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Journal of Physics and Chemistry of Solids 68 (2007) 2349–2352 www.elsevier.com/locate/jpcs
Microstructural studies of nanocrystalline a-alumina powder produced from Al13-cluster Megat Harun Al Rashid Megat Ahmada,, Abdul Aziz Mohameda, Azmi Ibrahimb, Che Seman Mahmooda, Edy Giri Rachman Putrac, Rafhayudi Jamroa, Razali Kasima, Muhammad Rawi Muhammad Zina a Materials Technology Group, Industrial Technology Division, Agensi Nuklear Malaysia, 43000 Kajang, Selangor, Malaysia Microelectronics and Nanotechnology Program, Telekom Malaysia R&D, UPM-MTDC, 43400 Serdang, Selangor, Malaysia c Neutron Scattering Laboratory, Centre of Technology for Nuclear Industrial Materials Technology, Badan Tenaga Nuklir Nasional (BATAN), Puspiptek Serpong, Tangerang 15314, Indonesia
b
Received 23 February 2007; received in revised form 6 July 2007; accepted 6 July 2007
Abstract Nanocrystalline alumina powder was produced from calcinations of Al13-oxalate precipitates at 1100 1C. A nearly normal distribution of agglomerated alumina powder was obtained with an average particle size of about 1 mm. XRD measurement confirmed that the alumina produced was of high purity and crystalline a-phase. Microstructural features of both the precipitates and alumina obtained were studied using the small angle neutron scattering (SANS) technique. SANS examinations show the formation of microstructures in the alumina powder of mass fractals type with dimension of 2.8 indicative of low intra-granular porosity. r 2007 Elsevier Ltd. All rights reserved. Keywords: A. Ceramics; A. Electronic materials; B. Chemical synthesis; C. Neutron scattering; D. Microstructure
1. Introduction Nanostructured materials contain nanoscale structures that can influence properties of the materials. These nanostructures can be adapted synthetically to produce materials with new and distinctive properties. Fully dense materials with nanosize features are most important for electronic, mechanical, magnetic, optical and thermodynamics applications [1]. In solid-state materials, nanosize grains increase the fraction of the structure related to interfacial regions and therefore increase the interfacial surface area. In the case of electronic properties, this relates to the space-charge region between the interfacial regions of the material grains. Therefore, small grain size spacecharge regions at interfaces can occupy a substantial
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volume fraction of the solid and can be produced to achieve specific electronic properties [2–4]. Alumina is widely used in catalysis, refractories, abrasive and structural applications. The a-phase of alumina is the most thermodynamically stable phase and is highly crystalline [5]. It is also one of the important and most researched ceramic materials for electronics. In particular as substrate in electronic packaging wherein grain characteristics can influence microwave dielectric properties such as dielectric loss that can be caused partly by higher levels of porosity [6,7]. Producing a-alumina with definite physical properties is of important interest, especially the nanocrystalline form. However, nanocrystalline a-alumina powder is very difficult to synthesize because the a-phase can only be achieved by high-temperature calcination at temperatures higher than 1200 1C [8,9]. This can cause grain growth and agglomeration because of the sintering effect [10,11]. These agglomerates can be viewed as fractals with porous network like structure. The characteristic of the agglomerates has a major effect on the powder
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properties particularly on the sintering process of the substrates. Various techniques have been developed for producing nanocrystalline a-alumina powder, which include gasphase synthesis [12,13], liquid-phase [14–16] and organicmedia reaction [17,18]. Wang and Muhammed [19] have demonstrated that nanoscale a-alumina can be produced from the precipitation of [AlO4Al12(OH)24(H2O)12]7+ or Al13-cluster at a relatively low temperature of 930 1C. This method is of great simplicity with the resultant a-alumina powder having particle sizes in the range 100–200 nm. The Al13-cluster can simply be produced by controlled hydrolysis of Al3+ aqueous solution using a base, with the final molar ratio of Al to the base between 2.2 and 2.6 [20–23]. In this work, we investigate the microstructural features and fractal behavior in alumina powder formed after calcinations of the complex Al13 precipitated with oxalic acid. The objectives are to synthesize an alumina precursor that can be transformed to a-alumina at relatively lower calcination temperature to obtain nanocrystallinity with low porosity, and to evaluate the powder properties for suitability as electronic packaging substrate. 2. Experimental procedure The Al13-cluster and resulting a-alumina were synthesized based on a method as reported in Ref. [19]. The Al13cluster solution was prepared using 100 mL of 0.25 M aluminum nitrate solution that was heated to 80 1C. A 240 mL aliquot of 0.25 M sodium hydroxide solution was then added drop-wise to the heated aluminum nitrate solution under vigorous stirring. Upon completion of adding the hydroxide solution, the resulting transparent mixture was left to cool to room temperature. The pH of this mixture was 3.90. Precipitation of the Al13-cluster was carried out using oxalic acid as the precipitating ligand. A 125 mL aliquot of 0.10 M oxalic acid solution was added drop-wise into the Al13-cluster solution under vigorous stirring. A white precipitate was formed. This precipitate was later filtered, dried overnight and eventually calcined at 1100 1C for 4 h to obtain the alumina powder. The particle size distributions of both precipitates and alumina powder were determined using the laser scattering technique (Microtrac X100). X-ray diffraction (XRD) analysis was carried out to confirm the existence of the aphase using a Bragg–Brantano (PANAnalytical) diffractometer with Cu anode. The instrument was operated in the following conditions—power: 40 kV, 40 mA; scanning parameters: 2y range from 21 to 1441; typical 2y step size of 0.021; measuring time of 25 s per step. Transmission electron microscope (TEM; Philips C12) with operating voltage 100 kV was used to image the alumina and the precipitates. Small angle neutron scattering (SANS) studies were carried out on samples of the alumina and the precipitates. Measurements were done using the 36 m SANS instrument (SMARTer) at Neutron Scattering Lab (NSL) at BATAN
