Zinc oxide ZnO

spore: profile
id: zinc-oxide
spore_version: 0.2
family: oxide-ceramics
name: Zinc oxide
abbreviation: ZnO
role: wide-bandgap oxide for thermal-control coatings, varistors and transparent conductors
category: ceramics
type: oxides
summary: |
  Zinc oxide (ZnO) is a white oxide that crystallizes in the hexagonal wurtzite structure. Its melting point is 1975 °C, and a review of its physical constants gives a density of 5.67 g/cm3 and a band gap of 3.37 eV at 300 K. In space it is used as a pigment in a potassium silicate binder, sold as Z-93. NASA flew that white coating on the ISS and on LDEF, and its solar absorptance stayed near 0.14 to 0.15 after years in orbit, unless silicone contamination was present. Ceramic zinc oxide is also the base of metal-oxide varistors for surge protection, with typical breakdown fields of 2 to 5 kV/cm. Aluminium-doped (AZO) and gallium-doped (GZO) zinc oxide films are transparent conductors: a 2004 sputtered AZO film reached 0.00037 ohm cm with more than 85% visible transmission, and a 2023 sputtered GZO film reached 29 ohm per square at 94% transmission.
best_for: Low-absorptance white thermal-control coating on spacecraft, where flight data show little change in solar absorptance over years when contamination is kept low
weakest_at: Silicone contamination, which raised the solar absorptance of a Z-93P coating from 0.16 to 0.23 in one flight test (POSA-I)
identity:
  formula: ZnO
  cas: 1314-13-2
  crystal_system: hexagonal
  space_group: P63mc
  source: arxiv-zno-2011
  where: Introduction, paragraph on the wurtzite structure (hexagonal cell, space group P63mc); CAS number printed in Section 1.1 of the U.S. Zinc SDS (sds-usz-zno)
tags:
  environments:
    - low-earth-orbit
  stresses:
    - atomic-oxygen
    - uv
  applications:
    - thermal-control
    - electronics-and-chips
properties:
  - benchmark: density
    label: Density, wurtzite ZnO (review value)
    value: 5.67
    unit: g/cm3
    scope: material
    conditions:
      crystal: wurtzite
      temperature: not stated in the review
    method: Value stated in an optical-properties review of ZnO
    status: ground
    source: arxiv-zno-2011
    where: Introduction, paragraph beginning "Other characteristics of ZnO"
  - benchmark: melting_point
    label: Melting point (review value)
    value: 1975
    unit: °C
    scope: material
    conditions:
      crystal: wurtzite
    method: Value stated in an optical-properties review of ZnO
    status: ground
    source: arxiv-zno-2011
    where: Introduction, paragraph beginning "Other characteristics of ZnO"
  - benchmark: melting_point
    label: Melting point, U.S. Zinc SDS (zinc oxide, all grades)
    value: 1975
    unit: °C
    scope: material
    conditions:
      grade: all product grades, as stated in the SDS
    method: Value printed in a supplier safety data sheet
    status: ground
    source: sds-usz-zno
    where: Section 9 (Physical and Chemical Properties), Melting point row
  - benchmark: vapor_pressure
    label: Vapour pressure of zinc oxide
    value: 12
    unit: mm Hg
    scope: material
    conditions:
      temperature: 1500 °C
    method: Value printed in a supplier safety data sheet
    status: ground
    source: sds-usz-zno
    where: Section 9 (Physical and Chemical Properties), Vapor Pressure row
  - benchmark: thermal_conductivity
    label: Thermal conductivity, wurtzite ZnO at room temperature
    value: 50
    unit: W/mK
    scope: material
    conditions:
      crystal: wurtzite, single crystal (first-principles lattice dynamics)
      temperature: room temperature
    method: Calculated in the paper by first-principles lattice dynamics; the abstract gives the value and compares it with 400 W/mK for wurtzite GaN
    status: calculated
    note: A calculated value, not a measurement. The paper says earlier literature values for ZnO disagree widely.
    source: srep-zno-2016
    where: Abstract (comparison with wurtzite GaN); main text, thermal conductivity results
  - benchmark: band_gap
    label: Band gap, ZnO at 300 K
    value: 3.37
    unit: eV
    scope: material
    conditions:
      temperature: 300 K
    method: Value stated in an optical-properties review of ZnO
    status: ground
    note: The same review gives 3.437 eV at 1.6 K.
