Microstructure and piezoelectric properties of CuO-doped 0.95(K0.5Na0.5)NbO3-0.05Li(Nb0.5Sb0.5)O3 lead-free ceramics

The effects of sintering temperature and the addition of CuO on the microstructure and piezoelectric properties of 0.95(K_0.5Na_0.5)NbO₃-0.05Li(Nb_0.5Sb_0.5)O₃ were investigated. The KNN-5LNS ceramics doped with CuO were well sintered even at 940 °C. A small amount of Cu²⁺ was incorporated into the KNN-5LNS matrix ceramics and XRD patterns suggested that the Cu²⁺ ion could enter the A or B site of the perovskite unit cell and replace the Nb⁵⁺ or Li⁺ simultaneously. The study also showed that the introduction of CuO effectively reduced the sintering temperature and improved the electrical properties of KNN-5LNS. The high piezoelectric properties of d₃₃ = 263 pC/N, k_p = 0.42, Q_m = 143 and tan δ = 0.024 were obtained from the 0.4 mol% CuO doped KNN-5LNS ceramics sintered at 980 °C for 2 h.     

Low temperature sintered MnZn ferrites for power applications at the frequency of 1 MHz

The low power loss Mn-Zn ferrites with fine grains were developed by the low-temperature-sintering ceramic process for power applications at a high frequency of 1 MHz. The LiBO₂ sintering aid was added to promote the low temperature sintering and densification. The effects of LiBO₂ on micromorphology and magnetic properties of the sintered Mn-Zn ferrites were investigated. With the aid of LiBO₂, sintering temperature could be reduced as low as 990 °C. The optimum sample was obtained by the addition of 500 ppm LiBO₂ sintered at 1020 °C. The average grain size of this sample is 2.78 μm, the density reaches 4.82 g/cm³, and the minimum power loss is 310 kW/m³ at 1 MHz/30 m T and 25 °C. This sample shows good wide-temperature stability of power loss. The mechanism of power loss affected by the LiBO₂ addition was also discussed. The ceramic sintering process combining the low temperature sintering and the sintering aid offers a new way to develop high-frequency Mn-Zn ferrites.     

Achieving both large transduction coefficient and high Curie temperature of Bi and Fe co-doped PZT piezoelectric ceramics

Achieving both the large transduction coefficient (the product of piezoelectric charge d₃₃ and voltage coefficients g₃₃) and high Curie temperature is very important to improve the power generation performance and their thermal stability of piezoelectric energy harvesters. It is difficult to improve the transduction coefficient of the commercial PZT based piezoelectric ceramics due to the same variation trend of piezoelectric charge coefficient and dielectric constant with chemical modifications. In this work, Bi₂O₃ and Fe₂O₃ co-modified ((Pb_1-xBi_x)((Zr_0.53Ti_0.47)_1-xFe_x)O₃) ceramics were prepared by conventional solid state reaction method, and their dielectric and piezoelectric properties were studied. The piezoelectric charge coefficient d₃₃ increases by Bi and Fe co-modifications due to the enlarged grain size and reduced lattice distortion, while the dielectric constant ε₃₃ deceases mainly owing to the increased micro-pores in grains, leading to the enhancement transduction coefficient d₃₃×g₃₃. The Curie temperature Tc and maximum transduction coefficient d₃₃×g₃₃ are 346 °C and 17169 × 10^?15 m²/N, respectively, which are both higher than those of commercial PZT and PZN-PZT based piezoelectric ceramics. This work provides a new way to enhance the transduction coefficient of PZT based ceramics for piezoelectric energy harvesters used in wide temperature range.     

Low temperature sintering of PLZST-based antiferroelectric ceramics with Al2O3 addition for energy storage applications

Antiferroelectric (AFE) materials have superior energy storage properties in high power multilayer ceramic capacitors (MLCCs). To adapt to the sintering temperature of inner metal electrodes with less palladium content, in this work, Al₂O₃ was added to Pb_0.95La_0.02Sr_0.02(Zr_0.50Sn_0.40Ti_0.10)O₃ (PLSZST) AFE ceramics, in an attempt to reduce the sintering temperature. Results of this study demonstrate that the optimal composition of PLSZST-0.8 wt% Al₂O₃ sintered at a lower temperature 1040 ℃, has a high recoverable energy density (W_re, 3.23 J/cm³) and a high efficiency (η, 90 %) at room temperature. It is also high in pulse discharge energy density (W_dis, 2.45 J/cm³), current density (1369 A/cm²), and has an extremely short period of discharge (less than 500 ns). In addition, both W_re and η demonstrate a good stability in temperature within a wide range of 30 ℃-100 ℃. In sum, this novel AFE composition has great potentials for energy storage applications such as high energy density MLCCs.     

