Tetragonal Zirconia Solid Solution Research Articles

Crystallization kinetics in a lithium alumosilicate glass using SnO2 and ZrO2 additives

The crystallization sequence, volume fractions and crystal sizes of nucleating agents, high-quartz (HQ-ss) and keatite (K-ss) solid solutions in a lithium alumosilicate glass with additions of SnO2 and ZrO2 were studied using high-temperature X-ray diffraction, differential thermal analysis and optical dilatometry. At low tin oxide fractions primary crystals were tetragonal zirconia solid solution, while at higher tin oxide fractions orthorhombic stannic srilankite solid solutions were detected. For continuously heated glasses we found that the substitution of zirconium by tin shifts in agreement with DTA and shrinkage curves the crystallization of HQ-ss and K-ss towards lower temperatures while only a minor effect on the glass transition temperature was evident. Isothermal annealing showed that the increase in the tin oxide fraction of the parent glass increases mainly the nucleation rate of HQ-ss (up to one order of magnitude) while the increase in the crystal growth rate is less pronounced. Dwelling of mixed ZrO2/SnO2-bearing glasses at the temperature of maximum nucleation results in fine-grained glass-ceramics (crystal size=50nm) of crystal number densities up to 1013mm−3.

Low-temperature sintering of yttria-stabilized zirconia produced from amorphous powders

Low-temperature sintering of a tetragonal zirconia solid solution proceeds through the fracture of all agglomerates during pressing of samples from hydroxide powder coprecipitated from an aqueous solution and through the increased reactivity of amorphous zirconium hydroxide and oxide. Thermal treatment at 1100°C for 1 h produces ceramics with relative density 0.928, grain size 120–135 nm, and pore size 50–75 nm. Sintering is most intensive in the temperature range 950–1150°C and is less active in the range 800–950°C.

Change in pore structure of yttria-stabilized zirconia during sintering

The behavior of coarse and fine pore channels and closed pores (pore space components) in temperature ranges of intensive (900–1150°C) and less active (1150–1400°C) sintering of a sample pressed from nanosized powder of tetragonal zirconia solid solution is examined. The change in the volume of the sample in the temperature range of intensive sintering is determined by approximately equal decrease in the volume of coarse and fine pore channels, which originate from channels between agglomerates and between aggregates in the powder agglomerates. Closed pores of the sample originating from closed pores of the powder make a minor contribution to the decrease in volume since they contain air. In the temperature range of less active sintering, all fine (200–300 nm) and coarse (0.8–1 μm) pore channels are broken and new closed pores are formed.

Aluminum doped zirconia nanopowders: Wet-chemical synthesis and structural analysis by Rietveld refinement

Alumina/zirconia nanopowders, with up to 20mol% Al2O3, were prepared by wet-chemical synthesis technique, using controlled hydrolysis of alkoxides. The as-synthesized powders are amorphous, have very high specific surface area and the corresponding particle size smaller than 4nm. Amorphous powders with 0, 10 and 20mol% Al2O3 crystallize at 460, 692 and 749°C, respectively, as a single-phase tetragonal zirconia, without any traces of alumina phases. Rietvled refinement of X-ray diffraction data, used for the detailed structural analysis of annealed nanopowders, showed that the high-temperature zirconia phase is stabilized due to the formation of ZrO2/Al2O3 solid solutions. High solubility of alumina in the tetragonal zirconia (up to 28.6at% Al3+) and stabilization of tetragonal zirconia solid solution up to high temperature (as high as 1150°C) were also confirmed.

