Greatly reduced leakage current and defect mechanism in atmosphere sintered BiFeO3–BaTiO3 high temperature piezoceramics

In current work, the effect of sintering atmospheres (N₂, air and O₂) on the structure, electrical properties, and defect mechanism of 0.8BiFeO₃–0.2BaTiO₃ lead-free piezoelectric ceramics has been investigated. X-ray diffractometer results indicated all the samples crystallized into the rhombohedrally distorted perovskite structure which was independent on the sintering atmospheres. Bi-containing impurity phases were observed in N₂ sintered samples while not appearing in other atmospheres. X-ray photoelectron spectrum analysis indicated more Fe²⁺ ions, which can result in high leakage current, were involved in N₂ sintered ceramics than that in O₂- and air sintered compositions. However, greatly reduced leakage currents were obtained in N₂ sintered ceramics which should be ascribed to the formation of secondary phases. The largest polarization and lowest leakage current were obtained in the sample sintered in N₂ (2 h), which owned the optimal ferroelectric, piezoelectric, and electromechanical properties with piezoelectric constant d ₃₃ = 98 pC/N, planar electromechanical coupling factors k _p = 26.1 %, remnant polarization P _r = 25.7 µC/cm², coercive field E _c = 74.6 kV/cm, and a high Curie temperature T _c = 632 °C, respectively.     

Comparison of sintering behavior and piezoelectric properties of (K,Na)NbO3-based ceramics sintered in conventional and microwave furnace

In this study (1 − x) K_0.48Na_0.48Li_0.04Nb_0.96Ta_0.04O₃ − xSrTiO₃ (0.0 ≤ x ≤ 0.10) ceramics were fabricated by sintering in microwave furnace for first time as well as in conventional furnace (either via single step or two-step procedures). Sintering behavior and piezoelectric properties of sintered samples were studied and compared. It was found that two-step sintering decreases sintering temperature effectively and enhances densification compared to single step sintering. Microstructure analysis revealed that, two-step sintering suppresses grain growth and promotes densification. On the other hand, microwave sintering enhanced densification more effectively and reduced sintering time and temperature. The maximum piezoelectric constants of ceramics were measured for those sintered in microwave furnace. Piezoelectric constant of the sample containing 1 mol% SrTiO₃ which was sintered in microwave furnace was measured 310 pC N⁻¹ while by sintering in conventional furnace via single and two-step sintering it was obtained 208 and 278 pC N⁻¹, respectively.     

Microstructure,ferroelectric, piezoelectric and ferromagnetic properties of BiFeO3–BaTiO3–Bi(Zn0.5Ti0.5)O3 lead-free multiferroic ceramics

Multiferroic ceramics of (0.70?x)BiFeO₃–0.30BaTiO₃–xBi(Zn_0.5Ti_0.5)O₃ + 1 mol% MnO₂ with perovskite structure were prepared by a conventional ceramic technique and the effects of Bi(Zn_0.5Ti_0.5)O₃ doping and sintering temperature on the microstructure, multiferroic and piezoelectric properties of the ceramics were studied. All the ceramics possess a pure perovskite structure and no second phases can be detected. After the addition of a small amount of Bi(Zn_0.5Ti_0.5)O₃ (x ≤ 0.05), the ferroelectric and piezoelectric properties of the ceramics are improved and the grain growth is promoted. However, excess Bi(Zn_0.5Ti_0.5)O₃ (x ≥ 0.10) retards the grain growth, degrades the ferroelectricity and piezoelectricity, and induces two dielectric anomalies at high temperature. The ceramics can be well sintered at the very wide range of low sintering temperatures (880–980 °C) and exhibit good densification (relative density: 96.2–98.4 %) and strong electric insulation. The increase in the sintering temperature promotes the grain growth and improves the ferroelectricity of the ceramics. The ceramic with x = 0.05 sintered at 880–980 °C possesses improved ferroelectric and piezoelectric properties with remanent polarizations P _r of 21.9–28.1 μm/cm², piezoelectric constants d ₃₃ of 125–139 pC/N and planar electromechanical coupling factors k _p of 30.1–32.4 %, and high Curie temperatures T _C of 523–565 °C. A weak ferromagnetism with remanent magnetizations M _r of 0.0411–0.0422 emu/g and coercive fields H _c of 1.70–1.99 kOe were observed in the ceramics with x = 0–0.025.     

