WO2012119994A2

WO2012119994A2 - Sensor material prepared of carbon-ionic liquid-polymer composite

Info

Publication number: WO2012119994A2
Application number: PCT/EP2012/053766
Authority: WO
Inventors: Alvo Aabloo; Urmas Johanson; Indrek MUST; Andres Punning
Original assignee: University Of Tartu
Priority date: 2011-03-05
Filing date: 2012-03-05
Publication date: 2012-09-13
Also published as: WO2012119994A3; EE05663B1; EE201100015A

Abstract

The present invention describes a sensor material used, for example, in robotics, biotechnology, biomedicine and medicine, whereas the sensor material is manufactured from carbon-ionic liquid-polymer composite, comprising at least two separator layers, which are made from carbon-ionic liquid-polymer composite material and comprises at least two electrode layers manufactured of carbon film and separated by a layer manufactured as porous polymer membrane comprising an ionic liquid not having electronic conductivity. At the same time the sensor material acts as actuator, which allows immediate feedback of the actuator curvature, its velocity and change of direction of movement.

Description

Sensor material prepared of carbon-ionic liquid-polymer composite Technical field

The present invention relates to sensors and actuators based on electro-active polymer composites which operate on bending, more specifically the invention relates to sensor material produced from carbon-ionic liquid-polymer composite. In addition, the present invention relates to a method for manufacturing the said laminated sensor material and the principles of selecting appropriate materials. The invention finds application in the fields of robotics, biotechnology, biomedicine, and medicine.

Background Art

In the prior art, actuators (US5268082) which are based on ionic polymer-metal composite (IPMC) materials and which comprise of two layers of conductive precious metal or electrically conductive polymer or carbonaceous electrodes and a ionic polymer layer (membrane) are known. An ion-conducting polymer layer containing water as a solvent bends or deforms due to electrical voltage applied to electrode layers.

IPMC materials can also be used in the manufacture of the so-called self-sensing actuators in which the position sensor and the actuator are made of the same material: [WO2007101448] and the publications "Self-sensing ionic polymer-metal composite actuating device with patterned surface electrodes" Kruusamae, K.; Brunetto, P.; Graziani, S.; Punning, A.; Di Pasquale, G.; Aabloo, A. Source: Polymer International, v 59, n 3, p 300-4, March 2010, "A self-sensing ion conducting polymer metal composite (IPMC) actuator"; Punning, A. (Inst, of Technol., Tartu Univ., Estonia); Kruusmaa, M.; Aabloo, A. Source: Sensors and Actuators A (Physical), v 136, n 2, pp. 656-64, 16 May 2007.

Major disadvantages of the IPMC actuators which use water as a solvent are the complexity of their manufacture, low resistance to repeated deformation and, in the case of operating in non-aqueous medium, evaporation of the solvent (water) containing in the polymer, as a result of which the actuator will stop operating. Therefore, non-aqueous actuators have been studied in which ionic liquids are used as a solvent. These actuators also work in conventional circumstances and are more stable in time. The use of ionic liquids in the manufacture of actuators which employ ionic polymer metal composites has been described in the patent application US2005/0103706 and in the following publications: B.J. Akle, M.D. Bennett and D.J. Leo, High-strain ionomeric-ionic liquid el ectroactive actuators, Sens. Actuators A: Phys. 126 (2006), pp. 173-181 ; M. D. Bennett and D. J. Leo, Ionic Liquids as Solvents for Ionic Polymer Transducers, Sensors and Actuators A: Physical, Vol. 1 15. pp. 79-90 (2004); Matthew D. Bennett, Donald J. Leo, Garth L. Wilkes, Frederick L. Beyer and Todd W. Pechar, A model of charge transport and electromechanical transduction in ionic liquid-swollen Nafion membranes, Polymer, Volume 47, Issue 19, 2006, pp. 6782-6796.

Also, actuators are known which act by bending or deforming their ionically conductive polymer membranes (US200701 141 16) and have electrode layers comprising of fine carbon particles (carbon black), which are connected with an ionic polymer or electron-conducting organic polymer (polypyrrole). With the aim to achieve better performance, the electrodes containing carbon powder (carbon black) are coated with a thin sheet of precious metal (gold or platinum).

