QUARTZ CRYSTAL
1. Introduction
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 in telecommunication
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 2649 kg/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.
2. The Basic Process of Growing Cultured Quartz
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.
|
