spore: profile
id: hafnium-diboride
spore_version: 0.2
family: metal-borides
name: Hafnium diboride (HfB2)
abbreviation: HfB2
role: ultra-high-temperature ceramic
category: ceramics
type: ultra-high-temperature-ceramics
summary: |
  Hafnium diboride (HfB2) is a hexagonal boride ceramic with an AlB2-type structure. Its melting point is listed as 3100 °C in a supplier table and as 3653 K in a NASA Ames table, and its density is listed as 10.5 g/cm3 by the supplier and as 11.212 g/cm3 in the NASA table. Oxidation and mechanical data for the pure material are thin in the sources read; most oxidation data are for HfB2-SiC composites.
best_for: Very high-temperature ceramic parts such as sharp leading edges, where the part is either kept out of hot oxidizing gas or made as an HfB2-SiC composite (NASA Ames describes the class as candidates for these uses)
weakest_at: Sintering to full density, and oxidation data for the pure material (the oxidation results found are for SiC-containing composites)
identity:
  formula: HfB2 (one hafnium atom and two boron atoms per formula unit; AlB2-type hexagonal structure)
  cas: 12007-23-7
  crystal_system: hexagonal
  space_group: P6/mmm
  source: lawson2011
  where: 'Section I (Introduction), second paragraph: AlB2-type C32 structure with space group P6/mmm; CAS number printed in the PubChem record (Other Identifiers)'
tags:
  environments:
    - re-entry
  applications:
    - heat-shields
    - propulsion-and-tanks
properties:
  - benchmark: density
    label: Density, experimental value tabulated for HfB2
    value: 11.212
    unit: g/cm3
    scope: material
    conditions:
      basis: room-temperature experimental value in Table I, row Hf B2 Exp.; the table caption refers to single-crystal ZrB2 and HfB2
      temperature: room temperature (table caption)
    method: Experimental value printed in a table of a NASA Ames paper
    status: ground
    source: lawson2011
    where: Table I, row Hf B2 Exp., column rho (g/cm3)
  - benchmark: density
    label: Density, powder listed by a supplier
    value: 10.5
    unit: g/cm3
    scope: material
    conditions:
      product: Hafnium Diboride Powder 99% (2N), max particle size 45 μm
      form: powder
      unit_in_source: printed as gcm⁻³
    method: Value in the properties table of a supplier product page
    status: ground
    note: Supplier listing for a powder. It is lower than the NASA table value, and the sources read do not explain the difference.
    source: goodfellow-hfb2
    where: Properties table, Density row
  - benchmark: density
    label: Bulk density of a pressureless-sintered monolithic sample
    value: 10.25
    unit: g/cm3
    scope: material
    conditions:
      form: monolithic HfB2 (sample HS0), pressureless sintered at 2050 °C for 4 h
      basis: bulk density of the sintered sample, not crystal density
    method: Bulk density printed in the sintering-condition table
    status: ground
    source: pmc8110836
    where: Table 3, row HS0, Bulk density (g/cm3) column
  - benchmark: density
    label: Calculated density, polycrystalline HfB2
    value: 11078
    unit: kg/m3
    scope: material
    conditions:
      basis: first-principles calculation for the AlB2-type structure; the table gives density in kg/m3
      form: polycrystalline
    method: Calculated by the authors from first principles; printed in the calculated column of the table
    status: calculated
    source: sheina2008
    where: Table V, rho (kg/m3) column, HfB2 row
  - benchmark: melting_point
    label: Melting point, powder listed by a supplier (table value)
    value: 3100
    unit: °C
    scope: material
    conditions:
      product: Hafnium Diboride Powder 99% (2N)
      form: powder
      unit_in_source: table header prints ( C )
    method: Value in the properties table of a supplier product page
    status: ground
    note: The same product page says in its description 'above 3200 °C'. The table and the text disagree, and both are kept as printed.
