Control of structural type and particle size in alumina synthesized by the spray pyrolysis method

SOLID STATE ELSEVIER

Solid State Ionics 101-103 (1997) 197-203

IONICS

Control of structural type and particle size in alumina synthesized by the spray pyrolysis method M. Vallet-Regf ~'b'*, L.M. Rodrfguez-Lorenzo a, C.V. Ragel a, A.J.

S a l i n a s a,

J . M . G o n z ~ , l e z - C a l b e t b'c aDpto. Qufmica lnorgdnica y Bioinorgdnica, Facultad de Farmacia. UCM. 28040 Madrid, Spain hlnst. Magnetismo Aplicado (RENFE-UCM), Apdo. Correos 155. Las Rozas, 28230 Madrid, Spain "Dpto. Qu[mica lnorgdnica, Facultad de Qu[micas. Universidad Complutense, 28040 Madrid. Spain

Abstract Fine particles of aluminum oxide (amorphous, a, 3' and 0) have been synthesized by spray pyrolysis as a function of both in situ and annealing temperatures and nature of precursors used in the preparation method. A scanning electron micrograph study shows as-received materials constituted by filled spherical particles, the ulterior annealing leading to the formation of small crystallites on the precursor spheres. Keywords: Alumina; Synthetic pathway; Spray pyrolysis; Ceramics Materials: AI20 ~

I. Introduction The general use of Al203 spreads at the present time into many areas of modern industry [1]. Alumina has been used for aluminum production, as an abrasive, in heterogeneous catalysis, as an adsorbent, as a biomaterial and as a ceramic material [2,3]. Porous alumina ceramics are considered to be very important because of high strength, high chemical resistance and high thermal resistance [4]. Presently, there are at least seven known metastable phases of alumina also designated as transition aluminas (~, X, K, xl, 0, ",/ and g), only one, the o~-form, being thermodynamically stable. The usual *Corresponding author. Fax: vallet @eucmax.sim.ucm.es

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0167-2738/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0 1 6 7 - 2 7 3 8 ( 9 7 ) 0 0 3 3 7 - 8

way to obtain o~-AI203 is by high temperature calcination of hydroxides, oxyhydroxides, or transition aluminas. Temperatures in the range 11001200°C [5] or even higher [61 are necessary to transform these aluminas into the stable form. The synthesis of metal oxides with specific properties begins with molecular precursors which must be transformed into the product. Aerosol decomposition or spray pyrolysis is a powerful route for ceramic powder production [7]. In previous works, Okada et al. [4,8] have prepared AI203 powders by the spray pyrolysis method with the furnace of the pyrolysis apparatus working in the range of 7001000°C. It is now assumed that the alumina support plays a decisive role in the activity displayed by the catalyst, the properties exhibited by the alumina support itself depending on both the nature of the

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Table 1 Pyrolysis conditions of samples obtained

precursor and thermal treatment since low temperatures are required to get a relatively high specific surface. On the other hand, high-density A1203 is a bioceramic widely used clinically for load-bearing hip prostheses and dental implants [3]. We describe in this paper, the synthesis of alumina by pyrolysis of an aerosol and the experimental conditions necessary to get either alumina at low temperature with relatively high specific surface or a-alumina at relatively high temperature with low specific surface.

Sample

Precursor solutions

Pyrolysis temperature (°C)

C900

AICI 3 • 6H20 (0.1 M) AIC13 - 6H20 (0.1 M) A12(SO4) 3 • 18H20 (0.05 M) AI(NO3) ~ • 9H20 (0.l M)

900

C400 $400 N400

400 400 400

2. Experimental 2.1. Particle preparation

The in situ materials so obtained were annealed by the following several thermal treatments at temperatures between 850°C and 1400°C.

A1203 fine particles were synthesized by pyrolysis of an aerosol generated by ultrahigh frequency spraying of different aqueous solutions: AICI 3 • 6H20 (0.1 M); A12(SO4)3.18H20 (0.05 M) and AI(NO3) 3 .9H20 (0.1 M) according to the following experimental conditions: furnace temperatures: 400°C and 900°C; gas flow: 4.5 l min ~; frequency: 850 kHz; aerosol flow: 0.8 ml min-l; carrier gas: pure air. The processing equipment is schematized in Fig. 1. A full description of this procedure is given in Ref. [9]. Table 1 presents the samples and the conditions of their elaboration.

2.2. Particle characterization Samples were characterized by powder X-ray diffraction (XRD) on a Siemens D-5000 diffractometer (CuKa radiation). The crystallite average size was deduced from the full width at half maximum (FWHM) of the XRD reflexions obtained in a X'Pert MPD diffractometer. Particle morphology, size and shape were analyzed by Scanning Electron Microscopy (SEM) on a JEOL 6400 microscope.

