Compositionally graded ceramics based on Ba<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>TiO<Subscript>3</Subscript> solid solutions

Homogeneous and compositionally graded barium strontium titanate (Ba_{1 − x} Sr _x TiO₃) ceramics have been prepared in the form of thick films by slip casting. The strontium content was varied across the graded films from 0 to 30 mol %. Microstructural examination and dielectric measurements have shown that, varying the composition and thickness of the layers in graded structures and the sintering conditions, one can control the behavior of their dielectric characteristics and obtain a small temperature coefficient of capacitance in a predetermined temperature range.     

Novel temperature stable Ba<Subscript>1−x</Subscript>Sr<Subscript>x</Subscript>V<Subscript>2</Subscript>O<Subscript>6</Subscript> microwave dielectric ceramics with ultra-low sintering temperature

New temperature stability Ba_1?xSr_xV₂O₆ (0.35?≤?x?≤?0.55) microwave dielectric ceramics prepared by the conventional solid-state route were investigated. X-ray diffraction confirmed that all the specimens formed a solid solution single phase with orthorhombic structure. The microwave dielectric properties strongly depended on the compositions, densification and microstructure of the specimens. Furthermore, partial Sr ions substitution for Ba ions in Ba_1?xSr_xV₂O₆ lattices not only successfully improved the temperature stability of BaV₂O₆-based ceramic but also promoted the sinterability of SrV₂O₆-based one. Out of these compositions, Ba_0.5Sr_0.5V₂O₆ sintered at 625?°C exhibited a near-zero τ_f together with a low permittivity ε_r?~?11.5 and a quality factor Q?×?f?~?14 100 GHz, which also showed good chemical compatibility with Al electrodes.     

Enhanced microwave dielectric properties of Ba<Subscript>0.40</Subscript>Sr<Subscript>0.60</Subscript>TiO<Subscript>3</Subscript>–Zr<Subscript>0.80</Subscript>Sn<Subscript>0.20</Subscript>TiO<Subscript>4</Subscript> composite ceramics

The (1−x) Ba_0.40Sr_0.60TiO₃ (BST)−xZr_0.80Sn_0.20TiO₄ (ZST) composite ceramics with x = 10, 20, 30, and 40 wt% were fabricated by conventional solid-state reaction method. With increasing of ZST content, the dielectric constant of composite ceramics was decreased and dielectric loss increases. The effect of ZnO addition to 70 wt% BST–30 wt% ZST composition on the microstructure and dielectric properties was investigated. The improvements in dielectric constant, dielectric loss, and microwave dielectric properties of composite ceramics can be achieved by ZnO addition. The sample with 98 wt% (70 wt% BST–30 wt% ZST)–2 wt%ZnO composition exhibits promising dielectric properties, with dielectric constant, loss tangent and tunability at 4 kV/mm, of 125, 0.0016 and 12%, at 10 kHz and room temperature. At ~2 GHz, it possesses a dielectric constant of 101 and a Q factor of 187, which makes it a good candidate for tunable microwave device applications.     

Microwave dielectric properties of temperature-stable (Mg<Subscript>0.95</Subscript>Co<Subscript>0.05</Subscript>)<Subscript>2</Subscript>TiO<Subscript>4</Subscript>–Li<Subscript>2</Subscript>TiO<Subscript>3</Subscript> composite ceramics for LTCC applications

