WO2009143857A2

WO2009143857A2 - Actuator

Info

Publication number: WO2009143857A2
Application number: PCT/EE2009/000008
Authority: WO
Inventors: Janno Torop; Jaan Leis; Mati Arulepp; Urmas Johanson; Alvo Aabloo
Original assignee: University Of Tartu; Carbon.Ee Oü
Priority date: 2008-05-30
Filing date: 2009-06-01
Publication date: 2009-12-03
Also published as: WO2009143857A3; EE200800042A

Abstract

Actuator comprises at least one layer, composed of nanoporous carbide derived carbon, and containing an electrolyte, such as ionic liquid. The actuator extends linearly when voltage is applied to the actuator. Nanoporous carbide derived carbon layer preferably contains 80 to 100 volume percent of a material having an average pore size is less than 1 nanometer and at least 50 per cent of the total pores volume is from the pores with the size less than 1.1 nanometers. Actuator may comprise two layers of nanoporous materials, separated by membrane of polymer material.

Description

ACTUATOR

TECHNICAL FIELD

The present invention belongs to the field of actuators composed of electroactive materials. The invention can be used in applications requiring an actuator with at least one- dimensional change of dimensions, which can be controlled with signals in the voltage range 0-50V. Such actuators, with displacement amplitude of less than 100 μm, can be used, for instance, in the manipulation of microfluidics, in biomedicine, medicament dosing and operation of optical devices.

BACKGROUND ART

Known prior art includes a multi-layer actuator based on an conductive polymer (polypyrrole) and carbon nanotubes (Polymethyl methacrylate, PMMA) gel electrolyte solution flanked on both sides by layers of polypyrrole and carbon nanotubes, covered by a layer of gold to improve conductivity, and protected by an external polymeric protective layer) (patent application US2007/0262677). Such low- voltage actuator operates through bending (i.e., the thin layered material bends if the surface-layer electrodes of the material are connected to an external power source). The electrodes are made of an organic polymer (polypyrrole), which includes carbon nanotubes.

There is a known actuator, which utilises the bending or deformation of an ionically conductive polymer membrane and comprises two conductive electrode layers, separated by a layer of ionically conductive polymer material (patent application US20070114116). Such low- voltage actuator also operates through bending. The electrodes are made of fine carbon powder (carbon black), which are bound to an ionically conductive polymer (resin) or an electroconductive organic polymer (polypyrrole). For better results, the electrode containing the carbon powder (carbon black) is covered with a thin sheet of precious metal (gold or platinum).

There is further a known actuator, which is based on an electric double-layer with an ionically conductive separator and two electrodes, implemented with carbon nanotubes (US6555945). Such low-voltage actuator also operates through bending. The electrodes are made of an ionic polymer, which includes carbon nanotubes (or carbon aerogel or highly porous graphite sheet).

There are further known actuators, which comprise various gel compositions of polymers, ionic liquids and carbon nanotubes in the electrode layer (US7315106). Such low-voltage multi-layer actuators also operate through bending. The electrodes are made of a polymer gel, which includes carbon nanotubes.

Even though reasonably consistent results have been achieved with such bending actuators, a major problem of such thin-membrane systems is the relatively low force generated by these actuators (generally only up to 0.05 N per actuator). Another serious problem with bending actuators is the difficulty of combining the forces of multiple actuators, i.e., using more than one actuator simultaneously to attain greater force. Various solutions have been proposed but they tend to overcomplicate the corresponding systems, particularly the controlling of the systems.

However, if the direction of actuation is perpendicular to the plane of the electrodes, force can be increased by increasing the surface area of the actuator. The amplitude of displacement can be increased by placing a larger number of actuators on top of each other.

Known prior art includes actuators, which are based on carbon materials with helicoid nanostructure (WO 2007/090639) and have also a layered structure and operate at low voltage. The electrodes are made of a carbon-containing ionic polymer with helicoid nanostructure.

Supercapacitors utilise microporous carbon materials as electrodes, but a problem is the expansion of such materials during the use of a supercapacitor. M. Hahn et al. (Hahn, M., Barbieri, O., Campana, M., Gallay, R., Koetz R.) Charge-induced dimensional changes in electrochemical double layer capacitors. Proc. 14th International Seminar on Double Layer Capacitors and Hybrid Energy Storage Devices, Dec. 4-6, 2004, Deerfield Beach, Florida) studied the expansion of microporous carbon electrodes of a supercapacitor (electrical double layer condensator, or EDLC) in 1.0M tetraethylammonium tetrafluoroborate acetonitrile (TEABF4/AN) electrolyte in relation to the applied potential. An expansion by ~1 μm was observed when the microporous electrode of 150 μm in thickness was charged at -1.5V of negative potential. M. Hahn et α/.(Hahn, M., Barbieri, O., Campana, F.P., Kδtz, R., Gallay, R. Carbon based double layer capacitors with aprotic electrolyte solutions: the possible role of intercalation/insertion processes. Applied Physics A 2006, 82, 633-638) have also recorded expansion of microporous carbon in 1.0M TEABF4 propylene carbonate electrolyte, observing expansion of 160 μm electrode by approximately 10 μm.

