Materials processing using chemically driven spherically symmetric implosions

A system and method for obtaining spherically symmetrical implosion of sample materials by directing radiant ignition energy onto a target which includes a spherically symmetrical core of selected sample material concentrically surrounded by a shell of high explosive material. The resulting implosive compression produces hydrodynamically controlled physical and/or chemical and/or metallurgical transformations of state in the sample material.

The present invention is directed to materials processing, and more 
particularly to methods and apparatus for the spherically symmetric 
implosive compression of matter. 
BACKGROUND OF THE INVENTION 
It has heretofore been proposed to induce polymorphic transition of 
graphite to diamond by subjecting a starting material to high pressure 
shock compression. The Garrett U.S. Pat. Nos. 3,499,732, 3,653,792 and 
3,659,972, for example, propose the use of shaped electrically detonated 
explosive charges to obtain spherically symmetrical implosion shock waves 
for forming diamonds from graphite or for sintering powdered metals. One 
significant problem associated with this technique is one of physical 
size: the mass of explosive material involved would be on the order of 
thirty kilograms. An explosion of this size requires extraordinary 
containment precautions, and also raises problems in connection with 
recovery and contamination of the sample. Smaller explosive masses cannot 
be uniformly ignited using the electrical detonation techniques proposed 
in the art. 
Another significant problem with the above proposed techniques is the lack 
of control of the implosive pressure afforded by shock compression 
techniques. Yet another problem with the electrically ignited explosive 
system is that of achieving a high degree of spherical symmetry in the 
detonating explosive. 
OBJECTS AND SUMMARY OF THE INVENTION 
It is an object of the present invention, therefore, to provide a system 
and method for implosive compression of materials which reduces the need 
for elaborate containment structures, which obtains uniform implosion of 
the sample material, which provides for more ready product recovery, which 
reduces the introduction of impurities in the sample, and/or which affords 
hydrodynamic control of the implosive pressures in the sample material. 
Another and more specific object of the invention is to provide a system 
and method for obtaining uniform implosion energies of reduced scale, and 
thereby alleviating the aforementioned deficiencies in the art. 
A further object of the invention is to provide a system and method of the 
described type which obtains increased pressure at low temperature from 
spherical shock compression, and thereby reduces retransformation of the 
product due to post-shock heating. 
Another object of the invention is to provide a system and means for 
accurately tailoring the pressure-time history of the implosively 
compressed sample to chemically and/or physically and/or metallurgically 
change the sample's final state. 
Briefly stated, the foregoing and other objects of the invention are 
obtained by uniformly illuminating a spherical target, including a sample 
material surrounded by explosive, with a pulsed laser, ion, electron or 
microwave beam to ignite the explosive and thereby obtain uniform 
spherical implosive compression of the material sample. In the preferred 
embodiments of the invention, the target comprises a thin ignition layer 
and a shell of high explosive concentrically surrounding a sample 
material, either with or without a surrounding transparent tamper shell. 
The target illumination system comprises lenses and/or reflectors for 
focusing ignition energy uniformly over the target surface. Uniform 
spherically symmetrical high explosive ignition is expected to be most 
easily attained using pulsed laser energy and an optical system designed 
to provide spherical illumination (hereafter denoted by "spherical optical 
system"). Hence, the combination of the pulsed energy and a spherical 
optical system is used in the preferred embodiment in the following. 
However, by using many beams, energetic electron, ion and microwave energy 
sources could be used in spherical ignition systems. Energetic, pulsed, 
incoherent light sources in combination with a spherical ignition system 
may also be used.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
FIG. 1 illustrates a target 10 of the present invention as comprising an 
inner shell 12 of sample material to be implosively compressed 
contiguously surrounded by a shell 14 of high explosive material. The 
interior 16 of shell 12 is preferably a vacuum or low pressure air. Each 
of the shells 12,14 is of radially uniform thickness, with the overall 
outside diameter of target 10 being in the range of several millimeters to 
several tens of centimeters. The dimensions shown in FIG. 1 are exemplary. 
The implosive compression of the spherical shell 12 is produced by the 
uniform laser ignition of the outer surface of the high explosive shell 
14, whereupon the outer region explodes, and the concomitant reaction 
force implodes and therefore compresses the "payload". (Here and in the 
following, "payload" refers to all of materials which are initially driven 
inward by the explosive forces). The uniform laser light ignition of 
spherical target 10 may be accomplished using a number of different 
methods. One method uses a number of individual laser beams and focusing 
lens, a second uses two laser beams and a spherical system of ellipsoidal 
mirrors (FIG. 2). 
