Microstructural analyses and improved ionic conductivity of La0.62Li0.16TiO3 ceramics prepared by a reactive-templated grain growth (RTGG) process

Impedance measurements were conducted for textured lithium lanthanum titanate (LLTO) ceramics prepared by the reactive-templated grain growth (RTGG) process. Four La_0.62Li_0.16TiO₃ cuboid ceramics with different degree of <110> crystal orientation were used for the measurement at temperatures from 298 to 423 K. The ionic conductivity of the specimen processed with 20% of the reactive template was one order of magnitude higher than that without a template. The high conductivity was attributed to the decrease of grain boundaries caused by the templated grain growth. No obvious anisotropic conductivity was detected for the specimens by the influence of grain and domain boundaries. Domain structures in the LLTO grains were observed by scanning electron microscopy (SEM) for all specimens.     

Improvement in electronic conductivity of perovskite electrolyte by variable-valence element doping

The perovskite electrolyte Li_0.33La_0.557TiO₃ (LLTO) exhibits high mechanical stability, high ionic conductivity, and low strain during cycling. The cycle stability has been significantly improved using LLTO as a coating material for cathode particles. However, the electronic conductivity of LLTO is very low, which affects electrochemical performances. In this study, variable-valence elements (Fe, Ni, Co, and Cr) are added to the perovskite to increase the electronic conductivity. The materials are fabricated using a high-temperature solid-state reaction method. The phase compositions are characterized by X-ray diffraction (XRD), while the microstructures are analyzed by scanning electron microscopy. The electronic conductivity of the doped sample is increased by three orders of magnitude, while the ionic conductivity is slightly reduced. The Cr-doped sample exhibits the highest electronic conductivity of 9.18 × 10⁻⁷ S cm⁻¹. The search for mixed conducting perovskites paves the way for the development of efficient coatings for cathode particles.     

Electrical Characterization of Pd-Doped CMAS-TiO2 Glass-Ceramics

Electrical characterization was performed on Pd-bearing and Pd-free, electrically conductive glass-ceramics in the CMAS-TiO₂ system. AC impedance plots clearly revealed both grain-core and grain-boundary semicircles. Grain boundaries had a lower magnitude of conductivity but a similar activation energy to grain-core conduction except when Pd was present, for which much lower activation energies were indicated. The lack of any frequency (f) dependence of conductivity for f < 100 Hz, while using noble metal electrodes, suggests the conduction was dominantly electronic in origin in these glass-ceramics. Time-dependent polarization measurements showed no change in electrical current over several days, also consistent with this conclusion. Overall, the temperature dependence of the conductivity was relatively low over a very large temperature range, but quite complex. The appearance of a sharp minimum in conduction seen near 600°C suggests some type of transition occurred with our glass-ceramics, possibly related to semiconductor-to-metal transitions seen with some transition-metal crystals. The available data suggest that Pd did have some effect on grain-boundary conduction, but that a percolating Pd network was not formed.     

Effect of sintering temperature on microstructure and transport properties of Li3xLa2/3−xTiO3 with different lithium contents

Li_3xLa_2/3−xTiO₃ (LLTO) powder with different lithium contents (nominal 3x = 0.03–0.75) was synthesized via a simple sol–gel route and then calcination of gel-derived precursor at 900 °C which was much below the calcination temperature required for synthesizing the LLTO powder via solid state reaction route. The LLTO powder of sub-micron sized particles, derived from such sol–gel method, showed almost no aggregation. Starting from the sol–gel-derived powder, the LLTO ceramics with different lithium contents were prepared at different sintering temperatures of 1250 and 1350 °C. It demonstrated that our sol–gel route is quite simple and convenient compared to the previous sol–gel method and requires lower temperature for the LLTO. Our results also illustrated that lithium content significantly affects the structure and ionic conductivity of the LLTO ceramics. The dependence of the ionic conductivity on the lithium content, lattice structure, microstructure and sintering temperature was investigated systematically.     

