Dielectric behavior and impedance spectroscopy in lead-free BNT–BT–NBN perovskite ceramics for energy storage

The dielectric behavior, impedance spectroscopy and energy-storage properties of 0.85[(1−x)Bi_0.5Na_0.5TiO₃–xBaTiO₃]–0.15Na_0.73Bi_0.09NbO₃ [(BNT–xBT)–NBN] ternary ceramics were investigated. Temperature dependent permittivity curves displayed two depressed anomalies, resulting in significantly improved dielectric temperature stability. (BNT–9BT)–NBN showed a permittivity of 1680 at 150 °C with Δε/ε_150 °C varying no more than ±10% up to 340 °C. From the complex impedance analysis, grain and grain boundary shared the same time constant. The high temperature resistivity followed the Arrhenius law with E_a=1.7–2.0 eV, suggesting intrinsic band-type electronic conduction. The maximum energy-storage density of all the samples reached 1.1–1.4 J/cm³, accompanied with good temperature stability in the range of 25–140 °C. These results indicate that (BNT–xBT)–NBN system should be a promising lead-free material for energy-storage capacitor applications.     

Cycling stability of lead-free BNT–8BT and BNT–6BT–3KNN multilayer actuators and bulk ceramics

This study presents the electromechanical properties and cycling stability of lead-free piezoelectric materials 0.92(Bi_1/2Na_1/2)TiO₃–0.08BaTiO₃ (BNT–8BT) and 0.91(Bi_1/2Na_1/2)TiO₃–0.06BaTiO₃–0.03(K_0.5Na_0.5)NbO₃ (BNT–6BT–3KNN). Both bulk samples as well as multilayer actuators (MLA) with internal Ag/Pd (70/30) electrodes were successfully processed from both materials. Electromechanical characteristics in the non-fatigued state and after different numbers of unipolar fatigue cycles are provided, representing the first direct comparison of the fatigue resistance of lead-free bulk ceramics and the corresponding MLAs. At a maximum field of 6 kV/mm and a frequency of 50 Hz, BNT–8BT MLA delivered a maximum strain of 0.07% and displayed excellent cycling stability. BNT–6BT–3KNN MLA provided a higher strain of 0.15% initially but degraded during cycling and exhibited break down after 10⁷ cycles. Furthermore, the frequency dependence of strain and the self-heating during cycling were investigated. The temperature increase is limited only to 2 °C in BNT–8BT MLA and 13 °C in BNT–6BT–3KNN MLA.     

Remarkably enhanced dielectric stability and energy storage properties in BNT–BST relaxor ceramics by A-site defect engineering for pulsed power applications

Lead-free bulk ceramics for advanced pulsed power capacitors show relatively low recoverable energy storage density(Wrec)especially at low electric field condition.To address this challenge,we propose an A-site defect engineering to optimize the electric polarization behavior by disrupting the orderly arrangement of A-site ions,in which Ba_0.105Na_0.325Sr_0.245−1.5x□_0.5xBi_0.325+xTiO₃(BNS_0.245−1.5x□_0.5xB_0.325+xT,x=0,0.02,0.04,0.06,and 0.08)lead-free ceramics are selected as the representative.The BNS_0.245−1.5x□_0.5xB_0.325+xT ceramics are prepared by using pressureless solid-state sintering and achieve large W_rec(1.8 J/cm³)at a low electric field(@110 kV/cm)when x=0.06.The value of 1.8 J/cm3 is super high as compared to all other W_rec in lead-free bulk ceramics under a relatively low electric field(<160 kV/cm).Furthermore,a high dielectric constant of 2930 within 15%fluctuation in a wide temperature range of 40–350℃is also obtained in BNS_0.245−1.5x□_0.5xB_0.325+xT(x=0.06)ceramics.The excellent performances can be attributed to the A-site defect engineering,which can reduce remnant polarization(P_r)and improve the thermal evolution of polar nanoregions(PNRs).This work confirms that the BNS_0.245−1.5x□_0.5xB_0.325+xT(x=0.06)ceramics are desirable for advanced pulsed power capacitors,and will push the development of a series of Bi0.5Na0.5TiO3(BNT)-based ceramics with high W_rec and high-temperature stability.     

