University, Diss., 2001--Uppsala.

Universiẗat, Diss., 2001--Frankfurt (Main).

Bachelor-Arbeit Univ. St. Gallen, 2006. / Buchhandelsausg. u.d.T.: Spannungsfeld Strommarkt.

Zugl.: Regensburg, Univ., Diss., 2008

Universiẗat, Diss., 1997--Kaiserslautern.

The present thesis investigates the grain refinement in solidifying Al-7wt%Si hypoeutectic alloy driven by electric currents. The grain size reduction in alloys generated by electric currents during the solidification has been intensively investigated. However, since various effects of electric currents have the potential to generate the finer equiaxed grains, it is still argued which effect plays the key role in the grain refinement process. In addition, the knowledge about the grain refinement mechanism under the application of electric currents remains fragmentary and inconsistent. Hence, the research objectives of the present thesis focus on the role of electric current effects and the grain refinement mechanism under the application of electric currents. Chapter 1 presents an introduction with respect to the subject of grain refinement in alloys driven by electric current during the solidification process in particular, including the research objectives; the research motivation; a brief review about the research history; a short introduction on the electric currents effects and a review relevant to the research status of grain refinement mechanism. Chapter 2 gives a description of research methods. This chapter shows the employed experiment materials, experimental setup, experimental procedure, the analysis methods of solidified samples, and numerical method, respectively. Chapter 3 focuses on the role of electric current effects in the grain refinement process. A series of solidification experiments are performed under various values of effective electric currents for both, electric current pulse and direct current. The corresponding temperature measurements and flow measurements are carried out with the increase of effective electric current intensity. Meanwhile, numerical simulations are conducted to present the details of the flow structure and the distribution of electric current density and electromagnetic force. Finally, the role of electric current effects is discussed to find the key effect in the grain refinement driven by electric currents. Chapter 4 investigates the grain refinement mechanism driven by electric currents. This chapter mainly focuses on the origin of finer equiaxed grain for grain refinement under the application of electric current on account of the importance of the origin for understanding the grain refinement mechanism. A series of solidification experiments are carried out in Al-7wt%Si alloy and in high purity aluminum. The main origin of equiaxed grain for grain refinement is concluded based on the experiment results. Chapter 5 presents three further investigations based on the achieved knowledge in chapter 3 and 4 about the role of electric current effects and the grain refinement mechanism. According to the insight into the key electric current effect for the grain refinement shown in chapter 3, this chapter presents a potential approach to promote the grain refinement. In addition, the solute distribution under the influence of electric current is examined based on the knowledge about the electric current effects. Moreover, the grain refinement mechanism under application of travelling magnetic field is investigated by performing a series of solidification experiments to compare with the experiments about the grain refinement mechanism driven by electric currents shown in chapter 4. Chapter 6 summarizes the main conclusions from the presented work.