    source: goodfellow-hfb2
    where: Properties table, Melting point row; product description text
  - benchmark: melting_point
    label: Melting point, quoted by a chemical listing
    value: 3250
    unit: °C
    scope: material
    conditions:
      basis: figure printed on a chemical-listing safety sheet; no test method given
    method: Value printed on a chemical listing page (second-hand)
    status: ground
    note: Second-hand figure from a chemical listing, not a manufacturer test report.
    source: chemicalbook-hfb2
    where: Physical data, melting point field of the safety sheet
  - benchmark: melting_point
    label: Melting temperature, experimental value tabulated for HfB2
    value: 3653
    unit: K
    scope: material
    conditions:
      basis: experimental value in Table I, row Hf B2 Exp.; the table caption gives room-temperature properties
    method: Experimental value printed in a table of a NASA Ames paper
    status: ground
    source: lawson2011
    where: Table I, row Hf B2 Exp., column Tmelt (K)
  - benchmark: debye_temperature
    label: Debye temperature, experimental value tabulated for HfB2
    value: 565
    unit: K
    scope: material
    conditions:
      basis: experimental value in Table I, row Hf B2 Exp.
    method: Value printed in a table of a NASA Ames paper
    status: ground
    source: lawson2011
    where: Table I, row Hf B2 Exp., column TD (K)
  - benchmark: heat_capacity_molar
    label: Molar heat capacity, experimental value tabulated for HfB2
    value: 49.5
    unit: J/(mol·K)
    scope: material
    conditions:
      temperature: room temperature (table caption)
      basis: experimental value in Table I, row Hf B2 Exp.
    method: Value printed in a table of a NASA Ames paper
    status: ground
    source: lawson2011
    where: Table I, row Hf B2 Exp., column C(J/(mol·K))
  - benchmark: thermal_conductivity
    label: Thermal conductivity, polycrystalline HfB2 at room temperature (literature value)
    value: 104
    unit: W/(m·K)
    scope: material
    conditions:
      form: polycrystalline HfB2
      temperature: room temperature
      basis: room-temperature measurement quoted from earlier studies by the NASA Ames paper; not measured by its authors
    method: Literature value quoted in the introduction of a NASA Ames paper
    status: ground
    source: lawson2011
    where: Section I (Introduction), paragraph on thermal conductivity of polycrystalline ZrB2 and HfB2
  - benchmark: thermal_conductivity
    label: Thermal conductivity, monolithic HfB2 sample (measured)
    value: 80.2
    unit: W/(m·K)
    scope: material
    conditions:
      form: monolithic HfB2 (sample HS0), pressureless sintered at 2050 °C for 4 h
      relative_density: 91.5%
    method: Measured value stated in the text of the sintering paper; the test method is not named in the text read
    status: ground
    note: The paper's own sintered sample, at 91.5% relative density, so it is not a fully dense crystal value.
    source: pmc8110836
    where: Thermal conductivity text, sentence giving the HS0 measurement
  - benchmark: thermal_conductivity
    label: Lattice thermal conductivity, in-plane (simulated)
    value: 74
    unit: W/(m·K)
    scope: material
    conditions:
      basis: lattice (phonon) part only, single crystal, 300 K, in-plane average
      method_detail: atomistic Green-Kubo simulation with Tersoff-type interatomic potentials
    method: Calculated by the NASA Ames authors from molecular dynamics
    status: calculated
    note: Simulated phonon part only. It is not a measured total thermal conductivity.
    source: lawson2011
    where: Results, paragraph after Fig. 3 (values integrated to 200 ps)
  - benchmark: thermal_conductivity
    label: Lattice thermal conductivity, normal to the planes (simulated)
    value: 63
    unit: W/(m·K)
    scope: material
    conditions:
      basis: lattice (phonon) part only, single crystal, 300 K, direction normal to the basal planes
      method_detail: atomistic Green-Kubo simulation with Tersoff-type interatomic potentials
    method: Calculated by the NASA Ames authors from molecular dynamics
    status: calculated
    note: Simulated phonon part only. It is not a measured total thermal conductivity.
