Studies on semiconductive (Ba0.8Sr0.2) (Ti0.9Zr0.1)O3 ceramics

In order to produce semiconductive (Ba_0.8Sr_0.2) (Ti_0.9Zr0.1)O₃ ceramics (BSZT), providing low resistivity for boundary-layer capacitor applications, a controlled valency method and a controlled-atmosphere method were applied and studied. In the controlled-valency method, trivalent ions (La³⁺ Sb³⁺) and pentavalent ions (Nb⁵⁺, Sb⁵⁺, Ta⁵⁺) were doped into BSZT ceramics, while in the controlled-atmosphere method, samples were sintered in air and a reducing atmosphere. The doped BSZT ceramics sintered in the reducing atmosphere showed much lower resistivities and smaller temperature coefficient of resistivity (TCR) than those sintered in air, indicating that low partial pressure of oxygen will increase the solubility of the donor dopant and enhance the grain growth. In addition, a small negative TCR at low temperature, as well as a small positive TCR at higher temperature, are also observed for specimens fired in a reducing atmosphere. The former is attributed to the semiconductive grain and the latter to the small barrier layer formed at the grain boundary.     

Dielectric,ferroelectric, and piezoelectric properties of Sb2O3-modified (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 lead-free ceramics

The microstructures, phase structure, and electrical properties have been investigated for the Sb₂O₃-modified (Ba_0.85Ca_0.15)(Zr_0.1Ti_0.9)O₃ lead-free piezoelectric ceramics. The grain size was strongly affected by Sb₂O₃, and the solid solutions show a single phase perovskite structure. The ceramics with x = 0.1 % exhibit enhanced electrical properties of d ₃₃ ～556 pC/N, k _p ～52 %, ε _r ～3,895, tan δ ～1.3 %, and P _r ～12.6 μC/cm² when sintered at a low temperature of ～1,350 °C. The high temperature stability of ferroelectric properties (P _r ～14.8 μC/cm²) was obtained in the temperature range from ?60 to 0 °C. And with the addition of small amount of Sb₂O₃, Curie temperature (T _c ) maintains virtually unaltered.     

Oxygen vacancy-related dielectric relaxation and electrical conductivity in La-doped Ba(Zr0.9Ti0.1)O3 ceramics

Ba_1?xLa_x (Zr_0.9Ti_0.1)_1?x/4O₃ (BLZT) ceramics with x = 0.02 (BLZT-1), 0.04 (BLZT-2), 0.06 (BLZT-3) and 0.08 (BLZT-4) were prepared by a solid-state reaction route. Crystal structure of the BLZT ceramics was determined using X-ray diffraction and Raman spectroscopy. While the ceramics for x ≤ 0.04 are pure phase with cubic perovskite structure, pyrochlore La₂Zr₂O₇ appears in the samples with x = 0.06 and 0.08. Dielectric properties as function of temperature and frequency showed more than one dielectric anomaly were found at high temperatures during heating but they weakened or disappeared during cooling. Both dielectric relaxation and electrical conductivity were taken into account in point defect mechanism. The double-ionized and short-range hopping of oxygen vacancy should be mainly responsible for the dielectric anomalies and conduction behavior. Activation energy of conductivity E _con is lower than half of the band gap E _g obtained by UV–Vis spectroscopy, which results from emergency of oxygen vacancies. In visible light region, the ceramics show a strong absorption with band gap of about 3.57 eV.     

(Ba0.85Ca0.15)(Ti0.9Zr0.1-xSnx)O3无铅压电陶瓷的相结构与压电性能

钙锆共掺钛酸钡陶瓷(BCZT)具有优异的介电性能和压电性能, 是一类具有发展潜力的无铅压电陶瓷, 但其压电性能仍无法与铅基陶瓷媲美。为提高压电性能, 本研究对陶瓷材料进行Sn元素掺杂改性((Ba_0.85Ca_0.15)- (Ti_0.9Zr_0.1-xSn_x)O₃, x=0.02~0.07))。晶体结构分析证实所有组分的陶瓷无杂相, 处于正交相与四方相两相共存状态, 并具有较大的c/a; 显微结构分析发现所有陶瓷都很致密, 且平均晶粒尺寸随着Sn含量的增加而增大。当x=0.04时, 陶瓷最致密, 且室温处于准同型相界附近, 因此拥有最佳的电学性能: d₃₃=590 pC•N ^-1, k_p=52.2%, tanδ=0.016, ε ^T₃₃=5372, d ^*₃₃=734 pm•V ^-1, IR=57.8 GΩ•cm。本研究表明: Sn掺杂的BCZT基无铅压电陶瓷具有优异的压电性能, 有望在换能器、机电传感器和驱动器等方面得到应用。     

