The ZrO2-rich region of the ZrO2-MgO system

The solubility limits of MgO in tetragonal zirconia were studied by combining the differential thermal analysis data and X-ray disappearing phase method. From these experiments a eutectoid reaction, tetragonal ZrO₂ solid solution monoclinic ZrO₂ solid solution + MgO, at 1120±10 °C and 1.6±0.2 mol% MgO was established. The solubility of MgO in tetragonal ZrO₂ diminished as the temperature increased, and at 1700 °C the solubility was less than 0.5 mol% MgO. The extent of the cubic zirconia solid solution single field was determined by using precise lattice parameter measurements and SEM observations. In this way an invariant eutectoid point, cubic ZrO₂ solid solution tetragonal ZrO₂ solid solution + MgO, was located at 1420±10 °C and 14.8±0.5 mol% MgO.     

Solid-state reaction,microstructure and phase relations in the ZrO2-rich region of the ZrO2-Yb2O3 system

Phase relations below 1700°C in the ZrO₂-rich region of the zirconia-ytterbia system have been established using thermal expansion, room-temperature X-ray diffraction, precision lattice parameter measurements and microscopic observations. The solubility limits of ytterbia in both monoclinic and tetragonal zirconia were determined. A eutectoid reaction, tetragonal zirconia solid solution monoclinic + cubic zirconia solid solutions at 400 ± 20°C and 2.4 mol % ytterbia was found. The left-hand boundary of the cubic zirconia solid-solution field was redetermined between room temperature and about 1700°C. Long-range ordering was present at 40 mol % ytterbia and the formation of an ordered phase, Zr₃Yb₄O₁₂, isostructural with M₇O₁₂-type compounds was found. Its thermal stability was established between room temperature and 1630 ± 10°C, in which it decomposes into cubic zirconia solid solution by an order-disorder reaction.     

A new tentative phase equilibrium diagram for the ZrO2-CeO2 system in air

The phase relationships over a wide range of temperature and compositions in the ZrO₂-CeO₂ system have been reinvestigated. From DTA results, thermal expansion measurements andK _IC determinations it was established that additions of CeO₂ to ZrO₂ decreases the monoclinic to tetragonal ZrO₂ transition temperature, from 990 ° C to 150 50 ° C, and an invariant eutectoid point at approximately 15 mol% CeO₂ exists. The extent of the different single- and two-phase fields were determined with precise lattice parameter measurements on quenched samples. Evidence for the existence of a binary compound Ce₂Zr₃O₁₀ (ø-phase) was obtained by X-ray diffraction. The ø-phase was stable below approximately 800 ° C, above which it decomposes into tetragonal zirconia + fluorite ceria solid solutions. Taking into account the polymorphic tetragonal-cubic transition and the narrowness of the two-phase tetragonal zirconia + fluorite ceria field above 2000 ° C, the existence of a new invariant eutectoid point was assumed, in which the metastable fluorite zirconia solid solution decomposes into tetragonal zirconia + fluorite ceria solid solutions. From the results obtained, the phase diagram also incorporates a eutectic point located at approximately 2300 ° C and 24 mol % CeO₂.     

