Scavenging effect on ionic conduction behaviour at the grain boundary of MgO partially stabilised zirconia

The scavenging effect of magnesium oxide (MgO) addition on electrical property of 9 mol-% MgO partially stabilised zirconia (Mg-PSZ) was investigated in terms of phase transformation and intergranular phase formation. The addition of MgO up to 5 mol-% caused a stabilisation of Mg-PSZ, which led to an increase in the cubic phase and a decrease in the monoclinic and tetragonal phases in Mg-PSZ. The Mg-PSZ with the addition of 5 mol-% of MgO also exhibited the maximum ionic conductivity (0.3915?S?cm^?1 at 1500°C) and forsterite (Mg₂SiO₄) was observed on the grain boundaries of Mg-PSZ. The intergranular phases, formed by reactions between the silicon in Mg-PSZ and MgO addition, reduced the grain boundary resistance, because the siliceous phase which is a hindrance for oxygen ion conduction was scavenged by the formation of Mg₂SiO₄.     

Effect of sintering temperature on phase transformation behavior and hardness of high-pressure high-temperature sintered 10 mol% Mg-PSZ

The sintering process of zirconia ceramics at high pressures is very different from that under normal pressure. In this paper, 10 mol% MgO partially-stabilized zirconia ceramics (Mg-PSZ) were synthesized under a pressure of 2.5 GPa at temperatures ranging from 1370 to 1610 °C. The effect of sintering temperature on the phase transformation behavior and hardness changes of Mg-PSZ were studied by X-ray diffraction, scanning electron microscopy, Raman spectrometry, and Vickers hardness tests. The optimal sintering conditions of 10 mol% Mg-PSZ were determined. At high pressures, the sintering time was shortened to 60 min, and the sintering temperature was reduced to 1530 °C, which indicates that the high pressure accelerated the sintering rate. Mg-PSZ reached a maximum Vickers hardness of 14.9 GPa at 1530 °C, but when the sintering temperature was further increased to 1610 °C, grain coarsening occurred, and the Vickers hardness decreased to 9.6 GPa.     

Structural parameters and oxygen ion conductivity of Y2O3–ZrO2 and MgO–ZrO2 at high temperature

The oxygen ion conductivity of zirconia-based solid electrolytes doped with 8 mol% Y₂O₃–ZrO₂ (YSZ) and 9 mol% MgO–ZrO₂ (Mg-PSZ) at high temperature was investigated in terms of their thermal behavior and structural changes. At room temperature, YSZ showed a single phase with a fluorite cubic structure, whereas Mg-PSZ had a mixture of cubic, tetragonal and some monoclinic phases. YSZ exhibited higher ionic conductivity than Mg-PSZ at temperatures from 600 °C to 1250 °C because of the existence of the single cubic structure and low activation energy. A considerable increase in the conductivity with increasing temperature was observed in Mg-PSZ, which showed higher ionic conductivity than YSZ within the higher temperature range of 1300–1500 °C. A monoclinic-to-tetragonal phase transformation was found in Mg-PSZ and the lattice parameter of the cubic phase increased at 1200 °C. The phase transformation and the large lattice free volume contributed to the significant enhancement of the ionic conductivity of Mg-PSZ at high temperatures.     

Pro- and Subeutectoid Behavior of the Tetragonal Phase in Magnesia-Partially-Stabilized Zirconia

Magnesia-partially-stabilized zirconia (Mg-PSZ) is industrially important because of transformable metastable tetragonal precipitates which interact with and arrest cracks. This work addresses the precipitation of tetragonal phases at one composition, 9.5 mol% MgO, throughout a range of temperatures. High-purity zirconia samples were sintered at 1700°C and rapidly quenched to heat-treatment temperatures ranging from 1600° to 1100°C, then quenched to room temperature. Cooling rates through the tetragonal + MgO and the monoclinic + MgO two-phase regions were found to affect the phase content. The kinetics of nonequilibrium phase transformation for high-purity Mg-PSZ are presented in terms of time-temperature-transformation diagrams.     

