Dissertations / Theses on the topic 'Gas barrier properties'

Microfibrillated cellulose (MFC) films have been prepared to demonstrate the potential of the material in order to see if it is a possibility to replace most parts of petroleum-based plastics. MFC is produced by mechanically delaminating the wood fiber cell wall into individual nano-fibrils. There are different ways to fabricate MFC at low energy consumption, thus keeping the cost down. The MFC films were produced using two different techniques; solvent-casting and by vacuum filtering, nanofiltration. The MFC films were characterized in terms of optical, mechanical and gas barrier properties. The results show that MFC (Generation 2) has superior mechanical properties and very promising oxygen barrier properties. MFC (Generation 2) has a more dense and homogenous nanocellulose film structure, thus very good oxygen permeability properties. These properties are good indications that MFC films have many suitable positive properties for use in e.g. the packaging industry. Incorporations of different components, such as nanoclays, latex or ionic crossed-linked improves the gas barrier properties for the MFC films.
Detta examensarbete handlar om filmer tillverkade av mikrofibrilär cellulosa (MFC). Syftet med studien var att se om det går att ersätta dagens petroleumbaserade plaster med MFC-filmer. Olika typer av MFC undersöktes, och detta gjordes genom olika tester av de mekaniska och optiska egenskaperna, samt en undersökning för att se permeabiliteten för syre genom MFC-filmer. MFC framställs genom att man mekaniskt bearbetar träfibrer och delaminerar på fibrillaggregaten som cellväggarna är delvis uppbyggda av. Det finns flera olika sätt att framställa MFC och hålla nere energikonsumtionen och därmed kostnaderna. Filmerna framställdes genom två olika sätt, det ena enklare genom att låta vattnet avdunsta i ett konditionerat rum, och den andra genom att vakuumfiltrera. Resultaten visar att typen, även kallad Generation 2, har bäst mekaniska egenskaper jämfört med alla andra generationer, samtidigt som den har bäst förmåga att motstå syrepermeabilitet. Generation 2 har en mer homogen filmstruktur, därav klarar den av att motstå permeabiliteten för syre. Behandlingen av olika preparat, så som nanoleror, latex och jonbyten kommer påverka egenskaperna för MFC filmerna.

As nanocomposites provide considerable improvements in material properties, scientists and engineers are focussing on biodegradable nanocomposites having superior material properties as well as degradability. This thesis has investigated the properties of biodegradable nanocomposites of the aliphatic thermoplastic polyester, poly (lactide acid) (PLA) and the synthetic biodegradable polyester, poly (butylene succinate) (PBS). To enhance the properties of this blend, nanometer-sized clay particles, have been added to produce tertiary nanocomposite. High aspect ratio and surface area of clay provide significant improvement in structural, mechanical, thermal and barrier properties in comparison to the base polymer. In this study, a series of PLA/PBS/layered silicate nanocomposites were produced by using a simple twin-screw extruder. PLA/PBS/Cloisite 30BX nanocomposites were prepared containing 1, 3, 5, 7 and 10 wt% of C30BX clay, while PLA and PBS polymers compositions were fixed at a ratio of 80 to 20. This study also included the validation of a gas barrier model for these biodegradable nanocomposites. WAXD indicated an exfoliated structure for nanocomposites having 1 and 3 wt% of clay, while predominantly development of intercalated structures was noticed for nanocomposites higher than 5 wt% of clay. However, TEM images confirmed a mixed morphology of intercalated and exfoliated structure for nanocomposite having 1 wt% of clay, while some clusters or agglomerated tactoids were detected for nanocomposites having more than 3 wt% of clay contents. The percolation threshold region for these nanocomposites lied between 3-5 wt% of clay loadings. Liquid-like behaviour of PLA/PBS blends gradually changed to solid-like behaviour with the increase in concentration of clay. Shear viscosity for the nanocomposites decreased as shear rate increased, exhibiting shear thinning non-Newtonian behaviour. Tensile strength and Young's modulus initially increased for nanocomposites of up to 3 wt% of clay but then decreased with the introduction of more clay. At high clay content (more than 3 wt%), clay particles tend to aggregate which causes microcracks at the interface of clay-polymer by lowering the polymer-clay interaction. Percentage elongation at break did not show any improvement with the addition of clay. PLA/PBS blends were considered as immiscible with each other as two separate glass transition and melting temperatures were observed in modulated differential scanning calorimetry (MDSC) thermograms. MDSC showed that crystallinity of the nanocomposites was not much affected by the addition of clay and hence some compatibilizer is required. Thermogravimetric analysis showed that the nanocomposite containing 3 wt% of clay demonstrated highest thermal stability compared to other nanocomposites. Decrease in thermal stability was noticed above 3 wt% clay; however the initial degradation temperature of nanocomposites with 5, 7 and 10 wt% of clay was higher than that of PLA/PBS blend alone. Gas barrier property measurements were undertaken to investigate the transmission of oxygen gas and water vapours. Oxygen barrier properties showed significant improvement with these nanocomposites, while that for water vapour modest improvement was observed. By comparing the relative permeabilities obtained from the experiments and the model, it was concluded that PLA/PBS/clay nanocomposites validated the Bharadwaj model for up to 3 wt% of clay concentration.

