Solar receiver tubes are key components of concentrating solar-thermal power (CSP) systems that harvest solar energy. For better efficiency, the Gen3 CSP receivers, which collect heat into a heat transfer fluid, require a temperature exceeding 700 degrees C during operation and need to perform under extreme conditions of high temperature and high thermal stress. Operators are seeking CSP designs using new high-temperature structural materials with high thermal conductivity and high creep resistance to achieve a design life of 30 years and thus help recover the plant capital cost sooner. MAX phase materials, which consist of an early transition metal element, an A-group element, and carbon or nitrogen, are expected to exhibit high creep resistance as well as high fracture toughness. In this paper, we describe fabricating both (1) dense Ti3SiC2 MAX phase disks and (2) short-length tubes using field-assisted sintering technology (FAST). First, the disk samples that we fabricated are fully dense and contain approximate to 90 % Ti3SiC2 MAX phase materials and approximate to 10 % TiC phase materials. We determined a flexure strength of 519 +/- 32 MPa by conducting a four-point bending test at room temperature with rectangular bar samples of approximate to 100 % density. The thermal conductivity of the Ti3SiC2 MAX phase samples, measured by lightflashing analysis, decreases linearly from a value of 41 W.m(-1).K-1 at room temperature to a value of 36 W.m(-1).K-1 at 650 degrees C. A solar reflectance measurement of the Ti3SiC2 MAX phase revealed that, temperature increases from 400 to 1400 degrees C, thermal emittance increases from 0.39 to 0.49, while selectivity decreases from 1.8 to 1.4, respectively. Whereas the surface oxidized MAX phase samples after 100 h exposure to air at 1000 degrees C exhibit that of SiC. Next, we discuss fabrication of the crack-free Ti3SiC2 MAX phase tubular structures accomplished by using FAST processing in graphite bedding. A Ti3SiC2 MAX phase content of > 95 % with traceable ti3% remaining TiC phase and ti15 % porosity were demonstrated after high-temperature annealing. An average fracture strength of ti250 MPa was determined with Ti3SiC2 MAX phase tubes of approximate to 85 % density by diametral compression testing at room temperature. Our work demonstrated that using FAST processing to produce Ti3SiC2 MAX phase tubular structures for CSP receiver applications is a viable approach.

In this study, additive-manufactured silicon carbide preforms were joined and densified by reaction bonding via liquid silicon infiltration. The silicon carbide preforms were first printed by binder jetting additive manufacturing. To demonstrate concurrent joining and densification, two preforms with carbon or parchment papers at the interface were concurrently joined and infiltrated by liquid silicon. Results showed a robust interface with thicknesses ranging from 150 to 500 mu m, depending on the paper type and the number of paper layers. High-energy synchrotron X-ray revealed that beta-phase silicon carbide was formed inside the interface. Finally, two additively manufactured samples with complicated channel geometry were successfully joined. Energy dispersive spectroscopy of the interface of the channeled samples showed a consistent and robust joining. This concurrent approach of joining and densification enables efficiency improvement of fabricating silicon carbide parts with complicated geometries and widens geometry freedom for additive manufacturing of silicon carbide.

The local structure and multiferroic properties of iron-doped lead titanate (PbTi1-xFexO3-delta) samples was investigated over the entire composition range (x = 0-1). Inherent polarization in PbTiO3 decreases due to Fe3+ incorporation up to the solubility limit (x similar to 0.3), although homogeneous doping persists only up to x = 0.1. Ti prefers highly distorted oxygen octahedra for any x value, while Fe prefers more symmetric O-deficient polyhedra (Fe-O-n). The charge compensating oxygen vacancies induce local tilting of the Fe-O-n polyhedra beyond a critical x value (x >= 0.2), promoting magnetic interaction between two adjacent Fe atoms. The strain induced by local heterogeneity could act as a coupling force between magnetic and ferroelectric properties. Fe-rich clusters evolve into ferromagnetic PbFe12O19 with increased Fe doping. PbTi1-xFexO3-delta ( x >= 0.3) samples therefore have separate origins for the ferroelectric (PbTi1-xFexO3-delta) and magnetic (PbFe12O19) phases.

