La-doped BaTiO3 Research Articles

Influence of Manganese on the Room-Temperature Resistivity of Lanthanum-Doped BaTiO3

The room-temperature resistivity of La-doped BaTiO3 increases with the increased Mn content. Some of the doped Mn ions will segregate at the grain boundaries and behave as acceptor levels. Thus, Mn will exihibit a grain growth inhibition effect which results in the increase of grain boundaries. Then, the surface charge density (N s) will be increased by the little doped Mn ions and result in a decreased effective donor concentration (N eff) and an enhanced barrier height at the grain boundaries. Thus, the semiconductivity of La-doped BaTiO3 is decreased by Mn addition.

THE DIELECTRIC PROPERTY AND DEFECTS OF La-DOPED BaTiO3 CERAMICS

The dielectric permittivity of La-doped BaTiO3 ceramics exhibits reverse U-shape variation with increasing La-content in the range of 0.4 at% La, and with increasing frequency f it decreases quickly for f≤1000 Hz and changes very slowly for f>1000 Hz. These variations of dielectric property result from variation of defects due to doping La as follows: isolated Ba-vacancy ( V″Ba)→ associated defect (LaBa·V″Ba)→(LaBa·V″Ba·LaBa).

Influence of Mn Addition on the Curie Point and the PTCR Effect of La-Doped BaTiO<sub>3</sub>

La, Mn-codoped BaTiO3 ceramics were prepared by using high purity starting materials to clarify the influence of Mn addition on the positive temperature coefficient of resistivity (PTCR) effect. No sintering aids such as SiO2 and excess TiO2 were used. Resistivity at room temperature, grain growth and Curie temperature (TC) were studied as functions of Mn content. The Curie temperature of La-doped BaTiO3 increased initially and then decreased with increasing Mn addition, and the Mn concentration of giving the maximun TC depended on the doping concentration of La. This newly observed phenomenon is interpreted in terms of the state of Mn in the compound in connection with the variation of its bulk and grain boundary resistivity. The largest PTCR effect was observed around the concentration of Mn giving the maximum TC.

Calculation of Ultraslow Dielectric Relaxation of Doped BaTiO3 Ceramics

Using a linear voltage ramp method, ultraslow dielectric relaxation processes of La-doped BaTiO3 boundary layer (BL) capacitors are measured at room temperature. For the electric polarization model of a double-layer dielectric, a procedure to calculate the electrical parameters of grain and grain boundary has been developed. Calculated results suggest that there is a shell on the surface of the grain. TEM images of the BL capacitor also show a lot of microregions with minor structural modifications. Relaxation times are estimated from the equivalent circuit and are in agreement with fitted results.

The influence of silicon on microstructure and electrical properties of La-doped BaTiO3 ceramics

BaTiO3 positive temperature coefficient of resistance ceramics were prepared with the general composition (La0.002Ca x Ba0.998−x ) (Ti1.01−y Mn y )O3 ·zSiO2 and sintered in air. The Ca content as well as the amount of doped Mn were varied ranging from 4–20 mol% and 0.02–0.04 mol%, respectively. Up to 4 mol% Si was added using a new method as well as by the classical means as solid silica. In the new method, the desired quantity of Si(OC2O5)4 (Si(OEt)4), contained in dry tetrahydrofurane (THF), was dropped under a slight stream of inert gas into the vigorously stirred aqueous slurry which was manufactured after the calcination process. The addition of Si by this technique was found to result in a homogenization of the microstructure and an improvement in electrical properties. The effect of Si on electrical properties is explained by the influence of the observed second phase on the equilibrium of Ba vacancies. The results of further variation of Ca and Mn contents are presented.

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.

Photoeffects in La3+-doped BaTiO3 ceramic electrodes

Photoelectrochemical conversion characteristics of semiconducting, polycrystalline BaTiO3 electrodes have been investigated as a function of sintering atmosphere (air/H2) and the amount of dopant (lanthanum). The photocurrents for specimens sintered in air were greater than those of specimens sintered in hydrogen. Also, as the amount of dopant in BaTiO3 sintered in air increased, the photocurrent increased up to 0.2 at. % addition and then decreased. However, for BaTiO3 sintered in hydrogen, the photocurrents decreased consistently with the addition of the dopant. These results could be explained in terms of ceramic microstructure and electrical resistivity. Regardless of the amount of lanthanum, the photoresponse, as the wavelength of the induced light that varied, began to appear around 420–430 and 780–800 nm, which corresponds to the energy band of La-doped BaTiO3 and the oxygen vacancy level within the gap, respectively.