Microstructure and Thermal Conductivity of Silicon Carbide with Yttria and Scandia

A fully dense SiC ceramic with high thermal conductivity was obtained by conventional hot pressing, with 1 vol% Y₂O₃–Sc₂O₃ additives. The ceramic had a bimodal microstructure consisting of large and small equiaxed SiC grains. Observation with high‐resolution transmission electron microscopy (HRTEM) showed two kinds of homophase (SiC/SiC) boundaries, that is crystallized and clean boundaries, and a fully crystallized junction phase. The thermal conductivity of the SiC ceramic was 234 W (m·K)^?1 at room temperature. The high thermal conductivity was attributed to a clean SiC lattice and good contiguity between SiC grains.     

Ionic conduction,colossal permittivity and dielectric relaxation behavior of solid electrolyte Li3xLa2/3-xTiO3 ceramics

Perovskite-type solid electrolyte lanthanum lithium titanate (LLTO), exhibiting high intrinsic ionic conductivity, has been attracting interests because of its potential use in all solid-state lithium-ion batteries. In this work, we prepared LLTO ceramics by solid state reaction method and studied their conductivity and dielectric properties systematically. It is found that the bulk conductivity of LLTO is several orders of magnitude higher than the grain boundary conductivity. In addition, colossal permittivity was observed in LLTO ceramics in wide frequency/temperature ranges. Two non-Debye type relaxation peaks were observed in the imaginary part of permittivity, resulting from Li⁺ ions motion and accumulation near interfaces of grains/grain boundaries/electrodes. It is suggested that colossal permittivity may originate from the lithium ion dipoles inside the samples and the interfacial polarization of lithium ion accumulation near the grain boundaries. These results clarify the relations among colossal permittivity, relaxation behavior and ionic conduction in solid ion conductor ceramics.     

Processing and mechanical response of highly textured Al2O3

Dense, high quality textured alumina was fabricated by templated grain with only 0.14 wt% (SiO₂ + CaO) and 1–15 wt% tabular alumina templates From stereological measurements, texture fraction was 95% and unaffected by template loading or dopant concentration. Due to the excellent template alignment during casting, the full-width at half the maximum (FWHM) of the rocking curve was exceptionally low at 4.6°. Samples with the largest templated grains (1% templated alumina) have much lower strength (300–320 MPa) than those with smaller templated grains (400–500 MPa). Predominantly transgranular crack paths were observed in fracture surfaces both parallel and perpendicular to the oriented grains. The fracture toughness was anisotropic with slightly higher toughness perpendicular to the basal surface than parallel the direction grain basal surface. Surprisingly, the lowest density sample (93%) tested, which started with 15 wt% templates had the highest strength of 511 MPa ± 0.45 and highest fracture toughness of 4.58 ± 0.44 MPa m^1/2 when measured perpendicular to the basal surface of the grains.     

Structural,optical, microstructure,and giant dielectric behavior of parent,Cu and Sr doped La0.55Li0.35TiO3-δ

Optimization of energy storage performance in dielectric ceramics has been a focus in recent decades due to the benefits of high energy storage density, efficiency, and exceptional temperature stability. In this work, we report huge dielectric constant in La_0.55Li_0.35TiO_3-δ and sharp decrease in its value with the substitution of Sr and Cu at Ti position. These samples La_0.55Li_0.35TiO_3-δ (LLTO), La_0.55Li_0.35Ti_0.9Cu_0.1O_3-δ (LLTCO) and La_0.55Li_0.35Sr_0.1Ti_0.9O_3-δ (LLSTO) were prepared by solid state reaction method. The interfacial polarization of lithium ion aggregation close to the grain boundaries and the dipoles of Li ions in the sample are suggested to be the source of the enormous dielectric values. Parent composition (LLTO) shows highest dielectric constant value (6.29 × 10⁵ at frequency 10 Hz, 7.30×10⁴ at 1 kHz) recorded at room temperature while the lowest dielectric loss value (0.124) was observed for LLSTO at frequency 1 kHz. Structural characterization has been done using X-ray diffraction (XRD) technique to investigate the crystal structure of the prepared compositions. The XRD patterns show the similar crystal structure for all the compositions with the parent composition LLTO. The optical band gap is calculated by Kulbeka Munk function and Tauc plot using UV–visible diffuse reflectance spectroscopy technique. The maximum band gap value (3.32 eV) is obtained for parent composition while doping of Cu and Sr at Ti site in La_0.55Li_0.35TiO_3-δ decreases the band gap value. Optical microscopy shows the micron size grains in these samples. Doping of Sr and Cu in perovskite structure of LLTO brings tunability in dielectric and optical properties.     

