Double-walled furnace tubes

For substances that require more than 50 °C for slow evaporation, a furnace must be attached directly to the vacuum shroud. A very simple solution, proposed by Tomioka, consists of a double-walled glass tube that fits into a steel tube... 

These seals permit the construction of complex glass-ceramic units including double-walled ceramic furnace tubes. In the latter, the annular space can be evacuated or used for introducing gases for permeability studies. 

Tests at900°C. The long period or equilibrium apparent leak rates of both the mullite and zircon double-walled furnace units are of the order of the limit of error of the author s measuring method, i.e., 1.7 to 3.4 X 10-9 l.-mm. of mercury per second, or 2 to 4 X 10-9 cc. at N.T.P. per second. These values were observed after 15 to 24 hours of pumping and after exposure to a dry gas atmosphere with the zirconium specimen removed. Control experiments were made without the furnace tube present. 

The vacuum microbalance and supplementary apparatus, including a double-walled mullite furnace tube, are used for all the measurements. The oxygen used was carefully freed from hydrogen, carbon monoxide, carbon dioxide, water, etc. (18). 

A refractory holder containing four specimens was located in a double-walled silica reaction tube, with a 6 mm cavity, containing alumina granules, between the two walls (Fig. 1). During an experiment, the mixed gas entered the reaction tube and flowed through the granules in the hot zone of the furnace, set at either 500 ( 3)°C or 700 ( 3)°C. This enabled the gas to approach equilibrium prior to passing into the inner tube and over the specimens. 

Figure 1. Tube furnace laser vaporization apparatus used by Haufler et al. (1991) to produce high yields of C 0F. The 532 nm doubled Nd YAG pulse vaporizes C from the rotating graphite target into the inert carrier gas stream. The fullerenes condense just outside the oven on the tube wall.

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