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<p>Fracture of silicon solar cells in photovoltaic (PV) modules are widely reported and a wellknown issue in the PV industry, since it is exposed to adverse climatic conditions and varying temperature loads. A commercial silicon solar cell is mainly composed of four different layers. This thesis investigates delamination failure and thermal fatigue failure due to alternating temperature loads using finite element method (FEM) simulation.</p> <p><br></p> <p>The delamination of the encapsulant (EVA) layer and the cell interface was simulated using</p> <p>finite element (FE) simulations in the COMSOL Multiphysics software. The adhesion between the</p> <p>layers were modeled using the cohesive zone model (CZM). The CZM parameters such as normal</p> <p>strength and penalty stiffness were used for the bilinear traction-separation law for the cohesive</p> <p>model in a 90-degree configuration. The critical energy release rate (𝐺𝐺𝑐𝑐) was experimentally calculated as one of the CZM parameters. A uniaxial tensile test of the upper layer of the cell was conducted to determine the material properties of the solar cell layers, and that information was</p> <p>later used for FE simulations. To validate the simulation, we compared the peeling force graph</p> <p>from the experiment and FE simulation, and it was found both graphs showed a maximum peeling</p> <p>force of 120 N.</p> <p><br></p> <p>Finite element simulations were also conducted to predict the stress variations in the silicon</p> <p>solar cell layer due to alternating temperatures. An alternating temperature function was developed</p> <p>using triangular waveform equations in the COMSOL Multiphysics software. For this simulation,</p> <p>a 3D model of the cell with a 90-degree peel arm was used, like in the peeling simulation. A</p> <p>maximum stress of 7.31 x 10−3 𝑁/𝑚𝑚2 was observed on the encapsulant (EVA)/cell layer, but no</p> <p>delamination was observed for the given temperature range. In future work, we plan to explore the</p> <p>calculation of fatigue life using thermal simulation to predict the reliability of a solar cell.</p>

学位プログラム名: 京都大学大学院思修館 / 京都大学 / 新制・課程博士 / 博士(総合学術) / 甲第23590号 / 総総博第22号 / 新制||総総||4(附属図書館) / 京都大学大学院総合生存学館総合生存学専攻 / (主査)准教授 趙 亮, 教授 積山 薫, 准教授 関山 健 / 学位規則第4条第1項該当 / Doctor of Philosophy / Kyoto University / DGAM

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The current state of the art in structural health monitoring is primarily reliant on sensing deformation of structures at discrete locations using sensors and detecting damage using techniques such as X-ray, microCT, acoustic emission, impedance methods etc., primarily employed at specified intervals of service life. There is a need to develop materials and structures with self-sensing capabilities such that deformation and damage state can be identified in-situ real time. In the current work, the inherent deformation and damage sensing capabilities of carbon nanotube (CNT) based nanocomposites are explored starting from the nanoscale electron hopping mechanism to effective macroscale piezoresistive response through finite elements based computational micromechanics techniques. The evolution of nanoscale conductive electron hopping pathways which leads to nanocomposite piezoresistivity is studied in detail along with its evolution under applied deformations. The nanoscale piezoresistive response is used to evaluate macroscale nanocomposite response by using analytical micromechanics methods. The effective piezoresistive response, obtained in terms of macroscale effective gauge factors, is shown to predict the experimentally obtained gauge factors published in the literature within reasonable tolerance. In addition, the effect of imperfect interface between the CNTs and the polymer matrix on the mechanical and piezoresistive properties is studied using coupled electromechanical cohesive zone modeling. It is observed that the interfacial separation and damage at the nanoscale leads to a larger nanocomposite irreversible piezoresistive response under monotonic and cyclic loading because of interfacial damage accumulation. As a sample application, the CNT-polymer nanocomposites are used as a binding medium for polycrystalline energetic materials where the nanocomposite binder piezoresistivity is exploited to provide inherent deformation and damage sensing. The nanocomposite binder medium is modeled using electromechanical cohesive zones with properties obtained through the Mori-Tanaka method allowing for different local CNT volume fractions and orientations. Finally, the traditional implementation of Material Point Method (MPM) is extended for composite problems with large deformation (e.g. large strain nanocomposite sensors with elastomer matrix) allowing for interfacial discontinuities appropriately. Overall, the current work evaluates nanocomposite piezoresistivity using a multiscale modeling framework and emphasizes through a sample application that nanocomposite piezoresistivity can be exploited for inherent sensing in materials. / Ph. D.

