Effects of additives Si on casting structure of ceramic liner produced by a centrifugal-thermit process

In order to control the quality of ceramic layer produced by a centrifugal—thermit process, effects of the amount of additives Si on phase constituents and casting structure of the layer are studied. The results show that silicon is mainly distributed over the matrix phase of the inner part of the layer, and the matrix belongs to a spinel“solid solution” having many Si⁴⁺ ions replacing Al³⁺. Because of its low melting point, the fluidity of liquid ceramic and thus the surface quality are improved. However, the effect of silicon is not always positive, since the dissolution of Si⁴⁺ ions enlarges the temperature range of freezing, thus it increases the tendency of “constitutional supercooling” and facilitates the development of equiaxed zone. In addition, with increasing silicon content the viscosity of liquid ceramic will increase as well. All these factors make the liquid feeding more difficult and lead to poor surface quality and high porosity of the layer. The changes in casting structure with various Si contents are discussed from the above point of view.     

Broken Symmetry Phase Transition in Solid HD: Quantum Behavior at Very High Pressures

The broken symmetry phase (BSP) transition in solid HD has been shown to be an example of quantum orientational melting. Anomalous features observed for the transition are a consequence of the symmetry properties of the system, namely, the fact that in HD all rotational states and transitions between them are allowed, in contrast to the behavior of the homonuclear H₂ and D₂.     

Modeling of ice formation in porous solids with regard to the description of frost damage

The freezing and thawing of liquid in porous media in connection with the question concerning the frost durability of solid materials is an important subject for discussion in civil engineering. Each construction or body which is in contact with liquid and frozen water is criticized by its resistance to the environment. The durability concerning frost attacks of a building material is affected by its porosity and the pore size distribution. The ice formation is a phenomenon of coupled heat and mass transport in freezing porous media, and is primarily caused by the expansion of ice in connection with hydraulic pressure. The volume increases due to the freezing front inside the porous solid. Taking into account the aforementioned effects in porous materials, a simplified macroscopic model within the framework of the Theory of Porous Media (TPM) for the numerical simulation of initial and boundary value problems of freezing and thawing processes of super saturated porous solids will be presented. The phase change between the ice and the liquid phase is modeled by different real densities of the phases.     

液态和非晶态Cu-Zr合金的研究进展

从性质、结构以及相图等方面论述了近年来液态和非晶态Cr Zr合金的研究现状。指出目前研究的主要内容集中在固态尤其是非晶态的结构和晶化以及相分离过程等方面 ,而对与凝固过程密切相关的液态Cu Zr合金的结构和性质方面的研究则涉及非常少。因而从液态Cu Zr合金的结构和性质入手 ,寻求液态、非晶态与晶态之间的相互关系 ,是一个值得研究的新课题     

Crystalline-amorphous phase transition of hyperbranched polyurethane phase change materials for energy storage

Hyperbranched polyurethane solid–solid phase change material (HB-PUPCM) has been prepared via a two-step process. The phase transition behaviors and morphologies of these HB-PUPCM films were investigated using differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD) and polarizing optical microscopy (POM). PEG soft segment in the polyurethane was found to be crystalline at room temperature. However, when the temperature was raised to PEG’s melting point, polyurethane did not melt into the liquid state as in the case of pure PEG but changed to an amorphous solid state. In HB-PUPCM, PEG’s molecules probably are tied to the hard segment chain so strongly by the chemical bonds that they cannot change to a liquid state but change to the amorphous state in the transition processing.     

Liquid oscillations in a circular cylindrical container with “sliding” contact line

Depending on the magnitude of the excitation response of the free liquid surface, the liquid and solid properties, the surface tension and the roughness or smoothness of the solid the contact line of the liquid surface to the solid container wall may slip, slide or be anchored. For an ideal liquid in a cylindrical tank the natural frequencies and response of the liquid have been determined for the cases. They are depending on the magnitude of the “sliding friction” coefficient larger than those of the freely slipping edge. The results for the anchored contact line exhibit the largest natural frequencies.     

