Synthesis of alumina powders by mechanical activation

A.S.A. Chinelatto1; C. Lago1; S. R. M. Antunes2; A. C. Antunes2 O. M. Cintho1; A. L. Chinelatto1

(1) Universidade Estadual de Ponta Grossa - Departamento de Engenharia de Materiais (2) Universidade Estadual de Ponta Grossa - Departamento de Química

adriana@uepg.br

Keywords: alumina, high-energy milling, mechanical activation, synthesis.

Abstract: Alumina is utilized in many areas of modern industry because of its unique mechanical, electrical and optical properties. Various methods are been employed for produce alumina for different end uses. The preparation of fine and sintering-reactive alumina powders is probably one of the most important steps for production alumina ceramics with controlled microstructure. In this work, it was studied the production of alumina powders by “Pechini” method associated to high-energy milling. For this, it was prepared the resin by Pechini method, using aluminum nitrate nona-hydrate. This resin was calcined at 500oC. Then, the calcined powders were submitted a high energy milling for different times. The powders mechanically activated were characterized by x ray diffraction, FT-IR and scanning electronic microscopic. After milling, the powders were calcined at 900oC. The results showed that the alumina phase transitions and powders characteristics were modified when the step of activation mechanical was introduced.

Introduction The production of dense ceramic components, with submicrometer grains or even

nanometrics and flawless microstructure is still one of the most challenging objectives in the development of ceramic materials. The possibility of improvement of the mechanical properties (mechanical resistance, and wear resistance) due to obtaining of these ceramic and the different routes of fine powder production or even nanometrics powders has been motivating the increase in the researches of these materials [1].

Alpha alumina (α-Al2O3) is one of the well-established ceramic materials for many structural and functional applications, because of its excellent mechanical, electrical, and chemically inert properties [2,3]. The performance of alumina ceramics in these applications is dependent on their microstructures, which are controlled by the processing routes and the processing parameters. The preparation of fine and sintering-reactive alumina powders is probably one of the most important steps for production alumina ceramics with controlled microstructure [2,4].

The obtaining of α− Al2O3 powders, in the past, were restricted to a defined size, due to the inherent growth of the crystal resulting from the phase transformation during the calcinations process [5] for the formation of the thermodynamically stable α-form. Various researches have been conducts to understand and identify various phases in alumina. It is widely believe that alumina exists in more than 15 different crystallography forms [6], and these forms can be undergo a variety of transitions until the stable phase α− Al2O3 is formed at high temperature [7]. The sequence of the transition alumina during heating depends strongly on the starting

materials and how this starting material was obtained [8]. High calcinations temperature results in a considerable degree of particle coarsening and particle agglomeration [2,9].

Recently, new routes of synthesis are been proposed for production the stable phase α− Al2O3 without crystals growth. Among these routes can be mentioned production of nanocrystalline α−Al2O3 using sucrose [7], synthesis of nanometric powders α−Al2O3 prepared by pyrolysis using triethanolamine [10], the preparation of nanometrics powders of α−Al2O3 obtained by the calcination controlled with addictive chemical [11], addition of a fraction of ultra fine alumina crystallites as seed materials [3] and use of mechanical activation [2].

The synthesis of fine or nanometric powders by mechanical milling, mechanical alloying and mechanochemical processing has been widely studied [12]. Benjamim [13] was the pioneer to establish mechanical alloying for the synthesis of metallic alloys and intermetallics. Such procedures have been exploited for several others materials [14,15,16]. In this process, collisions between the grinding media result in continual fracture and welding of the powder charge thus dynamically maintaining a high reaction interface area. As a consequence, solid state chemical reactions that would normally be inhibited by low diffusion rates can be promoted at ambient temperature without the need for externally applied heat [17]. There are many phenomena that can be associated with the mechanical alloying: refinement in crystallite size; creation of point, surface and lattice defects, amorphization of crystalline phases, phase transformation, crystallization for amorphous state and chemical reactions [2].

