IE42858B1 - High temperature, high pressure apparatus having a ductile driver element - Google Patents

Description

This invention relates to high pressure, high temperature piston-cylinder apparatus.

High pressure, high temperature pistol-cylinder apparatuses are used for a variety of purposes and, specifically, for the production of diamond crystals. In such apparatus, a core of charge material, in the case of diamonds, graphite and a carbon solvent metal; is confined within a reaction chamber in the form of a cylinder and is heated and subjected to pressure therein.

Between the cylinder and core of charge material, there is placed a surrounding sleeve of electrical insulating material, and the charge is heated by passing an electric current therethrough. The sleeve inhibits the transfer of heat outward from the core to the cylinder and surrounding apparatus and confines the heating current to the core.

The insulating material is generally cylindrical, fitting closely within the inside diameter of the cylinder and extending to near the ends of the.cylinder. The core of charge material is usually tightly fitted within the insulation material, and may also be cylindrical in shape. One end of the cylinder is closed and the piston fits in the open end of the cylinder and is advanceable into the cylinder in the direction toward the closed end of the cylinder. The inside diameter of the piston cylinder and the outside diameter of the piston that fits into the cylinder usually form a snug sliding fit with each other.

With the core of charge material placed in the cylinder and the insulation and piston in place, electrical heating Of the core of charge material takes place by passing electrical current from the closed end of the cylinder, through the core of charge material and on through the piston, thereby heating the core of charge material to any desired temperature upon command by the operator. Pressure in the cylinder is increased by advancing the piston into the cylinder at a controlled rate and with a known force exerted on the piston.

In the manufacture of diamonds, it is not only extremely important to know the precise temperature and pressure conditions on the core of charge material, but also to insure that the temperature and pressure conditions are constant throughout the entire core of charge material. It is known in the industry that if a core of graphite and carbon solvent is placed in the cylinder and subjected to sufficient pressure and temperature, diamond crystals will result. The size and quality of the diamond crystals can vary depending upon how well and uniformly the growth conditions of temperature and pressure are held within the cylinder.

One of the problems associated with the growth of diamonds in a piston-cylinder apparatus is the problem of compensation for the volume change that takes place as a result of the conversion of carbon from graphite form to diamond form. There is a significant difference in the density of diamond and the density of graphite. The density of diamond is about 3.5 while the density of graphite is only about 2.25.

Thus, when the sufficient temperature and pressure conditions are present and the graphite begins converting into diamond, there is a corresponding reduction of volume in the core of charge material. As mentioned earlier, since the pressure must be kept constant, then the piston has to be advanced into the cylinder as the volume reduction is occurring. - 3 In all prior art machines, the insulating material, is located between the inside diameter Cf the cylinder and the core of charge material. The insulating material is ideally of constant cross section throughout its length to maintain uniform current density through the charge and has uniform wall thickness such that no temperature gradients build up inside or along the core of charge material* If the above conditions are not maintained during the complete cycle of the process, the relative temperature in the core would vary and cold or hot spots will build up in the core of charge materials during the reaction and thereby reduce the quality of diamond crystals produced by the fact that the temperature is not held at a known value for a given period of time.

In these prior art machines, and by the very nature of the diamond crystals forming from graphite, the piston must advance into the cylinder. As the piston advances into the cylinder, since it is a snug sliding fit with the inside diameter of the cylinder, it necessarily begins to crush the insulating liner. The radially outward edges of the piston contact the insulating material and begin to buckle or compress it along with the core material.

When the buckling or compression of the insulating material occurs, uniform current density is not maintained through the core of charge material and the wall thickness of the insulating material between the core of charge material and the cylinder wall is not uniform. When the uniform conditions are not maintained, hot and cold spots now develop throughout the core of - 4 42858 charge material. This effect is necessarily detrimental to the controlled formation of uniform size and quality diamonds.

Xn the known belt type high-pressure apparatus, the pistons taper inwardly towards the charge and the ends of the cylinder in which the charge is disposed are correspondingly tapered. Gaskets formed of deformable electrical insulating material, such as pyrophyllite, are disposed between the tapered pistons and the tapered ends of the cylinder and seal the charge in the cylinder.

