Synthetic Quartz crystals are grown by hydrothermal method based on the temperature differences. In general, this process closely mimics how many crystals in nature originate from high temperature, high pressure water solutions. Hydrothermal method is also used in manufacturing high quality synthetic emeralds, rubies and sapphires. The ingredient materials are enclosed in a partially filled, sealed container of water solution called autoclave. Heated to high temperatures the feed materials are dissolved under high pressure and at point of cooling crystal growth is induced.
To grow quartz crystals an autoclave is filled with alkali solution of material such as Na2CO3 or NaOH. The high pressure required for growing is obtained by filling the alkaline solution up to the approximate 70-85% of the capacity of the autoclave and by rising the temperature. The ordinary growing temperature is approximately 350-360?C and the growing pressure is approximately 90-145Mpa. When the autoclave is put into service, the manufacturer performs a 'conditioning' run which allows a crystalline coating to form on the inside of the vessel. This coating is stable and prevents interaction between the autoclave and the chemical solution used in the vessel.
In the autoclave, the upper growing area and the lower dissolution area are partitioned by the perforated panel called the baffle. This baffle allows the bottom temperature and the top temperature to be significantly different and to help each zone have an isothermal characteristic. The baffle also restricts the convection flow and channels the convection flow into a desirable geometry.
The seed crystal is suspended in the upper growing area, and the raw material, Lasca, a fragment of the natural quartz crystal is placed in the lower dissolution area. A cap is screwed on and the autoclave suspended vertically, the lower half being inserted into an industrial furnace. The temperature in the lower part of the autoclave is higher than in the upper part. As a result, materials that dissolved in the lower part under high temperature and pressure are carried to the upper part by convection to deposit on seed crystals, creating synthetic quartz crystals. The growth cycle may vary from 40 to more than 200 days, time being a function of the size of the stone being grown. Different impurities are added to the growing solution to provide color to the quartz. In laboratories, smaller autoclaves for experimental work are used.
The main parameters that are monitored in the process are the temperature at various points on the vessel and the pressure of the vessel. These parameters are used to control the growth process. They have to vary during the process because at startup the majority of the quartz material is at the bottom of the autoclave. The top only contains the seed material. At the end of the process, most of the quartz in the bottom has dissolved and recrystallized on the stones in the upper growth zone. Moving this much material around greatly affects the hydrothermal flow conditions and the thermodynamic state of the vessel. For this reason, the control algorithms are designed to make continual changes during the process to compensate for these changing conditions.
Heating of the autoclave is accomplished by means of electric heaters fastened to the outside of the vessel. There are heaters throughout the length of the vessel with a higher density at the bottom. The temperature of both the bottom dissolving zone and the top growing zone has to be carefully controlled.
Around the autoclave and the heaters there is an exterior insulation package. This insulates the autoclave and helps isolate it from external temperature changes. There is usually an air space between the insulation package and the vessel. There are also vents at strategic places to allow the air flow to be controlled. Changing these vents and the power delivered to the heaters determine the thermodynamic condition of the autoclave.
PROPERTIES
Hardness (Mohs' scale): 7
Density: 2.65
Refractive index: 1.544 – 1.553
Dispersion: 0.013
Crystal system: trigonal
Chemical composition: SiO2 (with various additives to produce colour)
Dielectric constant: 4.5 |
