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The floating-zone melting of refractory metals by electron bombardment
Abstract
An apparatus which produces floating liquid zones in vertical rods of refractory metals is described. Movement of the zone along the specimen can purify it in three ways, namely, outgassing on vacuum fusion, evaporation of volatile impurities, and segregation of impurities in the liquid zone. Traversing speeds of approximately 1 cm/min have been used and the vacuum maintained between 10-5 and 5 × 10-4 mb. Power requirements are about 400 W or less. There is a theoretical upper limit of approximately 1 cm on the diameter of a specimen and a practical size limit on its length. Specimens as low as 2 mm in diameter have been zoned.
After zoning, many specimens are found to be single crystal.
... science and technology (Alonzo et al., 1995; Calverly et al., 1957; Glebovsky et al., 1998; Hay 37 et al., 1968; Liu & Zee, 1996; Moest et al., 1998). This necessitates both studying purification 38 processes and developing advanced techniques of growing single crystals of high-purity ...
... exclusively by EBFZM because of their extremely high melting temperatures and chemical 6 reactivity (Calverly et al., 1957; Hay et al., 1968; Alonzo et al., 1995; Moest et al., 1998; 7 Glebovsky et al., 1998). This necessitates both studying purification processes and ...
... The single crystals of the high-purity refractory metals are widely used in modern material science and technology (Alonzo et al., 1995;Calverly et al., 1957;Glebovsky et al., 1998;Hay et al., 1968;Liu & Zee, 1996;Moest et al., 1998). This necessitates both studying purification processes and developing advanced techniques of growing single crystals of high-purity refractory metals with modern electron beam (EB) guns (M. ...
... In the EBFZM method the liquid zone is held in place between two vertical collinear solid rods by its surface tension (Fig. 1). Single crystals of high-purity refractory metals can be grown exclusively by EBFZM because of their extremely high melting temperatures and chemical reactivity (Calverly et al., 1957;Hay et al., 1968;Alonzo et al., 1995;Moest et al., 1998;Glebovsky et al., 1998). This necessitates both studying purification processes and developing advanced methods of growing single crystals metals using modern electron beam guns (M. ...
... The main liquid-phase method for growing tungsten, rhenium, tantalum, molybdenum single crystals is a crucible-free zone melting. As a heating source, an electron beam was originally used (Calverly et al. 1957, Glebovsky 2019. One of the first options for implementing this method is shown in Figure 1a. ...
... On the other hand, some materials processing requires manual control according to information obtained during operation, and is difficult to automate. For example, in floating-zone (FZ) crystal growth, which is used to produce silicon wafers and various kinds of crystalline materials such as semiconductors, oxides, metals, and intermetallic compounds, an operator adaptively controls the input parameters to maintain preferred conditions for single-crystal growth by monitoring the status of the melt in the chamber [18][19][20][21][22][23][24][25][26][27][28] . In the present study, we aimed to construct a control model for automated operation of FZ crystal growth from a small number of operation trajectories. ...
The complete automation of materials manufacturing with high productivity is a key problem in some materials processing. In floating zone (FZ) crystal growth, which is a manufacturing process for semiconductor wafers such as silicon, an operator adaptively controls the input parameters in accordance with the state of the crystal growth process. Since the operation dynamics of FZ crystal growth are complicated, automation is often difficult, and usually the process is manually controlled. Here we demonstrate automated control of FZ crystal growth by reinforcement learning using the dynamics predicted by Gaussian mixture modeling (GMM) from small numbers of trajectories. Our proposed method of constructing the control model is completely data-driven. Using an emulator program for FZ crystal growth, we show that the control model constructed by our proposed model can more accurately follow the ideal growth trajectory than demonstration trajectories created by human operation. Furthermore, we reveal that policy optimization near the demonstration trajectories realizes accurate control following the ideal trajectory.
... Silicon wafers up to 200 mm diameter are manufactured by the FZ method using RF heating, for the production of semiconductors. Not only silicon crystals but also numerous other crystals such as semiconductors [27], metals [28], alloys [29], intermetallic compounds [30][31][32], oxides [33][34][35][36], borides [37,38], carbides [39] and silicides [40][41][42], especially those with a high melting temperature, have been grown by the optical FZ method [43]. Figure 1 shows a schematic illustration and photograph of a typical optical FZ apparatus. ...
