Conventional Solid-state Sintering Research Articles

Dense garnet-type electrolyte with coarse grains for improved air stability and ionic conductivity

Abstract Garnet-type electrolytes with high ionic conductivity and chemical stability against lithium metal show promise as solid-state electrolytes for lithium-ion batteries. However, a high concentration of pores and grain boundaries in air-processed polycrystalline electrolytes makes them prone to dendrite formation and reaction with atmospheric moisture, leading to electrochemical and mechanical instability. In this work, we illustrate that abnormal grain growth, an often-avoided phenomenon in conventional ceramic processing, can be employed as a unique approach to obtain extraordinarily large oligo crystals for minimal grain boundaries. Here we report a straightforward approach to develop a robust Ga-doped garnet, Li6.25Ga0.25La3Zr2O12 (LGLZO) electrolyte with conventional solid-state sintering in air. By preparing nanopowders without agglomeration through ball milling and freeze drying, we can control the microstructure of air-sintered electrolytes for desirable properties of a high density (98% of theoretical density) and an average grain size of 460 µm. The robust air-processed LGLZO electrolytes demonstrate high ionic conductivity, stability in air, and mechanical robustness relative to other garnet electrolytes offering promise as cost- and performance-competitive solid-state electrolytes for safe lithium-ion batteries.

Ce and W co-doped CaBi2Nb2O9 with enhanced piezoelectric constant and electrical resistivity at high temperature

Ce and W co-doped CaBi2Nb2O9 ceramics with chemical formula Ca0.96Ce0.04Bi2Nb2-xWxO9 (CCBN-W100x, x = 0–0.07) are fabricated via conventional solid state sintering method, to investigate the effect of W addition on the structure, electrical resistivity, dielectric and piezoelectric properties. A piezoelectric constant d33 of 13.4 pC/N is obtained in CCBN-W2 ceramics, >100% higher than that of pure CaBi2Nb2O9 (d33 = 5.8–6.4 pC/N). Of particular significance is that the electrical resistivity of CCBN-W2 ceramics (ρ = 3.7 × 109 Ω cm at 500 °C) is three orders of magnitude higher than pure CaBi2Nb2O9 (ρ = 2.9 × 106 Ω cm at same temperature). All these properties, together with its low dielectric loss (tanδ = 0.13%) and excellent d33 thermal stability up to 800 °C, merit the CCBN-W2 ceramics for high temperature piezoelectric sensing applications.

Balanced Development of Piezoelectricity and Curie Temperature in KNN-BC-xBNH Lead-Free Ceramics

In this work, (0.996 − x)(K0.48Na0.52)NbO3-0.004BiCoO3-xBi0.5Na0.5HfO3 (abbreviated as KNN-BC-xBNH) lead-free ceramics were prepared by the conventional solid-state sintering method. The effects of Bi0.5Na0.5HfO3 content on the microstructure and piezoelectric properties of ceramic crystals were systematically studied. The results of x-ray diffraction (XRD) and temperature dependence of the dielectric constant showed that with the increase of Bi0.5Na0.5HfO3 content, the rhombohedral–orthorhombic (TR–O) and orthorhombic–tetragonal (TO–T) phase-transition temperature gradually moved to the vicinity of room temperature. The KNN-BC-xBNH lead-free ceramics with x = 0.035 exhibited optimal electrical properties of d33 ∼ 272 pC/N, kp ∼ 0.47, Pr ∼ 25.63 μC/cm2, EC ∼ 13.31 kV/cm, Curie temperature Tc ∼ 333°C, Smax ∼ 0.15% and d33* ∼ 358 pm/V. These results suggest that the KNN-BC-0.035BNH ceramic is a promising ceramic system for industrial application.

