not bonded to one another when use is made of parallel separating strips approximately parallel bonding seams along which the neighboring foils are bonded to one another are formed by thermal compression as a result of such approximately parallel bonding seams the spaces between the foils are shaped as capillary tubes which extend approximately perpendicularly to the plane of drawing and parallel to the bonding seams the degree of expansion of the stack of foils determines in conjunction with the dimensions of the bonding seams and the spacing of the bonding seams the dimensions of the capillary tubes as a result of the expansion in the direction transversely of the foils the buffer members 12 are slightly contracted in the direction parallel to the foils it is thus achieved that the stack of foils is expanded mainly transversely of the foils and the size of the local spaces between the foils is uniform over the honey comb structure in order to realize the structured separating layer on one side of the foils there is provided a metal layer having a thickness of for example 20 nm this metal layer can be deposited for example by way of a vapor deposition process subsequently the metal is removed from the foil at the area of the bonding locations by means of for example laser ablation other possibilities for locally removing the material are for example wet chemical etching or the use of a shadow mask vapor deposition method in order to make the temperature adjustment even less critical during the heating and bonding of the stack of foils preferably both sides of the foils are provided with the structured separating layer fig3 shows the use of the honeycomb structure 30.