LV15570B - Flexoelectric materials from layered polymers - Google Patents

Description

DESCRIPTION OF THE INVENTION

[001] The invention relates to the field of intelligent materials and structures, specifically to materials for harvesting mechanical energy. Sources of mechanical energy are movements and vibrations in the environment, including sound. Using certain materials, it is possible to convert mechanical energy into electrical energy.

The known state of the art

[002] Flexoelectrics are materials that convert mechanical energy into electrical energy similar to piezoelectrics. Unlike piezoelectrics, which are made of special technical ceramics, flexoelectrics can be made of soft polymer materials, they are flexible and deformable, which allows them to be easily integrated into clothing [1].

[003] Flexoelectric properties are observed in multi-layered or porous polymer materials, which have charges of opposite signs on the opposite surfaces of the pores or layers, resulting in the formation of electric dipoles [1]. Electroconductive layers are placed on the outer surfaces of flexoelectric materials - electrodes connected to each other by an external electric circuit. Charges are induced on the conducting electrodes from the electric dipoles (electrostatic induction). As a result of external mechanical action, the material is deformed, which results in a change in the electrical potential between the conducting electrodes. When the electrical potential balance is restored, a current flow is observed in the external electrical circuit (electrical energy is generated).

[004] To obtain charges of opposite sign on opposite surfaces of pores or layers of flexoelectric material, they are subjected to a strong electric field. In this way, between the layers or in the pores, microplasma discharges occur and charges of the opposite sign are formed [1].

[005] An alternative method for creating charges of the opposite sign in the pores of a flexoelectric material is the deposition of different material particles selectively on one of the pore surfaces [2]. As a result of the deformation, the dissimilar materials on the pore walls come into contact and separate, resulting in the formation of triboelectric charges and dipole moments of opposite signs.

[006] The mentioned materials have significant disadvantages: (i) the charges created as a result of the microplasma discharge are not stable over time, and (ii) the selectively deposited particles from one pore wall are also transferred to the opposite pore wall with time, thus preventing the formation of charges of opposite signs on it and electric dipoles. The mentioned disadvantages are the reason that flexoelectric materials are not commercially available.

Purpose and essence of the invention

[007] The aim of the invention is to obtain durable flexoelectric materials whose performance does not decrease during operation.

[008] The objective was achieved by forming multilayered polymeric materials, where each layer is characterized by characteristics selected from the following group: layer porosity, layer surface roughness, cohesive energy, filler particle size (if added), filler particle shape, filler particle size concentration, moreover, the layers have different mentioned characteristics. The greater the difference between the characteristics of the layers, the greater the triboelectric charge at the interface during deformation, since the absolute deformation of each layer is different at the same force. The difference in different characteristics between the layers can be: porosity from 1 to 99%; surface roughness from 10 nm to =50 um Ra; cohesion energy from 50 to 800 kJ/mol; filler content from 1 to 99%; filler size differences from 10 nm to 5 mm. The filler particles can be, for example, in the form of plates or spheres or rods.

[009] It is known that upon contact and detachment (rubbing) of polymers with different cohesive energy, their contact electrification is observed [3]. When forming layered materials, where each polymer layer has a different cohesive energy, their mutual friction and triboelectrification are expected. Such a triboelectrification mechanism has been observed for all polymers [4], but it should be taken into account that the performance of the flexoelectric will depend on the electrification tendency of the polymers, which is different for each polymer.

[010] At least two polymer layers are used to form a multilayer flexoelectric material (the maximum number of layers depends on the maximum thickness of the sample), with at least one different characteristic (layer porosity, layer surface roughness, cohesive energy, filler particle size (if added), fillers particle shape, filler particle concentration), where each layer acquires a triboelectric charge of opposite sign when contacting each other. Fig. 1 this effect is shown for a multilayer flexoelectric material with three layers (A, B, and C). Thus, dipole moments are formed in the volume of the multilayer material and flexoelectric properties are given to the material.

[011] The invention has several advantages: (i) in order to obtain dipole moments in the material, it is not necessary to subject it to an electric field; (ii) there is no need to use complex materials and methods; (iii) the flexoelectric properties of the material are stable over time.

