Fusion-casting and crystallization of high temperature materials

Fusion-casting of high temperature materials results in a dense interlocking network of crystals. This method produces the highest strength and most corrosion resistant ceramic materials. Macrosructure and microstructure casting determine the properties of a fusion-cast material. Some of these can be predicted from phase diagrams. Non-equilibrium structures are, however, just as common. Examples of the various cases are discussed for typical industrially important compositions, not only in the oxide systems but in several non-oxide systems.     

Correlation between glass transition temperature and melting temperature in metallic glasses

We report that the glass transition temperature (T_g) of a variety of metallic glasses (MGs) correlates with the eutectic or peritectic temperature of two main components corresponding stoichiometric proportion in their binary phase diagram. The correlation suggests that the T_g of MGs is mainly determined by their solvent of two base components, which have composition close to the eutectic and peritectic points in the binary phase diagram and the weakest link in amorphous structure. The results have implications for understanding the structure and glass transition in MGs and for predicting and designing metallic glasses with a desirable T_g.     

Solidification and melting of high temperature materials: in situ observations by synchrotron radiation

Until recently understanding the solidification behavior of high temperature materials, including many intermetallic systems, required evaluation of a great number of individual solidification experimental results. An additional challenge was the reactivity of metallic melts at elevated temperatures. Alternative methods for in situ observation of solidification processes using the high-energy synchrotron X-ray diffraction, which came up in the last decade, are reviewed in the present work. Here, solidifying phases and transformation sequences are directly related to their X-ray diffraction pattern, which avoids any confusion caused by subsequent phase transformations especially in complex systems. By containerless processing with aerodynamic, electrostatic and electromagnetic levitation methods, adapted to the application at the synchrotron beamline, contamination of the melt with impurities is avoided, which can corrupt the results of solidification studies by conventional methods. To date, the majority of the studies is focused on metastable phase formation and the structure of undercooled melts. Current efforts on liquid–solid phase transformations under conditions close to the equilibrium, which provide a great potential for acquisition of phase diagram data of refractory and reactive alloys, are also addressed.     

The local-nonequilibrium temperature field around the melting and crystallization front induced by picosecond pulsed laser irradiation

The local-nonequilibrium model for heat transport around melting and crystallization zone induced by ultrafast laser irradiation is considered. The model predicts strong overheating during melting of the material near the interface. Moreover, the local-nonequilibrium effects lead to an interface temperature gradient steeper than expected from the classical heat flow calculations. Possible modification of the kinetics of melting to include the relaxation effects is also discussed.Paper presented at the Fourth International Workshop on Subsecond Thermophysics, June 27–29, 1995, Köln, Germany.     

High pressure melting and crystallization of Nylon-11

Differential thermal analysis (DTA), high pressure differential thermal analysis (HP-DTA), and high temperature X-ray studies are combined to elucidate the origin of the two melting peaks in Nylon-11. The results of the studies suggest that two species of crystals are involved in the melting of Nylon-11 for samples crystallized at atmospheric pressure or when the environmental pressure is below 4 kbar. At atmospheric pressure, the high melting species is predominant. However, under hydrostatic pressures, the high melting species undergoes phase transition to the low melting species before melting. The amount of the material involved in the transition depends on the pressure. At pressures of 4 kbar or greater, the entire high melting species transforms to the low melting species. The melting behaviour, at atmospheric pressure, of samples crystallized at high pressures also shows two melting peaks if the crystallization pressure is below 4 kbar. The amount of the low melting species increases with increasing pressure and, at 4 kbar or higher, only melting of the low melting species is observed. The X-ray photographs taken at room temperature suggest that samples crystallized between atmospheric pressure and 3 kbar contain both the-form and the-form crystals but the samples crystallized at 4 kbar and higher contain only the-form crystal. However, it appears from X-ray scans taken at high temperatures near melting that the low melting species is of the-form and the high melting species of the -form crystals for samples crystallized below 4 kbar. The-form crystals result from the- transition that occurs at 95° C. Moreover, the melting at high pressures (<4 kbar) of samples crystallized at atmospheric pressure also appears to involve a- transition. These results suggest that both the crystal forms, and, are stable at high temperatures, if the environmental pressure is below 4 kbar, and that only the-form crystals are stable up to melting at pressures greater than 4 kbar.     

Relationship between thermal diffusion and conductivity for two-component crystalline structures

This paper gives a more accurate form of Reinhold's formula, which connects the concentration thermal diffusion effect with the transport number and electrical conductivity. The obtained formula correctly predicts the direction of thermal diffusion and the order of magnitude, but the results are a little too high.     

Novel and durable composite phase change thermal energy storage materials with controllable melting temperature

The development of high temperature phase change materials(PCMs)with great comprehensive per-formance is significant in the future thermal energy storage system.In this study,novel and durable Al-Si/Al2O3-AlN composite PCMs with controllable melting temperature were successfully synthesized by using pristine Al powder as raw material and tetraethyl orthosilicate as SiO2 source.The Al2O3 shell and Al-Si alloy were in-situ produced via the substitution reaction between molten Al and SiO2.Impor-tantly,the crack caused by the incomplete encapsulation of the Al2O3 shell could repair itself by the nitridation reaction of internal molten Al and thereby forming a highly dense Al2O3-AlN composite shell.The produced dense Al2O3-AlN composite shell could significantly improve the thermal cycling stability of composite PCMs,and thus,the thermal storage density decrease of the Al-Si/Al2O3-AlN(59.8 J/g to 77.7 J/g)was far less than that of the Al-Si/Al2O3(118.5 J/g)after 3000 thermal cycles.Moreover,the syn-thesized Al-Si/Al2O3-AlN still exhibited a controllable melting temperature(571.5-637.9℃),relatively high thermal storage density(105.6-150.7 J/g),great dimensional stability and structural stability after 3000 thermal cycles.Hence,the synthesized Al-Si/Al2O3-AlN composite PCMs,as promising preferential thermal energy storage materials,can be stably used in the energy utilization efficiency improvement of various systems for more than 6 years.     

