Sensor for determining gases and method for manufacturing the sensor
A sensor for determining the concentration of a gas in gas mixtures, which has a measuring and a reference electrode as well as a polymer layer, which is in contact with the gas mixture and with the measuring electrode. A pH sensitive electrode is provided as the measuring electrode.
Optical sensors for determining the carbon dioxide content of the air are used, among other things, in fire detectors. Their function is based on the fact that a layer sensitive to carbon dioxide changes color reversibly upon contact with the gas to be determined. This change of color is monitored by a detector, and when a predetermined minimum concentration is exceeded, an alarm is triggered. This measuring method is relatively susceptible to contamination.
Optical CO2 gas sensors normally are made up of a polymeric matrix, e.g. ethylcellulose, a softener and solutizer, e.g. Brij 30, and a basic gas acceptor, e.g. tetraoctylammonium hydroxide. The sensor signal reveals a dependency on the moisture content of the surroundings.
U.S. Pat. No. 6,241,873 describes a carbon dioxide sensor which detects the carbon dioxide content of a surrounding atmosphere in a potentiometric manner. It features a measuring electrode and a reference electrode, which are applied on a substrate. The measuring electrode takes the form of a silver/silver carbonate electrode. The potential of this electrode is a direct function of the carbon dioxide concentration of the surroundings. A disadvantage of this measuring method is the fact that carbonate-containing electrodes are affected by weather influences and thus have only a low stability. Furthermore, the sensor is limited to measuring carbon dioxide.
The present invention is based on the objective of providing a gas sensor for determining different gases in a potentiometric and/or optical manner, which has a high stability and at the same time a high sensitivity.
A further objective lies in the compensation of the moisture-dependency of the sensor.
SUMMARY OF THE INVENTIONThe sensor according to the present invention has the advantage that its electrodes are stable over the long term and that its measuring electrode has a high sensitivity with respect to the gas to be determined. This is achieved in that a pH-sensitive electrode is used that detects the pH value of a surrounding polymer. Such pH electrodes have a sufficiently long service life and allow for the determination of various acidic and basic gases.
Thus, for example, the sensor can be designed in such a way that, in addition to measuring the pH value, the optical absorption and the conductivity can be measured as well.
An iridium oxide electrode is particularly suited as a measuring electrode since it is especially robust with respect to environmental influences and does not have to be provided in a pre-expanded state as do comparable glass electrodes.
In a particularly advantageous embodiment, the sensor includes a polymer that has a base or an acid since this results in a quick and effective absorption of the acidic or basic gas to be determined. This further raises the sensitivity and lowers the response time of the sensor.
BRIEF DESCRIPTION OF THE DRAWINGS
Sensor 10 represented in
Electrodes 14, 16 are preferably coated completely by a gas-sensitive and gas-permeable polymer layer 26, which functions as an electrolyte and which is represented in
The mode of operation of the sensor is based on the fact that a gas to be determined, for example carbon dioxide, is absorbed by polymer layer 26. The gas dissolves in the bound water of polymer layer 26 and changes the pH value of the latter. Since a pH sensitive electrode is used as measuring electrode 14, the change of the pH value results in a change of the potential at measuring electrode 14. The change in potential can be measured as a changing voltage between measuring electrode 14 and reference electrode 16. Measuring electrode 14 can take on any specific embodiment that is suited for detecting a change in the pH value of the surroundings with sufficient precision. Especially suited are conductive metal oxide pH electrodes that have for example a surface layer of mixed iridium oxides (IrO2) or ruthenium oxides (RuOx). However, platinum and rhodium electrodes are suitable as well.
At a high-impedance resistor 28, the voltage drop U produced by the potential difference of the sensor is measured. Alternatively, by switching the two switches 30, 32, it is possible to determine the conductivity or the resistance of the sensor element by applying a voltage (DC or AC voltage) and by measuring the current flowing through. By arranging two evaluation units at one pair of electrodes 14, 16, it is consequently possible to compensate the moisture-dependency of the sensor signal.