The gaskets must deform to permit the piston to advance into the cylinder to compact the charge, and the tapered configuration of the pistons and cylinder ends present a substantial area which absorbs load from the pistons while limiting the amount the pistons can advance into the cylinder.

According to one aspect of the present invention, there is provided a high-pressure, high-temperature piston-cylinder apparatus comprising a cylinder, means closing one end of said cylinder, said cylinder being adapted to receive a core of charge material therein, means for heating the core when in the cylinder, a sleeve of electrically insulating material fitted between the core and the cylinder, a piston movable in the cylinder to develop pressure on a core therein, and a driver element located in the cylinder between said piston and said sleeve, said driver element being made of a material which is ductile, electrically conductive and softer than the material of said sleeve under the conditions of pressure and temperature encountered in said cylinder in use, said driver element being operable as the piston advances into the cylinder towards the core in use, to deform into said sleeve and transmit pressure from the piston to the core without deforming said sleeve.

The driver element is preferably made of lead; however, any other suitable material may be used, but it must be softer - 5 Δ38&8 than the insulating material under the conditions of temperature and pressure in the cylinder in use» ductile and electrically conductive. The driver element should be so sized as to allow the piston maximum forward travel with5 out engagement with the insulating material. Xn the case of diamond making, as the conversion of graphite to diamond Occurs» the volume of charge shrinks and fche piston must be advanced to maintain constant pressure.

As the piston advances with the driver element pre40 ceding the piston, the driver element being softer than the insulating material» deforms to fit the inside diameter of the Insulating material and flows inside the insulating material rather than forcing deformation of the insulating material. Xn this way» tha insulating material remains dimen sionally stable around the core of charge» pinching and swelling do not occur and» therefore» no hot or cold spots develop in the charge.

The pressure in the charge is maintained very precisely and independently of the piston travel. Thus» with the pressure and electrical-thermal insulation being well-defined and not a function of piston advancement, better diamond synthesis conditions can ba maintained with the relatively large volume change taking place during diamond synthesis.

According to another aspect of the invention, there is provided a method of maintaining a desired pressure on an electrically heated charge which undergoes reaction and reduces in volume which comprises: confining the charge radially within a radially-supported sleeve of electrical insulating material» confining the charge axially at one end, advancing a pressure element towards the other end of the charge» and interposing a drive element between said other end ©f the charge and said pressure element which driver element will flow Into the adjacent end of the sleeve and — G — develop pressure on the charge without deformation of the sleeve during continued movement of said pressure element, said driver element being made of a material which is ductile, electrically conductive and softer than said electrically insulating material under the conditions of temperature and pressure to which it is exposed in said method. .

According to yet another aspect of the invention, there is provided a method of maintaining a more uniform Ί0 temperature along the length of an electrically heated charge which undergoes reaction and reduces in volume which comprises: confining the charge radially within a radially supported sleeve of electrically insulating material, confining the charge axially at one end, advancing a pressure element towards the other end of the charge, and interposing a driver element between said other end of the charge and the said pressure element which driver element will flow into the adjacent end of the sleeve and develop pressure on the charge without deformation of the sleeve during continued movement of said pressure element, said driver element being made of a material which is ductile, electrically conductive and softer than said electrically insulating material under the conditions of temperature and pressure to which it is exposed in said method.

The invention will now be further described with reference to the drawing, in which :Figure 1 is a side view of a prior art piston-cylinder arrangement before pressurisation; Figure 2 is a side view of the arrangement of Figure 1 after pressurisation; Figure 3 is a side view of a piston-cylinder arrangement according to the invention before pressurisation; and Figure 4 is a side view of the arrangement of Figure 3 after pressurisation. 2.8 5 8 Ση Figure 1 there is shown a prior art arrangement for heating and pressurising a core of charge material 22.

She core of.charge material 22 is placed in cylinder 12 having an immovable or stationary piston 18 having a central boss 18', the stationary piston 18 closing one end of cylinder 12 and insulated therefrom by a ring of electrical insulation-15-surrounding fche boss 18’. Between the core of charge material 22 and fche inside diameter of cylinder 12 insulation 14 is placed. Insulation 14 extends nearly the entire length of the inside diameter of cylinder 12 in a cylindrical form and has a thickness 28. Movable piston 10 just fits inside cylinder 12. The outside diameter of the insulation material 14 usually has a close slip fit with the inside diameter of cylinder 12.

