Influence of Processing Conditions for Nickel Particles Prepared by Pulsed Microwave Heating in Liquid

Abstract

A technique, pulsed microwave heating in liquid, is examined under the condition for time- and space-selective heating of nickel particles dispersed in liquid. This technique is modified from the pulsed laser melting in liquid for submicrometer spherical particle fabrication. When nickel particles that are 50 µm in size or larger were dispersed in low-dielectric-loss hexane with a concentration of 2 g L⁻¹ or higher, particle became large with smooth surface by microwave irradiation, suggesting that the particle surface temperature was elevated over the melting point. Pulsed microwave irradiation with short pulse width and large pulse frequency is effective for the fabrication of particles with smooth surfaces. Thus, pulsed microwave melting in liquid would provide a new processing technique at high temperatures in a liquid environment.

1. Introduction

A new technique called pulsed laser melting in liquid (PLML) has been developed for fabricating submicrometer spherical particles of various materials.^1–6) Submicrometer particles are relatively difficult to obtain by conventional particle fabrication techniques. Particles obtained by PLML show unique optical, medical, mechanical, and magnetic properties and have special applications.^7–9) Furthermore, the submicrometer spherical particles are highly crystalline despite the spherical shape. Therefore, the fabrication process of these particles is quite interesting from an engineering aspect.^10,11)

In PLML, raw nanoparticles are dispersed in liquid, and pulsed laser light is irradiated. During laser irradiation, instead of liquid, particles are selectively heated over the melting point of materials because of the difference in optical absorption efficiency, thus resulting in the formation of spherical particles due to the surface tension of melted particles. The principle of this technique is the space-selective pulsed heating of target materials.¹¹⁾ When particles become smaller than the laser wavelength, absorption efficiency would decrease to zero, and particles would not be heated. By contrast, when particles become large, their temperatures would not be high under the same laser irradiation condition because of the large heat capacity of particles. From the balance between these two effects, submicrometer-sized particles are generated because of their facile heating at this size range.

Pulsed laser is also another important factor because the energy required to melt one single particle has to be accumulated into the particle before the obtained energy dissipates to the surroundings. The time scale of heat dissipation is several hundreds of nanoseconds to microseconds. Therefore, nanosecond pulsed lasers or those with shorter pulse widths are effective as a tool for applying sufficient energy to particles before heat dissipation.^12–14) Thus, lasers play important roles in realizing space-selective pulsed heating for submicrometer spherical particles. This technique is an opposite approach for conventional large-size, long-time-scale heating processes such as mantle heating and furnace heating. However, the mass production of submicrometer spherical particles is difficult because of the small laser beam size with sufficient beam energy and expensive beam cost.

If space-selective pulsed heating can be realized using other energy sources, we may be able to produce spherical particles in large amounts in a mass production level. A microwave is basically reflected by bulk metals without energy transfer. In fact, it penetrates into the metal surface a few micrometers in depth while its energy is absorbed in the metal. Thus, metallic objects at a micrometer scale, such as thin films or particles, can be heated by microwave irradiation. On the basis of this effect, the microwave processing of metal-based materials has recently been studied intensively as a new approach for the thermal processing of metals.^15–18) However, the trial for the non-continuous pulsed microwave heating of metals in liquid has been rarely reported before.

Figure 1 presents the concept for pulsed microwave heating in liquid. Space-selective pulsed heating was realized using microwave irradiation. Space-selectivity can be achieved not by the size of the energy source but by the size of the energy absorbers. Microwave absorbing materials include metals, magnetic materials, and dielectric materials with different heating mechanisms by microwave, i.e., induction heating, magnetic heating, and dielectric heating. Even the surrounding liquid can absorb microwaves depending on permittivity or the dielectric loss of the liquid. Therefore, the selections of target materials and surrounding liquid are quite important for space-selective heating.

Fig. 1

Concept of space-selective pulsed microwave heating in liquid.