Thermal behavior of the amorphous precursors of the ZrO2-SnO2 system

Thermal behavior of the amorphous precursors of the ZrO₂-SnO₂ system on the ZrO₂-rich side of the concentration range, prepared by co-precipitation from aqueous solutions of the corresponding salts, was monitored using differential thermal analysis, X-ray powder diffraction, Raman spectroscopy, field emission scanning electron microscopy (FE-SEM) and energy dispersive X-ray spectrometry (EDS). The crystallization temperature of the amorphous precursors increased with an increase in the SnO₂ content, from 405 °C (0 mol% SnO₂) to 500 °C (40 mol% SnO₂). Maximum solubility of Sn⁴⁺ ions in the ZrO₂ lattice (∼25 mol%) occurred in the metastable products obtained upon crystallization of the amorphous precursors. A precise determination of unit-cell parameters, using both Rietveld and Le Bail refinements of the powder diffraction patterns, shows that the incorporation of Sn⁴⁺ ions causes an asymmetric distortion of the monoclinic ZrO₂ lattice. The results of phase analysis indicate that the incorporation of Sn⁴⁺ ions has no influence on the stabilization of cubic ZrO₂ and negligible influence on the stabilization of tetragonal ZrO₂. Partial stabilization of tetragonal ZrO₂ in products having a tin content above its solid-solubility limit was attributed to the influence of ZrO₂-SnO₂ surface interactions. In addition to phases closely structurally related to cassiterite, monoclinic ZrO₂ and tetragonal ZrO₂, a small amount of metastable ZrSnO₄ phase appeared in the crystallization products of samples with 40 and 50 mol% of SnO₂ calcined at 1000 °C. Further temperature treatments caused a decrease in and disappearance of metastable phases. The results of the micro-structural analysis show that the sinterability of the crystallization products significantly decreases with an increase in the SnO₂ content.     

Thermal barrier coatings from sol-gel-derived spray-grade Y2O3-ZrO2 microspheres

For the development of ceramic thermal barrier coatings, spray-grade yttria-stabilized zirconia microspheres were prepared by the sol-gel technique. Oxide microspheres were obtained by calcination of the corresponding gel spheres at 1000 °C. Scanning electron microscopic and optical microscopic observations revealed the material thus obtained to have a predominantly spherical morphology and the requisite size distribution (5–50 m). The dense, calcined microspheres showed good flowability. X-ray diffraction studies indicated the presence of the tetragonal polymorph of ZrO₂ as the major phase, in addition to about 14% monoclinic ZrO₂. The plasma-sprayed YSZ coatings made from the sol-gel-derived microspheres showed a further decrease in the monoclinic ZrO₂ content (6%). The coatings survived 40–50 thermal cycles (30 min at 1200 °C followed by a water quench), indicating good thermal shock resistance.     

Low temperature phase equilibria and ordering in the ZrO2-rich region of the system ZrO2-CaO

From co-precipitated powder samples, the solid state reactions occurring between room temperature and 1500° C in the ZrO₂-CaO system have been studied. At low temperatures, compositions containing < 25 mol% CaO show a complex picture of phase transformation and ordering in the system. From the obtained results the following singular reactions have been established. (i) Tetragonal zirconia solid solution decomposes eutectoidally at 7 mol% CaO and 1048 ± 4° C into monoclinic zirconia solid solution and calcium zirconate (CZ). (ii) Cubic zirconia solid solution undergoes a eutectoidal decomposition at 17.5 mol% Cao and 1080 ±20° C into tetragonal solid solution + calcium zirconate. (iii) The monoclinic ordered phase, CaZr₄O₉ (₁), ), undergoes an order-disorder transformation into cubic zirconia solid solution at 1232 ± 5° C. (iv) Cubic zirconia solid solution undergoes a eutectoidal decomposition into two ordered phases, ₁ + ₂ at 21 mol% CaO and 1200 ± 10°C. (v) Hexagonal ordered phase Ca₆Zr₁₉O₄₄ (₂) decomposes peritectoidally into cubic zirconia solid solution + calcium zirconate at 1360 ± 10° C. The two ordered phases ₁ and ₂ seem to be unstable below 1100° C. By using DTA, X-ray diffraction and SEM techniques, the extent of the tetragonal and cubic zirconia solid solution fields have been established. From the above experimental results a new tentative phase diagram is given for the ZrO₂-rich region of the system, ZrO₂-CaO.     

