Re - Ba - Co - O Materials, Oxygen Diffusion And Electrical Transport Properties

Nonstoichiometric oxides have been extensively studied due to their much unique properties. The existence of oxygen vacancies makes it possible that oxygen ions can be adsorbed into or desorbed from the surface of materials and diffuse in the bulk materials. Therefore, nonstoichiometric oxides have wide application in many fields such as gas separation membranes, solid state oxide fuel cells, and oxygen sensors.Previous studies have shown that double perovskite oxides RBaCo₂O_5+δ (R = rare earth element) have the high conductivity and fast oxygen diffusion ability, which means that they have potential application in the fields such as solid state oxide fuel cell and oxygen permeation membrane. In this paper, we selected three typical double perovskite oxides RBaCo₂O_5+δ (R = Pr, Gd, Y) and investigated their oxygen diffusion behaviors with thermogravimetric analysis method. Oxygen permeationflux J_O2, oxygen adsorption and desorption rate constants k_a , k_d , and difference of oxygen vacancy in oxygen and nitrogen atmosphereΔδ/ V_mol for these sampleswere calculated from experimental data. Our results show that oxygen adsorption rate constants of RBaCo₂O_5+δ are larger than oxygen desorption rate constants. Compared with the cubic perovskite oxides, the oxygen adsorption/desorption rate constants of these double perovskite oxides are markedly increased, indicating fast oxygen exchange ability for gas-solid interface. Since the difference of oxygen vacancy in oxygen and nitrogen is smaller than the commonly used cubic perovskite materials, their oxygen permeation flux is only approximately equal to that of cubic perovskite materials. Whereas, the large oxygen adsorption/desorption rate constants of these double perovskite oxides suggest that they are potential catalytic coating materials and can be used as surface modified materials on other membranes surfaces to improve the oxygen permeability by improving the velocity of oxygen between solid and gas phases. Recently, a new class of oxide RBaCo₄O₇ (R = rare earth element) has been synthesized. RBaCo₄O₇ has interesting oxygen diffusion properties. When heated in oxygen flow, it experiences two oxygen adsorption and desorption processes. One is in the temperature range of 200～450℃, and the other is 660～1050℃. The amount of changeable oxygen accounts for about 4% of the weight of the sample. This discovery means that the kind of materials may be used in the fields related to oxygen such as gas separation, oxygen storage, and oxygen supply. XRD results show that the oxygen ions adsorbed at the lower temperature do not change the crystal structure significantly and the oxygen adsorbed at the higher temperature destroys the structure and RBaCo₄O₇ decompose into other phases. All samples can be recovered to their original structures after releasing the adsorbed oxygen. The special oxygen adsorption properties of RBaCo₄O₇ should related to its special crystal structure and the changeable valence of Co ions. Zn substituted for Co will prevent oxygen adsorption because of unchangeable valence of Zn ion. Co substituted by Fe has little effect on the oxygen adsorption processes because Fe ion has similar changeable valence to Co ion.The studies on the electronic transport properties of RBaCo₄O₇ samples show that electrical resistivity is reduced with the increase of temperature and shows a typical semiconducting behavior in the investigated temperature region. Seebeck coefficients of all samples are positive in the whole temperature range measured, indicating p-type semiconductors, i.e., holes are major carrier. The conduction mechanism of RBaCo₄O₇ may be determined as a small polaron hopping model. When measured in the oxygen-containing atmosphere, oxygen adsorption and desorption influences the electronic transport properties of RBaCo₄O₇ markedly. Oxygen adsorption increases hole concentration, which results in the decrease of resistivity and Seebeck coefficients. However, oxygen desorption decreases hole concentration and resistivity and Seebeck coefficients rise consequently. Zn partially substituted for Co increases resistivity of YBaCo_4-XZn_XO₇, but Seebeck coefficient is reduced. Since Zn substituted for Co does not decrease hole concentration, so the carrier mobility should decrease with the increasing Zn concentration. Moreover, when Zn partially substituted for Co in the lattice, the cell parameters increase with the increasing Zn concentration and the distance of hopping also becomes larger, which increases the barrier height encountered by the hopping holes. The activation energy is, therefore, expected to increase with increasing Zn concentration. Power factors calculated from the data are large, indicating that RBaCo₄O₇ is potential oxide thermoelectric materials. Further study is needed to improve its thermoelectric performance.The large oxygen adsorption and desorption capacity at low temperature indicates that RBaCo₄O₇ is potential deoxidizer materials for nitrogen purification. In this paper, we selected YBaCo₄O₇ and investigated its performance as deoxidizer for nitrogen purification. Our results show that, after releasing oxygen at 500℃, 1kg YBaCo₄O₇ deoxidizer can transform about 160L nitrogen with purity of 98.6% to high purity nitrogen (purity >99.9999%) when work temperature is 300℃. Similarly, double perovskite oxides RBaCo₂O_5+δ are also potential deoxidizer materials because they possess fast surface oxygen exchange rate constant and large changeable oxygen content in the lattices. Experimental results show that, after releasing oxygen at 600℃, 1kg YBaCo₂O_5+δ deoxidizer can transform about 300L nitrogen with purity of 98.1% to high purity nitrogen (purity >99.9999%) when work temperature is 300℃. It is possible that the two kinds of oxides can be developed to new hydrogen-free deoxidizer. Of course, further study is necessary to achieve this aim.