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* '''Electronic Fillers''': This is the oldest approach to making an [[elastomeric]] material elastically stretchable. In principle, rigid and [[electrically conductive]] materials and mixed with an elastomeric [[polymer]] before curing to create stretchable composites. If the concentration of the electrically conductive filler is high enough they form a mesh-like [[percolation]] network that facilitates the free movement of charge carriers (ions, electrons) through contact junctions. The minimum concentration of the electronic filler material that is required to create conductive pathways for [[charge carrier]] transport through the elastomer<ref>{{cite journal |last1=Kyrylyuk |first1=Andriy V. |last2=van der Schoot |first2=Paul |title=Continuum percolation of carbon nanotubes in polymeric and colloidal media |journal=Proceedings of the National Academy of Sciences |date=17 June 2008 |volume=105 |issue=24 |pages=8221–8226 |doi=10.1073/pnas.0711449105 |doi-access=free |pmid=18550818 |pmc=2448818 }}</ref> is called the [[percolation threshold]].<ref>{{cite book |doi=10.1016/B978-1-895198-95-9.50011-X |chapter=Structure and Distribution of Non-Migrating Antistatics |title=Handbook of Antistatics |date=2016 |pages=117–127 |isbn=978-1-895198-95-9 |editor1-first=Jürgen |editor1-last=Pionteck |editor2-first=George |editor2-last=Wypych }}</ref> The [[percolation threshold]] is usually indicated as weight percentage (wt%) or volume percentage (vol%) of the filler material, and ranges from less than 1wt% for high aspect ration carbon [[nanotubes]] to over 15wt%. The type of filler materials ranges from metals in powder or [[nanowire]] form, [[carbon]] as [[graphite]] or [[nanotubes]], to electrically conducting polymers.
* '''‘Wavy’ [[Nanowires]] and Nanoribbons''': The spontaneous formation of wavy patterns of aligned [[buckles]] that is caused by the deposition of a thin gold film on the surface of the [[elastomer]] [[polydimethylsiloxane]] (PDMS) was first described by the group of George Whitesides at Harvard University in 2000.<ref>{{cite journal |last1=Huck |first1=Wilhelm T. S. |last2=Bowden |first2=Ned |last3=Onck |first3=Patrick |last4=Pardoen |first4=Thomas |last5=Hutchinson |first5=John W. |last6=Whitesides |first6=George M. |title=Ordering of Spontaneously Formed Buckles on Planar Surfaces |journal=Langmuir |date=April 2000 |volume=16 |issue=7 |pages=3497–3501 |doi=10.1021/la991302l }}</ref> The gold was deposited on warmed PDMS (100 °C), and, upon cooling and the associated thermal [[shrinkage]] of the elastomer, the gold film comes under compressive stress which is relieved by creating [[buckles]]. In subsequent years, the group of John Rogers at the University of Urbana Champaign (now at Northwestern University) has developed the technology to bond very thin silicon ribbons to a pre-stretched PDMS membrane. Upon relaxation of the per-stretch, the compressive [[mechanical stress]] in the [[silicon]] ribbons is relieved by creating wavy buckles in the PDMS. As silicon is a brittle material, the ribbons need to very thin (about 100 nm) to stay intact during buckling.<ref>{{cite journal |last1=Kim |first1=Dae-Hyeong |last2=Rogers |first2=John A. |title=Stretchable Electronics: Materials Strategies and Devices |journal=Advanced Materials |date=17 December 2008 |volume=20 |issue=24 |pages=4887–4892 |doi=10.1002/adma.200801788 }}</ref>
* '''Liquid Metals''': A [[metal]] or [[alloy]] that is liquid at room temperature can be enclosed in PDMS and used as a stretchable [[Electrical conductor|conductor]]. [[Mercury (element)|Mercury]] is the only pure metal that is liquid at room temperature but has limited application due to its [[neurotoxicity]]. Cesium melts at 28.5 °C, but reacts violently when exposed to air and is therefore not suitable for this application. Most researchers therefore use an [[eutectic]] mixture of Indium and Gallium, so called EGaIn, which has a melting point is 15.7 °C and consists of 75.5% Gallium and 24.5% Indium. A eutectic mixture of Ga (68.5%), In (21.5%) and Sn (10.0%), also known as [[Galinstan]], is another popular choice and has a melting point of 10.5 °C.
* '''Microcracked gold thin film''': When a thin gold film is deposited on PDMS under certain conditions,<ref>{{cite journal |last1=Graudejus |first1=Oliver |last2=Görrn |first2=Patrick |last3=Wagner |first3=Sigurd |title=Controlling the Morphology of Gold Films on Poly(dimethylsiloxane) |journal=ACS Applied Materials & Interfaces |date=28 July 2010 |volume=2 |issue=7 |pages=1927–1933 |doi=10.1021/am1002537 |pmid=20608644 }}</ref> the gold film adopts a microcracked morphology<ref>{{cite journal |last1=Lacour |first1=Stéphanie P. |last2=Chan |first2=Donald |last3=Wagner |first3=Sigurd |last4=Li |first4=Teng |last5=Suo |first5=Zhigang |title=Mechanisms of reversible stretchability of thin metal films on elastomeric substrates |journal=Applied Physics Letters |date=15 May 2006 |volume=88 |issue=20 |doi=10.1063/1.2201874 |url=http://nrs.harvard.edu/urn-3:HUL.InstRepos:41467478 }}</ref> which makes the gold stretchable. The maximum [[Strain (mechanics)|strain]] of the film decreases with the length and increases with the width of the conductor.<ref>{{cite journal |last1=Graudejus |first1=O. |last2=Jia |first2=Zheng |last3=Li |first3=Teng |last4=Wagner |first4=S. |title=Size-dependent rupture strain of elastically stretchable metal conductors |journal=Scripta Materialia |date=June 2012 |volume=66 |issue=11 |pages=919–922 |doi=10.1016/j.scriptamat.2012.02.034 |pmid=22773917 |pmc=3388513 }}</ref>
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