A thermoelectric generator, operating in a cryostat at liquid helium temperatures, is described. Energy to the generator is supplied via an external laser beam. For this prototype device the associated heat load at permanent operation is comparable with the heat load associated with power delivery via metallic wires. Estimates indicate that still better performance can be enabled with existing thermoelectric materials, thereby far exceeding efficiency of traditional cryostat wiring. We used a prototype generator to produce electric power for measuring critical currents in Nb3Sn-films at 4K

Gulian, Armen

Harutyunyan, S. R.

Kunii, S.

Kuzanyan, A. S.

Nikoghosyan, V. R.

Vardanyan, V. H.

Winzer, K.

Wood, K. S.

English

Chapman University Digital Commons

Chapman University Chapman University Digital Commons Mathematics, Physics, and Computer Science Faculty Articles and Research Science and Technology Faculty Articles and Research 11-4-2005 Laser-Powered Thermoelectric Generators Operating at Cryogenic Temperatures S. R. Harutyunyan National Academy of Sciences of Armenia V. H. Vardanyan National Academy of Sciences of Armenia A. S. Kuzanyan National Academy of Sciences of Armenia V. R. Nikoghosyan National Academy of Sciences of Armenia S. Kunii Tohoku University See next page for additional authors Follow this and additional works at: https://digitalcommons.chapman.edu/scs_articles  Part of the Condensed Matter Physics Commons, and the Other Physics Commons Recommended Citation Harutyunyan S.R., Vardanyan V.H., Kuzanyan A.S., Nikoghosyan V.R., Kunii S., Winzer K., Wood K.S., and Gulian A.M., Laser-powered thermoelectric generators operating at cryogenic temperatures. Appl. Phys. Lett., 2005, vol. 87, No. 19, art. 194114. https://doi.org/10.1063/1.2131202 This Article is brought to you for free and open access by the Science and Technology Faculty Articles and Research at Chapman University Digital Commons. It has been accepted for inclusion in Mathematics, Physics, and Computer Science Faculty Articles and Research by an authorized administrator of Chapman University Digital Commons. For more information, please contact laughtin@chapman.edu. Laser-Powered Thermoelectric Generators Operating at Cryogenic Temperatures Comments This article was originally published in Applied Physics Letters, volume 87, issue 19, in 2005. https://doi.org/10.1063/1.2131202 Copyright American Institute of Physics Authors S. R. Harutyunyan, V. H. Vardanyan, A. S. Kuzanyan, V. R. Nikoghosyan, S. Kunii, K. Winzer, K. S. Wood, and Armen Gulian This article is available at Chapman University Digital Commons: https://digitalcommons.chapman.edu/scs_articles/767 Laser-powered thermoelectric generators operating at cryogenictemperaturesS. R. Harutyunyan, V. H. Vardanyan, A. S. Kuzanyan, V. R. Nikoghosyan, S. Kunii et al.  Citation: Appl. Phys. Lett. 87, 194114 (2005); doi: 10.1063/1.2131202 View online: http://dx.doi.org/10.1063/1.2131202 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v87/i19 Published by the AIP Publishing LLC.  Additional information on Appl. Phys. Lett.Journal Homepage: http://apl.aip.org/ Journal Information: http://apl.aip.org/about/about_the_journal Top downloads: http://apl.aip.org/features/most_downloaded Information for Authors: http://apl.aip.org/authors Downloaded 30 Aug 2013 to 150.216.68.200. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissionsLaser-powered thermoelectric generators operating at cryogenictemperaturesS. R. Harutyunyan, V. H. Vardanyan, A. S. Kuzanyan, and V. R. NikoghosyanInstitute for Physics Research, National Academy of Sciences, Ashtarak 378410, ArmeniaS. KuniiDepartment of Physics, Tohoku University, Sendai 980-8578, JapanK. WinzerPhysics Institute I, University of Goettingen, D-37077 Goettingen, GermanyK. S. WoodNaval Research Laboratory, Washington, DC 20375A. M. GulianaPhysics Art Frontiers, Ashton, Maryland 20861-9747Received 21 July 2005; accepted 4 October 2005; published online 4 November 2005A thermoelectric generator, operating in a cryostat at liquid helium temperatures, is described.Energy to the generator is supplied via an external laser beam. For this prototype device theassociated heat load at permanent operation is comparable with the heat load associated with powerdelivery via metallic wires. Estimates indicate that still better performance can be enabled withexisting thermoelectric materials, thereby far exceeding efficiency of traditional cryostat wiring. Weused a prototype generator to produce electric power for measuring critical currents in Nb3Sn-filmsat 4 