Andreas Roosen

Rheological characterization of water-based slurries for the tape casting process

Rheological properties of aqueous tape casting slurries have been investigated as they strongly affect the tape casting process and the quality of the final product. The aqueous slurries consist of yttria stabilized zirconia and a polymeric latex emulsion as the binder. The viscosity, its time dependent behavior and the strength of the internal structure of the slurry were characterized by static and dynamic measurements using a cone-plate viscosimeter. The results were compared with the properties of the pure latex emulsion binder. The ceramic slurries exhibit the desired strong pseudoplastic, but non-thixotropic behavior. Due to the high solids content the slurries show distinctive elastic properties below a critical shear stress. Above the critical shear stress the internal structure is broken up and the viscous properties become predominant. The paper demonstrates that the measurement of the viscosity, its time dependent behavior and the strength of the internal structure of the slurry are excellent tools to characterize the suitability of a ceramic slurry for the tape casting process.

New lamination technique to join ceramic green tapes for the manufacturing of multilayer devices

Lamination is a quality determining processing step in the manufacturing of ceramic multilayer devices. The common method to join stacked ceramic green tapes is thermo-compression. The binder phases of two adjacent green tapes are joined together at elevated temperatures and pressures. This causes mass flow, which prevents the production of complex 3D structures or fine patterns. The new lamination technique is based on a temporary gluing step. The green tapes are stuck together by an adhesive tape at room temperature under very low pressures. Even though the tapes are separated by the adhesive tape, the ceramic green tapes are joined homogeneously after sintering. In contrast to the thermo-compression method, the new cold low pressure lamination technique reduces deformations, and has a high potential for the lamination of undercut, complex 3D structures with small lines and spaces of the metallization.

Low pressure lamination of ceramic green tapes by gluing at room temperature

An advanced method to laminate ceramic green tapes is described. In contrast to the well-known thermo-compression method, in which a junction is produced at elevated temperatures and pressures, this method allows the laminates to be produced at room temperature under very low pressures. To aid the joining of the tapes, an adhesive system was utilised: a double-sided adhesive tape, consisting of an acrylate adhesive and a carrier film made of polyethyleneterephthalate (PET). The studies were carried out on two alumina-based green tapes having different grain sizes. The laminates were characterised in the sintered state using a scanning electron microscope. The polymeric reactions occurring during heat treatment were revealed by optical microscopy experiments, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and viscosity measurements. It was found that for a successful joining, a low viscosity polymer melt is necessary. At elevated temperatures it functions as a flux for the ceramic particles, allowing them to rearrange. Due to capillary actions the flux creates a drag on the ceramic layers, leading to interpenetration and a homogeneous body.

Electrical and thermal conductivity of liquid phase sintered SiC

Commercial silicon carbide ceramics are semi-conductors. To obtain SiC ceramics with insulating properties, a synthesised SiC powder which was doped with vanadium and boron to get insulating behaviour due to compensating mechanism, and in addition a commercial powder was used. Both powders were pressureless sintered by liquid phase sintering. Different packing powders were used to influence the electrical behaviour of the dense SiC ceramics. The influence of these conditions on the electrical and thermal conductivity are described. Densification of SiC by a solution-precipitation mechanism in the temperature range around 1900°C results in a decrease of resistivity. The resistivity of the commercial powder could be improved by using a vanadium based packing powder.

Tape casting of piezo ceramic/polymer composites

Abstract 1-3 Piezoelectric composites are used as ultrasonic transducers for naval sonar devices, medical diagnostic systems, and non-destructive materials testing. Their excellent coupling behaviour to ultrasonic signals in water is due to the high fraction of polymer as the matrix and a very fine structured piezoelectric ceramic, which allows an effective transfer of the hydrostatic waves from the polymer to the ceramic. The aim is to obtain a composite of maximum hydrostatic performance characterised by dh x gh (dh: hydrostatic charge coefficient; gh: hydrostatic voltage coefficient), which allows a detection of signals even with low intensity. Different processing routes are known to manufacture such 1-3 composites, but there is a need for a flexible, low cost process for medium quantities. Tape casting is a very adaptable method to produce such composites. Strips were cut into PZT green tapes line by line. Subsequently, the sheets were sintered and stacked, by using spacers between the sheets. The free spaces are filled with polymer, and finally the edges of the composite are cut off to remove the frame of the PZT-sheets. Different techniques were investigated to structure the green tapes without damaging them and to preserve the fine structure of the tapes during firing by avoiding warpage.

