Dissertations / Theses on the topic 'Electrical conductivity mechanism'

In the process of studying the mechanisms of influence of structural defects on properties in scandium oxide, the thermal and electrical effects in the temperature range 25 – 1800 C in a vacuum, in the struc-ture was of detected a gradual phase transition of the ordered cubic type C in a disordered type C1 in the temperature range of 400 – 1000 C. Phase transformation is accompanied by the difference of the conduc-tivity type of the charge carriers. Anion conductivity of charges in the structure of scandium oxide exist there is to 400 С, with the energy of conductivity charges 2.46 eV. In scandium oxide in the interval tem-peratures 400 – 1000 C in case there is a mixture of two types of conductivity anion and electronic. In the interval of temperatures 1600 – 1800 C in case there is a change of the chemical composition of the oxide on the content of oxygen. With the help of the mathematical model of calculation of the elements of the structure of the change in the size of the radii of oxygen and anionic vacancies, which coincides with the change of parameters of elementary cells in the transition of the structure of C in C1. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/35390

Актуальність: оксид графену – це одношаровий графіен, у якому вуглецеві зв’язки на поверхні більшою мірою зв’язані з киснем. Такий матеріал просто отримати у водневому розчині і осадити на будь-яку підкладку. Відновлений оксид графену є двовимірним матеріалом, який перспективний для виготовлення на його основі різних типів сенсорів – від сенсорів інфрачервоного випромінювання до хімічних газових сенсорів. Тому отримання відновленого оксиду графена з високою електричною провідністю за низьких температур відпалу дозволяє мати базовий дешевий двовимірний матеріал для різних типів сенсорів на гнучких підкладках, що необхідно для медичної галузі, робототехніки і гнучкої мікро- та фотоелектроніки. Зв’язок роботи з науковими програмами, планами, темами кафедри: тема роботи відповідає пріорітетному науковому напрямку кафедри загальної фізики та фізики твердого тіла – «Фундаментальні наукові дослідження з найбільш важливих проблем розвитку науково-технічного, соціально-економічного, людського потенціалу для забезпечення конкурентоспроможності України у світі та сталого розвитку суспільства і держави». Об’єкт дослідження: дослідження оптичних та електрофізичних властивостей плівок оксиду графену після різних низькотемпературних методів відпалу. Предмет дослідження: оксид графену, відновлений за низької температури в ВЧ плазмовому розряді. Мета роботи: дослідження впливу плазмової обробки на фізикохімічні і електрофізичні властивості оксиду графену, порівняння їх з низькотемпературним термічним відновлення. Отримання базового матеріалу для газових та температурних сенсорів. Методи дослідження: інфрачервона спектроскопія, XPS спектроскопія, вольт-амперні характеристики, температурна і частотна залежності електричної провідності. Відомості про обсяг звіту, кількість ілюстрацій, таблиць, додатків і літературних найменувань за переліком використаних: звіт складається з переліку умовних позначень, символів, скорочень і термінів, вступу, основної частини (три розділі), висновків, переліку джерел посилання (72); містить 30 рисунків та 10 таблиць. Повний обсяг звіту – 88 сторінок. Мета індивідуального завдання, використані методи та отримані результати: метою індивідуального завдання є дослідження хімічних зв’язків та електрофізичних властивостей у плівках оксиду графену відновленого у ВЧ плазмовому розряді в атмосфері водню: провідність на змінному струмі, температурні залежності оксиду графену, визначення механізму провідності, температурного коефіцієнту питомого опору. Було показано, що низькотемпературна плазмова обробка оксиду графена в суміші азота і водню терміном 5 секунд призводить до значного зменшення електричного опору двовимірної плівки (до 8 порядків величини) значно більшого (до 2 порядків) ніж термічний відпал при 350ºС в вакуумі терміном 15 хвилин, що свідчить про вплив на плівку нетермічних факторів, які мають місце під час ВЧ плазмової обробки. Було встановлено, що механізм провідності плівки може бути описаний механізмом Мотта (стрибковою провідністю по пасткам, локалізованим біля рівня Фермі) на двох ділянках частотної і температурної залежностях провідності з різними параметрами, що свідчать про неоднорідність отриманої плівки. Плівки відновленого оксиду графена демонструють значний температурний коефіцієнт опору, значно кращий ніж золото і срібло, що дозволяє його пропанувати в якості сенсора температури в діапазоні від -50 до +100С. Новизна: вперше показано, що плівки оксиду графена можуть бути значно відновлені за допомогою низькотемпературного прямого впливу ВЧ плазмового розряду в атмосфері азотно-водневої суміші. Значний температурний коефіцієнт опору свідчить, що плівки відновленого графену можуть бути використані, як температурний сенсор на гнучкій пластиковій підкладці. Висновок: дослідження хімічних зв’язків у плівках оксиду графену методом ІЧ спектроскопії показують ефективне введення водневих і азотних зв’язків в графенову структуру під час обробки ВЧ плазмовим розрядом у формінг газі. Відпал при плазмовій обробці проводився при нижчих значеннях температури і тривалості, ніж термічний відпал однак провідність зразків після плазмової обробки є вищою на порядок, що свідчить про значний вплив на параметри матеріалу нетермічних факторів, що мають місце при плазмовій обробці матеріалу. Температурний коефіцієнт опору плазмово-відновленного оксиду графена значно вище, ніж у плівок золота, срібла та вуглецевих нанотрубок. Представлені результати свідчать, що відновлений за низьких температур оксид графену є дуже перспективним матеріалом для створення сенсорів температури на гнучких підкладках.
