Undoped BaTiO3 Research Articles

Influence of Shallow Levels in Charge Transport Process on Enhancement of Transient Diffraction Efficiency in BaTiO3 Crystal

A unique enhancement of the transient diffraction efficiency in an undoped BaTiO3 crystal is experimentally observed. The enhancement is a strong function of the switched-off time (or dark time) of the pulsed reading beam incident to a steady-state grating, formed in advance, in the crystal. This enhancement can be maximized for an optimum dark time, which reveals the presence of two shallow levels in the charge transport process at room temperature.

Effects of MgO-Doping on Electrical Properties and Microstructure of BaTiO3

Degradation behaviors of Mg-doped BaTiO3 have been studied in terms of oxygen vacancy concentration and microstructural development. Mg-doped BaTiO3 powders were precisely synthesized by the Pechini method. As MgO content was increased to 1.0 mol%, Curie points moved to lower temperatures and dielectric constants decreased. The specimens doped with MgO showed higher leakage currents under the continuously applied dc field at a high temperature (100°C) during the aging process, compared with the undoped BaTiO3. With low levels of MgO (<1.0 mol%), the microstructure is similar to that of undoped BaTiO3, whereas a significant reduction in grain size was observed with higher levels (>2.0 mol%).

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(Ti0.995 −xMg0.005Mn x )O2.995 −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− 10A. 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.

Deformed microstructure in pressureless-sintered barium titanate

The sintered microstructure of pressureless-sintered BaTiO 3 analysed by transmission electron microscopy (TEM) reveals that plastic deformation occurred at 1400 °C when the slip systems of 〈1 1 ̄ 0〉{1 1 0}, 〈1 0 0〉{1 1 0} and 〈100〉{0 0 1} were activated by the intrinsic sintering stress ( Σ). Dislocations with the Burgers vectors b 1 =〈1 1 0〉 and b 1 ′=〈1 0 0〉 have dissociated. The dissociation reactions of b 1= b 2+ b 3 are: [1 0 1] → [0 0 1] + [1 0 0] and [0 0 1] → 1 2 [1 0 1] + 1 2 [ 1 ̄ 0 1], respectively. Both dissociated dislocations contained a series of scallop-shaped half-loop partials with the Burgers vectors respectively of b 3 = [1 0 0] and b 3 ′= 1 2 [ 1 ̄ 0 1] . The dissociations took place by a climb mechanism. Polygonisation in dynamic recovery, necessitating both glide and climb of dislocations, occurred during sintering at 1400 °C, and which is clearly evidenced by sub-grain formation. Densification was contributed by dislocation mechanisms where the ceramic has been plastically deformed by the sintering stress. Densification of the undoped and Sr-doped BaTiO 3 compositions by pressureless-sintering in relation to the dislocation substructure is discussed.

Enhancement of the switched phase conjugate reflectivity in a BaTiO3 crystal

Abstract We experimentally investigated the response of the photorefractive phase conjugation to a pulsed reading beam in an undoped BaTiO 3 crystal with a four-wave mixing geometry. A single longitudinal-mode Ar + laser was used as a light source. The reading beam with a pulse width of ∼1 s was switched on after writing a steady-state grating in the crystal by two recording beams under a suitable condition and the generated phase conjugation was measured. The phase conjugate output was found to be increased by some factors of magnitude over the initial value for a given peak power and period of the reading pulses. A maximum of the enhanced reflectivity can be controlled by adjusting the interacting beam powers. An increase in the diffraction efficiency was also found with the same read-out process, which plays a key role to increase the phase conjugation as our knowledge.

Sintering of nanocrystalline BaTiO 3

Two-step sintering method was employed to consolidate nano-size BaTiO 3 powders. The two-step sintering method comprises the initial heating at relatively higher temperatures and the following low-temperature sintering for a long period of heating. Pressed green bodies of BaTiO 3 were first heated to 1300 °C to achieve an intermediate density (>75% of theoretical density), then cooled down and held at 1100 °C for 0–20 h until they became fairly dense. Slight grain growth progressed during the second step, whereas the density significantly increased. The average grain size of dense ceramics was around 1 μm. The dielectric constant of BaTiO 3 was much greater than that of the normal sintering method. Furthermore, Dy-doped BaTiO 3 showed much smaller grain sizes than the undoped BaTiO 3, which is comparable with the conventional sintering behavior of BaTiO 3.

