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Nanocrystallization has been an important approach for reducing thermal conductivity in thermoelectric materials due to limits on phonon mean-free path imposed by the characteristic structural size. We report on thermal conductivity as low as 0.3 Wm−1K−1 of nanocrystalline silicon thin films prepared by plasma-enhanced chemical-vapor deposition as grain size is reduced to 2.8 nm by controlling hydrogen dilution of silane gas during growth. A multilayered film composed by alternating growth conditions, with layer thicknesses of 3.6 nm, is measured to have a thermal conductivity 30% and 15% lower than its two constituents. Our quantitative analysis attributes the strong reduction of thermal conductivity with decreasing grain size to the magnifying effect of porosity which occurs concomitantly due to increased mass density fluctuations. Our results demonstrate that ultrasmall grain sizes, multilayering, and porosity, all at a similar nanometer-size scale, may be a promising way to engineer thermoelectric materials. Thermoelectric materials convert heat into electricity and their performance is determined by their figure of merit ZT, which is generally too small in many materials for practical applications. Here, the authors demonstrate that a reduction in grain size for nanocrystalline Si can reduce thermal conductivity and potentially be used as a method to engineer greater ZT in Si for thermoelectric applications.

Differential thermal analysis (DTA) and microstructural and microprobe measurements of DTA and as-cast Ni-Re alloys with compositions between 0.20 and 0.44 mass fraction Re provide information to resolve differences in previously published Ni-Re phase diagrams. This investigation determines that the peritectic invariant between liquid, Re-rich hexagonal close packed and Ni-rich face center cubic phases, L + HCP → FCC, occurs at 1561.1 °C ± 3.4 °C (1σ) with compositions of liquid, FCC and HCP phases of 0.283 ± 0.036, 0.436 ± 0.026, and 0.828 ± 0.037 mass fraction Re, respectively. Analysis of the microsegregation in FCC alloys yields a partition coefficient for solidification, k = 1.54 ± 0.09 (mass frac./mass frac.). A small deviation from Scheil behavior due to dendrite tip kinetics is documented in as-cast samples. No evidence of an intermetallic phase is observed.

Potential commercial applications for transition metal dichalcogenide (TMD) semiconductors such as MoS(2) rely on unique material properties that are only accessible at monolayer to few-layer thickness regimes. Therefore, production methods that lend themselves to scalable and controllable formation of TMD films on surfaces are desirable for high volume manufacturing of devices based on these materials. We have developed a new thermal atomic layer deposition (ALD) process using bis(tert-butylimido)-bis(dimethylamido)molybdenum and 1-propanethiol to produce MoS(2)-containing amorphous films. We observe self-limiting reaction behavior with respect to both the Mo and S precursors at a substrate temperature of 350 °C. Film thickness scales linearly with precursor cycling, with growth per cycle values of (≈)0.1 nm/cycle. As-deposited films are smooth and contain nitrogen and carbon impurities attributed to poor ligand elimination from the Mo source. Upon high-temperature annealing, a large portion of the impurities are removed, and we obtain few-layer crystalline 2H-MoS(2) films.

A 300 keV transmission electron microscope was modified to produce broadband pulsed beams that can be, in principle, between 40 MHz and 12 GHz, corresponding to temporal resolution in the nanosecond to picosecond range without an excitation laser. The key enabling technology is a pair of phase-matched modulating and de-modulating traveling wave metallic comb striplines (pulsers). An initial temporal resolution of 30 ps was achieved with a strobe frequency of 6.0 GHz. The placement of the pulsers, mounted immediately below the gun, allows for preservation of all optical configurations, otherwise available to the unmodified instrument, and therefore makes such a post-modified instrument for dual-use, i.e., both pulsed-beam mode (i.e., stroboscopic time-resolved) and conventional continuous waveform mode. In this article, we describe the elements inserted into the beam path, challenges encountered during integration with an in-service microscope, and early results from an electric-field-driven pump–probe experiment. We conclude with ideas for making this class of instruments broadly applicable for examining cyclical and repeatable phenomena.