Topicality: oxide graphene - a single layer of graphite where carbon bonds on the surface are more connected with oxygen. This material just get in the hydrogen solution and precipitate any substrate. Reduced graphene oxide is a two dimensional material that is promising for the manufacture of various types of sensors - from infrared sensors to chemical gas sensors. Therefore, obtaining reduced graphene oxide with high electrical conductivity at low annealing temperatures allows to have a basic cheap two-dimensional material for different types of sensors on flexible substrates, which is necessary for the medical industry, robotics and flexible micro- and photoelectronics. Relationship of work with scientific programs, plans, themes cathedra: оbject of research: The theme of the work corresponds to the priority scientific direction of the Department of General Physics and Solid State Physics - "Fundamental research of the most important issues of scientific, technical, socioeconomic, human potential to ensure Ukraine's competitiveness in the world and sustainable development of society and state." The goal of the work: research the effect of plasma treatment on the physicochemical and electrophysical properties of graphene oxide, comparing them with low-temperature thermal reduction. Obtaining basic material for gas and temperature sensors. Object of research: research optical and electrophysical properties of graphene oxide films after various low-temperature annealing methods. Subject of research: graphene oxide reduced at low temperatures in the RF plasma discharge. Research methods: infrared spectroscopy, XPS spectroscopy, volt-ampere characteristics, temperature and frequency dependence of electrical conductivity. Information about the volume of the report, the number of illustrations, tables, applications and literary names in the list of used ones: the report consists of a list of symbols, symbols, abbreviations and terms, introduction, main part (three sections), conclusions, list of reference sources (72); contains 29 figures and tables. Full report – 85 pages. The purpose of the individual task, the methods used and the results obtained: the purpose of the individual task is to study the chemical bonds and electrophysical properties in films of graphene oxide reduced in RF plasma discharge in a hydrogen atmosphere: conductivity on alternating current, temperature dependences of graphene oxide, determination of the conductivity mechanism, temperature resistivity. It was shown that low-temperature plasma treatment of graphene oxide in a mixture of nitrogen and hydrogen for 5 seconds leads to a significant reduction in electrical resistance of the two-dimensional film (up to 8 orders of magnitude) much greater (up to 2 orders of magnitude) than thermal annealing at 350 ° C in vacuum for 15 minutes. indicates the effect on the film of non-thermal factors that occur during RF plasma treatment. It was found that the mechanism of film conductivity can be described by the Mott mechanism (hopping conductivity on traps located near the Fermi level) in two sections of frequency and temperature dependences of conductivity with different parameters indicating the heterogeneity of the obtained film. The reduced graphene oxide films show a significant temperature coefficient of resistance, much better than gold and silver, which allows it to be propagated as a temperature sensor in the range from - 50 to + 100C. Novelty: for the first time it was shown that graphene oxide films can be significantly reduced by low-temperature direct exposure to RF plasma discharge in an atmosphere of nitrogen-hydrogen mixture. The significant temperature coefficient of resistance indicates that the films of reduced graphene can be used as a temperature sensor on a flexible plastic substrate. Conclusion: research of chemical bonds in graphene oxide films by IR spectroscopy show the effective introduction of hydrogen and nitrogen bonds into the graphene structure during the treatment of RF plasma discharge in the forming gas. Annealing in plasma modification was performed at lower values of temperature and duration than thermal annealing, but the conductivity of the samples after plasma treatment is higher by an order of magnitude, indicating a significant effect on material parameters of non-thermal factors occurring in plasma modification of material. The temperature coefficient of resistance of plasma-reduced graphene oxide is much higher than that of gold, silver and carbon nanotube films. The presented results show that graphene oxide reduced at low temperatures is a very promising material for creating temperature sensors on flexible substrates.

