Particle sizes effects on electrical properties and densification of laminated Ba1.002La0.003TiO3 ceramics

This paper investigated the influences of initial particle sizes on electrical properties and densification of laminated Ba_1.002La_0.003TiO₃ ceramics prepared by the reduction–reoxidation technique. Ba_1.002La_0.003TiO₃ powders with average size of∼820 nm and∼260 nm were prepared by the planet ball milling and the sand milling, respectively. For ceramic samples from ∼260 nm particles, the inflection point where the densification rate begins to decrease occurs at a higher sintering temperature than that of ceramic samples from∼820 nm particles. An abnormal growth of grains is observed in ceramic samples from∼820 nm particles, which resists reoxidation. Samples from∼260 nm particles are prone to be globally reoxidized and exhibit a much greater change in grain boundary resistance and RT resistivity after reoxidation. A possible mechanism of the oxygen diffusion in the reoxidation process is proposed, which verifies that samples with smaller grains as well as lower density are easily oxidized to a deeper degree.     

Electrical properties and microstructure of Y-doped BaTiO3 ceramics prepared by high-energy ball-milling

The effects of corn-starch content and high-energy ball-milling time on the microstructure and electrical properties of porous Y-doped (Ba, Sr)TiO₃ samples were investigated. All the (Ba, Sr)TiO₃ samples at room temperature crystallized in the tetragonal structure and the crystal structure was independent of the corn-starch content and ball-milling time. We found that the corn-starch additive and the ball-milling time affected the microstructure and the electrical properties. The (Ba, Sr)TiO₃ samples exhibited a large PTCR jump (>10⁵) due to the high porosity. The PTCR jump of the samples increased with increasing corn-starch content, while it decreased with increasing ball-milling time. In addition, the resistivity increased with increasing corn-starch content, while it decreased with increasing ball-milling time.     

Effects of CaTiO3 addition on the densification and microwave dielectric properties of BiSbO4 ceramics

In this study, the effects of CaTiO₃ addition on the sintering characteristics and microwave dielectric properties of BiSbO₄ were investigated. Pure BiSbO₄ achieved a sintered density of 8.46 g/cm³ at 1100 °C. The value of sintered density decreased with increasing CaTiO₃, and sintering at a temperature higher than 1100 °C led to a large weight loss (>2 wt%) caused by the volatile nature of the compound. Samples either sintered above 1100 °C or with a CaTiO₃ content exceeding 3 wt% showed poor densification. SEM micrographs revealed microstructures with bimodal grain size distribution. The size of the smaller grains ranged from 0.5 to 1.2 μm and that of the larger grains between 3 and 7 μm. The microwave dielectric properties of the (1−x) BiSbO_4−x CaTiO₃ ceramics are dependent both on the x value and on the sintering temperature. The 99.0 wt% BiSbO₄–1.0 wt% CaTiO₃ ceramic sintered at 1100 °C reported overall microwave dielectric properties that can be summarized as ε_r≈21.8, Q×f≈61,150 GHz, and τ_f≈−40.1 ppm/°C, all superior to those of the BiSbO₄ ceramics sintered with other additives.     

Effect of polyethylene glycol on the microstructure and PTCR characteristics of n-BaTiO3 ceramics

Porous BaTiO₃ (n-BaTiO₃) ceramics doped donor were fabricated by the addition of polyethylene glycol (PEG) into the n-BaTiO₃ powder. The effects of PEG on the microstructure and PTCR characteristics of the porous n-BaTiO₃ ceramics have been investigated. An endotherm was found at 60 °C, with strong exotherm at 262 °C, weight loss commenced at 165 °C and was virtually complete by 265 °C from the differential thermal analysis (DTA) and thermogravimetric analysis (TGA) of the PEG. It was also found that the porosity increased and the grain size decreased with increasing PEG. The crystalline structure of n-BaTiO₃ ceramics was independent on the PEG content and the n-BaTiO₃ ceramics containing PEG showed the presence of (Ba, Sr)TiO₃ peaks only from the XRD results. The n-BaTiO₃ ceramics containing PEG showed higher PTCR characteristics than that of the n-BaTiO₃ ceramics without PEG.     

