稀土—钡—钴—氧材料的氧扩散与电输运性能研究

【作者】 郝好山；

【导师】 胡行；

【作者基本信息】 郑州大学 ， 凝聚态物理， 2007， 博士

【摘要】 非化学计量性氧化物很多独特的特性使得它们受到了广泛研究。氧空位的出现导致氧离子能够在材料表面和内部以一定的速率吸附、脱附或扩散，因此，非化学计量性氧化物在气体分离膜，固体氧化物燃料电池，氧传感器，以及储氧、产氧等领域被广泛应用。先前的研究表明，双层钙钛矿结构材料RBaCo₂O_5+δ（R代表稀土元素）具有高电导率和快速氧扩散能力，在固体氧化物燃料电池以及透氧膜等领域具有潜在的应用价值。在本文中，我们选取了三个典型的双层钙钛矿结构氧化物RBaCo₂O_5+δ（R=Pr，Gd，Y），利用热重分析方法研究了它们的氧扩散行为，以获得其透氧性能、氧的脱附与吸附速率常数，以及氧空位浓度差之间的关系。实验结果表明，RBaCo₂O_5+δ材料的氧吸附速率常数k_α明显大于氧脱附速率常数k_d。与立方结构的钙钛矿材料相比，双层钙钛矿材料具有较大的氧吸附与脱附速率，这表明它们在气-固界面具有快速的氧交换能力。但是，它们在氧气与氮气下的质量差△δ／V_mol明显小于立方钙钛矿材料，导致双层钙钛矿材料的透氧量J_O2与立方钙钛矿材料基本相同。高的氧吸附与脱附速率常数表明双层钙钛矿材料是一种有前景的催化材料。它可以用作膜修饰材料，通过提高固相与气相间的氧交换速率来提高透氧膜的透氧量。RBaCo₄O₇（R表示稀土元素）是新近合成的一类氧化物材料。与一般的氧化物不同，RBaCo₄O₇显示出一种独特的氧吸附与脱附特性。当在含氧气氛中升温时，RBaCo₄O₇会经历两个氧吸附和脱附过程。其中一个大约在200～450℃之间，另一个在660～1050℃之间，氧量的变化会引起样品约4％的质量变化。这一发现意味着RBaCo₄O₇材料有可能在气体分离、储氧、产氧等涉氧领域获得应用。XRD结果显示，低温时所吸附的氧离子并没有明显改变其晶体结构，而高温时所吸附的氧离子几乎完全破坏了RBaCo₄O₇结构，使其分解为其它相。在被吸附的氧释放后，所有样品又能恢复到原来的结构。RBaCo₄O₇的氧吸附性能与其晶格结构以及Co离子的可变价性密切相关。元素替代的研究表明，由于Zn离子的不可变价性，Zn部分替代Co抑制了RBaCo₄O₇的氧吸附性能；由于Fe离子具有和Co离子相似的变价能力，Fe部分替代Co对RBaCo₄O₇的氧吸附性能几乎没有影响。电输运性能的研究表明，RBaCo₄O₇样品的电阻率随温度的升高而减小，表现为半导体导电特性。塞贝克系数为正值，说明RBaCo₄O₇是p型半导体，空穴是主要载流子。RBaCo₄O₇样品的导电机制可以确定为小极化子跃迁导电。在含氧气氛中，氧吸附与脱附显著影响了RBaCo₄O₇的电输运性能。氧吸附增大了空穴浓度，导致电阻与塞贝克系数降低；氧脱附减小了空穴浓度，导致电阻率和塞贝克系数增大。Zn部分替代Co增大了YBaCo_4-xZn_xO₇样品的电阻率但降低了塞贝克系数。由于Zn替代Co不会导致空穴浓度的下降，因此Zn替代样品的载流子迁移率应随Zn含量增加而降低。此外，Zn部分替代Co使得晶格参数增大，这就导致空穴跃迁所要克服的能量势垒随之增大，因此电导活化能随着Zn含量的增加而增加。功率因子的计算表明RBaCo₄O₇是性能较好的氧化物热电材料，其热电性能值得进一步研究。在较低温度下RBaCo₄O₇巨大的氧吸附与脱附能力使其具备了作为脱氧剂进行氮气纯化的条件。本文中，我们选取了YBaCo₄O₇作为脱氧剂材料进行了氮气纯化的性能研究。小批量的实验表明，在500℃脱氧后，1kg YBaCo₄O₇脱氧剂在300℃工作温度下，能够把160L纯度为98.6％的普氮转化为纯度大于99.9999％的高纯氮气。双层钙钛矿材料RBaCo₂O_5+δ具有较快的氧表面交换速率和较大的可变氧量，这意味着它同样可能是一种潜在的脱氧剂材料。实验表明，在600℃脱氧后，1kgYBaCo₂O_5+δ脱氧剂在300℃工作温度下，能够把300L纯度为98.1％的普氮转化为纯度大于99.9999％的高纯氮气。进一步的研究有可能把这两类氧化物开发成为新一代的无氢脱氧剂材料。更多还原

【Abstract】 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.更多还原