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.     

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.     

Suppressing resistance degradation in SrTiO3-based colossal permittivity capacitor material

At present, research on colossal permittivity materials is extensive but challenging to achieve simultaneous properties of colossal permittivity, low loss, and high resistivity. Resistance degradation also restricts industrial application of colossal permittivity materials. In this work, a new method has been proposed to improve resistivity of colossal permittivity ceramics by making metal ions diffuse on ceramic grain boundaries, thus inhibiting the diffusion of oxygen vacancies at grain boundaries. Sr_0.99La_0.01TiO₃(SLT10) ceramics were synthesized by traditional solid-state method, and then Bi₂O₃ (35%)-Al₂O₃ (10%)-MgO (20%)-CuO (25%)-SiO₂ (10%) mixed oxidant was selected to percolate into ceramics. The resistivity of SLT10 ceramics improved remarkably (from 2.1×10⁸ Ω cm to 1.23×10¹¹ Ω cm under DC 100 V) with a colossal permittivity (16695 @1 kHz) and a low dielectric loss (0.016 @1 kHz), as well as excellent frequency stability (20 Hz–2 MHz) and temperature stability (-170 °C to 375 °C). The source of high insulation resistivity of the SLT10 ceramic sample was discussed. Subsequent examination uncovered that grain in the SLT10 ceramics percolated with metal ions displayed semiconducting characteristics, wherein insulation grain boundaries significantly influenced the ceramic's resistivity and served as formidable potential barriers constraining long-range movement of charge carriers. Experimental analysis demonstrated that the resistance degradation behavior of the SLT10 ceramics was suppressed, the breakdown voltage was increased, and the service life was extended.     

Synergistic effect of cation ordered structure and grain boundary engineering on long-term cycling of Li0.35La0.55TiO3-based solid batteries

In this study, Li_0.35La_0.55TiO₃ (LLTO) was coupled with Al-doped lithium lanthanum zirconate (LLZO) to improve the grain boundary and total conductivity. The obtained ceramic pellets (LLTZO) demonstrated a recordable grain boundary and total conductivity of 3.41 × 10⁻⁴ and 3.03 × 10⁻⁴ S/cm, respectively. The obtained results establish that the heteroatoms can perturb the cation ordered structure and improve the 3D conductivity in grain bulk. In addition, the residual Al-doped LLZO on the grain boundary led to a decline in the boundary resistance. An LiFeCoPO₄ |Li cell was adopted to demonstrate the enhanced conductivity of LLTO. The solid state battery rendered a specific capacity of over 101.2 mAhg⁻¹ after 300 cycles at a relatively high rate of 0.5C. It is established from the experiments that manufacturing a solid battery using the all-coating technique provides a promising approach to achieve a practical application.     

Synthesis and peculiarities of electric properties of Li1.3Zr1.4Ti0.3Al0.3(PO4)3 solid electrolyte ceramics

The solid electrolyte Li_1.3Zr_1.4Ti_0.3Al_0.3(PO₄)₃ compound was synthesized by a solid-state reaction. The ceramic samples were sintered 1, 2 and 3 h and studied by X-ray and complex impedance spectroscopy in the frequency range from 10⁶ to 1.2 × 10⁹ Hz in temperature range from 300 to 600 K. The investigated compound at room temperature belongs to rhombohedral symmetry (s.g. ) with six formula units in the lattice. Two regions of relaxation dispersion were found. The dispersions are related to the fast Li⁺ ion transport in the grain and grain boundaries of ceramics. Varying of the sintering time affects the density of the ceramics, the values of total conductivity and its activation energy. The values of grain conductivity, its activation energy, and relaxation frequency in grain, dielectric permittivity and dielectric losses are independent from sintering duration of the ceramics. The value of activation energy of grain conductivity and activation energy of relaxation frequency is the same. That can be attributed to the fact that the temperature dependence of the grain conductivity is caused only by the mobility of Li⁺ ions, while a number of charge carriers remains constant with temperature.     

