Recording Phase Diagrams

There are basically two methods for recording phase diagrams the titration method and the constant composition method. Both have advantages and drawbacks. 

In the titration method a mixture is titrated by another. Typically, a mixture of two of the components is titrated by the third. The weight of titrant to reach a phase boundary is carefully recorded and plotted on the phase diagram. The process is then repeated to cover the whole domain to be investigated. Such a method is relatively fast and can give a good idea of the phase boundaries. 

There are two major drawbacks to this method. First, this method gives the phase diagram at one temperature only. To determine the phase diagram at another temperature, the process must be repeated. The temperature domain available with the titration method is limited for practical reasons, as all the components must be kept at the same temperature. 

The second drawback is that the method is usually used in out-of-equilibrium conditions. In some systems, such as those involving lyotropic liquid crystals, the time required to reach equilibrium can be very long metastable phases can also be encountered. 

A phase diagram recorded by the titration method should be used as a guide only and should never be applied for long-term stability prediction. 

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Amphiphilic compounds such as lipids are able to form monomolecular films when applied to a gas-water interface. These two-dimensional systems can be characterized recording phase diagrams in variation of surface-pressure, area, or temperature. These surface pressure-area diagrams are usually measured on a Langmuir film balance schematically illustrated in fig. 3. 

PHARMSEARCH Phase behavior Phase change recording Phase contrast Phase diagram Phase diagrams... 

In principle two similar crucibles containing respectively a sample of the alloy under investigation and an inert reference specimen are cooled (or heated) together in a furnace. By means of conveniently inserted thermocouples it is possible to follow (1) the continuous reference temperature trend of the reference specimen, (2) the temperature trend of the specimen under examination and (3) the temperature difference (A 7 ) between the two. If, at a certain temperature, there is a change in the structure of the sample (that is, if a boundary line in the corresponding phase diagram is crossed), a variation in the slope of the temperature vs. time curve is recorded. As a consequence the A7 vs. time curve (or vs. temperature) shows a deviation from the baseline. The temperatures at which such deviations are observed are used to build the phase diagram. 

Figure 10. Typical kinetic behavior in the zone B of the phase diagram ). The full line is the best fit of model (3). Only some points of the continuous ESR kinetic record have been represented.

Phase rule studies and describes the occurence of modifications and states of aggregation of pure substances or in mixtures in closed systems as well as the changes which occur in those systems when the pressure, temperature and composition of these substances in the system change. The behaviour of many pure substances and mixtures has thus been studied and recorded in diagrams. These diagrams constitute a vital aid for any scientist studying the development of materials, e.g. ceramics. 

Phase diagrams can either be calculated [15] or determined experimentally. On the experimental side, cooling curves have often been used in which a molten mixture at sufficiently high temperature is slowly cooled and its temperature recorded as a function of time. At the transformation temperature, the sample temperature will remain invariant until the transformation is complete. By comparison, cooling traces using DTA/DSC provide greater phase equilibria sensitivity, since signals from only transformation events are detected and amplified. 

This review has shown the role of phase diagrams and phase relations in fabricating five polycrystalline oxides to optical transparency. Most of the examples cited had a clear connection between phase relations and the first successes in reaching full density and optical transparency. A1203 was the only case history where there was not a clear record of the importance... 

It is relevant that attention should now be drawn to the recent work, by Patterson, Pyke, Reid, Tilley, McAteer, et al. (12-16), who have examined several aspects of the structural and catalytic character of coprecipitated tin-antimony oxides. The structure characterization by Pyke, Reid, and Tilley (12) by X-ray diffraction techniques clearly showed that bulk equilibrium is difficult to establish in this system. This observation alone places some earlier data in a different perspective and implicitly demands a high degree of caution in the interpretation of physical and spectroscopic data recorded from these materials. The phase diagram (Fig. 1) defines the... 

After the assessment of the thermodynamic properties of indium selenides was made by the review, the thermodynamic modelling of the In-Se system by Li, Record, and Tedenac [2003LI/REC] has appeared. The paper brings no new experimental information and does not affect the selection of the thermodynamic properties made by the review. However, the modelling resulted in thermodynamic properties of all condensed phases in the system and the values are given in the pertinent sections. Most of these data are not based on experimental thermodynamic information for individual phases, but are a result of the modelling of the thermodynamics and the phase diagram of the system as a whole. The data are included in the review for information only. 

If the temperature at which the phase diagram is recorded is above Ta (the haze point), a critical point CPa is present near the oil vertex (although pure amphiphile and pure oil are miscible, the presence of a small amount of water recalls the lack of compatibility between amphiphile and oil). If the temperature is below Ta, no critical point appears in the three-component phase diagram (it would be positioned at a negative water concentration). 

In WI and WII representations, the critical points CPp and CP are called plait points. If the difference between the temperature T at which the phase diagram is recorded and the critical point of the binary mixture, 7p or Ta, increases, the distance from the plait point to the oil-amphiphile axis for CPp and the water-amphiphile axis for CP also increases. An important characteristic of a ternary system is the line that links the plait points as a function of temperature. The plait point curve is really the trace of the partitioning of the amphiphile between oil and water. The closer the plait point is to the oil, the more water soluble the amphiphile, and vice versa. 

The chemical isomer shift of Sn has been measured over the entire range of composition of tin with the six metals Cd, In, Sb, Tl, Pb, and Bi [230]. Considerable correlation with the known phase diagrams of the binary alloys was found and in the cases of In, TI, and Pb, large changes in chemical isomer shift were recorded in the vicinity of phase boundaries. 

In the dropping ball method, one records the temperature (Tgs) which the gel can no longer bear the weight of the ball (left). The experiment is repeated at different concentrations, and from the variation of the Tgs with the concentration, one can construct the phase diagram (middle) and calculate AT/qs (left) using the van t Hoff equation. 

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