Zirconium

Zirconium — Material Page

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Zirconium (Zr, atomic number 40) is a Group 4 HCP transition metal with a melting point of 1,855 °C, density of 6.52 g/cm³, and the lowest thermal neutron absorption cross-section of any structural metal (σ_th ≈ 0.18 barn) — the property that makes it the indispensable cladding material for nuclear fuel rods in water-cooled reactors worldwide. Zr is relatively abundant (~130 ppm crustal), sourced as zircon (ZrSiO₄) from heavy mineral sand deposits in Australia, South Africa, and Mozambique, and is chemically inseparable from hafnium (Hf, ~1–3% in zircon) due to lanthanide contraction; nuclear-grade Zr requires Hf removal to <100 ppm (Hf σ_th ≈ 102 barn), accomplished by liquid-liquid extraction or fractional distillation of ZrCl₄/HfCl₄. Commercial Zr metal is produced by the Kroll process (ZrCl₄ + Mg, ~850 °C, Ar atmosphere) — the same process as Ti. Zr undergoes an α→β allotropic transformation at 863 °C (HCP→BCC), the basis of Zr alloy metallurgy.

Nuclear applications account for ~90% of Zr metal consumption (~6,000 tonnes/year), where Zircaloy-2 (Zr-1.5Sn-0.15Fe-0.10Cr-0.05Ni) and Zircaloy-4 (Zr-1.5Sn-0.20Fe-0.10Cr, Ni-free for PWR) clad UO₂ fuel pellets in light water reactors — over one million fuel rod lengths worldwide. Zr cladding provides structural containment, low neutron parasitic absorption, and adequate corrosion resistance in high-pressure (7–15 MPa) water at 300–350 °C. Advanced cladding alloys (ZIRLO, M5, E110) with lower Sn and higher Nb content offer improved corrosion and creep resistance for extended burnup. Zr hydride formation (ZrH₂, "hydrogen embrittlement") during reactor operation is a degradation mechanism managed by alloy composition, heat treatment, and water chemistry control.

Zirconia (ZrO₂) is the most important Zr compound by volume, with ~1 million tonnes/year consumed primarily as a refractory (monoclinic ZrO₂, mp 2,715 °C) for steelmaking furnace linings, and as stabilized ZrO₂ (yttria-stabilized, YSZ) for thermal barrier coatings, solid oxide fuel cell electrolytes, and structural ceramics. YSZ (8 mol% Y₂O₃) has among the highest fracture toughness of any ceramic (~6–10 MPa·m½ in TZP) via transformation toughening (tetragonal→monoclinic martensitic transformation at crack tips). YSZ thermal barrier coatings (TBCs, 100–300 µm, plasma-sprayed or EB-PVD) on Ni superalloy turbine blades reduce metal temperature by 100–200 °C in gas turbine hot sections, enabling higher firing temperatures and efficiency. Dental YSZ (3Y-TZP, 5Y-PSZ) is the dominant all-ceramic crown and bridge material due to its opacity-tunable translucency, high strength (~900–1,200 MPa), and excellent biocompatibility.

General Properties

Property	Value	Notes
Atomic Number	40	Group 4, Period 5; 4d²5s²; dominant oxidation state +4 (ZrO₂, ZrCl₄, zirconates); +3 is rare and strongly reducing. Zr⁴⁺ is a hard Lewis acid that coordinates strongly with O-donors — the basis of Zr MOF catalysts (UiO-66) and Zr-based phosphonate ion exchangers.
Atomic Mass	91.224 u	Five naturally occurring isotopes: ⁹⁰Zr (51.45%), ⁹¹Zr (11.22%, NMR-active), ⁹²Zr (17.15%), ⁹⁴Zr (17.38%, Stable), ⁹⁶Zr (2.80%, Stable). The Zr-Nb geochronometer (⁹²Zr from ⁹²Nb decay, t½ = 35 Myr) constrains early solar system nucleosynthesis timescales.
Density (20 °C)	6.52 g/cm³	Moderate density — comparable to vanadium (6.11) and lighter than most refractory metals. Zr's low density combined with adequate strength and exceptional neutron transparency makes it the only practical structural material for nuclear fuel cladding.
Melting Point	1,855 °C (2,128 K)	Refractory-class melting point; Zr is processed by vacuum arc remelting (VAR) or electron beam cold-hearth melting to avoid O, N, H contamination. Nuclear-grade Zr ingot must meet stringent Hf limits (<100 ppm) verified by ICP-MS before fabrication into cladding tube.
Boiling Point	4,409 °C	High boiling point; Zr is used as a getter material in vacuum tubes and specialty gas purification — hot Zr reacts with and getters O₂, N₂, H₂, and CO at temperatures above ~300 °C, similar to Ti and V getter applications.
Thermal Conductivity	22.7 W/m·K	Low thermal conductivity for a metal — similar to Ti (21.9 W/m·K). In nuclear fuel rod design, Zr cladding thermal conductivity must be accounted for in fuel temperature calculations; the limiting thermal resistance is the UO₂ fuel pellet (k ~ 3–8 W/m·K), not the Zr cladding.
Electrical Resistivity	421 nΩ·m (20 °C)	Source value of 42 nΩ·m is missing a digit — corrected to 421 nΩ·m per NIST data. High resistivity for a metal (comparable to Ti at 420 nΩ·m). Zr is a superconductor below Tc = 0.61 K, studied in early superconductor research but superseded by higher-Tc materials.
Crystal Structure	α-Zr: HCP, a = 3.232 Å, c = 5.147 Å (RT to 863 °C); β-Zr: BCC above 863 °C	The α→β transformation at 863 °C is the basis of Zircaloy texture control: thermomechanical processing in the α field develops strong basal-pole textures that minimize diametral creep of fuel cladding tubes during irradiation (important for reactor core geometry maintenance).