    source: arxiv-zno-2011
    where: Introduction, paragraph beginning "Other characteristics of ZnO"
  - benchmark: exciton_binding_energy
    label: Exciton binding energy, ZnO
    value: 60
    unit: meV
    scope: material
    conditions:
      crystal: wurtzite
    method: Value stated in an optical-properties review of ZnO
    status: ground
    source: arxiv-zno-2011
    where: Introduction, paragraph beginning "Other characteristics of ZnO"
  - benchmark: dielectric_constant
    label: Low-frequency dielectric constant, parallel to c axis
    value: 8.75
    unit: '-'
    scope: material
    conditions:
      direction: parallel to the c axis (printed as ||)
      frequency: low frequency
    method: Value stated in an optical-properties review of ZnO
    status: ground
    source: arxiv-zno-2011
    where: Introduction, paragraph beginning "Other characteristics of ZnO"
  - benchmark: dielectric_constant
    label: Low-frequency dielectric constant, perpendicular to c axis
    value: 7.8
    unit: '-'
    scope: material
    conditions:
      direction: perpendicular to the c axis (printed as perpendicular sign)
      frequency: low frequency
    method: Value stated in an optical-properties review of ZnO
    status: ground
    source: arxiv-zno-2011
    where: Introduction, paragraph beginning "Other characteristics of ZnO"
  - benchmark: dielectric_constant
    label: High-frequency dielectric constant, parallel to c axis
    value: 3.75
    unit: '-'
    scope: material
    conditions:
      direction: parallel to the c axis (printed as ||)
      frequency: high frequency
    method: Value stated in an optical-properties review of ZnO
    status: ground
    source: arxiv-zno-2011
    where: Introduction, paragraph beginning "Other characteristics of ZnO"
  - benchmark: lattice_constant_a
    label: Lattice constant a, wurtzite ZnO
    value: 3.25
    unit: Å
    scope: material
    conditions:
      crystal: wurtzite, hexagonal cell
    method: Value stated in an optical-properties review of ZnO
    status: ground
    source: arxiv-zno-2011
    where: Introduction, paragraph beginning "Other characteristics of ZnO"
  - benchmark: lattice_constant_c
    label: Lattice constant c, wurtzite ZnO
    value: 5.207
    unit: Å
    scope: material
    conditions:
      crystal: wurtzite, hexagonal cell
    method: Value stated in an optical-properties review of ZnO
    status: ground
    source: arxiv-zno-2011
    where: Introduction, paragraph beginning "Other characteristics of ZnO"
  - benchmark: solar_absorptance
    label: Solar absorptance, Z-93 zinc oxide coating, LDEF control and exposed samples
    value: 0.14
    value_high: 0.15
    bound: range
    unit: '-'
    scope: material
    conditions:
      coating: Z-93, zinc oxide pigment in potassium silicate binder (a coating, not bare ZnO)
      exposure: LDEF, about 5.8 years in space, sample S0069 (pre-flight 0.14, post-flight 0.15)
      contamination: minimal (300 Å or less, per the report)
    method: Value printed in the text of a NASA MISSE thermal-control review
    status: flight
    note: The report states that the Z-93 family showed very little change in solar absorptance and no change in infrared emittance after 5.8 years in space.
    source: nasa-misse-2008
    where: Section A (Zinc Oxide/Potassium Silicate), text on the LDEF samples
  - benchmark: solar_absorptance
    label: Solar absorptance, Z-93 on MISSE-2, MISSE-3 and MISSE-4, post-flight
    value: 0.14
    value_high: 0.15
    bound: range
    unit: '-'
    scope: material
    conditions:
      coating: Z-93 on aluminium, 5 mil layer (the report says normally applied to aluminium at 5 mils)
      exposure: ISS MISSE-2, MISSE-3 and MISSE-4, 1 to 4 years, UV 1,200 to 5,660 ESH
      pre_flight: 0.14
    method: Pre- and post-flight solar absorptance printed in Table 1
    status: flight
    source: nasa-misse-2008
    where: Table 1 (Alion Zinc Oxide/Potassium Silicate Coatings), Z-93 rows
  - benchmark: solar_absorptance
    label: Solar absorptance, Z-93P after silicone contamination (POSA-I)
    value: 0.23
    unit: '-'
    scope: material
    conditions:
      coating: Z-93P, zinc oxide pigment in potassium silicate binder with the Kasil 2130 binder
      contamination: about 5,000 Å of silicone on the space-facing side
      uv: 571 ESH
      pre_flight: 0.16
    method: Post-flight solar absorptance printed in the report text for the POSA-I sample
    status: flight
    note: The increase is a contamination effect; the same report says that with 300 Å or less of contamination, Z-93 keeps its beginning-of-life absorptance.
