Barium Titanate Film Interfaces for Hybrid Composite Energy

Jan 17, 2017 - (22) These BaTiO3 coated carbon fibers offer sufficient flexibility to integrate piezoelectric materials into flexible, functional fabr...
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Barium Titanate Film Interfaces for Hybrid Composite Energy Harvesters Christopher C. Bowland,† Mohammad H. Malakooti,‡ and Henry A. Sodano*,‡,§,∥ †

Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States Department of Aerospace Engineering, University of Michigan, Ann Arbor, Michigan 48109, United States § Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109, United States ∥ Macromolecular Science and Engineering Department, University of Michigan, Ann Arbor, Michigan 48109, United States ‡

S Supporting Information *

ABSTRACT: Energy harvesting utilizing piezoelectric materials has become an attractive approach for converting mechanical energy into electrical power for low-power electronics. Structural composites are ideally suited for energy scavenging due to the large amount of mechanical energy they are subjected to. Here, a multifunctional composite with embedded sensing and energy harvesting is developed by integrating an active interface into carbon fiber reinforced polymer composites. By modifying the composite matrix, both rigid and flexible multifunctional composites are fabricated. Through electromechanical testing of a cantilever beam of the rigid composite, it reveals a power density of 217 pW/cc from only 1 g root-mean-square acceleration when excited at its resonant frequency of 47 Hz. Electromechanical sensor testing of the flexible multifunctional composite reveals an average voltage generation of 23.5 mV/g at its resonant frequency of 96 Hz. This research introduces a route for integrating nonstructural functionality into structural fiber composites by utilizing BaTiO3 coated woven carbon fiber fabrics with power scavenging and passive sensing capabilities. KEYWORDS: multifunctional composite, barium titanate, energy harvesting, embedded sensing, carbon fibers

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piezoelectric materials, as seen with 1−3,11−13 active fiber,10,14 macro-fiber, 15−17 and hollow active fiber composites.18 However, due to shortcomings in maintaining the mechanical strength of the composites upon integrating these ceramic fibers, a new emphasis has been placed on using high-strength fiber cores surrounded by piezoelectric materials, as seen in active structural fibers.19−21 Recently, Malakooti et al. demonstrated the first high strength material with embedded energy scavenging capabilities through the deposition of zinc oxide (ZnO) nanowires on aramid fibers with subsequent integration into a carbon fiber composite.3 Additional research efforts demonstrated increased damping properties of ZnO nanowires synthesized on carbon

INTRODUCTION As high performance, fiber-reinforced polymer composites are experiencing widespread commercial adoption, researchers are now working toward developing the next generation of composites. These next generation composites go beyond providing structural support and focus on integrating additional functionalities into the composite thus leading to system-level performance enhancements.1 One way to fabricate these multifunctional composites is through the development of multifunctional fibers, which have led to novel composite properties, such as structural health monitoring,2 power scavenging,3 power storage,4,5 radar absorption,6 vibration damping,7 and actuation.8−10 A focus has been placed on integrating piezoelectric materials into these fiber reinforced composites due to the material’s ability to convert energy between the mechanical and electrical domains. The initial research efforts in this field illustrated fibers composed of © 2017 American Chemical Society

Received: November 22, 2016 Accepted: January 17, 2017 Published: January 17, 2017 4057

DOI: 10.1021/acsami.6b15011 ACS Appl. Mater. Interfaces 2017, 9, 4057−4065

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Schematic of the two-step hydrothermal reaction synthesizing TiO2 then converting it to BaTiO3 on a carbon fiber fabric. Layup of the energy harvesting composites consisting BaTiO3 coated carbon fibers are schematically shown for the (b) rigid composite and (c) flexible composite with (d) enhanced cross-sectional view of the layers. Embedded photographs show the rigidity and flexibility of the fabricated multifunctional composites.

