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Graphene aerogel templated fabrication of phase change microspheres as thermal buffers in microelectronic devices xuchun wang, Guangyong Li, Guo Hong, Qiang Guo, and Xuetong Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13969 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 10, 2017

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ACS Applied Materials & Interfaces

Graphene aerogel templated fabrication of phase change microspheres as thermal buffers in microelectronic devices Xuchun Wang, †,‡ Guangyong Li, † Guo Hong,

†

* Qiang Guo ‡ and Xuetong Zhang †*

§,

Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Suzhou, 215123, P. R.

China. ‡ School of Materials Science and Engineering, Shanghai University, Shanghai 200444, P. R. China. §

Institute of Applied Physics and Materials Engineering, University of Macau, Avenida da Universidade,

Taipa, Macau. E-mail: (X Zhang) [email protected], (G Hong) [email protected].

Abstract Phase change materials, changed from solid to liquid and vice versa, are capable of storing and releasing large amount of thermal energy during phase change, and thus hold promises for numerous applications including thermal protection of electronic devices. Shaping these materials into microspheres for additional fascinating properties is efficient but challenging. In this regard, a novel phase change microsphere with the design for electrical-regulation and thermal storage/release properties was fabricated via the combination of mono-dispersed graphene aerogel microsphere (GAM) and phase change paraffin. A programmable way, i.e., ink jetting ACS Paragon Plus Environment

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liquid marbling - supercritical drying (ILS) coupling techniques, was demonstrated to produce mono-dispersed graphene aerogel microspheres (GAMs) with precise size-control. The resulting GAMs showed ultra-low density, low electrical resistance, high specific surface area with only ca. 5 % diameter variation coefficient, and exhibited a promising performance in smart switches. The phase change microspheres were obtained by capillary filling of phase change paraffin inside the GAMs and exhibited excellent properties, such as low electrical resistance, high latent heat, well sphericity and thermal buffering. Assembling the phase change microsphere into the microcircuit, we found that this tiny device was quite sensitive and could respond for heat as low as 0.027 Joule. KEYWORDS: phase change microspheres, graphene, aerogel microspheres, Joule heat, heat flux stabilizer 1. Introduction Mono-dispersed functional microspheres, the micro- and nano-particles with spherical geometry and specific function, have shown extraordinary promise for micro/nanoreactors, energy storage, catalysis, sensors, biomedicine and environmental remediation due to their unique properties attributed to abundant compositions and various morphologies.1,2 On the base of microspheres’ geometry, they can be termed as solid microspheres, hollow microspheres and porous microspheres. Due to large porosity and pore volume in porous/hollow microspheres used as the storage for different cargoes, they can serve as drug delivery carriers, imaging contrast agents, electrodes for lithium ion batteries and lithium-sulphur batteries.1,

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Considering the

different compositions, they can also be divided into organic microspheres, inorganic microspheres and organic-inorganic composite microspheres. More specifically, the composition evolved from carbon, polymer, cellulose and silica to many other functional materials (metals, metal oxides and complex compounds, etc.).1,

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More importantly, the intrinsic properties of

different compositions, such as catalysis-, photo-, adsorb-, magnetic- and temperature/pHresponse, make microspheres a promising material for many applications, e.g. sensing, water ACS Paragon Plus Environment

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treatment, catalysis, and magnetic separation.1, 7-10 Furthermore, the structural and compositional turning offers additional possibilities for rational design of novel functional materials toward many desired applications. However, to the best of our knowledge, the mono-disperse phase change microspheres with excellent electrical conductivity, thermal energy storage/release and their application as thermal buffer in integrated circuits have not been reported. Phase change composites (PCCs) which can store and convert thermal energy electrically, were considered as an extremely promising way to utilize thermal energy in electronic device for thermal sensing and buffering of heat flux. Usually, electrical PCCs could be obtained by introducing an electrical host material with 3D conducting porous network into phase change materials. Cao11 introduced the carbon nanotube sponge into wax and obtained multifunctional PCCs in which the thermal energy storage can be driven by small voltage or light illumination with high electro-to-heat or photo-to-thermal storage efficiencies. Also they used a carbon nanotube array as a porous framework to encapsulate n-eicoasne and obtained the electrically phase change composite with tailored carbon nanotube density.12 Zhang13 made an multi-driven PCCs with anisotropic thermal/electric conductivities by using anisotropic graphene aerogels as host, which can be driven either by a small voltage (1-3 V) with high electro-to-heat efficiency (~85 %) or by irradiating with sunlight (0.8-1.0 sun) with high photo-heat efficiency (~77 %). Unfortunately, all of these electrical PCCs were monolith in the centimetre scale. Shaping these materials into microspheres may open up the potential applications and give them more fascinating properties14-16, such as high-sensitive for heat flux, electrical-regulation and resistance mutation. Aerogel microspheres are unique materials that can be used as templates for drug delivery, superabsorbent, synthesizing porous inorganic catalysts or functional composite particles, etc.14-16 Inspired by monolithic aerogel/sponge/foam, the aerogel microspheres with unique dimension may be an excellent host for phase change materials to prepare the electrical PCCs, due to their interconnected network and extraordinary capillary force which can prevent the leakage of melted phase change materials. So far three strategies have been adopted to synthesize aerogel ACS Paragon Plus Environment