Effect of extended annealing cycles on the thermal conductivity of AlN/Y2O3 ceramics

The effect of extended annealing cycles (up to 50 h at 1800°C) on the thermal conductivity of polycrystalline AlN, doped with 5 wt% Y₂O₃, has been studied. The microstructural evolution upon annealing has also been characterized in detail, using quantitative scanning electron microscopy (SEM) observation and energy dispersive X-ray analysis (EDX). As-sintered AlN/Y₂O₃ composites typically contained a dilute yttrium aluminate secondary phase well distributed and completely wetting the AlN grains. Upon annealing, the AlN matrix grains isotropically grew, while the grain-boundary yttrium aluminate phase tended to segregate to triple grain junctions. This segregation process produced a collapse of the grain-boundary film thickness, thus resulting in a completely different AlN microstructure dispersed with isolated yttrium aluminate grains. Equilibrium of the microstructural morphology was achieved after annealing times in the interval 5–10 h. As a consequence of microstructural changes, the thermal conductivity of the annealed AlN polycrystal exceeded that of the as-sintered material. A discussion is given about the variation of thermal properties in terms of both segregation to the triple-grain junctions of the intergranular Y₂O₃-phase and grain-growth of the bulk AlN grains.     

Influence of WO3‐Doping on the Microstructure and Electrical Properties of ZnO–Bi2O3 Varistor Ceramics Sintered at 950°C

The phase evolution, microstructure, and electrical properties of WO₃‐doped ZnO–Bi₂O₃‐based varistors were investigated for different amounts x (0 ≤ x ≤ 1.60 mol%) of the dopant. When x was less than 0.40, the dissolved W⁶⁺ in the β‐Bi₂O₃ acted as a donor in the grain boundaries and reduced the electrical properties of the ZnO varistors. However, when x was 0.40 mol%, which meant an amount of WO₃ equal to that of Bi₂O₃, the electrical properties dramatically increased, which means the W⁶⁺ donor effect is removed at the grain boundaries because a new Bi₂WO₆ phase was formed in the grain‐boundary regions. The Bi₂WO₆ phase has high oxygen conductivity at high temperatures; it transfers more oxygen to the grain boundaries in order to further enhance the electrical properties. For x values higher than 0.40 (i.e., an addition of WO₃ that is greater than the content of Bi₂O₃), the electrical properties were steadily reduced in comparison to the composition with x = 0.40. This could be explained by the reduced amount of Co, Mn, and Al at the grain boundaries and in the ZnO grains as a result of their incorporation into the ZnWO₄ phase. The electrical properties of the ZnO grains and the grain boundaries were in agreement with the results of the impedance spectroscopy analysis.     

Ionic conductivity and relaxations of In-doped GDC (gadolinium doped ceria) ceramics

The effect of In doping on the sintering behaviors and electrical properties of Gd_0.1Ce_0.9O_1.95 (Gd-doped ceria, or GDC) was investigated. The solubility limit of In in GDC was determined to be ~2 at%, and the lattice parameter of GDC was found to decrease from 5.417(7) Å to 5.416(5) Å with 2 at% In dopant. The mean grain size of the sintered body decreased with increasing In content. The concentration of In did not significantly affect the conductivity of the samples; however, undoped GDC showed the highest conductivity. Cole-Cole plots showed that the activation energies of the grain boundaries and grain interiors decreased and increased, respectively, as the In concentration increased to 1 at%. The decreased grain-boundary activation energy is attributed to the segregation of the negatively charged dopant at the grain boundaries, while the increased activation energy of the grain interiors is attributed to the decreases in both the lattice parameters and binding energies with In doping.     

