DE102005008406B4 - Conductive fluids for inclination and acceleration sensors - Google Patents

Abstract

Inclination and acceleration sensors or inclination switches, characterized in that they contain as conductive liquid one or more ionic liquids of the general formula aAm + bXn-, where n = 1 or n = 2 and m = 1 or m = 2 and a · m = b · n and the cation A is selected from quaternary ammonium cations of the general formula [R ''] [N +] ([R ']) ([R' ']) [R] quaternary phosphonium cations of the general formula [R '' '] [P +] ([R']) ([R '']) [R] Imidazolium cations of the general formula [R] N1C = C [N +] ([R ']) = C1, where the imidazolium nucleus may be substituted by at least one group selected from halides, hydroxyl, linear or branched substituted or unsubstituted alkyl groups of up to 20 carbon atoms which may be substituted with one or more groups selected from halides, hydroxyl, nitrile, amine, thiol; general formula - (RX) n R 'where n = 1-10, where R is a linear or branched alkyl group with 1 to 20 carbon atoms, R 'is hydrogen or a linear or branched alkyl group and X is an ether group, thioether group, ester group, siloxane group or amide group. Morpholinium cations of the general formula [R] [N +] 1CC [O] CC1, wherein the morpholinium nucleus may be substituted by at least one group selected from halides, hydroxyl, linear or branched substituted or unsubstituted alkyl radicals of up to 20 carbon atoms having one or more Groups selected from the halides, hydroxyl, nitrile, amine, thiol may be substituted ...

Description

background

The detection of measured variables by electrical current causes the sensors used to change their resistance or their capacity as a function of the measurand.

For the detection of the measurand "tilt" sensors are often used, which have a cavity, partially or completely with conductive liquid. The conductive liquid collects by gravity at the lowest point of the cavity. The position of the liquid can be detected by means of electrodes.

In the simplest case, the tilt sensor is a toggle switch 1 which occurs when the conductive liquid enters 1 ] between the electrodes [ 2 ] becomes conductive.

There are various arrangements of electrodes in various hollow bodies in circulation, which ultimately determine the position of a conductive liquid. The measuring principles of such resistive working sensors are described in "Sensor Report", 7, (1992), 5, pp. 34-36. In addition, other designs and evaluation methods are possible, for. B. a capacitive evaluation.

For each application, the conductive fluid must meet a specific requirement profile. Essential criteria here are the conductivity, the viscosity, the temperature stability, the surface tension and the vapor pressure.

State of the art

The conductive fluids used in tilt sensors are mercury and solutions of dissociated ions in molecular solvents. For the solutions of dissociated ions come as an ion source acids and alkalis and salts into consideration. The solvents used are organic solvents such as alcohols, ethers, etc. as well as water.

The use of mercury is problematic due to toxicity and high price. The freezing point of -38 ° C also limits the temperature range of such sensors.

When acids and alkalis are used, the corrosivity of the solutions poses a problem because the choice of materials for designing the sensor is limited.

If water is used as a solvent, the boiling point of 100 ° C or above is the upper limit for non-pressure sensors. The same applies to other solvents whose boiling point is below the intended maximum working temperature of the sensor. For such systems, the cavities of the sensors must be designed flameproof.

Another problem of the volatile solvents may be distillative processes within the sensor cavity. This can lead to local concentration changes of the solution, which adversely affect the measurement accuracy of the sensor.

Some volatile solvents also cause the problem of a relatively low flash point. This is especially detrimental to those sensors used in sources of ignition sources.

Another key issue is the diffusion of volatiles of the conductive fluid through sensor materials, especially polymer devices. To avoid this diffusion, glass bodies are often used. These are more expensive to manufacture than plastic body.

• – elektrische Leitfähigkeit in einem möglichst weiten Temperaturbereich ohne Unstetigkeiten.
• – Geringe Änderung der Leitfähigkeit mit der Temperatur (Verhältnis der Leitfähigkeiten über einen Bereich von ca. 120°C maximal 20)
• – Langfristig stabiles System mit den Elektroden und onstigen Sensorkomponenten, die mit der Flüssigkeit in Kontakt sind.
• – Geringe Korrosivität
• – Hoher Flammpunkt, niedriger Dampfdruck.
• – Geringe Diffusion durch verschiedene Werkstoffe wie z. B. Polymere.
• – Wenig toxisch, unproblematisch zu entsorgen.