Coatings are frequently applied on gas turbine components inorder to restrict surface degradation such as corrosion andoxidation of the structural material or to thermally insulatethe structural material against the hot environment, therebyincreasing the efficiency of the turbine. However, in order toobtain accurate lifetime expectancies and performance of thecoatings system it is necessary to have a reliableunderstanding of the mechanical properties and failuremechanisms of the coatings.

In this thesis, mechanical and fracture behaviour have beenstudied for a NiAl coating applied by a pack cementationprocess, an air-plasma sprayed NiCoCrAlY bondcoat, a vacuumplasma-sprayed NiCrAlY bondcoat and an air plasma-sprayed ZrO₂+ 6-8 % Y₂O₃topcoat. The mechanical tests were carried out ata temperature interval between room temperature and 860oC.Small punch tests and spherical indentation were the testmethods applied for this purpose, in which existing bending andindentation theory were adopted for interpretation of the testresults. Efforts were made to validate the test methods toensure their relevance for coating property measurements. Itwas found that the combination of these two methods givescapability to predict the temperature dependence of severalrelevant mechanical properties of gas turbine coatings, forexample the hardness, elastic modulus, yield strength, fracturestrength, flow stress-strain behaviour and ductility.Furthermore, the plasma-sprayed coatings were tested in bothas-coated and heat-treated condition, which revealedsignificant difference in properties. Microstructuralexamination of the bondcoats showed that oxidation with loss ofaluminium plays an important role in the coating degradationand for the property changes in the coatings.

Keywords:small punch test, miniaturised disc bendingtests, spherical indentation, coatings, NiAl, APS-NiCoCrAlY,VPS-NiCrAlY, mechanical properties

L’éthylène-alcool vinylique (EVOH) est un copolymère utilisé fréquemment dans les emballages alimentaires en raison de ses fortes capacités barrière aux gaz, qui sont directement liées à la structure cristalline et au réseau de liaisons hydrogènes du matériau, mais sont de ce fait très sensibles à la présence d’humidité. Les films pour l'emballage alimentaire sont ainsi classiquement formés par la combinaison de ce polymère avec un polymère peu sensible à l’eau pour le protéger (polyéthylène ou polypropylène généralement) sous forme d'une structure multicouche, formée par le procédé de coextrusion. L’objectif de ce travail de thèse a été d’étudier le comportement et plus précisément la cristallisation de l’EVOH dans un système multinanocouche, obtenu à l’aide d’un procédé innovant de coextrusion multinanocouches, qui peut amener un confinement de l’EVOH. Pour cela, nous avons tout d’abord étudié la cristallisation du matériau en masse, d’un point de vue morphologique et cinétique, en conditions isothermes et anisothermes. Par la suite, l’EVOH sous confinement a été étudié dans des films ultra minces et dans des films multinanocouches, en le combinant avec un copolymère d’oléfine cyclique (COC), dont la haute température de transition vitreuse a induit un confinement « dur » de l’EVOH. Nous avons finalement fabriqué deux types de films multinanocouches industriels dédiés à l’emballage alimentaire, à l’aide dans un premier temps d’une polyoléfine classique, PE linéaire à basse densité, et ensuite d’une combinaison de deux grades d’EVOH. Des films multinanocouches transparents avec des couches homogènes, continues et de très faibles épaisseurs (quelques dizaines de nanomètres) ont ainsi été fabriqués. Ces deux systèmes ont mis en évidence une amélioration substantielle de leur perméabilité à l’oxygène en conditions humides. Des études complémentaires ont été effectuées et plusieurs hypothèses formulées en vue d’expliquer l’origine de ces améliorations
Ethylene-vinyl alcohol (EVOH) is a copolymer commonly used in food packaging because of its high gas barrier properties, directly related to the crystalline structure and the hydrogen bonds network of the material, but therefore very sensitive to the presence of moisture. Films for food packaging are thus conventionally formed by the combination of EVOH with a water-insensitive polymer to protect it (usually polyethylene or polypropylene), in a multilayered structure fabricated by a coextrusion process. The aim of this work was to study the behavior and more precisely the crystallization of EVOH in a multinanolayered system, obtained thanks to an innovative multilayer process that can induce a confinement of EVOH. For this aim, we have first studied the crystallization of the bulk material, from a morphological and kinetic point of view under isothermal and non-isothermal conditions. Thereafter, EVOH behavior under confinement was studied in ultra-thin films and in multinanolayered films by combining it with a cyclic olefin copolymer (COC), whose high glass transition temperature induced a "hard" confinement of EVOH. We have finally manufactured two multinanolayered industrial systems, dedicated to food packaging, using, first, a conventional polyolefin (linear low-density PE) and then, a combination of two grades of EVOH. Transparent multinanolayered films with homogeneous, continuous and thin layers (several tens of nanometers) have thus been manufactured. These two systems have shown a significant improvement in their oxygen permeability under humid conditions. Complementary experiments have been performed and several hypotheses formulated in order to explain the origin of this improvement

