Enhanced electrical properties in lead-free NBT–BT ceramics by series ST substitution

Comprehensive electrical properties of 0.94(Na_1/2Bi_1/2)TiO₃–0.06BaTiO₃ lead-free ceramics by doping series SrTiO₃ were investigated. High piezoelectric constant of 205 pC/N and electromechanical coupling factor of 0.34 were obtained due to the forming of the rhombohedral–tetragonal morphotropic phase boundary at x=0.02–0.06. Very large recoverable strain of 0.34% was obtained at x=0.10 due to the coexistence of ferroelectric and relaxor pseudocubic phases. A large electrocaloric effect (ΔT_max=1.71 K and ΔT/ΔE=0.34 K mm kV⁻¹ at 50 kV cm⁻¹) which determined by indirect measurements method was obtained at 120 °C at x=0.02, which is significantly higher than that of lead-free ferroelectric ceramics reported so far. Moreover, lower operating temperatures of 50 °C and 30 °C were proposed when x=0.10 and 0.20 with ΔT_max=0.79 K and 0.6 K, respectively. These properties added together indicate a promising material for applications in cooling systems and actuators.     

Orientation related electrocaloric effect and dielectric phase transitions of relaxor PMN-PT single crystals

Orientation related phase transitions of 0.76Pb(Mg_1/3Nb_2/3)O₃-0.24PbTiO₃ single crystals are studied. Values of the relaxation parameters (γ = 1.87–1.97) estimated from linear fits of the modified Curie-Weiss law indicate the relaxor nature. For (110) oriented single crystal, typical oxygen vacancies related high temperature dielectric relaxation behaviors are observed and analyzed. Orientations dependent electrocaloric effects of 0.76Pb(Mg_1/3Nb_2/3)O₃-0.24PbTiO₃ single crystals are first studied and reported, large reversible adiabatic temperature change (0.80 K) and isothermal entropy change (0.66 J K⁻¹ kg⁻¹) are observed. Giant refrigeration efficiency (43.52) and high electrocaloric coefficient (0.041 K cm kV⁻¹) exceed the electrocaloric effect values in most bulk ferroelectrics in previous reports.     

Multilayer 0.9Pb(Mg1/3Nb2/3)O3–0.1PbTiO3 elements for electrocaloric cooling

Electrocaloric (EC) cooling elements in the form of multilayers (MLs) were prepared. The elements consist of five layers of the relaxor-ferroelectric 0.9Pb(Mg_1/3Nb_2/3)O₃–0.1PbTiO₃, about 60 μm thick, with internal platinum electrodes and exhibiting a dense, uniform microstructure with a grain size of 1.7 μm. The largest temperature change ΔT_EC of 2.26 K was achieved at an electric field (E) of 100 kV cm⁻¹ and at 105 °C, measured by a high-resolution calorimeter. These results agree well with the indirect measurements. The EC coefficient, ΔT_EC/ΔE, obtained for the MLs, is similar to the value obtained for bulk ceramics of the same composition. The ΔT_EC values above 2 K over a broad temperature range from 75 to 105 °C make the ML elements suitable candidates for EC cooling devices at significantly lower voltages than bulk ceramic plates with comparable dimensions and mass.     

Temperature-insensitive dielectric and piezoelectric properties in (1-x)K0.5Na0.5Nb0.997Cu0.0075O3-xSrZrO3 ceramics

Systematic investigation on phase transition, dielectric and piezoelectric properties of (1-x)K_0.5Na_0.5Nb_0.997Cu_0.0075O₃-xSrZrO₃ (x = 0, 0.03, 0.06, 0.09, 0.12, 0.15, abbreviated as KNNC-100xSZ) ceramics was carried out. Due to the coexistence of orthorhombic and tetragonal phase in a wide temperature range, a diffused polymorphic phase transition (PPT) region was achieved in KNNC with x ≥ 0.06. KNNC-12SZ ceramics exhibited high dielectric permittivity (∼1679), low dielectric loss (∼0.02) and small variation (Δe'/ε'_25 °C ≤ 15%) in dielectric permittivity from −78 °C to 237.3 °C. KNNC-6SZ ceramic possessed a high level of unipolar strain (∼0.15%) and maintained a smaller variation of ±12% under the corresponding electric field of 60 kV cm⁻¹ at 10 Hz from 25 °C to 175 °C. d₃₃^*, which was calculated according to the unipolar strain at 60 kV cm⁻¹, was 230 pm V⁻¹ and remained stable below 100 °C. Therefore, our work provided a new promising candidate for temperature-insensitive capacitors and piezoelectric actuators.     

