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Open Access Publications from the University of California

Scholarly Works (9 results)

  • Article
  • Peer Reviewed
UCLA Previously Published Works (2012)
Surface passivation of nanoporous crystalline silicon can reduce its thermal conductivity. This was established using equilibrium molecular dynamics simulations. The porosity varied from 8% to 38% while the pore diameter ranged from 1.74 to 2.93 nm. Hydrogen and oxygen passivation reduced thermal conductivity by 11% to 17% and 37% to 51% depending on porosity at 500 K, respectively. The hydrogen passivation effect decreased with increasing temperature. Vibrational spectra of oxygen overlapped with those of silicon at low frequencies. Therefore, oxygen passivation enhanced phonon scattering at solid matrix boundaries, resulting in stronger thermal conductivity reduction than that caused by hydrogen passivation.
    Cover page: Tuning thermal conductivity of nanoporous crystalline silicon by surface passivation: A molecular dynamics study
    • Article
    • Peer Reviewed
    UCLA Previously Published Works (2011)
    This study establishes that the effective thermal conductivity k eff of crystalline nanoporous silicon is strongly affected not only by the porosity f ν and the system’s length L z but also by the pore interfacial area concentration A i . The thermal conductivity of crystalline nanoporous silicon was predicted using non-equilibrium molecular dynamics simulations. The Stillinger-Weber potential for silicon was used to simulate the interatomic interactions. Spherical pores organized in a simple cubic lattice were introduced in a crystalline silicon matrix by removing atoms within selected regions of the simulation cell. Effects of the (i) system length ranging from 13 to 130 nm, (ii) pore diameter varying between 1.74 and 5.86 nm, and (iii) porosity ranging from 8% to 38%, on thermal conductivity were investigated. A physics-based model was also developed by combining kinetic theory and the coherent potential approximation. The effective thermal conductivity was proportional to (1 − 1.5f ν ) and inversely proportional to the sum (A i /4 + 1/L z ). This model was in excellent agreement with the thermal conductivity of nanoporous silicon predicted by molecular dynamics simulations for spherical pores (present study) as well as for cylindrical pores and vacancy defects reported in the literature. These results will be useful in designing nanostructured materials with desired thermal conductivity by tuning their morphology.
      Cover page: Law for Thermal Conductivity of Crystalline Nanoporous Silicon Using Molecular Dynamic Simulations
      • Thesis
      • Peer Reviewed
      UCLA Electronic Theses and Dissertations (2012)

      The present study investigates the complex relationship between nanostructures and microscale thermal transport in nanoporous thin films for energy applications. It experimentally and numerically demonstrates that the effective thermal conductivity of nanoporous materials can be tuned by controlling their nanoscale architectures including porosity, pore diameter, wall thickness, nanocrystal size, and crystallinity as well as surface passivation.

      This study reports measurements of the cross-plane thermal conductivity of nanoporous thin films with various architectures between 25 and 315 K. Physics-based models combining phonon transport theory and effective medium approximations were developed to interpret the experimental data. Ordered mesoporous titania and silicon thin films were prepared based on evaporation-induced self-assembly method. Pure silica zeolite films were produced by either in-situ growth or by spin coating a zeolite nanoparticle suspension followed by crystal growth upon heating. These synthesized thin films were systematically and fully characterized. They featured ordered nanopores with porosity, pore diameter, and film thickness ranging from 30% to 59%, 0.5 to 25 nm, and 120 to 370 nm, respectively. Their dense matrix was amorphous, polycrystalline, or consisted of an aggregate of nanocrystals. The thermal conductivity of all synthesized nanoporous films increased monotonically with temperature within the temperature range considered. At low temperatures, the nanoporous films behaved like amorphous or strongly disordered materials and their thermal conductivity was proportional to Tn with n varied between 1 and 2.3. At high temperatures, the thermal conductivity increased slowly with temperature or reached a plateau due to strong phonon Umklapp scattering and the saturation of phonon modes. The presence of pores in amorphous mesoporous thin films had a purely geometrical effect by reducing the cross-sectional area through which heat can diffuse. By contrast, in crystalline mesoporous thin films the presence of pores also increased phonon scattering. In addition, the film thickness generally did not affect the measured thermal conductivity. Indeed, phonon scattering by pores and by nanocrystal grain boundary dominated over boundary scattering and were identified as the dominant scattering mechanisms for nanoscale energy transport in the synthesized nanoporous films.