Colossal permittivity in Ta-doped SrTiO3 ceramics induced by interface effects and defect structure: An experimental and theoretical study

The lack of systematic research on the phase structure, defect structure, and polarization mechanism hinders the full comprehension of the colossal permittivity (CP) behavior for SrTiO₃-based ceramics. For this purpose, Ta-doped SrTiO₃-based ceramics were synthesized in an N₂ atmosphere with a traditional method. When the appropriate amount of Ta was doped, colossal permittivity (ԑ_r ∼ 62505), low dielectric loss (tanδ ∼ 0.07), as well as excellent temperature stability (−70 °C–180 °C, ΔC/C_25°C ≤ ±15%) were obtained in the Sr_0.996Ta_0.004TiO₃ ceramic. The relationship between Ta doping, polarization mechanism, and dielectric performance was systematically researched according to experimental analysis and theoretical calculations. The first-principle calculations indicate that the Ta⁵⁺ ion prefers to replace the Sr-site. The defect dipoles and oxygen vacancies formed by heterogeneous-ion doping play an active role in regulating the dielectric performance of ceramics. In addition, the interface barrier layer capacitance (IBLC) effect associated with semi-conductive grains and insulating grain boundaries is the primary origin of colossal permittivity for Sr_1-xTa_xTiO₃ ceramics. The polarization mechanism and defect structure proposed in the study can be extended to the research of SrTiO₃ CP ceramics. The results have a good development prospect in colossal permittivity (CP) materials.     

Increase of the ionic conductivity of perovskite-type lithium-ion conductor by bacterial cellulose templating

Solid-state lithium-ion conductors suffer the disadvantages of high grain-boundary resistance. Perovskite-type LLTO was prepared via bacterial cellulose templating in this paper. BET analysis indicated that a broad range of pores formed in the LLTO templated onto bacterial cellulose. XRD examinations showed that the crystallinity of LLTO reduced upon being templated onto bacterial cellulose. SEM observations indicated that grains of LLTO pellets with bacterial cellulose became much larger and well-connected. The boundaries between grains of LLTO with bacterial cellulose became much less sharp, and the grains were almost inseparable. Under the influence of BC, the average grain-boundary thickness decreased. LLTO with 2% bacterial cellulose exhibited the highest total conductivity, 9.38 × 10⁻⁵ S/cm, which increased by 146% compared with pure LLTO. Both the reduced crystallinity and the decreased grain-boundary thickness contributed to the increase of the total conductivity by facilitating the migration of lithium ions across the grain boundaries. An electrochemical test cell of Li₄Ti₅O₁₂//LLTO-2%BC//LiCoO₂ was demonstrated.     

Control of grain boundary in alumina doped CCTO showing colossal permittivity by core-shell approach

Grain boundaries of CaCu₃Ti₄O₁₂ (CCTO) materials have been shown to play leading role in colossal permittivity. Core-shell design is an attractive approach to make colossal dielectric capacitors by controlling the grain boundaries. Core-shell grains of CCTO surrounded by Al₂O₃ shell were synthesized by ultrasonic sol-gel reaction from alumina alkoxide precursor. The influence of alumina shell by comparison with bare CCTO grains was studied. Particularly, microstructure, dielectric and electric effects on sintered ceramics are reported. The average grain size and the density are increased compared to undoped CCTO leading to an improvement of permittivity from 58,000 to 81,000 at 1?kHz. Furthermore a decrease of dielectric loss is found in a frequency range of 10²–10³?Hz. Moreover, the activation energy of grain boundaries is increased from 0.55 to 0.73?eV and the electrical properties such as breakdown voltage, non-linear coefficient and resistivity are improved with the aim of making industrial capacitors.     

Study of the ionic conductivity of Li0.5La0.5TiO3 laser-sintered ceramics

This work explores a chemical synthesis route and, for the first time, laser processing of ionic conductor Li_0.5La_0.5TiO₃ (LLTO) ceramics. The laser sintering technique has been efficient in producing highly dense single-phase ceramics in just a few minutes, starting from an amorphous precursor powder. As comparison, conventionally sintered ceramics were also prepared. Both methods yield polycrystals with long-range structure compatible with a single cubic perovskite, as confirmed by Rietveld refinement of the powder XRD pattern. In contrast, Raman spectroscopy has revealed non-cubic symmetry, indicating the formation of ordered nanodomains. At room temperature, high ionic conductivity of ∼0.5 mS/cm was achieved for the bulk of laser and conventionally sintered samples. However, the grain boundary conductivity changed from 1⋅10⁻³ mS⋅cm⁻¹ (laser-sintered) to 6⋅10⁻³ mS⋅cm⁻¹ (conventionally sintered), which was attributed to changes in the microstructural characteristics of the ceramics.     

