Substrate Effect on the Magnetoelectric Behavior of Pb(Zr0.52Ti0.48)O3 Film-On-CoFe2O4 Bulk Ceramic Composites Prepared by Direct Solution Spin Coating

Magnetoelectric (ME) composite structures of Pb(Zr_0.52Ti_0.48)O₃ (PZT) and CoFe₂O₄ (CFO) were prepared by directly growing PZT films on highly dense CFO ceramics via a simple solution spin coating, rather than by conventional high-temperature cofiring. An obvious ME response, which had the same bias-dependent trend as the piezomagnetic coefficient of CFO ceramics, was observed in such film-on-bulk ceramic composites. It was found that the PZT films showed a good ferroelectric feature, and the ME response of the composites strongly depended on the resistivity of the CFO ceramics as both a substrate and a bottom electrode. The results suggest plenty of room for further enhancing the ME response of such films-on-ceramic substrate composites.     

Compensating Defects in Highly Donor-Doped BaTiO3

Nb-doped BaTiO₃ has been prepared with various Ba/(Ti + Nb) ratios such that single-phase products will be obtained if the charged donor center is assumed to be compensated in turn by Ba vacancies, Ti vacancies, equal concentrations of the two cation vacancies, oxygen interstitials, or electrons. For air-Fired samples, examination by transmission electron microscopy showed that only the composition adjusted for compensation by titanium vacancies was single phase. The other compositions contained a Ti-rich second phase in order to achieve a matrix with the appropriate concentration of titanium vacancies. When sintered in a reducing atmosphere, compensation was by electrons, and a Ba-rich second phase was present hi the composition adjusted to give compensation by titanium vacancies. The results indicate that for donor concentrations greater than ∼0.5 mol% in BaTiO₃, charge compensation is achieved by Ti vacancies under oxidizing conditions, and by electrons (as is well-known) under reducing conditions.' The effect of compensating defects on grain growth is also discussed.     

Resistance Degradation in Y(Cr,Mn)O3–Y2O3 Composite NTC Ceramics in Hostile Environments

A series of composite, negative temperature coefficient (NTC) ceramics were carefully processed with compositions based on the Y(Cr,Mn)O₃+Y₂O₃ system and these were investigated for resistance stability in hostile environments. This specific system is of interest for high-temperature automobile thermistors, however either through the processing or in use of these, materials can be exposed to reducing atmospheres at temperatures around 900°–1000°C. The thermochemical processes at intermediate temperatures and low     <10⁻¹⁰atm can influence the resistance of the given ceramics. Through an impedance analysis it is determined that the resistance increase is associated primarily with a grain boundary resistance increase. The grain and grain boundary elements are modeled through parallel constant phase element and resistance equivalent circuits connected in series. Possible origins of the defect chemistry being controlled through high-temperature processes at the sintering are partial Schottky reactions that are compensated through a superoxidation reaction on cooling and aging. The reduction process reversed the superoxidation reaction and transited the grain boundary surfaces to ionically compensated B-site vacancies with oxygen vacancies.     

Electrical Properties of Cerium-Doped BaTiO3

The high-temperature equilibrium electrical conductivity of Ce-doped BaTiO₃ was studied in terms of oxygen partial pressure, P (O₂), and composition. In (Ba_1−xCe _x )TiO₃, the conductivity follows the −1/4 power dependence of P (O₂) at high oxygen activities, which supports the view that metal vacancies are created for the compensation of Ce donors, and is nearly independent of P (O₂) where electron compensation prevails at low P (O₂). When Ce is substituted for the normal Ti sites, there is no significant change in conductivity behavior compared with undoped BaTiO₃, indicating the substitution of Ce as Ce⁴⁺ for Ti⁴⁺ in Ba(Ti_1−yCe _y )O₃. The Curie temperature ( T _c) was systematically lowered when Ce³⁺ was incorporated into Ba²⁺ sites, whereas the substitution of Ce⁴⁺ for Ti⁴⁺ sites resulted in no change in this parameter.     

