A self-consistent optimization of multicomponent solution properties: ab initio molecular dynamic simulations and the MgO-SiO_2 miscibility gap under pressure

We propose a new approach to parameterizing the Gibbs energy of a multicomponent solution as a function of temperature, pressure and composition. It uses the quasichemical model in the second nearest neighbour approximation and considers both a polynomial representation (for low pressure) and an exponential decay representation (for moderate-to-high pressure) of the excess molar volume v^(xs) to extend thermodynamic behaviour to elevated pressure. This approach differs from previous configuration-independent regular or associated solution-type models of multicomponent silicate liquids at elevated pressure and can account for any structural or short-range order data that may be available. A simultaneous least squares fit of the molar volume and the molar enthalpy of mixing data obtained from First Principles Molecular Dynamics (FPMD) simulations at various pressures enables complete parameterization of the excess thermodynamic properties of the solution. Together with consistently optimized properties of coexisting solids, this enables prediction of pressure-temperature-composition phase diagrams associated with melting. Although the method is extensible to natural multicomponent systems, we apply the procedure as a first test case to the important planetary model system MgO-SiO_2 using FPMD data found in the literature. One key result of this optimization, which depends only on the derived excess properties of the liquid phase, is that the consolute temperature of the SiO_2-rich miscibility gap is predicted to decrease with increasing pressure. This appears to be in disagreement with available experimental constraints and suggests possible thermodynamic inconsistency between FPMD data and experimental phase equilibrium data in the 0-5 GPa pressure range. We propose a new thermodynamic consistency criterion relating the signs of v^(xs) and other excess properties and discuss the need for precise calculations of derivatives of excess properties. Finally, the potential reappearance of the miscibility gap in the MgO-SiO_2 system above 5 GPa is discussed in light of this work

Thin Wall Structure by Welding

Manufacturing of thin wall structure by wire arc additive manufacturing (WAAM) is on the main application of additive manufacturing. 3D-printing technology has significant advantages over traditional milling and machining techniques or welded analogs. Thin wall structure constitutes an essential and growing proportion of engineering construction, within common areas as in structural aerospace and large scale-components. The dissertation utilized a layer-wise production technique known as gas arc tungsten arc welding (GTAW), performed by a programmed KUKA-30 robot. The thesis aspect of welded structures is the degree of how disposable the product is after manufactured, due to the different set of welding parameters. Therefore are an investigation of residual stresses and deformation implemented by different structure geometries. The research includes two practical and analytical experiment tests in addition to an FEA-simulation. The experiments involve; ultrasound measurement by a self-programmed measuring device developed by BiT, calculation due to measured deformation along the welding length and simulation performed in ANSYS. Findings of the methods implicate an estimated value of residual stresses and distortion in the thin wall structure and substrate. Through ten tests of the welding process can the technique of this technology state as slow with frequently sources of error, using the KUKA-30 robot welding system for a certain height. The level of residual stresses depends on the severity of the manufacturing process, which this research confirmed a generally low value along the length of the structural components and base plate due to the parameters developed in this study

Development, programming and start-up of an interchangeable 3D-printing module

This report has as main objective the development and application of a 3D printing module (additive manufacturing) in a computer numeric control (CNC) milling machine (subtractive manufacturing) creating a hybrid manufacturing environment that could offer the advantages of both methods. Every CNC milling machine using this 3D printing module could be converted in a 3D printer by changing from a regular tool to the 3D printing module which is applied in the spindle of the machine in a very simple process. This module is equipped with a system capable of reading the spindle rotation speed, and uses that information to set up different commands and actions. Focused in the development of a low-cost system, there is used an Arduino board to control all the systems needed to work with the module. Most of the parts of the module are printed in a 3D printer that uses the stereolithograph technology, being able to create parts with complex shapes, high precision and good surface finishing. The experimental results obtained in the first tests were not as expected. Many problems that haven’t been taken in consideration when the initial development of the module was done. Many solutions were found and some corrections were done to eliminate or minimize those problems. The temperature control system and the revolutions per minute reading system shown very good results in the experimental tests. The biggest issue faced was related with filament feeding system. Many structural modifications were implemented to improve it, with better performance, obtaining acceptable final results, however with significant possibilities for improvement

Thermodynamic modeling of the (Mg, Al)-Ca-Zn systems

Critical assessment of the experimental data and re-optimization of the binary Mg-Zn, Ca-Zn, Al-Zn, Al-Ca systems and the Laves phase in the Mg-Ca system have been performed. A Comprehensive thermodynamic database of the Mg-Ca-Zn and Al-Ca-Zn ternary systems is presented from the constituent binary systems using suitable extrapolation methods. All available as well as reliable experimental data both for the thermodynamic properties and phase boundaries are reproduced within experimental error limits. In the present assessment, the Modified Quasichemical Model in the pair approximation is used for the liquid phase to account for the presence of the short-range ordering properly. The intermediate solid solutions are modeled using the compound energy formalism. Since the literature included contradicting information regarding the ternary compounds in both ternary systems, thermodynamic modeling of phase equilibria is used to determine the most likely description of the two ternary systems and to exclude the self-contradicting experimental observations. The constructed database is used to calculate both the integral and partial thermodynamic properties of the constituent binary systems. Moreover, the liquidus projections, isothermal sections and vertical sections of the ternary systems are also calculated and the invariant reaction points are predicted using the constructed database

