Selenium Se

spore: profile
id: selenium
spore_version: 0.2
family: semiconductor-materials
name: Selenium
abbreviation: Se
role: chalcogen element; photoconductor and CIGS solar-cell component
category: metals
type: heavy-metals
summary: |
  Selenium is the element Se, a non-metal that is toxic by ingestion and inhalation. The supplier safety data sheet puts it in the Acute Tox. 3 class, and the USGS reports that most of it comes from the anode slimes of electrolytic copper refining. It is used in photocells, in copper-indium-gallium-diselenide (CIGS) thin-film solar cells, and in glass decolourising. A CIGS module made by Ascent Solar was launched to the International Space Station in November 2018 on the MISSE-X experiment, and the company reported in July 2026 that its CIGS products showed zero power loss after atomic-oxygen testing equal to six months at 400 km.
best_for: Thin-film photovoltaics and photoconductive devices where a selenium compound is the active absorber or photoconductor
weakest_at: 'Toxicity and reactivity: the supplier SDS classes it as toxic if swallowed or inhaled, it burns in air when heated, and it releases toxic hydrogen selenide if heated above 50 degC in water'
identity:
  formula: Se, elemental selenium (metallic grey hexagonal form; red and black amorphous forms also exist)
  cas: 7782-49-2
  crystal_system: hexagonal
  source: pubchem-se
  where: CAS number from the KGHM SDS section 1.1; molecular formula and molecular weight from PubChem CID 6326970; grey hexagonal crystal form from PubChem Hazards Summary (first entry)
tags:
  environments:
    - low-earth-orbit
  stresses:
    - atomic-oxygen
  applications:
    - solar-arrays
    - electronics-and-chips
properties:
  - benchmark: density
    label: Density at 20 degC
    value: 4.81
    unit: g/cm3
    scope: material
    conditions:
      temperature: 20 degC
      form: selenium as printed in the SDS; allotrope not stated
    method: Value printed in the supplier safety data sheet, physical and chemical properties section
    status: ground
    source: kghm-se-sds
    where: 'Section 9.1, item p (''Se density at temp. 20 C: 4.81 g/cm3'')'
  - benchmark: density
    label: Density of solid (WebElements)
    value: 4819
    unit: kg m-3
    scope: material
    conditions:
      basis: density of solid as printed; no temperature stated on the page
    method: Value printed on a reference property page
    status: ground
    source: webelements-se-physics
    where: Physical properties, Density properties, Density of solid
  - benchmark: melting_point
    label: Melting point (SDS)
    value: 221
    unit: degC
    scope: material
    conditions:
      basis: melting point as printed in the SDS; section 5.2 of the same SDS gives 220 degC
    method: Value printed in the supplier safety data sheet
    status: ground
    source: kghm-se-sds
    where: Section 9.1, item d
  - benchmark: melting_point
    label: Melting point (gray selenium)
    value: 220.8
    unit: degC
    scope: material
    conditions:
      form: gray selenium
      note_on_page: vitreous selenium changes to gray selenium at 180 degC; alpha form changes to gray above 120 degC
    method: Value printed in the PubChem property record, Melting Point section
    status: ground
    source: pubchem-se
    where: Melting Point section, entry for gray selenium
  - benchmark: boiling_point
    label: Boiling point
    value: 685
    unit: degC
    scope: material
    conditions:
      pressure: not stated in the SDS; PubChem gives 685 degC at 760 mmHg
    method: Value printed in the supplier safety data sheet
    status: ground
    source: kghm-se-sds
    where: Section 9.1, item e
  - benchmark: vapor_pressure
    label: Vapour pressure
    value: 1.33
    unit: Pa
    scope: material
    conditions:
      temperature: 234 degC
      note_on_page: the SDS also gives 13.3 Pa at 287 degC and 133 Pa at 348 degC
    method: Value printed in the supplier safety data sheet
    status: ground
    source: kghm-se-sds
    where: Section 9.1, item o
  - benchmark: water_solubility
    label: Solubility in water
    value: 3.774
    unit: µg/L
    scope: material
    conditions:
      temperature: 21.2 degC
    method: Value printed in the supplier safety data sheet
    status: ground
    note: The SDS gives this as the only water solubility figure; the PubChem record describes selenium as insoluble in water and alcohol.