Low temperature sintering and properties of lead-free (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 ceramics with Ba(Cu0.5W0.5)O3 addition

The low-temperature sintering behavior of (Ba_0.85Ca_0.15)(Zr_0.10Ti_0.90)O₃ (BCZT) piezoelectric ceramics with Ba(Cu_0.5W_0.5)O₃ (BCW) addition has been investigated. The addition of 0.1 wt% BCW promotes the sinterability of BCZT ceramics owing to the generation of a liquid phase, resulting in a reduction of sintering temperature from 1,540 to 1,350 °C. The piezoelectric coefficient (d ₃₃), and the electromechanical coupling factor (kp) of the BCZT ? 0.1 wt%BCW specimen sintered at 1,350 °C were 555 pC/N and 55 %, respectively, while Curie temperature (Tc) increases from 85 to 95 °C.     

Orientation-dependent phase transition and dielectric properties of Ba0.85Ca0.15Ti0.9Zr0.1O3 thin films

Polycrystalline Ba_0.85Ca_0.15Ti_0.9Zr_0.1O₃ (BCZT) thin films with (100) preferential and random orientation are prepared on the LaNiO₃ electrodes coated on Si substrates by chemical solution deposition process. The results show that the two types of films stabilize in the pure perovskite structure. The ferro-paraelectric phase transitions of two types of films occur at the temperature ranges of 280.2–297.8 K and 278.4–287.6 K, respectively. The dielectric constant, relaxation time and tunability of the random orientation film are smaller than those of the (100) preferential film but both almost have the similar leakage current characteristics. The mechanism associated with the change of the electrical properties of the thin films is also discussed.     

Improved energy storage and electrocaloric properties of lead-free Ba0.85Ca0.15Zr0.1Ti0.9O3 ceramic

Lead-free Ba_0.85Ca_0.15Zr_0.1Ti_0.9O₃ (BCZT) ceramic powders were synthesized using the sol–gel method. The ceramics thickness was reduced to achieve high-energy storage and large electrocaloric effect in bulk ceramics. Dielectric, ferroelectric, energy storage, and electrocaloric properties were investigated for BCZT ceramic with 400 µm. Here, pure crystalline structure and homogenous microstructure were identified by XRD analysis and SEM measurements, respectively. The dielectric measurements revealed a maximum dielectric constant associated with ferroelectric–paraelectric phase transition. The maximum of \(\varepsilon^{\prime}_{{\text{r}}}\) was 17841, around 352 K. Furthermore, the BCZT ceramic exhibited improved energy storage and electrocaloric properties. A high recoverable energy density W_rec of 0.24 J/cm³ and a total energy density W_total of 0.27 J/cm³ with an efficiency coefficient of?~?88% at 423 K under an electric field of 55 kV/cm were obtained. Besides, The maximum value of ΔT?=?2.32 K, the electrocaloric responsivity ζ?=?0.42 K mm/kV, the refrigeration capacity RC?=?4.59 J/kg and the coefficient of performance COP?=?12.38 were achieved around 384 K under 55 kV/cm. The total energy density W_total and the temperature change ΔT were also calculated by exploiting the Landau–Ginzburg–Devonshire (LGD) theory. The theoretical results matched the experimental findings. These results suggest that the synthesized BCZT ceramic with reduced thickness could be a promising candidate for energy storage and electrocaloric applications.

     

Lead-free (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3-Y2O3 ceramics with large piezoelectric coefficient obtained by low-temperature sintering

Lead-free (Ba_0.85Ca_0.15)(Ti_0.9Zr_0.1)O₃-xwt.%Y₂O₃ (BCZT-xY) piezoelectric ceramics have been synthesized using solid-state reaction technique and the effects of Y₂O₃ addition on the phase structure and piezoelectric properties of the ceramics have been studied. The results reveal that the addition of Y₂O₃ significantly improves the sinterability of BCZT ceramics, resulting in a reduction of sintering temperature from 1,540 to 1,350 °C, and an increase of the Curie temperature T _C from 85 to 95 °C. X-ray diffraction data shows that Y₂O₃ diffuses into the lattice of BCZT-xY ceramics and a pure perovskite phase forms in the ceramics. Scanning electron microscopy images indicate that a small amount of Y₂O₃ addition affects the microstructure, obviously. Main piezoelectric parameters of these ceramics are optimized around x = 0.06 wt % with a large piezoelectric coefficient (d ₃₃  = 560 pC/N), a high planar electromechanical coefficient (k _p  = 53 %) and a low dissipation factor (tan δ = 0.9 %) at 1 kHz. The results indicate that the BCZT-xY ceramics are promising lead-free materials for practical applications.     

