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

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 Ck having a liquidus temperature Tl(Ck),
• 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.

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 CKo having a liquidus temperature Ti(CKo), wherein the composition has a coefficient of thermal expansion α(CK), wherein α(CK)=m−α(K), wherein m<1.5, wherein α(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 CBO, wherein the composition CBO has a liquidus temperature Tl(CBO), which lies not less than 50 K, under the liquidus temperature Tl(CKO) of the average composition CKO of the core volume, wherein CBO:=(cBO1, . . . , cBON), wherein |CBO|=1, and wherein the cBOi are the stoichiometric fractions of the components Ki 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 CBO, 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.