Electrical Properties of Acceptor Doped BaTiO3

Electrical properties of acceptor (Mn, Mg or Mn+Mg)-doped BaTiO₃ ceramic have been studied in terms of oxygen vacancy concentration, various doping levels and electrical degradation behaviors. The solubility limit of Mn on Ti sites was confirmed to be close to or less than 1.0 mol%. Oxygen vacancy concentration of Ba(Ti_{0.995 –x}Mg_0.005Mn_x)O_{2.995 –y} (x = 0, 0.005, 0.01) was estimated to be 50 times greater than that of the un-doped BaTiO₃. The leakage current of 0.5 mol% Mn-doped BaTiO₃ was stable with time, which was much lower than that of the un-doped BaTiO₃. The BaTiO₃ specimen co-doped with 0.5 mol% Mg and 1.0 mol% Mn showed the lowest leakage current below 10^{– 10}A. It was confirmed that leakage currents of Mg-doped and un-doped BaTiO₃ under dc field are effectively suppressed by Mn co-doping as long as the Mn doping level is greater than Mg contents.     

The effect of alumina addition on the electrical conductivity of Gd-doped ceria

Acceptor doped-ceria is a possible electrolyte material for the IT-SOFC (intermediate temperature solid oxide fuel cell) due to its high oxygen-ion conductivity. However, its use has been limited by its mechanical weakness and the appearance of electronic conductivity in reducing condition. In this study, alumina was selected as an additive in the doped-ceria to see if it increases the oxygen-ion conductivity and mechanical strength. Effects of alumina addition in doped ceria were studied as a function of alumina content and acceptor (Gd) content. The electrical conductivity of (Ce_1−x Gd _x O_2−δ)_1−y + (Al₂O₃) _y (x = 0–0.35, y = 0–0.10) was measured by using impedance spectroscopy. The grain conductivity of Ce_0.8Gd_0.2O_2-δ (GDC20) with 5 mol% alumina increased ∼3 times from that of GDC20 at 300^∘C. The grain conductivity was even ∼2 times higher than that of Ce_0.9Gd_0.1O_2−δ (GDC10) at 300^∘C. The electrical conductivity of GDC20 without alumina addition, measured at 500^∘C in air, rapidly decreased after exposure to reducing condition (Po₂∼10⁻²² atm) at 800^∘C. However, the decrease was much slower in GDC20 with alumina addition, indicating the improved mechanical strength. Among the examined compositions, (Ce_0.75Gd_0.25 O_2-δ)_0.95 + (Al₂O₃)_0.05 (GDC25A5) showed the highest conductivity at most temperatures.     

Low-temperature sintering and electrical properties of (1−x)Pb(Zr0.5Ti0.5)O3-xPb(Cu0.33Nb0.67)O3 ceramics

Electrical properties and sintering behaviors of (1 − x)Pb(Zr_0.5Ti_0.5)O₃-xPb(Cu_0.33Nb_0.67)O₃ ((1 − x)PZT-xPCN, 0.04 ≤ x ≤ 0.32) ceramics were investigated as a function of PCN content and sintering temperature. For the specimens sintered at 1050^∘C for 2 h, a single phase of perovskite structure was obtained up to x = 0.16, and the pyrochlore phase, Pb₂Nb₂O₇ was detected for further substitution. The dielectric constant (ε _r), electromechanical coupling factor (K_p) and the piezoelectric coefficient (d ₃₃) increased up to x = 0.08 and then decreased. These results were due to the coexistence of tetragonal and rhombohedral phases in the composition of x = 0.08. With an increasing of PCN content, Curie temperature (T_c) decreased and the dielectric loss (tanδ) increased. Typically, ε_r of 1636, K_p of 64% and d₃₃ of 473pC/N were obtained for the 0.92PZT-0.08PCN ceramics sintered at 950^∘C for 2 h.     

