Porous silicon
Porous silicon ( abbreviated as PSI) is a form of the chemical element silicon. The eponymous peculiarity lies in the nanoporous structure, that is, the pores have a size in the range of 10-9 to 10-7 m. This results in a very high surface to volume ratio in the range of up to 500 m2/cm3. Due to its special optical and electrical properties of porous silicon for the manufacture of solar cells and batteries is suitable.
- 3.1 explosiveness
- 3.2 Optical Properties
History
Porous silicon was discovered in 1956 by Arthur Uhlir Jr. and Ingeborg Uhlir, who worked then at Bell Laboratories in the United States for a process by which the surface of silicon and germanium could be shaped and polished. They discovered that, given suitable conditions, a thick, black, red or brown film on the material surface. These results, however, were mentioned only in a laboratory notebook and not pursued.
Three decades later suspected Leigh Canham, who was busy at this time with the Defence Research Agency in England, in porous silicon quantum confinement effects, which could be verified experimentally in 1990. Only in this way the interest of science to the non -linear optical and electrical properties of the material was aroused.
Production
Anodization
An ability to produce porous silicon, the anodization. This typically platinum is used as a cathode material, silicon as the anode and hydrogen fluoride ( HF) as the electrolyte. While applying a direct current resulting in a homogenous layer of porous silicon is suitable alternating current for the formation of silicon wafers having a thickness of about 50 microns. By the formation of hydrogen gas greater inhomogeneities may occur in this process. To counteract this, the electrolyte ethanol ( min. 15 % ) was added. Thereby, the uniformity can be significantly improved.
Also, porous silicon can be made by etching with hydrofluoric acid (HF ), nitric acid (HNO3), and water. This method is particularly attractive because of its simplicity and the wide availability of the necessary materials. Also in the production of particularly thin films PSI this method is very useful; Coat thicknesses of only 25 angstroms are produced in this way.
Dry
With easy drying by evaporation due to contact of the capillary tension, which is proportional to the curvature of the interface, after a certain layer thickness cracks. Therefore, methods have been developed to minimize the risk for drying psi. Supercritical drying is considered as the most effective drying technique, since in the course of which the interface completely disappears, but is relatively expensive. In pentane drying the water is replaced only by pentane having a lower surface tension than water. During the subsequent drying, only minor tensions.
Properties
Explosiveness
2001, accidentally discovered a working group of the Technical University of Munich, that soaked with liquid oxygen hydrogenated pSi is highly explosive and his explosive power of TNT that exceeds .. Other oxidizing agents avoid the need for very low temperatures and make handling safer.
Optical Properties
The refractive index and the resulting optical properties of a material depend inter alia on the porosity and the medium within the pores from. The refractive index of porous silicon may thus differ significantly from the other silicon species.