Teruo Ohashi

Hydrogen sensor for molten metals usable up to 1500 K

Abstract A new hydrogen sensing system for molten metals usable up to 1500 K was developed by using two sort of galvanic cells employing proton conducting oxide CaZr0.9In0.1O3−δ and stabilized zirconia Zr0.91Mg0.09O2−δ respectively as the electrolytes. The system is founded on the principle that the accurate value of hydrogen potential at the measuring electrode of the galvanic cell based on CaZr0.9In0.1O3−δ can be obtained theoretically from its emf and the value of oxygen potential at the same electrode which is determined by an auxiliary galvanic cell based on Zr0.91Mg0.09O2−δ. The validity of the system was confirmed by experiments in a closed chamber using a controlled atmosphere. The performance of the system was checked in the practical melting processes of several industrial plants. These results showed that the system was very useful to monitor the hydrogen potentials in the plants.

Measurements of hydrogen permeation through fused silica and borosilicate glass by electrochemical pumping using oxide protonic conductor

Abstract The hydrogen diffusion parameters of fused silica and bolosilicate glass in the temperature range from 673 to 1073 K and 673 to 773 K, respectively, were studied by a measuring apparatus developed based on the coulometric titration of an electrochemical cell made of an oxide protonic conductor. The measured values were in good agreement with those reported in the literature. From the hydrogen activity dependencies of both permeability and diffusibility, it was concluded that in fused silica and borosilicate glass, most of the hydrogen permeation takes place in the form of hydrogen molecules passing through the large holes induced in the structure with random arrangements of SiO 4 tetrahedra. The solubility in the form of hydrogen molecules estimated by the permeability and diffusion coefficient values was two orders of magnitude smaller than that of hydrogen dissolved in the form of hydroxyl radicals. The migration rate of the hydrogen in the form of hydroxyl radicals is far smaller than the movement of hydrogen molecules. This fact is also suggested by the small H/D substitution rate observed by infrared absorption spectra.

Proton conductors of oxide and their application to research into metal-hydrogen systems

Abstract Hydrogen dissolves into oxides. In some acceptor-doped perovskite-type oxides, the solubilities rise to levels as much as some mole%. In these oxides, protonic conduction is observed within a certain temperature range. They may be used as proton conductive solid electrolytes at high temperature. In the present paper, the mechanism of hydrogen incorporation of these oxides is discussed on the basis of the thermodynamics of crystal imperfections. Further, the following important applications of these oxides to electrochemical devices for research into metal-hydrogen systems are shown; (1) a device for the real-time determination of hydrogen potentials in liquid metal, (2) a device to measure hydrogen contents in quenched metal, (3) a device to determine permeabilities and diffusivities of hydrogen in the materials. The significance of the phenomenon of protonic conduction in oxide is also discussed with reference to the absorption of hydrogen by metals.

Incorporation of hydrogen into magnesium aluminate spinel

In order to examine the possibility of using a spinel-type oxide as the base material of the high-temperature proton conductor, thephenomenonofincorporationofhydrogenintoasinglecrystalofstoichiometricandalumina-richmagnesiumaluminatespinel Mg1 � xAl2O4 � x (x=0, 0.1, 0.2, 0.3) was studied using IR absorption analysis. In the temperature range of 1373–1673 K, the reversible dissolution of hydrogen from the surrounding atmosphere was ascertained from the IR spectroscopy based on the absorption band attributed to the OH stretching vibration of the quenched samples. Using the data for the diffusion coefficient of hydrogen determined by the relaxation process of the incorporation and for that of the solubility of hydrogen, the proton conductivity was estimated to be more than two orders of magnitude lower than the total conductivity. It was found that the dominant charge carrier was a magnesium ion vacancy and that the contribution of a proton to the conductivity was very small. A new picture on the defects in the alumina-rich spinel was also given based on the data for hydrogen incorporation. D 2002 Elsevier Science B.V. All rights reserved.

