Cation grain‐boundary diffusivity in SiO2‐ and MgO‐doped Al2O3

Cation grain‐boundary diffusion in undoped and aliovalent‐doped Al₂O₃ is characterized using Cr₂O₃ as a chemical tracer. The compositional depth profiles measured by secondary ion mass spectrometry are fitted to the Whipple‐LeClaire model. The results indicate that cation grain‐boundary diffusivity is insensitive to MgO and SiO₂ dopants between 1100°C and 1300°C.     

Grain size effect and microstructure influence on the energy storage properties of fine‐grained BaTiO3‐based ceramics

The dependence of energy storage properties on grain size was investigated in BaTiO₃‐based ferroelectric ceramics. Modified BaTiO₃ ceramics with different grain size were fabricated by two‐step sintering method from BaTiO₃ powders doped with Al₂O₃ and SiO₂ by aqueous chemical coating. For samples doped with ZnO sintering aid in addition to Al₂O₃‐SiO₂, the density and breakdown strength increased significantly. In general, samples with smaller grains have lower polarization but higher energy storage efficiency. Al₂O₃‐SiO₂‐ZnO‐doped samples with average grain size of 118±2 nm have an energy density of 0.83±0.04 J/cm³. Obvious segregation of doping elements in second phase and grain boundary was observed by TEM‐EDS. Impedance spectroscopy further explains the relationship between microstructure and properties. Compared to common energy storage ceramics, the grain size of this low‐cost ceramics sintered at relatively low temperature is small, and the pilot scale production has been well completed. All these features make the utilization in multilayer devices and industrial mass production possible. In addition, the obtained rules are helpful in further development of energy storage ceramics.     

Microstructure and Li‐Ion Conductivity of Hot‐Pressed Cubic Li7La3Zr2O12

The effect of hot‐pressing temperature on the microstructure and Li‐ion transport of Al‐doped, cubic Li₇La₃Zr₂O₁₂ (LLZO) was investigated. At fixed pressure (62 MPa), the relative density was 86%, 97%, and 99% when hot‐pressing at 900°C, 1000°C, and 1100°C, respectively. Electrochemical impedance spectroscopy showed that the percent grain‐boundary resistance decreased with increasing hot‐pressing temperature. Hot pressing at 1100°C resulted in a total conductivity of 0.37 mS/cm at room temperature where the grain boundaries contributed to 8% of the total resistance; one of the lowest grain‐boundary resistances reported. We believe hot pressing is an appealing technique to minimize grain‐boundary resistance and enable correlations between LLZO composition and bulk ionic conductivity.     

The Effects of Na2O and SiO2 on Liquid Phase Sintering of Bayer Al2O3

To determine how grain‐boundary composition affects the liquid phase sintering of MgO‐free Bayer process aluminas, samples were singly or co‐doped with up to 1029 ppm Na₂O and 603 ppm SiO₂ and heated at 1525°C up to 8 h. Na₂O retards densification of samples from the onset of sintering and up to hold times of 30 min at 1525°C compared to the undoped samples, but similar to the as‐received, MgO‐free Al₂O₃, Na₂O‐doped samples sinter to 98% density with average grain sizes of ~3 μm after 8 h. Increasing SiO₂ concentration significantly retards densification at all hold times up to 8 h. The estimated viscosities (20?400 Pa·s) of the 0.3 to 1.8 nm thick siliceous grain‐boundary films in this study indicate that diffusion greatly depends on the composition of the liquid grain‐boundary phase. For low Na₂O/SiO₂ ratios, densification of Bayer Al₂O₃ at 1525°C is controlled by diffusion of Al³⁺ through the grain‐boundary liquid, whereas for high Na₂O/SiO₂ ratios, densification can be governed by either the interface reaction (i.e., dissolution) of Al₂O₃ or diffusion of Al³⁺. Increasing Na₂O in SiO₂‐doped samples increases diffusion of Al³⁺ and Al₂O₃ solubility in the liquid, and thus densification increases by 1%. Based on these findings, we conclude that Bayer Al₂O₃ densification can be manipulated by adjusting the Na₂O to SiO₂ ratio.     

