Diffusion of niobium in yttria-stabilized zirconia and in titania-doped yttria-stabilized zirconia polycrystalline materials

Bulk and grain boundary diffusion of Nb⁵⁺ cations in yttria-stabilized zirconia (YSZ, 8 mol% Y₂O₃–92 mol% ZrO₂) and in titania-doped yttria-stabilized zirconia (Ti–YSZ, 5 mol% TiO₂–8 mol% Y₂O₃–87 mol% ZrO₂) was studied in air in the temperature range from 900 to 1300 °C. Experiments were performed in the B-type kinetic region. Diffusion profiles were determined using the secondary ion mass spectrometry (SIMS). The temperature dependencies of the bulk diffusion coefficient D and the grain boundary diffusion parameter D′δs for both the materials were calculated. The activation energies of these transport processes in YSZ amounts to 258 and 226 kJ mol⁻¹, respectively, and 232 and 114 kJ mol⁻¹ in Ti–YSZ. The results were compared to the diffusion data of other cations previously obtained for the same material.     

Isothermal sintering kinetic of 3YTZ and 8YSZ: Cation diffusion

We have studied the sintering kinetic of 3 and 8 mol% of yttria stabilized zirconia under isothermal conditions. Sintering was performed in the temperature range between 1200 and 1450 °C. The sintering kinetic process was followed by measuring density as a function of sintering time. A model was applied to the first stage of densification. Sintering obeys to the grain boundary diffusion mechanism for both materials. It was possible to calculate the activation energy as well as the diffusion coefficients. 887 and 731 kJ mol^?1 were the activation energies for the initial stage of sintering for 3YTZ and 8YSZ respectively.Finally the diffusion activation energy was estimated for both materials. The diffusion coefficients were also estimated at 1400 °C in 4.05×10^?14 and 6.00×10^?11 cm² s^?1 for tetragonal and cubic zirconia respectively. The obtained results support the observations of a faster densification for 8YSZ.     

Grain-boundary cation diffusion in ceria tetragonal zirconia determined by constant-strain-rate deformation tests

Ceria tetragonal zirconia polycrystals with a content of 12 mol% ceria (CeTZP) have been tested in compression at constant strain rate between 1150 °C and 1300 °C. An accurate analysis of the stress–strain curves has permitted to determine the value of the grain boundary cation diffusion. The results are compared with those reported in literature for this alloy and yttria tetragonal zirconia polycrystals (YTZP). An isotopic effect is found to correlate both grain boundary diffusion coefficients.     

Ionic conductivity of tetragonal ZrO2 polycrystal doped with TiO2 and GeO2

The effects of the co-doping and the resultant co-segregation of 2 mol% TiO₂ and 2 mol% GeO₂ on the ionic conductivity and on the chemical bonding state in a tetragonal ZrO₂ polycrystal were investigated. The conductivity data and grain boundary microstructure showed that the doped Ti⁴⁺ and Ge⁴⁺ cations segregate along the grain boundary, and this segregation causes a reduction in the conductivity of both the grain interior and grain boundary and an increase in the activation energy of the grain boundary conductivity. Overall, the data indicate that the segregation retards the diffusion of oxygen anions. A first-principle molecular orbital calculation explains the retarded diffusion of the oxygen anion from a change in the covalent bonds around the dopant cations; an increase in the strength of the covalent bond between the oxygen and doped cation should work to suppress the diffusion of the oxygen anion.     

Fabrication of alumina-based ceramic foams utilizing superplasticity

Model ceramic foams were fabricated by expanding once-sintered dense shells utilizing the superplastic deformation of alumina dispersed with 3 mol% yttria-stabilized zirconia (3YSZ) or magnesia. The grain growth of alumina was suppressed by adding 3YSZ and the resultant grain size and amount of dispersion were closely related to the total porosity. Total porosity of the ceramic foam depended on the grain size and their distributions irrespective of the size of starting powders.     

