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Ultrasonic-Assisted Sintering of Silver Nanoparticles for Flexible Electronics Fuliang Wang, Nantian Nie, Hu He, Zikai Tang, Zhexi Chen, and Wenhui Zhu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09581 • Publication Date (Web): 04 Dec 2017 Downloaded from http://pubs.acs.org on December 4, 2017

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Ultrasonic-Assisted Sintering of Silver Nanoparticles for Flexible Electronics

Fuliang Wang,1,2 Nantian Nie,2 Hu He,1,2,* Zikai Tang,2 Zhexi Chen,2 Wenhui Zhu1,2

1

State Key Laboratory of High Performance complex Manufacturing, Changsha 410083, China

2

School of Mechanical and Electrical Engineering, Central South University, Changsha 410083,

China *Email: [email protected] *Tel: +86 186-7032-7639

1

ACS Paragon Plus Environment

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ABSTRACT: Silver nanoparticles sintering method is promising for the fabrication of flexible electronics. An ultrasonic-assisted sintering process is presented to obtain conductive patterns with low resistivity on paper-based substrate. Compared with conventional hot-press sintering process, the proposed method can efficiently fabricate densified microstructures with better electrical property at low temperature and pressure. In particular, the effects of ultrasonic effective time, entry time and amplitude were systematically analyzed. It was found that lower resistivity preferred larger ultrasonic amplitude. The ultrasonic effective time happened during the first 3 min of entire sintering process which implied ultrasonic synergetic effect occurred at the very beginning stage of sintering process. Additionally, the sooner ultrasonic entry time was, the better conductivity of sintered patterns was obtained. The mechanism of synergetic effect of ultrasonic in sintering was also discussed in terms of vibration and cyclic stress perspectives. The proposed ultrasonic assisted sintering method is believed to provide insights for the fabrication of flexible electronics.

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INTRODUCTION

Recently, flexible electronic devices have drawn remarkable attention, introducing large amounts of work on foldable displays, flexible batteries, and wearable sensors and high-performance electrodes.1-3 Due to cost-effective and process-efficient properties, sintering method is widely employed to fabricate the desired electrical patterns onto soft/flexible substrates in the last decades. However, high temperature and pressure are usually required in traditional sintering approaches.4-7 Unfortunately, most of the flexible substrates cannot tolerate with these process conditions. Therefore, compared with the traditional hot-pressuring approaches, several improved sintering methods, such as microwave-assisted, direct current-assisted and laser assisted sintering techniques8-10 were proposed to alleviate the requirements on high sintering temperature, pressure and time. Nevertheless, those techniques inevitably introduce the instant high temperature that would induce the early damages on some substrates such as paper and polymer. In addition, either expensive equipment or complex procedures is required for those methods. Although several room temperature and pressureless sintering methods have been also proposed,11,12 these methods mainly focus on self-sintering using chemical agents, with which it is difficult to optimize the conductive structure during the sintering process.13 Thus, the compactness of the sintered conductive pattern and the electrical as well as mechanical properties might not be configured. To avoid those issues, and inspired by the effect of ultrasonic during the flip chip bonding in microelectronic packaging,14 an ultrasonic was introduced into the sintering process to realize the sintering under low temperature and pressure. To our best knowledge, there is little work on sintering using a ultrasonic synergetic effect. Although Li et al.15 reported a sintering method with ultrasonic assistance, the ultrasonic was introduced prior to the sintering process. In fact, the 3



ultrasonic was isolated from the actual sintering process, which hindered the synergetic effect between ultrasonic and sintering processes. Our proposed method introduces ultrasonic during the sintering process which is fundamentally different from the prior work. In this work, the ultrasonic was introduced into the Ag NPs sintering process as an extra sintering driving force, whose performance was compared with that using traditional hot-pressing sintering. Additionally, the effects of time and amplitude of ultrasonic on the electrical resistivity of sintered structures were presented. Then, the entry time of ultrasonic was discussed with respect to the sintering performance. Finally, the potential improvement mechanism using ultrasonic during sintering was addressed.

