Jin Ho Kang,*,† Godfrey Sauti,† Cheol Park,*,‡ Vesselin I. Yamakov,† Kristopher E. Wise,‡ Sharon E. Lowther,‡ Catharine C. Fay,‡ Sheila A. Thibeault,‡ and Robert G. Bryant‡
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Multifunctional Electroactive Nanocomposites Based on Piezoelectric Boron Nitride Nanotubes †
National Institute of Aerospace, Hampton, Virginia 23666, United States and ‡Advanced Materials and Processing Branch, NASA Langley Research Center, Hampton, Virginia 23681-2199, United States
ABSTRACT
Space exploration missions require sensors and devices capable of stable operation in harsh environments such as those that include high thermal fluctuation, atomic oxygen, and high-energy ionizing radiation. However, conventional or state-of-the-art electroactive materials like lead zirconate titanate, poly(vinylidene fluoride), and carbon nanotube (CNT)-doped polyimides have limitations on use in those extreme applications. Theoretical studies have shown that boron nitride nanotubes (BNNTs) have strength-to-weight ratios comparable to those of CNTs, excellent high-temperature stability (to 800 °C in air), large electroactive characteristics, and excellent neutron radiation shielding capability. In this study, we demonstrated the experimental electroactive characteristics of BNNTs in novel multifunctional electroactive nanocomposites. Upon application of an external electric field, the 2 wt % BNNT/polyimide composite was found to exhibit electroactive strain composed of a superposition of linear piezoelectric and nonlinear electrostrictive components. When the BNNTs were aligned by stretching the 2 wt % BNNT/polyimide composite, electroactive characteristics increased by about 460% compared to the nonstretched sample. An all-nanotube actuator consisting of a BNNT buckypaper layer between two single-walled carbon nanotube buckypaper electrode layers was found to have much larger electroactive properties. The additional neutron radiation shielding properties and ultraviolet/visible/near-infrared optical properties of the BNNT composites make them excellent candidates for use in the extreme environments of space missions. KEYWORDS: boron nitride nanotubes . piezoelectric . electroactive . radiation shielding . nanocomposite
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lectroactive materials have been studied extensively for use in a variety of devices: electromechanical sensors and actuators, ultrasonic transducers, loudspeakers, sonar, medical devices, prosthetics, artificial muscles, and devices for vibration and noise control.17 Electroactive ceramics such as lead zirconate titanate (PZT), lead lanthanum zirconate titanate (PLZT), and lead niobium zirconate titanate (PNZT) have very high piezoelectric coefficients but exhibit poor toughness and high toxicity.1 KANG ET AL.
Compared with electroactive ceramics, electroactive polymers have numbers of advantages.27 For example, piezoelectric polymers (or ferroelectric polymer) such as poly(vinylidene fluoride) (PVDF) are lightweight, conformable, and have low acoustic impedance, which can be used for flexible sensors, sonars, and actuators.2,3 Ionic electroactive polymers such as an ionic polymermetal composite (IPMC), conductive polymers, and ionic gels are capable of response to low voltage (12 V) and show relatively large VOL. XXX
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* Address correspondence to
[email protected],
[email protected]. Received for review July 21, 2015 and accepted November 3, 2015. Published online 10.1021/acsnano.5b04526 This article not subject to U.S. Copyright. Published XXXX by the American Chemical Society
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ARTICLE Scheme 1. Future planetary exploration missions: (a) Conceptual Venus exploration. The planetary mobile platform mission requires actuators and sensors that are thermally stable to the hostile environment of Venus, where temperatures reach about 460 °C. (b) Conceptual Mars exploration. Robotic and human space missions on Mars and other deep space missions are in need of effective shielding materials against ionizing radiation that could cause catastrophic malfunctions and lethal dose issues. (Illustrations were modified from artist's conceptions at www.nasa.gov.)