Direct Observation of Cycled Phase Transformations in Zirconia

Martensitic transformations of zirconia precipitates in magnesia-partially-stabilized zirconia (Mg-PSZ) have been observed directly in a transmission electron microscope. By controlling the electron beam to produce heating and cooling cycles, the precipitates could be repeatedly transformed between monoclinic and tetragonal. Preliminary observations of the resultant precipitate microstructures are presented.     

Microstructure andphase stability of Y–PSZ codoped with MgO or CaO prepared via polymeric route

Abstract

Fully dense nanostructured Y-PSZ ceramics with microstructural uniformity and stable high temperature zirconia phases were prepared by hot pressing at 1400°C. The addition of 1 mol-% MgO or CaO as codopant has a significant influence on grain growth characteristics. Addition of MgO tends to promote densification during sintering and assists in the preparation of nanosized material (<200 nm grain size) with uniform microstructure. X-ray diffraction investigations of samples hot pressed at 35 MPa and 1400°C for 20 min and thermally etched at 1350°C for 3 h showed the presence of three zirconia phases, cubic, tetragonal, and monoclinic. The existence of the cubic phase with the two other phases is associated with a fine crystallite structure. T his desirable microstructure, combined with good stabilisation of the high temperature phases, was recorded in nanostructured samples codoped with 1 mol-% of either MgO or CaO.     

Phase Transformation and Densification of an Attrition-Milled Amorphous Yttria-Partially-Stabilized Zirconia–20 mol% Alumina Powder

Phase transformations during consolidation treatments of an attrition-milled amorphous yttria-partially-stabilized zirconia (Y-PSZ: ZrO₂–3 mol% Y₂O₃)–20 mol% Al₂O₃ powder and the resulting microstructures have been investigated. A metastable cubic phase ( c -ZrO₂ solid solution) together with an α-Al₂O₃ phase is formed in the amorphous matrix by consolidation at temperatures below 1204 K. The metastable cubic phase transforms to a stable tetragonal phase ( t -ZrO₂ solid solution) with an increase in the consolidation temperature. Fully dense bulk samples consisting of extremely fine tetragonal grains together with a small amount of α-Al₂O₃ particles could be obtained by consolidation at temperatures above 1432 K. Important features concerned with the densification behavior are as follows: (1) Marked increase in the relative density occurs after cubic crystallization and subsequent cubic-to-tetragonal transformation. (2) All of the consolidated bulk samples show extremely fine grain structure with grain sizes of several tens of nanometers, irrespective of the consolidation temperature. (3) The regularity of the lattice fringe contrast in each tetragonal grain seems to be kept in the vicinity of grain boundaries. These results suggest that densification of the attrition-milled amorphous powder proceeds via superplastic flow and/or diffusional creep, rather than viscous flow of the initial amorphous phase before crystallization.     

DC-assisted rapid wetting of 3Y-PSZ by molten 72Ag–28Cu and its application in joining

The effects of current polarity, temperature and current intensity on the wettability of 3 mol% yttria-partially stabilized zirconia (3Y-PSZ) by molten 72Ag–28Cu were investigated using a direct current (DC)-coupled sessile drop method. The wettability was significantly improved with the assistance of a DC, especially when 3Y-PSZ was connected to anode. Noticeably, the contact angle rapidly decreased from 143° to smaller than 20° when the current intensity was larger than +10 mA. The wettability also showed abnormal dependence on temperature, presumably due to the competition between nucleation and growth of the m(AgCu₄Zr) phase at the interface. The mechanisms for the significant improvement in the wettability under the DC application were ascribed to the formation of substoichiometric ZrO_2-δ, the release and adsorption of metallic Zr, and the formation of wettable m(AgCu₄Zr) phase at the interface. The DC-assisted wetting provides a new strategy for the rapid and robust joining of 3Y-PSZ to metals.     