Materiais nanoestruturados têm recebido destaque na comunidade científica, destacando-se, dentre eles, os nanocompósitos à base de polímeros e argila. Quando esses materiais são obtidos no estado líquido, ressalta-se também o uso de água em substituição a solventes orgânicos, devido a questões ambientais. Neste trabalho foram sintetizadas dispersões aquosas à base de poliuretanos (WPUs) e argila hidrofílica do tipo montimorilonita (MMT) de natureza sódica, com o objetivo de avaliar as propriedades de barreira a gases conferidas pela presença de argila e pela variação nas proporções entre os segmentos flexíveis poli(glicol propilênico) (PPG) e o copolímero em bloco à base de poli(glicol etilênico) e poli(glicol propilênico) (EG-b-PG). Os monômeros empregados na síntese foram: poli(glicol propilênico) (PPG); copolímero em bloco à base de poli(glicol etilênico) e poli(glicol propilênico) (EG-b-PG), com teor de 7% de EG; ácido dimetilolpropiônico (DMPA), diisocianato de isoforona (IPDI) e etilenodiamina (EDA), como extensor de cadeia. Foram sintetizadas dispersões aquosas com e sem a presença de argila, fixando-se a razão entre o número de equivalentes-grama de grupos diisocianato e hidroxila (razão NCO/OH) em 1,5. Nas formulações foi variado também o teor de argila em relação à massa de prepolímero em 0,5% e 1%. Foi adicionada uma etapa de agitação adicional com dispersor Turrax em algumas formulações. A argila foi previamente deslaminada em água deionizada e incorporada à formulação na etapa da dispersão do prepolímero. As dispersões foram avaliadas, quanto ao teor de sólidos totais, tamanho médio de partícula e viscosidade aparente. Os filmes vazados a partir das dispersões foram caracterizados por espectrometria na região do infravermelho (FTIR) e permeabilidade ao CO2. A resistência térmica dos filmes foi determinada por termogravimetria (TG). Foram observadas modificações nas propriedades dos filmes obtidos com a inserção da argila e com a variação no teor de segmentos à base de poli(glicol etilênico). A inserção da argila promoveu uma melhoria na resistência térmica das membranas bem como uma redução na permeabilidade das mesmas. Foi observado um aumento na permeabilidade das membranas obtidas a partir das formulações com maior percentual de copolímero (EG-b-PG), com e sem argila.
Nanostructured materials have been receiving increasing attention of scientific community, especially systems of nanocomposites based on polymer and clay. These materials can be obtained in aqueous state and the use of water, replacing organic solvents, is an important line of research due to environmental issues. In this work, dispersions based on waterbone polyurethanes and sodic hydrophilic clay montmorillonite (MMT) were synthesized, in order to evaluate the possible formation of nanocomposites (NWPUs). The monomers employed in the synthesis were: poly (propylene glycol) (PPG), block copolymer based on poly (ethylene glycol) and poly (propylene glycol) (EG-b-PG), containing 7% ethylene glycol; dimethylolpropionic acid (DMPA); isophorone diisocyanate (IPDI); and ethylenediamine (EDA) as chain extender. Aqueous dispersions with and without clay were synthesized by fixing the ratio between the equivalent-grams of diisocyanate and hydroxyl groups (NCO/OH ratio) in 1.5. The clay content also varied from 0.5% to 1% related to prepolymer initial mass. An additional stirring step with Turrax disperser was made in some formulations. The clay was previously delaminated in deionized water and incorporated at prepolymer dispersion in water-addition step. The dispersions were then evaluated in terms of solids content, particle size and apparent viscosity. The films cast from dispersions were characterized by infrared spectrometry (FTIR), thermogravimetry (TG) and permeability to CO2. Cast films were obtained and the gas barrier properties imparted by the clay were analyzed as well as the influence of poly (ethylene glycol) segments content. The insertion of clay imparted an improvement in thermal resistance properties and a decrease in membrane permeability. It was also observed an increase in permeability to CO2 of membranes obtained from formulations containing the higher amount of copolymer (EG-b-PG), independent of clay content.

Le manuscrit porte sur l'élaboration de couches minces ayant des propriétés barrières aux liquides, aux gaz et aux ultra-violets. Pour réaliser nos différents systèmes, la technologie plasma à décharge à barrière diélectrique à la pression atmosphérique (DBD) a été utilisée. Dans la première partie, des films polymère plasma à base de 1H, 1H, 2H, 2H, Perfluorodecyl acrylate ont été développé. En fonction des paramètres plasma une surface dîtes superhydrophobe en une étape a été obtenue grâce à l'obtention d'un film composé de nanoparticules fluorés. La seconde partie des travaux a consisté à développer des films barrières aux gaz à partir de l'hexamethyldisiloxane. Ainsi, des films minces SiOx et multicouches SiOxHyCz/SiOx ont pu être obtenue afin d'améliorer les performances barrières de substrat PET et PEN. Enfin, l'obtention de film barrière aux UV a consisté à une croissance in-situ de nanoparticules de dioxyde de titane (TiO2) à partir du film polymère plasma. La matrice polymère constitué d'une structure siloxane et aminée plasma joue le rôle de nano-réacteur pour la croissance de cristaux de TiO2.

