US 3461943 A
Description (OCR text may contain errors)
Aug. 19,1969" R. D. 5mm 3,461,943
I PROCESS FOR MAKI-NG FILAMENTARY MATERIALS 1 Filed Oct. 1'7, 1966 2 Sheets-Sheet 1 HIGH ' MOLTEN' /FILAMENT I INVENTOR RICHARD D. SCH/LE Aug. 19, 1969 R. D. scmus PROCESS FOR MAKING FILAMENTARY MATERIALS 2 Sheets-Sheet 2 Filed Oct. 17. 1966 c'RuciBLE Ev v 6L Mm ML m NM M MA m CORONA DISCHARGE POSITIVE EL ECTROOES STREAM 0F FLUX PARTICLES IN ,m ATTORNEYS INERT GAS ION/ZING EL 5c moo:
United States Patent 3,461,943 PROCESS FOR MAKING FILAMENTARY MATERIALS Richard D. Schile, Wethersfield, Conn., assignor to United Aircraft Corporation, East Hartford, Conn., 21 corporation of Delaware Filed Oct. 17, 1966, Ser. No. 587,009 Int. Cl. 322d 27/02, 25/00 U.S. Cl. 164-89 13 Claims ABSTRACT OF THE DISCLOSURE This application relates to a method of manufacturing fibers and filaments directly from a molten starting material using rapid cooling techniques.
This application relates to a method of manufacturing fibers and filaments directly from a molten starting material. More particularly, it relates to the rapid cooling of molten filaments to yield satisfactory, coherent fibers and filaments.
It is known that fibers and filaments of some materials can be produced by melting the desired material in a crucible and forcing the molten material through an orifice in the crucible. Heretofore, no attempt has been made to control the rate of cooling of the jet of molten material. With certain materials, generally low-melting metals and glasses, fibers can be formed by permitting the extruded jet of molten material to cool by free fall through an essentially stagnant gas atmosphere. However, with most materials, such a method will not produce fiber because the molten jet breaks up into droplets before it can be solidified. The cooling rate required to form fibers of a given material is a unique function of the physical properties of the material and of the diameter of the molten jet.
It has been found that for each material there is a critical cooling rate above which fiber is formed and below which shot is formed. In addition to more efiicient fiber formation, the use of the rapid cooling methods of this invention permits the carrying out of chemical reactions with the molten filament to produce coatings on or alloying or reactions with the molten filament prior to cooling. The use of the rapid cooling techniques in conjunction with the reaction techniques facilitates the control of the reaction.
Generally speaking, the cooling methods of this invention require that the molten filament be cooled at a rate between about 0.001 to 0.04 calories per second per square centimeter per degree Centigrade.
The required cooling rate can be determined more precisely but still only approximately by use of the following formula, it being understood that the precise rate is best determined through trial and error using the approximation as a convenient starting point:
H DV Patented Aug. 19, 1969 The following desirable cooling rates for specific materials is a useful guide:
Material h. cal./ sec. cm C. Aluminum 193 X 10- Iron 20 X 10 Boron 275 X 10- Silicon 365 X 10- Nickel 226 X 10" Chromium 184 x 10- Beryllium 187 X 10' Aluminum oxide 255x10" In describing the cooling methods of this invention it is to be understood that any means can be employed to form the molten metal filament. By filament is meant an infinite or if finite, a long strip of metallic material, having an overall circumference or perimeter not in excess of about 0.05 inch. Generally, the filaments used in this invention are produced by melting a metal, such as titanium, in a crucible provided with an orifice whose size and shape conform to the configuration desired of the filament and then pressuring the crucible with a suitable inert gas or fluid or other pressuring means, such as a mechanical piston, to thereby force molten metal through the orifice in a substantially downwardly and vertical direction.
