Surface hydration states of commercial high purity α-Al₂O₃powders evaluated by temperature programmed desorption mass spectrometry and diffuse reflectance infrared Fourier transform spectroscopy

Abstract

The surface of three different grades of commercial high-purity a-Al 2 O 3 powders produced by hydrolysis of aluminum alkoxide, which differ each other in SSA are evaluated by temperature programmed desorption mass spectrometry (TPDMS) and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. For the DRIFT evaluation the powders were heated in situ under vacuum from 25 to 700 8C. The TPDMS spectra of desorbed H 2 O were obtained by heating the samples under ultra high vacuum at a rate of 20 K min K1 up to 1200 8C.The presence of hydrogen bonded water molecules, amorphous Al(OH) 3 and AlOOH structures, as well as associated and isolated hydroxyl groups on the surface of all the a-Al 2 O 3 powders investigated is demonstrated. On the surface of one of the powders the presence of crystalline Al(OH) 3 structures, as evidenced by an additional sharp peak in the H 2 O TPDMS spectrum, is confirmed. q

Charging Behavior at the Alumina–Water Interface and Implications for Ceramic Processing

“…One can see that there are likely to be many singly coordinated surface hydroxyl groups. Investigators have identified the surface hydroxyl groups found on several types of submicron α‐alumina powders to be similar to those found on gibbsite and/or other aluminum hydroxides 51,52 …”

“…The surface of submicron powder is somewhat different than single crystal samples because the majority of the surface sites on the powders are defect sites 49,50 . Also, the surface of alumina powders prepared by grinding or milling (the majority of alumina is used after some milling) can have surface hydroxyl groups that are the same as those on gibbsite 51,52 …”

“…It is also possible to dehydrate a fully hydroxylated surface by thermal treatment either in vacuum or air 50,51,53 . The alumina surface dehydrates in a step‐wise fashion such that different type of surface hydroxyl groups are removed sequentially.…”

“…The last sites to dehydrate are the defect sites. No hydroxyl groups are found at temperatures above about 950°C 50,51,53 . The fully dehydroxylated alumina surface has a structure with oxygen anions bridging two aluminum cations 50 …”

“…is a schematic comparison of the hydrated α‐alumina surface and the gibbsite surface. The surface of hydrated α‐alumina powders prepared by grinding were also found to contain surface hydroxyl groups that are gibbsite like after hydrothermal hydration 51,52 …”