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京都大学 / 新制・課程博士 / 博士(工学) / 甲第23899号 / 工博第4986号 / 新制||工||1778(附属図書館) / 京都大学大学院工学研究科材料工学専攻 / (主査)教授 田中 功, 教授 中村 裕之, 教授 奥田 浩司 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

This work explores physics-based and data-driven methods for material property prediction for metallic microstructures while indicating the context and benefit for microstructure- sensitive design. From this, the use of shape moment invariants is offered as solution to quantifying microstructure topology numerically using images. This offers a substantial benefit for computational time since image data is converted to numeric values. The goal of quantifying the image data is to help index grains based on their crystallographic orientation. Additionally, individual grains are isolated in order to investigate the effect of their shapes. After the microstructures are quantified, two methods for identifying the grain boundaries are proposed to make a more comprehensive approach to material property prediction. The grain boundaries as well as the grains of the quantified image are used to train artificial neural networks capable of predicting the material properties of the material. This prediction technique can be used as a tool for a microstructure-sensitive approach to design subtractively manufactured and Laser Engineered Net Shaping (LENS)-produced metallic materials. / Master of Science / Material properties are dependent on the underlying microstructural features. This work pro- poses numerical methods to quantify topology and grain boundaries of metallic microstruc- tures by developing physics-based and data-driven techniques for subtractively manufactured and Laser Engineered Net Shaping (LENS)-produced materials.

In the first part of this dissertation, an analytical approach to n⁺/p solar cells is developed. Based on this model, the short circuit current, open circuit voltage and energy conversion efficiency of the cells as a function of doping concentration, junction depth, minority carrier surface recombination velocity and diffusion length are discussed. This model simplifies the analysis of the solar cell, and the calculated results agree closely with both experimental results and numerical analysis. The second part of the dissertation deals with grain boundary (GB) recombination, passivation, and characterization. A simple GB model is developed, and an expression for the GB barrier height under illumination is derived by introducing a quasi Fermi-function. By using this model, the dependencies of the minority carrier transport parameters on the illumination level, grain size, depth from the surface and trap state density are derived. The model is compared to the experimental results for GB capacitance and electron lifetime measurements. Hydrogen passivation of poly-Si solar cells was accomplished experimentally by hydrogen implantation. A systematic study of implantation parameters was conducted and an optimum condition was found for the samples under examination. Under this condition, 18 % improvement in cell efficiency was achieved for the cells, with initial efficiencies of about 7%. A transmission electron microscope (TEM) study of the hydrogen-dislocation interaction and hydrogen ion bombardment effects on the surface were also conducted. The GB recombination velocity and the hydrogen passivation effect were characterized by the electron beam induced current (EBIC) technique. A nonuniform passivation depth was observed. The nature of the passivation mechanism was addressed. It was found that hydrogen saturation of dangling bonds in Si grain boundaries is not the only mechanism for hydrogen passivation. / Ph. D.

