Description
Thermites are combustible substances, usually little known to the general public,prepared by physically mixing powders of metal oxide and metal. The particular chemical characteristic of thermites lies in the nature of their constituents, which are considered by common sense to be non-combustible. The displacement of the oxygen contained by metal oxides by aluminum was discovered by the Russian chemist Nikolay Beketov in 1865, but it was only between the late 19th and early 20th Century that the German chemist Johannes Wilhelm Goldschmidt patented the formulation of aluminothermic compounds, which were then intended for welding metal parts [GOL 07]. The mixtures prepared by Goldschmidt consisted of metal oxides or sulfides that were reduced by metals with a marked electropositive character, such as aluminum, calcium or magnesium.
It is worth noting that the first thermites were manufactured by the same industrial processes that made use of molten salts electrolysis to obtain the metals used as fuels in these compounds came to maturity. The Hall–Héroult process for producing aluminum by electrochemical reduction of a molten cryolite bath dates back to 1886. Several years later, in 1897, Herbert Henry Dow founded the famous “Dow Chemical Company”, which manufactured magnesium by the electrolysis of molten magnesiumchloride. The considerable amount of electrical power required for melting and breaking down the salts employed as reducing metal precursors required a source of abundant and inexpensive electricity. The invention of the dynamo in 1868 by the Belgian physicist Zénobe Théophile Gramme, and then the use by Aristide Bergès, in 1882, of “white coal” to activate it, marked the dawn of the age of industrial production of electricity.
The analysis of the historical context provides an explanation as to why thermites, despite their seeming chemical simplicity and the unsophisticated process used to prepare them by powder mixing, emerged quite late in the history of pyrotechnics.
The term “thermite” was coined by Goldschmidt to denote the reactive compositions he had developed. This term is justified by the very significant amount of heat released during these combustions. The Larousse dictionary defines thermite as “a mixture of metal oxides and fine-particle aluminum powder, whose highly exothermic combustion is used in aluminothermic welding”. This highly restrictive definition should be broadened to allow for taking into account the wide variety of compositions whose reaction modes are similar to aluminothermic reactions. In light of recent scientific advances in this field, thermites may be defined as “energetic compositions formed of reactive constituents that have a high proportion of metal elements, whose self-propagating reaction is accompanied by significant heat release”.
The classical definition of thermites reflects the fact that aluminothermic mixtures were for a long time the main representatives of this particular family of energetic materials. The mixtures of micron-sized powders of aluminum and metal oxides are insensitive to various forms of stress: flame, impact, friction and electrostatic discharge. It is very difficult to ignite micron-sized aluminothermic mixtures by means of a simple flame, and the only way to reliably and rapidly activate the reaction is to use a more sensitive pyrotechnic ignition composition [COM 06a]. The reaction is accompanied by a shower of sparks, but most of the combustion products remain in condensed form, either solid or liquid. Due to the difference in density, the melting metal separates from slags, which consist essentially of alumina. By cooling, the drop of metal forms a nugget that remains encased in its ceramic layer. Rail welding is done by means of a device that uses the effect of gravity to enable flowing of the molten metal resulting from the reaction.
Due to the transfer of the significant amount of heat stored in the liquid metal to the matter it is in contact with, micron-sized thermites can be used as incendiary substances. While flowing, the incandescent metal drops become subdivided into droplets whose oxidation in contact with air provides additional energy. The strong exothermicity of aluminothermic reactions is also taken advantage of in the field of demolition to perform thermal shearing of massive metal structures used as reinforcement. As these examples show, the uses of micron-sized thermites are quite limited and they mainly consist of using the significant amount of heat generated by the aluminothermic reaction in order to melt objects or set them on fire.