Dielectric and leakage current properties of Li doped (Ba,Sr)TiO3 thick film interdigital capacitors on the alumina substrates

Li doped (Ba,Sr)TiO₃ thick films were fabricated by employing the screen printing method on the alumina (Al₂O₃) substrates. Interdigital capacitor patterns with seven fingers of 200 μm gap, 250 μm length were designed and screen printed on the alumina substrates. Ba_0.5Sr_0.5TiO₃ materials, paraelectric state at the room temperature, have been chosen for the microwave devices due to high dielectric permittivity and low loss tangent, however, the sintering temperature of (Ba,Sr)TiO₃ is over 1350 °C. In order to lower the sintering temperature, Li (3 wt%) was added to the (Ba,Sr)TiO₃ materials. Li doped (Ba,Sr)TiO₃ thick films screen printed on the alumina (Al₂O₃) substrates were sintered at 900 °C for 1.5 h. The structural feature was analyzed with X-ray diffraction method. Temperature dependent dielectric properties were characterized from 303 to 403 K at 1 MHz. Within the ±100 V of bias voltage, current-voltage characteristics of Li doped (Ba,Sr)TiO₃ films were investigated from 303 to 403 K. Through the current-voltage characteristics, the resistivity of Li doped (Ba,Sr)TiO₃ films were calculated.In this paper, the significant negative temperature coefficient of resistance (NTCR) of Li doped (Ba,Sr)TiO₃ films will be presented through the activation energy fitting. Measured activation energy is approximately 0.366 eV.     

Sintering behavior and PTCR properties of stoichiometric blend BaTiO3

Donor doped positive temperature coefficient of resistivity barium titanate is highly sensitive to minor changes in processing conditions, Ba/Ti ratio, and dopant concentration. This leads to a lack of reproducibility of properties and microstructure. This study was performed in an effort to obtain a more microstructurally stable PTCR material. Barium titanate ceramics were prepared by blending Ba-excess BaTiO₃ powder with Ti-excess powder, in different ratios. Such donor modified blended systems display uniform, medium grain size (4-6 μm), high-density microstructures which are more stable to changes in processing parameters. The microstructures are characterized by flat grain edges, large grain-to-grain contact area and high degree of domain coherence across grain boundaries. The PTCR effect was, however, measured to be nominal in these samples. This has been attributed to the presence of a smaller barrier potential, and such microstructural features as strong domain coherence across grain boundaries, large grain-to-grain contact area, and high density. It was found that the simple act of blending donor doped BaTiO₃ powders of different Ba/Ti ratios drastically changes both microstructure and electrical properties. Blending results in the suppression of liquid-phase induced anomalous grain growth, suppressing grain growth processes and allowing sintering processes to go to a greater degree of completion. The proposed mechanism whereby this happens is that the presence of the donor in blended systems either changes the kinetics of liquid-phase formation and/or the wettability of grains, affecting liquid-phase distribution.     

Effect of a positive temperature coefficient of resistance in thin films of doped strontium-barium titanate

Thin films of doped semiconducting barium-strontium titanate (Ba,Sr)TiO₃ are prepared by pulsed laser ablation. It is shown that the crystal structure, morphology, and electrical properties of (Ba,Sr)TiO₃ thin films are determined primarily by the actual ablation conditions. The ablation regimes of deposition permitting preparation of uniform polycrystalline thin films with a composition close to that of the target and with grain sizes larger than 0.1 μm are established. These samples have a positive temperature coefficient of resistance in the phase transition region. The change in the resistivity can be as much as 100%.     

Microstructure and dielectric properties of Li2CO3 doped 0.7(Ba,Sr)TiO3–0.3MgO ceramics

Microstructure and dielectric properties of Li₂CO₃ doped 0.7(Ba,Sr)TiO₃–0.3MgO ceramics for the low temperature sintering and microwave applications will be presented. In these days, low temperature sintering process has been widely spread out for the integrated electronic modules for the communication systems such as front-end modules, antenna modules, and switching modules. We have added Li₂CO₃ and MgO to (Ba,Sr)TiO₃ material to reduce the sintering temperature and improve dielectric properties such as loss tangent, and frequency dispersion.In this paper, we have discussed the crystalline properties, dielectric properties, and the microstructures of Li₂CO₃ doped 0.7(Ba,Sr)TiO₃–0.3MgO ceramics. No pyro phase was observed in the X-ray diffraction method. Very weak frequency dispersion (<0.7%) of dielectric permittivity was observed from the 1 kHz to 1 MHz range. We found that the grain size of BST is around 2 μm, while the grain size of Li₂CO₃ dope 0.7BST–0.3MgO is around 4 μm from the SEM analysis.     

