Piezoelectric Assembly And Process Of Forming A Piezoelectric Assembly

Abstract

Piezoelectric assembly, which comprises a substrate of Ni, Cu, or steel, a first oriented layer assembled on the substrate and a piezoelectric layer on the oriented layer. The piezoelectric layer has a degree of (100) orientation with respect to the local surface normal of 90% or more.

Claims

1-24. (canceled)

25. A piezoelectric assembly comprising a base metal substrate having a main substrate surface; a first oriented layer arranged on the main substrate surface, the first oriented layer comprising a first material of crystalline perovskite structure which constitutes at least 90 wt % of the first oriented layer, the first oriented layer comprises a porous oriented sublayer which has a porous structure with a relative porosity of up to 70%; and a first piezoelectric layer arranged on the first oriented layer, the first piezoelectric layer comprises a piezoelectric material of crystalline perovskite structure comprising lead, the piezoelectric material having a larger longitudinal piezoelectric coefficient (d.sub.33) along a direction of unit cells of the piezoelectric material than the first material of the first oriented layer, and wherein a degree of orientation “o” of the piezoelectric material of the first piezoelectric layer is 90% or more, wherein the degree of orientation “o” is the average degree of alignment of the direction of the unit cells of the piezoelectric material with a local surface normal “N” of the main substrate surface.

26. The piezoelectric assembly according to claim 25, wherein the substrate has a second substrate surface opposite to the main substrate surface, and a layered structure comprising a piezoelectric layer on an oriented layer is arranged on the second substrate surface in analogy to the first piezoelectric layer on the first oriented layer on the main substrate surface.

27. The piezoelectric assembly according to claim 25, wherein a second oriented layer consisting of the first material is arranged above the first piezoelectric layer, a second piezoelectric layer is arranged on the second oriented layer, wherein the second piezoelectric layer comprises the same piezoelectric material with the same degree of orientation “o” of 90% or more as the first piezoelectric layer.

28. The piezoelectric assembly according to claim 25, wherein the first oriented layer comprises at least one dense oriented sub-layer, and the porous oriented sub-layer has an at least two times higher concentration of pores than the dense oriented sub-layer.

29. The piezoelectric assembly according to claim 28, wherein at least one dense oriented sub-layer is arranged directly on the main substrate surface and the porous oriented sub-layer is arranged on the dense oriented sub-layer.

30. The piezoelectric assembly according to claim 29, wherein a further dense oriented sub-layer is arranged on the porous oriented sub-layer.

31. The piezoelectric assembly according to claim 25, wherein an average pore size of the porous oriented sub-layer is 100 nm or below.

32. The piezoelectric assembly according to claim 25, wherein the first material of the first oriented layer is selected from the group consisting of LaNiO.sub.3, SrRuO.sub.3, and PbTiO.sub.3.

33. The piezoelectric assembly according to claim 25, wherein the first material of the first oriented layer has at least the same degree of orientation “o” of 90% or more as the crystalline perovskite structure of the first piezoelectric layer.

34. The piezoelectric assembly according to claim 25, wherein the first oriented layer has a thickness of 10 nm to 500 nm.

35. The piezoelectric assembly according to claim 25, wherein the substrate is titanium (Ti), aluminium (Al), nickel (Ni), copper (Cu), or a steel.

36. The piezoelectric assembly according to claim 25, wherein the piezoelectric material is represented by the formula [Pb.sub.1-y S.sup.1.sub.y][(Zr.sub.xTi.sub.1-x).sub.1-z S.sup.2.sub.z]O.sub.3, wherein S.sup.1 is a first substituent, S.sup.2 is a second substituent, 0.40≤x≤0.95, y<0.3, and z<0.15.

37. The piezoelectric assembly according to claim 25, wherein the first piezoelectric layer has a thickness of 0.1 μm to 5 μm.

38. A device comprising the piezoelectric assembly according to claim 25, wherein the piezoelectric assembly is attached to a surface of the device and configured to generate a haptic signal at the surface.