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microspheres: injection prilling, emulsion polymerization and spray granulation. Injection prilling is the first strategy to fabricate aerogel microspheres and easy to be applied to almost all aerogel systems. However, the resulting aerogel microspheres are usually in a larger size. Emulsion polymerization and spray granulation are used to make various aerogel microspheres by taking advantage of their high efficiency, but the resulting products possess wide size distribution with limited structural control. Recently, the graphene aerogel microspheres (GAMs) with centerdiverging microchannel structures were fabricated by a combination of electrospraying and freeze-casting.15 However, the size and distribution of aerogel microspheres could not be efficiently controlled. There are still great challenges towards well-controlled, mono-dispersed GAMs, such as fabrication of mono-disperse sol micro-droplets with resistance of amalgamation, and sol-gel transition of individual micro-droplets with thereafter capillary force reduction for drying.

Furthermore, traditional

strategies,

e.g.,

emulsion

polymerization

as well as

ultrasonic/electro spray, are not suitable for the fabrication of GAMs due to their long time aging process and poor control of droplet sizes.15, 17 The critical dimension of integrated circuit has been improved quickly, following the Moore’s law. The state of art commercial transistor is about 10 nm, rushing into the limit of silicon. Some promising nanomaterials have been proposed as the future alternatives, such as carbon nanotube18, graphene19 and transition metal dichalcogenides20. Although they have quite a lot of excellent properties, such as high electrical and thermal conductivity, the thermal capacity of these tiny devices is unfortunately too low for high frequency and large current operations because of the tiny material mass in a single device and the corresponding limited heat channel. As a result, undesired current/voltage fluctuation might induce an un-dispersible Joule heating which would burn down or damage the devices. Such feature requires an additional ability of the potential materials to sensitively detect the non-default working current and provide a compensable heat sink to temporarily buffer the additional energy. Herein, the phase change microsphere, one of the brand-new functional microspheres, was fabricated by combination of GAMs (with 3D electrically/thermally interconnected network) and ACS Paragon Plus Environment

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phase change materials. Firstly, we introduced a programmable way, i.e., ink jetting - liquid marbling - supercritical fluid drying (ILS) coupling techniques to produce uniform GAMs. Specifically, ink-jetting technology was used to make uniform GO droplets; liquid-marbling technology was used to keep the GO droplets from amalgamation and to confine sol-gel transition of them,21 and supercritical fluid drying technique was used to reduce surface tension of gel precursors between solid framework and liquid medium which can guarantee smallest shrinkage of the resulting aerogels. The size of aerogel microspheres can be easily controlled by adjusting the ink-jetting program. Such programmed GAMs have excellent properties (e.g., hydrophobicity, ultra-low density, low resistance, large specific surface area, monodisperse in diameter.) for electrical applications (tilting switch and liquid level switch). Furthermore, the phase change microspheres with good sphericity, high latent heat, excellent cyclic stability and fascinating electricity properties were successful prepared via the combination of phase change materials and GAMs, and were used as a heat flux detector to sensitively detect current fluctuation and buffer some extra Joule heat in the integrated microscale devices, which indicated a highly efficient way to protect the tiny devices in the future integrated circuits.

