NOVELTY - A thermal decomposition device has pressure vessel (1); reaction tube (2), an open end of which extends into the pressure vessel and the other end of which is located outside the pressure vessel; and gas outlet unit (7) equipped with gas guide (7a).    USE - Used for thermal decomposition of volatile compounds and deposition of ultrafine particles (claimed) from the gas phase, in particular of pulverulent silicon (claimed) for photovoltaic and electronic applications, silicon nitride and silica.    ADVANTAGE - The invention eliminates blocking of pipes and eliminates contamination of silicon that is formed.    DETAILED DESCRIPTION - The thermal decomposition device comprises pressure vessel; reaction tube, the open end of which extends into the pressure vessel and the other end of which is located outside the pressure vessel and is provided with gas feed, the longitudinal axis of the reaction tube is oriented in the direction of gravity and parallel to the longitudinal axis of the pressure vessel, and the reaction tube can be heated on the gas inlet side and cooled on the gas outlet side, the pressure vessel, in its lower part, has a collection cone (1a), the open end of the reaction tube(s) extending into the gas space (1b) of the collection cone, the collection cone is connected to an outlet lock for particles; and gas outlet unit, which is equipped with gas guide, the gas inlet region of which is in communication with the gas space of the collection cone, a filter system (8) and a gas outlet (9), which is located outside the pressure vessel.    An INDEPENDENT CLAIM is also included for a process for the thermal decomposition of volatile compound and deposition of particles which are then formed, using the inventive device, in which the corresponding reaction tubes are heated on the inlet side to the decomposition temperature of the volatile compound, and the lower region of the reaction tubes is cooled, the volatile, thermally decomposable compound is diluted with a substantially inert gas and this gas or gas mixture is fed to the reaction tubes via the corresponding gas feed, the particles which are formed during the decomposition and have gathered in the collection cone are discharged via the lock unit, and the gas or gas mixture which is formed during the decomposition reaction is discharged via the gas outlet, with the pressure in the pressure vessel being kept substantially constant.    DESCRIPTION OF DRAWING(S) - The figure shows a view of the inventive device.    Pressure vessel (1)    Collection cone (1a)    Gas space (1b)    Reaction tube (2)    Gas feed (3)    Outlet lock (6)    Gas outlet unit (7)    Gas guide (7a)    Gas inlet region (7b)    Filter system (8)    Gas outlet (9)