., ,-. :.. .,-:.,: ; , .: . . zyxwvu ..,j....~i zyxwvutsrqponmlkjihgfe :rib.:.,. J. Am. Cerom. zyx Soc.. zyxw 73 161 1504-509 (1990) Alcohol Interaction with Zirconia Powders Mary Sue Kaliszewski* and Arthur H. Heuer* Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106 z The mechanism by which alcohol washing of ceramic pow- ders produces “soft” agglomerates has been investigated by studying the interaction of ethanol with hydrous Zr02 pow- ders using Fourier transform infrared (FTIR) spectroscopy. Unambiguous evidence of ethoxide formation has been found, which apparently prevents bond formation between adjacent particles and thus the formation of “hard” agglom- erates. [Key words: zirconia, agglomerates, alcohol, par- ticles, powders.] I. Introduction GGLOMERATION of ceramic powders has long been of con- A, ern. Although agglomerates can enhance flow character- istics of ultrafine powders,’ the usually result in poor packing and incomplete densificat ion! However, some types of ag- glomerates are more detrimental than others. It is generally accepted that “soft” agglomerates, i.e., those which disinte- grate during green forming of compacts, do not impede densi- fication. On the other hand, “hard” agglomerates are not broken down during compaction and can lead to incomplete densification and/or strength-limiting processing flaws. The state of agglomeration is particularly important in ce- ramic powders synthesized by wet chemical means. Hard ag- glomerates are commonly found in Z r 0 2 powders synthesized by aqueous precipitation?-6 Since these precipitation tech- niques offer convenient routes to high-purity, ultrafine ce- ramic powders, a variety of methods have been used to avoid the formation of hard agglomerates, including hot kerosene ~ynthesis,~ citrate ~ynthesis,~ alkoxide ~ynthesis,~ and Haberko’s chloride synthesis followed by alcohol washing? All of these techniques are effective in producing soft agglomer- ates.’-” Alcohol has also been used to control agglomeration during wet milling of powders.I2 Although alcohol washing is commonly used, little work has been reported on the mechanism(s) involved in producing soft agglomerates. Aging of hydrous zirconia gels in water has been shown to lead to consolidation of the gel structure due to continued condensation reactions.’ It is possible’.” that the higher surface tension of water compared to alcohol leads to higher capillary forces and thus hard agglomerates in water- washed powders. However, as pointed out in Ref. 6, capillary forces vary inversely with particle size, and since the effect of alcohol washing does not appear to be size dependent, surface tension is probably not a controlling factor determining the strength of agglomerates. Likewise, solubility issues do not seem gerrnanc6 M. Sacks-contributing editor Manuscript No. 198172. Received August 29, 1989; approved December 12, 1989. This work was performed at Case Western Reserve University under financial support from National Science Foundation Grant No. DMR- 87-15622. M.S.K. received a travel grant from Magnesium Electron, Inc., and Norton zyxwvutsrqponmlk Co. which permitted her attendance at the Fourth International Conference zyxwvutsrqpon on the Science and Technology of Zirconia and the presentation of this work. *Member, American Ceramic Society. Lee and Readey’*(’ proposed that agglomerate strength is determined by the extent to which water molecules, hydrogen- bonded to surface hydroxyl groups, are able to form bridges between adjacent particles. During washing, ethanol was postulated to hydrogen bond to surface hydroxyls, but could not cause particle-particle interaction (Fig. 1). Thus, the possibility of any chemical bonds forming between particles during drying is significantly reduced, inhibiting formation of hard agglomerates. Jones and Norman13 showed that washing hydrous Z r 0 2 with methanol resulted in the removal of all nonbridging hy- droxyl groups and coordinated water. They proposed that methanol washing inhibited the condensation reaction Zr-OH + HO-Zr --* Zr-0-Zr + H20 (1) due to the loss of nonbridging hydroxyl groups, and it was this that prevented the formation of hard agglomerates. H H H H HH HH k0\ H HOOJ Fig. 1. Lee and Readey model for agglomerate formation in (A) water-washed zirconia gels and (B) ethanol-washed zirconia gels. Dashed lines indicate hydrogen bonds. 1504