Effects of Manufacturing Process on the States of Coordinatively Unsaturated Site (SUS) of Alumina Powder Surfaces

Takashi Shirai, Chanel Ishizaki, Masayoshi Fuji and Kozo Ishizaki

Submitted: 18 November 2011, Accepted: 13 January 2012/will be published online in full text at https://www.azom.com

Topics Covered

Abstract

Coordinatively unsaturated aluminum ions on the surface of commercially available high purity α-Al₂O₃ powders produced by three different processes were evaluated by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. Powders were heated in situ under vacuum up to 250 °C., Difference spectra between dry air conditions and 250 °C in vaccum presented bands centered around 1640, 1580, 1530, 1460 and 1380 cm^–1. The bands were interpreted to be related to water molecules, physically adsorbed and coordinated to unsaturated Al^VI, Al^V, Al^IV-Al^VI and/or Al^VI-Al^VI, and Al^IV, respectively, The difference spectra of powders produced by in-situ chemical vapor deposition, “A”, presented mainly physically adsorbed water, meanwhile powders produced by hydrolysis of aluminum alkoxide, “B”, and thermal decomposition of ammonium alum,“C”, showed mainly coordinated water bands. Thermal activation and dehydration processes appear to enhance coordinately unsaturated Al^IV and Al^V surface ions. Grinding a powder produced by the in-situ chemical vapor deposition increased the surface proportion of uncoordinated Al^V ions.

The main conclusion of the present work is that the nature and proportion of coordinately unsaturated aluminum ions present on the surface of high purity α-Al₂O₃ powders, intimately related to the manufacturing process, is responsible for the different hydration ability associated with each particular powder.

The surface proportion of coordinately unsaturated aluminum atoms presented on the surface of powders “B” and “C” was larger than for powders “A”. Furthermore, the surface proportion of uncoordinated Al atoms with coordination V was increased by grinding. From the results, the surface proportion of coordinately unsaturated aluminum ions such as Al^IV and Al^V might be affected by thermal activation and dehydration processes. Additionally, the grinding process appears to increase the proportion of coordinately unsaturated Al^V. It was possible that the different hydration ability of the powders might be linked to differences in the surface proportion of coordinately unsaturated Al^V atoms by different manufacturing processes.

Introduction

It has been shown that the surface state of α-Al₂O₃ powder cannot be regarded as α-Al₂O₃ but a hydrated state. Furtheremore, the nature of this hydrate cannot be considered universal among different α-Al₂O₃ even those produced by the same synthesized method and under the same identification code [1-5]. Surface condition differences influence the physical properties of the powders such as zeta potential [6], powder agglomeration [7] and rheological behavior [8].

Generally, Al₂O₃ powders are produced by calcinations of precursor materials such as aluminum hydroxide and alum [9-12]. In these processes the crystal transformation to α-phase is accompanied by growth of the particles with increasing calcination temperature of precursor materials. The transformation to α-Al₂O₃ proceeds at high temperatures usually exceeding 1200 °C, consequently, joints occur by calcination between produced α-Al₂O₃ particles [13, 14]. Therefore, in these manufacturing processes of α-Al₂O₃ grinding is necessary to break the joints and to control the particle size. Generally, in order to obtain mono-dispersed particles α-Al₂O₃ powders are ground by various grinding tecniques such as ball milling, vibration milling and jet milling, [14-20].

We believe that manufacturing conditions such as calcinations, dehydration and grinding, greatly affect the surface state of α-Al₂O₃ powders, but it has not been reported.

In this study, the effects of manufacturing process such as calcinations, dehydration and grinding are investigated on the Al₂O₃ surface states. Furthermore, we will show that certain manufacturing processes enhance the surface proportion of coordinatively unsaturated Al.

