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セラミックス基盤工学研究センター年報Ta (2009).kashi Shirai, Vol. 9,Hideo 23-31 Watanabe, Masayoshi Fuji and Minoru Takahashi 総説

Structural Properties and Surface Characteristics on Aluminum Powders

Takashi Shirai, Hideo Watanabe, Masayoshi Fuji and Minoru Takahashi

Ceramics Research Laboratory, Nagoya Institute of Technology Hon-machi 3-101-1, Tajimi, Gifu 507-0033, JAPAN

Abstract

a-Al2O3 is widely used and studied as high temperature structural material, electronic packaging, corrosion resistance ceramics

and translucent ceramics. The surface state of a-Al2O3 powders cannot be regarded as a-Al2O3 but a hydrated state, and that the

nature of this hydrate cannot be considered universal among different a-Al2O3, even produced by the same production method. One

of these reasons is that water can be incorporated in the a-Al2O3 structure resulting in the formation of aluminum

such as . Although much research has shed light on the Al2O3-water interface and charging of a-Al2O3 surfaces, there is

still much that we need to better understand a-Al2O3 structural property and surface characteristics. In this paper, general view

of crystal-structural properties, manufacturing methods of high purity a-Al2O3, and surface characteristics on a-Al2O3 powders are introduced.

1. Aluminum Oxide From crystalline structure difference, there are many

Aluminum oxide (alumina; Al2O3) has advantages forms of Al2O3 (a, c, h, d, k, q, g, r). An example of such as its thermal, chemical, and physical properties a-phase of Al2O3 is /. The other when compared with several ceramics materials, and is forms are frequently termed transition Al2O3 and arise widely used for firebricks, and integrated during the thermal decomposition of aluminum circuit (IC) packages. Industrially, more than about 45 trihydroxides under different conditions. a-Al2O3 is million tons of Al2O3 are produced in the world, which the most stable form of the compounds formed between are mainly manufactured by the Bayer method using aluminum and , and is the final product from , and about 40 million tons are consumed for thermal or dehydroxylation treatments of all the refining aluminum1) Furthermore about 5 million tons hydroxides. Nomenclatures of the aluminum 4) of Al2O3 are produced as chemical grade and used for hydroxides are listed in Table 2 various purposes. Moreover about 1.5 million ton The commonly-used Al2O3 is produced through the

Al2O3 is used as raw powder in the world. Other Al2O3 Bayer process starting from bauxite, which mainly is consumed for raw powder of aluminum , consists of hydrated aluminum. In the Bayer process, aluminum sulfate and polyaluminum chloride. The crushed bauxite is treated with caustic aluminate amount of Al2O3 powder used in Japan is about 350,000 solution containing soda. The dissolution reaction is tons, what is about 20% of the total quantity produced generally carried out under pressure at temperatures in the world. In order to produce Al2O3 powder with ranging from 140 to 280°C. The caustic solution reacts the quality necessary to be used as ceramic material, with the aluminum hydroxide so that the impurities can various manufacturing methods besides the Bayer method have been developed1), 2). Depending on the Table 1: Soda impurity level required for different applications3) use of Al2O3 in different applications, Al2O3 is classified into different grades. Table 1 shows the different soda level requirements in different applications.

Commercial grades of Al2O3 are often divided into: smelter, calcined (milled or unmilled), low soda, reactive, tabular, activated, catalytic, and high purity.

These differ in their particle size, morphology, a-Al2O3 content and impurities.

― 23 ― Structural Properties and Surface Characteristics on Aluminum Oxide Powders

Table 2: Nomenclatures of aluminum hydroxides4) mechano-chemical effects19),21) etc., has been reported. Therefore it may be inferred that grinding greatly affects 22)-26) the surface of high purity a-Al2O3 powders .

High purity a-Al2O3 powders are mainly produced by the following methods:

① Hydorlysis of Aluminum Alkoxides Aluminum alkoxide is obtained from a reaction be separated by sedimentation and filtration, leaving a between metallic aluminum and alcohol groups. clear solution. After precipitation of the hydroxide, Hydrolysis of aluminum alkoxides will produce

Al2O3 powders can be obtained through heat treatment at aluminum hydroxide, which can be transformed after 8)-10) their transition temperatures. heat treatment into Al2O3 powder . The following equations show the reactions of this a 2. Production Methods of High Purity -Al2O3 method, where R is hydrocarbon radical. Powder 3 Al ＋ 3ROH → Al(OR)3 ＋ ― H2 a-Al2O3 powders produced by the Bayer method have 2 maximum purity of 99.6-99.9%, which can be used for (alkoxide formation) manufacturing of refractories, spark plugs, and Al(OR)3 ＋ 3H2O → Al(OH)3 ＋ 3ROH substrates of integrated circuit. The demand of high (hydrolysis process) purity a-Al2O3 is increasing for electronic devices, such 2Al(OH)3 → Al2O3 ＋ 3H2O (thermal process) as YAG (Yttrium-Aluminum-Garnet) and

