Photothermally Driven Refreshable Microactuators Based on

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Photo-thermally driven refreshable microactuators based on graphene oxide doped paraffin Sichao Hou, Miao Wang, Shouwu Guo, and Ming Su ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08728 • Publication Date (Web): 18 Jul 2017 Downloaded from http://pubs.acs.org on July 23, 2017

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ACS Applied Materials & Interfaces

Photo-thermally driven refreshable microactuators based on graphene oxide doped paraffin Sichao Hou1, Miao Wang1, Shouwu Guo2, and Ming Su1* 1

Department of Chemical Engineering, Northeastern University, Boston, MA 02115 2

School of Electrical Engineering, Shanghai Jiaotong University, Shanghai, China

ABSTRACT Actuators based on phase change materials (paraffin) can simultaneously produce large stroke length and large force due to thermal expansion, but the low thermal conductivity of paraffin requires high power input and long actuation time. The graphene oxide (GO) doped paraffin dynamic actuator addresses the key challenges in the design of thermal phase change actuators: Thermal conductivity and light absorbing is increased and the response time is reduced compared to the standard phase change actuator designed with metal heating resistors. The thermal properties of GO-paraffin composites with varied loading amount are characterized to confirm the optimal loading amount of 1.0%. A multi-cell phase change actuator was integrated into a digital micromirror controlled optical system. A series of photo-thermally driven refreshable patterns were generated and confirmed with infrared imaging. Keywords: graphene oxide, phase change actuator, thermal conductivity, optical absorbance, refreshable Braille display E-mail: [email protected]

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1. INTRODUCTION Microactuator is a device that can supply and transmit a measured amount of energy to generate a desired action. The general standards for microactuators are large displacement,1high precision,2 fast switching,3 and low power consumption.4 A microactuator can be driven by electrostatic,5-7 piezoelectric,8 fluid,9 thermal10 or optical11 means. Among those, electrostatic and piezoelectric ones cannot offer large displacement and are difficult for miniaturization to satisfy needs at microscale;12 Fluid driven actuators, such as microvalves and pumps, tend to leak and rely on complex machining techniques.13 Thermally-driven phase-change actuator relies on the large volume change of a material such as paraffin during phase change process. Phase change microactuators are promising, not only because they are easy to be miniaturized, but also because they can be used to achieve a large volume expansion together with a large force in a relatively short time.14-15 For instance, paraffin goes through a 15%-25% volume expansion across its phase change region,16 and the melting of paraffin can induce a pressure as high as 20 MPa,17 which allows phase change system to achieve large stroke length and high force in a sealed architecture. Optical driving, one of the thermal driving methods, allows the heating to be conducted in the interested area, which facilitates the patterning in the phase change actuator arrays.18-19 PCM based actuators require high power consumption and long actuation time due to small contact area of resistive thin film heater,20 poor thermal conductivity21 and large latent heat22 of PCM. Materials with high thermal conductivity, for instance carbon black,23 graphite24 and graphene oxide,25-26 have been added to improve the thermal conductivity of paraffin, but the large loading amount (over 50% by weight) significantly reduces the volume of expansion and the stroke length of the actuator. Additionally, the resistive heating elements of PCM actuators require complicated circuitry structures, cannot be separated from PCMs or adaptively used on different system, and the residual heat on heating elements may affect response time in multi-cycle actuation. Since phase change involving gas phase can lead to leakage of gas through semi-permeable polymer membrane, a solid-to-liquid PCM is believed to have better manufacturability.


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Figure 1. Photo-thermally driven phase change microactuator based on graphene oxide (GO)paraffin composite. This article reports a photo-thermally driven graphene oxide (GO) doped paraffin actuators, where GO is used to improve the thermal conductivity and optical absorbance of paraffin. The photo heating is a promising driving method to reduce the direct contact resistance and improve the efficiency of doping material in enhancing the thermal properties of PCMs. Eicosane is chosen as the base material of the actuator, since its low melting temperature (37 °C) and high thermal expansion rate (15%) in phase change region enable a short actuation time and a large actuation distance.27 The thermal properties of GO-paraffin composites with varied loading amount from 0.1 to 10.0% are examined to optimize the loading amount. A multi-cell phase change actuator is fabricated and a digital micromirror device (DMD)-controlled system is built to control the actuation of each cell (Figure 1). DMD device allows remote control, and one DMD device can be used on multiple sets of PCM materials. An infrared camera is used to monitor the actuation time and distance by recording the displacement image and cell temperature as a function of time. The phase change actuators have the potential to be used as refreshable haptic device. 2. RESULTS AND DISCUSSION GO can enhance thermal conductivity of paraffin. Figure 2A-2C show the average thermal conductivities of GO-paraffin, activated carbon-paraffin and carbon nanotube-paraffin composites calculated from Differential Scanning Calorimetry (DSC) curves (inset) at a loading amount from 0 to 10.0% and at a constant ramp rate (10 oC/min). When the loading amount of the dopant increases, the width of the melting peak decreases and the peak height increases. The slope of the



