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Effect of nanopowder deagglomeration on the densities of nanocrystalline ceramic green bodies and their sintering behaviour
NanoStructured Materials, Vol. 11, No. 5, pp. 617– 622, 1999 Elsevier Science Ltd Copyright © 1999 Acta Metallurgica Inc. Printed in the USA. All rights reserved. 0965-9773/99/$–see front matter
Pergamon
PII S0965-9773(99)00348-7
EFFECT OF NANOPOWDER DEAGGLOMERATION ON THE DENSITIES OF NANOCRYSTALLINE CERAMIC GREEN BODIES AND THEIR SINTERING BEHAVIOUR H. Ferkel and R.J. Hellmig Institut fu¨r Werkstoffkunde und Werkstofftechnik, Technische, Universita¨t Clausthal, Agricolastr. 6, D-38678 Clausthal-Zellerfeld, Germany (Received April 5, 1999) (Accepted June 20, 1999) Abstract—The agglomeration of nanoscaled powders in general decreases the density of green bodies pressed from those nanopowders. Depending on the strength and morphology of the agglomerates different densities can be achieved. It is shown that in the case of ceramic nanopowders as alumina and yttria-stabilised zirconia which are produced by laser ablation technique or in the case of alumina also by physical vapour synthesis process the stronger agglomerates can easily be cracked by a short milling procedure. Green bodies pressed from the conditioned nanopowders exhibit up to 15% larger densities than bodies made from unmodified nanopowders. Due to the decreased porosity in those specimens the sintering activity is significantly increased and therefore ceramics of up to 99% of the theoretical density having median grain diameters less than 500 nm can be manufactured under simplified processing conditions. ©1999 Acta Metallurgica Inc.
Introduction For the consolidation of nanoscaled ceramic powders into nanostructured materials different techniques such as pressure assisted sintering (1), sinter forging (2) or compacting of the powders at enormous pressures of up to 8 GPa at elevated temperatures or at ambient temperatures followed by sintering in common furnaces are employed (1d-e). In general it was found that ceramic nanoparticles are difficult to compact at room temperature. Green densities of pellets pressed from alumina nanopowder higher than 60% of the theoretical density were not achieved at room temperature and at compacting pressures less than 1 GPa (1d-e). This maximum density is still lower than the density attainable with coarse grained alumina powder where densities close to the theoretical fractional density of spherical non agglomerated random dense packed particles of about 64% are achieved (3). In the case of agglomerated particles the packing density tends to decrease from the theoretical point of view (4) as well as from the experimental observation and can be correlated with increasing attraction between particles with decreasing particle size due to e.g. van-der-Waals interaction (4 – 6). Ceramic nanoparticles produced by gas condensation, physical vapour synthesis (PVS) or laser ablation process exhibit relatively weak agglomerates. However, a significant amount of strong agglomerates as particle groups which show necks between adjacent particles are still present. The bonding between the particles of these agglomerates are expected to be much stronger than the bonding between particles which are only attracted by van-der-Waals interaction. Whereas particles which are only attracted by weak interaction can more easily slide into voids of the green body during compaction, the particles in strong agglomerates have to experience large shearing forces necessary for neck cleavage to occur before sliding of these particles can take place. 617
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Therefore a modification of the nanopowder by reducing the amount of necks between particles before compaction will be a key to obtain larger green densities at lower compacting pressures. The goal of this work is to demonstrate a simple ball milling method which can be applied to nanopowders in order to crack strong agglomerates. The green densities obtained from alumina and yttria-stabilised zirconia nanopowders modified by this technique are compared with the green densities of the unmodified powders and are correlated with the observed sintering behaviour. Experimental Nanoscaled particles are produced by Nd:YAG-laser