Sapphire devices. High purity a-Al2O3 is ② Chemical Vapor Deposition indispensable in manufacturing substrate of SOS In the conventional chemical vapor deposition (CVD),

( on Sapphire) devices, high pressure sodium Al2O3 with small particle size is produced by a high 5) lamp, and bioceramics . High purity a-Al2O3 has also temperature reaction at 750-900°C between vaporized 6) 27) been applied for gas sensors . Because of its stability AlCl3 and water vapor and strength at high temperature, a-Al2O3 is a suitable 2AlCl ＋ 3H O → Al O ＋ 6HCl catalyst material at high temperature or catalyst 3 2 2 3 7) supports . For those purposes, the purity of a-Al2O3 The volatile AlCl3 is oxidized with oxygen or water should be higher than 99.99% (4N, four nine). vapor at 750-900°C, generating a homogenous particle

The manufacturing processes of high purity a-Al2O3 size of fine Al2O3 powder. Processing at temperatures by calcinations of starting materials, such as aluminum of 750-900°C can produce powders with particle size of 8), 9), 10), 11) hydroxide and alum are industrialized. In 50 nm, consisting of a mixture of g and a-Al2O3. To every process, crystal transformation to a-phase is increase the a-phase content, the powder must be heat- accompanied by growth of the particles at the time of treated at temperatures above 1200°C. Wong et al. high temperature calcination of precursor materials such reported that formation of dense deposits of a-Al2O3 as aluminum trihydroxides and alum12),13). The joints was favored by increasing temperature and decreasing 27) by sintering occur between produced a-Al2O3 particles, pressure Microstructure of the dense deposits 27) because transformation to a-Al2O3 advances at high showed long columnar grains temperatures exceeding 1200°C14),15). Consequently, in In-situ chemical deposition is a new patented CVD the manufacturing process of high purity a-Al2O3, method to produce nearly mono-dispersed single crystal 15),28) grinding is necessary to break joints and to control the a-Al2O3 powder The powder is named as particle size. Generally, high purity a-Al2O3 powders “Sumicorundum” by its manufacturer. The process has are ground by various grinders such as ball mill, the advantage of high crystal growth rate at lower vibration mill and jet mill, in order to obtain mono temperature than the conventional method. dispersed particles suitable for sintering14),16),17). Grinding processes by mechanical techniques to reduce ③ Thermal Decomposition of Aluminum Alum the particle size have been extensively investigated18)-21). Thermal decomposition of ammonium alum method The influence of grinding on powder characteristics21), has been used for the manufacturing of kidney jewelry,

― 24 ― Takashi Shirai, Hideo Watanabe, Masayoshi Fuji and Minoru Takahashi such as and sapphire. The equation of heat The aluminum hydroxides can exist in four well- decomposition is shown below. defined forms: the monohydrate AlOOH, as (g-monohydrate) and (a-monohydrate), and the 2NH4Al(SO4)2・12H2O → Al2O3 ＋ 2NH3 ＋ 4SO3 ＋ 25H2 trihydrate Al(OH)3, as gibbsite (g-trihydrate) and The ammonium alum is refined mainly by a bayerite (a-trihydrate). Of these, all but bayerite occur recrystallization method, weight decreases in 1/9 by the naturally in bauxite in profound amounts. At high heat decomposition reaction. Therefore, it is necessary temperatures, all of the heat treatment paths will to refine to the purity of ammonium alum one figure terminate in a-Al2O3. Figure 1 shows the paths of 29) higher than the target purity of Al2O3. Moreover, it transition Al2O3 during the heat treatment processes faces the problem about the exclusion processing of the a NH3 and SO3 gases which occur at the time of heat 3.2 of -Al2O3 2) decomposition The crystal structure of a-Al2O3, which is called corundum structure, ideally consists of close packed ④ Thermal Decomposition of Inorganic Aluminum planes (A and B planes) of large oxygen anions (radius Salts 0.14nm) stacked in the sequence30) as shown in Figure 2. Several inorganic aluminum salts, ammonium The aluminum cations (radius 0.053 nm) have valence aluminum carbonate hydroxide (AACH), NH4AlO (OH) of +3 and oxygen anions have valence of –2. There can 3+ 2– HCO3, for example, can be heated at 230°C to produce be only two Al ions for every three O ions to

Al2O3 through a thermal decomposition shown by the maintain electrical neutrality. Thus, the cations occupy following reaction11)

2NH4AlO(OH)HCO3 →Al2O3 ＋2NH3 ＋2CO2 ＋3H2O

Production condition of AACH will influence the sintering behavior of a-Al2O3. The process can generate a-Al2O3 powder with purity higher than 99.99% and 0.3-0.4 mm of particle size.

