3.1. Layer Structure and Morphological Analysis of the MFPs

SEM images in a cross-sectional view of Al/Al₂O₃/Al and TiO₂/Al/Al₂O₃/Al were obtained to understand the internal structure and component of MFPs and showed an equiaxial block growth with a tightly bound layered structure in Figure 2. Al and dense Al₂O₃ contacted closely and were demarcated clearly. The thickness of Al/Al₂O₃/Al (I) in each layer was 0.8 μm Al, 0.8 μm Al₂O₃, and 3.2 μm Al respectively; thus, a significant thickness and morphology difference can be seen among three layers from Figure 2(a). The SEM photograph of Al/Al₂O₃/Al (II) with 0.4 μm Al, 0.4 μm Al₂O₃, and 2.6 μm Al in the comparison group also showed obvious boundary and dense layer structure. In addition, the SEM image of the energetic flyer plates TiO₂/Al/Al₂O₃/Al (III) with 0.2 μm TiO₂, 0.2 μm Al, 0.4 μm Al₂O₃, and 2.6 μm Al also showed good compactness and clearly layered structure. The SEM analysis results showed that the internal structure and composition of the MFPs were the same as expected, indicating that the samples we prepared can be used as energy conversion microdevices with adjusted performance.

Figure 2: Cross-sectional SEM image of the MFPs. (a) Al/Al₂O₃/Al (I). (b) Al/Al₂O₃/Al (II). (c) TiO₂/Al/Al₂O₃/Al (III).

3.2. Energy Conversion Performance of MFPs

The dynamic process of laser-driven MFP acceleration was recorded simultaneously by the PDV system and ultrahigh-speed video. As shown in Figures 3 and 4, the time-velocity and time-displacement relationships of three types of flyer plates were obtained through the PDV system, and characteristic velocity curves in different acceleration stages were observed. The velocity curves’ variation of the flyer plates showed that the acceleration of the flyer plate can be divided into three stages: rapid acceleration stage, slow acceleration stage, and deceleration stage. The typical velocity curves of the flyer are shown in Figure 3, in which the excited laser energy was 43.1 J/cm². The rapid acceleration stage began when the flyer plate was driven away by the plasma and ended when the velocity increased to about three quarters of the final velocity. Rapid acceleration stage was characterized by a rapidly rising velocity of the flyer plate and the slowly decreasing acceleration, with a duration of about 20 ns. At the end of the rapid acceleration stage, the flyer entered the slow acceleration stage. The rising trend of the flyer velocity slowed down obviously at this time, and the acceleration decreased rapidly to zero. The slow acceleration stage lasted about three times as long as the rapid acceleration stage, which ended when the flyer plate reached the maximum velocity (final velocity). Finally, the flyer plate entered the deceleration stage, and its speed slowly decreased with the passage of time. It should be pointed out that, in the slow acceleration stage of Al/Al₂O₃/Al (II) and TiO₂/Al/Al₂O₃/Al (III), the phenomenon of flyer velocity bifurcation occurred, which was related to the material and structure of the flyer plates.

Figure 3: Time-velocity curves of MFPs with different types. (a) Al/Al₂O₃/Al (I). (b) Al/Al₂O₃/Al (II). (c) TiO₂/Al/Al₂O₃/Al (III).

Figure 4: Velocity and displacement histories of MFPs. (a) Al/Al₂O₃/Al (I). (b) Al/Al₂O₃/Al (II) and TiO₂/Al/Al₂O₃/Al (III).

