Synthesis of Y-Doped BaTiO3–(Bi1/2K1/2)TiO3 Lead-Free Positive Temperature Coefficient of Resistivity Ceramics and Their PTC Effects

As a lead-free positive temperature coefficient of resistivity (PTCR) material, (1– x mol%) BaTiO₃– x mol% (Bi_1/2K_1/2) TiO₃– y mol% Y₂O₃–0.5 mol% TiO₂ (BT– x BKT–2 y Y–0.5TiO₂) systems were prepared by the conventional solid-state reaction method. All samples containing <2 mol% BKT sintered in air possessed relatively low room-temperature resistivity (ρ₂₅) and high positive temperature coefficient (PTC) effect. However, when the BKT content exceeded 2 mol%, the sample was not semiconductive after sintering in air. The effects of sintering schedule on the properties of PTCR ceramics were discussed. The results showed that the optimum composition of BT–1BKT–0.2Y–0.5TiO₂, sintered at 1330°C for not-soaking and then fast quenched in air, achieved rather low ρ₂₅ of 28 Ω·cm and a high jump of resistivity (maximum resistivity [ρ_max]/minimum resistivity [ρ_min]) of 4.0 orders of magnitude with T _c about 155°C. The ρ₂₅ of the as-sintered sample could be further reduced to about 10 Ω·cm by annealing in N₂ at 450°C for 30 min, accompanied decrease on the PTC effect.     

Role of Liquid Phase in PTCR Characteristics of (Ba0.7Sr0.3)TiO3 Ceramics

The role of liquid phase in the enhancement of the PTCR (positive temperature coefficient of resistance) effect in (Ba_0.7Sr_0.3)TiO₃ (BST) with the addition of AST (4Al₂O₃· 9SiO₂· 3TiO₂) is investigated in this paper. The AST–BST samples were characterized with optical microscopy, transmission electron microscopy, energy-dispersive spectroscopy, and impedance spectroscopy. Microscopic observations showed that slower cooling might facilitate the precipitation of the (Ba,Sr)TiO₃ phase from the liquid phase on matrix grains since the amount of liquid phase was reduced with a decreasing cooling rate. Impedance spectroscopy indicated that this variation accompanied the change in the intrinsic properties of grain boundaries, which could not be explained by well-known oxidation effects. With the aid of a brick-layer model and high-resolution transmission electron microscopy (HRTEM), it appeared that the change in electrical characteristics of grain boundaries with decreasing cooling rate originated from the precipitation of (Ba,Sr)TiO₃. Finally, the effect of precipitated (Ba,Sr)TiO₃ on the PTCR characteristics is discussed in terms of the acceptor-state density and the polarization state at grain boundaries.     

Synthesis, Densification, and Phase Evolution Studies of Al2O3–Al2TiO5–TiO2 Nanocomposites and Measurement of Their Electrical Properties

Alumina–aluminum titanate–titania (Al₂O₃–Al₂TiO₅–TiO₂) nanocomposites were synthesized using alkoxide precursor solutions. Thermal analysis provided information on phase evolution from the as-synthesized gel with an increase in temperature. Calcination at 700°C led to the formation of an Al₂O₃–TiO₂ nanocomposite, while at a higher temperature (1300°C) an Al₂O₃–Al₂TiO₅–TiO₂ nanocomposite was formed. The nanocomposites were uniaxially compacted and sintered in a pressureless environment in air to study the densification behavior, grain growth, and phase evolution. The effects of nanosize particles on the crystal structure and densification of the nanocomposite have been discussed. The sintered nanocomposite structures were also characterized for dielectric properties.     