Piezoelectric properties of low temperature sintering in Pb(Zr,Ti)O3–Pb(Zn,Ni)1/3Nb2/3O3 ceramics for piezoelectric transformer applications

In this study, in order to develop low-temperature sintering ceramics for a multilayer piezoelectric transformer application, we explored CuO and Bi₂O₃ as sintering aids at low temperature (900 °C) sintering condition for Sb, Li and Mn-substituted 0.8Pb(Zr_0.48Ti_0.52)O₃–0.16Pb(Zn_1/3Nb_2/3)O₃–0.04Pb(Ni_1/3Nb_2/3)O₃ ceramics. These substituted ceramics have excellent piezoelectric and dielectric properties such as d₃₃  347 pC/N, k_p  0.57 and Q_m  1469 when sintered at 1200 °C. The addition of CuO decreased the sintering temperature through the formation of a liquid phase. However, the piezoelectric properties of the CuO-added ceramics sintered below 900 °C were lower than the desired values. The additional Bi₂O₃ resulted in a significant improvement in the piezoelectric properties. The composition Sb, Li and Mn-substituted 0.8Pb(Zr_0.48Ti_0.52)O₃–0.16Pb(Zn_1/3Nb_2/3)O₃–0.04Pb(Ni_1/3Nb_2/3)O₃ + 0.5 wt% CuO + 0.5 wt% Bi₂O₃ showed the value of k_p = 0.56, Q_m = 1042 (planar mode), d₃₃ = 350 pC/N, when it was sintered at 900 °C for 2 h. These values indicated that the newly developed composition might be suitable for multilayer piezoelectric transformer application.     

Effect of dual liquid phase sintering aids on the densification and microstructure of low temperature sintered alumina ceramics

Alumina ceramics was prepared by pressureless sintering technology in which a CuO–TiO₂–Bi₂O₃ mixture (0–4.0 wt% Bi₂O₃ and 4.0 wt% CuO and TiO₂) was added as dual liquid phase sintering aids. The phase compositions, microstructural feature, and sintering behaviour of the alumina ceramics were analyzed. The results showed that adding 2.5 wt% Bi₂O₃ to alumina ceramics can increase the contribution rate of initial stage of sintering to the sintering process. The relative density of the sample reached 97.63% after sintering at 1200 °C for 90 min. Measurements from differential scanning calorimetry, with the addition of CuO–TiO₂–Bi₂O_3, demonstrated the formation of two liquid phase points, 827.4 and 936.8 °C. Notably, the solid solution temperature of TiO₂ and Al₂O₃ ceramics diminished thanks to the dual liquid phase sintering aids, and at the same time the activation energy required also dropped from 368.96 to 137.31 kJ/mol. Research indicates that the combined action of dual liquid phase sintering and solid-state reaction sintering has promoted the densification of alumina ceramics during the sintering process while at the same time inhibiting the growth of abnormal grains so that a homogeneous microstructure can be formed.     

Controllable low-temperature flash sintering and giant dielectric performance of (Zn,Ta) co-doped TiO2 ceramics

In this study, the challenge of high-temperature and long-time sintering of (Zn, Ta) co-doped TiO₂ ceramics is solved successfully using flash sintering technology. Joule heating and a high heating rate make the sample compact rapidly at low temperatures (1050 °C in 24 min). When the electric field was equal to 200 V/cm, high permittivity (ε' ～ 1.32 × 10⁴), low dielectric loss tangent (tan δ ～ 0.27), and nonlinear coefficient ($\alpha$ ～ 5.8) values were obtained. Flash sintered samples have more free electrons, resulting in a high dielectric constant. Further, the higher the electric field, the smaller the grain resistance of the sample; this condition is conducive to reducing dielectric loss. giant dielectric performance is explained by the combined action of the electron-pinned defect dipole theory and the internal barrier layer capacitance effect. Therefore, this study provides a promising prospect for the green preparation of co-doped TiO₂ giant dielectric ceramics.     

