- Email: [email protected]
Effect of mechanical activation on phase transformations in transition aluminas
SOLID STATE Solid State Ionics
ELSEVIER
Effect of mechanical
0.V. Andryushkova”‘“,
101-103
(1997) 647-653
IONICS
activation on phase transformations transition aluminas
in
O.A. Kirichenko”, V.A. Ushakovb, VA. Poluboyarov”
“Institute of‘Solid State Chemistry SB RAS, Kutateladzr, 18, 630128 Novosibirsk, Russiu “Boreskov Institute of Catalysis SB RAS, Lmreqev Prosp., 5. 630090 Novosibirsk, Russia
Abstract The influence of mechanical activation (MA) on y- and X-Al,O, is studied by means of X-ray phase analysis. The change of specific surface of aluminium oxides after MA is estimated. It is shown that the mechanical treatment leads to the change of the sequence of solid-phase transitions in aluminium oxide and a decrease of cw-Al,O, formation temperature. Keywords:
Solid-phase
Materiuls:
a-, x-, F-, c, O-Al,O,
transformations;
Alumina;
Mechanical
activation
1. Introduction Aluminium oxides are widely used in industry as the initial raw material for the preparation of ceramic and composite materials, and as the carriers and catalysts in chemical synthesis. Different modifications of Al,O, are used, so the question of a series of factors influencing phase transitions in Al,O, is important for the technologies of ceramic and catalyst production. Existing methods of regulating the transformations of different modifications of aluminium oxide involve various additives, as well as the use of fine powder. Essentially, a different approach to the
*Corresponding author. Present address: Dept. of Total and Bioorganic Chemistry, Novosibirsk Medical Institute, Krasny Prospekt 52, Novosibirsk, Russia. Fax: +383 2 209405; e-mail: [email protected] 0167.2738/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SO167-2738(97)00379-2
solution of this problem is based on mechanical activation of the initial materials [l-4]. The goal of the present work was to study the influence of mechanical treatment in energy-strained planetary mills on the mechanism of phase transitions and on the changes of microstructure of y- and X-Al,O, powder.
2. Experimental Pseudobemite-derived y-A&O, and X-Al,O, obtained by annealing the corresponding hydroxides were used as an initial material in the present work. Mechanical activation was carried out in energystrained planetary mills with and without elimination of heat in steel drums with steel balls. Samples l-l .5 g in mass were thermally treated in a muffle furnace, the sample being placed in the furnace only when the
O.V. Andtyyushkova
648
et al.
I Solid
State
given annealing temperature was achieved. X-ray phase analysis was carried out with the help of a diffractometer HZG-4 using monochromatized Cu Kcx emission. Phase composition was determined according to state concisely the method [5]. Specific surface of the material was controlled according to the BET method by thermodesotption of argon.
Since it is known [6] that mechanical activation with and without elimination of heat leads to essentially different processes in the powder under activation, experiments were carried out with the oxides under investigation using mills both with and without aquatic cooling. The fragments of the diffraction pattern of x- and y-Al,O, before and after mechanical treatment are shown in Fig. 1 and Fig. 2. It should be stressed that the position of diffraction maxima characteristic of these phases [7] is not changed after MA. The intensity of the diffraction maximum in the region d/n = 2.12 A (20 = 42.5”) was found to be increased after MA of the oxides under investigation, both with and without elimination of heat. The maxima in the range d/n = 1.95 A (20 =46”) and d/n = 1.40 A (20 = 67”) were
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found to get flattened in both cases. After the activation of X-Al,O, for 15 min without elimination of heat, the X-ray pattern in the region 20 = 40-50” is somehow similar to the diffraction pattern characteristic of the product of thermal decomposition of gibbsite. Supposing that the change of the relation between depicts the process of the intensities N = H,,lH,,,, amorphization, we studied the dependence of this factor on the time of MA. We suppose that the change of the line d/n = 1.40 A directly depicts the changes in the structure of the oxide. Since the height of the background is also changed after MA, it is more correct to characterise the change of the line dln = 1.40 A with the relation F = H6,1HT2 involving the intensity (height) of the line in view of the background level to the background intensity. The change of the factor under consideration (F) with increasing the time of MA is complex by character and correlates with the changes of specific surface of the samples (Fig. 3) which is decreased by a factor of 5-6 during MA. This fact points to the interrelation between the intensity of the line d/n = 1.40 A and the deformation of the initial particles which results from the formation of dense aggregates. With increasing the time of MA, X-ray pattern of
3. Results
70
Ionics
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I
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Fig. 1. Diffraction pattern of the samples of initial X-AI,O, (1) and products of thermal decomposition treatment: (2) 30 min with heat elimination; (4) 15 min without heat elimination.
