Talking about Six-headed Hydraulic Electro-hydraulic Control Mode

This article tries to analyze the pressure and temperature field of the synthesis chamber and compare it with several commonly used control modes at present, and proposes an overall solution for the six-faced top hydraulic press that meets the requirements of the large cavity synthesis process.

Pressure control

The enlargement of the synthesis chamber inevitably increases the pressure difference between the shell and the core of the synthetic rod formed due to the pressure loss, that is, the pressure gradient of the pressure field increases; at the same time, the diamond growth process also leads to a change in the pressure field: , High-temperature and high-pressure graphite with pyrophyllite as the pressure-transforming medium converts diamond into a series of phase transformation process: pyrophyllite mineral phase transformation produces kyanite and coesite, graphite phase change produces diamond, due to the large product ratio of these phase transitions. The volumetric shrinkage before and after the phase change results in a decrease in the internal pressure of the synthesis chamber; moreover, since the pyrophyllite is rigid after the phase change due to the increase in both the friction coefficient and the strength, the effect of the transfer pressure and the pressure compensation is affected, and these all cause the cavity. The larger pressure gradient in the body, while the growth of high-quality diamond crystals requires relatively stable pressure conditions, therefore, how to better reduce the pressure gradient is a challenge facing the large cavity process.

The well-known ideal pressure control mode can be as shown in Figure 1:

1) The boost curve is controllable so that it can effectively control the pressure gradient with the heating curve

2) Increasing the pressure curve in the hold pressure phase, reducing the increase in the pressure gradient due to synthetic phase change

3) The pressure relief speed is controllable, taking into account the different requirements of the pressure relief speed at high and low pressures

At present, there are several pressure control modes in use:

1.1 The traditional pressure control mode

In this mode, the pressure fluctuations are quite large and it is completely an extensive control mode. It is not suitable for the large cavity synthesis process. as shown in picture 2.

1.2 Inverter pressure control mode

This mode maintains the constant pressure during the hold pressure phase, but neglects to compensate for the pressure gradient caused by the synthetic phase change. As shown in Figure 3.

1.3 Passive incremental pressure compensation mode

As shown in Figure 4. This is the original incremental pressure compensation mode, which is passive by setting the holding pressure to the set value and setting the incremental pressure. In fact, during the hold pressure stage, the number of compressor pressurization is limited, and it is related to the failure of the press high pressure seal. Therefore, the purpose of compensating the pressure gradient by increasing the pressure compensation is not really realized.

1.4 Active Incremental Packing Mode

As shown in Figure 5. The incremental pressure compensation is achieved by setting the pressure increment and time interval (the number of times of pressure compensation), so it is an active incremental pressure compensation mode. The use of variable pressure control can control the pressure drop at each set time interval to a minimum. However, because the pressure holding performance of the press is generally better, the pressure drop over a set time interval is negligible.

1.5 Pressure Control Mode with Proportional Valve

The principle of the proportional valve is to control the current or voltage according to the set curve, so that the thrust and displacement of the proportional electromagnet are proportionally and continuously controlled to achieve the purpose of controlling the pressure and flow of the system. Proportional pressure valves or proportional pumps are used in the system to not only achieve a continuous increase in system pressure during the hold pressure phase, but also to achieve continuous control of the boost pressure during the boost phase, ie to achieve full pressure curve control. It can be said that this is the most likely to be close to the ideal control mode, as shown in Figure 6.

2. Temperature control

The establishment of the temperature field in the synthesis chamber is achieved by direct current heating. Whether it is the control of the current or the voltage, the temperature rise is ultimately achieved by a change in the heating power. Since the heat is generated by the self-heating of the synthetic rod resistance, even the auxiliary measures such as the graphite liner only play the role of insulation. Therefore, the temperature gradient is generated due to heat dissipation, and the temperature should decrease outward from the rod core, and This temperature difference should be independent of the diameter of the composite rod under the same conditions of heating and heat dissipation. Based on this reasoning, the enlargement of the synthesis chamber provides an objective condition that can form a more balanced and stable temperature field, or that the proportion of space that meets the temperature conditions required for high quality diamond growth in large cavities will be greater.

The change in the temperature of the synthesis chamber is affected by two factors: one is heating and the other is heat dissipation.

At present, there are two control modes based on heating power control:

One is the constant power mode, and the heating power is in a constant state. As shown in Fig. 7, the corresponding heating curve is shown in Fig. 8. As time goes by, the heating amount increases proportionally;

The second is the variable power mode, and the heating power can be changed according to the set curve. As shown in Fig. 9, the corresponding heating curve is shown in Fig. 10. With the extension of time, the heating amount increases in a non-proportional manner.

With the change of the heating curve, there is a heat dissipation curve. In the case of free heating and heat dissipation, the two curves must intersect. That is, there is a balance point between heating and heat dissipation: after this point, heat does not increase, nor does it decrease. , In a warm state. As shown in Figure 11.

In the actual production of synthesis, there is often a phenomenon that the temperature difference between the core and the outside of the synthetic rod greatly influences the growth of the diamond, and the occurrence of such a heat preservation state will eliminate this temperature difference.

Adjusting the heating power in a timely and appropriate manner may cause or close to this equilibrium point during the synthesis cycle, and only an electronic control system with a variable heating output function provides such a condition.

3. Conclusion

Based on the above analysis and comparison, a six-head hydraulic press that meets the temperature and pressure conditions for the growth of high-quality diamond single crystals in large cavities should have at least the following functions:

1) It has active incremental pressure keeping function to effectively reduce the pressure gradient, and the control mode with proportional valve control is an ideal hydraulic control system.

2) The electric heating system has a wide variable heating output function, which provides conditions for adjusting the appearance of insulation.

3) It has at least 1800T synthetic tonnage to meet the pressure conditions of the 40 cavity synthesis process. After all, the natural temperature field advantage of the large cavity is unmatched by the small cavity.

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