June 20, 2011 - IMM@BUCT

SCIENCE & TECHNOLOGYMITCH JACOBY/C&ENTHERMOELECTRICSMAKE A COMEBACKNew concepts and materials invigorate acommercially active but obscure field specializingin HEATING, COOLING, AND POWER GENERATIONMITCH JACOBY , C&EN CHICAGO“TEN OR 15 YEARS AGO, nobody wantedto hear about thermoelectrics. Back then,people couldn’t even spell the word.”With playful exaggeration, MercouriG. Kanatzidis, a Northwestern Universitychemistry professor and materials specialist,makes the point that during the past decade,the field of thermoelectrics—whichencompasses a collection of heating, cooling,and power generation technologiesenabled by a unique class of semiconductors—isundergoing a renaissance.In the early 1960s, long after the curiouscollection of properties that definethermoelectric materials was discovered,manufacturers began producing specialtycooling and power-supply devices largelybased on the thermoelectric properties ofbismuth telluride, Bi 2 Te 3 . This niche market,which mainly served the military andaerospace industry, didn’t disappear, butthe next 30-plus years witnessed a steepdrop in interest in the topic.“In the mid-1990s, universities rarelytaught thermoelectrics in physics courses,”Kanatzidis points out. He adds, “It was aforgotten concept. Now it’s finally comingof age.”After years of sitting mostly unnoticedin a quiet corner of science, thermoelectricsis again drawing attention. In additionto supplying temperature managementand power products to the military andaerospace industry—for example as miniaturecoolers that chill the infrared detectorscentral to the imaging electronics inheat-seeking missiles and night-vision systems—thefew manufacturers in this areanow make millions of units each year fordown-to-earth civilian use. Their productsare found in climate-control automobileseats offered by major automakers, thermalcyclers for polymerase chain reactionsystems, and power generators for applicationsfar from an electrical grid.At the same time, researchers in industryand academia, motivated by recent fundamentalmaterials advances, are focusingtheir synthesis, analytical, and engineeringskills on discovering new thermoelectricmaterials and designing new ways to usethem. They hope these advances willprovide enhanced thermoelectric performanceand lead to a broader range ofproducts. Despite key advantages of thermoelectricpower and cooling systems relativeto conventional ones—for example,they have no moving parts, making themmechanically simpler, and they do not emitgreenhouse gases or depend on environmentallyharmful coolant fluids—only anarrow range of thermoelectric productsSHOP TALK Methodsto synthesize newthermoelectricsolids (samplein test tube) andanalyze them viaX-ray diffraction andother techniquesare key aspects ofresearch conductedby Northwestern’sKanatzidis (in vest)and coworkers.has been commercialized.One of thefield’s much talkedabout goals is tomake thermoelectricdevices thatwork well at hightemperature andcan generate electricitycost-effectivelyfrom wasteheat recovered, forexample, from industrialplants or automobile exhaust.The key observations that underpin thethermoelectric effect were made way backin the early 1800s. In 1821, German physicistThomas Johann Seebeck discoveredthat if a loop made from dissimilar metalsis exposed to a temperature gradient (oneside of the loop warm, the other cool) theloop can deflect a nearby compass needle.That set of conditions generates an electricalcurrent and a magnetic field.Another key observation was madeby French physicist Jean-Charles Peltierin 1834. Peltier found that if a current isapplied across a junction of dissimilarelectrically conductive materials, the junctioncan heat up or cool down. Reversingthe current flow, by switching the batteryhookups, for example, results in the oppositeheating or cooling effect.IN MOST MATERIALS, the properties thatcause a temperature change as a result ofan applied current are too weak to be usefulfor thermoelectric applications. The sameis true for those that generate an electricalcurrent in response to a temperaturegradient.In fact, materials that exhibit a pronouncedthermoelectric effect are ratherexotic, according to Lon E. Bell. Bell foundedthermoelectrics manufacturer Amerigon,based in Northville, Mich., and laterthe wholly owned thermoelectrics subsidiary,BSST, in Irwindale, Calif. For materialsto be useful thermoelectrically, he explains,they need to be good electrical conductorsand, at the same time, poor thermalconductors. They also must prominentlydisplay the effect observed by Seebeck.One of the challenges in finding usefulmaterials is that the desired qualities areinterrelated via the substance’s electronicproperties: Improvement in one propertyoften comes at the expense of another.In these materials, electrons serve as the“working fluid,” just as liquid refrigerantsWWW.CEN-ONLINE.ORG 33 JUNE 20, 2011