Significant differences exist among literature for thermal conductivity of various systems computed using molecular dynamics simulation. In some cases, unphysical results, for example, negative thermal conductivity, have been found. Using GaN as an example case and the direct non-* X. W. Zhou: xzhou@sandia.gov Both the direct and Green-Kubo approaches require long simulations (e.g. at least 1 ns) to reduce the uncertainty due to thermal fluctuations. For the direct method, another difficulty encountered is that the computed thermal conductivity depends strongly on the system length L along the propagation direction, which is typically limited to at most a few hundred nanometers. This means that for perfect bulk crystals the phonon mean free path is comparable to the system size and transport occurs in a partially ballistic regime [17,18,[21][22][23]. It follows from kinetic theory that the computed values of κ are smaller than that of a true bulk system. To obtain values that can be meaningfully compared with experiments, it is therefore necessary to perform several simulations for different cell lengths, and then

We report a systematic study that reveals the effect of composition ͑silicon-to-aluminum ratio͒ 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 26͒ and constant ͑h0l͒ 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 K͒ and Si/Al ratio. Detailed atomistic simulations ͑energy minimization and phonon dispersion calculations͒ 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 network͒, which in turn serve as model systems for tuning of phonon transport properties in complex materials.

A study of the thermal properties of the zeolite MFI by a combination of experimental measurements and lattice dynamical modeling is presented. Thermal conductivity data in the range of 150-400 K was obtained through 3w measurements on polycrystalline zeolite films. While Debye theory is inadequate in predicting the zeolite thermal properties, a detailed calculation of the specific heat using a full set of dispersion relations obtained from atomistic simulations gives excellent agreement with experiments. In addition, the thermal conductivity is successfully reproduced by a phonon relaxation time-based model. The results indicate the possibility of developing a predictive model of the thermal properties of complex zeolite materials.

Articles you may be interested inAnomalous thermal conductivity by surface phonon-polaritons of polar nano thin films due to their asymmetric surrounding media J. Appl. Phys. 113, 084311 (2013); 10.1063/1.4793498Extracting phonon thermal conductance across atomic junctions: Nonequilibrium Green's function approach compared to semiclassical methods An inelastic incoherent neutron scattering study of water in small-pored zeolites and other water-bearing mineralsWe 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 K͒. 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.

A theoretical study of the heat capacities of Quartz, zeolite MTT, and zeolite MFI is presented. The technique used to calculate the heat capacity can be applied to a many dielectrics, even those that are highly anisotropic, with complex crystal structures. For the aforementioned materials the assumptions used in Debye theory are too restrictive, the methodology presented in this paper uses a full set of dispersion relations obtained from atomistic simulation. Agreement with experiment for all materials studied is excellent, with the exceptions of temperature regions where phase transitions occur. The methodology that is used was developed with thermal conductivity in mind. It is hoped that this model will be smoothly incorporated into future conductivity models. Additionally, the effect of Al substitution on the heat capacity of MFI is investigated. It is predicted that aluminum substitution has a miniscule effect on the heat capacity of MFI.