CN113257991A

CN113257991A - 003-type flexible piezoelectric composite material, flexible multilayer actuator and preparation method

Info

Publication number: CN113257991A
Application number: CN202110708747.2A
Authority: CN
Inventors: 任丹阳; 施钧辉; 尹永刚; 陈睿黾; 李驰野
Original assignee: Zhejiang Lab
Current assignee: Zhejiang Lab
Priority date: 2021-06-25
Filing date: 2021-06-25
Publication date: 2021-08-13
Anticipated expiration: 2041-06-25
Also published as: CN113257991B

Abstract

The invention discloses a 003-type flexible piezoelectric composite material, a flexible multilayer actuator and a preparation method. The preparation method of the flexible piezoelectric composite material comprises the steps of doping graphene oxide with multiple functional groups in a PVDF-based polymer as a conductive phase, wherein the functional groups of the graphene oxide are-COOH, -OH, -CH (O) CH-and the like, and the functional groups can form hydrogen bonds with polar F atoms in a molecular chain of the PVDF-based polymer, so that the graphene oxide composite material becomes a link bridge between the graphene oxide with multiple functional groups and the polymer, the components among the composites are uniformly dispersed, the agglomeration phenomenon is avoided, the oriented arrangement of the molecular chain of the PVDF-based polymer can be realized, and the PVDF-based composite material with high oriented arrangement, high beta-phase content, high piezoelectric property and good flexibility is finally prepared and is finally applied to a flexible multilayer actuator, and the actuator has higher electromechanical coupling coefficient.

Description

003-type flexible piezoelectric composite material, flexible multilayer actuator and preparation method

Technical Field

The invention belongs to the technical field of piezoelectric materials, and particularly relates to a 003-type flexible piezoelectric composite material, a flexible multilayer actuator and a preparation method of the flexible multilayer actuator.

Background

Piezoelectric actuators are a class of driving devices made using the inverse piezoelectric effect of piezoelectric materials. The inverse piezoelectric effect means that when an electric field is applied in the polarization direction of the piezoelectric material, the piezoelectric material generates mechanical deformation or mechanical pressure in a certain direction, and when the applied electric field is removed, the deformation or stress disappears. The piezoelectric actuator is widely applied to industries such as precision driving, microelectronics and the like due to the advantages of small volume, large driving displacement, low driving voltage and the like. At present, most of the multilayer piezoelectric actuators are rigid devices, and in recent years, with the development of flexible electronic devices, flexible multilayer actuators have attracted much attention.

Disclosure of Invention

The invention aims to provide a 003-type flexible piezoelectric composite material, a flexible multilayer actuator and a preparation method thereof, aiming at overcoming the defects of the prior art. The polymer piezoelectric material/non-polymer piezoelectric material flexible composite piezoelectric film material added with the conductive phase has good flexibility, high piezoelectric performance and high electromechanical coupling coefficient, and improves the application of the material in the field of wearable devices, particularly flexible actuating devices.

The purpose of the invention is realized by the following technical scheme: a0-0-3 type flexible piezoelectric composite material is mainly obtained by compounding a polymer piezoelectric material, a non-polymer piezoelectric material and graphene oxide; the non-polymer piezoelectric material is piezoelectric ceramic or piezoelectric single crystal; the functional group of the graphene oxide is combined with the polar F atom in the polymer piezoelectric material in a hydrogen bond mode.

Further, the preparation method of the graphene oxide comprises the following steps: reacting potassium permanganate, concentrated sulfuric acid, sodium nitrate and graphite to obtain graphite oxide; and then, the hydrophilic functional groups on the surface of the graphite oxide act with water molecules to increase the lamellar gaps, and the graphene oxide lamellar is peeled off by adopting ultrasonic vibration, so that the graphene oxide is finally prepared.

Further, the polymer-based piezoelectric material is PVDF or PVDF-based copolymer.

Further, the PVDF-based copolymer includes P (VDF-TrFE), PVDF-HFP, P (VDF-CTFE), P (VDF-TrFE-CTFE).

Further, the piezoelectric single crystal is selected from PMN, PMN-PT, PZN-PT, lithium niobate and lithium tantalate; the piezoelectric ceramic is PZT.