Hierarchical Co3O4@ZnWO4 core/shell nanostructures on nickel foam: Synthesis and electrochemical performance for supercapacitors

To improve the electrochemical properties of Co₃O₄ for supercapacitors application, a hierarchical Co₃O₄@ZnWO₄ core/shell nanowire arrays (NWAs) material is designed and synthesized successfully via a facile two-step hydrothermal method followed by the heat treatment. Co₃O₄@ZnWO₄ NWAs exhibits excellent electrochemical performances with areal capacitance of 4.1 F cm⁻² (1020.1 F g⁻¹) at a current density of 2 mA cm⁻² and extremely good cycling stability (99.7% of the initial capacitance remained even after 3000 cycles). Compared with pure Co₃O₄ electrodes, the results prove that this unique hierarchical hybrid nanostructure and reasonable assembling of two electrochemical pseudocapacitor materials are more advantageous to enhance the electrochemical performance. Considering these remarkable capacitive behaviors, the hierarchical Co₃O₄@ZnWO₄ core/shell NWAs nanostructure electrode can be revealed promising for high-performance supercapacitors.     

Facile synthesis of functionalized porous carbon with three-dimensional interconnected pore structure for high volumetric performance supercapacitors

Functionalized porous carbon with three-dimensional (3D) interconnected pore structure has been successfully synthesized through direct heat-treatment of KOH-soaked soybeans. Benefiting from heteroatoms (N, O) doping, interconnected porous carbon framework with high surface area as well as high packing density (up to 1.1 g cm⁻³), the as-obtained porous carbon material exhibits high volumetric capacitance of 468 F cm⁻³, good rate capability and excellent cycling stability (91% of capacitance retention after 10,000 cycles) in 6 M KOH electolyte. More importantly, the as-assembled symmetric supercapacitor delivers high volumetric energy density of 28.6 Wh L⁻¹ in 1 M Na₂SO₄ aqueous solution.     

Low-cost and high-performance electrode materials based on BiCoO3 microspheres

Ternary metal oxides have great potential for chemical storage devices because of their outstanding synergistic effects as well as rich redox reactions. However, there are limited reports of 3D structure BiCoO₃ materials and relevant electrochemical properties. Meanwhile, the study of BiCoO₃ is reasonably important for underlying metal oxides researches. In this work, we have successfully developed a 3D urchin-like BiCoO₃ material without using any template and surfactant. For the supercapacitor application, the BiCoO₃ material showed a specific capacitance of 152 F g⁻¹ at the current density of 1 A g ⁻¹, and this value exhibited a rate capability of 82.3% at a high current density of 10 A g ⁻¹. Furthermore, the sample showed the ideal cycling stability (92.7% retention after 5000 times cycles at the current density of 1 A g ⁻¹ and nearly invariable specific capacitance during different current density cycles). These results suggest that the obtained urchin-like BiCoO₃ sample has superb electrochemical performances which suggest its promising applications as renewable and clean energy storage devices electrode materials in the future.     

Highly mesoporous LaNiO3/NiO composite with high specific surface area as a battery-type electrode

This study reports the fabrication and characterization of mesoporous LaNiO₃/NiO composite with a very high specific surface area for a battery-type electrode. The mesoporous LaNiO₃/NiO composite was synthesized via a sol–gel method by using silica gel as a template, the colloidal silica gel was obtained by the hydrolysis and polymerization of tetraethoxysilane in the presence of La and Ni salts. We investigated the structure and the electrochemical properties of mesoporous LaNiO₃/NiO composite in detail. The mesoporous composite sample showed a specific surface area of 372 m² g⁻¹ with 92.7% mesoporous area and displayed remarkable electrochemical performance as a battery-type electrode material for supercapacitor. The specific capacity values were found to be 237.2 mAh g⁻¹ at a current density of 1 A g⁻¹ and 128.6 mAh g⁻¹ at a high current density of 20 A g⁻¹ in 1 M KOH aqueous electrolyte. More importantly, this mesoporous composite also showed an excellent cycling performance with the retention of 92.6% specific capacitance after 60,000 charging and discharging cycles.     