Enhanced piezoelectricity and energy storage performances of Fe-doped BNT–BKT–ST thin films

Fe-doping is an effective way to improve physical performances of piezoelectric and ferroelectric materials. Under such circumstances, x mol% (x?=?0.0, 1.0, 1.5, 2.5) Fe-doped 0.72Bi_0.5Na_0.5TiO₃–0.18Bi_0.5K_0.5TiO₃–0.10SrTiO₃ (BNT–BKT–ST–xFe) thin films were prepared by sol-gel method and the relationships between the content of Fe and electromechanical properties of the films were studied. The BNT–BKT–ST–1.0Fe thin films exhibit the best electromechanical properties, whose S_max/E_max, W_rec, η, P_max, P_rem and ε_r of are 68.00?pm/V, 20.34?J/cm³, 65.17%, 71.5?μC/cm², 14.8?μC/cm², 868 respectively. These results indicate that BNT–BKT–ST–1.0Fe thin films are promising for applications for advanced piezoelectric materials and capacitors with high energy-storage density.     

Synthesis,dielectric and relaxation behavior of lead free NBT–BT ceramics

The 0.94(Na_0.5Bi_0.5TiO₃)–0.06BaTiO₃ ceramics have been prepared by the conventional solid state reaction method. Structural analysis of the prepared ceramic was made by means of room temperature XRD, FT-IR and Raman spectra. The formation of perovskite structure is confirmed by XRD and Raman studies. The dependence of dielectric constant on temperature for various frequencies (100 Hz–1.2 MHz) has been determined. The diffuse transition is observed in the variation of dielectric constant and it provides evidence for the relaxor characteristics. The relaxation mechanism of the prepared ceramic is also discussed in detail by using Debye, V–F and Power law relations and the suitable model was predicted by means of goodness of parameter. This is the first time the relaxation process is discussed for the lead free system to the best of our knowledge. High piezoelectric properties with d₃₃=206 pC/N are observed in the present system.     

Synthesis and characterization of magnesium–carbon compounds for hydrogen storage

Magnesium (Mg) and carbon (C) compounds were synthesized by ball-milling a mixture of Mg and different graphites with different crystallinities. The materials were characterized by X-ray diffraction, X-ray absorption spectroscopy, and X-ray total scattering techniques. Hydrogen storage properties were also investigated. In the case of the material using low-crystalline graphite, a Mg and C compound was formed as main phase, and its chemical bonding state was similar to that of magnesium carbide (Mg₂C₃). The hydrogen absorption reaction of the Mg–C compound occurred at around 400 °C under 3 MPa of hydrogen pressure to form magnesium hydride (MgH₂) and the C–H bonds in the carbon material. The hydrogenated Mg–C material desorbed about 3.7 mass% of hydrogen below 420 °C with two processes, which were the decomposition of MgH₂ and the subsequent reaction of the generated Mg and the C–H bonds. From the results, it is concluded that the Mg–C compound absorb and desorb about 3.7 mass% of hydrogen below 420 °C.     

Phase structure and energy storage performance for BiFeO3–BaTiO3 based lead-free ferroelectric ceramics

Recently, BiFeO₃–BaTiO₃ (BF-BT) lead-free ferroelectric ceramics have been widely concerned and deemed as one of the most promising candidates for lead-free energy-storage material because of their high spontaneous polarization and excellent energy storage properties. Herein, a series of Bi_1-zLa_zFeO₃-xBaTiO₃+yMnO₂ (BL_zF-xBT-yMn at 0.45 ≤ x ≤ 0.60 mol, 0.0 ≤ y ≤ 0.4% mol and z = 0 or 0.02 mol) ceramics were prepared to reveal their energy-storage performance. With increasing x, the breakdown strength (BDS) increases, while the maximum polarization (P_max), remanent polarization (P_r), and the difference value between P_max and P_r (ΔP) decrease. Because of the high BDS and ΔP, a large energy storage density W_re = 1.08 J/cm³ is achieved in BF-0.48BT ceramics as the electric field is 130 kV/cm. In addition, with increasing y and z, the increasing BDS and ΔP have been observed. Due to the improvement in BDS and ΔP, an excellent W_re = 1.22 J/cm³ was achieved in Bi_1-zLa_zFeO₃-xBaTiO₃+yMnO₂ ceramics at x = 0.48, y = 0.3% and z = 0.02. This work provides the clue for application of the high-power-energy BF-BT ceramics.     

