Sulfur pressure

AgaSI, is prepared at 210-215°C. In both cases, it takes several weeks to complete the reaction. The high-temperature modification a-AgaSI, which is stable above 235°C, can be obtained either by a treatment at 280°C instead of 210-215°C, as for /3-AgaSI, or by annealing of the latter for some hundreds of hours at 280°C and quenching to room temperature (317). a-AgaSI forms in only 17 hours if the reaction takes place at 550°C under various sulfur pressures (370). 

The sulfur pressure over liquid sodium polysulfides has been measured in the temperature range 400-1000 °C [26]. 

Rau studied the phase diagram of the Nii S phase in detail, i.e. the relation between the composition (the deviation from stoichiometry d, the equilibrium sulfur pressure and the temperature. He measured Ps. by following two methods. The procedure of the first method is as follows ... 

The value of sj v is almost constant (6-7 kcal mol" ) in the measured temperature range and the positive value means that the vacancy-vacancy interaction is repulsive. On the other hand, the value of (/iNis + ) changes sign from minus to plus with increasing temperature. Upon substituting eqns (1.145) for (jUnjs + fi ) and, from eqns (1.146) and (1.147), eqn (1.145) can be rewritten as the relation between y, Uj, and T, as shown in Fig. 1.36. The curves for phase boundaries (thicker curves), i.e. the upper curve for coexistent condensed phases (Ni. S phase + adjacent sulfur rich phase) and the lower curve for coexistent condensed phases (Ni. S phase + adjacent sulfur poor phase), were taken from Refs 26 and 27, in which the temperature dependence of Ps. for coexistent samples was investigated in detail. (As mentioned in Section 1.2, the relationship between the equilibrium sulfur pressure for coexistent condensed phases and temperature must show one to one correspondence. Rau calculated <5 in Nij S for the coexistent phases by substitution of the data from refs 26 and 27 for Os and T into eqn (1.145).)... 

Thus, we can prepare Nij. S with a predetermined composition by controlling the sulfur pressure, using the phase diagram shown in Fig. 1.36. 

Fig. 1.79 Equilibrium sulfur pressure of the V-S system as a function of composition at 800 Curve (a) is an experi-...

Thermodynamic analysis can be useful also in predicting the effect of gas-phase composition on defect concentrations in the solid and, implicitly, on the electrical properties of the deposited film (I, 95, 96). This technique has been used to predict the concentration and change in electrical carriers from electrons to holes in PbS with increasing sulfur pressure over the PbS crystal (96). 

At even higher temperatures, e.g., 1800 °C or above, the partial sulfur pressure of the molten Y-phase approaches or exceeds 1 atmosphere, and a vapor loss is possible, especially for prolonged or repeated heatings. The quenched products commonly consist of a sulfidic matrix with increased amounts of metallic molybdenum, as shown in Fig. 14. 

Due to their nonionic nature, metal sulfides are often nonstoichiometric rather than stoichiometric compounds in the classical sense. For most systems (Cr-S, Ni-S, Fe-S, Co-S, etc.), many phases occur, and each phase exists over a limited range of sulfur pressure and temperature. The nickel-sulfur system, for example, shows many phase transitions and bulk phases at high temperatures and large concentrations of H2S (13-16). Data from Rosenqvist (13) in Fig. 1 show that at temperatures of catalytic interest (673-773 K), nickel exists in the metallic state only at PHlS/Pm values below KT3-10-4. At higher H2S concentrations, the most stable bulk sulfide... 

Semenov and Rjabinin studied the oxidation of sulfur in a vessel containing solid sulfur at temperatures of 80-120 with O2 present at pressures under 20 torr. They found SO2 and SO3 as products, with the SO3 varying from 20 to 60 % of the total products. As in the case of phosphorus oxidation, the total pressure fell asymptotically to a minimum as the sulfur was oxidized. There was also a short induction period, the duration of which was temperature-dependent. Above a certain maximum and below a minimum value of sulfur pressure the oxidation was immeasurably slow. They concluded that either ozone or active S atoms initiated chain reactions by inducing the formation of chain carriers. Semenov and Rjabinin generally found no reaction between sulfur vapor and oxygen at 80-120° unless initiated by a pulsed electrical discharge. This reaction was accompanied by a luminescence, and its rate was almost independent of [O2] when this was below 1 torr, until the [O2] fell to about 0.2 torr, when the reaction ceased. The emission filled the vessel unless [O2] exceeded 20 torr, in which case it was confined to the surface of the sulfur. 

It is not easy to locate the origin of the deviations between the various sets of thermodynamic data listed in Table 1. Rau et al. mentioned that the enthalpies of all reactions except for the formation of S2 could be varied slightly without much effect on the total sulfur pressure which, on the other hand, was taken as the final proof of the consistency of the data set. In addition, the entropy of Sg used by these authors (430 J mol at 298 K) does... 

Sulfur pressure The following cell is suitable to control the chemical potentials and activities of silver and sulfur in Ag2S [31] ... 

Figures 11 and 12 show phase relations in the CrS-FeS section at 800 and 900°C in the S-pressure ranges of 10 -10 and 10°-10 Pa respectively [1987Nar]. With decreasing sulfur pressure, the Cr2FeS4 spinel transforms into the monoclinic and hexagonal sulfides. The sulfiu pressure gap in the two-phase field of the (Cr,Fe)3S4 and (Fe,Cr)i j,S decreases with decreasing temperature. The temperature at which the two-phase field disappears is estimated to be 763 °C.

The spinel-monoclinic transition of Cr2FeS4 was observed at sulfur pressures of 10 -10 Pa at 800°C and at 10°-10° Pa at 900°C [1987Nar]. The results of high-temperature XRD measurements and the phase composition data indicate the occurrence of a reversible solid-phase reaction Cr2FeS4 + Fe dCro.sFco.sS, coexisting with the first-order stmctural transition in Cr2FeS4 [1993Sok]. 

Fig. 11. Cr-Fe-S. Phase relations and stability fields at 800°C as a function of sulfur pressure...

The theory for high-temperature oxidation as described under Section II.C can be used also for sulfidation. Because of the high defect concentration in sulfides, the transport of material is mainly governed by lattice diffusion, so that the Wagner theory is probably more appropriate for many sulfides than for oxides, where grain boundary diHusion may be more important The sulfidation of Fe can be described very well with the Wagner theory. Experimental data are in excellent agreement with the partial sulfur pressure dependence (k = k - the reaction... 

The stationary spray nozzle has the advantage of simplicity and no moving parts (Sulfur Gun, 2011). The spinning cup atomizer has the advantages of lower input sulfur pressure (1 versus 10 bar), smaller droplets, easier flow rate adjustment, and a shorter furnace. 

Fd3m ) Z=8 Spinel-type. X, powder. Appropriate mixtures of the oxides heated under HjS at 650 < r<900°C, the product annealed at 600°C for 3d under sulfur pressure in closed silica tubes. 79 Nog 1... 

Substitute Sn" ions for ions into the preceding metal sulfide with the concentration C = 0.15 at tenqterature 1,246 K and under the di-sulfur pressure of 0.1 mmHg. Compute the concentrations of the various defects. 

On the left of the diagram, we can regard as prevalent the sulfur vacancies V° and electrons (of same concentrations), which consists, under these conditions, of metal sulfide, a n-type Wagner semiconductor with ionized anion vacancies and free electrons. The approximation is much better than that when the sulfur pressure is lower. 

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