Preparation and ionic conduction of Li1.5Al0.5Ge1.5(PO4)3 solid electrolyte using inorganic germanium as precursor

NASION-type Li_1.5Al_0.5Ge_1.5(PO₄)₃(LAGP) is prepared by a novel sol-gel method using low cost inorganic germanium (GeO₂) as the precursor. The composition and phase transformation during the heating of the LAGP precursors are analyzed using thermogravimetric-differential scanning calorimetry (TG-DSC) and X-ray diffraction (XRD). The structures and morphologies of the LAGP are characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that the LAGP annealed at 900 ℃ is partially crystallized and consists of a large number of nanoscale grains surrounded by amorphous regions. The LAGP particles present an irregular morphology with a large size distribution over a range of 0.2–1 μm. In addition, ionic conductivities of the prepared LAGP first increase and then decrease with an increase in the sintering temperature and time. A high ionic conductivity (4.18 × 10⁻⁴Scm^-1) with an activation energy of 0.30 eV are obtained for the LAGP sample sintered at 900 °C for 8 h.     

Cold sintering process of Li1.5Al0.5Ge1.5(PO4)3 solid electrolyte

The recently developed technique of cold sintering process (CSP) enables densification of ceramics at low temperatures, i.e., <300°C. CSP employs a transient aqueous solvent to enable liquid phase‐assisted densification through mediating the dissolution‐precipitation process under a uniaxial applied pressure. Using CSP in this study, 80% dense Li_1.5Al_0.5Ge_1.5(PO₄)₃ (LAGP) electrolytes were obtained at 120°C in 20 minutes. After a 5 minute belt furnace treatment at 650°C, 50°C above the crystallization onset, Li‐ion conductivity was 5.4 × 10^?5 S/cm at 25°C. Another route to high ionic conductivities ~10^?4 S/cm at 25°C is through a composite LAGP ‐ (PVDF‐HFP) co‐sintered system that was soaked in a liquid electrolyte. After soaking 95, 90, 80, 70, and 60 vol% LAGP in 1 M LiPF₆ EC‐DMC (50:50 vol%) at 25°C, Li‐ion conductivities were 1.0 × 10^?4 S/cm at 25°C with 5 to 10 wt% liquid electrolyte. This paper focuses on the microstructural development and impedance contributions within solid electrolytes processed by (i) Crystallization of bulk glasses, (ii) CSP of ceramics, and (iii) CSP of ceramic‐polymer composites. CSP may offer a new route to enable multilayer battery technology by avoiding the detrimental effects of high temperature heat treatments.     

Spark plasma sintering of Sm2O3-doped aluminum nitride

The high sintering temperature required for aluminum nitride (AlN) at typically 1800 °C, is an impediment to its development as an engineering material. Spark plasma sintering (SPS) of AlN is carried out with samarium oxide (Sm₂O₃) as sintering additive at a sintering temperature as low as 1500–1600 °C. The effect of sintering temperature and SPS cycle on the microstructure and performance of AlN is studied. There appears to be a direct correlation between SPS temperature and number of repeated SPS sintering cycle per sample with the density of the final sintered sample. The addition of Sm₂O₃ as a sintering aid (1 and 3 wt.%) improves the properties and density of AlN noticeably. Thermal conductivity of AlN samples improves with increase in number of SPS cycle (maximum of 2) and sintering temperature (up to 1600 °C). Thermal conductivity is found to be greatly improved with the presence of Sm₂O₃ as sintering additive, with a thermal conductivity value about 118 W m⁻¹ K⁻¹) for the 3 wt.% Sm₂O₃-doped AlN sample SPS at 1500 °C for 3 min. Dielectric constant of the sintered AlN samples is dependent on the relative density of the samples. The number of repeated SPS cycle and sintering aid do not, however, cause significant elevation of the dielectric constant of the final sintered samples. Microstructures of the AlN samples show that, densification of AlN sample is effectively enhanced through increase in the operating SPS temperature and the employment of multiple SPS cycles. Addition of Sm₂O₃ greatly improves the densification of AlN sample while maintaining a fine grain structure. The Sm₂O₃ dopant modifies the microstructures to decidedly faceted AlN grains, resulting in the flattening of AlN–AlN grain contacts.     