K. © 2005 American Institute of Physics. DOI: 10.1063/1.2131202Recently, it was noted that crystalline samples of CeB6Kondo metal have relatively high values of the figure ofmerit ZT0.2 at cryogenic temperatures.1 HereZT=TS2 /k, where T is the temperature, S is the Seebeckcoefficient, and  and k are electric and thermal conductivi-ties. Using this CeB6 thermoelectric, we have demonstratedon-demand generation of electric power on a cryogenic plat-form. There are no wires, but rather energy is being deliveredto a cold stage by a laser beam, and converted to electricityby the thermoelectric generator TEG. Doing this has manyadvantages including reduction and control over heat input,reduction of noise, and easy external manipulation.For a TEG the efficiency can be described by the Ioffeexpression2 =T1 − T0T1·1 + ZT1 + T0/2 − 11 + ZT1 + T0/2 + T0/T1· 100 % , 1which assumes that Z is not changing much between the coldT0 and hot T1 ends of the thermoelectric element. In caseZ varies, one can use for an estimate the mean value. AsFig. 1 demonstrates for crystals studied,1 the expected valuesof  can be as high as 3% at T=T1−T0=5 K at ambientliquid helium temperatures T04 K.For prototyping a laser-powered TEG Fig. 2 we use a110 oriented CeB6 crystal 8.1 mm1.46 mm0.93 mmin size.1,3,4 The sample is attached to a quartz holder by ametallic indium pad. This holder is placed on a copper baseand serves as a heat sink. The circuit consists of a supercon-ducting Nb3Sn foil critical temperature Tc=18 K, connect-ing the opposite ends of the CeB6 crystal via thin copperwires and rubbed-in In pads. The geometry of the SC foil isconstrained such that its critical current equals approximatelythe maximum current generated by the TEG. The resistanceof the crystal several m constitutes the internal resistancer of the TEG, while the resistance of the attached circuitcan be considered as a load resistance. The expression 1 isan optimization for Rload=r case.We excite the TEG by applying a diode laser=640 nm, or, sometimes, by resistive heating. The resis-tive heater is arranged directly on the free end of the crystal.The laser beam enters through a cryostat window and hits anabsorption cup mounted on the opposite surface of the crys-tal. A differential copper-constantan thermocouple is used tomeasure T=T1−T0. The absorbed radiation power Wh hasbeen determined comparing Tlaser with the Tresistive heater.The measurements were carried out at ambient temperaturesT0=3.5–6.5 K, in the ST405 “CRYOMECH” cryostat. Theabsolute accuracy of measurements is about 10%.aAuthor to whom correspondence should be addressed; electronic mail:armen.gulian@nrl.navy.milFIG. 1. Generator efficiency corresponding to Eq. 1 based on the values ofZT from the previous report see Ref. 1.APPLIED PHYSICS LETTERS 87, 194114 20050003-6951/2005/8719/194114/3/$22.50 © 2005 American Institute of Physics87, 194114-1Downloaded 30 Aug 2013 to 150.216.68.200. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissionsThe quantitative performance of the TEG is character-ized by using its power to transition superconducting films tothe normal state. Comparing critical currents for the super-conducting foil determined in the open-circuit I-V modewhen the current is applied externally with the critical cur-rent of the closed circuit when the current is generated bythe Seebeck effect, we determine the generated current. Spe-cial care is taken to correct for an additional contact resis-tance 0.5 m in case of the closed circuit. The I-V cali-bration of superconducting foil is performed in the open-circuit mode for all the values of the external heating powerWh and ambient temperatures T0, which subsequently wereused for closed-circuit measurements. The generated currentas a function of applied heating is shown in Fig. 3. As ex-pected, the maximal current I43.6 mA corresponds tothe minimal load 1 m. When the current is highenough, the foil goes to the normal state and the load resis-tance grows. Thus, Rload depends on the Wh as well as on T0.Meanwhile, the intrinsic resistance r also depends on T anddrops in the vicinity of T0 Kondo effect when the tempera-ture increases. As a result, Rload3.5 m is optimal at smallpowers Wh. At higher Wh optimal Rload is smaller. The maxi-mal output 4.1 W is generated at Rload2.5 m at theambient temperature T0=3.5 K.The efficiency of heat release from the crystal to the heatsink is essential to determining TEG efficiency. Given the0.5 m contact resistance, the thermal conductivity is0.55 mW cm−1 K−1, which is less than the thermal conduc-tivity of the crystal