Phase transformation and grain-boundary segregation in Al-Doped Li7La3Zr2O12 ceramics

Cubic phase garnet-type Li₇La₃Zr₂O₁₂ (LLZO) is a promising solid electrolyte for highly safe Li-ion batteries. Al-doped LLZO (Al-LLZO) has been widely studied due to the low cost of Al₂O₃. The reported ionic conductivities were variable due to the complicated Al³⁺-Li⁺ substitution and Li_xAlO_y segregation in Al-LLZO ceramics. This work prepared Li_7?3xAl_xLa₃Zr₂O₁₂ (x = 0.00~0.40) ceramics via a conventional solid-state reaction method. The AC impedance and corresponding distribution of relaxation times (DRT) were analyzed combined with phase transformation, cross-sectional microstructure evolution, and grain boundary element mapping results for these Al-LLZO ceramics to understand the various ionic transportation levels in LLZO with different Al-doping amounts. The low conductivity in low Al-doped (0.12~0.28) LLZO originates from the slow Li⁺ ion migration (1.4~0.25 μs) in the cubic-tetragonal mixed phase. On the other hand, LiAlO₂ and LaAlO₃ segregation occur at the grain boundaries of high Al-doped (0.40) LLZO, resulting in a gradual Li⁺ ion jump (6.5 μs) over grain boundaries and low ionic conductivity. The Li_6.04Al_0.32La₃Zr₂O₁₂ ceramic delivers the optimum Li⁺ ion conductivity of 1.7 × 10^?4 S cm^?1 at 25 °C.     

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.     

Impedance Spectroscopy and Dielectric Properties of Flash Versus Conventionally Sintered Yttria‐Doped Zirconia Electroceramics Viewed at the Microstructural Level

The defect chemistry‐modulated dielectric properties of dense yttria‐doped zirconia ceramics prepared by conventional sintering (at 1350°C–1500°C) and electric field‐assisted flash sintering (55 V/cm at 900°C) were studied by impedance spectroscopy. While the bulk dielectric properties from both sets of samples showed only small and insignificant changes in conductivity and permittivity, respectively, a huge increase of these properties was measured for the grain boundaries in the flash sintered specimens. A close analysis of these results suggests that flash sintering reduced grain‐boundary thickness (by about 30%), while increasing the concentration of oxygen vacancies near these interfaces (by about 49%). The underlying mechanism proposed is electric field‐assisted generation and accommodation of defects in the space‐charge layers adjacent to the grain surface. The changes in measured permittivity are attributed to the boundary thickness effect on capacitance, while conductivity involved variations in its defect density‐dependent intrinsic value, accounting for changes also observed in grain‐boundary relaxation frequencies. Therefore, in terms of modifications to the specific dielectric properties of these materials, the overall consequence of flash sintering was to considerably lower the semi‐blocking character of the grain boundaries.     

Abnormal dielectric relaxations and giant permittivity in SrTiO3 ceramic prepared by plasma activated sintering

In this study, the dielectric properties of SrTiO₃ ceramics prepared by plasma-activated sintering (PAS) were investigated. One of the striking findings is that the material exhibits giant room temperature permittivity (k∼3.5 × 10⁴) and low dielectric loss (∼0.05) at 1 kHz, with the permittivity exceeding that of the conventionally prepared SrTiO₃（ST） ceramics (k∼300) by two orders of magnitude. The enhancement of the polarizability was caused by the high concentration of defect dipoles. In this paper, two dielectric relaxation modes of the PAS ceramics below 0°C have been mainly discussed. One dielectric relaxation mode showed higher activation energy than that of the dielectric peak in the same temperature range for the conventional SrTiO₃-based ceramics. This mode was sensitive to humidity, and the strength of this mode was associated with the oxygen vacancies concentration in the ceramics. The other mode exhibited abnormal slowing down of relaxation rate with increasing temperature, which is contrary to the typical dielectric relaxation behavior, and the anomaly persisted over a narrow temperature range. Both modes were observed at the same interface between the grain and grain boundaries.     

