Electrical characterization of La9.33Si2Ge4O26 oxyapatite for prospective intermediate-temperature solid oxide fuel cells

La_9.33Si₂Ge₄O₂₆ materials have been fabricated from La₂O₃, SiO₂ and GeO₂ powders by high speed mechanical alloying followed by conventional and microwave hybrid sintering at different temperatures and holding times. XRD data showed that the apatite phase is formed after 1 h of mechanical alloying at 850 rpm. This phase remained stable after conventional sintering in an electric furnace with density increasing as sintering temperatures and holding times were increased. However, the highest density was achieved for samples sintered in the microwave furnace (5.44 g cm⁻³), corresponding to a relative density of 98%. The electrical conductivity of the samples microwave sintered at 700 and 800 ºC are 4.72×10⁻³ and 1.93×10⁻² S.cm⁻¹, respectively, with a correspondent activation energy of 0.952 eV.     

Electrically conductive SiC ceramics processed by pressureless sintering

Polycrystalline SiC ceramics with 10 vol% Y₂O₃-AlN additives were sintered without any applied pressure at temperatures of 1900-2050°C in nitrogen. The electrical resistivity of the resulting SiC ceramics decreased from 6.5 × 10¹ to 1.9 × 10⁻² Ω·cm as the sintering temperature increased from 1900 to 2050°C. The average grain size increased from 0.68 to 2.34 μm with increase in sintering temperature. A decrease in the electrical resistivity with increasing sintering temperature was attributed to the grain-growth-induced N-doping in the SiC grains, which is supported by the enhanced carrier density. The electrical conductivity of the SiC ceramic sintered at 2050°C was ~53 Ω⁻¹·cm⁻¹ at room temperature. This ceramic achieved the highest electrical conductivity among pressureless liquid-phase sintered SiC ceramics.     

Liquid‐phase sintering of highly Na+ ion conducting Na3Zr2Si2PO12 ceramics using Na3BO3 additive

Na₃Zr₂Si₂PO₁₂ (NASICON) is a promising material as a solid electrolyte for all‐solid‐state sodium batteries. Nevertheless, one challenge for the application of NASICON in batteries is their high sintering temperature above 1200°C, which can lead to volatilization of light elements and undesirable side reactions with electrode materials at such high temperatures. In this study, liquid‐phase sintering of NASICON with a Na₃BO₃ (NBO) additive was performed for the first time to lower the NASICON sintering temperature. A dense NASICON‐based ceramic was successfully obtained by sintering at 900°C with 4.8 wt% NBO. This liquid‐phase sintered NASICON ceramic exhibited high total conductivity of ~1 × 10^?3 S cm^?1 at room temperature and low conduction activation energy of 28 kJ mol^?1. Since the room‐temperature conductivity is identical to that of conventional high‐temperature‐sintered NASICON, NBO was demonstrated as a good liquid‐phase sintering additive for NASICON solid electrolyte. In the NASICON with 4.8 wt% NBO ceramic, most of the NASICON grains directly bonded with each other and some submicron sodium borates segregated in particulate form without full penetration to NASICON grain boundaries. This characteristic composite microstructure contributed to the high conductivity of the liquid‐phase sintered NASICON.     

Enhanced grain boundary ionic conductivity of LiTa2PO8 solid electrolyte by 75Li2O-12.5B2O3-12.5SiO2 sintering additive

LiTa₂PO₈(LTPO) has low electrolyte density and many pores at grain boundaries, and it is easy to precipitate dielectric phase LiTa₃O₈ at grain boundaries. The performance can be improved by adding 75Li₂O-12.5B₂O₃-12.5SiO₂ (LBS) sintering additive with low melting point during sintering. The effects of LBS addition on the microstructure and grain boundary ionic conductivity of LTPO electrolytes were studied. The results showed that the addition of LBS sintering additives reduced the sintering temperature, improved the density and stability of LTPO electrolyte samples, effectively inhibited the precipitation of LiTa₃O₈ phase, reduced the grain boundary impedance of samples, and improved the total ionic conductivity of electrolytes. When LBS was added at 0.4 wt%, the relative density of LTPO reached 93.54%, the grain boundary impedance decreased from 1243 Ω to 248.2 Ω, the total ionic conductivity increased from 1.55 × 10⁻⁴ S cm⁻¹ to 6.51 × 10⁻⁴ S cm⁻¹, and the ionic activation energy was 0.137 eV.     

