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Three-Dimensional Printed Thermal Regulation Textiles Tingting Gao,† Zhi Yang,‡ Chaoji Chen,† Yiju Li,† Kun Fu,† Jiaqi Dai,† Emily M. Hitz,† Hua Xie,† Boyang Liu,† Jianwei Song,† Bao Yang,‡ and Liangbing Hu*,† †

Department of Materials Science and Engineering and ‡Department of Mechanical Engineering, University of Maryland, College Park, Maryland 20742, United States S Supporting Information *

ABSTRACT: Space cooling is a predominant part of energy consumption in people’s daily life. Although cooling the whole building is an effective way to provide personal comfort in hot weather, it is energy-consuming and highcost. Personal cooling technology, being able to provide personal thermal comfort by directing local heat to the thermally regulated environment, has been regarded as one of the most promising technologies for cooling energy and cost savings. Here, we demonstrate a personal thermal regulated textile using thermally conductive and highly aligned boron nitride (BN)/poly(vinyl alcohol) (PVA) composite (denoted as a-BN/PVA) fibers to improve the thermal transport properties of textiles for personal cooling. The a-BN/PVA composite fibers are fabricated through a fast and scalable three-dimensional (3D) printing method. Uniform dispersion and high alignment of BN nanosheets (BNNSs) can be achieved during the processing of fiber fabrication, leading to a combination of high mechanical strength (355 MPa) and favorable heat dispersion. Due to the improved thermal transport property imparted by the thermally conductive and highly aligned BNNSs, better cooling effect (55% improvement over the commercial cotton fiber) can be realized in the a-BN/PVA textile. The wearable a-BN/PVA textiles containing the 3D-printed a-BN/PVA fibers offer a promising selection for meeting the personal cooling requirement, which can significantly reduce the energy consumption and cost for cooling the whole building. KEYWORDS: 3D printing, thermal regulation textiles, thermally conductive fiber, aligned BN nanosheets, energy efficiency

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level, which will limit its practical applications where the humidity level is low. Other technologies including cold pack textiles with phase change materials,9 air-cooled textiles,10 and liquid cooling textiles11 have their limitations, such as inconvenience from the bulky size of the cold pack, massive consumption of power, and high cost. To address these limitations, considerable efforts of developing thermal regulation textiles have been undertaken recently.12−17 Cui and co-workers reported on a mid-infrared transparent nanoporous polyethylene for efficient human body cooling.2 The same group further developed a wearable face mask based on a nanofiber/nanoporous polyethylene system with high infrared transparency of 92.1%, enabling an efficient radiative cooling.3 Yang and co-workers demonstrated a scalable manufactured randomized glass−polymer hybrid metamaterial that has excellent daytime radiative cooling effects.18 These versatile findings have inspired incredible approaches to incorporate thermal management materials into

ersonal cooling technologies have attracted increasing attention due to their capability of providing thermal comfort by locally controlling the temperature of an individual in a low-cost and energy-saving way.1−3 The combination of personal cooling with wearable textiles has been regarded as one of the most promising strategies to bring personal cooling into daily life.4,5 Wearable textiles with cooling function can provide the building occupants thermal comfort in hot weather via the localized cooling in wearable structure textiles and relax the temperature setting of the air conditioning system in buildings, resulting in an efficient reduction of the cost for cooling the whole building.6 These textiles have driven researchers in textile science and industry to devote continuous efforts to improve the thermal regulation properties of textiles, thus providing a comfortable and safe thermal microclimate to satisfy the cooling needs of the human body. Nowadays, there are several commercially available textiles which can provide different levels of personal thermal regulation. Moisture management textiles, as the most common thermal regulation technology in the industry, are able to cool the human body by removing excessive moisture.7,8 However, the thermal regulation mechanism in such textiles can only be triggered when the microclimate between body and fabric is at a high humidity © XXXX American Chemical Society

