The effect of ceria co-doping on chemical stability and fracture toughness of Y-TZP

The fracture toughness and ageing resistance of yttria, ceria-stabilized tetragonal zirconia polycrystals (Y, Ce-TZP) were evaluated as a function of grain size and ceria content. Very fine grained, fully dense materials could be produced by sinter forging at relatively low temperatures (1150–1200 °C). The ageing resistance in hot water (185 °C) of 2 mol% Y₂O₃-stabilized TZP is strongly enhanced by alloying with ceria. The ceria content necessary to avoid degradation completely, decreases with grain size. The toughness of fully dense Y, Ce-TZP is 7–9 MPa m^1/2 for grain sizes down to 0.2 m. No or very little transformation took place during fracturing and no clear variation with grain size was observed for the toughness at grain sizes up to 0.8 m. Reversible transformation and crack deflection may explain the observed toughness values.     

Size effect of trivalent oxides on low temperature phase stability of 2Y-TZP

2 mol% Y₂O₃stabilized-TZPs (2Y-TZPs) doped with oversized trivalent cations (Sc³⁺< Yb³⁺< Y³⁺< Sm³⁺< Nd³⁺< La³⁺) whose ionic radius is larger than Zr⁴⁺was sintered for 1 h at 1500°C over the range containing trivalent oxides from 0 to 2 mol% with 0.5 mol% interval to evaluate the effect of trivalent cation alloying on low temperature phase stability of 2Y-TZP by investigating the variation of Raman spectra and lattice parameters. For a given concentration of dopant, tetragonality (c/aaxial ratio) increases with raising the dopant size. However, monoclinic (m)-ZrO₂content for the specimens annealed for 500 h at 220°C in air firstly decreases with increasing dopant size and then increases as dopant size is greater than Y³⁺ion. Raman modes of Zr-O_II(260 cm^–1) and Zr-O_I(640 cm^–1) shift to higher wavenumbers only when Sm₂O₃, Nd₂O₃, and La₂O₃are added. Although full width at half maximum (FWHM) of 640 cm^–1is constant, FWHM of 260 cm^–1mode decreases with increasing dopant size, indicating that an ordered structure (pyrochlore phase) may be formed. Therefore, dopant size is dependent on phase stability of 2Y-TZP in this system.     

The effect of anelasticity and phase transformation on crack growth in Y-TZP ceramics

Crack growth behavior has been investigated under monotonic and cyclic loadings for Y-TZP that produces remarkable anelastic strain. Monotonic loading testing was carried out under the condition of various stress rates (8 × 10², 8 × 10^–1 and 8 × 10^–4 MPa/s) and temperatures (RT and 373 K). Resistance of crack propagation was observed at the lowest stress rate at elevated temperature. Cyclic fatigue crack growth rate was examined under the condition of different frequencies and stress waveforms. Crack growth rate clearly depended on stress waveform, which was explicable by exhaustion and restoration of anelasticity at the crack tip region. Experimental results make it clear that anelasticity works as strong resistance against crack growth. In this study, the effect of environment-induced tetragonal to monoclinic phase transformation on fracture strength was also investigated for pre-cracked sample. Aged (transformed) samples have shown extreme crack closure and considerable improvement in strength.     

通过等价和异价元素掺杂促进YbMg2Sb2热电性能提升（英文）

由于其电热输运性能便于调控,锑基Zintl相化合物AM2Sb2(A=Ca,Sr,Ba,Yb,Eu;M=Mg,Zn,Cd,Mn)被认为是一类重要的热电材料.本文通过在YbMg2Sb2中掺入等价的元素Zn和异价元素Ag,实现了声子和载流子的输运性能优化.首先,体系YbMg2-xZnxSb2室温热导率从1.96 W m^-1K^-1降低至1.15 W m^-1K^-1,这是由于形成YbMg2Sb2-YbZn2Sb2的固溶体带来的合金化散射效应.其次,掺杂Ag可以增加载流子浓度,其数值在室温下从0.42×10^19cm^-3提升至7.72×10^19cm^-3,从而有效地提高了电导率和功率因子.通过综合两方面的协同效应使得体系zT在703 K时达到0.48,比纯样品YbMgZnSb2高60%.     

The effect of dopants and organic washing on crystallization temperature of yttria- and ceria-doped zirconia gels

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Thermal stabilization of an active alumina and effect of dopants on the surface area

The -AI₂O₃ transformation of a monolithic active alumina has been increased from 1200 to 1380 C through structural incorporation of silica. This shift is significant since -Al₂O₃ transformation determines the limits of the usefulness of these materials as catalysts and catalyst carriers. The thermal stabilization effect is optimized at around 6% silica doping. At elevated temperatures, the material containing no silica rapidly loses surface area, primarily by -Al₂O₃ transformation, whereas the material containing excess silica loses surface area by classical sintering.     

The effect of surface GeO2-doping and CeO2-doping on the degradation of 2Y-TZP ceramic on annealing in water at 200°C

The bending strengths of 2Y-TZP ceramics treated by different methods were compared after annealing in water at 200°C. Surface GeO₂- and CeO₂-doped materials did not show the decrease in bending strength shown when GeO₂ and CeO₂ are doped through the bulk 2Y-TZP ceramic. This behavior is attributed to surface doping leading to an improvement in the resistance of Y-TZP to low temperature hydrothermal corrosion. The thermal stability of 2Y-TZP can be improved by surface doping without loss of fracture strength when the doped layer is less than the critical flaw length.     

