R. Ramprasad

University of Connecticut, Storrs, Connecticut, United States

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Publications (125)301.62 Total impact

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    Vinit Sharma · A. McDannald · M. Staruch · R. Ramprasad · M. Jain
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    ABSTRACT: Structural and magnetic properties of the doped terbium manganites (Tb,A)MnO3 (A = Gd, Dy and Ho) have been investigated using first-principles calculations and further confirmed by subse- quent experimental studies. Both computational and experimental studies suggest that compared to the parent material, namely, TbMnO3 (with a magnetic moment of 9.7 /muB for Tb3+) Dy- and Ho- ion substituted TbMnO3 results in an increase in the magnetic moment (< 10.6/muB for Dy3+ and Ho3+). The observed spiral-spin AFM order in TbMnO3 is stable with respect to the dopant substitutions, which modify the Mn-O-Mn bond angles and lead to stronger the ferromagnetic component of the magnetic moment. Given the fact that magnetic ordering in TbMnO3 causes the ferroelectricity, this is an important step in the field of the magnetically driven ferroelectricity in the class of magnetoelectric multiferroics, which traditionally have low magnetic moments due to the predominantly antiferromagnetic order. In addition, the present study reveals important insights on the phenomenological coupling mechanism in detail, which is essential in order to design new materials with enhanced magneto-electric effects at higher temperatures.
    Applied Physics Letters 05/2015; 107(1). DOI:10.1063/1.4926369 · 3.52 Impact Factor
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    V. Sharma · M. K. Mahapatra · P. Singh · R. Ramprasad
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    ABSTRACT: The surface cation chemistry in (La, A)MnO3 (A = Ca, Sr and Ba) is investigated using first-principles thermodynamics. We find that, all three dopants tend to segregate to the surface over a wide range of T– \( p_{{{\text{O}}_{2} }} \) conditions and the tendency for segregation increases with the increase in the dopant cationic size. Moving toward the low oxygen pressure, dopants prefer to remain in the surface regions accompanied by the appropriate number of charge compensating oxygen vacancies. The situation when dopants remain in the bulk regions tends to occur close to the thermodynamic conditions that also favor the decomposition of LaMnO3. The present work serves as an important step toward understanding of factors governing the cationic surface segregation in doped LaMnO3 and opens a pathway to study other important chemical environments (such as water- and CO2-containing air) which are crucially given the fact that the ‘real-world’ air enhances cationic segregation.
    Journal of Materials Science 04/2015; 50(8). DOI:10.1007/s10853-015-8861-z · 2.37 Impact Factor
  • Ying Sun · Steven Boggs · Ramamurthy Ramprasad
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    ABSTRACT: The introduction of dipoles into a polymer can enhance dipole-induced scattering which tends to "cool" hot electrons and thereby inhibits runaway of hot electrons. A theoretical analysis of electron scattering by dipoles and phonons is presented which provides the basis for explaining temperature dependence of breakdown field on the basis of the dominant scattering process as a function of temperature. By performing electron mobility calculation in non-polar and polar polymers, a quantitative correlation between chemical composition and intrinsic breakdown strength can be established. Calculation of dipole scattering limited electron mobility can be used to assess the effect of dipole scattering on the intrinsic breakdown field of polymers.
    IEEE Transactions on Dielectrics and Electrical Insulation 02/2015; 22(1):495-502. DOI:10.1109/TDEI.2014.004690 · 1.23 Impact Factor
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    A. Mannodi-Kanakkithodi · C. C. Wang · R. Ramprasad
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    ABSTRACT: Being in the group with the most diverse set of properties among all in the periodic table, the Group 14 elements (C, Si, Ge, Sn, and Pb) are particularly interesting candidates for structure–property investigation. Motivated by the need to create new insulators for energy storage and electronics applications, we study a few compounds based on Group 14 elements in this work, namely the dihydrides, dichlorides, and difluorides. Using density functional theory (DFT) calculations, we establish patterns in their properties, including favored coordination chemistry, stability, electronic structure, and dielectric behavior. While a coordination number (CN) of 4 is commonly associated with Group 14 elements, there is a significant deviation from it down the group, with CNs as high as 7 and 8 common in Pb. Further, there is an increase in the relative stability of the +2 oxidation state as opposed to +4 when we go from C to Pb, a direct consequence of which is the existence of the di-compounds of C and Si as polymers, whereas the compounds of Ge, Sn, and Pb are strictly 3D crystalline solids. The coordination chemistries are further linked with the band gaps and dielectric constants (divided into two components: the electronic part and the ionic part) of these compounds. We also see that the more stable difluorides and dichlorides have large band gaps and small electronic dielectric constants, and most of the Ge and Sn compounds have remarkably large ionic dielectric constants by virtue of having polar and more flexible bonds. The staggering variation in properties displayed by these parent compounds offers opportunities for designing derivative materials with a desired combination of properties.
