Kolloide und ihre Überstrukturen als Bausteine zur Herstellung funktioneller Materialien

In dieser Arbeit wird die Synthese von Polymerkolloiden mit unterschiedlichen Formen und Funktionalitäten sowie deren Verwendung zur Herstellung kolloidaler Überstrukturen beschrieben. Über emulgatorfreie Emulsionspolymerisation (SFEP) erzeugte monodisperse sphärische Kolloide dienen als Bausteine von Polymeropalen, die durch die Selbstorganisation dieser Kolloide über vertikale Kristallisation (mit Hilfe einer Ziehmaschine) oder horizontale Kristallisation (durch Aufschleudern oder Aufpipettieren) entstehen. Durch die Kontrolle der Kugelgröße über die Parameter der Emulsionspolymerisation sowie die Einstellung der Schichtdicke der Kolloidkristalle über die Anpassung der Kristallisationsparameter ist die Erzeugung von qualitativ hochwertigen Opalen mit definierter Reflektionswellenlänge möglich. Darüber hinaus kann die chemische und thermische Beständigkeit der Opale durch den Einbau von Vernetzern oder vernetzbaren Gruppen in die Polymere erhöht werden. Die Opalfilme können als wellenlängenselektive Reflektoren in auf Fluoreszenzkonzentratoren basierenden Solarzellensystemen eingesetzt werden, um Lichtverluste in diesen Systemen zu reduzieren. Sie können auch als Template für die Herstellung invertierter Opale aus verschiedenen anorganischen Oxiden (TiO2, Al2O3, ZnO) dienen. Über einen CVD-Prozess erzeugte ZnO-Replika besitzen dabei den Vorteil, dass sie nicht nur eine hohe optische Qualität sondern auch eine elektrische Leitfähigkeit aufweisen. Dies ermöglicht sowohl deren Einsatz als Zwischenreflektor in Tandemsolarzellen als auch die Herstellung hierarchischer Strukturen über die Elektroabscheidung von Nanokristallen. In einem weiteren Teil der Arbeit wird die Herstellung funktioneller formanisotroper Partikel behandelt. Durch die Entmischung von mit Monomer gequollenen vernetzten Partikeln in einer Saatpolymerisation sind mehrere Mikrometer große Kolloide zugänglich, die aus zwei interpenetrierenden Halbkugeln aus gleichen oder verschiedenen Polymeren bestehen. Dadurch sind unter anderem Glycidyl-, Alkin- und Carbonsäuregruppen in die eine oder die andere Halbkugel integrierbar. Diese funktionellen Gruppen erlauben die Markierung bestimmter Partikelhälften mit Farbstoffen, die Beschichtung von Partikelbereichen mit anorganischen Oxiden wie SiO2 sowie die Erzeugung amphiphiler formanisotroper Partikel, die sich an Grenzflächen ausrichten lassen. Das Synthesekonzept kann - ausgehend von mittels SFEP erzeugten stark vernetzten PMMA-Partikeln - auch auf kleine Kolloide mit Größen von mehreren hundert Nanometern übertragen werden.

Chemical exfoliation of graphene and its application in organic electronics and energy storage devices

Graphene, the thinnest two-dimensional material possible, is considered as a realistic candidate for the numerous applications in electronic, energy storage and conversion devices due to its unique properties, such as high optical transmittance, high conductivity, excellent chemical and thermal stability. However, the electronic and chemical properties of graphene are highly dependent on their preparation methods. Therefore, the development of novel chemical exfoliation process which aims at high yield synthesis of high quality graphene while maintaining good solution processability is of great concern. This thesis focuses on the solution production of high-quality graphene by wet-chemical exfoliation methods and addresses the applications of the chemically exfoliated graphene in organic electronics and energy storage devices.rnPlatinum is the most commonly used catalysts for fuel cells but they suffered from sluggish electron transfer kinetics. On the other hand, heteroatom doped graphene is known to enhance not only electrical conductivity but also long term operation stability. In this regard, a simple synthetic method is developed for the nitrogen doped graphene (NG) preparation. Moreover, iron (Fe) can be incorporated into the synthetic process. As-prepared NG with and without Fe shows excellent catalytic activity and stability compared to that of Pt based catalysts.rnHigh electrical conductivity is one of the most important requirements for the application of graphene in electronic devices. Therefore, for the fabrication of electrically conductive graphene films, a novel methane plasma assisted reduction of GO is developed. The high electrical conductivity of plasma reduced GO films revealed an excellent electrochemical performance in terms of high power and energy densities when used as an electrode in the micro-supercapacitors.rnAlthough, GO can be prepared in bulk scale, large amount of defect density and low electrical conductivity are major drawbacks. To overcome the intrinsic limitation of poor quality of GO and/or reduced GO, a novel protocol is extablished for mass production of high-quality graphene by means of electrochemical exfoliation of graphite. The prepared graphene shows high electrical conductivity, low defect density and good solution processability. Furthermore, when used as electrodes in organic field-effect transistors and/or in supercapacitors, the electrochemically exfoliated graphene shows excellent device performances. The low cost and environment friendly production of such high-quality graphene is of great importance for future generation electronics and energy storage devices. rn

