290 research outputs found
Vacancy-mediated complex phase selection in high entropy alloys
No full textPhase selection in Ti-Zr-Hf-Al high-entropy alloys was investigated by in-situ high-energy X-ray diffraction, single-crystal X-ray diffraction, and density-functional theory based electronic-structure methods that address disorder and vacancies, predicting formation enthalpy and chemical short-range order (SRO). Samples with varying Al content were synthesized, characterized, and computationally assessed to ascertain the composition-dependent phase selection, as increased Al content often acts as a stabilizer of a body-centered-cubic structure. Equiatomic TiZrHfAl was especially interesting due to its observed bcc superstructure – a variant of γ-brass with 4 vacancies per cell (not 2 as in γ-brass). We highlight how vacancy ordering mediates selection of this variant of γ-brass, which is driven by vacancy-atom SRO that dramatically suppress all atomic SRO. As vacancies are inherent in processing refractory systems, we expect that similar discoveries await in other high entropy alloys or in revisiting older experimental data.This is a manuscript of an article published as Singh, Prashant, Shalabh Gupta, Srinivasa Thimmaiah, Bryce Thoeny, Pratik K. Ray, A. V. Smirnov, Duane D. Johnson, and Matthew J. Kramer. "Vacancy-mediated complex phase selection in high entropy alloys." 194 Acta Materialia (2020): 540-546. DOI: 10.1016/j.actamat.2020.04.063. Posted with permission.</p
Multiple magnetic interactions and large inverse magnetocaloric effect in TbSi and TbSi0.6Ge0.4
No full textWe present a comprehensive investigation of the electronic structure, magnetization, specific heat, and crystallography of TbSi (FeB structure type) and TbSi0.6Ge0.4 (CrB structure type) compounds. Both TbSi and TbSi0.6Ge0.4 exhibit two antiferromagnetic (AFM) transitions at TN1≈ 58~K and 57~K, and TN2≈ 36~K and 44~K, respectively, along with an onset of weak metamagnetic-like transition around 6~T between TN1 and TN2. High-resolution specific heat (CP) measurements show the second- and first-order nature of the magnetic transition at TN1 and TN2, respectively, for both samples. However, in the case of TbSi, the low-temperature (LT) AFM to high-temperature (HT) AFM transition takes place via an additional AFM phase at the intermediate temperature (IT), where both LT to IT AFM and IT to HT AFM phase transitions exhibit a first-order nature. Both TbSi and TbSi0.6Ge0.4 manifest significant magnetic entropy changes (ΔSM) of 9.6 and 11.6~J/kg-K, respectively, for Δμ0H=7~T, at TN2. The HT AFM phase of TbSi0.6Ge0.4 is found to be more susceptible to the external magnetic field, causing a significant broadening in the peaks of ΔSM curves at higher magnetic fields. Temperature and field-dependent specific heat data have been utilized to construct the complex H-T phase diagram of these compounds. Furthermore, temperature-dependent x-ray diffraction measurements demonstrate substantial magnetostriction and anisotropic thermal expansion of the unit cell in both samples.This is a preprint from Kumar, Ajay, Prashant Singh, Andrew Doyle, Deborah L. Schlagel, and Yaroslav Mudryk. "Multiple magnetic interactions and large inverse magnetocaloric effect in TbSi and TbSi Ge ." arXiv preprint arXiv:2405.06777 (2024). doi: https://doi.org/10.48550/arXiv.2405.06777. Published as Kumar, Ajay, Prashant Singh, Andrew Doyle, Deborah L. Schlagel, and Yaroslav Mudryk. "Multiple magnetic interactions and large inverse magnetocaloric effect in TbSi and TbSi 0.6 Ge 0.4." Physical Review B 109, no. 21 (2024): 214410.
