Applications reactive elements doping

Although Y is one of the most often discussed reactive elements, its applicability to intermetaiiics is limited because of its low solubility. This tends to lead to the formation of Y-rich precipitates that can result in non-ideal behavior (e.g. Kuenzly and Douglass, 1974). Additions of Hf or Zr seem to be preferable. Adding too much RE can, however, be a problem when over-doping can lead to oxidation behavior worse than that of the undoped material (Doychak, 1994 Pint et al., 1996 Pint, 1998), e.g. when REs are added to improve mechanical properties. Addition of 0.2 at.% Hf to NiAl (Darolia and Walston, 1997) can result in initial improvements in oxidation behavior but could result in significant oxidation problems in long-term high-temperature oxidation because of internal oxidation of the Hf. 

Methods for electrochemical, catalytic (metal assisted), and deep reactive ion etching (DRIE) of silicon have been developed, which enable fabrication of arrays of deep cylindrical or modulated pores, walls, tubes, combinations of these, and other forms with vertical walls (Wu et al. 2010). As a rule, the regular arrays produced by electrochemical etching are characterized by constant porosity and pore depths (up to 500 pm) and form a planar front propagating into the substrate. Various devices and functional elements for micromechanics, photonics, chemical power sources, microfluidics, photovoltaics, etc. (see Porous Silicon Application Survey chapter), are commonly fabricated on the basis of these arrays by post-anodization treatment intended to modify the structure and properties of macroporous silicon to raise or reduce its porosity, change the shape of pores, transform the pore array into a column array, change the properties of the inner surface of pores, coat it with the film of a metal or insulator, open up pores, fill pores with various fillers, dope the silicon walls, etc. Some procedures can be performed locally, which requires formation of a pattern and subsequent structuring. 

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