Heusler compounds for thermoelectric applications

The demands for energy is leading to social and political conflicts in the world. For example, the limited resources of fossil fuels causing a dependence on the oil conveying countries in the world, leading to political discords. One way to save energy is to increase the efficiency of a process. In the field of thermoelectricity waste heat is used to produce electricity, this leads to an improvement of the efficiency. Heusler compounds with C1b structure with the general formula XY Z (X, Y = transition metal, Z = main group element) are in focus of the present thermoelectric research. Their mechanical and thermal stability is exceptional in comparison to the commonly used thermoelectric materials. The possibility to substitute small amounts of elements from the parent compound without destructing the lattice structure allows tuning the electronic properties. This tunability also allows to avoid the use of toxic and expensive elements. The reported thermoelectric Heusler compounds exhibit high electrical conductivity and moderate values of the Seebeck coefficients, which lead to a high powerfactor. The disadvantage of Heusler compounds is their high thermal conductivity. Introducing mass disorder on the X-site lattice is one effective way to produce additional phonon scattering and with it to decrease the thermal conductivity. Another approach is to implement a nano or micro structure in the thermoelectric material. This can be achieved by phase separation, composite materials, pulverization with additional spark plasma sintering or by a complex lattice structure. In the first part of this work, the influence of element substitutions on the Zr0.5Hf0.5NiSn system was investigated, to obtain the knowledge on how to optimize the electronic properties of the Heusler compounds with C1b structure. In line with this, the change of the electronic structure was investigated and a possible mechanism is predicted. In the second part of this work, the phenomenon of phase separation was investigated. First, by applying a phase separation in the well-known system Co2MnSn and subsequently by systematic investiga- tions on the TixZryHfzNiSn. In the third part, the results from the previous parts before were used to produce and explain the best reported Heusler compound with C1b structure exhibiting a Figure of Merit of ZT= 1.2 at 830 K.

Oxide thermoelectrics via a glass-ceramic route

Thermoelektrizität beschreibt die reversible Beeinflussung und Wechselwirkung von Elektrizität und Temperatur T in Systemen abseits des thermischen Gleichgewichtes. In diesen führt ein Temperaturgradient entlang eines thermoelektrischen Materials zu einem kontinuierlichen Ungleichgewicht in der Energieverteilung der Ladungsträger. Dies hat einen Diffusionsstrom der energiereichen Ladungsträger zum kalten Ende und der energiearmen Ladungsträger zum heißen Ende zur Folge. Da in offenen Stromkreisen kein Strom fließt, wird ein Ungleichgewicht der Ströme über das Ausbilden eines elektrischen Feldes kompensiert. Die dadurch entstehende Spannung wird als Seebeck Spannung bezeichnet. Über einen geeigneten Verbraucher, folgend aus dem Ohm'schen Gesetz, kann nun ein Strom fließen und elektrische Energie gewonnen werden. Den umgekehrten Fall beschreibt der sogenannte Peltier Effekt, bei dem ein Stromfluss durch zwei unterschiedliche miteinander verbundene Materialien ein Erwärmen oder Abkühlen der Kontaktstelle zur Folge hat. Die Effizienz eines thermoelektrischen Materials kann über die dimensionslose Größe ZT=S^2*sigma/kappa*T charakterisiert werden. Diese setzt sich zusammen aus den materialspezifischen Größen der elektrischen Leitfähigkeit sigma, der thermischen Leitfähigkeit kappa und dem Seebeck Koeffizienten S als Maß der erzeugten Spannung bei gegebener Temperaturdifferenz. Diese Arbeit verfolgt den Ansatz glaskeramische Materialien mit thermoelektrischen Kristallphasen zu synthetisieren, sie strukturell zu charakterisieren und ihre thermoelektrischen Eigenschaften zu messen, um eine Struktur-Eigenschaft Korrelation zu erarbeiten. Hierbei werden im Detail eine elektronenleitende (Hauptphase SrTi_xNb_{1-x}O_3) sowie eine löcherleitende Glaskeramik (Hauptphase Bi_2Sr_2Co_2O_y) untersucht. Unter dem Begriff Glaskeramiken sind teilkristalline Materialien zu verstehen, die aus Glasschmelzen durch gesteuerte Kristallisation hergestellt werden können. Über den Grad der Kristallisation und die Art der ausgeschiedenen Spezies an Kristallen lassen sich die physikalischen Eigenschaften dieser Systeme gezielt beeinflussen. Glaskeramiken bieten, verursacht durch ihre Restglasphase, eine niedrige thermische Leitfähigkeit und die Fermi Energie lässt sich durch Dotierungen in Richtung des Leitungs- oder Valenzbands verschieben. Ebenso besitzen glaskeramische Materialien durch ihre Porenfreiheit verbesserte mechanische Eigenschaften gegenüber Keramiken und sind weniger anfällig für den Einfluss des Sauerstoffpartialdruckes p_{O_2} auf die Parameter. Ein glaskeramisches und ein gemischt keramisch/glaskeramisches thermoelektrisches Modul aus den entwickelten Materialien werden konzipiert, präpariert, kontaktiert und bezüglich ihrer Leistung vermessen.

