A textured piezoelectric ceramic and its preparation method
By introducing magnetic field driving force and powder spreading roller shearing force during the 3D printing process, the orderly arrangement of sheet-like BaTiO3 powder is achieved, which solves the problems of poor template orderly arrangement and poor grain orientation in existing ceramic texturing, and improves the texture and electrical properties of textured piezoelectric ceramics.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- WUHAN UNIV OF TECH
- Filing Date
- 2024-04-07
- Publication Date
- 2026-06-30
Smart Images
Figure CN118359430B_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of piezoelectric material preparation and processing technology, specifically relating to a textured piezoelectric ceramic and its preparation method. Background Technology
[0002] The research and application of ceramic texturing technology has provided a new approach to further improve the performance of piezoelectric ceramic materials. The main method involves using powders with significant anisotropy as templates, mixing them with powders of common morphology as ceramic raw materials. Due to the surface free energy difference between the plate-like BaTiO3 powder and the grains, the grains preferentially grow along the template's arrangement direction, thereby optimizing the performance of piezoelectric ceramics in specific crystallographic directions. However, this method still suffers from problems such as poor template ordering, poor grain orientation, and low texture, which severely restrict the improvement of the electrical performance of piezoelectric ceramics. Summary of the Invention
[0003] The purpose of this invention is to provide a textured piezoelectric ceramic and its preparation method, which solves the problems of poor template ordering, poor grain orientation, and low texture in existing ceramic textured preparation methods.
[0004] In a first aspect, the present invention provides a method for preparing textured piezoelectric ceramics, comprising the following steps: mixing sheet-like BaTiO3 powder, piezoelectric ceramic powder, binder and sintering aid to obtain piezoelectric ceramic composite powder; subjecting the piezoelectric ceramic composite powder to 3D printing, wherein a magnetic field is provided during the 3D printing process, the direction of the magnetic field being parallel to the movement direction of the powder spreading roller shaft during the 3D printing process, to obtain a piezoelectric ceramic preform; subjecting the piezoelectric ceramic preform to debinding and sintering treatments in sequence to obtain textured piezoelectric ceramics.
[0005] In this invention, the inventors discovered that by introducing a magnetic field driving force during the 3D printing process, and utilizing the directional driving effect of the magnetic field driving force on the weakly magnetic sheet-like BaTiO3 powder, as well as the lateral shearing force of the powder spreading roller shaft in 3D printing on the orderly arrangement of the surface sheet-like BaTiO3 powder, the sheet-like BaTiO3 powder can be arranged in a highly ordered manner along the direction of the external magnetic field and the printing direction during the texturing process of piezoelectric ceramics, resulting in textured piezoelectric ceramics with high texture.
[0006] In some embodiments, during the preparation of the piezoelectric ceramic composite powder, the contents of the piezoelectric ceramic powder, binder, sintering aid, and flake BaTiO3 powder, by mass percentage, are 75%–85% (e.g., 75%, 77%, 79%, 81%, 83%, 85% or other values within this range), 10%–15% (e.g., 10%, 11%, 12%, 13%, 14%, 15% or other values within this range), 0.5%–1% (e.g., 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1% or other values within this range), and 4.5%–10% (e.g., 4.5%, 5%, 6%, 7%, 8%, 9%, 10% or other values within this range).
[0007] In some embodiments, the aspect ratio of the sheet-like BaTiO3 powder is >10, and the radial dimension is 2 to 15 μm, for example, 2 μm, 4 μm, 6 μm, 8 μm, 10 μm, 12 μm, 15 μm or other values within this range; the thickness is 0.5 to 1 μm, for example, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1 μm or other values within this range.
[0008] In some embodiments, the particle size of the piezoelectric ceramic powder is 500 nm to 600 nm, for example, it can be 500 nm, 520 nm, 540 nm, 560 nm, 580 nm, 600 nm or other values within this range; and it is selected from barium titanate or barium calcium zirconate titanate; the binder is epoxy resin or thermosetting phenolic resin; and the sintering aid is copper oxide.
