•Sublimation of Ag nano-cubes is used to measure specimen temperature to ±5 °C.•Temperatures in the range 700–850 °C are measured at high spatial resolution.•Consistent values are obtained from ...area-to-area and from one device to another.•Difference in device and specimen temperatures is due to low thermal conductivity.
Micro electro mechanical systems (MEMS) based TEM specimen heating holders exhibit excellent thermal stability and minimal specimen drift, which allows thermally-activated processes to be studied dynamically at high spatial resolution. The advantages of MEMS-based devices arise from the very small thermal masses of the sample studied, but this poses particular challenges for the precise measurement of specimen temperature. Previously, it has been proposed that the size-dependent sublimation behavior of Ag nanoparticles could be used to measure the specimen temperature by applying the Kelvin equation, but the effects of the capping ligands used in the nanoparticle synthesis and of electron beam heating have limited the application of such approaches. Here it is shown that for an appropriate choice of experimental parameters (nanoparticle size, loading, intermediate holding temperature, and illumination conditions) the sublimation of Ag nano-cubes can be used to measure the specimen temperature to an accuracy of ±5 °C, over the range 700–850 °C. The measurements are reproducible from area to area on the same MEMS chip, and from chip to chip of the same type. The values of specimen temperature obtained are consistently lower than the calibrated MEMS heater plate temperatures, and it is shown that this cannot be explained on the basis of random errors in the experimental measurements or systematic errors in the materials parameters used for the Kelvin equation analysis. It is proposed that this is instead due to the low thermal conductivity of the electron-transparent amorphous silicon nitride support membrane on the chip. As further evidence for this, it is shown that for a thicker crystalline Si support with a higher thermal conductivity, the magnitude of the difference is smaller. This approach could be extended to other temperature ranges by using nanoparticles of other metals with different vapor pressures and sublimation temperatures.
An experimental study has been performed on the grain growth in a model Ni-based superalloy, with particular emphasis on the pinning effect of the second-phase particles. Extensive annealing ...experiments were performed, and the microstructural development was evaluated in detail using scanning and transmission electron microscopy. It was found that grain growth was strongly inhibited by the pinning effect of both the coarse γ′ phase and the inert precipitates. For samples heat-treated at temperatures below the γ′ solvus (1143
°C), the limiting grain size was determined by the size and volume fraction of the γ′ phase. For samples heat-treated at super-solvus temperatures, the stability of the MC-type carbides and yttrium oxides led to a nearly constant Zener limiting grain size over the temperature range 1143–1244
°C. Samples heat-treated at 1270
°C had larger grain sizes due to the coarsening and/or dissolution of the MC-type carbides and yttrium oxides.
Ti-6Al-4V powders from six different vendors were compared with respect to their microstructures, size-distributions, chemistries, surface appearances, flow behavior, and packing densities. The ...analysis approaches followed closely ASTM F3049, the standard guide for characterization of additive manufacturing metal powders. Chemistries, including impurity content, agreed well with the standard requirements. Powder particle microstructures revealed acicular alpha prime for all vendors. Measurable differences were observed primarily in the size-distributions and the flow behavior.
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Mesoporous graphene aerogel (GA) supported Pt nanoparticles with narrow size distribution were prepared via supercritical deposition (SCD) using supercritical CO2 (scCO2). Pt(cod)me2 ...precursor was dissolved in scCO2 and adsorbed onto GA at 35°C and 10.7MPa. The Pt precursor was converted to its metal form under atmospheric pressure at various temperatures. The effects of precursor conversion temperature (400, 600, and 800°C) on the structural properties of the composites were investigated using Raman Spectroscopy, XRD, XPS, and TEM. The average particle size increased from 1.2 to 2.9nm when the conversion temperature was increased from 400 to 800°C. The electrocatalytic activity of the samples towards the Oxygen Reduction Reaction were evaluated using cyclic voltammetry (CV) and rotating disc electrode (RDE) measurements. SCD helped to preserve the textural properties of the GA after the Pt nanoparticle deposition, and thus Pt/GA converted at 600°C exhibited an enhanced mass activity of 30.6mAmgPt−1, outperforming the mass activities reported in the literature for Pt/GA electrocatalysts prepared using conventional routes.