Multiferroic Properties of Single-Phase Bi0.85La0.15FeO3 Lead-Free Ceramics

Single-phase and lead-free Bi_0.85La_0.15FeO₃ multiferroic ceramics were prepared by a rapid liquid sintering method to study their crystal structure and multiferroic behaviors. Rietveld refinement of X-ray diffraction data showed a pseudo-cubic structure plus a small triclinic distortion. The absence of Fe²⁺ ions suggested few oxygen vacancies. Large polarizations of 20.3 μC/cm² together with a piezoelectric d ₃₃ coefficient of ∼27.7 pC/N were achieved. This ferroelectricity originates from the stereochemical activity of the A-site Bi³⁺ lone electron pair. The remnant magnetizations of 0.0744–0.227 emu/g of Bi_0.85La_0.15FeO₃ permitted by the canting angle of canted antiferromagnetic order was released as a result of the collapse of the space-modulated spin structure of BiFeO₃. The observed dielectric anomalies proved the magnetoelectric couplings between ferroelectric order and canted antiferromagnetic order.     

Effect of MnO2 Addition on the Structure and Electrical Properties of Pb(Zn1/3Nb2/3)0.20(Zr0.50Ti0.50)0.80O3 Ceramics

Pb(Zn_1/3Nb_2/3)_0.20(Zr_0.50Ti_0.50)_0.80O₃ ceramics of pure perovskite structure were prepared by the two-stage method with the addition of 0–3.0 wt% MnO₂ and their piezoelectric properties were investigated systematically. The MnO₂ addition influences in a pronounced way both the crystal structure and the microstructure of the materials. The materials are transformed from the tetragonal to the rhombohedral structure, and the grain size is enhanced when manganese cations are added. The distortion of crystal structure for samples with MnO₂ addition can be explained by the Jahn–Teller effect. The values of electromechanical coupling factor ( k _p) and dielectric loss (tan δ) are optimized for 0.5-wt%-MnO₂-doped samples ( k _p= 0.60, tan δ= 0.2%) and the mechanical quality factor ( Q _m) is maximized for 1.0-wt%-MnO₂-doped samples ( Q _m= 1041), which suggests that oxygen vacancies formed by substituting Mn³⁺ and Mn²⁺ ions for B-site ions (e.g., Ti⁴⁺ and Zr⁴⁺ ions) in the perovskite structure partially inhibited polarization reversal in the ferroelectrics. The ceramics with 0.50–1.0 wt% MnO₂ addition show great promise as practical materials for piezoelectric applications.     

Decomposition Reactions in CaCu3Ti4O12 Ceramics

CaCu₃Ti₄O₁₂ (CCTO) ceramics sintered in air at 1115°C for 3 and 24 h have been heat treated in N₂ at 1000°C. Surface layers develop on the outer regions of the ceramics, and a combination of X-ray diffraction and analytical electron microscopy has been used to establish the phase content of the layers. A model to explain the formation of the surface layers is proposed based on decomposition of CCTO into a mixture of CaTiO₃, TiO₂, and Cu₂O. The role of limited decomposition in the development of electrically inhomogeneous CCTO ceramics prepared at elevated temperatures in air is discussed.     

Analysis of Phase Coexistence in Fe2O3-Doped 0.2PZN–0.8PZT Ferroelectric Ceramics by Raman Scattering Spectra

In this work, we suggested a method to evaluate quantitatively the effect of doping oxide on the phase coexistence of PbZr_{1− x} Ti _x O₃ (PZT)-based ceramics through the analysis of the Raman scattering spectra. Theoretically, the degenerated T _3u mode in the cubic phase of PZT will transform as A ₁(3) and E (4) modes in the tetragonal phase or as rhombohedral ( R ) modes in the rhombohedral phase below the Curie temperature, which set up the theoretical base to study the phase coexistence in ferroelectric materials. Through separation by fitting of the Raman bands, the shifts and intensities of different Raman vibration modes were determined. A calculation equation representing the phase coexistence was put forward based on the theoretical analysis of the degenerated T _3u modes. The results showed that a turning point appears at the Fe₂O₃ addition of 0.3%. The variation in the electrical properties of the Fe₂O₃-doped Pb(Zn_1/3Nb_2/3)O₃ (PZN)–PZT ceramics also affirmed the turning point of the phase evolution as the addition of Fe₂O₃.     

