Journal of the American Ceramic Society--Bitterlich and Heinrich 88,No.10 Table l. Si3N4 Powder Data(Silzot HQ, SKw, Germany) (um) As delivered (suppliers data) After milling Specific surface area(m/g) 4.5 9.2 阝 content(%) 12 Amount of nitrogen N(%) 38.6(40.2) O(%) 0.50(084) C(ppm) 0.085 0.119 Lasergranulometer (Typ LS 230, Coulter. Krefeld, Germany). Leco, TC-436. St Josephs, MI.Leco C400, St Josephs. MI with I mass% catalyst, Dicumyl peroxide( Aldrich Chemicals St. Louis, MO), a solvent (Terpineol, CioHiso, purum. Fluka Fig. 1. Surface of green tapes(left: top surface; right: bottom sur Chemie ag, Buchs, Switzerland), the powder mixture described face)(scanning electron microscopy, SEM). above, and I mass% of dispersant based on the powder content of polycarbonate(Trusan 450, Trukem Gmbl All processing steps and measurements were carried out in a Worms, Germany) in various ratios(see Table II glove box with controlled low oxygen and low water concen The pre-ceramic polymer was used as a binder for the powder tration to prevent any reactions of the pre-ceramic polymer. mixture. The solvent had to be added to optimize the viscosity of the compositions. In this work, terpineol was used because of its low evaporation rate and its chemical compatibility with the (2) Lamination and Thermal Treatment precursor. The dispersant was chosen after several screening Lamination was performed both by compression of green tapes tests to improve the stability of the pastes. These compor t 14 MPa for 20 s at room temperature and by using the pastes were homogenized using a triple-roller mill(Exakt 80S, Exakt consisting of the pre-ceramic polymer. The pastes were applied Apparatebau, Norderstedt, Germany), which is frequently used to the tapes by screen printing(mesh diameter=90 um, wire for high-viscosity mixtures thickness= 40 um). The laminated stack was heat treated in the During the screen-printing process, the shear rates in the paste change very fast. To analyze the behavior of the laminat Because of the observed shrinkage anisotropy of single tapes ing paste, this process was simulated in the viscosimeter(VT550, during sintering, 0 the alternating tapes were turned around by Haake, Karlsruhe, Germany) at 25"C using the following pro- 90 to their casting direction. Also, care was taken to ensure that cedure: after pre-shearing at 50 s for 30 s, a holding time of the bottom side of the green tapes always met the top side be 120 s followed. This pre-treatment is necessary to obtain repro- cause of their different surface properties( Fig. 1) Pyrolysis and binder burnout, respectively, were carried out during increasing shear rate in a logarithmic scale up to a max simultaneously under a flowing nitrogen atmosphere at a heat imum of 200s-l in 320s. Then the shear rate was decreased with ing rate of 5K/min and a soaking time of I h at a maximum the same ramp to observe any time-dependent behavior - Di- mperature of 750C. The pyrolyzed tapes were sintered rectly after this program, the shear rate was set to 30 s as fast in a gas pressure furnace(FPW, FCT, Rodental, Germany) as possible. After 30 s, the shear rate was decreased rapidly t 1800C for 60 min under a nitrogen pressure of 5 MPa. to only I s. The response of the viscosity was measured with The heating rate was 20 K/min up to 1100 C and 10 K/min up to 1800.C. The cooling rate was set to 10 K/min Composition of Pastes us heological examination an Amounts(vol%) Fraction of precursor to precursor plus powder Used for lamination In mass% Cer2041.8 14 11.0 45.8 Cer2041.0 Cer2040.6 Cer30424 424 37.2 Cer30419 419 14 38.0 Cer3041.3 41.3 1.3 38.8 Cer3040.2 1.3 40 Cer4041.9 41.9 1.3 16.8 Cer4040.8 40 1.3 28.4 9.4 Cer40-39.6 .3 274 31.7 Cer5042.1 42.1 14 43.8 12.7 23.3 Cer50-39.5 39.5 1.3 394 18.0 Cer50-394 394 1.3 39.5 18.2 38.5 1.3 60.2 31.3 0.3 3.2with 1 mass% catalyst,11 Dicumyl peroxide (Aldrich Chemicals, St. Louis, MO), a solvent (Terpineol, C10H18O, purum, Fluka Chemie AG, Buchs, Switzerland), the powder mixture described above, and 1 mass% of dispersant based on the powder content on the basis of polycarbonate (Trusan 450, Trukem GmbH, Worms, Germany) in various ratios (see Table II). The pre-ceramic polymer was used as a binder