39. A device comprising the piezoelectric assembly according to claim 25 and an energy storage element, wherein the piezoelectric assembly is configured to harvest electrical energy that is generated in the piezoelectric assembly during a mechanical deformation of the piezoelectric assembly, and wherein the energy storage element is configured to store the electrical energy harvested by the piezoelectric assembly.

40. A micromirror comprising the piezoelectric assembly according to claim 25, wherein the micromirror comprises a mirror surface on a piezoelectric assembly, and the piezoelectric assembly is configured to bend the micromirror upon voltage application to the piezoelectric assembly.

41. A process of forming a piezoelectric assembly, comprising: providing a base metal substrate having a main substrate surface; forming a first seed layer comprising a first material of crystalline perovskite structure represented by the general formula ABO.sub.3 on the main substrate surface, the forming comprising the acts of depositing a first seed layer solution comprising ions of A and B on the main substrate surface, wherein the first seed layer is prepared as a layered structure including the acts of depositing a first seed layer solution comprising ions of A and B, forming a dense seed sub-layer comprising a first pyrolysis procedure, characterized by a first pyrolysis temperature T1 and a first holding time t1 at the first pyrolysis temperature T1, depositing a second seed layer solution comprising ions of A and B, forming a porous seed sub-layer comprising a second pyrolysis procedure, characterized by a second pyrolysis temperature T2, and a second holding time t2 at the second pyrolysis temperature T2, and crystallization of the perovskite structure by a first annealing procedure, forming a first piezoelectric layer comprising a piezoelectric material of crystalline perovskite structure comprising lead, and comprising the acts of depositing a piezoelectric layer solution comprising lead-ions and further ions of the piezoelectric material, and crystallization of the perovskite structure comprising lead by a second annealing procedure.

42. The process of forming the piezoelectric assembly according to claim 41, wherein the dense seed sub-layer is prepared directly on the main substrate surface and the porous seed sub-layer is prepared on top of the dense seed sub-layer.

43. The process of forming the piezoelectric assembly according to claim 41, wherein T.sub.2<T.sub.1, and/or t.sub.2<t.sub.1.

44. The process of forming the piezoelectric assembly according to claim 41, wherein the second seed layer solution comprises a polymer, and the second pyrolysis procedure is carried out such that the polymer is not decomposed and dissipated completely.

45. The process of forming the piezoelectric assembly according to claim 41, wherein at least the first seed layer solution or the piezoelectric layer solution is deposited in a spin-coating method.

46. The process of forming the piezoelectric assembly according to claim 41, wherein at least the first seed layer solution or the piezoelectric layer solution is deposited by a printing method.

47. A piezoelectric assembly, comprising: a base metal substrate having a main substrate surface; a first oriented layer arranged on the main substrate surface, the first oriented layer comprises a first material of crystalline perovskite structure which constitutes at least 90 wt % of the first oriented layer; and a first piezoelectric layer is arranged on the first oriented layer, the first piezoelectric layer comprises a piezoelectric material of crystalline perovskite structure comprising lead, the piezoelectric material has a larger longitudinal piezoelectric coefficient (d33) along a direction of the unit cells of the piezoelectric material than the first material of the first oriented layer, and wherein a degree of orientation “o” of the piezoelectric material of the first piezoelectric layer is 90% or more, the degree of orientation “o” is the average degree of alignment of the direction of the unit cells of the crystalline piezoelectric material with a local surface normal “N” of the main substrate surface, and wherein the substrate has a second substrate surface opposite to the main substrate surface, and wherein a layered structure comprising a second piezoelectric layer on a second oriented layer is arranged on the second substrate surface in analogy to the first piezoelectric layer on the first oriented layer on the main substrate surface.

