Compositions – Piezoelectric – Lead – zirconium – titanium or compound thereof containing
Reexamination Certificate
2002-02-08
2003-12-02
Koslow, C. Melissa (Department: 1755)
Compositions
Piezoelectric
Lead, zirconium, titanium or compound thereof containing
C501S134000, C310S311000
Reexamination Certificate
active
06656378
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates, in general, to a piezoelectric ceramic composition reducing a leakage current and a piezoelectric device using the same and, in particular, to a piezoelectric ceramic composition in which a B-site of the PZT piezoelectric ceramic composition is substituted with a certain material, and chromium oxide is added, and a piezoelectric device using the piezoelectric ceramic composition. The piezoelectric ceramic composition has good heat resistance and frequency stability, and reduces a leakage current.
2. Description of the Prior Art
A recent piezoelectric sensor is widely used in a ceramic resonator, a ceramic filter, a piezoelectric transducer, a piezoelectric buzzer, an ultrasonic oscillator, or the like. For example, a kHz-range SMD (surface mounted device) type filter, which is an essential part in recent information technology, is commonly used to produce two layered IF filters in devices such as a pager, AMPS (Advanced Mobile Phone Service System), wireless transmitting/receiving apparatus, wire/wireless hands-free kit, and wireless network. Use of the piezoelectric sensor is continuously expanding, and more advanced electrical/piezoelectric properties of the piezoelectric sensor are required according to developments in sensitivity of piezoelectric sensors.
According to the great advances in the information technology, demand for a light weight, small-sized, and highly efficient electronic material and frequency stability of the electronic material is increased.
Particularly, when SMD type chip parts are used, they should be passed through a reflow process. Accordingly, a material is required which has piezoelectric properties unaffected by the reflow process, and resistance to high temperatures.
However, conventional piezoelectric compositions have low heat resistance owing to low Tc (phase transition temperature), and changes in their piezoelectric properties and frequency stability after the reflow process are large, so that a high value-added SMD type filter can hardly be produced from conventional piezoelectric compositions. Also, various electronic products cannot be produced and a competitive power of products is poor because the piezoelectric properties of the conventional piezoelectric composition are difficult to control.
To secure the heat resistance of the material and stabilize frequency and the piezoelectric properties of the material after the reflow process, studies have been conducted for stabilizing a heat resistance, as well as a frequency change, a capacitance change, and a coupling factor (k) change after the reflow process according to a oscillation mode by using a piezoelectric composition comprising Pb(Zr, Ti) O
3
as a main component and additives such as Mn, Y, Dy, Er, Ho, Lu, and Yb.
For example, U.S. Pat. No. 6,123,867 discloses a piezoelectric ceramic composition comprising Pb[(M, Nb) Zr, Ti]O
3
as a main component and additives, and a piezoelectric device using the piezoelectric ceramic composition, which can stabilize a heat resistance, as well as a frequency change ratio, a capacitance change ratio, and a coupling factor (k) change ratio after the reflow process.
Furthermore, a Pb(Zn Nb) (Sn, Nb) TiZrO
3
based composition disclosed in Japanese Patent Publication Nos. Sho. 52-17239 and Sho. 51-7318 and a Pb(Sn, Sb)TiZrO
3
based composition disclosed in Japanese Patent Publication Nos. Sho. 54-32516 and Sho. 54-36757 are suitable for use in a high frequency ceramic oscillator or filter, because of their excellent piezoelectricity and small size of crystal grains, as well as ease in removing impurities during sintering thereof. Therefore, these piezoelectric ceramic compositions are used in ceramic filters, ceramic oscillators, piezoelectric transducers, ceramic sensors or the like.
Additionally, Japanese Laid-Open Patent Publication Nos. Hei. 8-239269 and Hei. 9-142930 disclose materials that are not affected by heat, in which a composite oxide of Y, Nb or the like is the main component of PZT, or Cr is added thereto.
