Semiconducting Barium Titanate Ceramics Prepared by Boron-Containing Liquid-Phase Sintering

Semiconducting BaTiO₃ ceramics have been prepared by adding BN as a sintering aid. Density as high as 93% of theoretical and grain size as large as 16 μm are obtained after sintering at 1160°C. Most significant is that the semiconducting BaTiO₃ is obtained at sintering temperatures as low as 1100°C. The low-temperature-sintered BaTiO₃ exhibits a positive temperature coefficient. (PTC) anomaly above 120°C with a resistivity maximum at a temperature as high as 400°C, which is much higher than that of the conventional BaTiO₃. The incorporation of B into the perovskite structure is negligible. Also, the presence of B at a grain boundary after sintering is believed to enhance the PTC effect.     

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.     

Sol–Gel Coating of Powders for Processing Electronic Ceramics

A new technique (sol-gel coating of powders) was utilized to process electronic ceramics such as Sb-doped SnO₂ and positive-temperature-coefficient-of-resistance (PTCR) BaTiO₃ thermistors. Sol-gel coatings resulted in the densification via a liquid phase of Sb-doped SnO₂ powders. PTCR thermistors were developed from BaTiO₃ powders coated by a sol-gel composition containing dopants ( Y ), counterdopants (Mn, F), and additives (Si) with uniform microstructures and improved properties. Incorporation of F via the sol-gel coating increased the reliability of PTCR devices, making them less sensitive to reducing environments. This process may be extended to the processing of other electronic materials.     

Phase Transition and Physical Characteristics of Nanograined BaTiO3 Ceramics Synthesized from Surface-Coated Nanopowders

Nanograined BaTiO₃ ceramics prepared from 40-nm-size BaTiO₃ nanopowders exhibited the cubic as well as the tetragonal phase, while nanograined BaTiO₃ ceramics prepared from BaTiO₃ nanopowders coated with Mn had only the tetragonal phase. The dielectric constant of the latter was 10 times larger than that of the former; the latter exhibited PTCR behavior with a resistivity jump ratio of about 5.0 × 10⁴. These physical properties of the BaTiO₃ ceramics appeared to be significantly affected by the strain near grain boundaries; such strain resulted in a phase transition from the cubic to the tetragonal phase in the nanograined BaTiO₃ ceramics, even though the grain size was about 40 nm.     

Direct Examinations of PTC Action of Single Grain Boundaries in Semiconducting BaTiO3 Ceramics

R-T and I-V characteristics of single grains and grain boundaries in large-grained BaTiO₃ PTC ceramics were studied with a two-probe technique using a micromanipulator and fine Al wire. The PTC originates in the grain boundary only and behaves differently in each boundary. Even below T_c , the ceramic resistance depends almost entirely on the boundary. I-V characteristics in the boundary follow Ohm's law and conduction by a space-charge-limited current with a trap, using different applied voltages. The PTC anomaly relates to activation of the trap in the boundary, not to barrier height. A band model in the intergranular layer, with dielectric BaTiO₃ and the trap, is proposed.     

Phase Transitions in the System BaTiO3—KnbO3

Solid solution formation in the system BaTiO₃—KnbO₃ was established by X-ray diffraction and dielectric measurements. Solid solutions with cubic symmetry were observed in the composition range from 4 to 90 mole % KnbO₃ at room temperature. The lattice parameter for the BaTiO₃ solid solutions increased with increasing KNbO₃; that for the KnbO₃ solid solutions decreased with the addition of BaTiO₃. A distinct discontinuity in lattice parameter was observed at the composition containing about 65 mole % BaTiO₃. Dielectric measurements were made from-195° to 400°C. The cubic-tetragonal transition temperature of BaTiO₃ was rapidly lowered with increasing addition of KNbO₃, whereas the two lower phase transition temperatures were raised. All three phase transitions of KnbO₃ were rapidly lowered with increasing addition of BaTiO₃. The observed phase transitions, lattice parameters, and electron probe data suggest a complex region in the subsolidus which extends from 35 to about 75 mole % KNbO₃.     

