Research Article www.acsami.org
Carbonized Electrospun Nanofiber Sheets for Thermophones Ali E. Aliev,*,† Sahila Perananthan,‡ and John P. Ferraris†,‡ †
A. G. MacDiarmid NanoTech Institute, University of Texas at Dallas, Richardson, Texas 75083, United States Department of Chemistry and Biochemistry, University of Texas at Dallas, Richardson, Texas 75083, United States
‡
S Supporting Information *
ABSTRACT: Thermoacoustic performance of thin freestanding sheets of carbonized poly(acrylonitrile) and polybenzimidazole nanofibers are studied as promising candidates for thermophones. We analyze thermodynamic properties of sheets using transport parameters of single nanofibers and their aligned and randomly electrospun thin film assemblies. The electrical and thermal conductivities, thermal diffusivity, heat capacity, and infrared blackbody radiation are investigated to extract the heat exchange coefficient and enhance the energy conversion efficiency. Spectral and power dependencies of sound pressure in air are compared with carbon nanotube sheets and theoretical prediction. Despite lower thermoacoustic performance compared to that of CNT sheets, the mechanical strength and cost-effective production technology of thermophones make them very attractive for large-size sound projectors. The advantages of carbonized electrospun polymer nanofiber sheets are in the low frequency domain (50%), can be easily processed by hand, can be deployed (and removed) on any curved surfaces, is stretchable, and is tailored by shears. The freestanding sheet can withstand strong air flow, vibrations, and tensions. Unlike short carbon nanotubes (10 MΩ per square, further marked by □). The sheet resistance of a 20 μm-thick sheet is reduced by 3 orders of magnitude for 1 h of annealing at 800 °C in a nitrogen furnace (60 kHz, the largesize sheet creates strong interference of sound coming from the edges and central part of the mat. For smaller C-PAN mats, the interference region is shifted to higher frequencies. A 6-fold increase in sound pressure was observed when the thickness of C-PAN sheet was decreased to ∼20 μm. This thin transparent web-type sheet shows promise for wide-band TA projectors. The green dashed line in Figure 7a shows the ultimate theoretical limit calculated using reduced eq 1 for neglected heat capacity per unit area of TA heater (Ch*≪ Cp):4
Figure 6. Power required to heat the thick 120 μm C-PAN sheet to certain temperatures (open blue circles). The temperature was measured using thermocamera T650sc (FLIR Systems, Inc.) with preinstalled emissivity ε = 0.65. The dashed green line, showing linear Ph(T), was calculated for heat conduction to air using eq 2. The red dashed line shows the blackbody radiation part of the heat loss.
through the conduction. The above measured heat exchange coefficients and distribution of heat consuming channels in air are confirmed by measurements of power dependence of temperature evolution (dynamic (AC) and accumulated (DC) parts of temperature) in high vacuum, where the Pβ = 2SβTa part is totally canceled. Thermoacoustic Performance. Annealed Poly(acrylonitrile) Freestanding Sheets. Different thickness of CPAN mats, sheets, and thin transparent aerogel-type webs with electrical sheet resistances ranging from 100 Ω to 20 kΩ were
prms =
f 1 × × Ph 2 2 CpT0 r
(3)
Figure 7. (a) Sound pressure spectra of thick mats (bulk circles) and thin transparent sheets (open circles) of C-PAN nanofiber sheets annealed at 1000 °C. Power dependencies of sound pressure and temperature on the sheet surface for (b) a thick 120 μm mat (ρ = 0.17 g/cm3, ε = 0.65) and (c) a thin transparent sheet (ρ = 0.158 g/cm3, ε = 0.2). Top inset to panel b shows the splitting of thick layered C-PAN mat, and the bottom inset shows the resulted 60 μm film attached to copper electrodes using silver paste. Inset to panel c shows the 20 μm thin C-PAN sheet (15 × 15 mm2). prms( f) and prms(Ph) were measured in open air at T0 = 25 °C. (d) Sound pressure spectra for a 20 μm C-PAN sheet first annealed in a nitrogen furnace at 900 °C for 1 h and then resistively heated in vacuum at 1500 °C for 10 s. 31197