Effect of Fe2O3 doping on sintering of yttria-stabilized zirconia

This paper reports the effect of Fe₂O₃ doping on the densification and grain growth in yttria-stabilized zirconia (YSZ) during sintering at 1150 °C for 2 h. Fe₂O₃ doped 3 mol% YSZ (3YSZ) and 8 mol% YSZ (8YSZ) coatings were produced using electrophoretic deposition (EPD). For 0.5 mol% Fe₂O₃ doping, both 3YSZ and 8YSZ coatings during sintering at 1150 °C has similar densification. However, a significant grain growth occurred in 8YSZ during sintering, whereas grain size remains almost constant in 3YSZ. XRD results suggest that Fe₂O₃ addition substitutionally and interstitially dissolved into the lattice of 3YSZ and 8YSZ. In addition, colour of 3YSZ and 8YSZ changes differently with doping of Fe₂O₃. A Fe³⁺ ion interstitial diffusion mechanism is proposed to explain the densification and grain growth behaviour in the Fe₂O₃ doped 3YSZ and 8YSZ. A retard grain growth observed in the Fe₂O₃ doped 3YSZ is attributed to Fe³⁺ segregation at grain boundary.     

Thermodynamic calculations of t to m martenstic transformation of ZrO2–CaO binary system

The phase transformation of different polymorphs in zirconia is very important for the processing and mechanical properties of zirconia ceramics. This paper adopts thermodynamic model which is suitable for ceramic system to calculate the Gibbs free energy change of tetragonal and monoclinic phases in ZrO₂–CaO binary system. The difference of the Gibbs free energy between tetragonal and monoclinic phases in ZrO₂–CaO as a function of composition and temperature, namely t → m phase transformation driving force, is thermodynamically calculated from the binary systems. Furthermore, in 8.0 mol% CaO–ZrO₂, the equilibrium temperature between tetragonal and monoclinic phases, T₀, was obtained as 1270.3 K, and martensitic transformation starting temperature (M_s) for t → m transformation of this ceramic with a mean grain size of 2.0 mm was calculated as 805.9 K, which is good agreement with experiment one of 793 K with 12.9 K residual.     

High temperature creep behaviour of 4 mol% yttria tetragonal zirconia polycrystals (4-YTZP) with grain sizes between 0.38 and 1.15 μm

The high temperature mechanical behaviour of 4% mol yttria tetragonal zirconia polycrystals (4-YTZP) with different grain sizes (0.38 < d < 1.15 μm) has been analyzed by means of compression creep tests. The working temperature was 1350 °C and the strain rates ranged between 5 × 10⁻⁷ and 2 × 10⁻⁴ s⁻¹. Experimental results have been fitted to the conventionally accepted creep law for superplastic ceramics. Thus, stress exponents and activation energies have been measured as a function of the grain size. The dependence of strain rate on grain size has also been determined. The experimental data are discussed with respect to the existing theoretical models for these materials.     

Texturing of 3Y-TZP zirconia by electrophoretic deposition in a high magnetic field of 17.4 T

Ceramics can be textured during colloidal processing by aligning the suspended particles in a strong magnetic field and retaining this alignment in the green body. Attempts to align tetragonal zirconia particles however were not successful, not even in 12 T magnetic fields. In the current work, monoclinic zirconia was successfully aligned with its (1 0 0) plane perpendicular to the magnetic field direction by electrophoretic deposition (EPD) in a 17.4 T field. Moreover, textured tetragonal zirconia was developed by reactive sintering of undoped pure monoclinic zirconia and co-precipitated 8 mol% yttria-stabilized zirconia. The sintered tetragonal zirconia inherited the alignment of the monoclinic zirconia particle precursor and aligned with its (0 0 1) plane perpendicular to the magnetic field direction. The toughness of the (0 0 1)-oriented 3Y-TZP along the [0 0 1] direction of the textured zirconia was 65% higher than normal to it and 48% higher than the randomly oriented material.     

One-step alkali chloride-assisted solution combustion synthesis of 3YSZ nanopowders with ultrahigh specific surface area