[0009] In some implementations, during the preparation of the piezoelectric ceramic blank, before 3D printing the piezoelectric ceramic composite powder, a step of sieving the piezoelectric ceramic composite powder is included, wherein the sieve mesh has an aperture of 80-120 mesh, for example, 80 mesh, 90 mesh, 100 mesh, 110 mesh, 120 mesh or other values within this range.
[0010] In some implementations, the parameters for 3D printing during the preparation of the piezoelectric ceramic preform include: a scanning speed of 1500 mm / s to 2500 mm / s, for example, 1500 mm / s, 1700 mm / s, 1900 mm / s, 2100 mm / s, 2300 mm / s, 2500 mm / s, or other values within this range; a layer thickness of 50 to 100 μm, for example, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, or other values within this range; and a laser power of 6 to 10 W, for example, 6 W, 7 W, 8 W, 9 W, 10 W, or other values within this range.
[0011] In some preferred embodiments, the piezoelectric ceramic preform has a three-period minimal surface structure.
[0012] In some implementations, the magnetic field strength during the preparation of the piezoelectric ceramic preform is 8000 Gs to 20000 Gs, for example, it can be 8000 Gs, 10000 Gs, 12000 Gs, 14000 Gs, 16000 Gs, 18000 Gs, 20000 Gs or other values within this range.
[0013] It should be noted that the magnetic field can be set up using conventional methods. For example, the magnetic field can be composed of magnetic field generators on both sides of the printing platform, and the strength of the magnetic field generators can be controlled by an external electric field.
[0014] Specifically, during the printing process, the lateral shear force between the powder-spreading roller and the flake-like BaTiO3 powder causes the powder to align along the direction of the roller's movement on the surface of the composite material. Simultaneously, a magnetic field generator is activated, and by adjusting the current and frequency, the magnetic field strength is controlled within the range of 8000 Gs to 10000 Gs. The weakly magnetic flake-like BaTiO3 powder aligns orderly along the direction of the external magnetic field under its driving force. The combined effect of the directional magnetic field and the powder-spreading roller significantly improves the orderly alignment of the flake-like BaTiO3 powder along the roller's movement direction, resulting in a textured piezoelectric ceramic preform. This preform undergoes a two-step debinding and high-temperature sintering process to obtain a textured piezoelectric ceramic with high texture density.
[0015] In some embodiments, the degreasing treatment during the preparation of textured piezoelectric ceramics includes: heating from room temperature to 200°C, 300°C, 400°C, 450°C, 500°C, and 600°C respectively at a heating rate of 1°C / min to 3°C / min (e.g., 1°C / min, 1.5°C / min, 2°C / min, 2.5°C / min, 3°C / min or other values within this range) under an inert atmosphere, and holding at 200°C, 300°C, 400°C, 450°C, 500°C, and 600°C for 1h to 3h respectively (e.g., 1h, 1.5h, 2h, 2.5h, 3h or other values within this range). The piezoelectric ceramic preform undergoes a first debinding process, followed by cooling to room temperature, with the inert atmosphere selected from argon or nitrogen. Under air atmosphere, the preform is heated from room temperature to 600℃–650℃ at a heating rate of 1℃ / min–3℃ / min (e.g., 1℃ / min, 1.5℃ / min, 2℃ / min, 2.5℃ / min, 3℃ / min, or other values within this range), and held at 450℃ and 600℃ for 2h–5h respectively (e.g., 2h, 3h, 4h, 5h, or other values within this range). The piezoelectric ceramic preform undergoes a second debinding process after the first debinding, followed by cooling to room temperature to obtain the piezoelectric ceramic preform.
[0016] In this invention, the inventors further discovered that using a two-step degreasing process reduces the rate of gas generation inside the billet, which can significantly reduce cracking on the surface and inside of the billet, and is beneficial to the subsequent sintering process.
[0017] In some embodiments, the sintering process in the preparation of the textured piezoelectric ceramic includes: heating from room temperature to 1300°C to 1350°C at a heating rate of 3°C / min to 5°C / min (e.g., 3°C / min, 3.5°C / min, 4°C / min, 4.5°C / min, 5°C / min or other values within this range), for example, 1300°C, 1310°C, 1320°C, 1330°C, 1340°C, 1350°C or other values within this range; holding at this temperature for 2h to 5h, for example, 2h, 3h, 4h, 5h or other values within this range; sintering the piezoelectric ceramic preform; and then cooling it to room temperature to obtain the textured piezoelectric ceramic.
