Piezoelectric ceramic material

Abstract

Piezoelectric material comprising a ceramic material with the composition (1-x) ((Bi (a-y) RE y ) FeO3) - x (Ba b TiO3), where the molar fractions x and y satisfy the following conditions 0.28 ≤ x ≤ 0.34 and 0.0005 ≤ y ≤ 0.032 ; where RE is one or more elements from the group consisting of lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium and yttrium, where a = 1.04 and where b = 1.007, where the composition belongs to the perimeter and interior of a polygon which has the following eight points P (x; y) as vertices with respect to the values ​​of x and y: P1 = (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).

Description

[0001] The present invention relates to several aspects of a piezoelectric material and a ceramic composition.

[0002] The present invention focuses in particular on lead-free materials that can replace known materials such as PZT ceramics without compromising piezoelectric properties.

[0003] Furthermore, it is desirable that no DC bias voltage is required when using the piezoelectric material and that the piezoelectric properties are not affected or only slightly affected by thermal stresses in harsh environments between -50 °C and +200 °C.

[0004] The piezoelectric coefficients should be as high as possible.

[0005] Materials according to the state of the art are described, for example, in the Japanese unapproved publication JP 2009 298 621 A.

[0006] Another material known from the prior art is, for example, a piezoelectric ceramic 0.71 (Bi 1-x La x ) FeO3-0.29(BaTiO3) with x=0 - 0.05 and doping with 0.6 wt% MnO2 (see C. Zhou [et al.]: Dielectric, ferroelectric and piezoelectric properties of La-substituted BiFeO3-BaTiO3 ceramics. In: Ceramics International, Volume 39, Issue 4, 2013, Pages 4307-4311, ISSN 0272-8842).

[0007] Furthermore, a piezoelectric ceramic 0.7 (Bi 1-x Nd x )FeO3-0,3(BaTiO3) with a doping of 0.1 wt% MnO2, where x is varied between 0.005 and 0.05, is known (see D.Wang [et al.]: Temperature dependent, large electromechanical strain in Nd-doped BiFeO3-BaTiO3 lead-free ceramics. In: Journal of the European Ceramic Society 37 (2017) 1857-1860).

[0008] Also, a piezoelectric ceramic 0.7 (Bi 1-x re x )FeO3-0.29 (BaTiO3) -0.01Bi (Zn 0,5 Hf 0,5)O3 with Re = La and / or Sm, where x is varied between 0.02 and 0.1 is known (see D. Tai [et al.]: BiFeO3-BaTiO3 ferroelectrics: decrypting the mechanism of rare earth doping-induced electrical property discrepancy via scaling behavior and multi-level structure. In: Acta Mater., 262 (2024), Article 119411).

[0009] EP 2 953 177 B1 further discloses a piezoelectric ceramic with the composition (Ba 1-y Bi y ) a (Ti 1-x-z Zr x Fe z ) O3, where: 0.010 ≤ x ≤ 0.060, 0.001 ≤ y ≤ 0.015, 0.001 ≤ z ≤ 0.015, 0.950 ≤ y / z ≤ 1.050, and 0.986 ≤ a ≤ 1.020.

[0010] The present invention is defined in claim 1.

[0011] In particular, the present invention relates to a piezoelectric material comprising a ceramic material with the following composition: (1-x) ((Bi( a-y )RE y ) FeO3) - x (Ba b TiO3)

[0012] The ceramic material is a BF-BT material (also known as BFO-BT), a ceramic oxide material based on bismuth (Bi), iron (Fe), barium (Ba), and titanium (Ti). A specific proportion x of Fe in the composition is replaced by Ti.

[0013] The ceramic material further comprises a specific ratio of one or more rare earth elements, which may be lanthanum (La), any lanthanide, lutetium (Lu) or yttrium (Y), and which replace a specific amount y of bismuth in the ceramic material.

[0014] The lanthanide group includes 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).

[0015] In this composition, a, b, x and y are molar ratios.

[0016] According to the invention, the molar components x and y fulfill 0.28≤x≤0.34 and 0.0005≤y≤0.032.

[0017] The molar components a and b are limited according to the invention as follows.

[0018] The values ​​for a and b are within the ranges: 0.95≤a≤1.1 and 0.95≤b≤1.1; preferably 1 ≤ a ≤ 1.1 and 1 ≤ b ≤ 1.1; preferably 1 ≤ a ≤ 1.05 and 1 ≤ b ≤ 1.05.

