Ammonium Perchlorate as an Effective Addition for Enhancing the

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C: Physical Processes in Nanomaterials and Nanostructures

Ammonium Perchlorate as an Effective Addition for Enhancing the Combustion and Propulsion Performance of Al/CuO Nanothermites Ji Dai, Fei Wang, Chengbo Ru, Jianbing Xu, Chengai Wang, Wei Zhang, Yinghua Ye, and Ruiqi Shen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01514 • Publication Date (Web): 19 Apr 2018 Downloaded from http://pubs.acs.org on April 19, 2018

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The Journal of Physical Chemistry

Ammonium Perchlorate as an Effective Addition for Enhancing the Combustion and Propulsion Performance of Al/CuO Nanothermites Ji Dai, Fei Wang, Chengbo Ru, Jianbing Xu, Chengai Wang, Wei Zhang, Yinghua Ye*, Ruiqi Shen Department of Applied Chemistry, School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China.

Corresponding author: Yinghua Ye E-mail: [email protected].

Abstract Nanothermites are attracting much attention because of the high energy density, self-sustained exothermic reaction and high combustion temperature. However, they suffer from sintering and incomplete combustion, leading to poor reactivity and low energy utilization efficiency. In order to enhance the energy output and combustion performance of nanothermites, ammonium perchlorate (AP) was introduced into the Al/CuO nanothermits. The nanothermites with varied content of AP were prepared by electrospray. The morphological characterization of the nanothermites confirms that the nanoparticles are homogenously mixed without agglomeration. Heat release, specific impulse, and peak pressure of gaseous products exhibit remarkable enhancement with increasing AP content. Specifically, the nanothermites with 7.5 wt% 1

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AP produces specific impulse and heat energy of ~2.7 and ~1.4 times higher than those of Al/CuO-based nanothermites without AP. In addition, the ignition delay time of the nanothermites containing AP is not greatly increased, enabling fast response during practical applications. Thermal analysis implies that the thermite reaction between Al and CuO can be divided into two steps in the presence of AP: solid-solid phase and liquid-solid phase diffusion reaction. These results provide facile strategy to enhance the output performance of nanothermites, which may facilitate the practical propulsion and combustion applications of nanothermites.

1. Introduction Energetic materials (EMs) are a class of reactive materials that possess high chemical enthalpy. When initiated by external stimulus, they can release energy in a rapid manner.1 EMs are generally classified into propellants, explosives, and pyrotechnics in terms of their output performances and applications.2 In respect of chemical composition, there are commonly two types of conventional EMs, including monomolecular

energetic

materials

(e.g.

nitrocellulose,

nitroglycerine,

and

trinitrotoluene) and heterogeneous energetic materials (mixture of fuel and oxidizer).3 For years, EMs have been widely used in civilian and military applications since they are very attractive energy sources for generating gas, heat, and power. With the development of micro-electro-mechanical-systems (MEMS), interest in developing functional micro-device with EMs has recently increased, such as micro-thruster,4-6 heating and welding,7 and switching.8 However, heat loss is one of the most 2


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challenging issues to be solved in micro-device. Conventional EMs suffer from relatively low energy density and energy release rate, which may result in un-sustained combustion for microscale applications.9 Therefore, high energy density and high energy release rate are still important parameters for developing novel EMs. Various teams introduce high enthalpy metal powder into conventional EMs. Nevertheless, they have found it difficult to handle the powder and incorporate the powder into existing formulations.3,

10-12

Nanothermites consisting of metal fuel (Al) and oxidizer (Fe2O3, CuO, Bi2O3, MoO3, etc.) at nanoscale are attracting much attention.