Solid state amorphization of Mg-Zn-Ca system via mechanical alloying and characterization

Magnesium based bulk metallic glasses have attracted significant attention of researchers due to better mechanical and corrosion properties when compared to their crystalline counterparts especially for biomedical applications. Scaling up the part size and production volumes of such materials through liquid metallurgy route is challenging. In this work amorphous Ca₅Mg_60+xZn_35?x (X = 0, 3 and 7) alloys have been successfully synthesized through solid state amorphization using a high energy planetary ball mill. X-ray diffraction was used to identify the crystalline phases of the powder during reaction. Evolution of amorphous phase was analysed using a parameter involving the ratio of integral area of peaks to the integral area of background (I_PB) obtained from XRD patterns. Results showed reaction time increases with decreasing Zn content in Ca₅Mg_60+xZn_35?x (X = 0, 3 and 7) alloy to obtain maximum amorphous structure with a small amount of residual crystalline phase. Prolonged milling of these powders, to eliminate residual crystalline phases, resulted in the nucleation of Mg_102.08Zn_39.6 phase. The composition dependent characteristic temperatures and thermal stabilities were studied using differential scanning calorimetry.     

Rapid solidification of zirconium-based alloys

Zr based metal-metal binary and ternary alloys can be obtained in the amorphous state in very wide composition ranges. Several eutectic reactions and intermetallic compounds are present in these alloy systems which provide opportunities for examining the validity of different theories on glass formation. The amorphous phases in these alloys decompose by a variety of crystallization mechanisms. Instances of polymorphic, primary and eutectic crystallization have been encountered in these glasses. Zr-based metallic glasses possess excellent corrosion resistance and mechanical properties. In several studies their properties have been compared with that of their crystalline counterparts and interesting differences have emerged. In the solute lean Zr-based alloys very large freezing ranges are available for studying the liquid to solid transformation. It has been possible to study the formation of some of the low temperature phases directly from the liquid. This paper describes some of the aforementationed studies carried out on Zr-based amorphous and crystalline alloys.     

Phase-field modeling of eutectic structures on the nanoscale: the effect of anisotropy

A simple phase-field model is used to address anisotropic eutectic freezing on the nanoscale in two (2D) and three dimensions (3D). Comparing parameter-free simulations with experiments, it is demonstrated that the employed model can be made quantitative for Ag–Cu. Next, we explore the effect of material properties and the conditions of freezing on the eutectic pattern. We find that the anisotropies of kinetic coefficient and the interfacial free energies (solid–liquid and solid–solid), the crystal misorientation relative to pulling, the lateral temperature gradient play essential roles in determining the eutectic pattern. Finally, we explore eutectic morphologies, which form when one of the solid phases are faceted, and investigate cases, in which the kinetic anisotropy for the two solid phases is drastically different.     

Control of ice chromatographic retention mechanism by changing temperature and dopant concentration

A liquid phase coexists with solid water ice in a typical binary system, such as NaCl-water, in the temperature range between the freezing point and the eutectic point (t(eu)) of the system. In ice chromatography with salt-doped ice as the stationary phase, both solid and liquid phase can contribute to solute retention in different fashions; that is, the solid ice surface acts as an adsorbent, while a solute can be partitioned into the liquid phase. Thus, both adsorption and partition mechanisms can be utilized for ice chromatographic separation. An important feature in this approach is that the liquid phase volume can be varied by changing the temperature and the concentration of a salt incorporated into the ice stationary phase. Thus, we can control the relative contribution from the partition mechanism in the entire retention because the liquid phase volume can be estimated from the freezing depression curve. Separation selectivity can thereby be modified. The applicability of this concept has been confirmed for the solutes of different adsorption and partition abilities. The predicted retention based on thermodynamics basically agrees well with the corresponding experimental retention. However, one important inconsistency has been found. The calculation predicts a step-like discontinuity of the solute retention at t(eu) because the phase diagram suggests that the liquid phase abruptly appears at t(eu) when the temperature increases. In contrast, the corresponding experimental plots are continuous over the wider range including the subeutectic temperatures. This discrepancy is explained by the existence of the liquid phase below t(eu). A difference between predicted and measured retention factors allows the estimation of the volume of the subeutectic liquid phase.     

Homogeneous and heterogeneous melting behavior of bulk and nanometer-sized Cu systems: a numerical study

Molecular dynamics simulations have been used to investigate the solid–liquid transition of different Cu systems. These consisted of surface-free crystalline bulks and semi-crystals terminating with a free surface as well as of particles and wires with different shape and size in the mesoscale regime. The characteristic melting points of the various systems were attained by gradual heating starting from 300 K. Apart from surface-free bulk systems, where the phase transition at the limit of superheating is homogeneous, melting displays heterogeneous character. This is due to the existence of surface layers with structural and energetic properties different from the ones of bulk-like interior. Simulations point out a significant depression of both the melting point and latent heat of fusion for nanometer-sized systems respect to semi-crystals. Below the characteristic melting point, free surfaces are involved in pre-melting processes determining the formation of a solid–liquid interface. The onset of melting is related to the formation of a critical amount of lattice defects and this provides a common basis for the rationalization of homogeneous and heterogeneous melting processes despite their intrinsic differences.     

