Santanu Chaudhuri

Design of Anti-Icing Coatings Using Supercooled Droplets As Nano-to-Microscale Probes

A multiscale simulation-based approach is presented for predicting anti-icing properties of nanocomposite coatings. Development of robust anti-icing coatings is a challenging task. An anti-icing coating that can prevent in-flight icing is of particular interest to the aircraft industry. A multiscale simulations based approach is developed to provide insights into the complex effect of coating material and surface topology on the prevention of in-flight icing. Chemical properties of different coatings and kinetics of icing or inhibition of ice nucleation are calculated from nanoscale atomistic simulations. In addition, in-flight icing environments including impingement and rolling of supercooled microdroplet and nucleation of ice under wind shear have been implemented using fluid dynamics methodologies. A model for icing in nano-to-microscale for surfaces with known chemical composition and surface topology is used for developing predictive capabilities regarding anti-icing performance of potential coatings. In this work, fluorinated polyhedral oligomericsilsesquioxanes molecules have been used to increase nanoscale roughness when embedded in a polycarbonate polymeric matrix. The findings suggest that a successful anti-icing coating will require precise control over nanoscale and microscale roughness. The multiscale methodology presented therefore can potentially help in identifying coupled effects of material, surface topology, and icing environment for promising coatings before performing icing tunnel experiments.

Turning aluminium into a noble-metal-like catalyst for low-temperature activation of molecular hydrogen

Activation of molecular hydrogen is the first step in producing many important industrial chemicals that have so far required expensive noble-metal catalysts and thermal activation. We demonstrate here that aluminium doped with very small amounts of titanium can activate molecular hydrogen at temperatures as low as 90 K. Using an approach that uses CO as a probe molecule, we identify the atomistic arrangement of the catalytically active sites containing Ti on Al(111) surfaces, combining infrared reflection-absorption spectroscopy and first-principles modelling. CO molecules, selectively adsorbed on catalytically active sites, form a complex with activated hydrogen that is removed at remarkably low temperatures (115 K; possibly as a molecule). These results provide the first direct evidence that Ti-doped Al can carry out the essential first step of molecular hydrogen activation under nearly barrierless conditions, thereby challenging the monopoly of noble metals in hydrogen activation.

Ultrathin, transferred layers of thermally grown silicon dioxide as biofluid barriers for biointegrated flexible electronic systems

Significance A critical obstacle of flexible electronics for chronic implants is the absence of thin-film barriers to biofluids with multidecade lifetimes. Previously explored materials are unsuitable due to limitations of (i) extrinsic factors, such as the practical inability to avoid localized defects, and/or (ii) intrinsic properties, such as finite water permeability. The work presented here overcomes these challenges by combining pristine thermal SiO2 layers with processing steps for their integration onto flexible electronics. Experimental and theoretical studies reveal the key aspects of this material system. Accelerated immersion tests and cyclic bending measurements suggest robust, defect-free operation with various electronic components and an integrated system for multiplexed mapping of electrophysiological signals. The findings have broad relevance to diverse biointegrated electronics and optoelectronics. Materials that can serve as long-lived barriers to biofluids are essential to the development of any type of chronic electronic implant. Devices such as cardiac pacemakers and cochlear implants use bulk metal or ceramic packages as hermetic enclosures for the electronics. Emerging classes of flexible, biointegrated electronic systems demand similar levels of isolation from biofluids but with thin, compliant films that can simultaneously serve as biointerfaces for sensing and/or actuation while in contact with the soft, curved, and moving surfaces of target organs. This paper introduces a solution to this materials challenge that combines (i) ultrathin, pristine layers of silicon dioxide (SiO2) thermally grown on device-grade silicon wafers, and (ii) processing schemes that allow integration of these materials onto flexible electronic platforms. Accelerated lifetime tests suggest robust barrier characteristics on timescales that approach 70 y, in layers that are sufficiently thin (less than 1 μm) to avoid significant compromises in mechanical flexibility or in electrical interface fidelity. Detailed studies of temperature- and thickness-dependent electrical and physical properties reveal the key characteristics. Molecular simulations highlight essential aspects of the chemistry that governs interactions between the SiO2 and surrounding water. Examples of use with passive and active components in high-performance flexible electronic devices suggest broad utility in advanced chronic implants.

