US 3168509 A
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Feb. 2, 1965 H. JUEL 3,168,509 METHOD OF CONTROLLING ORIENTATION 0F EXTRUDED GRAPHITE CRYSTALLITES Filed Feb. 9. 1961 4 Sheets-Sheet 1 Feb. 2, 1965 JUEL 3,168,509
L. H. METHOD OF CONTROLLING ORIENTATION OF EXTRUDED GRAPHITE CRYSTALLITES Filed Feb. 9, 1961 4 Sheets-Sheet 3 I'll. H1" f fg MIMI 72" jjzz F I E. El
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METHOD OF CONTROLLING ORIENTATION 0F EXTRUDED GRAPHITE CRYSTALLITES 4 Sheets-Sheet 4 Filed Feb. 9, 1961 FIE-1U United States Patent 3,168,509 METHOD OF CONTROLLING ORIENTATION OF EXTRUDED GRAPHITE CRYSTALLITES Leslie H. Juel, Lewiston, N.Y., assignor to Great Lakes Carbon Corporation, New York, N.Y., a corporation of Delaware Fiied Feb. 9, 1961, Ser. No. 88,067 5 Claims. (Ci. 264-108) This invention relates to a special type extrusion die or apparatus designed to control the orientation of graphite crystallites or coke or carbonaceous particles in substantially platelet form during extrusion and to a process utilizing such die.
This invention has as one of its objects the making of core material bars or rods which, when graphitized, have maximum dimensional stability and minimum susceptibility to high temperature radiation damage in one critical direction when employed in atomic reactors.
The invention has an another object the control of the internal orientation of the carbonaceous particles used in making said bars or rods so as to achieve this maximum dimensional stability and minimum susceptibility to high temperature radiation damage in one critical direction.
It is a finding of this invention that the foregoing objectives may be realized by utilizing a die having the structural features hereinafter described, accompanied by various processing conditions, also hereinafter described.
The subject extrusion die, variations of which are shown in the attached drawings, is adapted to extrude green carbon bars, which when graphitized, have the foregoing desired properties.
These bars are formed from a charge consisting of graphite, coke or carbonaceous particles in a pitch or other suitable binder. Finely ground needle coke such as shown in US. Patent 2,775,549, decomposed silicon carbide and kish are examples of carbonaceous materials which may be employed with a binder such as pitch and processed and extruded through the die of the present invention to form rods having the desired properties. These carbonaceous particles are in substantially platelet form. 'When extruded through dies of this invention, these platelets become preferentially orientated in such a manner as to comprise lamellae disposed in a plurality of superimposed planes, a high percentage of which planes are substantially mutually parallel and approximately perpendicular to the parallel walls of the forming die. This preferential orientation or alignment of the lamellae or platelets obtains in a green extruded bar when, after baking and graphitizing said green bar, its physical properties, such as Coefiicientof Thermal Expansion, Resistivity, etc., measured along three mutually perpendicular axes corresponding to the two edges and length of a bar of rectangular cross section, approach exhibiting the same type of anisotropy characteristic of a single crystal of graphite. In other words, the magnitude of any given property measured along the axis of extrusion and in one direction, e.g., along the width perpendicular to the extrusion axis, will be substantially different from the magnitude of that same property measured in the direction (e.g., along the height) mutually perpendicular to the other two axes. These effects and their meaning will be made clearer by reference to FIG- URE 10 of the drawings and to the examples which follow.
The invention will become clearer after a consideration of the drawings wherein:
FIGURE 1 shows a front elevational view of a die.
FIGURE 2 is a sectional view taken on the line 22 of FIGURE 1;
FIGURE 3 is a plan elevational view taken on the line 33 of FIGURE 2;
FIGURE 4 is a vertical sectional view taken on the line 4-4 of FIGURE 2;
FIGURES 5, 6 and 7 show views corresponding to that of FIGURE 2 and represent various modifications of the die of FIGURE 1;
FIGURE 8 shows a front elevational view of a modified die wherein the lesser dirnension ,of' the material being extruded is further diminished as it passes through the die, whereas the larger dimension remains constant, rather than vice versa as with thedie of FIGURE 1;
FIGURE 9 is a sectional view of this modified die taken on the line 99 of FIGURE 8; and
FIGURE 10 is a perspective view of a portion of a neutronic reactor employing bars or rods produced in accordance with the teachings of this invention.