Pore orientation of the gadolinia‐doped ceria cathode interlayer for a tubular SOFC using dip‐coating

An active region of cathode interlayer in a tubular solid oxide fuel cell (SOFC) is structurally analyzed using a dual‐beam focused ion beam/scanning electron microscope (FIB/SEM). The GDC (10 mol% gadolinia‐doped ceria) cathode interlayer (about 1 μm in thickness) is dip‐coated, and then sintered on YSZ (8 mol% yttria‐stabilized zirconia) electrolyte. At 1150°C sintering temperature, the pores oriented more along the axial direction than the radial direction. The anisotropy of pore shape is accounted for the withdrawal force during the dip‐coating of the GDC interlayer.     

Structure and transformation of low‐temperature phases of 1,3‐distearoyl‐2‐oleoyl glycerol

Polymorphic transformations of 1,3‐distearoyl‐2‐oleoyl glycerol (SOS) at low temperatures (10 °C–30 °C) have been studied by time‐resolved synchrotron X‐ray diffraction (XRD) measurements at small and wide angles. Three structures have been identified: α‐phase structure forms by quenching to 10 °C from the liquid state, a metastable structure formed simultaneously with α‐phase and γ‐phase formed at 30 °C during α‐melt‐mediated crystallisation as well as at the pre‐melting temperature (22.5 °C) of the α‐phase when the coexistence of two phases (α and γ) is observed. The metastable structure produces three broad peaks in XRD patterns at small angles (one strong and two weak peaks corresponding to d‐spacings of ～4 nm, ～12 nm and ～1.5 nm respectively) and a peak in X‐ray diffraction patterns at wide angles (0.42 nm). A model is proposed for the metastable structure based on a combination of double‐ and triple‐chain packing of SOS molecules due to the similar length of alkyl chains (stearoyl and oleoyl) in the molecules.     

Hybrid FIB milling strategy for the fabrication of plasmonic nanostructures on semiconductor substrates

The optical properties of plasmonic semiconductor devices fabricated by focused ion beam (FIB) milling deteriorate because of the amorphisation of the semiconductor substrate. This study explores the effects of combining traditional 30 kV FIB milling with 5 kV FIB patterning to minimise the semiconductor damage and at the same time maintain high spatial resolution. The use of reduced acceleration voltages is shown to reduce the damage from higher energy ions on the example of fabrication of plasmonic crystals on semiconductor substrates leading to 7-fold increase in transmission. This effect is important for focused-ion beam fabrication of plasmonic structures integrated with photodetectors, light-emitting diodes and semiconductor lasers.     

Corrosion of zirconia ceramics in acidic solutions at high pressures and temperatures

Changes in microstructure and phase composition of ceria stabilized tetragonal zirconia polycrystals (Ce-TZP), magnesia and yttria partially stabilized zirconia [(Mg,Y)-PSZ] and magnesia partially stabilized zirconia (Mg-PSZ) were studied in diluted aqueous HCl, H₂SO₄ or H₃PO₄ solutions at a temperature of 390° C and a pressure of 27 MPa. Ce-TZP is corrosion resistant under these conditions in HCl, while Mg-PSZ is attacked severely and (Mg,Y)-PSZ undergoes a surface tetragonal to monoclinic phase transformation. All investigated zirconia ceramics suffer severe weight losses and transformation to the monoclinic phase on the surface in H₂SO₄. Only a small weight gain and a slight increase of m-phase on the surface of the ceramics is found in H₃PO₄.     

Effect of sintering temperature on mechanical properties of magnesia partially stabilized zirconia refractory

The optimized sintering conditions for a 3.5 wt% magnesia partially stabilized zirconia (Mg-PSZ) refractory were proposed in our recent research. The influence of the sintering temperature on the development of phase composition, microstructure, densification, thermal expansion and mechanical strength was studied in detail by X-ray diffraction (XRD), scanning electron microscope (SEM), He-pycnometer, high temperature dilatometry and three-point bending test. The samples sintered at 1670 °C had the highest bend strength, the maximum densification, the lowest thermal expansion coefficient (CTE), a homogeneous microstructure and a linear change in thermal expansion.     