ablation of solid alumina and of 3 mol% yttria-stabilised zirconia rods followed by condensation of the induced vapour in air. Details on the powder production are given elsewhere (7–9). The median particle diameter are 14 nm for alumina and 13 nm for the yttria-stabilised zirconia. Furthermore, commercially available alumina nanopowder produced by physical vapour synthesis (PVS) process having a median particle diameter of about 30 nm (NanoTek 0100, Nanophase Technologies Corporation) were also employed for the experiments. In order to reduce the amount of strong agglomerates, each nanopowder was milled for a maximum of 10 min at 150 rpm in a planetary ball mill (Retsch, PM400) in a sealed air atmosphere. The milling vessel had a volume of 500 ml and the milling balls had a diameter of 10 mm. In order to avoid contamination of the alumina and zirconia nanopowders during milling the vessel and milling balls were made of corundum and of zirconia, respectively. The weight ratio of ball-to-powder was 30:1. The modification of the nanopowder by the milling procedure was analysed by transmission electron microscopy (TEM) and by X-ray diffraction (XRD). For the TEM analysis some of the nanopowders were dispersed by ultrasonic treatment in distilled water. Subsequently, one droplet of the liquid was placed on carbon films, dried and inspected in the TEM. Green bodies of the as synthesised and milled nanopowders were uniaxially cold pressed at 440 MPa followed by density measurements of the pressed pellets. In order to investigate the sintering behaviour of the nanopowders the green bodies were heat treated between 1100°C and 1600°C for 60 min without applying compacting pressure. Furthermore, pressure assisted sintering in a uniaxial hot press (KCE HPW 150/400) was carried out at 80 MPa between 900°C and 1300°C for 90 min. Results and Discussion Figure 1 represents TEM pictures of NanoTek alumina nanopowder before and after milling. The particle size analysis of the powder before and after milling were carried out by determining the size distribution from ten TEM pictures of each powder sample. Averaging of the distributions of each batch reveal within the limit of errors the same log-normal distribution with a median particle diameter dm of 33 nm and a geometric standard deviation of 1.6. It can also clearly be seen that in the right part of Fig. 1 single nanoparticles in addition to particle groups are deposited on the TEM film whereas in the left part of Fig. 1 no isolated particles can be found. Evaluation of the median particle size from XRD spectra of both powders via the Scherrer formula yields in both cases within the limit of errors the same dm. From these results it can be concluded that no refinement of the (primary) particle size took place during milling whereas from the TEM pictures the extent of agglomeration seems to be less pronounced in the milled powder. Corresponding results are also obtained from the laser-generated nanopowders. Figure 2 shows the influence of the milling on the density of the green bodies. It can be seen that the milling procedure leads to an increase in green density of up to 15% related to the density of the bodies made from the unmodified powders. This behaviour can be explained by the decrease in the amount of strong agglomerates induced by the ball-milling procedure. Sliding of nanoparticles and the
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Figure 1. TEM-pictures of alumina nanopowder; (left) before and (right) after 10 min milling.
remaining smaller strong agglomerates into voids during compaction results obviously in an increase in green density which is close to the theoretical value for a random dense packing (3). Figure 3 compares the green densities with the density of the sintered alumina bodies. The effect of milling on the densification of the powders is evident. After sintering without (middle part of Fig. 3) and with (right part of Fig. 3) pressure assistance the effect of increased densification is even more pronounced. In the case of ball-milled nanopowders, unloaded sintering at 1600°C leads to bodies having a density of about 0.95 while the bodies from the non-milled nanopowders exhibit densities of
Figure 2. The densities of green bodies pressed uniaxially at 440 MPa from ball-milled and unmodified powder.
