Capacitive, ceramic pressure-measuring cell and method for the production thereof

Abstract

A pressure-measuring cell having a ceramic measuring membrane and a ceramic counter body, wherein the measuring membrane is joined to the counter body in such a way that a pressure chamber is formed between the measuring membrane and the counter body. The pressure-measuring cell also has a capacitive transducer for detecting a pressure-dependent deformation of the measuring membrane, which capacitive transducer has at least one membrane electrode arranged on the measuring membrane and at least one electrode on the counter body side, wherein according to the invention at least one membrane electrode comprises a titanium oxide.

Claims

1. A pressure-measuring cell, comprising: a ceramic measuring membrane; a ceramic counter body; and a capacitive transducer, wherein: said measuring membrane is joined in a pressure-tight manner with said counter body to form a pressure chamber between said measuring membrane and said counter body; said capacitive transducer detects a pressure-dependent deformation of said measuring membrane, and comprises at least one membrane electrode that is arranged on said measuring membrane and at least one electrode on said counter body side; and said at least one membrane electrode comprises a titanium oxide, wherein: the titanium oxide is arranged directly on the ceramic material of said measuring membrane; said measuring membrane is joined to said base body by means of an active brazing solder, wherein said at least one membrane electrode is in galvanic contact with the active brazing solder; said measuring membrane and said counter body comprise an aluminum oxide ceramic; and the active brazing solder comprises a nickel-titanium-zirconium-containing active brazing solder.

2. The pressure-measuring cell according to claim 1, wherein: said at least one membrane electrode comprises titanium oxide, which is non-stoichiometric, in particular non-stoichiometric forms of TiO.sub.2, Ti.sub.4O.sub.7, Ti.sub.5O.sub.9 or Ti.sub.6O.sub.11.

3. The pressure-measuring cell according to claim 1, wherein: said at least one membrane electrode comprises doped titanium oxide, which is doped in particular with Cr, Nb or W.

4. The pressure-measuring cell according to claim 3, wherein: the doping is up to about 10 atomic %, related to the Ti atoms.

5. The pressure-measuring cell according to claim 3, wherein: the doping is not more than 6 atomic %, related to the Ti atoms.

6. A method of manufacturing a capacitive pressure-measuring cell, comprising a ceramic measuring membrane; a ceramic counter body; and a capacitive transducer, wherein: said measuring membrane is joined in a pressure-tight manner with said counter body to form a pressure chamber between said membrane and said counter body; said capacitive transducer detects a pressure-dependent deformation of said measuring membrane, and comprises at least one membrane electrode that is arranged on said measuring membrane and at least one electrode on said counter body side, and said at least one membrane electrode comprises a titanium oxide, the method comprises the following steps: providing the measuring membrane and the counter body; preparing at least one membrane-side electrode on the measuring membrane, and at least one electrode on the counter body side on a surface thereof; and pressure-tight joining of the measuring membrane to the counter body, forming a pressure chamber between the measuring membrane and the counter body, wherein: at least the membrane electrode comprises titanium oxide; the titanium oxide is prepared directly on the ceramic material of the measuring membrane: and the joining of the measuring membrane with the counter body comprises the following steps: provision of the active brazing solder between the measuring membrane and the counter body in the surface areas of the measuring membrane and of the counter body to be wetted by the active brazing solder; heating of the measuring membrane, the counter body and the active brazing solder under vacuum up to a temperature at which the active brazing solder melts and reacts with the measuring membrane and the counter body: and letting the pressure-measuring cell formed by joining cool down.

7. The method according to claim 6, wherein: the membrane electrode is prepared by first sputtering a metal layer, namely a titanium layer or a titanium layer that is doped with, for example, Cr, Nb, and/or W, which is then thermally oxidized.

8. The method according to claim 7, wherein: said oxidation is carried out by heating in an oxygen-containing atmosphere, for example in air, said oxidation takes place by heating to a temperature of not less than 500 C.

9. The method according to claim 7, wherein: said oxidation is carried out by heating in an oxygen-containing atmosphere, for example in air, said oxidation takes place by heating to a temperature of not less than 600 C.

10. The method according to claim 6, wherein: the membrane electrode is prepared by reactive sputtering of titanium oxide, which may especially be doped, in particular with Cr, Nb and/or W.

11. The method according to claim 6, wherein: the joining of the ceramic measuring membrane and the ceramic counter body with the active brazing solder takes place at a temperature of not less than 800 C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is now disclosed with reference to the exemplary embodiment illustrated in the drawings. Illustrated are:

(2) FIG. 1: which is a longitudinal section of a pressure-measuring cell according to the invention; and

(3) FIGS. 2a-2d: which show a sequence of preparatory steps for manufacturing a pressure-measuring cell according to the invention.