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melting peak increases slightly when the GO loading amount increased from 0 to 0.5 wt%, and then increases rapidly from 0.5 to 1.0 wt%. The peaks become sharper, but the increase of the slope is slowed down when the loading amount further increases to 10 wt%. The same trend is observed in the activated carbon and carbon nanotube systems. The changes in the peak width, height and slope are directly affected by the thermal conductivity of the composites. The thermal conductivity of the samples can be calculated from the DSC melting curves based on the slope during the phase change of eicosane:28 =

∆ 2 =

(1)

where ∆ , and are the calibrated differential power, the sample temperature and the total thermal resistance. The total resistance R is the sum of three parts: thermal contact resistance between the sample and the sample pan (R1), thermal contact resistance between sample pan and heating surface (R2), and thermal resistance of sample (Rs). The thermal resistance of sample depends on its geometric dimension and the thermal conductivity in Eq. 4: =

ℎ ∙

(2)

where ℎ , and are the height, the horizontal cross section area and the thermal conductivity of the sample. So the slope can be expressed as follow by combining Eq. 3 and Eq. 4:

1 1 ℎ = ( + + ) 2 ∙

(3)

The thermal conductivity of the sample is derived by measuring the slope of sample with different geometric dimensions. In principle, the plot of 1/slope vs. hs/As should follow a linear relation and the slope of the straight line is the inverse of the thermal conductivity.


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Figure 2. Thermal and optical properties of carbon materials doped paraffin composite: thermal conductivities of GO-eicosane (A), activated carbon-eicosane (B) and carbon nanotube-eicosane (C), and according DSC curves (inset of each Figure); optical absorbance (D) of GO (red), activated carbon (blue) and carbon nanotube (black). The thermal conductivity of GO-eicosane composite increases slowly (from 0.374 to 0.525 W/m·K) when the loading amount increases from 0 to 0.5%. When the loading amount increases from 0.5 to 1.0%, the thermal conductivity jumps to 0.882 W/m·K. After reaching 1.0% loading, the thermal conductivity of GO-eicosane composite increases slowly with the increase of GO loading (Figure 2A). For activated carbon-eicosane and carbon nanotube-eicosane composites, the thermal conductivity increases relatively fast when the loading amount increases from 0 to 1.0%; but slows down after 1.0% loading (Figure 2B, 2C). The sudden change of thermal conductivity around certain loading amount is due to thermal percolation, where the dopant network forms a long-range connectivity that connects the paraffin system. The loading amount of 1.0 wt% is determined to be the percolation point for all the three dopants in the eicosane system, where the thermal conductivity enhancement has the highest efficiency. After percolation point, carbon material occupies more volume but provide less thermal conductivity enhancement, which impairs



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the actuation performance of the system. Compared the thermal conductivity at 1.0% loading, graphene has the largest thermal conductivity enhancement than the other two materials. Figure 2D indicates that both graphene oxide and active carbon have a large absorbance (3-4) in the range of 250-300 nm, and decreases to less than 1 in the range of 300-380 nm. For carbon nanotube, the absorbance maintains below 1 in the whole range. In the visible region, GO shows a stronger absorbance than active carbon and carbon nanotube from 380-600 nm, then slightly weaker between 600-760 nm. The average absorbance can be derived by integration over the whole wavelength range of 380-760 nm. In this range, the average absorbance of GO is higher than that of activated carbon and carbon nanotube by 0.09 and 0.02 in the range from 380 to 760 nm, which means the absorbance for GO is 23.0% and 5.1% higher than those of activated carbon and carbon nanotube. Assuming the scattering and reflection of all dopants are in the same level due to their similar physical surface property, the optical absorption ability of GO is stronger.

Figure 3. SEM images of graphene oxide (A), activated carbon (B) and carbon nanotube (C). The difference in the thermal conductivity and absorbance enhancement of the three dopants may result from their microscopic morphology. Figure 3 shows the scanning electronic microscopic (SEM) images of the three carbon materials. GO presents a wrinkled-sheet structure with a length of 10 µm consisting multiple randomly-aggregated layers (Figure 3A). Activated carbon presents a flake-like structure with a length varied from 1 to 20 µm (Figure 3B). Figure 3C shows a combination of fiber-like carbon nanotubes with a diameter from 50 to 1000 nm and a length of from 20 to 100 µm and unreacted carbon nanoparticles with a diameter from 100 to 500 nm. The layered structure of GO provides a larger surface area exposed to the incident light and surrounding paraffin wax, which increases its chance to absorb more light and conduct the heat to surrounding eicosane faster compared to activated carbon and carbon nanotube.29-30 Although the tube-like structure of carbon nanotube greatly facilitates the thermal conductivity along the axial