Phase evolution of sol-gel CaO-ZrO2 using sulfuric acid as hydrolysis catalyst

Several compositions in the CaO-ZrO2 system were synthesized from zirconium n-butoxide and calcium methoxide, by the sol-gel method. Hydrolysis and gelation occurred at pH 3, using H2SO4 as hydrolysis catalyst. Fresh gels were annealed in air at 100 to 900°C, in 100°C steps every 20 h, for a total annealing time of 140 h. Analysis by X-ray diffraction showed the formation of hydrated calcium sulfate together with amorphous zirconia up to 400°C. At the ZrO2 rich-end, tetragonal and monoclinic zirconia solid solutions were stabilized in the presence of Ca ions. When 20 and 30 wt% of CaO were added, cubic zirconia and CaZrO3 solid solutions were observed above 700°C. At the CaO rich-end, the coexistence of calcium carbonate polymorphs as vaterite and calcite were observed. Anhydrite was present across the entire range of compositions studied from 300 to 900°C.

ＺｒＯ２‐ＳｉＯ２‐ＣｅＯ２‐Ｙ２Ｏ３系アモルファスセラミックス粉末の創製と加圧焼結によるち密化

Mechanically alloyed amorphous ceramic powders with a chemical composition of (ZrO2-3 mol%Y2O3)-26.2 mol%SiO2-3.4 mol%CeO2 have been prepared by a planetary ball mill. Thermal differential analysis (DTA) experiments show that the onset temperature of crystallization increases from 1071 to 1105 K as the heating rate increases from 5 to 50 K/min. These amorphous powders have been consolidated at the heating rate of 480 K/min under the applied stress of 150 MPa in a chamber evacuated to 10 Pa, using an electrical discharge consolidation system. After the formation of tetragonal zirconia solid solution, marked increase in the relative density occurs in the narrow temperature range from 1178 to 1315 K. The fully dense bulk sample sintered at 1315 K consists of tetragonal (or tetragonal+cubic) zirconia solid solution together with amorphous silica. The average crystallite size increases from 10 to 15 nm as the consolidation temperature increases from 1223 to 1315 K. Moreover, the isothermal consolidation treatment for about 700 s causes further increase in the crystallite size by a factor of two, regardless of the consolidation temperature. These results indicate that the crystallite size can be widely changed (10~30 nm) by controlling the consolidation temperature and the isothermal consolidation period of time.

Structural characterization and mixed conductivity of TiO 2-doped ceria stabilized tetragonal zirconia

By using X-ray diffraction lattice parameter measurements and Raman spectroscopy studies, the solid solubility limits of titania in ceria tetragonal zirconia solid solutions (12 mol% CeO 2) have been established. Electrical properties of the mixed conductor TiO 2–Ce–TZP containing 5 and 10 mol% TiO 2 were measured at the 300 °C–900 °C temperature range. The total electrical conductivity in air of the TiO 2–Ce–TZP ceramics decreases with titania content due to Ti-Vö associations, which depresses the mobility of the oxygen vacancies during the conduction process. The electrical behaviour of undoped-Ce–TZP solid solutions as a function of the pO 2 and temperature showed that in oxidising conditions and up to an oxygen partial pressure of 10 −9 Pa the materials were nearly pO 2 independent. Under lower oxygen partial pressure the total electrical conductivity increases according to pO 2 –1/6 indicating that the n-type conduction in Ce–TZP is due to the electron hopping between Ce 4+ and Ce 3+ cations. In Ce–TZP containing 5 and 10 mol% titania the electrical behaviour was very different mainly under the most reducing conditions. Above 800 °C and with decreasing oxygen partial pressure below 10 −9 Pa, the theoretical correlation for electronic conduction was near to n ∝ pO 2 −1/4, which indicates that the conduction process is controlled by the Ce′ Zr and Ti′ Zr defect concentrations, by hopping of the electrons between Ce 4+ and Ce 3+ and Ti 4+ and Ti 3+ via a small polaron hopping mechanism.

