Influences of poling condition and sintering temperature on piezoelectric properties of (Na0.5Bi0.5)1 − xBaxTiO3 ceramics

The structure, ferroelectric characteristics and piezoelectric properties of (Na_0.5Bi_0.5)_1 − xBa_xTiO₃ (x = 0.04, 0.06, 0.10) ceramics prepared by conventional solid state method were investigated. The influences of poling condition and sintering temperature on the piezoelectric properties of the ceramics were examined. The piezoelectric properties of the ceramics highly depend on poling field and temperature, while no remarkable effect of poling time on the piezoelectric properties was found in the range of 5-25 min. Compared with (Na_0.5Bi_0.5)_0.96Ba_0.04TiO₃ and (Na_0.5Bi_0.5)_0.90Ba_0.10TiO₃, the piezoelectric properties of (Na_0.5Bi_0.5)_0.94Ba_0.06TiO₃ are more sensitive to poling temperature due to the relatively low depolarization temperature. Moderate increase of sintering temperature improved the poling process and piezoelectric properties due to the development of microstructural densification and crystal structure. With respect to sintering behavior and piezoelectric properties, a sintering temperature range of 1130-1160 °C was ascertained for (Na_0.5Bi_0.5)_0.90Ba_0.10TiO₃.     

Microwave dielectric properties of Ba2 − xSrxLa3Ti3NbO15 ceramics

High dielectric constant and low loss ceramics in the system Ba_2 − xSr_xLa₃Ti₃NbO₁₅ (x = 0-1) have been prepared by conventional solid-state ceramic route. Ba_2 − xSr_xLa₃Ti₃NbO₁₅ solid solutions adopted A₅B₄O₁₅ cation-deficient hexagonal perovskite structure for all compositions. The materials were characterized at microwave frequencies. They show a linear variation of dielectric properties with the value of x. Their dielectric constant varies from 48.34 to 43.03, quality factor Q_u × f from 20,291 to 39,088 GHz and temperature variation of resonant frequency from 8 to 1.39 ppm/°C as the value of x increases. These low loss ceramics might be used for dielectric resonator (DR) applications.     

Synthesis, low-temperature sintering and the dielectric properties of the ZnO-(1 − x)TiO2-xSnO2 (x = 0.04-0.2)

ZnO-(1 − x)TiO₂-xSnO₂ (x = 0.04-0.2) ceramics were prepared by conventional mixed-oxide method combined with a chemical processing. Fine particle powders were prepared by chemical processing to activate the formation of compound and to improve the sinterability. One wt.% of V₂O₅ and B₂O₃ with the mole ratios of 3:1 were used to lower the sintering temperature of ceramics. The effect of Sn content on phase structure and dielectric properties were investigated. The results show that the substituting Sn for Ti accelerates the hexagonal phase transition to cubic phase, and an inverse spinel structure Zn₂(Ti_1−xSn_x)O₄ solid solution forms. The best dielectric properties obtained at x = 0.12. The ZnO-0.88TiO₂-0.12SnO₂ ceramics sintered at 900 °C exhibit a good dielectric property: ?_r = 29 and tan δ = 9.86 × 10⁻⁵. Due to their good dielectric properties, low firing characteristics, ZnO-(1 − x)TiO₂-xSnO₂ (x = 0.04-0.2) can serve as the promising microwave dielectric capacitor.     

Preparation and characterization of Ba1 − xSrxTiO3 thin films deposited on Pt/SiO2/Si by sol-gel method

BST thin films have been investigated as potential candidates for use in frequency agile microwave circuit devices. Stoichiometric (Ba_1 − xSr_x)TiO₃ (BST) thin films have been prepared on Pt/SiO₂/Si substrates using sol-gel method. The BST films were characterized by X-ray fluorescence (XRF) spectroscopy analysis, X-ray diffraction (XRD), scanning electron microscope (SEM) and electrical measurements. The relationships of processing parameters, microstructures, and dielectric properties are discussed. The results show that the films exhibit pure perovskite phase through rapid thermal anneal at 700 °C and their grain sizes are about 20-40 nm. The dielectric constants of BST5, BST10, BST15 and BST20 are 323, 355, 382 and 405, respectively, at 80 kHz.     

