Znd. Eng. Chem. Res. 1996,34,2719-2727
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Unusual Sintering Behavior of Porous Chromatographic Zirconia Produced by Polymerization-Induced Colloid Aggregation C. Francisco LOrenzano-Porras,t David H. Reederl Michael J. Armen$# Peter W. Cam: and Alon V. McCormick*” Department of Chemical Engineering and Materials Science and Department of Chemistry and Znstitute for Advanced Studies in Bioprocess Technology, University of Minnesota, Minneapolis, Minnesota 55455
The effects of sintering temperature and duration on the pore structure of chromatographic zirconia particles produced by the controlled polymerization-induced aggregation of 1000 A colloids are studied with a n eye toward optimally strengthening the aggregates and eliminating small pores while preserving large pores. Nitrogen adsorption and mercury porosimetry are used to estimate the surface area, pore volume, and pore size distribution. Pulsed field gradient NMR measurements of solvent diffusion are used to estimate the diffusion tortuosity of the pore space. Initially of course, the pore volume and surface area decrease significantly, the decrease being more pronounced at higher temperatures. With prolonged sintering, the pore size, pore volume, and surface area change much more slowly, but the diffusion tortuosity seems to be minimized at a sintering temperature and time at which pores are allowed to redistribute so as to optimize large pores. The aggregates synthesized by this aggregation method apparently produce metastable large pores which are not easily collapsed.
Introduction The pore structure of ceramic supports for highperformance liquid chromatography (HPLC) significantly influences their chromatographic performance (Henry, 1991; Unger, 1990) and is largely influenced by the sintering process used t o strengthen the particles for packing and to eliminate small pores that may trap proteins (Lange, 1989;Wang et al., 1992). In a previous paper we discussed the effect of the aggregation mechanism on the particle arrangement and pore characteristics of ZrO2 colloidal aggregates produced for chromatography (Lorenzano-Porras et al., 1994). There we found that particles made by polymerization-induced colloid aggregation (PICA) had more pores of desirable size than similarly sintered particles made by more conventional oil emulsion aggregation. The most desirable pore structure of support particles for HPLC separation of proteins would have high porosity, high surface area, and a narrow pore size distribution with minimal contribution from pores less than -200 A in diameter (Henry, 1991;Unger, 1990). That study leads us to expect that the pores in the PICA materials might be unusually stable against collapse during sintering. In this paper we study the sintering behavior of porous zirconia samples formed by PICA (Annen et al., 1994; Sun et al., 1994). As we have shown previously, the PICA technique enables us to aggregate 1000 A spherical colloids of polycrystalline monoclinic ZrO2 into monodispersely sized spheres several microns in diameter that have porosities much higher than expected for randomly close packed colloids. Changes in the pore structure which take place upon sintering are subtle and not readily apparent by microscopy (see example in Figure 11, so here we use nitrogen sorption, mercury porosimetry, and pulsed field gradient NMR measure~~~~
~
* To whom correspondence
should be addressed. Department of Chemical Engineering and Materials Science. t Department of Chemistry and Institute for Advanced Studies in Bioprocess Technology. + Current address: Chemicals Group, Discovery Research, PPG Industries, Monroeville, PA 15146. +
ments of solvent diffisivity t o elucidate changes in properties of the pore structure with sintering. We are particularly interested in whether this material exhibits unusually stable pores. Theoretical and experimental studies have shown that the particle size distribution and pore coordination number (dictated by the particle packing arrangement) will influence the sintering behavior and associated physicochemical characteristics of a powder compact (Koplik, 1982;Lange, 1984,1989; Liniger and Raj, 1987; Montanaro and Negro, 1991; Reyes, 1985)and that thus we should expect that some aggregates might have resilient pores whereas others are more easily collapsed. Of course most of the literature focuses on the goal of achieving full density, but here we want to preserve large pores while eliminating small pores. In sintering, the contact regions between particles are filled by the transport of material driven by the differences in surface curvature near particle contacts (Lange, 19891, where material transport may occur by viscous flow, evaporatiodcondensation, or diffusion processes (including boundary motion) (Exner, 1979; Herring, 1950; Kinge et al., 1991). Since our colloid starts as dense 1000 polycrystalline (monoclinic)aggregates, we see no change in crystal structure upon sintering, though of course we expect grains to grow, so for our crystalline materials, viscous flow may not be a major contribution. Particularly for nonviscous mechanisms, it has been shown that neck growth and pore size redistribution will proceed quickly until a thermodynamically metastable pore configuration is reached, a t which point the pore coordination number is such that the work necessary for further growth of grain boundaries can no longer be balanced by the energy released by the further decrease of pore surface area. After this metastable state, further neck formation can occur only much more slowly as the pore coordination number changes with grain consolidation (Kellett and Lange, 1989a,b; Lange, 1989). Thus we expect two distinct mass transport stages: (1)initial sintering due to rapid transport to the contact region to form a metastable network and (2) secondary sintering controlled by slower grain coarsening (Kellett and Lange, 1989b). The excellent chromatographic
0 1995 American Chemical Society 0888-5885/95/2634-2719$09.00/0
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2720 Ind. Eng. Chem. Res., Vol. 34,No. 8,1995
Figure 1. Scanning electron micrographs of PICA ZrOa sintered for 12 h a t 750 "C (a and b, left) or 1050 "C (c and d, right). Samples were evaporatively coated with platinum, and images were acquired using a JEOL 840-11 SEM a t an accelerating voltage of 20 kV.