3. Structure of Aluminum Oxide 3.1 Structural Transformations

Beside a-Al2O3, there are other forms of metastable

Al2O3 structures, such as r, g, h, q, c and k-Al2O3.

Those kinds of transition Al2O3 can be produced from heat treatment of aluminum hydroxides or aluminum Figure 2: (a) Corundum structure in a-Al2O3, (b) top view of salts. the corundum structure, and (c) octahedral structure of a- 30) Al2O3 a Gibbsite Chi() Kappa () Alpha () A b a Theta Alpha B Boemite Gamma () Delta () () () b b A a Alpha Bayerite Eta () Theta () c () B a Diaspore Alpha Alumina () A b

0 100 200 300 400 500 600 700 800 900 1000 1100 1200 B ᯱᰜ Temperture / oC c A ᯿ Figure 1: Structure transformation of alumina and aluminum 29) 32) hydroxides Figure 3: Structure of a-Al2O3

― 25 ― Structural Properties and Surface Characteristics on Aluminum Oxide Powders only two-thirds of the octahedral sites of the basic array. This placement forms three different types of aluminum cation layers, named a, b, and c. In Figure 3, the complete stacking sequence of oxygen and aluminum layers will form A-a-B-b-A-c-B-a-A-b-B- c-A… One period in this sequence, i.e. from c-A to B-c, 32) forms a hexagonal unit cell of a-Al2O3

3.3 Crystal Structure of Transition Al2O3

Transition Al2O3 crystallizes in or similar to it with defect lattice. In the spinel structure, the oxygen ions form a face-centered cubic (FCC) lattice and Al3+ Figure 4: Structure of q-alumina in which half of the Al ions ions occupy tetrahedral and octahedral interstitial sites, are occupy tetrahedral sites32) as shown in Figure 4. FCC lattice is also formed by close packed plane, but the stacking sequence is 3.4 Crystal Structure of Aluminum Hydroxides designated as 3.4.1 Gibbsite Pauling first proposed the concept of the gibbsite A-B-C-A-B-C… structure39) Double layers of OH ions, with Al ions Since the Al ion favors octahedral coordination under occupying two-thirds of octahedral interstices within the normal circumstances, Saalfeld et al. assumed all layers, form the basic structural element. The octahedral sites to be occupied, and the cation vacancies hydroxyls of adjacent layers are situated, directly being confined to the tetrahedral sites33) John et al. opposite to each other, i.e., in a cubic packing. Thus the used solid-state nuclear magnetic resonance (NMR) sequence of OH ions in the direction perpendicular to with magic angle spinning to determine the coordination the planes is A-A-B-B (Figure 5). This superposition 34) of Al ions in the transition Al2O3 They found 65 and of layers and the hexagonal arrangement of Al ions lead 75% of the Al ions in the octahedral sites in h and g- to channels through the lattice parallel to the c-axis.

Al2O3, respectively. The results on h-Al2O3 agree with Hydrogen bridges OH groups of adjacent double layers. the X-ray powder diffraction pattern reported by From proton magnetic resonance measurements, Kroon Shirasuka et al.35) The cation vacancies, therefore, et al. has deduced a model of the spatial distribution of 40) appear to favor octahedral sites in h-Al2O3 but these H-bonds tetrahedral sites in g-Al2O3. The same workers showed that in q-Al2O3 prepared from both bayerite and 3.4.2 Bayerite boehmite, the Al ions were almost exclusively in Bayerite is rarely found in nature and produced octahedral coordination. This is in contrast to commercially for catalysts or other applications Saalfeld’s36) and Yamaguchi et al.37) structure analyses of requiring high quality for products. In laboratory, the q-Al2O3 in which both assumed half of the Al ions were trihydroxides can be prepared by treatment of aluminum occupying the tetrahedral sites. Wefer considered the q chloride solution with cold ammonium hydroxide, structure as an intermediate between the cubic close followed by aging at room temperature. According to packing of the low-temperature transition Al2O3 and the Fricke et al., bayerite is obtained by hydrolyzing hexagonally closed-packed corundum29) aluminum alcoholates at temperatures below 40°C41) Ratio of tetrahedral and octahedral sites of Al ions in Another preparation method is introduced by Torkar et transition Al2O3 still have contradiction between al., who produced extremely pure bayerite experimental results observed by several researchers. g- electrolytically, using cathode and H2O2 as an 42) Al2O3 has the complex Al-O infrared absorption bands electrolyte Bentor et al. reported the first occurrence between 350-1100 cm–1, which are interpreted under the of the structure verified by X-ray analysis43) The criteria for the band assignment of the . The structure of bayerite is similar to gibbsite which is built positions of the remainder hydroxyl groups are related by basic layers of Al-OH octahedra. The layers are, with the aluminum vacancies38) however, arranged in A-B-A-B-A-B sequence (Figure

