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Mechanical alloying of ceramics in zirconia-ceria system
Materials Science and Engineering, A 183 (1994) L 9 - L 12
L9
Letter
Mechanical alloying of ceramics in zirconia-ceria system
for the broader applications of mechanical alloying but also for the synthesis of new structure ceramic materials.
Y. L. Chen, M. Qi and D. Z. Yang Department of Materials Science and Engineering, Dalian University of Technology, Dalian 116023 (China)
K. H. Wu Department of Mechanical Engineering, Florida International University, University Park, Miami, FL (USA) (Received September 13, 1993; in revised form November 29, 1993)
Abstract In this work, the zirconia and ceria crystalline powders mixture with a composition of ZrO2-30mol.%CeO 2 has been subjected to high energy ball milling experiment to investigate whether mechanical alloying is possible in this ceramic system. The structure variation of the powders was followed mainly by X-ray diffraction technique. The result proves that mechanical alloying can occur in ceramic systems.
2. Experimental details
High purity ZrO2 (99.5%) and CeO2 (99.99%) powders were mixed to provide a composition of ZrOz-30mol.%CeO2. The powders were then charged in a stainless-steel columnar vial of 100 mm in diameter together with hardened steel balls as a milling media. The ball-to-powder weight ratio was 15:1. Mechanical alloying was carried out in a planetary ball mill at a rotating speed of 200 rpm. Milling was interrupted every 5 h for 1 h to prevent the powders' temperature from rising too high. After predetermined intervals of time, the milled powders were removed from the vial, and subjected to X-ray diffraction analysis. The X-ray diffraction experiments were made with a Shimadzu XD-3A diffractometer using Cu K a (2 = 0.15412 nm) radiation.
3. Results and discussion 1. Introduction
Mechanical alloying was first developed by Benjamin and co-workers in the late 1960s [1]. Since the successful preparation of the Ni60Nb40 metallic glass [2], mechanical alloying has attracted general attention. It has thus matured as a processing tool and has now been applied to a wide range of materials problems [3]. Almost all the research efforts of mechanical alloying so far have focused on metallic materials systems. Whether mechanical alloying of ceramics is possible is still a problem which needs consideration. Few materials show as much potential for advanced high temperature electrical and structural applications as zirconia-based ceramics. In the present paper, ZrO 2 and CeO 2 crystalline powder mixtures have been ball milled to investigate whether mechanical alloying of ceramics is possible. The primary results show that mechanical alloying, which has been extensively studied in metallic materials systems, can also occur in ceramic systems. The result may be important not only 0921-5093/94/$7.00 SSD10921-5093(93)02696-E
Figure 1 shows the X-ray diffractograms of the ZrO2-30mol.%CeO2 powders after milling for 0 h, 30 h and 60 h respectively. From Fig. l(a), it can be seen that the ZrO2 exists in monoclinic structure, and CeO2 in f.c.c, structure before milling. After 30 h milling, the sharp crystalline lines of the starting powders have disappeared and a relative broad diffuse amorphous-like halo shows up in the diffraction pattern. This is a clear indication that the crystal structures of the starting ingredients have been modified by high energy ball milling. This phenomena related to the fact that powder particles are fragmented by mechanical force during milling process, and the grain size decreases as a result, so the particle surface and grain boundary, both of which are lattice defects, will increase greatly. The intensity of stacking fault, dislocation and other point defects will also increase simultaneously because of the repeated fracture and severe deformation caused by ball-powder-ball and/or ballpowder-container collisions. It is well known that the atoms in crystal structure are arranged periodically in © 1994 - Elsevier Sequoia. All rights reserved
L 10
Letter
three dimensions and that the atoms in lattice defects will shift from the ideal crystal sites. When the intensity of the above mentioned defects increase to a certain degree, too many atoms will be deviated from their correct positions and the periodical structure of crystal will be destroyed. As shown in Fig. l(b), the halo is not wide enough to identify it as amorphous. It is not possible to clarify its structure exactly by X-ray diffraction technique in the present conditions. The X-ray diffraction pattern of the powder milled for 60 h reveals the formation of an f.c.c, phase with a=0.530 nm (calculated with (220) diffraction). It should be pointed out that these peaks coincide well with the main diffraction lines of the mixed compound Ce203"2ZrO2 reported by Casey et al. [4], but the following analysis proves that they are not the same. (i) The valence of element Ce in starting CeO2 powder is 4+ and that in Ce203.2ZrO 2 is 3 + , in the view of chemistry, a reducing reaction is needed for the formation of Ce203"2ZrO 2. In the system studied, only the iron contamination from the milling media can act as a reducing agent, but the wet chemical analysis indicates an impurity level of less than 1 at.% iron for the sample milled for 100 h (much longer than 60 h). This impurity content, being low, is not expected to support the reducing reaction. (ii) According to Casey et al. [4], annealing Ce203"2ZrO2 at 1000 °C for several hours results in the formation of another oxide compound CeO 2"ZrO2, but treating the 60 h milled sample in the same conditions gives a completely different result. Figure 2 is the X-ray diffraction pattern of the heattreated powder. An f.c.c, phase with a=0.536 nm precipitated from the original phase. It should be mentioned that the ( 111 ) diffractions of the two f.c.c, phases overlap because of the little difference between their lattice parameters.