    Journal of Materials Science 01/2015; 50(2). DOI:10.1007/s10853-014-8640-2 · 2.37 Impact Factor
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    Aaron F Baldwin · Rui Ma · Tran Doan Huan · Yang Cao · Ramamurthy Ramprasad · Gregory A Sotzing
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    ABSTRACT: High-dielectric constant materials are critical for numerous applications such as photovoltaics, photonics, transistors, and capacitors. There are numerous polymers used as dielectric layers in these applications but can suffer from having a low dielectric constant, small band gap, or ferroelectricity. Here, the structure-property relationship of various poly(dimethyltin esters) is described that look to enhance the dipolar and atomic polarization component of the dielectric constant. These polymers are also modeled using first principles calculations based on density functional theory (DFT) to predict such values as the total, electronic, and ionic dielectric constant as well as structure. A strong correlation is achieved between the theoretical and experimental values with the polymers exhibiting dielectric constants >4.5 with dissipation on the order of 10(-3) -10(-2) .
    Macromolecular Rapid Communications 11/2014; 35(24). DOI:10.1002/marc.201400507 · 4.61 Impact Factor
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    ABSTRACT: We demonstrate that weak chemical interactions between untethered moieties in molecular nanolayers on metal surfaces can strongly influence the effective work function Φeff. Electron spectroscopy shows that nanolayers of mercaptan-anchored organophosphonates on Au and Pt decrease Φeff. The measured Φeff shifts correlate with the chemical state of phosphonic acid moieties, and scale with molecular length. These results are contrary to predictions of ab initio calculations of monolayer-capped surfaces, but are consistent with calculations of bilayer-capped surfaces with face-to-face hydrogen-bonded phosphonic acid moieties. Our findings indicate that intra-layer bonding and layering in molecular nanolayers can be key to tailoring heterointerfacial electronic properties for applications.
    Applied Physics Letters 08/2014; 105(8):081601-081601-5. DOI:10.1063/1.4890486 · 3.52 Impact Factor
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    S.K. Yadav · R. Ramprasad · A. Misra · X.-Y. Liu
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    ABSTRACT: A first-principles computational scheme was applied for studying edge and screw dislocations in non-elemental systems for the first time. For the case of TiN as a model system, we established the preferred slip systems for edge and screw dislocations, with a Burgers vector of a /2〈1 1¯ 0〉 on the {0 0 1}, {1 1 0} and {1 1 1} slip planes. The simulations adopted periodically repeating triclinic supercells containing a dipole of dislocations arranged such that periodicity can be maintained without imposition of large spurious elastic stresses. It was determined that the Peierls stress is the smallest for slip along the {1 1 0} plane, and largest for slip along the {0 0 1} plane, for both edge and screw dislocations. The dislocation core structures and the Peierls stress results are discussed and compared to those in a purely ionic MgO system.
    Acta Materialia 08/2014; 74:268–277. DOI:10.1016/j.actamat.2014.04.047 · 4.47 Impact Factor
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    M. Staruch · V. Sharma · C. dela Cruz · R. Ramprasad · M. Jain
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    ABSTRACT: The structure and magnetic ordering of bulk TbMn0.5Cr0.5O3 was revealed through bulk magnetization and neutron diffraction measurements, and first-principles calculations, respectively. G-type antiferromagnetic ordering of Mn3+ and Cr3+ moments was observed in the neutron diffraction data below Néel temperature TN ∼ 84 K. In addition, below ∼40 K, short-range magnetic ordering was identified correlating to a ferromagnetic component due to the canting of the moments along the c-axis. The spin configuration is consistent with the first-principles calculations. The magnetic structure revealed in the present TbMn0.5Cr0.5O3 sample is distinct from that observed for both end members TbMnO3 and TbCrO3.