A preparation method of the 0-0-3 type flexible piezoelectric composite material comprises the following steps: preparing graphite oxide, and carrying out ultrasonic stripping to obtain graphene oxide; then adding the mixture into an organic dispersant, and oscillating to uniformly disperse the mixture to obtain a dispersant/graphene oxide solution; adding a polymer piezoelectric material, stirring, and uniformly dispersing to obtain a dispersing agent/graphene oxide/polymer piezoelectric material solution; and finally, adding a non-polymer piezoelectric material, stirring and ultrasonically dispersing to obtain a graphene oxide/polymer piezoelectric material/non-polymer piezoelectric material solution which is a 0-0-3 type flexible piezoelectric composite material.

Further, the polar organic dispersant is selected from DMF, DMAc, acetone.

A flexible multilayer actuator comprises the above-mentioned type 0-0-3 flexible piezoelectric composite material.

A preparation method of the flexible multi-layer actuator comprises the following steps: sputtering a metal layer on a silicon substrate to serve as a first electrode layer, then coating the 0-0-3 type flexible piezoelectric composite material on the electrode layer to form a piezoelectric film, and then sputtering the metal layer on the surface of the piezoelectric film to serve as an electrode on the other side; and then, sequentially and repeatedly coating and forming the electrodes to obtain piezoelectric films and sputtering the electrodes to finally obtain the flexible multilayer actuator.

Further, the 0-0-3 type flexible piezoelectric composite material is molded by a tape casting method or a spin coating method to obtain a piezoelectric film; the single electrode layer is a single-layer metal or a plurality of layers of metals, and each layer of metal is independently selected from gold, platinum, copper and titanium.

The invention has the beneficial effects that: the invention prepares the 0-0-3 piezoelectric composite material based on PVDF-based polymer, by adding graphene oxide (a large number of functional groups are connected on the main body structure of graphene) as a 0-dimensional conductive phase and PMN-PT as a 0-dimensional single crystal phase into the composite slurry, wherein the edge or the defect of the graphene oxide contains functional groups such as-COOH, -OH, -CH (O) CH-and the like, which can form hydrogen bonds with the molecular chain of the PVDF-based polymer, thereby realizing the oriented arrangement of PVDF molecular chains, finally preparing the PVDF-based composite material with high oriented arrangement, high beta phase content, high piezoelectric performance and good flexibility, meanwhile, the 0-0-3 type composite piezoelectric material has higher electromechanical coupling coefficient, so that the 0-0-3 type piezoelectric composite material is widely applied to the field of flexible devices, particularly the field of flexible multilayer actuators.

Drawings

FIG. 1 is a schematic diagram of a process for preparing a 0-0-3 flexible composite piezoelectric film according to the present invention;

FIG. 2 is a schematic flow chart illustrating the fabrication of a flexible multi-layer actuator according to an embodiment of the present invention;

FIG. 3 is a schematic diagram of an electrode layer sputtering structure according to an embodiment of the present invention;

FIG. 4 is a schematic flow chart illustrating the fabrication of a flexible multi-layer actuator according to an embodiment of the present invention;

FIG. 5 is a schematic diagram of a second electrode layer sputtering structure according to an embodiment of the present invention;

FIG. 6 is a schematic diagram of a second Au electrode layer according to an embodiment of the invention.

Detailed Description

In order to make the aforementioned objects, features and advantages of the present application more comprehensible, embodiments accompanying the present application are described in detail below with reference to the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present application. This application is capable of embodiments in many different forms than those described herein and those skilled in the art will be able to make similar modifications without departing from the spirit of the application and it is therefore not intended to be limited to the embodiments disclosed below.

The invention selects the conductive phase as a third phase material to form the flexible composite piezoelectric film material together with the non-polymer piezoelectric material and the polymer piezoelectric material. The communication mode of the 0-0-3 type piezoelectric composite material is that non-polymer piezoelectric material powder of 0 dimension and conductive phase powder of 0 dimension are dispersed in a three-dimensional communicated polymer matrix to form the 0-0-3 type piezoelectric composite material. The flexible composite piezoelectric film material with high piezoelectric performance and high electromechanical coupling coefficient can be prepared by adjusting the volume fractions of the conductive phase, the non-polymer piezoelectric material phase and the polymer phase. The non-polymer piezoelectric material is piezoelectric ceramic or piezoelectric single crystal. Preferably, the polymer matrix is PVDF-TrFE, the dispersed non-polymer piezoelectric material phase is PMN-PT piezoelectric single crystal powder, and the dispersed conductive phase is graphene oxide with a large number of functional groups on the surface.