for the powder mixture. The solvent had to be added to optimize the viscosity of the compositions. In this work, terpineol was used because of its low evaporation rate and its chemical compatibility with the precursor. The dispersant was chosen after several screening tests to improve the stability of the pastes. These components were homogenized using a triple-roller mill (Exakt 80S, Exakt Apparatebau, Norderstedt, Germany), which is frequently used for high-viscosity mixtures. During the screen-printing process, the shear rates in the paste change very fast. To analyze the behavior of the laminat￾ing paste, this process was simulated in the viscosimeter (VT550, Haake, Karlsruhe, Germany) at 251C using the following pro￾cedure: after pre-shearing at 50 s
1 for 30 s, a holding time of 120 s followed. This pre-treatment is necessary to obtain repro￾ducible results. After this, the viscosity values were recorded during increasing shear rate in a logarithmic scale up to a max￾imum of 200 s
1 in 320 s. Then the shear rate was decreased with the same ramp to observe any time-dependent behavior.12 Di￾rectly after this program, the shear rate was set to 30 s
1 as fast as possible. After 30 s, the shear rate was decreased rapidly to only 1 s
1 . The response of the viscosity was measured with time. All processing steps and measurements were carried out in a glove box with controlled low oxygen and low water concen￾tration to prevent any reactions of the pre-ceramic polymer. (2) Lamination and Thermal Treatment Lamination was performed both by compression of green tapes at 14 MPa for 20 s at room temperature and by using the pastes consisting of the pre-ceramic polymer. The pastes were applied to the tapes by screen printing (mesh diameter 5 90 mm, wire thickness 5 40 mm). The laminated stack was heat treated in the glove box at 1501C for 1 h. Because of the observed shrinkage anisotropy of single tapes during sintering,10 the alternating tapes were turned around by 901 to their casting direction. Also, care was taken to ensure that the bottom side of the green tapes always met the top side be￾cause of their different surface properties (Fig. 1). Pyrolysis and binder burnout, respectively, were carried out simultaneously under a flowing nitrogen atmosphere at a heat￾ing rate of 5 K/min and a soaking time of 1 h at a maximum temperature of 7501C. The pyrolyzed tapes were sintered in a gas pressure furnace (FPW, FCT, Ro¨dental, Germany) at 18001C for 60 min under a nitrogen pressure of 5 MPa. The heating rate was 20 K/min up to 11001C and 10 K/min up to 18001C. The cooling rate was set to 10 K/min Table I. Si3N4 Powder Data (Silzots HQ, SKW, Germany) Grain size (mm) As delivered (suppliers data) After milling D10 0.7 0.1w D50 1.6 0.4w D90 4.1 1.9w Specific surface area (m2 /g) 4.5 9.2 b-content (%) 12 Amount of nitrogen N (%) 38.6 (40.2z ) 38.2z Amount of contaminations O (%) 0.50 (0.84z ) 1.50z C (ppm) 0.085y 0.11y w Lasergranulometer (Typ LS 230, Coulter, Krefeld, Germany). z Leco, TC-436, St. Josephs, MI. y Leco, C-400, St. Josephs, MI. Table II. Composition of Pastes used for Rheological Examination and for Lamination Descriptionw Used for lamination Amounts (vol%) Fraction of precursor to precursor plus powder Powder Dispersant Precursor Solvent In vol% In mass% Cer 0 32.5 1.1 — 66.5 — — Cer 20–41.8 41.8 1.4 11.0 45.8 20 7.0 Cer 20–41.0 O 41.0 1.4 10.8 46.8 Cer 20–40.6 40.6 1.3 10.7 47.3 Cer 30–42.4 42.4 1.4 19.0 37.2 30 11.5 Cer 30–41.9 O 41.9 1.4 18.7 38.0 Cer 30–41.3 41.3 1.3 18.6 38.8 Cer 30–40.2 40.2 1.3 18.1 40.3 Cer 40–41.9 41.9 1.3 29.2 27.6 40 16.8 Cer 40–40.8 O 40.8 1.3 28.4 29.4 Cer 40–39.6 39.6 1.3 27.4 31.7 Cer 50–42.1 42.1 1.4 43.8 12.7 50 23.3 Cer 50–39.5 O 39.5 1.3 39.4 18.0 Cer 50–39.4 39.4 1.3 39.5 18.2 Cer 60 O 38.5 1.3 60.2 — 60 31.3 Cer 90 9.6 0.3 90.1 — 90 73.2 Cer100 — — 100 — 100 100 w The description contains (a) the precursor content in vol% based on precursor and powder and (b) the powder amount to distinguish pastes with a different solvent content. Fig. 1. Surface of green tapes (left: top surface; right: bottom sur￾face) (scanning electron microscopy, SEM).10 2714 Journal of the American Ceramic Society—Bitterlich and Heinrich Vol. 88, No. 10