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.     

Doping effect of divalent cations on sintering of polycrystalline yttria

The sintering behavior of Y₂O₃ doped with 1 mol% of Ca²⁺, Mg²⁺, Mn²⁺, Ni²⁺, Sr²⁺ or Zn²⁺ was investigated by pressureless sintering in air at a sintering temperature in the range 900–1600 °C. The sintering temperature required for full densification in Y₂O₃ was reduced by 100–400 °C by the cation doping, while undoped Y₂O₃ was densified at 1600 °C. The most effective dopant among the examined cations was Zn²⁺. The grain growth kinetics of undoped and cation-doped Y₂O₃ was described by the parabolic law. The grain boundary mobility of Y₂O₃ was accelerated by doping of the divalent cations. High-resolution transmission electron microscopy (HRTEM) observations and nano-probe X-ray energy dispersive spectroscopy (EDS) analyses confirmed that the dopant cations tended to segregate along the grain boundaries without forming amorphous layers. The improved sinterability of Y₂O₃ is probably related to the accelerated grain boundary diffusion owing to the grain boundary segregation of the dopant cations.     

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.     

Effect of cation doping on lattice and grain boundary diffusion in superplastic yttria-stabilized tetragonal zirconia

Lattice diffusion coefficients D_l and grain boundary diffusion D_gb coefficients of hafnium were studied for 0.5 and 1 mol% cation-doped yttria-stabilized tetragonal zirconia at the temperature range from 1283 to 1510 °C. The diffusion profiles were determined by two experimental techniques: secondary ion mass spectroscopy and electron microprobe analysis. Additionally the first principle calculations of the electronic states of Zr⁴⁺, dopant cations and O^2? anions and elastic properties in 3Y-TZP were performed. Superplastic strain rate versus stress and inverse temperature was also measured. For 1 mol% doped samples the significant increase of the grain boundary diffusion and superplastic strain rate was observed. Correlations between the calculated ionic net charges and D_gb indicate that enhancement of D_gb was caused by the reduction of ionic bonding strength between metal cation and oxygen anion in zirconia. The new constitutive equation for superplastic flow of yttria-stabilized tetragonal zirconia ceramics was obtained.     

Oxidation behaviour of pressureless liquid-phase-sintered α-SiC with additions of 5Al2O3 + 3RE2O3 (RE = La,Nd, Y,Er, Tm,or Yb)

The oxidation behaviour of pressureless liquid-phase-sintered (PLPS) α-SiC was investigated as a function of the sintering additives of 5Al₂O₃ + 3RE₂O₃ (RE = La, Nd, Y, Er, Tm, or Yb) by thermogravimetry experiments in oxygen at 1075–1400 °C for up to 22 h. It was found that the oxidation is in all cases passive and protective, with kinetics governed by the arctan-rate law. This is because the PLPS SiC ceramics develop oxide scales having no cracks or open porosity and accordingly prevent the parent material from direct contact with oxygen. In addition, these oxide scales crystallize gradually during the exposure to the oxidizing atmosphere with the attendant reduction in the amorphous cross-section available for oxygen diffusion. It was also found that the rate-limiting mechanism of the oxidation is outward diffusion of RE³⁺ cations from the intergranular phase into the oxide scale, and that the activation energy of the oxidation increases with increasing size of the RE³⁺ cation. It was also observed that the oxidation of PLPS SiC increases with increasing size of the RE³⁺ cation, an effect that is especially marked for cation sizes above 0.9 Å because the oxidation rate becomes several orders of magnitude faster. This trend is attributable to the oxide scales being more crystalline, and containing crystals that are more refractory and amorphous residual phases that are more viscous as the size of the RE³⁺ cation decreases. Finally, implications for the design of PLPS SiC ceramics with superior oxidation resistance are discussed.     

Doping effect on sinterability of polycrystalline yttria: From the viewpoint of cation diffusivity

The sintering behavior of Y₂O₃ doped with 1 mol% of a trivalent or tetravalent cation was investigated by pressureless sintering in air. Ga³⁺ or Ge⁴⁺-doped Y₂O₃ bodies exhibited higher relative densities than the undoped Y₂O₃, while the La³⁺ or Zr⁴⁺-doping suppressed the densification of Y₂O₃. An interdiffusion experiment was performed on the diffusion couples of polycrystalline Er₂O₃ and Y₂O₃ doped with Ni²⁺ or Zr⁴⁺, which are some of the most effective and least effective dopants for the improvement of the sinterability, respectively. The lattice and grain boundary diffusion coefficients of the Er³⁺ cation in Y₂O₃ were increased by the Ni²⁺-doping, but were decreased by the Zr⁴⁺-doping. High-resolution transmission electron microscopy observations and nano-probe X-ray energy dispersive spectroscopy analyses confirmed that the dopant cations segregate along the grain boundaries without forming an amorphous phase. The doping effect on the sinterability of Y₂O₃ must result from the change in the diffusivity in Y₂O₃.     