The effects of additive on microstructure and electrical properties of batio3 ceramics

In this paper comparative investigations of microstructure and dielectric properties of BaTiO3 ceramics doped with 1.0 wt% of Nb2O5, MnCO3 and CaZrO3 have been done. BaTiO3 samples were prepared using conventional method of solid state sintering at 13000C for two hours. Two distinguish micro structural regions can be observed in sample doped with Nb2O5. The first one, with a very small grained microstructure and the other one, with a rod like grains. In MnCO3 and CaZrO3 doped ceramics the uniform microstructure is formed with average grain size about 0.5- 2?m and 3-5?m respectively. The highest value of dielectric permittivity at room temperature and the greatest change of permittivity in function of temperature were observed in MnCO3/BaTiO3. In all investigated samples dielectric constant after initially large value at low frequency attains a constant value at f = 6kHz. A dissipation factor is independent of frequency greater than 10 kHz and, depending of systems, lies in the range from 0.035 to 0.25. At temperatures above Curie temperatures, the permittivity of all investigated samples follows a Curie- Weiss law. A slight shift of Curie temperature to the lower temperatures, in respect of Curie temperature for undoped BaTiO3, was observed in all investigated samples.

Transport numbers from hydrogen concentration cells over different oxides under oxidising and reducing conditions.

Partial hydrogen ion conductivity in acceptor doped CaTiO(3), BaCeO(3) and ZrO(2) and nominally undoped BaTiO(3) and TiO(2) has been investigated by transport number measurements using the hydrogen concentration cell/EMF method in wet atmospheres as a function of pO(2)(10(-20)-1 atm) at 800 or 1000 [degree]C. All oxides investigated, except ZrO(2), show minor proton conductivity in oxidising atmospheres. Earlier indications of apparent negative charge transport by hydrogen under reducing conditions and high temperatures in SrTiO(3) samples have been reproduced for all the investigated oxides including ZrO(2).

Comments on the Defect Chemistry of Undoped and Acceptor-Doped BaTiO3

In a recent study of the defect chemistry of undoped polycrystalline and acceptor-doped single crystalline BaTiO3 an attempt was made to obtain defect and thermodynamic parameters without any a priori assumptions, e.g. about the magnitude and temperature dependence of the carrier mobilities. The parameters found for the undoped sample differ significantly from those obtained from the acceptor-doped sample, and even more substantially from those reported from several earlier studies. It is suggested that these discrepancies result from the lack of sufficient data in certain critical regions of temperature and oxygen activity. As a result, the attempt to extract parameters from the equilibrium conductivity by multiparameter curve-fitting has considerable uncertainty. However, the electron and hole mobility products and ratios, μ n μ p and μ n /μp, were determined by linear extrapolation of the well-established log-log slopes of +1/4 and −1/4 for the equilibrium conductivities and oxygen diffusivities to their intersection. Both the products and the ratios were found to be independent of temperature, indicating that both mobilities are also independent of temperature. This important conclusion validates the thermodynamic parameters obtained in earlier studies in which it was assumed that the mobilities are temperature independent or nearly so, and that the carrier concentrations are proportional to the conductivities.

Photorefractive Properties of Two Series of BaTiO3 Crystals Annealed in Temperatures of 900°C and 1000°C

Two series of nominally undoped BaTiO3 crystals are fabricated and then systematically investigated by processing in a varying oxygen partial pressure atmosphere at 900°C and 1000°C. The photorefractive properties are sequentially controlled by the factors of dark decay, deep/shallow traps, deep/shallow traps with hole/electron competition, and then dark conductivity as the oxygen partial pressure is reduced. The properties of these two sample series show a similar trend, although the compensation point for 900°C samples occurs at an oxygen partial pressure of 10-10 atm and that for 1000°C samples at 10-7 atm. The two-beam coupling gains, effective trap densities, speeds, and dark conductivities for the two samples series are all at their smallest values around the compensation point. The range between the compensation point and the dark conductivity controlling regime decreases on the P side as the processing temperature increases, although it does not change on the N side.

Surface reaction kinetics in oxygen nonstoichiometry re-equilibration of BaTiO 3− δ

We report the (bare) surface redox-reaction rate constant k̃ that was determined, along with the chemical diffusivity D̃, by a conductivity relaxation technique on Al-doped single crystal and undoped polycrystal BaTiO 3− δ as a function of oxygen activity in its range of −16≤log a O 2 ≤0 at elevated temperatures of 800–1100 °C. It takes a value in the range of −4<log( k̃/cm s −1)≤−1, which is even larger than that of the oxides that are considered best as oxygen membranes. It has been found that the surface reaction step grows more rate controlling as the electronic transference number gets smaller or the electronic stoichiometric composition ( δ≈0) is approached. The oxygen potential drop due to the surface reaction was estimated by an oxygen concentration cell technique. The oxygen potential drop grows larger as the stoichiometric composition is approached, that is in accord with the variation of k̃ against oxygen activity.

