Thermal Conductivity of Carbon Aerogels as a Function of Pyrolysis Temperature

Abstract

Amorphous carbon samples with a total porosity of about 85% were synthesized via pyrolysis of sol-gel derived resin precursors. Since the pores in the samples investigated have dimensions of a few tens of nanometers only, the gaseous contribution to the thermal conductivity is largely suppressed at ambient pressure. Values for the total thermal conductivity as low as 0.054 W·m −1 ·K −1 at 300 • C are detected. However, the pyrolysis temperature has a great impact on the contribution of the solid backbone to the total thermal conductivity. From the same precursor a series of samples was prepared via pyrolysis at temperatures ranging from 800 to 2500 • C. The thermal conductivity of this series of carbons at 300 • C under vacuum increases by a factor of about 8 if the pyrolysis temperature is shifted from 800 to 2500 • C. To elucidate the reason for this strong increase, the infrared radiative properties, the electrical conductivity, the macroscopic density, the microcrystallite size, the sound velocity, and the inner surface of the samples were determined. Evaluation of the experimental data yields only a negligible contribution from radiative heat transfer and electronic transport to the total thermal conductivity. The main part of the increasing thermal conductivity therefore has to be attributed to an increasing phonon mean free path in the carbons prepared at higher pyrolysis temperatures. However, the phonon mean free path does not match directly the in-plane microcrystallite size of the amorphous carbon. Rather, the in-plane microcrystallite size represents an upper limit for the phonon mean free path. Hence, the limiting factor for the heat transport via phonons has to be defects within the carbon microcrystallites which are partially cured at higher temperatures.

Relationship between pore volumes and surface areas derived from the evaluation of N2-sorption data by DR-, BET- and t-plot

“…In contrast to Wiener et al [4], where the evaluation range was kept constant at relative pressures between 0.1 and 0.26, we here adjusted the parameter range according to the recommendations for microporous samples given in [7][8][9]. That means the relative pressure range was chosen such that -the data in the BET-plot showed a linear relationship, -the resulting parameter C BET is positive, -the intercept on the ordinate of the BET-plot is positive (negative intercepts would be outside the valid range of the BET equation).…”

“…synthetic porous carbon derived from porous resins (samples identical to the ones used by Wiener et al [4]). The synthesis is described in detail elsewhere [5].…”

“…The synthesis is described in detail elsewhere [5]. All samples were derived from the same parent carbon sample that had been pyrolyzed at 800°C in Argon; the samples were subsequently exposed to different temperatures ranging from 1000°C to 2500°C in an inert argon atmosphere [4], to vary the micropore volume accessible to nitrogen at 77 K at given meso-and macrostructure. The variation in microporosity is due to the ''falling card model" proposed by Buiel et al [6], where micropores become more and more inaccessible to the analysis gas as the pyrolysis temperature is increased.…”

“…The variation in microporosity is due to the ''falling card model" proposed by Buiel et al [6], where micropores become more and more inaccessible to the analysis gas as the pyrolysis temperature is increased. As the microporosity originally present and accessible to nitrogen in samples treated at 800 and 1000°C continuously vanishes with increasing treatment temperature, whereas the external surface area and macro-and mesopore structure are unaffected, the series of samples presented in Wiener et al [4] are ideal candidates to compare the characteristic structural parameters derived by the three different evaluation methods.…”

Microstructure of porous carbons derived from phenolic resin – Impact of annealing at temperatures up to 2000 °C analyzed by complementary characterization methods

“…Therefore it is surprising that this relationship is not valid for the samples investigated. While leaving the mechanical properties unaffected, the microstructure of the carbons analyzed has a huge impact on the macroscopic heat transport: Investigation of series B reveals an increase of the solid thermal conductivity by a factor of about 5 (see [27]) that is likely directly related to the changes of the microstructure shown above.…”