The structure and magnetotransport properties of La2/3Sr1/3MnO3/Ba(La)TiO3 composites

The effect of Ba(La)TiO₃ doping on the structure and magnetotransport properties of La_2/3Sr_1/3MnO₃(LSMO)/xBa(La)TiO₃ (x=0.0, 1.0, 5.0 mol%) have been investigated. The X-ray diffraction patterns and microstructural analysis show that BaTiO₃ and LSMO phases exist independently in BaTiO₃-doped composites. The metal-insulator transition temperature (T_MI) decreases whereas the maximum resistivity increases very quickly by the increase of BaTiO₃ doping level. The partial substitution of Ba by La(0.35 mol%) results in a decrease in resistivity of LSMO/xBa(La)TiO₃ composites. Magnetoresistance of BaTiO₃-doped composites decreases monotonously in the temperature range 200-400 K in a magnetic field of 5 T, which is completely different from that of LSMO compound. The value of MR decreases at low field (H<1 T) and increases at high fields (H>1 T) with increasing the BaTiO₃ doping level at low temperatures below 280 K. These investigations reveal that the magnetotransport properties of LSMO/xBa(La)TiO₃ composites are dominated by spin-dependent scattering and tunneling effect at the LSMO/BaTiO₃/LSMO magnetic tunnel junction.     

Nature of potential barrier in (Ca1/4,Cu3/4)TiO3 polycrystalline perovskite

The nonohmic electrical features of (Ca_1/4,Cu_3/4)TiO₃ perovskite ceramics, which have very strong gigantic dielectric is believed originate from potential barriers at the grain boundaries. In the present study, we used the admittance and impedance spectroscopy technique to investigate (Ca_1/4,Cu_3/4)TiO₃ perovskite ceramics with low nonohmic electrical properties. The study was conducted under two different conditions: on as-sintered ceramics and on ceramics thermally treated in an oxygen-rich atmosphere. The results confirm that thermal treatment in oxygen-rich atmospheres influence the nonohmic properties. Annealing at oxygen-rich atmospheres improve the nonohmic behavior and annealing at oxygen-poor atmospheres decrease the nonohmic properties, a behavior already reported for common metal oxide nonohmic devices and here firstly evidenced for the (Ca_1/4,Cu_3/4)TiO₃ perovskite related materials. The results show that oxygen also influences the capacitance values at low frequencies, a behavior that is indicative of the Schottky-type nature of the potential barrier.     

高温无铅BaTiO_3-(Bi_(1/2)Na_(1/2))TiO_3正温度系数电阻陶瓷阻抗和介电谱分析

采用固相反应法制备了Y2O3施主掺杂的92 mol%BaTiO3-8 mol%(Bi1/2Na1/2)TiO3(BBNT8)高温无铅正温度系数电阻(positive temperature coe?cient resistivity,PTCR)陶瓷.利用透射电镜观察材料的显微结构,发现陶瓷的显微结构主要包括晶粒和晶界两部分,观察不到明显的壳层结构.进一步利用交流阻抗谱研究了陶瓷的宏观电学性能,发现陶瓷的总电阻是晶粒和晶界两部分的贡献,而晶粒电阻很小,在居里温度以上变化不大,材料的PTCR效应主要是晶界部分的贡献.当温度高于居里温度时,随着温度的升高,晶界介电常数逐渐减小,导致势垒增加,晶界电阻增大,从而产生正温度系数效应.最后,通过测试材料的介电频谱特性,研究计算了陶瓷的室温电阻率.     