A multiscale model is proposed to study the influence of interfacial interactions at the nanoscale in carbon nanotube(CNT)-polymer nanocomposites on the macroscale bulk elastic material properties. The efficiency of CNT reinforcement in terms of interfacial load transferring is assessed for the non-functionalized and functionalized interfaces between the CNTs and polymer matrix using force field based molecular dynamic simulations at the nanoscale. Polyethylene (PE) as a thermoplastic material is adopted and studied first because of its simplicity. Characterization of the nanoscale load transfer has been done through the identification of representative nanoscale interface elements for unfunctionalized CNT-PE interface models which are studied parametrically in terms of the length of the PE chains, the number of the PE chains and the "grip" position. Referring to the non-functionalized interface, CNTs interact with surrounding polymer only through weakly nonbonded van der Waals (vdW) forces in our study. Once appropriate values of these parameters are deemed to yield sufficiently converged results, the representative interface elements are subjected to normal and sliding mode simulations in order to obtain the force-separation responses at 100K and 300K for unfunctionalized CNT-PE interfaces. To study the functionalization effects, atomistic interface representative elements for functionalized CNT-PE interface are built based on non-functionalized interface models by grafting functional groups between the PE matrix and the graphene sheet. This introduces covalent bonding forces in addition to the non-bonded vdW forces. A modified consistent covalent force field (CVFF) and adaptive intermolecular reactive empirical bond order (AIREBO) potentials, both of which account for bond breaking, are applied to investigate the interfacial characteristic of functionalized CNT-PE interface in terms of the force-separation responses at 100K in both normal opening and sliding mode separations. In these studies, the focus has been on the influence of the functionalization density on the load transfer at the nanoscale interface. As an important engineering material, Epon 862/DETDA epoxy polymer,a thermoset plastic, has also been used as the polymer matrix material in order to see the difference in interfacial load transfer between a network structured polymer and the amorphous entangled structure of the PE matrix. As for thermoset epoxy polymer, emphasis has been put on investigating the effects of the crosslink density of the epoxy network on the interfacial load transfer ability for both non-functionalized and functionalized CNT-Epoxy interface at different temperatures(100K and 300K) and on the functionalization effect influenceing the interfacial interactions at the functionalized CNT-Epoxy interface. Cohesive zone traction-displacement laws are developed based on the force-separation responses obtained from the MD simulations for both non-functionalied and functionalized CNT-PE/epoxy interfaces. Using the cohesive zone laws, the influence of the interface on the effective elastic material properties of the nanocomposites are observed and determined in continuum level models using analytic and computational micromechanics approaches, allowing for the assessment of the improvement in reinforcement efficiency of CNTs due to the functionalization. It is found that the inclusion of the nanoscale interface in place of the perfectly bonded interface results in effective elastic properties which are dependent on the applied strain and temperature in accordance with the interface sensitivity to those effects, and which are significantly diminished from those obtained under the perfect interface assumption for non-functionalized nanocomposites. Better reinforcement efficiency of CNTs are also observed for the nanocomposites with the functionalized interface between CNTs and polymer matrix, which results in large increasing for the effective elastic material properties relative to the non-functionalized nanocomposites with pristine CNTs. Such observations indicates that trough controlling the degree of functionalization, i.e. the number and distribution of covalent bonds between the embedded CNTs and the enveloping polymer, one can tailor to some degree the interfacial load transfer and hence, the effective mechanical properties. The multiscale model developed in this study bridges the atomistic modeling and micromechanics approaches with cohesive zone models, which demonstrates to deepen the understanding of the nanoscale load transfer mechanism at the interface and its effects on the effective mechanical properties of the nanocomposites. It is anticipated that the results can offer insights about how to engineer the interface and improve the design of nanocomposites. / Ph. D.

At the turn of the 20th century two contemporaries in their respective fields teamed up to develop a solution to an acoustic problem with the hard-surfaced vaulted ceilings being installed in many large spanning rooms being built at the time. In the spirit of their ingenuity, this research explores a 21st century solution to a similar problem in contemporary buildings; the desire for a durable, hard surface wall or ceiling material treatment that is more sound absorbent than other common surface treatments. To find a material answer to this desire an impedance tube was used to analyze the mid-frequency octave band absorption coefficients of various re-purposed existing materials and tiles created utilizing 3D print technology and Helmholtz resonators. Additionally, an empirical study of Helmholtz resonator geometry was performed by analyzing the sound absorption of resonant cavity shape changes. Finally, plots of the absorption coefficients for each material tested were created to provide a visual comparison against two common surface treatment materials, tectum and gypsum wall board. / Master of Architecture