PYROLYSIS METHOD PULVERIZA'flONZONE 2

PYROLYSISZONE

F-.]OiAUS1 ZONE

4

V/////////////////A

1.- Ultrasonic 2.- Constant 3.-

Air

4.-

Electric

pulverization level container

flow

pot and solution

6.- Tungsten wire 7.- Tungsten wire holder 8.- Stainless steel collsotlon plate 9.- Heater

furnace

5.- Quartz tube

Fig. l. Schematic representation of the processing equipment.

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Chemical analysis was carried out by Energy Dispersive Spectroscopy (EDS) on a LINK AN 10000 system. Thermal studies (TGA and DTA) were carried out on a TG-DTA 320 Seiko thermobalance with a heating rate of 5°C rain ~, in the range 30-1250°C under still air. Alumina crucibles were used with Al203 as reference. The specific surface area was obtained by the BET method on a Porosimeter 4000 (Milestone 200 report generator).

3. Results and discussion

3.1. Pyrolysis temperature influence fi~r aluminum chloride precursor XRD data of both C400 and C900 in situ samples are characteristic of amorphous materials (Fig. 2). However, the diagram of the material obtained at 900°C also shows a unique maximum corresponding to the most intense diffraction peak of a metastable alumina. When C400 was used as starting material, crystalline and well characterized ~x-A1203 was obtained after annealing at 1200°C for 1 h. However, the C900 sample is a bad precursor for the synthesis of pure ~-AI_,O 3 (see Table 2). It is worth recalling that Okada et al. [8] have stated that 3'-A1203 can be obtained either at a pyrolysis temperature of 900°C or by annealing, at 840°C, a sample obtained at a pyrolysis temperature of at least 700°C. In our case, the structural study, as a function of the annealing temperature, indicates that 3'-A1203 can be stabilized as a single phase (see Table 2) starting from a pyrolysis temperature of only 400°C. Ulterior annealing shows that a 3'--->c~ structural transition starts at l l00°C, but a single c¢-A1203 phase is obtained only at 1200°C. A different behaviour is observed in the case of the C900 sample. After the decomposition temperature, i.e., 900°C, a 3' and 0-A1203 mixture phase appears. At 1150°C, the o~ phase also appears (see Table 2) but only a 0 and ~x-Al203 mixture phase remains after 1250°C. It is worth mentioning that up to now the 0-phase has been described as a meta-

Fig. 2. Scanning electron micrographs of (a) C400 and (b) C900 in situ samples. The insets show the powder XRD pattern of each one (20 = 5-80).

stable phase, but in this case the increasing of the annealing temperature leads to the appearance of such a phase, although a single phase has not been isolated. On the other hand, chemical analysis by EDS indicates the presence of chlorine for both samples but with a smaller amount in C900. The thermogravimetric analysis shows that the precursor decomposition occurs at 800°C for the C400 sample and 900°C for the C900 material, the amount of precursor in the in situ sample being smaller in C900 since the TG curve shows a weight loss of only 6% while in C400 such a weight loss is 29%. According to these data, the EDS results for C900 could be surprising since a certain amount of chlorine still

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Table 2 Phases identified by X-ray diffraction Sample

XRD 'in situ'

C900 C400 $400

Amorphous Amorphous Amorphous + Alz(SO4)3 Amorphous

N400

A1203 phases identified by XRD after annealing for 1 h at different temperatures (°C) 850

900

1100

1150

1200

1250

3'

3' 3' 3'

3' + (0)~ 3' + (c0 3'

(3') + 0 +c~ (3') + a 3'

0+a a 3' + (ct)

(0)+a a ct

3' + a

Minority phases are indicated in brackets. remains at the decomposition temperature, but this is because the aerosol permanence time in the furnace is lower than 2 s. The SEM study of in situ materials C400 and C900, obtained from aluminum chloride precursor but with different pyrolysis temperature, shows that they are constituted by full spherical particles with a diameter ranging from 0.1 to 2 Ixm (Fig. 2). However, the particles obtained at 900°C present a great number of surface irregularities as a consequence of the higher pyrolysis temperature (Fig. 2b). The smaller pyrolysis temperature presents obvious advantages from the technical point of view. In addition, as it can be observed in Table 2, the necessary temperature to obtain e~-A120 3 as a pure phase is lower for the C400 sample than for C900. So, 400°C has been chosen as the adequate temperature to study, in a comparative way, the influence of different precursors on the textural properties of the materials. 3.2. I n f l u e n c e o f p r e c u r s o r

