Yasuhiro Awakura

Structural and Electrical Characterizations of Electrodeposited p-Type Semiconductor Cu2O Films

The p-type semiconductor cuprous oxide (Cu 2 O) film has been of considerable interest as a component of solar cells and photodiodes due to its bandgap energy of 2.1 eV and high optical absorption coefficient. We prepared Cu 2 O films on a conductive substrate by electrodeposition at 318 K from an aqueous solution containing copper sulfate and lactic acid. The structural and electrical characterizations of the resulting films were examined by X-ray diffraction, X-ray photoelectron spectroscopy, and X-ray absorption measurements, and the Hall effect measurement, respectively. The resistivity varied from 2.7 × 10 4 to 3.3 X 10 6 Ω cm, while the carrier density was from 10 1 2 to 10 1 4 cm - 3 and the mobility from 0.4 to 1.8 cm 2 V - 1 s - 1 , depending on the preparation conditions, i.e., solution pH and deposition potential. The carrier density was sensitive to the atomic ratio of Cu to O in the films and the mobility to the grain size.

The leaching of chalcopyrite with ferric sulfate

The leaching kinetics of natural chalcopyrite crystals with ferric sulfate was studied. The morphology of the leached chalcopyrite and the electrochemical properties of chalcopyrite electrodes also were investigated. The leaching of chalcopyrite showed parabolic-like kinetics initially and then showed linear kinetics. In the initial stage, a dense sulfur layer formed on the chalcopyrite surface. The growth of the layer caused it to peel from the surface, leaving a rough surface. In the linear stage, no thick sulfur layer was observed. In this investigation, chalcopyrite leaching in the linear stage was principally studied. The apparent activation energy for chalcopyrite leaching was found to range from 76.8 to 87.7 kJ mol−1, and this suggests that the leaching of chalcopyrite is chemically controlled. The leaching rate of chalcopyrite increases with an increase in Fe(SO4)1.5 concentration up to 0.1 mol dm−3, but a further increase of the Fe(SO4)1.5 concentration has little effect on the leaching rate. The dependency of the mixed potential upon Fe(SO4)1.5 concentration was found to be 79 mV decade−1 from 0.01 mol dm−3 to 1 mol dm−3 Fe(SO4)1.5. Both the leaching rate and the mixed potential decreased with an increased FeSO4 concentration. The anodic current of Fe(II) oxidation on the chalcopyrite surface in a sulfate medium was larger than that in a chloride medium.

A kinetic study on nonoxidative dissolution of sphalerite in aqueous hydrochloric acid solutions

The nonoxidative leaching of sphalerite in aqueous acidic solutions was studied from a kinetic point of view. Also the selective nonoxidation leaching in a hydrochloric acid solution containing a large amount of sodium chloride was examined for a Pb-Zn sulfide bulk concentrate. The dissolution rates of sphalerites from five different mines appeared to be controlled by a chemical reaction on the surface of sphalerite. The dissolution rate of sphalerite is of the first order with respect to the hydrogen ion activity of the solutions. It is also considerably affected by the iron content of the sphalerite sample; a linear relationship was observed between iron content of the sphalerite and its dissolution rate. The addition of sodium chloride to the hydrochloric acid solutions greatly enhanced dissolution rates. Compared to the dissolution rates of galena, which were reported in a previous paper, the dissolution rates of sphalerite were found to be far slower. The difference in the dissolution rates between these two minerals becomes greater with the addition of sodium chloride to the hydrchloric acid solutions. Based on these findings, the selective leaching of Pb-Zn bulk concentrate in a hydrochloric acid solution containing a large amount of sodium chloride was examined. The experimental results clearly showed that the galena was selectively leached, leaving a residue of sphalerite.

Electrodeposition of CdTe Films from Ammoniacal Alkaline Aqueous Solution at Low Cathodic Overpotentials

Cathodic electrodeposition of CdTe films was studied using aqueous ammonia-alkaline electrolytic baths (pH 10.7; temperature 343 K) in which Cd(II) and Te(IV) species were dissolved to form Cd(NH{sub 3}){sub 4}{sup 2+} and TeO{sub 3}{sup 2{minus}} ions, respectively. From the solution, 60 mM Cd(II)-10 mM Te(IV)-4.0 M NH{sub 3}-1.0 M NH{sub 4}{sup +} (M = mol dm{sup {minus}3}), a flat and smooth polycrystalline CdTe film (thickness, ca. 1 {micro}m) with nearly stoichiometric composition was deposited at a constant cathode potential, ranging from {minus}0.70 to {minus}0.30 V vs. SHE, whereas dendrite CdTe accompanying elemental cadmium was obtained at {minus}0.80 V. The deposition behavior was fully explained by an underpotential deposition mechanism taking the calculated redox potentials of Te{sup 0}/Te{sup IV}O{sub 3}{sup 2{minus}} and Cd{sup 0}/Cd{sup II}(NH{sub 3}){sub 4}{sup 2+} pairs into consideration. During electrodeposition of nearly stoichiometric crystalline CdTe, the current density was decreasing monotonously.