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Figure 3. Densities of alumina bodies; green density (left), after sintering for 60 min at 1600°C without load (middle) and after sintering for 90 min at 1300°C and a pressure of 80 MPa (right).
about 0.80. Moreover, pressure assisted sintering of the modified nanopowder at 80 MPa and a temperature of 1300°C results in a fully dense sintered body whereas in the case of an unmodified powder a sintered density of less than 0.70 could only be achieved. These results also demonstrate the benefit of the milling step during processing. Figure 4 shows SEM micrographs of the sintered bodies prepared at 1300°C and at a pressure of 80 MPa. In the left part of the figure a fracture surface of a sample prepared from the unmodified alumina nanopowder is seen. The grain size is about 90 nm and the specimen exhibits a significant amount of porosity. On the right side of Fig. 4 a fracture surface of a sample prepared from the ball-milled alumina nanopowder is presented. The material is almost fully dense. The grain size is about 400 nm indicating the higher sintering activity of the previously ball-milled nanopowders. Both specimens are completely transformed from ␥ to ␣ alumina as determined from XRD measurements. Sintering of zirconia nanopowder confirmed the observations described above. This can be seen in Fig. 5. The density achieved after a sintering step of 1300°C without load is about 0.92 for the sample made from the ball-milled zirconia nanopowder while the sample made from the non-milled powder had a sintered density of 0.82 for the same consolidation conditions. These results again show the advantages of the introduced milling step. Additionally, pressure assisted sintering at a lower temperature had been carried out. At 1100°C with a load of 80 MPa the sintered density of ball-milled zirconia reached almost 0.96. The results demonstrate that the short milling procedure significantly helps to produce full dense ultra–fine grained ceramics from nanoscaled powders at simplified compacting conditions. However, to produce nanostructured materials a second phase has to be incorporated in the material
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Figure 4. SEM micrographs of alumina bodies sintered at 1300°C and 80 MPa; without ball-milling (left) and with the milling step (right).
acting as a grain size stabiliser. If then the milling step is applied to those nanopowder mixtures it can be expected that full dense nanostructured composites can be produced at simplified compacting conditions.
Figure 5. Densities of zirconia bodies; green density (left), after sintering for 60 min at 1300°C without load (middle) and after sintering for 90 min at 1100°C and a pressure of 80 MPa (right).
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Conclusions It was shown that a short ball-milling procedure decreases the amount of strong agglomerates in alumina and zirconia nanopowders produced with PVS and laser ablation technique. Green bodies pressed from the modified powders exhibit up to 15% higher densities than bodies prepared from the unmodified material. It is also demonstrated that the sintering activity of the modified nanopowder is significantly increased leading to almost dense material at simplified sintering conditions. Due to the improved sinter activity significant grain growth is observed leading to a microscaled structure with a grain size of about 400 nm. From the microstructure of the alumina ceramics sintered from the nanopowders in the modified and the as produced state a significant difference in the sintering behaviour can be concluded. Acknowledgement The authors would like to thank Prof. B.L. Mordike for his support. References 1.
2. 3. 4. 5. 6. 7. 8. 9.
For review see: (a) H. Gleiter, Prog. Mat. Science. 33, 223 (1989); (b) R. W. Siegel, Cluster assembly of nanophase materials, in Materials Science and Technology Vol. 15, edited by R. W. Cahn, P. Haasen, and E. J. Kramer, VCH-Verlag (1991) p 583– 614; (c) C. Suryanarayana, Intern Mater. Rev. 40, 41 (1995) and references therein; and for recent works see: (d) S.-C. Liao, Y.-J. Chen, B. H. Kear, and W. E. Mayo, Nanostructured Materials. 10, 1063 (1998); (e) S. M. Sweeney and M. J. Mayo, Nanostructured Materials. (1999), in press. H. Hahn, J. A. Eastman, and R. W. Siegel, Ceramic Transactions 1b, Ceramic Powder Science. 1115 (1988). R. M. German, Particle Packing Characteristics, Metal Powder Industries Federation, Princeton, NJ (1989). R. F. Fedors and R. F. Landel, Powder Technology. 23, 201 (1978). M. D. Sacks, in Science of Ceramic Chemical Processing, ed. L. L. Hench and R. Ulrich, pp 522–538, Wiley, New York (1986). D. M. Newitt and J. M. Conway-Jones, Trans Institution Chem. Eng. 36, 422 (1958). H. Ferkel and W. Riehemann, Nanostructured Materials. 7, 835 (1996). H. Ferkel, J. Naser, and W. Riehemann, Nanostructured Materials. 8, 457 (1997). J. Naser and H. Ferkel, Nanostructured Materials. (1999), in press.
