Porous silicon as a matrix for energy-related applications

Porous materials based on silicon are of great general interest, with a focus on energy-related topics. The energy released as a result of an oxidation reaction between porous silicon (PSi) and a solid oxidizer embedded within its pores is of interest as a potential new type of energetic material for various applications. In this report, we describe a lift-off method for forming suspended flakes of porous silicon subsequently oxidized and analyzed within a bomb calorimeter. The PSi samples’ mesopores (with a 30 ± 10 nm diameter) were electrochemically produced in an HF/ethanol solution. Two other electro-less etching methods are discussed (galvanic and stain etching) and compared to the electrochemical pathway. The temperature dependence of metal-assisted etching is reported here for the first time. The oxidizing material was primarily sodium perchlorate NaClO4, embedded within the pores. The energy released was analyzed per unit weight of the PSi + NaClO4 mixture at a stoichiometric ratio, revealing that the pure, suspended PSi reacted to provide 32 KJ/g of Si and 8.5 KJ/g of (Si + NaClO4) when activated in pure oxygen. This is about twice as high as the energy released by 1 gram of TNT. Similar results are obtained when technical Si powder is stain-etched, characterized, and activated. Replacing the rather sensitive NaClO4 oxidizer with a perfluoro-polymer retains the high energy release but is much less sensitive to activation tests, such as a falling hammer, thus categorizing it as a secondary explosive material that can be well controlled. Simple thermodynamic calculations of the expected energy release upon the oxidation reaction quantitatively explain the experimentally obtained values.