The present thesis investigates the grain refinement in solidifying Al-7wt%Si hypoeutectic alloy driven by electric currents. The grain size reduction in alloys generated by electric currents during the solidification has been intensively investigated. However, since various effects of electric currents have the potential to generate the finer equiaxed grains, it is still argued which effect plays the key role in the grain refinement process. In addition, the knowledge about the grain refinement mechanism under the application of electric currents remains fragmentary and inconsistent. Hence, the research objectives of the present thesis focus on the role of electric current effects and the grain refinement mechanism under the application of electric currents. Chapter 1 presents an introduction with respect to the subject of grain refinement in alloys driven by electric current during the solidification process in particular, including the research objectives; the research motivation; a brief review about the research history; a short introduction on the electric currents effects and a review relevant to the research status of grain refinement mechanism. Chapter 2 gives a description of research methods. This chapter shows the employed experiment materials, experimental setup, experimental procedure, the analysis methods of solidified samples, and numerical method, respectively. Chapter 3 focuses on the role of electric current effects in the grain refinement process. A series of solidification experiments are performed under various values of effective electric currents for both, electric current pulse and direct current. The corresponding temperature measurements and flow measurements are carried out with the increase of effective electric current intensity. Meanwhile, numerical simulations are conducted to present the details of the flow structure and the distribution of electric current density and electromagnetic force. Finally, the role of electric current effects is discussed to find the key effect in the grain refinement driven by electric currents. Chapter 4 investigates the grain refinement mechanism driven by electric currents. This chapter mainly focuses on the origin of finer equiaxed grain for grain refinement under the application of electric current on account of the importance of the origin for understanding the grain refinement mechanism. A series of solidification experiments are carried out in Al-7wt%Si alloy and in high purity aluminum. The main origin of equiaxed grain for grain refinement is concluded based on the experiment results. Chapter 5 presents three further investigations based on the achieved knowledge in chapter 3 and 4 about the role of electric current effects and the grain refinement mechanism. According to the insight into the key electric current effect for the grain refinement shown in chapter 3, this chapter presents a potential approach to promote the grain refinement. In addition, the solute distribution under the influence of electric current is examined based on the knowledge about the electric current effects. Moreover, the grain refinement mechanism under application of travelling magnetic field is investigated by performing a series of solidification experiments to compare with the experiments about the grain refinement mechanism driven by electric currents shown in chapter 4. Chapter 6 summarizes the main conclusions from the presented work.:Abstract VII Contents IX List of figures XI List of tables XVII 1. Introduction 1 1.1 Research objectives 1 1.2 Research motivation 2 1.3 Research history 5 1.4 Electric currents effects 9 1.4.1 Some fundamentals 10 1.4.2 Role of electric currents effects in grain refinement 12 1.5 Grain refinement mechanism 13 1.5.1 Nucleation theory 13 1.5.2 Equiaxed grain formation without the application of external fields 18 1.5.3 Grain refinement mechanism under the application of electric currents 23 1.5.4 Grain refinement mechanism under the application of magnetic field 29 2. Research methods 31 2.1 Introduction 31 2.2 Experimental materials 31 2.2.1 Solidification 31 2.2.2 Similarity of GaInSn liquid metal and Al-Si melt 32 2.3 Experimental setup 33 2.3.1 Solidification 33 2.3.2 Flow measurements 35 2.3.3 External energy fields 36 2.4 Experimental procedure 38 2.4.1 Solidification 38 2.4.2 Flow measurements 39 2.5 Metallography 39 2.6 Numerical method 41 2.6.1 Numerical model 41 2.6.2 Numerical domain and boundary conditions 42 3. Role of electric currents effects in the grain refinement 45 3.1 Introduction 45 3.2 Experimental parameter 45 3.3 Results 46 3.3.1 Solidified structure 46 3.3.2 Forced melt flow 50 3.3.3 Temperature distribution 58 3.4 Discussion 61 3.5 Conclusions 67 4. Grain refinement mechanism driven by electric currents 69 4.1 Introduction 69 4.2 Experimental parameter 69 4.3 Results 73 4.3.1 Solidified structure of Al-Si alloy 73 4.3.2 Cooling curves of Al-Si alloy 77 4.3.3 Solidified structure of high purity aluminum 78 4.4 Discussion 80 4.5 Conclusions 83 5. Supplemental investigations 85 5.1 A potential approach to improve the grain refinement 85 5.1.1 Introduction 85 5.1.2 Experimental parameter 86 5.1.3 Results and discussion 87 5.2 Macrosegregation formation 90 5.2.1 Introduction 90 5.2.2 Experimental parameter 91 5.2.3 Results and discussion 92 5.3 Grain refinement driven by TMF 97 5.3.1 Introduction 97 5.3.2 Experimental parameter 97 5.3.3 Results and discussion 98 5.4 Conclusions 102 6. Summary 103 Bibliography 105

