Effects of some oxide additions on the thermal length changes of zirconia

Abstract

The oxides of cerium, yttrium, silicon, magnesium, and calcium were added in various proportions to zirconia of 99-percent purity, and the effects of these additions, combined with preheating at various temperatures ranging from 1,450° to 1,950° C, on thermal length changes of the zirconia during heating and cooling between room temperature and a maximum of 1,700° C were observed. The results are compared with similar observations on zirconia of 96-, of 98-, and of 99-percent purity without oxide additions.The results show that the irregular thermal lcngth changes accompanying phase transformations in zirconia may be prevented by changing the crystal to the stable cubic form. This was accomplished by (a) 11.5-and IS-percent additions of Y20a and heating at 1,700 0 C or higher; (b) 8-and 15-percent additions of MgO and heating at 1,550 0 C or higher, but only for the range from room temperature to 1,200° C; and (c) 5-, 6-, 8-, and IS-percent additions of CaO and heating at 1,550° C or higher, but additions of more than 6 percent caused the specimens to be very porous and proportionately weak. In a ll cases, however, the expansion during heating and contraction during cooling was relatively high. The coefficients of linear thermal expa nsion of the specimens that were all, or nearly all, cubic ranged from 8.8 to 11.8 X 10-6 • Also, zirconia has a low thermal conductivity relative to such materials of high thermal expansion as alumina, magnesia, and beryllia (unpublished data).

Studies of binary and ternary combinations of magnesia, calcia, baria, beryllia, alumina, thoria and zirconia in relation to their use as porcelains

“…About 14 g of the starch solution was used for 100 g of dry batch, and the forming pressure was 9,300 Ib/in 2 • Later, the method was improved by machine-blunging the slip with a chemical-Iaboratory-type stirrer, and by vibrating the dampened mixture in the steel moulds before pressing. The pressed specimens were matured in an electric furnace, heated with eight Th02-Y20a resistors, described in Bureau Research Paper RP1662 [12]. The heating and cooling rates were maintained very close to 3°0 per min, and the holding time at the maximum temperature usually was 1 hr.…”

“…It was not observed in the cubic habit. The other end members of the system, BeO and A120 3, fuse at about 2,520° 0 [16] and 2,015° ± 15° 0 [12] , respectively. The lowest liquidus point in the Th02-BeO system is reported [16] to be a.t 2,200° 0 and about 20 percent by weight of BeO.…”

“…Tetragonal zirconia in the exsolution texture may invert in part to the monoclinic form during cooling, and some amoeboid grains of tetra~onal Zr02 contain lathlike crystals of lower refractive index and higher birefringence than the enclosing grains. This evidence is assumed to indicate that solid solution and exsolution occur within the range of temperatures in which tetra.s:onal ZrOz is the stable phase, probably between 1,000° and 1,500° v [12].…”

Thermal length changes of some refractory castables

“…Expansion values above 1,000 0 C lVere extrapolated . Th e difference betw?en these l'esults and those obtained by others [4] did no t exceed 0.0002 in. jin .…”

“…Except for the usc of a differen tial transformer in place of a dial indicator, the entire assembl~' is similar to that described by Geller and Yavorsky [4]. The transformer is preferable for tris application b ecause of its automatic recording featur es.…”