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1.
The grain-growth behavior and grain-boundary structure in titanium-excess BaTiO3 depend on the amount of excess titanium at 1250° and 1300°3C. With excess titanium, abnormal grain growth (AGG) occurs and the grain boundaries are mostly flat or faceted with hill-and-valley shapes. With 0.5 at.% excess titanium, the large grains have flat {111} faces forming singular grain boundaries parallel to {111} double twins. With excess-titanium content between 0.1 and 0.3 at.%, the abnormal grains appear to have polyhedral shapes with {100} faces. These flat or faceted grain boundaries are expected to have singular structures, and hence AGG can occur by the step growth mechanism. When the excess-titanium content is decreased to 0, the grain boundaries become curved, indicating a rough atomic structure, and normal grain growth occurs.  相似文献   

2.
Microstructural characterizations using transmission electron microscopy on 0.95(Na0.5K0.5)NbO3–0.05BaTiO3 ceramics sintered at 1030°–1150°C for 2 h were carried out. The liquid phase was found at the triple junction of the grains in all specimens and abnormal grain growth occurred in the presence of the liquid phase. Abnormally grown grains whose shapes were cuboidal were well developed. Anisotropically faceted amorphous liquid phase pockets were observed inside the grain in a specimen sintered at 1060°C for 2 h. The interface between the grain and the liquid matrix was flat and some were identified to be {100} planes of the grains. A certain amount of liquid at the sintering temperature of 1060°C enhanced the abnormal grain growth and contributed to the improvement of the piezoelectric properties.  相似文献   

3.
The effect of grain boundary structure, either rough or faceted, on diffusion-induced grain boundary migration (DIGM) has been investigated in BaTiO3. SrTiO3 particles were scattered on the polished surfaces of two kinds of BaTiO3 samples with faceted and rough boundaries and annealed in air for the samples with faceted boundaries and in H2 for those with rough boundaries. In the BaTiO3 samples with rough boundaries, an appreciable grain boundary migration occurred. In contrast, grain-boundary migration hardly occurred in the BaTiO3 samples with faceted boundaries. The migration suppression observed in the sample with faceted boundaries was attributed to a low boundary mobility. The present experimental results show that DIGM is strongly affected by the boundary structure and can be suppressed by structural transition of boundaries from rough to faceted.  相似文献   

4.
Dislocation loops in pressureless-sintered undoped BaTiO3 ceramics have been analyzed by transmission electron microscopy. The Burgers vector of the loops and its sense b =+1/2[010] were determined using the g·b =0 invisibility criteria, combined with the inside–outside contrast technique using ( g·b ) s g >0 or<0, keeping the deviation parameter s g >0. The edge-vacancy nature was further ascertained by determining the loop habit plane normal n =[0 1 0]. Weak-beam dark-field imaging reveals that loops contained no stacking fault fringes; they are edge-vacancy partial dislocation loops lying in {020} or {010} where parts of the TiO2 or BaO layer are vacant. It is suggested that the extrinsic defects of both cations and oxygen vacancies generated by non-stoichiometry have condensed during sintering in air and are responsible for the formation of such vacancy loops.  相似文献   

5.
The grain growth of donor-doped BaTiO3 at different oxygen partial pressures was studied. Results showed that the oxygen pressure had a pronounced influence on the grain growth and related effects. A model for the grain size anomaly during sintering of donor-doped BaTiO3 in the presence of a TiO2-rich liquid phase is proposed.  相似文献   

6.
The grain boundaries in BaTiO3 with excess Ti of 0.5, 0.3, and 0.1 at.% sintered at 1300° or 1250°C have been examined by scanning electron microscopy (SEM), electron backscattered diffraction pattern (EBSP), and transmission electron microscopy (TEM). In the 0.1% Ti-excess specimen, large grains growing abnormally form high-angle grain boundaries when they impinge on each other as verified by EBSP. A large fraction of these grain boundaries are faceted with hill-and-valley shapes. In the 0.5% Ti-excess specimen, large grains growing abnormally are elongated in the directions of their {111} double twins. These grains often form flat grain boundaries parallel to their {111} planes with the fine matrix grains, and the grain-boundary segments between the large impinging grains with high misorientation angles are often also parallel to the {111} planes of one of the grains. These grain boundaries are expected to be singular. Most of the grain boundaries between the randomly oriented fine-matrix grains in the 0.3 at.% Ti-excess specimen are also faceted with hill-and-valley shapes at finer scales when observed under TEM. The facet planes are parallel to {111}, {011}, and {012} planes of one of the grain pairs and are also expected to be singular. These high-angle grain boundaries lying on low index planes of one of the grain pairs are similar to those observed in other oxides and metals.  相似文献   