Basic properties of Sr1 − xBaxSi2

Basic properties, such as the phase relationship, crystal structure, and energy gap E_g, have been investigated in Sr-rich Sr_1 − xBa_xSi₂. Sr_1 − xBa_xSi₂ (0 ≤ x ≤ 1.0) has two phases: one with the SrSi₂-type structure and another with the BaSi₂-type structure. The SrSi₂ phase exists at x ranging from 0 to 0.13, and the BaSi₂ phase exists at x ranging from 0.24 to 1.0. The volume increases with x in both the SrSi₂ and BaSi₂ phases. A volume jump of 13.7% appears at the structural phase transition from the SrSi₂ phase to the BaSi₂ phase. E_g increases with x in SrSi₂-phase Sr_1 − xBa_xSi₂ but E_g decreases with x in the BaSi₂-phase Sr_1 − xBa_xSi₂. In Sr-rich BaSi₂-phase Sr_1 − xBa_xSi₂, Ba atoms at a specific crystallographic site, the A1 site, are preferentially substituted by Sr atoms, as well as in Ba-rich BaSi₂-phase Sr_1 − xBa_xSi₂.     

Synthesis and luminescence properties of color-tunable Ba1−ySryLa4−xTbx(WO4)7 (x = 0.02-1.2, y = 0-0.4) phosphors

Ba_1−ySr_yLa_4−xTb_x(WO₄)₇ (x = 0.02-1.2, y = 0-0.4) phosphors were prepared via a solid-state reaction and their photoluminescence properties were investigated. An analysis of the decay behavior indicates that the energy migration between Tb³⁺ ions is conspicuous in the ⁵D₃ → ⁷F₄ transition due to the cross-relaxation in BaLa₄(WO₄)₇. A partial substitution of Ba²⁺ by Sr²⁺ can not only enhance the emission intensity but also increase the solid solubility of Tb³⁺ in Ba_1−ySr_yLa_4−xTb_x(WO₄)₇. The emission intensity of the ⁵D₄ → ⁷F_J (J = 4, 5, 6) transitions can be enhanced by increasing Sr²⁺ and Tb³⁺ concentrations, with the optimal conditions being x = 1.2, y = 0.4 (Ba_0.6Sr_0.4La_2.8Tb_1.2(WO₄)₇). Under near-UV excitation at 379 nm, the CIE color coordinates of Ba_1−ySr_yLa_4−xTb_x(WO₄)₇ vary from blue (0.212, 0.181) at x = 0.04, y = 0, to green (0.245, 0.607) at x = 1.2, y = 0.4.     

Large area Ba1 − xSrxTiO3 thin films for microwave applications deposited by pulsed laser ablation

Large area Ba_1 − xSr_xTiO₃ (BST) thin films with x = 0.4 or x = 0.5 were deposited on 75 mm diameter Si wafers in a pulsed laser deposition (PLD) chamber enabling full-wafer device fabrication using standard lithography. The deposition conditions were re-optimized for large PLD chambers to obtain uniform film thickness, grain size, crystal structure, orientation, and dielectric properties of BST films. X-ray diffraction and microstructural analyses on the BST films grown on Pt/Au/Ti electrodes deposited on SiO₂/Si wafers revealed films with (110) preferred orientation with a grain size < 100 nm. An area map of the thickness and crystal orientation of a BST film deposited on SiO₂/Si wafer also showed (110) preferred orientation with a film thickness variation < 6%. Large area BST films were found to have a high dielectric tunability of 76% at an electric field of 400 kV/cm and dielectric loss tangent below 0.03 at microwave frequencies up to 20 GHz and a commutation quality factor of ~ 4200.     

Synthesis of (1 − x)Ca2/5Sm2/5TiO3-xLi1/2Nd1/2TiO3 ceramic powder via ethylenediaminetetraacetic acid precursor

(1 − x)Ca_2/5Sm_2/5TiO₃-xLi_1/2Nd_1/2TiO₃ (CSLNT) ceramic powder was prepared by a liquid mixing method using ethylenediaminetetraacetic acid (EDTA) as the chelating agent. TG, DTA, XRD and TEM characterized the precursors and derived oxide powders. When x = 0.3, perovskite CSLNT was synthesized at 1000 °C for 3 h in air. The CSLNT (x = 0.3) ceramics sintered at 1200 °C for 3 h show excellent microwave dielectric properties of ?_r = 99, Q_f = 6200 GHz and τ_f = 9 × 10⁻⁶ °C⁻¹.     