Mecanisme du dopage par des cations organiques et cinetique d'insertion des cations dans le film. Pour un meme taux de dopage, la conductivite diminue lorsque la taille du cation insere augmente. La stabilite intrinseque des complexe (ch)::(x)/r::(4)n**(+), bonne a temperature ordinaire, est fortement activee a 100**(o)c (mecanisme de degradation du type hofman)

The electrical and optical properties of boron doped hydrogenated amorphous silicon thin films (a-Si) were investigated to determine the effect of boron and hydrogen incorporation on carrier transport. The a-Si thin films were grown by plasma enhanced chemical vapor deposition (PECVD) at various boron concentrations, hydrogen dilutions, and at differing growth temperatures. The temperature dependent conductivity generally follows the hopping conduction model. Above a critical temperature, the dominant conduction mechanism is Mott variable range hopping conductivity (M-VRH), where p = ¼, and the carrier hopping depends on energy. However, at lower temperatures, the coulomb interaction between charge carriers becomes important and Efros-Shklosvkii variable hopping (ES-VRH) conduction, where p=1/2, must be included to describe the total conductivity. To correlate changes in electrical conductivity to changes in the local crystalline order, the transverse optical (TO) and transverse acoustic (TA) modes of the Raman spectra were studied to relate changes in short- and mid-range order to the effects of growth temperature, boron, and hydrogen incorporation. With an increase of hydrogen and/or growth temperature, both short and mid-range order improve, whereas the addition of boron results in the degradation of short range order. It is seen that there is a direct correlation between the electrical conductivity and changes in the short and mid-range order resulting from the passivation of defects by hydrogen and the creation of trap states by boron. This work was done under the ARO grant W911NF-10-1-0410, William W. Clark Program Manager. The samples were provided by L-3 Communications.

Preparation de couches minces par pelliculage sur substrat de verre metallique ou semiconducteur. L'adherence se fait par des liaisons chimiques; les films resistent a la gravure par plasma fluore. Mecanisme de la conduction electrique

L'elaboration d'une nouvelle classe d'heterocycles soufres 2 12, bases sur le motif tetrathiapentalenes (ttp) est decrite. La synthese des tetrathiapentalenes disubstitues se deroule en une seule etape par couplage de la thiapendione avec dix differents aldehydes (rcho, r = phenyle, thienyle, furyle, etc) dans le phosphite de triethyle. Les monomeres sont caracterises par leurs proprietes spectroscopiques : uv-visible, ir, rmn, rpe, spectroscopie de masse, et leur proprietes electrochimiques. Excepte pour le compose 10, l'oxydation electrochimique des composes 2 12 conduit a la formation de polymeres a hauts potentiels. Les composes 2 (r = ph) et 7 (r = th) ont ete plus particulierement etudies. Les polymeres correspondants ont ete obtenus, ici encore en une seule etape, soit par voie chimique soit par voie electrochimique. Ces polymeres sont conducteurs : le poly-2 a une conductivite de 10##5 s/cm et le poly-7 a une conductivite de 10##2 cm. L'ensemble des proprietes spectroscopiques (uv-visible, ir, rpe) indique que le poly-2 est un polymere base sur le motif ttf vinylogue. Le mecanisme d'electropolymerisation du poly-2 met en jeu la dimerisation du monomere et la formation a haut potentiel du radical trication du dimere. Des experiences de voltamperometrie cyclique couplees d'une part a la rpe et d'autre part a la conductivite ont montre que l'oxydation du poly-2 donne naissance a trois systemes polarons bipolarons et que l'etat conducteur correspond au polymere a demi-charge presentant le maximum de spins. En revanche, le poly-7, motif alterne ttp et bithienyle, a montre deux systemes polarons bipolarons. Enfin, les etudes saxs (source synchrotron esrf) ont mis en evidence la nature polymerique de ces materiaux.