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.     

Sintering temperature dependency on sodium-ion conductivity for Na2Zn2TeO6 solid electrolyte

We investigated the sintering temperature dependency on the properties of Na₂Zn₂TeO₆ (NZTO) solid electrolyte synthesized via a conventional solid-state reaction method. Sintering temperature of calcined NZTO powder, which was obtained by the calcination of precursor at 850℃, was changed in the range from 650 to 850℃. X-ray diffraction analysis showed that P2-type layered NZTO phase was formed in all sintered samples without forming any secondary phases. The relative densities of sintered NZTO samples were approximately 83%−85% for the samples sintered at 700℃ or higher. The all sintered samples showed sodium-ion conductivity above 10⁻⁴ S cm⁻¹ at room temperature and the highest conductivity of 4.0 × 10⁻⁴ S cm⁻¹ in the sample sintered at 750℃. The sintering temperature to obtain the highest room temperature conductivity is 100℃ lower than that used in previous works. Such low sintering temperature compared to other Na-based oxide solid electrolytes could be useful for co-sintering with electrode active materials for fabrication of all-solid-state sodium-ion battery.     

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.     

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.     

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.     

Synthesis and sodium ionic conductivity for perovskite-structured Na0.25La0.25NbO3 ceramic

In this work, a perovskite-structured sodium ion conductor, Na_0.25La_0.25NbO₃ (NLNO) was developed from analogous Li_0.25La_0.25NbO₃ ceramic. NLNO ceramic was successfully synthesized by solid state reaction. The sodium ionic conduction in Na_0.25La_0.25NbO₃ ceramic was studied and the effect of sintering temperature on the microstructure, phase structure, density and sodium ionic conductivity for Na_0.25La_0.25NbO₃ was also discussed. Single phase of perovskite was successfully obtained from NLNO sintered at 1200 °C and 1250 °C, and the result shows high sintering temperature leads to a large grain size, large lattice parameters and high density. With an increase of sintering temperature from 1150 °C to 1250 °C, the conductivity of samples increases gradually. NLNO sintered at 1250 °C presents a high sodium ionic conductivity of 1.06 × 10^?5 S cm^?1 at 30 °C, which is much higher than that of electronic conductivity in NLNO sintered at 1250 °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.     

Densification and properties of transition metal borides-based cermets via spark plasma sintering

Engineering borides like TiB₂ and ZrB₂ are difficult to sinter materials due to strong covalent bonding, low self-diffusion coefficient and the presence of oxide layer on the powder particles. The present investigation reports the processing of hard, tough and electrically conductive transition metal borides (TiB₂ and ZrB₂) based cermets sintered with 6 wt.% Cu using spark plasma sintering (SPS) route. SPS experiments were carried out with a heating rate of 500 K/min in the temperature range of 1200–1500 °C for a varying holding time of 10–15 min and the optimization of the SPS conditions is established. A maximum density of ∼95% ρ_th in ZrB₂/Cu and ∼99% ρ_th in TiB₂/Cu is obtained after SPS processing at 1500 °C for 15 min. While the optimized TiB₂/Cu cermet exhibits hardness and fracture toughness of ∼17 GPa and ∼11 MPa m^1/2, respectively, the optimized ZrB₂/Cu cermet has higher hardness of ∼19 GPa and fracture toughness of ∼7.5 MPa m^1/2, respectively. High electrical conductivity of ∼0.20 MΩ⁻¹ cm⁻¹ (TiB₂/Cu) and ∼0.15 MΩ⁻¹ cm⁻¹ (ZrB₂/Cu) are also measured with the optimally sintered cermets.     