    source: kghm-se-sds
    where: Section 9.1, item m
  - benchmark: thermal_conductivity
    label: Thermal conductivity
    value: 0.52
    unit: W m-1 K-1
    scope: material
    conditions:
      temperature: not stated on the page
    method: Value printed on a reference property page
    status: ground
    source: webelements-se-physics
    where: Heat and conduction, Thermal conductivity
  - benchmark: elastic_modulus
    label: Young's modulus
    value: 10
    unit: GPa
    scope: material
    conditions:
      temperature: not stated on the page
    method: Value printed on a reference property page
    status: ground
    source: webelements-se-physics
    where: Elastic properties, Young's modulus
  - benchmark: shear_modulus
    label: Rigidity modulus (shear)
    value: 3.7
    unit: GPa
    scope: material
    conditions:
      temperature: not stated on the page
    method: Value printed on a reference property page, listed as rigidity modulus
    status: ground
    source: webelements-se-physics
    where: Elastic properties, Rigidity modulus
  - benchmark: bulk_modulus
    label: Bulk modulus
    value: 8.3
    unit: GPa
    scope: material
    conditions:
      temperature: not stated on the page
    method: Value printed on a reference property page
    status: ground
    source: webelements-se-physics
    where: Elastic properties, Bulk modulus
  - benchmark: poissons_ratio
    label: Poisson's ratio
    value: 0.33
    unit: '-'
    scope: material
    conditions:
      temperature: not stated on the page
    method: Value printed on a reference property page (no unit printed)
    status: ground
    source: webelements-se-physics
    where: Elastic properties, Poisson's ratio
  - benchmark: heat_of_combustion
    label: Heat of combustion in oxygen
    value: 223
    unit: kJ/g
    scope: material
    conditions:
      atmosphere: oxygen
    method: Value printed in the supplier safety data sheet
    status: ground
    source: kghm-se-sds
    where: Section 9.2
  - benchmark: crustal_abundance
    label: Average crustal abundance
    value: 0.05
    unit: ppm
    scope: material
    conditions:
      basis: average abundance in the Earth's crust, as tabulated by USGS
    method: Value printed in the USGS table of crustal abundances
    status: ground
    source: usgs-pp1802a
    where: Table A1, Selenium row (Se 0.05)
corrosion:
  - environment: Air, heated
    headline: Heated selenium reacts with air and can ignite and burn, forming toxic selenium dioxide.
    effect: The SDS says that when heated, selenium reacts with atmospheric oxygen, which can lead to fire and explosion, and that in air it combusts with a blue flame. Combustion forms selenium dioxide, which is toxic. WebElements gives the equation Se8 + 8O2 gives 8SeO2 and says selenium burns in air, with no temperature.
    rating: moderate
    conditions:
      basis: Rating is the assistant's reading of the SDS warning; no temperature or rate is printed in the cited section
    source: kghm-se-sds
    where: Sections 5.2 and 10.1
  - environment: Water above 50 degC
    headline: Heating selenium in water above 50 degC releases toxic hydrogen selenide.
    effect: The SDS says not to heat selenium above 50 degC because of the reaction 3Se + 3H2O = H2SeO3 + 3H2Se. The products are toxic, and the smell is that of rotten radish, characteristic of hydrogen selenide. At room temperature selenium is insoluble in water (3.774 µg/L at 21.2 degC).
    rating: moderate
    conditions:
      basis: temperature threshold as printed; the rating is the assistant's reading
    source: kghm-se-sds
    where: Sections 7.1, 9.1 (item m) and 10.4
  - environment: Concentrated nitric acid
    headline: Nitric acid is listed as a dangerous reaction partner for selenium.