In this paper, the effects of Li₂O–B₂O₃–Bi₂O₃–SiO₂ (LBBS) glass on the phase formation, sintering characteristic, the microstructure and microwave dielectric properties of temperature-stable (Mg_0.95Co_0.05)₂TiO₄–Li₂TiO₃ ceramics were investigated. (Mg_0.95Co_0.05)₂TiO₄–Li₂TiO₃ powders were obtained by using the traditional solid-state process. A small amount of LBBS doping can effectively reduce sintering temperature and promote the densification of the ceramics. X-ray diffraction analysis revealed not only the primary phase (Mg·Co)₂TiO₄ associated with Li₂TiO₃ minor phase but also a third phase (Mg·Co)TiO₃. The dielectric constant and Qf values vary with the doping amount of LBBS and sintering temperatures. With the compensation of the positive temperature coefficient (τ _f ) of Li₂TiO₃ and the negative τ _f of (Mg_0.95Co_0.05)₂TiO₄, the τ _f of the specimens fluctuates around zero. The (Mg_0.95Co_0.05)₂TiO₄ ceramic with 2.5 wt% LBBS addition and sintering at 900?°C for 4 h exhibited excellent microwave dielectric properties: ? _r ?=?19.076, Qf?=?126100 GHz, and τ _f ?=?0.98 ppm/°C.     

Preparation,characterization and dielectric properties of Ba<Subscript>3−x</Subscript>Sr<Subscript>x</Subscript>LiM<Subscript>3</Subscript>Ti<Subscript>5</Subscript>O<Subscript>21</Subscript>[M=Nb and Ta,x = 0 to 3] ceramics

The ceramic compositions Ba_3−xSr_xLiM₃Ti₅O₂₁[M=Nb and Ta, x = 0 to 3] were prepared through conventional solid state ceramic route. A detailed study has been carried out to correlate the structure of Ba_3−xSr_xLiM₃Ti₅O₂₁[M=Nb and Ta, x = 0 to 3] with respect to their dielectric properties. The structure and microstructure of ceramic samples were studied using powder X-ray diffractometer and Scanning Electron Microscopic techniques. The dielectric properties of the sintered ceramic compacts have been studied. The Ba-rich compositions were identified as promising candidates for high frequency applications whereas the Sr-rich compositions were excellent ionic conductors and can be commercially exploited for applications in solid-state batteries.     

Phase formation and dielectric properties of Ba<Subscript>0.5</Subscript>Sr<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript> by slow injection sol–gel technique

A simple sol–gel process incorporating slow precursor injection technique was employed to synthesize homogeneous Ba_0.5Sr_0.5TiO₃ nano powders. The Ba_0.5Sr_0.5TiO₃ samples were subjected to calcination temperatures from 600 to 1,100 °C and sintering temperatures from 1,250 to 1,350 °C for the study of phase formation, crystallite size, particle distribution, and dielectric properties. Single phase Ba_0.5Sr_0.5TiO₃ with a cubic perovskite structure was successfully synthesized after calcination at 800 °C. The average size of the nano particles is 42 nm with a narrow size distribution, and a standard deviation of 10%. The highest values recorded within the investigated range for dielectric constant, and dielectric loss measured at 1 kHz are 1,164 and 0.063, respectively, for Ba_0.5Sr_0.5TiO₃ pellets calcined at 800 °C and sintered at 1,350 °C. Leakage current density measured at 5 V for the Ba_0.5Sr_0.5TiO₃ pellet was found to be 49.4 pA/cm².     

Dielectric and impedance studies of (Pb<Subscript>1−x</Subscript>Ca<Subscript>x</Subscript>)(Fe<Subscript>0.5</Subscript>Nb<Subscript>0.5</Subscript>)O<Subscript>3</Subscript> dielectric ceramics

Ceramic samples of (Pb_1?xCa_x)(Fe_0.5Nb_0.5)O₃ with x = 0.20, 0.40, 0.45, 0.50, 0.55 and 0.60 were obtained by columbite precursor method. All the synthesized samples have perovskite structure with pseudo-cubic symmetry. Dielectric properties of all the samples were measured as a function of frequency from room temperature up to 573 K. Two dielectric anomalies were observed in ε_r–T plots at about 400 and 500 K. The impedance analysis depicts a single relaxation process. Activation energies obtained from temperature dependence of relaxation frequency, f₀ and grain resistance, R_g were found to be more or less comparable. The observed relaxation in all the samples seems to be due to electron relaxation associated with oxygen vacancies.     