Figure 2. Micrographs of the synthesized BaTiO3 film on carbon fiber; (a) uniform growth of the BaTiO3 film at cross section of a plain weave carbon fabric and (b) full coverage of the BaTiO3 film on individual carbon fibers. Characteristics of synthesized TiO2 and BaTiO3 films: (c) XRD patterns for the TiO2 and BaTiO3 films with a magnified view of the 45° peak for the BaTiO3 film. (d) EDS spectra for the TiO2 and BaTiO3 films.

fiber and integrated into a carbon fiber composite.7 To further enhance the electromechanical performance of multifunctional fibers, it was shown in prior work that barium titanate (BaTiO3), which has higher piezoelectric coupling than ZnO, could be deposited on carbon fiber tows. These multifunctional fibers demonstrated ferroelectric behavior that could be used for vibration sensing using a single fiber.4,21,22 However, composite integration has yet to be demonstrated in the literature for BaTiO3 coated carbon fiber. Consequently, this

work demonstrates the first synthesis of BaTiO3 on woven carbon fiber fabric with subsequent integration into carbon fiber composites. In addition to rigid structural multifunctional composites, recent research efforts have also demonstrated the drive to create functional textiles for applications where flexibility is required.23,24 Research in the literature has illustrated many examples of functional textiles fabricated through the use of nonwoven fabrics of piezoelectric nanofibers, with the current 4058

DOI: 10.1021/acsami.6b15011 ACS Appl. Mater. Interfaces 2017, 9, 4057−4065

Research Article

ACS Applied Materials & Interfaces popular material of choice being a flexible piezoelectric polymer called polyvinylidene fluoride.25−28 Piezoelectric ceramic fibers have seen limited use in this flexible fabric field due to their brittle nature. However, prior work demonstrated the ability to adhere a piezoceramic, BaTiO3, to the surface of carbon fibers.22 These BaTiO3 coated carbon fibers offer sufficient flexibility to integrate piezoelectric materials into flexible, functional fabrics. Therefore, through the development of these BaTiO3 coated carbon fiber fabrics, not only are structural, multifunctional composites capable of being fabricated but flexible multifunctional composites are attainable as well. This work establishes the ability to synthesize BaTiO3 textured films on carbon fiber fabrics and their integration into a carbon fiber composite with power generation and sensing capabilities using different matrices to achieve variable composite flexibility. As schematically shown in Figure 1a, a BaTiO3 film is deposited on a carbon fiber woven fabric using a two-step hydrothermal reaction. The first step in the process deposits a rutile TiO2 film on the carbon fiber surface, and the second step converts the TiO2 to BaTiO3. The synthesis of the TiO2 and BaTiO3 films is confirmed through X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDS). These BaTiO3 coated fabrics are then integrated into a carbon fiber composite structure using two different matrices to produce a rigid and a flexible composite (Figure 1b,c). Subsequent electromechanical characterization is performed by sectioning the composites into beam structured and subjecting it to a base acceleration after fixing one end of the beam on a permanent magnet shaker. By measuring the electrical output of the BaTiO3 film under different acceleration conditions, it is shown that both the rigid and flexible composites output an electrical signal that could be used for either power scavenging or sensing purposes. These results show efficient energy conversion in both rigid and flexible composites through the integration of BaTiO3 coated carbon fibers as embedded energy harvesting and sensory units. This work offers the first demonstration of BaTiO3 deposited on a carbon fiber woven fabric with subsequent integration into both a rigid and flexible carbon fiber composite.