2. Experimental 2.1. Materials Graphite powder was purchased from Qingdao Tianheda Graphite Co., Ltd, Qingdao, China. P2O5, K2S2O7, 30% H2O2, HF (aq) and ethanol were obtained from Sinopharm Chemical Reagent Company. Vitamin C and hydrophobic fumed silica nanoparticles (HFSNs) were purchased from Aladdin Reagent (Shanghai) and used without further purification. Paraffin was obtained from ShangHai Joule Wax Co. Ltd. 2.2. Preparation of GO aqueous dispersion GO aqueous dispersion was prepared from graphite powder by a modified Hummers method reported in our previous studies.22, 23 The concentration of GO was 5 mg/mL by centrifugation. 2.3. Preparation of GAMs ACS Paragon Plus Environment

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Ethanol and ultrapure water was pumped increasingly into the sample pipe of microarrayer to wash the pipeline. A square plastic culture dish (127.8 mm × 85.5 mm) was used as a base, which filled with HFSNs (diameter 7-40 nm, specific surface area 150 m2/g). After that, the volume of each GO droplet was set by changing the jetting program. The jetting process was automated by controlling the route of spray-head based on the size of the square plastic culture dish. As a result, Monodisperse GO dispersion (5 mg/mL) droplets came out of the pipette and arranged in a matrix with the space of 1.5 mm between droplets. After the program finished, the base was removed and a thin-layer of HFSNs was equably sprinkled on the matrix to protect the GO droplets and make sure every single GO droplet become a liquid marble. Then the whole base was sealed by using plastic wrap and put in a thermotank for gelation. 24 h later, the monodisperse graphene hydrogel microspheres (GHMs) formed, which were then poured into 500 mL ethanol (75 %) to remove the un-bonded HFSNs. The precipitate was treated with hydrofluoric acid and deionized water in sequence to clean the GHMs. Finally the GAMs were obtained by CO2 supercritical drying after solvent exchange of GHMs with absolute ethanol for one week. 2.4. Fabrication of GAMs ball switch and liquid level switch The diameter of the GAMs used for ball switch was about 700 µm. A quartz glass tube (inner diameter: 2 mm, external diameter: 4mm) was used as a shell frame. First, two wedge-shape copper electrodes were inserted in one end of the quartz glass tube, and a polyethylene film was put inside the two electrodes for insulation. Then, an individual graphene aerogel microsphere was put in the tube. Finally, a GAM ball switch was developed using a laminating film to seal the other end of the quartz glass tube. The fabrication of GAM liquid level switch was similar to the GAM ball switch, the only difference was the absence of laminating film to seal the other end of quartz glass tube, which was immersed into the water alternatively. 2.5. Preparation of GAM-paraffin phase change microspheres GAMs were treated at 600 °C for 3h in argon atmosphere. After that, both GAMs and paraffin were heated to 80 °C (above the melting point of the paraffin, 48 °C) in an oven and placed for 2h ACS Paragon Plus Environment

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for the GAMs to be infused with paraffin. After the GAMs were precipitated down and filled with paraffin, the liquid phase was filtered at 65 °C, and the residue was transferred to a hot plate with a filter to absorb extra paraffin. The GAM-paraffin phase change microspheres were obtained by cooling in ambient conditions. 2.6. Characterization Raman spectra were recorded on a LabRAM HR Raman spectrometer with a 50 mW He-Ne laser operating at 632 nm with a Charge-coupled Device (CCD) detector. Powder X-ray diffraction (XRD) patterns were recorded on a D8 advanced spectrometer with a scanning rate of 0.05 /s over an angular range of 5-65˚ (2θ). The morphology of the GAMs was examined by scanning electron microscope (SEM, Hitachi S-4800) with the acceleration voltage of 5-15 KV. where the GAMs were coated with Au nano-powder under current of 30 mA for 2 min. Transmission Electron Microscopy (TEM) measurement was carried on a Tecnai G2F20 S-Twin with an acceleration voltage of 300 kV, the TEM sample with about 40nm thickness was prepared by cryo-cutting. The pore size distribution and average pore diameter of the aerogels were analyzed by the Barret– Joyner–Halenda (BJH) nitrogen adsorption and desorption method (ASAP 2020, Micromeritics, USA). The specific surface area of the aerogels was determined by the Brunauer-Emmett-Teller (BET) method, based on the amount of N2 adsorbed at pressures 0.05 < P/P0 < 0.3. Thermal Gravimetric Analyzer (TGA) was carried out using a TG 209F1 Libra (NETZSCH) analyzer with a heating rate of 10 K/min in a nitrogen atmosphere. Differential scanning calorimetry (DSC) analysis was performed on a DSC 200F3 NETZSCH with a heating and cooling rate of 10 K/min. The Fourier transform infrared (FTIR) spectra of GO, graphene aerogel monolith and GAM were obtained on a Nicolet 6700 spectrometer between 4000 and 400 cm-1, using KBr pellets for all the samples. The diameter of GHMs was observed from optical microscope with a camera system. The performance of heat flux stabilizer was tested by Keithley 2636B digital source meter. 3. Result and discussion 3.1. Mono-dispersed GAMs ACS Paragon Plus Environment