A preferred method of rapid cooling in accordance with this invention involves the use of a plurality of gas jets located such that gas impinges laterally in one direction on the molten filament, coupled with a means for electrostatically attracting the filament toward the gas source. Lateral gas flow has been found to be the only type of flow which does not appreciably disturb the molten metal jet and cause it to disintegrate int-o droplets. A flow of cooling gas longitudinally along the axis of the molten jet causes rapid droplet formation. A convenient means for eelctrostatic attraction is to cause the jet to bear an electrical charge of one polarity and to cause a fixed object located apart therefrom, such as the gas blowing means, to bear a charge of the opposite polarity. In operation, the gas jets would push the molten filament in one direction (i.e., away from the gas source), while the electrostatic charge would cause the filament to be pushed in the 0pposite direction (i.e., toward the gas source). The electrostatic force of attraction is required in order to maintain the molten jet in a fixed position with respect to the gas discharge, otherwise the jet would be displaced to a region of lower gas velocity and lower cooling rate. By correlating the magnitude and location of the electrical charge with the gas velocity and volume, conditions can be balanced so as to avoid any lateral displacement of the filament as it travels downwardly through the cooling zone. In such circumstances, the gas serves to cool the filament rgpidly without, at the same time, causing disintegration of t e jet.
Gas jets located opposite each other and impinging on both sides of the molten filament would not be suitable, even if they were balanced to prevent lateral displacement, because the net gas velocity at the filament would be substantially Zero, due to the balancing of velocities in each direction. The atmosphere in the immediate vicinity of the filament would be substantially stagnant, thereby resulting in little or no heat dissipation.
In general, any inert gas which is non-reactive with the filament under the conditions of operation can be employed as the cooling medium for use in the gas jets.
Where rapid oxidation is not a problem, air is suitable. In other instances various inert gases, such as helium, argon, hydrogen and the like, can be employed. The use of helium or hydrogen is particularly advantageous because of the very high thermal conductivities of these gases. It is desirable that the coolant gas be provided in a plurality of air streams transverse to the filament, one above the other in the same vertical direction along the axis of the molten jet. The length of the gas stream (i.e., the number of individual jets located one above the other) depends to a large extent upon the rapidity of cooling desired and the temperatures involved as well as the practical problem of being able to conveniently maintain an adequate electrical charge over the entire distance of cooling. A cooling distance of from about six inches to four feet can be employed satisfactorily. The velocity of the cooling gas impinging on the molten filament can vary with the degree of cooling required and the various heat transfer coefiicients, but will generally be in the range from about ten to fifty feet per second.
A suitable means of applying the cooling gas is to cause the gas to flow, under suitable pressure through a metal tube having a porous wall opposite the filament.
The radial gas velocity through a porous tube decreases with the distance from the tube surface in such a way that the aerodynamic drag, in conjunction with the electrostatic force acting in the molten jet allows a stable balancing of the two forces. In this way, slight accidental variations in the position of the molten jet automatically give rise to corrective forces which return the jet to the desired position. The combination of a porous cylindrical gas nozzle and electrostatic field is unique in this respect.
The electrostatic force is produced by applying a direct current voltage between the molten filament and the surface of the metal tube through which the cooling gas flows. Electrical connection to the filament is preferably made through the melt in the crucible.
The precise voltage for use with any air velocity and volume can easily be determined by conventional calculations or by trial and error through varying impressed voltage until a stable position of the filament is achieved.
Another means of cooling the molten filament involves the use of a corona discharge. A high direct current voltage of between about 10,000 and 30,000 volts is placed across the filament and one or more metallic surfaces surrounding the filament. Electrical contact to the filament may be made through the melt in the crucible or otherwise. When the voltage is sufiiciently high so that a cloud of positive and negative ions is produced about the filament which is preferably negatively charged, a stream of negative ions of inert gas in the vicinity of the filament flows from the filament to the positive electrode. Since these ions have mass, this ion flow results in a circulation of cooling gas between the filament and the electrode. Convection cells are thus set up which have a substantial cooling effect on the molten filament.
This method of cooling is most advantageous in forming filaments of very high temperature materials. Filament formation is best accomplished if the cooling takes place as close to the crucible orifice as possible. Since the cooling gas may be circulated by the electric field generated by a very fine wire electrode, the problems associated with the use of gas nozzles and other cooling equipment at very high temperatures is avoided. In addition, if the material which is to be converted into fiber is heated by means of an induction coil, all parts in the vicinity of the coil must be nonconductors of electricity in order to avoid undesirable heating of these parts and loss of power from the coil. The corona generating electrode, however, may be placed even within the induction coil with no heating whatsoever.