Influence of rare earth Tb4O7 addition on the densification,abrasion resistance and microstructure of alumina ceramics

This study aimed to improve the purity and performance of alumina ceramics used as ball milling media. High-alumina ceramics (>?96?wt% Al₂O₃), with high densification and excellent abrasion resistance, were fabricated by the cold isostatic pressing method. The effects of adding the rare earth Tb₄O₇ on the densification, abrasion resistance, crystalline phase, micro-morphology and grain size of the ceramics were studied. The experiment results showed that the densification and abrasion resistance of the samples increased with Tb₄O₇ addition. The sample with 0.8?wt% Tb₄O₇ sintered at 1625?°C exhibited the best performance, with a linear shrinkage, relative density and abrasion rate of 22.28%, 95.70% and 0.103‰^, respectively. The abrasion resistance improved by 27.5% compared with the sample without Tb₄O₇. X-ray diffraction analysis indicated that the primary phases of the samples were corundum, spinel, CaAl₁₂O₁₉ and α-quartz, and a small quantity of Tb₃Al₅O₁₂ was generated when more than 0.4?wt% Tb₄O₇ was added. Furthermore, some Mg²⁺ and Ca²⁺ ions in the liquid phases dissolved into Tb₃Al₅O₁₂ crystalline grains during the sintering process, which enhanced the grain boundary cohesion of the materials. Scanning electron microscopy indicated that the existence of Tb₃Al₅O₁₂ at grain boundaries reduced the average size of the corundum grains. This helped transform inter-granular fractures into trans-granular fractures, thereby improving the abrasion resistance of the ceramic materials.     

Effect of MgO addition on the microstructure and dielectric properties of BaTiO3 ceramics

MgO-doped BaTiO₃ (BaTiO₃/MgO) ceramics were prepared by a solid-state sintering method. The effects of MgO doping on the dielectric properties of BaTiO₃/MgO were investigated in terms of its microstructural development. The BaTiO₃/MgO was characterized by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and x-ray powder diffraction. Results show that grain growth of the BaTiO₃/MgO during sintering was inhibited by adding MgO at least 0.5 mol%. It resulted in a high resistance of the BaTiO₃/MgO sintered at high temperature. The BaTiO₃/MgO possessed a broad temperature stability and met Electronic Industries Association (EIA) ×7R specification. The improved dielectric properties of the BaTiO₃/MgO are attributed to the decreased tetragonality of BaTiO₃ lattice due to Mg²⁺ substitute for Ti⁴⁺.     

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.     

Vanadium doping effects on microstructure and dielectric properties of barium titanate ceramics

V-doped barium titante ceramics were prepared by conventional solid state reaction method. XRD patterns show that V⁵⁺ ions have entered into the tetragonal perovskite structure of solid solution to substitute for Ti⁴⁺ ions on the B sites. Addition of vanadium accelerates grain growth of BTO ceramics and there is abnormal grain growth of barium titanate ceramics with higher vanadium concentration. Vanadium doping can increase the Curie temperature and decrease the dielectric loss of barium titanate ceramics. As vanadium concentration increases, the remnant polarization of V-doped BTO ceramics begins to increase and reaches the maximum and then decreases. The coercive electric field for V-doped barium titanate ceramics decreases with the increasing of vanadium concentration. As temperature rises, the remnant polarization and the coercive electric field of V-doped barium titanate ceramics decrease simultaneously.     

Engineering grain size and electrical properties of donor-doped barium titanate ceramics

A semiconducting lanthanum-doped barium titanate ceramic has been fabricated for battery safety applications by simple means from nanoparticles prepared at room temperature by kinetically controlled vapor diffusion catalysis. The material, characterized by electron microscopy, X-ray diffraction and electrical measurements, exhibits a difficult to achieve combination of submicron grain size (∼500 nm) and attractive electrical properties of room temperature resistivity below 100 Ω cm and a 12-fold increase in resistivity through the Curie temperature (positive thermal coefficient of resistivity, PTCR). Systematic investigation of sintering conditions revealed that a short period of heating at 1350 °C under air is necessary to suppress abnormal grain growth, while precise control of the cooling rate is needed to achieve the targeted electrical properties. Cooling must be sufficiently fast to avoid complete back-oxidation, yet slow enough to facilitate oxygen adsorption at the grain boundaries to produce the thin oxide layer apparently responsible for the observed PTCR.     

