UU. S. NAVAL RESEARCH LABORATORY
Washington, DC 20375-5320
NRL/MR/7165- -20-10,123
3D Printed PVDF A LEC I KEI J AMES W ISSMAN (F ORMER P OSTDOC )C HARLES R OHDE
Acoustic Signal Processing and Systems BranchAcoustics Division G REGORY Y ESNER (C URRENT P OSTDOC ) Physical Acoustics BranchAcoustics Division
February 26, 2021
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PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ORGANIZATION.1. REPORT DATE (DD-MM-YYYY)
September 30, 2020
2. REPORT TYPE
NRL Memorandum Report
3. DATES COVERED (From – To)
4. TITLE AND SUBTITLE
3D Printed PVDF
NISE
6. AUTHOR(S)
Alec Ikei, James Wissman, Charles Rohde, and Gregory Yesner
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Naval Research Laboratory4555 Overlook Avenue, SWWashington, DC 20375-5320
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NRL/MR/7165--20-10,123
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Naval Research Laboratory4555 Overlook Avenue, SWWashington, DC 20375-5320
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NRL-NISE
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Distribution A: Approved for public release: distribution unlimited
13. SUPPLEMENTARY NOTES
Karles Fellowship
14. ABSTRACT
In this paper we report on the 3D printing and testing of the piezoelectric polymer polyvinylidene difluoride (PVDF).Samples of PVDF were fabricated using a fused deposition modeling (FDM) 3D printer and then activated using a coronapoling process. The d33 piezoelectric coefficient, which is related to the overall piezoelectric performance, was exper-imentally measured using a d33 meter to be 6 pC/N. While less than commercially available PVDF fabricated usingtraditional techniques (which can have a d33 of 10 – 40 pC/N), the value of 6 pC/N achieved in this work is severalorders of magnitude larger than comparable previously published results for 3D-printed PVDF, and as a result representsa significant step in the 3D printing of piezoelectric polymers.
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18. NUMBER OFPAGES Alec Ikei (Include area code)
Standard Form 298 (Rev. 8/98)Prescribed by ANSI Std. Z39.18L A TEX iiihis pageintentionallyleft blankiv
ONTENTS
EXECUTIVE SUMMARY................................................................................................... E-11. BACKGROUND ........................................................................................................... 11.1 Piezoelectric Devices .............................................................................................. 11.2 PVDF Manufacturing ............................................................................................. 11.3 3D Printed Sensors ................................................................................................. 22. RESULTS.................................................................................................................... 32.1 Additive Manufacturing........................................................................................... 32.2 Piezoelectric Poling and Benchmarking ...................................................................... 52.3 Simultaneous Poling and Printing .............................................................................. 73. SUMMARY ................................................................................................................. 8REFERENCES .................................................................................................................. 9APPENDIX A—Appendix One ............................................................................................. 11v
IGURES α phase of PVDF. Net dipole moment transverse to carbon chain directionis zero. (b) Monomer of β phase of PVDF. Non-zero dipole moment transverse to the carbonchain, which is necessary for poling. .......................................................................... 22 PVDF pellets (Raw Material) are processed into filament (Filament Extrusion), which isthen used as feed material in 3D printers. .................................................................... 33 Shear occurs through restriction of the feed material’s cross-sectional area in the print head,and during deposition into thin layers. ........................................................................ 44 Printed PVDF. (a) While the overall geometry was correct, there was significant porosity inthe print. (b) Printed PVDF part, with ”PVDF” embossed on the surface. ........................... 45 PVDF layer on the bottom with carbon PLA layer on top................................................ 56 (a) An attempt to print a carbon PLA raft, with PVDF and a carbon PLA layer on top. Therewas significant delamination of the PVDF layer and the carbon PLA layer. (b) Anotherattempt with different parameters. Intact lamination of all layers, but with visible warping. .... 57 Oil bath with lid and heated fluid circulator. Bath constructed out of polypropylene, withclamps and fittings made out of ABS and PTFE (Teflon) respectively. ............................... 68 (a) Parallel bimorph setup for LDV cantilever deflection measurement, where polarizationis in the same direction. (b) Input voltage (blue) vs cantilever actuation (orange). ................ 69 Corona poling setup. High voltage was applied to a tungsten needle to ionize the air. ThePVDF was stretched while under high temperature and high electrical fields. ...................... 710 In-situ poling setup. Print bed is charged instead of print head to reduce risk of damage tothe printer. ............................................................................................................ 8vi ABLES
