Design of alumina bodies with directional porosity by a freeze-casting method C. Tallón, R. Moreno and M. I. Nieto

Transcription

1 Design of alumina bodies with directional porosity by a freeze-casting method C. Tallón, and M. I. Nieto Institute of Ceramics and Glass, CSIC, Madrid, Spain

2 INTRODUCTION MANUFACTURE OF POROUS MATERIALS FIRST POSSIBILITY SPOUNGE IMPREGNATION P SUSPENSION STARCH CONSOLIDATION HEATING RIGID NETWORK OTHER POSSIBILITY FREEZE CASTING SOAP BUBBLES SURFACTANT

3 Synthesis of nanopowders Synthesis of mixed oxides Synthesis of POROUS materials FREEZE-DRYING Solution Freezing Nozzle Freeze-Drying Vacuum sublimation at low T Thermal treatment N 2 (g) Solution Refrigerant Porous, soft granules Suspension Freeze-Casting Shaping method Porous bodies

4 FREEZE-CASTING Suspensions Organics Camphene Warm milling Sublimation at T room Aqueous Freezing Sublimation under vacuum (T < 0º C) Ice formation and growth Crystal size = f (T freezing, additive) CRYOPROTECTOR

5 Decrease in T freezing 10 wt% -1.6ºC 20 wt% -4.8ºC 30 wt% -9.5ºC FREEZE-CASTING CRYOPROTECTOR PVA Glycerol Bonding to water molecules Interaction with dispersant forming micelles Ice crystallization Open microstructure Higher homogeneity Cracking avoiding Increased strength

6 OBJECTIVE PREPARATION OF ALUMINA BODIES WITH ALIGNED POROSITY BY FREEZE-CASTING Influence of the solid content of the starting suspension Addition of glycerol as cryoprotector Pore size and distribution Microstructure Influence of freezing conditions

7 EXPERIMENTAL Suspension Al 2 O 3 Condea HPA05 (USA), d 50 = 0.35 µm, S S =9.5 m 2 /g Dispersant: poly(acrylic acid, PAA (Duramax D3005, Rohm/Haas, USA) Ball milling 6h Cryoprotector, Glycerol Procedure Suspension Freezing Drying Frozen Sample 20ºC -50ºC 0.05mBar Sintering 2ºC/min: 250ºC/30min 5ºC/min: 1500ºC/2h 10ºC/min: 1200ºC 5ºC/min: T room Characterization Flow behaviour (CR): Haake RS50 (sensor DC60/2) FE-SEM Mercury Intrusion Porosimetry Green and sintered densities

8 EXPERIMENTAL Directional Freezing Suspension Plastic mold Frozen sample Ice growth Metallic base Freezing Refrigerant Demoulding Short secondary freezing Bulk Freezing Suspension Plastic mold Frozen sample Freezing Refrigerant Demoulding

9 EXPERIMENTAL EXPERIMENTAL CONDITIONS Solids content Cryoprotector (Glycerol) 20, 35, 50 vol% 50, 68, 80 wt% 0, 10, 20 wt% (respect to water) Freezing device bulk directional Freezing rate N 2 (l) (-198ºC) instantaneous freezer (-20ºC) slow

10 RHEOLOGY OF FREEZING SUSPENSIONS EFFECT OF GLYCEROL AND SOLIDS LOADING vol% 50vol%/20wtlycerol 20vol% 20vol%/20wtlycerol vol% 35 vol% 50 vol% Stress (Pa) 40 Stress (Pa) Shear rate (s -1 ) Shear rate (s -1 )

11 DIRECTIONAL FREEZE CASTING EFFECT OF SOLIDS CONTENT 20 vol% 35 vol% 50 vol% 50 µm 50 µm 50 µm 10 µm 10 µm 10 µm

12 DIRECTIONAL FREEZE CASTING EFFECT OF SOLIDS CONTENT 0,25 1,2 Cumulative Volume (cm 3 /g) 0,20 0,15 0,10 0,05 20 vol% 35vol% 50 vol% dv/dlog(d) (cm 3 /g) 1,0 0,8 0,6 0,4 0,2 20 vol% 35 vol% 50 vol% 0,00 1E-3 0,01 0, Pore size (µm) 0,0 1E-3 0,01 0, Pore size (µm) Properties of sintered bodies Suspension (vol %) Relative density (% TD) Porosity (%) Total Open 20 vol% vol% vol%

13 DIRECTIONAL FREEZE CASTING EFFECT OF CRYOPROTECTOR ADDITION 35 vol% 10 wt% 20 wt% 50 µm 50 µm BETTER FORMATION OF CHANNELS 10 µm 10 µm

14 DIRECTIONAL FREEZE CASTING EFFECT OF CRYOPROTECTOR ADDITION 35 vol% 0,25 35 vol% 1,2 Cumulative Volume (cm 3 /g) 0,20 0,15 0,10 0,05 10 wt% glycerol dv/dlog(d) (cm 3 /g) 1,0 0,8 0,6 0,4 0,2 35vol% 10 wt% glycerol 0,00 1E-3 0,01 0, Pore size (µm) 0,0 1E-3 0,01 0, Pore size (µm) Properties of sintered bodies Glycerol (wt %) Relative density (% TD) Green Sintered Porosity (%) Total Open

15 FREEZE CASTING EFFECT OF FREEZING DEVICE Directional 35vol% Bulk 50 µm 50 µm NO PREFERRED ORIENTATION LOWER PORE SIZE 10 µm 10 µm

16 FREEZE CASTING EFFECT OF FREEZING DEVICE 0,20 35vol% 1,2 Cumulative Volume (cm 3 /g) 0,15 0,10 0,05 Directional Bulk dv/dlog(d) (cm 3 /g) 1,0 0,8 0,6 0,4 0,2 Directional Bulk 0,00 1E-3 0,01 0, Pore size (µm) 0,0 1E-3 0,01 0, Pore size (µm) Properties of sintered bodies Freezing Relative density (% TD) Green Sintered Porosity (%) Total Open Directional Bulk

17 BULK FREEZE CASTING EFFECT OF FREEZING RATE 35vol% Liquid nitrogen (-198ºC) Freezer (-20ºC) 50 µm 50 µm SLOWER RATE LEADS TO LARGER ICE CRYSTALS 10 µm 10 µm

18 BULK FREEZE CASTING EFFECT OF FREEZING RATE Cumulative Volume (cm 3 /g) 0,25 0,20 0,15 0,10 0,05 35 vol% Liquid nitrogen freezing chamber 35vol% dv/dlog(d) (cm 3 /g) 1,2 1,0 0,8 0,6 0,4 0,2 35vol% 20wt%glycerol liquid nitrogen freezing chamber 0,00 1E-3 0,01 0, Pore size (µm) 0,0 1E-3 0,01 0, Pore size (µm) Properties of sintered bodies Freezing Relative density (% TD) Green Sintered Porosity (%) Total Open N 2 -liq Freezer

19 CONCLUSIONS Freeze-casting has been successfully used to shape porous bodies of alumina with aligned porosity in the direction of the ice growth The total porosity decreases as the solids loading increases Higher content of glycerol is necessary to achieve open, aligned pore channels Freezing device does not change the total porosity, but narrower pore size distribution is obtained for bulk freezing The higher is the freezing rate the lower is the pore size, although the total porosity is nearly the same Particles forming the inner walls of the pores or channels are well sintered 5 µm

20 THANK YOU VERY MUCH