[0019] According to the invention, a = 1.04 and b = 1.007.

[0020] According to the invention, the composition includes the perimeter and interior of a polygon with the following eight points P1 to P8 (x; y) as vertices with respect to the values ​​of x and y: P1 = (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).

[0021] The corresponding polygon is in Fig. Figure 1 is shown. Points P1 to P8 are labelled with the numbers 1 to 8.

[0022] Here, x corresponds, as also defined above, to the molar ratio of Ti to the sum of iron and titanium in the ceramic composition: x=Ti / (Fe+Ti)

[0023] Here, y corresponds, as also defined above, to the molar ratio of La to Fe in the ceramic composition: y=La / Fe

[0024] As described later, a ceramic material with the composition defined above has advantages in terms of high coercivity, suppression of reversal, and good elongation properties.

[0025] In one embodiment, RE is lanthanum, as this further improves the piezoelectric properties.

[0026] In one embodiment, the ceramic material additionally comprises manganese (Mn) as a dopant.

[0027] Preferably, the ceramic material comprises between 0.05 and 0.2 wt% MnO2 as a dopant, which can contribute to improving the piezoelectric properties of the ceramic material.

[0028] According to one embodiment, a certain molar amount of the sum of Fe and Ti in the ceramic composition can also be replaced by zirconium (Zr, see also aspect 2).

[0029] In one embodiment, the piezoelectric material consists of a ceramic material as defined above.

[0030] The piezoelectric material according to the invention can advantageously have a coercive force (= coercive field) Ec of 2.00 kV / mm or higher, as can also be seen from Table 1.

[0031] This is achieved in particular by a composition according to the polygon defined above, where RE is La.

[0032] Advantageously, there can be no reversal of the polarity of the piezoelectric material when an alternating electric field E between -2 kV / mm (inclusive) and +2 kV / mm (inclusive, i.e. with an amplitude of |2| kV / mm) is applied to the piezoelectric material, since the coercive force is high enough.

[0033] This is achieved in particular by a composition according to the polygon defined above, where RE is La.

[0034] Advantageously, the maximum strain of the piezoelectric material in at least one spatial direction can be at least 0.15% or more when an alternating electric field E between -2 kV / mm and +2 kV / mm (limits included) is applied or vice versa.

[0035] The high elongation is a particular advantage over undoped BF-BT materials without RE.

[0036] It is advantageous that no DC bias voltage is applied to the piezoelectric material, or that such a voltage does not need to be applied, since the coercive force is high enough and no polarity reversal takes place.

[0037] Since no DC bias voltage needs to be applied, energy can be saved.

[0038] In other words, the magnitude of the strain is preferably independent of the polarity of the applied electric field. This is a particular advantage over PZT ceramics (Pb-Zr-TiO3, lead zirconium titanate), where the application of negative electric fields should be avoided.

[0039] These properties are achieved in particular by a composition according to the polygon defined above, where RE is La.

[0040] Advantageously, the piezoelectric coefficient d33 (nonlinear behavior) is not constant between -2 kV / mm and +2 kV / mm, but changes slightly depending on the applied electric field. This allows for a higher strain of the piezoelectric material compared to a material with a constant d33 (linear behavior).

[0041] To further illustrate the invention, Table 1 shows the eleven specific examples, Example 1 to Example 11, of piezoelectric materials consisting of the ceramic material according to the invention and without the addition of dopants. The molar amounts x and y are defined in the table. The value of a is 1.04 and b is 1.007. As can be seen from the table, the coercive force Ec is 2 kV / mm or more in each case. Furthermore, the maximum strain of the material when an alternating current field is applied in the range between -2 and +2 kV / mm (including the limits) is 0.15% or more. No polarity reversals occurred during the measurements.