The crossover of (Ba<Subscript>1−x</Subscript>Ca<Subscript>x</Subscript>)(Ti<Subscript>0.9</Subscript>Sn<Subscript>0.1</Subscript>)O<Subscript>3</Subscript> piezoelectric ceramics from single-phase to composite with studying the structural and dielectric properties

Lead-free (Ba_1?xCa_x)(Ti_0.9Sn_0.1)O₃ perovskite ceramics (x?=?0.02–0.5) (BCTS) were synthesized using the solid-state reaction technique. X-ray diffraction was used to identify the formed phases of the prepared compositions. The morphology of ceramics has been studied using a scanning electron microscope equipped with an energy dispersive spectrometer. Field emission scanning electron microscope was used to examine the morphology of sensing film calcined powder. The crossover from BCTS single-phase (x?=?0.02) to BCTS composite(x?=?0.5) was obtained via coexistence of both (x?=?0.3) of Ca addition. The composite powder was sintered at higher temperature rather than the single-phase powder. The calcined powder sensing film was prepared by the screen-printing technique as humidity sensors. Thereafter, DC resistance measurements were performed in the presence of relative humidity RH at room temperature. All the compositions exhibited a poor sensitivity toward the humidity sensing in the range of 0–98% RH. The compositions 0.02 and 0.06 have shown orthorhombic–tetragonal phase transition (T_O?T) below the room temperature, while the other compositions have shown a pure tetragonal phase. The highest value of permittivity at Curie temperature (ε?=?29241 at 100 Hz) and piezoelectric coefficient (d₃₃?=?495 pC/N) at room temperature were obtained at Ca?=?0.06 due to present polymorphic phase transition. The effect of frequency on the dielectric constant and dielectric loss at room temperature were investigated. All the prepared compositions exhibited small values of dielectric loss from 50 Hz up to 100 KHz, which indicates a good reliability for electronic applications such as capacitors or memory devices.     

Electrical and oxygen sensing properties of Nd<Subscript>1−x</Subscript>Ba<Subscript>x</Subscript>CoO<Subscript>3</Subscript> ceramics

Nd_1?xBa_xCoO₃ (0?≤?x?≤?0.2) ceramics was synthesized by solid state reaction. All the samples have an orthorhombic perovskite structure (Space group P n m a). The electrical transport property indicates that Ba doped NdCoO₃ ceramics goes through semiconductor–metal phase transition. The electrical resistivity of Nd_1?xBa_xCoO₃ (0?≤?x?≤?0.15) ceramics decreases, while the electrical resistivity of Nd_0.8Ba_0.2CoO₃ ceramics increases with the increase of temperature. The chemical-sensing property shows that Nd_1?xBa_xCoO₃ ceramics is very sensitive to oxygen. Also, increasing Ba²⁺ doping concentration can reduce the oxygen desorption rate and increase the sensitivity of resistivity. These results indicate that Ba²⁺ doped NdCoO₃ ceramics is not only the good candidate of the cathode materials of solid fuel cells but also the good materials of gas sensor devices.     

Microstructure,dielectric and photoluminescence properties of Tm-doped Ba<Subscript>0.8</Subscript>Sr<Subscript>0.2</Subscript>TiO<Subscript>3 </Subscript>thin films fabricated by sol–gel method

Ba_0.8Sr_0.2TiO₃ thin films doped by Tm from 0 to 7 mol% were fabricated by sol–gel method on silicon and Pt/Ti/SiO₂/Si substrates. X-ray diffraction, scanning electron microscopy, and Raman spectroscopy have been used to study variations of crystal structure, surface morphologies, and phase stability of Tm-doped BST films, respectively. The residual stress in BST films on silicon substrates can be reduced by Tm doping, as demonstrated by the blueshift of phonon peaks in Raman spectra. The dielectric measurements were conducted on metal-insulator-metal capacitors at the frequency from 1 kHz to 1 MHz. The grain size and dielectric constant decreased with increasing Tm concentration. While the variation of dielectric loss, tunability and the figure of merit were nonlinear with increasing Tm concentration. In addition, the photoluminescence property of 0.2 mol% Tm-doped BST was also studied. The effect of Tm doping on the microstructure, dielectric and photoluminescence properties were analyzed.     