Electrical conductivity characteristics of Sr substituted layered perovskite cathode (SmBa0.5Sr0.5Co2O5+d) for intermediate temperature-operating solid oxide fuel cell

The high electrical conductivity of the cathode is one of the important factors for reducing the polarization resistance. For this reason, we here report the electrical conductivity characteristics of SmBa_0.5Sr_0.5Co₂O_5+δ (SBSCO) as a function of sintering temperature and current ranges. Calcined SBSCO samples were sintered at 1000, 1050, 1100, and 1150 °C. The current ranges applied in the process of measuring electrical conductivity were subdivided as 1.0A [0.05step], 0.5A [0.025step], and 0.1A [0.005step]. It was found that the sintering temperature affected the electrical conductivity in the following way: when the sintering temperature increases, an increase in the observed electrical conductivity is the result. However, as the current range decreases, it was found that the electrical conductivity would increase. The maximum and minimum conductivities of SBSCO sintered at 1150 °C were 2263S?cm^?1 at 50 °C and 382 S?cm^?1 at 900 °C with metallic behavior in air condition. When a current of 0.1A was applied to SBSCO sintered at 1150 °C, the electrical conductivity at the 800 °C was 1377.15 S/cm. It can be determined that the increase in the internal charge carrier flux of the SBSCO is associated with the decrease in the overall electrical conductivity of the Co-based metallic electrical conductivity. These results show that the high sintering temperature and low current range enable higher electrical conductivity at high operating temperature.     

Solvent-deficient method lowers grain-boundary resistivity of doped ceria

Nanoparticles of gadolinium-doped cerium oxide (GDC) were synthesized using solvent-deficient method and their sinterability and electrical properties were investigated using the powder and cold sintering process. The GDC powder was uniaxially pressed into cylindrically-shaped pellets with a mixture of nitric acid and hydrogen peroxide at 200°C to encourage particle arrangement during forming process. These bulk samples were annealed using two different temperature profiles: at 800°C for 5 hours and at 1300°C for 1 minute—800°C for 5 hours. The samples produced using HNO₃/H₂O₂ mixture showed higher relative density than ones without it. Ionic conductivity of the sample sintered through the two-step profile was obtained from electrochemical impedance spectroscopy. Although the grain conductivity for the samples (8.0 × 10⁻³ S cm⁻¹ at 500°C, and 3.3 × 10⁻² S cm⁻¹ at 700°C) is on par with a conventionally sintered sample, the measured total conductivity (3.9 × 10⁻³ S cm⁻¹ at 500°C, and 2.5 × 10⁻² S cm⁻¹ at 700°C) is about 10 times higher than the conventionally sintered one and is comparable to the values seen in the previous studies for GDC which employed higher sintering temperature, pointing to the effectively lower grain-boundary impedance. This result could be attributed to no significant phase segregation along grain boundaries due to the low-temperature processing.     

Effect of sintering and annealing on electrochemical and mechanical characteristics of Na3Zr2Si2PO12 solid electrolyte