Enhancing the microwave dielectric properties of Nd (Mg0.5Sn0.5)O3 ceramics by substituting Nd3+ with Ca2+

The microwave dielectric properties of Nd_(1?2x/3)Ca_x(Mg_0.5Sn_0.5)O₃ ceramics were examined to evaluate their exploitation for mobile communication. Nd_(1?2x/3)Ca_x(Mg_0.5Sn_0.5)O₃ ceramics were prepared by the conventional solid-state method with various sintering temperatures. The X-ray diffraction patterns of the Nd_2.9/3Ca_0.05(Mg_0.5Sn_0.5)O₃ ceramics revealed no significant variation of phase with the sintering temperature. Nd_2.9/3Ca_0.05(Mg_0.5Sn_0.5)O₃ ceramics that were sintered at 1,550 °C for 4 h had the following properties: a density of 6.86 g/cm³, a dielectric constant (ε_r) of 19.3, a quality factor (Q × f) of 99,000 GHz, and a temperature coefficient of resonant frequency (τ _f ) of ?65 ppm/°C.     

Influence of different dopant sites by lanthanum on the dielectric properties of CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> ceramics

The effects of La³⁺ doped in calcium copper titanate (CCTO) at Ca²⁺ site and Cu²⁺ site were examined. The doped compositions, La_0.1Ca_0.85Cu₃Ti₄O₁₂ (LCCTO) ceramics and CaLa_0.1Cu_2.85Ti₄O₁₂ (CLCTO) ceramics were prepared by the solid-state method. The microstructure, dielectric properties, complex impedance and nonlinear I–V characteristics were studied. And it was found that La³⁺ doped at Ca²⁺ site achieved lower sintering temperatures than that doped at Cu²⁺ site in CCTO ceramics. The dielectric loss (tan δ) of LCCTO ceramics was about 0.05 at 40 kHz when the sample was sintered at 1080 °C. Dielectric constant (ε′) of LCCTO ceramics was about 3.2 × 10⁴ when the sample was sintered at 1100 °C, which was larger than CLCTO ceramics examined under the same process condition with sintering temperatures vary. The impedance analysis revealed that LCCTO ceramics had an influence of resistance of grain boundaries, which was stronger than that of CLCTO ceramics. Meanwhile, both LCCTO ceramics and CLCTO ceramics had a nonlinear-Ohmic property.     

Effect of Ba(Cu1/3Nb2/3)O3 content on multiferroic properties in BiFeO3 ceramics

The crystal structure, microstructural evolution, and ferroelectric and magnetic properties of BiFeO₃ (BFO) ceramic materials, widely known as room temperature multiferroic materials, were investigated with various contents of Ba(Cu_1/3Nb_2/3)O₃ (BCN). The ceramics were synthesized through the solid state reaction and pressureless sintering in air. By adding tetragonal structured BCN phase to the rhombohedral structured BFO phase, a solid solution was formed. The rhombohedral crystal structure and lattice parameter, densification behavior, and microstructure of BFO were noticeably changed upon the addition of BCB. The electrical properties of BFO were slightly enhanced by the addition of BCN. However, the magnetic properties of BFO which is the most critical issue in BFO multiferroics were drastically changed and the 20 mol% BCN added BFO ceramics showed extremely enhanced magnetization characteristics compared to the other BFO–BCN ceramics. It is expected that BFO–BCN ceramics with this composition could be one of the most promising multiferroic materials for future multiferroic applications.     

