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Scandium

Scandium — Material Page
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Scandium (Sc, atomic number 21) is a lightweight (2.985 g/cm³), silvery-white HCP transition metal with a melting point of 1,541 °C, Young's modulus of 74 GPa, and a 3d¹4s² configuration that gives it purely trivalent chemistry (Sc³⁺ only) analogous to the rare-earth elements despite its position in Group 3. It is monoisotopic — ⁴⁵Sc (100%, stable, I = 7/2, NMR-active) is the only natural isotope. Despite a crustal abundance of ~26 ppm (comparable to cobalt), Sc occurs in dispersed form without concentrated ore deposits, making it one of the most expensive metals per kilogram; primary production is dominated by extraction from uranium process tailings and from the rare Sc-silicate thortveitite.

The dominant application of Sc is as an alloying addition to aluminium: Al-Sc alloys (0.1–0.5 wt% Sc) undergo precipitation hardening via nanoscale Al₃Sc dispersoids (L1₂ structure, coherent with the Al matrix) that pin grain boundaries, increase tensile strength by 50–150 MPa without reducing ductility, suppress recrystallisation during welding and elevated-temperature service, and markedly improve fatigue life. These properties are exploited in aerospace structural components (Russian MiG and Sukhoi aircraft use Al-Sc-Mg alloys extensively), high-performance sports equipment (bicycle frames, baseball bats), and, increasingly, in metal additive manufacturing where Al-Sc powder (e.g. Scalmalloy: Al-4.5Mg-0.66Sc-0.38Zr) enables printing of high-strength fine-grained aluminium parts that cannot be achieved with conventional Al-Si AM alloys.

ScSZ (scandia-stabilised zirconia, Sc₂O₃ + ZrO₂, typically 9–11 mol% Sc₂O₃) has the highest ionic conductivity of any zirconia electrolyte — approximately 4× higher than yttria-stabilised zirconia (YSZ) at 800 °C — enabling SOFC operation at lower temperatures (650–800 °C vs. 800–1,000 °C for YSZ-based cells), which reduces balance-of-plant costs and thermal degradation. Sc₂O₃ is also used as a stabiliser in high-temperature refractory ceramics, as a sintering aid for Si₃N₄, and in ScAlMgO₄ (SCAM) substrates for GaN power electronics epitaxy.

General Properties

PropertyValueNotes
Atomic Number21 3d¹4s²; +3 the only stable oxidation state in all conditions. Sc³⁺ (ionic radius 0.745 Å, 6-coordinate) is close in size to Y³⁺ (0.900 Å) and the lighter lanthanides, giving Sc geochemical behaviour intermediate between transition metals and REEs. ⁴⁵Sc (I = 7/2) is NMR-active; ⁴⁵Sc NMR characterises Sc³⁺ coordination in ScSZ electrolytes and Al-Sc alloy grain boundary phases.
Atomic Mass44.956 uMonoisotopic: ⁴⁵Sc (100%, stable). Single clean ICP-MS signal at m/z 45 with no natural isobaric interference. ⁴⁶Sc (t½ = 83.8 days, β⁻ + γ) is produced by neutron activation of ⁴⁵Sc and used as a radiotracer in wear studies of Al-Sc alloy components.
Density (20 °C)2.985 g/cm³Lowest density of any first-row transition metal; comparable to Al (2.70 g/cm³). The low density combined with high Young's modulus (74 GPa vs. Al's 70 GPa) gives Al-Sc alloys an excellent specific stiffness, relevant to aerospace structural design.
Melting Point1,541 °C (1,814 K)High melting point for a low-density metal. Sc is produced by metallothermic reduction of ScF₃ with Ca, or by electrolysis of ScCl₃/KCl/LiCl melts; processed under Ar or vacuum to avoid oxide contamination.
Boiling Point2,836 °CHigh boiling point; Sc evaporation sources are used for MBE deposition of Sc-doped oxide films (ScAlN piezoelectric layers for BAW/FBAR resonators).
Thermal Conductivity15.8 W/m·KModerate conductivity. Relevant to thermal modelling of Al-Sc alloy heat exchangers and to ScSZ electrolyte thermal conductivity in SOFC stack design.
Electrical Resistivity61 nΩ·m (20 °C)Moderate for a transition metal. Sc becomes superconducting below ~0.05 K. Sc additions to Al increase resistivity slightly, which is acceptable for structural aerospace alloys where conductivity is not the design parameter.
Crystal StructureHCP, a = 3.309 Å, c = 5.273 ÅHCP at RT; transforms to BCC above ~1,337 °C. The Al₃Sc precipitate in Al-Sc alloys has the cubic L1₂ structure with a lattice parameter closely matched to the Al matrix (~0.4% misfit), giving coherent, low-energy interfaces that maximise strengthening per unit Sc addition.