Die vorliegende Arbeit beschäftigt sich mit der elektrischen Charakterisierung von Zinkoxid-Nanodrähten, die mittels gepulster Laserablation (PLD) hergestellt wurden. Ausgehend von den so generierten ZnO-Nanodraht-Ensembles werden Methoden zu deren elektrischer Untersuchung diskutiert und auf praktische Anwendbarkeit hin verglichen. Die entwickelten Methoden werden auf Ensembles von auf n-leitenden ZnO- und ZnO:Ga-Dünnschichten aufgewachsenen Phosphor-dotierten ZnO-Nanodrähten angewendet. Deren reproduzierbares, in Strom–Spannungs- (I–U-) Kennlinien beobachtetes diodenartiges Verhalten wird genauer beleuchtet. Im Zusammenhang mit der elektrischen Charakterisierung einzelner ZnO-Nano-drähte werden experimentelle Methoden zur Vereinzelung und zur Kontaktierung der vereinzelten ZnO-Nanodrähte diskutiert. Dabei werden sowohl etablierte Methoden wie Elektronenstrahllithographie (EBL) als auch neue Techniken wie elektronen- und ionenstrahlinduzierte Deposition (EBID/IBID) und Strom–Spannungs-Rastersondenmikroskopie (I-AFM) behandelt und ihre Eignung für eingehende elektrische Untersuchungen und reproduzierbare Messungen analysiert. Die geeignetsten Methoden werden schließlich eingesetzt, um spezifischen Widerstand sowie Ladungsträgermobilität und -dichte sowohl in nominell undotierten als auch in Aluminium-dotierten ZnO-Nanodrähten zu untersuchen und zu vergleichen. In der Ableitung der physikalischen Materialparameter aus den Messdaten wird dabei besonderes Augenmerk auf die Einbeziehung der geometrischen Besonderheiten der Nanodrähte gegenüber Volumenmaterial- und Dünnschichtproben gelegt. Im Zuge dessen wird unter anderem ein Modell für den elektrischen Widerstand in Nanodrähten mit ihrer Länge nach veränderlichem Querschnitt abgeleitet.

Die vorliegende Arbeit beschäftigt sich mit der elektrischen Charakterisierung von Zinkoxid-Nanodrähten, die mittels gepulster Laserablation (PLD) hergestellt wurden. Ausgehend von den so generierten ZnO-Nanodraht-Ensembles werden Methoden zu deren elektrischer Untersuchung diskutiert und auf praktische Anwendbarkeit hin verglichen. Die entwickelten Methoden werden auf Ensembles von auf n-leitenden ZnO- und ZnO:Ga-Dünnschichten aufgewachsenen Phosphor-dotierten ZnO-Nanodrähten angewendet. Deren reproduzierbares, in Strom–Spannungs- (I–U-) Kennlinien beobachtetes diodenartiges Verhalten wird genauer beleuchtet. Im Zusammenhang mit der elektrischen Charakterisierung einzelner ZnO-Nano-drähte werden experimentelle Methoden zur Vereinzelung und zur Kontaktierung der vereinzelten ZnO-Nanodrähte diskutiert. Dabei werden sowohl etablierte Methoden wie Elektronenstrahllithographie (EBL) als auch neue Techniken wie elektronen- und ionenstrahlinduzierte Deposition (EBID/IBID) und Strom–Spannungs-Rastersondenmikroskopie (I-AFM) behandelt und ihre Eignung für eingehende elektrische Untersuchungen und reproduzierbare Messungen analysiert. Die geeignetsten Methoden werden schließlich eingesetzt, um spezifischen Widerstand sowie Ladungsträgermobilität und -dichte sowohl in nominell undotierten als auch in Aluminium-dotierten ZnO-Nanodrähten zu untersuchen und zu vergleichen. In der Ableitung der physikalischen Materialparameter aus den Messdaten wird dabei besonderes Augenmerk auf die Einbeziehung der geometrischen Besonderheiten der Nanodrähte gegenüber Volumenmaterial- und Dünnschichtproben gelegt. Im Zuge dessen wird unter anderem ein Modell für den elektrischen Widerstand in Nanodrähten mit ihrer Länge nach veränderlichem Querschnitt abgeleitet.