Enhanced Thermal Conductivity of Individual ... - ACS Publications

Subscriber access provided by UNIV OF ARIZONA

Article

Enhanced Thermal Conductivity of Individual Polymeric Nanofiber Incorporated with Boron Nitride Nanotubes Dukeun Kim, Myungil You, Jae Hun Seol, Sumin Ha, and Yoong Ahm Kim J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00047 • Publication Date (Web): 09 Mar 2017 Downloaded from http://pubs.acs.org on March 9, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 17

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Enhanced Thermal Conductivity of Individual Polymeric Nanofiber Incorporated with Boron Nitride Nanotubes Dukeun Kim,†, ‡ Myungill You,§ Jae Hun Seol,§,* Sumin Ha, ‡ and Yoong Ahm Kim†, ‡,* †Alan G. MacDiarmid Energy Research Institute & School of Polymer Science and Engineering, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju 61186, Republic of Korea ‡Department of Polymer Engineering, Graduate School, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju 61186, Republic of Korea §School of Mechanical Engineering, Gwangju Institute of Science & Technology, 123 Cheomdangwagiro, Buk-gu, Gwangju, 61005, Republic of Korea *Email: [email protected] and [email protected]

ABSTRACT. Thermal conductivity of individual polyvinyl pyrrolidone (PVP) nanofibers embedding boron nitride nanotube (BNNT) fillers has been measured. The PVP nanofibers were electrospun on suspended micro-devices in order to better understand the effect of BNNT fillers on the thermal conductivity of polymeric nanofibers. Various material characterization methods provided evidences that ketone group in the PVP interacted with the surface of BNNTs via strong intermolecular forces, thereby resulting in an effective heat transfer between the polymer

1 Environment ACS Paragon Plus


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

matrix and BNNTs. The individual PVP nanofiber containing 30 wt% of BNNTs exhibited approximately two-fold higher thermal conductivity than that of the bulk PVP.

INTRODUCTION Despite their low price and high manufacturability, polymers found very limited use in heat dissipating applications due to their low thermal conductivity. The low thermal conductivity mainly originates from their structure characterized as randomly oriented and entangled molecular chains which makes polymers thermally insulating.1 In order to overcome this drawback, a great deal of studies has been attempted to improve the thermal conductivity of polymers. One typical way is to introduce thermally conducting fillers into polymer matrix. Thermally conducting fillers can be categorized into carbon-based, metallic, and ceramic ones.2 In addition, thermal conductivity of polymers can be improved via alignment of polymer chains through mechanical stretching.3-4 With increasing popularity of nanomaterials, carbon nanotubes (CNTs) gained attention as a novel carbon-based filler due to its incomparably high thermal conductivity. The theoretical thermal conductivity of single-walled CNTs were estimated to be about 6000 W/mK,5 and that of multi-walled CNTs were measured to be 3075 W/mK at room temperature.6 However, the thermal conductivity of polymeric composite was not governed by the rule of mixture in a simple manner, but largely affected by the interface resistance7 between the polymer matrix and the fillers, the dispersion8 and alignment9 of fillers. Especially, due to the metallic or semiconducting nature of CNTs, their industrial applications have been restricted for cases where electrical insulation of a CNT/polymer composite is necessary. As a replacement of CNTs for this purpose, boron nitride nanotube (BNNTs), which have a tubular structure similar to that of the CNTs, is


Page 2 of 17

Page 3 of 17

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


considered as a promising candidate because they have great thermal and mechanical properties10-11 like CNTs, but unlike CNTs; for example, the BNNTs have a large band gap of 5.5 eV, leading to dielectric properties,12-14 and possesses superb oxidation resistance.13 Also the thermal conductivity of BNNTs was theoretically estimated as 6600 W/mK, which was as high as that of CNTs.15 However, the measured value was still 200-300 W/mK at room temperature.10, 16

Recently, several studies have been conducted to achieve high thermal conductivity of

polymeric composites loaded with BNNTs as follows. The thermal conductivity of the polymeric composite film with 10 wt. % of BNNT fillers was remarkably enhanced by more than a factor of 20.17 Subsequently, the thermal conductivity of a BNNTs/polymer composite, which was synthesized with axially aligned electrospun fibers, was greatly improved along the direction of the fiber alignment.18 Such indisputable effects of BNNTs in polymeric composites demand a more fundamental study on the thermal conductivity of an individual polymeric fiber itself. In the current work, we prepared individual electrospun nanofibers on suspended micro-devices with a view to comparing the change in the thermal conductivity of a polymeric nanofiber before and after the addition of BNNTs. Our result not only showed the effect of BNNT fillers on enhancing the thermal conductivity of a polymeric nanofiber, but also enlightens the mechanism of the thermal conductivity enhancement in regard to molecular interactions between the matrix polymer and the fillers. EXPERIMENTAL SECTION Individual Polymeric Fibers on Suspended Micro-devices. BNNTs (nanotube’s diameter and length were ~5 nm and up to the 200 µm, respectively, BNNT Ltd.) were purchased and purified



