Calcium disilicide (
Ca Si2) is an inorganic compound, a
silicide of
calcium. It is a whitish or dark grey to black solid matter with melting point 1033 °C. It is insoluble in water, but may decompose when subjected to moisture, evolving
hydrogen and producing
calcium hydroxide
Calcium hydroxide (traditionally called slaked lime) is an inorganic compound with the chemical formula Ca( OH)2. It is a colorless crystal or white powder and is produced when quicklime (calcium oxide) is mixed or slaked with water. It has m ...
. It decomposes in hot water, and is flammable and may ignite spontaneously in air.
Industrial calcium silicide usually contains
iron and
aluminium as the primary contaminants, and low amounts of
carbon and
sulfur
Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...
.
Properties
At ambient conditions calcium disilicide exists in two
polymorphs, hR9 and hR18; in the hR18 structure the hR9
unit cell is stacked twice along the c axis. Upon heating to 1000 °C at a pressure of ca. 40 kBar, calcium disilicide converts to a (semi-stable)
tetragonal phase.
The tetragonal phase is a
superconductor with a transition temperature of 1.37 K to 1.58 K. Although there is no observable superconducting transition temperature for the trigonal/rhombohedral (i.e. hR9 and hR18 unit cells) at ambient pressure, under high pressure (>12 GPa/120 kbar) this phase has been observed exhibit superconducting transition. When the trigonal phase is placed under pressures exceeding 16 GPa, there is a phase transition to an AlB
2-like phase.
Uses
Alloys
Calcium silicide is used for manufacture of special metal
alloys, e.g. for removing
phosphorus and as a
deoxidizer.
Pyrotechnics
In
pyrotechnics
Pyrotechnics is the science and craft of creating such things as fireworks, safety matches, oxygen candles, explosive bolts and other fasteners, parts of automotive airbags, as well as gas-pressure blasting in mining, quarrying, and demolition. ...
, it is used as fuel to make special mixtures, e.g. for production of
smokes, in flash compositions, and in
percussion caps. Specification for pyrotechnic calcium silicide is MIL-C-324C. In some mixtures it may be substituted with
ferrosilicon. Silicon-based fuels are used in some time delay mixtures, e.g. for controlling of
explosive bolts, hand grenades, and infrared decoys. Smoke compositions often contain
hexachloroethane; during burning they produce
silicon tetrachloride, which, like
titanium tetrachloride used in
smoke-screens, reacts with air moisture and produces dense white fog.
Gum arabic is used in some mixtures to inhibit calcium silicide decomposition.
Heating food
Self-heating can
Self-heating food packaging is active packaging with the ability to heat food contents without external heat sources or power. Packets typically use an exothermic chemical reaction. Packets can also be self-cooling. These packages are useful fo ...
s of military food rations developed during WWII used a
thermite-like mixture of 1:1
iron(II,III) oxide
Iron(II,III) oxide is the chemical compound with formula Fe3O4. It occurs in nature as the mineral magnetite. It is one of a number of iron oxides, the others being iron(II) oxide (FeO), which is rare, and iron(III) oxide (Fe2O3) which also occur ...
and calcium silicide. Such mixture, when ignited, generates moderate amount of heat and no gaseous products.
[Calvert, J. B. (2004]
Flash! Bang! Whiz! An introduction to propellants, explosives, pyrotechnics and fireworks
University of Denver
References
{{Silicides
Alkaline earth silicides
Calcium compounds
Deoxidizers
Pyrotechnic fuels