Investigations on pure and Ag doped lithium lanthanum titanate (LLTO) nanocrystalline ceramic electrolytes for rechargeable lithium-ion batteries

The nano-crystalline Li_0.5La_0.5TiO₃ (LLTO) was prepared as an electrolyte material for lithium-ion batteries. The effect of Ag⁺ ion doping in three different concentrations were investigated: Ag_0.1Li_0.4La_0.5TiO₃, Ag_0.3Li_0.2La_0.5TiO₃, and Ag_0.5La_0.5TiO₃ along with Li_0.5La_0.5TiO₃. The prepared pure and Ag⁺ doped LLTO were subjected for structural, morphological, electrical and optical characterizations. The cubic superlattice structure of LLTO nano-powder was altered due to the Ag⁺ substitution tending towards a tetragonal phase. Increasing Ag⁺ substitution a complete tetragonal phase occurs in Ag_0.5La_0.5TiO₃. The average particle size of the prepared ceramic electrolyte ranged between 80 nm and 120 nm. The photoluminescence study reveals that the LLTO and Ag doped LLTO gives a blue emission peak. The size effect on grain and grain boundary resistance was observed and reported. With Ag⁺ substitution, the conductivity got decreased due to the impedance caused by Ag⁺ ions in the conducting path of Li⁺ ion. Among all the samples, Ag_0.5La_0.5TiO₃ shows maximum conductivity of the order of 10^?3 S cm^?1.     

The combined influence of Mg and Ca on microstructural evolution of alumina

The solubility limits of Ca and Mg co-doped in alumina at 1600°C were determined by equilibrating alumina saturated with Ca and Mg. This resulted in the formation of MgAl₂O₄ (Mg spinel), CaO·6Al₂O₃ (CA6), Ca₂Mg₂Al₂₈O₄₆ (CAM-II), and alumina grains saturated with Mg and Ca. Under these conditions, the amount of Ca and Mg in the alumina grains represents the solubility limits. The solubility limits were measured using a fully standardized wavelength dispersive spectrometer mounted on a scanning electron microscope. In the co-doped state, the solubility limit of Ca in alumina was 32 ± 13 ppm, and the solubility limit of Mg in alumina was 210 ± 43 ppm. The presence of Ca results in an increase of the solubility limit of Mg in alumina from 132 to 210 ppm, suggesting that the increased Mg in solution results in more Mg excess at the alumina grain boundaries, thus contributing to a decreased grain-boundary mobility by solute-drag.     

A comparative investigation on protonic ceramic fuel cell electrolytes BaZr0.8Y0.2O3-δ and BaZr0.1Ce0.7Y0.2O3-δ with NiO as sintering aid

Solid oxide fuel cells based on proton-conducting ceramic electrolytes, i.e., protonic ceramic fuel cells (PCFCs), are promising in operating at intermediate to low temperature. BaZr_0.8Y_0.2O_3-δ (BZY) and BaZr_0.1Ce_0.7Y_0.2O_3-δ (BZCY) are two typical electrolyte materials for PCFCs. However, there is still a lack of basis for making a choice between the two materials. In this paper, we present a comparison investigation on practical BZY and BZCY electrolytes with NiO of 2 mol.% as sintering aid. Their crystal structure, sinterability, microstructure, and electrical conductivity in humid air and hydrogen (3% H₂O) are measured and analyzed. Anode-supported PCFCs based on the two electrolyte materials are prepared and their electrochemical performances are tested and analyzed in association with an examination on their microstructure. The results show that both materials can be densified after sintered with NiO aid at 1400 °C for 6h. Ni is doped into the interstitial of BZY while it occupies the B site of perovskite lattice of BZCY. The sintered BZY has small grains and many grain boundaries while BZCY has large grains and much fewer grain boundaries, resulting in lower conductivity of BZY than that of BZCY. A PCFC with BZY electrolyte gives a peak power density of 360 mW cm^?2 at 700 °C, while this value for a PCFC with BZCY is 855 mW cm^?2. Although the performances of BZCY seems much better than those of BZY, a stability test in 10% CO₂-containing Ar at 650 °C shows BZY is stable while BZCY reacts with CO₂ to form BaCO₃ and CeO₂.     