2c,d. The XRD patterns for the TiO2 film in Figure 2c show strong peaks at 27°, 36°, and 55°, which clearly match the characteristic peaks for rutile TiO2 as established in Joint Committee on Powder Diffraction Standards (JCPDS) Card No. 65-0190. The BaTiO3 film XRD pattern shows a reduction of the TiO2 peaks and reveals peaks that are indicative of BaTiO3, as established by JCPDS Card No. 05-0626. An additional plot in Figure 2c shows an enlarged view of the 45° peak from the BaTiO3 film XRD pattern and shows evidence of a shoulder appearing in the peak. In a perovskite structure, the shape of the 45° peak illustrates the phase, in that a single peak indicates a cubic phase while a split peak indicates the tetragonal phase. The cubic phase consists of lattice parameters of equal values resulting in the overlap of the (002) and (200) reflections in the diffraction pattern. For the tetragonal structure, the c lattice parameter becomes larger than the a lattice parameter thus resulting in a difference in (002) and (200) reflections.37−39 A peak split is clearly seen in the XRD pattern so the tetragonal phase was synthesized thus signifying that ferroelectric behavior should be present. While the BaTiO3 pattern matches the cataloged pattern for BaTiO3, further characterization was required to verify that it was BaTiO3 and not a different perovskite crystal structure. Therefore, EDS was performed on both films. These spectra can be seen in Figure 2d indicating the presence of titanium, oxygen and carbon in the TiO2 film and the addition of barium peaks in the BaTiO3 film. These EDS spectra showed that the second hydrothermal reaction did in fact diffuse barium through the TiO2 film. Therefore, the XRD patterns and EDS spectra confirmed that the two-step hydrothermal reaction successfully synthesized rutile TiO2 and subsequently converted it to BaTiO3. Composite Design and Fabrication. Upon synthesizing and confirming that BaTiO3 films were deposited on the carbon fiber woven fabric, they were integrated into carbon fiber composites. Two different matrix materials were used to generate both rigid and flexible multifunctional composites as seen in Figure 1b,c. First, to create the rigid composite, the BaTiO3 coated fabric was placed between three layers of bare carbon fiber fabric. Two layers were placed under and one layer placed on top of the BaTiO3 coated fabric. Additionally, a layer of aluminum foil was situated between the BaTiO3 and the top layer of bare carbon fiber fabric to reduce the probability of shorting due to stray fibers. The general configuration for this rigid multifunctional composite is shown in Figure 1b, and an enhanced view of the placement of the aluminum foil is shown in Figure 1d. Upon stacking these fabrics, it was infiltrated with a commercial epoxy using a conventional vacuum-assisted resin transfer molding (VARTM) technique thus resulting in a rigid multifunctional composite after the epoxy was cured. The embedded photograph in Figure 1c shows a fabricated rigid composite with integrated energy harvesting functionality. Prior to characterization of these multifunctional composite, electroding of the specimen was achieved by contacting the aluminum foil and the carbon fiber cores of the BaTiO3 carbon fiber fabric as schematically illustrated in Figure 1d. To fabricate the flexible, multifunctional composite, a different composite configuration was used as shown in Figure 1c. A noticeable difference is the lack of carbon fiber layers added to the composite. To maintain flexibility, only one bare carbon fiber layer was added below the BaTiO3 coated carbon fiber and no bare carbon fiber layer was added above the aluminum foil. The electroding scheme remained the same as for the rigid composite. However, instead of using a commercial epoxy,

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RESULTS AND DISCUSSION Material Synthesis and Characterization. Due in part to their inherent scalability, hydrothermal synthesis methods were utilized in this work to deposit BaTiO3 films on carbon fiber woven fabrics (Hexcel, IM7). Using a procedure established in prior research and described in further detail in the Experimental Methods section, woven carbon fiber fabrics were coated with a conformal layer of BaTiO3.4,22,29−31 The synthesis procedure is a two-step hydrothermal reaction that is schematically illustrated in Figure 1a. This two-step reaction initially creates a textured film of rutile TiO2 on the carbon fiber surface during the first hydrothermal reaction step. The second hydrothermal reaction uses the TiO2 film as a template and diffuses Ba2+ ions into the crystal structure resulting in a BaTiO3 film as shown in Figure 2. This conversion step involves a dissolution/reprecipitation reaction that has been extensively studied.32−36 Thermogravimetric analysis data included in the Supporting Information, Figure S1, indicates that the BaTiO3 films account for 50−60 wt % of the total weight of the composite. To confirm the synthesis of TiO2 and conversion to BaTiO3, XRD and EDS were performed on the films as seen in Figure 4059

DOI: 10.1021/acsami.6b15011 ACS Appl. Mater. Interfaces 2017, 9, 4057−4065

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Figure 3. Electromechanical characteristics of the rigid multifunctional composites: the frequency response function (a) magnitude and (b) phase plots of the voltage response of the beam under white noise excitation indicating a resonant frequency of 47 Hz. (c) Voltage and (d) current response of the beam from a 1 g root-mean-square acceleration input of a 47 Hz sine wave. (e) The acceleration and current response showing a 90° phase lag in the current response due to the excitation being at the resonant frequency of 47 Hz.