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To obtain mono-dispersed GAMs as templates for fabrication of phase change microspheres, the following requirements should be strictly met in sequence: 1) the sol droplets with controlled narrow size-distribution need to be initially fabricated, 2) the individual sol droplets are resistant to collision and amalgamation, 3) sol-gel transition of the precursors must occur in each individual droplet synchronously, and 4) capillary force of the existing liquid needs to be carefully addressed during drying process of wet gel droplets into aerogel microspheres. Briefly, GAMs were first fabricated by the ILS approach, and GAM-paraffin composite microspheres were then obtained by immerging GAMs in melt wax for 2h under 80 °C as shown in Figure 1a-b. Valve ink-jetting24, 25, 26 was applied for the purpose of obtaining uniform sol droplets, because of its advantage in precise control of droplets with much wider size window. The spray head was connected with a solenoid valve, and the volume of droplets was controlled by the pulse signal. Typically, GO/Vitamin C (VC) aqueous dispersion (i.e. ink) was first pumped into the sample pipe and then mono-dispersed sol droplets would jet out from the spray head and drop orderly into a square plastic container filled with hydrophobic fumed silica nanoparticles (HFSNs) (Supporting Information Movie S1) to form GO liquid marble matrix (Figure S1, Supporting Information) under the control of the predesigned program (Supporting Program S1). The distance between each liquid marble can be as short as 750 µm without undesired amalgamation. Such a liquid marbling technique21,

27, 28

would keep individual GO droplets separated from each other and

maintain their integrity. With the help of liquid marbling technique, we can easily realize the preservation and movement of the GO droplets (Figure S2, Supporting Information). Once the injection was completed, the whole container was sealed to prevent the solvent evaporation and kept in a controlled environment for gelation. For specific application, larger GO liquid marbles can also be realized using larger hydrophobic nanoparticles, such as polystyrene particles and silica aerogel powders (Figure S3, Supporting Information). Furthermore, the factor which can affect the sphericity of liquid marble is the volume of droplets (Figure S4, Supporting Information), while the droplets used in our case was so small (micro scale) that the influence can be ignored. ACS Paragon Plus Environment

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Figure 1. Schematic of Ink jetting-Liquid marbling-Supercritical drying (ILS) coupling approach to make graphene aerogel microspheres and phase change microspheres. (a) Schematic of ink jetting of sol droplet precursor. The sol precursor is GO aqueous dispersion mixed with VC, which is 4 times of the mass of GO. The size of sol droplet and the movement regulation of spray head was set precisely before jetting. (b) Schematic of liquid marbling of sol droplet precursor for gelling, drying, and filling paraffin in GAMs. The HFSNs were washed and etched by ethanol and hydrofluoric acid after the sol liquid marble gelled into graphene hydrogel microspheres. GAMs were obtained by supercritical CO2 drying, and paraffin was filled by soak GAMs in melt wax for 2h under 80 °C. (c) Low-resolution and (d) high-resolution SEM image of obtained GAM. (e) Low-resolution and (f) high-resolution SEM image of GAM-paraffin phase change microsphere.

GO liquid marbles will transform into graphene hydrogel microspheres (GHMs) under 40 °C for 24h sol-gel transition. After that, hydrofluoric acid was used to etch HFSNs, and ethanol was then used to clean the samples to get purified GHMs. A series of GHMs with average diameters of 170, 200 and 230 µm were shown in Supporting Information Figure S5 a-c. It was found that the diameter of the GHMs was mostly dependent on the size of GO droplets, which can be controlled by the pulse signal of the ink-jetting process. The sphericity of these GHMs was also better than those reported elsewhere, because no forces were acting to droplets during the fabrication process. Supporting Information Figure S5 d-f are the Gauss fitting of their size distribution. The calculated variation coefficients were 6.4%, 4.9% and 5.6 % for 170, 200 and 230 µm hydrogel microspheres, respectively. These results indicated that the resulting GHMs are well monodispersed. The result GAMs were characterized by field emission scanning electron microscopy (FESEM). As shown in Figure 1c-d， the GAM exhibited a good sphericity and interconnected ACS Paragon Plus Environment

