Pengrong Ren

Structure, Phase Transition Behaviors and Electrical Properties of Nd Substituted Aurivillius Polycrystallines Na0.5NdxBi2.5–xNb2O9 (x = 0.1, 0.2, 0.3, and 0.5)

New high temperature Aurivillius piezoelectrics Na0.5Nd(x)Bi(2.5-x)Nb2O9 (NDBNx, x = 0.1, 0.2, 0.3, and 0.5) with Nd substitution for Bi at the A site were synthesized using a solid state reaction process. Crystal structures of NDBN0.2 and NDBN0.5 were refined with the Rietveld method with powder X-ray diffraction, and they crystallized in the orthorhombic space group A21am [a = 5.48558(8) Å, b = 5.46326(9) Å, c = 24.8940(4) Å, and Z = 4 for NDBN0.2 and a = 5.46872(5) Å, b = 5.46730(5) Å, c = 24.80723(25) Å, and Z = 4 for NDBN0.5], at room temperature. The refinement results and Raman spectroscopy of NDBNx verified that Nd occupied both the A site in the perovskite layers and the cation site in the (Bi2O2)(2+) layers. The Nd substitution induced an enhancement in cation disordering between the A site and the (Bi2O2)(2+) layer and an increase in the degree of the relaxation behavior for NDBNx. The ferroelectric to paraelectric phase transition temperature (Tc) of NDBNx ranged from 735 to 764 °C. Furthermore, the isovalent substitution of Nd for Bi had a great influence on microstructure (grain size and shape), defect concentration (mainly oxygen vacancies), preferred grain orientation (texture), and distortion of the octahedron. The coaction between these effects determined the structure characteristics, phase transition behaviors, and electrical properties of NDBNx.

Carbon coated SnO2: synthesis, characterization, and photocatalytic performance

A carbon coated SnO2 photocatalyst was prepared by using sucrose as a carbon source and the microwave hydrothermal method at the temperature as low as 180 °C. The resulting carbon coated SnO2 was characterized by XRD, Raman, TEM, TG, UV-Vis adsorption and XPS. The carbon layer was found to have multiple functions, increase of the adsorption capability for organic dyes, enhancement of visible light absorption, and facilitating the separation of photogenerated charges. The prepared carbon coated SnO2 exhibits high and stable photocatalytic activity for the degradation of rhodamine B (RhB) under UV-Vis irradiation. Moreover, hydroxyl radicals (˙OH) are found to be the main active species generated in the oxidation reaction of RhB over the carbon coated SnO2 photocatalyst. It is believed that this synthesis method can be extended to prepare a wide variety of functional nanohybrids for different applications.

A candidate for lead-free ultrahigh-temperature piezoelectrics: the excellent electro-mechanical properties of Aurivillius oxides, Ca1−5xLi2xNd2x□xBi2Nb2−2xScxWxO9−1.5x

Modified ultrahigh-temperature piezoelectric ceramics, Ca1−5xLi2xNd2x□xBi2Nb2−2xScxWxO9−1.5x (CBNO-LiNd-ScW-x, x = 0, 0.01, 0.02, 0.03, 0.04; □ represented A-site vacancies), were synthesized by a traditional solid-state reaction process. XRD Rietveld refinement, TEM, HRTEM and Raman spectroscopy were used to characterize the crystal structures and domain structures of the prepared ceramics. The lithium/neodymium (Li/Nd) and scandium/tungsten (Sc/W) co-substitution as well as the existence of small amounts of A-site vacancies significantly enhanced the ferroelectric and electro-mechanical properties of the CBNO-LiNd-ScW-x ceramics. The Curie temperature (Tc) decreased from 941 °C to 832 °C as the x values increased in CBNO-LiNd-ScW-x, because of an enhancement in the cation disordering between the A site and the bismuth oxide layer. The CBNO-LiNd-ScW-0.01 ceramic with an ultrahigh Tc of 908 °C had a high piezoelectric activity (d33) of 20.6 pC N−1, a large field-induced strain (S33) of 0.27 × 10−3 and a high remnant polarization (Pr) of 6.1 μC cm−2, due to the perfect growth in the ferroelectric domains and the contribution from the reversal of the defect dipoles (VCa′′–VO•• and ScNb′′–VO••); interestingly, it still has a high d33 of 19.5 pC N−1 after up to 800 °C thermal annealing. In addition, the CBNO-LiNd-ScW-0.01 ceramic had a relatively high resistivity of >106 Ω cm at 600 °C and >105 Ω cm at 700 °C.

