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Preparation of pristine graphene paste for screen printing patterns with high conductivity
Accepted Manuscript Research paper Preparation of pristine graphene paste for screen printing patterns with high conductivity Minyan Lin, Yanzhe Gai, Ding Xiao, Huijun Tan, Yaping Zhao PII: DOI: Reference:
S0009-2614(18)30831-5 https://doi.org/10.1016/j.cplett.2018.10.022 CPLETT 36008
To appear in:
Chemical Physics Letters
Received Date: Accepted Date:
5 August 2018 8 October 2018
Please cite this article as: M. Lin, Y. Gai, D. Xiao, H. Tan, Y. Zhao, Preparation of pristine graphene paste for screen printing patterns with high conductivity, Chemical Physics Letters (2018), doi: https://doi.org/10.1016/j.cplett. 2018.10.022
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Preparation of pristine graphene paste for screen printing patterns with high conductivity Minyan Lin, Yanzhe Gai, Ding Xiao, Huijun Tan, Yaping Zhao * School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University Shanghai 200240, P. R. China Corresponding author (E-mail: [email protected])
Abstract Graphene conductive pastes are poised to apply in making flexible electronics via a screen-printing way because of its excellent electrical and mechanical properties. In this paper, a new formulation of a pristine graphene-based paste was presented consisting of the conductive filler of graphene, a mixed solvent of terpineol, dipropylene glycol methyl ether, cyclohexanone, and the auxiliaries of ethyl cellulose, Span-85 and 1,4-butyrolactone. The paste containing 20 wt% of the graphene was prepared and showed excellent dispersibility and stability. The screen-printed patterns on a polyimide substrate achieved a high conductivity of 9×104 S/m and had excellent mechanical flexibility. Keywords: graphene, conductive paste, resistivity 1. Introduction The conductive pastes are garnering more and more attention because they have wide applications in constructing the electronics. A lot of electronics, such as sensors[1,
2], solar cells[3, 4], flexible electrodes[5, 6], solders in microelectronics[7] and radio frequency identification devices [8, 9], can be easily formulated via the paste using a screen-printing method. The composition of the paste plays an important role in the quality of screen-printed patterns. Though the silver-based paste has excellent electrical conductive performance[4, 10, 11], it is desired to be replaced due to its high cost. The copper-based paste is a potential substitute for the silver-based because the copper has similar conductivity to the silver but much low price[8, 12]. However, the copper-based paste is limited because it is easily oxidized damaging its conductive properties severely. The graphene-based paste is considered as a promising alternative to metal-based pastes because the graphene has remarkable electronic conductivity, chemical stability, and mechanical flexibility[13-16]. Thus far, graphene has become a sort of important filler in making conductive ink/paste [17-21]. However, the electrical patterns obtained by screen-printing of graphene paste still have the shortcoming of low conductivity limiting their applications in electrical devices requiring high conductivity, such as Radio Frequency Identification. The major reason is that the pristine graphene sheets are easily aggregated resulting in low graphene loading concentration of the paste. Also, the stability of the pastes is poor. To deal with this issue, Heiner Friedrich[22] prepared a concentrated and colloidally stable graphene paste based on the gelation dispersion by a dipropylene glycol-exchange procedure, in which the graphene loading concentration reached to ~5 wt%. The screen-printed patterns exhibited sheet resistances of 30 Ω·□-1 at 25 μm. Francis[23] applied the ethyl cellulose (EC) and terpineol to make the
graphene paste in which the graphene loading concentration reached 8 wt%. The highest conductivity of the screen-printed pattern was 1.86 × 104 S·m-1. It is expected that the conductivity of printed films could be improved with the increasing of the graphene loading concentration of the paste. The precondition to prepare the paste with high graphene loading is to solve its issue of the dispersion and stability. It involves the optimization of the composition of the organic vehicle including solvent, binder and surfactant, et al. Thus, the purpose of this paper is to formulate a conductive paste with high pristine graphene loading concentration, by which the screen-printed pattern achieves high electrical conductivity. The ratio among terpineol, cyclohexanone, and dipropylene glycol methyl ether (DPM) was first studied by a volatility test to determine the ternary solvent. Then, the rheological properties of solution using ternary solvent added with EC (as a binder), Span-85 (as a dispersant) and 1,4-butyrolactone (as a rheological agent), respectively, were tested to prepare the organic vehicle in terms of the printability and stability of paste. Finally, the pristine graphene was mixed with the organic vehicle by ball milling to make the paste used to fabricate conductive pattern on the polyimide (PI) substrate by a screen-printing method. The printed pattern with excellent conductivity was obtained. 2. Experimental section 2.1. Materials and reagents Ethyl cellulose (EC; 0.04-0.1 Pa·s viscosity for 5% (w/v) in 1:4 ethanol/toluene)
was purchased from Shanghai Shiyi Chemical Reagent Co., Ltd. Terpineol and 1,4-butyrolactone were bought from Shanghai Lingfeng Chemical Reagent Co., Ltd. Cyclohexanone, Dipropylene glycol methyl ether (DPM) and Span-85(the main ingredient is sorbitan trioleate) were purchased from Sinopharm Group Chemical Reagent Co., Ltd. All reagents were used as received. 2.2. Preparation of pristine graphene paste and its characterization The pristine graphene paste was prepared by mixing the organic vehicle and pristine graphene in a vertical planetary ball mill (Changsha Tianchuang Powder Technology Co., Ltd.) at 400 rpm for 4 hours (as shown in Fig S1). The structure and morphology of the graphene were characterized by atomic force microscopy (AFM, NanoNavi/E-Sweep, SII NanoTechnology, Inc.), field-emission scanning electron microscope (FE-SEM, Nova NanoSEM 450, FEI Company, USA) and transmission electron microscope (TEM, JEOL JEM-2100F). The rheology of the paste was measured by HAAKE Mars III Rheometer (Thermo Scientific, Germany). The surface tension of the paste was measured by surface tensiometer (K100, Kruss). The thermal stability of the EC and paste was analyzed by thermal gravimetric analysis (TGA, Perkin-Elmer, Inc., Wellesley, MA) performed at a heating rate of 10 ℃/min to 600 ℃ in the air. The FT-IR spectrometer （Spectrum 100, Perkin Elmer, Inc., USA）was used to characterize the thermal decomposition of EC. The organical vehicle was prepared by mixing the terpineol-cyclohexanone-DPM ternary solvent, Span-85,1,4-butyrolactone and EC according to the certain mixing ratio
which was determined by rheological test. The test was performed by a digital rotational viscometer (SNB-1A, Shanghai Fangrui Instrument Co., Ltd.) at 25℃. The ratio of the terpineol, cyclohexanone, and DPM in the ternary solvent was determined by a volatility test. The test was carried out in an electric thermostatic blast box （DHG-9035A, Shanghai Yiheng Scientific Instrument Co., Ltd.）and the Volatility (η) was calculated as formula 2-1. η = (m0 – m1)/m0×100%.
