DE4019707A1

DE4019707A1 - Controlling gas transport to gas sensors - using liq. film over gas inlet orifice

Info

Publication number: DE4019707A1
Application number: DE19904019707
Authority: DE
Inventors: Karoly Dr Dobos; Maria Dobosne Gyulai
Original assignee: Maria Dobosne Gyulai
Current assignee: Endress and Hauser Gastec GmbH and Co
Priority date: 1990-06-21
Filing date: 1990-06-21
Publication date: 1992-01-02

Abstract

Gas barrier for use with gas sensors comprising a liq. film which seals the whole cross section of the gas inlet orifice. The gas barrier film is mounted e.g. on a hydrophobic or hydrophilic support, mounted between two or more hydrophobic and/or hydrophilic layers, or mounted on a hydrophilic support which is in turn placed between two or more hydrophobic layers. The hydrophobic layers are e.g. PTFE, polyethylene, polypropylene or porous plastic film and the hydrophilic layers are e.g. of glass wool, glass paper, glass frit or hydrophilic plastics layers e.g. polyethylene or polypropylene. Pref. the hydrophilic support is porous and the pores are full or only partially filled. The carrier for the liq. phase is e.g. the hydrophobic teflon or other electrode support film for the measuring electrode of the electrochemical cell. USE/ADVANTAGE - The liq. gas barrier film controls passage of gas to be determined in the atmos. to a gas sensor of any type, such as semiconductor oxide sensors, pellistors, MOSFET-gas sensors, electrochemical sensors, etc., by physical means such as diffusion, dissolution, etc.. Controlling the flow of gas to the sensor surface causes the sensor to retain its catalytic activity for longer periods so that its sensitivity and output signal are retained for a long time.

Description

Gassensoren aller Art (Halbleiter-Oxid-Sensoren, Pellistoren, MOSFET-Gassensoren, elektrochemische Sensoren usw.) haben die Aufgabe Gaskonzentrationen in der Umgebung zu messen. Werden die Sensoren den vorhandenen Gasen in der atmosphärischen Umgebung direkt ausgesetzt, wird das Signal meistens sehr stark von der Reaktionsgeschwindigkeit der Sensoroberfläche (meistens von deren katalytische Aktivität) abhängen.Gas sensors of all kinds (semiconductor oxide sensors, pellistors, MOSFET gas sensors, electrochemical sensors, etc.) have them Task to measure gas concentrations in the area. Will the Sensors the gases present in the atmospheric environment directly exposed, the signal is mostly very strongly from the Response speed of the sensor surface (mostly from its catalytic activity).

Diese Reaktionsfähigkeit (katalytische Aktivität) wird mit der Zeit abgebaut. Dies hat zur Folge, daß die Empfindlichkeit (Sensorsignal), welche anfangs sehr groß ist, mit der Zeit nachläßt.This reactivity (catalytic activity) is with the Time reduced. As a result, the sensitivity (Sensor signal), which is initially very large over time subsides.

Unser Lösungsweg schlägt vor, daß vor dem Sensor eine Gasbarriere eingebaut wird. Durch diese Gasbarriere kann das zu messende Gas durch einen physikalischen Prozeß zu dem eigentlichen Sensorelement gelangen. Als physikalischer Prozeß eignet sich die Auflösung und/oder Diffusion durch einen Flüssigkeitsfilm am besten. Wenn als Gasbarriere eine stabile Flüssigkeit gewählt wird, wie Wasser, Silikonöl, Säuren usw., die in der normalen Atmosphäre eine lange Beständigkeit haben, wird die Gasauflösung und der Diffusionsprozeß durch diese Barriere immer nur von physikalischen Parametern, wie Temperatur, Druck usw. abhängen. Diese Parameter sind aber konstant zu halten, meßbar und sind keinem Alterungsprozeß unterworfen.Our solution suggests that a gas barrier in front of the sensor is installed. The gas to be measured can pass through this gas barrier through a physical process to the actual Arrive sensor element. Suitable as a physical process the dissolution and / or diffusion through a liquid film on best. If a stable liquid is chosen as the gas barrier will, like water, silicone oil, acids, etc., which in the normal Atmosphere will have a long life Gas dissolution and the diffusion process through this barrier always only physical parameters such as temperature, pressure, etc. depend. However, these parameters must be kept constant and measurable and are not subject to any aging process.

Die technische Schwierigkeit ist dabei, daß die Schichtdicke des Flüssigkeitsfilmes absolut konstant bleiben muß. Dies kann dadurch geschehen, daß dieser Flüssigkeitsfilm auf einen Träger aufgebracht wird. Hier eignen sich verschiedene poröse Träger, wie poröse Folien aus hydrophilen und hydrophoben Kunststoffen, Glaswolle, Glaspapier, Glasfritten und Filtermaterial.The technical difficulty is that the layer thickness of the Liquid film must remain absolutely constant. This can happen that this liquid film on a support is applied. Various porous supports are suitable here, like porous foils made of hydrophilic and hydrophobic plastics, Glass wool, glass paper, glass frits and filter material.