Stability and electrical properties of MoSi2‐ and WSi2‐oxide electroconductive composites

MoSi₂‐ and WSi₂‐based electroconductive ceramic composites were fabricated using 40‐80 vol% fine‐ and coarse‐Al₂O₃, and ZrO₂ particles (refractory oxides) after sintering in argon. Their chemical and thermal stability was tested between 1400°C‐1600°C for up to 48 hours. X‐ray diffraction analysis showed the formation of secondary 5‐3 metal silicide (Mo₅Si₃, W₅Si₃) and silica phases on the grain boundaries and surface. The fraction of the W₅Si₃ (11.4‐38.8 vol%) was significantly higher than that of the Mo₅Si₃ (3.3‐7.3 vol%) in the composites after annealing at 1400°C for 48 hours. The rates of grain growth in the composites (0.013‐0.023 μm/h) were highly decreased by a grain‐boundary pinning effect. This effect was relatively better with the addition of the coarse‐grained oxides due to their more homogeneous distribution throughout the microstructure. The 20–80 vol% MoSi₂‐Al₂O₃ (fine‐grained) composite exhibited an electrical conductivity of 8.8 S/cm at 900°C. At the 60 vol% silicide content, MoSi₂–Al₂O₃ (coarse‐grained) and WSi₂–Al₂O₃ (fine‐grained) showed higher electrical conductivity (126‐128 S/cm) at 900°C. The density, porosity level, particle distribution, intrinsic conductivity of silicide phase, particle size, and fraction of the secondary 5‐3 silicide phase highly influenced their electrical properties.     

Increased electrical conductivity and the mechanism of samarium‐doped ceria/Al2O3 nanocomposite electrolyte

To further enhance the electrical conductivity of doped ceria, the samarium‐doped ceria (SDC)/Al₂O₃ nanocomposites were prepared through sintering the coprecipitated powders in 1100°C‐1300°C. The grain sizes of all composites are less than 100 nm and decrease with alumina addition. Besides the main phases of SDC and Al₂O₃, the SmAlO₃ can precipitate in the composites if sintered at higher temperatures or for longer dwell time. The deviations of SDC diffraction peak positions demonstrate the solid solution of alumina into SDC lattice. The total electrical conductivities of the composites increase with alumina content until 30% alumina is added. The SDC/30%Al₂O₃ presents the higher total conductivity than the pure SDC by about five times. Specifically, the grain interior conductivity generally decreases with the alumina addition while the grain‐boundary conductivity increases with that. The introduction of the conductive SDC/Al₂O₃ interface can contribute to the rise of total conductivity, yet the excessive alumina addition also blocks the oxygen ion conduction. The SmAlO₃ precipitation is detrimental to the ion conduction for it consumes part of alumina and leads to the decrement in Sm concentration of SDC grain. Appropriate alumina addition not only enhances the conductivity of SDC but also lowers the material cost.     

High‐Selectivity Y2O3‐Doped SiO2 Nanocomposite Membranes for Gas Separation in Steam at High Temperatures

Asymmetric structures were fabricated by depositing Y₂O₃‐doped SiO₂ (Si/Y) membranes onto γ‐Al₂O₃ supported by tubular α‐Al₂O₃. The thickness of the Y₂O₃‐doped SiO₂ deposits was approximately 100 nm. The deposits/membranes have micropores with a pore diameter ~ <0.40–0.55 nm. Pore size distribution measurements were conducted directly on the membranes before and after hydrothermal treatment with a nano‐permporometer. The gas permeance properties of the membranes were measured in the temperature range 100°C–500°C. The Y‐doped SiO₂ membrane (Si/Y = 3/1) was found to exhibit asymptotically stable permeances of 2.39 × 10^?7 mol/m²/s/Pa for He and 6.19 × 10^?10 mol/m²/s/Pa for CO₂, with a high selectivity of 386 (He/CO₂) at 500°C for 20 h in the presence of steam. The Y‐doped silica membranes exhibit very high gas permeances for molecules with smaller kinetic diameters. The apparent activation energies of the H₂ permeance at 400°C were 24.2 ± 0.2 and 21.3 ± 0.7 kJ/mol for SiO₂ and Si/Y, respectively.     