Microwave dielectric properties of Ca[(Li1/3Nb2/3)1−x Tix]O3−δ ceramics with glass

The effect of the addition of glass on the densification, low temperature sintering, and microwave dielectric properties of the Ca[(Li_1/3Nb_2/3)_1−x Ti_x]O_3−δ(CLNT) was investigated. Addition of glass (B₂O₃-ZnO-SiO₂-PbO system) improved the densification and reduced the sintering temperature from 1150^∘C to 900^∘C of Ca[(Li_1/3Nb_2/3)_1−x -Ti_x]O_3−δ microwave dielectric ceramics. As increasing glass contents from 10 wt% to 15 wt%, the dielectric constants (ε_r) and bulk density were increased. The quality factor (Q⋅f₀), however, was decreased slightly. The temperature coefficients of the resonant frequency (τ_f) shifted positive value as increasing glass contents over Ti content is 0.2 mol. The dielectric properties of Ca[(Li_1/3Nb_2/3)_0.75Ti_0.25]O_3−δ with 10 wt% glass sintered at 900^∘C for 3 h were ε_r = 40 Q·f₀ = 11500 GHz, τ_f = 8, ppm/°C. The relationship between the microstructure and dielectric properties of ceramics was studied by X-ray diffraction (XRD), and scanning electron microscope (SEM).     

Effect of partial substitution of Mg for Co in Mn1.4Ni1.2Co0.4O4 NTC thermistors on electrical stability

Sintered Mn_1.4Ni_1.2Co_{0.4 − x} Mg_xO₄ samples were thermal constrained by heating at 850^∘C followed by air cooling. As-sintered and thermal constraint samples were composed of Mn- and Ni-rich phases with a cubic spinel structure. The substituted Mg suppressed the separation of Ni-rich phase in a Mn_1.4Ni_1.2Co_{0.4 − x} Mg_xO₄ solid solution and resulted in a more stable spinel structure. In particular, the substituted Mg led to a significant decrease in the resistance drift of the Mn_1.4Ni_1.2Co_{0.4 − x} Mg_xO₄ NTC thermistors after thermal constraint. This indicates that the Mg substituted Mn_1.4Ni_1.2Co_{0.4 − x} Mg_xO₄ NTC thermistors have good electrical stability in comparison with the Mg-free Mn_1.4Ni_1.2Co_0.4O₄ thermistors. We strongly recommended the substitution of Mg for Co in Mn-Ni-Co-O NTC thermistors for stabilizing their resistivity drift, i.e., ageing.     

Temperature Sensitive Properties of the La(Ti<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>)O<Subscript>3</Subscript> System

The electric mechanisms of perovskite-type LaMnO₃ was investigated with B-site substitution in this paper. Samples of La(Ti_xMn_{1 − x})O₃ (0.1 ≰ x ≰ 0.7) were sintered at different temperature. The voltage-temperature (V-T) curves of the samples were tested from room temperature (25^∘C) to 300^∘C, then the electric properties were measured and analyzed. The experimental results showed that the resistivity-temperature (ρ-T) curves of the samples matched NTC characteristic. The resistivity increased slightly with the increase of Ti amount as x was less than 0.5, however, it rose greatly after x exceeded 0.5; The sintering temperatures have a little influence on the resistivity, except for the sample with x = 0.7.     

Control of 0.2CaTiO3-0.8Li0.5Nd0.5TiO3 microwave dielectric ceramics by additions of Bi2Ti2O7

Ceramics of 0.2CaTiO₃-0.8Li_0.5Nd_0.5TiO₃) have been prepared by the mixed oxide route using additions of Bi₂O₃-2TiO₂ (up to 15 wt%). Powders were calcined 1100^∘C; cylindrical specimens were fired at temperatures in the range 1250–1325^∘C. Sintered products were typically 95% dense. The microstructures were dominated by angular grains 1–2 μm in size. With increasing levels of Bi₂O₃-2TiO₂ additions, needle and lath shaped second phases developed. For Bi₂Ti₂O₇ additions up to 5 wt%, the relative permittivity increased from 95 to 131, the product of dielectric Q value and measurement frequency increased from 2150 to 2450 GHz and the temperature coefficient of resonant frequency (τ _f ) increased from −28pp/^∘C to +22pp/^∘C. A product with temperature stable τ _f could be obtained at ∼2 wt% Bi₂Ti₂O₇ additions. For high levels of additives, there is minimal change in relative permittivity, the Qxf values degrade and τ _f becomes increasingly negative.     

Dielectric properties of nanocrystalline TiO2 prepared using spark plasma sintering

Ultra fine rutile powders (below 50 nm) were prepared via the sol-gel process and bulk type TiO₂ specimens were fabricated using spark plasma sintering (SPS). The TiO₂ specimen sintered at a low temperature (720^∘C) exhibited a highly relative density (97%) and a nano-sized grain structure (200 nm). Dielectric properties of spark plasma sintered TiO₂ specimens including dielectric constants (k) and losses (tan δ) were measured. The TiO₂ specimen, obtained by SPS, showed a high dielectric constant (∼780) and a low tan δ (∼0.005), and a relaxation behavior at 1 MHz. After the subsequent annealing process of the TiO₂ specimen in O₂ flow, the dielectric constants remarkably decreased (k = 100s). These dielectric properties of nanocrystalline TiO₂ specimens prepared by SPS were discussed in terms of space charges produced by the reduction of Ti⁴⁺ ions and crystallographic orientations of grains.     