When

compared

with

conventional EMs, nanothermites demonstrate higher energy density and higher reaction rate, which have potential applications for MEMS.13-20 In addition, the self-sustained exothermic reactions are suitable heat sources to numerous engineering applications and can be applied in advanced materials synthesis. 21-23 The interfacial contact and diffusion length between the fuel and oxidizer have great influence on the energy output and reactivity of nanothermites. Various preparation methods have been employed to prepare well mixed nanothermites, such as

sonic

wave-assisted

physical

mixing,24-27

sol-gel

chemistry,28-29

and

electrospray.30-35 However, the pre-reaction sintering of nanoparticles and the oxide shell (Al2O3) passivated on the Al decrease the reactivity and energy output in actual combustion of nanothermites.16,34,35 As an oxidizer, ammonium perchlorate (AP) has been commonly used in traditional solid propellant, resulting in high burning rate and large amount of gaseous 3



products.36-39 The AP also can be introduced into nanothermites to tune the micro structure and thermodynamic performance, which may further enhance the energy output and energy release rate. In this paper, AP was introduced into Al/CuO-based nanothermites by electrospray method. Nitrocellulose (NC) was employed as an energetic binder. The ignition delay time, thermodynamic performance and propulsion properties of the nanothermites were characterized. The heat release, specific impulse and volume of gaseous products of nanothermites were employed for estimating the output performance of the nanothermites. The feasibility of AP enhancing the combustion and propulsion properties of Al/CuO-based nanothermites was evaluated.

2. Experimental 2.1.

Materials

Aluminum nanoparticles (Al NPs, ~100 nm) and copper oxide (CuO, ~40 nm) were commercially available from Haotian Nano Technology Corporation (Shanghai) and used as received. NC with nitrogen content of 12.5% was used as binder for electrospray formation of nanothermites. AP (99.0 wt%) was introduced to the nanothermites, as the gas-generator and oxidizer agent. The mixture solvent of acetone (99.7%) and N, N-dimethylformamide (DMF, 99.5%) (volume ratio: 4:1) was employed to dissolve the AP and NC, forming AP/NC solution. The mixture solvent of ethanol (99.9%) and diethyl ether (99.7%) (volume ratio: 3:1) was used to form NC solution. 4


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2.2.

Preparation of precursor solution

AP was introduced into Al/CuO based nanothermites in the ratios of 2.5, 5.0, and 7.5%. The mass content of NC binder was kept constant at 2.5% in each formulation. AP and NC were firstly dissolved in the mixture solvent of acetone/DMF and ultra-sonicated for 0.5 h to obtain AP/NC solution. Afterwards, Al NPs and CuO NPs were dispersed in the solution to form stable suspension, and the suspension was magnetically stirred for 24 h enabling sufficient mixing of the substances. The equivalence ratio (Φ) of nanothermite was fixed at 1.8 (mass ratio of Al/CuO was 28.8/71.2) with the existence of alumina shell. The process of preparing precursor solution for Al/CuO/NC without AP was similar to that described above, except that the mixture solvent used to dissolve NC was ethanol/ether.

2.3.

Preparation of nanothermites

In this study, electrospray was employed to prepare Al/CuO based nanothermites. Electrospray can be developed as a liquid atomization method by applying high electric field to overcome the surface tension at the solution interface. Well-mixed reactive particles with decreased size can be obtained by electrospray, which efficiently enhance the output performance of the reactive particles.32,33,35 As illustrated in Figure 1, the precursor solution was loaded into a syringe, driving by a syringe pump at the flow rate of 1.5 mL/h. The voltage applied between needle and substrate charged was 18 kV. High electric field between the nozzle and substrate (with the distance of 8.0 cm) was used to overcome the surface tension at the 5



precursor interface, and then form the micro-droplets with Al/CuO nanoparticles wrapped in NC/AP solution. During the micro-droplets flying to the substrate, the solution was rapidly evaporated and the nanothermites can be collected from the substrate. Any operation was carefully processed with using protective gear in case of high voltage.

Figure 1. Schematic of electrospray formation of nanothermites.

2.4.

