Crystal Industry Innovation
Quartz is a piezoelectric material. A thin wafer of quartz, with electrodes attached to opposing surfaces, vibrates mechanically when voltage is applied to the two electrodes. Frequency of vibration is primarily a function of wafer dimensions. The wafers, called crystal resonators when suitably mounted with electrodes attached, have long been used for controlling frequency of radio transmitters, and it has been an essential component intelecommunication communication equipment where its piezoelectric properties are used in filters, oscillators and other devices. Now quartz crystals time and coordinate signals for microprocessors, computers, programmable controllers, watches, and other digital equipment such as various DSP.
Quartz is a crystalline form of silicon dioxide (SiO2). It is a hard, brittle, transparent material with a density of 2649kg/m3 and a melting point of 1750° C. Quartz is insoluble in ordinary acids, but soluble in hydrofluoric acid and in hot alkalis. When quartz is heated to 573° C, its crystalline form changes. The stable form above this transition temperature is known as high-quartz or beta-quartz, while the stable form below 573° C is known as low-quartz or alpha-quartz. For resonator applications, only alpha-quartz is of interest and unless stated otherwise the term quartz in the sequel always refers to alpha-quartz. Quartz is an abundant natural material, but considerable labor is required to separate good quality from poor-quality natural quartz. Although silicon (mainly in the form of dioxide, and generally as small quartz crystallites) comprises approximately one third of earth’s crust, natural quartz of size and quality suitable for use in devices employing its piezoelectric properties, has been found principally in Brazil. Natural quartz is also costly to process because it occurs in random shapes and sizes.
Moreover, some segments of poor-quality quartz are discovered only after partial processing. And widespread impurities in natural quartz often make cutting of small wafers impractical. The first major step in the development of cultured quartz was in 1936 when the US Army Signal Corps gave a contract to Brush Laboratories under the direction of Drs. Jaffe, Hale, and Sawyer. This was done due to the pending scarcity of natural quartz with good piezoelectric quality, customarily purchased from Brazil.
Today, quartz is now grown artificially to specified dimensions. Crystal orientation is controlled, and purity is uniformly high. Standard sizes reduce the cost of cutting wafers, and impurities are widely dispersed, making possible small resonators requiring low driving power.
Cultured quartz is grown in a large pressure vessel known as an autoclave (see the following schematic drawing).
The autoclave is a metal cylinder, closed at one end, capable of withstanding pressures up to 30,000 pounds per square inch with internal temperature of 700 to 800° F. It usually stands from 12 to 20 feet high and 2 to 3 feet in diameter.
Small chips of pure but un-faced quartz (1 to 1.5 inch in size), called "lascas or nutrient", are placed in a wire mesh basket and lowered into the bottom half of the vessel. A steel plate with prearranged holes, called a "baffle", is set on top of the basket. The baffle is used to separate the growth (seed) region and the nutrient region, and to help establish a temperature differential between the two regions. Suitably oriented single crystal plates (either natural or cultured), called "seed", are mounted on a rack and suspended on top of the baffle in the upper half of the vessel. The autoclave is then filled with an aqueous alkaline solution (Sodium Carbonate or Sodium Hydroxide) to approximately 80% of its free volume to allow for future liquid expansion, and it is sealed with a high-pressure closure.
The autoclave is then brought to operating temperature by a series of resistive heaters attached to the exterior circumference of the cylinder. As the temperature increases, the pressure begins to build within the autoclave.
A temperature of 700 to 800° F is attained in the lower half of the vessel while the top half is maintained at 70 to 80°F cooler than the bottom half.
At operating pressure and temperature, the lascas dissolves in the heated solution in the lower half of the vessel, which then rises. As it reaches the cooler temperature of the upper part of the vessel, the solution becomes supersaturated, causing the dissolved quartz within the lascas to re-crystallize onto the seed. The cooled spent solution then returns to the lower half of the vessel to repeat the cycle until the lascas is depleted and the cultured quartz stones have reached the desired size. This so-called "Hydrothermal Process" time ranges from 25 to 365 days, depending upon the desired stone size, properties, and the process type - Sodium Hydroxide or Sodium Carbonate.