Ce travail concerne l'étude des modifications de nanocharges lamellaires synthétiques (α-ZrP) et de leur influence sur les propriétés mécaniques et barrière aux gaz de nanocomposites à matrice élastomère (SBR). Cette étude s'inscrit dans le cadre de l'amélioration de l'étanchéité des pneumatiques. L'une des originalités de ce travail a résidé dans l'introduction des nanocharges hydrophiles par le biais d'une dispersion aqueuse (slurry), dans la matrice SBR hydrophobe. La première phase de ce travail a consisté à entreprendre plusieurs types de modification des nanocharges afin d'étudier les mécanismes d'intercalation et/ou d'exfoliation des ces dernières dans le slurry. Ces différentes familles de charges modifiées ont été utilisées pour réaliser des nanocomposites selon différentes voies de mise en oeuvre : principalement solvant et latex. Nous avons ensuite étudié l'influence, (i) de la nature des intercalants, (ii) des distances interfoliaires initiales des nanocharges et (iii) des procédés de mise en oeuvre des nanocomposites, sur la morphologie et les propriétés finales des matériaux. Cette étude a montré la synergie de ces trois paramètres et mis en évidence l'importance du contrôle des interactions charges modifiées/matrice sur les propriétés de transport de gaz. Parmi l'ensemble des matériaux synthétisés, nous avons pu mettre en avant une formulation, permettant d'atteindre des propriétés mécaniques et barrière intéressantes. Cette formulation, en voie latex, est basée sur l'utilisation de la charge modifiée aminosilane et de l'agent de couplage Si69

L’application de la technologie OLED (diode électroluminescente organique) pour l’affichage est actuellement principalement portée par le marché des écrans de télévision et des smartphones. Les matériaux et les substrats utilisés permettent de produire des dispositifs légers, compacts, voire flexibles, possédant un excellent contraste image et une fréquence de rafraîchissement élevée. Les contraintes technologiques limitant l’industrialisation concernent la durée de vie des dispositifs. D’une part, les matériaux utilisés dans les structures OLEDs sont dégradés en présence d’eau et d’oxygène, il est donc essentiel de passiver le dispositif avec une structure de haute qualité barrière à l’atmosphère. D’autre part, il est nécessaire d’ajouter une protection mécanique en surface, sous forme de capot ou de protection monolithique, modulable en fonction de l’application visée. Ces travaux de thèse visent à développer une nouvelle conception de packaging, intégrant des nanocomposites organiques-inorganiques au sein de l’architecture d’encapsulation multicouche développée au CEA-LETI, et appliquée aux écrans de petite taille (microdisplays) OLED réalisés sur substrat silicium. Ces matériaux permettent de cumuler des propriétés d’ordinaire antagonistes ; celles de la phase organiques (flexible, déposable par voie liquide) et celles de la phase inorganique (barrière au gaz, résistance mécanique et chimique). Les résultats reposent sur le contrôle fin de la morphologie ; le procédé sol-gel a donc été choisi pour sa versatilité. Nous avons montré qu’il était possible de définir plusieurs formulations composites. L’une en particulier, basée sur des nanoparticules de silice intégrées dans une matrice polymère, s’est révélée compatible avec l’encapsulation monolithique des substrats OLED, permettant notamment la reprise de contact électrique. L’ajout de ce matériau au sein de l’architecture de passivation augmente la résistance barrière au gaz et, par extension, la durabilité des dispositifs en conditions climatiques sévères. Utilisé en tant que protection mécanique de surface, cette solution en couche mince n’atteint pas la résistance mécanique des capots de verre, mais permet malgré tout de protéger les substrats pendant toutes les phases de fabrication et de manipulation. L’avantage principal de cette solution packaging réside dans la diminution de l’épaisseur totale, augmentant le contraste en limitant fortement les pertes lumineuses, et ouvrant la porte aux substrats et dispositifs flexibles
Due of the ongoing growth of smartphones and TVs displays markets, the application of OLED (Organic Light Emitting Diode) technology for displays has become a major center of interest. The materials and substrates used in such architectures allow to develop lightweight, compact and even foldable displays, demonstrating an excellent image quality and fast refresh rates. Currently, the technological drawbacks restricting the exploitation on industrial scale mainly concern the lifespan of the devices. First, materials used in OLED architecture are highly sensitive to moisture and oxygen ingress and require a high barrier encapsulation. In addition, a specific protection needs to be included to secure the device from mechanical failures. As so various options from glass lids to flexible barriers are likely to be considered depending on the intended use. This work deals with the production of OLED microdisplays deposited on silicon substrates, and aims to develop an alternative packaging solution, based on organic-inorganic nanocomposite layers, both on top and embedded into the multi-barrier passivation architecture previously developed at the CEA-LETI. Synergistic properties can be obtained from composite materials, enhancing the advantages of both the organic (flexibility, processability) and inorganic phase (barrier properties, mechanical and chemical resistance). As a high control on the morphology in required, the sol-gel process was therefore selected for its versatility. Several composite materials were designed. One selected formulation, based on silica nanoparticles dispersed in a polymer matrix, proved to be fully compatible with the monolithic encapsulation of OLED circuits, including, among other properties, the recovery of the electrical bonding. Passivation architectures using the composite as interface layer showed improved barrier properties as well as an enhanced durability of devices stored in warm and damp environment. Obviously, a thin hard-coat layer does not equal a glass lid in terms of mechanical resistance, yet our formulation provided a sufficient protection during the overall process and handling of the displays. The main advantages of this alternative packaging rely on the reduced thickness, increasing the contrast by minimizing the loss of luminous efficacy through guided mode and offering the prospect of flexible substrate manufacturing