Effects of preparation method on the microstructure and electrical properties of tungsten bronze structure Sr2NaNb5O15 ceramics

Herein, Sr₂NaNb₅O₁₅ (SNN) ceramics with a filled tungsten bronze structure were synthesized by the various methods of conventional mixed-oxide (CMO), two-step sintering (TSS), reactive sintering (RS) and molten salt synthesis (MSS). It was found that varying the preparation method could result in significant differences in the ceramic morphology, relative density, and electrical properties. The TSS, RS and MSS methods all produced a pure tungsten bronze phase SNN ceramic, though the CMO method could not. Further characterization revealed a sharp drop in the Curie temperature and significant deterioration of the dielectric properties in the ceramics prepared by MSS compared to the other methods, likely owing to residual K⁺ from the molten salt. Systematic comparison of the electrical properties of the ceramics produced by the RS and TSS method found that the RS method was the most suitable preparation method for SNN ceramics. The superior ferroelectric and piezoelectric properties in the sample produced by the RS method were attributed to the highly distorted NbO₆ octahedron, as suggested by Raman spectroscopy.     

The effect of Ga addition on the sinterability and microwave dielectric properties of RE3Al5O12 (Tb,Y, Er and Yb) garnet ceramics

The RE₃Al₅O₁₂ (RE=Tb, Y, Er, Yb) ceramics have been prepared by the mixed oxide route and the influence of Ga³⁺ doping on their properties is investigated. The intrinsic Y₃Al₅O₁₂ (YAG) ceramic sintered at 1650 °C for 4 h showed good dielectric properties; (ε_r=10.1, Q_u×f=65,000 GHz, τ_f=−45 ppm/°C). Addition of Ga₂O₃ was found to be beneficial in improving the densification of Tb₃Al₅O₁₂, Er₃Al₅O₁₂ and Yb₃Al₅O₁₂ except Y₃Al₅O₁₂ where Nb₂O₅ is the better choice. Among Ga³⁺ added samples, the composition Yb₃Al₅O₁₂+1 wt% Ga₂O₃ showed good microwave dielectric properties: ε_r=10.3, Q_u×f=50,000 GHz, τ_f=−58 ppm/°C. The Y₃Al₅O₁₂ doped with 1 wt% Nb₂O₅ has ε_r=10.7, Q_u×f=120,000 GHz and τ_f=−45 ppm/°C. The ceramics have good thermal properties (CTE=2–3 ppm/°C, λ=2–12 W/m K).     

Effects of Al2O3 addition on the sintering behavior and microwave dielectric properties of CaSiO3 ceramics

The effects of Al₂O₃ addition on the densification, structure and microwave dielectric properties of CaSiO₃ ceramics have been investigated. The Al₂O₃ addition results in the presence of two distinct phases, e.g. Ca₂Al₂SiO₇ and CaAl₂Si₂O₈, which can restrict the growth of CaSiO₃ grains by surrounding their boundaries and also improve the bulk density of CaSiO₃-Al₂O₃ ceramics. However, excessive addition (≥2 wt%) of Al₂O₃ undermines the microwave dielectric properties of the title ceramics since the derived phases of Ca₂Al₂SiO₇ and CaAl₂Si₂O₈ have poor quality factor. The optimum amount of Al₂O₃ addition is found to be 1 wt%, and the derived CaSiO₃-Al₂O₃ ceramic sintered at 1250 °C presents improved microwave dielectric properties of ?_r = 6.66 and Q × f = 24,626 GHz, which is much better than those of pure CaSiO₃ ceramic sintered at 1340 °C (Q × f = 13,109 GHz).     

Effects of CaSiO3 addition on sintering behavior and microwave dielectric properties of Al2O3 ceramics

The effects of CaSiO₃ addition on the sintering behavior and microwave dielectric properties of Al₂O₃ ceramics have been investigated. The addition of CaSiO₃ into Al₂O₃ ceramics resulted in the emergence of Ca₂Al₂SiO₇ and CaAl₂Si₂O₈, which acting as liquid sintering aids can effectively lower the sintering temperature of Al₂O₃ ceramic. The Q × f value of Al₂O₃-CaSiO₃ ceramics decreased with the CaSiO₃ addition increasing because of the lower Q × f value of Ca₂Al₂SiO₇ and CaAl₂Si₂O₈. Compared with the pure CaSiO₃ ceramic, the Al₂O₃-CaSiO₃ ceramic with 20 wt% CaSiO₃ addition possessed good dielectric properties of ?_r = 9.36 and Q × f = 13,678 GHz at the similar sintering temperature.     