Ce travail est consacre a l'etude des proprietes de transport, conductivite et pouvoir thermoelectrique, de plusieurs polymeres conducteurs electroniques. Le suivi de l'evolution de ces proprietes en fonction d'un seul parametre de structure, a savoir: le desordre cree par le vieillissement dans le polypyrrole ou dans la polyaniline protonee a l'acide camphre sulfonique, le taux de reticulation dans des reseaux a base de polyoctylthiophene, ou la composition dans des composites polyaniline-nafion ou polyaniline-polyoxyde d'ethylene, nous a permis de preciser la nature du processus de conduction dans chacun de ces materiaux, et de degager des caracteristiques communes a ces systemes. Dans les polymeres conducteurs tres desordonnes, on observe un processus de sauts limite par l'energie de charge entre ilots conducteurs polaroniques separes par des barrieres isolantes. Nous montrons que la variation thermique de la conductivite observee experimentalement peut etre obtenue par des simulations numeriques a partir de methodes de milieu effectif. Dans les reseaux de polyoctylthiophene, une correlation est fortement suggeree entre ilots conducteurs et ilots cristallins. Dans les melanges a base de polyaniline, les ecarts observes par rapport a la theorie classique de la percolation sont attribues a ces processus de hopping. Pour des niveaux de conduction croissants, interviennent un effet tunnel entre gros ilots induit par les fluctuations thermiques puis des processus de conduction de type metallique, lies a la structure interne des grains conducteurs et de nature quasi-unidimensionnel. Les mesures de pouvoir thermoelectrique confirment cette image heterogene de la structure. De facon generale, a un terme lineaire en fonction de la temperature, de type metallique, se superpose, une contribution de type semi-conducteur amorphe. Dans les polymeres conducteurs, c'est la microstructure, les heterogeneites de dopage ou les zones plus ou moins cristallines qui determinent en premier lieu les proprietes de transport macroscopiques. La structure locale des chaines intervient dans les processus de conduction a l'interieur des ilots conducteurs et en filigrane dans les proprietes de transport macroscopiques. Dans cette perspective, le vieillissement peut etre decrit comme un grignotage d'ilots conducteurs au profit de barrieres isolantes

Isotropically conductive adhesives (ICAs) are promising as a lead-free interconnect material; However, ICAs have a higher resistivity compared to tin/lead solder. The higher resistivity of ICAs results from the large contact resistance between conductive fillers. Several novel approaches to engineer the interface between electrically conductive fillers were studied to develop highly conductive ICAs. Shown in this dissertation are three methodologies to reduce contact resistance: low temperature sintering, fast sintering and in-situ reduction. Furthermore, two approaches, surface modification and in-situ protection, were developed to prevent oxidation and corrosion of silver-coated copper flakes to produce low cost ICAs. The findings and insights in this dissertation significantly contribute to (1) understanding of filler-filler, filler-polymer and structure-property relationships of ICAs; (2) the structural design and formulation of high performance ICAs; and (3) the wider use of ICAs in emerging applications such as printed electronics and solar cells.

Negli ultimi anni l’impiego sempre più diffuso di energie rinnovabili e la necessità di una rete sempre più robusta ed interconnessa ha reso fondamentale l’impiego di linee in HVDC. All'interno di queste linee i componenti più soggetti a guasti sono giunti e terminali, i quali presentano una zona particolarmente critica: la zona interfacciale. Quest’ultima è soggetta ad un elevato campo elettrico tangenziale, il quale cambia la sua distribuzione a seconda di vari fattori. Questo lavoro ha dunque come obiettivo quello di indagare a fondo su tale zona, per scoprirne i meccanismi di guasto e per trovare dei marker diagnostici per poter perfezionare il design dei giunti e monitorare il loro stato. L'analisi è stata svolta utilizzando elettrodi con profilo Rogowski sputterati su dei fogli di XLPE ed inseriti in apposite presse meccaniche. Sono state svolte due tipologie di prove: una in cui si è misurata la conducibilità dei provini a vari campi elettrici e una in cui si è monitorato la carica di spazio accumulata all’interno dei provini; l’attività di scariche parziali è stata monitorata durante entrambe le prove. Tra i possibili marker diagnostici di pre-breakdown non è stata rilevata l’utilità del monitoraggio di scariche parziali, che sembrano poter essere escluse dai meccanismi di guasto. Si nota d’altro canto un importante contributo della ridistribuzione di carica all’interno del provino, che deforma notevolmente il campo elettrico al suo interno, ed è responsabile di un forte aumento di conducibilità qualche ora prima del breakdown; venendo quindi proposto come possibile parametro di monitoraggio.