Elements IS located at respective ends of the core 22 are current and pressure transmitters to facilitate electrical heating of the core of charge material 22. An electric current is passed through stationary piston 18 to the current and pressure transmitter 16. Bach element 16 has a central pact 16’ made of a highly electrically-conductive material such as graphite, so that a high central current density is produced which allows better control of the temperature of the core.

The current passes from upper transmitter IS through the core of charge material 22 to the lower current and pressure transmitter. The current then further passes through movable piston 10 to complete the circuit.

Movable piston 10 is aligned with the inside diameter of cylinder 12 and provides a small clearance fit with the inside diameter of the cylinder. As 3oen from Figure 1, the insulating material 14 extends nearly all the way to the face of both pistons 10 and 18 to prevent any excess heat loss. - 8 43858 Pressurisation is achieved by advancing movable piston 10 into cylinder 12 towards stationary piston 18.

The insulating material 14 has a uniform thickness throughout as pictured in 28. The core of charge material is then heated to the required temperature for the reaction to occur.

Referring to Figure 2, the condition of the core of charge material 22 and the insulating material 14 may be seen after the reaction has taken place. In the usual formation of diamond crystals in apparatus of the type illustrated, the core of change njaterial will be a mixture of graphite and a carbon solvent and the insulating material could, for example, be pyrophyllite. The sequence of events that could take place would be pressurization of the core 22 to a sufficient pressure above the diamond graphite equilibrium line and then heating of the core of charge material 22 to a sufficient temperature to start the conversion from graphite to diamond.

The temperature is very important depending on the quality of diamond crystals desired. Therefore, what occurs in Figure 2 is that the movable piston 10 is advanced into cylinder 12 until the desired pressure is reached. The movement or advancement of piston 10 initiates the deforming and buckling of insulating material 14. Once sufficient pressure is reached, the core is heated to sufficient temperature to start the conversion of graphite to diamond, and a constant force is kept on piston 10 to maintain a steady pressure. Inside the core of charge material, during the reaction, volume is being reduced because diamonds are about 56 per cent more dense than graphite.

Therefore, as the reaction occurs, piston 10 advances further into the cylinder 12 because a constant driving force is being kept on it. This movement further deforms insulating 43858 material 14 such that it bulges as at 24 and is thinner as at 26. The bulging at 24 increases the current density through the core and reduces the amount of heat carried out through the walls of the cylinder, causing the relative temperature in the core to become hotter in the region of 24.

In the meantime, the relative thinness of the insulation material at 26 decreases the current density through the core and allows more rapid heat transfer out to the walls of the cylinder 12 thereby reducing the relative temperature of the core 22 in region· 26.

These are the regions of hot' and cold spots, respectively, that the present invention avoids or minimizes to a great extent. Usually, in presses of these kinds, it is Very difficult to measure temperature directly, so the temperature is usually estimated from past experience, and the amount of heat input into the core of charge material. If these cold and hot spots develop, it becomes impossible to determine the temperature of the reaction in order to control the quality of the end product of diamond crystals.

In Figure 3 is shown the arrangement of the present invention before pressurization takes place. After pressurization, heating is accomplished as described previously by passing current through stationary piston 18, current and pressure transmitter, core of charge material 22, driver element 30 and movable piston 10. It is necessary for the driver element to be electrically conductive in order to complete the circuit required for heating the core of charge material.

Originally, the driver element 30 is in solid state and has a cylindrical configuration with its diameter approximately equal to the inside diameter of cylinder 12. An additional property - 10 42858 of lead and one which is extremely favourable here is that the material has a very low thermal conductivity factor. Thus, it additionally helps contain heat in the core of charge material 22 rather than help dissipate the heat outward.