Thermoelectric performance of p-type TiCoSb half-Heusler compounds - Intrinsic phase separation and charge carrier concentration optimization as key to high efficiency

The world's rising demand of energy turns the development of sustainable and more efficient technologies for energy production and storage into an inevitable task. Thermoelectric generators, composed of pairs of n-type and p-type semiconducting materials, di¬rectly transform waste heat into useful electricity. The efficiency of a thermoelectric mate¬rial depends on its electronic and lattice properties, summarized in its figure of merit ZT. Desirable are high electrical conductivity and Seebeck coefficients, and low thermal con¬ductivity. Half-Heusler materials are very promising candidates for thermoelectric applications in the medium¬ temperature range such as in industrial and automotive waste heat recovery. The advantage of Heusler compounds are excellent electronic properties and high thermal and mechanical stability, as well as their low toxicity and elemental abundance. Thus, the main obstacle to further enhance their thermoelectric performance is their relatively high thermal conductivity.rn rnIn this work, the thermoelectric properties of the p-type material (Ti/Zr/Hf)CoSb1-xSnx were optimized in a multistep process. The concept of an intrinsic phase separation has recently become a focus of research in the compatible n-type (Ti/Zr/Hf)NiSn system to achieve low thermal conductivities and boost the TE performance. This concept is successfully transferred to the TiCoSb system. The phase separation approach can form a significant alternative to the previous nanostructuring approach via ball milling and hot pressing, saving pro¬cessing time, energy consumption and increasing the thermoelectric efficiency. A fundamental concept to tune the performance of thermoelectric materials is charge carrier concentration optimization. The optimum carrier concentration is reached with a substitution level for Sn of x = 0.15, enhancing the ZT about 40% compared to previous state-of-the-art samples with x = 0.2. The TE performance can be enhanced further by a fine-tuning of the Ti-to-Hf ratio. A correlation of the microstructure and the thermoelectric properties is observed and a record figure of merit ZT = 1.2 at 710°C was reached with the composition Ti0.25Hf0.75CoSb0.85Sn0.15.rnTowards application, the long term stability of the material under actual conditions of operation are an important issue. The impact of such a heat treatment on the structural and thermoelectric properties is investigated. Particularly, the best and most reliable performance is achieved in Ti0.5Hf0.5CoSb0.85Sn0.15, which reached a maximum ZT of 1.1 at 700°C. The intrinsic phase separation and resulting microstructure is stable even after 500 heating and cooling cycles.

Morphological and optical properties of SiON thin films for photovoltaic applications

In the last years technologies related to photovoltaic energy have rapidly developed and the interest on renewable energy power source substantially increased. In particular, cost reduction and appropriate feed-in tariff contributed to the increase of photovoltaic installation, especially in Germany and Italy. However, for several technologies, the observed experimental efficiency of solar cells is still far from the theoretical maximum efficiency, and thus there is still room for improvement. In this framework the research and development of new materials and new solar devices is mandatory. In this thesis the morphological and optical properties of thin films of nanocrystalline silicon oxynitride (nc-SiON) have been investigated. This material has been studied in view of its application in Si based heterojunction solar cells (HIT). Actually, a-Si:H is used now in these cells as emitter layer. Amorphous SiO_x N_y has already shown excellent properties, such as: electrical conductivity, optical energy gap and transmittance higher than the ones of a-Si:H. Nc-SiO_x N_y has never been investigated up to now, but its properties can surpass the ones of amorphous SiON. The films of nc-SiON have been deposited at the University of Konstanz (Germany). The properties of these films have been studied using of atomic force microscopy and optical spectroscopy methods. This material is highly complex as it is made by different coexisting phases. The main purpose of this thesis is the development of methods for the analyses of morphological and optical properties of nc-SiON and the study of the reliability of those methods to the measurement of the characteristics of these silicon films. The collected data will be used to understand the evolution of the properties of nc-SiON, as a function of the deposition parameters. The results here obtained show that nc-SiON films have better properties with respect to both a-Si:H and a-SiON, i. e. higher optical band-gap and transmittance. In addition, the analysis of the variation of the observed properties as a function of the deposition parameters allows for the optimization of deposition conditions for obtaining optimal efficiency of a HIT cell with SiON layer.