B. Akle et al have proposed the so-called Direct Assembly Process for manufacturing actuators, which allows the use of various materials of high specific surface area (ruthenium(IV)oxide, carbon nanotubes, carbon black, etc.) in the composition of I PMC electrodes. The process of direct assembly in the production of such actuators has been described in the patent application US2006026642 and the publications B.J. Akle, M.D. Bennett, D.J. Leo, K.B. Wiles, J.E. McGrath, Direct assembly process: A novel fabrication technique for large strain ionic polymer transducers, Journal of Materials Science 42 (16) (2007) 7031 -7041 ; B.J. Akle, M.D. Bennett and D.J. Leo, High-strain ionomeric-ionic liquid electroactive actuators, Sens. Actuators A: Phys. 126 (2006), pp. 173-181 ; B. Akle, S. Nawshin, D. Leo, Reliability of high strain ionomeric polymer transducers fabricated using the direct assembly process, Smart Materials and Structures 16 (2007) S256- S261. According to this method, electrode layers are deposited onto a polymer membrane containing ionic liquid by spraying, followed by hot-pressing of the material. Generally, an additional metal layer (such as gold foil) is deposited onto the electrode surface in the stage of hot pressing. Also, the polymer membrane may be treated with ionic liquid before or after hot pressing. Also, a method is known for preparing thin films comprising ionic liquid, polymer, and carbon nanotubes and for constructing laminated actuators from the said films, which is described in the patent application US7315106 and the articles by K. Mukai, K. Asaka, T. Sugino, K. Kiyohara, I. Takeuchi, N. Terasawa, D. N. Futaba, K. Hata, T. Fukushima, T. Aida, Adv. Mater. 20 (2009) 1 -4; I. Takaeuchi, K. Asaka, K. Kiyohara, T. Sugino, N. Terasawa, K. Mukai, T. Fukushima, T. Aida, Electromechanical behavior of fully plastic actuators based on bucky gel containing various internal liquids, Elecrochimica Acta 54 (2009) 1762-1768.

Patent application US6555945 describes actuators based on double-layer induced charge injection in nanotubes of high specific areas. This actuator operating at low voltages has layers in which carbon nanotubes are used as electron-conducting material. The synthesis of the carbon nanotubes, however, cannot be easily controlled and the products of synthesis contain carbon nanotubes of varied dimensions which requires the employment of costly separation techniques so as to select nanotubes with appropriate properties.

Methods for the synthesis of carbide-derived nanoporous carbon (CDC) and the use of films prepared from such powders for manufacture of supercapacitors is described in patent applications US11/407202; WO 2005/118471 ; WO 2004/094307, and in the article by Gogotsi, Y., Nikitin, A, Ye, H., Zhou, W., Fischer, J. E., Yi, B., Foley, H. C, Barsoum, M. W. Nanoporous carbide-derived carbon with tunable pore size, Nature Materials 2003, 2, 591. Carbide-derived carbon is a nanostructural carbonaceous material made of metal or non-metal carbide with a large specific surface area (800-2000 m²/g, after postprocessing up to 2500 m²/g) and an average pore size within the range of 0.3-2 nm (classified as the class of microporous materials according to lUPAC classification), the macro- and microstructure of which follows the shape and size of starting carbide. During the production process of carbide-derived carbon, the size distribution of nanopores (starting from 0.6-0.7 nm) can be varied by alternating controllable parameters., Carbide-derived carbon electrodes have a characteristically large and stable electrical double-layer capacitance and electro-active properties when external electric field is applied.

The use of CDC in actuators has been described in the patent application WO2009143857 and the article by Janno Torop, Mati Arulepp, Jaan Leis, Andres Punning, Urmas Johanson, Viljar Palmre, Alvo Aabloo (2010) Nanoporous Carbide- Derived Carbon Material-Based Linear Actuators, Materials, 3(1 ), 9-25. The use of ionic liquids and various carbonaceous materials as mechanical sensor materials has been described in patent applications US20100288635A1 and US20090243428.

