We have studied the behavior of gallium in porous glass. Except for a few lines of ␣-Ga, the main pattern of the x ray does not fit in with any previously reported phase of gallium. By resistivity measurements, just above a superconducting transition, an anomalous peak at 6.3 K was observed. Lack of x-ray patterns of -Ga and ␦-Ga suggests that the 6.4 K transition might be due to a new phase of gallium or a  phase in strong disorder. ͓S0163-1829͑96͒02830-5͔Porous glass filled with different materials plays an important role in many aspects of science and technology. Properties of materials in confined geometries can differ significantly from those of bulk samples. 1,2 In this paper, we will discuss the properties of gallium in a porous glass.The porous matrix was prepared from a sodium borosilicate glass. The pores had a rather narrow size distributions, and 95% of the pore diameters were lying within Ϯ4 Å of the average value. The pores together with narrow necks, which connect pores, form the random interconnected network in a glass bulk. The average distance between pores, according to small-angle diffractometry and electron microscopy, can be estimated to be 90 Å, about twice as large as the pore diameter. Before porous glass was filled with gallium, it was cleaned by H 2 O 2 and heated up to 130°C for the inner water evaporation. The liquid gallium was embedded into glass under high pressure up to 9 Kbar at 35°C. The purity of gallium was 99.9%. The density of porous glass with gallium was 2.83 g cm Ϫ3 corresponding to about 80% filled total void volume. Figure 1 gives the temperature dependences of resistance between 2 and 300 K. Electrical resistance was measured by a four-probe method. The schematic representation of the structure of porous glass with gallium and four probes is shown in the inset of Fig. 2. The sample was fixed tightly in the sample holder and four probes were pinned into the sample. A very small current 0.5 mA was used to measure the resistance. No self-heat effect was observed. The resistance of a sample was measured by averaging the voltages obtained with the current in the forward and reverse directions. As shown in Fig. 1, the resistance fluctuates between about 290 and 160 K. Except this temperature range, the resistance measurements are reversible and repeatable. Figure 2 gives the temperature dependence of resistance R between 2 and 12 K. A very sharp superconducting transition is observed at 7Ϯ0.01 K. At 7 K the resistance is near but still not zero. As shown in Fig. 2, there are many microchannels between A and B. As long as gallium in any one of the channels becomes superconductive, it will cause zero resistance between A and B.
The melting and freezing processes and the structure of gallium in an opal were studied using x-ray powder diffraction within the temperature range 10 to 320 K. Four different modifications of solid confined gallium were found. Two of these modifications did not coincide with any known gallium structures; another two coincided with and disordered . The broadening of the total melting and freezing processes and reduction of the phase transition temperatures compared to bulk phase were obtained. The size of confined gallium crystallites corresponding to the different modifications was estimated for cooling and warming. The reproducible freezing was treated as a result of the steep temperature dependence of the nucleation rate in supercooled melts. Additional measurements of resistance for the opal filled with gallium revealed that it was sensitive mainly to the melting and freezing within the gallium modification coinciding with disordered . Melting-freezing hysteresis for this modification was found to depend on temperatures of pre-warming. It was suggested that the freezing of this modification is driven by the amount of frozen crystallites of the phase with the highest onset of freezing. The melting broadening and the lowering of the phase transition temperatures for the confined gallium modifications were discussed.
The crystallization behavior of the electroless Ni-P deposit was investigated in detail with the aid of SEM, TEM, XRD, and electron diffraction techniques. The as-deposited film was shown to exhibit amorphous structure with XRD (x-ray diffraction) and electron diffraction. The XRD and EDAX results show that heat-treatment at temperatures above 300~ gives rise to Ni and Ni3P phases. The fcc Ni and tetragonal Ni3P grains of the film heated at 800~ are identified with electron diffraction. The occurrence of the maximum hardness achieved upon heating of the electroless Ni-P plating is explained in terms of crystallization, grain coarsening, and defect change. The grain coarsening is also responsible for the random distribution of the phosphorus content across the deposit at temperatures above 600~ABSTRACT By the application of well-known thermodynamics, growth kinetics, diffusion processes, and experimental results, a reasonable explanation for the behavior of silica in contact with liquid silicon is presented. Decomposition of silica to solid SiO, and diffusion of silicon and oxygen ions through solid SiO, play important parts in the discussion. The consequences for Czochralski growth of silicon crystals are outlined.
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