Thermal exposure effects on the long‐term behavior of a mullite fiber at high temperature

The behavior of an oxide fiber at elevated temperatures was analyzed before and after thermal exposures. The material studied was a mullite fiber developed for high‐temperature applications, CeraFib 75. Heat treatments were performed at temperatures ranging from 1200°C to 1400°C for 25 hours. Quantitative high‐temperature X‐ray analysis and creep tests at 1200°C were carried out to analyze the effect of previous heat treatment on the thermal stability of the fibers. The as‐received fibers presented a metastable microstructure of mullite grains with traces of alumina. Starting at 1200°C, grain growth and phase transformations occurred, including the initial formation of mullite, followed by the dissociation of the previous alumina‐rich mullite phase. The observed transformations are continuous and occur until the mullite phase reaches a state near the stoichiometric 3/2 mullite. Only the fibers previously heat treated at 1400°C did not show further changes when exposed again to 1200°C. Overall, the heat treatments increased the fiber stability and creep resistance but reduced the tensile strength. Changes observed in the creep strain vs. time curves of the fibers were related to the observed microstructural transformations. Based on these results, the chemical composition of the stable mullite fiber is suggested.     

On Structural Features Responsible for the Mechanical Properties of Ceramics from Zirconia Partially Stabilized with Y3+ and Mg2+ Ions

It is shown that Y-PSZ ceramics mainly fracture by an intercrystalline mechanism and the grain size fluctuates from 0.2 to 0.8 μm, whereas Mg-PSZ ceramics mainly fracture by a transcrystalline mechanism and the grain size fluctuates from 30 to 80 μm. The high level of mechanical properties is provided by compressive stresses that appear in the transformation of the β′-phase into the a-phase. In the Y-PSZ ceramics the transformation concerns individual grains, and in the Mg-PSZ ceramics lenticular segregations of the β′-phase are positioned in the γ-phase. The transformation is induced by tensile stresses of the elastic field of a crack, i.e., can be classified as transformation toughening.     

Enhanced ionic conductivity and thermal shock resistance of MgO stabilized ZrO2 doped with Y2O3

The present work focused on the effect of Y₂O₃ co-doping on the phase composition, microstructure, ionic conductivity and thermal shock resistance of 8 mol% MgO stabilized ZrO₂ (Mg-PSZ) electrolyte ceramics for high temperature applications. The addition of Y₂O₃ could promote the process of monoclinic-to-cubic/tetragonal phase transformation and became the metastable phase at room temperature. Meanwhile, the grain size of Mg-PSZ decreased. It was demonstrated that an appreciable increase in the ionic conductivity and compressive strength occurred on substituting MgO with Y₂O₃ in the Mg-PSZ electrolyte ceramics across the measured temperature range. Moreover, the Y₂O₃ addition could restrain the adverse effect of the cyclic thermal shock on the ionic conductivity and compressive strength of Mg-PSZ. The main reason was that the increase of the amount of monoclinic phase caused by cubic/tetragonal-to-monoclinic phase transformation by the cyclic thermal shock was restrained after the Y₂O₃ addition.     

Effect of zirconia content on mechanical and thermal properties of mullite–zirconia composite

