Assembly having at least two ceramic bodies joined with one another, especially a pressure measuring cell, and method for joining ceramic bodies by means of an active hard solder, or braze

Abstract

An assembly comprising: two ceramic bodies, which are joined by means of a joint of an active hard solder, or braze, wherein the active hard solder, or braze, has a continuous core volume, which is spaced, in each case, from the ceramic bodies by at least 1 m, and an average composition C.sub.K with a liquidus temperature T.sub.l(C.sub.K), wherein the composition C.sub.K has a coefficient of thermal expansion (C.sub.K), wherein (C.sub.K)=m.Math.(K), wherein m1.5, especially m1.3 and preferably m1.2, wherein (K) is the average coefficient of thermal expansion of the ceramic material of the ceramic bodies, wherein the joint has boundary layers, which border on the ceramic body, wherein at least one of the boundary layers, which lies outside of the core volume, has an average composition C.sub.B with a liquidus temperature T.sub.l(C.sub.B), which lies not less than 50 K, preferably not less than 100 K, and especially preferably not less than 200 K, under the liquidus temperature T.sub.l(C.sub.K) of the average composition C.sub.K of the core volume.

Claims

1. An assembly, comprising: a first ceramic body: a second ceramic body; and a joint for connecting said first ceramic body to said second ceramic body, wherein: said joint contains an active hard solder, or braze; said active hard solder, or braze, averaged over a continuous core volume, which is spaced from said first ceramic body and from said second ceramic body, in each case, by at least 1 m, has an average composition C.sub.K having a liquidus temperature T.sub.l(C.sub.K); the composition C.sub.K has a coefficient of thermal expansion (C.sub.K), wherein (C.sub.K)=m.Math.(K), wherein m1.5, wherein (K) is the average coefficient of thermal expansion of the ceramic material of said first and said second ceramic bodies; said joint has a first boundary layer and a second boundary layer of said active hard solder, or braze, which border on said first ceramic body and said second ceramic body, respectively; wherein at least one of said boundary layers, which lies outside of the core volume, has an average composition C.sub.B having a liquidus temperature T.sub.l(C.sub.B), which lies not less than 50 K, under the liquidus temperature T.sub.l(C.sub.K) of the average composition C.sub.K of the core volume.

2. The assembly as claimed in claim 1, wherein: said at least one boundary layer has a thickness of no more than 3 m.

3. The assembly as claimed in claim 1, wherein: the composition C.sub.B has a liquidus temperature T.sub.l(C.sub.B), which lies no more than 300 K above the liquidus temperature T.sub.l(C.sub.e) of the eutectic point, respectively of the nearest intersection with a eutectic valley having a composition C.sub.e in the composition space.

4. The assembly as claimed in claim 1, wherein: the alloy of the joint at the eutectic point, respectively at the nearest intersection with a eutectic valley, has a composition C.sub.e in the composition space, wherein C.sub.e:=(c.sub.e1, . . . , c.sub.eN), wherein |C.sub.e|=1, wherein the c.sub.ei are the stoichiometric fractions of the components K.sub.i with i=1, . . . , N at the eutectic point, respectively at the nearest intersection with a eutectic valley, wherein the difference between the composition C.sub.e and the composition C.sub.B is describable with a normalized vector difference D.sub.eB, wherein: C.sub.e=C.sub.B+a.sub.eB*D.sub.eB, with D.sub.eB|=1, wherein the difference between the composition C.sub.K and the composition C.sub.B is describable with a normalized vector difference D.sub.KB, wherein: C.sub.K=C.sub.B+a.sub.KB*D.sub.KB, with |D.sub.KB|=1, wherein a.sub.eB and a.sub.KB are positive scalars, and wherein for the scalar product s.sub.eK:=D.sub.eB.Math.D.sub.KB:s.sub.eK<0.

5. The assembly as claimed in claim 1, wherein: said first ceramic body and/or said second ceramic body comprise Al.sub.2O.sub.3.

6. The assembly as claimed in claim 1, wherein: said active hard solder, or braze, comprises Zr, Ni and Ti.

7. The assembly as claimed in claim 6, wherein: the composition C.sub.K contains essentially zirconium and titanium, with (50+x) atom-% titanium and (50x) atom-% zirconium, wherein x<10, and wherein the composition C.sub.K is present in the -(Zr, Ti) phase.

8. The assembly as claimed in claim 1, wherein: the composition C.sub.K has a coefficient of thermal expansion (C.sub.K), wherein (C.sub.K)10.Math.10.sup.6/K.