High piezoelectric properties of BaTiO3-xLiF ceramics sintered at low temperatures

BaTiO₃-xLiF ceramics were prepared by a conventional sintering method using BaTiO₃ powder about 100 nm in diameter. The effects of LiF content (x) and sintering temperature on density, crystalline structure and electrical properties were investigated. A phase transition from tetragonal to orthorhombic symmetry appeared as sintering temperatures were raised from 1100 °C to 1200 °C or as LiF was added from 0 mol% to 3 mol%. BaTiO₃-6 mol% LiF ceramic sintered at 1000 °C exhibited a high relative density of 95.5%, which was comparable to that for pure BaTiO₃ sintered at 1250 °C. BaTiO₃-4 mol% LiF ceramic sintered at 1100 °C exhibited excellent properties with a piezoelectric constant d₃₃ = 270 pC/N and a planar electromechanical coupling coefficient k_p = 45%, because it is close to the phase transition point in addition to high density.     

Structures and electrical conductivities of Gd3+ and Fe3+ co-doped cerium oxide electrolytes sintered at low temperature for ILT-SOFCs

Gd³⁺ and Fe³⁺ co-doped cerium oxide electrolytes, Ce_0.9Gd_0.1‐xFe_xO_2-δ (x?=?0.00, 0.01, 0.03, 0.05, 0.07, 0.10), were prepared by co-precipitation for ultrafine precursor powders and sintering for densified ceramic pellets. The crystal and microscopic structures were characterized by XRD, FESEM and Raman spectroscopy and their electrical properties were studied by AC impedance spectroscopy and the measurement of single cell's outputs. In comparison with Ce_0.9Gd_0.1O_1.95, the ceramic pellets of Ce_0.9Gd_0.1‐xFe_xO_2-δ with a relative density of 95% can be obtained after sintered at 1000?°C for 5?h, showing a remarkably enhanced sintering performance with a sintering temperature reduction of 500?°C, which might be ascribed to the highly activated migration of constituent species in the cerium oxide lattice doped with Gd³⁺ and Fe³⁺ions. Moreover, the electrical conductivity of Ce_0.9Gd_0.1‐xFe_xO_2-δ can be significantly enhanced depending on the mole fraction x, with Ce_0.9Gd_0.07Fe_0.03O_1.95 exhibiting the highest electrical conductivity of 38 mS/cm at 800?°C, about 36% higher than that of Ce_0.9Gd_0.1O_1.95 electrolyte sintered at 1500?°C for 5?h. So, The Gd³⁺ and Fe³⁺ co-doped cerium oxide would be an excellent candidate electrolyte for ILT SOFCs due to its prominent sintering performance and enhanced electrical conductivity.     

Fabrication and characterization of low temperature sintered hard piezoelectric ceramics for multilayer piezoelectric energy harvesters

In this study, 0.65Pb(Zr_0.465Ti_0.545)O₃-0.35Pb(Zn_1/6Ni_1/6Nb_2/3)O₃, doped with 3.0 mol% MnCO₃ and 1.0 mol% CuO (M3.0C1.0PZT-PZNN), was investigated as a hard piezoelectric ceramic for multilayer ceramic piezoelectric energy harvesters (MLC-PEHs). These PEHs showed a high output performance that was partly ascribed to the hardening effect of MnCO₃. In contrast, a single-layer ceramic piezoelectric energy harvester (SLC-PEH), containing M3.0C1.0PZT-PZNN, exhibited a higher output power density than that containing an analogous non-Mn-doped soft piezoelectric ceramic (C1.0PZT-PZNN), especially at high accelerations. In the fabricated MLC-PEH, containing an M3.0C1.0PZT-PZNN-based five-layer ceramic and a pure Ag inner electrode, no interdiffusion was observed between the electrode and the ceramic layers, and the corresponding interface was clear and smooth. This MLC-PEH, which exhibited a high output power density and a relatively large current, was used to charge a 0.22 F capacitor at its resonance frequency and an acceleration of 1.5 G, achieving a charging rate higher than that of the SLC-PEH.     

Effects of Li2O-B2O3-SiO2 glass on the low-temperature sintering of Zn0.15Nb0.3Ti0.55O2 ceramics

The influences of Li₂O-B₂O₃-SiO₂ glass (LBS) on the activation energy, phase composition, the stability of the structure and microwave dielectric properties of Zn_0.15Nb_0.3Ti_0.55O₂ ceramics have been systematically investigated. LBS glass acted as flux former and contributed to the reactive liquid-phase sintering mechanism, which remarkably lowed the sintering temperature from 1150?°C to 900?°C and enhanced the shrinkage and densification of ceramic at the low sintering temperatures. The ceramics with 1.5?wt% LBS glass sintered at 900?°C for 3?h show great properties: ε_r = 73.59, Q × f = 8024?GHz, τ_f = 270.54?ppm/°C.     

