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1.
The influence was studied of grain boundary layer modifiers on the impedance spectra of semiconducting BaTiO3 ceramics processed
in various atmospheres. Ceramic samples of undoped BaTiO3 annealed in an N2+H2 atmosphere showed a single semicircle in the
Cole–Cole plot when ohmic Ni electrodes were used. Impedance spectra of donor-doped specimens sintered in air showed overlapping
semicircles which may indicate relaxational processes within the grains, e.g. electron trapping and detrapping at the deep
traps. This is strongly supported by the minima shown by the grain resistance (Rg) around the Curie point (Tc), whereas the
behaviour of the grain boundary resistance (Rgb) with temperature is similar to the d.c. resistance, i.e. a positive temperature
coefficient of resistance (PTCR). Electron paramagnetic resonance (EPR) results indicate that the charge trapping is caused
by the redistribution of electrons at native and extrinsic acceptor states during phase transitions. Thus, the neutral barium
vacancies (VXBa) present in the tetragonal and orthorhombic phases change into singly ionized barium vacancies (V′Ba) in the
cubic and rhombohedral phases. From the correlation between the resistivity changes and the charge redistribution at the trap
states, it is envisaged that the PTCR effect is related to charge trapping at the acceptor states having higher concentration
in the regions of the grain boundary layer. The impedance spectral data support this conclusion.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
2.
Modifications in the positive temperature coefficient in resistance (PTCR) of n-BaTiO 3 ceramics are brought about by specific additives such as Al 2O 3, B 2O 3 or SiO 2, leading to the segregation of secondary phases such as BaAl 6TiO 12, BaB 6TiO 12 or BaTiSi 3O 9 at the grain boundaries. Segregation of barium aluminotitanates resulted in broad PTCR curves, whereas B 2O 3 addition gave rise to steeper jumps and SiO 2 addition did not result in much broadening compared with donor-only doped samples. Microstructural studies clearly show the formation of a structurally coherent expitaxial second phase layer of barium aluminotitanate surrounding the BaTiO 3 grains. Electron paramagnetic resonance investigations indicated barium vacancies, V Ba, as the major electron trap centres which are activated across the tetragonal-to-cubic phase transition according to the process V X
Ba + e V Ba. The grain size dependence of the intensity of the V Ba signal indicated the concentration of these trap centers in the grain-boundary layer (GBL) regions. Further, the charge occupancy of these centres is modified by the secondary phases formed through grain-boundary segregation layers. BaAl 6TiO 12 gave rise to Al-O – hole centres whereas no paramagnetic centres corresponding to boron could be detected on B 2O 3 addition. Such secondary phases, forming epitaxial layers over the BaTiO 3 grains, modify the GBL region, rich in electron traps, surrounding the grain core. The complex impedance analyses support this three-layer structure, showing the corresponding contributions to the total resistance which can be assigned as R
g, R
gb and R
secondary phase. The epitaxial second phase layers bring about inhomogeneity in the spatial distribution of acceptor states between the grain boundary and the grain bulk resulting in extended diffuse phase transition characteristics for the GBL regions in n-BaTiO 3 ceramics. This can cause the GBL regions to have different transition temperatures from the grain bulk and a spread in energy levels of the associated GBL trap states, thus modifying the PTCR curves. An attempt has been made to explain the results based on the vibronic interactions applied to the mid-band-gap states in n-BaTiO 3. 相似文献
3.
The influence of B 2O 3, and Al 2O 3 as segregative additives in modifying the ρ– T characteristics has been studied in BaTiO 3 ceramics with positive temperature coefficient of resistance (PTCR). Reaction of Al 2O 3 at the grain boundary regions of BaTiO 3 ceramics leads to the segregation of the secondary phase, BaAl 6TiO 12 resulting in broad PTCR jump, whereas B 2O 3 addition gives rise to steeper resistivity jump. Microstructure studies by SEM reveal the formation of coherent second phase
layer of barium aluminotitanate surrounding the BaTiO 3 grains. The EDX results shows varying Al to Ti ratio in the early stage of phase formation in BaAl 6TiO 12 resulting in electrically active layer around the BaTiO 3 grains. The TiO 2-excess melt formation results in lower resistivity for 2–4% Al 2O 3 containing n-BaTiO 3 ceramics whereas at higher alumina contents, BaAl 6TiO 12 phase becomes dominant leading to higher resistivity in the sample. Complex impedance analyses support the three-layer regions,
corresponding to the contributions from grain interior resistance ( R
g), grain boundary resistance ( R
gb), and that from secondary phase ( R
sec). Electron paramagnetic resonance spectroscopy (EPR) indicated barium vacancies, V
Ba
/
as the major electron trap centers which are activated across the tetragonal-to-cubic phase transition. A charge trapping
mechanism is proposed wherein the segregation of secondary phases bring carrier redistribution among the various acceptor
states thereby affecting the electrical conductivity of n-BaTiO 3 ceramics. The presence of Al 3+–O −–Al 3+ or Ti 4+–O −–Al 3+ type hole centers at the grain boundary layer (GBL) regions results in charge redistribution across the modified phase transition
temperature due to symmetry-related vibronic interactions resulting in broad PTCR characteristics extending to higher temperatures. 相似文献
5.