    source: lawson2011
    where: Results, paragraph after Fig. 3 (values integrated to 200 ps)
  - benchmark: hardness
    label: Hardness, literature value quoted for HfB2
    value: 21.2
    unit: GPa
    scope: material
    conditions:
      basis: literature value cited in the discussion, not measured on the paper's own monolithic sample
      test: method and load not stated
    method: Value quoted in the discussion of a sintering paper, citing earlier work
    status: ground
    note: Second-hand figure. The paper gives no hardness test on its own monolithic HfB2 sample.
    source: pmc8110836
    where: Discussion, hardness paragraph (value cited to Cutler and Munro)
  - benchmark: relative_density
    label: Relative density, monolithic HfB2 after pressureless sintering
    value: 91.5
    unit: '%'
    scope: material
    conditions:
      form: monolithic HfB2 (sample HS0), pressureless sintered at 2050 °C for 4 h
      basis: bulk density divided by the theoretical density printed in the same table
    method: Measured value printed in the sintering table and in the text
    status: ground
    source: pmc8110836
    where: Table 3, row HS0, Relative density (%) column; Physical properties paragraph
  - benchmark: bulk_modulus
    label: Bulk modulus, polycrystalline HfB2 (calculated)
    value: 278.8
    unit: GPa
    scope: material
    conditions:
      basis: Voigt-Reuss-Hill average of first-principles elastic constants
      form: polycrystalline ceramic
    method: Calculated by the authors from first principles (Voigt-Reuss-Hill approximation)
    status: calculated
    source: sheina2008
    where: Table V, B_VRH column, HfB2 row
  - benchmark: shear_modulus
    label: Shear modulus, polycrystalline HfB2 (calculated)
    value: 239.5
    unit: GPa
    scope: material
    conditions:
      basis: Voigt-Reuss-Hill average of first-principles elastic constants
      form: polycrystalline ceramic
    method: Calculated by the authors from first principles (Voigt-Reuss-Hill approximation)
    status: calculated
    source: sheina2008
    where: Table V, G_VRH column, HfB2 row
  - benchmark: elastic_modulus
    label: Young's modulus, polycrystalline HfB2 (calculated)
    value: 558.6
    unit: GPa
    scope: material
    conditions:
      basis: Voigt-Reuss-Hill average of first-principles elastic constants
      form: polycrystalline ceramic
    method: Calculated by the authors from first principles (Voigt-Reuss-Hill approximation)
    status: calculated
    source: sheina2008
    where: Table V, Y_VRH column, HfB2 row
  - benchmark: poissons_ratio
    label: Poisson's ratio, polycrystalline HfB2 (calculated)
    value: 0.166
    unit: '-'
    scope: material
    conditions:
      basis: Voigt-Reuss-Hill average of first-principles elastic constants
      form: polycrystalline ceramic
    method: Calculated by the authors from first principles (Voigt-Reuss-Hill approximation)
    status: calculated
    source: sheina2008
    where: Table V, nu column, HfB2 row
  - benchmark: molar_mass
    label: Molar mass (computed)
    value: 200.11
    unit: g/mol
    scope: material
    conditions:
      basis: molecular weight of HfB2 computed from the formula
    method: Computed molecular weight printed in the PubChem record
    status: calculated
    source: pubchem-hfb2
    where: Chemical and Physical Properties > Computed Properties > Molecular Weight
corrosion:
  - environment: Hot oxidizing gas (air, or supersonic CO2 jets with and without laser heating), tested on an HfB2-SiC composite
    headline: The HfB2-SiC composite forms a layered oxide surface region in hot oxidizing gas, and laser heating deepens it. This is a composite result, not a monolithic HfB2 result.
    effect: The oxidation study found a multilayer degraded region near the surface, with an outer layer of HfO2 over a SiC-depleted zone. The source describes the degradation as typical of HfB2-SiC ceramics in air above about 1750 to 1800 °C. Short laser heating of the surface roughly doubled to tripled the thickness of the degraded area.