MATERIALS AND METHODS

Materials and Preparation of Conductive Ink: Ag NPs whose purity is 99.9% and average size is 20 nm were ordered from Shanghai Chao Wei Technology Co., Ltd. The organic solvent, including PEG-600 as the solvent, hydrogenated castor oil as the stabilizer and glycerol as the thickener, was purchased from Sinopharm Chemical Reagent Co., Ltd. EPSON photo papers (S042551) were employed as the substrate. Regarding to the preparation of conductive ink, Ag NPs (4 g) were first placed into the mixed solvent consisting of PEG-600 (5.5 g) and hydrogenated castor oil (0.5 g). Then, a few droplets of Glycerol were added into the mixture to adjust the wettability and surface tension of the prepared Ag NPs conductive ink. Finally, the prepared ink was stirred using agitator for 3 min and ultrasonic process for 10 min to obtain the ready-to-use condition. Instrumentation: The desired pattern was obtained using a 3D printing system that consists of a power and writing device as shown in Figure 1a. The printed conductive lines on the substrate were 4


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dried at the room temperature prior to sintering process. In order to study the effect of ultrasonic during the hot-pressing sintering process, a dedicated platform (as shown in Figure 1b) was designed, which can control the sintering temperature, pressure as well as the ultrasonic amplitude simultaneously. In our experiment, the frequency of ultrasonic was a constant (40 K Hz) due to the equipment limitation. The microstructures of sintered Ag NPs tracks were observed using field emission scanning electron microscopy (SEM) and the resistance was measured using a four-probe DC low resistance meter.

Figure 1. (a) 3D printing system to print conductive patterns. (b) Ultrasonic-assisted hot-pressing to sinter conductive structures.

Calculation of Resistivity. In order to exclude the bias of geometrical variation of printed tracks, the resistivity (ρ) was calculated using the formula (1) to evaluate the electrical performance. 5



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The direct resistance measurement process was addressed in Figure S1 (Supporting Information).

(1)

where ρ is the resistivity, R is the resistances of sintered, L is the length of sintered Ag NPs lines and A is the cross-sectional area of sintered Ag NPs lines. To obtain accurate resistivity, the actual cross-sectional area was required. The typical sintered sample is shown in Figure 2a. Figure 2b illustrates one of the cross-sectional microstructures of sintered lines. Image processing algorithm from Matlab was applied to compute the exact area of cross-section, as shown in Figure 2c. For the statistical purpose, all the data under different experimental conditions were collected at least 5 times. Note that the variation of cross-sectional area of sintered tracks in our experiments with or without ultrasonic is insignificant. More details can be referred in Figure S2 (Supporting Information).

Figure 2. (a) A sample with sintered Ag NPs lines on Epson paper. Scale bar = 1 cm. (b) Cross-sectional SEM image of a sintered Ag NPs line. Scale bar = 200 µm. (c) Extracted cross-sectional area of sintered Ag NPs line using image processing with Matlab software. Scale bar = 200 µm.

RESULTS AND DISCUSSION

The Impact Effects of Ultrasonic Time and Amplitude. To find out the effect of ultrasonic 6


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during the sintering process, the printed Ag NPs lines were first sintered under the same temperature (120 ℃) and pressure (10 MPa) with different sintering time using traditional hot-press sintering method. While in comparison group, the ultrasonic whose amplitude is 12 µm was employed during the sintering process. Figure 3a shows the average resistivity of sintered samples using conventional hot-pressing process and ultrasonic-assisted sintering method. In addition, the resistivity difference is illustrated in Figure 3a. Obviously, the ultrasonic decreased the average resistivity of sintered Ag NPs lines compared with that using hot-pressing sintering process. It implies that ultrasonic-assisted sintering process can improve electrical properties of sintered Ag NPs lines. Interestingly, the resistivity difference varies insignificantly after 3 min, which indicates extending ultrasonic time might not be necessary. In our experiment, the effective ultrasonic time is approximate 3 min. In other word, the synergetic effect of ultrasonic is insignificant after 3 min. With respect to the effect of ultrasonic amplitude on the resistivity of sintered Ag NPs lines, an array of Ag NPs lines was sintered using ultrasonic-assisted method under the same hot-pressing conditions (10 MPa in pressure and 120 ℃ in temperature) for 7 min with different ultrasonic amplitude. The amplitude of employed ultrasonic was configured as 3 µm, 6 µm, 9 µm and 12 µm, respectively. As shown in Figure 3b, the average resistivity of sintered Ag NPs lines followed a downward trend with the growth of ultrasonic amplitude. Meanwhile, the SEM images of sintered lines (More SEM images of our sintered samples are shown in Figure S3, Supporting Information) indicate that larger ultrasonic amplitude would lead to denser microstructure (Figure S4, Supporting Information, demonstrates the relationship between ultrasonic amplitude and porosity), which is consistent with the electrical resistivity pattern. Therefore, larger ultrasonic amplitude is benefit for the electrical conductivity of sintered Ag NPs lines. 7