Effects of Cd-Substitution Site on the Electrical Conductivities in Pb(Ni1/3Nb2/3)O3-PbZrO3-PbTiO3 Ceramics

The effects of Cd²⁺ substitution for Pb²⁺ and Ni²⁺ ions (on the A-site and B-site of the ABO₃ perovskite structure, respectively) on the electrical conductivities in Pb(Ni_1/3-Nb_2/3)O₃-PbZrO₃-PbTiO₃ (PNN-PZ-PT) ceramics have been investigated. Generally, the compounds that contain PbO show p -type conductivity, because of PbO evaporation. Thus, the conductivities are known to be proportional to the PbO evaporation. However, PNN-PZ-PT ceramics exhibit a reciprocal relationship between the conductivities and the evaporation of PbO. Generally, no vacancy change is observed with the substitution of the same-charge valence ion. However, in the PNN-PZ-PT ceramic, lead vacancies could be created or annihilated by replacing Pb²⁺ and Ni²⁺ ions with Cd²⁺ ions, because of incomplete substitution. The electrical conductivities are influenced by this incomplete substitution.     

Defect Structure of Ta2O5

Electrical conductivity, thermoelectric power, and weight change were measured for polycrystalline Ta₂O₅ from 900° to 1400°C. The predominant ionic and electronic defects in this temperature range are oxygen vacancies and electrons. The oxygen-vacancy and electron mobilities are 8.1 × 10³exp (−1.8 eV/ k T) and ∼0.05 cm²/V-s, respectively. At O₂ partial pressures near 1 atm, the ionic-defect concentration is essentially fixed by the presence of lower-valence cation impurities, and the total electrical conductivity is predominantly ionic, whereas at low P _o2's the conductivity is electronic and proportional to P P _o2^−1/6.     

Surface Decomposition of Strontium-Doped Soft PbZrO3–PbTiO3

Sintering temperature has a pronounced effect on perovskite phase stability at the surface of Pb_0.88Sr_0.12Zr_0.54Ti_0.44Sb_0.02O₃ (PSZT) soft piezoelectric ceramics ( d ₃₃≈ 600 pC/N). After sintering 4 h at 1070°C, XRD reveals only perovskite PSZT peaks in the bulk and at the surface. As sintering temperature increases, XRD from the ceramic surface reveals a second-phase peak at ∼27° (2θ), 0.316 nm ( d -spacing). After 4 h at 1280°C, further second-phase peaks are observed, confirming it to be monoclinic ZrO₂, accompanied by a strong increase in the degree of tetragonality of the perovskite phase. These observations are consistent with decomposition of the PSZT to ZrO₂ and tetragonal PZT (PbZrO₃–PbTiO₃) associated with PbO loss. SEM and cross-sectional TEM indicated that surface decomposition had progressed ∼0.5 mm into the sample after 4 h at 1280°C.     

Fabrication of Na0.5Bi0.5TiO3–BaTiO3-Textured Ceramics Templated by Plate-Like Na0.5Bi0.5TiO3 Particles

Textured 0.94Na_0.5Bi_0.5TiO₃–0.06BaTiO₃ (NBT–6BT) ceramics were fabricated by templated grain growth (TGG) using anisotropically shaped Na_0.5Bi_0.5TiO₃ (NBT) templates. Platelet NBT was synthesized by the topochemical technique, using precursor Na_0.5Bi_4.5Ti₄O₁₅ (NBIT). The NBT particles have an average length of 10–15 μm and a thickness of 1 μm, which are suitable templates for obtaining textured ceramics (especially NBT-based ceramics) by the TGG process. This study revealed that the NBT templates are effective in inducing grain orientation in NBT–6BT ceramics. For NBT–6BT ceramics textured with 5 vol% NBT templates, a Lotgering factor of 0.87 and a d ₃₃ of 299 pC/N are given.     

Incorporation of Er3+ into BaTiO3

The incorporation of Er³⁺ into BaTiO₃ ceramics was investigated on samples containing 0.25, 0.5, 1, 2, 8, and 10 at.% of dopant, after sintering at 1350–1550°C in air. For Er³⁺ concentrations ≤1 at.%, dense and large-grained ceramics with low room-temperature resistivity (10²–10³Ω·cm) were obtained. The observed properties are largely independent of stoichiometry. Simultaneous substitution of Er³⁺ at both cation sites, with higher preference for the Ba site, is proposed. The behavior of heavily doped ceramics depends on stoichiometry. When Ba/Ti < 1, the electrical properties change from slightly semiconducting to insulating as Er concentration increases from 2 to 8 at.%. The ceramics have tetragonal perovskite structure and contain a large amount of Er₂Ti₂O₇ pyrochlore phase. On the other hand, when Ba/Ti > 1, the ceramics are insulating, fine-grained, and single phase. In this case, incorporation of Er³⁺ predominantly occurs at the Ti site, with oxygen vacancy compensation. Incorporation is accompanied by a significant reduction of tetragonality and by expansion of the unit cell. The different results indicate that Er³⁺ solubility at the Ba site does not exceed 1 at.%, whereas solubility at the Ti site is at least 10 at.%. However, the incorporation of Er³⁺ and the resulting properties are also strongly affected by sintering conditions.     