Low temperature sintering and microwave dielectric properties of Ba3Ti5Nb6O28 with B2O3 and CuO additions

The low sintering temperature and the good dielectric properties such as high dielectric constant (ε _r ), high quality factor (Q × f), and small temperature coefficient of resonant frequency (TCF) are required for the application of chip passive components in wireless communication low temperature co-fired ceramics (LTCC). In the present study, the sintering behaviors and dielectric properties of Ba₃Ti₅Nb₆O₂₈ ceramics were investigated as a function of B₂O₃-CuO content. The pure Ba₃Ti₅Nb₆O₂₈ system showed a high sintering temperature (1250^∘C) and had the good microwave dielectric properties: Q × f of 10,600 GHz, ε _r of 37, TCF of −12 ppm/^∘C. The addition of B₂O₃-CuO was revealed to lower the sintering temperature of Ba₃Ti₅Nb₆O₂₈, 900^∘C and to enhance the microwave dielectric properties: Q × f of 32,500 GHz, ε _r of 40, TCF of 9 ppm/^∘C. From the X-ray photoelectron spectroscopy (XPS) and X-ray powder diffraction (XRD) studies, these phenomena were explained in terms of the reduction of oxygen vacancies and the formation of secondary phases having the good microwave dielectric properties.     

Ni–Mn–Fe–Cr–O negative temperature coefficient thermistor compositions: Correlation between processing conditions and electrical characteristics

A study has been carried out to correlate the effect of sintering temperature on the microstructural, electrical and reliability aspects of Ni_0.75Mn_{(2.25−x−y)}Cr _x Fe _y O₄ (x = 0 to 0.3 and y = 0 to 0.3) negative temperature coefficient thermistor compositions prepared by solid-state route. The calcined and sintered compositions were characterized by X-ray diffraction and Scanning Electron Microscopy. The existence of cubic spinel single-phase region was determined by sintering Ni_0.75Mn_{(2.25−x−y)}Cr _x Fe _y O₄ samples in air at temperatures 1150 to 1250 °C. X-ray diffraction patterns of samples sintered above 1200 °C shows additional Bragg reflections of a rock salt structured NiO phase besides normal cubic spinel. A maximum B-value of 4044 K was obtained for Ni_0.75Mn_1.95Cr_0.25Fe_0.05O₄ composition at a sintering temperature 1250 °C/3 h. The reliability of the thermistor compositions were evaluated by performing accelerated ageing based on thermal cycling test. We found that chromium enhances the reliability of Ni_0.75Mn_{(2.25−x−y)}Cr _x Fe _y O₄ (x = 0 to 0.3 and y = 0 to 0.3) based NTC thermistor compositions. A maximum reliability of +0.25% resistance drift was observed at sintering temperature 1200 °C for 0.25 mol% chromium content. Excellent reliability of Ni_0.75Mn_{(2.25−x−y)}Cr _x Fe _y O₄ NTC thermistor compositions makes it ideal candidates for high-performance thermal sensor applications.     

The effect of Co addition on the electrical conductivity of Sr- and Mg-doped LaAlO3

Co was added to see its effect on the electrical conductivity of Sr- and Mg-doped LaAlO₃ (La_0.9Sr_0.1 Al_0.9Mg_0.1O₃, LSAM). Electrical conductivities of La_0.9Sr_0.1(Al_0.9Mg_0.1)_{1− x}Co_xO₃ (LSAMC) for x = 0–0.20 were measured using 2-probe a.c. and 4-probe d.c. method at temperature between 300 and 1300^∘C in air, and as a function of Po ₂ (1–10⁻¹⁵ atm) at 1200^∘C. Electrical conductivities in air increased with increasing Co content, while their activation energy decreased. From the impedance spectroscopy analysis, it was found that both the grain and the grain boundary conductivities of LSAMC samples increased rapidly with Co-addition. LSAMC samples were oxygen ion conductors in low Po₂ and mixed conductors in high Po ₂ up to x = 0.1 just like LSAM. With Co-doping, p-type conductivities increased, however, ionic conductivities remained nearly constant.     