Growth and microstructure of Ba β-alumina films by laser chemical vapor deposition

Ba β-alumina films were prepared by laser chemical vapor deposition. Mostly single-phase Ba β-alumina films were obtained at 1125–1200 K and for an Al/Ba molar ratio of 12.4–16.6. BaAl₂O₄ and α-Al₂O₃ were codeposited with Ba β-alumina under Ba- and Al-rich conditions, respectively. The Ba β-alumina films consisted of hexagonal grains, and the (1 1 0)-oriented Ba β-alumina films had a fin-like columnar structure. The highest deposition rate reached 120 μm h^?1 at around 1200 K. A thin layer of Ba-rich superstructure was formed on the surface of the (1 1 0)-oriented columnar grains.     

The effect of silica doping on neodymium diffusion in yttrium aluminum garnet ceramics: implications for sintering mechanisms

The Nd³⁺ cation diffusion into transparent polycrystalline YAG (Y₃Al₅O₁₂) was investigated as a function of temperature and silica content. Thin neodymium oxide layers were deposited on sintered YAG substrates prior to annealing under air at temperatures from 1400 to 1600 °C. Bulk and grain boundary neodymium diffusion coefficients were measured by secondary ion mass spectrometry. The experimental results show that silica addition increases the diffusivity of Nd³⁺ by a factor 10 whatever the diffusion path, probably as a result of extrinsic point defects formation, especially rare-earth vacancies.The experimental diffusion data were used to elucidate the sintering mechanism of Nd:YAG ceramics in the temperature range 1450–1550 °C. Firstly, it appeared that the intermediate stage of solid-state sintering should be controlled by the rare-earth diffusion along the grain boundary with an activation energy of about 600 kJ mol^?1. Secondly, grain growth mechanism at the final stage of liquid-phase sintering was investigated for silica-doped Nd:YAG samples. Thus, the grain growth should be limited by the reaction at interfaces at a temperature lower than 1500 °C, with an activation energy of about 880 kJ mol^?1. At higher temperature, it seems to be limited by the ionic diffusion through the intergranular liquid phase, with an activation energy of 250 kJ mol^?1.     

Role of vermiculite and zirconium–vermiculite on the formation of zircon–cordierite nanocomposites

The Zr^4 +–vermiculites were studied in their new role of the zircon precursor in the clay minerals mixtures which were prepared for firing of the zircon–cordierite nanocomposites. Currently there is a lack of data available about the structure of Zr^4 +–vermiculites, on which this study was performed. The modeling of the arrangement of interlayer material in the Zr^4 +–vermiculite led to new findings that water molecules are attracted more strongly by Mg^2 + cations than by Zr-tetrameric cations, and that both the tetrameric cations [Zr₄(OH)₁₄(H₂O)₁₀]^2 + and [Zr₄(OH)₈(H₂O)₁₆]^8 + may be present in the interlayer space of the Zr^4 +–vermiculites. Vermiculites from two different localities Czech Republic (Verm1) and from Brazil (Verm2) were intercalated using the zirconyl chloride (ZrOCl₂–30% solution in HCl) and the prepared Zr^4 +–vermiculites were designated as Zr^4 +–Verm1 and Zr^4 +–Verm2, respectively. Influence of the Zr^4 +–Verm1 and Zr^4 +–Verm2 in the mixtures of clay minerals on the properties of zircon–cordierite nanocomposites were investigated by their comparison with the properties of the zircon–cordierite nanocomposites, which were prepared using saturation of the clay minerals mixtures containing Verm1 and Verm2 with the zirconyl chloride (ZrOCl₂–30% solution in HCl). The zircon–cordierite nanocomposites fired from the clay mineral mixtures containing Zr^4 +–Verm1 and Zr^4 +–Verm2 showed a maximum porosity of P = 58 and 60%, skeletal density SD = 3.2 and 3.6, and the smallest pores with a median pores diameter MDP = 18 and 15 μm, respectively, in comparison with the zircon–cordierite nanocomposites fired from the clay mineral mixtures containing Verm1 and Verm2 and saturated with zirconyl chloride solution. The type of vermiculite Verm1 or Verm2 in the clay mineral mixtures did not affect the contents of the crystalline mineral phases in cordierite and zircon–cordierite nanocomposites.     

Deformation behaviour of iron-doped alumina

Compressive creep tests in air were carried out on 1 cat.% Fe-doped alumina at a temperature T=1400 °C. Iron doping affected the plastic deformation by different ways in relation with Fe²⁺ cations population. Fe²⁺ cations sped up the deformation rates. FeAl₂O₄ spinel precipitates were identified and they were found (i) to interact with alumina grain boundaries (ii) to limit the grain growth within a range of strain. The Fe²⁺ cations underwent oxidation and this resulted in the dissolution of the some precipitates and in the decrease of deformation rates. It was suggested that deformation sped up this evolution through mass transport and that time was not a dominating parameter.     

