June 20, 2011 - IMM@BUCT

+ MANUFACTURING EXCELLENCE +NANO LETT.SLOPPY CRYSTALS The randomorientation of microscopic grains(colored patches at left) in this bismuthantimony telluride specimen andnanometer-wide bismuth-rich regionsin between the grains (yellow area atright) play a key role in this material’sthermoelectric properties.both sets of lattices match perfectly in 3-D. As reported in a paperpublished earlier this year, the structure and composition of these“perfect” synthetic lattices block the propagation of phonons, butthey do not impede the transport of charge typically caused byembedded crystallites. This “rational design” approach led to a ZTvalue of 1.7 near 500 °C ( Nat. Chem., DOI: 10.1038/nchem.955).Investigations by Kanatzidis, Dresselhaus, and others havestimulated a recent rise in thermoelectrics research that is helpinguncover new structural and electronic phenomena as well asnovel types of promising materials. For example, in a TEM studyof bismuth antimony telluride samples with a ZT of 1.4, YuchengLan and Zhifeng Ren of Boston College, working with MIT’s GangChen, found that the bulk material contains numerous randomlyoriented nanosized grains dotted with precipitates. They also detectedbismuth-rich regions several nanometers thick in betweenthe grains ( Nano Lett., DOI: 10.1021/nl803235n).And just recently, a group led by California Institute of Technologymaterials science professor G. Jeffrey Snyder showed thatZT values as high as 1.8 could be coaxed from lead telluride (oftencharacterized by ZT below 1.0) by selectively doping the materialwith sodium and selenium in a way that customizes the material’selectronic structure. Specifically, the group reports that samplesof Pb 0.98 Na 0.02 Te 1– x Se x benefit from a high “valley degeneracy.” Interms of the electrons-in-the-corridor analogy, this material can bethought of as having multiple parallel corridors, which helps avoidelectronic congestion (C&EN, May 9, page 38).IN ADDITION to the commonly studied materials—ones basedon bismuth telluride, lead telluride, and related compounds—other classes of materials also figure into today’s research onthermoelectrics. At the University of California, Berkeley, forexample, chemistry professor Peidong Yang has shown thatIn 2- x Ga x O 3 (ZnO)n nanowires and holey silicon membranes, bothof which can be prepared via straightforward synthesis methods,show promise as thermoelectric materials.Yet another group of materials, skutterudites, which are cobaltantimony-basedcompounds containing rare-earth elements, figureprominently into research aimed at generating electricity fromwaste heat. Skutterudites appear to be well suited for this applicationbecause, unlike bismuth telluride, they have high ZT s at the hightemperatures typical of industrial and automobile exhaust systems.As part of a multiyear project partly sponsored by the Departmentof Energy, Gregory P. Meisner, James R. Salvador, and coworkers atGeneral Motors have been developing prototype devices that use automobileexhaust heat to produce electricity for onboard use.The team’s work, carried out withvarious partners including Dallas-basedMarlow Industries, shows that such devicescan readily provide fuel-economyimprovements of several percent. Withfurther development of hybrid cars andother types of electric vehicles, even greater benefits can be realizedfrom such a system, Meisner says.“Discovery of new materials has really propelled this field forward,”Meisner asserts. He adds that progress is now being madequickly across all of thermoelectrics. Yet he injects a note of cautionby pointing out that key engineering problems still need to be solved.In the case of exhaust-heat recovery, for example, the thermoelectricsystem must be highly robust and heat resistant. Furthermore, itsweight, volume, and cost all must be optimized, he adds.As Amerigon and BSST’s founder, Bell, sees it, if those types of limitationscan be overcome, “thermoelectrics could well play a crucialrole in addressing some of the sustainability issues we face today.” ■100% cGMP Manufacturedin the U.S.A.JOST CHEMICAL CO.PhosphatesUÊAmmonium Phosphate Monobasic/Dibasic NF/ACSUÊPotassium Phosphate Monobasic/Dibasic USP/NF/EP/BP/ACSUÊSodium Phosphate Monobasic USP/BP/ACSUÊSodium Phosphate Dibasic USP/EP/ACSUÊSodium Phosphate Tribasic ACSCarbonatesUÊSodium Carbonate NF/EP/JP/ACSJost Chemical Co. manufactures over 250high purity chemical salts that meet yourspecifications. All of our products are BSE/TSEfree and available in custom packaging.Pharmaceutical / Biotech Product OfferingsSulfatesUÊAmmonium Sulfate NF/ACSUÊCupric Sulfate USPUÊFerrous Ammonium Sulfate PurifiedUÊManganese Sulfate USP/EP/ACSUÊPotassium Sulfate EP/ACSUÊSodium Sulfate USP/ACSWWW.CEN-ONLINE.ORG 35 JUNE 20, 2011