bending via transport or diffusion of ions.4,5 Electroactive shape memory polymers (SMP) show shape recovery behavior from a temporarily programed shape to the original shape by applying electric energy.6,7 Unfortunately, they have relatively low electroactive coefficients (piezoelectric polymers), poor thermal and mechanical properties (ionic electroactive polymer), and relatively slow response time (SMP).27 Amorphous piezoelectric polyimides containing polar functional groups have been developed for potential use as high-temperature sensors.8 The piezoelectric responses of these polyimides are, however, an order of magnitude less than that of PVDF. Although the imide ring structure contributes to their hightemperature performance, it restricts chain rotational mobility. This prevents complete alignment of dipolar groups during poling and limits their piezoelectric response. Attempts have been made to circumvent this limitation by synthesizing new polymers from monomers that contain highly polar groups, by carefully controlling the poling process, and by adding electroactive nanoinclusions into the polymers.913 Despite these efforts, a number of problems remain that prevent the widespread use of electroactive polyimide composites. For example, while carbon nanotube (CNT)-doped polyimides exhibit significant electrostrictive actuation, their piezoelectric response is limited.13 Additionally, because CNTs are either conductors or narrow-band-gap semiconductors, leakage currents limit their use as high-voltage dielectric devices. Finally, CNTs are chemically reactive at elevated temperatures and are easily oxidized above 350 °C, which limits their use in the harsh environments of future planetary missions where oxygen is present.1416 Space exploration missions require sensors and devices capable of stable operation in harsh environments such as high thermal fluctuation, ultraviolet (UV), corrosive gas molecules, and high-energy ionizing radiation.1517 For example, Venus planetary exploration missions (i.e., Venus Mobile Explorer15,16) will experience a high-temperature environment of KANG ET AL.
about 460 °C (Scheme 1a). In addition, robotic and human space missions on Mars and other deep space missions are exposed to high-energy ionizing radiation, causing catastrophic malfunctions and lethal dose issues (Scheme 1b). It has been a challenge to develop reliable sensors and devices for prolonged stable operation in the harsh space environment. Experimental and theoretical studies have shown that boron nitride nanotubes (BNNTs) have a strengthto-weight ratio comparable to that of CNTs, excellent high-temperature stability (to 800 °C in air), and excellent neutron radiation shielding capability,1820 which mitigates harsh environmental effects for future space missions. The increased availability of highly crystalline BNNTs makes them leading candidates as enabling materials for sensors, devices, and integrated structures capable of extended operation in harsh environments. Theoretical studies predict that they should also have piezoelectric coefficients higher than PVDF and poly(vinylidene fluoride trifluoroethylene), P(VDF-TrFE),21,22 and they may be useful as a neutron radiation shielding material owing to the presence of boron. This report seeks to confirm these predictions by studying the electroactive properties of BNNTs in both buckypaper and composite forms and by evaluating their radiation shielding effectiveness using neutron activation analysis. Other properties relevant to aerospace applications;mechanical stiffness, thermal stability, and optical absorbance;are also discussed. RESULTS AND DISCUSSION The high-temperaturepressure synthesized BNNTs used for the study appeared in transmission electron microscopy (TEM) images as highly crystalline, long, thin nanotubes with typical diameters in the range of 110 nm (Figure 1), with from 1 to 10 walls (Figure 1bd). The highly crystalline and long BNNTs can be expected to fulfill theoretical predictions of desirable mechanical, optical, and piezoelectric properties.18,20 Electrical and dielectric properties of the BNNT composites were studied because these parameters VOL. XXX
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ARTICLE Figure 1. (a) TEM image of boron nitride nanotubes showing (b) a single wall, (c) double walls, and (d) 10 walls. TEM images show highly crystalline, long, thin nanotubes with typical diameters in the range of 110 nm.
