abstract: The stability of nanocrystalline microstructural features allows structural materials to be synthesized and tested in ways that have heretofore been pursued only on a limited basis, especially under dynamic loading combined with temperature effects. Thus, a recently developed, stable nanocrystalline alloy is analyzed here for quasi-static (<100 s-1) and dynamic loading (103 to 104 s-1) under uniaxial compression and tension at multiple temperatures ranging from 298-1073 K. After mechanical tests, microstructures are analyzed and possible deformation mechanisms are proposed. Following this, strain and strain rate history effects on mechanical behavior are analyzed using a combination of quasi-static and dynamic strain rate Bauschinger testing. The stable nanocrystalline material is found to exhibit limited flow stress increase with increasing strain rate as compared to that of both pure, coarse grained and nanocrystalline Cu. Further, the material microstructural features, which includes Ta nano-dispersions, is seen to pin dislocation at quasi-static strain rates, but the deformation becomes dominated by twin nucleation at high strain rates. These twins are pinned from further growth past nucleation by the Ta nano-dispersions. Testing of thermal and load history effects on the mechanical behavior reveals that when thermal energy is increased beyond 200 °C, an upturn in flow stress is present at strain rates below 104 s-1. However, in this study, this simple assumption, established 50-years ago, is shown to break-down when the average grain size and microstructural length-scale is decreased and stabilized below 100nm. This divergent strain-rate behavior is attributed to a unique microstructure that alters slip-processes and their interactions with phonons; thus enabling materials response with a constant flow-stress even at extreme conditions. Hence, the present study provides a pathway for designing and synthesizing a new-level of tough and high-energy absorbing materials. / Dissertation/Thesis / Doctoral Dissertation Mechanical Engineering 2017

As superligas à base de cobalto são bem conhecidas por sua excelente resistência ao desgaste. Muitas pesquisas reportadas na literatura abordam o comportamento do desgaste destas ligas, seja no desgaste por deslizamento, erosivo ou abrasivo. Não obstante, o desgaste microabrasivo destas ligas não tem sido muito investigado, apesar dos danos causados por este tipo de solicitação. O comportamento do desgaste microabrasivo de três superligas à base de cobalto: a) 48% Co, 29 %Cr, 19 % Fe; b) uma liga com composição química próxima à da liga comercial Tribaloy T400 (Co 56 %, Cr 8.5%, Mo 29% Si 3.3 %) e c) uma liga com composição próxima à da liga comercial Stellite 6 (Co 64%, Cr 24 %, W 4.2 %, C 2,3%) foram investigadas. Os ensaios de microabrasão foram conduzidos com três abrasivos SiO2, Al2O3, e SiC em suspensão em água destilada, com concentração de 0,1 g/cm3. A carga aplicada foi de 0,3 N, a velocidade angular 20 rpm e a distância total de deslizamento, 48 metros. A análise das superfícies desgastadas por microscopia óptica, eletrônica de varredura e por perfilometria de contato mostraram que o tamanho, forma e dureza dos abrasivos podem influenciar significativamente os coeficientes de desgaste. Os ensaios conduzidos com partículas abrasivas de SiC e Al2O3 apresentaram maiores coeficientes de desgaste que os conduzidos com partículas de SiO2. A Liga Co-Cr-Fe mostrou os maiores coeficientes de desgaste quando comparada com as demais ligas, devido à baixa fração volumétrica de partículas de segunda fase, duras, precipitadas em sua microestrutura. Durante os ensaios, as três ligas, ensaiadas com os três diferentes abrasivos, apresentaram coeficientes de desgaste crescentes com o aumento da dureza do abrasivo; observou-se uma variação linear dos coeficientes de desgaste com a razão entre a dureza do abrasivo (Ha) e a dureza composta da liga (Hs), com R2 = 0.74. O micromecanismo dominante em todos os ensaios foi o desgaste abrasivo a dois corpos (grooving wear). A liga com composição próxima à da liga comercial Tribaloy T400, contendo fases de Laves dispersas em sua microestrutura, apresentou uma transição de micromecanismo de desgaste dúctil para frágil, quando