Mechanical Properties

PropertyValueNotes
Tensile Strength250 MPaModerate for a pure metal; Al-Sc alloys in peak-aged condition reach 400–550 MPa tensile strength depending on composition and processing, comparable to 7xxx-series Al alloys.
Young's Modulus74 GPaComparable to Al (70 GPa) but in a metal with 10% lower density. Al-Sc alloy modulus is essentially that of the Al matrix; Sc additions improve strength and fatigue life without significantly affecting stiffness.
Hardness~100–120 HB (annealed)Moderately hard for a low-density metal. Sc can be machined under standard conditions — unlike most reactive rare-earth metals it is stable enough in dry air for short-duration machining without inert atmosphere.
Elongation at Break~20%Good ductility in high-purity form. Sc foil and rod are used as alloying master alloy precursors and for Al-Sc alloy research sample preparation.
Poisson's Ratio0.28Typical for an HCP metal.

Chemical Properties

PropertyValue / BehaviorNotes
Oxidation States+3 only (Sc³⁺: ScCl₃, Sc₂O₃, Sc(OTf)₃)Sc(OTf)₃ (scandium triflate) is a water-stable Lewis acid catalyst for Diels-Alder, Mukaiyama aldol, Mannich, and allylation reactions — one of the most effective Lewis acid catalysts for aqueous organic synthesis. Sc³⁺'s high charge-to-radius ratio gives it stronger Lewis acidity than most lanthanide triflates.
Corrosion ResistanceGood; forms adherent Sc₂O₃ passive layer in air and dilute oxidising media; attacked by HF and concentrated acidsSc develops a yellowish-pink surface tint on prolonged air exposure from the Sc₂O₃ layer. More stable in air than the lanthanides; Sc metal rod and foil can be handled briefly in air without significant degradation. Al-Sc alloys have comparable or slightly better corrosion resistance than the base Al alloy due to Sc's role in suppressing sensitisation.
Surface OxideSc₂O₃ (cubic C-type) forms in airSc₂O₃ (mp ~2,485 °C) is a refractory oxide used as a ZrO₂ stabiliser in ScSZ SOFC electrolytes, as a sintering aid for Si₃N₄ and AlN ceramics, and as a high-κ gate dielectric candidate (κ ~ 14). Also used in ScAlN piezoelectric thin films (sputtered from Sc + Al targets) for high-frequency BAW/FBAR resonators in 5G RF front-end filters.
IdentifierValue
SymbolSc
Atomic Number21
CAS Number7440-20-2
UN NumberNot classified
EINECS Number231-129-2
IsotopeTypeNotes
⁴⁵ScStable100% natural abundance; I = 7/2, NMR-active. Sc is monoisotopic — ⁴⁵Sc is the only natural isotope. ⁴⁵Sc NMR characterises Sc³⁺ coordination in ScSZ electrolytes, ScAlN piezoelectric films, and Sc(OTf)₃ catalyst–substrate complexes in solution. The single ICP-MS signal at m/z 45 makes Sc a reliable trace-element monitor in geological and metallurgical analysis, though isobaric overlap with ⁴⁵Ti⁺ requires correction at high Ti concentrations. σ(thermal) = 27.2 barn; ⁴⁵Sc(n,γ)⁴⁶Sc (t½ = 83.8 days, β⁻ + 1.12/1.37 MeV γ) is used as a radiotracer in wear and corrosion studies of Al-Sc alloy components.

Scientific & Research Applications

Use CaseForm Typically UsedDescription
Al-Sc Alloy DevelopmentSc metal (99–99.9%+) as Al-Sc master alloy (2% Sc); Scalmalloy powder for AMResearch focuses on Al₃Sc precipitate optimisation (size, spacing, coherency), Sc-Zr co-additions (Zr retards Al₃Sc coarsening for improved thermal stability), and processing routes — conventional casting, FSW, and powder-bed fusion AM. Scalmalloy (Al-Mg-Sc-Zr) achieves ~520 MPa UTS in L-PBF parts, exceeding wrought 6061-T6.
ScSZ SOFC Electrolyte ResearchSc₂O₃ (99.9%+) co-precipitated with ZrO₂; 9–11 mol% Sc₂O₃ target compositionScSZ's cubic-phase ionic conductivity (~0.3 S/cm at 800 °C, ~4× YSZ) enables intermediate-temperature SOFC operation at 650–800 °C, reducing Ni-YAG anode coarsening, cathode delamination, and sealing material degradation. Research addresses the cubic→rhombohedral phase transition in ScSZ at low temperatures (causing conductivity drop) and stabilisation with Ce or Bi co-dopants.
ScAlN Piezoelectric Thin FilmsSc-Al alloy sputtering targets (Sc content 15–43 at%); reactive sputtering in N₂/ArSc substitution in AlN (Al₁₋ₓScₓN, x = 0.15–0.43) increases the piezoelectric coefficient e₃₃ by 3–5× vs. pure AlN without significantly degrading electromechanical coupling. ScAlN is the dominant piezoelectric layer in next-generation BAW and FBAR resonators for 5G sub-6 GHz RF front-end filters (Band n77/n78/n79), replacing conventional AlN in high-volume smartphone filter production.
Sc(OTf)₃ Lewis Acid CatalysisSc(OTf)₃ (scandium trifluoromethanesulfonate); prepared from Sc₂O₃ + HOTfSc(OTf)₃ catalyses a broad range of carbon–carbon bond forming reactions in water or aqueous solvent mixtures — a major advantage over water-sensitive Lewis acids (BF₃, TiCl₄, AlCl₃). Applications include Diels-Alder cycloadditions, Mukaiyama aldol reactions, allylations, Mannich reactions, and ring-opening polymerisation of lactide.