AC Impedance Spectroscopy of CaF2‐doped AlN Ceramics

The electrical conductivity of CaF₂‐doped aluminum nitride (AlN) ceramics was characterized at high temperatures, up to 500°C, by AC impedance spectroscopy. High thermal conductive CaF₂‐doped AlN ceramics were sintered with a second additive, Al₂O₃, added to control the electrical conductivity. The effects of calcium fluoride (CaF₂) on microstructure and related electrical conductivity of AlN ceramics were examined. Investigation into the microstructure of specimens by TEM analysis showed that AlN ceramics sintered with only CaF₂ additive have no secondary phases at grain boundaries. Addition of Al₂O₃ caused the formation of amorphous phases at grain boundaries. Addition of Al₂O₃ to CaF₂‐doped AlN ceramics at temperatures 200°C–500°C revealed a variation in electrical resistivity that was four orders of magnitude larger than for the specimen without Al₂O_3. The amorphous phase at the grain boundary greatly increases the electrical resistivity of AlN ceramics without causing a significant deterioration of thermal conductivity.     

Perfect High‐Temperature Plasticity Realized in Multiwalled Carbon Nanotube‐Concentrated α‐Al2O3 Hybrid

We investigate the high‐temperature compressive deformation behavior of a novel, fully dense and structurally uniform, 20 vol% multiwalled carbon nanotube (MWCNT)–α‐Al₂O₃ matrix hybrid, which has a strong room‐temperature interfacial shear resistance (ISR) and a unique MWCNT‐concentrated grain‐boundary (GB) structure. We realized a perfect plastic deformation at 1400°C and a rather high initial strain rate of 10^?4 s^?1 by a low ~30 MPa flow stress, which is contrary to the strain hardening response of fine‐grain monolithic Al₂O₃. This unique performance in CNT–ceramic system in compression is explained as follows: the concentrated network of individual MWCNTs perfectly withstands the high‐temperature and shear/compressive forces, and strongly preserves the nanostructure of Al₂O₃ matrix by preventing the dynamic grain growth, even during a large ~44% deformation. Furthermore, the presence of large amount of radially soft/elastic, highly energy‐absorbing MWCNTs in the GB and specially multiple junction areas, and a potentially weak 1400°C‐ISR, could greatly facilitate the GB sliding process (despite the hybrid's strong room‐temperature ISR), as evidenced by the formation of some submicrometer‐scale MWCNT aggregates in GB area, the equiaxed grains and dislocation‐free nanostructure of the deformed hybrid. The results presented here could be attractive for the ceramic forming industry and could be regarded as a reference for oxide systems in which, the GB areas are occupied with soft/elastic, highly energy‐absorbing nanostructures.     

Improved Crack Healing and High‐Temperature Oxidation Resistance of Ni/Al2O3 by Y or Si Doping

Recovery of mechanical strength was investigated for 5 vol% Ni/α‐Al₂O₃ nanocomposites that had improved resistance to high‐temperature oxidation by doping with Y or Si (Ni/Al₂O₃‐Y and Ni/Al₂O₃‐Si). Surface cracks disappeared completely because of the oxidation product, NiAl₂O₄. The fraction of crack disappearance was comparable between Ni/Al₂O₃‐Y and Ni/Al₂O₃‐Si. The apparent activation energy of crack healing is similar to the grain‐boundary diffusion of Ni ions in an Al₂O₃ matrix. The rate‐controlling process of crack healing is the grain‐boundary diffusion of cations in an internally oxidized zone (IOZ) of the Ni/Al₂O₃ system. The bending strengths of the as‐sintered and as‐cracked Ni/Al₂O₃‐Y samples were 561 and 232 MPa, respectively. Heat treatment at 1200°C for 6 h resulted in a recovery of the bending strength up to 662 MPa for Ni/Al₂O₃‐Y as well as 606 MPa for Ni/Al₂O₃‐Si. Y and Si dopants were segregated into the Al site at the Al₂O₃ grain boundaries, and then, enhanced covalent bonding occurred with neighboring oxygen. While the flux of Ni ions was retarded slightly by doping with Y and Si, a shorter IOZ provided enough Ni ions to form NiAl₂O₄ on the surface. Ni/Al₂O₃‐Y and Ni/Al₂O₃‐Si have the desirable properties of crack healing and resistance to high‐temperature oxidation.     