However, we cannot conclude that the milling formed f.c.c, phase is a new phase based on the above analysis. It has been proven that high temperature stable phase can be obtained during high energy ball milling process. Bakker and Di [5] pointed out that A15 compound V3Ga, which is stable up to about 1300 °C could be transformed to a high temperature stable phase--a solid solution of Ga in b.c.c. V--by high energy ball milling. Koyano et al. [6] reported that a high-temperature a phase of the solid solution can be obtained by milling from respective Fe and Cr elemental powders. Referring to Fig. 3, the latest equilibrium diagram for ZrO2-CeO 2 binary system given by
X
%.
~
I
,
40
3O
I
50 Degree(20)
J
0
I
60
,
O
I
70
Fig. 2. X-ray diffraction pattern for the 60 h milled powder after heat treatment.
2 f \~\h ~ssS---
Cs s
L~ 6
"zr°2 OCeO2
i ..... i: sl ! . . . . \... ...... ! ........
\
',
\
I I A I
I
30
i
I
40
I
I
50 Degree(20)
i
I
60
i
i
I
I
70
Fig. 1. X-ray diffraction patterns of ZrO2-30mol.%CeO 2 powders after milling for: (a) 0 h, (b) 30 h and (c) 60 h respectively.
ZrO 2
20
40 60 80 CeO 2 c o n t e n t ( m o l g )
100
Fig. 3. Equilibrium diagram forthe ZrO2-CeO2system given by Duran etal. {7].
Letter
Duran et al. [7], we found that the stable phase above 1700 °C at the composition of ZrO2-30mol.%CeO 2 is a high temperature solution of ZrO2 in CeO2, whose lattice cell is f.c.c, structure. Thus the f.c.c, phase mentioned above is possibly a high-temperature solid solution of ZrO2 in CeO2. Since the structure of the f.c.c. C e O 2 has been destroyed by 30 h milling, the formation of the solid solution must have gone through a very complicated process. Figure 4 shows the result of continued milling of the f.c.c, phase. Figure 4(a)-4(d) are all characteristic f.c.c. patterns, but the peaks shift to higher diffraction angles (or to smaller lattice spacing) as milling time is extended. It is obvious that the f.c.c, structure remains stable but the lattice parameters reduce as milling goes on. Calculation show that the lattice constants are a = 0 . 5 3 0 nm, 0.526 nm, 0.521 nm and 0.518 nm for Fig. 4(a), 4(b), (4c) and 4(d) respectively. These results are shown in Table 1. What causes the lattice reduction is a problem which needs further consideration. As mentioned above the f.c.c, phase is a solid solution of ZrO2 in CeO2. It is well known that the change of solute content in solid solution will result in the
f ~
I 40
~
I 50
~
I 60
;
I 70
Degree(20 )
Fig. 4. X-ray diffractograms of the ZrO2-30mol.% CeO2 mixed powders milled for: (a) 60 h, (b) 70 h, (c) 80 h, (d) 90 h, (el 100 h and (f) 125 h respectively. TABLE 1. Lattice parameters and its corresponding zirconia solute content after milling for different times Milling time (h) 60 Lattice parameters (nm) Corresponding solute content (mol.%)
0.530 34.2
change of lattice parameters. It is probable that the solute content has been changed during milling. Kim [8] gave an empirical equation for the lattice parameter change of C e O 2 solid solution as a function of solute content:
dce=O.5413+~(O.O220Ark+O.OO15Azk)m k
70 0.526 46.4
80 0.521 61.5
90 0.518 70.6
(1)
k
where d (in nanometers) is the lattice constant of the fluorite CeO 2 solid solution at room temperature, Ar k (in nanometers) is the difference in ionic radius (r k - rh) of the kth dopant (rk) and the host cation (rh) , Azk is the valency difference (zk - zh), and mk is the mole per cent of the kth dopant in the form of MO,, which can be represented by