    effect: The SDS lists nitric acid among substances that dangerously react with selenium, and gives it in the incompatible-materials list. No rate or temperature is given.
    rating: severe
    conditions:
      basis: Rating is the assistant's reading of the word dangerously; no reaction rate was read
    source: kghm-se-sds
    where: Sections 10.3 and 10.5
  - environment: Dilute non-oxidizing acids
    headline: Selenium does not react with dilute non-oxidizing acids.
    effect: WebElements says selenium does not react with dilute non-oxidizing acids. The page gives no figure for oxidizing acids.
    rating: low
    conditions:
      basis: Rating is the assistant's reading of the single statement
    source: webelements-se-chem
    where: Reaction of selenium with acids
  - environment: Halogens (fluorine, chlorine, bromine)
    headline: Selenium reacts with fluorine, chlorine and bromine, and fluorine can make it burn.
    effect: WebElements gives selenium reacting with F2 to form SeF6 and burning in fluorine, and gives the chloride and bromide products Se2Cl2 and Se2Br2. The SDS says selenium reacts violently with halogens with a large release of heat. Fluorine reacts at 0 degC to form SeF4, and the page is the only source for that temperature.
    rating: severe
    conditions:
      basis: Rating is the assistant's reading of the reactions; the SDS calls the reaction with halogens violent
    source: webelements-se-chem
    where: Reaction of selenium with the halogens
risks:
  hazards:
    - text: The supplier SDS for technical selenium (KGHM, revision 18, 2021) classifies selenium as Acute Tox. 3 for oral and inhalation exposure, STOT RE 2 and Aquatic Chronic 4, with the signal word DANGER.
      ghs:
        - 'H301: Toxic if swallowed'
        - 'H331: Toxic if inhaled'
        - 'H373: May cause damage to organs through prolonged or repeated exposure'
        - 'H413: May cause long lasting harmful effects to aquatic life'
      source: kghm-se-sds
      where: Section 2 (Hazard statements and classification)
    - text: PubChem's ECHA aggregate, built from 1063 company reports, gives H301 (100%), H331 (99.8%), H373 (99.7%) and H413 (95.8%), with the signal word Danger.
      ghs: []
      source: pubchem-se
      where: Safety and Hazards, GHS Classification, ECHA C&L Notifications Summary (1063 reports)
    - text: The SDS notes that hydrogen selenide creates an explosive mixture with air.
      ghs: []
      source: kghm-se-sds
      where: Section 2.3 (Other hazards)
    - text: 'Exposure limits for selenium compounds as Se: 0.1 mg/m3 TWA and 0.3 mg/m3 STEL in Poland (SDS section 8.1); PubChem gives a US PEL of 0.2 mg/m3 as an 8-hour time-weighted average, which also applies to other selenium compounds except the hexafluoride.'
      ghs: []
      source: kghm-se-sds
      where: Section 8.1 (Poland TLV table)
  failure_risks:
    - mode: Ignition of heated selenium
      trigger: Heating in air, or contact with oxygen at elevated temperature
      consequence: The SDS says that selenium can lead to fire and explosion when heated in air, and that combustion forms toxic selenium dioxide.
      severity: high
      source: kghm-se-sds
      where: Sections 5.2 and 10.1
    - mode: Release of hydrogen selenide
      trigger: Heating selenium above 50 degC in contact with water
      consequence: The reaction 3Se + 3H2O = H2SeO3 + 3H2Se releases toxic hydrogen selenide, which the SDS says has an odour of rotten radish.
      severity: high
      source: kghm-se-sds
      where: Section 10.4
    - mode: Long-term exposure (selenosis)
      trigger: Repeated or long-term exposure to selenium or its compounds
      consequence: The SDS lists stomach and intestine disorders, organ injury, nervous system disorders, anaemia, hair and nail loss and tooth decay as symptoms of selenosis.
