TENSOR LEED FOR THE GEOMETRICAL AND CHEMICAL STRUCTURE OF ALLOY SURFACES

1996 ◽  
Vol 03 (05n06) ◽  
pp. 1651-1661 ◽  
Author(s):  
K. HEINZ ◽  
R. DÖLL ◽  
M. KOTTCKE
Keyword(s):  
Direct Method ◽  
Easy Access ◽  
Disordered Alloys ◽  
Ordered Alloys ◽  
Low Energy Electron ◽  
The One ◽  
Further Development ◽  
A Direct Method ◽  
Structure Of Alloy ◽  
Surface Unit Cell

Low energy electron diffraction is still the most frequently used technique to retrieve the structure of surfaces. Multiple scattering of electrons on the one hand provides high structural precision but on the other hand also complicates the structural analysis and limits the complexity of accessible structures. This particularly applies to alloy surfaces with their enlarged variety of possible structures which comes by the increased size of the surface unit cell (chemically ordered alloys) or the unknown layer-dependent stoichiometry (disordered alloys). A way out of that dilemma comes by application and further development of the perturbation method Tensor LEED. We show that it can be extended to treat efficiently and accurately the chemical substitution of atoms. This allows easy access to the layer-dependent stoichiometry of chemically disordered alloys. Also, by periodic substitution of atoms, intensities calculated for an elemental crystal can be perturbed to yield the intensities for an ordered alloy. Moreover, we propose and test the application of a direct method which gives the deviations from bulk stoichiometry directly, i.e. without the usual trial and error procedure.

scholarly journals On a direct method for the determination of an exact invariant for the time-dependent harmonic oscillator

1977 ◽  
Vol 20 (1) ◽  
pp. 97-105 ◽  
Author(s):  
P. G. L. Leach
Keyword(s):  
Direct Method ◽  
Dynamical Symmetry ◽  
Time Dependent ◽  
Symmetry Groups ◽  
New Approach ◽  
One Dimensional ◽  
The One ◽  
A Direct Method ◽  
Exact Invariant

AbstractAn exact invariant is found for the one-dimensional oscillator with equation of motion . The method used is that of linear canonical transformations with time-dependent coeffcients. This is a new approach to the problem and has the advantage of simplicity. When f(t) and g(t) are zero, the invariant is related to the well-known Lewis invariant. The significance of extension to higher dimension of these results is indicated, in particular for the existence of non-invariance dynamical symmetry groups.

Atomic Structure of Binary Alloy Surfaces

1986 ◽  
Vol 83 ◽  
Author(s):  
H. L. Davis ◽  
J. R. Noonan
Keyword(s):  
Atomic Structure ◽  
Binary Alloys ◽  
Low Energy ◽  
Metallic Surfaces ◽  
Surface Layers ◽  
Low Energy Electron Diffraction ◽  
Specific Effects ◽  
Ordered Alloys ◽  
Low Energy Electron ◽  
Structure Of Alloy

ABSTRACTAfter discussions of multilayer relaxation for monatomic metallic surfaces and low-energy electron diffraction (LEED) procedures, some available results concerning the atomic structure of alloy surfaces are reviewed briefly. Results are discussed only for surfaces of ordered binary alloys and crystalline, substitutionally random binary alloys. Surfaces for which detailed results are available are highlighted as examples which illustrate some general crystallographic effects that occur at alloy surfaces. Specific effects included in the examples are the rippling of atomic constituents in the surface layers of ordered alloys, the preferential termination of alloys by a specific type of layer where more than one type of layer exists in the bulk, a mixture of two possible terminations, and the alternating enrichment and depletion of an atomic constituent in the surface layers of alloys which are disordered in the bulk.

Migration and US agricultural competitiveness

2013 ◽  
Vol 10 (2) ◽  
pp. 159-179 ◽  
Author(s):  
Philip L. Martin
Keyword(s):  
Farm Workers ◽  
Easy Access ◽  
Foreign Born ◽  
Guest Workers ◽  
Long Run ◽  
Short Run ◽  
Lower Wage ◽  
The Government ◽  
The One

Agriculture has one of the highest shares of foreign-born and unauthorized workers among US industries; over three-fourths of hired farm workers were born abroad, usually in Mexico, and over half of all farm workers are unauthorized. Farm employers are among the few to openly acknowledge their dependence on migrant and unauthorized workers, and they oppose efforts to reduce unauthorized migration unless the government legalizes currently illegal farm workers or provides easy access to legal guest workers. The effects of migrants on agricultural competitiveness are mixed. On the one hand, wages held down by migrants keep labour-intensive commodities competitive in the short run, but the fact that most labour-intensive commodities are shipped long distances means that long-run US competitiveness may be eroded as US farmers have fewer incentives to develop labour-saving and productivity-improving methods of farming and production in lower-wage countries expands.

scholarly journals Plastic waste to fuels by hydrocracking at mild conditions

2021 ◽  
Vol 7 (17) ◽  
pp. eabf8283
Author(s):  
Sibao Liu ◽  
Pavel A. Kots ◽  
Brandon C. Vance ◽  
Andrew Danielson ◽  
Dionisios G. Vlachos
Keyword(s):  
Direct Method ◽  
Acid Sites ◽  
Liquid Fuels ◽  
High Yield ◽  
Tandem Catalysis ◽  
Hy Zeolite ◽  
Environmental Threat ◽  
Single Use ◽  
Initial Activation ◽  
A Direct Method

Single-use plastics impose an enormous environmental threat, but their recycling, especially of polyolefins, has been proven challenging. We report a direct method to selectively convert polyolefins to branched, liquid fuels including diesel, jet, and gasoline-range hydrocarbons, with high yield up to 85% over Pt/WO3/ZrO2 and HY zeolite in hydrogen at temperatures as low as 225°C. The process proceeds via tandem catalysis with initial activation of the polymer primarily over Pt, with subsequent cracking over the acid sites of WO3/ZrO2 and HY zeolite, isomerization over WO3/ZrO2 sites, and hydrogenation of olefin intermediates over Pt. The process can be tuned to convert different common plastic wastes, including low- and high-density polyethylene, polypropylene, polystyrene, everyday polyethylene bottles and bags, and composite plastics to desirable fuels and light lubricants.

Sign in / Sign up

Export Citation Format

Share Document