Dielectric relaxation, impedance spectra, temperature stability and electrical properties of Sr2MnSbO6-modified KNN ceramics

Lead-free (1−x)K0.5Na0.5NbO3-x mol%Sr2MnSbO6 (abbreviated as KNN–xSMS, x = 0, 1 and 2) piezoelectric ceramics were prepared using the conventional solid-state sintering method. The effects of SMS on the microstructure and electrical properties of the KNN ceramics are investigated. 1 mol% SMS can greatly enhance the densification and increase the piezoelectric properties. Higher doping amounts will decrease the grain size and reduce electrical properties. The x = 0 and 1 samples show normal ferroelectric phase transition behavior, while the x = 2 sample exhibits diffuse phase transition behavior. Besides, after doping 1 mol% SMS, the field-induced unipolar strain and the remnant polarization vary less than undoped KNN ceramics in the temperature range of 30–160 °C. Therefore, doping SMS is an effective way to promote the temperature stability and electrical properties of KNN piezoelectric materials.

Electronic structure, optical and chemical bonding properties of strontium doped Barium Titanate

In this work, Strontium doped Barium Titanate (Ba1-xSrxTiO3 (x = 0.00, 0.05, 0.1)) ceramic solid solutions were synthesized by high temperature conventional solid state sintering process. Rietveld profile refinement was done by PXRD data and the synthesized samples were crystallized by perovskite type-tetragonal structure without additional phase. Powder X-ray diffraction studies were used to investigate the structural and chemical bonding properties of the constituent atoms through electron density distribution. The chemical bonding between Ba and O ions and Ti and O ions possess ionic and covalent in nature, respectively. Egap values evaluated from UV–vis absorption spectra. Scanning electron microscopic images showed that the particles with regular shapes and well defined clear grain boundaries. The phase purity of the samples is confirmed by EDS qualitative spectral analysis.

Crystal structures, dielectric properties, and lattice vibrational characteristics of (1-x)CaWO4-xTiO2 composite ceramics

(1-x)CaWO4-xTiO2 (CWT) ceramics were fabricated by conventional solid-state sintering method at 1200 °C for 4 h. X-ray diffraction data show that CaWO4 and TiO2 coexist as composite ceramics and Rietveld refinement reveals the change in crystal structures of CWT system. There are seven Raman vibrational modes for CaWO4, two Raman modes for TiO2, and eight infrared vibrational modes for CaWO4, whose intensities decrease with increasing TiO2 content. The theoretical properties obtained from four-parameter semiquantum model coincide with those calculated from C-M and damping equations. The structure-property relationships of CWT system were established based on the lattice vibrational modes. When x = 0.15, the sample achieves the optimum dielectric properties with εr = 10.86, Q × f = 16046 GHz (f = 15.51 G), τf = −15.99 ppm/°C.

Experimental and Theoretical Analysis of Li-Stuffed Garnet-Type Electrolytes

Rechargeable lithium-ion batteries (LIBs) are one of the dominant technologies for electrochemical energy storage. They are widely used in portable electronics, electrical vehicles, and large-scale energy storage systems. Current LIBs use organic polymer based electrolytes and they have several problems such as dendrite formation, flammability and leakage.1 Therefore, there is much attention on developing a solid-state Li-ion electrolyte that is thermally and chemically stable. Among the various inorganic solid Li-ion conducting electrolytes, garnet structured compounds have shown great promise as potential electrolytes for all-solid-state lithium batteries due to their high ionic conductivities (10-4 S/cm), compatibility with Li metal anode, and wide electrochemical window (~ 6V vs Li/Li+).2 The present study reports a comparison of the experimental and theoretical data of garnet type Li-ion solid electrolytes. The solid electrolytes, Li6.5La2.9A0.1Ta0.6Zr1.4O12 (A = Ca, Sr, Ba) were prepared by conventional solid state sintering and spark plasma sintering method. The formation of the ‘‘single-phase’’ garnet-type structure was analysed by powder X-ray diffraction (PXRD). Li-ion conductivity was determined using electrochemical impedance spectroscopy (EIS) and all the members have exhibited ionic conductivity in the order of 104 S/cm at 25 °C. Interfacial resistance of the garnet samples and Li metal was measured using EIS and the interfacial stability of the same was confirmed by DC galvanostatic cycling experiments. Li-ion conductivity and activation energy of the prepared compounds were also calculated using a statistical mechanical approach and the estimated values are in good agreement with the experimental values.

Electrical Properties of Ba0.96Ca0.04Ti0.90Sn0.10O₃ Lead-Free Ceramics with the Addition of Nano-CeO₂.