Measurement of Al Freezing-Point Temperature: Effect of Initiation Process

In CCT documents it is stated that “...for the freezing curves of the metallic fixed points, the maximum observed temperature on the plateau should be taken as the best approximation of the liquidus temperature. The fixed points should be realized with the inner and outer liquid-solid interfaces and extend past the maximum by 10 % to 20 % of the fraction frozen, to clearly establish the value of the maximum and the resolution of its determination.” Also, it is accepted that “...the inner interface is essentially static. It is the temperature of the inner liquid/solid interface that is measured by the thermometer.” The analysis of freezing curves obtained by the standard method of fixed-point realization shows that the parameters of the initial part of the freezing curve, the mean temperature value of which is usually taken as the liquidus temperature, depend on how the inner interface is initiated. Variations in the duration and intensity of initiation cause changes in the initial part of the freezing curve and in the resulting SPRT measurement. Moreover, the relation between the duration of the initial section of the plateau with a minor temperature change and the duration of its final section with a significant slope also depend on the initiation method used and on the furnace temperature. The effect of freezing initiation conditions on the measurement result is individual for each fixed point because of the differences in thermophysical properties of metals and in conditions of the heat transfer from the liquid–solid interface to the thermometer. Aluminum has a maximum value of the melting specific heat in comparison with other metals used in ITS-90 fixed points; in the present study, the effect of the intensity and duration of the inner liquid–solid interface initiation was investigated both experimentally and through calculation.     

The description of morphologically stable regimes for steady state solidification based on the maximum entropy production rate postulate

The maximum entropy production rate (MEPR) in the solid–liquid zone is developed and tested as a possible postulate for predicting the stable morphology for the special case of steady state directional solidification (DS). The principle of MEPR states that, if there are sufficient degrees of freedom within a system, it will adopt a stable state at which the entropy generation (production) rate is maximized. Where feasible, the system will also try and adopt a steady state. The MEPR postulate determines the most probable state and therefore allows pathway selections to occur in an open thermodynamic system. In the context of steady state solidification, pathway selections are reflected in the corresponding morphological selections made by the system in the solid–liquid (mushy) zone in order to cope with the required entropy production. Steady state solidification is feasible at both close to, and far from equilibrium conditions. Based on MEPR, a model is proposed for examining the stability of various morphologies that have been experimentally observed during steady state directional solidification. This model employs a control volume approach for entropy balance, including the entropy generation term (S _gen), which depends on the diffuse zone and average temperature of the solid–liquid region within the control volume. In this manner, the model takes a different approach from the successful kinetic models that have been able to predict key features of stable morphological patterns. Unstable planar interfaces, faceted cellular arrays, cell–dendrite transitions, half cells both faceted and smooth, and other transitions such as the absolute stability transition at high solid/liquid velocities are examined with the model. Uncommon solidification morphological features such as non-crystallographic dendrites and discontinuous cell-tip splitting are also examined with the model. The preferred morphological change-direction for the emergence of the stable morphological feature is inferred with the MEPR postulate in a manner analogous to the free energy minimization principle(s) when used for predicting phase stability and metastable phase formation. Aspects of mixed-mode order transformation characteristics are also discussed for non-equilibrium solidification containing a diffuse interface, in contrast to classifying solidification as purely a first order transformation. The MEPR model predictions are shown to follow the experimental transitions observed to date in several historical studies.     

Liquid-crystal state in the system polysaccharide-mesophasogenic solvent

Data on the influence of vapors of nitromethane — a mesophasogenic solvent, i.e., a solvent forming with a polymer a lyotropic liquid-crystal phase — on the structure of acetate fibers are presented. It has been established that nitromethane in the vaporous state initiates in the polymer matrix orientation processes: induced anisotropy, spontaneous elongation of the fiber (which, in terms of Flory, is considered to be the transition to the nematic phase), as well as the process inverse to the self-elongation discovered for the first time, etc. It has been shown that the realization of both the direct and inverse processes of spontaneous deformation of cellulose acetate in mesophasogenic solvent vapors is associated with the optical asymmetry, i.e., the optical activity of the polysaccharide. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 79, No. 1, pp. 139–147, January–February, 2006.     