Fabrication of solid propellant micro-thruster (SPM)

A 10×10 SPM array (as shown in Figure 2) was fabricated by MEMS compatible technology containing four main parts (from bottom to top): nozzle layer, semiconductor bridge (SCB) igniter, combustion chamber, and seal layer. The SPM is similar to our previous work.33 The nozzle layer and 10 × 10 SCB igniters (as shown in Figure 2a) were fabricated on a 500 µm thick silicon wafer. The SCB on SiO2 insulation layer was designed as a symmetrical “double-V” (with the angles of 140.7°) shaped bridge to improve its output performance with the size of 118 µm (length) × 20 µm (width) × 2.5 µm (thickness), the resistance of 15.9 Ω, as shown in Figure 2b and 2c. Conductive Au was deposited to connect the SCB, forming the ignition circuit. The nozzle array was etched by deep reactive ion etching (DRIE) technology, starting from the backside of the SCB igniter layer. The combustion chamber with inner 6


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diameter of 1.0 mm and depth of 1.5 mm was obtained by precise machining. The ignition circuit layer was then bound to combustion chamber layer. Afterwards, the nanothermites were charged in the combustion chamber and sealed with silicon wafer. The side and top-down views of the SPM were shown in Figure 2d and 2e, respectively.

Figure 2. (a) Diagram of the ignition circuit. (b) Optical microscope image of the SCB igniter. (c) Cross-sectional diagram of the SCB igniter. (d) Photograph of side view of the SPM. (e) Photograph of top view of the SPM.

2.5.

Characterization of the nanothermites

The morphology of the nanothermites was examined by scanning electron microscopy (SEM, Hitachi, S-4800II). The exothermic reaction properties of the samples were investigated by differential scanning calorimetry (DSC, NETZSCH STA 449 C). The samples were placed in alumina crucible and the analysis was conducted under flowing argon (30 mL·min−1) at the heating of 5 °C·min−1, 10 °C·min−1, 15 °C·min−1 and 20 °C·min−1, respectively. The combustion performance of nanothermites and the propulsion properties of SPM were 7



characterized by a firing system, as shown in Figure 3. The firing system mainly consists of a constant voltage power supply (E3634A, Agilent), a mercury switch (with characteristic rise time below 200 ns), voltage probe (PP011, LeCroy), current probe (AP015, LeCroy), Si based photo detector (Thorlabs, DET02AFC) and the digital phosphor oscilloscope (LeCroy, 44Xs). In the process of experiment, the SCB igniter was initiated by electric energy from power supply. After a short delay, the nanothermites were ignited by the SCB and the thrust was generated by the rapid combustion reaction of the nanothermites. The voltage and current signals were recorded by the oscilloscope, and the combustion process was recorded synchronously using a high-speed camera (HG-100 K) at 20000 frames per second. The corresponding impulse was measured by a home-made impulse testing system as described in our previous work.33 In addition, the volume of gaseous products of nanothermites was evaluated by measuring the pressure output in confined constant-volume cell. A fixed mass (25 mg) of each thermite sample was weighed out and placed in the confined cell with a constant volume of 13 mL. After ignition, the induced pressure was detected by a pressure sensor (PCB Piezotronics, 113B26). Through a signal processor (PCB, 482A20), the corresponding data was collected by the oscilloscope.

8


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Figure 3. Schematic of the firing system used to measure the propulsion performance of SPM and electrical characteristics of SCB.

3. Results and discussion 3.1.

Ignition performance of nanothermites

The ignition process involves complex physical and chemical changes under constant voltage. Typical curves of voltage and current under 12 V for SCB only and SCB with nanothemrites are presented in Figure 4a and 4b, respectively. According to the previous works,40,41 the functional process of SCB can be generally divided into the following several stages: the temperature rising stage, melting stage, vaporing stage and outbreak for ionization stage. According to the current curve, the current starts from the turn-on time (t0) and rapid cross through the SCB (t1), as shown in Figure 4a. From t1 to t2, the physical changes including melting and vaporing was carried out. The ionization (or electric explosion) occurred at t2 and then rapid dissipated, leading to the disconnection of the electric circuit (t3). The plasma generated at t2 is attributed to the ignition of various 9