      This study further establishes that the effective thermal conductivity keff of crystalline nanoporous silicon was strongly affected not only by the porosity fv and the system's length Lz but also by the pore interfacial area concentration Ai. A modified effective medium approximation combining kinetic theory and the coherent potential approximation suggested that keff was proportional to (1-1.5fv) and inversely proportional to the sum (Ai/4+1/Lz). This scaling law was in excellent agreement with the thermal conductivity of nanoporous silicon predicted by molecular dynamics simulations for spherical pores as well as for cylindrical pores and vacancy defects.

      Finally, this study demonstrated, using equilibrium molecular dynamics simulations, that surface passivation added another parameter for reducing the thermal conductivity of nanostructured materials. To do so, there should be strong acoustic vibrational modes coupling between surface and passivation atoms. For example, oxygen passivation reduced the thermal conductivity of nanoporous crystalline silicon. In addition, the effect of passivation reduced with temperature because of increasing contribution of Umklapp scattering.

      These results could help establish new strategies to control the thermal conductivity of nanoporous materials for a wide range of applications including thermoelectric devices, supercapacitors, dye-sensitized solar cells, and hydrogen storage devices.

        Cover page: Thermal Transport in Nanoporous Materials for Energy Applications
        • Article
        • Peer Reviewed
        UCLA Previously Published Works (2011)
        This study reports, for the first time, non-equilibrium molecular dynamics (MD) simulations predicting the thermal conductivity of amorphous nanoporous silica. The heat flux was imposed using the Müller-Plathe method and interatomic interactions were modeled using the widely used van Beest, Kramerand van Santen potential. Monodisperse spherical pores organized in a simple cubic lattice were introduced in an amorphous silica matrix by removing atoms within selected regions. The simulation cell length ranged from 17 to 189 Å, the pore diameter from 12 to 25 Å, and the porosity varied between 10% and 35%. Results establish that the thermal conductivity of nanoporous silica at room temperature was independent of pore size and depended only on porosity. This qualitatively confirms recent experimental measurements for cubic and hexagonal mesoporous silica films with pore diameter and porosityranging from 3 to 18 nm and 20% to 48%, respectively. Moreover, predictions of MD simulations agreed well with predictions from the coherent potential model. By contrast, finite element analysis simulating the same nanoporous structures, but based on continuum theory of heat conduction, agreed with the well-known Maxwell Garnett model.
          Cover page: Molecular Dynamic Study of Thermal Conductivity of Amorphous Nanoporous Silica
          • Article
          • Peer Reviewed
          UCLA Previously Published Works (2010)
          Pyroelectric energy conversion offers a way to convert waste heat directly into electricity. It makes use of the pyroelectric effect to create a flow of charge to or from the surface of a material as a result of heating or cooling. However, existing pyroelectric energy converter can only operate at low frequency due to relatively small convective heat transfer rate between the pyroelectric materials and the working fluid. On the other hand, energy transfer by thermal radiation between two semi-infinite solids is nearly instantaneous and can be enhanced by several orders of magnitude from the conventional Stefan-Boltzmann law as the gap separating them becomes smaller than the Wien's displacement wavelength. This paper explores a novel way to harvest waste heat by combining pyroelectric energy conversion and nanoscale thermal radiation. A new device was investigated numerically by accurately modeling nanoscale radiative heat transfer between a pyroelectric element and hot and cold plates. Silica absorbing layers on top of every surface were used to further increase the net radiative heat fluxes. Temperature oscillations with time and performances of the pyroelectric converter were predicted at various frequencies. The device using 60/40 P(VDF-TrFE) achieved 0.2% efficiency and 0.84 mW/cm2 electrical power output for the cold and hot sources at 273 and 388 K, respectively. Better performances could be achieved with 0.9PMN-PT namely efficiency of 1.3% and power output of 6.5 mW/cm2 between the cold and hot sources at 283 and 383 K, respectively. These results are compared with alternative technologies and suggestions are made to further improve the device.
            Cover page: Harvesting nanoscale thermal radiation using pyroelectric materials
            • Article
            • Peer Reviewed
            UCLA Previously Published Works (2012)