Bulk conduction and nonlinear behaviour in multiferroic YMnO3

Electrical conductivity of YMnO3 in O2, air, and N2 has been investigated by impedance spectroscopy. Within the investigated temperature range, three types of electrical conductivity were distinguished. Below 300 °C, the conductivity is dominated by p-type conductivity, while between 300 and 500 °C, the conductivity is in the so called saturation region. Above 500 °C, n-type conductivity occurred, probably due to the loss of oxygen. The air-processed YMnO3 is characterized by the presence of mixed Mn3+/Mn4+ and electrical heterogeneous structure, which are responsible for both, dielectric anomaly observed between 200 and 300 °C and nonlinear behavior under high electric field.

Mixed oxide ion and proton conduction and p-type semiconduction in BaTi0.98Ca0.02O2.98 ceramics

Ca-doped BaTiO3, BaTi0.98Ca0.02O2.98 is primarily an oxide ion conductor with activation energy 0.77(1) eV when annealed at 700 °C and measured in an atmosphere of dry N2. On exposure of this sample to moisture, proton conduction dominates the conductivity at low temperatures with activation energy 0.42(1) eV but the sample reverts to mainly oxide ion conduction above ∼500 °C as water is lost from the sample. When samples are annealed at 700 °C and measured in an atmosphere of dry O2, p-type semiconduction with activation energy 0.70(2) eV dominates. The nature of the conducting species was determined by a combination of conductivity measurements as a function of oxygen partial pressure and characteristic features in low frequency impedance data.

Microstructure and tunable dielectric properties of Ba0.6Sr0.4TiO3/Y2O3 composite ceramics

Abstract(100−x) at.% Ba0.6Sr0.4TiO3−x at.% Y2O3 (BST/Y2O3) composite ceramics were prepared by conventional solid-state reaction methods. Phase constitution, microstructure, and element distribution of the composite ceramics were characterized, and the dielectric properties of BST/Y2O3 composite ceramics were measured. The results revealed that some of Y3+ were dissolved into the lattice of BST and the rest remained at grain boundaries in oxide form of Y2O3. With the increase of the content of Y2O3, Curie peak of BST/Y2O3 composite ceramics moved to a lower temperature, the dielectric permittivity and dielectric loss of BST/Y2O3 composite ceramics decreased significantly, meanwhile, the dielectric tunability still maintains a relatively high value. Furthermore, a multipolarization mechanism model is employed to describe the field dependence of the dielectric permittivity in BST/Y2O3 composite, which suggests that the existence of the second phase of Y2O3 diminishes the contribution of polar clusters in BST matrix, leading to the decrease of dielectric tunability. P–E loop measurement indicates that the breakdown strength of BST/Y2O3 ceramics increases during addition of Y2O3 in BST matrix. Therefore, the BST/Y2O3 composites with good tunable dielectric properties and enhanced breakdown strength can be exploited for phase shifter and capacitor device applications.

Large nonlinear dielectric behavior in BaTi 1−x SnxO 3

BaTi1−xSnxO3 (BTSn, 0 ≤ x ≤ 0.30) ceramics were prepared by both the conventional sintering (CS) and sparking plasma sintering (SPS). Composition, temperature and grain size dependences of the nonlinear dielectric behaviors were systematically studied. BTSn was found to have especially large tunability (≥90%), which is larger than most other Pb-free systems, and is comparable to Pb-based relaxors. The high dielectric tunability in BTSn is attributed to its specific domain structures. Besides, temperature dependent tunability of BTSn presents a dispersed behavior and the dispersion is enhanced with the increase of Sn4+ concentrations, which is explained by the compositional fluctuation model.

Anomaly Diffuse and Dielectric Relaxation of Barium Titanate at High Temperatures

The relaxation behaviors of barium titanate ceramics were investigated in the temperature range of 200–450°C. The diffuse dielectric anomalies were described by the universal dielectric relaxation law (UDR) and polaron theory. It is indicated the dielectric relaxation of barium titanate at high temperatures is associated with the hopping localized oxygen vacancies.