Effect of Sintering Additives on Relative Density and Li‐ion Conductivity of Nb‐Doped Li7La3ZrO12 Solid Electrolyte

Lithium ion conductors with garnet‐type structure are promising candidates for applications in all solid‐state lithium ion batteries, because these materials present a high chemical stability against Li metal and a rather high Li⁺ conductivity (10^?3–10^?4 S/cm). Producing densified Li‐ion conductors by lowering sintering temperature is an important issue, which can achieve high Li conductivity in garnet oxide by preventing the evaporation of lithium and a good Li‐ion conduction in grain boundary between garnet oxides. In this study, we concentrate on the use of sintering additives to enhance densification and microstructure of Li₇La₃ZrNbO₁₂ at sintering temperature of 900°C. Glasses in the LiO₂‐B₂O₃‐SiO₂‐CaO‐Al₂O₃ (LBSCA) and BaO‐B₂O₃‐SiO₂‐CaO‐Al₂O₃ (BBSCA) system with low softening temperature (<700°C) were used to modify the grain‐boundary resistance during sintering process. Lithium compounds with low melting point (<850°C) such as LiF, Li₂CO₃, and LiOH were also studied to improve the rearrangement of grains during the initial and middle stages of sintering. Among these sintering additives, LBSCA and BBSCA were proved to be better sintering additives at reducing the porosity of the pellets and improving connectivity between the grains. Glass additives produced relative densities of 85–92%, whereas those of lithium compounds were 62–77%. Li₇La₃ZrNbO₁₂ sintered with 4 wt% of LBSCA at 900°C for 10 h achieved a rather high relative density of 85% and total Li‐ion conductivity of 0.8 × 10^?4 S/cm at room temperature (30°C).     

Oxygen‐ion conduction in scandia‐stabilized zirconia‐ceria solid electrolyte (xSc2O3–1CeO2–(99−x)ZrO2, 5 ≤ x ≤ 11)

Solid oxide fuel cells (SOFCs) operating at intermediate temperature (500°C‐700°C) provide advantages of better durability, lower cost, and wider target application market. In this work, we have studied Sc₂O₃ (5‐11 mol%) stabilized ZrO₂–CeO₂ as a potential solid electrolyte for application in IT‐SOFCs. Lower Sc₂O₃ doping range than the traditional 11 mol% Sc₂O₃‐stabilized ZrO₂ is an interesting research topic as it could potentially lead to an electrolyte with reduced oxygen vacancy ordering, lower cost, and higher mechanical strength. XRD and Raman spectroscopy was used to study the phase equilibrium in ZrO₂–CeO₂–Sc₂O₃ system and impedance spectroscopy was done to estimate the grain, grain boundary, and total ionic conductivities. Maximum for the grain and grain‐boundary conductivities as well as the tetragonal‐cubic phase boundary was found at 8‐9 Sc₂O₃ mol% in ZrO₂‐1 mol% CeO₂ system. It is suggested that the addition of 1 mol% CeO₂ in the ZrO₂ host lattice has improved the phase stability of high‐conductivity cubic and tetragonal phases at the expense of low‐conductivity t′‐ and β‐phases.     

Preparation of Nitrogen‐Doped Mullite Fiber by Nitridation of Silica–Alumina Gel Fiber in Ammonia

Nitrogen‐doped mullite fibers were first synthesized through the nitridation of Al₂O₃–SiO₂ gel fibers in NH₃. The results showed that nitrogen take‐up began at 800°C, reached the maximum at 900°C, and then decreased with increasing temperature. The ceramic fibers nitridated at 900°C were essentially amorphous, but contained a small amount of nano‐sized Al–Si spinel crystals. Mullite was formed after nitridation at 1200°C, accompanied by crystallization of χ‐SiAlON and δ‐Al₂O₃. The incorporation of nitrogen resulted in the formation of a variety of nitrogen‐containing crystalline phases. The grain size of the mullite fibers can be adjusted by changing of the nitrogen content.     