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porous structure. Typically, the GAMs have a uniform size distribution (the average diameter 150 µm, Supporting Information Figure S6 a-c) and a lot of macro-pores in a diameter of submicrometers to several tens of micrometers located in the resulting GAMs and the macroporous walls of the microspheres were composed by graphene layers (Supporting Information Figure S6 e-f). Most importantly, we found that there was no obvious difference for the superficial and internal morphologies of the obtained GAMs as shown in Supporting Information Figure S6f and Figure S7, which indicated that the gel process between surface and inner parts was almost the same. A series of GAMs with different diameters could be obtained (Figure S8, Supporting Information). To understand the sol-gel transition of the microspheres, Fourier transform infrared spectrometer (FTIR) spectra of GO and GAM were acquired and compared (Figure S9a, Supporting Information). For the curve of GO, the strong peak at 3000-3700 cm-1 belongs to OH stretching vibration29, the peak at 1062 cm-1 belongs to vibration absorption of C-O-C and the peak nearby 869 cm-1 is a characteristic of epoxy group30. After reduced by VC, a narrow and small peak appeared in 3000-3700 cm-1. This may be associated to a small amount of un-reduced OH. The signal in the vicinity of 1620 cm-1 was the C=C absorption and the peak at 1720 cm-1 represented C=O stretching reduced significantly. All of these released a conclusion that the vast majority of oxygen-containing groups have been removed. Raman spectra of the GO and GAMs were shown in Figure S9b, which exhibited typical Raman spectra with a D-band (ascribed to in plane sp2 domain defect) and a G-band (ascribed to graphitic structure)31. The intensity ratio of

D/G of the GAMs (1.22) is higher than that of GO (0.99). This increment suggested the decrease in the average sp2 domain size upon chemical reduction, which can be probably ascribed to the formation of large amounts of new smaller graphitic domains and indicated recovery of the conjugated structure via the reduction process32. The X-ray diffraction (XRD) patterns revealing crystal forms of GO and GAMs were depicted in Figure S9c. The diffraction peak of GO centered at 10.7° corresponded to the interlayer spacing of 0.77 nm, which was much larger than that of pristine graphite (0.335 nm). This difference probably originated from the grafting of oxygen ACS Paragon Plus Environment

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containing functional groups of GO.33 For GAMs, the peak located at 10.7 ° disappeared, which indicated the successful reduction of GO by VC. Instead, it displayed a broad diffraction peak at 24.5°, indicating that the interlayer spacing of GAMs reduced to 0.36 nm by the removal of the oxygen containing functional groups. Even so, the spacing of GAMs was still larger than that of pristine graphite because of the residual oxygen-containing functional groups. In addition, the diffraction peak of GAMs was quite broad, suggesting that the graphene nanosheets of GAMs were poorly ordered along their stacking direction. More detailed information was determined by thermal gravimetric analyzer (TGA) in Figure S9d, which showed a two-step weight loss. It can be seen that the first step showed different mass losses below 100 °C due to volatilization of water absorbed in the products. In the case of GO, the second step was at 150 - 250 °C and had nearly 30 % weight loss which was attributed to the decomposition of oxygen-containing groups of GO.34 While for GAMs, about 15 % weight loss occurred at around 200 °C, which was half of the GO and indicates the reduction of GAMs. The physicochemical properties of the resulting GAMs had been further disclosed. The water contact angle was measured up to 137.5° as shown in Figure S10, higher than that reported elsewhere,15 which indicated the hydrophobicity of the resulting GAMs. The electrical resistance of GAMs with different size was measured by electrochemical workstation. As shown in Supporting Information Figure S11, the resistance of GAMs increased along with the average diameter of the GAMs. What’s more, it was quite interesting that the gelation temperature had played an important role in determining the porous structure of the resulting GAMs as shown in Supporting Information Figure S12. The average pore diameter decreased obviously from 30.6 nm at 20 °C to 29.2 nm at 25 °C and to 23.0 nm at 30 °C. The pore volume had the same tendency (Table S1, Supporting Information). Besides, the gelation temperature also caused vigorous decrease of BET surface area, with 444.3 m2/g at 20 °C, 330.9 m2/g at 25 °C and 209.8 m2/g at 30 °C, respectively. The density of the resulting GAM was in the range of 37-62 mg/cm3, and the higher the sol-gel transition temperature, the higher the density.

