Electromagnetic Convection Research Articles

Effects of coil frequency on the carbon transport in the top-seeded solution growth of SiC single crystal

The flow pattern and carbon transport in the solution for the solution growth of silicon carbide (SiC) in an induction-heating system can be effectively controlled by adjusting the coil frequency to obtain a relatively uniform distribution of growth rate. In this paper, a global heat and mass transfer model was first verified by the SiC crystal growth experiment results at the coil frequency of 8 kHz. Then, the influence of coil frequency on the fluid flow, carbon concentration, and growth rate for the growth of SiC crystals was numerically investigated. The results indicated that electromagnetic convection (at low frequencies) greatly enhances the upward convection beneath the crystal and weakens the adverse effects of Marangoni convection, resulting in a uniform distribution of radial temperature along the growth interface. Consequently, the carbon transport from the crucible wall to the crystal center is strengthened, leading to a uniform region of high carbon supersaturation with a lower coil frequency. The optimal radial uniformity of the growth rate along the growth interface is achieved at the coil frequency of 3 kHz.

25-26 Ağustos 2018 Jeomanyetik Fırtına Sırasında Afrika Bölgesi Üzerindeki Manyetik Eşlenik Çiftleri Üzerindeki TEC Değişikliklerinin İncelenmesi

In this study, the electron transport process resulting from electromagnetic drift between two magnetic conjugate pairs over the African region during the August 25-26 2018 geomagnetic storm was investigated. The effects of geomagnetic conditions presented with Dst index and IMF Bz values on Total Electron Content (TEC) values at conjugate stations were compared separately for stormy and quiet periods. During the storm period, the effect of TEC values at stations in the northern hemisphere (Haifa and Djibouti) on the TEC values at stations in the southern hemisphere (Ambalavao and Malindi) is greater than the effect of TEC values in the southern hemisphere on TEC values in the northern hemisphere. According to this result, it can be said that the south-directed electromagnetic convection was more than the north-directed convection in the examined dates. When the coefficients are examined, it can be said that the interaction is more in the magnetic conjugate pair that is closer to the equator during the storm period, and the interaction is more in the magnetic conjugate pair that is far from the equator during the silent period. Considering the coefficients calculated for Dst and IMF Bz, it is seen that the TEC values are very small compared to their coefficients. From this it can be concluded that the effect of Dst and IMF Bz is much smaller than the effect of TEC values at a station on TEC values at its magnetic conjugates.

Influence of an external alternating electromagnetic field on convective flows in the bath of an ore-thermal furnace during the melting of oxide refractory materials

Electric arc melting of oxide materials under the influence of an external electromagnetic field is a high-current and high-temperature technological process, accompanied by powerful electric and magnetic fields and complex structural motion of the melt. In the general case, the movement of the melt in the bath of an electric arc installation is determined by a combination of thermocapillary, thermal concentration, thermal and electromagnetic convections. In this work, based on the analysis of convective flows and the calculation of the integral vortex field of Archimedes forces and electromagnetic forces in the bath, using the OKB-2126A ore-thermal furnace for melting oxide refractory materials as an example, an assessment of thermal and electromagnetic convections was carried out. For the first time it has been established that the value of the integral vorticity of the field of electromagnetic forces in the depth of the bath exceeds the integral vorticity of the field of Archimedes forces by 3-4 times. In real objects, this difference is even greater, since, compared to those accepted in the calculations, the actual current density in the bath is higher, and the temperature difference is lower. Thus, electromagnetic convection in ore-thermal furnaces for melting oxide materials significantly exceeds thermal convection and makes a decisive contribution to the formation of the structure and intensity of melt movement.

Macrosteps formation on 4H-SiC surfaces via Si melting within a sandwich configuration

With a view to designing a homogeneous and controlled step-bunched morphology on large monocrystalline 4H-SiC(0001) surfaces, a sandwich configuration was investigated. A piece of silicon was melted between two 4H-SiC 4° off wafers, allowing a better spreading of the liquid than with a Si drop approach. Using as low liquid Si thickness as 30 µm, homogeneously highly step-bunched surfaces were obtained, though with irregular step shapes. This was found to be the result of time dependent dissolution of the SiC-bottom wafer under the natural vertical thermal gradient of the setup. The most parallel step and terrace structures were generated using 400 µm thick liquid Si though this led to long range inhomogeneities associated with complex mass transport. The proposed mechanism includes the combined effects of H2 etching at the edges acting as C pump and electromagnetic convection rolls for long range transport of these dissolved C atoms.