have a strong influence on electroactive characteristics. The real dielectric constants and conductivities of the BNNT composites as a function of applied frequency (1 102 to 1 106 Hz) at room temperature are shown in Figure 2a. The measured real dielectric constant of a 2 wt % BNNT/polyimide composite was higher than that of the pristine polyimide across the entire frequency range. This increase may originate from a higher dielectric constant of BNNTs or may be due to interfacial polarization at the BNNTmatrix interface as the difference in the dielectric constant at lower frequency was slightly larger than that at higher frequency. A similar observation has been reported elsewhere for related materials.23 Very little difference was observed between the AC conductivity of the pristine polymer and that of the 2 wt % BNNT/ polyimide composite. This is not surprising as both the polyimide matrix and the BNNTs are wide-bandgap insulators.24 Both the low dielectric constant and high resistivity (or low conductivity) of the BNNT composite mean that it can transfer the electric field without significant loss and support the creation of highly efficient sensors.1,2 Ultraviolet/visible/near-infrared (UV/vis/NIR) spectra (2002500 nm) of the pristine polymer and the 2 wt % BNNT/polyimide composite are shown in Figure 2b. Sample thicknesses are about 40 μm for each sample. The transmittance in the vis/NIR range is lower for the BNNT composite, but it remains about 70% transparent at a 650 nm wavelength. The change of transmittance in the visible range (at 500 and 550 nm in wavelength) upon adding the BNNT was greater than that in the IR range (at 2000 nm in wavelength) (inset of Figure 2b); KANG ET AL.
this may enable the use of BNNT composites in selective wavelength filters. Both samples were opaque below 400 nm, which would make them useful in UV shielding applications. The opacity below 400 nm seems to originate from both the polyimide matrix and BNNTs. The BNNTs can effectively block light from 500 nm down into the UV (below 400 nm in wavelength) (Figure S1, Supporting Information). Neutron shielding effectiveness of the BNNT composites was studied using neutron activation analysis. As shown in Figure 2c, the pristine polyimide has a neutron cross section of 0.021 cm1. For comparison, pristine polyethylene, a good moderator but poor absorber of neutrons, has a cross section of 0.03 cm1. Addition of the 2 wt % BNNT/polyimide composite results in an increase in cross section to 0.047 cm1, owing primarily to neutron capture by boron and, to a smaller extent, nitrogen. This shielding effectiveness is nearly 60% greater than that of polyethylene and more than 120% higher than that of the pristine polyimide. Note that the boron and nitrogen in the BNNTs are present in their natural isotopic ratios (19.6% 10B and 80.4% 11B; 99.63% 14N and 0.37% 15N). Using these ratios in combination with the isotopic absorption cross sections, the effective cross section (σa) of boron was calculated as 767 barns (σa of 10B: 3835 barns; σa of 11 B: 0.00350 barns), and that of nitrogen was calculated as 1.90 barns (σa of 14N: 1.91 barns; σa of 15N: 0.000024 barns). Table 1 summarizes the optical and neutron shielding properties of the pristine polyimide and the 2 wt % BNNT/polyimide composite and also shows that addition of 2 wt % BNNT to the polyimide matrix results in VOL. XXX
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S3 ¼ d33 E3 þ M33 E3 2 þ :::
Figure 2. Electrical, optical, and radiation shielding properties of the pristine polyimide and the 2 wt % BNNT/polyimide composite. (a) Comparison of real dielectric constant and AC electrical conductivity. (b) Ultraviolet/visible/near-infrared (UV/vis/NIR) spectra of both pristine polyimide and BNNT/ polyimide composites. Note that the change of transmittance in the visible range (at 500 and 550 nm in wavelength) was greater than that in the IR range (at 2000 nm in wavelength). (c) Macroscopic thermal neutron cross sections. Note that the 2 wt % BNNT/polyimide composite showed a higher cross section, 0.047 cm1, than did pristine polyethylene, which has a cross section of 0.03 cm1.
