Liquid-solid Transformation Research Articles

Influence of an axial high magnetic field on the liquid–solid transformation in Al–Cu hypoeutectic alloys and on the microstructure of the solid

The influence of an axial high magnetic field (up to 10 T) on the liquid–solid interface morphology and microstructure of the solid has been investigated experimentally during Bridgman growth of Al–Cu hypoeutectic alloys. It is found that the field causes the interface to become destabilized and irregular, and promotes planar–cellular and cellular–dendritic transformation. The field has a great influence on the cellular and dendrite array morphology. Indeed, the field causes severe distortion in the cellular array and enhances cell branching. The field makes the morphology of the dendrite array more complex and, with the increase of the magnetic field intensities and decrease of the growth velocities, the dendrites become broken and orientate with the 〈1 1 1〉-direction along the solidification direction instead of the 〈1 0 0〉-direction. Furthermore, the field also enlarges the primary dendrite spacing and promotes the branching of the dendrites to form higher-order arms. The above phenomena may be attributed to the change of the equilibrium partition coefficient k and the liquidus slope m L caused by the field, the magnetic anisotropy of the α-Al crystal and the flow created by the thermoelectromagnetic convection.

Size, shape and stress effects on the melting temperature of nano-polyhedral grains on asubstrate

During the first steps of vacuum deposition of thin films, grains appear randomly onthe substrate as isolated islands. To characterize the formation of grains in thinfilms, the melting temperature of grains is studied theoretically as a functionof their size and shape. Assuming a liquid grain that is a portion of a sphere,during the liquid–solid transformation the nano-grains could solidify adopting asphere, portion of sphere, tetrahedron, or parallelepiped shape. These cases areinvestigated here. The effect of stress on the melting temperature is also considered.

Modeling the interdependence of processing and alloy composition on the evolution of microstructure in Sn-based lead-free solders in fine pitch flip chip

Market demands and legislation are driving the electronics-manufacturing sector to move rapidly toward a lead-free future, with Pb-containing electronics products to be banned in Europe after 2006. Although the related scientific research has been undertaken for a decade, a number of technical complications still exist, which are further exacerbated due to the concurrent developments in miniaturization and multifunctionality of microelectronic products. As the packaging joint geometry shrinks toward a microscopic scale, the joint fabrication and reliability become extremely sensitive to the composition and resulting microstructure generated from the chosen joining process and materials. The current level of understanding of such issues is still in its infancy and therefore requires further fundamental study. Thermodynamic modeling is employed in this work as a computational tool to study the sensitivity of processing ranges (e.g., reflow temperature) and the resultant reliability of the microjoints by changing the alloying elements and their content in Sn-based lead-free systems. The work is implemented using the MTDATA program developed by the National Physical Laboratory. With a newly developed database containing critically assessed thermodynamic data appropriate for lead-free solder systems, MTDATA allows the prediction of the liquid-solid transformation and phase formation, for example, as a function of chemical composition and temperature. The paper emphasizes the formation and mass fraction of intermetallic precipitates of different phases in the bulk solder joints and the modeling is also validated through experimental work and recent literature. The results are expected to assist the optimization of processing parameters and cost-effective production using lead-free solders.

Liquid Ga Penetration along Al Grain Boundaries: Effect of External Stress and Ga Undercooling

The effects of compressive stress and undercooling of Ga to liquid gallium penetration along grain boundaries (GBs) of aluminum were investigated. It was shown that the penetration rate does not change with the temperature if gallium is in liquid state. The effect of compressive stress applied to Al samples was demonstrated. The time to wetting of all aluminum GBs increased several orders of magnitude if the compressive stress was between 0.1 and 6 MPa. It was proved that solid-liquid transformation does not take place during penetration process.

A comparison of numerical models for one-dimensional Stefan problems

In this paper, we present a critical comparison of the suitability of several numerical methods, level set, moving grid and phase field model, to address two well-known Stefan problems in phase transformation studies: melting of a pure phase and diffusional solid-state phase transformations in a binary system. Similarity solutions are applied to verify the numerical results. The comparison shows that the type of phase transformation considered determines the convenience of the numerical techniques. Finally, it is shown both numerically and analytically that the solid-solid phase transformation is a limiting case of the solid–liquid transformation.