Known prior art includes methods of production of carbide-derived carbon (CDC) (USl 1/407202, WO2005/118471, WO2004/094307). Carbide-derived carbon is a nanostructural carbon material extracted from metal carbides or non-metal carbides. The macro- and microstructure of the carbide-derived carbon is similar to the precursor carbide in shape and size. Carbide-derived carbon has a high specific surface area (100 to 2000 m²/g, and up to 2500 m²/g with post-processing). Adjustment of controllable parameters during the production of nanoporous CDC enables to vary the nanostructure of the carbon material and to fine-tune the size of nanopores (from 6-7 A) and the distribution of pore sizes. CDC is characterised by very high and stable electric double-layer capacitance and is suitable for utilisation in supercapacitors.

The objective, in the context of developing supercapacitors, has been to develop materials, which would undergo only minimal expansion during the use of a supercapacitor to ensure maximum service life and stability over time of the electrodes and the supercapacitor. Utilisation of such materials in actuators has not been studied or implemented. DISCLOSURE OF INVENTION

The invention is a multi-layer actuator, which generates linear movement (expansion), perpendicular to the layers, as a result of voltage applied to the actuator. For this purpose, the actuator comprises at least one layer of nanoporous material, such as carbide-derived carbon, containing a suitable solvent and electrolyte or ionic liquid. The actuator may also comprise two layers of nanoporous material, separated by a porous polymeric membrane.

hi order to operate the actuators, signals in the voltage range of 0-50V are applied to the layer(s) of nanoporous material through terminal(s) attached to the material. Such actuators can be used in various microactuators with displacement amplitude of less than 100 μm, which can be utilised in the manipulation of microfiuids, in biomedicine, medicament dosing and operation of optical devices. BRIEF DESCRIPTION OF DRAWINGS

Fig. 1 is a cross-section of one actuator cell.

Fig. 2 is a scheme of an actuator assembled from actuator cells.

Fig. 3 illustrates the operating principle of the actuator.

Fig. 4 illustrates the voltage and current applied to the actuator in relation to time and the actuation and speed in relation to time according to embodiment 9 of the invention.

Fig. 5 illustrates the correlation between the charge applied to the actuator and the actuation according to embodiment 12 of the invention (TiC 800 °C/EMITf (I= 40OmA, Umax= 3V)).

Fig. 6 illustrates the voltage and current applied to the actuator in relation to time and the actuation and speed in relation to time according to embodiment 13 of the invention.

Fig. 7 illustrates the change in the pore size of the carbon material in response to the electric field applied to ions.

MODES FOR CARRYING OUT THE INVENTION

An actuator according to one embodiment of the invention is shown on Fig. 1 and comprises nanoporous carbon films 3 and 5, covered by metal layers 1 and 7, and separated by polymer membrane 4. In preferred embodiments of the invention, the nanoporous carbon film is made of carbide-derived carbon material. External force is applied to the actuator, e.g., by putting a weight on the top of the actuator. A vertical displacement Δh of the layers occurs upon charging the actuator through terminals 2 and 6 (see Fig. 3). In one embodiment, the metal layers 1 and 7 can be made of aluminium.

In order to measure the displacement of the actuator, a package according to Fig. 2, comprising nanoporous carbon films 9 and 11, separators 10 and internal conductors 12, was assembled and charged through terminals 8 and 13.

Vertical displacement was determined by placing the actuator in a hermetically sealed and spring-pressurised measuring cell. The system was subjected to constant potential, which causes linear expansion of the material if in the range of 1-3 V. The movement of the actuator as a result of the expansion of the material is transferred to a mirror system, which projects a laser beam onto a screen as a moving spot. A high-speed camera (max 200 frames/s) registers any changes in the position of the mirror, which represent the movement of the actuator. The actuator was gauged at a micro-scale using an adjustment screw (e.g., by Thorlabs).

Table 1. Characteristics of the nanoporous carbide-derived carbon material used in the invention

Microporosity

SBET Vμ Vtot [volume %] APS

CDC [m²/g] [cm³/g] [cm³/g] [A]

< 2.0 nm < l.l nm

TiC-Cl 600 °c 1150 0.487 0.534 91 70 9.3 TiC-Cl 800 °c 1470 0.594 0.712 83 50 9.7

S_BET- specific surface area, determined by low temperature nitrogen adsorption

V_μ — volume of micropores V_tot - total volume of pores

APS - average pore size, calculated according to the formula APS = 2 ^• V_tot / S_BET

Embodiments 1-4 describe the production of an actuator according to the invention.