Positive Temperature Coefficient of Resistivity Effect in Undoped, Atmospherically Reduced Barium Titanate

A sharp positive temperature coefficient of resistivity effect for undoped, fluorinated BaTiO3−x samples was observed. The qualitative agreement between the behavior of BaTiO3−x and La‐doped BaTiO3, with respect to the trap density at the boundaries, suggests that in the absence of counterdopants such as Mn, the main origin of surface acceptor states is chemisorbed gases. Impedance analysis indicates that, under the conditions of the present work, fluorine seems not to diffuse in significant amounts into the lattice. The treatment of the fluorinated samples in inert/reducing atmospheres did not markedly decrease the resistivity jump, suggesting that fluorine might be chemisorbed more strongly than oxygen. The composition of two different liquid phases, produced by excess titania and silica additions, did not have an important effect on the resulting properties.

Barium titanate-based actuator with ceramic internal electrodes

Abstract Electrostriction and mechanical properties of multilayer ceramic actuators with ceramic internal electrodes were investigated. Insulative BaTiO3 and La-doped semiconductive BaTiO3 were laminated by means of a conventional green sheet method and co-fired into the multilayer structure. The strain induced by an electric field in the multilayer was measured using a differential transformer method. The 4 × 10−4 magnitude of strain at 15 kV/cm is sufficiently applicable to fine movement control devices. Mechanical strength obtained was 50 MN/m2, about three times larger than that for a conventional multilayer ceramic actuator with Pd internal electrodes.

Oxygen activity dependence of the defect structure of La-doped BaTiO3

A model for the behaviour of donor-doped BaTiO3 as a function of oxygen activity is proposed. Thermogravimetric measurements show good agreement with the proposed model. The results show that donor-doped BaTiO3 is stoichiometric with electronic compensation of the donor at low oxygen activity, but with increasing oxygen activity, compensation becomes ionic as excess oxygen is absorbed. The possible compensation mechanisms are discussed.

Electrical Properties of Composite Ceramics with Texture Modified II

La-doped BaTiO3-Bi2O3 composite ceramics prepared by a soaking method (sintered bodies of La-doped BaTiO3 being dipped into an alcoholic solution of BiCl3 for several days, then fired to give rise to Bi2O3 matrix along the grain boundaries) exhibit a two-step anomalous increase of resistivity around room temperature and above the Curie point, when non-ohmic contact electrodes such as Ag are applied. A surface barrier-layer built in the interface between the non-ohmic electrode and the sintered body with a modified texture gives rise to an anomaly not only in the resistivity-temperature characteristic but also in the capacitance (C)-voltage (V) characteristic.The plots of 1/C2 vs. V deduced from the C-V characteristics under reverse bias to the surface barrier suggest a constant value of ca. 1.3V for the built-in voltage in the temperature range between room temperature and 58°C (at 1kHz) and in the frequency range between 0.5 and 30kHz (at 1°C). Each straight line in the 1/C2 vs. V plot reaches the maximum at the critical reverse bias, which shifts to higher voltages with temperature and frequency. It is demonstrated that the type of the surface barrier-layer is the one of modified MOS type.

Two-Step Anomalous Increase of Resistivity on La-Doped BaTiO3-Bi2O3 Composite Ceramics with Surface Barrier-Layer

Two-step anomalous increase of resistivity is observed on La-doped BaTiO3–Bi2O3 composite ceramics with glazed silver paste as electrodes, where the first increase appears around a room temperature and the second above the Curie temperature. On the other hand, when indium-base alloy electrodes are used to obtain ohmic contacts, the former disappears and the latter diminishes. The existence of barrier-layers between opposing electrodes and pellet surfaces is demonstrated by resistivity measurements using asymmetric electrodes. It seems that Bi2O3 forms a barrier-layer.