48. A piezoelectric assembly, comprising a base metal substrate having a main substrate surface; a first oriented layer arranged on the main substrate surface, the first oriented layer comprises a first material of crystalline perovskite structure which constitutes at least 90 wt % of the first oriented layer; and a first piezoelectric layer is arranged on the first oriented layer, the first piezoelectric layer comprises a piezoelectric material of crystalline perovskite structure comprising lead, the piezoelectric material has a larger longitudinal piezoelectric coefficient (d33) along a direction of the unit cells of the piezoelectric material than the first material of the first oriented layer, and wherein a degree of orientation “o” of the piezoelectric material of the first piezoelectric layer is 90% or more, the degree of orientation “o” is the average degree of alignment of the direction of the unit cells of the crystalline piezoelectric material with a local surface normal “N” of the main substrate surface, and wherein a second oriented layer consisting of the first material is arranged above the first piezoelectric layer, a second piezoelectric layer is arranged on the second oriented layer, wherein the second piezoelectric layer comprises the same piezoelectric material with the same degree of orientation “o” of 90% or more as the first piezoelectric layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0125] In the following the invention is explained in greater detail on the basis of exemplary embodiments and the associated figures.

[0126] The figures serve solely to elucidate the invention and are therefore only illustrated schematically and not in a manner true to scale. Individual parts may be illustrated in an enlarged manner or in a distorted manner in terms of the dimensions. Therefore neither absolute nor relative dimensional specifications can be inferred from the figures. Identical or identically acting parts are provided with identical reference signs.

[0127] In the figures:

[0128] FIG. 1 shows a first exemplary embodiment of a piezoelectric assembly in schematic cross-section;

[0129] FIG. 2 shows a second exemplary embodiment of a piezoelectric assembly in schematic cross-section;

[0130] FIG. 3 shows a third exemplary embodiment of piezoelectric assembly in schematic cross-section;

[0131] FIG. 4 shows a scanning electron microscopy image of a cross-section of a part of the third exemplary embodiment of a piezoelectric assembly;

[0132] FIG. 5 shows a fourth exemplary embodiment of a piezoelectric assembly in schematic cross-section;

[0133] FIG. 6 shows the fourth exemplary embodiment of a piezoelectric assembly in schematic cross-section in a bent state;

[0134] FIG. 7 shows a fifth exemplary embodiment of a piezoelectric assembly in schematic cross-section;

[0135] FIG. 8 shows a sixth exemplary embodiment of a piezoelectric assembly in schematic cross-section;

[0136] FIG. 9 shows an x-ray diffraction pattern of a piezoelectric assembly on a 20 μm thick substrate and on a 500 μm thick substrate;

[0137] FIG. 10 shows the electric field dependence of the electrical polarization (FIG. 10A) and the real part of the dielectric permittivity at different DC bias fields (FIG. 10B) for a piezoelectric assembly with a 20 μm thick stainless steel substrate; and

[0138] FIG. 11 shows the electric field dependence of the relative strain (S) for a 500 μm thick substrate (FIG. 11A) and a 20 μm thick substrate (FIG. 11C) and the longitudinal piezoelectric coefficient (d33) at different DC bias fields for a piezoelectric film on a 500 μm (FIG. 11B) and a 20 μm (FIG. 11D) thick substrate.

DETAILED DESCRIPTION OF THE DRAWINGS

[0139] FIG. 1 shows a schematic cross-section of a first exemplary embodiment of a piezoelectric assembly 1.

[0140] The piezoelectric assembly 1 comprises a nickel-chromium-based austenitic steel substrate 2. The substrate 2 can be, for example, a steel such as EN 1.4310, EN 1.4404, EN 1.4541 or EN 1.4845. These steels have a preferential oxidation resistivity. The substrate 2 comprises a main substrate surface 21 which preferably has an arithmetical mean deviation of the surface roughness (Ra) of 200 nm or below.

[0141] A first oriented layer 3 is arranged directly on the main substrate surface 21. The first oriented layer 3 comprises lanthanum nickelate (LaNiO3) as the first material of the first oriented layer 3. The perovskite crystallites of the first material have a degree of orientation o of 90% or more, preferentially of 95% or more and most preferably of 99% or more.

[0142] As indicated in FIG. 1, the main substrate surface can be flat, therefore the direction of all local surface normals is the same.

[0143] The higher the degree of orientation of the templating first oriented layer 3, the higher can be the degree of orientation o of the first piezoelectric layer 4 assembled on the first oriented layer 3 can.

