Piezoelectric ceramic composition and piezoelectric element containing the same

Abstract

The present invention provides a piezoelectric ceramic composition that can be manufactured easily by an ordinary sintering method, is free from lead, and has a large frequency constant, a small grain size, and a high mechanical Q. This composition is expressed by a formula of (1−v−w)[(Li1-x-yNaxKy)zRO3].vLMnO3.wBi½Na½TiO3. The present invention also provides a piezoelectric ceramic composition, including at least one selected from Mn, Cr, and Co that is added to a composition expressed by a formula of (Li1-x-yNaxKy)zRO3 so as to have a ratio thereof to the whole of 0.01 to 1 wt % in terms of MnO2, Cr2O3, and CoO, respectively. In the above-mentioned formulae, 0≦v≦0.05, 0≦w≦0.2, v+w>0, 0.2≦x≦0.97, 0≦y≦0.8, x+y≧0.8, 0.98≦z≦1, L indicates at least one element selected from the rare earth elements, and R denotes at least one element selected from Nb, Ta, and Sb.

Description

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates generally to piezoelectric ceramic compositions that are useful as materials for piezoelectric elements such as piezoelectric ceramic filters, piezoelectric ceramic vibrators, actuators, piezoelectric transformers, various types of sensors, or the like.

[0003] 2. Related Background Art

[0004] Conventionally, as piezoelectric ceramic materials, lead titanate-based ceramics containing PbTiO3 as a main component, lead zirconate titanate-based ceramics containing Pb(Ti, Zr)O3 as a main component, and a multicomponent piezoelectric ceramic that includes some kinds of complex perovskite compositions, for example, Pb(Mg⅓Nb⅔)O3, Pb(Ni⅓Nb⅔)O3 have been used. With these compositions, piezoelectric ceramics having different characteristics in accordance with the intended uses can be obtained through the suitable selection of a composition ratio of the components thereof. These piezoelectric ceramics are used for ceramic filters, piezobuzzers, piezoelectric spark plugs, ultrasonic piezoelectric transducers, or the like.

[0005] However, conventional lead zirconate titanate piezoelectric ceramics have a small frequency constant that is about 2000 Hz·m. Hence, when using such a piezoelectric ceramics, a resonator in which thickness longitudinal vibration occurs in the frequency band of about 10 MHz or higher has an element thickness of 0.2 mm or less, which makes the manufacture thereof difficult. In addition, the above-mentioned conventional materials contain a large amount of lead as a main component, which is not desirable from the viewpoint of environmental protection. The above-mentioned conventional materials have large grain sizes and thus it is difficult to use them for vibrators used at high frequencies. Furthermore, it is desired that piezoelectric ceramics have a high mechanical quality factor Qm (hereinafter referred to as “mechanical Q”).

SUMMARY OF THE INVENTION

[0006] Hence, an object of the present invention is to provide a piezoelectric ceramic composition that is manufactured easily by an ordinary sintering method, is free from lead, and has a large frequency constant, a small grain size, and a high mechanical Q at least in its preferable embodiments. Another object of the present invention is to provide a piezoelectric element including this piezoelectric ceramic composition.

[0007] From one aspect of the present invention, a first piezoelectric ceramic composition is expressed by a formula of (1−v−w)[(Li1-x-yNaxKy)zRO3].vLMnO3.wBi½Na½TiO3.

[0008] In the above formula, 0≦v≦0.05, 0≦w≦0.2, v+w>0, 0.2≦x≦0.97, 0≦y≦0.8, x+y≧0.8, 0.98≦z≦1, L indicates at least one element selected from the rare earth elements (Sc, Y, and lanthanide series elements), and R denotes at least one element selected from Nb, Ta, and Sb.

[0009] From another aspect of the present invention, a second piezoelectric ceramic composition includes at least one selected from Mn, Cr, and Co that is added to a composition expressed by a formula of (Li1-x-yNaxKy)zRO3 so as to have a ratio thereof to the whole of 0.01 to 1 wt % in terms of MnO2, Cr2O3, and CoO, respectively.

[0010] In the formula, x, y, x+y, and z are values in the same range as in the above, and L and R are the same elements as those described above.