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

by acid and heating at 700 oC for 2 h to remove impurities, such as catalyst and h-BN powder. We used polyvinyl pyrrolidone (PVP, Mw = 1300000, Sigma Andrich) as polymer matrix and ethanol as a solvent. To prepare PVP-BNNT single-fiber, purified BNNTs (0, 5 and 30 wt. % of PVP, named as BNNT-0, BNNT-5, and BNNT-30, respectively) were first dispersed in ethanol (9 g) under tip-sonication (Sonics & Materials Inc., output power = 350 W) with a water-cooling system (controlling temperature 10 ± 2 oC) for 3 h. The PVP solid (1 g) was then added to ethanol containing BNNTs, followed by stirring for 2 days at room temperature. Polymeric fiber was prepared via electrospinning process (Fig. S1). In this process, resulting composite solutions were sprayed as polymeric nanofibers when a high voltage of 15 kV was applied between a needle and a suspended micro-device. The suspended micro-device was fabricated based on a previous report19 and attached on an aluminum foil (TLC = 10 cm, injection flow = 4 ml/min). Subsequently, unnecessary polymeric fibers suspended on the device were eliminated by focused ion beam (FIB, NOVA200, FEI), leaving only a single fiber. The ends of the single fiber were fixed to metal electrodes of the suspended micro-device via platinum deposition (Fig. 1). For Pt deposition, focused electron beam was used instead of focused ion beam. Any exposure of the fiber to the electron beam was avoided in order to minimize the possible thermal conductivity reduction by the exposure. Considering the comparatively low thermal conductance of the polymeric fiber, differential bridge circuit technique was applied rather than 4-point IV one.4 Characterizations. To determine the presence of BNNTs in PVP matrix, Fourier transform infrared spectroscopy (FT-IR, Nicolet iS10 spectrometer using ATR accessory, Thermo scientific Ltd., Japan) and X-ray photoelectron spectroscopy (XPS, MultiLab2000, VG Ltd., UK) were used. To investigate the morphologies of the PVP nanofibers with and without BNNT, Field emission scanning electron microscopy (FE-SEM, S-4700, Hitachi Ltd., Japan) with an


Page 4 of 17

Page 5 of 17

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


operating voltage of 15 kV was employed. Field emission transmission electron microscopy (FETEM, JEM-2100F, JEOL Ltd., Japan) was performed at an operating voltage of 100 kV. For preparation of TEM sample, ethanol suspensions (50 µl) of both pristine PVP and BNNT/PVP fibers were first made by bath-sonication (Branson 3210, Branson Ultrasonics corp.) for 1hr, followed by placing the suspensions on carbon-coated nickel grid. The ethanol was evaporated at room temperature. Thermogravimetric analysis (TGA, TGA-50, Shimadzu Ltd., Japan) and differential scanning calorimetry (DSC, DSC-60, Shimadzu Ltd., Japan) were conducted at a heating rate of 10 oC/min under aerobic condition. RESULTS AND DISCUSSIONS Before the thermal conductivity analysis of PVP single fibers with and without various concentrations of BNNTs, electrospun PVP fibers were structurally characterized. Electrospun pristine PVP fibers with mean diameter of ~ 467 nm show a smooth and uniform morphology with random orientation (Fig. 2 (a)). Although the PVP fibers containing 5 wt. % BNNTs (Fig. 2 (b)) displayed similar morphology as that of the pure polyvinyl alcohol (PVA) fibers, we observed a slight increase in the mean diameter (~ 620 nm), probably due to the embedded BNNTs. We directly identified embedded BNNTs within the PVP fiber based on TEM image (see, inset in Fig. 2 (b)) as compared with that of the pristine BNNTs (Fig. S2). In order to investigate the interaction between BNNTs and PVP, FT-IR (Fig. 3 (a)) and XPS (Fig. 3 (b)) spectra of electrospun PVP fibers with and without BNNTs was measured. A typical FT-IR spectrum of pure PVP fibers exhibited absorption peaks of vinyl groups at 2952 cm-1 (CH asymmetric stretching) and 1427 cm-1 (CH2 bending vibration), and of pyrrolidinyl groups at 1652 cm-1 (C=O stretching) and 1018cm-1 (C-N stretching).20-21 On comparing with the