Liquid Si-Induced 4H-SiC Surface Structuring Using a Sandwich Configuration

In view of obtaining a step bunched morphology on large 4H-SiC surfaces, a sandwich configuration is investigated. A piece of silicon is melted between two 4H-SiC 4° off wafers, allowing a better spreading of the liquid than a Si drop approach. This successfully leads to highly step-bunched surfaces, though with irregular steps. The most regular step and terrace stuctures were found to be the result of epitaxial growth via a dissolution-precipitation process occuring from the edges to the center of the wafers. This is probably caused by radio-frequency induced electromagnetic convection within liquid Si. This process is quenched when using smaller liquid thickness.

Evolution of the microstructure and solute distribution of Sn-10wt% Bi alloys during electromagnetic field-assisted directional solidification

The effects of forced flows at different velocities on microstructure and solute distribution during the directional solidification of Sn-10 wt% Bi alloys under a simultaneous imposition of a transverse static magnetic field (TSMF) and an external direct current (DC) have been investigated experimentally and numerically. The experimental results show that the solid-liquid interface will gradually become sloping with the increase of the forced flow velocity when the thermoelectric magnetic convection (TEMC) dominates the forced flow at solidification front. However, the interface will gradually become planar as the flow velocity further increases when the electromagnetic convection (EMC) dominates the forced flow. Moreover, when the flow velocity gradually increases, the primary dendrite spacing decreases from 384 to 105 μm accordingly. The simulation results show that the solute distribution at the two sides of the sample can be significantly changed by the forced flow at solidification front. The rejected solute will be unidirectionally transported to one side of the sample along the TEMC (a low-velocity forced flow), thereby causing the formation of a sloping interface. However, the rejected solute will be returned back along the EMC (a higher-velocity force flow), which results in a planar interface. Furthermore, the solute content at the two sides of the sample under the forced flows at different velocities was measured. The results are in good agreement with the simulation results, which shows that the solute content difference between the two sides of the sample reaches the maximum when a 0.5 T TSMF is applied, while the solute content difference decreases to zero with a simultaneous application of a 0.5 T TSMF and a 1.6 × 105 A/m2 external DC.

Grain refinement of as cast superalloy K4169 solidified with low voltage pulsed magnetic field

The solidification behaviour and the as cast structure of superalloy K4169 ingots solidified with a low voltage pulsed magnetic field (LVPMF) were investigated by experiments and modelling. The experimental results showed that complete fine equiaxed grains in ingots of diameter 95 mm were obtained by applying an LVPMF. Dendrite tips in the as cast structure were rounded due to the release of latent heat and the concentration of the Joule heating effect on the dendrite tips. The solidification procedure was simulated in order to investigate the grain refinement mechanism. The simulation results show that the LVPMF has electromagnetic convection and electromagnetic vibration effects on the alloy melt. Grain refinement was considered as the result of the magnetic force and melt flow during solidification. Based on the experimental and simulation results, a refining mechanism was proposed for nucleus multiplication by melt vibration and convection, which contributes to heterogeneous nucleation enhancement and leads to high nucleation rate and grain refinement.

Refinement mechanism of low voltage pulsed magnetic field on solidification structure of silicon steel

The grain refinement of a silicon steel solidified with a low voltage pulsed magnetic field (LVPMF) was investigated by experiments and modeling. The experiment results show that complete fine equiaxed grains are acquired by applying the LVPMF. The effects of process parameters, such as the melt cooling rate and superheating on the solidification structure, were also studied. Complete fine equiaxed grains can be obtained under a larger range of cooling rate or superheating when the LVPMF is applied. The magnetic force and the melt flow during solidification were modeled and simulated to reveal the grain refinement mechanism. The simulation results show that the LVPMF has a double role in electromagnetic convection and electromagnetic vibration on the alloy melt. We propose the refining mechanism: The melt vibration and convection can promote nucleus multiplication, which contributes to heterogeneous nucleation enhancement and leads to a high nucleation rate and grain refinement.

Electromagnetic Enhancement of Carbon Transport in SiC Solution Growth Process: A Numerical Modeling Approach

The carbon distribution and its transport in the liquid from the source to the crystal directly affect the control of parasitic nucleation, the growth front stability, and the growth rate during SiC solution growth. Controlling the carbon transport is one of the key issues for understanding and improving the process. In this paper, numerical modeling by finite element method is used to describe the complex convective flow pattern in the melt. We focus on electromagnetic convection and investigate the effect of coil frequency, keeping a simple and technologically realistic crucible design. We show that below a critical value of frequency, the carbon transport can be controlled by the electromagnetic convection, giving rise to significant growth rate enhancement.

Solution growth of SiC from silicon melts: Influence of the alternative magnetic field on fluid dynamics

We studied numerically the fluid dynamics of the silicon melt in the high temperature solution growth of silicon carbide (SiC) with the presence of alternative magnetic fields. A 2D-axisymmetric model for 2in SiC crystal growth was used for this study. The results revealed that the melt convection is strongly affected by the coil position and the applied frequency. Results on the effect of electromagnetic convection in the presence of buoyancy convection are also given in this paper.