Embodiment 1

The preparation started with 1.0 g of carbon, synthesised at 600°C from titanium carbide, with specific surface area 1150 m²/g, average pore size 9.3 A and volume of micropores with size less than 1.1 run constituting 70% of the total volume of pores according to the BJH method. The carbon powder was mixed with 10 ml of ethanol and 0.18 g of polytetrafluoroethylene (PTFE) suspension in water (Aldrich, 60% wt.) was added. The resulting mixture was mixed carefully until it achieved a plastic consistency. The mixture was then pressed into wet cakes, which were gradually rolled under a press until a uniform thickness of 100 μm was achieved. One side of these carbon firms, was coated with approximately 2 μm of aluminium, using plasma activated physical vapour deposition. Ten discs with a diameter of 0=17mm were cut from the resulting carbon film. A polymeric separation cellulose membrane (e.g., by Nippon Kodoshi) of 30μm in thickness was placed between two discs. Five such pairs of discs were piled on top of each other and connected into a parallel electrical connection by using aluminium foil, as shown on Fig. 2.

The package shown on Figure 2 was hermetically sealed in a container with a vertically moveable pressurised cover. Vacuum was then created inside the container and the container was filled with a solution of 1.0 M tetraethylammonium tetrafluoroborate in propylene carbonate (e.g., by "Honeywell, Digirena ®" TEABF₄/PC).

Embodiment 2

An actuator according to embodiment 1, but the carbon used in the actuator was produced from titanium carbide at 800°C and had a specific surface area 1470 m²/g, average pore size 9.7 A and volume of micropores with size less than 1.1 nm constituting 50% of the total volume of pores according to the Barrett- Joiner- Halenda (BJH; see ) method.

Embodiment s

Same as embodiment 1 but characterised in that the actuator was filled with C₇HnF₃N₂O₃S (l-ethyl-3-methylimidazolium trifluoromethanesulfonate; EMITf) ionic liquid (Fluka, CAS: 145022-44-2).

Embodiment 4

Same as embodiment 2 but characterised in that the actuator was filled with C₇H₁₁F₃N₂O₃S, (EMITf) ionic liquid (Fluka, CAS: 145022-44-2).

Embodiments 5-13 describe the use of an actuator according to the invention.

Embodiment 5

The test cell of the actuator according to embodiment 1 was charged with direct current of 400 mA up to the voltage of 2.0 V and was maintained at that potential for 5 minutes. The measurements taken during charging included the vertical displacement Δh, which is expressed in Table 1, and the average displacement speed DΔh/t, which is expressed in Table 2. Embodiment 6

The test cell of the actuator according to embodiment 1 was charged with direct current of 400 mA up to the voltage of 3.0 V and was maintained at that potential for 5 minutes. The measurements taken during charging included the vertical displacement Δh, which is expressed in Table 1, and the average displacement speed DΔh/t, which is expressed in Table 2.

Embodiment 7

The test cell of the actuator according to embodiment 2 was charged with direct current of 400 mA up to the voltage of 2.0 V and was maintained at that potential for 5 minutes. The measurements taken during charging included the vertical displacement Δh, which is expressed in Table 1, and the average displacement speed DΔh/t, which is expressed in Table 2.

Embodiment 8

The test cell of the actuator according to embodiment 2 was charged with direct current of 400 mA up to the voltage of 3.0 V and was maintained at that potential for 5 minutes. The measurements taken during charging included the vertical displacement Δh, which is expressed in Table 1, and the average displacement speed DΔh/t, which is expressed in Table 2.

Embodiment 9

The test cell of the actuator according to embodiment 3 was charged with direct current of 400 mA up to the voltage of 2.0 V and was maintained at that potential for 5 minutes. The measurements taken during charging included the vertical displacement Δh, which is expressed in Table 1, and the average displacement speed DΔh/t, which is expressed in Table 2. The voltage and current in relation to time and the actuation and speed in relation to time are shown in the chart on Fig. 4.

Embodiment 10

The test cell of the actuator according to embodiment 3 was charged with direct current of 400 mA up to the voltage of 3.0 V and was maintained at that potential for 5 minutes. The measurements taken during charging included the vertical displacement Δh, which is expressed in Table 1, and the average displacement speed DΔh/t, which is expressed in Table 2.

Embodiment 11

The test cell of the actuator according to embodiment 4 was charged with direct current of 400 mA up to the voltage of 3.0 V and was maintained at that potential for 5 minutes. The measurements taken during charging included the vertical displacement Δh, which is expressed in Table 1, and the average displacementspeed DΔh/t, which is expressed in Table 2.