at least a factor of 10. For this reason, atlarge heat loads there is a large temperature differencebetween the cold end and the heat sink. As a result, theoperational range of the TEG shifts to a non optimalZTT regime. This reduces the device efficiency. Thevalues = WR /Wh100% are plotted in Fig. 3. HereWR= I2Rload. The maximum value of 0.22% is reachedat Rload3.5 m at Wh=1.57 mW and T0=3.5 K. Note theapplied heat load Wh is not the absorbed heat load Wa. Thisvalue for the given case can be estimated as Wa640 W,hence, parasitic losses heat flow via contact wires, thermo-couple, superconducting foil, etc. are huge. In accordancewith the equation V=ST= IR+r, the Seebeck voltage isV200 V which corresponds to T2 K at theRload=3.5 m. Since the temperature of the hot end is8.5 K, the temperature of the cold end is 6.5 K ratherthan 3.5 K. Correspondingly, it follows 0.55% for theefficiency defined as = WR /Wa100%. This value corre-sponds well to the theoretical estimate 0.7%, which fol-lows from Eq. 1 at ZT0.12—which is the average valuenear T=7.5±1 K.In practice, higher values of efficiency close to the the-oretical limits are achievable at higher values of T, which ispossible if the sample has an optimal geometry a thin rodwith a large base to allow the required heat outflow. Also,parasitic heat losses should be minimized. Of course, it isbetter to work with the higher values of ZT. It should benoticed in this respect that there are published data on CeB6crystal5 with values of the Seebeck coefficient twice as largeas ours. Such crystals, provided  /k is as high as ours, canquadruple TEG efficiency.Another interesting opportunity is related to the Kondocrystals La1−xCexB6 at x0.01 Ref. 6 with values ofS100 V/K at T0.3–0.8 K.7 We measured heat andelectric conductivity of these crystals8 to make sure thatk /T reaches the Lorenz number L025 nW  /K2 belowFIG. 4. Efficiency and figure of merit for La1−xCexB6 crystals at x=0.02and 0.005 of prospective thermoelectric generators.FIG. 2. Color online Schematic of the generator experiment.FIG. 3. Generated current solid lines and efficiency dotted lines atdifferent values of external resistive loads and ambient temperature 3.5 K.194114-2 Harutyunyan et al. Appl. Phys. Lett. 87, 194114 2005Downloaded 30 Aug 2013 to 150.216.68.200. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissions4 K temperatures, so that one can plot the figure of meritZTT Fig. 4. Relying on this curve and the Eq. 1, effi-ciencies 10% and higher could be achieved at 0.3 K am-bient temperatures.In conclusion, we have demonstrated that thermoelectricmaterials can supply electric power at cryogenic tempera-tures, with power supplied by laser beams. No wiring is re-quired. The estimates show that at efficiencies better than0.5% thermoelectric generators provide better performancein terms of the heat load/electric current than existing me-tallic wire solutions.9 We reached this limit, and it can clearlybe surpassed by existing technologies. Note, that one canalso activate different circuits by switching on the corre-sponding generator in multiple beam/generator arrangement.The laser delivery of electric power can be performed selec-tively in time on demand, so that there is no permanentheat load to the cold finger of the cryostat. It may be essen-tial in cases when the long cryogenic lifetimes are manda-tory, for example, in space-based applications, and a perma-nent power supply is not required.1S. Harutyunyan, V. Vardanyan, A. Kuzanyan, V. Nikoghosyan, S. Kunii,K. Wood, and A. Gulian, Appl. Phys. Lett. 83, 2142 2003.2A. F. Ioffe, Semiconductor Thermoelements and Thermo-Electric CoolingInfosearch, London, 1957.3S. Kunii, J. Phys. Soc. Jpn. 57, 361 1988.4A. Tomita and S. Kunii, J. Magn. Magn. Mater. 108, 165 1992.5N. Ali and S. B. Woods, J. Appl. Phys. 57, 3182 1985.6K. Winzer, Solid State Commun. 16, 521 1875.7H. J. Ernst, H. Gruhl, T. Krug, and K. Winzer, in Proc. LT-17, edited byU. Eckern, A. Schmid, W. Weber, and H. Wuehl Elsevier, New York,1984, pp. 137–138.8S. R. Harutyunyan, V. H. Vardanyan, A. S. Kuzanyan, and V. R. Nikog-hosyan, S. Kunii, K. Winzer, K. S. Wood, and A. M. Gulianunpublished.9The authors are grateful to Dr. A. Kadin who outlined this point to them.194114-3 Harutyunyan et al. Appl. Phys. Lett. 87, 194114 2005Downloaded 30 Aug 2013 to 150.216.68.200. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissions

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Laser-Powered Thermoelectric Generators Operating at Cryogenic Temperatures

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