Person: Lee, Dongwoo
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Dongwoo
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Lee, Dongwoo
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Publication Scanning AC Nanocalorimetry Study of Zr/B Reactive Multilayers(American Institute of Physics, 2013) Lee, Dongwoo; Sim, Gi-dong; Xiao, Kechao; Seok Choi, Yong; Vlassak, JoostThe reaction of Zr/B multilayers with a 50 nm modulation period has been studied using scanning AC nanocalorimetry at a heating rate of approximately \(10^3 K/s\). We describe a data reduction algorithm to determine the rate of heat released from the multilayer. Two different exothermic peaks are identified in the nanocalorimetry signal: a shallow peak at low temperature (200–650°C) and a sharp peak at elevated temperature (650–800°C). TEM observation shows that the first peak corresponds to heterogeneous inter-diffusion and amorphization of Zr and B while the second peak is due to the crystallization of the amorphous Zr/B alloy to form \(ZrB_2\).Publication Spectral descriptors for bulk metallic glasses based on the thermodynamics of competing crystalline phases(Nature Publishing Group, 2016) Perim, Eric; Lee, Dongwoo; Liu, Yanhui; Toher, Cormac; Gong, Pan; Li, Yanglin; Simmons, W. Neal; Levy, Ohad; Vlassak, Joost; Schroers, Jan; Curtarolo, StefanoMetallic glasses attract considerable interest due to their unique combination of superb properties and processability. Predicting their formation from known alloy parameters remains the major hindrance to the discovery of new systems. Here, we propose a descriptor based on the heuristics that structural and energetic ‘confusion' obstructs crystalline growth, and demonstrate its validity by experiments on two well-known glass-forming alloy systems. We then develop a robust model for predicting glass formation ability based on the geometrical and energetic features of crystalline phases calculated ab initio in the AFLOW framework. Our findings indicate that the formation of metallic glass phases could be much more common than currently thought, with more than 17% of binary alloy systems potential glass formers. Our approach pinpoints favourable compositions and demonstrates that smart descriptors, based solely on alloy properties available in online repositories, offer the sought-after key for accelerated discovery of metallic glasses.Publication Kinetics of solid-gas reactions characterized by scanning AC nano-calorimetry with application to Zr oxidation(AIP Publishing, 2014) Xiao, Kechao; Lee, Dongwoo; Vlassak, JoostScanning AC nano-calorimetry is a recently developed experimental technique capable of measuring the heat capacity of thin-film samples of a material over a wide range of temperatures and heating rates. Here, we describe how this technique can be used to study solid-gas phase reactions by measuring the change in heat capacity of a sample during reaction. We apply this approach to evaluate the oxidation kinetics of thin-film samples of zirconium in air. The results confirm parabolic oxidation kinetics with an activation energy of 0.59 ± 0.03 eV. The nano-calorimetry measurements were performed using a device that contains an array of micromachined nano-calorimeter sensors in an architecture designed for combinatorial studies. We demonstrate that the oxidation kinetics can be quantified using a single sample, thus enabling high-throughput mapping of the composition-dependence of the reaction rate.Publication Low-Temperature Synthesis of Ultra-High-Temperature Coatings of ZrB 2 Using Reactive Multilayers(American Chemical Society (ACS), 2014) Lee, Dongwoo; Sim, Gi-Dong; Xiao, Kechao; Vlassak, JoostWe demonstrate a route to synthesize ultra-high-temperature ceramic coatings of ZrB2 at temperatures below 1300 K using Zr/B reactive multilayers. Highly textured crystalline ZrB2 is formed at modest temperatures because of the absence of any oxide at the interface between Zr and B and the very short diffusion distance that is inherent to the multilayer geometry. The kinetics of the ZrB2 formation reaction is analyzed using high-temperature scanning nanocalorimetry, and the microstructural evolution of the multilayer is revealed using transmission electron microscopy. We show that the Zr/B reaction proceeds in two stages: (1) interdiffusion between the nanocrystalline Zr and the amorphous B layers, forming an amorphous Zr/B alloy, and (2) crystallization of the amorphous alloy to form ZrB2. Scanning nanocalorimetry measurements performed at heating rates ranging from 3100 to 10000 K/s allow determination of the kinetic parameters of the multilayer reaction, yielding activation energies of 0.47 and 2.4 eV for Zr/B interdiffusion and ZrB2 crystallization, respectively.Publication Nanocalorimetry Experiments and First-Principles Theoretical Studies of Solid-State Reactions in Nanolaminates(2016-05-04) Lee, Dongwoo; Vlassak, Joost; Spaepen, Frans; Clarke, David; Aizenberg, JoannaThe extraordinary sensitivity and extremely small thermal mass of chip-based nanocalorimetry sensors allow the study of reactions in thin films over a broad range of heating rates, from isothermal to 10^5 K/s. First-principles calculations provide insight in the phase transformation and diffusion behavior of a material at the atomistic scale. Combination of nanocalorimetry and first-principles, therefore, is highly efficient and reliable to determine the atomistic-to-macroscopic response of materials. This thesis explores, through use of this combined approach, reactions in reactive multilayers to synthesize ultra-high temperature ceramic coatings. We employ scanning AC and DC calorimetry techniques to investigate the synthesis of ZrB2 and carbon-doped ZrB2 using Zr/B and Zr/B4C multilayered reactive nanolaminates (MRNL). The solid-state reactions in these multilayers are shown to proceed in two distinct steps: an interdiffusion/amorphization step followed by a crystallization step. Measurements performed at heating rates ranging from 1,000 to 55,000 K/s allows determination of the kinetic parameters of the multilayer reactions, such as the activation energies of interdiffusion and crystallization. Low activation energies in the interdiffusion processes in the Zr/B MRNLs are found and amorphization is shown to facilitate fast transport of B atoms into Zr lattice. It has also been shown that C impurity atoms in the Zr/B4C MRNLs further reduce activation energies of interdiffusion and crystallization. First-principles theoretical modeling provides insight in the amorphization processes in the Zr/B MRNLs and confirms the relatively low activation energies associated with the processes. The simulations further elucidate the effects of concentration (ZrBx, 0