Photorefractive Optical Image Transfer in Feedback Signal Communication

An optical setup and relevant communication schemes interconnecting optical signals among mutually coherent optical channels are proposed to enhance the functionality of optical devices used for communication purposes. Based on self-pumped phase conjugation and two-wave mixing mechanisms in an undoped BaTiO3 photorefractive crystal, an optical image can be transferred from the signal-input channel to the signal-requiring channel of the emitting station. To overcome noise generated during communication, two different approaches are presented and the associated experimental results also demonstrated. We also discuss how to extend the use of the device to store and retrieve interconnected signals, to prevent communication interruption.

Electrical conductivity of hexagonal Ba(Ti0.94Ga0.06)O2.97 ceramics

Abstract Impedance spectroscopy is used to estimate the bulk conductivity, σ b , of hexagonal (6H)-Ba(Ti 0.94 Ga 0.06 )O 2.97 ceramics in air, N 2 and 5%H 2 /95%N 2 between 400 and 1000 °C. Isothermal plots of log σ b vs log pO 2 in the temperature range between 700 and 1000 °C show the presence of a p–n transition with slopes of ∼−1/4 and +1/4 in the n- and p-type regions indicating that the conductivity obeys the ‘extrinsic’ model proposed by Smyth and co-workers for undoped and acceptor-doped cubic BaTiO 3 -based materials [ J. Am. Ceram. Soc . 64 (1981) 556; J. Am. Ceram. Soc . 65 (1982) 167]. The activation energy E a for oxidation in the p-type region to produce free holes is similar for 6H-Ba(Ti 0.94 Ga 0.06 )O 2.97 and cubic BaTiO 3 -based ceramics with an estimated value from σ b data of ∼0.8 eV. The band gap for 6H-Ba(Ti 0.94 Ga 0.06 )O 2.97 ceramics is ∼3.2 eV.

Analysis of point defects in polycrystalline BaTiO 3 by electron paramagnetic resonance

Several paramagnetic point defects in BaTiO 3 polycrystals were detected and identified using the electron paramagnetic resonance technique. Polycrystalline samples sintered in a reducing atmosphere showed a broad signal with a giromagnetic constant of 1.932. This signal, observed only in conducting ceramics, was assigned to Ti 3+ and its related complexes. V Ba and V Ti defects were found at giromagetic constants of 1.974 and 2.004, respectively. The EPR intensity of these defects increases after oxidising at T>1000 °C. The appearance of the V Ba and V Ti signals in EPR spectra of oxidized ceramics correlates with the onset of the PTCR effect. At donor dopant levels ≥0.3 at.%, the concentration of V Ba and V Ti shows good agreement with the defect compensation mechanism of donor doped BaTiO 3. However, small amounts of V Ba and V Ti were also detected in undoped BaTiO 3, contrary to the generally accepted defect model.

Electrical properties of MgO-doped BaTiO3

The defect chemistry of Mg-doped BaTiO3 has been quantitatively studied. BaTi1−xMgxO3−x samples (x = 0, 0.001, 0.005, 0.01, 0.012, 0.02) have been precisely synthesized by the Pechini method. The concentration of oxygen vacancies was estimated from the location of the oxygen partial pressure P°(O2) at which the minimum point of equilibrium electrical conductivities occurs. Below 1.0 mol% Mg substitution for Ti sites, the experimental result is in good agreement with the theoretical calculation if the concentration of oxygen vacancies is assumed to be 100 ppm for undoped BaTiO3, where the oxygen vacancies stem from the background acceptor impurities. The solubility limit of Mg on Ti sites was confirmed to close to 1.0 mol%.

Nonstoichiometry and defect structure of Mn-doped BaTiO 3− δ

Oxygen nonstoichiometry ( δ) of 1 m/o Mn-doped polycrystalline BaTiO 3− δ has been measured as a function of oxygen partial pressure in the range of 10 −16≤ P O 2 /atm≤0.1 at elevated temperatures (900≤ T/°C≤1100) by a solid-state coulometric titration technique. The extent of nonstoichiometry of Mn-doped BaTiO 3 is much larger than that of undoped BaTiO 3, which is attributed to defect-chemical role of Mn as acceptors. The nonstoichiometry isotherms indicate that Mn-ions change their valence from +4 (or Mn Ti x ) to +3 (or Mn Ti′) to +2 (or Mn Ti″) with decreasing P O 2 .