Sintering Behavior and Surface Microstructure of PbO-Rich PbNi1/3Nb2/3O3–PbTiO3–PbZrO3 Ceramics

The sintering behavior and surface microstructure of PbNi_1/3Nb_2/3O₃–PbTiO₃–PbZrO₃ (PNiNb-PT-PZ) ceramics were investigated. The PNiNb-PT-PZ ceramics with the stoichiometric composition and the addition of excess lead oxide (PbO-rich ceramics) were sintered by liquid-phase sintering in accordance with the solution-reprecipitation mechanism at temperatures below the melting point of PbO. The temperature at which the liquid phase forms fell to near the eutectic point of the PbO–Nb₂O₅ and the PbO–TiO₂ system (868°C) with the addition of 5 mol% PbO. As the calcination temperature influenced the sinterability of the stoichiometric PNiNb-PT-PZ ceramic, unreacted PbO was considered to be the source of the liquid phase in the sintering of the stoichiometric powder. The secondary phase was observed at the surface of PbO-rich ceramics and was suggested to be a liquid phase expelled from inside the ceramic. A sintering scheme of PNiNb-PT-PZ ceramics was proposed, and the high sinterability of PNiNb-PT-PZ ceramics was attributed to the low formation temperature of the liquid phase.     

Transmission Electron Microscopy Microstructure of 0.95(Na0.5K0.5)NbO3–0.05BaTiO3 Ceramics

Microstructural characterizations using transmission electron microscopy on 0.95(Na_0.5K_0.5)NbO₃–0.05BaTiO₃ ceramics sintered at 1030°–1150°C for 2 h were carried out. The liquid phase was found at the triple junction of the grains in all specimens and abnormal grain growth occurred in the presence of the liquid phase. Abnormally grown grains whose shapes were cuboidal were well developed. Anisotropically faceted amorphous liquid phase pockets were observed inside the grain in a specimen sintered at 1060°C for 2 h. The interface between the grain and the liquid matrix was flat and some were identified to be {100} planes of the grains. A certain amount of liquid at the sintering temperature of 1060°C enhanced the abnormal grain growth and contributed to the improvement of the piezoelectric properties.     

Microstructure and Piezoelectric Properties of (1−x)(Na0.5K0.5)NbO3–xLiNbO3 Ceramics

(1− x )(Na_0.5K_0.5)NbO₃– x LiNbO₃ [(1− x )NKN– x LN] ceramics were produced by the conventional solid-state sintering method, and their microstructure and piezoelectric properties were investigated. The formation of the liquid phase and K₆Li₄Nb₁₀O₃₀ second phase that were observed in the (1− x )NKN– x LN ceramics was explained by the evaporation of Na₂O during the sintering. A morphotropic phase boundary (MPB) was observed in the specimens with 0.05< x <0.08. Promising piezoelectric properties were obtained for the specimens with x =0.07. Therefore, the piezoelectric properties of this 0.93NKN–0.07LN ceramic were further investigated and were found to be influenced by their relative density and grain size. In particular, grain size considerably affected the d ₃₃ value. Two-step sintering was conducted at different temperatures to increase the grain size. Piezoelectric properties of d ₃₃=240 (pC/N) and k _p=0.35 were obtained for the 0.93NKN–0.07LN ceramics sintered at 1030°C and subsequently annealed at 1050°C.     

Sintering and Crystallization of CaO–Al2O3–ZrO2–SiO2 Glasses Containing Different Amount of Al2O3

In this work several complementary techniques have been employed to carefully characterize the sintering and crystallization behavior of CaO–Al₂O₃–ZrO₂–SiO₂ glass powder compacts after different heat treatments. The research started from a new base glass 33.69 CaO–1.00 Al₂O₃–7.68 ZrO₂–55.43SiO₂ (mol%) to which 5 and 10 mol% Al₂O₃ were added. The glasses with higher amounts of alumina sintered at higher temperatures (953°C [lower amount] vs. 987°C [higher amount]). A combination of the linear shrinkage and viscosity data allowed to easily find the viscosity values corresponding to the beginning and the end of the sintering process. Anorthite and wollastonite crystals formed in the sintered samples, especially at lower temperatures. At higher temperatures, a new crystalline phase containing ZrO₂ (2CaO·4SiO₂·ZrO₂) appeared in all studied specimens.     