Mechanical Properties

Property	Value	Notes
Tensile Strength	370–500 MPa	Adequate strength for nuclear cladding and chemical process applications; strength is strongly affected by interstitial O content (O is an α-stabilizer that significantly solid-solution strengthens Zr). Zircaloy-4 has ~450 MPa UTS in the cold-worked stress-relieved condition used for LWR cladding.
Yield Strength	240–380 MPa	Sufficient for pressure-vessel and cladding applications at reactor operating temperatures (~300–350 °C). Irradiation hardening in-reactor raises yield strength significantly but reduces ductility — a key consideration in spent fuel storage and transport safety assessments.
Young's Modulus	88 GPa	Lower modulus than Ti (116 GPa) and most refractory metals. The low modulus combined with HCP anisotropy means Zircaloy cladding deformation under internal fuel pellet pressure (pellet-cladding interaction, PCI) is an important reactor safety consideration.
Hardness	~150 HV (annealed CP Zr)	Source value of "Vickers ~560" is inconsistent with published data for Zr (typical ~100–160 HV for annealed CP Zr; 560 HV is closer to ZrN or ZrC hard coatings) — corrected here. Hardness increases with O content and cold work; Zircaloy in the cold-worked condition reaches ~200–220 HV.
Elongation at Break	15–45%	Good ductility in high-purity form, maintaining toughness to cryogenic temperatures. Ductility is reduced by hydrogen pickup (hydride embrittlement) during reactor operation — a key degradation mechanism managed by alloy composition and reactor water chemistry.
Poisson's Ratio	0.34	Used in finite element models of fuel cladding mechanical behavior under pellet-cladding interaction loads and in stress analysis of Zr chemical process vessels under pressure and thermal cycling.

Chemical Properties

Property	Value / Behavior	Notes
Oxidation States	+4 (dominant: ZrO₂, ZrCl₄, zirconates); +3 rare	Zr⁴⁺ is a hard Lewis acid with strong affinity for hard O-donors — the basis of Zr MOF catalysts (UiO-66, Zr₆O₄(OH)₄ nodes) with exceptional hydrothermal stability, and Zr-based phosphonate/sulfonate ion exchange materials. Zirconates (BaZrO₃, SrZrO₃) are proton-conducting ceramics for high-temperature fuel cells.
Corrosion Resistance	Excellent; forms stable adherent ZrO₂ layer; resistant to most acids (except HF and hot concentrated H₂SO₄), alkalis, and seawater; resistant to high-pressure hot water to ~350 °C	Zr's corrosion resistance in hot water to 350 °C and high-pressure steam is uniquely suited to LWR reactor environments; uniform corrosion of Zircaloy cladding (~1–3 µm/year) is acceptable over fuel cycle lifetimes. Zr is attacked by HF (forms soluble ZrF₄²⁻ complexes) — used in Zr chemical processing and dissolution.
Surface Oxide	Zirconium dioxide (ZrO₂); monoclinic at RT, tetragonal above ~1,170 °C, cubic above ~2,370 °C	The ZrO₂ polymorphic transformations (monoclinic↔tetragonal, ~4% volume change) cause cracking in pure ZrO₂ ceramics on cooling — stabilized with Y₂O₃, MgO, or CaO to retain the tetragonal or cubic phase at room temperature for structural and electrolyte applications.