Because the dies of FIGURES 1 and 8 have analogous counterparts, the same numbers are employed to represent corresponding parts, with the exception that in the case of the die of FIGURE 8 (or FIGURE 9) a small a is added after these numbers.
The portion of a neutronic reactor shown in perspective in FIGURE 10 shows bars or rods 20 in their extruded green (viz. formed but unbaked), condition, having lamellae 22 disposed in a plurality of superimposed, substantially mutually parallel planes. The rods 20 of this reactor are also all properly placed with respect toeach other. Cyiindrically shaped holes 21 are centrally placed within each of the rods or bars to accommodate fuel elements. (It is to be understood that the rods will be graphitized before their placement in the reactor with the green state orientation above described for illustrative purposes preserved.) When these conditions-prevail, the CTE (coefficient of thermal expansion) and resistivity of the rods when graphitized are considerably greater in the X direction than they are in the Y or Z directions. This results in the minimization of radiation damage in the X'direction and the substantial reductionof downward slumping caused thereby when such graphitized rods or bars are used in the reactors. The elimination or minimization of damage in the X direction is much more critical or important that it is in the horizontal or 2 direction, or lengthwise of the rods or in the Y direction. This is because the base of the reactor remains substantially firm and mechanically sound even with damage in the Z or Y directions, whereas slumping in the X direction soon creates mechanical inoperability and damage throughout the entire reactor.
The die, which will bring about the aforedescr ibed orientation and which is referred to generally as 1, possesses a substantially rectangular inlet 2 and a substantially rectangular outlet 3, each having one substantially equal and corresponding dimension such as each having approximately the same or equal breadth or Width. The die possesses two substantially parallel walls 4 and 5 which are spaced apart a distance equal to the Width or breadth, and two converging walls 6 and 7 which are adapted to compress the platelet-like carbonaceous particles-binder mixture as it passes through the inlet 2 toward the outlet 3. Walls 6 and 7 should preferably continuously converge toward each other most generally with a continuously diminishing slope or curvature until the desired rectangular dimension of the mass being extruded is reached, after which these walls too become substantially parallel. Thus, when using the die of FIG- URE 1 or the die of FIGURE 8, one dimension of the mass always remains substantially constant while the other dimension of the mass is substantially diminished as it goes from the inlet of the die to the outlet. The dimension of the mass which changes may sometimes be larger than the constant dimension of the inlet as is the case when the dies of FIGURES 1-7 are used, and at other times may be less than the constant dimension, as
L0 when the die of FIGURE 8 or 9 is used. The selection of the particular type of die employed may depend on the shape of product desired or on other factors, the die of FIGURE 1 producing rectangular rods which are substantially of equal side-length or square in cross-section and the die of FIGURE 8 producing highly oriented extruded rectangular slabs of unequal side-length. The dies to produce such slabs may sometimes, if desired, be so shaped or dimensioned that the slabs can be further cut lengthwise into a niunber of rods substantially square in cross-section.
In a preferred embodiment, one or more blade-like vanes 8 may be employed near the entrance to the die. The vane or vanes may be entirely behind the inlet or opening as with the dies shown in FIGURES l, 2, 3, 7, 8 and 9, or may straddle the inlet as with the die of FIGURE 6. These vanes may be mounted in the dies by means of flanges 9 inserted in slots in the parallel walls 4 and 5 of the die as shown in FIGURES 1 and 8. Alternatively, the vane or vanes may be mounted in a separate member leading into the die. The vane or vanes 8 act in conjunction with the curvature of the walls 6 and 7 to assist in setting up shear forces in the mixture being extruded as it passes adjacent thereto and these forces tend to align the platelet-like particles of the carbonaceous mixture in planes substantially parallel to the vane and to the axis of the die and approximately perpendicular to the parallel walls 4 and 5 thereof.
The dies typically may be made from two main sections which may be coupled together by any suitable means such as by threading screws 10 into threaded holes 11. Wells may be provided in one of the sections to receive heads 12 of the screws.
A flange 13 having holes 14 may be employed near the inlet end of the die if it is desired to mechanically couple the die by means of nuts and bolts or other suitable means to an introductory member through which the extrudable mixture is brought up to the entrance or inlet 2 of the die 1.
