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Coprecipitation conditions and compaction behaviour of Y-TZP nanometric powders
Ceramics International 20 (1994) 85-89
Coprecipitation Conditions and Compaction Behaviour of Y-TZP Nanometric Powders Z. Pqdzich & K. Haberko Department of Special Ceramics, University of Mining and Metallurgy, 30-059 Cracow, AI. Mickiewicza 30, Poland (Received 2 May 1993; accepted 27 May 1993)
Abstract: The concentration of the starting ZrOC12 + YC13 solution influences
the coprecipitated gel morphology. The gel prepared from the highly concentrated solution shows a loose microstructure. Its calcination results in a 3Y-TZP powder of relatively weak agglomerates. The latter collapse during uniaxial pressing under 200 MPa in contrast to 400 MPa pressure observed in the case of a powder derived from the gel coprecipitated from the diluted ZrOC12 + YC13 solution. Compacts of the powder composed of weak agglomerates attain higher densities during both cold pressing and sintering.
1 INTRODUCTION
methods were used in the case of zirconia solid solution powders. The coprecipitated hydroxides ZrOz-Y203, ZrOz-Nd203 and ZrOz-CaO systems, washed with the liquids of low surface tension resulted in the fluffy gels. 5'6 Their calcination gave powders composed of highly porous and mechanically weak agglomerates. Similar results were achieved by the freeze drying of the coprecipitated gels7 and their subsequent calcination. Crystallization of the zirconia solid solutions under hydrothermal conditions results also in the mechanically weak agglomerates8-1°. This is mainly due to the weak, second-order bonds between the particles crystallized in the presence of water, which is the liquid perfectly wetting zirconia and penetrating the interparticle contacts. Precipitating conditions can influence the properties of the resulting zirconia gel. For example, Mamott e t al. ~ have studied the effect of the pH of the liquid surrounding the zirconium hydroxide precipitate. The observed relationship between the precipitation conditions and the onset temperature of crystallization as well as the rate of crystallization and crystal growth have been interpreted in terms of the degree of gel ordering. The objective of the present study was to determine the effect of the starting solution concentrations on the resultant gel and crystalline powder morphology.
Among various methods of preparation of ceramic oxide multi-component nanometric powders, the coprecipitation technique seems to be the most frequently used. However, the high specific surface area of these powders leads to problems with their agglomeration. Large pores can be formed in a green compact among strong agglomerates. A large pore can be thermodynamically stable depending on the dihedral angle and the pore size:grain (crystallite) size ratio. ~-3 For a given dihedral angle there is a critical pore size, as measured by the number of crystallites surrounding the pore, above which the pore becomes stable (and cannot sinter) and below which it is unstable (and can sinter). In order to overcome these problems it is necessary to control the agglomerate strength and/ or size. Mechanically weak agglomerates can easily be ground and/or collapse under the applied compaction pressure. Both processes decrease the fraction of the overcritical pores in the green compact or even reduce it to zero. According to Rumpf ~ the strength of powder agglomerates composed of crystallites of a given size can be reduced by increasing the agglomerate porosity and/or decreasing the force necessary to break the link between the two crystallites. Both 85
Ceramics Internationa10272-8842/94/$7.00 © 1994 Elsevier Science Limited, England and Techna S.r.l., Italy. Printed in Great Britain
86 2 EXPERIMENTAL
The TZP composition of Y203 of 3 mol% and of Z r O 2 of 97 mol% was selected for the present study. Two aqueous solutions of ZrOC12 + YCI 3 of different concentrations were prepared; a highly concentrated solution with a total salt content of 0.8940 mol/litre and a 20 times diluted solution with a salt concentration of 0.0447 mol/litre. Both solutions were hydrolyzed by pouring them into a vigorously stirred ammonium hydroxide solution. The solution with the higher salt concentration was poured into highly concentrated NH4OH (5 mole/litre) and the solution with the low salt concentration was added to diluted ammonium hydroxide (1.67 mole/litre). The volume rate of pouring of the ZrOC12 + YC13 solution into the NHaOH one was the same in both cases (100 cm3/ min). The final pH of thesuspension was 9.0. The materials were then carefully washed with distilled water by decantation. Washing was continued until the filtrate electrical conductivity dropped down to the level of distilled water (~10 /zS at room temperature). Drying of the gels was performed at 120°C. The gels were ground in an agate mortar under the same and strictly controlled conditions. The gels were calcined under isothermal conditions using temperatures within the range of 450-1000°C and with 30 min soaking at each of the selected temperatures. One part of the experiment was performed with the nonisothermally calcined gels at 700°C for 30 min. In