Apparatus such as a furnace muffle (11) for use in a CVI / CVD furnace. The apparatus includes a bottom (12), a top (13), and an outer wall (3) defining an
interior space (1) in the apparatus, and a passive
heat distribution element (7, 9) located within the
interior space (1) and apart from the outer wall (3). Preferably, the bottom (12) and top (12) include perforated plates and the outer wall (3) is cylindrical in shape and all are made of
graphite or carbon-carbon
composite material and the passive
heat distribution element (7, 9) is cylindrical in shape and includes
graphite or carbon-carbon composite discs having no spacers therebetween. Also, a method for densifying a
porous carbon preform (5), which method includes the steps of: (a) providing the apparatus (11); (b) charging the apparatus (11) with a plurality of stacks of annular
porous carbon preforms (5), the preforms being separated from one another by spacers (15); (c) locating the charged apparatus (11) in a furnace at a temperature in the range of 950-1100° C. and a pressure in the range of 5-40
torr; and (d) circulating a
natural gas reactant blended with up to 15%
propane through the apparatus for 150-900 hours. Also, a batch of carbon-carbon composite preforms made by the method, wherein the density of the batch of preforms is at least 0.5 g / cc higher than the density of a batch of preforms made by an otherwise identical process in which the apparatus does not contain a passive
heat distribution element located within its interior. The preforms may be configured as
aircraft landing system brake discs or racing car
brake discs.