In still another alternative method of cooling, similar to the foregoing, flux particles rather than inert gas can 4 be placed in the vicinity of the filament. These particles can be ionized and caused to be attracted to the filament, in which case upon contact with the filament, the cooler flux particles absorb heat from the filament.
The flux particles must be electrically charged by passing them through a region containing unipolar ions. This may be accomplished by placing a fine wire adjacent the jet and connecting a high voltage D.C. power supply between the melt and this elect-rode such that the electrode (charging electrode) is negative. The voltage is then adjusted so that a corona discharge occurs at the negative electrode but not at the molten jet. A stream of negative gas ions then flows across the space between the charging electrode and the jet. Flux particles are introduced into this ionization region, are negatively charged by collision with the gas ions and are attracted to and deposited on the jet. Alternatively, the flux particles can be charged by passing them through an ionization region between two fixed electrodes and then collected on the molten jet which is maintained at a lower positive potential in order to prevent back-ionization at the jet.
Representative fiux particles include the following:
Flux- Melting point, C.
A1 0 2050 its $388 MgO+Al O 1995 CaO+MgO 2370 CaO+Al O 1400 Na O+SiO 800 MgO+SiO 1543 MgO+TiO 1600 PbO+SiO 715 Al O +SiO 1600 The flux particles can be introduced to the ionization zone in the form of a fine powder sprayed periodically or continuously into the ionization region in the form of a suspension in an inert gas.
The flux particles, if they are to be employed solely for cooling purposes, should be inert to the filament material and also should not ionize at the filament temperature. The flux should have a melting point somewhat lower than that of the filament forming material in order that the flux particles melt on contact with the molten jet and thus absorb from the jet the heat equal to the latent heat of the flux. Good deposition is obtained if the flux particles have an average diameter of 20 microns or smaller. For some purposes, the inert flux particles can be permitted to remain on the filament surface. For other purposes, they can be removed by a suitable chemical aftertreatment.
For some purposes it is convenient to have the flux particles reactive with the filament, as indicated below so that the flux particles act as both reactant and coolant.
Prior to cooling with subsequent solidification of the molten filament, the filament can be contacted with a reactant at a concentration and of a type adequate to form a refractory fiber or other reaction product. Reactants thus employed can be either a gas or a finely divided solid, e.g., boron carbide filaments can be made by forming a filament of molten boron and then introducing graphite in the form of a suspension of powdered graphite in an inert gas. Graphite reacts with the molten boron very rapidly under these conditions to form boron carbide. The same reaction can occur if a gaseous reactant such as methane is employed in place of the graphite. Similarly, titanium boride filaments can be made by using as the molten material either boron or titanium and then introducing correspondingly either titanium or boron to the reaction zone as a finely divided powder.
The speed of the reaction can be accelerated if a corona discharge as indicated previously is simultaneously caused to occur about the molten filament. The use of the corona discharge will at this time in addition to accelerating the reaction, also serve to cool the filament thereby acting as a composite reaction and cooling step. If desired, an inert flux can the deposited to cool the reaction product and/ or stop the reaction at the desired time.
Suitable metals that can be employed as the filamentary material in the practice of the invention include any fiber forming metal including preferably, nickel, chromium and chromium alloys, stainless steel, beryllium, boron, titanium, and non-metallic fibrous materials such as aluminum oxide alone or in mixture With magnesium oxide and/or silicon dioxide or with calcium oxide and silicon dioxide.
The reaction plus cooling concept is particularly applicable when the molten material is a eutectic alloy of two (or more) metals and the material which is electrostatically deposited on the molten jet is either one of these metals or a compound of both of them. The electrostatically deposited material dissolves in and/or reacts with the molten metal in the jet to produce an alloy which has a higher liquidus temperature than the metal in the crucible. In some cases, the final product is an alloy which could not have been melted and contained in any existing crucible. Some examples are given below.
Melting point, Molten metal C. Reactant Ttntalum+20 atom percent 1, 775 Boron or tantalum boride.
Vanadium+15 atomic Boron or vanadium boride.
percent Boron. Yttrium+25.5 atomic percent boron.
1, 290 Boron or yttrium boride.
