October 2005 laminated silicon nitride stacks down to 1500.C. At this temperature, the furnace was switched pressureless method using lamination aids. Use of a slurry, how- ever, with the same components as that of the original tape casting slurry, did not show success d lamination of (3) Characterization of the Multi-Layer Stacks the silicon nitride green tapes. After applying the slurry onto the The density of the sintered stacks was measured by the Ar- green tape, the layer dries rapidly because the porous green tape paks up the water immediately. The dry binder emulsion does chimedes principle in water with a wetting agent to avoid air not have the ability to join a second tape anymore. Thus, scanning electron microscopy(sEM, CamScan, Cambridge lamination with this system is not possible. This problem can UK). The 4-point bending strength was measured according be overcome by using terpineol as a viscous organic solvent to DIN/EN 843, part I with an sample geometry of 2.5 mm x 2 or the powder suspension. The processing is further improved mm×30mm. These les were prepared parallel and pe when pastes containing the liquid precursor are used because dicular to the direction of the laminated layers. In the fo the precursor increases the adhesion between the laminated owing text, parallel" and"perpendicular" are used when the resulting tensile stress is parallel and perpendicular, respective An efficient way to apply a thin layer of a slurry onto a ubstrate is screen printing. For this process, the pastes must to the stacking direction during strength measurement(compare exhibit a certain rheological behavior: the viscosity should be within certain limits and the pastes should be shear thinning Thermal diffusivity was measured in the Netzsch application To optimize the rheological behavior, different pastes ranging lab using the laser-Flash method (LFA 427, Netzsch). Disks from precursor contents of 0% to 100% with varying solvent having a diameter of 12 mm and a thickness of approximately 2 contents have been investigated (Fig 3). Because the pastes con- mm were used. These samples were cut out of sintered stacks arallel and perpendicular to the laminated layers. The specific sist of several components, a complex rheological behavior can heat was evaluated by differential sc be expected. The composition of Cer100(i. e. pure liquid pre- 404. Netzsch). The thermal conductivity i was calculated from cursor) was analyzed to understand the effects of the single the diffusivity a, density p, and specific heat cp using the fol components. The liquid polymer shows Newtonian behavior lowing equation: i=apcp. X-ray diffraction patterns of the viscosity does not change with the applied shear rate(at low planes parallel to and perpendicular to the casting plane shear rates, the limit of the equipment was reached, resulting in determined by a Philips diffractometer(PW 1050, Almelo, great scattering of the measured values). The viscosity is only 0. 15 Pa s at 25C and therefore the pure polymer cannot be used for screen printing. On adding powder, the viscosity in- (Cer90, i.e. 10 vol% er). The composition with 60 II. Results and discussion vol% of precursor( Cer60)still does not contain an additional solvent. At this higher solid content, the rheological behavior () Lamination and Thermal Treatment changes to slightly shear thinning and additional shear thicken- Unstructured green tapes can be joined at room temperature at ing at higher shear rates. Both effects become stronger with in- a pressure of 14 MPa without any additional aid. By the applied creased solids content( Cer40 and Cer 20). If more solvent pressure, the tapes are pressed into each other and a homoge- added, it only acts as a diluent and reduces the effects. It can also neous material is obtained(Fig. 2). Because of its low glass be observed that in the shear-thickening range, a hysteresis de- transformation temperature of-6C, the binder in the tapes can velops: the measured viscosities during increasing shear rates are be easily deformed plastically. Because of the tape-casting higher than that of the ramp downward. The slurry without the process, the "bottom" surface, which had contact with the pre-ceramic polymer(CerO), i.e. only the powder dispersed in the solvent, shows strong shear thinning and no further shear ). Lamination is not possible when two rough top surfaces thickening, but a slight hysteresis. This thixotropic behavior meet during joining. Because of the compression during lami- typical for powder dispersions nation. a shrinkage in the thickness direction of about 8% takes Shear thinning can be explained by breaking up an internal lace,which slightly