Key Engineering Materials Vols. 602-603

Abstract: Sr-doped lanthanum manganite (La_1-xSr_xMnO₃) is characteristic of thermochromic, which can act as a smart thermal control material used in the variable-emittance devices. In the present study, La_1-xSr_xMnO₃ thin films were prepared on MgO(100) substrates by pulsed laser deposition, and the effect of Sr-doping (x = 0 ~ 0.4) on the structure and infrared emissivity was investigated. Single-phased La_1-xSr_xMnO₃ films with (100)-orientation were obtained, which showed a dense texture with smooth surface. The ratio of Mn⁴⁺/Mn³⁺ in the films was increased with increasing Sr doping, leading to the enhancement in double-exchange interaction and electrical conductivity. As a result, the phase transition from metal to insulator was observed with the increasing of test temperature. For the La_0.8Sr_0.2MnO₃ thin film, a large value of emittance (De = 0.28) was obtained, indicating good variable-emittance by appropriate Sr doping.

Abstract: We report on the effect of Ni doping on the thermoelectric properties of p-type BiCuSeO oxyselenide, with layer structure composed of conductive (Cu₂Se₂)^2- layers alternately stacked with insulating (Bi₂O₂)²⁺ layers along c axis. After doping with Ni, enhanced electrical conductivity coupled with a moderate Seebeck coefficient leads to a power factor of ~231 μwm^-1K^-2 at 873 K. Coupled to low thermal conductivity, ZT at 873 K is increased from 0.35 for pristine BiCuSeO to 0.39 for Bi_0.95Ni_0.05CuSeO. However, the efficiency of Ni doping in the insulating (Bi₂O₂)²⁺ layer is low, and this doping only leads to a limited increase of the hole carriers concentration. Therefore Ni doped BiCuSeO has relatively low electrical conductivity which makes its thermoelectric figure of merit much lower than that of Ca, Sr, Ba and Pb doped BiCuSeO.

Abstract: TiB₂ doped B₄C composite which TiB₂ contents were 12.4% and 25.4% have been fabricated by the combination of the ball milling process and hot pressing technology. The phase analysis and microstructure of the composite materials are investigated through X-ray diffraction and scan electron microscope, respectively. In addition, thermoelectric properties of the composite materials are studied. The results show that the microcracks are easy to form in the interface between B₄C and TiB₂. It is also found that, the electrical and thermal conductivity increase wi2. Edit Paperth the increase of TiB₂ doping, while the Seebeck coefficient decreases with the increase of TiB₂ doping. The thermoelectric figure of merit of the B₄C+25.4%TiB₂ composite material is 50% higher than that of B₄C at 573K, while the ZT of B₄C+25.4% TiB₂ composite material is lower than that of B₄C at 873K.

Abstract: The effect of Zr/Sn ratio on the dielectric and energy storage properties of lanthanum modified lead zirconate stannate titanate (PLZST) ceramics with compositions located near the boundary between antiferroelectric and ferroelectric phases was studied. Microstructural observation indicated that all the samples had a uniform morphology with pure perovskite phase and the average grain size reduced obviously with increasing Zr/Sn ratio. As the Zr/Sn ratio increased, the dielectric constant increased and the transition temperature T_m shifted to higher temperature. All the PLZST ceramics exhibited double hysteresis loops. The maximum polarization increased, while the switching field decreased when the Zr/Sn ratio increased. The variations of Zr/Sn ratio had little impact on remanent polarization. As a result, both charged energy density and discharged energy density increased with increasing Zr/Sn ratio. A high energy storage density of 1.75 J/cm³ was achieved in the PLZST ceramics with the Zr/Sn ratio of 82.5/7.5 at 9 kV/mm.

Abstract: In this paper, dry ice is converted into few-layer graphene, which can be dispersed stably in N, N-Dimethylformamide (DMF) by adding pyrene-1-boronic as a stabilizer that non-covalently functionalizes the surface of graphene to obtain homogeneous colloidal suspensions. Moreover, we make use of vacuum filtration transferring for fabricating transparent conducting graphene films by incorporating multiwalled carbon nanotubes (MWNTs). The increased conductivity is ascribed to the formation of a more efficient network. Here a transmittance of 81% at 550 nm and a sheet resistance as low as 38.17 KΩ/sq are obtained.