― 26 ― Takashi Shirai, Hideo Watanabe, Masayoshi Fuji and Minoru Takahashi

by double molecules of AlOOH which extend in the direction of the a-axis. The double layers are linked by hydrogen bonds between hydroxyl ions in neighboring planes. Average O-O distance of the hydrogen bridges is 0.27 nm29) If the excess water is very high (typically contains > 15 wt% excess water), a, b, c distances in 3 dimensional directions of crystallographic dimension become longer, and produce pseudo-boehmite32) X-ray diffraction pattern of pseudo-boehmite is similar to boehmite with a broad peak. Papee et al. postulated that the excess water is not merely adsorbed on crystallite surface, but is located between boehmite-like layers as molecular water45) Heating pseudo-boehmite

results in the formation of transition Al2O3 in a sequence 36) Figure 5: Structure of aluminum trihydroxide; (a) Top view of similar to that associated with bayerite the gibbsite, (b) gibbsite, and (c) bayerite, respectively. The structures have different layer packing32)

5)32)

3.4.3 Nordstrandite Nordstand et al. published the X-ray difractogram of a trihydroxide which differed from the diffraction patterns of gibbsite and bayerite. They obtained their trihydroxide by precipitating a gel from aluminum chloride or nitrate solutions with ammonium hydroxide44) Figure 7: A boehmite structure which consists of double layers There are two kinds of arrangement for the Al-OH and hydrogen bonds in between the layers32) octahedra layer, as shown in Figure 6.

4. Reported Al2O3 Surfaces B Surface of a solid crystal is regarded as a truncated A area of the crystal consisting of coordinately unsaturated A site (cus) anions and cations. For this reason, when it is exposed to the atmosphere, all solids become covered B A with various types of adsorbed species. In the case of C B metal , the outer layer is usually made up of different adsorbed species. The most abundant B component of the surface layer of oxides is water. A Aᰜ Besides physisorbed water, absorbed water can be B present at the surface in the form of hydrogen bonded or

A OH coordinated molecular H2O, and in a dissociated form as surface hydroxyl46) Figure 6: Structure of Nordstrandite44) In this regard, infrared spectroscopy has been a

commonly used technique to study the surface of Al2O3. By infrared spectroscopy, the O-H stretching

3.4.4 Boehmite fundamental vibrations (nOH) of OH groups located at The structure of Boehmite consists of double layers in the surface of oxides can be observed in the high which oxygen ions are in cubic packing, as shown in frequency region (wavenumber higher than 2500 cm–1). Figure 7. These layers are composed of chains formed A very broad band centered at around 3300 cm–1 has

― 27 ― Structural Properties and Surface Characteristics on Aluminum Oxide Powders been assigned to the stretching modes of molecular transition aluminas. 47) water hydrogen bonded on an Al2O3 surface and a band at 3500 cm–1 to hydrogen bonded hydroxyl groups. At 870 K, the hydrogen bonds no longer exist and only free hydroxyl groups remain46) The knowledge gained so far about the OH band assignments for transition Al2O3 is helpful to understand a-Al2O3 powders. The vibrational spectrum from the surface hydroxyl of the transition Al2O3 is complex and typical. Table 3 tabulates the bands and assignment proposed for surface hydroxyl species on transition 46) Al2O3 so far.

