Materials Science and Engineering, A181/A 182 (1994) 1227-1231 1227 Comparison of the transformation sequence from ),-A1OOH (boehmite) to o~-A12O 3 (corundum) induced by heating and by ball milling A. Tonejc, A. M. Tonejc and D. Bagovid Department of Physics, Faculty of Science, PO Box 162, 41001 Zagreb (Croatia) C. Kosanovid Ruder Bo~kovi( Institute, PO Box 1016, 41001 Zagreb (Croatia) Abstract This paper presents the X-ray diffraction investigation of ball-milling-induced transformation of boehmite (y-AIOOH) and of gibbsite (AI(OH).d to corundum (a-A1203). The results are compared with the transformation observed in heating experiments, and the conclusion is drawn that equivalence exists between the high energy ball milling and the thermal process. 1. Introduction In a recent publication [1] we have shown that the well-known temperature-induced transition from V- boehmite (y-A1OOH) to corundum (a-A1203) [2-6] could also be obtained by high energy ball milling. Since, upon heating, the transformation of boehmite to corundum occurs over a series of transition phases, in the present experiment an attempt is made to compare the heating- and milling-induced transition sequences. Heating of boehmite or of gibbsite (AI(OH)3) at over 1000 °C yields a variety of transition aluminas before the stable corundum is formed [2-6]: > 170 °C > 500 °C > 825 °C AI(OH)3 ~ z-A1203 --~ 7¢-A1203 ~ R-AI203 (1) (gibbsite) > 351) °C > 61111 °C ~ 9(11) °C 7-AI(OOH) ~ y-Al203 ~ 6-A1203 --~ 0-A1203 (boehmite) > 1000 °C a-Al203 (2) The transition temperatures are somewhat approxi- mate, but it is worth remembering that, although all transition aluminas are stable at room temperature, they are normally activated only at above-indicated temperatures and cannot be retraced in reverse with decreasing temperature. 2. Experimental procedure A commercial boehmite Karlsruhe) and a synthetic 0921-5093/94/$7.00 SSDI 0921-5093(93)05661-8 powder (Ventron, gibbsite (Aluminium Factory, Mostar) were used as starting materials. The heating was performed in a well-temperature- controlled furnace, and the milling in a Fritsch planetary ball mill. Both the vial and the balls were made of tungsten carbide (WC). A 1 g mass of powder and a set of ten balls with a diameter of 10 mm were used in all experiments. For every milling experiment a new charge of starting powder was used and then milled uninterruptedly for the imposed time intervals. Before and after the heating or milling, the powders were examined using a Philips PW 1820 X-ray diffractometer with Cu Ka monochromated radiation. Transmission electron microscopy (TEM) observations were performed on a JEOL JEM 2010 microscope. 3. Results 3.1. Heating The diffuse characteristic and the similarity of X-ray diffraction (XRD) patterns of non-milled alumina powders were the main reason that we also performed the heating experiments in order to be able to deter- mine which of the transition aluminas appeared in the milling-induced boehmite- or gibbsite-to-corundum transition. We performed several heating experiments 200 h long which confirmed the facts [5, 6] that the transition phases of the aluminas are stable below the transition temperature and that diffuse broad lines persist even after long heating. In Fig. 1, those XRD patterns which we found to be the most important in identifying the phases in milled powders are shown. The boehmite and the gibbsite © 1994 - Elsevier Sequoia. All rights reserved