Received: September 4, 2017 Accepted: October 26, 2017 Published: October 26, 2017 A

DOI: 10.1021/acsnano.7b06295 ACS Nano XXXX, XXX, XXX−XXX

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Figure 1. Schematic illustration of the thermal regulation textile. The thermal regulation textile is made of thermally conductive composite fibers with well-aligned and interconnected BNNSs embedded in the PVA polymer matrix. The a-BN/PVA textile can release the extra heat produced by the human body along the fiber into the ambient environment, thus providing a thermally comfortable microclimate to the human body for personal cooling. The highly aligned BNNSs in the microfibers act as efficient heat transfer pathways.

Figure 2. Processing of a-BN/PVA nanocomposite fiber. (a) Schematic of the fabrication process of a-BN/PVA composite fiber. (b) Photo image of DMSO/BNNS dispersion and PVA/DMSO/BNNS dispersion after standing for 1 week, suggesting that the addition of PVA helps to homogeneously disperse the BNNSs in DMSO solution. (c) Photo image showing the a-BN/PVA fiber was prepared by printing the uniform dispersion into cooled methanol through a 3D printing machine. (d) Photo image showing the a-BN/PVA fiber winding on a yarn bobbin, indicating the scalability of the a-BN/PVA fiber preparation. (e) Photo image showing the textile woven by the a-BN/PVA composite fibers. Optical images of the a-BN/PVA fabrics with different structures: (f) plain woven and (g) knitted fabric. Insets are the structure diagrams of fabric construction.

textiles for effective personal cooling. Despite the tremendous efforts dedicated to developing personal cooling textiles through infrared thermal radiation as mentioned above, limited success has been achieved with personal cooling textiles that use direct thermal conduction from the hot body to the ambient environment. Among the three main ways (conduction, convection, and radiation) for thermal transmission in thermal regulated textiles, heat conduction is as significant as

the other two ways. When the heat is transferred by conduction, the energy is dissipated outward from the human body through clothing to the external environment.19−21 Therefore, thermally conductive textiles can be attractive for personal cooling. Nevertheless, conventional textile fibers usually have a low thermal conductivity, which hinders the body-generated heat from escaping to the environment, leading to an unsatisfactory cooling effect. B

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Figure 3. Morphological characterizations of the a-BN/PVA composite. (a) Optical image of a-BN/PVA fiber shows the diameter of the finished fiber is about 95 μm (scale bar is 500 μm). (b−d) SEM images of the a-BN/PVA fiber show the alignment of the BNNSs along the fiber direction after the hot-drawing. (e,f) Low- and high-resolution TEM images of the BNNSs. In (e), inset is the SAED pattern of the BNNS. (g) TEM image of a slice peeled from a-BN/PVA fiber. 2D small-angle X-ray diffraction patterns of the as-printed fiber before (h) and after hot-stretching (i). (j) Profiles of the scattering intensity as a function of azimuthal angle (φ).

Here, we demonstrate a thermal regulation textile based on thermally conductive and highly aligned boron nitride (BN)/ poly(vinyl alcohol) (PVA) composite fibers synthesized by 3D printing for efficient personal cooling (Figure 1). 3D printing, as an efficient additive manufacturing technique, can fast and accurately fabricate an arbitrary and complicated structure that is desired for optimizing its performance by successive printing.22−27 BN has been traditionally considered as an effective material in thermal management applications due to its high thermal conductivity yet electrical insulation.28−30 With a two-dimensional structure, the BN nanosheets (BNNSs) have a high in-plane thermal conductivity of up to 2000 W/(m·K).31,32 To take advantage of the in-plane thermal performance of BNNSs, the sheets must be well oriented and uniformly dispersed.33−35 Because BNNSs can be sterically stabilized by absorbed polymer during ultrasonication in a solution of PVA, a homogeneous dispersion can be achieved.36 Meanwhile, a good orientation of BNNSs in nanocomposite fiber was introduced by uniaxial elongational flow during fiber printing and further hot-drawing processing, which can form energetic pathways for phonon conduction. Highly orientated BNNSs effectively enhance the thermal performance of the a-BN/PVA composite fiber by providing numerous thermally conductive pathways along the aligned and interconnected BNNSs. A wearable textile based on the a-BN/PVA composite fibers thus can effectively direct the body-generated heat away from the skin to