Effect of sintering temperature on electrical properties, dielectric characteristics, and aging behavior of ZnO–V2O5-based varistor ceramics modified with Bi2O3

The effect of sintering temperature on microstructure, electrical properties, dielectric characteristics, and aging behavior of ZnO–V₂O₅–MnO₂–Nb₂O₅–Bi₂O₃ varistor ceramics was systematically investigated at 875–950 °C. The sintered density decreased from 5.50 to 5.34 g/cm³ and the average grain size increased from 5.4 to 15.0 μm with an increase in the sintering temperature. The breakdown field (E_B) decreased from 5,785 to 1,181 V/cm with an increase in the sintering temperature. The varistor ceramics sintered at 900 °C exhibited a surprisingly high nonlinear coefficient (α = 61). The donor concentration (N_d) increased from 2.08 × 10¹⁷ to 4.64 × 10¹⁷ cm^?3 with an increase in the sintering temperature and the barrier height (Φ_b) exhibited 1.08 eV as the maximum value at 900 °C. Concerning stability, the varistors sintered at 950 °C exhibited the strongest accelerated aging characteristics, where %ΔE_B = ?1.4 % and %Δα = ?14.6 % for DC accelerated aging stress of 0.85 E_B/85 °C/24 h.     

Dielectric characteristics of Ba(Mg1/3Ta2/3)O3 ceramics sintered at low temperatures

Dielectric characteristics of Ba(Mg_1/3Ta_2/3)O₃ ceramics (BMT ceramics) sintered at low temperatures with 2–3 wt% NaF additives were determined. A dielectric constant of 25 and extremely low dielectric loss (< 0.0001) were measured at 100 kHz and 1 MHz in BMT ceramics sintered under these conditions, and no frequency dependence of the dielectric constant was observed. This suggested that NaF as sintering additive had no harmful influence on the dielectric properties of the ceramics.     

Microstructure and properties of spark plasma sintered AlN ceramics

Spark plasma sintering (SPS) is a newly developed technique that enables poorly sinterable aluminum nitride (AlN) powder to be fully densified. It is addressed that pure AlN sintered by SPS has relatively low thermal conductivity. In this work, SPS of AlN ceramic was carried out with Y₂O₃, Sm₂O₃ and Li₂O as sintering aids. Effects of additives on AlN densification, microstructure and properties were investigated. Addition of sintering aids accelerated the densification, lowered AlN sintering temperature and was advantageous to improve properties of AlN ceramic. Thermal conductivity and strength were found to be greatly improved with the present of Sm₂O₃ as sintering additive, with a thermal conductivity value about 131 Wm⁻¹K⁻¹ and bending strength about 330 MPa for the 2 wt% Sm₂O₃-doped AlN sample SPS at 1,780 °C for 5 min. XRD measurement revealed that additives had no obvious effect on the AlN lattice parameters. Observation by SEM showed that AlN ceramics prepared by SPS method manifested quite homogeneous microstructure. However, AlN grain sizes and shapes, location of secondary phases varied with the additives. The thermal conductivity of AlN ceramics was mainly affected by the additives through their effects on the growth of AlN grain and the location of liquid phases.     

Spark plasma sintering of BiFeO3

Dense BiFeO₃ ceramics were prepared by a novel spark plasma sintering (SPS) technique. The sintering was conducted at temperatures ranging from 675 to 750 °C under 70 MPa pressure. A bulk density value up to 96% of theoretical density was achieved in the process. This contrast to around 90% of the theoretical density achieved by conventional sintering at around 830 °C. It was found that the tendency to form unwanted Bi₂Fe₄O₉ phase is higher at a high sintering temperature for SPS. The dielectric and ferroelectric properties also improved (with respect to conventionally sintered sample) for spark plasma-sintered samples.     

