Impedance spectroscopy and conduction mechanism of magnetoelectric hexaferrite BaFe10.2Sc1.8O19

The dielectric relaxation and conduction properties of hexaferrite BaFe_10.2Sc_1.8O₁₉ (BFSO) ceramics have been investigated by impedance spectroscopy (1 Hz-2 GHz) at various temperatures (253-473 K). The frequency dependence of impedance and modulus spectra of BFSO shows that its dielectric responses are thermally activated. The scaling behaviors of impedance spectra indicate that the distribution of dielectric relaxation times in BFSO is temperature independent. The frequency-dependent conductivity spectra follow the universal-power-law at high temperatures but deviate slightly at low temperatures. An enormous increase in relaxation/conduction activation energies of BFSO above 413 K is also observed in both impedance and conductivity spectra. This indicates that at high temperatures, relaxation/conduction processes may be contributed mainly by the movable oxygen vacancies, whereas at low-temperature electron hopping dominates. The conductivity fitting results further suggest that electron/oxygen vacancy-related small polaron hopping should be the most probable conduction mechanism for BFSO.     

Impedance spectroscopy studies on samarium and sodium substituted strontium bismuth titanate (SBTi)

Sr_1-x Sm_xNa_xBi₄Ti₄O₁₅ lead-free piezoelectric ceramics are prepared by solid-state sintering method using high energy Ball- Mill and ground for 10 h. The single phase of the compound is confirmed by X-ray Diffraction and highest intensity peak (1 1 0) is observed at 30°. Diffraction peaks shifted towards higher angles with an increasing concentration of Sm³⁺ and Na¹⁺ of SBT. Grains orientation and grain size are analyzed by Scanning Electron Microscope. Temperature and frequency dependent AC conductivity studies are carried out using impedance spectroscopy in the temperature range of 30 °C–650 °C using Wayn-Kerr LCR meter. At lower frequencies, conductivity is independent of frequency. The variation of Z″ with frequency shows peaks shifting towards higher frequencies with an increase of temperature. The resistance of grains (R_g), grain boundaries (R_gb) are calculated and it is found that grain resistance is greater than grain boundary resistance (R_g > R_gb). Relaxation times of grain and grain boundaries are calculated.     

Impedance spectroscopy on pH-sensors with lithium lanthanum titanate sensitive material

The ceramic Li_3xLa_2/3−x,_1/3−2xTiO₃ is used, in an all-solid-state configuration, as pH-sensor in aqueous buffer solution. This ceramic displays a pH sensitivity comparable to the one obtained with a commercial glass electrode and does not show any sensitivity to the redox potential of the solution. This is one of the remarkable properties of this ceramic pH-sensor. It is shown in this paper that the reproducibility and the behaviour of the pH-response depend on the morphology of the grain boundaries particularly on the size of the grains. A so-called ‘good’ pH-response is obtained with a ceramic showing big and homogeneous grains. Complex impedance spectroscopy reveals that such ceramic has a high grain boundary resistance. Furthermore, this electrochemical technique allows us to determine that the interface reaction involved in the pH detection has a time constant of the order of the second. Several assumptions such as ion exchange or acid-base reaction are proposed to explain the sensitivity of the ceramic material to the pH of the solution.     

纳米钛酸钡的掺杂对陶瓷性能的影响

采用掺杂纳米级钛酸钡和碳酸锰的方法,观察了掺杂不同量纳米级钛酸钡和碳酸锰后的钛酸钡坯片烧结所得陶瓷表面的显微组织形貌的变化,研究了单一掺杂纳米级钛酸钡、单一掺杂碳酸锰、复合掺杂碳酸锰+纳米级钛酸钡对陶瓷制品晶粒尺寸与介电性能的影响。结果表明：掺杂纳米级钛酸钡对钛酸钡陶瓷制品的介电性能有显著的提升,但是随着掺杂量的进一步增加,其介电性能的变化不大;掺杂碳酸锰对钛酸钡陶瓷晶粒的细化效果优于掺杂纳米级钛酸钡的效果;复合掺杂1%（质量分数,下同）碳酸锰+1%纳米级钛酸钡所得陶瓷的致密性高于单一掺杂1%碳酸锰的效果。     

Characterization of Lanthanum-Doped Barium Titanate Ceramics Using Impedance Spectroscopy

The electrical properties of two single-phase, lanthanum-doped BaTiO₃ compositions, x = 0.03 and x = 0.20, in Ba_{1– x} La _x Ti_{1– x /4}O₃ were investigated by impedance spectroscopy after heat treatment in oxygen, argon, and air at 1350°C. Samples heated in oxygen were electrically insulating, whereas those heated in argon lost oxygen and were semiconducting at room temperature, irrespective of x . Samples heated in air showed intermediate electrical properties and also were electrically inhomogeneous; the two compositions showed different electrical behaviors, and a model for each, based on oxygen nonstoichiometry within the ceramics, is proposed. Oxygen deficiency in samples sintered in air was avoided by heating at 1200°C, instead of 1350°C. Alternatively, oxygen lost from ceramics heated in air at 1350°C was regained by postannealing in oxygen at 1350°C.     