this case the rate of temperatui'e increase was 6°C/min. Specific surface area (Sw) measurements of the crystalline powders were performed by the BET method (Carlo Erba, Sorpty 1750). On this basis the BET particle size (DBET) w a s calculated using the relation assuming spherical or cubic shape of the particles: DBET = 6 / p S w, where p is the X-ray density. Crystallite size measurements were based on the X-ray line broadening of the (l 1l) tetragonal reflection (CuKa). Mercury porosimetry (Carlo Erba, Porosimeter 2000) was helpful in the determination of the inter- and intra-agglomerate porosity in the powder compacts. The X-ray density of the tetragonal 3Y-TZP (6.044 g/cm3) was calculated on the basis of the generally accepted zirconia solid solution models and the unit cell sizes determined by Scott. 12 This value was used throughout the work to assess densification of the compacts, agglomerates and sintered samples. 3 RESULTS A N D DISCUSSION
The dry gel precipitated from the concentrated solution is porous, 'chalklike' and mechanically
Z. Pcdzich, K. Haberko
weak. The gel prepared from the diluted solution is dense, opalescent and mechanically strong. Infrared spectroscopic examination of the gels revealed no difference between their spectra. This seems to indicate that the two different coprecipitation conditions applied in the present work have no influence on the gel short range order. This conclusion seems to be corroborated by the DTA measurements. They show the characteristic exothemic peak attributed to the crystallization of the zirconia solid solution. 13 No difference of the crystallization temperature (450°C) between the gels is observed. In Fig. 1, the crystallite size (D1,) and the BET particle size (DBE'r) as a function of isothermal calcination temperature, are shown. The Din values of the both powders do not differ up to the calcination temperature of 600°C. This suggests that the sizes of the ordered areas in both kinds of gels are comparable. According to Mamott et al. ~ within such areas the primary crystallites are formed during the gel calcination. At higher temperatures the crystallite growth becomes more distinct especially in the samples derived from the diluted solution. This fact can be plausibly explained if we realize that the starting gel in this case is more dense than the one precipitated from the concentrated solution, as suggested by the gel morphology. The crystallite growth occurs by the movement of the inter-crystalline boundaries. Thus, it should be faster in the envi30
a
_,ffi 25 o
20
i
r
160 140
120 o o
100
80 60 40
20 7 7 0 50O 400
i
i
i
t
600
700
800
900
1000
Temperature [C]
Fig. 1. Crystallite size ( D u O a n d B E T particle size (DBET)
of the powders as functions of calcination temperature. (A) and (B) correspond to the powders derived from the gels coprecipitated from the concentratedsolution and the diluted one, respectively.
87
Coprecipitation and compaction of Y-TZP nanometric powders ronment in which each particle contacts with the larger number of the neighbouring crystallites. The mean number of contacts per one crystallite (LK) can be assessed on the basis of the Rumpf's approximate relation: 4 LK . Vp -" "/T, where Vp is the pore volume fraction. It will be shown later in the text that the agglomerate porosities of the powders calcined at 700°C are 61% and 51% for the samples derived from the concentrated and diluted solutions, respectively. This leads to the 20% difference in the mean number of the neighbours per particle. It seems to substantiate the observed difference between the Dl~t values between the samples under discussion. Specific surface area changes, as reflected by the changes of the BET particle size, with the calcination temperature (Fig. 1) can be explained by the crystallite growth and/or by the decrease of the inter-crystalline surface area. Most probably the latter process becomes active at the higher calcination temperatures, as is indicated by an increase in the discrepancy between the D l l I and DBET sizes with the calcination temperature. The marked difference between the DBE-r values of both kinds of powders, observed in the samples calcined at 1000°C, indicates that this process is much faster within agglomerates of higher density. The powders calcined (nonisothermally) at 700°C were selected for the further experiments. Their basic characteristics are given in Table 1. The plots of the relative density of the green compacts as a function of log compaction pressure are shown in Fig. 2. The breaking point between the straight lines is strongly dependent on the preparation method of the starting gel. In the case of the powder derived from the dense gel (i.e. the gel coprecipitated from the diluted solution) the breaking point occurs at the compaction pressure of 400 MPa, compared to 200 MPa for the second powder. According to the previous investigations on the zirconia nanometric powder compaction, 5'6 the breaking point corresponds to the decohesion of the contacts between crystaUites within the agglomerates. Thus, the observed behaviour shows the marked influence of the coprecipitation conditions on the agglomerate strength in the resultant powders.