Synthesis and Electrical Properties of Dense Ce0.9Gd0.1O1.95 Ceramics

Uniform spherical powders of Ce_0.9Gd_0.1O_1.95 with an average diameter of 250 nm were obtained at 700°C from a sol-gel process of mixing nitrates and ethylene glycol. Broadening of the X-ray peaks of the fluorite structure reveals a small crystallite size within the powders. Sintering in air of pressed pellets of the powders at 1585°C gives ceramics of 99% theoretical density with grain sizes 1-10 µm and a cubic unit cell a = 5.422 ± 0.034 Å. The electrical conductivity σ=σ_o+σ_e has two components. In air or argon, the electronic component σ_e is negligible and the oxide-ion conductivity σ_o is not described by a classical Arrhenius equation; a pronounced curvature at T similar/congruent T * has been observed in the Arrhenius plot of the bulk conductivity. The system could be modeled by a condensation of mobile oxygen vacancies into ordered clusters below a temperature T * similar/congruent 583 ± 45°C and a motional enthalpy Δ H _m= 0.63 ± 0.01 eV for the vacancies. A measured trapping energy Δ H _t(1 - T/T *) has Δ H _t= 0.19 ± 0.01 eV = 2.57 kT *. In a reducing atmosphere, σ_e exhibits a small-polaron motional enthalpy Δ H _p= 0.40 ± 0.08 eV for transfer of a 4 f electron from a Ce³⁺ to a Ce⁴⁺ ion and a Δ H _p+Δ H _pt= 0.51 ± 0.04 eV at T < T *.     

Effect of Alkali Elements Content on the Structure and Electrical Properties of (K0.48Na0.48Li0.04)(Nb0.90Ta0.04Sb0.06)O3 Lead-Free Piezoelectric Ceramics

In this communication, we report on the effects of alkaline elements (K, Na, and Li) content on the electrical properties of (K_0.48Na_0.48Li_0.04)(Nb_0.90Sb_0.06Ta_0.04)O₃ ceramics. Because of the high volatility, the alkaline elements evaporate during the high-temperature sintering process, generating vacancy defects in the ceramics. As a result, the ferroelectric and piezoelectric properties of the ceramics become weakened. Our results reveal that the volatilization loss can be effectively compensated by a 1 mol% excess of alkaline elements. The remanent polarization P _r, piezoelectric coefficient d ₃₃, and electromechanical coupling coefficient k _p of the compensated ceramics are improved by 15%, 16%, and 20%, respectively, having a value of 30 μC/cm², 310 pC/N, and 0.6.     

Surface Chemical and Leakage Current Density Characteristics of Nanocrystalline Ag–Ba0.5Sr0.5TiO3 Thin Films

Nanocrystalline x Ag–(1− x )Ba_0.5Sr_0.5TiO₃ (Ag–BST, 0≤ x ≤0.1, where x is the mole fraction of Ag) thin films have been deposited on Pt/Ti/SiO₂/Si substrates by a sol–gel method. The films have been characterized by X-ray diffraction (XRD), scanning electron microscopy, and X-ray photoelectron spectroscopy (XPS). The core-level XPS of oxygen (O1 s ) of the Ag–BST films indicate that an optimum amount of Ag ( x =0.02 or 2 mol%) enhances the binding energy of oxygen, possibly through a mechanism in which the electrophilic oxygen dissociates from the Ag surface and fills the oxygen vacancies. Similarly, the binding energy of Ag (Ag3 d ) shows a shift toward a higher value with increasing Ag up to 4 mol%, probably because of the chemical shift of Ag in BST along the surface layers, surface relaxation, or changes in the Fermi level of small Ag particles in the solid solution range of Ag in BST films ( x ≤0.04). The leakage current density of 2 mol% Ag-added BST (∼10⁻⁶ A/cm²) is less by about an order of magnitude than pure BST at an electric field of 200 kV/cm. A defect model is proposed to explain the observed leakage current density of Ag–BST films satisfactorily.     

