Preparation and properties of BaTi1−xSnxO3 multilayered ceramics

BaTi_1−xSn_xO₃ (BTS) powders, with x ranging from 0 to 0.15, were synthesized by solid-state reaction technique. The powders were pressed and sintered at 1370 °C. Obtained BTS ceramics were investigated by X-ray diffraction and dielectric properties measurements. It is found that Curie temperature decreases while dielectric constant increases with increasing of tin content. A monolithic multilayered ceramics with up to five layers of BTS with different amounts of Sn were prepared. Their dielectric properties were examined. Relatively high dielectric constants in a wide temperature range were obtained. It is noticed that BTS mono- and multilayered ceramics have better dielectric properties if they are additionally treated in microwave oven for 10 min, after sintering at 1370 °C.     

Effects of phase constitution and microstructure on energy storage properties of barium strontium titanate ceramics

Barium strontium titanate (Ba_0.3Sr_0.7TiO₃, BST) ceramics have been prepared by conventional sintering (CS) and spark plasma sintering (SPS). The effects of phase constitution and microstructure on dielectric properties, electrical breakdown process and energy storage properties of the BST ceramics were investigated. The X-ray diffraction analysis and dielectric properties measurements showed that the cubic and tetragonal phase coexisted in the SPS sample while the CS sample contained only tetragonal phase. Much smaller grain size, lower porosity, fewer defects and dislocation were observed in SPS samples, which greatly improved the electrical breakdown strength of the Ba_0.3Sr_0.7TiO₃ ceramics. The enhanced breakdown strength of the SPS samples resulted in an improved maximum electrical energy storage density of 1.13 J/cm³ which was twice as large as that of the CS sample (0.57 J/cm³). Meanwhile, the energy storage efficiency was improved from 69.3% to 86.8% by using spark plasma sintering.     

Influence of oxygen pressure on microstructure and dielectric properties of lead-free BaTi0.85Sn0.15O3 thin films prepared by pulsed laser deposition

Lead-free barium tin titanate BaTi_0.85Sn_0.15O₃ (BTS) ferroelectric thin films have been deposited on Pt/Ti/SiO₂/Si substrates by pulsed laser deposition. The structure and dielectric properties of thin films deposited at various oxygen pressures are investigated systematically. By optimizing the oxygen pressure during the deposition, the structure and dielectric properties are improved. The thin films grown at 15 Pa have the best crystal quality and the largest grain size, which result in the enhancement of the dielectric properties. The dielectric constant and loss tangent show the similar trend in the entire oxygen pressure range. The influence mechanisms of the oxygen pressure on the structure and dielectric properties are proposed. The BTS thin films deposited at 15 Pa with large figure of merit (FOM) of 81.1, high tunability of 72.1%, moderate dielectric constant of 341, low loss tangent of 0.009 are considered to be appropriate as a field tunable ferroelectric material for electrically tunable devices.     

Significant enhancement in breakdown strength and energy density of the BaTiO3/BaTiO3@SiO2 layered ceramics with strong interface blocking effect

The BaTiO₃/BaTiO₃@SiO₂ (BT/BTS) ceramics with layered structure, where grain size was about 1–2 μm in the BT layer while it was about 300–400 nm in the BTS layer, were fabricated by the tape-casting and lamination method. With the increasing of SiO₂ content in the BTS layer, the dielectric constant decreased gradually, and the breakdown strength was remarkably improved. Compared to the SiO₂-added BaTiO₃ bulk ceramics, the layered ceramics displayed significant enhancements in dielectric properties, breakdown properties and energy storage properties. The enhancement in dielectric properties was mostly attributed to the diluting effects created by this structure to SiO₂. Based on the finite element analysis with the dielectric breakdown mode, it was regarded that the electric field redistribution and the interface blocking effect led to the enhancement of breakdown strength. Finally, the maximum energy density of 1.8 J/cm³ was obtained at a breakdown strength of 301.4 kV/cm for the BT/BTS3 ceramic.     