We have applied a Gaussian mixture regression to the prediction of operation dynamics in floating zone crystal growth as an example of a materials process. From only five demonstration trajectories, we successfully predicted the operating dynamics using the Gaussian mixture model with better precision than obtained by using linear regression or neural networks. The current results indicate that the Gaussian mixture regression is suitable for predicting the operation dynamics of materials processes in which it is preferable to avoid large changes from stable operating conditions. Furthermore, precise prediction by the Gaussian mixture regression will lead to the optimization of operation trajectories and automatic control of materials processes.
... Many metallic impurities associated with tungsten have comparably higher vapor pressure, and purification from them is largely by evaporation. Savitskii and Burkhanov [1] and Calverley et al. [43] pointed out that effective removal of impurities in electron beam melting occurs when the vapor pressure of the impurity is a factor of 10 higher than the vapor pressure of the metal base. Stawovy [16] observed recently something similar in electron beam additive manufacturing of molybdenum using a PM and an LCAC manufactured filler wire. ...
In present study, the feasibility of wire-based additive manufacturing of commercially pure tungsten using electron beam technique could be demonstrated. Three different representative volumetric AM structures were built and subsequently characterized. The parts show a sound visual appearance with the absence of macroscopic cracks or severe distortion. The fabricated parts exhibit high density and the value depends on the welding sequence applied; while the thin-and medium-walled structure has a relative density of ~100% and 99.875%, the measured relative density of the volumetric structure is slightly reduced to ~99.131% due to the smaller periodic bonding defects. However, a higher density could be achieved compared to powder-based processes on refractory metal. The mean hardness value of the fabricated AM structures is approx. 366-380 HV1 and is in the range of approx. 89-93% of the conventionally fabricated substrate of 410 ± 39 HV1. A coarsening of the grains from the bottom to the top and a change in morphology can be noted for all AM structures. While the coarsening is quite severe for the thin-walled structure, it is moderate for the volumetric AM structures due to the change of the thermal boundary conditions. Caused by the deposition process, the microstructure in the substrate also changes and exhibits a coarse-grained heat-affected zone. Nevertheless, the grain size is still smaller compared to the AM bulk material.
... In the EBFZM melting 3 method the liquid zone is held in place between two vertical collinear solid rods by its 4 surface tension (Fig. 1). Single crystals of high-purity refractory metals can be grown 5 exclusively by EBFZM because of their extremely high melting temperatures and chemical 6 reactivity (Calverly et al., 1957;Hay et al., 1968;Alonzo et al., 1995;Moest et al., 1998; 7 Glebovsky et al., 1998). This necessitates both studying purification processes and 8 developing advanced methods of growing single crystals metals using modern electron 9 beam guns (M. ...
The dislocation substructure of single crystals of molybdenum and tungsten is characterized by significant similarity and remains virtually unchanged at the growth rates of 0.5-5 mm/min. Significant changes in the substructure, reflected in increasing fragmentation of subgrains and the misorientation angles of up to 3-40 of an arc, take place when the growth rates increase to 10-20 mm/min and above. After sudden increase of the growth rates from 2 mm/min to 40 mm/min, the dislocation density inside subgrains increases on an order of magnitude, reaching 5x106 cm-2. Due to the high temperature gradients near the crystallization front and the high cooling rates, the growth of the perfect single crystals of molybdenum and tungsten from the melt is impossible. Even using the perfect seed crystal, free from the small-angle boundaries with the misorientation angles 3’, the method does not allow growing crystals of satisfactory structural quality. The numerical estimates show that the cooling rates of single crystals can reach 104 K/s. This results in small increase in the dislocation density in the thin surface layer during cooling, which is quite acceptable. Therefore, to cool single crystals slowly after growing by EBFZM is impractical from all points of view. The conditions of the growth of single crystals with the ultimately low dislocation density and the small subgrains spread are revealed. The relatively low dislocation density and the lack of the small-angle boundaries are achieved by recrystallization. The optimal procedure involves the 6% deformation of the single crystalline specimen with the <111> growth axis by rolling in vacuum along the <112> plane and in-situ high-temperature annealing. To monitor the subgrain substructure of the tungsten single crystals, the anomalous X-ray transmission method is effective. The Borrmann effect is observed in the recrystallized perfect tungsten single crystals. The dislocation density determined by diffraction data is close to that determined by etch pits (~5x10-4 cm-2). This opens potentialities for controlling the dislocation density by the X-ray diffraction techniques. The perfect single crystals can be employed as the novel crystalline deflectors to monitor the beams of relativistic charged particles and for other applications. The recrystallization example shows how perspective and reliable is this way in obtaining structurally perfect single crystals of tungsten and other refractory metals as well.
