Phase Evolution in Manganese-Germanium System During Mechanical Alloying by Vamsi Madhukar Meka A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Engineering (Mechanical Engineering) in the University of Michigan–Dearborn 2017 Master Thesis Committee: Assistant Professor Tanjore V. Jayaraman, Chair Professor Pravansu Mohanty Associate Professor German Reyes-Villanueva ACKNOWLEDGEMENTS I would like to extend my deepest gratitude towards my thesis advisor, Prof. Tanjore V. Jayaraman, for his continual support, encouragement, and expert guidance throughout the research work. This opportunity to work with him has enhanced my technical, ethical, and soft skills. I would also like to thank Prof. Pravansu Mohanty and Prof. German Reyes-Villanueva for sparing their time to evaluate my thesis and for being a part of the thesis committee. I would like to thank the Mechanical Engineering Department, College of Engineering and Computer Science at the University of Michigan in Dearborn for their support in completing my thesis and the Department of Material Science and Engineering at The University of Michigan at Ann Arbor for providing access to some of their equipment. I would also like to thank Tim Chambers, Chris Cristian, Ying Qi, and Erik Kirk for their assistance with equipment and safety training and, special thanks to Cory Sayle from Ann Arbor for opening the laboratory door every time we were early. Lastly, I would like to thank my family Mr. Vijaya Bhaskar Meka, Ms. Naga Mani Meka, Ms. Pranathi, friends, and the close ones for supporting me throughout this journey. ii ABSTRACT In this thesis work, the phase evolution during the synthesis of metastable binary germanides of Mn-Ge system was investigated. A thorough literature review on the various metallic germanides was performed. Attempts were made to synthesize various metastable manganese germanides by mechanical alloying. The phase evolution during the synthesis was investigated by x-ray diffraction and scanning electron microscopy. Powders of MnGe (an equiatomic metastable phase of Mn-Ge system) were successfully synthesized. The structural characterization revealed the lattice parameter and the particle size to be 0.4798 ± 0.0008 nm and ~1-3 μm, respectively. The magnetic characterization showed that MnGe was paramagnetic at room temperature, antiferromagnetic at sub-ambient temperatures with Neel temperature estimated as ~162 K, and the magnetization (at 1 Tesla) was estimated to be ~ 3 emu/g. In the case of MnGe, based on the phase evolution, attempts were made to reduce the synthesis time by varying appropriate processing parameters. During the synthesis of Ge-rich metastable manganese-germanides, the evolution of the respective phases (Mn3Ge5, MnGe2, and MnGe4) was always accompanied with MnGe. The metastable MnGe synthesized (in powder form), in this work, at ambient temperature and pressure conditions had a considerably high yield and reproducibility, unlike in the past synthesized by high-pressure/high-temperature technique (in bulk form) and thin- film deposition technique (thin film) available in the literature. Future study would involve the synthesis of other metastable compound by isolating MnGe during their evolution. iii TABLE OF CONTENTS ACKNOWLEDGEMENTS ....................................................................................................... ii ABSTRACT ............................................................................................................................. iii LIST OF FIGURES .................................................................................................................. vi LIST OF TABLES .................................................................................................................. xiv CHAPTER 1. INTRODUCTION ........................................................................................ 1 Diluted Magnetic Semiconductors .............................................................................. 1 Non-Equilibrium Processing ....................................................................................... 2 CHAPTER 2. LITERATURE REVIEW AND BACKGROUND ...................................... 5 Metallic Germanides ................................................................................................... 5 Transition metal Germanides ...................................................................................... 7 Rare Earth Germanides ............................................................................................. 54 Rare Earth-Transition Metal Germanides ................................................................. 65 Mechanical Alloying ................................................................................................. 72 2.5.1. Type of Mill ....................................................................................................... 73 2.5.2. Milling Container ............................................................................................... 75 2.5.3. Milling Speed ..................................................................................................... 76 2.5.4. Milling Time ...................................................................................................... 76 2.5.5. Type, Size and Size Distribution of Grinding Medium ..................................... 76 2.5.6. Ball to Powder Ratio .......................................................................................... 77 2.5.7. Extent to Which the Vial Is Filled ..................................................................... 77 2.5.8. Milling Atmosphere ........................................................................................... 78 2.5.9. Process Control Agent ....................................................................................... 78 2.5.10. Temperature of Milling .................................................................................. 79 Contaminations in Mechanical Alloying................................................................... 79 2.6.1. Contamination from Milling Tools .................................................................... 79 2.6.2. Contamination from Atmosphere ...................................................................... 80 2.6.3. Contamination from Process Control Agents .................................................... 80 Key Points from Literature Review: ......................................................................... 81 CHAPTER 3. MOTIVATION ........................................................................................... 83 CHAPTER 4. EXPERIMENTAL PROCEDURE ............................................................. 84 iv Material Synthesis ..................................................................................................... 84 Materials Characterization ........................................................................................ 87 CHAPTER 5. RESULTS AND DISCUSSIONS .............................................................. 89 Phase evolution during the synthesis of MnGe ......................................................... 89 5.1.1. Adopting the Prevalent Technique ..................................................................... 89 5.1.2. Adopting the Ideal Technique ............................................................................ 92 5.1.3. Magnetization of MnGe ................................................................................... 118 5.1.4. Synthesis Process Comparison: ....................................................................... 118 Process Optimization for synthesis of MnGe .......................................................... 119 5.2.1. Using ideal technique with intermediate addition of stearic acid .................... 119 5.2.2. Using Ideal technique, with increased ball to powder ratio ............................. 124 5.2.3. Synthesis of MnGe by alloying powders in atmosphere. ................................ 126 5.2.4. Study of Effect of Different Types of PCA’s .................................................. 128 Phase evolution during the synthesis of Mn3Ge5 .................................................... 130 Phase evolution during the synthesis of MnGe2 ...................................................... 131 5.4.1. Adopting Ideal alloying technique ................................................................... 131 5.4.2. Adopting prevalent alloying technique ............................................................ 132 Phase evolution during the synthesis of MnGe4. ..................................................... 135 CHAPTER 6. CONCLUSION ........................................................................................ 136 REFERENCES ...................................................................................................................... 138 v LIST OF FIGURES Figure 1 The basic concept of "energize and quench" to synthesize non-equilibrium materials [1] ............................................................................................................................................... 2 Figure 2 Periodic Table with metals, rare-earths, and transition metals highlighted ................ 5 Figure 3 Temperature dependence a) electrical resistivity and b) thermoelectric power measured from 500 to 90K of SrGe5.6 [9] .................................................................................