[0018] In this invention, the inventors further discovered that controlling the sintering temperature within a specific range can yield textured piezoelectric ceramics with better performance. If the sintering temperature is too low, the prepared ceramic is difficult to densify; if the sintering temperature is too high or the holding time is too long, abnormal grain growth may occur, which is detrimental to the stability of the ceramic properties.
[0019] In a second aspect, the present invention provides a textured piezoelectric ceramic prepared using any of the above-described preparation methods.
[0020] The beneficial effects of this invention are as follows: Unlike the prior art, this invention introduces a magnetic field driving force during the 3D printing process. By utilizing the directional driving effect of the magnetic field driving force on the weakly magnetic sheet-like BaTiO3 powder and the lateral shearing force of the powder spreading roller shaft in 3D printing on the orderly arrangement of the surface sheet-like BaTiO3 powder, the sheet-like BaTiO3 powder is arranged in a highly ordered manner along the direction of the external magnetic field and the printing direction during the texturing process of piezoelectric ceramics, resulting in textured piezoelectric ceramics with high texture. Attached Figure Description
[0021] Figure 1 This is a schematic diagram illustrating the principle of directional texturing using 3D printing equipment under magnetic field conditions according to the present invention.
[0022] Figure 2 This is a schematic diagram illustrating the grain growth principle after directional texturing sintering according to the present invention.
[0023] Figure 3 This is a microscopic electron microscope image of the sheet-like BaTiO3 powder used in this invention;
[0024] Figure 4 The images show the XRD patterns of the textured piezoelectric ceramics prepared in this invention, wherein (a) is the XRD pattern of the textured piezoelectric ceramics prepared in Example 1 and Comparative Example 1 of this invention, and (b) is the XRD pattern of the textured piezoelectric ceramics prepared in Example 2 and Comparative Example 2 of this invention.
[0025] Figure 5 The piezoelectric coefficient results of the textured piezoelectric ceramics prepared in this invention are shown in the figure. (a) is the piezoelectric coefficient result of the textured piezoelectric ceramics prepared in Example 1 and Comparative Example 1 of this invention, and (b) is the piezoelectric coefficient result of the textured piezoelectric ceramics prepared in Example 2 and Comparative Example 2 of this invention.
[0026] Figure 6 The diagram shows the hysteresis loop results of the textured piezoelectric ceramics prepared in this invention, wherein (a) is the hysteresis loop result of the textured piezoelectric ceramics prepared in Example 1 and Comparative Example 1 of this invention, and (b) is the piezoelectric coefficient result of the textured piezoelectric ceramics prepared in Example 2 and Comparative Example 2 of this invention. Detailed Implementation
[0027] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.
[0028] Experimental methods not specified in the examples are generally performed under conventional conditions and as described in the manual, or as recommended by the manufacturer. Unless otherwise specified, the general equipment, materials, reagents, etc. used are commercially available.
[0029] Please see Figure 1 This is a schematic diagram illustrating the principle of directional texturing using 3D printing equipment under magnetic field conditions. The magnetic field generator is positioned on both sides of the printing platform. When 3D printing is performed with the printing parameters set, the transverse shear force between the powder-spreading roller (shown as a scraper in the diagram; both can be used interchangeably in this paper) and the sheet-like BaTiO3 powder causes the sheet-like BaTiO3 powder to align along the direction of the powder-spreading roller on the surface of the composite material. Simultaneously, the magnetic field generator is activated, and by adjusting the current and frequency, the magnetic field strength is controlled within a certain range. The weakly magnetic sheet-like BaTiO3 powder will then align orderly along the direction of the external magnetic field under the driving force of the magnetic field.
[0030] Continue reading Figure 2 The diagram shows the grain growth principle after directional texturing sintering according to the present invention. It can be seen that the grains grow in a dense and orderly manner after directional texturing.