Measuring apparatus for hydrogen permeation using oxide proton conductor

Abstract A new type of measuring apparatus for hydrogen permeation has been developed based on coulometric titration utilizing the electrochemical cell: (ref.:-)Pt, Ar + 1% H 2 / CaZrO 3 (+ In 2 O 3 )/ Ar + H 2 , Pt (+:mes.). This is a modified apparatus of the hydrogen analyzer developed previously by the present authors. A very small permeation of hydrogen (to the order of 10 −12 mol/s(H 2 )) could be measured by this apparatus. A permeability of hydrogen of silica glass was measured at temperatures between 773 and 1073 K. The measured values were close to the previous values reported by Lee et al.

Decomposition characteristics of Al-Mn-Zr alloys rapidly-quenched from the melt

The rapidly-quenched structures of liquid Al-Mn-Zr ternary alloys and Al-Mn binary alloys and their decomposition behaviors were investigated by hardness tests, X-ray diffraction analyses, and TEM observations. The solid solubility of Mn can be extended to about 10 wt pct irrespective of whether the alloys contain 1 wt pct Zr or not. The solidification structures are composed of fine dendritic cells and with increasing Mn content, interdendritic precipitates gather volume and branch out into the cell grains. The decomposition of rapidly-quenched alloys takes place during aging for 1 hour at temperatures of 300 to 350 °C, and the dependence of the decomposition temperatures on the Mn content varies in this range. Precipitation hardening of the ternary alloys is intensified by the addition of Mn up to about 7 wt pct and proceeds in a two-step manner during aging in the temperature range examined (350 to 450 °C). It is suggested that the precipitates contributing to the maximum hardening are pseudomorphous to the Al6Mn equilibrium phase.

Hydrogen concentration cell using α-Al2O3 as a solid electrolyte

Abstract The following gas concentration cell was constructed and the electromotive force was measured, ( ref .,−) Pt ,p H 2 ′,p O 2 ′| α - Al 2 O 3 |p H 2 ″,p O 2 ″, Pt (+, work .) When a difference in oxygen partial pressures was applied to the above cell while maintaining the hydrogen partial pressure on both electrodes at the same value, no significant electromotive force was observed in the experimental temperature range (1273–1673 K). On the application of a difference in hydrogen partial pressures while maintaining both oxygen partial pressures constant, the electromotive force was not observed at high oxygen partial pressure conditions. At low oxygen partial pressure conditions, however, a stable electromotive force depending on the difference in hydrogen partial pressures was clearly observed. The relation between the electromotive force and the hydrogen partial pressures is represented by the following Schmalzried-type equation, E=− RT F ln p″ H 2 1/2 +A p′ H 2 1/2 +A where A is a parameter which depends only on the temperature. In this study, it was clarified that α-Al 2 O 3 could be used as a solid electrolyte for a galvanic cell-type hydrogen sensor at elevated temperature.

Analysis of Defect Structure of the Proton-Conducting Oxide CaZr0.9In0.1 O 3 − δ by a DC Polarization Technique

In order to clarify the defect structure of the proton-conducting oxide, CaZr 0.9 In 0.1 O 3-δ , the dc polarization characteristics of the following cell (irreversible:-)Pt,Ar + H 2 (O 2 ) + H 2 O/CaZr 0.9 In 0.1 O 3-δ /Ar + H 2 (O 2 ) + H 2 O,Pt(+:reversible) was studied at 1173 K and a defect model for this oxide was derived from the relation between the applied voltage and the external current and its dependence on the surrounding atmosphere. Using the equilibrium constants of the defect reactions and the mobility of the oxide ion vacancy as fitting parameters, a simulation of the measured current-voltage characteristics was carried out. It was found that an excellent simulation was possible based on the proposed model. The solubility of the proton in moles per one mole of unit formula was estimated to be about 0.0089 under the condition p H2O = 0.01 and r O2 = 1 at 1173 K by this calculation (p x is the activity of gas species X, represented by the ratio of partial pressure of X to the standard pressure, 101,325 Pa). This value is significantly larger than what has been supposed for this oxide. Therefore, the assumption that the hydrogen solubility is very small and the excess charge of the dopant is compensated mainly by oxide ion vacancies was found as a valid first approximation only in the relatively higher temperature regions.

Effects of solidification control on structure and mechanical properties of cast Zn-12Al-xCu alloy blocks

ZA12 alloy is known as a high strength version of zinc alloys; however, it is difficult to produce sound cast blocks from the alloy for moulding tools by the conventional sand cast method because o...