Microstructure Characterization of the (1−x)La2/3TiO3·xLaAlO3 System

Microstructural characterizations on the (1− x )La_2/3TiO₃· x LaAlO₃ (LTLA) system were conducted using transmission electron microscopy. The presence of La₂Ti₂O₇ and La₄Ti₉O₂₄ phases in pure La_2/3TiO₃ is confirmed by the electron diffraction pattern. When x = 0.1, the ordering due to the A-site vacancies could be confirmed by the presence of antiphase boundaries (APBs) and return ½(100) superlattice reflection. As x increases, the ordering decreases and finally disappears when x = 0.6. The tilting of oxygen octahedra could be demonstrated by the presence of the ferroelastic domains in the matrix and return ½(111) and return ½(110) superlattice reflections in selected area electron diffraction patterns. In pure LaAlO₃, only the antiphase tilting of oxygen octahedra is present due to the presence of return ½(111) superlattice reflection. In the LTLA system of x = 0.1, both the antiphase and in-phase tiltings of the oxygen octahedra are involved; however, in the range of x from 0.3 to 0.9, the antiphase tilting of oxygen octahedra has appeared. The growth of the ferroelastic domains is influenced by the APBs in the matrix.     

Reversible Weight Change of Acceptor-Doped BaTiO3

The oxygen vacancy concentration of BaTiO₃ doped with acceptors (Cr to Ni) is determined gravimetrically as a function of the O₂ partial pressure during and after annealing at 700° to 1300°C. The oxygen vacancy concentration of these materials is larger than that of undoped and donor-doped BaTiO₃. The oxygen vacancies are doubly ionized and they compensate the acceptors of lower valence. Both the vacancy concentration and the valence of the acceptor dopants depend on the annealing conditions. The electronic energy levels of the acceptors within the BaTiO₃ band gap are derived from the gravimetric measurements. The electrical properties of the acceptor-doped ceramics are favorable for base-metal-electrode multilayer capacitors, which require sintering in reducing atmospheres.     

Processing and Piezoelectric Properties of (Na0.5K0.5)0.96Li0.04(Ta0.1Nb0.9)1−xCuxO3−3x/2 Lead-Free Ceramics

The Cu-modified (Na_0.5K_0.5)_0.96Li_0.04Ta_0.1Nb_0.9O₃ lead-free piezoelectric ceramics were fabricated at relatively low temperatures by ordinary sintering. The results indicate that the addition of copper oxide (CuO) does not change the crystal structure and the dielectric–temperature characteristic, but tends to slightly increase the loss tangent and significantly modify the ferroelectric and electromechanical properties. Moreover, the grains get clearly coarsened with increasing CuO content. When doped with <0.25% CuO, the materials get softer with slightly decreased coercive fields ( E _c) and increased maximum electric field-driven strains ( S _m), and thus own enhanced piezoelectric properties; however, as the doping level becomes higher, the materials get harder, possessing larger E _c and reduced remanent polarization and S _m. The change in the electrical properties can be attributed to both the formation of oxygen vacancies by Cu²⁺ replacing Nb⁵⁺ and the modification of densification.     

Ag2O-Doped Bi2Sr2Ca1Cu2Ox Superconductors Prepared via Melt-Quenching

Ag₂O-doped superconducting Bi₂Sr₂Ca₁Cu₂O _x ceramics were prepared by a melt-quenching–reheating method. It is found that the Ag₂O-doped, as-cast specimens exhibit superconductivity ( T _c= around 80 K) by heat treatment at temperatures around 800°C even in an evacuated and sealed silica glass tube, while the undoped specimens do not and vaporize by the corresponding heat treatment. Conversion of the Ag₂O-doped, as-cast specimens into superconducting ceramics when heated in an evacuated vessel is explained in terms of the oxygen donor of Ag₂O in the specimen. This finding enables us to fabricate a desired shape of superconducting Bi₂Sr₂Ca₁Cu₂O _x ceramics sealed in metals or glasses. The addition of Ag₂O to Bi₂Sr₂Ca₁Cu₂O _x melt, however, had deleterious influences on the superconducting properties ( T _c and J _c) of the resultant ceramics when obtained by heat treatment in air.