Ultrafast field-driven monochromatic photoemission from carbon nanotubes

Ultrafast electron pulses, combined with laser-pump and electron-probe technologies, allow for various forms of ultrafast microscopy and spectroscopy to elucidate otherwise challenging to observe physical and chemical transitions. However, the pursuit of simultaneous ultimate spatial and temporal resolution has been largely subdued by the low monochromaticity of the electron pulses and their poor phase synchronization to the optical excitation pulses. State-of-the-art photon-driven sources have good monochromaticity but poor phase synchronization. In contrast, field-driven photoemission has much higher light phase synchronization, due to the intrinsic sub-cycle emission dynamics, but poor monochromaticity. Such sources suffer from larger electron energy spreads (3 - 100 eV) attributed to the relatively low field enhancement of the conventional metal tips which necessitates long pump wavelengths (> 800 nm) in order to gain sufficient ponderomotive potential to access the field-driven regime. In this work, field-driven photoemission from ~1 nm radius carbon nanotubes excited by a femtosecond laser at a short wavelength of 410 nm has been realized. The energy spread of field-driven electrons is effectively compressed to 0.25 eV outperforming all conventional ultrafast electron sources. Our new nanotube-based ultrafast electron source opens exciting prospects for attosecond imaging and emerging light-wave electronics

Flt3L controls the development of radiosensitive dendritic cells in the meninges and choroid plexus of the steady-state mouse brain

As shown by analyses of morphology, gene expression, antigen-presenting function, and Flt3 dependence, the steady-state mouse brain contains a population of DCs that exhibits similarities to splenic DCs and differences from microglia

Greenhouse gas emissions from U.S. crude oil pipeline accidents:1968 to 2020

Abstract Crude oil pipelines are considered as the lifelines of energy industry. However, accidents of the pipelines can lead to severe public health and environmental concerns, in which greenhouse gas (GHG) emissions, primarily methane, are frequently overlooked. While previous studies examined fugitive emissions in normal operation of crude oil pipelines, emissions resulting from accidents were typically managed separately and were therefore not included in the emission account of oil systems. To bridge this knowledge gap, we employed a bottom-up approach to conducted the first-ever inventory of GHG emissions resulting from crude oil pipeline accidents in the United States at the state level from 1968 to 2020, and leveraged Monte Carlo simulation to estimate the associated uncertainties. Our results reveal that GHG emissions from accidents in gathering pipelines (~720,000 tCO2e) exceed those from transmission pipelines (~290,000 tCO2e), although significantly more accidents have occurred in transmission pipelines (6883 cases) than gathering pipelines (773 cases). Texas accounted for over 40% of total accident-related GHG emissions nationwide. Our study contributes to enhanced accuracy of the GHG account associated with crude oil transport and implementing the data-driven climate mitigation strategies

Failure Behaviour of Jointed Rock Masses with 3D Nonpenetrating Joints under Uniaxial Compression: Insights from Discrete Element Method Modelling

It is well known that joints or fissures have an important effect on the failure mechanism of natural rocks. Previously, many numerical and experimental papers have been carried out to study the strength anisotropy and failure characteristics of jointed rocks. However, few studies have been carried out on the failure mechanism of nonpersistent jointed rock masses with different persistence, especially for nonpersistent joints in three dimensions. In the present study, the failure characteristics of a 3D nonpersistent jointed rock mass with different inclinations (θ) and persistence (K) are studied by numerical simulation. For the 3D digital elevation model (DEM), the linear parallel bond model (LPBM) and smooth-joint model (S-J) were used to model the rock-like material and joint interface, respectively. The connections between the geometric parameters of joints and peak strength are revealed. For the peak strength, the joint persistence only plays a minor role in specimens with inclinations of 0° and 90°, and its influence on strength is mainly reflected in the specimens with shear failure (θ = 45°, 60°, and 75°). Based on microcrack accumulation and evolution, four typical failure processes (shear failure, split failure, mixed failure, and intact failure) are analysed from the micro perspective. The shear stress evolution process on the 3D nonpersistent joint of the specimen with different inclinations under K1 = 0.42 was monitored by the measurement circle, and it was found that the distribution of shear stress inside the rock bridge is related to the failure mode of the specimen. For the specimens with θ = 0° and 90°, the shear stress had little change, indicating that there is slight shear slip behaviour on the joint surface. When the inclination is 45°, 60°, and 75°, the shear stress changes obviously during loading, indicating that the shear action is strong in this failure mode