[0144] The overall thickness of the first oriented layer 3 may be in the range between 10 nm to 500 nm. For example, the thickness of the oriented layer may be in the range between 100 and 200 nm, for example 150 nm.

[0145] The first piezoelectric layer 4 consists of a piezoelectric material of crystalline perovskite structure. The piezoelectric material of this present exemplary embodiment is a lead zirconate titanate based material, for example a lanthanum-doped lead zirconate titanate, such as (La0.02Pb0.98)(Zr0.52Ti0.48)O3.

[0146] The piezoelectric material of the first piezoelectric layer 4 has a degree or orientation o of 90% or more. Even more preferably, the degree or orientation o of the piezoelectric material of the first piezoelectric layer 4 is 95% or more, or better 99% or more.

[0147] The higher the degree or orientation o the higher the values for the longitudinal piezoelectric coefficient d33 of the overall layer can be. Values for d33 of up to 300 pm/V can be achieved with a degree or orientation o in the range of 99%.

[0148] The thickness of the first piezoelectric layer may be between 100 nm and 5 μm, for example the thickness can be in the range between 600 and 900 nm.

[0149] A top electrode 5 is arranged on top of the first piezoelectric layer. The top electrode 5 may consist of any suitable conductive material, for example it can consist of a metal or of a conductive oxide or ceramic material. For example, the top electrode may consist of copper, nickel, gold, silver, platinum, or lanthanum nickelate, or be a layered electrode such as Cr/Ni/Ag, Cr/Ni/Au, Ni/Au, Cr/Au or Ti/Au.

[0150] The substrate 2 together with the conducting first oriented layer 3 can act as a base electrode, when electrically contacted externally.

[0151] The first exemplary embodiment of a piezoelectric assembly 1 can be produced by any suitable process. It can be produced, for example, by the following process.

[0152] First, the austenitic steel substrate 2 is provided. The main substrate surface 21 is cleaned by an ultrasonic cleaning method in acetone, isopropanol and deionized water. It is dried in a flow of nitrogen. Subsequently it is treated under UV and ozone to remove organic components. Other cleaning methods may be applied. It is important to achieve a nearly atomically clean surface, which shows suitable wettability by the seed-layer solution.

[0153] On the main substrate surface 21, then the first seed layer 3 is prepared, for example by a spin-coating method or a screen-printing method. In the fully processed assembly the resulting layer can be identified as the first oriented layer 3.

[0154] The first seed layer 3 can be formed by a repeated deposition, drying, pyrolysis and annealing procedure.

[0155] For example a seed layer of 150 nm thickness can be prepared as follows: First lanthanum acetate and nickel nitrate is dissolved in 2-ethoxyethanol forming a solution of 0.5 mol/l concerning La- and Ni-ions. The mixture is heated for 2 h at 80° C. under reflux yielding a clear solution. After cooling back to room temperature a first seed layer solution with a concentration of 0.2 mol/l is formed by adding solvent.

[0156] To prepare a first seed layer 3 with a thickness of approximately 150 nm a six times repeated spin-coating deposition, drying, pyrolysis and annealing procedure can be performed. One preparation cycle comprises: Deposition of the first seed layer solution on the main substrate surface 21 in a spin-coating process with a rotation speed of 3000 rpm for 30 s. Subsequently, the solvent is removed by drying at around 100° C. to 200° C. Subsequently a pyrolysis step (first pyrolysis procedure) is performed to remove other organic components at the first pyrolysis temperature of 300° C. to 450° C. A first holding time at the first pyrolysis temperature may be in the range of 0.1 min to 10 min, typically of 0.5 min to 5 min. Thereby an amorphous preliminary seed sublayer is formed. Subsequently, the film is annealed at a temperature of 600 to 750° C. to crystalize it. The first temperature increase rate is typically 10 K/s or above, such as for example 30 K/s, preferably, as it facilitates growth of the first seed layer in 100 crystal orientation. The annealing can be performed in a nitrogen oxygen mixture with a nitrogen-to-oxygen ratio of 4:1, to ensure a stoichiometric oxygen content in the seed layer.