The composite electrodes according to the present inventions, which serve as sensors, contain, differently from solutions known in the state of art, appropriate amounts of carbide-derived nanoporous carbon. Composite material used as electrode is composed of carbide-derived carbon powder, ionic liquid and a porous polymer. Producing carbide-derived nanoporous carbon is essentially easier, can be controlled more precisely and requires fewer resources as compared to the production of carbon nanotubes. In addition, the material can be used in preparing self-sensible actuators in which the same material acts as an actuator and a sensor at the same time.

Disclosure of Invention

The objective of the present invention is to provide a mechanoelectrical sensor. Bending of this mechanoelectrical sensor generates voltage and current hence it can be applied as a motion sensor and/or as an energy harvesting unit. The aim of the invention is achieved by providing a sensor according to the present invention which comprises a laminate, which contains at least two electrode layers (carbon films) composed of carbide-derived carbon (for example highly homoporous carbide derived carbon) and ionic liquid, and separated by a separator layer. The electrode layer may contain porous polymer or carbon nanotubes or other additional carbonaceous materials. Electrodes may be coated with an additional conductive electrode layer. Electrode layers are separated by an electronically non-conductive and ionically conductive porous polymer membrane containing ionic liquids. Carbon content of the described electrode layers is within the range from 5 to 95 per cent by weight. Larger carbon density in the electrodes allows achieving a more sensitive sensor and more powerful actuator, whereas a smaller density allows greater bending speed. The composite sensor/actuator according to the present invention is manufactured from independent carbon films and a membrane by means of a process such as hot pressing.

By measuring voltage and electric current at terminals the radius of curvature of the sensor as well as the rate and direction of its variation can be determined. By combining the actuator described in the previous patent application and the object of the present invention it is possible to achieve an actuator which gives feedback on its curvature, velocity and change of direction of movement.

The device generates electric current and voltage, and it is possible to use the device as a power source.

Brief Description of Drawings

The present invention will be illustrated by means of the following figures wherein: Figure 1 illustrates a three-layer sensor according to the present invention;

Figure 2 illustrates a sensor in a bent state or in working conditions after motion or after acting as actuator;

Figure 3 is a diagram of a measuring device used for measuring the current generated in the sensor material; and

Figure 4 is a schematic diagram representing the action of the measured voltage and electric current in the case of mechanical bending and calculated charge values from electrodes.

Figure 5 is a schematic representing the transient course of ion diffusion and double-layer rearrangement processes of ionic liquid in the carbon - polymer - ionic liquid composite after straining (bending) the laminate at time t=0.

Figure 6 is a SEM image of the three-layer laminate after hot-pressing.

Figure 7 is a SEM image of the same spot on three-layer laminate after bending to 10³ m- .

Best Mode for Carrying Out the Invention

Carbon films according to the present invention may be manufactured with various methods.

In one embodiment of the present invention, carbon films are prepared from a liquid suspension which is dried in a vacuum oven after casting. As a result, a thin porous carbon film containing ionic liquid is achieved which is suitable for hot pressing without any further treatment. The said suspension contains not only carbon film components but also necessary amounts of an appropriate solvent which will be later removed by evaporation in a vacuum oven.

In another embodiment of the present invention, thin carbon films are formed from a more viscous paste which also contains all components necessary for preparing carbon films. A homogenous layer of the said paste is applied on a glass base using a specific applicator, and the produced electrode layer is then dried. The described method guarantees films with a uniform thickness.

In the applications according to the present invention, carbon films are prepared from carbide-derived carbon material (CDCs), which is obtained from TiC at temperatures 400 °C, 600 °C, 800 °C, 850 °C, and 950 °C and which has a varied pore distribution in carbon material. Carbon films may also be prepared from B₄C and M02C and from materials synthesised from carbon aerogel. So as to aggregate carbon material in the electrode layer, poly(vinylidene difluoride- hexafluoropropylene) (PVdF(HFP)) (Sigma Aldrich) was used as a binder, and to dissolve the latter, Ν,Ν-dimethylacetamide was used as a solvent (DMAc) (Sigma Aldrich). 1-ethyl-3-methyl-imidasolium tetrafluoroborate (EMIBF₄) (Sigma Aldrich) was used as ionic liquid.

