Modeling of Compositional Grading in Heavy Oil Fields

Field data for the compositional variation with depth in five petroleum reservoirs have been analyzed including one rich in asphaltenes with a very high compositional gradient. The data follows the well-known pattern with an increasing concentration of heavy components with depth. The observed compositional variation is higher than what could be explained by gravity segregation alone. An enthalpy term was added to take into account that the components have different preferences for residing at a higher or a lower temperature. It was found necessary to modify the enthalpy term to account for the impact of fluid viscosity and concentration of aromatic components on the relative rate at which smaller and larger molecules move in a fluid column. With this modification, a good match was seen of the compositional variation in the five reservoirs initially analyzed. The method was successfully tested on three other reservoirs, not part of the initial analysis, one of which had a fluid column of 970 m.

Recent Advances in the Modeling of Strain Relaxation and Dislocation Dynamics in InGaAs/GaAs (001) Heterostructures

In this paper we describe state-of-the-art approaches to the modeling of strain relaxation and dislocation dynamics in InGaAs/GaAs (001) heterostructures. Current approaches are all based on the extension of the original Dodson and Tsao plastic flow model to include compositional grading and multilayers, dislocation interactions, and differential thermal expansion. Important recent break-throughs have greatly enhanced the utility of these modeling approaches in four respects: i) pinning interactions are included in graded and multilayered structures, providing a better description of the limiting strain relaxation as well as the dislocation sidewall gettering; ii) a refined model for dislocation-dislocation interactions including zagging enables a more accurate physical description of compositionally-graded layers and step-graded layers; iii) inclusion of back-and-forth weaving of dislocations provides a better description of dislocation dynamics in structures containing strain reversals, such as strained-layer superlattices or overshoot graded layers; and iv) the compositional dependence of the model kinetic parameters has been elucidated for the InGaAs material system, allowing more accurate modeling of heterostructures with wide variations in composition. We will describe these four key advances and illustrate their applications to heterostructures of practical interest.

Thermodynamic stability, phase separation and Ag grading in (Ag,Cu)(In,Ga)Se2 solar absorbers

Herein, we prove that (Ag,Cu)(In,Ga)Se2 alloy system has a wide miscibility gap, which can induce compositional grading and cause phase separation in thin-film solar absorbers.

Sub–single-exciton lasing using charged quantum dots coupled to a distributed feedback cavity

Colloidal semiconductor quantum dots (QDs) are attractive materials for realizing highly flexible, solution-processable optical gain media, but they are difficult to use in lasing because of complications associated with extremely short optical-gain lifetimes limited by nonradiative Auger recombination. By combining compositional grading of the QD’s interior for hindering Auger decay with postsynthetic charging for suppressing parasitic ground-state absorption, we can reduce the lasing threshold to values below the single-exciton-per-dot limit. As a favorable departure from traditional multi-exciton–based lasing schemes, our approach should facilitate the development of solution-processable lasing devices and thereby help to extend the reach of lasing technologies into areas not accessible with traditional, epitaxially grown semiconductor materials.