Experimental details

To fabricate continuous nanocomposites filament for printing, commercial PVDF powder (MW~534,000; Sigma-Aldrich, USA), BT powder (700 nm; Inframat^®, USA), and MWCNTs powder (diameter: 8–15 nm, length: 10–50 µm, Cheaptubes^®, USA) are mixed with N-dimethylformamide solvent (DMF, OmniSolv^®, USA) via the solvent-casting method. As a schematic illustration of the synthesis process is shown in Fig. 1(a), the BT and PVDF powder are mixed via a solvent-casting process which is an alternative method of mechanical mixing for uniform distribution of BT nanoparticles. The solution is prepared by dissolving PVDF powder in DMF solvent (1:10 weight ratio). The solution is then placed in a water bath at 80 °C and is stirred using a magnetic stir bar at 300 rpm for approximately 30 min. After the PVDF powder fully dissolves, BT powder is introduced to the solution. This attained nanocomposite solution is then heated and stirred using the same parameters for approximately 15 min. BT build up at the bottom of the solution is addressed by ultra-sonication (Branson Sonifier 450) for 20 min. DMF solvent is then evaporated by dispersing nanocomposites solution onto a glass substrate and heated to a temperature of 80 °C for 12 h. The procedure yields a thin sheet of BT/PVDF nanocomposite, and these casted nanocomposites are sliced down to be easily extruded by filament extruder machine (Filabot). The diameter of the extruded filament and extruding temperature are 2.9 mm and 195 °C, respectively. These resultant thin sheets of nanocomposites are sliced down to be easily extruded by a filament extruder machine (Filabot^®) and the nanocomposite filament [see Fig. 1(b)-i] is used to 3D print a thick film by a FDM 3D printer (Lulzbot^® Taz 5) for a pressure sensor application [see detail in online Supplementary Method S1 and Figs. 1(b)-ii and 1(b)-iii]. This filament extrusion and 3D printing processes are proved in the literature to provide better dispersion of nanoparticles in the matrix than solvent-casting, therefore enhancing piezoelectric and mechanical properties.^[ Reference Kim, Fernando, Li, Lin and Tseng ^{8 ]}

Figure 1.

(a) Schematic illustration of the synthesis process for MWCNT/BT/PVDF nanocomposites; (b) (i) extruded nanocomposite filament, (ii) 3D printed nanocomposite film device covered with silver paste, and (iii) FDM 3D printer; (c) schematic illustration of the printed nanocomposite sensor device, and SEM images of (d) BT nanoparticles (the inset shows XRD analysis for BT nanoparticles), (e) MWCNT (the inset shows XRD analysis on MWCNT), (f) 0.4 wt%-MWCNT/PVDF, and (g) 0.4 wt%-MWCNT/18 wt%BT/PVDF.

Results and discussion

The printed nanocomposites film is depicted in Fig. 1(c). BT particles and PVDF polymer are not compatible with each other because of their respective hydrophobic and hydrophilic characteristics, so the combinations of these two offer a low-stress transfer efficiency between the ceramic nanoparticles and the polymer matrix.^[ Reference Kim, Torres, Villagran, Stewart, Lin and Tseng ^{9 ,} Reference Shen, Chang, Hsieh, Li, Chiang, Yang and Yip ^{14 ]} MWCNTs are dispersed in this nanocomposites for stress reinforcing and conducting functional agents between BT particles and the PVDF polymer.^[ Reference Park, Lee, Liu, Moon, Hwang, Zhu, Kim, Kim, Kim and Wang ^{11 ,} Reference Mandal and Nandi ^{15 ]} Although MWCNTs are still hydrophobic, active mechanical stress reinforcing between the BT and PVDF will be significantly caused when applying external stress.^[ Reference Park, Lee, Liu, Moon, Hwang, Zhu, Kim, Kim, Kim and Wang ^{11 ,} Reference Kim, Zhu, Qu, Aaronson, McCall, Chen and Sirbuly ^{12 ]} These agents can efficiently load external stress onto BT particles through the PVDF matrix and the piezoelectric charges generated by BT particles are conducted through MWCNTs.^[ Reference Ahn, Lim, Hong, Lee, Ha, Choi and Seo ^{10 ]}

The morphology and crystalline structure of the materials are analyzed by scanning electron microscopy (SEM, TM-1000, Hitachi) and x-ray diffraction (XRD, D8 Discover diffractometer, Bruker). SEM images and spectra in Figs. 1(d) and 1(e) demonstrate commercially purchased pure BT nanoparticles and MWCNTs. Figures 1(f) and 1(g) indicate the well-distributed MWCNTs and agglomeration of BT nanoparticles are observed to increase as its percentage increases in the PVDF matrix (see details in Fig. S2).

