Variation with stoichiometry

Little information is available on homogeneity ranges and defect structures in the dodecaborides. The only variation from stoichiometry in these borides is for YB,2i the limiting phase determined by density measurements is Yq92B,2. This result can be attributed to the size of Y which is the maximum for metals that form the dodecaborides. Attempts to prepare DyB,2 with a nonstoichiometric composition are conclusive. ... 

Complex Intermetallic Compounds with Significant Variation in Stoichiometry... 

An interesting effect was observed by varying the pressure between 80 and 400 mTorr. At pressures of 180 mTorr and above, the deposition rate jumped from 600 to 2000 A/min. At the same time, resistivity rose as high as 4000 pi2-cm. The variation with pressure is shown in Figure 12. Apparently, the stoichiometry changed dramatically at pressures of 180 mTorr and higher. In fact, there is very little Ta in the film created at 400 mTorr (about 13%), so this is mostly a polysilicon film. 

Two series of sulfides obtained in this manner were prepared they are differentiated only by the temperatures of preparation, 1000° and 800° C. The experimental results are collected in Table I. Lattice constants and density increase linearly with a decreasing S/Ti ratio (Figures 3 and 4). Calculating the mass of the unit cell and studying its variation as a function of composition indicate the nature of the lattice defects responsible for the variations from stoichiometry ... 

The stoichiometry shows some variations with the peroxodisulphate and alcohol concentrations. It is given approximately by... 

SiC has fascinated material scientists not only for its extreme mechanical, thermal and electronic properties but also because of its unusual structural properties. The basic unit of SiC consists of a covalently bonded tetrahedron of Si (or C) atoms with a C (or Si) at the centre. The identical polar layers of Si4C (or C4Si) are continuously stacked and the permutation of stacking sequences allows an endless number of different one-dimensional orderings (polytypes) without variation in stoichiometry. 

Fig. 6. Variation of stoichiometry of Lao9Sro iMn 0(3 5) with temperature and oxygen partial pressure (after Ref. 14).

No effect of the addition of a-CD on the absorption or the emission of an aqueous solution of 15 was found [121], while the addition of p-CD caused a slight variation of the absorption [121-123], which allowed an equilibrium constant for the 1 1 complex, formation to be calculated. The fluorescence emission was monomeric and its yield was scarcely affected by the presence of the CD. The fluorescence decay was practically monoexponential with a lifetime of 72 ns. The addition of [y-CD] = 5 x 10 M to [15] = 2.5 X 10 M caused strong variations in the absorption spectrum, the appearance of a new, excimeric emission concomitant with the decrease of the monomer fluorescence. The lifetimes of the two fluorescences were 60 ns, excimeric, and 50 ns, monomeric. The fluorescence excitation spectrum of the two emissions differed and that of the monomer was very similar to that of 15 in pure water. The plots of the excimer emission increase and of the absorbance decrease with [y-CD] are sigmoids. They were fitted by the sequential formation of complexes with stoichiometry 1 1 [Ki = 1 ), 1. 2 (A2 = 10 M ), and 2 2 (A3 = 2 x 10 M ), as calculated by a computer simulation of the variations [121]. 

For CyD complexes a number of stoichiometric ratios has been observed [2]. The most commonly reported ratios are H G = 1 1 and H G = 2 1. However, other stoichiometries as well as ternary CyD-containing complexes [47] are known. An example of 2 1 stoichiometry is the camphor-a-CyD complex in which the guest molecule is embedded inside a capsule formed by two host molecules [48]. Fenbu-fen (y-oxo-[l,l -biphenyl]-4-butanoic acid) is an interesting example of a compound which shows stoichiometry dependence on the CyD cavity size. It does not form an inclusion complex with a-CyD, but displays H G = 1 1 stoichiometry with f-CyD and H G = 1 2 stoichiometry with y-CyD [49, 50]. Metoprolol is another such compound which forms 1 1 complexes with a-CyD and f-CyD but with y-CyD it forms an H G = 1 2 complex [51]. A similar phenomenon detected using HPLC for a complex with a first-generation dendrimer is presented in Chapter 5 [52]. On the other hand, 1-adamantanecarboxylic acid and f-CyD form a complex with temperature-dependent stoichiometry, H G = 1 1 at 25 °C and H G = 1 2 at 0 °C [28]. For the complexation of dodecyltrimethylammonium bromide with a-CyD two competing associations with stoichiometries of H G = 1 1 and H G = 2 1 have been reported [53]. Use of the method of continuous variations in such situations becomes questionable and information about the complex stoichiometry is revealed directly from the titration measurement described in Section 9.2.3. 

In the calculations which follow, the equivalence of the additives, magnesium carbonate and sodium oxalate, as part of the overall equivalence of the base fire, is small and may be ignored. The simplest way to perform the adjustment exactly, would be to blend the mixtures of the additives and sulfur of the percentage composition given in the cheat sheet. In practice, this is rarely necessary, as so many other factors are used to vary the effect that small variations in stoichiometry are of small impact on the effect the star will produce. With this simplification the calculations for the mix become ... 

In order to detect small deviations from stoichiometric composition, physical properties such as electrical, magnetic, or optical characteristics provide us a valuable information. The measurements of physical properties described above are commonly used with many other systems where variations in stoichiometry appear in compounds as well as with rare earth oxides. 

However, a major limitation of this model is the impossibility of fitting cloud-point curves for polydisperse systems. Moreover, it cannot deal with the fractionation effect accompanying phase separation, i.e. the dispersed phase will be enriched in the highest molar-mass fractions of modifier but in the lowest molar-mass fractions of the growing thermosetting polymer. This may produce variations in stoichiometry and conversion between both phases. These phenomena can be conveniently treated taking polydispersity of constituents into account. 

Recently, the variation of mechanical properties with stoichiometry has been investigated. An increase in fracture toughness from 3 MPa m at B4C to 5-5.5 MPa m / approaching the composition of B13C2 has been observed by Chheda et al. [533]. Whilst Young s modulus slightly decreased, the modulus of rupture (MOR) increased from 387 to 557 MPa. 

Cava et al. (1990). 4 days at ddO C with Zr foil variation of stoichiometry with the amount of Zr. 

Although the described spectroscopic methods for quantitative in-line analysis can in principle be applied to all kinds of screening experiments that comprise systematic variations of process parameters, this approach has to be handled with care since some of the parameters have an undesired non-linear effect on a spectrum. For instance, in NIR spectroscopy, an increase in temperature can cause band shifts and influence the peak height. Variations in stoichiometry, which can be easily achieved in a microreaction process by changing the reactant flows, have a direct impact on the analyzed compound concentration. However, such modifications can be calculated subsequent to the reaction, if concentrations are in the linear detection range or the detection range was calibrated in advance. 

The variation of combustion rate with stoichiometry at 6 MPa is shown in Figure 8.10. 

Figure 10.35 Variation of static radiant intensity with stoichiometry at both sea level and 65 kft [52].

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