FREIRE, Márcio de Melo. Funções de Green em Mecânica Estatística. 2014. 56 f. Dissertação (Mestrado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2014.
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Neste trabalho estabeleceremos as definições das funções de Green em mecânica estatística e suas propriedades básicas. Estas funções dependem duplamente do tempo e da temperatura. Isto pode ser observado por meio de suas definições, onde aparecem os valores médios dos produtos de operadores. Neste caso a média é feita sobre o ensemble grão-canônico. Os operadores envolvidos nestas funções satisfazem a equação de movimento de Heisenberg, o que nos permite descrever as equações de evolução para as funções de Green. Por meio da representação espectral das funções de correlação temporal, que é feita através da introdução de uma transformada de Fourier para mudar o sistema do espaço dos tempos para o espaço das frequências, podemos obter as representações espectrais para as funções de Green retardada, avançada e causal. Por último, faremos o uso da função de Green retardada para descrever a condutividade elétrica de um sistema de elétrons submetido a um campo elétrico externo dependente de tempo, em outras palavras, descreveremos o tensor de condutividade elétrica em termos da função de Green retardada e, por último, calcularemos a condutividade elétrica de um sistema de elétrons e fônons.

Conselho Nacional de Desenvolvimento CientÃfico e TecnolÃgico
Neste trabalho estabeleceremos as definiÃÃes das funÃÃes de Green em mecÃnica estatÃstica e suas propriedades bÃsicas. Estas funÃÃes dependem duplamente do tempo e da temperatura. Isto pode ser observado por meio de suas definiÃÃes, onde aparecem os valores mÃdios dos produtos de operadores. Neste caso a mÃdia à feita sobre o ensemble grÃo-canÃnico. Os operadores envolvidos nestas funÃÃes satisfazem a equaÃÃo de movimento de Heisenberg, o que nos permite descrever as equaÃÃes de evoluÃÃo para as funÃÃes de Green. Por meio da representaÃÃo espectral das funÃÃes de correlaÃÃo temporal, que à feita atravÃs da introduÃÃo de uma transformada de Fourier para mudar o sistema do espaÃo dos tempos para o espaÃo das frequÃncias, podemos obter as representaÃÃes espectrais para as funÃÃes de Green retardada, avanÃada e causal. Por Ãltimo, faremos o uso da funÃÃo de Green retardada para descrever a condutividade elÃtrica de um sistema de elÃtrons submetido a um campo elÃtrico externo dependente de tempo, em outras palavras, descreveremos o tensor de condutividade elÃtrica em termos da funÃÃo de Green retardada e, por Ãltimo, calcularemos a condutividade elÃtrica de um sistema de elÃtrons e fÃnons.

