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Characterization of high-energy milled alumina powders

Caracterização de pós de alumina submetidos a moagem de alta energia

Abstracts

The utilization of reactive high-energy milling has been reported for the synthesis of ceramic powders namely, metal oxides, carbides, borides, nitrides or mixtures of ceramics or ceramic and metal compounds. In this work, high-energy milling was used for reduction of alumina powders to nanometric particle size. The ceramic characteristics of the powders were analyzed in terms of the behavior during deagglomeration, compaction curves, sintering and microstructure characterization. It was observed that the high energy milling has strong effect in producing agglomeration of the nanosized powders. This effect is explained by the high-energy impact of the balls, which may fracture particles or just cause the particles compacting. In this case, strong agglomerates are produced. As the powder surface area increases, stronger agglomerates are produced.

alumina; nanometric particle; high-energy milling


Tem sido amplamente divulgada a utilização da moagem reativa de alta energia para a síntese de pós cerâmicos de óxidos de metais, carbetos, boretos, nitretos ou misturas de compostos cerâmicos ou compostos cerâmicos e metálicos. Neste trabalho, a moagem de alta energia (não reativa) foi utilizada para a redução de pós de alumina para partículas de dimensões nanométricas. As características cerâmicas dos pós obtidos foram analisadas a partir de resultados de comportamento durante a desaglomeração, curvas de densificação, sinterização e caracterização de microestrutura. Observou-se que a moagem de alta energia tem forte efeito de aglomeração dos pós com partículas em dimensões nanométricas. Esse efeito é explicado pelo impacto de alta energia das bolas, os quais podem fraturar as partículas ou apenas causar a compactação das mesmas. Nesse último caso, que sempre ocorre, são formados aglomerados de alta resistência. O aumento da área superficial do pó produz aglomerados mais resistentes.

alumina; partículas nanométricas; moagem de alta energia


Characterization of high-energy milled alumina powders

(Caracterização de pós de alumina submetidos a moagem de alta energia)

Laudo Reis, Walter J. Botta Fo

Departamento de Engenharia de Materiais, Universidade Federal de S. Carlos,

Rod. Washington Luiz, Km 235, C.P. 676, 13565-905, S. Carlos, SP, Brazil

email: tomasi@power.ufscar.br

Abstract

The utilization of reactive high-energy milling has been reported for the synthesis of ceramic powders namely, metal oxides, carbides, borides, nitrides or mixtures of ceramics or ceramic and metal compounds. In this work, high-energy milling was used for reduction of alumina powders to nanometric particle size. The ceramic characteristics of the powders were analyzed in terms of the behavior during deagglomeration, compaction curves, sintering and microstructure characterization. It was observed that the high energy milling has strong effect in producing agglomeration of the nanosized powders. This effect is explained by the high-energy impact of the balls, which may fracture particles or just cause the particles compacting. In this case, strong agglomerates are produced. As the powder surface area increases, stronger agglomerates are produced.

Keywords: alumina, nanometric particle, high-energy milling.

Resumo

Tem sido amplamente divulgada a utilização da moagem reativa de alta energia para a síntese de pós cerâmicos de óxidos de metais, carbetos, boretos, nitretos ou misturas de compostos cerâmicos ou compostos cerâmicos e metálicos. Neste trabalho, a moagem de alta energia (não reativa) foi utilizada para a redução de pós de alumina para partículas de dimensões nanométricas. As características cerâmicas dos pós obtidos foram analisadas a partir de resultados de comportamento durante a desaglomeração, curvas de densificação, sinterização e caracterização de microestrutura. Observou-se que a moagem de alta energia tem forte efeito de aglomeração dos pós com partículas em dimensões nanométricas. Esse efeito é explicado pelo impacto de alta energia das bolas, os quais podem fraturar as partículas ou apenas causar a compactação das mesmas. Nesse último caso, que sempre ocorre, são formados aglomerados de alta resistência. O aumento da área superficial do pó produz aglomerados mais resistentes.

Palavras-chave: alumina, partículas nanométricas, moagem de alta energia.

INTRODUCTION

The use of high-energy reactive milling for the synthesis ceramic powders, namely metal oxides, carbides, borides, nitrides or mixtures of ceramics or ceramic-metal compounds has been well reported in the literature [1-8]. Important characteristics of the obtained powders, as consequence of the milling process, include small crystallite size, high specific surface area, high deformation and amorphization. Such characteristics are also present in high-energy ball-milled single phase powders. For a brittle material such as alumina, the process of fracture of primary particles are expected to predominate over deformation and forging (present in mechanical alloying), producing nanometric particles and crystallites.

The use of nanometric powders in ceramics is becoming increasingly attractive due to the interesting properties associated to the nanostructure of dense ceramic or ceramic matrix composites obtained from these powders [9-11] as well as the improvement of sinterability [12].