Theoretical study of elementary steps in the reactions between aluminum and teflon fragments under combustive environments.

Gas-phase reactions between aluminum particles and Teflon fragments were studied to develop a fundamental understanding of the decomposition reactions and combustion processes of the Al-Teflon composites. The reactions were investigated theoretically using ab initio calculations at the MP2/aug-cc-pVDZ level, with the final thermokinetic data obtained with coupled cluster theory (CCSD(T)/aug-cc-pVTZ). Among reactions under oxygen-lean conditions, CF(3) + Al --> CF(2) + AlF channel is the fastest, followed by the CF(2) + Al --> CF + AlF and CF + Al --> C + AlF channels. Under oxygen-rich conditions, reactions of COF with aluminum are probed to be faster than those involving COF(2) species. Reaction path multiplicity has been considered. Our results show that multiplicity plays a very important role in determining the reaction order, that is first order or addition-elimination reactions of Al with CF(3) are predicted to be faster than those proceeding through direct abstraction or second order. In addition, the present kinetic model suggests that CF(3) + Al --> CF(2) + AlF with m = 1 and COF + Al --> CO + AlF channels are very competitive under the same thermal conditions. The computed enthalpies of reaction are systematically compared with the available literature. The predicted kinetic model and its time constants (tau) are in good qualitative agreement with experimental observations of the reactions between Al nanoparticles and Teflon for the 500-1200 K temperature range.

Effect of A-site cation radius on ordering of BX6 octahedra in (K,Na)MgF3 perovskite

Abstract We present a structural model for (K,Na)MgF3 perovskite using results from high-resolution synchrotron X-ray powder diffraction and nuclear magnetic resonance (NMR) spectroscopy. (K,Na)MgF3 perovskite is found to transition from orthorhombic (Pbnm) to tetragonal (P4/mbm) to cubic (Pm3̅m) as potassium concentration is increased. These phase transitions are not accompanied by a discontinuity in pseudo-cubic unit-cell volume and occur close to compositions (K0.37Na0.63)MgF3 and (K0.47Na0.53)MgF3, respectively. 19F NMR spectra indicate that the Na+ and K+ cations do not occupy the A cation site at random and end-member local environments are favored for all compositions. Based on results from both X-ray diffraction and NMR, we propose that diffuse diffraction is the result of strain between coexisting regions of different octahedra (MgF6) tilts brought about by the ionic radius mismatch of Na+ and K+ cations. We suggest A-site cations group with like cations as neighbors to reduce excess volume and total strain.

Graphene-Assisted Solution Growth of Vertically Oriented Organic Semiconducting Single Crystals

Vertically oriented structures of single crystalline conductors and semiconductors are of great technological importance due to their directional charge carrier transport, high device density, and interesting optical properties. However, creating such architectures for organic electronic materials remains challenging. Here, we report a facile, controllable route for producing oriented vertical arrays of single crystalline conjugated molecules using graphene as the guiding substrate. The arrays exhibit uniform morphological and crystallographic orientations. Using an oligoaniline as an example, we demonstrate this method to be highly versatile in controlling the nucleation densities, crystal sizes, and orientations. Charge carriers are shown to travel most efficiently along the vertical interfacial stacking direction with a conductivity of 12.3 S/cm in individual crystals, the highest reported to date for an aniline oligomer. These crystal arrays can be readily patterned and their current harnessed collectively over large areas, illustrating the promise for both micro- and macroscopic device applications.

Formation and bonding of alane clusters on Al(111) surfaces studied by infrared absorption spectroscopy and theoretical modeling

Alanes are believed to be the mass transport intermediate in many hydrogen storage reactions and thus important for understanding rehydrogenation kinetics for alanates and AlH3. Combining density functional theory (DFT) and surface infrared (IR) spectroscopy, we provide atomistic details about the formation of alanes on the Al(111) surface, a model environment for the rehydrogenation reactions. At low coverage, DFT predicts a 2-fold bridge site adsorption for atomic hydrogen at 1150 cm(-1), which is too weak to be detected by IR but was previously observed in electron energy loss spectroscopy. At higher coverage, steps are the most favorable adsorption sites for atomic H adsorption, and it is likely that the AlH3 molecules form (initially strongly bound to steps) at saturation. With increasing exposures AlH3 is extracted from the step edge and becomes highly mobile on the terraces in a weakly bound state, accounting for step etching observed in previous STM studies. The mobility of these weakly bound AlH3 molecules is the key factor leading to the growth of larger alanes through AlH3 oligomerization. The subsequent decomposition and desorption of alanes is also investigated and compared to previous temperature programmed desorption studies.