Dielectric and tunable properties of Pb0.25BaxSr0.75−xTiO3 thin films fabricated by a modified sol-gel method

A modified sol-gel method was used to fabricate (Pb_0.25Ba_x Sr_0.75−x)TiO₃ (PBST) thin films with x = 0.05,0.1,0.15 and 0.2 on Pt/TiO₂/SiO₂/Si substrate. The structure, surface morphology, dielectric and tunable properties of PBST thin films were investigated as a function of barium content (x). X-ray diffraction and scanning electron microscopy analysis showed that we could get pure PBST perovskite phase and relative fine density thin films with smooth surface. It was found that the crystal lattice constant, grain size, room temperature dielectric constant, dielectric loss and tunability of Ba solutionizing PST thin films increased with the increase in Ba content. For (Pb_0.25Ba_0.2Sr_0.55)TiO₃ thin film, it had the highest dielectric constant of 1390 and the largest tunability of 80.6%. The figure of merit parameter reached a maximal value of 28.9 corresponding to the (Pb_0.25Ba_0.05 Sr_0.7)TiO₃ thin film, whose dielectric constant, dielectric loss and tunability measured at 1 MHz were 627, 0.024 and 69.4%, respectively.     

Synthesis of perovskite-type (La1−xCax)FeO3 (0 ≤ x ≤ 0.2) at low temperature

Gel formation was realized by adding citric acid to a solution of La(NO₃)₃·5H₂O, Ca(NO₃)₂·4H₂O, and Fe(NO₃)₂·9H₂O. Perovskite-type (La_1−xCa_x)FeO₃ (0 ≤ x ≤ 0.2) was synthesized by firing the gel at 500 °C in air for 1 h. The crystallite size (D_1 2 1) decreased with increasing x, while the specific surface area was 6.8-9.4 m²/g and independent of x. The XPS measurement of the (La_1−xCa_x)FeO₃ surface indicated that the Ca²⁺ ion content increased with increasing x, while the Fe ion content was independent of x. Catalytic activity for CO oxidation increased with increasing x.     

Structure and negative thermal expansion of Pb1 −xBixTiO3

Pb_1-xBi_xTiO₃ (x = 0.0-0.1) compounds were prepared to study the unique dopant effect of bismuth in PbTiO₃. Their thermal expansions and structures were investigated by high-temperature X-ray diffraction and X-ray Rietveld method. The results indicated that Bismuth substitution evidently weakened the tetragonality of PbTiO₃ solid solution, but increased the spontaneous polarization. Both the enhanced spontaneous polarization and the decreased tetragonality led to small volume shrinkage with temperature rising, where the average volumetric thermal expansion coefficient changed from − 1.99 × 10^− 5/°C for pure PbTiO₃ to − 0.56 × 10^− 5/°C for Pb_0.90Bi_0.10TiO₃. The Curie point of Pb_1 − xBi_xTiO₃ was slightly raised compared to PbTiO₃ and permitted one to use it in a wide temperature range.     

BaxSr1 − xTi1.02O3 metal-insulator-metal capacitors on planarized alumina substrates

Nanocrystalline barium strontium titanate (Ba_xSr_1 − xTi_1.02O₃) thin films with a barium content of x = 0.8, 0.9 and 1 have been fabricated in a metal-insulator-metal configuration on glass-planarized alumina substrates. Cost-effective processing measures have been utilized by using poly-crystalline alumina substrates, wet-chemical processing of the dielectric, and by a small physical area of the ferroelectric capacitors (as low as 50 µm² for radio frequencies measurements). Glass-planarization on alumina ceramic substrates enables barium strontium titanate films with high quality and homogeneity. We mainly focus on fine-tuning the electrical performance in the low gigahertz range (< 10 GHz). Extensive micro-structural and electrical characterization has been performed. Micro-structural information is obtained by: Transmission Electron Microscopy, Scanning Electron Microscopy and X-ray diffraction. The dielectric response is investigated as a function of temperature, frequency and electric field for each sample. We measured a relatively constant permittivity for typical operating temperatures of applications. The quality factor Q is between 21 and 27 at 1 GHz at zero DC bias and the tuning ratio η between 1.8 and 2.2 at |E| = 0.4 MV/cm.     