A critical challenge in the predictive capability of materials deformation behavior under extreme environments is the availability of computational methods to model the microstructural evolution at ...the mesoscale. The capability of the recently-developed quasi-coarse-grained dynamics (QCGD) method to model mesoscale behavior is demonstrated for the phenomenon of supersonic impact of 20 µm sized Al particles on to an Al substrate at various impact velocities and over time and length scales relevant to cold spray deposition. The QCGD simulations are able to model the kinetics related to heat generation and dissipation, and the pressure evolution and propagation, during single particle impact over the time and length scales that are important experimentally. These simulations are able to unravel the roles of particle and substrate deformation behavior that lead to an outward/upward flow of both the particle and the substrate, which is a likely precursor for the experimentally observed jetting and bonding of the particles during cold spray impact.
Supercritical deposition was used to synthesize Pt/carbon aerogel (Pt/CA) electrocatalysts. CAs with average pore diameters of 6, 8 and 19 nm (CA6, CA8 and CA19, respectively) were synthesized and ...impregnated with Pt(cod)me2 precursor using supercritical carbon dioxide followed by the thermal conversion of the Pt precursor into its metallic form under N2 atmosphere at various temperatures between 200 and 1000 °C. All of the prepared CAs have high surface areas with very sharp pore size distributions. XRD and TEM results show increased Pt particle size with increasing conversion temperature with a homogenous distribution of nanoparticles on the CA supports. Cyclic voltammetry was used to determine the effect of CA pore properties on electrocatalytic activity. At a conversion temperature of 400 °C, the highest and lowest electrochemical surface area values were obtained for Pt/CA19 and Pt/CA6 of (126 and 36 m2/g, respectively). Furthermore, Pt/CA19 showed good mass activity whereas Pt/CA6 and Pt/CA8 had lower activity values towards the Oxygen Reduction Reaction (ORR). The mass activity values for Pt/CA19 increased with increasing conversion temperature, except for the sample converted at 1000 °C which exhibited the lowest mass activity. The specific activity increased significantly with the conversion temperature up to 600 °C which gave a value six times that obtained at 200 °C. At 800 °C, the specific activity decreased slightly, probably due to a change in the CA structure at this elevated conversion temperature. The Pt/CA19 sample converted at 600 °C exhibited the best performance with a mass activity of 0.1 A/mgPt and a specific activity of 0.24 mA/cm2.
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•Carbon aerogels with different average pore sizes were loaded with Pt nanoparticles via supercritical deposition method.•The average Pt particle size could be controlled by changing the conversion temperature.•Pt nanoparticles were homogeneously distributed on the surface of carbon aerogels with a narrow particle size distribution.•High ESA and electrocatalytic activity were obtained for carbon aerogel with the larger average pore size.•Highest electrocatalytic activity was obtained for Pt nanoparticle size of 1.9 ± 0.2 nm.
Magnesium is widely regarded as an excellent structural material, primarily because it forms the basis for a range of light-weight high-strength alloys. Recently, high-strain rate deformation of ...magnesium has received a great deal of attention due to the complicated deformation modes that involve combinations of dislocation slip and deformation twinning. In this study, single crystal magnesium samples were shock-compressed along the c- and a-axis, then released back to ambient conditions. Post-mortem transmission electron microscopy revealed that extension twins developed for both c- and a-axis shock loading. Also, the nanoindentation hardness values for these shocked samples were compared to those for samples compressed under quasi-static conditions; it was found that the hardness decreased with increasing strain rate for both c- and a-axis loading. Molecular dynamics simulations were performed to elucidate the detailed mechanisms of deformation twinning in terms of inertial confinement of sample geometry and different stress relaxation speed between impact and lateral directions. The conversion from work-done to heat was discussed to explain the influence of shock-induced heating on the residual hardness. These results give new insights into the residual mechanical response in shock-compressed materials and may help to develop a more fundamental understanding of shock phenomena in metallic materials.