the cool ambient environment, resulting in an attractive cooling effect for personal cooling (Figure 1). The personal cooling technique using the wearable cooling textile is an effective way to promote the thermal comfort for building occupants in hot weather, which can significantly reduce the energy required to cool the building itself.

RESULTS AND DISCUSSION Figure 2a briefly illustrates the 3D printing fabrication process of the a-BN/PVA composite fibers. Liquid-phase exfoliated BNNSs were first prepared by sonicating the raw hexagonal boron nitride (h-BN) powders in isopropyl alcohol solution. The raw h-BN bulk powders with a size of over 30 μm can be exfoliated to BN flakes with a size of less than 1 μm (Figure S1). Figure S2a shows a typical Raman spectrum of h-BN. After exfoliation, the (100), (101), and (102) peaks of h-BN bulk disappear, which is attributed to the decrease of the thicknessto-size ratio (Figure S2b).35 In other words, the h-BN micropowders were successfully exfoliated to thin BN nanosheets. The uniform BN/PVA suspensions were then obtained by dispersing the liquid-phase exfoliated BNNSs into PVA/ dimethyl sulfoxide (DMSO) solution using sonication. The above-prepared homogeneous BN/PVA dispersion solution was injected into a coagulation bath from a needle through the 3D printer to fabricate the continuous as-printed fibers. The asprinted fibers were then hot-drawn under high temperature C

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Figure 4. Mechanical and thermal properties of the a-BN/PVA composite fiber. (a) Typical stress−strain curves of the prepared a-BN (45 wt %)/PVA fiber and pure PVA; the tensile strength of 350 MPa was achieved by incorporation of 45 wt % BNNS loading. (b) Stiffness (Young’s modulus) of the pure PVA and a-BN/PVA fibers are 216 MPa and 12.38 GPa, respectively. (c) Comparison of our data of tensile strength with results reported elsewhere. Our a-BN/PVA fiber even with high loading of BNNSs exhibits a tensile strength better than that of other polymer-based composites. (d) Schematic illustration of the proposed thermal conduction model of the a-BN/PVA fiber. The aligned BNNSs form continuous thermally conductive pathways along the fiber direction. The red lines indicate heat transfer paths. (e) Temperature distribution on the cotton yarn, PVA fiber, as-printed BN/PVA fiber without BNNS alignment, and a-BN/PVA fiber.

(200 °C) until a draw ratio of 4 was reached. The PVA fibers were condensed, and the randomly dispersed BNNSs inside the fibers became highly aligned under hot-drawing. Note that the well-dispersed BNNSs’ ink can guarantee a fiber with a uniform distribution of BNNSs during 3D printing. Photographs of the dispersions containing 0.5 wt % BNNSs with and without 5 wt % PVA are shown in Figure 2b. With the addition of 5 wt % PVA, BNNSs can be well-dispersed in the printing ink without aggregation for a long time. In contrast, aggregation of BNNSs occurs in a short time without the addition of PVA, indicating the poor dispersion of BNNSs in pure DMSO solution. The uniform and stable dispersion of BNNSs in the PVA/DMSO solution can be ascribed to the steric stabilization of BNNSs by the absorbed polymer molecules through hydrogen bonding during the ultrasonication process. The well-dispersed BN/ PVA solution has a suitable viscosity to be easily printed from a metal needle into a cooled methanol solution by a programcontrolled 3D printing machine (Figure 2c). By using the 3D printing fabrication method, scalable fibers can be quickly and cost-effectively produced (Figure 2d). The prepared a-BN/PVA composite fibers can be woven into fabrics with different structures, which makes the cooling textile based on the a-BN/ PVA fibers possible (Figure 2e−g). To understand the orientation of BNNSs in fibers, we investigated the microscale morphology of the a-BN/PVA composite fiber. Figure 3a and Figure S3a show that the diameter of the stretched fiber is 3 times smaller than that of asprinted fiber before hot-drawing. The as-printed BN/PVA composite fiber before hot-drawing is about 300 μm in diameter (Figure S3a). Meanwhile, the BNNSs in the fiber are disorderedly embedded in the polymer matrix (Figure S3b,c). Further hot-drawing dramatically decreases the diameter of the BN/PVA fiber from 300 to 95 μm, resulting

