Fabrication and dielectric properties of textured Na<Subscript>0.5</Subscript>Bi<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript>-BaTiO<Subscript>3</Subscript> ceramics by RTGG method

Grain-oriented Na_0.5Bi_0.5TiO₃-BaTiO₃ (NBTBT) ceramics were fabricated by reactive-templated grain growth using plate-like Bi_2.5Na_3.5Nb₅O₁₈ (BNN) as templates. The specimens are composed of NBTBT perovskite phase and BNN lay-structured phase. Textured ceramics have a brick-wall microstructure with strip-like grains aligning in the direction parallel to the casting plane and exhibit an {h00} preferred orientation. The texture fraction increases initially, and then decreases with increasing sintering temperature. The optimal sintering temperature is 1,185 °C where the texture fraction has a maximum value of 0.58 and d₃₃ is 98 pC/N. The textured NBTBT ceramics show evidence of relaxor ferroelectrics with diffuse phase transition and frequency dispersion because of composite biphasic structure.     

High-temperature phase transition of Tm<Subscript>2</Subscript>Ti<Subscript>2</Subscript>O<Subscript>7</Subscript>

The high-temperature (>1600°C) order—disorder phase transition of Tm₂Ti₂O₇ is shown to be irreversible. The 740°C ionic conductivity of nanocrystalline Tm₂Ti₂O₇ ceramics synthesized at 1670°C is 2 × 10^-3 S/cm and remains unchanged upon heat treatment in air at 860°C for 240 h. The conductivity of the high-temperature (disordered pyrochlore) phase of Tm₂Ti₂O₇ is independent of grain size in the range 20–30 nm.Translated from Neorganicheskie Materialy, Vol. 40, No. 12, 2004, pp. 1495–1500.Original Russian Text Copyright © 2004 by Shlyakhtina, Knotko, Larina, Borichev, Shcherbakova.     

Microwave dielectric properties and its compatibility with silver electrode of LiNb<Subscript>0.6</Subscript>Ti<Subscript>0.5</Subscript>O<Subscript>3</Subscript> with B<Subscript>2</Subscript>O<Subscript>3</Subscript> and CuO additions

The influences of B₂O₃ and CuO (BCu, B₂O₃: CuO = 1:1) additions on the sintering behavior and microwave dielectric properties of LiNb_0.6Ti_0.5O₃ (LNT) ceramics were investigated. LNT ceramics were prepared with conventional solid-state method and sintered at temperatures about 1,100 °C. The sintering temperature of LNT ceramics with BCu addition could be effectively reduced to 900 °C due to the liquid phase effects resulting from the additives. The addition of BCu does not induce much degradation in the microwave dielectric properties. Typically, the excellent microwave dielectric properties of ε_r = 66, Q × f = 6,210 GHz, and τ _f  = 25 ppm/^oC were obtained for the 2 wt% BCu-doped sample sintered at 900 °C. Chemical compatibility of silver electrodes and low-fired samples has also been investigated.     

Dielectric properties of B<Subscript>2</Subscript>O<Subscript>3</Subscript> doped Sm(Co<Subscript>1/2</Subscript>Ti<Subscript>1/2</Subscript>)O<Subscript>3</Subscript> ceramics at microwave frequency

The microwave dielectric properties and the microstructures of Sm(Co_1/2Ti_1/2)O₃ ceramics with B₂O₃ additions (0.25 and 0.5 wt%) prepared by conventional solid-state route have been investigated. The prepared Sm(Co_1/2Ti_1/2)O₃ exhibited a mixture of Co and Ti showing 1:1 order in the B-site. Doping with B₂O₃ (up to 0.5 wt%) can effectively promote the densification of Sm(Co_1/2Ti_1/2)O₃ ceramics with low sintering temperature. It is found that Sm(Co_1/2Ti_1/2)O₃ ceramics can be sintered at 1,260 °C due to the grain boundary phase effect of B₂O₃ addition. At 1,290 °C, Sm(Co_1/2Ti_1/2)O₃ ceramics with 0.5 wt% B₂O₃ addition possess a dielectric constant (ε _r) of 27.7, a Q × f value of 33,600 (at 9 GHz) and a temperature coefficient of resonant frequency (τ_f) of −11.4 ppm/ °C. The B₂O₃-doped Sm(Co_1/2Ti_1/2)O₃ ceramics can find applications in microwave devices requiring low sintering temperature.     

