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Nair,
Sankar
Nair,
Sankar
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ItemEffects of nonframework metal cations and phonon scattering mechanisms on the thermal transport properties of polycrystalline zeolite LTA films(Georgia Institute of Technology, 2010-03) Greenstein, Abraham ; Hudiono, Yeny ; Graham, Samuel ; Nair, SankarWe present a systematic study to investigate the effects of nonframework cations and the role of phonon scattering mechanisms on the thermal transport properties of zeolite LTA, via experiment and semiempirical lattice dynamics calculations. Our study is motivated by the increasing interest in accurate measurements and mechanistic understanding of the thermal transport properties of zeolite materials. The presence of a nanostructured pore network, extra-framework cations, and tunable framework structure and composition confer interesting thermophysical properties to these materials, making them a good model system to investigate thermal transport in complex materials. Continuous films of zeolite LTA with different nonframework cations (Na⁺, K⁺, and Ca⁺²) were synthesized and characterized. The thermal conductivity was measured using the three-omega method over a wide range of temperature (150–450 K). These are the first thermal conductivity measurements performed on bulk LTA, so they are more accurate than previous measurements, which involved the use of compacted zeolite powders. Our data showed significant dependence of the thermal conductivity on the extra-framework cations as well the temperature. The thermal conductivities of the zeolite LTA samples were modeled with the relaxation time approximation to the Boltzmann transport equation. The full phonon spectra for each type of LTA zeolite were calculated and used in conjunction with semiempirical relaxation time expressions to calculate the thermal conductivity. The results both validated, and suggested the limitations of, this modeling approach. Optical phonons dominated the thermal conductivity and boundarylike scattering was found to be the strongest phonon scattering mechanism, as also observed in MFI zeolite.
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ItemNanoscopic Metal Oxide Objects via Controlled Creation and Rearrangement of Amorphous Nanoparticles(Georgia Institute of Technology, 2007-12-11) Nair, SankarMetal oxide materials offer a vast range of tunable physical and chemical properties, accessible by low-temperature liquid-phase chemistry. In the form of nanoscopic objects, these materials have a variety of potential applications in future nanoscale devices and nanostructured assemblies. Our research over the last 3 years at Georgia Tech has addressed several fundamental questions pertaining to the synthesis of metal oxide nano-objects with complex structure and controllable dimensions. The ultimate objective is to develop a generalizable set of "design rules" to engineer the "shape and size", "curvature", chemical composition, and ordered internal structure of ultra-small nanoscopic objects. In this talk, he described insights into the kinetic and thermodynamic principles underlying the formation of complex nano-objects such as single-walled metal oxide nanotubes whose lengths can be tuned from ultra-short (20 nm) to about 100 nm and whose diameters can be tuned with Angstrom-level precision through manipulation of interatomic potential energies. A proposal is made for a mechanistic paradigm that we refer to as "amorphous nanoparticle condensation followed by internal rearrangement", and some of its broader technological implications were discussed.
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ItemEffects of composition and phonon scattering mechanisms on thermal transport in MFI zeolite films(Georgia Institute of Technology, 2007-09) Hudiono, Yeny ; Greenstein, Abraham ; Saha-Kuete, Carine ; Olson, Brandon ; Graham, Samuel ; Nair, SankarWe report a systematic study that reveals the effect of composition (silicon-to-aluminum ratio) and the role of different phonon scattering processes on thermal transport in the nanoporous zeolite MFI. This is accomplished via synthesis of a series of films with graded compositions, thermal property measurements, and lattice dynamical modeling in the framework of the Boltzmann equation. MFI films with different Si/Al ratios (from infinity to 26) and constant (h0l) out-of-plane orientation were successfully synthesized by a seeded hydrothermal process. Three-omega measurements on these films allowed us to obtain comprehensive information on the thermal conductivity of MFI as a function of temperature (150-450 K) and Si/Al ratio. Detailed atomistic simulations (energy minimization and phonon dispersion calculations) were carried out for the MFI crystal structure with different Si/Al ratios and incorporated into a Boltzmann transport model along with approximate theoretical expressions for describing the rate of phonon scattering through umklapp, defect, and boundary scattering processes. The model predicts the observed thermal conductivity behavior very well across the entire range of temperature and composition investigated, with only a small number of fitting parameters of physical significance which allow us to distinguish the contributions of the different phonon scattering mechanisms. In particular, our results strongly suggest that the upper limit of thermal conductivity is defined by boundary-like scattering associated with the pore structure of the material. Below this limit, silicon substitution with aluminum allows considerable suppression of thermal conductivity by point defect scattering and a decrease in phonon velocity. These findings are important from the point of view of developing a robust platform for understanding thermal transport in complex crystalline materials with nanostructural features (such as an ordered nanopore network), which in turn serve as model systems for tuning of phonon transport properties in complex materials.