    source: nasa-misse-2008
    where: Section A (Zinc Oxide/Potassium Silicate), text on Z-93P from POSA-I; Figure 3
  - benchmark: optical_transmittance
    label: Average visible transmission, Al-doped ZnO film (AZO)
    value: 85
    bound: lower
    unit: '%'
    scope: material
    conditions:
      range: visible, 300 to 800 nm
      film: the same optimized AZO film
    method: Value printed in the abstract of a 2004 sputtering study
    status: ground
    source: stdj-azo-2004
    where: Abstract
  - benchmark: sheet_resistance
    label: Sheet resistance, Ga-doped ZnO film (GZO), after post-annealing at 450 °C
    value: 29
    unit: Ω/square
    scope: material
    conditions:
      dopant: gallium
      film_thickness: 100 nm
      deposition: RF sputtering, then post-annealing under nitrogen and hydrogen forming gas
      anneal_temperature: 450 °C
    method: Value printed in the abstract of a 2023 study on ZnO:Ga films
    status: ground
    note: Sheet resistance, not resistivity. The paper compares it with commercial indium tin oxide.
    source: mdpi-gzo-2023
    where: Abstract
  - benchmark: optical_transmittance
    label: Transparency, Ga-doped ZnO film (GZO), after post-annealing at 450 °C
    value: 94
    bound: lower
    unit: '%'
    scope: material
    conditions:
      film_thickness: 100 nm
      anneal_temperature: 450 °C
    method: Value printed in the abstract of a 2023 study on ZnO:Ga films
    status: ground
    source: mdpi-gzo-2023
    where: Abstract
  - benchmark: breakdown_field
    label: Typical varistor breakdown field, ZnO-based metal-oxide varistors
    value: 2
    value_high: 5
    bound: range
    unit: kV/cm
    scope: material
    conditions:
      material: ZnO-based metal-oxide varistor ceramics, breakdown voltage at 1 mA
      basis: typical range stated in the review
    method: Range stated in an open-access review of ZnO-V2O5 varistors (the review cites earlier work for it)
    status: ground
    source: mdpi-varistor-2023
    where: Section 1.1 (Specific Features of ZnO-Based Varistors), voltage rating paragraph
  - benchmark: nonlinear_coefficient
    label: Nonlinearity coefficient, doped ZnO varistor example
    value: 50
    unit: '-'
    scope: material
    conditions:
      composition: ZnO with 0.5% Bi2O3, 1% Sb2O3, 0.5% CoO, 0.5% MnO2 and 0.5% Cr2O3 (as printed in the review)
      sintering: 1350 °C
      breakdown_field: 1.35 kV/cm (as printed in the review)
    method: Value quoted in an open-access review, which cites the study it describes
    status: ground
    note: One multi-oxide example from the review, not a typical value for all varistors.
    source: mdpi-varistor-2023
    where: Section 1.2 (Brief History of ZnO-Based Varistors), example of the best nonlinearity
corrosion:
  - environment: Atomic oxygen and ultraviolet in low Earth orbit (Z-93 coating on LDEF and ISS)
    headline: Z-93 kept its solar absorptance through years of exposure, because atomic oxygen can reverse the darkening that ultraviolet causes on oxides.
    effect: On LDEF, Z-93 and related zinc oxide coatings showed very little change in solar absorptance and no change in infrared emittance after 5.8 years. On MISSE-2, -3 and -4, Z-93 went from 0.14 to 0.14 or 0.15. The report says that atomic oxygen may reverse the ultraviolet darkening of oxides, while it erodes polymer binders.
    rating: low
    conditions:
      exposure: LDEF about 5.8 years; MISSE-2 to MISSE-4 on the ISS, up to about 5,660 ESH of UV (as printed in Table 1)
      contamination: minimal, 300 Å or less
    source: nasa-misse-2008
    where: Section A (Zinc Oxide/Potassium Silicate), text and Table 1
  - environment: Silicone contamination with ultraviolet in orbit (Z-93P on POSA-I)
    headline: Silicone deposits turned the zinc oxide coating yellow or tan and raised its solar absorptance.
    effect: Z-93P on the space-facing side of POSA-I got about 5,000 Å of silicone contamination and 571 ESH of UV, and its solar absorptance rose from 0.16 to 0.23. The report says the contaminated inorganic white coatings became yellowed or tan, and that ground tests of contaminated zinc oxide gave much higher absorptance with more UV.
