Non-destructive technique to pole BaTiO3 crystal

Details Non-destructive technique to pole BaTiO3 crystal Non-destructive technique to pole BaTiO3 crystal

1999-05-28

2001-09-11

Vargot, Mathieu D. (Department: 1732)

Plastic and nonmetallic article shaping or treating: processes

Optical article shaping or treating

Nonresinous material only

C264S001360, C264S435000, C264S442000, C264S002700

Reexamination Certificate

active

06287486

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The non-destructive technique to pole BaTiO
3
crystal calls for a slight stress to force the domains to re-orient, which then eliminates the 90° domains in the crystal. By adjusting the voltage and frequency this method may can be applied to the lump crystals of BaTiO
3
.
In the solidification process of BaTiO
3
, the temperature continues to fall. When the temperature drops to Curie Temperature (about 130° C.), the crystal structure will change from cubic to tetragonal, and the corresponding point group from 3 mm to 4 mm. In the crystal's changing process, the barium and titanium atoms displace position opposite to the oxygen atoms in the [001] direction. The movement will force the titanium atom, originally situated at the center of TiO
6
−8
octagon, to deviate towards a certain oxygen atom's position, resulting in the positive and negative atoms' gravity center to fall on different points in producing dipole moment. At the same time, the direction movement also creates a spontaneous polarization, endowing the crystal with ferroelectric effect. However, since the polarization may be oriented in any of the six pseudodoubic [001] directions of the cubic structure, in the cooling process of crystal, crystals at the different positions will have varied polarization directions, leading to the domains of the entire crystal under room temperature to be non-single domain. It forms the 90° and 180° domains
1
,
2
as shown in FIG.
1
. In the 90° domains
2
, the c-axis of each domain is vertical with each other; therefore the reflection indices in the same direction are inconsistent, resulting in scattering when light passes the crystal. Consequently, the 90° domains
2
can be easily seen as shown in FIG.
2
. Nonetheless, for 180° domains, the c-axis in each domain is antiparallel with each other. Thus, unless an etching method is used to reveal different degrees of etching for the positive and the negative domains, it is not discernible by sight.
2. Description of the Prior Art
As the 90° domains
2
in the crystal scatter the light and the 180° domains offset the electro-optic effect, the efficacy of optical experiments is considerably reduced. Consequently, before conducting optical test, it is imperative to turn these multi-domains into a single one. The customary method for eliminating 180° domains
1
is to apply an electric field to the c-face of a BaTio
3
with temperature slightly below the curie point. In terms of the 90° domains
2
, three methods are available presently for the elimination. One of which is uniaxial pressure. The method administers pressure to the a-axis of the crystal, forcing the domain walls to move towards the crystal surface. The crystal surface can be polished to remove the multiple domains on the outer layer, as the 90° domains
2
generally cannot be completely eliminated by one pressure. The step has to be repeated until all 90° domains are removed, which wears off the crystal.
Moreover, the pressure administered must be great. In 1992, NaKao, et al. (Jpn. J. Appl. Phys. Vol. 31, Pt. 1-9B, pp. 3117), the pressure reported was as high as 2×10
7
Newton/m
2
. Thus, once a crystal contains any internal flaw, the crystal is likely to fracture, especially for crystals with a diameter of less than 1 mm. A little force will break the crystal into little pieces. This makes the uniaxial method inappropriate for the current materials.
Furthermore, in 1991, Garrett, et. al. (Ferroelectrics, Vol. 120, pp. 167) had also used immersed etching to remove the 90° domains
2
. The concept of the method arises from using acidic solution to remove the strain layer of the crystal surface created by machine cutting and polishing (The layer is likely to impede domain movement). This enables electrical poling to easily move the atom position, and along with electric field direction, turn it into a single domain. The advantage of the method is that it does not require great stress, and eliminates the occurrence of a flaw in the crystal. However, during the corrosion process, the crystal surface is worn off and turned coarse, making it not suitable for a crystal fiber.
In 1992, F. Ito (Appl. Phys. Lett. Vol. 6131-18, pp. 2144) proposed another method to eliminate the 90° domains. The method is quite simple. It calls for placing crystal on the electrodes, and subjecting it to high voltage for tens of hours. The greatest advantage of the method is its capacity to turn both 90° and 180° domains to a single domain. The drawback is that the 90° domain walls are difficult to move, and therefore difficult to eradicate completely. From the above description, it is evident that the elimination of 90° domains has been a problematic issue for the field.
SUMMARY OF THE INVENTION
The new technique proposes using a high-frequency vibration theory to remove the internal stress of the crystal, and incorporating this with uniaxial pressure to use slight stress for achieving the goal of eliminating 90° domains.
The non-destructive technique to pole BaTiO
3
crystal comprises the following steps:
a) The cut BaTiO
3
crystal is subjected to polishing liquid to polish the two ends of the crystal;
b) Use oscillation transducer to push against the crystal, and secure the other end;
c) Turn on oscillation transducer to produce ultrasonic vibration, and administer to the crystal for removing the 90° domains.
The oscillation transducer is made of two piezoelectric materials and a steeped horn; the length of the crystal is 3 to 5 mm, and set against the crystal's a-axis and negative a-axis direction. The ultrasonic vibration is produced by function generator, generating tens of thousand hertz of ultrasonic frequency; and an amplifier enlarges the voltage discharged by the function generator to tens of volts.
The characteristic of the new technique is that it is a non-destructive, repetitious vibration theory to administer pressure onto the crystal's a-axis direction for the release and dissipation of the stress of the domain movement, and drives the movement of the atoms at the same time. The reciprocal force is different from the customary uniaxial pressure method that administer continual pressure to the crystal for cumulating the forces produced. Consequently, only slight stress is required to force the domains to re-orient, and thus eliminate the 90° domains.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.


REFERENCES:
patent: 3306835 (1967-02-01), Magnus
Jpn. J. Appl. Phys., vol. 31, Nakao et al, “Influence of Poling Conditions . . . ”, Part I, No. 9B, Sept. 1992, pp. 3117-3119.
Ferroelectrics, vol. 120, Garrett et al, “A Method for Poling Barium Titanate BaTiO3”, 1991, pp. 167-173.
Appl. Phys. Lett. 61 (18), Ito et al, “Observation of the photorefractive effect . . . ”, Nov. 2, 1992, pp. 2144-2146.

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