a) Preparation of electrodes

Electrodes according to the exemplary embodiment of the present invention contain 35% (by weight) of PVdF(HFP), 10% (by weight) of EMIBF₄ and 30% (by weight) of carbon material. In order to form electrodes, 0.1 g of the polymer PVdF(HFP) was weighed and dissolved in 1.5 ml of DMAc. Amounts of carbide- derived carbon and ionic liquid (EMIBF₄) corresponding to the amount of polymer were weighed, 0.5 ml of DMA was added and the mixture was stirred in an ultrasonic bath for 25 minutes. After that, the previously prepared polymer solution was added to the suspension of CDC carbon and ionic liquid. The resulting mixture was stirred on a magnetic stirrer and sonicated again for 20 minutes. After formation of a uniform suspension, the mixture was poured into a polytetrafluoroethylene (PTFE) mould and was dried in a vacuum oven,

b) Preparation of polymer membrane The polymer membrane is composed of 50% (by weight) of PVdF(HFP) and 50% (by weight) of EMIBF₄. For preparing the membrane, 0.15 g of PVdF(HFP) was weighed and dissolved in 1.5 ml of DMAc. After that, an amount of ionic liquid corresponding to that of the polymer was added to the dissolved polymer and the mixture was treated in an ultrasonic bath for 30 minutes. The resulting mixture was poured into a polytetrafluoroethylene (PTFE) mould and was left there to dry.

c) Hot-pressing of the material

The obtained polymeric films were arranged as follows: the polymeric membrane was sandwiched between the carbon electrodes and assembled by hot-pressing at a temperature of 120 °C and pressure of -20 MPa for 10 seconds. The edges of the resulting composite laminated material were smoothed in order to prevent the generation of short-circuit at electrodes. In addition, a thin metal layer may be pressed on both sides of the composite. The said metal may be selected from a group consisting of gold (Au), platinum (Pt), also aluminium (Al) and titanium (Ti). Essentially, the above described and prepared composite acts as a sensor as well as an actuator operating on bending. When voltage is applied to the prepared carbon film the composite acts as a shape-modifying actuator. The described material may also be used as a capacitor because the carbon material of large specific area and the ionic liquid containing in the film together form an electric double-layer capacitor. The formation of electric double-layer on ionic-liquid- carbon boundary allows depositing remarkable amounts of electric power. Such capacitors may self-discharge much faster than special electric double-layer capacitors, but they operate on the same principles. The described material can also be used to produce a small amount of electric power, because mechanical straining of the material causes changes in double-layer, which results in formation of electric current and potential between differently strained parts of composite. In addition to measuring the deformation extent, the generated electric power can be collected and used to power electric devices. The material also has vibration damping properties. Figure 1 is a three-layer sensor according to the present invention, wherein 1 and 5 represent terminals, 2 and 4 represent CDC+IL+polymer layers and 3 represents the separator or polymer+IL (ionic liquid). Figure 2 illustrates the same sensor in its strained state after bending motion. Figure 3 is a device for measuring the voltage and electric current generated in the sensor and Figure 4 is a schematic diagram showing the transient course of measured voltage and electric current under mechanical bending. In addition, charge values collected from sensor have been calculated. Figure 5 is a schematic representing the transient course of ion diffusion and double-layer rearrangement processes of ionic liquid in the carbon - polymer - ionic liquid composite after straining (bending) the laminate at time t=0. Figure 6 is a SEM image of the three-layer laminate after hot-pressing. Figure 7 is a SEM image of the same spot on three-layer laminate after bending to 10³ nr¹.

Options for selecting materials suitable for the preparation of the sensor.

In preparing a composite sensor/actuator according to the present inventor, the carbide-derived carbons (SiC-CDC, TiC-CDC, Mo₂C-CDC, AI₄C₃-CDC, B₄C-CDC, VC-CDC, NbC-CDC, etc.) may be used as carbonaceous material, but carbon aerogels are also suitable. With the aim to improve electron-conductive properties, both sides of a composite may be coated with a thin metal layer. As additional carbonaceous material, single-wall carbon nanotubes of high purity and metallic conductivity may be used as well as commercially available TIMCAL SUPER R carbonaceous material.