Preparation of high pure α-Al2O3 nanoparticles at low temperatures using Pechini method

A Pechini process was successfully used to synthesize alpha-alumina (98.95% mass fraction) at relatively low calcination temperature (925 °C). The synthesis of these nanoparticles was carried out using a polymer prepared from citric acid and ethylene glycol by the melt blending method. This polymer worked as a chelating agent for aluminum cations. The final products were produced after a dual-stages thermal treatment. The resulting α-alumina consisted of nanoparticles of 8–16 nm in diameters with a surface area (～8 m² g^?1). The mass fraction of α-alumina was dependent on the concentration of aluminum salt and polymer precursor's solutions, while the surface area of the final product was dependent on the mass fraction of θ-alumina.     

Densification of nanocrystalline Y2O3 ceramic powder by spark plasma sintering

Nanocrystalline Y₂O₃ powders with 18 nm crystallite size were sintered using spark plasma sintering (SPS) at different conditions between 1100 and 1600 °C. Dense specimens were fabricated at 100 MPa and 1400 °C for 5 min duration. A maximum in density was observed at 1400 °C. The grain size continuously increased with the SPS temperature into the micrometer size range. The maximum in density arises from competition between densification and grain growth. Retarded densification above 1400 °C is associated with enhanced grain growth that resulted in residual pores within the grains. Analysis of the grain growth kinetics resulted in activation energy of 150 kJ mol^?1 and associated diffusion coefficients higher by 10³ than expected for Y³⁺ grain boundary diffusion. The enhanced diffusion may be explained by combined surface diffusion and particle coarsening during the heating up with grain boundary diffusion at the SPS temperature.     

Solubility limit of Si in YAG at 1700 °C in vacuum

Measurement of the solubility limit of Si in yttrium aluminum garnet (YAG-Y₃Al₅O₁₂) is crucial for understanding the mechanisms by which Si influences grain boundary mobility, and the mechanisms by which grain boundaries migrate. In the present work, the solubility limit of Si in YAG at 1700 °C in vacuum (5 × 10⁻⁶ Torr), which are the most common sintering temperature and environment for YAG, was measured for the first time. Measurements were conducted by wavelength dispersive spectroscopy (WDS), using polished YAG specimens with 3700 ppm Si (0.8 wt% SiO₂). Si content to ensure saturation with Si. The accuracy of the WDS result was confirmed by using a series of doped specimens and by comparing to inductively coupled plasma mass spectrometry ICP-MS results. The results indicate that the solubility limit of Si in YAG at 1700 °C (5 × 10⁻⁶ Torr), is 980 ± 60 ppm. The measured Si solubility was found to significantly depend on the cooling rate, where for furnace cooled specimens the measured Si solubility was 650 ± 60 ppm. A second phase in triple junction was repeatedly observed when higher content of Si was used, confirming this work results.     

Highly porous α-Al2O3 ceramics obtained by sintering atomic layer deposited inverse opals

A process for generating highly porous α-Al₂O₃ ceramics has been developed. In this paper, a combination of self-assembly and atomic layer deposition is demonstrated as a means to fabricate inverse alumina opals, which have their structures transformed via sintering. The resulting highly porous structure is stable even after a 4 h dwell time at 1400 °C, in contrast to structures generated by conventional powder metallurgy, sol-gel or colloidal powder suspension infiltration methods. TEM analysis reveals that the structure consists of single grain domains of up to 3 µm, each containing a randomly interconnected network of alumina ligaments that share a common crystalline orientation, suggesting a different mechanism of grain boundary migration during sintering. These highly porous α-alumina ceramics are considered to be ideal for filtration or catalysis applications.     

Microstructure,mechanical, thermal,and oxidation properties of a Zr2[Al(Si)]4C5–SiC composite prepared by in situ reaction/hot-pressing

The microstructure, mechanical and thermal properties, as well as oxidation behavior, of in situ hot-pressed Zr₂[Al(Si)]₄C₅–30 vol.% SiC composite have been characterized. The microstructure is composed of elongated Zr₂[Al(Si)]₄C₅ grains and embedded SiC particles. The composite shows superior hardness (Vickers hardness of 16.4 GPa), stiffness (Young's modulus of 386 GPa), strength (bending strength of 353 MPa), and toughness (fracture toughness of 6.62 MPa m^1/2) compared to a monolithic Zr₂[Al(Si)]₄C₅ ceramic. Stiffness is maintained up to 1600 °C (323 GPa) due to clean grain boundaries with no glassy phase. The composite also exhibits higher specific heat capacity and thermal conductivity as well as better oxidation resistance compared to Zr₂[Al(Si)]₄C₅.     