    rating: moderate
    conditions:
      material: HfB2 with 30 vol% SiC and 1 vol% graphene (composite)
      environment_detail: supersonic dissociated CO2 jet, with and without ytterbium laser heating
      rating_basis: Space Mushroom judgement from the degraded-layer description; the source does not print a rating
    source: pmc10488200
    where: Abstract; Results and discussion on the multilayer degraded region
  - environment: Air, monolithic HfB2 (no oxidation test read)
    headline: No oxidation onset, oxidation rate or mass change for monolithic HfB2 was found in the sources read.
    effect: The sintering paper states that HfB2 combines good thermophysical, mechanical and oxidation properties, and that these are superior to ZrB2 and TiB2. It gives no test temperature, time or result for that claim.
    rating: unknown
    conditions:
      basis: general statement in the introduction; no test conditions given
    source: pmc8110836
    where: Introduction, sentence comparing diborides
risks:
  hazards:
    - text: PubChem's GHS record for hafnium boride (CID 6336857) gives H315 (causes skin irritation), H319 (causes serious eye irritation) and H335 (may cause respiratory irritation). Most company reports carry these statements, but a minority of reports say the chemical does not meet GHS hazard criteria.
      ghs:
        - H315
        - H319
        - H335
      source: pubchem-hfb2
      where: Safety and Hazards > Hazards Identification > GHS Classification
    - text: A chemical-listing safety sheet for hafnium boride (CAS 12007-23-7) gives the same three H-codes, H315, H319 and H335, and precautionary statements that include P261, P271, P280, P405 and P501 (waste disposal).
      ghs:
        - H315
        - H319
        - H335
      source: chemicalbook-hfb2
      where: Hazard statements (H) and safety statements (P) sections
    - text: PubChem also lists regulatory exposure limits for airborne hafnium (written as Hf). The values are not copied into this profile, so check the limit that applies where you work before handling dust.
      ghs: []
      source: pubchem-hfb2
      where: Safety and Hazards > Exposure Control and Personal Protection
  failure_risks:
    - mode: Incomplete densification in pressureless sintering
      trigger: Pressureless sintering of monolithic HfB2 at 2050 °C for 4 h
      consequence: The sintered monolithic sample reached 91.5% relative density. The paper attributes the poor sinterability of HfB2 to its covalent bonding.
      severity: moderate
      source: pmc8110836
      where: Table 3, row HS0; Physical properties paragraph
    - mode: Surface degradation in hot oxidizing gas (composite)
      trigger: Supersonic CO2 jet, with or without laser heating of the surface, tested on an HfB2-SiC composite
      consequence: A multilayer degraded region forms near the surface. Laser heating of the surface increased its thickness two to threefold in the test.
      severity: high
      source: pmc10488200
      where: Abstract; Results and discussion
  side_effects: []
  handling:
    - text: Keep the powder in a closed container in a well-ventilated area, and avoid breathing the dust. The chemical listing gives the precautionary statements P261 and P271 for this.
      source: chemicalbook-hfb2
      where: Precautionary statements (prevention)
    - text: Wear gloves and eye protection when handling the powder, as the listing advises under P280.
      source: chemicalbook-hfb2
      where: Precautionary statements (prevention)
  mitigations:
    - text: Adding SiC in situ during pressureless sintering improved the hardness and fracture toughness of the HfB2-based composites in this study. These are composite results, not monolithic HfB2 data.
      source: pmc8110836
      where: Abstract
uses:
  - application: heat-shields
    device: Sharp leading edges of hypersonic vehicles (candidate use)
    example: NASA Ames describes HfB2 and ZrB2 as candidate materials for high-temperature environments such as sharp leading edges of hypersonic vehicles.
    source: lawson2011
    where: Section I (Introduction), first paragraph
  - application: heat-shields
    device: Wing and nose sharp edges of high-speed flying vehicles (research, HfB2-SiC composite)
    example: A 2023 study of HfB2-SiC under supersonic CO2 jets says such materials are interesting for the most thermally loaded parts of high-speed aircraft, such as sharp wing and nose edges, and for high-speed vehicles in CO2 atmospheres such as Venus and Mars. The study is laboratory testing, not a flight.