The controlled fabrication of monodisperse yttria-stabilized zirconia powders with high specific surface area (SSA) is crucial for their application performances to be bought into full play. Herein, the 3 mol% yttria stabilized zirconia (3YSZ) with ultrahigh SSA was synthesized by an one-step alkali chloride-assisted solution combustion process, employing urea as fuel and easily available ZrOCl₂·8H₂O as zirconium source. The effects of the alkali chloride amount/sort, urea amount and ignition temperature (T_ig) on the microstructure of the resultants were systematically investigated. The results show that the 3YSZ derived from KCl-assisted solution combustion consists of monodisperse nanocrystals with 3.2 nm in average size (D_TEM) and 378 m² g⁻¹ in SSA under the condition of stoichiometric urea-to-nitrate ratio (α = 1), 4:1 M ratio of alkali chloride to metal ions of 3YSZ (β = 4) and T_ig = 500 °C. The introduction of alkali chlorides into the combustion mixture solution plays a critical part in the formation of well-dispersed 3YSZ nanocrystals with ultrahigh SSA. The amount/sort of alkali chloride, urea amount and ignition temperature also have significant influence on the SSAs of the resultants. In summary, the approach offers a facile, efficient, scalable and cost-effective strategy for fabrication of zirconia-based oxides with ultrahigh SSA.     

Two-Step Sintering of oxide ceramics with various crystal structures

The influence of Two-Step Sintering (TSS) process on the final microstructure of oxide ceramic materials with three different crystal structures was studied. Two kinds of alumina (particle size 100 nm resp. 240 nm) as well as tetragonal zirconia (stabilized with 3 mol% Y₂O₃, particle size 60 nm) and cubic zirconia (8 mol% Y₂O₃, 140 nm) powders were cold isostatically pressed and pressurelessly sintered with different heating schedules. The microstructures achieved with TSS method were compared with microstructures achieved with conventional Single-Step Sintering (SSS) schedule. The results showed that the efficiency of the TSS of these oxide ceramics was more dependent on their crystal structure than on their particle size and green body microstructure. The method of TSS brought only negligible improvement of the microstructure of tetragonal zirconia and hexagonal alumina ceramics. On the other hand, TSS was successful in the sintering of cubic zirconia ceramics; it led to a decrease in grain size by a factor of 2.     

Phase-transformation and grain-growth kinetics in yttria-stabilized tetragonal zirconia polycrystal doped with a small amount of alumina

Microstructure development during sintering in 3 mol% Y₂O₃-stabilized tetragonal zirconia polycrystal doped with a small amount of Al₂O₃ was investigated in the isothermal sintering conditions of 1300–1500 °C. At the low sintering temperature at 1300 °C, although the density was relatively high, the grain-growth rate was much slow. In the specimen sintered at 1300 °C for 50 h, Y³⁺ and Al³⁺ ions segregated along grain boundaries within the widths of about 10 and 6 nm, respectively. In grain interiors, the cubic-phase regions were formed by not only a grain-boundary segregation-induced phase-transformation mechanism but also by spinodal decomposition. The grain-growth behavior was kinetically analyzed using the grain-size data in 1300–1500 °C, which indicated that the grain-growth rate was enhanced by Al₂O₃-doping. These phase-transformation and grain-growth behaviors are reasonably explained by the diffusion-enhanced effect of Al₂O₃-doping.     

Phase stability and mechanical properties of TZP with a low mixed Nd2O3/Y2O3 stabiliser content

The phase assembly of 1.0–5.0 mol% Nd₂O₃-doped ZrO₂ sintered at 1400 °C revealed that the tetragonal ZrO₂ phase could not be completely stabilised. Co-stabilising of 0.5–2.5 mol% Nd₂O₃ with 0.5–1.0 mol% Y₂O₃, however, allowed the preparation of fully dense (Nd,Y)-TZP ceramics by pressureless sintering in air at 1450 °C. The mixed stabiliser monoclinic zirconia nanopowder starting material was synthesized from a suspension of neodymium nitrate, yttrium nitrate and monoclinic zirconia powder in an alcohol/water mixture. A HV₃₀ hardness of 10 GPa combined with an excellent indentation toughness of 13 MPa m^1/2 could be achieved for the (1.0Nd,1.0Y)- and (1.5Nd,1.0Y)-TZP ceramics. The influence of the mixed stabiliser content on the phase stability and mechanical properties are investigated and discussed.     

