Effect of Bi and Li co-substituted SrTiO<Subscript>3</Subscript> ceramics on structural and dielectric properties

Polycrystalline Sr_(1–x)(Bi, Li)_xTiO₃ ceramics (x = 0 and 0.02) are prepared by a microwave processing method. The effect of co-substitution on structural, dielectric properties and ac-conductivity were investigated. The XRD of ceramics shows single phase with cubic structure. The lattice parameter of the compounds is estimated from the XRD patterns which confirm the incorporation of Bi and Li in SrTiO₃ ceramics. Studies revealed that the dielectric constant and dielectric loss increased with an increase of temperature and decreased with an increase in frequency. The maximum dielectric constant obtained at room temperature is around 570 and increased to around 10⁴ at 600 °C measured at 1 kHz frequency whereas the maximum dielectric loss measured was 0.048 at 600 °C and the loss measured at room temperature is 0.02. The activation energy of the samples were investigated using Arrhenius plots.     

Effect of BaTiO<Subscript>3</Subscript> heat treatment on the microstructure and dielectric properties of BaTiO<Subscript>3</Subscript>-based ceramics

We have studied the effect of heat treatment of the starting BaTiO₃ powder on the dielectric properties and microstructure of X7R-type BaTiO₃-based ceramics. The results demonstrate that annealing of BaTiO₃ stabilizes the degree of tetragonality in the crystal lattice of the ceramics. Microstructural analysis shows that the annealing temperature has no effect on the average grain size of the ceramics. Increasing the BaTiO₃ annealing temperature increases the dielectric permittivity of the core phase and reduces the temperature coefficient of capacitance (TCC). We obtained an X7R-type BaTiO₃-based ceramic material (BaTiO₃ annealing temperature, 1150°C; firing temperature, 1160°C) with the following properties: ɛ_25°C = 2230, TCC = ±12% (−55 to 125°C), and tanδ_25°C = 0.013.     

Grain-size effects on thermal properties of BaTiO<Subscript>3</Subscript> ceramics

Dense nanocrystalline BaTiO₃ ceramics are successfully prepared by the high pressure assisted sintering. Microstructures are observed by scanning electronic microscopes. The grain sizes are estimated to be about 30 and 150 nm. In comparison, BaTiO₃ ceramics with the grain size of 600 nm and 1.5 μm are fabricated by conventional pressure-less sintering. The thermal properties of BaTiO₃ ceramics with different grain sizes are investigated by differential scanning calorimetry and thermal expansion. The results suggest that the enthalpy values for the tetragonal-cubic transition decreased and the thermal expansion values increased with decreasing grain size. Furthermore, the Curie temperature shifts to lower temperature with decreasing grain size.     

Influence of 3d-elements on dielectric properties of BaTiO<Subscript>3</Subscript> ceramics

The microstructure and dielectric properties of Yb-Mn- and Yb-Ni-substituted BaTiO₃ ceramics are investigated in this paper. Both Yb-Mn- and Yb-Ni-substituted BaTiO₃ ceramics satisfy the X8R specification (−55 ^∘C to 150 ^∘C, Δ C = ±15% or less) for automotive application when CaZrO₃ is incorporated in the formulations. It is found that both Mn and Ni ions can suppress the diffusion of Yb and CaZrO₃ into BaTiO₃ grains, resulting in formation of core-shell structures in the grains. It is found that Mn is more favorable to stabilize the core-shell structure in BaTiO₃ ceramics as compared with Ni.     

Effects of addition of BiFeO<Subscript>3</Subscript> on phase transition and dielectric properties of BaTiO<Subscript>3</Subscript> ceramics

Samples of xBiFeO₃–(1 − x)BaTiO₃ (x = 0, 0.02, 0.04, 0.06, 0.07 and 0.08) were synthesized by solid state reaction technique and sintered in air in the temperature range 1,220–1,280 °C for 4 h. X-ray diffraction data showed that 2–8 mol% BiFeO₃ can dissolve into the lattice of BaTiO₃ and form single perovskite phase. The crystal structure changes from tetragonal to cubic phase at room temperature when 8 mol% of BiFeO₃ was added into BaTiO₃. Scanning electron microscope images indicated that the ceramics have compact and uniform microstructures, and the grain size of the ceramics decreases with the increase of BiFeO₃ content. Dielectric constants were measured as functions of temperatures (25–200 °C). With rising addition of BiFeO₃, the Curie temperature decreases. For the sample with x = 0.08, the phase transition occurred below room temperature. The boundary between tetragonal and cubic phase of the BiFeO₃–BaTiO₃ system at room temperature locates at a composition between 7 and 8 mol% of BiFeO₃. The diffusivity parameter γ for compositions x = 0.02 and x = 0.07 is 1.21 and 1.29, respectively. The relaxor-like behaviour is enhanced by the BiFeO₃ addition.     