Leukocyte populations and mRNA expression of inflammatory factors in quarter milk fractions at different somatic cell score levels in dairy cows

The effect of somatic cell count (SCC) and milk fraction on milk composition, distribution of cell populations, and mRNA expression of various inflammatory parameters was studied. Therefore, quarter milk samples were defined as cisternal (C), first 400 g of alveolar (A1), and remaining alveolar milk (A2) during the course of milking. Quarters were assigned to 4 groups according to their total SCC: 1) <12 x 10(3)/mL, 2) 12 to 100 x 10(3)/mL, 3) 100 to 350 x 10(3)/mL, and 4) >350 x 10(3)/mL. Milk constituents of interest were SCC, fat, protein, lactose sodium, and chloride ions as well as electrical conductivity. Cell populations were classified into lymphocytes, macrophages, and neutrophils (PMN). The mRNA expression of the inflammatory factors tumor necrosis factor-alpha, interleukin-1beta, cyclooxygenase-2, lactoferrin, and lysozyme was measured via real-time, quantitative reverse transcription PCR. Somatic cell count decreased from highest levels in C to lowest levels in A1 and increased thereafter to A2 in all groups. Fat content increased from C to A2 and with increasing SCC level. Lactose decreased with increasing SCC level but remained unchanged during milking. Concentrations of sodium and chloride, and electrical conductivity increased with increasing SCC but were higher in C than in A1 and A2. Protein was not affected by milk fraction or SCC level. The distribution of leukocytes was dramatically influenced by milk fraction and SCC. Lymphocytes were the dominating cell population in group 1, but the proportion of lymphocytes was low in groups 2, 3, and 4. Macrophage proportion was highest in group 2 and decreased in groups 3 and 4, whereas that of PMN increased from group 2 to 4. The content of macrophages decreased during milking in all SCC groups whereas that of PMN increased. The proportion of lymphocytes was not affected by milk fraction. The mRNA expression of all inflammatory factors showed an increase with increasing SCC but minor changes occurred during milking. In conclusion, milk fraction and SCC level have a crucial influence on the distribution of leukocyte populations and several milk constituents. The surprisingly high content of lymphocytes and concomitantly low mRNA expression of inflammatory factors in quarters with SCC <12 x 10(3)/mL indicates a different and possibly reduced readiness of the immune system to respond to invading pathogens.

Investigation into the enhancement of polycarbonate with conductive nanomaterials

Polymers are typically electrically and thermally insulating materials. The electrical and thermal conductivities of polymers can be increased by the addition conductive fillers such as carbons. Once the polymer composites have been made electrically and thermally conductive, they can be used in applications where these conductivities are desired such as electromagnetic shielding and static dissipation. In this project, three carbon nanomaterials are added to polycarbonate to enhance the electrical and thermal conductivity of the resulting composite. Hyperion Catalysis FIBRILs carbon nanotubes were added to a maximum loading of 8 wt%. Ketjenblack EC-600 JD carbon black was added to a maximum loading of 10 wt%. XG Sciences xGnP™ graphene nanoplatelets were added to a maximum loading of 15 wt%. These three materials have drastically different morphologies and will have varying effects on the various properties of polycarbonate composites. It was determined that carbon nanotubes have the largest effect on electrical conductivity with an 8 wt% carbon nanotube in polycarbonate composite having an electrical conductivity of 0.128 S/cm (from a pure polycarbonate value of 10-17 S/cm). Carbon black has the next largest effect with an 8 wt% carbon black in polycarbonate composite having an electrical conductivity of 0.008 S/cm. Graphene nanoplatelets have the least effect with an 8 wt% graphene nanoplatelet in polycarbonate having an electrical conductivity of 2.53 x 10-8 S/cm. Graphene nanoplatelets show a significantly higher effect on increasing thermal conductivity than either carbon nanotubes or carbon black. Mechanically, all three materials have similar effects with graphene nanoplatelets being somewhat more effective at increasing the tensile modulus of the composite than the other fillers. Carbon black and graphene nanoplatelets show standard carbon-filler rheology where the addition of filler increases the viscosity of the resulting composite. Carbon nanotubes, on the other hand, show an unexpected rheology. As carbon nanotubes are added to polycarbonate the viscosity of the composite is reduced below that of the original polycarbonate. It was seen that the addition of carbon nanotubes offsets the increased viscosity from a second filler, such as carbon black or graphene nanoplatelets.