increases the green density of the whole structure that builds up when the suspension is in rest. The stack. The applied pressure also influences the shrinkage during strength of the internal structure is dependent on the attraction tering in the x-y and the z direction. and repulsion forces between the particles. The more this struc- Complex-shaped geometries, where no pressure can be ap- ture is destroyed, i.e. the higher the applied shear rate, the lower plied because of expected deformation, can be produced by a he viscosity. When the shear rate decreases again, the structure begins to build up again. If this buildup need time, a hysteresis will be observed. This is the case for th der suspension without a precursor(Fig 3). Slurries containing the pre-ceramic polymer also show shear thinning at low shear rates. But because of the long molecules, hich are present between the powder particles, the strength of he internal structure is lower, leading to a lower viscosity in the condition of rest and to a less steep decrease in viscosity wit increasing shear rate. At slightly higher shear rates. however, the viscosity increases again. Such shear thickening can be explained by a buildup of a new internal structure under the influence of e applied shearing. The resulting viscosity is determined by the competing processes of structure destruction and buildup The viscosity measurements comprise a second part to ob- serve the response of viscosity to a simulated screen-printing process. During screen printing, the paste is placed onto the screen. Then, the paste is abruptly sheared by the blade. The neasurements show that the pastes in this work respond with some delay to a sudden change in shear rate(Fig 4). The sus- pension without a precursor, which exhibits a strong shear-thin- Fig. 2. Fracture surface of green tapes, laminated by compression at ning behavior (Fig. 3), needs almost 10 s to gain a constan room temperature(13 layers)(scanning electron microscopy, SEM); bar viscosity. This confirms the time-dependent behavior( thixotro- on the right top corner represents the thickness of a single layer. py) of this slurry. Because of the strong shear thinning, the vis-down to 15001C. At this temperature, the furnace was switched off. (3) Characterization of the Multi-Layer Stacks The density of the sintered stacks was measured by the Ar￾chimedes principle in water with a wetting agent to avoid air bubbles in the samples. The microstructure was investigated by scanning electron microscopy (SEM, CamScan, Cambridge, UK). The 4-point bending strength was measured according to DIN/EN 843, part 1 with an sample geometry of 2.5 mm 2 mm 30 mm. These samples were prepared parallel and per￾pendicular to the direction of the laminated layers. In the fol￾lowing text, ‘‘parallel’’ and ‘‘perpendicular’’ are used when the resulting tensile stress is parallel and perpendicular, respectively, to the stacking direction during strength measurement (compare Fig. 11). Thermal diffusivity was measured in the Netzsch application lab using the Laser–Flash method (LFA 427, Netzsch). Disks having a diameter of 12 mm and a thickness of approximately 2 mm were used. These samples were cut out of sintered stacks parallel and perpendicular to the laminated layers. The specific heat was evaluated by differential scanning calorimetry (DSC 404, Netzsch). The thermal conductivity l was calculated from the diffusivity a, density r, and specific heat cp using the fol￾lowing equation: l 5 arcp. X-ray diffraction patterns of the planes parallel to and perpendicular to the casting plane were determined by a Philips diffractometer (PW 1050, Almelo, The Netherlands). III. Results and Discussion (1) Lamination and Thermal Treatment Unstructured green tapes can be joined at room temperature at a pressure of 14 MPa without any additional aid. By the applied pressure, the tapes are pressed into each other and a homoge￾neous material is obtained (Fig. 2). Because of its low glass transformation temperature of
