Dielectric properties of Ba(Ti1 − xZrx)O3 solid solutions

This paper reports the structural and dielectric properties of Ba(Ti_1 − xZr_x)O₃ (x = 0-0.3) ceramics. Single-phase solid solutions of the samples were determined by X-ray diffraction. Microscopic observation by scanning electron microscope revealed dense, single-phase microstructure with large grains (20-60 μm). The evolution of dielectric behavior from a sharp ferroelectric peak (for x ≤ 0.08) to a round dielectric peak (for 0.15 ≤ x ≤ 0.25) with pinched phase transitions and successively to a ferroelectric relaxor (for x = 0.3) was observed with increasing Zr concentration. Compared with pure BaTiO₃, broaden dielectric peaks with high dielectric constant of 25,000-40,000 and reasonably low loss (tanδ: 0.01-0.06) in the Ba(Ti_1 − xZr_x)O₃ ceramics have been observed, indicating great application potential as a dielectric material.     

Phase relations and microwave dielectric properties of vanadium modified Ba5Nb4O15 ceramics

Microwave dielectric ceramics of Ba₅Nb_4−xV_xO₁₅ (x = 0-1) were prepared by a solid-state reaction method. Vanadium substitution can markedly lower the sintering temperature of Ba₅Nb₄O₁₅ from 1450 to 1100 °C. The X-ray powder diffraction analysis reveals the multiphase nature of this system. A hexagonal-to-orthorhombic phase transition was also observed for the BaNb₂O₆ secondary phase. The microwave dielectric properties, such as τ_f, ε_r and Q × f value, decreased with increasing vanadium content for samples sintered at 1100 °C. There was an apparent increase in τ_f and Q × f value for samples (x ≥ 0.5) sintered at 1200 °C due to the hexagonal-to-orthorhombic phase transition of the BaNb₂O₆ phase. These results suggested that the microwave dielectric properties of multiphase ceramics strongly depended on the phase compositions and the phase transitions.     

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.     

Structural, magnetic and dielectric properties of Bi5−xLaxTi3Co0.5Fe0.5O15 ceramics

The Bi_5−xLa_xTi₃Co_0.5Fe_0.5O₁₅ (0 ≤ x ≤ 0.4) ceramics were successfully synthesized by a modified Pechini process. The samples were characterized by X-ray diffraction and no impurity phase has been detected. The cell volume of the composites increases monotonously with the increase of La content, which indicates that La ions have been incorporated into the lattice of Bi₅Ti₃Co_0.5Fe_0.5O₁₅. The magnetic measurements show that La doping on Bi sites has enhanced the magnetization of Bi_5−xLa_xTi₃Co_0.5Fe_0.5O₁₅ (0 ≤ x ≤ 0.4). Both the dielectric constants and loss tangent of all the samples decrease on increasing frequency and then become almost constant at room temperature. The La doped Bi₅Ti₃Co_0.5Fe_0.5O₁₅ samples exhibit improved dielectric and ferroelectric properties, with higher dielectric constant enhanced remnant polarization and lower losses at room temperature.     

High piezoelectric d33 coefficient in (Ba1 − xCax)(Ti0.98Zr0.02)O3 lead-free ceramics with relative high Curie temperature

Lead-free (Ba_1 − xCa_x)(Ti_0.98Zr_0.02)O₃ (x = 0-0.04) ceramics were prepared successfully using a solid-state reaction technique. The polymorphic phase transitions (PPT) from orthorhombic to tetragonal phase around room temperature were identified in the composition range of 0 < x < 0.03. High piezoelectric coefficient of d₃₃ = 375 pC/N and planar electromechanical coupling factor of k_p = 44.1% were obtained for the samples at x = 0.01. With the increase of Ca content, the orthorhombic-tetragonal phase transitions shifted towards room temperature, while relative high Curie temperature (T_C) was still maintained about 115 °C.     