submetida a ensaios com partículas abrasivas de Al2O3. Assim sendo, o volume de material removido nesta liga foi ligeiramente maior que o observado no ensaio com partículas de SiC. Na liga contendo baixa fração volumétrica de partículas de segunda fase, com matriz constituída por Co (CFC), observou-se uma camada subsuperficial nanocristalina de aproximadamente 1 µm de espessura, severamente deformada, imediatamente abaixo da superfície desgastada. Concluiu-se que o desgaste microabrasivo induziu a recristalização a frio do material encruado, com formação de grãos equiaxiais de dimensões nanométricas. / Cobalt alloys are well known for their excellent wear resistance. Many investigations are reported in literature related to the behavior of erosive, abrasive or sliding wear of these alloys. Nevertheless, the micro-abrasive wear of these alloys has not been thoroughly investigated, despite the damage caused by this type of wear. The microabrasive wear behavior of three cobalt alloys: a) 48 wt.% Co, 29 wt.% Cr, 19 wt.% Fe; b) an alloy with chemical composition close to Tribaloy T400 (56 wt.% Co, 8.5 wt.% Cr, 29% wt. Mo, 3.3 wt. %Si) and c) an alloy with chemical composition close to Stellite 6 (64wt.% Co 24 wt.% Cr, 4.2 wt.% W, 2,3 wt.% C were investigated. The tests were carried out using three 0,1 g/cm3 slurries composed by SiO2, Al2O3, and SiC particles, in suspension in distilled water. The applied load was 0.3 N, the rotational speed 20 rpm and the total sliding distance 48 m. Analysis of the worn surfaces of the tested alloys by Optical Microscopy, Scanning Electron Microscopy and Contact Stylus Profilometry showed that abrasive size, shape and hardness could significantly influence the wear coefficients. The tests carried out with SiC and Al2O3 slurries resulted in greater wear rates than those carried out in SiO2 slurry. Stellite 250, showed the greatest wear coefficient, compared to the two other experimental alloys, due to a very low volume fraction of hard second phase particles in the microstructure. Wear coefficients decreased with increasing abrasive particles hardness. An approximate linear correlation with the ratio between the hardness of the abrasives (Ha) and the compound hardness of the alloys (Ha) with a correlation factor R2= 0.74. The dominant wear micromechanism observed in all tests was two-body abrasion (grooving wear). The modified T400 alloy, containing Laves phase showed a transition from ductile to brittle wear mechanisms when testing with alumina slurries. The worn volume was slightly greater than the one observed with SiC. A severely deformed nanocrystalline layer was identified, immediately below the worn surface. It was concluded that cold recrystallization of the work-hardened material occurred, with the formation of nano sized equiaxed grains.

Le diamant de synthèse présente un intérêt croissant pour des applications diverses dans les domaines de l’optique, la catalyse, la biologie ou encore l’électronique. Par dépôt chimique en phase vapeur (CVD) ou par haute pression et haute température (HPHT), il peut être synthétisé sous forme de films. Les particules de diamant sont généralement produites par détonation ou par broyage de diamant massif. Cependant, il n’existe pas actuellement de particules de diamant combinant à la fois sphéricité, monodispersité et qualité cristalline contrôlée. Dans ce contexte, l’objectif de ce travail de thèse est d’élaborer un matériau diamant répondant à ces critères. Pour cela, des cœur-coquilles de diamant nanocristallin sphériques et monodisperses de taille micrométrique ont été synthétisés à partir de billes de silice ensemencées par des nanodiamants. Le revêtement de diamant nanocristallin a été obtenu dans un réacteur CVD spécifique dédié au traitement de poudres. En faisant varier la composition du mélange gazeux, la nature du revêtement a pu être modifiée, du diamant nanocristallin à un matériau hybride composé de nanodiamants enrobés d’une matrice graphitique. Des méthodes de caractérisations complémentaires comme la spectroscopie Raman et le HR-TEM ont permis de déterminer la structure cristalline de ces différents revêtements. Un traitement d’oxydation des cœur-coquilles a permis de les disperser en suspension colloïdale dans l’eau. En utilisant