According to the 0-0-3 type flexible piezoelectric composite material, graphene oxide with a large number of active functional groups (carboxyl, hydroxyl, epoxy and the like) is selected as a third phase to form the graphene oxide/PVDF-TrFE/PMN-PT piezoelectric composite material, wherein the functional groups of the graphene oxide can be combined with polar F atoms in a PVDF-based polymer in a hydrogen bond mode, so that the combining capacity of the graphene oxide and the PVDF-based polymer is improved, the components of the composite material can be uniformly dispersed, the required polarization voltage can be reduced, the graphene oxide/PVDF-TrFE/PMN-PT piezoelectric composite material is more sufficiently polarized, and the flexible 0-0-3 type piezoelectric composite film with higher piezoelectric performance is finally prepared. The forming method of the 0-0-3 type piezoelectric composite film material is a tape casting method or a spin coating method. According to the invention, the multifunctional group graphene oxide is introduced, so that the piezoelectric property of the PVDF-based polymer is obviously improved, and the PVDF-based polymer is applied to a flexible multilayer actuator.

In the 0-0-3 type flexible piezoelectric composite material, the selected polymer piezoelectric material can be PVDF and PVDF-based copolymer, wherein the PVDF-based copolymer comprises P (VDF-TrFE), PVDF-HFP, P (VDF-CTFE), P (VDF-TrFE-CTFE) and the like. The piezoelectric single crystal selected for use in the present invention may be

(PMN-PT), PMN, PZN-PT, LiNbO₃Lithium tantalate LiTaO₃And the like. Hair brushThe piezoelectric ceramic used in the present invention may be PZT or the like. Preferably, the polymer-based piezoelectric material is polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE), and the non-polymer-based piezoelectric material is piezoelectric single crystal powder

(PMN-PT), the conductive phase is polyfunctional group graphene oxide, and the conductive phase, the polyfunctional group graphene oxide and the conductive phase are compounded into a 0-0-3 type composite piezoelectric film material; the PMN-PT and the graphene oxide are dispersed in the PVDF-TrFE substrate in a 0-dimensional mode, so that the prepared flexible composite film material has high piezoelectric performance.

As shown in fig. 1, an embodiment of a method for manufacturing a 0-0-3 type flexible composite piezoelectric film according to the present invention specifically includes the following steps: firstly, preparing multifunctional group graphite oxide, and ultrasonically stripping to obtain graphene oxide; then adding the solution into a polar organic dispersant solution, and oscillating to uniformly disperse the solution to obtain a DMF/graphene oxide solution; adding a proper amount of PVDF-TrFE, stirring, and uniformly dispersing to obtain a DMF/graphene oxide/PVDF-TrFE solution; and finally, adding a proper amount of PMN-PT, stirring and ultrasonically dispersing to obtain a graphene oxide/PVDF-TrFE/PMN-PT solution, coating the graphene oxide/PVDF-TrFE/PMN-PT solution on a casting plate, drying and hot-pressing to obtain the 0-0-3 type graphene oxide/PVDF-TrFE/PMN-PT flexible composite piezoelectric film. In the prepared graphene oxide/polymer piezoelectric material/non-polymer piezoelectric material solution, the ratio of each substance is as follows: when the piezoelectric single crystal is adopted, 0.0001-15 wt% of graphene oxide, 50-75 wt% of polymer piezoelectric material and 20-40 wt% of piezoelectric single crystal; when the piezoelectric ceramic is adopted, 0.0001-15 wt% of graphene oxide, 50-75 wt% of polymer piezoelectric material and 24-48 wt% of piezoelectric ceramic are adopted. The polar organic dispersant is selected from N, N Dimethylformamide (DMF), dimethylacetamide (DMAc) and acetone.

The flexible multilayer actuator comprises a 0-0-3 type flexible piezoelectric composite material and has a high electromechanical coupling coefficient.

