Yb3+ doping effects on thermal conductivity and thermal expansion of Yttrium aluminium garnet

Yttrium Aluminium Garnet (YAG) is an attractive candidate as thermal barrier material used for turbine blade in aero engines, due to its relatively low thermal conductivity, low oxygen diffusivity and good phase stability at high temperature. YAG has a complex crystal structure, in which Y3+ ions locate in dodecahedron and Al3+ ions in octahedron and tetrahedron. Replacing the host cations with rare earth elements can cause the structure change which influences the thermal properties of YAG. Because the space inside the octahedron is relatively small, Yb3+ ions which have the smallest ionic radial size in the lanthanide series, have been selected and attempted to be doped on dodecahedral and octahedral sites to investigate the effects on thermal conductivity and thermal expansion. The variation of lattice constant indicates that Yb3+ ions are located on the dodecahedron or octahedron. In addition, when Yb3+ ions replace Al3+ ions on octahedral sites, the thermal conductivity at room temperature is dramatically reduced and the coefficient of thermal expansion is over 10×10−6 K−1 at high temperature, which results from the expansion of octahedron due to the much larger radius of Yb3+ ion compared with the host cation (Al3+ ion). On the contrary, replacing Y3+ ions with Yb3+ ions in dodecahedron, the thermal conductivity also gradually reduces to the similar value but the coefficient of thermal expansion is getting smaller, due to the relatively small ionic radius of Yb3+ causing the contraction of the dodecahedron. Therefore, a dopant with much larger radius would be preferred in both dodecahedron and octahedron to significant reduce thermal conductivity as well as increase coefficient of thermal expansion of YAG, by introducing large radial difference between the dopant and the host cations

On the Microstructural Evolution and Failure Mechanism in Laser Powder Bed Fusioned Ti-6Al-4V during Low Cycle Fatigue at Room and Elevated Temperatures

Microstructural features and their evolution during cyclic deformation directly impact the low cycle fatigue (LCF) life of additively manufactured Laser Powder Bed Fusion (LPBF) Ti-6Al-4V. Tensile and strain controlled LCF tests were performed at room (RT) and elevated temperature (ET, @ 400 °C) to study the cyclic softening behaviour and failure mechanism of LPBF Ti-6Al-4V. The evolution of α’ grains and free dislocation density were studied using Electron Back Scatter Diffraction (EBSD). LPBF Ti-6Al-4V has greater tensile strengths than the conventionally manufactured wrought Ti-6Al-4V due to its microstructure with fine α’ needles which provide small slip lengths. For cyclic loading at ET, the interaction between the dislocations increases which in-turn increase the ability of material to overcome the obstacles to dislocation motion, resulting into higher cyclic softening compared to the RT test. During cyclic deformation, evolution of dislocation substructures takes place to subsequently produce Low Angle Boundaries (LABs) inside the prior α’ grains. The LABs progressively lead to nucleation and coalescence of voids with fatigue cycles, eventually leading to fracture. An increase in strain range (i.e. plasticity levels) causes more significant dislocation pile-up contributing to a greater amount of cyclic softening. The lack of fusion voids or pores, present at or near the surface, and microcracks, present at the rough surface, act as the crack initiation locations which propagate to cause fracture of the LPBF specimens/components under LCF loading, where the primary mode of fatigue fracture observed is intergranular

Toward mid-infrared, subdiffraction, spectral-mapping of human cells and tissue: SNIM (scanning near-field infrared microscopy) tip fabrication

Scanning near-field infrared microscopy (SNIM) potentially enables subdiffraction, broadband mid-infrared (MIR:3–25-μm wavelength range) spectral-mapping of human cells and tissue for real-time molecular sensing, with prospective use in disease diagnosis. SNIM requires an MIR-transmitting tip of small aperture for photon collection. Here, chalcogenide-glass optical fibers are reproducibly tapered at one end to form a MIR transmitting tip for SNIM. A wet-etching method is used to form the tip. The tapering sides of the tip are Al-coated. These Al-coated tapered-tips exhibit near-field power-confinement when acting either as the launch-end or exit-end of the MIR optical fiber. We report first time optimal cleaving of the end of the tapered tip using focused ion beam milling. A flat aperture is produced at the end of the tip, which is orthogonal to the fiber-axis and of controlled diameter. A FIB-cleaved aperture is used to collect MIR spectra of cells mounted on a transflection plate, under illumination of a synchrotron- generated wideband MIR beam