The ceramic processing of nanometric powders has also beneficial peculiarities due to the high specific surface area and the increased importance of surface tension. Among these are the increase in surface and grain boundary diffusion and in viscous flow. The consequence is an increased kinetics of solid state reaction, sintering, recrystallization and grain growth. Detrimental peculiarities come from the strong particles agglomeration before and during forming, friction during pressing, surface contamination and decrease of pressureless sinterability of composites with nanosized inclusions [10,12].

Nanosized powders obtained from reactive milling or non-reactive high-energy milling are usually characterized in terms of phase transformations and crystallite size reduction, produced by the milling process [1-6]. The ceramic processing and characterization of powders obtained by reactive milling has shown [7,8] that, besides the above mentioned agglomeration in nanosized powders, high energy milling has additional effects in producing strong agglomeration. One of these effects is due the reaction during reactive milling [7,8]. The other possible agglomeration mechanism is due to the impacts of high energy. For mainly brittle particles, balls collisions may fracture primary particles as well as compact clusters of particles. As surface area increases stronger agglomerates are produced [8]. The goal of the present study is to carry on an experimental observation of the effect of high-energy milling on the characteristics and ceramic processing behavior of a single phase ceramic powder.

EXPERIMENTAL PROCEDURE

The precursor ceramic powder used in this study was a Alcoa A1000 alumina. The high-energy milling processing of the dry powder was performed in a high-energy vibratory ball mill (Spex 8000 mixer/mill), using hardened steel vials and balls as milling media in air. The use of alumina as milling media was discontinued due its high rate of wear. Different powder samples were prepared from different milling conditions: the milling power (MP), given by the ball/material mass ratio of 3:1, 5:1 and 15:1, and milling default font of 1 h, 2 h and 4 h. After high-energy milling the powder samples were treated in hydrochloric acid solution for iron contamination removal and washed with de-ionized water. The powders were separated from water by sedimentation, dispersed in isopropyl alcohol and dried at 70 °C.

The crystallite size was determined by X-ray diffraction analysis, after the a-alumina peak broadening. The change of the specific surface area of the powder due to the comminution process, was determined by the BET gas adsorption method. The state of agglomeration of the powders was evaluated from particle size distribution curves obtained by sedimentation in a Micromeritics Sedigraph 5100 apparatus. For the determination of the particle size distribution, that actually corresponds to size distribution of agglomerates, each sample was dispersed in water at fixed concentration, by stirring and ultrasound pulses, with ammonia polyacrylate as deflocculant. Other experiment carried out to characterize the state of agglomeration was done by monitoring the decrease of agglomerates size with ball milling time.

Compaction curves, i.e., the compact density (in % of theoretical density) as a function of applied pressure (in MPa), were determined by direct measurements of applied force and die punch dislocation under uniaxial pressing, in a Instron test machine. The compaction tests were carried out in a cylindrical die with 10 mm diameter and for final compact height of about 4 mm. The specimens obtained from compaction tests and others formed with the same dimensions as above, by uniaxial pressing followed by isostatic pressing at 280 MPa were sintered at 1450 °C for 2 h and 1600 °C for 3 h, in air.

After sintering, apparent density and porosity of the specimens were determined following the Archimedes method and the pores distribution observed by scanning electron microscopy (SEM) of polished cross sections. In order to keep better definition of pores, the polished samples used for microscopy observation of the pores were not etched.

RESULTS AND DISCUSSION

The X-ray diffraction of the high-energy milled alumina powders, Fig. 1, showed evident broadening of the a-alumina phase peaks. For brittle materials, one may consider the broadening due to the decrease of crystallite size, as a result of milling. Values of the calculated crystallite sizes, in Table I, show a significant difference between the original alumina and the milled samples. For milled samples the observed decrease of crystallite size is negligible, regardless the change in the milling power (MP). The increase of specific surface area (Table I), on the other hand, is quite significant, as well as the decrease of calculated equivalent spherical particle size of the surface area, coherently with the brittle fracture of powder particles. The significant difference between X-ray diffraction crystallite size and the equivalent spherical particle size of the surface area for such range of particle size may be due to different factors affecting measurements. Three of these factors are: large aspect ratio of the primary particles, large contact area or grain boundary between particles, and strong plastic deformation. The aspect ratio and contact area may affect the values of surface area and plastic deformation may decrease values of crystallite size.

Figure 1:
X-ray diffraction patterns of alumina powders: (a) A1000 Alcoa alumina; (b) after high-energy milling with milling power 5:1, for 4 h and (c) with milling power 15:1, for 4 h.
Table I:
X-ray diffraction crystallite size, BET specific surface area and calculated particle size of alumina (A1000) before and after high-energy milling under different conditions of MP and milling time.

