The purpose of this work is to optimize the chemical composition as well as the heat treatment for improving the mechanical performance of the TRIP steel by employing the theoretical models. TRIP steel consists of the microstructure with ferrite, bainite, retained austenite and minor martensite. Austenite contributes directly to the TRIP effect as its transformation to martensite under the external stress. In order to stabilize austenite against the martensitic transformation through the heat treatment, the two-step heat treatment is broadly applied to enrich the carbon and stabilize the austenite. During the first step of the heat treatment, intercritical annealing (IA), a dual phase structure (ferrite+austenite) is achieved. The austenite can be initially stabilized because of the low carbon solubility of ferrite. The bainite isothermal treatment (BIT) leads to the further carbon enrichment of IA-austenite by the formation of carbon-free ferrite. Comparing to the experiments, the thermodynamic and kinetic models are the lower and upper bounds of the carbon content of retained austenite. The mechanical properties are predicted using the swift model based on the predicted microstructure. In this work, a theoretical approach is coupled to a Genetic Algorithm-based optimization procedure to design (1) the heat treated temperatures to maximize the volume fraction of retained austenite in a Fe-0.32C-1.42Mn-1.56Si alloy and the chemical composition of (2) Fe-C-Mn-Si and (3) Fe-C-Mn-Si-Al-Cr-Ni alloy. The results recommend the optimum conditions of chemical composition and the heat treatment for maximizing the TRIP effect. Comparing to the experimental results, this designing strategy can be utilized to explore the potential materials of the novel alloys.

No description available.

Conventional energetic materials which are based on monomolecular compounds such as trinitrotoluene (TNT) have relatively low volumetric energy density. The energy density can be significantly enhanced by the addition of metal particulates. Among all metals, aluminum is popular because of its high oxidation enthalpy, low cost, and relative safety. Micron-sized aluminum particles, which have relatively high ignition temperatures and burning times, have been most commonly employed. Ignition of micron-sized aluminum particles is typically achieved only upon melting of the oxide shell at 2350 K, thereby resulting in fairly high ignition delay. Novel approaches to reduce the ignition temperatures and burning times and enhance the energy content of the particle are necessary. Recently, there has been an enormous interest in nano-materials due to their unique physicochemical properties such as lower melting and ignition temperatures and shorter burning times. Favorably, tremendous developments in the synthesis technology of nano-materials have also been made in the recent past. Several metal-based energetic materials with nano-sized particles such as nano-thermites, nano-fluids, and metalized solid propellants are being actively studied. The “green” reactive mixture of nano-aluminum particles and water/ice mixture (ALICE) is being explored for various applications such as space and underwater propulsion, hydrogen generation, and fuel-cell technology. Strand burning experiments indicate that the burning rates of nano-aluminum and water mixtures surpass those of common energetic materials such as ammonium dinitramide (ADN), hydrazinium nitroformate (HNF), and cyclotetramethylene tetranitramine (HMX). Sufficient understanding of key physicochemical phenomena is, however, not present. Furthermore, the most critical parameters that dictate the burning rate have not been identified. A multi-zone theoretical framework is established to predict the burning properties and flame structure by solving conservation equations in each zone and enforcing the mass and energy continuities at the interfacial boundaries. An analytical expression for the burning rate is derived and physicochemical parameters that dictate the flame behavior are identified. An attempt is made to elucidate the rate-controlling combustion mechanism. The effect of bi-modal particle size distribution on the burning rate and flame structure are investigated. The results are compared with the experimental data and favorable agreement is achieved. The ignition and combustion characteristics of micron-sized aluminum particles can also be enhanced by replacing the inert alumina layer with favorable metallic coatings such as nickel. Experiments indicate that nickel-coated aluminum particles ignite at temperatures significantly lower than the melting point of the oxide film, 2350 K due to the presence of inter-metallic reactions. Nickel coating is also attractive for nano-sized aluminum particles due to its ability to maximize the active aluminum content. Understanding the thermo-chemical behavior of nickel-aluminum core-shell structured particles is of key importance to both propulsion and material synthesis applications. The current understanding is, however, far from complete. In the present study, molecular dynamics simulations are performed to investigate the melting behavior, diffusion characteristics, and inter-metallic reactions in nickel-coated nano-aluminum particles. Particular emphasis is on the effects of core size and shell thickness on all important phenomena. The properties of nickel-coated aluminum particles and aluminum-coated nickel particles are also compared. Considerable uncertainties pertaining to the ignition characteristics of nano-aluminum particles exist. Aluminum particles can spontaneously burn at room temperature, a phenomenon known as pyrophoricity. This is a major safety issue during particle synthesis, handling, and storage. The critical particle size below which nascent particles are pyrophoric is not well known. Energy balance analysis with accurate evaluation of material properties (including size dependent properties) is performed to estimate the critical particle size for nascent particles. The effect of oxide layer thickness on pyrophoricity of aluminum particles is studied. The ignition delay and ignition temperature of passivated aluminum particles are also calculated. Specific focus is placed on the effect of particle size. An attempt is made to explain the weak dependence of the ignition delay on particle size at nano-scales.

