Piezoelectric ceramic material

Abstract

The present invention relates to a piezoelectric material comprising a ceramic material with the composition (1-x)((Bi(a-y)REy)FeO3) – x(BabTiO3), wherein the molar ratios x and y fulfill 0.28 ≤ x ≤ 0.34 and 0.0005 ≤ y ≤ 0.032; and to a ceramic material with the composition (BiaBab)(FecTidZre)O3, wherein the molar ratios a, b, c, d and e fulfill 0.600 ≤ a ≤ 0.780, 0.245 ≤ b ≤ 0.410, 0.590 ≤ c ≤ 0.752, 0.0003 ≤ e ≤ 0.0153 and c+d+e = 1.

Description

[0001] Description

[0002] Piezoelectric ceramic material

[0003] The present invention concerns several aspects of a piezoelectric material and a ceramic composition.

[0004] In particular, the present invention is focused on lead-free materials which may substitute known materials such as PZT ceramics without losses in the piezoelectric properties.

[0005] Furthermore, it is desired that the application of a direct current bias is not required when applying the piezoelectric material and that the piezoelectric properties are not or only hardly influenced by thermal stress in harsh environments between -50 °C and +200 °C.

[0006] The piezoelectric coefficients shall be as high as possible.

[0007] State of the art materials are described in, e.g., Japanese unexamined publication JP 2009 298 621 A.

[0008] The main features of the present invention are defined by the claims .

[0009] In particular, an aspect of the present invention concerns a piezoelectric material comprising a ceramic material with the following composition:

[0010] (1-x) ( (Bi(a-y)REy) FeO3) - x(BabTiO3)

[0011] The ceramic material is a BF-BT material (also known as BFO- BT) , which is a ceramic oxide material based on bismuth (Bi) , iron (Fe) , barium (Ba) and titanium (Ti) . A defined amount x of Fe in the composition is substituted by Ti.

[0012] The ceramic material comprises further a defined ratio of one or several rare earth element (s) , which may be lanthanum (La) , any lanthanoid, lutetium (Lu) or yttrium (Y) , and which substitutes a defined amount y of bismuth in the ceramic material .

[0013] The group of lanthanoids comprises Cerium (Ce) , Praseodymium (Pr) , Neodymium (Nd) , Promethium (Pm) , Samarium (Sm) , Europium (Eu) , Gadolinium (Gd) , Terbium (Tb) , Dysprosium (Dy) , Holmium (Ho) , Erbium (Er) , Thulium (Tm) and Ytterbium (Yb) .

[0014] In this composition, a, b, x and y are molar ratios.

[0015] According to the invention, the molar ratios x and y fulfill 0.28 < x < 0.34 and 0.0005 < y < 0.032.

[0016] The molar ratios a and b are not further limited according to the invention.

[0017] Preferable ranges for a and b, according to the invention, are : 0.95 < a < 1.1 and 0.95 < b < 1.1; preferably 1 < a d 1.1 and 1 < b < 1.1; more preferably 1 < a < 1.05 and 1 < b < 1.05.

[0018] In a preferred embodiment according to the invention a = 1.04 and / or b = 1.007. Preferably, the composition belongs to the perimeter and the interior of a polygon having the following eight points Pl to P8 (x; y) as vertices with respect to the values of x and y: Pl = (0.301; 0.032) ; P2 = (0.304; 0.004) ; P3 = (0.304; 0.0315) ; P4 = (0.310; 0.003) ; P5 = (0.310; 0.029) ; P6 = (0.314; 0.026) ; P7 = (0.327; 0.005) ; P8 = (0.329; 0.0005) .

[0019] The corresponding polygon is shown in figure 1. The points Pl to P8 are labeled by the numbers 1 to 8.

[0020] Herein, x, as also defined above, corresponds to the molar ratio of Ti versus the sum of iron and titanium in the ceramic composition: x = Ti / (Fe+Ti)

[0021] Herein, y, as also defined above, corresponds to the molar ratio of La versus Fe in the ceramic composition: y = La / Fe

[0022] As described later, a ceramic material with the above-defined composition has advantages in terms of high coercive force, suppression of polarization reversal and good strain properties .

[0023] In an embodiment RE is lanthanum, since the piezoelectric properties can be further improved. In an embodiment the ceramic material comprises further Manganese (Mn) as a dopant.

[0024] Preferably, the ceramic material comprises between 0.05 and 0.2 weight-% of MnCt as a dopant, which can help to improve the piezoelectric properties of the ceramic material.

[0025] According to an embodiment, a defined molar amount of the sum of Fe and Ti in the ceramic composition may also be substituted by Zirconium (Zr, see also aspect 2) .

[0026] In an embodiment, the piezoelectric material consists of the ceramic material as defined before.

[0027] The piezoelectric material according to the invention may advantageously have a coercive force (= coercive field) Ec of 2.00 kV / mm or higher as is also shown by table 1 below.

[0028] This is particularly achieved by a composition according to the polygon as defined above, wherein RE is La.