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slight improvements in thermal and mechanical properties relative to the pristine polymer. It is noteworthy that the addition of only 2 wt % BNNT enhanced the neutron radiation shielding effectiveness approximately 120% without degrading the thermal and mechanical properties of the polyimide. The electric-field-induced out-of-plane strain (S3) was measured using fiber-optic sensors while applying a 1 Hz AC electric field on each sample. As shown in Figure 3a, the pristine, unpoled polyimide showed no electroactive response under any electric field (E3) up to 11 MV/m. This agrees with previous work using the same polymer, which found no piezoelectric response3,12,23 without poling. In contrast, the 2 wt % BNNT/polyimide composite showed a significant electric-field-induced strain (S3) (Figure 3b) without poling. The relative contributions of the piezoelectric (PE) and electrostrictive (ES) effects may be determined by fitting S3 to a power series as a function of the applied electric field (E3):25 (1)
In eq 1, written using Voigt notation, d33 and M33 are piezoelectric (linear response) and electrostrictive (quadratic nonlinear response) coefficients, respectively. The piezoelectric (red), electrostrictive (blue), and total (black) strains are shown in Figure 3 as a function of applied electric field. From the linear term (Figure 3c), the piezoelectric coefficient, d33, of the 2 wt % BNNT/polyimide composite was found to be 0.84 pm/V. Although a piezoelectric response can be induced in the neat polymer by poling, that observed value is smaller in magnitude than 0.84 pm/V, even when the polymer is treated with poling fields as high as 80 MV/m.8,26 It is noted that the 2 wt % BNNT/ polymer composite exhibits piezoelectric strain without poling. Because of this, it appears that the linear piezoelectric response originates from the presence of the BNNTs, which are intrinsically piezoelectric.21 A similar procedure was followed to fit the quadratic component of the strain response to the square of the electric field, as shown in Figure 3d. From this fit, the electrostrictive coefficient (M33) of the 2 wt % BNNT composite sample is found to be 3.07 1019 m2/V2. Previous computational studies of piezoelectricity in BNNTs used a theoretical quantum mechanical model,27 ab initio calculations,21 tight-binding methods,28 hybrid
TABLE 1. Physical Properties of BNNT/Polyimide Composites optical transmittance (%)
pristine polyimide 2% BNNT/polyimide composite KANG ET AL.
macroscopic thermal neutron
glass transition
Young's modulus
@ 500 nm
@ 2000 nm
cross section (cm1)
temperature (°C)
(GPa)
77.0 54.1
87.3 84.6
0.021 0.047
274.3 275.2
2.3 2.5
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ARTICLE Figure 3. (a) Electric-field-induced out-of-plane strain of pristine polyimide. Note that there was no noticeable response under an electric field of 11 MV/m. (b) Electric-field-induced out-of-plane strain (black) of the 2 wt % BNNT/polyimide composite, which was a sum of piezoelectric strain (red) and electrostrictive strain (blue). (c) Piezoelectric strain of the 2 wt % BNNT/polyimide composite showing linear response to an electric field. (d) Electrostrictive strain of the 2 wt % BNNT/ polyimide composite showing quadratic nonlinear response to an electric field.
density functional theory,29 and a semiempirical quantum-chemical method.30 Based on these calculations, the piezoelectric polarization of BNNTs under various deformations was studied using an empirical molecular dynamics model.31 As a result of the symmetry of the arrangement of partially charged boron and nitrogen atoms in the BNNT, the piezoelectric response in stretching of an individual tube depends on the cosine of the chiral angle, cos j. It is largest for zigzag tubes (j = 0) and is directed along the tube axis.31 The 2 wt % BNNT composite samples in this study were prepared by film-casting, so the BNNTs should have no preferential in-plane orientation. The random alignment of the tubes leads to a zero mean polarization and a net polarization, p, equal to the standard deviation of a Gaussian ensemble (i.e., |p| µ c1/2, where c is the BNNT concentration in the composite). This net polarization accounts for the significant piezoelectricity of the BNNT composite without poling. The electrostrictive strain does not average to zero and is expected to be proportional to c. This explains its stronger contribution to the total electroactive response as indicated in Figure 3bd. KANG ET AL.