Structural characterization of dense reduced BaTiO3 and Ba0.95La0.05TiO3 nanoceramics showing colossal dielectric values

BaTiO_3−x and Ba_0.95La_0.05TiO_3−x nanoceramics showing colossal permittivity values have been characterized. While starting powders are of cubic symmetry, X-ray and Neutron Diffraction techniques and Raman Spectroscopy measurements show that the one-step processed ceramics obtained by Spark Plasma Sintering (SPS) contain cubic and tetragonal phases. Rather large oxygen deficiency determined in such ceramics by Electron Micro Probe analysis and Electron Energy Loss Spectroscopy analyzes is explained by the presence of Ti³⁺, as evidenced by X-ray Photoelectron Spectroscopy measurements. Transmission Electron Microscopy and High Resolution Transmission Electron Microscopy show that these ceramics contain 50-300 nm grains, which have single-domains, while grain boundaries are of nanometer scale. Colossal permittivity values measured in our dense nanoceramics are explained by a charge hopping mechanism and an interfacial polarization of a large number of polarons generated after sample reduction in SPS apparatus.     

Physical properties of double perovskite compounds ALaVMoO6 (A=Ca, Sr, Ba)—a possible half-metallic antiferromagnetic system

Double perovskite compounds ALaVMoO₆ (A=Ca, Sr, Ba) have been synthesized and their electrical and magnetic properties have been investigated. Magnetization measurements have indicated the possible antiferromagnetic transitions at 120 and 130 K for A=Ca and Sr samples, respectively. Electrical resistivity ρ for this system shows metallic temperature dependence from 300 to 20 K, though the sample with A=Ca shows weak semiconducting behavior in the low temperature region (<70 K). Considering the magnetic and electrical properties and assuming the V³⁺S=1 and Mo⁴⁺S=1 valence and spin states, the samples with A=Ca and Sr can be promising candidates for half-metallic antiferromagnets.     

The effect of compound addition Dy2O3 and Sn on the structure and properties of NdFeNbB magnets

NdFeNbB with the additions of Dy₂O₃ and Sn permanent magnets have been attained by means of powder-blending technique, and their magnetic properties, temperature performance and microstructure were studied in this paper. The addition of just 2.0 wt% Dy₂O₃ or 0.3 wt% Sn proved to be very effective in improving the permanent magnetic properties of NdFeNbB magnets. Dy₂O₃ additions result in the increase in the H_ci and temperature dependence due to the increase of T_c, formation of (NdDy)-rich phase and grain refinement of Φ phase. This improvement of the coercivity stability of the magnets from the addition of Sn is attributed to the smoothing effect of the Sn addition at the grain boundaries. The magnetic properties, the temperature dependence and Curie temperature of NdFeNbB with Dy₂O₃ and Sn combined addition were found to be considerably improved. From the X-ray diffraction, SEM-EDAX studies and the thermo-magnetic study, the improved properties due to the solution of Dy and Sn to the Φ phase, the reduced N_eff and the smaller Φ phase.     

Synthesis and characterization of zinc titanate doped with magnesium

Zinc titanate crystals doped with magnesium have been grown by conventional solid state reaction technique using metal oxides. It is shown that they are semiconductors. The characteristics of zinc titanate samples were found to depend on the heating conditions and the amounts of additions. Our studies revealed that magnesium can replace the zinc ion and forms a solid solution in the ZnTiO₃ phase. The electrical resistivity of (Zn,Mg)TiO₃ varied with sintering temperature, and has a minimum when sintered at 900 °C. Increasing amounts of magnesium will also decrease the resistivity. A V-shaped temperature dependence of resistivity was observed. Furthermore, the dielectric constant increased with sintering temperature and decreased with increasing amounts of magnesium. It also shows a maximum Q factor at a frequency of 8 GHz for the sample of (Zn_0.9, Mg_0.1)TiO₃ sintered at 900 °C.     

