Thermal methods, phase transitions

Voigt [1705] prepared transparent Ge02 glasses on hydrolysis of Ge(OEt)4 the temperature of glass formation is about l(XfC lower in comparison with that used in conventional methods. On crystallization of amorphous oxide prepared on hydrolysis of Ge(OPri)4, a mixture of two phases, crystoballite and a-quartz, is formed. On further thermal treatment, phase transition of crystoballite to a-quartz occurs, while grinding in air results in the transition to a rutile-type structure on treatment at temperatures higher than 1050°C all phases are tranferredto a-quartz [1766]. 

Before computers and efficient simulation methods became available, the canonical approach was the only way for a theoretical discussion of thermodynamic phenomena. On the experimental side, it was appealing to use the heat bath temperature as an external control parameter. Because of this, in equilibrium, temperature seemed to be an easily accessible parameter to control the macrostate of the system, and transition points of thermally driven phase transitions are typically defined by transition temperatures. The canonical analysis of phase transitions has been extremely successful and it enabled the introduction of fundamental physical concepts such as universality. 

The mobile phase in LC-MS may play several roles active carrier (to be removed prior to MS), transfer medium (for nonvolatile and/or thermally labile analytes from the liquid to the gas state), or essential constituent (analyte ionisation). As LC is often selected for the separation of involatile and thermally labile samples, ionisation methods different from those predominantly used in GC-MS are required. Only a few of the ionisation methods originally developed in MS, notably El and Cl, have found application in LC-MS, whereas other methods have been modified (e.g. FAB, PI) or remained incompatible (e.g. FD). Other ionisation methods (TSP, ESI, APCI, SSI) have even emerged in close relationship to LC-MS interfacing. With these methods, ion formation is achieved within the LC-MS interface, i.e. during the liquid- to gas-phase transition process. LC-MS ionisation processes involve either gas-phase ionisation (El), gas-phase chemical reactions (Cl, APCI) or ion evaporation (TSP, ESP, SSI). Van Baar [519] has reviewed ionisation methods (TSP, APCI, ESI and CF-FAB) in LC-MS. 

Phase transitions, whether first-order or second-order, are potent sources of instability of solid drugs and can usually be detected and studied by thermal methods of analysis (e.g., DSC, TGA, TMA, ODSC, DMA, DEA). In crystalline solids, typical first-order transitions are polymorphic or desolvation transitions. In amorphous solids, second-order transitions, such as glass transitions, are common. 

The sample temperature is increased in a linear fashion, while the property in question is evaluated on a continuous basis. These methods are used to characterize compound purity, polymorphism, solvation, degradation, and excipient compatibility [41], Thermal analysis methods are normally used to monitor endothermic processes (melting, boiling, sublimation, vaporization, desolvation, solid-solid phase transitions, and chemical degradation) as well as exothermic processes (crystallization and oxidative decomposition). Thermal methods can be extremely useful in preformulation studies, since the carefully planned studies can be used to indicate the existence of possible drug-excipient interactions in a prototype formulation [7]. 

It is important to ascertain whether the solid phase of the solute changes during equilibration to produce a different polymorph or solvate, by analyzing the solid phase (using either chemical or thermal analysis, or x-ray diffraction). If a solid-solid phase transition occurs during equilibration, the measured equilibrium solubility will be that of the new solid phase of the solute. Methods of circumventing this problem have been proposed and evaluated [26]. 

Abstract. We review the recent development of quantum dynamics for nonequilibrium phase transitions. To describe the detailed dynamical processes of nonequilibrium phase transitions, the Liouville-von Neumann method is applied to quenched second order phase transitions. Domain growth and topological defect formation is discussed in the second order phase transitions. Thermofield dynamics is extended to nonequilibrium phase transitions. Finally, we discuss the physical implications of nonequilibrium processes such as decoherence of order parameter and thermalization. 

We applied the Liouville-von Neumann (LvN) method, a canonical method, to nonequilibrium quantum phase transitions. The essential idea of the LvN method is first to solve the LvN equation and then to find exact wave functionals of time-dependent quantum systems. The LvN method has several advantages that it can easily incorporate thermal theory in terms of density operators and that it can also be extended to thermofield dynamics (TFD) by using the time-dependent creation and annihilation operators, invariant operators. Combined with the oscillator representation, the LvN method provides the Fock space of a Hartree-Fock type quadratic part of the Hamiltonian, and further allows to improve wave functionals systematically either by the Green function or perturbation technique. In this sense the LvN method goes beyond the Hartree-Fock approximation. 