Pergamon
PII S0965-9773(99)00348-7
EFFECT OF NANOPOWDER DEAGGLOMERATION ON THE DENSITIES OF NANOCRYSTALLINE CERAMIC GREEN BODIES AND THEIR SINTERING BEHAVIOUR H. Ferkel and R.J. Hellmig Institut fu¨r Werkstoffkunde und Werkstofftechnik, Technische, Universita¨t Clausthal, Agricolastr. 6, D-38678 Clausthal-Zellerfeld, Germany (Received April 5, 1999) (Accepted June 20, 1999) Abstract—The agglomeration of nanoscaled powders in general decreases the density of green bodies pressed from those nanopowders. Depending on the strength and morphology of the agglomerates different densities can be achieved. It is shown that in the case of ceramic nanopowders as alumina and yttria-stabilised zirconia which are produced by laser ablation technique or in the case of alumina also by physical vapour synthesis process the stronger agglomerates can easily be cracked by a short milling procedure. Green bodies pressed from the conditioned nanopowders exhibit up to 15% larger densities than bodies made from unmodified nanopowders. Due to the decreased porosity in those specimens the sintering activity is significantly increased and therefore ceramics of up to 99% of the theoretical density having median grain diameters less than 500 nm can be manufactured under simplified processing conditions. ©1999 Acta Metallurgica Inc.
Introduction For the consolidation of nanoscaled ceramic powders into nanostructured materials different techniques such as pressure assisted sintering (1), sinter forging (2) or compacting of the powders at enormous pressures of up to 8 GPa at elevated temperatures or at ambient temperatures followed by sintering in common furnaces are employed (1d-e). In general it was found that ceramic nanoparticles are difficult to compact at room temperature. Green densities of pellets pressed from alumina nanopowder higher than 60% of the theoretical density were not achieved at room temperature and at compacting pressures less than 1 GPa (1d-e). This maximum density is still lower than the density attainable with coarse grained alumina powder where densities close to the theoretical fractional density of spherical non agglomerated random dense packed particles of about 64% are achieved (3). In the case of agglomerated particles the packing density tends to decrease from the theoretical point of view (4) as well as from the experimental observation and can be correlated with increasing attraction between particles with decreasing particle size due to e.g. van-der-Waals interaction (4 – 6). Ceramic nanoparticles produced by gas condensation, physical vapour synthesis (PVS) or laser ablation process exhibit relatively weak agglomerates. However, a significant amount of strong agglomerates as particle groups which show necks between adjacent particles are still present. The bonding between the particles of these agglomerates are expected to be much stronger than the bonding between particles which are only attracted by van-der-Waals interaction. Whereas particles which are only attracted by weak interaction can more easily slide into voids of the green body during compaction, the particles in strong agglomerates have to experience large shearing forces necessary for neck cleavage to occur before sliding of these particles can take place. 617
618
SINTERING NANOPOWDERS
Vol. 11, No. 5
Therefore a modification of the nanopowder by reducing the amount of necks between particles before compaction will be a key to obtain larger green densities at lower compacting pressures. The goal of this work is to demonstrate a simple ball milling method which can be applied to nanopowders in order to crack strong agglomerates. The green densities obtained from alumina and yttria-stabilised zirconia nanopowders modified by this technique are compared with the green densities of the unmodified powders and are correlated with the observed sintering behaviour. Experimental Nanoscaled particles are produced by Nd:YAG-laser ablation of solid alumina and of 3 mol% yttria-stabilised zirconia rods followed by condensation of the induced vapour in air. Details on the powder production are given elsewhere (7–9). The median particle diameter are 14 nm for alumina and 13 nm for the yttria-stabilised zirconia. Furthermore, commercially available