Phase transformations, detection

Phase transformations in the solid state (like those in iron), too, have latent heats. They may be small, but with sensitive equipment for measuring cooling curves or heating curves, they are easily detected. 

If the component to be encaged is a liquid at the temperature of investigation, the composition of the equilibrium clathrate cannot easily be determined in the way described previously. In this case the pressure of the system is not sensitive enough for detecting phase transformations so that another property has to be used. 

Secondary processing does not always lead to phase transformations, as was shown during studies of the polymorphs of ranitidine hydrochloride [92]. No solid-solid transformation could be detected during either the grinding or compression of metastable Form I, stable Form II, or of a 1 1 mixture of these forms. The dissolution rates of both forms were found to be equivalent, and the solution-mediated transformation of Form I to Form II was observed to be slow. 

Topsee and coworkers—in situ XRD synchrotron studies indicate well-dispersed metallic Cu particles upon activation ZnO observed to strain Cu particles by EELS. Topsoe and coworkers,264 utilizing in situ XRD with synchrotron radiation, demonstrated that the Cu phase transforms primarily to a crystalline metallic Cu phase from CuO precursor during activation. Smaller particles were detected when the ternary A1203 component was present (9.5 nm versus 14 nm for the binary Cu/Zn catalyst), indicating that alumina acts primarily as a structural stabilizer, a spacer for well-dispersed Cu particles, which assists in minimizing sintering. 

The equilibrium solubility of an Fe oxide can be approached from two directions -precipitation and dissolution. The first method involves precipitating the oxide from a supersaturated solution of ions with stepwise or continuous addition of base und using potentiometric measurements to monitor pH and calculate Fej- in equilibrium with the solid phase until no further systematic change is detected. Alternatively the oxide is allowed to dissolve in an undersaturated solution, with simultaneous measurement of pH and Fejuntil equilibrium is reached. It is essential that neither a phase transformation nor recrystallization (formation of larger crystals) occurs during the experiment this may happen with ferrihydrite which transforms (at room temperature) to a more condensed, less soluble phase. A discussion of the details of these methods is given by Feitknecht and Schindler (1963) and by Schindler (1963). 

All of our observations lead us to believe that the crystallization of VPI-5 involves a solid phase reordering process. This solid phase transformation does not involve TBA as either a template or a space-filler. TBA may be present in the solid phase in very low concentrations that are below detection limits by IR and NMR and may still play some role in the crystallization mechanism. Also, it may serve to moderate the pH of the reaction mixture during its formulation and heating such that certain aluminophosphate precursor species are present in the solid phase. Here we will explore these two possibilities in order to further understand the crystallization process for forming VPI-5. 

No phase transformation of zirconia was detected, using x-ray diffraction analysis, in the hot-pressed sinter at any of the HAP/PSZ ratios used in this study. 

We will discuss some preliminary results, which have been performed recently l01). In Fig. 39a the results for polymer No. 2d of Table 10 are shown, which were obtained by torsional vibration experiments. At low temperatures the step in the G (T) curve and the maximum in the G"(T) curve indicate a p-relaxation process at about 120-130 K. Accordingly the glass transition is detected at about 260 K. At 277 K the nematic elastomer becomes isotropic. This phase transformation can be seen only by a very small step in G and G" in the tail of glass transition region, which is shown in more detail in Fig. 39 b. From these measurements we can conclude, that the visco-elastic properties are largely dominated by the properties of the polymer backbone the change of the mesogenic side chains from isotropic to liquid crystalline acts only as a small disturbance and in principle the visco-elastic behavior of the elastomer... 