Mixed-conducting ceramic oxides have potential uses in high-temperature electrochemical applications such as solid oxide fuel cells, batteries, sensors, and oxygen-permeable membranes. The Sr-Fe-Co-O system combines high electronic/ionic conductivity with appreciable oxygen permeability at elevated temperatures. Dense ceramic membranes made of this material can be used to separate high-purity oxygen from air without the need for external electrical circuitry, or to partially oxidize methane to produce syngas. Samples of Sr2Fe3-xCoxOy (with x = 0, 0.6, 1.0, and 1.4) were prepared by solid-state reaction method in atmospheres with various oxygen partial pressures (pO(2)) and were characterized by X-ray diffraction, scanning electron microscopy, and electrical conductivity testing. Phase components of the sample are dependent on cobalt concentration and pO(2). Electrical conductivity increases with increasing temperature and cobalt content in the material.

By electron-beam (e-beam) melting, we prepared 0.4 wt% carbon-infused copper (CuCv4), and a copper control without carbon addition (CuCvO). Scanning electron microscopy and helium ion microscopy (HIM) were performed on the as-solidified surface, fracture surface, and ion-polished surface of the CuCv4 sample. The results revealed that graphitic carbon flakes cover the as-solidified surface, and carbon nanoparticles and clusters exist in the fracture and ion-polished surfaces. HIM on the ion-polished surface revealed a unique ripple-shaped feature, which is possibly associated with the infusion of carbon nanoribbons in the copper matrix. The bulk densities were measured to be 8.86 and 8.53 g/cm^3, which correspond to relative densities of 98.9% and 96.4% for the CuCvO and CuCv4 samples, respectively. In addition, apparent electrical conductivities were measured to be 56.9 and 57.5 MS/m, respectively, for the e-beam melted CuCvO and CuCv4 samples. These values correspond to true electrical conductivities of 100.5% IACS (International Annealed Copper Standard) and 107.4% IACS after correction for the porosity. Our results reveal remarkable promise of using covetic copper for the next generation conductors in energy applications from microelectronic devices to high-power transmission cables.

Energy storage capability of dense PLZT film capacitor fabricated by aerosol deposition.

We prepared 0.4 wt. % carbon infused copper (i.e., copper covetic, CuCv) by electron-beam melting and deposited approximate to 18 nm-thick films of the CuCv on a substrate by electron-beam evaporation. Helium ion microscopy (HIM) and scanning transmission electron microscopy (STEM) were used to study the characteristics of the carbon nanostructure in the CuCv bulk and thin films. HIM observation of the fracture and ion-polished surface of bulk CuCv revealed ripple structures that are associated with carbon nanoribbons formed in the copper matrix. STEM high angle annular dark field imaging and energy dispersive spectroscopy mapping indicated that carbon nanoparticles and carbon-rich pathways are interconnected to form a carbon-rich network in the CuCv films. High-resolution transmission electron microscopy and STEM electron energy loss spectroscopy suggest that the carbon nanoparticles are composed of highly distorted graphenic carbon sheets that are bonded to the copper matrix and likely provide means for superior electrical and thermal conduction. In this letter, we report structural hierarchy and representations of carbon nanostructures in copper covetics at different scales from a few micrometers to subnanometers. Published by AIP Publishing.