铜离子注入镧掺杂BaTiO3半导体陶瓷的阻抗谱研究

本文应用宽频率范围的阻抗谱测量,分析了铜离子注入(110kev,6×10~(15)和1×10~(17)ions/cm~2)镧掺杂BaTiO_3半导瓷在不同温度时的晶粒电阻,晶界电阻和电极与材料界面电阻。提出了相应的等效电路。同时,讨论了离子注入剂量的影响,表明铜离子注入剂量较低时,可以提高材料的PTCR效应。此外,也应用阻温特性测量分析了铜离子注入样品的PTCR效应。     

Impedance and modulus spectroscopy characterization of lead free barium titanate ferroelectric ceramics

In this work, dense BaTiO₃ ceramics were successfully prepared by the solid state reaction process. X-ray diffraction technique showed single-phase polycrystalline sample with perovskite structure. Dielectric behavior and the impedance relaxation were investigated in a wide range of temperature (room temperature (RT) – 40 °C) and frequency (1 kHz–1 MHz). A broad dielectric constant peak was observed over a wide temperature range around the phase transition temperature. The complex impedance plot exhibited two impedance semicircles identified over the frequency range of 1 kHz–1 MHz, which is explained by the grain and grain boundary effects. The presence of non-Debye type of relaxation has been confirmed by the complex impedance analysis. The centers of the impedance semicircles lie below the real axis, which indicates that the impedance response is a Cole–Cole type relaxation.     

Preparation of 0.95Bi1/2Na1/2TiO3·0.05BaTiO3 Ceramics by an Aqueous Citrate-Gel Route

This report details development of a route to solution-derived (1− x )Bi_1/2Na_1/2TiO₃· x BaTiO₃ powders. The method developed was the citrate-gel method—an evaporative, aqueous technique. When applied to 0.95Bi_1/2Na_1/2TiO₃·0.05BaTiO₃ (BNBT-5), the method produced perovskite phase powders that readily densified in the temperature range of 1000°C. The grain size of the sintered materials was on the order of 1 μm, and the weak-field dielectric properties at 1 MHz were similar to those reported for conventionally prepared materials sintered at higher temperatures (e.g., 1200°C).     

Structural,optical, and Raman studies of Gd doped sodium bismuth titanate

The effects of gadolinium (Gd) on lead free sodium bismuth titanate (Na_0.5Bi_0.5TiO₃, NBT) ceramics are investigated. X-ray diffraction (XRD) studies indicate that perovskite phase (rhombohedral, R3c) is formed for all Gd doped NBT (Gd_x:NBT) compositions (x?=?0, 0.02, 0.04, 0.06, 0.08). XRD peak shifts to the higher angles for all compositions except for x?=?0.08. Amphoteric nature of Gd in NBT sites are observed i.e., it occupies Bi-site up to x?=?0.06 and then distributes to Ti-site. Octahedral distortion (c/a) increases in the range 0.02?≤?x?≤?0.06 and then decreases. Raman spectra suggest that the introduction of Gd⁺³ ion induces structural changes without disturbing the long range order. The material can be readily excited using UV (360?nm) and shows emission peaks at ~592?nm and 687?nm. Optical property evaluation indicates that the lowest band gap (E_g = 2.78?eV) is observed at x?=?0.08. When x?>?0.04, the photoluminescence (PL) intensity decreases indicating the onset of concentration quenching. The critical energy distance (found to be 14?Å) and Dexter's theory based analysis indicate that concentration quenching is attributable to multipole-multipole (specifically dipole-dipole) interactions in the system. Commission International de Eclairage (CIE) chromatic color coordinates are reported for all doped systems; the observed patterns mirror PL analysis results. For instance, PL intensity shrinks beyond 4?at%; this corresponds to a regression in the CIE trajectory with respect to concentration.     