Highly Aligned Ni-Decorated GO–CNT Nanostructures in Epoxy with Enhanced Thermal and Electrical Properties

In this study, graphene oxide–carbon nanotubes nanostructures decorated with nickel nanoparticles (NiGNT) were prepared through the molecular-level-mixing method, followed by a reduction process, and then applied as reinforcements to enhance the epoxy resin matrix. The ferromagnetism of the Ni nanoparticles allowed NiGNT nanostructures to be vertically aligned within the composite with the assistance of a magnetic field. Due to the alignment distribution of the NiGNT, the composites demonstrated enhanced anisotropic thermal and electrical conduction performances, compared with pure epoxy and randomly distributed composites. The aligned distribution of NiGNT–epoxy composites displayed 2.7 times higher thermal conductivity and around 104 times better electrical conduction performance, compared with pure epoxy. The thermal expansion of NiGNT–epoxy composite was also restricted in the aligned direction of NiGNT nanostructures. Thus, NiGNT–epoxy composites show great potential as future aerospace, aviation, and automobile materials

Enhanced thermal and electrical properties by Ag nanoparticles decorated GO-CNT nanostructures in PEEK composites

A nanostructure of graphene oxide (GO) and carbon nanotubes (CNTs) decorated with silver nanoparticles (AgGNT) has been prepared via a molecular-level-mixing (MLM) method followed by a subsequent freeze-drying and reduction process. The obtained well-dispersed AgGNT nanostructures were then applied as fillers to reinforce the poly(ether ether ketone) (PEEK) matrix. AgGNT-PEEK composites have then demonstrated excellent electrical and thermal conduction performances as well as high thermal durability compared with pure PEEK and its pure Ag or GO-CNT (GNT) enhanced composites. Owing to the unique morphology of AgGNT nanostructures, which made them uniformly dispersed in the PEEK matrix and formed a 3D network structure, the AgGNT-PEEK composites displayed 60% higher thermal conductivity and around 109 times better electrical conduction performance than pure PEEK, and superior thermal durability even above the melting temperature of pure PEEK. Thus, the AgGNT-PEEK composites have shown great potential for applications such as semiconductors, high-temperature electrical applications, aerospace, and automobile materials

Investigation on time-dependent wetting behavior of Ni-Cu-P ternary coating

Hydrophobic metallic coatings have attracted increasing interest in the recent years due to their excellent mechanical durability. But the wetting behavior and hydrophobic mechanism of metallic coatings are far from clear. In this work, Ni-Cu-P ternary coatings with hierarchical structure were prepared on 304 stainless steel by electrodeposition method. The surface morphologies, phase compositions, and wetting behavior were studied systemically. Time-dependent wetting behaviour of Ni-Cu-P coatings had been clearly observed, and the surface of the as-deposited coatings changed from hydrophilic state to hydrophobic state after aging in ambient air. The related surface wetting mechanism was investigated with the assistance of plasma cleaning to study the possible surface adsorption contributing to the time-dependent wetting behavior. The variations of the surface species were analyzed using X-ray photoelectron spectroscopy (XPS), showing the composition change of both carbon and the oxygen. The atomic ratio of hydrocarbon on the Ni-Cu-P coating first increased from 78.7% to 86.5% when stored in ambient air and then decreased from 82.3% to 65.9% after the plasma cleaning treatment; while the variation of oxygen content was an opposite trend. The results indicated that the observed time-dependent wettability was a combined result of the adsorption of airborne hydrocarbon and the change of lattice oxygen on the coating surface

Electric Field Induced Biomimetic Transmembrane Electron Transport Using Carbon Nanotube Porins

