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NaTaOj

najta^o,,

o-TaVOg

nr = no reaction

Fig. 7. Reaction behavior of ceramic oxides and vanadium compounds. (Taken from ref. 21; used with permission.)

vice versa, but no reaction occurs between compounds of comparable acid-base nature. (Note that NaV03 acts as an acid with basic Y203 but as a base with acidic GeOz or Ta205.) Fig. 7 indicates also that Zr02 is essentially nonreactive with vanadium compounds (the V205 reaction is very slow)

because of its particular acid-base -,, character, ¡-and that a highly vanadate- h; , resistant^stabilized zirconia might be possible, if an effective stabilizer .having ; a more acidic nature than Y203 (or MgO and CaO) could be found.

One such potential stabilizer is CeOz (which in fact has been patented as a sulfate/vanadate, .resistant stabilizer (22)), but in tests; (23) with NaV03, CeOr stabilizedr Zr02 was destabilized to virtually) the same extent as Y203- , stabilized ZrOz, even though pure Ce02 and Zr02 had been found (Fig. 7) not to react with NaV03. this paradox, which brings doubt on the possibility of ceramic design by oxide acid-base theory, has yet to be resolved./ Ceria has (23) a physical solubilityivin NaV03 of about 0.1 mol-% at 700C -to 1:; mol-% at 1000C, but it is not certain how much >this contributes to the destabilizationf process.

Perhapsmorc relevant was the showing that reaction of.-,V205 with Zr02-(20wgt-%)Cc02 produced a CeV04 surface phase (Fig. 8a,b), whereas reaction of NaVOj under the

Fig. 8a. CeV04 crystallites produced by ; V205»fonTZr02-(20wgt-%)Ce02. (Taken from ref. 23; used with permission.)

same conditions yielded surface crystals of segregated Ce02 (Fig. 9a,b). This may indicate a -flaw in the approach^ taken in Figiî 7; i.e:, the assumption that the ac. d base reaction tendencies ibfi -pyrejoxides iarev unchanged when the oxide is incorporated as a minority ^component in : aoceramip structure. Experience with glass and solid state catalysis-suggest that acid-base reactivities- of:, ¡incorporated ¿oxides;: may be v modified sorithatif;e;g;i ;;GeOt2 under the influence. of a Zr02 matrix might react with NaV.03, although pure Ce02 alone would not.

Fig. 8b. Ce-V Xray map of Fig. 8a. (Taken from ref. 23; used with permission.)
28KAJfc . 1 79KX K.,8989
Fig. 9a and b.. Micrograph and Xray map of Ce02 crystals produced. by NaV03 on Zr02-(20wgt-%)Ce02. (Taken from ref. 23; used by permission.)

One promising approach for studying stabilizing oxide-zirconia lattice oxide interactions (which is possibly the key for the development of corrosion-resistant zirconias) is the "optical basicity" technique of Duffy (2). This technique measures the shift in energy of electronic transitions of "probe ions" (e.g., Pb2+, Fig. 10) as these energies are affected by

50,000 40,000 30,000

Frequency (cm-1)

Fig. 10. Absorption spectrum of Pb2+ in (a) sodium borate glass vs. (b) in 11 M HC1. (From ref. 2; used by permission.)

50,000 40,000 30,000

Frequency (cm-1)

Fig. 10. Absorption spectrum of Pb2+ in (a) sodium borate glass vs. (b) in 11 M HC1. (From ref. 2; used by permission.)

electron charge donation (i.e., the factor determining Lewis acid-base nature) from oxygen atoms in the oxide matrix lattice, with the relative "basicity" of the glass or other oxide material being calculated from the energy shift measurements. Application to zirconia may be hindered because Zr02 has a strong adsorption peak at 43,500 cm"1, but peak deconvolution may be possible.

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