(2-1)
Wherein, m0 is the initial weight of the sample while m1 is the final weight. 2.3.Fabrication and characterization of graphene patterns Screen-printed graphene patterns were fabricated using the graphene paste by a manual screen printer (ZYS-3024, Dongguan Electromechanical Technology Co., Ltd. The paste was transferred to the polyimide (PI, DuPont Kapton, 125 μm) substrate (5cm × 5cm) through a stencil screen with a designed pattern. Then the printed patterns were carried out by a series of post-treatment. The wet film was first dried at 70℃ for an hour and then annealed in an electrical resistance furnace (YT-K60X300, Shanghai Y-Feng electrical furnace CO., LTD). Finally, the film was mechanical pressed by a desktop powder tablet machine (FY-40, Tianjin Sichuang Jingshi technology development co., Ltd.). The thickness and the resistivity of the obtained screen-printed pattern were measured, respectively, by a micrometer and a four-point probe resistivity measurement system with silver paint electrodes (SX1944, Suzhou Baisheng Technology Co., Ltd.,
Suzhou, China). The morphology of the printed pattern was characterized by FE-SEM operated at 10 kV and 5 μA. Its flexibility was evaluated by measuring its resistivity when folding it at 180 ° and repeating 1000 cycles. 3. Results and discussions 3.1. Formulation of graphene conductive paste 3.1.1. Formulation of organic vehicle The organic vehicle consists of solvent, binder, dispersant, and rheological agent, and its formulation is determined according to its volatility, viscosity, and rheology. The solvent as a diluent plays a key role in letting the paste pass through the screen to the substrate. Also, its volatility can affect the surface of the printed pattern. Stepwise volatilization is required to make the uniform and continuous surface of the printed pattern [24, 25]. Considering the desired properties of the solvent including its ability to dissolve EC, terpineol, cyclohexanone, and DPM were chosen to form a ternary solvent. This solvent can be volatilized in a step-by-step mode because the boiling points of cyclohexanone, DPM, and terpineol are 155.6 ℃, 190 ℃, and 214.0-224.0 ℃, respectively. The volatility of the single solvent is related to its boiling point as shown in Fig.1a. According to Henry's law [26], the volatilization of the ternary solvent can be adjusted by changing the ratio of its component. Four groups of the ternary solvent with different ratios of terpineol, cyclohexanone, and DPM were designed as shown in Table 1. Fig.1b showed the volatility change of the ternary solvents with increasing temperature. All ternary solvents were volatilized in a step-by-step mode, but the
changing trend of No.1 and No.3 were different with No.2 and No.4. The No.1 and No.3 exhibited lower volatility than the No.2 and No.4 from 120℃ to 180℃. It can be attributed to the higher content of terpineol. Given lower volatility below 120℃ that is favorable for the stable storage of paste and higher volatility between 120-180 °C which is beneficial for solvents to volatilize quickly after printing, and terpineol has suitable surface tension in screen printing[27, 28], the No.2 is better than the No.4. Thus, the preferred ternary solvent was selected as 50 wt% of terpineol, 40 wt% of DPM, and 10 wt% of cyclohexanone, which was a main fraction of the organic vehicle. Table 1 Different ratio of components of the ternary solvent No.
Terpineol（%）
Cyclohexanone（%）
DPM（%）
1
60
10
30
2
50
10
40
3
50
0
50
4
30
10
60
Fig. 1. The volatility curves of the solvent at different temperatures. (a) Terpineol, DPM, and cyclohexanone, respectively; (b) Four groups of ternary solvent made from
terpineol-cyclohexanone-DPM as shown in Table 1. After determining the ternary solvent, the EC, Span-85 and 1,4-butyrolactone were investigated as a binder, a dispersant, and a rheological agent, respectively. The influence of the EC content on the rheological property of ternary solvent was shown in Fig.2. It can be seen that the viscosity of the ternary solvent (8.87 mPa·s) increased greatly with adding EC (3 wt% to 7 wt%) and decreased with increasing rotating speed. It indicates that the ternary solvent had good rheological property when a certain amount of EC was added. When the EC content was between 5 wt% and 6 wt%, the viscosity fell to the range of 3000-4000 mPa·s that was suitable for the screen-printing process. Considering the solubility of EC, 5 wt% EC was selected in the organic vehicle later. A surfactant can increase the loading concentration of stabilized graphene flakes in the solvent by dispersing and stabilize the graphene [29]. Span-85 is a kind of suitable surfactant because it did not influence the viscosity and rheology of the ternary solvent added with EC severely even though Span-85 was added from 2 wt% to 8 wt% as shown in Fig.3a. In view of that Span-85 could reduce the conductivity of printed patterns due to its inherent insulation characteristics, 2 wt% of Span 85 was selected in the organic vehicle later. 1,4-butyrolactone was selected as a rheological agent to improve the rheological properties of the paste. As shown in Fig. 3(b), the viscosity of the ternary solvent added with EC and Span-85 decreased gradually with increasing of 1,4-butyrolactone from 1
wt% to 4 wt%, and the most reduced deviation value reached 372 mPa•s (from 2701 mPa·s to 2329 mPa·s) when the content was fixed at 2 wt%, and the rotating speed increased from 2 r/min to 10 r/min. So, the 2 wt% of 1,4-butyrolactone was selected in the organic vehicle later. In conclusion, the organic vehicle was consisted of ternary solvent (50 wt% of terpineol, 40 wt% of DPM and 10 wt% of cyclohexanone) and 5 wt% of EC, 2 wt% of Span-85 and 2 wt% 1,4-butyrolactone (the content of the later three auxiliaries were respected to the ternary solvent).