Failure Behaviour of Jointed Rock Masses with 3D Nonpenetrating Joints under Uniaxial Compression: Insights from Discrete Element Method Modelling

It is well known that joints or fissures have an important effect on the failure mechanism of natural rocks. Previously, many numerical and experimental papers have been carried out to study the strength anisotropy and failure characteristics of jointed rocks. However, few studies have been carried out on the failure mechanism of nonpersistent jointed rock masses with different persistence, especially for nonpersistent joints in three dimensions. In the present study, the failure characteristics of a 3D nonpersistent jointed rock mass with different inclinations (θ) and persistence (K) are studied by numerical simulation. For the 3D digital elevation model (DEM), the linear parallel bond model (LPBM) and smooth-joint model (S-J) were used to model the rock-like material and joint interface, respectively. The connections between the geometric parameters of joints and peak strength are revealed. For the peak strength, the joint persistence only plays a minor role in specimens with inclinations of 0° and 90°, and its influence on strength is mainly reflected in the specimens with shear failure (θ = 45°, 60°, and 75°). Based on microcrack accumulation and evolution, four typical failure processes (shear failure, split failure, mixed failure, and intact failure) are analysed from the micro perspective. The shear stress evolution process on the 3D nonpersistent joint of the specimen with different inclinations under K1 = 0.42 was monitored by the measurement circle, and it was found that the distribution of shear stress inside the rock bridge is related to the failure mode of the specimen. For the specimens with θ = 0° and 90°, the shear stress had little change, indicating that there is slight shear slip behaviour on the joint surface. When the inclination is 45°, 60°, and 75°, the shear stress changes obviously during loading, indicating that the shear action is strong in this failure mode

Effect of organic additives on mechanical properties of SiC ceramics prepared by a modified gelcasting method

A novel and simple gel system of isobutylene and maleic anhydride (PIBM) was used to prepare SiC ceramics. The rheological behaviour of the SiC slurries was investigated as function of organic additives. The SiC slurries with 0.2 wt.% PIBM and 0.2 wt.% tetramethylammonium hydroxide (TMAH) showed low viscosity, which was favourable for casting SiC green bodies. In order to obtain homogeneous green bodies, polyvinyl alcohol (PVA) was used to assist the dispersion of carbon black in the slurries, and polyethylene glycol (PEG) was added to inhibit the surface exfoliation of green bodies. The content of PVA was controlled carefully to avoid the warpage of green bodies during the drying process. Finally, homogeneous defect-free SiC green bodies were successfully fabricated via aqueous gelcasting. The SiC ceramics sintered at 2100 °C (prepared from slurries with solid content of 60 wt.%) showed an average flexural strength of 305.7 MPa with porosity of 19.92%

Porous SiC ceramics fabricated by quick freeze casting and solid state sintering

Porous SiC ceramics with uniform microstructure were fabricated by quick freezing in liquid nitrogen and solid state sintering. Poly (vinyl alcohol) (PVA) was added as binder and pore morphology controller in this work. The microstructure and mechanical properties of porous SiC ceramics could be controlled by the composition of the aqueous slurries. Both solid content of the slurries and PVA content impacted on the pore structures and mechanical properties of the porous SiC ceramics. The solid content of slurries and PVA content varied from 60 to 67.5 wt% and 2–6 wt%, respectively. Besides, the grain morphology of ceramics was also tailored by changing the sintering temperature from 2050 to 2150 °C. Porous SiC ceramics with an average porosity of 42.72%, flexural strength of 59.28 MPa were obtained at 2150 °C from 67.5 wt% slurries with 2 wt% PVA