[0031] Continue reading Figure 3 The image shown is a microscopic electron microscope image of the sheet-like BaTiO3 powder used in this invention, which shows that the sheet-like BaTiO3 powder is arranged in a disordered state.
[0032] Example 1
[0033] A method for preparing textured piezoelectric ceramics includes the following steps:
[0034] 1) By mass percentage, 85% barium titanate ceramic powder, 10% epoxy resin, 0.5% copper oxide and 4.5% flake BaTiO3 powder were placed in a vertical powder mixer, the speed was set to 180 r / min and the mixing time was 4 h to obtain mixed powder. The mixed powder was then sieved through a 100 mesh sieve to remove large particles and obtain piezoelectric ceramic composite powder for 3D printing.
[0035] 2) Import the 6.25mm×6.25mm×6.25mm spiral icosahedral structure model into the 3D printer and set the printing parameters as follows: layer thickness 100μm, scanning speed 1500mm / s, laser power 6.5W; place the piezoelectric ceramic composite powder obtained in step 1) into the powder cylinder of the 3D printer and start printing. During the printing process, the lateral shear force of the powder spreading roller on the sheet-like BaTiO3 powder causes it to deflect along the direction of roller movement. At the same time, turn on the magnetic field generator and adjust the current to control the magnetic field strength to 8000Gs. The magnetic field direction is consistent with the direction of roller movement. Under the combined action of the external magnetic field driving force and the lateral shear force of the powder spreading roller, the sheet-like BaTiO3 powder is arranged in an orderly manner to obtain the piezoelectric ceramic preform.
[0036] 3) The printed spiral icosahedral piezoelectric ceramic preform is degreased using a nitrogen-air two-step degreasing process, the specific steps of which are as follows:
[0037] Under a nitrogen atmosphere, the piezoelectric ceramic preform was heated from room temperature to 200℃, 300℃, 400℃, 450℃, 500℃, and 600℃ respectively at a heating rate of 3℃ / min, and held at 200℃, 300℃, 400℃, 450℃, 500℃, and 600℃ for 2 hours respectively to remove the binder once, and then cooled to room temperature.
[0038] In an air atmosphere, the temperature was increased from room temperature to 650℃ at a heating rate of 3℃ / min, and held at 450℃ and 600℃ for 2 hours respectively. The piezoelectric ceramic preform after the first debinding was then debinded a second time, and then cooled to room temperature to obtain the piezoelectric ceramic preform.
[0039] The piezoelectric ceramic preform was sintered by heating from room temperature to 1350℃ at a heating rate of 5℃ / min and holding at that temperature for 2 hours. Then it was cooled to room temperature to obtain textured piezoelectric ceramic.
[0040] Example 2
[0041] A method for preparing textured piezoelectric ceramics includes the following steps:
[0042] 1) By mass percentage, 78% barium titanate ceramic powder, 15% epoxy resin, 0.5% copper oxide and 6.5% flake BaTiO3 powder were placed in a vertical powder mixer, the speed was set to 180 r / min and the mixing time was 4 h to obtain mixed powder. The mixed powder was then sieved through a 100 mesh sieve to remove large particles and obtain piezoelectric ceramic composite powder for 3D printing.
[0043] 2) Import the 20mm×20mm×20mm body-centered cubic structure model into the 3D printer and set the printing parameters as follows: layer thickness 100μm, scanning speed 2500mm / s, laser power 7.5W; place the piezoelectric ceramic composite powder obtained in step 1) into the powder cylinder of the 3D printer and start printing. During the printing process, the lateral shear force of the powder spreading roller on the sheet-like BaTiO3 powder causes it to deflect along the direction of roller movement. At the same time, turn on the magnetic field generator and adjust the current to control the magnetic field strength to 10000Gs. The magnetic field direction is consistent with the direction of roller movement. Under the combined action of the external magnetic field driving force and the lateral shear force of the powder spreading roller, the sheet-like BaTiO3 powder is arranged in an orderly manner to obtain the piezoelectric ceramic preform.