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•Samples of single crystal magnesium were shock-compressed and released along both the a- and c- axes.•Extension twinning was observed using transmission electron microscopy.•Shock-induced softening was observed using nano-indentation.•Analytical model and molecular dynamics simulations were used to examine the evolution of pressure profiles and extension twinning during the shock-recovery process.
Additively manufactured (AM) components usually have nonequilibrium microstructures. Post-built heat treatments are recommended for AM components to achieve homogenous microstructures. In this study, ...the effects were investigated of conventional solutionizing and precipitation hardening (H-900) heat treatments on the microstructure evolution of 17-4PH AM and wrought components. Microstructural characterization techniques including SEM, TEM and EBSD analysis were used on 17-4PH AM and wrought components to obtain quantitative information about the microstructure and phase evolution during these heat treatments. These microstructural studies demonstrate that 17-4PH AM components can achieve microstructures and hardnesses similar to those of wrought samples by post-built heat treatments.
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•As-built 17-4 PH has steel an inhomogeneous columnar microstructure composed of ferrite, martensite, and some austenite.•Solution heat treatments homogenized the grain structure and eliminated the {100} texture observed in as-built samples.•Oxide inclusions pinned grain boundaries during heat treatment thereby refining the grain structure.•Precipitation heat treatment yielded Cu-rich nano-precipitates without further changes in the grain structure.•Heat treatments can achieve comparable microstructures and hardness in additively manufactured and wrought 17-4 PH samples.
The compound Ag
3
Sn adopts the ordered orthorhombic D0
a
Cu
3
Ti-type structure. It exhibits an unusual low yield stress and high ductility for an intermetallic compound, but the reasons for these ...effects are not clear. Here, we report an electron microscopy study on the defects present in solution-grown Ag
3
Sn single crystals that have deformed during the decanting and subsequent handling processes. It is found that the crystals contain two types of lenticular deformation twins: {011}-type and {211}-type. These twins interpenetrate with no evidence of cracking at the intersections. The crystals also contain high densities of dislocations including long straight dipoles with
b
=
± 010 and shorter curved segments and loops with
b
=
10
2
¯
and 001. It is inferred that the dipoles are artifacts of specimen preparation that climb in from the cross-sectional sample surfaces, whereas the shorter segments are deformation debris. If a combination of twinning and dislocation glide of the types observed here were to form concurrently during general deformation of Ag
3
Sn, then they could provide the necessary number of independent deformation modes to accommodate an arbitrary plastic strain, which might help to explain the unusual ductility of this compound.
Pt/Al2O3 catalysts prepared via supercritical deposition (SCD), with supercritical CO2, wet impregnation (WI) methods and a selected benchmark catalyst, were evaluated for the dehydrogenation of ...perhydro-dibenzyltoluene (H18-DBT) at 300 °C in a batch reactor. After ten dehydrogenation runs, the average performance of the catalyst prepared using SCD was the highest compared to the benchmark and WI-prepared catalysts. The pre-treatment of the catalysts with the product (dibenzyltoluene) indicated that the deactivation observed is mainly due to the adsorbed H0-DBT blocking the active sites for the reactant (H18-DBT). Furthermore, the SCD method afforded a catalyst with a higher dispersion of smaller sized Pt particles, thus improving catalytic performance towards the dehydrogenation of H18-DBT. The particle diameters of the SCD- and WI-prepared catalysts varied in the ranges of 0.6–2.2 nm and 0.8–3.4 nm and had average particle sizes of 1.1 nm and 1.7 nm, respectively. Energy dispersive X-ray spectroscopy analysis of the catalysts after ten dehydrogenation runs revealed the presence of carbon. In this study, improved catalyst performance led to the production of more liquid-based by-products and carbon material compared to catalysts with low catalytic performance.
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