      severity: moderate
      source: kghm-se-sds
      where: Section 4 (Long-term exposure)
    - mode: Supply interruption
      trigger: Lower output from copper refineries, since selenium is recovered as a by-product of copper refining
      consequence: USGS says supply of selenium is directly affected by supply of copper. US net import reliance was above 50% in every year from 2021 to 2025e, and US imports rose to an estimated 400 t of selenium in 2025.
      severity: moderate
      source: usgs-mcs2026-selenium
      where: Events, Trends, and Issues; Salient Statistics
  side_effects: []
  handling:
    - text: Keep containers tightly closed and identified. Avoid open flame and temperatures above 50 degC near the substance. Store at room temperature and do not let it stay in continuous contact with air.
      source: kghm-se-sds
      where: Sections 7.1 and 7.2
    - text: Keep selenium away from oxygen, halogens, nitric acid, chlorates, bromates, carbides, alkali metals and sodium peroxide, which the SDS lists as incompatible.
      source: kghm-se-sds
      where: Section 10.5
    - text: Do not use water or water-containing extinguishing agents on a selenium fire.
      source: kghm-se-sds
      where: Section 5.1 (Unsuitable extinguishing media)
  mitigations:
    - text: Use local exhaust ventilation with a housing wherever dust is produced, and wear goggles against fine dust.
      source: kghm-se-sds
      where: Sections 8.2.1 and 8.2.2
uses:
  - application: electronics-and-chips
    device: Photocells and photoconductive electronics
    example: USGS lists selenium in photocells and solar cells for its photovoltaic and photoconductive properties, and lists its use in pigments, plating solutions and rubber compounding.
    source: usgs-mcs2026-selenium
    where: Domestic Production and Use
  - application: solar-arrays
    device: Flexible CIGS thin-film module flown on the ISS (MISSE-X experiment)
    example: The article reports that the MISSE-X experiment, carrying Ascent Solar's flexible CIGS module, launched on 17 November 2018 aboard the NG CRS-10 mission for an attached period of a year or more on the ISS. It reports no performance results.
    source: ascent-misse-x-2018
    where: Article body (launch and deployment paragraphs)
  - application: solar-arrays
    device: CIGS thin-film PV products, atomic-oxygen tested on the ground (company-reported)
    example: Ascent says its CIGS products, protected by a fluorinated ethylene propylene film, could withstand atomic-oxygen exposure equal to six months in orbit at 400 km and showed zero loss of power afterward.
    source: pvmag-ascent-2026
    where: Article body, atomic oxygen test paragraph
supply:
  - subject: Selenium (refined)
    abundance: 0.05 ppm average crustal abundance (USGS PP 1802-A, Table A1)
    producers:
      - country: China
        share_percent: 53
        year: 2025
    world_output: 'USGS refinery production 2025: 3,800 t of selenium content in total; China 2,000 t, Japan 640 t, Russia 320 t, Belgium 200 t, Canada 130 t (USGS table; 2024 world total 3,670 t)'
    us_position: 'US imports for consumption: 225 t in 2024 and an estimated 400 t in 2025. Import sources 2021 to 2024 for selenium: Philippines 25%, Mexico 14%, Chile 12%, Poland 11%. US average price was an estimated 28 USD/kg in 2025 (99.5% minimum purity, Argus).'
    critical_lists:
      - list: USGS 2025 List of Critical Minerals
        listed: false
      - list: EU critical raw materials list (European Commission page)
        listed: false
    recycling: 'USGS: insignificant. Most scrap from electronic materials was exported for recovery of contained selenium.'
    substitutes:
      - 'USGS: amorphous silicon and cadmium telluride are the two main competitors with CIGS in thin-film PV; silicon is the major substitute in low- and medium-voltage rectifiers.'