Dielectric behaviors of Nb2O5–Co2O3 doped BaTiO3–Bi(Mg1/2Ti1/2)O3 ceramics

Nb₂O₅ and Nb–Co doped 0.85BaTiO₃–0.15Bi(Mg_1/2Ti_1/2)O₃ (0.85BT–0.15BMT) ceramics were investigated. From XRD patterns, undesired phase was observed when the (Nb₂O₅/Nb-Co) doping levels exceed 3 wt.%/2 wt.%, giving rise to the deteriorate dielectric constant. The 0.85BT–0.15BMT ceramics doped with 2 wt.%Nb₂O₅ was found to possess a moderate dielectric constant (? ～ 1000) and low dielectric loss (tan δ = 0.9%) at room temperature and 1 kHz, showing flat dielectric behavior over the temperature range from ?55 to 155 °C. It was found that the formation of core–shell structure in the BT based ceramics is controlled by the doping sequence of Nb- and Bi-oxides.     

The master sintering curves for BaTi0.975Sn0.025O3/BaTi0.85Sn0.15O3 functionally graded materials

The most important aim in the design and processing of functionally graded materials (FGMs) is to produce devices free from any deformation. Smart choices of different combination of graded layers, as well as the heating rate during sintering, are important for the fabrication of high-quality FGMs. In this study, BaTi_0.975Sn_0.025O₃/BaTi_0.85Sn_0.15O₃ (noted as BTS2.5 and BTS15, respectively) FGM was used as a model system for the construction of master sintering curves (MSCs) and estimation of the effective activation energies of sintering for different BTS graded layers. The MSCs were constructed, for BTS2.5 and BTS15 graded layers in FGMs, using shrinkage data obtained by a heating microscope during sintering at four constant heating rates, 2, 5, 10 and 20 °/min. The effective activation energies were determined using the concept of MSC; values of 359.5 and 340.5 kJ/mol were obtained for graded layers BTS2.5 and BTS15, respectively. A small difference of the effective activation energies of chosen powders made it possible for us to prepare high-quality FGMs, without delamination, distortion or other forms of defects.     

Effects of Sn on the microstructure and dielectric properties in BaTiO3-based ceramics

Ba_0.985Bi_0.01TiO₃–BaTi_1?xSn_xO₃ powders were synthesized by a two-step soft chemical method. Ceramics with core–shell structure could be easily obtained by using these uniformly distributed powders. The ceramics not only satisfied the requirement of EIA-X8R specification, but also were near to that of EIA-X9R specification. Microstructural evaluation conducted by X-ray diffraction and scanning electron microscopy confirmed the hierarchical structure of the ceramic grains. The shape of the ε–T curves near the dielectric peak became broad when x increased from 0.001 to 0.02. The permittivity of Ba_0.985Bi_0.01TiO₃–BaTi_0.98Sn_0.02O₃ ceramic was ～23,000, ΔC/C_20 °C was ?15%, 14.4% and ?15% at ?55 °C, 120 °C and 170 °C, respectively, and the dielectric loss was 0.5. The results showed that the content of Sn had a strong impact on the diffusion and the dielectric properties of the ceramics.     

Study of the pyroelectric profile in BTS piezoceramics by the single frequency thermal square wave method

Lead-free BaTi_1?xSn_xO₃ (BTS) piezoceramics finding its use in actuators and microsensors was studied with the aid of the thermal square wave method (TSWM). Functionally graded BTS with tin concentration of 0.075 ≤ x ≤ 0.15 varying over the sample thickness were investigated. The samples were fabricated by sintering of the bi-, tri- and tetramorph green bodies. The results were compared with the pyroelectric profiles of BTS samples without tin gradient with x = 0.075, 0.10, 0.125 and 0.15.     

Dielectric properties of barium titanate–molybdenum composite

The barium titanate–molybdenum composites were prepared through solid state reaction method in argon atmosphere. The microstructure, resistivity, and dielectric properties of the composites were investigated. XRD results indicated that chemical reactions between barium titanate (BaTiO₃:BT) and molybdenum (Mo) have taken place during sintering, resulting in the formation of BaMoO₄ (BM) and BaTi₂O₅ (BT₂). The resistivity decreased with the increasing amount of Mo in the composites. The composites (when x = 5 and 20 wt.%) showed lower dielectric constant than pure BaTiO₃, especially, the dielectric constant (when x = 20 wt.%) reached a minimum value (<10⁴), while composites (when x = 10 and 15 wt.%) showed rather high dielectric constant at temperatures range from 25 °C to 160 °C. The dielectric constant of the composite gradually decreased with increase in frequency at the room temperature. The dielectric constant of composite (when x = 5 wt.%) comes up to 10⁴, and the T_c (Curie temperature) of the composite was relatively higher than that of BT (120 °C).     