Effects of glass additions on the microstructure and dielectric properties of barium strontium titanate (BST) ceramics

Commercial glass frits (lead borosilicate glasses) were employed as the sintering aids to reduce the sintering temperatures of BST ceramics. The effects of the glass content and the sintering temperature on the microstructures, dielectric properties and tunabilities of BST ceramics have been investigated. Densification of BST ceramics of 5 wt% glass content becomes significant from sintering temperature of 1000 °C. The glass content shows a strong influence on the Curie temperature T_c, permittivity and the diffuse transition. X-ray results show all BST ceramics exhibit a perovskite structure and also the formation of a secondary phase, Ba₂TiSi₂O₈. The shift of BST diffraction peaks towards higher angle with increasing the glass content indicates the substitution of Pb²⁺ in Ba²⁺ site, which mainly accounts for the diffuse transition observed in these BST ceramics. BST ceramics with 10 wt% glass additives possess the highest tunability at all four sintering temperatures. A tunability of 12.2% at a bias field of 1 kV/mm was achieved for BST ceramics with 10 wt% glass content sintered at 900 °C.     

Effects of Al2O3 addition on the microstructure and microwave dielectric properties of Ba4Nd9.33Ti18O54 ceramics

Ba₄Nd_9.33Ti₁₈O₅₄·x wt%Al₂O₃ (BNT-A) ceramics (x=0, 0.5, 1.0, 1.5, 2.0, 2.5) were prepared by the conventional solid state reaction. The effects of Al₂O₃ on the microstructure and microwave dielectric properties of Ba₄Nd_9.33Ti₁₈O₅₄ (BNT) ceramics were investigated. X-ray diffraction and backscatter electronic images showed that the Al₂O₃ additive gave rise to a second phase BaAl₂Ti₅O₁₄ (BAT). The formation mechanism and grain growth of the BAT phase were first discussed. Dielectric property test revealed that the Al₂O₃ additive had improved the dielectric properties of the BNT ceramics: increased the Q×f value from 8270 to 12,180 GHz and decreased the τ_f value from 53.4 to 11.2 ppm/°C. A BNT-A ceramic with excellent dielectric properties: ε_r=70.2, Q×f=12,180 GHz, τ_f=20 ppm/°C was obtained with 2.0 wt% Al₂O₃ added after sintering at 1320 °C for 4 h.     

Dielectric properties and microstructures of CaSiO3 ceramics with B2O3 addition

The effects of B₂O₃ additives on the sintering behavior, microstructure and dielectric properties of CaSiO₃ ceramics have been investigated. The B₂O₃ addition resulted in the emergence of CaO–B₂O₃–SiO₂ glass phase, which was advantageous to lower the synthesis temperature of CaSiO₃ crystal phase, and could effectively lower the densification temperature of CaSiO₃ ceramic to as low as 1100 °C. The 6 wt% B₂O₃-doped CaSiO₃ ceramic sintered at 1100 °C possessed good dielectric properties: _r = 6.84 and tan δ = 6.9 × 10⁻⁴ (1 MHz).     

Influence of cooling mode on the electrical properties and microstructure of Ba1.022−xSmxTiO3 ceramics sintered in a reducing atmosphere

We investigated the effects of the Sm-dopant content and the cooling rate on the electrical properties and microstructure of Ba_1.022−xSm_xTiO₃ (BST) ceramics, which were sintered at 1200 °C for 30 min in a reducing atmosphere and then reoxidized at 800 °C for 1 h. The results indicated that the cooling rate affected the electrical properties and the microstructure of the BST samples, whose room-temperature resistivity increased with increasing cooling rate. The semiconducting BST ceramics showed a pronounced positive temperature coefficient of resistivity effect, with a resistance jump greater by 3.16 orders of magnitude, along with a low room-temperature resistivity of 157.4 Ω cm at a cooling rate of 4 °C/min. The room-temperature resistivity of the specimen was lower after sintering for 30 min at 1150 °C during cooling.     