[0157] By adapting the number of deposition cycles, the thickness of the first seed layer 3 can be varied.

[0158] On the as prepared, highly oriented seed layer 3, an also highly oriented first piezoelectric layer 4 can be prepared.

[0159] The preparation of the first piezoelectric layer 4 is in principle very similar to the preparation of the first seed layer 3. It also can be based on a repeated deposition of a piezoelectric layer solution, drying, pyrolysis and annealing procedure.

[0160] The piezoelectric layer solution can be prepared by: First a solution is prepared comprising the ions of the lead zirconate titanate piezoelectric material. For example, for a piezoelectric material of the composition (La0.02Pb0.98) (Zr0.52Ti0.48)O3 a solution of zirconium n-propoxide (0.52 mol/l), titanium isopropoxide (0.48 mol/l), anhydrous lead acetate (1.127 mol/l), and anhydrous lanthanum acetate (0.02 mol/l) is prepared in 2-methoxyethanol. In this solution the lead is present in a 15% molar excess in relation to the desired stoichiometric ratio in the piezoelectric material, to compensate the lead-losses during heating steps in the preparation procedure. A clear solution was produced by heating to around 120° C. for 2 h under reflux and after distillation. A piezoelectric layer solution of 0.5 mol/l in terms of (La0.02Pb0.98) (Zr0.52Ti0.48)O3 with 15% molar excess of lead is prepared by adding of solvent.

[0161] The piezoelectric layer can be produced in a repeated deposition, drying, pyrolysis and annealing procedure. For example for a layer thickness in the order of 780 nm the following scheme can be applied: First, the piezoelectric layer solution is deposited in a spin-coating process with a rotation speed of 3000 rpm applied for 30 s. Subsequently, the deposited solution is dried at temperatures between 100 and 200° C. Then a pyrolysis step is applied at temperatures between 300 to 450° C. to remove organic components. This produces an amorphous preliminary piezoelectric sub layer. The procedure up to this point can be repeated up to three times. This means that three amorphous partial layers can be assembled one upon another. Subsequently an annealing procedure at 500 to 700° C. in a nitrogen oxygen mixture with a nitrogen-to-oxygen ratio of 4:1 is performed for an annealing time below 15 minutes. For example the annealing time can be 1 to 5 minutes. This procedure may be repeated four times, which means that the overall first piezoelectric layer of 780 nm is formed from 12 preliminary sub-layers.

[0162] By adapting the number of deposition cycles, the thickness of the first piezoelectric layer 4 can be varied.

[0163] The outer electrode 5 can be deposited by any suitable technique, for example sputtering or a burn-in procedure may be applied.

[0164] FIG. 2 shows a schematic cross-section of a second exemplary embodiment of the piezoelectric assembly 1.

[0165] The second exemplary embodiment can be identical to the first exemplary embodiment except for the first oriented layer 3, which has a layered structure.

[0166] The first oriented layer 3 consists of a dense oriented sub-layer 31 and a porous oriented sub-layer 32. Both consist of the same first material as the first oriented layer 3 in the first exemplary embodiment.

[0167] However, the dense oriented sub-layer 31 has an at least two times higher density, or in other words a two times lower porosity than the porous oriented sub-layer 32.

[0168] The porous oriented sub-layer 32 consists of a porous material with average pore diameters below 100 nm and preferentially in the range between 1 nm to 20 nm.

[0169] This assembly has the advantage that the dense oriented sub-layer 31 assembled directly on the austenitic steel substrate 2 can be an efficient buffer layer, which prevents interdiffusion between the first piezoelectric layer 4 and the substrate 2.

[0170] The porous oriented sub-layer 32 can provide stress relaxation between the substrate 2 and the first piezoelectric layer 4. Thereby, the substrate clamping effect can be reduced.

[0171] In such an assembly the relative strain S of the piezoelectric material can exceed 0.7%. This is a value comparable to piezoelectric thin-films achieved on expensive silicon wafer substrates.

[0172] The preparation procedure for the second exemplary embodiment can be identical to the procedure for the first exemplary embodiment, however with adaptions for the first seed layer 3.