制备工艺对(Ba0.85Ca0.15)(Ti0.9Zr0.1)O3无铅压电陶瓷结构与性能的影响

采用液相混合与固相烧结相结合的方法制备了(Ba_0.85Ca_0.15)(Ti_0.9Zr_0.1)O₃ (BCTZ) 无铅压电陶瓷, 系统研究了烧结保温时间对其相结构、介电、压电和铁电性能的影响以及电学性能随温度的变化。研究结果表明: 制备的陶瓷样品具有单一的四方钙钛矿结构。当烧结温度为1540℃时, 随着保温时间的延长, 样品晶粒尺寸变大, 居里温度(T_c)升高, 压电性能提高, 电致伸缩性能下降。当保温时间为24 h时, BCTZ陶瓷综合性能最为优异: T_c ~90℃, tanδ < 0.05, k_p ~ 0.46, d₃₃ ~ 540 pC/N, P_s ~17 μC/cm²。陶瓷电学性能随温度变化测试结果又表明, BCTZ陶瓷的电学性能具有很强的温度依赖性, 随着温度的升高其电学性能逐渐下降。     

Fabrication of One-Transistor-Capacitor Structure of Nonvolatile TFT Ferroelectric RAM Devices Using Ba(Zr 0.1Ti 0.9)O 3 Gated Oxide Film

In this study, the Ba(Zr_0.1Ti_0.9)O₃ (BZ1T9) thin films have been well deposited on the Pt/Ti/SiO₂/Si substrate. The optimum radio frequency (RF) deposition parameters are developed, and the BZ1T9 thin films deposition at the optimum parameters have the maximum capacitance and dielectric constant of 4.4 nF and 190. As the applied voltage is increased to 8 V, the remnant polarization and coercive field of BZ1T9 thin films are about 4.5 muC/cm² and 80 kV/cm. The counterclockwise current hysteresis and memory window of n-channel thin-film transistor property are observed, and that can be used to indicate the switching of ferroelectric polarization of BZ1T9 thin films. One-transistor-capacitor (1TC) structure of BZ1T9 ferroelectric random access memory device using bottom-gate amorphous silicon thin-film transistor was desirable because of the smaller size and better sensitivity. The BZ1T9 ferroelectric RAM devices with channel width = 40 mum and channel length = 8 mum has been successfully fabricated and the I_D-V_G transfer characteristics also are investigated in this study.     

High-K (Ba0.8Bi0.2)(Zn0.1Ti0.9)O3 ceramics for high-temperature capacitor applications

Solid solutions of BaTiO(3)-Bi(Zn(1/2)Ti(1/2))O(3) were investigated for high-temperature capacitor applications. Compositions close to 0.8BaTiO(3)-0.2Bi(Zn(1/2)Ti(1/2))O(3) revealed pseudo-cubic symmetry and showed a linear dielectric response. The existence of a nearly flat temperature dependence of the relative permittivity over the temperature range of 100 to 350°C was also obtained. In this study, the effects of cation non-stoichiometry and doping were investigated in an attempt to optimize the insulation resistance for high-temperature applications. The dielectric response of (Ba(0.8)-xBi(0.2))(Zn(0.1)Ti(0.9)) O(3) ceramics where 0 ≤ X ≤ 0.08, as well as ZrO2- and Mn(2)O(3)-doped ceramics were studied. The optimum compositions exhibited a relative permittivity in excess of 1150 with a low loss tangent (tan δ < 0.05) that persisted up to a temperature of 460δC. The temperature dependence of resistivity also revealed the improved insulation resistance of Ba-deficient compositions. Additionally, we suggest that an ionic conduction mechanism is responsible for the degradation of resistivity at high temperatures. The temperature coefficient of permittivity ((τ)K) and the RC time constant were also investigated.     

Fabrication of 3-dimensional PbZr(1-x)Ti(x)O3 nanoscale thin film capacitors for high density ferroelectric random access memory devices

PbZr(x)Ti(1-x)O3 (PZT) thin films were deposited on 3-dimensional (3D) nano-scale trench structures for use in giga-bit density ferroelectric random access memories. PZT thin films were deposited by liquid delivery metalorganic chemical vapor deposition using Pb(thd)2, Zr(MMP)4, and Ti(MMP)4 precursors dissolved in ethyl cyclohexane. Iridium thin films were deposited by atomic layer deposition, and they exhibited excellent properties for capacitor electrodes even at a thickness of 20 nm. The trench capacitor was composed of three layers, viz. Ir/PZT/lr (20/60/20 nm), and had a diameter of 250 nm and a height of 400 nm. Almost 100% step coverage was obtained at a deposition temperature of 530 degrees C. The PZT thin film capacitors with a thickness of 60 nm on a planar structure exhibited a remnant polarization (Pr) of 28 microC/cm2, but the Pr value of the 3D PZT capacitors decreased slightly with decreasing 3D trench pattern size. The transmission electron microscope analysis indicated that the PZT thin films had compositional uniformity in the 3D trench region. Both columnar and granular grains were formed on the sidewalls of the trench capacitors, and their relative proportion exhibited strong size dependence. The trench capacitors with more columnar PZT grains showed good switching behavior under an external bias of 2.1 V and had a remnant polarization of 19-24 microC/cm2.     