碩士
國立勤益科技大學
化工與材料工程系
102
In recent years, it indicated that the electronic display panel is gradually from the plane toward curved panels. the flexible display is one of the main research and development themes. First, the most important question is how to find the appropriate flexible substrates. The types of substrates can be divided into ultra-thin glass, metal foil, and plastic substrates on the market. In this study, we used polyethersulphone (PES) as flexible substrate. Because of its lightweight, thin, flexible and impact-resistant properties, it can be used on portable or flexible panels. Part A：Characteristics of graphene or graphene oxide / Polyethersulfone nanocomposite films In this study, we added the graphene oxide (GO) and graphene (RG) into PES was to produce a series of GO/PES and RG/PES composite films. GO was made by Hummer’s methods, and then RG was obtained after thermal reduction process. The optical properties, the transmittance of 100GO and 100RG still maintained at 90%. The GO/PES and RG/PES composite film can enhance the pure PES film Young’s modulus. When the contents were added to 0.1% (100GO, 100RG), they have the best tensile strength. Finally, the water vapor transmission rate (WVTR) of 100GO (62.76 g-mil/m2-day) and 100RG (53.64 g-mil/m2-day) have the best values respectively. It decreased by 64% for 100GO and 69% for 100RG than pure PES. In summary, RG/PES composite films not only in mechanical properties but also in water resistance effects are more excellent than GO/PES composite films. Therefore, the proportion of the most appropriate was 100RG composite film. Part B：The moisture barrier properties of polyethersulfone films enhanced by sputtering We used aluminum nitride (AlN) as the target in the RF magnetron sputtering. We plated with a thin layer of AlN that used single power and two kinds of power on the PES films. Then we discussed the difference among the sample of the optical properties, the surface structure, and water vapor transmission rate. On the single power, the sample of 80W showed the best barrier properties (WVTR=25.98 g-mil/m2-day). When the power is more than 80W, the sputtering rate is increased, but the film of surface smoothness and moisture barrier properties are showing a downward trend. In this study, we observed that dual powers films have a lower water vapor transmission rate compared to a single power films. Water vapor transmission rate of A8 series films were also lower than the B8 series of films. In those films, the B6A8’s water vapor transmission rate reached the lowest 13.61 g-mil/m2-day. B6A8 decreased by 92% compared to pure PES film.

碩士
國立臺灣科技大學
材料科學與工程系
105
Because of graphene oxide mechanical, and thermal properties, it is being explored for various applications and has attracted enormous academic and industrial interest. In this study, the preparation of polyurethane (PU) nanocomposites with graphene oxide (GO) using a simple solution dispersion processing method with surfactant poly(oxyethylene)-segmented urea (POEU). Well-dispersed graphene oxide/PU nanocomposties have good gas barrier properties, mechanical properties and thermal properties. The gas barrier properties of the graphene oxide-based nanocomposite with molecule-level dispersion are significantly improved. Graphene layers in the polymer matrix are capable of producing a tortuous path, which acts as a barrier for gases. A high tortuosity leads to superior barrier properties and lower permeability of PU. PU/GO have a 44.31% increase in gas barrier properties. Furthermore, a 507% increase in tensile strength and a 96.26% improvement of toughness and a 19.75% increase of water contact angle and a 14.5% increase of degradation temperature are achieved by addition of 3wt% of GO 4% which oxygen content is 4%. The improved properties of the composite film could lead to potential applications in food package.

碩士
國立勤益科技大學
化工與材料工程系
100
The transparent polyimide (PI) filmswere synthesized from an alicyclic dianhydride (BCDA) and aromatic diamine (3,4'-ODA) in the cosolvent of DMAc and GBL via one-step process. For the application in flexible displays, the transparency and water barrier property of PI films were enhanced by blending with graphene (RG) or graphene oxide (GO) and by deposition of silicon nitride. Part I: Synthesis and characteristics of transparent polyimide/graphene or graphene oxide nanocomposite films Graphene oxide (GO) was first synthesized by the oxidation process of natural graphite. Graphene (RG) was then obtained after thermal reduction process and the successful exfoliation was confirmed by XRD and SEM results A series of PI nanocomposite films were prepared by adding different proportions of RG or GO into PI solution. Adding 0.01 wt% of GO or RG, the storage modulus (E') of PI/GO-0.01 and PI/RG-0.01 was 2474 MPa and 3138 MPa, respectively, indicating enhanced mechanical strength. The coefficient of thermal expansion (CTE) reduced to 41.4 ppm/oC for PI/GO-0.001 indicating good thermal dimensional stability. Moreover, all PI nanocomposite films with various contents of GO or RG exhibit excellent thermal stability. PI/RG nanocomposite films show better water barrier property than PI/GO. The water vapor transmission (WVTR) of PI/GO-0.001 was 31.71 g-mil/m2-day and that of PI/RG-0.001 was only 13.09 g-mil/m2-day, which is 92.7 % lower than that of pure PI (181.33 g-mil/m2-day). The UV-Vis spectra show the optical transmittance at 550 nm was 97% and 85%, respectively, for PI nanocomposites containing 0.001 wt% of GO and RG. . Based on the above experimental results, adding a small amount of GO or RG in PI, the resultant PI nanocomposite maintain high optical transmittance and, at the same time, effectively extend the moisture penetration path and thus the water barrier property was significantly improved. Part II: Deposition of a silicon nitride gas barrier layer on the transparent polyimide/graphene and graphene oxide nanocomposite films via RF magnetron sputtering Based on the result from Part I, PI nanocomposite films with improved water barrier property and, simultaneously, remaining high optical transmittance were deposited with silicon nitride on the surface to study the WVTR values. The Si2p and N1s XPS analysis showed the binding energy of Si3N4 at 102.6 and 397.5 eV, respectively, indicating the presence of Si3N4 structure in the deposited barrier layer. The presence of Si-C bonding at the interface of PI and deposited barrier layer was confirmed from the corresponded depth profile of Si2p and N1s spectra. FE-SEM and AFM images show that the deposited barrier layer has the denser packing of particles and the lower surface roughness (less than 1 nm) at the working pressure of 4 m Torr, sputtering power of 80W and the deposition thickness of 30 nm. Hence, the PI nanocomposites deposited with the barrier layer under those parameters show the best water barrier property. The WVTR of 30nm-deposited PI/GO-0.001reduced to 0.17 g-mil/m2-day, which is 99.9% reduction of pure PI before deposition (181.33 g-mil/m2-day). Notably, this barrier layer deposited PI/GO-0.001 nanocomposite film (thickness: 21 μm) exhibits high optical transmittance at 550 nm (81%), and improved barrier property thus was potential substrate material for flexible electronics.

