High temperature NTC BaTiO3-based ceramic resistors

In our research it was firstly found that BaTiO₃ materials doped BaBiO₃ compounds only showed NTC effect in wide temperature scale. The effect of the composition on the microstructure and electrical properties of BaTiO₃-based NTC thermistors was studied. Major phases present in the sintered bodies were the solid solutions of BaTiO₃ compounds with a perovskite structure and BaBiO₃ compounds with a monoclinal structure. The average grain size of the samples increased with increasing BaBiO₃ contents. It is apparent that the samples' ρ₄₅₀ and B_450 / 600 constant decreased with an increase in BaBiO₃ content.     

Electrical transport and magnetoresistance in La0.67Ca0.33MnO3/BaTiO3 composites

The results of the structure, electrical transport and magnetoresistance of a ferromagnet-ferroelectric-type La_0.67Ca_0.33MnO₃ (LCMO)/BaTiO₃ composites fabricated by the sol-gel method are presented. The structure and morphology characterization indicates no apparent variations in morphology and particle size in spite of the existence of BaTiO₃. The insulator-metal transition temperature (T_IM) is shifted to a higher temperature and resistivity decreases with the increase of low content BaTiO₃. Magnetoresistance (MR) of the composites is enhanced over the whole temperature range as a result of the introduction of BaTiO₃. By calculating in terms of a ferromagnetic grain coupling model, we attribute these transport properties to the enhancement of the ferromagnetic coupling between the neighboring grains, which could be explained by the increase of the carrier concentration at the grain boundary due to the introduction of BaTiO₃ and the associated magnetoelectric coupling effect.     

BaTiO<Subscript>3</Subscript> and SrBiO<Subscript>2.5</Subscript> two phase system NTC thermistors

BaTiO₃-based materials doped with SrBiO_2.5 only show negative temperature coefficient effect over a wide temperature range. X-ray diffraction analysis displays that major phases present in the sintered bodies were the Ba_0.5Sr_0.5TiO₃ compounds with a cubic structure and the BaBiO₃ compounds with a monoclinic structure. The mean grain size of samples remains unchanged with an increase from 20 to 50 in SrBiO_2.5 content. The values of ρ ₂₅ and B _25/125 constants of thermistors decreased with an increase in SrBiO_2.5 content. The resistivity measured at other temperatures showed basically the same behavior as that at room temperature, irrespective of the measuring temperature.     

PTCR effect in BaBi<Subscript>2</Subscript>Nb<Subscript>2</Subscript>O<Subscript>9</Subscript>-doped BaTiO<Subscript>3</Subscript> ceramics

The influences of BaBi₂Nb₂O₉ content on the electrical property and the microstructure of BaTiO₃-based materials have been studied. With an increase in BaBi₂Nb₂O₉ content the grain size decreases. All the prepared BaBi₂Nb₂O₉ doping BaTiO₃-based thermistors show typical PTC effect. As the amount of BaBi₂Nb₂O₉ added in BaTiO₃-based ceramics increases, resistivity appears to exhibit a minimum value. At high BaBi₂Nb₂O₉ content (≥0.0875), the resistivity increased again with increasing BaBi₂Nb₂O₉ content. At a given content of BaBi₂Nb₂O₉, the influence of sintering temperature on the electrical properties of samples has been investigated. A minimum of room temperature resistivity is obtained at the sintering temperature equal to 1,290 °C at a given content of BaBi₂Nb₂O₉.     

The effect of sintering conditions on the grain growth of the BaTiO3-based GBBL capacitors

BaTiO₃-based grain-boundary barrier layer capacitors are prepared by a conventional ceramic fabrication process with SiO₂, Dy₂O₃ and Nb₂O₅ dopants. A two-stage sintering process is employed to investigate the influence of the sintering conditions on the grain size of the BaTiO₃ ceramics. The reducing sintering atmosphere or reducing catalytic dopant decreases the apparent eutectic temperature of the BaTiO₃ ceramics. The decrease in the eutectic temperature associated with abnormal grain growth is related to the oxygen partial pressure during sintering. By properly employing the sintering temperature profile and the sintering atmosphere, a desirable microstructure with controlled grain growth could be achieved.     

Influence of La2O3 loading on SnO2 based sensors

Influence of La₂O₃ loading on the structural and gas sensing properties of SnO₂ have been studied. The effect of different weight percentages of La₂O₃ (1–10 wt.%) in SnO₂, and the effect of calcination temperature on the sensitivity to various reducing gases like LPG, H₂ and CH₄ has been studied. The structural characteristics have been investigated by X-ray diffraction, and the corresponding crystallite size estimated. In addition to the systematic variation in the crystallite size as a function of the sintering temperature, the role of La₂O₃ as an effective grain growth inhibitor has been confirmed. Increasing the percentage of La₂O₃ above 2 wt.% has no added advantage in terms of improving the gas sensing characteristics and also in stabilizing the SnO₂ surface. Once the La₂Sn₂O₇ formation temperature is reached, the sensitivity of the sensor decreases marginally. At this temperature the crystallite size also increases very abruptly. All these have been correlated with the formation of the La₂Sn₂O₇, a new phase with standard pyrochlore structure.     