The influences of firing temperatures and excess PbO on the crystal structure and microstructure of (Pb<Subscript>0.25</Subscript> Sr<Subscript>0.75</Subscript>)TiO<Subscript>3</Subscript> ceramics

Polycrystalline samples of (Pb_0.25Sr_0.75)TiO₃ (PST75) were prepared by the solid-state reaction method. The effects of firing temperatures and excess PbO on PST75 ceramics were investigated. The PST75 was calcined between 600 and 1000 °C for 3 h and the sintering temperature ranged between 1050 and 1250 °C for 2 h. The optimized calcination and sintering conditions were identified as 950 and 1250 °C, respectively. The lattice parameter c increased, while the lattice parameter a decreased with increased firing temperatures. The average particle size and average grain size increased with increased firing temperatures. After the addition of PbO—excess 0, 1, 3, 5, and 10 wt%—in the PST75 samples, the lattice parameter a decreased. The average particle size and the average grain size increased with the increase of PbO. The porous microstructure slightly decreased with an increasing amount of PbO—up to 3 wt%—then slightly increased with the higher excess PbO. The density was improved by adding 3 wt% of excess PbO. A low dielectric loss was observed from the 3 wt% excess PbO sample.     

Structure and properties of Ba(Zr<Subscript>0.2</Subscript>Ti<Subscript>0.8</Subscript>)O<Subscript>3</Subscript> ceramics prepared by spark plasma sintering

Ba(Zr_0.2Ti_0.8)O₃ (BZT) ceramics are prepared from spray-dried powder by spark plasma sintering (SPS) and by normal sintering. By the application of SPS, ceramics with >96% relative densities could be obtained by sintering at 1,100 °C for 5 min in air atmosphere. The pellet as sintered by SPS at 1,100 °C was black and conductive. Although SPS was carried out in air atmosphere, the samples were deoxidized by heating the carbon die. By post-annealing at 1,000 °C for 12 h in air, the pellet was oxidized and became white and insulating. Grain growth was suppressed in the ceramics prepared by SPS, and the average grain size was 0.52 μm. The starting powder contained 1.90% carbon, mainly as binder, and the SPS-prepared ceramics and ordinary prepared ceramics contained 0.15 and 0.024% carbon, respectively. The BZT ceramics obtained by SPS and the subsequent annealing at 1,000 °C for 12 h exhibited a mild temperature dependence of their dielectric constant. The field-induced displacement of the BZT ceramics was less hysteretic and smaller than that of the ceramics sintered by the conventional method.     

Effect of segregative additives on the positive temperature coefficient in resistance characteristics of n-BaTiO3 ceramics

The influence of B₂O₃, and Al₂O₃ as segregative additives in modifying the ρ–T characteristics has been studied in BaTiO₃ ceramics with positive temperature coefficient of resistance (PTCR). Reaction of Al₂O₃ at the grain boundary regions of BaTiO₃ ceramics leads to the segregation of the secondary phase, BaAl₆TiO₁₂ resulting in broad PTCR jump, whereas B₂O₃ addition gives rise to steeper resistivity jump. Microstructure studies by SEM reveal the formation of coherent second phase layer of barium aluminotitanate surrounding the BaTiO₃ grains. The EDX results shows varying Al to Ti ratio in the early stage of phase formation in BaAl₆TiO₁₂ resulting in electrically active layer around the BaTiO₃ grains. The TiO₂-excess melt formation results in lower resistivity for 2–4% Al₂O₃ containing n-BaTiO₃ ceramics whereas at higher alumina contents, BaAl₆TiO₁₂ phase becomes dominant leading to higher resistivity in the sample. Complex impedance analyses support the three-layer regions, corresponding to the contributions from grain interior resistance (R _g), grain boundary resistance (R _gb), and that from secondary phase (R _sec). Electron paramagnetic resonance spectroscopy (EPR) indicated barium vacancies, V _Ba ^/ as the major electron trap centers which are activated across the tetragonal-to-cubic phase transition. A charge trapping mechanism is proposed wherein the segregation of secondary phases bring carrier redistribution among the various acceptor states thereby affecting the electrical conductivity of n-BaTiO₃ ceramics. The presence of Al³⁺–O⁻–Al³⁺ or Ti⁴⁺–O⁻–Al³⁺ type hole centers at the grain boundary layer (GBL) regions results in charge redistribution across the modified phase transition temperature due to symmetry-related vibronic interactions resulting in broad PTCR characteristics extending to higher temperatures.     