    rating: moderate
    conditions:
      contamination: about 5,000 Å silicone
      uv: 571 ESH
    source: nasa-misse-2008
    where: Section A (Zinc Oxide/Potassium Silicate), text on POSA-I and Figure 3
  - environment: Acids and bases (chemical attack)
    headline: The supplier SDS lists acids and bases as suitable solvents for zinc oxide, and gives no rate of attack.
    effect: The SDS states that zinc oxide is insoluble in water in practice (solubility negligible) and lists acids and bases as suggested solvents. It gives no corrosion rate, so the rate of attack in a given acid or base is not known from this source.
    rating: unknown
    conditions:
      form: zinc oxide powder, all grades (as stated in the SDS)
    source: sds-usz-zno
    where: Section 9 (Physical and Chemical Properties), Solubility in water and Suggested Solvents rows
risks:
  hazards:
    - text: The U.S. Zinc SDS classifies zinc oxide as Aquatic Chronic 1 with H410 (very toxic to aquatic life with long lasting effects) and the signal word Warning, with P273, P391 and P501. PubChem's record for CID 14806 reports it as not classified, based on one company report.
      ghs:
        - H410
      source: sds-usz-zno
      where: Section 2 (Hazards Identification), GHS Classification and Labeling
    - text: PubChem's GHS summary for CID 14806 reports that zinc oxide does not meet GHS hazard criteria, from 1 of 1 company reports, so the classification differs between sources.
      ghs: []
      source: pubchem-zno
      where: Safety and Hazards, GHS Classification
    - text: At high temperatures the SDS lists zinc oxide fumes as a decomposition product in a fire, and it lists heated magnesium and chlorinated rubbers above 215 °C as incompatible.
      ghs: []
      source: sds-usz-zno
      where: Section 5.2 (Special hazards) and Section 10 (Conditions to avoid)
  failure_risks:
    - mode: Trade-off between breakdown field and nonlinearity in varistors
      trigger: Raising the breakdown voltage of a ZnO varistor by design or processing
      consequence: The review says that an increase in breakdown field can decrease nonlinearity and increase the leakage currents, so a higher-voltage varistor clamps less sharply.
      severity: moderate
      source: mdpi-varistor-2023
      where: Section 1.1 (Specific Features of ZnO-Based Varistors), text on E_B and nonlinearity
    - mode: Darkening and absorptance rise from contamination
      trigger: Silicone or other molecular contamination on a Z-93 type coating, with ultraviolet exposure
      consequence: Solar absorptance rose from 0.16 to 0.23 on a contaminated Z-93P sample. Heavier contamination turns the coating yellow or tan, which raises the temperature of the spacecraft surface.
      severity: moderate
      source: nasa-misse-2008
      where: Section A (Zinc Oxide/Potassium Silicate), text on POSA-I
  side_effects: []
  handling:
    - text: Avoid creating airborne dust. Use adequate exhaust ventilation, gloves and eye protection when dust may form. The SDS says to wash hands and forearms after handling.
      source: sds-usz-zno
      where: Section 7.1 (Precautions for Safe Handling); Section 8.2 (Exposure Controls)
    - text: Clean up spills without dry sweeping. Vacuum or wet-sweep the powder into a closable, labelled container, and keep waste out of drains and surface water.
      source: sds-usz-zno
      where: Section 6.3 (Spill Cleanup Recommendation); Section 6.2 (Environmental Precautions)
    - text: Store in a cool, dry, well-ventilated space, sealed in the original container, and use the product within 1 year.
      source: sds-usz-zno
      where: Section 7.2 (Conditions for Safe Storage)
  mitigations:
    - text: 'Keep the exposure to the OSHA permissible exposure limit: 5 mg/m3 for fumes and 15 mg/m3 total dust (5 mg/m3 respirable dust), 8-hour TWA. The NIOSH IDLH is 500 mg/m3.'
      source: sds-usz-zno
      where: Section 8.1 (Control Parameters)
    - text: Keep contamination of the coating at 300 Å or less, which the report says keeps Z-93 close to its beginning-of-life absorptance. The report says this needs materials selection, venting design and other contamination controls.
      source: nasa-misse-2008
      where: Section A (Zinc Oxide/Potassium Silicate), text on contamination levels
uses:
  - application: thermal-control
    device: Z-93 white thermal-control coating on spacecraft radiators (ISS)
    example: NASA's MISSE report says the inorganic zinc oxide white coating was selected for use on the ISS radiators, based on its performance on LDEF.