The simulation for a high-efficiency millimeter wave microstrip antenna by low dielectric loss and wide temperature stable lithium-based microwave dielectric ceramics for LTCC applications

As a common flux agent, B₂O₃–CuO was introduced into Li₂TiO₃ system to reduce the sintering temperature for the requirements of LTCC applications. The optimal mass ratio of CuO to B₂O₃ was innovatively explored. When the mass ratio of CuO to B₂O₃ increased to 1.2:1.0, excellent microwave dielectric properties were obtained in LTMF&LTZN_0.892+CB_1.2 ceramic of ε_r = 13.23, Q × f = 62,749 GHz, τ_f = -2.48 ppm/°C and the sintering temperature was reduced from 1300 to 930 °C. In a wide temperature range, the sample still maintain high temperature stability of |τ_f| < 5 ppm/°C (-40–120 °C). Based on the LTMF&LTZN_0.892+CB_1.2 substrate, a millimeter wave microstrip antenna resonated at 30.12 GHz was designed with a considerably high radiation efficiency of 93.94% and a signal gain of 4.87 dB. Comprehensive microwave dielectric properties make LTMF&LTZN_0.892+CB_1.2 become a candidate material for LTCC applications.     

Cold sintering of the ceramic potassium sodium niobate, (K0.5Na0.5)NbO3, and influences on piezoelectric properties

Cold Sintering was applied to densify a Potassium-Sodium Niobate solid solution composition, 0.5KNbO₃-0.5NaNbO₃ (KNN); the process uses a transient chemical sintering aid, moderate pressure (400 MPa), and temperatures between 230–300 °C to obtain ceramics of ～92 to 96 % theoretical density. Typically, sintering temperatures between ～1000?1050 °C are required to density KNN using conventional methods. In this paper, the densification was investigated during heating, particularly the shrinkage in the first 60 min of the cold sintering process. The low-field dielectric and electrical properties of the resulting ceramics were found to be comparable to conventionally sintered KNN. Electric fields up to 80 kV/cm could be applied, however the ceramics showed pinched hysteresis loops, even after poling over a wide range of temperatures and electric fields. A Rayleigh analysis was used to investigate domain dynamics and high reversible permittivity. The irreversible behavior was an order of magnitude lower than in conventionally sintered KNN, likely associated with defect pinning of ferroelectric domains. A Transmission Electron Microscopy (TEM) study revealed a high density of line defects in most grains; dislocations in the grains limit poling and domain wall movement, thus suppressing both the piezoelectric properties and the hysteresis. Furthermore, TEM observations indicated crystalline grain boundaries that were faceted with terrace kink ledges. These observations point to the importance of the initial powder optimization and grain boundary diffusion when using cold sintering to prepare ceramics that are intended to show bulk cooperative properties such as ferroelectricity. The impact of limited high temperature homogenization of bulk diffusional processes is discussed.     

Cold-sintered V2O5-PEDOT:PSS nanocomposites for negative temperature coefficient materials

A low temperature sintering method, namely cold sintering process, was used to prepare 97 vol%V₂O₅-3 vol% PEDOT:PSS (Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate ceramic-polymer nanocomposites. The density, phase purity, microstructure, elemental distribution and electrical properties of sintered tape-cast films were investigated. The composition with 97 vol%V₂O₅-3 vol% PEDOT:PSS ceramic-polymer nanocomposites can be densified (∼90%) after a cold sintering of 140 °C for 45 min under a uniaxial pressure of 300 MPa. The Transmission Electron Microscopy (TEM) microstructure shows that a ∼10 nm thick intergranular polymer of PEDOT:PSS has been distributed around the V₂O₅ grains after cold sintering. The resistivity decreases with temperature increasing, indicating a typical negative temperature coefficient (NTC) characteristic. The resistivity at 25 °C, temperature coefficient α at 25 °C, and B coefficient (material constant) are 6.34 Ωm, −2.4% K⁻¹ and 2153 K, respectively. The V₂O₅-PEDOT:PSS nanocomposite materials are suitable for new NTC devices, with properties that are comparable to traditional NTC materials that are sintered at much higher temperatures and with much more complexed process and compositions.     