nkm, m, = 100 + ~. (n - 1) x 100 (2) k where nk is the number of cations in the kth solute oxide and M k is the mole per cent of the kth dopant oxide (e.g. n k = 2 and M k = 3 for doping with 3 mol.%Y203). Since there is only one solute Zr02 in the present system, the equation can be expressed as follows: d = 0.5413 + 0.0220(r z - r,)m
o
I 30
L 11
(3)
where d (in nanometers) is the lattice constant of the f.c.c, solid solution of ZrO 2 in CeO2, r, = 0.087 and r,.=0.102 nm are radius of Zr 4+ and Ce 4+ respectively; and m is the mole per cent of solute Z r O 2. Since the lattice parameters have been calculated earlier, we can use eqn. (3) to predict the Z r O 2 solute content. The results are presented in Table 1 together with their corresponding lattice parameters. Obviously, the Z r O 2 solute content increase with milling time, and the highest Z r O 2 content, 70.6%, is in good agreement with the percentage of the respective constituents in the initial charge. Based on the above analysis, we find that although it seems from Fig. 4(a) that only f.c.c, phase exists in the 60 h milled powder, in fact a certain amount of Z r O 2 have not dissolved in the lattice, still remaining in the powder. The question is why these Z r O 2 give no diffraction line. The similar phenomena have been observed by Sui et al. [9] in A1-Co binary system. Referring to ref. 9, we suggest that these Z r O 2 exist as very small subunits in the particle surfaces and grain boundaries. Since the subunits are very very small and randomly arranged, there should be no Bragg diffraction line from them. The energy supplied by continued milling make the Z r O 2 existing in particle surfaces and grain boundaries come into the f.c.c. lattice gradually. Since the radius of Zr 4+ is smaller than that of Ce 4+, the lattice constant will decrease as ZrO2 solute becomes higher.
L 12
Letter
As shown in Fig. 3, the solubility of ZrO 2 in f.c.c. CeO2 is only about 6 mol.% in equilibrium condition at room temperature [7], however, almost 70 mol.% ZrO2 has dissolved in the f.c.c, lattice during milling as described earlier. Thus we conclude that a supersaturated solid solution of oxide ceramics has been obtained by mechanical alloying. The present paper should be the first report of super-saturated solid solution in ceramic systems induced by mechanical alloying. Since all the ZrO2 have dissolved in the lattice by 90 h milling, prolonged milling will not reduce the lattice parameters further as expected, but results in a decrease of the diffraction intensity gradually to obtain a characteristic amorphous pattern at 125 h instead (see Fig. 4(e) and 4(f)). Ball-milling-induced amorphization has been studied thoroughly in many material systems since the discovery by Koch et al. [2]. It is believed that ball milling will increase the energy storage in the milled material. It will result in amorphization if the storage energy exceeds the free energy of the amorphous state [10]. The precise mechanism of mechanical alloying is not understood so far, especially for the brittle-brittle systems [3, 11]. Despite this fact, the above results prove that mechanical alloying can certainly occur in ceramic systems. Since mechanical alloying has been applied to a wide range of materials problems, and the technique is known as a versatile tool for producing materials far from thermodynamic equilibrium [12], it should be expected that mechanical alloying will not only be widely used to improve the application limit of ceramics, but also to synthesize new structure ceramic materials.
