Coefficient of thermal expansion of pentaerythritol tetranitrate and hexahydro-1,3,5-trinitro-s-triazine (RDX)

Abstract

T h e thermal expansion coefficient is a second-order symmetric tensor and hence contains six independent constants for a triclinic crystal. This number is reduced for crystals with higher symmetry. Hexahydro-l,3,5-trinitro-s-triazine (RDX, C3H6N606) crystallizes with orthorhombic symmetry, and its thermal expansion is expressed by three coefficients of linear expansion measured along the principal crystallographic axes. Pentaerythritol tetranitrate (PETN, CSHsK4012) is tetragonal, and two coefficients, one along the unique crystallographic axis and one perpendicular to it, describe the thermal expansion, The direction of measurement of the coefficient of linear thermal expansion is indicated by a subscript giving the crystal face normal to the direction of measurement-e.g., C U ( ~ Z O is measured in a direction perpendicular to the (120) crystal face. It happens, for these materials, that a(1oo) is measured parallel to the a crystallographic axis, and similarly ~ ~ ( 0 1 0 ) and