Niobium-Doped Barium Titanate Research Articles

Processing and characterization of calcined and sintered niobium doped barium titanate

ABSTRACTProcessing and characterization of niobium doped barium titanate was accomplished through mixed oxides route via calcination based on the formula BaTi1-xNbxO3 where x = 0.004, 0.008 and 0.016. Perovskite structure was observed at calcination temperature of 1300°C, confirmed by X-ray diffraction (XRD) data. Interestingly, XRD peaks corresponding to (002) and (200) planes insisted that increasing Nb doping level reduced the tetragonality of barium titanate as found by plotting c/a ratio against doping level. Submicron-size particles were observed for all compositions, though size varied significantly due to Nb. Dielectric constant values were apparently small at higher doping concentration.

Microstructure and Electrical Properties of Niobium Doped Barium Titanate Ceramics

The effects of Nb addition on phase, microstructure and electrical properties of barium titanate ceramics were investigated. Nb-doped barium titanate ceramics (BaTi1-xNbx)O3, x = 0 to 0.02) were prepared by conventional mixed-oxide method. All barium titanate compositions show tetragonal perovskite structure. Low Nb concentration (x = 0.005) results in abnormal grain growth in barium titanate. However, the grain size becomes uniform and decreases as Nb content increases, suggesting Nb inhibits grain growth in the studied composition range. The dielectric constant increases as the amount of Nb increases. The temperature of tetragonal to cubic phase transition shifts to lower temperature and the temperature curves become broadening with increasing of Nb content. Measurement of ferroelectric P-E hysteresis loop at 50 Hz and applied electric field of 15 kV shows that the remanent polarization of barium titanate ceramics tends to decrease as Nb content increases. Change in dielectric properties and the P-E hysteresis loop did not show large aging effect in the Nb-doped barium titanate ceramics.

Preparation and characterization of niobium-doped barium titanate nanocrystalline powders and ceramics

Nb-doped BaTiO 3 nanocrystalline powders and ceramics were prepared by a simple sol–gel process, which used H 3[Nb(O 2) 4] as a precursor. The powders and ceramics were characterized by methods of XRD, SEM and TEM, while dielectric properties of the ceramics were also determined. The results indicated that the powders synthesized by sol–gel process were in nanometer scale, which were mainly composed of cubic BaTiO 3 with small amount of BaCO 3. After sintering, only BaTiO 3 phase could be found and there was no evidence of a second phase. Compared to the traditional solid-state method, the sintering temperature of the ceramics prepared by sol–gel route was reduced obviously due to the good sinterability of nanopowders. And doped niobium could make Curie temperature shift to low temperature. Nb-doped BaTiO 3 ceramics calcined at 1000 °C for 2 h and sintered at 1250 °C for 2 h had a regular microstructure with the maximal dielectric constant of 10,298. The methodology is a simple process with low cost, by which materials of BaTiO 3 powders doped with high concentration of niobium, even up to 10 mol%, can be prepared.

Microscopic Origin of the PTC Effect in Niobium-Doped Barium Titanate

The influence of bulk conductivity on the PTC effect of Nb-doped barium titanate is investigated. Both macroscopic and microscopic resistance measurements are performed as a function of temperature. The macroscopic measurement refers to the whole sample while the microscopic one, obtained by photolithography techniques, concerns only several grains. The results show that the increase of resistivity occurs in two steps: first, an abrupt variation at the Curie temperature T-c which is ascribed to the phase transition in the bulk; second, a continuous increase above T-c which is usually explained from grain boundary potential barriers only. However, our results suggest that a nonnegligible contribution of the bulk is also involved in this increase above T-c.

Positive temperature coefficient of resistivity effect in niobium-doped barium titanate ceramics obtained at low sintering temperature

A positive temperature coefficient of resistivity (PTCR) effect was observed for low temperature sintered Nb-doped BaTiO 3 ceramics using YB 6 as a sintering aid. A semiconducting BaTiO 3 with good PTCR property was obtained at a low sintering temperature of 1150 °C because of the addition of YB 6. Addition of 0.2 mol% Nb 2O 5 and 1 mol% of YB 6 led to a low room-temperature resistivity and a resistivity jump of about 10 4. Room-temperature resistivity of the ceramics was controlled by the grain boundary resistivity which was related to the added amount of YB 6 and the sintering temperature. YB 6 only worked as a sintering aid to enhance the densification of semiconducting BaTiO 3 ceramics at low sintering temperature.

Size effect on structure and dielectric properties of Nb-doped barium titanate

It is known that the dielectric properties of BaTiO 3 (BT) are strongly dependent on its grain size. Coarse-grained ceramics of pure BT showed lower dielectric constant at room temperature then fine grained. Many authors considered that when the grain size is lower than 700 nm, the lattice of BT changes from tetragonal to pseudocubic, and the dielectric constant value is very low. In the doped BT this effect is more complex, because it is necessary to consider also the influence of dopants. The grain size effect on the structure and dielectric properties of niobium-doped barium titanate was investigated. Niobium-doped barium titanate was prepared from powders obtained by doping of commercial barium titanate and from organometallic complex using citrates as precursors (Pechini procedure). The crystal and microstructure of sintered niobium-doped barium titanate were determined. Dielectric constant and dissipation factor were measured. The observation confirmed that the structure and properties are strongly dependent on grain size.

