Synthesis and characterization of nanocrystalline ferroelectric Sr0.8Bi2.2Ta2O9 by conventional and microwave sintering: A comparative study

In recent years mechanical activation technique has been utilized to synthesize the nanocrystalline form of compounds resulting in enhancement in the properties. Also, microwave sintering is being preferred over conventional sintering due to rapid processing and uniform temperature distribution throughout the specimen. In the present work, nanocrystalline non-stoichiometric strontium bismuth tantalate (SBT) of the composition Sr_0.8Bi_2.2Ta₂O₉ ferroelectric ceramics were synthesized by microwave sintering process (with sintering temperatures of 1000 °C and 1100 °C) and conventional solid state reaction process (with sintering temperature of 1100 °C) with an objective of comparing the properties of the synthesized specimens by the two processes. X-ray diffraction analysis shows the formation of single phase layered perovskite structure formation by both the processes. Scanning electron microscopy reveals the formation of a finer granular microstructure in the specimen synthesized by microwave sintering compared to that in the specimen prepared by conventional sintering. The specimen prepared by microwave sintering process exhibits improved electrical properties with higher dielectric constant, higher piezoelectric and pyroelectric coefficients and lower dielectric loss.     

直径对玻璃包覆非晶合金微丝磁性的影响

采用Taylor-Ulitovsky方法制备了直径分别在6.3～28.0μm、20.2～28.0μm和14.0～35.2μm之间的玻璃包覆非晶态FeCuNbVSiB、FeBSiCMn和CoNiFeSiB微丝。通过X射线衍射、扫描电镜、振动样品磁强计分别测试了玻璃包覆微丝的组织结构、微观形貌和磁性,研究了不同成分玻璃包覆磁性合金微丝的玻璃包覆层厚度、合金芯直径对微丝磁性能的影响。结果表明了,玻璃包覆磁性合金微丝的磁性能的影响因素由大到小依次为：饱和磁致伸缩系数、微丝成分和微丝尺寸。轴向磁化时随着微丝直径及玻璃包覆层厚度的增大,3 种微丝的径向饱和场强度降低,FeCuNbVSiB和FeBSiCMn微丝的轴向矫顽力先分别由508 A/m和390 A/m降低到486 A/m和278 A/m后再升高到2570 A/m和342 A/m,CoNiFeSiB微丝的轴向矫顽力由171 A/m降低到63 A/m。     

Grain boundary effects on dielectric,infrared and Raman response of SrTiO3 nanograin ceramics

Dielectric, infrared reflectivity and Raman measurements were carried out on dense undoped SrTiO₃ ceramic with the average grain size of 150 nm processed by spark-plasma sintering. The results were compared with our earlier data on conventional ceramics (grain size 1500 nm), see Petzelt et al., Phys. Rev. B 2001, 64, 184111, and on single crystals. Permittivity is dramatically reduced at low temperatures, the soft mode correspondingly stiffened and in Raman spectra the forbidden infrared modes are more pronounced compared to conventional ceramics. All the effects can be accounted for by the existence of polar dead layers with smaller permittivity at grain boundaries. Two models are suggested to explain the data quantitatively. The first model assumes that the dead layer is caused by grain-boundary dipole moment, which penetrates into the grain bulk with the polarization correlation length. Its estimate, using the soft-phonon branch curvature from inelastic neutron data and Landau theory, yields 1.2 nm at 120 K and 5.6 nm at 5 K. Fitting the data with brick-wall model to this dead-layer thickness required also temperature dependence of its permittivity (ɛ ∼12 at 120 K and ∼56 at 5 K). Comparably good fit of both ceramics can be obtained with temperature-independent dead-layer parameters. The latter model is supported by strongly reduced local refractive index at room temperature in the grain-boundary region (von Benthem et al., Phys. Rev. Lett. 2004, 93, 227201).     

Magnetic and magnetocaloric properties of partially disordered RFeAl (R = Gd,Tb) intermetallic

Magnetic and magnetocaloric properties of TbFeAl and GdFeAl were studied in a wide temperature region in magnetic field up to 10 T. They order magnetically below T_c = 196 K and 259 K for TbFeAl and GdFeAl, respectively. The temperature change of ΔT = 1.4 K and 1.6 K was observed in GdFeAl and TbFeAl respectively for a field change of 4 T. Interestingly wide temperature region of significant magnetocaloric effect was observed in both compounds which give rise to relative cooling power of 350 J kg⁻¹ for TbFeAl and 348 J kg⁻¹ for GdFeAl for 4 T field span.     

Effect of microstructure on dielectric and fatigue strengths of BaTiO3

In the present study, bulk barium titanate ceramic specimens with bimodal microstructures are prepared and their dielectric and fatigue strengths under an alternating electric field are investigated. It is found that both the dielectric and fatigue strengths decrease with increasing amount of coarse grains. The scatter of the fatigue strength is characterized with the Weibull statistics. The extent of scatter of the fatigue strength data correlates strongly with the size distribution of the coarse grains. Such correlation is resulted from the presence of intrinsic defects within the microstructure. Direct microstructure evidences are provided.     

Superior hybrid improper ferroelectricity and enhanced ferromagnetism of Ca3(Ti1-xCox)2O7 ceramics through the superexchange interaction

Ca₃(Ti_1-xCo_x)₂O₇ ceramics were prepared by a tartaric acid sol-gel method and sintered in an oxygen atmosphere. The introduction of Co²⁺/Co³⁺ as acceptor dopants leads to the formation of more oxygen vacancies and defect dipoles in Ca₃(Ti_1-xCo_x)₂O₇ ceramics. Oxygen vacancy and defect dipoles lead to the transition of dielectric, leakage, and ferroelectric behaviors of Ca₃(Ti_1-xCo_x)₂O₇ ceramics. The coexistence of hybrid improper ferroelectricity and ferromagnetism at room temperature in Ca₃(Ti_1-xCo_x)₂O₇ ceramics has been successfully realized through the superexchange interaction of Co–O–Co. Ca₃(Ti_1-xCo_x)₂O₇ ceramics exhibit superior ferroelectricity (the remnant polarization is 3.29 μC/cm²) and enhanced ferromagnetism (the remnant magnetization reaches 6.4×10^?3 emu/g). This strategy based on the introduction of transition metal ions with unfilled 3d shells at B sites is an important approach to realize novel room-temperature single-phase multiferroic materials for Ca₃Ti₂O₇-based materials.     

Preparation and magnetic properties of anisotropic Nd_2Fe_(14)B/Sm_2Co_(17) hybrid-bonded magnets

In the present work, anisotropic Nd_2 Fe_(14) B/Sm_2 Co_(17) hybrid-bonded magnets were prepared with different Nd-Fe-B contents. It is found that the particle distributions and ratios between the two magnetic phases have important roles in the magnetic properties, microstructures and thermal stability of the magnets. With increase of Nd-Fe-B content, the saturation magnetization of the anisotropic hybrid magnet increases significantly, however, coercivity decreases, and the demagnetization curves show magnetically single-phase behavior. The anisotropic Nd_2 Fe_(14) B/Sm_2 Co_(17) hybrid-bonded magnets exhibit a maximum energy product and remanence of 14.15 MGOe and 99.53 A·m~2/kg, respectively, when the NdFe-B content is 70 wt% at room temperature. Furthermore, the hybrid magnets also have better thermal stability at elevated temperatures due to the interaction between the two magnetic particles.