Effects of talc and clay addition on pressureless sintering of porous Si<Subscript>3</Subscript>N<Subscript>4</Subscript> ceramics

Porous Si₃N₄ ceramics were successfully synthesized using cheaper talc and clay as sintering additives by pressureless sintering technology and the microstructure and mechanical properties of the ceramics were also investigated. The results indicated that the ceramics consisted of elongated β-Si₃N₄ and small Si₂N₂O grains. Fibrous β-Si₃N₄ grains developed in the porous microstructure, and the grain morphology and size were affected by different sintering conditions. Adding 20% talc and clay sintered at 1700°C for 2 h, the porous Si₃N₄ ceramics were obtained with excellent properties. The final mechanical properties of the Si₃N₄ ceramics were as follows: porosity, P ₀ = 45·39%; density, ρ = 1·663·g·cm⁻³; flexural strength, σ _b (average) = 131·59 MPa; Weibull modulus, m = 16·20.     

Electron transport behaviors across single grain boundaries in <Emphasis Type="Italic">n</Emphasis>-type BaTiO<Subscript>3</Subscript>, SrTiO<Subscript>3</Subscript> and ZnO

In some electroceramic materials, their unique electrical properties are due to potential barriers, i.e., double Schottky barriers (DSBs), formed at grain boundaries. So far, some researchers have revealed that the electrical properties of DSB are closely related to grain boundary characters, especially grain boundary coherency. For example, highly coherent boundary does not give PTCR or varistic property, while random types exhibit clear resistivity jump or abrupt current increment. Therefore, a concept of grain boundary design will be required for future device manufacturing, even in bulk materials. But it has not been clarified yet why the electron transport behaviors depend on them. In order to address this question, it is necessary to carry out a systematic experiment focusing on single grain boundaries using well-defined bicrystals.In the present study, we have summarized our studies with a special interest in electron transport behavior across single grain boundaries for n-type BaTiO₃, SrTiO₃ and ZnO.     

The effect of sintering temperature on magnetic and dielectric properties of Ho<Subscript>3</Subscript>Fe<Subscript>5</Subscript>O<Subscript>12</Subscript> ceramics

Ho₃Fe₅O₁₂ ceramics were fabricated by the solid-state reaction method. The results revealed an increase of the grain size, dielectric constant, and dielectric loss, while a decrease of the remnant magnetization and coercive field with increasing sintering temperature. A dielectric relaxation behavior was observed, which might be associated with the charge carrier hopping between Fe²⁺ and Fe³⁺. A cole–cole fitting to loss peaks revealed a dependence of the activation energy and the broaden factor on the relative density of the samples. Furthermore, at appropriate frequencies, the 1250 °C-sintered samples showed high dielectric constant, low dispassion, and good temperature stability around room temperature.     

Microstructure and electrical properties of Pr<Subscript>6</Subscript>O<Subscript>11</Subscript> doped ZnO-Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-based varistors

The microstructure and electrical properties of ZnO-Bi₂O₃-based varistor ceramics doped by Pr₆O₁₁ in the content range of 0–5.49 wt% were investigated at different sintering temperatures (1,100, 1,150, 1,175, 1,200 °C). The increase of sintering temperature leads to more dense ceramics, which increases the nonlinear property, whereas it decreases the voltage-gradient and leakage current. With increasing Pr₆O₁₁ content, the breakdown voltage increases because of the decreases of ZnO grain size. The improvement of non linear coefficient together with the decrease of leakage current are related to the uniformly distribution of secondary phases along the grain boundaries of the ZnO. The varistors sintered at 1,175 °C with the 3.37 wt% Pr₆O₁₁ doping possess the best electrical properties: the varistor voltage, nonlinear coefficient, and leakage current are 340 V/mm, 46 and 0.63 μA, respectively.     

