Possibility of the Formation of Mesostructured Silica Titania Mixed-Oxide Thin Films with High Titania Load Demonstrated
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researchers annealed the samples in 1 atm flowing oxygen at 900°C. Following the implantation with one, two, and then all of the elements, secondary ion mass spectrometry depth profiles were taken of both the annealed and unannealed samples. The researchers found that when magnesium was implanted on its own, it did not diffuse after annealing. They also found that the magnesium moved most when it was co-implanted with just sodium, and less when aluminum was also implanted. The researchers said, “These findings corroborate the roles of sodium, magnesium, and aluminum in the oxidation behavior of NBD 200 silicon nitride ceramic.” PAMELA JOHNSON
Protein Promotes Nucleation in Water Microdroplets Ice crystallization in micro-sized water droplets plays an important role in nature. Researchers at the National University of Singapore have made a quantitative measurement of the interfacial effect of impurities on ice nucleation kinetics. Using microdroplet suspension to minimize the effects of the container and foreign particles, Du Ning and X.Y. Liu found that the protein lysozyme promotes freezing as a result of its effects on the interfacial kinetics of nucleation. The crystallization of microdroplets has significant implications for natural processes or systems such as climatic systems and the freezing and antifreezing processes within plants and animals. As described in the July 15 issue of Applied Physics Letters, a 0.4-µl droplet of deionized water was suspended between two layers of oil of different densities. To study the nucleation kinetics, the researchers measured the amount of time it took for an observable amount of the new phase to appear at several different temperatures. This induction time included both the time for crystals to nucleate and the time for them to grow to an observable size. However, because ice crystals grow much faster than they nucleate, the growth time can be neglected, and the time measured in the experiment was considered to be the nucleation time only. At a constant nucleation rate, a smaller water droplet has a longer induction time. In bulk water, a small change in volume during the experiment has a negligible effect, since the total volume is so large. However, for microdroplets, the change in volume can double the induction time. The researchers included the size effects by multiplying the induction time by the volume of the water droplet, allowing a precise and reproducible measurement of the nucleation kinetics. The effects of pro586
teins on ice nucleation kinetics was also studied.The researchers found that the addition of lysozyme at 0.05 wt% to the droplets had two effects. First, it raised the energy barrier for removing impurity molecules from the surface, which inhibited nucleation. Second, the lysozyme decreased the interfacial free energy. This made the nucleation time increase less rapidly with the supercooling temperature and promoted nucleation. Because this latter effect is dominant, the net effect of the lysozyme is to promote nucleation. ELIZABETH A. SHACK
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