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University of California UNIVERSITY OF CALIFORNIA Los Angeles Origin of Superhardness in Metallic Tungsten Monoboride A dissertation submitted in partial satisfaction of the requirements for the degree Doctor of Philosophy in Chemistry by Michael Tyrone Yeung 2016 © Copyright by Michael Tyrone Yeung 2016 ABSTRACT OF THE DISSERTATION Origin of Superhardness in Metallic Tungsten Monoboride by Michael Tyrone Yeung Doctor of Philosophy in Chemistry University of California, Los Angeles, 2016 Professor Richard B. Kaner, Chair Conventional superhard materials (Vicker’s hardness Hv ≥ 40 GPa) are typically found in highly covalent systems such as diamond and cubic boron nitride. Here, we demonstrate that even materials dominated by metallic bonding can be made to be superhard. A solid solution between tungsten monoboride and tantalum monoboride, i.e. W1-xTaxB, produces a material that is both superhard (Hv = 42.8 ± 2.6 GPa) and ultra-incompressible (bulk modulus = 337 ± 3 GPa). Note that this new superhard material is derived from two non-superhard parents. The hardness of W1-xTaxB increases linearly with increasing tantalum concentration up to 50%, strongly suggesting that the increased hardness comes from solid-solution hardening of the metallic bilayer. Further evidence for this hypothesis comes from high-pressure radial diffraction. Tantalum-substituted tungsten monoboride represents the newest superhard member in the tungsten-boron system. ii Next, we demonstrate that the superhard metal W0.5Ta0.5B can be prepared as nanowires through flux growth. The primary focus of superhard materials development has relied on chemical tuning of the crystal structure. While these intrinsic effects are invaluable, there is a strong possibility that hardness can be dramatically enhanced using extrinsic effects. The aspect ratios of the nanowires are controlled by the concentration of boride in molten aluminum, and the nanowires grow along the boron-boron chains, confirmed via electron diffraction. This morphology inherently results from the crystal habit of borides and can inspire the development of other nanostructured materials. Finally, we study the role of inorganic crystal structure towards surface area through the model system, tungsten trioxide. High surface area in h-WO3 has been verified from the intracrystalline tunnels. This bottom-up approach differs from conventional top-down templating-type methods. The 3.67 Å diameter tunnels are characterized by low-pressure CO2 adsorption isotherms with non-local density functional theory fitting, transmission electron microscopy, and thermal gravimetric analysis. These open and rigid tunnels absorb H+ and Li+, but not Na+ in aqueous electrolytes without inducing a phase transformation, accessing both internal and external active sites. Moreover, these tunnel structures demonstrate high specific pseudocapacitance and good stability in an H2SO4 aqueous electrolyte. Thus, the high surface area created from 3.67 Å diameter tunnels in h-WO3 shows potential applications in electrochemical energy storage, selective ion transfer and selective gas adsorption. iii The dissertation of Michael Tyrone Yeung is approved. Xiangfeng Duan Qibing Pei Richard B. Kaner, Committee Chair University of California, Los Angeles 2016 iv Table of Contents Title of Dissertation i Abstract of Dissertation ii List of Figures vi Acknowledgements vii Vita x Chapter 1 1 Chapter 2 12 Chapter 3 42 Chapter 4 58 Chapter 5 79 Chapter 6 92 Conclusions 119 v List of Figures Figure 1: The techniques for measuring hardness 2 Figure 2: Schematic of a diamond anvil cell 6 Figure 3: Design rules for superhard metals 16 Figure 4: Rhenium Diboride 19 Figure 5: Hardness of Rhenium Diboride 21 Figure 6: Tungsten Tetraboride 23 Figure 7: Chromium and Manganese Tetraboride 26 Figure 8: Rhenium nitrides 32 Figure 9: Materials Characterization of W1-xTaxB 47 Figure 10: Grain size of W1-xTaxB 50 Figure 11: Hardness of W1-xTaxB 51 Figure 12: Differential strain of W1-xTaxB 54 Figure 13: Incompressibility of W1-xTaxB 56 Figure 14: TGA of W1-xTaxB 58 Figure 15: Electrical resistance vs temperature of W1-xTaxB 59 Figure 16: pXRD of nanostructured W1-xTaxB 66 Figure 17: SEM micrographs of nanostructured W1-xTaxB 69 Figure 18: TEM micrographs of nanostructured W1-xTaxB 71 Figure 19: Growth direction of nanostructured W1-xTaxB 72 Figure 20: Hierarchical Assembly of W1-xTaxB Nanowires 73 Figure 21: Crystal Structure of WO3 94 Figure 22: Characterization of h-WO3 96 vi Figure 23: BET Isotherms of WO3 98 Figure 24: Thermogravimetric Analysis of WO3 100 Figure 25: Powder X-Ray diffraction of WO3 103 Figure 26: Volumetric Capacitance 104 Figure 27: Cyclic Voltammograms before and