ABSTRACT
Ductile metals such as Ni and Cu can become brittle when certain impurities (e.g., Bi) diffuse and segregate into their grain boundaries (GBs). Using first-principles calculations, we investigate the microscopic origin of the Bi-induced loss of cohesion of Ni and Cu GBs. We find that the Bi bilayer interfacial phase is the most stable impurity phase under the Bi-rich condition, while the Bi monolayer phase is a metastable phase regardless of the value of the Bi chemical potential. Our finding is consistent with the recent experimental observation for Ni GBs [Luo et al. Science 333, 1730 (2011)]. The electric polarization effect of the Bi bilayer substantially enhances the strength of the Bi-metal interfacial bonds, stabilizing the bilayer phase over other phases. The Bi-Bi interlayer bonding is significantly weakened in the GBs, leading to a factor of 20 to 50 decrease in the GB cohesion, which has strong implications for the understanding of Bi-induced intergranular fracture of Ni and Cu polycrystals.
ABSTRACT
When compressive strain is applied to a single-layered material, the layer generally ripples along the third dimension to release the strain energy. In contrast, such a rippling effect is not favored when it is under tensile strain. Here, using first-principles density-functional calculations, we show that molecular adsorption on boron-doped graphene (BG) can be largely tuned by exploiting the rippling effect of the strained graphene. Under tensile strain, the adsorption energy of K2CO3, NO2, and NH3 on BG, for which the molecular adsorption is a chemisorption characterized by a covalent B-molecule bond, exhibits a superlinear dependence on the applied strain. In contrast, when microscopic ripples are present in the BG under compressive strain, the adsorption strength is significantly enhanced with increasing the strain. Such a nonlinear and asymmetric effect of strain on the molecular adsorption is a characteristic of two-dimensional systems, because a general elastic theory of molecular adsorption on three-dimensional systems gives a linear and symmetric strain effect on the adsorption strength. We provide the underlying mechanism of the anomalous strain effect on the chemical molecular adsorption on BG, in which the microscopic rippling of the graphene and the creation of the π-dangling bond state near the Dirac point play an important role. Our finding can be used to modify chemical reactivity of graphene with a wide range of application.
ABSTRACT
The atomic and electronic structures of the liquid Al/(0001) α-Al(2)O(3) interfaces are investigated by first-principles molecular dynamics simulations. Surprisingly, the formed liquid-solid interface is always atomically abrupt and is characterized by a transitional Al layer that contains a fixed concentration of Al vacancies (~10 at.%). We find that the self-regulation of the defect density in the metal layer is due to the fact that the formation energy of the Al vacancies is readjusted in a way that opposes changes in the defect density. The negative-feedback effect stabilizes the defected transitional layer and maintains the atomic abruptness at the interface. The proposed mechanism is generally applicable to other liquid-metal/metal-oxide systems, and thus of significant importance in understanding the interface structures at high temperature.
ABSTRACT
Researchers from the National Renewable Energy Laboratory recently conducted a pilot-scale study at McClellan Air Force Base (AFB) in Sacramento, CA. The objective of the test was to determine the effectiveness of an ambient-temperature, solar-powered photocatalytic oxidation treatment unit for destroying emissions of chlorinated organic compounds from an air stripper. This paper reports test results and discusses applications and limitations of the technology. A 10-standard-cubic-foot-per-minute (SCFM) (28.3 L/min) slip stream of air from an air stripper at Operative Unit 29-31 at McClellan AFB was passed through a reactor that contained a lightweight, perforated, inert support coated with photoactive titanium dioxide. The reactor faced south and was tilted at a 45° angle from vertical so that the light-activated catalyst received most of the available sunlight. An online portable gas chro-matograph with two identical columns simultaneously analyzed the volatile organic compounds contained in the reactor inlet and outlet air streams. Summa canister grab samples of the inlet and outlet were also collected and sent to a certified laboratory for U.S. Environmental Protection Agency Method TO-14 analysis and verification of our field analyses. Three weeks of testing demonstrated that the treatment system's destruction and removal efficiencies (DREs) are greater than 95% at 10 SCFM with UV intensities at or greater than 1.5 milliwatts/square centimeter (mW/cm2). DREs greater than 95% at 20 SCFM were obtained under conditions where UV irradiation measured at or greater than 2 mW/cm2. In Sacramento, this provided 6 hours of operation per clear or nearly clear day in April. A solar tracking system could extend operating time. The air stream also contained trace amounts of benzene. We observed no loss of system performance during testing.