Phase field modeling of self-assembling nanostructures in constrained films

We present a thermodynamic analysis and phase field modeling of self-assembled multiphase nanostructures produced by phase transformations in constrained layers. Due to coherency between the phases, the elastic interactions between them and between each phase and the substrate layer play an important role in the formation of the nanostructures. It has been shown that a variety of morphologies of heterophase nanostructures can be obtained depending on the crystallographic characteristics of transformations, elastic properties of the phases, relative fractions of the phases, and the thickness of the film. The results obtained by phase-field modeling agree well with predictions of an analytical thermodynamic model. The final equilibrium structures are determined by thermodynamic parameters and do not depend on the transformation path and, therefore, the phase-field approach developed in this paper can be expanded to finding equilibrium multiphase coherent nanostructures created as a result of solid–solid or solid–liquid transformations as well as during co-deposition on a substrate.

Particle in cell simulation of combustion synthesis of TiC nanoparticles

A coupled continuum-discrete numerical model is presented to study the synthesis of TiC nanosized aggregates during a self-propagating combustion synthesis (SHS) process. The overall model describes the transient of the basic mechanisms governing the SHS process in a two-dimensional micrometer size system. At each time step, the continuum (micrometer scale) model computes the current temperature field according to the prescribed boundary conditions. The continuum system is discretized with a desired number of uniform computational cells. Each cell contains a convenient number of computational particles which represent the actual particles mixture. The particle-in-cell (discrete) model maps the temperature field from the (continuum) cells to the particles. Depending on the temperature reached by the cell, the titanium particles may undergo a solid-liquid transformation. If the distance between the carbon particle and the liquid titanium particles is within a certain tolerance they react and a TiC particle is formed in the cell. Accordingly, the molecular dynamic method updates the location of all particles in the cell and the amount of transformation heat accounted by the cell is entered into the source term of the (continuum) heat conduction equation. The new temperature distribution progresses depending on the cells which undergo the chemical reaction. As a demonstration of the effectiveness of the overall model some examples are shown.

Large-scale synthesis of antimony nanobelt bundles

The large-scale synthesis of antimony nanobelt bundles has been facilely realized in existence of polyethylene glycol (PEG) by a hydrothermal reduction method using aluminum powder as reducing agent. These nanobelts have width in the range of 200–600 nm and length up to several micrometres, and the width-to-thickness ratio is ca. 10. FESEM images show individual nanobelts have multi-layered structures. The size of nanobelts can be varied by adjusting the molecular weights of PEG. Based on a series of comparative experiments under different reaction conditions, the probable formation mechanism of antimony nanobelt bundles is proposed to be a solid–liquid–solid transformation and surfactant-assisted directional growth process.

Numerical Modeling of the Transient Liquid-phase Diffusion Bonding Process of Al Using Cu Filler Metal

A numerical modeling of dissolution and isothermal solidification during the transient liquid-phase (TLP) diffusion bonding process of Al using pure Cu filler metal based on a diffusion-controlled model was carried out. In the modeling, both the changes in volume accompanying interdiffusion between the base metal (All and the filler metal (Cu) and the solid-liquid transformation were taken into account by using variable grids. The effect of a load applied to the base metal was also examined by considering simple force balance among the surface and interface energies of the base metal and liquid formed in the bonding region. The early dissolution process simulated by the developed model agreed with the experimental results, and the predicted isothermal solidification time of a sample with an applied load also agreed with the experimental results.

Investigation of the polymorphic transformations from glassy nifedipine

The polymorphic behaviour of nifedipine was investigated using differential scanning calorimetry (DSC), modulated temperature DSC (MTDSC) and hot stage microscopy (HSM). DSC and MTDSC scans of glassy nifedipine samples revealed the presence of several heating rate-dependent events occurring prior to the main melting endotherm (172 °C) including solid–solid recrystallisation on MTDSC curves. These events were related to the recrystallisation and transformation between different polymorphs of nifedipine. Recrystallisation, solid–solid and solid–liquid–solid transformation processes occurred in glassy nifedipine when heated from 45 °C to its final melting point of 172 °C, including a glass transition at ∼48 °C. It was concluded that there were probably four polymorphic forms of nifedipine. The chemical stability of nifedipine during DSC scanning was established by high performance liquid chromatography (HPLC) with diode array UV detection.