The XRD pattern of the N400 sample is characteristic of an amorphous material (Fig. 3a, inset). The thermogravimetric study shows a weight loss of 31% from 30°C up to l l 0 0 ° C , where, in agreement with previous data [10], the precursor decomposition is complete. The XRD pattern of the $400 sample shows an amorphous background together with the diffraction maxima characteristic of anhydrous aluminum sulphate (Fig. 3b, inset). This is confirmed by chemical analysis (EDS) which indicates the presence of sulphur in this sample. From the TG curve corresponding to $400, it can be observed that the total

Fig. 3. Scanning electron micrographs of (a) N400 and (b) $400 in situ samples. The insets show the powder XRD pattern of each one (20 = 5-80).

precursor decomposition takes place in the range 30-900°C with a weight loss of 73%, which supposes a very low yield in the alumina synthesis process.

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Once again, in situ $400 and N400 samples were annealed in order to follow both the textural evolution and structural transition. The powder XRD study shows (see Table 2) that after the precursor decomposition temperature of $400 (900°C), the yAI203 phase is obtained, a y - ~ t x structural transition appearing at 1200°C. It is worth mentioning that the ~/--~et transition has been described at different temperatures as a function of the previous material conditions [2,5,6,11,12]. At 1250°C, only et-A1203 is obtained. Moreover, the annealing of N400 indicates that a y and et mixture phase appears at I100°C, a e~-A1203 single phase being stabilized at 1150°C (see Table 2). To determine the morphology of the particles of the samples $400 and N400, a SEM study was performed. It can be seen (Fig. 3) that they are formed by filled spherical particles showing a dispersion of the average diameter ranging between 0.1 and 2 ~m, which are the same values observed for C400 sample (Fig. 2a). It can be noted that N400 spherical particles seem to be more dense. 3.3. S i n t e r i n g p r o c e s s

The powder XRD study shows that the annealing at 1250°C of C400, $400 and N400 samples leads, in all cases, to et-A120 3 (Fig. 4, insets). Fig. 4 shows the SEM micrographs corresponding to these samples. A similar situation can be observed in both C400 and $400 treated at 1250°C, since the sintering process leads to the formation of small crystallites formed on the precursor sphere. However, this process seems to be slower in the N400 sample which can be attributed to the different starting texture (see Fig. 3a) of the in situ samples at 400°C where, as previously mentioned, more dense particles were obtained by using nitrate as precursor. On the other hand, the annealing of the C900 sample at 1250°C leads to spheres formed by small crystallites (Fig. 5) as previously observed in Fig. 4a Fig. 4b, but also other kind of spheres, marked with arrows, showing a different texture can be seen. The sintering process still progresses as a function of the annealing temperature. As an example, we will show here the results obtained for the C400 sample. When this sample was heated at 1400°C for 1 h, an

Fig. 4. Scanning electron micrographs of (a) C400, (b) $400 and (c) N400 samples annealed at 1250°C/1 h. The insets show the powder XRD pattern of each one (20 = 5-80).

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Fig. 5. Scanning electron micrograph of the C900 sample annealed at 1250°C/1 h. The inset shows the powder XRD pattern (20 = 5 -

80).

evolution of the crystal size was clearly observed (Fig. 6a) but some of the particles still kept their spherical form. Finally, when C400 was annealed at 1400°C for 24 h (Fig. 6b), the cx-A1203 crystallites were found to be bigger and practically independent. The crystallization process has also been followed by XRD. From the width of oL-A1203 reflexion (300) corresponding to the C400 sample annealed at 1250°C/1 h and 1400°C/24 h, a crystallite size three times higher for the sample annealed at 1400°C/24 h can be deduced. Thus, an average size of the crystallites of 0.29 ~m is obtained when the annealing temperature is 1250°C and 0.96 ~zm at 1400°C for 24 h. It can be observed that with this experimental procedure, the crystallite size can be easily fitted to the optimum size for a given application. The evolution of both particle and crystallite size is reflected in the specific surface area of every sample since it decreases with annealing temperature increasing. Thus, a value of 36 m 2 g-J is obtained for the C400 in situ sample, while the sample annealed at 1250°C for 1 h, i.e., when the a-phase is stabilized, shows a specific surface area of 9 m 2 g - ~. This suggests that alumina prepared at low temperatures can be a good candidate for applications in catalysis, while higher temperatures are required for alumina used as either dense ceramic or biomaterial.

Acknowledgements i ¸¸

/

Financial support of CICYT through research projects No. MAT 95-0642 and MAT 96-0919 is acknowledged. We also thank A. Rodrfguez (Centro Microscopfa Electrrnica, U.C.M.) for valuable technical assistance.

References

Fig. 6. Scanning electron micrographs of the C400 sample annealed at (a) 1400°C/1 h and (b) 1400°C/24 h.

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