    source: usgs-mcs2026-selenium
    where: World Refinery Production table (2025, metric tons of selenium content, China share 53% in the text); Salient Statistics; Import Sources; Recycling; Substitutes; EU list checked on the European Commission page
media: []
documents:
  - title: USGS Mineral Commodity Summaries 2026, Selenium
    type: report
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-selenium.pdf
    year: 2026
  - title: 'KGHM Polska Miedz, Material Safety Data Sheet: Technical selenium (revision 18, 02.09.2021)'
    type: sds
    url: https://kghm.com/sites/default/files/en_hmg_msds_technical_selenium_02_09_2021_18.pdf
    year: 2021
  - title: 'PubChem: Selenium (CID 6326970), property records and GHS classification'
    type: database
    url: https://pubchem.ncbi.nlm.nih.gov/compound/6326970
    year: 2026
  - title: USGS Professional Paper 1802-A, Critical Mineral Resources of the United States, Table A1
    type: report
    url: https://pubs.usgs.gov/pp/1802/a/pp1802a.pdf
    year: 2017
gaps:
  - No flight or vacuum test of selenium itself was found; the only space items are the MISSE-X launch record (2018, no results) and a company-reported atomic-oxygen test (2026).
  - The Ascent Solar 92% power-retention figure from a 2021 press release could not be read (the publisher returned HTTP 403), so it is not used.
  - Electrical resistivity and band gap of selenium were not given cleanly by WebElements (the page prints a garbled range), so no value is given.
  - Specific heat and heat capacity were not printed on the pages read.
  - 'Speed of sound (3350 m/s, WebElements) and the Mohs hardness (2.0) are left out: the first is not in property-canon.yaml and the second has no unit.'
  - 'Non-canonical property names used: vapor_pressure, water_solubility, heat_of_combustion and crustal_abundance. None of them is in spores/property-canon.yaml.'
  - Rates of reaction with air and water (times, humidity dependence) are not given in any source read.
  - Toxicology numbers (LD50, LC50) are in the SDS but were not checked against the page in this run, so none are used.
  - The PubChem ECHA aggregate and the SDS classification come from different notifications; the SDS is one supplier only.
  - Selenium is not on the 2025 USGS critical minerals list or the EU CRM list, as checked on the two pages read.
  - Melting point is given three ways in the sources (220, 220.8 and 221 degC); the SDS value of 221 degC is used and the others are kept as separate rows.
sources:
  usgs-mcs2026-selenium:
    title: Mineral Commodity Summaries 2026, Selenium (USGS, February 2026)
    id: USGS MCS 2026
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-selenium.pdf
    year: 2026
    kind: report
    license: public domain (USGS)
  kghm-se-sds:
    title: 'Material Safety Data Sheet: Technical selenium (KGHM Polska Miedz, revision 18, 02.09.2021)'
    id: CAS 7782-49-2; revision 18, 02.09.2021
    url: https://kghm.com/sites/default/files/en_hmg_msds_technical_selenium_02_09_2021_18.pdf
    year: 2021
    kind: datasheet
    license: supplier document, linked not reproduced
  pubchem-se:
    title: PubChem compound record, Selenium (CID 6326970), property and GHS records
    id: CID 6326970; CAS 7782-49-2
    url: https://pubchem.ncbi.nlm.nih.gov/rest/pug_view/data/compound/6326970/JSON
    year: 2026
    kind: database
    license: public domain (NCBI)
  webelements-se-physics:
    title: 'WebElements, Selenium: physics properties'
    id: WebElements, selenium physics page
    url: https://www.webelements.com/selenium/physics.html
    year: 2026
    kind: web
    license: page content, cited as published
  webelements-se-chem:
    title: 'WebElements, Selenium: chemistry (reactions with air, halogens, acids)'
    id: WebElements, selenium chemistry page
    url: https://www.webelements.com/selenium/chemistry.html
    year: 2026
    kind: web
    license: page content, cited as published
  usgs-pp1802a:
    title: 'Critical Mineral Resources of the United States: An Introduction (Professional Paper 1802-A), Table A1, crustal abundances'
    id: USGS PP 1802-A