Radiative melting of crystalline ruthenium oxide nanorods

In this investigation, crystalline ruthenium oxide square nanorods have been observed via scanning electron microscopy (SEM) to melt at significantly lower temperatures than the melting temperature of bulk ruthenium oxide (1200?°C) at a measured substrate temperature of only 180?°C. The heating and subsequent melting of these nanorods occurs as a result of the combined effects of enhanced infrared (IR) absorption by the surface plasmon resonance and the inability of the nanorods to radiate at long wavelengths. This can result in the transfer of energy from a lower temperature body to a higher temperature body. This observation does not violate any thermodynamic laws as the entropy of the system is reduced with the concurrent input of energy.     

Nanoscale rapid melting and crystallization of semiconductor thin films

The current study details nanosecond laser-based rapid melting and crystallization of thin amorphous silicon (a-Si) films at the nanoscale using two different optical near-field processing schemes. Both apertureless and tapered fiber near-field scanning optical microscope probes were utilized to deliver highly confined irradiation on the target surface. The various modification regimes produced as a result of the rapid a-Si melting and crystallization transformations were shown to critically depend on the applied laser fluence. Consequently, the crystallized pattern morphology and feature size could be finely controlled. High energy density was observed to impart ablation surrounded by a narrow melt ring. At much lower incident laser energy density, single nanostructures with a lateral dimension of approximately 90 nm were defined.     

Mesogenic molecular crystalline materials

Liquid crystals are increasingly accepted as a separate, fourth, state of matter. Apart from being the foundation of the modern flat panel non-emissive display industry, liquid crystals pervade all forms of molecular materials. Recent research has generated new mesophases in antiferroelectric systems and for molecules with banana-shaped architectures. The disciplines of dendrimers, self-assembling aggregates and co-polymers already have their range of mesogenic forms, and bio-systems are being actively investigated.     

Dynamics of melting and crystallization processes in CdTe under laser irradiation

Numerical modeling results indicate that cadmium vaporization caused by nanosecond pulses of a ruby laser has a significant effect on the dynamics of phase transitions in the near-surface region of CdTe and leads to surface cooling of the material, resulting in a nonmonotonic temperature profile, with a maximum at a depth of about 20 nm. At incident energy densities above the threshold for CdTe melting, the molten zone forming below the surface layer extends both toward the surface and into the bulk of the semiconductor. Cd vaporization and the diffusion of Cd and Te in the melt give rise to tellurium enrichment in the near-surface region. Taking into account the dependences of the crystallization temperature and the latent heat of the phase transition on the Cd and Te concentrations in the melt, we achieved reasonable agreement with experimental data on the effect of incident energy density on the time during which a molten layer is present in CdTe.     

Relationship between static and cyclic fatigue behavior in ceramic materials

International Journal of Fracture -     

Disorder-induced localization in crystalline phase-change materials

Localization of charge carriers in crystalline solids has been the subject of numerous investigations over more than half a century. Materials that show a metal-insulator transition without a structural change are therefore of interest. Mechanisms leading to metal-insulator transition include electron correlation (Mott transition) or disorder (Anderson localization), but a clear distinction is difficult. Here we report on a metal-insulator transition on increasing annealing temperature for a group of crystalline phase-change materials, where the metal-insulator transition is due to strong disorder usually associated only with amorphous solids. With pronounced disorder but weak electron correlation, these phase-change materials form an unparalleled quantum state of matter. Their universal electronic behaviour seems to be at the origin of the remarkable reproducibility of the resistance switching that is crucial to their applications in non-volatile-memory devices. Controlling the degree of disorder in crystalline phase-change materials might enable multilevel resistance states in upcoming storage devices.     

Resonant bonding in crystalline phase-change materials

The identification of materials suitable for non-volatile phase-change memory applications is driven by the need to find materials with tailored properties for different technological applications and the desire to understand the scientific basis for their unique properties. Here, we report the observation of a distinctive and characteristic feature of phase-change materials. Measurements of the dielectric function in the energy range from 0.025 to 3 eV reveal that the optical dielectric constant is 70-200% larger for the crystalline than the amorphous phases. This difference is attributed to a significant change in bonding between the two phases. The optical dielectric constant of the amorphous phases is that expected of a covalent semiconductor, whereas that of the crystalline phases is strongly enhanced by resonant bonding effects. The quantification of these is enabled by measurements of the electronic polarizability. As this bonding in the crystalline state is a unique fingerprint for phase-change materials, a simple scheme to identify and characterize potential phase-change materials emerges.     

Nanoparticles in low melting amorphous selenite materials

The aim of the present investigation is to establish the appropriate routе of nanoparticles formation during heat treatment of selected selenite glasses. Multicomponent compositions containing SeO₂, V₂O₅, TeO₂, MoO₃, ZnO and Ag₂O have been selected. Different preparation methods of the initial glass samples have been combined with heat treatment to influence the glass microstructure and formation of different types of microheterogeneites. TEM and SEM have been used to prove the formation of nanosized particles, randomly distributed in the amorphous matrix volume. Samples containing above 50 wt% Ag₂O show the formation of elementary silver with an average particle size of 50–100 nm. Glass-ceramic materials have been obtained after a long thermal treatment. The main crystal phases detected are Ag₂SeO₃, Ag₂TeO₃ and TeO₂.