Fig.2. Influence of the EC content on the viscosity of the ternary solvent at different rotating speed: (a) 3 wt% EC; (b) 4 wt% EC; (c) 5 wt% EC; (d) 6 wt% EC; (e) 7 wt% EC.
Fig.3. Influence of the Span-85 and 1,4-butyrolactone content on the viscosity of the ternary solvent added by 5 wt% EC at different rotating speed. (a) influence of Span-85; (b) influence of 1,4-butyrolactone. 3.1.2. Preparation of pristine graphene The pristine graphene was taken as a conductive filler of the paste in this work and was prepared using supercritical CO2 associated with ultrasonic according to our previously published papers[30, 31]. The structure and morphology of graphene were shown in Fig.4. The lateral size of the graphene sheets was around 2-5 microns as determined by AFM (Fig.4a), which can also be seen from SEM images (Fig.4b). The step height of 2-3 nm of the graphene sheets (Fig.4d) indicates the sheets were around 6-9 layers of numbers which is consistent with the TEM images(Fig.4c). The graphene with good quality provides the fundamental for high conductivity of printed patterns.
Fig.4. The characterizations of pristine graphene. (a) SEM images of graphene; (b) the lateral size distribution of the graphene sheets; (c) TEM of graphene; (b) AFM of graphene. 3.1.3. Rheological property of the graphene paste The graphene paste was formulated by mixing the obtained organic vehicle with the prepared pristine graphene in a ball-milling way at 400 rpm for 4 hours (as shown in Table S1 and Fig.S2). Usually, high loading of the graphene sheets in the paste favors high conductivity of the printed pattern. However, it was found that the paste could not be formed when the loading was 30%. It can be attributed to that the organic vehicle cannot fully infiltrate the graphene. In order to evaluate the printability of the paste, the rheological property of the organic vehicle and the paste with 20 wt% graphene (as a representative) was examined. As shown in Fig.5, the static viscosity of the organic vehicle was 1.92 Pa·s, while the paste was 19.14 Pa·s. It means that the paste had a
higher viscosity than the organic vehicle. Also, the shear stress of the paste reduced under increased shear rate as shown in Fig.5(c), which suggests that the graphene paste was a pseudoplastic fluid. It can be easy to flow through the screen, and the formed pattern would not deform on the substrate during screen-printing. Also, the surface tension of the paste with 20wt% graphene was 30.78 mN/m, which suggested that the paste had good wettability to the surface of PI substrates[32]. The influence of the graphene loading concentration on the performance of the paste was discussed by the resistivity of the screen-printed patterns in the next part.
Fig.5. The rheological properties of the organic vehicle and the graphene conductive paste:(a) the viscosity of the organic vehicle at a different shear rate;(b) photograph of the organic vehicle;(c) the viscosity of the paste at a different shear rate; (d)photograph of the paste. 3.2. The resistivity of the screen-printed patterns The resistivity was applied as the index to evaluate the performance of the printed patterns. The effects of the process parameter on the resistivity were investigated in this paper.
3.2.1. Influence of graphene loading concentration on the resistivity The influence of the graphene loading concentration on the resistivity of the paste was shown in Fig.6a. It can be seen that the resistivity decreased from 0.0185 to 1.8×10-5 Ω·m when the graphene loading concentration increased from 10 to 20 wt% and leveled off beyond 20 wt%. This result can be attributed to the amount of graphene in the pattern. When the graphene loading concentration was low, a conductive network could not be formed in the pattern resulting in poor conductivity. Fig.6b displays that there were many gaps in the pattern. When the amount of the graphene increased, the conductive network of the graphene was gradually formed due to more contacts between the graphene flakes. However, the conductivity would not increase with increasing the graphene loading concentration when the graphene flakes have been fully contacted, such as the case when the loading was 20 wt%. Fig.6c displays that the printing pattern was uniform and did not have the gap on the pattern. Therefore, the 20 wt% of graphene loading of the paste was enough.
Fig.6. The influence of the graphene loading concentration on the conductivity of printed films. (a) The resistivity of films printed by using the pastes with different graphene loading concentrations; (b, c) SEM images of the printed films with (b) 10 wt%
and (c) 20 wt% graphene. 3.2.2. Influence of annealing and mechanical pressing conditions on the resistivity The annealing temperature and time have significant influences on the electrical performance of the printed pattern. As shown in Fig.7a, the resistivity sharply decreased from 4.2×10-5 to 1.6×10-5 Ω·m when the annealing temperature increased from 200 to 350℃. It can be ascribed to two main reasons. One is that the residual solvent was removed at high temperature. The other is that the aromatic compound formed due to the thermal decomposition of EC at high temperature (Fig.7c) [33], which can be verified as shown in Figure 7(d). There were stronger C=O and C=C stretching vibrations at 1724 and 1613 cm-1 indicating the formation of aromatic components[34].
