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Zirconia nano-powder – a useful precursor to dense polycrystals
Ceramics International (xxxx) xxxx–xxxx
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Zirconia nano-powder – a useful precursor to dense polycrystals Radosław Lach, Kamil Wojciechowski, Dariusz Zientara, Krzysztof Haberko, Paweł Rutkowski, ⁎ Mirosław M. Bućko AGH University of Science and Technology, Faculty of Materials Science and Ceramics, al. Mickiewicza, 30, 30-059 Krakow, Poland
A R T I C L E I N F O
A BS T RAC T
Keywords: A. Sintering B Microstructure-final D. ZrO2
The main problem of utilization of ceramic nanopowders consists in their tendency to form hard agglomerates. The inter-agglomerate space forms pores surrounded by numerous elementary particles (crystallites). Such pores cannot be easily removed from the system during pressureless sintering. So, the obvious strategy for a technologist is to eliminate such pores at the level of shaping operations. Dry pressing is a frequently applied shaping technique of ceramic materials. That is why mechanical strength of agglomerates should be as low as possible. It enables the elimination of inter-agglomerate pores to occur under moderate pressures, applied during the shaping process. This problem will be illustrated using the really nanometric (below 10 nm) zirconiayttria solid solution powder. The methods of effecting agglomerate strength will be shown. One of the elaborated powder preparation rout results in extremely soft agglomerates.
1. Introduction Numerous measurements indicate that the grain boundary surface energy is essentially lower than the energy of free surface of grains. Since surface energy is a part of free enthalpy of a system, the indicated difference is, from a thermodynamic point of view, the driving force of sintering of a powder compact. Capillary forces are responsible for the mass transfer during sintering. These two factors indicate that the finer is a ceramic powder the higher is thermodynamic tendency and the higher are capillary forces operating during densification of a powder compact. That is why a ceramic nano-powder, i.e. a powder of particle sizes about 10 nm, should be recognized as especially attractive from this point of view. There are two ways of producing of fine ceramic powders; one is based on the breaking down solid grains by their comminution. The other way consists in the build-up (crystallization) process. Particle sizes, manufactured by the former method, are limited to 0.1–1 µm. The build-up process can be arrested at the crystallite size level even below 10 nm and specific surface area of the resulting powder surpassing 100 m2/g. Small particle (crystallite) sizes are not the only feature of the particles prepared by the crystallization process. Another one is the narrow particle size distribution, important for the powder processing behaviour. This can be illustrated by the plots of the particle size distribution, presented in the Rosin-Rammler coordinates (Fig. 1). One of them was prepared by the ball milling (BM) and the other one by the
⁎
8 mol% Y2O3-ZrO2 solid solution powder, crystallized under hydrothermal conditions (H). The details of the hydrothermal process are described elsewhere [1–4]. Note that particle sizes of the BM powder are expressed in micrometers and, in the case of the hydrothermally crystallized (H) one, in nanometers. The higher is the slope ratio (n) of the straight line, the narrower is the particle size distribution. The main problem in application of such fine powders (H) as the one shown in Fig. 1 is their tendency to form hard agglomerates. If they survive compaction of a green sample the final microstructure is far from full densification. This is illustrated in Fig. 2. According to Kingery's and Francoise [5] concept the reason of such a phenomenon can be related to the existence of pores characterized by the overcritical coordination number, i.e. surrounded by a number of grains leading to pore shapes such as those shown in Fig. 3a. The critical coordination number depends on the dihedral angle (ψ). The pores of under-critical coordination number (cf. Fig. 3b) shrink during sintering. Basing on the thermodynamic calculations by Kellet and Lange [6,7] show that the pores characterized by the overcritical coordination number shrink only to an equilibrium level. Their further elimination is possible when such pores become under-critical due to the grain growth phenomenon. Unfortunately, larger grain sizes lead to the elongated diffusion distances and by themselves to slowing-down of the pore elimination process. That is why much a better strategy consists in limiting the volume fraction of large pores at the level of a green body preparation. Large pores, surrounded by numerous grains (crystal-
Corresponding author. E-mail address: [email protected] (M.M. Bućko).
http://dx.doi.org/10.1016/j.ceramint.2016.12.097 Received 7 June 2016; Received in revised form 19 December 2016; Accepted 19 December 2016 0272-8842/ © 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Lach, R., Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.12.097
Ceramics International (xxxx) xxxx–xxxx
R. Lach et al.
Fig. 4. Powder preparation routes. Fig. 1. Particle size distribution of the ball milled (BM) glass powder and the 8 mol. % Y2O3-ZrO2 solid solution powder crystallized under hydrothermal conditions (H) at 240 °C for 4 h. d – particle size, R(d) – mass fraction of the particles larger than the size d.