However, the characteristics of the conventional piezoelectric ceramic composition change because of poor heat resistance, and drift occurs upon heating. Therefore, the piezoelectric device has poor reliability and disadvantages in mass production. In other words, when a piezoelectric device formed of the conventional piezoelectric ceramic composition is heated at 150° C. for 1 hour, the resonance frequency fr changes by several % immediately after the heat treatment, when compared to that before the heat treatment.
Additionally, a reflow temperature is higher and a temperature of an element heated is also higher because SMT type electronic parts such as ceramic filters and ceramic oscillators are commonly used. Accordingly, the piezoelectric properties of the piezoelectric ceramics before the heat treatment are different from those after the heat treatment. That is to say, there is a problem that the piezoelectric properties—such as resonance frequency—of the piezoelectric ceramics before the heat treatment are different from those after the heat treatment, when such piezoelectric ceramics are heated to a temperature of about 250° C. and then cooled to room temperature in order to reflow the piezoelectric device.
Furthermore, there is another problem that a leakage current becomes large and thus a voltage is nonuniform when the conventional piezoelectric composition is polarized, therefore dispersion is large and productivity is poor while the piezoelectric device is produced.
SUMMARY OF THE INVENTION
Therefore, it is an object of the present invention to provide a piezoelectric ceramic composition with an excellent heat resistance and frequency stability, which is low in leakage current.
It is another object of the present invention to provide the piezoelectric ceramic composition which has a minimum dispersion owing to a reduction of the leakage current and which can be manufactured to piezoelectric devices at high production yield.
It is yet another object of the present invention to provide a piezoelectric device using the piezoelectric ceramic composition, in which the piezoelectric ceramic composition has an excellent heat resistance and frequency stability and reduces a leakage current.
DETAILED DESCRIPTION OF THE INVENTION
According to an embodiment of the present invention, a piezoelectric ceramic composition is provided, which comprises a main component expressed by Pb[(Co
1/2
W
1/2
)
x
Ti
1−x−y
Zr
y
]O
3
(wherein, 0.001≦x≦0.04, and 0.35≦y≦0.55), Cr
2
O
3
in an amount of 0.01 to 2 wt %, MnO
2
in an amount of 0.1 to 0.5 wt %, and an additive in an amount of 0.0l to 2.0 wt %, based on the total weight of the piezoelectric ceramic composition. The additive is selected from the group consisting of CoO, MgO, ZnO, Al
2
O
3
, Fe
2
O
3
, Sb
2
O
3
, SnO
2
, CeO
2
, Nb
2
O
5
, V
2
O
5
and WO
3
, and mixtures thereof.
According to another embodiment of the present invention, a piezoelectric device based on the piezoelectric ceramic composition of the present invention is provided.
Hereafter, a detailed description of the present invention will be provided.
A composite oxide, a main component of the piezoelectric ceramic composition according to the present invention is a PZT based composite oxide, a B-site of which is partially substituted with cobalt and tungsten, and which can be expressed by formula 1, below.
Pb [(Co
1/2
W
1/2
)
x
Ti
1−x−y
Zr
y]
O
3
Formula 1
wherein, 0.001≦x≦0.04, and 0.35≦y≦0.55
In addition, the piezoelectric ceramic composition comprises Cr
2
O
3
in an amount of 0.01 to 2 wt %, MnO
2
in an amount of 0.1 to 0.5 wt %, and an additive in an amount of 0.01 to 2.0 wt %, based on the total weight of the piezoelectric ceramic composition, in which the additive is selected from the group consisting of CoO, MgO, ZnO, Al
2
O
3
, Fe
2
O
3
, Sb
2
O
3
, SnO
2
, CeO
2
, Nb
2
O
5
, V
2
O
5
and WO
3
, and mixtures thereof.
Mostly comprising P
Hong Jong Kuk
Hur Kang Heon
Kwon Sang Koo
Seo Dong Hwan
Koslow C. Melissa
Lowe Hauptman & Gilman & Berner LLP
S-Cera Co., Ltd.
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