Sintering and properties of highly donor-doped barium titanate ceramics

The electrical properties of donor (La³⁺) doped BaTiO₃ samples with a donor concentration in the range from 0.3 to 1.5 mol.% of La were studied. Samples were sintered at a low partial pressure of oxygen in order to facilitate anomalous grain growth and donor incorporation. In order to optimise the PTCR anomaly, the samples were annealed in air at 1100°C. Results show that with the use of a specific sintering profile PTCR ceramics containing an amount of donor dopant >0.3 mol.%, can be prepared. Heavily doped samples which do not exhibit anomalous grain growth show a core shell structure.     

M<Subscript>23</Subscript>C<Subscript>6</Subscript> precipitation behavior in a Ni-base superalloy M963

The M₂₃C₆ precipitation behavior in a cast Ni-base superalloy M963 was investigated after tensile creep testing at 800°C and strain-controlled low cycle fatigue testing at 700–950°C. During high temperature creep and low cycle fatigue, the primary MC decomposed into M₆C continuously, and a great amount of secondary carbide, chromium-rich M₂₃C₆, precipitated preferentially in the periphery of MC and γ +γ′ eutectic at grain boundaries. M₂₃C₆ was rarely present in grain interior, indicating that grain boundary promoted M₂₃C₆ carbide precipitation. The M₂₃C₆ precipitation was closely dependent on the stress state and testing temperature, seemed to be independent of the total strain amplitude. M₂₃C₆ is unstable during low cycle fatigue testing. The occurrence of M₂₃C₆ precipitation was sharply reduced during low cycle fatigue testing at 950°C. Crack was easily initiated at interface between MC and matrix, while fine M₂₃C₆ was effective to prevent grain boundary migration.     

Preparation of fine-grained ceramics by hot-pressing of Ce<Subscript>0.09</Subscript>Zr<Subscript>0.91</Subscript>O<Subscript>2</Subscript>/MgO/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanopowder

We have studied the effect of hot-pressing conditions (different pressure rise rates and isothermal holding temperatures in the range 1450–1550°C) on the microstructure of ceramics produced from nanopowder with the composition Ce_0.09Zr_0.91O₂/MgAl₆O₁₀/γ-Al₂O₃ (20.6, 37.4, and 42.0 wt %, respectively). Firing at 1450°C for 1 h made it possible to obtain fine-grained ceramics with less than 3 μm in grain size. The compaction pressure rise rate was shown to be a key parameter under such thermal conditions (20 + 10°C/min → 1450°C). Grain growth was prevented most effectively when the maximum load, 30 MPa, was reached at a temperature of 1000°C. Under such conditions, the grain size was 0.4–0.8 μm and the relative density reached 98.8%.     

Effect of partially oxidized Ti powders on the microstructure and PTCR characteristics of (Ba,Sr)TiO3

Rutile TiO₂ (a=4.594 å and c=2.958 å) phase was formed on the outer region of Ti powders after oxidation at 600 °C for 1–300 h. Porous (Ba,Sr)TiO₃ ceramics were fabricated by adding partially oxidized Ti powders (4–8 vol %) into (Ba,Sr)TiO₃ powders, and showed excellent positive temperature coefficient of resistivity (PTCR) characteristics after paste-baking treatment at 580 °C in air. The PTCR characteristics of the porous ceramics were mainly attributed to the adsorption of oxygen at the grain boundaries. The microstructure and electrical properties of the porous (Ba,Sr)TiO₃ ceramics containing the partially oxidized Ti powders oxidized at 600 °C for different oxidation times (1–300 h) were investigated.     

Microstructure,electrical properties of CeO<Subscript>2</Subscript>-doped (K<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>)NbO<Subscript>3</Subscript> lead-free piezoelectric ceramics

CeO₂-doped K_0.5Na_0.5NbO₃ lead-free piezoelectric ceramics have been fabricated by a conventional ceramic fabrication technique. The ceramics retain the orthorhombic perovskite structure at low doping levels (<1 mol.%). Our results also demonstrate that the Ce-doping can suppress the grain growth, promote the densification, decrease the ferroelectric–paraelectric phase transition temperature (T _C), and improve the dielectric and piezoelectric properties. For the ceramic doped with 0.75 mol.% CeO₂, the dielectric and piezoelectric properties become optimum: piezoelectric coefficient d ₃₃ = 130 pC/N, planar electromechanical coupling coefficient k _p = 0.38, relative permittivity ε_r = 820, and loss tangent tanδ = 3%.     

