Laijun Liu

Electrostrictive and relaxor ferroelectric behavior in BiAlO3-modified BaTiO3 lead-free ceramics

The crystal structure of (1−x)BaTiO3-xBiAlO3 (x = 0, 0.02, 0.05, 0.08, and 0.1) ceramics was determined using X-ray diffraction and Raman spectroscopy at room temperature, which revealed a phase transition from tetragonal to rhombohedral with increasing x. The dielectric properties were studied as a function of temperature at different frequencies, which indicated that the phase transition temperature (Tm) decreased with increasing x. The relaxor behavior was observed by frequency and temperature dependent dielectric permittivity. The Lorenz-type quadratic law was used to characterize the dielectric permittivity peaks near Tm of high-temperature slopes at 1 MHz. The temperatures Tm of dielectric permittivity peaks fit very well with the Vogel-Fulcher law in x = 0.05 and x = 0.1. The polarization hysteresis loops and electrostrictive were displayed at room temperature. The sample for x = 0.1 exhibits a slim loop with negligible hysteresis and a subtle linear feature, which is a promising transducer materia...

Grain selection during casting Ni-base,single-crystal superalloys with spiral grain selector

The behavior of grain selection in a spiral grain selector during investment casting of a Ni-base, single-crystal (SX) superalloy, DD3, has been investigated by electron backscattered diffraction (EBSD) techniques and optical microscopy. The results indicated that the main function of starter block is to optimize the crystal orientation. During the process of grain selection in spiral passage, the grain near the inner wall of spiral passage was usually selected as the final single crystal. It was found that the dendrites near the inner wall could develop new tertiary dendritic arms that paralleled the primary dendrites from the secondary dendritic arms to overgrow the dendrites far away from the inner wall. The crystal orientation that was examined by X-ray diffraction revealed that (1) the crystal orientation did not change obviously with increasing spiral thickness or angle and (2) the crystal orientation could be optimized by increasing the withdrawal rate and ceramic mold temperature. The influence of pouring temperature on crystal orientation was also discussed.

Dc-bias-field-induced Dielectric Relaxation and AC Conduction in CaCu3Ti4O12 Ceramics

The dielectric relaxation and ac conduction of CaCu3Ti4O12 (CCTO) ceramics were investigated at different temperatures under a dc bias. The dc bias gives rise to space charge accumulation, i.e. an electrode response, resulting in the significant increase of dielectric permittivity and dielectric loss tangent. Two Debye-like relaxations, arising from electrode and grain boundary responses, are present at low frequency with an increase of the dc bias. The electrode and grain boundary relaxations are distinguished according to the impedance spectroscopy and the frequency-dependent ac conductivity. The relaxation times of electrode and grain boundary relaxation are 0.955 ms and 0.026 ms, respectively, with a dc bias of 10 V at 328 K.

Effect of holding time on the dielectric properties and non-ohmic behavior of CaCu3Ti4O12 capacitor-varistors

Giant dielectric ceramics CaCu3Ti4O12 (CCTO) with non-ohmic electrical properties were prepared by a sol–gel processing method. Crystal structure and microstructure of the ceramics have been characterized using X-ray diffraction and Scanning electron microscopy. The effects of sintering duration and cooling rate on electrical properties of the ceramics were investigated by measuring the properties of permittivity, I–V and grain-boundary barriers. Prolonging holding time led to substantial improvement in permittivity, furthermore, the quenched sample showed larger dielectric permittivity than the furnace-cooling one. The non-ohmic behavior relating current density (J) to the applied electric field (E) at different temperatures was characterized. A low-voltage and giant dielectric permittivity CCTO varistor with breakdown voltages in the range of Eb = 0.2–3 kV cm−1, nonlinearity coefficient α = 2–6 and dielectric permittivity ε = 4,000–30,000 was obtained. A linear relationship between ln(J) and E1/2 indicated that a Schottky barrier should exist at the grain boundary.

Influence of interface point defect on the dielectric properties of Y doped CaCu3Ti4O12 ceramics

CaCu3Ti4−xYxO12 (0≤x≤0.12) ceramics were fabricated with conventional solid-state reaction method. Phase structure and microstructure of prepared ceramics were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The impedance and modulus tests both suggested the existence of two different relaxation behavior, which were attributed to bulk and grain boundary response. In addition, the conductivity and dielectric permittivity showed a step-like behavior under 405K. Meanwhile, frequency independence of dc conduction became dominant when above 405K. In CCTO ceramic, rare earth element Y3+ ions as an acceptor were used to substitute Ti sites, decreasing the concentration of oxygen vacancy around grain-electrode and grain boundary. The reason to the reduction of dielectric behavior in low frequencies range was associated with the Y doping in CCTO ceramic.