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

corresponding specific absorption peaks of pristine PVP fibers, the FT-IR spectrum of PVP containing BNNTs revealed that although the peaks of vinyl groups of PVP remained unchanged, the pyrrolidinyl peaks were largely shifted; for example, in the PVP with 5 wt. % BNNTs, the absorption peaks corresponding to the C=O and C-N stretching vibration were observed at 1654 cm-1and 1016 cm-1, respectively. Furthermore, in the PVP with 30 wt. % BNNTs, C=O stretching vibration was blue-shifted to 1656 cm-1and C-N stretching vibration was red-shifted to 1008 cm-1. Thus, the shift of C=O and C-N stretching vibration imply a strong interaction between BNNTs with PVP in close proximity to C=O and C-N bonding. To further understand the interaction between PVP and BNNTs, XPS spectra of PVP fibers with and without BNNTs were analyzed. The C 1s spectrum of PVP fibers revealed four peaks observed at 283.5, 284.5, 285.6, and 287.4 eV (Fig. S3), which corresponds to C-H, C-C, C-N and C=O bonding, respectively.21 Most of the specific peaks in PVP fibers shifted slightly after interaction with BNNTs. By contrast, the C=O peak showed a large shift toward the low binding energy with increasing concentration of BNNTs. The O 1s spectra of PVP fibers with and without BNNTs were compared (Fig. 3 (b)). From pristine PVP fiber, the peaks attributed to O-H, oxide, and C=O were confirmed at 529.6, 530.7, and 531.5 eV, respectively. Among the specific peaks, the C=O peak is shifted toward high binding energy when 5 wt. % BNNT was used. In the PVP fiber containing 30 wt. % BNNTs, a newly appeared peak coming from the C=O centered at 533.2 eV can be explained as a coordination effect between O atom and the BNNTs. The XPS results indicated that BNNTs interacted with PVP molecules not only via physical adsorption between C=O of PVP and the BNNTs, but also through chemical coordination by the O atoms in PVP. The effect of embedded BNNTs on the thermal property of PVP fibers was evaluated using TGA (Fig. S4) and DSC (Fig. 4) tools. The TGA and DTG curves of pure PVP fiber


Page 6 of 17

Page 7 of 17

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


indicated that the first weight loss of ~10.7% occurred up to 100 oC, which was attributed to the loss of residual water and/or impurities (Fig. S4 (a)). The second and third decomposition points can be related to the removal of functional groups in PVP and to polymer backbones, totally 85.8 %, were confirmed at 334 oC and 518 oC, respectively. Finally, the PVP fibers decomposed completely below 600 oC. After the addition of BNNTs into PVP fibers (Fig. S4 (b, c)), the major weight loss was 82.8 % in PVP fibers with 5 wt. % BNNTs and 71.3 % in fibers with 30 wt. % BNNTs. We observed large change in decomposition temperatures; 325 and 499 oC in PVP-5 wt. % BNNTs, and at 341 and 517 oC in PVP-30 wt. % BNNTs. On incorporation of BNNTs into PVP, the decrease in the weight loss of the fiber can be explained by strong intermolecular interaction between PVP and BNNTs. We obtained DSC curves of PVP fibers with and without BNNTs (Fig. 4). The PVP fibers revealed two specific exothermic and endothermic peaks at 99.4 oC with a heat of fusion of -406.7 J/g and at 361.8oC with a heat of fusion of 130.0 J/g, respectively. These were attributed to the glass transition (Tg) and decomposition temperature (Td), respectively. The exothermic Tg of PVP fibers was shifted to low temperature at 99.0 oC after the addition of 5 wt. % BNNTs and at 87.1 oC after addition of 30 wt. % BNNTs, whereas the endothermic Td (361.8 oC) was shifted to high temperatures at 370.6 oC and 377.6 oC on addition of 5 wt. % and 30 wt. % of BNNTs, respectively. Such change in the PVP containing BNNTs, as compared to that of pure PVP, was not only due to good miscibility of BNNT with the PVP chains, but also because of intermolecular forces between the PVP and BNNTs. The thermal conductivity of the PVP fibers containing BNNTs increased with increasing the weight fraction of BNNTs (Fig. 5 (a)). The similar trend was observed in a polymer composite with aligned CNT fillers22 as well as hot-pressed electrospun BNNT/polymer fibers,18



