DE19725948A1 - Production of joint between e.g. ceramic or powdered metal green product - Google Patents

Abstract

Production of a joint between two or more initial products, namely ceramic or powdered metal green products or pre-sintered products comprises adhering with a film with adhesive on both sides.

Description

field of use

The invention relates to a method for producing a material connection between ceramic or powder metallurgical green bodies, so that these after sintering are firmly connected. The method is particularly suitable for laminating ceramic green foils for the production of functional and engineering ceramics such as B. Housing, capacitors, actuators, sensors, HT fuel cells, heat exchangers u. a.

State of the art

The basis of the ceramic laminating technology are ceramic green foils, which through the Foil casting can be made. The green foils are up to 2 mm thick and consist of Ceramic powder, which is in a polymer matrix, often based on polyvinyl butyral (PVB), is embedded. Added plasticizers give the film a certain flexibility. Before the The individual layers of the green foils to be laminated can be laminated depending on the application be structured, d. H. with bushings or other recesses or structures be provided and printed with metal pastes. These layers are then under Pressures between 15 MPa / 86 Boc / and 100 MPa / 95 Tuf / at temperatures between 60 ° C / 86 Boc / and approx. 110 ° C / 97 Cha / pressed. The elevated temperature is to soften the Binder (polymers) necessary. Pressure and temperature are interdependent. A low one Temperature requires higher pressure; vice versa, the same applies. After Hellebrand / 96 Hel / is important for a good, flawless bond that the Reorient powder particles in the touching green film surfaces so that it becomes a Interlocking of the powder particles comes. The driving force for the rearrangement of the powder grains is the external pressure associated with the softening of the polymer. To one  To enable reorientation, there are some requirements for the green film: the relative volumes of the pores, the powder and the binder must be according to / 96 Hel / in one certain relationship to each other.

Disadvantages of the prior art

Laminating under pressure and temperature is called thermocompression. This Technology has some key disadvantages. The process is time consuming and costly, because the pressure, depending on the number of layers, can be reached for a while at elevated temperature the thermal equilibrium must be kept within the films to be laminated. This precludes a continuous process from the outset. Increased pressure and Thermocompression temperatures lead to deformations of the original geometry. Therefore, to avoid deformation of the laminate in length, width and thickness Molding tools are used.

In the production of cavity structures via the layered structure of structured laminates (heat exchanger / 87 Hei /) there are further problems. At the Cut-outs are punched into the green foils to create layer structures. By Layering of the individual layers and their connection via lamination are created then z. B. in the case of the heat exchanger duct systems. When using thermal compression The inhomogeneous pressure distribution in the laminate is created by the cavities in the green foils with partially insufficient pressure penetration, which leads to insufficient lamination and this leads to undesired delamination, d. H. the foil layers are not after sintering consistently interconnected, the damage zones lead to Characteristics changes, whereby the part represents committee. It is particularly problematic to realize fine structures, for example honeycombs, in the laminate, as this is due to the applied pressure can be destroyed.

So far, attempts have been made to overcome these problems with laminating aids which are used in the contain essential solvents and binders. They have the function of green foils to dissolve on the surface so that lower pressures and temperatures are used can. The disadvantage is that the application of the laminating aid is an additional step requires and that the laminate can be deformed more easily by dissolving the binder. The above Temperature and pressure ranges also refer to the use of Laminating aids.

Recently there have been so-called laminating foils that use an adhesive to bond the foils together Gluing, which also require elevated temperatures and therefore not the above. Solve problems.  