Microwave dielectric properties of Mg1.8Ti1.1O4 ceramics

Low-loss Mg_1.8Ti_1.1O₄ ceramics were prepared by the conventional solid-state route and their microwave dielectric properties were investigated for the first time. The forming of tetragonal-structured Mg_1.8Ti_1.1O₄ main phase associated with a second phase MgTiO₃ were confirmed by the X-ray diffraction patterns. However, the presence of the second phase would cause no significant variance in the dielectric properties of the specimen because the second phase properties are very similar to that of the main phase. A fine combination of microwave dielectric properties (?_r ∼ 15.74, Q × f ∼ 141,000 GHz at 10.57 GHz, τ_f ∼ − 52.4 ppm/°C) was achieved for Mg_1.8Ti_1.1O₄ ceramics sintered at 1450 °C for 4 h.     

Microwave driven hydrothermal synthesis of Ba1−xSrxTiO3 nanoparticles

Ba_1−xSr_xTiO₃ (BST) nanopowders with composition x = 0.1-0.4 have been prepared using microwave driven hydrothermal synthesis (MDHS). A low temperature process has been chosen in order to avoid high temperature heat treatment leading to particle growth and agglomeration. MDHS method allows obtaining the nanocrystalline powder samples during relatively short period of time (10 min) and therefore MDHS was optimized. In case of the phase evolution studies the XRD measurements were performed. The average size of crystallites was estimated using Scherrer equation. TEM and SEM images were taken for the detailed analysis of the grain size, surface and morphology.     

Temperature stable high-Q microwave dielectric ceramics in (1 − x)BaTi4O9-xBaZn2Ti4O11 system

The microwave dielectric properties of (1 − x)BaTi₄O₉-xBaZn₂Ti₄O₁₁ ceramics were investigated by solid-state reaction technique for obtaining high-Q dielectric ceramics in BaO-ZnO-TiO₂ system. And they were strongly determined by the chemical composition. As x was increased from 0.05 to 0.50, BaZn₂Ti₄O₁₁ phase formed more and more. Therefore, the ε_r decreased from 37.3 to 32.8 and the Q × f values first raised from 45,300 GHz to 60,600 GHz (x = 0.30) and then started to decline to 58,700 GHz (x = 0.40), and the τ_f values varied gradually from 12 ppm/°C to − 13 ppm/°C. 0.7BaTi₄O₉-0.3BaZn₂Ti₄O₁₁ ceramics sintered at 1240 °C for 3 h had excellent comprehensive microwave dielectric properties: ε_r = 34.2, Q × f = 60,600 GHz and τ_f = − 2 ppm/°C.     

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.     

Low loss Sr1−xCaxLa4Ti5O17 microwave dielectric ceramics

Microwave dielectric ceramics in the Sr_1−xCa_xLa₄Ti₅O₁₇ (0 ≤ x ≤ 1) composition series were prepared through a solid state mixed oxide route. All the compositions formed single phase ceramics within the detection limit of in-house X-ray diffraction when sintered in the temperature range 1450-1580 °C. Theoretical density and molar volume decreased due to the substitution of Ca²⁺ for Sr²⁺ which was associated with a decrease in the dielectric constant (?_r) and temperature coefficient of resonant frequency (τ_f) but an increase in quality factor, Qf_o. Optimum properties were achieved for Sr_0.4Ca_0.6La₄Ti₅O₁₇ which exhibited, ?_r ∼ 53.7, Qf_o ∼ 11,532 GHz and τ_f ∼ −1.4 ppm/°C.     

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.     

Preparation and characterization of La-doped Ba(1 − x)SrxTiO3 powders and thin films