For piezoelectric property analysis, piezoelectric output voltage and current in a fatigue load frame (Bose ElectroForce-BioDynamic, TA Instruments) are measured by Voltmeter (InstruNet i-400), and Pico ammeter (Keithley 6485) as shown in Fig. 2(a)-ii.^[ Reference Kim, Torres, Wu, Villagran, Lin and Tseng ^{7 ,} Reference Kim, Fernando, Li, Lin and Tseng ^{8 ]} The sample can be fabricated with a large area, but due to the geometry of the fatigue grip, nanocomposites samples were printed with dimensions of 6 × 35 × 0.55 mm³ as shown in Fig. 2(a)-i. To quantify the piezoelectric property of the printed film, samples are prepared with silver conductive paint electrodes on both surfaces. The electrodes are then attached with copper (Cu) tape to allow a proper connection to the Pico ammeter as shown in Fig. 2(a)-ii. Cyclic force is applied on the printed nanocomposites to measure periodic output current and voltage; the fatigue load frame generated 30 cyclic loads on the sample at 0.5–4 Hz, while the Pico ammeter measured the current output.^[ Reference Kim, Fernando, Li, Lin and Tseng ^{8 ]} To prevent noise and artifacts from the fatigue machine during measurement, the two grips handling the film are covered with electrical insulating tape.

Figure 2.

(a) Schematic illustration of (i) sample design and (ii) experimental setup for piezoelectric output measurement, (b) the measured (i) voltage and (ii) current output results generated by 0.4 wt%-MWCNT/18 wt%-BT/PVDF under 40 N before and after electric poling, (c) voltage output results generated by (i) various MWCNTs content of 12 wt%-BT/PVDF and (ii) various BT particle contents of 0.4 wt%-MWCNT/PVDF under 40 N, and (d) voltage output results generated by 0.4 wt%-MWCNT/18 wt%-BT/PVDF under various forces (10, 20, 30, 40, 60, and 80 N).

Electric poling is applied under 5.4 MV/m for 15 h to the printed nanocomposite films after 3D printing fabrication. Note that the three samples for each nanocomposite are tested for FTIR analysis and output electricity. Figure 2(b)-i and ii show output voltage and current produced by the printed nanocomposites film before and after the electric poling process. The amplitude of voltage outputs generated from the nanocomposite films before and after poling treatment increases from ±0 to ±340 mV and current outputs from ±0.084 to ±0.94 nA. BT nanoparticles dispersed in the matrix should be polarized through the post-poling process. This result indicates that electric poling has activated piezoelectricity of the BT particles and that the BT particles and MWCNTs are mechanically and electrically interacting with each other. Charges generated from BT particles and PVDF due to the MWCNTs interaction can be efficiently transferred to external electrodes through the conductive channel of MWCNTS. Figures 2(c)-i and 2(c)-ii indicate results of voltage outputs produced by various MWCNTs and BT nanoparticles percentages. In Fig. 2(c)-i, 12 wt%-BT/PVDF nanocomposites film is characterized without MWCNTs to study the role of MWCNTs within the printed nanocomposites. No reliable output signal is observed. However, with an increase in MWCNT content, the output voltages increased by ±192 mV under 0.4 wt%-MWCNTs, which is the maximum percentage over which electrical breakdown occurs during electrical poling. For electrical poling process, the least amount of coercive electric field for BT particles about 0.5–1.6 MV/m is required.^[ Reference Kim, Torres, Villagran, Stewart, Lin and Tseng ^{9 ,} Reference Nagata and Kiyota ^{16 ]} However, the breakdown strength lowers down as MWCNTs increase and 0.4 wt%-MWCNTs content is a limitation before experiencing a breakdown. This increase of the voltage amplitude indicates that MWCNTs play a significant role as a stress reinforcing agent within the nanocomposites. In Fig. 2(c)-ii, when there are 0.4 wt%-MWCNTs in the PVDF matrix and no BT nanoparticle content, the piezoelectric output voltage generated is ±49 mV. This is because PVDF β-phase was formed on high-specific surface areas of MWCNTs.^[ Reference Ahn, Lim, Hong, Lee, Ha, Choi and Seo ^{10 ]} As BT particle content increases, the voltage output increases up to ±340 mV at 18 wt%-BT, which is the maximum percentage of BT content that can be extruded in the FDM 3D printer nozzle. With 0.4 wt%-MWCNTs/18 wt%-BT/PVDF film (which showed the highest generated output voltage), output voltages were measured under various external forces applied by fatigue load frame. Figure 2(d) shows that a higher amplitude of output voltages was produced at higher external forces, increased by ±725 mV under 80 N.

