Hafnium carbide

{{Use dmy dates|date=July 2020}}

{{chembox

| Verifiedfields = changed

| Watchedfields = changed

| verifiedrevid = 437634684

| Name = Hafnium carbide

| ImageFile =NaCl polyhedra.png

| ImageName = Hafnium carbide

| OtherNames =

|Section1={{Chembox Identifiers

| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}

| ChemSpiderID = 17340381

| EINECS = 235-114-1

| InChI = 1/C.Hf/q-1;+1/rCHf/c1-2

| SMILES = [Hf+]#[C-]

| InChIKey = NVDNLVYQHRUYJA-GLWNXBRTAK

| StdInChI_Ref = {{stdinchicite|correct|chemspider}}

| StdInChI = 1S/C.Hf/q-1;+1

| StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}

| StdInChIKey = NVDNLVYQHRUYJA-UHFFFAOYSA-N

| CASNo_Ref = {{cascite|correct|CAS}}

| CASNo = 12069-85-1

| PubChem = 16212551

}}

|Section2={{Chembox Properties

| Formula = HfC

| Hf=1|C=1

| Appearance = black odorless powder

| Density = 12.2 g/cm³Physical Constants of Inorganic Compounds in {{RubberBible86th|pages=4–44 ff.}}

| Solubility = insoluble

| MeltingPtC = 3958

| MeltingPt_ref =

| BoilingPt =

}}

|Section3={{Chembox Structure

| CrystalStruct = Cubic crystal system, cF8

| SpaceGroup = Fm3m, No. 225

}}

|Section7={{Chembox Hazards

| GHSPictograms = {{GHS02}}

| GHSSignalWord = Warning

| HPhrases = {{H-phrases|228}}

| PPhrases = {{P-phrases|}}

| NFPA-H = 2

| NFPA-R = 1

| NFPA-F = 2

}}

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Hafnium carbide ({{chem2|HfC|auto=1}}) is a chemical compound of hafnium and carbon. Previously the material was estimated to have a melting point of about 3,900 °C. More recent tests have been able to conclusively prove that the substance has an even higher melting point of 3,958 °C exceeding those of tantalum carbide and tantalum hafnium carbide which were both previously estimated to be higher.{{cite journal | doi=10.1038/srep37962 | title=Investigating the highest melting temperature materials: A laser melting study of the TaC-HFC system | year=2016 | last1=Cedillos-Barraza | first1=Omar | last2=Manara | first2=Dario | last3=Boboridis | first3=K. | last4=Watkins | first4=Tyson | last5=Grasso | first5=Salvatore | last6=Jayaseelan | first6=Daniel D. | last7=Konings | first7=Rudy J. M. | last8=Reece | first8=Michael J. | last9=Lee | first9=William E. | journal=Scientific Reports | volume=6 | page=37962 | pmid=27905481 | pmc=5131352 | bibcode=2016NatSR...637962C}} However, it has a low oxidation resistance, with the oxidation starting at temperatures as low as 430 °C.{{cite journal|doi=10.1111/j.1151-2916.1992.tb05487.x | title=Oxidation Kinetics of Hafnium Carbide in the Temperature Range of 480° to 600°C | journal=Journal of the American Ceramic Society | volume=75 | issue=10 | pages=2671–2678 | first=Shiro | last=Shimada|date=October 1992}} Experimental testing in 2018 confirmed the higher melting point yielding a result of 3,982 (±30°C) with a small possibility that the melting point may even exceed 4,000°C.{{cite journal |last1=Ushakov |first1=Sergey V. |last2=Navrotsky |first2=Alexandra |last3=Hong |first3=Qi-Jun |last4=van de Walle |first4=Axel |title=Carbides and Nitrides of Zirconium and Hafnium |journal=Materials |date=26 August 2019 |volume=12 |issue=17 |pages=2728 |doi=10.3390/ma12172728 |pmid=31454900 |pmc=6747801 |bibcode=2019Mate...12.2728U |doi-access=free}}