Properties of a new nanosized tin sulphide phase obtained by mechanochemical route

The paper deals with the properties of a new 4H-SnS2 phase (N-phase) synthesized together with berndtite 4H-SnS2 by a mechanochemical route from elemental tin and sulphur. The surface and bulk properties of SnS2 phases were verified by surface area measurements, particle size analysis, electron microscopy, XRD, Mössbauer spectroscopy and chemical dissolution methods. The particles of synthesized product are 12–18 nm in size and during milling, secondary particles (aggregates) are formed that are 20–60 μm in size. The particle size and a high developed surface area (7–21 m2 g−1) greatly influence the solid–liquid reaction and solid transformations of mechanosynthesized tin sulphide products.

Influence of an electric or magnetic field on the liquid–solid transformation in materials and on the microstructure of the solid

The influence of an electric or magnetic field on the liquid–solid transformation in materials and the microstructure of the resulting solid is reviewed. In the case of metals, electromigration in the liquid produced substantial changes in composition due to differences in ionic mobility. Further, both a small, continuous d.c. current (∼0.1 A cm −2) and high density (∼10 3 A cm −2) electropulsing refined the microstructure of castings. A magnetic field (>1 T) applied during directional solidification significantly reduced the convective flow in the melt and distorted the cellular array, but did not affect the dendritic array, nor the macrosegregation. The mechanisms by which these effects on the microstructure occur are not clear. In the case of electropulsing, the influence of the current appears to be to enhance the nucleation rate and at the higher current densities to deform and fracture dendrites by the pinch effect. The influence of a magnetic field appears to be largely due to the Lorentz forces which are established between the motion of the conductive melt and the applied field. Regarding semiconductors, an electric current of 1–10 A cm −2 enhanced the growth rate of GaAs single crystals on a substrate and reduced the dislocation density in the product. Theoretical treatments of these effects are in good accord with the experimental results. In the case of polymers, an electric field of ∼1 V cm −1 ‘pulled’ a camphor single crystal from a solution of camphor in CCl 4. The mechanism by which this interesting phenomenon occurs still needs to be established.

A new type of phenolic lignin-based network polymer with the structure-variable function composed of 1,1-diarylpropane units

Phenolic lignin-based network polymers with structure-variable func-tion have been designed, and a reaction process has been developed for synthesizing them directly from native lignins. The process includes a phase-separation reaction system composed of phenol derivatives and concentrated acid. The key point of the process is that lignin and carbohydrates, which form an interpenetrating polymer network within the cell wall, are separated into different phases and their modifications are controlled individually. The resulting lignin-based polymers (lignophenol derivatives) have unique functions, which conventional lignins do not have, in spite of retention of the original interunit linkages: these include highly phenolic property, no conjugated system, light colour comparable with native lignin, structure-variable function and solid–liquid transformation. These original functions are due to the selective hybridization of lignin structural units with monomeric phenol derivatives at Cα positions, leading to network structures composed mainly of 1,1-diarylpropane-type units. New types of applications of lignophenol derivatives using their original functions are described (biocatalyst system and lignin–fibre composites). © 1998 Society of Chemical Industry

A new type of phenolic lignin‐based network polymer with the structure‐variable function composed of 1,1-diarylpropane units

Phenolic lignin-based network polymers with structure-variable func-tion have been designed, and a reaction process has been developed for synthesizing them directly from native lignins. The process includes a phase-separation reaction system composed of phenol derivatives and concentrated acid. The key point of the process is that lignin and carbohydrates, which form an interpenetrating polymer network within the cell wall, are separated into different phases and their modifications are controlled individually. The resulting lignin-based polymers (lignophenol derivatives) have unique functions, which conventional lignins do not have, in spite of retention of the original interunit linkages: these include highly phenolic property, no conjugated system, light colour comparable with native lignin, structure-variable function and solid–liquid transformation. These original functions are due to the selective hybridization of lignin structural units with monomeric phenol derivatives at Cα positions, leading to network structures composed mainly of 1,1-diarylpropane-type units. New types of applications of lignophenol derivatives using their original functions are described (biocatalyst system and lignin–fibre composites). © 1998 Society of Chemical Industry

