Thermal shock fracture behaviour of ZrO2 based ceramics

Thermal shock fracture behaviour of alumina, mullite, silicon carbide, silicon nitride and various kinds of zirconia based ceramics, such as magnesia partially stabilized zirconia (Mg-PSZ), yttria and ceria doped tetragonal zirconia polycrystals (Y-TZP and Ce-TZP), Y-TZP/Al₂O₃ composites and yttria doped cubic stabilized zirconia (Y-CSZ), was evaluated by the quenching method using water, methyl alcohol and glycerin as quenching media. Thermal shock fracture of all materials seemed to proceed by the thermal stress due to convective heat transfer accompanied by boiling of the solvents under the present experimental conditions. Thermal shock resistance of zirconia based ceramics increased with increasing the fracture strength, but that of Y-TZP and Y-TZP/Al₂O₃ composites was anormalously lower than the predicted value.     

Creep of polycrystalline alumina,pure and doped with transition metal impurities

Grain size effects were used to evaluate the relative contributions of aluminium lattice and oxygen grain boundary diffusion to the high temperature (1350 to 1550° C) steady state creep of polycrystalline alumina, pure and doped with transition metal impurities (Cr, Fe). Divalent iron in solid solution was found to enhance both aluminium lattice and oxygen grain-boundary diffusion. Large concentrations of divalent iron led to viscous Coble creep which was rate-limited entirely by oxygen grain-boundary diffusion. Nabarro-Herring creep which was rate-limited by aluminium lattice diffusion was observed in pure and chromium-doped material. Chromium additions had no effect on diffusional creep rates but significantly depressed non-viscous creep modes of deformation. Creep deformation maps were constructed at various iron dopant concentrations to illustrate the relative contributions of aluminium grain boundary, aluminium lattice, and oxygen grain-boundary diffusion to the diffusional creep of polycrystalline alumina.     

The effect of tetravalent dopants on the unit cell volume of 2Y-TZP and 8Y-SZ

The effects of tetravalent cations on tetragonal zirconia and cubic zirconia were evaluated by investigating lattice parameter variation. XRD observation was made on 2Y-TZP (2 mol% Y₂O₃-stabilized tetragonal zirconia polycrystals) and 8Y-SZ (8 mol% Y₂O₃-stabilized zirconia) doped with CeO₂, SnO₂ and TiO₂. The contents of the tetravalent dopants increased within the solubility limits of the dopants in 2Y-TZP and 8Y-SZ. The ionic radii of the Ti⁴⁺, Sn⁴⁺, Zr⁴⁺ and Ce⁴⁺ for eight-fold coordination with oxygen ions are 0.74, 0.81, 0.84 and 0.97 Å, respectively. The unit cell volumes of cubic zirconias are in proportion to the ionic radii of dopant cations but the unit cell volumes of tetragonal zirconias are not.     

Fractographic study of the alumina and zirconia particles embedded in mullite prepared by reaction sintering

Four different mullite-alumina-zirconia composites have been prepared by reaction sintering between alumina and zircon powders using magnesia or spinel (MgAl₂O₄) to increase the sintering and reaction rates. The microstructure of these materials can be described as composed of two parts: the first one is the mullite matrix containing various kinds of zirconia and alumina particles, whereas the second part is an amorphous phase in which alumina submatrices, zirconia and spinel particles are embedded. Examination of fracture surfaces allows one to identify the crack paths and shows that the main differences are related to zirconia inclusions. Analysis of mechanical properties and fracture features leads to the conclusion that crack deflection and microcracking are operative toughening mechanisms for the various materials. Moreover, a crack bowing mechanism is proposed to explain the higher modulus of rupture found for the series of materials prepared with magnesia as a reaction sintering aid. On leave from Instituto de Ceramica Y Vidrio, CSIC, Arganda del Rey, Madrid, Spain.     

Formation of diffusionlessly transformed tetragonal phases by rapid quenching of melts in ZrO2-RO1.5 systems (R = rare earths)

Rare-earth (Nd, Sm, Er, Yb, Sc)-doped zirconia was melted using an arc-imaging furnace, and rapidly quenched with a hammer and anvil apparatus (cooling rate >10⁵ K sec^–1). These ZrO₂-RO_1.5 samples were investigated by X-ray diffraction and transmission electron microscopy. The existing region of metastable tetragonal zirconia is from 2 to 14 mol% of RO_1.5 regardless of the species of dopants, RO_1.5. In the lattice parameters of the tetragonal phases, the unit cell volume corresponds to the ionic radii of dopants, whereas the tetragonality (c/a) is independent of the species of dopants, but dependent on the content of dopants.     