Single crystals of refractory metals with a given orientation are demanded in both the power-optics and high-power laser applications. The evolution of liquid-phase methods for growing such refractory metals as the molybdenum and tungsten single crystals is analysed. As shown, the methods for growing crystals with a single heating source cannot solve the problem of fabrication of industrial-sizes’ crystals. The superlarge single crystals of various config-urations, such as ingot and plates, can be grown using the combined plasma-induction method. The single-crystalline quality of the ingots and the preservation of the orientations of the selected crystallographic axes are con-firmed by the x-ray method according to Laue images. The repeatability of physical and mechanical properties is shown by the indentation test.
The complete automation of materials manufacturing with high productivity is a key problem in some materials processing. In floating zone (FZ) crystal growth, which is a manufacturing process for semiconductor wafers such as silicon, an operator adaptively controls the input parameters in accordance with the state of the crystal growth process. Since the operation dynamics of FZ crystal growth are complicated, automation is often difficult, and usually the process is manually controlled. Here we demonstrate automated control of FZ crystal growth by reinforcement learning using the dynamics predicted by Gaussian mixture modeling (GMM) from small numbers of trajectories. Our proposed method of constructing the control model is completely data-driven. Using an emulator program for FZ crystal growth, we show that the control model constructed by our proposed model can more accurately follow the ideal growth trajectory than demonstration trajectories created by human operation. Furthermore, we reveal that policy optimization near the demonstration trajectories realizes accurate control following the ideal trajectory.
Additive manufacturing (AM) technologies offer novel opportunities for processing difficult to cast refractory materials. Electron beam melting (EBM) AM is particularly attractive as the rapidly moving electron beam can be utilized to heat the powder bed which mitigates against some process induced cracking mechanisms. A great deal of prior work has been done to investigate laser based processing of molybdenum but little EBM focused work currently exists. In this work we investigate EBM processed molybdenum and observe sharp 001,111, and mixed 001 & 111 crystallographic fibers in the build direction. The apparent preference between these build direction fibers is dependent on the imposed energy density and this is likely explained by the weld pool shape. Detailed microscopy reveals that the observed columnar grains consist of much finer equiaxed low angle boundary subgrains suggesting large process induced stresses leading to appreciable plastic deformation. The implications resulting from this work are that molybdenum may be processed crack-free via EBM AM and that fiber-switching may be controlled, and exploited, towards fabricating components with optimized performance.
The effects of purity on the properties for refractory metals, including the chemical impurities and defects, have been discussed. Several promising purifying techniques, such as electron beam floating zone melting, and plasma arc melting have been mainly investigated and compared. Some available refining methods have been suggested in this paper. Copyright © 2014, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.
Nb-Si based high temperature alloys possess higher melting points, relatively lower densities and attractive high temperature strength compared with Ni based superalloys. Therefore, they are expected to be promising candidate materials used at high temperatures beyond the maximum operating temperature of the Ni based superalloys. Directional solidification could improve both the room temperature ductility and high temperature strength of Nb-Si based high temperature alloys. Up to now, the directional solidification of Nb-Si based high temperature alloys has been conducted by cold crucible Czochralski method, floating zone melting techniques and integrally directional solidification process. In this paper, a review of directional solidification for Nb-Si based high temperature alloys is present, and the prospective research effort about directional solidification is also put forward. © 2014, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.
Refractory metals and alloys with outstanding properties play an unsubstituted role in the fields of national defence, aerospace, electronics, energy sources, chemical engineering, metallurgy and nuclear industry, and attract much more attention recently. The application and development of the refractory metals and alloys are reviewed. As an example of monocrystal W, Mo and alloys, the development of the directional solidification technology with electron beam floating zone melting for the preparation of refractory metal monocrystal is summarized. The processing and microstructure of molybdenum produced by electron beam zone melting is also preliminarily investigated.