Pc =
9 1 − Vp ⋅ ⋅LK FK 8 πd 2
(1)
where: Vp – is pore fraction in an agglomerate, LK – mean number of contacts per particle, FK -force necessary to separate two particles in contact and d - particle size. Our studies are based on the application of really nanometric 8 mol % Y2O3-ZrO2 powders manufactured by calcination of the co-precipitated zirconia-yttria gel or by its crystallization under hydrothermal conditions. The details of such powder preparation conditions are described in [1–4,9–11]. In the present work we focused on overcoming the problems encountered in the application of nanometric powders of the crystallite sizes of about 10 nm. 2. Experimental Fig. 4 shows the powder preparation routs, applied by us. Aqueous solution of indicated salts was introduced to the vigorously stirred solution of ammonia. The co- precipitated gel was washed with distilled water in order to remove the soluble by-products of the reaction. A part of the gel was additionally washed with ethyl alcohol, dried and calcined (route 1). Washing with the liquid of low surface tension, in this case with ethyl alcohol, leads to the less compacted gel and hence to the highly porous powder agglomerates, resulting from the calcination of such a gel [9,10]. Calcination temperature was selected in order to lead to the crystallite sizes, close to those which result from the route 2 and 3. It should be emphasized that during calcination strong bonds develop among the ZrO2-Y2O3 solid solution crystallites. Due of the factor FK (Eq. (1)), agglomerates of increased strength should be expected. Routes 2 and 3 were based on our experience on the crystallization of zirconia solid solutions under hydrothermal conditions [1–4,11]. The co-precipitated X-ray amorphous zirconia gel transforms to the
Fig. 2. ZrO2-Y2O3 solid solution powder compacted under 200 MPa and sintered at 1500 °C. In this case hard agglomerates survived compacting pressure. Density of this material corresponds to 86%.
lites), result from the agglomerates which survived the shaping process. Dry compaction is the frequently used method of the green body shaping. Therefore mechanical strength of agglomerates is of the utmost importance for the final material microstructure build up. In the case of ceramic nanopowders the problem is especially acute, since agglomerate strength is strongly dependent on the crystallite sizes which form agglomerates. According to Rumpf [8], tensile strength (Pc) of the agglomerate is given by the relation:
Fig. 3. Real and model over-critical (a) under-critical (b) and pores in not fully densified 8 mol.% Y2O3-ZrO2 solid solution polycrystal. Insets show idealized pores.
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crystalline form when treated in an autoclave under autogenous water vapour pressure. Hydrothermal crystallization leads to the formation of weak, coagulation inter-crystalline bonds. Therefore the reduced strength of agglomerates should be expected. Route 3 differs from route 2 in freeze drying of the diluted hydrothermally crystallized powder in order to obtain agglomerates of high porosity. Factor (Vp) in Eq. (1) should be operating in the decreasing strength of agglomerates. Further experiments will demonstrate the effect of the applied preparation routes. The X-ray diffraction analysis (Empyrean, PANalytical) allowed us to determine the powders phase composition and powder crystallite sizes on the basis of the X-ray (111) line broadening of the cubic phase. CuKα radiation was applied. Pore size distribution in the green samples was measured by mercury porosimetry (PoreMaster 60 Quantachrome Instruments). The powder compaction behaviour was studied either by compacting samples of 10 mm diameter under selected pressures and measuring their weight and sizes (route 1 and 2) or by using the testing machine (Zwick/Roell Z020) and the die of 10 mm diameter (route 3). A certain amount of the tested powder was placed in a die. During the processing cycle the ram travel and transmitted load were automatically recorded in the computer memory. Knowing the geometry of the die and the final dimensions of the compact, it was possible to backcalculate the pressure-density response from the load-displacement curve. Dilatometric measurements (NETSCH DIL 402 C) were used to follow shrinkage of powder compacts vs. temperature. The temperature arrested at the preselected levels allowed us to manufacture samples of different densification and hence characterized by the different pore size distributions. This part of the work was limited to the compacts made of the powder prepared, according to route 3 only.
Fig. 6. Relative density vs. compaction pressure of the powders prepared according to route 3. Table 1 Characteristics of the starting powders and agglomerates. Properties
Route 1
Route 2
Route 3
Agglomerate porosity [%] Breaking point [MPa] D(111) [nm]
59 95 10.5
56 40 11.4
82 1 7.6
3. Results and discussion The well-established powder characteristic's method is based on the observation of its behaviour vs. compaction pressure. In one of the first publications on this problem [12] it was demonstrated that on the plot of green density vs. logarithm of compaction pressure the straight line segments could be observed. The fundamental interpretation of this behaviour was suggested by Niesz et al.[13]. The point of intersection between straight line segments corresponds to the pressure under which inter-particle contacts break [10,13]. To some extent this pressure is a measure of agglomerate strength. The load-density response of the powders prepared along route 1 and 2 are shown in Fig. 5 while in Fig. 6 the one which results from the route 3 is presented. The essential difference between all three powders is constituted by the pressure corresponding to the breaking point of the straight line segments. It indicates that strength of agglomerates is
Fig. 7. Cumulative curves illustrating pore size distribution in the powder compacts prepared according to route 1, 2 and 3. Compaction pressure 200 MPa.