The temperature-dependent piezoelectric and electromechanical properties of cobalt-modified sodium bismuth titanate

Lightly cobalt-modified, Aurivillius-type, sodium bismuth titanate (Na_0.5Bi_4.5Ti₄O₁₅, NBT) ceramics were synthesized by substituting a small amount of cobalt ions onto the Ti⁴⁺ sites using conventional solid-state reaction. X-ray photoelectron spectroscopy (XPS) analysis coupled with bond valence sum calculations show that the dopant cobalt ions substitute for Ti⁴⁺ ions in the form of Co³⁺. The resultant cobalt-modified NBT ceramics (NBT-Co) exhibit better piezoelectric and electromechanical properties by comparison with pure NBT. With only 0.3 wt% Co³⁺ substitution, the piezoelectric properties of the NBT-Co ceramics are optimal, exhibiting a high piezoelectric coefficient (d₃₃~33 pC/N), a low dielectric loss tan δ (~0.1% at 1 kHz), a high thickness planar coupling coefficient (k_t~34%) as well as a high Curie temperature (T_c~663 °C). Such NBT-Co ceramics exhibit nearly temperature-independent piezoelectric and electromechanical properties up to 400 °C, suggesting that these cobalt-modified NBT ceramics are promising materials for high temperature piezoelectric applications.     

钛酸铋纳米粉体的水热合成与分散

以四氯化钛和硝酸铋为原料,氢氧化钾为矿化剂,在220~260℃水热合成6 h,制备出发育良好的纳米级钛酸铋粉体,并对所得粉体进行了不同方式的分散。借助XRD,SEM,TEM及FT-IR等手段对钛酸铋的晶相组成、粒度、形貌等进行了分析。研究结果表明:所得钛酸铋晶粒的形状为矩形片状,宽度在30 nm左右,长度大于100 nm;未经分散的钛酸铋粉体有团聚现象,团聚体的直径可达500 nm左右,将钛酸铋粉体分散于由正硅酸乙酯、丙烯酸、无水乙醇按一定的配比配成的混合液中,然后用高剪切分散乳化机分散处理,可破坏粉体的团聚,具有很好的分散效果。     

Electrical Properties of PTCR Barium Titanate

When properly doped, barium titanate ceramics display positive temperature coefficient resistance (PTCR) behavior. This has been proved to be a Schottky barrier type of grain-boundary effect. However there has not yet been a complete point-to-point comparison between the experimental data and theory for the entire set of the material nonlinear dielectric properties. In this study, a methodology has been developed which allows the study of the depletion layer dielectric properties while the PTCR effect is being investigated. An equivalent dielectric constant, the value of which is to be determined from this experiment, is treated as an average of the dielectric properties of the depletion layer and is used to analyze the grain-boundary resistance and capacitance data based on a simple double-depletion-layer model. The theoretical relationship between this equivalent dielectric constant and the material dielectric properties is also explored in this study.     

Crystal structure and electrical properties of bismuth sodium titanate zirconate ceramics

Lead-free bismuth sodium titanate zirconate (Bi_0.5Na_0.5Ti_1-xZr_xO₃ where x = 0.20, 0.35, 0.40, 0.45, 0.60, and 0.80 mole fraction) [BNTZ] ceramics were successfully prepared using the conventional mixed-oxide method. The samples were sintered for 2 h at temperatures lower than 1,000°C. The density of the BNTZ samples was at least 95% of the theoretical values. The scanning electron microscopy micrographs showed that small grains were embedded between large grains, causing a relatively wide grain size distribution. The density and grain size increased with increasing Zr concentration. A peak shift in X-ray diffraction patterns as well as the disappearance of several hkl reflections indicated some significant crystal-structure changes in these materials. Preliminary crystal-structure analysis indicated the existence of phase transition from a rhombohedral to an orthorhombic structure. The dielectric and ferroelectric properties were also found to correlate well with the observed phase transition.     

Defect mechanisms in BaTiO3‐BiMO3 ceramics

Often, addition of BiMO₃ to BaTiO₃ (BT) leads to improvement in resistivity with a simultaneous shift to n‐type conduction from p‐type for BT. In considering one specific BiMO₃ composition, that is, Bi(Zn_1/2Ti_1/2)O₃ (BZT), several prospective candidates for the origin of this n‐type behavior in BT‐BZT were studied—loss of volatile cations, oxygen vacancies, bismuth present in multiple valence states and precipitation of secondary phases. Combined x‐ray and neutron diffraction, prompt gamma neutron activation analysis and electron energy loss spectroscopy suggested much higher oxygen vacancy concentration in BT‐BZT ceramics (>4%) as compared to BT alone. X‐ray photoelectron spectroscopy and x‐ray absorption spectroscopy did not suggest the presence of bismuth in multiple valence states. At the same time, using transmission electron microscopy, some minor secondary phases were observed, whose compositions were such that they could result in effective donor doping in BT‐BZT ceramics. Using experimentally determined thermodynamic parameters for BT and slopes of Kröger‐Vink plots, it has been suggested that an ionic compensation mechanism is prevalent in these ceramics instead of electronic compensation. These ionic defects have an effect of shifting the conductivity minimum in the Kröger‐Vink plots to higher oxygen partial pressure values in BT‐BZT ceramics as compared to BT, resulting in a significantly higher resistivity values in air atmosphere and n‐type behavior. This provides an important tool to tailor transport properties and defects in BT‐BiMO₃ ceramics, to make them better suited for dielectric or other applications.     

