Ultra-high pressure system with variable lateral anvil support

Abstract

The generation of ultra-high pressures by a pair of opposed Bridgeman-type nvils is improved by surrounding the major portions of each anvil with a frustro-conical segmented jacket in position to transmit vertical forces thereon to the anvils in an axial direction and at the same time induce lateral compressive stresses therein for increasing the resistance thereof to brittle failure. Additional support is provided to the pressure-face ends of the anvils by a die ring laterally disposed therebetween in position to be circumferentially stressed by a segmented die ring which is, in turn, similarly compressed by a band of pressure-transmitting metal subjected to lateral extrusion by an annular piston enclosing the pressure system. The displacement of the piston is adjustably controlled in accordance with the size of the anvils and the axial forces thereon to provide optimum support to the die ring.

Description

BACKGROUND OF THE INVENTION

This invention relates to apparatus for generating ultra-high static pressures and is more specifically directed to means for improving the well-known Bridgeman system of providing massive support to a pair of opposed pressure anvils.

As a result of the successful conversion of graphite into industrial diamonds, considerable interest has been aroused in the development of technology which will provide substantially greater pressures than heretofore attainable with existing apparatus for other promising applications such as the compaction of porous materials down to their theoretical density or the synthesis of entirely new metallic compounds. The generation of pressures by commercial equipment has generally been under 100 kilobars (1 .times. 10.sup.10 Pa) due to the inadequate strength of the load-carrying elements in direct contact with the high pressure region surrounding the workpiece to which the pressure is being applied and by the tendency for the high pressure to flow or extrude to a region of lower pressure. These limitations have been recently overcome, to some extent, by designing the anvils so that the load and resulting stresses thereon will decrease inversely with the distance from the high pressure region. Such arrangement reduces the shear stress and tensile stress components on the pressure anvils and consequently permits the fabrication thereof from materials such as cemented tungsten carbide which, while relatively brittle, nevertheless possess unusually high compressive strength. Additionally, the gradual decrease in pressure on the anvil surfaces which slope away from the working faces thereof allows maximum utilization of the internal and surface friction characteristics of the pyrophyllite which acts as a gasket between the anvils for preventing extrusion of the workpiece toward the region of lower pressure.

While there are several ultra-high pressure systems wherein the pressures generated between the opposing faces of Bridgeman-type anvils are distributed along the sloping nonworking surfaces thereof in a gradually decreasing fashion, the most successful, insofar as the attainment of pressures in excess of 100 kilobars (1 .times. 10.sup.10 Pa) is concerned, is the one taught by A. S. Balchan and H. G. Drickamer in a paper published March 1961 in The Review of Scientific Instruments, Vol. 32, No. 3. In this arrangement, the workpiece is seated in a solid "pellet" or gasket of pyrophyllite located in the space between the angular non-working surfaces of the pressure anvils and contained by a thick-walled cylinder of high strength metal. While this design effectively prevents the extrusion of whe workpiece into the lower pressure regions along the non-working surfaces of the anvils, the magnitude and distribution of the support pressure given to the anvils and the gasket material therebetween cannot be increased since it is determined by the physical dimensions of the parts and the elastic expansion of the thick-walled cylinder. Accordingly, such development is not commercially attractive inasmuch as a desired increase in the pressure generated against the workpiece would require a corresponding increase in the size of the anvils and containing cylinder. There are other systems, such as the one shown in U.S. Pat. No. 3,150,412 to Donald H. Newhall which utilizes a fairly large pressure cavity generally symmetrical in all three dimensions and employs a plurality of laterally disposed wedges which are hydrostatically functioned to cooperate with the anvils in the application of pressures to the workpiece, either simultaneously or alternately. The use of hydrostatic fluids limits the pressure which can be attained to maximum of 100 kilobars (1 .times. 10.sup.10 Pa). Furthermore, the environment in which such system can be usefully employed is limited to the minimum and maximum temperatures which can be tolerated by the particular fluids utilized.

SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to provide a completely mechanical system for generating ultra-high pressures on a workpiece in excess of 300 kilobars (3 .times. 10.sup.10 Pa).

It is another object of this invention to provide a system, as aforesaid, wherein lateral support is imparted to a pair of Bridgeman-type pressure anvils independently of the compressive axial load imparted thereto.

An additional object of the present invention resides in the provision of a pressure generating system, as aforesaid, wherein the lateral support imparted to the pressure anvils can be readily varied by a corresponding adjustment of the force being transmitted to the pyrophyllite gasket disposed between the pressure faces of the anvils.

A further object of this invention is to provide an ultra-high pressure system, as aforesaid, which will operate with equal effectiveness in either a high-temperature or a cryogenic environment.

Still another object of this invention is to provide a pressure system, as aforesaid, which will permit direct access to the ends of the anvils remote from the pressure faces thereof for attachment thereto of the instrumentation required to indicate the extent of the pressure generated on the workpiece.

It has been found that the foregoing objects can be effectively achieved by a pressure-generating assembly incorporating a pair of Bridgeman-type anvils fabricated of cemented tungsten carbide and shaped to include frustoconical sections of dissimilar angularity integrally joined at the larger diameter ends thereof. Each anvil is oriented so that the truncated surface of the conical section with the shorter vertical axis serves as one of the pressure faces between which the workpiece is compressed. The workpiece is preferably press-fitted in a pyrophyllite ring which is, in turn, similarly disposed in a tungsten carbide die ring shaped to provide a spaced fit between the surfaces which slope away from the pressure faces of the anvils. Additional pyrophyllite extends into a portion of the gap between the die ring and the sloped anvil surfaces to serve as a gasket therebetween. The presence of the die ring between the sloped anvil surfaces insures a more effective control of the pressure imparted to the pyrophyllite therebetween than would be possible with pyrophyllite alone. The die ring is concentrically fitted into a segmented support ring which is, in turn, encircled by an annular band of pressure-transmitting metal, such as indium, disposed in the path of an annular piston surrounding the upper anvil and the holder therefor. Extrusion of the annular band outwardly of the exterior periphery of the piston is prevented by a surrounding massive support ring provided in the tandem press utilized to separately impart different vertical forces to the anvil assembly and to the annular piston. Thus, the force applied to the piston can be independently adjusted to compress the segmented die ring support for stressing the die ring to the extent required to impart optimum support to the anvils in accordance with the cross-sectional size thereof and the axial forces imparted thereto.

Additional anvil support is provided by a hollow segmented conical frustrum of sufficient height to serve as a jacket surrounding the conical anvil section with the longer vertical axis. The segmented structure of this support jacket produces a more uniform, and consequently more effective, compressive stress in the entire anvil which avoids the prior art solution of enlarging the cross-section thereof to compensate for the tendency of the material to fail by brittle fracture under pressures in excess of 100 kilobars (1 .times. 10.sup.10 Pa). Furthermore, this additional anvil support is achieved in a manner which permits direct access to the end face of each anvil remote from the pressure face thereof for the attachment of the instrumentation required to monitor the response of the workpiece to the pressure generated thereagainst.

BRIEF DESCRIPTION OF THE DRAWINGS

The exact nature of the invention, as well as other objects and advantages thereof, will be readily apparent from consideration of the following specification relating to the annexed drawings wherein:

FIG. 1 is a vertical section through the pressure-generating assembly;

FIG. 2 is a perspective elevational view of the assembly with portions of the components cut away to show the interior structure thereof;

FIG. 3 is an enlarged perspective view in partial section of the retaining ring and gasket structure utilized to hold the workpiece between the pressure anvils;

FIG. 4 is an additionally enlarged view, shown in vertical section, of an alternate workpiece arrangement of the type required for electrical conductivity measurement; and