Insbesondere sind solche Gasbarrieren für elektrochemische Zellen interessant, die in einem Gasdiffusionsbegrenzungsmodus arbeiten sollen.Such gas barriers are in particular for electrochemical cells interesting that work in a gas diffusion limitation mode should.

example 1

Ein elektrochemischer Sensor zum Nachweis von Kohlenmonoxid, bestehend aus drei Platin-Katalysatorschichten, aufgebracht auf hydrophober Teflonfolie, wobei die Arbeitselektrode von außen mit einer porösen hydrophilen sehr dünnen Schicht beschichtet wird. Diese Beschichtung kann z. B. durch Vakuum-Aufdampf-Verfahren, Ionen-Beschuß oder die Aufbringung einer nicht löslichen Metallschicht erfolgen. Die elektrochemische Zelle enthält einen Elektrolyt aus Schwefelsäure, wobei der Elektrolytkanal so ausgefüllt wird, daß er an der äußeren Fläche der Arbeitselektrode durch einen elektrolytleitenden Kanal mit dem Elektrolyt in Kontakt kommen kann. Über dieser Arbeitselektrode ist jetzt noch eine weitere poröse Teflonfolie auszuspannen, damit die Elektrolytschicht, die an der äußeren Oberfläche der Arbeitselektrode gleichmäßig verteilt ist, nicht direkt mit der Außenwelt in Berührung kommt. Durch diesen einige µm dünnen Elektrolytfilm kann das Gas nicht ungehindert auf die Arbeitselektrode gelangen, sondern muß sich zuerst in der Flüssigkeit auflösen, um dann in die Poren der Elektrode einzudringen. Durch diese Auflösung und dem Austritt aus dem Elektrolyt wird die Konzentration drastisch reduziert und die Gaszufuhr durch diesen Transporteffekt bestimmt. An electrochemical sensor for the detection of carbon monoxide, consisting of three platinum catalyst layers applied to hydrophobic Teflon film, with the working electrode from the outside a porous hydrophilic very thin layer is coated. This coating can e.g. B. by vacuum evaporation method, Ion bombardment or the application of a non-soluble Metal layer done. The electrochemical cell contains one Electrolyte from sulfuric acid, the electrolyte channel so is filled in that it is on the outer surface of the Working electrode through an electrolyte-conducting channel with the Electrolyte can come into contact. About this Working electrode is now another porous Teflon film unclamp so the electrolyte layer attached to the outer Surface of the working electrode is evenly distributed, not comes into direct contact with the outside world. Through this some µm thin electrolyte film, the gas can not freely on the Working electrode, but must first in the Dissolve liquid, then into the pores of the electrode to penetrate. Through this dissolution and exit from the The concentration is drastically reduced and the electrolyte Gas supply determined by this transport effect.

Gas sensor arrangement

Eine Anordnung zur Begrenzung des Gaseintritts bei Gassensoren aller Art (wie elektrochemische Gassensoren, Pellistoren, Halbleiter, Halbleiter-Oxid-Gassensoren und optische Gassensoren) dadurch gekennzeichnet, daß das Gas das in den Sensor eintritt oder einströmt vorher mit einer Gasbarriere aus flüssigem Material aller Art (wie fließende Flüssigkeit, Lösungen, Mischungen, gelartige Flüssigkeiten, erstarrte Flüssigkeiten, gummiartige-, Silicone-, polymere Flüssigkeiten) in Kontakt tritt.An arrangement for limiting gas entry in gas sensors of all kinds (such as electrochemical gas sensors, pellistors, Semiconductors, semiconductor oxide gas sensors and optical gas sensors) characterized in that the gas entering the sensor or flows in beforehand with a gas barrier made of liquid All kinds of material (such as flowing liquid, solutions, Mixtures, gel-like liquids, solidified liquids, rubber-like, silicone, polymeric liquids) in contact occurs.

Claims

Claim 1, characterized in that the gas barrier is a liquid film which blocks the entire cross section of the gas inlet opening.

Claim 2, characterized in that this gas film on a hydrophobic or hydrophilic carrier is applied.

Claim 3, characterized in that this liquid film between two or more hydrophobic and / or hydrophilic layers are upset.

Claim 4 characterized in that the gas barrier liquid phases a hydrophilic carrier is applied and this carrier is placed between two or more hydrophobic layers.

Claim 5 characterized in that the hydrophobic layers are Teflon, Polyethylene, polypropylene, etc. are porous plastic films and the hydrophilic layers of glass wool, glass paper, glass frits or hydrophilic plastic layers such as polyethylene, polypropylene are.

Claim 6, after 1-5, characterized in that the hydrophilic carrier is porous and the pores are fully or partially filled.

Claim 7, according to 1-6, characterized in that the carrier for the liquid phase of the hydrophobic Teflon or other electrode carrier foil Measuring electrode of the electrochemical cell.

Claim 8, according to claim 7, characterized in that the carrier of the liquid phase hydrophobic support material for the electrochemical electrode Cell is, with the upper layer of the carrier material partially was converted into hydrophilic layers or an additional Layer is applied, and on this layer or in the pores or partly only contain the liquid phase in these pores is.

Claim 9 characterized in that the liquid phase is the same Material like the electrolyte of the electrochemical cell.

Claim 10 characterized in that the liquid phase gas barrier with a liquid conductor, with the electrolyte reservoir electrochemical cell is connected.

Claim 11 characterized in that the liquid gas barrier a Is liquid through which the gas to be measured must diffuse.

Claim 12 characterized in that the liquid phase gas barrier Gas to be measured is absorbed on the outside of the sensor and on desorbed the inside of the sensor.

Claim 13, characterized in that the liquid phase gas barrier Dissolves gas and through this dissolution a gas vapor pressure inside the sensor.

Claim 14, according to Claim 12, characterized in that the liquid phase does not cover the entire Fills the cross section of the gas inlet, but between the Liquid phase (on carrier material) formed small gas channels and the gas to be measured from these gas channels into the Liquid phase is absorbed and then desorbed and settled plays chromatographic effect.

Claim 15, according to claims 12 and 14, characterized in that this chromatographic effect after appropriate choice of absorption liquids to a Leads to improvement in cross-sensitivity.