Ternary eutectics in the systems FeO-UO2-ZrO2 and Fe2O3-U3O8-ZrO2

The systems FeO-UO₂-ZrO₂ (in inert atmosphere) and Fe₂O₃-U₃O₈-ZrO₂ (in air) were studied. For the FeO-UO₂-ZrO₂ system, the eutectic temperature was found to be 1310°C, with the following component concentrations (mol %): 91.8 FeO, 3.8 UO₂, and 4.4 ZrO₂. For the Fe₂O₃-U₃O₈-ZrO₂ system, the eutectic temperature was found to be 1323°C, with the following component concentrations (mol %): 67.4 FeO_1.5, 30.5 UO_2.67, and 2.1 ZrO₂. The solubility limits of iron oxides in the phases based on UO₂(ZrO₂,FeO) and UO_2.67(ZrO₂,FeO_1.5) were determined.     

Low-temperature ageing map for 3mol% Y2O3-ZrO2

The ageing behaviour of 3mol% Y₂O₃-ZrO₂ at 100–500° C in a water-containing atmosphere was studied. A critical grain size of ～ 0.37 μm and a lower temperature limit of ～80°C for retaining the tetragonal symmetry were deduced from the kinetic study. An “ageing map” constructed on the grain size (G) against ageing temperature (T) plot is proposed to describe the low-temperature ageing behaviour of 3mol% Y₂O₃-ZrO₂.     

Sintering and compensation effect of donor- and acceptor-codoped 3mol% Y2O3-ZrO2

Addition of 0.15–0.5 mol% acceptor oxide, Al₂O₃, to 3 mol% Y₂O₃-ZrO₂ results in enhanced densification at 1350 °C. The enhancement is accounted for by a liquid phase sintering mechanism. The addition of donor oxide, Ta₂O₅, of 0.15–2.5 mol % at 1300–1600 °C results in the destabilization of tetragonal (t-) phase and the decrease of final density in 3 mol% Y₂O₃-TZP (tetragonal ZrO₂ polycrystals). X-ray diffractometry (XRD) reveals that the Ta₂O₅-added 3 mol% Y₂O₃-ZrO₂ contains monoclinic (m-) ZrO₂ and a second phase of Ta₂Zr₆O₁₇. The decreasing in final density is attributed to the increase of m-ZrO₂ content. Complete destabilization of t-ZrO₂ to m-ZrO₂ in samples added with 2.5 mol% Ta₂O₅ is interpreted by the compensation effect based on donor- and acceptor-codoping defect chemistry.     

Sintering and compensation effect of donor and acceptor codoped 3 mol% Y2O3-ZrO2

Addition of 0.15–0.5 mol% acceptor oxide, Al₂O₃, to 3 mol% Y₂O₃-ZrO₂ results in enhanced densification at 1350°C. The enhancement is accounted for by a liquid phase sintering mechanism. While the addition of donor oxide, Ta₂O₅, of 0.15–2.5 mol% at 1300–1600°C results in the decrease of final density and in the destabilization of the tetragonal (t) phase of the 3 mol% Y₂O₃-t-ZrO₂ (TZP). X-ray diffractometry (XRD) reveals that the Ta₂O₅-added 3 mol% Y₂O₃-ZrO₂ contains monoclinic (m) ZrO₂ phase and a second Ta₂Zr₆O₁₇ phase. The decrease is attributed to the increase of m-ZrO₂ content in these samples. Complete phase transformation from t-ZrO₂ to m-ZrO₂ observed in samples added with 2.5 mol% Ta₂O₅ is interpreted by the compensation effect based on donor and acceptor codoping defect chemistry.     