The effect of CuO doping on the microstructures and dielectric properties of BaTiO3 ceramics

(1 − x) BaTiO₃/xCuO ceramic pellets with x = 0, 0.2, 0.4, 0.6, and 0.8% respectively were prepared by the traditional solid-state reaction method. The effect of CuO doping on the microstructure and dielectric properties of BaTiO₃ ceramics has been investigated. SEM and XRD results at room temperature show that the grain size grows with the increase of CuO content under the same sintering conditions and the crystal structure undergoes the mixed phases (pseudocubic/tetragonal) to tetragonal phase transition with the growth of grain size. Regular shape grains with average grain size ~2 μm are detectable in the specimens as CuO dopant content adds up to 0.8% and the crystal structure has completely changed into tetragonal phase. The permittivity increases markedly for CuO dopant content x = 0.2 ~ 0.4% and the dielectric loss decreases significantly after being doped by CuO and down to a minimum value for x = 0.8%. In addition, the permittivity and dielectric loss display a good stability in a broad frequency range comparing that of pure BaTiO₃ ceramics.     

Effect of La2O3 doping on the microstructure and electrical properties of a SnO2-based varistor

The effect of La₂O₃ addition on the densification and electrical properties of the (0.9895-xSnO₂+0.01CoO+0.0005Nb₂O₅+x La₂O₅ system, where x=0.0005 or 0.00075, was considered in this study. The samples were sintered at 1300 °C for 2 and 4 h and a single SnO₂ phase was identified by X-ray diffraction. Microstructure analysis by scanning electron microscopy showed that the affect of La₂O₃ addition is to decrease the SnO₂ grain size. J versus E curves indicated that the system exhibits a varistor behavior and the effect of La₂O₃ is to increase both the non-linear coefficient () and the breakdown voltage (E₂). Considering the Schottky thermionic emission model the potential height and the width were estimated. The addition of small amounts of La₂O₃ to the basic system increases the potential barrier height and decreases both grain size and potential barrier width. © 2001 Kluwer Academic Publishers     

Microstructural and dielectric properties of donor doped (La3+) CaCu3Ti4O12 ceramics

The effect of La³⁺ doping on Ca²⁺ sites in CaCu₃Ti₄O₁₂ (CCTO) was examined. Polycrystalline samples in the chemical formula Ca_(1-x)La_(2/3)x Cu₃Ti₄O₁₂ with x = 0, 0.5, 1 were synthesized via the conventional solid state reaction route. X-ray powder diffraction analysis confirmed the formation of the monophasic compounds and indicated the structure to be remaining cubic with a small increase in lattice parameter with increase in La³⁺ doping. The dielectric and impedance characteristics of Ca_(1-x)La_(2/3)x Cu₃Ti₄O₁₂ were studied in the 100 Hz–10 MHz frequency range at various temperatures (100–475 K). A remarkable decrease in grain size from 50 μm to 3–5 μm was observed on La³⁺ substitution. The dielectric constant of CaCu₃Ti₄O₁₂ decreased drastically on La³⁺ doping. The frequency and temperature responses of dielectric constant of La³⁺ doped samples were found to be similar to that of CaCu₃Ti₄O₁₂. The effects of La³⁺ doping on the electrical properties of CaCu₃Ti₄O₁₂ were probed using impedance spectroscopy. The conducting properties of grain decreased while that of the grain boundary increased on La³⁺ doping, resulting in a decrease of the internal barrier layer effect. A decrease in grain boundary capacitance and stable grain response in La³⁺ doped CCTO ceramics were unambiguously established by modulus spectra studies.     

Effect of Mn doping on the dielectric properties of BaTi<Subscript>0.9</Subscript>Sn<Subscript>0.1</Subscript>O<Subscript>3</Subscript> ceramics

Pure and Mn-doped BaTi_0.9Sn_0.1O₃ ceramics are prepared via the conventional solid state reaction method. The microstructures, dielectric properties, and diffuse transition of BaTi_0.9Sn_0.1O₃ ceramics with 0, 0.2, 1 and 2 at.% Mn have been investigated. The results indicate that manganese ions enter the unit cell maintaining the perovskite structure of solid solution. Grain size of BaTi_0.9Sn_0.1O₃ ceramics sharply decreases after doping MnO₂. However, once MnO₂ content is more than 0.2 at.%, the average grain size of Mn-doped BaTi_0.9Sn_0.1O₃ ceramics increases with the increasing of MnO₂ content. The addition of manganese leads to the increase of the Curie temperature. The diffuseness of the phase transition of Mn-doped BTS ceramics decreases with the increase of Mn content, which may be due to grain size effect.     