    source: nasa-misse-2008
    where: Section A (Zinc Oxide/Potassium Silicate), first paragraph
  - application: thermal-control
    device: Z-93 coating on ISS experiments (MISSE-2 to MISSE-4)
    example: Z-93 flown on MISSE-2, MISSE-3 and MISSE-4 kept its solar absorptance at 0.14 pre-flight and 0.14 to 0.15 post-flight, for UV exposure up to about 5,660 ESH.
    source: nasa-misse-2008
    where: Table 1 (Alion Zinc Oxide/Potassium Silicate Coatings)
  - application: electronics-and-chips
    device: Metal-oxide varistors for surge arresters and surge protection
    example: A 2023 review of ZnO-V2O5 varistors gives typical breakdown fields of 2 to 5 kV/cm for ZnO varistors, and says that 1000 kV surge arresters need a breakdown field above 4 kV/cm.
    source: mdpi-varistor-2023
    where: Section 1.1 (Specific Features of ZnO-Based Varistors), text on ZnO systems for 1000 kV surge arresters
  - application: electronics-and-chips
    device: Transparent conducting films, Al-doped (AZO) and Ga-doped (GZO) zinc oxide
    example: A 2004 sputtered AZO film reached 0.00037 ohm cm with more than 85% visible transmission. A 2023 RF-sputtered GZO film, annealed at 450 °C, reached 94% transparency and 29 ohm per square at 100 nm thickness, which the authors compare with commercial indium tin oxide.
    source: stdj-azo-2004
    where: Abstract (AZO); mdpi-gzo-2023 Abstract (GZO)
supply:
  - subject: Zinc (feedstock for zinc oxide)
    producers:
      - country: China
        share_percent: 31.5
        year: 2025
      - country: Peru
        share_percent: 11.5
        year: 2025
      - country: Australia
        share_percent: 8.5
        year: 2025
      - country: India
        share_percent: 6.7
        year: 2025
      - country: Mexico
        share_percent: 6
        year: 2025
      - country: United States
        share_percent: 5.2
        year: 2025
    critical_lists:
      - list: USGS 2025 List of Critical Minerals (zinc is on the final list)
        listed: true
    recycling: 'USGS zinc chapter: refined zinc is recovered from secondary materials, including galvanizing residues and crude zinc oxide recovered from electric arc furnace dust.'
    substitutes:
      - 'USGS zinc chapter: many elements are substitutes for zinc in chemical, electronic and pigment uses; the chapter does not name a substitute for zinc oxide.'
    source: usgs-mcs2026-zinc
    where: 'World Mine Production and Reserves table, mine production 2025e (world total 13,000 thousand t zinc content, rounded; shares calculated by Space Mushroom against that total, excluding the other countries); Recycling; Substitutes. Zinc is the feedstock, not zinc oxide itself: the USGS does not give a zinc oxide production table. The critical-list entry is confirmed on the USGS 2025 list page (usgs-crit2025).'
documents:
  - title: 'U.S. Zinc safety data sheet: Zinc Oxide (CAS 1314-13-2, Rev. 1.2, 2026; effective 16 July 2015)'
    type: sds
    url: https://sds.chemicalstore.com/uszincoxide.pdf
    year: 2026
  - title: 'PubChem: Zinc oxide (CID 14806), GHS classification and properties'
    type: database
    url: https://pubchem.ncbi.nlm.nih.gov/compound/14806
    year: 2026
  - title: Optical and Luminescence Properties of Zinc Oxide (Rodnyi and Khodyuk, arXiv 1203.4366; published in Optics and Spectroscopy 2011)
    type: paper
    url: https://arxiv.org/pdf/1203.4366
    year: 2011
  - title: Thermal Conductivity of Wurtzite Zinc-Oxide from First-Principles Lattice Dynamics (Scientific Reports, 2016)
    type: paper
    url: https://www.nature.com/articles/srep22504
    year: 2016
  - title: MISSE Thermal Control Materials with Comparison to Previous Flight Experiments (Finckenor, Pippin, Frey; NASA MSFC; NTRS 20080031037)
    type: report
    url: https://ntrs.nasa.gov/api/citations/20080031037/downloads/20080031037.pdf
    year: 2008
  - title: Studying and making transparent conducting ZnO:Al thin films by magnetron DC sputtering (VNUHCM Journal of Science and Technology Development, 2004)
    type: paper
    url: https://stdj.scienceandtechnology.com.vn/index.php/stdj/article/view/3212
    year: 2004
  - title: Effects of Post-Annealing on the Properties of ZnO:Ga Films with High Transparency (94%) and Low Sheet Resistance (29 Ω/square) (Materials, 2023; PMC10573347)
    type: paper
    url: https://pmc.ncbi.nlm.nih.gov/articles/PMC10573347/
    year: 2023
  - title: 'Effects of Dopants and Processing Parameters on the Properties of ZnO-V2O5-Based Varistors Prepared by Powder Metallurgy: A Review (Materials, 2023; PMC10221863)'
    type: paper
    url: https://pmc.ncbi.nlm.nih.gov/articles/PMC10221863/
    year: 2023
  - title: 'USGS Mineral Commodity Summaries 2026: Zinc'
    type: report
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-zinc.pdf
    year: 2026
gaps:
  - The AZO resistivity (0.00037 ohm cm, 2004 sputtering study) is printed with a decimal comma and a superscript exponent, so the value check cannot match it. It is kept out of the properties and stated only in the text, until it is checked by hand against the abstract.