alumina nanopowder produced by physical vapour synthesis (PVS) process having a median particle diameter of about 30 nm (NanoTek 0100, Nanophase Technologies Corporation) were also employed for the experiments. In order to reduce the amount of strong agglomerates, each nanopowder was milled for a maximum of 10 min at 150 rpm in a planetary ball mill (Retsch, PM400) in a sealed air atmosphere. The milling vessel had a volume of 500 ml and the milling balls had a diameter of 10 mm. In order to avoid contamination of the alumina and zirconia nanopowders during milling the vessel and milling balls were made of corundum and of zirconia, respectively. The weight ratio of ball-to-powder was 30:1. The modification of the nanopowder by the milling procedure was analysed by transmission electron microscopy (TEM) and by X-ray diffraction (XRD). For the TEM analysis some of the nanopowders were dispersed by ultrasonic treatment in distilled water. Subsequently, one droplet of the liquid was placed on carbon films, dried and inspected in the TEM. Green bodies of the as synthesised and milled nanopowders were uniaxially cold pressed at 440 MPa followed by density measurements of the pressed pellets. In order to investigate the sintering behaviour of the nanopowders the green bodies were heat treated between 1100°C and 1600°C for 60 min without applying compacting pressure. Furthermore, pressure assisted sintering in a uniaxial hot press (KCE HPW 150/400) was carried out at 80 MPa between 900°C and 1300°C for 90 min. Results and Discussion Figure 1 represents TEM pictures of NanoTek alumina nanopowder before and after milling. The particle size analysis of the powder before and after milling were carried out by determining the size distribution from ten TEM pictures of each powder sample. Averaging of the distributions of each batch reveal within the limit of errors the same log-normal distribution with a median particle diameter dm of 33 nm and a geometric standard deviation of 1.6. It can also clearly be seen that in the right part of Fig. 1 single nanoparticles in addition to particle groups are deposited on the TEM film whereas in the left part of Fig. 1 no isolated particles can be found. Evaluation of the median particle size from XRD spectra of both powders via the Scherrer formula yields in both cases within the limit of errors the same dm. From these results it can be concluded that no refinement of the (primary) particle size took place during milling whereas from the TEM pictures the extent of agglomeration seems to be less pronounced in the milled powder. Corresponding results are also obtained from the laser-generated nanopowders. Figure 2 shows the influence of the milling on the density of the green bodies. It can be seen that the milling procedure leads to an increase in green density of up to 15% related to the density of the bodies made from the unmodified powders. This behaviour can be explained by the decrease in the amount of strong agglomerates induced by the ball-milling procedure. Sliding of nanoparticles and the
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Figure 1. TEM-pictures of alumina nanopowder; (left) before and (right) after 10 min milling.
remaining smaller strong agglomerates into voids during compaction results obviously in an increase in green density which is close to the theoretical value for a random dense packing (3). Figure 3 compares the green densities with the density of the sintered alumina bodies. The effect of milling on the densification of the powders is evident. After sintering without (middle part of Fig. 3) and with (right part of Fig. 3) pressure assistance the effect of increased densification is even more pronounced. In the case of ball-milled nanopowders, unloaded sintering at 1600°C leads to bodies having a density of about 0.95 while the bodies from the non-milled nanopowders exhibit densities of
Figure 2. The densities of green bodies pressed uniaxially at 440 MPa from ball-milled and unmodified powder.
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Figure 3. Densities of alumina bodies; green density (left), after sintering for 60 min at 1600°C without load (middle) and after sintering for 90 min at 1300°C and a pressure of 80 MPa (right).