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targets. Therefore, t0-t2 is commonly defined as electric explosion delay time. As for the SCB with nanothermites loaded, the electric properties were similar to that of the SCB without nanothermites, except that the current changed at t4. This was attributed to the ignition of nanothermites with some of the conductive combustion products promoting the connection of the circuit for a while. The ignition of nanothermites was also validated by the optical signal (or flame) rising at t4, as shown in Figure 4b. The results indicate that the nanothermites can be ignited by the SCB reliably. Therefore, t0-t4 is employed to estimate the ignition delay time of the nanothermites. The ignition results of different samples are presented in Table 1 and Figure 4c. In addition, the scatter of the ignition properties of the nanothermites also can be found in Figure 4c. The results indicated that the explosion of SCB was accomplished rapidly (about 27 µs) under 12 V. The nanothermites have little effect on the explosion performance of SCB (in consideration of data scatter), indicating the good compatibility between the SCB and nanothemrites. Compared with Al/CuO/NC, the ignition delay of the nanothermties containing AP increased, especially that of the Al/CuO/NC/AP (2.5 wt%). Table 1. Electrical explosion delay time of SCB and ignition delay time of nanothermites Materials

Electrical explosion

Ignition delay

delay time/µs

time/µs

Al/CuO/NC

27.3

40.9

Al/CuO/NC/AP (2.5 wt%)

25.9

64.8


19.4

45.5


21.9

42.1

10


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MSCB only

27.2

/

Figure 4. Typical curves of voltage and current through SCB under constant voltage of 12 V, (a) without nanothermites, (b) with nanothermites. (c) Electrical explosion delay time of SCB and ignition delay time of the nanothermites with varied AP content.

3.2.

Thermodynamic performance of nanothermites

Thermal analysis was conducted to examine the effect of AP on the reactivity and energy output of the nanothemrites. The experiments were conducted under argon flow with the temperatures ranging from 30 to 900 ℃. The activation energy of thermite reaction was determined according to the DSC results at various heating rates (from 5 to 20 ℃·min−1). The DSC curves of nano-Al/CuO/NC/AP at a heating rate of 5 ℃·min−1 are shown in Figure 5. As for the Al/CuO/NC, the exothermic reaction was caused by the thermite reaction of Al and CuO with peak temperature of 549.0 ℃ (below the melting point of Al and CuO). The result implies that the reaction of Al and CuO is most probably a solid-solid diffusion reaction, benefiting from the enhanced interfacial

11



contact and reduced diffusion distance (nanoscale) between the Al and CuO nanoparticle.26,28 After introducing AP into the nanothermites, the heat release was divided into two stage. The first exothermic reaction was still accomplished below the melting point of Al (660 ℃). The first heat release peak shifted toward low temperatures. Specifically, increasing AP content from 0 wt% to 7.5 wt% decreases the peak temperature from 549.0 ℃ to 536.2 ℃, suggesting that AP has influence on the thermal properties of the nanothermites. This can be explained that the chloride ions in AP have great effect on Al2O3 and partially remove (or break) the oxide layer on Al core, promoting the heat and mass transfer between Al and CuO. The second heat release peak with the onset temperature of about 660 ℃ was attributed to the reaction of melting Al and solid CuO (only observed for the nanothermites containing AP), implying a solid-liquid reaction. As the reaction of Al/CuO (without AP) is carried out, more Al2O3 is formed on the surface of Al core. The Al in core cannot totally reacts with CuO in the presence of thick Al2O3 barrier, leading to insufficient reaction. As described above, the AP may partially remove or break the Al2O3 layer and enable the reaction of Al core and CuO. Especially, after the melting point of Al, the melted Al can flow out of the broken Al2O3 layer and further react with CuO, resulting in the second heat release peak. It can be speculated that the second heat release peak is the result of efficiently employing the core Al in the thermite reaction. In addition, heat release values of the nanothermites were quantified based on the DSC measurements, as shown in Figure 5. The total heat release for nanothermites containing AP is ~1.4 times higher than that of the sample without AP (1113.0 J/g for 12


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Al/CuO/NC/AP7.5% and 792.1 J/g for Al/CuO/NC2.5%), The results suggest that the energy output can be greatly enhanced by introducing AP into the Al/CuO nanothermites.

Figure 5. DSC curves of nanotherimites with different AP content at heating rate of 5 ℃·min−1.