50 46 42 o
38 34
o
30 50
100
200 300 400 600 MPa
Log Compaction Pressure Fig. 2. Compaction diagrams of the powders calcined at 700°C (for details of (A) and (B) see Fig. 1).
In order to understand the compaction process, performed under much higher pressures than those applied in previously reported investigations, 5,6'8 pore size distribution measurements were performed. Figure 3 demonstrates the cumulative pore size distribution curves in the samples compacted under the indicated pressures. Generally, two different-sized pore populations are observed: small, inter-agglomerate pores and much bigger intra-agglomerate pores. The curves for the two kinds of the powders differ in shape. In the case of the strong agglomerates (B), the transition between the inter- and intra-agglomerate porosity is very sharp, in contrast to that of the weak agglomerates. Most probably in the latter case the pore volume separating the inter- and intra-agglomerate porosity is filled with the small fragments of the agglomerates. The latter are created during the agglomerate rearrangement under the pressures up to the breaking point in the compaction diagram (Fig. 2). This process is less distinct in the case of the dense and strong agglomerates. On the basis of the pore size distribution curves and the total porosity of the compacts, the interand intra-agglomerate porosity could be assessed. The results versus compaction pressure are shown in Fig. 4. They indicate that under the low pressures 300 240
E oJ E 180
>o
Table 1. Characteristics of the noniaothermally calcined powders Powder
Sw (m2/g)
DBET (nm)
D l l l (nm)
A B
66.6 62-4
14.9 + 0-6 15.9 _+0.6
14-6 + 0.6 17.5 + 0.8
.~ 120
E E 60
d
0
. . . . . . . .
;
10
. . . . . . . .
i
. . . . . . . .
100 Pore radius (nm)
i
1000
.
.
.
.
....
10000
Fig. 3. Cumulative pore size distribution curves of the
A, Powder derived from the concentrated solution. B, Powder derived from the diluted solution.
samples compacted under 100 MPa (A1 and B1) and 600 MPa (A2 and B2) (for details of (A) and (B) see Fig. 1).
88
Z. Pcdzich, K. Haberko 35O
n
A
T
3O0
1
n
250 E
200
.~-
150
s.
o
100
la
[]
L]
5O i
100
200
300
400
500
600
700
Compaction Pressure [MPa] 350
B
3OO AT
"--- 250
200
•
I .t
A
150 0
la
I00
50 0
,
0
i
i
100
200
,
i
300
,
i
400
,
i
i
500
600
,
700
C o m p a c t i o n Pressure [MPa]
Fig. 4. Total porosity (T), inter- (I) and intra-agglomerate (Ia) porosity as a function of compaction pressure (for details of (A) and (B) see Fig. 1). (up to the breaking point) decrease of the total porosity occurs by the agglomerate rearrangement. This is substantiated by the constant interagglomerate porosity and the decrease of the intra-agglomerate space with pressure. Under the higher pressures, but close to the breaking point, reduction of the total porosity is realized by the agglomerate deformation. The agglommerates fill the intra-agglomerate space. This is possible because of the decohesion of the intercrystalline bonds within the agglomerates. During this process, abrupt decease of the intraagglomerate porosity versus pressure occurs, but the inter-agglomerate porosity remains constant. 100 ~
9O
e~
~
8O
~
70
~
6o
~
50
B
0
~
100
±
200
300
400
500
600
700
C o m p a c t i o n Pressure [MPa]
Fig. 5. Relative density of the samples sintered at 1200°C for 1 h as a function of compaction pressure (for details of (A) and (B) see Fig. 1).
Fig. 6. Fracture surface of the sintered samples. Compaction pressure 600 MPa; (A) and (B) correspond to strong and weak agglomerates respectively. Porosity of sample (A) is 10% and sample (B) is 29.5%.