    Journal of Applied Physics 07/2014; 116(3):033919-033919-5. DOI:10.1063/1.4890637 · 2.19 Impact Factor
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    ABSTRACT: A series of large scale Mx Co3-x O4 (M=Co, Ni, Zn) nanoarray catalysts have been cost-effectively integrated onto large commercial cordierite monolithic substrates to greatly enhance the catalyst utilization efficiency. The monolithically integrated spinel nanoarrays exhibit tunable catalytic performance (as revealed by spectroscopy characterization and parallel first-principles calculations) toward low-temperature CO and CH4 oxidation by selective cation occupancy and concentration, which lead to controlled adsorption-desorption behavior and surface defect population. This provides a feasible approach for scalable fabrication and rational manipulation of metal oxide nanoarray catalysts applicable at low temperatures for various catalytic reactions.
    Angewandte Chemie International Edition in English 07/2014; 53(28). DOI:10.1002/anie.201403461 · 13.45 Impact Factor
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    S K Yadav · R Ramprasad · J Wang · A Misra · X-Y Liu
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    ABSTRACT: Using density functional theory based first-principles calculations, we show that the preferred interfacial plane orientation relationship is determined by the strength of bonding at the interface. The thermodynamic stability, and the ideal tensile and shear strengths of Cu(1 1 1)/TiN(1 1 1) and Al(1 1 1)/TiN(1 1 1) interfaces are calculated. While there is a strong orientation relation preference for the Al/TiN interface, there is no orientation relation preference for the Cu/TiN interface. Both the ideal tensile and shear strengths of Cu/TiN interfaces are lower than those of bulk Cu and TiN, suggesting such interfaces are weaker than their bulk components. By comparison, the ideal strengths of the Al/TiN interface are comparable to the constituents in the bulk form. Such contrasting interfaces can be a test-bed for studying the role of interfaces in determining the mechanical behavior of the nanolayered structures.
    Modelling and Simulation in Materials Science and Engineering 03/2014; 22(3):035020. DOI:10.1088/0965-0393/22/3/035020 · 1.49 Impact Factor
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    ABSTRACT: This work examines the complex nature of SOx (x = 0–4) interaction and oxidation on Pt(111) and Pd(111) surfaces using density functional theory (DFT) calculations coupled with microkinetic modeling. Thermodynamic and kinetic analyses suggest similar adsorption and oxidation behaviors for SOx species on both metal surfaces, although the observed greater tendency of Pd to undergo sulfating is borne out by the present results. Selected quantities computed using DFT, when used in a previously developed microkinetic model, are shown to predict SO2 conversion as a function of temperature in excellent agreement with available experimental data.
    The Journal of Physical Chemistry C 03/2014; 118(13). DOI:10.1021/jp501538v · 4.77 Impact Factor
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    C. C. Wang · G. Pilania · S.A. Boggs · S. Kumar · C. Breneman · R. Ramprasad
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    ABSTRACT: The present contribution provides a perspective on the degree to which modern computational methods can be harnessed to guide the design of polymeric dielectrics. A variety of methods, including quantum mechanical ab initio methods, classical force-field based molecular dynamics simulations, and data-driven paradigms, such as quantitative structure-property relationship and machine learning schemes, are discussed. Strategies to explore, search and screen chemical and configurational spaces extensively are also proposed. Some examples of computation-guided synthesis and understanding of real polymer dielectrics are also provided, highlighting the anticipated increasing role of such computational methods in the future design of polymer dielectrics.