Dielectric data of ceramic substrates at high frequencies

The dielectric constants of standard ceramic substrates like alumina and LTCC have been determined in the frequency range from 1 to 50 GHz. For the measurements cylindrical disk resonators and printed microstrip line resonators on substrates have been prepared. The RF-data have been measured with network analyzers and suitable wafer probe stations, the calculations of the dielectric data have been done on basis of two different simulation programs. The preparation of the samples and the test conditions of both resonator methods have proven to yield reliable results over a wide frequency range.

Conceptional design of nano-particulate ITO inks for inkjet printing of electron devices

This manuscript presents the conceptional design of indium tin oxide inkjet inks for the manufacture of electron devices. For this purpose, the process window of the printer used is identified and the inks are conceived to meet the requirements. The nano-particles are effectively stabilized in different dispersion media. The rheological, the wetting and the drying behavior of the inks are adapted to the inkjet process and the substrates to be coated. To assemble a field effect transistor (FET), the most suitable ink is chosen and source and drain contacts are printed. In the device, a nano-particulate ZnO layer acts as semiconducting layer and the gate electrode as well as the dielectric layer is formed by a thermally oxidized silicon wafer. The electron device assembled shows the typical FET characteristic proving its functionality.

3-D Structures via Tape Casting and Lamination

Miniaturisation and multi-functionality are the driving forces in the field of many technical market segments. Ceramic multilayer technology via tape casting and lamination is one of the key technologies to generate complex, highly integrated devices for future applications. The technique offers a tremendous potential for the integration of different materials combined with the possibility to form complex 3-D structures by layered object manufacturing. The ceramic multilayer technology is based on shaping, screen-printing and stacking of individual ceramic green tapes, which are laminated and co-fired. The tapes can offer ferroelectric, piezoelectric and magnetic behaviour, insulating and semi-conducting properties, voltage-, temperature- and atmosphere-dependent resistances, high thermal conductivity, corrosion resistance, or bio inert behaviour. For the integration of additional features like reactors or sensors, the described layered manufacturing method allows to introduce simultaneously microchannels and other cavities. - New developments in processing and in materials concepts are addressed to overcome limitations and to take maximum advantage of this technique. The technique is illustrated considering e.g. microwave circulators for communication, sensors for automotive applications, and piezoelectric actuators as examples, which demonstrate the potential of the technology for the manufacture of highly integrated multifunctional systems.

Processing of preceramic paper and ceramic green tape derived multilayer structures

Abstract Multilayer laminates with gradients in material composition and porosity were fabricated by the combination of ZrO2, Al2O3–ZrO2 and Al2O3–MgAl2O4 preceramic papers with MgO–MgAl2O4, Al2O3 and MgAl2O4 ceramic green tapes. A ZrO2–loaded adhesive based on an aqueous dispersion of copolymerised polyvinyl acetate served as the interface adhesive. The shrinkage behaviour of the individual layers was adapted by combining coarse and fine grained ceramic powders in order to avoid crack formation and delamination during firing. Defect-free multilayer laminates were obtained after sintering at 1700°C for 5 h which offer a high potential for application in refractory functional components.

Hermetic glass sealing of AlN packages for high temperature applications

Abstract In this study the feasibility of glass soldering of aluminium nitride (AlN) for the package of silicon carbide semiconductors with application temperatures up to 600°C was evaluated. The components used were various commercially available solder glasses and a crystallising three-component glass. Chemical compatibility between the glass and AlN was investigated by means of both thermodynamic calculations and experimental work on solder joints. The PbO-free three-component glass was stable against AlN. Lead oxide containing glasses showed deleterious reaction with AlN, which can be inhibited by a passivating layer. Additionally the influence of the processing parameters atmosphere and mechanical pressure was investigated. Hermeticity of sealed packages could be proved for three glasses allowing high temperature application between 500 and 600°C.