    source: pmc10488200
    where: Abstract and Introduction, first paragraph
  - application: propulsion-and-tanks
    device: Propulsion systems and refractory crucibles (candidate uses of the diboride class)
    example: NASA Ames lists propulsion systems and refractory crucibles among candidate applications for the high-melting diborides.
    source: lawson2011
    where: Section I (Introduction), first paragraph
supply:
  - subject: Hafnium
    producers:
      - country: Germany
        share_percent: 54
        year: 2024
      - country: China
        share_percent: 21
        year: 2024
      - country: France
        share_percent: 12
        year: 2024
      - country: United Kingdom
        share_percent: 8
        year: 2024
    critical_lists:
      - list: USGS 2025 List of Critical Minerals (hafnium on the final list)
        listed: true
    recycling: 'USGS: hafnium metal recycling was minimal.'
    substitutes:
      - 'USGS: boron or cadmium-silver-indium alloys can substitute for hafnium metal in nuclear control rods, and zirconium can be used interchangeably with hafnium in certain superalloys. These are uses of the metal, not of the diboride.'
    source: usgs-mcs2026-zirconium-hafnium
    where: 'Import Sources (2021-24) for unwrought hafnium: these are US import shares for 2021 to 2024, recorded here with year 2024, not world production shares. USGS states that world primary hafnium production data were not available. Recycling; Substitutes; Events, Trends, and Issues. Critical-list status from the USGS 2025 critical minerals page (usgs-crit2025).'
  - subject: Boron (feedstock for the diboride)
    producers:
      - country: Turkey
        year: 2025
      - country: Bolivia
        year: 2025
      - country: Chile
        year: 2025
      - country: China
        year: 2025
    critical_lists:
      - list: USGS 2025 List of Critical Minerals (boron on the final list)
        listed: true
    recycling: 'USGS: boron recycling is ''Insignificant.'''
    substitutes:
      - 'USGS: substitution for boron is possible in detergents, enamels, insulation and soaps; the chapter does not discuss boride or ceramic uses.'
    source: usgs-mcs2026-boron
    where: World Production and Reserves table, 2025 column (refined borates from Turkey, ulexite from Bolivia and Chile, boric oxide equivalent from China; world total printed as XX, so no shares are calculated); Recycling; Substitutes; critical status from usgs-crit2025.
media: []
documents:
  - title: 'Lawson, Daw and Bauschlicher: Lattice thermal conductivity of ultra high temperature ceramics ZrB2 and HfB2 from atomistic simulations (NASA NTRS 20110015597)'
    type: paper
    url: https://ntrs.nasa.gov/api/citations/20110015597/downloads/20110015597.pdf
    year: 2011
  - title: 'Goodfellow: Hafnium Diboride Powder 99% (2N) product page'
    type: datasheet
    url: https://www.goodfellow.com/usa/hafnium-diboride-powder-group
    year: 2026
  - title: 'PubChem: Hafnium boride (CID 6336857), computed properties, GHS classification and regulatory summaries'
    type: database
    url: https://pubchem.ncbi.nlm.nih.gov/rest/pug_view/data/compound/6336857/JSON
    year: 2026
  - title: 'ChemicalBook safety data sheet: Hafnium boride (CAS 12007-23-7)'
    type: sds
    url: https://www.chemicalbook.com/msds/hafnium-boride.htm
    year: 2026
  - title: Improvement of mechanical properties of HfB2-based composites by incorporating in situ SiC reinforcement through pressureless sintering (PMC open access)
    type: paper
    url: https://pmc.ncbi.nlm.nih.gov/articles/PMC8110836/
    year: 2021
  - title: Oxidation of Ceramic Materials Based on HfB2-SiC under the Influence of Supersonic CO2 Jets and Additional Laser Heating (PMC open access)
    type: paper
    url: https://pmc.ncbi.nlm.nih.gov/articles/PMC10488200/
    year: 2023
  - title: 'Sheina and Ivanovskii: Elastic properties of mono- and polycrystalline hexagonal AlB2-like diborides of s, p and d metals from first-principles calculations (arXiv:0804.0897)'
    type: paper
    url: https://arxiv.org/pdf/0804.0897
    year: 2008
  - title: 'USGS Mineral Commodity Summaries 2026: Zirconium and Hafnium'
    type: report
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-zirconium-hafnium.pdf
    year: 2026
  - title: 'USGS Mineral Commodity Summaries 2026: Boron'
    type: report
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-boron.pdf
    year: 2026
gaps:
  - No flight, vacuum, radiation or other space-environment result for HfB2 was found in the sources read. The only space-related statement is the NASA Ames description of the class as a candidate for sharp leading edges of hypersonic vehicles.