DOI: 10.1021/acsami.6b08717 ACS Appl. Mater. Interfaces 2016, 8, 31192−31201
Research Article
ACS Applied Materials & Interfaces
electrospun fiber sheet was carbonized under a helium environment (flow rate of 200 mL/min and a ramp rate of 5 °C/min) at 900 and 950 °C for 1 h. The tested thin (h ∼ 2 μm) and thick (h ∼ 25 μm) C-PBI sheets have demonstrated transport properties close to those of C-PAN sheets (see Sections S5 and S7 in the Supporting Information). The thermal conductivity of sheets resistively heated up to 1000 °C increases to 2.5 W/m·K (see Figure S10 in the Supporting Information); this corresponds to the thermal conductivity of individual fibers of ∼125 W/m·K. At low frequencies, the sound pressure of aligned PBI sheets is close to the theoretical level for the case of negligibly small heat capacity of the TA heater, Ch = 0. Figure 9a shows that at higher frequencies the slope of prms( f) declines from linear increase to prms ∝ f 0.8. The TA performance of random and aligned sheets with the same thickness is similar. The fiber alignment achieved by relatively high rotating speed of the collector drum (1500 rpm) is ∼70% (Figure 8b). In the sheet with aligned nanofibers, most of the fibers are involved in the resistive heating by applied current, whereas in randomly deposited sheets, the fibers perpendicular to the current pathway do not contribute directly. They are heated through the high thermal conductivity of fibers. Thus, the similar performance confirms the high thermal conductivity of individual nanofibers. The power dependencies of the sound pressure and temperature shown in Figure 9b demonstrate high thermal exchange with air and low heat accumulation, which resulted in higher sound pressure and lower temperatures (i.e., higher exchange coefficient) compared to those of C-PAN sheets. Table 1 shows some physical and chemical properties of studied C-PAN and C-PBI sheets and their TA performance in comparison with multiwalled carbon nanotube (MWNT) sheets. The sound pressure is shown for frequency f = 3 kHz (r = 3 cm), which is most sensitive to our ears. The far-field measurements used in this study (r ≥ 10DR, where DR = πa2/λ is the Rayleigh distance) are less sensitive to the microphone aperture and the size of flat TA source.
This dependency has been confirmed for freestanding CNT sheets, both single- and multiwalled (see Figure S12 in the Supporting Information).2 The sound pressure versus applied AC power for thick 120 μm porous mats shown by open blue circles in Figure 7b follows along with the temperature profile shown by open red squares ( f = 3 kHz). A more linear prms(Ph) dependence for the thin sheet shown in Figure 7c suggests better heat exchange with surrounding air. The enhanced TA performance of thin PAN sheet carbonized at T = 1500 °C is shown in Figure 7d in comparison with the same sheet annealed at T = 900 °C. Apparently, the reduced diameter of fibers, reduced heat capacity, and enhanced porosity of individual fibers at carbonization all together contribute to the high sound generation efficiency. The low-density, aerogel-like C-PAN sheet shown in the inset of Figure 7c demonstrates high TA performance in open air up to 140 kHz. Despite the high thermal inertia of large diameter PAN fibers (170−230 nm), the open aerogel structure of the thin PAN sheet provides enhanced heat exchange and a broad power range where sound pressure linearly increases with applied power. Annealed Polybenzimidazole (PBI) Nanofiber Sheets. PBI is another strong and thermally stable thermoplastic polymer whose solubility in dimethyl-acetamide (DMAc) permits the