in a much denser structure than the as-printed fiber without stretching (Figure 3a,b). Figure 3c,d shows that a high alignment of BNNSs along the length direction of fiber is obtained after the hot-drawing process. The element mapping results indicate that BNNSs are uniformly embedded in the polymer matrix (Figure S4). The microstructure, phase structure, and orientation of the BNNSs are further characterized by transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS). Figure 3e shows the TEM image of the BNNSs, which reveals the two-dimensional morphology and thin thickness of the BNNSs. The inset of selected area electron diffraction (SAED) pattern in Figure 3e shows the single-crystal structure of the BNNS.37,38 Figure 3f reveals that the thickness of the BNNS is around 3 nm, which corresponds to 9 layers of single BNNSs. The TEM image on the edge of the composite fiber displays the alignment of BNNSs, in good agreement with the scanning electron microscopy (SEM) observation of the cross section of the aBN/PVA composite fiber (Figure 3g). Enhanced alignment of the BNNSs after hot-drawing treatment is further confirmed using SAXS. Due to the weak X-ray scattering of PVA, the diffraction peaks mainly originated from the BNNSs.39 For the as-printed fiber before hot-stretching, no sharp scattering peak is observed in the elliptical two-dimensional small-angle X-ray scattering (2D-SAXS) pattern, indicating a poor alignment of BNNSs (Figure 3h,j). For the a-BN/PVA fiber after hotdrawing, a sharp scattering peak can be clearly observed, suggesting the well-aligned BNNSs along the length direction of the fiber (Figure 3i,j).40−42 Moreover, the monotonic intensity drop with no visible peak also indicates the uniform dispersion of BNNSs in the fiber after hot-stretching (Figure S5). D

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Figure 5. Thermal properties and cooling effects of various fabrics. (a) IR images showing the temperature distributions of various textiles. (b) Maximum surface temperature of the cotton, PVA, and a-BN/PVA fabrics under different input laser powers. (c) Measurements of the thermal conductivities of the cotton, PVA, and a-BN/PVA fabrics, which are sandwiched between two aluminum (Al) blocks. Same laser intensity was applied on the top surface of an Al block. (d) Modeling and calculation of the maximum temperature of various fabrics above the human skin (the temperature of human skin is set to be 37 °C). The a-BN/PVA fabric displays the highest surface temperature, indicating the best cooling effect among the three fabrics.

of the loading energy.36,50 With a further increase of the loading strength, the link between PVA and BNNSs is broken, resulting in the pulling out of BNNSs. Another benefit of the dense composite fiber with aligned BNNSs is the enhanced thermal conductivity and heat spreading along the fiber.51 As illustrated in Figure 4d, the alignment of BNNSs can facilitate the in-plane heat transfer by forming a rapid thermal path along the fiber. The thermal properties of various fiber samples with the same diameter were qualitatively characterized by a laser-IR camera system. Under the same laser input power (1 mm diameter spot size), different maximum local temperature of the fiber samples can qualitatively reveal different heat conduction properties. Fiber with higher thermal conductivity conducts the heat more effective from the heat spot generated by a laser, resulting in a lower maximum temperature at the heat source. On the contrary, the fiber with low thermal conductivity will lead to the heat accumulation around the heat spot due to insufficient heat dissipation from a hot source to surroundings. The maximum temperature of the a-BN/PVA composite fiber (39.8 °C) is much lower than those of the commercial cotton yarn (55.9 °C), pure PVA fiber (48 °C), and BN/PVA composite fiber without BNNS alignment (43.6 °C), suggesting highest thermal conductivity of the a-BN/PVA composite fiber (Figure 4e).