Microwave-assisted wet chemical (MAWC) synthesis of lithium iron phosphate

LiFePO4 is a Co-free battery material. Its advantages of low cost, non-toxic and flat discharge plateau show promising for vehicle propulsion applications. A major problem associated with this material is its low electrical conductivity. Use of nanosized LiFePO4 coated with carbon is considered a solution because the nanosized particles have much shorter path for L+ ions to travel from the LiFePO4 crystal lattice to electrolytes. As other nano material powders, however, nano LiFePO4 could have processing and health issues. In order to achieve high electrical conductivity while maintaining a satisfactory manufacturability, the particles should possess both of the nano- and the microcharacteristics correspondingly. These two contradictory requirements could only be fulfilled if the LiFePO4 powders have a hierarchical structure: micron-sized parent particles assembled by nanosized crystallites with appropriate electrolyte communication channels. This study addressed the issue by study of the formation and development mechanisms of the LiFePO4 crystallites and their microstructures. Microwaveassisted wet chemical (MAWC) synthesis approach was employed in order to facilitate the evolvement of the nanostructures. The results reveal that the LiFePO4 crystallites were directly nucleated from amorphous precursors by competition against other low temperature phases, Li3PO4 and Fe3(PO4)2•8H2O. Growth of the crystalline LiFePO4 particles went through oriented attachment first, followed by revised Ostwald ripening and then recrystallization. While recrystallization played the role in growth of well crystallized particles, oriented attachment and revised Ostwald ripening were responsible for formation of the straight edge and plate-like shaped LiFePO4 particles comprised of nanoscale substructure. Oriented attachment and revised Ostwald ripening seemed to be also responsible for clustering the plate-like LiFePO4 particles into a high-level aggregated structure. The finding from this study indicates a hope for obtaining the hierarchical structure of LiFePO4 particles that could exhibit the both micro- and nano- scale characteristics. Future study is proposed to further advance the understanding of the structural development mechanisms, so that they can be manipulated for new LiFePO4 structures ideal for battery application.

Experimental and theoretical study of microstructure effect on piezoelectric property of one dimensional ZnO nanostructures

Emerging nanogenerators have attracted the attention of the research community, focusing on energy generation using piezoelectric nanomaterials. Nanogenerators can be utilized for powering NEMS/MEMS devices. Understanding the piezoelectric properties of ZnO one-dimensional materials such as ZnO nanobelts (NBs) and Nanowires (NWs) can have a significant impact on the design of new devices. The goal of this dissertation is to study the piezoelectric properties of one-dimensional ZnO nanostructures both experimentally and theoretically. First, the experimental procedure for producing the ZnO nanostructures is discussed. The produced ZnO nanostructures were characterized using an in-situ atomic force microscope and a piezoelectric force microscope. It is shown that the electrical conductivity of ZnO NBs is a function of applied mechanical force and its crystalline structure. This phenomenon was described in the context of formation of an electric field due to the piezoelectric property of ZnO NBs. In the PFM studies, it was shown that the piezoelectric response of the ZnO NBs depends on their production method and presence of defects in the NB. Second, a model was proposed for making nanocomposite electrical generators based on ZnO nanowires. The proposed model has advantages over the original configuration of nanogenerators which uses an AFM tip for bending the ZnO NWs. Higher stability of the electric source, capability for producing larger electric fields, and lower production costs are advantages of this configuration. Finally, piezoelectric properties of ZnO NBs were simulated using the molecular dynamics (MD) technique. The size-scale effect on piezoelectric properties of ZnO NBs was captured, and it is shown that the piezoelectric coefficient of ZnO NBs decreases by increasing their lateral dimensions. This phenomenon is attributed to the surface charge redistribution and compression of unit cells that are placed on the outer shell of ZnO NBs.

Processing and characterization of PbSnTe-based thermoelectric materials made by mechanical alloying