Enhanced spontaneous polarization in Sr and Ca co-doped BaTiO3 ceramics

The dielectric and ferroelectric properties of (Ba_xSr_1−x)_0.77Ca_0.23TiO₃ ceramics with x=1 to 0.7 were studied and compared with those of Ba_xSr_1−xTiO₃ and Ba_0.77Ca_0.23TiO₃ ceramics. It has been found that Sr doping of the Ba_0.77Ca_0.23TiO₃ ceramics causes a drastic decrease of the Curie temperature, just like Sr doping of pure BaTiO₃ ceramics, demonstrating a cell volume effect. However, the (Ba_xSr_1−x)_0.77Ca_0.23TiO₃ ceramics with x=0.9 and 0.8 have larger spontaneous polarization than those of the corresponding Ba_xSr_1−xTiO₃ and Ba_0.77Ca_0.23TiO₃ ceramics, along with sufficient insulating properties. The enhancement of their polarization was explained by the increase of the lattice parameter c/a ratio due to the lattice distortion and strain developed in the ceramics.     

Preparation of transparent conductive TiO2:Nb thin films by pulsed laser deposition

This study investigated the optical and electrical properties of Nb-doped TiO₂ thin films prepared by pulsed laser deposition (PLD). The PLD conditions were optimized to fabricate Nb-doped TiO₂ thin films with an improved electrical conductivity and crystalline structure. XRD analyses revealed that the deposition at room temperature in 0.92 Pa O₂ was suitable to produce anatase-type TiO₂. A Nb-doped TiO₂ thin film attained a resistivity as low as 6.7 × 10⁻⁴ Ω cm after annealing at 350 °C in vacuum (<10⁻⁵ Pa), thereby maintaining the transmittance as high as 60% in the UV-vis region.     

Microstructure and piezoelectric properties of Bi0.5(Na0.82K0.18)0.5TiO3−NaSbO3 ceramics

Lead-free piezoelectric ceramics (1−x)Bi_0.5(Na_0.82K_0.18)_0.5TiO₃−xNaSbO₃ have been prepared by a conventional ceramics technique, and their microstructure and electrical properties have been investigated. The addition of NaSbO₃ has no remarkable effect on the crystal structure within the studied doping content; however, an obvious change in microstructure took place. With increase in NaSbO₃ content, the temperature from a ferroelectric to antiferroelectric phase transition increases, and the temperature for a transition from antiferroelectric phases to paraelectric phases changes insignificantly. Simultaneously, the temperature range between the rhombohedral phase transition point and the Curie temperature point becomes smaller. The piezoelectric properties significantly increase with increase in NaSbO₃ content and the piezoelectric constant and electromechanical coupling factor attain maximum values of d₃₃=160 pC/N and k_p=0.333 at x=0.01. The results indicate that (1−x)Bi_0.5(Na_0.82K_0.18)_0.5TiO₃−xNaSbO₃ ceramic is a promising lead-free piezoelectric candidate material.     

NTCR behavior of Sm-substituted barium zirconium titanate nanocrystalline solid solution

Ceramic solid solution of nanocrystalline barium zirconium titanate in the form of Ba(Zr_0.52Ti_0.48)O₃ substituted by samarium (Sm³⁺) was prepared using the conventional solid state reaction method. The phase assemblage analyzed by the X-ray diffraction technique was fitted for cubic-crystal-symmetry. The change in the grain size depicted the influence of Sm³⁺ ions on the microstructure. The electrical behavior was studied in the temperature range from 323 to 773 K. The sintered samples exhibited a negative temperature coefficient of resistance (NTCR) and superior semiconducting behavior above 513 K. Addition of Sm³⁺ increased the room temperature resistivity of Ba(Zr_0.52Ti_0.48)O₃ solid solution. The results obtained from the thermoelectric power measurement confirm electrons as the majority charge carriers.     

Guidelines for improving resistance to CO2 of materials for solid state electrochemical systems

Alkali earth oxides are present in many ionic and mixed conductors with a potential application in solid state electrochemical or electroceramic devices. Hence, study has been made of the relative risks of carbonation of selective materials based on thermodynamic predictions, with emphasis on ATiO₃ electroceramics, with A = Ca, Sr, Ba, and proton conductors based on AMO₃ perovskites with A = Sr or Ba and M = Ce or Zr. Predictions show that slight A-site deficiency lowers the activity of alkali earth oxide and enhances resistance to CO₂; this is confirmed by experimental findings. Thermodynamic predictions also indicate that the high stability of (Ba,Sr)TiO₃ and (Ba,Sr)ZrO₃ perovskites contributes to attain reasonable resistance to CO₂, in spite of the ready carbonation of BaO and SrO. However, these materials show wide gap between the upper and lower limits of resistance to CO₂. This gap provides interpretation for the effects of powder preparation and/or ceramic processing on resistance to CO₂. It is also shown that the lower and upper limits of resistance to CO₂ correlate with the structural tolerance factor of these perovskites.     

