**5.3 Confinement effects**

As already stated, in order to produce monodispersed nanoparticles, it is important to confine the plume within the converging shock waves. To confirm the importance of

Thermodynamics of Nanoparticle Formation in Laser Ablation 141

mm, as shown in Figure 13(b), some aggregation is observed. The result in Figure 13(c), where *x* = 2.0 mm and *θ*= 2.5°, shows the appearance of fine nanoparticles, similar to the normal case (Figure 13(a)). The mainly small and uniformly sized nanoparticles shown in Figure 13(d) formed under conditions of *x* = 2.0 mm and *θ* = 5.0°. In contrast, when *x* = 2 mm, *θ* = 7.5°, secondary particles were generated by nanoparticle aggregation (Figure 13(e)). Although the position of the laser spot is shifted and also the density of laser energy is slightly changed (Figures 13(c) and 13(d)) relative to the normal case (Figure 13(a)), the sizes of the resulting nanoparticles were found to be finely dispersed, similar to the normal case. The confinement effect of the plume by the converging shock wave plays a role in these cases, because the plume ejection is approximately directed to the focal point of the ellipsoidal cell. The result of Figure 13(e) indicates that the residence time of nanoparticles in the ellipsoidal cell increased due to circulation by a vortex flow resulting from the shifted

As mentioned above, nanoparticle size was found to be monodispersed in the ellipsoidal cell under appropriate conditions. We will now discuss a case in which the monodispersed nanoparticles were sintered under low-temperature conditions. This low-temperature

direction of the plume ejection relative to the focal point.

sintering procedure could serve as a metal bonding technique.

Fig. 14. Two gold nanoparticles forming a neck and binding to each other.

The bonding of metal is an important process for the construction of fine mechanical parts and heat sinks. Conventional bonding methods such as diffusion bonding, melted alloy bonding, hot isostatic pressing and silver brazing cause thermal stress at the interface between two metals because of differences in thermal expansion between the bonded parts. This thermal stress in turn causes warping of the bonded material. Therefore, lowtemperature metal bonding is desired to overcome these problems. Since the melting point of metals decreases with decreasing particle size, metal nanoparticle paste has been used as

**5.4 Low temperature sintering** 

confining the plume at the focal point of the ellipsoidal cell, further nanoparticle formation experiments were carried out.

Figure 12 is a schematic diagram of the apparatus with an ellipsoidal cell. The laser spot is intentionally shifted by a distance, *x*, from the central axis of the ellipsoidal cell, while the target surface is also intentionally inclined by an angle, *θ*, against a plane perpendicular to the central axis. Figure 13 shows some of the results for nanoparticles produced as a result of changing these parameters. The experimental results shown in Figure 13(a), which are obtained under the conditions *x* = 0.0 mm and *θ* = 0.0 °, represent monodispersed nanoparticles. When the target surface has no inclination but the laser spot is shifted *x* = 2

Fig. 12. Schematic of experiment demonstrating the importance of confinement

Fig. 13. Influence of shock wave confinement on deposited nanoparticles morphology in the ellipsoidal cell (field of view:200×200nm)

mm, as shown in Figure 13(b), some aggregation is observed. The result in Figure 13(c), where *x* = 2.0 mm and *θ*= 2.5°, shows the appearance of fine nanoparticles, similar to the normal case (Figure 13(a)). The mainly small and uniformly sized nanoparticles shown in Figure 13(d) formed under conditions of *x* = 2.0 mm and *θ* = 5.0°. In contrast, when *x* = 2 mm, *θ* = 7.5°, secondary particles were generated by nanoparticle aggregation (Figure 13(e)). Although the position of the laser spot is shifted and also the density of laser energy is slightly changed (Figures 13(c) and 13(d)) relative to the normal case (Figure 13(a)), the sizes of the resulting nanoparticles were found to be finely dispersed, similar to the normal case. The confinement effect of the plume by the converging shock wave plays a role in these cases, because the plume ejection is approximately directed to the focal point of the ellipsoidal cell. The result of Figure 13(e) indicates that the residence time of nanoparticles in the ellipsoidal cell increased due to circulation by a vortex flow resulting from the shifted direction of the plume ejection relative to the focal point.