Preparation and thermal evolution of sol-gel derived transparent ZrO2 and MgO-ZrO2 gel monolith

Transparent gel monoliths of pure and MgO-doped zirconia having dopant concentrations in the range 0 to 15 mol % were prepared by chemical polymerization of zirconium n-propoxide and magnesium acetate tetrahydrate using 2-methoxy ethanol as solvent. The thermal evolution of amorphous gels was studied by differential thermal analysis, X-ray diffraction and transmission electron microscopy. The crystallization of pure and doped zirconia gels occurred in the temperature range 360 to 450° C. The first crystalline phase to appear is tetragonal for pure and 2 mol % doped zirconia, and cubic for 3 to 15 mol % doped samples. Both crystallization and decomposition temperatures are found to increase with increasing dopant concentration, approaching a saturation value for 10 mol % doped samples. It has been established that the transformation of the cubic to the monoclinic phase takes place via a metastable tetragonal phase. A linear relationship between the lattice parameter of cubic zirconia and MgO concentration has been established. X-ray diffraction studies have also revealed that the entire amount of MgO used in preparing doped zirconia gels remains in a single MgO-ZrO₂ crystalline phase formed initially by thermal treatment.[/p]     

Electrochemical ZrO2 and Al2O3 coatings on SiC substrates

SiC was electrochemically coated with ZrO₂ and with Al₂O₃ from 0.1 m aqueous solutions of metal-nitrate-hydrates with ethanol added. Amorphous zirconia and alumina coatings were formed with current densities from 10 to 70 mA cm^–2, and deposition durations of 1–60 min. The as-deposited coatings contained microcracks caused by drying shrinkage. Sintering of zirconia at 900 °C for 1 h and of alumina at 1200 °C for 2 h in air was accompanied by crystallization to a mixture of tetragonal and monoclinic phases in the former and to -alumina in the latter. The absence of intermediate phases between the coatings and the substrates and the good adherence of the sintered coatings indicate the high-temperature stability of these coatings.     

The modulated structure formed by isothermal ageing in ZrO2-5.2 mol % Y2O3 alloy

The modulated structure produced by isothermal ageing of ZrO₂-5.2 mol % Y₂O₃ alloy was examined mainly by electron microscopy. It was found that the modulated structure was formed at ageing temperatures between 1400 and 1600° C, but not at 1700° C. The structure is developed by spinodal decomposition, which produces compositional fluctuation in the elastically soft 111 direction in cubic zirconia. The hardness increase caused by the development of modulated structure during ageing is larger than the hardening by precipitation of tetragonal phase in the cubic matrix.Graduate Student, Tohoku Univerisy, Sendai, Japan.     

Preparation and study of YSTZ-Al2O3 nanocomposites

Yttria stabilized zirconia-alumina (YSTZ-Al₂O₃) nanocomposite system with various Al₂O₃ concentrations has been synthesized by sol-gel route. The experimental techniques XRD, DTA, TGA, FT-Raman, FT-IR, SEM, Vickers hardness measurements, density measurements and Impedance spectroscopy were used to characterize the synthesized specimens. DTA result shows two exothermic reactions: one around 760°C and another around 960°C. XRD results confirm that the specimen starts to crystallize on heating above 750°C. Well resolved XRD reflections corresponding to tetragonal (t) ZrO₂ were obtained after the specimens were heated at 1000°C. FT-Raman results confirmed that the crystallites developed above 750°C was t-ZrO₂. It was observed from the XRD and DTA results that the bulk and grain boundary region crystallize independently in two different temperatures with a difference in temperature of about 200°C. The crystallization temperatures increase with Al₂O₃ contents. At 1300°C, the pure YSTZ and 5 and 10 wt % Al₂O₃ added YSTZ specimens underwent structural transformation from tetragonal to monoclinic ZrO₂. But, the tetragonal symmetry remains stable at 1300°C with an addition of 15 wt % Al₂O₃. The system which retain its tetragonal symmetry at its processing temperature (1300°C) gives high hardness and maximum density values. Almost 100% theoretical density value was obtained at 1300°C with an addition of 15 wt % of Al₂O₃.     