Ba_(1 − x)Sr_xTiO₃ powders with different Ba/Sr ratios (x = 0.10, 0.25, 0.40, 0.55, 0.70) and La-doped Ba_0.9Sr_0.1TiO₃·yLa powders (y = 0.002, 0.004, 0.006, 0.008, 0.010) have been prepared by sol-gel technology using dehydrated barium-acetate, strontium-carbonate, lanthanum-nitrate, and titanium-isopropoxide as raw materials. The experimental results show that the dielectric properties of Ba_(1 − x)Sr_xTiO₃ powders depend on the Ba/Sr ratios. When the Sr fraction is 0.10, the dielectric constant is relatively higher and the dielectric loss is relatively lower, which are more than 2000 and less than 2.0 × 10^− 2 at 1000 Hz, respectively, the most important is that this kind of powder has better frequency stability. La-doping can increase the dielectric constant distinctly, but the dielectric loss can also be increased. Their dielectric properties at 1.0 × 10³ Hz are better than those at 1.0 × 10⁵ Hz. At 1.0 × 10³ Hz the dielectric constant is much higher, while the dielectric loss is much lower. The dielectric constant of different La-doping contents is nearly 3.5 × 10⁴ and the dielectric loss is less than 0.20 when La fraction is 0.008. The La-doped BST sample also has better frequency stability, especially at high frequency. La-doped BST thin films are successfully deposited on mild steel substrates by using plasma spray system with suspension precursors of Ba_0.90Sr_0.10TiO₃·0.8La powders. The XRD patterns of Ba_0.90Sr_0.10TiO₃ and Ba_0.90Sr_0.10TiO₃·0.8La powders are almost the same. No new peaks appear after La-doping, but the peaks move slightly to a larger degree, which indicates that the element La has entered the lattice of the Ba_0.90Sr_0.10TiO₃ and has made the constant of the crystal cell reduce. The XRD pattern of the thin films is just like that of the Ba_0.90Sr_0.10TiO₃·0.8La powders except a peak corresponding to Fe substrate. The SEM results show that the thin films have a uniform and smooth surface. The morphology of cross-section shows a columnar grain structure indicating smooth surface and uniform thickness of the film. The thickness of the film is about 15 um. The thin films obtained are expected to be prospective material for applications in tunable microwave devices.     

Phase evolution and microwave dielectric characteristics of Ti-substituted Mg2SiO4 forsterite ceramics

In the present paper, the phase evolution and microstructures of Ti-substituted Mg₂SiO₄ forsterite ceramics with nominal composition Mg₂(Si_1 − xTi_x)O₄ were investigated together with their microwave dielectric characteristics. MgSiO₃ secondary phase was observed in Mg₂SiO₄ ceramics, and it could be suppressed by Ti-substitution. However, Mg₂TiO₄ and MgTi₂O₅ appeared gradually with increasing Ti-substitution amount. The dielectric constant slightly increased from 6.8 to 8.1 with Ti-substitution, and the Qf value was improved significantly and reached the maximum at x = 0.1 where the optimum combination of microwave dielectric characteristics were achieved: ε_r = 7.4, Qf = 73,760 GHz at 15 GHz, τ_f = − 60 ppm/°C.     

Magnetic and electrical properties of Ba2CrMo1 − xWxO6 double perovskite

Effects of the partial substitution of W^5+,6+ for Mo^5+,6+ on the structural and physical properties of Ba₂CrMoO₆ have been investigated. Polycrystalline Ba₂CrMo_1 − xW_xO₆samples have been prepared by sol-gel method in a stream of 5% Ar/H₂ gas at various sintering temperatures. Rietveld analysis of X-ray diffraction patterns shows a partial disorder of Mo/W and Cr on the B sites of the double perovskite, which plays a dominant role in the structural and magnetic properties of these compounds. The symmetry is cubic (Fm3?m) for all samples, and no phase transition was detected for Ba₂CrMo_1 − xW_xO₆. The Curie temperature T_C has been analyzed by two methods: a linear extrapolation of M(T) to zero magnetization and the thermodynamic model. The experimental results indicate that T_C decrease from 335 K (x = 0) to 285 K (x = 0.5) with increasing W substitution independently of the method used to obtain T_C. A systematic decrease in saturation magnetization, M_s with increasing W substitution has been observed in this solid solution series. This decrease of magnetization arises from the disorder at the Cr and Mo/W sites. Electrical properties change as well strongly along the series.     

Ba3ZnTa2−xNbxO9 and Ba3MgTa2−xNbxO9 (0≤x≤1): synthesis, structure and dielectric properties

Oxides belonging to the families Ba₃ZnTa_2−xNb_xO₉ and Ba₃MgTa_2−xNb_xO₉ were synthesized by the solid state reaction route. Sintering temperatures of 1300°C led to oxides with disordered (cubic) perovskite structure. However, on sintering at 1425°C hexagonally ordered structures were obtained for Ba₃MgTa_2−xNb_xO₉ over the entire range (0≤x≤1) of composition, while for Ba₃ZnTa_2−xNb_xO₉ the ordered structure exists in a limited range (0≤x≤0.5). The dielectric constant is close to 30 for the Ba₃ZnTa_2−xNb_xO₉ family of oxides while the Mg analogues have lower dielectric constant of ∼18 in the range 50 Hz to 500 kHz. At microwave frequencies (5-7 GHz) dielectric constant increases with increase in niobium concentration (22-26) for Ba₃ZnTa_2−xNb_xO₉; for Ba₃MgTa_2−xNb_xO₉ it varies between 12 and 14. The “Zn” compounds have much higher quality factors and lower temperature coefficient of resonant frequency compared to the “Mg” analogues.     