Characterization and electrical conductivity of La<Subscript>1−x</Subscript>Sr<Subscript>x</Subscript>CrO<Subscript>3</Subscript> NTC ceramics

The La_1?xSr_xCrO₃ (x?=?0–0.1) negative temperature coefficient (NTC) ceramics have been prepared by the traditional solid state reaction method. X-ray diffraction (XRD) analysis has revealed that the as-sintered ceramics crystallize in a single perovskite structure. Scanning Electron Microscope (SEM) images show that the doped Sr²⁺ contributes to in the decrease in porosity. X-ray photoelectron spectroscopy (XPS) analysis indicates the existence of Cr³⁺ and Cr⁶⁺ ions on lattice sites, which result in hopping conduction. The presence of the Cr⁶⁺ is one of the key factors that affect the electrical conductivity of La_1?xSr_xCrO₃. Resistance–temperature characteristics were studied in the range of ?80 to 10?°C for the ceramic samples, the electrical characterizations show that the electrical resistivity and material constant B decrease with the increase of the strontium content.     

Synthesis and piezoelectric properties of (Na<Subscript>0.5</Subscript>Bi<Subscript>0.5</Subscript>)<Subscript>0.94</Subscript>Ba<Subscript>0.06</Subscript>TiO<Subscript>3</Subscript> ceramics prepared by sol–gel auto-combustion method

In this study, NaNO₃, Bi(NO₃)₃·5H₂O, Ba(NO₃)₂, Ti(OC₄H₉)₄ and citric acid were successfully introduced to fabricate lead-free piezoelectric (Na_0.5Bi_0.5)_0.94Ba_0.06TiO₃ [NBBT] nanopartical powders by a novel modified sol–gel auto-combustion method. The resultant products were characterized by the X-ray diffraction analysis and transmission electron microscope method. (Na_0.5Bi_0.5)_0.94Ba_0.06TiO₃ + Mn(NO₃)₂ [NBBTM] can be sintered by the traditional solid-state reaction, and the effects of NBBT doped different amounts of Mn(NO₃)₂ at various sintering temperatures upon phase formation, microstructure as well as piezoelectric properties were further studied. The experimental results show that it was helpful to control their chemical ingredients and microstructure to prepare nanocrystalline single phase NBBT powders. Where is the X-ray diffraction result of the corresponding ceramics to prove the existence of the mixing between rhombohedral and tetragonal phases at the MPB compositions. Doping 0.015 mol% Mn(NO₃)₂ into NBBT at 1,090 °C, piezoelectric constant (d ₃₃) and relative dielectric constant (ε_r) reach the superior value of 159pC/N and 1,304, respectively, and dielectric loss (tan δ) and electromechanical coupling factor (K _t) are 2.5% and 65%, respectively.     

Microwave dielectric properties of Ba<Subscript>4</Subscript>LaTiNb<Subscript>3−<Emphasis Type="Italic">x</Emphasis></Subscript>Ta<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>15</Subscript> ceramics

High dielectric constant and low loss ceramics in the system Ba₄LaTiNb_3−x Ta _x O₁₅ (x = 0–3) have been prepared by conventional solid-state ceramic route. Ba₄LaTiNb_3−x Ta _x O₁₅ 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 53.1 to 42.3, quality factor Q_u × f from 18,790 to 28,070 GHz and temperature variation of resonant frequency from +94.3 to +33.1 ppm/°C as the value of x increases.     