I
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of gibbsite
2e” (3) and mechanical
0.V.
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Andryushkova
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et ul. I Solid
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Fig. 2. Diffraction pattern of the samples of initial y-Al,O, elimination: (3) 30 min with heat elimination.
1
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Ionics
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(I) and the products of its mechanochemical
the samples treated for 5 min and longer with the activator with elimination of heat, demonstrate several lines in the positions correlating with the most intensive lines of a-Al,O,. This process is more clearly expressed in the activation of x-form. Similar changes were observed in the X-ray pattern of yAl,O, after the treatment with shock waves equivalent to the pressure of 40 kbar [8]. Its amount is then stabilised. It should be noted that the milling of iron in the samples of y- and X-Al,O, activated without cooling was 0.62 and 0.22% correspondingly, as analytical data show. This level of iron impurity is known [9] to have no influence on phase transitions in y-Al,O, but causes a slight elevation of a-AlzO, content after calcination at 1200°C [lo]. MA of X-Al,O, for several minutes results in a sharp decrease of the dimensions of oxide particles to 0.06-0.09 pm from 0.3-0.5 pm. Heating of the sample at 850” leads to the occurrence of single crystals of o-Al,O, as smooth and non-porous particles. It is known [ 111 that (Y- and X-Al,O, are similar in structure, being characterised by hexagonal packing of anions. The difference in lattice parameters is 12.5 and 3.34% for a, and cO, correspondingly. So, the conditions exist for the orientation nucleation to occur. From this point of view, the turn
649
activation:
(2)
2ec 15 min without heat
and shift of microcrystal blocks in the structure of particles of initial oxide, being the result of MA, must be accompanied by the changes of corresponding distances which, in turn, seems to promote the formation of wAl,O,. The results of X-ray diffraction studies of thermally treated samples are ranges according to the initial forms subjected to MA and to the intermediate high-temperature forms which appear as a result of thermal treatment. They are shown in Fig. 4. It is shown that X-Al,O, content within the temperature range of thermal treatment under consideration (800-1050°C) is determined mainly by the treatment temperature. ~-Form content is decreased with the duration of mechanical treatment nearly according to the exponential law, so the formation of K-form is quenched after 10 min MA in the whole range of thermal treatment. At the same time, its content grows with increasing the calcination temperature and reaches its maximum at 1000°C (Fig. 4b). a-Al,O, content is determined by the duration of mechanical treatment and the temperature of thermal treatment. For each temperature value, there is a characteristic time of MA, so that if the duration of MA exceeds this value, the correlation between 01Al,O, content and the duration of MA is practically
650
0.K Andryushkova et al. I Solid State Ionics 101-103
(1997) 647-653
observe the presence of 8-Al,O, in the studied samples. The change of o-oxide content is similar by character to that of the ‘x-row’ (Fig. 4f). After mechanochemical treatment for more than 10 min, the formation of o-form is observed at a temperature as low as 850°C. The character of operation during MA was changed as follows: time was packed by stages, treatment for 15 min, cooling of the rollers for 15 min, then again activation for 15 min. This caused the changes in the sequence of polymorphic transitions of y-Al,O,. It follows from the data given in Table 1 that the chosen type of time ‘packing’ has caused the change of MA influence on phase transitions. Coincidently obtained data on the specific area after milling and sintering, on the powder morphology and phase composition indicate that the formation of the low surface area aggregates consisted of arbitrarily oriented nanocrystallites proceeds to (Yformation.
4. Discussion
0
2b
‘MA’
’
30
Fig. 3. Dependence of the relation between diffraction line intensity and specific area on MA duration and mill type (TV=H,, / HJ2 5, F=H,,lH,2). (a) and (b) - activation, with heat elimination, correspondingly. (c) and (d) - activation of y- and X-AJO,, without heat elimination, y- and X-Al,O,, correspondingly.
linear. In other words, there is a definite ‘induction’ period of the sensitivity to the influence of MA duration on the phase transition to a-Al,O, (Fig. 4~). Change of &A&O, content is a complicated characteristic. It is determined by the calcinating temperature (Fig. 4e). At the temperature higher than 900°C the type of influence of mechanochemical treatment on S-form content is similar to that of k-form, but the formation of &form is completely quenched only at the mechanical treatment longer than 15 min. It should be emphasized that we did not
According to the results of the present work, mechanochemical treatment for 15 min (Table 2) is accompanied with the following sequences of formation and changes of temperature for transition forms of oxides during short-scale heating: x “‘2
x + (Y ‘“z’ cu-Al,O,
y “‘F
y + (Y“‘%c o-Al,O,.