碩士
國立臺灣科技大學
應用科技研究所
105
Emerging two-dimensional (2D) materials such as Molybdenum disulfide (MoS2) offer opportunities to tailor the mechanical properties and gas barrier of polymeric materials. In this study, MoS2 was exfoliated to a monolayer. The thickness of the MoS2 monolayer was 0.7 nm for MoS2-ethanethiol and 1.1 nm for MoS2-nonanethiol. MoS2 monolayers were added to butyl rubber to prepare a MoS2-butyl rubber nanocomposite at concentrations of 0.5, 1, 3, and 5 phr. The tensile stress showed a maximum enhancement of about 30.7% for MoS2-ethanethiol-butyl rubber and 34.8% for MoS2-nonanethiol-butyl rubber compared to pure butyl rubber. In addition, the gas barrier increased by 53.5% in MoS2-ethanethiol-butyl rubber and 49.6% in MoS2-nonanethiol-butyl rubber. MoS2 nanosheets enhanced the mechanical properties and gas barrier of butyl rubber when dispersed in butyl rubber. And the nanocomposites used to manufacture pharmaceutical stoppers with high mechanical properties and gas barrier.

碩士
國立臺灣大學
材料科學與工程學研究所
100
This study investigates effects of cross-linking on the gas-barrier property of branched polyethyleneimine (BPEI) / montmorillonite (MMT) nano-composite films prepared by layer-by-layer (LbL) self-assembly, with the goal of reducing the hygroscopic nature of the films and improving their moisture-barrier performance. The process for fabricating before-cross-linking nano-composite films was first optimized by tuning processing parameters including molecular weight of BPEI, concentration and pH value of the BPEI solutions. It was determined that a BPEI molecular weight of 25000, a concentration of 0.1 wt%, and a pH value of 10 resulted in a regularly aligned MMT, low gas permeability and excellent optical transparancy. Three cross-linking agents were applied to the LbL nano-composite films, including glutaraldehyde (GA), adipic acid, and oxalic axid. Contrary to previous reports on the effects of these cross-linking agents on LbL BPEI / MMT nano-composite films, we found that the cross-linking agents caused swelling instead of densification of the nano-composite films, and the gas permeability of the films significantly increased. This was attributed to the cross-linking agents swelling the BPEI phase, causing disordering of the MMT platelets which in turns created fast permeation passageways through the films. Our findings indicate that the approach of using cross-linking agents to improve gas-barrier property of LbL nano-composite films may be implausible, and that use of polymer electrolytes with cross-linkable functional groups may be a possible solution.

碩士
國立勤益科技大學
化工與材料工程系
98
In this study, composite membranes with the function of gas barrier were developed. A silicon nitride (Si3N4) thin film, as a gas barrier layer, was deposited on polyimide (PI) or polyimide/aluminium oxide (PI/Al2O3) hybrid substrates by RF magnetron sputtering. The parameters of sputtering process were optimized and the characteristics of the resultant composite membranes were studied in order to obtain a gas barrier thin film on PI/Al2O3 hybrid substrates. First, Si3N4 was deposited on PI substrates synthesized from 4,4'-oxydianiline (ODA) and 3,3'-oxydiphalic anhydride (ODPA) via polycondensation reaction. Different working pressures (4~8m Torr) and sputtering powers (40~100W) were performed to obtain various thicknesses or structures of Si3N4 thin films on PI substrates. XPS results confirmed the chemical composition of all deposited film on PI substrate was Si3N4. With a lower working pressure (4m Torr) and higher sputtering power (100W), the deposited Si3N4 thin film exhibits denser structure (by FE-SEM), lower RMS surface roughness (by AFM) and lower transmission rate of water vapor (WVTR, 0.88 g-mil/m2-day). A critical Si3N4 thickness of 100 nm was observed to obtain a WVTR as low as 5.4 g-mil/m2-day, when the sputtering power and working pressure were controlled at 100W and 6m Torr, respectively. The thickness of Si3N4 has no effect on the optical transmittance at 550 nm, Td5 and Tg. They were over 86 %, around 538 oC and 268 oC, respectively, for all synthesized composited membranes under various sputtering procedures. In addition, a Si3N4 thin film with a thickness of 100 nm was deposited on various PI/Al2O3 hybrid substrates under a controlled sputtering condition, 4m Torr and 100W. These hybrid substrates with different Al2O3 contents were prepared via sol-gel procedure and showed higher thermal stability, better dimensional stability and stronger mechanical property than pure PI substrate. The bending tests suggested the composite membrane with a hybrid substrate of PI/Al2O3 with 10 wt. % Al2O3 has better hinder-resistant for crack. XPS’s composition-depth profiles analysis confirm the existence of Al-N and Al-O-Si bonding at the interface of Si3N4 thin film and PI/Al2O3 hybrid substrate.

