Vibration-Induced Property Change in the Melting and Solidifying Process of Metallic Nanoparticles
© The Author(s). 2017
Received: 16 March 2017
Accepted: 16 April 2017
Published: 26 April 2017
Tuning material properties in the 3-D printing process of metallic parts is a challenging task of current interests. Much research has been conducted to understand the effects of controlling parameters such as the particle geometry (size and shape), heating, and cooling ways on the outcome of the printing process. However, nothing has been done to explore the system vibration effect. This letter reports our findings on the vibration-induced property change in the melting and solidifying process of silver nanoparticles with the use of molecular dynamics simulation. We find that the increase of system vibration magnitude would increase the number fraction of disordered atoms, which in turn changes the nanostructure of solidified products. For a given system vibration magnitude, the number fraction of disordered atoms reaches the maximum around the system natural frequency so that the stiffness of solidified products becomes the minimum. Since this trend is not affected by the system size, the above findings reveal a feasible path toward the real-time tuning of material properties for advancing additive manufacturing.
Various 3-D printing technologies have been developed in recent years . Among them, the 3-D printing methods based on the laser melting and laser sintering techniques can effectively process nanocrystalline metals [2, 3], which are playing an increasingly important role in the rapid processing and manufacturing of advanced micro-/nano-devices. Due to the small characteristic size and light weight of metallic nanoparticles, however, the sintering behavior and mechanism of these particles have fundamental differences from those of the conventional metallic particles. In addition, they can also be influenced by various environmental factors (such as heating and cooling rates, system vibration), which will eventually alter the physico-mechanical properties of the final sintered products. Thus, studies on the sintering behavior and mechanism of metallic nanoparticles are of great scientific significance and practical value.
To understand the sintering, including the melting and solidifying, process of nanoparticles, extensive experimental studies have been conducted worldwide. For examples, Link et al. found experimentally the morphological changes of spheroidal gold nanoparticles or nanorods into nanospheres via melting at moderate laser energies [4, 5]. Kim and Jang observed that picosecond laser pulses can induce the nanowelding of gold nanoparticles and the formation of single-phased nanocontact, which is useful for the fabrication of ohmic contact . Ko et al. demonstrated that the low-temperature metal deposition as well as high resolution pattern can be achieved via the laser sintering of inkjet-printed metal nanoparticles, which overcomes the resolution limitation of the current inkjet direct writing processes . Moreover, molecular dynamics (MD) simulations have also been widely used to investigate the sintering behavior of various metallic nanoparticles. Raut et al. studied the sintering of aluminum nanoparticles by MD simulations, and they found that the increase of the particle size may slow down the sintering kinetics . Zeng et al. performed MD simulations to study the surface energies, grain boundary mobility, and sintering of copper and gold nanoparticle arrays at different temperatures, and their results show unexpected contributions from plastic deformation, mechanical rotation, amorphization, and ultrarapid diffusion effects . Koparde and Cummings demonstrated that the dipole-dipole interaction between sintering titanium dioxide nanoparticles plays a very important role at the temperature far from the melting point . Wang et al. studied the ultrafast laser interaction with free gold nanoparticles using the MD and two-temperature models, and they found that a nonhomogeneous surface premelting mechanism is dominant at a low laser intensity and the appearance of a contiguous surface liquid layer is size dependent . Jiang et al. studied the necking growth behavior in laser sintering of hollow silver nanoparticles under different heating rates, and they revealed that the melting temperature vs. heating rate shows an inverse trend in all the hollow nanoparticle pairs at an ultrahigh heating rate as compared to that in the solid particle cases . Moreover, Arcidiacono et al. demonstrated the reliability of MD simulations for the sintering of metallic nanoparticles based on the results for the necking growth obtained from both simulations and theoretical predictions . Although many efforts have been made to understand the sintering behavior of metallic nanoparticles, the previous studies were mainly focused on the melting process. To the authors’ knowledge, little has been done to explore the environmental effects on the sintering behavior and corresponding mechanical features of the final products. However, the environmental factors, such as the vibration of the experimental platform, might greatly affect the melting and solidifying process and eventually the microstructure of the final products. Furthermore, it is the physico-mechanical properties of the sintered products that are of practical importance.
In this paper, the environmental effects on the melting and solidifying behaviors and corresponding material properties of sintered products obtained from silver nanoparticles are investigated using MD simulations. The influences of the temperature conditions (including the final heating temperature and heating rate), vibration factors (including amplitude and frequency), and the size of nanoparticles are reported.