9. The assembly as claimed in claim 1, wherein: said at least one boundary layer has a thickness of no more than 1 m.

10. The assembly as claimed in claim 1, wherein: the composition C.sub.B has a liquidus temperature T.sub.l(C.sub.B), which lies no more than 150 K above the liquidus temperature T.sub.l(C.sub.e) of the eutectic point, respectively of the nearest intersection with a eutectic valley having a composition C.sub.e in the composition space.

11. The assembly as claimed in claim 1, wherein: the composition C.sub.B has a liquidus temperature T.sub.l(C.sub.B), which lies no more than 50 K above the liquidus temperature T.sub.l(C.sub.e) of the eutectic point, respectively of the nearest intersection with a eutectic valley having a composition C.sub.e in the composition space.

12. A pressure measuring cell, comprising: a first ceramic body, said first ceramic body is a membrane body of a measuring membrane of the pressure measuring cell; a second ceramic body, said second ceramic body being a platform of the pressure measuring cell; and a ring-shaped joint for connecting said first ceramic body to said second ceramic body, wherein: said joint contains an active hard solder, or braze, said active hard solder, or braze, averaged over a continuous core volume, which is spaced from said first ceramic body and from said second ceramic body, in each case, by at least 1 um has an average composition C.sub.k having a liquidus temperature T.sub.l(C.sub.k), the composition CK has a coefficient of thermal expansion *(CK), wherein *(CK)=m**(K), wherein m*1.5, wherein *(K) is the average coefficient of thermal expansion of the ceramic material of said first and said second ceramic bodies; said joint has a first boundary layer and a second boundary layer, which border on said first ceramic body and said second ceramic body, respectively; and at least one of said boundary layers, which lies outside of the core volume, has an average composition CB having a liquidus temperature Tl(CB), which lies not less than 50 K under the liquidus temperature Tl(CK) of the average composition CK of the core volume.

13. A method for manufacturing an assembly comprising a first ceramic body and a second ceramic body joined by means of an active hard solder, or braze, comprising the steps of: providing the active hard solder, or braze, between the ceramic bodies, wherein the active hard solder, or braze, has, averaged over a continuous core volume, an average composition C.sub.Ko having a liquidus temperature T.sub.i(C.sub.Ko), wherein the composition has a coefficient of thermal expansion (C.sub.K), wherein (C.sub.K)=m(K), wherein m<1.5, wherein (K) is the average coefficient of thermal expansion of the ceramic material of said first and second ceramic bodies, wherein the active hard solder, or braze, has, on at least one of its surfaces facing said ceramic bodies, a boundary layer having an average composition C.sub.BO, wherein the composition C.sub.BO has a liquidus temperature T.sub.l(C.sub.BO), which lies not less than 50 K, under the liquidus temperature T.sub.l(C.sub.KO) of the average composition C.sub.KO of the core volume, wherein C.sub.BO:=(c.sub.BO1, . . . , c.sub.BON), wherein |C.sub.BO|=1, and wherein the c.sub.BOi are the stoichiometric fractions of the components K.sub.i i=1, . . . , N of the average composition of the active hard solder, or braze, in said boundary layer; and heating the ceramic body and the active hard solder, or braze, in a vacuum soldering, brazing process, up to a melting of the composition C.sub.BO, wherein: the boundary layer develops a melt and said melt mixes in the transition to the core region with the material of the core volume, whereby the liquidus temperature of the boundary layer is increased, so that the boundary layer at least partially isothermally solidifies or becomes more viscous.

14. A method as claimed in claim 13, wherein: the providing of the active hard solder, or braze, includes that a solder preform, which has the composition C.sub.K0, is coated by means of gas phase deposition, by sputtering, on at least one surface with a boundary layer, which has the composition C.sub.B0.

15. A method as claimed in claim 13, wherein: the providing of the active hard solder, or braze, includes that at least one surface section of a ceramic body is coated with a boundary layer, which has the composition C.sub.B0, wherein the coating occurs by sputtering.

16. The method as claimed in claim 15, wherein: there is arranged between the ceramic bodies provided with the boundary layer a solder preform, which has a core volume with the composition C.sub.K0, and which is coated with a boundary layer of composition C.sub.B0.

17. The method as claimed in claim 13, wherein: the composition C.sub.K contains essentially zirconium and titanium with (50+x) atom-% titanium and (50x) atom-% zirconium, wherein x<10 wherein the composition C.sub.K is present in the -(Zr, Ti) phase.