Dehydration behavior of the superprotonic conductor CsH2PO4 at moderate temperatures: 230 to 260 °C

The dehydration behavior of caesium dihydrogen phosphate CsH2PO4 was investigated in the temperature range of 230 °C to 260 °C under high humidity, conditions of particular relevance to the operation of fuel cells based on this electrolyte. The onset temperature of dehydration was determined from changes in ionic conductivity on heating and confirmed by weight change measurements under isothermal conditions. The relationship between the onset temperature of dehydration (Tdehy) and water partial pressure (pH2O) was determined to be log(pH2O/atm = 6.11(±0.82) − 3.63(±0.42) × 1000/(Tdehy/K), from which the thermodynamic parameters of the dehydration reaction from CsH2PO4 to CsPO3 were evaluated. The dehydration pathway was then probed by X-ray powder diffraction analysis of the product phases and by thermogravimetric analysis under slow heating. It was found that, although the equilibrium dehydration product is solid caesium metaphosphate CsPO3, the reaction occurs via two overlapping steps: CsH2PO4 → Cs2H2P2O7 → CsPO3, with solid caesium hydrogen pyrophosphate, Cs2H2P2O7, appearing as a kinetically favored, transient phase.

Concentration of uranyl sulfate solution by an emulsion-type liquid membrane process

Abstract The applicability of the emulsion-type liquid membrane process using tri-n-octylamine (TNOA) as an extractant to concentrate uranium from sulfuric acid solutions was investigated. It was found that this liquid membrane process yields much higher extraction percentages than the conventional solvent extraction process using the same amount of TNOA. The liquid membrane process is particularly suitable for dilute solutions such as leach and wash solutions of uranium ores. Effects of various factors on the stability of emulsions and the extraction rate and percent extraction of U(VI) were examined. Among various agents tested, Na 2 CO 3 was found to be preferable to strip sulfato-complexes of U(VI) in the internal phase. Batch-type extractions of U(VI) by the emulsion-type liquid membrane method were performed to simulate a two-stage counter current extraction, and the experimental results suggest that the U(VI) concentration in the final raffinate can be lowered to below 1 g m −3 when the feed solution containing 1 kg m −3 U(VI) is treated by a two-stage extraction.

Acid dissolution of cupric oxide

The rates of dissolution of synthetic cupric oxide in solutions containing perchloric, sulfuric, nitric or hydrochloric acid were studied using sintered disks. In each case, the dissolution rate increased with elapsed retention time until an essentially constant value was reached. This phenomenon can be attributed to an increase in the disk’s effective surface area. The dissolution rate is of the first order with respect to aH+ for perchloric, nitric, and hydrochloric acids, while it is of a half order for sulfuric acid. High activation energies, ranging from 12.4 to 20.5 kcal/mol, and the independence of agitation speed on cupric oxide dissolution reaction rate suggest that chemical reactions are the major determinants of dissolution rates. The addition of electrolytes having anions common with the acids resulted in an acceleration of the dissolution rate due to increases in aH+ values. However, the addition of electrolytes of noncommon anions revealed a quite different effect on dissolution rate. This suggests that the adsorption and/ or complexing of anions on the cupric oxide surface may have had a significant role in the determination of the dissolution rates. The type of acid used determined the identity of the adsorbed anion.

Potential‐pH Diagram of the Cd ‐ Te ‐ NH 3 ‐ H 2 O System and Electrodeposition Behavior of CdTe from Ammoniacal Alkaline Baths

A potential‐pH diagram of the system was constructed based on diagrams of the and systems and discussed in connection with the redox behavior of an ammonia‐alkaline CdTe electrolytic bath with pH 10.7. CdTe has a wide domain of stability throughout the acidic and alkaline regions, and the redox behavior was well explained with the diagram. The diagram indicated that the cathodic electrodeposition of CdTe occurs across a domain of stability of tellurium metal, i.e., at lower potentials than the deposition potential of bulk Te and higher than that of bulk Cd, with respect to the bath with pH < ca. 11.5, while in the higher pH region, CdTe is expected to deposit directly from Te(IV) and Cd(II) ions. The deposition mechanism is considered as follows: (i) deposition of tellurium layer followed by (ii) an immediate underpotential deposition of Cd on it, which prevents the bulk deposition of tellurium. It can be considered that the stoichiometric CdTe is more easily electrodeposited from alkaline baths, since the domain for tellurium metal is narrower in the alkaline region compared to the conventionally employed acidic region with pH 0–2. Therefore, the bulk deposition of elemental tellurium is less apt to occur from an alkaline bath.

