Steam Explosions of Single Drops of Molten Silicon-Rich Alloys
Commercial quantities of ferrosilicon alloys and silicon metal in the form of granules are available. They are produced in water granulation, a process involving the risk of large-scale steam explosions. To learn about these steam explosions, we have investigated single drops of molten alloys when released into water. When single drops of molten ferrosilicon alloy with 75 wt.% Si (FeSi75) or silicon metal at its liquidus or melting temperature were released into cold, room temperature or hot water, spontaneous explosions never occurred. We could, however, initiate steam explosions by exposing the drops to upwarddirected pressure transients generated in the water with an encapsulated impactor. For water at room temperature, we determined (a) the strength of the pressure transients needed to initiate the steam explosion of a single drop, and (b) the range of fall depths over which the drop can be triggered to explode with this transient. When varying the water temperature we learned that the triggered explosions became less vigorous as the temperature increased. The explosions became more vigorous also when the Al and/or Ca content in the molten drop decreased. Photographic images of the bubbles generated during the steam explosions indicate that the maximum energies released correspond to about 1 % to 2% (dependent on drop size) of the total enthalpy of the molten ferrosilicon at its liquidus temperature. During the explosions, tiny bubbles of a permanent gas, presumably hydrogen, were generated. Video images show that ferrosilicon drops with diameters between 3 mm and 11 mm fall through water at about 0.5 m/s. Shutter wheel photographs confirm these fall rates and also indicate the onset of solidification of the drops and show that steam explosions can occur in partially solidified drops.
Oxidation of Liquid Silicon-Reaction Mechanisms and Kinetics
When liquid silicon reacts with water, an oxide layer forms on the surface of the melt. The composition and structure of this oxide layer is determined by the initial compo9tion of the melt, the temperature and the flow conditions at the surface. We have used two different techniques to oxidise molten silicon, with varying content of impurities (Al and Ca). The results show that the initial oxidation product is gaseous SiO. This gas will not necessarily deposit on the surface of the silicon melt, but may be carried away depending on the local flow conditions. Impurities in the melt will result in a slag film on the surface of the melt. This slag consists of a mixture of Si02' Alz03 and CaO. Depending on the composition and temperature, this slag mixture may be solid or liquid. As a consequence of this slag formation, more hydrogen is produced, which has a desirable effect as to prevent large-scale steam explosions.
Collagen dynamics in articular cartilage under osmotic pressure
Cartilage is a complex biological tissue consisting of collagen, proteoglycans and water. The structure and molecular mobility of the collagen component of cartilage were studied by 13C solid‐state NMR spectroscopy as a function of hydration. The hydration level of cartilage was adjusted between fully hydrated (∼80 wt% H2O) and highly dehydrated (∼30 wt% H2O) using the osmotic stress technique. Thus, the conditions of mechanical load could be simulated and the response of the tissue macromolecules to mechanical stress is reported. From the NMR measurements, the following results were obtained. (i) Measurements of motionally averaged dipolar 1H–13C couplings were carried out to study the segmental mobility in cartilage collagen at full hydration. Backbone segments undergo fast motions with amplitudes of ∼35° whereas the collagen side‐chains are somewhat more mobile with amplitudes between 40 and 50°. In spite of the high water content of cartilage, collagen remains essentially rigid. (ii) No chemical shift changes were observed in 13C cross‐polarization magic angle spinning spectra of cartilage tissue at varying hydration indicating that the collagen structure was not altered by application of high osmotic stress. (iii) The 1H–13C dipolar coupling values detected for collagen signals respond to dehydration. The dipolar coupling values gradually increase upon cartilage dehydration, reaching rigid limit values at ∼30 wt% H2O. This indicates that collagen is essentially dehydrated in cartilage tissue under very high mechanical load, which provides insights into the elastic properties of cartilage collagen, although the mechanical pressures applied here exceed the physiological limit. Copyright © 2006 John Wiley & Sons, Ltd.