The design and operation of equipment to purify and to grow single crystals of gallium arsenide are described. Features of the design include a demountable quartz system, adjustment of the relative position of the ends of the gallium arsenide rod during operation, excellent visibility, a traversable heater system and close control of the arsenic pressure in the tube. The latter feature has reduced the problem of ejection of molten material from the zone found in previous work.
A simple electron focusing device for floating-zone electron-bombardment furnaces is described. The filament is surrounded by and electrically connected to a water-cooled copper cage, which focuses the electron beam some 3 cm vertically below the filament. The anode is thus largely protected from evaporation from the filament, the molten zone is not obscured and evaporation and spluttering from the zone do not contaminate the filament. The sharpness of the focused beam may be altered by varying the filament diameter and position. Rods of between 3 mm and 12 mm diameter have been successfully zoned in a cage of the dimensions given.
The production of molten drops by bombarding the lower end of a vertically suspended rod with electrons has enabled the drop-weight method to be applied to the measurement of the surface tension γ of liquid tungsten. The mean value obtained is 2300 dyn cm-1 at the melting temperature of 3380°C in a vacuum of approximately 10-4 mb.
An improved apparatus for floating-zone melting by electron bombardment is described. Provision is made for zoning speeds of 0-350 cm/h with bombarding voltages up to 10 kV at 500 mA. The operating vacua are of the order 10-6 mb or better. Up to 22 cm of specimen length can be zoned.
A small electron focusing cage for use in electron bombardment floating zone melting is described. The design eliminates contamination of the specimen by the filament. The filament, which is protected from material ejected by the molten zone, is not visible to the operator and does not interfere with observation of the zone. A grid electrode has been used; this resulted in a reduction of the power required to form a given zone.
Simple experimental arrangements are described for the evaporation of tantalum and zirconium from a pendant drop, maintained molten by electron bombardment. The method is clearly of general application and has advantages over existing techniques. Since there is no boat or container there is no possibility of solution of the boat material and consequent contamination of the evaporant. The material to be evaporated is used in wire or rod form; these may be very easily interchanged.
Stabilization requirements and techniques for electron-bombardment heating are considered, and basic design formulae are presented for a flexible control system applicable to temperature-limited emission furnaces. Power handling capacity, loop stability and speed of response are discussed, and full circuit details of practical stabilization systems for a 250 W and a 6 kW furnace are given. It is concluded that emission stabilization is always beneficial, and sometimes essential, and that emission stabilities of better than 0.5% at powers up to 20 kW are practicable.
Die Stabilitätsverhältnisse stationärer senkrechter Schmelzzonen. wie sie insbesondere beim tiegelfreien Zonenziehen auftreten, wurden theoretisch untersucht. Solange keine elektromagnetischen Kräfte wirken, ist die Form der Schmelzzonenoberfläche aus dem Gleichgewicht zwischen der Oberflächenspannung und dem von den Randbedingungen abhängigen hydrostatischen Innendruck bestimmt. Bei großem Unterschied zwischen oberem und unterem Stabradius tritt ein nahezu horizontaler Bereich der Flüssigkeitsoberfläche auf. Hierdurch ist der Innendruck festgelegt, so daß sich die maximale Länge der Schmelzzone in guter Näherung als Funktion der Stabradien angeben läßt. Komplizierter liegen die Verhältnisse bei gleichem oberen und unteren Stabradius; doch konnte durch verschiedene Näherungsmethoden ein guter Überblick über den Existenzbereich stabiler Schmelzzonen in Abhängigkeit vom Stabradius gewonnen werden.
DOI:https://doi.org/10.1103/PhysRev.89.1297
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The present paper seeks to describe the shapes of liquid floating zones between solid rods of silicon and of germanium and to discuss stability conditions. The shapes were deduced from the equation given by Laplace, p=γ(1/R1+1/R2),where p indicates the pressure difference across the surface membrane of a liquid at any point, R1 and R2 are the principal radii of curvature at that point, and γ is the surface tension of the liquid.
The floating liquid zone is analyzed within a wide stability range. Its dimensions were theoretically determined and experimentally confirmed.
Equipment to carry out recrystallization and zone melting of silicon from a floating liquid zone is described. In the first design, silicon was melted by a tungsten ring. In an improved apparatus, no heater element inside the melting chamber is necessary, since the silicon is melted by induction heating. The system is capable of producing crystals of extreme purity. Silicon single crystals of resistivities up to several hundred ohm cm have been grown with the described equipment.