Structure and Properties of Donor Doped Barium Titanate Prepared by Citrate Process

Niobium doped barium titanate was prepared using two procedures. First, doped barium titanate was prepared starting from citrate solutions of all components and second, pure barium titanate powder was obtained from the citrate solutions and after that doped. Besides niobium, a small amount of manganese, as acceptor dopant was added. Phase composition, crystal structure, microstructure and dielectric properties were reported. The influence of powder processing on the properties of niobium doped barium titanate was analysed. The grain growth and the concentration of dopants on the dielectric properties were considered.

Secondary phases in Nb-doped BaTiO 3 ceramics

In this work, the effect of Nb 2O 5 addition on the microstructure of BaTiO 3 was studied. From XRD diagrams, a diminution in tetragonality parameters with an increase in dopant concentration was observed. In order to determine morphology and composition of secondary phases in niobium-doped barium titanate, EDAX and SEM analyses were carried out. It was found that a concentration of dopant higher than 0.15 mol% leads to fine-grained BaTiO 3 without abnormal grain growth. Otherwise, compositions of secondary phases correspond to the titanium-rich region in the BaO–TiO 2 phase diagram. Besides, the titanium content in the precipitate increases with the Nb 2O 5 addition.

Positive Temperature Coefficient of Resistance Effect in Heavily Niobium-Doped Barium Titanate by the Growth of the Double-Twinned Seeds

Positive temperature coefficient of resistivity (PTCR) was attained in heavily niobium-doped BaTiO3 through the addition of BaTiO3 seed particles. The seed particles containing a double twin grew during heat treatment, and a uniform microstructure composed of large, coarse grains was obtained. The incorporation of niobium into the grain via grain growth probably caused the semiconducting and PTCR character of the niobium-doped BaTiO3.

Formation of Potential Barrier Related to Grain‐Boundary Character in Semiconducting Barium Titanate

Resistance–temperature (R–T) characteristics were measured directly at single‐grain boundaries in 0.1‐mol%‐niobium‐doped barium titanate bicrystals that had been fabricated from polycrystalline sinters, to determine a geometrical grain‐boundary character dependence of the positive temperature coefficient of resistivity (PTCR) effect. Both random boundaries and low‐Σ boundaries exhibit a similar grain‐boundary character dependence of the PTCR effect through a simple geometrical analysis, using the coincidence of reciprocal lattice points. Differences of the R–T characteristics in individual boundaries have been explained in terms of the formation of a potential barrier that is associated with the oxidation of grain boundaries during cooling, after sintering or annealing. The grain‐boundary character is likely to affect the diffusivity of O2− ions and, hence, is crucial to the formation of the potential barrier.

Effect of Acceptors on the Segregation of Donors in Niobium‐Doped Barium Titanate Positive Temperature Coefficient Resistors

Small amounts of niobium (donor) doping in barium titanate (BaTiO3) cause semiconductivity in this material. When the amount of niobium is increased slightly, the BaTiO3 is converted back to an insulator. This phenomenon is attributed to donor segregation. Such an insulating material can recover its semiconductivity when specific amounts of acceptors are introduced. The space‐charge segregation theory explains that such acceptors can prevent the segregation of donors. Impedance/modulus analysis of BaTiO3 with a slightly increased niobium content and with no or a minimal amount of magnesium (acceptor) show a response of a third resistance–capacitance (RC) network, in addition to the two that have been thought to be caused by the grains and grain boundaries. The response of the third RC network is not observed at greater magnesium concentrations that give a recovery of the semiconductivity. This study suggests that acceptors can inhibit the donor segregation.

Grain boundary electrical barriers in positive temperature coefficient thermistors

Resistance–temperature, current–voltage, and capacitance–voltage characteristics have been measured in niobium-doped barium titanate bicrystals. The analysis by device simulation reveals that potential barrier with a semiconductor–insulator–semiconductor structure can consistently interpret the observed characteristics rather than the double Schottky barrier. Absence of sub-ohmic behavior in the current–voltage characteristics and excess low-frequency capacitance at zero bias are caused by the spatially distributed traps at the insulating layer.

