New 3-D printed ‘meta­ma­te­rial’ de­fy­ing the laws of physics

China Daily (USA) - - ACROSS AMERICA - Chris Davis Con­tact­thewrit­er­atchris­davis@chi­nadai­

Any­one who’s ever had gram­mar school sci­ence knows that heat makes things ex­pand. Rub­ber, glass, gran­ite or steel, heat it up and it grows in vol­ume. Right?

There are in na­ture a few very rare cases of ma­te­ri­als that buck this ther­mo­dy­namic rule and ac­tu­ally shrink when heated. Not many. Cold wa­ter, when heated from 0 to 4 de­grees Cel­sius, will ac­tu­ally con­tract be­fore ex­pand­ing.

Thanks to decades-old the­o­ries and 3-D printer tech­nol­ogy, a team of engi­neers at MIT and USC is adding to this odd-ball group of heat-shrink­ing ma­te­ri­als.

Led by Ni­cholas X. Fang, an as­so­ciate pro­fes­sor of me­chan­i­cal en­gi­neer­ing at MIT who got his master’s in physics at Nan­jing Uni­ver­sity, the team has man­u­fac­tured tiny, star­shaped struc­tures out of in­ter­con­nected beams and trusses, each about the size of a sugar cube, that quickly shrink when heated to about 540 de­grees Fahren­heit.

Each truss is made from ma­te­ri­als that ex­pand with heat, but Fang and his col­leagues re­al­ized that th­ese trusses, if ar­ranged in par­tic­u­lar ar­chi­tec­tural con­fig­u­ra­tion, could ac­tu­ally pull the over­all struc­ture in on it­self, caus­ing it to shrink like a Hober­man sphere — those col­lapsi­ble toy balls made of ge­o­desic dome­like lat­tices and joints.

Dub­bing the struc­ture a “meta­ma­te­rial” — a com­pos­ite that ex­hibits strange, coun­ter­in­tu­itive prop­er­ties not nor­mally found in na­ture — the sci­en­tists say that just by not ex­pand­ing when heated it could be es­pe­cially use­ful for com­puter chips, which can warp and de­form when heated over long pe­ri­ods of time.

“Printed cir­cuit boards can heat up when there’s a CPU run­ning, and this sud­den heat­ing could af­fect their per­for­mance,” Fang says. “So you re­ally have to take great care in ac­count­ing for this ther­mal stress or shock.”

The re­sults of their ex­per­i­ments, funded in part by the Pen­tagon, were pub­lished in the re­cent edi­tion of the jour­nal Phys­i­cal Re­view Let­ters.

The the­ory of neg­a­tive ther­mal ex­pan­sion, or NTE, goes back to the 1990s when sci­en­tists the­o­rized about build­ing three-di­men­sional, lat­tice-like struc­tures from two types of ma­te­ri­als, each with dif­fer­ent ex­pan­sion rates. When heated, one ma­te­rial should ex­pand faster, pulling the other in­ward and shrink­ing the whole as a re­sult.

“Th­ese the­o­ret­i­cal pa­pers were talk­ing about how th­ese types of struc­tures could re­ally break the con­ven­tional limit of ther­mal ex­pan­sion,” Fang says. “But at the time, they were lim­ited by how things were made. That’s where we saw this as a very good op­por­tu­nity for mi­cro­fab­ri­ca­tion to demon­strate this con­cept.”

Fang’s lab pi­o­neered a 3-D print­ing tech­nique called mi­crostere­olithog­ra­phy, which uses light from a pro­jec­tor to print very small struc­tures in liq­uid resin, layer by layer.

“We can now use the mi­crostere­olithog­ra­phy sys­tem to cre­ate a ther­mo­me­chan­i­cal meta­ma­te­rial that may en­able ap­pli­ca­tions not pos­si­ble be­fore,” said team mem­ber Christo­pher Spadac­cini, who is di­rec­tor of the Lawrence Liver­more Na­tional Lab­o­ra­tory’s cen­ter for en­gi­neered ma­te­ri­als and man­u­fac­tur­ing. “It has ther­mo­me­chan­i­cal prop­er­ties not achiev­able in con­ven­tional bulk ma­te­ri­als.”

“We can take the same idea as an inkjet printer, and print and so­lid­ify dif­fer­ent in­gre­di­ents, all on the same tem­plate,” Fang says.

Fang and his col­leagues printed small, three-di­men­sional, star-shaped struc­tures made from in­ter­con­nect­ing beams. They fab­ri­cated each beam from one of two in­gre­di­ents: a stiff, slow-ex­pand­ing cop­per ma­te­rial for the out­side, and a more elas­tic, faster-ex­pand­ing poly­mer for the in­side.

“If we have proper place­ment of th­ese beams and lat­tices,” Fang said, “once we in­crease the tem­per­a­ture, they in­ter­act with each other and pull in­ward, so the over­all struc­ture’s vol­ume de­creases.”

The re­searchers put their com­pos­ite struc­tures to the test by plac­ing them within a small glass cham­ber and slowly in­creas­ing the cham­ber’s tem­per­a­ture, from room tem­per­a­ture to about 540 de­grees Fahren­heit. At first it main­tained its shape, then grad­u­ally shrunk.

Not by much — about 0.6 per­cent to be pre­cise, but that is still sig­nif­i­cant. “The very fact that it shrinks is im­pres­sive,” Fang said, for most ap­pli­ca­tions, some­thing that doesn’t ex­pand un­der heat would be enough.

“There is room to ex­per­i­ment with other ma­te­ri­als,” Fang said. “Now we can have more fun in the lab ex­plor­ing th­ese dif­fer­ent struc­tures.”

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