    url: https://pubs.usgs.gov/pp/1802/a/pp1802a.pdf
    year: 2017
    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)
  ec-crm:
    title: Critical raw materials (European Commission, list of critical raw materials under Regulation (EU) 2024/1252)
    id: Regulation (EU) 2024/1252
    url: https://single-market-economy.ec.europa.eu/sectors/raw-materials/areas-specific-interest/critical-raw-materials_en
    year: 2024
    kind: web
    license: European Commission web page
  ascent-misse-x-2018:
    title: Ascent CIGS technology goes into space (Compound Semiconductor, 2018)
    id: Compound Semiconductor article 105783
    url: https://compoundsemiconductor.net/article/105783/Ascent_CIGS_technology_goes_into_space
    year: 2018
    kind: web
    license: page content, cited as published
  pvmag-ascent-2026:
    title: Thin-film solar products from Ascent Solar show resilience in simulated low earth orbit tests (pv magazine USA, 6 July 2026)
    id: pv magazine USA, 2026/07/06
    url: https://pv-magazine-usa.com/2026/07/06/thin-film-solar-products-from-ascent-solar-show-resilience-in-simulated-low-earth-orbit-tests/
    year: 2026
    kind: web
    license: page content, cited as published
  wikipedia-se:
    title: Wikipedia, Selenium (history, discovery and allotropes only)
    id: en.wikipedia.org/wiki/Selenium
    url: https://en.wikipedia.org/wiki/Selenium
    year: 2026
    kind: web
    license: CC BY-SA 4.0
license: CC-BY-4.0
updated: '2026-10-26'
review:
  status: draft
  method: assistant
  checked_values: 14
  caveats: 2
  note: Read by an AI assistant on 2026-10-26 from the sources listed in this file. Values were copied from pages read in this run; verify-values has not yet been run on this file.
story:
  recipe:
    line: 'Selenium is the element Se, with the molar mass given in PubChem. It is recovered mainly as a by-product of electrolytic copper refining, from the anode slimes that collect in copper refinery residues (USGS: more than 80% of selenium comes from anode slimes).'
    parts:
      - name: Selenium
        symbol: Se
        share_percent: 100
        role: The element. It is used as the pure element in thin-film solar cells and photocells, and also converted to selenium dioxide and other compounds for glass, pigments and catalysts.
        source: pubchem-se
        where: Molecular Weight (CID 6326970)
  risk_groups:
    - title: For people
      lead: Selenium is not a material to handle casually. The supplier SDS classes it as toxic if swallowed or inhaled and as causing organ damage with repeated exposure.
      level: 2
      verdict: Toxic by ingestion and inhalation
      points:
        - label: Classification
          line: The SDS classes it as Acute Tox. 3 (H301 and H331), STOT RE 2 (H373) and Aquatic Chronic 4 (H413), with the signal word DANGER. PubChem's aggregate of company reports agrees.
        - label: Exposure limit
          line: The Polish limit for selenium compounds as Se is 0.1 mg/m3 TWA and 0.3 mg/m3 STEL. PubChem gives a US PEL of 0.2 mg/m3.
        - label: Long-term effects
          line: The SDS lists selenosis as the long-term syndrome, with stomach and intestine disorders, organ injury, nervous system disorders, anaemia, hair and nail loss and tooth decay.
    - title: If it is heated
      lead: The main hazards in the sources read are combustion in air and release of hydrogen selenide in hot water.
      level: 2
      points:
        - label: In air
          line: Heated selenium reacts with atmospheric oxygen and can lead to fire and explosion. Combustion forms selenium dioxide, which is toxic (SDS 5.2).
        - label: In water
          line: Heating above 50 degC in water releases toxic hydrogen selenide, which smells of rotten radish (SDS 10.4).
        - label: Fire fighting
          line: Do not use water on a selenium fire (SDS 5.1).
    - title: Handling and storage
      points:
        - label: Storage
          line: Keep containers tightly closed, away from open flame and temperatures above 50 degC, and do not let it stay in continuous contact with air (SDS 7.1 and 7.2).