Indiana University-Purdue University Indianapolis (IUPUI)
Zirconia (ZrO2) is an important ceramic material with a broad range of applications. Due to its high melting temperature, low thermal conductivity, and high-temperature stability, zirconia based ceramics have been widely used for thermal barrier coatings (TBCs). When TBC is exposed to thermal cycling during real applications, the TBC may fail due to several mechanisms: (1) phase transformation into yttrium-rich and yttrium-depleted regions, When the yttrium-rich region produces pure zirconia domains that transform between monoclinic and tetragonal phases upon thermal cycling; and (2) cracking of the coating due to stress induced by erosion. The mechanism of erosion involves gross plastic damage within the TBC, often leading to ceramic loss and/or cracks down to the bond coat. The damage mechanisms are related to service parameters, including TBC material properties, temperature, velocity, particle size, and impact angle. The goal of this thesis is to understand the structural and mechanical properties of the thermal barrier coating material, thus increasing the service lifetime of gas turbine engines. To this end, it is critical to study the fundamental properties and potential failure mechanisms of zirconia. This thesis is focused on investigating the structural and mechanical properties of zirconia. There are mainly two parts studied in this paper, (1) ab initio calculations of thermodynamic properties of both monoclinic and tetragonal phase zirconia, and monoclinic-to-tetragonal phase transformation, and (2) image-based finite element simulation of the indentation process of yttria-stabilized zirconia. In the first part of this study, the structural properties, including lattice parameter, band structure, density of state, as well as elastic constants for both monoclinic and tetragonal zirconia have been computed. The pressure-dependent phase transition between tetragonal (t-ZrO2) and cubic zirconia (c-ZrO2) has been calculated using the density function theory (DFT) method. Phase transformation is defined by the band structure and tetragonal distortion changes. The results predict a transition from a monoclinic structure to a fluorite-type cubic structure at the pressure of 37 GPa. Thermodynamic property calculations of monoclinic zirconia (m-ZrO2) were also carried out. Temperature-dependent heat capacity, entropy, free energy, Debye temperature of monoclinic zirconia, from 0 to 1000 K, were computed, and they compared well with those reported in the literature. Moreover, the atomistic simulations correctly predicted the phase transitions of m-ZrO2 under compressive pressures ranging from 0 to 70 GPa. The phase transition pressures of monoclinic to orthorhombic I (3 GPa), orthorhombic I to orthorhombic II (8 GPa), orthorhombic II to tetragonal (37 GPa), and stable tetragonal phases (37-60 GPa) are in excellent agreement with experimental data. In the second part of this study, the mechanical response of yttria-stabilized zirconia under Rockwell superficial indentation was studied. The microstructure image based finite element method was used to validate the model using a composite cermet material. Then, the finite element model of Rockwell indentation of yttria-stabilized zirconia was developed, and the result was compared with experimental hardness data.

The electrically active defects introduced in GaAs by electron beam deposition (EB) of Ta were characterised. The effect of electron beam deposition on the electrical properties of GaAs was evaluated by current-voltage (I-V), capacitance¬voltage (C- V) and deep level transient spectroscopy (DL TS). However, when electronic devices are formed by EB, defects may be introduced into the semiconductor material, depending on the properties of the metal being deposited. Depending on the application, these defects may have either advantages or detrimental effects on the performance of such a device. I-V measurements indicated that the EB induced damage results in an increase in ideality factor and decrease in the barrier height with increasing the applied substrate bias, while C- V measurements showed that EB deposition also caused a decrease in the barrier height. DL TS studies on the same material in the temperature range of 20 - 350 K showed that at least three electrically active defects are introduced during EB deposition, with energies (0.102 ± 0.004, 0.322 ± 0.014 and 0.637 ± 0.029 eV) within the band gap. DL TS data was used to construct concentration profiles of these defects as a function of depth below the surface. It was found that the defect concentration increases with increasing substrate bias during the deposition, irrespective of the direction of the applied bias. This may be related to the I-V characteristics of the SBDs. The SBDs investigated by IV measurements showed that GaAs yields SBDs with poorer characteristic. The influence of EB deposition on the device properties of SBDs fabricated on GaAs is presented. These device properties were monitored using a variable temperature I-V and C- V apparatus. In order to have an understanding of the change in electrical properties of these contacts after EB deposition, it is necessary to characterise the EB induced defects. DL TS was used to characterise the defects in terms of their D L TS signature and defect concentration.
Dissertation (MSc (Physics))--University of Pretoria, 2006.
Physics
unrestricted

This thesis presents an experimental and theoretical study of some of the electronic properties and device applications of GaAs/Al_xGa_1-xAs single and double barrier tunnel structures. In Chapter 2, energy band diagrams are calculated for heterostuctures in which tunneling occurs between two degenerately doped electrodes separated by a single quantum barrier. When a bias voltage is applied to a structure, the energy band profile gives the voltage drop distribution in the cladding layers as well as in the barrier. This distribution may differ significantly from that based on the commonly made assumption that the entire applied voltage drops linearly across the barrier layer. It is shown that band bending effects become more important for larger applied voltages, thicker barriers, smaller electrode doping densities and larger barrier doping concentrations. Energy band diagrams are found to be useful for calculating tunneling currents and determining what the dominant low temperature current transport mechanisms occurring in these structures are. In some cases, they reveal that these mechanisms are different from those predicted when band bending is neglected.