61C, the binder in the tapes can be easily deformed plastically.10 Because of the tape-casting process, the ‘‘bottom’’ surface, which had contact with the tape carrier, is much smoother than the ‘‘top’’ surface (see Sec￾tion II). Lamination is not possible when two rough top surfaces meet during joining. Because of the compression during lami￾nation, a shrinkage in the thickness direction of about 8% takes place, which slightly increases the green density of the whole stack. The applied pressure also influences the shrinkage during sintering in the X–Y and the Z direction.13 Complex-shaped geometries, where no pressure can be ap￾plied because of expected deformation, can be produced by a pressureless method using lamination aids. Use of a slurry, how￾ever, with the same components as that of the original tape￾casting slurry, did not show any success during lamination of the silicon nitride green tapes. After applying the slurry onto the green tape, the layer dries rapidly because the porous green tape soaks up the water immediately. The dry binder emulsion does not have the ability to join a second tape anymore. Thus, lamination with this system is not possible. This problem can be overcome by using terpineol as a viscous organic solvent for the powder suspension. The processing is further improved when pastes containing the liquid precursor are used because the precursor increases the adhesion between the laminated tapes. An efficient way to apply a thin layer of a slurry onto a substrate is screen printing. For this process, the pastes must exhibit a certain rheological behavior: the viscosity should be within certain limits and the pastes should be shear thinning.14 To optimize the rheological behavior, different pastes ranging from precursor contents of 0% to 100% with varying solvent contents have been investigated (Fig. 3). Because the pastes con￾sist of several components, a complex rheological behavior can be expected. The composition of Cer100 (i.e. pure liquid pre￾cursor) was analyzed to understand the effects of the single components. The liquid polymer shows Newtonian behavior: the viscosity does not change with the applied shear rate (at low shear rates, the limit of the equipment was reached, resulting in great scattering of the measured values). The viscosity is only 0.15 Pa s at 251C15 and therefore the pure polymer cannot be used for screen printing. On adding powder, the viscosity in￾creases (Cer90, i.e. 10 vol% powder). The composition with 60 vol% of precursor (Cer60) still does not contain an additional solvent. At this higher solid content, the rheological behavior changes to slightly shear thinning and additional shear thicken￾ing at higher shear rates. Both effects become stronger with in￾creased solids content (Cer40 and Cer20). If more solvent is added, it only acts as a diluent and reduces the effects. It can also be observed that in the shear-thickening range, a hysteresis de￾velops: the measured viscosities during increasing shear rates are higher than that of the ramp downward. The slurry without the pre-ceramic polymer (Cer0), i.e. only the powder dispersed in the solvent, shows strong shear thinning and no further shear thickening, but a slight hysteresis. This thixotropic behavior is typical for powder dispersions.14 Shear thinning can be explained by breaking up an internal structure that builds up when the suspension is in rest. The strength of the internal structure is dependent on the attraction and repulsion forces between the particles. The more this struc￾ture is destroyed, i.e. the higher the applied shear rate, the lower the viscosity. When the shear rate decreases again, the internal structure begins to build up again. If this buildup needs some time, a hysteresis will be observed. This is the case for the pow￾der suspension without a precursor (Fig. 3). Slurries containing the pre-ceramic polymer also show shear thinning at low shear rates. But because of the long molecules, which are present between the powder particles, the strength of the internal structure is lower, leading to a lower viscosity in the condition of rest and to a less steep decrease in viscosity with increasing shear rate. At slightly higher shear rates, however, the viscosity increases again. Such shear thickening can be explained by a buildup of a new internal structure under the influence of the applied shearing.14 The resulting viscosity is determined by the competing processes of structure destruction and buildup. The viscosity measurements comprise a second part to ob￾serve the response of viscosity to a simulated screen-printing process. During screen printing, the paste is placed onto the screen. Then, the paste is abruptly sheared by the blade. The measurements show that the pastes in this work respond with some delay to a sudden change in shear rate (Fig. 4). The sus￾pension without a precursor, which exhibits a strong shear-thin￾ning behavior (Fig. 3), needs almost 10 s to gain a constant viscosity. This confirms the time-dependent behavior (thixotro￾py) of this slurry. Because of the strong shear thinning, the vis￾Fig. 2. Fracture surface of green tapes, laminated by compression at room temperature (13 layers) (scanning electron microscopy, SEM); bar on the right top corner represents the thickness of a single layer. October 2005 Laminated Silicon Nitride Stacks 2715