Kissinger method was utilized to estimate the activation energy (Ea) of the first exothermic peak. DSC curves of the nanothermites at various heating rates (from 5 to 20 oC·min−1) and the fitting curves obtained from Kissinger method are presented in Figure 6. The activation energy is commonly defined as the minimum energy needed to initiate the reaction. Higher Ea value implies more input energy is required to ignition the sample. The activation energy of nano-Al/CuO/NC composites without AP was estimated to be 131.5 kJ/mol. While the activation energy reached 216.4 kJ/mol for the nanothemites containing 2.5 wt% AP, which was much higher than that of Al/CuO/NC. When the addition of AP further increases, the activation energy decreases and reaches 167.3 kJ/mol for the nanothemites containing 7.5 wt% AP. The 13



results exhibit a similar trend versus AP content as ignition delay (mentioned above), indicating that the activation energy has intimate correlation with ignition sensitivity (or reactivity). The activation energy along with the ignition properties indicate that the addition of AP has great influence on the reactivity of Al/CuO based nanothermites. Although the addition of AP inhibits the initiation of thermites reaction, the energy release has been greatly enhanced.

14


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Figure 6. DSC plots of nanothermites at different heating rates and the fitting curves used for calculating activation energy of the first exothermic peak by Kissinger method: nanothermites 15



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with AP content of (a) 0.0 wt%, (b) 2.5 wt%, (c) 5.0 wt%, (d) 7.5 wt% (here, β is the heating rate, Tp and Ea are the temperature and activation energy of the first exothermic peak, respectively).

3.3.

Propulsion performance of nanothermites To further evaluate the output performance and the potential application in

micro-thruster, the total impulse (I), specific impulse (Isp) and volume of gaseous products of the nanothermites were measured. The measurements of propulsion properties were conducted by charging the nanothermites to the combustion chamber in SPM and initiating the SPM. Relevant propulsion parameters including the thrust force (F, N) mass flow rate (ṁ, kg·s-1), Isp, I and their relationship can be written as follows: F = ṁ·ve + (pe - pa )·Ae

(1)

tb

I = F dt

(2)

= I / (m·g)

(3)

0

Here, pe is the pressure at exit of nozzle (Pa), ve is the exhaust gas velocity (m·s-1), pa is the atmospheric pressure, m and g are mass of the nanothermites and gravity acceleration, respectively. The impulse testing system with a displacement sensor (resolution value of 0.46 µs) was used to measure propulsion parameters. The combustion process of the nanothermites was recorded by high-speed camera at 20000 fps. The principle of testing system was similar to leverage effect. The displacements of SPM was tracked by displacement sensor, as shown in Figure 7a. The calculation method of Isp and the relationship of total impulse (I) versus 16


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displacement (A) of SPM were written as follows:

Jω0 ·A α∙A α·A Isp = I / (m·g) = m·g I =

(4) (5)

Here, A is the amplitude of the displacement curve, as shown in Figure 7a. As for the testing system, α has been calibrated to be 1.20 by standard component. The obtained propulsion parameters for different nanothermites were presented in Table 2 and Figure 7b. The total impulse of the SMP with different samples was in the range of 0.283 mN·s to 0.957 mN·s. The results show that AP addition has great influence on the propulsion performance of Al/CuO nanothermites. The specific impulse (key propulsion parameter) of nano-Al/CuO/NC was estimated for 22.2 s. When introducing AP into Al/CuO nanothermites, the Isp increases with increasing AP content. In particular, the nanothermites contained AP of 7.5% produced the specific impulse of 61.0 s, which was ~2.7 times higher than that of the nanothermites without AP. The mass of nanothermites charged in the SPM was different. The average mass of nanothermites without AP was about 1.3 mg, while, the mass for the sample containing AP reached 1.6 mg. Although it is not appropriate to directly compare the total impulse of different nanothermites due to the mass variation, the total impulse may give output magnitude of the micro-thruster and present the strategy to improve the impulse where the volume of combustion chamber is fixed. The combustion duration in the propulsion process was also estimated and presented. With increasing the amount of AP in the Al/CuO based nanothermiets, the combustion duration time gradually decreases. The result suggests that the combustion is enhanced in the 17