Dielectric properties and energy storage density in ZnO-doped Ba0.3Sr0.7TiO3 ceramics

ZnO additions to Ba_0.3Sr_0.7TiO₃ ceramics have been studied in order to determine the role of this dopant on dielectric property and energy storage density development. The temperature and frequency dependences of dielectric constant, the breakdown strength, and the dielectric dissipation were measured. The dependence of dielectric constant to applied DC field was used to evaluate the energy storage densities. The crystalline structure and morphology were also investigated by XRD and SEM, respectively. Experimental results show that the stored energy density for the sample with 1.6 wt% ZnO addition is the highest among all the compositions. At 40 kV mm^?1 electric fieled, its stored energy density can reach 3.9 J cm^?3. Moreover, the ceramic with this composition has higher dielectric constant and breakdown strength, lower loss and dependency characteristic of dielectric constant to applied electric field.     

Influence of sintering aids and excess Lithium on the conductivity of perovskite-type Li3/8Sr7/16Zr1/4Nb3/4O3 solid electrolyte

Perovskite-structured Li_3/8Sr_7/16Zr_1/4Nb_3/4O₃ solid-state Lithium-conductors were prepared by conventional solid-state reaction method. Influence of sintering aids (Al₂O₃, B₂O₃) and excess Lithium on structure and electrical properties of Li_3/8Sr_7/16Zr_1/4Nb_3/4O₃ (LSNZ) has been investigated. Their crystal structure and microstructure were characterized by X-ray diffraction analysis and scanning electron microscope, respectively. The conductivity and electronic conductivity were evaluated by AC-impedance spectra and potentiostatic polarization experiment. All sintered compounds are cubic perovskite structure. Optimal amount of excess Li₂CO₃ was chosen as 20 wt% because of the total conductivity of LSNZ-20% was as high as 1.6×10⁻⁵ S cm⁻¹ at 30 °C and 1.1×10⁻⁴ S cm⁻¹ at 100 °C, respectively. Electronic conductivity of LSNZ-20% is 2.93×10⁻⁸ S cm⁻¹, nearly 3 orders of magnitude lower than ionic conductivity. The density of solid electrolytes appears to be increased by the addition of sintering aids. The addition of B₂O₃ leads to a considerable increase of the total conductivity and the enhancement of conductivity is attributed to the decrease of grain-boundary resistance. Among these compounds, LSNZ-1 wt%B₂O₃ has lower activation energy of 0.34 eV and the highest conductivity of 1.98×10⁻⁵ S cm⁻¹ at 30 °C.     

High-energy storage performance in lead-free (1-x)BaTiO3-xBi(Zn0.5Ti0.5)O3 relaxor ceramics for temperature stability applications

In this paper, we prepared lead-free (1-x)BaTiO₃-xBi(Zn_0.5Ti_0.5)O₃ (x=0.04, 0.08, 0.10, and 0.14) ceramics by a conventional solid-state reaction technique. Pure perovskite structures and dense microstructures were demonstrated for all the compositions. Interestingly, it was found that the sintering temperature tended to decrease with increasing the Bi(Zn_0.5Ti_0.5)O₃ content. It should be stressed that a low sintering temperature of 1050 °C was utilized for the composition of x=0.14. Moreover, the dielectric permittivity-temperature curve became more flat and the relaxor degree became stronger with the augment in Bi(Zn_0.5Ti_0.5)O₃ content. We also conducted a detailed study on the energy storage performance for all the compositions from 25 °C to 180 °C.We found that relatively temperature-stable energy storage performance could be obtained in the compositions with x=0.08, 0.10 and 0.14 regardless of the evolution of dielectric constant during the test temperature range. In particular, due to a higher field of 12 MV m⁻¹, the discharge energy storage densities of x=0.14 could reach 0.81 J cm⁻³, 0.80 J cm⁻³, 0.78 J cm⁻³, 0.72 J cm⁻³, and 0.67 J cm⁻³ with high efficiencies of 94%, 92%, 94%, 88% and 77% at 25 °C, 50 °C, 100 °C, 150 °C, and 180 °C, respectively. All these results demonstrate the (1-x)BaTiO₃-xBi(Zn_0.5Ti_0.5)O₃ ceramics are quite promising for temperature-stable energy storage applications.     