polydimethylsiloxane (PDMS) was used as the matrix which allowed the composite to flex after curing. After both composite configurations were fabricated, each one was cut into a beam structure to undergo electromechanical characterization. Rigid Multifunctional Composite. The power scavenging capabilities of the rigid, multifunctional composite was investigated through base vibration of the cantilevered composite beam and measurement of the power output. By mounting the beam on a permanent magnet shaker and adding a 20 g tip mass, mechanical vibrations could be induced at the base of the cantilever thus causing the free end of the beam to flex. This mechanical deformation of the beam creates stress in the BaTiO3 layer resulting in a voltage and current output. During the initial tests, the cantilever was excited with a white Gaussian noise signal to perform a frequency response function (FRF) characterization. This revealed the resonant frequency of the beam where the cantilever beam experiences the highest bending deformation. By exciting the beam with a sine wave input at the resonant frequency, the maximum voltage output for the composite at a specific acceleration was generated due to maximum strain occurring at the resonant frequency. Measuring this voltage output across load resistors placed in

the circuit revealed the power output of the beam according to the equation below PL = IL(RMS)

⎛ VRMS ⎞2 VL(RMS)2 RL = ⎜ ⎟ RL = RL ⎝ Zs + RL ⎠

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where P, V, and R are power, voltage, and resistance with the subscript L denoting the variables with load resistance in the circuit, and Zs is the source impedance, which does not need to be directly measured in this case because the VL and RL are sufficient to calculate the power output. Using these classic vibrational characterization methods, the electromechanical properties of the rigid and the capability of the multifunctional composite as energy harvesting materials are determined. Under white Gaussian noise excitation, a resonant frequency of 47 Hz was observed for the rigid composite with a tip mass as illustrated in the magnitude and phase responses of the voltage output Figure 3a,b, respectively. This cantilever configuration excited on the shaker is categorized as a dynamically forced single degree-of-freedom system, which can be thought of as a mass-spring system subjected into a harmonic base vibration.40 Based on the steady-state response of these systems, a 180° shift in the phase comparing the 4060

DOI: 10.1021/acsami.6b15011 ACS Appl. Mater. Interfaces 2017, 9, 4057−4065

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ACS Applied Materials & Interfaces

Figure 4. (a) Frequency response function of the voltage response magnitude of the rigid composite under white noise excitation with an enhanced view of the resonant frequency peak and various load resistances. (b) RMS voltage output and power density calculation of for the rigid composite with various load resistances at a constant RMS acceleration of 1 g from a 47 Hz sine wave.

Figure 5. Frequency response function plots of the (a) magnitude and (b) phase of the voltage response of the flexible composite beam under white noise excitation showing a resonant frequency of 96 Hz. (c) Acceleration of the shaker with a square wave input with a 1% duty cycle and the corresponding voltage response as the beam dampens to its steady state.

outputs. The magnitude of the voltage relative to the excitation amplitude measured for each resistance is shown in the FRF plot in Figure 4a where the resonant frequency peak at 47 Hz is magnified. This illustrates the expected response of increasing voltage output as the resistance is increased from 50 kΩ to open circuit conditions. The power characterization was then performed by exciting the beam with a 47 Hz sine wave and holding the input acceleration constant at 1 g as the voltage output was measured with various load resistors in the circuit. The RMS voltage output shown in Figure 4b shows a rapid increase in voltage output followed by saturation as the resistance increases, representing a peak power output at a matched impedance. The power density, plotted in Figure 4b, shows a maximum of 217 pW/cc at a resistance of 250 kΩ. This result demonstrates that a woven carbon fiber fabric coated with BaTiO3 can be integrated into a carbon fiber composite using a conventional composite fabrication technique with commercial epoxy to create a rigid, multifunctional composite that can produce power in response to an applied force. Despite