[0011] The present invention provides a piezoelectric element including the piezoelectric ceramic composition.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIG. 1 is a plan view showing an example of a piezoelectric element according to the present invention.

[0013] FIG. 2 is a sectional view showing an example of a piezo-resonator according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0014] Preferable embodiments of the present invention are described as follows.

[0015] In the first piezoelectric ceramic composition of the present invention, when 0.001≦v≦0.05, an effect of improving the mechanical Q can be obtained to a sufficiently high degree. In a use, for example, in a vibrator, it is desirable that the mechanical Q be high to reduce losses. The value of v is further preferably at least 0.005 and particularly preferably 0.03 or lower. When v>0, w may be zero (w=0).

[0016] When 0.001≦w≦0.2, the effect of improving the mechanical Q can be obtained to a sufficiently high degree as in the above. The value of w is further preferably at least 0.02 and particularly preferably 0.1 or lower. When w>0, v may be zero (v=0). In addition, the values of v and w may be 0.001≦v≦0.05 and 0.001≦w≦0.2.

[0017] As alkali elements, Na is essential, but only either Li or K may be contained. When K is not contained (y=0), 0.80≦x≦0.97 is preferable in Li1-xNax. This is because the sintering characteristics may deteriorate slightly when the value of x is out of this range. The value of x is further preferably at least 0.83 and particularly preferably 0.93 or lower.

[0018] When Li is not contained (x+y=1), 0.2≦x≦0.8 and 0.2≦y≦0.8 are preferable in NaxKy. This is because the sintering characteristics may deteriorate slightly when the values of x and y are out of these ranges. Both values of x and y are further preferably at least 0.4 and particularly preferably 0.6 or lower.

[0019] When the value of z is lower than 0.98, the effect of improving mechanical Q is not obtained to a sufficiently high degree. Hence, the value of z was set to be at least 0.98. In this case, the value of z is set to 1 or lower, and thereby it is intended to produce no excess alkali compound. Moisture resistance of ceramics to be obtained is influenced on their electrical properties if all of the alkali components cannot be included as a solid solution with a perovskite structure.

[0020] Preferably, L indicates at least one element selected from Y, Er, Ho, Tm, Lu, and Yb. Preferably, R includes at least Nb and further preferably includes Nb as a main component. Here, the “main component” denotes an element accounting for the highest ratio among the elements to be considered R.

[0021] Similarly in the second piezoelectric ceramic composition, it is preferable to set the values of x and y for defining the ratios of alkali elements to be in the same ranges as those described above. In addition, with respect to the elements L and R, the same elements as those described above are preferable.

[0022] Furthermore, at least one selected from Mn, Cr, and Co may be added to the first piezoelectric ceramic composition so as to have a ratio thereof to the whole of 0.01 to 1 wt % in terms of MnO2, Cr2O3, and CoO, respectively. The amount of such metal to be added is further preferably at least 0.1 wt % and particularly preferably 0.5 wt % or less.

[0023] A composition of the present invention can be used, for instance, for an energy-trapping-type resonator as shown in FIG. 1. This piezoelectric element 1 is composed of a piezoelectric ceramics 2 and electrodes 3. For instance, Cr—Au electrodes may be used as the electrodes. Thus, a piezoelectric element used in a high frequency band can be provided.

[0024] A composition of the present invention also may be used for a piezo-resonator as shown in FIG. 2. This piezo-resonator 4 includes a capacitative element 5 together with a piezoelectric ceramic 2 and they are connected to each other with an electrically conductive adhesive 6 in the state where electrodes 3 are formed. For example, a barium titanate ceramic can be used as the capacitative element. An adhesive mainly containing silver powder and epoxy resin may be used as the electrically conductive adhesive, for instance. The variation in temperature change rate of capacitance of the capacitive element allows the temperature characteristic of resonant frequency of the piezo-resonator to be controlled. Here, the operation mode of the above-mentioned piezoelectric element and piezo-resonator are not limited to a thickness extensional vibration mode but may be a thickness shear mode.