The dense structure and well-aligned BNNSs are expected to improve the mechanical properties of the a-BN/PVA composite fiber. Typical stress−strain curves for the pure PVA fiber and aBN/PVA composite fiber are shown in Figure 4a. A high tensile strength of 355 MPa is achieved for the a-BN/PVA composite fiber with 45 wt % BNNS loading, which is ∼3 times higher than that of pure PVA fiber (122 MPa). The a-BN/PVA composite fiber also demonstrates a significantly improved stiffness of 12.38 GPa, ∼57 times higher than that of pure PVA fiber (Figure 4b). The high mechanical tensile strength of the aBN/PVA composite fiber surpasses most of the existing BNbased fibers, demonstrating superior mechanical properties for wearable textile applications (Figure 4c).43−49 The enhanced mechanical tensile strength should be ascribed to the structure of the aligned BNNSs, where efficient load transfer across the filler−matrix interface may occur during the tensile process. Meanwhile, the great interfacial compatibility between PVA and BNNSs may also contribute to the enhanced mechanical strength.36 As shown in Figure S6, the fractured BNNSs linked with PVA bridges can be observed in the fracture surface area after tensile test, confirming the strong interaction between the PVA molecules and BNNSs. After the force loading, the BNNSs first slide against each other, and subsequently, the PVA long chains acting as bridges between BNNSs are stretched along the sliding direction to dissipate a large amount E

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composite fibers are fabricated through the low-cost, fast, and scalable 3D printing technique followed by a hot-drawing treatment. The dense structure of fibers and uniform dispersion and high alignment of BNNSs can be achieved during the process of fiber fabrication, leading to excellent mechanical strength (355 MPa), uniform heat dispersion, and high thermal conductivity. As a result, the fabric based on the a-BN/PVA fibers has a high thermal conductivity of 0.078 W/(m·K), which is 1.56 and 2.22 times higher than those of PVA fabrics and cotton fabrics, respectively. Through the finite element thermal simulation, we found that the cooling effect of a-BN/PVA fabric is 55% higher than that of commercial cotton fabric, further confirming the excellent cooling ability of the a-BN/PVA fabric. The wearable cooling textile consisting of 3D-printed a-BN/ PVA fibers provides an effective choice for personal cooling of the building occupants in hot weather, thus decreasing the demand for indoor temperature regulation and significantly reducing the energy and cost for cooling the building itself.