In this study a semi-passive pulse thermal loop (PTL) was designed and experimentally validated. It provides improved heat transfer over passive systems such as the loop heat pipe in the moderate to high heat flux range and can be a sustainable alternative to active systems as it does not require an electric pump. This work details the components of the engineering prototype and characterizes their performance through the application of compressible and two-phase flow theory. A custom LabVIEW application was utilized for data acquisition and control. During operation with refrigerant R-134a the system was shown to be robust under a range of heat loads from 100 W to 800 W. Operation was achieved with driving pressure differentials ranging from 3 bar to 12 bar and pulse frequencies ranging from 0.42 Hz to 0.08 Hz. A smaller pressure differential and an increased pulse frequency results in improved heat transfer at the boilers. An evolution of the PTL is proposed that incorporates a novel, ejector-based pump-free refrigeration system. The design of the pulse refrigeration system (PRS) features valves at the outlet of two PTL-like boilers that are alternately actuated to direct pulses of refrigerant through an ejector. This is intended to entrain and raise the pressure of a secondary stream of refrigerant from the cooling loop, thereby replacing the compressor in a conventional vapor-compression cycle. The PRS is therefore characterized by transient flow through the ejector. An experimental prototype has been constructed which is able to operate as a conventional PTL when the cooling section is bypassed, although full operation of the refrigeration loop remains to be demonstrated. The design of the ejector is carried out using a one-dimensional model implemented in MATLAB that accounts for compressibility effects with NIST REFPROP vapor data sub-routines. The model enables the analysis of ejector performance in response to a transient pressure wave at the primary inlet. The high driving pressures provided by the PTL permit operation in a micro-gravity environment with minimal power consumption. Like the PTL, the proposed PRS is therefore well suited to terrestrial and aerospace applications where it could be driven by waste heat from electronics or solar thermal energy. As a novel semi-passive thermal management system, it will require complex control of the valves. Further analysis of the transient thermodynamic cycle is necessary in order to characterize and effect successful operation of the PRS. / Thesis (M.Sc.)-University of KwaZulu-Natal, Durban, 2013.

No description available.

No description available.

There are many sources of free energy available in the form of heat that is often simply wasted for want of an effective way to convert it into useful energy such as electricity. The aim of this research project is to design and build a low temperature differential Stirling engine capable of generating electric power from heat sources such as waste hot water or geothermal springs. The engine that has been developed is a research prototype model of a new type of design featuring a rotating displacer which is actuated by a pair of stepper motors. The rotating displacer design enables the use of readily available and comparatively cheap and robust steam pipe as the housing for the engine, and it also avoids problems associated with sealing and heat exchange that would be present in a large engine of a more traditional configuration. Owing to the fact that this engine is a research prototype, it has the ability to have some of its critical operating parameters such as phase angle and stroke length adjusted to investigate the effects on performance. When the next phase of development takes place most of these parameters will be fixed at the optimum values which will make manufacture cheaper and easier. Unfortunately, construction of the prototype engine has not been completed at the time of writing so no power producing results have been achieved; however thorough results are presented on the operation of the control system for the stepper motors which actuate the displacer. Additionally, after a thorough history and background of Stirling engines was researched, the understanding gained of how these engines work has enabled a design process to take place which has hopefully led to a successful design. Analysis of various aspects of the engine have been carried out and results look promising for the engine to produce around 500 Watts of electrical power output whilst running on hot water up to around 90°C.