frequencies of the input and output signals is expected as the input force frequency passes the beam’s resonant frequency. At this frequency, when the forcing frequency is equal to the resonant frequency, the peak in the magnitude response corresponds to a 90° phase shift. By exciting the beam with a 47 Hz sine wave, a peak-to-peak voltage (Vpp) of 19 mV and rootmean-square voltage (VRMS) of 5.15 mV was produced using a root-mean-square acceleration of only 1 g as illustrated in Figure 3c. At the same frequency and same base acceleration of 1 g, the composite generated a peak-to-peak current (Ipp) of 160 nA, shown in Figure 3d. The acceleration input and short circuit current are shown simultaneously in Figure 3e. The plot clearly shows a 90° phase lag of the electrical response in relation to the acceleration input, which is a typical observation for a piezoelectric beam vibrating at its resonant frequency.3,41 The characterization of this beam was taken one step further by calculating the power output of the beam across a resistive load. Initially, white Gaussian noise acceleration was input to the beam with various load resistances resulting in different voltage 4061

DOI: 10.1021/acsami.6b15011 ACS Appl. Mater. Interfaces 2017, 9, 4057−4065

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Figure 6. (a) Acceleration of the flexible composite beam with a 96 Hz sine wave and corresponding voltage output. (b) Voltage output of the beam under different acceleration inputs with the (c) root-mean-square voltage values plotted as a function of the root-mean-square acceleration input. (d) The root-mean-square displacement of the beam tip at the free end of the beam as a function of root-mean-square acceleration input at the fixed end of the beam.

the relatively low power density of this composite as compared to energy harvesters in the literature that can achieve power densities into the mW/cc range, this composite design has the advantage of integrating energy harvesting functionality into structural components that would eliminate the need for subcomponents in the system.42 Simply replacing batteries with nonstructural energy harvesting devices does not improve the system as a whole due to the increased weight and complexity of the system. Flexible Multifunctional Composite. Similar to the rigid composite, the flexible composite was diced into a beam with a 1 g mass and mounted on a permanent magnet shaker for electromechanical testing. Accelerating the beam with white noise excitation revealed a resonant frequency of 96 Hz as identified from the peak in the magnitude and 180° phase change in the FRF plots in Figure 5a,b. An additional resonant frequency identification technique was employed for this beam. As shown in Figure 5c, a square wave acceleration input with a 1% duty cycle was used to excite the beam with a peak acceleration of approximately 3 g. This excitation waveform provided pulses to the beam resulting in a sine wave voltage output that slowly damped and the frequency of this sine wave reveals the beam’s resonant frequency as the beam dampens to its steady state. It should be noted that the resonant frequency of this flexible energy harvester is higher than that of the rigid energy harvesting beam. This is due to the large difference in tip masses added to the beams. The rigid composite is capable of supporting a 20 g mass and still having a stable dynamic response while the flexible composite can support only 1 g. This significant difference in tip mass leads to this apparent resonant frequency discrepancy. Upon identification of the resonant frequency, the beam was excited with a 96 Hz sine wave to produce the maximum voltage output. Figure 6a shows

that an RMS acceleration of 0.64 g produced a RMS voltage output of 15.9 mV and that the acceleration input phase led the voltage output phase by about 90°. Further testing at this frequency showed that the voltage increased as the acceleration increased as illustrated in Figure 6b,c, which is indicative of a piezoelectric response. In Figure 6c, the RMS voltage increased in an approximately linear fashion as a function of increasing RMS acceleration. These RMS values equate to an average electrical output of 23.5 mV/g at its resonant frequency. This sensitivity lies within the range that is useful for passive sensing purposes. For comparison, the reference accelerometer used in the experimental setup operates at a sensitivity of 10 mV/g. The tip displacement of the beam under varying base acceleration is measured to confirm that the increasing voltage as a function of increasing acceleration is due to the larger strain induced in the beam. As shown in Figure 6d, the RMS displacement of the tip of the beam increased as the RMS acceleration increased in a similar fashion as the RMS voltage increased with increasing RMS acceleration in Figure 6c. All of the above testing verified that BaTiO3 coated carbon fiber fabrics can function as a flexible fabric and create voltage outputs in response to acceleration inputs thus demonstrating a flexible, multifunctional fabric that can perform as a passive sensing composite.