Acoustic emission study of phase relations in low-Y2O3 portion of ZrO2-Y2O3 system

The (metastable) tetragonal phase in 3–4 mol% Y₂O₃-ZrO₂ alloys undergoes a transition to the monoclinic form in the 200–300 °C temperature range. Microcracking due to the volume change at this transition has been detected in these compositions by sharp acoustic emission during heating. The phase change was confirmed by X-ray diffraction, dilatometry and scanning electron microscopy. The monoclinic tetragonal transition in ZrO₂-1 mol% Y₂O₃ alloy at 850–750 °C and the same phase change in 2, 3, 4 and 6 mol% Y₂O₃ compositions at the eutectoid temperature of about 560 °C was also clearly signalled by the acoustic emission counts during heating and cooling. There was no significant acoustic emission activity on heating and cooling the 9 and 12 mol% Y₂O₃ compositions, which are cubic. The acoustic emission data thus confirm the phase relations in the 1–12 mol% Y₂O₃ region, established by conventional methods such as differential thermal analysis, dilatometry and X-ray diffraction.     

Formation and hot isostatic pressing of ZrO2 solid solution in the system ZrO2-Al2O3

In the system of ZrO₂-Al₂O₃, cubic ZrO₂ solid solutions containing up to 40 mol% Al₂O₃ crystallize at low temperatures from amorphous materials prepared by the simultaneous hydrolysis of zirconium and aluminium alkoxides. At higher temperatures, they transform into tetragonal solid solutions. Metastable ZrO₂ solid solution powders containing 25 mol% Al₂O₃ have been sintered at 1000–1150 °C under 196 M Pausing the hot isostatic pressing technique. The solid solution ceramics consisting of homogeneous microstructure with an average grain size of 50 nm exhibited a very high fracture toughness of 23 MN m ^–1.5. They have been characterized by X-ray diffraction and electron probe surface analyses.     

Formation of diffusionlessly transformed tetragonal phases in rapid quenching of ZrO2-Y2O3 melts

ZrO₂ polycrystals, partially stabilized by 2 to 7 mol% Y₂O₃, were arc-melted and rapidly quenched using an arc-imaging furnace with a hammer-anvil unit. Some of the specimens were further annealed at 1700° C for 3 h in air. The phases and the microstructures of these ZrO₂-Y₂O₃ polycrystals were examined through X-ray diffraction and transmission electron microscopy. Special emphasis was placed upon the examination of the microstructure of the metastable tetragonal phase (t phase) which was formed by a diffusionless transformation of the high-temperature cubic phase. It was found that the t phase exhibits a twinned and mosaic structure made of alternating layers of twin-related variants. A comparison of the present experimental results with other related works has also been made.     

Role of some rare earth (RE) ions (RE = La, Pr, Nd, Sm, Gd and Dy) in crystal and mechanical behaviours of sol-gel derived ZrO2-2 mol% REO2O3 spun fibres calcined at 1300°C

Crystal behaviours such as crystallization temperature (amorphous to tetragonal (t) zirconia), tendency of phase transformation (tetragonal to monoclinic (m) zirconia) and lattice strain were studied with mechanical property e.g. tensile strength of sol-gel derived ZrO₂-2 mol% RE₂O₃ (RE = La, Pr, Nd, Sm, Gd and Dy) spun fibres. Rare earth cations of varying sizes played a significant role in changing the above mentioned properties of ZrO₂-2 mol% RE₂O₃ fibres. It was found that with decreasing the ionic size difference between the zirconium and RE ions, crystallization temperature (amorphous →)t-ZrO₂) decreased, the probability of phase transformation (t→m) decreased, lattice strain which is related to lattice distortion decreased and tensile strength increased.     

Effects of ZrO2 precursors on the synthesis of V-ZrSiO4 solid solutions by the sol-gel method

The preparation of V-ZrSiO₄ solid solutions starting from different ZrO₂ precursors by using sol-gel methods is reported. The starting materials were hydrolysed and the dried gels were fired at a temperature between 500 and 900 °C with soaking times of 12h. The organic character of zirconia precursors was stronger, i.e. the starting material had more carbon atoms, a higher temperature was necessary to make the first crystalline phase appear (ZrO₂(tetragonal)) and the temperature range for the whole phase transformation was narrower. In all dried gel samples the presence of infrared bands which might be associated with either Si-O-Zr or Si-O-V was not observed. On the other hand, some bands could be attributed to a silica network and ZrO₈ groups. The main steps in V-ZrSiO₄ solid solutions were confirmed. ZrO₂(tetragonal) is crystallized on heating from an amorphous sample. The ZrO₂(tetragonal) ZrO₂(monoclinic) phase transformation then occurs and immediately afterwards the zircon formation begins. Finer textures in samples were obtained from polymeric gels rather than for colloidal gel samples, as seen from the scanning electron micrographs.     

