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We describe a new class of the periodic banding of Ag2O precipitates through reaction–transport–reaction processes in an agarose-hydrogel column; the gel contains NaNO3 as an additive and is inserted between Ag and Ti rods with an applied constant voltage of 4.0–7.0 V. Submillimeter-scale periodic dark-brown precipitation bands were transiently generated with high reproducibility in this system, in which (i) electrochemical reactions at the metal rods to generate Ag+ and OH−, (ii) transportation of the reactant ions by the electric field, and (iii) reactions to produce Ag2O are expected to couple. The bands successively emerged mainly toward the cathode, accompanied by substantial changes in the electric current through the gel. The periodic banding depended on the applied voltage, duration of application, and concentration of NaNO3 initially loaded in the gel. The banding was most clearly observed in a 2.0 mass% agarose gel column containing 0.005 M NaNO3 under a constant applied voltage of ∼5.5 V for ∼3 h. This optimal condition resulted in more than ten periodic bands, of which the bandwidth (w) and the distance between the adjacent bands (d) were randomly distributed around the average values: for the bands formed up to 2 h, w = 0.25 ± 0.04 mm and d = 0.42 ± 0.05 mm; for the bands formed after 2 h, w = 0.42 ± 0.11 mm and d = 0.68 ± 0.07 mm. The generated periodic bands were gradually painted out with time to form an almost continuous broad band, even after the applied voltage was discontinued, although immersion of the gel in deionized water for 3 h could suppress this uniformization. Observations using optical and scanning electron microscopes with an energy-dispersive X-ray detector suggested that the periodic bands were composed of gelatinous Ag2O and micrometer-scale (1–50 μm) clusters, which comprised Ag-abundant nanoparticles with considerable size dispersion (10–500 nm). The relationship between the periodic banding and inhomogeneous distribution of the reactant ions and potential applications of the observed banding phenomena are discussed.

This paper reports a new class of periodic banding of Fe(OH)3 precipitate through reaction–diffusion–reaction (RDR) processes in agarose gel sandwiched between two metal rods (Ti and Fe) with cyclic alternating voltages. In the RDR processes, electrochemical reactions at metal rods to generate reactant ions, diffusion of the ions influenced by the electric field in the gel, and reactions of the ions to form precipitates were coupled to generate periodic bands of Fe(OH)3 precipitates at the cathode side. The banding morphologies, spatial Fe distribution, and microscopic morphologies of the precipitates were investigated by visual inspection, Fe Kα intensity distribution measurements, and scanning electron microscopy observations. The periodic banding strongly depended on the applied voltages, the periods for voltage alternation, the cycle number (NC), and the length of the gel column. Periodic bands resembling Liesegang bands were most clearly observed with high reproducibility for 50 mm gel columns under the applied cyclic alternating voltages of 3 and 1 V for 1 and 4 h, respectively. The number of the periodic bands formed in this system was generally given by NC—NC1 + 1, where NC1 is the cycle number where the first band emerges. These periodic bands contained significant amounts of Fe atoms that were almost uniformly distributed in the agarose gel, strongly supporting the formation of gelatinous Fe(OH)3 precipitates in the bands.

In agarose gel containing [Fe(CN)6]3‒ ions and sandwiched between two metal rods (Ti, Fe, or Cu) with a voltage of 1‒5 V applied for 20–100 h, reaction–diffusion–reaction (RDR) processes (that is, electrochemical reactions at metal rods to generate reactant ions, diffusion of the reactant ions influenced by the electric field in agarose gel, and reactions of the reactant ions to form/decompose precipitates) were coupled to generate diverse precipitation patterns of Prussian blues (PB) or Cu–Fe-based Prussian blue analogs (Cu–Fe PBA). These patterns strongly depended on the type of metal electrode, applied voltage, initial [Fe(CN)6]3‒ concentration, and elapsed time after voltage application. Under the application of 2 V for 20/50 h, the PB/Cu–Fe PBA formed a discrete precipitation band on the anode/cathode side in an agarose gel containing 0.050 M [Fe(CN)6]3‒ ions. In the Cu–Fe PBA system, a relatively long precipitation band of Cu(OH)2 was also generated on the anode side by OH− ions produced on the cathode as a byproduct. Longer voltage applications promoted propagation of the Cu–Fe PBA band to the anode side and caused the discrete PB band to disappear. Higher initial [Fe(CN)6]3‒ concentrations deepened the color of the generated patterns. Higher voltage applications suppressed the propagation of the Cu–Fe PBA band to the anode side and caused the PB band to disappear. Experiments using a Ti cathode suggested that the formation and subsequent decomposition of PB or Cu–Fe PBA at the cathode surface are important for forming precipitation band(s) in the gel near the cathode. The application of cyclic alternating voltages (particularly, 4 V for 1 h and 1 V for 4 h) was effective in generating Liesegang-band-like periodic bands, particularly for the Cu–Fe PBA system.

