within the quartz tube 12 is an inner quartz tube 18 running coaxially therewith from the lower end cap 16 on which it is supported and stopping short of the upper end cap 14 approximately half way along its length the inner quartz tube 18 has a disc 20 of porous silica frit bridging across its bore an inlet tube 22 for the introduction of a mixture of gaseous carbon source and diluent gas extends through the lower end cap 16 axially into the lower end of the inner quartz tube 18 an outlet tube 24 for venting gas from the reactor extends from the annular space between quartz tubes 12 and 18 through the lower end cap 16 an inlet tube 26 extends axially through the upper end cap 14 to reach down into the upper part of the inner quartz tube 18 a hopper 28 for the gravity feed of substrate particles is connected via a ball valve 30 to a port at the top of a horizontal run of the tube 26 and a side arm of said tube leading to said port is connected to a supply 32 of carrier gas in use the furnace is heated to heat the quartz tubes 12 and 16 to the desired nanotubes forming reaction temperature and a flow of carbon containing gas and diluent gas mixture is established through inlet 22 thereafter substrate particles are dropped from the hopper 28 and displaced by a flow of carrier gas from the side arm of tube 26 to fall into the reaction zone where they are supported on the frit 20 and form a fluidised bed 34 carbon.