FIG. 5 is a graph showing the relationship between the applied force and the pressure generated at the transition points of selected materials.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As best shown in FIG. 1, wherein similar reference characters are utilized to designate corresponding parts throughout, the pressure-generating system of the present invention includes a pair of Bridgeman-type anvils 12 each fabricated of a material with an unusually high compressive strength, such as cemented tungsten carbide, and shaped to form oppositely extending frustro-conical sections 14 and 16 integrally joined at the larger diameter ends thereof. Anvils 12 are vertically oriented so that the apices of the axially shorter sections 14 serve as the upper and lower working faces 18 which transmit the generated pressure to a workpiece 20 placed therebetween. While the slope of the non-working faces 22 relative to horizontal pressure faces 18 will vary in accordance with the compressive strength of the material from which anvils 12 are fabricated, the angle of such slope should be small enough to insure a maximum of massive support to faces 18 and yet not so small as to deform under the force applied thereto whereby the resulting increase in the area thereof produces a corresponding decrease in the pressure being applied to workpiece 20. Experience has shown that optimum results are achieved when the slope of the non-working surfaces 22 relative to the horizontal falls within a 20.degree.-30.degree. range. Although the height of the axially longer section 16 will vary in accordance with the instrumentation area desired at the exposed end face 24 thereof, the junction thereof with conical section 14 is designed to provide a 90.degree. angle in order to maximize the load transmitted to anvils 12.

Workpiece 20 which may be rectangular or in the form of a flat disc, as best shown in FIG. 3, is, in all instances, of no greater diameter than pressure faces 18 of anvils 12 and is centrally press-fitted into a retaining ring 26 of pyrophyllite, a natural mineral, which is, in turn, concentrically disposed in a radially beveled annulus 28 of pyrophyllite inserted into the center of a die ring 30. However, in the event information concerning the electrical resistivity of workpiece 20 is required, the latter is preferably in the form of a wafer 27 which, as shown in FIG. 4, is completely embedded in a cylinder 29 of silver chloride or pyrophyllite inserted into retaining ring 26. Suitable tabs or strips 31 of copper are seated in opposite faces of cylinder 29 and extend therein into electrical contact with wafer 27. If desired, ring 26 and annulus 28 may be integrally joined to form a single retaining configuration. The opposite faces of die ring 30 are sloped in spaced relation to the corresponding non-working surfaces 22 of anvils 12. Although FIG. 1 shows a uniform gap between the faces of die ring 30 and anvil surfaces 22, the present invention also contemplates the situation where the gap varies inversely as the distance from annulus 28. However, regardless of the particular configuration thereof, the gap between die ring 30 and anvil surfaces 22 is filled with pyrophyllite to serve as a gasket 32 for preventing lateral extrusion of the workpiece 20 or similar displacement of anvil surfaces 22 adjacent thereto. The outer periphery of gasket 32 is subject to excessive deformation and possible fracture during the tremendous compression imparted thereto and is, therefore, supported by a concentric ring 34 of a plastic material with a high compressive strength, such as Teflon, which also imparts a small amount of support to anvils 12.

Conical sections 16 of anvils 12 are coextensively jacketed by a plurality of tungsten carbide segments 36, preferably six in number, which mate to form a frustro-conical hollow support jacket 38 terminating in a dished configuration 40 at the exposed upper end thereof. Each combined anvil 12 and hollow support jacket 38 is seated in a corresponding conical cavity 42 extending into one end of a cylindrical anvil holder 44 preferably fabricated of maraging steel. When anvil holders 44 are subjected to a force F.sub.1, segments 36 of support jacket 38 transmit uniform pressure to anvils 12 which counteracts the shear and tensile stresses therein thereby improving the resistance thereof to brittle fracture. The opposite end of each holder 44 is provided with an axial hole 46 which intersects cavity 42 to expose the uppper end 24 of anvil section 16 for connection thereto of the electrical leads 48 from suitable instrumentation capable of monitoring the response of workpiece 20 to the pressures imparted thereto. In order to facilitate the proper exit of leads 48 from the upper anvil holder 44, the latter is provided with a cylindrical extension 50 containing oppositely disposed radial passages 52 in intersection with hole 46. Each anvil assembly is adapted to be inserted into a hydraulic press (not shown) in position to receive a vertical force F.sub.1 applied to extension 50. Removal of anvils 12 from the press to permit the insertion of another workpiece 20 is accomplished by a suitable rod (not shown) arranged to be threadably engaged with a threaded portion 56 of axial hole 46.