Fig. 8. Cumulative curves of the pore size distribution curves of the powder prepared according to route 3. Compaction pressures, in MPa, indicated. Fig. 5. Relative density vs. compaction pressure of the powders prepared according to route 1 (calcined) and route 2 (hydrothermal).
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Fig. 9. Intra-agglomerate porosity vs. compaction pressure.
Fig. 11. Dilatometric plot of the powder compacted under 300 MPa. Rate of temperature increase 10 °C/min. Selected temperatures at which sintering process was interrupted and corresponding to these temperatures shrinkage values indicated.
strongly dependent on the powder processing method. Table 1 summarizes the results of these observations and additionally the porosity of agglomerates and X-ray determined crystallite sizes are pointed out. The porosity of agglomerates was extracted from the cumulative pore size distribution curves, corresponding to the compacted pressure lower than the indicated points of intersection of the straight line segments. Table 1shows that the powders manufactured according to route 1 and 2 do not differ essentially in relation to their crystallite sizes and agglomerate porosity. So, the distinct difference of the pressure under which agglomerates start to break should be attributed to the nature of the inter-crystalline bonds. Strong bonds build up during the calcination process and only weak (coagulation) bonds occur in the hydrothermally processed powder. An essential difference between both powders is constituted by the pressure under which inter-crystalline bonds break. From the point of view of Rumpf's equation a further decrease of this pressure is possible if the volume fraction of intra-agglomerate porosity is increased. It was achieved by freeze drying of diluted (8 vol %) suspension of the hydrothermally crystallized powder. The results are shown in Table 1 and Fig. 6. The point of intersection of the two straight sections corresponds to the pressure below 1 MPa. This is a very low value corresponding rather to the de-cohesion pressure of the spray dried granules [14]. Cumulative curves in Fig. 7 illustrate a different behaviour of the powders prepared according to routes 1, 2 and 3, compacted under
200 MPa. Only in the case of route 1 a trace of inter-agglomerate porosity is visible. It is due to the highest strength of these agglomerates as it is indicated by the highest pressure under which these agglomerate start to break (cf Fig. 5 and Table 1). Much weaker agglomerates of the powders prepared by route 2 and 3 become totally destroyed, so the only inter-crystalline space is visible in Fig. 7. As it was pointed out previously, porosity of agglomerates could be assessed on the basis of the pore size distribution curves in the powder compacts. This is shown in Fig. 8 of the powder prepared according to route 3. The points indicated separate inter- and intra-agglomerate pore volumes in the plots. Inter-agglomerate pores lose their identity (become invisible) in the samples compacted under pressures > 15 MPa. Up to this pressure we are able to follow changes of intraagglomerate porosity vs. compaction pressure. We can observe (Fig. 9) that it drops down under the pressures corresponding to the break of the inter-crystallite contacts. So the total porosity decrease vs. pressure occurs due to the decrease of both types of pore volumes (inter- and intra-agglomerate ones). Such a behaviour can be related to the extremely low strength of agglomerates due to their high porosity and weak inter-crystalline bonds. Pressure increase influences distinctly the sintered material density and its microstructure. Fig. 10 shows fracture surface of the powder (route 3) sample, compacted under 0.5 MPa and 300 MPa and sintered at 1150 °C for 2 h. Since 0.5 MPa compaction pressure is lower than that indicated by the breaking point of the straight line segments (Fig. 6), we can clearly
Fig. 10. Fracture surface of the samples sintered at 1150 °C for 2 h. a) powder compaction pressure 0.5 MPa, density 63.4%, b) powder compaction pressure 300 MPa, density 96.2%,.
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Fig. 12. Pore size distribution in the samples heated up to different temperatures indicated in Fig. 11. a) cumulative curves, b) differential curves.
3. Strength of agglomerates is the key problem in application of nanometric powders. It depends on the nature of the inter-crystalline bonds, porosity of agglomerates and on particle sizes. Freeze drying of the diluted powder suspension allowed us to receive powder composed of agglomerates of about 80% porosity whose de-cohesion starts under pressure as low as < 1 MPa. 4. A strong effect of the pore size distribution in the powder compact on density and microstructure of the sintered material is evident. 5. The analysis of the pore size distribution shows its shift towards higher pore sizes. However, distribution of pore sizes becomes narrow within the investigated system densification.
Table 2 Changes of the powder compacts (route 3) on non-isothermal sintering. Temperature/ properties
D (111) [nm]
ΔL/ Lo [%]
Rel. density* [%]
Modal pore size [nm]
Relative pore size distribution [%]
green body 850 °C 950 °C 1000 °C 1050 °C
7.6 11.3 15.4 32.4 34.0
– 2.08 3.64 5.30 7.53
48.01 49.02 53.90 57.09 62.02
4.7 8.1 11.7 13.7 15.7
45.2 35.8 30.7 28.7 25.5
* ) Relative density calculated applying X-ray density of the applied powder 5.9979 g/ cm3[15].