XECUTIVE SUMMARY
The purpose of this project is to leverage 3D printing to produce complex structures for the fabrication ofacoustic metamaterials. To accomplish this goal, this project’s approach is to 3D print piezoelectric devices.PVDF (polyvinylidene difluoride) was chosen for its relatively strong piezoelectric response (10-40 pCN )when prepared properly, as well as its relatively high resistance to corrosion and environmental friendlinesswhen compared to ceramics like PZT (lead zirconate titanate). Another key feature is that it also possibleto print it on an FDM (fused deposition modeling) based 3D printer. To prepare it properly, a sufficientamount of shear stress must be applied while heating and applying a large electrical field across the material.The shear stress and electric field serve to align the polymer chains and electric dipoles in a particulardirection, while the heat provides mobility to do so. The approach used in this work applies high electricalpotential difference between the print head and the bed of the FDM machine in-situ to accommodate theseneeds. In this work, 3D printed PVDF that was stretched and poled post-process was shown to exhibit 6 pCN ,performing better than other efforts of FDM manufacture of PVDF.E-1his pageintentionallyleft blankE-2
D PRINTED PVDF
1. BACKGROUND1.1 Piezoelectric Devices
Piezoelectric devices are commonly used in acoustic applications as transducers and sensors. The piezo-electric phenomenon converts strain into a potential difference between the surfaces of the sample. Thisrelationship is defined in Eq. 1, where X is strain, d is the piezoelectric coefficient, E is the electric field,and M is the electrostrictive coefficient [1]. The d tensor is compressed into a second rank tensor throughsymmetry arguments [1] and the non-linear electrostrictive contribution, present in all materials, is smallin piezoelectric materials at low fields (Eq. 2) [2]. Additionally, convention dictates that the “1” directionrefers to the stretch direction and the “3” direction refers to the polarization direction. As an example, thismeans that if a sample is pulled in the 1 direction, the electric response in the 3 direction is related by thed component. X kl = ∑ i d ikl E i + ∑ i , j M i jkl E i E j (1) X j = ∑ i d i j E i (2) In many applications, ceramic piezoelectrics have been favored due to their better piezoelectric perfor-mance when compared to PVDF (polyvinylidene difluoride) (PZT 501A and PVDF have a d value of 400 pCN and 20 pCN respectively [1]). However, PVDF is printable through FDM (fused deposition modeling),does not contain lead, is corrosion resistant, relatively cheap and is not brittle.PVDF is a fluorinated hydrocarbon, and has either two hydrogen or two fluorine atoms connected to eachinterior carbon atom (CF CH ) n , where the n denotes that the elements inside the parenthesis are repeatedto comprise the polymer. It has several phases that refer to the orientation of its molecular structure, whichare the basis of its piezoelectric properties. If the fluorine atoms and hydrogen atoms are on separate sidesof the chain, it creates a dipole moment for each monomer and is called the β phase [1]. When cooled fromthe liquid phase, it forms the α phase, where the dipoles within the monomer oppose each other and cancelout, which makes each monomer and the polymer non-polar (Figure 1). Therefore, it must be mechanicallystretched to convert it to the polar β phase to prepare for poling [1] [3]. In the poling step, an electric field Manuscript approved September 30, 2020. Alec Ikei [et al.] is applied to the β phase PVDF to orient the dipoles in bulk. When strain is applied to the crystal it deformsthe molecular structure, bringing together or pushing apart the dipoles of the monomers. This changesthe density of dipoles and is observed macroscopically as a development of electric potential between thecrystal’s surfaces i.e. the piezoelectric effect.Many commercially produced piezoelectric PVDF films are made by extruding it in a relatively thicksheet, which is then stretched to around 400-500% of its original length [4], electroded and corona poled.The commercial sample used in this work is a product of this process (Component Distributors Inc., part β phase contentexist, such as high pressure treatment, annealing, ultra-rapid cooling, electrospinning, or crystallization fromsolution [5]. Fig. 1—(a) Monomer of α phase of PVDF. Net dipole moment transverse to carbon chain direction iszero. (b) Monomer of β phase of PVDF. Non-zero dipole moment transverse to the carbon chain, whichis necessary for poling.
