Quartz Resonator & Oscillator Tutorial

Electrical twinning

Optical Twinning

· The X-axes of quartz, the electrical axes, are parallel to the line bisecting

adjacent prism faces; the +X-direction is positive upon extension due to

tension.

· Electric twinning (also called Dauphiné twinning) consists of localized reversal

of the X-axes. It usually consists of irregular patches, with irregular

boundaries. It can be produced artificially by inversion from high quartz,

thermal shock, high local pressure (even at room temperature), and by an

intense electric field.

· In right-handed quartz, the plane of polarization is rotated clockwise as seen

by looking toward the light source; in left handed, it is counterclockwise.

Optically twinned (also called Brazil twinned) quartz contains both left and

right-handed quartz. Boundaries between optical twins are usually straight.

· Etching can reveal both kinds of twinning.

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Quartz Twinning


	Natural quartz often consists of partly right handed and partly left handed quartz. Such crystals are said to be optically twinned. Optical twins are growth twins, i.e., if absent from a crystal when the crystal is grown, they cannot be induced subsequently without breaking silicon-oxygen bonds. Electrical twins, on the other hand, can be produced in quartz by the application of mechanical and thermal stress. The higher the temperature, the less stress is required to produce this type of twinning. The atomic shifts necessary to produce electrical twinning (~0.03 nm) are less than one-tenth of the lattice spacings; no breaking of bonds takes place.
	Twinned plates are not useful for frequency control applications. It is, therefore, necessary to avoid high stresses during fabrication, especially at elevated temperatures. Processes such as the thermocompression bonding of mounting clips to the edges of quartz plates can readily produce twinning, if not done carefully. Twinning has also produced failures in quartz pressure transducers used at high pressures and temperatures, e.g., in oil wells.
	It has been observed that it takes less stress to produce twinning in very high Q, high purity quartz than in lower quality quartz. One company specifies both a minimum and a maximum material Q; a minimum to ensure that the material does not limit the overall electrical Q performance, and a maximum to minimize the incidence of twinning*.


	C. Frondel, The System of Mineralogy, Vol. III, “Silica Minerals”, John Wiley and Sons, Inc., New York, 1962.

	T. L. Anderson, R. E. Newnham and L. E. Cross, "Coercive Stress for Ferrobielastic Twinning in Quartz," Proc. 31st Annual Symposium on Frequency Control, pp. 171-177, 1977, AD-A088221.

	Jack Kusters, Hewlett-Packard Co., private communication circa 1985, confirmed in 1999.