Low-Temperature Sintering and Piezoelectric Properties of CuO-Added 0.95(Na0.5K0.5)NbO3–0.05BaTiO3 Ceramics

The sintering temperature of 0.95(Na_0.5K_0.5)NbO₃–0.05BaTiO₃ (NKN–BT) ceramics needs to be decreased below 1000°C to prevent Na₂O evaporation, which can cause difficulties in poling and may eventually degrade their piezoelectric properties. NKN–BT ceramics containing CuO were well sintered at 950°C with grain growth. Poling was easy for all specimens. Densification and grain growth were explained by the formation of a liquid phase. The addition of CuO improved the piezoelectric properties by increasing the grain size and density. High piezoelectric properties of d ₃₃=230 pC/N, k _p=37%, and ɛ₃^T/ɛ₀=1150 were obtained from the specimen containing 1.0 mol% of CuO synthesized by the conventional solid-state method.     

Processing and Properties of TiB2 with MoSi2 Sinter-additive: A First Report

The densification of non-oxide ceramics like titanium boride (TiB₂) has always been a major challenge. The use of metallic binders to obtain a high density in liquid phase-sintered borides is investigated and reported. However, a non-metallic sintering additive needs to be used to obtain dense borides for high-temperature applications. This contribution, for the first time, reports the sintering, microstructure, and properties of TiB₂ materials densified using a MoSi₂ sinter-additive. The densification experiments were carried out using a hot-pressing and pressureless sintering route. The binderless densification of monolithic TiB₂ to 98% theoretical density with 2–5 μm grain size was achieved by hot pressing at 1800°C for 1 h in vacuum. The addition of 10–20 wt% MoSi₂ enables us to achieve 97%–99%ρ_th in the composites at 1700°C under similar hot-pressing conditions. The densification mechanism is dominated by liquid-phase sintering in the presence of TiSi₂. In the pressureless sintering route, a maximum of 90%ρ_th is achieved after sintering at 1900°C for 2 h in an (Ar+H₂) atmosphere. The hot-pressed TiB₂–10 wt% MoSi₂ composites exhibit high Vickers hardness (∼26–27 GPa) and modest indentation toughness (∼4–5 MPa·m^1/2).     

Evolution of Microstructure and V-Shaped Positive Temperature Coefficient of Resistivity of (Pb0.6Sr0.4)TiO3 Materials

The effects of various liquid-phase sintering aids on (Pb_0.6SrO_0.4)TiO₃ ceramics have been investigated. The relationships between electrical properties and microstructures have been scrutinized. It has been found that, among the sintering aids studied, only SiO₂ exhibits a significant effect on the grain growth of (Pb_0.6SrO₄)TiO₃. The optimum firing profiles for sound microstructure and good electrical properties of (Pb_0.6SrO_0.4)TiO₃+ 5.0 mol% SiO₂ have been established. The V-shaped electrical behavior is prominent, and a PTCR jump of about 10^2.9 is observed. The formation of cation vacancies may increase the resistivity of the over-fired specimens. Various milling methods to pulverize the calcined powder and the optimum amount of packing protection powder during sintering are also discussed.     

Effect of TiO2 on Initial Sintering of Al2O3

Alpha alumina with additions of TiO₂ sintered more rapidly than "pure" alumina. The rate of initial sintering increased approximately exponentially with titania concentration up to a percentage beyond which the rate of sintering remained approximately constant or decreased slightly with additional titania. The concentration which produces the maximum rate of sintering is thought to be the solubility limit of TiO₂ in Al₂O₃. For alumina particles larger than about 2 μm, the kinetic process was mainly grain-boundary diffusion. With smaller particles, volume diffusion increased. The "solubility limit" increased with decreasing particle size, indicating an excess surface concentration of TiO₂. The data may be interpreted in terms of a region of enhanced diffusion at the grain boundary that increases with TiO₂ concentration. With small alumina particles, this region is large enough to become a significant portion of the volume of the particle, and the small particles appear to sinter by volume diffusion kinetics, but the diffusion coefficient corresponds to an enhanced diffusion coefficient.     