Pulsed laser deposition BTS thin films: The role of substrate temperature

BaSn_0.15Ti_0.85O₃ (BTS) thin films were deposited on Pt/Ti/SiO₂/Si(1 0 0) substrate by pulsed laser deposition and the effects of substrate temperature on their structure, dielectric properties and leakage current density were investigated. The results indicate that the substrate temperature has a significant effect on the structural and dielectric properties of the BTS thin films which exhibit a polycrystalline perovskite structure if the substrate temperature ranges within 550–750 °C. The dielectric constant and loss tangent of the BTS thin films deposited at 650 °C are 341 and 0.009 at 1 MHz, respectively, the tunability is 72.1% at a dc bias field of 400 kV/cm, while the largest figure of merit (FOM) is 81.1. The effect of the substrate temperature on the leakage current of the BTS thin films is discussed.     

Microwave dielectric properties of BaTi4O9 thin film

BaTi₄O₉ thin films have been prepared by RF magnetron sputtering on the Pt/Ti/SiO₂/Si substrates and the dielectric properties of the BaTi₄O₉ film have been investigated at microwave frequency range. The homogeneous BaTi₄O₉ thin film was obtained when the film was grown at 550 °C and rapid thermal annealed (RTA) at 900 °C for 3 min. The circular-patch capacitor (CPC) was used to measure the microwave dielectric properties of the film. The dielectric constant (ɛ_r) and the dielectric loss (tan δ) were successfully measured up to 6 GHz. The ɛ_r of the BaTi₄O₉ thin film slightly increased with the increase of the film thickness. However, the tan δ decreased with increasing the thickness of the film. The ɛ_r of BaTi₄O₉ thin film was similar to that of the BaTi₄O₉ ceramics, which is about 36–39. The tan δ of the film with 460 nm thickness was very low approximately, 0.0001 at 1–3 GHz. Since BaTi₄O₉ film has a high ɛ_r and a low tan δ, the BaTi₄O₉ film can be used as the microwave devices.     

Microwave dielectric properties of high dielectric tunable - low permittivity Ba0.5Sr0.5TiO3–Mg2(Ti0.95Sn0.05)O4 composite ceramics

Ba_0.5Sr_0.5TiO₃–Mg₂(Ti_0.95Sn_0.05)O₄ composite ceramics have been synthesized by the solid-state reaction. Phase structure, microstructure and microwave dielectric properties have been systematically characterized. The permittivity is tailored to a certain extent with the addition of Mg₂(Ti_0.95Sn_0.05)O₄. Both X-ray diffraction (XRD) and back electric image (BEI) analysis show the co-existence of two-phase structures of ABO₃ perovskite and A₂BO₄ spinel structure. A high dielectric tunablity can be obtained and a high Q value can be achieved at microwave frequency. The composition 30 wt.%Ba_0.5Sr_0.5TiO₃–70 wt.%Mg₂(Ti_0.95Sn_0.05)O₄ exhibits good dielectric properties with ? of 79, Q of 152 (at 2.997 GHz) and T of 15.8% (30 kV/cm & 10 kHz) at room temperature, which make it a promising candidate for tunable microwave device applications in the wireless communication system.     

Influence of glass compositions on the microstructure and dielectric properties of low temperature fired BaTi4O9 microwave material with copper electrodes in reducing atmosphere

BaTi₄O₉ (BT4) dielectric ceramics, containing 10 wt% BaO–ZnO–B₂O₃–SiO₂ (BZBS) glass frit as sintering aid were co-fired with copper paste under reducing atmosphere at 950 °C and investigated on microstructures and dielectric properties. Experimental results show the microstructures and microwave dielectric properties of BT4–BZBS glass composite materials are strongly dependent on the Ba–Zn–B–Si ratio, especially for the content of SiO₂ addition, in BZBS glass. XRD and EDS results indicate that Cu reacts with BZBS glass-added BT4 ceramics seriously to form complicated reaction products such as BaCuO₂ phase, when the BZBS glass composition with lower content of SiO₂ (less than 20 wt%), and transformation of some BaTi₄O₉ into barium titanate with various ratios of Ba/Ti, which, fortunately, does not seriously degrade the dielectric properties.     

