Abstract:
A design for high pressure/high temperature apparatus and reaction cell to achieve ˜30 GPa pressure in ˜1 cm volume and ˜100 GPa pressure in ˜1 mm volumes and 20-5000° C. temperatures in a static regime. The device includes profiled anvils ( 28 ) action on a reaction cell ( 14, 16 ) containing the material ( 26 ) to be processed. The reaction cell includes a heater ( 18 ) surrounded by insulating layers and screens. Surrounding the anvils are cylindrical inserts and supporting rings ( 30-48 ) whose hardness increases towards the reaction cell. These volumes may be increased considerably if applications require it, making use of presses that have larger loading force capability, larger frames and using larger anvils.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of the filing date of U.S. provisional patent application Ser. No. 60/226,318 filed Aug. 21, 2000. 

   STATEMENT OF GOVERNMENT SUPPORT OF THIS INVENTION 
   This invention was supported under the Department of Energy, Grant No, DE-FG02-96ER 82154 

   BACKGROUND AND SUMMARY OF THE INVENTION 
   This invention relates to apparatus for providing high pressure and temperature for use in the formation of minerals and new materials. 
   Conventional high pressure units enable pressures of ˜15 GPa (using WC/Co anvils) and ˜100 GPa (for diamond anvils) in a working volume of ˜1 μm 3 , but with a limitation in temperature of about 2,000° C. However, if a reaction cell could be made with a larger working volume and even higher temperature capabilities, there is then the possibility of synthesizing diamonds directly from molten carbon in a relatively short time. Such synthesized diamond pieces will have fine grain size or single crystal structures, depending on the solidification rate. 
   Previous apparatus for achieving high pressures and temperatures may be found in: P. W. Bridgman  Scientific American,  Novermber 1955, p.42;. U.S. Pat. No. 2,941,248 to H. T. Hall “High temperature-high pressure apparatus”; and. U.S. Pat. No. 3,746,484 to L. F. Vereshchagin et al “Apparatus for achieving high pressure and high temperature”. 
     FIG. 1  shows schematically the key components of the high pressure/high temperature apparatus of the present invention, and the corresponding reaction cell (which holds the material to be processed), designed and implemented for the hot-pressing of carbon-based and other materials. The high pressure/high temperature apparatus consists of two profiled anvils  1  and three supporting steel rings  2 - 4  supporting each anvil. The anvils  1  squeeze a container  5  made of plastic stone and a reaction cell  6  that resides within the container. Cylindrical inserts  7  and  8  are disposed above and below profiled anvils  1  and are constructed from WC/6 wt % Co, which are supported by steel rings which are described in detail with below. The hardness of the supporting rings decreases from the center of the apparatus to the periphery. Reaction cell  6  consists of a graphite crucible that serves as a heater when electrical current is passed therethrough. 
   Supporting steel rings are used to increase the allowed load exerted on the anvils and inserts. In effect, they provide side-supporting pressure, which increases the effective fracture strength of the anvils under compression. A set of such supporting rings is needed, since the maximal supporting pressure that a multilayer cylinder can bear is twice the maximum pressure that can be achieved in a monolayer cylinder:
 
 P   o(max) ≈2σ ts /√{square root over (3)}  (1)
 
where σ ts  is the ultimate tensile strength of the steel. It is ˜2.0 GPa for hardened steel. This scheme permits a maximal working pressure in the RC (P Wmax ) that is higher than the compressive fracture strength of the anvils; however, this pressure is always less than the Vickers hardness (H V ) of the anvils:
 
σ fs   ≦P   Wmax   ≦H   V   (2)
 
The maximal working volume (V max ) that can be achieved under pressure depends on σ fs , maximal loading force (F max ) of press, size of frame window (a j ) and size of anvils used (V a ):
 
V max= V max (σ fs , F max , a j , V a )  (3)
 
According to theory, the fracture compressive strength of a brittle material is inversely proportional to the sample volume:
 
σ fs =ησ cs V a   −γ   (4)
 
where η=η 0 V 0a   γ  is constant, η 0  is dimensional constant that is typical of a given material, V 0a  is the volume of a standard sample for measuring compressive strength (σ cs ) and exponent γ is a typical value for a given material (γ˜ 1/15 for regular WC/Co). The values of σ cs  and H V  in formula (2) are also interrelated. The H V   st  is ˜2.5σ cs   st  for hardened steel that has some plasticity. The H V   cer  is ˜7σ os   cer  for brittle rocks, stones and ceramics. The H V   com  is about from 3 to 5σ cs   st  for composite materials with brittle skeleton and plastic matrix, such as materials of the WC/Co type. The degree of sensitivity of the compressive fracture strength on sample volume depends on porosity, crystallite size, and value of side supporting pressure (P ss ):
 
γ=γ(ρ A   , d, P   ss )  (5)
 
where ρ A  is apparent density, d is typical size of crystallites.
 