[34] . Therefore the decomposition resulted in a π-π bond stack between the aromatic compound and graphene, which provided efficient charge transport and decreased resistivity through the graphene network[25]. However, the resistivity raised when the annealing temperature was over 350℃. It might be that the removed EC residue damaged the graphene network. Annealing time had a similar influence. The resistivity decreased from 2.2×10-5 to 1.3×10-5 Ω·m when the annealing time increased from 10 to 30 min as shown in Fig.7b. When the annealing time was over 50 min, the resistivity increased slightly. It can be attributed to the slight peeling of the film layer during prolonged annealing period.
Mechanical pressing can increase the conductivity of the printed pattern further because the distance between two graphene layers was reduced. As shown in Fig.7e, the resistivity decreased from 1.7×10-4 to 1.2×10-5 Ω·m when the pressing pressure increased from 0 to 20MPa. Also, 1 min of the pressing period was enough to decrease resistivity as shown in Fig.7f.
Fig.7. Influence of annealing and mechanical pressing conditions on the resistivity of the printed films. (a, b) the resistivity of the printed films plotted against (a) annealing temperature for a fixed annealing time of 30 min and (b) annealing time at a fixed annealing temperature of 350 ℃;(c) TGA curves of EC, an organic vehicle, and graphene conductive paste; (d)FTIR curves of the EC before and after thermal annealing in different temperature; (e, f) the resistivity of printed films plotted against (e) pressing pressure for a fixed pressing time of 1 min and (f) pressing time at a fixed pressing pressure of 20 MPa. 3.2.3. Influence of screen-printing conditions on the resistivity
The influence of screen-printing times on the resistivity was shown in Fig.8a. It can be seen that the resistivity sharply decreased from 5.4×10-4 to 1.5×10-5 Ω·m and gradually leveled off when the screen-printing times increased from 1 to 10 times. It can be ascribed to the improved continuity of the printed patterns with increasing the printing times, as shown in Fig.8e-f. High uniform and continuous patterns with low resistivity were obtained after 4 printing times. The line width of patterns also influenced resistivity. The lines with different width were obtained by printing 4 times, as shown in Fig.8c. The resistivity decreased from 3.2×10-4 to 2.7×10-5 Ω·m when the width increased from 0.5 to 2 mm. Therefore, the line width should not be less than 1.5 mm to obtain good conductivity.
Fig.8. Influence of screen-printing conditions on the resistivity of the printed films. (a, b) the resistivity of the printed films plotted against (a) print times and (b) the width of the line at a fixed print time of 4 times. (c) Photograph of printed patterns of the lines with different width by 4 times screen printing; (d, e) SEM images of printed films after (d)1 time printing and (e) 4 times printing. And flexibility assessment of the printed graphene electrodes on PI substrates:(f) the resistivity of the printed pattern after different bending cycles; (g) photograph of the pattern folded inward 180 °.
By previous experiments, a pattern was made using the paste containing 20 wt% pristine graphene at the conditions of screen-printing for four times, drying at 70 ℃ for 1h, annealing at 350 ℃ for 30 min and mechanical pressing at 20 MPa for 1 min. The resistivity of the pattern was 1.04×10-5 Ω·m at the thickness of 10 μm, and the conductivity was 9*10^4 S/m, which is the highest value achieved so far using screen-printing of the pristine graphene [9, 22, 23, 35]. The paste has good stability because it would not precipitate even though after storing for two months. Also, the resistivity of the printed pattern made from the paste stored for two months was 2.83×10-5 Ω·m at the thickness of 7 μm and is similar to that of the pattern made from the initial paste. Also, the printed graphene patterns on PI substrates had excellent mechanical flexibility as shown in Fig.8g-f. Even though the pattern was folded at 180° for 1000 cycles, the resistivity only increased from initial 1.9×10-5 Ω·m to 3.2×10-5 Ω·m, and no obvious cracks were observed on the patterns. 4. Conclusions A conductive paste with high pristine graphene loading concentration was formulated. It demonstrates that the formulated paste can be screen-printed on the PI substrate to form flexible patterns with high conductivity. The paste was stable for at least two months without apparent precipitation. The formulated graphene paste is expected to have significant potential applications in fabricating flexible electric devices. Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grants No. 21576165). We are thankful for Ms. Huiqin Li of Instrumental Analysis Center of SJTU for analysis. Declarations of interest: none References [1] Chang W Y FTH, Lin H J, et al. . A Large Area Flexible Array Sensors Using Screen Printing Technology. Journal of Display Technology. 5（2009）178-83. [2] Karuwan C, Wisitsoraat A, Phokharatkul D, Sriprachuabwong C, Lomas T, Nacapricha D, et al. A disposable screen printed graphene–carbon paste electrode and its application in electrochemical sensing. RSC Advances. 3（2013）25792. [3] Liu HW, Liang SP, Wu TJ, Chang H, Kao PK, Hsu CC, et al. Rapid atmospheric pressure plasma jet processed reduced graphene oxide counter electrodes for dye-sensitized solar cells. ACS applied materials & interfaces. 6（2014）15105-12. [4] Xu C FM, Willenbacher N. Impact of Wall Slip on Screen Printing of Front-Side Silver Pastes for Silicon Solar Cells. IEEE Journal of Photovoltaics. PP（2017）1-7. [5] Ahn HY, Kim J-G, Gong M-S. Preparation of flexible resistive humidity sensors with different electrode gaps by screen printing and their humidity-sensing properties. Macromolecular Research. 20（2012）174-80. [6] Wang S, Liu N, Yang C, Liu W, Su J, Li L, et al. Fully screen printed highly conductive electrodes on various flexible substrates for asymmetric supercapacitors. RSC Advances. 5（2015）85799-805.
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Highlights 1.
A conductive paste has been formulated based on high pristine graphene loading
2.