[012] The invention is explained with the following drawings:

Fig. 1 Flexoelectric material from three layers A, B and C, where each layer rubs against each other and acquires a triboelectric charge e ^- of the opposite sign.

Fig. 2 Current measurement at 100 N compression force for a flexoelectric multilayer material made of polyvinylidene fluoride (PVDF) membranes with different pore sizes of 0.10 μm, 0.45 μm, and 0.22 μm, respectively.

Fig. 3 Current measurement at 100 N compressive force for a flexoelectric multilayer material made of layers of the polymers polyvinylidene fluoride (PVDF), ethylene-vinyl acetate copolymer (EVA) and low-density polyethylene (LDPE) with different cohesive energy.

Fig. 4 Current measurement at 100 N compressive force for a flexoelectric multilayer material made of ethyl cellulose (EC) films with different surface roughness.

Fig. 5 Current measurement at 100 N compressive force for a flexoelectric multilayer material made of polydimethylsiloxane (PDMS) films with different BaTiOs filler content (10, 25 and 50 wt %).

Fig. 6 Current measurement at 100 N compressive force for a flexoelectric multilayer material made of ethylene-vinyl acetate copolymer (EVA) films with different sizes of TiO2 filler (100 nm, 50 nm and 10 nm).

Fig. 7 Current measurement at 100 N compressive force of a flexoelectric multilayer material made of ethylene-vinyl acetate copolymer (EVA) films with different forms of hematite filler (nanorods, nanospheres and nanoplates).

Fig. 8 Current measurement at 100 N compressive force for a flexoelectric multilayer material made of 3 layers of polystyrene polymer, where each layer is porous on one side and smooth on the other.

Fig. 9 Current measurement at 100 N compression force for a flexoelectric material consisting of two PDMS bilayers with different cohesive energies.

Examples of implementation of the invention

[013] Example 1:

Polyvinylidene fluoride (PVDF) polymer membranes with different pore sizes of 0.10 μm, 0.22 μm and 0.45 μm are stacked on top of each other so that the membrane with a pore size of 0.45 μm is in the middle. Then the membranes are compressed with a force of 500 N and held for 5 minutes. This step ensures that the layers are interconnected by both open pore mechanical adhesion and molecular forces. In the next step, the layers are rolled between 100 μm thick interconnected polydimethylsiloxane (PDMS) films. The PDMS protective layer ensures that the individual layers in the flexoelectric material do not delaminate. Conductive electrodes are painted on the PDMS surface using silver varnish. The current generated from such a composite under the influence of a mechanical force of 100 N is shown in Fig. 2.

[014] Example 2:

Polymer membranes with different cohesive energies. Polyvinylidene fluoride (PVDF) polymer film is obtained from a solution of PVDF (15%) in dimethylformamide (DMF). The PVDF solution is applied to a polyethylene terephthalate (ITO/PET) substrate coated with electroconductive indium-doped tin oxide (ITO/PET) using a rotating equipment. After application, the PVDF film is dipped in an antisolvent (methanol) for 10 seconds, then dried with compressed air. The PVDF film is then removed from the ITO/PET to obtain a thin film with a cohesive energy of 99.52 kJ/mol. An ethylene-vinyl acetate copolymer (EVA) film is obtained from a mixture of EVA in toluene in a similar manner to the PVDF film, and a film with a cohesive energy of 27.47 kJ/mol is obtained. On the other hand, low-density polyethylene (LDPE) film is obtained by dissolving LDPE in xylene and then substituting in methanol to obtain a film with a cohesive energy of 55.93 kJ/mol. The layers are stacked one above the other so that the layer with the lowest cohesive energy - EVA - is in the middle, then the layers are compressed with a force of 100 N and held for 1 minute. This step ensures mutual bonding of the layers both by mechanical adhesion caused by surface roughness and by molecular forces. In the next step, the layers are rolled between 100 μm thick interconnected polydimethylsiloxane (PDMS) films. The PDMS protective layer ensures that the individual layers in the flexoelectric material do not delaminate. Conductive electrodes are painted on the PDMS surface using silver varnish. The current generated from layers with different cohesive energy is shown in Fig. 3.