The preparation method of the flexible multi-layer actuator comprises the following steps: sputtering an Au layer on a silicon substrate to serve as a first electrode layer, casting 0-0-3 type flexible piezoelectric composite slurry on the electrode layer to obtain a piezoelectric film, sputtering the Au layer on the surface of the film to serve as an electrode on the other side, casting on the electrode in sequence to obtain the piezoelectric film, and then sputtering the electrode again. The electrode layer can be made of metal such as gold, platinum, copper, titanium and the like, or any of a plurality of metal layers.

The first embodiment is as follows:

as shown in fig. 2, an embodiment of a method of manufacturing a flexible multi-layer actuator of the present invention includes the steps of:

1) and sputtering an Au layer on the silicon substrate to serve as an electrode, and simultaneously obtaining the Si/Au substrate, wherein the schematic diagram of the electrode layer sputtering structure is shown in FIG. 3, and the insulating part of the interdigital electrode is a cured PDMS film.

2) The added multifunctional group graphene oxide is prepared by reacting potassium permanganate, concentrated sulfuric acid, sodium nitrate and graphite to obtain graphite oxide, then, the hydrophilic functional groups on the surface of the graphite oxide act with water molecules to increase the lamellar gaps, and then, the graphite oxide lamellar is peeled off by ultrasonic vibration, and finally, the graphene oxide is prepared.

3) Weighing a proper amount of multifunctional group graphene oxide, dissolving the multifunctional group graphene oxide in a N, N Dimethylformamide (DMF) solution, and performing ultrasonic oscillation for a certain time, wherein the oscillation time is 3 hours in the embodiment.

4) Adding a proper amount of PVDF-TrFE into the DMF/graphene oxide solution prepared in the step 3), and then stirring for several hours until the mixture is uniformly stirred to obtain a DMF/graphene oxide/PVDF-TrFE composite solution.

5) Adding a proper amount of PMN-PT powder into the composite solution prepared in the step 4), stirring, and ultrasonically dispersing for a certain time until the dispersion is uniform, so as to obtain the graphene oxide/PVDF-TrFE/PMN-PT composite slurry. In the prepared graphene oxide/PVDF-TrFE/PMN-PT composite slurry, the ratio of each substance is as follows: 1wt% of graphene oxide, 59wt% of PVDF-TrFE and 40wt% of PMN-PT.

6) Coating the composite slurry obtained in the step 5) on a horizontally placed Si/Au substrate to enable the solution to flow uniformly, then putting the solution into an oven, adjusting the temperature and drying. In the embodiment, the temperature of the oven is 80-800 ℃, and the drying time is 2.5 hours, so that the dry cast film is obtained.

7) And (3) carrying out hot press molding on the casting film obtained in the step 6) to obtain a graphene oxide/PVDF-TrFE/PMN-PT/Au flexible composite structure. In the embodiment, the hot pressing temperature is 80-150 ℃, the pressure is 0.01-10MPa, and the time is 0.5-3 h.

8) And 7) sputtering Au on the composite structure obtained in the step 7) to be used as an inner interdigital electrode layer to obtain a Si/Au// graphene oxide/PVDF-TrFE/PMN-PT// Au structure, namely a Si/Au//0-0-3 type piezoelectric film// Au structure, wherein the insulating part of the interdigital electrode is a solidified PDMS film.

9) Coating the composite slurry obtained in the step 5) on the horizontally placed Si/Au//0-0-3 type piezoelectric film// Au structure obtained in the step 8), enabling the solution to flow uniformly, then putting the solution into an oven, adjusting the temperature and drying. In the embodiment, the temperature of the oven is 80-800 ℃, and the drying time is 2.5 hours, so that the dry cast film is obtained.

10) And (3) carrying out hot-press molding on the casting film obtained in the step (9) to obtain a Si/Au//0-0-3 type piezoelectric film// Au//0-0-3 type piezoelectric film flexible composite structure. In one embodiment, the hot pressing temperature may be 80-150 deg.C, the pressure may be 0.01-10MPa, and the time may be 0.5-3 h.

11) And sputtering Au on the composite structure obtained in the step 10) to be used as an inner interdigital electrode layer to obtain a Si/Au//0-0-3 type piezoelectric film// Au multilayer structure, wherein the interdigital electrode insulation part is a solidified PDMS film.

12) Coating the composite slurry obtained in the step 5) on the horizontally placed Si/Au//0-0-3 type piezoelectric film// Au multilayer structure obtained in the step 11), enabling the solution to flow uniformly, then putting the solution into an oven, adjusting the temperature and drying. In the embodiment, the temperature of the oven is 80-800 ℃, and the drying time is 2.5 hours, so that the dry cast film is obtained.