Peritectic Alloy Rapid Solidification with Electromagnetic Convection

Peritectic Alloy Rapid Solidification with Electromagnetic Convection

Development of an advanced A-TIG (AA-TIG) welding method by control of Marangoni convection

A new type of tungsten inert gas (TIG) welding has been developed, in which an ultra-deep penetration is obtained. In order to control the Marangoni convection induced by the surface tension gradient on the molten pool, He gas containing a small amount of oxidizing gas was used. The effect of the concentration of O 2 and CO 2 in the shielding gas on the weld shape was studied for the bead-on-plate TIG welding of SUS304 stainless under He–O 2 and He–CO 2 mixed shielding gases. Because oxygen is a surface active element for stainless steel, the addition of oxygen to the molten pool can control the Marangoni convection from the outward to inward direction on the liquid pool surface. When the oxygen content in the liquid pool is over a critical value, around 70 ppm, the weld shape suddenly changes from a wide shallow shape to a deep narrow shape due to the change in the direction of the Marangoni convection. Also, for He-based shielding gas, a high welding current will strengthen both the inward Marangoni convection on the pool surface and the inward electromagnetic convection in the liquid pool. Accordingly, at a welding speed of 0.75 mm/s, the welding current of 160 A and the electrode gap of 1 mm under the He–0.4%O 2 shielding, the depth/width ratio reaches 1.8, which is much larger for Ar–O 2 shielding gas (0.7). The effects of the welding parameters, such as welding speed and welding current were also systematically investigated. In addition, a double shielding gas method has been developed to prevent any consumption of the tungsten electrode.

Effect of oxygen in ambient atmosphere on penetration characteristics in single yttrium–aluminum–garnet laser and hybrid welding

Welding with a yttrium–aluminum–garnet (YAG) laser only or with hybrid source combined with a YAG laser and a TIG arc was carried out on Type 304 stainless steel plate with low S content in the air or in the chamber with Ar gas including 0%–15% oxygen gas. It was confirmed in both laser and hybrid welding that the penetration and geometry of welds depended upon the volume of the oxygen in the ambient atmosphere, and that a pronounced “nail head” disappeared in a weld metal containing a higher content of oxygen. An x-ray transmission real-time imaging apparatus revealed that melt flows depended on the volume ratio of oxygen in the atmosphere and there was a strong melt flow from the keyhole inlet to the surrounding pool near the top surface in pure Ar atmosphere. It is therefore considered that the melt flows induced by surface tension produces the “nail head” in pure Ar shielding atmosphere and the resulting weld becomes a wine glass shape. During YAG laser and hybrid welding in the air or in the Ar gas with the oxygen gas increased, the downward melt flows along the keyhole wall was more dominant to produce slightly deeper welds with narrower surface bead widths due to the effect of the surface tension with the positive gradient. On the other hand, the formation of shallower YAG-TIG weld beads produced in air was attributed to the mechanism that the Marangoni convection and electromagnetic convection induced by TIG arc constriction in the central part of the molten pool prevented downward melt flows from a keyhole tip resulting in deeper penetration than the tip. It was moreover confirmed that the porosity reduction in hybrid welding at a high current was due to the reduced formation of bubbles.

Short-Range Correlations and Meson Exchange Currents in Photonucleon Emission

One-nucleon emission processes induced by photon absorption are studied by considering short-range correlation effects. At energies above the giant resonance region the validity of the direct knock-out model has been tested by comparison with continuum random phase approximation results. Nucleon re-scattering effects have been considered by using an optical potential. The role of the electromagnetic convection, magnetization, and meson exchange currents has been investigated as a function of both excitation energy and momentum transfer. The short-range correlation effects have been studied by using various correlation functions. We found that the nucleon photo-emission cross section is rather sensitive to the presence of short-range correlations at large values of nucleon emission angle. In this region, however, the effects of meson exchange currents are even larger than those produced by short-range correlations.