un traitement basique, des sphères creuses ont pu être obtenues et mises en suspension. Des études préliminaires des performances de ces différents matériaux ont ensuite été menées : les propriétés photo-électrocatalytiques pour la réduction du CO2 et la cytotoxicité in vitro pour des applications en biologie. La méthode d’élaboration des cœur-coquilles de diamant mise au point a été enfin étendue à des cœurs magnétiques de maghémite. / Synthetic diamond is now considered in various fields of applications like optics, catalysis, biology or even electronic. Thin films can be synthesized by Chemical Vapor Deposition (CVD) or by High Pressure/ High Temperature (HPHT), while particles are produced by detonation synthesis or milling of bulk diamond. Nevertheless, among all these diamond materials, there is no material available combining sphericity, monodispersity and crystalline quality. This is the purpose of this thesis work. Core-shell systems made of nanocrystalline diamond shell surrounding a silica core have been synthesized, starting from nanodiamond-seeded silica particles. These particles have been grown in a dedicated home-made CVD reactor, specifically developed to treat powders. Varying the gas composition, the nature of the coating has been tuned, from nanocrystalline diamond to a hybrid material made of nanodiamond particles surrounded by organized graphite. Complementary techniques such as Raman spectroscopy and High Resolution Transmission Electronic Microscopy (HR-TEM) have been used to characterize the crystalline structures. Colloidal suspensions were also obtained with these new diamond core-shells, by oxidation of their surface. Dissolving the silica core, diamond shells were also synthesized, exhibiting stable colloidal properties. Preliminary studies on diamond core-shells performances are also presented in this manuscript: their photocatalytic properties toward CO2 reduction and their in-vitro cytotoxicity considering further biological applications. Finally, the manuscript also reports on the extension of the process to magnetic silica cores for the synthesis of magnetic diamond core-shells.

Amorphous materials are used as soft magnetic materials and also as surface coatings to improve the surface properties. Furthermore, the nanocrystalline materials derived from their amorphous precursors show superior soft magnetic properties than amorphous counter parts for transformer core applications. In the present work, laser based processing of amorphous materials will be presented. Conventionally, the nanocrystalline materials are synthesized by furnace heat treatment of amorphous precursors. Fe-based amorphous/nanocrystalline materials due to their low cost and superior magnetic properties are the most widely used soft magnetic materials. However, achieving nanocrystalline microstructure in Fe-Si-B ternary system becomes very difficult owing its rapid growth rate at higher temperatures and sluggish diffusion at low temperature annealing. Hence, nanocrystallization in this system is achieved by using alloying additions (Cu and Nb) in the ternary Fe-Si-B system. Thus, increasing the cost and also resulting in reduction of saturation magnetization. laser processing technique is used to achieve extremely fine nanocrystalline microstructure in Fe-Si-B amorphous precursor. Microstructure-magnetic Property-laser processing co-relationship has been established for Fe-Si-B ternary system using analytical techniques. Laser processing improved the magnetic properties with significant increase in saturation magnetization and near zero coercivity values. Amorphous materials exhibit excellent corrosion resistance by virtue of their atomic structure. Fe-based amorphous materials are economical and due to their ease of processing are of potential interest to synthesize as coatings materials for wear and corrosion resistance applications. Fe-Cr-Mo-Y-C-B amorphous system was used to develop thick coatings on 4130 Steel substrate and the corrosion resistance of the amorphous coatings was improved. It is also shown that the mode of corrosion depends on the laser processing conditions. The microstructure evolution and the corrosion mechanisms operating are evaluated using post processing and post corrosion analysis.