Well dispersed CeO₂ nanoparticles are prepared by azeotropic co-precipitation method. (Ba0.96Ca0.04)(Ti0.90Sn0.10)O₃ lead-free piezoelectric ceramics doped with nano-CeO₂ (x =0 mol%, 0.03 mol%, and 0.07 mol%) and micro-CeO₂ (x = 0.03 mol%) are prepared at 1430 °C for 2 h by the conventional solid state sintering method. XRD diffraction indicates that all components have typical perovskite structure. Both doping of nano-CeO₂ and micro-CeO₂ can inhibit grain growth. And the average grain size decreased apparently with the increase of nano-CeO₂ amount. All the samples exhibit typical diffuse phase transition behavior. The optimized electrical performances are obtained at x = 0.03 mol% with d33 = 512 pC/N, kp = 41.5%, and Pr = 14.00 μC/cm².

Anomalous in Vitro and in Vivo Degradation of Magnesium Phosphate Bioceramics: Role of Zinc Addition.

In vitro and in vivo degradation behavior and biocompatibility of magnesium phosphate (MgP) bioceramics and the potential role of zinc (Zn) ondegradation were compared. Samples were prepared by conventional solid-state sintering at 1200 °C for 2h. Zn-doped MgP (0.5 wt %) showed 50% less degradation than that of pure MgP after immersion into simulated body fluid (SBF)for 8 weeks. Osteoblast-like cell (MG-63) proliferation was evident in MgP ceramics, which was significantly enhanced upon Zn addition. Both Alamar Blue assay and Live/Dead imaging showed the highest cell attachment and proliferation for 0.5 wt % Zn-doped MgP. In vivo biocompatibility of these MgP ceramics were studied after implantation in the rabbit femur. The micro computed tomography (μ-CT) analysis showed that in vivo degradability increased with the increase in the Zn content which is in contradiction to in vitro degradability. Histological evaluation showed large influx of osteoclast cells to the implantation site for Zn-doped MgP samples compared to that of undoped MgP, which is the primary reason of increased degradability of these samples. After 90 days of implantation, large sections of 0.5 wt % Zn-doped MgP samples were replaced by newly formed bones. Fluorochrome labeling showed 78 ± 3% new bone formation for 0.5 wt % Zn-doped MgP ceramics compared to 56 ± 3% for pure MgP samples. Our findings suggest that the addition of Zn in MgP ceramics alters their sintering and degradation kinetics that leads to decreased in vitro degradation, however, when Zn-doped MgP ceramics were implanted in rabbits, higher degradability was observed due to lower Mg2+ ion concentration in the degradation media.

Effect of Ta-doping on functional properties of K0.51Na0.49NbO3

K.51Na.49Nb1−xTaxO3 (0.05 ≤ x ≤ 0.30) ceramics were prepared using a conventional solid-state sintering route. XRD of this system showed the crystal structure shifted gradually from orthorhombic to tetragonal phase. SEM images revealed that grain morphology remained the same in all compounds. At room temperature, relative permittivity has its highest value at x = 0.20, while tanδ remained less than 5% for all compositions at 100 kHz. For x > 0.05, double peaks were observed in relative permittivity versus temperature data. Saturated and symmetrical ferroelectric loops were obtained at an applied electric field of 60 kV cm−1. Remanent polarization (Pr) decreased with increasing x, however coercive field (Ec) was almost independent of composition. Isovalent doping of Ta5+ () marginally improved d33 at low concentrations (i.e., maximum d33 = 135pC/N, at x = 0.10). However, the broadening of peaks observed in dielectric data due to inhomogeneity may open new applications in microwave dielectrics or relaxor type applications in smart ceramics.

