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.     

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.     

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.     

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.     

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.     

Grain orientation evolution and multi-scale interfaces enhanced thermoelectric properties of textured Sr0.9La0.1TiO3 based ceramics

Sr_0.9La_0.1TiO₃ based textured ceramics (SLTT-S3T) with a texture fraction of 0.81 are successfully fabricated by the reactive template grain growth method, in which Sr_0.9La_0.1TiO₃/20 wt%Ti was used as matrix and 10 wt% plate-like Sr₃Ti₂O₇ template seeds were used as templates. The phase transition, microstructure evolution, and the anisotropic thermoelectric properties of SLTT-S3T ceramics were investigated. The results show that the ceramics are mainly composed of Sr_0.9La_0.1TiO₃ and rutile TiO₂ phases. Grains grow with a preferred orientation along (h00). A maximum ZT of 0.26 at 1073 K was achieved in the direction perpendicular to the tape casting direction. The low lattice thermal conductivity of 1.9 W/(m K) at 1073 K was obtained decreased by 34%, 40%, and 38% compared with non-textured, SrTiO₃ and Sr_0.9La_0.1TiO₃ ceramics prepared by the same process, can be attributed to the enhanced phonon scattering by the complex multi-scale boundaries and interfaces. This work provides a strategy of microstructural design for thermoelectric oxides to decrease intrinsic lattice thermal conductivity and further regulate thermoelectric properties via texture engineering.     

Subcritical crack growth in Al2O3 with submicron grain size

Gelcast sintered α-Al₂O₃ (corundum) ceramics were developed with a sub-μm grain size at densities >99%. Highly perfect samples with a minimum of flaws were prepared by an approach that maintains the high purity of the raw powder >99.99% Al₂O₃ throughout processing. As a consequence, all grain boundaries are free of even thinnest amorphous interface films, amorphous triple junctions are ⩽150 nm, and their frequency is low. Subcritical crack growth was investigated by an approach recording growth rates as low as 10⁻¹³ m/s. The outstanding purity of grain boundaries gives rise to a resistance against subcritical crack growth which is similar or even below that of coarser conventional alumina ceramics. No significant promotion of subcritical crack growth by water was observed for the new gelcast high-purity ceramics with grain sizes <1 μm, and there is no indication of a threshold K_I0 below which no crack growth would occur. The results suggest that in sintered alumina ceramics with a given purity of grain boundaries the subcritical crack-growth mechanism of stress corrosion is independent of the grain size. With their high mechanical reliability, these corundum grades are promising candidates for the use in new prostheses for joints with a high load bearing capability and with small calliper sizes.     

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.     

Sintering and conductivity of Sc-doped CaZrO3 with Fe2O3 as a sintering aid

This paper reports the effect of 0.1–0.5 wt% Fe₂O₃ addition on sintering and electrical properties of CaZr_0.95Sc_0.05O_3-δ ceramics synthesized by combustion method. Addition of the sintering aid was shown to enhance ceramic densification and grain coarsening at a reduced sintering temperature and a shorter holding time (1430 °C, 2 h). Effect of the sintering aid on electrical conductivity of the ceramics was investigated using impedance spectroscopy. The highest total conductivity was achieved for the composition with 0.5 wt% Fe₂O₃; it was about an order of magnitude higher than that of the composition without Fe₂O₃. The effect of Fe₂O₃ addition on the conductivity of the grain interior and grain boundaries has been discussed. It was concluded that ceramic densification, grain coarsening and formation of small amounts of calcium ferrite at the grain boundaries upon Fe₂O₃ addition were responsible for the conductivity enhancement.     

Low temperature sintering of LaNbO4 proton conductors from freeze-dried precursors

A synthesis method based on freeze-dried precursors was used to obtain nanocrystalline powders of pure and Ca-doped LaNbO₄ at 800 °C. Dense ceramics were prepared at temperature as low as 1100 °C. The LaNbO₄ ceramics were examined by scanning electron microscopy (SEM) to study the microstructure evolution with the sintering temperature. The densification and grain growth rate are lower in Ca-doped samples. The bulk and grain boundary contributions to the overall conductivity were studied by impedance spectroscopy under different gases. Samples sintered at low temperature and with smaller grain size exhibit higher grain boundary resistance and consequently lower total conductivity.     

Effects of LiF addition on microstructure and dielectric properties of CaCu3Ti4O12 ceramics

The effects of small amounts of lithium fluoride sintering aid on the microstructure and dielectric properties of CaCu₃Ti₄O₁₂ (CCTO) ceramics were investigated. CCTO polycrystalline ceramics with 0.5 and 1.0 mol% LiF, and without additive were prepared by solid state synthesis. Good densification (>90% of the theoretical density) was obtained for all prepared materials. Specimens without the sintering aid and sintered at 1090 °C exhibit secondary phases as an outcome of the decomposition reaction. The mean grain size is controlled by the amount of LiF in specimens containing the additive. Impedance spectroscopy measurements on CaCu₃Ti₄O₁₂ ceramics evidence the electrically heterogeneous nature of this material consisting of semiconductor grains along with insulating grain boundaries. The activation energy for grain boundary conduction is lower for specimens prepared with the additive, and the electric permittivity reached 53,000 for 0.5 mol% LiF containing CCTO.     