4. Summary The high energy ball milling experiment has been performed on ZrO2-30mol.%CeO2 mixed powder in
the present paper. The results show that mechanical alloying is also possible in ceramic systems. The mechanical alloying of ceramics will open up a wide range of possibilities for the synthesis of new ceramic materials. The crystalline structures of the starting components are destroyed by 30 h milling. A f.c.c. phase with a = 0 . 5 3 0 nm, which is considered as a super-saturated solid solution of ZrO2 in CeO2, is formed after 6 0 h milling, its structure remains unchanged but lattice parameter decrease gradually to 0.518 nm when more ZrO2 is dissolved in the lattice and the f.c.c, phase amorphizes after 125 h milling.
Acknowledgment This work is supported by the doctoral student foundation of the National Education Committee of China. This financial support is gratefully acknowledged.
References 1 J.S. Benjamin, Metall. Tram., 1 (1970) 2943. 2 C. C. Koch, O. B. Cvin, C. G. Mckamery and J. D. Scarbrough, Appl. Phys. Lett., 43 (1983) 1017. 3 C.C. Koch, Annu. Rev. Mater. Sci., 19 (1989) 121. 4 J. J. Casey, L. Katz and W. C. Orr, J. Am. Chem. Soc., 27 (1955) 2187. 5 H. Bakker and C. M. Di, Mater. Sci. Form., 88-90 (1992) 27. 6 T. Koyano, T. Takizawa, T. Fukunaga and U. Mizutani, J. Appl. Phys., 73 (1993) 429. 7 P. Duran, M. Gonzalez, C. Moure, J. R. Jurado and C. Pascual, J. Mater. Sci., 25 (1990) 5001. 8 D.J. IClm, J. Am. Ceram. Soc., 72(1989) 1415. 9 H. X. Sui, M. Zhu, M. Qi and D. Z. Yang, J. Appl. Phys., 71 (1992) 2945. 10 A.W. Weeker and H. Bakker, Physica B, 153 (1988) 93. 11 R. M. Davis, B. Mcdermott and C. C. Koch, Metall. Trans. 19A (1988) 2867. 12 J. S. Benjamin, Sci. Am., 40 (1976) 234.
L9
Letter
Mechanical alloying of ceramics in zirconia-ceria system
for the broader applications of mechanical alloying but also for the synthesis of new structure ceramic materials.
Y. L. Chen, M. Qi and D. Z. Yang Department of Materials Science and Engineering, Dalian University of Technology, Dalian 116023 (China)
K. H. Wu Department of Mechanical Engineering, Florida International University, University Park, Miami, FL (USA) (Received September 13, 1993; in revised form November 29, 1993)
Abstract In this work, the zirconia and ceria crystalline powders mixture with a composition of ZrO2-30mol.%CeO 2 has been subjected to high energy ball milling experiment to investigate whether mechanical alloying is possible in this ceramic system. The structure variation of the powders was followed mainly by X-ray diffraction technique. The result proves that mechanical alloying can occur in ceramic systems.
2. Experimental details
High purity ZrO2 (99.5%) and CeO2 (99.99%) powders were mixed to provide a composition of ZrOz-30mol.%CeO2. The powders were then charged in a stainless-steel columnar vial of 100 mm in diameter together with hardened steel balls as a milling media. The ball-to-powder weight ratio was 15:1. Mechanical alloying was carried out in a planetary ball mill at a rotating speed of 200 rpm. Milling was interrupted every 5 h for 1 h to prevent the powders' temperature from rising too high. After predetermined intervals of time, the milled powders were removed from the vial, and subjected to X-ray diffraction analysis. The X-ray diffraction experiments were made with a Shimadzu XD-3A diffractometer using Cu K a (2 = 0.15412 nm) radiation.