  - No thermal expansion coefficient for bulk ZnO was found in a source read in this run.
  - Atomic weights for zinc and oxygen were not read in a source opened for this profile, so the recipe parts carry no atomic_weight.
  - No zinc oxide production table by country was found; the USGS zinc chapter gives zinc metal mine production, and the producer shares above are for zinc, not zinc oxide.
  - Crustal abundance of zinc was not read in a source opened for this profile.
  - The EU critical raw materials list was not read for zinc, so it is marked not listed here only because it was not checked; this entry should be confirmed.
  - No sensor source was read, so the sensing use named in the brief is not included.
  - Z-93 is a zinc oxide pigment in a potassium silicate binder. Its absorptance values describe that coating, not pure ZnO, and they are kept with scope material and a coating condition.
  - The thermal conductivity of 50 W/mK is a calculated value from the Scientific Reports paper. No measured bulk value from a source read in this run was found.
  - Carrier concentration, mobility and the gallium content of the GZO film were not read in a source opened for this profile.
  - A vacuum outgassing (TML or CVCM) result for ZnO or Z-93 was not found in a source read in this run.
  - Band gap and lattice constants are from one optical-properties review; density-functional theory values were not used.
  - Hazard data rest on one supplier SDS and the PubChem summary; other supplier SDSs, including one with a different H-statement list, were not read for this profile.
  - The varistor breakdown range and nonlinearity example come from a review that cites earlier work; the original studies were not read.
sources:
  arxiv-zno-2011:
    title: Optical and Luminescence Properties of Zinc Oxide (P. A. Rodnyi and I. V. Khodyuk; published in Optics and Spectroscopy, vol. 111, no. 5, 2011)
    id: arXiv 1203.4366
    url: https://arxiv.org/pdf/1203.4366
    year: 2011
    kind: paper
    license: arXiv preprint (cited, not reproduced)
  srep-zno-2016:
    title: 'Thermal Conductivity of Wurtzite Zinc-Oxide from First-Principles Lattice Dynamics: a Comparative Study with Gallium Nitride (Scientific Reports)'
    id: Sci Rep, published 1 March 2016 (srep22504)
    url: https://www.nature.com/articles/srep22504
    year: 2016
    kind: paper
    license: open access (cited, not reproduced)
  sds-usz-zno:
    title: 'U.S. Zinc safety data sheet: Zinc Oxide, all product grades (Rev. 1.2, 2026)'
    id: CAS 1314-13-2; document USZ_ZincOxideSDS_GLOBAL_Rev8
    url: https://sds.chemicalstore.com/uszincoxide.pdf
    year: 2026
    kind: datasheet
    license: supplier document, linked not reproduced
  pubchem-zno:
    title: PubChem compound record, Zinc oxide (CID 14806)
    id: CID 14806; CAS 1314-13-2; molecular formula OZn
    url: https://pubchem.ncbi.nlm.nih.gov/compound/14806
    year: 2026
    kind: database
    license: public domain (NCBI)
  usgs-mcs2026-zinc:
    title: Mineral Commodity Summaries 2026, Zinc
    id: USGS MCS 2026, February 2026
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-zinc.pdf
    year: 2026
    kind: report
    license: public domain (USGS)
  usgs-crit2025:
    title: About the 2025 List of Critical Minerals (USGS, final list of 60 minerals)
    id: Final 2025 List of Critical Minerals, November 2025
    url: https://www.usgs.gov/programs/mineral-resources-program/science/about-2025-list-critical-minerals
    year: 2025
    kind: web
    license: public domain (USGS)
  nasa-misse-2008:
    title: MISSE Thermal Control Materials with Comparison to Previous Flight Experiments (NASA Marshall Space Flight Center; Boeing Phantom Works)
    id: NTRS 20080031037
    url: https://ntrs.nasa.gov/api/citations/20080031037/downloads/20080031037.pdf
    year: 2008
    kind: report
    license: public domain (NASA)
  stdj-azo-2004:
    title: Studying and making transparent conducting ZnO:Al thin films by magnetron DC sputtering (Le Tran, Nguyen Huu Chi, Tran Tuan, Ho Van Binh; VNUHCM Journal of Science and Technology Development, Vol. 7 No. 6)