Microstructure and dielectric properties of SrTiO3 ceramics by controlled growth of silica shells on SrTiO3 nanoparticles

SrTiO₃@SiO₂ nanopowder was synthesized via a core-shell nano-scale technique that is known as the Stöber process. The effect of the SiO₂ concentration on microstructure, dielectric response and energy storage properties of SrTiO₃@SiO₂ ceramics was investigated. Transmission electron microscopy (TEM) results confirmed the formation of core–shell nanostructures with controlled shell thicknesses between 2 nm and 13 nm. After increasing SiO₂, a secondary phase with Sr₂TiSi₂O₈ appeared due to inter-diffusion reactions between the SrTiO₃ core and SiO₂ shells during the sintering process. The results show that both breakdown strength and energy density improved apparently. The homogeneous coating of silica on ST cores is considered to dominate the contribution to improved breakdown strength. The composition for SrTiO₃ coated with 2.5 wt% SiO₂ shows the maximum energy storage density (1.2 J/cm³) and a breakdown strength of 310 kV/cm. The former is higher than for pure SrTiO₃ (0.19 J/cm³). Measurements of the dielectric performance indicate that the SrTiO₃@SiO₂ ceramics possess good bias stabilities compared to pure ST ceramics.     

Lead-free BaTiO3-Bi0.5Na0.5TiO3-Na0.73Bi0.09NbO3 relaxor ferroelectric ceramics for high energy storage

A series of (1-x)(0.65BaTiO₃-0.35Bi_0.5Na_0.5TiO₃)-xNa_0.73Bi_0.09NbO₃ ((1-x)BBNT-xNBN) (x = 0–0.14) ceramics were designed and fabricated using the conventional solid-state sintering method. The microstructure, dielectric property, relaxor behavior and energy storage property were systematically investigated. X-ray diffraction results reveal a pure perovskite structure and dielectric measurements exhibit a relaxor behavior for the (1-x)BBNT-xNBN ceramics. The slim polarization electric field (P-E) loops were observed in the samples with x ≥ 0.02 and the addition of Na_0.73Bi_0.09NbO₃ (NBN) could decrease the remnant polarization (P_r) of the (1-x)BBNT-xNBN ceramics obviously. The sample with x = 0.08 exhibits the highest energy storage density of 1.70 J/cm³ and the energy storage efficiency of 82% at 172 kV/cm owing to its submicron grain size and high relative density. These results show that the (1-x)BBNT-xNBN ceramics may be promising lead-free materials for high energy storage density capacitors.     

Control of coring effect in BaTi4O9 microwave dielectric ceramics by doping with Mn4+

In the present work, we have attempted to reduce the effect of coring effect in the titanate ceramic system BaTi₄O₉ (BT4) by doping it with Mn⁴⁺. The microwave dielectric BaTi₄O₉ ceramics doped with 0, 0.5 and 1.0 mol% Mn⁴⁺ were synthesized by conventional ceramic processing route. The XRD studies confirmed a single phase crystalline structure for all the ceramic samples studied. The SEM micrographs of the ceramics reveal a microstructural change leading towards a more uniform grain size distribution as the Mn⁴⁺ content increases to 1.0 mol%. In the low frequency region (100 Hz to 1 MHz), the temperature stability of dielectric properties exhibits a marked improvement with the increasing amount of Mn⁴⁺ in the ceramic system. In the microwave frequency region (9.3 GHz), Q-factor increases from 11,625 GHz to 46,500 GHz for BaTi₄O₉ ceramic doped with 1.0 mol% Mn⁴⁺. The present paper reveals that the commonly observed degradation of dielectric properties due to coring effect in the BaTi₄O₉ ceramic system can be controlled by doping it with an appropriate quantity of Mn⁴⁺.     