  - No measured oxidation onset, oxidation rate or mass change for monolithic HfB2 was found. All oxidation results read are for HfB2-SiC composites.
  - No measured fracture toughness, and no hardness test on monolithic HfB2, was found. The hardness value recorded is a second-hand literature figure.
  - 'Melting point and density disagree across sources: the supplier table, the chemical listing and the NASA Ames table each give a different value. All are recorded with their sources, and none has been reconciled.'
  - The chemical listing gives a density of 10.5 with no unit printed. It is left out.
  - The sintering paper's electrical conductivity values are printed with an exponent, so they are left out.
  - Debye temperature and molar heat capacity are recorded under the snake_case names debye_temperature and heat_capacity_molar, because neither name is in spores/property-canon.yaml.
  - Atomic weights of hafnium and boron were not printed in a source read in this run, so the recipe lists atom counts only.
  - No UNS, EN, DIN, AISI or other standard designation for HfB2 was printed in a source read in this run, so no designations are listed.
  - USGS states that world primary hafnium production data were not available. The producer entries are US import shares for 2021 to 2024, not world production, and no world producer shares are given.
  - The USGS chapter does not quantify hafnium resources, so no crustal abundance is recorded.
  - The EU critical raw materials list was not checked for hafnium or boron in this run.
  - No supplier safety data sheet for the specific powder was read. The hazard entries rest on the PubChem GHS record and a chemical listing's safety sheet.
  - No thermal expansion coefficient for HfB2 was found in a fetched source in this run.
  - The lattice constants calculated in the Sheina and Ivanovskii table were not recorded, because the row layout in the extracted text was unclear.
sources:
  lawson2011:
    title: Lattice thermal conductivity of ultra high temperature ceramics ZrB2 and HfB2 from atomistic simulations
    id: NASA Ames Research Center, NTRS citation 20110015597 (J. W. Lawson, M. S. Daw, C. W. Bauschlicher Jr.)
    url: https://ntrs.nasa.gov/api/citations/20110015597/downloads/20110015597.pdf
    year: 2011
    kind: paper
    license: public domain (NASA)
  goodfellow-hfb2:
    title: Hafnium Diboride Powder 99% (2N), Goodfellow product page
    id: CAS 12007-23-7
    url: https://www.goodfellow.com/usa/hafnium-diboride-powder-group
    year: 2026
    kind: datasheet
    license: supplier product page, cited not reproduced
  pmc8110836:
    title: Improvement of mechanical properties of HfB2-based composites by incorporating in situ SiC reinforcement through pressureless sintering
    id: PMC8110836 (received 2020 Nov 4; accepted 2021 Apr 14)
    url: https://pmc.ncbi.nlm.nih.gov/articles/PMC8110836/
    year: 2021
    kind: paper
    license: open access (PMC), cited not reproduced
  pmc10488200:
    title: Oxidation of Ceramic Materials Based on HfB2-SiC under the Influence of Supersonic CO2 Jets and Additional Laser Heating
    id: PMC10488200 (published 2023 Sep 4; 24(17):13634)
    url: https://pmc.ncbi.nlm.nih.gov/articles/PMC10488200/
    year: 2023
    kind: paper
    license: open access (PMC), cited not reproduced
  sheina2008:
    title: Elastic properties of mono- and polycrystalline hexagonal AlB2-like diborides of s, p and d metals from first-principles calculations
    id: arXiv:0804.0897 (I. R. Sheina, A. L. Ivanovskii)
    url: https://arxiv.org/pdf/0804.0897
    year: 2008
    kind: paper
    license: arXiv, cited not reproduced
  pubchem-hfb2:
    title: PubChem compound record, Hafnium boride (CID 6336857)
    id: CID 6336857; CAS 12007-23-7
    url: https://pubchem.ncbi.nlm.nih.gov/rest/pug_view/data/compound/6336857/JSON