Simultaneously, the increased slope of the compaction diagram is observed (see Fig. 2). Such a behaviour was previously observed in zirconia nanometric powders, 5'6 However, under still higher pressures (>250 MPa and >500 MPa for the A and B powders, respectively) the reduction of the total porosity occurs by the two competing processes: filling up the intra-agglomerate porosity and compression of the inter-agglomerate space. This is substantiated by the volume reduction of both kinds of pores. In compacts of powder A, the intra-agglomerate porosity becomes undiscernible from the inter-agglomerate one under the pressures of 500 MPa and higher (see Figs 3 and 4). In Fig. 5 the relative density of the samples sintered at 1200°C for 1 h is shown. The marked difference between the two kinds of the samples can plausibly be explained on the basis of the critical coordination pore number concept. 2,3 Since the powder composed of the weak agglomerates densities better in the whole compaction pressure
89
Coprecipitation and compaction o f Y - T Z P nanometric powders
range, the volume fraction of the undercritical pores should be higher in this case. Hence, densification of this system during sintering should be better, which is the case. This is illustrated by the microstructure o f the sintered samples (Fig. 6).
2.
3.
4 CONCLUSIONS 4.
The results presented indicate that the concentration of the starting solutions strongly influences the m o r p h o l o g y of the coprecipitated gels in the ZrO2-Y20 3 system. The gel derived from the diluted solution is dense compared to that prepared from the concentration solution. Since the ordered areas within the both gels are probably o f the same size, one must conclude that their compaction is dependent on the precipitation conditions. When the process is performed with the concentrated solutions it is fast and results in the fluffy gel microstructure. Densification of the coprecipitated gels influences the microstructure o f the resultant crystalline powder. The powder derived from the dense gel should be c o m p o s e d of the agglomerates of lower porosity, which is really the case. The agglomerate porosity affects the powder behaviour under the compaction pressure. The powder derived from the gel prepared from the concentrated solutions reaches higher densities during uniaxial pressing and sintering.
11.
REFERENCES
12.
1. KINGERY, W.D. & FRANCOIS, B., Sintering of crystalline oxides. I. Interreactions between grain boundaries and pores. In Sintering and Related Phenomena, ed. G.C.
5. 6.
7.
8.
9.
10.
13.
Kuczynski, N.A. Hooton, & G.F. Gibbon. Gordon Breach, New York, 1967, pp.471-98. KELLETT, B.J. & LANGE, F.F., Thermodynamics of densification: I, Sintering of simple particle arrays, equilibrium configurations, pores stability and shrinkage. J. Am. Ceram Soc., 72 (1989) 725-34. LANGE, F.F. & KELLETT, B.J., Thermodynamics of densification: II, Grain growth in porous compacts and relation to densification. J. Am. Ceram Soc., 72 (1989) 735-41. RUMPF, H., Principles and method of granulation. Chem. Ing. Technol., 30 (1958) 144-58. HABERKO, K., Characteristics and sintering behaviour of zirconia ultrafine powders. Ceramurgia International, 5 (1979) 148-54. HABERKO, K., Preparation and properties of zirconia micropowders. Scientific Bulletins of the University of Mining and Metallurgy, No. 931, CERAMICS 47, Cracow, Poland, 1983 (in Polish). ROOSEN, A. & HAUSNER H., Sintering kinetics of ZrO 2 powders. In Advances in Ceramics 1Ioi. 12, Science and Technology of Zirconia II, ed. N. Claussen, M. ROhle & A.H. Heuer. The American Ceramic Society, Columbus, OH, 1984, pp. 714-26. HABERKO, K. & PYDA, W., Preparation of Ca-stabilized ZrO2 micropowders by a hydrothermal method. In Advances in Ceramics, Vol. 12, Science and Technology of Zirconia II, ed. N. Claussen, M. Rt~hle & A.H. Heuer. The American Ceramic Society, Columbus, OH, 1984 pp. 774-83. PYDA, W., HABERKO K. & BUCKO, M.M., A study on preparation of tetragonal zirconia polycrystals (TZP) in the TiO2-Y203-ZrO 2 system, Ceramics Int., 18 (1992) 321-6. PYDA, W., HABERKO, K. & ZUREK Z., Zirconia stabilized with a mixture of the rare earth oxides. J. Europ. Cer. Soc., 10 (1992)453-9. MAMOTT, G.T, BARNES, P., TARLING, S.E., JONES, S.L. & NORMAN, C.J., Dynamic studies of zirconia crystallization. J. Mat. Sci., 26 (1991) 4054-61. SCOTT, H.G., Phase relationship in the zirconia-yttria system, J. Mat. ScL, 10 (1975) 1527-35. HABERKO, K., Some properties of zirconia obtained obtained by coprecipitation with different oxides, Rev. Int. Htes. Temp. et Refract, 14 (1977) 217-24.