properties of even highly sintered PICA (McNeff et aZ., 1994) suggest that in this material the first stage may end quickly. On the other hand, given this possibility we will 51so be concerned as to whether small pores (
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Figure 10. Relative diffusion coefficient from SEPFG-NMR for PICA ZrO2 samples after sintering for 3-12 h a t 900 and 1050 "C.
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Sintering Time (h) Figure 9. Change with sintering time of pore volume within pores of different size ranges calculated by the method of Cranston and Inkley from nitrogen adsorption data for PICA ZrO2 sintered at temperatures of (a) 750, (b) 900, and (c) 1050 "C.
be associated with features of the pore space reported in the previous section. The high diffisivity of the samples sintered 6 h at 900 "C seems to be related to the shrinkage of small pores with growth of intermediate pores and the maintenance of large pores. The very low diffusivity for sintering 9 h at 1050 "C seems to be related t o the collapse of the intermediate pores.
Discussion Kellett and Lange (Kellett and Lange, 1989a,b;Lange and Kellett, 1986) have shown that the sintering of
aggregates can be divided into two main stages: (1) rapid neck growth in which the main contributions are by surface diffusion, evaporatiodcondensation, and volume diffusion, and (2) slow grain growth due t o coarsening from interparticle diffision and grain boundary diffusion. They have also shown that the metastable structure set up at the end of the first stage is " governed ' ~ in part by the pore coordination number. During the first stage, sintering may produce little or no net shrinkage. Instead, the most important effect at this stage can be the redistribution of pore sizes t o reach a thermodynamically metastable configuration. Pores with a lower coordination number will experience greater compressive stresses than will highly coordinated pores. Larger, highly coordinated pores may actually grow until a more stable configuration is reached (Lange, 1984, 1989; Lange and Kellett, 1986). At 750 "C, even prolonged sintering beyond 3 h of PICA ZrO2 causes no significant reduction in either volume or surface area. Prolonged sintering at 900 "C beyond 3 h causes no significant reduction in volume, but the surface area decreases as the pore size distribution is shifted t o increase the volume contribution of intermediate pores (-200 to -400 A). The pore structure changes to a metastable configuration without appreciably modifying the total pore volume. This redistribution seems to improve the pore fluid diffisivity at 9 h. Sintering at 1050 "C fails to show the advantages seen at 900 "C, since 1050 "C results in too great a reduction in volume and surface area even in the first 3 h. Intermediate pore sizes no longer seem stable; they shrink at higher rates than do the remaining small pores. However, even at 1050 "C prolonged sintering somewhat increases the volume of larger pores, possibly by pore coalescence. Lange's observations help t o rationalize these results. It is possible that the "metastable" pore structure that is reached a t 750 "C has too many small throats and that the structure reached at 1050 "C has too few intermediate pores (and too little pore volume of any size). At 900 "C, though, the "metastable" pore structure seems closer to optimal and is reached at an intermediate sintering time. That such favorable pore redistribution is possible for this material might be
2726 Ind. Eng. Chem. Res., Vol. 34, No. 8, 1995
thanks in part to a fortuitous initial colloid arrangement set up by this synthesis technique. The favorable rearrangement of pore volume toward large pores observed in PICA ZrO2 with prolonged sintering is not seen in ZrO2 materials made by all colloid aggregation methods. In materials produced from the same colloid by oil emulsion aggregation, we have seen that the fractional volume contribution of pores in each size range changes little with prolonged sintering and unacceptably small pores (