3D printing has become popular as a way to rapidly prototype and create complicated designs, becauseit does not rely on machining material away from parts. The rapid prototyping makes the design processquicker, smoother and less expensive. Incorporating active elements into 3D printed designs allows thesensor design process to reap these benefits. One application of interest is in 3D printed acoustic sensors.Acoustic sensors have been 3D printed that sense through bionic, capacitive, or piezo-resistance methods[6].3D printers have also been used to print piezoelectric devices. Kim et. al implemented a way to 3Dprint piezoelectric films using corona poling to induce piezoelectric structure [7]. This process relies on theshear from printing to transform the melt into the β phase and applies high voltage to the print head andscans over the layer after it has been printed. The voltage is high enough to ionize the air surrounding thenozzle (corona field). The electric field of the ionized particles deposited on the surface of the print andthe grounded base plate beneath the printed layer induce the dipole orientation necessary for piezoelectricresponse in PVDF. The print was determined to have a d value of 4.8 x 10 − . While Kim et. al relyon a corona field to pole their print, in this work the voltage was applied directly to the print head to supplythe necessary electric field. Additionally, with post process poling the sample was able to achieve a higherpiezoelectric response of 6 pCN .The mechanical compliance of PVDF has made it a good material for 3D printed soft robotics. 3Dprinted soft robots have also used other polymers, hydrogels and elastomers and have been actuated throughthermal, electrical, pH-based, light-based and magnetic means [8]. Soft robotics are good at handlingdelicate materials and conforming to space restrictions, which make them useful in many industrial, medicaland human interaction settings. D Printed PVDF
2. RESULTS2.1 Additive Manufacturing
Kynar 740 PVDF pellets and a Filabot EX2 filament extruder were purchased to create the filament.Through control of the extrusion parameters (temperature, extrusion speed, cooling, drawing speed), theamount of air bubbles was decreased, and cross sectional geometry and consistency of the filament diameterwas adjusted to create filament that was suitable for 3D printing in an Ultimaker 3. Even while keepingall the parameters consistent, however, the inconsistencies of the extruder’s motor resulted in it over/underextruding. A paper shredder was used to reprocess over/undersized filament that was not within the diameterspecifications of the printer. Although the melting point of PVDF is approximately 177 ◦ C, extruding at thistemperature it appears to cause additional air bubbles and diameter inconsistency. In general, best resultswere achieved when extruding in the 200-220 ◦ C range.
Fig. 2—PVDF pellets (Raw Material) are processed into filament (Filament Extrusion), which is thenused as feed material in 3D printers.
In the printing setup, Cura (v2.2.6) was used to slice the models into gcode (movement code), which wasrun on an Ultimaker 3. FDM type 3D printers, like the Ultimaker 3, drive filament into a print head, whichheats and extrudes it to build a model layer by layer. To apply the shear stress and required heat, thin layersof PVDF were printed using a small diameter nozzle (0.4mm) (Figure 3). Tuning the printer parameters wasa crucial step. In initial attempts to print, the PVDF extruded from the nozzle would roll up into a ball or notextrude from the nozzle at all. After troubleshooting from the failed prints (e.g. Figure 4 (a)), the samplein Figure 4 (b) was printed by modifying following settings from the pre-set “Extra Fine” printing profile,listed in Table 1. The settings log is listed in the appendix A.
Alec Ikei [et al.]
Fig. 3—Shear occurs through restriction of the feed material’s cross-sectional area in the print head, andduring deposition into thin layers.
Table 1—Print parameter deviations from ”Extra Fine” printing profile inCura Print Temperature 220 ◦ C-240 ◦ CInfill Percentage 100Line Width 0.3mmMaterial Diameter 2.55mmLayer Thickness 0.1mm-0.15mm
Fig. 4—Printed PVDF. (a) While the overall geometry was correct, there was significant porosity in theprint. (b) Printed PVDF part, with ”PVDF” embossed on the surface.
To create an electrode for evenly applying a potential across the surface of the PVDF, layers of carboninfused PLA (polylactic acid) were printed above and below the PVDF layer. To start off, PVDF was printeddirectly onto the build plate, with carbon PLA on top in Figure 5. However, it was significantly more difficultto make a print with a layer of carbon PLA on the bottom. This was due to the PVDF peeling off the carbonPLA during the print, which was a result of thermal contraction and poor bonding between the two materialtypes (Figure 6). Including additional PLA around the bottom layer (i.e. adding a raft), as well as increasingprint temperature and lowering the printing speed improved its ability to stay relatively flat and successfullyprint the three layers, using the parameters from Table 1.
D Printed PVDF Fig. 5—PVDF layer on the bottom with carbon PLA layer on topFig. 6—(a) An attempt to print a carbon PLA raft, with PVDF and a carbon PLA layer on top. There wassignificant delamination of the PVDF layer and the carbon PLA layer. (b) Another attempt with differentparameters. Intact lamination of all layers, but with visible warping.