Selective Corrosion Preparation and Sintering of Disperse α‐Al2O3 Nanoparticles

Disperse fine equiaxed α‐Al₂O₃ nanoparticles with a mean particle size of 9 nm and a narrow size distribution of 2–27 nm were synthesized using α‐Fe₂O₃ as seeds and isolation via homogeneous precipitation‐calcination‐selective corrosion processing. The presence of α‐Fe₂O₃ acting as seeds and isolation phase can reduce the formation temperature to 700°C and prevent agglomeration and growth of α‐Al₂O₃ nanoparticles, resulting in disperse fine equiaxed α‐Al₂O₃ nanoparticles. These α‐Al₂O₃ nanoparticles were pressed into green compacts at 500 MPa and sintered first by normal sintering to study their sintering behavior and finally by two‐step sintering (heated to 1175°C without hold and decreased to 1025°C with a 20 h hold in air) to obtain nanocrystalline α‐Al₂O₃ ceramics. The two‐step sintered bodies are nanocrystalline α‐Al₂O₃ with an average grain size of 55 nm and a relative density of 99.6%. The almost fully dense nanocrystalline α‐Al₂O₃ ceramic with finest grains achieved so far by pressureless sintering reveals that these α‐Al₂O₃ nanoparticles have an excellent sintering activity.     

Anisotropic Grain Boundary Mobility in Undoped and Doped Alumina

The grain boundary mobility of polycrystalline alumina (α‐Al₂O₃), and the effective grain boundary mobility of the basal (0001) plane as it grew into polycrystalline alumina, was determined for undoped alumina, alumina doped with 23 ppm MgO, and alumina doped with 13 ppm CaO at 1600°C. Doping with MgO at a level below the solubility limit decreased the grain‐boundary mobility from 2.7 × 10^?15 to 1.5 × 10^?15 m²/s, and doping with CaO at a level below the solubility limit increased the mobility to 3.5 × 10^?15 m²/s. For the undoped samples at 1600°C, the activation energy for the average grain boundary mobility was 372 ± 39 kJ/mol. The mobility of the (0001) plane growing into alumina doped with MgO at a level below the solubility limit decreased to 1.1 × 10^?15 m²/s compared with the mobility of the (0001) plane growing into undoped alumina (2.5 × 10^?15 m²/s), and the mobility of the (0001) plane growing into alumina doped with CaO (below the solubility limit) increased to 3.2 × 10^?15 m²/s. The activation energy for the mobility of the (0001) plane was 483 ± 76 kJ/mol. Although a measured Ca excess of 2.6 Ca/nm² at the boundary between the (0001) plane and CaO‐doped alumina is correlated with an increased mobility, the platelike morphology of CaO‐doped polycrystalline alumina is associated with an increased mobility of nonbasal planes.     

Liquid‐phase sintering,microstructural evolution,and microwave dielectric properties of Li2Mg3SnO6–LiF ceramics

The liquid‐phase sintering behavior and microstructural evolution of x wt% LiF aided Li₂Mg₃SnO₆ ceramics (x = 1‐7) were investigated for the purpose to prepare dense phase‐pure ceramic samples. The grain and pore morphology, density variation, and phase structures were especially correlated with the subsequent microwave dielectric properties. The experimental results demonstrate a typical liquid‐phase sintering in LiF–Li₂Mg₃SnO₆ ceramics, in which LiF proves to be an effective sintering aid for the Li₂Mg₃SnO₆ ceramic and obviously reduces its optimum sintering temperature from ~1200°C to ~850°C. The actual sample density and microstructure (grain and pores) strongly depended on both the amount of LiF additive and the sintering temperature. Higher sintering temperature tended to cause the formation of closed pores in Li₂Mg₃SnO₆‐x wt% LiF ceramics owing to the increase in the migration ability of grain boundary. An obvious transition of fracture modes from transgranular to intergranular ones was observed approximately at x = 4. A single‐phase dense Li₂Mg₃SnO₆ ceramic could be obtained in the temperature range of 875°C‐1100°C, beyond which the secondary phase Li₄MgSn₂O₇ (<850°C) and Mg₂SnO₄ (>1100°C) appeared. Excellent microwave dielectric properties of Q × f = 230 000‐330 000 GHz, ε_r = ~10.5 and τ_f = ~?40 ppm/°C were obtained for Li₂Mg₃SnO₆ ceramics with x = 2‐5 as sintered at ~1150°C. For LTCC applications, a desirable Q × f value of ~133 000 GHz could be achieved in samples with x = 3‐4 as sintered at 875°C.     