Conventional liquid lubricants are faced with limitations under extreme cyclic operating conditions, such as in applications that require lubrication when changing from atmospheric pressure to ultrahigh vacuum and ambient air to dry nitrogen (e.g., satellite components), and room to elevated (>500°C) temperatures (e.g., aerospace bearings). Alternatively, solid lubricant coatings can be used in conditions where synthetic liquid lubricants and greases are not applicable; however, individual solid lubricant phases usually perform best only for a limited range of operating conditions. Therefore, solid lubricants that can adequately perform over a wider range of environmental conditions are needed, especially during thermal cycling with temperatures exceeding 500°C. One potential material class investigated in this dissertation is lubricious oxides, because unlike other solid lubricant coatings they are typically thermodynamically stable in air and at elevated temperatures. While past studies have been focused on binary metal oxide coatings, such as ZnO, there have been very few ternary oxide and no reported quaternary oxide investigations. The premise behind the addition of the third and fourth refractory metals Ti and Zr is to increase the number of hard and wear resistant phases while maintaining solid lubrication with ZnO. Therefore, the major focus of this dissertation is to investigate the processing-structure-tribological property relations of composite ZnO, TiO2 and ZrO2 phases that form ternary (ZnTi)xOy and quaternary (ZnTiZr)xOy nanocrystalline coatings. The coatings were processed by atomic layer deposition (ALD) using a selective variation of ALD parameters. The growth structure and chemical composition of as-deposited and ex situ annealed ternary and quaternary oxide coatings were studied by combined x-ray diffraction/focused ion beam microscopy/cross-sectional transmission electron microscopy, and x-ray photoelectron spectroscopy/Auger electron spectroscopy, respectively. It was determined that the structure varied from purely nanocrystalline (ternary oxides) to composite amorphous/nanocrystalline (quaternary oxides) depending on ALD parameters and annealing temperatures. In particular, the ZnTiO3 ilmenite phase with (104) textured nanocolumnar grains, exhibiting high stacking fault/partial dislocation densities >1012/cm2, was responsible for the excellent tribological behavior. Steady-state sliding friction coefficients down to 0.12 in humid air and 0.2 in dry nitrogen were measured along with sliding and fretting wear factors in the range of 10-6 to 10-7 mm3/N·m, even after ex situ annealing to 550°C. Additionally, the quaternary oxide phase Zn(Ti,Zr)O3 in solid solution exhibited a low fretting wear rate of 1x10-6 mm3/N·m. In contrast, certain phases, such as Zn2TiO4 cubic spinel, that form at annealing temperatures >550°C were responsible for high friction and wear. Mechanistic studies using the above techniques revealed low friction and wear-reducing surfaces and subsurfaces were due to different velocity accommodation modes (VAM). In the case of the ternary system, sliding-induced plastic deformation was possible when ZnTiO3 (104) stacking faults, bordered by partial dislocations, serve as a pathway for the dislocations to glide parallel to the sliding direction and hence achieve low friction and wear via an intrafilm shear VAM. It was evident that the individual nanocolumnar ZnTiO3 grains were plastically sheared as opposed to being fractured during wear. Conversely for the quaternary system, an interfacial sliding VAM between the counterface and a mechanically mixed layer (tribofilm) composed of the refined coating and counterface material, that also served as a source for the formation of cylindrical rolls, was responsible for wear reduction. Therefore, these lubricious oxides are a potential candidate for solid lubrication at high temperatures (up to 550 °C) and in space environments.

In the past decade, the use of nanotechnology as a tool to develop and fabricate new structures and devices for biological sensing and energy applications has become increasingly widespread. In this work, a systematic study has been performed on one-dimensional nanomaterials, with a focus on the development of miniaturized devices with a "bottom up" approach. First, members of the nano - carbon family are utilized for biosensing applications; in particular, carbon nanotubes as well as nitrogen - doped and boron - doped nanocrystalline diamond (NCD) films. These carbon - based materials possess several unique electrochemical properties over other conductive materials which make them suitable for biosensing applications. Single walled carbon nanotubes were deposited on a glass carbon electrode and modified for the detection of Salmonella DNA hybridization. Electrochemical impedance spectroscopy (EIS) was used as the method of detection and a detection limit of 10-9 M was achieved. Nanocrystalline diamond was grown using a microwave enhanced plasma chemical vapor deposition method. The diamond electrodes were doped with either boron or nitrogen to provide substrates and characterization was performed using scanning electron microscopy, atomic force microscopy, Raman spectroscopy, Fourier transform infrared spectroscopy, UV-vis spectroscopy, as well as by electrochemical methods. Modified boron - doped NCD was able to detect Salmonella DNA hybridization via EIS and fluorescent microscopy. The detection limit for these genosensors was found to be 0.4 micrometer complementary DNA. Boron - doped and nitrogen - incorporated nanocrystalline diamond also served as functionalized electrodes for lactic acid detection. It was found that the boron - doped electrodes could detect 0.5 mM lactic acid in a phosphate buffer solution. Second, bismuth antimony nanowires were grown in an anodized alumina template for the fabrication of a thermoelectric cooling device. Bismuth antimony nanowires were chosen due to their high thermoelectric efficiency compared to their bulk material counterpart. The development of a successful anodized template was achieved and EIS was used to diagnose the optimal etch parameters of the barrier oxide layer for nanowire growth. Bismuth antimony nanowires were grown directly on a silicon substrate and a thermoelectric cooling device was fabricated. The nanowires exhibited a thermoelectric efficiency of 0.18 at room temperature.

