Stoichiometric compositions

The initial basis for the development of a material in the lithium disilicate system were compositions that were derived from the stoichiometric composition of phyllosilicate crystals, The structure is shown in Appendix 

As shown by the binary phase diagram in Fig. 2-1 according to Levin et al. 

Barret and Hench (1980) and Wu (1985) improved the chemical durability of lithium disilicate glass-ceramics to a significant extent by incorporating additions such as AiPj and K2O to the stoichiometric base glass. The objective of improving the chemical durability of this glass-ceramic was to render the material suitable for use as a biomaterial in human medicine and, in particular, as a restorative material in dentistry. 

---

Manganese Oxides. Manganese(IV) dioxide rarely corresponds to the expected stoichiometric composition of Mn02, but is more reahsticaHy represented by the formula MnO y 2 q, because invariably contains varying percentages of lower valent manganese. It also exists in a number of different crystal forms, in various states of hydration, and with a variety of contents of foreign ions. 

This excess hydrogen is normally carried forward to be compressed into the synthesis loop, from which it is ultimately purged as fuel. Addition of by-product CO2 where available may be advantageous in that it serves to adjust the reformed gas to a more stoichiometric composition gas for methanol production, which results in a decrease in natural gas consumption (8). Carbon-rich off-gases from other sources, such as acetylene units, can also be used to provide supplemental synthesis gas. Alternatively, the hydrogen-rich purge gas can be an attractive feedstock for ammonia production (9). 

FAR or AFR. The composition of a mixture of fuel and air or oxidant is often specified according to the Fuel to Air Ratio (FAR), and can be expressed on a mass, molar, or volume basis. The FAR is normalized to the stoichiometric composition by defining the equivalence ratio ( ) as in equation 1, where = mass of fuel, kg and = mass of oxidizer, kg. 

Dq as die stoichiometric composition is approached. The experimental results for TiC may be represented by die equation... 

The value of the activation energy approaches 50000 near the stoichiometric composition. This diffusion process therefore approximates to the selfdiffusion of metals at stoichiometty where the vacancy concentration on the carbon sub-lattice is small. 

In all of these oxide phases it is possible that departures from the simple stoichiometric composition occur dirough variation of the charges of some of the cationic species. Furthermore, if a cation is raised to a higher oxidation state, by the addition of oxygen to tire lattice, a conesponding number of vacant cation sites must be formed to compensate tire structure. Thus in nickel oxide NiO, which at stoichiomen ic composition has only Ni + cations, oxidation leads to Ni + ion formation to counterbalance the addition of extra oxide ions. At the same time vacant sites must be added to the cation lattice to retain dre NaCl sUmcture. This balanced process can be described by a normal chemical equation thus... 

The sulphur pressure of tire sulphides is a sensitive function of die tnetal/sulphur ratio around the stoichiometric composition, rising sharply when the sulphur content exceeds this limit. It follows from die Gibbs-Duhem equation... 

The stoichiometric flame temperature ( Tg ) is used to characterize the burning gas surrounding the droplets because combustion naturally predominates at a distance where the fastest burning mixture is produced. This mixture approximates to the stoichiometric composition. The selection of the droplet surface temperature BP is discussed below. The enthalpy change for vaporization AH is given by... 

When the overall reaction includes more than two elementary steps, the situation may not be easy to analyze. The product of the nth step is the reactant of the (n -I- l)st step, but in order for these two states to be represented by the same free energy they must have the same composition this means that the stoichiometric composition must be constant throughout the entire series of reactions. Suppose that it has been possible to construct the free energy reaction coordinate. Murdoch gives this method for identifying the rds ... 

Conversely, if the pressure of O2 above a crystalline oxide is decreased below the equilibrium value appropriate for the stoichiometric composition, oxygen boils out of the lattice leaving supernumerary metal atoms or lower-valent ions in interstitial positions, e.g. ... 

However, it is not yet clear why the ener es of the SISF and the twin boundary increase with increasing A1 concentration. To find a clue to the problem, it would be needed to make out the effects of the short-range ordering of A1 atoms in excess of the stoichiometric composition of the HAl phase on the energies of planar faults and the stmcture of dislocation cores in the Al-rich HAl phase. 

Very close to the stoichiometric composition, where the thermal defects dominate the constitutional defects, the results are more complicated. 

To a good approximation this structure can be regarded as f.c.c. based with (002) planes composed alternatively of Tl and Al the cubic crystallographic notation Is. therefore, commonly used. If we denote as c and a the lattice parameters In the [001) and 11001/1010) directions, respectively, the c/a ratio is 1.02 at the stoichiometric composition and Increases to 1.03 at Al-rich compositions (Duwez and Taylor 1952 Huang, et al. 1987). 

Cjt = stoichiometric composition of combustionable vapor in air, expressed as a volume percent. 

The speed of a combustion reaction will be at a maximum at a certain fuel-air ratio that is generally close to the stoichiometric composition. It will be lower, however, for compositions closer to each of the explosive limits. The... 

Qt = stoichiometric composition of combustible vapor in air expressed as a volume percent Co = sonic flow discharge orifice constant, varying VvTth Reynolds number... 

Absolute method. A synthetic sample containing known amounts of the constituents in question is used. Known amounts of a constituent can be obtained by weighing out pure elements or compounds of known stoichiometric composition. These substances, primary standards, may be available commercially... 

In battery electrodes, the stoichiometric composition is usually not accomplished, and oxidation ends at a composition of about PbO, 9g [7]. 

For SNG manufacture, it is necessary to reduce the residual hydrogen to a minimum in order to achieve a high calorific value. This is best realized if the synthesis gas, instead of having a stoichiometric composition, contains a surplus of C02 which can be utilized to reduce the H2 content by the C02 methanation reaction to less than 1% according to equilibrium conditions. The surplus C02 must be removed at the end of the process sequence. It is, of course, also possible to operate a methanation plant with synthesis gas of stoichiometric composition then there is no need for a final C02 removal system. The residual H2 content will be higher, and therefore the heating value will be lower (cf. the two long term runs in Table II). 

---