Figure 4 shows the velocity curves of three types of flyer plates over time. As can be seen, the three stages of the flyer plate acceleration process were clearly visible. For the rapid acceleration stage, the velocity of Al/Al₂O₃/Al (I), Al/Al₂O₃/Al (II), and TiO₂/Al/Al₂O₃/Al (III) increased from 0 to 3500 m/s within 26 ns, 0 to 4100 m/s within 20 ns, and 0 to 4400 m/s within 24 ns, respectively, which was due to the high-pressure shock wave generated by the strong constrained expansion of the plasma plume at the initial moment when the flyer plate was sheared off. The shock wave formed by the rapid expansion of the plasma plume propagated back and forth in the Al flyer layer, which accelerated the flyer layer and made the velocity of the flyer plate rise very rapidly. At this time, the plasma just broke through the bound of the Al flyer layer from the confinement state and drove the flyer plate to accelerate rapidly in the barrel. However, the flyer plates moved about 55 μm, 46 μm, and 58 μm, respectively, in this stage, which accounted for 12.7%, 14.4%, and 20.7% of the displacement at the corresponding maximum velocity. From the velocity curve and flight distance, it can be concluded that the speed of the energetic flyer plate increased more dramatically in the rapid acceleration stage. When the velocity increased slowly in the barrel, the flight process of the flyer plate entered the second stage: slow acceleration stage. At this time, the acceleration of the flyer plate decreased rapidly, which reduced the velocity increment of the flyer plate. However, due to the high velocity level of the flyer plate and the long flight duration time, the flyer plate had a large displacement at this stage. At this stage, the maximum velocity of Al/Al₂O₃/Al (I) reached 4201.0 m/s (432.7 μm flight distance), while the maximum velocities of Al/Al₂O₃/Al (II) after fragmentation were 4609.8 m/s (257.9 μm flight distance) and 4988.1 m/s (320.0 μm flight distance), and the maximum velocities of TiO₂/Al/Al₂O₃/Al (III) after fragmentation were 5182.2 m/s (274.6 μm flight distance) and 5386.7 m/s (280.5 μm flight distance), respectively.

It can be inferred from the velocity variation curves of Al/Al₂O₃/Al (II) and TiO₂/Al/Al₂O₃/Al (III) in the slow acceleration stage that the flyer plate layer fragmentation occurred, as shown in Figures 3(b), 3(c), and 4(b), which did not appear in the rapid acceleration stage. As time goes by, the flyer flew in the slow acceleration stage which was constantly eroded by the high temperature and high pressure from the plasma plume and the shock wave. Due to the thin Al₂O₃ insulation thickness of the flyer plate, it was unable to effectively isolate the shock and ablation of the plasma plume. Therefore, the breakage phenomenon appeared at the weak point of the flyer layer, and the velocity differentiation occurred in the slow acceleration stage as shown in the velocity curve. Therefore, the thicker Al₂O₃ insulating layer can prevent the flyer plate from breaking under the shock and ablation of high-temperature and high-pressure plasma. During the slow acceleration stage, the reason for the slow increase of the flyer velocity was that the energy used to support the shock wave in the plasma plume became less. As the flyer plate was completely sheared, the plasma plume lost strong constraint, causing the plasma plume to expand very rapidly. The rapid expansion of the plume reduced the pressure of the plasma, resulting in a reduction in the pressure that drove the flyer plate. Therefore, the energy obtained by the shock wave in the flyer layer and the acceleration effect of the shock wave on the flyer were reduced, which was shown as the acceleration slowing down in the curve.

When the flyer plate reached its maximum velocity, the flight of the flyer plate moved from the second stage to the third stage: the deceleration stage. For the Al/Al₂O₃/Al (I) flyer plate, the velocity of the flyer plate at this stage showed a trend of slow decline and lasted for a long time, reached about 776 ns, but the velocity of the flyer plate still remained at a high level. As a result, the total flight distance of the flyer plate reached 3.72 mm. As for the flyer plates of Al/Al₂O₃/Al (II) and TiO₂/Al/Al₂O₃/Al (III), the velocity evolution curves of the two types of flyer plates in the deceleration stage became shorter because the flyer plates were broken in the slow acceleration stage. Thus, the displacements of Al/Al₂O₃/Al (II) and TiO₂/Al/Al₂O₃/Al (III) reached only 817.2 μm and 984.9 μm, respectively. In addition, by reducing the thickness of the MFPs, the velocity of the flyer plate was also effectively improved, which was mainly caused by the reduction of the mass of the flyer plate. Among them, TiO₂/Al film as the ablative layer of MFPs had the highest final velocity, which was due to the increased heat release and laser absorption of the thermite system. It was also found that the sufficient insulation layer thickness of the flyer plate can improve effectively the integrity of the flyer plate so that the flyer plate can still remain intact when flight a long distance.

