress, and implement hyperspectral electroluminescence imaging to identify electric field nonunifornities. We investigate plasma enhanced chemical vapor deposition (PECVD) SiN, a common GaN HEMT passivation method, and demonstrate how to optimize the deposition conditions to effectively passivate the HEMT. Generally, a mixed frequency PECVD SiN process is used, where a combination of low frequency (LF) e.g. 100-360 kHz and high frequency (HF) 13.56 MHz plasmas are used to create the SiN film. However, the LF step could create undesirable damage to the surface of the AlGaN barrier from N+ ion bombardment, degrading device performance. We developed a bilayer PECVD process, where a 10 nm thick layer is deposited using HF plasma only in order to protect the AlGaN surface, followed by a HF/LF mixed frequency SiN deposition. Annealing the 10 nm thick HF layer densified the film and resulted in a HEMT with significantly improved current collapse. On the other hand, SiN layers grown in situ during MOCVD growth of the GaN/AlGaN layers have also been demonstrated to serve as an effective protective layer for subsequent HF/LF PECVD SiN deposition, yielding HEMTs with low current collapse. Atomic layer epitaxy (ALE) deposition of a thin (10 nm) AlN layer is evaluated, and is shown to serve well as a passivation layer. This is particularly advantageous, since the thin layer is beneficial for reducing gate capacitance for high frequency operation. Finally, a nanocrystalline diamond (NCD) layer is incorporated on the surface of the GaN HEMT, primarily to improve thermal management, but this also is effective as a surface passivation layer, mitigating surface charge trapping. Effective passivation of the AlGaN/GaN HEMT surface is essential for mitigating current collapse, and we demonstrate recent advancements in passivation technology.

In order to reveal the failure mechanism of coating,both scratch test on Ni-P-Al2O3 complex coating surface and the simulation of scratch process with finite element method are carried out. Scratch test shows that transverse surface cracks in an interval between two neighboring cracks appear on the scratch groove surface of Ni-P-Al2O3 complex coating. Finite element simulation of scratch process indicates distributing rules of stress in the coating surface and interface at different stages of scratch process,and reveals coating surface cracks generating mode and forming mechanism. Scratch process is divided into three stages that are stylus tip indenting coating surface,slipping on coating surface and rising from coating surface. In former two stages the scratch groove is formed on the coating surface and the surface cracks are induced by stylus tip acting on coating surface. The stress analysis shows that there are two ways of forming coating surface cracks. One is that coating interface cracks are firstly induced,thus they are propagated to the coating surface,and finally the surface cracks are formed. Other is that cracks are directly formed in the coating surface. Because they are induced by maximum tensile stress,surface cracks are modeⅠ. After stylus tip rising from coating surface,biggish residual stresses remain in the coating. Maximum residual tensile stress is first residual principal stress at the coating interface below the contacting central. Those will supply the basis of coating design and application.

The aim of the paper is the numerical investigation of the stability of two-layered shells under the action of imposed azimuthal strain on the external surface. Although this type of loading is not common in engineering practice, it appears often in biomedicine (for example buckling of esophagus, asthmatic airways, gastrointestinal tract etc). The differential stability equations are discretised using the finite volume method and the resulting generalised eigenvalue problem is solved using the QZ decomposition technique. The results show that the buckling behaviour under circumferential loading is entirely different compared to hydrostatic pressure loading. More specifically, it is well known that in the latter case the number of folds with the smallest critical load is equal to 2. In the former case however it depends on the thickness of each layer and their moduli of elasticity. Comparison with experimental measurements shows good agreement. The thickness of the inner layer significantly affects the number of folds and the critical load (it was found that the number of folds is reduced with increasing layer thickness). Comparison of the eigenfunctions of radial and azimuthal displacements also shows more complex behavior compared to pressure loading.