Acknowledgment
observe undestroyed agglomerates of this powder. This is not the case when high compaction pressure is applied. Both samples differ essentially in their densification. Fig. 11 shows shrinkage of the samples (route 3) compacted under 300 MPa sintered in the dilatometer. Rate of the temperature increase 10 °C/min was applied. Pore size distributions in the samples heated up to the indicated temperatures are shown in Fig. 12. The cumulative (Fig. 12a) and differential (Fig. 12b) plots indicate shift of the pore sizes towards higher values vs. sintering temperature. Simultaneously, porosity of the compacts decreases. The analyses of the data are collected in Table 2. As it should be expected, we can observe the increase crystallite sizes vs. temperature. It was assessed on the basis of the X-ray line broadening (D(111)). Also linear shrinkage (ΔL/Lo) of the samples is given as well as their relative density. The latter was calculated assuming their uniform shrinkage. Increased modal pore sizes with sintering temperature occur. It should be emphasized that these sizes are essentially smaller than the crystallite ones. It seems to indicate that the distinctly narrow pore size distribution throughout the investigated density range occur. This tendency is corroborated by the decreasing relative pore size distribution with temperature. The latter is defined as the pore size range at the half width of their distribution (Fig. 12b), divided by the modal pore size. Such a behaviour should not be expected if traces of agglomerates could survive the compaction procedure of the powder. In other words, the described phenomenon points out that the over-critical pore volume fraction in the starting compact is extremely low.
The work was financially supported by the Polish Nacional Science Centre under grant DEC-2011/03/B/ST8/06286. References [1] K. Haberko, W. Pyda, Preparation of Ca-stabilized zirconia micropowders by hydrothermal method, Science and Technology of Zirconia II, Advances in Ceramics 12, The American Ceramic Society, Columbus, OH, 1984, pp. 774–783. [2] W. Pyda, K. Haberko, M.M. Bućko, Hydrothermal crystallization of zirconia and zirconia solid solutions, J. Am. Ceram. Soc. 74 (1991) 2622–2629. [3] M. Bućko, K. Haberko, M. Faryna, Crystalisation of zirconia under hydrothermal conditions, J. Am. Ceram. Soc. 78 (1995) 3397–3400. [4] M.M. Bućko, K. Haberko, Mechanism of the hydrothermal crystallisation of zirconia, Key Eng. 132–136 (1997) 2072–2075. [5] W.D. Kingery, B. Francoise, Sintering of crystalline oxides, I: Interaction between grain boundaries and pores, in: G.C. Kuczyński, N.A. Hooten, G.N. Gibson (Eds.), Sintering and Related Phenomena, Gordon and Breach, New York, 1967, pp. 471– 498. [6] B.J. Kellet, F.F. Lange, Thermodynamics of densification: I, sintering of simple particle arrays, equilibrium configuration, pore stability and shrinkage, J. Am. Ceram. Soc. 72 (1989) 725–734. [7] F.F. Lange, B.J. Kellet, Thermodynamics of densification: II, Grain growth in porous compacts and relation to densification, J. Am. Ceram. Soc. 72 (1989) 735–741. [8] H. Rumpf, Grundlagen und Methoden des Granulieren, Chem. Ing. Tech. 30 (1958) 144–158. [9] K. Haberko, Some properties of zirconia obtained by co-precipitation with different oxides, Rev. Int. Htes Temp. Et. Refract. 14 (1977) 217–224. [10] K. Haberko, Characteristics and sintering behaviour of zirconia ultrafine powders, Ceramurg. Int. 5 (1979) 145–148. [11] K.Haberko, M.Bućko, M.Haberko, M.Jaśkowski, W.Pyda, Preparation of ceramic micropowders by hydrothermal treatment, in: Freiberger Foschungshefte A779, Teil 2, Herstellen und Charakteristiken Feinster Pulver, VEB Deutscher Verlag fur Grundstatt – Industriepp. 71–83, 1988. [12] C.A. Bruch, Problems in dye pressing submicron size alumina powder, Ceram. Age 83 (1967) 44–53. [13] D.E. Niesz, R.B. Bennet, M.J. Snyder, Strength characterization of powder aggregates, Am. Ceram. Soc. Bull. 51 (1972) 677–680. [14] G.L. Messing, C.J. Markhoff, L.G. McCoy, Characterization of ceramic powder compaction, Am. Ceram. Soc. Bull. 61 (1982) 857–860. [15] R. Lach, M.M. Bućko, K. Haberko, M. Sitarz, K. Cholewa-Kowalska, From nanometric powder to transparent polycrystal, J. Eur. Ceram. Soc. 34 (2014) 4321–4326.
4. Conclusions 1. Really nanometric powders can be manufactured by the bottom-up techniques. 2. Such powders are characterized not only by the nanometric particle (crystallite) sizes but also by the very narrow particle size distribution.