[0044] 3) The printed spiral icosahedral piezoelectric ceramic preform is degreased using a nitrogen-air two-step degreasing process, the specific steps of which are as follows:
[0045] Under a nitrogen atmosphere, the piezoelectric ceramic preform was heated from room temperature to 200℃, 300℃, 400℃, 450℃, 500℃, and 600℃ respectively at a heating rate of 2℃ / min, and held at 200℃, 300℃, 400℃, 450℃, 500℃, and 600℃ for 2 hours respectively to remove the binder once, and then cooled to room temperature.
[0046] In an air atmosphere, the temperature was increased from room temperature to 600℃ at a heating rate of 3℃ / min, and held at 450℃ and 600℃ for 3 hours respectively. The piezoelectric ceramic preform after the first debinding was then debinded a second time, and then cooled to room temperature to obtain the piezoelectric ceramic preform.
[0047] The piezoelectric ceramic preform was sintered by heating from room temperature to 1300℃ at a heating rate of 3℃ / min and holding at that temperature for 2 hours. Then it was cooled to room temperature to obtain textured piezoelectric ceramic.
[0048] Comparative Example 1
[0049] The preparation method of the textured piezoelectric ceramic in this comparative example is basically the same as that in Example 1, except that no magnetic field is added.
[0050] Comparative Example 2
[0051] The preparation method of the textured piezoelectric ceramic in this comparative example is basically the same as that in Example 2, except that no magnetic field is added.
[0052] Performance testing
[0053] XRD tests were performed on the textured piezoelectric ceramics prepared in Examples 1 and 2 and Comparative Examples 1 and 2, and the results are as follows: Figure 4 As shown.
[0054] From Figure 4 It can be seen that the prepared textured piezoelectric ceramics are of a pure perovskite structure, and no diffraction peaks of impurity phases are observed. Compared with the textured piezoelectric ceramics in Comparative Examples 1 and 2, the diffraction intensities of the main peaks of the textured piezoelectric ceramics in Examples 1 and 2 along the (110) direction are significantly increased, indicating that the preferred orientation of the piezoelectric ceramics along the (110) direction is more obvious under the magnetic field assistance.
[0055] Generally, the texture degree of textured piezoelectric ceramics is often expressed by the Lotgering factor f (0 < f < 1). f(110) represents the texture degree of the piezoelectric ceramics in the <110> direction. The higher f(110) is, the higher the texture quality and the better the piezoelectric properties. In this invention, the Lotgering factor was used to calculate the texture degrees of the piezoelectric ceramics in Examples 1, 2 and Comparative Examples 1, 2 respectively. The texture degrees f(110) of the piezoelectric ceramics in Examples 1 and 2 are 52.17% and 48.35% respectively, and the texture degrees f(110) of the piezoelectric ceramics in Comparative Examples 1 and 2 are 10.32% and 5.32% respectively. The results further show that after adding the magnetic field, the texture degree of the piezoelectric ceramics can be significantly improved.
[0056] The piezoelectric coefficient comparison tests were carried out on the textured piezoelectric ceramics prepared in Examples 1, 2 and Comparative Examples 1, 2, and the results are as Figure 5 shown.
[0057] From Figure 5 it can be seen that the piezoelectric coefficients d 33 of the piezoelectric ceramics in Examples 1 and 2 are 253 pC / N and 236 pC / N respectively, and the piezoelectric coefficients d 33 of the piezoelectric ceramics in Comparative Examples 1 and 2 are 185 pC / N and 163 pC / N respectively. The results further show that after adding the magnetic field, the piezoelectric coefficient of the piezoelectric ceramics can be significantly improved.
[0058] The hysteresis loops of the textured piezoelectric ceramics prepared in Examples 1, 2 and Comparative Examples 1, 2 were tested, and the results are as Figure 6 shown.
[0059] Generally, the remanent polarization intensity P r in the hysteresis loop represents the ability of the piezoelectric ceramics to still maintain piezoelectric properties when the electric field intensity drops to zero. The larger its value, the more the number of reversible electric domains. From Figure 6 [[ID=3i]]it can be seen that the P r of the piezoelectric ceramics in Examples 1 and 2 are 3.25 μC / cm 2 and 2.87 μC / cm 2 respectively, and the P r of the piezoelectric ceramics in Comparative Examples 1 and 2 are 2.58 μC / cm 2 and 1.94 μC / cm 2The results further show that adding a magnetic field can significantly improve the piezoelectric properties of piezoelectric ceramics.
