Paper PS-ThA2
High Rate Production of Silicon Nanoparticles Through a Microwave Torch Production Process

Thursday, November 13, 2014, 2:40 pm, Room 305

Nanometer sized aluminum particles are currently utilized as an energetic additive in propellant, pyrogen, and explosives formulations. Nano-silicon is potentially an attractive replacement for nano-aluminum in these applications since it has a similar energy density while being less sensitive, and thus safer to handle. In addition silicon forms a thin passivation layer which makes it a stable additive compared to aluminum which can oxidize in-depth. Incorporating nano-silicon into energetic formulations is currently limited by the high cost of the material which is generally formed in low production rate, batch processes.

This paper describes the development of a scalable, continuous (non-batch) high production rate method for nano-silicon utilizing a microwave driven plasma torch-based process. Silane (SiH₄) is injected near the throat of the supersonic output nozzle of the torch where it dissociates in the near atmospheric pressure nitrogen plasma formed by the microwave discharge. The local gas temperature in the torch plenum is approximately 2800 K which is sufficient to produce greater than 0.9 moles of silicon atoms for each mole of SiH₄ (>90% efficiency). The resulting silicon atom rich gas is rapidly quenched (~2x10⁸ K/s) in a supersonic expansion into a vacuum chamber (~ 50 torr) producing the seed particles that then grow to 10 – 20 nm in approximately 0.1 ms. The process meets the criteria specified by Kodas and Frielander for producing monodisperse particles in a flow reactor by namely: 1) Separating seed particle production in the supersonic expansion from subsequent growth of the seeds downstream of the nozzle, 2) Providing a flat velocity profile in the growth region enabling a uniform residence time for the particles in the growth region, and 3) Utilizing a high velocity jet which results in a short residence time in the growth region to minimize particle agglomeration.

Growth conditions will be described that produce 6 g/min of 10 -20 nm diameter silicon particles with a 140 m²/g surface area from a 2 kW microwave discharge. Material analysis will be described including SEM and TEM to assess the particle morphology and size distribution, single point and BET surface area measurements, and EDX, differential scanning calorimetry, and electrochemical performance to assess purity and energetic characteristics of the material. Future efforts will extend the growth region of the process to enable larger particle sizes and scale up the microwave system to 5 kW to enable 15 – 30 g/min production rates.