Red iron(III) oxide (Fe2O3, commonly known as rust) is the most common iron oxide used in thermite. Magnetite also works. Other oxides are occasionally used, such as MnO2 in manganese thermite, Cr2O3 in chromium thermite, or copper(II) oxide in copper thermite, but only for specialized purposes. All of these examples use aluminium as the reactive metal. Fluoropolymers can be used in special formulations, Teflon with magnesium or aluminium being a relatively common example. Magnesium/teflon/viton is another pyrolant of this type.
In principle, any reactive metal could be used instead of aluminium. This is rarely done, however, because the properties of aluminium are nearly ideal for this reaction:
- It is by far the cheapest of the highly reactive metals;
- It forms a passivation layer making it safer to handle than many other reactive metals.
- Its relatively low melting point (660 °C (1,220 °F)) means that it is easy to melt the metal, so that the reaction can occur mainly in the liquid phase and thus proceeds fairly quickly.
- Its high boiling point (2,519 °C (4,566 °F)) enables the reaction to reach very high temperatures, since several processes tend to limit the maximum temperature to just below the boiling point. Such a high boiling point is common among transition metals (e.g., iron and copper boil at 2,887 °C (5,229 °F) and 2,582 °C (4,680 °F) respectively), but is especially unusual among the highly reactive metals (cf. magnesium and sodium which boil at 1,090 °C (1,990 °F) and 883 °C (1,621 °F) respectively).
- Further, the low density of the aluminium oxide formed as a result of the reaction tends to cause it to float on the resultant pure metal. This is particularly important for reducing contamination in a weld.
Although the reactants are stable at room temperature, they burn with an extremely intense exothermic reaction when they are heated to ignition temperature. The products emerge as liquids due to the high temperatures reached (up to 2,500 °C (4,530 °F) with iron(III) oxide)—although the actual temperature reached depends on how quickly heat can escape to the surrounding environment. Thermite contains its own supply of oxygen and does not require any external source of air. Consequently, it cannot be smothered and may ignite in any environment, given sufficient initial heat. It will burn well while wet and cannot be easily extinguished with water, although enough water will remove heat and may stop the reaction. Small amounts of water will boil before reaching the reaction. Even so, thermite is used for welding underwater.
The thermites are characterized by almost complete absence of gas production during burning, high reaction temperature, and production of molten slag. The fuel should have high heat of combustion and produce oxides with low melting point and high boiling point. The oxidizer should contain at least 25% oxygen, have high density, low heat of formation, and produce metal with low melting and high boiling point (so the energy released is not consumed in evaporation of reaction products). Organic binders can be added to the composition to improve its mechanical properties, however they tend to produce endothermic decomposition products, causing some loss of reaction heat and production of gases.
The temperature achieved during the reaction determines the outcome. In ideal case, the reaction produces a well-separated melt of metal and slag. For this, the temperature has to be high enough to melt both the reaction products, the resulting metal and the fuel oxide. Too low temperature will result in a mixture of sintered metal and slag, too high temperature – above boiling point of any reactant or product – will lead to rapid production of gas, dispersing the burning reaction mixture, sometimes with effects similar to a low-yield explosion. (Moisture in composition or adjanced materials can result in similar effect, a steam explosion.) In compositions intended for production of metal by aluminothermic reaction, these effects can be counteracted. Too low reaction temperature (e.g. when producing silicon from sand) can be boosted with addition of a suitable oxidizer (e.g. sulfur in aluminium-sulfur-sand compositions), too high temperatures can be reduced by using a suitable coolant and/or slag flux. The flux often used in amateur compositions is calcium fluoride, as it reacts only minimally, has relatively low melting point, low melt viscosity at high temperatures (therefore increasing fluidity of the slag) and forms a eutectic with alumina. Too much of flux however dilutes the reactants to the point of not being able to sustain combustion. The type of metal oxide also has dramatic influence to the amount of energy produced; the higher the oxide, the higher the amount of energy produced. A good example is the difference between manganese(IV) oxide and manganese(II) oxide, where the former produces too high temperature and the latter is barely able to sustain combustion; to achieve good results a mixture with proper ratio of both oxides should be used.
The reaction rate can be also tuned with particle sizes; coarser particles burn slower than finer particles. The effect is more pronounced with the particles requiring being heated to higher temperature to start reacting. This effect is pushed to the extreme with nano-thermites.
The temperature achieved in the reaction in adiabatic conditions, when no heat is lost to the environment, can be estimated using the Hess's law – by calculating the energy produced by the reaction itself (subtracting the enthalpy of the reactants from the enthalpy of the products) and subtracting the energy consumed to heating the products (from their specific heat, when the materials only change their temperature, and their enthalpy of fusion and eventually enthalpy of vaporization, when the materials melt or boil). In real conditions, the reaction loses heat to the environment, the achieved temperature is therefore somewhat lower. The heat transfer rate is finite, so the faster the reaction is, the closer to adiabatic condition it runs and the higher is the achieved temperature.
Read more about this topic: Thermite
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