One-step microwave-assisted synthesis of Sb2O3/reduced graphene oxide composites as advanced anode materials for sodium-ion batteries

Sb₂O₃/reduced graphene oxide (RGO) composites were prepared through a facile microwave-assisted reduction of graphite oxide in SbCl₃ precursor solution, and investigated as anode material for sodium-ion batteries (SIBs). The experimental results show that a maximum specific capacity of 503 mA h g⁻¹ is achieved after 50 galvanostatic charge/discharge cycles at a current density of 100 mA g⁻¹ by optimizing the RGO content in the composites and an excellent rate performance is also obtained due to the synergistic effect between Sb₂O₃ and RGO. The high capacity, superior rate capability and excellent cycling performance of Sb₂O₃/RGO composites demonstrate their excellent sodium-ion storage ability and show their great potential as electrode materials for SIBs.     

Signature of lattice defects in far infrared reflectivity of Ba(Mg1/3Ta2/3)O3

Far infrared reflectivity spectra of Ba(Mg_1/3,Ta_2/3)O₃ prepared at several sintering temperatures were measured, and the eigenfrequencies and damping constants of the TO modes were determined. The reflectivity spectra were fitted with the four-parameter semi-quantum model. The variation in the E_u(O_II) at 222 cm⁻¹ and A_2u(O_II) at 238 cm⁻¹ modes in well ordered ceramics was attributed to the variation in the concentration of the B site defects. It was also found that the change in the oxygen partial pressure of the sintering atmosphere causes a change in the seventh (316 cm⁻¹) and eighth (352 cm⁻¹) modes. We attribute these changes to the oxygen site defect although we cannot evaluate the concentration of this defect at this moment. From the behavior of the damping constants it is suggested that the Ba(Mg_1/3Ta_2/3)O₃ (BMT) attains equilibrium defect density at the heat treatment temperature of more than 1630 °C (120 h).     

Enhanced rate capability of nanostructured three-dimensional graphene/Ni3S2 composite for supercapacitor electrode

Three-dimensional graphene/Ni₃S₂ (3DG/Ni₃S₂) composite electrodes were produced by a facile two-step synthesis route involving chemical vapor deposition (CVD) growth of graphene foam and in situ hydrothermal synthesis of Ni₃S₂. The porous structure of the prepared 3DG is ideal for use as a scaffold for fabricating monolithic composite electrodes. The relative content of Ni₃S₂ initially increased and then decreased with increasing hydrothermal reaction time. The basal surface of the electrode was completely covered after 6 h of hydrothermal reaction. The size of the Ni₃S₂ microspheres also increased with increasing hydrothermal reaction time. The composite electrodes exhibited good specific capacitance (11.529 F cm⁻² at 2 mA cm⁻², i.e., 2611.9 F g⁻¹ at 5 mV s⁻¹) and cyclability (retention of 88.97% capacitance after 1000 charge/discharge cycles at 20 mA cm⁻²). These results are attributed to the fact that the uniform distribution of the Ni₃S₂ microspheres increased the specific surface area of the electrode and facilitated electron transfer and ion diffusion. The 3D multiplexed and highly conductive pathways provided by the defect-free graphene foam also ensured rapid charge transfer and conduction to improve the rate capability of the supercapacitors.     

Fast discharge and high energy density of nanocomposite capacitors using Ba0.6Sr0.4TiO3 nanofibers

Nanocomposites combining high breakdown strength (BDS) polymer and high dielectric permittivity ceramic fillers have shown great potential for pulsed power application. Here a new composite material based on surface-functionalized Ba_0.6Sr_0.4TiO₃ nanofibers/poly(vinylidene fluoride) (BST NF/PVDF) has been prepared by solution casting. The nanocomposites containing 2.5 vol% isopropyl dioleic(dioctylphosphate) titanate (NDZ 101)-functionalized BST NF (N-h-BST NF) have large energy density of 6.95 J cm⁻³ at 380 MV m⁻¹, which is 1.85 times larger than that of the pure PVDF at the same electric field. Also, the discharge speed of the nanocomposites containing 7.5 vol% N-h-BST NF is approximately 0.11 μs. The good properties, together with the large energy density and fast discharge speed, make this material a promising candidate for pulsed power capacitor.     

Preparation of on chip,flexible supercapacitor with high performance based on electrophoretic deposition of reduced graphene oxide/polypyrrole composites

A new concept is introduced to fabricate flexible, on-chip supercapacitors by electrophoretically depositing highly dispersed reduced graphene oxide/polypyrrole on interdigital-like electrodes. By the unique method, the deposited films could construct on the substrate facilely and uniformly. The prepared all-solid-state device demonstrates high volumetric capacitance (about 147.9 F cm⁻³), high energy density (13.15 mWh cm⁻³ at a power density of 1300 mW cm⁻³) and excellent cycling stability (approximately 71.7% of the initial capacitance retained after 5000 cycles). Compared with other supercapacitor, the device demonstrated here is lightweight, flexible and inexpensive.     