Collinear partial dislocations in barium titanate perovskite

Dislocation substructures in CO 2-laser-sintered undoped BaTiO 3 have been investigated via transmission electron microscopy. Deformation of BaTiO 3 at 1390 °C occurs when the slip system of 〈100〉{001} is activated by the intrinsic thermodynamic driving force for sintering. Dissociation of dislocations with Burgers vector b 1=[001] into two collinear partials of b 2=1/2[001] and b 3=1/2[001] has been identified unambiguously using weak-beam dark-field imaging. Part of the b 3 partial further changes to a scallop-shape half-loop of b 3′=1/2[100] by a non-conservative loop formation mechanism of mixed climb and glide in the {111} plane. Results suggest that plastic flow by dislocation mechanisms contribute to the densification of BaTiO 3 at 1390 °C. It takes place by a climb-controlled glide mechanism, which is rate-determined kinetically by the lattice diffusion of Ba 2+.

Oxygen Defects Related to Electrical Properties of La-doped BaTiO3

The oxygen diffusion coefficients of undoped and La-doped BaTiO3 were evaluated by secondary ion mass spectrometry (SIMS). The La-doped BaTiO3 has markedly lower oxygen diffusion coefficient than the undoped BaTiO3. Regarding the effect of the amount of La doped, the volume diffusion coefficient of oxygen was found to be dependent on the amount of La doped. The oxygen diffusion behavior of samples synthesized by various cooling processes was also compared. The samples that were very slowly cooled (2°C/h from 1250 to 700°C) showed markedly low oxygen diffusion coefficient in the low-temperature region. It was found that the difference in cooling rate during the calcination process has significant effect on the oxygen diffusion behavior of La-doped BaTiO3. Regarding the electrical properties of samples, the dielectric properties were strongly influenced by the cooling rate, whereas the dc resistivities were not influenced by the cooling rate.

The influence of grain boundary impedances on the p-type conductivity of undoped BaTiO3 ceramics

Impedance spectroscopy is used to deconvolute the dc conductivity (σ) of undoped BaTiO3 ceramics (∼95% of the theoretical X-ray density) into bulk (σb) and grain boundary (σgb) components at two oxygen partial pressures, PO2 ∼10 Pa (N2) and ∼0.21 MPa (air). At 900°C, σ∼σb in both atmospheres, however, at lower temperatures Z∗ plots are dominated by the grain boundary component and σ∼σgb. The temperature of the switch from σ∼σb to σ∼σgb is different in the two atmospheres and occurs at ∼850°C in air and ∼650°C in N2. Isothermal plots of log σb vs log PO2 in the temperature range 450–900°C show the expected oxygen partial pressure dependence with a gradient of +1/4. In contrast, σgb is relatively insensitive to PO2 and log σgb vs log PO2 plots have gradients < +1/4 with values as low as ∼+1/14.0. In general, isothermal log σ vs log pO2 plots have gradients <+1/4 as σ is dominated by the grain boundary component. This may explain the wide range of gradients (∼1/4–1/9) reported in the literature for isothermal dc conductivity measurements on polycrystalline BaTiO3 in the p-type regime.

Oxygen nonstoichiometry of undoped BaTiO 3− δ

Oxygen nonstoichiometry ( δ) of “undoped”, polycrystalline BaTiO 3− δ (but with two different nominal purity) has been measured as a function of oxygen partial pressure in the range of 10 −15≤ P O 2 /atm≤0.1 at elevated temperatures (1073≤ T/K≤1273) by a solid-state coulometric titration technique. Each isotherm varies sine-hyperbolically against log( P O 2 ) with an inflexion point therein (corresponding to the electronic stoichiometric composition or δ=0). The nonstoichiometry variation is attributed to oxygen vacancies compensated mostly by background and/or intrinsic acceptors in the higher P O 2 region and by electrons in the lower P O 2 region. The purity effect on nonstoichiometry is discussed. The intrinsic electronic excitation equilibrium constant is evaluated and compared exhaustively with the reported. The relative partial molar enthalpy and entropy of component oxygen are also evaluated as functions of the nonstoichiometry.