Direct Evidence for Low-Density Regions inCompacted Spray-Dried Powders

Yutaka Saito,* Satoshi Tanaka,* Nozomu Uchida, and Keizo Uematsu*Department of Chemistry, Nagaoka University of Technology, 1603-1 Kamitomioka, Nagaoka, Niigata, Japan

Pore structure in a ceramic green body was directly observedwith a novel characterization tool to examine the detrimentaleffect of a binder on the microstructure of ceramics for powdercompaction processing. The binder segregated on the surfaceof granules and formed a low-density region at the boundariesof granules in the compact after binder removal. A network ofpores with round shapes was produced in the sintering processfrom the low-density region. This paper also explains a newcharacterization technique, confocal laser scanning fluorescentmicroscopy.

I. Introduction

ABINDER has a detrimental effect on the quality of ceramics. Ittends to form defects in the microstructure and reduces the

quality of ceramics. However, the mechanism of the detrimentaleffect is not well understood. In our recent paper on the processingof alumina ceramics, we showed that the binder was segregated atthe surface region of granules and formed a nonuniform greenstructure in the compact.1 The binder hindered the rearrangementof powder particles and thus the densification during the compac-tion process. The boundary region of granules should have a lowdensity, and may develop defects in the sintering process.2 Theformation of this low-density region should be examined directlyto understand the detrimental role of a binder in the processing ofceramics.

A new characterization tool was applied to visualize thedistribution of the packing density of powder particles in acompact. The tool is based on confocal laser scanning fluorescencemicroscopy (CLSFM). With this tool, the ceramic green body isfirst made transparent/translucent with an immersion liquid con-taining a fluorescent dye. Then the distribution of the immersionliquid and thus the fluorescent dye in the compact is visualizedwith a confocal laser scanning florescence microscope, providinga detailed negative image of the packing structure of powderparticles. A region of low density appears bright in the micrograph,since the quantity of the immersion liquid per volume is large andthus the light emitted from the region is high in this region. Aregion rich in binder appears dark, since the void space is low andthus only little liquid (and dye) is accommodated there. It shouldbe emphasized that the structure can be observed down to �50 �mfrom the surface. Very high spatial resolution of CLSFM mayprovide a detailed image of green microstructure three-dimensionally.

This paper presents explicitly the distribution of a binder in thegreen compact and nonuniform packing density of powder parti-cles near the boundary region of granules after binder removal.

II. Experimental Procedure

The starting materials used in the present experiments wereindustrial-grade commercial Al2O3 (Al160SG-1, Showa DenkoCo., Ltd., Tokyo, Japan) and poly(vinyl alcohol) (PVA205, KurareCorp., Tokyo, Japan) as a binder. According to the supplier, theaverage particle size of the Al2O3 powder was 0.6 �m. The degreeof polymerization and the degree of hydrolysis of the binder were500 and 88%, respectively. The starting materials were mixed withdistilled water (solids:water � 1:1) in a ball-mill for 16 h toprepare slurries with binder contents of 2 mass%. No dispersantwas used in this slurry. A spray dryer (Model No. SD-13, MitsuiKozan, Tokyo, Japan) was used to prepare granules at inlet andoutlet air temperatures of 200° and 100°C, respectively. Granuleswere pressed with hardened steel dies at 20 MPa. Then, compactwas stored at 22°C and 85% relative humidity for 30 h and CIPedat 100 MPa. They were heated to 1000°C at a heating rate of5°C/min and kept for 1 h in air to remove binder. The specimenwas cut parallel to the pressing direction and thinned to 0.2 mmwith a grinding paper. An immersion liquid with fluorescent dyewas prepared by dissolving a small amount of fluorescent reagent(Nile red, Molecular Probes, Inc., USA) in methylene iodide.Thinned specimens were immersed in the liquid and evacuated for5 min. A confocal laser scanning fluorescence microscope withan argon ion laser (wavelength 488 nm, FLUOVIEW, OlympusCo., Japan,) was used to observe the internal structure of thespecimens. The compact was sintered at 1600°C for 1 h. Thesintered specimen was thinned (�40 �m) and polished for bothsides. Internal structure was examined using a normal opticalmicroscope.

III. Results and Discussion

Figure 1 shows the internal structure of an alumina compactexamined by confocal laser scanning fluorescence microscopy.This image was taken �10 �m below the surface of the specimen.A dark network is clearly noted, representing the nonuniform localdistribution of fluorescent dye, and thus of binder. The darknetwork clearly corresponds to the region rich in binder, and thuspoor in fluorescent dye. Each region surrounded by the darknetwork corresponds to a granule. Clearly, all granules are de-formed significantly in the compact. The granules are deformed tofill the interstices of granules. Many small dark features are alsonoted. They are tentatively ascribed to large particles, agglomer-ates, or segregated binder. A large dark trace at the upper right inthis micrograph (shown by an arrow) likely corresponds to a lumpof binder dissolved in the slurry incompletely. This featuredisappeared after thermal treatment. Binder segregation at granulesurfaces has been shown in several studies.1,3,4

Figure 2 shows the internal structure of an alumina compactafter binder removal. The micrograph shows reduced density at the

J. W. Halloran—contributing editor

Manuscript No. 187768. Received June 7, 2001; approved June 29, 2001.This work was supported by a Grant-in-Aid for Scientific Research from the

Ministry of Education, Culture, Sport and Technology and also by a NEDO Grant.*Member, American Ceramic Society.