Ferroelectric and dielectric properties of sol-gel derived BaxSr1−xTiO3 thin films

Ferroelectric thin films of Ba_0.7Sr_0.3TiO₃, Ba_0.8Sr_0.2TiO₃, Ba_0.9Sr_0.1TiO₃ and BaTiO₃ were fabricated by a modified sol-gel technique on Pt/Ti/SiO₂/Si substrates. All the compositions crystallized in perovskite structure and consist of well-defined grains. As the value of x increases grain size of the Ba_xSr_1−xTiO₃ thin films increases while the dielectric permittivity decreases. Ba_0.7Sr_0.3TiO₃ composition possesses the highest dielectric permittivity of 748 (at 100 kHz). Hysteresis loops measured at room temperature for all compositions showed that BaTiO₃ has the largest remnant polarization of 4.8 μC/cm². The dielectric and ferroelectric properties of the sol-gel derived Ba_xSr_1−xTiO₃ thin films are strongly dependent on the Sr content and the grain size.     

Properties of the PLZTN x/54/46 (0.4 ≤ x ≤ 1.4) ceramic system

Even though the PZT ceramic system has been thoroughly studied and modified with different additives, no numerous reports have been published focusing on the PZT double ‘soft’ modification with La³⁺ and Nb⁵⁺ (PLZTN). In this paper, we explore the structural, morphological, dielectric, ferroelectric and piezoelectric properties of the PLZTN system for different doping levels (x_La3+ = x_Nb5+ = 0.4, 0.6, 0.8, 1.0 and 1.4 at.%) synthesized by conventional powders reaction. The temperature dependence of the piezoelectric response of poled ceramic disks is also analyzed and, according to its overall features, an optimum modification is proposed in order to develop PZT-based sensors for several applications.     

KxNa1 − xNbO3 powder synthesized by molten-salt process

K_xNa_1 − xNbO₃ ceramic powders have been successfully synthesized in different salts (NaCl, KCl, NaCl-KCl). Our results reveal that K_xNa_1 − xNbO₃ powders with single-phase perovskite structure can be formed at a low temperature such as 750 °C. The type of salts has significant effects on the morphology and chemical composition of the powders. As Na⁺ has a higher diffusing rate and occupies the A-site in the perovskite structure more easily as compared to K⁺, the powder contains only a small amount of K⁺ (x ∼ 0.10) when it is synthesized according to formula K_0.5Na_0.5NbO₃ and in a flux containing the same molar content of Na⁺ and K⁺. By using a NaCl or KCl salt, the K⁺ concentration x can be adjusted to almost 0 and 0.77, respectively.     

Sintering behavior, microstructure and microwave dielectric properties of Li2+xTiO3 (0 ≤ x ≤ 0.2)

Sintering behavior, microstructure and microwave dielectric properties of Li_2+xTiO₃ (0 ≤ x ≤ 0.2) ceramics have been studied by X-ray diffraction (XRD), scan electron microscopy (SEM), Raman spectra, dilatometery and microwave resonant measurement in this research. Homogeneous non-stoichiometric composition with rock salt structure existed for Li_2+xTiO₃ (0 ≤ x ≤ 0.2) ceramics. The sintering temperature was successfully reduced and highly densified sample could be obtained with appropriate excessive amount of lithium (x = 0.08). A transient reactive liquid phase sintering mechanism was proposed. The preferred orientation of grain growth and micro-cracks existed in the Li₂TiO₃ (x = 0) sample disappeared in the lithium excessive samples with x ≥ 0.08. The microwave dielectric properties varied significantly with the excessive amount of lithium. Optimized microwave dielectric properties were obtained for the x = 0.08 composition: ?_r = 24.6, Q × f = 66,000 GHz, and τ_f = 22.1 ppm/°C.     

