3.1 Microstructure of the absorption layer
Fig. 3(a-f) shows the microstructure of the nanostructured absorption layer of Sample A, B and C, respectively. Since Sample A was irradiated by ultrafast lasers in one direction with a scanning speed of vx=0 and vy=1000mm/s, the surface of sample A exhibits semi-periodic structures, as shown in Fig.3a. A nano-spherical structure was observed for Sample A in Fig. 3d. The nano-spheres with about 50-200nm diameter were covered with smaller nano-spheres whose diameters were less than 10 nm. Sample B and C were irradiated in both directions and their scanning speed are much higher than Sample A, no evident periodic structures were observed on their surfaces, as shown in Fig. 3b and Fig. 3c. As for Sample B, many particles in the scale of micrometers were observed on its surface(Fig. 3b), and the particles were composed of cauliflower nanostructures(Fig. 3e). Since Sample C were irradiated and scanned by an even higher speed compared with Sample A and B, the accumulation of nanoparticles was much faster and the heat effect were more prominent. Consequently, much thicker nanosheet and nanoparticle aggregations were observed in Fig. 3c and Fig. 3f. And multiple cracks occurred on the surface because relatively high stress was emerged during the cooling process due to prominent heat input.
Fig. 3(g-i) are the energy dispersive X-ray analysis (EDX) results for Sample A, B and C, respectively. The EDX showed the presence of Al2O3 oxides in the composition of nanostructures. The oxides were formed due to the oxidation of aluminum during laser writing process. The oxygen contents of Sample A, B and C were 2.2, 8.4 and 22.9 atom%, respectively. Apparently, Sample B and C had much higher oxygen content compared to Sample A, while the irradiation laser power for Sample B(13.82W) was lower than that of Sample A(22.60W) and the irradiation laser power for Sample A and C were identical, indicating that the scanning speed and scanning period significantly influence the heat generation and dissipation during direct laser writing. And the oxidation increases with the increase of scanning speed and decrease of the scanning period.
3.2 Light absorption of the samples
Fig. 4 a shows the optical microscope appearances of Al foil and the flyers with nanostructured absorption layer. The color of Al foil is silvery white. With the addition of nanostructured absorption layer, Sample A, B and C exhibits grey, black and dark black colors, indicating that more light can be absorbed with the absorption layer. The reflectance of Al foil and Sample A, B and C is tested by a spectrophotometer, and the measurements is repeated two times for each sample. Fig. 4b shows the reflectance spectrum of Al foil and the aluminum flyer with nanostructure absorption layer. Since the transmitting thickness of infrared light through metals often varies from a few tens of nanometers to several hundreds nanometers [24], thus none of the light was transmitted through the Al foil samples whose thickness were 50μm. And the scattered light was included in the reflected light in the measurement using an integral sphere. Consequently, the absorption could be calculated by 1-R(reflectance). Differences were evident between Al foil and the aluminum flyer with nanostructured absorption layer. The reflectance of Al foil was 81.3% at the laser wavelength of 1064 nm, indicating that 81.3% of incidence light was reflected. The average reflectance can be reduced to 50.5%, 31.5%, and 9.8% for Samples A, B and C, respectively. Therefore, light absorption can be effectively enhanced with the nanostructure absorption layer prepared by direct laser writing. Sample C has the strongest absorption (90.2%) at 1064nm compared to Sample A and B. Aside from the effect of the nanostructures, we believe that the aluminum oxide presented in the nanostructures also tremendously influences the light absorption of the flyer. Generally, Al2O3 is transparent and doesn’t absorb light, however, in a direct laser writing process, it is highly possible for the generated Al2O3 and aluminum particles to form a metal-dielectric-metal structure. The structure behaves as an F-P cavity which will in turn enhance the surface plasmon resonance and increase the light absorption [25]. As the oxygen concentrations of Sample A and B are far less than that of Sample C, implicating that the Al2O3 particles is richer in Sample C than other samples, resultantly, a more enhanced surface plasmon resonance effect and far stronger absorption can be achieved.
3.3 Velocity of the flyer
Fig. 5 shows the flyer velocities of Al foil, Sample A, B and C. At the beginning of 30 ns, the flyer velocity increases sharply. Afterward, the flyer velocity gradually increases starting from 30 ns to 200 ns, and hardly changes when the time exceeds 200 ns. The terminal flyer velocity for Sample A, B and C is 1083m/s, 1173m/s and 1110m/s, respectively, which is about 1.30, 1.41, and 1.33 times higher than that of the Al foil (831m/s). These results confirmed that the addition of an in-situ nanostructured layer can not only enhance the light absorption but also promote the flyer velocity. It’s worth mentioning that the flyer velocity for Sample B is higher than Sample C whilst Sample C has the strongest light absorption. The reason is that Sample C has a far richer Al2O3 content compared with Sample B. Ionic bond and metal bond was formed in Al2O3 and Al, respectively. And it was known that ionic bond was far stronger than metal bond, which makes the vaporization point and melting point for Al2O3 higher than Al. The melting point and vaporization point for Al2O3 is 2054℃ and 2980℃, while the melting point and vaporization point for Al is 660℃ and 2519℃, respectively. Additionally, the thermal conductivity is 29.3W/m·K and 237W/m·K for Al2O3 and Al. Hence, it is more difficult for Al2O3 to vaporize and form plasma at the incident pulsed laser due to its high melting point and low thermal conductivity compared to pure aluminum [26]. Therefore, although the light absorption is enhanced by Al2O3 in Sample C, in the meantime, Al2O3 consumes some of the incident laser energy while it doesn’t help driving the flyer.
The kinetic energy of the flyers can be obtained by the following relationship.
[Please see the supplementary files section to view the equation.] (1)
Where mf is the original flyer mass; ma represents the ablated flyer mass. Moreover, we assume the flyer keeps an integrated state during the flying process. The ablated flyer mass can be evaluated according to Lawrence and Trott model [27].
[Please see the supplementary files section to view the equation.] (2)
Where r is the radius of the flyer; μeff is the effective absorption index; I0 is the incident laser intensity; k is the energy loss index; εd is the vaporization energy.
The energy conversion efficiency of the flyer can be denoted using the following equation.
[Please see the supplementary files section to view the equation.] (3)
The calculated results of the flyer kinetic energy and energy conversion efficiency were illustrated in Fig. 6. The energy conversion efficiency for Sample A, B and C is 36.8%, 43.2% and 38.6%, which is 1.70, 1.99 and 1.78 times that of the Al foil(21.7%). In this work, when a nanostructured absorption layer is added on Al foil, the highest energy conversion efficiency almost doubled. The experimental results are summarized in Table 1. Therefore, the in-situ fabrication of a nanostructured absorption layer on the surface of a flyer provides a new method to significantly improve the energy conversion efficiency of a LDF.