Cells modulate their homeostasis through the control of redox reactions via transmembrane electron transport systems. These are largely mediated via oxidoreductase enzymes. Their use in biology has been linked to a host of systems including reprogramming for energy requirements in cancer. Consequently, our ability to modulate membrane redox systems may give rise to opportunities to modulate underlying biology. The current work aimed to develop a wireless bipolar electrochemical approach to form on-demand electron transfer across biological membranes. To achieve this goal, we show that using membrane inserted carbon nanotube porins that can act as bipolar nanoelectrodes, we could control electron flow with externally applied electric fields across membranes. Before this work, bipolar electrochemistry has been thought to require high applied voltages not compatible with biological systems. We show that bipolar electrochemical reaction via gold reduction at the nanotubes could be modulated at low cell-friendly voltages, providing an opportunity to use bipolar electrodes to control electron flux across membranes. We provide new mechanistic insight into this newly describe phenomena at the nanoscale. The results presented a give rise to a new method using CNTPs to modulate cell behavior via wireless control of membrane electron transfer

Efficient dye-removal via Ni-decorated graphene oxide-carbon nanotube nanocomposites

The nickel nanoparticles decorated graphene oxide-carbon nanotubes nanocomposite has been prepared through a novel molecular-level-mixing method followed by a freeze-drying and subsequent reduction process. The resulting products showed a well-dispersed 3D structure and demonstrated excellent performance in removing Rhodamine B (RhB) from the aqueous solution through a synergistic effect of physical adsorption and photo-degradation. The physical adsorption of RhB by this nanocomposite followed the pseudo-second-order and could be ascribed to its high surface area. The nanocomposite containing 30 wt.% of Ni exhibited the highest removal capacity of ca. 41.5 mg g-1 in 5 ppm RhB solution. Meanwhile, because of the ferromagnetism of Ni nanoparticles coated, the nanocomposite could be easily collected and recycled after the removal of dye via magnetic separation. Thus, the nanocomposite have demonstrated great potentials as an efficient adsorbent with good photocatalytic performance for removing organic dyes from wastewater

Flame spheroidisation of dense and porous Ca2Fe2O5 microspheres

Compositionally uniform magnetic Ca2Fe2O5 (srebrodolskite) microspheres created via a rapid, single-stage flame spheroidisation (FS) process using magnetite and carbonate based porogen (1:1 Fe3O4:CaCO3) feedstock powders, are described. Two types of Ca2Fe2O5 microsphere are produced: dense (35 - 80 µm), and porous (125 - 180 µm). Scanning electron microscopy (SEM) based techniques are used to image and quantify these. Complementary high-temperature X-ray diffraction (HT-XRD) measurements and thermogravimetric analysis (TGA) provide insights into the initial process of porogen feedstock decomposition, prior to the coalescence of molten droplets and spheroidisation, driven by surface tension. Evolution of CO2 gas (from porogen decomposition) is attributed to the development of interconnected porosity within the porous microspheres. This occurs during Ca2Fe2O5 rapid cooling and solidification. The facile FS-processing route provides a method for the rapid production of both dense and porous magnetic microspheres, with high levels of compositional uniformity and excellent opportunity for size control. The controllability of these factors make the FS production method useful for a range of healthcare, energy and environmental remediation applications

Evolution of carbon nanotubes and their metallurgical reactions in Al-based composites in response to laser irradiation during selective laser melting

Aluminium-based composites reinforced with carbon nanotubes are widely sought for their outstanding metallurgical and structural properties that largely depend on the manufacturing route. In this work, the process-structure-property relationship for a composite made from high-energy-ball-milled pure Al and multi-walled carbon nanotubes (MWCNT) processed by laser powder-bed-fusion additive manufacturing was investigated. The response of MWCNT to laser irradiation and their interfacial reactions with Al were probed in a holistic investigation. X-ray diffraction confirmed the partial transformation of C into Al-carbides in addition to the presence of some nano-crystalline graphitic materials. Microscopy revealed evidence of carbides segregation at the melt pool boundaries as well as migration along the build direction. Raman spectroscopy showed that laser irradiation promoted re-graphitisation in MWCNT, reducing the amount of defects introduced by milling. Two types of Al4C3 formed as a result of the metallurgical reaction between Al and MWCNT. These were needle-like and hexagonal Al4C3 and their mechanisms of formation, direct precipitation and dissolution-precipitation, respectively, were explained in light of the thermal profile experienced by the material during melting and solidification. Large scale electron backscatter diffraction showed that there is no distinctive texture developing during melting and solidification. Micro- and nano-indentation testing showed uniform mechanical properties