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 17

of which thermal conductivities were measured using the laser flash method. The thermal conductivity of pristine PVP fiber was higher than that of bulk PVP, which was 0.27 W/mK at room temperature,23 by 19 %, which indicated that the electrospinning process may axially align the molecular chains in the PVP fibers, as reported earlier.3 The thermal conductivity of the fibers containing 5 and 30 wt. % BNNTs were higher than that of bulk PVP by 31 % and 110 % at room temperature, respectively. The strong molecular interaction between the BNNTs and the PVP possibly aligned the BNNTs along the molecular chains of the PVP. In order to better understand the experimental results, thermal conductivity of the composite fibers was calculated based on Lewis-Nielsen model,24 which is given as

=

∅

∅

(1)

where, the parameters, A, B and ψ, are defined as,

A = − 1 (2a)

B =

(2b)

(1 − ∅ ) Ψ=1+ ∅ ∅ (2c)

In the above equations, k , and ! are the effective thermal conductivities of the composite, matrix, and fillers, respectively. Also, A is a constant involved in generalized Einstein coefficient ( ), " and " are the volume fraction of the fillers and the maximum packing fraction, 8 Environment ACS Paragon Plus

Page 9 of 17

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


respectively. When the value of was same as the thermal conductivity of the pristine PVP

fiber and # and ∅ are taken to be 28 and 0.82,24 this model matched well with the measurement

extracted ! was calculated to be 1.72 W/mK, which was reduced by a factor of ~116 under the assumption that thermal conductivity of BNNTs is 200 W/mK at room temperature. The interface resistance played a major role in the reduction of the extracted filler thermal conductivity. Moreover, it was expected that the strong molecular interaction between BNNTs and PVP, which reduced the interface resistance between BNNTs and PVP, helped the thermally conducting fillers to increase the effective thermal conductivity of the BNNT/PVP fibers.25 The thermal conductivity of BNNT-30, of which the BNNT volume fraction was equivalent to 20 vol. %,

was approximately same as that of electrospun and subsequently hot-pressed

BNNT/PVA composites with less than 5 vol.% of BNNTs (Fig. 5 (b)).18 Additionally, the thermal conductivity of BNNT-30 was five-fold lower than that of a BNNT/polymer composite with 15-21 vol. % of BNNTs, which was synthesized using the hot-pressing technique,17 respectively. This discrepancy between this work and the previous studies might be due to the reinforced alignment of the fillers and the reduced interfacial resistance between the polymer matrix and the fillers when the hot-pressing technique was applied. However, this lower thermal conductivity also suggests that there would exist a large potential for the thermal conductivity of an individual BNNT/polymer fiber to increase by enhancing the alignment of fillers and diminishing the interfacial resistance between matrix and fillers. CONCLUSIONS In conclusion, thermal conductivity of individual electrospun BNNT/PVP fibers was measured with suspended micro-devices. The thermal conductivity of the PVP fiber with 30 wt. % BNNTs was increased by a factor of two compared to that of bulk PVP. This enhancement was 9 Environment ACS Paragon Plus


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 17

attributed to the high thermal conductivity of BNNTs and the reduced interface thermal resistance between PVP and BNNTs. Our result proposed that the thermal conductivity of a BNNT/polymer fiber can be enhanced further by optimizing the geometries of BNNT fillers and dispersion conditions. ASSOCIATED CONTENT Supporting Information Schematic experimental process, TEM images, XPS spectra, thermogravimetric and their derivative thermogravimetric graphs were recorded. This material is available free of charge via the Internet at http://pubs.acs.org.” AUTHOR INFORMATION Corresponding Author *J. H. Seol. E-mail: [email protected] *Y. A. Kim. E-mail: [email protected] Author Contributions D.K. designed the research and wrote the paper; M.Y. and J.H.S. performed the computationally thermal conductivity research and analyzed the data; Y.A.K., S.H. and J.H.S. participated in discussion of the study; All authors read and approved the final manuscript Notes The authors declare no competing financial interest


Page 11 of 17

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ACKNOWLEDGMENTS Y.A.K. acknowledges the financial support from the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. NRF-2014R1A2A1A10050585) and from Nano·Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2016M3A7B4021149).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 17