Density Functional Theory Calculations of Pressure Effects on the Structure and Vibrations of 1,1-Diamino-2,2-dinitroethene (FOX-7)

Pressure effects on the Raman vibrations of an energetic crystal FOX-7 (1, 1-diamino-2, 2-dinitroethene) were examined using density functional theory (DFT) calculations. High accuracy calculations were performed with a periodic plane-wave DFT method using norm-conserving pseudopotentials. Different exchange-correlation functionals were examined for their applicability in describing the structural and vibrational experimental data. It is shown that the PBE functional with an empirical dispersion correction by Grimme, PBE-D method, reproduces best the molecular geometry, unit cell parameters, and vibrational frequencies. Assignments of intramolecular Raman active vibrations are provided. The calculated pressure dependence of Raman shifts for the intramolecular and lattice modes were found to be in good agreement with the experimental data; in particular, the calculations predicted correctly a decrease of frequencies for the NH2 stretching modes with pressure. Also, in accord with experiments, the calculations indicated some instances of modes mixing/coupling with increasing pressure. This work demonstrates that the dispersion-corrected PBE functional can account for the structural and vibrational properties of FOX-7 crystal at ambient and high pressures.

Local bonding and atomic environments in Ni-catalyzed complex hydrides.

The local bonding and atomic environments in the Ni-catalyzed destabilized system LiBH4/MgH2 and the quaternary borohydride-amide phase Li3BN2H8, were studied by x-ray absorption spectroscopy. In both cases the Ni catalyst was introduced as NiCl2 and a qualitative comparison of the Ni K-edge near-edge structure suggests the Ni2+ is reduced to primarily Ni0 after ball milling. The extended fine structure of the Ni K edge indicates that the Ni is coordinated by approximately 3 boron atoms with an interatomic distance of approximately 2.1 A and approximately 11 Ni atoms in a split shell at around 2.5 and 2.8 A. These results, and the lack of long-range order, suggest that the Ni is present as a disordered nanocluster with a local structure similar to that of Ni3B. In the fully hydrogenated phase of LiBH4/MgH2 a small amount Mg2NiHx was also present. Surface calculations performed using density functional theory suggest that the lowest kinetic barrier for H2 chemisorption occurs on the Ni3B(100) surface.

Finite size effects on aluminum/Teflon reaction channels under combustive environment: A Rice–Ramsperger–Kassel–Marcus and transition state theory study of fluorination

The effect of particle size on combustion efficiency is an important factor in combustion research. Gas-phase aluminum clusters in oxidizing environment constitute a relatively simple and extensively studied system. In an attempt to underscore the correlation between electronic structure, finite size effect, and reactivity in small aluminum clusters, reactions between aluminum, [Al(13)](-) cluster, and Teflon decomposition fragments were studied using theoretical calculations at the density functional theoretical level. The unimolecular rate constants calculated using transition state and Rice-Ramsperger-Kassel-Marcus theory show that reactions with COF and CF(2) species with aluminum are faster than those involving CF(3) and COF(2). The results show that the kinetic barriers along different exothermic reaction channels correlate with the trends in HOMO(R)-HOMO(TS) (HOMO denotes highest occupied molecular orbital) energy gap and related shifts of the HOMO levels of reactants. Overall reactions involving carbonyl fluoride species (COF and COF(2)) lead to CO elimination and fluorination of the Al cluster. The CF(3)/CF(2) fragments lead to stable multicenter Al-C bond formation on the fluorinated Al cluster surface. Temperature-, energy-, and pressure-dependent rate constants are provided for extrapolating the expected reaction kinetics to conditions similar to known combustion reactions.