Antonio Feteira

Influence of Mn doping on the semiconducting properties of CaCu3Ti4O12 ceramics

Comparison of impedance spectroscopy data for undoped CaCu3Ti4O12 (CCTO) and CaCu2.94Mn0.06Ti4O12 CCMTO ceramics shows Mn doping to suppress the room temperature bulk semiconductivity of ∼0.01Scm−1 for CCTO by six orders of magnitude. The magnitude of and activation energy for bulk conduction in CCMTO are similar to those of the insulating grain boundaries in CCTO ceramics. A model to explain the suppression of the bulk conductivity in CCMTO ceramics is proposed; the grain and grain boundary regions in CCTO ceramics may consist of the same phase but with slightly different compositions.

Origin(s) of the apparent high permittivity in CaCu3Ti4O12 ceramics: clarification on the contributions from internal barrier layer capacitor and sample-electrode contact effects

CaCu3Ti4O12 ceramics with a range of resistivities have been prepared using both conventional sintering and spark plasma sintering. For all cases, the high effective permittivity is associated primarily with an internal barrier layer capacitor mechanism. Additional polarization associated with the electrode-sample interface may appear but its visibility depends on the grain boundary resistivity (Rgb) of the ceramic. If the Rgb is large, the electrode polarization is obscured by sample-related effects; if the Rgb is small, a separate impedance associated with the electrode polarization may be seen. Discrepancies in the literature regarding the magnitude and origin of the high effective permittivity are attributed to a combination of differences in processing conditions, electrode contact material and measuring frequency range.

Remarkably high-temperature stable piezoelectric properties of Bi(Mg0.5Ti0.5)O3 modified BiFeO3–BaTiO3 ceramics

Dense (1-y)BiFe1−x(Mg0.5Ti0.5)xO3 − yBaTiO3 (BFMT-BT, y = 0.29, x = 0-0.12) ceramics with a pseudocubic perovskite crystal structure exhibit remarkably high-temperature stable piezoelectric properties. In particular, x = 0.03 BFMT-BT ceramics, with a Curie temperature, Tc, of ∼425 °C, show a stable piezoelectric coefficient, d33, of ∼155 pC/N up to a depolarization temperature, Td, of ∼400 °C. This Td is higher than Tc for Pb(Zr,Ti)O3, therefore suggesting BFMT-BT ceramics to be promising high-temperature Pb-free piezoelectric materials.

Relaxor ferroelectric-like high effective permittivity in leaky dielectrics/oxide semiconductors induced by electrode effects: A case study of CuO ceramics

The electrical behavior of copper oxide (CuO) ceramics sintered at 920 °C has been characterized by a combination of fixed, radio frequency (rf) capacitance measurements, and impedance spectroscopy (IS). Fixed rf capacitance measurements on ceramics with sputtered Au electrodes revealed a temperature- and frequency-dependent high effective permittivity of ∼104 in the temperature range of 150–320 K. The response is similar to that observed for relaxor-ferroelectrics, however, the magnitude of the effect can be suppressed by thermal annealing of the ceramics with Au electrodes in air at 300 °C or by changing the work function of the electrode material by using In–Ga as opposed to Au. IS data analysis revealed the ceramics to be electrically heterogeneous semiconductors with a room temperature dc resistivity <104 Ω cm, consisting of semiconducting grains with relative permittivity, er, <10 and slightly more resistive grain boundaries with “effective” permittivity, eeff, of ∼110. Samples with Au electrodes ex...

Comment on the use of calcium as a dopant in X8R BaTiO3-based ceramics

BaTi1−xCaxO3−x ceramics, where 0⩽x⩽0.04, prepared by the mixed oxide route at 1500°C display a strong suppression of the Curie temperature and a significant increase in grain conductivity with increasing x. The ramifications of these results on the applicability of using Ca2+ as a dopant to achieve X8R specification in BaTiO3-based formulations are discussed.