An eye-bolt 15 may be threaded into a threaded Well in one of the sections of the die in order to provide convenient means for lifting and locating the device. A circumferential heating chamber 16 may surround the entry portion of the die in order to assist in keeping the extrudable carbonaceous mass at an optimum fluidity level for obtaining maximum particle orientation. Steam or other heat transfer means may be employed in this region.
The employment of one or more vanes or blade like members 8 and the particular placing of same and configuration of said members selected are all variables which may depend on the characteristics of the mix being extruded, temperatures and pressures employed, etc. They will of course be so chosen and designed as to obtain a maximum amount of orientation of the platelet like particles, consistent with minimum counter pressures, good production rates and absence of any cleavage lines in the final, extruded green bars or rods. It should be understood that in some cases a vane may not be required and that a die such as shown in FIGURE 5 is suflicient to obtain the required degree of orientation. Its need is also somewhat dependent on the amount of converging taking place in the die which will in turn depend greatly on the ratio of the diminishing dimension at the inlet to its dimension at the outlet. These factors apply also to the use of a vane with a die of the type shown in FIGURE 8.
It should be understood that although the dies will generally be comprised of or assembled from two main sections, this is not strictly necessary. The die may exist in a singly fabricated apparatus having a geometrically shaped chamber previously described.
The relationship of one dimension of the die remaining substantially constant from inlet to outlet while the other dimension is substantially reduced is an essential feature of the invention and is necessary to bring about the desired preferential orientation of the platelet particles present in the carbonaceous mixture being extruded.
The following examples are set forth in order to more fully describe the invention.
Example 1 An extrudable carbonaceous mass was prepared from a mixture of approximately 37 par-ts of coal tar pitch binder and parts of needle coke such as shown in US. Patent 2,775,549. The particles of needle coke were of such a size that at least 55% passed through a 200 mesh screen and substantially all passed through a 20 mesh screen. This extrudable mixture was mixed at a temperature of approximately C., cooled to ap proximately 100 C. and then extruded through the die shown in FIGURE 2. The platelet like carbonaceous particles of this mixture after their passage through the die were oriented in lamellae disposed a plurality of superimposed planes, a high percentage of which planes were substantially mutually parallel and approximately perpendicular to the parallel faces of the die. The ratio of the height of the inlet of this die to the height of the outlet of said die was about 2 to 1. In other words, one dimension of the extrudable mass changed or was reduced by this amount while the other dimension remained substantially constant. The extruded green carbon rod product was rectangular (and substantially square) in cross-section and possessed the platelet orientation previously described. This green carbon rod was baked and graphitized in accordance with conventional techniques and was then, except for final machining and boring, ready for placement and use in an atomic reactor.
Example 2 The procedure of Example 1 was repeated employing approximately 40 parts of pitch binder and 100 parts of needle coke of such a particle size that at least 55% passed through a 200 mesh screen and substantially all passed through a 20 mesh screen. This mixture was extruded through the die shown in FIGURE 2. The green car bon rod produced, when baked and graphitized, possessed a CTE (l/ C. 10") of 10.4 in the Z direction, 21.4 in the X direction and 8.2 in the Y direction, as these direc tions are indicated in FIGURE 10. It can be seen from this that the magnitude of the CTE along the axis of extrusion and along the width perpendicular to the extrusion axis are substantially different from the magnitude of the CTE measured along the X direction or the heighth of the rod, which is mutually perpendicular to the other two directions.
Example 3 Example 1 was repeated employing approximately 43 parts of pitch binder and 100 parts of needle coke of such a size that at least 55 passed through a 200 mesh screen and substantially all passed through a 20 mesh screen. The green carbon rod produced when baked and graphitized possessed a CTE in the Z direction of 11.0, a CTE of 19.2 in the X direction and a CTE of 7.1 in the Y direction.
These examples show some of the variations possible with respect to such matters as proportions of materials, etc. in carrying out the practices of this invention.
Example 4 The process of Example 2 was repeated employing the extrusion die of FIGURE 5, having no vanes. When baked and graphitized the CTE of the rod produced was 19.7 in the Z direction, 33.6 in the X direction and 10.1 in the Y direction. The resistivity (ohm/in. l0+ of this rod was 39.6 in the Z direction, 57.6 in the X direction and 37.1 in the Y direction.