Use of welded titanium alloy to aluminium alloy structures in the aerospace industry has a number of potential benefits for both cost and weight saving by enabling titanium to be used only in the most critical parts, with the cheaper and lighter aluminum alloy making up the rest of the structure. However, due to the formation of brittle intermetallic compounds (IMC) at interface and the enormous gap in melting point, the welding of titanium to aluminium remains a major challenge. Solid state welding processes are most likely to be successful since they do not involve any melting, and so issues associated with the large difference in melting point and the high reaction rate of the liquid phase are avoided. In this study, an emerging low energy input solid state welding process - high-power ultrasonic spot welding (USW) was applied to weld Al and Ti (AA6111-T4/Ti6Al4V and AA2139-T8/Ti6Al4V combinations). No obvious intermetallic reaction layer was observed on the Al/Ti interface even using transmission electron microscopy. As a result, the maximum joint strength measured reached the same level as similar Al-Al (AA6111) welds and greatly exceeded those observed in Al-Fe and Al-Mg joints made using the same technique, in which a brittle reaction layer forms rapidly. However, the Al/Ti welds always failed at the weld interface after natural ageing, which is not desirable due to the low fracture energy associated with interfacial fracture mode. By using high resolution STEM-EDS, residual oxides and Si segregation were detected on the as-welded Al/Ti interface, which are thought to be factors that result in the no reaction layer Al/Ti interface. The Si segregation is predicted to be able to increase the weld interface cohesion through thermodynamic calculation. A series of prolonged heat treatment experiments were performed to understand the Al-Ti reaction layer growth kinetics and to explain the lack of reaction layer in as-welded Al-Ti joint. Al3Ti (D022 structure) was the only Al-Ti intermetallic phase observed in the reaction layer (IMC layer). In pure Al/Ti joints, it is found that the very long slow-growth stage of IMC layer is probably caused by the residual oxides on the interface. Calculations show that grain boundary (GB) diffusion makes the major contribution to the effective diffusion coefficient in the Al3Ti layer. In AA2139/Ti joints, the IMC layer growth is significantly slower than that in pure Al/Ti joints. The effects of alloying elements on the IMC layer growth was studied in detail. Cu was observed to segregate on both the Al3Ti grain boundaries and the Al3Ti/Ti interface. Si also segregated on the the Al3Ti/Ti interface and enriched in the Al3Ti layer. Both Cu and Si are thought to retard IMC layer growth. Interestingly small patches of Al were found trapped in the IMC layer; its formation mechanism is discussed. In pure Al/Ti6Al4V joints, the IMC layer growth rate did not change significantly. The presence of V greatly retarded the Al3Ti grain growth at high annealing temperature (630 °C) and suppressed the anisotropic growth of Al3Ti at 600 °C. Overall this study successfully joined Al/Ti by USW and systematically investigated the grain size effect and alloying effects on the Al3Ti layer growth. The present study for the first time: (a) observed the no-IMC-layer Al/Ti weld interface; (b) observed Cu segeration on Al3Ti GBs; (c) quantitatively studied the grain size effect on Al3Ti layer growth kinetics; (d) observed the orientation relationship between trapped Al islands and the adjacent Al3Ti grains; (e) observed that V greatly retarded the anisotropic growth of Al3Ti grains.