[0025] The piezoelectric ceramic compositions of the present invention are particularly suitable for low-loss resonators used at high frequencies since they have a high mechanical Q and a small grain size. In addition, the frequency constant reaches about 1.5 times that of a conventional lead-zirconate-titanate-based piezoelectric ceramic composition. Hence, in a resonator that is operated in a thickness extensional vibration mode at the same frequency as that used by a conventional resonator, its thickness increases to about 1.5 times the conventional one. Thus, a particularly useful effect can be obtained in the application of the piezoelectric ceramic compositions according to the present invention in a high-frequency band. The piezoelectric ceramic compositions of the present invention have a high coupling coefficient and thus are particularly suitable for various sensors such as an acceleration sensor, actuators, or the like. Furthermore, the piezoelectric ceramic compositions of the present invention do not contain lead and therefore also are preferable from the viewpoint of environmental protection.

EXAMPLE

Example 1

[0026] Initially, Nb2O5, Li2CO3, Na2CO3, K2CO3, Y2O3, Er2O3, Ho2O3, Tm2O3, Lu2O3, Yb2O3, CoO, Cr2O3, and Mn3O4 were prepared as raw materials, and powders of raw materials suitably selected from them were weighed to have composition ratios shown in Tables 1 and 2. Next, the raw material powders thus weighed were mixed with ethanol for 20 hours using a ball mill, and then the mixture was dried and was calcined at 750 to 1100° C. for two hours. After being coarsely pulverized, the calcined body thus obtained was further pulverized with ethanol for 15 hours in a ball mill, which then was dried. Afterward, an organic binder was added thereto and the mixture was granulated, which then was formed under a pressure of 70 MPa into a disc-like green compact with a diameter of 13 mm and a thickness of 1 mm. This was sintered at a temperature between 900° C. and 1250° C. for one hour. After sintering, each of the ceramics with the highest density among the respective compositions was ground to have a thickness of 0.35 mm and then Cr—Au was deposited on both faces thereof to form electrodes. This element thus obtained was subjected to polarization through application of a direct electric field of 3 to 7 kV/mm between the electrodes in silicone oil with a temperature of 100 to 200° C. for 30 minutes.

[0027] The ceramic compositions prepared by the steps described above were subjected to measurements of average grain size, dielectric constant, thickness-longitudinal-vibration electromechanical coupling coefficient kt, and mechanical Q. The results are shown in Tables 1 and 2.

[0028] In the present example, since the ceramic compositions are free from Bi½Na½TiO3 (w=0), the compositions can be expressed by a formula of (1−v)[(Li1-x-yNaxKy)zRO3].vLMnO3. Tables 1 and 2 show the results with respect to a composition containing Li and Na as alkali elements (y=0) and the results with respect to a composition containing Na and K as alkali elements (x+y=1), respectively. The added materials shown in the tables were added so that their ratios to the whole were equal to the values indicated in the tables.

TABLE 1
(1 − v)[(Li1 · xNax)RO3] · vLMnO3
	Type of
	Added	Grain	Dielec-	Coupling	Mechan-
Sample	Composition	Material	Size	tric	Coefficient	ical
Nos.	z	x	L	v	(wt %)	[μm]	Constant	kt	Q
1*	0.99	0.88	—	—	—	46	116	0.32	290
2	0.99	0.88	Y	0.001	—	13	121	0.35	420
3	0.99	0.88	Y	0.005	—	2.3	125	0.38	810
4#	0.99	0.75	Y	0.02	—	1.5	113	0.22	160
5	0.99	0.80	Y	0.02	—	1.4	118	0.29	310
6	0.99	0.83	Y	0.02	—	1.4	124	0.41	430
7	0.99	0.88	Y	0.02	—	1.5	133	0.44	920
8	0.99	0.93	Y	0.02	—	1.6	127	0.42	670
9	0.99	0.97	Y	0.02	—	1.7	119	0.38	460
10*	0.99	0.99	Y	0.02	—	2.4	105	0.22	180
11	0.99	0.88	Y	0.05	—	1.4	244	0.35	450
12*	0.99	0.88	Y	0.08	—	1.7	228	0.34	290
13	0.99	0.88	Er	0.01	—	3.7	111	0.39	550
14	0.99	0.88	Er	0.02	—	1.1	122	0.45	860
15	0.99	0.88	Ho	0.005	—	5.6	115	0.41	510
16	0.99	0.88	Ho	0.02	—	1.3	113	0.48	790
17	0.99	0.88	Tm	0.01	—	2.5	115	0.42	1020
18	0.99	0.88	Lu	0.01	—	5.1	123	0.34	740
19	0.99	0.88	Yb	0.02	—	2.3	119	0.48	780
20*	1.02	0.88	Y	0.02	—	6.2	121	0.26	280
21	1.00	0.88	Y	0.02	—	4.5	127	0.42	820
22	0.98	0.88	Y	0.02	—	5.2	117	0.39	570
23*	0.96	0.88	Y	0.02	—	5.5	109	0.23	170
24	0.99	0.88	—	—	MnO2(0.2)	5.1	117	0.34	420
25	0.99	0.88	—	—	Cr2O3(0.2)	2.3	119	0.37	370
26	0.99	0.88	—	—	CoO (0.2)	7.3	113	0.40	380
27	0.99	0.88	Y	0.001	MnO2(0.2)	1.1	117	0.40	1280
28	0.99	0.88	Er	0.005	Cr2O3(0.2)	0.9	116	0.39	1190
29	0.99	0.88	Yb	0.02	CoO (0.2)	1.2	121	0.42	1120