Unusual Strong Incommensurate Modulation in a Tungsten-Bronze-Type Relaxor PbBiNb5O15.

Pb- or Bi-based perovskite oxides have been widely studied and used because of their large ferroelectric polarization features induced by stereochemically active 6s(2) lone pair electrons. It is intriguing whether this effect could exist in other related systems. Herein, we designed and synthesized a mixed Pb and Bi A site polar compound, PbBiNb5O15, with the TTB framework. The as-synthesized material turns out to be a relaxor with weak macroscopic ferroelectricity but adopts strong local polarizations. Whats more, unusual five orders of incommensurate satellite reflections with strong intensities were observed under the electron diffraction, suggesting that the modulation is highly developed with large amplitudes. The structural modulation was solved with a (3 + 1)D superspace group using high-resolution synchrotron radiation combined with anomalous dispersion X-ray diffraction technique to distinguish Pb from Bi. We show that the strong modulation mainly originates from lone-pair driven Pb(2+)-Bi(3+) ordering in the large pentagonal caves, which can suppress the local polarization in x-y plane in long ranges. Moreover, the as-synthesized ceramics display strong relaxor ferroelectric feature with transition temperature near room temperature and moderate dielectric properties, which could be functionalized to be electromechanical device materials.

Analysis of the alternating current conductivity and magnetic behaviors for the polycrystalline Y-type Ba0.5Sr1.5Co2(Fe1-xAlx)12O22 hexaferrites

Co2Y hexaferrites have attracted intensive interests due to its potential high temperature magnetoelectricity as the single phase multiferroics. Numerous efforts have been paid to enhance their magnetoelectric properties at high temperatures through increasing the magnetic transition temperature and decreasing the conductivity. In this work, we investigated the conductivity and magnetic properties of the polycrystalline Ba0.5Sr1.5Co2(Fe1−xAlx)12O22 (0 ≤ x ≤ 0.12) hexaferrites and found that Al-doping has important effects on both the conductivity and magnetic properties. The underlying physical mechanisms were also systematically analyzed. Most importantly, a very much enhanced resistivity (over 10 MΩ cm), and a high magnetic transition temperature (∼346 K) have been obtained at a doping amount of x = 0.04. These improvements are very promising for achieving significant magnetoelectric effect at room temperature. The current research can provide the basic understanding of the Y-type ferrites for future ap...

Quantitative Description of the Diffuse Phase Transition of BNT-NKN Ceramics

A perovskite solid solution of (1-x)(Bi0.5Na0.5)TiO3-x(Na0.5K0.5)NbO3 (BNT-xNKN) (x = 0–0.08) synthesized by solid state reaction technique exhibits diffuse phase transition (DPT) behavior. Power law, Gauss-type quadratic law and Lorenz-type quadratic law was employed to characterize the dielectric permittivity peaks near Tm . The diffuseness parameters γ, δG and δA increase with the increase in the value of x, which implies a composition-induced diffuse transition. Among these laws simulating dielectric response in diffusion ferroelectric phase transition, the Lorenz-type quadratic law is the best to describe the characterization of DPT behavior in such a wide temperature range as observed in our experiment.

Ferroic properties of Fe-doped and Cu-doped K0.45Na0.49Li0.06NbO3 ceramics

Ferroelectric and magnetic properties of Fe-doped K0.45Na0.49Li0.06NbO3 (KNLN) and Cu-doped KNLN ceramics were investigated. Comparing with pure KNLN ceramic, Fe-doped KNLN shows ferromagnetic and ferroelectric properties simultaneously while Cu-doped KNLN shows stronger diamagnetic and ferroelectric properties. The magnetic property in KNLN matrix are believed to be induced by point defects. The ferromagnetism of Fe-doped KNLN can be explained by the F-center exchange mechanism, while the antiferroelectric-like loop and diamagnetism of Cu-doped KNLN was explained by the special defect structure and Cu1+ ion at A site.

Copper doped EuMnO3: synthesis, structure and magnetic properties

A solid solution EuMn1−xCuxO3−δ (0 ≤ x ≤ 0.316) was synthesized by a traditional solid state reaction at 1150 °C. It crystallized into the Pnma space group, which was confirmed by powder X-ray and selected area electron diffraction. Magnetic susceptibility measurements showed that the two magnetic transitions for these materials occurred from 52 to 22 K and 45 to 20 K.