The observed quantitative relations between the transition forms of two different rows (Fig. 4) show that the type of dependence of the decrease of low-temperature oxide forms (y and x) and the increase of high-temperature form, Al,O,, on MA time and heating temperature is similar. Differences are essential for the transition forms 6 and K. These differences are likely to be linked with the mode of interconnection between the structures of the transition forms.
0.V Andryushkova
et al. I Solid State tonics 101-103
651
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b
io
i
i5
min
Fig. 4. Dependence of phase content [X (a), K (b), (Y (c), y (d), 6 (e), (Y (f)] on MA duration 8OO”C, (2) 85O”C, (3) 9OO”C, (4) 950°C (5) lOOO”C, (6) 1050°C.
Table 1 The effect of step time packing annealing on y-Al,O,. Treatment
conditions T(“C)
15+ 15
850 900
y-Al&L y-AlzO, +u-AIZO,
Table 2 The effect of mechanical Sample
X-Al,O,
T(“C)
0 1120 1170 1220 1270
Y-A~~O,
0 1120 1170 1220 1270
treatment
and
Phase content
MA (min)
15+ 15
of mechanical
treatment
0 min s
x
Y
y,6 y.6 f3 0
time and annealing
10 min
PC
s
220 94 80 43 31
X
180 53 58 39 18
310 160
Y
64 69
x> K(lO), a(121 X, K(20), o( 19) K, o(44) K. o(68)
Y. 604) Y. N20) 0, o(l) 0, o(5)
210 170 120 86 68
PC X
X x* o(7) X, a(37). o(31) K, o(59) Y
Y. 603) y. 6(46) Y. %44) 0, o(2)
of the further roasting:
(1)
In the case of ‘x-Al,O,’ row, consequent transitions occur between the forms characterised by hexagonal packing of the anion shell (x, K, a) [ 11,121. In the case of ‘y-A&O, row, the structures of transition forms are different: the lattice of yoxide is cubic, that of &form is practically cubic, while cr-Al,O, possesses hexagonal packing of the anion sub-lattice [ 111. The influence of MA is in this
on phase content (PC, weight percentage)
5 min
PC
X X K K
(less than 1%)
and temperature
and specific surface area (S, m’/g)
15 min S
30 min
PC 95 65 56 34 17
200 110 90 100 53
S
x x. o(1) x. o(5) X. ~(34). ~(27) a, o(59) y Y. %28), ~3 %16), Y. %20), Y. 6(29),
o(l) a(l) o(6) (~(5 1)
PC 90 63 61 32 18
260 100 80 86 36
X
X? o(5) x, o(l1) x. o(51) x. (~(81) Y
Y, W28). o( 1) y, %26), o(9) Y, %26), o(6) _
S 80 63 40 22 16 240 67 54 40 _
652
0.V Andryushkova
et al. I Solid State lonics 101-103
case more complicated by character, depending both on the duration of MA and, which is more important, on the sintering temperature. In order to interpret the reasons of the observed significant influence of MA (Table 3) on phase transitions of aluminium oxides, it is necessary to create a notion of the mechanism of these transitions. The analysis of literature data reveals that the mechanisms of solid-phase transitions of low-temperature y- and X-Al,O, into a-Al,O, are poorly studied. It is a common supposition that the formation of a-form proceeds via either two inseparable stages, namely, agglomeration accompanied by synchroshift, or the diffusion mechanism of transformation including the stages of nucleation and growth of the particles of o-form. Some authors [9,13,14] propose that the first mechanism is preferable, and synchroshift occurs when the agglomerated clusters (aggregates) achieve critical size. Complete transformation of y- and x-A&O, into cu-AI,O, includes not only structural transformation but also the removal of OH-groups which stabilise the lattice of low-temperature aluminium oxide. If low-temperature dehydration is supposed to occur under the action of MA, this would result in the formation of new Al,O, clusters containing no OH-groups. These clusters can substantially differ in structure from 6-, 8-, K-forms due to the low probability (especially at low temperature) of the diffusion of Al”+ cations, which is needed for the transitions in the chains y-+6+0 and X+K to take place. This may be the observed amorphous and vitreous phase. These clus-