Effect of Pr6O11 doping on the microstructure and electrical properties of ZnO varistors

ZnO-Pr6O11 based varistors have anticipated application prospect in electrical system, but the study of the detailed microstructure is still poor. In this paper, the effect of Pr6O11 doping on the microstructure (such as element distributions and grain boundary characteristics) and electrical properties of the ZnO-Pr6O11–Co2O3–Cr2O3–Y2O3 varistors manufactured by the conventional solid-state sintering procedure was studied detailedly. Without Pr6O11 doping, the elements have the similar distributions in the interior and grain boundaries of ZnO phase, and the double Schottky barrier cannot be formed. With Pr6O11 doping, Pr6O11 liquid phase dissolving a large number of Zn, Cr, and Y forms during sintering and Pr6O11 solid phase forms in the ZnO grain boundaries during cooling, so the double Schottky barriers form. The voltage gradients E1 mA and nonlinearity coefficients α of the samples with Pr6O11 contents of 0.25–1 mol% are much larger than those of the sample without Pr6O11 doping, and the values of α and E1 mA increase with Pr6O11 content increasing from 0 to 1 mol%. The values of α and E1 mA simultaneously depend on the double Schottky barrier height ϕb and the CPE exponent αCPE. Finally, the ZnO varistor with a voltage gradient of 333.6 V/mm and a nonlinearity coefficient of 24.4 was obtained by Pr6O11 doping of 1 mol%. The detailed study of the microstructure and related mechanism is of great importance for preparing ZnO-Pr6O11 based varistors with excellent electrical properties.

Tailoring properties of (Bi0.51Na0.47)TiO3 based dielectrics for energy storage applications

Lead free dielectric ceramics of 0.65Bi0.51Na0.47Ti1-5x/4NbxO3- 0.35Ba(Ti0.7Zr0.3)O3 (BB35-100xNb, x = 0.00, 0.01, 0.02, 0.04 and 0.08) are fabricated by conventional solid-state sintering method for potential energy storage applications. Benefited from the coexistence of relaxor and antiferroelectric features, a high recoverable energy storage density of 3.2 J/cm3, together with high energy efficiency of 93%, is simultaneously achieved in bulk BB35-1Nb ceramic at the critical electric field of 280 kV/cm. The studied BB35-1Nb ceramic exhibits a wide temperature usage range of 20–160 °C with energy density variation below 3%, and excellent cycling reliability with both energy density and efficiency variations less than 4% over 106 cycles, together with its fast discharge time of ˜1.2 μs, making BB35-1Nb ceramic promising candidate for high-temperature, high power energy storage applications.

Influence of co-modification with tungsten and tantalum on the crystal structure and electrical properties of bismuth titanate ceramics

Bismuth titanate Bi4Ti3O12 (BIT) based lead-free piezoelectric ceramics with the formula Bi4Ti3-2xWxTaxO12 (abbreviated as BITWT-x, x = 0, 0.015, 0.03, 0.04, 0.045, 0.05) were fabricated by the conventional solid-state sintering method. The effect of W and Ta doping on the crystal structure and electrical properties of BITWT-x ceramics were investigated. BITWT-x ceramics had an orthogonal phase, and crystal structural distortion of W/Ta co-doped BITWT-x based ceramics was higher than that of BITWT-0 ceramics, which improved the ferroelectricity and the piezoelectric property. The ferroelectricity (Pr = 7.78 μC/cm2) of BITWT-0.03 ceramics was better than that (Pr = 0.84 μC/cm2) of BITWT-0 ceramics. The piezoelectric property (d33) was enhanced, and the highest d33 of BITWT-0.04 ceramics was 21.4 pC/N. Moreover, the d33 of the BITWT-x ceramics at 600 °C still kept 80% of its original d33 when the value of x was in the range of 0.03–0.045. Donor ions W6+ and Ta5+ reduced ionic conductivity (oxygen vacancies) and electron conductivity (holes), then increased the resistivity of BITWT-x ceramics (ρ400 °C = 1.82 × 108 Ω cm, x = 0.03). However, the enhanced effect reached a saturated state when x was 0.03. Thus, the BITWT-x ceramics were suitable for the high-temperature application.