In order to more intuitively observe the morphologic changes of the flyer plate in the flight process, ultrahigh-speed photography was used to shoot the flight process of the three types of MFPs, and the results are shown in Figure 5. As can be seen from the figure, Al/Al₂O₃/Al (I) maintained a relatively complete morphology under the plasma drive within 700 ns, which was due to its thick Al₂O₃ insulation layer that could effectively isolate the impact ablation of high-temperature and high-pressure plasma, while Al/Al₂O₃/Al (II) and TiO₂/Al/Al₂O₃/Al (III) were broken and separated under the impact ablation of the laser-induced plasma within 100 ns. These characteristics were consistent with the time-velocity curves of the three kinds of MFPs. Al/Al₂O₃/Al (I) maintained a relatively complete appearance during the laser driving process, so the obtained time-velocity curve was relatively complete. However, the time-velocity curves of Al/Al₂O₃/Al (II) and TiO₂/Al/Al₂O₃/Al (III) showed a bifurcation phenomenon, and the duration was also shorter. These results indicated that the 0.4 μm-thick Al₂O₃ insulation layer cannot guarantee the integrity of the flyer plate, so the design of the insulation layer should be thicker with 43.1 J/cm².

Figure 5: Ultrahigh-speed photography of the flight process for various flyer plates. (a) Al/Al2O3/Al (I). (b) Al/Al2O3/Al (II). (c) TiO₂/Al/Al₂O₃/Al (III).

3.3. Probing the Pyrolysis Mechanism of TiO₂/Al

The MFPs based on the TiO₂/Al thermite film had the highest final velocity, which may be related to the laser absorption and the exothermic reaction of the thermite film. In order to verify the influence of the TiO₂/Al thermite film on the laser conversion efficiency of MFPs, the laser reflectivity was tested. Figure 6 shows the laser reflectivity of Al and TiO₂/Al films. It can be seen from the figure that the laser reflectivity of the Al film was between 80 and 90%, while that of TiO₂/Al was relatively low, between 70 and 80%. Under the irradiation of the 1064 nm laser, the laser reflectivity of the Al film was 80.55%, and that of TiO₂/Al was 72.13%. The absorptivity of the TiO₂/Al film to the 1064 nm laser was higher than that of the Al film, so it can be inferred that more laser energy was used as ablation.

Figure 6: The laser reflectivity of ablation films.

The differential thermal analysis (DTA) instrument was used to characterize the exothermic behavior of the TiO₂/Al ablative layer at different heating rates (from 5 to 20°C/min) in order to obtain the energy release law of the films under thermal stimulation. The thermal analysis experiments were conducted to study the heat release of the TiO₂/Al layer under the N2 flow with the temperatures ranging from 30 to 900°C. As can be seen from Figure 7, the exothermic reaction of the TiO₂/Al film between 300 and 800°C was divided into two stages [Reference Guo, Chang, Cao, Wu, Shen and Ye22]. The remarkable exothermic peaks of TiO₂/Al were 653.19, 674.00, 692.95, and 702.43°C at heating rates from 5 to 20°C/min, respectively. The smaller exothermic peaks were shown at 345.64, 354.73, 384.85, and 393.14°C, corresponding to heating rates from 5 to 20°C/min, respectively. According to the heat release curves, the heat release in the high-temperature section was greater than that in the low-temperature section. With the decreased heating rate, the peak temperature and heat release of the exothermic reaction decreased. The Kissinger method was utilized to estimate the activation energy (Ea) of the two exothermic peaks of the TiO₂/Al film [Reference Guo, Chang, Cao, Wu, Shen and Ye22, Reference Sinditskii, Burzhava and Sheremetev23]. The activation energy of the first exothermic peak was estimated to be 23.81 kJ/mol, while the activation energy reached 93.70 kJ/mol for the second exothermic peak, which was much higher than that of the first exothermic peak. With the decreased heating rates, the initial reaction temperature of the TiO₂/Al film decreased, which was consistent with the change of reaction process curves, as shown in Figure 8. The temperature range of the first exothermic peak of 30% conversion was 365–378°C, and that of the second exothermic peak of 30% conversion was 660–695°C.

