Bulk Melt Research Articles

Observations of the columnar-to-equiaxed transition in stainless steels

Observations are reported for the columnar-to-equiaxed transition (CET) in stainless steel bars which have been solidified slowly and progressively in a horizontal configuration. For ferritic, austenitic, and ferritic/austenitic stainless steels containing more than 0.085 wt pct carbon, CETs occur at about the same distance from the start of solidification at a given growth rate. With increasing growth rates, the transition occurs closer to the start of solidification. At low carbon levels, near 0.02 wt pct carbon, the ferritic/austenitic steel is entirely columnar, in most cases. Adding nickel to the ferritic/austenitic steel, which makes the leading phase austenitic, produces a CET with small equiaxed grains. This suggests that different particles which are effective with austenitic growth become operative as nucleants. The transition from a columnar to an equiaxed structure occurs abruptly across the diameter of the sample. There is extensive fluid flow in the bulk melt, which produces shallow temperature gradients in the melt prior to the onset of solidification. The bulk melt flow does not appear to interact significantly with the melt in the interdendritic region or the melt immediately ahead of this region. The width of the solid/liquid region in the present experiments is observed to be between 10 and 20 mm, depending on the growth velocity and the distance from the start of solidification.

Modelling Intercalation Kinetics of Polymer Silicate Nanocomposites

AbstractPolymer layered silicates form an important class of nanocomposite materials. During their formation by melt intercalation, polymer molecules from a bulk fluid move into the galleries of layered silicates. An essential feature of this process is the flow of macromolecules from a bulk fluid to a confined environment. To model this phenomenon, we have performed molecular dynamics simulations of the flow of polymer molecules from a bulk melt into a rectangular slit. The simulations are consistent with a diffusive description of the transport, and show qualitative agreement with time-dependent x-ray diffraction measurements of intercalation kinetics in layered nanocomposites.

Characteristics of polymer light conduits from different manufacturing processes

The characteristics of polymer light conduits (PLCs) prepared from three different processes are compared: (1) a bulk polymerization and melt spinning process (PLC1), ( 2 ) a thermal polymerization (PLC2 ), and (3 ) a UV curing photopolymerization (PLC3-PLC4) of a monomer mixture in a fluorinated ethylene propylene tube. PLC1 and PLC2 were obtained from Toray and Lumenyte, respectively. New PLCs, PLC3 and PLC4, were prepared from the monomer mixtures consisted of methyl methacrylate, 2-ethyl-hexyl acrylate, and the crosslinking agent polyethylglycol 200 diacrylate and diallyl phthalate, respectively. All four polymer light conduits contain the acrylic moiety. PLC2 may also contain the plasticizer additives, which resulted in an excellent flexibility but with a poor thermal stability. The flexibilities of PLC2 and PLC3 were significantly improved from that of PLC1 due to the insertion of a plasticizer and the long alkyl chain monomer 2-EHA, respectively. The much larger numerical apertures of PLC2-PLC4 than that of PLC1 suggest that PLC2-PLC4 can have a wide angle application. The transmitted distance of PLC1 is larger than those of PLC2-PLC4. However, the illumination intensity of PLC1 is worse than those of PLC2-PLC4. This result indicates that PLC1 is suitable for the applications as PLCs with the characteristics of long distance and low illumination intensity, while PLC2-PLC4 are suitable for short distance and high illumination intensity applications.

The fabrication and properties of single domain YBCO disk

Bulk melt textured YBCO materials in the shape of disks have been developed. It is found that the top seeded method is efficacious to fabricate single domain material, which is required to achieve the best magnetic properties. Both trapped-field mapping and X-ray Ω-scan techniques have been used to evaluate the single domain nature of the YBCO disks. A levitation force of 2N/g has been achieved for this kind of bulk.

Eutectic solidification processing via bulk melt undercooling

The control of eutectic microstructures is generally achieved via controlling the growth velocity, thermal gradients and the alloy composition. While current efforts in producing in-situ nanocomposites are centered on conventional rapid solidification processing (RSP), we describe here a more non-traditional processing of eutectics by RSP via bulk melt undercooling of near-eutectic compositions. It will be shown that a wide variety of microstructures can be produced in alloys with the same composition. These include the formation of metastable phases, growth of coupled eutectic compositions and the formation of mixed metastable/stable eutectics. These effects are discussed using examples of rapidly solidified Nb-Si alloys processed in a containerless environment.

