T. Chartier T. Rou./ 3 Results and discussion VISCOSITY (mPa. s) 3.1 Tape casting of layers 1500 30vo.%Ak23 3.1. I Selection of the parameters of phosphate esters The effectiveness of the different phosphate esters HLB 1< HLB 2< HLB 3 HLB 4 was evaluated using the viscosity of the alumina HLB 2 dispersant/solvent systems. The phosphate esters used were prepared by reaction between phospho- B3HB4 cid and an ethoxylate. The ethoxy 500 obtained by condensation of ethylene oxide in alcohol. The esters contain a combination of mono- and diesters and the remains of the ethoxylate not combined with the phosphoric acid. The chemical ESTER/AL2O3(wt %) structure of a typical phosphate ester is shown in Fig. 4. Apparent viscosity of a 30 volo alumina susper Fig. 3. The infuence of four parameters was stud versus dispersant concentration for phosphate esters different hlb value ied, namely (i)the molecular structure(aliphatic or aromatic), 5( i) the monoester/diester ratio, (ii) the degree of phosphatization and (iv) the the unreacted ethoxylate disturbs ceramic/disper Hydrophile/Lipophile Balance(HLB). The addition sant interactions and increases the viscosity, and of phosphate ester to alumina results in lowering (iv) a high hLB because adsorption on charged the pH, indicating that the dispersant partially dis- ceramic particles is favoured by more hydrophilic phosphate ester, the alumina dispersants(Fig 4) powder exhibited a slightly negative surface The most efficient phosphate ester, fulfilling all positive after addition of phosphate ester suggest- cialized( Beycostat C213, CECA, France s mmer- MEK/EtOH. The surfacc reversed to these characteristics, was synthesized and ce g that the H*ions liberated on dissociation were adsorbed onto alumina particles. Phosphate esters 3. 1.2 Selection of the slurry formulation act by a combination of electrostatic and steric For Al2O3 +10 vol% ZrO2 green sheets, cracking repulsion. The steric hindrance prevents contact develops for a volume solids ratio(X) higher than between particles. The double layer, which may be 0. 75 and for a binder/plasticizer ratio (r) higher due to net charge on the particle surface and/ than 0-7(Table 1). Cracking depends on shrinkage or charges coming from the dissociation of the rate, which depends itself on the composition of adsorbed polymer, provides repulsion by a poten- the slurry. The maximum shrinkage rate decreases tial energy barrier at larger distances when the volume solids ratio decreases. The high The best dispersion (i.e. the lowest viscosity) is content of organic phase slows down the motion achieved with a phosphate ester having (i)an of particles, then reduces the shrinkage rate aliphatic molecule, (ii)a high diester concentra- Binders are polymeric molecules which adsorb tion, as diesters contain two lipophilic tails, each on the particle surfaces and form organic bridges able to extend into the solvent for steric sta biliza I'able 1. Cracking during drying (C: cracking, N C:non tion. (ii)a high degree of phosphatization because cracking) and green density, excluding the organic phase of alumina+10 vol zirconia tapes (theoretical density = 4. 19 g MONOESTER Composition Cracking Green densit OH HH HH Y HO-P-0-C-C-0)(C)-C-H HH HYDROPHILE LIPOPHILE < CRACKING LIMIT DIESTER OH 2.64 RO-P-OR RACKING LIMIT 9 4 Fig. 3. Typical chemical structure of a phosphate ester.302 T. Churtier. T. Rouse1 3 Results and Discussion 3.1 Tape casting of layers 3. I. 1 Selection of’ the parameters oj’phosphate esters The effectiveness of the different phosphate esters was evaluated using the viscosity of the alumina/ dispersant/solvent systems. The phosphate esters used were prepared by reaction between phospho￾ric acid and an ethoxylate. The ethoxylate is obtained by condensation of ethylene oxide in alcohol, The esters contain a combination of mono￾and diesters and the remains of the ethoxylate not combined with the phosphoric acid. The chemical structure of a typical phosphate ester is shown in Fig. 3. The influence of four parameters was stud￾ied, namely (i) the molecular structure (aliphatic or aromatic),15 (ii) the monoester/diester ratio, (iii) the degree of phosphatization, and (iv) the Hydrophile/Lipophile Balance (HLB). The addition of phosphate ester to alumina results in lowering the pH, indicating that the dispersant partially dis￾sociates. Without phosphate ester, the alumina powder exhibited a slightly negative surface charge in MEWEtOH. The surface reversed to positive after addition of phosphate ester suggest￾ing that the H’ ions liberated on dissociation were adsorbed onto alumina particles. Phosphate esters act by a combination of electrostatic and steric repulsion. The steric hindrance prevents contact between particles. The double layer, which may be due to net charge on the particle surface and/ or charges coming from the dissociation of the adsorbed polymer, provides repulsion by a poten￾tial energy barrier at larger distances. The best dispersion (i.e. the lowest viscosity) is achieved with a phosphate ester having (i) an aliphatic molecule, (ii) a high diester concentra￾tion, as diesters contain two lipophilic tails, each able to extend into the solvent for steric stabiliza￾tion, (iii) a high degree of phosphatization because MONOESTER R I 0,H 7 lyi 7 c;’ ’ HO-P-0-(C-C-0)-U-C-H c; AA “A’A i._ _I I J HYDROPHILE LIPOPHILE DIESTER OIH RO-P-OR t; Fig. 3. Typical chemical structure of a phosphate ester. VISCOSITY 1mPa.s) 2.000 ,,500_ -2 3Ovol.% A’2O3 1 HLE 1 < HLB 2 < “LB 3 < HLB 4 0 0.2 O-4 0.6 0.8 ESTER/Al,O,(wt.%) Fig. 4. Apparent viscosity of a 30 vol’%, alumina suspension versus dispersant concentration for phosphate esters with different HLB values. the unreacted ethoxylate disturbs ceramicidisper￾sant interactions and increases the viscosity, and (iv) a high HLB because adsorption on charged ceramic particles is favoured by more hydrophilic dispersants (Fig. 4). The most efficient phosphate ester, fulfilling all these characteristics, was synthesized and commer￾cialized (Beycostat C213, CECA, France).” 3.1.2 Selection of the slurry ,formulation For Al,O,+ 10 vol’/o ZrO, green sheets, cracking develops for a volume solids ratio (X) higher than 0.75 and for a binder/plasticizer ratio (Y) higher than 0.7 (Table 1). Cracking depends on shrinkage rate, which depends itself on the composition of the slurry. The maximum shrinkage rate decreases when the volume solids ratio decreases. The high content of organic phase slows down the motion of particles, then reduces the shrinkage rate. Binders are polymeric molecules which adsorb on the particle surfaces and form organic bridges Table 1. Cracking during drying (C.: cracking, N.C.: non￾cracking) and green density, excluding the organic phase, of alumina+ 10 vol”% zirconia tapes (theoretical density = 4.19 g cm ‘) for various values of X and Y X1 O-6 0.7 N.C. 3.78 _- x3 0.7 0.7 N.C. 2.48 x4 0.75 0.7 N.C. 2.52 X5< CRACKING LIMIT > 0.8 0.7 c. 2.56 x7 0.9 0.7 c. 2.60 Yl o-7 0.3 N.C. 2.64 Y2 o-7 o-7 N.C. 2.48 Y3< CRACKING LIMIT > o-7 1.1 C. 2.45 Y4 0.7 I.5 C. 2.43 Y5 o-7 I .9 C. 2.40