Low sintering temperature,large strain and reduced strain hysteresis of BiFeO3–BaTiO3 ceramics for piezoelectric multilayer actuator applications

BiFeO₃–BaTiO₃ solid solutions with pseudo-cubic phase have received a lot of attention because of their large strain for potential piezoelectric multilayer actuator applications. However, the high sintering temperature, large dielectric loss and severe strain hysteresis hindered their real applications. In this work, Li₂CO₃ sintering aid modified 0.64BiFeO₃-0.36BaTiO₃ (BF-BT-L) ceramics were prepared by the high-temperature sintering method, and their phase structure, microstructure, electric properties as well as strain were investigated. With the increase of Li₂CO₃ content, the sintering temperature decreases down to 900 °C, and the relative density increases up to 96%. Of particular importance is that the dielectric loss and strain hysteresis of BF-BT-L ceramics are reduced by 40% and 47%, respectively, while BF-BT-L ceramics shows a large strain of 0.3% (60 kV/cm). The sintering temperature, relative density, strain, large-signal d₃₃* and strain hysteresis of BF-BT-L ceramics are 900 °C, 96.5%, 0.3%, 500 pm/V, and 25% respectively. The reduced dielectric loss and strain hysteresis result from the enhanced relative density, decreased concentration of defects and partial phase transition from partial pseudo-cubic to rhombohedral one by Li₂CO₃ additions. Furthermore, BF-BT-L ceramics show positive temperature dependent strain, with large d₃₃* and strain hysteresis of 675 pm/V and 15% at 200 °C respectively. The simple composition, low sintering temperature, large strain and reduced strain hysteresis of BF-BT-L ceramics indicate that they are promising candidates for high-performance and lead-free piezoelectric multilayer actuator applications.     

Reduced leakage current and enhanced piezoelectricity of BNT–BT–BMO thin films

BiMeO₃ (where Me denotes a transition metal) is often used as a chemical modifier to form the Bi_0.5Na_0.5TiO₃-based solid solutions and to improve the electromechanical properties of the materials. In this study, BiMnO₃ was selected as a chemical modifier, and (1 − x)(0.94Bi_0.5Na_0.5TiO₃-0.06BaTiO₃)–xBiMnO₃ thin films with x = 0, 0.005, 0.01, and 0.015 were fabricated using the metal organic decomposition method to study the contributions of the third end-member BiMnO₃ to the reduction in the leakage current and the enhancement of the piezoelectric properties of Bi_0.5Na_0.5TiO₃-BaTiO₃ thin films. Thin films with 1 mol% BiMnO₃ exhibit a lower leakage current, and a better piezoelectricity and ferroelectricity, whose S_max/E_max, P_max, 2E_c, and ε_r are 100.4 pm/V, 48.0 μC/cm², 54.9 kV/cm, and 942, respectively.     

Phase development and dielectric responses in PMN–BNT ceramics

(1 − x)Pb(Mg_1/3Nb_2/3)O₃–x(Bi_0.5Na_0.5)TiO₃ ceramics were prepared by the conventional mixed-oxide method. All compositions show complete perovskite solid solutions and the structure to change from cubic to rhombohedral at x = 0.5. The dielectric constant and dielectric loss tangent were measured as a function of both temperature and frequency. The results indicated a relaxor ferroelectric behavior for all ceramics. The temperature at maximum of the dielectric constant of PMN–BNT ceramics were seen to increase with increasing BNT content. Moreover, the broadest dielectric peak occurs at x = 0.9, which leads to a morphotropic phase boundary in this system.     