In summary, the following requirements are placed on a conductive fluid for use in inclination sensors:

• - Electrical conductivity in the widest possible temperature range without discontinuities.
• - Low change of conductivity with temperature (ratio of conductivities over a range of approx. 120 ° C maximum 20)
• - Long-term stable system with the electrodes and on-line sensor components in contact with the fluid.
• - Low corrosivity
• - High flash point, low vapor pressure.
• - Low diffusion through different materials such. B. polymers.
• - Low toxicity, easy to dispose of.

Electrically conductive liquids are z. In DE 4244724 described. These consist of lower alcohols and salts dissolved therein, preferably alkali metal sulfates. The alcohols used have flash points of less than 15 ° C.

US 4975806 describes conductive liquids based on aprotic solvents such as formamide, N-alkylpyrrolidone, etc. with the conductive salt ammonium borosalicylate. Many of the solvents used have flash points below 90 ° C, as well as many of the solvents have melting points around the freezing point.

DE 19507610 describes the use of oligoethylene glycol dimethyl ethers mixed with alcohols as solvents for salts. These mixtures require the addition of additives to adjust higher viscosities; In addition, the solvent mixtures used are often incompatible with polymers.

The invention as a problem solution

After the previously described is to find a conductive liquid that largely meets the requirements and represents an alternative to the previously used mixtures.

Since the end of the forties, ionic liquids have been known. These are liquid salt melts at room temperature and below, representing a novel class of non-molecular, ionic solvents. A common definition of ionic liquids with demarcation against the known salt melts is a melting point of less than 80 ° C. Other places here call a melting point below room temperature. In the context of this patent, ionic liquids are to be understood as meaning those salts which, when in the pure state, have a melting point of below 80 ° C., preferably below room temperature. The mixtures of ionic liquids have known from other substances melting point, so that melting points of below -60 ° C can be achieved.

Typical cation / anion combinations leading to ionic liquids are e.g. As dialkylimidazolium, pyridinium, ammonium and phosphonium with halide, tetrafluoroborate, methyl sulfate. In addition, many other combinations of cations and anions are conceivable that lead to low-melting salts.

• – Elektrische Leitfähigkeit der Reinsubstanzen von unter 0.1 mS/cm bis über 20 mS/cm bei Raumtemperatur.
• – Praktisch kein Dampfdruck unterhalb der Zersetzungstemperatur (bis 400°C).
• – Unbrennbarkeit.
• – großer Flüssigbereich von –60°C bis 400°.
• – weites elektrochemisches Fenster bis über 5 V, damit hohe Reduktions- und Oxidationsbeständigkeit.

With regard to the use as conductive liquid in inclination sensors, the following basic properties of this material class are of interest:

• - Electrical conductivity of the pure substances from below 0.1 mS / cm to over 20 mS / cm at room temperature.
• - Virtually no vapor pressure below the decomposition temperature (up to 400 ° C).
• - incombustibility.
• - large liquid range from -60 ° C to 400 °.
• - Wide electrochemical window to over 5 V, so high reduction and oxidation resistance.

Overviews of the ionic liquids, their production, properties and use can be found for. In: Ionic Liquids in Synthesis, P. Wasserscheid, T. Welton (eds), Wiley; Green Industrial Applications of Ionic Liquids (NATO Science Series, Ii. Mathematics, Physics and Chemistry, 92); Ionic Liquids: Industrial Applications for Green Chemistry (Acs Symposium Series, 818) by Robin D. Rogers (Editor).

Ionic liquids generally show a continuous dependence of the conductivity on the temperature within their liquid range, since no components can precipitate or evaporate. 2 shows the conductivity of some ionic liquids as a function of the temperature, the numerical values are given in Tab. 1. The substance names correspond to the examples. T [° C] BMIM OTf EMIM-NTf ₂ EMMIM-NTf ₂ EMMIM-NTf ₂ 30 2.9 9.1 3.4 3.8 80 12.8 25.0 13 7 19.4 120 24.5 42.0 29.7 40.4 160 44.2 59.8 44 6 63.8 200 62.2 79.8 63.0 87.8 230 74.8 95.4 75.3 107 6 F (230-30) 25.8 10.5 22.1 28.3 F (160-30) 15.2 6.6 13.1 16.8 Tab. 1: Temperature dependence of the conductivity, values: mS / cm