Role of amorphous boundary layer in enhancing ionic conductivity of lithium-lanthanum-titanate electrolyte

The low ionic conductivity is a bottleneck of the inorganic solid state electrolyte used for lithium ion battery. In ceramic electrolytes, grain boundary usually dominates the total conductivity. In order to improve the grain boundary effect, an amorphous silica layer is introduced into grain boundary of ceramic electrolytes based on lithium-lanthanum-titanate, as evidenced by electron microscopy. The results showed that the total ionic conductivity could be to be enhanced over 1 × 10⁻⁴ S/cm at room temperature. The reasons can be attributed to removing the anisotropy of outer-shell of grains, supplement of lithium ions in various sites in grain boundary and close bindings among grains by the amorphous boundary layer among grains.     

Low-temperature sintering of Al2O3 ceramics doped with 4CuO-TiO2-2Nb2O5 composite oxide sintering aid

Dense alumina ceramics doped with 5 wt% 4CuO-TiO₂-2Nb₂O₅ composite sintering aids were obtained at low sintering temperatures of 950∼975 °C. The ceramic sintered at optimal condition shows good microwave dielectric properties (ε_r = 12.7, Q × f = 7400 GHz), high thermal conductivity (18.4 W/m K) and high bending strength (320 MPa). TEM and EDS analysis revealed that amorphous Cu-Ti-Nb-O interfacial films with nanometer thickness formed at the grain boundaries, which could provide paths of mass transportation for densification. Al³⁺ ions may be involved in mass transportation through substitution by Ti³⁺ and Ti⁴⁺ ions near the grain boundary during the sintering process. The accumulation of copper ions at the trigeminal grain boundary was observed. The migration and reaction of copper ions in grain boundaries may also play an important role in promoting mass transportation and low-temperature densification of alumina ceramics.     

Oxygen Ion Conduction in Bulk and Grain Boundaries of Nominally Donor‐Doped Lead Zirconate Titanate (PZT): A Combined Impedance and Tracer Diffusion Study

Oxygen ion conduction in Nd³⁺‐doped Pb(Zr_xTi_1?x)O₃ (PZT) was investigated by impedance spectroscopy and ¹⁸O‐tracer diffusion with subsequent secondary ion mass spectrometry (SIMS) analysis. Ion blocking electrodes lead to a second relaxation feature in impedance spectra at temperatures above 600°C. This allowed analysis of ionic and electronic partial conductivities. Between 600°C and 700°C those are in the same order of magnitude (10^?5–10^?4 S/cm) though very differently activated (2.4 eV vs. 1.2 eV for ions and electron holes, respectively). Oxygen tracer experiments showed that ion transport mainly takes place along grain boundaries with partly very high local ionic conductivities. Numerical analysis of the tracer profiles, including a near‐surface space charge zone, revealed bulk and grain‐boundary diffusion coefficients. Calculation of an effective ionic conductivity from these diffusion coefficients showed good agreement with conductivity values determined from impedance measurements. Based on these data oxygen vacancy concentrations in grain boundary and bulk could be estimated. Annealing at high temperatures caused a decrease in the grain‐boundary ionic conductivity and onset of additional defect chemical processes near the surface, most probably due to cation diffusion.     