Sintering ofβ-sialon with 5mol% Y2O3-ZrO2 additives

The sintering behaviour ofβ-Sialon composition powders with 5 mol% Y₂O₃-ZrO₂ additives at 1750°C for 1.5 h in nitrogen or argon atmospheres was studied.β-Sialon composition powders could be pressureless-sintered to about 93% theoretical density by the addition of 5 wt% 5 mol% Y₂O₃-ZrO₂. By HIPing the pressureless-sintered bodies the density was increased to higher than 98% theoretical density, and uniform submicrometre ZrO₂ particles were homogeneously dispersed in theβ-Sialon matrix, resulting in an increase of fracture toughness,K _1c, from 5.1 to about 5.7 MN m^−1.5. Increasing the amount of tetragonal ZrO₂ transformable to monoclinic phase in theβ-Sialon matrix increasedK _1c.     

Crystallization of gels in the SiO2-ZrO2-B2O3 system

The preparation of glasses from gels in the ternary system of SiO₂-ZrO₂-B₂O₃ has been investigated. The crystallization character of ZrO₂ particles and high-temperature structure stability in the SiO₂-ZrO₂-B₂O₃ gels were analyzed. The effect of B₂O₃ as the inhibitor of crystallization was illustrated. Finally, the result that the purified DD3 single crystal superalloy melt can retain high undercooling in the SiO₂-ZrO₂-B₂O₃ coating mold indicates that the coating has an ideal non-catalytic nucleation property.     

Crystallization of gels in the SiO2-Al2O3-ZrO2 system

The preparation of glasses and ceramics from gels in the ternary system of SiO₂-Al₂O₃-ZrO₂ has been investigated. The phase relations in this system suggest the existence of a stable joint between mullite and ZrO₂ in addition to the joint between mullite and zircon. The gel technique allows metastable ZrO₂ particles to be readily dispersed in alumino-silicate matrices.     

Microstructure and mechanical properties of Al2O3-Cr2O3-ZrO2 composites

Al₂O₃ is a popular ceramic and has been used widely in many applications and studied in many aspects. On the other hand, zirconia-toughened alumina (ZTA) is a desirable material for engineering ceramics because of its high hardness, high wear resistance and high toughness. In the present research, Al₂O₃-Cr₂O₃-ZrO₂ composites were produced by hot-pressing in order to harden the Al₂O₃ matrix in ZTA. Its microstructure and mechanical properties were studied by SEM, ESCA, XRD, Vickers hardness and bending strength test. It was found that addition of ZrO₂ inhibited the grain growth of Al₂O₃-Cr₂O₃ and the grain growth of ZrO₂ proceeded with increasing amounts of ZrO₂ in the Al₂O₃-Cr₂O₃-Zr₂ composite. The formation of solid solution Al₂O₃-Cr₂O₃ was also confirmed by XRD, and monoclinic ZrO₂ increased on addition of Cr₂O₃. Maximum hardness was at Al₂O₃-10wt% Cr₂O₃ with 10 vol% ZrO₂ and a stress-induced transformation was confirmed on the fracture surface of the specimen after the bending test.     

The eutectic and liquidus in the Al2O3-ZrO2 system

A high-temperature X-ray diffraction method was employed to establish,in situ, the eutectic temperature (1910±20° C) and the liquidus line in the Al₂O₃-ZrO₃ binary system. The eutectic composition was determined by optical microscopy to be 42.5±1 wt% ZrO₂. No evidence was found for the existence of a reported-Al₂O₂ high temperature phase.     

Microwave and conventional sintering of rapidly solidified Al2O3-ZrO2 powders

The Al₂O₃-ZrO₂ eutectic composition was rapidly solidified, forming amorphous and crystalline structures. The as-quenched material was crushed and pressed into pellets which were sintered conventionally or with microwaves. Conventional and microwave sintering at temperatures up to 1600 °C resulted in a microstructure where 100–200 nm ZrO₂ grains were present intergranularly in the -Al₂O₃ grains. Larger ZrO₂ grains (1 m) were found intergranularly. The as-quenched lamellar structure spheroidized during sintering at high temperatures. Boron contamination of the powders resulted in more homogeneous and dense as-fired samples but promoted the ZrO₂ tetragonal-to-monoclinic transformation, which was attributed to increased grain boundary diffusivity. Conventional sintering at low temperatures resulted in the formation of rods of an Al₂O₃-rich phase which grew from a low-melting B₂O₃-rich liquid.     