[0173] Typically a dense seed sub-layer 31 can be prepared by the deposition, drying, pyrolysis and annealing procedure described for the preparation of the entire first seed layer 3 of the first exemplary embodiment. This procedure can be repeated, for example, three times.

[0174] For a porous seed sub-layer 32 two principal preparation techniques can be applied. In the first option, first a second seed layer solution is prepared identically to the first seed layer solution, except for a lower concentration in the range of 0.1 to 0.15 mol/l concerning Ni- and La-ions. The deposition, drying and annealing can be identical to the dense seed sub-layer 31 preparation. For the porous seed sub-layer 32, a second pyrolysis procedure is applied. The second pyrolysis procedure is characterized by a second pyrolysis temperature T2 and a second holding time t2. To cause a porosity of the porous seed sub-layer 32 to be higher than the porosity of the dense seed sub layer 31 either the second pyrolysis temperature T2 must be smaller than the first pyrolysis temperature or the second holding time t2 is chosen shorter than the first holding time. For example in a case in, which T1=T2, t2 may be below 2 min, for example 0.5 min, and t1 may be higher than 2 min, for example 5 min.

[0175] This procedure can be repeated to achieve a desired thickness of the porous seed sub-layer 32.

[0176] A second option for producing the porous seed sub-layer 32 based on the use of a polymer. The polymer can be added to the above-described second seed-layer solution. The polymer may be, for example, polyvinlypyrrolidone which may be added in an amount of 1 g/5 ml. The solution as prepared may be stirred for 24 hours and be filtered by a 0.2 μm filter. In principle other polymers, such as polymethylmethacrylate or polyethylene glycol can also be used, depending for example on the solubility in the solvent and the conditions of the pyrolysis step.

[0177] The porous seed sub-layer 32 can be formed using the second seed sublayer solution containing the polymer similarly to the first sublayer solution. However, the temperature of the pyrolysis step has to be chosen such that the polymer is not decomposed and dissipated completely. The residues of the polymer remain in the formed amorphous porous seed sub-layer. These residues prevent formation of a closed and dense seed sub-layer. Only during the third annealing procedure the polymer is fully dissociated and dissipated. At these temperatures the perovskite material can be crystalized efficiently. However, hardly any sintering takes place under these conditions.

[0178] In the completed piezoelectric assembly, the dense seed sub-layer 31 can be identified as the dense oriented sublayer 31 and the porous seed sub-layer 32 can be identified as the porous oriented sublayer 32.

[0179] FIG. 3 shows a third exemplary embodiment of a piezoelectric assembly 1. It is identical to the first exemplary embodiment of the piezoelectric assembly except for the first oriented layer 3.

[0180] The first oriented layer 3 comprises a dense oriented sub-layer 31 directly attached to the main substrate surface 21. It comprises a porous oriented sub-layer 32 directly above the dense oriented sub-layer 31. Further, it comprises a further dense oriented sub-layer 33 which is directly arranged on the porous oriented sub-layer 32. The preparation procedure of the porous and dense oriented sub-layers can be performed analogously as described for the second exemplary embodiment shown in FIG. 2.

[0181] Preferably the dense oriented sub-layers 31 and 33 are thinner than the porous oriented sub-layer 32. Thereby a chemical buffering can be achieved by the lower dense oriented sub-layer 31. Further a smooth continuous surface is formed by the upper dense oriented sub-layer 33 on which high quality piezoelectric films can be formed. Further, the comparatively thick porous oriented sub-layer 32 allows for an efficient stress release between the substrate 2 and the first piezoelectric layer 4. For example a thickness ratio between the lower dense oriented sub-layer 31, the porous oriented sub-layer 32 and the upper dense oriented sub-layer 33 may be 1:4:1. For example, an overall thickness may be 150 nm.

[0182] FIG. 4 shows a cross-sectional scanning electron microscopy (SEM) image of the piezoelectric assembly 1 as shown in FIG. 3, except for the outer electrode 5, which is not present in the sample of the SEM image.

