128 Search Results

Materials that exhibit negative thermal expansion (NTE) are of fundamental interest due to their rarity and counterintuitive behavior. While research in this area has been directed toward the discovery of materials that display NTE and explaining its origin, there has been less attention to describing the complexities of a secondary phase or other guests that could influence the magnitude and mechanism of NTE. We report herein that zeolite-templated carbon (ZTC), a soft carbonaceous framework solid with ordered microporosity, exhibits a large and widely tunable thermal expansion in the presence of adsorbed guests. For ZTC in the presence of CO₂ atmore » 1 bar, the largest coefficient of isotropic NTE ever observed (−8.4 × 10^–4 K^–1) is measured between 200 and 220 K. These results comprise a tunable mechanism of thermal expansion based on the interaction between two independent, positively expanding phases that together give rise to an anomalous guest-induced NTE under certain conditions.« less

Oxygen-terminated diamond has a wide breadth of applications, which include stabilizing near-surface color centers, semiconductor devices, and biological sensors. Despite the vast literature on characterizing functionalization groups on diamond, the chemical composition of the shallowest portion of the surface (<1 nm) is challenging to probe with conventional techniques like XPS and FTIR. In this work, we demonstrate the use of angleresolved XPS to probe the first ten nanometers of both oxygen and hydrogen terminated (100) single-crystalline diamond grown via chemical vapor deposition (CVD). With the use of consistent peakfitting methods, the peak identities and relative peak binding energies were identifiedmore » for sp² carbon, ether, hydroxyl, carbonyl, and C−H groups for both of these diamond surface terminations. For the oxygen-terminated sample, we also quantified the thickness of the sp² carbon layer situated on top of the bulk sp³ diamond bonded carbon to be 0.3 ± 0.1 nm, based on the analysis of the Auger electron spectra and D-parameter calculations. These results indicate that the majority of the oxygen is bonded to the sp² carbon layer on the diamond, and not directly to the sp³ diamond bonded carbon.« less

Direct air capture (DAC) systems often consist of packing material wetted by a capture fluid that reacts with CO₂ in the airstream. The efficiency of the contactor is determined by a complex relationship of fluid dynamics, heat and mass transfer, contactor geometry, and chemical properties. The efficiency of the contactor must be balanced with other factors, primarily pressure drop through the system. Triply periodic minimal surfaces (TPMS) are a class of differential surfaces that have been explored in multiple engineering applications and have been shown to exhibit excellent performance when used in heat exchangers. Their tortuous path provides a highmore » surface-to-volume ratio and favorable trade-off between contact area and pressure drop. In this work, a gyroid-type TPMS contactor was evaluated using computational fluid dynamics for a variety of geometric parameters to explore the potential benefit of TPMS shapes for DAC applications. A thin-film model was employed to model the flow and distribution of the capture solvent, allowing efficient simulations of TPMS structures at scale by eliminating the need for a computationally intensive interface capturing method. A liquid-gas mass transfer model was implemented in the commercial software STAR-CCM+ and used to predict the CO₂ capture efficiency and study the trade-off between capture performance and pressure drop through analysis of capture rates, mass transfer coefficients, and other relevant variables. TPMS contactors with a variety of geometric parameters and two capture solvent options were investigated to determine the effect of design choices on the operational performance of DAC systems. In conclusion, results showed that while contactor geometry is the dominant factor in efficiency and pressure drop, the physiochemical properties of the solvent are an important secondary influence on the contactor performance.« less

This report reviews the tribological and tribocorrosion performance of steels that are case-hardened via boriding, chromizing, carburizing, nitriding, nitrocarburizing, and carbonitriding. Case-hardening is commonly used to improve the hardness, impact durability, wear resistance, and corrosion resistance of steel alloys and has been successfully applied in various industries, providing a cost-effective, high-throughput solution for applications involving contact and sliding interfaces in complex service environments. This article summarizes the literature results of the wear and friction behavior of common case-hardening methods for steel alloys under various conditions, including corrosive environments. Special attention is given to the influences of case-hardening process parameters andmore » alloy composition on tribological performance. Furthermore, by discussing key findings from the literature, this review provides insights into optimizing case-hardening processes for improving the tribological and tribocorrosion performance of steel alloys.« less

Stacking and twisting 2D van der Waals (vdW) layers have become versatile platforms to tune the electron correlation. These platforms rely on exfoliating vdW materials down to a single vdW layer and a few vdW layers. We calculate the intrinsic layer-dependent surface and exfoliation energies of typical vdW materials such as graphite, h-BN, black P, MX₂ (M = Mo or W; X = S, Se, or Te), MX (M = Ga or In; X = S, Se, or Te), Bi₂Te₃, and MnBi₂Te₄ using density functional theory. For exchange-correlation functionals with explicit vdW interaction, a single vdW layer always has themore » smallest surface energy, giving a surface energy reduction when compared to that of thicker vdW layers. Furthermore, the magnitude of this surface energy reduction quickly decreases with an increase in the number of atomic layers inside the single vdW layer for different vdW materials. Such atomic-layer dependence in surface energy reduction helps explain the different effectiveness of exfoliation for different vdW materials down to a single vdW layer.« less