   The high pressure/high temperature apparatus of the present invention enables a maximum possible static pressure over the range 1-100 GPa Hereafter, we will call the range 1-10 GPa “very high pressure” and the range 10-100 GPa “super high pressure”. Even higher pressures in large volume can, in principle, be achieved with the help of dynamic methods. We will call this pressure range (P&gt;100 GPa) “ultra high pressure”. 
   Let us now consider how to achieve very high temperatures in the high pressure/high temperature apparatus. The necessary high temperature is best realized by passing an electric current directly through the graphite container. The thermal regime of the reaction cell and its container may be computed from the following equation:
 
 Wdt=∫cρ·dT·dV +(           λ·grad T·dS ) dt   (6)
 
where W=qdV is power, q is power emitted in unit volume, c is specific heat capacity, ρ is density, λ is thermal conductivity. This equation in the static state may be represented as:
 
div(λ·grad T )=0  (7)

   An approximate solution of equation (6) for spherical thermal conductivity provides an opportunity to determine the thickness of thermal insulation and the relaxation time that is needed for the reaction cell to respond to a power change and to achieve steady state: 
                   T   =       T   max     ⁡     (     1   -     e     -     i   τ           )         ⁢     
     ⁢         where   ⁢           ⁢     T   max       =     W   /   v       ;           ⁢     τ   =     η   /   v       ;     ⁢     
     ⁢   v   =     ∫       div   ⁡     (     λ   ⁢           ⁢   grad   ⁢           ⁢     ψ   ⁡     (   r   )         )       ⁢     ⅆ   V           ;     ⁢     
     ⁢       η   =     ∫       c   ·     ρψ   ⁡     (   r   )         ⁢     ⅆ   V           ;             (   8   )             
 
and ψ(r) is a function typical of a specific container. If the energy released is not uniform over the entire volume of the sample, some interval of time will be necessary to heating the center of the sample to a temperature close to that of the heater, T max . A typical relaxation time (τ 0 ) depends on the materials properties and size of samples: 
               τ   0     =       1   3     ⁢         c   0     ⁢     ρ   0         λ   0       ⁢     r   0   2               (   9   )             
 
where c 0 , ρ 0 , λ 0  are heat capacity, density and thermal conductivity of sample and r 0  is radius of sample.
 
   A temperature range of 100-2000° C. is achievable using graphite heaters. In the temperature range 2000-4000° C., the carbon does not melt, but reacts with all elements and compounds, with the exception of inert gases. The very high temperature range, up to 4000-5000° C., is difficult to obtain, especially under high pressure, because the efficiency of thermal insulation is limited. 
   Thermal flow from the heater is proportional to the first power of temperature, but thermal flow by radiation is proportional to the fourth power of temperature:
 
 dE   e =σ ST             β T ( r )α T ( r )T 4   dSdt   (10)
 
where α T  is blackness, σ ST  is Stefan-Boltzman constant, β T  is integral coefficient of reflection of electromagnetic waves from the surface. The energy dE e  added to the right side of eq. (6) increases the heat transfer at very high temperature and leads to a situation where a small increase in temperature demands a large increment in heater power.