3.2. The smart switches based on GAMs ACS Paragon Plus Environment

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Before making phase change microspheres, the highly porous templates were investigated as the key component in the smart switches. Due to the good sphericity and low electrical resistance of GAM (~73 Ω, in comparison the electrical resistance of the monolithic graphene aerogel was ~1.06 Ω), it could be a qualified candidate for ball switch. Figure 2a was the illustration of the GAMs unidirectional ball switch. In order to improve the sensitivity of ball switch, we chose a quartz glass tube as a shell frame, because the rolling angle of GAM on glass was the smallest (Figure S13, Supporting Information). Figure 2b was the performance test of our GAM ball switch compared with the commercial ball switch (SW-200DB, SAIA). It can be found that the voltage of GAM ball switch was much more stable than that of the commercial one. The failure rate of commercial switch was 4 times higher than that of the GAM ball switch within 100 times tests. This may be assigned to the fact that the surface of ball in commercial switch was too smooth and easy to miss contact with the electrodes. The corresponding voltage of GAM ball switch was also higher when the switch was on, due to the relatively high resistance of GAMs. Partial enlargement picture of the performance test was shown in Figure 2c. Because the flow ability of ball in commercial switch was better than that of GAMs, the response time of the GAM ball switch was about 0.7s compared with 0.4 s for the commercial one.

Figure 2. (a) Illustration of the GAM unidirectional ball switch, inset on the left is the photo of our GAM ball switch, and inset on the right is the circuit schematic of performance test. (b) Performance test of our GAM ball switch and commercial ball switch. Both tests were carried through hand to tilted the switch, and the collect signal is the voltage on the switch. (c) Partial enlargement picture of the performance test, and the response time of both switch can be read from this picture.

However, unidirectional ball switch can’t meet the demand on some specific applications.35 We fabricated a bidirectional ball switch, as shown in Figure 3a. The performance of different ends was tested by the same method as unidirectional ball switch. As shown in Figure 3b, with the ACS Paragon Plus Environment

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same deflection angle, the two curves were almost overlapped, which means that the performance of the two ends was consistent. The response time was 0.45s and 0.43s for negative and positive direction, respectively (Figure 3c).

Figure 3. (a) Illustration of the GAM bidirectional ball switch. (b) Performance test of the bidirectional GAM ball switch. Two curves represent two directions. We define one side is positive direction, and the deflection angle that can prompt the GAM rolling is about 7°, same as the unidirectional ball switch. (c) Partial enlargement picture of the performance test of GAM bidirectional ball switch, and the response time is about 0.9 s.

In addition to good sphericity and low electrical resistance, the GAMs possessed excellent hydrophobic property. To this end, liquid level switch was fabricated. Figure 4a was the illustration of the GAM liquid level switch. When immersed the bottom of switch in water, the GAM would float on the water surface. With the water injected to the beaker, GAM would rise when the water level raised. Finally, the GAM contacted with two copper electrodes and the circuit was connected and lights on the LED (Figure 4b). At the same time, voltage change of level switch was collected and shown in Figure 4c. The signal started with a slightly voltage drop, which was induced by the contact of water and electrodes due to its wedge-shape. After that, momentary voltage dip can be observed when the switch was on, with the response time of only 0.14 s. Benefit from the micro size, the GAM used in ball switch was 124 times smaller and 330 times lighter than the ball used in the commercial ball switch. The GAM in liquid level switch was 1.2 × 105 times smaller and 1.6 × 105 times lighter than the ball used in the commercial ones (EM15-2, ELECALL). This may point out the direction of the miniaturization and weightlightening for both ball switches and liquid level switches.

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Figure 4. (a) Illustration of the GAM liquid level switch. (b) Photos of the liquid level switch when it switches on and switches off, the liquid inject into beaker by a watering can. (c) Voltage change when the liquid level switch is on.

3.3. Phase change microspheres The fabrication and characterization of the phase change microspheres templated by the asmade graphene aerogel microspheres would be investigated in sequence. The size of the template was nearly no change after filled with paraffin (Figure S14, Supporting Information). As shown in Figure 1e and 1f, the GAM-paraffin composite microspheres also exhibited good sphericity, and the porous structure of GAMs was fully filled by the crystals of paraffin: some lump crystals of paraffin can be observed in the pore (Figure S15a, b, d), further confirmed by the TEM image (Figure 5 and Figure S16 in Supporting Information). Furthermore, combined with the N2 adsorption-desorption isotherms of GAM-paraffin (Figure S17 in Supporting Information), the meso-pore and macro-pore all were full-filled by paraffin. The crystallization of paraffin in composite spheres was confirmed via XRD patterns (Figure S18, Supporting Information). Thermal stability of paraffin in GAMs was measured by TGA as shown in Figure 6a, the degradation step occurred at temperature range between 200 °C and 450 °C belonged to the thermal degradation of paraffin, and the remaining could be attributed to the GAM framework. The paraffin loading percentage in GAMs was up to 91.3% as calculated from the TGA result, which indicated that there was a high loading percentage for the composite spheres. In addition, the GAM-paraffin composite microspheres showed very stable performance during repeated phase change process. The melting and freezing enthalpies of GAM-paraffin could be calculated from the DSC curves as shown in Figure 6b. For our sample, freezing enthalpy (ΔHf=144.4 J/g) was higher than the melting enthalpy (ΔHm=136.6 J/g, which exhibited similar behaviour to most ACS Paragon Plus Environment