    Polymer 02/2014; 55(4):979–988. DOI:10.1016/j.polymer.2013.12.069 · 3.77 Impact Factor
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    ABSTRACT: We use simulations and experiments to delineate the mechanism by which the addition of a small number of polar −OH groups to a nonpolar polymer increases the static relative permittivity (or dielectric constant) by a factor of 2, but more importantly while keeping the dielectric loss in the frequency regime of interest to power electronics to less than 1%. Dielectric properties obtained from experiments on functionalized polyethylenes and polypropylenes as a function of −OH doping are in quantitative agreement with one another. Molecular dynamics simulations for the static relative permittivity of “dry” −OH functionalized polyethylene (in the absence of water) are apparently in quantitative agreement with experiments. However, these simulation results would further imply that there should be considerable dielectric loss beyond simulation time scales (>0.1 μs). Since there are minimal experimentally observed dielectric losses for times as short as a microsecond, we believe that a small amount of adsorbed water plays a critical role in this attenuated loss. We use simulations to derive the water concentration at saturation, and our results for this quantity are also in good agreement with experiments. Simulations of the static relative permittivity of PE–OH incorporating this quantity of hydration water are found to be in quantitative agreement with experiments when it is assumed that all the dipolar relaxations occur at time scales faster than 0.1 μs. These results suggest that improved polymeric dielectric materials can be designed by including −OH groups on the chain, but the mechanism requires the presence of a stoichiometric quantity of hydration water.
    Macromolecules 01/2014; 47(3). DOI:10.1021/ma402220j · 5.93 Impact Factor
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    ABSTRACT: Background Cationic surface segregation under oxygen environment (O 2 and water vapor reservoirs) Model Details Computational Methods • Density Functional Theory (DFT) as implemented in VASP was used. • The fully relaxed bulk structure of LaMnO 3 is used to construct the (La)O-terminated surfaces (001). • The slab model was ~16 Å thick and contains 5 atomic symmetric layers, in which the bottom three atomic layers were always fixed at their respective bulk positions in the cubic perovskite phase. • Using a 4×4×1 Monkhorst-Pack k-point mesh, all other substrate atoms were allowed to fully relax until the absolute value of all corresponding forces dropped below 0.05 eV/Å. • Only doped LaO-terminated (001) surfaces are considered as they are more stable as compare to MnO 2-terminated (001) surface. • (La,M)MnO 3 and (La,M)CoO 3 are commonly found in several catalytic and electrochemical devices, for example, as cathodes in solid oxide fuel cells (SOFCs). 1 • Under the prevailing oxygen-rich and moisture-rich conditions that such oxides encounter in typical SOFCs, preferential segregation of the M atoms to the surfaces is commonly observed. • This surface segregation is detrimental to the overall performance as it leads to reduced oxygen reduction activity and cathode stability. • The surface cation chemistry in (La,A)MnO 3 (A = Ca, Sr and Ba) has been studied using first-principles thermodynamics. 2 • Over a range of temperature-pressure conditions, the effect of different chemical environments (presence of O 2 and water vapor reservoirs) is explored. 2,3 (a) Dopant selection • The stability and the site preference are mainly dominated by the mismatch of the ionic size and oxidation state of the dopant with respect to the host atom. (b) Surface in contact with O 2 reservoir • Surface remains significantly enriched with dopants under all realistic conditions. • Over a wide range of T-pO 2 conditions the cation surface segregation is favored. • Under low O 2 pressures and/or high T that the segregation behavior is suppressed. • With increasing dopant size the tendency for cation surface segregation increases. (c) Future directions • Preliminary results indicates that in the presence of moisture, the surface is predicted to be covered with dissociated water species. • Effect of water on cation segregation and role of oxygen vacancies is under investigation. • Understanding of the other factors/chemical environments that lead to segregation and consequent degradation (e.g., in the presence of CO 2).
    Solid State Energy Conversion Alliance, Pittsburgh; 01/2014
  • Clive R Bealing · R Ramprasad
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    ABSTRACT: A microscopic mechanism governing the initiating step in the high-field aging of crystalline polyethylene is proposed, based on density functional calculations and ab initio molecular dynamics simulations. It is assumed that electrons, holes, and excitons are present in the system. While the additional individual electrons or holes are not expected to lead to significant degradation, the presence of triplet excitons are concluded to be rather damaging. The electron and hole states of the exciton localize on a distorted region of polyethylene, significantly weakening nearby C-H bonds and facilitating C-H bond scission. The barrier to cleavage of the weakened C-H bonds is estimated and is comparable to the thermal energy, suggesting that this mechanism may be responsible for the degradation of polyethylene when placed under electrical stress, e.g., in high-voltage cables.
