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Within this work, the growth of out-of-plane Si and SiᵪGe₁₋ᵪ and in-plane Ge nanowire have been investigated. For this purpose, nanowires have been grown ccording to the vapor-liquid-solid mechanism by means of molecular beam epitaxy with Au as metallic solvent on Si(111) and nano-structured Si(001)/SiO₂ substrates. Each stage from surface preparation to final nanowire growth has been examined in order to determine relevant parameters which influence the nucleation process nanowire elongation. Particular attention has been given to describe the variation of Au droplet size distribution on Si(111) to evaluate the subsequent nanowire growth and to obtain optimal growth conditions for an site-selective nucleation on the nano-structured substrates.
Due to increased surface diffusion velocity with raising temperature, the droplet diameter distribution shifts from a positive skewed distribution with a high fraction of smaller droplets to a negative skewed distribution with a high fraction of larger droplets. The temperature dependency of the most probable droplet diameter and the number of Au droplets per area has been determined, which can be applied to predict the resulting nanowire diameters.
The out-of-plane Si and SiᵪGe₁₋ᵪ on Si(111) predominantly grow along the surface normal, exhibiting the characteristic sawtooth-like sidewall faceting. During the nanowire formation, the Au surface diffusion velocity increased and causes an increase of the most frequently observed diameter. Furthermore, the total number of droplets/nanowires decreases by a constant factor due to an increase of the contact angle during Si/Ge deposition. The specific diameter range for an possible nanowire formation is reduced by the incorporation of Ge into SiᵪGe₁₋ᵪ nanowires.
To obtain a regular and uniform nanowire growth, a nano-structured substrate consisting of Si(001) terminated pillars surrounded by a SiO₂ matrix has been utilized. The initial growth of Ge nanowire starting from Si-Au droplets with SiᵪGe₁₋ᵪ nucleation from ternary alloy is discussed from a thermodynamic point of view and a model based on the Si-Ge-Au ternary phase diagram has been developed to predict the SiᵪGe₁₋ᵪ concentration gradient in the nanowire base. The fully relaxed in-plane Ge nanowires occur within one of the four distinct in-plane ⟨110⟩ directions and nanowires are mainly bounded by two 55° inclined {111} facets and a less pronounced planar (001) top facet.
High-resolution scanning X-ray diffraction microscopy reveals a slightly tilted growth of individual nanowires with respect to each other, causing an abrupt change in the orientation at junction points of interconnected nanowires.
Research on energy saving technologies surged in the last decades. One especially relevant technology regards thermal energy storage via phase change materials, or PCM. These materials function as regenerative thermal batteries that can absorb and release thermal energy via the latent heat associated with a phase change, while temperature is kept constant. The advantage of this technology is that due to the latent heat effect the energy density is very high, which reduces the required size of the medium and makes it easier to be coupled with heat loss sources, both in industrial and household applications. The challenge lies, however, in identifying correct PCMs for specific operation temperatures. The goal of the thesis is, then, to develop a novel thermodynamic database that describes the thermodynamic properties of salt mixtures with potential as phase change materials, both for high (up to 800 ℃) and low temperature (up to 100 ℃) applications; and, then, to perform a screening to identify potential PCM compositions in the database.
The database is created with FactSage, a Calphad software, and the systems covered are the CaCl₂-Ca(NO₃)₂-KCl-KNO₃-NaCl-NaNO₃, for high temperature PCMs, and the hydrated Mn(NO₃)₂-H₂O, Zn(NO₃)₂-H₂O, MgSO₄-H₂O and ZnSO₄-H₂O for low temperature PCMs. The liquid solution in all systems is modelled with the non-ideal associates model and, therefore, no aqueous solution model is required. The experimental data used for the assessments come from the literature and from new measurements performed by the partners of the PCM-Screening project (FKZ 03ET1441).
A new program called DataOptimizer has been developed to assist with the optimisation of thermodynamic databases. Relying on the ChemApp software and the NOMAD optimizer, DataOptimizer overcomes many shortcomings of similar database optimisation programs. A graphical user interface featuring a real-time plotting output is also implemented, which allows for a much easier and user-friendly experience. Details about the implementation and features of the program are given.
Finally, the identification of PCM candidates is performed using both phase diagrams calculated with FactSage and a new numerical screening algorithm, which relies on ChemApp. The screening algorithm proves to be capable of identifying eutectics in multicomponent systems automatically without the need for phase diagrams. As a result, twenty-two PCM candidates are identified for high temperature applications within the anhydrous system and, fourteen candidates, for low temperature applications within the hydrated systems.