doi: https://doi.org/10.1103/PhysRevB.109.214410
Smart nanocomposites: Harnessing magnetically recoverable MWCNT-CF for efficient organic dyes reduction in water quality monitoring applications
No full textThe accelerating use of organic dyes in various industries has led to a surge in water pollution, especially from non-biodegradable dye effluents discharged into water resources. This study addresses the critical issue of catalyzing the reduction of two prevalent dyes, methylene blue (MB) and rhodamine-B (RhB), using a multiwalled carbon nanotube-cobalt ferrite (MWCNT-CF) nanocomposite. The synthesized nanocomposite demonstrates exceptional catalytic activity, stability, and recyclability. Conventional methods for treating dye-containing wastewater often prove expensive. This study explores the efficacy of catalytic reduction, a relatively fast process facilitated by semiconductor nanoparticles. Structural analyses using X-ray diffraction and high-resolution transmission electron microscopy (HRTEM) confirm the formation of the nanocomposite, revealing unsaturated surface bonds and chains conducive to adsorption. The nanocomposite exhibits a remarkable reduction in both dyes, with easy recyclability for multiple cycles. Magnetization studies confirm the ferrimagnetic nature of the nanocomposite, facilitating its efficient separation from the reaction mixture using a magnet. The study delves into the kinetics of the catalytic reduction following pseudo-first-order kinetics. The surface modifications of the nanocomposite, as revealed by TEM, contribute to enhanced adsorption and catalytic efficiency. Notably, the MWCNT-CF nanocomposite demonstrates negligible loss of catalytic activity during recycling, highlighting its potential for cost-effective and sustainable applications in dye reduction across various industries
Predictive design of novel nickel-based superalloys beyond Haynes 282
No full textNickel-based superalloys are in great demand for harsh-service conditions involving high temperatures and oxidative environments. Haynes 282 stands out due to its excellent high-temperature properties and easy fabricability. However, the upper operation temperature of Haynes 282 is limited due to its relatively low liquidus temperature. Equipped with high-fidelity density-functional theory calculations and high-throughput experimentation methodology, we explored new compositional spaces that exhibit higher liquidus temperature and higher strength. While maintaining the manufacturability, the newly designed alloy shows improved strength and ductility at room temperature and better oxidation resistance up to 800°C. The new compositions showcase a minor change in the refractory and metalloid content can significantly impact the mechanical and oxidation performance of superalloys.This is a pre-proof version of the article Published as Ouyang, Gaoyuan, Olena Palasyuk, Prashant Singh, Pratik K. Ray, Vinay Deodeshmukh, Jun Cui, Duane D. Johnson, and Matthew J. Kramer. "Predictive design of novel nickel-based superalloys beyond Haynes 282." Acta Materialia (2024): 120045. doi: https://doi.org/10.1016/j.actamat.2024.120045. © 2024 Elsevier.
CC BY-NC-N
Density-functional theory of material design: fundamentals and applications-I
No full textThis article is part-I of a review of density-functional theory (DFT) that is the most widely used method for calculating electronic structure of materials. The accuracy and ease of numerical implementation of DFT methods has resulted in its extensive use for materials design and discovery and has thus ushered in the new field of computational material science. In this article, we start with an introduction to Schrödinger equation and methods of its solutions. After presenting exact results for some well-known systems, difficulties encountered in solving the equation for interacting electrons are described. How these difficulties are handled using the variational principle for the energy to obtain approximate solutions of the Schrödinger equation is discussed. The resulting Hartree and Hartree–Fock theories are presented along with results they give for atomic and solid-state systems. We then describe Thomas–Fermi theory and its extensions which were the initial attempts to formulate many-electron problem in terms of electronic density of a system. Having described these theories, we introduce modern DFT by discussing Hohenberg–Kohn theorems that form its foundations. We then go on to discuss Kohn–Sham (KS) formulation of DFT in its exact form. Next, local density approximation (LDA) is introduced and solutions of KS equation for some representative systems, obtained using the LDA, are presented. We end part-I of the review describing the contents of part-II.This article is published as Singh, Prashant, and Manoj K. Harbola. "Density-functional theory of material design: fundamentals and applications-I." Oxford Open Materials Science 1, no. 1 (2021): itab018.
DOI: 10.1093/oxfmat/itab018.
Copyright 2021 The Author(s).
Attribution 4.0 International (CC BY 4.0).
DOE Contract Number(s): AC02-07CH11358.