Figure 3. (a) Average resistivity of sintered Ag NPs lines against different sintering time using conventional hot-pressing method and the proposed ultrasonic assisted hot-pressing method. The sintering temperature and pressure are set as 120 ℃ and 10 MPa, respective. The black plot is the resistivity difference of samples using hot-pressing and ultrasonic-assisted method. (b) Average resistivity of sintered Ag NPs lines versus different ultrasonic amplitude under the same sintering temperature (120 ℃), pressure (10 MPa) and time (7 min). Insets are SEM images corresponding to microstructures under the ultrasonic amplitude as 0 µm, 6 µm and 12 µm, respectively. The scale bar is 1 µm for all SEM images. Scale bar = 1 µm. Analysis of The Action Mechanism of Ultrasonic. To understand the synergetic effect of ultrasonic during the sintering process, the potential mechanism was discussed in terms of vibration and stress perspectives. It is well known the sintering process is the atom diffusion driven by internal and external energy or force.16 Denser microstructure would improve the electrical conductivity, which indicates the densified sintering is the key to secure better electrical property. As shown in Figure 4, ultrasonic induced vibration would potentially redistribute the Ag NPs uniformly which increases the effective contact area between Ag NPs in comparison to conventional hot-pressing sintering. Thus, more sufficient atom diffusive channels would lead to denser microstructure with 8


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less pores using ultrasonic assisted sintering. Additionally, larger ultrasonic amplitude indicating larger vibrational energy would intensively enhance the activation of Ag NPs which furtherly promotes effective contact area that eventually improves the sintered lines with denser structures. Therefore, better electrical conductivity can be obtained under higher ultrasonic amplitude.

Figure 4. The schematic of the comparison between hot-pressing and ultrasonic-assisted sintering methods. Scale bar = 1 µm.

In addition to the vibrational effect of ultrasonic, ultrasonic energy introduces varied stress between particles which would induce more crystal defects of Ag NPs, such as dislocation. The dislocation provides fast atom diffusive channels.17-19 With respect to ultrasonic assisted sintering method, the conventional hot-pressing sintering introduces approximate constant stress between Ag NPs which generates less dislocation defects. As shown in Figure 5, molecular dynamics simulations using LAMMPs software were conducted to demonstrate the sintering process under hot-pressing as well as ultrasonic assisted conditions. To simulate the sintering process of the Ag NPs, the Ag-Ag EAM (embedded atom method) based on Chen et al.20 modified EAM potential21 was employed to model the interactions of Ag atoms. In the ensemble, the initial velocity of atoms in the Ag NPs was 9