Characteristics of highly (001) oriented (K,Na)NbO3 films grown on LaNiO3 bottom electrodes by RF magnetron sputtering

(K,Na)NbO₃ ferroelectric films were grown on LaNiO₃ coated silicon substrates by RF magnetron sputtering. The conductive LaNiO₃ films acted as seed layers and induced the highly (001) oriented perovskite (K,Na)NbO₃ films. Such films exhibit saturated hysteresis loops and have a remnant polarization (2P_r) of 23 μC/cm², and coercive field (2E_c) of 139 kV/cm. The films showed a fatigue-free behavior up to 10⁹ switching cycles. A high tunability of 65.7% (@300 kV/cm) was obtained in the films. The leakage current density of the films is about 6.0×10^?8 A/cm² at an electric field of 50 kV/cm.     

High performance Na3V2(PO4)3 cathode prepared by a facile solution evaporation method for sodium-ion batteries

Based on its abundance and low cost, sodium based batteries have aroused extensive attention for large scale energy-storage systems. In the current work, Na₃V₂(PO₄)₃ prepared by a facile solution evaporation method (denoted as NVP-SE) is used as cathode materials for sodium ion battery, with a control sample by solid state method. Raman spectrum and TEM are used to study the carbon layer coated on NVP-SE. The results show a highly graphitization and well-coated carbon layer, which is predominant by sp² carbon. Graphitized carbon leads to high electrical conductivity, which can improve the rate performance of Na₃V₂(PO₄)₃ materials. Besides, GITT tests show high Na-ion diffusion coefficient. Even at 30 C, the NVP-SE cathode still delivers a capacity of 70 mAh g⁻¹. Moreover, the material also shows great long term cycling performance. After 500 cycles at 1 C rate and 1000 cycles at 5 C, its discharge capacities are still 103.3 mAh g⁻¹ and 85.4 mAh g⁻¹, which maintain 92.6% and 85.0% of its initial capacity. Thus, simple preparation process and excellent electrochemical performance for Na₃V₂(PO₄)₃/C extend it as a potential material for high power applications.     

Enhanced energy storage properties in La(Mg1/2Ti1/2)O3-modified BiFeO3-BaTiO3 lead-free relaxor ferroelectric ceramics within a wide temperature range

A new ternary lead-free (0.67-x)BiFeO₃-0.33BaTiO₃-xLa(Mg_1/2Ti_1/2)O₃ ferroelectric ceramic exhibited an obvious evolution of dielectric relaxation behavior. A significantly enhanced energy-storage property was observed at room temperature, showing a good energy-storage density of 1.66 J/cm³ at 13 kV/mm and a relatively high energy-storage efficiency of 82% at x = 0.06. This was basically ascribed to the formation of a slim polarization-electric field hysteresis loop, in which a high saturated polarization P_max and a rather small remnant polarization P_r were simultaneously obtained. Particularly, its energy storage properties were found to depend weakly on frequency (0.2 Hz–100 Hz), and also to exhibit a good stability against temperature (25 °C–180 °C). The achievement of these characteristics was attributed to both a rapid response of the electric field induced reversible ergodic relaxor to long-range ferroelectric phase transition and a typical diffuse phase transformation process in the dielectric maxima.     

Porous carbon spheres as anode materials for sodium-ion batteries with high capacity and long cycling life

Porous carbon spheres (PCSs) with high surface area were fabricated by the reaction of D-Glucose monohydrate precursor with sodium molybdate dihydrate (Na₂MoO₄·2H₂O) via a facile hydrothermal method followed by carbonization and aqueous ammonia solution (NH₃·H₂O) treatment. The as-prepared PCSs exhibit a highly developed porous structure with a large specific surface area and show an excellent electrochemical performance as anode material of sodium-ion batteries (SIBs). A reversible capacity of 249.9 mA h g⁻¹ after 50 cycles at a current density of 50 mA g⁻¹ and a long cycling life at a high current density of 500 mA g⁻¹ are achieved. The excellent cycling performance and high capacity make the PCSs a promising candidate for long cycling SIBs.