La faible taille des nanoparticules permet d’améliorer significativement la biodisponibilité des médicaments et faciliter leur administration par voie parentérale sous forme de particules sans risque de bouchage des vaisseaux sanguins. Cependant, cette faible taille peut être un facteur d’instabilité physique pendant le stockage ou/et pendant l’administration. Le principal défi lié à la fabrication des nano-suspensions réside dans leur stabilisation ainsi que dans le choix de la technologie appropriée pour leur fabrication. La présente thèse est centrée sur la stabilisation et la fabrication des suspensions nano-crystallines. Pour la partie formulation, la thèse décrit une approche systématique pour le choix des stabilisants. Ceux-ci peuvent être soit des dispersants ou soit une combinaison de dispersants et d’agents mouillant. La méthode proposée pour le criblage des stabilisants est rapide et efficace. Elle permet d’investiguer un grand nombre de stabilisant en se basant sur des principes physico-chimiques et en utilisant une approche étape par étape. Cette méthodologie a été divisée en deux parties : la première partie a été centrée sur un criblage qualitatif dont l’objectif est de sélectionner une tête de série à investiguer dans la seconde partie. La seconde partie est centrée sur un criblage quantitative qui a pour objectif d’optimiser le ratio est la quantité d’agent mouillant et de dispersant. Les résultats obtenus ont montré clairement que le système SDS/PVP 40/60 % (m/m) à la concentration de 1.2 % est le stabilisant optimum qui a permis de stabiliser la suspension à température ambiante. Par ailleurs, la robustesse de la formulation a été évaluée en mesurant l’agrégation sous cisaillement ainsi qu’en fonction de la force ionique, la température et du pH. Les résultats ont montré que la concentration critique de coagulation du système étudiée est cinq fois plus importante que celle décrite dans la littérature pour des nano-suspensions concentrées et stables. En revanche, l’agrégation des nanoparticules a été observée à haute température et à haut taux de cisaillement. Pour la partie développement du procédé de fabrication, la thèse est centrée sur la comparaison technologique de l’homogénéisation haute pression (HHP) par rapport au broyage à billes. L'impact de la formulation de la suspension, des paramètres du procédé et de la configuration technologique sur la qualité physique de la suspension produite par chacune des technologies ont été investigués. En outre, les deux technologies ont été comparées en termes de leur robustesse et de leur conformité à la réglementation pharmaceutique. Les conclusions principales de cette étude ont mis en évidence que le broyage à billes conduit à une suspension plus fine que celle obtenue par le HHP. De plus, il a été montré pour les 2 technologies que la relation d90 versus d50 est décrite par une seule courbe maîtresse indépendamment des paramètres du procédé et de l’échelle utilisée. Ceci a clairement mis en évidence que la HPH conduit à une distribution granulométrique plus resserrée que celle obtenue par broyage à billes. Finalement, nous avons modélisé le procédé de broyage à bille en adaptant les équations de Tanka et Chodakov. Le modèle conçu a permis un lissage précis des cinétiques de broyage ainsi que la simulation et la prédiction du procédé de broyage. / The small particle size of nano-crystalline suspension increases the bioavailability of water insoluble drug and facilitates their direct injection into the systemic circulation as particles. However, this small particle size can be responsible for their physical instability during storage and/or administration. The main challenges related to the production of nano-crystalline suspensions are to select the best stabilizer agent to ensure process-ability and to stabilize the API. Moreover, it is necessary to select the suitable technology for the suspension manufacturing taken into account the industrial constraints (scale-down/scale-up, process robustness and equipment reliability). The present thesis is focused on the formulation and the process development of nano-crystalline suspension. For the formulation part, we described a systematic approach to select optimum stabilizer for the preparation of nano-suspensions of given drug. The stabilizer can be either a dispersant or a combination of dispersant and wetting agents. The proposed screening method is a fast and efficient way to investigate a large number of stabilizers based on the principles of physical-chemistry and employs a stepwise approach. The methodology has been divided in two main parts; the first part focused on qualitative screening with the objective of selecting the appropriate candidate(s) for