Douglas S. Dudis

Molecular dynamics of rigid rod polymers

Abstract Molecular dynamics (MD) calculations have been used to study the behaviour of isolated rigid rod molecules of poly( p -phenylene), poly( p -phenylene benzobisthiazole) and poly( p -phenylene benzobisoxazole). The molecular mechanics force field was initially modified to improve agreement between minimized structural geometries and available X-ray data, as well as results from semiempirical molecular orbital calculations. The MD simulations show the molecules to be surprisingly flexible, with changes in end-to-end distances as large as 16%. An examination of the energies (calculated by various methods) associated with out-of-plane bending deformation, suggests that the rigid rod polymers may in fact be even more flexible than the simulations indicate. The results provide rationalizations for the relatively short persistence lengths measured in solution and for bending observed in high-resolution electron micrographs of these materials.

High Seebeck effects from hybrid metal/polymer/metal thin-film devices.

Thermoelectrics is defi ned as the phenomenon in which temperature and electrical potential differences can be mutually converted through a material medium. [ 1 , 2 ] When temperature difference is used to generate an electrical potential difference, this thermoelectric phenomenon is called the Seebeck effect. The Seebeck coeffi cient S is a critical parameter to determine the thermoelectric effi ciency given by dimensionless fi gure of merit (ZT), as shown in Equation (1) ,

Vibronic contribution to static molecular hyperpolarizabilities

A simplified semiempirical scheme (AM1) is developed as a handy tool to calculate vibrational corrections to hyperpolarizabilities of moderately large planar conjugated systems. In formulating the scheme Born-Oppenheimer (BO) approximation is assumed and only those normal modes are considered which correspond to vibration along the direction of the field. Ab initio calculations are performed to calibrate and justify the various steps involved in the scheme. Using this approach vibronic corrections to the third-order polarizabilities γ are computed for a number of systems including the polyenes C 2 n H 2 n +2 , n = 3, 4 and 5. The corrections are large and may explain some of the discrepancies that are found to exist between experiment and theory.

Measurement of Thermal Conductivity of PbTe Nanocrystal Coated Glass Fibers by the 3ω Method

Fiber-based thermoelectric materials can conform to curved surfaces to form energy harvesting devices for waste heat recovery. Here we investigate the thermal conductivity in the axial direction of glass fibers coated with lead telluride (PbTe) nanocrystals using the self-heated 3ω method particularly at low frequency. While prior 3ω measurements on wire-like structures have only been demonstrated for high thermal conductivity materials, the present work demonstrates the suitability of the 3ω method for PbTe nanocrystal coated glass fibers where the low thermal conductivity and high aspect ratio result in a significant thermal radiation effect. We simulate the experiment using a finite-difference method that corrects the thermal radiation effect and extract the thermal conductivity of glass fibers coated by PbTe nanocrystals. The simulation method for radiation correction is shown to be generally much more accurate than analytical methods. We explore the effect of nanocrystal volume fraction on thermal conductivity and obtain results in the range of 0.50-0.93 W/mK near room temperature.

Modeling, synthesis, and characterization of third-order nonlinear optical salts

A number of dithioacetate and dithiolate mono- and dianions have been synthesized and characterized through Z-scan measurements, with some showing significant third order nonlinear optical (NLO) behavior. Tetralkylphosphonium cations were utilized in tandem with the nonlinear anions so as to minimize electrostatic interactions within the salt, consequently resulting in the materials being room temperature ionic liquids (RTILs), which have numerous advantages over typical organic-based materials. Anions composed of metal-ligand systems were also tested for NLO behavior as components of novel ionic liquid materials. These RTILs introduce a new class of materials with potential applications in optical limiting and other all-optical devices.

A theoretical model for excited state absorption

For long incident pulses, one-photon absorption in the excited state formed following a two-photon absorption can play a very significant role in some organic materials. We develop here a quantitative model to determine the contribution of excited state absorption in the interpretation of the two-photon absorption data for such materials. We present representative calculations of this effect for some selected systems.

Barrier to phenyl rotation in poly(p-phenylene benzobisthiazole) from ab initio molecular orbital calculations

Abstract Optimal molecular geometry and the barrier to internal phenyl rotation were obtained from ab initio molecular orbital calculations of model compounds of poly( p -phenylene benzobisthiazole). Bond lengths and angles are in good agreement with those found from a previous X-ray crystallographic study of a similar model compound. Based on the calculated torsional potential, the discrepancy between theoretical and experimental phenyl torsions is attributed to crystal packing forces. The calculated torsion barrier is three to five times greater than in previous semi-empirical AM1 molecular orbital calculations. Calculations were performed with the 6 – 31 G ∗ basis set and the rotation barrier was corrected for electron correlation employing second-order Moller-Plesset perturbation theory.