The research reported in this dissertation investigates the processes required to mechanically alloy Pb1-xSnxTe and AgSbTe2 and a method of combining these two end compounds to result in (y)(AgSbTe2)–(1 - y)(Pb1-xSnxTe) thermoelectric materials for power generation applications. In general, traditional melt processing of these alloys has employed high purity materials that are subjected to time and energy intensive processes that result in highly functional material that is not easily reproducible. This research reports the development of mechanical alloying processes using commercially available 99.9% pure elemental powders in order to provide a basis for the economical production of highly functional thermoelectric materials. Though there have been reports of high and low ZT materials fabricated by both melt alloying and mechanical alloying, the processing-structure-properties-performance relationship connecting how the material is made to its resulting functionality is poorly understood. This is particularly true for mechanically alloyed material, motivating an effort to investigate bulk material within the (y)(AgSbTe2)–(1 - y)(Pb1-xSnx- Te) system using the mechanical alloying method. This research adds to the body of knowledge concerning the way in which mechanical alloying can be used to efficiently produce high ZT thermoelectric materials. The processes required to mechanically alloy elemental powders to form Pb1-xSnxTe and AgSbTe2 and to subsequently consolidate the alloyed powder is described. The composition, phases present in the alloy, volume percent, size and spacing of the phases are reported. The room temperature electronic transport properties of electrical conductivity, carrier concentration and carrier mobility are reported for each alloy and the effect of the presence of any secondary phase on the electronic transport properties is described. An mechanical mixing approach for incorporating the end compounds to result in (y)(AgSbTe2)–(1-y)(Pb1-xSnxTe) is described and when 5 vol.% AgSbTe2 was incorporated was found to form a solid solution with the Pb1-xSnxTe phase. An initial attempt to change the carrier concentration of the Pb1-xSnxTe phase was made by adding excess Te and found that the carrier density of the alloys in this work are not sensitive to excess Te. It has been demonstrated using the processing techniques reported in this research that this material system, when appropriately doped, has the potential to perform as highly functional thermoelectric material.

Analysis of transport properties of mechanically alloyed [Pb1-xSnxTe]

The work described in this thesis had two objectives. The first objective was to develop a physically based computational model that could be used to predict the electronic conductivity, Seebeck coefficient, and thermal conductivity of Pb1-xSnxTe alloys over the 400 K to 700 K temperature as a function of Sn content and doping level. The second objective was to determine how the secondary phase inclusions observed in Pb1-xSnxTe alloys made by consolidating mechanically alloyed elemental powders impact the ability of the material to harvest waste heat and generate electricity in the 400 K to 700 K temperature range. The motivation for this work was that though the promise of this alloy as an unusually efficient thermoelectric power generator material in the 400 K to 700 K range had been demonstrated in the literature, methods to reproducibly control and subsequently optimize the materials thermoelectric figure of merit remain elusive. Mechanical alloying, though not typically used to fabricate these alloys, is a potential method for cost-effectively engineering these properties. Given that there are deviations from crystalline perfection in mechanically alloyed material such as secondary phase inclusions, the question arises as to whether these defects are detrimental to thermoelectric function or alternatively, whether they enhance thermoelectric function of the alloy. The hypothesis formed at the onset of this work was that the small secondary phase SnO2 inclusions observed to be present in the mechanically alloyed Pb1-xSnxTe would increase the thermoelectric figure of merit of the material over the temperature range of interest. It was proposed that the increase in the figure of merit would arise because the inclusions in the material would not reduce the electrical conductivity to as great an extent as the thermal conductivity. If this were to be true, then the experimentally measured electronic conductivity in mechanically alloyed Pb1-xSnxTe alloys that have these inclusions would not be less than that expected in alloys without these inclusions while the portion of the thermal conductivity that is not due to charge carriers (the lattice thermal conductivity) would be less than what would be expected from alloys that do not have these inclusions. Furthermore, it would be possible to approximate the observed changes in the electrical and thermal transport properties using existing physical models for the scattering of electrons and phonons by small inclusions. The approach taken to investigate this hypothesis was to first experimentally characterize the mobile carrier concentration at room temperature along with the extent and type of secondary phase inclusions present in a series of three mechanically alloyed Pb1-xSnxTe alloys with different Sn content. Second, the physically based computational model was developed. This model was used to determine what the electronic conductivity, Seebeck coefficient, total thermal conductivity, and the portion of the thermal conductivity not due to mobile charge carriers would be in these particular Pb1-xSnxTe alloys if there were to be no secondary phase inclusions. Third, the electronic conductivity, Seebeck coefficient and total thermal conductivity was experimentally measured for these three alloys with inclusions present at elevated temperatures. The model predictions for electrical conductivity and Seebeck coefficient were directly compared to the experimental elevated temperature electrical transport measurements. The computational model was then used to extract the lattice thermal conductivity from the experimentally measured total thermal conductivity. This lattice thermal conductivity was then compared to what would be expected from the alloys in the absence of secondary phase inclusions. Secondary phase inclusions were determined by X-ray diffraction analysis to be present in all three alloys to a varying extent. The inclusions were found not to significantly degrade electrical conductivity at temperatures above ~ 400 K in these alloys, though they do dramatically impact electronic mobility at room temperature. It is shown that, at temperatures above ~ 400 K, electrons are scattered predominantly by optical and acoustical phonons rather than by an alloy scattering mechanism or the inclusions. The experimental electrical conductivity and Seebeck coefficient data at elevated temperatures were found to be within ~ 10 % of what would be expected for material without inclusions. The inclusions were not found to reduce the lattice thermal conductivity at elevated temperatures. The experimentally measured thermal conductivity data was found to be consistent with the lattice thermal conductivity that would arise due to two scattering processes: Phonon phonon scattering (Umklapp scattering) and the scattering of phonons by the disorder induced by the formation of a PbTe-SnTe solid solution (alloy scattering). As opposed to the case in electrical transport, the alloy scattering mechanism in thermal transport is shown to be a significant contributor to the total thermal resistance. An estimation of the extent to which the mean free time between phonon scattering events would be reduced due to the presence of the inclusions is consistent with the above analysis of the experimental data. The first important result of this work was the development of an experimentally validated, physically based computational model that can be used to predict the electronic conductivity, Seebeck coefficient, and thermal conductivity of Pb1-xSnxTe alloys over the 400 K to 700 K temperature as a function of Sn content and doping level. This model will be critical in future work as a tool to first determine what the highest thermoelectric figure of merit one can expect from this alloy system at a given temperature and, second, as a tool to determine the optimum Sn content and doping level to achieve this figure of merit. The second important result of this work is the determination that the secondary phase inclusions that were observed to be present in the Pb1-xSnxTe made by mechanical alloying do not keep the material from having the same electrical and thermal transport that would be expected from “perfect" single crystal material at elevated temperatures. The analytical approach described in this work will be critical in future investigations to predict how changing the size, type, and volume fraction of secondary phase inclusions can be used to impact thermal and electrical transport in this materials system.

