The thermal flash method was developed to characterize the thermal diffusivity of micro/nanofibers without concern for thermal contact resistance, which is commonly a barrier to accurate thermal measurement of these materials. Within a scanning electron microscope, a micromanipulator supplies instantaneous heating to the micro/nanofiber, and the resulting transient thermal response is detected at a microfabricated silicon sensor. These data are used to determine thermal diffusivity. Glass fibers of diameter 15 mu m had a measured diffusivity of 1.21x10(-7) m(2)/s; polyimide fibers of diameters 570 and 271 nm exhibited diffusivities of 5.97x10(-8) and 6.28x10(-8) m(2)/s, respectively, which compare favorably with bulk values

Abramson, Alexis R.

Cakmak, Miko

Dai, Zhenting

Demko, Michael T.

King, William P.

Yan, Han

English

The University of Akron

The University of AkronIdeaExchange@UAkronCollege of Polymer Science and Polymer Engineering3-19-2009Application of the Thermal Flash Technique forLow Thermal Diffusivity Micro/NanofibersMichael T. DemkoCase Western Reserve UniversityZhenting DaiUniversity of Illinois at Urbana-ChampaignHan YanUniversity of Akron Main CampusWilliam P. KingUniversity of Illinois at Urbana-ChampaignMiko CakmakUniversity of Akron Main Campus, cakmak1@uakron.eduSee next page for additional authorsPlease take a moment to share how this work helps you through this survey. Your feedback will beimportant as we plan further development of our repository.Follow this and additional works at: http://ideaexchange.uakron.edu/polymer_ideasPart of the Polymer Science CommonsThis Article is brought to you for free and open access by IdeaExchange@UAkron, the institutional repository ofThe University of Akron in Akron, Ohio, USA. It has been accepted for inclusion in College of Polymer Science andPolymer Engineering by an authorized administrator of IdeaExchange@UAkron. For more information, pleasecontact mjon@uakron.edu, uapress@uakron.edu.Recommended CitationDemko, Michael T.; Dai, Zhenting; Yan, Han; King, William P.; Cakmak, Miko; and Abramson, Alexis R.,"Application of the Thermal Flash Technique for Low Thermal Diffusivity Micro/Nanofibers" (2009). College ofPolymer Science and Polymer Engineering. 1.http://ideaexchange.uakron.edu/polymer_ideas/1AuthorsMichael T. Demko, Zhenting Dai, Han Yan, William P. King, Miko Cakmak, and Alexis R. AbramsonThis article is available at IdeaExchange@UAkron: http://ideaexchange.uakron.edu/polymer_ideas/1Application of the thermal flash technique for low thermal diffusivitymicro/nanofibersMichael T. Demko,1 Zhenting Dai,2 Han Yan,3 William P. King,2 Miko Cakmak,3 andAlexis R. Abramson1,a1Department of Mechanical and Aerospace Engineering, Case Western Reserve University,Cleveland, Ohio 44106, USA2Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign,Champaign, Illinois 61801, USA3Department of Polymer Engineering, University of Akron, Akron, Ohio 44325, USAReceived 31 October 2008; accepted 1 February 2009; published online 19 March 2009The thermal flash method was developed to characterize the thermal diffusivity of micro/nanofiberswithout concern for thermal contact resistance, which is commonly a barrier to accurate thermalmeasurement of these materials. Within a scanning electron microscope, a micromanipulatorsupplies instantaneous heating to the micro/nanofiber, and the resulting transient thermal response isdetected at a microfabricated silicon sensor. These data are used to determine thermal diffusivity.Glass fibers of diameter 15 m had a measured diffusivity of 1.2110−7 m2 /s; polyimide fibersof diameters 570 and 271 nm exhibited diffusivities of 5.9710−8 and 6.2810−8 m2 /s,respectively, which compare favorably with bulk values. © 2009 American Institute of Physics.DOI: 10.1063/1.3086310Micro- and nanofibers are one-dimensional structureswith diameters ranging from several nanometers i.e., nanofi-bers to 100 m i.e., microfibers. These materials offerpromise for use in nanolasers,1 photonics,2 photovoltaics,3thermoelectrics,4 sensors and actuators,5 functional semicon-ductor electronic components,6 and