DETAILED DISCUSSION IN CONJUNCTION WITH THE DRAWINGS

(4) The pressure-measuring cell 1 shown in FIG. 1 comprises a measuring membrane 2, which has high-purity (>99.9%) aluminum oxide, and a counter body 4, which also comprises aluminum oxide. The counter body 4 may have the same purity as the measuring membrane, wherein this is not absolutely necessary because the counter body is not in contact with the medium on the one hand, so that the requirements for corrosion resistance are lower and is not exposed to mechanical stresses like the measuring membrane 2 on the other hand. The measuring membrane is joined pressure-tight with the counter body along a circumferential joint 6, which has a ZrNiTi-containing active solder, thereby forming a pressure chamber between the measuring membrane 2 and the counter body 4.

(5) For detecting a pressure-dependent deformation of the measuring membrane, the pressure-measuring cell 1 comprises a differential capacitor, which is formed by a membrane electrode 8 that is arranged on the measuring membrane 2, a central circular disk-shaped measuring electrode 10 on the counter body side and a reference electrode 12 surrounding the measuring electrode. Ideally, the capacitance value between the measuring electrode 10 and the membrane electrode 8 is equal to the capacitance between the reference electrode 12 and the membrane electrode 8, when the measuring membrane 2 is in the rest position. The membrane electrode 8 comprises titanium oxide that is prepared directly on the ceramic material of the measuring membrane 2.

(6) The titanium oxide preferably includes a dopant with a few atomic % of Nb, whereby the conductivity of the electrode material is significantly improved.

(7) The reference electrode 12 and the measuring electrode 10 may also comprise titanium oxide on the one hand, which is optionally doped, or, platinum or tantalum on the other. If tantalum electrodes are used, they must preferably be stabilized by thermal oxidation.

(8) The membrane electrode 8 is electrically contacted via the joint 6, and an electrical feedthrough 20 that extends along the radial area of the joint 6 through the counter body. The measuring electrode 10 and the reference electrode 12 are contacted directly by the counter body 4 via electrical feedthroughs 22, 24. The electrical feedthroughs 20, 22, 24 comprise, for example tantalum pins, which are soldered pressure-tight in boreholes through the counter body 4 by means of an active brazing solder.

(9) The membrane electrode 8 also acts as a solder stop, which prevents the active brazing solder from flowing radially inwardly out of the edge area into the pressure chamber during soldering of the counter body 4 and the measuring membrane 2.

(10) Referring to FIGS. 2a-2d, the manufacturing steps for producing the pressure-measuring cell according to the invention are now briefly described.

(11) As shown in FIG. 2a, a circular disk-shaped measuring membrane 2 made of high-purity corundum and a stiffer, circular disk-shaped counter body 4 made of corundum are first provided.

(12) Then, as shown in FIG. 2b, the surfaces of the membrane electrode 8, the measuring electrode 10 and the reference electrode 12 are prepared on the counter body 4 and the measuring membrane 2.

(13) The membrane electrode 8 and any other electrodes on the basis of titanium oxide comprise highly oxidized titanium, for example, TiO.sub.2 and/or Ti.sub.4O.sub.7, which is prepared by sputtering of titanium and optionally doping metals (Cr, Nb, W) onto the ceramic material of the measuring membrane 2 or the counter body 4, followed by thermal oxidation at 600 C. in air.

(14) In an alternative preparation, instead of the metallic titanium, titanium oxide can be deposited directly by reactive sputtering, wherein the subsequent oxidation step can be omitted.

(15) To prepare the joining of the measuring membrane 2 with the counter body 4, the two elements to be joined are coaxially stacked with an annular solder preform 5 in between, as shown in FIG. 2c. The annular solder preform 5 has a height of, for example, about 30 to 50 microns.

(16) Finally, the components are soldered in a high-vacuum solder process at temperatures, for example 950 C., wherein the molten active brazing solder reacts with the ceramic surfaces of the measuring membrane 2 and the counter body 4, but it cannot flow over the edge of the membrane electrode 8 into the pressure chamber, since the oxygen from the titanium oxide at least partly enters the solder, and thus slags the solder so that it solidifies or becomes highly viscous and does not flow further into the pressure chamber. Nevertheless, a galvanic contact is established between the joint 6 and the membrane electrode 8, so that a metallic coating on an outer surface of the pressure-measuring cell can be brought into contact with the membrane electrode 8 via the joint.

(17) This results in the pressure-measuring cell shown in FIG. 2d.

(18) Of course, this also includes the electrical feedthroughs described in connection with FIG. 1, which have however been omitted in FIGS. 2a to 2d for reasons of clarity. The feedthroughs are also soldered into the high-vacuum solder process.

(19) In particular, drawing to scale has been omitted for the height ratios in the drawings to allow illustration of the layers, in particular. The pressure-measuring cell has a radius of about 10 mm. The height or axial thickness of the counter body is, for example, 3 to 15 mm. The thickness of the measuring membrane is, for example, not less than 100 microns, and no more than 2,000 microns. The layer thickness of the electrodes is about 100 nm. These dimensions are merely illustrative and are not to be considered for the definition or strict interpretation of the invention.