3. Results and discussion 1. Introduction
Mechanical alloying was first developed by Benjamin and co-workers in the late 1960s [1]. Since the successful preparation of the Ni60Nb40 metallic glass [2], mechanical alloying has attracted general attention. It has thus matured as a processing tool and has now been applied to a wide range of materials problems [3]. Almost all the research efforts of mechanical alloying so far have focused on metallic materials systems. Whether mechanical alloying of ceramics is possible is still a problem which needs consideration. Few materials show as much potential for advanced high temperature electrical and structural applications as zirconia-based ceramics. In the present paper, ZrO 2 and CeO 2 crystalline powder mixtures have been ball milled to investigate whether mechanical alloying of ceramics is possible. The primary results show that mechanical alloying, which has been extensively studied in metallic materials systems, can also occur in ceramic systems. The result may be important not only 0921-5093/94/$7.00 SSD10921-5093(93)02696-E
Figure 1 shows the X-ray diffractograms of the ZrO2-30mol.%CeO2 powders after milling for 0 h, 30 h and 60 h respectively. From Fig. l(a), it can be seen that the ZrO2 exists in monoclinic structure, and CeO2 in f.c.c, structure before milling. After 30 h milling, the sharp crystalline lines of the starting powders have disappeared and a relative broad diffuse amorphous-like halo shows up in the diffraction pattern. This is a clear indication that the crystal structures of the starting ingredients have been modified by high energy ball milling. This phenomena related to the fact that powder particles are fragmented by mechanical force during milling process, and the grain size decreases as a result, so the particle surface and grain boundary, both of which are lattice defects, will increase greatly. The intensity of stacking fault, dislocation and other point defects will also increase simultaneously because of the repeated fracture and severe deformation caused by ball-powder-ball and/or ballpowder-container collisions. It is well known that the atoms in crystal structure are arranged periodically in © 1994 - Elsevier Sequoia. All rights reserved
L 10
Letter
three dimensions and that the atoms in lattice defects will shift from the ideal crystal sites. When the intensity of the above mentioned defects increase to a certain degree, too many atoms will be deviated from their correct positions and the periodical structure of crystal will be destroyed. As shown in Fig. l(b), the halo is not wide enough to identify it as amorphous. It is not possible to clarify its structure exactly by X-ray diffraction technique in the present conditions. The X-ray diffraction pattern of the powder milled for 60 h reveals the formation of an f.c.c, phase with a=0.530 nm (calculated with (220) diffraction). It should be pointed out that these peaks coincide well with the main diffraction lines of the mixed compound Ce203"2ZrO2 reported by Casey et al. [4], but the following analysis proves that they are not the same. (i) The valence of element Ce in starting CeO2 powder is 4+ and that in Ce203.2ZrO 2 is 3 + , in the view of chemistry, a reducing reaction is needed for the formation of Ce203"2ZrO 2. In the system studied, only the iron contamination from the milling media can act as a reducing agent, but the wet chemical analysis indicates an impurity level of less than 1 at.% iron for the sample milled for 100 h (much longer than 60 h). This impurity content, being low, is not expected to support the reducing reaction. (ii) According to Casey et al. [4], annealing Ce203"2ZrO2 at 1000 °C for several hours results in the formation of another oxide compound CeO 2"ZrO2, but treating the 60 h milled sample in the same conditions gives a completely different result. Figure 2 is the X-ray diffraction pattern of the heattreated powder. An f.c.c, phase with a=0.536 nm precipitated from the original phase. It should be mentioned that the ( 111 ) diffractions of the two f.c.c, phases overlap because of the little difference between their lattice parameters.
However, we cannot conclude that the milling formed f.c.c, phase is a new phase based on the above analysis. It has been proven that high temperature stable phase can be obtained during high energy ball milling process. Bakker and Di [5] pointed out that A15 compound V3Ga, which is stable up to about 1300 °C could be transformed to a high temperature stable phase--a solid solution of Ga in b.c.c. V--by high energy ball milling. Koyano et al. [6] reported that a high-temperature a phase of the solid solution can be obtained by milling from respective Fe and Cr elemental powders. Referring to Fig. 3, the latest equilibrium diagram for ZrO2-CeO 2 binary system given by
X
%.
~
I
,
40
3O
I
50 Degree(20)
J
0
I
60
,
O
I
70
Fig. 2. X-ray diffraction pattern for the 60 h milled powder after heat treatment.
2 f \~\h ~ssS---
Cs s
L~ 6
"zr°2 OCeO2
i ..... i: sl ! . . . . \... ...... ! ........