Pour étendre l'utilisation des composites dans des applications plus variées (applications intelligentes et multifonctionnelles), l'une des barrières est leur faible conductivité électrique et thermique. Dans le cas de composites renforcés par des fibres de carbone, la matrice organique est responsable des propriétés isolantes du composite résultant. L'une des solutions pour améliorer les conductivités des matériaux est l'utilisation des nanocharges conductrices. L'amélioration des propriétés électriques et thermiques des polymères nanochargés est une problématique récurrente dans la littérature. Cependant, étudier les propriétés des composites à fibre de carbone continue et nanochargés est moins abordée. Ce travail porte sur la fabrication et la caractérisation des composites nanochargés par du noir de carbone et des nanotubes de carbone. Tout d'abord, un intérêt particulier a été accordé à la phase délicate de la fabrication. Comme mentionné ci-dessus, la mise en œuvre des composites à renfort continu et matrice nanochargée implique des problèmes liés à l'agglomération et à la dispersion inhomogène des nanocharges dans le composite final. Pour résoudre ces problèmes, le choix de la matrice thermoplastique (Polyamide 6) était judicieux. En fait, la dispersion des nanocharges a été faite par extrusion bi-vis qui est connue comme l'une des voies les plus efficaces de séparation d'agglomérats. De plus, la méthode de fabrication à base de films de Polyamide 6, appelée film stacking, assure une partition homogène dès le début du processus. Des observations MEB ont été effectuées pour localiser les nanoparticules. Ceux-là ont montré que les particules pénétraient dans la zone des fibres. En effet, en atteignant le cœur des torons, les nano-charges ont créé un réseau de connectivité entre les fibres pour le passage de courant. Ceci explique l'amélioration constatée de la conductivité électrique des composites en présence de noir de carbone et des nanotubes de carbone. Ces essais ont été réalisés avec la méthode à 4 points. La conductivité électrique du composite à matrice « pure » est passée de 20S / cm à 80S / cm en ajoutant 8% en poids de noir de carbone et à 15S / cm en ajoutant 18% en poids de la même charge nanométrique. Pour les nanotubes de carbone, avec 2,5% en poids, la conductivité était d'environ 150S / cm. Pour les propriétés thermiques, des tests basés sur l'effet Joule ont été réalisés. L'augmentation de la température a été enregistrée en utilisant une caméra IR. Les résultats obtenus sont en accord avec ceux de la conductivité électrique, montrant une amélioration du comportement thermique en présence de nanocharges. Grâce à ces résultats, l'utilisation de ces composites comme outil de suivi d’endommagement était possible. Par ailleurs, la méthode de variation de la résistance électrique a été effectuée. Les matériaux nanochargés ont montré une meilleure sensibilité aux endommagements. Les résultats ont été comparés aux outils classiques de suivi d’endommagement. A la fin, plusieurs applications « intelligentes » ont été testées telles que : le composite à gradients de propriétés et des matériaux nanochargés cousus
To extend the use of composites in more varied application (smart applications, multifunctional issues), one of the actual barrier is their poor electrical and thermal conductivities. In the case of carbon fiber reinforced composites, organic matrix are in charge of the insulating properties of the resulting composite. One of the solutions to enhance conductivities of materials is the use of conductive nanofillers. Improving the electrical and thermal properties of nanofilled polymers has been investigated in several studies. However, studiing the properties of continuous carbon fiber nano-filled composites is less approached. This work tends to fabricate and characterize carbon black and carbon nanotubes nano-filled composites. First of all, special interest was given to the delicate phase of manufacturing. As mentioned before, processing continuous fiber reinforced nanofilled polymers implies issues related to nanofillers agglomeration and inhomogeneous dispersion in the final composite. To resolve these problems, the choice of the thermoplastic (Polyamide6) matrix seemed preferable. In fact, the dispersion of nanofillers was made by twin screw extrusion which is known as one of the most effective agglomeration separation ways. Adding to this, the fabrication method based on Polyamide 6 shects called film stacking, ensure a homogeneous partition at the beginning of the process. SEM observations were performed to localize the nano-particles. It showed that particles penetrated on the fiber zone. In fact, by reaching the fiber zone, the nano-fillers created network connectivity between fibers which means an easy pathway for the current. It explains the noticed improvement of the electrical conductivity of the composites by adding carbon black and carbon nanotube. This test was performed with the 4 points electrical circuit. It shows that electrical conductivity of 'neat' matrix composite passed from 20S/cm to 80S/cm by adding 8wt% of carbon black and to 15S/cm by adding 18wt% of the same nano-filler. For carbon nanotubes, with '2.5wt% the conductivity was around 150S/cm. For the thermal properties, tests based on Joule's effect were performed. The rise of temperature was recorded using IR camera. Results obtained are in agreement with the electrical conductivity ones, showing enhancement of the thermal behavior in presence of nanofillers. Thanks to these results, the use of these composites as a damage-monitoring tool was possible. By the way, the electrical resistance change method was performed. Nanofilled materials showed better sensitivity to damage. Results were compared with classical damage monitoring tools. At the end, several 'smart' applications were tested such as graded functionalities composite and stitched nanofilled materials

碩士
國立清華大學
化學系
104
We have fabricated small PbS nanocrystals (cubes: 21 nm; octahedra: 57 nm) in aqueous solution using an one-step approach. We gain knowledge in manipulating their shape and size by clear exploration of the relation between nucleation and crystal growth. This enables us to scale up the production using a more intuitive method. Also, the refined seeding growth method allows us to enlarge the size of PbS polyhedra, fulfilling single particle electrical conductivity measurements. Facet-dependent electrical conductivity of Cu2O crystals has been previously revealed. In this study, we have again successfully demonstrated the facet-dependent electrical property of PbS using face-raised cubes, edge-and corner-truncated cubes, edge- and corner-truncated octahedra and pristine octahedra with the size of several hundreds of nanometers. Two tungsten probes are manipulated to contact the opposite faces of a single particle. The {110} facets showed the highest electrical conductivity, followed by the {100} facets and the {111} facets were observed to be the least conductive. The {111} facets are 500 times less conductive than the {110} faces at an applied voltage of 5 V. This study suggests that facet-dependent electrical conductivity should be regarded as an intrinsic property for semiconductors.