Microwave dielectric properties of ZnO–Nb<Subscript>2</Subscript>O<Subscript>5</Subscript>–<Emphasis Type="Italic">x</Emphasis>TiO<Subscript>2</Subscript> ceramics prepared by reaction-sintering process

The ZnO–Nb₂O₅–xTiO₂ (1 ≤ x ≤ 2) ceramics were fabricated by reaction-sintering process, and the effects of TiO₂ content and sintering temperature on the crystal structure and microwave dielectric properties of the ceramics were investigated. The XRD patterns of the ceramics showed that ZnTiNb₂O₈ single phase was formed as x ≤ 1.6 and second phase Zn_0.17Nb_0.33Ti_0.5O₂ appeared at x ≥ 1.8. With the increase of TiO₂ content and sintering temperature, the amount of the second phase Zn_0.17Nb_0.33Ti_0.5O₂ increased, resulting in the increase of dielectric constant, decrease of Q × f value, and the temperature coefficient of resonant frequency (τ _f ) shifted to a positive value. The optimum microwave dielectric properties were obtained for ZnO–Nb₂O₅–2TiO₂ ceramics sintered at 1075 °C for 5 h: ε _r  = 45.3, Q × f = 23,500 GHz, τ _f  = +4.5 ppm/°C.     

Microstructure and properties of the Ba<Subscript>0.75−x</Subscript>Ca<Subscript>x</Subscript>La<Subscript>0.25</Subscript>Fe<Subscript>11.6</Subscript>Co<Subscript>0.25</Subscript>Ti<Subscript>0.15</Subscript>O<Subscript>19</Subscript> hexaferrites

In the case of Ti⁴⁺ remain unchanged, the Ca²⁺ substituted Ba_0.75?xCa_xLa_0.25Fe_11.6Co_0.25Ti_0.15O₁₉ (0?≤?x?≤?0.05) were prepared by conventional solid-state reaction method at temperature of 1280 °C. A ball-to-power weight ratio of 10:1. Their crystal structure and magnetic properties were mainly investigated. The results show that the single magnetoplumbite phase structure transformed into the multiphase structure. Meanwhile, the small amount of α-Fe₂O₃ phase existed in M-type phase. The micrographs were observed by a field emission scanning electron microscopy (SEM). Vibrating sample magnetometer (VSM) was used to analyze the magnetic properties. The saturation magnetization (M _s ) first increases then decreases when x from 0 to 0.03. But, when x from 0.03 to 0.05, the saturation magnetization (M _s ) first increases then decreases too. The maximum value is at x?=?0.04 (M _s ?=?70.73 emu/g). The value of coercivity (H _c ) first increases then decreases when x from 0 to 0.04. But, the value increased when x from 0.04 to 0.05. The maximum value is at x?=?0.02 (H _c ?=?1691 Oe).     

Effects on dielectric and magnetic properties of Cr-doped Bi<Subscript>0.9</Subscript>Ba<Subscript>0.1</Subscript>Fe<Subscript>1−x</Subscript>Cr<Subscript>x</Subscript>O<Subscript>3</Subscript> ceramic

In this paper, Cr-doped Bi_0.9Ba_0.1Fe_1?xCr_xO₃ (x = 0.00, 0.05, 0.10, 0.15, 0.20) ceramic materials were prepared by traditional state solid synthesized method, and the effects of Cr³⁺ ion on magnetic and dielectric properties were investigated. All samples showed BiFeO₃ phase formation were successful synthesized. The SEM images showed the shape of samples changed from regular to irregular shape. With increasing of Cr²⁺ ions, Saturation magnetization (M _s) increased from 5.24 to 8.6 emu/g, and then decreased to 7.31 emu/g, and coercivity (H _c) increased from 110.66 to 256.49 Oe. All the samples showed high dielectric constants at low frequency and the values of dielectric constants decreased slightly with frequency increasing. Delectric loss (tanδ) values kept a steady in a wide range frequency of 10–600 MHz. They ranged in tanδ from 0.01 to 0.07, which was a low dielectric loss in Bi_0.9Ba_0.1Fe_1?xCr_xO₃ ceramics.     

