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Surface segregation and solute trapping during planar film growth is one of the important issues in molecular beam epitaxy, yet the study on surface composition has been largely restricted to experimental work. This paper introduces some mathematical models of surface composition during planar film growth. Analytical solutions are obtained for the...
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Context 1
... c satisfies c 1 to make sure that x = kx x 0 for 0 x 1. Figure 5 illustrates fx for different c. ...
Context 2
... u = 4vãds4vãds v ˜c + 1+v˜11+v˜1−c 2 and C 1 is a constant of integration. Figure 6 shows how the results for step modulation dop- ing from Fig. 5 are modified using the nonlinear model. For this comparison, we pick c so that the nonlinear model has the same effective for x 1 as in the results in Fig. 3. In Fig. 6, x always stays 1 as expected. Also, since when x is relatively small, fx is close to the linear function with slope 1 − c, the model for different could correspond to ...
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Citations
... The rocksalt phase is not the only structure we have found experimentally with a composition far from conventional thermodynamic expectations. For nickel-chromium-iron alloys and nickelchromium-molybdenum-alloys an oxide with the corundum structure ( 3 � ) has been reported 21,26,[35][36][37][38] , and assumed to be chromia. We can verify the existence of an oxide with a corundum structure consistent with the existing literature, in some cases it is chromia Cr2O3 but in other cases careful compositional analysis demonstrates a composition of NixCr2-xO3-y with x~1 as shown in Fig. 2, here for a sample oxidized in-situ at 700 o C in 1x10 -4 torr of oxygen. ...
... In the general case, the partitioning across a moving interface is well documented for the solidification case (e.g. 5,7,34 ) and has been evaluated for a at least one related cases, namely during thin film growth 35 . The partitioning of a solute between the liquid and solid as a function of the velocity of the interface ( ) can be written as 7 ...
If all humans vanished tomorrow, almost every metal structure would collapse within a century or less, the metal converting to an oxide. In applications ranging from the mature technology of nuts and bolts to high technology batteries, nuclear fuels and turbine engines, protective oxide films are critical to limiting oxidation. To date models of these oxide films have assumed that they form thermodynamic equilibrium stable or metastable phases doped within thermodynamic solubility limits. Here we demonstrate experimentally and theoretically the formation of unusual non-equilibrium oxide phases, that can be predicted using a scientific framework for solute capture at a moving oxide/substrate interface. The theory shows that solute capture is likely a generic process for many electrochemical processes, and suggests that similar phenomena yielding non-equilibrium phases can occur and be predicted for a wide range of other processes involving solid-fluid and solid-solid chemical reactions.
... The papers Spencer & Blanariu (2005), Tu & Tersoff (2004) and Spencer et al. (2001) address theory for intermixing but have some limitations that the composition is not tracked during the whole process of island growth. The paper Han & Spencer (2007) starts to study surface composition analytically and introduces some mathematical models of surface composition and intermixing with the substrate during planar film growth. The focus of the present paper is to develop a model for the composition of growing islands. ...
... Following a similar idea to that of Han & Spencer (2007), pure atoms B are deposited on the substrate A ( Fig. 1), forming a wetting layer. At the same time, interlayer diffusion between the wetting layer and its adjacent substrate causes the intermixing of the substrate with the wetting layer. ...
... We use the same nomenclature as Han & Spencer (2007) and consider the concentration of A and B in the wetting layer (x A σ and x B σ , respectively). The processes affecting the wetting layer are absorption from the vapour, diffusional intermixing with the substrate and surface diffusion of the wetting layer to the island. ...
This paper describes a simple mathematical model for the development of non-uniform composition during the growth of quantum dot (QD) islands (solid drops) on a substrate. The purpose of the model is to quantify a known phenomenon in such systems: deposition of pure germanium onto a silicon substrate results in a QD, which is not pure germanium. The intermixing is due to the interdiffusion between the wetting layer surrounding the island and the underlying substrate. Growth of the island occurs by surface diffusion of the intermixed wetting layer to the island surface, resulting in the growth of compositionally non-uniform islands. In this paper, we develop a simple model for the time evolution of composition in the wetting layer, which can then be used to determine the island composition. The resulting system is a history-dependent integrodifferential equation for the composition of the wetting layer. Numerical solutions were found and verified to be in good qualitative agreement with experimental results.
We report experimental results on the composition and crystallography of oxides formed on NiCrMo alloys during both high-temperature oxidation and aqueous corrosion experiments. Detailed characterization using transmission electron microscopy and diffraction, aberration-corrected chemical analysis, and atom probe tomography shows unexpected combinations of composition and crystallography, far outside thermodynamic solubility limits. The results are explained using a theory for nonequilibrium solute capture that combines thermodynamic, kinetic, and density functional theory analyses. In this predictive nonequilibrium framework, the composition and crystallography are controlled by the rapidly moving interface. The theoretical framework explains the unusual combinations of composition and crystallography, which we predict will be common for many other systems in oxidation and corrosion, and other solid-state processes involving nonequilibrium moving interfaces.
Control over structure and composition of (ABO3) perovskite oxides offers exciting opportunities since these materials possess unique, tunable properties. Perovskite oxides with cobalt B-site cations are particularly promising, as the range of the cation’s stable oxidation states leads to many possible structural frameworks. Here, we report growth of strontium cobalt oxide thin films by molecular beam epitaxy, and conditions necessary to stabilize different defect concentration phases. In situ X-ray scattering is used to monitor structural evolution during growth, while in situ X-ray absorption near-edge spectroscopy is used to probe oxidation state and measure changes to oxygen vacancy concentration as a function of film thickness. Experimental results are compared to kinetically-limited thermodynamic predictions, in particular, solute trapping, with semi-quantitative agreement. Agreement between observations of dependence of cobaltite phase on oxidation activity and deposition rate, and predictions indicates that a combined experimental/theoretical approach is key to understanding phase behavior in the strontium cobalt oxide system.
A liquid quinary alloy with composition Ni–5%Cu–5%Fe–5%Sn–5%Ge has been prepared from a containerless state by undercooling. Dendritic growth of α-Ni phase took place with a velocity of 28 m s−1 at the maximum degree of undercooling, which was as high as 405 K (0.24T L). All of the four solute elements Cu, Fe, Sn and Ge exhibited a significant solute trapping effect during the rapid dendrite growth. Segregation-less solidification is consequently realized when the degree of undercooling is sufficiently large. The lattice constant of α-Ni solid solution phase is found to increase with the amount of multicomponent solute trapping.
Aim of this research is to propose an accurate model of measuring business excellence by applying the theory of System Dynamics. This theory defines the interaction of system variables giving the opportunity of a reliable and fair score as well as objective decision-making with regard to the areas that are susceptible of continuous improvement. This paper utilises the principles of the System Dynamics in order to propose a new method for measuring business excellence and thus contributing to a thorough evaluation of an enterprise which implements a Business Excellence Model. The proposed business excellence system is adopted and applied as the Hellenic National Quality Award scheme. The study only adopts empirical data from the application of the proposed business excellence system due to the early stages of implementation in the market. Future research may use more data to better tune the proposed method. Based on the more accurate analytical results achieved by the proposed model, the Hellenic national authority of managing the Hellenic scheme of business excellence might use the proposed model for measuring business excellence and to drive continuous improvement and competitiveness according to its strategy.