本篇論文藉由探討主題推進類型 (thematic progression patterns)與凝結關係 (cohesive ties) 在高中英文教科書課文的呈現以及在英文大學入學考試試題 (綜合測驗，文意選填，篇章結構) 上的應用，來提倡篇章結構的閱讀方法；希望在英文閱讀方面能有啟發的功效。首先，高中英文教科書中敘述文和說明文類型的文章被挑選出來作分析。為了解釋文章的主題發展，功能語法觀點 (Functional Sentence Perspective) 的語言學家Daneš提出四個主題推進類型，分別為Type 1: Simple Linear TP，Type 2: TP with a continuous (constant) theme，Type 3: TP with derived T’s，Type 4: Exposition of a Split Rheme。Cloran另外建議兩個主題推進類型Type 5: Theme > Rheme 和Type 6: Rheme > Rheme。這六種主題推進類型再加上由Halliday and Hasan所提出的五種凝結關係 (cohesive ties, i.e. reference, substitution, ellipsis, conjunction, and lexical cohesion) 被用來分析上述的文章並且應用在英文大學入學考試試題的解題。研究發現除了上述六種預設的主題推進類型之外，第七種類型被歸為Referential Type，涉及指稱詞 this和that 的使用。其他的發現敘述如下：在高中英文教科書文章分析方面，Type 1和Type 2出現的頻率最多，第二多是Type 5 and Type 6；Referential Type排名出現頻率的第三名，而Type 4少見，Type 3最罕見.。另外，在英文大學入學考試試題的應用方面，結果亦大致符合上述。至於凝結關係 (cohesive ties) 的頻率，指稱詞 (reference)，尤其是人稱代名詞出現最多次，字彙 (lexicon) 次之，然而大部份都是相同字 (same word or repetition) 的一再重複出現；其他字彙的呈現，如同義詞 (synonym)、反義詞 (antonym)、搭配詞 (collocation)、統領詞 (superordinate) 等稍嫌不足。 / Because little attention has been paid to the explicit teaching of text structure in local senior high schools, this present study analyzes the reading texts to explore how students are exposed to expositive and narrative text types and how the text is structured. Reading texts are selected from textbooks for senior high school students and then theme categories, thematic progression types and cohesive devices are analyzed. Combined Daneš’s theory with Cloran’s suggestion, the following six types of thematic progression (TP) are identified: Type 1, Rheme>Theme pattern (> means ‘followed by’); Type 2, Theme>Theme pattern; Type 3, Split Theme; Type 4, Split Rheme; Type 5, Theme>Rheme pattern; and Type 6, Rheme>Rheme pattern. Besides, five cohesive ties (i.e. reference, substitution, ellipsis, conjunction, and lexical cohesion) proposed by Halliday and Hasan are identified as clue to trace the progression. Next, evidence is found to decode gapped passages in Integrative Test (綜合測驗, i.e. Cloze Test), Semantic Choice (文意選填), and Discourse Structure (篇章結構). Besides the presupposed six progression patterns mentioned above, one more type is found and categorized as Referential Type. The progression of Referential Type involves the use of cohesive device this or that. The findings are presented as follows: In the analysis of reading texts in EFL textbooks, thematic progression of Type 1 R>T and Type 2 T>T predominates the frequency of occurrence, while thematic progression of Type 5 T>R and Type 6 R>R follows behind, and Referential Type ranks third in terms of frequency, followed by Type 4 Split R outnumbering Type 3 Split T. Furthermore, while applying to the analysis of test passages, the outcome of progression types is roughly correspondent with what is mentioned above. As for cohesive ties, reference predominates the frequency of occurrence, followed by lexicon. Of all the cohesive devices, items of personal reference are found to appear most frequently, and then the second most are items of the same word. Through the exploration and analysis of thematic progression and cohesive devices, it is hoped that students’ awareness of textual organization will be enhanced and thus help activate efficient reading.

Multi-layered electronic packages increase in complexity with demands for functionality. Interfacial delamination remains a prominent failure mechanism due to mismatch of coefficient of thermal expansion (CTE). Numerous studies have investigated interfacial cracking in microelectronic packages using fracture mechanics, which requires knowledge of starter crack locations and crack propagation paths. Cohesive zone theory has been identified as an alternative method for modeling crack propagation and delamination without the need for a pre-existing crack. In a cohesive zone approach, traction forces between surfaces are related to the crack tip opening displacement and are governed by a traction-separation law. Unlike traditional fracture mechanics approaches, cohesive zone analyses can predict starter crack locations and directions or simulate complex geometries with more than one type of interface. In a cohesive zone model, cohesive zone elements are placed along material interfaces. Parameters that define cohesive zone behavior must be experimentally determined to be able to predict delamination propagation in a microelectronic package. The objective of this work is to study delamination propagation in a copper/mold compound interface through cohesive zone modeling. Mold compound and copper samples are fabricated, and such samples are used in experiments such as four-point bend test and double cantilever beam test to obtain the cohesive zone model parameters for a range of mode mixity. The developed cohesive zone elements are then placed in a small-outline integrated circuit package model at the interface between an epoxy mold compound and a copper lead frame. The package is simulated to go through thermal profiles associated with the fabrication of the package, and the potential locations for delamination are determined. Design guidelines are developed to reduce mold compound/copper lead frame interfacial delamination.