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Cadmium
Cadmium (Cd, atomic number 48) is a soft, ductile, bluish-white post-transition metal in Group 12 of the periodic table, sitting below zinc and above mercury in the same column and sharing many of their chemical characteristics. With a density of 8.65 g/cm³ and a melting point of 321.1 °C, cadmium is significantly denser than zinc (7.13 g/cm³) yet melts at a comparably low temperature. It adopts a hexagonal close-packed (HCP) crystal structure with a notably elongated c/a ratio (1.886 vs. the ideal 1.633), reflecting distorted interlayer bonding. Cadmium has eight stable isotopes spanning mass numbers 106–116, the widest stable isotope spread of any element, and ¹¹³Cd has a thermal neutron capture cross-section of 20,600 barn — the highest of any stable isotope — making cadmium the most effective thermal neutron absorber per atom of any naturally occurring element. Its electrochemical standard potential (–0.40 V vs. SHE) places it between zinc and nickel, underpinning its use in electroplating and battery electrochemistry.
Cadmium's most commercially significant properties are its corrosion resistance in alkaline and marine environments, its electrochemical reversibility, and its ability to form II-VI compound semiconductors with exceptional optoelectronic properties. Cadmium telluride (CdTe) has a direct bandgap of 1.45 eV — essentially ideal for single-junction solar energy conversion under the AM1.5 solar spectrum — and CdTe thin-film modules are the second most deployed photovoltaic technology globally after silicon, with record cell efficiencies above 22%. Cadmium sulfide (CdS) and cadmium selenide (CdSe) quantum dots exhibit size-tunable fluorescence across the visible spectrum with narrow emission linewidths, exploited in biological imaging, display technology (QLED), and single-photon sources. Cadmium zinc telluride (CdZnTe, CZT) is the leading room-temperature semiconductor radiation detector material for gamma-ray and X-ray spectroscopy in medical imaging, nuclear security, and astrophysics.
Despite increasing regulatory pressure from RoHS, REACH, and similar legislation limiting cadmium in consumer products, its unique combination of properties sustains demand in several irreplaceable applications. Nickel-cadmium (NiCd) batteries remain the chemistry of choice in aircraft emergency systems, power tools, and industrial UPS applications where wide operating temperature range (–40 to +60 °C), overcharge tolerance, and cycle life exceeding 1,000 deep cycles are required. Cadmium electroplating (typically 5–25 µm) provides sacrificial corrosion protection with exceptional performance in salt spray, marine, and hydrogen-embrittlement-prone environments — it remains the aerospace standard for high-strength steel fasteners where zinc plating is inadequate. Cadmium sulfoselenide and cadmium mercury sulfide pigments (Pigment Yellow 35, Orange 20, Red 108) offer outstanding lightfastness, heat stability to 600 °C, and acid resistance for artists' colors, engineering plastics, and high-temperature ceramic glazes where no organic pigment can match their durability.
General Properties
| Property | Value | Notes |
|---|---|---|
| Atomic Number | 48 | Group 12, Period 5; post-transition metal; chemically similar to zinc (above) and mercury (below) in the same group |
| Atomic Mass | 112.414 u | Eight stable isotopes from ¹⁰⁶Cd to ¹¹⁶Cd — the widest stable isotope spread of any element; ¹¹⁴Cd is the most abundant (28.73%) |
| Density (20 °C) | 8.65 g/cm³ | ~21% denser than zinc (7.13 g/cm³); ~12× lighter than mercury; used as a dense sacrificial coating for steel rather than a structural metal |
| Melting Point | 321.1 °C (594.2 K) | Low melting point accessible with standard electric furnaces; enables low-energy casting and electrodeposition from aqueous solution at room temperature |
| Boiling Point | 767 °C (1,040 K) | Narrow liquid range (~446 °C); cadmium vapor is highly toxic (IDLH 9 mg/m³) — fume extraction essential during any high-temperature processing or welding of cadmium-plated parts |
| Thermal Conductivity | 96.6 W/m·K | Moderate — about 40% that of copper; adequate for thermal management in battery electrodes and electroplated heat-sink coatings |
| Electrical Resistivity | 72.7 nΩ·m (20 °C) | Higher than zinc (59 nΩ·m) and copper (17 nΩ·m); sufficient for electrode and contact applications where conductivity is secondary to corrosion resistance |