Phase diagram of the system: ZrO 2–Cr 2O 3

The ZrO 2–Cr 2O 3 system was studied by differential thermal analysis in the composition range from 0 to 40 mass% Cr 2O 3. The formation of extensive solid solutions (25 mass% Cr 2O 3 in cubic zirconia and 40 mass% ZrO 2 in Cr 2O 3 at the eutectic temperature) and the eutectic point coordinate: 1950°C and 50 mass% of ZrO 2 are reported. Cubic zirconia solid solution decomposes at 1840°C, 25 mass% Cr 2O 3, by eutectoid reaction forming tetragonal zirconia solid solution with 9 mass% Cr 2O 3. This phase transforms at 1115°C to monoclinic zirconia solid solution. The enthalpies of these transformations are: 610±60 J/g for eutectic fusion, 110±10 J/g for eutectoid at 1840°C, 25 mass% Cr 2O 3, 22±5 J/g for eutectoid at 1115°C, 9 mass% Cr 2O 3 and 42±5 J/g for pure zirconia at 1165°C.

Crystallization behaviour and microstructural development in ZrSiO4 and V-ZrSiO4 solid solutions from colloidal gels

Zircon and vanadium-doped zircon blue pigments were prepared by heat treatment of gel precursors. Gels with nominal compositions Vx-ZrSiO4 with x=0.0, 0.002, 0.004, 0.02 and 0.2 were prepared by formation of a silica coating on zirconia colloidal particles previously obtained. The crystallization behavior and microstructural evolution were studied using X-ray powder diffraction (XRD), energy dispersive X-rays microanalysis (EDX), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and selected area electron diffraction (SAED). The results indicated that the vanadia loading in the precursor gels speeds up the crystallization of the vanadium-containing tetragonal zirconia solid solutions and their transformation to the monoclinic form. The overall conversion rate of gel precursors to pigmenting powders increased when the vanadia content was higher. Microstructural data revealed that the used procedure for the preparation of vanadium-zircon pigments allowed high-purity and non-agglomerated powders with controlled particle size to be obtained.

Structure–electrical properties relationships in TiO 2-doped stabilized tetragonal zirconia ceramics

The tetragonal zirconia solid solution field in the ternary system ZrO 2–Y 2O 3–TiO 2 has been established, and the influence of TiO 2 addition on the electrical properties of the formed ternary Ti-YTZP solid solutions has also been studied. The unit cell parameters determined from the X-ray diffraction patterns of the sintered samples, and the data obtained from X-ray absorption (XANES and EXAFS) were used to provide information on the environment of Ti atoms. The electrical conductivity of the Ti-YTZP tetragonal solid solutions decreases with increasing titania concentration. EXAFS and XANES results show that as the TiO 2 dissolves into the tetragonal zirconia YTZP matrix, a displacement of Ti 4+ ions from the center of symmetry seems to take place which leads to a state of non-random substitution of Ti 4+ ions on Zr 4+ lattice sites. Ti–O bond distances derived from EXAFS indicate that Ti ions can be in a square-pyramidal arrangememt, i.e. five-fold oxygen coordinated. As consequence, two kinds of cation–oxygen vacancy associations with different diffusion dynamics are created. This results in a decrease in the global concentration of moving oxygen vacancies and, therefore, in a decreasing of the ionic conductivity. ©

Structure and electrical behaviour in air of TiO 2-doped stabilized tetragonal zirconia ceramics

The data obtained from X-ray absorption (XANES and EXAFS) were used to provide information on the environment of Ti atoms in the TiO 2-doped stabilized tetragonal zirconia solid solutions. The electrical conductivity of the Ti–YTZP ceramics decreases with increasing TiO 2 content. From the EXAFS results, i.e. Ti–O and Ti–Ti distances, the decrease in conductivity was attributed to the formation of two kinds of cation–oxygen vacancy associations with different diffusion dynamics, leading to a decrease in the global concentration of moving oxygen vacancies.

The solid solubility limit of TiO 2 in 3Y-TZP studied by Raman spectroscopy

Raman spectra of tetragonal zirconia solid solutions have been studied for the ZrO 2–Y 2O 3–TiO 2 system. A linear relationship between the frequency of the Raman lines associated to the Zr–O I and ZrO II stretching modes, as a function of the TiO 2 content seems to exist. The constancy in the frequency of the two Raman modes beyond a determined TiO 2 concentration was taken to establish the solid solubility limit of TiO 2 into tetragonal zirconia (3Y-TZP) which, on the other hand, was in close agreement with that obtained by precision lattice parameter measurements. The influence of the TiO 2 addition on the Zr–O I and Zr–O II bond lengths in 3Y-TZP is also reported.

