Mechanisms of Al and titania hydrogel complex formation via a mechanical route
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Mechanisms of Al and titania hydrogel complex formation via a mechanical route Yuumi Kojima, Tetsuhiko Isobe, and Mamoru Sennaa) Faculty of Science and Technology, Keio University, Hiyoshi, Yokohama 223, Japan
Takeshi Shinohara, Sakai Ono, Kenji Sumiyama, and Kenji Suzuki Institute for Materials Research, Tohoku University, Katahira, Sendai 980, Japan (Received 25 May 1995; accepted 16 January 1996)
The process of complex formation has been studied during milling a powder mixture of metallic Al and titania hydrogel, TiO2 ? H2 O. Solid state 27 Al NMR and ESR are employed together with conventional x-ray diffractometry and thermal analyses. Al atoms partly changed from metallic to oxide state. 27 Al NMR analyses indicate that two metallic Al states are allotted to distorted and undistorted lattices, while the other to AlOx units in the oxide state, where x is 4, 5, or 6. As detected by ESR, Ti41 was partly reduced to Ti31 , suggesting a redox reaction during milling. A vigorous thermit reaction was detected during subsequent heating of the mixture after prolonged milling. These results indicate the formation of Al –O–Ti bonds during milling, leading to Al2 TiO5 on subsequent heating.
I. INTRODUCTION
In the course of a systematic study on the incipient chemical interaction between solid species under mechanical stressing, we have found that two different metallic species are combined as a result of stressing, either directly or abridged by oxygen or °°OH groups.1 Solid state NMR is a very versatile tool for discovering reasons for this reaction process.2 As a matter of fact, we have found that during grinding of the Al –Ti metallic mixture, electrons transfer from Al to Ti, leading to a formation of an intermetallic compound AlTi.2 In a hydrogel mixture, atomically intimate mixing of AlO6 and TiO6 units have been verified.2 Although the mechanisms are not always similar, formation of precursors of the complex oxides through mechanical routes has been confirmed. Similar findings have been made for different systems such as Me(OH)2 –SiO2 (Me Mg, Ca, and Sr)3 or hydrogels of titanium and zirconium.4 In the case of a mixture of metal and oxide or hydroxide, different electron flow and different mechanism of complex formation may be anticipated, because of strong electron affinity of metallic species. For instance, a thermit reaction is enhanced by oxygen atoms supplied from the oxide if the absolute value of the standard free energy of formation is smaller than that of Al2 O3 . Such thermit reactions are known to occur during grinding.5 Mechanical milling of metals and oxide mixtures is very attractive from the viewpoint of materials synthesis because it may synthesize complex oxides that are
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Author to whom all correspondence should be addressed. J. Mater. Res., Vol. 11, No. 5, May 1996
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otherwise prepared only via laborious routes. Takacs has shown the reactive milling of several systems.6–8 The purpose of the
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