Silicone ratio

Copper—chromium and copper—nickel—silicon—chromium alloys are also precipitation hardenable. The precipitates are nickel sdicides, chromium silicides, and elemental chromium. If conductivity is critical, the chromium—silicon ratio should be held at 10 1 so that appreciable amounts of either element are not left in soHd solution in the copper after aging. Lithium can be used as a deoxidizer in copper alloys when conductivity is important. For a discussion of the principle of age- or precipitation-hardening copper alloys, see Copperalloys,wrought copperalloys. 

The molecular structure of Li-, Na-, and K-silicates in 0.2 to 3 mole SiOj/L aqueous solutions has been investigated by FTIR and Raman spectroscopy to help exploring their solidification process. These silicates were found to be only partially dissociated and their average molecular weight (AMW) varies with the type of the alkaline ion, the alkaline/silicon ratio, and the concentration. It is demonstrated that these differences are associated with differences in the Qn connectivity ratios of [Si04] tetrahedra and in the dominating siloxane ring structures which can be identified by their vibrational spectra. 

Chondrite-normalized element/silicon ratios measured in chondritic porous IDPs. Cl chondrite ratios are shown by the vertical dashed line. Modified from Schramm et al. (1989). 

The rule reqiiires that in silicates the silicon tetrahedra share no elements with each other if the oxygen-silicon ratio is equal to or greater than four. This is found t o be vrue in general (topaz, zircon, olivine, other orthosilicates), Most of the few exceptional substances now known contain extra oxygen atoms in the form of hydroxide ions. These include the clay minerals, micas, and chlorites, discussed above, and also the mineral hemimorphite, Zn[Pg.561]

The molecular structure depends on the oxygen-to-silicon ratio. If O/Si = 2, each oxygen is covalently bonded to two tetrahedra so a three-dimensional network is formed. At the other extreme, if the O/Si = 4, none of the four oxygen atoms is shared by another tetrahedra, and isolated molecules are formed. Figure 17.2 shows many of the possibilities. 

Figure 14. Oxygen-to-silicon ratio profile for the data presented in Fig. 13. (Reproduced, with permission, from Ref. 20. Copyright 1980, PNL Report 3465.)...

Rebrov et al. [158] synthesized ZSM-5 zeolite in micro channels. The main focus of this work was to assess the performance benefits of zeolitic coatings in micro channels compared with conventional zeolite-based pellets and powders. The coatings were performed in a sandwich of two plates of 1 cm length and width at a thickness of 2 mm. The plates carried seven channels each, which were 1 cm long and had a diameter of500 pm. The plates were positioned at a distance of280 pm from each other in the housing of the reactor. A zeolitic film of one crystal thickness was formed under the optimum synthesis conditions, which were determined as a water/silicon ratio of 130 and a template/aluminum ratio of 2 at a temperature of 130 °C after 35 h on a flat plate. The Si/Al ratio of the zeolite, which also lowers the crystal size when decreased, was optimized to a value of 28. 

To obtain a uniform distribution of the zeolite in the micro channels, a two-step procedure was developed, including nucleation growth at high temperature at the horizontally oriented plates followed by a growth period at the vertically oriented plates, which was performed at lower temperature and water/silicon ratio. The crystals were oriented parallel to the surface of the carrier. Nitrogen adsorption revealed the typical micropore distribution of ZSM-5 for the coating. Thermal cycling... 

Silyl manganese pentacarbonyl decomposes in a more complex way to give a mixture of two phases, "MnSi (actually a silicon-rich nonstoichiometric phase of approximate composition MnSii.25) and stoichiometric Mn5Si3, such that the overall metal silicon ratio is 1 1. The analogous rhenium system behaves similarly. 

These examples illustrate the important general point that the metal silicon ratio of the silicide film is always the same as that in the volatile precursor. 

