The effect of porosity on energetic porous silicon solid propellant micro-propulsion

Abstract

Energetic porous silicon is investigated as an actuator for micro-propulsion based on thrust and impulse measurements for a variety of porous silicon porosity conditions. Porosity of 2 mm diameter, porous silicon microthruster devices was varied by changing the concentration of hydrofluoric acid and ethanol in an etch solution, by changing porous silicon etch depth, and by changing the resistivity of silicon wafers used for the etch process. The porosity varied from 30% to 75% for these experiments. The highest mean thrust and impulse values measured with a calibrated Kistler 9215 force sensor were 674 mN and 271 μN s, respectively, with a 73% porosity, 2 mm diameter porous silicon device etched in a 3 : 1 etch solution on a 3.6 Ω cm wafer to a target etch depth of 30 μm. As a result of changing porosity, a 23× increase in thrust performance and a 36× increase in impulse performance was demonstrated. Impulse values were also validated using a pendulum experiment in which the porous silicon microthruster was unconstrained, but several non-linearities in the pendulum experimental setup resulted in less consistent data than when measured by the force sensor for microthrusters at this size scale. These thrust and impulse results complement previous work in determining the effect of porosity on other porous silicon reaction metrics such as flame speed.

Energy Harvesting from Energetic Porous Silicon

“…Electrical energy conversion relies heavily on the mechanical force output of the PSi devices. While some samples did indeed provide the approximately 600-mN force as Churaman et al 3 saw with their samples, other samples provided much lower outputs. Testing of our samples on a load cell showed that device outputs varied from 100 mN to 600 mN.…”

“…Energy is also proportional to the integral of the force moment on the end of the cantilever, M, which is equal to the force multiplied by the distance between the applied force to the fixed point of the beam. For example, based on data from Churaman et al, 3 we expect that these 2-mm devices with 22-µm PSi depth should output 600 mN of force. The strain energy in the cantilever under this force, assuming the moment is acting at 2.5 cm from the fixed point of the cantilever, will thus equal (according to Eq.…”

Factors Affecting Substrate Heating with Printed Thermites

“…Typically, the output of the reactive material, usually in the form of hot particles and gases, is employed to actuate a mechanical response or initiate a subsequent chemical reaction. Many such hot particle/gas-driven applications have been reported including the following: micro thrusters [3][4][5]; fuses/ignitors [6,7]; safe and arm devices [8,9]; delay lines [10,11]; MEMS-based injectors [12]; and MEMS balloon/pneumatic actuators [13][14][15]. While the thermal output of thermites has traditionally been used for macro-scale applications such as welding (e. g., railroad rails), metal cutting [16,17], and IR flares; and while it is often suggested that the thermal output could be utilized on the microscale; few micro applications have been realized.…”