Die ring 30 is concentrically surrounded by a plurality of segments 58, preferably six in number, which combine to form a support ring 60 laterally disposed between anvil holders 44 in coextensive peripheral alignment therewith. A sleeve 62, preferably of beryllium copper, is fitted around the oppositely facing ends of holders 44 and the exterior periphery of support ring 60. Sleeve 62 is, in turn, retained in place by the upper end of a tubular housing 64 seated on base 54 in position to surround the lower anvil holder 44 and by the lower end of a hollow piston 66 surrounding the upper anvil holder 44 in opposed axial alignment with housing 64. The opposing ends of housing 64 and piston 66 are preferably wedge-shaped to mate with correspondingly contoured metallic sealing rings 68 and 69 disposed in concentric juxtaposition. The space between each pair of rings 68 and 69 is filled with an annular band 70 of a pressure-transmitting metal, such as indium. A massive support ring 72 is seated on a shoulder 74 formed at the upper end of housing 64 and extends upwardly in contact with the exterior peripheries of band 70 and the lower end of hollow piston 66. Anvils 12 and die ring 30 are electrically insulated from each other by a layer 76 of of glass-epoxy composite material of about 0.03 inch (76mm) thickness between anvil supports 38 and anvil holders 44.

Accordingly, when a secondary force F.sub.2, as shown in FIG. 1, is applied to the upper end of piston 66 substantially simultaneously with the application of force F.sub.1, against anvil holder extension 50, the band 70 of indium is subjected to plastic displacement which, due to the massive nature of support ring 72 and anvil holders 44, is confined to the surface in contact with sleeve 62 therebetween. The resulting compressive reduction in the interior diameter of sleeve 62 is converted into a corresponding radial displacement of segments 58 of die ring support 60. The segmented configuration thereof effectively triples the pressure imparted to die ring 30 thereby producing sufficient compressive stress therein to improve the resistance thereof to displacement of gaskets 32. As a result, die ring 30 prevents any extrusion or lateral deformation of workpiece 20 away from pressure faces 18 during the volume change incurred under the ultra-high pressure generated thereagainst and at the same time provides support to non-working faces 22. Since force F.sub.2 on piston 66 can be applied independently of force F.sub.1 on anvils 12, the internal stress of die ring 30 can be controlled to provide a slightly compressive tangential or circumferential strain therein which will impart optimum support to anvils 12. It has been found that such condition is achieved for the particular anvil configuration described herein when the ratio of F.sub.1 to F.sub.2 is on the order of 1.25 to 1.

The pressure generated against workpiece 20 is measured by a "through-the-anvil" technique wherein current and voltage leads 48 are soldered to anvil faces 24 and connected to a microvoltmeter (not shown) coupled to a recorder (not shown). In order to achieve reliable readings, the system was calibrated in accordance with the well-established resistance transitions of bismuth, iron, and lead which, in accordance with the paper of H. G. Drickamer entitled "Revised Calibration For High Pressure Electrical Resistance Cell" published in the 1970 Review of Scientific Instruments No. 41 page 1667, were rated at 74, 112, and 130 kilobars, respectively. Additional calibration points were obtained from iron-cobalt alloys as a linear function of the percentage of cobalt therein as demonstrated by F. P. Bundy in the article entitled "Fe-Co and FeV Alloys for Pressure Calibration in the 130 to 300 Kilobar Region" published in 1967 in the Journal of Applied Physics, Vol 38, No. 6, pg. 2446. When using iron-cobalt alloys with 10% and 20% cobalt, the respective values of 205 and 280 kilobars were adjusted in accordance with the teachings of Drickamer to 160 and 212 kilobars respectively. A plot of these calibration points was taken as a function of the force applied to anvil holder extension 50 and is shown in FIG. 5 wherein each point is the average of two to four separate tests. Since a pressure of about 12 KSI (83 .times. 10.sup.6 Pa) was imparted to anvil holders 44, an extrapolation of the calibration curve would indicate that a pressure of 400 kilobars (4 .times. 10.sup.10 Pa) was attained.