polymer electronics.7Accurate thermal characterization of micro/nanofibers iscritical for their integration into these and other applications.The development of a rapid, simple, accurate, and reliablemethod to measure their thermal properties would facilitatetheir use and expand fundamental understanding of nano-scale thermal transport.For measurement of the thermal properties of micro/nanostructures, various steady-state methods have beenproposed.8,9 These techniques use the steady-state flux ofheat through a nanowire from a heater to a sensor to deter-mine thermal conductivity. However, thermal contact resis-tances in these systems are difficult to accurately estimate.10Some transient thermal measurements have been employedin which an estimation of thermal contact resistance isunnecessary.11–14 These methods use the change in electricalresistance of a thermal sensor to measure the transient ther-mal response to a sudden application of heat. This approach,however, requires the micro/nanowires to be electricallyconductive or to be coated with a thin, electrically conduc-tive material. The positioning and attachment of micro/nanostructures to required sensors further complicate thisapproach.This work reports on the development of a transient ther-mal measurement technique for one-dimensional micro/nanostructures which is administered within a scanning elec-tron microscope SEM. For this thermal flash method,thermal contact resistance is inconsequential and variouscomplexities typically associated with this type of measure-ment are minimized. While this method examines the ther-mal diffusivity rather than conductivity, the conductivity maybe obtained though the knowledge on the material densityand specific heat. In the particular case of micro/nanofibers,if it can be assumed that the density and specific heat aresimilar to their corresponding bulk values, then the thermaldiffusivity can be used to calculate thermal conductivity. Thetechnique builds on previous work to measure the mechani-cal properties of micro/nanowires15 and expands upon thelaser flash method in which a sudden application of radiantheat is applied to a sample after which a transient change intemperature is detected.16 A wire-wrapped micromanipulatorsupplies instantaneous heating to one end of the micro/nanostructure; the temperature response is then detectedalong the micro/nanofibers using a microfabricated sensor.17The transient electrical resistance of the sensor allows calcu-lation of thermal diffusivity. The technique is validated forlow diffusivity materials by measuring the properties of bothglass and polyimide micro/nanofibers.Figure 1 shows the experimental configuration and Fig.2 depicts a SEM image of the setup for the case of a poly-imide microfiber. A doped silicon cantilever acts as the ther-mal sensor17 and is employed in a Wheatstone bridge circuitalong with three precision potentiometers; the bridge is bal-anced prior to the measurement. Inside the SEM FEI XTNova nanoLab 200, a micro/nanofiber is positioned betweenthe thermal sensor and an aluminum prop, which serves tokeep the sample suspended in vacuum, with the help of amicromanipulator Kleindiek MM3A. If necessary, electronbeam ion deposition of platinum is performed using a mi-crodelivery gas-injection system to form a bond between themicro/nanostructure and the sensor. The micromanipulator,which is wrapped with a 0.127 mm diameter polyimide-aElectronic mail: alexis.abramson@case.edu.REVIEW OF SCIENTIFIC INSTRUMENTS 80, 036103 20090034-6748/2009/803/036103/3/$25.00 © 2009 American Institute of Physics80, 036103-1coated nichrome wire through which a current is sent, is thenmoved close to the micro/nanofiber. The electron beam of theSEM is turned off and the micromanipulator is heated to atemperature not exceeding the glass transition temperature ofthe polyimide fiber and then brought into sudden contactwith the suspended structure. The relatively large size andhigh thermal conductivity of the micromanipulator justify theapproximation that it can be modeled as a high temperaturethermal reservoir, therefore maintaining a constant