[0060] It should be noted that all the above embodiments belong to the same inventive concept, and the descriptions of each embodiment have different focuses. Where the description in a particular embodiment is not detailed, please refer to the description in other embodiments.
[0061] The embodiments described above are merely illustrative of implementation methods of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.
Claims
1. A method for preparing textured piezoelectric ceramics, characterized in that, Includes the following steps: The flake-shaped BaTiO3 powder, piezoelectric ceramic powder, binder and sintering aid are mixed to obtain piezoelectric ceramic composite powder; The piezoelectric ceramic composite powder is subjected to 3D printing, and a magnetic field is provided during the 3D printing process. The direction of the magnetic field is parallel to the movement direction of the powder spreading roller during the 3D printing process, to obtain a piezoelectric ceramic preform. The piezoelectric ceramic blank is subjected to degreasing and sintering treatments in sequence to obtain the textured piezoelectric ceramic; In the preparation process of the piezoelectric ceramic composite powder, the contents of the piezoelectric ceramic powder, the binder, the sintering aid, and the flake BaTiO3 powder, by mass percentage, are 75%~85%, 10%~15%, 0.5%~1%, and 4.5%~10%, respectively. During the preparation of the piezoelectric ceramic preform, the strength of the magnetic field is 8000 Gs to 20000 Gs.
2. The method for preparing textured piezoelectric ceramics according to claim 1, characterized in that, The sheet-like BaTiO3 powder has an aspect ratio >10, a radial dimension of 2~15μm, and a thickness of 0.5~1μm.
3. The method for preparing textured piezoelectric ceramics according to claim 1, characterized in that, The piezoelectric ceramic powder has a particle size of 500nm~600nm and is selected from barium titanate or barium calcium zirconate titanate. The adhesive is epoxy resin or thermosetting phenolic resin. The sintering aid is copper oxide.
4. The method for preparing textured piezoelectric ceramics according to claim 1, characterized in that, In the preparation process of the piezoelectric ceramic blank, before the piezoelectric ceramic composite powder is 3D printed, a step of sieving the piezoelectric ceramic composite powder is included, wherein the sieve mesh has an aperture of 80-120 mesh.
5. The method for preparing textured piezoelectric ceramics according to claim 1, characterized in that, In the preparation process of the piezoelectric ceramic preform, the parameters of the 3D printing process include: scanning speed of 1500mm / s to 2500mm / s, layer thickness of 50 to 100μm, and laser power of 6 to 10W.
6. The method for preparing textured piezoelectric ceramics according to claim 1, characterized in that, In the preparation process of the textured piezoelectric ceramic, the degreasing treatment includes: Under an inert atmosphere, the piezoelectric ceramic preform is heated from room temperature to 200℃, 300℃, 400℃, 450℃, 500℃, and 600℃ respectively at a heating rate of 1℃ / min to 3℃ / min, and held at 200℃, 300℃, 400℃, 450℃, 500℃, and 600℃ for 1h to 3h respectively to remove the binder once, and then cooled to room temperature. The inert atmosphere is selected from argon or nitrogen. In an air atmosphere, the temperature is increased from room temperature to 600℃~650℃ at a heating rate of 1℃ / min~3℃ / min, and held at 450℃ and 600℃ for 2h~5h respectively. The piezoelectric ceramic preform after the first debinding is then debinded a second time, and then cooled to room temperature to obtain the piezoelectric ceramic preform.
7. The method for preparing textured piezoelectric ceramics according to claim 1, characterized in that, In the preparation process of the textured piezoelectric ceramic, the sintering treatment includes: heating the piezoelectric ceramic preform from room temperature to 1300℃~1350℃ at a heating rate of 3℃ / min~5℃ / min, holding it at that temperature for 2h~5h, sintering the piezoelectric ceramic preform, and then cooling it to room temperature to obtain the textured piezoelectric ceramic.
8. A textured piezoelectric ceramic prepared by the preparation method according to any one of claims 1-7.