GB2301439A

GB2301439A - Sensor for measuring partial pressures of oxygen

Info

Publication number: GB2301439A
Application number: GB9611101A
Authority: GB
Inventors: Dieter Thumfart; Walter Findl
Original assignee: Electrovac AG
Current assignee: Electrovac AG
Priority date: 1995-05-29
Filing date: 1996-05-28
Publication date: 1996-12-04
Also published as: GB9611101D0; FR2734907A1; ITMI961093A1; IT1283078B1; DE19621433A1; ITMI961093A0

Description

2301439 SENSOR FOR MEASURING PARTIAL PRESSURES OF OXYGEN The invention

relates to a sensor for the measurement of oxygen partial pressures based on the nlimiting current" principle, comprising an oxygen pumping cell which is gastightly joined to a gas diffusion barrier.

in oxygen sensors based on the nlimiting currentu principle, electrodes are disposed at two opposite sides on a zirconium oxide element and voltage is applied to the zirconium oxide cell produced in this way, as a result of which oxygen is "pumped" from the cathode to the anode of the cell since the charge transport in the interior of the cell is carried out by oxygen ions. if the applied voltage is increased, the current reaches a saturation value which depends on the oxygen content of the atmosphere surrounding the cell.

In the case of this simple -embodiment of the sensor, the saturation current is unstable since it depends directly on the decomposition of the cathode.

The cathode is therefore surrounded by a diffusion barrier. This is of gastight design and is provided with a small bore through which the atmosphere can reach the cathode. The bore is of such small design that the saturation current reached depends solely on the gas diffusion through said bore. A slight decomposition of the cathode does not affect the value of the saturation current under these circumstances.

Since the oxygen ion conductivity of the cell is strongly dependent on the temperature,. a heater is provided for the sensor in almost all zirconium oxide oxygen sensors.

in the embodiments hitherto known of such oxygen sensors.. the gas diffusion barrier is also composed, like the oxygen pumping cell, of zirconium oxide. The price of this material is, however, high and it is difficult to machine, in particular it is difficult to manufacture a bore having a sufficiently small diameter.

For these reasons, the diffusion barrier, which has to be of substantially larger volume than the oxygen pumping cell makes the sensory relatively expensive.

The object of the invention is to providea sensor of the type mentioned at the outset in which the gas diffusion barrier in easy to machine, in particular can be provided in a simple manner with a bore having a small diameter.

According to the invention, this is achieved in that the gas diffusion barrier is composed of a material selected from the group comprising glass, metal and glass ceramic.

The price of said materials is substantially lower than that of zirconium oxide and they are easier to machine, in particular smaller bores can be manufactured than in zirconium oxide. As a result, the operating temperature of the sensor according to the invention can be kept lower than in the case of a sensor having zirconium oxide diffusion barrier and, consequently, a lower power consumption and an -increase in the reliability can be achieved..:- The materials enumerated nevertheless have, however, all the properties necessary for the formation as gas diffusion barrier.

in this connection, in a further development of the invention, provision can be made that the oxygen pumping cell is gastightly joined to the diffusion barrier by means of glass.

An absolutely gastight joint between oxygen pumping cell and diffusion barrier can thereby be provided in a simple manner.

in a preferred embodiment of the invention, provision can be made that, as heater for the sensor, platinum tracks provided with platinum connecting wires are disposed on the outside of the diffusion barrier.

In this way, the heater can be produced in a simple manner and permits an effective heating of the sensor owing to the direct arrangement on the diffusion barrier.

in a further development of the invention it may be advantageous that, an pull relief for the platinum connecting wires, elements enclosing the platinum connecting wires in each case and composed of glass paste are disposed on the surface of the diffusion barrier or on the surface of the oxygen pumping cell.

Said pull relief is easy to produce and of f era an effective protection against unacceptably high pulling loads.

in this connection, a further embodiment of the invention may consist in that, to match the material properties of the glass and the glass paste to each other, the same are formed from a mixture of various glass powders and organic binders. As a result of this measure, the two materials can be processed jointly.

A preferred embodiment of the invention may is consist in that the glass and the glass paste are composed of materials selected from the group comprising SiO., Na203. BaO, X20, A1,03 and B203.

This composition produces a glass having a coef f icient of therma 1 expansion which is matched to that of the oxygen pumping cell formed from zirconium oxide.

The invention is described below by reference to a preferred embodiment, shown in section in the drawing, of an oxygen sensor according to the invention.

