Impact of Impurities on the Mechanical Strength of Multicrystalline Silicon

The usage of more inexpensive silicon feedstock for crystallizing mc-Si blocks promises cost reduction for the photovoltaic market. For example, less expensive substrates of upgraded metallurgical silicon (UMG-Si) are used as a mechanical support for the epitaxial solar cell. This feedstock has higher content of impurities which influences cell performance and mechanical strength of the wafers. Thus, it is of importance to know these effects in order to know which impurities should be preferentially removed or prevented during the crystallization process. Metals like aluminum (Al) can decrease the mechanical strength due to micro-cracking of the silicon matrix and introduction of high values of thermal residual stress. Additionally, silicon oxide (SiOx) lowers the mechanical strength of mc-Si due to thermal residual stresses and stress intensification when an external load is applied in the surrounding of the particle. Silicon carbide (SiC) introduces thermal residual stresses and intensifies slightly the stress in the surrounding of the particle but can have a toughening effect on the silicon matrix. Finally, silicon nitride (Si3N4) does not influence significantly the mechanical strength of mc- Si and can have a toughening effect on the silicon matrix.

Influence of high aluminium content on the mechanical properties of directionally solidified multicrystalline silicon

The purpose of this research is the mechanical characterisation of multicrystalline silicon crystallised from silicon feedstock with a high content of aluminium for photovoltaic applications. The mechanical strength, fracture toughness and elastic modulus were measured at different positions within the multicrystalline silicon block to quantify the impact of the segregation of impurities on these mechanical properties. Aluminium segregated to the top of the block and caused extensive micro-cracking of the silicon matrix due to the thermal mismatch between silicon and the aluminium inclusions. Silicon nitride inclusions reduced the fracture toughness and caused failure by radial cracking in its surroundings due to its thermal mismatch with silicon. However, silicon carbide increased the fracture toughness and elastic modulus of silicon.

Short-Lived and Phosphorylated Proteins Contribute to Carrier-Mediated Efflux, but Not to Influx, of Auxin in Suspension-Cultured Tobacco Cells1

Auxin is transported across the plasma membrane of plant cells by diffusion and by two carriers operating in opposite directions, the influx and efflux carriers. Both carriers most likely play an important role in controlling auxin concentration and distribution in plants but little is known regarding their regulation. We describe the influence of modifications of the transmembrane pH gradient and the effect of agents interfering with protein synthesis, protein traffic, and protein phosphorylation on the activity of the auxin carriers in suspension-cultured tobacco (Nicotiana tabacum L.) cells. Carrier-mediated influx and efflux were monitored independently by measuring the accumulation of [14C]2,4-dichlorophenoxyacetic acid and [3H]naphthylacetic acid, respectively. The activity of the influx carrier decreased on increasing external pH and on decreasing internal pH, whereas that of the efflux carrier was only impaired on internal acidification. The efflux carrier activity was inhibited by cycloheximide, brefeldin A, and the protein kinase inhibitors staurosporine and K252a, as shown by the increased capability of treated cells to accumulate [3H]naphthylacetic acid. Kinetics and reversibility of the effect of brefeldin A were consistent with one or several components of the efflux system being turned over at the plasma membrane with a half-time of less than 10 min. Inhibition of efflux by protein kinase inhibitors suggested that protein phosphorylation was essential to sustain the activity of the efflux carrier. On the contrary, the pharmacological agents used in this study failed to inhibit [14C]2,4-dichlorophenoxyacetic acid accumulation, suggesting that rapidly turned-over proteins or proteins activated by phosphorylation are not essential to carrier-mediated auxin influx. Our data support the idea that the efflux carrier in plants constitutes a complex system regulated at multiple levels, in marked contrast with the influx carrier. Physiological implications of the kinetic features of this regulation are discussed.

Beryllium--fabrication and joining : a literature search /

This search, containing 128 references, was prepared to serve as a guide to the report literature on the fabrication and joining of beryllium. Studies on compacts and powder metallurgy are included. Articles selected from scientific journals and which appear in Nuclear Science Abstracts (NSA) are included. Reports are listed alpha-numerically under the issuing agency.

Special occupational hazard review and control recommendations for nickel carbonyl /

Mode of access: Internet.

Evaluation of Horizons, Inc. Th metal powder : progress report no. 1 for February 18, 1954-March 4, 1954 /

Three samples of thorium powder as supplied by Horizons, Inc. of Cleveland, Ohio were compared. The powder was processed entirely without the use of protective atmospheres with the exception of the sintering which was performed in a tank argon atmosphere with a dew point of approximately -80°C and the hot-pressing which was carried out in vacuum. The equipment and procedures used for cold-pressing and sintering were identical to those used in the evaluation of Sylvania produced thorium powders. The hot-pressing was done with standard uranium hot-pressing equipment.

An investigation of the slip-casting mechanism as applied to stainless steel powder /

The slip-casting technique, which is successfully employed in the ceramic industry, was utilized to produce sintered stainless steel components experimentally. The procedure used is described, along with an evaluation of the physical and mechanical properties of the samples produced. Specimens were made with properties comparable to those of wrought stainless steel and of cold pressed and sintered steel powder.

Consumable electrode arc-melting of cold-compacted thorium powder /

The consumable electrode arc melting of cold-compacted thorium powder has been developed to produce dense, clean and soft ingots which can be hot rolled without cracking. By using a water-cooled copper crucible, contamination has been prevented and resultant ingots cleaner than the original powder have been produced.

Isostatic pressing of UO₂ in high temperature metals /

The hot isostatic pressing process has been applied at temperatures up to 1500°C for the fabrication of high temperature fuel rods composed of UO₂ clad in columbium and UO₂ in iron-aluminum type alloy. The fused UO₂ powder apparently becomes quite plastic at temperatures above 1200°C and can be isostatically compacted at 1500°C to 98% of its theoretical density. Columbian tubes particularly lend themselves to the fabrication of fuel rods by simultaneously compacting and cladding UO₂ powders in the tubes, but the cast iron-aluminum type alloy that was used was unsatisfactory because of its brittleness.

Alloys of chromium, copper and nickel,

Published also as University of Illinois Engineering Experiment Station bulletin no. 93 with title: A preliminary study of the alloys of chromium, copper, and nickel, by D.F. McFarland and O.E. Harder.

Transmission electron microscopy sample preparation technique for sintered alloys

Powder metallurgy alloys are typically inhomogeneous with a significant amount of porosity. This complicates conventional transmission electron microscopy sample preparation. However, the use of focused ion beam milling allows site specific transmission electron microscopy samples to be prepared in a short amount of time. This paper presents a method that can be used to produce transmission electron microscopy samples from an Al-Cu-Mg PM alloy. (C) 2003 IoM Communications Ltd. Published by Maney for the Institute of Materials, Minerals and Mining.

Crystallographyof Mg17Al12 precipitates in AZ91D alloy

Kikuchi diffraction was used to accurately determine the orientation relationship (OR) between Mg17Al12 precipitates and matrix in an AZ91D alloy. For both continuous and discontinuous precipitations, the Burgers OR and the Potter OR were equally observed. The lattice parameter of Mg17Al12 associated with the former is bigger than that of the latter. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.