FIG. 2 illustrates an illumination system 20 for directing laser ignition 
energy uniformly over the surface of target 10. System 20 includes a pair 
of centrally apertured concave ellipsoidal reflectors 22,24 disposed on a 
common axis 26 such that the near focus of each reflector is disposed on 
axis 26 at or near the midpoint between the reflectors and the far focus 
of each reflector is disposed on axis 26 at the central opening of the 
opposing reflector. A target 10 is suspended by a thin wire 28 on axis 26 
at the common reflector focus. A pair of lenses 30,32 are disposed 
externally of reflectors 22,24 on axis 26 for focusing associated 
collimated beams of laser energy 34,36 through the adjacent reflector 
aperture to the far focus of the opposing reflector, and thence onto the 
opposing reflector surface and over target 10. The incoming laser beams 
34,36 may be generated by a suitable pulsed laser amplifier system 60 
which directs energy onto a 50/50 beamsplitter/reflector 62. The split 
beams are directed along paths of equal length to lenses 30,32 by the 
mirrors 64. 
The system 20 thus directs pulsed laser energy substantially uniformly over 
the target surface. The basic system 20, to the extent thus far described, 
is similar to that disclosed in Thomas, "Laser Fusion Illumination 
System," Applied Optics, 14, 6 (June 1975) pp. 1267-1273. The Glass U.S. 
Pat. No. 4,017,163, Sigler U.S. Pat. Nos. 4,084,887 and 4,136,926, and 
Thomas et al U.S. Pat. No. 4,161,351, and Brueckner et al, "Ellipsoidal 
Illumination System Optimization for Laser Fusion Experiments," Applied 
Optics, 14, 6 (June 1975) pp. 1274-1278 discloses improvements which may 
be incorporated into the basic Thomas system. 
In operation, opposing collimated beams of pulsed laser energy are directed 
with spherical uniformity around the target surface and ignite high 
explosive shell 14. A spherically symmetrical detonation is launched in 
shell 14, which both implodes and compresses the material of shell 12. The 
implosive detonation accelerates the matter of shell 12 toward the center 
of symmetry, impacting shell 12 upon itself, and thereby raising the 
density, pressure and temperature in such a way as to induce physical 
and/or chemical and/or metallurgical changes of state. An important 
advantage of spherical implosion techniques of the type described lies in 
the strong increase in density and pressure obtained from spherical 
convergence at relatively low temperatures, as has been determined by 
Guderley, Luffahrtforschung 19, 302, (1942). This lower temperature, 
obtained using spherical shock compression in accordance with the present 
invention, helps avoid retransformation of the product material back to 
its initial form. Furthermore, the high explosive material will expand 
radially outwardly during the compression of the payload. Thus, no 
high-density material remains in thermal contact with the compressed 
material sample. This helps prevent contamination and aids in cooling. A 
cup 38 is positioned beneath reflectors 22,24 for collecting the 
compressed sample. Cup 38 may include an oil or water quench further to 
reduce retransformation. 
Target 10 is constructed using standard machining and/or molding 
techniques. High explosive layers can be cast and/or machined to size. For 
the formation of diamond from an implosion-induced polymorphic transition 
in graphite, shell 12 may comprise graphite in pure form or with catalytic 
materials as in Cowan U.S. Pat. No. 3,401,019. Ultra-hard boron nitride 
for industrial machining may be formed by implosion-induced polymorphic 
transition of cubic and/or wurtzite forms of boron nitride. The process of 
the invention may also be employed for implosion-induced polymerization, 
sintering of ceramic or refractory metals, compaction of metal alloys, and 
possibly for creation of metallic hydrogen. 
Explosive shell 14 may be constructed of any of a variety of explosives, 
for example, those known as HMX, PETN, TATB, etc. (See B. M. Dobratz, 
Lawrence Livermore Laboratory, Report #UCRL-52997). Such explosives 
typically possess an energy density of about 5 kJ/g and a mass density of 
1 to 2 g/cc. For a shell 14 having an ID of 6 mm and an OD of 1 cm, 
explosive energy released would be about 5 kJ. Such an explosion may be 
easily contained, and yet will support a "payload" having an ID of 2 mm 
and a mass of about 1 g. The peak pressure at the target surface can be as 
high or higher than 200 kiloatmospheres, while the "stagnation pressure" 
at a distance of 13 cm, i.e. at the reflector surfaces, would be only 
about 10 atmospheres. The pressure impulse would have a decay time of 
several microseconds. In order to protect the reflector surfaces, 
sacrificial liners 40,42 made, for example, of soft transparent plastic 
may be employed. 
