No abstract

The Eastern Institute of Refractories has designed a resistance heater from stabilized zirconium dioxide for use in oxidizing atmospheres at temperatures of up to 2500~ [1][2][3], ana has designed furnaces based on it [4][5][6][7].These furnaces are particularly valuable for firing ceramics, growing single crystals, and forming quartz fiber [8][9][10].Experience with these furnaces has shown that to reduce the inertia and increase the working reliability of the heaters it is extremely important to use linings which possess a range of properties which are, in fact, very difficult to combine: application temperature up to 2500~ high resistance to creep, good thermal-shock resistance, and low thermal conductivity and specific heat.Materials based on magnesium and aluminum oxides are unsuitable for these purposes, so zirconia was used to make high-temperature insulation; this has a high refractoriness and low thermal conductivity.Previously [Ii], it was shown that stabilized zirconia obtained by the combined precipitation of components from aqueous solutions, together with a high homogeneity and sintering activity, possesses cementing properties, which means it can be used for making refractory concretes.The technology involved the use of the principles for obtaining porous ceramic concretes [12,13], foamed ceramic from Zr02 [14], and the use of foamed polystyrene to obtain spherical pores in the ceramic products [15].The technology for producing porous ceramic concrete from zirconia is shown in Fig. i. To produce the filler, the foam briquets, made of Zr02 by a method described in [16], were ground in a jaw crusher, and the crushed material was screened into fractions of large and fine fillers.The suspension was prepared from powdered Zr02 stabilized with 15% (weight parts) Y203, obtained by the combined precipitation of components from solutions of chlorides [ii]. The suspension has a particle size of the order of micrometers, with a density of 2.3-2.6 g/cm 3 and pH = 2.3. The polystyrene grade PSV was foamed in 40 min in boiling water.

No abstract

Zirconium dioxide stabilized with yttrium oxide is used to obtain refractory ceramics for various purposes [i]. It is obtained by mixing the components, followed by calcination, ensuring the formation of a solid solution of Y203 in Zr02. To obtain highly homogeneous material we blend solutions of the salts, followed by combined precipitation of the components, normally in the form of hydroxides [2].The high cost of yttrium compounds makes it necessary to carry out combined precipitation with zirconia in a schedule ensuring the minimum loss of this component.We carried out a study to determine the parameters of such a schedule.The method was as follows.In a solution containing 0.5 mole/liter of ZrOCI2 and 2 mole/liter of HCI, we dissolved YCI3 in such quantity that its concentration in the solution was 0.iiii mole/liter.To the resulting solution we added, with continuous stirring, a 12% solution of NH~OH (direct precipitation); or, conversely, to this ammoniac solution we added a solution of salts (reverse precipitation) to the attainment of the prescribed pH, which was checked with the 1-120 ionomer.The suspension was filtered on a vacuum-filter to complete suction of the mother liquor.In it we determined the content of yttrium and zirconia using the methods of [3] and [4], respectively.The results are shown in Table i.To determine the optimum final pH value with the combined precipitation of the hydroxides we calculated the outlays connected with the consumptions on compensating for the loss of yttrium and the outlay on the precipitant.The computations used the actual prices for ammonia (GOST 9-77, grade A, sort I) and yttrium oxide (sort O). The price of 1 ton of ammonia is 27.5 rubles, and yttrium oxide 135,000 rubles.The results are shown in Fig. I.The expenditures on the precipitant sharply increase if the precipitation is completed when pH > 8.5. With direct precipitation, the outlays on compensating the loss of yttrium sharply increase when pH < 8.5, and for the reverse precipitation --when pH < 7.5.Consequently, the optimum pH for direct precipitation is only 8.5, and for reverse precipitation the optimum range of pH values is 7.5-8.5 (striated region).The displacement of the curve 4 relative to curve 3 in Fig. 1 is connected with the reduction in the consumption of precipitant on account of the underprecipitation of the yttrium hydroxide (see Table I).With direct precipitation at first the zirconium hydroxide comes down into the precipitate, and then the yttrium hydroxide; accordingly, the yttrium hydroxide is dissolved earlier with the reverse precipitation.Hence, with direct precipitation a homogeneous product of coprecipitation is not obtained, since the components are thrown down one after the other.On the other hand, with the reverse variant of the process the components are precipitated simultaneously, and the range of acceptable pH values of the mother solution is extended.Therefore, it is desirable to use the reverse precipitation in technology.However, we must remember that in this version when ...

No abstract