High intrinsic permittivity in Na1/2Bi1/2Cu3Ti4O12

The electrical properties of Na1∕2Bi1∕2Cu3Ti4O12 (NBCTO) ceramics between ∼8 and 400K are electrically inhomogeneous, consisting of semiconducting grains and insulating grain boundaries, similar to isostructural CaCu3Ti4O12 (CCTO); however, the intrinsic permittivity of NBCTO of ∼250 at 10K is much higher compared to ∼110 for CCTO. Possible explanations for this high intrinsic permittivity are discussed.

Origin of the high permittivity in (La0.4Ba0.4Ca0.2 ) (Mn0.4Ti0.6)O3 ceramics

Single-phase cubic (La0.4Ba0.4Ca0.2)(Mn0.4Ti0.6)O3 (LBCMT) was reported by Jha et al. [J. Solid State Chem. 177, 2881 (2004)] to exhibit an extraordinarily high effective permittivity (eeff∼6980 and 590 at 1 and 100kHz, respectively) at room temperature. According to x-ray-diffraction analysis, the structure of this perovskite can be fully described on the centrosymmetric space-group Pm3¯m, which precludes the occurrence of ferroelectricity. In the present work, impedance spectroscopy has shown that LBCMT exhibits a bulk permittivity, er∼40–45, in accordance with that expected for a cubic, centrosymmetric perovskite. LBCMT is a semiconductor with a bulk resistivity of ∼3kΩcm at 300K and an activation energy for conduction of ∼0.20eV which is consistent with an electron hopping conduction mechanism. Finally, it is demonstrated that the high effective permittivity eeff reported previously is not an intrinsic bulk effect arising from hopping conduction between Mn+3 and Mn+4, but is ultimately associated with...

In situ Raman spectroscopy of A-site doped barium titanate

Raman spectroscopy has been used to study the ferroelectric tetragonal to paraelectric cubic phase transition Tc in undoped BaTiO3 (BT) and (Ba0.92A0.08)TiO3 [where A=Ca (BCT), Sr (BST), and Pb (BPT)]. In BT and BPT, mode characteristic of the tetragonal phase at 303 and 710cm−1 persisted until >50°C above Tc, indicating the presence of local dynamic polar clusters well above the displacive phase transitions, ∼120 and ∼149°C, respectively. However, in BCT and BST, the Raman spectra exhibited only broad second order bands above Tc, suggesting that Ca and Sr, A-site dopants supress local dynamic polar clusters.

Incipient ferroelectricity and microwave dielectric resonance properties of CaCu2.85Mn0.15Ti4O12 ceramics

Mn-doped CaCu3Ti4O12 ceramics with nominal composition CaCu2.85Mn0.15Ti4O12 (CCMTO) have been prepared and their radio- and micro-wave dielectric properties established. At 300 K, dense CCMTO ceramics have appreciable grain resistivity, ∼20MΩcm, and exhibit microwave dielectric resonance with er∼93, Qf∼3950GHz (at 3.95GHz) and temperature coefficient of resonant frequency ∼+657ppm∕K. er decreases smoothly from ∼150 to 90 in the range of 10–300K, obeys the Curie-Weiss law in the range of ∼60–180K, and is consistent with incipient ferroelectricity. The source of the incipient ferroelectricity and potential applications of CCMTO ceramics are discussed.

Bismuth Sodium Titanate Based Materials for Piezoelectric Actuators

The ban of lead in many electronic products and the expectation that, sooner or later, this ban will include the currently exempt piezoelectric ceramics based on Lead-Zirconate-Titanate has motivated many research groups to look for lead-free substitutes. After a short overview on different classes of lead-free piezoelectric ceramics with large strain, this review will focus on Bismuth-Sodium-Titanate and its solid solutions. These compounds exhibit extraordinarily high strain, due to a field induced phase transition, which makes them attractive for actuator applications. The structural features of these materials and the origin of the field-induced strain will be revised. Technologies for texturing, which increases the useable strain, will be introduced. Finally, the features that are relevant for the application of these materials in a multilayer design will be summarized.