Electrical properties of gadolinia doped ceria electrolytes fabricated by infiltration aided sintering

Common solid oxide fuel cell (SOFC) electrolyte materials (e.g., gadolinia doped ceria – GDC) demand temperatures exceeding 1400 °C for densification by conventional solid state sintering. It is very desirable to reduce the densification of the SOFC electroltytes to i) avoid microstructural coarsening of the composite anode layers, which are co-sintered with the electolyte layer in the anode supported SOFC fabrication scheme and ii) reduce energy consumption during SOFC manufacturing. We have recently demostrated a novel infiltration-aided sintering route to densify GDC ceramics at 1200 °C. In the present work, we present the electrical properties of GDC ceramics fabricated thusly. Comparison of high density (≥95%) samples fabricated by conventional or infiltration-aided sintering reveal that at 700 °C, similar total electrical conductivities are obtained, while at 300 °C, specific grain boundary resistivity is smaller in the latter. Bulk (grain) conductivity is higher in porous GDC ceramics (relative density ≤ 90%) fabricated by infiltration-aided sintering than the conventionally sintered ones with similar porosities. Finally, open circuit voltage of 0.84 V at 700 °C, obtained under dilute hydrogen and stagnant air conditions suggests that GDC ceramics densified by infiltration-aided sintering are suitable for use as SOFC electrolytes.

Anti-reductive characteristics and dielectric loss mechanisms of Ba2ZnSi2O7 microwave dielectric ceramic

Abstract Ba2ZnSi2O7 ceramic was prepared by conventional solid-state method and sintering under four different atmospheres (air, O2, N2, and N2–1 vol% H2). The Ba2ZnSi2O7 ceramic exhibited a single phase under air, O2, and, N2, whereas two phases (including BaSiO3 impurity) were observed under the N2–1 vol% H2 atmosphere due to Zn2+ evaporation. Dielectric loss was proportional to oxygen partial pressure due to the conduction mechanism of dominant holes. The Ba2ZnSi2O7 ceramic exhibited good microwave dielectric properties (er = 8.3, Q × f = 27200 GHz, τf = −41.5 ppm/°C) under the N2–1 vol% H2 atmosphere, making it a novel candidate with good anti-reductive characteristics for base metal electrode–multilayer ceramic capacitor applications.

Fabrication, microstructure and optical properties of large-sized Nd:YAG and composite Yb:YAG transparent ceramic slabs

Abstract Large-sized Nd:YAG and composite Yb:YAG transparent ceramic slabs of high quality were fabricated by the conventional solid-state reaction sintering technique. The Yb:YAG transparent ceramic slabs were surrounded by undoped YAG transparent ceramics and this composite structure was built in the process of ceramic shaping. The difference in microstructure between Yb:YAG and YAG transparent ceramics was not observed and the composite Yb:YAG slabs showed fully dense and pore-free microstructure. The in-line transmittances at 400 nm reached 81.7% and 79.7 % for YAG and Yb:YAG transparent ceramics, respectively, indicating good optical quality of the composite ceramic slab. And the optical transmission micrograph was very homogeneous with only a few isolated scattering centers. These results suggested that building composite structure in the process of ceramic shaping was a simple and reliable technical scheme. Moreover, the large-sized Nd:YAG transparent ceramic slab with size of 150.0 mm × 50.0 mm × 4.0 mm was also prepared. The calculated absorption coefficient was about 0.002 cm-1 @1064 nm, which suggested that the optical quality of this Nd:YAG transparent ceramic slab had reached the level of Nd:YAG crystals.

Effect of BiScO3 doping on the structure and properties of BiFeO3-BaTiO3 piezoelectric ceramics

High temperature lead-free piezoelectric ceramics 0.67BiFeO3–0.33BaTiO3 doped with 0.35 mol% MnO2 and xBiScO3 (x = 0, 0.005, 0.01, 0.015, 0.02) were prepared by conventional solid-state sintering method. The effects of BiScO3 doping on the crystal structure, piezoelectric, ferroelectric and dielectric properties of BiFeO3-BaTiO3 piezoelectric ceramics were investigated. The results show that the BiScO3 doping can not change the crystal structure of the ceramics and the grain size of ceramics increases first and then decreases with the BiScO3 doping content increasing. The optimum piezoelectric properties d33 = 165 pC/N, kp = 0.349 and Qm = 52.132 were obtained when x = 0.01, with the highest remanent polarization Pr = 37.1 μC/cm2. The ceramics become normal ferroelectrics determined by temperature spectrum with the increase of x, and the Curie temperature and depolarization temperature of 425 and 408 °C are respectively obtained when x = 0.01. In particular, a small amount of BiScO3 doping is beneficial to the improvement of the temperature stability. Moreover, a perovskite solid solution forms between the BiScO3 and BiFeO3-BaTiO3 ceramics, which can improve the electrical properties of BF-BT ceramic system.