Industrial & Commercial Applications

SectorForm / Grade UsedDescription
Al-Sc Alloys (Aerospace & AM)Al-Sc master alloy (2% Sc in Al, 99%+ Sc); Scalmalloy powder (Al-4.5Mg-0.66Sc-0.38Zr)Al-Sc alloys are used in Russian military aircraft (MiG-29, Su-27 airframes), commercial aerospace structures, bicycle frames, baseball bats, and kayak paddles. Scalmalloy powder is commercially produced for L-PBF and DED additive manufacturing of high-strength lightweight aerospace brackets, heat exchangers, and topology-optimised structural parts.
ScSZ SOFC ElectrolytesScSZ tape-cast electrolyte sheets (9–11 mol% Sc₂O₃, 99.9%+ Sc₂O₃ precursor)ScSZ electrolytes are used in commercial IT-SOFC stacks (Siemens, Bloom Energy, and Asian manufacturers) where reduced operating temperatures enable lower-cost metallic interconnects and simplified balance-of-plant. Each kW of SOFC capacity requires ~5–10 g of Sc₂O₃.
High-Intensity Discharge LampsScI₃ (scandium iodide) as metal halide lamp filler (99.9%+ ScI₃)Sc iodide metal halide lamps achieve a high colour-rendering index (CRI >90) and daylight-balanced colour temperature (5,500–6,500 K), making them the standard for film, broadcast, and stadium lighting. ScI₃ combined with NaI gives the characteristic spectral output of professional HID lamps. Being replaced by LED in some markets but still dominant in large-venue broadcast and outdoor applications.
5G RF Filter Piezoelectrics (ScAlN)Sc-Al alloy sputtering targets (15–40 at% Sc, 99.9%+ purity); BAW/FBAR filter wafersScAlN BAW/FBAR filters are produced at high volume by Broadcom, Qorvo, and TDK for 5G smartphone RF front-ends. Each smartphone contains multiple BAW filter modules; at ~20–40 mg Sc per wafer, global Sc demand from the 5G filter market is growing rapidly and represents an emerging critical demand driver alongside Al-Sc alloys and SOFCs.
PurityApplicationsNotes
96% (1N6)Alloying with aluminum for general-purpose structural applications.Economical grade, often sufficient for commercial lightweighting uses.
99% (2N)Higher-spec aluminum-scandium alloys for improved corrosion resistance.Industrial-grade purity suitable for aerospace and sporting goods.
99.9% (3N)Research, fuel cells, and advanced electronics development.High-purity option for scientific and high-performance applications.
99.95% (3N5)Specialized semiconductor and optical materials research.Ultra-high purity for critical experimental and clean energy applications.
Synonym / Alternative NameContext
ScChemical symbol; from Scandia (Latin for Scandinavia), predicted by Mendeleev as eka-boron and discovered by Lars Fredrik Nilson in 1879. Used in Al-Sc alloy datasheets, ScSZ electrolyte specifications, and ICP-MS trace-element databases (m/z 45).
Sc metalCommercial form designation for ingot, rod, foil, or powder. Used in Al-Sc master alloy procurement specifications and ScAlN sputtering target datasheets.
Sc elementScientific designation distinguishing elemental Sc from Sc compounds; used in ⁴⁵Sc NMR spectroscopy literature and condensed matter physics publications on Sc electronic structure.
Scandium metalFull commercial designation in REACH/RoHS documentation, ASTM standards, and procurement documents for Sc additions to Al alloys and SOFC electrolyte fabrication.
Scandium elementUsed in academic databases, Sc(OTf)₃ catalysis literature, and geochemistry texts discussing Sc as a mantle-compatible trace element and its use as a geochemical proxy for mafic/ultramafic source rocks.
Scandium transition metalClassification designating Sc as a Group 3 transition metal; used in inorganic chemistry and materials science texts distinguishing Sc from the lanthanides (which it resembles chemically) and from typical d-block transition metals (which it resembles structurally).
Scandium rare earth elementTrade and regulatory classification; Sc is grouped with the REEs on EU and US critical materials lists due to supply concentration risk (primary production from limited uranium tailings and thortveitite sources) and growing demand from Al-Sc, ScSZ, and ScAlN applications.
Element 21Periodic table designation used in XRF/ICP-MS software, nuclear data libraries (⁴⁵Sc thermal neutron cross-section and ⁴⁶Sc activation tracer data), and reactor physics codes.
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