    id: doi:10.32508/stdj.v7i6.3212
    url: https://stdj.scienceandtechnology.com.vn/index.php/stdj/article/view/3212
    year: 2004
    kind: paper
    license: open access (cited, not reproduced)
  mdpi-gzo-2023:
    title: Effects of Post-Annealing on the Properties of ZnO:Ga Films with High Transparency (94%) and Low Sheet Resistance (29 Ω/square) (Li-Wen Wang et al.; Materials)
    id: PMID 37834601; PMC10573347
    url: https://pmc.ncbi.nlm.nih.gov/articles/PMC10573347/
    year: 2023
    kind: paper
    license: open access (cited, not reproduced)
  mdpi-varistor-2023:
    title: 'Effects of Dopants and Processing Parameters on the Properties of ZnO-V2O5-Based Varistors Prepared by Powder Metallurgy: A Review (Materials)'
    id: doi:10.3390/ma16103725; PMC10221863
    url: https://pmc.ncbi.nlm.nih.gov/articles/PMC10221863/
    year: 2023
    kind: paper
    license: open access (cited, not reproduced)
license: CC-BY-4.0
updated: '2026-10-26'
review:
  status: draft
  method: assistant
  checked_values: 20
  unreachable: 2
  caveats: 5
  note: 'Read by an AI assistant on 2026-10-26 from the pages and PDFs fetched in that run. MDPI''s own page returned HTTP 403 and PubChem''s experimental-properties page returned a verification screen, so the PubChem melting point and density were not used. The AZO and GZO values come from one paper each, not from a review. The thermal conductivity is a calculated value. The EU critical-list entry was not checked. Caveats: the Z-93 values describe a coating, not bare ZnO; the varistor range and nonlinearity example come from a review; the SDS hazard classification differs from PubChem''s.'
story:
  recipe:
    line: ZnO combines one zinc atom with one oxygen atom. Pure zinc oxide crystallizes in the hexagonal wurtzite structure, in which each zinc atom is surrounded by four oxygen atoms at the corners of a nearly regular tetrahedron. For transparent conductors the oxide is doped with small amounts of aluminium or gallium, which provide extra electrons.
    parts:
      - name: Zinc
        symbol: Zn
        role: The metal cation. Each zinc ion sits in a tetrahedron of four oxygen anions in the wurtzite structure.
      - name: Oxygen
        symbol: O
        role: The anion. Each oxygen sits at a corner of the tetrahedron around one zinc ion.
      - name: Aluminium (dopant, AZO)
        symbol: Al
        role: A dopant added to make the film conductive. The 2004 study used a sputtering powder target of ZnO with 2 wt% Al2O3.
      - name: Gallium (dopant, GZO)
        symbol: Ga
        role: A dopant in sputtered GZO films. The Ga content of the 2023 film was not read in this run.
  risk_groups:
    - title: For people
      level: 1
      verdict: Dust and fumes; aquatic H410
      lead: The supplier SDS classifies zinc oxide as hazardous only to aquatic life, but it gives exposure limits for dust and fume. PubChem's record of one company report says it is not classified.
      points:
        - label: The powder
          line: The U.S. Zinc SDS gives the signal word Warning and the H410 statement for aquatic life. It gives no skin or eye hazard statement.
        - label: Dust and fume
          line: The OSHA permissible exposure limit is 5 mg/m3 for fumes and 15 mg/m3 for total dust. Fumes can form from a hot fire.
        - label: Aquatic life
          line: Keep the powder out of drains and surface water.
    - title: If the coating is contaminated
      lead: These results come from flight and ground tests of the coating, not from incidents on the spacecraft.
      points:
        - label: Contamination
          line: Silicone contamination of about 5,000 Å raised the solar absorptance of a Z-93P sample from 0.16 to 0.23 after 571 ESH of UV on POSA-I.
        - label: Clean coatings
          line: With 300 Å or less of contamination, Z-93 kept its solar absorptance close to its starting value on LDEF and MISSE.
        - label: Varistors
          line: In a varistor, a higher breakdown field can reduce nonlinearity and raise leakage current, according to the review.
