Editing Sintering (section)

== Electric current assisted sintering ==
These techniques employ electric currents to drive or enhance sintering.<ref>{{cite journal|title=Consolidation/synthesis of materials by electric current activated/assisted sintering|url=https://www.sciencedirect.com/science/article/abs/pii/S0927796X08000995|doi=10.1016/j.mser.2008.09.003|volume=63 |issue=4–6 |journal=Materials Science and Engineering: R: Reports |pages=127–287|date=February 2009 |last1=Orrù |first1=Roberto |last2=Licheri |first2=Roberta |last3=Locci |first3=Antonio Mario |last4=Cincotti |first4=Alberto |last5=Cao |first5=Giacomo }}</ref><ref>{{Cite journal |last1=Grasso |first1=Salvatore|last2=Sakka |first2=Yoshio |last3=Maizza |first3=Giovanni |date=October 2009|title=Electric current activated/assisted sintering (ECAS): a review of patents 1906–2008|journal=Science and Technology of Advanced Materials |volume=10 |issue=5|page=053001 |doi=10.1088/1468-6996/10/5/053001 |issn=1468-6996 |pmc=5090538 |pmid=27877308}}</ref> English engineer A. G. Bloxam registered in 1906 the first [[patent]] on sintering powders using [[direct current]] in [[vacuum]]. The primary purpose of his inventions was the industrial scale production of filaments for [[Incandescent light bulb|incandescent lamp]]s by compacting [[tungsten]] or [[molybdenum]] particles. The applied current was particularly effective in reducing surface [[oxide]]s that increased the [[emissivity]] of the filaments.<ref name=grasso/>

In 1913, Weintraub and Rush patented a modified sintering method which combined electric current with [[pressure]]. The benefits of this method were proved for the sintering of [[refractory metals]] as well as conductive [[carbide]] or [[nitride]] powders. The starting [[boron]]–[[carbon]] or [[silicon]]–carbon powders were placed in an [[Insulator (electrical)|electrically insulating]] tube and compressed by two rods which also served as [[electrode]]s for the current. The estimated sintering temperature was 2000&nbsp;°C.<ref name=grasso/>

In the United States, sintering was first patented by Duval d'Adrian in 1922. His three-step process aimed at producing heat-resistant blocks from such oxide materials as [[Zirconium dioxide|zirconia]], [[Thorium dioxide|thoria]] or [[Tantalum pentoxide|tantalia]]. The steps were: (i) [[Molding (process)|molding]] the powder; (ii) [[Annealing (metallurgy)|annealing]] it at about 2500&nbsp;°C to make it conducting; (iii) applying current-pressure sintering as in the method by Weintraub and Rush.<ref name=grasso/>

Sintering that uses an [[Electric arc|arc]] produced via a [[capacitance]] discharge to eliminate oxides before direct current heating, was patented by G. F. Taylor in 1932. This originated sintering methods employing pulsed or [[alternating current]], eventually superimposed to a direct current. Those techniques have been developed over many decades and summarized in more than 640 patents.<ref name=grasso>{{cite journal|journal=Sci. Technol. Adv. Mater.|volume= 10|year=2009|page=053001|title=Electric current activated/assisted sintering (ECAS): a review of patents 1906–2008|doi= 10.1088/1468-6996/10/5/053001|issue=5|pmc=5090538|last1= Grasso|first1= S|last2= Sakka|first2= Y|last3= Maizza|first3= G|pmid=27877308}}</ref>

Of these technologies the most well known is resistance sintering (also called [[hot pressing]]) and [[spark plasma sintering]], while [[electro sinter forging]] is the latest advancement in this field.

=== Spark plasma sintering ===
In [[spark plasma sintering]] (SPS), external pressure and an electric field are applied simultaneously to enhance the densification of the metallic/ceramic powder compacts. However, after commercialization it was determined there is no plasma, so the proper name is spark sintering as coined by Lenel. The electric field driven densification supplements sintering with a form of hot pressing, to enable lower temperatures and taking less time than typical sintering.<ref name = Tuan>{{Cite book|last1 = Tuan|first1 = W.H.|last2 =Guo|first2 =J.K.|publisher =Springer|year = 2004 |isbn = 3-540-40516-X|title = Multi-phased ceramic materials: processing and potential}}</ref> For a number of years, it was speculated that the existence of sparks or plasma between particles could aid sintering; however, Hulbert and coworkers systematically proved that the electric parameters used during spark plasma sintering make it (highly) unlikely.<ref>{{cite journal | last1 = Hulbert | first1 = D. M. | display-authors = etal | year = 2008 | title = The Absence of Plasma in' Spark Plasma Sintering' | doi = 10.1063/1.2963701 | journal = Journal of Applied Physics | volume = 104 | issue = 3| pages = 033305–033305–7 | bibcode = 2008JAP...104c3305H | s2cid = 54726651 | url = http://www.escholarship.org/uc/item/2c14z63t }}</ref> In light of this, the name "spark plasma sintering" has been rendered obsolete. Terms such as field assisted sintering technique (FAST), electric field assisted sintering (EFAS), and direct current sintering (DCS) have been implemented by the sintering community.<ref>Anselmi-Tamburini, U. et al. in Sintering: Nanodensification and Field Assisted Processes (Castro, R. & van Benthem, K.) (Springer Verlag, 2012).</ref> Using a [[direct current]] (DC) pulse as the electric current, spark plasma, spark impact pressure, joule heating, and an electrical field diffusion effect would be created.<ref name=Palmer>{{Cite book|last1 = Palmer|first1 = R.E.|last2 = Wilde|first2 = G.|title = Mechanical Properties of Nanocomposite Materials|publisher = Elsevier Ltd.|date = December 22, 2008|location =EBL Database|isbn = 978-0-08-044965-4}}</ref> By modifying the graphite die design and its assembly, it is possible to perform [[pressureless sintering]] in spark plasma sintering facility. This modified die design setup is reported to synergize the advantages of both conventional pressureless sintering and spark plasma sintering techniques.<ref>{{cite journal
|author=K. Sairam |author2=J.K. Sonber |author3=T.S.R.Ch. Murthy |author4=A.K. Sahu |author5=R.D. Bedse |author6=J.K. Chakravartty
|title=Pressureless sintering of chromium diboride using spark plasma sintering facility
|journal=International Journal of Refractory Metals and Hard Materials
|year=2016 |volume=58 |pages=165–171
|doi=10.1016/j.ijrmhm.2016.05.002
}}</ref>

=== Electro sinter forging ===
[[Electro sinter forging]] is an electric current assisted sintering (ECAS) technology originated from [[capacitor discharge sintering]]. It is used for the production of diamond metal matrix composites and is under evaluation for the production of hard metals,<ref>Fais, A. "Discharge sintering of hard metal cutting tools". ''International Powder Metallurgy Congress and Exhibition'', Euro PM 2013</ref> [[NiTiNOL|nitinol]]<ref>{{cite journal|doi=10.1016/j.intermet.2015.08.016|title=Electro-sinter-forged Ni–Ti alloy|journal=Intermetallics|volume=68|pages=31–41|year=2016|last1=Balagna|first1=Cristina|last2=Fais|first2=Alessandro|last3=Brunelli|first3=Katya|last4=Peruzzo|first4=Luca|last5=Horynová|first5=Miroslava|last6=Čelko|first6=Ladislav|last7=Spriano|first7=Silvia}}</ref> and other metals and intermetallics. It is characterized by a very low sintering time, allowing machines to sinter at the same speed as a compaction press.