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Zirconia nano-powder – a useful precursor to dense polycrystals Radosław Lach, Kamil Wojciechowski, Dariusz Zientara, Krzysztof Haberko, Paweł Rutkowski, ⁎ Mirosław M. Bućko AGH University of Science and Technology, Faculty of Materials Science and Ceramics, al. Mickiewicza, 30, 30-059 Krakow, Poland
A R T I C L E I N F O
A BS T RAC T
Keywords: A. Sintering B Microstructure-final D. ZrO2
The main problem of utilization of ceramic nanopowders consists in their tendency to form hard agglomerates. The inter-agglomerate space forms pores surrounded by numerous elementary particles (crystallites). Such pores cannot be easily removed from the system during pressureless sintering. So, the obvious strategy for a technologist is to eliminate such pores at the level of shaping operations. Dry pressing is a frequently applied shaping technique of ceramic materials. That is why mechanical strength of agglomerates should be as low as possible. It enables the elimination of inter-agglomerate pores to occur under moderate pressures, applied during the shaping process. This problem will be illustrated using the really nanometric (below 10 nm) zirconiayttria solid solution powder. The methods of effecting agglomerate strength will be shown. One of the elaborated powder preparation rout results in extremely soft agglomerates.
1. Introduction Numerous measurements indicate that the grain boundary surface energy is essentially lower than the energy of free surface of grains. Since surface energy is a part of free enthalpy of a system, the indicated difference is, from a thermodynamic point of view, the driving force of sintering of a powder compact. Capillary forces are responsible for the mass transfer during sintering. These two factors indicate that the finer is a ceramic powder the higher is thermodynamic tendency and the higher are capillary forces operating during densification of a powder compact. That is why a ceramic nano-powder, i.e. a powder of particle sizes about 10 nm, should be recognized as especially attractive from this point of view. There are two ways of producing of fine ceramic powders; one is based on the breaking down solid grains by their comminution. The other way consists in the build-up (crystallization) process. Particle sizes, manufactured by the former method, are limited to 0.1–1 µm. The build-up process can be arrested at the crystallite size level even below 10 nm and specific surface area of the resulting powder surpassing 100 m2/g. Small particle (crystallite) sizes are not the only feature of the particles prepared by the crystallization process. Another one is the narrow particle size distribution, important for the powder processing behaviour. This can be illustrated by the plots of the particle size distribution, presented in the Rosin-Rammler coordinates (Fig. 1). One of them was prepared by the ball milling (BM) and the other one by the
⁎
8 mol% Y2O3-ZrO2 solid solution powder, crystallized under hydrothermal conditions (H). The details of the hydrothermal process are described elsewhere [1–4]. Note that particle sizes of the BM powder are expressed in micrometers and, in the case of the hydrothermally crystallized (H) one, in nanometers. The higher is the slope ratio (n) of the straight line, the narrower is the particle size distribution. The main problem in application of such fine powders (H) as the one shown in Fig. 1 is their tendency to form hard agglomerates. If they survive compaction of a green sample the final microstructure is far from full densification. This is illustrated in Fig. 2. According to Kingery's and Francoise [5] concept the reason of such a phenomenon can be related to the existence of pores characterized by the overcritical coordination number, i.e. surrounded by a number of grains leading to pore shapes such as those shown in Fig. 3a. The critical coordination number depends on the dihedral angle (ψ). The pores of under-critical coordination number (cf. Fig. 3b) shrink during sintering. Basing on the thermodynamic calculations by Kellet and Lange [6,7] show that the pores characterized by the overcritical coordination number shrink only to an equilibrium level. Their further elimination is possible when such pores become under-critical due to the grain growth phenomenon. Unfortunately, larger grain sizes lead to the elongated diffusion distances and by themselves to slowing-down of the pore elimination process. That is why much a better strategy consists in limiting the volume fraction of large pores at the level of a green body preparation. Large pores, surrounded by numerous grains (crystal-
Corresponding author. E-mail address: [email protected] (M.M. Bućko).
http://dx.doi.org/10.1016/j.ceramint.2016.12.097 Received 7 June 2016; Received in revised form 19 December 2016; Accepted 19 December 2016 0272-8842/ © 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Lach, R., Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.12.097
Ceramics International (xxxx) xxxx–xxxx
R. Lach et al.
Fig. 4. Powder preparation routes. Fig. 1. Particle size distribution of the ball milled (BM) glass powder and the 8 mol. % Y2O3-ZrO2 solid solution powder crystallized under hydrothermal conditions (H) at 240 °C for 4 h. d – particle size, R(d) – mass fraction of the particles larger than the size d.
