טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentEvgenia Golda Kishilev
SubjectInvestigation of Nano-Porous Silicon Based Energetic
Materials
DepartmentDepartment of Aerospace Engineering
Supervisor Professor Emeritus Gany Alon


Abstract

This study investigates a unique energetic material based on nano-porous silicon fuel filled with an oxidizer. This energetic material is very promising due to the high energetic performance as compared to TNT and its integration potential with microelectronics and MEMS (Micro-Electro-Mechanical-Systems) devices, which are realized using the same raw material - silicon. Among the potential applications of this energetic material are actuation and propulsion of MEMS devices, ignition of airbag gas generators, and self-destructing chips for data protection.

Nano-porous silicon is an inert material that can be fabricated by anodization of silicon in a hydrofluoric acid solution. The inert nano-porous silicon structure is transformed into an energetic material by impregnation of a dissolved oxidizer into the nano-pores and evaporation of the solvent. The impregnation process creates a mixture of a fuel (silicon) and an oxidizer in an almost molecular level due to the unique nano-porous structure of the silicon. This energetic mixture can be triggered using different energy sources such as heat, friction, and focused light.

During this research a thorough investigation of the nano-porous silicon fabrication process was conducted. The correlations between different process parameters such as electric current density, hydrofluoric acid concentration, silicon resistivity, and process duration, and nano-porous silicon properties such as pore size, pore depth, porosity, and specific surface area, were established. It was found that the silicon fuel properties can be varied by orders of magnitude by changing the etching process parameters. Additionally, the impregnation method of the nano-pores was optimized by investigation of different impregnation techniques.

The energetic performance of nano-porous silicon with different oxidizers was characterized using DSC (Differential Scanning Calorimeter). DSC thermal analysis showed a similar qualitative behavior of two unique exothermic reaction peaks for the nano-porous silicon with sodium perchlorate, lithium perchlorate, and calcium perchlorate oxidizers. These oxidizers were chosen due to their high potential energetic performance with silicon. Energy outputs for all compositions were quantified by integrating the exothermic peak areas in the DSC plots. Energy outputs of 4.8 - 9.3 kJ/g, up to twice the value for TNT, have been measured. The highest energetic outputs were achieved for nano-porous silicon impregnated with calcium perchlorate (tetra-hydrate). Comparison between the measured energy outputs and the theoretical ones obtained from thermochemical calculations showed a good correlation for all compositions with different oxidizer to fuel ratios. In addition, different thermal ignition techniques were investigated using a hot plate, a hot wire, a Semi-Conductor-Bridge device and a standard initiator.

Finally, the sensitivity of the new energetic material was characterized by performing standard sensitivity tests for friction, impact, and electrostatic discharge. The composition was found to be most sensitive in all tests, suggesting that nano-porous silicon based energetic is a very sensitive primary material. Implementation of this energetic material in industrial applications requires a reduction of its sensitivity without significantly degrading its energetic performance. A method for sensitivity reduction by passivation of the nano-porous silicon surface was suggested.