Abstract: In this paper, a series of Al₂O₃/Ni composites with various content of Ni (0 vol%, 10 vol%, 20 vol%, 30 vol%, 40 vol% and 50 vol%) were prepared, and the effect of the methods and compositions on the density and electromagnetic properties was investigated. The results show that the density of the composites increases with the increasing sintering temperature as well as the increasing content of Ni. Moreover, the dielectric constant of the composites increases remarkably with the sintering temperature and the content of Ni, while the permeability shows an almost linear enhancement with the increase of Ni content. This suggests that the Al₂O₃/Ni composite can be expected to be a promising substrate material with certain electromagnetic compatibility.

Abstract: Cd²⁺-substituted NiCuZn ferrites with compositions of Ni_0.24-yCd_yCu_0.21Zn_0.55 Fe_1.92O_3.88 (y = 0.00, 0.002, 0.005, 0.007, 0.01) were prepared by a solid state reaction method. The effects of Cd²⁺ substitution on phase, microstructure feature and magnetic properties of samples were discussed. The results showed that NiCuZn ferrites with Cd²⁺ substitution were single spinel phases. The lattice parameter decreased first and subsequently increased due to the difference of ions radius and incomplete substitution. The density of sintered samples had similar trend as well. The saturation magnetization M_S decreased with the increase of Cd²⁺ ions. The highest initial permeability μ_i and lowest relative loss factor tanδ/μ_i were found in the composition with Cd content y=0.007 attributed to the decrease of crystal magnetic anisotropy constant K₁ of the ferrites.

Abstract: In this work the results of investigations for (BiFeO₃)_x(BaTiO₃)_1-x and Bi_1-xNd_xFeO₃ solid solutions are described. Samples were prepared by the conventional solid-state sintering method. X-ray diffraction, ⁵⁷Fe Mössbauer spectroscopy, and magnetoelectric effect measurements were applied as complementary methods to determine the structure and magnetic properties of materials. For (BiFeO₃)_x(BaTiO₃)_1-x solid solutions Mössbauer spectroscopy revealed the relationship between the content of BiFeO₃ and the magnetic properties of the samples. Moreover, the presence of magnetoelectric coupling in (BiFeO₃)_x(BaTiO₃)_1-x solid solutions was registered at room temperature for the materials sintered at various temperatures. The maximum value of magnetoelectric voltage coefficient was achieved for 0.7(BiFeO₃)0.3(BaTiO₃) sintered at 1153K. Structure of Bi_1-xNd_xFeO₃ solid solutions was investigated in the whole range of concentration. Hyperfine interactions parameters were determined for the first time for these solid solutions.

Abstract: This study reports the synthesis of bismuth ferrite ceramics (BiFeO₃, BFO) by the sol-gel method and the effect of different sintering temperatures (800 °C, 820 °C, 850 °C, 880 °C, 900 °C) on the crystal structure, morphology, dielectric properties and ferroelectric properties of the bismuth ferrite ceramics. It is observed that the bismuth ferrite phase are formed when the molar ratio of bismuth and ferric is 1.025 : 1, and the bismuth ferrite ceramics have less impurity phase at all the five sintering temperature. The morphology studies revealed that the grain of bismuth ferrite ceramics is spherical and the grain size increases with the increasing of the sintering temperature. When the bismuth ferrite ceramics with excessive bismuth sintered at 820 °C, it exhibits the higher dielectric constant and the lower dielectric loss, the remnant polarization and the coercive field are 0.35 μC/cm² and 4.03kV/cm, respectively. The observed ferroelectricity in bismuth ferrite ceramics owes to the point defects associated with oxygen vacancy defects as well as the smaller size effect.

Abstract: BiFeO₃ nanoparticles were successfully synthesized by a hydrothermal method by a mineralizer (KNO₃). Structural characterization was performed by thermal analysis, powder X-ray diffraction (XRD) and scanning electron microscopy (TEM).The results showed that the products were perovskite structure BiFeO₃ powders. Optimal conditions for the synthesis of single-phase BiFeO₃ ceramics were obtained.