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 hydrothermally synthesized Co7(μ3-OH)8(μ-ox)3(μ-ppz)3 contains heptanuclear hydroxo-bridged Co(II) clusters in the form of corner-fused double cubanes. Each cluster is further bridged by six oxalate (ox) and six piperazine (ppz) ligands with 12 other clusters into a 3-D structure.
The melting and freezing phase transitions of mercury in a porous glass were studied by NMR and acoustic techniques. The NMR measurements provided direct information on the total amount of liquid mercury versus temperature. A depression of the phase transition temperatures and pronounced hysteresis between melting and freezing were found. Acoustic measurements showed that the freezing process was irreversible while the melting process consisted of reversible and irreversible temperature ranges. The use of longitudinal and transverse acoustic waves made it possible to obtain information about the origin of reversible and irreversible behavior upon melting. In particular, we found that the complete melting of confined mercury can be acoustically detected using only longitudinal and not transverse waves. The broadening of melting is explained by the formation of a liquid layer on the mercury solid surface, and freezing was driven by the pore geometry with no visible precursor effects. ͓S0163-1829͑98͒06033-0͔
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