[0183] In the SEM image in FIG. 4 the layers of the piezoelectric assembly 1 can be clearly identified and are labelled accordingly. Interestingly a chromium-enriched layer 22 has formed in the vicinity of the main substrate surface 21 and is visible as a darkened layer in the piezoelectric assembly 1. It is believed that the chromium enrichment takes place during pyrolysis or annealing procedures. By Energy-dispersive X-ray spectroscopy (EDX) the absence of Cr or other elements form the substrate in the first piezoelectric layer was proven. Thus EDX could prove that the first oriented layer provides can exhibit an efficient buffering function.

[0184] FIG. 5 shows a schematic cross-section of a fourth exemplary embodiment of a piezoelectric assembly 1.

[0185] The layers assembled above the main substrate surface 21 of the substrate 2 can be identical to the first exemplary embodiment shown in FIG. 1. Further, in principle the oriented layer 3 can be realized also in analogy to the second or third exemplary embodiment.

[0186] In this fourth exemplary embodiment the substrate 2 comprises a second substrate surface 21′ opposite to the main substrate surface 21. On the second substrate surface 21′ an oriented layer 3′ which can be identical to the first oriented layer 3 is assembled. Further a piezoelectric layer 4′ which may be identical to the first piezoelectric layer 4 is assembled on the oriented layer 3′. Further, a second outer electrode 5′ is assembled on the piezoelectric layer 4′.

[0187] This means that a symmetrical stacking on the main substrate surface 21 and on the second substrate surface 21′ is realized.

[0188] In this arrangement the substrate 2 and the oriented layers 3 and 3′ can form an internal electrode.

[0189] The preparation procedure can be analogous to the preparation procedure discussed for the first exemplary embodiment shown in FIG. 1.

[0190] FIG. 6 shows the fourth exemplary embodiment of a piezoelectric assembly in a bent state. It can be realized on a bent substrate or it can be arranged in an application in which it becomes bent. This means that the symmetry described for the embodiment shown in FIG. 5 is maintained only along the local surface normal of the main substrate surface, which means for each point along the direction of the local surface normal each layer is a mirror equivalent on both sides of the substrate 2.

[0191] FIG. 7 shows a fifth exemplary embodiment of a piezoelectric assembly in schematic cross-section.

[0192] It is based on the first exemplary embodiment of a piezoelectric assembly as shown in FIG. 1. However, on the electrode 5, which was the outer electrode 5 in the first exemplary embodiment, further layers are arranged.

[0193] A second oriented layer 6 is arranged directly on the electrode 5, which thus becomes an internal electrode. In principle the second oriented layer 6 can be identical to the first oriented layer 3. On the second oriented layer 6 a second piezoelectric layer 7 is arranged which may be identical to the first piezoelectric layer 4. This means in particular the composition and the degree of orientation o of the second oriented layer 6 and the second piezoelectric layer 7 may be identical to the first oriented layer 3 and the first piezoelectric layer 4, respectively. An outer electrode 8 is arranged on the second piezoelectric layer 7.

[0194] The preparation procedure can be analogous to the preparation procedure discussed for the first exemplary embodiment shown in FIG. 1.

[0195] In such an assembly the substrate 2 with the first oriented layer 3 can serve as a base electrode. The electrode 5 together with the second oriented layer 6 can serve as an internal electrode.

[0196] FIG. 8 shows a sixth exemplary embodiment of a piezoelectric assembly 1. It is identical to the fifth exemplary embodiment of a piezoelectric assembly shown in FIG. 7, except for the internal electrode 5, which is not present in the sixth exemplary embodiment. Instead, the second oriented layer 6 is arranged directly on the first piezoelectric layer 4.

[0197] In such an arrangement the second oriented layer 6 can act as an internal electrode, when electrically contacted.

[0198] FIG. 9 shows an X-ray diffraction patterns of the piezoelectric layer assemblies similar to the one shown in FIG. 4. Both samples comprise a nickel-chromium-based austenitic steel substrate, one with a thickness of 20 μm and a second with a thickness of 500 μm.