The influence of titanium on the properties of zinc oxide films deposited by radio frequency magnetron sputtering

TiO₂-doped zinc oxide thin films were deposited on glass substrates by radio frequency (RF) magnetron sputtering with TiO₂-doped ZnO targets in an argon atmosphere. The structural properties of TiO₂-doped ZnO films doped with different TiO₂ contents were investigated. The experimental results show that polycrystalline TiO₂-doped ZnO films had the (0 0 2) preferred orientation. The deposition parameters such as the working pressure and substrate temperature of TiO₂-doped ZnO films were also investigated. The crystalline structure of the TiO₂-doped ZnO films gradually improved as the working pressure was lowered and the substrate temperature was raised. The lowest electrical resistivity for the TiO₂-doped ZnO films was obtained when the Ti addition was 1.34 wt%; its value was 2.50 × 10⁻³ Ω cm, smaller than that found in previous studies. The transmittance of the TiO₂-doped ZnO films in the visible wavelength range was more than 80%. The optical energy gap was related to the carrier concentration, and was in the range of 3.30-3.48 eV.     

Optical and electric properties of Nb-doped anatase TiO2 single crystal

Nb-doped anatase TiO₂ single crystal has been grown by chemical vapour transport method. Raman spectra shows that the obtained crystal with Nb of 0.08 wt% has typical anatase structure. An absorption band was observed at around 2.2 eV, which seems to be due to the d-d transition in the conduction band. The electron paramagnetic resonance and electric resistivity measurements show that the doped niobium makes quite shallow donor level whose orbital is d_xy-like centered at the titanium position of anatase.     

Influence of porous morphology on optical dispersion properties of template free mesoporous titanium dioxide (TiO2) films

This paper focuses the influence of porous morphology on the microstructure and optical properties of TiO₂ films prepared by different sol concentration and calcination temperatures. Mesoporous TiO₂ thin films were prepared on the glass substrates by sol-gel dip coating technique using titanium (IV) isopropoxide. Porous morphology of the films can be regulated by chemical kinetics and is studied by scanning electron microscopy. The optical dispersion parameters such as refractive index (n), oscillator energy (E_d), and particle co-ordination number (N_c) of the mesoporous TiO₂ films were studied using Swanepoel and Wemple-DiDomenico single oscillator models. The higher precursor concentration (0.06 M), films exhibit high porosity and refractive index, which are modified under calcination treatment. Calcinated films of low metal precursor concentration (0.03 M) possess higher particle co-ordination number (N_c = 5.05) than that of 0.06 M films (N_c = 4.90) due to calcination at 400 °C. The lattice dielectric constant (E_∞) of mesoporous TiO₂ films was determined by using Spintzer model. Urbach energy of the mesoporous films has been estimated for both concentration and the analysis revealed the strong dependence of Urbach energy on porous morphology. The influence of porous morphology on the optical dispersion properties also has been explained briefly in this paper.     

A strong correlation of crystal structure and Curie point of barium titanate ceramics with Ba/Ti ratio of precursor composition

A series of barium titanate powders were synthesized from precursors BaCO₃ and TiO₂ with Ba/Ti ratio ranged from 0.96 to 1.04. For the ceramics sintered at 1300 °C for 2 h, with increasing Ba/Ti ratio from 0.96 to 1.04, the tetragonal distortion of perovskite barium titanate phase was decreased continuously and the Curie point was decreased monotonously from 122.9 to 98.0 °C. At the same time, the content of secondary phases was very low. This correlation of crystal structure and Curie point of barium titanate ceramics was explained by composition variation of the perovskite phase itself rather than by the influence from secondary phases. Due to very controversial results reported by different groups, it was proposed that BaTiO₃ has some different states with different solubilities for BaO and TiO₂. Further investigations should be conducted on the preparation and the properties of BaTiO₃ with much BaO or TiO₂ dissolved.