Noriaki Kurita

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.

Hydrogen sensor based on oxide proton conductors and its application to metallurgical engineering

Galvanic cell-type hydrogen sensor for high temperature use is now available thanks to the discovery of oxide-type proton conducting solid electrolyte such as acceptor-doped perovskite-type oxides. In order to develop the practically functional hydrogen sensor based on these electrolytes, the electrochemical properties of these oxides should be understood as a function of hydrogen and oxygen potentials at the temperature range where the sensor is to be used. In this review, the properties of indium-doped calcium zirconate (represented by the composition CaZr0.9In0.1O3–δ) known as, among these oxides, the most chemically stable and mechanically strong were explained briefly on the basis of the thermodynamics of the crystal imperfections. Then the structure of hydrogen sensor for molten aluminum and that for molten copper, which were designed to make the best use of the properties of the electrolyte for the respective process conditions, were shown. Lastly, the performance of these sensors examined in the laboratory scale and the practical industrial conditions was overviewed.

Incorporation of hydrogen into magnesium-doped α-alumina

Abstract The phenomenon of the incorporation of hydrogen into a single crystal of magnesium-doped α-alumina grown by the Verneuil method was studied using IR absorption and conductivity measurements. In the as-grown sample, a large wide band of IR absorption was observed in the range 2500–3500 cm −1 . This band was attributed to the stretching vibration of the OH bond formed by the incorporated hydrogen and host oxide ion. The reversible dissolution of hydrogen was confirmed by the equilibrium experiment in the hydrogen-containing atmosphere. The equilibrium amount of hydrogen evaluated by the integral absorbance for the specimen annealed in x % H 2 O–1% H 2 –Ar was proportional to the half power of the partial pressure of water. The chemical diffusion of hydrogen was determined from the relaxation time of the dissolution phenomenon. The transport number of the proton roughly estimated from the H/D isotope effect observed in the electric conductivity showed that the proton was the major charge carrier in the specimen equilibrated with the gas mixture 20% H 2 O–1% H 2 –Ar at 1373 K. The anisotropy observed in the IR absorption was found to be due to the fact that hydrogen enters between the neighboring O–O pair with a specific spacing and makes a hydrogen bond with them. Based on the experiment using polarized light, it was concluded that, among four kinds of O–O pairs with the different oxide ion octahedron spacings, two having a large angle to the basal plane were preferred for the site of hydrogen. The anisotropy observed in the chemical diffusion coefficient of hydrogen and in the electrical conductivity were reasonably explained based on the above configuration of the incorporated hydrogen.

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.

Electrical Conductivity of Barium-Doped α-Alumina

The phenomenon of the incorporation of hydrogen into a single crystal of 0.1 mol% Ba-doped α-alumina was studied using IR absorption analysis. A large wide band of IR absorption was observed. This band was attributed to the stretching vibration of the OH bond. The electrical conductivity of the specimens has been measured by the two-probe ac technique in the temperature range 1073-1673K. The H/D-isotope effect on the conductivity was observed. It was found that the dominant charge carrier of barium doped α -alumina is proton under the reducing atmosphere containing hydrogen.