Removal of inclusions from silicon

The removal of inclusions from molten silicon is necessary to satisfy the purity requirements for solar grade silicon. This paper summarizes two methods that are investigated: (i) settling of the inclusions followed by subsequent directional solidification and (infiltration by ceramic foam filters. Settling of inclusions followed by directional solidification is of industrial importance for production of low-cost solar grade silicon. Filtration is reported as the most efficient method for removal of inclusions from the top-cut silicon scrap.     

Wettability of Aluminum on Alumina

The wettability of molten aluminum on solid alumina substrate has been investigated by the sessile drop technique in a 10⁻⁸ bar vacuum or under argon atmosphere in the temperature range from 1273 K to 1673 K (1000 °C to 1400 °C). It is shown that the reduction of oxide skin on molten aluminum is slow under normal pressures even with ultralow oxygen potential, but it is enhanced in high vacuum. To describe the wetting behavior of the Al-Al₂O₃ system at lower temperatures, a semiempirical calculation was employed. The calculated contact angle at 973 K (700 °C) is approximately 97 deg, which indicates that aluminum does not wet alumina at aluminum casting temperatures. Thus, a priming height is required for aluminum to infiltrate a filter. Wetting in the Al-Al₂O₃ system increases with temperature.     

The Ranawat Award. Comparison of surgical approaches in total knee arthroplasty

A series of 123 revision total knee replacements performed at three centers and followed up for 2 to 4 years was evaluated. In cases in which exposure could be obtained with undue tension on the patellar tendon, the surgical approach was modified using either quadriceps turndown (14 cases) or tibial tubercle osteotomy (15 cases). The remaining 94 patients underwent a standard operative approach that consisted of a medial parapatellar capsular incision which in 31 cases was combined with a quadriceps snip. The results were compared using the Knee Society clinical score, a patient satisfaction survey, and a patellofemoral questionnaire. Postoperatively, the group of patients who had a quadriceps snip was equivalent to the group of patients who underwent a standard approach in every parameter measured, and the groups therefore were combined for comparison purposes. The patients who had quadriceps turndown and the tibial tubercle osteotomy had equivalent scores postoperatively both of which were significantly lower than the standard group. The group of patients who had quadriceps turndown had a significantly greater increase in arc of motion than the tibial tubercle osteotomy group. The tibial tubercle osteotomy group had a lower degree of extension lag but a higher percentage of patients who had difficulty with kneeling and stooping and a higher percentage of patients who had difficulty with kneeling and stooping and a higher percentage of patients who thought the surgery was unsuccessful in relieving pain and unsuccessful in returning them to normal daily activities.     

Radiographic evaluation of penetration by the femoral head into the polyethylene liner over time

We examined the pattern of temporal penetration by thirty-two-millimeter-diameter femoral heads into polyethylene liners in a group of 105 hips (103 patients) in which an Arthropor metal-backed cup had been implanted. Each patient was evaluated radiographically and clinically at a minimum of four different postoperative intervals. The initial evaluation was performed a mean of 2.9 weeks (range, one to fifteen weeks) postoperatively, and the latest evaluation was performed a mean of 7.9 years (range, five to ten years) postoperatively. Two-dimensional wear - that is, penetration by the femoral head into the ultra-high molecular weight polyethylene liner - was determined from anteroposterior radiographs of the pelvis with a computer analysis system that calculated the change in the position of the center of the head relative to the center of the cup. Three new findings are reported. First, there was a large difference (mean, 1.1 millimeters) between the center of the head and that of the cup as measured on the initial postoperative radiographs. This difference underscores the need for researchers to consider the initial displacement of the head when measuring and reporting polyethylene wear. Second, although there was wide variation in responses among individuals, temporal examination of the data revealed a trend toward a decreasing rate of penetration with time. Moreover, the rate of penetration appeared to reach a steady-state value after the sixth postoperative year and remained nearly constant until the ninth postoperative year. Third, by comparing the subsets of patients who had the greatest and the least initial penetration by the head, we found that penetration behavior, although remarkably different between the groups in the first three years postoperatively, became similar with time. CLINICAL RELEVANCE: When making decisions regarding individual patients or hip systems that demonstrate penetration by the femoral head into the polyethylene liner, clinicians should consider the patterns of penetration over time. Measurements of the amount and rate of penetration that are based solely on the most recent radiograph do not represent the full clinical picture. We advocate more frequent radiographic follow-up and, when available, analysis of serial radiographs for patients who have excessive penetration by the femoral head into the acetabular liner.     

