Baroemf in O₂ – ZrO₂ – YBa₂CuO_7-x (YBCO) system

Rahimbekov A.Zh., Azimzhan A.N.

I.Zhansugurov University, 040008 Taldykorgan, Kazakhstan

We have studied baroemf of Pt, O₂ │ZrO₂│Pt, O₂, YBCO cell induced by load applied to YBCO sample in equilibrium with one of the gas electrodes. Superionik - this is a solid superionic compound, one of the sublattices, which, figuratively speaking, melted. Issue is devoted to the superionic state is now a lot of theoretical and experimental studies. More work is associated with the creation of a variety of devices that use a high ionic conductivity superionikov [1] . In the superionic state atomic lattice does not form a periodic potential inherent in the crystal. This leads to new problems still unresolved. For example, electrons and holes are not ordinary kvachastitsami and their energy states are no longer determined by conventional Brillion zone. The lattice vibrations superionika not describe, using the representation of phonons. New problems arise in the description of contact phenomena. On the border with superionika electrode, in addition to the contact potential difference, there are mechanical stresses. This fact leads to a kind of potential barrier, whose influence on ion transport and is considered in this paper [2]. It is known that YBCO compound is easily equilibrated at comparatively low temperatures with gas phase by absorption of liberation of pxygen. In this case the sample volume changes the oxygen absorption resulting in decrease an not increase of volume. Therefore, according to the Le Chatelier principle, the decrease of the sample volume induced by an external load should lead to adsorption of oxygen by the sample and6 consequently, to rise of its weight and change in chemical potential of atomic oxygen. As a result baroemf E= / 2e appears at the cell electrodes (e is the electron charge, - the increment of chemical potential of atomic oxygen in the sample and gas phase environment induced by load) [3].

Let us consider two particular cases corresponding to conditions: c>>v and v << v, where v is the gas volume around the sample and v is the change of this volume at constant gas pressure produced by the action of the load. In the first case the loading would lead to an increase of oxygen content in the sample but the oxygen pressure around the sample would remain practically constant and baroemf will be close to zero. In the second case the oxygen content in the sample would remain practically constant (since there is not enough oxygen in the cell for significant change of oxygen content to occur) but the oxygen pressure around the sample would change and baroemf appear. We shall take into account that chemical potential of the oxygen atoms in YBCO depends in the lattice constant a, b, c, which depend in their turn on the load. We shall assume also that polycrystalline sample consists of there equal parts, the direction of load action coinciding with the direction of “a” axis of microcrystal in the first part, with the direction of “b” axis in the second part, and with the direction of “c” in the third part. Then in the first approximation we can write

(1)

where is stress, //,/ components of contribution of oxygen atoms into deformation potential and da/d,db/d, dc/d - values of linear compressibility which can be obtained from experimental dependences of the lattice constants on the oxygen pressure and on stress, respectively [4]. The dependence (x) calculated from (1) has singularities at the points x=0.2 and x=0.5 connected with “phase separation” and transition to tetragonal phase, correspondingly. Experimental studies have shown that YBCO compound absorbs actually oxygen under the load. The baroemf values observed are in fair agreement with the estimates from (1)

Literature
1. Chandra S. Superionic Sol., North-Holland, 1981. 885 p.
2. Phys., Superionic Conductors / ed. M. B. Salamon, Springer - Verlag, Berlin-Heidelberg-New York, 1979. 364 p.
3. Ukshe EA, NG Bukun Solid electrolytes. Moscow: Nauka, 1977. 146 with.
4. Chebotin V., M. Perfiliev Electrochemistry of solid electrolytes. Moscow: Khimiya, 1978. 345.