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direction, the low thermal conductivity along the radius direction makes its overall performance worse than that of GO.31-32 The enhancement efficiency of GO in both thermal conductivity and absorbance are higher than the other two carbon based materials. The temperature increase rate of carbon-paraffin composites is tested by heating the composites on a hot plate or remotely using a light source and measured using an infrared camera. Figure 4A shows an example of the temperature profiles of GO-eicosane composites with the loading amount varying from 0 to 10.0 wt% heated by a hot plate (other data not shown), where the temperature increase rate increases with the increase of the loading amount. The average temperature increase rate is calculated from the time needed for each sample to increase its temperature from 30 to 45 °C. Figure 4B shows the average temperature increase rates of GOeicosane, activated carbon-eicosane and carbon nanotube-eicosane composites using contact heating and remote heating. The temperature increase rate rises quickly before 1.0% loading (percolation point) and slows down after in both situations. For contact heating, the statistical results indicates that all three carbon materials can enhance the thermal performance. At the loading amount of 10.0 wt%, GO has a 114.3% increase in the temperature increase rate compared to control experiment while the activated carbon and carbon nanotube have 94.5% and 105.9%, respectively. For remote heating, all three dopants have significant enhancement of the thermal radiant absorption, while GO performs better than the other two at each loading. At the loading amount of 10.0%, GO has a 136.4% increase in the temperature increase rate, while the activated carbon and carbon nanotube have 101.7% and 117.7%, respectively. The better performance in remote heating is resulted from the enhanced optical absorbance and thermal conductivity in GOeicosane composites. In contrast, only the enhanced thermal conductivity is played a role in contact heating. The opposite effects of loading amount in thermal conductivity enhancement and actuation performance requires a compromise in GO-eicosane composites. 1.0 wt% is chosen as the optimal loading amount and remote heating is used to construct photo-thermal actuators as follows.



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Figure 4. Surface temperatures of GO-eicosane composite as a function of time with various GO loading amount during contact heating (A); loading amount dependent ramp rates of surface temperature from 30 to 45 oC of GO-eicosane composite (green), activated carbon-eicosane composite (red) and carbon nanotube-eicosane composite (blue) during remote optical heating and contact heating (B). Figure 5A shows the design of phase change actuator, which is consisted of three parts: an aluminum base, a cylindrical polymethyl methacrylate (PMMA) sidewall and a silicone thin film cover. The three parts encircle an ink-bottle shaped hole with a base diameter of 30 mm and a height of 5 mm, and a neck region of 10 mm diameter and 3 mm height. The neck region is used to amplify the thermal expansion of the phase change material in the base during the melting process. PMMA is transparent (transmittance >90%) in visible region and does not adsorb much incident light. The cylindrical hole is filled with liquid eicosane to avoid air gap between solid particles. After eicosane naturally cools to solid phase, the void space generated by phase change is filled with liquid eicosane until full. The three parts are tightly connected to avoid any leakage when eicosane melts. The silicone thin film (50 µm thick) is elastic and strong enough to withstand thermal expansion of eicosane during melting process (Figure 5B). The GO-eicosane actuators are placed vertically under a remote heat source at a fixed distance of 10 cm. The actuation process is recorded using a video camera. Figure 5B-5D show the actuation distances of films after remote heating for 60 seconds. For controlled sample without GO, the height of the film is 10.1 mm (not shown); for samples with GO of 0.1, 0.5 to 1.0%, the height of the film increases from 14.3, 15.2 to 16.8 mm due to partial melting of eicosane to a variable extent. Figure 5E shows the single cycle response of GO-eicosane actuator, where the maximum distance is achieved after 3 minutes’ illumination. The response times for GO-eicosane


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actuator are 205, 135, 100, and 70 s for the corresponding loading amount 0, 0.1, 0.5 and 1.0%, respectively. The maximum actuation distances of all these actuators are in the range of 1.69 to 1.76 mm. The result reveals that with the increase of the loading amount from 0 to 1.0%wt, the response time decreased. Figure 5F shows multi-cycle response curves of GO-eicosane actuator, indicating the repeatability and stability of the actuator. Each cycle lasts for 10 mins: 1 min for heating and 9 mins for cooling. The actuation distance of phase change actuator without doping increases in the first 3 cycles and reaches the steady state. For GO-eicosane actuator, it reaches the steady state and becomes stable after 3 cycles when the loading amount is 0.1%, while only 2 duty cycles are needed to be stable and no fatigue is observed in the third cycle for 0.5%. When the loading amount increases to 1.0%, the actuation distance reaches its maximum height after the first cycle and without any fatigue after 3 cycles. The settlement of GO is the key to the repeatability of GOeicosane system, since the distribution of GO affected the response time. The settlement of graphene oxide will lead to a decrease in the thermal conductivity, and lengthen the actuation time. For the equal-time cycle, actuation distance is a reflection of actuation time: if the thermal conductivity decreases, the actuation distance will decrease as well for the same heating time. Figure 5G shows the actuation distance of GO-eicosane actuator with different loading amount from 0 to 1.0 wt% for 50 cycles. The result indicates that for all loading amounts, the actuation distance keeps in the same level after reaching the steady-state, proving the settlement of GO will not significantly affect the actuation performance within 50 cycles. Settlement time of graphene oxide in liquid eicosane is calculated theoretically using Stokes’ Law, assuming the settlement is extremely slow (Reynold number

Photothermally Driven Refreshable Microactuators Based on

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