Electrical conduction in La<Subscript>0.9</Subscript>Ba<Subscript>0.1</Subscript>Er<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Mg<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>3−α</Subscript> ceramics

Doubly doped LaErO₃ ceramics, La_0.9Ba_0.1Er_1−x Mg _x O_3−α (x = 0.05, 0.10, 0.15, 0.20), were synthesized by solid-state reaction method and characterized by X-ray diffraction (XRD). The samples have a single orthorhombic perovskite-type structure. The conduction behavior was investigated using various electrochemical methods including AC impedance spectroscopy, gas concentration cell, isotope effect of hydrogen, and hydrogen electrochemical permeation (pumping) in the temperature range of 500–1000 °C. The results indicated that specimens were pure ionic conductors under low oxygen partial pressure (about 10⁻⁷–10⁻²⁰ atm) and mixed conductors of proton, oxide ion, and electron hole under high oxygen partial pressure (about 10⁻⁵–1 atm). The pure ion conduction of the ceramics in hydrogen atmosphere was confirmed by electromotive force method of hydrogen concentration cell, and the observed emf values coincided well with the theoretical ones. The conductivity in H₂O–Ar atmosphere was higher than that in D₂O–Ar atmosphere, exhibiting an obvious isotope effect and proton conduction in water vapor containing atmosphere. It has been confirmed by electrochemical hydrogen permeation (hydrogen pumping) experiment that the ceramics were mainly proton conductors in hydrogen containing atmosphere. Whereas in dry oxygen-containing atmosphere, observed emf values of the oxygen concentration cell were far lower than the theoretical ones, indicating that the ceramics were mixed conductors of electron hole and oxide ion.     

Influence of Dy-doping on ferroelectric and dielectric properties in Bi<Subscript>1.05−x</Subscript>Dy<Subscript>x</Subscript>FeO<Subscript>3</Subscript> ceramics

Bi_1.05−xDy_xFeO₃ (BDFO) (x = 0−0.2) ceramics were synthesized by solid-state reaction method. The influence of Dy dopant on crystal structural, dielectric and ferroelectric properties was investigated. The lattice parameter and the Curie temperature of BDFO were degraded continuously with increasing contents of Dy³⁺ cations. Leakage current density, ferroelectric polarization and dielectric loss were improved by appropriate Dy doping. When x = 0.1, BDFO showed the best electric properties. At applied electric field of 53 kV/cm, the remnant polarization (2P _r ) was 12.2 μC/cm².These improvements in electric properties in BDFO ceramics could have resulted from the relatively low oxygen vacancy concentration and structural distortion.     

Influence of sintering conditions on Ni internal electrode and PTCR effect of the multilayer (Ba<Subscript>1.005−<Emphasis Type="Italic">x</Emphasis></Subscript>Y<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)TiO<Subscript>3</Subscript> ceramics

The influence of sintering conditions on the positive temperature coefficient of resistance (PTCR) characteristics and the Ni internal electrode in multilayer (Ba_1.005?x Y _x )TiO₃ (BYT) ceramics is investigated. The BYT ceramics were fired at 1160–1240?°C for 2 h in a reducing atmosphere and then reoxidised at 500–900?°C for 1–2 h. Results indicate that the room-temperature (RT) resistance for BYT ceramics—which were reoxidised at 750?°C for 2 h after sintering at different temperatures—decreased quickly when the sintering temperature was increased from 1160 to 1240?°C. As the firing temperature increased, the sample resistance jump [Lg(R _max/R _min)] increased initially and then decreased. In addition, the reoxidation temperature effect on the electrical properties and the PTCR effect on the specimens were also studied. Finally, the BYT ceramics showed a significant PTCR characteristic. They obtained a resistance jump greater than 3.06 orders of magnitude and a low RT resistance of 0.25 Ω at a 750?°C reoxidated temperature for 2 h after sintering at 1180?°C for 2 h in a reducing atmosphere. In addition, the sintering temperature influence on the Ni internal electrode surface microstructure is investigated using FSEM.