Dense sintered bodies of proton conducting BaZrO 3 (BZ) and Y-doped BaZrO 3 (BZ-Y) were obtained at 1600℃ for a short sintering time of 5 hours, by the addition of NiO as a sintering promotion agent. The relative density and grain growth of samples, Ni-doped BaZrO 3 (BZ-N) and Ni, Y co-doped BaZrO 3 (BZ-NY), were increased with increasing Ni addition. The sinterability of BZ-NY was greatly improved just to add only 0.6 mol% Ni and the relative density of this sample was more than 98%, in contrast to that of 60% at most for BZ-Y without Ni addition. Electrical conductivity of BZ-NY added Ni 1.0 mol%, BaZr 0.91Ni 0.01Y 0.08O 3-α, was more than 10 -3 S.cm -2 at 900℃ in a wet 1% hydrogen atmosphere, which value was 10 times higher than that of BZ-Y. In addition, the kind of electrical conduction carrier and an ionic transport number were also examined by employing various concentration cells. It was found that the proton conduction was dominant for both BZ-N and BZ-NY samples, although BZ-NY showed scarcely oxygenion conduction approximately 10% in a high temperature range higher than 800℃. From these results, as mall amount of Ni addition found to be effective for improvement of both the sinterability and the electrical conductivity.
The electrochemical device employing proton conducting perovskite-type oxide has a great potential for practical applications, such as a large scale fuel cell and hydrogen sensor etc. [
In this study, the sinterability and the electrical properties of BaZr1-x(Ni, Y)xO3-α co-doped with mixture of Ni and Y are investigated. Their proton conductivities are also examined.
The powder for bulk samples was synthesized by an ordinary solid state reaction method. Two kinds of sample with NiO additive representing the chemical formula of BaZr1-aNiaO3-α (BZ-N) and BaZr0.92-bNibY0.08O3-α (BZ-NY) were prepared by using commercially available BaCO3, ZrO2, Y2O3 and NiO as starting reagents. These reagents were mixed with an organic solvent by using a planetary-motion ball-milling machine. After drying at 60˚C for 30 min, the mixed powders were calcined at 1200˚C for 5 h in air. Two times of ball-milling operation was repeated to make the size of the particle smaller. By using the uniaxial pressing and cold isostatic pressing (CIP), thus prepared calcined powders were molded into a pellet of 20 mm in diameter by adding molding pressure of 200 MPa for 2 min. The green pellets were sintered at 1600˚C for 5 h in air at a heating rate of 200˚C/h from room temperature to 1300˚C and 50˚C/h to 1600˚C/h and cooling rate of 50˚C/h from 1600˚C to 1400˚C/h and 200˚C/h to room temperature. Differential thermal analysis (DTA), differential scanning calorimetry (DSC) and thermomechanical analysis (TMA) was used to study transition and melting behavior of samples at a heating and cooling rate of 10˚C/min.
After heat treatment, the crystal structure of sample was analyzed using X-ray diffractometer (XRD). The microstructural observation and the chemical composition were examined by a scanning electron microscope (SEM) and an energy dispersive X-ray analyser (EDXA). The relative density measurement by the Archimedes method was performed as evaluation of sinterability. For the measurements of electrical properties, porous platinum mesh was attached on both sides of pellet using Pt paste as electrodes. Electrical conductivities were measured at temperature range of 500˚C - 900˚C in wet 1% H2 + Ar atmosphere by complex impedance method. Furthermore, in order to investigate the proton contribution to conduction, an electromotive force (EMF) was also measured at temperature range of 500˚C - 900˚C by using some kind of gas concentration cell, such as a hydrogen gas concertation cell and a water vapor concentration cell.
After sintering, we examined the crystal phases of the samples by XRD analysis. The diffraction patterns indicated that all samples doped with Ni lower than 0.05 mol showed highly crystalline BaZrO3 phase, but the reflection peaks of secondary NiO phase were also detected in samples which added Ni more than amolar ratio of 0.05. From SEM observation and EDX analysis, NiO precipitates and the related Ni-rich regions were confirmed on surface of these samples. Therefore, samples less than Ni adding 1.0 mol% are subsequently investigated.
In the case of BZ-NY heat-treated at rather low temperature, the BZ-NY with Ni 0.006 mol showed dense structure even when at low sintering temperature around 1300˚C. Thermal analysis using TG and DSC was performed to investigate the thermal behaviour. The powdery sample was BaCO3 and NiO which were mixed by the molar ratio of 1:1. TG and DSC profiles are shown in
Although we did not show it in the figure, the following experimental result
about dispersibility and solid solubility limit of Ni was also acquired. SEM observation showed that NiO particles were dispersed in the grain boundary for BZ-N when the Ni doping level was higher than a molar ratio of 0.01. Furthermore, due to the detection of impurities phases of XRD analysis for this sample, we derived that the solubility limit of Ni to BZ sample was under 0.01 mol. Same results have been reported that a small amount of Ni addition around 0.004mol into BaCe0.9Y0.1O3-α is effective on the improvement of sinterability [
Effect of Ni addition on the electrical conductivity of BZ-N and BZ-NY was investigated.
These solid electrolytes under wet hydrogen atmosphere have the possibility of candidate of the electrical conducting carrier that H+, O2−, h+, e− and metal cations of the constituent elements, such as Ba2+, Zr4+, Y3+ and Ni2+ contribute to conduction.
To investigate what kind of electrical conductive species affects electrical conductivity, we measured EMF of hydrogen concentration cell.
To estimate proton conduction regarding ion conduction in samples, we calculated oxygen ion transport number (to) by measuring EMF of water vapour concentration cell. Proton transport number (tp) in samples is obtained from the formula (1) shown below.
tp = 100 − to (1)
The relative density and grain growth of BZ-N and BZ-NY samples were increased with increasing Ni doping. Ni addition leaded both the decrease of the grain boundary resistance and high conductivity. The relative density hither than 99% was obtained in the samples with Ni-doping in amount of more than 0.008 mol, even when the sample treated at 1600˚C for rather short time sintering. On the other hand, NiO precipitation was observed in BZ-N doped with Ni 0.01 molar ratio. The solid solubility limit of Ni is assumed to be lower than 0.01 molar ratio.
The EMF of both BZ-N and BZ-NY samples accorded with the theoretical value regardless of measurement temperature, ion conduction of proton and oxygen ion is dominant to all that in wet hydrogen atmosphere. From the measurement result of hydrogen and water vapor concentration cell, BZ-N electrolyte indicates the proton transport number higher than 95%. It is suggested that the proton conduction is dominant for both BZ-N and BZ-NY samples, though BZ-NY has a few oxygen conduction at high temperature range higher than 800˚C.
Morishita, H., Ikebe, Y. and Ban, E. (2018) Proton Conductivity of Ni, Y Co-Doped BaZrO3. Journal of Materials Science and Chemical Engineering, 6, 19-27. https://doi.org/10.4236/msce.2018.66002