Box furnaces (also called Ash furnaces) have many applications in modern research and chemistry laboratories. These furnaces are used to determine the amount of non-combustible and non-volatile (ash) material in a sample. To determine the ash quantity, a sample is placed in the furnace and exposed to high temperatures (typically up to 1,100°C) for a given period of time. The combustible and volatile material in the sample is burned off and removed from the furnace, typically as gas.
The furnace is placed in a fume hood during operation to allow safe venting of the gas. The remaining material in the furnace after the procedure is complete consists entirely of ash which is not burned off at high temperatures. This process is commonly used for coal and petroleum coke ashing procedures.
A muffle furnace is used for many of the same types of protocols as an ashing furnace. The use of mechanical convection in these ovens directs airflow out of an exhaust muffle, which typically eliminates the need to place the furnace under a fume hood.
Common applications for a muffle furnace include high-temperature applications such as fusing glass, creating enamel coatings, ceramics and soldering and brazing articles. Also, advances in materials used for heating elements, such as non-flammable molybdenum disilicide, can now produce working temperatures up to 1,800 degrees Celsius (3,272 degrees Fahrenheit), which facilitate more sophisticated metallurgical applications.
Tube furnaces are used to synthesize and purify compounds, primarily inorganic. These furnaces consist of a cylindrical cavity that is heated via one or more heating elements outside of the chamber. The cavity’s temperatures can reach up to 1,100°C. Additionally, tube furnaces typically have one (or more) heating cavities that can be controlled via thermocouple feedback, exposing materials to different temperatures for varying periods of time. Transport reactions, for example, requiring multiple temperature zones within the same compartment, can be performed in tube furnaces. The production of crystals also results from transport reactions.
An example of a material prepared using a tube furnace is the superconductor Yttrium barium copper oxide (YBa2Cu3O7), a mixture of CuO, BaO, and Y2O3. The concoction is heated in a tube furnace at several hundred degrees using oxygen to help achieve the desired result. Other superconductors are created using specific tube furnace “recipes,” depending on their individual reaction characteristics and control criteria.
Other factors will influence your selection of a furnace, whether for metallurgic, semiconductor or chemicals processes. Most importantly, you will want to select a furnace that offers the appropriate temperature range for your application. In addition, many experiments require the introduction of a process gas for transport processes. If your application requires an inert gas, you will want to select a furnace that provides a warming gradient. Lastly, the configuration of the furnace will need to be determined: muffle, box or tubing furnaces all are offered in a wide range of shapes and sizes.