    year: 2026
    kind: database
    license: public domain (NCBI)
  chemicalbook-hfb2:
    title: 'ChemicalBook safety data sheet: Hafnium boride'
    id: CAS 12007-23-7
    url: https://www.chemicalbook.com/msds/hafnium-boride.htm
    year: 2026
    kind: datasheet
    license: chemical listing, cited not reproduced
  usgs-mcs2026-zirconium-hafnium:
    title: Mineral Commodity Summaries 2026, Zirconium and Hafnium
    id: USGS MCS 2026, February 2026
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-zirconium-hafnium.pdf
    year: 2026
    kind: report
    license: public domain (USGS)
  usgs-mcs2026-boron:
    title: Mineral Commodity Summaries 2026, Boron
    id: USGS MCS 2026, February 2026
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-boron.pdf
    year: 2026
    kind: report
    license: public domain (USGS)
  usgs-crit2025:
    title: About the 2025 List of Critical Minerals (USGS)
    id: Final 2025 List of Critical Minerals
    url: https://www.usgs.gov/programs/mineral-resources-program/science/about-2025-list-critical-minerals
    year: 2025
    kind: web
    license: public domain (USGS)
license: CC-BY-4.0
updated: '2026-11-02'
review:
  status: draft
  method: assistant
  checked_values: 20
  unreachable: 1
  caveats: 3
  note: 'Read by an AI assistant on 2026-11-02: 20 property values taken from fetched pages; one source (Springer, sintering strength) could not be opened; conflicts between supplier, chemical-listing and NASA values are kept as printed.'
story:
  recipe:
    line: HfB₂ is a hafnium boride. Each formula unit combines one hafnium atom with two boron atoms, which form graphitic boron layers in the AlB₂-type hexagonal structure. In the sintering study, monolithic HfB₂ powder was pressureless-sintered at 2050 °C for 4 h.
    parts:
      - name: Hafnium
        symbol: Hf
        role: The metal atoms sit in closed-packed hexagonal planes between the boron layers, and they are the heavy element of the compound.
      - name: Boron
        symbol: B
        role: Forms open, graphite-like hexagonal layers that carry the covalent bonding of the crystal.
  risk_groups:
    - title: For people
      level: 1
      verdict: Irritant under GHS (H315, H319, H335)
      lead: The powder is classed as an irritant of skin, eyes and airways in the GHS records read. These come from a database summary and a chemical listing, not from a full toxicology study.
      points:
        - label: Skin and eyes
          line: PubChem's GHS record gives H315 (skin irritation) and H319 (serious eye irritation). The chemical listing gives the same codes.
        - label: Breathing
          line: H335 (may cause respiratory irritation) means dust should be kept out of the air. The listing advises use in a well-ventilated area (P271) and avoiding breathing dust (P261).
        - label: Exposure limits
          line: PubChem lists regulatory exposure limits for airborne hafnium. Check the limit that applies where you work before handling dust.
    - title: In hot oxidizing gas
      lead: The oxidation results read are for an HfB2-SiC composite heated in a supersonic CO2 jet, not for the pure material.
      points:
        - label: Composite in a jet
          line: The composite formed a multilayer degraded region near the surface, with a hafnium oxide outer layer over a silicon-depleted zone. Laser heating of the surface made the degraded layer two to three times thicker in the test.
        - label: Pure material
          line: No source read gives an oxidation onset for monolithic HfB2, so the composite result should not be assumed to apply to the pure material.
    - title: Handling and storage
      points:
        - label: Storage
          line: The listing advises a closed container and locking the material up (P405). Keep the powder dry and away from heat.
