Piezoelectric actuator

1. The piezoelectric actuator including a piezoelectric element that has a pair of electrodes formed on the surface of a sheet of piezoelectric ceramic, and a holding member that holds the piezoelectric element, wherein:

x, y, and z included in the general formula {Li x (K 1-y Na y ) 1-x }{Nb 1-z-w Ta z Sb w }O 3 representing a grain-oriented piezoelectric ceramic satisfy the relationships expressed by the following equations (3) and (4):

9 x −5 z −17 w ≧−318  (3)

−18.9 x −3.9 z −5.8 w ≦−130  (4).

2. The piezoelectric actuator according to claim 1 , wherein when the piezoelectric actuator is driven under such a driving condition that a driving voltage shall have a constant amplitude in electric field strength of 100 V/mm or more, the following requirements (f) and (g) are satisfied:

(f) the magnitude of dynamic strain D33 calculated by dividing a strain exhibited in an electric field-applied direction by the piezoelectric actuator by the electric field strength shall be equal to or larger than 250 pm/V over the temperature range from −30° C. to 80° C.; and

(g) the width of variation W D33 in the magnitude of dynamic strain D33 derived from a change in temperature, which is expressed by the following equation (1), shall fall within ±14% over the temperature range from −30° C. to 80° C.:

W D33 (%)=[{2 ×D 33 max /( D 33 max +D 33 min )}−1]×100  (1)

where D33 max denotes the maximum value of the magnitude of dynamic strain observed at the temperatures ranging from −30° C. to 80° C., and D33 min denotes the minimum value of the magnitude of dynamic strain observed at the temperatures ranging from −30° C. to 80° C.

3. The piezoelectric actuator according to claim 1 , wherein when the piezoelectric actuator is driven under such a driving condition that a driving voltage shall have a constant amplitude in electric field strength of 100 V/mm or more, the following requirements (h) and (i) are satisfied:

(h) the magnitude of dynamic strain D33 calculated by dividing a strain exhibited in an electric field-applied direction by the piezoelectric actuator by the electric field strength shall be equal to or larger than 250 pm/V over the temperature range from −30° C. to 160° C.; and

(i) the width of variation W D33 in the magnitude of dynamic strain derived from a change in temperature, which is expressed by the following general equation (2), shall fall within ±14% over the temperature range from −30° C. to 160° C.:

W D33 (%)=[{2 ×D 33 max /( D 33 max +D 33 min )}−1]×100  (2)

where D33 max denotes the maximum value of the magnitude of dynamic strain observed at the temperatures ranging from −30° C. to 160° C., and D33 min denotes the minimum value of the magnitude of dynamic strain observed at the temperatures ranging from −30° C. to 160° C.

4. The piezoelectric actuator according to claim 1 , wherein the piezoelectric actuator is adapted to a fuel injection valve.

5. The piezoelectric actuator according to claim 1 , wherein the piezoelectric actuator is adapted to any of a sonar, an ultrasonic motor, and a piezoelectric ultrasonic transducer.

6. The piezoelectric actuator according to claim 1 , wherein the piezoelectric actuator is adapted to a piezoelectric sounding part.

7. The piezoelectric actuator according to claim 1 , wherein the piezoelectric element is a laminated piezoelectric element having sheets of piezoelectric ceramic and electrodes stacked alternately.

8. The piezoelectric actuator according to claim 1 , wherein the piezoelectric ceramic is a piezoelectric ceramic that does not contain lead.

9. The piezoelectric actuator according to claim 1 , wherein the orientation degree of the pseudo-cubic {100} planes of grains constituting the grain-oriented piezoelectric ceramic, which is calculated according to a Lotgering's method, is equal to or larger than 30%, and the crystal system is the tetragonal system over a temperature range from 10° C. to 160° C.

10. The piezoelectric actuator according to claim 2 , wherein when the piezoelectric actuator is driven under such a driving condition that a driving voltage shall have a constant amplitude in electric field strength of 100 V/mm or more, the following requirements (h) and (i) are satisfied:

(h) the magnitude of dynamic strain D33 calculated by dividing a strain exhibited in an electric field-applied direction by the piezoelectric actuator by the electric field strength shall be equal to or larger than 250 pm/V over the temperature range from −30° C. to 160° C.; and

(i) the width of variation W D33 in the magnitude of dynamic strain derived from a change in temperature, which is expressed by the following general equation (2), shall fall within ±14% over the temperature range from −30° C. to 160° C.:

W D33 (%)=[{2 ×D 33 max /( D 33 max +D 33 min )}−1]×100  (2)

where D33 max denotes the maximum value of the magnitude of dynamic strain observed at the temperatures ranging from −30° C. to 160° C., and D33 min denotes the minimum value of the magnitude of dynamic strain observed at the temperatures ranging from −30° C. to 160° C.

11. The piezoelectric actuator according to claim 10 , wherein the orientation degree of the pseudo-cubic {100} planes of grains constituting the grain-oriented piezoelectric ceramic, which is calculated according to a Lotgering's method, is equal to or larger than 30%, and the crystal system is the tetragonal system over a temperature range from 10° C. to 160° C.

12. The piezoelectric actuator according to claim 1 , wherein

the piezoelectric ceramic satisfies at least one of the following requirements (a) to (e):

(a) the bulk density shall be equal to or smaller than 5 g/cm 3 , and the Young's modulus Y 11 E calculated according to a resonance-antiresonance method shall be equal to or larger than 90 GPa;

(b) the coefficient of thermal conductivity shall be equal to or larger than 2 Wm −1 K −1 ;

(c) the coefficient of thermal expansion shall be equal to or larger than 3.0 ppm/° C. over a temperature range from −30° C. to 160° C.;

(d) the pyroelectric coefficient shall be equal to or smaller than 400 μCm −2 K −1 over the temperature range from −30° C. to 160° C.; and

(e) the mechanical quality factor Qm calculated according to the resonance-antiresonance method shall be equal to or smaller than 50 over a temperature range from −30° C. to 80° C.