tempera-ture at the point of contact. Heat flows along the micro/nanofiber to the sensor, causing the Wheatstone bridge tobecome unbalanced. The transient voltage at the center of thebridge is monitored with a nanovoltmeter Keithley 2182A.The amount of heat flowing along the micro/nanofiber in theother direction toward the aluminum prop does not influencethe final determination of thermal diffusivity since knowl-edge on the absolute magnitude of the heat flux is not re-quired for the analysis as described below.An analytical solution representative of the experimentwas derived from the heat equation and used to determinethe thermal diffusivity of the micro/nanofibers from the data.The normalized temporal variation in temperature T¯ t at thecontact point between the micro/nanofiber and the sensor canbe expressed as18T¯ t = 1 + 2n=1− 1nefn22t/lf2. 1Here, the subscript f refers to the micro/nanofiber; lf is itslength, and  f is its thermal diffusivity. The analytical modelassumes that at time t equals zero, the heated micromanipu-lator touches the micro/nanofiber, resulting in an instanta-neous temperature rise at the point of contact. The micro/nanofiber is modeled as being in perfect contact with thesilicon thermal sensor, which itself is affixed to a large sili-con body that is assumed to be at constant temperature.Equation 1 was simplified from its original form by assum-ing thatkskf fs 1 and  fslfls, 2where the subscript s now refers to the sensor and k is ther-mal conductivity. These assumptions are applicable for lowthermal conductivity/diffusivity materials and for micro/nanofibers of sufficiently long length. Since the transientvariation in temperature is required to extract thermal diffu-sivity and not the magnitude of temperature itself, an offsetdue to thermal contact resistance will not influence the finalresult. Experimentally, the voltage measured at the center ofthe Wheatstone bridge also follows the change in tempera-ture experienced by the sensor. Consequently, the normalizedvoltage difference measured as a function of time may alsobe modeled by Eq. 1, and the analytical solution can beused in conjunction with the normalized experimental volt-age data to extract the thermal diffusivity of the micro/nanofiber. A curve fitting procedure minimizes the sum of thesquared errors between the analytical solution and the experi-mental data.The experimental technique was verified usingphosphate-based glass available commercially Mo-SciCorp. and polyimide micro/nanofibers. The polyimidenanofibers were prepared by first electrospinning thepolyamic acid precursor solution DuPont. The electrospin-ning apparatus was operated with 10 kV electrical potentialapplied between the spinning nozzle and grounded collectorthat are separated by 15 cm air gap. The collected fibers werefound to be mostly devoid of solvents. The completion ofsolvent removal and imidization reaction was carried out bystepwise heating of the fibers in vacuum at 50 °C for 24 h,100 °C for 2 h, 175 °C for 1 h, and 350 °C for 1 h. Anexample of the experimental data used to extract thermaldiffusivity is shown in Fig. 3 for the case of the glass mi-crofibers. Here, the exponential rise in voltage versus time isshown to be well modeled by the analytical solution dis-cussed previously. The thermal diffusivity data are summa-rized in Fig. 4. For the 15 m diameter glass fibers withlengths varying from 403 to 490 m, thermal diffusivitywas determined to be 1.210.1610−7 m2 /s. The errorwas taken to be the standard deviation of the eleven mea-surements made. Note that even potential inaccuracies asso-ciated with the length measurement of the microfiber of10% still resulted in a thermal diffusivity within the errorbars given. As a comparison, a bulk form of the glass samplewas measured using the numerical mirage method,19 giving aFIG. 1. Color online A schematic of the thermal flash method.FIG. 2. Color online An SEM image of the thermal flash measurement fora polyimide micro/nanofiber.036103-2 Demko et al. Rev. Sci. Instrum. 