Microstructure evolution and reaction mechanism of Pb(Zr1/2Ti1/2)O3-Pb(Zn1/3Nb2/3)O3–Pb(Ni1/3Nb2/3)O3 piezoelectric ceramics with plate-like PbTiO3 template

0.83 Pb(Zr_1/2Ti_1/2)O₃-0.11Pb(Zn_1/3Nb_2/3)O₃-0.06Pb(Ni_1/3Nb_2/3)O₃ (PZNNT) samples with plate-like PbTiO₃ (PT) template were prepared using tape casting technology. The microstructure evolution and reaction mechanism between the matrix and PT template was investigated systematically. The quench heat treatment experiment was designed and the microstructure was evaluated. The results showed that the plate-like PT template has relatively low thermal stability which would decompose to form Pb-rich liquid phase and Ti-rich region at the sintering temperature of 900 °C–1050 °C. Plate-like PT template reacted with the PZNNT matrix materials during the sintering process, which did not contribute to the grain growth orientation for PZNNT matrix. Finally, the mechanism of grain growth for the PZNNT ceramics with plate-like PT template is clarified. This work demonstrated that the thermal stability of plate-like template is one of the key factors for fabricating textured piezoelectric ceramics.     

Sintering behavior and properties of lithium stabilized sodium β'' - alumina ceramics with YSZ addition

In this study, investigations of sintering behavior and properties were performed on lithium-stabilized Na-β''-alumina (LiSBA) ceramics with and without 15?wt% 8?mol% Y₂O₃ stabilized ZrO₂ (8YSZ) addition synthesized by solid phase reaction. Changes of phase composition, relative density, and grain size in the ceramics sintered at different temperature were analyzed. It was shown that phase transformation in sintering ceramics was controlled by relationship between the Na₂O evaporation and Li⁺ ions stabilization, while microstructure evolution was controlled by pore-boundary interaction. LiSBA with YSZ addition (Zr-LiSBA) showed more significant variation of β'' phase fractions, slower grain growth and faster densification with increasing sintering temperature, which were caused by enhanced Na₂O evaporation and gas transport by oxygen ion conductor ZrO₂ as well as the drag effect by second phase particles of YSZ in Zr-LiSBA ceramics. Zr-LiSBA specimens sintered at optimized condition achieved higher Vickers microhardness and intermediate Na⁺ ion conductivity.     

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.     

Electron-hole transport in (La0.9Sr0.1)0.98Ga0.8Mg0.2O3−δ electrolyte: effects of ceramic microstructure

The oxygen ion transference numbers of a series of (La_0.9Sr_0.1)_0.98Ga_0.8Mg_0.2O_3−δ (LSGM) ceramics with different microstructures, prepared by sintering at 1673 K for 0.5-120 h, were determined at 973-1223 K by a modified Faradaic efficiency technique, taking electrode polarization into account. In air, the transference numbers vary in the range 0.984-0.998, decreasing when temperature or oxygen partial pressure increases. Longer sintering times lead to grain growth and to the dissolution of Sr-rich secondary phases and magnesium oxide, present in trace amounts at the grain boundaries, into the major perovskite phase. This is accompanied with a slight decrease of the total grain-interior resistivity and thermal expansion, while the boundary resistance evaluated from impedance spectroscopy data decreases 3-7 times. The electron-hole transport in LSGM ceramics was found to decrease when the sintering time increases from 0.5 to 40 h, probably indicating a considerable contribution of acceptor-enriched boundaries in the hole conduction. Due to reducing boundary area in single-phase materials, further sintering leads to higher p-type conductivity. The results show that, as for ionic conductivity, electronic transport in solid electrolytes significantly depends on ceramic microstructure.     

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.     

Oxygen Migration in Dense Spark Plasma Sintered Aluminum‐Doped Neodymium Silicate Apatite Electrolytes

Neodymium silicate apatites are promising intermediate temperature (500°C–700°C) electrolytes for solid oxide fuel cells. The introduction of Al promotes isotropic percolation of O^2?, and at low levels (0.83–2.0 wt% Al) enhances bulk conductivity. To better understand the effect of Al‐doping on intrinsic conductivity, and the impact of grain boundaries on the transport, dense Nd_9.33+x/3Al_xSi_6?xO₂₆ (0 ≤ x ≤ 2) pellets were prepared by spark plasma sintering. Phase purity of the products was established by powder X‐ray diffraction and the microstructure examined by scanning electron microscopy. The ionic conductivity measured by AC impedance spectroscopy for the spark plasma sintered ceramics were compared with transport in single crystals of similar composition. Intermediate Al‐doping (0.5 ≤ x ≤ 1.5) delivered superior overall conductivity for both the polycrystalline and single crystal specimens.     

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.     

Effects of carbothermal reduction on the thermal and electrical conductivities of aluminum nitride ceramics

This study attempts to control the oxygen content by adding various amounts of yttria sintering additives or by introducing an in situ carbothermal reduction using a carbon addition during the Power processing step. While both yttria and the carbon addition increased the thermal conductivity and electrical resistivity, carbon addition was more effective in increasing the DC volume resistivity for specimens that had 3 or 5 wt% Y₂O₃. Among several elements of the electrical conductivities of the grains and grain boundaries, the conductivity of the grains appeared to be more relevant to the thermal conductivities of AlN ceramics. In addition, yttrium enrichment was observed in the grain boundary region. We also found that an in situ carbothermal reduction resulted in a small amount of yttrium-aluminate second phases, which are beneficial to the plasma resistance of the AlN ceramics.     

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.