ALTERNATING CURRENT DIELECTROPHORESIS OF CORE-SHELL NANOPARTICLES: EXPERIMENTS AND COMPARISON WITH THEORY

Nanoparticles are fascinating where physical and optical properties are related to size. Highly controllable synthesis methods and nanoparticle assembly are essential [6] for highly innovative technological applications. Among nanoparticles, nonhomogeneous core-shell nanoparticles (CSnp) have new properties that arise when varying the relative dimensions of the core and the shell. This CSnp structure enables various optical resonances, and engineered energy barriers, in addition to the high charge to surface ratio. Assembly of homogeneous nanoparticles into functional structures has become ubiquitous in biosensors (i.e. optical labeling) [7, 8], nanocoatings [9-13], and electrical circuits [14, 15]. Limited nonhomogenous nanoparticle assembly has only been explored. Many conventional nanoparticle assembly methods exist, but this work explores dielectrophoresis (DEP) as a new method. DEP is particle polarization via non-uniform electric fields while suspended in conductive fluids. Most prior DEP efforts involve microscale particles. Prior work on core-shell nanoparticle assemblies and separately, nanoparticle characterizations with dielectrophoresis and electrorotation [2-5], did not systematically explore particle size, dielectric properties (permittivity and electrical conductivity), shell thickness, particle concentration, medium conductivity, and frequency. This work is the first, to the best of our knowledge, to systematically examine these dielectrophoretic properties for core-shell nanoparticles. Further, we conduct a parametric fitting to traditional core-shell models. These biocompatible core-shell nanoparticles were studied to fill a knowledge gap in the DEP field. Experimental results (chapter 5) first examine medium conductivity, size and shell material dependencies of dielectrophoretic behaviors of spherical CSnp into 2D and 3D particle-assemblies. Chitosan (amino sugar) and poly-L-lysine (amino acid, PLL) CSnp shell materials were custom synthesized around a hollow (gas) core by utilizing a phospholipid micelle around a volatile fluid templating for the shell material; this approach proves to be novel and distinct from conventional core-shell models wherein a conductive core is coated with an insulative shell. Experiments were conducted within a 100 nl chamber housing 100 um wide Ti/Au quadrapole electrodes spaced 25 um apart. Frequencies from 100kHz to 80MHz at fixed local field of 5Vpp were tested with 10-5 and 10-3 S/m medium conductivities for 25 seconds. Dielectrophoretic responses of ~220 and 340(or ~400) nm chitosan or PLL CSnp were compiled as a function of medium conductivity, size and shell material.