The efficiency of multicrystalline (mc) silicon solar cells depends strongly on the fraction of recombination active crystal defects. This work focuses on a systematic analysis of how the area fraction of recombination active crystal defects and thus the solar cell efficiency is af-fected by the grain structure of mc-silicon wafers, i.e. grain size, grain orientation and type of the grain boundaries between adjacent grains. For that purpose a new characterization method was developed which allows the measurement of the grain orientation and grain boundary type of full 156x156 mm² mc-silicon wafers. The results of the grain structure analysis were correlated with the etch pit density, the recombination active area fraction measured by photo-luminescence imaging, and the solar cell efficiency in order to quantify the most important features of the grain structure, which were relevant to obtain high quality mc-silicon wafer material. For the determination of the grain orientation and grain boundary type two metrology sys-tems were combined. The so-called grain detector determines the geometrical data of each grain (size and form) by a reflectivity measurement. Afterwards the wafer with the geomet-rical information of all grains is transferred into the so-called Laue Scanner. This system irra-diates each grain larger 3 mm² with white x-rays and creates a backscatter diffraction pattern (Laue pattern) for each grain. From this Laue pattern the grain orientation and the grain boundary type of neighboured grains is calculated and statistically analysed in combination with the geometrical data of the grain detector. In this work the grain structure of twelve industrially grown mc-silicon bricks, which were produced by different manufacturers, and two laboratory grown bricks were investigated. Seven of these bricks show a fine grain structure. This material named class F is considered to be typical for so-called High Performance Multi (HPM) silicon. The other bricks show a coarse-grained structure. This grain structure was called class G and corresponds to the con-ventional mc-silicon material. The results show that the grain structures of the start of the crystallization process differ sig-nificantly between class F and class G. The class F mc-silicon wafers have a uniform initial grain size (characterized by coefficient of variation CV¬KG < 2.5) and grain orientation (charac-terized by coefficient of variation CVKO < 1.5) distribution with a small mean grain size (< 4 mm²) and a high length fraction of random grain boundaries (> 60 %) in comparison to the class G wafers. Despite the totally different initial grain structure for the class F and class G bricks, the grain structure of the wafers which represent the end of the crystallization process is more or less comparable. It can be concluded that the development of the grain structure along the crystal height of the class F bricks is driven by an energy minimization due to the surface energy and the grain boundary energy, that means that the share of (111) oriented grains having the lowest surface energy and the share of ∑3 grain boundaries having the lowest interface energy increase from the start of crystallization to the end. This phenomenon could not be observed for the class G bricks, which show a decreasing ∑3 length fraction and a decreasing area fraction of {111} oriented grains. This energetically unfavourable grain structure development is not clear so far but it means another kind of energy minimization effect must exist within class G. This could be for instance the formation of dislocations. The grain structure investigations show clearly that especially the initially fine-grained struc-ture of the class F bricks, i.e. at the start of crystallization, influences beneficially the area fraction of recombination active defects and the solar cell efficiency subsequently. This ob-servation can be explained as follows. Reduced dislocation cluster formation: • The small grain sizes in combination with the low length fraction of ∑3 grain bounda-ries capture the dislocations within a grain. Dislocations are not able to move across the grain boundaries which have not the ∑3-type within moderate stress and tempera-ture fields. This prohibits the formation and expansion of large dislocation cluster. • The previously described energetically driven grain selection and the continuously in-creasing grain size from bottom to top leads to an overgrowth of grains. This means that also dislocated grains will disappear which also prohibits the formation of large dislocation cluster. Reduced possibility of dislocation formation: • Compared to the class G bricks the area fraction of {111} oriented grains is reduced. Therefore, the possibility of the formation of dislocations is reduced, because they would be activated first in {111} oriented grains taking the Schmidt factor in account which is lowest for {111} oriented grains. After the dislocation generation within a {111} oriented grain, the dislocation can move forward on 3 of 4 possible {111} slip planes which have an angle of 19.5° with regard to the growth direction. No other ori-entation has more slip planes for the dislocation movement which have an angle smaller 20° with regard to the growth direction. These arguments in combination with the high reproducibility of the characteristic initial class F structure can explain the observed low recombination active area fraction from start to end of crystallization which was smaller 5 % and especially the low variation of 2 % of the electrical active wafer area in between the class F bricks. One can also easily explain the higher recombination active area fraction up to 14 % and the large variation of 10 % between the class G bricks due to the obtained grain structure data. These differences in the recombination active area fractions are reflected in the solar cell efficiency which is 0.4 % higher for the class F bricks compared to the class G bricks. In consideration of the above mentioned reasons it is not beneficial for the industrial ingot production technology to increase the ingot height further, due to the fact that the advanta-geous initial grain structure properties of class F bricks disappear with increasing crystal height.

This thesis consists of a thermodynamically based kinetic model that more accurately predicts grain boundary mobility (GBM) over a large range of thermodynamic states including changes in temperature, pressure and shear stress. The form of the model was validated against calculated GBM values for Al bicrystals via molecular dynamics (MD) simulations. A total of 98,786 simulations were performed (164 different GBs, each with a minimum of 250 different thermodynamic states, and 2 different driving forces). Methodology for the computation of the GBM via MD simulations is provided. The model parameters are directly linked to extensive thermodynamic quantities and suggest potential mechanisms for GBM under combined thermal and triaxial loads. This thesis also discusses the influence of GB character on the thermodynamic mobility parameters. The resulting insights about GB character and thermodynamic state on GBM suggest an opportunity to achieve designed microstructures by controlling thermodynamic state during microstructure evolution.