further investigations. The second part discussed quantitative screening with the objective of optimizing the ratio and the amount of wetting and dispersing agents. The results clearly showed that SDS/PVP as wetting/dispersing 40/60% (w/w) at a total concentration of 1.2% was the optimum stabilizer composition, at which the resulting nano-suspensions were stable at room temperature. Furthermore, the formulation robustness was assessed by measuring the rate of perikinetic, orthokinetic aggregation rate as a function of ionic strength (using NaCl), temperature and of pH variation. The results show that using the SDS/PVP mixture, the critical coagulation concentration is about five times higher than that observed in the literature for suspension colloidaly stable at high concentration. The nano-suspension was also found to be very stable at different pH conditions. Desorption test confirmed the high affinity between API and wetting/dispersing agent. However, the suspension undergoes aggregation at high temperature due to desorption of the wetting/dispersing agent and disaggregation of SDS micelles. Furthermore, aggregation occurs at very high shear rate by overcoming the energy barrier responsible for colloidal stability of the system. With respect to process engineering, a technological comparison (high pressure homogenization (HPH) versus bead milling) for the manufacturing of nano-suspension was carried out in terms of impact of suspension formulation, process parameters and technological configuration on the physical quality of the produced suspension. In addition both technologies were compared in terms of their robustness and their compliance to pharmaceutical regulations. The main findings are that, insofar the formulation is sufficiently robust and stable, both technologies are suitable for processing nano-crystalline suspensions. Bead milling is more powerful than HPH. It leads to a limit of milling (d50) lower than that obtained with HPH (100 vs 200 nm). In addition, for both technologies, one can observe that regardless, the process parameters used for milling and the scale, the relationship of d90 versus d50 could be described by a unique master curve (technology signature of milling pathway) outlining that the HPH leads to a tighten particle size distributions. Finally, a modelling approach based on adapted Tanka and Chodakov’s equation was proposed for modelling the milling kinetic. The generated model provides an accurate fit of milling kinetic, to predict and to simulate the milling manufacturing process.

This thesis focuses on developing an appropriate cathode material throughnanotechnology as a key component for a promising alternative of renewable energygenerating systems, Intermediate Temperature Solid Oxide Fuel Cells (IT-SOFC).Aiming at a working cathode material for IT-SOFC, a recently reported capable oxideperovskite material has been synthesized through two different chemical methods.BaxSr1-xCoyFe1-yO3−δ (BSCF) with y =0.8 and x =0.2 was fabricated in nanocrystallineform by a novel chemical alloying approach, co-precipitation- as well as conventionalsol-gel method to produce oxide perovskites. The thermal properties, phase constituents,microstructure and elemental analysis of the samples were characterized by TG-DSC,XRD, SEM and EDS techniques respectively. Thermodynamic modeling has beenperformed using a KTH-developed software (Medusa) and Spark Plasma Sintering (SPS)has been used to obtain pellets of BSCF, preserving the nanostructure and generatingquite dense pellets for electrical conductivity measurements.The results show that the powders synthesized by solution co-precipitation have cubicperovskite-type structure with a high homogeneity and uniform distribution and meanparticle size of 50-90 nm range, while sol-gel powders are not easy to form a pure phaseand mostly the process ends up with large particle containing two or three phases.Finer resultant powder compared to sol-gel technique and earlier research works onBSCF has been achieved in this project using oxalate co-precipitation method. Topreserve nanoscaled features of BSCF powder which possess a significant increase ofelectrical conductivity due to decrease the electrical resistivity of grain boundaries, forthe sample synthesized through co-precipitation, ~92% dense pellet sintered by SPS atV1080 °C and under 50 MPa pressure and its electrical conductivity has been measuredfrom room temperature to 900 °C.Specific conductivity values were precisely measured and the maximum of 63 S.cm-1 at430 °C in air and 25 S.cm-1 at 375°C in N2 correspondingly are two times higher thanconventional BSCF implying a high pledge for nano-BSCF as a strong candidate ascathode material in IT-SOFC.