[0199] The intensities for the (100) and (200) diffraction peaks for both samples are compared to the intensity of the (110) and (111) peaks and are evaluated in terms of a Lotgering factor (LF100), which has found to be above 99% for both samples. This is equivalent to a degree of orientation o of the crystalline perovskite structure of the first piezoelectric layer of 99% or above.

[0200] This proves that highly oriented piezoelectric films can be prepared with the above-described procedure.

[0201] FIG. 10 provides an analysis of the electric properties of the assembly discussed for FIG. 9 with the 20 μm thick substrate. For the measurements a sputtered gold electrode 5 with a diameter of 500 μm was formed on the first piezoelectric layer 3.

[0202] The measurements were carried out with a Aixacct TF2000FE using a sinusoidal test signal of 100 Hz.

[0203] FIG. 10A shows the electric field dependence of the electrical polarization at different DC bias fields.

[0204] A typical ferroelectric square-like hysteresis loop is observed in the polarization electric field measurements, with a saturation polarization being over 40 μC/cm2. A coercive field of 50 kV/cm is observed.

[0205] FIG. 10B shows the dependence of the real part of the dielectric permittivity on the DC bias field.

[0206] The real part of the complex dielectric permittivity at 0 DC fields approaches 600 and it approaches 700 at the coercive field.

[0207] These values are considerably larger than values reported previously for piezoelectric thin-films prepared on metal foils, for example compared to the film of EP 1 282 901 B1 where around only a dielectric permittivity of 150 was measured. This shows that superior piezoelectric assemblies can be formed with the above-described methods.

[0208] FIG. 11 shows the results of double-beam laser interferometry measurements recorded with a frequency of 1 kHz on both samples addressed in the context of FIG. 9. Both the 20 μm and the 500 μm substrate sample have a gold electrode with a diameter of 500 μm arranged on the first piezoelectric layer.

[0209] FIG. 11A shows the electric field dependence of the relative strain (S) for the piezoelectric assembly with a 500 μm thick substrate. FIG. 11B shows the, longitudinal piezoelectric coefficient d33 in dependence on the electric field for the piezoelectric assembly on a 500 μm thick substrate.

[0210] The equivalent curves for the sample with the 20 μm thick substrate are shown in FIGS. 11C and 11D, respectively.

[0211] Relative strain (S) can be understood as the relative displacement, perpendicular to the film normal and induced by an electric field in the piezoelectric assembly. S was measured with a sinusoidal test signal at 100 Hz. The longitudinal piezoelectric coefficient d33 was extracted from field-induced displacement measurements using an AC signal of 0.5 V at 1 kHz.

[0212] For the assembly with a 500 μm thick substrate an S value of 0.15% is exceeded. This equals a peak value for the longitudinal piezoelectric coefficient d33 of above 70 pm/V.

[0213] For the assembly on the 20 μm thick substrate even higher values are achieved. A relative strain S of 0.7% is exceeded. This equals a d33 value of 300 pm/V, approximately.

[0214] Note that the undulating signal measured for the hysteresis curve of FIG. 11D towards low or high electric field, may be caused by minor deviations in the thickness of the substrate, which affect the measurements accordingly during piezoelectric bending of the substrate

[0215] These values are to the best of the author's knowledge the highest reported for piezoelectric thin-films on steel substrate. In this context compare, for example, non-patent literature 2. The achieved values are even comparable to piezoelectric thin-film elements prepared on standard silicon wafer substrates comprising noble electrode materials. In this context compare, for example, non-patent literature 1.

REFERENCE LIST

[0216] 1 piezoelectric assembly [0217] 2 substrate [0218] 21 main substrate surface [0219] 21′ second substrate surface [0220] 3 first oriented layer [0221] 3′ oriented layer [0222] 31 dense oriented sub-layer [0223] 32 porous oriented sub-layer [0224] 33 dense oriented sub-layer [0225] 4 first piezoelectric layer [0226] 4′ piezoelectric layer [0227] 5 electrode [0228] 5′ electrode [0229] 6 second oriented layer [0230] 7 second piezoelectric layer [0231] 8 electrode