The phase transition from amorphous to crystalline can sometimes be promoted by thermal treatment (annealing) [ 1.45]. In a laboratory scale, this can be done relatively simple. In a production scale the process must be proven as reproducible and reliable by a validation process, which is time consuming. It is therefore recommended, that a search for CPAs and process conditions, which would lead to crystallization be carried out, using methods such as DTA, DSC, ER and DRS (see Section 1.1.5) also see Yarwood [1.46. If this is not successful, time and temperature for TT should be chosen in such away, that the tolerances for time and temperature are not to narrow, e. g. -24.0 °C 0.5 °C and 18 min 1 min are difficult to operate, while -30 °C 1.5 °C and 40 min 2 min might be easier to control. 

Photochemical switching of the phase transition is also found in the polyion complex film. Figure 29 shows reversible cycles of the absorption at 370nm by the coupling of the thermal and photoinduced phase transition of the complex film with carboxymethylcellulose 8. In conclusion, we indicate that the immobilized bilayer membranes containing the azobenzene chromophore are available to the erasable memory materials based on the phase transition triggered by thermal and photochemical processes. The polyion complex technique is clearly shown to be a very useful method for materialization of the immobilized bilayer membranes. 

Typical applications of x-ray powder diffraction methodology include the evaluation of polymorphism and solvatomorphism, the study of phase transitions, and evaluation of degrees of crystallinity. More recently, advances have been made in the use of powder diffraction as a means to obtain solved crystal structures. A very useful complement to ordinary powder x-ray diffraction is variable temperature x-ray diffraction. In this method, the sample is contained on a stage that can be heated to any desired temperature. The method is extremely useful for the study of thermally induced phenomena, and can be a vital complement to thermal methods of analysis. 

The method of latent heat storage based on liquid-solid phase transition is available to make smaller the volume of heat storage tank, because of its higher thermal density than that of sensible heat storage. Therefore, a substance which has a large amount of latent heat of fusion is more profitable as a heat storage material. 

In DSC the sample is subjected to a controlled temperature program, usually a temperature scan, and the heat flow to or from the sample is monitored in comparison to an inert reference [75,76], The resulting curves — which show the phase transitions in the monitored temperature range, such as crystallization, melting, or polymorphic transitions — can be evaluated with regard to phase transition temperatures and transition enthalpy. DSC is thus a convenient method to confirm the presence of solid lipid particles via the detection of a melting transition. DSC recrystaUization studies give indications of whether the dispersed material of interest is likely to pose recrystallization problems and what kind of thermal procedure may be used to ensure solidification [62-65,68,77]. 

The fabrication process of vanadium oxide (VO2) has also been studied using RBS/C. Since optieal and electrical properties of VO2 are dramatically changed at 68°C due to phase transition, VO2 is regarded as one of the candidates for thermally activated electronic or optical switching devices for optieal fibers or sensors. To obtain the desired properties, the development of the fabrication process for very thin films, without crystalline defects on various substrates, is required. Single-crystalline VO2 thin films on (0001) plane of a sapphire substrate have been synthesized by a laser ablation method. The quality of VO2 was examined by X-ray diffraction and RBS/C method. The eleetrieal resistanee and the optical transmittance of the VO2 film were measured under inereasing and deereasing temperatures. At a temperature of 68 °C, an abrupt transition of resistanee from metal to... 

Phase transitions Differential thermal analysis (DTA) or differential scanning calorimetry (DSC), light scattering and various spectroscopic techniques, diffraction methods (especially X-ray diffraction), measurement of thermal expansion and any other property changing with the transition... 

Yoshida et al. recently disclosed an alternative method that allowed them to produce stable suspensions of gold nanoparticles (1-2 nm in diameter) in nematic liquid crystals [315]. They used a simple sputter deposition process, which allowed them to prepare thin liquid crystal films of well-dispersed gold nanoparticles in both 5CB and E47 (available from Merck) with a nanoparticle size depending on the used nematic liquid crystal. Unfortunately, the authors did not provide any details on whether the nanoparticles were capped with a ligand or bare, non-coated particles, which makes it difficult to assess and compare the reported thermal as well as electro-optic data. However, very similar effects were found as a result of nanoparticle doping, including lower nematic-to-isotropic phase transition temperatures compared to the used pure nematics as well as 10% lower threshold voltages at nanoparticle concentrations below 1 wt% [315]. 

---