13) Carrying out hot-press molding on the casting film obtained in the step 12) to obtain a Si/Au//0-0-3 type piezoelectric film// Au//0-0-3 type piezoelectric film flexible composite structure. In one embodiment, the hot pressing temperature may be 80-150 deg.C, the pressure may be 0.01-10MPa, and the time may be 0.5-3 h.

14) And sputtering Au on the composite structure obtained in the step 13) to be used as an inner interdigital electrode layer to obtain a multilayer structure of Si/Au//0-0-3 type piezoelectric film// Au, wherein the insulating part of the interdigital electrode is a solidified PDMS film.

15) Etching off the silicon substrate in the multilayer structure obtained in the step 14) to obtain an Au//0-0-3 type piezoelectric film// Au multilayer structure.

16) And sputtering Au electrode connecting layers on the left side and the right side of the multilayer structure obtained in the step 15).

And (3) polarizing the multilayer structure obtained in the first embodiment at the required polarizing temperature of 80-150 ℃, the polarizing electric field intensity of 10-100 KV/cm, and the required polarizing time of 5-30 min. And voltage is applied to the electrode positions on the two sides, so that the displacement thrust can be obtained by utilizing the inverse piezoelectric effect, and the actuating effect is generated.

Example two:

as shown in fig. 4, an embodiment of a method of manufacturing a flexible multi-layer actuator of the present invention includes the steps of:

1) and sputtering an Au layer on the silicon substrate to serve as an electrode, and simultaneously obtaining the Si/Au substrate, wherein the schematic diagram of the electrode layer sputtering structure is shown in FIG. 5, and the electrode insulation part is a cured PDMS film. As shown in fig. 6, the Au electrode layer is divided into four parts and numbered.

3) Weighing a proper amount of multifunctional group graphene oxide, dissolving the multifunctional group graphene oxide in N, N Dimethylformamide (DMF) solution, and performing ultrasonic oscillation for a certain time, wherein the oscillation time is 3 hours in the embodiment.

5) Adding a proper amount of PMN-PT powder into the composite solution prepared in the step 4), stirring, and ultrasonically dispersing for a certain time until the dispersion is uniform, so as to obtain the graphene oxide/PVDF-TrFE/PMN-PT composite slurry. In the prepared graphene oxide/PVDF-TrFE/PMN-PT composite slurry, the ratio of each substance is as follows: 5wt% of graphene oxide, 75wt% of PVDF-TrFE and 20wt% of PMN-PT.

8) And 7) sputtering Au on the composite structure obtained in the step 7) as an inner electrode layer to obtain a Si/Au// graphene oxide/PVDF-TrFE/PMN-PT// Au structure, namely a Si/Au//0-0-3 type piezoelectric film// Au structure, wherein the electrode insulation part is a solidified PDMS film.

10) And (3) carrying out hot-press molding on the casting film obtained in the step (9) to obtain a Si/Au//0-0-3 type piezoelectric film// Au//0-0-3 type piezoelectric film flexible composite structure. In the embodiment, the hot pressing temperature is 80-150 ℃, the pressure is 0.01-10MPa, and the time is 0.5-3 h.

11) And sputtering Au as an inner electrode layer on the composite structure obtained in the step 10), so as to obtain a Si/Au//0-0-3 type piezoelectric film// Au multilayer structure, wherein the electrode insulation part is a cured PDMS film.

12) Etching off the silicon substrate in the multilayer structure obtained in the step 11) to obtain an Au//0-0-3 type piezoelectric film// Au multilayer structure.

13) Sputtering Au electrode connecting layers around the multilayer structure obtained in the step 12).