Cerium dioxide (ceria) is an excellent catalytic material due to its ability to both facilitate oxidation/reduction reactions as well as store/release oxygen as an oxygen buffer. The traditional approach to assess and improve ceria's catalytic behavior focuses on how efficiently O-vacancies can be generated and/or annihilated within the material, and how to extend established understandings of "bulk" ceria to further explain the greatly enhanced catalytic behavior of ultra-small ceria nanoparticles (uCNPs) with sizes less than 10 nm. Here, using density functional theory (DFT) calculations, we reexamine the atomic and electronic structures of uCNPs, especially their surface configurations. A unique picture dissimilar to the traditional point of view emerges from these calculations for the surface structure of uCNPs. uCNPs similar to those obtained by experimental synthesis and applied in catalytic environments exhibit core-shell like structures overall, with under-stoichiometric, reduced CNP "cores" and over-stoichiometric, oxidized surface "shell" constituted by various surface functional groups, e.g.,-Ox and/or -OH surface groups. Therefore, their catalytic behavior is dominated by surface chemistry rather than O-vacancies. Based on this finding, reaction pathways of two prevalent catalytic reactions, namely CO oxidation and the water-gas shift reaction over uCNPs are systematically investigated. Combined, these results demonstrate an alternative understanding of the surface structure of uCNPs, and provide new avenues to explore and enhance their catalytic behavior, which is likely applicable to other transition metal oxide nanoparticles with multivalent ions and very small sizes.

The approach advocated in this work implements energy/exergy analysis and indirectly an irreversibility evaluation to a continuous manufacturing process involving discrete net shape production of compact heat exchangers through a complex controlled atmosphere brazing (CAB) process. The system under consideration involves fifteen cells of a continuous ramp-up heating, melting, reactive flow, isothermal dwell, and rapid quench solidification processing sequence during a controlled atmosphere brazing of aluminum compact heat exchangers. Detailed mass, energy, and exergy balances were performed. The irreversibility sources were identified and the quality of energy utilization at different processing steps determined. It is demonstrated that advanced thermodynamics metrics based on entropy generation may indicate the level of sustainable energy utilization of transient open systems, such as in manufacturing. This indicator may be related to particular property uniformity during materials processing. In such a case, the property uniformity would indicate systems distance from equilibrium, i.e., from the process sustainable energy utilization level. This idea is applied to net shape manufacturing process considered. A metric based on exergy destruction is devised to relate the heat exchanger temperature uniformity and the quality. The idea advocated in this thesis will represent the coherent framework for developing energy efficient, economically affordable and environmentally friendly manufacturing technology.