   The apparatus to be described below permits the achievement of static super high pressure in combination with static very high temperature. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the invention, reference is made to the following drawings which are to be taken in conjunction with the detailed description to follow in which: 
       FIG. 1  illustrates the key components of the high pressure and temperature apparatus of the present invention; 
       FIG. 2  illustrates in detail the high pressure and temperature apparatus of the present invention; 
       FIG. 3  is a sectional view of a reaction cell for use in the 20-2000° C. temperature range; 
       FIG. 4  is a sectional view of a reaction cell for use in the 20-5000° C. temperature range; and 
       FIG. 5  illustrates schematically the control arrangement of the high pressure and temperature apparatus of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 2  illustrates the high pressure and temperature apparatus  10  of the present invention which is shown disposed between the upper  11  and lower  12  steel contact plates of a conventional hydraulic press. The central unit consists of upper and lower profiled anvils  14 ,  16  and supporting annular steel rings  18 ,  20 ,  22  which surround the periphery of each anvil  14 ,  16 . Anvils  14 ,  16  press on a container  24  made of plastic stone and a reaction cell  26  that resides within container  24 . Upper and lower intermediate cylindrical inserts  28  are disposed above and below profiled anvils  14 ,  16  and outer cylindrical inserts  30  are located between inserts  28  and contact plates  11  and  12  of the hydraulic press. The materials used to construct anvils  14 ,  16  and cylindrical inserts  28 ,  30  are chosen so that the hardness increases vertically towards reaction cell  26 . Contact plates  11 ,  12  are made of soft steel, outer inserts  30  are constructed of hardened steel and intermediate inserts  28  are made of a hard alloy. Anvils  14 ,  16  are manufactured either from diamond (HK=100 GPa) or from carbides (TiC or SiC) with a hardness of HV=30 GPa. Profiled anvils made from fine-structured TiC can achieve a pressure of 30 GPa in a volume of ˜1 cm 3 , and profiled anvils made from fine-structured Diamond can achieve a pressure of 100 GPa in a volume of ˜1 mm 3 . The inserts and anvils thus form a pyramidal structure such that the pressure decreases from a maximum inside reaction cell  26  to a pressure of &lt;1 GPa on the interface between the outer inserts  30  and the plates  11 ,  12  of soft steel. The above mentioned volumes may be increased by the use of a press with a larger frame, maximal loading force and larger anvils. The maximal loading force will rise as F max ˜P max ·V 2/3 . 
   Container  24  of plastic stone is located between two profiled anvils  14 ,  16 . Some part of the container flows out of the cavity to fill the clearance between the anvils when the loading force is increased, thus fixing the pressure gradient from maximum inside the reaction cell to ambient outside the container. The clearance between outer part of anvils  14 ,  16  and supporting rings  18 ,  20  and  22  may be vacuum, air (gas) or a polymeric material with high compressibility (such as rubber). Such a rubber ring placed around the container serves to regulate the pressure gradient. The hardness of the plastic stone of container  24  is 1 to 3 on the Mohs scale. It is generally the same for all volumes of containers or gradually decreases from the reaction cell to the periphery. 
   Anvils and inserts can be made from, for example, fine-structured W—C—Co, W—Ti—C—Co—Fe—Ni, Ti—C, Si—C, Si—W—Ti—C—Co—Fe—Ni, and C. In addition to the fact that the hardness of the anvils and the cylindrical inserts increases towards the reaction cell, the anvils themselves can have a functionally graded hardness, wherein the hardness gradually decreases from the portion contacting reaction cell  26  towards inserts  28 . By way of example, profiled functionally graded anvils can be made from WC—TiC—Co alloy, where the quantity of TiC decreases from 100% on the side contacting reaction cell  26  to 5% on the outer part; with the corresponding Co content increasing up to 10% on the outer part of the cell. Such profiled anvils are capable of maintaining static pressure up to 100 GPa ( if the anvils are constructed of diamond) inside the reaction cell, with decrease to ambient pressure outside the reaction cell. 
   Steel annular support rings  32 ,  34 ,  36  surround the periphery of intermediate inserts  28  and steel annular support rings  38 ,  40  and.  42  surround the periphery of outer inserts  30 . Support rings  18 ,  20 ,  22  which surround anvils  14 ,  16 ; support rings  32 ,  34 ,  36  which surround intermediate inserts  28  and support rings  38 ,  40 ,  42  which surround outer inserts  30  are made from hardened alloyed steel. The heat treatment of the support rings is done in such a way that hardness and ultimate tensile strength decrease from center to periphery, but the plasticity of the rings increases. The calculation of stresses in the rings is done so that all the rings work in the elastic range and the maximal stress in any one ring does not exceed the yield strength (σ ys ) of the material. Outer safety rings  44 ,  46 ,  48  are made from soft, non heat-treated steel. Centering rings  50 ,  52 ,  54 ,  56 ,  58  and  60  disposed outside safety rings  44 ,  46 ,  48  are made from non-conducting polymeric materials and are used for precisely locating the axes of top and bottom parts of the apparatus. 