Sintering behavior of calcium lanthanum sulfide ceramics in field‐assisted consolidation

In this study, calcium lanthanum sulfide (CaLa₂S₄, CLS) ceramics with the cubic thorium phosphate structure were sintered at different temperatures by field‐assisted sintering technique (FAST). Densification behavior and grain growth kinetics were studied through densification curves and microstructural characterizations. It was determined that the densification in the 850°C‐950°C temperature range was controlled by a mixture of lattice or grain‐boundary diffusion, and grain‐boundary sliding. It was revealed that grain‐boundary diffusion was the main mechanism controlling the grain growth between 950°C and 1100°C. The infrared (IR) transmittance of the FAST‐sintered CLS ceramics was measured and observed to reach a maximum of 48.1% at 9.2 μm in ceramic sintered at 1000°C. In addition, it was observed that the hardness of the CLS ceramics first increased with increasing temperature due to densification, and then decreased due to a decrease in dislocations associated with grain growth.     

Determination of Chemical Tracer Diffusion Coefficients for the La‐ and Ni‐site in La2NiO4+δ Studied by SIMS

Chemical (impurity) tracer diffusion of Pr, Nd, and Co into polycrystalline La₂NiO_4+δ was done at 950°C–1350°C in air, argon, and intermediate pO₂ (5.5 × 10^?3 atm O₂), and diffusion coefficients were extracted from depth profiles determined by Secondary Ion Mass Spectrometry (SIMS). The Pr and Nd profiles have only one broad region, corresponding to bulk diffusion, whereas the Co tracer depth profile has two distinct regions with different slopes, where the outer shallow region represents bulk diffusion and the inner region with deep penetration depths represents grain‐boundary diffusion. It is thus concluded that the diffusivity on the Ni‐site is enhanced by grain‐boundary diffusion. The bulk diffusion was evaluated using the solution of Fick's second law for thin‐film source, and the grain‐boundary diffusion was evaluated according to Whipple‐Le Claire's equation. The average apparent activation energies for Pr and Nd bulk diffusion are 165 ± 15 kJ/mol, for Co bulk diffusion 295 ± 15 kJ/mol, and for Co grain‐boundary diffusion 380 ± 20 kJ/mol. Qualitatively, the diffusivities and activation energies follow levels and trends in agreement with those from other experimental techniques. The apparent lack of—in fact reverse—correlation between activation energy and level of diffusivity is discussed in terms of a possibility that the faster species (Ni) reach equilibrium defect concentrations while the slower (La) is in effect frozen in.     

Effect of High‐Energy Ball Milling and Silica Fume Addition in BaCO3–Al2O3. Part I: Formation of Cementing Phases

The effects of high‐energy ball milling and subsequent calcination on the formation of barium aluminate cementing phases using mixtures of Al₂O₃ and BaCO₃ were investigated. Silica fume was further added in the raw mixtures to observe its role on the cementing phase formation. Results indicated that the decomposition temperature of BaCO₃ lowered remarkably with the increase in milling time. Barium aluminate cements with grain size in nanometer range were obtained from high‐energy ball‐milled raw mixtures. X‐ray diffraction (XRD) results confirmed several crystalline barium‐silicate and barium aluminate phases present. Formation of crystalline BaO·Al₂O₃ phase was observed between 1000°C and 1100°C in the raw mixtures, which were obtained in amorphous state after milling for 5 h. This temperature is at least 300°C lower than that used in the traditional solid‐state method. Fume SiO₂ additions resulted in BaO·Al₂O₃·2SiO₂ (celsian) formation which acted as a retarder, provides more workability and mechanical strength.