Fabrication and Electrical Properties of Mn–Ni–Co–Cu–Si Oxides Negative Temperature Coefficient Thermistors

The microstructure and electrical properties of Mn–Ni–Co–Cu–Si oxides negative temperature coefficient (NTC) thermistors were studied. The as-sintered (Mn_1.62Ni_0.72Co_{0.57− x} Cu _x Si_0.09)O₄ (0≤ x ≤0.12) and (Mn_1.2Ni_0.78Co_{0.87− x} Cu_0.15Si _x )O₄ (0≤ x ≤ 0.15) ceramics showed the solid solutions of Mn–Ni–Co–Cu–Si oxides with a cubic spinel structure. The addition of SiO₂ led to an increase in the temperature coefficient of resistivity. This demonstrates that the SiO₂ addition is desirable for developing highly sensitive NTC thermistors. In addition, the resistivity and the temperature coefficient of resistivity for (Mn_1.62Ni_0.72Co_{0.57− x} Cu _x Si_0.09)O₄ and (Mn_1.2Ni_0.78Co_{0.87− x} Cu_0.15Si _x )O₄ NTC thermistors were controlled by changing the composition.     

Defect Structure and Dielectric Properties of Nd2O3-Modified BaTiO3

The defect structure and dielectric properties of BaTiO₃ with 1 to 10 mol% Nd₂O₃ additions were studied. The results indicated that neodymium occupies the barium site and charge compensation takes place by creation of titanium vacancies. The dependence of inverse electric susceptibility, spontaneous polarization, and specific heat on temperature for samples containing more than 2 mol% of Nd₂O₃ were characteristic of a diffuse transformation resulting from a disordering of defects. The addition of Nd2O3 leads to a very drastic sfiift in the Curie temperature ( Tc ) of BaTiO₃; 3 mel% Nd₂O₃ addition moves Tc below room temperature.     

Experimental Evaluation of the Acceptor-States Compensation in Positive-Temperature-Coefficient-Type Barium Titanate

Experimental evidence shows that the acceptor-state levels in Sb-doped positive-temperature-coefficient-type BaTiO₃ are compensated up to a critical acceptor-state density. Using the slope of the natural logarithm of the resistivity with respect to 1/ T , instead of maximum resistivity as a measure for the acceptor-state density, it is possible to estimate this critical value. The value obtained (4.2 × 10¹⁷ m⁻²) is believed to be the first reported estimate based on experimental data. It is in good agreement with the estimate of 6 × 10¹⁷ m⁻² (first reported by Jonker) obtained from the spontaneous polarization of BaTiO₃. This shows that the ferroelectric behavior of BaTiO₃ is indeed a feasible explanation for the low resistivity below the Curie point, as proposed by Jonker.     

Determination of Inversion Temperature of Sb2O3-Doped BaTiO3 Positive Temperature Coefficient of Resistivity (PTCR) Ceramics by the Finite Difference Method

The effect of the cooling rate on the PTCR (positive temperature coefficient of resistivity) characteristics of 0.1 mol% Sb₂O₃-doped BaTiO₃ ceramics has been investigated. Resistances both below and above the Curie temperature were increased by slow cooling, which indicated that the resistive layer width at the grain boundary increased as the cooling rate decreased. Concentration profiles of the Ba vacancies as a function of distance from the grain boundary have been simulated by the finite difference method. The inversion temperature of the 0.1 mol% Sb₂O₃-doped BaTiO₃ system was determined to be 1160°C from the measured electrical properties and computed concentration profiles.     

Grain-Boundary Migration and Dielectric Properties of Semiconducting SrTiO3 in the SrTiO3-BaTiO3-CaTiO3 System

Chemically induced grain-boundary migration and its effects on the interface and dielectric properties of semiconducting SrTiO₃ have been investigated. Strontium titanate specimens that had been doped with 0.2 mol% of Nb₂O₅ were sintered in 5H₂/95N₂. The sintered specimens were diffusion annealed at 1400°C in 5H₂/95N₂ with BaTiO₃ or 0.5BaTiO₃-0.5CaTiO₃ (mole fraction) packing powder. The grain boundaries of the annealed specimens were oxidized in air. In the case of BaTiO₃ packing, grain-boundary migration occurred with the diffusion of BaTiO₃ along the grain boundary. The effective dielectric constant of the specimen decreased gradually as the temperature increased but showed two peaks, possibly because of barium enrichment at the grain boundary and an oxidized Sr(Ba)TiO₃ layer. In the case of 0.5BaTiO₃-0.5CaTiO₃ packing, although barium and calcium were present at the grain boundary of the specimen, no boundary migration occurred, as in a previous investigation. With the diffusion of barium and calcium, the resistivity of the specimen increased and the variation of the effective dielectric constant with temperature was much reduced, in comparison to those without solute diffusion. These enhanced properties were attributed to the solute enrichment and the formation of a thin diffusional Sr(Ba,Ca)TiO₃ layer at the grain boundary.     

