Bismuth

Bismuth (symbol Bi) is a brittle, crystalline metal with a high metallic luster with a distinctive pinkish tinge. The metal is easily cast but not readily formed by working. Within a narrow range of temperature, around 225°C, it can be extruded. Its crystal structure is rhombohedral.

It is one of the few metals that expand on solidification; the expansion is 3.3%. The thermal conductivity of bismuth is lower than that of any metal, with the exception of mercury.

Bismuth is the most diamagnetic of all metals (mass susceptibility of -1.35 x 106). It shows the greatest Hall effect (increase in resistance under influence of a magnetic field). It also has a low capture cross section for thermal neutrons (0.034 barn).

Bismuth is inert in dry air at room temperature, although it oxidizes slightly in moist air. It rapidly forms an oxide film at temperatures above its melting point, and it burns at red heat, forming the yellow oxide, Bi2O3. The metal combines directly with halogens and with sulfur, selenium, and tellurium; however, it does not combine directly with nitrogen or phosphorus.

Bismuth is not attacked at ordinary temperatures by air-free water, but it is slowly oxidized at red heat by water vapor. Bismuth does not dissolve in nonoxidizing acids, but does dissolve in HNO3 and in hot concentrated H2SO4. The formation of intermetallic compounds involves mainly the strongly electropositive metals.

Bismuth combined with a number of metallic elements forms a group of interesting and useful low-melting alloys. Some of the lowest melting of these are as follows:

Bismuth combined with a number of metallic elements forms a group of interesting and useful low-melting alloys. Some of the lowest melting of these are as follows:

The fusible alloys are used in many ingenious ways, e.g., sprinkler-system triggering devices, bending pipes, anchoring tools during machining, accurate die patterns, etc.

Bismuth metal (0.1%) is also added to cast iron and steel to improve machinability and mechanical properties. An alloy of 50% bismuth and 50% lead is added to aluminum for screw machine stock, to increase machinability.

A permanent magnet (bismanol) with excellent resistance to demagnetization is produced from manganese and bismuth.

The development of refrigerating systems depending on the Peltier effect for cooling uses a bismuth-tellurium or selenium alloy for thermocouples. Bismuth telluride is used extensively for thermoelectric cooling and for low-temperature thermoelectric power production.

Bismuth is playing an important role in nuclear research. Its high density gives it excellent shielding properties for gamma rays while its low thermal neutron capture cross section allows the neutrons to pass through. For investigations in which it is desired to irradiate objects, i.e., animals, with neutrons but protect them from gamma rays, castings of bismuth are used as neutron windows in nuclear reactors.

Bismuth has been proposed as a solvent-coolant system for nuclear power reactors. The bismuth dissolves sufficient uranium so that, when the solvent and solute are pumped through a moderator (graphite), criticality is reached and fission takes place. The heat generated from the fission reaction raises the temperature of the bismuth. The heated bismuth is then pumped to conventional heat exchangers producing the steam power required for eventual conversion to electricity.

The advantages of such a reactor are that (1) it has potential for producing low-cost power, (2) it has an integrated fuel processing system, and (3) it converts thorium to fissionable uranium.

Another important use of bismuth is in the manufacture of pharmaceutical compounds. Various bismuth preparations have been employed in the treatment of skin injuries, alimentary diseases, such as diarrhea and ulcers, and syphilis. The oxide and basic nitrate are perhaps the most widely used compounds of bismuth. The trioxide is used in the manufacture of glass and ceramic products, and the basic nitrate is used in the porcelain painting to fire on gilt decoration.