Al2O3掺杂对YSZ固体电解质烧结及电性能的影响

研究了用常规共沉淀法掺杂Al2O3对YSZ固体电解质的烧结及电性能的影响．结果表明：适量的Al2O3能提高YSZ材料的烧结性能，促使其致密化，但过量的Al2O3对材料的致密化不利；同时，材料的晶界电导随Al2O3含量的增大表现出先增大后减小的变化趋势，这与Al2O3对YSZ晶界两方面的不同影响有关，Al2O3偏析于晶界一方面能清除晶界上对氧离子电导不利的SiO2，但另一方面也会降低晶界空间电荷层中的自由氧离子空穴的浓度．     

The comparison of alumina and zirconia fibres using simple thermal and mechanical techniques

The laboratory development of sol/gel ceramic fibres requires rapid objective means of assessing the mechanical and thermal properties of a product at the earliest stages of preparation. The merits of a simple tumbling test, leading to a fibre friability index, and the Bend Stress Relaxation test, which gives a high temperature creep rating, are demonstrated on commercial Saffil and Safimax alumina fibres and a development Saffil zirconia fibre, each in staple blanket form. Measurements on Altex continuous alumina/silica fibre and Nicalon are also presented.Standard and off specification alumina fibres are readily distinguished by their friability indices which correlate with the fibre strain to break.Saffil and Safimax alumina are comparable to Nicalonin terms of creep and superior to Altex. Saffil zirconia approaches alumina creep performance after post firing to 1250°C.     

Studies on crystallization behaviour of 3Y-TZP/Al2O3 composite powders

The present work deals with the characterization of 3.0 mol.% yttria stabilized zirconia (3Y-TZP) powders prepared as composite precursors containing up to 20.0 vol.% of alumina. Following the wet-chemistry route of co-precipitation, a homogeneous dispersion of component hydroxides was obtained. Effect of calcination treatment on t-phase and primary particle size evolution of zirconia was investigated by XRD technique. A marked difference with respect to inhibition of crystallization reaction of 3Y-TZP was observed in the presence of alumina. This is attributed to the increased diffusion lengths of the two developing phases. The powders could be retained in the ultrafine and quasi-amorphous state by a suitable choice of alumina content and calcination treatment.     

Processing of alumina-based composites via conventional sintering and their characterization

The present work relates to the processing of dense alumina-based composites, their microstructural characterization and study of mechanical properties. Alumina ceramic material and alumina-based composites with m-Zirconia and Ceria addition are sintered at 1600°C, 1650°C and 1700°C temperatures via conventional sintering. Solid-state diffusion during sintering led to volume diffusion in alumina, and volume and grain boundary diffusion in alumina composite. In the present sintering conditions alumina is found to be the least dense as improper solid-state diffusion resulted in porosity, whereas alumina–zirconia composite achieved the highest density of 97%. Scanning electron microscope (SEM) micrograph shows homogeneous distribution of fine zirconia particles inside the alumina matrix, filling the voids of the alumina skeletal structure. Zirconia connects to alumina particles, restricting its abnormal grain growth. It results in strong bonding and grain refinement. Alumina–zirconia composite exhibits the highest hardness and fracture toughness of 14.37?GPa and 4.6?MPa?·?m^1/2 at 1700°C. Alumina suppresses the transformation of m-t zirconia, resulting in high toughness of alumina composites. Alumina–zirconia–ceria composite revealed the presence of porosity, which led to less densification and low mechanical properties.     

Computer Modeling of Ionic Conductivity in Low Temperature Doped Ceria Solid Electrolytes

Solid oxides, such as ceria (CeO₂) doped with cations of lower valance, are potential electrolytes for future solid oxide fuel cells. This is due to the theoretically high ionic conductivity at low operation temperature. This paper investigates the feasibility of two potential electrolytes which are samarium-doped ceria (SDC) and gadolinium-doped ceria (GDC) to replace the traditional yttria-stablized zirconia (YSZ). Molecular simulation techniques were employed to study the influence of different dopant concentrations at different operation temperatures on the ionic conductivity from the atomistic perspective. Simulation results show that the optimized ionic conductivity occurs at 11.11mol% concentration using both dopants of Gd₂O₃ and Sm₂O₃. The temperature effect was also examined under a fixed concentration simulation to check how low temperature they still function. The predicted ionic conductivities have been verified with published experimental results and show reasonable agreements. This simulation technique reveals a clear picture with qualitative and quantitative connection between the choice of the dopant and the improvement of the ionic conductivity of fuel cell electrolytes.     