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of phase change materials.36 (By the way, the melting enthalpy of the monolith graphene/paraffin was ~193 J/g.) Also, the GAM-Paraffin microsphere showed a lower melting onset (47.8 °C) compared to paraffin (49.3 °C), and the delayed peak temperature of heating and cooling can be observed (Figure 6b) mainly due to the interaction of graphene network and paraffin[13]. Furthermore, the thermal stability of the GAM-paraffin composite microspheres was tested for 50 cycles by differential scanning calorimetry. As shown in Figure 6c, the 50 curves were virtually identical and the melting latent heat and freezing latent heat all maintained very well, which indicated the excellent reversibility of the phase change microspheres.

Figure 5. TEM images of GAM-Paraffin composite microsphere. The graphene sheets were wrapped by paraffin; the hole was formed by split away of the lump crystal of paraffin.

The electrical property of graphene/paraffin phase change microsphere was investigated by current-voltage (I-V) test as shown in Figure 6d and Figure S19 in Supporting Information. Paraffin is a well-known excellent insulator with a nearly infinity electrical resistance. It was found the electrical resistance of graphene/paraffin phase change microsphere was about 1370 Ω (a lower electric resistance of ~1.22 Ω for the monolithic graphene aerogel) below room temperature (15 °C)，which increased for about 20 times compared to GAM (73.2 Ω), an moderate electrical resistance for covert electric to thermal energy. On the other hand, the phase change microsphere was transferred on a hot plate to melt the wax inside the GAM and it was ACS Paragon Plus Environment

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found that the electrical resistance of GAM-paraffin at 65 °C was 4850 Ω, about 4 times higher than that at 15 °C. This should be attributed to the stretched space of graphene sheets by the volume change of paraffin after melting, and it promised to be an excellent candidate to serve as a sensor for temperature and to buffer the non-default heat fluctuation.

Figure 6. (a) TGA curves of the pure paraffin and the Paraffin-GAMs (The GAMs were treated at 873K for 3h under Ar atmosphere before TG test and filling paraffin. All of TGA spectra of these three samples was obtained by using N2 the as protection gas), (b) DSC melting and freezing curves of pure paraffin (1st and 2nd), Paraffin-GAMs, (c) the DSC curves of Paraffin-GAMs tested for 50 cycles (the inset shows the measured latent heat of Paraffin-GAMs during 50 melting-freezing cycles), d) the I-V curves of Paraffin-GAM in different temperature.

3.4. The thermal buffer based on phase change microsphere The existence of phase change materials can provide a compensable heat sink for the microscale devices. A single GAM-paraffin phase change microsphere with 700 µm in diameter was selected to assemble the heat flux buffer. The schematic of the thermal buffer was shown in Figure 7a, where the inset was the optical image of our device. In order to precisely calculate the required energy for the phase changing of paraffin, we applied a linear voltage to the device and ACS Paragon Plus Environment

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found that the output current first increased correspondingly and suddenly dropped when the voltage was up to 0.87 V (Figure 7b). Meanwhile the Joule heat produced at this circumstance was 0.027 J (little bigger than DSC value 0.02 J, the mass of paraffin in a single GAM was about 0.148 mg), which could be calculated from the integral area under the curve, and this value was so small which can cause resistance mutation. It indicated that the phase change microsphere can serve as an excellent candidate to detected the temperature and heat flux.

Figure 7. (a) Illustration of the testing process of heat sensor. The single phase change microsphere was put on a hollowed-out mood. The inset is the device under a magnifier. (b) I-E curves of heat flux stabilizer. The voltage range is 0-1.5 V, and current mutation appears at 0.87 V.