Thus, there is here provided a system wherein Bridgeman-type anvils can be laterally supported in direct proportion to the axial forces thereon. Such arrangement permits the application of extremely large axial forces without the necessity for increasing the size of the anvils to avoid compressive failure. Moreover, since the lateral support is accomplished by mechanical means, the pressure-generating system of this invention can be utilized at elevated temperatures above 700.degree.F as well as at cryogenic temperatures as low as 77.degree. on the Kelvin scale.

The foregoing disclosure and description of the invention is illustrative only. Various changes may be made within the scope of the appended claims without departing from the spirit of the invention.

Claims

I claim:

1. In a system for subjecting a workpiece to ultra-high pressures, the combination of,

a pair of oppositely disposed Bridgeman-type anvils having frustro-conical ends terminating in pressure faces for compressing the workpiece therebetween,

a plurality of segments surrounding each of said anvils adjacent said frustro-conical ends thereof to form a conical jacket for transmitting axial forces to said anvils while imparting compressive lateral stresses thereto to increase the internal resistance thereof to brittle failure,

a die ring spaced between said frustro-conical ends of said anvils in position to surround the workpiece,

a compressible gasket disposed between the workpiece and said die ring and extending outwardly into the space between said die ring and the conical anvil surfaces adjacent said pressure faces,

a segmented support ring surrounding the exterior periphery of said die ring, and

means for imparting compressive lateral forces to said support ring to stress said die ring for increasing the internal resistance thereof to the compression of said gasket whereby the correspondingly increased support imparted to said anvils prevents extrusion of the workpiece during the reduction in volume produced in response to the pressures thereagainst.

2. The system defined in claim 1 wherein said anvils, said jackets, said die ring, and said support ring are fabricated of cemented tungsten carbide and said gasket consists of pyrophyllite.

3. The system defined in claim 1 wherein the opposite faces of said die ring are sloped to conform to said frustro-conical ends of said anvils.

4. The system defined in claim 1 wherein said means for imparting compressive lateral forces to said die support ring comprises,

a sleeve fitted around said die support ring,

a band of a pressure-transmitting metal softer than the material of said sleeve in surrounding contact therewith,

a massive support ring encircling said band, and

piston means for extruding said band in a lateral direction to compress said sleeve and thereby reduce the interior diameter thereof for imparting similar displacement to said die support ring.

5. In a system for subjecting a workpiece to a pressure in excess of 300 kilobars (3 .times. 10.sup.10 Pa), the combination of,

a pair of oppositely disposed Bridgeman-type anvils each consisting of coextensive frustro-conical sections of different axial length integrally joined at the bases thereof and oriented so that the ends of the axially shorter sections serve as pressure faces for compressing the workpiece therebetween,

a plurality of conically tapered segments coextensive with the axially longer of said anvil sections to form a jacket for transmitting axial forces to said anvils while imparting compressive lateral stresses thereto to increase the internal resistance thereof to brittle failure,

a die ring laterally disposed between said axially shorter of said frustro-conical ends in position to surround the workpiece in spaced relation thereto, said ring having conical opposite faces of the same slope as the conical surfaces of said axially shorter frustro-conical anvil ends disposed in spaced relation thereto,

a ring of pyrophyllite surrounding the workpiece for retention thereof in said die ring,

a pyrophyllite gasket surrounding said workpiece retaining ring and extending into the space between said opposed faces of said die ring and said corresponding anvil surfaces,

a plurality of radial segments surrounding the exterior periphery of said die ring to provide a support ring therefor, and

piston means for imparting compressive forces to said die support ring to thereby stress the interior of said die ring for increasing the ability thereof to resist the forces imparted thereby to said gasket whereby said die ring acts to support said anvil surfaces adjacent said pressure faces and to prevent lateral extrusion of the workpiece during the reduction in volume thereof produced in response to the pressures thereagainst.