STUDY OF THE CRYSTALLIZATION OF NODULES CONTAINING A PHASE CHANGE MATERIAL FOR COOL THERMAL STORAGE

Air conditioning and refrigeration have an increasing technological importance and the use of storage process is economically a good solution. The presented cool thermal storage process consists in the use of the solid-liquid transformation of Phase Change Materials (PCM) encapsulated in spherical nodules. These nodules fill a tank placed in the refrigeration loop. Laboratory experiments allow to know exactly the crystallization process of one nodule or of a small number of nodules and to specify the nature and the importance of the undercooling phenomenon (delay of crystallization). The erratic character of the crystallizations is particularly in evidence. A model, based on a combination of the energy equation and nucleation theories, is validated by the experiments and gives information on the kinetics of phase transformations. Copyright © 1996 Elsevier Science Ltd.

Synthesis of ZSM-5 Using a TBP Template and an Oligomeric Silicic Ester

Abstract The syntheses of pentasil zeolites were carried out using the system xNa2O·y(TBP)2O·Al2O3·zSiO2·1050H2O, keeping z = 50—600, y/z = 0.03 to 0.1, and x/z = 0.01 to 0.05 at 448 K. Pure ZSM-5 phase is found to form for any value of z in the range specified when y/z = 0.02 and x/z = 0.01. When these compositional ranges and ratios are altered either ZSM-11 or intergrowth of ZSM-5/11 or formation of ZSM-5 or ZSM-11 and quartz takes place. The decomposition of the templates, from the channels of ZSM-5 of different SiO2/Al2O3 ratio, showed differences in their patterns. After calcination of the samples phosphorus is retained in the zeolite channels in the form of a surface complex. Thermal analysis and FTIR studies support this observation. SEM pictures drive to the conclusion that a liquid–solid transformation mechanism operates in the crystallization of ZSM-5 under the conditions of the synthesis studied.

Microstructural development in rapidly solidified TiNi alloys

The TiNi binary system under non-equilibrium conditions offers an exciting range of metastable microstructures including nanodispersions in glassy and crystalline matrices. We report a systematic study of these microstructures and phase stability for a wide range (10–90 at.%) of rapidly solidified TiNi binary alloys. This study has revealed the nature of the metastable phase equilibria and kinetic pathways for liquid-solid transformation in undercooled TiNi melts during rapid solidification.

Crystal Data and Solid-Liquid Transformation Study by DSC of 2 Hydroxymethyl-2 methyl-1,3 propanediol tetrahydrate

Abstract2 Hydroxymethyl-2 methyl-1,3 propanediol tetrahydrate has been crystallised from aqueous solutions in the form of transparent tetragonal thin plates. The cell parameters of the unit cell were determined as a = 9.102(1) Å, c = 47.654(6) Å and the space group is I41/ a.Solid liquid and reverse transformations have been studied by DSC, and a melting enthalpy of 185 kJ.kg−1 at 29.8° C determined.

Whiteness in zeolite A prepared from Shirasu volcanic glass

Fine white crystals of zeolite A were formed from iron-containing water glass and Na 2 SiO 3 ·9 H 2 O extracted from Shirasu volcanic glass. The whiteness in the products was affected by the degree of crystallinity and alkalinity of the batch composition as well as by the iron content of the silica source. Zeolite A·that was suitable as a detergent builder was prepared under highly alkali-rich conditions because the faster rate of crystal growth produced a greater amount of amorphous solid, which contained the iron complex, within the crystal aggregates. Also, the alkali-rich mother liquor could release greater amounts of iron from the solid. From the whiteness, chemical composition, and SEM observations, a two-step liquid-solid transformation mechanism was suggested.

Determination of surface areas by thermoporometry

An original method, previously designed for the determination of pore size distributions, has been improved for the determination of the surface areas of various materials. This method, called thermoporometry, is based on the thermodynamic conditions of the liquid-solid transformation of a capillary condensate inside a porous body. Very good agreement is found between the surface areas measured by thermoporometry and by the BET method. One of the advantages of the new method is the possibility of determining the surface areas of samples while they are still in the aqueous medium in which they are used.