Improvement microwave dielectric properties of Zn2SnO4 ceramics by substituting Sn4+ with Si4+

The microwave dielectric properties of Zn₂(Sn_(1?x)Si_x)O₄ ceramics were examined with a view to their exploitation for mobile communication. The Zn₂(Sn_(1?x)Si_x)O₄ ceramics were prepared by the conventional solid-state method with various sintering temperatures. The X-ray diffraction patterns of the Zn₂(Sn_(1?x)Si_x)O₄ ceramics revealed no significant variation of phase with sintering temperatures. A maximum density of 6.24 g/cm³ was obtained for Zn₂(Sn_0.93Si_0.07)O₄ ceramic, sintered at 1,175 °C for 4 h. Dielectric constant (? _r ) of 8.12, quality factor (Q × f) of 55,500 GHz, and temperature coefficient of resonant frequency (τ _f ) of ?119.3 ppm/°C were obtained for Zn₂(Sn_0.93Si_0.07)O₄ ceramics that were sintered at 1,175 °C for 4 h.     

Effect of grain width and aspect ratio on mechanical properties of Si<Subscript>3</Subscript>N<Subscript>4</Subscript> ceramics

Sintering additives Y₂O₃ and Al₂O₃ with different ratios ((Y₂O₃/Al₂O₃) from 1 to 4) were used to sinter Si₃N₄ to high density and to induce microstructural changes suitable for raising mechanical properties of the resultant ceramics. The sintered Si₃N₄ ceramics have bi-modal microstructures with elongated β-Si₃N₄ grains uniformly distributed in a matrix of equiaxed or slightly elongated grains. Pores were found within the grain boundary phase at the junction regions of Si₃N₄ grains. The highest average aspect ratio (length/width of the grains) of ∼4.92 was found for Y₂O₃/Al₂O₃ ratio of 2.33 with fracture toughness and strength values of ∼7 MPam^1/2 and 800 MPa, respectively. The effect of microstructure, specifically grain morphology, on mechanical properties of sintered Si₃N₄ were investigated and found that the aspect ratio of the elongated grains is the most important microstructural feature which controls mechanical properties of these ceramics.     

Decomposition behavior and dielectric properties of Ti-doped BiFeO3 ceramics derived from molten salt method

The effects of Ti-doping on the decomposition behavior, crystal structure, sintering behavior and dielectric properties have been investigated for the Ti-doped BiFeO₃ ceramics derived from molten salt method. XRD reveals the almost pure phase BiFeO₃ is synthesized in the Ti-doped BiFeO₃ powders. And the particle size of Ti-doped BiFeO₃ powers obviously decreases with the increase of Ti content. However, the sintering temperature elevates significantly after Ti-doping. The DC resistivity can enhance by up to four orders of magnitude (from 10⁴ to 10⁸?Ωm) with only 5 at% Ti doping. But the dielectric constant is suppressed from 10⁴ to 10², and dielectric loss obviously reduces with a small amount of Ti doping. The variation of dielectric properties has been discussed from the decomposition of BiFeO₃ phase. The Ti-doping can effectively suppress the decomposition reaction in Ti-doped BiFeO₃ ceramics.     

Ferroelectric and piezoelectric properties of high temperature perovskite-type 0.69BiFeO3–0.02Bi(Mg1/2Ti1/2)O3–0.29BaTiO3 ceramics

Lead-free high-temperature piezoceramics of MnO₂-doped 0.69BiFeO₃–0.02Bi(Mg_1/2Ti_1/2)O₃–0.29BaTiO₃ (BF69–BMT2–BT29) were prepared with solid state reaction method. Those ceramics were crystallized in a pure pseudocubic perovskite-structured phase. In contrast to high dielectric loss BF–BT binary solid solution ceramics, low dielectric loss ceramics were obtained here with combined strategies of refined electroceramic processing, Mn-doping and adding BMT third member. Ferroelectric and piezoelectric properties were systematically measured. Here, the 0.30 wt% MnO₂-doped BF69–BMT2–BT29 ceramics sintered at 900 °C for 10 h showed a better combination of ferroelectric and piezoelectric properties than those BiFeO₃–BaTiO₃ piezoceramics.     