1 Data not available.
There are a number of useable metal alloys and mixtures of oxides which have important electrical properties. For example, fibers formed from Mn-l-Bi when fiberized at high cooling rates have a fine enough grain structure so that subsequent heat treatment of the fiber will result in the conversion of the fibers to Cb Sn or MnBi, respectively. Other compounds that can be formed through the rapid cooling techniques of this invention include BaTiO and KCbO as well as the eutectic between BaFe O and BaFe O This invention is not limited .to those metals which cannot conveniently be formed into filaments at low cooling rates. Certain metals which form coherent oxide films in air can be made into wire at low cooling rates. However, the use of the higher cooling rates of this invention results in smoother wire and fewer kinks, bumps and bends. Also, the grain size decreases as the cooling rate is increased, resulting in a more homogeneous product.
An over-all understanding of the invention in several of its aspects can be obtained from the drawings which are intended to be schematic representations. In the drawings:
FIGURE 1 represents a schematic view of a cooling operation performed on a molten filament using the balanced system of gas flow and electrical attraction.
FIGURE 2 represents a schematic View of a cooling operation performed on a molten filament using a corona discharge.
FIGURE 3 represents a schematic view of a cooling or combined cooling reacting operation performed on a molten filament using the introduction of flux particles and a corona discharge.
In FIGURE 1, there is shown a porous metal tube with solid nonporous end caps through which an inert gas is caused to flow under a desired pressure.
A high voltage supply is arranged so that the crucible and through it the molten filament is negatively charged while the porous metal tube is positively charged. The voltage employed is correlated with the gas fiow to prevent lateral displacement of the filament, i.e., the lateral translation of the filament to the right due to the gas flow must equal its lateral translation to the left due to the electrical attraction, resulting in a net translation of zero.
As shown in FIGURE 2, electrodes are provided of a polarity opposite to that of the filament with a voltage high enough to cause a corona discharge.
In FIGURE 3 a corona discharge occurs at the negative electrode with the introduction of fiux particles which are ionized at the ionizing wire as described above.
The following examples in the opinion of the inventor represent the best mode of carrying out the invention.
EXAMPLE 1 A cooling tube was prepared having an outside diameter 1 /2 inches, wall thickness of inch and a length of 24 inches. This tube was hot pressed from micron diameter stainless steel powder and seam welded into a porous tubular form. Air was supplied through this \tube by means of /2 horse power centrifugal blower operating at 12,000 r.p.m.
A crucible was prepared containing molten tin, maintained at a temperature above the melting point of tin, and provided with a small aperture through which molten tin was forced in the form of a downwardly flowing molten filament of diameter .005". The velocity of the downwardly moving tin filament was approximately 600 feet per minute. The top of the cooling tube was located 4 inches below the orifice of the crucible. Air was caused to impinge on the tin jet from the cooling tube at a rate of 53 cubic feet per minute. Simultaneously, electrical contact was made at the crucible and at the cooling tube with 1,100 volts of direct current. The electrodes were set so that the cooling tube was positively charged and the jet making electrical contact through the crucible was negatively charged. The jet was located approximately inch from the surface of the cooling tube as this was found to be its stable position. The jet was observed to flow vertically without any lateral movement due to the air blast.
The same experiment was repeated using instead of air a helium jet at the rate of 66 cubic feet per minute.
EXAMPLE 2 A jet of molten boron of .003" diameter is caused to fiow from a crucible. The jet is charged positively through electrical connections at the crucible. A negative electrode is provided spaced 1% inch from the jet and 12 inches below the crucible. A mixture of propane and boron trichloride is injected just below the crucible, causing a layer of boron carbide to be chemically deposited on the surface of the boron jet. At 12 inches below the crucible an aerosol of finely powdered quartz suspended in a gaseous mixture of helium and boron trichloride is injected. At an electrode potential of 6000 volts, the silica powder is deposited on the jet, stopping the deposition of boron carbide and cooling the jet. The product is boron carbide fiber with a silica coating.
EXAMPLE 3 A jet of molten chromium of .002" diameter is caused to flow from a crucible. The jet is positively charged and a negative electrode 12" long is positioned just below the crucible as described in Example 2. At 2 inches below the crucible, an aerosol of finely powdered silica suspended in helium is injected so as to flow parallel to the jet. At an electrode potential of 6000 volts, the silica powder is deposited on the jet, cooling it and at the same time forming a glass coating over the chromium.