Investigation of reactions between BaTiO3 and La(Ni,Co)O3 by diffusion couples

The diffusion of cations and the formation of compounds at the interface of diffusion couples of BaTiO₃ and LaNi_0.6Co_0.4O₃ have been investigated by scanning electron microscopy and energy dispersive X-ray spectroscopy. The firing conditions have been varied with respect to firing temperature, firing time and heating rate. At the interface between BaTiO₃ and LaNi_0.6Co_0.4O₃ a layer of a perovskite type compound with varying composition is formed. Ti and La ions diffuse preferentially towards the reaction layer from the BaTiO₃ and the LaNi_0.6Co_0.4O₃, respectively. A consequence of the preferred diffusion of Ti towards the reaction layer is a depletion of Ti and the enrichment of Ba within the BaTiO₃ adjacent to the interface causing the formation of overstoichiometric BaTiO₃ (with respect to Ba), which is known to exhibit pronounced grain growth inhibition. The zone of reduced grain size is found to reach approximately 200 m into the BaTiO₃ bulk.     

Preparation and characterization of Mn-doped BaTiO<Subscript>3</Subscript> thin films by magnetron sputtering

Barium titanate (BaTiO₃) thin films doped with Mn (0.1–1.0 at%) were prepared by r.f. magnetron sputtering technique. Oxygen/argon (O₂/Ar) gas ratio is found to influence the sputtering rate of the films. The effects of Mn doping on the structural, microstructural and electrical properties of BaTiO₃ thin films are studied. Mn-doped thin films annealed at high temperatures (700 °C) exhibited cubic perovskite structure. Mn doping is found to reduce the crystallization temperature and inhibit the grain growth in barium titanate thin films. The dielectric constant increases with Mn content and the dielectric loss (tan δ) reveals a minimum value of 0.0054 for 0.5% Mn-doped BaTiO₃ films measured at 1 MHz. The leakage current density decreases with Mn doping and is 10⁻¹¹ A/cm⁻² at 6 kV/cm for 1% Mn-doped thin films.     

Microstructure and dielectric properties of BaTiO<Subscript>3</Subscript>-CeO<Subscript>2</Subscript>-SnO<Subscript>2</Subscript> ceramics

We have studied the effect of codoping with CeO₂ and SnO₂ (2 to 3.5 wt %) on the microstructure and dielectric properties of BaTiO₃. Doping with CeO₂ and SnO₂ inhibits grain growth in BaTiO₃ and enables the fabrication of ceramic materials with a grain size below 1 μm. The temperature coefficient of permittivity of the ceramics increases with CeO₂ + SnO₂ content, firing temperature, and firing time.     

Solid state sintering of lime in presence of La<Subscript>2</Subscript>O<Subscript>3</Subscript> and CeO<Subscript>2</Subscript>

The sintering of lime by double calcination process from natural limestone has been conducted with La₂O₃ and CeO₂ additive up to 4 wt.% in the temperature range 1500–1650° C. The results show that the additives enhanced the densification and hydration resistance of sintered lime. Densification is achieved up to 98.5% of the theoretical value with La₂O₃ and CeO₂ addition in lime. Grain growth is substantial when additives are incorporated in lime. The grain size of sintered CaO (1600°C) with 4 wt.% La₂O₃ addition is 82 μm and that for CeO₂ addition is 50 μm. The grains of sintered CaO in presence of additive are angular with pores distributed throughout the matrix. EDX analysis shows that the solid solubility of La₂O₃ and CeO₂ in CaO grain is 2.9 and 1.7 weight %, respectively. The cell dimension of CaO lattice is 4.803 %C. This value decreases with incorporation of La₂O₃ and CeO₂. The better hydration resistance of La₂O₃ added sintered lime compared to that of CeO₂ added one, is related to the bigger grain size of the lime in former case.     

Low-field magnetoresistance in La0.7Sr0.3MnO3/BaTiO3 composites

A manganite composite series of (1 ? x)La_0.7Sr_0.3MnO₃/xBaTiO₃ (x = 0, 0.06, 0.12, and 0.18) has been fabricated by solid-state reaction combined with a high-energy mechanical milling method. Experimental results revealed that the insulator–metal transition temperature was shifted towards lower temperatures, and resistivity increases with increasing BaTiO₃ content in (1 ? x)La_0.7Sr_0.3MnO₃/xBaTiO₃. Meanwhile, the ferromagnetic–paramagnetic transition temperature was almost unchanged. The increase in magnetoresistance was observed in the all composites at whole measurement temperatures under an applied magnetic field of 3 kOe. Here, temperature dependences of magnetoresistance display a Curie–Weiss law-like behavior. The nature of this phenomenon is explained in detail.     