Effect of MnCO3 on the electrical properties of PZT-based piezoceramics sintered at low temperature

In this work, the piezoelectric ceramics Pb0.95Ba0.01Sr0.04(Zr0.53Ti0.47)O3-1wt.%LiBiO2-0.06 wt.%CuO-xwt.%MnCO3 (PBSZT-LBCu-xwt.%MnCO3) were prepared by the conventional solid-state sintering method. The effect of MnCO3 on addition phase structure, microstructure, ferroelectric performances, piezoelectric and dielectric properties were systematically studied. It was found that the addition of MnCO3 can highly increase the mechanical quality factor (Qm) from a small value (<80). High Qm of ∼400–560, moderate piezoelectric constant (d33∼250–270 pC/N), and high converse piezoelectric constant (d33*∼360–510 pm/V) were realized in compositions from x = 0.10 to x = 0.30. Meanwhile, Pr and εr also maintain relatively high values. Moreover, the addition of oxides, i.e., LiBiO2 and CuO, can significantly reduce the sintering temperature of ceramics, and all specimens were sintered at a low temperature around 900 °C.

Phase characteristics, microstructure, and electrical properties of (1-x)BaZr0.2Ti0.8O3-(x)(Ba0.7Ca0.3)0.985La0.01TiO3 ceramics

Abstract In this study, (1-x)BaZr0.2Ti0.8O3-(x)(Ba0.7Ca0.3)0.985La0.01TiO3 ((1-x)BZT-(x)BCLT) ceramics, where x = 0.3, 0.4, 0.5, and 0.6, were prepared employing a conventional solid-state sintering technique. X-ray diffraction patterns and dielectric measurements indicated three phase regions at room temperature, including a single rhombohedral (x = 0.3), a phase coexistence of rhombohedral and tetragonal (x = 0.4), and a single tetragonal structure (x ≥ 0.5). X-ray photoemission spectra at the surface of ceramics confirmed the oxidation state of Ba2+, Ca2+, Ti4+, and Zr4+ ions. Upon BCLT addition, the reduction of the average grain size and the presence of the tetragonal structure significantly affected the dielectric, ferroelectric, and piezoelectric properties of these ceramics. With these results, the composition x = 0.3 showed maximum er′ and em′, whereas the composition x = 0.5 showed maximum Pr, Ec, d33, kp, and d∗33 factors. These results suggest a new phase diagram for the (1-x)BZT-(x)BCLT system, which could be tuneable by BCLT concentration and might be useful as an alternative material in dielectric, ferroelectric, and piezoelectric devices.

A novel Na0.5K0.5NbO3–La(Zn0.5Ti0.5)O3 ceramics with excellent dielectric properties at wide temperature range

Abstract Ceramic-based dielectrics are considered as the best candidates for high temperature capacitors because of their outstanding mechanical and electrical properties. Nevertheless, conventional barium titanate-based capacitors show narrow operating temperature ranges owing to the low tetragonal-cubic phase transition temperature. In order to increase the working temperature and relative permittivity, a novel (1-x)Na0.5K0.5NbO3- xLa(Zn0.5Ti0.5)O3 (NKN-xLZT) ceramics were chosen to meet the targets in this work. The NKN-xLZT ceramics with sub-micrometer grains (0.2–0.4 μm) were synthesized via a conventional solid-state sintering route. A relative permittivity (e’ = 1560 ± 15%) with low loss tangent over wide temperature range from 96 °C to 350 °C was obtained in the x = 0.02 ceramics. Additionally, the crystal structure distortion and conduction behaviors of the NKN-xLZT ceramics were systematically studied. The decrease of oxygen octahedron distortion induced a weak polarization, and the high resistance (9 × 106 Ωcm at 400 °C) greatly suppressed the long-term migration of defective ions in the ceramics. Therefore, the low loss tangent and high permittivity were still stabilized at the high temperature. It believes that the NKN-xLZT ceramic system in this work will become one of the most promising candidates for high-temperature capacitor devices.