[015] Example 3:

Polymer membranes with a smooth and porous surface. Ethyl cellulose (EC) polymer films are obtained from a solution of EC (15%) in dimethylformamide (DMF). The EC solution is applied to the ITO/PET substrate with the help of a rotary method equipment. To obtain a porous membrane after application, the EC film is dipped in an antisolvent (methanol) for 10 seconds, while to obtain a smooth film, it is left to air dry after application. The EC layers are then removed from the ITO/PET and stacked on top of each other with a porous layer in the middle. Next, the layers are compressed with a force of 100 N and held for 1 minute. This step ensures that the layers are interconnected by both mechanical adhesion and molecular forces. In the next step, the layers are rolled between 100 μm thick interconnected polydimethylsiloxane (PDMS) films. The PDMS protective layer ensures that the individual layers in the flexoelectric material do not delaminate. Conductive electrodes are painted on the PDMS surface using silver varnish. The current generated from such layers under the influence of a mechanical force of 100 N is shown in Fig. 4.

[016] Example 4:

Polymer membranes with different concentrations of fillers. Filler-filled polydimethylsiloxane (PDMS) films (BaTiO3) with particle size >100 nm are obtained by adding different amounts of BaTiO3 filler (10, 25 and 50 wt%) to PDMS. The PDMS BaTiO3 mixture is deposited on the ITO/PET substrate with the help of a rotary method equipment. After deposition, the PDMS films are cross-linked for 30 minutes at 100 °C. The PDMS layers are then removed from the ITO/PET and stacked so that the layer with 10 wt% BaTiO3 is in the middle between the layers with 25 and 50 wt%. Then the layers are compressed with a force of 100 N and held for 1 minute. This step ensures that the layers are interconnected by both mechanical adhesion and molecular forces. In the next step, the layers are rolled between 100 μm thick interconnected polydimethylsiloxane (PDMS) films. The PDMS protective layer ensures that the individual layers in the flexoelectric material do not delaminate. Conductive electrodes are painted on the PDMS surface using silver varnish. The current generated from PDMS films with different amounts of fillers is shown in Fig. 5.

[017] Example 5:

Polymer membranes with different particle sizes of fillers. Ethylene and vinyl acetate copolymer (EVA) polymer layers with different sizes of fillers are obtained by dissolving EVA in a solvent - toluene and adding TiO2 particles. Particle sizes in membranes 100 nm, 50 nm and 10 nm. The particles are mixed into the polymer mixture at a volume concentration of 10%. The EVA (TiO2) solution is poured into a Petri dish to evaporate the solvent. After evaporation, EVA (TiO2) polymer is obtained, which is pressed with a hot press at 70 °C with a force of 30 kN for 5 min. EVA (TiO2) films 100 μm thick are obtained. The layers are stacked one on top of the other so that the layer with 10 nm particles is in the middle, then the layers are compressed with a force of 100 N and held for 1 minute. This step ensures that the layers are interconnected by both mechanical adhesion and molecular forces. In the next step, the layers are rolled between 100 μm thick interconnected polydimethylsiloxane (PDMS) films. The PDMS protective layer ensures that the individual layers in the flexoelectric material do not delaminate. Conductive electrodes are painted on the PDMS surface using silver varnish. The current generated from films with different filler sizes is shown in Fig. 6.

[018] Example 6:

Polymer membranes with different forms of fillers. Ethylene-vinyl acetate copolymer (EVA) polymer films with different filler forms are obtained by dissolving EVA in toluene solvent and adding hematite nanorods, hematite nanospheres, and hematite nanoplates. The particles are mixed into the polymer mixture with a volume concentration of 10%. The polymer solution is poured into a Petri dish to evaporate the solvent. After evaporation, an EVA polymer with hematite particles is obtained, which is pressed with a hot press at 70 °C with a force of 30 kN for 5 min. EVA films filled with hematite particles 100 μm thick are obtained. The layers are stacked one above the other so that the layer with spherical hematite is in the middle, then the layers are compressed with a force of 100 N and held for 1 minute. This step ensures that the layers are interconnected by both mechanical adhesion and molecular forces. In the next step, the layers are rolled between 100 nm thick interconnected polydimethylsiloxane (PDMS) films. The PDMS protective layer ensures that the individual layers in the flexoelectric material do not delaminate. Conductive electrodes are painted on the PDMS surface using silver varnish. The current generated from films with different shapes of fillers is shown in Fig. 7.