Dislocations are attracting increasing attention for their potential to tune a whole range of functional properties in ceramics. As one-dimensional line defects, they influence their environment by both their elastic strain field as well as their charged core with compensating space charge. This can be a competing approach to point defect doping. Their geometric versatility and localized impact simultaneously allow new design dimensions and pose new layers of complexity. Dislocations also severely impact mechanical properties, which demands an interrelated understanding of both their mechanics and their impact on each functional property, severely complicating research efforts. In particular, the detailed mechanisms by which dislocations impact functional properties, such as electrical and thermal conductivity, and the respective design parameters are not fully explored. Furthermore, ways to introduce dislocations into polycrystalline samples need to be developed. And lastly, the role of dislocations on the fracture toughness of ceramics needs to be revisited. Literature on all of these three questions is abundantly available. However, it is scattered over decades and a range of different materials, which impedes overview and quick identification of critical questions to be addressed. In this work, key potential is identified by first presenting a simple perspective on dislocation mechanics in ceramics. This leads to a longer discussion on room temperature plasticity and fracture toughness: Dislocations are mobile at ambient temperature in a broad range of ceramics. While these single crystals are ductile, their respective polycrystals not ductile and both behave brittle despite their ductility. A coherent explanation for this seemingly contradicting mechanical behavior is attempted while suggesting that dislocations can unexpectedly enhance the fracture toughness of ceramics substantially. Furthermore, dislocation structures are introduced into polycrystals in compression tests at elevated temperatures. This allows the rationalization of the high-temperature deformability and designing dislocation structures in polycrystals. Lastly, a conceptual framework is developed to simplify pointed discussions on the impact of dislocations on functional properties such as conductivity. In each of the three topics discussed, 1) room temperature plasticity and fracture toughness, 2) high temperature plasticity, and 3) conductivity, attempts will be made to fully embrace the mechanistic complexity. Simpler perspectives will be derived allowing easier and more accurate judgement of the potential in each of the three fields.

博士
國立成功大學
材料科學及工程學系碩博士班
98
NiO has the rock salt structure of p-type semiconductors. Its electrical conductivity has made NiO useful in a wide range of applications, such as transparent conductive films, solar cells, electrochromic devices, and gas sensors. Although a number of reports have discussed the properties of sputtered NiO films for various parameters, some electrical properties of sputtered NiO are still unclear. The dominant point defects in sputtered NiO films are unknown. According to defects chemistry, the major carriers of a p-type NiO semiconductor come from the vacancies of metal ions or the interstitial oxygen ions formed in NiO crystal. It is unclear which one is dominant. Sputtered NiO film experiences electrical aging. The resistivity of the sputtered film increases with time exposed to air. Electrical aging is not common in n-type semiconductors and it has been rarely discussed in p-type semiconductors. However, it greatly affects the longevity and applications of devices. Therefore, the study of its mechanism and suppression is very important. Finally, we discuss some reduction properties of NiO film annealed in a vacuum environment. Although reduction has been previously reported, its mechanism is still ambiguous. To investigate the electrical conduction mechanism, an RF sputter and a NiO target were used to deposit NiO films. The composition of double layer films were adjusted by changing the working gas and the in-depth composition were measured by SIMS and XPS. The results show that the degree of non-stoichiometry of NiO films was determined by Ni content. Also, the coordination numbers of the annealed NiO films were measured by X-ray absorption spectroscopy to confirm the in-depth composition data. The results show that non-stoichiometry NiO contains more oxygen atoms than stoichiometric NiO. When the composition was made close to that of stoichiometry via the annealing process, the first coordination number shell does not change while the second coordination number shell increased, which implies that the composition change results from a change in Ni atoms. It was concluded that Ni vacancies are the dominant point defects that result in the electrical conductivity of NiO films. To investigate electrical aging, stability tests were performed in various dry atmospheres (H2, CO, O2, CO2, N2 and Ar) and in a humid Ar environment. The stability was determined by measuring the resistance change with time. The results show that electrical aging was caused by the adsorption of reduction gas and water vapor, which injected electrons into the NiO film. These electrons then counteracted with the redundant electric holes and attenuated the charge carriers in the NiO film. This reaction lowered the carrier concentration; the electrical conductivity of NiO film subsequently decayed with time. Electrical aging is due to gas adsorption; which greatly affected by film structure. In this study, the substrate temperature and Li doping were used to the suppress aging. The results show that an increase of substrate temperature changes the crystal structure. The preferred orientation changes from polar (111) into non-polar (200); this change starts from the top surface to the substrate. The dangling bonds on (111) surface increase the aging phenomena. The formation of non-polar (200) can decrease the aging rate of sputtered NiO films. For Li doping, the concentration of Li in the thin films was adjusted by placing 0~15 Li2O disks on the target surface. The Li concentration in the films varied from 0 to 16.29 at.%, as determined by WDS and ICP-MS. The results show that the doped Li ions occupy crystal defect sites such as vacancies or segregate on the film surface. Initially, doped Li occupied the Ni vacancies in the film, decreasing electrical conductivity. When the Li concentration was further increased, some Li segregated on the film surface and formed bulges at high Li concentrations. These Li-rich oxides which covered the film surface served as partitions between the film and moisture from the atmosphere. Also, the doping process decreased the (111) peak, which means that it suppresses the formation of the (111) plane. As a result, the Li-doped NiO films show a relatively high arrestment to electrical resistance aging. The effects of annealing temperature, substrate material, and gas atmosphere on reduction properties were studied. The results show that reduction is also related to the non-stoichiometry of the sputtered NiO films. The reduction occurs from the film surfaces and the reduction depth is about 100 nm.