Fig. 2(a) shows the effect of high-energy milling on the agglomeration state of the powders. The agglomerate size increases with milling time and MP. This is probably due to the greater difficult for powder dispersion in water, for the particle size distribution determination, as described above. Fig. 2(b) shows the deagglomeration evolution during ball milling of a high-energy milled powder for 4 h, with MP=3:1. The particle size distribution presents values comparable with the original powder only after 10 h of milling. The ball milling deagglomeration behavior of the different powders obtained under different conditions of high energy milling were similar to that shown in Fig. 2(b). Fig. 2(c) shows that a higher amount of deflocculant is necessary for dispersion of the high-energy milled powder with MP=15:1, for 4 h and deagglomerated by 10 h of ball milling. The shape of curve 3 in Fig. 2(c) is typically found in flocculated powder. The increase of the amount of deflocculant brings the curve of particle size distribution closer to that of the original powder. The increase of the amount of deflocculant is a consequence of the higher surface area of this powder. Therefore, curve 5 in Fig. 2(c) does not represent a completely deagglomerated powder.

Figure 2:
Particle size distribution after dispersion in water with deflocculation: (a) effect of high-energy milling: (1) original powder; (2) as high-energy milled with MP=3:1, 1 h; (3) MP=3:1, 4 h and (4) MP=15:1, 4 h; (b) ball milling de-agglomeration of a high-energy milled (MP=3:1, 4 h) powder: (1) original powder; (2) high energy milled; (3) ball milled for 2 h and (4) ball milled for 10 h; (c) Deflocculation of a high-energy milled powder with MP=15:1, 4 h: (1) original powder; (2) high energy milled (3) ball milled for 10 h in water with 6 drops of deflocculant; (4) with 12 drops of deflocculant and (5) with 24 drops of deflocculant.

Fig. 3 shows the densification curve for the as-received A1000 alumina and a typical densification curve for the alumina powder after high-energy milling. For the as-received powder, the relatively well dispersed powder produced a linear increase in apparent density of the compact due to particles sliding and rearrangement with the increase on applied pressure. For the alumina powders as obtained from high-energy milling, large agglomerates are obtained, as shown above, and the densification curve shows, in Fig. 3(b), a change of curve inclination. This is typically due to the presence of granules or agglomerates, changing the compaction stage. At low pressures, the first stage occurs by flow and rearrangement of agglomerates. A second stage begins with deformation and fracture of the agglomerates. For the present case, the second stage begins at relatively high pressure and continues up to the maximum applied pressure. These results show that agglomerates produced during high-energy milling are very strong and are not completely destroyed during pressing.

Figure 3:
Compaction curves in percentage of theoretical density (TD) as a function of applied pressure: (a) for the as-received A1000 alumina and (b) alumina powder after high-energy milling with MP=5:1, 1 h.

The microstructure analysis of polished cross section of sintered specimens, by SEM, shows heterogeneous distribution of pores, with large inter-agglomerate pores (Fig. 4). For the low sintering temperature of the specimen of Fig. 4(a), the larger amount of pores shows clearly the agglomerates outline. Sintering at 1600 °C made all samples of agglomerated high energy milled powder to present almost the same pore distribution, remaining only large pores, as shown in Figs. 4b and 4c. These observations are confirmed by the apparent densities of the specimens before and after sintering. As can be seen in Table II, the apparent density before sintering of specimens prepared with high energy milled powders are higher than that of specimens prepared with original powder. Probably, densities of compacts with the milled powders depend on the high density of agglomerates. On the other hand, these specimens sintered at 1600 °C have densities of about 96% of the theoretical density. The remaining porosity is probably due to the large inter-agglomerate pores shown in microstructure analysis.

Figure 4:
SEM of the polished cross section (X1000) of sintered specimens: (a) from powder MP=5:1/1 h, sintered at 1450 ° C/2 h; (b) MP= 5:1/1 h 1600 ° C/3 h and (c) MP= 5:1/4 h, 1600 ° C/3 h.
Table II:
Apparent density of compacted and sintered specimens obtained from powders milled at different conditions.

We suggest that high energy milling has additional effects in producing strong agglomeration in nano-sized powders. For non-reactive high-energy milling of brittle particles, balls collisions may fracture particles as well as compact agglomerates of particles. The main difference for the present case is that the high energy of ball collisions makes the agglomerates dense and exceptionally strong. Therefore, agglomerates are formed and destroyed during all milling process and are always present in the final product. As surface area increases, stronger agglomerates are produced.

CONCLUSION

It has been shown that high-energy milling is able to produce nano-sized particles in brittle ceramic powder. However, the formation of strong agglomerates is intrinsic to the method and the powder utilization in ceramic processing requires a previous deagglomeration.

ACKNOWLEDGMENTS

The authors would like to thank FAPESP (Projeto Temático de Equipe) and CNPq for financial support.

(Rec. 02/98, Ac. 03/98)

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Publication Dates

  • Publication in this collection
    12 June 2000
  • Date of issue
    Oct 1998

History

  • Received
    Feb 1998
  • Accepted
    Mar 1998
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