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presence of AP, which enables fast thrust and rapid energy release. In addition, the magnitude of gaseous products (or pressure output) was evaluated by constant-volume combustion tests. After ignited, the nanothermites rapidly generated pressure output. Peak pressure is generally associated with the amount of gas production. As shown in Figure 7b, the peak pressure increases with the AP content increases. In particular, the nanothermites containing 7.5 wt% of AP generated the peak pressure to be 1.381 MPa instead of 0.789 MPa for the nanothermites without NC. The result can be employed to explain the enhancement of specific impulse, and further validate that the output performance of nanothermites can be greatly enhanced with the addition of AP.

Figure 7. (a) Displacement curves of propulsion processes tracked by displacement sensor. (b) Output performance of the nanothermites with different AP content. Table 2. Test results of propulsion and combustion Materials

Peak pressure,

Specific

Total impulse,

Combustion

MPa

impulse, s

mN·s

duration, ms

Al/CuO/NC

0.789

22.2

0.283

1.73


1.126

33.5

0.525

1.45


1.242

48.3

0.757

1.33

18


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1.381

61.0

0.957

0.73

Figure 8 presents the SEM images and typical combustion processes of nano-Al/CuO/NC/AP with varied AP content. The nanothermites prepared by electrospray were found to be highly uniform distributed and mixed at nanoscle. Adding AP to the nanothermites did not lead to agglomeration. The result suggests that the AP is compatible with the nanothermites and introducing AP into the Al/CuO based nanothermites is a facile strategy to enhance the combustion and propulsion performance.

Figure 8. SEM images and combustion processes of nano-Al/CuO/NC/AP with different AP content: (a) 0.0 wt%, (b) 2.5 wt%, (c) 5.0 wt%, and (d) 7.5 wt%.

4. Conclusions Al/CuO-based nanothermites with varied content of AP (0.0 to 7.5 wt%) were 19



successfully prepared by electrospray method. The nanothermites with AP were highly uniform distributed without agglomeration. A 10 × 10 solid propellant micro-thruster (SPM) array was fabricated to characterize the combustion and propulsion performance of the prepared nanothermites. The ignition delay time, thermodynamic properties, heat release, specific impulse (Isp) and peak pressure were measured and systematically analyzed to evaluate the output performance of the nanothermites. The results indicate that the addition of AP greatly enhances the combustion and propulsion performance of Al/CuO nanothermites, which may facilitate the practical propulsion and combustion applications of nanothermites.

Acknowledgement This work was supported by the Space Technology Innovation Foundation (CASC150710).

References (1) Ahn, J. Y.; Kim, S. H. Encapsulation of Aluminum Nanoparticles within Copper Oxide Matrix for Enhancing their Reactive Properties. Chem. Eng. J. 2017, 325, 249-256. (2) Rossi, C.; Zhang, K.; Esteve, D.; Alphonse, P.; Tailhades, P.; Vahlas, C. Nanoenergetic Materials for MEMS: A Review. J. Microelectromech. S. 2007, 16 (4), 919-931. (3) Teipel, U., Energetic materials: particle processing and characterization. John Wiley & Sons: 2006. (4) Lee, J.; Kim, T. MEMS Solid Propellant Thruster Array with Micro Membrane Igniter. Sensor. Actuat. A-Phys. 2013, 190 (1), 52-60. (5) Zhang, K. L.; Chou, S. K.; Ang, S. S. Development of a Low-temperature Co-fired Ceramic Solid Propellant Microthruster. J. Micromech. Microeng. 2005, 15 (5), 944-952. (6) Rossi, C.; Orieux, S.; Larangot, B.; Do Conto, T.; Esteve, D. Design, Fabrication and Modeling of Solid Propellant Microrocket-application to Micropropulsion. Sensor. Actuat. A-Phys. 2002, 99 (1), 125-133. (7) Stewart, D. S. Miniaturization of Explosive Technology and Microdetonics. Mechanics of Century 2005, 379-385. 20


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