Experimental study of thermal conductivity of solid and liquid phases at the phase transition

An experimental method for the determination of the thermal conductivity of a pure material at its freezing melting point has been developed. In the present investigation. this method is discussed further by studying the effective thermal conductivity of the frozen as well as the thawed state of a wet porous material. and of solid and liquid benzene at the interface. The method involves the study of the transient propagation of the freezing or melting zone through the specimen as well as the steady state of the freezing melting process. The method makes use of the heat of transition as the heat flow, and it enables the interlace to act as a heat transfer surface, thus incorporating realistic features of the phase change process. Compared with data from the literature. the experimental results for benzene agree within 2% for the solid phase and within 3% for the liquid phase. when some precautions are made to avoid free convection. The experimental effective thermal conductivity of the packed bed is compared with data from a numerical analysis: the results agree within 4%, indicating that the method is applicable also for measurements on heterogeneous materials.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, U.S.A.On leave from Central South University of Technology, 410083 Changsha, P.R. China.     

A finite temperature continuum theory based on interatomic potential in crystalline solids

A finite temperature continuum theory of crystalline solid based on an approximate Helmholtz free energy expression is proposed. The free energy expression is specifically derived for simple implementation in atomistic-based continuum methods (i.e. quasicontinuum method) via the Cauchy–Born rule at finite temperature. It is obtained by the method of statistical moments via the quasi-harmonic approximation together with Taylor series expansion of a given interatomic potential. The phonons are assumed to follow the Bose–Einstein distribution so that the quantum effects at low temperature are accounted for. The resulting free energy is in terms of a given interatomic potential and a simple function of displacement that accounts for thermal expansion. It is employed to formulate two finite temperature continuum methods via Cauchy–Born rule and via the virtual atomic cluster (VAC). It is validated through comparison with experimental results of various thermodynamic quantities. In the case of fcc metals, the proposed free energy expression is shown to be valid for a wide range of temperatures above 50 K.     

On reducing the size of liquid separators for injector circulation plate freezers

Liquid separators for injector plate freezers are large because the liquid level rises towards the end of the freezing process. To calculate the volume of liquid being collected in the separator during freezing the freezing time and the heat removed must be evaluated. A simple method of freezing time estimation based on the progression of a phase change front is proposed. The size of separators can be reduced considerably by letting part of the liquid feed by-pass the injector during initial freezing. With this arrangement the injector dimensions are based upon a refrigerating capacity lower than the maximum.     

Preparation of SiC hollow particles by gas-phase reaction in the SiH4-CH4-H2 system

The formation of SiC hollow particles by gas-phase reaction in the silane-methane-hydrogen system was studied at temperatures from 1200 to 1400° C. Synthesized powders were analysed by means of the thermogravimetry, X-ray diffraction, transmission electron microscopy, infrared irradiation, etc. The powders synthesized at 1200 to 1300° C consisted ofβ-SiC and silicon phases, but they became almost hollowβ-SiC particles at 1400° C. The composition, particle size, and shell thickness of the synthesized particles were dependent on the reaction conditions. From lattice parameter measurements, a certain amount of excess silicon was verified to be incorporated into theβ-SiC lattice. On the other hand, excess carbon existed, for the most part, as an amorphous phase not forming solid solutions withβ-SiC. Transmission electron microscopy observations and infrared absorption measurements have shown that excess carbon is contained within the shells of hollow particles, while unreacted excess silicon exists as a crystalline phase mostly in the cores of the particles.     

Excitations and Stability of 2D Liquid 4He

The excitation modes of two-dimensional liquid ⁴He are approached from two angles: Firstly, the phonon-roton spectrum is calculated and the related transition currents examined to gain insight into the detailed microscopic structure of the excitations and to look for possible evidence of the proposed spontaneous formation of vortex-antivortex pairs at low densities. The roton excitation is interpreted as a resonance effect in which the wavelength of the long-range density fluctuation matches favorably with the short-range oscillations caused by two-particle correlations. Contrary to the 3D case, no backflow rolls are observed in the two-body current at high momenta. The calculations reproduce reasonably the density dependence of the spectrum, and the saturation of the high-momentum part following from the decay processes of excitations is satisfactorily predicted. The stability of the liquid ground state is then studied by searching for soft modes. It is found that at densities near the expected liquid-solid phase transition the energy of the liquid can be lowered by changing the symmetry of the pair distribution function from spherical to non-spherical. This two-body structure indicates the point-group symmetry of the emerging solid phase to be hexagonal.