Abstract

Mullite–zirconia composites were prepared by adding various zirconia contents in the mullite ranging from 0 to 30 wt-% and sintering at 1400–1600°C for 2 h. The phase composition examined by X-ray diffraction showed that mullite was the major phase combined with developed t-ZrO₂ and m-ZrO₂ phase as a function of zirconia content, especially at 1600°C, wherein m-ZrO₂ predominated. Density increased when the zirconia content and sintering temperature were increased ranging from 2·2 to 3·53 g cm^?3. The morphology of mullite grain showed elongated grains, whereas dispersed zirconia showed equiaxed and intergranular grains. Flexural strength was continuously improved by adding zirconia during the sintering temperature ranging from 1400 to 1500°C, whereas flexural strength was initially improved up to 5 wt-% of zirconia addition and deteriorated with more than 5 wt-% of zirconia content during sintering between 1550 and 1600°C. The maximum strength, 190 MPa, was obtained when sintering mullite with 30 wt-% of zirconia content at 1500°C. The degradation of strength at high sintering temperature may be a result from more occurrence of m-ZrO₂ phase. Thermal expansion of sintered specimens indicated linear change and hysteresis loop change. The hysteresis loop obtained with increased zirconia content resulted in the t–m phase transformation. Martensitic start temperature M_s was determined to be 530°C for 15 wt-% zirconia sintered at 1500°C, implying that the t–m phase transformation occurred.     

MgO calcination for easy direct coagulation casting of aqueous alumina slurries

The reactivity of MgO with ammonium poly(acrylate) and diammonium hydrogen citrate dispersants was decreased by high-temperature calcination which enabled easy preparation of direct coagulation casting slurries without cooling. The decrease in reactivity of MgO with an increase of calcination temperature (30–1200?°C) was due to the decrease of surface area (52.7–0.7?m²/g) as a result of an increase of average particle size (285–2075?nm) as well as a change of particle morphology from flaky to near spherical. The MgO calcined at a temperature of 1000?°C and above provided sufficient time for mixing with aqueous alumina slurries by ball milling at room temperature (~30?°C) without producing an adverse increase in viscosity before casting. The setting time of 55?vol% alumina slurries was in the ranges of 260–1070 and 10–50?min at room temperature and at 70?°C, respectively, at MgO concentrations in the range of 0.1123–1.2?wt%. The faster setting at 70?°C was due to a combination of faster dispersant-MgO reaction, faster hydration of MgO and high valance counter ion effect.     

Utilizing TiO2 amorphous precursors for polymorph selection: An in situ TEM study of phase formation and kinetics

Selective synthesis of metastable polymorphs requires a fundamental understanding of the complex energy landscapes in which these phases form. Recently, the development of in situ high temperature and controlled atmosphere transmission electron microscopy has enabled the direct observation of nucleation, growth, and phase transformations with near atomic resolution. In this work, we directly observe the crystallization behavior of amorphous TiO₂ thin films grown under different pulsed laser deposition conditions and quantify the mechanisms behind metastable crystalline polymorph stabilization. Films deposited at 10 mTorr chamber oxygen pressure crystallize into nanocrystalline Anatase at 325°C, whereas films deposited at 2 mTorr crystallize into significantly larger needle-like grains of Brookite and Anatase at 270°C. Increasing film deposition rate by a factor of 4 results in a 10× increase in the crystalline growth front velocity as well as a decrease in crystallization temperature from 270°C to 225°C. Engineering the amorphous precursor state through deposition conditions therefore provides routes to microstructure control and the accessibility of higher energy metastable phases.     

β-TCP coatings on zirconia bioceramics: The importance of heating temperature on the bond strength and the substrate/coating interface

β-tricalcium phosphate (β-TCP) coatings were synthesized on tetragonal zirconia (Y-TZP) discs by heating the apatite coating between 800?°C and 1200?°C. The study results suggest that heating temperature has a strong influence on the coating bond strength and microstructure of the substrate/coating interface. The β-TCP coatings fired at 800?°C and 900?°C exhibited excellent tensile bond strength (～50?MPa) while heating at 1100?°C and 1200?°C led to decreased bond strength (～30?MPa) as the result of substantial structural and microstructural changes: diffusion of Y³⁺ from the zirconia substrate in the coating resulting in partial crystal transformation (t-m) of zirconia, formation of surface uplifts and nanoporosity in zirconia, as well as generation of large residual thermal stresses leading to microcracking of the β-TCP coatings. However, these structural changes did not have any measurable effect on the flexural strength of the bulk zirconia substrates.