The graphene paste shows good rheology and stability for screen-printing
3.
The screen-printed pattern has a high conductivity of 9×104 S/m and excellent mechanical flexibility
Graphical abstract
S0009-2614(18)30831-5 https://doi.org/10.1016/j.cplett.2018.10.022 CPLETT 36008
To appear in:
Chemical Physics Letters
Received Date: Accepted Date:
5 August 2018 8 October 2018
Please cite this article as: M. Lin, Y. Gai, D. Xiao, H. Tan, Y. Zhao, Preparation of pristine graphene paste for screen printing patterns with high conductivity, Chemical Physics Letters (2018), doi: https://doi.org/10.1016/j.cplett. 2018.10.022
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Preparation of pristine graphene paste for screen printing patterns with high conductivity Minyan Lin, Yanzhe Gai, Ding Xiao, Huijun Tan, Yaping Zhao * School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University Shanghai 200240, P. R. China Corresponding author (E-mail: [email protected])
Abstract Graphene conductive pastes are poised to apply in making flexible electronics via a screen-printing way because of its excellent electrical and mechanical properties. In this paper, a new formulation of a pristine graphene-based paste was presented consisting of the conductive filler of graphene, a mixed solvent of terpineol, dipropylene glycol methyl ether, cyclohexanone, and the auxiliaries of ethyl cellulose, Span-85 and 1,4-butyrolactone. The paste containing 20 wt% of the graphene was prepared and showed excellent dispersibility and stability. The screen-printed patterns on a polyimide substrate achieved a high conductivity of 9×104 S/m and had excellent mechanical flexibility. Keywords: graphene, conductive paste, resistivity 1. Introduction The conductive pastes are garnering more and more attention because they have wide applications in constructing the electronics. A lot of electronics, such as sensors[1,
2], solar cells[3, 4], flexible electrodes[5, 6], solders in microelectronics[7] and radio frequency identification devices [8, 9], can be easily formulated via the paste using a screen-printing method. The composition of the paste plays an important role in the quality of screen-printed patterns. Though the silver-based paste has excellent electrical conductive performance[4, 10, 11], it is desired to be replaced due to its high cost. The copper-based paste is a potential substitute for the silver-based because the copper has similar conductivity to the silver but much low price[8, 12]. However, the copper-based paste is limited because it is easily oxidized damaging its conductive properties severely. The graphene-based paste is considered as a promising alternative to metal-based pastes because the graphene has remarkable electronic conductivity, chemical stability, and mechanical flexibility[13-16]. Thus far, graphene has become a sort of important filler in making conductive ink/paste [17-21]. However, the electrical patterns obtained by screen-printing of graphene paste still have the shortcoming of low conductivity limiting their applications in electrical devices requiring high conductivity, such as Radio Frequency Identification. The major reason is that the pristine graphene sheets are easily aggregated resulting in low graphene loading concentration of the paste. Also, the stability of the pastes is poor. To deal with this issue, Heiner Friedrich[22] prepared a concentrated and colloidally stable graphene paste based on the gelation dispersion by a dipropylene glycol-exchange procedure, in which the graphene loading concentration reached to ~5 wt%. The screen-printed patterns exhibited sheet resistances of 30 Ω·□-1 at 25 μm. Francis[23] applied the ethyl cellulose (EC) and terpineol to make the
graphene paste in which the graphene loading concentration reached 8 wt%. The highest conductivity of the screen-printed pattern was 1.86 × 104 S·m-1. It is expected that the conductivity of printed films could be improved with the increasing of the graphene loading concentration of the paste. The precondition to prepare the paste with high graphene loading is to solve its issue of the dispersion and stability. It involves the optimization of the composition of the organic vehicle including solvent, binder and surfactant, et al. Thus, the purpose of this paper is to formulate a conductive paste with high pristine graphene loading concentration, by which the screen-printed pattern achieves high electrical conductivity. The ratio among terpineol, cyclohexanone, and dipropylene glycol methyl ether (DPM) was first studied by a volatility test to determine the ternary solvent. Then, the rheological properties of solution using ternary solvent added with EC (as a binder), Span-85 (as a dispersant) and 1,4-butyrolactone (as a rheological agent), respectively, were tested to prepare the organic vehicle in terms of the printability and stability of paste. Finally, the pristine graphene was mixed with the organic vehicle by ball milling to make the paste used to fabricate conductive pattern on the polyimide (PI) substrate by a screen-printing method. The printed pattern with excellent conductivity was obtained. 2. Experimental section 2.1. Materials and reagents Ethyl cellulose (EC; 0.04-0.1 Pa·s viscosity for 5% (w/v) in 1:4 ethanol/toluene)
was purchased from Shanghai Shiyi Chemical Reagent Co., Ltd. Terpineol and 1,4-butyrolactone were bought from Shanghai Lingfeng Chemical Reagent Co., Ltd. Cyclohexanone, Dipropylene glycol methyl ether (DPM) and Span-85(the main ingredient is sorbitan trioleate) were purchased from Sinopharm Group Chemical Reagent Co., Ltd. All reagents were used as received. 2.2. Preparation of pristine graphene paste and its characterization The pristine graphene paste was prepared by mixing the organic vehicle and pristine graphene in a vertical planetary ball mill (Changsha Tianchuang Powder Technology Co., Ltd.) at 400 rpm for 4 hours (as shown in Fig S1). The structure and morphology of the graphene were characterized by atomic force microscopy (AFM, NanoNavi/E-Sweep, SII NanoTechnology, Inc.), field-emission scanning electron microscope (FE-SEM, Nova NanoSEM 450, FEI Company, USA) and transmission electron microscope (TEM, JEOL JEM-2100F). The rheology of the paste was measured by HAAKE Mars III Rheometer (Thermo Scientific, Germany). The surface tension of the paste was measured by surface tensiometer (K100, Kruss). The thermal stability of the EC and paste was analyzed by thermal gravimetric analysis (TGA, Perkin-Elmer, Inc., Wellesley, MA) performed at a heating rate of 10 ℃/min to 600 ℃ in the air. The FT-IR spectrometer （Spectrum 100, Perkin Elmer, Inc., USA）was used to characterize the thermal decomposition of EC. The organical vehicle was prepared by mixing the terpineol-cyclohexanone-DPM ternary solvent, Span-85,1,4-butyrolactone and EC according to the certain mixing ratio
which was determined by rheological test. The test was performed by a digital rotational viscometer (SNB-1A, Shanghai Fangrui Instrument Co., Ltd.) at 25℃. The ratio of the terpineol, cyclohexanone, and DPM in the ternary solvent was determined by a volatility test. The test was carried out in an electric thermostatic blast box （DHG-9035A, Shanghai Yiheng Scientific Instrument Co., Ltd.）and the Volatility (η) was calculated as formula 2-1. η = (m0 – m1)/m0×100%.