In demonstration of the feasibility of poling 3D printed PVDF with contact electrodes, a post-processpoling setup was devised. A heated, circulating fluid pump was purchased to provide silicone oil at aconstant temperature in a dielectric bath. The dielectric bath was constructed out of polypropylene, with theclamps and fittings chosen to provide additional electrical insulation (Figure 7). With this heat bath, polingof a PVDF sample was attempted, but it did not yield a measurable piezoelectric coefficient. This was dueto early dielectric breakdown of the PVDF layers, which was likely caused by local variation in the printthickness and squeezing from the thermal expansion of the clamps.
In preparation of future needs to characterize piezoelectric films, a cantilever test was performed, usingcommercially supplied piezoelectric sheets (Component Distributors Inc., part value. The tip deflection was measuredusing a Laser Doppler Vibrometer (LDV) (CLV-2534, Polytec) with its data acquired by an oscilloscope. Alec Ikei [et al.]
Fig. 7—Oil bath with lid and heated fluid circulator. Bath constructed out of polypropylene, with clampsand fittings made out of ABS and PTFE (Teflon) respectively.
The piezoelectric sheets consisted of a 110 µ m thick PVDF layer sputtered with 700 ˚A of copper and 100 ˚Aof nickel on both sides. The sample measured 82.87mm in length.From Figure 8b, the difference in the LDV output voltage indicates that the actuation distance was 32 µ m,as calculated through Eq. 3. In the setup shown in Figure 8, the appropriate equation for a parallel configu-ration bimorph is described in Eq. 3, where δ is tip deflection, d is the piezoelectric strain coefficient, L islength, t is thickness of the bimorph (i.e. twice the sheet/layer thickness) and V is applied voltage [1]. Thebimorph consisted of two strips of the piezoelectric film epoxied together with their polarization orientedin same direction (parallel configuration). This gives twice the actuation for the sample applied voltage incomparison to the antiparallel configuration, where the strip polarization are oriented in the opposite direc-tion [1]. 0.32V corresponds to a d value of 15 pCN . The manufacturer specifies a piezoelectric coefficientof 23 pmV , which is equivalent to 23 pCN . δ = d ( L / t ) V (3) Fig. 8—(a) Parallel bimorph setup for LDV cantilever deflection measurement, where polarization is inthe same direction. (b) Input voltage (blue) vs cantilever actuation (orange).
D Printed PVDF Local variation in the thickness of the print lead to early dielectric breakdown through the PVDF in theoil bath, so to verify that it was possible to pole the samples that were 3D printed on the Ultimaker 3, acorona poling method was designed and made to combat the issues previously encountered. In this setup, agrounded metal plate was placed under an acrylic enclosure to prevent external leakage of charge. A highvoltage power supply (PS300, SRS) was used to apply up to 20kV to a tungsten needle, placed at the top ofthe enclosure. A grounded piece of metal was placed under the needle to direct the ionized air towards thesample.Many corona poling setups also apply elevated temperatures and high levels of strain to the PVDF film,to lower the coercive field strength and align the polymer backbone respectively (the coercive field is theelectric field needed to reverse the piezoelectric polarization of the material). To do this to the printed parts, aheater and film stretcher were constructed mostly out of parts from an old 3D printer (Figure 9). The heatingelement was placed in an aluminum block and a thermistor provided the feedback control to keep the blockat a set temperature. The stretching mechanism utilized a 3D printed worm gear and a small stepper motor,operated by a control board from a 3D printer to apply 400-500% strain at 80 ◦ C. Using this setup, a 3Dprinted piezoelectric film sample with a d value of 6 pCN was achieved. The d measurement was taken bya d meter a day after the poling process took place, to ensure that the remnant polarization was measured. Fig. 9—Corona poling setup. High voltage was applied to a tungsten needle to ionize the air. The PVDFwas stretched while under high temperature and high electrical fields.
As mentioned in the background, it is necessary to apply strain, high electric fields and elevated temper-atures to convert α phase PVDF to its piezoelectric form. It has been shown that it is possible to do these Alec Ikei [et al.] processes in series in Kim et al., but the current work was done to do these processes simultaneously. Toreduce the costs involved if the printer were to break from having high voltage applied, an inexpensive FDM3D printer (Ender 3) was purchased. A steel plate was placed on the print bed to serve as a high voltageelectrode. The ground lead was attached to the top of the print head.