Preparations and characterizations of Ca doped Ni–Mg–Mn nanocrystalline ferrites for switching field high-frequency applications

Ca-doped Ni–Mg–Mn spinel ferrites with compositions of Ni_0·5Mg_0·3Mn_0.2Ca_xFe_2-xO₄ (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) were prepared via sol-gel auto-ignition technique. TGA/DTA, FTIR, XRD, FESEM, and VSM were employed to evaluate the thermal, spectral, structural, morphological, and magnetic features of Ca-doped Ni–Mg–Mn spinel ferrites. TGA/DTA curves show the weight loss in the sample. This weight loss was attributed to the oxidation and decomposition of the sample contents at a temperature of 500 °C. XRD reveals a single-phase structure of the Ni–Mg–Mn nano ferrites. A single-phase orthorhombic structure was confirmed for Ca-doped Ni–Mg–Mn ferrites. Structural parameters such as lattice parameter, ‘da’, ‘db’, ‘dc’, and ‘dv’ were evaluated using unit cell software. The absorption peaks at 427 to 538 cm^?1 confirmed the spinel structure, which was evaluated using FTIR. FESEM analyses showed that the agglomerations increased with the doping of Ca in Ni–Mg–Mn ferrites. Remanence, Y–K angles, saturation, coercive force, magnetic squareness, magnetic moment, and anisotropy constant were determined for Ca-doped Ni–Mg–Mn spinel ferrite samples. It is noticed that saturation increases from 29.157 to 51.322 emu/g, whereas remanence increased from 5.34 to 9.40 emu/g, respectively. The permeability, anisotropy constant, and magnetic moments were also found to increase with Ca doping. However, the Y–K angles increased with Ca concentration in Ni–Mg–Mn nano ferrites. In addition, the switching field distribution (SFD) and high-frequency response of all the Ca-doped Ni–Mg–Mn samples were also evaluated. Ca-doped Ni–Mg–Mn samples are suggested to be suitable for switching, filters, inductors, and microwave absorption applications because of the superparamagnetic nature of the prepared spinel ferrites.     

Synthesis of ZnS–Mn nano-luminescent pigment for ink applications

In the present study, ZnS–Mn nano-luminescent pigments were synthesized, using co-precipitation method. Polyvinylpyrrolidine (PVP) surface modifier and Mn dopant concentrations were considered as affecting parameters. The luminescent ink was loaded with two different concentrations of pigments. The obtained ink was silk-screened on different types of fabrics mainly treated cotton, cotton and nylon. Structure, microstructure, luminescent properties of nano-pigments, inks and fabrics and also rheological properties of the inks were investigated. The results showed that the ceramic ink prepared with nano-luminescent pigment had high photoluminescence (PL) intensity. Moreover, the optimum concentrations of Mn and PVP for obtaining maximum PL intensity were found as 2 and 5 wt%, respectively. SEM images of fabrics indicated that nanoparticles were loaded, nonuniformly, on the fibers. The treated linen and nylon fabrics showed maximum and minimum PL intensity, respectively, due to ink penetration depth in the fabrics. Furthermore, washing fastness estimated for all fabrics was in the proper range.     

Preparation of erythritol–graphite foam phase change composite with enhanced thermal conductivity for thermal energy storage applications

Three-dimensional interconnected graphite composite foam as a heat conductive matrix was fabricated by using low cost polymeric precursors and polyurethane (PU) foam as carbon source and sacrificial macroporous template, respectively. Erythritol–graphite foam as a stable composite phase change material (PCM) was obtained by incipient wetness impregnation method. The thermophysical properties such as thermal diffusivity, specific heat, thermal conductivity and latent heat of the erythritol–graphite composite foam were measured. From the results, it was found that the thermal conductivity of the erythritol–graphite composite foam (3.77 W/mK) was enhanced 5 times as compared with that of pristine erythritol (0.72 W/mK). This enhancement can significantly reduce the charging and discharging times of the PCM storage system. There is no chemical reaction between erythritol and graphite as confirmed by X-ray diffractometer (XRD). The PCM/foam composite has a melting point of 118 °C and latent heat of 251 J/g which corresponds to the mass percentage (75 wt.%) of the erythritol within the composite foam. The obtained results confirmed the feasibility of using erythritol–graphite foam as a new phase change composite for thermal energy storage (TES) applications, thus it can contribute to the efficient utilization and recovery of solar heat or industrial waste heat.     

