Piezoelectric Effect of Quartz Crystal and Its Application
In 1880, Pierre Curie and his older brother Paul-Jacques Curie discovered the piezoelectric effect of quartz crystal. After breakout of World War II, as strategic materials, natural quartz (α-SiO2) crystals (natural piezoelectric crystals) were in short supply, thus driving mass production of man-made quartz. Man-made crystals are manufactured by using hydrothermal process in an autoclave, where the temperature in upper part must be lower than that in lower part. Raw materials are dissolved in the lower part at high temperature and pressure, move to the upper part through convection/evaporation, and are deposited on seed crystals, thus growing man-made quartz crystals. High quality of quartz crystals comes from the crystals with high Q value.
Impact of Quartz Crystals on Quality of Piezoelectric Crystals
The quality of quartz crystals has high impact on the parameters of quartz crystal components:
- A quartz crystal with defects (primarily bi-crystals and big inclusions) will lead to inconsistent frequency, worse frequency-temperature characteristics and lower activity;
- A quartz crystal with bad surface smoothness will lead to variable frequency, more aging, lower activity and unacceptable DLD. In the cases that pits, marks, broken edges, broken corners and cracks occur on the surface of a quartz crystal, it will also reduce the crystal’s activity and degrade the DLD performance, especially when the quartz crystal is thin (high-frequency crystal).
- A quartz crystal with bad surface parallelism and planarity will lead to higher resistance, worse temperature characteristics, lower activity or no oscillation, and more parasitic oscillation.
- A quartz crystal with improper or erroneous cutting angle will cause the changes in frequency-temperature characteristics, zero temperature coefficient point and frequency constant, thus leading to inconsistent frequency.
- A quartz crystal with improper side ratio will lead to increased coupling of parasitic oscillation and distorted curve of frequency-temperature characteristics.
- In the case of a change in the radius of curvature of a quartz crystal, the frequency-temperature characteristics will vary with it. When the radius of curvature decreases, the optimal cutting angle becomes smaller; when the diameter of platform decreases, the cutting angle will decrease correspondingly; larger diameter of platform leads to higher equivalent resistance; if more margin is reserved, the equivalent resistance will increase.
Testing on Quality of Quartz Crystal
1. Testing on Quality of Quartz Crystal
(1) Measurement of Dimensions of Quartz Crystal
The measurement of dimensions of a quartz crystal includes length, width, thickness, surface parallelism and planarity, and radius of curvature etc. To measure the dimensions, planarity and parallelism of a quartz crystal blank or final product, an appropriate micrometer or caliper should be selected based on the requirements. In the processing of a quartz crystal with medium or high accuracy, they should be measured with a vertical optical micrometer, whose accuracy is up to 0.001mm. To ensure perpendicularity in length and width of a quartz crystal, a right angle gauge or universal bevel protractor is often used for measurement.
(2) Measurement of Cutting Angle
The cutting angle of a quartz crystal is measured using an X-ray machine. Once a quartz crystal is finished, it is difficult to measure the cutting angle. So a final product manufacturer will take certain number of quartz crystals, prepare specimens and test their frequency-temperature characteristics in order to determine if the cutting angle complies with the design requirements and to give corrective suggestions on cutting angle.
(3) Measurement of Frequency
In the processing of some quartz crystals, frequency may be measured. It is not only helpful to processing, but also helpful to assembling of components. For example, a thickness-shear or face-shear quartz crystal can be processed while the frequency is monitored. Today, automatic measurement of frequency is mostly performed using “excitation in air gap” technique, which has high accuracy and efficiency.
Quality of Piezoelectric Quartz Crystal with High Q Value:
The quality and economic effect of a piezoelectric quartz crystal depend on the orientation of seed crystal. Different cutting face forms different seed veil, which directly affects the product quality. Q value of piezoelectric quartz crystal was the earliest quality parameter of man-made crystal. Later, it was replaced by the fifth overtone of Q value, i.e. 5MHz surface acoustic wave resonator. This parameter can more accurately reflect inner loss of a man-made quartz crystal, and does not affect the processing of crystal. As a piezoelectric quartz crystal is processed/manufactured, Q value is estimated by measuring the infrared absorption α value. If the measured Q value is of high class, it is equivalent to excellent quality with no difference from natural high-purity crystal.