[0029]

TABLE 2
(1 − v)[NaxKy)zRO3] · vLMnO3 (x + y = 1)
	Type of
	Added	Grain	Dielec-	Coupling	Mechan-
Sample	Composition	Material	Size	tric	Coefficient	ical
Nos.	z	y	L	v	(wt %)	[μm]	Constant	kt	Q
30*	0.99	0.45	—	—	—	17	479	0.32	87
31	0.99	0.45	Y	0.001	—	1.4	465	0.36	220
32	0.99	0.45	Y	0.005	—	1.0	473	0.41	430
33#	0.99	0.10	Y	0.02	—	1.8	152	0.18	140
34	0.99	0.20	Y	0.02	—	1.2	232	0.31	370
35	0.99	0.40	Y	0.02	—	0.8	392	0.35	380
36	0.99	0.45	Y	0.02	—	0.6	454	0.42	510
37	0.99	0.60	Y	0.02	—	1.1	385	0.32	360
38	0.99	0.80	Y	0.02	—	1.5	214	0.25	310
39#	0.99	0.90	Y	0.02	—	2.2	146	0.22	280
40	0.99	0.45	Y	0.05	—	1.2	483	0.38	380
41	0.99	0.45	Er	0.01	—	1.8	492	0.46	330
42	0.99	0.45	Er	0.02	—	0.5	481	0.41	420
43	0.99	0.45	Ho	0.01	—	1.9	509	0.37	220
44	0.99	0.45	Ho	0.02	—	0.9	501	0.35	310
45	0.99	0.45	Tm	0.01	—	1.5	488	0.42	450
46	0.99	0.45	Lu	0.02	—	1.1	494	0.37	390
47	0.99	0.45	Yb	0.02	—	0.8	499	0.38	380
48	1.00	0.45	Yb	0.02	—	2.5	512	0.36	220
49	0.98	0.45	Yb	0.02	—	4.1	487	0.34	230
50	0.99	0.45	—	—	MnO2(0.2)	2.8	475	0.37	420
51	0.99	0.45	—	—	Cr2O3(0.2)	2.1	491	0.34	230
52	0.99	0.45	—	—	CoO (0.2)	3.3	499	0.38	260
53	0.99	0.45	Y	0.02	MnO2(0.2)	0.6	487	0.44	780
54	0.99	0.45	Er	0.02	CrO3(0.2)	0.7	501	0.43	650
55	0.99	0.45	Yb	0.02	CoO (0.2)	0.6	512	0.42	510

[0030] In Tables 1 and 2, the samples indicated with the mark * are piezoelectric ceramic compositions of comparative examples that are out of the range of the present invention. In the samples indicated with the mark #, the values of x and y are out of the preferable ranges.

[0031] All the above-mentioned samples had a frequency constant Nt of thickness longitudinal vibration in a range of 2700 to 3200 Hz·m. This corresponds to about 1.5 times the value of conventional lead-zirconate-titanate-based piezoelectric ceramics.