Microstructure and dielectric properties of BaTiO<Subscript>3</Subscript>-CeO<Subscript>2</Subscript>-SnO<Subscript>2</Subscript> ceramics

We have studied the effect of codoping with CeO₂ and SnO₂ (2 to 3.5 wt %) on the microstructure and dielectric properties of BaTiO₃. Doping with CeO₂ and SnO₂ inhibits grain growth in BaTiO₃ and enables the fabrication of ceramic materials with a grain size below 1 μm. The temperature coefficient of permittivity of the ceramics increases with CeO₂ + SnO₂ content, firing temperature, and firing time.     

Structural,dielectric and impedance study of Bi and Li co-substituted Ba<Subscript>0.50</Subscript>Sr<Subscript>0.50</Subscript>TiO<Subscript>3</Subscript> ceramics for tunable microwave devices applications

In this article, the structural, dielectric and electrical properties of Bi and Li co-substituted (Ba, Sr) site in Ba_0.50Sr_0.50TiO₃ ceramics are presented. Four different compositions of Ba_0.50Sr_0.50TiO₃, (Ba_0.50Sr_0.50)_0.98(Bi, Li)_0.02TiO₃, (Ba_0.50Sr_0.50)_0.96(Bi, Li)_0.04TiO_3, and (Ba_0.50Sr_0.50)_0.92(Bi, Li)_0.08TiO₃ were synthesized using solid-state reaction with microwave heating of starting materials. Phase detection for all samples has been examined by XRD along with Rietveld refinement analyses, and the results show the formation of single phase without observation of any secondary phase. However, a decrease in crystallite size, lattice parameters, and unit cell volume has been observed with the increase of Bi and Li concentration. A Dense microstructure with different grains sizes and shapes has been obtained by scanning electron microscopy. Impedance spectroscopy in the temperature range of 30–300 °C and frequency range of 60 Hz–1 MHz has been used to study the dielectric properties. The result shows that the Bi and Li co-substituted Ba_0.5Sr_0.5TiO₃ ceramics exhibit very interesting features, such as enhanced dielectric constant with low loss which make it suitable for microwave tunable devices applications. An electric impedance analysis was carried out at different temperatures namely (400, 450, 500, and 550 °C). A single semicircular arc with single relaxation process has been observed in all studied samples which suggest that the grains contribute to the total resistance in these materials. The activation energy was obtained from the impedance analysis using Arrhenius plot of grain conductivity.     

Microstructures and dielectric properties of Y/Zn codoped BaTiO<Subscript>3</Subscript> ceramics

The microstructures and dielectric properties of Y/Zn codoped BaTiO₃ ceramics sintered in a reducing atmosphere were investigated. XRD analysis indicated the crystal structure of samples change from tetragonal to pseudocubic with increasing Y₂O₃ and ZnO content. SEM micrographs showed Y₂O₃ can suppress grain growth more effectively compared with ZnO, which is ascribed to the presence of second phase Y₂Ti₂O₇. Proper amount of Y₂O₃ and ZnO can significantly improve the dielectric temperature characteristics due to the formation of grain core-shell structure. The high performance dielectrics meeting the X7R code were achieved by codoping 1.5 mol% Y₂O₃ and 3.0 mol% ZnO.     

Synthesis and microstructure of SrTiO<Subscript>3</Subscript> and BaTiO<Subscript>3</Subscript> ceramics by a reaction-sintering process

Strontium titanate and barium titanate ceramics prepared by a reaction-sintering process were investigated. The mixture of raw materials of stoichiometric SrTiO₃ and BaTiO₃ was pressed and sintered into ceramics without any calcination stage involved. A density 4.99 g/cm³ (97.5% of the theoretic value) was found in SrTiO₃ after 6 h sintering at 1,370 °C. Grains less than 1.5 μm were formed at 1,300–1,330 °C and became 2.2–3.3 μm at 1,350–1,370 °C SrTiO₃. A density 5.89 g/cm³ (97.9% of the theoretic value) was found in BaTiO₃ after 6 h sintering at 1,400 °C. Merged grains were observed in BaTiO₃ and were less than 10 μm after sintered at 1,400 °C.     

