Microstructure and characterization of Ag1−xLixNbO3 ceramics

Silver–lithium niobate, Ag_1−xLi_xNbO₃, is a promising candidate for lead-free piezoelectrics. Ceramic samples for x equal 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.08 and 0.1 were synthesized by solid state reaction. Room temperature X-ray studies indicate that all samples consist mainly of perovskite phase. Only small amounts of secondary phases, namely Ag₂Nb₄O₁₁, Ag and LiNbO₃, appear. For x≈0.06 the orthorhombic–rhombohedral morphotropic phase boundary is observed. Excellent piezoelectric properties, recently reported for x>0.1, may be the result of proximity of morphotropic phase boundary. Scanning electron microscopy and secondary ion mass spectroscopy investigations suggest that the secondary phases occur chiefly at the grain boundaries. Dielectric measurements, carried out in a wide temperature range, revealed that Ag_1−xLi_xNbO₃, for x≤0.05, undergoes the same complex sequence of phase transitions as AgNbO₃. On the other hand, for x>0.06, the temperature dependence of dielectric permittivity exhibits only two maxima, related to transitions from the ferroelectric R phase to the antiferroelectric M phase and followed by the paraelectric O phase. The dielectric results obtained confirm the existence of the morphotropic phase boundary.     

Structural and electric properties of polycrystalline Bi1−xErxFeO3 ceramics

Polycrystalline Bi_1?xEr_xFeO₃ ceramics were synthesized by the solid state reaction method followed by rapid liquid phase sintering. The effects of Er substitution on the structure, morphology and electrical properties of the BiFeO₃ multiferroic ceramics were investigated. X-ray diffraction and Raman studies reveal that the structure of BiFeO₃ is changed from rhombohedral to orthorhombic in the Er concentration range of 0.10–0.15, and the impurity phases decrease both due to Er substitution. The X-ray photoelectron spectroscopy shows that Fe²⁺ could be suppressed by Er substitution. The SEM investigations suggest that the Er substitution could significantly reduce the grain sizes and increase the density of the samples. The leakage current is found to be decreased with increasing Er concentration. The dielectric and ferroelectric measurements show that dielectric constant, dielectric loss and ferroelectric properties are strongly dependent on the Er concentration. Er substitution can significantly improve the dielectric constant and remnant polarization, and decrease the dielectric loss by reducing the leakage current.     

Preparation and electrical properties of (1−x)SrBi2Nb2O9−xBiFeO3 lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics, (1−x)SrBi₂Nb₂O₉−xBiFeO₃ [(1−x)SBN−xBFO] (x=0.0, 0.03, 0.05, 0.07, 0.10) were prepared by a conventional solid-state reaction method. The crystal structure, microstructure and electrical properties were systematically investigated. All compositions formed layered perovskite structure without any detectable secondary phases. Plate-like morphology of the grains which is characteristic for layer-structure Aurivillius compounds was clearly observed. The excellent electrical properties (e.g., d₃₃~19 pC/N, 2P_r~18.8 μC/cm²) and a high Curie temperature (e.g., T_c~449 °C) were simultaneously obtained in the ceramics with x=0.05. Additionally, thermal annealing studies indicated that the BFO modified SBN ceramics system possessed stable piezoelectric properties, demonstrating that the modified SBN-based ceramics are the promising candidates for high-temperature applications.     

Thermoelectric properties and figure of merit of perovskite-type Sr1−xLaxSnO3 ceramics

The thermoelectric properties of perovskite-type Sr_1−xLa_xSnO₃ ceramics with x=0.01–0.05 were evaluated from the Seebeck coefficient S, electrical conductivity σ, and thermal conductivity κ measured at high temperatures. The La-doped ceramics were n-type semiconductors exhibiting thermally activated electrical conduction behaviors in the temperature range of 473–1073 K. Eelectron carriers were introduced into the conduction band from doped La atoms up to x=0.03, which was the solubility limit of La at Sr site. The temperature dependence of the κ values for the ceramics was unaffected by both the La content and the microstructures. Estimations of the electronic thermal conductivities by the Wiedemann-Franz law revealed that the phonon thermal conductivities were dominant for all ceramics. The dimensionless figure of merit ZT increased with increasing temperature for all ceramics and reached 0.02–0.05 at 1073 K. In contrast to cubic Ba_1−xLa_xSnO₃ ceramics, bending of the Sn–O–Sn bonds due to octahedral tilting distortion in Sr_1−xLa_xSnO₃ lowered the electron mobility, decreasing the power factor S²σ and ZT values, although it effectively reduced the phonon mean free path, decreasing the κ values.     