LoJacono et al. (108) also utilized X-ray diffraction methods to study the structural and phase transformations which occurred in the Bi-Fe-Mo oxide system. They detected two ternary compounds containing bismuth, molybdenum, and iron. One of the compounds formed when the atomic ratio Bi/Fe/Mo = 1 1 1 the other formed when the atomic ratio Bi/Fe/Mo = 3 1 2. The X-ray data indicated a close structural relationship of the bismuth iron molybdate compounds with the scheelite structure of a-phase bismuth molybdate. Moreover, their structures were similar to compound X. The structure of the Bi/Fe/Mo = 3 1 2 compound was identical to the compound reported by Sleight and Jeitschko (107). The authors proposed that the structures of both of the compounds could be viewed as resulting from the substitution of Fe3+ in the a-phase lattice. In the Bi/Fe/Mo = 1 1 1 compound, 1 Mo6+ ion is replaced by 2 Fe3+ ions one Fe3+ ion occupies a Mo6+ site the other Fe3+ ion occupies one of the vacant bismuth sites. In the Bi/Fe/Mo = 3 1 2 compound, the Fe3+ ion replaces one Mo6+ ion while the additional Bi3+ ion occupies one of the vacant bismuth sites. 

Figure 7.8 indicates the effect of a phase transition on the large signal strain. Specimens without a detectable phase transformation (pzt (53/47) and (55/45)) reveal a similar slope, whereas a significant change could be observed for pzt (54/46) in the temperature range... 

Li, et al. (2003), Phase Transformation in the Surface Region of Zirconia and Doped Zirconia Detected by UV Raman Spectroscopy , Phys. Chem. Chem. Phys., 5, 5326-5332. 

Differential Scanning Calorimetry (DSC) is a sensitive way of detecting phase transformations of a bulk material [85,86]. Monitoring the thermal behavior of a crystal or a powder as a function of its conversion to product can give important information. This technique can verify whether a reaction occurs in a purely solid phase or whether there may be liquid phases involved at a given temperature. Melting point depression can be monitored as product appears, and the characteristic melting of a new phase can be detected if one is formed. DSC can reveal whether or not a eutectic transition attributable to a mixture of phases is present. We have also used DSC in our lab to monitor the thermal stability of reactive crystals. 

When the sample undergoes a transformation, it will either absorb (endothermic) or release (exothermic) heat. For example, the melting of a solid material will absorb heat, where that thermal energy is used to promote the phase transformation. The instrument will detect that the sample is cooler than the reference, and will indicate the transformation as an endotherm on a plot of differential temperature (AT) versus time.2 Figure 3.2 shows a typical DTA trace of the decomposition of dolomite. If the sample and reference are exposed to a constant heating rate, the x-axis is often denoted as tem-... 

Figure 6. Effect of reaction time on zeolite phase transformation. Keys to symbols indicate species detected after 1 hour.

It is significant that the second calorimetric peak and the associated isotherm sub-step were detectable only if the graphitized thermal black had been heated at temperatures above 1700°C. These results suggest that the 2-D phase transformation is very sensitive to the perfection of the surface basal planes and this is a further indication that the phase change leads to the development of a commensurate structure. 

From the experimental standpoint, the use of a.c. techniques offers many advantages. Sensitivity is much higher than in d.c. measurements, since phase-sensitive detection can be used and very small probe signals can be employed ( 5mV). The technique is therefore a truly equilibrium one, unlike cyclic voltammetry. An alternative approach to the commonly used sinusoidal signal superimposed on the selected d.c. potential is to use a potential step and to employ Laplace transform methods. Instrumentally, this is rather more demanding and the advantages are not clear [51]. Fourier transform methods have also been considered and their use will have advantages in terms of the time-scale for an experiment, especially at very low frequencies. 

During thermal treatment, a-Mg(BH4)2 first undergoes a phase transformation at about 190 °C before it further decomposes in several steps to MgH2, Mg, and MgB2 (Eqs. (5.10) and (5.11)) [33]. The weight loss between 290 and 500 °C is 13 wt.% H2 with a maximum release between 300 and 400 °C. Small amounts of B2H6 were detected by mass spectrometric measurements. The presence of TiCl3 decreases the... 

Phase transformation and hydrogen release were detected by DSG, as shown in Figure 9.10. Mg2NiH4 was used as a reference. 

---