We prepared graphite-infused aluminum composites by electron-beam vacuum melting and electromagnetic stirring with 0% and 4 wt% graphite addition. Bulk density of 2.69 g/cm 3 was measured for samples with 0% graphite addition (AlCv0) and 2.66 g/cm 3 for samples with 4 wt% graphite addition (AlCv4). Both are > 99% of their theoretical values. X-ray diffraction indicated that AlCv0 is phase pure, while the AlCv4 sample is a composite consisting of aluminum, Al 4 C 3 , and graphite. Electron microscopy and energy-dispersive X-ray spectroscopy revealed pockets of carbon-rich phase of 10–50 μm in sizes dispersed in the Al matrix in AlCv4. Scanning transmission electron microscopy showed that Al 4 C 3 crystallites of 1–2 μm sizes are mostly located near the outer shell of the pockets of carbon-rich phase and with undissolved graphite at the center. Thermal conductivity of 107.8 W/m K was measured for the AlCv4 and 226.7 W/m K for the AlCv0. Our work demonstrated the feasibility of producing pore-free high-density graphite-infused aluminum composite materials by electron-beam melting and electromagnetic stirring in vacuum.

Magnetoelectric effect, arising from the interfacial coupling between magnetic and electrical order parameters, has recently emerged as a robust means to electrically manipulate the magnetic properties in multiferroic heterostructures. Challenge remains as finding an energy efficient way to modify the distinct magnetic states in a reliable, reversible, and non-volatile manner. Here we report ferroelectric switching of ferromagnetic resonance in multiferroic bilayers consisting of ultrathin ferromagnetic NiFe and ferroelectric Pb0.92La0.08Zr0.52Ti0.48O3 (PLZT) films, where the magnetic anisotropy of NiFe can be electrically modified by low voltages. Ferromagnetic resonance measurements confirm that the interfacial charge-mediated magnetoelectric effect is dominant in NiFe/PLZT heterostructures. Nonvolatile modification of ferromagnetic resonance field is demonstrated by applying voltage pulses. The ferroelectric switching of magnetic anisotropy exhibits extensive applications in energy-efficient electronic devices such as magnetoelectric random access memories, magnetic field sensors, and tunable radio frequency (RF)/microwave devices.

We report the growth of ferroelectric Pb0.92La0.08Zr0.52TiO4O3 (PLZT) thick films using a poly(1-vinylpyrrolidone-co-vinyl acetate) (PVP/VA)-modified sal-gel process. A per-coating thickness of approximate to 0.66 mu m has been demonstrated using PVP/VA-modified solution, which is more than doubled that of the PLZT films grown by PVP-modified method, and nearly 6 times the per-coating thickness of films prepared by conventional sal-gel process. PLZT thick films grown on LNO/Ni substrates exhibited denser microstructure, higher remanent polarization (11 mu C/cm(2)) and dielectric tunability (45%), lower leakage current density (approximate to 1.2 x 10(-8) A/cm(2)), and higher breakdown strength (approximate to 1.6 MV/cm) than those for the samples grown on PtSi substrates. These results demonstrated great potential of using PVP/VA-modified sol-gel process for high power film capacitor applications. (C) 2015 Published by Elsevier Ltd.

High-quality epitaxial Pb0.92La0.08Zr0.52Ti0.48O3 (PLZT) films of thickness of ∼880 nm were fabricated using pulsed laser deposition on (001) Nb doped SrTiO3 (Nb:STO) substrates. Besides a confirmation of the epitaxial relationship [100]PLZT//[100]Nb:STO and (001)PLZT//(001)Nb:STO using X-ray diffraction, a transmission electron microscopy study has revealed a columnar structure across the film thickness. The recoverable energy density (Wrec) of the epitaxial PLZT thin film capacitors increases linearly with the applied electric field and the best value of ∼31 J/cm3 observed at 2.27 MV/cm is considerably higher by 41% than that of the polycrystalline PLZT film of a comparable thickness. In addition to the high Wrec value, an excellent thermal stability as illustrated in a negligible temperature dependence of the Wrec in the temperature range from room temperature to 180 °C is achieved. The enhanced Wrec and the thermal stability are attributed to the reduced defects and grain boundaries in epitaxial PLZT thin films, making them promising for energy storage applications that require both high energy density, power density, and wide operation temperatures.