NASICON (Sodium superionic conductor) type Na₃Zr₂Si₂PO₁₂ (NZSP) has received a lot of interest as the solid electrolyte for all-solid-state sodium-ion batteries (ASSIBs). The electrolyte has superior interfacial characteristics, high thermal stability, and good ionic conductivity. Because of their higher energy density, improved mechanical stability, no liquid leakage problem, and higher operating voltages, All solid-state batteries are expected to replace liquid electrolyte-based batteries in many applications. The solid electrolyte also acts as a separator, and hence additional separator is not required for cell operations. Because of its 3D open architecture and continuous diffusion channels, NZSP is considered a better solid electrolyte. The NZSP solid electrolyte has been synthesized by spark plasma sintering (SPS) followed by annealing the sintered materials. The SPS method leads the material to have higher density and ionic conductivity. Conventional sintering of the materials requires a temperature as high as 1225°C; however, the temperature required for the SPS is as low as 1050°C. Moreover, conventional sintering yields samples of relative density up to 91%, while SPSed samples have achieved a maximum density of around 98%. The ionic conductivity of solid electrolyte SPSed at 1050°C for 10 min is found to be 3.5 × 10⁻⁴ S/cm with an activation energy of 0.27 eV. The annealing of the SPSed samples improves the ionic conductivity of the SPS1050-20mins sample to roughly double the value obtained from the as-prepared SPS sample because there are fewer secondary phases and a structural change from a rhombohedral to a monoclinic system. To ascertain the samples' crystal structure, particle shape, and ionic conductivity, materials were characterized using X-ray diffraction, scanning electron microscopy, and electrochemical impedance spectroscopy. The samples' mechanical characteristics, for example, the hardness and fracture toughness of the samples, were also determined.     

Effect of sintering temperature on the structural,electrical and electrochemical properties of novel Mg0.5Si2 (PO4)3 ceramic electrolytes

The objective of the present study is to investigate the effect of sintering temperature on the structural, electrical and electrochemical properties of novel Mg_0.5Si₂ (PO₄) ₃ NASICON structured compound prepared via sol gel method. X-ray diffraction was used to study the structural properties such as crystalline phase and lattice parameters of the solid electrolytes. Electrical properties of the compound were measured using impedance spectroscopy while the electrochemical stability was investigated by linear sweep voltammetry. All the sintering temperatures yielded compounds consisted of monoclinic crystalline phase with a space group of P1 2₁/c1. Lattice parameters for Mg_0.5Si₂ (PO₄) ₃ samples increased from the sintering temperature at 700–800 °C but decreased for sintering temperature at 900 °C. The sample sintered at 800 °C showed the highest total conductivity of 1.83×10⁻⁵ S cm⁻¹ and the highest value of ions mobility, µ of 6.17×10¹⁰ cm² V⁻¹ s⁻¹ which was attributed to the optimum size of migration channel indicated by its unit cell volume. Linear sweep voltammetry result showed that the Mg_0.5Si₂ (PO₄)₃ powder was electrochemically stable up to 3.21 V.     

Li2ZrO3 based Li-ion conductors doped with halide ions & sintered in oxygen-deficient atmosphere

All-solid-state batteries have recently attracted much attention for their high energy density and safety. Li₂ZrO₃-based Li-ion conductors with high electrochemical stability have potential applications for electrolytes in all-solid-state batteries. In this work, comparative investigations of Li₂ZrO₃ and halogen doped Li₂ZrO₃ ceramics were conducted by sintering at 700 °C in air or in oxygen-deficient atmosphere which was induced by a simple setup covering with corundum crucible. The analysis of phase composition reveals that the undoped Li₂ZrO₃ ceramic sintered in air contains pure monoclinic phase, while halogen-doped Li₂ZrO₃ sintered in air and all ceramics sintered in oxygen-deficient atmosphere are simultaneously composed of monoclinic and tetragonal phases. Li₂ZrO₃ ceramic with tetragonal phases has higher conductivity (0.28 mS cm⁻¹ for undoped Li₂ZrO₃) than the pure monoclinic Li₂ZrO₃ (0.07 mS cm⁻¹), and halogen doping can further enhance the conductivity of Li₂ZrO₃ ceramics higher than 0.5 mS cm⁻¹ at room temperature.     