COMPUTATIONAL STUDY OF MICROSTRUCTURE-PROPERTYMECHANISM RELATIONS IN FERROIC COMPOSITES

Ferroic materials, as notable members of smart materials, have been widely used in applications that perform sensing, actuation and control. The macroscopic property change of ferroic materials may become remarkably large during ferroic phase transition, leading to the fact that the macroscopic properties can be tuned by carefully applying a suitable external field (electric, magnetic, stress). To obtain an enhancement in physical and/or mechanical properties, different kinds of ferroic composites have been fabricated. The properties of a ferroic composite are determined not only by the properties and relative amounts of the constituent phases, but also by the microstructure of individual phase such as the phase connectivity, phase size, shape and spatial arrangement. This dissertation mainly focuses on the computational study of microstructure – property – mechanism relations in two representative ferroic composites, i.e., two-phase particulate magnetoelectric (ME) composite and polymer matrix ferroelectric composite. The former is a great example of ferroic composite exhibiting a new property and functionality that neither of the constituent phases possesses individually. The latter well represents the kind of ferroic composites having property combinations that are better than the existing materials. Phase field modeling was employed as the computing tool, and the required models for ferroic composites were developed based on existing models for monolithic materials. Extensive computational simulations were performed to investigate the microstructure-property relations and the underlying mechanism in ferroic composites. In particulate, it is found that for ME composite 0-3 connectivity (isolated magnetostrictive phase) is necessary to exhibit ME effect, and small but finite electrical conductivity of isolated magnetic phase can beneficially enhance ME effect. It is revealed that longitudinal and transverse ME coefficients of isotropic 0-3 particulate composites can be effectively tailored by controlling magnetic domain structures without resort to anisotropic two-phase microstructures. Simulations also show that the macroscopic properties of the ferroelectricpolymer composites critically depend on the ferroelectric phase connectivity while are not sensitive to the sizes and internal grain structures of the ceramic particles. Texturing is found critical to exploit the paraelectric«ferroelectric phase transition and nonlinear polarization behavior in paraelectric polycrystal and its polymer matrix composite. Additionally, a Diffuse Interface Field model was developed to simulate packing and motion in liquid phase which is promising for studying the fabrication of particulatepolymer composites.

SYNTHESIS OF GRAPHENE AND ITS APPLICATIONS FOR DYE-SENSITIZED SOLAR CELLS

Graphene, which is a two-dimensional carbon material, exhibits unique properties that promise its potential applications in photovoltaic devices. Dye-sensitized solar cell (DSSC) is a representative of the third generation photovoltaic devices. Therefore, it is important to synthesize graphene with special structures, which possess excellent properties for dye-sensitized solar cells. This dissertation research was focused on (1) the effect of oxygen content on the structure of graphite oxide, (2) the stability of graphene oxide solution, (3) the application of graphene precipitate from graphene oxide solution as counter electrode for DSSCs, (4) the development of a novel synthesis method for the three-dimensional graphene with honeycomb-like structure, and (5) the exploration of honeycomb structured graphene (HSG) as counter electrodes for DSSCs. Graphite oxide is a crucial precursor to synthesize graphene sheets via chemical exfoliation method. The relationship between the oxygen content and the structures of graphite oxides was still not explored. In this research, the oxygen content of graphite oxide is tuned by changing the oxidation time and the effect of oxygen content on the structure of graphite oxide was evaluated. It has been found that the saturated ratio of oxygen to carbon is 0.47. The types of functional groups in graphite oxides, which are epoxy, hydroxyl, and carboxylgroups, are independent of oxygen content. However, the interplanar space and BET surface area of graphite oxide linearly increases with increasing O/C ratio. Graphene oxide (GO) can easily dissolve in water to form a stable homogeneous solution, which can be used to fabricate graphene films and graphene based composites. This work is the first research to evaluate the stability of graphene oxide solution. It has been found that the introduction of strong electrolytes (HCl, LiOH, LiCl) into GO solution can cause GO precipitation. This indicates that the electrostatic repulsion plays a critical role in stabilizing aqueous GO solution. Furthermore, the HCl-induced GO precipitation is a feasible approach to deposit GO sheets on a substrate as a Pt-free counter electrode for a dye-sensitized solar cell (DSSC), which exhibited 1.65% of power conversion efficiency. To explore broad and practical applications, large-scale synthesis with controllable integration of individual graphene sheets is essential. A novel strategy for the synthesis of graphene sheets with three-dimensional (3D) Honeycomb-like structure has been invented in this project based on a simple and novel chemical reaction (Li2O and CO to graphene and Li2CO3). The simultaneous formation of Li2CO3 with graphene not only can isolate graphene sheets from each other to prevent graphite formation during the process, but also determine the locally curved shape of graphene sheets. After removing Li2CO3, 3D graphene sheets with a honeycomb-like structure were obtained. This would be the first approach to synthesize 3D graphene sheets with a controllable shape. Furthermore, it has been demonstrated that the 3D Honeycomb-Structured Graphene (HSG) possesses excellent electrical conductivity and high catalytic activity. As a result, DSSCs with HSG counter electrodes exhibit energy conversion efficiency as high as 7.8%, which is comparable to that of an expensive noble Pt electrode.