Magnetocaloric properties in Ln0.67Ba0.33Mn1 − xFexO3 (Ln = La or Pr) manganites

We have investigated the magnetocaloric properties of Ln_0.67Ba_0.33Mn_1 − xFe_xO₃ (Ln = La or Pr) manganites with x = 0 and 0.05. All compounds present a maximum and large magnetocaloric effect near the Curie temperature (T_C). The associated maximum value of the magnetic entropy change, at 5 T applied change in the magnetic field, is 4.37 J.kg^− 1.K^− 1 for Pr_0.67Ba_0.33MnO₃ manganite with a T_C value of 205 K. The corresponding relative cooling power (RCP) reaches 230 J.kg^− 1. All the samples present similar RCP values that are relatively high and are promising materials to be used in ecologically friendly magnetic refrigeration technology. Iron doping reduces both T_C and ΔS_M^max and spreads the temperature working range with an almost constant RCP and can then be used to tune the working conditions of a refrigerator device.     

Morphologies and crystal structures of nano-sized Ba1−xSrxTiO3 primary particles prepared by flame spray pyrolysis

Nano-sized Ba_1−xSr_xTiO₃ (BST) powder was prepared by flame spray pyrolysis using “CA-assisted” spray solution. The effects of the mole ratios of Ba to Sr components on the mean sizes, morphologies, and crystal structures of the BST powder prepared by flame spray pyrolysis were investigated. The precursor powders obtained by flame spray pyrolysis had large size, fractured and hollow structures irrespective of the mole ratios of Ba to Sr components. The post-treated BST powders had slightly aggregated morphology of the primary particles with nanometer sizes. The slightly aggregated BST powders turned to nano-sized primary particles by a simple milling process. The milled BaTiO₃ particles post-treated at temperature of 1000 °C had spherical-like shape. On the other hand, the milled Ba_0.5Sr_0.5TiO₃ and SrTiO₃ particles had square shape. The mean sizes of the milled BaTiO₃, Ba_0.5Sr_0.5TiO₃ and SrTiO₃ particles were each 110, 32, and 48 nm. Phase pure BST powder was obtained at a post-treatment temperature of 1000 °C irrespective of the mole ratios of Ba to Sr components. The BaTiO₃ powder had tetragonal crystal structure. On the other hand, the BST except for the BaTiO₃ composition had cubic crystal structures at post-treatment temperature of 1000 °C. The mean crystallite sizes of the milled Ba_1−xSr_xTiO₃ primary particles were changed from 29 to 37 nm according to the mole ratios of Ba to Sr components.     

Oxide-ion conductivity in CuxCe1−xO2−δ (0 ≤ x ≤ 0.10)

Up to 10 at.% of copper readily substitutes for cerium in ceria. It is found that at oxygen partial pressures between 0.21 atm and 10⁻⁵ atm, Cu_xCe_1−xO_2−δ (0 ≤ x ≤ 0.10) solid solution behave as an oxide-ion electrolyte. Interestingly, Cu_0.10Ce_0.90O_2−δ exhibits the oxide-ion conductivity of ca. 10⁻⁴ Ω⁻¹ cm⁻¹ at 600 °C at an oxygen partial pressure of 10⁻⁵ atm.     

Oxygen vacancy ordering in strontium doped rare earth cobaltate perovskites Ln1−xSrxCoO3−δ (Ln = La, Pr and Nd; x > 0.60)

A family of Sr-doped perovskite compounds Ln_1−xSr_xCoO_3−δ (Ln = La³⁺, Pr³⁺ and Nd³⁺; x > 0.60), were prepared by sol-gel chemistry and reaction at 1100 °C under 1 atm of oxygen. This structural family has been shown to be present only for rare earth ions larger than Sm³⁺ and an upper limit of Sr²⁺ solubility in these phases was found to exist between x = 0.90 and 0.95. X-ray diffraction shows oxygen-deficient, simple cubic (Pm-3m) perovskite crystal structures. The combination of electron and powder neutron diffraction reveals that oxygen vacancy ordering occurs, leading to a tetragonal (P4/mmm) superstructure and a doubling of the basic perovskite unit along the crystallographic c-axis. No additional Ln³⁺/Sr²⁺ cation ordering was observed.