Embodiment 12

The test cell of the actuator according to embodiment 4 was charged with direct current* of 400 mA up to the voltage of 3.0 V and was maintained at that potential for 5 minutes. The measurements taken during charging included the vertical displacement Δh, which is expressed in Table 1, and the average displacement speed DΔh/t, which is expressed in Table 2. The correlation between the charge applied to the actuator and the actuation is shown in the chart on Fig. 5.

Embodiment 13

The test cell of the actuator according to embodiment 4 was charged for 60 seconds with direct current of 1000 mA up to the voltage of 3.0 V, after which the system was short- circuited for 60 seconds and the cycle was repeated. The measurements taken during charging included the vertical displacement Δh, which is expressed in Table 1, and the average displacement speed DΔh/t, which is expressed in Table 2. The voltage and current in relation to time and the actuation and speed in relation to time are shown in the chart on Fig. 6 Table 2. Actuator displacement Δh and change of displacement over time Δh/t according to preferred embodiments 5-13

Embodiment Carbon (ionic Voltage Displaceme Average displacement speed conductor) nt DΔh Δh/t (μm/s)

(V)

(μm)

5 TiC 600 (TEA/PC) 2.0 3.4 0.013

6 TiC 600 (TEA/PC) 3.0 10.2 0.04

7 TiC 800 (TEA/PC) 2.0 3.2 0.013

8 TiC 800 (TEA/PC) 3.0 9.8 0.038

9 TiC 600 (EMITf) 2.0 12.1 0.049

10 TiC 600 (EMITf) 3.0 16.2 0.063

11 TiC 800 (EMITf) 2.0 2.3 0.009

12 TiC 800 (EMITf) 3.0 13.5 0.053

13 TiC 800 (EMITf) 3.0 12.0 0.2

The presented embodiments demonstrate that the amplitude of the vertical displacement of the actuator depends on both the carbon material and the ionic conductor used in the actuator. The effect of the carbon material ensues from its porous structure. The present invention indicates that a carbon material with smaller pores ensures greater displacement of the actuator in both ionic liquids and solutions of salt in solvents. This can be explained by the change of ion radiuses in a potential field and the increasing size of carbon pores as a result of the impact of ions at higher voltages. This effect is schematically depicted on Fig. 7. hi an uncharged state of the actuator, at voltage 0 V, the carbon nanopores are free. If voltage is increased, the ions are drawn, or adsorbed, into the pores, whereas the ion radius (di) remains relatively similar to the initial size of the ions. If voltage increases to 3 volts, more ions are drawn from the electrolyte to the carbon surface and the dimensions of the ions change on the surface under the influence of the potential field. The size of the ions increases (d₂) according to the principle of tighter fit and the dimensions of the nanopores change accordingly. This, in turn, has an effect on the dimensions of the material.

Considering the fact that different conductive mediums have different ion sizes and that the ions in ionic liquids are larger than those in salt and solvent systems, it follows that vertical displacement of materials is larger in ionic liquids.

It is important to note that the higher the number of pores in carbon the more it is able to accommodate tightly fitting ions and the larger the amplitude of the resulting macroscopic actuation.

Claims

1. An actuator comprising at least one electrode of porous carbon material, containing an electrolyte, characterised in that the pore size of the porous material is selected to match the size of the electrolyte particles for enabling the actuator to change its linear dimensions when voltage is applied to said electrode.

2. An actuator according to claim 1, wherein said electrode of carbon material contains between 80 and 100 volume % of carbon material with average pore size less than 1 nanometer and at least 50 % of the volume of pores is comprised of micropores with the size of up to 1.1 nanometer.

3. An actuator according to claims 1 and 2, wherein the carbon material of the carbon electrode is a carbide-derived carbon material, obtained by extracting the element, which forms carbide upon carbonisation of a metal or non-metal, from the crystal lattice of carbide.

4. An actuator according to claims 1 to 3, wherein an outer surface of the carbon electrode is coated with a conductive layer.

5. An actuator according to claim 4, wherein said conductive layer comprises a metal.

6. An actuator according to claims 1 to 5, wherein said electrolyte is a protonic or non- protonic solution of a salt or salts, which produce ion pairs.

7. An actuator according to claims 1 to 6, wherein said electrolyte contains between 1 and 100 volume % of ionic liquid.

8. An actuator according to claims 1 to 7, wherein dimensions of said electrolyte ions are from 1 to 1.5 times smaller than an average pore size of said carbon material of the electrodes.

9. An actuator according to claims 1 to 8, wherein said ionic liquid is polytetrafluoroethylene suspension or l-ethyl-3-methylimidazolium trifluoromethanesulfonate ionic liquid.

10. An actuator according to claims 1 to 9, wherein said actuator comprises at least two porous carbon electrodes, separated by a layer of dielectric polymeric material.