Microstructure and dielectric properties of low temperature sintered ZnNb2O6 microwave ceramics

Low sintering temperature ZnNb₂O₆ microwave ceramics were prepared by doping with mixed oxides of V₂O₅–Bi₂O₃ and V₂O₅–Bi₂O₃–CuO. The effects of additives on the microstructure and dielectric properties of the ceramics were investigated. The results show that doping with V₂O₅–Bi₂O₃ can reduce the sintering temperature of ZnNb₂O₆ from 1150 °C to 1000 °C due to the formation of V₂O₅ and Bi₂O₃ based eutectic phases. The combined influence of V₂O₅ and Bi₂O₃ resulted in rod-like grains. Co-doping CuO with 1 wt.% V₂O₅–1 wt.% Bi₂O₃ further lowered the sintering temperature to 880 °C, because eutectic phases could be formed between the CuO, V₂O₅ and Bi₂O₃. A second phase of (Cu₂Zn)Nb₂O₈ also forms when the content of CuO is greater than 2.5 wt.%. A pure ZnNb₂O₆ phase can be obtained when the amount of CuO was 1.0–2.5 wt.%. The Q × f values of ZnNb₂O₆ ceramics doped with V₂O₅–Bi₂O₃–CuO were all higher than 25,000 GHz. The dielectric constants were 22.8–23.8 at microwave frequencies. In addition, theτ_f values decreased towards negative as the content of CuO increased. The ceramic with composition of ZnNb₂O₆ + 1 wt.%V₂O₅ + 1 wt.% Bi₂O₃ + 2.5 wt.% CuO sintered at 880 °C exhibited the optimum microwave dielectric properties, is 23.4, Q × f is 46,975 GHz, and τ_f is −44.89 ppm/°C, which makes it a promising material for low-temperature co-fired ceramics (LTCCs).     

Electromechanical modeling and experimental analysis of a compression-based piezoelectric vibration energy harvester

Over the past few decades, wireless sensor networks have been widely used in the field of structure health monitoring of civil, mechanical, and aerospace systems. Currently, most wireless sensor networks are battery-powered and it is costly and unsustainable for maintenance because of the requirement for frequent battery replacements. As an attempt to address such issue, this article theoretically and experimentally studies a compression-based piezoelectric energy harvester using a multilayer stack configuration, which is suitable for civil infrastructure system applications where large compressive loads occur, such as heavily vehicular loading acting on pavements. In this article, we firstly present analytical and numerical modeling of the piezoelectric multilayer stack under axial compressive loading, which is based on the linear theory of piezoelectricity. A two-degree-of-freedom electromechanical model, considering both the mechanical and electrical aspects of the proposed harvester, was developed to characterize the harvested electrical power under the external electrical load. Exact closed-form expressions of the electromechanical models have been derived to analyze the mechanical and electrical properties of the proposed harvester. The theoretical analyses are validated through several experiments for a test prototype under harmonic excitations. The test results exhibit very good agreement with the analytical analyses and numerical simulations for a range of resistive loads and input excitation levels.     

Radiation-grafted materials for energy conversion and energy storage applications

Polymer electrolyte membranes are key components in electrochemical power sources that are receiving ever-growing demand for the development of more efficient, reliable and environmentally friendly energy systems. Ongoing research is focusing on materials with high ionic conductivity and stability, at low cost. Among different methods, radiation-induced grafting is a universal attractive method for preparation of polymer electrolyte materials with tunable properties for various energy conversion and energy storage applications. This review addresses recent advances in the application of radiation-induced grafting techniques for the preparation of polymer electrolyte membranes/separators for emerging electrochemical devices such as fuel cells, batteries and supercapacitors. The challenges associated with the current state-of-the-art materials are highlighted, together with new directions that should be considered for future research.     

Enhanced gyromagnetic properties of Cu-substituted LiZnTi ferrites for LTCC applications

In this work, we prepared a type of LiZnTiCu ferrite with a large grain size and enhanced magnetic properties at ～900 °C by substituting Cu²⁺ ions and doping LBSCA glass. Rietveld refinement of XRD patterns indicates that the Fe³⁺ ions in B sites are partially replaced by the Cu²⁺ ions, which causes the monotone increase of lattice constant. SEM results show that the interaction of CuO nanoparticles and LBSCA glass causes two changes in the grain growth of the LiZnTiCu ferrites. The grain growth is suppressed when the amount of CuO nanoparticles is less than a threshold value (x = 0.05 for 900 °C; x = 0.20 for 875 °C). However, when enough CuO nanoparticles are added, the ferrites possess a large and compact microstructure. The variation of magnetic hysteresis (M-H) loops confirms that M_s follows the Néel's collinear spinel model with the increasing number of CuO nanoparticles (x ≤ 0.25). Finally, a type of LiZnTiCu ferrite (x = 0.15) with uniform large grains (average size >10 μm) and good magnetic parameters (4πM_s = 3457.59 G, H_c = 224.4 A/m and B_s = 236.4 mT) is obtained at ～900 °C.