7.
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.  相似文献   

8.
Equilibrium electrical conductivity data for large-grained, poly crystalline, undoped BaTiO3, as a function of temperature, 750° to 1000°C, and oxygen partial pressure, 10−20< P O2<10−1 MPa, were quantitatively fit to a defect model involving only doubly ionized oxygen vacancies, electrons, holes, and accidental acceptor impurities. The latter are invariably present in sufficient excess to control the defect concentrations through the compensating oxygen vacancies, except under the most severely reducing conditions. Singly ionized oxygen vacancies play no discernible role in the defect chemistry of BaTiO3 within this experimental range. The derived accidental acceptor content has a slight temperature dependence which may reflect some small amount of defect association. Deviation of the conductivity minima from the ideal shape yields a small P O2-independent conductivity contribution, which is tentatively identified as oxygen vacancy conduction.  相似文献   

9.
The high-temperature equilibrium electrical conductivity of Ce-doped BaTiO3 was studied in terms of oxygen partial pressure, P (O2), and composition. In (Ba1−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(Ti1−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.  相似文献   

10.
The equilibrium electrical conductivity of polycrystalline, calcium-doped BaTiO3 was studied over the oxygen partial pressure range 10-13 to 105 Pa and the temperature range 800° to 1000°C. There is little effect if CaO is substituted for a corresponding amount of BaO, i.e., Ba, 1-xCaxTiO3. If CaO is substituted for a corresponding amount of the TiO2 content, i.e., BaTi1-xCaxO3-x, the equilibrium conductivity shows strong evidence of acceptor-doped behavior. If the corresponding amount of excess CaO is added to stoichiometric BaTiO3, i.e., BaCaxTiO3+x, the conductivity profiles are very close to those for samples with TiO2 replaced by CaO, and show highly acceptor-doped behavior. This is in agreement with the replacement of a small amount of Ti by Ca2+ on the octahedral B-sites of BaTiO3, where it acts as an acceptor center, CaT  相似文献   

11.
The purpose of this study was to try to find out why BaTiO3 (essentially a cubic structure) can consistently be grown from KF solutions as large flat plates when a more equant morphology is expected. It is suggested that the size, shape, and perfection of the platelike "butterfly" twin crystals can be qualitatively related to the re-entrant angles which result from {111} twinning of crystals of {100} habit. The re-entrant angles form permanent steps for both the advancing crystal tip and for the growth layers of the wings. The amount of twinning is related to the cooling rate and to the presence of certain impurities. Thus it can be shown that increasing the cooling rate increases the number of {111} twins (essentially a hexagonal stacking fault); the extreme case is the metastable formation of hexagonal barium titanate by quenching. Evidence is presented for the stabilization of the Ti2O9 group of the hexagonal structure by substitution of ions of lower polarizing power.  相似文献   

12.
This paper demonstrates a number of features that can contribute to BaTiO3 being perturbed from an equilibrium condition. The BaTiO3 synthesis study was conducted with a modified citrate process, but the kinetic aspects limiting the equilibrium state are believed to be general to any processing route. The equilibrium conditions can be determined through a detailed analysis of the paraelectric–ferroelectric phase transition data measured by differential scanning calorimeter. Broadened and multiple latent heat peaks are found indicating a distribution of slightly different ferroelectric transition temperatures for nonequilibrated BaTiO3 materials. A number of factors have been found that limit the equilibrium conditions, and these include time at formation conditions of temperature and oxygen partial pressure, a second phase BaCO3, and the rapid nucleation of a BaTi2O5 phase on quenching to room temperature from temperatures in and around 1250°C. All of these nonequilibrium factors that occurs at high temperature lead to the ferroelectric phase transition having multiple phase transitions, owing to regions of the BaTiO3 having different partial Schottky concentrations.  相似文献   

13.
Solid solution formation in the system BaTiO3—KnbO3 was established by X-ray diffraction and dielectric measurements. Solid solutions with cubic symmetry were observed in the composition range from 4 to 90 mole % KnbO3 at room temperature. The lattice parameter for the BaTiO3 solid solutions increased with increasing KNbO3; that for the KnbO3 solid solutions decreased with the addition of BaTiO3. A distinct discontinuity in lattice parameter was observed at the composition containing about 65 mole % BaTiO3. Dielectric measurements were made from-195° to 400°C. The cubic-tetragonal transition temperature of BaTiO3 was rapidly lowered with increasing addition of KNbO3, whereas the two lower phase transition temperatures were raised. All three phase transitions of KnbO3 were rapidly lowered with increasing addition of BaTiO3. The observed phase transitions, lattice parameters, and electron probe data suggest a complex region in the subsolidus which extends from 35 to about 75 mole % KNbO3.  相似文献   

14.
BaTiO3 nanofibers were prepared by electrospinning. The morphology of synthesized BaTiO3 nanofibers was investigated under different heat treatment conditions. The phase transformations in BaTiO3 nanofibers were monitored using Raman spectroscopy. It has been found that the Curie temperature of BaTiO3 nanofibers increased to 220°C, which is notably higher than the bulk BaTiO3 ceramics.  相似文献   