Structure and electrical behavior in air of TiO 2 -doped stabilized tetragonal zirconia ceramics

-doped YTZP ([%mol]3 Y2O3) compositions sintered in the temperature range of 1300 to 1450 °C, the tetragonal zirconia solid solutions field for the ZrO2-Y2O3-TiO2 system was established. The solubility of TiO2 in YTZP was found to be about 12–[%mol]14 at 1450 °C. Structural characterization of the Ti-YTZP tetragonal zirconia solid solutions was carried out using X-ray absorption spectroscopy (EXAFS and XANES) to provide information on the environment of Ti atoms. The electrical behavior in air of the TiO2-doped tetragonal zirconia solid solutions was studied by impedance spectroscopy in the temperature range of 300 to 800 °C, and it was found that the ionic conductivity decreases with increasing titania content. EXAFS and XANES results show that as the Ti4+ ions dissolve into the tetragonal zirconia YTZP matrix, a displacement of Ti ions from the center of symmetry takes place, leading to a non-random substitution of Ti4+ ions on Zr4+ lattice sites. Ti-O bond distances derived from EXAFS indicate that the Ti ion can be in a square-pyramidal arrangement, i.e., fivefold oxygen-coordinated. As a consequence two kinds of cation–oxygen vacancy associations are created; the high-mobility oxygen-vacancy–eightfold-coordinated cation (Zr4+) and the low-mobility oxygen-vacancy–fivefold-coordinated cation (Ti4+). This results in a decrease in the global concentration of moving oxygen vacancies and, therefore, in a decrease of the electrical conductivity.

Fracture Toughness, Ionic Conductivity, and Low‐Temperature Phase Stability of Tetragonal Zirconia Codoped with Yttria and Niobium Oxide

Tetragonal zirconia (t‐ZrO2) solid solutions were prepared with additions of up to 1.5 mol% of niobium oxide (Nb2O5) into 3‐mol%‐yttria‐stabilized t‐ZrO2 (3Y‐TZP). The influence of pentavalent cation doping on fracture toughness, ionic conductivity, and the tetragonal‐to‐monoclinic phase transformation in the temperature range of 120°‐210°C was investigated. Fracture toughness and ionic conductivity increased and decreased, respectively, as the Nb2O5 content increased, which indicated that the annihilation of oxygen vacancies in 3Y‐TZP was responsible for the instability of the t‐ZrO2 lattice. The activation enthalpy related to the conductivity was ~83 kJ/mol, regardless of the dopant content, which was consistent with that for the low‐temperature degradation of 3Y‐TZP doped with Nb2O5. Degradation under an applied electric field occurred only on the specimen surface that was in contact with the anode, which suggests that depletion of the oxygen vacancies led to the degradation. The identical activation enthalpies and the involvement of the vacancy migration in both processes fortified the belief that the low‐temperature degradation of yttria‐stabilized t‐ZrO2 is attributed to oxygen vacancy diffusion.

Thermodynamic assessment of the ZrO2-AlO1.5 system

The ZrO2-AlO1.5 quasibinary system has been assessed by means of the CALPHAD (CALculation of PHAse Diagram) technique. The liquid phase, cubic (fluorite-type) zirconia solid solution and tetragonal zirconia solid solution are described by a regular solution model. The monoclinic zirconia and α-AlO1.5 are treated as stoichiometric phase. A consistent set of optimized parameters describing the system have been obtained to agree with almost all of the avaiable experimental data. Comparisons between the assessed and experimental data are presented. It is shown that further studies are needed for equilibrium solubilities of AlO1.5 in ZrO2 phases.