And (3) polarizing the multilayer structure obtained in the second embodiment, wherein the required polarizing temperature is 80-150 ℃, the polarizing electric field intensity is 10-100 KV/cm, the required polarizing time is 5-30 min, and the polarizing directions are shown by arrows in fig. 4, namely 3 is the same as 4, 1 is the same as 2, and 3, 4 are opposite to 1 and 2. When the flexible multi-layer actuator is built, the 1 electrode and the 3 electrode are connected in series and lead out a cable, and the 2 electrode and the 4 electrode are connected in series and lead out a cable. Since the 0-0-3 type flexible composite piezoelectric material is polarized in the thickness direction, the polarization direction of the portions (3 and 4) is opposite to that of the other portions (1 and 2). When exciting a 1, 3 series cable, if part 1 of the resonant actuator is shortened, part 3 will be lengthened and vice versa. This is due to the opposite polarization between section 1 and section 3. Thus, a bending in the y-direction is generated and the tip at the resonant actuator will have a vertical displacement component. Thus, when the 2, 4 series cables are excited, a bend is produced in the x-direction and the tip has a horizontal displacement component. If the 1, 3 and 2, 4 wires are excited simultaneously with a phase difference of 90 deg., the two orthogonal bending modes are excited simultaneously with a phase difference of 90 deg.. If a block is stuck on the plane of 1234, the stuck block excites an elliptical motion under simultaneous excitation of 90 ° phase difference, which can be used to drive the slide to move linearly.

Claims

1. A0-0-3 type flexible piezoelectric composite material is characterized in that the composite material is mainly obtained by compounding a polymer piezoelectric material, a non-polymer piezoelectric material and graphene oxide; the non-polymer piezoelectric material is piezoelectric ceramic or piezoelectric single crystal; the functional group of the graphene oxide is combined with the polar F atom in the polymer piezoelectric material in a hydrogen bond mode.

2. The type 0-0-3 flexible piezoelectric composite material of claim 1, wherein the graphene oxide is prepared by a method comprising: reacting potassium permanganate, concentrated sulfuric acid, sodium nitrate and graphite to obtain graphite oxide; and then, the hydrophilic functional groups on the surface of the graphite oxide act with water molecules to increase the lamellar gaps, and the graphene oxide lamellar is peeled off by adopting ultrasonic vibration, so that the graphene oxide is finally prepared.

3. The type 0-0-3 flexible piezoelectric composite of claim 1, wherein the polymer-based piezoelectric material is PVDF or PVDF-based copolymer.

4. The type 0-0-3 flexible piezoelectric composite of claim 3, wherein the PVDF-based copolymer comprises P (VDF-TrFE), PVDF-HFP, P (VDF-CTFE), P (VDF-TrFE-CTFE).

5. The flexible piezoelectric composite material of type 0-0-3 as claimed in claim 1, wherein the piezoelectric single crystal is selected from the group consisting of PMN, PMN-PT, PZN-PT, lithium niobate, lithium tantalate; the piezoelectric ceramic is PZT.

6. A method for preparing the type 0-0-3 flexible piezoelectric composite material according to claim 1, comprising the steps of: preparing graphite oxide, and carrying out ultrasonic stripping to obtain graphene oxide; then adding the mixture into an organic dispersant, and oscillating to uniformly disperse the mixture to obtain a dispersant/graphene oxide solution; adding a polymer piezoelectric material, stirring, and uniformly dispersing to obtain a dispersing agent/graphene oxide/polymer piezoelectric material solution; and finally, adding a non-polymer piezoelectric material, stirring and ultrasonically dispersing to obtain a graphene oxide/polymer piezoelectric material/non-polymer piezoelectric material solution which is a 0-0-3 type flexible piezoelectric composite material.

7. The method of claim 6 wherein the polar organic dispersant is selected from the group consisting of DMF, DMAc, acetone.

8. A flexible multilayer actuator comprising a flexible piezoelectric composite of type 0-0-3 according to claim 1.

9. A method of making the flexible multilayer actuator of claim 8, comprising the steps of: sputtering a metal layer on a silicon substrate to serve as a first electrode layer, then coating the 0-0-3 type flexible piezoelectric composite material on the electrode layer to form a piezoelectric film, and then sputtering the metal layer on the surface of the piezoelectric film to serve as an electrode on the other side; and then, sequentially and repeatedly coating and forming the electrodes to obtain piezoelectric films and sputtering the electrodes to finally obtain the flexible multilayer actuator.

10. The method for manufacturing a flexible multilayer actuator according to claim 9, wherein the 0-0-3 type flexible piezoelectric composite is formed into a piezoelectric film by a casting method or a spin coating method; the single electrode layer is a single-layer metal or a plurality of layers of metals, and each layer of metal is independently selected from gold, platinum, copper and titanium.