The influences of particle sizes on the dielectric properties of PEI/(Ba0.8Sr0.2)(Ti0.9Zr0.1)O3 composites

In order to investigate the embedded capacitors with higher capacitance value in the future, two technologies of increasing the dielectric constant of filler and decreasing the thickness of capacitors can be combined to develop a best method. For that, the high dielectric constant of (Ba0.8Sr0.2)(Ti0.9Zr0.1)O3 (BSTZ) ceramic powders with three different sizes (3 microm, 200 nm, and 100 nm) are used as the filler to mix with polyetherimide (PEI) to form the PEI/BSTZ composites. The dielectric constants and loss tangents of PEI/BSTZ composites with different contents of BSTZ ceramic powders are measured using the plate method. As the contents of BSTZ powders increase from 10 wt% to 70 wt%, the dielectric constants and loss tangents of PEI/BSTZ composites increase with the increase content of BSTZ ceramic powders, independent on the particle sizes. As the BSTZ content is less than and equal to 50 wt% and same loading ratio is used, the dielectric constants of PEI/200 nm-BSTZ and PEI/100 nm-BSTZ composites are large than those of PEI/3 microm-BSTZ composites. In this study, the Lichtenecker logarithmic mixing rule is also used to fit the measured results and predict the dielectric constants of PEI/BSTZ composites.     

Tailoring the multiferroic properties in Ba0.85Ca0.15Zr0.1Ti0.9O3/La0.7Sr0.3MnO3 bilayer heterostructures using residual strain

Ba_0.85Ca_0.15Zr_0.1Ti_0.9O₃/La_0.7Sr_0.3MnO₃ (BCZT/LSMO) bilayer multiferroic heterostructures with different BCZT layer thickness are fabricated by pulsed laser deposition technique. Structural characterization of XRD and TEM reveals the epitaxial growth of the bilayer heterostructures, whose residual strain reduces with increasing thickness. The room temperature multiferroic characteristics of the bilayer heterostructures are demonstrated by the simultaneously observed ferroelectric and ferromagnetic properties as well as the magnetoelectric effect. Due to the varying residual strain, the multiferroic properties of the bilayer heterostructures show strong dependence on the BCZT layer thickness, which significantly improves with increasing thickness. The largest magnetoelectric coefficient with α_E31 value of 355.2 mV/cm·Oe is achieved in BCZT/LSMO bilayer heterostructures with BCZT layer thickness of 150 nm.

     

Effects of La3+ addition on the phase transition,microstructure, dielectric and piezoelectric properties of Ba0.9Ca0.1Ti0.9Zr0.1O3 ceramics prepared by hydrothermal method

Lead-free Ba_0.9Ca_0.1Ti_0.9Zr_0.1O₃–xLa (x = 0.000, 0.005, 0.010, 0.015, 0.020, 0.025) ceramics were prepared by hydrothermal method and were assisted by fast microwave sintering. The effects of La³⁺ addition on the phase transition, microstructure, dielectric and piezoelectric properties were investigated. Single orthorhombic phase was observed in the composition of x ≤ 0.010 and the coexistence of orthorhombic and tetragonal phase were detected at x = 0.015 and 0.020, which is characterized by the broadening of (002)/(200) peaks at around 2θ ~ 45°. The ceramics show cubic phase when the x was increased to 0.025. The grain size was affected significantly by the La³⁺ content, reaching a maximum value of ~8.2 μm at x = 0.010. The dielectric constant decreases with the addition of small amount of La³⁺ and then increases sharply at x ≥ 0.015, exhibiting a PTC behavior at x = 0.020, while no T _C was observed at x = 0.025. The Curie temperature (T _C ) always decrease with the introduction of La³⁺ and the dielectric loss shows a minimum value at x = 0.015. The piezoelectric constant (d ₃₃ ) measured at 50 °C of the ceramics were increased after the partial substitution of La³⁺ because of the enhancement of coexistence of orthorhombic and tetragonal phases at this temperature. Ferroelectric and piezoelectric properties can not be detected when x was increased to 0.02 while they were observed again with further addition.