Synthesis and characterization of thiophene-based donor–acceptor type polyimide and polyazomethines for optical limiting applications

In this communication we describe the design and synthesis of four new conjugated polymers (P1–P4) based on 3,4-ditetradecyloxythiophene. The required diamine monomer was prepared by a unique catalyst-free reduction process using hydrazine hydrate. The structures of the intermediates and polymers were established by FTIR, ¹H NMR spectroscopy. Molecular weights of polymers were determined by gel permeation chromatographic (GPC) method. Their electrochemical properties were investigated by cyclic voltammetry and linear optical properties were determined by UV–Visible absorption and fluorescence emission spectroscopic techniques. Further, their nonlinear optical properties were evaluated by Z-scan technique using Nd:YAG laser. These polymers showed strong optical limiting behavior with two-photon absorption (2PA) coefficients of the order of 10^?10 m/W, which are comparable to that of good optical limiting materials reported in the literature. Also, it has been observed that the optical nonlinearity enhanced with the increase in donor–acceptor strength of the polymer backbone.     

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.     

Synthesis,processing and properties of TaC–TaB2–C ceramics

Multi-phase ceramics in the TaC–TaB₂–C system were prepared from TaC and B₄C mixtures by reactive pressureless sintering at 1700–1900 °C. The pressureless densification was promoted by the use of nano-TaC and by the presence of active carbon in the reaction products. The presence of TaB₂ inhibited grain growth of TaC and increased the hardness compared to pure TaC. If a coarse TaC powder was used, the compositions did not densify. In contrast, pure nano-TaC was pressureless sintered at 1800 °C by the addition of 2 wt.% carbon introduced as carbon black or graphite. The introduction of carbon black resulted in fully dense TaC ceramics at temperatures as low as 1500 °C. The grain size of nominally pure TaC ceramics was a strong function of carbon stoichiometry. Enhanced grain size in sub-stoichiometric TaC, compared to stoichiometric TaC, was observed. Additional work is necessary to optimize processing parameters and evaluate the properties of ceramics in the TaC–TaB₂–C system.     

Microwave- or conventional–hydrothermal synthesis of Co-based materials for electrochemical energy storage

Crystalline Co₃O₄ and Co(OH)₂ were synthesized using Co(NO₃)₂ as a precursor by conventional–hydrothermal and microwave–hydrothermal routes, respectively. The Co₃O₄ phase showed cubic morphologies while the β-Co(OH)₂ phase exhibited plate-like shapes. The electrochemical performances of Co₃O₄ and Co(OH)₂ phases were evaluated as electrode materials for lithium-ion battery anodes, cathodes and supercapacitors. Both Co₃O₄ and Co(OH)₂ phases showed pseudocapacitive performances in Li₂SO₄ and KOH electrolytes. The Co₃O₄ and Co(OH)₂ phases were found to be more promising as anodes than as cathodes in lithium-ion batteries. The Co(OH)₂ electrodes showed higher specific capacitances than those of Co₃O₄ materials.     

Influences of crystallization temperature on the structure,dielectric, and energy storage characteristics of KBaSrNb5O15-based glass–ceramics

Glass–ceramics capacitors have great application potential in pulsed power systems, due to ultrafast discharge speed and high dielectric breakdown strength (BDS). Here, lead-free niobate glass–ceramic dielectric materials were synthesized, and the effects of heat treatment temperature on the dielectric, ferroelectric, and energy storage properties of glass–ceramics were investigated comprehensively. The results exhibit that the dielectric permittivity first increases and then decreases as the crystallinity increases; however, the dielectric BDS diminishes. At the optimum crystallization temperature of 740°C, the maximum value of discharge energy density is 2.2 J/cm³ at 600 kV/cm, which is about 7.6 times that of mother glass. Furthermore, an ultrahigh power density of about 380.9 MW/cm³ and ultrafast discharge speed of about 11.2 ns were achieved simultaneously. Meanwhile, great thermal stability of charge–discharge property was verified in this glass–ceramics. According to P–E loops and dielectric test result, a high dielectric constant (∼207) and low dielectric loss (<0.005) as well as high energy storage efficiency of about 94.9% were achieved for G740 sample. The previous results make the obtained glass–ceramic as potential candidates in dielectric capacitors.