18. The method as claimed in claim 13, wherein: the composition C.sub.B0 comprises 42 to 52 atom-% Zr, 23 to 28 atom-% Ni and 24 to 30 atom-% Ti.

19. The assembly as claimed in claim 1, wherein: said at least one boundary layer has a thickness of no more than 2 m.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be explained based on the example of an embodiment illustrated in the drawing, the figures of which show as follows:

(2) FIG. 1 is a simplified diagram for the ternary system NiTiZr; and

(3) FIG. 2 is a partial longitudinal section through a pressure measuring cell of the invention.

DETAILED DISCUSSION IN CONJUNCTION WITH THE DRAWINGS

(4) The diagram shown in FIG. 1 for a ternary metal system, namely the NiTiZr system is based on data of Gupta (Journal of Phase Equilibria, 20(4), pages 441-448, August 1999). It shows the position of the eutectic point E and various eutectic valleys. The arrows in the eutectic valleys point toward lower liquidus temperature.

(5) According to the invention, a core volume of an active hard solder, or braze, is provided, which determines the mechanical properties of a joint formed therewith, having a composition C.sub.K0, for example, as a solder preform, wherein the surfaces of the core volume are coated with a boundary layer of a composition C.sub.B0, wherein the last named composition has a significantly lower melting point than the composition of the core volume.

(6) The composition C.sub.K0 of the core volume is preferably so selected that the coefficient of thermal expansion of the composition deviates as little as possible from the coefficient of thermal expansion of the ceramic material of the ceramic bodies to be joined. The coefficient of thermal expansion was ascertained for different compositions. In such case, compositions of zirconium and titanium with (50+x) atom-% titanium and (50x) atom-% zirconium, wherein x<10, especially x<5, especially in the -(Zr, Ti) phase proved especially suitable as composition of the core volume C.sub.K0 for joining of corundum. For the coefficient of thermal expansion (C.sub.K), (C.sub.K)9.5.Math.10.sup.6/K, especially (C.sub.K)9.2.Math.10.sup.6/K.

(7) As shown in FIG. 1, the composition C.sub.B0 of the boundary layer can be selected to be at or near the eutectic point, such as indicated in FIG. 1. A suitable composition C.sub.B0 comprises, for example, 47 atom-% Zr, 26 atom-% Ni and 27 atom-% Ti. The associated liquidus temperature amounts, for instance, to 770 C.

(8) The liquidus temperature of a composition of the core volume C.sub.K0 with 55 atom-% Zr and 45 atom-% Ni amounts, in contrast, to more than, for instance, 1200 C.

(9) Correspondingly, the boundary layer can be reliably melted at a soldering temperature of 800 C. to 850 C., for example, without melting the core volume of the active hard solder, or braze.

(10) As a result, the fine crystalline, respectively amorphous, structure of the core volume can be retained in the soldering. Solely at the interface between the boundary layer and the core volume is there, in given cases, an exchange of materials between the core volume and the boundary layer, such that the boundary layer experiences, sectionally, an increase of the liquidus temperature, which, depending on the selected soldering temperature, effects that regions of the boundary layer become isothermally viscous or solidify. In any case, however, the structure of the core volume scarcely changes.

(11) As an example of application of this procedure, the components of a pressure measuring cell are joined. FIG. 2 shows the arrangement before the joining. The pressure measuring cell includes a ceramic platform 1 and a measuring membrane 2. Each of these is composed of aluminum oxide. The measuring membrane 2 and the platform are to be joined by means of an active hard solder, or braze, wherein the active hard solder, or braze, is provided as a annular solder preform 3 with a thickness of, for example, 20 m, wherein on both end faces of the solder ring a boundary layer 4, 5 is deposited by sputtering-on a thickness of 1 m to 2 m.

(12) The solder preform has the above described composition C.sub.K0 of the core volume, thus Zr and Ti in the stoichiometric ratio of, for instance, 3 to 1. The boundary layer has, in contrast, a composition C.sub.B0, which lies near or at the eutectic point E.

(13) By soldering in high vacuum at, for example, 850 C., the boundary layers 4, 5 react with the platform and with the measuring membrane 1, 2, so that a joint is formed, wherein the core volume of the active hard solder, or braze, does not melt and essentially retains its amorphous structure. The measuring membrane and the platform each bear an electrode 7, 6 of a capacitive transducer, wherein the electrodes can be prepared, for example, by depositing Ni.