80, 036103 2009result of 1.20.310−7 m2 /s, providing confidence inthe technique. Two different sets of polyimide fibers of570 and 271 nm diameter were also measured with lengthsvarying from 163 to 272 m. The resulting thermal diffu-sivities were determined to be 5.970.7110−8 and6.280.6310−8 m2 /s for the 570 and 271 nm fibers,respectively. It should be noted that the 271 nm fibers are notexpected to exhibit significant effects of enhanced thermalboundary scattering or other nanoscale mechanisms thatmight be present in sub-100 nm fibers, and therefore theirthermal diffusivity should be comparable to that of the570 nm diameter fibers. A bulk form of the polyimide wasmeasured using the dynamic plane source technique,20resulting in a thermal diffusivity of 6.530.2210−8 m2 /s, which compares well with the measurementsobtained using the thermal flash method.The successful application of the thermal flash methoddescribed herein validates this simple, reliable, and accuratetechnique for characterization of low thermal diffusivity,one-dimensional micro/nanostructures; the measurementsconducted on phosphate glass and polyimide micro/nanofibers concur with bulk characterization of similar ma-terials. This technique is attractive because thermal contactresistance is inconsequential and, compared to various othermethods, many complexities are minimized. Although mea-surements thus far have been restricted to low thermal diffu-sivity materials, the analysis may be appropriately rederivedfor high thermal diffusivity micro/nanofibers.The authors are grateful to Dr. Mariano Velez andMo-Sci Co. for providing the glass microfibers studied inthis work. This research was supported by NSF Grant No.0438389 and State of Ohio Third frontier funding from OhioDepartment of Development on Thermal Managementproducts.1 M. H. Huang, S. Mao, H. Feick, H. Q. Yan, Y. Y. Wu, H. Kind, E. Weber,R. Russo, and P. D. Yang, Science 292, 1897 2001.2 C. Yeh, F. Shimabukuro, P. Stanton, V. Jamnejad, W. Imbriale, and F.Manshadi, Nature London 404, 584 2000.3 W. U. Huynh, J. J. Dittmer, and A. P. Alivisatos, Science 295, 24252002.4 L. D. Hicks and M. S. Dresselhaus, Phys. Rev. B 47, 12727 1993.5 Y. Cui, Q. Q. Wei, H. K. Park, and C. M. Lieber, Science 293, 12892001.6 Y. Cui and C. M. Lieber, Science 291, 851 2001.7 A. G. MacDiarmid, Synth. Met. 84, 27 1997.8 P. Kim, L. Shi, A. Majumdar, and P. L. McEuen, Phys. Rev. Lett. 87,215502 2001.9 D. Y. Li, Y. Y. Wu, P. Kim, L. Shi, P. D. Yang, and A. Majumdar, Appl.Phys. Lett. 83, 2934 2003.10 C. H. Yu, S. Saha, J. H. Zhou, L. Shi, A. M. Cassell, B. A. Cruden, Q.Ngo, and J. Li, ASME Trans. J. Heat Transfer 128, 234 2006.11 J. Q. Guo, X. W. Wang, and T. Wang, J. Appl. Phys. 101, 063537 2007.12 J. Q. Guo, X. W. Wang, L. J. Zhang, and T. Wang, Appl. Phys. A: Mater.Sci. Process. 89, 153 2007.13 J. B. Hou, X. W. Wang, and J. Q. Guo, J. Phys. D 39, 3362 2006.14 T. Wang, X. Wang, J. Guo, Z. Luo, and K. Cen, Appl. Phys. A: Mater. Sci.Process. 87, 599 2007.15 U. Singh, V. Prakash, A. R. Abramson, W. Chen, L. T. Qu, and L. M. Dai,Appl. Phys. Lett. 89, 073103 2006.16 W. J. Parker, R. J. Jenkins, C. P. Butler, and G. L. Abbott, J. Appl. Phys.32, 1679 1961.17 J. Lee, T. Beechem, T. L. Wright, B. A. Nelson, S. Graham, and W. P.King, J. Microelectromech. Syst. 15, 1644 2006.18 H. S. Carslaw and J. C. Jaeger, Conduction of Heat in Solids, 2nd ed.Oxford University Press, New York, 1959 pp. 319–324.19 M. T. Demko, S. R. Hostler, and A. R. Abramson, Rev. Sci. Instrum. 79,044902 2008.20 E. Karawacki and B. M. Suleiman, Meas. Sci. Technol. 2, 744 1991.FIG. 3. Color online Voltage vs time for a thermal measurement on a15 m diameter glass microrod, showing the experimental data after acurve fit with the analytical solution.FIG. 4. Color online Thermal diffusivity obtained in the thermal flashmeasurements vs length of wire used for various materials.036103-3 Notes Rev. Sci. Instrum. 80, 036103 2009

Application of the Thermal Flash Technique for Low Thermal Diffusivity Micro/Nanofibers

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Application of the Thermal Flash Technique for Low Thermal Diffusivity Micro/Nanofibers

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