Fabrication and properties of transparent Tb:YAG fluorescent ceramics with different doping concentrations

Terbium doped yttrium aluminum garnet (Tb:YAG) transparent ceramics with different doping concentrations were fabricated by the solid-state reaction method using commercial Y₂O₃, α-Al₂O₃ and Tb₄O₇ powders as raw materials. Samples sintered at 1750 °C for 20 h were utilized to observe the optical transmittance, microstructure and fluorescence characteristics. It is found that all the Tb: YAG ceramics with different doping concentrations exhibit homogeneous structures with grain size distributions around 22–29 µm. For the 5 at% Tb:YAG transparent ceramics, the grain boundaries are clean with no secondary phases. The photoluminescence spectra show that Tb:YAG ceramics emit predominantly at 544 nm originated from the energy levels transition of ⁵D₄→⁷F₅ of Tb³⁺ ions, and the intensity of the emission peak reaches a maximum value when the Tb³⁺ concentration is 5 at%. The in-line transmittance of the 5 at% Tb:YAG ceramics is 73.4% at the wavelength of 544 nm, which needs to be further enhanced by optimizing the fabrication process. We think that Tb:YAG transparent ceramics may have potential applications in the high-power white LEDs.     

Fabrication of submicron alumina ceramics by pulse electric current sintering using M2+ (M = Mg,Ca, Ni)-doped alumina nanopowders

Dense submicron-grained alumina ceramics were fabricated by pulse electric current sintering (PECS) using M²⁺(M: Mg, Ca, Ni)-doped alumina nanopowders at 1250 °C under a uniaxial pressure of 80 MPa. The M²⁺-doped alumina nanopowders (0–0.10 mass%) were prepared through a new sol–gel route using high-purity polyhydroxoaluminum (PHA) and MCl₂ solutions as starting materials. The composite gels obtained were calcined at 900 °C and ground by planetary ball milling. The powders were re-calcined at 900 °C to increase the content of α-alumina particles, which act as seeding for low-temperature densification. Densification and microstructural development depend on the M²⁺ dopant species. Dense alumina ceramics (relative density ≥99.0%) thus obtained had a uniform microstructure composed of fine grains, where the average grain size developed for non-doped, Ni-doped, Mg-doped and Ca-doped samples was 0.67, 0.67, 0.47 and 0.30 μm, respectively, showing that Ca-doping is the most promising method for tailoring of nanocrystalline alumina ceramics.     

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.     

Improved structural stability and ionic conductivity of Na3Zr2Si2PO12 solid electrolyte by rare earth metal substitutions

Sodium zirconium silicon phosphorus with the composition of Na₃Zr₂Si₂PO₁₂ (NZSP) was prepared by a facile solid state reaction method. The effects of the calcination temperature and rare earth element substitution on the structure and ionic conductivity of the NZSP material were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and AC impedance measurement. The results show that the microstructure and ionic inductivity of the NZSP was strongly affected by the aliovalent substitution of Zr⁴⁺ ions in NZSP with rare earth metal of La³⁺, Nd³⁺ and Y³⁺. At room temperature, the optimum bulk and total ionic conductivity of the pure NZSP solid electrolyte sintered under different conditions were 6.77×10⁻⁴ and 4.56×10⁻⁴ S cm⁻¹, respectively. Substitution of La³⁺, Nd³⁺ and Y³⁺ in place of Zr⁴⁺ exhibited higher bulk conductivity compared with that of pure NZSP. Maximum bulk and ionic conductivity value of 1.43×10⁻³ and 1.10×10⁻³ S cm⁻¹ at room temperature were obtained by Na_3+xZr_1.9La_0.1Si₂PO₁₂ sample. The charge imbalance created by aliovalent substitution improves the mobility of Na⁺ ions in the lattice, which leads to increase in the conductivity. AC impedance results indicated that the total ionic conductivity strongly depends on the substitution element and the feature of the grain boundary.     

Constrained sintering of 8 mol% Y2O3 stabilised zirconia films

Constrained sintering kinetics of 8 mol% Y₂O₃/92 mol% ZrO₂ (8YSZ) films approximately 10–15 μm thick screen-printed on dense YSZ substrates, and the resulting stress induced in the films, were measured in the temperature range 1100–1350 °C. The results are compared with those reported earlier for 3YSZ films.Both materials behave similarly, although there are differences in detail. The constrained densification rate was greatly retarded compared with the unconstrained densification rate due to the effect of the constraint on the developing anisotropic microstructure (3YSZ) and, in the case of 8YSZ, considerable grain growth. The stress generated during constrained sintering was typically a few MPa. The apparent activation energies for free sintering, constrained sintering, creep and grain growth are found to cover a wide range (135–670 kJ mol^?1) despite all probably being mainly controlled by grain boundary cation diffusion.