production of random or highly aligned freestanding sheets using the electrospinning technique. Due to its high stability, PBI is used to fabricate high-performance protective apparel such as a firefighter’s gear, astronaut space suits, hightemperature protective gloves, welders’ apparel, and aircraft wall fabrics. Carbonization at high temperatures (800−1000 °C) gives about 50% yield without significant shrinkage.15,20 PBI cross-linked with polybenzoxazine showed enhanced mechanical properties of nanofibers and resistance to solvents. The improved mechanical properties can also be obtained by modification with sulfonated poly(2,6-dimethyl-1,4-phenylene oxide).21 Here, for the first time, we fabricated and characterized freestanding, highly aligned carbonized PBI (C-PBI) sheets with diameter of nanofibers of ∼200 nm and optical transparency of ∼50% (Figure 8). A 20 wt % solution was prepared using commercially available LiCl doped (0.4%) PBI in DMAc and electrospun by applying 18 kV to the needle, which was connected to the polymer solution-loaded syringe. Fibers were collected in the rotating collector drum at a speed of 1500 rpm at a distance of 18 cm from the needle tip. The obtained
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CONCLUSIONS The carbonized PAN and PBI sheets have several attractive features as an alternative material for thermoacoustic applications: (i) the freestanding sheets have a web-type structure with high surface area and enhanced heat exchange coefficient with the surrounding air, (ii) carbonization with further graphitization reduces the density of individual fibers and makes them porous with low heat capacity, which substantially enhances the TA performance, (iii) no bundling issues were observed in studied samples, and (iv) the technology is low cost and easy to handle. On the other hand, the high intrinsic thermal conductivity of individual nanofibers is an important feature for TA performance, aiming for homogeneous dynamic distribution of heat across and along the sheet. Among the reported electrospun polymers, not all of them are suitable for annealing at high temperatures and carbonization. Nevertheless, the list of possible candidates is large: PAN, PBI, polyimides, lignin, cellulose, and others. The pyrolysis of electrospun polymer sheets in inert gases, which preserves mechanical strength, increases the electrical and thermal conductivities and decreases the heat capacity of fibers, is a promising route to improve the TA performance of these new materials in the field. However, at the current state, the
Figure 8. (a) Randomly electrospun PBI sheet placed 3 cm from the microphone (on the left), and a high-speed infrared detector (on the right) PDA10PT (ThorLab Inc.) to detect the sound pressure and modulated part of the temperature. (b) Highly aligned PBI sheets. Dimensions of the sheets are 1 × 1.6 cm2 and 1 × 0.8 cm2. 31198
DOI: 10.1021/acsami.6b08717 ACS Appl. Mater. Interfaces 2016, 8, 31192−31201
Research Article
ACS Applied Materials & Interfaces
Figure 9. (a) Sound pressure spectra of aerogel-type aligned carbonized nanofiber sheet of PBI (solid green circles) measured at a distance of 3 cm in air at T0 = 25 °C. The sound pressure is normalized to applied ac power Ph = 1.2 W. For comparison, the randomly deposited PBI sheets of the same thickness, h ∼ 25 μm, annealed at 900 °C (open red squares) and 950 °C (open blue circles) are shown as well. The green dashed line shows the theoretical limit for Ch = 0 and prms ∝ f, and the black dotted line shows the fitting of experimental results for C-PBI annealed at 950 °C, prms ∝ f 0.8 . (b) Sound pressure (blue open circles) versus applied ac power measured at f = 3 kHz. Solid red circles show the temperature on the surface of the PBI sheet versus applied power. Inset shows the thermal image of the sample taken by thermocamera T650sc.