Identifier	Value
Symbol	Zr
Atomic Number	40
CAS Number	7440-67-7
UN Number	UN2858 (dry solid); UN2008 (dry powder); UN1932 (scrap); UN1358 (wet powder — at least 25% water)
EINECS Number	231-176-9

Isotope	Type	Notes
⁹⁰Zr	Stable	51.45% natural abundance — the most abundant Zr isotope; I = 0. ⁹⁰Zr has a large neutron capture cross-section compared to other Zr isotopes (σ_th = 0.014 barn for ⁹⁰Zr vs. 0.18 barn average), contributing disproportionately to the total low absorption. Used as the primary reference isotope in Zr isotope ratio measurements.
⁹¹Zr	Stable	11.22% natural abundance; I = 5/2, NMR-active. ⁹¹Zr NMR (chemical shift range ~700 ppm; quadrupolar broadening in low-symmetry environments) characterizes Zr coordination in MOFs (UiO-66 node geometry), zirconates, Zr-containing glasses, and sol-gel ZrO₂ precursors. Solid-state ⁹¹Zr NMR requires high-field instruments (>14 T) due to the low gyromagnetic ratio.
⁹²Zr	Stable	17.15% natural abundance; I = 0. Radiogenic ⁹²Zr (from now-extinct ⁹²Nb, t½ = 34.7 Myr) measured in early solar system materials constrains nucleosynthesis timescales and Nb/Zr fractionation in planetary mantles. ⁹²Zr is used as a reference isotope in Zr-Nb isotope geochemistry.
⁹⁴Zr	Stable*	17.38% natural abundance; I = 0; Stable* — double beta decay to ⁹⁴Mo is energetically allowed (Q = 1.144 MeV); t½ not yet measured but predicted >10²⁰ yr. ⁹⁴Zr(n,γ)⁹⁵Zr (σ = 0.050 barn) produces ⁹⁵Zr (t½ = 64.0 days), a fission product monitored in nuclear safeguards and atmospheric nuclear test fallout analysis.
⁹⁶Zr	Stable*	2.80% natural abundance; I = 0; Stable* — double beta decay to ⁹⁶Mo measured: 2νββ t½ = 2.0 × 10¹⁹ yr (NEMO-3 experiment). ⁹⁶Zr is the target of the ZICOS and SuperNEMO experiments searching for 0νββ decay (Q = 3.350 MeV, one of the highest Q-values among 2νββ nuclei, favorable for background suppression).

Scientific & Research Applications

Use Case	Form Typically Used	Description
Nuclear Cladding & Reactor Research	Zircaloy-2, Zircaloy-4, ZIRLO, M5 tubing; CP Zr (99.2%+) for irradiation studies	Zircaloy cladding research targets in-reactor corrosion, hydrogen uptake, irradiation growth, and pellet-cladding interaction under simulated LWR conditions. Out-of-pile autoclave testing in 360 °C water and 400 °C steam, complemented by in-reactor loop experiments, validates new alloy compositions for extended burnup operation.
ZrO₂ Ceramic & YSZ Research	Zr sputtering targets (99.2%+), ZrO₂/YSZ powder, Zr alkoxide sol-gel precursors	YSZ thin-film electrolytes (0.5–10 µm, deposited by ALD, PLD, or RF sputtering) are studied for micro-solid oxide fuel cells operating at 350–600 °C. YSZ thermal barrier coating research uses EBSD, TEM, and synchrotron diffraction to characterize phase stability, sintering, and thermally grown oxide (TGO) failure under cyclic thermal loading.
Zr MOF Catalysis Research	ZrCl₄ (precursor for UiO-66 and UiO-67 synthesis); Zr metal (99.2%) for dissolution	UiO-66 (Zr₆O₄(OH)₄(BDC)₆) and its derivatives have exceptional thermal stability (>500 °C) and hydrolytic stability — enabling catalytic applications in water. Research targets Lewis acid catalysis (epoxide ring-opening, Meerwein-Ponndorf-Verley reduction), photocatalytic CO₂ reduction with linker-sensitized Zr nodes, and drug loading/release from defect-engineered UiO-66.
Thin-Film & Sputtering Research	Zr sputtering targets (99.2–99.8%), Zr evaporation pellets	Zr thin films are deposited for ZrO₂ high-k dielectric gate oxides (k ~ 25, studied as HfO₂ alternatives in CMOS), ZrN hard coatings (PVD, ~2,300 HV, gold-colored), and ZrB₂ ultra-hard coatings for cutting tools. Zr also serves as a diffusion barrier and adhesion layer in metal interconnect stacks.
⁹⁶Zr Double Beta Decay Research	Enriched ⁹⁶Zr (depleted of other isotopes); ZrO₂ crystals or ZrCl₄ solutions	⁹⁶Zr is a 0νββ search target due to its high Q-value (3.350 MeV) and measured 2νββ t½. The ZICOS experiment uses ⁹⁶ZrO₂ crystals as scintillating bolometers at millikelvin temperatures; SuperNEMO investigates ⁹⁶Zr thin foils using tracking calorimetry to reconstruct individual electron trajectories from ββ decay events.

