Meng Songhe

Thermal Stresses and Cracks During the Growth of Large-sized Sapphire with SAPMAC Method

Abstract The finite-element method has been used to study the thermal stress distribution in large-sized sapphire crystals grown with the sapphire growth technique with micro-pulling and shoulder-expanding at cooled center (SAPMAC) method. A critical defect model has been established to explain the growth and propagation of cracks during the sapphire growing process. It is demonstrated that the stress field depends on the growth rate, the ambient temperature and the crystallizing direction. High stresses always exist near the growth interfaces, at the shoulder-expanding locations, the tailing locations and the sites where the diameters undergo sharp changes. The maximum stresses always occur at the interface of seeds and crystals. Cracks often form in the critical defect region and spread in the m-planes and a-planes under applied tensile stresses during crystal growth. The experimental results have verified that with the improved system of crystal growth and well-controlled techniques, the large-sized sapphire crystals of high quality can be grown due to absence of cracks.

Critical Biot's number for Determination of the Sensitivity of Spherical Ceramics to Thermal Shock

A critical Biot number, which determines both the sensitivity of spherical ceramics to quenching and the durations of the temperature-wave propagation and the thermal stresses in the ceramics subjected to thermal shock, is theoretically obtained. The results prove that once the Biot number of a ceramic sphere is greater than the critical number, its thermal shock failure will be such a rapid process that the failure only occurs in the initial regime of heat conduction, whereas the thermal shock failure of the ceramic sphere is uncertain in the course of heat conduction. The presented results provide a guide to the selection of the ceramics applied in the thermostructural engineering with thermal shock.

Field emission from amorphous carbon nitride films deposited on silicon tip arrays

Amorphous carbon nitride films (a-CNx) were deposited on silicon tip arrays by rf magnetron sputtering in pure nitrogen atmosphere. The field emission property of carbon nitride films on Si tips was compared with that of carbon nitride on silicon wafer. The results show that field emission property of carbon nitride films deposited on silicon tips can be improved significantly in contrast with that on wafer. It can be explained that field emission is sensitive to the local curvature and geometry, thus silicon tips can effectively promote field emission property of a-CNx films. In addition, the films deposited on silicon tips have a smaller effective work function (F = 0.024 eV) of electron field emission than that on silicon wafer (F = 0.060 eV), which indicates a significant enhancement of the ability of electron field emission from a-CNx films.

Study on Cure-Induced Residual Stresses for Fibre Metal Laminate

During autoclave curing process, because of the mismatch between coefficients of thermal expansion of fibre layers and metal layers, residual stresses are developed in Fibre Metal Laminates (FML). In this paper, a three-dimensional finite element model was developed to predict cure-induced residual stresses for FML. And then FML-2/1 and FML-3/2 structures (plate and L-shape) were studied with the finite element model. The results show that residual tensile stress is in metal layers and residual compressive stress is in fibre layers. Residual stresses in Fibre Metal Laminates are bigger enough to affect the deformation and fatigue performance.

A rigid line in a confocal elliptic inhomogeneity embedded in an infinite medium under antiplane shear

An effective method is developed and used to investigate the antiplane problem of a rigid line in a confocal elliptic inhomogeneity embedded in an infinite medium. The analytical solution is obtained. The proposed method is based upon the use of conformal mapping and the theorem of analytic continuation. Special solutions which are verified by comparison with existing ones are provided. Finally, the characteristics of stress singularity at the tip of the rigid line inhomogeneity are analyzed and the extension forces for the crack and the rigid line inhomogeneity are derived.