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.     

CuAlO2 thermoelectric compacts by SPS and thermoelectric performance improvement by orientation control

We fabricated CuO/Al₂O₃ green compacts from plate-like Al₂O₃ and granular CuO powders by multi-press forming and investigated the alumina orientation using Lotgering's method. The results showed that Al₂O₃ particles preferentially aligned perpendicular to the pressure direction and the orientation degree increased as the forming pressure was increased. We proposed a model describing the movement of the alumina particles to explain the pressure effect on their orientation. The orientation calculation was in good agreement with those by Lotgering's method. Furthermore, we prepared the CuAlO₂ compacts by regular or spark plasma sintering (SPS). However, the compacts sintered by SPS exhibited higher orientation degree and density than those produced by regular sintering. The electrical conductivity values of the orientation-controlled compacts sintered by SPS reached 770 S m⁻¹ at 928 K, which was close to that of CuAlO₂ single crystal. The power factor of the CuAlO₂ compacts with highest orientation degree is as high as 5.95 × 10⁻⁵ W m⁻¹ K⁻¹ at 928 K. Therefore, we can conclude that orientation control is an effective method to enhance the thermoelectric performance of compact polycrystalline CuAlO₂ bulks.     

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.     

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.     

High-strength AlN ceramics by low-temperature sintering with CaZrO3–Y2O3 co-additives

Aluminum nitride (AlN) ceramics with the concurrent addition of CaZrO₃ and Y₂O₃ were sintered at 1450-1700 °C. The degree of densification, microstructure, flexural strength, and thermal conductivity of the resulting ceramics were evaluated with respect to their composition and sintering temperature. Specimens prepared using both additives could be sintered to almost full density at relatively low temperature (3 h at 1550 °C under nitrogen at ambient pressure); grain growth was suppressed by grain-boundary pinning, and high flexural strength over 630 MPa could be obtained. With two-step sintering process, the morphology of second phase was changed from interconnected structure to isolated structure; this two-step process limited grain growth and increased thermal conductivity. The highest thermal conductivity (156 Wm⁻¹ K⁻¹) was achieved by two-step sintering, and the ceramic showed moderate flexural strength (560 MPa).     

Mechanical properties and thermal conductivity of dense β-SiAlON ceramics fabricated by two-stage spark plasma sintering with Al2O3-AlN-Y2O3 additives

Fully dense β-SiAlON ceramics with excellent mechanical properties and good thermal conductivity were fabricated by two-stage spark plasma sintering (SPS) processes without and with applying pressure respectively, using α-Si₃N₄ powder and 6 Al₂O₃-3 AlN-6 Y₂O₃ (in wt.%, label with 636), 424 and 422 additives. In the first stage SPS process without pressure, the relative dense β-SiAlON ceramics with interlock microstructures of elongated grains and density of 3.14˜3.18 g cm⁻³, hardness of 14.00˜14.82 GPa and fracture toughness of 6.00˜6.63 MPa m^1/2 were obtained by sintering at about 1600 °C for 20 min. In the second stage SPS process at about 1425 °C for 5 min under pressure of 24 MPa, the fully dese β-SiAlON ceramics with density of 3.22˜3.24 g cm⁻³, high hardness of 15.68˜15.95 GPa, high fracture toughness of 6.38˜7.03 MPa m^1/2 and thermal conductivity of 13.5˜19.6 Wm^-1K^-1 were obtained. The reaction between the samples and the graphite mold can be avoided in this fabrication method.     