Figure 7: (a) The reaction exothermic curve and (b) the activation energy of TiO₂/Al.

Figure 8: The reaction progress curves of the two exothermic peaks. (a) First exothermic peak. (b) Second exothermic peak.

The first exothermic reaction occurred in the ranges of 300–450°C before Al melted, at which time, the aluminothermic film released less heat. The results showed that the solid-solid diffusion reaction was most likely to occur in the interface layer between Al and TiO₂, as shown in Figure 9, which was attributed to the reduction of the diffusion distance between Al and TiO₂ layers prepared by magnetron sputtering, and the Al₂O₃ barrier layer formed between the film layers was thin or defective. The solid-solid reaction gradually formed a relatively complete Al₂O₃ barrier layer between the film layers, preventing Al and TiO₂ from further reacting. The second heat release peak occurred after Al melting (melting point of the Al film was 645°C), which was a solid-liquid reaction between liquid Al and solid TiO₂. Different from the reaction of the TiO₂/Al layer in the first stage, a relatively complete and thick Al₂O₃ barrier layer was formed between Al and TiO₂. As temperature rose, melted Al can partially break the Al₂O₃ layer and flow out, resulting in the reaction with TiO₂. It can be inferred that the second exothermic peak as the main exothermic reaction was the result of the full reaction between melted Al and TiO₂.

Figure 9: Schematics of the main reaction mechanism of the TiO₂/Al film. (a) As-deposited. (b) Solid-solid reaction. (c) Al₂O₃ barrier. (d) Molten Al. (e) Solid-liquid reaction.

The activation energies of the two exothermic peaks corresponded to the solid-solid and solid-liquid chemical reactions at two temperature stages, respectively, and also corresponded to the difficulty of the two reactions. The first solid-solid reaction between 300°C and 450°C was relatively easy to occur, while the second exothermic reaction required molten Al to break through the Al₂O₃ barrier layer; thus, two kinds of reaction mechanisms have been speculated for the TiO₂/Al film [Reference Guo, Wu and Meng20, Reference Zhang, Rossi, Alphonse, Tenailleau, Cayez and Chane-Ching24–Reference Xu, Shen and Wang27]. The solid-solid reaction was condensed state of free molecular oxygen reaction process equation (1), in which free oxygen firstly reacted with Al and then produced the Al₂O₃ barrier in the interface. The solid-liquid reaction was gas state reaction process equation (2), in which molten Al breached the Al₂O₃ barrier layer and reacted with the decomposition oxygen molecules of the oxidizer.

(1) $\begin{array}{}\text{Al}+{\text{O}}_x⟶{\text{Al}}_2{\text{O}}_3,\end{array}$

(2) $\begin{array}{}\left(\begin{array}{c}{\text{MO}}_x⟶{\text{MO}}_{x-1}+\frac{1}{2}{\text{O}}_2,\\ \text{Al}+{\text{MO}}_{x-1}⟶{\text{Al}}_2{\text{O}}_3+\text{M.}\end{array}\right)\end{array}$

In general, the energy release from the TiO₂/Al film under thermal stimulation contributed to the improvement of the output capacity of the system. Therefore, combined with the velocity of flyer plate results, it can be found that using the laser to stimulate a thermite reaction can also increase the energy of the plasma, thus improving the flyer output capacity. In addition, the peak temperature and heat release of the exothermic peak presented with increasing heating rate tended to increase. The second exothermic by the thermite film was higher than that of the first exothermic reaction.