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The design of a vertical-tube furnace for use with a transmission-mode X-ray powder diffractometer is discussed in detail. The X-ray beam passes through a 3 mm slit cut perpendicular to the furnace-tube axis. Two Pt-Pt/10%Rh thermocouples are located within the pyrophyllite furnace-tube wall at the top and bottom of the 3 mm slit. The potential difference between these thermocouples is used to adjust the power supplied to the upper and lower halves of the furnace. By selecting the desired potential difference at each measuring temperature, gradients from 0 to 1 K across the 3 mm slit can be easily achieved over the entire temperature range of the furnace (i.e. 323 to 1323 K). The furnace is housed in a cooling jacket that can be adjusted vertically to optimize the diffraction geometry. Samples are loaded in capillaries of 0.3 mm outer diameter, mounted on a rotating goniometer head, and centred horizontally and vertically. This sample geometry yields very low sample-position errors. The design also allows 180° coverage in 20 and variable sample environments (i.e. flowing gas, air and vacuum).