FIG. 3 illustrates a basic target 10a in accordance with the invention as 
comprising a core 13 consisting of a solid sphere of sample material 12. 
Core 13 is contiguously and entirely surrounded by a shell 14 of explosive 
material. The outer surface of core 13 and the thickness of shell 14 are 
of respectively uniform radial dimension. In the target 10b of FIG. 4, the 
core 13 includes a shell of sample material 12 surrounding a mandrel 17. 
Mandrel 17 is a sphere of solid material, such as aluminum, which prevents 
implosion pressures in sample material 12 from becoming too large and 
helps control the pressure wave profile in time. FIG. 5 illustrates the 
target 10 of FIG. 1 wherein the sample material shell 12 of core 13 
surrounds a void 16. 
In the target 10c of FIG. 6, a malleable wave shaping layer or "pusher" 
shell 48 of iron, for example, is positioned between explosive shell 14 
and core 13. The function of pusher shell 48 is to obtain desired 
tailoring of the hydrodynamic pressure and temperature profiles during 
implosive compression of sample core 13 by extending the time that peak 
pressure is applied to the sample. In the target 10d of FIG. 7, pusher 48 
consists of multiple contiguous concentric shells 50 of materials having 
differing density for obtaining more complex pressure and temperature 
profile tailoring. This design allows a more nearly isentropic sample 
compression, as has been suggested for planar geometry by Lyzenga and 
Ahrens (in Shock Waves in Condensed Matter, American Institute of Physics 
Conference Proceedings #78, 1981). Isentropic compression reduces the 
sample temperature rise induced by shock waves. Core 13 in FIGS. 6 and 7 
may, of course, be any of those illustrated in FIGS. 3-5. 
In order to ignite explosive shell 14, laser energy at a fluence density of 
up to 100 joule/cm.sup.2 from a 25 ns Nd-glass or ruby laser may be 
employed. A given explosive material in shell 14 will require a certain 
amount of ignition energy per unit area, which is readily determined by 
experimentation. FIG. 8 illustrates a modified target 10e for lowering the 
required ignition energy by providing a thin (on the order of 1000 .ANG.) 
coating or ignition layer 44 of aluminum, for example, over the explosive 
shell 14 or a thin layer of chemical explosive having low ignition 
sensitivity. Use of a thin ignition layer is described in connection with 
planar geometries by Yang et al, Applied Physics Letters, 19, 473 (1971) 
and in U.S. Pat. No. 3,812,783. Layer 44 may be formed by standard vacuum 
deposition or chemical evaporation techniques. For this target design, 
ignition energy of up to 10 joules/cm.sup.2 and laser pulse length of 10 
ns may be required. FIG. 9 illustrates another modified design 10f wherein 
the ignition layer 44 is disposed between explosive shell 14 and core 13, 
and explosive shell 14 is transparent for admitting laser energy. The 
design of FIG. 9 has the advantage that the unburned mass of explosive 
shell 14 operates to "tamp" the explosive energy. Again, any of the cores 
13 in FIGS. 3-5 may be employed in FIGS. 8-9. Likewise, pusher shells 48 
(FIGS. 6-7) may be positioned between explosive shell 14 and core 13 in 
FIG. 8, or between layer 44 and core 13 in FIG. 9. 
FIGS. 10 and 11 illustrate "tamped" target designs 10g and 10h. In both 
FIGS. 10 and 11, the outer shell or layer (44 in FIG. 11, 14 in FIG. 10) 
is surrounded by a spherically continuous transparent glass or plastic 
tamper shell 46. Most preferably, tamper shell 46 is sufficiently thick 
and resilient to absorb the explosive energy without rupture, which 
greatly facilitates both product collection and protection of the 
illumination optics. SYLGARD 184 marketed by Dow-Corning may be 
appropriate for construction of shell 46. Such a shell is formed by 
standard molding techniques. 
It is an important feature of this invention to provide for hydrodynamic 
control of the pressure and temperature time-histories through the use of 
layered target structures as shown at 10 through 10h. This procedure is 
similar in character to the hydrodynamic control used to design inertial 
fusion targets (see Lindl, U.S. Pat. No. 4,272,320 and Brueckner, U.S. 
Pat. No. 4,297,165), although the end result is quite different. In the 
present invention, the sample material is implosively processed and then 
recovered, whereas inertial fusion targets are totally destroyed. In the 
present invention, such hydrodynamic control is employed to obtain desired 
physical, and/or chemical and/or metallurgical changes of state in the 
recovered sample material.