Coprecipitation Conditions and Compaction Behaviour of Y-TZP Nanometric Powders Z. Pqdzich & K. Haberko Department of Special Ceramics, University of Mining and Metallurgy, 30-059 Cracow, AI. Mickiewicza 30, Poland (Received 2 May 1993; accepted 27 May 1993)
Abstract: The concentration of the starting ZrOC12 + YC13 solution influences
the coprecipitated gel morphology. The gel prepared from the highly concentrated solution shows a loose microstructure. Its calcination results in a 3Y-TZP powder of relatively weak agglomerates. The latter collapse during uniaxial pressing under 200 MPa in contrast to 400 MPa pressure observed in the case of a powder derived from the gel coprecipitated from the diluted ZrOC12 + YC13 solution. Compacts of the powder composed of weak agglomerates attain higher densities during both cold pressing and sintering.
1 INTRODUCTION
methods were used in the case of zirconia solid solution powders. The coprecipitated hydroxides ZrOz-Y203, ZrOz-Nd203 and ZrOz-CaO systems, washed with the liquids of low surface tension resulted in the fluffy gels. 5'6 Their calcination gave powders composed of highly porous and mechanically weak agglomerates. Similar results were achieved by the freeze drying of the coprecipitated gels7 and their subsequent calcination. Crystallization of the zirconia solid solutions under hydrothermal conditions results also in the mechanically weak agglomerates8-1°. This is mainly due to the weak, second-order bonds between the particles crystallized in the presence of water, which is the liquid perfectly wetting zirconia and penetrating the interparticle contacts. Precipitating conditions can influence the properties of the resulting zirconia gel. For example, Mamott e t al. ~ have studied the effect of the pH of the liquid surrounding the zirconium hydroxide precipitate. The observed relationship between the precipitation conditions and the onset temperature of crystallization as well as the rate of crystallization and crystal growth have been interpreted in terms of the degree of gel ordering. The objective of the present study was to determine the effect of the starting solution concentrations on the resultant gel and crystalline powder morphology.
Among various methods of preparation of ceramic oxide multi-component nanometric powders, the coprecipitation technique seems to be the most frequently used. However, the high specific surface area of these powders leads to problems with their agglomeration. Large pores can be formed in a green compact among strong agglomerates. A large pore can be thermodynamically stable depending on the dihedral angle and the pore size:grain (crystallite) size ratio. ~-3 For a given dihedral angle there is a critical pore size, as measured by the number of crystallites surrounding the pore, above which the pore becomes stable (and cannot sinter) and below which it is unstable (and can sinter). In order to overcome these problems it is necessary to control the agglomerate strength and/ or size. Mechanically weak agglomerates can easily be ground and/or collapse under the applied compaction pressure. Both processes decrease the fraction of the overcritical pores in the green compact or even reduce it to zero. According to Rumpf ~ the strength of powder agglomerates composed of crystallites of a given size can be reduced by increasing the agglomerate porosity and/or decreasing the force necessary to break the link between the two crystallites. Both 85
Ceramics Internationa10272-8842/94/$7.00 © 1994 Elsevier Science Limited, England and Techna S.r.l., Italy. Printed in Great Britain
86 2 EXPERIMENTAL
The TZP composition of Y203 of 3 mol% and of Z r O 2 of 97 mol% was selected for the present study. Two aqueous solutions of ZrOC12 + YCI 3 of different concentrations were prepared; a highly concentrated solution with a total salt content of 0.8940 mol/litre and a 20 times diluted solution with a salt concentration of 0.0447 mol/litre. Both solutions were hydrolyzed by pouring them into a vigorously stirred ammonium hydroxide solution. The solution with the higher salt concentration was poured into highly concentrated NH4OH (5 mole/litre) and the solution with the low salt concentration was added to diluted ammonium hydroxide (1.67 mole/litre). The volume rate of pouring of the ZrOC12 + YC13 solution into the NHaOH one was the same in both cases (100 cm3/ min). The final pH of thesuspension was 9.0. The materials were then carefully washed with distilled water by decantation. Washing was continued until the filtrate electrical conductivity dropped down to the level of distilled water (~10 /zS at room temperature). Drying of the gels was performed at 120°C. The gels were ground in an agate mortar under the same and strictly controlled conditions. The gels were calcined under isothermal conditions using temperatures within the range of 450-1000°C and with 30 min soaking at each of the selected temperatures. One part of the experiment was performed with the nonisothermally calcined gels at 700°C for 30 min. In this case the rate of temperatui'e increase was 6°C/min. Specific surface area (Sw) measurements of the crystalline powders were performed by the BET method (Carlo Erba, Sorpty 1750). On this basis the BET particle size (DBET) w a s calculated using the relation assuming spherical or cubic shape of the particles: DBET = 6 / p S w, where p is the X-ray density. Crystallite size measurements were based on the X-ray line broadening of the (l 1l) tetragonal reflection (CuKa). Mercury porosimetry (Carlo Erba, Porosimeter 2000) was helpful in the determination of the inter- and intra-agglomerate porosity in the powder compacts. The X-ray density of the tetragonal 3Y-TZP (6.044 g/cm3) was calculated on the basis of the generally accepted zirconia solid solution models and the unit cell sizes determined by Scott. 