Pc =
9 1 − Vp ⋅ ⋅LK FK 8 πd 2
(1)
where: Vp – is pore fraction in an agglomerate, LK – mean number of contacts per particle, FK -force necessary to separate two particles in contact and d - particle size. Our studies are based on the application of really nanometric 8 mol % Y2O3-ZrO2 powders manufactured by calcination of the co-precipitated zirconia-yttria gel or by its crystallization under hydrothermal conditions. The details of such powder preparation conditions are described in [1–4,9–11]. In the present work we focused on overcoming the problems encountered in the application of nanometric powders of the crystallite sizes of about 10 nm. 2. Experimental Fig. 4 shows the powder preparation routs, applied by us. Aqueous solution of indicated salts was introduced to the vigorously stirred solution of ammonia. The co- precipitated gel was washed with distilled water in order to remove the soluble by-products of the reaction. A part of the gel was additionally washed with ethyl alcohol, dried and calcined (route 1). Washing with the liquid of low surface tension, in this case with ethyl alcohol, leads to the less compacted gel and hence to the highly porous powder agglomerates, resulting from the calcination of such a gel [9,10]. Calcination temperature was selected in order to lead to the crystallite sizes, close to those which result from the route 2 and 3. It should be emphasized that during calcination strong bonds develop among the ZrO2-Y2O3 solid solution crystallites. Due of the factor FK (Eq. (1)), agglomerates of increased strength should be expected. Routes 2 and 3 were based on our experience on the crystallization of zirconia solid solutions under hydrothermal conditions [1–4,11]. The co-precipitated X-ray amorphous zirconia gel transforms to the
Fig. 2. ZrO2-Y2O3 solid solution powder compacted under 200 MPa and sintered at 1500 °C. In this case hard agglomerates survived compacting pressure. Density of this material corresponds to 86%.
lites), result from the agglomerates which survived the shaping process. Dry compaction is the frequently used method of the green body shaping. Therefore mechanical strength of agglomerates is of the utmost importance for the final material microstructure build up. In the case of ceramic nanopowders the problem is especially acute, since agglomerate strength is strongly dependent on the crystallite sizes which form agglomerates. According to Rumpf [8], tensile strength (Pc) of the agglomerate is given by the relation:
Fig. 3. Real and model over-critical (a) under-critical (b) and pores in not fully densified 8 mol.% Y2O3-ZrO2 solid solution polycrystal. Insets show idealized pores.
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crystalline form when treated in an autoclave under autogenous water vapour pressure. Hydrothermal crystallization leads to the formation of weak, coagulation inter-crystalline bonds. Therefore the reduced strength of agglomerates should be expected. Route 3 differs from route 2 in freeze drying of the diluted hydrothermally crystallized powder in order to obtain agglomerates of high porosity. Factor (Vp) in Eq. (1) should be operating in the decreasing strength of agglomerates. Further experiments will demonstrate the effect of the applied preparation routes. The X-ray diffraction analysis (Empyrean, PANalytical) allowed us to determine the powders phase composition and powder crystallite sizes on the basis of the X-ray (111) line broadening of the cubic phase. CuKα radiation was applied. Pore size distribution in the green samples was measured by mercury porosimetry (PoreMaster 60 Quantachrome Instruments). The powder compaction behaviour was studied either by compacting samples of 10 mm diameter under selected pressures and measuring their weight and sizes (route 1 and 2) or by using the testing machine (Zwick/Roell Z020) and the die of 10 mm diameter (route 3). A certain amount of the tested powder was placed in a die. During the processing cycle the ram travel and transmitted load were automatically recorded in the computer memory. Knowing the geometry of the die and the final dimensions of the compact, it was possible to backcalculate the pressure-density response from the load-displacement curve. Dilatometric measurements (NETSCH DIL 402 C) were used to follow shrinkage of powder compacts vs. temperature. The temperature arrested at the preselected levels allowed us to manufacture samples of different densification and hence characterized by the different pore size distributions. This part of the work was limited to the compacts made of the powder prepared, according to route 3 only.
Fig. 6. Relative density vs. compaction pressure of the powders prepared according to route 3. Table 1 Characteristics of the starting powders and agglomerates. Properties
Route 1
Route 2
Route 3
Agglomerate porosity [%] Breaking point [MPa] D(111) [nm]
59 95 10.5
56 40 11.4
82 1 7.6
3. Results and discussion The well-established powder characteristic's method is based on the observation of its behaviour vs. compaction pressure. In one of the first publications on this problem [12] it was demonstrated that on the plot of green density vs. logarithm of compaction pressure the straight line segments could be observed. The fundamental interpretation of this behaviour was suggested by Niesz et al.[13]. The point of intersection between straight line segments corresponds to the pressure under which inter-particle contacts break [10,13]. To some extent this pressure is a measure of agglomerate strength. The load-density response of the powders prepared along route 1 and 2 are shown in Fig. 5 while in Fig. 6 the one which results from the route 3 is presented. The essential difference between all three powders is constituted by the pressure corresponding to the breaking point of the straight line segments. It indicates that strength of agglomerates is
Fig. 7. Cumulative curves illustrating pore size distribution in the powder compacts prepared according to route 1, 2 and 3. Compaction pressure 200 MPa.