Surprisingly, it turns out that the change in the conductivity with the temperature is small. If a factor F (T) is calculated as F (ΔT) = (conductivity at upper temperature T1 / conductivity at lower temperature T2), it turns out that the factor F over the range of 30 ° C to 230 ° C for the Ionic liquids BMIM-Otf, EMMIM-NTf ₂ and EMMIM-NTf _{2 is} between 20 and 30, so it is easy to detect for evaluation electronics. The ionic liquid EMIM-NTf ₂ is characterized by a very low value for F of only 10.5, which allows simple electronics for a wide temperature range. The values for the temperature range from 160 ° C to 30 ° C are all well below the value of about 20 required for a span of 130 ° C. With regard to the temperature dependence of the conductivities, ionic liquids are outstandingly suitable as conductive liquid over a wide temperature range to become.

Ionic liquids are well tolerated by many electrode materials, so there is a wide range of materials available. In addition to carbon and platinum, silver, various stainless steels, tin, nickel and conductive polymers may be used as the electrode material. The compatibility of the ionic liquids with polymers is good. For example, the ionic liquid BMIM-OTf is compatible with polypropylene, polyoxymethylene, polystyrene, polyurethane and polyamides up to temperatures of 150 ° C.

The corrosivity of most ionic liquids is classified as low. The ionic liquid BMIM NTF _2, for example, attacks in an anhydrous state z. As aluminum, nickel, silver and stainless steels, even over long periods of time.

The low aggressiveness of the ionic liquids compared to electrode and other sensor materials ensures that the entire system can remain stable for a long time.

Due to their negligible vapor pressure below their decomposition temperature, ionic liquids also have no flash point there. Only above the decomposition temperature arise flammable gases. Since the decomposition temperatures of ionic liquids are generally well above 150 ° C, in some cases even above 400 ° C, they are superior to most organic solvents in terms of flash point.

Another advantage of the negligible vapor pressure is the possibility to seal the cavities of the sensors under vacuum. As a result, no pressure load of the sensor housing occurs when the sensor warms up.

The diffusion of ionic liquids through polymeric materials, metals and glass is negligible. Passage of the ionic liquids BMIM-BF ₄ and BMIM-NTf ₂ through membranes of polyethylene, polypropylene, polyoxymethylene and polyurethane can not be detected. Similarly, diffusion of ionic liquids through metal or glass is not known. This makes it possible to construct sensor components such as the liquid container from a variety of materials, without being limited by incompatibilities.

Finally, the high thermal stability of the ionic liquids, in addition to the use of the sensors produced in them in high-temperature environments, also enables the production of the sensors using elevated temperatures. This can be z. B. be advantageous in the sealing of glass bodies with glass solder.

Pure ionic liquids are preferably used according to the invention, the term "pure" meaning here the absence of molecular, nonionic compounds. This also includes mixtures of ionic liquids. These mixtures are advantageously used to obtain by lowering the melting point ionic liquids that are suitable for low temperatures.

It is also possible according to the invention to add the ionic liquids or their mixtures with other salts which are solid at room temperature or other molecular liquids in order to modulate the conductivity and viscosity.

Example 1:

N-ethyl-N'-methylimidazolium bis (trifluoromethanesulfone) imide (EMIM-NTf ₂ )

• Conductivity: s. 2 ,
• Viscosity: 34 mPa / s at 25 ° C
• Change in conductivity after AC operation 2 V / 1 kHz / 150 ° C / 300 h: 0.2%
• Electrochemical window (on glassy carbon): 4.5 V
• Decomposition temperature:> 400 ° C
• Freezing point: -3 ° C

Example 2: N-butyl-N'-methylimidazolium trifluoromethanesulfonate (BMIM-OTf)

• Conductivity: s. 2 ,
• Viscosity: 89 mPa / s at 25 ° C
• Change in conductivity after AC operation 2 V / 1 kHz / 150 ° C / 300 h: 0.4%
• Electrochemical window (on glassy carbon): 4.4 V
• Decomposition temperature:> 300 ° C
• Freezing Point:

Example 3: N-ethyl-N'-methyl-2-methylimidazolium bis (trifluoromethanesulfone) imide (EMMIM-NTf ₂ )

• Conductivity: s. 2 ,
• Viscosity: 85 mPa / s at 25 ° C
• Change in conductivity after AC operation 2 V / 1 kHz / 150 ° C / 300 h: 0.1%
• Electrochemical window (on glassy carbon): 4.7 V
• Decomposition temperature:> 300 ° C
• Freezing Point:

Example 4: N-butyl-N'-methylimidazolium tetrafluoroborate (BMIM-BF ₄ )

• Conductivity: s. 2 ,
• Viscosity: 142 mPa / s at 25 ° C
• Change in conductivity after AC operation 2 V / 1 kHz / 150 ° C / 300 h: 0.6%
• Electrochemical window (on platinum): 4.1 V
• Decomposition temperature:> 300 ° C
• Freezing Point:

Example of use:

The ionic liquids described in the invention can be used as electrolyte liquids in inclination sensors advantageous everywhere where temperatures above 150 ° C is expected, z. B. near the engine in the automotive sector, in mechanical engineering, etc.

Claims (5)

Inclination and acceleration sensors or inclination switches, characterized in that they contain as conductive liquid one or more ionic liquids of the general formula aA ^{m +} bX ^n- , where n = 1 or n = 2 and m = 1 or m = 2 and a · m = b · n and the cation A is selected from quaternary ammonium cations of the general formula [R '] [N +] ([R']) ([R ']) [R] quaternary phosphonium cations of the general formula [R '] [P +] ([R']) ([R ']) [R] Imidazolium cations of the general formula [R] N 1 C = C [N +] ([R ']) = C 1, wherein the imidazolium nucleus may be substituted by at least one group selected from halides, hydroxyl, linear or branched substituted or unsubstituted alkyl radicals to 20 Carbon atoms which may be substituted by one or more groups selected from the halides, hydroxyl, nitrile, amine, thiol, Radicals of the general formula - (RX) _n -R 'where n = 1-10, where R is a linear or branched alkyl group having 1 to 20 carbon atoms, R' is hydrogen or a linear or branched alkyl group and X is an ether group, thioether group, Ester group, siloxane group or amide group. Morpholinium cations of the general formula [R] [N +] 1CC [O] CC1, wherein the morpholinium nucleus may be substituted by at least one group selected from halides, hydroxyl, linear or branched substituted or unsubstituted alkyl radicals of up to 20 carbon atoms having one or more Groups selected from the halides, hydroxyl, nitrile, amine, thiol can be substituted, radicals of the general formula - (RX) _n -R 'with n = 1-10, wherein R is a linear or branched alkyl group having 1 to 20 carbon atoms, R 'is hydrogen or a linear or branched alkyl group and X is an ether group, thioether group, ester group, siloxane group or amide group. Oxazolinium cations of the general formula [R] [N +] 1 = COCC1, wherein the oxazolinium nucleus may be substituted by at least one group selected from halides, hydroxyl, linear or branched substituted or unsubstituted alkyl radicals of up to 20 carbon atoms selected from one or more groups may be substituted from the halides, hydroxyl, nitrile, amine, thiol, radicals of the general formula - (RX) _n -R 'where n = 1-10, where R is a linear or branched alkyl group having 1 to 20 carbon atoms, R' Is hydrogen or a linear or branched alkyl group and X is an ether group, thioether group, ester group, siloxane group or amide group. Pyridinium cations of the general formula [R] [N +] 1 = CC = CC = C1, wherein the pyridinium nucleus may be substituted by at least one group selected from halides, hydroxyl, linear or branched substituted or unsubstituted alkyl radicals of up to 20 carbon atoms may be substituted with one or more groups selected from the halides, hydroxyl, nitrile, amine, thiol, radicals of the general formula - (RX) _n -R 'where n = 1-10, where R is a linear or branched alkyl group having 1 to 20 carbon atoms, R 'is hydrogen or a linear or branched alkyl group and X is an ether group, thioether group, ester group, siloxane group or amide group. Pyrrolidinium cations of general formula [R] [N +] 1 ([R'1] CCCC1, wherein the pyrrolidinium nucleus may be substituted by at least one group selected from halides, hydroxyl, linear or branched substituted or unsubstituted alkyl radicals of up to 20 carbon atoms which may be substituted by one or more groups selected from the halides, hydroxyl, nitrile, amine, thiol, radicals of the general formula - (RX) _n -R 'where n = 1-10, where R is a linear or branched alkyl group with 1 to 20 carbon atoms, R 'is hydrogen or a linear or branched alkyl group and X is an ether group, thioether group, ester group, siloxane group or amide group. Pyrazolium cations of the general formula [R] [N +] 1C = CC = N1, where pyrazolium nucleus may be