Silicon oxidation basic system

The acidic character of silica is shown by its reaction with a large number of basic oxides to form silicates. The phase relations of numerous oxide systems involving silica have been summarized (23). Reactions of silica at elevated temperatures with alkali and alkaline-earth carbonates result in the displacement of the more volatile acid, C02, and the formation of the corresponding silicates. Similar reactions occur with a number of nitrates and sulfates. Silica at high temperature in the presence of sulfides gives thiosilicates or silicon disulfide, SiS2. 

Oscillations have been observed in chemical as well as electrochemical systems [Frl, Fi3, Wol]. Such oscillatory phenomena usually originate from a multivariable system with extremely nonlinear kinetic relationships and complicated coupling mechanisms [Fr4], Current oscillations at silicon electrodes under potentio-static conditions in HF were already reported in one of the first electrochemical studies of silicon electrodes [Tul] and ascribed to the presence of a thin anodic silicon oxide film. In contrast to the case of anodic oxidation in HF-free electrolytes where the oscillations become damped after a few periods, the oscillations in aqueous HF can be stable over hours. Several groups have studied this phenomenon since this early work, and a common understanding of its basic origin has emerged, but details of the oscillation process are still controversial. 

Although few applications have so far been found for ceramic matrix composites, they have shown considerable promise for certain military applications, especially in the manufacture of armor for personnel protection and military vehicles. Historically, monolithic ("pure") ceramics such as aluminum oxide (Al203), boron carbide (B4C), silicon carbide (SiC), tungsten carbide (WC), and titanium diboride (TiB2) have been used as basic components of armor systems. Research has now shown that embedding some type of reinforcement, such as silicon boride (SiBg) or silicon carbide (SiC), can improve the mechanical properties of any of these ceramics. 

Functional oxide materials play an important role for applications in microwave communication and sensor systems. Whereas silicon and GaAs represent the basic materials for the digital part of communication and sensor systems, the analogue parts require high quality factors and low losses, which cannot be fulfilled by semiconductors. Oxide insulators provide extremely low microwave losses expressed by the value of its loss tangent tan 5 = Im(er)/Re(er). The functionality of oxides in microwave devices or circuits can be classified as follows ... 

The mechanism of silane alcoholysis has been the focus of discussions over many years.10a 14 15 17 25 30 A general mechanism is outlined below. This mechanism begins with oxidative addition of the metal into the Si-H bond to form either a r 2 complex (la) or a silyl hydride (lb) (Figure 5). The alcohol then coordinates to the silicon forming a new complex (II) which can lose silyl ether to form a metal dihydrogen complex (HI). The catalyst is regenerated when another silane displace molecular hydrogen from the catalyst. There are several minor variations of this mechanism however this basic mechanism is believed to hold for many catalytic systems. 

Oxidative addition of the silane to the metal is fast and reversible 30 therefore unless the pentacoordinated silane drastically slows down the oxidative addition process, pentacoordination will not alter the rate of the reaction at this stage of the cycle. The increased reactivity of le may be explained by the attack of the alcohol on the pentacoordinated silane that would form after oxidative addition (Figure 9A). The rate of the alcohol addition is increased by the higher reactivity of the pentacoordinated silicon center. This may explain the slower reactivity for those alkoxysilanes that cannot form this intramolecular coordination complex due to the absence of a nearby Lewis basic atom. We had observed during the comparison of aliphatic alcohol to benzyl alcohol that the nucleophilicity of the alcohols has an effect on the rate of the reaction. This is evidence that the alcohol and the silane are involved in the rate-determining step with 10 % Pd/C catalytic system. 

According to Schwartz and Robbins,the etching of silicon in the HF-HNO3 system follows a chemical process with two basic reaction steps. In the first step, sihcon is oxidized by HNO3 which is followed by dissolution of the oxidized Si by HF. The ovCTaU reaction is... 

This model has a basic assumption that the electrochemical reactions involved in the dissolution of the silicon surface operate in microscopic units. These reaction units have a temporal and a spatial distribution in number and in the state of activity. The formation of pores is due to the synchronization of these operational units at certain time and geometrical scales. It is further assumed that the state of reactivity of these units oscillates by the same mechanism as the oscillation involved in the oxide formation-dissolution in HF at anodic potentials (see Section 5.10). Due to the nature of the oscillation the unit on any specific position of the electrode surface can be silent or burst into action resulting in an increase in ciurent. More specifically. Foil et al. assumed the system to have the following features ... 

The complexity of the system implies that many phenomena are not directly explainable by the basic theories of semiconductor electrochemistry. The basic theories are developed for idealized situations, but the electrode behavior of a specific system is almost always deviated from the idealized situations in many different ways. Also, the complex details of each phenomenon are associated with all the processes at the silicon/electrolyte interface from a macro scale to the atomic scale such that the rich details are lost when simplifications are made in developing theories. Additionally, most theories are developed based on the data that are from a limited domain in the multidimensional space of numerous variables. As a result, in general such theories are valid only within this domain of the variable space but are inconsistent with the data outside this domain. In fact, the specific theories developed by different research groups on the various phenomena of silicon electrodes are often inconsistent with each other. In this respect, this book had the opportunity to have the space and scope to assemble the data and to review the discrete theories in a global perspective. In a number of cases, this exercise resulted in more complete physical schemes for the mechanisms of the electrode phenomena, such as current oscillation, growth of anodic oxide, anisotropic etching, and formation of porous silicon. 

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