[0029] Advantageously, no polarization reversal of the piezoelectric material may occur when an electric alternating current (AC) field E between -2 kV / mm (inclusive) and +2 kV / mm (inclusive, in other words with an amplitude of | 2 | kV / mm) is applied to the piezoelectric material, since the coercive force is high enough .

[0030] This is particularly achieved by a composition according to the polygon as defined above, wherein RE is La. Advantageously, a maximum strain of the piezoelectric material in at least one spatial direction may amount at least 0 . 15 % or more when an electric AC field E between -2 kV / mm and +2 kV / mm (boundaries inclusive ) is applied or vice versa .

[0031] The high strain is in particular an advantage compared to undoped BF-BT materials without RE .

[0032] Herby, advantageously, no direct current bias is or has to be applied to the piezoelectric material since the coercive force is high enough and no polari zation reversal occurs .

[0033] Since no direct current bias has to be applied, energy can be saved .

[0034] In other words , the amount of the strain is preferably independent of the polarity of the applied electric field . This is in particular an advantage compared to PZT ( Pb-Zr- TiOa, lead- zirconium-titanate ) ceramics , where the application of negative electric fields should be avoided .

[0035] These properties are particularly achieved by a composition according to the polygon as defined above , wherein RE is La .

[0036] Advantageously, the piezoelectric coef ficient d33 (non-linear behavior ) is not constant between - 2 kV / mm and +2 kV / mm but changes slightly depending on the applied electric field . Thus , a higher strain of the piezoelectric material can be achieved compared to a material with constant d33 ( linear behavior ) . For further illustration of the invention, table 1 shows 11 particular examples Example 1 to Example 11 of piezoelectric materials consisting of the ceramic material according to the invention and without added doping materials. The molar amounts x and y are defined in the table. The amount of a is 1.04 and b is 1.007. As can be seen in the table, the amount of the coercive force Ec is every time 2 kV / mm or higher. Further, when applying an electric alternating current field in the range between -2 and + 2 kV / mm (boundaries inclusive) the maximum strain of the material is 0.15 % or higher. During the measurements, no polarization reversals occurred.

[0037] Besides, Table 1 shows 6 comparative examples Comparative 1 to Comparative 6. As can be seen from table 1, for the comparative examples, the coercive force Ec is lower than 2 kV / mm or the maximum strain of the material at an application of - / +2 kV / mm is lower than 0.15 %. Since in several comparative examples Ec is lower than 2, several polarization reversals occur.

[0038] Table 1 :

[0039] Exemplary curves, showing the dependence of the strain S of the electric field E are shown in figures 2 and 3.

[0040] Figure 2 shows for comparison a typical curve of a PZT material when a direct current bias is applied. Figure 3 shows a curve according to the invention, when no direct current bias (DC-bias) is applied.

[0041] Figure 4 shows a simple schematic example of a piezoelectric material. The material forms a body 1 with a cylindrical shape. When an electric AC field is applied to the body in the z direction, a strain S of the body in the z direction occurs (d33-coupling) . In the meaning of the present text, the strain S can comprise an elongation or a compression. Besides also a strain in the x direction vertical to the z direction may occur (not shown, d31-coupling) .

[0042] A second aspect of the invention concerns a piezoelectric material comprising a ceramic material which is defined by the composition (BiaBab) ( FecTidZre) O3.

[0043] The ceramic material is a BF-BT material, which is a ceramic oxide material based on bismuth (Bi) , iron (Fe) , barium (Ba) and titanium (Ti) .

[0044] In the formula above, the molar ratios a, b, c, d and e fulfill 0.600 < a < 0.780, 0.245 < b < 0.410, 0.590 < c < 0.752, 0.0003 < e d 0.0153 and c+d+e = 1. In particular, a defined molar amount e of the sum of Fe and Ti (c+d) is substituted by Zirconium (Zr) .

[0045] According to an embodiment, the ceramic material may further comprise Lanthanum (La) as a dopant. Preferably, the ceramic material comprises not more than 1 weight-% of Lanthanum as a dopant .

[0046] According to embodiment, the ceramic material may further comprise a defined ratio of one or several rare earth element (s) , which may be lanthanum (La) , any lanthanoid, lutetium (Lu) or yttrium (Y) , and which substitutes a defined amount of bismuth in the ceramic material, according to aspect 1 (see above) . Aspect 1 and 2 may concern the same ceramic composition.

[0047] According to an embodiment, the piezoelectric material may consist of the ceramic material.

[0048] Advantageously, the piezoelectric coefficient -d31 of the piezoelectric material amounts at least 50 pm / V or more as can be seen in table 2. In other words, the amount of -d31 is equal to or higher than 50 pm / V.

[0049] This is particularly achieved by a ceramic composition according to the second aspect as defined above.

[0050] Advantageously, a change in the amount of the piezoelectric coefficient -d31 is 15 % or less, when a thermal shock test is applied to the piezoelectric material. During the thermal shock test the ambient temperature of the piezoelectric material is changed 90 times from -50 ° C to +200 ° C and back from +200 ° C to -50 ° C .