12 This value was used throughout the work to assess densification of the compacts, agglomerates and sintered samples. 3 RESULTS A N D DISCUSSION
The dry gel precipitated from the concentrated solution is porous, 'chalklike' and mechanically
Z. Pcdzich, K. Haberko
weak. The gel prepared from the diluted solution is dense, opalescent and mechanically strong. Infrared spectroscopic examination of the gels revealed no difference between their spectra. This seems to indicate that the two different coprecipitation conditions applied in the present work have no influence on the gel short range order. This conclusion seems to be corroborated by the DTA measurements. They show the characteristic exothemic peak attributed to the crystallization of the zirconia solid solution. 13 No difference of the crystallization temperature (450°C) between the gels is observed. In Fig. 1, the crystallite size (D1,) and the BET particle size (DBE'r) as a function of isothermal calcination temperature, are shown. The Din values of the both powders do not differ up to the calcination temperature of 600°C. This suggests that the sizes of the ordered areas in both kinds of gels are comparable. According to Mamott et al. ~ within such areas the primary crystallites are formed during the gel calcination. At higher temperatures the crystallite growth becomes more distinct especially in the samples derived from the diluted solution. This fact can be plausibly explained if we realize that the starting gel in this case is more dense than the one precipitated from the concentrated solution, as suggested by the gel morphology. The crystallite growth occurs by the movement of the inter-crystalline boundaries. Thus, it should be faster in the envi30
a
_,ffi 25 o
20
i
r
160 140
120 o o
100
80 60 40
20 7 7 0 50O 400
i
i
i
t
600
700
800
900
1000
Temperature [C]
Fig. 1. Crystallite size ( D u O a n d B E T particle size (DBET)
of the powders as functions of calcination temperature. (A) and (B) correspond to the powders derived from the gels coprecipitated from the concentratedsolution and the diluted one, respectively.
87
Coprecipitation and compaction of Y-TZP nanometric powders ronment in which each particle contacts with the larger number of the neighbouring crystallites. The mean number of contacts per one crystallite (LK) can be assessed on the basis of the Rumpf's approximate relation: 4 LK . Vp -" "/T, where Vp is the pore volume fraction. It will be shown later in the text that the agglomerate porosities of the powders calcined at 700°C are 61% and 51% for the samples derived from the concentrated and diluted solutions, respectively. This leads to the 20% difference in the mean number of the neighbours per particle. It seems to substantiate the observed difference between the Dl~t values between the samples under discussion. Specific surface area changes, as reflected by the changes of the BET particle size, with the calcination temperature (Fig. 1) can be explained by the crystallite growth and/or by the decrease of the inter-crystalline surface area. Most probably the latter process becomes active at the higher calcination temperatures, as is indicated by an increase in the discrepancy between the D l l I and DBET sizes with the calcination temperature. The marked difference between the DBE-r values of both kinds of powders, observed in the samples calcined at 1000°C, indicates that this process is much faster within agglomerates of higher density. The powders calcined (nonisothermally) at 700°C were selected for the further experiments. Their basic characteristics are given in Table 1. The plots of the relative density of the green compacts as a function of log compaction pressure are shown in Fig. 2. The breaking point between the straight lines is strongly dependent on the preparation method of the starting gel. In the case of the powder derived from the dense gel (i.e. the gel coprecipitated from the diluted solution) the breaking point occurs at the compaction pressure of 400 MPa, compared to 200 MPa for the second powder. According to the previous investigations on the zirconia nanometric powder compaction, 5'6 the breaking point corresponds to the decohesion of the contacts between crystaUites within the agglomerates. Thus, the observed behaviour shows the marked influence of the coprecipitation conditions on the agglomerate strength in the resultant powders.
