What Is Bismuth?

Bismuth is a heavy post-transition metal with atomic number 83 and the chemical symbol Bi, belonging to group 15 of the periodic table. It is the most naturally diamagnetic element and displays the highest Hall coefficient of any metal, properties that arise from its unusual electronic band structure, in which the Fermi surface consists of very small electron and hole pockets. Bismuth was among the first materials identified as exhibiting significant thermoelectric coefficients, a finding that dates to the mid-nineteenth century and that motivated decades of research before compound semiconductors displaced it as the preferred thermoelectric material in the 1950s. Interest in elemental bismuth and its alloys has renewed substantially with the advent of nanostructured materials and topological insulator physics.

Bismuth occupies a semimetal position in band theory, with a band overlap of approximately 38 meV between the valence and conduction bands in bulk crystals. Its carrier concentration is several orders of magnitude lower than that of typical metals, giving it electrical properties intermediate between semiconductors and metals. The long mean free path of charge carriers in bismuth, sometimes exceeding several millimeters at low temperatures, makes it sensitive to quantum effects and useful for studying magnetotransport phenomena. These characteristics are examined in IEEE conference publications on bismuth as a thermoelectric material, which trace its history and prospects alongside competing materials.

Electronic and Thermoelectric Properties

The thermoelectric efficiency of a material is characterized by the dimensionless figure of merit ZT, equal to the Seebeck coefficient squared times electrical conductivity, divided by thermal conductivity. Bismuth has a large Seebeck coefficient and low thermal conductivity relative to most metals, but its high carrier mobility limits ZT in bulk form to values well below unity at room temperature. Quantum confinement in bismuth nanowires and thin films modifies the band structure, pushing the semimetal toward a semiconducting state and potentially enhancing ZT substantially. Research demonstrated in the 1990s that reducing one or more physical dimensions of bismuth could open a bandgap and increase the density of states near the Fermi level, a prediction confirmed in bismuth nanowire arrays and thin-film superlattices. The PMC-hosted review of bismuth telluride and related alloys for thermoelectric generation places elemental bismuth in the broader context of compound thermoelectric development.

Bismuth Compounds and Related Materials

Bismuth forms a range of technologically significant compounds. Bismuth telluride (Bi2Te3) is the preeminent room-temperature thermoelectric material, used in solid-state cooling modules (Peltier coolers) and low-grade heat recovery generators. Bismuth selenide (Bi2Se3) and bismuth telluride are prototypical three-dimensional topological insulators, materials in which the bulk is electrically insulating but the surface hosts topologically protected metallic states arising from strong spin-orbit coupling. These surface states, which exhibit spin-momentum locking, are of interest for spintronic devices and fault-tolerant quantum computing platforms. Bismuth oxide (Bi2O3) finds use as a phase stabilizer in yttria-stabilized zirconia and as a component of varistor ceramics. Bismuth-based superconductors of the BSCCO family (bismuth strontium calcium copper oxide), with critical temperatures up to 110 K, were among the early high-temperature superconductors characterized following the 1986 cuprate discoveries. Properties of these compounds and their synthesis routes are catalogued in ScienceDirect review articles on bismuth telluride thermoelectric devices.

Applications

Bismuth and its compounds have applications in a wide range of fields, including:

• Solid-state thermoelectric coolers for electronics thermal management and point-of-use refrigeration
• Topological insulator research for spintronics and quantum information platforms
• High-temperature superconductor tapes and coils for MRI magnets and power cables
• Varistor and piezoelectric ceramics in electronic protection circuits
• Low-toxicity replacement for lead in solders, radiation shielding, and pigments