Atomistic simulations conducted in 2015 predicted that a similar compound, hafnium carbonitride (HfCN), could have a melting point exceeding even that of hafnium carbide.{{cite journal|last1=Hong|first1=Qi-Jun|last2=van de Walle|first2=Axel|year=2015|title=Prediction of the material with highest known melting point from ab initio molecular dynamics calculations|journal=Physical Review B|volume=92|issue=2|page=020104|doi=10.1103/PhysRevB.92.020104|bibcode=2015PhRvB..92b0104H|issn=1098-0121|doi-access=free}} Experimental evidence gathered in 2020 confirmed that it did indeed have a higher melting point exceeding 4,000 °C,{{cite web | url=https://www.forbes.com/sites/allisongasparini/2020/05/31/scientists-create-worlds-most-heat-resistant-material-with-potential-use-for-spaceplanes/?sh=79f5452b4f7e | title=Scientists Create World's Most Heat Resistant Material with Potential Use for Spaceplanes | website=Forbes}} with more recent ab initio molecular dynamics calculations predicting the {{chem2|HfC0.75N0.22}} phase to have a melting point as high as 4,110 ± 62 °C, highest known for any material.{{Cite journal |last1=Dai |first1=Yu |last2=Zeng |first2=Fanhao |last3=Liu |first3=Honghao |last4=Gao |first4=Yafang |last5=Yang |first5=Qiaobin |last6=Chen |first6=Meiyan |last7=Huang |first7=Rui |last8=Gu |first8=Yi |date=2023-10-15 |title=Controlled nitrogen content synthesis of hafnium carbonitride powders by carbonizing hafnium nitride for enhanced ablation properties |url=https://linkinghub.elsevier.com/retrieve/pii/S0272884223022666 |journal=Ceramics International |language=en |volume=49 |issue=20 |pages=33265–33274 |doi=10.1016/j.ceramint.2023.08.035 |issn=0272-8842 |eissn=1873-3956 |oclc=9997899259 |s2cid=260672783|url-access=subscription }}

Hafnium carbide is usually carbon deficient and therefore its composition is often expressed as {{chem2|HfC_{x}|}} (x = 0.5 to 1.0). It has a cubic (rock-salt) crystal structure at any value of x.{{cite journal|title=Electronic structure of cubic Hf_xTa_1–xC_y carbides from X-ray spectroscopy studies and cluster self-consistent calculations |doi=10.1016/j.jallcom.2007.08.018 |date=2008-08-26|last1=Lavrentyev|first1=A.A.|last2=Gabrelian|first2=B.V.|last3=Vorzhev|first3=V.B.|last4=Nikiforov|first4=I.Ya.|last5=Khyzhun|first5=O.Yu.|last6=Rehr|first6=J.J.|authorlink6=John J. Rehr|journal=Journal of Alloys and Compounds|volume=462|pages=4–10|issue=1–2}}

Hafnium carbide powder is obtained by the reduction of hafnium(IV) oxide with carbon at 1,800 to 2,000 °C. A long processing time is required to remove all oxygen. Alternatively, high-purity HfC coatings can be obtained by chemical vapor deposition from a gas mixture of methane, hydrogen, and vaporized hafnium(IV) chloride.

Because of the technical complexity and high cost of the synthesis, HfC has a very limited use, despite its favorable properties such as high hardness (greater than 9 Mohs{{cite book|title=CRC Materials Science and Engineering Handbook|year=2001 |publisher=CRC Press|editor1=James F. Shackelford|editor2=William Alexander|isbn=978-0-849-32696-7|edition=3rd}}) and melting point.{{cite book|author=Harry Julius Emeléus|author-link=Harry Julius Emeléus|title=Advances in Inorganic Chemistry and Radiochemistry|chapter-url=https://books.google.com/books?id=-SnCsg5jM_kC&pg=PA169|year=1968|publisher=Academic Press|isbn=978-0-12-023611-4|pages=169–170|chapter=Metal Carbides}}

The magnetic properties of {{chem2|HfC_{x}|}} change from paramagnetic for x ≤ 0.8 to diamagnetic at larger x. An inverse behavior (dia-paramagnetic transition with increasing x) is observed for Tantalum carbide, despite its having the same crystal structure as {{chem2|HfC_{x}|}}.{{cite book|author1=Aleksandr Ivanovich Gusev|author2=Andreĭ Andreevich Rempel|author3=Andreas J. Magerl|title=Disorder and Order in Strongly Nonstoichiometric Compounds: Transition Metal Carbides, Nitrides, and Oxides|url=https://books.google.com/books?id=jc2D7TGZcyUC&pg=PA513|year=2001|publisher=Springer|isbn=978-3-540-41817-7|pages=513–516}}