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Quantitative phase field modeling of directional solidification of a binary alloy
No full text當任何合金物質往單一方向進行由液相變成固相的程序，我們稱之為單向固化。在單向固化的過程之中，如果因拉速過快使組成過冷克服溫度梯度和表面張力所提供的穩定因子，界面附近會產生局部過飽和而使之崩潰，並依程度的多寡生長出淺細胞、深細胞，到樹枝狀的界面。冶金學家可以利用固化界面型態的控制，來達到其所希望的合金機械強度；物理學家也發現這些界面所表現出來的波長結構與控制變因（例如拉速和溫度梯度）有許多非線性的關係，基於這些理由，合金單向固化問題在過去幾十年來一直是相當熱門的課題。
在本論文中，我們使用相場模式（Phase field model）來模擬此問題。此模式雖然在處理許多複雜界面型態的問題時，表現出相當強的能力，然而在模擬合金固化時，因為溶質包覆（Solute trapping）的發生，使得定量計算非常困難，為了要解決這個問題，我們除了使用Karma所提出的模式來處理低濃度的問題外，更提出了另一個簡單的界面修正法來處理高濃度問題，這些物理模式的修正讓我們完成首次定量的合金凝固模擬，對相場模式的開發有著重大的意義。
基於以上的模式，我們成功觀察到許多不曾模擬出來的界面形態，並和許多古典理論呈現了良好的一致性，其中也發現許多實驗上所無法確切觀察的長時間尺度的現象。由此可知，藉由模式和計算方法的改進，和實驗一對一的定量相場模擬已經不再是一個遙不可及的夢想，而基於我們以上的努力，本篇論文對日後合金凝固理論的發展，開闢了一條新的道路。Directional solidification is a process describing that an alloy solution undergoes phase transformation from melt to solid toward a specific direction. During this process, if the solidification speed is high enough to overcome the stabilizing effects of the thermal gradient and interfacial tension, the planar interface becomes unstable and wrinkled. As observed in the experiments, once the instability starts, shallow cellular structures grow initially followed by deep cells and then dendrites with the increasing speed. By controlling the interfacial morphology during freezing of an alloy, one can design its mechanical properties for many industrial applications. Also, in terms of theoretical aspects, the structural wavelength shows some nonlinear dependence with the control parameters. So far, the directional solidification of an alloy is a popular topic both in physics and material science in past few decades.
The purpose of the research presented in this thesis is to simulate alloy solidification by using the phase field model. The phase field model has emerged as a powerful tool to simulate microstructure evolution in solidification. However, limited by computation and inherent numerical nature, the phase field model has encountered many difficulties to perform quantitative simulation; one of them is the effect of solute trapping due to the diffusive interface. To mend this discrepancy, the anti-trapping current proposed by Karma is added to the standard WBM model to model solidifications of lower concentration. For ones with higher concentration, we introduce a simple interface model to restore the local equilibrium, and the solute trapping is totally eliminated. By using these modifications, the phase field modeling of alloy solidification is simulated quantitatively for the first time.
Based on the models presented here, we observe many interesting morphological evolutions, which have never been simulated. In addition, they also fit nicely to the classical theories. A long-time scale structural transition, which is extremely difficult to observe in real experiments, is also been performed. Consequently, a one-to-one comparison with experiments is quite promising in the future. Our work contributed in this thesis indeed opens a window for the research of alloy solidification.Abstract (Chinese) I
Abstract (English) III
Table of Contents V
Nomenclature IX
List of Tables XIII
List of Figures XV
Chapter 1 Introduction 1
Chapter 2 Theory of alloy solidification 5
2-1 Solute partition and phase equilibrium 5
2-2 Alloy directional solidification system 6
2-3 Interface instability during directional solidification 8
2-4 Theory of dendritic growth 17
2-5 Non-equilibrium and solute trapping 21
Chapter 3 Phase-field simulation 25
3-1 Modeling solidifications 25
Front-tracking method 25
Level-set method 26
Phase-field method 27
3-2 Derivation of phase field model 28
Thermodynamic treatment 28
Geometrical description 31
3-3 Quantitative phase-field modeling 33
Anti-trapping current (ATC) 34
Simple interface model (SIM) 35
3-4 Numerical solutions 36
Adaptive mesh refinement (AMR) 37
Finite Volume Method (FVM) 39
3-5 Computational domain and dimensionless equations 42
Chapter 4 Results and Discussion 49
4-1 SCN/Acetone dilute alloy system 49
4-2 Simulations of lower concentration 52
Planar interface 54
lc lc/2 cellular transitions 56
Region for Vl2~constant 61
Multiplets 65
4-3 Simulations of higher concentration 71
Morphological transition near the onset of instability 77
The formation of deep cells 85
Onset of initial instability 95
The Stephan effect 99
Chapter 5 Conclusions and Future directions 103
Bibliography 10
Scalable nanopatterning of lead halide perovskite quantum dots and their assemblies for directional light emission
Full text linkLAUREA MAGISTRALEI punti quantici (QDs) costituiti da perovskiti a base di alogenuro di piombo (LHP)
sono emersi come un materiale promettente per dispositivi optoelettronici come diodi
ad emissione luminosa (LEDs), fotodetettori e celle solari. Il loro successo è legato alla
semplice sintesi in soluzione, all’intervallo di banda facilmente variabile in tutto lo
spettro del visibile ed allo stretto e luminoso spettro di fotoluminescenza. Negli ultimi
anni sono statti fatti molti sforzi per: migliorarne la stabilità lavorando sulla
composizione e sulla chimica superficiale; sviluppare semplici protocolli per regolare
la loro emissione variando il contenuto di alogeni; facilitarne l’integrazione in
dispositivi. Tuttavia, a causa della loro instabilità e non compatibilità con i processi
litografici tradizionali, lo sviluppo di un protocollo per il loro “patterning” a livello
nanometrico rimane una sfida.
