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DB1, the HPHT press used, was originally built by Vector S.A. for the synthesis of diamond. It was refurbished and modified for the Diamond Trading Company, to be used for research into the effects of HPHT annealing on defects in diamond. It was then donated to the University of Warwick to continue this work. The system is comprised of three elements:

ˆ The 500 tonne hydraulic press

ˆ The electrical power system

ˆ The pressure and power feedback control

4.1.1.1 The hydraulic system

The hydraulic system controls both the pressure and flow of hydraulic fluid within a closed loop circuit. These two parameters are controlled by an electronic ampli- fier card and via a pressure transducer, oil pressure and flow values are transmitted

Figure 4-2: Configuration of four anvils.

as voltage feedback values to the system. A valve acts as a flow controller when the set point value is in excess of the current pressure and a regulator when the de- sired set point has been achieved. A Bosch radial piston pump is pressure and flow compensated, with the pump delivery responding to system parameter changes as required. A high pressure relief valve protects the system, ensuring that the pis- ton pump will compensate at high pressure loads. An accumulator, located in the main pressure line, helps to cushion any shock changes.

The ram is driven by supplying a voltage to the pressure and flow controls at the proportional valve. The speed of movement is governed by the signal from the flow controller, until the point where contact is made and pressure begins to build. The load, provided by the ram, is then governed by the pressure build up. The load will remain at a constant value until the voltage governing the load is changed. In order for the ram to drop, the system pressure must be slowly released.

4.1.1.2 The anvils

Anvils are formed from tungsten carbide, held in a steel binding ring with a hollow steel ring surrounding for the purpose of water cooling. Two of the four anvils have an electrical terminal attached, to provide an electrical circuit when contacted with the sample cell. This was used for the resistive heating of the sample. Three of the anvils were machined to fit inside thegun barrel cone with an inclined angle

Figure 4-3: Configuration of the lower three anvils within the gun barrel. Labelled are the electrodes and the water supply tubes for the cooling of the anvils.

of 18.5○_{, with the fourth top anvil having a flat face for planar contact with the}

load plate (Figure 4-3).

In preparation for running, the gun barrel was lined with Teflon and Melinex sheeting to electrically insulate the anvils from the gun barrel and provide a surface of reduced friction and the smooth and even displacement of the anvils under pressure. Teflon buttons 0.25 cm thick, were placed in four places evenly around one face of the three base anvils and all three faces of the top anvil to prevent contact between them. These were mounted using double-sided adhesive tape. The slanting faces, 0.5 cm from the flat triangular face, were painted with ferric oxide to increase the friction between the extruding pyrophyllite and the tungsten carbide anvil face and encourage the formation of gaskets. In experiments where a thermocouple was used, the non-electrode connected base anvil had additional Teflon sheeting on the uppermost face, over which the thermocouple wires passed.

4.1.1.3 The high pressure cell

The sample cell was machined from pyrophyllite (Figure 4.4(b)), an easily machin- able material with excellent thermal stability. It is widely used in high pressure experiments as both a pressure transmitting medium and a gasket material. In this system, it fulfills both functions.

(a)

(b)

Figure 4-4: The design of the tetrahedral sample cell for HPHT annealing. (a) shows an exploded view of the tetrahedron, highlighting the graphite contact rings, discs and pyrophyllite plugs. (b) shows a cross section through the tetrahedron, indicating the positioning of the thermocouple junction and the graphite heating rod and crucible [4].

(a) (b)

Figure 4-5: Estimates of the heat dissipation across a sample chamber [6]. (a) illustrates a vertical cross section of the graphite heater crucible with a set point of 1600‰for 10 minutes. (b) indicates the circular cross section under the same environmental parameters.

large pressure variations within the sample region. Therefore, before use, the pyrophyllite was baked at 500‰ for 8 hours and then kept in a furnace at 80‰ until required, to stabilise its properties and minimise its water content.

Two chambers are drilled into the tetrahedron. One is filled with a solid graphite rod and the other with a crucible style graphite sample holder. This creates the conductive path for resistive heating of the sample. A graphite disc and ring with a pyrophyllite plug seal the crucible once the sample has been inserted.

Within the sample crucible, the sample is placed centrally and packed with hexagonal boron nitride powder. The low shear strength of boron nitride allows it to flow freely around the material and there is no reported evidence of any interaction between boron nitride and diamond [5].