(Serpong, Indonesia). Samples were packed into quartz cells of 2 mm thickness, and a small circular sample area of 10 mm in diameter was exposed to the neutron beam. The scattered elastic part, due to inhomogeneities in the sample microstructures, was registered on a two-dimensional position sensitive detector (128 128 pixels). Data were collected on a momentum transfer range from about Q ¼ 0.06 to 2.5 nm1 using neutron with wavelength of 0.39 nm in three samples to detect positions of 1.3, 4.0 and 13.0 m (Q ¼ 4p sin 2y/l, where 2y is the scattering angle, and l is the wavelength of incident neutron radiation). The efficiency of the detector was calibrated using water as scattering standard. The scattering data were corrected for empty beam, background counts and detector efficiency, and then radially summed with Q ¼ 0 as the center. 3. Results and discussion The agglomerated particle size distribution of the alumina powder and of the Al13-oxalate precipitates by laser light scattering technique are shown in Fig. 1. The particle distribution of Al13-oxalate precipitates shows that the agglomerated particle size range is about 0.6–9.0 mm, with an average particle size of about 2.45 mm. A shoulder peak was found at about 1.19 mm with the principal peak at about 2.77 mm. The distribution of alumina also shows a similar profile of two peaks with the shoulder peak found at about 0.43 mm and the principal peak at about 1.12 mm with distribution range of 0.15–7.00 mm. The alumina agglomerated particles sizes mostly fall in the range that is usually used for electronic substrate with low dielectric loss [24,25]. The shift of alumina distribution shape to a
Fig. 1. Agglomerated size distribution of Al13-cluster oxalate precipitates and alumina.
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lower size range indicates the reduction of agglomerated particle size after calcinations and this signifies that major agglomeration came into effect during the precipitation process itself. Fig. 2 shows the XRD pattern of alumina, obviously confirming the a-phase with sharp peaks and demonstrating that the alumina produced is highly crystalline. Average grain size of alumina calculated from XRD peak broadening was found to be about 42.4 nm. Fig. 3 shows TEM images of both the Al13-oxalate precipitates and alumina confirming that the magnitude of these agglomerated sizes is consistent with the results from particle size analyzer, and particularly the individual particle size of alumina is found smaller and less agglomerated. Fig. 4 shows the SANS profiles on log–log scale of intensity against scattering vector, Q, for both the alumina and Al13-oxalate precipitates. The scattering intensity of alumina is about hundred times higher in magnitude than the intensity of the Al13-oxalate precipitates. Fig. 4 also shows that alumina displays linearity over a range of Q with a break at Q near 0.14 nm1, roughly marked by the change of slope in the curve. The change in linearity at Q ¼ 0.14 nm1 is characteristic of an agglomeration of particles with sizes of the order 2p/Q ¼ 44.9 nm, which value is close to the XRD measurement.
Fig. 2. XRD pattern of alumina powders obtained.
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As there is no other alumina phase significantly present in the sample, the shift to higher Q value for alumina when compared to the Al13-oxalate precipitates indicates the formation of additional grain/pore morphology [26,27] with most range with the profile following the Q4 Porod regime. At low Q in Fig. 4, power-law dependence (I–QD) is observed (Q3 line shown as guidance) with fractals dimension D2.8 suggesting the formation of intragranular porosity and mass fractals [28,29]. Intra-granular porosity relates to pore formation between grains of alumina whereas mass fractals are structures that are formed by the aggregation or packing of particles in such a way that the structure exhibits invariance in scale giving fractal characters [30]. Herewith, SANS measurement gives a semi-quantitative assessment on the intra-granular porosity of the alumina powder, which cannot be accessed by conventional methods like BET gas adsorption and mercury porosimetry. In this case, the fractal primary particles are the alumina grains. The value of D is usually
Fig. 4. The log–log plot of SANS profile for both Al13-cluster oxalate precipitates and alumina.
Fig. 3. TEM image of of Al13-cluster oxalate precipitates (A) and alumina (B).
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between 1 and 3, and the higher the value of D, the less porous the structure. 4. Conclusions High-purity nanocrystalline a-alumina can be produced from calcinations of complex Al13-oxalate precipitates at much lower calcination temperature of 1100 1C. This resultes in almost normal distribution of agglomerated alumina powder in a reasonably narrow particles size range as lower calcination temperature reduces the sintering effect. Even though intra-granular pore formation occurred after calcinations, the alumina produced has low fractal structure with fractal dimension of about 2.8, indicating very low porosity. The alumina powder, is therefore, considered suitable to be used as nanoscale dielectric materials in electronic materials substrate as higher level of porosity results in dielectric loss. Acknowledgments The authors would like to thank Telekom Malaysia Research & Development Sdn. Bhd. and Ministry of Science, Technology and Innovation, Malaysia, for financing this research work through the Forum for Nuclear Cooperation Asia, Japan network. References [1] C.Q. Sun, Prog. Solid State Chem. 35 (2007) 1. [2] H. Gleiter, J. Weissmu¨ller, O. Wollersheim, R. Wu¨rschum, Acta Mater. 49 (2001) 737. [3] H. Gleiter, Nanostruct. Mater. 6 (1995) 3. [4] H. Gleiter, Acta Mater. 48 (2000) 1.
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