Dendritic growth velocities of the intermetallic compounds Ni 2TiAl, NiTi, Ni 3Sn, Ni 3Sn 2 and FeAl

The dendritic growth velocities of melts of several ordered intermetallic compounds were measured during rapid solidification. Bulk melt undercoolings were achieved by the application of the electromagnetic levitation and the glass fluxing technique. The results obtained on Ni 2TiAl, NiTi, Ni 3Sn, Ni 3Sn 2 and FeAl allow comparisons with respect to the dependence of the crystal growth behaviour on the complexity of the superlattice structure and on the predominant type of bonding. Disorder trapping in FeAl was strongly indicated by the observation of large growth velocities up to 7 m s −1, whereas all other investigated alloys solidified chemically ordered even at deep undercoolings up to 300 K. A dendrite growth model was applied to analyse the experimental data. The results reveal the diffusion-controlled growth kinetics of ordered intermetallic compounds in contrast to collision-limited growth conditions in pure metals and solid solutions.

Enhanced critical current density in melt textured YBCO via Au ion surface irradiation

We irradiated both side of bulk melt textured YBCO with 235 MeV Au ions. The thickness of the specimens is significantly larger than the penetration range of the ions so that two surface layers of columnar defects are created. Critical current density enhancements were obtained in temperature and field ranges that are useful for applications. The boundary between reversible and irreversible regime is shifted toward higher temperatures. At higher fields the increase of the relaxation rate with temperature is less steep after irradiation. The general trends are independent of the thickness of the samples, at least in the range from 5% to 13% of volume affected by irradiation.

Melt textured YBCO samples for trapped field magnets and levitating bearings

Bulk melt textured YBCO materials in the shape of disks are being developed for use as trapped field magnets and in superconducting bearings. The trapped field in the gap between two disks of 26 mm in diameter reached maximum values of 1 T at 77 K and of 8.5 T at 51.5 K. A superconducting magnetic bearing has been built up of two YBCO rings and two concentric NdFeB ring magnets between them. A static levitation force of 25 N and values of the axial and radial stiffnesses of 15 N/mm and 10 N/mm, respectively, have been achieved for this bearing in the working position. A high speed motor with two superconducting magnetic bearings of this kind has been tested at 77 K.

Nucleation and growth of the orthorhombic phase in melt textured YBa 2Cu 3O 6+δ

The nucleation and growth of the orthorhombic phase by transformation through oxygen ordering from the tetragonal phase in melt textured YBa 2Cu 3O 6+δ (123) are investgated by polarized light microscopy. The samples for observation were collected from the interior of partially oxygenated bulk melt textured material. The orthorhombic phase is observed to nucleate basically at three different sites, viz. along domain boundaries, at platelet boundaries and around macroscopic cracks. The nuclei at domain boundaries are observed to grow along the platelet boundary gaps with a cellular morphology with sharp tips, whereas the growth front of the phase growing from the boundaries of platelets into their interior is planar. The growth of the phase from the domain boundaries into the domains differs depending on the orientation of the platelets at the point of nucleation. The origin of inter-platelet gaps in the melt processed material is discussed in the light of the observations made during this work. The ratio of growth rates of the orthorhombic phase along the gaos and into the platelets is estimated.

Nuclear magnetic resonance and acoustic investigations of the melting - freezing phase transition of gallium in a porous glass

The melting - freezing phase transition of gallium in a porous glass was studied by NMR and acoustical techniques. A depression of the freezing and melting phase transition temperatures and a pronounced hysteresis in the melting - freezing processes were found. An intricate NMR lineshape for liquid gallium was observed with a temperature coefficient of the Knight shift more than twice that measured for the bulk melt. The results are discussed on the basis of the Gibbs - Thompson equation and by means of a geometric freezing model.