6. The system defined in claim 5 wherein the joinder of said frustro-conical sections defines a right angle therebetween and said conical surfaces of said frustro-conical ends slope at an angle of between 20.degree. and 30.degree. to the horizontal.

7. The system defined in claim 5 wherein said pyrophyllite gasket is supported by a surrounding gasket ring of a plastic material having a greater compressibility than pyrophyllite.

8. A system for generating pressures in excess of 300 kilobars (3 .times. 10.sup.10 Pa) against a test specimen, comprising,

a pyrophyllite cylinder having the test specimen centrally embedded therein,

a pair of oppositely disposed Bridgeman-type anvils each formed by a first frustro-conical section and a second frustro-conical section of greater axial length integrally joined at the larger diameter ends thereof in coextensive relation, said anvils being oriented so that said first frustro-conical sections serve as pressure faces for compressing the pyrophyllite cylinder containing the test specimen,

a plurality of conically tapered segments disposed around each of said second frustro-conical sections to form a frustro-conical jacket for transmitting axial forces to said anvils while simultaneously imparting lateral compressive forces thereto for increasing the internal stresses therein to prevent brittle failure,

a pair of oppositely disposed cylindrical anvil holders having frustro-conical openings extending into the oppositely facing ends thereof for receiving said anvil jackets,

a die ring spaced between said first frustro-conical sections in axial alignment with said pressure faces on said anvils,

a pyrophyllite ring surrounding said specimen-embedding cylinder and fitted into said die ring in retaining contact therewith,

a pyrophyllite gasket surrounding said retaining ring and extending outwardly therefrom into the space between said die ring and said first frustro-conical anvil sections,

a plurality of radial segments surrounding the exterior periphery of said die ring to provide a support ring therefor, and

piston means for imparting compressive forces to said die support ring to thereby stress the interior of said die ring for increasing the ability thereof to resist the forces imparted thereto by said gasket in contact therewith whereby said die ring acts to support said anvil surfaces adjacent said pressure faces and to prevent lateral extrusion of said test specimen cylinder during the reduction in volume thereof in response to the anvil pressure thereagainst.

9. The system defined in claim 8 wherein the opposite ends of said test specimen cylinder include copper strips extending into electrical contact with the opposite sides of the test specimen and wherein electrical leads are connected to the end faces of said second frustro-conical sections to provide for the passage of a flow of current to the test specimen.

10. The system defined in claim 8 wherein said anvils, said segmented jackets, said die ring, and said die ring support are fabricated of cemented tungsten carbide and said anvil holders, said housing, said massive support ring and said piston are fabricated of maraging steel.

11. The system defined in claim 8 including,

a sleeve of beryllium copper encircling said die support ring and the adjacent ends of said anvil holders,

a stationary base having a cylindrical housing extending upwardly therefrom to receive one of said anvil holders and a portion of said sleeve,

a hollow piston encircling the other of said anvil holders in circumferential alignment with said cylindrical housing,

a band of indium encircling said sleeve between said housing and said piston, and

a massive support ring surrounding said indium band and the adjacent end portions of said housing and said piston whereby vertical displacement of said piston compresses said indium band to extrude laterally against said sleeve and thereby impart corresponding displacement to said segmented die support ring for compressing said die ring to induce internal stresses therein.

12. The system defined in claim 10 wherein the axial forces imparted to said hollow piston are controlled relative to the forces imparted to said anvil holders to maintain a slightly compressive strain in said die ring.