Experimental

Materials

Eight commercially available sub-micron high purity α-Al₂O₃ powders produced by three different processes; in-situ chemical vapor deposition; “A” powders (A1, A2 and A3, Sumitomo Chemical Co., Ltd., Japan), hydrolysis of aluminum alkoxide; “B” powders (B1, B2 and B3, ditto) and thermal decomposition of ammonium alum; “C” powders (C1 and C2, Showa Denko K. K., Japan) methods are used for this study. Transition Al₂O₃ (γ-Al₂O₃ and θ-Al₂O₃, Sumitomo Chemical Co., Ltd., Japan) are used as reference materials.

Production process and characteristics of the α-Al₂O₃ powders supplied by the manufacturers complemented with the data obtained in the present work are shown in Table 1. The characteristics of the transition γ and θ-Al₂O₃ are also included in the same table. All powders have low impurity content. The values of impurity content are lower than the impurity in aluminum oxide produced from the Bayer process, which usually have high NaO impurity content. For the in-situ chemical vapor deposition process (“A” powders), the growth of a particle occurs through the gas phase, besides, transformation to α-Al₂O₃ proceeds by low temperatures between 800-1100 °C, and therefore, joints of particles produced by calcination do not occur [12, 21]. Consequently, for the in-situ chemical vapor deposition powders the grinding process is not necessary [21].

On the other hand, hydrolysis of aluminum alkoxide (“B” powders) and thermal decomposition of aluminum alum (“C” powders), the crystal transformation to α-phase is accompanied by the growth of particles with increasing calcination temperature of precursor materials such as aluminum trihydroxides and alum. The joints of particles by calcination occur because transformation to α-Al₂O₃ proceeds at high temperature usually exceeding 1200 °C [14, 21]. Consequently, these manufacturing processes need grinding to break these joints and to control the particle size. All “B” powders have been ground by the manufacturer. In contrast, for the “C” powders ground or not ground (before grinding) powders are available. A not ground powder “C1” and a ground powder “C2” are included in this study.

DRIFT Procedures

Diffuse reflectance infrared Fourier transform (DRIFT) spectra were recorded by using FTIR spectrometer (Shimadzu Corp., FTIR 8300) equipped with a triglycine sulfate (TGS) detector, DRIFT accessory (Spectra Tech, Inc., Model #0030-0XX) and data processing software (Shimadzu Corp., Hyper IR) with 4 cm^-1 of resolution and 256 scanning times. The samples and backgrounds spectra were collected in power mode and stored as single beam spectra for further processing. To increase the spectrum signal from the Al₂O₃ powder and to avoid influences of water absorbed on KBr, the investigated powder was placed in a micro sample holder without diluting in KBr. The DRIFT spectra of all powders recorded under dry air atmosphere and after heating in situ under vacuum up to 250 °C in the OH bending (1800-1200 cm^-1) wave number regions. The spectral bands were separated and quantified using a Jandel Peak Separation and analysis software in log (1/R) units.

Table 1. Production method and specifications of the powders.

Results and Discussion

Water Absorption on Coordinately Unsaturated Al Atoms

The difference of the DRIFT spectra recorded in dry air atmosphere and after heating in situ under vacuum up to 250 °C for all the as-received α-Al₂O₃ powders is presented in figure 1. Figure 2 shows the difference of the spectra recorded in dry air atmosphere and after heating in situ under vacuum up to 250 °C for transition Al₂O₃ (γ and θ) powders. Profound differences are observed in the difference spectra for the powders produce by the three methods and among powders produced by the same production method in the bending OH vibration frequencies.

Vlaev et al. studied the nature and reactivity of the hydrate coverage on the surface of γ-alumina [22]. It physically or coordinately bonded to definite incompletely coordinated aluminum ions on the oxide surface. Furthermore, specific assignments were made. It was stated that: water molecules coordinated on tetrahedral or octahedral aluminum ions are characterized by absorption bands at 1380 and 1580 cm^-1, respectively, whereas those forming the pairs Al^IV - Al^VI and Al^VI – Al^VI have a band at 1460 cm^-1. Physically adsorbed water in the form of associations of molecules gives an absorption band at 1640 cm^-1. Shirai et al. have reported about the state of adsorbed molecular water on the surface of as-received commercial high purity α-Al₂O₃ powders [2]. A visible band at 1530 cm^-1 in the difference spectra of the powders produced by hydrolysis of aluminum alkoxide, was assigned to water molecules coordinated on Al^V ions [2].