Pulsed heating (time selectivity) is also another important point for pulsed microwave heating in liquid. Various types of lasers with different pulse widths from milliseconds to femtoseconds and with differences of 12 orders of magnitude are available. However, nanosecond lasers and picosecond lasers are typically effective for a thermal process such as PLML because of the electron-phonon coupling period of around several hundreds of femtoseconds to several picoseconds.¹⁹⁾ PLML as a thermal process has to be longer than this period. By contrast, heat dissipation increases at a longer pulse width than at several hundreds of nanoseconds.¹²⁾ Lasers with longer pulse widths than this value are not effective for heating only particles even though the energy is space-selectively applied.

Pulsed microwave power supplies are only recently becoming available in the market and are still quite limited. Furthermore, the pulse width for such a power supply is only approximately several microseconds or longer; this pulse width is longer than the ideal case in PLML. In this study, we experimentally clarified the possibility of pulsed microwave heating in liquid for the fabrication of metal particles on the basis of the space-selective pulsed heating of suspended particles in liquid.

2. Experimental Procedure

2.1 Materials

Raw nickel particles with different sizes of <1 µm (Sigma-Aldrich 268283), <50 µm (Sigma-Aldrich 266981), and <150 µm (Sigma-Aldrich 266965); were dispersed in hexane (Kanto Chemical Co.) by using ultrasonic bath. Other solvents (toluene, acetone, ethanol, and water) were also used for comparison.

2.2 Microwave irradiation

A single-mode resonator microwave equipment with a magnetron oscillator was used by combining it with a function generator (Gw INSTEK, AFG-2105) for pulsation control. The important specifications for this equipment included an output of 0–1500 W, frequency of 2.45 GHz, pulse frequency of 0–20 kHz, and duty of 0%–100%.

The suspensions containing raw particles were filled in a test tube (inner diameter: 15 mm) made of quartz that did not absorb microwave. The test tube was placed at the E_max position in the resonator. Microwave was irradiated under various conditions of power, pulse frequency, and duty according to the change in particle size, raw particle concentration, and solvent. Microwaves were directly irradiated onto liquid containing raw particles without any interference by the container wall.

3. Results and Discussions

3.1 Effect of raw particle size and concentration

Figure 2 summarizes the effects of raw particle size and concentration on the morphologies of products by continuous wave (CW) microwave irradiation to check the space-selective heating effect. In this study, we selected hexane as a solvent with low absorptivity to microwave. From the SEM images of raw nickel particles with different sizes (1, 50, and 150 µm), the images with 1 µm particles were almost always composed of single grains, whereas those with 50 and 150 µm particle sizes were aggregated polycrystalline particles. At the nickel concentration of 0.2 g L⁻¹ after microwave irradiation, negligible morphological changes were observed for all particles with different sizes even though this concentration is a typical concentration for PLML when obtaining submicrometer spherical particles in high yield.^1–4) By increasing the concentration to 2 g L⁻¹, the surfaces of raw particles with 50 and 150 µm became smooth, thus indicating that the particles melted during microwave irradiation. By further increasing the concentration to 20 g L⁻¹, the surfaces of all particles from different-sized raw particles became smooth. Therefore, large particles in high concentrations are needed for effective microwave absorption. A suitable concentration range for melting by microwave irradiation could be two orders of magnitude larger than that for melting by laser irradiation.

Fig. 2

Effects of raw particle size and concentration on the products. Microwave irradiation conditions: 200 W, 20 min CW. Solvent: hexane. Scale bar: 1 µm.