        - label: Incompatible
          line: Keep it separate from oxygen, halogens, nitric acid, chlorates, bromates, carbides, alkali metals and sodium peroxide (SDS 10.5).
  why: Selenium is a chalcogen. Its main technical roles are as a photoconductor and as the Se part of CIGS thin-film solar cells, and it is produced in quantity as a by-product of copper refining. The metallic grey hexagonal form is the one most often named in the sources. Wikipedia credits its discovery to Berzelius and Gahn and notes that it forms several allotropes that interconvert with temperature.
  uses:
    - where: Photocells and solar cells
      line: USGS lists photocells and solar cells among the uses of selenium, for its photovoltaic and photoconductive properties.
    - where: Thin-film CIGS solar cells in orbit
      line: Ascent Solar's flexible CIGS module was launched to the International Space Station on the MISSE-X experiment on 17 November 2018. The source reports the launch only, with no performance result.
    - where: Atomic-oxygen testing (company-reported)
      line: In July 2026 Ascent Solar reported that its CIGS products, protected by an FEP film, showed zero power loss after atomic-oxygen exposure equal to six months in orbit at 400 km. This is a company claim, not a NASA result.
  can_do:
    - label: Density
      benchmark: density
      value: 4.81 g/cm3
      line: The SDS gives 4.81 g/cm3 at 20 degC. WebElements gives 4819 kg m-3 for the solid, without a temperature.
    - label: Melting point
      benchmark: melting_point
      value: 221 degC
      line: The SDS gives 221 degC, and PubChem gives 220.8 degC for gray selenium. Selenium melts at this temperature in the SDS and PubChem records.
    - label: Heat conduction
      benchmark: thermal_conductivity
      value: 0.52 W m-1 K-1
      line: WebElements gives a thermal conductivity of 0.52 W m-1 K-1, a low figure that means selenium spreads heat poorly.
    - label: Scarcity
      benchmark: crustal_abundance
      value: 0.05 ppm
      line: The average crustal abundance is 0.05 ppm (USGS PP 1802-A), so selenium is rare in the crust and is recovered as a by-product.
  compare: Selenium melts at 221 degC and has a density of 4.81 g/cm3, but its thermal conductivity is only 0.52 W m-1 K-1, which makes it a poor conductor of heat for its density. The sources read here do not compare it directly with the other metals in this collection, so the comparison is limited to the figures above.
  compare_rows:
    - scope: material
      label: Density
      benchmark: density
    - scope: material
      label: Melting point
      benchmark: melting_point
    - scope: material
      label: Thermal conductivity
      benchmark: thermal_conductivity
  handling:
    - The powder is toxic if swallowed or inhaled. Use local exhaust ventilation where dust forms, and wear goggles against fine dust.
    - Keep it dry and closed, away from heat above 50 degC, oxygen, halogens and nitric acid.
    - Do not use water on a selenium fire. Heated selenium releases toxic hydrogen selenide in water.
  good_for:
    - 'Thin-film photovoltaics: selenium is the Se part of CIGS cells, and an Ascent Solar CIGS module flew on the ISS in 2018 (launch record only).'
    - 'Photoconductive devices: USGS lists photocells among the main uses of selenium.'
  watch_out:
    - 'Toxicity: the SDS classes selenium as toxic if swallowed or inhaled, with a DANGER signal word.'
    - 'Heat and water: above 50 degC in water it releases hydrogen selenide, and it burns in air when heated.'
    - 'Supply: selenium is a by-product of copper refining, and China produced an estimated 53% of refined selenium in 2025.'
  verdict: Selenium is useful for thin-film photovoltaics and photocells, and its CIGS form has the strongest space record found here, though that record is a launch with no published result plus a company-reported atomic-oxygen test. The toxicity and reactivity data in the supplier SDS are the main constraint. Several basic properties (electrical, optical and specific heat) were not read cleanly in this run and are left out.