Posted with permission
High resolution X-ray diffraction datasets of cytosinium chloride
No full textSubatomic resolution X-ray measurements were performed at 90 K on an Agilent Technologies SuperNova four-circle diffractometer equipped with a micro-focus sealed tube and Eos CCD detector
Effect of Zn Addition on Phase Selection in AlCrFeCoNiZn High-Entropy Alloy
No full textThe addition of Zn to AlCrFeCoNi high-entropy alloy (HEA) poses intriguing questions as to how it would affect phase evolution. On one hand, Zn has high valence-electron count that should stabilize a face-centered-cubic phase, while, on the other, it exhibits ordering (strong clustering) tendency with Ni and Co (with Cr and Fe), hinting towards phase separation. Here, the phase evolution in AlCrFeCoNiZn was studied using a combination of experimental techniques (XRD, SEM, EDS and DSC) and computational (density-functional theory (DFT), calculation of phase diagrams (CALPHAD), and machine-learning) methods. Mechanically alloyed (MA) and spark plasma sintered (SPS) AlCrFeCoNiZn assumes a metastable single-phase, body-centered-cubic (BCC) structure that undergoes diffusion-controlled phase separation upon subsequent heat treatment to form separate (Al, Cr)-rich, (Fe, Co)-rich and (Zn, Ni)-rich phases. The formation of (Al, Cr)-rich phase, not reported previously in AlCrFeCoNi-based HEAs, is attributed to strong clustering tendency of (Cr-Zn) and (Cr-Ni) pairs, combined with the strong ordering of (Zn-Ni) pair, driving out Cr that in turn combines with Al to form a (Al, Cr)-rich phase. DFT results show the formation of thermodynamically stable L12 phase wherein Cr-Fe-Zn [Al-Ni-Co] preferably occupy1a (000) [3c (0 ½ ½)] positions. The sluggish diffusional transformation to L12 phase from BCC precursors is attributed to the small stacking-fault energy of AlCrFeCoNiZn. The equilibrated HEA exhibits a high microhardness of 8.24 GPa with an elastic modulus of 184 GPa.This is a preprint from Shivam, Vikas, Dishant Beniwal, Yagnesh Shadangi, Prashant Singh, V. S. Hariharan, Gandham Phanikumar, Duane D. Johnson, Pratik K. Ray, and N. K. Mukhopadhyay. "Effect of Zn addition on phase selection in AlCrFeCoNiZn high-entropy alloy." Available at SSRN 4263461 (2022). doi: https://dx.doi.org/10.2139/ssrn.4263461
Computational electronic structure studies of novel condensed matter phases
No full textThis dissertation compiles the bulk of my work as a PhD student in the research group of Professor Prashant K. Jain at University of Illinois at Urbana-Champaign. My research was exclusively in the field of theoretical chemistry and materials science: I employed high-performance computing tools to perform electronic structure investigations of novel crystalline materials synthesized, some for the very first time, in the group. My placement in the experimentally-focused Jain group afforded multiple opportunities in which the discoveries of my fellow group members prompted me to conduct stand-alone or collaborative theoretical investigations of new nanomaterials. A summary of the experimental backdrop to my work is presented in Chapter 1, along with a description of the theoretical methods that were the mainstay of my PhD research. Chapter 2 presents work in which my density functional theory (DFT) calculations improved our understanding of the metastability of a previously unobserved vacancy ordering in a Cu2Se. Chapter 3 presents a different direction of investigations that we conducted on Cu2Se, this time into its superionic properties. The nucleation, kinetics, and correlation of lattice strain to the order-disorder superionic phase transition were explored through a combination of transmission electron microscopy and DFT. The correlation between lattice strain and superionicity is expanded upon in Chapter 4 where Prashant and I developed a theoretical basis on which to understand compressively strain-stabilized superionicity in Cu2Se and Li2Se. Chapter 5 shifts away from Cu2Se on to HgSe. Additionally, the focus changes from the structure and transport of cations to the structure and transport of electrons, specifically the electron-conducting surface states found in topological phases of matter. Bulk band-structure calculations and charge density character analysis that I carried out led us to hypothesize that a hexagonal phase of HgSe newly-synthesized bny the Jain group was a 3-D topological insulator. The unique topological surface states (TSS) of HgSe and their dependence on strain, crystallographic symmetry, and surface faceting are determined by DFT and presented in Chapter 6. Particularly, the effect of lattice strain on the dispersion and spin texture circles back to the central theme in the studies of super-ionic crystals: that small amounts of strain can significantly alter the charge transport properties of a material.Submission published under a 24 month embargo labeled 'Closed Access', the embargo will last until 2022-05-01The student, Daniel Dumett Torres, accepted the attached license on 2020-04-21 at 14:56.The student, Daniel Dumett Torres, submitted this Dissertation for approval on 2020-04-21 at 15:04.This Dissertation was approved for publication on 2020-04-24 at 13:06.DSpace SAF Submission Ingestion Package generated from Vireo submission #15022 on 2020-08-25 at 17:40:29Made available in DSpace on 2020-08-27T00:49:58Z (GMT). No. of bitstreams: 2