50 46 42 o
38 34
o
30 50
100
200 300 400 600 MPa
Log Compaction Pressure Fig. 2. Compaction diagrams of the powders calcined at 700°C (for details of (A) and (B) see Fig. 1).
In order to understand the compaction process, performed under much higher pressures than those applied in previously reported investigations, 5,6'8 pore size distribution measurements were performed. Figure 3 demonstrates the cumulative pore size distribution curves in the samples compacted under the indicated pressures. Generally, two different-sized pore populations are observed: small, inter-agglomerate pores and much bigger intra-agglomerate pores. The curves for the two kinds of the powders differ in shape. In the case of the strong agglomerates (B), the transition between the inter- and intra-agglomerate porosity is very sharp, in contrast to that of the weak agglomerates. Most probably in the latter case the pore volume separating the inter- and intra-agglomerate porosity is filled with the small fragments of the agglomerates. The latter are created during the agglomerate rearrangement under the pressures up to the breaking point in the compaction diagram (Fig. 2). This process is less distinct in the case of the dense and strong agglomerates. On the basis of the pore size distribution curves and the total porosity of the compacts, the interand intra-agglomerate porosity could be assessed. The results versus compaction pressure are shown in Fig. 4. They indicate that under the low pressures 300 240
E oJ E 180
>o
Table 1. Characteristics of the noniaothermally calcined powders Powder
Sw (m2/g)
DBET (nm)
D l l l (nm)
A B
66.6 62-4
14.9 + 0-6 15.9 _+0.6
14-6 + 0.6 17.5 + 0.8
.~ 120
E E 60
d
0
. . . . . . . .
;
10
. . . . . . . .
i
. . . . . . . .
100 Pore radius (nm)
i
1000
.
.
.
.
....
10000
Fig. 3. Cumulative pore size distribution curves of the
A, Powder derived from the concentrated solution. B, Powder derived from the diluted solution.
samples compacted under 100 MPa (A1 and B1) and 600 MPa (A2 and B2) (for details of (A) and (B) see Fig. 1).
88
Z. Pcdzich, K. Haberko 35O
n
A
T
3O0
1
n
250 E
200
.~-
150
s.
o
100
la
[]
L]
5O i
100
200
300
400
500
600
700
Compaction Pressure [MPa] 350
B
3OO AT
"--- 250
200
•
I .t
A
150 0
la
I00
50 0
,
0
i
i
100
200
,
i
300
,
i
400
,
i
i
500
600
,
700
C o m p a c t i o n Pressure [MPa]
Fig. 4. Total porosity (T), inter- (I) and intra-agglomerate (Ia) porosity as a function of compaction pressure (for details of (A) and (B) see Fig. 1). (up to the breaking point) decrease of the total porosity occurs by the agglomerate rearrangement. This is substantiated by the constant interagglomerate porosity and the decrease of the intra-agglomerate space with pressure. Under the higher pressures, but close to the breaking point, reduction of the total porosity is realized by the agglomerate deformation. The agglommerates fill the intra-agglomerate space. This is possible because of the decohesion of the intercrystalline bonds within the agglomerates. During this process, abrupt decease of the intraagglomerate porosity versus pressure occurs, but the inter-agglomerate porosity remains constant. 100 ~
9O
e~
~
8O
~
70
~
6o
~
50
B
0
~
100
±
200
300
400
500
600
700
C o m p a c t i o n Pressure [MPa]
Fig. 5. Relative density of the samples sintered at 1200°C for 1 h as a function of compaction pressure (for details of (A) and (B) see Fig. 1).
Fig. 6. Fracture surface of the sintered samples. Compaction pressure 600 MPa; (A) and (B) correspond to strong and weak agglomerates respectively. Porosity of sample (A) is 10% and sample (B) is 29.5%.