The interactions between silver atoms are described by the embedded atom method . The simulations are conducted by using the open-source large-scale MD simulator LAMMPS . The nanoparticles are initially relaxed at 0 K separately using the conjugated gradient method and then are put together to form the simulated systems (as shown in Fig. 1a). During the laser sintering process, the system is first thermally equilibrated at 298 K using the Nosé-Hoover thermostat [15, 16] for 1 ns to mimic the solid-state sintering. The system temperature is then increased linearly via the velocity rescaling method with a constant heating rate to model the laser sintering process. After reaching a specified temperature, the system is equilibrated at that temperature for another 1 ns. Subsequently, the system temperature is decreased linearly to 298 K with a cooling rate that is the same as the heating rate. Finally, the cooled specimen is equilibrated at 298 K for 1 more nanosecond to obtain the final sintered product. During the laser sintering process, the effects of the heating/cooling rate and system vibration on the sintering behavior are explored. To evaluate the mechanical properties of the sintered product, compressive tests with a planar indenter are carried out, as shown in Fig. 1b. For all simulations, a time step of 2 fs is used. The common neighbor analysis method [17, 18] is used to determine the local crystalline order of silver atoms in order to identify the dislocation core, stacking fault, deformation twin, and nanostructure evolution [19, 20] which can be visualized by OVITO .
Results and Discussion
Melting and Solidifying Process of Silver Nanoparticles
Number fraction of atoms with different local crystalline orders at 1098, 738, 668, and 298 K during the cooling process
Mechanical Properties of Sintered Nanoparticles
Number fraction of atoms with different local crystalline orders in the products sintered under different heating rates
Heating rate (K/ps)
Number fraction of atoms with different local crystalline orders in the products sintered under different vibration amplitudes
Number fraction of atoms with different local crystalline orders in the products sintered under different vibration frequencies (R = 5a)
Number fraction of atoms with different local crystalline orders in the products sintered under different vibration frequencies (R = 10a)
In this work, atomistic simulations have been conducted to investigate the melting and solidifying behaviors of silver nanoparticles and the mechanical properties of the final products sintered under different environments. The influences of the heating/cooling rate and the vibration amplitude and frequency have been explored. Major findings are summarized as follows: (1) Solid-state sintering occurs when nanoparticles are relaxed at the room temperature and the adjacent particles are close to each other, which is accompanied by rotation and eventually forming dumbbell-like solid products. With the increase of temperature, dumbbell-like products begin to melt. When the maximum temperature exceeds 1098 K, silver nanoparticles completely melt and form a single liquid nanoparticle. In the cooling process, the liquid particle gradually solidifies and this process consists of two main phases, i.e., the nucleation phase and crystal growth phase. The final products equilibrated at the room temperature are mostly polycrystalline materials with grains separated by grain boundaries, stacking fault, twin boundary, etc. (2) The higher the heating rate, the later the initial fusion of nanoparticles and the lesser the time of fusion stage. (3) External vibration could affect the evolution of nanostructures of the sintered products. The number fraction of disordered atoms in the sintered products increases monotonically with the increase of the vibration amplitude for a specific frequency, and reaches the maximum as the vibrational frequency approaches the natural frequency of the system for a given amplitude. As a result, the material properties of sintered products depend on both vibration frequency and amplitude. (4) The plastic deformation of sintered products under compression is mainly dominated by grain boundary and partial dislocation activities. (5) The above phenomena appear to be independent of the simulation system size. Although the present results provide many insights to understand the sintering behaviors of more realistic multiple-particle systems, it should be noted that only limited ranges of particle size and number, heating/cooling rate, vibration frequency, and amplitude have been considered in the present work. In the future, multiscale investigation is required to design a feasible path toward the real-time tuning of material properties in the 3-D printing process of metallic parts.
The authors would like to thank the Program for New Century Excellent Talents in University (NCET-13-0088), the National Natural Science Foundation of China (Nos. 11672062, 11232003), and the 111 Project (B08014) for their support, and the Fundamental Research Funds for the Central Universities are gratefully acknowledged.
ZC and YGZ conceived this study; YGZ, LQD, and HFY conducted the simulations; YGZ, LQD, HFY, and ZC analyzed the data, discussed the results, and wrote the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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