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CONCLUSION A two-step hydrothermal reaction was used to deposit a conformal BaTiO3 film on a woven carbon fiber fabric. These fabrics with an active coating were fabricated into multifunctional composites using two different matrices in order to create both a rigid energy harvesting composite and flexible energy harvesting composite. Vibration testing of cantilever beams of these hybrid composites revealed their electromechanical 4062

DOI: 10.1021/acsami.6b15011 ACS Appl. Mater. Interfaces 2017, 9, 4057−4065

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within a few minutes. The epoxy matrix consisted of a 100:35 weight ratio of Epon 862 and Epikure 3230. Upon complete infiltration, the composite was left under vacuum and allowed to cure at room temperature for 10 h. It was then released from the vacuum and cured at 80 °C for 2 h followed by heating at 125 °C for 3 h. Upon cooling to room temperature, the composite was sectioned into a beam with dimensions of 20 mm × 10 mm for electromechanical testing. To measure the voltage and current outputs of this beam, electrodes were connected to the top and bottom carbon fiber fabrics. This was achieved by using a razor to remove the top layer of epoxy to expose the carbon fiber and attaching 33 gauge wires to the carbon fiber with silver paint. Once electroded, the composite was poled before power scavenging measurements. This was achieved via a conventional poling technique in which the two electrodes were connected to a 5 V DC power supply and held at this voltage for 12 h at room temperature. Flexible Composite Fabrication. The flexible composite was fabricated by placing one bare carbon fiber fabric under the BaTiO3 coated carbon fiber fabric. This composite was infiltrated with polydimethylsiloxane (PDMS, Dow Corning Sylgard 184). The two parts of PDMS were mixed and applied to the BaTiO3 coated fabric providing the mixture sufficient time to infiltrate the fabric. Before curing, aluminum foil was placed on top of the PDMS to act as the top electrode, and the bottom bare carbon fiber fabric acted as the bottom electrode. Electroding was achieved by attaching 33 gauge wires with silver paint to the bottom bare carbon fiber fabric and the aluminum foil. The composite was cured at 150 °C for 10 min resulting in a flexible composite. This composite was sectioned in the same way as the rigid composite to create a beam for electromechanical testing. Composite Vibration Testing. Both the rigid and flexible composite beams were tested in the same way. The beams were mounted in a cantilever confirguration so that one end was fixed and the other end was free to vibrate. A base acceleration was provided to the fixed end via a LDS permanent magnet shaker, and this acceleration was measured with a commercial shear accelerometer (PCB 352C22) that was placed on the clamped end of the beam. The tip displacement of the vibrating beam for the flexible composite was measured by using a laser displacement sensor (Keyence, LK− H057S). To induce larger displacements of the free end of of the beams, lumped masses were added. A 20 g mass was placed on the rigid composite and a 1 g mass was placed on the flexible composite. The voltage and current outputs of the composites were measured using an electrometer (Keithly 6514). Power measurements were made by inserting load resistances into the circuit and measuring the voltage output. NI SignalExpress Software in conjunction with a DAQ board (NI USB 4431) were used to generate the excitation waveforms and simultaneously acquire the output voltages and currents.

performance. The rigid energy harvesting composite was able to produce an RMS voltage of 5.15 mV from 1 g RMS acceleration when excited with a sine wave at its resonant frequency of 47 Hz. This functional composite had a power density of 217 pW/cc from only 1 g RMS acceleration. The flexible multifunctional composite showed an electrical output of 23.5 mV/g when excited at its resonant frequency of 96 Hz. Clearly, the lower flexural rigidity of the flexible composite compared to the rigid composite leads to larger bending deformation of the cantilever beam resulting in enhanced output voltage from the flexible composite. The agreement between the increasing output voltage and tip-displacement of the flexible composite as a function of acceleration input further confirms the enhanced voltage response in flexible energy harvesting composite is due to the increased displacement of the cantilever beam. Therefore, this research demonstrated that piezoelectric interfaces integrated into fiber reinforced polymer composites enable power scavenging and passive sensing capabilities in structural materials, which offers a further advancement to the field of multifunctional composites.