Regardless of the lamination technique, it is known that green film production tries to get by with the lowest possible amount of binder / plasticizer / 88 Roo /, because this must then be burned out again during the compaction to form the ceramic. / 89 Cim / have the heating of the organic components of a green sheet and the movement of the softening binder described in the green body structure.

literature

/ 86 Boc / P. Boch, C. Chartier, M. Huttepain, "Tape Casting of Al ₂

O ₃

/ ZrO ₂

Laminated Composites ", J. Am. Ceram. Soc. 69 [8] (1986) C191-C192
/ 87 Hei / J. Heinrich, J. Huber, H. Schelter, R. Ganz, R. Golly, S. Foerster, P. Quell, "Compact Ceramic Heat-Exchangers: Design, Fabrication and Testing", Brit. Ceram. Trans. J. 86 [6] (1987) 178-182
/ 88 Roo / A. Roosen, Basic requirements for tape casting of ceramic powders, in Ceram. Transactions. Vol. 1, Part B, Ceramic Powder Science, Am. Ceram. Soc., Inc., Columbus, OH, 1988, pp. 675-692
/ 89 Cim / MJ Cima, M. Dudziak, JA Lewis, "Observation of Poly (vinyl Butyral) -Dibutyl Phthalate Binder Capillary Migration", J. Am. Ceram. Soc., 72 [6] (1989), 1087-1090
/ 95 Tuf / S. Tuffé, DS Wilkinson, "MoSi ₂

-Based Sandwich Composite Made by Tape-Casting ", J. Am. Ceram. Soc., 78 [11] (1995), 2967-2972
/ 96 Hel / H. Hellebrand. "Tape casting"; pp 189-265 in Materials Science and Technology, vol. 17 A, Processing of ceramics. Part 1, ed RJ Brook, VCH publishing company, Weinheim, FRG 1996
/ 97 Cha / T. Chartier T. Rouxel, "Tape-Cast Alumina-Zirconia Laminates: Processing and Mechanical Properties" J. Eur. Ceram. Soc. 17 [2-3] (1997), 299-308

Technical task

The object of the invention is to offer a lamination method with which green foils lowest possible temperatures can be combined with very low pressures without that errors, e.g. B. delaminations or areas of increased porosity at the seams of the Green foils remain after sintering.

Solution of the task

By gluing the foils together using double-sided adhesive tape (adhesive on both sides) the task was surprisingly achieved according to the invention. It turned out out that the carrier film allows the green foils to be joined to densely sintered bodies without that the lamination of the particles as required at / 96 Hel / is present when laminating a corresponding external pressure and temperature is reached during lamination. In contrast to this, in the case of the invention, the green foils are through after the gluing a polymeric carrier film separated from each other. This carrier film is responsible for that there is a bond between the two films in the further baking and sintering process. He becomes a thin melt at higher temperatures. The melt is through the Capillarity of the ceramic body in which the other organic auxiliaries already exist are largely heated, sucked into the pore ducts. The occurring Capillary forces pull the two bodies together. In the liquid polymer melt can  the ceramic powder particles slide into each other, it comes to the required Gearing. To do this, the ceramic powder particles must be able to move freely organic additives that are contained in the green film, as far as possible and there must be no neck formation during sintering, which also increases mobility would belittle.

The method demonstrated here using ceramic green foils can generally be used Connection of ceramic or powder metallurgical green bodies or presintered ones Bodies with sufficient residual porosity can be used. The process consists of two essential steps: 1. applying the adhesive tape and gluing the to connecting body and 2. the temperature treatment to drive out the binder, glue and the carrier film of the adhesive and the subsequent sealing sintering of the body.

Advantages of the invention

The lamination step has thus been greatly simplified. The essentially new and What is surprising about this method is that the pressure required to mesh the individual ceramic film layers not from the outside by pressing while increasing Temperature must be applied, but between the layers through the Capillary forces that result from the formation of polymer melt in the green structure, d. H. is generated in situ. Here the mobility of the powder particles in the Melt plays a crucial role in reorientation.

The new method has significant advantages over the previous technology. So it can Laminating can be done at room temperature with very low pressures. This is time and energy saving because the pickling and holding at an elevated temperature not applicable. The binder of the green sheet has not yet softened at room temperature, so that Laminating can be done without a mold. This allows Tool costs can be saved. It can also be complicated, three-dimensional Realize cavity structures because the structures remain when the green foils are connected received without deformation. Because the pressure to interlock the film layers by capillary forces generated, it is distributed evenly throughout the laminate. Therefore none arise Delamination or seams in the material.