Tailored electrical properties in ternary BiScO3-PbTiO3 ceramics by composition modification

Ternary 0.99(0.36BiScO₃-0.64PbTiO₃)-0.01Bi(M₁M₂)_0.5O₃ (BS-PT-BM₁M₂) ceramics were prepared by the solid-state reaction method, where M₁ and M₂ respectively stand for bivalent and quadrivalent elements (M₁=Sn, Pb, Ni, Sr, Ba, Ca, Cu, Mg and Mn, M₂ = Hf, Sn, Zr, Si, Ce and Mn). Effects of different elements on their structure and electrical properties were studied in detail. It was found that the formation of MPB by optimizing the doped elements can enhance electrical properties (d₃₃ = 500 pC/N, ?_r = 2013, tan δ = 0.024 at 100?kHz). Interestingly, different electrical properties can be induced by choosing the doped elements. For example, a high d₃₃ can be realized by doping M₁ = Sn, Pb or Sr (M₂ = Ti) as well as M₂ = Hf, Sn or Zr (M₁ = Zn), and the dielectric loss can be suppressed by doping Ce or Mn. In addition, large bipolar strain (S = 0.25–0.46%) as well as high remanent polarization (P_r = 34.7–46.4?µC/cm²) can be observed for all doped elements, which were superior to pure BS-PT (P_r = 32?µC/cm² and S = 0.18%), and high T_C (T_C = 417–443?°C) can be attained in all the ceramics. We believe that the addition of ABO₃-type compounds with optimum elements can enhance electrical properties of BS-PT ceramics.     

Influence of Al2O3 addition on microstructure,defects level and magnetic properties of LiTiZn ferrite ceramics

In the present work, the results of the influence of diamagnetic additives on the defects level of ferrite ceramics, its microstructure and magnetic properties are presented. A method based on a mathematical analysis of the experimental temperature dependences of the initial permeability was used for estimation of the defects level in the samples. Model samples containing a controlled amount of the diamagnetic additive Al₂O₃ served to test the possibility of monitoring this method of nonmagnetic phases of ferrite ceramics. It was shown that with an increase in the concentration of the Al₂O₃ additive in the range of (0–0.5) wt%, a significant increase in the defects level was observed almost 6-fold. The data from SEM micrographs showed that the addition of Al₂O₃ affects the type of grains of ferrite ceramics, but does not affect their grain size. Grains are highly agglomerated and show large grain size dispersion and also pore. Obtained data were compared to hysteresis loop parameters. It is shown that with an increase in the concentration of the Al₂O₃ addition, there is a regular decrease in the residual induction and an increase in the coercive force. However, such changes in hysteresis loop parameters are small in comparison to defects level. Investigations of the true physical broadening of the diffraction reflections were performed for the same model samples in order to compare the change in the defects level to the direct X-ray diffraction measurements of micro deformations. The defects level as a characteristic of the elastic stress of a ferrite ceramics is proposed. This assumption follows from a linear relationship between the defects level and the width of the diffraction reflections. The consistency of the obtained results made it possible to evaluate the high efficiency and sensitivity of the method for defects level estimating.     

Effect of BaTiO3 addition on the microstructure and relaxor ferroelectric properties of Sr0.7Ba0.3Nb2O6 ceramics

A conventional solid-state reaction was used to synthesize (1-x) Sr_0.7Ba_0.3Nb₂O₆–xBaTiO₃ (0.00≤x≤0.10) ceramics. The phase structure, microstructure, and dielectric and relaxor ferroelectric properties of these ceramics were investigated. Tungsten bronze structure can be observed in ceramics, and addition of BaTiO₃ can make the grain size decrease and the porosity increase. The dielectric characteristics show diffuse phase transition phenomena for all samples, which were demonstrated by a linear fit of the modified Curie-Weiss law with γ varying between 1.54 and 1.88. As the BaTiO₃ content increases, the transition temperature (T_C) decreases gradually and has a minimum value of 37.53 °C at composition x=0.06, and the maximum dielectric constant (ε_max) increases gradually from 66 to 3309 and subsequently decreases to 1625 at x=0.10. In addition, the relaxor ferroelectric properties of these ceramics at x=0.8 are consistent with the Volgel-Fulcher relationship; polarization versus electric field (P-E) loops were measured at a different temperature.