| Crystal Structure | Hexagonal Close-Packed (HCP) | Distorted HCP with c/a = 1.886 (ideal: 1.633); the elongated axis reflects weaker interlayer bonding and contributes to cadmium's softness and easy cleavage parallel to the basal plane |
Mechanical Properties
| Property | Value | Notes |
|---|---|---|
| Hardness | Brinell ~20 HB; Mohs 2.0 | Very soft — easily cut with a knife; comparable to tin and lead; not used structurally, but softness aids deposition in smooth, adherent electroplated coatings |
| Elastic (Young's) Modulus | 50 GPa | Low stiffness — about ¼ that of steel; highly compliant thin coatings conform to substrate surface irregularities, contributing to good adhesion in electroplated layers |
| Poisson's Ratio | 0.30 | Typical for metallic systems; relevant to stress analysis of cadmium-plated fatigue-critical aerospace components |
| Tensile Strength | ~65 MPa (annealed) | Very low — cadmium is used exclusively as a coating or electrode material, never as a load-bearing structural component |
Thermal & Environmental Properties
| Property | Value | Notes |
|---|---|---|
| Corrosion Resistance | High in alkaline and marine environments | Cadmium plating provides sacrificial galvanic protection to steel in salt spray, marine, and high-humidity environments; superior to zinc plating in alkaline conditions and at elevated temperatures; standard aerospace fastener coating per MIL-STD-870 |
| Oxidation States | +2 (exclusively) | Cd²⁺ is the only stable oxidation state; no +1 or higher states under normal conditions; d¹⁰ electronic configuration gives stable closed-shell chemistry analogous to zinc |
| Neutron Capture | ¹¹³Cd: 20,600 barn (thermal) | Highest thermal neutron capture cross-section of any stable nuclide; the ¹¹³Cd(n,γ)¹¹⁴Cd reaction is used in nuclear reactor control rods and neutron shielding; 1 mm of cadmium sheet attenuates essentially all thermal neutrons |
| Toxicology | IARC Group 1 carcinogen; highly toxic | Cadmium accumulates in kidneys causing nephrotoxicity (Itai-itai disease was caused by Cd contamination of rice in Japan); TLV-TWA 0.01 mg/m³ (ACGIH); cadmium compounds are restricted by RoHS, REACH, and numerous national regulations in consumer products |
Chemical Properties
| Property | Value / Behavior | Notes |
|---|---|---|
| Surface Oxide | CdO (brown-black, rock salt structure) | Cadmium oxide forms a thin protective layer in air; CdO is an n-type semiconductor (Eg ~2.2 eV) with high carrier mobility used in transparent conducting electrodes and gas sensors |
| Acid Resistance | Dissolves in dilute HCl, H₂SO₄, HNO₃ | Reacts with mineral acids forming Cd²⁺ salts; resistant to alkalis — does not dissolve in NaOH unlike zinc (no amphoteric behavior); resistant to dry organic acids |
| Electrochemical Potential | –0.40 V vs. SHE (Cd²⁺/Cd) | Standard electrode potential places cadmium between zinc (–0.76 V) and nickel (–0.26 V); the basis of NiCd battery chemistry (1.2 V nominal cell voltage) and cadmium's role as a sacrificial anode for steel protection |
| Semiconductor Compounds | CdTe (Eg 1.45 eV), CdS (2.42 eV), CdSe (1.74 eV) | II-VI compound semiconductors with direct bandgaps spanning the visible and near-IR; CdTe is the basis of commercial thin-film solar cells; CdSe quantum dots exhibit size-tunable fluorescence; CdZnTe is the leading room-temperature gamma-ray detector material |
| Identifier | Value |
|---|---|
| Symbol | Cd |
| Atomic Number | 48 |
| CAS Number | 7440-43-9 |
| UN Number | UN2570 (powder) |
| EINECS Number | 231-152-8 |
| Isotope | Type | Notes |
|---|---|---|
| ¹⁰⁶Cd | Stable | 1.25% natural abundance; lightest stable cadmium isotope; used in enriched form for isotope dilution mass spectrometry (IDMS) as a spike isotope in environmental cadmium analysis |
| ¹⁰⁸Cd | Stable | 0.89% natural abundance; least abundant stable Cd isotope; used in MC-ICP-MS isotope ratio measurements for geochemical tracing |
| ¹¹⁰Cd | Stable | 12.49% natural abundance; I = 0; used in nuclear structure research as a benchmark for vibrational and rotational collective nuclear models |
| ¹¹¹Cd | Stable | 12.80% natural abundance; I = 1/2, NMR-active; ¹¹¹Cd NMR is a sensitive probe of cadmium coordination environments in metalloproteins and inorganic complexes |
| ¹¹²Cd | Stable | 24.13% natural abundance; most abundant stable isotope after ¹¹⁴Cd; used in Mössbauer spectroscopy studies via its daughter ¹¹²Sn |
