5.2.5 Superconductors

It is well known that YBa2Cu3Ox (123) is unstable with respect to reaction with carbon dioxide or water vapor. This reaction is related to the large enthalpies of formation of barium carbonate and barium hydroxide, which are -64.4 and -35.4 kcal, respectively. Davison et al. [5.166] reported the formation of barium carbonate when 123 was held over water for 48 hr. Yan et al. [5.167] reported the formation of Cu(OH)2 when 123 was exposed to 85°C and 85% relative humidity for 90 min. After exposing 123 to 80°C and 100% relative humidity for times ranging from 15 min to 24 hr, Fitch and Burdick [5.168] reported the formation of Y2BaCuOx, BaCO3, CuO, Cu(OH)2, and the possible formation of Y(OH)3 and BaO. Fitch and Burdick, who noticed that corrosion was visibly present by a significant expansion of their samples, concluded that barium was leached first then reacted with atmospheric CO2 to form BaCO3 on the surface.

5.3 ATTACK BY SOLIDS

The stability of various materials to graphite is a good example of a solid-solid reaction. In this case, however, at least one of the products is a gas. The stability of a few selected refractory oxides in contact with graphite increases in the order TiO2, Al2O3, ThO2, MgO, MgAl2O4, SiO2, and BeO, as reported by Klinger et al. [5.169].

5.3.1 Silica

Miller et al. [5.170] have shown that carbon reacts with SiO2 to form the intermediate phase SiC, which then reacts with silica to form the gaseous phase SiO. The following equations were given to represent the reaction:

(5.52)

(5.53)

They stated that these reactions were sufficiently rapid at 1000°C and, in the presence of iron, which acts as a catalyst for the

Copyright © 2004 by Marcel Dekker, Inc.

reduction of silica by SiC, caused failure of silicate refractories in coal gasification atmospheres.

Probably one of more severe reactions of the past that has taken place in commercial glass furnaces is that between silica and alumina or alumina-containing refractories. When these two materials are in direct physical contact at high temperature, an interface of mullite forms. This reaction is accompanied by a substantial volume increase that tends to push the two original materials apart. Separation of silica and alumina by the more neutral material, zircon, has prevented this deleterious reaction in modern furnaces.

5.3.2 Magnesia

Magnesia vaporization is important in basic refractories where it migrates to form a region rich in magnesia by vaporization and condensation, and leaving behind a region of high porosity. The zone of high porosity causes a mechanically weak area that may crack or spall. Vaporization and condensation of magnesia can also occur in silicon nitride where it is used as a sintering aid (see discussion on page 203).

In pitch-containing high magnesia refractories, it has been found [5.171] that the carbon in the refractory can react with the magnesia to form magnesium gas according to the following equation:

(5.54)

This magnesium gas is then transported to the hot-face of the refractory where it can react with FeO in the slag forming iron metal liquid and a dense solid magnesia layer according to:

(5.55)

5.3.3 Superconductors

The YBa2Cu3Ox (123) superconductors, where x=6.5–7.0, have been reported to exhibit reaction and/or decomposition when

Copyright © 2004 by Marcel Dekker, Inc.

in contact with various materials. This has presented researchers with the problem of sample holders for the production of 123 materials. Williams and Chandhury [5.172] have conducted a thermodynamic study of the various materials that might react with 123. Based upon the heat of formation of CuO of -18.6 kcal/g atom and the following equations:

(5.56)

or

(5.57)

they reported that the nine elements Ru, Rh, Pd, Ag, Os, Ir, Pt, Au, and Hg should not react with CuO, and most likely would not react with 123. Murphy et al. [5.47] reported that 123 was nonreactive toward silver and to a lesser extent, gold.

One of the potential applications of superconductors is that of thin films on a semiconductor substrate; however, the most widely used semiconductor substrate material, silicon, reacts with 123. An examination of the various phase equilibria indicated that BaSi2O5 does not react with 123, because these two materials form a stable tie-line in the BaO–Y2O3–CuO–

SiO2 quaternary system. Thus this barium silicate could be used as a buffer layer for production purposes or during manufacture of thin films on semiconducting substrates.

Mikalsen et al. [5.173] reported that no reaction occurred between thin film superconductors in the Bi–Sr–Ca–Cu–O system and MgO substrates even after annealing at 850°C for 30 min. Thin films on Al2O3, however, reacted and became insulating and transparent. Abe et al. [5.174] and Ibara et al. [5.175] have reported that melts of BiSrCaCu2Ox or BiPbySr CaCu2Ox reacted with alumina crucibles contaminating their samples.

5.3.4 Attack by Metals

Because of the prevalence of platinum metal in various research and manufacturing operations, the reactions of various refractory

Copyright © 2004 by Marcel Dekker, Inc.