[0032] As is apparent from the aforementioned results, with the application of the present invention, piezoelectric ceramic compositions were obtained that had an average grain size of 15 μm or less, preferably 10 μm or less, and a mechanical Q of at least 200, preferably at least 300.

Example 2

[0033] Initially, Li2CO3, Na2CO3, Nb2O5, Y2O3, Er2O3, Ho2O3, Tm2O3, LU2O3, Yb2O3, Bi2O3, TiO2, CoO, Cr2O3, and Mn3O4 were prepared as raw materials, and powders of raw materials suitably selected from them were weighed to have composition ratios shown in Table 3. Next, the raw material powders thus weighed were mixed with ethanol for 20 hours using a ball mill, and then the mixture was dried at 120° C. for 15 hours and was calcined at 1000° C. for two hours. After being coarsely pulverized, the calcined body thus obtained was further pulverized with ethanol for 15 hours in a ball mill, which then was dried at 120° C. for 15 hours. Afterward, an organic binder was added thereto and the mixture was granulated, which then was formed under a pressure of 70 MPa into a disc-like green compact with a diameter of 13 mm and a thickness of 1 mm. This was sintered at a temperature between 1000° C. and 1250° C. for one hour. After sintering, each of the ceramics with the highest density among the respective compositions was ground to have a thickness of 0.35 mm and then Cr—Au was deposited on both faces thereof to form electrodes. This element thus obtained was subjected to polarization through application of a direct electric field of 4 kV/mm between the electrodes in silicone oil with a temperature of 150° C. for 30 minutes.

[0034] The piezoelectric ceramic compositions prepared by the steps described above were subjected to measurements of average grain size, dielectric constant, thickness-longitudinal-vibration electromechanical coupling coefficient kt, and mechanical Q. The results are shown in Table 3. The added materials shown in the table were added so that their ratios to the whole were equal to the values indicated in the table.

TABLE 3
(1 − v − w)[Li1 · x · yNaxKy)zRO3] ·
vLMnO3 · wBi1/2TiO3
	Type of
	Added	Grain	Dielec-	Coupling	Mechan-
Sample	Composition	Material	Size	tric	Coefficient	ical
Nos.	z	x	W	L	v	(wt %)	[μm]	Constant	kt	Q
56*	0.99	0.88	—	—	—	—	46	116	0.32	290
57	0.99	0.88	0.001	—	—	—	13	109	0.32	340
58	0.99	0.88	0.005	—	—	—	6.5	119	0.33	380
59	0.99	0.88	0.02	—	—	—	4.5	186	0.34	420
60	0.99	0.88	0.05	—	—	—	2.5	228	0.37	450
61	0.99	0.88	0.1	—	—	—	2.1	213	0.35	610
62	0.99	0.88	0.2	—	—	—	0.9	280	0.33	520
63*	0.99	0.88	0.25	Y	0.02	—	1.3	354	0.32	260
64	0.99	0.88	0.1	Y	0.001	—	1.6	192	0.36	740
65	0.99	0.88	0.1	Y	0.005	—	1.2	208	0.38	1050
66	0.99	0.88	0.1	Y	0.02	—	1.0	221	0.40	1100
67	0.99	0.88	0.1	Y	0.05	—	1.1	246	0.36	630
68	0.99	0.88	0.1	Er	0.01	—	1.0	223	0.40	870
69	0.99	0.88	0.1	Er	0.02	—	0.8	241	0.43	930
70	0.99	0.88	0.1	Ho	0.01	—	1.7	214	0.38	890
71	0.99	0.88	0.1	Ho	0.02	—	1.2	257	0.46	1110
72	0.99	0.88	0.1	Tm	0.01	—	1.6	222	0.43	1420
73	0.99	0.88	0.1	Lu	0.01	—	1.5	235	0.37	850
74	0.99	0.88	0.1	Yb	0.02	—	1.0	231	0.46	1030
75	1.00	0.88	0.05	—	—	—	2.4	209	0.33	430
76	0.98	0.88	0.05	—	—	—	2.1	212	0.35	520
77	0.99	0.88	0.1	—	—	MnO2(0.2)	0.9	215	0.34	1150
78	0.99	0.88	0.1	—	—	Cr2O3(0.2)	1.3	224	0.36	850
79	0.99	0.88	0.1	—	—	CoO (0.2)	1.7	237	0.40	890
80	0.99	0.88	0.05	Y	0.02	MnO2(0.2)	1.1	171	0.41	1430
81	0.99	0.88	0.05	Er	0.02	Cr2O3(0.2)	1.0	232	0.41	1280
82	0.99	0.88	0.05	Yb	0.02	CoO (0.2)	1.4	263	0.40	1320