Similar results are obtained when nickel is substituted for chromium.
Having thus described the invention that which is desired to be claimed by Letters Patent is as follows:
1. A method of producing a continuous filament comprising forcing molten material through an orifice having substantially the diameter desired of the filament to form a jet of approximately the desired filament diameter and rapidly cooling the molten jet to produce a solid filament without causing appreciable oxidation by unidirectionally impinging a gas upon the molten pet in a direction substantially perpendicular thereto while simultaneously creating an electrostatic charge whose tendency is to push the jet in a direction opposite to the direction of gas flow, and correlating the rate of gas fiow with the magnitude of the electrostatic charge to substantially prevent lateral displacement of the jet.
2. A method as in claim 1 wherein the molten jet is caused to flow generally downwardly and the gas is impinged thereon in a substantially horizontal direciton.
3. A method of producing a continuous filament comprising forcing molten material through an orifice having substantially the diameter desired of the filament to form a jet of approximately the desired filament diameter and rapidly cooling the molten jet to produce a solid filament without causing appreciable oxidation by causing charged particles to impinge upon the surface of the molten jet, the rate of cooling being sufliciently rapid to prevent breakup of the filament and consequent droplet formation.
4. A method of producing a continuous filament comprising forcing molten material through a norifice having substantially the diameter desired of the filament to form a jet of approximately the desired filament diameter and rapidly cooling the molten jet to produce a solid filament without causing appreciable oxidation by causing charged particles to impinge on the surface of the molten jet by causing the jet to bear an electrical charge of one polarity and supplying flux particles to an ionization zone located between the said molten jet and a surface bearing an electrical charge of an opposite polarity, whereby said flux particles become electrically charged and are attracted to the jet.
5. A method as in claim 4 wherein the flux particles are introduced to the ionization zone in admixture with an inert gas.
6. A method as in claim 4 wherein the flux particles are introduced to the ionization zone in the form of a fine powder.
7. A method of producing a continuous filament comprising forcing molten material through an orifice having substantially the diameter desired of the filament to form a jet of approximately the desired filament diameter and rapidly cooling the molten jet to produce a solid filament without causing appreciable oxidation by causing the formation of a corona discharge about the molten jet, in a substantially inert atmosphere, thereby causing a gas flow in the vicinity of the jet and the consequent removal of heat by convection from the jet.
8. A method as in claim 7 wherein the corona discharge is created by imparting a negative electrical charge to the jet which is maintained in an atmosphere of an inert gas and spaced apart from an electrode to which is imparted a corresponding positive charge, the resulting negative charge causing negative ions of the inert gas to flow away from the jet toward the electrode, thereby setting up convection currents.
9. A method of producing a continuous filament of refractory fiber comprising forcing molten material through an orifice having substantially the diameter desired of the filament to form a jet of approximately the desired filament diameter, contacting the jet with a reactant of a type and at a concentration capable of reacting thcrewith to form a refractory material and thereafter rapidly cooling the molten jet to produce a solid filament without causing appreciable oxidation.
10. A method as in claim 9 wherein the reactant is in the form of a gas.
11. A method as in claim 10 wherein the molten jet is passed through the gas while a corona discharge is produced about the jet.
12. A method as in claim 9 wherein the reactant is in the for of a finely divided solid.
13. A method as in claim 9 wherein the reaction is cooled by the impinging of inert flux particles.
References Cited UNITED STATES PATENTS 705,691 7/1902 Morton. 2,048,651 7/1936 Norton. 2,108,3 61 2/ 193 8 Asakawa. 2,336,745 12/ 1943 Manning 26410 2,338,570 1/1944 Childs 264-10 2,879,566 3/1959 :Pond 16482 2,907,082 10/ 1959 Pond 164-66 3,218,681 11/1965 Ditto 164-49 FOREIGN PATENTS 70,395 12/ 1941 Czechoslavakia.
J. SPENCER OVERHOLSER, Primary Examiner R. S. ANNEAR, Assistant Examiner US. Cl. X.R.