For simulation of the actual status of thermal buffer in use, the pulse currents with different pulse width were used as the input signal. As shown in Figure 8a, when the pulse width was 1 ms (black curve), as the impact by pulse current, the corresponding voltage raised rapidly in the first 0.01s and slowly after 0.02s (red curve), indicating that the voltage of phase change microsphere cannot response the pulse current and the resistance of phase change microsphere has a rise tendency. When the width was 2.5 ms (Figure S20a, Supporting Information), the trend of voltage was similar to the 1.5 ms width, but in the later period, the voltage exhibited the pulse tendency couple with pulse current. Increasing the width to 10 ms, 50 ms, 500ms, after the rapid rise of the voltage in the early stage, the voltage exhibited a similar pulse trend to current (Figure S20b-d, Supporting Information). These results indicated that, for low-frequency, the electrical resistance of phase change microsphere will be increased in the initial impact stage via timely phase change; for high-frequency, the phase change microsphere can protect the electric circuit via increasing the electrical resistance itself by absorbing the Joule heat.


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Figure 8. (a) The black curve is the input pulse current signal for pulse test and the pulse width is 1ms. The red curve is output voltage-time for pulse test with a pulse with of 1 ms. (b) The black curve is the input pulse current signal for pulse test. The initial current was a constant value of 0.5 mA apply for 0.1 s, then the pulse current was applied for another 0.1 s with 1 ms pulse width. After that, the current back to its initial constant value, where we repeated for two 500-Hz pulses. The red curve is the output voltage-time curve for pulse test, which corresponds to the input pulse current.

To further simulate the actual status under the cross impact between high-frequency impulse and constant current, two 500-Hz pulse currents were used as the input signal as shown in Figure 8b and the inset showed details with enlarged time scale (for simplicity only 2 - 3 crests and troughs were shown), where the pulse width was 1 ms. In the Figure 8b, the red curve was the corresponding voltage-time output of the black curve. When the input was a constant current at 0.5 mA, the voltage output maintained a stable value (ca. 0.7 V). Such output would rise immediately when the 500-Hz pulse current was acting. During the 500-Hz pulse, the output voltage raised slowly because the Joule heat generated in the crest period could not be dissipated immediately in the following trough period. When the 500-Hz pulse stopped and the input became constant again, the voltage output decreased to the initial value slowly. The repeatable phenomenon was observed by increasing the 500-Hz pulse times, which represented the outstanding durability when used for heat flux buffer. With the super heat sensitive and stable performance, the GAM-paraffin phase change microspheres, so called thermal buffer, might help those promising and frontier semiconductor materials work better in transistors.

4. Conclusions ACS Paragon Plus Environment

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Phase change microspheres with fascinating electrical properties and thermal storage/release functionality were fabricated by the combination of graphene aerogel templates and phase change materials, and successfully used as thermal buffer for electric circuit in microelectronic devices. Mono-dispersed graphene aerogel microspheres with uniform size distribution and perfect sphericity were prepared through the ILS coupling techniques. The size of the aerogel microspheres can be easily controlled by adjusting the program of ink-jetting process. The Brunauer-Emmett-Teller surface area decreases vigorously with the gelation temperature from 20 °C for 444.3 m2/g to 25 °C for 330.9 m2/g and 30 °C for 209.8 m2/g, respectively. Further tests revealed that the electrical resistance of GAMs could be as low as 8.79 Ω, and the water contact angle of GAMs could be as high as 137.5°. Due to these excellent properties, GAMs were used in ball switch and liquid level switch with comparable or even better performance than commercial counterparts. Furthermore, the mono-dispersed graphene aerogel microsphere was used as a template, where paraffin was filled into the cavities. Such phase change microsphere was made into a heat flux buffer. Due to existence of micro/nano pore channels in GAMs, the highcrystalline, high latent heat, stable shape of paraffin can be well maintained and optimized. The composite microsphere has stable thermal performance during phase change process. Because of its micro size, the heat contributed to the phase changing of paraffin was very small. It was found that only 0.027 J heat was enough to initial the phase change and trigger electrical resistance mutation.

ASSOCIATED CONTENT Supporting information. Digital photograph of sol droplet matrix, the more structure characterization results of the liquid marble, graphene hydrogel/aerogel microspheres, the more physicochemical characterization results of the graphene aerogel microspheres, the optical microscope image, XRD, I-V curve and pulse test of the phase change microspheres. This material is available free of charge via the Internet at http://pubs.acs.org ACS Paragon Plus Environment

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AUTHOR INFORMATION Corresponding Author

*To whom correspondence should be addresses. Xuetong Zhang: [email protected]; Guo Hong: [email protected]. Notes

The authors declare no competing financial interest.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (51572285 and 21373024) and the National Key Research and Development Program of China (2016YFA0203301).

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