Influence of Ba/Ti Ratio on the Positive Temperature Coefficient of Resistivity Characteristics of Ca-Doped Semiconducting BaTiO3 Fired in Reducing Atmosphere and Reoxidized in Air

The positive temperature coefficient of resistivity (PTCR) characteristics of donor-doped BaTiO₃ fired in a reducing atmosphere and reoxidized in air are investigated. The result reveals that conventional semiconducting BaTiO₃ ceramics fired in a reducing atmosphere and reoxidized at a low temperature of 800°C in air show minimal PTCR characteristics, as reported earlier; however, Ca-doped BaTiO₃ with compositions in the range of 1.005≤(Ba+Ca+La)/Ti≤1.010 exhibit pronounced PTCR characteristics, even when reoxidized at such a low temperature. The semiconducting BaTiO₃ ceramics with {(Ba+Ca+La)/Ti}=1.005 and Ca-doped to 20 mol% exhibit remarkable PTCR characteristics with a resistivity jump of two orders of magnitude when they have been reoxidized at 800°C after firing in a reducing atmosphere.     

Lanthanum-Magnesium and Lanthanum-Manganese Donor-Acceptor-Codoped Semiconducting Barium Titanate

BaTiO₃ powder doped with La donor and codoped with Mn or Mg acceptor was sintered at 1350°C/1 h in air. For Ladoped BaTiO₃, the room-temperature resistivity decreased to a minimum at [La³⁺] ∼ 0.15 mol%. For La-Mn-codoped BaTiO₃, the minimum resistivity occurred at [La³⁺] - 2[Mn²⁺] ∼ 0.15 mol%. When the ceramic was changed to a fine-grained insulator by high donor doping ([La³⁺] >0.15 mol%), its semiconductivity was restored, and the relatively homogeneous, coarse-grained microstructure recurred by codoping with either Mg or Mn acceptor, with the transition at [La³⁺] - 2[Mg²⁺] = 0.15 mol% or [La³⁺] - 2[Mn²⁺] = 0.15 mol%. The analogy of a compensation effect between La-Mn- and La-Mg-codoped BaTiO₃ suggested that Mn acceptor added to BaTiO₃ exists as Mn²⁺ ion in the bulk grain region; its influence on the positive temperature coefficient of resistivity behavior is then discussed.     

Effect of Grain Boundary Structure on Diffusion-Induced Grain Boundary Migration in BaTiO3

The effect of grain boundary structure, either rough or faceted, on diffusion-induced grain boundary migration (DIGM) has been investigated in BaTiO₃. SrTiO₃ particles were scattered on the polished surfaces of two kinds of BaTiO₃ samples with faceted and rough boundaries and annealed in air for the samples with faceted boundaries and in H₂ for those with rough boundaries. In the BaTiO₃ samples with rough boundaries, an appreciable grain boundary migration occurred. In contrast, grain-boundary migration hardly occurred in the BaTiO₃ samples with faceted boundaries. The migration suppression observed in the sample with faceted boundaries was attributed to a low boundary mobility. The present experimental results show that DIGM is strongly affected by the boundary structure and can be suppressed by structural transition of boundaries from rough to faceted.     

Compensation Effect in Semiconducting Barium Titanate

Donor-doped, stoichiometric BaTiO₃ sintered at 1350°C for 1 h exhibits a maximum room-temperature conductivity at [La³⁺]∼0.15 mol%. Elements of lower valence than Ba²⁺ or Ti⁴⁺, when incorporated into semiconducting BaTiO₃, are regarded as poisoning impurities, i.e., acceptors. They tend to increase the room-temperature resistivity of the semiconducting BaTiO₃. For insulating BaTiO₃ resulting from high Mg²⁺ acceptor doping levels, the semiconductivity can be restored by introducing higher La³⁺ donor-dopant concentrations. This behavior is interpreted as a compensation effect based on the defect chemistry of the acceptor- and donor-doped BaTiO₃.     

Positive Temperature Coefficient of Resistance Effect in Heavily Niobium-Doped Barium Titanate by the Growth of the Double-Twinned Seeds

Positive temperature coefficient of resistivity (PTCR) was attained in heavily niobium-doped BaTiO₃ through the addition of BaTiO₃ seed particles. The seed particles containing a double twin grew during heat treatment, and a uniform microstructure composed of large, coarse grains was obtained. The incorporation of niobium into the grain via grain growth probably caused the semiconducting and PTCR character of the niobium-doped BaTiO₃.     