Dielectric dispersion and impedance spectroscopy of B3+-doped Ba(Ti0.9Sn0.1)O3 ceramics

In this study, the B₂O₃ doped Ba(Ti_0.9Sn_0.1)O₃ ceramics were prepared by using a solid state reaction method. Wide ranges of frequency (0.1 Hz to 1 MHz) and temperature (20–280 °C) dependence of the impedance relaxation were investigated. The impedance study indicates the presence of both dielectric relaxation in bulk and grain boundary effects in the material. The relaxation times for grain and grain boundary estimated from Cole–Cole plots varied with temperature according to the Arrhenius relation. The activation energy for grain and grain boundary were estimated to be 0.73 and 0.85 eV, respectively.     

Synthesis and characterization of Bi1.5Zn0.92Nb1.5−xSnxO6.92−x/2 pyrochlore ceramics

The morphological, compositional, structural, dielectric and electrical properties of Bi_1.5Zn_0.92Nb_1.5?xSn_xO_6.92?x/2 ceramics have been investigated by means of scanning electron microscopy (SEM), X-ray energy dispersion spectroscopy (EDS), X-ray diffraction (XRD), temperature and frequency dependent dielectric constant and temperature dependent conductivity measurements for Sn-contents in the range of 0.00 ≤ x ≤ 0.60. It was shown that single phase of the pyrochlore ceramics can only be obtained for x ≤ 0.25. Above this value a ZnO phase appeared in the XRD patterns and SEM micrographs as well. An increase in the lattice constant and in the temperature coefficient of dielectric constant and a decrease in the dielectric constant values with increasing Sn content was observed for the ceramics which exhibited a single phase formation. A temperature dependent but frequency invariant dielectric constant was observed for this type of ceramics. The lowest electrical conductivity and highest dielectric constant was observed for the sample which contains 0.06 Sn. The Bi_1.5Zn_0.92Nb_1.5?xSn_xO_6.92?x/2 pyrochlore ceramic conductivities are thermally active above 395 K. For temperatures greater than 395 K, the conductivity activation energy which was found to be 0.415 eV for the pure sample increased to 1.371 eV when sample was doped with 0.06 Sn.     

Fabrication of sub-micrometer MgO transparent ceramics by spark plasma sintering

Transparent MgO ceramics were fabricated by spark plasma sintering (SPS) of the commercial MgO powder using LiF as the sintering additive. Effects of the additive amount and the SPS conditions (i.e., sintering temperature and heating rate) on the optical transparency and microstructure of the obtained MgO ceramics were investigated. The results showed that LiF facilitated rapid densification and grain growth. Thus, the MgO ceramics could be easily densified at a moderate temperature and under a low pressure. In addition, the transparency and microstructure of the MgO ceramics were found to be strongly dependent on the temperature and heating rate. For the MgO ceramics sintered at 900 °C for 5 min with the heating rate of 100 °C/min and the pressure of 30 MPa from the powders with 1 wt% LiF, the average in-line transmittance reached 85% in the range of 3 – 5 μm, and the average grain size is ∼0.7 μm.     

Anti-reduction of Ti4+ in Ba4.2Sm9.2Ti18O54 ceramics by doping with MgO,Al2O3 and MnO2

The anti-reduction of Ti⁴⁺ ions in Ba_4.2Sm_9.2Ti₁₈O₅₄ (BST) ceramics at high sintering temperature over 1300 °C was investigated. MgO, Al₂O₃ and MnO₂ were added separately to suppress the reduction of Ti⁴⁺ ions so as to improve the microwave dielectric properties of BST ceramics. The microstructure of BST ceramics was analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). X-ray photoelectron spectroscopy (XPS) was used to study the electroconductivity of BST ceramics and valency changes of Ti ions. The results showed that MgO or Al₂O₃, when acting as an acceptor, could effectively suppress the reduction of Ti⁴⁺ ions and significantly improve the Q × f values of BST ceramics at the cost of dielectric constant. Meanwhile, MnO₂ as an oxidant had also improved the Q × f values but with no decrease in dielectric constant. Excellent microwave dielectric properties were achieved in Ba_4.2Sm_9.2Ti₁₈O₅₄ ceramics doped with 0.2 wt.% Al₂O₃ sintered at 1340 °C for 3 h: ?_r = 76.9, Q × f = 10,120 GHz and τ_f  = ?22.7 ppm/°C.