        - label: Densification
          line: Full density needs very high sintering temperatures. The monolithic sample sintered at 2050 °C for 4 h reached 91.5% relative density.
  why: 'Hafnium diboride (HfB2) is a hexagonal boride ceramic that NASA Ames researchers describe as a candidate for sharp leading edges of hypersonic vehicles, propulsion systems and refractory crucibles. Its melting point is given as 3100 °C in a supplier table and as 3653 K in a NASA table. Its density is given as 10.5 g/cm3 by the supplier and as 11.212 g/cm3 in the NASA table. One practical limit in the sources is sintering: a monolithic sample pressureless-sintered at 2050 °C for 4 h reached 91.5% relative density, and the paper attributes the difficulty to the covalent bonding of HfB2.'
  uses:
    - where: Sharp leading edges of hypersonic vehicles
      line: NASA Ames describes HfB2 and ZrB2 as candidate materials for high-temperature environments such as sharp leading edges of hypersonic vehicles. This is a candidate use, not a flown part.
    - where: Wing and nose edges of high-speed vehicles (laboratory research)
      line: A 2023 study of an HfB2-SiC composite under supersonic CO2 jets says such ceramics are interesting for the most thermally loaded parts of high-speed aircraft, such as sharp wing and nose edges. The study is laboratory testing.
    - where: Propulsion and crucibles
      line: NASA Ames lists propulsion systems and refractory crucibles among candidate applications for the diboride class. No test on the pure grade for these uses was found.
  can_do:
    - label: Carry heat well for a ceramic
      line: a literature value of 104 W/(m·K) at room temperature is quoted for polycrystalline HfB2, and a measured 80.2 W/(m·K) was reported for one sintered sample at 91.5% relative density.
    - label: Stay stiff on paper
      line: the first-principles calculation gives a bulk modulus of 278.8 GPa and a Young's modulus of 558.6 GPa for polycrystalline HfB2. No measured Young's modulus for the pure material was found.
    - label: Resist indentation
      line: a literature hardness of 21.2 GPa is quoted for HfB2. The paper that quotes it did not test its own monolithic sample.
    - label: Be dense
      line: 11.212 g/cm3 is the experimental value in the NASA table, while a supplier powder listing gives 10.5 g/cm3.
  compare: In the NASA Ames table, HfB2 is listed with a higher density than zirconium diboride (ZrB2), and its simulated lattice thermal conductivity at 300 K is higher than that of ZrB2 in the same paper. Its Debye temperature in the table is lower than that of ZrB2. The calculated elastic values here are the only stiffness data in this profile, and no measured Young's modulus for the pure material was found. Compared with ZrB2 and TiB2 in the sintering study, the authors call the oxidation resistance of HfB2 superior, but they give no test data to support that claim.
  compare_rows:
    - label: Density
      benchmark: density
    - label: Melting point
      benchmark: melting_point
    - label: Thermal conductivity
      benchmark: thermal_conductivity
  handling:
    - The powder is classed as an irritant in the GHS records read. Wear gloves and eye protection.
    - Avoid breathing the dust, and use it in a well-ventilated area.
    - Keep the material in a closed container, in a dry place, and locked up.
    - Do not assume the composite oxidation result applies to the pure material; check the grade before use in hot gas.
  good_for:
    - Very high-temperature ceramic parts in candidate roles such as sharp leading edges, where NASA Ames reports the class as a candidate.
    - 'Parts that need the high thermal conductivity typical of this class: 104 W/(m·K) at room temperature is quoted for polycrystalline HfB2.'
  watch_out:
    - Sintering to full density is hard. The monolithic sample reached 91.5% relative density at 2050 °C for 4 h.
    - Oxidation data found are for HfB2-SiC composites. No oxidation onset for the pure material was found in the sources read.
    - 'Sources disagree on melting point and density: 3100 °C against 3653 K, and 10.5 g/cm3 against 11.212 g/cm3. Check the certificate of analysis for the lot you buy.'
  verdict: HfB2 is a very high-melting ceramic with a clear high-temperature use case. Its best-supported data are literature and calculated values, and the oxidation and mechanical data for the pure grade are thin.