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Metal matrix syntactic foams are promising materials with high energy absorption capability. To study the effects of matrix strength on the quasistatic compressive properties of syntactic foams using SiC hollow particles as reinforcement, matrices of Al-A206 and Mg-AZ91 were used. Because Al-A206 is a heat-treatable alloy, matrix strength can be varied by heat treatment conditions, and foams in as-cast, T4, and T7 conditions were tested in this study. It is shown that the peak strength, plateau strength, and toughness of the foams increase with increasing yield strength of the matrix and that these foams show better performance than other foams on a specific property basis. High strain rate testing of the Mg-AZ91/SiC syntactic foams showed that there was little strain rate dependence of the peak stress under strain rates ranging from 10−3/s to 726/s.
Hydrogen is commonly introduced into silicon solar cells to reduce the deleterious effects of defects and to increase cell efficiency. When hydrogen is introduced into multicrystalline Si that is often used for the fabrication of solar cells, the H atoms become trapped by carbon impurities to produce defect structures known at H2*(C). These defects act as both a source and a sink for hydrogen in H-related defect reactions. IR spectroscopy has been used to determine what H- and C-related defects are formed in multicrystalline Si when the carbon concentration is varied. A process that is used by industry to introduce hydrogen into Si solar cells is the post-deposition annealing of a hydrogen-rich SiNx layer. The H2*(C) defects provide a strategy for estimating the concentration and penetration depth of the hydrogen that is introduced by this method.
A 29-year-old man presented with sudden onset of severe pain in his throat, difficulty breathing and a hoarse voice, following an episode of vomiting.
Initial laboratory tests were normal. The patient underwent fibre-optic nasendoscopy, which demonstrated a haematoma in the piriform fossa. Lateral neck radiography and subsequent computed tomography scanning confirmed a 2 cm, loculated, gas-containing collection at the level of the vallecula in the right posterolateral wall, extending to the false vocal folds and communicating between the right parapharyngeal space and the right carotid sheath. Water-soluble contrast swallow confirmed the diagnosis.
Contained oesophageal perforation.
Conservative treatment was adopted involving nil orally, intravenous antibiotics and nasogastric feeding. The patient made an uneventful recovery.
Hydrogen is commonly introduced into silicon solar cells to reduce the deleterious effects of defects and to increase cell efficiency. We have developed strategies by which hydrogen in silicon can be detected by IR spectroscopy with high sensitivity. The introduction of hydrogen into Si by the post-deposition annealing of a hydrogen-rich, SiNx coating has been investigated to determine hydrogen's concentration and penetration depth. Different hydrogenation processes were studied so that their effectiveness for the passivation of bulk defects could be compared. The best conditions investigated in our experiments yielded a hydrogen concentration near 1015 cm-3 and a diffusion depth consistent with the diffusivity of H found by Van Wieringen and Warmoltz.
We report on spectroscopic photoluminescence (PL) mapping of solar-cell-grade
mc-Si ribbon wafers. We observe under UV excitation a broad visible emission
from the SiNx:H layer. The samples covered with SiNx:H were subjected
to rapid thermal processing at different temperatures, and PL maps were
measured before and after treatment. We observed that after RTP treatment the
intensity of the PL band was significantly increased, which indicates further
reduction of non-radiative defects. The magnitude of these increases depends
on the RTP conditions. The visible PL band exhibits a reversible photo
quenching of the intensity under 325 nm HeCd laser excitation. The PL
intensity can be recovered by annealing with the rate exhibiting a thermally
activated behavior. The observed PL photo quenching reveals a metastable
process in the SiNx:H film under UV excitation. We demonstrate the
possibility of creating a reversible luminescence micro-pattern on the
SiNx:H layer using a focused UV laser beam.
Deep level Transient Spectroscopy (DLTS), Electron Beam Induced Current (EBIC), EBIC Diffusion Length Mapping (EBIC-DLM) and contactless Photoconductive Decay (PCD) were used to characterize both bulk substrates and epitaxially grown Silicon Carbide films. Traps as deep as 0.93 eV were observed via DLTS. These traps may play a role in the persistent photoconductivity effect. EBIC reveals the electrical activity of the well known triangular defects. However, only some of these defects display electrical activity consistent with that of 3C-SiC inclusions, others do not. Additionally, not all defects identified in the EBIC images are observable in the topographic SEM image, possibly indicating a new, yet unidentified defect. EBIC revealed the electrical activity of defects including micro-pipes, dislocations (or possibly growth step edge decoration), surface polish damage, and bulk defects. Diffusion length maps of SiC indicate wide variations in diffusion length on both microscopic and macroscopic scales. EBIC-DLMindicatedepitaxial 4H SiC resulted in diffusion lengths from 0.1 to 3 μm, while bulk values were less than 0.07 μm. PCD measurements indicate tens of nanosecond to microsecond variations in lifetime. Lifetime verses injection level variations are observed and explained on the basis of trap energy. The injection level dependence of lifetime was observed at various nitrogen doping concentrations. Finally, electron beam annealing is found to dramatically improve the minority carrier lifetime in epitaxial SiC.