Microwave Dielectric Properties of 5Li2O–0.583Nb2O5–3.248TiO2 Ceramics with V2O5

The effects of V₂O₅ addition on the sintering behavior, microstructure, and the microwave dielectric properties of 5Li₂O–0.583Nb₂O₅–3.248TiO₂ (LNT) ceramics have been investigated. With addition of low-level doping of V₂O₅ (≤2 wt%), the sintering temperature of the LNT ceramics could be lowered down to around 920°C due to the liquid phase effect. A secondary phase was observed at the level of 2 wt% V₂O₅ addition. The addition of V₂O₅ does not induce much degradation in the microwave dielectric properties but lowers the τ_f value to near zero. Typically, the excellent microwave dielectric properties of ɛ_r=21.5, Q × f =32 938 GHz, and τ_f=6.1 ppm/°C could be obtained for the 1 wt% V₂O₅-doped sample sintered at 920°C, which is promising for application of the multilayer microwave devices using Ag as an internal electrode.     

Microstructures and Piezoelectric Properties in the Li2O-Excess 0.95(Na0.5K0.5)NbO3–0.05LiTaO3 Ceramics

As a candidate for lead-free piezoelectric materials, Li₂O-excess 0.95(Na_0.5K_0.5)NbO₃–0.05LiTaO₃ (NKN–5LT) ceramics were developed by a conventional sintering process. The sintering temperature was lowered by adding Li₂O as a sintering aid. Abnormal grain growth in NKN–5LT ceramics was observed with varying Li₂O content. This grain-growth behavior was explained in terms of interface reaction-controlled nucleation and growth. In the 1 mol% Li₂O excess NKN–5LT samples sintered at 1000°C for 4 h in air, the electromechanical coupling factor and the piezoelectric constant of NKN–5LT ceramics were found to reach the highest values of 0.37 and 250 pC/N, respectively.     

Effect of PO2 on Bulk and Grain Boundary Resistance of n-Type BaTiO3 at Cryogenic Temperatures

Complex impedance analysis at cryogenic temperatures has revealed that the bulk and grain boundary properties of BaTiO₃ polycrystals are very sensitive to the oxygen partial pressure during sintering. Polycrystals sintered at P _O2 as low as 10⁻¹⁵ atm were already electrically heterogeneous. The activation energy of the bulk conductivity in the rhombohedral phase was found to be close to that of the reduced undoped single crystal (i.e., 0.093 eV). The activation energy of the grain boundary conductivity increases with the temperature of the postsinter oxidation treatment from 0.064 to 0.113 eV. Analysis of polycrystalline BaTiO₃ sintered in reducing atmosphere and then annealed at P _O2= 0.2 atm has shown that the onset of the PTCR effect occurs at much higher temperatures than expected in the framework of the oxygen chemisorption model. The EPR intensity of barium and titanium vacancies increases after oxidation at T > 1000°C. A substantial PTCR effect is achieved only after prolonged annealing of the ceramic in air at temperatures as high as 1200–1250°C. This result suggests that the PTCR effect in polycrystalline BaTiO₃ is associated with interfacial segregation of cation vacancies during oxidation of the grain boundaries.     

A Study of the Phase Relations of ZrO2–TiO2 and ZrO2–TiO2–SiO2

The phase relations of the systems ZrO₂–TiO₂ and ZrO₂–TiO₂–SiO₂ were investigated. X-ray diffraction techniques served as the principal means of analysis. The binary system ZrO₂–TiO₂ was found to be one of partial solid solutions with no intermediate compounds. A eutectic point was found to exist at 50 to 55 weight % ZrO₂ and 1600°C. A preliminary investigation of the ternary system ZrO₂–TiO₂–SiO₂, although not extensive, resulted in a better understanding of this system, with a fairly accurate location of some of its boundary lines. A eutectic point was located at 2% ZrO₂, 10% TiO₂, and 88% SiO₂ at approximately 1500°C.     