Preparation of BaTiO<Subscript>3</Subscript>-based ceramics by nanocomposite doping process

BaTiO₃-based ultrafine nonreducible dielectrics for multilayer ceramic capacitors were prepared by a newly developed nanocomposite doping process. According to TG-DTA, XRD and TEM analysis, the nanocomposite dopants via sol–gel method were uniform and well dispersive. The micromechanism was investigated based on comparing conventional process with nano-doping process. It indicated that due to the special nano-effect, doping effect of additives became more effective and the microstructure and dielectric properties of ceramics were improved. The results showed that high performance dielectrics satisfying X8R specification were achieved, with high dielectric constant of 2,900, low dielectric loss of 0.6% and large insulation resistivity of 10¹² Ω cm.     

Synthesis and characterization of BaTiO<Subscript>3</Subscript>-based X9R ceramics

The effects of (Na_0.5Bi_0.5)TiO₃ (NBT) and MgO addition on the dielectric properties and microstructures of BaTiO₃ (BT) ceramics were investigated. NBT was first added to Nb₂O₅-doped BT system. As NBT content increases from 0 to 0.2 mol, the Curie temperature of the systems shifts to high temperatures and dielectric constant peak at T _c is suppressed evidently. The variation of capacity (ΔC/C _20 °C (%)) of the system at 200 °C decreases with increasing NBT content from 0.1 to 0.2 mol, but that of −55 and 125 °C increases monotonously. The stable temperature characteristics of the dielectric properties improved by NBT doping would be connected with the distortion and deformation of the structure induced by substitution of Na⁺ and Bi³⁺ into Ba sites. MgO was employed to further flatten the ΔC/C _20 °C–T curve. It is very helpful for this ceramic system to satisfy the requirement of EIA-X9R specification on ΔC/C _20 °C and still keep a satisfied dielectric constant. The addition of MgO improved effectively the temperature stability of the dielectric properties. Changes of the crystalline structure and microstructure induced by MgO doping might contribute to these improvements.     

Structural and dielectric properties of Bi doped Ba<Subscript>0.6</Subscript>Sr<Subscript>0.4</Subscript>TiO<Subscript>3</Subscript> ceramics

The dielectric properties of 0.1–15% mol bismuth doped Ba_0.6Sr_0.4TiO₃ (BST) ceramics have been investigated systematically. The solubility limit of bismuth is determined as about 10 mol% by means of both X-ray diffraction and scanning electron microscopy, which is further verified by the fact that the lattice constant of the samples above 10 mol% is almost invariable. The temperature dependence of the dielectric permittivity suggest that the ferroelectric behavior transit to relaxor ferroelectric type when impurity concentration reaches 5 mol%, and further to relaxor behavior for samples above 10 mol% Bi content, which is verified by the absence of a hysteresis loop. Thermal expansion results show differences between 5 and 10 mol% doped samples. Dielectric tunability at room temperature decreases with bismuth content increasing. The variation of properties was attributed to the impurity induced polar regions and former long-order structure.     

PTCR effect in BaBi<Subscript>2</Subscript>Nb<Subscript>2</Subscript>O<Subscript>9</Subscript>-doped BaTiO<Subscript>3</Subscript> ceramics

The influences of BaBi₂Nb₂O₉ content on the electrical property and the microstructure of BaTiO₃-based materials have been studied. With an increase in BaBi₂Nb₂O₉ content the grain size decreases. All the prepared BaBi₂Nb₂O₉ doping BaTiO₃-based thermistors show typical PTC effect. As the amount of BaBi₂Nb₂O₉ added in BaTiO₃-based ceramics increases, resistivity appears to exhibit a minimum value. At high BaBi₂Nb₂O₉ content (≥0.0875), the resistivity increased again with increasing BaBi₂Nb₂O₉ content. At a given content of BaBi₂Nb₂O₉, the influence of sintering temperature on the electrical properties of samples has been investigated. A minimum of room temperature resistivity is obtained at the sintering temperature equal to 1,290 °C at a given content of BaBi₂Nb₂O₉.     

Effect of the crystallinity of BaTiO<Subscript>3</Subscript> powders prepared using the merker method on the properties of PTCR ceramics

Barium titanate powders differing in particle size (110–740 nm) were prepared by calcining barium titanyl oxalate precipitated by the Merker method. The powders were sintered to produce PTCR ceramics with the composition 100(Ba_0.89Ca_0.08Pb_0.03)TiO₃ + ^0.8TiO₂ + ^0.7Y + ^0.1Mn + ^2.5SiO₂ and electrical properties of the ceramics were studied. The results demonstrate that improving the crystallinity of the barium titanate powder suppresses recrystallization of the ceramics and has a significant effect on their resistance ratio and electric strength. We found the optimal range of calcination temperatures (950–1000°C) for barium titanyl oxalate which ensures the highest electric strength of thermistors with a resistance of 31 Ω. The average crystallite size of the parent barium titanate powder is ∼250–320 nm.     