Preparation and microwave dielectric properties of Ca0.6La0.8/3(SnxTi1−x)O3 ceramics

Ca_0.6La_0.8/3(Sn_xTi_1−x)O₃ ceramics were prepared via a conventional solid state reaction method, and the effect of Sn doping on their crystal phase structure and microwave dielectric properties was investigated. Results showed that Sn doping could hinder the formation of the rutile TiO₂ detrimental phase of Ca_0.6La_0.8/3TiO₃ ceramic. Also, the Q×f₀ value was enhanced and the τ_f value was lowered by Sn doping. The best microwave dielectric properties, i. e. ε_r=113 and Q×f₀=8487 GHz were obtained for a Sn doping content of 0.02. The mechanism of the improved properties deriving by Sn doping is discussed.     

Synthesis,structural and electrical properties of Bi1−xDyxFeO3 multiferroic ceramics

Polycrystalline ceramic samples of dysprosium (Dy³⁺) doped bismuth ferrite of general formula Bi_1?xDy_xFeO₃ (x=0.00, 0.01, 0.05 and 0.1) have been prepared by standard solid state reaction method. Powder X-ray diffraction (XRD) analysis reveals that all the samples crystallize in the rhombohedral structure with noncentrosymmetric R3c space group. The refined lattice parameters decrease with the increase of Dy concentration within the same structure symmetry. The bond lengths among atoms for all the compounds were calculated by the Rietveld analysis. The frequency and temperature dependent dielectric constants (real and imaginary parts) have been measured. The real part of dielectric constant reveals that the Neel temperature decreases with the increase of Dy-substitution down to ～200 °C for 10% substitution to the Bi site.     

Enhanced electrocaloric,pyroelectric and energy storage performance of BaCexTi1−xO3 ceramics

BaCe_xTi_1−xO₃ (BCT) ceramics with compositions x = 0, 0.1, 0.12 and 0.15 were synthesized using conventional solid state reaction route. Systematic exploration of enhancing electrocaloric effect (ECE) in BaTiO₃ by rare earth dopant Ce is presented. BaCe_0.12Ti_0.88O₃ exhibited an electrocaloric strength of ∼0.35 K m/MV at 351 K, which caters the need for a series of high-level ECE material. Further, the temperature dependence of pyroelectric coefficient is established for all compositions. The pyroelectric figure of merits (FOMs) for current responsivity (F_i), voltage responsivity (F_v), detectivity (F_d) and energy harvesting (F_e and F_e^*) are calculated and the results reveal that x = 0.1 could be a technologically superior candidate for pyroelectric devices. Further, BaCe_0.15Ti_0.85O₃ exhibited highest electrical energy storage performance of 115 kJ/m³ compared with 71 kJ/m³ in BaTiO₃. Our findings in this work may provide a better understanding for developing high ECE materials combined with pyroelectric and energy storage performance of Ce substituted BaTiO₃ ceramics.     

Preparation and characterization of BaCe0.95Tb0.05O3−α hollow fibre membranes for hydrogen permeation

BaCe_0.95Tb_0.05O_3?α (BCTb) perovskite hollow fibre membranes were fabricated by spinning the slurry mixture containing 66.67 wt% BCTb powder, 6.67 wt% polyethersulphone (PESf) and 26.67 wt% N-methyl-2-pyrrolidone (NMP) followed by sintering at elevated temperatures. The influence of sintering temperature on the membrane properties was investigated in terms of crystal phase, morphology, porosity and mechanical strength. In order to obtain gas-tight hollow fibres with sufficient mechanical strength, the sintering temperature should be controlled between 1350 and 1450 °C. Hydrogen permeation through the BCTb hollow fibre membranes was carried out between 700 and 1000 °C using 50% H₂–He mixture as feed on the shell side and N₂ as sweep gas in the fibre lumen. The measured hydrogen permeation flux through the BCTb hollow fibre membranes reached up to 0.422 μmol cm^?2 s^?1 at 1000 °C when the flow rates of the H₂–He feed and the nitrogen sweep were 40 mL min^?1 and 30 mL min^?1, respectively.     

X-ray diffraction,dielectric and Raman studies of the Ba1−xNaxTi1−x(Nb1−ySby)xO3 ceramics

Ba_1−xNa_xTi_1−x(Nb_1−ySb_y)_xO₃ (BNTNSO) ceramics with compositions (x=0.05; y=0; 0.10; 0.20 and 0.30) have been prepared by conventional solid-state method and sintered in the temperature range 1350–1400 °C. Phase purity and structure are investigated using X-ray diffraction (XRD) data. The structural study showed that our synthesized compounds are single phase and crystallize in the tetragonal system with P4mm space group at room temperature. Based on a phenomenological model, dielectric and Raman properties of BNTNSO compounds have been explored. Referring to this model, dielectric properties of ceramics have been investigated in broad ranges of temperature (100–500 K) and frequency 1–10³ kHz). The dielectric permittivity evolution as a function of temperature and frequency has exhibited a classical ferroelectric character for 0≤y≤0.20 and a relaxor type behavior for y=0.30. The investigation of Raman spectra as a function of temperatures and compositions, confirmed the dielectric behavior. These results indicate that the y=0.20 composition is of extreme significance as far as its technological and industrial applications are concerned, which refers basically to its interesting physical properties and environmentally friendly characters, especially as its transition temperature is equal to the room temperature. The used samples show that the substitution of Nb by Sb favors and maintains the relaxor characters without changing the transition temperature.     