In order to be able to absorb acidic gases—that is, gases such as carbon dioxide, nitrogen oxides or sulfur oxides which in contact with water result in an acidic solution—as quickly as possible and in sufficient quantity in polymer layer 26, the latter preferably contains a strong base such as tetraalkyl ammonium hydroxides or tetraalkyl ammonium hydrogen carbonates. These increase the solubility of the acidic gases in water that is bound in polymer layer 26 by removing the acid produced in the dissolving process.
For determining gases that react in a basic manner such as ammonia, an acid such as a sulphonic acid, for example, is preferably added to polymer layer 26. This promotes the solubility of basic gases in polymer layer 26. Moreover, polymer layer 26 may contain homogenization agents such as tensides for example.
For manufacturing sensor 10, an electrode paste preferably containing ceramic and metallic components, a so-called cermet, is applied onto substrate 12 and is sintered with the ceramic substrate 12. Polymer layer 26 is applied onto the electrode set-up in that a solution containing the polymer, a base or acid and other additives is deposited or imprinted and the solvent is removed. The polymer layer has a layer thickness of 10 to 100 μm, preferably between 20 and 40 μm.
Electrodes 14, 16 are contacted via circuit traces 22, 24, which are either formed also as cermet in one step together with electrodes 14, 16 or by imprinting a solution containing a hardenable resin and a precious-metal component and subsequent hardening of the solution. The use of a silver-containing epoxy resin is preferred.
The present invention is not limited to the exemplary embodiment described, but other specific embodiments in addition to the sensor described are conceivable as well.
Thus, for example, an activated carbon layer can be provided on polymer layer 26 to prevent the entry of gases that damage polymer layer 26 such as nitrogen oxides or sulfur oxides. Furthermore, a temperature measuring unit may be additionally provided for compensating temperature influences on the measured potential differences.
Claims
1. A sensor for determining a concentration of a gas in a gas mixture, comprising:
- a measuring electrode, the measuring electrode being a pH sensitive electrode;
- a reference electrode; and
- a polymer layer in contact with the gas mixture and the measuring electrode.
2. The sensor according to claim 1, wherein, in addition to measuring a pH value, a conductivity of the polymer layer is measured as well.
3. The sensor according to claim 1, wherein the measuring electrode contains an iridium oxide.
4. The sensor according to claim 1, wherein the measuring electrode is at least substantially shielded from the gas mixture by the polymer layer.
5. The sensor according to claim 1, wherein at least one of the measuring electrode and the reference electrode is an interdigital electrode.
6. The sensor according to claim 1, wherein the polymer layer includes a base.
7. The sensor according to claim 6, wherein the polymer layer contains a quaternary ammonium compound as the base.
8. The sensor according to claim 1, wherein the polymer layer contains an acid.
9. The sensor according to claim 8, wherein the polymer layer contains a sulphonic acid as the acid.
10. The sensor according to claim 1, wherein the polymer layer contains ethylcellulose.
11. The sensor according to claim 1, wherein the sensor is a sensitive element in a fire detector.
12. The sensor according to claim 1, wherein the sensor is a sensitive element in an air-quality sensor.
13. The sensor according to claim 1, wherein the sensor is a sensitive element for detecting ammonia.
14. A method for manufacturing a sensor for determining a concentration of a gas in a gas mixture having at least two electrodes that are deposited on a ceramic substrate and a polymer layer, the method comprising:
- in a first step, producing the electrodes on the ceramic substrate by depositing an electrode paste and a subsequent heat treatment; and
- in a second step, depositing the polymer layer by applying a polymer solution on at least one of the ceramic substrate and the electrodes and subsequently removing a solvent.
15. The method according to claim 14, wherein the polymer layer has a thickness of 10 to 50 μm.
16. The method according to claim 14, further comprising:
- in a third step, contacting the electrodes by a hardenable, precious metal-containing polymer.
Type: Application
Filed: Nov 30, 2004
Publication Date: Jul 21, 2005
Inventors: Thomas Brinz (Bissingen A.D. Teck), Jane Lewis (Stuttgart), Claus Heppel (Stuttgart)
Application Number: 11/000,387