about 0.80. Moreover, pressure assisted sintering of the modified nanopowder at 80 MPa and a temperature of 1300°C results in a fully dense sintered body whereas in the case of an unmodified powder a sintered density of less than 0.70 could only be achieved. These results also demonstrate the benefit of the milling step during processing. Figure 4 shows SEM micrographs of the sintered bodies prepared at 1300°C and at a pressure of 80 MPa. In the left part of the figure a fracture surface of a sample prepared from the unmodified alumina nanopowder is seen. The grain size is about 90 nm and the specimen exhibits a significant amount of porosity. On the right side of Fig. 4 a fracture surface of a sample prepared from the ball-milled alumina nanopowder is presented. The material is almost fully dense. The grain size is about 400 nm indicating the higher sintering activity of the previously ball-milled nanopowders. Both specimens are completely transformed from ␥ to ␣ alumina as determined from XRD measurements. Sintering of zirconia nanopowder confirmed the observations described above. This can be seen in Fig. 5. The density achieved after a sintering step of 1300°C without load is about 0.92 for the sample made from the ball-milled zirconia nanopowder while the sample made from the non-milled powder had a sintered density of 0.82 for the same consolidation conditions. These results again show the advantages of the introduced milling step. Additionally, pressure assisted sintering at a lower temperature had been carried out. At 1100°C with a load of 80 MPa the sintered density of ball-milled zirconia reached almost 0.96. The results demonstrate that the short milling procedure significantly helps to produce full dense ultra–fine grained ceramics from nanoscaled powders at simplified compacting conditions. However, to produce nanostructured materials a second phase has to be incorporated in the material
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Figure 4. SEM micrographs of alumina bodies sintered at 1300°C and 80 MPa; without ball-milling (left) and with the milling step (right).
acting as a grain size stabiliser. If then the milling step is applied to those nanopowder mixtures it can be expected that full dense nanostructured composites can be produced at simplified compacting conditions.
Figure 5. Densities of zirconia bodies; green density (left), after sintering for 60 min at 1300°C without load (middle) and after sintering for 90 min at 1100°C and a pressure of 80 MPa (right).
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Conclusions It was shown that a short ball-milling procedure decreases the amount of strong agglomerates in alumina and zirconia nanopowders produced with PVS and laser ablation technique. Green bodies pressed from the modified powders exhibit up to 15% higher densities than bodies prepared from the unmodified material. It is also demonstrated that the sintering activity of the modified nanopowder is significantly increased leading to almost dense material at simplified sintering conditions. Due to the improved sinter activity significant grain growth is observed leading to a microscaled structure with a grain size of about 400 nm. From the microstructure of the alumina ceramics sintered from the nanopowders in the modified and the as produced state a significant difference in the sintering behaviour can be concluded. Acknowledgement The authors would like to thank Prof. B.L. Mordike for his support. References 1.
2. 3. 4. 5. 6. 7. 8. 9.
For review see: (a) H. Gleiter, Prog. Mat. Science. 33, 223 (1989); (b) R. W. Siegel, Cluster assembly of nanophase materials, in Materials Science and Technology Vol. 15, edited by R. W. Cahn, P. Haasen, and E. J. Kramer, VCH-Verlag (1991) p 583– 614; (c) C. Suryanarayana, Intern Mater. Rev. 40, 41 (1995) and references therein; and for recent works see: (d) S.-C. Liao, Y.-J. Chen, B. H. Kear, and W. E. Mayo, Nanostructured Materials. 10, 1063 (1998); (e) S. M. Sweeney and M. J. Mayo, Nanostructured Materials. (1999), in press. H. Hahn, J. A. Eastman, and R. W. Siegel, Ceramic Transactions 1b, Ceramic Powder Science. 1115 (1988). R. M. German, Particle Packing Characteristics, Metal Powder Industries Federation, Princeton, NJ (1989). R. F. Fedors and R. F. Landel, Powder Technology. 23, 201 (1978). M. D. Sacks, in Science of Ceramic Chemical Processing, ed. L. L. Hench and R. Ulrich, pp 522–538, Wiley, New York (1986). D. M. Newitt and J. M. Conway-Jones, Trans Institution Chem. Eng. 36, 422 (1958). H. Ferkel and W. Riehemann, Nanostructured Materials. 7, 835 (1996). H. Ferkel, J. Naser, and W. Riehemann, Nanostructured Materials. 8, 457 (1997). J. Naser and H. Ferkel, Nanostructured Materials. (1999), in press.