Figure 1. Hydroxyl bending absorption frequency region showing the difference between the DRIFT spectra recorded in dry air atmosphere and after heating in situ under vacuum up to 250 °C for all the as-received α-Al₂O₃ powders, (a) powders “A”, (b) powders “B”, and (c) powders “C”.

Figure 2. Hydroxyl bending absorption frequency region showing the differential DRIFT spectra in dry air atmosphere and after heating in situ under vacuum up to 250 °C for transition Al₂O₃ powders.

Quality and Quantity of Coordinately Unsaturated Al Atoms

Figure 3 shows a comparison of physically adsorbed and coordinated water molecules intensity fractions for all the powders investigated. The quantities presented in this figure were obtained from the difference spectra shown in figure 1 and 2, which were deconvoluted using a Jandel Peak fit and analyusis software in log(1/R) units, considering the 1640 cm^-1 associated to physically adsorbed water molecules and all the other bands to molecular water coordinated to coordinately unsaturated surface Al ions. Powders “A” mainly show physically adsorbed water. For all the “B” powders, produced by the hydrolysis of aluminum alkoxide, and “C” powders, produced by thermal decomposition of ammonium alum, the surface population of coordinately unsaturated Al ions is larger than for the “A” powders, produced by in situ chemical vapor deposition and very similar to the surface of the θ and γ transition alumina powders.

Several studies by Fripiat and co-authors have showed that the presence of coordinately unsaturated sites, such as Al^V and distorted Al^IV, on the surface of transition alumina powders is linked to the presence of Lewis sites [23-25]. These coordination defects and /or the presence of Lewis sites are said to result from thermal activation and dehydration processes and play a key role in the surface and bulk rehydration. Furthermore it was shown that the partial and slow hydrolysis of aluminum alkoxide increases the Al^V content [23]. For the in-situ chemical vapor deposition process, “A” powders, since the growth of the particles occurred through the gas phase, the transformation to α-Al₂O₃ proceeds at low temperatures; 800-1100 °C [12]. On the other hand, for the hydrolysis of aluminum alkoxide process, powders “B”, and thermal decomposition of aluminum alum process, powders “C”, the crystal transformation to α-phase is accompanied by the growth of the particles with increasing calcination temperatures of precursor materials. The transformation to α-Al₂O₃ proceeds by high temperature usually exceeding 1200 °C by heat treatment [9-11]. It is considered that the coordination defects present on the “B” and “C” powder surfaces, result from the thermal activation and dehydration processes in manufacturing process. Thereby, for all the “B” and “C” powders, the surface proportion of coordinately unsaturated Al atoms is

Figure 3. Comparison of physically adsorbed and coordinated water molecules intensity fraction, from the spectra given in Figures 1 and 2. For all the “B” and “C” powders, the surface proportion of coordinately unsaturated Al atoms is larger than for the “A” powders.

Figure 4 shows the intensity fraction of water molecules coordinated to different CUS taken from the spectra given in figures 1 and 2, which were deconvoluted using a Jandel Peak fit and analyusis software in log(1/R) units. For all the “A” powders, which were not ground in the manufacturing process, surface coordinately unsaturated Al^V ions are not present. On the other hand, all the “B” powders, which were ground in the manufacturing process, mainly show coordinately unsaturated Al^V ions. For the C2 powder, which was ground in the manufacturing process, the surface population of incompletely coordinated Al^V is lager than for the not ground powder C1.