Analogiskt tänkande är en central kognitiv förmåga som vi använder i vardagslivet, såväl som i mer formella sammanhang, såsom i forskning och undervisning. Föreliggande avhandling behandlar hur analogier och analogiskt tänkande, uppmärksamhet på semantik och förståelse för vetenskaplig modellering kan användas för att hantera utmaningar i naturvetenskapsundervisningen, särskilt inom området termodynamik. Dessutom presenteras ett teoretiskt ramverk över hur analogiskt tänkande förhåller sig till semantik och vetenskaplig modellering, tre ämnesområden som alla utgår ifrån att finna motsvarigheter mellan två olika domäner. Mot denna bakgrund fokuserar avhandlingen på följande forskningsfrågor: I vilken utsträckning används analogier för att koppla olika representationer av ett fenomen till varandra och till det representerade fenomenet? Hur relaterar självgenerade analogier till vetenskaplig modellering? Avhandlingen består av fyra publicerade tidskriftsartiklar och en kappa. Den första artikeln är en semantisk utredning av ordet ’entropi’, den andra artikeln är en empirisk undersökning av synen på vetenskaplig modellering i olika kunskapstraditioner, och de tredje och fjärde artiklarna är empiriska undersökningar av fysiklärarstudenters respektive förstaklassares självgenererade analogier för termiska fenomen. Från ett metodperspektiv utfördes de empiriska studierna i en huvudsakligen kvalitativ tradition, där centrala resonemang exemplifieras genom analys av dialogutdrag. I de två studierna av självgenererade analogier fick deltagarna olika former av stöttning i form av social interaktion med varandra, gemensam erfarenhet av naturfenomen och diskussion kring deras representationer av fenomenen. I kappan utvecklas det teoretiska ramverket och mot den bakgrunden görs en omanalys av artiklarnas resultat. En central ståndpunkt i avhandlingen är att varje fenomen kan representeras på många olika sätt, som alla kan vara lämpliga och användbara i olika sammanhang med tyngdpunkt på olika aspekter av fenomenet. Rörande analogiskt tänkande anförs att elever och studenter kan skapa flera egna analogier för att få en rikare, kompletterande bild av ett fenomen, snarare än att undervisas utifrån en enda förment bästa analogi. Med utgångspunkt från vetenskaplig modellering kan olika representationer eller modeller lyfta fram olika aspekter av ett fenomen, med olika grad av idealisering och inom olika kunskapstraditioner. Slutligen, från ett semantiskt perspektiv kan ett ord svara mot flera, distinkta, men relaterade betydelser – fenomenet polysemi. Dessa tre perspektiv kan erbjuda konstruktivistiska ansatser till begreppsförståelse inom naturvetenskapsundervisningen, genom att elever och studenter uppmuntras att i dialog knyta till sin vardagsföreställning av de begrepp och fenomen de möts av, snarare än att byta ut den mot ett enda, förmodat korrekt vetenskapligt begrepp. Dessutom hävdas att den naturvetenskapsdidaktiska forskningen kan komma långt med ett strukturellt fokus på analogiskt tänkande och vetenskaplig modellering, där man försöker finna motsvarigheter mellan domäners beståndsdelar och deras relationer och helst isomorfism, en perfekt  överensstämmelse, men att beaktande av andra dimensioner, såsom en insikt i kognitionens förankring i kroppen och varseblivningen, de pragmatiska, kontextuella sammanhangen mot vilken bakgrund tänkande sker och språkets särskilda karaktär, krävs för en mer heltäckande bild. / Analogical reasoning is a central cognitive ability that is used in our everyday lives, as well as in formal settings, such as in research and teaching. This dissertation concerns how analogies and analogical reasoning, attention to semantics and insight into scientific modelling may be recruited in order to come to terms with challenges in science education, in particular within the field of thermodynamics. In addition, it provides a theoretical framework of how analogy relates to semantics and the practice of scientific modelling, three fields of study which all strive to map correspondences between two different domains. In particular, the dissertation addresses the following research questions: To what degree is analogy involved in connecting different representations of a phenomenon to each other and to the represented phenomenon? How do students’ selfgenerated analogies relate to the practice of scientific modelling? The dissertation comprises four published journal articles and a cover story. The first article is a semantic investigation of the word ‘entropy’, the second article is an empirical study of the view on scientific modelling in different traditions of knowledge, and the third and fourth articles are empirical studies of self-generated analogies for thermal phenomena among preservice physics teachers and first-graders, respectively. From a methodological point of view, the empirical studies were conducted in a primarily qualitative tradition, where central lines of reasoning are exemplified by analysis of dialogue excerpts. The two studies on self-generated analogies provided the participants with extensive scaffolding in the form of social interaction among peers, interaction with physical phenomena and discussion of their representations of the phenomena. The theoretical framework is developed in the cover story, which provides a background to the individual studies and reanalyses of the findings. A key claim of the dissertation is that any phenomenon can be represented in many different ways, all potentially adequate and useful in different contexts, emphasising different aspects of the phenomenon. Applied to the field of analogical reasoning, it is argued that students can generate several analogies themselves in order to get a richer, complementary view of a phenomenon, as opposed to be provided with a presumed best analogy. As for scientific models, many different representations or models may bring across different aspects of a phenomenon at varying degrees of idealisation and within different traditions of knowledge. Finally, in semantics, one word may correspond to several distinct, yet related, meanings: the phenomenon of polysemy. These three perspectives may provide constructivist approaches to conceptual development in science teaching, in which students are encouraged to connect to and enrich their everyday understanding of encountered concepts and phenomena in dialogue, rather than merely abandoning them for one single, supposedly correct, scientific concept. In addition, science education research can come quite far with structural approaches to analysing analogical reasoning and scientific modelling, establishing correspondences between entities in different domains, ultimately striving for isomorphism, perfect matches, but other dimensions, such as the perceptual, embodied nature of our cognition, the pragmatic, contextual circumstances in which any act of reasoning is performed, and the specificities of language, should also be taken into account for a fuller view.