Influence of Ba2+ substitution on the microwave dielectric properties of Nd(Mg0.5Sn0.5)O3 ceramics

The microwave dielectric properties of Nd(Mg_0.5?xBa_xSn_0.5)O₃ ceramics were examined with a view to their exploitation in mobile communication. The Nd(Mg_0.5?xBa_xSn_0.5)O₃ ceramics were prepared by the conventional solid-state method with various sintering temperatures. The X-ray diffraction patterns of the Nd(Mg_0.47Ba_0.03Sn_0.5)O₃ ceramics revealed no significant variation of phase with sintering temperatures. A density of 6.91 g/cm³, a dielectric constant (ε _r ) of 19.14, a quality factor (Q × f) of 97,500 GHz, and a temperature coefficient of resonant frequency (τ _f ) of ?65.4 ppm/°C were obtained for Nd(Mg_0.47Ba_0.03Sn_0.5)O₃ ceramics that were sintered at 1,600 °C for 4 h.     

Effects of Li2O3–B2O3–SiO2 addition on dielectric properties and microstructure of MgO–TiO2–ZnO–CaO ceramics

The effect of Li₂O₃–B₂O₃–SiO₂ (LBS) liquid-phase additives on the sintering, microstructures, and dielectric properties of MgO–TiO₂–ZnO–CaO (MTZC) ceramics was investigated. It was found that the sintering temperature could be lowered easily, and the dielectric properties of MTZC ceramics could be greatly improved by adding a small amount of LBS solution additives. With the addition of 10 wt% LBS, the ceramics sintered at 900 °C showed favorable dielectric properties with ε_r = 21.7, Qf = 5.0 × 10⁴ GHz, and TCF = ?21.6 ppm/ °C. The distructive physical analysis showed an excellent co-firing interfacial behavior between the MTZC ceramic and the Ag electrode. It indicated that MTZC ceramics with LBS solution additives have a number of potential applications on passive integrated devices based on the low-temperature co-fired ceramics technology.     

Effects of glass additions on the dielectric properties and energy storage performance of Pb0.97La0.02(Zr0.56Sn0.35Ti0.09)O3 antiferroelectric ceramics

In this work, CdO–Bi₂O₃–PbO–ZnO–Al₂O₃–B₂O₃–SiO₂ low softening point glass powders were prepared and employed as sintering aid to improve the dielectric breakdown strength and reduce the sintering temperature of Pb_0.97La_0.02(Zr_0.56Sn_0.35Ti_0.09)O₃ antiferroelectric ceramics. The effects of glass content and sintering temperature on the densification, microstructure, dielectric properties and energy storage performance of Pb_0.97La_0.02(Zr_0.56Sn_0.35Ti_0.09)O₃ antiferroelectric ceramics have been investigated. With inclusion of glass, sintered densities comparable to those obtained by conventional sintering are achieved at only 1,050 °C. The breakdown strength of glass-added samples was notably improved due to the reduction of the grain size. The antiferroelectric to ferroelectric switching field and the ferroelectric to antiferroelectric field both increased with increasing glass content. The dielectric constant and dielectric loss decreased gradually with increasing glass content. As a result, the highest recoverable energy density of 3.3 J/cm³ with an energy efficiency of 80 % was achieved in 4 wt% glass-added sample sintered at 1,130 °C.     