Si3N4-ZrO2 composites with small Al2O3 and Y2O3 additions prepared by HIP

Si₃N₄-ZrO₂ composites have been prepared by hot isostatic pressing at 1550 and 1750 °C, using both unstabilized ZrO₂ and ZrO₂ stabilized with 3 mol% Y₂O₃. The composites were formed with a zirconia addition of 0, 5, 10, 15 and 20 wt%, with respect to the silicon nitride, together with 0–4 wt% Al₂O₃ and 0–6 wt% Y₂O₃. Composites prepared at 1550 °C contained substantial amounts of unreacted -Si₃N₄, and full density was achieved only when 1 wt% Al₂O₃ or 4 wt % Y₂O₃ had been added. These materials were generally harder and more brittle than those densified at the higher temperature. When the ZrO₂ starting powder was stabilized by Y₂O₃, fully dense Si₃N₄-ZrO₂ composites could be prepared at 1750 °C even without other oxide additives. Densification at 1750 °C resulted in the highest fracture toughness values. Several groups of materials densified at 1750 °C showed a good combination of Vickers hardness (HV10) and indentation fracture toughness; around 1450 kg mm^–2 and 4.5 MPam^1/2, respectively. Examples of such materials were either Si₃N₄ formed with an addition of 2–6 wt% Y₂O₃ or Si₃N₄-ZrO₂ composites with a simultaneous addition of 2–6 wt%Y₂O₃ and 2–4 wt% Al₂O₃.     

Pressureless-sintering behavior of nanocrystalline ZrO2–Y2O3–Al2O3 system

ZrO₂–Y₂O₃–Al₂O₃ nanocrystalline powders have been synthesized using chemical coprecipitation method. Nano-powders were compacted uniaxially and densified in a muffle furnace. Densification studies showed that a fully dense pellet of ZrO₂(3Y) and a 99% relative density for 5 mol% Al₂O₃ doped ZrO₂(3Y) were obtained after sintering at 1200 °C. The presence of Al₂O₃ inhibits grain growth and suppresses the densification process. Full densification and the maximum microhardness of 17.8 GPa were achieved for the ZrO₂(3Y)/5 mol% Al₂O₃ composites sintered at 1250 °C.     

Microstructure of sol–gel synthesized Al2O3–ZrO2(Y2O3) nano-composites studied by transmission electron microscopy

The Al₂O₃–ZrO₂(Y₂O₃) composite powder was synthesized through a sol–gel process using aluminum sec-butoxide and zirconium butoxide as precursors. The as-received powders in an amorphous phase were crystallized with c-ZrO₂ at around 980 °C. As the calcination temperature increased, the c-ZrO₂ crystalline phase was transformed to t-ZrO₂ at about 1200 °C. However, the Al₂O₃ phase in the Al₂O₃–ZrO₂(Y₂O₃) composite powders still existed in an amorphous phase up to 1050 °C. In the sintered body using the calcined powders at 400 °C, the Al₂O₃ phase was crystallized in an α-phase at 1200 °C during the sintering for 2 h. Using the sol–gel Al₂O₃–ZrO₂(Y₂O₃) powder, a typical nano-composite having a nano-crystalline phase (less than 20 nm) can be successfully obtained by a pressureless-sintering process even at 1200 °C for 2 h.Using the sol–gel Al₂O₃–ZrO₂(Y₂O₃) powder, a typical nano-composite having a nano-crystalline phase (less than 20 nm) can be successfully obtained by a pressureless-sintering process even at 1200 °C for 2 h. The values of relative density and Vickers hardness were comparatively high value with about 96.2% and 1100 Hv, respectively, even though it was made at low temperature. In the composite sintered at 1400 °C, the hardness value was saturated with 1570 Hv and the values of fracture toughness were almost same with about 6 MPa m^1/2.     

