DEVELOPMENT OF A CAPSULE-TYPE ELECTROCHEMICAL

A ZrO 2 -based solid-electrolyte ceramic was developed in the course of engineering sensors. This ceramic is characterized by high strength and thermal shock resistance. This ceramic was used to fabricate sensitive elements in the form of capsules. Sital was developed for an air-tight and strong ceramic‐metal bond. Different modifications of sensors for the thermodynamic activity of oxygen for different conditions were designed and built. The sensor was certified by Gosstandart of Russia, entered into the State registry of means for performing measurements, and approved for application. Interest in lead‐bismuth and lead coolants has increased in the last ten years. The content of dissolved oxygen in lead or lead‐bismuth melt determines the direction of the basic physicochemical processes associated with the stability of structural steel and formation of solid oxide products on various surfaces, including heat-transfer surfaces, so that attention is focused on measuring the thermodynamic activity of oxygen. The sensors developed previously in  do not meet the requirements of the new problems with respect to certain characteristics: working temperature range, thermal shock resistance, strength, and other characteristics. The technology for fabricating them is complicated and largely lost. A complex of scientific investigations involving monitoring oxygen impurity in the coolant must be performed in order to implement new designs. In this connection, it is important to develop simple, inexpensive, and reliable sensors for the thermodynamic activity of oxygen that can withstand high temperatures, thermal shocks, and hydraulic loads. To develop a new sensor, it is necessary to develop for the ceramic sensitive element a material which has suitable electrophysical, mechanical, and thermal properties and which can be fabricated efficiently. The form must be chosen in accordance with prescribed mechanical loads and a method must be developed for air-tight and thermally stable assembly. The construction and materials must satisfy the following requirements: the measurement range for oxygen activity a = 1‐10 ‐7 ,w orking temperature range 623‐923 K, working pressures up to 1.5 MPa, heat shocks 20‐100 K/sec, and coolant velocity up to 1 m/sec. Development of the Electrolyte Material. Solid electrolytes operating at high temperatures under heat and hydraulic shocks in lead and lead‐bismuth melts must possess stable electrophysical and mechanical properties and they must withstand, without breaking down, thermal stresses with severalfold variations of the temperature and low gas permeability during their entire service life with multiple thermocycles. The first two characteristics are influenced primarily by the electrolyte composition, while gas permeability depends primarily on the method used to prepare the ceramic. Ceramic materials based on uranium dioxide are now available. Examples are ZrO 2 ‐Y 2 O 3 . These materials are completely stabilized, contain only a cubic phase, and are used as solid electrolytes in metal melt. However, they have a low strength and are sensitive to thermal shocks. Ceramic based on partially stabilized zirconium dioxide, containing a mixed crystalline phase (monoclinic, tetragonal, and cubic), possesses high strength and stability to thermal shocks. The thermal shock resistance of the ceramic is determined to a large extent by its microstructure. There are several methods for increasing the thermomechanical stability of ceramic materials ‐ combining coarsely and finely-dispersed compo