In this work it was studied the possibility the production of α−Al2O3, without grain growth, using the Pechini method associated to mechanical activation. The Pechini method consists of the formation of a polymeric resin between a metallic acid chelate and polyhydroxide alcohol by polyesterification and in the combustion of the same ones for removal of the organic material, what results in a powder finely divided [18]. Experimental Procedure

In this study, aluminum nitrate (Al(NO3)3.9H2O was used as the starting material. Aluminum nitrate was dissolved in citric acid, in the proportion of 3:1, in constant agitation and heating. Then, it was added ethylene glycol, in the ratio of 40:60 (citric acid: ethylene glycol).

The resin obtained was calcined at 500oC for 1 hour and then it was characterized through simultaneous differential thermal analysis (DTA) and thermogravimetric analysis (TG) (model STA409EP – Netzsch), at a heating rate of 2oC/min, using flux of nitrogen, until the temperature at 1300oC.

The powders obtained after calcination were submitted to mechanical activation. For this, the powder was loaded in a hardened steel vial with hardened steel balls of 10 mm in diameter. The vial was placed in a glove box of nitrogen and then sealed after the air was drawn out and the vial was subsequently filled with purified nitrogen. The mechanical activation was performed in a high energy shaker mill (model SPEX 8000 mixer/mill) using a ball to powder mass ratio of 8:1, for various time periods (5, 10 and 15 hours). The mechanical activated powders were characterized by X-ray diffraction (Model XRD-6000, Shimadzu) for phase analysis, scanning electron microscopy (Model SSX-550, Shimadzu) to observe particle morphology and infrared spectroscopy (Model 4700, Nicolet) to examine thermal-bonding changes. The mechanical activated powders and the powders obtained after calcination were calcined in different temperatures, for 2 hours, for comparison.

Results and Discussion The differential thermal analysis (DTA) and thermogravimetric analysis (TG) for the resin

calcined at 500oC are showed in figure 1. It is observed an endothermic peak in temperatures at around 80-100oC, that is associated a weight loss. This peak is caused by the evaporation of absorbed water and it is not pronounced, because the resin was calcined at 500oC. At temperature around 510oC, there is a sharp peak exothermic accompanied of a great weight loss that is probably caused by burning off of organic materials. The two exothermic peaks, at 830 and 1100oC, that it is not accompanied by loss weight, can be associated to alumina phase transformations, where the first is probably the formation of the γ-Al2O3 and the other it is the transformation for α-Al2O3. These observations can be confirmed by X-ray diffraction, as shown in figure 2, when resin was calcined at 500, 900 e 1100oC, for two hours. At 500oC, the calcined resin presents an amorphous structure. After calcination at 900oC, it is observed the presence of γ-Al2O3 and at 1100oC, the stable α-Al2O3 is established.

200 400 600 800 1000 1200

0,0

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DTA

(µV/

mg)

Temperature(oC)

50

60

70

80

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100

exo

weight (%

)

10 20 30 40 50 60 70 80

γ−Al2O3

1100oCInte

nsity

2θ

500oC

900oC

α−Al2O3

Figure 1. DTA/TG curves of the resin Figure 2. X-ray diffraction of calcined

calcined at 500oC. powders at 500, 900 and 1100oC.

The figure 3 shows X-ray diffraction of powders calcined at 500oC and submitted at mechanical activation for various time periods. It ca be observed that the mechanical activation promote the crystallization and the formation to α-Al2O3 but it is possible to still observe peaks of γ-Al2O3. It can also be seen that, with the increase of the time of milling, the peaks become a little more intense and narrow, what can mean an increase in the growth of the crystals and a larger crystallization of the powders.