Fig. 10—In-situ poling setup. Print bed is charged instead of print head to reduce risk of damage to theprinter.
In normal use, the print bed of a 3D printer is heated to around the glass transition temperature (around55 ◦ C for PLA). However, the print bed was instead cooled using liquid nitrogen, dried and placed back onthe printer. This was done to help set in place the dipole alignment and material phase through the rapidcooling of the sample. Making the print bed extremely cold mitigates the chance of reversion to the unpoledstate due to retained heat. This method has not yet yielded a measurable piezoelectric coefficient in anysamples, but further work is planned to improve these results, which is noted in the summary section below.
3. SUMMARY
The ability to take pellets of PVDF stock, create a workable filament, and print it with conductivematerial has been shown. While early attempts to extrude PVDF proved more challenging than extrudingwith more conventional materials like PLA, tuning the extrusion parameters based on failed filament andfailed printed parts improved further attempts. Multi material samples were printed, which would enablefully printing sources and sensors.To characterize piezoelectrics, the performance of a bimorph cantilever can be used to determine the d piezoelectric coefficient. The setup in this work used an LDV to measure the deflection and a waveformgenerator to supply voltage to the bimorph. This was performed on commercially obtained PVDF sheets,which showed that the commercial sheet had 68% of the expected piezoelectric value.Both a heated dielectric bath and a corona poling setup were made to pole printed parts ex-situ. Whilethe heated bath proved to be unreliable, the corona poling showed that the printed and stretched parts wereable to reach d values of 6 pCN . A high voltage in-situ printing setup was made, but has not yet yielded ameasurable piezoelectric coefficient. In future work, a PVDF copolymer (PVDF-TrFE) will be used insteadas the starting stock. This co-polymer forms β phase during simultaneous heating and poling [9], withoutthe use of mechanical stretching. This would eliminate the shear force requirement, and may produce abetter result when printing and poling simultaneously. D Printed PVDF REFERENCES
1. K. Uchino,
Designing with Materials and Devices and Fabrication Processes (CRC Press, Boca Raton,FL, 1994).2. R. E. Newnham,
Properties of Materials Anisotropy, Symmetry, Structure (Oxford University Press,New York, 2005).3. J. Gomes, J. Serrado-Nunes, V. Sencadas, and S. Lanceros-Mendez, “Influence of the β -phase contentand degree of crystallinity on the piezo and ferroelectric properties of poly(vinylidene fluoride),” SmartMaterials and Structures , 065010 (2010).4. C. Lee and J. A. Tarbutton, “Electric poling-assisted additive manufacturing process for PVDF polymer-based piezoelectric device applications,” Smart Materials and Structures , 095044 (2014).5. F. Lederle, C. H¨arter, and S. Beuermann, “Inducing β phase crystallinity of PVDF homopolymer, blendsand block copolymers by anti-solvent crystallization,” Journal of Fluorine Chemistry (109522)(2020).6. Y. Xu, X. Wu, X. Guo, B. Kong, M. Zhang, X. Qian, S. Mi, and W. Sun, “The Boom in 3D-PrintedSensor Technology,”
Sensors (5), 1166 (2017).7. H. Kim, F. Torres, Y. Wu, D. Villagran, Y. Lin, and T. L. Tseng, “Integrated 3D printing and corona pol-ing process of PVDF piezoelectric films for pressure sensor application,” Smart Materials and Structures (8) (2017).8. A. Zolfagharian, A. Z. Kouzani, S. Y. Khoo, A. A. A. Moghadam, I. Gibson, and A. Kaynak, “Evolutionof 3D printed soft actuators,” Sensors and Actuators A: Physical , 258–272 (2016).9. Z. Pi, J. Zhang, C. Wen, Z. bin Zhang, and D. Wu, “Flexible piezoelectric nanogenerator made ofpoly(vinylidenefluoride-co-trifluoroethylene) (PVDF-TrFE) thin film,”
Nano Energy , 33–41 (2014).his pageintentionallyleft blank10 ppendix AAPPENDIX ONE Example Cura Profile Settings for PVDF Extrusion ultimaker3_pvdf[general]version = 3name = pvdfdefinition = ultimaker3[metadata]setting_version = 4quality_type = fasttype = quality_changes[values]adhesion_extruder_nr = 0adhesion_type = skirtdefault_material_bed_temperature = 100layer_height_0 = 0.15ultimaker3_extruder_right_