Effect of dopants on alumina grain boundary sliding: implications for creep inhibition

We investigate by means of periodic density functional theory the mechanism of grain boundary sliding along the α-alumina Σ11 tilt grain boundary. We identify minimum and maximum energy structures along a preferential sliding pathway for the pure grain boundary, as well as for grain boundaries doped with a series of early transition metals, as well as barium, gadolinium, and neodymium. We predict that the segregation of those dopants results in a considerable increase in the grain boundary sliding barrier. Grain boundary sliding occurs by a series of bond breaking and forming across the grain boundary. Our results suggest that the presence of large cations inhibits the regeneration of bonds during sliding, which results in a decrease in total number of bonds across the grain boundary interface, thereby raising the barrier to sliding. Trends in predicted grain boundary sliding energies are in good agreement with recently measured creep activation energies in polycrystalline alumina, lending further credence to the notion that grain boundary sliding plays a dominant role in alumina creep.     

A study of grain-boundary structure in rare-earth doped aluminas using an EBSD technique

Oversized rare-earth dopant ions such as Y³⁺, Nd³⁺, and La³⁺ segregate to grain boundaries and reduce the tensile creep rate of -Al₂O₃ by 2-3 orders of magnitude. It has been speculated that these dopant ions can modify the grain boundary structure in alumina by promoting the formation of special grain boundaries. If this were indeed the case, it would provide a possible explanation for the aforementioned creep rate retardation. In order to test this hypothesis, electron backscatter diffraction (EBSD) has been used to assess both the proportion of coincidence-site lattice boundaries, and the grain boundary misorientation distribution, in aluminas doped with various ions (Zr, Y, Nd, La, Nd/Zr). The results show that the grain boundary structure in alumina is not significantly altered by the addition of the above dopants, implying that the change in grain boundary chemistry is primarily responsible for the observed creep behavior.     

Nanocomposite Al2O3–ZrO2 thin films grown by reactive dual radio-frequency magnetron sputtering

Crystalline alumina–zirconia nanocomposites have been synthesized at 450 °C and 750 °C with reactive magnetron sputtering using radio-frequency power supplies. The composition of the films ranged from pure alumina to pure zirconia as measured by ion beam techniques. Microstructural characterization showed the presence of monoclinic zirconia in the pure zirconia films and γ-alumina in the pure alumina films while the nanocomposites contained either an amorphous compound, γ-alumina, cubic zirconia or a mixture of these. The grain size was  5 nm for the nanocomposite compared to larger grains in the pure oxide films. Electron energy loss spectroscopy showed a clear progression from the pure alumina to the pure zirconia.     

Preparation and properties evaluation of zirconia-based/Al2O3 composites electrolytes for solid oxide fuel cell systems

The compaction behaviour of ultrafine yttria-doped zirconia powders (6–8 nm) without and with alumina additions (0 to 20 wt%) has been studied. From the pore size distribution and using isothermal and nonisothermal techniques, the sintering behaviour of zirconia compacts in the temperature range 800–1500 °C was studied. It was found that alumina additions (up to 10 wt%) enhanced the zirconia compacts' densification process and, above that alumina content, that process was retarded. Alumina additions did not affect the grain grown process in tetragonal zirconia samples. However, this was strongly hindered in the fully stabilized zirconia ones. The results were compared with those obtained in the same experimental conditions on a commercial zirconia powder.     

Size-controlled synthesis and optical properties of doped nanoparticles prepared by soft solution processing

In this review, we outline the synthesis and luminescence properties of metal-ion-incorporated doped nanoparticles and surface-passivated doped nanoparticles. The synthetic routes we describe are limited to those involving soft solution processing. The doping effects are discussed in this review on the semiconductor nanoparticles confining the size range near to the 'quantum dot size.' The effects on luminescence with respect to ionic valance of dopants and the luminescence phenomena on mismatching of ionic radii between the host-guest are also provided. In addition, we discuss the role of passivated organic surfactants and the necessity of surface passivation of doped or undoped nanoparticles with other semiconductor materials that possess larger band gaps. Biocompatible semiconductor nanoparticles and some of their applications are also mentioned briefly.     

Rare-earth doped zirconia fibres by sol-gel processing

Polycrystalline zirconia fibres, doped with 2–8 mol% of oxides of trivalent lanthanum, praseodymium, neodymium, samarium, gadolinium, and dysprosium (in decreasing cation size), were prepared by spinning of acetate-derived sols and baking the gel fibres thus obtained at 900–1300 °C for 1 h. The larger sized dopants lanthanum, praseodymium and neodymium (Group A) gave rise to tetragonal zirconia, with or without cubic zirconia, at 900 °C which converted partly or fully to monoclinic zirconia, in certain cases accompanied by a cubic zirconate phase at higher temperatures. The smaller sized dopants samarium, gadolinium and dysprosium (Group B) generated only tetragonal or cubic, or both polymorphs of zirconia, depending on the cation type, concentration and temperature. This stabilization of higher symmetry polymorphs with Group B dopants was associated with relatively large crystallite size (especially when calcined at 1300 °C). The maximum tensile strength values of usable fibres calcined at 1300 °C were found to decrease with increasing size in dopant dysprosium > gadolinium > samarium > neodymium, praseodymium, lanthanum=0). Although all the dopant cations were larger in size than Zr⁴⁺ (in the same oxygen coordination), the relative closeness in size of Group B cations with Zr⁴⁺ was considered to be the reason behind the obtained differences in properties.     