Depletion-layer Dielectric Properties of Positive Temperature Coefficient of Resistance Barium Titanate

For pure and impurity-added positive temperature coefficient of resistance (PTCR) barium titanate ceramic samples, a −11°C shift of the Curie point at the grain-boundary/depletion-layer region was observed. This result is obtained by fitting the PTCR grain-boundary resistance and capacitance data to a theory which combines a double-depletion-layer model with the Devonshire thermodynamic theory of barium titanate. The parameters used in the fitting are obtained from independent experiments. The shift of the Curie point is believed to result from the grain-boundary clamp ing effect near the cubic-tetragonal phase transition point.     

XPS Analysis of Submicrometer Barium Titanate Powder

A commercial submicrometer BaTiO₃ powder was analyzed using X-ray photoelectron spectroscopy. The analysis revealed the powder surfaces to be covered with a layer of physisorbed H₂O and chemisorbed –OH ions. A BaCO₃ residual not detected with XPS was shown to be present in the powder using X-ray diffraction, suggesting that the carbonate takes the form of discrete particles rather than of a continuous surface layer. A relaxed surface phase detected in previous XPS analyses of bulk BaTiO₃ was also shown to be present. Depth profiling revealed the powder surfaces to be Ti-rich, confirming the presence of a phase, or phases, to stoichiometrically balance the barium carbonate.     

Preparation of bioactive sodium titanate ceramics

The in vitro bioactivity of sol–gel derived sodium titanates with different sodium contents was investigated. Calcined sodium titanates show bioactive behaviour when exposed to simulated body fluid (SBF). A higher sodium content entails higher bioactivity since a higher amount of sodium ions is available for the exchange with H₃O⁺ ions from SBF, which results in the formation of Ti–OH groups and subsequently in the formation of an intermediate calcium titanate. The local pH increase triggers the formation of a biomimetic calcium phosphate layer on the surface. Sintering the samples leads to more crystalline sodium tri- and hexatitanate structures and a decrease of the sodium content. The amount of Na⁺ ions that can be released from the sodium titanate surface decreases due to a decreasing surface area and the reduced sodium content. As a result bioactivity is lost. Bioactive behaviour can be re-achieved by a subsequent chemical treatment of sintered sodium titanate ceramics in an aqueous NaOH solution.     

Defect structure‐electrical property relationship in Mn‐doped calcium strontium titanate dielectric ceramics

Ca_0.6Sr_0.4TiO₃ (CST) ceramics with different amounts of Mn dopant (0‐2.0 mol%) were prepared by solid‐state reaction method. The electric field and temperature stability of energy storage performance was found to be greatly enhanced with moderate doped level of 0.5 mol%. The dielectric loss‐frequency spectra revealed the existence and evolution of defect dipoles at elevated temperature, which was confirmed directly by electron paramagnetic resonance (EPR) spectra. The response of defect dipoles was characterized by thermally stimulated depolarization current (TSDC), where the activation energy and the concentration evolution of defect dipoles were calculated, with the highest values observed for 0.5% doped samples. The dissociation of defect dipoles and the movement of free were analyzed by high‐temperature impedance spectra analysis, with the activation energy of 1.04‐1.60 eV, and 0.5% doped samples also demonstrated the highest E_a. The relationship between microscopic defect structure and macroscopic electrical behavior was established in this work.     

Composition-driven broad phase boundary for optimizing properties and stability in lead-free barium titanate ceramics

A new piezoelectric system of (1−x−y)BaTiO₃-yCaZrO₃-xBaSnO₃ (BT-yCZ-xBS) was designed to achieve enhanced piezoelectric/strain properties and temperature stability. First, the relationships between composition, phase, and electrical properties are systematically investigated. The broad phase boundary with successive rhombohedral-orthorhombic (R-O) and orthorhombic-tetragonal (O-T) was obtained in 0.04 ≤ x ≤ 0.05 and 0.04 ≤ y ≤ 0.07 by tailoring the relationship of composition and phase structure, confirmed by X-ray diffraction, temperature-dependent dielectric constants, and Raman spectra. The optimized piezoelectric coefficient of d₃₃ = 560 pC/N, high strain of >0.20%, and large converse piezoelectric coefficient of d₃₃* = 1170 pm/V were realized. Second, the optimized piezoelectricity both demonstrate a stable performance with fluctuation <8% for d₃₃* and 20% for d₃₃ between 22 and 60°C since the broad phase boundary is exhibited in this temperature range. We believe that this work is a successful example to optimize piezoelectric properties and enhance the stability for piezoceramics.