Effect of dopant concentration on electrical conductivity in the Sc2O3-ZrO2 system

Electrical conductivity measurements have been made as a function of dopant concentration (4 to 8 mol% Sc₂O₃) in the scandia-zirconia system, All the compositions studied had a tetragonal structure. The hombohedral phase was present only in samples prepared from mechanical mixtures of Sc₂O₃ and ZrO₂. In specimens prepared by coprecipitation, no phase lines were observed and the monoclinic zirconia (m-ZrO₂) phase was present for only Sc₂O₃ contents 5 mol %. The conductivity of Sc₂O₃-ZrO₂ decreased continuously with time up to 300 h anneal time between 700 and 1000° C. X-ray diffraction of coprecipated specimens of 7.8 mol % Sc₂O₃-ZrO₂ composition annealed at 1000° C (28 days), 750° C (42 days) or 460° C (189 days) did not reveal any changes to account for this. However, transmission electron microscopy showed that changes associated with the formation of very fine precipitates had occurred. The activation energy for conduction in the low-temperature region decreased monotonically with decrease in the scandia content. Jumps in the conductivity curves and hysterisis effects were observed in specimens containing m-ZrO₂.     

One-pot synthesis of Al2O3 modified mesoporous ZrO2 with excellent thermal stability and controllable crystalline phase

In this paper, a series of Al₂O₃ modified mesoporous ZrO₂ (M-ZrAl) materials are designed and achieved through a one-pot EISA strategy. As proved by different characterizations, the introduced Al₂O₃ species exist as highly dispersed states, and the crystalline phase (t-ZrO₂ and m-ZrO₂) of M-ZrAl material could be accurately controlled through adjusting the content of Al₂O₃ species. Moreover, the M-ZrAl-10 material exhibits excellent textural properties (specific surface area (56?m²·g^?1), pore size (8.2?nm) and pore volume (0.13?cm³·g^?1)) even treated at 800?°C. The obtained materials are employed as support for MoO₃/M-ZrAl solid acid catalyst, and the relationship between catalytic performance (Friedel-Crafts alkylation, acetalization and esterification) and crystalline structure of support is specially explored. The result shows that the support with t-ZrO₂ phase is beneficial for the formation of acid sites and enhancement of catalytic performance for the MoO₃/M-ZrAl solid acid catalyst.     

Phase relations in the systems ZrO2Y2O3Nd2O3 and ZrO2Y2O3CeO2

The phase relations of ZrO₂Y₂O₃Nd₂O₃ and ZrO₂Y₂O₃CeO₂ systems have been studied at 1100–1600°C. The single region of the fluorite phase was intensively examined using the relation between lattice parameter and composition. In the ZrO₂Y₂O₃Nd₂O₃ system, 37 mol% Nd₂O₃ is soluble in Y₂O₃-stabilized zirconia (fluorite phase) at 1100°C and 42 mol% Nd₂O₃ at 1600°C. In the ZrO₂Y₂O₃CeO₂ system, 40 mol% CeO₂ dissolves into the stabilized zirconia at 1600°C.     

Effect of powder treatment on the crystallization behaviour and phase evolution of Al2O3–High ZrO2 nanocomposites

Al₂O₃–ZrO₂ composites containing nominally equal volume fraction of Al₂O₃ and ZrO₂ have been synthesized through combined gel-precipitation technique. Subsequently the gels were subjected to three different post gel processing treatments like ultrasonication, ultrasonication followed by water washing and ultrasonication followed by alcohol washing. It was observed that while in unwashed samples crystallization took place at low temperature, crystallization was delayed in the washed gels. The phase transition of ZrO₂ in the calcined gels followed the sequence; amorphous → cubic ZrO₂ → tetragonal ZrO₂ → monoclinic ZrO₂. On the other hand, phase transition in alumina followed the sequence amorphous to γ-Al₂O₃, the transition taking place at 650 °C. No α-Al₂O₃ could be detected even after calcination at 950 °C. However, all the sintered samples had α-Al₂O₃. In spite of high linear shrinkage (19–21%) during sintering, the sintered sample had density of only above 70% for all the four varieties of the powders. However, in spite of the low sintered density of the pellets, 31% tetragonal zirconia could be retained after sintering at 1400 °C and it reduced to about 16% at 1600 °C.     