氧化钇含量对Al_2O_3／Y－TZP复相陶瓷的影响

本文以ＺｒＯＣｌ２８Ｈ２Ｏ、Ａｌ２Ｏ３及Ｙ（ＮＯ３）３为原料，用共沉淀法合成Ｙ２Ｏ３含量不同的ＺｒＯ２－Ａｌ２Ｏ３复合粉体，并采用热压工艺制备复相陶瓷．研究了氧化钇含量对复相陶瓷力学性能及应力诱导下氧化锆相变能力的影响．结果表明，氧化钇含量为１．８ｍｏｌ％时，复相陶瓷中氧化锆仍能全部保持为四方相，且在应力诱导下可相交量高达６４．６％，使材料呈现优良的室温和较好的高温力学性能．     

The effect of loaded volume and stress gradient on the fatigue limit

In this paper an investigation of multiaxial stress based criteria and evaluation methods is presented. The criteria are used with the point, gradient and volume methods. The purpose is to determine the combination of criteria and methods that is best suited for design against the fatigue limit. The evaluation is based on elastic FE-analysis of 15 geometries for which the fatigue limit loads are known. The point method is based on the maximum values of the fatigue stress in each specimen. With the gradient method, the fatigue stress is adjusted with the relative or absolute gradient of the fatigue stress itself. With the volume method, a statistical size effect is considered, by use of a weakest link integral. Thus, the probability of fatigue depends on the fatigue stress distribution. Also, the gradient and volume methods are combined. The results show that the point and gradient methods are not good for prediction of the fatigue limit. It is recommended to use the volume method in fatigue design. It is accurate enough for prediction of the fatigue limit, straightforward to use and easy to interpret. The choice of method is much more important than the choice of criteria.     

The effect of dopants on the morphology, microstructure and electrical properties of transparent zinc oxide films prepared by the sol-gel method

The influence of doping on the morphology, physical and electrical properties of zinc oxide produced by the sol-gel method was examined. Undoped zinc oxide was observed to form relatively porous films. Addition of an Al dopant influenced the sheet resistance, but did not result in a change in morphology, examined by atomic force microscopy when compared to undoped films. In the case of electrical measurements, undoped ZnO films were extremely resistive. A minimum dopant concentration of 2 at.%. Al was required to produce materials which were more conductive, as observed by sheet resistance measurements, which were shown to vary with annealing temperature. The versatile nature of sol-gel processing was demonstrated by selective ink-jet deposition of sol-gel droplets which were annealed to form oxide materials.     

The effects of copper and iron oxide additions on the sintering and properties of Y-TZP

Yttria-tetragonal zirconia polycrystals (Y-TZP) with a range of yttria contents were prepared from powders neutralized during processing with ammonium hydroxide and sintered at temperatures of 1300–1700 °C. Iron or copper oxide was ádded and studies made of body characteristics, mechanical properties and ageing resistance. Densification was aided by higher yttria concentrations. The effects of the oxide additives were dependent on amounts present and sintering conditions, including ramp rates and holding temperatures. Ageing resistance was significantly improved for both oxide additions fired to lower temperatures but rapid transformation to monoclinic phase was observed for materials with larger grain sizes associated with higher sintering temperatures.     

Grain-size dependence of toughness and transformability of 2mol% Y-TZP ceramics

Journal of Materials Science Letters -     

The effect of copper oxide on sintering, microstructure, mechanical properties and hydrothermal ageing of coated 2.5Y-TZP ceramics

Copper oxide dopants in amounts up to 1 wt% were added to 2.5 mol% yttria-coated zirconia powders in studies of sintering, microstructure, mechanical properties and hydrothermal ageing behaviour. High densities (>6 Mgm^–3), high tetragonal phase content (>95%), and phenomenal fracture toughness values (>17 MPam^1/2), were obtained for lower dopant levels. Grain sizes of 0.13 to 0.25 m were measured for all samples sintered at 1300°C. Rounded pores in some doped samples indicated that a liquid phase was involved during sintering. Copper oxide additions aid low temperature sintering and offer potential for property enhancement with a particularly high toughness being measured as well as improving resistance to structural degradation in 180°C hydrothermal ageing.     

The role of dopants on the formation and defect structures of the M-type of calcium ferrite

The formation mechanisms of M-type calcium ferrite were further assessed by doping Y₂O₃, BaCO₃ and SrCO₃. It was found that the ionic radius, rather than the valence is the predominant factor in the formation of the magnetoplumbite phase of calcium ferrite. The CaO/sd2Fe₂O₃ phase which played a precursor role in the formation of the M-type calcium ferrite was further verified. The defect structures based on the substitution of Ca²⁺ by La³⁺, the charge compensation by Fe²⁺, and release of oxygen were supported by DTA-TGA and conductivity data. The conductivity was assumed to, occur through a hopping mechanism as it increased with increasing temperature. The estimated values of the activation energy based on the small-polaron conduction were 0.35 to 0.40 eV in the high temperature region and 0.027 to 0.054 eV in the low temperature region. Moreover, the preexponential factor in the conductivity equation is an exponential function of the fraction of the M phase in the specimen.