Relationship of nanostructure and thermo-chemical response/thermal ablation of carbon aerogels

ABSTRACT

In this work, the interfacial mass balance relations combined with the non-parametric kinetic (NPK) analysis results were used for evaluating the thermo-chemical ablation process and oxidation mechanism of carbon aerogels with various porous structure. It was found that the two-parameter model of Nomen–Sempereis was able to describe the kinetics of the oxidation reaction and to reveal the structure-dependent contribution of two main processes with chemical and physical nature. The porosity of the carbon aerogel, rather than the other microstructural features, was realized more effective on the rate of ablation.

KEYWORDS:

• Carbon aerogel
• pore structure
• thermal analysis
• ablation process
• thermal response

Nomenclature

${\rho }_a$	=	Density of the carbon aerogel (kg m−3)
$v_f$	=	The velocity of the overrunning air flow (ms−1)
$T_0$	=	Ambient Temperature (K)
${\vartheta }_j$	=	Stoichiometric coefficient of jth component
$D_j^g$	=	Diffusion coefficient of jth component in gas phase (m2s−1)
$D_j^a$	=	Diffusion coefficient of jth component in aerogel phase (m2s−1)
$C_j^g$	=	Mass concentration of jth component in gas phase (kg m−3)
$C_j^a$	=	Mass concentration of jth component in aerogel phase (kg m−3)
$K_j^g$	=	Mass transfer coefficient of jth component in gas phase (ms−1)
$J_j$	=	Mass flux of jth component (kg m−2s−1)
$M_j$	=	Molecular mass of the jth component (kg mol−1)
$R_s$	=	The molar rate of oxidation reaction (mol m−2 s−1)
$v_s$	=	Ablation rate (m s−1)
$c_f$	=	Specific heat capacity of air (J kg−1 K−1)
$k_f$	=	Thermal conductivity of air (W m−1 K−1)
${\rho }_f$	=	Density of air (kg m−3)
$h_f$	=	Heat transfer coefficient of air (W m−2 K−1)
$R$	=	Universal gas constant (J K−1 mol−1)
$C_j^{bl}$	=	Mass concentration of jth component in boundary layer (kg m−3)
$K\left(T\right)$	=	Rate constant of the oxidation reaction (s−1)
$\epsilon$	=	Porosity of the carbon aerogel
$\vartheta$	=	Air kinematic viscosity (m2 s−1)
$E_a$	=	Activation energy (J mol−1)
$A$	=	Pre-exponential factor (s−1)
$T_m$	=	Reference temperature in Vogel–Fulcher relation (K)
$S$	=	Interfacial surface of the aerogel (m2)

Acknowledgments

The authors thank Tarbiat Modares University and Iran Nanotechnology Initiative Council (INIC) for supporting this research work.