One drawback of the laser powder-bed fusion additive manufacturing (AM) technique is the build-up of residual stresses during processing that requires a stress-relief heat treatment prior to components being removed from the build plate. Here, we demonstrate the coprecipitation of the γ″-phase alongside the known δ-phase, during stress-relief annealing of AM Inconel 625 at 870 °C. The unexpected precipitation of γ″ in the AM material is attributed to the local solute enrichment to the interdendritic regions of the as-built solidification microstructure. Dissolution of the δ and γ″ phases is achieved after annealing for 15 min at 1150 °C.

For most of recorded history, poverty reflected God’s will. The poor were always with us. They were not inherently immoral, dangerous, or different. They were not to be shunned, feared, or avoided. In the late eighteenth and early nineteenth centuries, a harsh new idea of poverty and poor people as different and inferior began to replace this ancient biblical view. In what ways, exactly, are poor people different from the rest of us became – and remains – a burning question answered with moral philosophy, political economy, social science, and, eventually, biology. Why did biological conceptions of poverty wax and wane over the last century and a half? What forms have they taken? What have been their consequences?

The microstructural evolution of laser powder-bed additively manufactured Inconel 625 during a post-build stress-relief anneal of 1 hour at 1143 K (870 °C) is investigated. It is found that this industry-recommended heat treatment promotes the formation of a significant fraction of the orthorhombic D0a Ni3Nb δ-phase. This phase is known to have a deleterious influence on fracture toughness, ductility, and other mechanical properties in conventional, wrought Inconel 625; and is generally considered detrimental to materials’ performance in service. The δ-phase platelets are found to precipitate within the inter-dendritic regions of the as-built solidification microstructure. These regions are enriched in solute elements, particularly Nb and Mo, due to the micro-segregation that occurs during solidification. The precipitation of δ-phase at 1073 K (800 °C) is found to require up to 4 hours. This indicates a potential alternative stress-relief processing window that mitigates δ-phase formation in this alloy. Ultimately, a homogenization heat treatment is recommended for additively manufactured Inconel 625 because the increased susceptibility to δ-phase precipitation increases the possibility for significant degradation of materials' properties in service.

A small-angle tilting technique, applied to scanning transmission electron microscopy (STEM), was used together with multislice image simulation to reveal new atomic-scale information about the structural evolution of single-crystalline Pt-doped CaTiO3 thin films, grown by pulsed laser deposition, that occurs in response to reduction and reoxidation treatments. Specifically, we were able to confirm that Pt atoms are randomly dispersed throughout the as-grown film, most often occupying Ti sites, show that the smallest (∼1 nm) Pt-rich clusters embedded within CaTiO3 after reduction have a structure consistent with metallic Pt, and demonstrate that the Pt atoms from these clusters occupy mainly Ca sites, in the form of ordered Pt-atom arrays, after reoxidation.

Surface phonon polaritons (SPhPs), the surface-bound electromagnetic modes of a polar material resulting from the coupling of light with optic phonons, offer immense technological opportunities for nanophotonics in the infrared (IR) spectral region. However, once a particular material is chosen, the SPhP characteristics are fixed by the spectral positions of the optic phonon frequencies. Here, we provide a demonstration of how the frequency of these optic phonons can be altered by employing atomic-scale superlattices (SLs) of polar semiconductors using AlN/GaN SLs as an example. Using second harmonic generation (SHG) spectroscopy, we show that the optic phonon frequencies of the SLs exhibit a strong dependence on the layer thicknesses of the constituent materials. Furthermore, new vibrational modes emerge that are confined to the layers, while others are centered at the AlN/GaN interfaces. As the IR dielectric function is governed by the optic phonon behavior in polar materials, controlling the optic phonons provides a means to induce and potentially design a dielectric function distinct from the constituent materials and from the effective-medium approximation of the SL. We show that atomic-scale AlN/GaN SLs instead have multiple Reststrahlen bands featuring spectral regions that exhibit either normal or extreme hyperbolic dispersion with both positive and negative permittivities dispersing rapidly with frequency. Apart from the ability to engineer the SPhP properties, SL structures may also lead to multifunctional devices that combine the mechanical, electrical, thermal, or optoelectronic functionality of the constituent layers. We propose that this effort is another step toward realizing user-defined, actively tunable IR optics and sources.