3.2 Effect of surrounding liquid

Figure 3 shows the surrounding liquid effect on the products by microwave irradiation on Ni powders (20 g L⁻¹) dispersed in various liquids with different permittivities. The microwave irradiation conditions were 200 W for 20 min CW. Particles became large and smooth in hexane and toluene, thus clearly indicating that particles were melted. By contrast, particle morphologies in acetone, ethanol, and water did not change greatly, although some parts of the particle surface appeared to be slightly smooth. Table 1 summarizes the parameters for the dielectric properties of the used organic solvents. The above experimental results of a clear morphological difference between toluene and acetone appear to be related to the difference in the imaginary part of complex permittivity, the dielectric loss term. These results suggest that particles dispersed in liquid with low dielectric loss were selectively heated over the melting point, whereas those with large dielectric loss remained at lower temperatures than the melting point. In the case of a liquid with large dielectric loss, the applied energy by microwave irradiation was absorbed near the liquid surface and mainly heated the surrounding liquid. Therefore, the temperature distribution was not space selective compared with that of low-dielectric-loss liquid. By choosing appropriate raw particle sizes, concentrations, and surrounding solvents, the space-selective heating of Ni particles could be achieved.

Fig. 3

Effect of solvent on the products. Microwave irradiation conditions: 200 W at 20 min CW. Raw particles: Ni (50 µm), 20 g L⁻¹.

Table 1 Real (ε′) and imaginary (ε′′) parts of complex permittivity, as well as permittivity loss (tan δ) of organic solvents and water at 2.45 GHz.²⁰⁾

3.3 Effect of microwave pulse frequency

For pulsed heating, microwave output has to be pulsed by controlling the pulse frequency and duty (output time fraction in a single cycle) independently. Figure 4 shows the effect of microwave pulse frequency on product morphology by fixing the duty at 10% by using the same raw particles. In this case, the microwave power that was actually applied to the particle-dispersed hexane was fixed at 20 W. Thus, the pulse width and pulse power were changed to (a) 200 µs and 40 mJ, (b) 10 µs and 2 mJ, and (c) 5 µs and 1 mJ in Fig. 4. By using this change in microwave irradiation condition, the product morphologies varied from a stick-like shape in (a) to a spherical shape in (b) and (c). This suggests that a long pulse width induced the melting and merging of spherical particles to form rod-like array structures, whereas a short heating time was suitable for isolated spherical particles.

Fig. 4

Effect of pulse frequency on the products. Microwave power: 200 W for 30 min. Duty was fixed at 10%. Pulse power (pulse width): 40 mJ (200 µs) for (a), 2 mJ (10 µs) for (b), and 1 mJ (5 µs) for (c). Raw particles: Ni (50 µm), 20 g L⁻¹ in hexane.

3.4 Effect of microwave pulse duty

Figure 5 shows the effect of microwave pulse duty on the product morphology by fixing the pulse frequency at 15 kHz by using the same raw particles. In this case, microwave pulse power and pulse width were changed to (a) 10 mJ and 50 µs, (b) 6.7 mJ and 33 µs, and (c) 2 mJ and 10 µs; under these conditions, the corresponding duty was 75% for (a), 50% for (b), and 15% for (c), respectively. By changing such microwave irradiation conditions, the product morphologies varied from a stick-like shape in (a) to a spherical shape in (b) and (c). Therefore, long pulse widths induced the merging of spherical particles to form rod-like structures. Short heating times were suitable for isolated spherical particles.

Fig. 5

Effect of duty on the products. Microwave power: 200 W for 30 min. Pulse frequency was fixed at 15 kHz. Pulse power (pulse width): 10 mJ (50 µs) for (a), 6.7 mJ (33 µs) for (b), and 2 mJ (10 µs) for (c). Raw particles: Ni (50 µm), 20 g L⁻¹ in hexane.

3.5 Conditions for spherical particle formation

Table 2 summarizes the conditions for the formation of spherical particles (S) and rod-like particles (R) for various pulse frequencies and duties. This clearly indicates that high pulse frequencies and small duties are suitable for the formation of spherical particles, i.e., short heating times and long cooling times are required.

Table 2 Product morphologies depending on pulse frequency and duty. R: rod-like structure. S: sphere-like structure. Microwave power: 200 W for 30 min. Raw particles: Ni (50 µm), 20 g L⁻¹ in hexane.

In PLML, the typical heating time is approximately picoseconds to nanoseconds. The interval time for cooling is over microseconds depending on the type of laser. A heating time shorter than several tens of nanoseconds is advantageous because the required energy to melt a single particle is instantaneously supplied without heat dissipation to the surrounding liquid.^12–14) An interval longer than milliseconds for cooling induces the complete decrease to room temperature and the independent heating cycle without heat accumulation in the particle.