Expendable High Energy Density Thermal Management Material: Ammonium Carbamate

I NCREASING power loads for various electronic devices have created a demand for novel thermal management (TM) technologies that allow these devices to operate in their ideal temperature ranges in order to ensure device efficiency and lifetime. Most electronic devices need to operate between 20 and 100 C. Cooling these devices with air or conventional liquid coolants can be energy intensive, require a large TM system, or even be impossible when dealing with high thermal fluxes [1]. Traditionally, cooling in these devices is accomplished using the sensible heat associated with a temperature change of a fluid such as air or water. Air cooling systems are the simplest but cannot handle high thermal loads. The higher specific heat of water allows it to handle greater thermal loads than air. Nevertheless, the specific heat of water is low relative to that of its phase changes, which cannot be used for the temperature range of interest. Since latent heats accompanying phase changes are fundamentally higher than sensible heats on a mass basis, phase change materials (PCMs) have garnered intense interest for TM. Additionally, as PCMs offer the ability to maintain a constant temperature, they are being explored for the temperature range of interest. One class of PCMs being explored for this temperature range is graphitic or metallic foams impregnated with paraffin wax [1–6]. Waxes are advantageous since their melting temperatures can be tuned between 5 and 95 C, but they suffer from low specific energy densities on the order of 200 kJ=kg for the purematerial. The systemlevel properties (specific energy density and thermal power) are much lower since the low thermal conductivity of waxes, which is around 0:2 W=m K, necessitates system-level architectures that enhance the heat transfer rate in order to achieve a practical thermal power rating [7]. Much of the work in using waxes as PCMs has concentrated on incorporating them into thermally conductive carbon or metal matrices in order to improve the thermal conductivity. This has shown success but lowers the system-specific properties to less than half those of the neat waxes [1,2,4,8]. TM systems based on chemical reactions, rather than sensible or latent heats, are another class of materials being explored. In principle, chemical reaction systems can yield the highest gravimetric TM capacities since the energies associated with changes in chemical bonding are intrinsically greater than those of physical phase changes. One class of chemical reaction systems involves a solid that exothermically reacts with a gas to form a complex. The complex can then be disassociated in an endothermic process, which can serve as a heat sink.Ammoniates are representative of this type of system. They work by using ammonia gas and a salt (normally period 4, 5, and 6 metal chlorides) to form complexes with ammonia [9–11]. The problem with these systems is that they require high pressures and large amounts of the salts, which lead to low specific properties and makes them unsuitable for applications in which specific energy and specific thermal power are key drivers at the system level. Metal hydride-based chemical reaction systems function by using hydrogen gas that exothermically reacts with metals to form a metalhydrogen complex from which the hydrogen can then be endothermically released. Metal hydrides can have very high gravimetric thermal energy densities, and there are many possible metal-hydrogen storage material systems that have been reported elsewhere [12,13]. Much of the research into metal hydrides is being conducted for the purpose of hydrogen storage in fuel cell vehicles. This application requiresminimization of the thermal load associated with the sorption process. However, metal hydride systems with a high thermal load may be good candidates for TM. The Mg=MgH2 system is well characterized with a thermal storage density of 1850 kJ=kg, while LiAlH4 has been reported to have the extremely Presented as Paper 2011-3950 at the 42nd AIAA Thermophysics Conference, Honolulu, HI, 27–30 June 2011; received 21 June 2011; revision received 11 August 2011; accepted for publication 5 September 2011. This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States. Copies of this paper may be made for personal or internal use, on condition that the copier pay the

An AM1 study of the two-photon absorption in bis(styryl)benzene derivatives

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Electronic and Structural Consequences of n-Doping: Bithiazole Oligomers and Partially Reduced Bithiazolium Cations

10.00 in correspondence with the CCC. Research Engineer, Materials and Manufacturing Directorate, Thermal Sciences and Materials Branch, 2977 Hobson Way. Principle Research Chemist, Materials and Manufacturing Directorate, Thermal Sciences and Materials Branch, 2977 Hobson Way (Corresponding Author). Professor of Chemistry, Department of Science and Mathematics, 251 N. Main Street. JOURNAL OF THERMOPHYSICS AND HEAT TRANSFER Vol. 26, No. 2, April–June 2012