Experimental and theoretical Raman study on the structure and microstructure of Li0.30La0.57TiO3 electrolyte prepared by the sol-gel method in acetic medium

In this report, we prepare LLTO ceramics by the sol-gel method in acetic medium. Raman spectroscopy showed the formation of lanthanum and titanium acetates precursors, which after calcination, lead to formation of the LLTO nanoparticles. Raman spectra were scanned directly over the LLTO pellets and the disappearance of impurities was observed during the microstructure evolution with increasing sintering temperature. X-ray diffraction characterization, including full pattern profile fitting refinements, showed no drastic changes in the unit cell parameters of the LLTO perovskite, but a large increase in the crystallite size domain was observed with increasing sintering temperature. Additionally, an interesting structural phase transition for the Li_0.30La_0.57TiO₃ perovskite structure was observed, from tetragonal P4/mmm to distorted-cubic Pm-3m spacegroup, for the highest sintering temperature (Ts=1300 °C). Experimental and theoretical simulations of Raman spectroscopy confirmed the formation of a distorted-cubic phase and confocal Raman spectroscopy showed the presence of traces of impurities at the grain boundary region. In spite of the low total lithium conductivity observed, the electrochemical impedance spectroscopy analysis showed a remarkable increase in the lithium bulk conductivity for Ts=1300 °C. This fact could be attributed to the structural phase transition from tetragonal to the cubic crystal system.     

Electrochemical promotion of a platinum catalyst supported on the high-temperature proton conductor La0.99Sr0.01NbO4−δ

The recently discovered, high-temperature proton conductor, La_0.99Sr_0.01NbO_4−δ, was used as a support for the electrochemical promotion of a platinum catalyst. Ethylene oxidation was used as a probe reaction in the temperature range 350–450 °C. Moderate non-Faradaic rate modification, attributable to a protonic promoting species, occurred under negative polarisation; some permanent promotion was also observed. In oxidative atmospheres, both the p_O2 of the reaction mixture and the temperature influenced the type and magnitude of the observed rate modification. Rate-enhancement values of up to ρ = 1.4 and Faradaic-efficiency values approaching Λ = −100 were obtained. Promotion was observed under positive polarisation and relatively dry, oxygen-rich atmospheres suggesting that some oxygen ion conductivity may occur under these conditions. Impedance spectroscopy performed in atmospheres of 4 kPa O₂/N₂ and of 5 kPa H₂/N₂ under dry and slightly humidified (0.3 kPa H₂O) conditions indicated that the electrical resistivity is heavily dominated by the grain-boundary response in the temperature range of the EPOC studies; much lower grain-boundary impedances in the wetter conditions are likely to be attributable to proton transport.     

A novel approach toward microstructure evaluation of sintered ceramic materials through image processing techniques

In this paper, an image processing technique is introduced to measure the grain size and their distributions from the SEM image of copper oxide (CuO) and titanium dioxide (TiO₂) doped sintered alumina ceramics accurately. The noise present in SEM image is removed by applying low pass Gaussian filter followed by suppression of regional minima over a threshold. The clarity of individual grains and grain boundaries have been done by applying Watershed transform to this preprocessed SEM image. Morphological operations like dilation and erosion are used to make the grain-boundary edges clear and continuous. The individual grain size in µm scale is measured from the pixel length of the rectangular bounding box drawn around the segmented grain. The normal Gaussian type distribution of grain size is observed in both CuO- and TiO₂-doped grains in SEM image. The average grain size of CuO-doped alumina grains (2.24 µm) is very close to G₅₀ value (2.17 µm), but G₅₀ value of TiO₂-doped grains (8.59 µm) is slightly higher than its average grain size (7.96 µm). The proposed algorithm is compared with linear intercept method and the grain sizes obtained are very close to each other.     

