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Boiling heat transfer provides a means of removing high heat fluxes at low temperature differences in many applications in the power and process industries. A strong interest has been also developed for the cooling of silicon-based devices, such as electronic chips. However, a complete model to describe the processes involved has not been developed as yet. This PhD project focused on the study of nucleate pool boiling via numerical simulations for a solid plate horizontally immersed in a saturated liquid with a large number of potential nucleation sites. The simulations were developed by a FORTRAN code based on a hybrid approach, combining the 3-dimensional time-dependent solution for the temperature field on the substrate with semi-empirical models for phenomena occurring on the liquid side. The starting point of the project was the modification of a previous version of the code in order to reduce the computational time (in collaboration with Dr. Nelson at Los Alamos National Laboratory) and improve the modelling of the physics of the processes. One of the key features of the code is the flexibility in adapting to different conditions. In fact the code was used to study bubble growth, site activation frequency and superheat variations, as well as the interactions between nucleation sites. The differences in behaviour between very thin metal foils immersed in water and thicker silicon substrates in FC-72 were studied. The results were compared with experimental results produced at the University of Edinburgh and the University of Ljubljana, both partners of this project. Both the numerical and physical modifications introduced made it possible to have simulations for a large number of sites, of the order of 100, in reasonable times, of the order of days, so that the code can be now used as a tool for the design of new test sections.

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Low voltage power electronics are made from dislocation free silicon heavily doped with arsenic or antimony to provide low electrical resistivity. Attempts to grow crystals with decreased resistivity have led to a higher probability of twinning during growth, so that the crystal no longer possesses the required crystallographic orientation for device fabrication. The source of the twins must be identified so that crystal growth process conditions can be designed to eliminate this defect mechanism, allowing lower resistivity crystals to be grown reliably. In lightly doped crystals, twinning was ascribed to presence of carbon impurity or a low probability atomic stacking accident, neither of which should be affected by increased concentration of arsenic or antimony. Crystals that twinned during growth were characterized by resistivity, Laue back-reflection x-ray diffraction, optical and scanning electron microscopy, energy dispersive x-ray spectroscopy, spreading resistance, x-ray computed tomography and electron backscatter diffraction. The twin nucleation site of silicon crystals that were grown heavily doped with arsenic or antimony were compared to lightly doped crystals which twinned, and crystals that exhibited other defects. The initial twinning in the <100> orientation heavily doped crystals occurred from small gas bubbles bursting at a {111} facet at the three phase boundary, and forming a twin orientation domain on that facet. The gas bubbles likely consist of argon, the process gas used during solidification to remove silicon monoxide gas from the growth system. The higher levels of arsenic or antimony dopant may have changed the silicon surface tension, or provided additional impurities into the liquid silicon. Either effect may have changed the number or size of argon bubbles in the liquid silicon, leading to a higher incidence of gas bubbles near the {111} facet during solidification. Similar but smaller crater features were observed on two lightly boron-doped silicon crystals that twinned. Two other lightly doped crystals formed twins from carbon inclusions, consistent with carbon as a cause. Some heavily-doped twinned samples also show high concentrations of metals at the twin nucleation site, which could affect surface energy. Measurement of the geometry of crystal surface-to-facet radius eliminated a recently-proposed twin nucleation theory from consideration. Constitutional supercooling was demonstrated to not be a major contributing factor to twin nucleation. It was shown that deliberately introducing additional arsenic dopant during solidification would nucleate twins, but twins did not occur if only elemental carbon was introduced.

