the fire assay reloaded

The fire assay process is still the most accurate and precise method for measuring the gold content in gold alloys. Scanning electron microscopy and transmission electron microscopy have been applied to observe the change in microstructure of the samples undergoing the fire assay process. The performed observations reveal that the microstructure of the specimen is more complex than expected. Before the parting stage, the specimen is not a perfect gold–silver binary alloy but contains also copper–silver oxides and other residual compounds. The parting stage appears to be a dealloying process leading to a nanoporous gold nanostructure. What observed after partition explains the evolution of the shape and colour of the specimen and may allow for a better comprehension of the procedure and an improvement in the method

Evolving surface finite element method for the Cahn-Hilliard equation

We use the evolving surface finite element method to solve a Cahn- Hilliard equation on an evolving surface with prescribed velocity. We start by deriving the equation using a conservation law and appropriate transport for- mulae and provide the necessary functional analytic setting. The finite element method relies on evolving an initial triangulation by moving the nodes according to the prescribed velocity. We go on to show a rigorous well-posedness result for the continuous equations by showing convergence, along a subse- quence, of the finite element scheme. We conclude the paper by deriving error estimates and present various numerical examples

Numerical simulation of dealloying by surface dissolution via the evolving surface finite element method

In this article we formulate a numerical method for the simulation of dealloying of a binary alloy by the selective removal of one component via electrochemical dissolution such that there is phase separation of the other component at the solid/electrolyte interface. The evolution of the interface is modelled by a forced mean curvature flow strongly coupled to the solution of a Cahn-Hilliard equation modelling surface phase separation. The method is based on a triangulated hypersurface whose evolution is calculated as well as the solution of the Cahn-Hilliard equation by the evolving surface finite element method (ESFEM). The numerical experiments simulate complex morphology and concentration patterns providing evidence that the mathematical model may describe the formation of nanoporosity. (C) 2008 Published by Elsevier Inc

Digitalization and computational thinking in lower secondary science education using the example of paper chromatography

Paper chromatography is a simple and harmless experiment suitable for lower secondary education. Besides the training of subject-specific contents, paper chromatography allows the application of modelling and simulation since the submicroscopic processes can be simplified in a way that they become understandable in lower secondary education. Here, the application of digital tools in addition to the experimental performance of paper chromatography in a compulsory STEM course in grade 6 is reported. Following a series of experiments, chromatography is at first simulated by a board game and then by computer simulations. Such simulation program allows the variation of the parameters in order to explore the model and to introduce the particle concept of matter and molecular interactions. The implementation with a graphical programming environment such as Scratch also allows the programming by pupils in this age group. In this way, it is possible to introduce modelling and simulation in an early stage as important contributions to scientific work in natural sciences