[019] Example 7:

Polymeric membranes, where each membrane is porous on one side and smooth on the other. Polystyrene (PS) membranes with one surface smooth and the other porous are obtained by spin-coating a 15% PS solution in dimethylformamide (DMF) onto a porous gelatin substrate. The applied layer is dipped in an antisolvent - methanol for 30 seconds to obtain a PS coating. Then the gelatin base is dissolved in water. After dissolving the gelatin, a free PS layer is obtained with one smooth and the other porous surface. The resulting PS layers are stacked one above the other so that the smooth and porous surface are in contact. Then the layers are compressed with a force of 100 N and held for 1 minute. This step ensures that the layers are interconnected by both mechanical adhesion and molecular forces. In the next step, the layers are rolled between 100 nm thick interconnected polydimethylsiloxane (PDMS) films. The PDMS protective layer ensures that the individual layers in the flexoelectric material do not delaminate. Conductive electrodes are painted on the PDMS surface using silver varnish. The current generated from the mentioned material is shown in Fig. 8.

[020] Example 8:

The flexoelectric, consisting of two polymer layers, is made from polydimethylsiloxane (PDMS). The layers are prepared using the casting method, which produces bilayers consisting of variously cross-linked PDMS layers. Bislanis is prepared in two stages. The first layer is prepared from PDMS with a cross-linking ratio (prepolymer to cross-linking agent) of 15:1, the polymer layer is poured into a polystyrene container and cross-linked for 30 min at 100 °C. A 100 μm thick PDMS layer is obtained. After the PDMS layer has cooled, a second PDMS layer is applied with a cross-stitching ratio of 5:1 and cross-stitched for 30 minutes at 100 °C. A bilayer with a total thickness of 200 μm is obtained. After obtaining the bislayer, it is cut into 2.5x2.5 cm sizes and two bislayers are placed between the ITO/PET electrodes. The layers are stacked one on top of the other in such a way that a contact is formed between the PDMS cross-linked with a ratio of 15:1 to 5:1, thus achieving that the polymers with different cohesive energy are in contact with each other. Then the layers are compressed with a force of 100 N and held for 1 minute. This step ensures that the layers are interconnected by both mechanical adhesion and molecular forces. In the next step, the layers are rolled between 100 μm thick interconnected polydimethylsiloxane (PDMS) films. The current generated from layers with different cohesive energy is shown in Fig. 9.

Used information sources

[1] Q. Deng, L. Liu, P. Sharma, Electrets in soft materials: Nonlinearity, size effects, and giant electromechanical coupling, Phys. Rev. E, 2014, 90, 012603.

[2] J. Chun, J.W. Kim, W.-S. Jung, C.-Y. Kang, S.-W. Kim, Z.L. Wang, J.M. Baik, Mesoporous pores impregnated with Au nanoparticles as effective dielectrics for enhancing triboelectric nanogenerator performance in harsh environments, Energy Environ. Sci., 2015, 8, 3006-3012.

[3] A. Šutka, K. Mālnieks, L. Lapčinskis, P. Kaufelde, A. Linarts, A. Bērziņa, R. Zābels, V. Jurķāns, I. Gorņevs, J. Blums, M. Knite, The role of intermolecular forces in contact electrification on polymer surfaces and triboelectric nanogenerators, Energy Environ. Sci., 2019, 12, 2417-2421.

[4] A. Šutka, K. Mālnieks, L. Lapčinskis, M. Timusk, K. Kalniņš, A. Kovaļovs, J. Grunlan, Contact electrification between identical polymers as the basis for triboelectric/flexoelectric materials. Phys. Chem. Chem. Phys, 2020, 22(23), 13299-13305.

Claims (2)

1. A flexoelectric material comprising at least two polymer layers, characterized in that each layer is characterized by characteristics selected from the group consisting of: porosity, surface roughness, cohesive energy, filler content, filler size, filler particle shape, and layers are different from each other in the mentioned characteristics. 2. Material according to claim 1, characterized in that the shape of the filler particles is plates or spheres or rods.