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Tracking the evolution of photoexcitations in strongly light absorbing systems
No full textAuthor requested closed access (OA after 2yrs) in Vireo ETD systemLimitedThis dissertation consists of the work done towards a Ph.D. degree in the research group of Professor Prashant K. Jain at the University of Illinois at Urbana-Champaign. Here, I describe the study of the conversion of light energy using hybrid perovskite and noble metal-based semiconductor nanoparticles. The large surface-area-to-volume ratios and superlative ability to absorb visible light make these materials worthy candidates for solar energy harvesting. The primary questions I have asked in my research are: what is the fate of photoexcitation in a nanostructured material and how can we channel such photo-excitations in an efficient and selective manner? Chapter 1 of this dissertation introduces some of the theoretical backdrops to my studies of strongly light-absorbing plasmonic nanoparticulate systems. This chapter elucidates what follows and introduces the concepts and terms used. Chapter 2 presents my investigation of hybrid organic-inorganic perovskite materials for potential uses towards light trapping and emission. We discovered that commonly observed luminescence from microcrystals of these materials showed a spectrum that varied with sample morphology and location on the sample. The origin of this spectral heterogeneity was then traced to the phenomenon of luminescence self-absorption, which is prevalent due to the overlapping absorption and emission, i.e., small Stokes-shift, in these materials. Then we explored light-to-chemical-energy conversion in perovskite materials, but they proved to be photochemically unstable; so, we turned our attention to noble metal nanoparticles, which have strong plasmon resonance absorption and high photostability. Chapter 3 describes the investigation of light-to-chemical energy conversion in colloidal gold (Au) nanoparticles In particular, we studied the effect of visible-light excitation of Au nanoparticles in the presence of an electron acceptor (HAuCl4) and a hole acceptor (short-chain alcohol). This led to the discovery of a hitherto unknown photoreaction, which involves the splitting and chlorination of the alcohol
generating a chloroalkane and an aldehyde. This reaction was found to take place with several alcohols, which led us to a general reaction mechanism that is catalyzed synergistically by the photoexcited nanoparticle and the Lewis acidic HAuCl4. In the specific case of 2-butanol as the hole acceptor, we found a substantial difference between the product distributions of the light-driven reaction as compared to a thermal reaction. This finding represents an example of light-driven-control of catalytic selectivity. Finally, as presented in Chapter 4, the insights gained from the study described in Chapter 3 led me to a new, simple chemical process for low-temperature chlorination of methane in a non-corrosive aqueous environment. The kinetics and mechanism of this reaction were studied. Methane chlorination is at the heart of natural-gas upgradation, so this new finding represents an ideal culmination of my dissertation. An outlook and potential future directions are presented in Chapter 5.Submission published under a 24 month embargo labeled 'Closed Access', the embargo will last until 2022-12-01The student, Varun Mohan, accepted the attached license on 2020-12-01 at 18:31.The student, Varun Mohan, submitted this Dissertation for approval on 2020-12-01 at 18:50.This Dissertation was approved for publication on 2020-12-04 at 10:01.DSpace SAF Submission Ingestion Package generated from Vireo submission #16022 on 2021-03-04 at 16:33:11Made available in DSpace on 2021-03-05T21:47:26Z (GMT). No. of bitstreams: 8
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Polymer-mediated synthesis of γ-Fe2O3 nano-particles
No full textNano-particles of γ-Fe2O3 were synthesized by reacting polyethylene oxide–FeCl3 complex with NH4OH. These were characterized by X-ray diffraction (XRD), scanning electron miscroscopy (SEM), selected area electron diffraction (SAED) and transmision electron microscopy (TEM). The average particle size was found to be 10 nm, as determined from the line broadening of the main XRD peak. The crystalline phase was a spinel-type tetragonal structure, which was confirmed from the electron diffraction pattern. The zero field cooled magnetization of samples with varying γ-Fe2O3 content as a function of temperature was measured using a vibrating sample magnetometer. The magnetization curves show a peak at low temperature (15 K) corresponding to the blocking temperature TB. The value of TB was found to decrease with decreasing particle size. The magnetization measurements with respect to field at 5 and 170 K confirmed the transition from superparamagnetic to spin-glass state at TB, as evidenced from the remanence and hysteresis. These results can be explained on the basis of Néel's theory of superparamagnetism as applied to nano-particles
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