This driver element in its solid state is confined between the movable piston 10 and the insulating material 14 and current and pressure transmitter 16. When pressurization begins by movement of the piston 10 inward, there is no axial force exerted on the ends radially outward of insulating material 14 sufficient to deform the material. The driver element deforms around the edges of the sleeve of insulating material 14 and into the center of the sleeve. The important feature here is that any material chosen, while it must be electrically conductive, must also have lower flow strength than that of the insulating material of the sleeve, i.e. be softer than the insulating material at the conditions of temperature and pressure present.

In Figure 4 is shown the arrangement of the present invention after the total reaction has taken place. Movable piston 10 has moved inward into proximity with the bottom of the sleeve 28 and the driver element 30 has been deformed into the center of the sleeve 14. What is shown is, of course, an extreme position of the piston. The piston may normally not have to travel into contact or even near the bottom of the sleeve. The amount of lead to be used or required can be predetermined by one skilled in the art 'depending upon the dimensions of the apparatus to be used.

The main feature, however, is that the sleeve of insulating material 14 will have a nearly uniform thickness 28 throughout after the reaction instead of a buckled and pinched configuration as shown in Figure 2, and the core of the charge material will 43858 have a uniform cross sectional area throughout its length.

Further, the piston 10 can follow the volume change of the charge and thereby maintain constant reaction pressure on the charge for a prolonged period of time.

Further modifications may be made within the scope of the appended claims.

Claims (10)

1. A high pressure, high temperature pistoncylinder apparatus comprising a cylinder, means closing one end of said cylinder, said cylinder being adapted to 5 receive a core of charge material therein, means for heating the core when in the cylinder, a sleeve of electrically insulating material fitted between the core and the cylinder, a piston movable in the cylinder to develop pressure on a core therein, and a driver element 10 located in the cylinder between said piston and said sleeve, said driver element being made of a material which is ductile, electrically conductive and softer than the material of said sleeve under the conditions of pressure and temperature encountered in said cylinder in use, said 15 driver element being operable as the piston advances into the cylinder towards the core in use, to deform into said sleeve and transmit pressure from the piston to the core without deforming said sleeve.

2. Apparatus as claimed in Claim 1, wherein said 20 driver element is made of a metallic material.

3. Apparatus as claimed in Claim 2, wherein said driver element is of lead.

4. Apparatus as claimed in any one of Claims 1 to 3, wherein said core is provided with respective conductive 25 disc elements at the ends, the length of said core being about equal to fhe length of said sleeve, said sleeve being shorter than said cylinder, and said driver element being confined in said cylinder between said piston and the end of said sleeve and core facing said piston.

5. « Apparatus as claimed in any one of fche preceding Claims, wherein said sleeve is formed of substantially rigid material.

6. A method of maintaining a desired pressure on an electrically heated charge which undergoes reaction and reduces in volume which comprisest confining the charge radially within a radially supported sleeve of electrically insulating material, confining the charge axially at one end, advancing a pressure element towards the other end of the charge, and interposing a driver element between said other end of the charge and the said pressure element which driver element will flow into the adjacent end of fche sleeve and develop pressure on fche charge without deformation of the sleeve during continued movement of said pressure elements said driver element being made of a material which is ductile, electrically conductive and softer than said electrically insulating material under fche conditions Of temperature and pressure to which it is exposed in said method.

7. O A method of maintaining a more uniform temperature along the length of an electrically heated charge which undergoes reaction and reduces in volume which comprises? confining the charge radially within a radially supported sleeve of electrically insulating material, confinihg the charge axially at one end, advancing a pressure element towards the other end of the charge, and interposing a driver element between said other end of the charge and the said pressure elanent which driver element will flow into the adjacent end of the sleeve and develop pressure on the charge without _ 14. deformation of the sleeve during continued movement of said pressure element, said driver element being made of a material which is ductile, electrically conductive and softer than said electrically insulating material under 5 the conditions of temperature and pressure to which it is exposed in said method.

8. A method as claimed in Claim 6 or Claim 7, wherein the driver element is confined radially between the pressure element and the adjacent end of the sleeve.

9. 10 9. A high temperature, high pressure pistoncylinder apparatus substantially as hereinbefore described with reference to and as shown in Figure 3 and 4 of the accompanying drawings. 10. A method as claimed in Claim 6 or Claim 7, substantially as hereinbefore described with reference to

10. 15 the accompanying drawings.