The Otto Aufranc Award. Skeletal response to well fixed femoral components inserted with and without cement

Previous studies evaluating femoral remodeling after total hip arthroplasty have used clinical radiographs and dual energy xray absorptiometry. Limitation of these techniques make it impossible to quantify the magnitude of bone loss in terms of cortical thinning and cortical bone area and bone mineral density changes. Femoral cortical bone remodeling after cemented and cementless replacement was quantified and possible determinants of bone remodeling in terms of clinical and radiographic variables were evaluated. Forty-eight anatomic specimen femora from 24 patients with unilateral cemented and cementless hip replacements were analyzed. Cortical thickness, cortical bone area, and bone mineral density was assessed in 4 quadrants at 5 discrete levels. The maximum cortical bone loss by level was at the middle section for the cemented femurs and at the midproximal and middle sections for the cementless femurs. However, if one examines individual quadrants, the proximal medial cortex still represents the specific region of maximal bone loss for both types of implant fixation. The posterior cortex had substantially more bone loss, even in the diaphyseal levels, than had been previously appreciated. A strong correlation was noted between the bone mineral density of the control femur and the percentage decrease of bone mineral density in the remodeled femur. Based on this data, it seems that the less dense the bone is before hip replacement surgery, the greater the extent of bone loss after total hip arthroplasty regardless of the fixation type.     

Bubbling at high flow rates in inviscid and viscous liquids (slags)

The behavior of gas discharging into melts at high velocities but still in the bubbling regime has been investigated in a laboratory modeling study for constant flow conditions. Air or helium was injected through a vertical tuyere into water, zinc-chloride, and aqueous glycerol solutions. High speed cinematography and pressure measurements in the tuyere have been carried out simultaneously. Pressure fluctuations at the injection point were monitored and correlated to the mode of bubble formation. The effects of high gas flow rates and high liquid viscosities have been examined in particular. Flow rates were employed up to 10^-3 m³/s and viscosity to 0.5 Ns/m₂. In order to attain a high gas momentum, the tuyere diameter was only 3 x 10^-3 m. The experimental conditions and modeling liquids were chosen with special reference to the established practice of submerged gas injection to treat nonferrous slags. Such slags can be highly viscous. Bubble volume is smaller than that calculated from existing models such as those given by Davidson and Schüler10,11 due to the effect of gas momentum elongating the bubbles. On the other hand, viscosity tends to retard the bubble rise velocity, thus increasing volumes. To take elongation into account, a mathematical model is presented that assumes a prolate ellipsoidal shape of the bubbles. The unsteady potential flow equations for the liquid are solved for this case. Viscous effects are taken into account by noting that flow deviates from irrotational motion only in a thin boundary layer along the surface of the bubble. Thus, drag on the bubble can be obtained by calculating the viscous energy dissipation for potential flow past an ellipse. The time-dependent inertia coefficient for the ellipsoid is found by equating the vertical pressure increase inside and outside the bubble. This pressure change in the bubble is obtained by assuming that gas enters as a homogeneous jet and then calculating the stagnation pressure at the apex of the bubble.     

Fluid dynamics in bubble stirred ladles: Part I. experiments

Air is supplied through a porous plug placed in the center axis of a cylindrical perspex-water model of a ladle. A Laser-Doppler system is employed to measure radial and axial mean and fluctuating velocities. Velocities in the two-phase bubbly region can also be determined. Velocities are measured near the bottom, half-way up, and near the free surface. It is shown that the bubbles contribute to production of turbulence. The ladle has recirculation zones near the bottom, where the mean velocities are very low. Close to the free surface the radial mean and turbulent velocities are high, promoting mass transfer through the interface. The present measured velocity profiles cannot be reduced to a single profile by employing similarity scaling.     