In Chapter 3, elastic and inelastic tunneling processes are investigated in GaAs-AlAs-GaAs single barrier heterostructures grown on [100]-oriented substrates. The GaAs electrodes are degenerately doped n-type with Se, and the AlAs quantum barriers are doped either p-type with Mg or n-type with Se. In p-type barrier structures, low temperature current transport is found to be dominated by elastic and inelastic electron tunneling through the AlAs band gap at the Γ-point and at the X-point. Anomalous zero-bias conductances obtained from several of the samples are also discussed. A theoretical model, which treats trap levels in the AlAs barrier as intermediate states for two-step tunneling processes shows that impurity-assisted tunneling becomes more important as the tunnel barrier is made thicker. In heterostructures in which the n-type barrier layers are thick enough and/or sufficiently doped, the AlAs conduction band at the X-point is not totally depleted of electrons. The dominant low temperature current transport mechanism is then tunneling through two reduced AlAs X-point barriers separated by a bulk region of AlAs. When the n-type AlAs barrier layer is sufficiently thin, the AlAs conduction band remains fully depleted of carriers. As a result, electrons tunnel through the AlAs band gap at the X-point and/or at the Γ-point in a one-step process. In these structures, it is found that plasmons located near the GaAs/AlAs interfaces interact with GaAs and AlAs longitudinal optical (LO) phonons when the doping density in the n-type GaAs electrodes is such that the plasma frequency becomes comparable to the LO phonon frequencies.

Chapter 4 presents a study of resonant tunneling in GaAs/Al_xGa_1-xAs double barrier heterostructures grown epitaxially in the [100]-direction. In these structures, electrons tunnel through two Al_xGa_1-xAs quantum barriers separated by a thin GaAs layer forming a quantum well. The resonant energy levels in the GaAs well which produce negative differential resistances in the experimental I-V characteristics are identified by calculating the energy band diagrams of the structures. In samples having pure AlAs barrier layers, tunneling via resonant states confined in the well by the AlAs Γ-point potential energy barriers is often inconsistent with experimental results. However, the experimental data can usually be explained by tunneling via quasi-stationary levels confined in the well by the AlAs X-point potential energy barriers as well as the AlAs Γ-point barriers. The relative contributions of tunneling via resonant Γ- and X-states in the well are found to depend upon the samples studied and sometimes upon the sign of the applied bias. Resonant tunneling is also investigated in double barrier heterostructures in which a low doped GaAs buffer layer is grown before the first Al_xGa_1-xAs barrier. As a result of this structural asymmetry, the peaks in current corresponding to a given resonant state in the quantum well may be observed in the experimental I-V characteristics at very different applied voltages in reverse bias than in forward bias.

In Chapter 5, we propose and analyze two types of three-terminal devices based upon resonant tunneling through quantum well and quantum barrier heterostructures. The first type includes two configurations in which a base voltage controls the emitter-collector tunneling current by shifting the resonances in a quantum well. In the proposed devices, the relative positions of the base and collector are interchanged with respect to the conventional emitter-base-collector sequence as a means for obtaining negligible base currents and large current transfer ratios. The second type of three-terminal devices includes three configurations in which the current through a double barrier structure is modulated by a Schottky barrier gate placed along the path of the electrons. These devices feature, in their output current-voltage (I_D-V_D) curves, negative differential resistances controlled by a gate voltage.

Chapter 6 presents a growth uniformity study performed on several of the heterostructures discussed in the thesis. First, the reproducibility and uniformity of the electrical characteristics of GaAs/AlAs tunnel structures are used to show that the doping concentrations and layer thicknesses are uniform across the samples under test. Secondly, discrete fluctuations in layer thicknesses are discussed in GaAs/Al_0.35Ga_0.65As double barrier heterostructures. These fluctuations are manifested by non-uniform experimental results and by sequences of negative differential resistances in the I-V characteristics of many devices.

碩士
國立交通大學
電子物理系所
101
Optical properties of type-II InAs/GaAsSb quantum dots (QDs) with different inserted GaAs thickness are investigated by photoluminescence (PL) and time-resolved PL measurements. With increasing GaAs thicknesses, the QD emission shows a blueshift in energy and a lengthening in lifetime. These two phenomena are caused by enhanced compressive strain and spatial separation of electron-hole wave function. Theoretical calculations based on eight-band k‧p model indicates that quantum confinement of hole states and their wave function distribution are sensitive to the thickness of inserted GaAs. We demonstrate that controlling the thickness of the inserted GaAs thickness is a feasible way to manipulate the carrier lifetime for QD-based intermediate-band solar cell applications. Furthermore, type-I transitions can be observed at high temperature. Comparing the PL spectra, we found two conditions. First, only the type-II transition can be observed at low temperature. Second, both transitions can be observed at high temperature due to the thermal excitation and tunneling effect. In InAs/GaAsSb system, the hole is thermal excited from GaAsSb layer into InAs QD, and carriers recombine in the InAs QDs. In InAs/GaAs/GaAsSb system, hole has to tunnel from GaAsSb layer into the InAs QDs in addition to these thermal excitation.