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A tube furnace is an electric heating device capable of reaching high temperatures in a controlled manner, widely used in chemistry for the synthesis of various inorganic and organic compounds.
The most common type of tube furnace consists of a cylindrical tube (reactor) which is surrounded by a "mantle", which consists of heating coils embedded in a thermally insulating matrix. The tube is made from a variety of materials, such as quartz, alumina, graphite, magnesia, zirconia, as well as refractory alloys and metals such as Kanthal, nickel alloys, tantalum, etc. The type of tube limits the maximum temperature the furnace can reach. A control panel is used to set the parameters and to display the environment from inside the tube. Temperature can be controlled via feedback from a thermocouple. Some types of tube furnaces can also rotate.
Tube furnaces can be purchased from various lab and industrial suppliers, though most entities may not sell them to the general public. They are also not cheap.
Tube furnaces can also be found on eBay and Amazon.
DIY tube furnace
Tube furnaces can be made wrapping heating element wire (Kanthal, Nichrome) and an insulating fabric around a carefully cut quartz tube, either bought from suppliers or from a space heater. By using a thermocouple hooked to an electronic display you can control the desired temperature inside the tube.
If you're using a graphite tube as reactor, you won't need heating wire as graphite is electrically conductive. However graphite may ignite at high temperatures in an oxidizing atmosphere, which limits its use in many reactions.
A crude, yet very simple tube furnace can be done by simply placing a quartz tube on two supports at both ends, and carefully heating it its middle from the outside using a blowtorch. While this does carry the risk of fire if you use flammable reagents, it's very easy to do and with the right tubing and stoppers, you can limit or completely eliminate the fire hazard.
Tube furnaces can reach very high temperatures and in the event the tube gets damaged during operation, very hot gasses will be released which may ignite flammable nearby objects or people.
Tube furnaces are power hogs and there's the risk of blowing your fuses if your home wiring is bad.