15.
Polycrystalline barium titanate fired in nitrogen at 1300° to 1400°C accommodates up to 3 mole % UO2 in solid solution; its structure is then cubic at room temperature. With BaUO3 additions the structure becomes disordered and quasi-cubic. In air, about 1 mole % UO2 goes into solid solution in BaTiO3 but the structure remains tetragonal. Diffraction peaks of a new phase, possibly a ternary oxide of barium, uranium, and titanium, appear in patterns of specimens containing more than 2 mole % UO2. The dielectric constant of BaTiO3 ceramics fired in air, steam, or oxygen increases with up to about 0.5 mole % UO2 but declines rapidly above this level. The dielectric constant of BaUO3 is about two orders of magnitude lower than that of BaTiO3, and additions of BaUO3 invariably lower the dielectric constant of BaTiO3.  相似文献   

16.
Scanning tunneling microscopy (STM) and spectroscopy (STS) have been applied to study the surface electronic properties of n -type BaTiO3 ceramics under ultrahigh vacuum and at various oxygen partial pressures. I – V tunneling characteristics of vacuum-annealed BaTiO3 do not exhibit rectifying behavior, implying that the Fermi level is pinned at the surface. The surface band gap of BaTiO3 annealed under vacuum at 540°C is equal to 1 eV. The top edge of the surface valence band is located 0.7 eV below the Fermi level. Hysteresis in the I – V characteristics has been observed at high oxygen partial pressures. Dosing of the BaTiO3 with oxygen increases the surface band gap and unpins the Fermi level. As a result, the I – V characteristics acquire rectifying features similar to those observed for BaTiO3 Schottky-type diodes. Hysteresis in the I – V spectra observed at high oxygen partial pressures is attributed to the changes of the surface potential barrier due to adsorption/desorption of oxygen modulated by the tip-sample potential difference.  相似文献   

17.
The properties and composition of glasses suitable for crystallization of BaTiO3 are described. The crystallization of certain glasses results in a nearly complete recovery of BaTiO3, besides the feldspar BaAl2SiO3 as a minor phase. The mechanism of crystallization was investigated by thermal analysis, viscosity, and grainsize measurements as a function of the temperature whereas density data were used for evaluation of the BaTiO3 content. Within the range 30 to 60% by volume of BaTiO3 at about 1μ grain size, the measured dielectric constant increased from 100 to 1200. The calculated partial dielectric constant of the Titanate phase at this grain size was about 3500. As the grain size approached 0.1μ, the dielectric constant decreased and became nearly independent of the temperature because of the predominance of surface states. Other effects were attributed to special structural characteristics, such as absence of porosity and clamping of the titanate particles within the microcrystalline matrix. Data are also presented on dielectric constant and loss tangent at different frequencies, dc breakdown strength, dc resistivity, and ferroelectric properties as a function of the grain size of the crystallized material.  相似文献   

18.
Defects in the paraelectric phases of BaTiO3 doped with Bi2O3 were analyzed by transmission electron microscopy under two-beam conditions. (111) twin structures were characterized by selected area diffraction and bright-field images. The orientation relationships of the (111) twins were determined using stereograms. Lamella-twinned crystallites included in the paraelectric phases were found in this system. Pure wedge fringes were analyzed in these grains using electron diffraction and imaging techniques. Double diffraction was observed in the overlapped regions of the matrix and the microtwin in the [113] direction, and high-density dislocation loops were seen in some grains. Weak-beam dark-field microscopy techniques were used to observe the dislocation loops, which predominately lay on {100} crystal planes with Burgers vectors a 〈100〉, and were found to be pure edge dislocations. Some dislocations were transformed into crystallographic shear planes.  相似文献   

19.
(111) twins are found frequently in the microstructure of BaTiO3 ceramics prepared by the mixed-oxide technique. The processing steps by which the (111) twins are formed are in question. A BaCO3-TiO2 powder mixture is milled and then Calcined at 1100°C for 2 h, where BaTiO3 is formed by the release of CO2. After the powder is calcined, it is analyzed by TEM. Image and diffraction pattern data are combined to prove the presence of (111) twins in powder particles; the formation of (111) twins occurred frequently during calcination. The formation mechanism of such twins and their role during anomalous grain growth are discussed.  相似文献   

20.
Complex impedance analysis at cryogenic temperatures has revealed that the bulk and grain boundary properties of BaTiO3 polycrystals are very sensitive to the oxygen partial pressure during sintering. Polycrystals sintered at P O2 as low as 10−15 atm were already electrically heterogeneous. The activation energy of the bulk conductivity in the rhombohedral phase was found to be close to that of the reduced undoped single crystal (i.e., 0.093 eV). The activation energy of the grain boundary conductivity increases with the temperature of the postsinter oxidation treatment from 0.064 to 0.113 eV. Analysis of polycrystalline BaTiO3 sintered in reducing atmosphere and then annealed at P O2= 0.2 atm has shown that the onset of the PTCR effect occurs at much higher temperatures than expected in the framework of the oxygen chemisorption model. The EPR intensity of barium and titanium vacancies increases after oxidation at T > 1000°C. A substantial PTCR effect is achieved only after prolonged annealing of the ceramic in air at temperatures as high as 1200–1250°C. This result suggests that the PTCR effect in polycrystalline BaTiO3 is associated with interfacial segregation of cation vacancies during oxidation of the grain boundaries.  相似文献   

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