The oxygen pumping cell 1 has a preferably cylindrical plate 20 formed from an oxygen- ion- conducting material, for example zirconium oxide stabilized with yttrium oxide. Disposed on the upper side and lower side of said plate 20 are platinum electrodes 4, 5 which are connected via platinum wires 6, 12 to a voltage source 11. Information about the magnitude of the oxygen partial pressure to be measured in obtained by evaluating the current which the voltage source 11 supplies to the sensor. in the embodiment of the invention shown in the drawing, this evaluation is carried out with the aid of an ammeter 13.

The oxygen pumping cell 1 is joined gastightly to a diffusion barrier 2 by means of a glass 3.

The diffusion barrier 2 comprises a gastight 4 material formed with a bore 7. Preferably, this material is a glass ceramic. in principle, materials which can be used in the context of the invention are those which have:

approximately the same coefficient of thermal expansion as the oxygen pumping cell 1; 2.) a heat resistance of up to approximately 700OC; 3.) a complete gastightness; 4.) an easy machinability; S.) the possibility of manufacturing bores having very small diameter and 6.) a low price.

Glass ceramic fulfils these requirements in a particularly satisfactory manner, in particular, in the is case of this material, bores can be manufactured which have smaller diameter than in the case of zirconium oxide; alternatively, however, glass and metal fulfil the requirements imposed.

Glass ceramic additionally has a particularly good wettability with the glass 3, as a result of which possible leaks between the glass 3 and the diffusion barrier 2 are avoided.

The heater for the sensor is disposed on the outside of the diffusion barrier 2. Like the electrodes of the oxygen pumping cell 1, it comprises platinum tracks 8 which are connected to a voltage source via platinum wires 9.

An pull relief for the platinum connecting wires 9, 6, elements enclosing the platinum connecting wires 6, 9 in each case and composed of glass paste 10 are disposed on the surface of the diffusion barrier 2 or on the surface of the oxygen pumping cell 1.

The composition of the glass 3 and of the glass paste 10 forming the pull relief a in chosen in such a way that similar thermal properties of said materials are present which are matched to the glass ceramic. This achieves the result that the two materials, glass paste 10 and glass 3, can be processed together, i.e. fused, in - one production step.

The glass 3 and the glass paste 10 are f ormed f rom a mixture of various glass powders and organic binders.

it has proved beneficial that the glass 3 and the glass paste 10 are composed of materials selected f rom the group comprising SiO., Na203, BaO, F'20, A1203 and B203,, a mixing ratio of 55% SiO2r 5% Na.03. 17% BaO, 6% 1C20, 3% A1203J' 14% B203 yielding a glass whose coefficient of thermal expansion is particularly well matched to the coefficient of thermal expansion of the oxygen pumping cell. Approximately 5% of Plextol is added as binder to render said powder mouldable.

Claims

Patent Claims

1. Sensor for the measurement of oxygen partial pressures based on the "limiting currentu principle, comprising an electrochemical oxygen pumping cell (1) which is gastightly joined to a gas diffusion barrier (2), characterized in that the gas diffusion barrier (2) is composed of a material selected from the group comprising glass, metal and glass ceramic.

Sensor for the measurement of oxygen partial pressures according to Claim 1, characterized in that the oxygen pumping cell (1) in gastightly fused to the diffusion barrier (2) by means of glass (3).

3. Sensor for the measurement of oxygen partial pressures according to Claim 1 or 2, characterized in that, as heater for the sensor, platinum tracks (8) provided with platinum connecting wires (9) are disposed on the outside of the diffusion barrier (2).

4. Sensor for the measurement of oxygen partial pressures according to one of Claims 1 to 3, characterized in that, as pull relief for the platinum connecting wires (9, 6) elements enclosing the platinum connecting wires (6, 9) in each case and composed of glass paste (10) are disposed on the surface of the diffusion barrier (2) or on the surface of the oxygen pumping cell (1).

5. Sensor for the measurement of oxygen partial pressures according to one of Claims 1 to 4, characterized in that, to match the material properties of the glass (3) and the glass paste (10) to each other, the same are formed from a mixture of various glass powders and organic binders.

6. Sensor for the measurement of oxygen partial pressures according to Claim 5, characterized in that the glass (3) and the glass paste are composed of materials selected from the group comprising S'02, Na203, BaO, X200 A1203 and B203