ADJOINT VARIABLE METHOD FOR THE THERMAL DESIGN OF EUTECTIC DIRECTIONAL SOLIDIFICATION PROCESSES IN AN OPEN-BOAT CONFIGURATION

A computational method for the inverse design of a directional solidification process of a near-eutectic binary alloy driven by the coupled action of buoyancy, thermocapillary, and electromagnetic convection is presented. The objective is to calculate the mold cooling/heating conditions such that a stable desired interface growth with growth velocity vf and thermal gradient G is achieved. The interface velocity vf and thermal gradient G are chosen such that a diffusion-based growth is obtained in the presence of melt convection. Morphological stability is enforced by imposing an appropriate magnitude of G/|vf|, which is determined a-priori based on the constitutional stability criterion. The design problem is posed as a functional optimization problem. The cost functional is defined so as to represent the deviation of the freezing interface thermal conditions from thermodynamic equilibrium. An appropriate continuum adjoint problem is defined such that an analytical expression for the gradient of the objective function is obtained. The conjugate gradient method coupled with the finite element solutions of the continuum direct, sensitivity, and adjoint problems is employed for solving the inverse problem. The method is demonstrated with an example of calculating the boundary thermal fluxes for the directional growth of an Sb-8.6% Ge melt in an open-boat configuration under the influence of an external horizontal magnetic field such that a stable vertical interface advances from left to right with a desired growth velocity.

Interface shape, heat transfer and fluid flow in the floating zone growth of large silicon crystals with the needle-eye technique

A computer simulation is carried out to study the interface shape, heat transfer and fluid flow in the floating zone (FZ) growth of large (> 100 mm) Si crystals with the needle-eye technique and with feed/crystal rotation. Natural convection, thermocapillary convection, electromagnetic (EM) forces and rotation in the melt are considered. The unknown shape of the molten zone is calculated as a coupled thermal-electromagnetic-hydrodynamic problem and compared with that observed during experiments. The effects of the growth rate and the process stage on the shape of the interface are demonstrated. It was observed that natural convection and rotation dominate over thermocapillary and EM convection, at least for conditions corresponding to the industrial FZ Si production with the needle-eye technique. It is shown that under these conditions the rotation destabilizes the flow and only unsteady flows exist in the molten zone. The calculated distributions of the oscillation amplitude of the tangential velocity at the growing interface correspond to the radial resistivity distributions measured in the single crystal by the photo-scanning method.

低速ＤＣ・ＴＩＧ溶接法の研究 （第３報） フィラーメタルにフラックス入りワイヤを用いるＤＣ・ＴＩＧアーク溶接の溶込み形態

Previous report has clarified that in DC-TIG arc welding of SUS 304 stainless steel the penetration ratio (penetration depth/bead width=D/W) can be improved by using flux cored stainless steel wire for CO2 gas welding.In this report an effect of flux cored wire on penetration is further examined, and at the same time the mechanism of the penetration forming is investigated paying attention on changes of arc phenomena.The report shows that shrinked anode is formed directly under electrode by using flux cored wire and weld with large D/W is obtained without influence of the penetration caused by quite small quantites of elements in the base metal. Meanwhile, the idea that the primary factor of the large D/ W is due to electro-magnetic convection likely generated by electric current concentrating in shrinked anode is made clear in this welding method.For stabilized welding with large D/W penetration, a method to supply intermittently flux cored wire immediately before starting of step-movement welding is proposed.

Penetration form of DC TIG arc welding using flux‐cored wire filler. Study of low‐speed DC TIG welding (3rd Report)

Summary Previous reports have shown that the penetration ratio (penetration depth/bead width = D/W) during DC TIG arc welding of SUS304 stainless steel can be improved by using flux‐cored stainless steel wire for CO2 gas welding. The present paper further examines the effect of flux‐cored wire on penetration, considering the mechanism of penetration forming with regard to changes in arc phenomena. The paper shows that a shrunken anode is formed just under the electrode when flux‐cored wire is used. High D/W welds are then obtained without any penetration effect being caused by small quantities of elements in the base metal. The notion of the high D/W being mainly due to electromagnetic convection generated by electric current concentrating in the shrunken anode is also clarified by the experimental approach. For stable welding with high D/W penetration, the paper proposes a method of intermittent supply of flux cored wire just before step‐movement welding starts.

Drift-dissipative instability of the electromagnetic convection mode in a low-temperature plasma

The convection mode in a collisional plasma is investigated, with the inclusion of electromagnetic effects. It is shown that a dissipative instability can occur. The relationship to several well-known modes as well as applications to typical low-temperature plasmas are discussed.

Suppression of thermal convection and of similar types of fluid streaming by an electromagnetic force field

A method is described for increasing the stability of a fluid column against convection (such as thermal or electromagnetic convection) by superimposing upon a stabilizing vertical density gradient a parallel gradient of electromagnetic force density. The latter gradient is generated by creating a gradient of electrical conductivity concomitant with the vertical density gradient and maintaining at right angles to this gradient electric current traversed by a perpendicular homogeneous magnetic field. To obtain a stable column, one must choose the directions of the current and the magnetic field such as to make the electromagnetic force vector point in the direction of increasing electrical conductivity. Conditions under which the stability of a density gradient is reinforced, diminished or neutralized are considered as well as the engendering of instability by electromagnetic forces in a column stabilized by a density gradient.