Effect of Zr on dielectric, ferroelectric and impedance properties of BaTiO3 ceramic

A polycrystalline sample of Zr-doped barium titanate (BaTiO₃) was prepared by conventional solid state reaction method. The effect of Zr (0·15) on the structural and microstructural properties of BaTiO₃ was investigated by XRD and SEM. The electrical properties (dielectric, ferroelectric and impedance spectroscopy) were measured in wide range of frequency and temperature. With substitutions of Zr, the structure of BaTiO₃ changes from tetragonal to rhombohedral. Lattice parameters were found to increase with substitution. The room temperature dielectric constant increases from ～ 1675 to ～ 10586 and peak dielectric constant value increases from ～ 13626 to ～ 21023 with diffuse phase transition. Impedance spectroscopy reveals the formation of grain and grain boundary in the material and found to decrease with increase in temperature.     

Fabrication of (BiNa)0.5TiO3–BaTiO3 textured ceramics by tape casting

The textured (BiNa)_0.5TiO₃–BaTiO₃ ceramic with a preferred 〈h00〉 orientation was fabricated by tape casting with the plate-like Bi_2.5Na_3.5Nb₅O₁₈ (BINN5) particles as template. Effects of processing parameters and the content of template on the microstructure and the texture fraction of NBT-based ceramics were investigated. The results show that the texture fraction increases with increasing sintering temperature and the increasing content of BINN5. The increased sintering temperature and BINN5 content result in the grain size and the grain shape changing from irregular to plate like. These factors were responsible for the increased degree of orientation. The texture fraction had a maximum value when the sintering temperature is 1225 °C and the optimal content of template is 20 wt.%.     

Electronic properties of BaTiO3/4H-SiC interface

The possibility of barium titanate (BaTiO₃) application in silicon carbide (SiC) technology has been elaborated in terms of the dielectric film quality and properties of the BaTiO₃/4H-SiC interface. High resistivity, high-k thin films containing La₂O₃ admixture were applied as gate insulator of metal-insulator-semiconductor (MIS) structure. The thin films were deposited by means of radio frequency plasma sputtering (RF PS) of sintered BaTiO₃ + La₂O₃ (2 wt.%) target on 8° off-axis 4H-SiC (0001) epitaxial layers doped with nitrogen. The results of current-voltage and capacitance-voltage measurements are presented for MIS capacitors.     

Microstructures and dielectric properties of Y/Zn codoped BaTiO<Subscript>3</Subscript> ceramics

The microstructures and dielectric properties of Y/Zn codoped BaTiO₃ ceramics sintered in a reducing atmosphere were investigated. XRD analysis indicated the crystal structure of samples change from tetragonal to pseudocubic with increasing Y₂O₃ and ZnO content. SEM micrographs showed Y₂O₃ can suppress grain growth more effectively compared with ZnO, which is ascribed to the presence of second phase Y₂Ti₂O₇. Proper amount of Y₂O₃ and ZnO can significantly improve the dielectric temperature characteristics due to the formation of grain core-shell structure. The high performance dielectrics meeting the X7R code were achieved by codoping 1.5 mol% Y₂O₃ and 3.0 mol% ZnO.     

Room temperature mechanical properties and high temperature oxidation behavior of MoSi2 matrix composite reinforced by adding La2O3 and Mo5Si3

MoSi₂ matrix composites containing 0.8 wt.%La₂O₃ and different volume fractions of Mo₅Si₃ were synthesized by self-propagating high temperature synthesis. The room temperature mechanical properties and high temperature oxidation behavior at 1200 °C were studied. Results showed that La₂O₃ and Mo₅Si₃ caused the grain size to decrease of the MoSi₂ matrix composite. The flexure strength and fracture toughness are improved compared with pure MoSi₂. The strengthening mechanism of La₂O₃–Mo₅Si₃/MoSi₂ is fine-grain strengthening, and its toughening mechanisms are fine-grain toughening, crack deflection, crack branching and crack bridging. With an increase in Mo₅Si₃ content, the oxidation resistance gradually decreased. This is attributed to the poor oxidation resistance of Mo₅Si₃, grain refinement and relative density decrease of the composites. In this experiment, a La₂O₃–Mo₅Si₃/MoSi₂ composite was found to have optimal mechanical properties and high temperature oxidation resistance after adding 0.8 wt.%La₂O₃ and 16.3 wt.% Mo₅Si₃.