Effects of Ba6Ti17O40 on the Dielectric Properties of Nb-Doped BaTiO3 Ceramics

The dielectric properties, including the DC breakdown strength, of 1 mol% Nb⁵⁺-doped BaTiO₃ ceramics with different quantities of excess TiO₂ have been investigated. The breakdown strength was found to decrease with increasing TiO₂ content, but could not be readily explained by relative density and grain size effects. The decrease in the breakdown strength from a stoichiometric BaTiO₃ composition to samples with excess TiO₂ is believed to be due to the field enhancement effect (up to a factor of 1.40) at the BaTiO₃ matrix because of the presence of a Ba₆Ti₁₇O₄₀ second phase. The thermal expansion coefficient mismatch between the BaTiO₃ matrix phase and the Ba₆Ti₁₇O₄₀ phase may also result in a low breakdown strength. The dielectric properties of the pure Ba₆Ti₁₇O₄₀ phase were also investigated and are reported herein.     

High-Energy Density Capacitors Utilizing 0.7 BaTiO3–0.3 BiScO3 Ceramics

A high, temperature-stable dielectric constant (∼1000 from 0° to 300°C) coupled with a high electrical resistivity (∼10¹²Ω·cm at 250°C) make 0.7 BaTiO₃–0.3 BiScO₃ ceramics an attractive candidate for high-energy density capacitors operating at elevated temperatures. Single dielectric layer capacitors were prepared to confirm the feasibility of BaTiO₃–BiScO₃ for this application. It was found that an energy density of about 6.1 J/cm³ at a field of 73 kV/mm could be achieved at room temperature, which is superior to typical commercial X7R capacitors. Moreover, the high-energy density values were retained to 300°C. This suggests that BaTiO₃–BiScO₃ ceramics have some advantages compared with conventional capacitor materials for high-temperature energy storage, and with further improvements in microstructure and composition, could provide realistic solutions for power electronic capacitors.     

Mixed Oxide Capacitor of CuO—BaTiO3 as a New Type CO2 Gas Sensor

An oxide capacitor consisting of BaTiO₃ and an oxide is studied as a new type CO₂ sensor based on capacitance change. Sensitivity to CO₂, as well as the optimum operating temperature, was strongly dependent on the particular oxide mixed with BaTiO₃. Among the elements investigated in this study, CuO–BaTiO₃ exhibited the highest sensitivity to CO₂. In particular, the CuO–BaTiO₃ mixed oxide at the equimolar composition is highly sensitive to CO₂. The optimum operating temperature and frequency for CuO–BaTiO₃ are 729 K and 100 Hz, respectively, and the 80% response time to 2% CO₂ is within 25 s. The equimolar mixture of CuO and BaTiO₃ can measure the CO₂ concentration from 100 to 60 000 ppm. Carbonation of oxide seems to play a key role for the detection of CO₂ on these mixed oxide capacitors. The optimum operating temperature of these mixed oxide capacitors for CO₂ detection, therefore, correlates with the decomposition temperature of the carbonate corresponding to the oxide mixed with BaTiO₃. The capacitance increase of CuO–BaTiO₃ upon exposure to CO₂ seems to result from the elevated height of the potential barrier at the grain boundary between CuO and BaTiO₃. Carbonation of CuO in the element seems to bring about the elevation in the height of the potential barrier.     

Grain-Boundary Chemistry of Barium Titanate and Strontium Titanate: II, Origin of Electrical Barriers in Positive-Temperature-Coefficient Thermistors

Scanning transmission electron microscopy (STEM) of positive-temperature-coefficient (PTC) BaTiO₃ thermistors shows that the grain-boundary oxygen content in as-received (oxidatively cooled) materials is slightly enriched compared to quenched samples, and the acceptor-rich space-charge present at high temperatures is retained upon cooling. The defect density of the space charge is approximately equal to the acceptor state density at PTC boundaries determined by electrical measurements. Accordingly, it is proposed that the electrical barrier forms when acceptor defects already segregated in the ionic space charge at high temperature become active interface states when compensating donor defects in the grain-boundary core are oxidized. These acceptor defects appear to be primarily barium vacancies, but need not form upon cooling in the manner proposed by Daniels and Wernicke. Acceptor solutes when present can also contribute to barrier formation through space-charge segregation; the increase in interface state density upon addition of Mn is consistent with the magnitude of the expected segregation.