Microstructure and electrical properties of K<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>NbO<Subscript>3</Subscript> lead-free piezoelectric ceramics sintered in low pO<Subscript>2</Subscript> atmosphere

Pure K_0.5Na_0.5NbO₃ lead-free piezoelectric ceramics without any dopants/additives were sintered at various temperatures (950–1125 °C) in low pO₂ atmosphere (pO₂?~?10^?6 atm). All ceramics exhibit high relative densities (>?94%) and low weight loss (<?0.6%). Compared to the ceramics sintered in air, the ceramics sintered in low pO₂ exhibit improved electrical properties. The piezoelectric constant d₃₃ and converse piezoelectric constant d₃₃* are 112 pC/N and 119 pm/V, respectively. The ceramics show typical ferroelectric behavior with the remnant polarization of 21.6 µC/cm² and coercive field of 15.5 kV/cm under measurement electric field of 70 kV/cm. The good electrical properties of the present samples are related to the suppression of volatility of the alkali cations during the sintering process in low pO₂ atmosphere.     

Varistor properties and aging behavior of ZnO-V<Subscript>2</Subscript>O<Subscript>5</Subscript>-Mn<Subscript>3</Subscript>O<Subscript>4</Subscript> ceramics with sintering process

The microstructure, electrical properties, and DC-accelerated aging behavior of the ZnO-V₂O₅-Mn₃O₄ ceramics were investigated at different sintering temperatures of 850–925°C. The microstructure of the ZnO-V₂O₅-Mn₃O₄ ceramics consisted of ZnO grain as a primary phase, and Zn₃(VO₄)₂ which acts as a liquid-phase sintering aid, in addition to Mn-rich phase as secondary phases. The maximum value (3,172 V/cm) and minimum value (977 V/cm) of breakdown field were obtained at sintering temperature of 850 and 900°C, respectively. The nonlinear coefficient exhibited the highest value, reaching 30 at 925°C and the lowest value, reaching 4 at 850°C. The optimum sintering temperature was 900°C, which exhibited not only high nonlinearity with 24 in nonlinear coefficient, but also the high stability, with %ΔE_1mA = −0.9% and %∆α = −12.5% for DC-accelerated aging stress of 0.85 E_1mA/85°C/24 h.     

Dielectric characteristics of Pb(Zn<Subscript>1/3</Subscript>Ta<Subscript>2/3</Subscript>)O<Subscript>3</Subscript>-BaTiO<Subscript>3</Subscript> ceramics with/without PbTiO<Subscript>3</Subscript> modification

Stabilization tendencies of the perovskite structure in a Pb(Zn_1/3Ta_2/3)O₃-BaTiO₃ pseudobinary system with/without compositional modification by 20 mol% PbTiO₃ introduction were compared. In order to promote perovskite phase formation, the B-site precursor method (which is conceptually similar to the columbite process) was employed in this study. Dielectric properties of sintered samples were investigated as functions of composition and measurement frequency. Dielectric constant spectra, in the paraelectric temperature region, were further analyzed in terms of diffuseness. Microstructures of sintered specimens were also investigated and correlated with perovskite stabilization.     

Superionic Conductors in the Bi<Subscript>2</Subscript>WO<Subscript>6</Subscript>-Bi<Subscript>2</Subscript>VO<Subscript>5.5</Subscript> System

Bi₂V_xW_{1 − x} O_{6 − y} ceramics are synthesized, and their structure and electrical properties are studied. The results indicate that the Bi₂WO₆-Bi₂VO_5.5 system contains Bi₂WO₆- and Bi₂VO_5.5-based solid solutions in the ranges 0 < x ≤ 0.2 and 0.75 ≤ x < 1, respectively. Tungsten stabilizes the high-temperature, tetragonal phase γ-Bi₂VO_5.5, which persists down to room temperature at 0.75 ≤ x ≤ 0.84. In the range 350–550°C, the electrical conductivity of the bismuth-vanadate-based solid solutions exceeds that of Bi₂VO_5.5 by about one order of magnitude. The conductivity of the Bi₂WO₆-based solid solutions is also higher than that of the host phase.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 7, 2005, pp. 866–870.Original Russian Text Copyright © 2005 by Voronkova, Yanovskii, Kharitonova, Rudnitskaya.