[0051] This is particularly achieved by a ceramic composition according to the second aspect as defined above .

[0052] Thus , the piezoelectric material according to the second aspect is particularly advantageous for the usage in applications under high temperature stress like automotive sensors .

[0053] For further illustration of the invention, table 2 shows 16 particular examples Example 1 to Example 16 of piezoelectric materials consisting of the ceramic material according to the invention and without added doping materials . The molar amounts a to e are defined in the table . As can be seen in the table , the amount of the piezoelectric coef ficient -d31 is every time 50 pm / V or higher and the ratio of change of - d31 after thermal shock test is every time 15 % or lower .

[0054] Besides , Table 2 shows 8 comparative examples Comparative 1 to Comparative 8 . As can be seen from table 2 , for the comparative examples , the amount of the piezoelectric coef ficient -d31 is every time less than 50 pm / V or the ratio of change of -d31 after thermal shock test is higher than 15

[0055] Table 2 :

[0056] Reference signs

[0057] 1 Piezoelectric body E electric field

[0058] S strain

[0059] DC-bias direct current bias

Claims

Patent claims1. Piezoelectric material comprising a ceramic material with the composition (1-x) ( (Bi(a-y)REy) FeO3) - x(BabTiO3) , wherein the molar ratios x and y fulfill 0.28 < x < 0.34 and 0.0005 < y < 0.032; wherein RE is one or more elements of the group of Lanthanum, Cerium, Praseodymium, Neodymium, Promethium, Samarium, Europium, Gadolinium, Terbium, Dysprosium, Holmium, Erbium, Thulium, Ytterbium, Lutetium and Yttrium, and wherein the molar ratios a and b are not further limited.

2. Piezoelectric material according to claim 1 wherein 0.95 < a < 1.1 and wherein 0.95 < b < 1.1.

3. Piezoelectric material according to one of claims 1 or 2, wherein the composition belongs to the perimeter and the interior of a polygon having the following eight points P (x; y) as vertices with respect to the values of x and y:Pl (0.301; 0.032) ;P2 (0.304; 0.004) ;P3 (0.304; 0.0315) ;P4 (0.310; 0.003) ;P5 (0.310; 0.029) ;P6 (0.314; 0.026) ;P7 (0.327; 0.005) ;P8 (0.329; 0.0005) .

4. Piezoelectric material according to one of claims 1 to 3, wherein RE is Lanthanum (La) .

5. Piezoelectric material according to one of claims 1 to 4, wherein the ceramic material comprises further Manganese (Mn) as a dopant.

6. Piezoelectric material according to claim 5, wherein the ceramic material comprises between 0.05 and 0.2 weight-% of MnO2as a dopant.

7. Piezoelectric material according to one of claims 1 to 6, wherein a = 1.04.

8. Piezoelectric material according to one of claims 1 to 7, wherein b = 1.007.

9. Piezoelectric material according to one of claims 1 to 8 consisting of the ceramic material.

10. Piezoelectric material according to one of claims 1 to 9, wherein the coercive force Ec > 2.00 kV / mm.

11. Piezoelectric material according to one of claims 1 to10, wherein no polarization reversal of the piezoelectric material occurs when an electric field E between -2 kV / mm and +2 kV / mm inclusive is applied to the piezoelectric material.

12. Piezoelectric material according to one of claims 1 to11, wherein a maximum strain S of the piezoelectric material in at least one spatial direction amounts at least 0.15 % or more when an electric alternating current field E with an amplitude of | 2 | kV / mm is applied.

13. Piezoelectric material according to claim 12, wherein no direct current bias is applied to the piezoelectric material.

14. Piezoelectric material according to one of claims 1 to 13, wherein the piezoelectric coefficient d33 is not constant between - 2 kV / mm and +2 kV / mm but changes depending on the applied electric field.

15. Piezoelectric material comprising a ceramic material with the composition (BiaBab) ( FecTidZre) O3, wherein the molar ratios a, b, c, d and e fulfill0.600 < a < 0.780, 0.245 < b < 0.410, 0.590 < c < 0.752, 0.0003 < e d 0.0153 and c+d+e = 1.

16. Piezoelectric material according to claim 15, wherein the ceramic material comprises further Lanthanum (La) as a dopant .

17. Piezoelectric material according to claim 16, wherein the ceramic material comprises not more than 1 weight-% of Lanthanum as a dopant.

18. Piezoelectric material according to one of claims 15 to 17 consisting of the ceramic material.

19. Piezoelectric material according to one of claims 15 to18, wherein the piezoelectric coefficient -d31 amounts at least 50 pm / V or more.

20. Piezoelectric material according to one of claims 15 to19, wherein a change in the amount of the piezoelectric coefficient -d31 is 15 % or less, when a thermal shock test is applied to the piezoelectric material, wherein during the thermal shock test the ambient temperature of thepiezoelectric material is changed 90 times between -50 °C and +200 °C and back.

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