I have worked on many projects, but there are several things that they all had in common. First, nearly all projects involved searching for the structural parameters that governed the macroscopic properties of the polymers and composite materials. A second common denominator is that even though my work was performed in an “academic context”, the goals were targeted toward industrial needs. Lastly, the methods and procedures were similar; they were all based on experimental results obtained for various scales of measurement (see Fig. 1). Hence, multi-scale modeling was very useful and beneficial for these projects. The models developed (mainly numerical and sometimes analytical) were initially derived from experimental evidence and then validated and improved with further experimentation. The refined models provided an efficient means of: (i) optimizing the composites according to specific needs, (ii) better understanding the hierarchical relations between the different scales, (iii) controlling the micro or meso structure and thereby the macroscopic properties. This study of the relations between structure and properties was performed on a wide variety of physical properties and materials. However, the electric and dielectric properties of composites constituted the major- ity of it and will be presented in this report. The remaining property investigations provided supplemental but valuable information. This work often requires altering various conventional experimental techniques or using well-known techniques for new purposes. I also developed, when needed, several unconventional but necessary measurement techniques. This report contains two major parts which are separated according to the nature of the fillers: Part I : Conducting fillers. In the first part, the main interest both for application and fundamental point of view, is related to the changes in properties in the vicinity of the sharp percolation transition. After a brief introduction to the percolation theory, this part will be subdivided in three chapters: Chapter 1. presents a numerical method that correlates the mesostructure to the macroscopic electrical properties both in two and three dimensions. Chapter 2. will show that an external variable (the mechanical stress) may largely alter the microstruc- ture of the percolating network within composites as revealed the macroscopic conductivity. The understanding of the mesoscale changes will be based on the chemical structure of the polymer matrix. Chapter 3. is devoted to the description of a unique case in term of percolation behavior, which made possible the control of the phase arrangement within the composite and thereby the control of the macroscopic resistivity. p. 2 Multiscale relationships in polymer–based heterogeneous systems. . . Part II : Insulating fillers. In the second part, the main interest is to obtain good electrical insulators, i.e. that can withstand large electric fields. This part thus starts with a brief introduction to the common failure mechanisms, associated with the dielectric breakdown and is also divided in three chapters: Chapter 4. is devoted to the description of a numerical simulation of the relationships between mesostructure and dielectric breakdown. Chapter 5. reveals the influence of the processing conditions of a composite utilized in the industry on the microstructure and the quantitative consequences on breakdown properties. Chapter 6. presents the aging of these composites under “real word” conditions which will further be compared to accelerated aging performed in controlled conditions, in the laboratory. A comparison of the two aging situations will furnish a quantitative understanding of the relative influence of the chemical and physical contributions to the aging process. This report will then be concluded with a description of the current and future projects.