[0042] Furthermore, Table 1 shows six comparative examples, from Comparison 1 to Comparison 6. As can be seen from Table 1, in these comparative examples the coercive force Ec is less than 2 kV / mm or the maximum elongation of the material at an application of - / +2 kV / mm is less than 0.15%. Since Ec is less than 2 in several comparative examples, multiple polarity reversals occur. Table 1: x y Ec[kV / mm] Elongation [%] at - / +2 kV / mm Polarization reversal Example 1 0,301 0,0320 2,17 0,150 No Example 2 0,304 0,0040 2,40 0,150 No Example 3 0,304 0,0315 2,13 0,150 No Example 4 0,310 0,0030 2,32 0,150 No Example 5 0,310 0,0290 2,06 0,150 No Example 6 0,314 0,0260 2,02 0,150 No Example 7 0,327 0,0050 2,02 0,150 No Example 8 0,329 0,0005 2,03 0,150 No Example 9 0,310 0,0170 2,18 0,230 No Example 10 0,320 0,0100 2,08 0,215 No Example 11 0,320 0,0150 2,03 0,185 No Comparison 1 0,300 0,0100 2,41 0,080 No Comparison 2 0,300 0,0400 2,11 0,110 No Comparison 3 0,330 0,0100 1.92 0,015 Yes Comparison 4 0,330 0,0400 1.62 0,001 Yes Comparison 5 0,320 0,0190 1.99 0,150 Yes Comparison 6 0,304 0,0350 2,09 0,140 No

[0043] Exemplary curves showing the dependence of the strain S on the electric field E are in the Fig. 2 and Fig. 3 shown.

[0044] Fig. Figure 2 shows, for comparison, a typical curve of a PZT material when a DC bias is applied. Fig. Figure 3 shows a curve according to the invention when no DC bias voltage is applied.

[0045] Fig.Figure 4 shows a simple schematic example of a piezoelectric material. The material forms a body 1 with a cylindrical shape. When an alternating electric field is applied to the body in the z-direction, a strain S of the body occurs in the z-direction (d33 coupling). For the purposes of this text, the strain S can include stretching or compression. Furthermore, a strain in the x-direction perpendicular to the z-direction can also occur (not shown, d31 coupling).

[0046] A second aspect, which is not part of the present invention, relates to a piezoelectric material comprising a ceramic material characterized by the composition (Bi a Ba b ) (Fe c Ti d Zr e ) O3 is defined.

[0047] The ceramic material is a BF-BT material, a ceramic oxide material based on bismuth (Bi), iron (Fe), barium (Ba) and titanium (Ti).

[0048] In the formula above, the molar components a, b, c, d and e satisfy 0.600 ≤ a ≤ 0.780, 0.24 5 ≤ b ≤ 0.410, 0.590 ≤ c ≤ 0.752, 0.0003 ≤ e ≤ 0.0153 and c+d+e = 1.

[0049] In particular, a certain molar amount e of the sum of Fe and Ti (c+d) is replaced by zirconium (Zr).

[0050] According to one embodiment, the ceramic material may further comprise lanthanum (La) as a dopant. Preferably, the ceramic material comprises no more than 1% by weight of lanthanum as a dopant.

[0051] According to one embodiment, the ceramic material may further comprise a specific ratio of one or more rare-earth elements, which may be lanthanum (La), any lanthanide, lutetium (Lu), or yttrium (Y), and which replace a specific amount of bismuth in the ceramic material according to aspect 1 (see above). Aspects 1 and 2 may relate to the same ceramic composition.

[0052] According to one embodiment, the piezoelectric material can consist of the ceramic material.

[0053] Advantageously, the coefficient -d31 of the piezoelectric material is at least 50 pm / V or more, as shown in Table 2. In other words, the value of -d31 is equal to or greater than 50 pm / V.

[0054] This is achieved in particular through a ceramic composition according to the second aspect as defined above.

[0055] Advantageously, the change in the piezoelectric coefficient -d31 is 15% or less when the piezoelectric material is subjected to a thermal shock test.

[0056] 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.

[0057] This is achieved in particular through a ceramic composition according to the second aspect as defined above.

[0058] According to the second aspect, this makes the piezoelectric material particularly advantageous for use in high-temperature applications, such as sensors in the automotive sector.

[0059] To further illustrate the invention, Table 2 lists the 16 specific examples (Examples 1 to 16) of piezoelectric materials that, according to the invention, consist of the ceramic material and contain no dopants. The molar amounts a to e are defined in the table. As can be seen from the table, the magnitude of the piezoelectric coefficient -d31 is in each case 50 pm / V or more, and the ratio of the change in -d31 after the thermal shock test is in each case 15% or less.