For the microwave process, the possible pulsed heating time is a microsecond order, whereas the cooling time is at most a 100 µs order. A long heating time induces heat dissipation to the surroundings even during heating. Therefore, liquid temperature is evidently increased. A short cooling time brings incomplete particle cooling, thus leading to the heat accumulation and temperature increase of particles by plural pulse irradiation.

3.6 Mechanism on particle melting

In the case of PLML, there is almost no photoemission from particles during the process, although the temperature of the particles is estimated to be 2000–3000 K during the pulse duration. This result possibly indicates that the formation process of particles is mostly governed by a simple thermal process observed in a conventional thermal processing technique. By contrast, photoemissions from the dispersed particles are observed during the pulsed microwave heating in hexane, as shown in Fig. 6(a). The possible mechanism of photoemission and particle melting is schematically illustrated in Fig. 6(b). First, the Ni particles dispersed in hexane are space-selectively heated. Hexane is vaporized accordingly. When the interparticle distance among heated Ni particles decreases because of convectional flow in hexane, the microwave electric field is concentrated among them and generates hot spots to heat them over the melting point of nickel at 1455°C. Photoemission is possibly caused by the plasma discharge in hexane vapor at hot spots. The red and blue plasma colors observed in Fig. 6(a) are probably caused from H_α and CH plasma emitted from the decomposed species of hexane. This scheme can also explain the facile melting at high concentration of raw particles because of the short interparticle distance.

Fig. 6

(a) Photoemission during microwave irradiation. (b) Schematic illustration of space-selective microwave melting in liquid.

3.7 Required improvement for spherical particle formation

By pulsed microwave heating in hexane, the surface of dispersed particles became smooth, indicating that the temperature of surface was over the melting point of the particles. However, isolated particles were seldom observed, and most particles formed sphere-like or rod-like structure of particle aggregates.

One of the reason for such aggregation is the required particle concentration for pulsed microwave heating that is about two orders of magnitude larger than that for pulsed laser heating. This requirement is due to the difference in the wavelength of microwave and laser, and thus larger particles over micrometers might be effective for particle melting by microwave energy absorption. The high concentration of particles brings the short inter-particle distance and increases the chance of particle encountering by convectional mixing during heating, resulting in aggregated particle formation. Thus, the selection of microwave absorber particles with suitable concentration would be important.

Another reason for the aggregation during pulsed microwave heating is long time duration for heating and cooling. Due to the limitation of pulsed microwave power supply, possible heating time is a microsecond order, and the cooling time is at most a 100 µs order. Comparing with the high-pulse-power laser, heating time is too long and cooling time is too short. This indicates that the time for aggregating and merging of multiple particles are much longer than pulsed laser heating case. This effect appears to be very large to form aggregated particle formation, and will be greatly improved by the development of short-pulse microwave power supply for isolated spherical particle fabrication.

4. Conclusion

In this study, pulsed microwave heating in liquid was tested under the condition of the time- and space-selective heating of nickel particles dispersed in liquid. If nickel particles with 50 µm or larger were dispersed in low-dielectric-loss hexane with the concentration of 2 g L⁻¹ or higher, the particle surface became smooth by microwave irradiation. Thus, the temperature on the particle surface increased over the melting point. Pulsed microwave irradiation with short pulse widths and large pulse frequencies was effective for obtaining less aggregated and more isolated particles. Therefore, pulsed microwave melting in liquid would provide a new approach for fabricating spherical particles at a micrometer level using microwave power supply with further shorter pulse width.

Acknowledgments

This work was partially supported by JSPS KAKENHI (Grant Nos. 26289266 and 26870908). The authors would like to thank Dr. Kazuaki Senda (Fuji Electric Industrial Co., Ltd.) for the helpful discussions and Prof. Koichi Sasaki (Hokkaido University) for providing the microwave apparatus.

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