   High pressure and temperature apparatus  10  has a cylindrical axis of symmetry with the top and bottom parts are electrically insulated from the press. As is seen, the hardness of the inserts and anvils increases in a vertical direction towards the reaction cell. Furthermore, the hardness of the supporting rings increases in the radial direction towards the reaction cell. The power supply is connected to the top and bottom parts of the high pressure and temperature apparatus  10  by copper cables. The electrical voltage can be applied to the top and bottom parts of the high pressure and temperature apparatus  10 . The heater of the reaction cell is connected to the anvils by contacting units. 
   The details of a reaction cell  26  for material processing in the 20-2000° C. temperature range and its relationship to anvils  14 ,  16  is shown in FIG.  3 . The profiled walls  70 ,  72  of anvils  14 ,  16  form container  24  in which reaction cell  26  is located together with “plastic stone” which is thermally and electrically non-conductive material which can be, for example fine limestone (calcite), pyrophyllite, talc, clay, gypsum, or combination of that, or a mixture of clay and sand, or other non conductive material. The edges of anvils  14 ,  16  can be sealed with an elastic, plastic or rubber ring  76 . The profiled walls  70 ,  72  of anvils  14 ,  16  form a central cavity  78  for receiving reaction cell  26  which is constructed of graphite ceramic and which has a cylindrical axis of symmetry in which the material to be processed  80  is placed. The surfaces of anvils are spherically shaped, so that the shape of cavity  78  between the two anvils is close to spherical. This shape along with the centering rings assures centering of the container material. The graphite ceramic forming reaction cell  26  becomes heated when an electrical current is passed therethrough. The conductivity of the graphite ceramic material is considerably higher (σ Ω ≅10 +3  (ohm·cm) −1 ) than that of the container material (σ Ω &lt;10 −12 (ohm·cm) −1 ). Reaction cell  26  is electrically connected to anvils by contacting units  82  which are also formed from graphite ceramics or from metal foil. 
   The design of a reaction cell for processing in the 20-5000° C. temperature range is shown in FIG.  4 . The material  86  to be processed is located inside a cylindrical graphite ceramic heater  88  which is surrounded by a layer  90  of pure diamond powder, which insulates heater  88  from a first cylindrical graphite ceramic screen  92  which is concentric with heater  88 . Screen  92  is insulated from a second graphite ceramic screen  94  by a layer of carbide powder  95 , such as SiC or B 4 C, which does not react with carbon over the temperature range existing between screens  92 ,  94  under steady state conditions. Screens  92 ,  94  serve to reflect the radiant heat while the diamond  90  and carbide  95  layers serve as electrical and chemical insulators preventing current passing trough the screens and chemical reactions between container  24  and heater  88  up to 5000° C. Screen  94  is located inside container  24  which again is made from plastic stone. A copper foil  96  is placed on the anvil walls  70 ,  72  and provides the electrical contact to heater  88  by means of graphite ceramic contacting units  97 . A rubber ring  98  around the outside of container  24  regulates the pressure gradient in the container. This reaction cell permits melting material such as carbon under super high pressure in a static regime and in a relatively large volume. 
   The overall design of a high pressure/high temperature apparatus  99  is shown in  FIG. 5 ; it consists of reaction cell  26  (as described above) inserted within container  24  of plastic stone disposed between anvils with supporting steel rings  100 ; inserts with supporting rings  108  and insulating layers  109 . Apparatus  99  includes a frame  102 , a hydraulic ram  106  with its associated oil tank  116  joined by an electrically operated valve  107 . Hydraulic ram  106  is powered by an oil pump  104  operated by an electrical motor  103 . Also acting on hydraulic ram  106  is a smaller (up to 500 bar oil pressure capabilities) oil pump  114  operated by an associated electrical motor  113 . The uses of large and small oil pumps permits precision control of the operating pressure which is difficult with only a single pump. The apparatus is controlled by a controller  105  (such as a programmed logic controller) which in turn is operated by a computer  115 . Operation of the apparatus is monitored by electrical multimeters  101  and  110  (such as Hewlett Packard HP 34401-A multimeters) and an oil pressure gauge  112 . A 10 kilowatt power supply  117  operating through a high current 0.1 milliohm shunt  111  supplies the power to heat reaction cell  26  and oil pump motors  103 ,  113  and the other components are simply powered by 115 volt AC. Motors  103 ,  113 , valve  107 , oil pressure gauge  112 , multimeters  101 ,  110  and power supply  117  are electrically joined with computer  115  through controller  105 . Computer  115  monitors oil pressure-voltage-current-time parameters and provides control signals to motors  103 ,  113 ; valve  107 , and power supply  117 . 
   The present apparatus may also be used at pressures less than 1 Gpa depending on the needs of the material to be processed. Furthermore, the operating pressure can be increased to above 100 Gpa and the operating temperature increased above 5000° C. by dynamic methods. 
   The invention has been described with respect to preferred embodiments. However, as those skilled in the art will recognize, modifications and variations in the specific details which have been described and illustrated may be resorted to without departing from the spirit and scope of the invention as defined in the appended claims