Soil organic matter (SOM) is a key reservoir for global carbon (C), supporting soil fertility and influencing greenhouse gas emissions. Microbial residues, composed of dead cells and cellular fragments, are major contributors to SOM formation. Yet, mechanisms by which minerals enhance the accumulation of microbial residues remain poorly understood. Here, we used ¹³C-labeled glucose in a year-long incubation to trace microbial residue in sandy and silty soils. Across both soils, approximately 89% of retained microbial ¹³C was recovered in the fine (<53 μm) mineral-associated organic matter (MAOM) pool. Within this pool, the light MAOM fraction, enriched in poorly crystalline Femore » minerals, held 4.3 times more ¹³C than the heavy, phyllosilicate-dominated MAOM fraction, despite accounting for only 17.2% of the total MAOM mass and 12.3% of the total soil mass. Along with ¹³C enrichment, the light MAOM fraction showed greater abundance of N-containing groups, e.g., (amides and amino groups), indicative of microbial-derived compounds like proteins and amino sugars. Fe oxides in light MAOM from both soils were spatially dispersed. Microbial residue accumulation was greater in finer-textured silty soil. These findings demonstrate that mineral composition and texture jointly regulate microbial necromass accrual, highlighting light MAOM as a key pool for enhancing soil C storage.« less

Air-processed carbon-based perovskite solar cells (C-PSCs) offer scalable and cost-effective photovoltaic manufacturing but face efficiency loss compared to metal-contact perovskite solar cells. Surface passivation of three-dimensional (3D) perovskites with two-dimensional (2D) perovskite layers has emerged as a promising strategy to enhance device performance. However, the mechanisms by which 2D perovskites more effectively improve C-PSC efficiency and stability remain underexplored. This study investigates the efficacy of 2D/3D heterostructures using n-hexylammonium bromide (C6Br), phenethylammonium iodide (PEAI), and n-octylammonium iodide (OAI) as surface passivators for C-PSCs. C-PSCs treated with C₆Br achieved a champion power conversion efficiency (PCE) of 21.0%. This enhancement is attributedmore » to superior defect passivation, improved charge extraction, and suppressed non-radiative recombination. Transient ion-drift characterization demonstrates that C₆Br and OAI reduce ionic conductivity by 2–3 orders of magnitude, correlating with enhanced operational stability under continuous illumination. Our findings highlight the role of short-chain bromide cations (C₆Br) in optimizing halide-mediated defect healing and interfacial band alignment, positioning 2D-passivated C-PSCs as viable competitors to conventional metal-contact perovskite solar cells.« less

The formation of carbon–carbon interlinkage bonds (CCIBs) via the chemical binding of interlayer carbon atoms of many sp²-bonded carbon precursors is an essential step for synthesizing various diamond and diamond-like materials. Although the existence of CCIBs may be reasonably assumed under high-pressure conditions, direct experimental evidence has been scarce. Micro-Raman spectroscopy is here employed to track in situ the evolution of C–C bonds in a pressure range from ambient to 54 GPa. A pressure-induced two-stage (polynomial and linear) shift of the G peak and new generation of the CCIB peak at about 1550 cm^–1 are observed in multiple types ofmore » layer-structured carbon precursors, including glassy carbon, natural graphite, and carbon nanotubes. In conclusion, the experimental discovery of CCIBs holds significance in comprehending phase transitions of sp²-bonded carbon materials and has implications for the advancement of novel carbon structures.« less

Generating hydrogen from local energy resources such as solar or wind would unlock a low-carbon energy carrier that could be used to reduce greenhouse gas emissions in sectors such as industry and transportation. Yet, the allocation of new or existing power generation solely to hydrogen production remains contentious due to disputes regarding emissions accounting. Photocatalytic (PC) hydrogen production technologies offer a unique solution, as hydrogen is produced directly from solar energy and water, without the need for electricity generation. However, cost projections for all photocatalytic designs to date have suggested that they are not cost competitive compared to conventional electrolysismore » systems manufactured at scale. Herein, we offer the first illustrative benchmark of cost and carbon intensity of hydrogen produced in a type 2 “Z-scheme” photocatalytic reactor design, which employs suspended semiconducting nanoconductor particles organized into two stacked volumes in a raceway design. The “Z-scheme” system utilizes two separate photoabsorber particles, tuned to drive either the hydrogen evolution reaction or the oxygen evolution reaction individually, connected via a reversible, charge transfer redox couple in solution. Furthermore, the results suggest a highly competitive and scalable technology, that justifies further experimental validation and prototyping in the field.« less

An elucidation of the underlying formation pathways to acyclic hydrocarbons such as polyynes (C_nH₂), cumulenes (C_nH₂), and linear resonantly stabilized linear radicals (l-C_nH) is indispensable to understand the hydrocarbon chemistry in extreme low and high temperature environments. In this study, we exploited the crossed molecular beam technique to investigate the reaction of tricarbon C₃(X¹Σ_g⁺) with diacetylene (butadiyne; HCCCCH; X¹Σ_g⁺) at a collision energy of 47 ± 1 kJ mol^⁻1. The experimental data were merged with ab initio calculations of the singlet C₇H₂ potential energy surface (PES) revealing that the reaction is initiated via the formation of an initial van dermore » Waals reactant complex in the entrance channel. Subsequent rearrangements lead to various carbene-type and cyclic intermediates via ring-opening, ring-closure, and hydrogen migration processes eventually forming acyclic C₇H₂ isomers prior to their barrierless unimolecular decomposition to the most stable linear isomer, heptatriynylidyne (C₇H, X²Π) in an overall endoergic reaction (+57 kJ mol^⁻1). The reaction exhibits strong similarities to the tricarbon – acetylene (C₃ – C₂H₂). Furthermore, the significant energy threshold suggests that the tricarbon reaction with (poly)acetylenes forming resonantly stabilized linear radicals are open in high-temperature environments such as combustion flames and circumstellar envelopes of carbon stars and planetary nebulae as their descendants; however, these reactions are closed in low-temperature environments as in cold molecular clouds and hydrocarbon-rich atmospheres of planets and their moons such as in Titan.« less