Microstructure and ionic conductivity of Li1.5Al0.5Ge1.5(PO4)3 solid electrolyte prepared by spark plasma sintering

In this paper, the microstructure and ionic conductivity of Li_1.5Al_0.5Ge_1.5(PO₄)₃ (LAGP) solid electrolytes prepared by spark plasma sintering (SPS) were investigated by XRD, SEM, TEM and EIS, respectively. The results showed that as the sintering temperature was increased, both the relative density and the ionic conductivity of the sintered LAGP samples first increased and then decreased, achieving a maximum value of 97% and 2.12 × 10⁻⁴ S cm⁻¹ simultaneously at 700 °C. At the same time, the crystallinity of the sintered samples was improved, while a few impurity phases, such as AlPO₄ and GeO₂, appeared in the samples. It was also found that carbon contamination and oxycarbide gas was be brought in during SPS. Carbon contamination could produce an extra grain boundary impedance to the samples and could be removed by annealing at 500 °C in an air atmosphere. Oxycarbide gas could affect the relative density of the sintered LAGP samples and could be mitigated by choosing a suitable SPS process. Moreover, the shear modulus of the sintered LAGP was measured to be 49.6 GPa, which exceeded the minimum value of 8.5 GPa that was necessary to suppress Li dendrite growth.     

Lithium mobility along conduction channels of ceramic LiTa2PO8

In the next generation of lithium-ion batteries, the liquid electrolyte is considered to be replaced by its solid counterpart. Recently, a novel Li-ion conductor based on metal oxides emerged – LiTa₂PO₈. Due to the high value of bulk conductivity of ca. 10⁻³ S∙cm⁻¹, it is believed to be a potential candidate for application as a solid electrolyte in all-solid-state battery technology. In this work, we investigate LiTa₂PO₈ ceramics synthesized by a conventional solid-state reaction method with an excess of the lithium-containing substrate to compensate for the loss of Li⁺ during sintering. The properties of LiTa₂PO₈ ceramics were studied using X-ray diffractometry (XRD), ⁶Li and ³¹P magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR), thermogravimetry (TG), scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS), impedance spectroscopy (IS), DC potentiostatic polarization technique and density method. Referring to the experimental results, increasing of the Li⁺ content above the stoichiometric one lowers the total ionic conductivity. The reasons for the deterioration and correlations between microstructure, phase composition, and ionic conductivity are presented and discussed. The MAS NMR spectroscopy has been used to explain high bulk ionic conductivity of LiTa₂PO₈ ceramics. A maximum value of total ionic conductivity, 4.5 × 10⁻⁴ S∙cm⁻¹, was obtained at room temperature for the sample without any excess of Li⁺ source.     

Influence of processing conditions on the ionic conductivity of holmium zirconate (Ho2Zr2O7)

Nanopowders of holmium zirconate (Ho₂Zr₂O₇) synthesised through carbon neutral sol-gel method were pressed into pellets and individually sintered for 2 h in a single step sintering (SSS) process from 1100 °C to 1500 °C at 100 °C interval and in a two step sintering (TSS) process at (I) −1500 °C for 5 min followed by (II) - 1300 °C for 96 h. Relative density of each of the sintered pellet was determined using the Archimedes’ technique and the theoretical density was calculated from crystal structure data. Grain size was obtained from SEM micrographs using ImageJ. Pellets processed by TSS have been found to be denser (98 %) with less grain growth (1.29 μm) as compared to the pellets processed using SSS process. Ionic conductivity of Ho₂Zr₂O₇ pellets sintered by two different processes was measured using ac impedance spectroscopy technique over the temperature range of 350 °C–750 °C in the frequency range of 100 mHz–100 MHz for both heating and cooling cycles. The temperature dependence of bulk (2.67⨯10⁻³ Scm⁻¹) and grain boundary (2.50⨯10⁻³ Scm⁻¹) conductivities of Ho₂Zr₂O₇ prepared by TSS process are greater than those processed by SSS process suggesting the strong influence of processing conditions and grain size. Results of this study, indicates that the TSS is the preferable route for processing the holmium zirconate as it can be sintered to exceptionally high densities at lower temperature, exhibits less grain growth and enhanced ionic conductivity compared with the samples processed by SSS process. Hence, holmium zirconate can be considered as a promising new oxide ion conducting solid electrolyte for intermediate temperature SOFC applications between 350 °C and 750 °C temperature range.     