Lead-free Ag1−3xLaxNbO3 antiferroelectric ceramics with high-energy storage density and efficiency

Environmentally benign lead-free bulk ceramics with high recoverable energy density (W_rec) are very attractive in advanced pulsed power capacitors. In this work, composition engineering was adopted by La³⁺ modification to improve the energy storage performance of Ag_1−3xLa_xNbO₃ ceramics. It was found that the antiferroelectric (AFE) phase was stabilized after La³⁺ substitution, as a result of the reduced tolerance factor t. Significant improvement of W_rec and energy storage efficiency (η) were achieved with value of 3.12 J/cm³ and 0.63 for x = 0.02 at an electric field of 230 kV/cm, more than 1.5 times the value of pure AgNbO₃ (W_rec = 1.9 J/cm³, η = 0.40). The excellent energy storage properties are resulted from the increased antiferroelectric-ferroelectric phase transition electric field (E_F), ferroelectric-antiferroelectric phase transition electric field (E_A), and breakdown electric field (E_b). The enhanced E_b was ascribed to the decreased grain size and increased electrical resistivity upon La³⁺ modification. The feature makes Ag_1−3xLa_xNbO₃ a potential candidate for energy storage applications.     

Preparation and characterisation of LaNixCo1−xO3 thin films on polycrystalline Al2O3-substrates

The perovskite LaNi_xCo_1−xO₃ exhibits metallic conductivity with a change from p- to n-type conduction around x=0·5, thus being a candidate for electrodes or buffer layers in thin film technology. Thin films of LaNi_xCo_1−xO₃ have been grown onto polycrystalline Al₂O₃ substrates by Chemical Solution Deposition (CSD) of nitrate solutions in ethanol/butylacetate. The solutions were applied by dip-coating. After pyrolysis the compounds are formed in air at temperatures between 600°C and 750°C. Formation of the perovskite phase was confirmed by grazing angle X-ray diffraction. Electron micrographs revealed that the obtained films were smooth and crack-free and consisted of nanocrystalline LaNi_xCo_1−xO₃ particles. The thickness of the films was between 200 nm and 400 nm, depending on the conditions of the dipcoating procedure. Specific conductivities of the film were measured using the van der Pauw-method and were found to be around 400 S/cm for LaNiO₃ and around 1 S/cm for LaCoO₃ at room temperature.     

Synthesis and thermoelectric properties of (NaxCa1−x)3Co4O9 ceramics

(Na_xCa_1−x)₃Co₄O₉ (x=0.05−0.2) ceramics with a layered crystal structure were prepared by a sol–gel method followed by a low-temperature sintering procedure. The electrical conductivity and Seebeck coefficient of the complex oxide ceramics were measured from 400 to 900 °C. Their electrical conductivity and power factor increase with increasing temperature, while the thermal conductivity is very weakly dependant on the temperature. Na dopant amount has a remarkable effect on electrical and thermal transport properties. The figure of merit in the ceramic samples is smaller than that of traditional thermoelectric alloys.     

Diffuse phase transition and high electric field properties of BaCeyTi1−yO3 relaxor ferroelectric ceramics

Pure perovskite BaCe_yTi_1−yO₃ ceramics with compositions y=0.10, 0.20 and 0.30 have been prepared by solid state reaction. The temperature and frequency dependence of dielectric properties (permittivity, dielectric loss and dielectric modulus) of the ceramics has been investigated. A diffuse phase transition is typical for all the compositions, with a reduction of the Curie temperature with increasing Ce addition. If in the case of the sample with 0.10 Ce content, no frequency shift of the phase transition temperature (T_m) is noticed, a relaxor-like ferroelectric character become predominant for concentrations 0.20 and 0.30. The ceramic with y=0.20 presents higher tunability, a reduced hysteretic behavior and reasonable low dielectric losses at room temperature, which makes this composition a very good candidate for tunable capacitors applications.     