REFERENCES 1. Burger, N.; Laachachi, A.; Ferriol, M.; Lutz, M.; Toniazzo, V.; Ruch, D. Review of Thermal Conductivity in Composites: Mechanisms, Parameters and Theory. Pro. Polym. Sci. 2016, 61, 1-28. 2. Han, Z.; Fina, A. Thermal Conductivity of Carbon Nanotubes and Their Polymer Nanocomposites: A Review. Pro. Polym. Sci. 2011, 36, 914-944. 3. Shen, S.; Henry, A.; Tong, J.; Zheng, R. T.; Chen, G. Polyethylene Nanofibres with Very High Thermal Conductivities. Nat. Nanotechnol. 2010, 5, 251-255. 4. Wingert, M. C.; Chen, Z. C. Y.; Kwon, S.; Xiang, J.; Chen, R. Ultra-sensitive Thermal Conductance Measurement of One-dimensional Nanostructures Enhanced by Differential Bridge. Rev. Sci. Instrum. 2012, 83, 024901 (1-7). 5. Berber, S.; Kwon, Y.-K.; Tománek, D. Unusually High Thermal Conductivity of Carbon Nanotubes. Phys. Rev. Lett. 2000, 84, 4613-4616. 6. Kim, P.; Shi, L.; Majumdar, A.; McEuen, P. L. Thermal Transport Measurements of Individual Multiwalled Nanotubes. Phys. Rev. Lett. 2001, 87, 215502 (1-4). 7. Shenogin, S.; Xue, L.; Ozisik, R.; Keblinski, P.; Cahill, D. G. Role of Thermal Boundary Resistance on the Heat Flow in Carbon-nanotube Composites. J. Appl. Phys. 2004, 95, 81368144. 8. Song, Y. S.; Youn, J. R. Influence of Dispersion States of Carbon Nanotubes on Physical Properties of Epoxy Nanocomposites. Carbon 2005, 43, 1378-1385. 9. Sihn, S.; Ganguli, S.; Roy, A. K.; Qu, L.; Dai, L. Enhancement of Through-thickness Thermal Conductivity in Adhesively Bonded Joints Using Aligned Carbon Nanotubes. Compo. Sci. Technol. 2008, 68, 658-665. 10. Chang, C. W.; Han, W.-Q.; Zettl, A. Thermal Conductivity of B–C–N and BN Nanotubes. Appl. Phys. Lett. 2005, 86, 173102 (1-3). 11. Chopra, N. G.; Zettl, A. Measurement of the Elastic Modulus of A Multi-wall Boron Nitride Nanotube. Solid State Commun. 1998, 105, 297-300. 12. Khoo, K. H.; Mazzoni, M. S. C.; Louie, S. G. Tuning the Electronic Properties of Boron Nitride Nanotubes with Transverse Electric Fields: A Giant Dc Stark Effect. Phys. Rev. B 2004, 69, 201401 (1-4). 13. 13. Kim, D.; Sawada, T.; Zhi, C.; Bando, Y.; Golberg, D.; Serizawa, T. Dispersion of Boron Nitride Nanotubes in Aqueous Solution by Simple Aromatic Molecules. J. Nanosci. Nanotechnol. 2014, 14, 3028-3033. 14. Kim, D.; Nakajima, S.; Sawada, T.; Iwasaki, M.; Kawauchi, S.; Zhi, C.; Bando, Y.; Golberg, D.; Serizawa, T. Sonication-assisted Alcoholysis of Boron Nitride Nanotubes for Their Sidewalls Chemical Peeling. Chem. Commun. 2015, 51, 7104-7107. 15. Xiao, Y.; Yan, X. H.; Cao, J. X.; Ding, J. W.; Mao, Y. L.; Xiang, J. Specific Heat and Quantized Thermal Conductance of Single-walled Boron Nitride Nanotubes. Phys. Rev. B 2004, 69, 205415 (1-5). 16. Chang, C. W.; Fennimore, A. M.; Afanasiev, A.; Okawa, D.; Ikuno, T.; Garcia, H.; Li, D.; Majumdar, A.; Zettl, A. Isotope Effect on the Thermal Conductivity of Boron Nitride Nanotubes. Phys. Rev. Lett. 2006, 97, 085901 (1-4). 17. Zhi, C.; Bando, Y.; Terao, T.; Tang, C.; Kuwahara, H.; Golberg, D. Towards Thermoconductive, Electrically Insulating Polymeric Composites with Boron Nitride Nanotubes as Fillers. Adv. Funct. Mater. 2009, 19, 1857-1862.


