Thermal Properties and Lattice Dynamics of Polycrystalline MFI Zeolite Films

Abstract

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.

Temperature Dependent Thermal Conductivity of Pure Silica MEL and MFI Zeolite Thin Films

“…Figure 2 shows that the calculated thermal conductivity from Equations (1) to (3) agreed within 6% of the experimental data for the PSZ MFI film for all temperatures between 30 and 315 K. It establishes that using the Debye dispersion relation instead of the complete phonon dispersion was sufficient to predict the thermal conductivity of MFI zeolite films. In addition, phonon boundary scattering was found to dominate over phonon Umklapp scattering in MFI zeolite as discussed in the literature [14,[16][17][18]. In fact, the predicted thermal conductivity was insensitive to the Umklapp scattering relaxation time τ U since τ U ≫ τ B and τ U ≫ τ D for all temperatures considered and all phonon frequencies up to the Debye cut-off frequency of ω D /2π = 8 THz.…”

“…The thermal conductivity of the in situ MFI film increased from 0.05 to 1.2 W/m·K as temperature increased from 30 to 315 K. In addition, it was about 10% to 15% larger than that of calcined MFI zeolite measured by Greenstein et al [16] between 150 and 315 K. However, the thermal conductivity of uncalcined MFI [16] was about 20% to 40% larger than that measured in the present study [16]. This was likely due to the fact that the uncalcined films measured by Greenstein et al [16] were denser (2.1 g/cm 3 ) than the in situ MFI film (1.7 g/cm 3 ) investigated in the present study. Moreover, the measured thermal conductivity of our MFI films was comparable to or smaller than that of the amorphous silica [31] despite its crystalline nature.…”

“…By analogy with other studies [14,16,17,30,32], the parameters A = 1.38 × 10 −42 s 3 , B = 4.24 × 10 −21 s/K, and l B = 0.95 nm in Equation (3) were obtained by fitting the predictions of Equations (1) to (3) to the experimental data over the entire temperature range explored. Figure 2 shows that the calculated thermal conductivity from Equations (1) to (3) agreed within 6% of the experimental data for the PSZ MFI film for all temperatures between 30 and 315 K. It establishes that using the Debye dispersion relation instead of the complete phonon dispersion was sufficient to predict the thermal conductivity of MFI zeolite films.…”

“…Figure 2 shows the measured thermal conductivity of the PSZ MFI and MEL zeolite thin films as a function of temperature, along with data reported in the literature for calcined and uncalcined MFI zeolite [16] and amorphous silica [31]. The four MEL thin films differ from one another by their relative crystallinity.…”

“…The measured thermal conductivity of (h0l)-oriented PSZ MFI films varied from 1.0 to 1.4 W/m·K in the temperature range considered [17]. That of calcined and uncalcined c-oriented PSZ MFI films deposited on silicon substrates was found to range from 0.75 to 1.1 W/m·K and 1.0 to 1.6 W/m·K, respectively [16]. More recently, Coquil et al [18] measured room temperature thermal conductivity of PSZ MFI and MEL zeolite thin films.…”