BaTiO 3 ceramics for positive temperature coefficient resistance (PTCR) were fabricated via tape casting technique and reduction/reoxidation process. A novel method was used to regulate the density of the ceramics via changing binder amount in tape casting green bodies. The influences of different amounts of the binder on density, electrical properties, and microstructure of ceramics were investigated. The BaTiO 3 ceramic monolith without internal electrodes with an addition of binder of 0.84 wt% was sintered in reducing atmosphere at 1150 °C and reoxidized at 800 °C for 1 h showed a resistivity of 56 Ω·cm and a PTCR jump about 3.2 orders of magnitude. The green and sintered density of the samples decreased with the increasing binder amount. The room temperature resistivity increased monotonically with the binder amount. However, the correlation of the PTCR jump with binder amount showed a contrary tendency. These electrical properties can be explained by the Jonker model. The influences of sintering temperature on the density of BaTiO 3 ceramics with different binder amount were also discussed. In addition to the ceramic monoliths without internal electrodes, multilayer elements with internal electrodes were also prepared. 相似文献
6.
In this paper, room temperature multiferroic behaviour in Ni (Nickel)-doped NBT (Na0.5Bi0.5TiO3) ceramics synthesized using solid state reaction technique have been investigated. Structural, dielectric, ferroelectric, magnetic and magneto-dielectric properties were consistently probed with the increment in transition metal doping. XRD peaks were indexed for the monoclinic Cc phase. SEM micrographs clearly depicted the reduction of grain size with addition of Ni content. Ferroelectric polarization (P) vs applied electric field (E) hysteresis curves shows an increase in lossy behaviour with an increase in Ni content. The room temperature magnetization (M) vs applied magnetic field (H) curves depict the weak ferromagnetic ordering on increasing the Ni doping. Enhanced magneto-dielectric change of 1.26% was observed in 25% Ni-doped NBT ceramic, which may be useful in the development of novel non-volatile lead-free multiferroic memory devices. 相似文献
7.
There have been studied single crystals of undoped and doped Bi 12TiO 20 with two concentrations of W 5+ (2.62 × 10 17 cm −3 and 2.62 × 10 18 cm −3). There have been obtained absorption spectra in the energy range of 10,482–15,408 cm −1 by classical measurements. There have been determined the cross-section ( σa) of the impurity absorption and the oscillator strength of d– d transitions. There have been calculated the refractive index of doped crystals and the concentration of Ti 3+ ions in an undoped sample through an experiment. 相似文献
10.
Ceramics with simplified compositions of ZnO + Bi 2O 3 + CoO or MnO show non-linearity coefficients () of 40–65 provided the concentration of transition metal ions is > 1.5mol.... Samples doped with Co have higher non-linearity coefficients than those with Mn. This is attributed to the stability of multiple oxidation states of Co(III) + Co(II) in contrast to Mn(II) as the stable species in sintered ZnO ceramics. Electron paramagnetic resonance and diffuse reflectance spectral studies establish this fact. Low-signal capacitance-voltage measurements show that donor density ( N
d) in these ceramics ranges from 0.3 to 1.8 × 10 19 cm –3, which is comparatively larger than those of commercial varistors. The barrier height ( b) reaches up to 0.66 eV and the breakdown voltage is around 3.4–3.7 eV. Admittance spectroscopy and isothermal capacitance transient spectroscopy (ICTS) are used for characterizing the bulk traps originating from the transition metal dopants. Capacitance-voltage analyses above the breakdown voltages show negative capacitance, indicative of oscillatory charge redistribution involving multivalent states of Co and the shallower interface states. Multiple trapping relaxations are evident from the complex-plane capacitance studies. 相似文献
11.
Donor doped BaTiO 3 (n-BaTiO 3) ceramics were fabricated by adding polyethylene glycol (PEG) at 20 wt %. The effects of reducing and oxidizing atmospheres on the PTCR characteristics of the porous n-BaTiO 3 ceramics were investigated. The PTCR characteristics of the porous n-BaTiO 3 ceramics is strongly affected by chemisorbed oxygen at the grain boundaries and are recovered as the atmosphere is changed from the reducing gas to oxidizing gas. The low room-temperature resistivity of the porous n-BaTiO 3 ceramics in reducing atmospheres may be caused by the decrease in potential barrier height, which originates from an increase in the number of electrons owing to the desorption of chemisorbed oxygen atoms at the grain boundaries. In addition, the high room-temperature resistivity of the porous n-BaTiO 3 ceramics in oxidizing atmospheres may be caused by the increase in potential barrier height, which results from the adsorption of chemisorbed oxygen atoms at the grain boundaries. 相似文献
13.