In tunnel construction in gravel-mixed ground, it is extremely important to predict the risk of gravel dropout and the accompanying large deformation of the substrate ground. The distribution and shape of gravel vary, and it is difficult to reproduce the situation depending on the individual site. Therefore, in this study, we created a model of the gravel-mixed ground using the Voronoi partition model. Then, by mathematical analysis, the gravel shape that appears in the actual phenomenon could be reproduced in consideration of randomness and inhomogeneity. In addition, it was clarified that the gravel extrusion phenomenon and the large deformation of the ground can be expressed.

We propose a simple and novel system to form precipitation patterns of Cu-Fe-based Prussian blue analogues (Cu-Fe PBA) in agarose gel through coupled electrochemical reactions, reactant ion diffusion influenced by electric field, and precipitation reactions. The spatiotemporal evolution, spatial distribution, and crystallite morphologies of the precipitates were investigated by visual inspection, Fe Kα intensity distribution measurements, and optical and scanning electron microscope observations. The observed precipitation patterns and their evolution depended on the applied voltage. Multicolored periodic precipitation bands were stochastically formed under cyclic alternating voltage (4 V for 1 h and then 1 V for 4 h per cycle). The distances between adjacent bands were randomly distributed (0.30 ± 0.25 mm). The sizes and shapes of the crystallites generated in the gel were position-dependent. Cubic but fairly irregular crystallites (0.1–0.8 μm) were formed in the periodic bands, whereas definitely cube-shaped crystallites (1–3 μm) appeared close to the anode. These cube-like reddish–brown crystallites were assigned to Cu-FeII PBA. In some periodic bands, plate-like blue crystallites (assigned to Cu(OH)2) were also present. Future issues for potential applications of the observed periodic banding for selective preparation of Cu-Fe PBA crystallites were discussed.

We have developed a hybrid-type temperature sensor using thin-film transistors. First, we evaluate temperature dependences of transistor characteristics and find that the temperature dependence of the off-leakage current is much larger than that of the on current. Next, we combine a transistor, capacitor, and ring oscillator to develop the hybrid-type temperature sensor and detect the temperature by measuring the oscillation frequency. The large temperature dependences of the off-leakage currents can be utilized, and simultaneously only a digital circuit is required to count the digital pulse.

We describe a new class of the periodic banding of Ag2O precipitates through reaction-transport-reaction processes in an agarose-hydrogel column; the gel contains NaNO3 as an additive and is inserted between Ag and Ti rods with an applied constant voltage of 4.0-7.0 V. Submillimeter-scale periodic dark-brown precipitation bands were transiently generated with high reproducibility in this system, in which (i) electrochemical reactions at the metal rods to generate Ag+ and OH-, (ii) transportation of the reactant ions by the electric field, and (iii) reactions to produce Ag2O are expected to couple. The bands successively emerged mainly toward the cathode, accompanied by substantial changes in the electric current through the gel. The periodic banding depended on the applied voltage, duration of application, and concentration of NaNO3 initially loaded in the gel. The banding was most clearly observed in a 2.0 mass% agarose gel column containing 0.005 MNaNO3 under a constant applied voltage of ~5.5 V for ~3 h. This optimal condition resulted in more than ten periodic bands, of which the bandwidth (w) and the distance between the adjacent bands (d) were randomly distributed around the average values: for the bands formed up to 2 h, w = 0.25 ± 0.04mmand d = 0.42 ± 0.05 mm; for the bands formed after 2 h, w = 0.42 ± 0.11mmand d = 0.68 ± 0.07 mm. The generated periodic bands were gradually painted out with time to form an almost continuous broad band, even after the applied voltage was discontinued, although immersion of the gel in deionized water for 3 h could suppress this uniformization. Observations using optical and scanning electron microscopes with an energy-dispersive X-ray detector suggested that the periodic bands were composed of gelatinous Ag2O and micrometer-scale (1-50 μm) clusters, which comprised Ag-abundant nanoparticles with considerable size dispersion (10-500 nm). The relationship between the periodic banding and inhomogeneous distribution of the reactant ions and potential applications of the observed banding phenomena are discussed. [ABSTRACT FROM AUTHOR]