[0060] Furthermore, Table 2 shows 8 comparison examples from comparison example 1 to comparison example 8. As can be seen from Table 2, in the comparison examples the magnitude of the piezoelectric coefficient -d31 is in each case less than 50 pm / V or the ratio of the change of -d31 after the thermal shock test is higher than 15 %. Table 2: Bi Ba Fe Ti Zr |-d31|[pm / V] |-d31|Change [%] after thermal shock test a b c d e Example 1 0,6300 0,4100 0,6000 0,3970 0,0030 54 13,1 Example 2 0,6195 0,4018 0,5900 0,4069 0,0031 52 14,2 Example 3 0,6000 0,3920 0,6000 0,3970 0,0030 53 13,9 Example 4 0,6656 0,3600 0,6400 0,3573 0,0027 64 12,6 Example 5 0,7180 0,3350 0,6650 0,3325 0,0025 69 9, 2 Example 6 0,7436 0,2907 0,7150 0,2829 0,0021 77 3,0 Example 7 0,7800 0,2550 0,7500 0,2481 0,0019 66 4,0 Example 8 0,7595 0,2480 0,7520 0,2455 0,0025 61 5,7 Example 9 0,7650 0,2450 0,7500 0,2475 0,0025 65 4,3 Example 10 0,7089 0,3050 0,6950 0,3047 0,0003 88 14,0 Example 11 0,7089 0,3050 0,6950 0,3044 0,0006 85 12,5 Example 12 0,7089 0,3050 0,6950 0,3035 0,0015 84 7, 3 Example 13 0,7089 0,3050 0,6950 0,3020 0,0031 81 8, 0 Example 14 0,7089 0,3050 0,6950 0,2989 0,0061 67 11,8 Example 15 0,7089 0,3050 0,6950 0,2958 0,0092 65 12,0 Example 16 0,7089 0,3050 0,6950 0,2898 0,0153 53 13,4 Comparison 1 0,5500 0,4410 0,5500 0,4466 0,0034 43 20,5 Comparison 2 0,7950 0,2575 0,7500 0,2481 0,0019 45 3,0 Comparison 3 0,7575 0,2376 0,7500 0,2498 0,0003 46 4, 1 Comparison 4 0,6360 0,4160 0,6000 0,3960 0,0040 41 17,0 Comparison 5 0,6413 0,3969 0,5950 0,3969 0,0081 45 16,7 Comparison 6 0,7540 0,2485 0,7540 0,2455 0,0005 47 3, 3 Comparison 7 0,6900 0,3100 0,6900 0,3100 0,0000 87 16,2 Comparison 8 0,6900 0,3100 0,6900 0,2883 0,0217 69 21,0 Reference sign 1 Piezoelectric body E Electric field S stretching DC bias

Claims

[1] Piezoelectric material comprising a ceramic material having the composition (1-x) ((Bi (a-y) RE y ) FeO3) - x (Ba b TiO3), where the molar fractions x and y satisfy the following conditions 0.28≤x≤0.34 and 0.0005≤y≤0.032; where RE is one or more elements from the group consisting of lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium and yttrium, where a = 1.04 and where b = 1.007, where the composition belongs to the perimeter and interior of a polygon which has the following eight points P (x; y) as vertices with respect to the values ​​of x and y: P1 = (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). [2] Piezoelectric material according to claim 1, wherein RE is lanthanum (La). [3] Piezoelectric material according to one of claims 1 or 2, wherein the ceramic material additionally comprises manganese (Mn) as a dopant. [4] Piezoelectric material according to claim 3, wherein the ceramic material comprises between 0.05 and 0.2 wt% MnO2 as a dopant. [5] Piezoelectric material according to any one of claims 1 to 4, consisting of the ceramic material. [6] Piezoelectric material according to any one of claims 1 to 5, wherein the coercive force Ec ≥ 2.00 kV / mm. [7] Piezoelectric material according to any one of claims 1 to 6, wherein no reversal of polarity 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. [8] Piezoelectric material according to any one of claims 1 to 7, wherein a maximum strain S of the piezoelectric material in at least one spatial direction is at least 0.15% or more when an alternating electric field E with an amplitude of |2| kV / mm is applied. [9] Piezoelectric material according to claim 8, wherein no DC bias voltage is applied to the piezoelectric material. [10] Piezoelectric material according to any one of claims 1 to 9, wherein the piezoelectric coefficient d33 is not constant between - 2 kV / mm and +2 kV / mm, but changes depending on the applied electric field.

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