硅對現今科技至關重要。而有機物吸附於硅表面可能大大擴展硅的用途。在本論文中，我採用密度泛函理論研究了六種分子於硅表面的反應。本論文大致可分為四部分。 / 第一部分包含四個分子（吡嗪、吡啶、嘧啶與1,3,5-三氮嗪）的吸附研究。它們都擁有類似於苯的結構而環中又包含氮原子。分子可以用氮原子與表面形成配位鍵並作為進一步反應的前體故其反應更為容易預測。而反應通常只消除π 鍵而不會使分子分解這使得分子可以在平衡狀態下與表面生成最為穩定的結構。此兩種特性使它們成為硅表面吸附的絶佳研究對象。不同的實驗都顯示出吡嗪於硅表面只會生成Cross-row bridge 結構加熱硅表面更會促進納米線的生成。過往的計算無法解釋實驗結果而我的計算表明范德華力對導出正確計算結果非常重要加入范德華力的修正可以同時解釋單一產物與納米線的生成。吡啶的吸附實驗結果相比之下較為複雜。低溫下的吸附與飽和只生成兩種產物可是室溫下的實驗卻顯示多種產物並存於表面。加入范德華力的計算結果指出覆蓋度和溫度與吸附密切相關並指出對低溫下形成的飽和表面加溫可能促使納米線的生成。而於室溫下吡啶可進入多條反應路徑致使多種產物於表面並存。基於范德華力對這兩個分子的顯著影響我又將此效應加諸於嘧啶與1,3,5-三氮嗪的吸附研究。結果表明於低覆蓋度與低溫下它們只生成Double-dative 產物。然而增加覆蓋度會使Double-dative 的能量高於其它產物導致多種產物並存。由於嘧啶與1,3,5-三氮嗪都可生成一種特別的tight-bridge 結構加溫會使它們輕易解離。對此四種分子的研究顯示出氮原子的數量與位置如何影響吸附過程。 / 第二部分包含對1,2-環己二酮的吸附研究。於室溫下此分子會以互變異構體形式存在。之前的實驗指出它會先變回二酮才進行後續反應這種硅表面輔助互變反應的現象是前所未見的卻無後續研究以驗証其說。我的計算結果指出雖然互變反應的能壘被硅表面大幅降低可是仍不足以與其他反應路徑相匹敵。其中三種新產物既與實驗譜相吻合而反應能壘和能量都低故比之前所提產物更為可取。 / 第三部分包含對氰基苯的吸附研究。之前的三個實驗給出完全不同的結論。探明其與覆蓋度和溫度的關係後我提出氰基苯於低溫下飽和只會生成一種產物。而在室溫下的吸附會先生成valley-bridge 結構增加覆蓋度將使多種產物並存。計算結果不但表明有序吸附的可能性同時又解釋為何不同實驗給出看似不相容的結論。 / Silicon is an important material and substrate in modern technologies. Adsorption of organic molecules on silicon surface has attracted much attention due to the possibility of functionalizing the surface. In this thesis, density functional theory has been applied to study the adsorption of several organic molecules on Si(100) surface. / Results on the adsorption of benzene-like N-heteroaromatic molecules on Si(100)surface, including pyrazine, pyridine, pyrimidine and s-triazine, are presented in Chapters 2-4. This class of molecules, containing nitrogen atoms, can datively attach to dimer and act as precursor for further reaction. Thus, their reactions on surface can be easily predicted. Their reaction with surface would sacrifice π bond rather than dissociation under very mild condition such that the most stable adsorption product can be obtained under thermal equilibrium. These two properties of N-heteroaromatic compounds makes them good models for the study of reaction on surface Experiments repeatedly showed that pyrazines formed cross-row bridge structures on Si(100) under various conditions, and thermal annealing led to the formation of ordered nano-lines. Previous calculations did not explain these observations. This thesis demonstrates that addition of van der Waals (vdW) correction is critical to explain the formation of both the cross-row bridge and the nano-lines. For pyridine, the situation is more complex. Only two products were observed in low temperature adsorption experiments. In contrast, room temperature experiments yielded a mixture of products. With vdW corrections added, results show that pyridine adsorption is dependent on both coverage and temperature. Adsorption and saturation started at low temperature and followed by annealing may produce nano-lines consisted of cross-row bridge structures. Adsorption at room temperature opens up all reaction channels such that nano-line formation becomes difficult, which explains the discrepancy among experimental studies. In light of these results, the pyrimidine and s-triazine adsorption on Si(100) are also explored. The results show that they form double-dative structures at low coverage and low temperature. Increasing coverage will destabilize it and lead to mixture of products. These structures decompose easily upon heating due to the formation of a particular tight-bridge structure. These results show that the position and number of heteroatom are important to explain the difference in reaction selectivity among N-heteroaromatic molecules. For all four molecules, cooperative effect and the addition of vdW correction, which are almost neglected in similar study, are shown to be important in surface adsorption process. / In Chapter 5, the adsorption of 1,2-cyclohexanedione (1,2-CHD) on surface is studied. Gas molecule of 1,2-CHD exists in the tautomeric form (2-hydroxylcyclohex-2-ene-1-one) at room temperature. Previous experimental studies claimed that surface assisted tautomerization back to 1,2-CHD took place before further reaction, although there was no follow-up study. Computational results showed that although surface adsorption lowers the barrier for tautomerization, the barrier is still at least 5 times larger than other reaction channels. Three new structures are proposed, which are in better agreement with previous spectroscopic results. / In the final chapter, the adsorption of benzonitrile on Si(100) is studied. Three previous experimental studies presented conflicting results. With the coverage and temperature dependence effects clarified, my results suggest that adsorption at saturation and low temperature leads to a single product. Adsorption at room temperature first leads to a valley-bridge structure, while a mixture of adsorption structures is produced upon increasing coverage, which explained the conflicting experimental results. The last two chapters show the importance of computational modeling for interpretation and summarizing experimental results. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Ng, Kwok Hung. / Thesis (Ph.D.) Chinese University of Hong Kong, 2014. / Includes bibliographical references. / Abstracts also in Chinese. / Ng, Kwok Hung.

by Kim-chung Koon. / Chinese title: / Bibliography: leaves 108-112 / Thesis (M.Phil.)--Chinese University of Hong Kong, 1983

by Leung Yat-chor, Calvin. / Includes bibliographical references / Thesis (M.Ph.)--Chinese University of Hong Kong, 1984