X-ray diffraction and Rutherford backscattering spectrometry of Ba1NbxTi1−xO3 thin films synthesized by laser ablation

Niobium doped barium titanate Ba1NbxTi1−xO3 thin films have been successfully synthesized by laser ablation on sapphire (000̄1) and (100) SrTiO3 substrates, for the growth of hexagonal and cubic phases of this system. These films were synthesized with different concentrations of niobium under various conditions. X-ray diffraction measurements showed that the films were well aligned along the c-axis of the sapphire substrate for the hexagonal phase, while perpendicular to the SrTiO3 substrate surface for the cubic phase of this system. These films were deposited at substrate temperatures of 700 and 650 °C for the hexagonal and cubic phases, respectively, both in oxygen and nitrogen atmospheres. At room temperature these films have a very high resistance and large negative thermopower ≃300 mV/K. Rutherford backscattering spectrometry (RBS) was performed on very thin films of this system for different concentrations of niobium deposited in the N2 atmosphere. We have found a deficiency of Ti in these films by RBS analysis, which along with other antisite defects and oxygen deficiency, may be a cause of high resistance and large thermopower in these films.

Grain Orientation Dependence of the PTCR Effect in Niobium‐Doped Barium Titanate

The positive temperature coefficient of resistivity (PTCR) effect is directly measured in single grain boundaries in 0.1‐mol%‐Nb‐doped BaTiO3 with 1 mm coarse grains. The PTCR effect largely depends on grain boundary structure. Random grain boundaries exhibit the PTCR effect as in polycrystalline samples, but the PTCR effect does not appear in highly coherent boundaries such as small‐angle boundaries, twin boundaries, and coincidence site lattice (CSL) boundaries with low Σ values. For Σ= 3 boundaries, the resistance increase above the Curie temperature is a function of deviation angle. A small PTCR effect is observed in Σ= 3 boundaries with a deviation angle of about 9° in contrast with ideal Σ= 3 boundaries and boundaries with a deviation of about 4°.

Transition of Compensating Defect Mode in Niobium‐Doped Barium Titanate

The influence of Nb‐doping from 1.0 to 8.0 at.% on grain size, ferroelectric phase transition, lattice strain, and electrical properties of BaTiO3 Cerumics has been studied. A change in the Nb‐doping effect was observed at compositions around 3.5–4.5 at.% Nb. The impedance behavior revealed that specimens of low Nb‐doping had an activation energy of 1.85 eV for carrier conduction either in bulk or at grain boundaries, but specimens having a high Nb‐doping content showed an energy of 1.30 eV in bulk and 1.85 eV at grain boundaries. The capacitance–voltage relation also disclosed different influences of Nb‐doping on the potential barrier height at grain boundaries. The above results are explained by the transition of a compensating defect mode from pure barium vacancies to a combination of titanium vacancies in grain interiors and barium vacancies at grain boundaries as the Nb‐doping content in BaTiO3 is increased.

Niobium-Doped Barium Titanate—A Chemically Processed Material with High Dielectric Constant

Various mol% of Nb2O5 were incorporated into BaTiO3 during its preparation in the form of BaTiO(C2O4)2.4H2O. For a critical concentration of 0.25 mol% Nb2O5, the dielectric constant of the material...

Pyroelectricity and dielectric anisotropy in polarized and niobium modified barium titanate ceramics

The effect of time of soaking on bulk density and dielectric constant has been measured on pure and niobium-doped barium titanate pyroelectric specimens. The temperature dependence of the dielectric constant for both pure and niobium-doped barium titanate samples has been investigated before and after poling. The percentage dielectric anisotropy and ferroelectricity was also calculated for various pure and modified barium titanate pyroelectric materials. Our results can be explained in terms of the redistribution of the barium titanate lattice in a different crystallographic orientation, polarization, as well as the existence of lattice imperfection accompanying temperature, poling and niobium doping. The obtained results were in good conformity, and correlated with the increased proportion of the remnant 90° domains (induced electrically charged vacancies as a result of Nb 3+ substitution of Ti 4+ in octahedral lattice sites). It was concluded that poling of Nb-doped barium titanate samples produces the best quality ceramic pyroelectrics.

Piezoelectricity, pyroelectricity and thermoelectricity of polymer films: Takeo Furukawa, Yutaka Uemastsu, Kiyoshi Asakawa, Yasaku Nada Journal of Applied Polymer Science, Vol 12, No 12 (December 1968) p 2675 (1017)

The effect of time of soaking on bulk density and dielectric constant has been measured on pure and niobium-doped barium titanate pyroelectric specimens. The temperature dependence of the dielectric constant for both pure and niobium-doped barium titanate samples has been investigated before and after poling. The percentage dielectric anisotropy and ferroelectricity was also calculated for various pure and modified barium titanate pyroelectric materials. Our results can be explained in terms of the redistribution of the barium titanate lattice in a different crystallographic orientation, polarization, as well as the existence of lattice imperfection accompanying temperature, poling and niobium doping. The obtained results were in good conformity, and correlated with the increased proportion of the remnant 90° domains (induced electrically charged vacancies as a result of Nb3+ substitution of Ti4+ in octahedral lattice sites). It was concluded that poling of Nb-doped barium titanate samples produces the best quality ceramic pyroelectrics.

Electrical Conductivity of Niobium-Doped Barium Titanate

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