Simultaneously, the increased slope of the compaction diagram is observed (see Fig. 2). Such a behaviour was previously observed in zirconia nanometric powders, 5'6 However, under still higher pressures (>250 MPa and >500 MPa for the A and B powders, respectively) the reduction of the total porosity occurs by the two competing processes: filling up the intra-agglomerate porosity and compression of the inter-agglomerate space. This is substantiated by the volume reduction of both kinds of pores. In compacts of powder A, the intra-agglomerate porosity becomes undiscernible from the inter-agglomerate one under the pressures of 500 MPa and higher (see Figs 3 and 4). In Fig. 5 the relative density of the samples sintered at 1200°C for 1 h is shown. The marked difference between the two kinds of the samples can plausibly be explained on the basis of the critical coordination pore number concept. 2,3 Since the powder composed of the weak agglomerates densities better in the whole compaction pressure
89
Coprecipitation and compaction o f Y - T Z P nanometric powders
range, the volume fraction of the undercritical pores should be higher in this case. Hence, densification of this system during sintering should be better, which is the case. This is illustrated by the microstructure o f the sintered samples (Fig. 6).
2.
3.
4 CONCLUSIONS 4.
The results presented indicate that the concentration of the starting solutions strongly influences the m o r p h o l o g y of the coprecipitated gels in the ZrO2-Y20 3 system. The gel derived from the diluted solution is dense compared to that prepared from the concentration solution. Since the ordered areas within the both gels are probably o f the same size, one must conclude that their compaction is dependent on the precipitation conditions. When the process is performed with the concentrated solutions it is fast and results in the fluffy gel microstructure. Densification of the coprecipitated gels influences the microstructure o f the resultant crystalline powder. The powder derived from the dense gel should be c o m p o s e d of the agglomerates of lower porosity, which is really the case. The agglomerate porosity affects the powder behaviour under the compaction pressure. The powder derived from the gel prepared from the concentrated solutions reaches higher densities during uniaxial pressing and sintering.
11.
REFERENCES
12.
1. KINGERY, W.D. & FRANCOIS, B., Sintering of crystalline oxides. I. Interreactions between grain boundaries and pores. In Sintering and Related Phenomena, ed. G.C.
5. 6.
7.
8.
9.
10.
13.
Kuczynski, N.A. Hooton, & G.F. Gibbon. Gordon Breach, New York, 1967, pp.471-98. KELLETT, B.J. & LANGE, F.F., Thermodynamics of densification: I, Sintering of simple particle arrays, equilibrium configurations, pores stability and shrinkage. J. Am. Ceram Soc., 72 (1989) 725-34. LANGE, F.F. & KELLETT, B.J., Thermodynamics of densification: II, Grain growth in porous compacts and relation to densification. J. Am. Ceram Soc., 72 (1989) 735-41. RUMPF, H., Principles and method of granulation. Chem. Ing. Technol., 30 (1958) 144-58. HABERKO, K., Characteristics and sintering behaviour of zirconia ultrafine powders. Ceramurgia International, 5 (1979) 148-54. HABERKO, K., Preparation and properties of zirconia micropowders. Scientific Bulletins of the University of Mining and Metallurgy, No. 931, CERAMICS 47, Cracow, Poland, 1983 (in Polish). ROOSEN, A. & HAUSNER H., Sintering kinetics of ZrO 2 powders. In Advances in Ceramics 1Ioi. 12, Science and Technology of Zirconia II, ed. N. Claussen, M. ROhle & A.H. Heuer. The American Ceramic Society, Columbus, OH, 1984, pp. 714-26. HABERKO, K. & PYDA, W., Preparation of Ca-stabilized ZrO2 micropowders by a hydrothermal method. In Advances in Ceramics, Vol. 12, Science and Technology of Zirconia II, ed. N. Claussen, M. Rt~hle & A.H. Heuer. The American Ceramic Society, Columbus, OH, 1984 pp. 774-83. PYDA, W., HABERKO K. & BUCKO, M.M., A study on preparation of tetragonal zirconia polycrystals (TZP) in the TiO2-Y203-ZrO 2 system, Ceramics Int., 18 (1992) 321-6. PYDA, W., HABERKO, K. & ZUREK Z., Zirconia stabilized with a mixture of the rare earth oxides. J. Europ. Cer. Soc., 10 (1992)453-9. MAMOTT, G.T, BARNES, P., TARLING, S.E., JONES, S.L. & NORMAN, C.J., Dynamic studies of zirconia crystallization. J. Mat. Sci., 26 (1991) 4054-61. SCOTT, H.G., Phase relationship in the zirconia-yttria system, J. Mat. ScL, 10 (1975) 1527-35. HABERKO, K., Some properties of zirconia obtained obtained by coprecipitation with different oxides, Rev. Int. Htes. Temp. et Refract, 14 (1977) 217-24.