Rapid thermal processing is fast emerging as a vital low thermal budget processing technique. Use of photons of wavelengths less than 800 nm in conjunction with infrared and visible photons in RTP resulted in the reduction of microscopic defects and processing time. Screen printed back surface field (BSF) contacts and ohmic contacts which are an integral part of solar cells were processed and Schottky barrier diodes were made. Cycle time was reduced from 172 see's to 108 see's in the case of back surface field contacts and from 162 see's to 122 see's for the ohmic contacts. The Schottky diodes were characterized for electrical data. The structural properties of the metal silicon interface have direct correlation with the electrical properties of the device.
Several efficiency influencing factors in MOCVD-grown CdTe/CdS solar cells, including preferential crystal orientation of CdTe layers, CdTe grain size and surface roughness, interfacial mixing, and surface and interface geometrical morphology, are studied. X-ray diffraction (XRD) shows that polycrystalline CdTe/CdS solar cells with higher efficiencies tend to have more (111) planes of CdTe parallel to the macro-surface. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) analysis reveal the relationship between the grain size/surface roughness and cell efficiency. Secondary ion mass spectroscopy (SIMS) and Auger electron spectroscopy (AES) depth profiling show that the interfacial geometrical morphology has a significant influence on the efficiency of CdTe/CdS solar cells. Finally it is shown that interfacial mixing reduces the number of interfacial states and recombination centers and the energy loss due to internal reflectance, enhancing the performance of the solar cells.
Quantum photoeffects associated with photons of wavelength less than 0.8 micron lead to higher bulk and surface diffusion coefficients. We have exploited this fundamental property in designing rapid isothermal processing (RIP) systems for shallow junction formation in silicon. A detailed comparative study of diffusion, metallization and CVD with and without high energy photons has been carried out. The results show that microscopic defects, cycle time and processing temperature is lower than what can be achieved byconventional methods is realized by using photons in the ultra violet (UV) and vacuum ultra violet (VUV) spectrum.
The growth and advancement of the electronic and photonic industry in the 21 st century hinges on revolutionary new processing techniques that will overcome some of the most fundamental limitations of conventional methods. Rapid isothermal processing (RIP) based on incoherent radiation as the source of optical and thermal energy can play a major role in designing processing systems that offer the tight process control, low thermal budgets, low microscopic defects, high throughput and high yields required for almost every semiconductor device. Conventional RIP can be further optimized by fully exploiting the contribution of quantum photoeffects. The improved performance and reliability offered by RIP will make it the mainstream technology for the green manufacture of microelectronics, optoelectronics, solar cells, flat panel displays and microelectromechanical systems. Key issues related to the cost of ownership, design of RIP system based on the full utilization of photo–thermal effects and model based control systems are described. New experimental results for a number of processing steps are provided. These results demonstrate the importance of advanced RIP systems in providing better performance and lower defects for future devices.
The potential use of porous silicon as an antireflective coating on solar cells has recently been recognized. This study investigates the effect of current density, anodization time, and surface conditions on the reflectance of porous silicon which was fabricated by anodizing (100) float zone single crystal Si wafers. The wafers were coated on one side with Al prior to anodization, and a HFbased solution was used as the electrolyte. Current densities of 5 – 100 mA/cm2 were used to anodize both polished and unpolished wafers over time intervals ranging from 2sec - 30 minutes. Reflectance properties were tested over the 400 - 1100 nm range, and minimum reflectances of 3 – 5% were achieved. The reflectance of the best porous Si sample normalized with respect to the sun's spectrum compares favorably with the reflectance of a double layer ZnS/ MgF2 with prior texturing.
A new type of HF solution, HF-acetonitrile (MeCN), has been employed to produce 10-30 μm thick porous silicon (P-Si) layers by photoelectrochemical etching of different types of Si wafers, Si(100), Si(111) and polycrystalline Si, with different resistivities. A combined optical, surface and nuclear microscopic assessment of these P-Si layers was performed using photoluminescence (PL), Raman scattering, X-ray photoelectron spectroscopy (XPS) and Rutherford backscattering spectroscopy (RBS). The PL emission intensities, Raman line shapes and structural features are strongly dependent on the properties of the substrates such as the crystallinity and resistivity of the Si wafers used for forming P-Si. With increasing resisitivity of the Si(100) wafers, the resulting P-Si layers show a slight blue-shift of their visible light emission peak energy, an up-shift of the peak position and a narrowing of the band width of the dominant Raman band, and a decrease in the amount of residual elemental Si on the surface. Those Si(l 11) wafers, etched in HF-MeCN, showed no porous structures and no visible light emission.
Consider a k-variable normal distribution Ν (μ,Σ where mgr; = (μ1,μ2, … μk)' and Σ is diagonal matrix of unknown elements >0,i = 1,2, … k. The problem of sequential estimation of = 1 αiμi is considered. The stopping rule is shown to have some interesting limiting properties when the σi's become infinite.