The sole example of a silicon-platinum cluster is the compound in entry 24 its structure has been noted in Section IV,A. It seems very likely that many further cluster systems await discovery, particularly with iridium, platinum, and gold, and that this represents an important future area of research. One obvious application is as precursors to metal silicides with high metal silicon ratios using c.v.d. techniques (compare Section V,A). 

Figure 2.5 Correlation between argon solubility (in natural logarithm of the Henry s law constant) in silicate melts and various parameters NBO/Si (no bridging oxygen to silicon ratio), molar volume, ionic porosity, and density. Reproduced from Shibata et al. (1998).

The pore structure of two types of catalyst support material were studied 7-alumina and silica aerogel. The alumina samples were commercial catalyst supports made in 1/8 inch diameter pellet form by Harshaw Chemical. Silica aerogels were prepared from silica gels synthesized by a two step acid/base catalyzed procedure employing TE0S with a water to silicon ratio equal to 3.7 [17] and ammonium hydroxide concentration of 0.005 M (sample A) or 0.01 M... 

Many of the inorganic chars are thermally unstable at elevated temperatures, generally as a result of poor control of stoichiometry, that is, high oxygen content and excessively high car-bon/silicon ratios. 

Their abundances are in most cases below solar, i.e., they have lower element/silicon ratios than the Sun or Cl chondrites, they are depleted (see below). In Figure 2, abundances of moderately volatile elements in CVS meteorites relative to those in Cl meteorites are plotted. Increasing depletions correlate with decreasing condensation temperatures but are independent of the geochemical properties of the elements. Depletions of moderately volatile elements in meteorites are produced by incomplete condensation. The amount and the relative abundances of these elements in meteorites are probably the result of removal of volatiles during condensation (Palme et al., 1988). 

Figure 12 Cl chondrite-normalized element to silicon ratios for CS and CP IDPs. The solid line represents frequency of CS IDPs and the dotted line frequency of CP IDPs. Numbers in upper right of each histogram are the number of CS and CP IDPs, respectively, with element to silicon ratios >3 CL CS IDPs are systematically depleted in calcium and magnesium while CP IDPs are only slightly depleted in calcium, aluminum, sulfur, and iron relative to Cl (vertical dotted line) (source Schramm et al., 1989).

Precursors that release stoichiometric silicon ifitride in an inert gas atmosphere and which therefore avoid segregation of silicon metal can be obtained by coammonolysis of dichlorosilane and trichlorosilane. - Such polymers exhibit nitrogen silicon ratios greater than 4 3. Compared to [SiH2NH] copolymers [SiH2NH] ,[SiH(NH)i 5] are more highly cross-linked, but in contrast to [Si(NH)2l , they are processable. 

In other respects, we can consider zeohte membranes as pertaining to the ceramic material category. Indeed zeolites are classified for the most part as microporous, crystalline silico-aluminate stmctures with different alumininum/silicon ratios. Thus, the chemical compositions are close to those of ceramic oxide membranes, in particular of microporous silica and alumina membranes. On the other hand, zeohtes are crystalline materials and they have a structural porosity very different from microporous amorphous silica [124]. Zeohte membranes are well adapted to the separation of gases, in particular H2 from hydrocarbons, but these membranes are not very selective for the separation of mixtures of noncondensable gases. 

For each silicide phase investigated a v-shaped activation energy pattern was found (Fig. 2) [7]. The temperature at the minimum of the rate constants for a given silicide is designated as the reaction start temperature, Tstart- The reaction start temperature, which depends on the nature of the metal, the metal-to-silicon ratio of the silicide phase, and the hydrogen chloride partial pressure, is... 

While the chemistry of other types of silanols has been explored in somewhat detail [30], the reactivity studies of silanetriols, until recently, have mainly been limited to a few esterification reactions [31], The recent objective in this area is to see whether the three OH groups on silicon can be reacted in concert with suitable metal and metalloid precursors to result in three-dimensional polyhedral cages containing a high metal to silicon ratio. 

---