Creep in pure and two phase nickel-doped alumina

Creep in pure and two phase nickel-doped alumina has been investigated in the stress range 0.70 to 4.57 kgf mm^–2 (1000 to 6500 psi), and temperature range 1450 to 1800° C, for grain sizes from 15 to 45 m (pure alumina) and 15 to 30 um, (nickel-doped alumina). The effect of stress, grain size and temperature on the creep rate suggests that diffusion controlled grain-boundary sliding is the predominant creep mechanism at low stresses and small grain sizes. However, the stress exponents show that some non-viscous boundary sliding occurs even at the lowest stresses investigated. This mechanism is confirmed by metallographic evidence, which shows considerable boundary corrugation in the deformed aluminas. At higher stresses and larger grain sizes the localized propagation of microcracks leads to high stress exponents in the creep rate equation. The nickel dopant, which introduces an evenly distributed spinel second phase into the alumina matrix, increases the creep rate and enhances boundary sliding and localized crack propagation. The weakening effect of the second phase increases with grain size, and tertiary creep occurs at strains of 0.5% and below in large grained material.     

Modeling the final sintering stage of doped ceramics: mutual interaction between grain growth and densification

Applying the thermodynamic extremal principle, a model for grain growth and densification in the final stage of sintering of doped ceramics was derived, with segregation-dependent interfacial energies and mobilities (or diffusivities). The model demonstrated an interdependence between the driving forces of grain growth and densification during sintering evolution, observed because the surface energy contributes positively to the driving force of grain growth while the GB energy negatively to the driving force of densification. The model was tested in alumina as a host system, and calculations demonstrate that dopants with more negative GB (or surface) segregation enthalpy or which cause lower GB diffusion coefficient can induce higher relative densities at a given grain size. Comparatively studying yttria- and lanthana-doped alumina, the lanthana doping showed significantly enhanced sintering attributed to the larger La³⁺ radius causing a more negative GB segregation energy. This present model is expected to help dopant designing to improve control over sintering.     

RETRACTED ARTICLE: Determination of dopant of ceria system by density functional theory

Oxides with the cubic fluorite structure, e.g., ceria (CeO₂), are known to be good solid electrolytes when they are doped with cations of lower valence than the host cations. The high ionic conductivity of doped ceria makes it an attractive electrolyte for solid oxide fuel cells, whose prospects as an environmentally friendly power source are very promising. In these electrolytes, the current is carried by oxygen ions that are transported by oxygen vacancies, present to compensate for the lower charge of the dopant cations. Ionic conductivity in ceria is closely related to oxygen-vacancy formation and migration properties. A clear physical picture of the connection between the choice of a dopant and the improvement of ionic conductivity in ceria is still lacking. Here we present quantum-mechanical first-principles study of the influence of different trivalent impurities on these properties. Our results reveal a remarkable correspondence between vacancy properties at the atomic level and the macroscopic ionic conductivity. The key parameters comprise migration barriers for bulk diffusion and vacancy–dopant interactions, represented by association (binding) energies of vacancy–dopant clusters. The interactions can be divided into repulsive elastic and attractive electronic parts. In the optimal electrolyte, these parts should balance. This finding offers a simple and clear way to narrow the search for superior dopants and combinations of dopants. The ideal dopant should have an effective atomic number between 61 (Pm) and 62 (Sm), and we elaborate that combinations of Nd/Sm and Pr/Gd show enhanced ionic conductivity, as compared with that for each element separately. An erratum to this article can be found at     

Gadolinia doped ceria/yttria stabilised zirconia electrolytes for solid oxide fuel cell applications

Solid oxide fuel cells are currently constructed using a yttria stabilised zirconia electrolyte membrane. However, zirconia has a number of disadvantages associated with its use, such as the high operational temperatures required for it to exhibit acceptable levels of ionic conductivity. Alternative ceramics such as doped cerium oxide show promise as electrolytes capable of operating at reduced temperatures, but introduce additional problems such as electronic conduction and inferior mechanical properties. This paper describes the manufacture and characterisation of a number of prototype electrolytes consisting of a mixture of yttria stabilised zirconia and gadolinium doped ceria. Traditional ceramic processing techniques were used to produce the samples, which were then examined using dilatometry, impedance spectroscopy and X-ray diffraction. Results show a lowering of the ionic conductivity of zirconia with the addition of doped ceria. X-ray diffraction patterns obtained from the samples suggested that this effect could be attributed to the formation of a solid solution of ceria in zirconia.