Fig. 8. Cumulative curves of the pore size distribution curves of the powder prepared according to route 3. Compaction pressures, in MPa, indicated. Fig. 5. Relative density vs. compaction pressure of the powders prepared according to route 1 (calcined) and route 2 (hydrothermal).
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Fig. 9. Intra-agglomerate porosity vs. compaction pressure.
Fig. 11. Dilatometric plot of the powder compacted under 300 MPa. Rate of temperature increase 10 °C/min. Selected temperatures at which sintering process was interrupted and corresponding to these temperatures shrinkage values indicated.
strongly dependent on the powder processing method. Table 1 summarizes the results of these observations and additionally the porosity of agglomerates and X-ray determined crystallite sizes are pointed out. The porosity of agglomerates was extracted from the cumulative pore size distribution curves, corresponding to the compacted pressure lower than the indicated points of intersection of the straight line segments. Table 1shows that the powders manufactured according to route 1 and 2 do not differ essentially in relation to their crystallite sizes and agglomerate porosity. So, the distinct difference of the pressure under which agglomerates start to break should be attributed to the nature of the inter-crystalline bonds. Strong bonds build up during the calcination process and only weak (coagulation) bonds occur in the hydrothermally processed powder. An essential difference between both powders is constituted by the pressure under which inter-crystalline bonds break. From the point of view of Rumpf's equation a further decrease of this pressure is possible if the volume fraction of intra-agglomerate porosity is increased. It was achieved by freeze drying of diluted (8 vol %) suspension of the hydrothermally crystallized powder. The results are shown in Table 1 and Fig. 6. The point of intersection of the two straight sections corresponds to the pressure below 1 MPa. This is a very low value corresponding rather to the de-cohesion pressure of the spray dried granules [14]. Cumulative curves in Fig. 7 illustrate a different behaviour of the powders prepared according to routes 1, 2 and 3, compacted under
200 MPa. Only in the case of route 1 a trace of inter-agglomerate porosity is visible. It is due to the highest strength of these agglomerates as it is indicated by the highest pressure under which these agglomerate start to break (cf Fig. 5 and Table 1). Much weaker agglomerates of the powders prepared by route 2 and 3 become totally destroyed, so the only inter-crystalline space is visible in Fig. 7. As it was pointed out previously, porosity of agglomerates could be assessed on the basis of the pore size distribution curves in the powder compacts. This is shown in Fig. 8 of the powder prepared according to route 3. The points indicated separate inter- and intra-agglomerate pore volumes in the plots. Inter-agglomerate pores lose their identity (become invisible) in the samples compacted under pressures > 15 MPa. Up to this pressure we are able to follow changes of intraagglomerate porosity vs. compaction pressure. We can observe (Fig. 9) that it drops down under the pressures corresponding to the break of the inter-crystallite contacts. So the total porosity decrease vs. pressure occurs due to the decrease of both types of pore volumes (inter- and intra-agglomerate ones). Such a behaviour can be related to the extremely low strength of agglomerates due to their high porosity and weak inter-crystalline bonds. Pressure increase influences distinctly the sintered material density and its microstructure. Fig. 10 shows fracture surface of the powder (route 3) sample, compacted under 0.5 MPa and 300 MPa and sintered at 1150 °C for 2 h. Since 0.5 MPa compaction pressure is lower than that indicated by the breaking point of the straight line segments (Fig. 6), we can clearly
Fig. 10. Fracture surface of the samples sintered at 1150 °C for 2 h. a) powder compaction pressure 0.5 MPa, density 63.4%, b) powder compaction pressure 300 MPa, density 96.2%,.
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Fig. 12. Pore size distribution in the samples heated up to different temperatures indicated in Fig. 11. a) cumulative curves, b) differential curves.
3. Strength of agglomerates is the key problem in application of nanometric powders. It depends on the nature of the inter-crystalline bonds, porosity of agglomerates and on particle sizes. Freeze drying of the diluted powder suspension allowed us to receive powder composed of agglomerates of about 80% porosity whose de-cohesion starts under pressure as low as < 1 MPa. 4. A strong effect of the pore size distribution in the powder compact on density and microstructure of the sintered material is evident. 5. The analysis of the pore size distribution shows its shift towards higher pore sizes. However, distribution of pore sizes becomes narrow within the investigated system densification.
Table 2 Changes of the powder compacts (route 3) on non-isothermal sintering. Temperature/ properties
D (111) [nm]
ΔL/ Lo [%]
Rel. density* [%]
Modal pore size [nm]
Relative pore size distribution [%]
green body 850 °C 950 °C 1000 °C 1050 °C
7.6 11.3 15.4 32.4 34.0
– 2.08 3.64 5.30 7.53
48.01 49.02 53.90 57.09 62.02
4.7 8.1 11.7 13.7 15.7
45.2 35.8 30.7 28.7 25.5
* ) Relative density calculated applying X-ray density of the applied powder 5.9979 g/ cm3[15].