A strong effort of the Mn 3+(d 4) ions to remove their orbitally degenerated ground state is a motivating power to lower the symmetry of their surroundings. Besides this tendency the real effect depends also on a number of additional contributions, above all the fundamental structural matrix and compositional influences. They are analyzed on examples of three structural types derived from the close packed arrangement of oxygen anions, namely spinel, magnetoplumbite and Perovskite structures. 相似文献
15.
Journal of Materials Science: Materials in Electronics - In this work, (1-x)Bi1+yFeO3-xBaTiO3 (x?=?0.25, 0.30, 0.35, 0.40; y?=?? 0.02, 0.00, 0.02, 0.04)... 相似文献
16.
Positive temperature coefficient of resistivity (PTCR) ceramics with low resistivities at room temperature were obtained by using oxalate-derived barium titanate powders. The average room-temperature resistivity of the PTCR ceramics was 4 cm, and the magnitude of their PTCR jump was around four orders with a voltage proof of more than 50 Vmm –1. These PTCR properties are significantly influenced by the calcination temperature of the starting materials and by the resultant properties of the ceramic bodies. The microstructure of such-PTCR ceramics with a low room-temperature resistivity produced in this study was found to be rather heterogeneous. Complex impedance measurements revealed that the resistivity of the present PTCR materials was determined predominantly by the grain-boundary resistance even at room temperature. 相似文献
17.
Infrared (IR) detecting elements were prepared using positive temperature coefficient (PTC) thermistors with large temperature
coefficients of resistivity (α). Their compositions were denoted as Ba 1−x
Sr
x
Nb 0.003Ti 0.997O 3 + 1 mol % TiO 2 + 0.07 mol %MnO ( x=0, 0.2), and their temperature coefficients of resistivity were 78 and 50% K −1, respectively. Their IR sensing properties were measured under the self-regulating heating conditions, and were compared
with those of a detector with small α (18 % K −1). It was shown that large α was effective for controlling the element temperature by self-regulating heating and for improving
sensitivity. The responsivity, R
v
of the element with x=0.2 was 980 VW −1, and was as large as those of pyroelectric detectors. Expressions which normalize the sensitivity and the thermal time constant
were derived. From these expressions, criteria for improving some IR sensing properties were obtained. 相似文献
18.
Ca 1−2xDy xBi xMnO 3 ( x = 0.02, 0.05, 0.08 and 0.10) ceramics have been synthesized by solid state reaction method. Samples with relative densities up to 98% have been obtained. Microstructures have been characterized by XRD and SEM. Thermoelectric properties are evaluated between 300 and 1100 K. Both electrical resistivity and Seebeck coefficient decrease with increasing Dy and Bi content. Thermal conductivity increases with doping, increasing up to x = 0.08. The highest power factor of 420 μW (K 2 m) −1 is obtained at x = 0.02, resulting in the highest dimensionless figure of merit (ZT) of 0.21 at 973 K. This value is four times as much as that of undoped CaMnO 3 ceramics. All examples show good self-compatibility with weak temperature dependence. 相似文献
20.
The effect of A-site substitution of Ce 3+ in tungsten bronze structured PBN ceramics with the general formula, Pb (x − 3y/2)Ce
y
Ba (1 − x)Nb 2O 6 and the stoichiometric chemical formula, Pb (0.65 − 3y/2)Ce
y
Ba 0.35Nb 2O 6, where y = 0, 2, 4, 6, 8 and 10 mol% ceramic compositions synthesized through solid state reaction method are reported. The X-ray
diffraction studies exhibited the presence of an orthorhombic phase, and its intensity increased with the increasing Ce 3+ content up to y = 6 mol% or A 3 composition. The lattice parameters, unit cell volume and density as a function of Ce 3+ concentration are discussed. It is observed that increasing Ce 3+ content in A-site influenced the dielectric properties. The optimum dielectric properties of room temperature dielectric
constant (ε RT) and dielectric maximum
() are observed in y = 6 mol% or A 3 composition while Curie temperature ( T
c) and dielectric loss (tan δ) constantly decreased from undoped to y = 10 mol%, and thus A 3 composition or 6 mol% Ce modified tungsten bronze structured-PBN could be suitable for capacitor applications. 相似文献
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