Hydrogenation induced transformation of paramagnetism to room-temperature ferromagnetism in co-doped TiO2 ceramics

Effects of Co _x Ti _1−x O _2−δ on the sinterability and the ferromagnetism properties of Co ₂sO ₃/TiO ₂ (0.0 < x < 0.06) ceramics are investigated in this paper. It is found that the Co-doped Ti O ₂ ceramics transform from paramagnetism to room-temperature ferromagnetism (RTFM) after hydrogenation. With annealing temperatures at 600^∘ C and 1000^∘ C, these as-prepared samples present anatase and rutile structures respectively, which are analyzed with X-ray diffraction (XRD). After hydrogenation, the relation between temperature variations and the magnetic susceptibility for the hydrogenated samples were measured under zero-field-cooled and field-cooled conditions by using SQUID magnetometer. And the hysteresis loops are observed. These ferromagnetism resonance data suggest that the observed RTFM is at least partly due to the Cobalt nano-particles in our hydrogenated samples.     

Synthesis and properties of ferroelectric Si-doped (Bi, Nd)4Ti3O12 thin films by chemical solution deposition

Ferroelectric Si-doped (Bi,Nd)₄Ti₃O₁₂ thin films have been prepared on Pt/TiO_x/SiO₂/Si substrates through metal-organic compounds by the chemical solution deposition. The Bi_3.25Nd_0.75Ti_2.9Si_0.1O₁₂ (BNTS) precursor films were found to crystallize into the Bi-layered perovskite Bi₄Ti₃O₁₂ single-phase above 600^∘C. The synthesized BNTS films revealed a random orientation having a strong 117 reflection. The BNTS thin films prepared between 600^∘C and 700^∘C showed well-saturated P-E hysteresis loops with P _r of 13–14 μ C/cm² and E _c of 100–110 kV/cm at an applied voltage of 5 V. The surface roughness of the BNTS thin films was improved by Si doping compared with that of undoped Bi_3.35Nd_0.75Ti₃O₁₂ films.     

Ferroelectric properties of heterolayered lead zirconate titanate thin films

Heterolayered Pb(Zr_{1 − x} Ti _x )O₃ thin films consisting of alternating PbZr_0.7Ti_0.3O₃ and PbZr_0.3Ti_0.7O₃ layers were successfully deposited via a multistep sol-gel route assisted by spin-coating. These heterolayered PZT films, when annealed at a temperature in the range of 600–700^∘C show (001)/(100) preferred orientation, demonstrate desired ferroelectric and dielectric properties. The most interesting ferroelectric and dielectric properties were obtained from the six-layered PZT thin film annealed at 650^∘C, which exhibits a remanent polarization of 47.7 μC/cm² and a dielectric permittivity of 1002 at 100 Hz. Reversible polarization constituents a considerably high contribution towards the ferroelectric hysteresis of the heterolayered PZT films, as shown by studies obtained from C-V and AC measurement.     

Electrical properties of Pb0.98Eu0.02(ZryTi1−y)0.995O3 prepared by sol–gel processing

A Pb_0.98Eu_0.02(Zr_yTi_1−y )_0.995O₃ compound series, with y = 0.60, 0.53 and 0.45 was prepared. PZT samples were synthesized by sol–gel technique. The crystallization and quality of the compounds were analyzed by powder X-ray diffraction and electron microscopy. The shape of the ε′(ω) vs T curves can be considered typical of a ferro-paraelectric transition. The ferro-paraelectric transition temperature for each composition was 348, 328 and 307°C, for the y = 0.45, 0.53 and 0.60, respectively. σ′(ω) is strongly influenced by short range processes. For the logσ′(ω) curves as function of temperature, there is evidence of a non dispersive dc-conductivity component for the high temperature region. The associated dc-activation energies are larger than those calculated for the ac region (at lower temperatures).     