Another important aspect is that the new process offers the possibility of continuous production of laminates. Both the green film and the double-sided adhesive tape can be removed from rolls. The individual green film webs can first be stuck with double-sided adhesive tape using rollers. After peeling off the silicone paper from the double-sided adhesive tape, the individual layers can then be connected using pressure rollers. An endless laminate is created, which can be divided into small units by punching. This enables high quantities, which increases efficiency and reduces unit costs. Fig. 1 shows the scheme of such a process.

In addition, other ceramic green bodies can also be used with this method connect.

example

The new lamination method is based on gluing the green film layers with one suitable double-sided adhesive tape, such as. B. Tesafix 4972 from Beiersdorf AG, Hamburg Room temperature. This double-sided adhesive tape is made up of several layers. It consists of one Backing film made of polyethylene terephthalate (PET) with an acrylic adhesive on both sides is coated. The total thickness of the double-sided adhesive tape is approx. 48 µm. It lies wound up in the form of rolls. One side of the tape is covered with silicone paper. This paper allows the double-sided tape to be applied and pressed on (low pressure) on the green sheet. The silicone paper must be connected to another layer of green film subtracted from.

A number of lamination tests were carried out with the double adhesive tape Tesafix 4972 been. The green film used is based on aluminum oxide with a medium one Grain size of 3.5 µm and polyvinyl butyral as a binder. The composition of the Slip systems are given in Table 1. This slip becomes a green sheet Foil casting and drying. The cast film is 725 µm thick and has one mean pore size of 0.6 µm. The film became squares for the lamination tests cut with an edge length of 30 mm. In the experiments, laminates were made from three Layers.

Fig. 2 illustrates the lamination process with the new technology . For the lamination, the double-sided adhesive tape was stuck on. After the silicone paper had been peeled off, the individual layers were stacked on top of one another and pressed together under slight pressure. A laminate was made by gently pressing the film layers by hand. Further laminates were realized by pressing with pressures of 2.5 MPa and 5 MPa. The laminates were then baked and sintered using the furnace program shown in Fig. 3. Sections of the sintered laminates showed no defects or seams in the laminates in all three cases.

The structure of the double-sided adhesive tape is decisive for the effectiveness of the new technology. The acrylic adhesive ensures the green foils are fixed when laminating. The adhesive is sensitive to pressure, which is why a connection is possible even at low contact pressures. Therefore, the tests showed no dependence of the laminate quality on the pressing pressures. The acrylic adhesive is burned out at the same time as the binder in the green sheet when it is baked out, ie when the temperature rises during the fire. The PET of the carrier film remains stable up to approx. 250 ° C. Fig. 4 shows the already decomposed and z. T. cross-linked structure of the acrylic adhesive, as well as the beginning liquefaction of the PET film. The PET then forms a low-viscosity melt, the viscosity of which decreases with increasing temperature due to chain breakage of the polymer molecules ( Fig. 5). At 300 ° C dynamic viscosities of the PET of approx. 30 Pas could be measured (e.g. see Cima / 89 Cim / shows polyvinyl butyral at 160 ° C a viscosity of approx. 5000 Pas). The PET melt is broken down by flowing through the pores of the ceramic layers and by evaporating short-chain cracked products.

The polymer melt leads to capillary forces in the porous capillary system of the green sheet, which in turn exert a suction on the polymer melt as capillary pressure. This Capillary forces pull the individual layers of the ceramic foils towards each other. In the low-viscosity melt, the near-surface powder particles can reorient so that as the individual layers approach each other, they become interlocked and become one homogeneous composite comes.