Effects of ultrasonic assisted processing and clay nanofiller on dielectric properties and lithium ion transport mechanism of poly(methyl methacrylate) based plasticized polymer electrolytes

Lithium ion conducting solid polymer electrolyte (SPE) films consisted of poly(methyl methacrylate) (PMMA) matrix with lithium perchlorate as a dopant ionic salt, poly(ethylene glycol) as plasticizer and montmorillonite clay as inorganic nanofiller have been prepared by classical solution casting and high intensity ultrasonic assisted solution casting methods. The X‐ray diffraction study confirmed the amorphous structure of all these PMMA‐based solid electrolytes and the clay nanosheets existed in exfoliated form in their amorphous phase. Dielectric relaxation spectroscopy had been employed for the investigation of complex dielectric function, ac electrical conductivity, electric modulus, and impedance spectra of these electrolytes over the frequency range from 20 Hz to 1 MHz. It was observed that the dielectric properties and ionic conductivity of the electrolytes strongly depended on the sample preparation methods, and also had changes with addition of the clay nanofiller. Temperature‐dependent dielectric study of the electrolyte films confirmed that their dc ionic conductivity and conductivity relaxation time values obeyed the Arrhenius behavior. This study also revealed that the lithium ion transportation in the ion–dipolar complexes of these electrolytes occurred through hopping mechanism and it was correlated with the conductivity relaxation time. Preparation of these electrolyte films through ultrasonic assisted solution casting method increased the ionic conductivity by more than one order of magnitude in comparison to that of the classical solution casting method, which revealed that the former was a novel method for the preparation of these SPEs of relatively enhanced ionic conductivity. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42188.     

Li_2O掺杂Li_(3x)La_(2/3-x)TiO_3固体电解质的制备及性能

用高温固相法合成了一系列不同组成的固体电解质Li3xLa2/3-xTiO3(LLTO,x=0.06,0.10,0.12,0.16,摩尔分数),研究了不同Li2O掺杂量对LLTO显微结构和电导率的影响。对样品进行X射线衍射和扫描电子显微镜分析,用交流阻抗技术测试其电导率。结果表明:LLTO为超结构的立方晶体,在LLTO(x=0.12)陶瓷中有Li0.485La0.505TiO3相产生;1150℃烧结的样品晶粒分布较均匀且大部分为球形,1250℃烧结的样品致密度较高,晶粒的形状均匀,为片状,x=0.06时,LLTO的电导率最大,其室温电导率为1.1×10-6S/cm。     

Microstructure,conductivity and mechanical properties of calcia stabilized zirconia ceramics obtained from nanosized precursor and reduced graphene oxide doped precursor powders

In the work 12CaO-88ZrO₂ (12CSZ, mol%) ceramics was manufactured both from nanopowder, obtained via cryochemical technique, and composite precursor 12CSZ?+?0.25?wt% rGO (reduced graphene oxide). Via SEM, XRD and Raman spectroscopy the detailed investigation of the effect of the precursor type and intermediate processing on the microstructure and electrical conductivity of ceramics was carried out. It was shown that rGO is completely removed during the annealing at 1550?°C for 3?h in air with no effect on the high ionic conductivity of ceramics. The use of nanosized powder and the additional processing step results in vacuum dense solid electrolytes characterized by well-formed cubic zirconia based solid solution, thin discontinuous grain boundaries and rather high ionic conductivity. The addition of rGO leads to slight microhardness (HV) decrease comparing to ceramics manufactured from the nanosized precursor. As a result, a new technique for zirconia based solid electrolytes having both high electrical conductivity at high temperatures and sufficient mechanical properties was suggested.     

Dielectric Properties of Pure and Gd‐Doped HfO2 Ceramics

The pure, 2 at.%, and 20 at.% Gd‐doped HfO₂ ceramics were prepared by the standard solid‐state reaction technique. Dielectric properties of these ceramics were investigated in the temperature range 300–1050 K and frequency range 20–5 × 10⁶ Hz. Our results revealed an intrinsic dielectric constant around 20 in the temperature below 450 K for all tested ceramics. Two oxygen‐vacancy‐related relaxations R1 and R2 were observed at temperatures higher than 450 K, which were identified to be a dipolar relaxation due to grain response and a Maxwell–Wagner relaxation due to grain‐boundary response, respectively. The dielectric properties of the pure and slightly doped (2 at.%,) samples are dominated by the grain‐boundary response, which results in a colossal dielectric behavior similar to that found in CaCu₃Ti₄O₁₂. The doping level of 20 at.% leads to the structural transformation from monoclinic phase to cubic phase. The dielectric properties of the heavily doped HfO₂ are dominated by the grain response without any colossal dielectric behavior.