Influence of TiO2 Solid Solution on the Thermal Property and Ionic Conductivity Of Partially Stabilized Zirconia

TiO₂(0–20 mol%)-3 mol% yttria-stabilized zirconia (3YSZ) ceramics were prepared by a solid-state reaction. With increasing TiO₂ content in 3YSZ, the structure of the main phase changed from a monoclinic, tetragonal, and cubic mixture to a tetragonal single phase. Increasing TiO₂ content in 3YSZ caused an increase in the average grain size of these ceramics. The thermal conductivity decreased from 4.1 to 2.1 at room temperature with an increase in the TiO₂ content. The specific heat of non-TiO₂-doped 3YSZ was slightly larger than all the doped TiO₂–3YSZ at room temperature. When the TiO₂ content was >8 mol% in 3YSZ, no abrupt expansion, shrinkage, or cracks were observed on heating and cooling these samples; thus, the thermal stability of 3YSZ was improved by TiO₂ solid solution. The ionic conductivity of the samples decreased with increasing TiO₂ solid solution.     

Effects of Sintering Atmosphere on Densification Behavior and Piezoelectric Properties of Pb(Ni1/3Nb2/3)O3–PbZrO3 Ceramics

Densification of polycrystalline Pb(Ni_1/3Nb_2/3)O₃–PbTiO₃–PbZrO₃ (PNN–PT–PZ) specimens was enhanced as the partial pressure of O₂ in the sintering atmosphere was increased. This observation was attributed to the increase in the internal pressure of a closed pore due to the thermal decomposition of PbO at a low partial pressure of O₂. The relative dielectric permittivity (ε_r), d ₃₃, k _p, and grain size of sintered specimens were also increased as the partial pressure of O₂ in the sintering atmosphere was increased. The observed dependence of piezoelectric properties on the partial pressure of O₂ was discussed in terms of the enhanced formation of the A-site vacancy ( V "_Pb) or the suppression of the B-site defect ( V ¨_O) as the oxygen potential increased.     

Critical Current Densities in Thin Ceramic Tapes of Superconducting Ba2YCu3O7

Tape-cast slurries of Ba₂YCu₃O₇ powders offer a convenient means of preparing sintered ceramic samples for critical current density (J_c) measurements where the transport cross section is small and the current electrode areas are large. Samples were sintered from 900° to 1000°C and characterized for bulk density, grain size, phase composition, T_c, and J_c. Bulk density and grain size both increase with sintering temperature while all samples were single-phase perovskite except for those sintered at 900°C. The onset temperature for superconductivity is constant at about 93 K while the transition sharpens to R=0 at about 92 K for the densest samples. J_c rises with sintering temperature to a maximum of ∼10³ A/cm². A linear relationship between J_c and bulk density is predicted from microstructural considerations.     

Subsolidus Phase Relationships in the System Fe2O3–Al2O3–TiO2 between 1000° and 1300°C

Subsolidus phase equilibria in the system Fe₂O₃–Al₂O₃–TiO₂ were investigated between 1000° and 1300°C. Quenched samples were examined using powder X-ray diffraction and electron probe microanalytical methods. The main features of the phase relations were: (a) the presence of an M₃O₅ solid solution series between end members Fe₂TiO₅ and Al₂TiO₅, (b) a miscibility gap along the Fe₂O₃–Al₂O₃ binary, (c) an α-M₂O₃( ss ) ternary solid-solution region based on mutual solubility between Fe₂O₃, Al₂O₃, and TiO₂, and (d) an extensive three-phase region characterized by the assemblage M₃O₅+α-M₂O₃( ss ) + Cor( ss ). A comparison of results with previously established phase relations for the Fe₂O₃–Al₂O₃–TiO₂ system shows considerable discrepancy.     

Oxygen Partial Pressure and Grain Growth in Donor-Doped BaTiO3

The grain growth of donor-doped BaTiO₃ at different oxygen partial pressures was studied. Results showed that the oxygen pressure had a pronounced influence on the grain growth and related effects. A model for the grain size anomaly during sintering of donor-doped BaTiO₃ in the presence of a TiO₂-rich liquid phase is proposed.