Aluminosilicate melts: structure, composition and temperature

The anionic structure of aluminosilicate melts of intermediate degree of polymerization (NBO/T = 0.5) and with along the composition join (LS4-LA4) has been examined in-situ to ˜1480 °C, and compared with recent data for melts along the analog composition join and with less polymerized melts along the join and O_5. With , the anionic equilibrium, (1) , adequately describes the structure. With , a second expression, (2) , is required because an additional structural unit, Q1, is stabilized in the melts. The enthalpy, , of reaction (1) increases from − 36 ±4 kJ/mol in the absence of aluminum to 34± 5 kJ/mol at and 64 ± 4 kJ/mol at Al/(Al + Si) = 0.45. Similar trends are reported for other alkali aluminosilicate melts. Least-squares fitting of abundance of structural units as a function of temperature and bulk composition has been conducted. The unit abundance is dominantly a function of temperature, Al/(Al +Si), and bulk melt polymerization. Configurational entropy and heat capacity of mixing of melts above their glass transition temperatures have been calculated with the aid of the least-squares fitted equations. The values of these parameters indicate that as the ionization potential of the metal cations increases, configurational heat capacity of alkali aluminosilicate melts becomes temperature dependent. As a result, transport properties (viscosity, diffusivity, and conductivity) of such melts will not show Arrhenian behavior even in the high-temperature range. Further, discontinuous changes in entropy and heat capacity of mixing results from temperature-induced changes in types of structural units in the melts. Such discontinuous changes would also be reflected in discontinuous changes of temperature-dependent transport properties.

Assessment of mathematical models for the flow in directional solidification

In a binary solution unidirectionally solidified from below, the bulk melt and the eutectic solid is separated by a dendritic mushy zone. The mathematical formulation governing the fluid motion shall thus consist of the equations in the bulk melt and the mushy zone and the associated boundary conditions. In the bulk melt, assuming that the melt is a Newtonian fluid, the governing equations are the continuity equation, the Navier-Stokes equations, the heat conservation equation, and the solute conservation equation. In the mushy layer, however, the formulation of the momentum equation and the associated boundary conditions are diversified in previous investigations. In this paper, we discuss three mathematical models, which had been previously applied to study the flow induced by the solidification of binary solutions cooling from below. The assessment is given on the bases of the stability characteristics of the convective flow and the comparison between the numerical and experimental results.

High trapped fields in bulk YBa2Cu3O7−δ samples at temperatures around 50 K

Bulk melt textured YBa2Cu3O7−δ samples with single grains of about 24 mm diameter were obtained by use of SmBa2Cu3O7−x seed crystals. The maximum trapped field B0 in the gap between two samples was investigated as function of temperature. B0 increased from 1 T at 77 K to 8.5 T at 51.5 K, which is the highest trapped field achieved in nonirradiated samples. At low temperatures, cracking of the samples was observed under magnetic pressure. In this temperature range, the trapped field is limited by the mechanical strength of the samples, for which a value of 25 MPa was estimated.

Melt textured YBCO: High trapped fields and superconducting magnetic bearings

Bulk melt textured YBCO samples in the shape of discs were prepared by a top seeding method using Sm-123 crystals. The field distribution of the trapped field on the surface of the samples was measured at 77 K by means of a Hall probe. In single grains with diameter of 20 mm the trapped field reached maximum values up to 0.56 T at 77 K. Investigating the temperature dependence of the maximum trapped field, a value of 5.5 T at 30 K was found for the best sample. At the same temperature, degradation of the trapped field by a factor of about 10 was observed for a second sample after field cooling in a field of 7 T. This degradation is explained by damage to the sample due to magnetic stresses. At the centre of the sample a magnetic stress of 11 MPa is estimated to be responsible for this damage. Additionally YBCO material has been tested in a high speed motor with superconducting magnetic bearings (SMB) in order to demonstrate the passive stabilization properties of SMB. In first tests the shaft rotated with a speed of 12 000 rpm limited by air drag.

Material analysis and characterization on zone refined and zone leveled vertical zone melt GaAs for radiation spectrometers

GaAs is a wide band gap (1.42 eV) semiconductor that has shown promise as a room temperature operated γ-ray detector. A practical γ-ray detector would be large in volume, hence the resistivity of the material must be high to ensure large depletion volumes and low leakage currents. Commercially available semi-insulating (SI) bulk GaAs is compensated by a balance between native defect deep donors (EL2) and residual dopant impurities. The high concentrations of electrically active deep and shallow levels are believed to contribute to electric field distortions observed in γ-ray detectors fabricated from SI bulk GaAs. Hence, the controlled reduction of native defects and contaminant impurities may yield improved bulk GaAs for γ-ray detectors. Custom grown vertical zone melt (VZM) bulk GaAs is presently under investigation as an alternative material for room temperature operated γ-ray detectors. The VZM technique allows for zone refinement (ZR) and zone leveling (ZL) of the GaAs ingots. Custom growth of the material allows for controlled changes in the bulk crystal, including deliberate reductions in EL2 and impurity concentrations. Comparisons are made to commercial vertical gradient freeze (VGF) and liquid encapsulated Czochralski (LEC) bulk GaAs.