The essentially new and surprising thing about this method is that the necessary pressure to Do not interlock the individual ceramic film layers from the outside by pressing at the same time increased temperature must be applied, but between the layers by the capillary forces that arise from the formation of the melt in Green body structure, d. H. is generated in situ. Here the mobility of the Powder particles in the melt play a crucial role for reorientation.

For this purpose, polymers can be used which have a liquid phase in the baking process form such a temperature range by the organic used in the green body Auxiliaries have already largely decomposed or liquefied. This melt is in the porous bodies are drawn in due to their capillarity, so that those occurring Capillary forces pull the bodies to be joined together. This allows the Interface between the two bodies of powder particles due to reorientation in interlink the liquid phase. This leads the subsequent Temperature treatment to densify the body and to eliminate the interface between the bodies, whereby the bodies are firmly connected. - The im Organic additives used in green bodies must be below the Liquefaction temperature of the carrier film of the adhesive film in the respective green bodies  begin to decompose or liquefy so that these organic additives liquefy the carrier film of the adhesive no longer fix the powder particles so that they can move freely are. - The ceramic or powder metallurgical green body must have a porosity of <10 Vol% have with respect to the inorganic grains and the individual powder particles must are isolated from each other, d. H. there must not have been any sinter neck formation yet, so that the powder particles still before the final burnout of the organic additives are mutually displaceable.

The method demonstrated here using ceramic green foils can generally be used Production of a cohesive connection of ceramic or powder metallurgical Green bodies or pre-sintered bodies with sufficient residual porosity are used will. The process consists of two main steps: 1. applying the Adhesive tape and the gluing of the body to be connected and 2. the Temperature treatment to drive out the binder, glue and the carrier film of the glue and the subsequent dense sintering of the body.

The method is particularly suitable for the lamination of ceramic green foils Production of functional and engineering ceramics such as B. housing, capacitors, Actuators, sensors, HT fuel cells, heat exchangers u. a.  

Components of the film casting slip in mass%

Components of the film casting slip in mass%

Claims (16)

1. A method of connecting two or more Intermediate products, namely ceramic or powder-metallurgical green bodies or pre-sintered bodies, by gluing the preliminary products with a double-sided adhesive film. 2. The method according to claim 1, wherein the green body is organic additives, such as B. Binder made of polyvinyl butyral or polyvinyl alcohol. 3. The method according to any one of the preceding claims, wherein the porosity of the green body is greater than 10% by volume with respect to the inorganic grains. 4. The method according to any one of the preceding claims, wherein the Green bodies made of inorganic grains isolated from each other is formed. 5. The method according to any one of the preceding claims, wherein the Green body is a ceramic film. 6. The method according to any one of the preceding claims, wherein according to the Glue a first heating step to heat the double-sided adhesive film and optionally the binder or other organic additives is carried out. 7. The method of claim 6, wherein the temperature of the first heating step is a maximum of 650 ° C. 8. The method according to claim 6 or 7, wherein after the first heating step second heating step for sealing sintering is carried out.   9. The method according to any one of claims 6 to 8, wherein the maximum Temperature of the second heating step is greater than the maximum temperature of the first heating step. 10. The method according to any one of the preceding claims, wherein the bilateral adhesive film a double-sided tape with one on both sides with a Acrylic adhesive coated, preferably made of polyethylene terephthalate manufactured, carrier film is. 11. The method according to any one of the preceding claims, wherein the Melting point of the carrier film above the temperature of the thermal degradation any organic additives present in the green bodies. 12. The method according to any one of the preceding claims, wherein the Melting point of the carrier film is above a temperature of 250 ° C. 13. The method of claim 10 or 12, wherein the thickness of the Double adhesive tape is about 48 microns. 14. The method according to any one of the preceding claims, wherein the Bonding of the bodies to be joined at temperatures above -20 ° C takes place. 15. The method according to any one of the preceding claims, wherein the Bonding of the bodies to be connected takes place at pressures above 0.1 MPa. 16. The method according to any one of the preceding claims, wherein the Process operated as a continuous process.