Property Degradation of Tetragonal Zirconia Induced by Low-Temperature Defect Reaction with Water Molecules

Abstract

Tetragonal ZrO 2 exhibits good ionic conductivity, high strength, and fracture toughness. But while annealing at relatively low temperatures (63-400°C), tetragonal ZrO 2 spontaneously transforms to a monoclinic one, and its electrical and mechanical properties degrade severely. The phenomenological observations of the low-temperature degradation of tetragonal ZrO 2 are summarized, and major degradation mechanisms are critically reviewed. It is crucial to maintain sufficient oxygen vacancy concentration to stabilize the tetragonal structure; excess reduction of the oxygen vacancy concentration causes the tetragonal to monoclinic transformation. Water molecules can be incorporated into the ZrO 2 lattice by filling oxygen vacancies, which leads to the formation of proton defects. Experimental and theoretical evidence support such a defect reaction between oxygen vacancies and water molecules. And a degradation mechanism based on this defect reaction satisfactorily explains all the phenomenological observations. The diffusion rate of oxygen vacancies at low temperatures is not high enough to cause the observed degradation depth; therefore, the relatively fast diffusion of proton defects most probably controls the degradation process.

Effect of Titania Doping on Phase Stability of Zirconia Bioceramics in Hot Water

“…11,[27][28][29][30][31][32][33] Among the models, a model proposed by Guo seems to be most commonly accepted. 32,33) That model consists of the following steps: (i) chemical adsorption of H 2 O on the ZrO 2 surface, (ii) reaction of H 2 O with O 2À on the ZrO 2 surface to form hydroxyl ions OH À , (iii) penetration of OH À into the inner part by grain boundary diffusion, (vi) filling of oxygen vacancies by OH ions, and therefore the formation of proton defects, and (v) occurrence of a tetragonal to monoclinic transformation when the oxygen vacancy concentration is reduced to the extent that the tetragonal phase is no longer stable. In this reaction, the interaction between dopant ions and oxygen vacancies is very important.…”

Degradación ambiental de las propiedades mecánicas de fibras monocristalinas de circona dopada con erbia

“…Las teorías que trataban de explicar esta desestabilización postulaban la formación de hidróxidos (Zr-OH e Y-OH) por adsorción de agua en la superficie, lo que aumentaría las tensiones en la superficie y la nucleación de la transformación martensítica [10]; o bien el lixiviado del itrio por el agua, que conduciría a la desestabilización de los granos y su paso a la fase monoclínica [16]. Más recientemente, Guo [17][18][19] propuso un nuevo mecanismo según el cual reacciones superficiales con el ion O 2-darían lugar a iones OH -, que penetrarían en el material e irían aniquilando las vacantes de oxígeno. Si la concentración de estas vacantes cayera por debajo de determinado umbral, la fase tetragonal dejaría de ser estable y nos encontraríamos con la degradación por transformación de fase del material.…”

The Balance Between Reactivity and Stability of Modified Oxide Surfaces Illustrated by the Behavior of Sulfated Zirconia Catalysts

“…O 2 is not a product of this reaction. Oxygen vacancies V O°°, which are known to be present in tetragonal zirconia [27], require the presence of negative charges, that is, electrons, for charge balance somewhere in the crystal; these electrons can, for example, be located in the vacancy (V O° The TG data indicate that sulfate is much more strongly bound than water -the products are volatile in both cases. At the end of the TG analysis, the samples were largely monoclinic.…”

“…Guo [27] attempted to mold the multiple mechanisms discussed and the available data into a consistent picture and proposed that first water adsorbs on the surface and OH -are formed, which then move along the grain boundaries and fill oxygen vacancies until the vacancy concentration is below that necessary for stabilization of the tetragonal phase. However, a phase transition could also be envisioned as a result of surface effects alone.…”

“…Transformation of the tetragonal to the monoclinic phase is easily initiated by elevated temperatures of 336-673 K, particularly in the presence of water vapor [27] and by mechanical stress in the form of grinding, milling, or pressing [28].…”