Synthesis and sintering properties of (La0.8Ca0.2−x Sr x )CrO3 perovskite materials for SOFC interconnect

Synthesis and sintering properties of the (La_0.8Ca_0.2−x Sr _x )CrO₃ samples doped by two alkaline earth metals in comparison to the doped only by one alkaline earth metal were evaluated by phase analysis, sintering properties, thermal expansion behaviors, and electrical conductivity. The sintered (La_0.8Ca_0.2−x Sr _x )CrO₃ (x = 0, 0.05, and 0.1) and (La_0.8Ca_0.2−x Sr _x )CrO₃ (x = 0.2) were found to have orthorhombic and rhombohedral symmetries, respectively. Relative density of the (La_0.8Sr_0.2)CrO₃ sample sintered at 1500^∘C for 5 h was lower than that of the (La_0.8Ca_0.2−x Sr _x )CrO₃ (x = 0, 0.05, and 0.1) sample. TECs of the (La_0.8Ca_0.2−x Sr _x )CrO₃ (x = 0, 0.05, 0.1, and 0.2) in air were 11 × 10⁻⁶/^∘C, 11.2 × 10⁻⁶/^∘C, 11.2 × 10⁻⁶/^∘C, and 11.3 × 10⁻⁶/^∘C, respectively. The electric conductivity of the (La_0.8Ca_0.2−x Sr _x )CrO₃ sample was determined.     

Application of spark plasma sintering for growing dense Pb(Mg1/3Nb2/3)O3–35 mol% PbTiO3 single crystal by solid-state crystal growth

This study examined the effect of spark plasma sintering (SPS) on the densification behavior and resulting dielectric and piezoelectric properties of Pb(Mg_1/3Nb_2/3)O₃–35 mol% PbTiO₃ ceramics with a 5 mol% excess of PbO. Through normal sintering at 1200^∘C, the density of the specimen reached only 92% of the theoretical density (TD). However, with the SPS treatment, the density of the PMN-PT ceramics increased to more than 99% of the TD at 900^∘C, and maintained over 98% of the TD during subsequent heat-treatment at 1200^∘C for 10 h. The increased density of the Pb(Mg_1/3Nb_2/3)O₃–35 mol% PbTiO₃ ceramics resulted in an improvement in the dielectric and piezoelectric properties. The SPS treatment was also successfully applied to the densification of a PMN-PT single crystal grown on a BaTiO₃ seed crystal using a solid-state crystal growth (SSCG) process.     

Piezoelectric properties of a Ba(Zr0.075Ti0.925)O3 single crystal induced by poling treatment

Piezoelectric properties in a Ba(Zr_0.075Ti_0.925)O₃ single crystal were investigated with a consideration that its phase transition lies on room temperatures and the transition can affect piezoelectric properties. The poling treatments were carried with bias fields ranging 2.2 kV/cm∼11 kV/cm at room temperature. The crystal possesses the phase transition from orthorhombic to rhombohedral, lying on room temperature. At 37^∘C, the crystal exhibited the excellent piezoelectric properties, a d₃₁ coefficient of over 500 pC/N and a k₃₁ coefficient of 0.38, being caused by the phase transition.     

Gas sensors: New materials and processing approaches

To satisfy demands for detecting chemicals in the environment, versatile sensors are required to detect a rapidly growing range of chemical species. In this paper, focus is directed on progress being made to develop temperature-independent oxygen sensors based on the perovskite solid solution system SrTi_1−x Fe _x O_3−δ,and on improving the sensitivity of thin film gas sensors integrated on Si-based self heated substrates. A novel strategy to produce macroporous films with high surface area for enhanced chemical activity is described, and how this processing strategy results in markedly improved sensitivity of gas sensors based on a novel material, CaCu₃Ti₄O₁₂, is demonstrated.     

Ni-Cu-Zn Ferrites for Low Temperature Firing: I. Ferrite Composition and its Effect on Sintering Behavior and Permeability

Ni-Cu-Zn ferrites of composition Ni_{1 − x − y}Cu_yZn_xFe₂O₄ with 0.4 ≰ x ≰ 0.6 and 0 ≰ y ≰ 0.25 were prepared by standard ceramic processing routes. The density of samples sintered at 900^∘C increases with copper concentration y. Dilatometry reveals a significant decrease of the temperature of maximum shrinkage with y. The permeability has maximum values of μ = 500–1000 for x = 0.6. The Curie temperature is sensitive to composition and changes form about 150^∘C for x = 0.6 to T_c > 250^∘C for x = 0.4, almost independent on the Cu-content. A small iron deficiency in Ni_0.20Cu_0.20Zn_{0.60 + z}Fe_{2 − z}O_{4 − (z/2)} with 0 ≰ z ≰ 0.06 significantly enhances the density of samples sintered at 900^∘C. The maximum shrinkage rate is shifted to T < 900^∘C. These compositions are therefore appropriate for application in low temperature co-firing processes. The permeability is reduced with z, hence a small z = 0.02 seems to be the optimum ferrite composition for high sintering activity and permeability.