Oxidation kinetics of metallic powders

Aluminum and magnesium are widely used in pyrotechnic formulations and other energetic materials; they are also common components of reactive alloys, e.g., Al-Mg and B-Mg, and others, which are potential fuels for explosives and propellants. Reaction mechanisms and oxidation kinetics of aluminum, magnesium and Al-Mg alloy powders in different oxidizing environments are investigated using thermo-analytical measurements. New methods of data processing are developed, relying on measured particle size distributions of the reactive spherical powders. It became possible to identify the reaction interface location for many heterogeneous metal oxidation processes; for several reactions, detailed kinetic descriptions are obtained. For aluminum powders, location of the reaction interface is established for oxidation in steam and liquid water. Stage-wise oxidation behavior is observed and interpreted. The oxidation of aluminum covered by a thin natural oxide layer in oxygen occurring at relatively low temperatures is quantitatively characterized using different types of thermo-gravimetric (TG) measurements with increased amount of powder for greater sensitivity. Activation energy and the pre-exponent are determined as a function of reaction progress using isoconversion processing and assuming a diffusion-limited reaction mechanism. The reaction kinetics is also established for aluminum nanopowders. It is shown that the oxidation mechanism established for micron sized aluminum remains valid for particles as small as 10 nm. Aluminum oxidation model is combined with a heat transfer model to describe ignition of aluminum particles exposed to a heated oxidizing environment. For magnesium powders, their oxidation by both oxygen and steam was studied by thermo-analytical measurements for micron-sized powders. The location of reaction interface is identified using experiments with spherical powders with different but overlapping particle size distributions. The reaction is found to occur at the interface of metal and the growing oxide layer for all oxidizing conditions. Thus, the reaction is rate limited by diffusion of oxidizer to the metal surface. Reaction rates for low and elevated temperatures are quantified using heat flow calorimetry and TG measurements, respectively. Simplified diffusion-limited reaction models are developed for oxidation of magnesium in both oxygen and steam. The models enable one to predict both pre-ignition reactions and the time of Mg powder aging when exposed to moisture or oxygen at different temperatures. Finally, the mechanisms of low-temperature, heterogeneous oxidation of differently prepared Al-Mg alloy powders in oxygen are studied using thermo-gravimetric measurements. Fully and partially oxidized samples are recovered and characterized using scanning electron microscopy and x-ray diffraction. Voids grow within oxidized alloy powders for both atomized and mechanically alloyed powders. Two oxidation stages are identified for both alloy powders. Both magnesium and aluminum are oxidized at first oxidation stage, producing MgO and amorphous alumina. Spinel MgAl2O4 is produced during the second stage. The reaction is found to occur at the internal surface of the oxide shell as determined by matching the oxidation dynamics for particles with the same size but belonging to powders with different particles size distributions. Apparent activation energies for both oxidation stages are obtained as a function of the thickness of the growing oxide layer. The switchover between oxidation stages occurs when the spinel structure starts forming

Micro/Nano Manufacturing

Micro manufacturing involves dealing with the fabrication of structures in the size range of 0.1 to 1000 µm. The scope of nano manufacturing extends the size range of manufactured features to even smaller length scales—below 100 nm. A strict borderline between micro and nano manufacturing can hardly be drawn, such that both domains are treated as complementary and mutually beneficial within a closely interconnected scientific community. Both micro and nano manufacturing can be considered as important enablers for high-end products. This Special Issue of Applied Sciences is dedicated to recent advances in research and development within the field of micro and nano manufacturing. The included papers report recent findings and advances in manufacturing technologies for producing products with micro and nano scale features and structures as well as applications underpinned by the advances in these technologies

Surface Studies of Model Systems relevant for Pd and Ag Catalysts

This PhD thesis reports on investigations of the atomic scale structure of model catalysts relevant for the catalytic oxidation of CO and methane over Pd, and NOx reduction over silver-alumina, important reactions in automotive catalysts. By using complementary experimental techniques such as X-ray Photoelectron Spectroscopy, High Pressure X-ray Photoelectron Spectroscopy, Surface X-ray Diffraction, Low Energy Electron Diffraction, Infrared Spectroscopy, and Scanning Tunneling Microscopy, combined with theoretical calculations, the work presented in this thesis aims at an atomistic characterization of the active sites for silver-alumina and Pd model catalysts which may help to the design of new and improved catalysts. The results highlight the complexity of the different structures that may form upon oxygen interaction with Ag and Pd single crystal surfaces. The gas adsorption studies (CO, NO) on clean and oxidized silver surfaces give insights into the chemical properties of the surfaces, information important for the understanding of the reaction mechanism for the NOx reduction. We have been able to propose models for the adsorption sites for CO and NO on oxidized Ag surface and to propose a model system for the NOx reduction over silver-alumina. A particular surface orientation of bulk Pd oxide, the PdO(101) was found to be active for the oxidation of methane. Our studies allow us to reveal the active sites on the surface, information important for the design of better catalysts. Moreover, the results from the CO oxidation over Pd(100) give new insights into the reaction mechanism. The active phase switches between a surface with chemisorbed oxygen to a surface oxide when the oxygen partial pressure is increased