Total hip arthroplasty with porous-coated prostheses fixed without cement in patients who are sixty-five years of age or older

We reviewed the results of 212 total hip arthroplasties performed without cement in 203 unselected, consecutive patients who were sixty-five years of age or older. The outcome was known for 196 hips, thirty-seven of which had been followed until the death of the patient and 159 of which had been followed for a minimum of five years. A reoperation was done in 4 percent (seven) of the 196 hips. These reoperations included one revision because of loosening of the stem and five revisions of the cup. Three of the acetabular revisions were done because of fracture due to polyethylene wear; one, because of recurrent dislocation; and one, because of polyethylene wear and a fracture due to lysis. In the seventh reoperation, a cup and stem were exchanged because of infection. The probability that a hip prosthesis would survive twelve years without a reoperation was 0.92 (95 percent confidence interval, 0.85 to 0.99). We evaluated the clinical results for 152 patients who had not had a reoperation and had been followed for at least five years (mean, 8.5 years; range, five to fourteen years). Of these patients, thirteen (9 percent) had pain that limited activity: five (3 percent) had pain in the thigh (four patients) or groin (one patient), and eight (5 percent) had trochanteric pain and tenderness. One hundred and forty-four (95 percent) of the patients noted an improvement in overall function, and 149 (98 percent) reported satisfaction with the outcome. One hundred and thirty-five hips that were not revised were followed radiographically for at least five years (mean, 8.2 years; range, five to fourteen years). In this group, osteolysis was observed in three hips (2 percent); loosening of the cup, in three (2 percent); and loosening of the stem, in one (1 percent). Stress-shielding was seen on the two-year postoperative radiographs of forty-five (26 percent) of the 174 hips that were followed for at least that duration. The prevalence of osteolysis, loosening of a component, and reoperations was no greater in this subgroup than in the overall group. These results indicate that total hip arthroplasty without cement can be successful in older patients. This study also provides a reference for comparison with the results of total hip arthroplasty performed with cement and those of so-called hybrid total hip arthroplasty (an acetabular component fixed without cement and a femoral component fixed with cement) in patients who are sixty-five years of age or older.     

Wetting Properties of Molten Silicon with Graphite Materials

The wetting behavior of refractory materials by molten silicon is important in the refining and casting of silicon with respect to production of low-cost solar cells. Here we studied the wetting properties of several graphite materials by molten silicon. These materials are used in the photovoltaic industry. The sessile drop method is employed to measure the contact angles. Initially, molten silicon does not wet graphite materials. The initial contact angles measured are approximately 120 deg. Molten silicon will react with C to form β-SiC and to infiltrate the refractory. Because losses of Si should be minimized, infiltration of Si into the refractory also is a problem. Surface roughness increases the contact area between Si and refractory and thus the loss of Si. Equilibrium wetting angles of 0 deg to 31 deg are measured. With increasing surface roughness, the equilibrium wetting angles decrease. The results show that the infiltration depth of molten silicon increases with the average pore size of graphite materials.     

Sharp DNA bends as landmarks of protein-binding sites on straightened DNA

We have developed a fluorescence-based method for mapping single or multiple protein-binding sites on straightened, large-size DNA molecules (> 5 kbp). In the described method, protein-DNA complexes were straightened and immobilized on a flat surface using surface tension. A fraction of the immobilized complexes displayed a sharp DNA bend with two DNA segments extending from the apex. The presence of DNA-binding proteins at the apex was verified by atomic force microscopy. The position of protein binding relative to the ends of the DNA molecule was determined by measuring the length of two DNA segments using fluorescence microscopy. We demonstrate the potential of the fluorescence-based method to localize protein-binding sites on the DNA template and to evaluate relative binding affinity. The proposed protein-binding-site mapping technique is simple and easy to perform. Practical applications include screening for DNA-binding proteins and the localization of protein-binding sites on large segments of DNA.