\
',
\
I I A I
I
30
i
I
40
I
I
50 Degree(20)
i
I
60
i
i
I
I
70
Fig. 1. X-ray diffraction patterns of ZrO2-30mol.%CeO 2 powders after milling for: (a) 0 h, (b) 30 h and (c) 60 h respectively.
ZrO 2
20
40 60 80 CeO 2 c o n t e n t ( m o l g )
100
Fig. 3. Equilibrium diagram forthe ZrO2-CeO2system given by Duran etal. {7].
Letter
Duran et al. [7], we found that the stable phase above 1700 °C at the composition of ZrO2-30mol.%CeO 2 is a high temperature solution of ZrO2 in CeO2, whose lattice cell is f.c.c, structure. Thus the f.c.c, phase mentioned above is possibly a high-temperature solid solution of ZrO2 in CeO2. Since the structure of the f.c.c. C e O 2 has been destroyed by 30 h milling, the formation of the solid solution must have gone through a very complicated process. Figure 4 shows the result of continued milling of the f.c.c, phase. Figure 4(a)-4(d) are all characteristic f.c.c. patterns, but the peaks shift to higher diffraction angles (or to smaller lattice spacing) as milling time is extended. It is obvious that the f.c.c, structure remains stable but the lattice parameters reduce as milling goes on. Calculation show that the lattice constants are a = 0 . 5 3 0 nm, 0.526 nm, 0.521 nm and 0.518 nm for Fig. 4(a), 4(b), (4c) and 4(d) respectively. These results are shown in Table 1. What causes the lattice reduction is a problem which needs further consideration. As mentioned above the f.c.c, phase is a solid solution of ZrO2 in CeO2. It is well known that the change of solute content in solid solution will result in the
f ~
I 40
~
I 50
~
I 60
;
I 70
Degree(20 )
Fig. 4. X-ray diffractograms of the ZrO2-30mol.% CeO2 mixed powders milled for: (a) 60 h, (b) 70 h, (c) 80 h, (d) 90 h, (el 100 h and (f) 125 h respectively. TABLE 1. Lattice parameters and its corresponding zirconia solute content after milling for different times Milling time (h) 60 Lattice parameters (nm) Corresponding solute content (mol.%)
0.530 34.2
change of lattice parameters. It is probable that the solute content has been changed during milling. Kim [8] gave an empirical equation for the lattice parameter change of C e O 2 solid solution as a function of solute content:
dce=O.5413+~(O.O220Ark+O.OO15Azk)m k
70 0.526 46.4
80 0.521 61.5
90 0.518 70.6
(1)
k
where d (in nanometers) is the lattice constant of the fluorite CeO 2 solid solution at room temperature, Ar k (in nanometers) is the difference in ionic radius (r k - rh) of the kth dopant (rk) and the host cation (rh) , Azk is the valency difference (zk - zh), and mk is the mole per cent of the kth dopant in the form of MO,, which can be represented by
nkm, m, = 100 + ~. (n - 1) x 100 (2) k where nk is the number of cations in the kth solute oxide and M k is the mole per cent of the kth dopant oxide (e.g. n k = 2 and M k = 3 for doping with 3 mol.%Y203). Since there is only one solute Zr02 in the present system, the equation can be expressed as follows: d = 0.5413 + 0.0220(r z - r,)m
o
I 30
L 11
(3)
where d (in nanometers) is the lattice constant of the f.c.c, solid solution of ZrO 2 in CeO2, r, = 0.087 and r,.=0.102 nm are radius of Zr 4+ and Ce 4+ respectively; and m is the mole per cent of solute Z r O 2. Since the lattice parameters have been calculated earlier, we can use eqn. (3) to predict the Z r O 2 solute content. The results are presented in Table 1 together with their corresponding lattice parameters. Obviously, the Z r O 2 solute content increase with milling time, and the highest Z r O 2 content, 70.6%, is in good agreement with the percentage of the respective constituents in the initial charge. Based on the above analysis, we find that although it seems from Fig. 4(a) that only f.c.c, phase exists in the 60 h milled powder, in fact a certain amount of Z r O 2 have not dissolved in the lattice, still remaining in the powder. The question is why these Z r O 2 give no diffraction line. The similar phenomena have been observed by Sui et al. [9] in A1-Co binary system. Referring to ref. 9, we suggest that these Z r O 2 exist as very small subunits in the particle surfaces and grain boundaries. Since the subunits are very very small and randomly arranged, there should be no Bragg diffraction line from them. The energy supplied by continued milling make the Z r O 2 existing in particle surfaces and grain boundaries come into the f.c.c. lattice gradually. Since the radius of Zr 4+ is smaller than that of Ce 4+, the lattice constant will decrease as ZrO2 solute becomes higher.