substituted by at least one group selected from halides, hydroxyl, linear or branched substituted or unsubstituted alkyl radicals of up to 20 carbon atoms having one or more carbon atoms a plurality of groups selected from the halides, hydroxyl, nitrile, amine, thiol may be substituted radicals of the general formula - (RX) _n -R 'where n = 1-10, wherein R is a linear or branched alkyl group having 1 to 20 carbon atoms R 'is hydrogen or a linear or branched alkyl group and X is an ether group, thioether group, ester group, siloxane group or amide group. Triazolium cations of the general formulas [R] [N +] 1 ([R ']) N = CC = N1 or [R] [N +] 1 ([R']) C = NC = N1, where the triazolium nucleus may be substituted at least one group selected from halides, hydroxyl, linear or branched substituted or unsubstituted alkyl radicals of up to 20 carbon atoms which may be substituted by one or more groups selected from the halides, hydroxyl, nitrile, amine, thiol radicals of the general formula (RX) _n -R 'where n = 1-10, wherein R is a linear or branched alkyl group having 1 to 20 carbon atoms, R' is hydrogen or a linear or branched alkyl group and X is an ether group, thioether group, ester group, siloxane group or amide group is. Guanidinium cations of general formula [R '] N ([R]) C (N ([R''])[R'']) = [N +] ([R''']) [R '''] wherein the guanidinium nucleus may be substituted by at least one group selected from halides, hydroxyl, linear or branched substituted or unsubstituted alkyl groups of up to 20 carbon atoms substituted with one or more groups selected from halides, hydroxyl, nitrile, amine, thiol may be radicals of the general formula - (RX) _n -R 'where n = 1-10, where R is a linear or branched alkyl group having 1 to 20 carbon atoms, R' is hydrogen or a linear or branched alkyl group and X is an ether group, Thioether group, ester group, siloxane group or amide group. and in the general formulas the radicals R, R ', R'',R''' are independently selected from hydrogen; halides; hydroxyl; linear or branched substituted or unsubstituted alkyl radicals of up to 20 carbon atoms which may be substituted by one or more groups selected from the halides, hydroxyl, nitrile, amine, thiol; Radicals of the general formula - (RX) _n -R 'where n = 1-10, where R is a linear or branched alkyl group having 1 to 20 carbon atoms, R' is hydrogen or a linear or branched alkyl group and X is an ether group, thioether group, Ester group, siloxane group or amide group. and the anion X ^{- is} selected from the group consisting of the halides, tetrafluoroborate, hexafluorophosphate, hexafluoroantimonate, sulfate, phosphate, RR'PO ₄ ^- , dicyanamide, thiocyanate, cyanide, carboxylate R-COO ^- sulfonate R-SO ₃ ^- , benzenesulfonate , Toluenesulfonate, organic sulfates RO-SO ₃ ^- , bis (sulfone) imides R-SO ₂ -N'-SO ₂ -R ', imides of the structure [R'] S ([N-] C ([R]) = O) (= O) = O, where R and R 'independently of one another are a linear or branched aliphatic or alicyclic alkyl or C5-C15-aryl, C5-C15-aryl-C1-C6-alkyl containing 1 to 20 carbon atoms or C 1 -C 6 -alkyl-C 5 -C 15 -aryl radical or a radical of the general formula - (RX) _n -R 'where n = 1-10, where R is a linear or branched alkyl group having 1 to 20 carbon atoms, R 'is hydrogen or a linear or branched alkyl group and X is an ether group, thioether group, ester group, siloxane group or amide group, which may be substituted by halogen atoms and / or hydroxyl groups may be stituiert, and the single pure ionic liquid has a melting point of less than 80 ° C, preferably below 25 ° C and more preferably below 0 ° C. Inclination and acceleration sensors according to claim 1, characterized in that a solid at room temperature salt is dissolved in the ionic liquid. Inclination and acceleration sensors according to claim 1 or 2, characterized in that the ionic liquid, an organic solvent is added to a volume fraction of less than 20%, preferably less than 10% and particularly preferably less than 5%. The inclination and acceleration sensors defined in the preceding claims, which determine acceleration or inclination by capacitive evaluation. The inclination and acceleration sensors defined in one of the preceding claims, which determine acceleration or inclination by resistive evaluation.

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