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EXPERIMENTAL METHODS

Film Synthesis. A BaTiO3 textured film was grown on unsized woven carbon fiber fabrics (Hexcel, IM7) consisting of 5.2 μm diameter fibers via a two-step hydrothermal reaction. Prior to synthesis, the fabrics were functionalized. The functionalization process involved submerging the fabrics in nitric acid (Fisher, 70%) and heating it to 83 °C for 4 h and then washing it with DI water in a Soxhlet extraction process for 8 h. The carbon fiber fabrics were then seeded with TiO2 nanoparticles through a dip-coating process using a titanium sol−gel consisting of hydrochloric acid (HCl, Fisher, 35%), isopropanol, and titanium isopropoxide (Alfa Aesar, VERTEC TIPT, 97+%). The fabrics were dipped in the solution and then heat treated at 120 °C for 1 h to create a TiO2 nanoparticle seed layer to assist in the nucleation of the TiO2 film during the hydrothermal synthesis. A hydrothermal growth solution was prepared with a 1:2 volumetric ratio of titanium tetrachloride (Alfa Aesar, 99.0%) and titanium isopropoxide (Alfa Aesar, VERTEC TIPT, 97+%) in a 1:1 volumetric ratio of HCl (Fisher, 35%) and DI water. This solution and the fabric were combined in a sealed acid digestion bomb (Parr Instrument Company) and heated to 180 °C for 3 h. After allowing the reactors to cool to room temperature, the fabric was removed from the solution and rinsed with water and allowed to dry at room temperature. The second hydrothermal reaction was prepared and consisted of 1.23 M BaCl2·2H2O (Fisher, 99.5%) and 0.75 M NaOH (EMD Millipore, 99.5%) dissolved in DI water. The TiO2 coated fabrics were then placed in this solution in the acid digestion bomb, and the reaction was heated to 225 °C for 24 h. Upon cooling to room temperature, the BaTiO3 coated carbon fiber fabric was rinsed in a dilute HCl solution and then dried at room temperature. The BaTiO3 film was 10.5 μm thick resulting in an increase of the carbon fiber fabric from 250 to 430 μm. Film Characterization. The crystal structures of the TiO2 and BaTiO3 films were investigated with XRD (PANalytical X’Pert Powder) using Cu Kα radiation over 2θ range of 20° to 60°. XRD samples were prepared for each sample using a mortar and pestle to create a powder from the woven fabrics. The elemental composition was confirmed with energy-dispersive X-ray spectroscopy (EDS, JEOL-7800FLV FE SEM). The morphologies of the BaTiO3 films on woven carbon fiber fabrics were investigated using a scanning electron microscope (SEM, TESCAN VEGA3 LM) Rigid Composite Fabrication. The rigid multifunctional composite was fabricated by placing the BaTiO3 coated fabric on top of two layers of bare carbon fiber fabric, and aluminum foil and one layer of carbon fiber fabric was placed on top of the BaTiO3 coated fabric. Using vacuum-assisted resin transfer molding (VARTM), the epoxy was able to completely infiltrate the fabrics

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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/acsami.6b15011. TGA data for the BaTiO3 coated carbon fiber fabric (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Christopher C. Bowland: 0000-0002-1229-4312 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge finanical support from National Science Foundation (Award No. CMMI-1333818), Air Force Office of Scientific Research (Contract No. FA95504063

DOI: 10.1021/acsami.6b15011 ACS Appl. Mater. Interfaces 2017, 9, 4057−4065

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16-1-0087 and Contract No. FA9550-12-1-0132), and Air Force Research Laboratory (Award No. FA8651-08-D-0108) for this research.

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DOI: 10.1021/acsami.6b15011 ACS Appl. Mater. Interfaces 2017, 9, 4057−4065

Research Article

ACS Applied Materials & Interfaces (42) Wang, Z. L.; Wu, W. Nanotechnology-Enabled Energy Harvesting for Self-Powered Micro-/Nanosystems. Angew. Chem., Int. Ed. 2012, 51, 11700−11721.

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DOI: 10.1021/acsami.6b15011 ACS Appl. Mater. Interfaces 2017, 9, 4057−4065