(2-1)
Wherein, m0 is the initial weight of the sample while m1 is the final weight. 2.3.Fabrication and characterization of graphene patterns Screen-printed graphene patterns were fabricated using the graphene paste by a manual screen printer (ZYS-3024, Dongguan Electromechanical Technology Co., Ltd. The paste was transferred to the polyimide (PI, DuPont Kapton, 125 μm) substrate (5cm × 5cm) through a stencil screen with a designed pattern. Then the printed patterns were carried out by a series of post-treatment. The wet film was first dried at 70℃ for an hour and then annealed in an electrical resistance furnace (YT-K60X300, Shanghai Y-Feng electrical furnace CO., LTD). Finally, the film was mechanical pressed by a desktop powder tablet machine (FY-40, Tianjin Sichuang Jingshi technology development co., Ltd.). The thickness and the resistivity of the obtained screen-printed pattern were measured, respectively, by a micrometer and a four-point probe resistivity measurement system with silver paint electrodes (SX1944, Suzhou Baisheng Technology Co., Ltd.,
Suzhou, China). The morphology of the printed pattern was characterized by FE-SEM operated at 10 kV and 5 μA. Its flexibility was evaluated by measuring its resistivity when folding it at 180 ° and repeating 1000 cycles. 3. Results and discussions 3.1. Formulation of graphene conductive paste 3.1.1. Formulation of organic vehicle The organic vehicle consists of solvent, binder, dispersant, and rheological agent, and its formulation is determined according to its volatility, viscosity, and rheology. The solvent as a diluent plays a key role in letting the paste pass through the screen to the substrate. Also, its volatility can affect the surface of the printed pattern. Stepwise volatilization is required to make the uniform and continuous surface of the printed pattern [24, 25]. Considering the desired properties of the solvent including its ability to dissolve EC, terpineol, cyclohexanone, and DPM were chosen to form a ternary solvent. This solvent can be volatilized in a step-by-step mode because the boiling points of cyclohexanone, DPM, and terpineol are 155.6 ℃, 190 ℃, and 214.0-224.0 ℃, respectively. The volatility of the single solvent is related to its boiling point as shown in Fig.1a. According to Henry's law [26], the volatilization of the ternary solvent can be adjusted by changing the ratio of its component. Four groups of the ternary solvent with different ratios of terpineol, cyclohexanone, and DPM were designed as shown in Table 1. Fig.1b showed the volatility change of the ternary solvents with increasing temperature. All ternary solvents were volatilized in a step-by-step mode, but the
changing trend of No.1 and No.3 were different with No.2 and No.4. The No.1 and No.3 exhibited lower volatility than the No.2 and No.4 from 120℃ to 180℃. It can be attributed to the higher content of terpineol. Given lower volatility below 120℃ that is favorable for the stable storage of paste and higher volatility between 120-180 °C which is beneficial for solvents to volatilize quickly after printing, and terpineol has suitable surface tension in screen printing[27, 28], the No.2 is better than the No.4. Thus, the preferred ternary solvent was selected as 50 wt% of terpineol, 40 wt% of DPM, and 10 wt% of cyclohexanone, which was a main fraction of the organic vehicle. Table 1 Different ratio of components of the ternary solvent No.
Terpineol（%）
Cyclohexanone（%）
DPM（%）
1
60
10
30
2
50
10
40
3
50
0
50
4
30
10
60
Fig. 1. The volatility curves of the solvent at different temperatures. (a) Terpineol, DPM, and cyclohexanone, respectively; (b) Four groups of ternary solvent made from
terpineol-cyclohexanone-DPM as shown in Table 1. After determining the ternary solvent, the EC, Span-85 and 1,4-butyrolactone were investigated as a binder, a dispersant, and a rheological agent, respectively. The influence of the EC content on the rheological property of ternary solvent was shown in Fig.2. It can be seen that the viscosity of the ternary solvent (8.87 mPa·s) increased greatly with adding EC (3 wt% to 7 wt%) and decreased with increasing rotating speed. It indicates that the ternary solvent had good rheological property when a certain amount of EC was added. When the EC content was between 5 wt% and 6 wt%, the viscosity fell to the range of 3000-4000 mPa·s that was suitable for the screen-printing process. Considering the solubility of EC, 5 wt% EC was selected in the organic vehicle later. A surfactant can increase the loading concentration of stabilized graphene flakes in the solvent by dispersing and stabilize the graphene [29]. Span-85 is a kind of suitable surfactant because it did not influence the viscosity and rheology of the ternary solvent added with EC severely even though Span-85 was added from 2 wt% to 8 wt% as shown in Fig.3a. In view of that Span-85 could reduce the conductivity of printed patterns due to its inherent insulation characteristics, 2 wt% of Span 85 was selected in the organic vehicle later. 1,4-butyrolactone was selected as a rheological agent to improve the rheological properties of the paste. As shown in Fig. 3(b), the viscosity of the ternary solvent added with EC and Span-85 decreased gradually with increasing of 1,4-butyrolactone from 1
wt% to 4 wt%, and the most reduced deviation value reached 372 mPa•s (from 2701 mPa·s to 2329 mPa·s) when the content was fixed at 2 wt%, and the rotating speed increased from 2 r/min to 10 r/min. So, the 2 wt% of 1,4-butyrolactone was selected in the organic vehicle later. In conclusion, the organic vehicle was consisted of ternary solvent (50 wt% of terpineol, 40 wt% of DPM and 10 wt% of cyclohexanone) and 5 wt% of EC, 2 wt% of Span-85 and 2 wt% 1,4-butyrolactone (the content of the later three auxiliaries were respected to the ternary solvent).
