Thermal conductivity measurement of the epoxies and composite material for low temperature superconducting magnet design

The thermal conductivities of solid materials were measured by a G-M cryo-cooler based apparatus in the temperature range of 2.6–21.0 K. The performance of this apparatus was verified by measuring the thermal conductivity of 304-stainless steel, and good reproducibility as well as accuracy was shown when compared with the certified values. The thermal conductivities of EC1017 and Stycast2850FT and a composite material were measured. Similar behavior to amorphous materials was shown for EC1017 and Stycast2850FT in that there was an apparent plateau, which could be ascribed to a very lower crystallinity of epoxy. An equivalent model was proposed to predict the thermal conductivity of the composite material.     

Hydrodynamic and thermal development of compressible oscillatory flow inside circular channel

In order to study the hydrodynamic and thermal development of compressible oscillatory flow inside a circular channel, the coefficient of hydrodynamic development (CHD) and the coefficient of thermal development (CTD) are proposed as index parameters, based on the temporal–spatial profile characteristics of the velocity and the temperature field. CHD expresses the variation rate of velocity profile along the axial direction at the phase of centerline velocity reaching its maximum, while CTD denotes the variation rate of temperature profile along the axial direction at the phase of maximum temperature difference between the centerline fluid and the wall. Numerical computations are conducted for a given physical model to validate the effectiveness of these two index parameters. The CHD and CTD as a function of the normalized distance from the entrance into the channel are computed and presented. CHD = 0.1 and CTD = 0.1 are suggested to determine the hydrodynamic and thermal entrance lengths, respectively, which is proved acceptable by comparing the velocity and temperature profiles at various positions along the channel axis.     

α-MnO2 nanowires transformed from precursor δ-MnO2 by refluxing under ambient pressure: The key role of pH and growth mechanism

α-MnO₂ nanowires were obtained by reflux treatment of precursor δ-MnO₂ in acidic medium under ambient pressure. The great effects of pH on the transformation of δ-MnO₂ to α-MnO₂ and the concentration of coexistent cations (K⁺, Mn²⁺) was investigated in systematically designed experiments by using powder X-ray diffraction and atomic absorption spectrometry analysis. The specific surface area of the products could be simply controlled by adjusting the initial pH value of the suspension. The micro-morphologies during the transition process from the precursors to final products were characterized by SEM and TEM. A dissolution–recrystallization mechanism was proposed to describe the growth process of the one-dimensional nanowire. MnO_x units or MnO₆ octahedra was formed firstly from the dissolution of outmost surfaces of δ-MnO₂, followed by a rearrangement/crystallization to form one-dimensional α-MnO₂ nanowire. In addition, the time-dependent process of dissolution would take place gradually from the external to internal of the precursor.     

Phase transition characteristics and dielectric properties of rare-earth (La,Pr, Nd,Gd) doped Ba(Zr0.09Ti0.91)O3 ceramics

A-site deficient rare-earth doped barium zirconate titanate (BZT) ceramics (Ba_1−yLn_2y/3)Zr_0.09Ti_0.91O₃ (Ln = La, Pr, Nd, Gd) are obtained by a modified solid-state reaction method. Perovskite-like single-phase compounds were confirmed from X-ray diffraction data. Morphological analysis on sintered samples shows that the addition of rare-earth ions inhibits the growth of the grain and remarkably changes the grain morphology. The effect of rare-earth addition to BZT on phase transition and dielectric properties is analyzed. A dramatic fall in the transition temperature occurs when BZT ceramic is doped with rare-earths. Moreover, diffusivity degree of the phase transition increases and a relaxor-type behaviour is induced due to both the increment of the lanthanide content and the increase of the ionic radius of the dopant element. High values of dielectric tunability are obtained for lanthanum doped BZT. A direct relation between transition temperature and tunability is discussed. Conclusively, low permittivity and high tunability materials can be obtained by the adequate substitution of rare-earths into BZT ceramics.     

Carbon Nanotubes: Biocompatibility of Immobilized Aligned Carbon Nanotubes (Small 8/2011)

In vivo host responses to an electrode‐like array of aligned carbon nanotubes (ACNTs) embedded within a biopolymer sheet are reported. This biocompatibility study assesses the suitability of immobilized carbon nanotubes for bionic devices. Inflammatory responses and foreign‐body histiocytic reactions are not substantially elevated when compared to negative controls following 12 weeks implantation. A fibrous capsule isolates the implanted ACNTs from the surrounding muscle tissue. Filamentous nanotube fragments are engulfed by macrophages, and globular debris is incorporated into the fibrous capsule with no further reaction. Scattered leukocytes are observed, adherent to the ACNT surface. These data indicate that there is a minimal local foreign‐body response to immobilized ACNTs, that detached fragments are phagocytosed into an inert material, and that ACNTs do not attract high levels of surface fouling. Collectively, these results suggest that immobilized nanotube structures should be considered for further investigation as bionic components.     

Large-scale synthesis of hierarchical alpha-FeOOH flowers by ultrasonic-assisted hydrothermal route

In this paper, we report a facile, an environmental friendly ultrasonic-assisted hydrothermal route for preparation of goethite flower structures using Fe nanopowders at low temperature (85°C). The flower structure consisted of tens of hundreds of nanowires and such structures can further self-assemble with the flake with micro size area. Structural, morphological, and elemental analysis revealed that the products consisted of flower-like structures with high structural uniformity, good crystal quality, and high yields. Influencing factors such as the reaction temperature, pH value, and the deposition time were systematically investigated. A possible formation mechanism was proposed on the basis of the experimental results. Magnetic measurements showed that the as-obtained goethite flowers exhibited weakly ferromagnetic characteristics at room temperature, which were quite different from those of the corresponding bulk materials.     

Effect of Bi content on spalling behavior of Sn–Bi–Zn–Ag/Cu interface

The effect of the Bi content on the formation of intermetallic compounds (IMCs) layers between the Sn-xBi-0.9Zn-0.3Ag lead-free solder (with x = 1, 2, 3 and 4, in weight percent, hereafter) and Cu substrate was investigated. The structure of the IMC layer in the soldered interface varies apparently with increasing the Bi content. When the Bi content is 1 wt%, the interface soldered is consisted of CuZn and Cu₆Sn₅ IMC layers, which are separated by an intermediate solder layer. As the Bi content increases, the spalling phenomenon tends to disappear. Moreover, the layer between the Sn-2Bi-0.9Zn-0.3Ag solder and Cu substrate is thicker than others. The evolution of the soldered interfacial structure could be attributed to the existence of Bi.     

Synthesis and characterization of core–shell SiO2@(Er3+,Yb3+):Lu2O3

We synthesized crystalline Erbium Er³⁺ and Ytterbium Yb³⁺ codoped -Lu₂O₃ nanolayers on SiO₂ microspheres using the modified Pechini method. Two different kinds of precursors, nitrates and chlorides, have been used leading to a layer-to-layer morphology and necklaces structures, respectively. In both cases, the size of nanocrystallites constituting the optical active layer is around 5 nm. We performed X-ray powder diffraction to confirm the cubic crystalline structure of the sesquioxides layer. High resolution transmission electron microscopy analyses corroborate the crystalline nature of the layer. The optical emission of Er³⁺ in the visible range has been recorded.