Proportion of Coordinately Unsaturated AlV Ions and Grinding

To clarify the effects of grinding on the proportion of coordinately unsaturated Al^V ions, the A1 powder, not ground, in the manufacturing process, was subjected to grinding in dry process for a period of 30 min by a planetary ball mill. The difference of the spectra recorded in dry air atmosphere and after heating in situ under vacuum up to 250 °C for the as-received and as-ground A1 powders are presented in figure 5. Figure 6 shows the intensity fraction of water molecules coordinated to coordinately unsaturated Al of different coordination, from the spectra given in figure 5, which were deconvoluted using a Jandel Peak fit and analyusis software in log(1/R) units. For the as-received A1 powder, which was not ground in the manufacturing process, coordinately unsaturated Al^V ions are not present. On the other hand, the as-ground A1 powder, mainly show coordinately unsaturated Al^V ions.

From these results, the surface proportion of coordinately unsaturated aluminum ions such as Al^IV and Al^V might be affected by thermal activation and dehydration processes. Furthermore, the grinding process specially affects the surface proportion of coordinately unsaturated Al^V. It is therefore possible to link the different hydration ability of the powders [3] to differences in the surface proportion of coordinately unsaturated aluminum ions resulting from differences in the manufacturing process.

Figure 4. Intensity fraction of water molecules coordinated to coordinately unsaturated Al of different coordination, from the spectra given in Figures 1 and 2. All powders “B” and powder “C2”, which were ground in the manufacturing process, the surface population of incompletely coordinated Al^V are larger than for the not ground powders all “A” and “C1”.

Figure 5. The comparison of as-received and as-ground A1 powders on the difference of the spectra recorded in dry air atmosphere and after heating in situ under vacuum up to 250 °C for A1 powders.

Figure 6. Intensity fraction of water molecules coordinated to coordinately unsaturated Al of different coordination, from the spectra given in Figure 5. For the as-received A1 powder, which was not ground in the manufacturing process, coordinately unsaturated Al^V atoms are not present. On the other hand, the as-ground A1 powder, mainly shows coordinately unsaturated Al^V atoms.

Conclusions

The nature and quantity of coordinatively unsaturated Al atoms present on the surface of commercially available high purity α-Al₂O₃ powders produced by three different manufacturing processes were evaluated by DRIFT spectroscopy. The main observations and conclusions can be summarized as follows:

1. Difference of spectra taken in dry air conditions and 250 °C in vaccum of all powders presented bands centered around 1640, 1580, 1530, 1460 and 1380 cm^–1.
2. These bands are interpreted to be related to water molecules, physically adsorbed and coordinated to unsaturated Al^VI, Al^V, Al^IV-Al^VI and/or Al^VI-Al^VI, and Al^IV surface ions, respectively,.
3. The difference spectra of powders produced by in-situ chemical vapor deposition, “A”, presented mainly physically adsorbed water molecules.
4. The powders produced by hydrolysis of aluminum alkoxide, “B”, and thermal decomposition of ammonium alum, “C”, showed mainly coordinated water molecules.
5. Thermal activation and dehydration processes appear to enhance coordinately unsaturated Al^IV and Al^V surface ions.
6. Grinding a powder produced by the in-situ chemical vapor deposition increased the surface proportion of uncoordinated Al^V ions.
7. It is therefore concluded that the grinding process also affects the quality and quantity of surface.
8. The main conclusion of the present work is that the nature and proportion of coordinately unsaturated aluminum atoms present on the surface of high purity α-Al₂O₃ powders, intimately related to the manufacturing process, is responsible for the different hydration ability associated with each particular powder.

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Contact Details

Takashi Shirai, Chanel Ishizaki ², Masayoshi Fuji ¹ and Kozo Ishizaki ³

¹ Ceramics Research Laboratory, Nagoya Institute of Technology, 3-101-1, Honmachi, Tajimi, Gifu, 507-0033, Japan
² Nano Tem Co., Ltd. Shimogejo 1-485, Nagaoka, Niigata, 940-0012, Japan
³ Nagaoka University of Technology (Nagaoka Gijutsu-Kagaku Daigaku) Niigata, 940-2188, Japan

This paper was also published in print form in "Advances in Technology of Materials and Materials Processing", 13[2] (2011) 85-92.