Preparation of β"-Al2O3 electrolytes by freeze-drying and SPS

Na-ionic conductivity and mechanical strength are two important parameters for β"-Al₂O₃ ceramics used as electrolyte materials. In this study, the ice-templating and freeze-drying processes were used to prepare uniformly porous Al₂O₃ powder with a certain degree of crystallographic orientation. With the help of spark plasma sintering (SPS) process, solid samples were fabricated from the porous power at a low temperature (1100 °C). Finally, highly conductive and highly strong β"-Al₂O₃ electrolytes were prepared after the solid samples were subject to further treatment at a high temperature (1600 °C). The scanning electron microscopy (SEM) results indicate that integration of the initial SPS process and further high-temperature sintering treatment have turned the freeze-dried Al₂O₃ powder to β"-Al₂O₃ samples whose grain structure is similar to the “brick-bridge-mortar” structure. Regarding such β"-Al₂O₃ samples, the bending strength parallel to the pressure direction is up to 273 MPa; the degree of crystallographic orientation, 0.25, and the ionic conductivity perpendicular to the pressure direction, 0.239 S cm^?1 at 350 °C. Therefore, it can be concluded that the combination of freeze-drying, SPS and an additional high-temperature sintering process has helped generate a structure that improves the ion mobility and bending strength of the β"-Al₂O₃ ceramic, and thereby improving its strength and ionic conductivity.     

Electrical conductivity of spark plasma sintered Al2O3–SiC and Al2O3-carbon nanotube nanocomposites

The electrical conductivity of alumina-silicon carbide (Al₂O₃–SiC) and alumina-multiwalled carbon nanotube (Al₂O₃-MWCNT) nanocomposites prepared by sonication and ball milling and then consolidated by spark plasma sintering (SPS) is reported. The effects of the nanophase (SiC and MWCNTs) and SPS processing temperature on the densification, microstructure, and functional properties were studied. The microstructure of the fabricated nanocomposites was investigated using field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). The phase evolution was determined using X-ray diffraction (XRD). The room-temperature direct current (DC) electrical conductivity of the monolithic alumina and nanocomposites was determined using the four-point probe technique. The EDS mapping results showed a homogenous distribution of the nanophases (SiC and MWCNTs) in the corresponding alumina matrix. The room-temperature DC electrical conductivity of monolithic alumina was measured to be 6.78 × 10⁻¹⁰ S/m, while the maximum electrical conductivities of the alumina-10 wt%SiC and alumina-2wt%MWCNT samples were 2.65 × 10⁻⁵ S/m and 101.118 S/m, respectively. The electrical conductivity increased with increasing nanophase concentration and SPS temperature. The mechanism of electrical conduction and the disparity in the electrical performance of the two investigated nanocomposite systems (alumina-SiC and alumina-MWCNT) are clearly described.     

Influences of Bi0.75Y0.25O1.5 addition on the microstructure and ionic conductivity of Ce0.8Y0.2O1.9 ceramics

A second phase of Y₂O₃-stabilized Bi₂O₃ (Bi_0.75Y_0.25O_1.5,YSB) is introduced into Y₂O₃-doped CeO₂ (Ce_0.8Y_0.2O_1.9,YDC) as a sintering additive and the composite ceramics of YDC-xYSB (x = 0, 5, 10, 20, 30, 40 wt%) are prepared through sintering at 1100°C for 6 h in air atmosphere. The YDC-xYSB ceramics are composed of both YDC and YSB with cubic fluorite structure, and no other impurity phases are detected in XRD patterns. The relative density of YDC-xYSB rises firstly for x ≤5 wt%, and then it declines with YSB addition from 5 to 40 wt%. The average grain size of YDC decreases from 270 nm to 85.7 nm with YSB addition from 0 to 40 wt%. The YSB phase segregates at the grain boundaries of YDC based on the TEM analysis result. The ionic conductivity of YDC-xYSB (x ≥5 wt%) is lower than that of YDC in the test temperature of 200°C–500°C, while it gradually exceeds that of YDC in 500°C–750°C. At 750°C, the conductivity of YDC-30%YSB (6.22 × 10⁻² S/cm) is 1.35 times higher than that of YDC (4.6 × 10⁻² S/cm). The YSB addition can improve the ionic conductivity of YDC in 500°C–750°C and decrease its sintering temperature.