    - title: Handling and storage
      points:
        - label: Dust
          line: Avoid creating dust. Use exhaust ventilation and gloves, and clean up spills with a vacuum or by wet sweeping.
        - label: Storage
          line: Store in a cool, dry, ventilated space in sealed containers, and use the product within 1 year.
        - label: Incompatible
          line: Keep the powder away from heated magnesium and from chlorinated rubbers above 215 °C.
  why: 'Zinc oxide is a white, inexpensive oxide with a wide band gap of 3.37 eV at 300 K. Its wurtzite structure and its chemistry give it three uses in this collection: a space-proven white thermal-control coating (Z-93, a zinc oxide pigment in a potassium silicate binder), a ceramic base for surge-protection varistors, and a transparent conductor when it is doped with aluminium or gallium. The flight data are the strongest part of the record: a coating that kept its solar absorptance through years of ultraviolet and atomic oxygen exposure, as long as silicone contamination was kept out.'
  uses:
    - where: Thermal-control coatings in orbit
      line: NASA chose the zinc oxide white coating Z-93 for the ISS radiators after its performance on LDEF. On MISSE-2 to MISSE-4 it kept a solar absorptance of 0.14 before flight and 0.14 to 0.15 after exposure.
    - where: Surge-protection varistors
      line: ZnO varistors have a typical breakdown field of 2 to 5 kV/cm, according to a 2023 review, which also says that 1000 kV surge arresters need more than 4 kV/cm.
    - where: Transparent conductors
      line: Al-doped ZnO (AZO) films sputtered in 2004 reached 0.00037 ohm cm with more than 85% visible transmission. Ga-doped ZnO (GZO) films annealed at 450 °C in 2023 reached 94% transparency and 29 ohm per square at 100 nm.
  can_do:
    - label: Thermal control
      value: Solar absorptance 0.14 to 0.15
      line: Z-93 kept a solar absorptance of 0.14 to 0.15 in flight on LDEF and MISSE, with little change in infrared emittance after 5.8 years on LDEF.
    - label: Surge protection
      value: 2 to 5 kV/cm breakdown field
      line: A ZnO varistor with a typical breakdown field of 2 to 5 kV/cm clamps overvoltages, and one multi-oxide example in the review reached a nonlinearity coefficient of 50.
    - label: Transparent conduction
      value: 29 ohm per square at 94%
      line: A 100 nm GZO film annealed at 450 °C reached 29 ohm per square and 94% transparency. The 2004 AZO film reached 0.00037 ohm cm.
    - label: Heat conduction
      value: 50 W/mK (calculated)
      line: A first-principles calculation gives about 50 W/mK for wurtzite ZnO at room temperature, against 400 W/mK for wurtzite GaN.
  compare: The thermal-control coating and the transparent-conductor films use zinc oxide in different forms, so their numbers are not directly comparable. Compared with wurtzite gallium nitride, the calculated thermal conductivity of ZnO is about one-eighth. Its melting point of 1975 °C is high, but the coating and varistor data show that its real limits are contamination and the trade-off between breakdown field and nonlinearity. The table gives ZnO's melting point, thermal conductivity and density.
  compare_rows:
    - label: Melting point
      benchmark: melting_point
    - label: Heat conduction
      benchmark: thermal_conductivity
    - label: Density
      benchmark: density
  handling:
    - Avoid creating dust. Use exhaust ventilation and gloves, and keep to the OSHA limit of 5 mg/m3 for fumes.
    - Clean up spills by vacuum or wet sweeping, and keep the powder out of drains.
    - Store cool, dry and sealed, and use within 1 year.
  good_for:
    - Low-absorptance white coatings in orbit. Z-93 kept a solar absorptance of 0.14 to 0.15 in flight when contamination was kept low.
    - Surge protection. Varistors made from ZnO have a typical breakdown field of 2 to 5 kV/cm.
    - Low-cost transparent conductors. Doped films reach 29 ohm per square at 94% transparency (GZO) or 0.00037 ohm cm (AZO).
  watch_out:
    - Silicone contamination. A contaminated Z-93P sample rose from 0.16 to 0.23 in solar absorptance.
    - The H410 aquatic hazard, and the dust and fume limits in the SDS.
    - Calculated versus measured data. The thermal conductivity is calculated, and the AZO and GZO values come from single laboratory films.
  verdict: Zinc oxide is a proven low-absorptance white coating in orbit, as long as contamination is controlled, and a useful base for varistors and cheap transparent conductors once doped. Its weak points are the contamination and the coating-versus-bulk gap, and the flight record is for a coating, not a bulk part.