Improvements in microstructural and mechanical properties of ZrO2 ceramics after addition of BaO

The effects of the addition of BaO on the sinterability, phase balance, microstructure, and mechanical properties of 8 mol% yttria-stabilized cubic zirconia (8YSZ) were investigated using scanning electron microscopy, X-ray diffraction (XRD) analyses, and micro-hardness testing. The 8YSZ powder was doped with 0–15 wt% BaO using a colloidal process. The undoped and BaO-doped 8YSZ specimens were sintered at 1550 °C for 1 h. The XRD analyses results showed that the specimens doped with up to 1 wt% BaO did not exhibit BaO-related peaks, indicating that BaO was completely solubilized in the 8YSZ matrix. However, when more than 1 wt% BaO was added, BaZrO₃-related peaks appeared, suggesting that the overdoped BaO did not dissolve in the 8YSZ matrix but formed a secondary phase of BaZrO₃ at high temperatures. Grain size measurements showed that the grain size of 8YSZ decreased with an increase in the amount of BaO added. The decrease in the grain size was owing to the fact that the grains of BaZrO₃, which precipitated at the grain boundaries and grain junctions of 8YSZ, increased the grain boundary cohesive resistance because of the pinning effect. This resulted in a decrease in the grain boundary mobility, and an increase in the grain boundary energy. Furthermore, while the addition of BaO to 8YSZ caused a slight decrease in the hardness of 8YSZ, the fracture toughness of 8YSZ increased from 1.64 MPa m^1/2 to 2.08 MPa m^1/2, owing to the resulting decrease in the grain size.     

The influence of grain boundary misorientation on ionic conductivity in YSZ

Molecular dynamics (MD) was used to investigate the structure and ion transport properties of three interfaces in 8 mol% yttria-stabilized zirconia (YSZ); namely the Σ5 (310)/[001] and Σ13 (320)/[001] tilt grain boundaries and Σ5 (111) 60° twist grain boundary. Atomic interactions were described by a potential function of the Buckingham form. Diffusion rates of oxide ions in the grain boundary containing systems showed that the tilt grain boundaries reduce the overall ionic conductivity relative to a single crystal of 8 mol% YSZ, while the Σ5 twist boundary is able to support rapid diffusion and increases the total conductivity. The effect of segregation of dopant ions to the boundary regions was also investigated.     

Effect of two-step sintering on the hydrothermal ageing resistance of tetragonal zirconia polycrystals

The effects of two-step sintering (TSS) on the mechanical properties and hydrothermal ageing resistance of yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) were investigated. In TSS, the first step involved heating the samples up to 1400 °C at a heating rate of 10 °C/min and holding the samples at this temperature for 1 min. The second step involved sintering by cooling the samples down to 1200 °C and holding the samples at this temperature for various holding times (t) ranging from 0 to 30 h before cooling to room temperature. Moreover, TSS promoted densification with increasing holding time without sacrificing the mechanical properties of the sintered body and causing abnormal grain growth. The average grain size was found not to be affected by the long holding times, and the final microstructure composed of a uniformly distributed tetragonal grain having sizes ranging from 0.24 to 0.26 µm. The beneficial effect of TSS in suppressing the hydrothermal ageing of Y-TZP has been revealed in the present work. In particular, samples sintered at t=20 and 30 h exhibited excellent resistance to low-temperature degradation when exposed to superheated steam at 180 °C, attributed mainly to the enhance densification of the sintered bodies.     

Manipulating microstructure and mechanical properties of CuO doped 3Y-TZP nano-ceramics using spark-plasma sintering

Nano-powder composites of 3Y-TZP doped with 8 mol% CuO were processed by spark-plasma sintering (SPS). A 96% dense composite ceramic with an average grain size of 70 nm was obtained by applying the SPS process at 1100 °C and 100 MPa for 1 min. In contrast to normal, pressureless, sintering during SPS reactions between CuO and 3Y-TZP were suppressed, the CuO phase was reduced to metallic Cu, while the 3Y-TZP phase remained almost purely tetragonal. Annealing after SPS results in grain growth and tetragonal to monoclinic zirconia phase transformation. The grain size and monoclinic zirconia phase content are strongly dependent on the annealing temperature. By combining the processing techniques studied in this work, including traditional pressureless sintering, properties of the composite ceramic can be tuned via manipulation of microstructure. Tuning the mechanical properties of dense 8 mol% CuO doped 3Y-TZP composite ceramic by utilising different processing techniques is given as an example.     