after acid washing 105 Figure 28: Electrochemicstry Characterizations 106 Figure 29: Cyclic Voltammograms at different scan rates 108 Figure 30: Cyclic Performance 111 Figure 31: Specific Capacitance 112 Figure 32: Nyquist plots 113 vii Acknowledgements There are advisors in the world who trust their students. These few believe in doing the best things for their students, and more importantly, they are there to lend their students a hand should their students fall. There is no question that Ric is one of these advisors. Throughout my graduate career, I have been given the freedom to study the most interesting of topics, and the freedom to dictate my experiments. Whenever I failed, Ric was always there to pick me off the ground, dust me off, and send me off on my merry way. Quite frankly, I would not have done as well as I could had it not been for my boss (and probably would have amounted to nothing without him). I cannot thank Ric enough for trusting me throughout these years. I would like to extend a deep debt of gratitude to the Old Guard of the Kaner Lab. Whenever I was about to do something stupid, Jessica would be there to stop me. She is one of my good friends in lab, and she has always ensured that I did the right thing no matter how difficult the problem may be. I will always remember the lighthearted approach you took to things, and…the K-jokes….ugh. She is the best the Kaner lab has to offer in both pure talent and character, and I am very fortunate to have overlapped with her. She will best all of us one day. And Reza, you were the one that sent me on my path; had it not been for my good friend, I would have been working on a dead project. You’ve also spent hours giving me advice on everything. I remember the late nights in lab we spent, XRD-ing those damn samples. To Jessica and Reza, I owe and thank you two immensely for fixing me both inside and outside of lab. Sergey is a man of his word, and I very much admire that. He and I were forced to do our oral exams together, and that lead to a great understanding between him and I. Danny and I have struggled together, and without him it would have been much more difficult. viii To the New Guard of the Kaner Lab, it was my great pleasure to have served with you guys. I value the friendship I have with Cheng-Wei, and I see the drive and the intellect that he possesses. The fact that only he and I were in lab on New Year’s should have already been enough to say how impressive he will be. Also, he routinely beats me in any of our bets and competitions (I’ll beat him one day, I might have to cheat to do so). Jialin, thank you for all the discussions and arguments we had. Georgiy is the next hope the Kaner lab has to offer, and he will be better than all previous superhard members. By far he is one of the smartest people I have ever met, and he is incredibly dedicated. We spent hours in the Ben Wu lab poking a “that’s nice” sample while discussing all manners of things. Wanmei always prodded me throughout my graduate career, ensuring that I had the most difficult time and contributing extensively to my drinking problem (I’m joking). She created some of the most interesting encounters in lab which led me here today, thank you Wanmei. Rebecca and I are often the only people in lab sometimes. She would hit me with a stick whenever I was not productive. Maybe if I can take her around wherever I go in life, I’ll stay focused from fear of SMACK SMACK SMACK. Finally, to the grad students I hang out with outside of research. I thank Chain for saving my @#$ during that superacid misadventure. Logan has made my life infinitely more interesting from all the adventures on the side. Bootsie has served as my foil and my lawyer for many a litigation (he is also one of the smartest people I have ever met). Erik’s sharp wit has stung me many a times in the campaign. Abraham and I have spent many dinners arguing about Fate. Renato may be the only vegetarian I have met that can handedly beat the $%#@ out of me. Matthew, you ruined my campaign sometimes with your perception (I hope you are proud of yourself). Prof Lin and Prof Jung, you guys were the ones that made me me. I would like to say I did this by my own ability, but I didn’t. Thank you all. ix Chapter 1 and 2 is split from the following reference: Michael T. Yeung, Reza Mohammadi, Richard B. Kaner. “Ultraincompressible, Superhard Materials” Annual Reviews of Materials Research 2016, 46 (1) Link Chapter 5 is abridged from the following reference: Georgiy Akopov, Michael T. Yeung; “Rediscovering the Crystal Chemistry of Borides” Under Review, Advanced Materials Chapter 4 is from the following reference: Michael T. Yeung, Jialin Lei, Reza Mohammadi, Christopher L.
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