L 12
Letter
As shown in Fig. 3, the solubility of ZrO 2 in f.c.c. CeO2 is only about 6 mol.% in equilibrium condition at room temperature [7], however, almost 70 mol.% ZrO2 has dissolved in the f.c.c, lattice during milling as described earlier. Thus we conclude that a supersaturated solid solution of oxide ceramics has been obtained by mechanical alloying. The present paper should be the first report of super-saturated solid solution in ceramic systems induced by mechanical alloying. Since all the ZrO2 have dissolved in the lattice by 90 h milling, prolonged milling will not reduce the lattice parameters further as expected, but results in a decrease of the diffraction intensity gradually to obtain a characteristic amorphous pattern at 125 h instead (see Fig. 4(e) and 4(f)). Ball-milling-induced amorphization has been studied thoroughly in many material systems since the discovery by Koch et al. [2]. It is believed that ball milling will increase the energy storage in the milled material. It will result in amorphization if the storage energy exceeds the free energy of the amorphous state [10]. The precise mechanism of mechanical alloying is not understood so far, especially for the brittle-brittle systems [3, 11]. Despite this fact, the above results prove that mechanical alloying can certainly occur in ceramic systems. Since mechanical alloying has been applied to a wide range of materials problems, and the technique is known as a versatile tool for producing materials far from thermodynamic equilibrium [12], it should be expected that mechanical alloying will not only be widely used to improve the application limit of ceramics, but also to synthesize new structure ceramic materials.
4. Summary The high energy ball milling experiment has been performed on ZrO2-30mol.%CeO2 mixed powder in
the present paper. The results show that mechanical alloying is also possible in ceramic systems. The mechanical alloying of ceramics will open up a wide range of possibilities for the synthesis of new ceramic materials. The crystalline structures of the starting components are destroyed by 30 h milling. A f.c.c. phase with a = 0 . 5 3 0 nm, which is considered as a super-saturated solid solution of ZrO2 in CeO2, is formed after 6 0 h milling, its structure remains unchanged but lattice parameter decrease gradually to 0.518 nm when more ZrO2 is dissolved in the lattice and the f.c.c, phase amorphizes after 125 h milling.
Acknowledgment This work is supported by the doctoral student foundation of the National Education Committee of China. This financial support is gratefully acknowledged.
References 1 J.S. Benjamin, Metall. Tram., 1 (1970) 2943. 2 C. C. Koch, O. B. Cvin, C. G. Mckamery and J. D. Scarbrough, Appl. Phys. Lett., 43 (1983) 1017. 3 C.C. Koch, Annu. Rev. Mater. Sci., 19 (1989) 121. 4 J. J. Casey, L. Katz and W. C. Orr, J. Am. Chem. Soc., 27 (1955) 2187. 5 H. Bakker and C. M. Di, Mater. Sci. Form., 88-90 (1992) 27. 6 T. Koyano, T. Takizawa, T. Fukunaga and U. Mizutani, J. Appl. Phys., 73 (1993) 429. 7 P. Duran, M. Gonzalez, C. Moure, J. R. Jurado and C. Pascual, J. Mater. Sci., 25 (1990) 5001. 8 D.J. IClm, J. Am. Ceram. Soc., 72(1989) 1415. 9 H. X. Sui, M. Zhu, M. Qi and D. Z. Yang, J. Appl. Phys., 71 (1992) 2945. 10 A.W. Weeker and H. Bakker, Physica B, 153 (1988) 93. 11 R. M. Davis, B. Mcdermott and C. C. Koch, Metall. Trans. 19A (1988) 2867. 12 J. S. Benjamin, Sci. Am., 40 (1976) 234.