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boundaries of granules due to the presence of binder for the firsttime. A network similar to Fig. 1 but of opposite optical contrastis noted in this micrograph. The boundary region of granulesappears bright in the micrograph (shown by arrow A). Clearly, thepacking density of powder particles is low in the boundary regionof granules. The binder segregated at the surface of granuleshindered the densification of primary particles and formed alow-density region. Careful examination of the micrograph showsthat the brightness also varied locally other than with the abovenetwork. The variation of brightness directly corresponds todensity variation in the specimen. Bright and dark regions corre-spond to regions of relatively low density, which accommodates alarge quantity of fluorescent immersion liquid and of high density,respectively. Small dark features correspond to large particles oragglomerates, which contain no fluorescent dye or only a smallamount (shown by arrow B). Note that particularly bright spots arefound in some of the triple junctions of granules (shown by arrowC). These regions should have especially low density. A smallspace was left in these regions by incomplete deformation ofgranules in the compaction period. Spotted small bright features inother regions are tentatively ascribed to large pores originatingfrom air bubbles or/and undissolved binder (shown by arrows D).

Fig. 1. Internal structure of an as-pressed compact using a confocal laser scanning fluorescence microscope: (a) low magnification, (b) high magnification.

Fig. 2. Internal structure of pores after binder removal using a confocal laser scanning fluorescence microscope: (a) low magnification, (b) highmagnification.

Fig. 3. Internal structure of sintered alumina (1600°C, 1 h).

October 2001 Communications of the American Ceramic Society 2455

Figure 3 shows the structure of a sintered specimen observed ona thinned specimen with a normal optical microscope in thetransmission mode. Many round features are noted in the micro-graph (shown by arrows). They are traces of granules. Theboundaries of granules clearly have lower density than otherregions. A dark region has low density since the scattering of lightincreases with increasing porosity, or with decreasing density.

The present results are consistent with general beliefs aboutceramics; i.e., the structure of ceramics is governed by the greenstructure and no healing of defects is expected in the densificationprocess.5–9 They also show the detrimental effect of a binder onthe structure of ceramics.

IV. Conclusion

A new characterization method based on confocal laser scanningfluorescent microscopy was applied to examine the mechanism of anydetrimental effect of a binder on the compaction processing ofceramics. Segregated binder on the surface of granules formed regionsof low density at the boundaries of granules in the compact. Insintering, the nonuniformity of the packing structure was exaggeratedin the densification process, and formed regions of low density atcorresponding areas in the ceramics.

References

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2N. Shinohara, M. Okumiya, T. Hotta, K. Nakahira, M. Naito, and K. Uematsu,“Morphological Changes in Process-Related Large Pores of Granular Compacted andSintered Alumina,” J. Am. Ceram. Soc., 83 [7] 1633–40 (2000).

3Y. Zhang, N. Uchida, and K. Uematsu, “Direct Observation of NonuniformDistribution of PVA Binder in Alumina Green Body,” J. Mater. Sci., 30, 1357–60(1995).

4Y. Zhang, X.-X. Tang, N. Uchida, and K. Uematsu, “Binder SurfaceSegregation during Spray Drying of Ceramic Slurry,” J. Mater. Res., 13 [7]1881– 87 (1998).

5W. D. Kingery and B. Francois, “The Sintering of Crystalline Oxides: I,Interactions between Grain Boundaries and Pores”; pp. 471–98 in Sintering andRelated Phenomena. Edited by G. C. Kuczynski, N. A. Hooton, and G. N. Gibbon.Gordon and Breach, New York, 1967.

6B. J. Kellett and F. F. Lange, “Thermodyamics of Densification: I, Sintering ofSimple Particle Arrays, Equilibrium Configurations, Pore Stability, and Shrinkage,”J. Am. Ceram. Soc., 72 [5] 725–34 (1989).

7E. B. Slamovich and F. F. Lange, “Densification of Large Pores: I, Experiments,”J. Am. Ceram. Soc., 75 [9] 2498–508 (1992).

8E. B. Slamovich and F. F. Lange, “Densification of Large Pores: II, DrivingPotential and Kinetics,” J. Am. Ceram. Soc., 76 [6] 1584–90 (1993).

9K. Uematsu, M. Miyashita, J.-Y. Kim, and N. Uchida, “Direct Study of theBehavior of Flaw-Forming Defect in Sintering,” J. Am. Ceram. Soc., 75 [4] 1016–18(1992). �

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