Fig.2. Influence of the EC content on the viscosity of the ternary solvent at different rotating speed: (a) 3 wt% EC; (b) 4 wt% EC; (c) 5 wt% EC; (d) 6 wt% EC; (e) 7 wt% EC.
Fig.3. Influence of the Span-85 and 1,4-butyrolactone content on the viscosity of the ternary solvent added by 5 wt% EC at different rotating speed. (a) influence of Span-85; (b) influence of 1,4-butyrolactone. 3.1.2. Preparation of pristine graphene The pristine graphene was taken as a conductive filler of the paste in this work and was prepared using supercritical CO2 associated with ultrasonic according to our previously published papers[30, 31]. The structure and morphology of graphene were shown in Fig.4. The lateral size of the graphene sheets was around 2-5 microns as determined by AFM (Fig.4a), which can also be seen from SEM images (Fig.4b). The step height of 2-3 nm of the graphene sheets (Fig.4d) indicates the sheets were around 6-9 layers of numbers which is consistent with the TEM images(Fig.4c). The graphene with good quality provides the fundamental for high conductivity of printed patterns.
Fig.4. The characterizations of pristine graphene. (a) SEM images of graphene; (b) the lateral size distribution of the graphene sheets; (c) TEM of graphene; (b) AFM of graphene. 3.1.3. Rheological property of the graphene paste The graphene paste was formulated by mixing the obtained organic vehicle with the prepared pristine graphene in a ball-milling way at 400 rpm for 4 hours (as shown in Table S1 and Fig.S2). Usually, high loading of the graphene sheets in the paste favors high conductivity of the printed pattern. However, it was found that the paste could not be formed when the loading was 30%. It can be attributed to that the organic vehicle cannot fully infiltrate the graphene. In order to evaluate the printability of the paste, the rheological property of the organic vehicle and the paste with 20 wt% graphene (as a representative) was examined. As shown in Fig.5, the static viscosity of the organic vehicle was 1.92 Pa·s, while the paste was 19.14 Pa·s. It means that the paste had a
higher viscosity than the organic vehicle. Also, the shear stress of the paste reduced under increased shear rate as shown in Fig.5(c), which suggests that the graphene paste was a pseudoplastic fluid. It can be easy to flow through the screen, and the formed pattern would not deform on the substrate during screen-printing. Also, the surface tension of the paste with 20wt% graphene was 30.78 mN/m, which suggested that the paste had good wettability to the surface of PI substrates[32]. The influence of the graphene loading concentration on the performance of the paste was discussed by the resistivity of the screen-printed patterns in the next part.
Fig.5. The rheological properties of the organic vehicle and the graphene conductive paste:(a) the viscosity of the organic vehicle at a different shear rate;(b) photograph of the organic vehicle;(c) the viscosity of the paste at a different shear rate; (d)photograph of the paste. 3.2. The resistivity of the screen-printed patterns The resistivity was applied as the index to evaluate the performance of the printed patterns. The effects of the process parameter on the resistivity were investigated in this paper.
3.2.1. Influence of graphene loading concentration on the resistivity The influence of the graphene loading concentration on the resistivity of the paste was shown in Fig.6a. It can be seen that the resistivity decreased from 0.0185 to 1.8×10-5 Ω·m when the graphene loading concentration increased from 10 to 20 wt% and leveled off beyond 20 wt%. This result can be attributed to the amount of graphene in the pattern. When the graphene loading concentration was low, a conductive network could not be formed in the pattern resulting in poor conductivity. Fig.6b displays that there were many gaps in the pattern. When the amount of the graphene increased, the conductive network of the graphene was gradually formed due to more contacts between the graphene flakes. However, the conductivity would not increase with increasing the graphene loading concentration when the graphene flakes have been fully contacted, such as the case when the loading was 20 wt%. Fig.6c displays that the printing pattern was uniform and did not have the gap on the pattern. Therefore, the 20 wt% of graphene loading of the paste was enough.
Fig.6. The influence of the graphene loading concentration on the conductivity of printed films. (a) The resistivity of films printed by using the pastes with different graphene loading concentrations; (b, c) SEM images of the printed films with (b) 10 wt%
and (c) 20 wt% graphene. 3.2.2. Influence of annealing and mechanical pressing conditions on the resistivity The annealing temperature and time have significant influences on the electrical performance of the printed pattern. As shown in Fig.7a, the resistivity sharply decreased from 4.2×10-5 to 1.6×10-5 Ω·m when the annealing temperature increased from 200 to 350℃. It can be ascribed to two main reasons. One is that the residual solvent was removed at high temperature. The other is that the aromatic compound formed due to the thermal decomposition of EC at high temperature (Fig.7c) [33], which can be verified as shown in Figure 7(d). There were stronger C=O and C=C stretching vibrations at 1724 and 1613 cm-1 indicating the formation of aromatic components[34].
