Norihiko Fukatsu

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.

Electronic conductivity measurements of 5 mol% TiO2-doped YSZ by a d.c.-polarization technique

The partial conductivities of electrons and holes in yttria stabilized zirconia doped with 5 mol% of TiO2 (5TD-YSZ) have been measured by the d.c.-polarization method using Hebb-Wagners asymmetric cell. The partial conductivities of holes, σp, and electrons, σn, were found to be proportional to po214 and po2−14, respectively, except for σn in the low po2 regime. The σn values in 5TD-YSZ were greater than those in non-doped ones reported by several investigators. In contrast, the σp values in 5TD-YSZ were almost the same with those in the non-doped ones. The activation energy for σn at the fixed po2 of 1.0 atm in 5TD-YSZ was smaller than that of the non-doped one, while that for σp in 5TD-YSZ was almost the same.

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.

Electrochemical Determination of Standard Gibbs Energies of Formation of Rare‐Earth Oxysulfides and Oxysulfates

Mesures de 1200 a 1500 K, en utilisant la pile: Au, oxysulfure, oxyde ou oxysulfate, gaz S-O/ZrO 2 (+CaO)/air, Pt