Comparison of vertical gradient freeze bulk GaAs and custom grown vertical zone melt bulk GaAs as radiation spectrometers

Custom grown vertical zone melt (VZM) ingots of GaAs have been zone refined and zone leveled. The crystallinity, impurity concentrations, defect concentrations, and electrical properties of the ingots have been studied. Radiation detectors fabricated from the ingots have been compared to radiation detectors fabricated from commercially available vertical gradient freeze (VGF) material. Preliminary results suggest that electrical homogeneity may be a major factor in determining the performance of GaAs gamma ray detectors. Deep level EL2 concentrations were reduced in the VZM material, however gamma ray detectors fabricated from the material exhibited inferior resolution. Commercial semi-insulating bulk VGF GaAs material demonstrated much better resolution than the VZM detectors, giving best energy resolutions of 7.8% FWHM for 122 keV gamma rays and 5% FWHM for 356 keV gamma rays.

Analysis of Ge distribution in Si 1 − xGe x single crystal fibers by the micro-pulling down method

The Ge distribution in both melt and crystal was analyzed for the growth of Si 1 − x Ge x ( x = 0.05) single crystal fibers by the micro-pulling down (μ-PD) method. The solute accumulation volume in the vicinity of the interface comprises the molten zone (zone I) next to the interface and the capillary zone (zone II) leading to the bulk melt. A steady-state solution was given for each of these two zones and the initial transient analysis lead to the equilibrium partition coefficient, k 0 and diffusion constant, D, for Ge in the Si 1 − x Ge x ( x = 0.05) melt to be 0.43 and 5.6 × 10 −5cm 2s −1, respectively. A sudden change of either the growth velocity or the molten zone volume size during steady-state growth yields an abrupt change of Ge distribution, and its change mode is interpreted in terms of the degree of imbalance of the Ge mass flow between zone I and zone II at the zone boundary. It is, therefore, suggested that the desired impurity distribution in a Si single crystal fiber can be designed by the manipulation of the imbalance of the impurity mass flow at the zone boundary.

High trapped fields in melt-textured YBCO samples

Bulk melt textured YBCO samples in the shape of disks were developed for use as trapped field magnets. The trapped field in single grains with diameter of 24 mm reached maximum values up to 0.56 T at 77 K and a value of 5.5 T at 30 K. Degradation of the trapped field was observed in samples after the activation process at 20 K in a high applied magnetic field. This is interpreted in the framework of a model of magnetic stress induced damage.

Modeling of a continuous rotating disk polycondensation reactor for the synthesis of thermoplastic polyesters

A dynamic multicompartment model is proposed for a continuous flow rotating disk reactor for the finishing stage melt polycondensation of poly(ethylene terephthalate). In the multicompartment reactor model, ethylene glycol is removed from both the bulk melt phase and the film phase formed on the rotating disks. The specific interfacial area for the film phase is estimated using the empirical correlation for polymer film thickness, and the mass transfer coefficient is calculated using the penetration theory. The mass transfer enhancement factor is introduced to account for the increased interfacial area due to ethylene glycol bubbles. The effects of reactor design and operating parameters on molecular weight and ethylene glycol removal have been investigated through model simulations. In particular, a detailed analysis is presented on the ethylene glycol removal rate from the two phases. © 1996 John Wiley & Sons, Inc.

Numerical simulation of fluid flow in electromagnetic centrifugal casting

A two-dimensional model is presented to compute the steady-state flow of an Al melt in centrifugal casting with electromagnetic stirring. The distributions of flow velocity, body forces and pressure were studied by means of numerical simulations. It was found that the tangential flow was strongest near the mold/melt interface, while the radial flow velocity increased with a decrease in the distance to the interface. The azimuthal distributions showed angle-dependent fluctuations. The flows were dramatically influenced by the rotation rate of the mold and the intensity of the imposed magnetic field. The distributions and variations of melt flow were related to the body forces discussed in the present paper. The existence and increase in magnetic field caused a decrease in the pressure in the bulk melt; this problem in practical casting might be solved by increasing the rotation rate of the mold.