The surface- and grain boundary composition of Y, Ce and Ti doped zirconia were studied by X-ray Photoelectron Spectroscopy and Auger Electron Spectroscopy/Scanning Auger Microscopy. The grain boundaries and free surfaces showed the same enrichment levels. After heat treatment ≥ 1000 ‡C all yttria doped samples showed yttrium enrichment. In the ZrO2-Y2O3 system the yttrium enrichment did not depend on the bulk concentration and amounted 30–34 mol% YO1.5 in all cases. As a consequence the segregation factor increases with decreasing solute concentration in the bulk. The thickness of the segregation layer was about 2–4 nm. In the ternary Y doped systems yttrium is the main segregant. In ceria-doped tetragonal zirconia polycrystals (Ce-TZP) systems significant segregation of cerium starts atT≥1300‡C and is mainly attributed to Ce3+. In Y,Ti-TZP systems also strong segregation of Ti4+ occurs. The absolute value of the increase of the surface concentration in fine grained material is smaller than in coarse grained material. This is mainly due to depletion of the bulk.

/pdf/surface-and-grain-boundary-analysis-of-doped-zirconia-4g6rv5ja9p.pdf

Surface and grain boundary analysis of doped zirconia ceramics studied by AES and XPS

Zirconia samples doped with 3 mol% yttria were prepared by gel precipitation from a metal chloride solution and their sintering behaviour compared with that of a commercial powder. Dense (relative density 97%) nanoscale ceramics with a mean grain size of 60 nm are obtained after sintering at 1050°C for 7h. Important densification mechanisms in the initial sintering stage are grain boundary sliding and grain boundary diffusion. Grain growth in the final sintering stage seems to be impurity drag controlled. Extremely low activation energies are obtained for both densification and grain growth in the initial sintering stages. Special attention has been paid to the effect of aggregate size of the precursor powder on the final grain size.

https://ris.utwente.nl/ws/files/6715808/Theunissen93sintering.pdf

Sintering kinetics and microstructure development of nanoscale Y-TZP ceramics

Powder synthesis and ceramic processing methods have to be improved to take full advantages of new, improved properties of nanocrystalline ceramics. Sintered nanocrystalline ceramics of pure, undoped zirconia are formed from nanocrystalline powder of optimized quality obtained by the chemical vapor synthesis (CVS) method. The as-synthesized CVS ZrO2 powder is nonagglomerated with a crystallite size of about 5 nm, narrow size distribution, and high crystallinity. On uniaxial compaction a transparent green body of ultrafine, uniform microstructure and narrow pore size distribution corresponding to the grain size distribution is formed, which is sintered under vacuum at 950°C into a transparent, fully dense ZrO2 ceramic with a grain size of 60 nm.

Sintering Behavior of Nanocrystalline Zirconia Prepared by Chemical Vapor Synthesis

Powders of yttria-doped tetragonal zirconia (3 mol%) with a narrow pore size distribution and ultrafine particle size (~ 9 nm) have been prepared by the mixed organic + inorganic precursors coprecipitation method. The compaction behaviour of almost agglomerate-free calcined powders was studied, and their sintering behaviour using both isothermal and non-isothermal techniques was evaluated. Fully dense nanoscale ceramics with an average grain size below 95 nm were obtained after sintering at 1200 °C for 20 min or at 1150 °C for 4 h. Several stages were identified in the whole densification process: it was found that a particle rearrangement process assisted by sintering pressure is the densification mechanism in the earlier sintering step (up to 800 °C), and grain boundary diffusion was the dominant mechanism for densification up to 1180 °C. Activation energy values of 130 ± 20 and 300 ± 40 kJ mol−1, respectively, were calculated for these densification mechanisms. The almost complete absence of agglomerates in the calcined powders and the homogeneous pore structure of the green compacts are the two main factors leading to lowtemperature fully densified Y-TZP bodies.

Low-temperature sintering and microstructural development of nanocrystalline Y-TZP powders

The mechanical properties of tetragonal zirconia (TZP) materials doped with Y, Ce or Ti were studied as a function of temperature and grain size. Fine grained Y-TZP (grain size < 0.3 mgrm) shows values for fracture toughness and strength at room temperature, which are comparable with the coarse grained transformation toughened materials, despite lacking transformation toughening. The morphology of the fracture surface points to crack deflection as the most important toughening mechanism. At 800 °C fracture toughness and strength are higher than in coarse grained Y-TZP materials. Doping Y-TZP with Ce or Ti results in a similar trend in mechanical properties, for fine grained material, as for the Y-TZP materials.

/pdf/mechanical-properties-of-ultra-fine-grained-zirconia-779tym8bke.pdf

Mechanical properties of ultra-fine grained zirconia ceramics

Two wet chemical preparation methods are described for yttria-doped tetragonal zirconia powders. Both methods yield powders with an extremely small crystallite size (8 nm) and a narrow size distribution. The agglomerate and aggregate structure of these powders have been investigated by several techniques. Gel precipitation from an alkoxide solution in water (ldquoalkoxiderdquo synthesis) results in a ceramic powder with irregular-shaped weak and porous agglomerates, which are built up from dense aggregates with a size of 18 nm. Gel precipitates formed from a metal-chloride solution in ammonia (ldquochloriderdquo synthesis) do not contain aggregates. Both types of agglomerate are fractured during isostatic compaction. Hydrolysis and washing under (strong) basic conditions probably decrease the degree of aggregation. The aggregate morphology and structure are key parameters in the microstructure development during sintering of a ceramic. Several characteristics of these powders are compared with those of a commerical one (Toyo Soda TZ3Y).

/pdf/powder-preparation-and-compaction-behaviour-of-fine-grained-49u0sqnp47.pdf

Powder preparation and compaction behaviour of fine-grained Y-TZP

Yttria-doped zirconia powders with an extremely small crystallite size (8 nm) and a narrow size distribution have been prepared by gel precipitation methods. Precipitation of an alkoxide solution in water (“alkoxide” synthesis) results in weaker agglomerates when compared with a precipitation of a metal chloride solution in ammonia (“chloride” synthesis). Both types of agglomerate are fractured during isostatic compaction. The “alkoxide” agglomerates consist of strong small aggregates (15–20 nm) while within the “chloride” agglomerates no, or only a few, aggregates are present

/pdf/microstructure-characteristics-of-ultra-fine-zro2y2o3-1nkhqgwz4o.pdf

Microstructure characteristics of ultra-fine ZrO2Y2O3 ceramic powders

Ultra-fine grained, weakly agglomerated Y2O3 doped tetragonal ZrO2 powders with a primary particle size of about 8 nm were prepared by means of two different hydrous-gel precipitation techniques. These methods are respectively the hydrolysis of a metal alkoxide (“alkoxide” method) or a metal chloride (“chloride” method) solution. The sintering behaviour of these powders is compared with a commercial powder (Tosoh TZ3Y). After isostatic compaction at 400 MPa the chloride, alkoxide and TZ3Y compacts densify to 97% relative density after 10 hours sintering at 1050, 1200 and 1100°C respectively. The change in crystallite size and pore morphology has been studied as function of time. The sintering kinetics are probably determined by the aggregate structure within the green compact which is different for the investigated powders. A nanoscale ceramic can be obtained (grain size 53 nm) by sintering a chloride compact during 6.5 hr at 1044°C.

W.F.M. Groot Zevert

Papers

Powder preparation and compaction behaviour of fine-grained Y-TZP

Microstructure characteristics of ultra-fine ZrO2Y2O3 ceramic powders

Microstructure Development During Sintering of Ultra Fine Grained Y-TZP