Different microstructure and dielectric properties of Ba1−xCaxTiO3 ceramics and pulsed-laser-ablated films

A comparative study of the microstructure and dielectric properties between Ba_1−xCa_xTiO₃ (BCT) ceramics and films were performed in the whole Ca concentration range of x = 0-1. The ceramics were prepared by conventional solid-state reaction technique and the films by the method of pulsed-laser deposition. X-ray diffraction (XRD) study of the BCT ceramics exhibited a pure tetragonal phase for x = 0-0.25, a tetragonal-orthorhombic diphase for x = 0.25-0.85 and a pure orthorhombic phase for x = 0.90-1.00. And the dielectric phase transition temperature from tetragonal to cubic was marginally affected by the Ca doping into BaTiO₃. However, BCT films deposited on Pt/Si/SiO₂/Si substrates showed a different microstructure and dielectric properties. Tetragonal-orthorhombic diphase was not found in the BCT films for x = 0.25-0.85, and a large decrease of the Curie point and diffuse phase transition were observed in the BCT films. Based on the compositional analysis, such phenomena were ascribed to the occupancy of some Ca²⁺ to the Ti⁴⁺ sites in the BCT films.     

Large-distance rf- and dc-sputtering of epitaxial La1 − xSrxMnO3 thin films

Studies on large-distance sputtering as an effective alternative to molecular beam epitaxy, pulsed laser deposition or off-axis sputtering for the deposition of epitaxial La_1 − xSr_xMnO₃ (LSMO) thin films, are reported. The focus of this study is on the quality of the samples and their structural and magnetic properties. The dependence of the characteristics of the LSMO films on the sputtering mode (rf, dc) and the sputtering parameters, in particular on the oxygen partial pressure is established and discussed. It is shown that large-distance sputtering can provide high quality LSMO thin films without the need for post-annealing.     

Compositional dependence on the optical properties of amorphous As2 − xS3 − xSbx thin films

In this paper, we report results of the optical properties of thermally deposited As_2 − xS_3 − xSb_x thin films with x = 0.02, 0.07, 0.1 and 0.15. We have characterized the deposited films by Fourier Transform Infrared, Raman and X-ray photoelectron spectroscopy (XPS). The relationship between the structural and optical properties and the compositional variation were investigated. It was found that the optical bandgap decreases with increase in Sb content. The XPS core level spectra show a decrease in As₂S₃ percentage with increase in Sb content. This is confirmed from the shifting of the Raman peak from AsS₃ vibrational mode towards SbS₃ vibrational mode.     

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 electrochemical properties of Ti doped Li3 − xFe2 − xTix(PO4)3/C cathode materials

Li_3 − xFe_2 − xTi_x(PO₄)₃/C (x = 0-0.4) cathodes designed with Fe doped by Ti was studied. Both Li₃Fe₂(PO₄)₃/C (x = 0) and Li_2.8Fe_1.8Ti_0.2(PO₄)₃/C (x = 0.2) possess two plateau potentials of Fe³⁺/Fe²⁺ couple (around 2.8 V and 2.7 V vs. Li⁺/Li) upon discharge observed from galvanostatic charge/discharge and cyclic voltammetry. Li_2.8Fe_1.8Ti_0.2(PO₄)₃/C has higher reversibility and better capacity retention than that of the undoped Li₃Fe₂(PO₄)₃/C. A much higher specific capacity of 122.3 mAh/g was obtained at C/20 in the first cycle, approaching the theoretical capacity of 128 mAh/g, and a capacity of 100.1 mAh/g was held at C/2 after the 20th cycle.