Monolithic quantum-dot distributed feedback laser array on silicon

Electrically-pumped lasers directly grown on silicon are key devices interfacing silicon microelectronics and photonics. We report here, for the first time, an electrically-pumped, room-temperature, continuous-wave (CW) and single-mode distributed feedback (DFB) laser array fabricated in InAs/GaAs quantum-dot (QD) gain material epitaxially grown on silicon. CW threshold currents as low as 12 mA and single-mode side mode suppression ratios (SMSRs) as high as 50 dB have been achieved from individual devices in the array. The laser array, compatible with state-of-the-art coarse wavelength division multiplexing (CWDM) systems, has a well-aligned channel spacing of 20 0.2 nm and exhibits a record wavelength coverage range of 100 nm, the full span of the O-band. These results indicate that, for the first time, the performance of lasers epitaxially grown on silicon is elevated to a point approaching real-world CWDM applications, demonstrating the great potential of this technology

Anisotropic thermal transport in MOF-5 composites

a b s t r a c t Metal-organic frameworks (MOFs) are a new class of porous, crystalline materials with applications in the capture, storage, and separation of gasses. Although much effort has been devoted to understanding the properties of MOFs in powder form, in a realistic system the MOF media will likely be employed as dense compacts, such as pucks or pellets, to maximize volumetric efficiency. In these applications efficient transport of the heat of adsorption/desorption is an important design consideration. Consequently, densified composites consisting of a physical mixture of a MOF and expanded natural graphite (ENG) have been proposed as a means to enhance the intrinsically low thermal conductivity of these materials. Here we demonstrate that the high-aspect ratio of ENG particles, combined with uni-axial compression, results in anisotropic microstructural and thermal transport properties in composite MOF-5/ENG pellets. Microscopy of pellet cross-sections revealed a textured microstructure with MOF particle boundaries and ENG orientations aligned perpendicular to the pressing direction. This anisotropy is manifested in the thermal conductivity, which is two to four times higher in directions perpendicular to the pressing direction. We further demonstrate that this anisotropy can be exploited using two processing techniques. First, a custom die and densification process allows for reorientation of the preferred heat flow pathway. Second, a layered MOF-5/ENG microstructure increases the thermal conductivity by an order of magnitude, with only minor ENG additions (5 wt.%). These results reveal that anisotropic thermal transport in MOF composites can be tailored using a judicious combination of second phase additions and processing techniques

Thermophysical properties of MOF-5 powders

a b s t r a c t We present a comprehensive assessment of the thermophysical properties of an industrial, pilot-scale version of the prototype adsorbent, metal-organic framework 5 (MOF-5). These properties are essential ingredients in the design and modeling of MOF-5-based hydrogen adsorption systems, and may serve as a useful starting point for the development of other MOF-based systems for applications in catalysis, gas separations, and adsorption of other gasses or fluids. Characterized properties include: packing density, surface area, pore volume, particle size distribution, thermal conductivity, heat capacity, stability against hydrolysis, differential enthalpy of H 2 adsorption, and Dubinin-Astakhov isotherm parameters. Hydrogen adsorption/desorption isotherms were measured at six temperatures spanning the range 77-295 K, and at pressures of 0-100 bar

Tailoring a Three-Phase Microenvironment for High-Performance Oxygen Reduction Reaction in Proton Exchange Membrane Fuel Cells

Despite tremendous progress in catalyst development for rate-limiting cathodic oxygen reduction reaction (ORR), reducing Pt usage while meeting performance requirements in practical proton exchange membrane fuel cells (PEMFCs) remains a challenge. The ORR in PEMFCs occurs at a catalyst–electrolyte–gas three-phase interface. A desirable interface should exhibit highly active and available catalytic sites, as well as allow efficient oxygen and proton feeding to the catalytic sites and timely removal of water to avoid interface flooding. Here, we report the design of a three-phase microenvironment in PEFMCs, showing that carbon surface chemistry can be tuned to modulate its interaction with the ionomers and create favorable transport paths for rapid delivery of both reactants and products. With such an elaborate interfacial design, for the first time we have demonstrated PEMFCs with all key ORR catalyst performance metrics, including mass activity, rated power, and durability, surpassing the US Department of Energy targets

Tuning Nb–Pt Interactions To Facilitate Fuel Cell Electrocatalysis

High stability, availability of multiple oxidation states, and accessibility within a wide electrochemical window are the prime features of Nb that make it a favorable candidate for electrocatalysis, especially when it is combined with Pt. However, Nb has been used as a support in the form of oxides in all previously reported Pt–Nb electrocatalysts, and no Pt–Nb alloying phase has been demonstrated hitherto. Herein, we report a multifunctional Pt–Nb composite (PtNb/NbO_<i>x</i>-C) where Nb exists both as an alloying component with Pt and as an oxide support and is synthesized by means of a simple wet chemical method. In this work, the Pt–Nb alloy phase has been firmly verified with the help of multiple spectroscopic methods. This allows for the experimental evidence of the theoretical prediction that Pt–Nb alloy interactions improve the oxygen reduction reaction (ORR) activity of Pt. In addition, such a combination of multiphase Nb brings up myriad features encompassing increased ORR durability, immunity to phosphate anion poisoning, enhanced hydrogen oxidation reaction (HOR) activity, and oxidative carbon monoxide (CO) stripping, making this electrocatalyst useful in multiple fuel cell systems

Tailoring a Three-Phase Microenvironment for High-Performance Oxygen Reduction Reaction in Proton Exchange Membrane Fuel Cells

Despite tremendous progress in catalyst development for rate-limiting cathodic oxygen reduction reaction (ORR), reducing Pt usage while meeting performance requirements in practical proton exchange membrane fuel cells (PEMFCs) remains a challenge. The ORR in PEMFCs occurs at a catalyst–electrolyte–gas three-phase interface. A desirable interface should exhibit highly active and available catalytic sites, as well as allow efficient oxygen and proton feeding to the catalytic sites and timely removal of water to avoid interface flooding. Here, we report the design of a three-phase microenvironment in PEFMCs, showing that carbon surface chemistry can be tuned to modulate its interaction with the ionomers and create favorable transport paths for rapid delivery of both reactants and products. With such an elaborate interfacial design, for the first time we have demonstrated PEMFCs with all key ORR catalyst performance metrics, including mass activity, rated power, and durability, surpassing the US Department of Energy targets