Dielectric properties of Ba1−xSrxTiO3 ceramics prepared by microwave sintering

A comparative study on the dielectric properties of Ba_1?xSr_xTiO₃ (x=0.1–0.6) ceramics prepared by microwave sintering (MS) and conventional sintering (CS) has been done. It was found that MS samples need lower temperature and much shorter time than CS samples to obtain the same degree of densification. Compared with CS samples, MS samples possessed smaller grain size, better densification and more uniform grain growth. The dielectric properties of the samples were measured as a function of temperature. It was observed that the dielectric constant was higher for MS samples compared with that of CS samples especially in the ferroelectric phase.     

Enhanced thermoelectric properties and electronic structures of p-type BiCu1−xAgxSeO ceramics

The thermoelectric properties and electronic structures were investigated on p-type BiCu_1-xAg_xSeO (x=0, 0.02, 0.05, 0.08) ceramics prepared using a two-step solid state reaction followed by inductively hot pressing. All the samples consist of single BiCuSeO phase with lamella structure and no preferential orientation exists in the crystallites. Upon replacing Cu⁺ by Ag⁺, maximum values of electrical conductivity of 36.6 S cm⁻¹ and Seebeck coefficient of 350 μV K⁻¹ are obtained in BiCu_0.98Ag_0.02SeO and BiCu_0.92Ag_0.08SeO, respectively. Nevertheless, a maximum power factor of 3.67 μW cm⁻¹K⁻² is achieved for BiCu_0.95Ag_0.05SeO at 750 K owing to the moderate electrical conductivity and Seebeck coefficient. Simultaneously, this oxyselenide exhibits a thermal conductivity as low as 0.38 W m⁻¹ K⁻¹ and a high ZT value of 0.72 at 750 K, which is nearly 1.85 times as large as that of the pristine BiCuSeO. The enhancement of thermoelectric performance is mainly attributed to the increased density of states near the Fermi level as indicated by the calculated results.     

Preparation and thermoelectric properties of Bi2SexTe3−x bulk materials

Bi₂Te₃ and Bi₂Se₃ nanoplates were synthesized by a microwave-assisted wet chemical method, and Bi₂Se_xTe_3−x (x=1, 2, 3) bulk nanocomposites were then prepared by hot pressing the Bi₂Te₃ and Bi₂Se₃ nanoplates at 80 MPa and 723 K in vacuum. The phase composition and microstructures of the bulk samples were characterized by powder X-ray diffraction and field-emission scanning electron microscopy, respectively. The electrical conductivity of the Bi₂Se_xTe_3−x bulk nanocomposites increases with increasing Se content, and the Seebeck coefficient value is negative, showing n-type conduction. The absolute Seebeck coefficient value decreases with increasing Se content. A highest power factor, 24.5 µWcm⁻¹ K⁻², is achieved from the sample of x=1 at 369 K among the studied samples.     

Fabrication of transparent electro-optic (K0.5Na0.5)1−xLixNb1−xBixO3 lead-free ceramics

Lead-free transparent electro-optic ceramics (K_0.5Na_0.5)_1?xLi_xNb_1?xBi_xO₃ have been fabricated by hot-press sintering. Owing to the effective suppression of grain growth, the Li and Bi co-modified ceramics generally possess a dense and fine-grained structure. The co-modification also causes the ceramics to transform into a nearly cubic structure with minimal optical anisotropy. A diffuse phase transformation is also induced, causing the ceramics to become more relaxor-like and contain more polar nano-regions. These would reduce the light scattering by the grains, at the grain boundaries and at the domain walls, respectively, and thus making the ceramics become optically transparent. For the ceramic modified with 5 mol% Li⁺ and Bi⁵⁺, the optical transmittance reaches a high value of 60% in the near-IR region. The ceramics also exhibit a strong linear EO response, giving a large effective linear EO coefficient in the range of 120–200 pm/V.     

Microstructure of single-phase cobalt and manganese oxide spinel Mn3−xCoxO4 ceramics

This paper reports microstructural studies of single-phase Mn_3−xCo_xO₄ (0.98 ≤ x ≤ 2.93) spinel ceramics using transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX). These ceramics were obtained by conventional sintering or by spark plasma sintering (SPS) of powders prepared by thermal decomposition of coprecipitated oxalate precursors. For x < 1.78 or x ≥ 1.78, the monophasic ceramics correspond respectively to quadratic (Q) or cubic (C) spinel structure. The ferroelastic character of the structural phase transition from C to Q is highlighted by specific microstructural features. The effect of chemical composition and heat treatment conditions on the microstructure and essentially on the presence and the characteristics of twins were investigated. The coherent twin interfaces are parallel to (1 1 2) planes in the Q cell. Twins can correspond to: tweeds, single lamellae (widths: 5–306 nm) arranged parallel to each other, large lamellae (widths: 69–928 nm) internally twinned and sometimes arranged in cyclic forms (triangular shapes).