Suitable polymers are selected according to their solubility in the selected solvent, their chemical stability in a given system and the mechanical properties of a polymer, also the porosity of the polymer-based membrane. In the exemplary embodiment of the present invention, PVdF(HFP), a polymer belonging to a large family of fluoropolymers was used, but another member of the same family, KYNAR 2801 , has also been tested. For use as a membrane only, cellulose-based polymers (e.g. products by NIPPON KODOSHI 5) may be used.

Suitable ionic liquids are the liquids that are capable of remaining liquid at the operating temperature of the sensor and in a given composite sensor/actuator system. In preferred applications of the present invention, ionic liquids are of low viscosity (less than 1 Pa), have a low melting point and high ionic conductivity. In suitable ionic liquids, cations are various alkyl-substituted ammonium ions, alkyl- substituted pyrrolidonium ions, alkyl-substituted imidasolium ions, alkyl-substituted piperidinium ions, etc., and anions are trifluoromethanesulphonate ions, bis(trifluoromethanesulfonyl)imide ions, bis(fluorosulfonyl)imide ions, bis(pentafluoroethanesulfonyl)imide ions, tetrafluoroborate, hexafluorophosphate, etc.

Claims

1. Sensor material prepared of carbon-ionic liquid-polymer composite comprising at least two electrode layers manufactured from carbon film and separated by a separator layer and a separator layer manufactured as a porous polymer membrane containing an ionic liquid not having electron conductivity, wherein the sensor material is intended to act same time as an actuator.

2. Sensor material according to claim 1 , characterised in that the carbon film is manufactured from a liquid suspension or a paste-like mixture of carbide-derived carbon, ionic liquid and polymeric material, wherein first a homogeneous suspension or paste-like mixture is prepared from said components whereafter a layer of uniform thickness is formed and finally said layer is dried to form a homogeneous carbon film.

3. Sensor material according to claim 1 , characterised in that a polymer membrane is prepared from the mixture of a polymer and ionic liquid, wherein a polymer is first dissolved in a solvent, then ionic liquid is added to the resulting polymer solution, and finally, the resulting mixture is dried.

4. Sensor material according to claim 1 , characterised in that a polymeric membrane is sandwiched between carbon film electrodes in order to form a composite laminate and all layers are then hot-pressed at temperatures between 70 and 130 °C and at a pressure between 10-25 MPa for 5-20 seconds.

5. Sensor material according to any of the claims 1 to 3, characterised in that the ionic liquid is selected from ionic liquids in which cations are various alkyl- substituted ammonium ions, alkyl-substituted pyrrolidonium ions, alkyl-substituted imidasolium ions, or alkyl-substituted piperidinium ions, and anions are trifluoromethanesulphonate ions, bis(trifluoromethanesulfonyl)imide ions, bis(fluorosulfonyl)imide ions, bis(pentafluoroethanesulfonyl)imide ions, tetrafluoroborate, or hexafluorophosphate.

6. Sensor material according to claim 2, characterised in that the carbide-derived carbon material for use in manufacturing carbon films is selected from a group containing SiC-CDC, TiC-CDC, M02C-CDC, AI₄C₃-₅-CDC, B₄C-CDC, VC-CDC, NbC-CDC, or from carbon aerogels.

7. Sensor material according to claim 3, characterised in that the polymer is selected from a group containing KYNAR 2801 , PVdF, PVdF(HFP).

8. Sensor material according to claim 1 , characterised in that a metal layer is pressed onto both sides of the carbon-ionic liquid-polymer composite, wherein the metal layer is formed from a metal selected from a group containing gold (Au), platinum (Pt), aluminium (Al), titanium (Ti).

9. Sensor material according to any of claims 1 to 8, characterised in that the composite material comprises a number of electrodes made from carbon film whereas polymeric non-conductive membranes are placed between the electrodes and the external electrodes of said composite material are coated with a thin metal layer.