Improved structural stability and ionic conductivity of Na3Zr2Si2PO12 solid electrolyte by rare earth metal substitutions

Sodium zirconium silicon phosphorus with the composition of Na₃Zr₂Si₂PO₁₂ (NZSP) was prepared by a facile solid state reaction method. The effects of the calcination temperature and rare earth element substitution on the structure and ionic conductivity of the NZSP material were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and AC impedance measurement. The results show that the microstructure and ionic inductivity of the NZSP was strongly affected by the aliovalent substitution of Zr⁴⁺ ions in NZSP with rare earth metal of La³⁺, Nd³⁺ and Y³⁺. At room temperature, the optimum bulk and total ionic conductivity of the pure NZSP solid electrolyte sintered under different conditions were 6.77×10⁻⁴ and 4.56×10⁻⁴ S cm⁻¹, respectively. Substitution of La³⁺, Nd³⁺ and Y³⁺ in place of Zr⁴⁺ exhibited higher bulk conductivity compared with that of pure NZSP. Maximum bulk and ionic conductivity value of 1.43×10⁻³ and 1.10×10⁻³ S cm⁻¹ at room temperature were obtained by Na_3+xZr_1.9La_0.1Si₂PO₁₂ sample. The charge imbalance created by aliovalent substitution improves the mobility of Na⁺ ions in the lattice, which leads to increase in the conductivity. AC impedance results indicated that the total ionic conductivity strongly depends on the substitution element and the feature of the grain boundary.     

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.     

High Li-ion conductivity of Al-doped Li7La3Zr2O12 synthesized by solid-state reaction

Li₇La₃Zr₂O₁₂ (LLZO) has cubic garnet type structure and is a promising solid electrolyte for next-generation Li-ion batteries. In this work, Al-doped LLZO was prepared via conventional solid-state reaction. The effects of sintering temperature and Al doping content on the structure and Li-ion conductivity of LLZO were investigated. The phase composition of the products was confirmed to be cubic LLZO via XRD. The morphology and chemical composition of calcined powders were investigated with SEM, EDS, and TEM. The Li-ion conductivity was measured by AC impedance. The results indicated the optimum sintering temperature range is 800–950 °C, the appropriate molar ratio of LiOH·H₂O, La(OH)₃, ZrO₂ and Al₂O₃ is 7.7:3:2:(0.2–0.4), and the Li-ion conductivity of LLZO sintered at 900 °C with 0.3 mol of Al-doped was 2.11×10⁻⁴ S cm⁻¹ at 25 °C.     

Signature of multiferroicity and pyroelectricity close to room temperature in BaFe12O19 hexaferrite

In this paper, we have reported the signature of multiferroicity and pyroelectricity in BaFe₁₂O₁₉ hexaferrite close to room temperature. The BaFe₁₂O₁₉ hexaferrite samples are synthesized by co-precipitation method at different sintering temperature ranging from 800 to 1200 °C and study their structural, ferroelectric, magnetic, magnetoelectric and pyroelectric properties. X-ray Diffraction patterns show the pure phase formation for all samples. Morphological changes are examined through the scanning electron microscope. The maximum ferroelectric polarization (0.66 μC/cm²) is observed for the sample sintered at 1200 °C, however maximum magnetic polarization 74 emu/g is observed for sample sintered at 1000 °C. Magneto-electric coupling measurements are also performed through dynamic method and average magneto-electric coupling coefficient (~ 7.05 × 10⁻⁷ mV/cm Oe²) is observed at room temperature for the sample sintered at 1200 °C. Furthermore, maximum pyroelectric constant (147 × 10⁻¹³C/cm² °C) is observed at 75 °C for BaFe₁₂O₁₉ samples sintered at 1200 °C. The observation of both multiferroicity and pyroelectricity close to room temperature in BaFe₁₂O₁₉ hexaferrite is interesting and useful for multifunctional devices.     