Table 1. Comparison of Some Physical Parameters Useful for TA Applications for Pristine Bulk and Carbonized (Ta = 1000 °C) Electrospun Sheets (Fibers) of PAN, PBI, and MWNT sheets (T = 295 K) o
melting point ( C) density (g/cm3) heat capacity (J/kg·K) thermal conductivity (W/m·K) thermal diffusivity (mm2/s) tensile strength (MPa) sound pressure (Pa/W, r = 3 cm, f = 3 kHz)
PAN
C-PAN
PBI
C-PBI
MWNT sheet
22
>2000 0.158 775 ± 50 0.04 (28) 0.56 (0.66) 220 ± 20 0.047
>540 (760) 1.319 930 0.41
>2000 0.23 700 ± 50 0.74 (32) 3.7 (45) 360 ± 50 0.05
>3000 ∼10−3 ∼700 ± 10 5024 4524 150 ± 15 0.1179
378 1.184 128022 0.30423 53 ± 10
was measured using the heat capacity option of a Physical Property Measurement System (PPMS, Quantum Design Inc.) and at high temperatures (200−750 K) using a differential scanning calorimeter (Q2000, TA Instruments Inc.). The values were then compared with calculated values obtained from thermal conductivity, thermal diffusivity, and density measurements (UMX2 Ultramicrobalance, Mettler Toledo Inc.). The mechanical properties of pristine and carbonized sheets were studied using an Instron 5848 Microtester. The average temperature on the surface of freestanding sheets was measured using infrared thermocamera T650SC (FLIR System, Inc.). The emissivity of porous freestanding sheets was obtained for each sample by preliminary temperature calibration using a supported sample. The amplitude of the modulated part of the temperature was measured using high-speed IR detector PDA10PT (ThorLab Inc.) (see Section S10 in the Supporting Information for details). Thermal Conductivity and Thermal Diffusivity Measurements: 3-Omega Method. The nanoscale dimension of C-PAN fibers interwoven in a thin (∼20 μm) aerogel network of sheets suggests the use of the self-heating 3-omega technique to determine the thermal transport along the narrow strip of freestanding sheet or individual fiber. In this case, the one-dimensional heat flow along the sheet (fiber) can be expressed in the terms of the third-harmonic (3ω) voltage signal U3ω induced by an AC current Iosin(ωt) applied through the elongated sample (the aspect ratio of sample should be >100). The AC current with frequency ω creates a temperature fluctuation in the specimen at double the driving frequency (2ω). The interaction of the 2ω modulated resistance R with the 1ω current creates a third harmonic response on the potential electrodes:25
diameter distribution and inhomogeneity of local density of fibers in electrospun sheets still hinder a wide application of TA projectors: large diameter fibers, owing low electrical resistance, create the current redistribution, as in bundled CNT sheets, a higher local density of fibers creates accumulation of heat in those areas with relatively higher averaged temperatures, leading to further reduction of resistance and current redistribution. The primary target of electrospun carbonized polymer fibers is a low frequency (1 kW) TA projector for noise cancellation from industrial sources and vehicles and for underwater locations. The highly aligned freestanding sheets attached to the patterned array of electrodes can serve as a passive-addressed matrix array for acoustic field manipulation of suspended objects and high-resolution acoustical holograms. The encapsulated TA projectors with cantilevered vibrating plates are another potential application for electrospun carbonized polymer fibers, where a large displacement of air is needed (fire extinguisher). Further work is needed to improve the performance of polymer-derived TA heaters, namely, the diameter of fibers should be reduced twice by the proper choice of solution and applied electrostatic potential. The resistance of fibers can be decreased further by increasing the carbonization temperature in the furnace and by doping.
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16021
EXPERIMENTAL SECTION
U3ω ,rms =
Characterization. The structural density and morphology of studied nanostructures were characterized by an ultrahigh-resolution scanning electron microscope (SEM) LEO 1530 VP (LEO Electron Microscopy Ltd.) and UV−vis optical spectrometer Lambda-900 (PerkinElmer Inc.). The heat capacity at low temperatures (2−350 K)
4I 3RR′ L × × S π 4κ
e−i(3ω − φ) 1 + (2ωγ )2
(4)
where I = U/R is the rms value of applied AC current, R is the specimen resistance between the current electrodes, R′ = (dR/dT) is the temperature gradient of the resistance at temperature of interest, L 31199
DOI: 10.1021/acsami.6b08717 ACS Appl. Mater. Interfaces 2016, 8, 31192−31201
Research Article
ACS Applied Materials & Interfaces
= 12.5 mm for an ACO Pacific Model 7046 (2 Hz − 20 kHz, S = 54.3 mV/Pa), A = 6.25 mm for an ACO Pacific Model 7016 (5 Hz −120 kHz, S = 5.43 mV/Pa), and A = 3.1 mm for a B&K pressure-field microphone 4138-A-015 (6 Hz −140 kHz, S = 0.543 mV/Pa). The sound pressure level (SPL) in air was measured using an impulse SPLmeter (Quest Technologies, Model 2700) provided with a QC-10 acoustic calibrator. The charge conditioning preamplifier 5 (B&K Type 2670 or “Sound Connect” (Listen Inc.)), which receives the 2f acoustical signal via the 4138 microphone, amplifies the signal and sends it to another highgain low-noise amplifier marked 6, which increases the detected signal further and sends it to the lock-in amplifier 7 (SR830, Stanford Research Instruments Inc.). The rms amplitude and phase delay of the second harmonic signal is read from the lock-in amplifier display. Simultaneously, the signal goes to a four-channel digital oscilloscope 8 (Tektronix, TDS-210) for visualization and recording of the shape of the received signal. The detection of the second harmonic signal using the phase-sensitive lock-in amplifier (7) substantially enhances the signal/noise ratio. The reference resistor Rref = 1 Ω is used to control the applied current (power) and measure the dynamic resistance (impedance) of TA projector 3.