The figure 4 presents the infrared spectra of the calcined powder at 500oC and submitted at mechanical activated for 5, 10 and 15 hours. Santhija et al. [19] attribute the 3451 cm-1 broad bands to hydrogen-bonded surface hydroxyl groups. The absorption band at 1646 cm-1 is attributed to moisture in the sample. Intense broad bands appear in the vicinity of 600-800 cm-1 and a band appears at 467 cm-1, with the AlO stretching vibration. These spectra shows that the mechanical activated of the powders promote a phase transformation of the α-Al2O3 for the γ-Al2O3, according to table I.

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non milled

γ-Al2O3

α-Al2O3

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10 hours

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Inte

nsity

2θ

4000 3000 2000 1000

15 hours

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% T

wavenumber(cm-1) Figure 3. X-ray diffraction of powders calcined Figure 4. FTIR spectra of the powders calcined at 500oC and submitted at mechanical activated at 500oC and submitted at mechanical activated for 5, 10 and 15 hours. for 5, 10 and 15 hours.

Figure 5 show the X-ray diffraction of powders submitted a high energy milling for 5 and 10 hours and calcined at 900oC and a powder not milled calcined at 900oC. It can be observed that, after calcination there are no more γ-Al2O3 in the powders that were submitted the mechanical activation and the peaks of α-Al2O3 are well defined.

10 20 30 40 50 60 70 80

γ−Al2O3

Inte

nsity

2θ

non milled

5 hours

10 hours

T=900oCα−Al2O3

Figure 5. X- ray diffraction of powders not milled and milled for 5 and 10 hours calcined at

900oC.

A possible explication for the phase transformation of the alumina using mechanical activation is given for Wang and Wong [2] which suggested that the repeated mechanical impacts led to a strained framework of cubic close-packed O atoms which a disordered Al-atom sublattice, which can easily be rearranged into the hexagonal close-packed O-atom framework of α-Al2O3. This observation is consistent with what has been suggested by Liao and Senna [22] that the mechanochemical dehydration of hydroxide-ion bearing (OH-bearing) compounds is initiated by mechanical energy.

Table I. IR Spectra of transformation of γ-Al2O3 to α-Al2O3. Phase/Transformation IR spectra mode

Bound hydroxyls (O-H groups)

Al-O vibration

γ-Al2O3 [20] 3780-3880, 3740-3750, 3730-3740, 3700-3720, 3690-3710, 3660-3580,

~3360 cm-1

Al(IV)-O - 700-200 cm-1 Al(VI)-O – 900-700 cm-1

α-Al2O3 450-460 cm-1 [19, 21] (isolated AlO6 octrahedra in

530-400cm-1) 640-590 cm-1

(two strong bands near 650 and 600 cm-1 are condensed AlO6

octahedra)

In the figure 6, it is showed micrographs obtained in scanning electronic microscopy of the powder that was subjected to 5, 10 and 15 hours of mechanical activated and a micrograph of a α-Al2O3 powder not milled and calcined at 1100oC. It is possible to observe in the micrograph of the powders mechanically activated, the presence of agglomerates of spherical particles of α-Al2O3 in the range of 80-90 nm. In the powder obtained by Pechini method, not submitted the activation mechanical and calcined at 1100oC (figure 6(d)), it is observed particles partially sintered, forming necks among them. Calcination at such high temperature resulted in extensive particle coarsening and agglomeration, as evidence by the formation of strong necking between neighboring alumina powders [2].

(a) (b)

(c) (d)

Figure 6. SEM micrographs of powders calcined at 500oC and after milled for: (a) 5 hours; (b) 10 hours, (c) 15 hours and (d) powder obtained by Pechini method and calcined at 1100oC

Conclusion Using the high energy milling for mechanical activation of the powders obtained by the calcination of the Pechini resin at 500oC, it was possible to promote the phase transformation for the phase stable α-Al2O3, without calcination need at high temperatures. The obtaining of α-Al2O3 powders starting from the Pechini method occurred just after calcination to 1100oC. The alumina powder submitted the mechanical activation exhibited agglomerates with refined particle size in the range of 80-90nm, while powders of alumina resultants of the direct calcination resulted in highly aggregated particles.

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