Laser-IR camera test systems were built to investigate the thermal properties of the a-BN/PVA, PVA, and cotton fabrics. The measurement details (temperature distribution on fabrics) are shown in the supplementary methods S1 and Figure S7. For the same fabric sample, when the laser power input to the fabric increased, a higher maximum temperature was observed on the fabric. From the comparison of three different fabric samples, the a-BN/PVA composite fabric has the lowest maximum temperature at all laser power inputs of P = 0.047, 0.079, and 0.096 W, indicating the best heat dissipation property (Figure 5a,b). We further measured the thermal conductivities of the aBN/PVA, pure PVA, and cotton fabrics. Figure 5c shows the measurement device of the thermal conductivity of fabric in our work. The fabrics were sandwiched between two Al blocks, and the laser was applied on the top surface of Al block. The generated heat from the top Al block conducts to the bottom Al block through the fabrics in the middle. We can clearly observe that, under the same laser power, the Al block on the aBN/PVA fabric has the lowest temperature, which indicates the best thermal conductivity of the a-BN/PVA fabric. Through simulation on ANSYS and calculation (supplementary methods S2 and Figure S8), we can conclude that the a-BN/PVA fabric possesses a high thermal conductivity of 0.078 W/(m·K), which is ∼2.2 and ∼1.6 times higher than that of the cotton fabric and PVA fabric, respectively (Figure 5c) and superior than many commercial fabircs (Table S1). A finite element model in ANSYS was built to demonstrate the cooling effect of the three fabrics with different thermal properties for personal cooling applications. In the simulation model, fabric is exposed to ambient conditions on the top surface (skin) of a human body, which is considered a thermal barrier between the human body and the ambient environment. The temperature of skin and ambient environment was set to 37 and 25 °C, respectively. The heat generated by the human body is transferred through the fabric to the environment. During the heat transfer process from the body to the environment, the thermal conductivity of the fabric plays a vital role to maintain thermal comfort of the human body. High thermal conductivity of fabrics contributes to unimpeded heat dissipation from the body to the environment (high heat flux). From the results of simulation, the a-BN/PVA composite fabric yields the highest fabric outer temperature of 36.2 °C and heat flux of 58.4 W/m2. For the pure PVA and cotton fabric, the fabric outer temperature and heat fluxes are 35.7 °C and 55.99 W/m2, 35.2 °C and 53.35 W/m2, respectively. The cooling effect of the a-BN/PVA composite fabric is 55% greater than the commercial cotton fabric, which displays a better thermal management capacity for personal cooling. Note that in real applications, the cooling textile contacts the human body with different angles between the skin and the fibers, causing a temperature nonuniformity along the fabric, which will cause heat transfer along the fiber-length direction and radial direction (Figure 1). Overall, the cooling mechanism for such a thermal regulation textile modulates the heat transfer path from the human body directly to the environment by changing its properties, such as thermal conductivity for heat conduction, porosity for convection, radiation, and moisture transfer.

METHODS Preparation of BNNSs. In brief, commercial h-BN micropowder (2 g, 30 μm in size, Momentive Inc.) was dispersed in isopropyl alcohol (IPA) (300 mL, purity ≥99.5%). The dispersion was sonicated for 48 h in a sonic bath and then centrifuged at 2000 rpm for 20 min. After centrifugation of the dispersions, the supernatant was decanted. The supernatant of the BN/IPA solution from the centrifugation process was then filtered via vacuum filtration. After filtration, the obtained BN cake was dried for 4 h in an oven at 60 °C, and this dried powder was used as the filler material for composite fiber formation. Suspension Preparation and Fiber Printing. PVA solution with 5 wt % PVA concentration was used as surfactant to obtain uniformly dispersed BNNS powders. First, the 0.5 g PVA chip was dissolved into the 9.5 g of DMSO at 100 °C with agitation. Then, the prepared BNNSs were dispersed in the PVA solution by sonication for 4 h and high energy probe sonication for 1 h. During the sonication process, sonication bath temperature was maintained close to room temperature. After that, PVA chips (lab grade, Fisher Scientific, USA) with desired weight were gradually added into the prepared solution. This mixture was subjected to sonication and further stirring for 8 h by heated to 95 °C. The optically homogeneous BN/PVA dispersion thus obtained was ready for fiber printing. 3D printing fabrication was conducted using a 3D printer (Fisnar F4200n), which is controlled by programmed procedures. The BN/PVA ink was injected at 1 mL/min using a 20 gauge needle into a cooled methanol bath at a temperature of 0 °C. The as-printed fibers were then collected and kept rotating in methanol at room temperature for more than 24 h and subsequently dried at ambient conditions. The hot-drawn process of the fiber was subsequently conducted by heating and stretching the as-printed fiber through a hot zone at a temperature of 200 °C (on a hot plate). The stretching ratio was controlled by a feeding roll with a low speed and a stretching reel with a high speed (Figure S9). The drawn ratio of the samples was 4. Characterization. The morphology was studied via SEM and TEM using Hitachi SU-70 field emission scanning electron microscopy and a JEOL JEM 2100 TEM, respectively. To determine the thickness, BN nanosheets were spin-coated on Si wafers for atomic force microscopy (AFM) measurement. AFM data were taken and analyzed by Veeco Multimode AFM with a Nanoscope III controller, 180 × 180 μm scanner, Nanoscope and Gwyddion software. The Raman spectra of the h-BN surface were measured by a Horiba JobinYvon Raman spectrometer with laser wavelength of 532 nm. The X-ray diffraction pattern was collected by D8 Advanced (Bruker AXS, WI, USA) using a Cu Kα radiation source. SAXS was performed to characterize the fiber alignment with an X-ray wavelength of λ = 0.957 Å and sample-to-detector distance of 8.422 mm. The beam size was 24 × 11 mm (horizontal × vertical), and a single-photon counting