assigned according to Maxwell-Boltzmann distribution and the temperature was calculated using Nose-Hoover thermostat22. Additionally, the Verlet algorithm was applied as the integration algorithm for efficient calculation. Periodic boundary condition and NVT ensemble with the step as 0.002 ps were used in our simulation. Figure 5a shows two Ag NPs with diameter as 20 nm are aligned together before sintering. Only a half of particle is modelled for computation efficiency. In Figure 5b, two 20 nm Ag NPs have been sintered under constant stress condition representing hot-pressing sintering. During the hot-pressing simulation, the Ag NPs model was relaxed 40000 steps at 300 K to achieve a balance state followed by a sintering process under 425 K. Then, a constant speed was loaded to one Ag NP, while the other Ag NP was fixed until the stress between the Ag NPs reach to 10 MPa. While in Figure 5c, cyclic stress in a range from 0-10 MPa is applied to simulate the ultrasonic assisted sintering. To simulate the vibration of ultrasonic, the same relaxation and loading process in the hot-pressing simulation were carried when the stress between Ag NPs reach to 10 MPa. In the following, the stress between two Ag NPs was unloaded until the stress between the Ag NPs decreases to 0 MPa. The load and unload process were repeated then with the cyclic frequency as 3.168×107 Hz (In our MD simulation, one cycle of load and unload process costs 31680 ps). Compared with constant stress, cyclic stress introduces more dislocation defects between Ag NPs which enhances atom diffusion towards denser microstructure. Furthermore, the dislocation lines in Figure 5b, mainly situating inside of Ag NPs, are short and discontinuous. In contrast, the dislocation lines concentrate across the contact area between two Ag NPs, which indicates ultrasonic promotes more atom diffusive channels preferring densified sintering. Thus, lower resistivity is obtained when the ultrasonic is present during sintering. In addition, larger ultrasonic amplitude would enhance dislocation generation. Consequently, denser microstructure of sintered lines is 10


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formed.

Figure 5. Ag NPs sintering process simulation using MD method. (a) The initial condition of Ag NPs before sintering, (b) the stage of sintered Ag NPs with constant stress after 100 simulation timestamps and (c) the stage of sintered Ag NPs with cyclic stress after 100 simulation timestamps.

The Impact Effects of Ultrasonic Entry Time As mentioned, the synergetic effect of ultrasonic is insignificant after 3 min. In our experiment, the effective ultrasonic time is approximate 3 min. It is suggested that longer ultrasonic time beyond 3 min is not necessary, which implies the capability of atom diffusion during sintering is saturated when the effective ultrasonic time is reached. When the entire sintering time is greater than 3 min, it is significant to determine the optimal time window to introduce ultrasonic in sintering process. As shown in Figure 6, the entire sintering time was set as 7 min, and the ultrasonic entry time was configured to be the 0, 2 and 4, respectively. Specifically, ultrasonic was introduced into the sintering at the very beginning, the second minute and the fourth minute, respectively. The later the entry time is, the larger resistivity obtains. This trend is supported by corresponding SEM images, which indicates denser microstructure is obtained when the ultrasonic is employed earlier. Prior to 11



introducing ultrasonic, conventional hot-pressing sintering dominates the procedure. The synergetic effect of ultrasonic degrades with respect to partially sintered structures. In other word, the formed pores in the partially sintered structures during conventional hot-pressing sintering cannot be eliminated, which suggests the ultrasonic requires to be introduced at the very beginning to increase the effective contact area between Ag NPs.

Figure 6. The average resistivity of sintered Ag NPs lines under the same conditions (sintering temperature as 120 ℃, sintering pressure as 10 MPa, sintering time as 7 min and ultrasonic time as 3 min with the amplitude as 12 µm) with different ultrasonic entry time. Insets are SEM images corresponding to microstructures under the ultrasonic entry time as 0, 2 and 4, respectively. The scale bar is 1 µm for all SEM images. Scale bar = 1 µm.

CONCLUSIONS

In summary, an ultrasonic-assisted sintering method was presented to improve the electrical 12


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conductivity in comparison to conventional hot-pressing sintering. The synergetic effect of ultrasonic during sintering was discussed in terms of vibration and stress perspectives. Vibration energy promotes the uniform distribution of Ag NPs which provides larger effective contact area between Ag NPs. Cyclic stress induces more dislocation defects along the boundary between Ag NPs which offers more atom diffusive channels. Additionally, the effective ultrasonic time and the entry time of ultrasonic were also discussed. It is believed that the proposed sintering method is significant for the fabrication of conductive patterns which is useful for flexible electronics.

ASSOCIATED CONTENT Supporting Information The method of resistance measurement; average cross-sectional area of samples sintered under different conditions; SEM images of different samples; Method to obtain porosity of different samples.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors are grateful to the National Natural Science Foundation of China (51605497) and the 13



National Basic Research Program (2015CB057202) and Fundamental Research Funds for the Central Universities of Central South University (2017zzts398) for their supports.

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Ultrasonic-Assisted Sintering of Silver ... - ACS Publications

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