[34] . Therefore the decomposition resulted in a π-π bond stack between the aromatic compound and graphene, which provided efficient charge transport and decreased resistivity through the graphene network[25]. However, the resistivity raised when the annealing temperature was over 350℃. It might be that the removed EC residue damaged the graphene network. Annealing time had a similar influence. The resistivity decreased from 2.2×10-5 to 1.3×10-5 Ω·m when the annealing time increased from 10 to 30 min as shown in Fig.7b. When the annealing time was over 50 min, the resistivity increased slightly. It can be attributed to the slight peeling of the film layer during prolonged annealing period.
Mechanical pressing can increase the conductivity of the printed pattern further because the distance between two graphene layers was reduced. As shown in Fig.7e, the resistivity decreased from 1.7×10-4 to 1.2×10-5 Ω·m when the pressing pressure increased from 0 to 20MPa. Also, 1 min of the pressing period was enough to decrease resistivity as shown in Fig.7f.
Fig.7. Influence of annealing and mechanical pressing conditions on the resistivity of the printed films. (a, b) the resistivity of the printed films plotted against (a) annealing temperature for a fixed annealing time of 30 min and (b) annealing time at a fixed annealing temperature of 350 ℃;(c) TGA curves of EC, an organic vehicle, and graphene conductive paste; (d)FTIR curves of the EC before and after thermal annealing in different temperature; (e, f) the resistivity of printed films plotted against (e) pressing pressure for a fixed pressing time of 1 min and (f) pressing time at a fixed pressing pressure of 20 MPa. 3.2.3. Influence of screen-printing conditions on the resistivity
The influence of screen-printing times on the resistivity was shown in Fig.8a. It can be seen that the resistivity sharply decreased from 5.4×10-4 to 1.5×10-5 Ω·m and gradually leveled off when the screen-printing times increased from 1 to 10 times. It can be ascribed to the improved continuity of the printed patterns with increasing the printing times, as shown in Fig.8e-f. High uniform and continuous patterns with low resistivity were obtained after 4 printing times. The line width of patterns also influenced resistivity. The lines with different width were obtained by printing 4 times, as shown in Fig.8c. The resistivity decreased from 3.2×10-4 to 2.7×10-5 Ω·m when the width increased from 0.5 to 2 mm. Therefore, the line width should not be less than 1.5 mm to obtain good conductivity.
Fig.8. Influence of screen-printing conditions on the resistivity of the printed films. (a, b) the resistivity of the printed films plotted against (a) print times and (b) the width of the line at a fixed print time of 4 times. (c) Photograph of printed patterns of the lines with different width by 4 times screen printing; (d, e) SEM images of printed films after (d)1 time printing and (e) 4 times printing. And flexibility assessment of the printed graphene electrodes on PI substrates:(f) the resistivity of the printed pattern after different bending cycles; (g) photograph of the pattern folded inward 180 °.
By previous experiments, a pattern was made using the paste containing 20 wt% pristine graphene at the conditions of screen-printing for four times, drying at 70 ℃ for 1h, annealing at 350 ℃ for 30 min and mechanical pressing at 20 MPa for 1 min. The resistivity of the pattern was 1.04×10-5 Ω·m at the thickness of 10 μm, and the conductivity was 9*10^4 S/m, which is the highest value achieved so far using screen-printing of the pristine graphene [9, 22, 23, 35]. The paste has good stability because it would not precipitate even though after storing for two months. Also, the resistivity of the printed pattern made from the paste stored for two months was 2.83×10-5 Ω·m at the thickness of 7 μm and is similar to that of the pattern made from the initial paste. Also, the printed graphene patterns on PI substrates had excellent mechanical flexibility as shown in Fig.8g-f. Even though the pattern was folded at 180° for 1000 cycles, the resistivity only increased from initial 1.9×10-5 Ω·m to 3.2×10-5 Ω·m, and no obvious cracks were observed on the patterns. 4. Conclusions A conductive paste with high pristine graphene loading concentration was formulated. It demonstrates that the formulated paste can be screen-printed on the PI substrate to form flexible patterns with high conductivity. The paste was stable for at least two months without apparent precipitation. The formulated graphene paste is expected to have significant potential applications in fabricating flexible electric devices. Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grants No. 21576165). We are thankful for Ms. Huiqin Li of Instrumental Analysis Center of SJTU for analysis. Declarations of interest: none References [1] Chang W Y FTH, Lin H J, et al. . A Large Area Flexible Array Sensors Using Screen Printing Technology. Journal of Display Technology. 5（2009）178-83. [2] Karuwan C, Wisitsoraat A, Phokharatkul D, Sriprachuabwong C, Lomas T, Nacapricha D, et al. A disposable screen printed graphene–carbon paste electrode and its application in electrochemical sensing. RSC Advances. 3（2013）25792. [3] Liu HW, Liang SP, Wu TJ, Chang H, Kao PK, Hsu CC, et al. Rapid atmospheric pressure plasma jet processed reduced graphene oxide counter electrodes for dye-sensitized solar cells. ACS applied materials & interfaces. 6（2014）15105-12. [4] Xu C FM, Willenbacher N. Impact of Wall Slip on Screen Printing of Front-Side Silver Pastes for Silicon Solar Cells. IEEE Journal of Photovoltaics. PP（2017）1-7. [5] Ahn HY, Kim J-G, Gong M-S. Preparation of flexible resistive humidity sensors with different electrode gaps by screen printing and their humidity-sensing properties. Macromolecular Research. 20（2012）174-80. [6] Wang S, Liu N, Yang C, Liu W, Su J, Li L, et al. Fully screen printed highly conductive electrodes on various flexible substrates for asymmetric supercapacitors. RSC Advances. 5（2015）85799-805.
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Highlights 1.
A conductive paste has been formulated based on high pristine graphene loading
2.
The graphene paste shows good rheology and stability for screen-printing
3.
The screen-printed pattern has a high conductivity of 9×104 S/m and excellent mechanical flexibility
Graphical abstract