Crystallization process of cubic ZrO2 in calcium acetate solution

Ultrafine powders of cubic ZrO₂ were obtained at about 270° C by heating hydrated amorphous ZrO₂ in greater than 0.2 molal calcium acetate solutions. Ca²⁺ ions played a role as nucleii for crystallization and were introduced into distinct sites of the crystalline phases, that is, substituted for Zr⁴⁺ ions. Mn²⁺ ions produced almost the same effects on the crystallization of ZrO₂·EPR spectra for powder samples containing Mn²⁺ ions apparently showed two types as follows: for tetragonal ZrO₂ with a trace of monoclinic ZrO₂, the central fine structure transitions (M=+1/2–1/2) showed a well-resolved hyperfine structure. In addition to the m=0 transition, forbidden m=±1 transitions were observed. For cubic ZrO₂, the broad underlying response was observed as well as the hyperfine structure composed of six main peaks.     

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.     

Effects of TiO2 addition on the sintering of ZrO2·TiO2 compositions and on the retention of the tetragonal phase of zirconia at room temperature

The production of tetragonal zirconia polycrystalline (TZP) ceramics and the identification of factors controlling retention of the tetragonal phase in the ZrO₂·TiO₂ system have been investigated. In this binary system, it was not possible to retain tetragonal zirconia polycrystals at room temperature for a range of compositions sintered above 1200 °C. A decrease in the martensitic transformation temperature of zirconia with titania addition was observed, but the effect was insufficient to retain the tetragonal phase at room temperature. In solid solution, the TiO₂ additions act to suppress ZrO₂ densification, this leading to grain growth when attempts are made to attain higher densities. The use of fine powders, fast firing or sintering in reducing conditions altered densification but was not able to generate a final grain size sufficiently small to avoid spontaneous tetragonalmonoclinic transformation on cooling. Based on the results obtained for ZrO₂·MO_x systems, the main factors involved in the retention of tetragonal zirconia at room temperature are discussed in an attempt to incorporate thermodynamical and the stress field effects.     

Fracture of metastable tetragonal zirconia crystals

Crystals of single-phased metastable tetragonal zirconia (MTZ) were prepared from skullmelting at the composition ZrO₂-3 mol % M₂O₃ (M = Y, Yb or Gd). The fracture of the tetragonal crystals occurred differently than for the cubic stabilized zirconia ones. The toughness was much higher for the tetragonal specimens and the cleavage planes were not the same as for the fluorite crystals. The results were interpreted by considering the domain microstructure induced by the cubic → tetragonal phase transformation undergone by the crystals.     

Solid solutions of metastable tetragonal ZrO2 and Ce3ZrO8 in the system ZrO2-CeO2

In the system ZrO₂-CeO₂, metastable t-ZrO₂ solid solutions containing up to 30 mol% CeO₂ crystallize at temperatures of 385–430 °C from amorphous materials prepared by the hydrazine method. Crystalline Ce₃ZrO₈ solid solutions are formed in as-prepared powders between 30–75 mol % CeO₂. The variation of the lattice parameters of both solid solutions is determined as a function of CeO₂ content. The value of the lattice parameter of pure Ce₃ZrO₈ (cubic) is a = 0.5342 nm. Detailed characterization of the Ce₃ZrO₈ powder has been performed. Crystallite size and particle size are strongly dependent on the heating temperature. Specific surface areas do not drop below 40 m²g^–1 until the heating temperature is above 1000°C.