A thermocouple was frequently used within the chamber. A tungsten iridium thermocouple was originally chosen, which had the benefit of a small EMF below 50‰and so did not require a cold junction calibration. However, at high pressures, the wires were often damaged by compression from the anvils. Thinner K-type thermocouples were subsequently used, which had an upper operating temperature

limit of 1250‰. Extrapolation of the input electrical power with temperature permitted conversions at higher powers.

It has been shown that the presence of a thermocouple within a chamber can act as a heat sink, creating a temperature gradient across the sample region (Fig. 4- 5) [6]. This was measured by finding the ratio of luminescence of the 2.464 (H3 defect) and 1.945 eV (NV−_{defect) sidebands which provided an indication of tem-}

perature at various points across a compacted diamond powder at 9.5 GPa. A similar temperature gradient would be expected for boron nitride.

4.1.1.4 The controllers

The existing hardware controllers of the press system were outdated and required replacement. Two new Eurotherm 2704 controllers were installed, providing the user interface with the hydraulic and power systems. These controllers were soft and hard wired to suit our requirements, providing control over output commands, feedback controls, PID settings and permitted the integration of a number of safety features based upon inputs from various hardware components.

Within the power controller, a power demand (set point) was made, which by a micropower control card, was separated into distinct current and voltage values according to internal settings. Via a transformer, these were then transmitted to anvil electrodes, producing a sufficient voltage to resistively heat the sample to the desired temperature. A monitor on the feedback current and voltage, com- municated a process variable (working value) via a multiplier, so as to display a power value to the user.

A number of hardware monitors and the input and output variables of the controllers were integrated into the safety system, controlled by the Eurotherm controllers. Optical sensors detected the current height of the ram which then was used to identify afast approach andslow approach regimes as the ram approached the load plate. These two states then throttled the flow voltage between maximum and half maximum to control the speed of the ram. Check signals identified if water flow was present around the anvils before heating was permitted and communication between the two controllers using the soft wiring options, ensured that pressure ramping was completed before heating could begin and that the

power programme had reach anend statebefore the pressure would ramp down. A final safety check was the introduction of Hall sensors in the hardware to detect the feedback current and voltage. On occasion, where a short circuit had developed, normally due to contacting anvils, the current would rise rapidly with no associated resistance. From a number of successful experiments across the operating range of the press, it was noted that the current would never rise above 300 A. Therefore, if the Hall sensor detected a current greater than 300 A, a signal was sent to the controller, soft wired to be understood as an abort command and would end the power ramping programme.

To preserve the anvils, load ramping was limited to 3 tonnes per minute and un- loading to 1 tonne per minute. For heating, this was limited to 50 Watts per minute to prevent excessive heating strains to the sample cell.

4.1.1.5 Calibration of sample cell environment

Thermometry with this system proved difficult. The pressure exerted on the wires by the extruding gasket was sufficient that wires frequently fractured. Thicker thermocouples did not fracture so easily but were too large to form a neat, non–shorting join in the graphite sample chamber or alternatively were pinched and fractured by the compressing anvils. Lower pressure thermometry at temper- atures less than 1250‰was possible (<4 GPa), which could then be extrapolated for higher temperatures.

An EMF is generated along the length of the two dissimilar metals that are subjected to a temperature gradient. For most metals used for thermocouples, the output voltage increases linearly with temperature difference. The EMF of the thermocouple was recorded and was translated into a temperature value based upon the materials of the thermocouple. The resulting calibration equation was calculated as:

Temperature(○

C) =Power(W) ×1.53(4) (4-2) The cell pressure versus the applied load was calibrated by measuring the change in electrical resistance of several well known phase transitions of bis- muth [7, 8]. At room temperature, load was applied to the system at a rate of 1 tonne per minute and the resistance of the bismuth wire measured. The re-

Table 4-1: Phase transitions of bismuth under pressure [7, 8].

Transition Pressure (GPa)

III 2.54

IIIII 2.7

High Bismuth 7.7

sistance data were correlated to the process variable and set point data from the pressure controller, to find a resistance versus load relation and therefore a con- version factor between the two quantities. Observing the high bismuth transition was not possible, resulting in having only two points at similar pressures to form the calibration from. Repeated calibrations using this technique have resulted in similar relations between load and pressure such that:

Pressure(GPa) =0.037(4) × Load(T) (4-3) Hysteresis is evident in the system. As the load is applied, the tetrahedra cell is compressed to a volume approximately half of its original. Therefore, as load is removed, the relative pressure is substantially higher. Experiments performed at a range of pressures were completed sequentially as load on the cell was increased.