Effect of Tetravalent Dopants on Raman Spectra of Tetragonal Zirconia

Tetragonal zirconia solid solutions were prepared by the additions of ceria, titania, germania, and cassiterite into 2‐mol%‐Y2O3‐stabilized t‐ZrO2 (2Y‐TZP), and the effect of the dopant size on the changes in the Raman spectra of t‐ZrO2 was investigated. The frequencies of the Raman modes that were associated with Zr‐O bond stretching decreased as doping with the oversized cation (Ce4+) increased and increased linearly as doping with the undersized dopants (Ti4+, Ge4+, Sn4+) increased within the solubility limits of the dopants in 2Y‐TZP. The frequency of the 640 cm−1 Raman band decreased as the square root of the unit‐cell volume increased, suggesting that the ionic size of dopants governs the Raman mode that is related to the short ZrO bond between two ZrO bonds in t‐ZrO2 solid solutions. Although there was no consistent relationship between the change in the tetragonality of t‐ZrO2 solid solutions and the tetravalent cation size, the intensity ratio of the Raman modes at 609 and 640 cm−1 (I609/I640) decreased as the ionic size decreased.

Effect of Dopants on Zirconia Stabilization—An X‐ray Absorption Study: III, Charge‐Compensating Dopants

X‐ray absorption spectra at the Zr‐, Y‐, and Nb‐K edges of ZrO2‐YNBO4 solid solutions have been measured at 10 K to determine the local atomic structures. Both Y and Nb cations substitute for Zr in the cation network but maintain rather different local oxygen coordination from Zr. In tetragonal zirconia solid solutions, Y adopts a YO8 structure with a bond length of 2.32 Å. This is the same as the structure found in ZrO2–Y2O3, solid solutions, confirming our previous conclusion that Y is not associated with oxygen vacancies. Nb has an NbO4 structure with a bond length of 1.90 Å. This is shorter than the Zr–O, distance of 2.10 Å. The strong Nb–O coordination increases the bonding disparity between Zr–O layers, thus increasing tetragonality. This is similar to the trend previously established for ZrO2‐GeO2 solid solutions. Severe distortion of neighboring cations around the undersized Nb, similar to that previously found for undersized Fe3+ and Ga3+, is also observed. At higher temperatures, local Y–Nb cation ordering occurs at a concentration below the solubility limit, similar to the Zr–Ge ordering reported previously. This cation ordering mechanism allows the charge‐compensating Y‐Nb pair to stabilize the tetragonal structure but increase tetragonality.

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.

Thermodynamic Assessment of the ZrO2─YO1.5 System

An optimal set of thermodynamic functions for the ZrO2─YO1.5 system are obtained using phase diagram and thermodynamic data. The liquid is described by a subregular solution model. Both cubic ZrO2 and YO1.5 solid solutions are regarded as one cubic solution, which is also treated as a subregular solution. The ordered Zr3Y4O12 phase is treated as a stoichiometric compound. A regular solution model is applied to the other solid solutions. Tentative equilibrium boundaries between monoclinic and tetragonal ZrO2 solid solutions are evaluated from information about the T0 line. The calculated phase diagram and thermodynamic functions agree well with experimental data.

Preparation, Sintering, and Properties of Translucent Er2O3‐Doped Tetragonal Zirconia

Tetragonal zirconia doped with 3 mol% Er2O3 was prepared by the gel‐precipitation wet‐chemical method. The compaction response of ultrafine (∼8.5 nm), calcined, deagglomerated powders was studied. Initial sintering was studied using both isothermal and nonisothermal techniques, and an activation energy of 270 ± 40 kJ/mol was obtained; therefore, grain‐boundary diffusion was probably the predominant mechanism in the sintering stage. The microstructural development in the high‐temperature‐aged, sintered samples and its effect on density and mechanical properties were also studied. A theoretically dense body of tetragonal zirconia solid solution of 3 mol% Er2O3–ZrO2, obtained from sintering below 1400°C, was translucent.