Effect of Pb(Ti,Sn)O<Subscript>3</Subscript> on the dielectric properties of high dielectric constant X8R BaTiO<Subscript>3</Subscript>-based ceramics

The high dielectric constant X8R dielectric materials could be sintered at 1,240 °C by doping 2.5 mol% Pb(Ti,Sn)O₃ additives into the BaTiO₃ ceramics, with a dielectric constant greater than 3,400 at 25 °C, dielectric loss lower than 2.0% and temperature coefficient of capacitance (TCC) less than ±15% from −55 to 150 °C, which satisfied X8R specification. The effects of Pb(Ti,Sn)O₃ on the microstructure and dielectric properties of BaTiO₃-based ceramics were investigated. Doped with Pb(Ti,Sn)O₃ additives, the partial solid solution was formed between Pb(Ti,Sn)O₃ and BaTiO₃. Due to the high Curie point of Pb(Ti,Sn)O₃, the Curie point of the ceramics was markedly shifted to higher temperature about 150 °C, and the temperature coefficient of capacitance curves was flattened. The increase of the tetragonality (c/a ratio) and the fine microstructure were resulted in the increase of dielectric constant. With Pb(Ti, Sn)O₃ content up to 3 mol%, the depression of Ti⁴⁺’s polarization and the decrease of the tetragonality (c/a ratio) were resulted in the decrease of dielectric constant.     

Preparation and properties of BaTiO<Subscript>3</Subscript>/porous oxide composites

Barium titanate has been synthesized in pores of oxide matrices using barium titanyl oxalate and barium titanyl peroxide (peroxotitanate) by a sol-gel process and by dispersing the precursors together with pyrogenic oxides in water. The composites were characterized by X-ray diffraction, differential thermal analysis, nitrogen adsorption measurements, transmission electron microscopy, dielectric spectroscopy, and photocatalytic activity measurements. The results demonstrate that the pore structure of the composites forms concurrently with their crystal structure. The composite produced using pyrogenic alumina has high dielectric permittivity.     

Effects of manganese additive on piezoelectric properties of (Bi<Subscript>1/2</Subscript>Na<Subscript>1/2</Subscript>)TiO<Subscript>3</Subscript>–BaTiO<Subscript>3</Subscript> ferroelectric ceramics

The microstructure, piezoelectric and dielectric properties of piezoelectric ceramics (Bi_1/2Na_1/2)_0.94Ba_0.06TiO₃+xwt%MnCO₃ (BNBT6–xMn) with x ranging from 0 to 1.5 were studied. X-ray diffraction patterns showed the samples exhibited perovskite structure and Mn additive turned the structure from tetragonal phase to rhombohedral phase. The results of SEM indicated the Mn additive promoted the sintering which was beneficial to the synthesis of the perovskite. With the amount of Mn additive up to 0.1 wt.%, the value of d ₃₃, k _p ,and ε were improved; While Q _m also was increased until 0.8 wt%. The Mn additive acted on the “soft” and “hard” function simultaneously; When the amount of Mn additive more than 0.8 wt%, the performance drops.     

Effect of Mn<Superscript>2+</Superscript> doping on the temperature coefficient of capacitance of TiO<Subscript>2</Subscript>/SiO<Subscript>2</Subscript>-doped BaTiO<Subscript>3</Subscript> ceramics

The effect of Mn²⁺ on the temperature coefficient of capacitance (TCC) of TiO₂/SiO₂-doped BaTiO₃ ceramics has been investigated. The experiment has shown that the high temperature peak of TCC exhibited a continuous enhancement when Mn²⁺ concentration increased and X8R specification was gradually met. The secondary phase Ba₂TiSi₂O₈ was found in all samples. SEM and XRD analyses have proved that Mn²⁺ could depress the crystallization of TiO₂/SiO₂ in BaTiO₃ ceramics. The microstrain study through MAUD analysis depicted that the high temperature peak of TCC was dependent on the microstrain of samples to a certain extent. The Mn²⁺ could be a useful dopant for ameliorating the TCC of TiO₂/SiO₂-doped BaTiO₃ ceramics. The text was submitted by the authors in English.     

Effect of mechanical activation on physical properties of relaxor ferroelectric Pb<Subscript>2</Subscript>ScNbO<Subscript>6</Subscript> ceramics

Relaxor ferroelectric lead scandium niobate Pb₂ScNbO₆ (PSN) ceramics was obtained by solidstate synthesis and a modified ceramic technology, whereby the sintering stage was preceded by room-temperature compression and shear straining of the synthesized PSN powder in Bridgman anvils. It is established that this mechanical activation leads to the development of dynamic recrystallization processes in PSN grains, which significantly influence the physical properties of the final ferroelectric ceramic material.