            This paper reports the cross-plane thermal conductivity of ordered mesoporous nanocrystalline silicon thin films between 25 and 315 K. The films were produced by evaporation induced self-assembly of mesoporous silica followed by magnesium reduction. The periodic ordering of pores in mesoporous silicon was characterized by X-ray diffraction and direct SEM imaging. The average crystallite size, porosity, and film thickness were about 13 nm, 25-35%, and 140-340 nm, respectively. The pores were arranged in a face-centered cubic lattice. The cross-plane thermal conductivity of the mesoporous silicon thin films was measured using the 3ω method. It was between 3 and 5 orders of magnitude smaller than that of bulk single crystal silicon in the temperature range considered. The effects of temperature, film thickness, and copolymer template on the thermal conductivity were investigated. A model based on kinetic theory was used to accurately predict the measured thermal conductivity for all temperatures. On the one hand, both the measured thermal conductivity and the model predictions showed a temperature dependence of k proportional to T2 at low temperatures, typical of amorphous and strongly disordered materials. On the other hand, at high temperatures the thermal conductivity of mesoporous silicon films reached a maximum, indicating a crystalline-like behavior. These results will be useful in designing mesoporous silicon with desired thermal conductivity by tuning its morphology for various applications.
              Cover page: Thermal conductivity of ordered mesoporous nanocrystalline silicon thin films made from magnesium reduction of polymer templated silica
              • Article
              • Peer Reviewed
              UCLA Previously Published Works (2012)
              This paper reports the temperature dependent cross-plane thermal conductivity of pure silica zeolite (PSZ) MFI and MEL thin films measured using the 3ω method between 30 and 315 K. PSZ MFI thin films were b-oriented, fully crystalline, and had a 33% microporosity. PSZ MEL thin films consisted of MEL nanoparticles embedded in a non-uniform and porous silica matrix. They featured porosity, relative crystallinity, and particle size ranging from 40% to 59%, 23% to 47%, and 55 to 80 nm, respectively. Despite their crystallinity, MFI films were found to have thermal conductivity smaller than that of amorphous silica due to strong phonon scattering by micropores. In addition, the effects of increased relative crystallinity and particle size on thermal conductivity of MEL thin films were compensated by the simultaneous increase in porosity. Finally, thermal conductivity of MFI zeolite was predicted and discussed using the Callaway model based on the Debye approximation.
                Cover page: Temperature dependent thermal conductivity of pure silica MEL and MFI zeolite thin films
                • Article
                • Peer Reviewed
                UCLA Previously Published Works (2011)
                This paper reports the cross-plane thermal conductivity of highly-ordered amorphous and crystalline templated mesoporous titania thin films measured by the 3ω method from 30 to 320 K. Both sol-gel and nanocrystal-based films were synthesized by evaporation-induced self-assembly, with average porosity of 30% and 35%, respectively. The pore diameter ranged from 14 to 25 nm. The size of crystalline domains in polycrystalline mesoporous films was 12 to 13 nm while the nanocrystals in the nanocrystal-based film were 9 nm in diameter. At high temperatures, the thermal conductivity of amorphous dense and mesoporous films showed similar trends with respect to temperature. This was attributed to the fact that the presence of pores had a purely geometrical effect by reducing the cross-sectional area through which heat can diffuse. By contrast, the thermal conductivity of polycrystalline dense and mesoporous films behave differently with temperature due to phonon scattering by pores and crystalline nanosize domains. In addition, at low temperatures, the presence of pores caused the thermal conductivity of mesoporous films to be less temperature dependent than their dense counterparts. Despite its crystallinity, the thermal conductivity of the nanocrystal-based film was about 40% less than that of the polycrystalline mesoporous films. This was mainly attributed to its larger porosity, smaller crystal size, and strong phonon scattering at the poorly interconnected nanocrystal boundaries. These results suggest various ways to control the thermal conductivity of mesoporous materials for various applications.
                  Cover page: Thermal conductivity of highly-ordered mesoporous titania thin films from 30 to 320 K
                  • Article
                  • Peer Reviewed
                  UCLA Previously Published Works (2018)
                  Transfer of molecular chirality to supramolecular chirality at nanoscale and microscale by chemical self-assembly has been studied intensively for years. However, how such molecular chirality further transfers to the macroscale along the same path remains elusive. Here we reveal how the chirality from molecular level transfers to macroscopic level via self-assembly. We assemble a macrostripe using enantiomeric camphorsulfonic acid (CSA)-doped polyaniline with hierarchical order. The stripe can twist into a single-handed helical ribbon via helical self-motion. A multi-scale chemo-mechanical model is used to elucidate the mechanism underlying its chirality transfer and induction. The molecular origin of this macroscopic helical chirality is verified. Results provide a comprehensive understanding of hierarchical chirality transfer and helical motion in self-assembled materials and even their natural analogues. The stripe exhibits disparate actuation behaviour under stimuli of enantiomeric amines and integrating such chiral perception with helical self-motion may motivate chiral biomimetic studies of smart materials.
                    Cover page: Macroscopic helical chirality and self-motion of hierarchical self-assemblies induced by enantiomeric small molecules
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