Example 5 Example 3 was repeated employing the vaneless extrusion die shown in FIGURE 5. The CTE of the rod produced when baked and graphitized was 20.0 in the Z direction, 35.5 in the X direction and 10.2 in the Y direction. The resistivity of this rod was 41.2 in the Z direction, 55.2 in the X direction and 45.2 in the Y direction.
Examples 4 and 5 show that the desired orientation may be obtained not only with dies which have vanes, but also with vaneless dies.
Example 6 An extrudable carbonaceous mass was prepared of a mixture of approximately 40 parts of pitch binder and 100 parts of needles coke having a particle size such that at least 55% pass through a 200 mesh screen and substantially all passed through a 20 mesh screen and this mass was extruded according to the process of Example 1, but employing a standard type of extrusion die rather than one having the characteristics of this invention. In other words the die did not possess two Walls which were substantially parallel along the length of the die, but instead was so constructed that compression of the mass took place from all four sides. That is the mass was compressed and reduced in dimension between the side walls as well as between the top and bottom walls. The green carbon rod produced, when baked and graphitized, possessed a CTE in the Z direction of 29.5, a CTE in the X direction of 29.5 and a CTE in the Y direction of 6.5. It can be concluded from this example that the desired preferential orientation was not achieved, nor is it achievable when using a standard type of extrusion die.
It should be appreciated from the foregoing description and examples that a wide variation in the processing conditions, starting materials and apparatus features are possible and contemplated when carrying out the practices of this invention. For example, resins or suitable hydrocarbon binders may be employed as well as pitch. The amount of binder employed and the particle sizes and types of starting carbonaceous platelets may all be varied. For example, the amount of pitch used when it is employed as a binder will generally vary from about 30 to about 45 parts per one hundred parts of carbonaceous material. The pressures and temperatures employed may be varied greatly. The ratio of the dimension of the inlet which is compressed to its reduced dimension at the outlet of the die may vary considerably such as from about 2:1 to 5:1 with suitable modifications in the die contour. The employment of one or more vanes in conjunction with all of the foregoing variables and the possible varied locations of same, all taken together, make it possible to form the rods having the desired characteristics previously described for use in neutronic reactors and are contemplated as being employed in the present invention. These variations and selection of the desired 6 starting materials and equipment employed, etc, are considered within the skill of a man working in the art once the main features of this invention are before him.
I therefore do not wish to be limited except as defined comprises extruding a mixture of binder and carbonaceous particles in substantially platelet form through an extrusion die having a substantially rectangular inlet and a substantially rectangular outlet, one dimension of the inlet being substantially the same as the corresponding dimension of the outlet, the other dimension of the inlet being from about 2 to about 5 times greater than the corresponding dimension of the outlet, said die having two substantially parallel Walls and two converging walls which are adapted to compress the mixture as it passes therebetween from the inlet to the outlet.
2. A process according to claim 1 wherein at least one blade-like vane is situated near the inlet and extends between the parallel walls thereof.
3. A process for producing a green, rectangular crosssectioned carbonaceous bar having lamellae disposed in a plurality of superimposed planes a high percentage of which planes are substantially mutually parallel which comprises extruding a mixture of binder and carbonaceous particles in substantially platelet form through an extrusion die having a substantially rectangular inlet and a substantially rectangular outlet, the breadth of the inlet being substantia ly the same as the breadth of the outlet and the height of the inlet being from about 2 to about 5 times greater than the height of the outlet, said die having two substantially parallel walls and two converging walls which are adapted to compress the mixture as it passes therebetween from the inlet to the outlet.
4. A process according to claim 1 wherein the dimensions of the outlet are of substantially equal length.
5. A process according to claim 1 wherein one dimension of the outlet is substantially greater than the other dimension of the outlet.
References fitted in the file of this patent UNITED STATES PATENTS 1,245,898 Gates Nov. 6, 1917 1,411,170 Kahr Mar. 28, 1922 1,906,744 Frandsen May 2, 1933 1,952,038 Fischer Mar. 20, 1934 2,209,643 Chamblin July 30, 1940 FOREIGN PATENTS 51,167 Denmark Jan. 20, 1936