Basic Knowledge of Piezoelectric Quartz Crystal

1. Chemical and physical characteristics

(1) The composition of crystal SiO2, at different temperatures under atmospheric pressure, the structure of quartz crystal is different, temperature T < 573℃ α quartz crystal, when 573℃ < T < 870℃ β quartz crystal, melting point is 1750℃, we usually say the piezoelectric quartz crystal refers to α quartz crystal.

(2) With piezoelectric characteristics:
Piezoelectric effect:
Some media due to external mechanical action (such as compression, stretching and so on) and in its internal polarization, the phenomenon of surface charge is called piezoelectric effect.
Inverse piezoelectric effect:
Some media placed in the external electric field, due to the action of the electric field, will cause the displacement of the center of positive and negative charge inside the medium, resulting in the deformation of the medium, this effect is called the inverse piezoelectric effect.
When the force along the X (or Y) axis acts on the quartz crystal, the piezoelectric effect occurs in the X direction, while the Y and Z directions do not. The X axis is called the electric axis, and the Y axis is called the mechanical axis.

(3) With anisotropy: Quartz crystal is a kind of good insulation, near room temperature, thermal conductivity along the Z axis is perpendicular to the Z axis direction of 2 times the left and right sides, along the Z axis direction a3 approximately linear expansion coefficient of linear expansion coefficient along the direction perpendicular to the Z axis a1 1/2, the dielectric coefficient of epsilon, piezoelectric coefficient d, etc. Its values vary with the direction of, At different temperatures, the values of thermal conductivity K and expansion coefficient A are also different.

(4) Piezo quartz is the highly symmetric shape of the single crystal, is characterized by the arrangement of atoms and molecules have rules well developed quartz crystal, appearance is the most significant features rule configuration, the crystal quartz crystal crystal face a total of 30, 6 m (cylindrical), six R surface (large surface edges) six R surface (surface) edges six s surface (three-way partial cone), Six X planes (tripartite partial planes), the clip Angle of adjacent M plane is 60°, the Angle between adjacent M plane and R plane and adjacent M plane and R plane is 38°13 ', and the Angle between adjacent S plane and X plane is 25°57 '.
Quartz crystal has a cubic axis of symmetry C and three axes A, B and D that are 120° each other. When discussing the physical properties of quartz crystal, it is more convenient to adopt the rectangular coordinate system as shown in the following figure. Axis C is z axis, axis A (or B, D) is X axis, and axis Y is perpendicular to X axis and Z axis. The orientation is determined according to the 1949 IRE standard, and the right-handed cartesian coordinate system is adopted for both right-handed and right-handed rotation crystals.
As shown in figure: A, B, C and D are crystal coordinate systems
X, Y, and Z are rectangular coordinates

(5) With birefringence:
But when the light along the z-axis direction does not occur birefringence, so it is also called the z-axis as the optical axis.

The density of quartz crystal ρ=2.65g/cm2, the hardness is Mohs hardness 7, insoluble in triacid (HCL,H2SO4,HNO3) at room temperature and pressure, belongs to the very small solubility of substances, but hydrofluoric acid and ammonium hydrogen fluoride is a good solution of quartz crystal, its chemical reaction equation
SiO2 hf + 4 = SiF4 + 2 h2o (3 + 3 h2o SiF4 = H2SiO3 h2sif6 + 2)
SiO2+4HF+2NH4F= (NH4) 2SiF6+2H2O
Its properties are used for quartz chip corrosion.


2. Cutting type of quartz wafer

The cutting of quartz wafer to a certain orientation of the crystal coordinate axis is called the cutting pattern of quartz wafer. Due to the anisotropy of quartz crystal, the electrical and thermal properties of quartz slices are different due to their different elastic properties, piezoelectric properties and temperature properties.

There are two ways to express the cut symbol of quartz crystal, one is the symbol representation prescribed by IRE standard, the other is the custom symbol representation peculiar to quartz crystal. IRE (cut type prescribed by the symbol (XYZlWt) with a set of letters and Angle, said with XYZ three letters in order to represent the chip thickness and length along the axis of the original location, with t (thickness), l (length), W (width) to represent the rotating direction, Angle counterclockwise said plus and minus sign said clockwise.
Example: (Yxl) 35° Original wafer Angle: Y direction
Original wafer length: X direction
(XYtl) 5°/-50° thickness T (X axis) 5° counterclockwise rotation, and then around the length L (Y axis) 50° clockwise rotation, is the quartz chip cutting azimuth.
Most of the customary symbols of quartz crystal are represented by two English capital letters. For example, the customary symbols of 35° cut type (YXl) are represented by AT, and 5°/-50° (XYtl) are represented by NT.