Electrical conductivity of Sc2O3-ZrO2 compositions by 4-probe d.c. and 2-probe complex impedance techniques

The ionic conductivities of several samples in the Sc₂O₃-ZrO₂ system (Sc₂O₃: ～ 8 mol %) have been measured using 4-probe d.c. and 2-probe complex impedance dispersion techniques. Samples which contained monoclinic zirconia showed hysteresis effects and S-shaped Arrhenius conductivity plots. This behaviour was assigned to the m-ZrO₂ ? t-ZrO₂ transformation. In samples which were free of monoclinic ZrO₂, contributions from the grain boundary resistance were relatively small. The Arrhenius plots of their conductivity showed a distinct change in the slope around 600° C towards higher activation energy and this was attributed to vacancy trapping. The 4-probe d.c. data could be fitted to an equation of the formρ=A ₁ T exp (E ₁/RT)+A ₂ T exp (E ₂/RT). The process which dominated the conduction mechanism at lower temperatures had an activation energy of 130 to 140 kJ mol^?1. The activation energy for the migration of oxygen ion vacancies within the bulk of the grain was 64 to 70 kJ mol^?1.     

A structural study of metastable tetragonal zirconia in an Al2O3-ZrO2-SiO2-Na2O glass ceramic system

The structural and microstructural properties (crystalline system at the beginning of crystallization, lattice disorder and crystallite size) of metastable zirconia have been studied by an X-ray line broadening analysis using simplified methods based on suitably assumed functions describing the diffraction profiles. Metastable tetragonal zirconia has been crystallized at 970, 1000 and 1050° C, respectively, starting from an Al₂O₃-ZrO₂-SiO₂Na₂O glassy system with a chemical composition very close to that of well known electromelted refractory materials. In the present work we have definitely shown the presence, inside the crystallized zirconia phase, of internal microstrains having values ranging approximately between 2 and 4×10^–3. Moreover, we have confirmed the peculiar smallness in size of precipitated zirconia crystallites ( 200 Å). Therefore, in the present system, the stabilization of the tetragonal form of ZrO₂ with respect to the stable monoclinic one can be explained in terms of a contribution to the amount of free energy due to strain energy, in addition to the previously hypothesized surface energy. The observed strong line broadening for some samples treated at lower temperatures (970 and 1000° C) gives rise to an apparent cubic lattice pattern; but the asymmetry of each apparent single line masks unequivocally a tetragonal doublet. This latter conclusion disagrees with some hypotheses on the existence of a cubic metastable form of ZrO₂ which could originate at the beginning of zirconia crystallization.     

Developments in highly toughened CeO2-Y2O3-ZrO2 ceramic system

A wet-chemical approach has been applied to derive fine powders with various ceria and yttria compositions in the CeO₂-Y₂O₃ ZrO₂ ceramic system by the co-precipitation method. The characteristics of the as-derived powders have been evaluated through differential thermal analysis, thermogravimetric analysis, BET surface-area analysis, and inductively coupled plasma technique. The hardness and fracture toughness of the as-sintered specimens were evaluated by the indentation method. A highly toughened ceramic withK _IC>25 MPa m^1/2 was achieved with the composition 5.5 mol% CeO₂-2 mol% YO_1.5-ZrO₂. The relationship between the mechanical properties and the compositions of stabilizers, CeO₂ and Y₂O₃ is discussed with respect to the degree of tetragonal to monoclinic transformation as well as the grain size of the as-sintered ceramics.