Electrical Properties of CdTe Layers Electrodeposited from Ammoniacal Basic Electrolytes

CdTe is a promising material for solar cell application because its bandgap of 1.44 eV at room temperature is suitable for energy conversion from sunlight to electricity. In addition to dry processes such as screen printing and close-spaced sublimation, electrodeposition 1-3 has been investigated for the preparation of polycrystalline CdTe layers, and thin layered n-CdS/p-CdTe heterojunction solar cells have already been manufactured industrially. Although aqueous sulfate electrolytes with pH 1-2 have historically been studied for CdTe electrodeposition, we have proposed that ammoniacal basic aqueous electrolytes are also suitable, because basic solutions have a relatively high solubility of Te ~IV! species. 4-9 From the ammoniacal basic electrolytes, we successfully obtained smooth and flat polycrystalline CdTe deposits with a nearly stoichiometric composition at potentials positive to that of bulk-Cd deposition. 6 Furthermore, it turned out that the deposition rate was considerably increased by photoirradiation of the cathode surface during the electrodeposition. 10 The mechanism of the CdTe deposition is considered to be ~i! cathodic electrodeposition of surface tellurium atoms (TeO32 1 6H 1 1 4e ! Te(ads) 1 3H2O), followed by ~ii! an adsorption of Cd~II! ions on the tellurium, and ~iii! underpotential deposition of the Cd~II! ions to form CdTe @Cd(II) 1 Te(ads) 1 2e ! CdTe#. 9 It is known that electrodeposited CdTe from acidic sulfate electrolytes without intentional doping are n-type and that a heattreatment in air is necessary to obtain p-type. If as-deposited p-type CdTe can be obtained by electrodeposition from aqueous electrolytes without the heat-treatment process, it will lead to a less energyconsuming fabrication of p-n junction solar cells. Therefore, it is important to investigate the electrical properties, including conduction type, of electrodeposited CdTe. Generally, undoped CdTe with a nearly stoichiometric composition is known to have a high resistivity, or a low carrier density. We previously tried conventional MottSchottky plots using CdTe layers electrodeposited from an ammoniacal basic electrolyte to determine the majority carrier type and carrier density of the CdTe. However, reliable data could not be derived from the results, since the CdTe/electrolyte interface capacitance was almost constant and independent of the electrode potential. This is attributable to a high internal resistivity of the CdTe layer. In the present study, we tried to determine the majority carrier type of the electrodeposited CdTe layer by means of photoelectrochemical investigation using an aqueous solution containing sulfite ion as a hole scavenger and, then, resistivity and Hall effect measurements were carried out. The Hall effect measurement is a standard, reliable, and more direct method for obtaining fundamental electrical properties such as carrier type, carrier density, and mobility. Nevertheless, limited numbers of Hall effect measurements on electrodeposited materials have so far been carried out. As for the electrodeposited undoped CdTe, for example, the Hall effect for the deposits from an organic bath 11,12 has only been reported and there have been no papers regarding that from aqueous media. A possible reason for there being few reports is that the sample preparation for Hall effect measurements is difficult because the conducting substrate must be removed from the electrodeposited layer, while the CdTe layer is maintained intact. In the present work, we employed a method in which the CdTe layer was mechanically transferred from the conducting substrate onto a nonconductive epoxy resin without the formation of cracks. Experimental

A kinetic study of nonoxidative dissolution of galena in aqueous acid solution

This paper presents the results obtained through a kinetic study of the nonoxidative dissolution of natural galena in hydrochloric acid and perchloric acid solutions with and without the addition of sodium chloride. Under the experimental conditions employed in this study, the dissolution rates were controlled by a chemical reaction on the surface of the galena sample. The galena dissolution rate is of the first order with respect to hydrochloric ion activity in hydrochloric acid and perchloric acid solutions. The addition of sodium chloride to the acid solutions greatly enhanced the dissolution rate. The effect of sodium chloride has two possible interpretations: First, it may be the result of an increase in hydrogen ion activity. Second, the enhancement of the dissolution rate observable at high sodium chloride concentration may be due to the specific adsorption of chloride ions or the surface complexing of chloride ions on the galena surface.