Investigations on the structure,defects, electrical and magnetic properties of Ni-substituted BiFeO<Subscript>3</Subscript> ceramics

Polycrystalline BiFe_1?x Ni _x O₃ (x = 0.00–0.30) ceramics were synthesized by solid state reaction method followed by rapid liquid phase sintering. The effect of Ni substitution on the structure, defects, electrical and magnetic properties of BiFeO₃ ceramics was investigated. X-ray diffraction measurements reveal that Ni-substituted samples exhibit distorted rhombohedral perovskite structure as that of unsubstituted BiFeO₃, and the impurity phases such as Bi₂Fe₄O₉ decrease due to Ni substitution. The changes in the phonon frequencies in Raman spectra reveal the lattice distortion in Ni-substituted samples which are in agreement with the XRD analysis. Positron annihilation lifetime measurements reveal that cation vacancy-type defects exist in all samples, the vacancy concentration increases with increasing Ni content from 0.00 to 0.30. The enhancement in leakage current and dielectric properties is observed in Ni-substituted samples due to the suppression of impurity phases and Fe²⁺. Magnetic measurements indicate that the Ni-substituted BiFeO₃ samples exhibit weak ferromagnetism. The magnetic properties are found to increase with increasing Ni concentration due to the internal structural distortion, cation vacancy, the ferromagnetic exchange between the neighboring Fe³⁺ and Ni²⁺ ions and the change in Fe–O–Fe bond angle.     

Microwave dielectric properties and microstructures of Nd(Mg0.5?xCoxSn0.5)O3 ceramics

This study elucidates the microwave dielectric properties and microstructures of Nd(Mg_0.5?xCo_xSn_0.5)O₃ ceramics with a view to their potential for microwave devices. The Nd(Mg_0.5?xCo_xSn_0.5)O₃ ceramics were prepared by the conventional solid-state method with various sintering temperatures. The X-ray diffraction patterns of the Nd(Mg_0.45Co_0.05Sn_0.5)O₃ ceramics revealed no significant variation of phase with sintering temperatures. A dielectric constant (? _r ) of 19.2, a quality factor (Q?×?f) of 68,900?GHz, and a temperature coefficient of resonant frequency (τ _f ) of ?67?ppm/°C were obtained for Nd(Mg_0.45Co_0.05Sn_0.5)O₃ ceramics that were sintered at 1,550?°C for 4?h.     

Grain growth,densification and electrical properties of lead-free piezoelectric ceramics from nanocrystalline (Ba0.85Ca0.15)(Ti0.90Zr0.10)O3 powder by sol–gel technique

The sintering behavior revealed in the sintering processes of the conventional and a two-step process and electrical properties of the (Ba_0.85Ca_0.15)(Ti_0.90Zr_0.10)O₃ ceramics from the nanocrystalline powders synthesized by a sol–gel technique were systematically studied. It was found that the sintering process of the (Ba_0.85Ca_0.15)(Ti_0.90Zr_0.10)O₃ ceramics made from nanocrystalline powders was significantly improved, the sintering temperature was reduced markedly from 1,540 to 1,280 °C, as well as a high relative density (>97 %) was obtained in the conventional sintering. Under the two-step sintering conditions, the full densification and the most suppression of grain growth was achieved simultaneously. The (Ba_0.85Ca_0.15)(Ti_0.90Zr_0.10)O₃ ceramics from nanocrystalline powders sintered by the two-step sintering technique (sintered at T ₁ of 1,300 °C for 1 min and T ₂ of 1,150 °C for 20 h) exhibited the optimum average grain size of 700 nm and a high relative density of 98 %. The electrical properties of the (Ba_0.85Ca_0.15)(Ti_0.90Zr_0.10)O₃ ceramics were greatly influenced by the grain size and phase structure formed under the both sintering conditions, with sintering temperature and grain size increased, the electrical properties of the (Ba_0.85Ca_0.15)(Ti_0.90Zr_0.10)O₃ ceramics, which made from nanocrystalline powders, shows an enhancing trend: d ₃₃ ~100 pC/N, k _p ~53.3 % for the specimen sintered at 1,300 °C for 1 min and 1,150 °C for 20 h, d ₃₃ ~310 pC/N, k _p ~53.3 % for the specimen sintered at 1,350 °C for 2 h respectively.