    The Journal of Chemical Physics 11/2013; 139(17):174904. DOI:10.1063/1.4824386 · 3.12 Impact Factor
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    S. K. Yadav · X.-Y. Liu · J. Wang · R. Ramprasad · A. Misra · R. G. Hoagland
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    ABSTRACT: In this paper, the generalized stacking fault (GSF) energies in different slip planes of TiN and MgO are calculated using highly reliable first-principles density functional theory (DFT) calculations. During DFT calculations, the issue of different ways to calculate the GSF energetics in ceramic materials containing more than one element was addressed and applied. For < 110 >/{111} slip, a splitting of saddle point in TiN was observed. For < 112 >/{111} slip, a stable stacking fault at a(0)/3 < 112 > displacement was formed in TiN. For synchroshear mechanism where the slip was accompanied by a cooperative motion of the interfacial nitrogen atoms within the slip plane, a second stable stacking fault was formed at a(0)/6 < 112 > displacement. The energy barrier for the shuffling of nitrogen atoms from one state to another is calculated to be 0.70eV per atom. In contrast, such features are absent in MgO. These differences highlight the influence of complex bonding nature (mixed covalent, ionic, and metallic bondings) of TiN, which is substantially different than that in MgO (simple ionic bonding) on GSF shapes.
    Philosophical Magazine 11/2013; 94(5-5):464-475. DOI:10.1080/14786435.2013.856525 · 1.43 Impact Factor
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    ABSTRACT: A parameter-free, quantitative, first-principles methodology to determine the environment-dependent interfacial strength of metal-metal oxide interfaces is presented. This approach uses the notion of the weakest link to identify the most likely cleavage plane, and first principles thermodynamics to calculate the average work of separation as a function of the environment (in this case, temperature and oxygen pressure). The method is applied to the case of the Pt-HfO2 interface, and it is shown that the computed environment-dependent work of separation is in quantitative agreement with available experimental data.
    Journal of Applied Physics 10/2013; 114(16):163503-163503-4. DOI:10.1063/1.4826528 · 2.19 Impact Factor
  • L. Autry · R. Ramprasad
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    ABSTRACT: Using density functional theory, the mechanism of surface segregation of N to the [001] Fe surface was studied. The formation and migration energies were decomposed into chemical bonding and strain energy components. While the segregation energy was determined to be -1.1 eV, the bonding and strain energy components for segregation were -0.5 and -0.6 eV, respectively. The results indicate that strain energy relaxation plays a major role in surface segregation, and that there is approximately a 7-layer transient region, which separates bulk and surface environments. The role of the strain energy on interstitial migration barriers was also critically evaluated.
    Journal of Materials Science 10/2013; 48(19):6542-6548. DOI:10.1007/s10853-013-7450-2 · 2.37 Impact Factor
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    ABSTRACT: The materials discovery process can be significantly expedited and simplified if we can learn effectively from available knowledge and data. In the present contribution, we show that efficient and accurate prediction of a diverse set of properties of material systems is possible by employing machine (or statistical) learning methods trained on quantum mechanical computations in combination with the notions of chemical similarity. Using a family of one-dimensional chain systems, we present a general formalism that allows us to discover decision rules that establish a mapping between easily accessible attributes of a system and its properties. It is shown that fingerprints based on either chemo-structural (compositional and configurational information) or the electronic charge density distribution can be used to make ultra-fast, yet accurate, property predictions. Harnessing such learning paradigms extends recent efforts to systematically explore and mine vast chemical spaces, and can significantly accelerate the discovery of new application-specific materials.
    Scientific Reports 09/2013; 3:2810. DOI:10.1038/srep02810 · 5.58 Impact Factor

Publication Stats

903 Citations
301.62 Total Impact Points

Institutions

  • 2005–2015
    • University of Connecticut
      • • Institute of Materials Science
      • • Department of Chemistry
      • • Department of Chemical and Biomolecular Engineering
      • • Department of Materials Science and Engineering
      Storrs, Connecticut, United States
  • 2008–2010
    • University of Toronto
      • The Edward S. Rogers Sr. Department of Electrical and Computer Engineering
      Toronto, Ontario, Canada