If the observed piezoelectric response of the BNNT composite arises from the presence of BNNTs, a greater piezoelectric response would be expected if the tubes could be aligned. To test this idea, an aligned 2 wt % BNNT/polyimide composite was prepared by stretching an as-prepared sample by 100% at 225 °C. The degree of alignment of BNNT by stretching was estimated using scanning electron microscopy (SEM) (Figure S2). For comparison, a pristine polyimide film was also stretched by 100% under the same conditions. As shown in Figure 4, stretching the composite significantly increased the absolute magnitudes of its electroactive properties (compare Figure 3). The piezoelectric coefficient, d33, was calculated to be about 4.71 pm/V versus 0.84 pm/V for the unstretched sample;a 460% increase (Figure S3a). A similar magnitude of improvement was found for the electrostrictive coefficient, M33, which increased to 1.79 1018 m2/V2 (Figure S3b). However, the stretched pristine polyimide film did not produce any electroactive response (Figure S4). This confirms that the piezoelectric response of the 2 wt % BNNT/polyimide composite is due to the presence of the piezoelectric BNNTs. VOL. XXX
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Finally, to unambiguously separate the contributions of the BNNTs and the polyimide matrix to the total electroactive response, a sample with an active layer composed of pure BNNTs (a BNNT buckypaper) was fabricated based on a similar device with a singlewalled carbon nanotube (SWCNT)/polyimide active layer.32 To produce the all-nanotube actuator, a BNNT layer was sandwiched between two conductive SWCNT layers that serve as conductive electrodes (Figure 5). A non-piezoelectric polyurethane (PU) resin was infused into the actuator to improve its durability. The free-standing flexible all-nanotube actuator is shown in Figure 5d. The SEM image (Figure 5e) shows the cross section of a prototype BNNT actuator. Here, the top and bottom electrode layers are composed of SWCNTs and PU (about 4.5 μm thick), and the middle section is the BNNT/PU active layer (about 13 μm thick). The concentration of BNNT in the active layer was about 40 wt %. The sample was placed under a 1 Hz AC electric field, and the in-plane strain (S1) was measured with a fiber-optic sensor using a dilatometry method.32,33 The strain as a function of electric field (black solid squares in Figure 5f) was deconvoluted into a linear response (red circles) and a nonlinear response (blue triangles). From a linear fit of the data, the piezoelectric coefficient, d13, was determined to be about 14.41 pm/V (Figure S5a). This is comparable to the values (2028 pm/V) of commercially available piezoelectric polymers such as PVDF.2,34,35 The
MATERIALS AND METHODS Sample Preparation. Boron nitride nanotubes were synthesized through a high-temperaturepressure method18 and used as produced. BNNT/polyimide nanocomposite films were
KANG ET AL.
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Figure 4. Electroactive (piezoelectric and electrostrictive) properties of the stretched 2 wt % BNNT/polyimide composite. The 2 wt % BNNT/polyimide composite was stretched by 100% at 225 °C to align the BNNTs, and the stretched sample showed a piezoelectric response higher than that of the nonstretched sample.
electrostrictive coefficient (M13) of the BNNT/PU active layer was determined by fitting the measured strain (S1) to the square of electric field strength (E2), S1 = M13 E2. From this, M13 was found to be 2.05 1016 m2/V2 (Figure S5b). This value is several orders of magnitude higher than those of electrostrictive polyurethanes (4.6 1018 to 7.5 1017 m2/V2)36,37 and irradiated PVDF-TrFE (0.37 1018 to 5.1 1018 m2/V2).38,39 Other physical processes, such as thermal and viscoelastic effects and the Maxwell stress, can contribute to the nonlinear electroactive behavior.23 However, under the conditions used, these effects are negligible. Joule heating is insignificant because the sample is an insulator. In addition, the contribution of the Maxwell effect is small (