Page 13 of 17

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


18. Terao, T.; Zhi, C.; Bando, Y.; Mitome, M.; Tang, C.; Golberg, D. Alignment of Boron Nitride Nanotubes in Polymeric Composite Films for Thermal Conductivity Improvement. J. Phys. Chem. C 2010, 114, 4340-4344. 19. Shi, L.; Li, D.; Yu, C.; Jang, W.; Kim, D.; Yao, Z.; Kim, P.; Majumdar, A. Measuring Thermal and Thermoelectric Properties of One-Dimensional Nanostructures Using a Microfabricated Device. J. Heat Trans. 2003, 125, 881-888. 20. Koczkur, K. M.; Mourdikoudis, S.; Polavarapu, L.; Skrabalak, S. E. Polyvinylpyrrolidone (PVP) in Nanoparticle Synthesis. Dalton Trans. 2015, 44, 17883-17905. 21. Liu, H.; Zhang, B.; Shi, H.; Tang, Y.; Jiao, K.; Fu, X. Hydrothermal Synthesis of Monodisperse Ag2Se Nanoparticles in the Presence of PVP and KI and Their Application as Oligonucleotide Labels. J. Mater. Chem. 2008, 18, 2573-2580. 22. Ghose, S.; Watson, K. A.; Working, D. C.; Connell, J. W.; Smith Jr, J. G.; Sun, Y. P. Thermal Conductivity of Ethylene Vinyl Acetate Copolymer/Nanofiller Blends. Compo. Sci. Technol. 2008, 68, 1843-1853. 23. Xie, X.; Li, D.; Tsai, T.-H.; Liu, J.; Braun, P. V.; Cahill, D. G., Thermal Conductivity, Heat Capacity, and Elastic Constants of Water-Soluble Polymers and Polymer Blends. Macromolecules 2016, 49, 972-978. 24. Nielsen, L. E. The Thermal and Electrical Conductivity of Two-Phase Systems. Ind. Eng. Chem. Fundam. 1974, 13, 17-20. 25. Yang, K.; Gu, M.; Guo, Y.; Pan, X.; Mu, G. Effects of Carbon Nanotube Functionalization on the Mechanical and Thermal Properties of Epoxy Composites. Carbon 2009, 47, 17231737.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 17

Figure 1. SEM image of electrospun PVP with 5 wt. % BNNTs suspended on micro-fabricated device after FIB treatments. Note that unnecessary polymeric fibers suspended on the device were eliminated by focused ion beam, leaving only a single fiber. The ends of the single fiber were fixed to metal electrodes of the suspended micro-device via platinum deposition.

Figure 2. SEM images of electrospun PVP nanofiber web before and after the addition of 5 wt. % of BNNTs. Insets are their corresponding TEM images.


Page 15 of 17

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. (a) FT-IR spectra and (b) O 1s x-ray photoelectron spectra of PVP fibers produced by electrospinning with 0, 5, and 30 wt. % of BNNTs (enlarged portion show the shifting of C=O stretching vibration).

Figure 4. Differential scanning calorimetric curves of electrospun PVP fibers containing 5 wt. % BNNT and 30 wt. % BNNTs.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 17

Figure 5. (a) Measured thermal conductivity of individual PVP fibers with varying BNNT concentrations (0, 5, 30 wt. %) as a function of temperature and (b) Thermal conductivity enhancement compared to a reference matrix ( $% ) as a function of volume fraction. The Nielsen model agreed well with the measurement result of this work by adjusting the thermal conductivity of fillers (! ). In this work, the thermal conductivity of BNNT-30 was enhanced by 110 and 73 % compared with those of bulk PVP and the pristine PVP fiber, respectively. Zhi et al. reported approximately a twenty-fold increase in the thermal conductivity of a hot-pressed BNNT/polymer composite compared with that of bulk polymer.17 On the contrary, Terao et al. reported the three-fold thermal conductivity enhancement of a BNNT/PVA composite, which was made of hot-pressed electrospun fibers, when the electrospun fibers were aligned uniaxially.18 Note that 5 and 30 wt. % of BNNTs in this work correspond to volume fractions of 2.9 and 19.7 vol. %, respectively. The symbols in the legend, ∥ and ', denote the thermal

conductivity values parallel and perpendicular to the uniaxially aligned electrospun fibers, respectively.


Page 17 of 17

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC Graphic Heating membrane

Sensing membrane


Enhanced Thermal Conductivity of Individual ... - ACS Publications

Recommend Documents