Acknowledgment
observe undestroyed agglomerates of this powder. This is not the case when high compaction pressure is applied. Both samples differ essentially in their densification. Fig. 11 shows shrinkage of the samples (route 3) compacted under 300 MPa sintered in the dilatometer. Rate of the temperature increase 10 °C/min was applied. Pore size distributions in the samples heated up to the indicated temperatures are shown in Fig. 12. The cumulative (Fig. 12a) and differential (Fig. 12b) plots indicate shift of the pore sizes towards higher values vs. sintering temperature. Simultaneously, porosity of the compacts decreases. The analyses of the data are collected in Table 2. As it should be expected, we can observe the increase crystallite sizes vs. temperature. It was assessed on the basis of the X-ray line broadening (D(111)). Also linear shrinkage (ΔL/Lo) of the samples is given as well as their relative density. The latter was calculated assuming their uniform shrinkage. Increased modal pore sizes with sintering temperature occur. It should be emphasized that these sizes are essentially smaller than the crystallite ones. It seems to indicate that the distinctly narrow pore size distribution throughout the investigated density range occur. This tendency is corroborated by the decreasing relative pore size distribution with temperature. The latter is defined as the pore size range at the half width of their distribution (Fig. 12b), divided by the modal pore size. Such a behaviour should not be expected if traces of agglomerates could survive the compaction procedure of the powder. In other words, the described phenomenon points out that the over-critical pore volume fraction in the starting compact is extremely low.
The work was financially supported by the Polish Nacional Science Centre under grant DEC-2011/03/B/ST8/06286. References [1] K. Haberko, W. Pyda, Preparation of Ca-stabilized zirconia micropowders by hydrothermal method, Science and Technology of Zirconia II, Advances in Ceramics 12, The American Ceramic Society, Columbus, OH, 1984, pp. 774–783. [2] W. Pyda, K. Haberko, M.M. Bućko, Hydrothermal crystallization of zirconia and zirconia solid solutions, J. Am. Ceram. Soc. 74 (1991) 2622–2629. [3] M. Bućko, K. Haberko, M. Faryna, Crystalisation of zirconia under hydrothermal conditions, J. Am. Ceram. Soc. 78 (1995) 3397–3400. [4] M.M. Bućko, K. Haberko, Mechanism of the hydrothermal crystallisation of zirconia, Key Eng. 132–136 (1997) 2072–2075. [5] W.D. Kingery, B. Francoise, Sintering of crystalline oxides, I: Interaction between grain boundaries and pores, in: G.C. Kuczyński, N.A. Hooten, G.N. Gibson (Eds.), Sintering and Related Phenomena, Gordon and Breach, New York, 1967, pp. 471– 498. [6] B.J. Kellet, F.F. Lange, Thermodynamics of densification: I, sintering of simple particle arrays, equilibrium configuration, pore stability and shrinkage, J. Am. Ceram. Soc. 72 (1989) 725–734. [7] F.F. Lange, B.J. Kellet, Thermodynamics of densification: II, Grain growth in porous compacts and relation to densification, J. Am. Ceram. Soc. 72 (1989) 735–741. [8] H. Rumpf, Grundlagen und Methoden des Granulieren, Chem. Ing. Tech. 30 (1958) 144–158. [9] K. Haberko, Some properties of zirconia obtained by co-precipitation with different oxides, Rev. Int. Htes Temp. Et. Refract. 14 (1977) 217–224. [10] K. Haberko, Characteristics and sintering behaviour of zirconia ultrafine powders, Ceramurg. Int. 5 (1979) 145–148. [11] K.Haberko, M.Bućko, M.Haberko, M.Jaśkowski, W.Pyda, Preparation of ceramic micropowders by hydrothermal treatment, in: Freiberger Foschungshefte A779, Teil 2, Herstellen und Charakteristiken Feinster Pulver, VEB Deutscher Verlag fur Grundstatt – Industriepp. 71–83, 1988. [12] C.A. Bruch, Problems in dye pressing submicron size alumina powder, Ceram. Age 83 (1967) 44–53. [13] D.E. Niesz, R.B. Bennet, M.J. Snyder, Strength characterization of powder aggregates, Am. Ceram. Soc. Bull. 51 (1972) 677–680. [14] G.L. Messing, C.J. Markhoff, L.G. McCoy, Characterization of ceramic powder compaction, Am. Ceram. Soc. Bull. 61 (1982) 857–860. [15] R. Lach, M.M. Bućko, K. Haberko, M. Sitarz, K. Cholewa-Kowalska, From nanometric powder to transparent polycrystal, J. Eur. Ceram. Soc. 34 (2014) 4321–4326.
4. Conclusions 1. Really nanometric powders can be manufactured by the bottom-up techniques. 2. Such powders are characterized not only by the nanometric particle (crystallite) sizes but also by the very narrow particle size distribution.
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