In questo lavoro viene presentato un approccio per la formazione di array di superreticoli di LHP scalabili. In particolare, i substrati vengono fabbricati tramite litografia
con nanostencil. La crescita dei sper-reticoli all’interno dei nanopattern è guidata dalla
morfologia dei substrati. Con questo approccio è possibile ottenere degli array con
diverse geometrie e su diversi substrati, con dimensione minima di ~400 nm e
distanziamento di ~200 nm. Il processo è compatibile con LHP QDs senza
compromettere le loro proprietà optoelettroniche. È stata poi analizzata l’emissione
direzionale degli array, i quali hanno mostrato una emissione angolare anisotropa e
dipendente dallo specifico arrangiamento in array 2D. L’emissione angolare può
migliorare l’efficienza di accoppiamento esterno in LHP LEDs.Lead halide perovskites (LHP) quantum dots (QDs) have emerged as an attractive
material for optoelectronic devices such as light emitting diodes (LEDs),
photodetectors and solar cells. Their success is linked to their facile solution-based
synthesis, their easily tuneable bandgap in the whole visible spectral range and their
narrow and bright emission spectra. In the recent years, many efforts have been done
in: improving their stability by working on their surface chemistry and their
composition; development of facile protocols for tuning their emission by adjusting
the halogen content; and device integration. However, due to their instability and
incompatibility with conventional lithographic processes the development of a
protocol for their nanoscale patterning is challenging.
In this work is presented an approach for the formation of scalable arrays of LHP QDs
superlattices. In this approach, nanopatterned substrates are fabricated via nanostencil
lithography. The LHP QDs superlattices growth inside the nanopatterns is driven by
the morphology of the substrates. With this approach arrays with different shapes and
sizes are obtained on different substrates with the smallest feature size ~400 nm and
spacing ~200nm. The process is compatible with LHP QDs without compromising
their optoelectronic properties. The arrays were further used to analyse their
directional emission. The arrays showed anisotropic angular emission dependent on
their arrangement in 2D lattices. The angular emission can improve the extern
outcoupling efficiency of LHP LEDs
Novel synthesis routes to colloidal nanocrystals of semiconductive metal pnictides and metal halides
No full textPrecise emission wavelength adjustment in lead halide perovskites using a gas-solid phase reaction with Parylene-C
Full text linkLAUREA MAGISTRALEQuesta tesi esplora la precisa regolazione della lunghezza d’onda di emissione nei perovskiti alogenuri di piombo (LHP) mediante una reazione in fase solido-gas facilitata da parylene-C. I nanocristalli (NC) di perovskite alogenuri di piombo sono messi in evidenza per le loro eccezionali proprietà fotoluminescenti e per le potenziali applicazioni in optoelettronica, tra cui LED, fotodetettori e laser. Il lavoro si propone di ottenere uno spostamento controllato dall’emissione verde a quella blu attraverso un innovativo processo di scambio ionico in fase solido-gas durante la deposizione del parylene.
I risultati chiave includono l’identificazione delle limitazioni dovute alla diffusione e le relative soluzioni, nonché lo sviluppo di un modello numerico predittivo per ottimizzare il processo di scambio ionico. La ricerca dimostra significativi progressi nella stabilità e nell’efficienza dei NC LHP a emissione blu, aprendo la strada a una loro più ampia applicazione in dispositivi optoelettronici ad alte prestazioni e basso costo.This thesis explores the precise emission wavelength adjustment in lead halide perovskites (LHPs) using a gas-solid phase reaction facilitated by parylene-C. Lead halide perovskite nanocrystals (NCs) are highlighted for their exceptional photoluminescent properties and potential applications in optoelectronics, including LEDs, photodetectors, and lasers. The work aims to achieve a controlled shift from green to blue emission through a novel solid-gas-phase ion-exchange process during parylene deposition.
Key findings include the identification of diffusion limitations and their solutions, as well as the development of a numerical prediction model to optimize the ion exchange process. The research demonstrates significant advancements in the stability and efficiency of blue-emitting LHP NCs, paving the way for their broader application in cost-effective, high-performance optoelectronic devices
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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