FTIR (Agilent Technologies Cary 630 ATR-IR) analyses are performed in the 600–1600 cm⁻¹ wavenumber range at room temperature to characterize the degree of crystallinity of α- and β-phases of the PVDF before and after the electric poling process.^[ Reference Cai, Lei, Sun and Lin ^{17 –} Reference Matsushige, Nagata, Imada and Takemura ^{19 ]} IR absorption bands at 766 and 840 cm⁻¹, which are, respectively, characteristic of α- and β-phases, are observed for analysis of the phase transformation. Based on the results of the IR absorption bands (see Fig. S3), the β-phase contents of each printed nanocomposite film are calculated as described in online Supplementary Method S2. In Fig. 3(a), β-phase contents significantly increased after electric poling and as MWCNTs increased by 70.3% of β-phase at 0.4 wt%-MWCNTs/12 wt%-BT/PVDF. It is certain that MWCNTs led to a high degree of conversion from α- to β-phases. Under the same amount of MWCNTs (0.4 wt%), the analysis of β-phase contents showed an increasing trend as BT particles were increased.

Figure 3.

(a) β-Phase contents of the printed nanocomposites with various MWCNTs and BT particle contents before and after electric poling process and (b) piezoelectric coefficients (d ₃₁) of the printed nanocomposites with various MWCNTs and BT particles contents.

Based on the voltage output results of Figs. 2(c)-i and 2(c)-ii, piezoelectric coefficients (d ₃₁) of each printed nanocomposite film are calculated with various compositions using measured relative permittivity (Table S1). The detailed calculation procedure is described in online Supplementary Method S3. In Fig. 3(b), d ₃₁ has significantly increased from 0.002 to 0.058 pC/N when MWCNTs’ content increases under 12 wt%-BT. This indicates that the addition of MWCNTs enhances the performance of stress reinforcing between BT particles and the matrix. Under the same amount of MWCNTs (0.4 wt%), d ₃₁ is tested with various percentage of BT particles. The result indicates that the d ₃₁ increases from 0.022 (0 wt%-BT) to 0.13 pC/N (18 wt%-BT) as BT particles increase, a 492% increase.

To further demonstrate the piezoelectric sensor application, the feasibility of using printed piezoelectric nanocomposites for pressure sensor application was demonstrated. The output voltage signal is measured when bending (90°) and pressing (40–50 N) by human fingers shown in Fig. 4(a)-i shows the printed nanocomposites film and hands covered, respectively, by Kapton film and gloves, to exclude external electrostatic charges. Two Cu wires attached on both silver electrodes are extended to connect to the data acquisition machine. In Fig. 4(a)-ii, upon bending the sample, the output voltage was measured to be ±120 mV. Upon pressing in the middle of the sample, the amplitude of the output voltage increased to ±435 mV at most. In Fig. 4(c), durability test is implemented at 4 Hz frequency for 2000 cycles to confirm the mechanical stability of the printed nanocomposite film. The amplitude of voltage output remains constant after 2000 cycles. The printed film also shows the same piezoelectric stability after 3 days. The bottom inset of Fig. 4(c) shows the magnified voltage signal from 250 to 260 s.

Figure 4.

(a) Optical images of the printed nanocomposite sensor device in (i) original, (ii) bending, and (iii) pressing states; (b) the measured voltage output signals of 0.4 wt%-MWCNT/18 wt%-BT/PVDF film generated by (i) bending and (ii) pressing motions by human fingers, (c) the durability test result of 0.4 wt%-MWCNT/18 wt%-BT/PVDF film under 2000 periodic cycles (4 Hz) using fatigue load frame.