is the distance between potential electrodes, S is the cross-sectional area of the sample, κ is the thermal conductivity, γ = L2/π2α is the characteristic thermal time constant of the specimen for the axial thermal process, and α = κ/ρCh is the thermal diffusivity. The diffusivity can be obtained from the frequency dependence of the phase lag of the third-harmonic signal tan φ ≈ 2ωγ =
2ωL2 π 2α
(5)
or from the frequency dependence of U3ω using eq 2. For thin and long specimens in the low frequency limit (2ωγ ≪ 1), one obtains the thermal conductivity
κ=
4I 3RR′ L × 4 S π U3ω
(6)
To extract intrinsic thermal conductivity of fibers and sheets, all measurements were done under high vacuum (P < 10−4 Torr) to reduce the heat loss through gas convection. The schematic diagram of setup for acquisition of the third harmonic signal is shown in Figure S8 in the Supporting Information. To minimize the static blackbody heat current from the specimen, we used a heat shield of aluminum foil to cap the sample (Figure S9 in the Supporting Information). To check the validity of the 3-omega method, we performed a set of preliminary feasibility measurements for the pitch-derived carbon fibers (Du-Pont de Nemours, E-75). The measured values of κ = 110 W/m·K and D = 77 mm2/s are within 2−3% of the values obtained by the laser flash methods and the values reported in literature. Acoustic Measurements. The schematic diagram of the setup shown in Figure 10 works as follows: A sinusoidal AC signal of
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publication Website at DOI: 10.1021/acsami.6b08717. Additional graphs, tables, and useful pictures (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by Office of Naval Research (Grant N00014-14-1-0152) and the Army Research Office (STTR Contract W911NF-15-P-0023).
■ Figure 10. (a) Schematic diagram of the experimental setup, including the AC generator (1), high-power amplifier (2), TA projector (3), microphone (4) combined with build-in preamplifier, conditioning amplifier (5), low-noise amplifier with tunable bandwidth (6), lock-in amplifier (7) for harmonic selection and measurement of the amplitude and phase of the acoustic signal, and a four-channel digital oscilloscope (8) for visualization of the input and acoustical signals. frequency f generated by a Digital Function/Arbitrary Waveform Generator 1 (Agilent 33250A) is amplified by a high-power amplifier 2 and applied to the TA projector 3, which is grounded through a reference resistor (Rref = 1 Ω). Two high-power amplifiers were used in this work for different experimental conditions: (1) The TREK PZD350-M/S is a wide frequency range (DC to 250 kHz) highvoltage (±350 V) piezo-driver amplifier for high-impedance projectors (R0 = 100 Ω −100 kΩ, output current ±0.4 A). (2) An AE-Techron 8102 power supply amplifier with a frequency bandwidth from DC to 5 kHz and maximum power output ∼1100 W was used for lowimpedance projectors (2−62 Ω). The thermoacoustically generated sound pressure, where frequency is doubled, was measured using precision microphones 4 with different apertures A and sensitivities: A
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ABBREVIATIONS CNT, carbon nanotube MWNT, multiwalled carbon nanotube SONAR, sound navigation and ranging TA, thermoacoustic C-PAN, carbonized poly(acrylonitrile) C-PBI, carbonized polybenzimidazole DMAc, dimethyl-acetamide AC, alternative current DC, direct current CVD, chemical vapor deposition SEM, scanning electron microscope PPMS, physical property measurement system TCR, temperature coefficient of resistivity SPL, sound pressure level REFERENCES
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