Low temperature degradation resistant nanostructured yttria-stabilized zirconia for dental applications

When used in prosthetic dentistry, zirconia encounters severe durability issues due to low temperature degradation: exposure to humidity results in a transition from tetragonal to monoclinic phase, associated to disruptive integrity loss. Recently it has been shown that size-induced stabilization helps maintaining zirconia in tetragonal form, when the grain size is reduced to the nano-range. Objective of this work is to demonstrate the applicability of High Pressure Field Assisted Sintering (HP-FAST) to the preparation of dense, nanostructured samples of tetragonal yttria stabilized zirconia, with yttria content between 0.5 and 3 mol% and showing resistance to low temperature degradation. The yttria stabilized zirconia nanopowders were prepared by a hydrothermal method. Sintering by HP-FAST was performed at 900 °C in 5 min, under a pressure of 620 MPa. Resistance to low temperature degradation was tested at 134 °C, under vapor pressure, for up to 40 h. Both pristine and aged samples were characterized by X-ray diffraction, high-resolution scanning electron microscopy and nanoindentation tests in continuous stiffness measurement mode. The sintered samples presented a grain size between 20 and 30 nm and low or null monoclinic content. Both parameters resulted unaffected by ageing. The best results in terms of phase composition and mechanical properties have been obtained with the material containing 1.5 mol% of yttria. These results induce to reconsider the use of yttria stabilized zirconia as material for dental prosthetic systems requiring long-term durability.     

Barium modified lead lanthanum strontium zirconium niobium titanate for dielectric and piezoelectric properties

The isovalent Ba²⁺ modified lead lanthanum strontium zirconium niobium titanate (PLSZNT) solid solutions were prepared by solid-state reaction method with the compositional formula [Pb_{1−x−w−y}La_xSr_wBa_y][(Zr_zTi_1−z)_{(1−(x/4)−(5/4)k)}Nb_k]O₃ where Ba content, y, was varied from 0, 0.5, 1 and 1.5 mol%. X-ray diffraction (XRD) studies revealed that all the samples have ferroelectric tetragonal structure (FE_TET). The presence of multiple ions at Pb-site enhanced grain growth and further addition of Ba concentration resulted in restrained grain growth. Dielectric studies suggested that the ɛ_RT increased up to 1 mol% Ba while T_c has shown a continuous decreasing trend throughout the series. The piezoelectric parameters as a function of grain growth were characterized. The optimum piezoelectric charge coefficient (d₃₃ = 538 pC/N) and piezoelectric planar coupling coefficient (k_p = 0.521) was found to be in 1 mol% Ba modified PLSZNT ceramic, respectively, and this composition may be suitable for possible sensor and actuator applications.     

Thermal expansion of EB-PVD yttria stabilized zirconia

Yttria stabilized zirconia (YSZ) layers, suited for thermal barrier coatings (TBCs) in turbine engines, were produced by electron-beam physical vapour deposition (EB-PVD) on metallic (Ni-based alloys) ceramic (Al₂O₃) substrates (typically at 1000 °C) using ingot compositions of about 4 mol% Y₂O₃–ZrO₂. Employing powdered samples, removed from the surface of the as-deposited YSZ layers, precision X-ray diffraction data sets were recorded between 60 and 900 °C and the lattice parameters of the metastable, tetragonal (t′) and the cubic (c) phases were refined using Rietveld's method. It was unambiguously verified that phase (c) was present in all investigated samples as a minority phase already in the as-deposited state.The thermal expansion coefficients of the tetragonal phase (t′) (α₁₁ = 9.3(2) × 10^?6 K^?1, α₃₃ = 10.8(1) × 10^?6 K^?1) and the cubic phase (c) (α₁₁ = 8.5(2) × 10^?6 K^?1) were evaluated from the refined temperature dependent lattice parameter values of this study, and turned out to be close to each other, but significantly different from the coefficient of the monoclinic phase (m) (α₁₁ = 9.0 × 10^?6 K^?1, α₂₂ = 1.2 × 10^?6 K^?1, α₃₃ = 11.9 × 10^?6 K^?1, α₁₃ = 0.0 × 10^?6 K^?1), which was derived from temperature dependent lattice parameter measurements published by Touloukian et al.⁵ The expansion coefficient of the monoclinic phase (m) exhibited a very pronounced anisotropy in contrast to the tetragonal phases (t′) and (t). Our values for (t′) and (c) were in excellent agreement with that of other tetragonal and cubic phases from the literature with quite different Y₂O₃ contents.