Effect of mixing glass frits on electrical property and microstructure of sintered Cu conductive thick film

The resistivity of the sintered Cu thick film decreases with the weight percentage of the SiO₂–ZnO–B₂O₃ additive in the mixing glass frits up to 50 wt%. As the weight percentage of the SiO₂–ZnO–B₂O₃ additive in the mixing glass frits is over 50 wt%, the resistivity of the sintered Cu thick films is quite similar. The lowest resistivity (6.62 × 10⁻⁶ Ω-cm) of the sintered Cu thick films occurs at 75 wt% of the SiO₂–ZnO–B₂O₃ additive. Also, we observe the extensive glass phase framing around the large Cu grains in the Cu thick films sintered with low SiO₂–ZnO–B₂O₃ additives (less than 50 wt%) narrows the cross-section area of the electrical path. On the contrary, the round-shaped glass phase solidified among the small Cu grains allows a larger cross-section of the electrical path (a possible lower resistivity) for the Cu thick films sintered with higher SiO₂–ZnO–B₂O₃ additives (larger than 50 wt%). The above results imply that the resistivity of the sintered Cu thick film correlates well with the microstructure (Cu grain size and the glass/Cu composite structure) of the sintered Cu thick films. Twin grain boundaries can clearly be observed in the sintered Cu thick films, especially for the Cu thick film sintered with the higher SiO₂–ZnO–B₂O₃ additives. Owing to small Cu grains size and high density of Cu grain boundary, the probability of the grain boundaries with a high grain-boundary energy in the Cu thick film sintered with high SiO₂–ZnO–B₂O₃ additive would be much larger, comparing to that in the Cu thick film sintered with low SiO₂–ZnO–B₂O₃ additive. Thus, more annealing twin boundaries formed in the Cu thick film sintered with high SiO₂–ZnO–B₂O₃ additive. Hence, the formation of the twin boundary in the sintered Cu thick film helps reducing the resistivity of the sintered Cu films.     

Electrical resistivity at the micron scale in a polycrystalline SiC ceramic

Grain boundaries typically dominate the electrical properties of polycrystalline ceramics. To understand the effect of grain boundaries on the electrical conductivity of SiC ceramics sintered with 2000 ppm Y₂O₃, the electrical resistivity of individual grains and multi-grains across boundaries at the micron scale was measured using a nano-probing system equipped with nano-manipulators. The results revealed that grain resistivity was bimodal because of the existence of a core/rim structure in grains, and the electrical resistivity of multigrain samples slowly increased with an increase in the number of grain boundaries crossed. Specifically, the electrical resistivity of a grain without a core, a grain with a core, a bicrystal with a single boundary, a sample crossing three boundaries, and a bulk polycrystalline sample were 2.36 × 10^-1, 5.05 × 10^-1, 4.80 × 10^-1, 5.04 × 10^-1, and 5.84 × 10^-1 Ω cm, respectively. The results suggest that the electrical resistivity of polycrystalline SiC ceramics is primarily influenced by the presence of a grain boundary or core and secondarily by the number of boundaries.     

Electrochemical and mechanical stability of LixLa0.557TiO3-δ perovskite electrolyte at various voltages

Perovskite Li_xLa_0.557TiO₃ electrolytes in all-solid-state lithium batteries are operated under a voltage gradient, which potentially induces the electrochemical and mechanical instability. To simulate the properties of Li_xLa_0.557TiO₃ (LLTO) under application relevant conditions, samples are charged (or discharged) to 0.2 V, 3.2 V, 4.0 V, and 4.5 V, respectively. The Li ion conductivity is 9.55 × 10⁻⁵ S/cm at 3.2 V and decreases obviously to 2.54 × 10⁻⁵ S/cm as the voltage increases to 4.5 V, whereas the value of the LLTO-0.2 V is between that of LLTO-3.2 V and LLTO-4.0 V. In terms of mechanical behavior, elastic modulus (E), hardness (H), and fracture toughness (K_IC) of LLTO operated at different voltages are also tested using depth-sensitive indentation. The results can be used in the designing, monitoring and also improving of the battery cells.