Preparation of dual-phase composite BaCe0.8Y0.2O3/Ce0.8Y0.2O2 and its application for hydrogen permeation

Dual-phase ceramic membranes composed of BaCe_0.8Y_0.2O₃ (BCY) and Ce_0.8Y_0.2O₂ (CYO) were successfully synthesized by solid state reaction method for hydrogen permeation. The influences of the BCY/CYO volume ratios on phase composition, microstructure, chemical stability and electrical property were investigated. The hydrogen permeation of the dual-phase composite was characterized as a function of temperature and feed side hydrogen partial pressure. The results showed that there was no reaction between the two constituent oxides observed under the preparation conditions. The dual-phase composite with different BCY/CYO volume ratios after sintering at 1550 °C exhibited dense structure, as well as good stability in 4% H₂/Ar, wet Ar and pure CO₂ atmosphere. The conductivity of the dual-phase composite increased with the content of CYO increasing and 30BCY–70CYO exhibited the highest total conductivity of 2.6×10⁻² S cm⁻¹ at 800 °C in 4% H₂/Ar. The hydrogen permeability of 30BCY–70CYO sample was improved as the temperature and the hydrogen partial pressure in feed gas increased. The hydrogen permeation flux of 1.7 μmol cm⁻² s⁻¹ was achieved at 850 °C.     

Rapid crystallization of Li1.5Al0.5Ge1.5(PO4)3 glass ceramics via ultra-fast high-temperature sintering (UHS)

Lithium aluminum germanium phosphate solid electrolyte is a promising candidate for all-solid-state batteries. The currently available processing techniques based on melting-quenching require a rather lengthy crystallization step lasting up to 8 hours. In this work, the newly emerged ultra-fast high-temperature sintering (UHS) was proposed to achieve in a single step reactive consolidation of powder mixture (GeO₂, Al(PO₃)₃, LiPO₃) and crystallization of the electrolyte within 16 minutes. Samples produced using UHS had a better phase purity, reduced Li loss, and improved ionic conductivity when compared to the counterparts prepared using conventional crystallization at 850°C for 6 h. In fact, specimens prepared by UHS achieved a crystallinity of 90% and ionic conductivity as high as 1.31 × 10⁻⁴ S cm⁻¹.     

Sol–gel synthesis of Li1.5Al0.5Ge1.5(PO4)3 solid electrolyte

The effect of calcination on Li ion conductivity of Li_1.5Al_0.5Ge_1.5(PO₄)₃ (LAGP) solid electrolyte prepared by a sol–gel method is examined. The Li ion conductivity of LAGP increases with calcination temperature. After reaching maximum conductivity at 850 °C, the conductivity decreases with increase of the calcination temperature. The calcination holding time also strongly affects Li ion conductivity of LAGP. The conductivity increases with holding time until 12 h and then decreases. It is found that the control of crystallization rate is critical to obtain bulk LAGP with high Li ion conductivity. The highest bulk and total conductivities at 30 °C are 9.5×10⁻⁴ and 1.8×10⁻⁴ S cm⁻¹, respectively, obtained for the bulk LAGP calcined at 850 °C for 12 h.     

Fabrication of lightweight ZrO2 fiberboards using hollow ZrO2 fibers

ZrO₂ fiberboards with ultra-low densities (0.34–0.40 g/cm³) were fabricated using biomorphic ZrO₂ hollow fibers, which have a lower density and better thermal insulation than traditional ZrO₂ solid fibers. The effects of sol binder content, sintering temperature, and proportion of solid fibers on the density, microstructure, compressive strength, linear shrinkage, and thermal conductivity of lightweight ZrO₂ fiberboards were investigated. The results showed that the hollow features of biomorphic ZrO₂ fibers were successfully maintained after they were made into ZrO₂ fiberboards, which made them less dense and thermally conductive. The best conditions were found to be a sol binder content of 30 vol%, sintering temperature of 1400 °C, and 20 wt% sintered solid fibers to balance thermal insulation and compressive strength. The results show that the density and thermal conductivity of lightweight ZrO₂ fiberboard gives it obvious advantages as a heat-insulating ceramic. Specifically, when the sintering temperature was 1400 °C, the sample had an ultra-low density of 0.34–0.40 g/cm³, a thermal conductivity of 0.101–0.116 W/(m·K) (at 500 °C), a compressive strength of 0.05–0.24 MPa, and a linear shrinkage of 9.4–13%.