[0035] All the above-mentioned samples had a frequency constant Nt of thickness longitudinal vibration in a range of 2700 to 3200 Hz·m. This corresponds to about 1.5 times the value of conventional lead-zirconate-titanate-based piezoelectric ceramics.

[0036] As is apparent from the aforementioned results, with the application of the present invention, piezoelectric ceramic compositions were obtained that had an average grain size of 13 μm or less, preferably 10 μm or less, and a mechanical Q of at least 300.

[0037] The invention may be embodied in other forms without departing from the spirit or essential characteristics thereof. The embodiments disclosed in this application are to be considered in all respects as illustrative and not limiting. The scope of the invention is indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein.

Claims

1. A piezoelectric ceramic composition, expressed by a formula of (1−v−w)[(Li1-x-yNaxKy)zRO3].vLMnO3.wBi½Na½TiO3, wherein 0≦v≦0.05, 0≦w≦0.2, v+w>0, 0.2≦x≦0.97, 0≦y≦0.8, x+y≧0.8, 0.98≦z≦1, L indicates at least one element selected from rare earth elements, and R denotes at least one element selected from Nb, Ta, and Sb.

2. The piezoelectric ceramic composition according to claim 1, wherein 0.0010≦v≦0.05.

3. The piezoelectric ceramic composition according to claim 2, wherein w=0.

4. The piezoelectric ceramic composition according to claim 1, wherein 0.001≦w≦0.2.

5. The piezoelectric ceramic composition according to claim 4, wherein v=0.

6. The piezoelectric ceramic composition according to claim 1, wherein 0.001≦v≦0.05 and 0.001≦w≦0.2.

7. The piezoelectric ceramic composition according to claim 1, wherein 0.80≦x≦0.97 and y=0.

8. The piezoelectric ceramic composition according to claim 1, wherein 0.2≦x≦0.8, 0.2≦y≦0.8, and x+y=1.

9. The piezoelectric ceramic composition according to claim 1, wherein L indicates at least one element selected from Y, Er, Ho, Tm, Lu, and Yb.

10. The piezoelectric ceramic composition according to claim 1, wherein R comprises Nb.

11. The piezoelectric ceramic composition according to claim 1, wherein the piezoelectric ceramic composition comprises at least one selected from Mn, Cr, and Co that is added thereto so as to have a ratio thereof to the whole of 0.01 to 1 wt % in terms of MnO2, Cr2O3, and CoO, respectively.

12. A piezoelectric ceramic composition, comprising at least one selected from Mn, Cr, and Co that is added to a composition expressed by a formula of (Li1-x-yNaxKy)zRO3 so as to have a ratio thereof to the whole of 0.01 to 1 wt % in terms of MnO2, Cr2O3, and CoO, respectively, wherein 0.2≦x≦0.97, 0≦y≦0.8, x+y≧0.8, 0.98≦z≦1, L indicates at least one element selected from rare earth elements, and R denotes at least one element selected from Nb, Ta, and Sb.

13. The piezoelectric ceramic composition according to claim 12, wherein 0.80≦x≦0.97 and y=0.

14. The piezoelectric ceramic composition according to claim 12, wherein 0.2≦x≦0.8, 0.2≦y≦0.8, and x+y=1.

15. The piezoelectric ceramic composition according to claim 12, wherein L indicates at least one element selected from Y, Er, Ho, Tm, Lu, and Yb.

16. The piezoelectric ceramic composition according to claim 12, wherein R comprises Nb.

17. A piezoelectric element, comprising a piezoelectric ceramic composition according to claim 1.

18. The piezoelectric element according to claim 17, further comprising a capacitative element.

19. A piezoelectric element, comprising a piezoelectric ceramic composition according to claim 12.

20. The piezoelectric element according to claim 19, further comprising a capacitative element.