| ¹¹³Cd | Stable* | 12.22% natural abundance; thermal neutron capture cross-section 20,600 barn — the highest of any stable nuclide; the basis of cadmium's use in nuclear reactor control rods and neutron shielding; technically beta-radioactive (t½ = 7.7 × 10¹⁵ yr) but treated as stable |
| ¹¹⁴Cd | Stable | 28.73% natural abundance; most abundant stable isotope; I = 0; used as the primary reference isotope in cadmium isotope ratio studies by MC-ICP-MS |
| ¹¹⁶Cd | Stable* | 7.49% natural abundance; double beta decay candidate (t½ > 2.8 × 10¹⁹ yr); used in neutrino physics experiments (CAMEO, COBRA) searching for neutrinoless double beta decay to probe the Majorana nature of neutrinos |
| ¹⁰⁹Cd | Radioactive | t½ = 461.4 days (electron capture); emits 88 keV gamma via ¹⁰⁹Ag daughter; widely used as an X-ray fluorescence excitation source in portable XRF analyzers and as an energy calibration standard for gamma-ray detectors |
Scientific & Research Applications
| Use Case | Form Typically Used | Description |
|---|---|---|
| CdTe & CdZnTe Radiation Detectors | CdZnTe single crystals, CdTe wafers | Cadmium zinc telluride (CZT) is the leading room-temperature semiconductor radiation detector for gamma-ray and X-ray spectroscopy. With a bandgap of ~1.6 eV, high atomic number (Z = 48/52), and electron mobility-lifetime product adequate for sub-cm drift lengths, CZT enables compact, high-resolution gamma cameras for nuclear medicine (SPECT), nuclear security portal monitors, and hard X-ray astrophysics instruments (NuSTAR, INTEGRAL). |
| Quantum Dot Research & Displays | CdSe/ZnS core-shell nanocrystals, CdS quantum dots | CdSe quantum dots exhibit size-tunable photoluminescence (450–650 nm) with quantum yields exceeding 90% in core-shell (CdSe/ZnS) architectures. Used extensively in bioimaging as fluorescent labels, in QLED display color-conversion layers, and as model systems in quantum optics. Emission linewidths of 20–30 nm FWHM enable precise color rendering beyond NTSC gamut in display applications. |
| Neutron Absorption & Shielding | Cd sheet, Cd-lined collimators, Cd foil | ¹¹³Cd's thermal neutron capture cross-section of 20,600 barn makes 1 mm cadmium sheet essentially opaque to thermal neutrons. Used in neutron beam collimators, Cd-difference activation analysis (to separate thermal from epithermal neutron contributions), neutron radiography beam stops, and as a thermal neutron cutoff filter in neutron scattering instruments at research reactors. |
| XRF Excitation Sources | ¹⁰⁹Cd sealed radioactive source | ¹⁰⁹Cd (t½ = 461 days) decays by electron capture to ¹⁰⁹Ag*, emitting an 88 keV gamma ray and Ag Kα/Kβ X-rays at 22–25 keV — ideal for exciting K-lines of elements from Ca to Sn in portable XRF analyzers used in mining, environmental monitoring, and art authentication. |
| Photovoltaic Research | CdTe sputtering targets, CdS buffer layer precursors | CdTe (Eg = 1.45 eV, direct bandgap) is among the most theoretically efficient single-junction photovoltaic absorbers under AM1.5 illumination. Research focuses on grain boundary passivation, back-contact ohmic junction formation, and CdSeTe alloying to raise Voc. Laboratory cells have exceeded 22% efficiency; commercial module efficiencies typically 18–20%. |
| Isotope Geochemistry | Enriched Cd isotope spike solutions, high-purity Cd standards | Cadmium isotope fractionation (δ¹¹⁴/¹¹⁰Cd) measured by MC-ICP-MS is a tracer of biological cadmium cycling in the ocean — cadmium is a micro-nutrient surrogate for zinc in phytoplankton, and Cd isotope ratios in sediment cores record past ocean productivity and zinc availability, providing a paleoproductivity proxy. |
Industrial & Commercial Applications
| Sector | Form / Compound Used | Description |
|---|---|---|
| Aerospace Electroplating | Electrodeposited Cd (5–25 µm), LHE Cd plating | Cadmium electroplating provides sacrificial galvanic protection to high-strength steel fasteners, landing gear components, and hydraulic fittings in aerospace and defense applications. Superior to zinc plating in alkaline environments, at elevated temperatures, and in preventing hydrogen embrittlement of high-strength steels (per MIL-DTL-14072 and AMS 2400). Low-hydrogen embrittlement (LHE) cadmium processes are specified for steels above 220 ksi tensile strength. |