京都大学 / 新制・課程博士 / 博士(地球環境学) / 甲第23352号 / 地環博第210号 / 新制||地環||40(附属図書館) / 京都大学大学院地球環境学舎地球環境学専攻 / (主査)教授 舟川 晋也, 教授 德地 直子, 准教授 渡邉 哲弘 / 学位規則第4条第1項該当 / Doctor of Global Environmental Studies / Kyoto University / DFAM

Many modern technologies are enabled by the use of thin films and/or nanostructured composite materials. For example, many thermoelectric devices, solar cells, power electronics, thermal barrier coatings, and hard disk drives contain nanostructured materials where the thermal conductivity of the material is a critical parameter for the device performance. At the nanoscale, the mean free path and wavelength of heat carriers may become comparable to or smaller than the size of a nanostructured material and/or device. For nanostructured materials made from semiconductors and insulators, the additional phonon scattering mechanisms associated with the high density of interfaces and boundaries introduces additional resistances that can significantly change the thermal conductivity of the material as compared to a macroscale counterpart. Thus, better understanding and control of nanoscale heat conduction in solids is important scientifically and for the engineering applications mentioned above. In this dissertation, I discuss my work in two areas dealing with nanoscale thermal transport: (1) I describe my development and advancement of important thermal characterization tools for measurements of thermal and thermoelectric properties of a variety of materials from thin films to nanostructured bulk systems, and (2) I discuss my measurements on several materials systems done with these characterization tools. First, I describe the development, assembly, and modification of a time-domain thermoreflectance (TDTR) system that we use to measure the thermal conductivity and the interface thermal conductance of a variety of samples including nanocrystalline alloys of Ni-Fe and Co-P, bulk metallic glasses, and other thin films. Next, a unique thermoelectric measurement system was designed and assembled for measurements of electrical resistivity and thermopower of thermoelectric materials in the temperature range of 20 to 350 °C. Finally, a commercial Anter Flashline 3000 thermal diffusivity measurement system is used to measure the thermal diffusivitiy and heat capacity of bulk materials at high temperatures. With regards to the specific experiments, I examine the thermal conductivity and interface thermal conductance of two different types of nanocrystalline metallic alloys of nickel-iron and cobalt-phosphorus. I find that the thermal conductivity of the nanocrystalline alloys is reduced by a factor of approximately two from the thermal conductivity measured on metallic alloys with larger grain sizes. With subsequent molecular dynamics simulations performed by a collaborator, and my own electrical conductivity measurements, we determine that this strong reduction in thermal conductivity is the result of increased electron scattering at the grain boundaries, and that the phonon component of the thermal conductivity is largely unchanged by the grain boundaries. We also examine four complex bulk metallic glass (BMG) materials with compositions of Zr₅₀Cu₄₀Al₁₀, Cu_46.25Zr_44.25Al_7.5Er₂, Fe₄₈Cr₁₅Mo₁₄C₁₅B₆Er₂, and Ti_41.5Zr_2.5Hf₅Cu_42.5Ni_7.5Si₁. From these measurements, I find that the addition of even a small percentage of heavy atoms (i.e. Hf and Er) into complex disordered BMG structures can create a significant reduction in the phonon thermal conductivity of these materials. This work also indicates that the addition of these heavy atoms does not disrupt electron transport to the degree with which thermal transport is reduced. / Ph. D.