CONCLUSION In our work, we demonstrated a personal thermal regulation textile based on thermally conductive and highly aligned BN/ PVA fibers for personal cooling applications. The a-BN/PVA F

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ACS Nano detector (Pilatus 1 M, Dectris) having a pixel size of 172 × 172 μm2 was used to record the scattering patterns. Mechanical Property Characterization. The tensile strength of the fibers was tested using a dynamic mechanical analysis machine (Q800, TA Instruments, USA) in tension mode. Each fiber was 20 mm long, and the strain ramp was set at 5% min−1. All tests were conducted under laboratory environment at room temperature (25 ± 0.2 °C). Twenty specimens were tested for each type of the fibers. The forces obtained by DMA were converted into stress values, dividing by the cross-sectional area of the fiber. The average diameters of fibers for tensile testing were measured by optical microscopy.

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ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b06295. SEM images and X-ray diffraction profiles of the h-BN powders and BNNSs; Raman spectra of the BNNSs, morphological structure of the BN/PVA fiber before hotstretching; elemental mappings of the cross section of the a-BN/PVA composite fiber, profiles of scattering intensity (I × q2) as a function of scattering; SEM images of the tensile fractured surface of the a-BN/PVA fiber; thermal conductivity comparison of our a-BN/PVA composite textile with other commercial fabrics; schematic illustration of the device for temperature distribution measurement, of the setup for thermal conductivity measurement, and of hot-drawn process of the fiber (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Yiju Li: 0000-0001-9240-5686 Liangbing Hu: 0000-0002-9456-9315 Author Contributions

L.H., T.G., and C.C. designed the experiments. Z.Y. and B.Y. measured the thermal conductivity and conducted IR measurements. J.D. created the 3D illustrations. Y.L. provided characterization via SEM and TEM. C.C. performed mechanical measurements and the corresponding analysis. K.F., J.S., E.H., H.X., and B.L. contributed material characterizations. Z.Y. and B.Y. were responsible for the thermal modeling. L.H., T.G., Y.L., and C.C. collectively wrote the paper. All authors commented on the final manuscript. T.G., Z.Y., and C.C. contributed equally to this work. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS L.H. acknowledges the financial support from the Office of Naval Research Young Investigator Program (ONR YIP, Prougram Manager: Sarwat Chappell) and the Advanced Research Projects Agency-Energy (ARPA-E), U.S. Department of Energy. Z.Y. and B.Y. acknowledge the financial support from the Advanced Research Projects Agency-Energy (ARPAE), U.S. Department of Energy. We acknowledge the support of the Maryland Nanocenter, its Surface Analysis Center and AIMLab. T.G. and Y.L. acknowledge the financial support by China Scholarship Council (CSC). G

DOI: 10.1021/acsnano.7b06295 ACS Nano XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsnano.7b06295 ACS Nano XXXX, XXX, XXX−XXX