| Nickel-Cadmium Batteries | Cd(OH)₂ / Cd powder electrode | NiCd batteries offer a flat 1.2 V discharge curve, tolerance of overcharge and over-discharge, a wide operating temperature range (–40 to +60 °C), and cycle life exceeding 1,000 deep cycles — making them the chemistry of choice for aircraft emergency systems (FAA-required), industrial UPS, power tools, and portable medical equipment. Cadmium hydroxide forms the negative electrode active material via the reversible Cd(OH)₂ + 2e⁻ ⇌ Cd + 2OH⁻ reaction. |
| CdTe Thin-Film Solar Modules | CdTe / CdS bilayer (vapor transport deposition) | CdTe modules (produced commercially by First Solar and others) are the second most deployed PV technology globally, with ~5% world market share. The CdS/CdTe heterojunction is formed by close-space sublimation or vapor transport deposition; module manufacturing costs are among the lowest in the PV industry. Life-cycle analyses show that cadmium in CdTe modules is more environmentally benign than many alternatives due to the high stability of CdTe. |
| Cadmium Pigments | CdS (yellow), CdSe (red/orange), CdS/CdSe solid solutions | Cadmium sulfide (Pigment Yellow 35) and cadmium sulfoselenide (Pigment Orange 20, Red 108) offer outstanding lightfastness (ASTM I), heat stability to 600 °C, and resistance to acids and alkalis. Used in artists' oil and acrylic colors, high-temperature engineering plastics (polyamides, polyesters), ceramic glazes and glass colorants, and automotive coatings where organic pigments fade or degrade. Subject to increasing regulatory restriction in consumer goods but remain approved for professional and industrial use. |
| Low-Melting Fusible Alloys | Cd-Bi-Sn-Pb alloys (Wood's metal variants) | Cadmium-containing fusible alloys melt at 60–90 °C and are used in fire sprinkler heads, boiler safety plugs, and prototype tooling. Cadmium's contribution — typically 10–17 wt% — lowers the melting point and improves castability. Use is declining due to toxicity regulations, with bismuth-based alloys replacing many applications. |
| Bearing Alloys | Cd-Ni, Cd-Cu, Cd-Ag alloys | Cadmium-based bearing alloys (e.g. Cd-1.5%Ni, Cd-0.75%Cu-0.75%Ag) offer low friction, high fatigue strength, and resistance to seizure under high load and temperature — used in aircraft engine bearings and heavy-duty diesel engine applications where conventional babbitt metals are inadequate. |
| Purity | Main Use |
|---|---|
| 99.7% | General industrial use, pigments, and electroplating — suitable for cadmium sulfide and sulfoselenide pigment synthesis, bulk electroplating bath preparation, and low-melting fusible alloy production where trace impurities do not significantly affect end performance |
| 99.9% | Battery electrodes and precision coatings — appropriate for NiCd battery electrode paste production and aerospace electroplating processes where controlled impurity levels ensure consistent electrochemical behavior and coating adhesion |
| 99.95% | Semiconductors and optoelectronic devices — used in CdS and CdSe synthesis for photodetectors and quantum dot precursors where sub-100 ppm impurities are required to avoid non-radiative recombination centers and maintain photoluminescence quantum yield |
| 99.99% | Photovoltaics and thin-film deposition — the standard purity for CdTe and CdZnTe sputtering targets and evaporation sources; minimizes electrically active impurities that would reduce minority carrier lifetime and degrade solar cell open-circuit voltage and fill factor |
| 99.999% | Analytical standards and neutron absorption studies — used in preparation of primary standard solutions for ICP-MS and atomic absorption spectrometry, CdZnTe single-crystal growth for radiation detectors, and isotopically enriched ¹¹³Cd neutron absorber fabrication where maximum purity is essential |
| Synonym / Alternative Name | Context |
|---|---|
| Cd | Chemical symbol |
| Cadmium metal | Standard commercial and regulatory designation for the elemental form |
| Elemental cadmium | General scientific term distinguishing the pure metal from cadmium compounds (CdTe, CdS, CdO, Cd(OH)₂, etc.) |
| Kadmium | German and Scandinavian language equivalent; the name derives from the Latin cadmia (calamine, ZnCO₃), reflecting cadmium's discovery as an impurity in zinc carbonate ores by Stromeyer and Hermann in 1817 |
| Cadmio | Spanish and Italian language equivalent |
| Cadmium blanche | Historical French term for cadmium sulfate used as a white pigment precursor; distinguished from cadmium jaune (yellow, CdS) in historical pigment literature |