Industrial & Commercial Applications

Sector	Form / Grade Used	Description
Nuclear Fuel Cladding	Zircaloy-2 (BWR), Zircaloy-4 (PWR), ZIRLO, M5, E110 seamless tubing (~99.8% Zr, <100 ppm Hf)	Zircaloy cladding tubes (outer diameter ~9.5–11.2 mm, wall ~0.57–0.64 mm) contain UO₂ fuel pellets in LWR fuel assemblies — over 250,000 km of tubing in global nuclear fleet. The low neutron capture cross-section (~0.18 barn) reduces parasitic neutron loss, improving reactor efficiency. Hf removal to <100 ppm is mandatory for nuclear-grade Zr.
Chemical Processing Equipment	CP Zr (99.2%, ASTM B550/B551), Zr-702, Zr-705 plate, tube, fittings	Zr Gr.702 (CP Zr) and Gr.705 (Zr-2.5Nb) are used for reactors, heat exchangers, and piping in HCl, H₂SO₄, and organic acid environments where titanium and stainless steels corrode unacceptably. Zr offers corrosion rates <0.025 mm/year in boiling 20% HCl and is resistant to urea and acetic acid synthesis environments.
Zirconia Refractories & Ceramics	Monoclinic ZrO₂ (fused or sintered); YSZ (3–8 mol% Y₂O₃) powder/coating	Fused ZrO₂ refractories line steel continuous casting tundishes and slide gates where resistance to molten steel and slag at >1,600 °C is required. YSZ thermal barrier coatings (APS or EB-PVD) on Ni superalloy turbine blades reduce metal temperature by 100–200 °C in gas turbine hot sections, enabling higher combustion temperatures and fuel efficiency.
Dental & Biomedical Ceramics	3Y-TZP, 5Y-PSZ dental discs (zirconia milled by CAD/CAM); Zr implant abutments	Dental zirconia (3Y-TZP, ~900–1,200 MPa flexural strength) is machined by CAD/CAM milling from pre-sintered blanks for all-ceramic crowns, bridges, and implant-supported prostheses. 5Y-PSZ offers higher translucency (cubic-rich phase) for anterior esthetic restorations. Zr biocompatibility matches Ti for implant applications with the added advantage of a tooth-colored appearance.
Alloying & Surface Coatings	Zr metal additions; ZrN/ZrO₂ PVD coatings	Small Zr additions (<0.5 wt%) refine grain size and improve creep resistance in Mg alloys (ZK60, ZE41) and Cu alloys; Zr is a strong carbide/nitride former used as a microalloying element in high-strength steels. ZrN coatings (PVD, ~2,300 HV, gold color) are used on cutting tools, medical instruments, and decorative hardware as a TiN alternative.

Purity	Description
99.2% (2N2)	Standard purity zirconium used for general industrial applications, corrosion-resistant alloys, and moderate research use.
99.8% (2N8)	High-purity zirconium ideal for demanding applications such as nuclear-grade materials, advanced coatings, and biomedical devices.

Synonym / Alternative Name	Context
Zirconium metal	Commercial designation for elemental Zr in rod, tube, sheet, sponge, or target form; used in ASTM standards (B550 bar, B551 sheet, B523 tube), nuclear fuel cladding procurement specifications, and chemical process equipment documentation (ASME UNS R60702).
Zr metal	Abbreviated commercial designation used interchangeably with "zirconium metal" in materials datasheets, sputtering target catalogs, and nuclear industry supply chain documentation where brevity is preferred.
Elemental Zirconium	Scientific designation distinguishing the pure element from ZrO₂, ZrCl₄, zirconates, and Zircaloy alloys; used in nuclear physics literature (neutron cross-section data), isotope geochemistry, and materials science research specifying the metallic form.
Element 40	Periodic table designation; used in XRF/ICP-MS analytical software and nuclear engineering databases (ENDF/B nuclear data library) where the atomic number is the primary element identifier.
Zirkonium	German language name; used in German scientific literature, DIN standards, and industrial documentation in German-speaking markets with significant nuclear and chemical industries (Germany, Austria, Switzerland).
Zirconio	Spanish and Italian language name; used in scientific literature and industrial documentation in Spanish- and Italian-speaking markets; Italy has significant zirconia ceramics and dental materials manufacturing.

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