Table 3: OH Band assignment proposed for transition aluminas46)

Figure 8: Five types (A-E) of isolated hydroxyl ions (+ denotes Al3+ in lower layer). The remaining hydroxyl ions, covering 9.6% of the lattice, are found on five types of sites on which they have from zero to four nearest oxide neighbors, as illustrated and identified48)

Actually the spectra of transition Al2O3 consist of a II: Tsyganenko’s model (1973) mixture of OH bands that closely overlap with each other. By considering the most probable terminations of the Since transition aluminas are commonly used as crystallites and the geometry of the OH groups in these catalysts or support of catalyst, their surfaces have been surface termination groups, Tsyganenko et al. came to attempted to characterize. Several models have been the conclusion that the number of nearest neighbors has proposed to explain the reactivity of the surface. a negligible effect on the frequency of OH species. The Several models as summarized by Morterra are determining factor is the number of lattice metal atoms described below46) that the OH groups are attached to. On the basis of the electronic characteristics of I: Peri’s model (1965) oxygen, there can be three types of OH groups, termed This model advocates that there are five possible free OH groups of type I, II, and III, respectively, depending 48) 49) OH configurations on g-Al2O3 surfaces These are on the coordination number of the OH group The depicted in Figure 8 and their symbol and frequency are three types of hydroxyls are shown in Figure 9. shown also in the third column of Table 3. The major limit of Peri’s model consists in the assumption of the (100) crystal face as the only possible termination for aluminas crystallites. This assumption yields an oversimplified picture of the spinel structure, in that only AlVI ions would result present in the uppermost layer and all OH groups in the fully hydrated surface (located on top of equivalent cations) would results to be equivalent. In these conditions, the adoption of random Figure 9: Three types of hydroxyl groups are possible at the elimination of water from adjoining OH groups is surface of transition alumina. M denotes metal ions in a metal correct and no acidity-basicity concepts associated with oxide49) different surface OH species needs to be involved. The model is thus valid, in principle, but gives only a partial description of the structurally complex situation of

― 28 ― Takashi Shirai, Hideo Watanabe, Masayoshi Fuji and Minoru Takahashi

III: Morterra’s model (1976) (ii) the spectral characteristics of metal-hydroxy Morterra and co-workers considered the coordination complexes having non-H-bonded OH groups with of the Al cations, rather than the coordination of the OH different coordination numbers. group, as the most important factor in determining the The conclusions reached, that are schematically OH frequencies50)-52) According to their approach, that reported also in the last column of Table 3, reassign the did not consider the actual crystallographic termination various OH species of aluminas as reported in Figure 11. of transition Al2O3 and of the other Al-containing systems, the assignment of the various OH species of

Al2O3 could be made only in very general terms by comparing the OH spectra of different Al oxides. The assignment of OH group is as described in Figure 10.

Figure 11: Possible OH stractures, and vOH frequencies, at the surface of defective spinel transition aluminas (Symbol; □ Figure 10: Schematic distribution of OH bands in transition stands for a cation vacancy)46) aluminas and other Al-containing oxidic systems52) VI: Morterra’s defect model (1994) IV. Knozinger’s model (1978) Finally, Morterra et al. attributed the 3775 cm–1 OH The OH configurations, frequencies and net charges band to hydroxyl group coordinated on tetrahedral Al at Al and OH according to this model are given in Table ion (AlIV-OH groups) present in portions of surface 4. belonging to crystallographically defective The net charge is obtained as the sum of the negative configurations (i.e. stepped terminations) which charge of the anion and the sum of the strength of the frequently occur in porous system of high surface area electrostatic bonds (=cation charge divided by and poor crystallinity56) coordination number) to the anion from adjacent Enormous amount of infrared spectroscopic works cations53) have been devoted to understand the surface behavior of

Al2O3, since Peri proposed a model of Al2O3 about thirty V: The possible role of cation vacancies years ago. Most of the works have been related to

The role played by cation vacancies is very difficult to transition Al2O3. In spite of some controversial aspects take into account, as there is no way to determine them and some remaining uncertainties of the results, the directly. The possible surface occurrence of cation overall picture obtained for the surface properties of the vacancies is one further parameter, besides OH and transition Al2O3 may be useful for understanding the cations coordination, that has been considered in the surface behavior of a-Al2O3 in this study. most recent model for OH of aluminas, proposed by Busca et al.7), 54) and Della Gata et al.55) 5. Concluding Remarks As a starting point, Busca’s model uses the same Crystal structural properties, manufacturing methods general criteria successfully adopted by Knozinger and of high purity Al2O3, and surface characteristics on reported above, but the assignment is also based, on a Al2O3 oxide powders are described. phenomenological ground, on ; The surface chemical characteristics of materials play (i) the observation of the OH spectral patterns of several an important role in many technological processes and normal spinels (MgAl2O4, ZnAl2O4). Inverse spinels applications. Differences in surface conditions of the

(NiAl2O4) and defective spinels (transition phase d- powders can influence their physical properties such as

Al2O3); zeta potential, powder agglomeration and sintering

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