Particle precipitation-aided chemical vapor deposition (PP-CVD) is a modification of the conventional CW process, where an aerosol is formed in the pas phase at an elevated temperature, and particles are deposited on a cooled substrate. The synthesis of titanium nitride (TiN), using titanium tetrachloride vapor (TiCl4), nitrogen (N-2), ammonia (NH3), and hydrogen (H-2), by the PP-(SVD process is studied. TiN is formed by a heterogeneous reaction, using TiCl4, N-2, H-2, whereas simultaneously TiCl4 and NH3 react to form an aerosol. The activation energy of this homogeneous reaction is on the order of 100 kJ/mol. The powder formation process is determined by the dissociation of a titanium containing intermediate species. At low temperature differences between substrate and pas phase (i.e., <2 K), only dense columnar microstructures, with growth rates of around 20 mu m/h, are observed. At these temperature differences no particle deposition is observed. The layers are formed by a molecular diffusion controlled CVD growth mechanism. Porous coherent layers are found in experiments, where intermediate temperature differences are applied (i.e., approximately 2-10 K). The observed interconnection of the particles has to originate from a heterogeneous reaction. Apparently, under these conditions the heterogeneous reaction is fast enough, with respect to the particle precipitation rate, to interconnect the precipitated particles. A further increase in temperature difference between the susceptor and the gas phase only leads to loose powder deposits. In principle, the PP-CVD process is a suitable method for the synthesis of thin porous layers of ceramics. To obtain uniform coherent porous layers two separate reaction mechanisms are required under the same experimental conditions. There should be a homogeneous reaction in the gas phase as well as a heterogeneous reaction, which is controlled by surface kinetics, in order to interconnect precipitated particles to obtain a coherent porous layer. Porous ceramic layers ean be formed as long as the particle precipitation rate is slow enough with respect to the heterogeneous reaction rate.