Table 3 The effect of initial composition transition aluminas and MA conditions (I -- 10 W/g; 2 - 25 W/g) on specific surface area (S, m’/g) and o-phase content (cu, weight percentage)
I
Sample
2
s
cy
s
(Y
X K
64 50 37 20 23
7 0 59 58 _
_ _ _ _
_ _ _ _
K(0.2)f8(0.8)
36
II
x(0.2)
45
_
35
36
47
_
50
36
;(0.8) + X(0.2) %0.2)+X(0.8)
0
+ O(O.8)
6.2
11 81
(1997) 647-653
ters can serve as the nuclei of the direct c-w-Al,O, formation avoiding intermediate forms. However, dehydration of y- and x-forms with simultaneous formation of amorphous aluminium oxide is not the only reason for o-A120, occurrence. Mechanical loading of porous materials similar in structure to the aggregates of y- and X-Al,O, causes simultaneous occurrence of compressing, stretching and shift tensions. Therefore, the most favourable conditions are created in the present case for the realization of diffusion creep and dislocation slipping. Moreover, the shift of 02- layers can occur if the applied shift tension reaches sufficiently high values. The following conclusions can be made from the consideration of the observed peculiarities of thermal behaviour of mechanically treated aluminium oxides: the formation of a-Al,O, in mechanically activated oxides takes place in the temperature range 850900°C characterised by corundum agglomeration according to the mechanism of surface diffusion. This points to the possibility of synthesis of porous corundum on the basis of mechanochemical technique, without using toxic mineralizers; it is principally possible to obtain, on the basis of mechanochemical technologies, low-temperature corundum with developed specific area.
References [I] F.W. Dynys, J.W. Halloran, J. Am. Ceram. Sot. 65(9) (1982) 442. [2] SM. Paramzin, B.P. Zolotovsky. O.P. Krivoruchko et al.. Izv. SO AN SSSR. Ser. Khim. Nauk I (1989) 33. [3] I.J. Lin, S. Nadiv, P. Bar-On, Thermochim. Acta 148 (1989) 301. [4] B.B. Bokhonov, I.G. Konstanchuk, VV Boldyrev. Mater. Res. Bull. 30(10) (1995) 1277. 151 R.A. Shkrabina, Yu.K. Vorob’ev, E.M. Moroz et al.. Kinetika i Kataliz 22 (1981) 1080. [6] L.T. Menzheres, VP. Isupov, N.P. Kotsupalo, Izv. SO AN SSSR. Ser. Khim. Nauk 3 (1988) 53. [7] VA. Ushakov, E.M. Moroz, React. Kinet. Catal. Lett. 24( I2) (1984) 113. [8] B.C. Adamenko, P.O. Pashkov, L.N. Tambovtseva. Poroshkovaya Metallurgiya 10 (1978) 93. [9] G.C. Bye, G.T. Simpkin, J. Am. Ceram. Sot. 57(8) (1974) 367. [ IO] VI. Vereshchagin, VYu. Zelinsky. T.A. Khabas et al., ZhPKh 55(9) (1982) 1946.
O.V. Andryushkova
et al. I Solid State Ionics 101-103
[I l] B.K. Lippens, Y.Y. Steggerda, Stroenie i Svoystva Adsorbentov i Katalizatorov, Mir, Moscow, 1973. [12] N.A. Toropov et al. (Eds.), Diagrammy Sostoyaniya Silikatnykh Sistem: Spravochnik, Issue 2, Nauka, Leningrad, 1970.
(1997) 647-653
653
[13] V.A. Gagarina, V.N. Kuklina, L.G. Khomyakova et al., Kinetika i Kataliz 13 (1972) 174. 1141 S.V. Raman, R.H. Doremus, R.M. German, Mater, Sci. Res. 16 (1984) 253.
ELSEVIER
Effect of mechanical
0.V. Andryushkova”‘“,
101-103
(1997) 647-653
IONICS
activation on phase transformations transition aluminas
in
O.A. Kirichenko”, V.A. Ushakovb, VA. Poluboyarov”
“Institute of‘Solid State Chemistry SB RAS, Kutateladzr, 18, 630128 Novosibirsk, Russiu “Boreskov Institute of Catalysis SB RAS, Lmreqev Prosp., 5. 630090 Novosibirsk, Russia
Abstract The influence of mechanical activation (MA) on y- and X-Al,O, is studied by means of X-ray phase analysis. The change of specific surface of aluminium oxides after MA is estimated. It is shown that the mechanical treatment leads to the change of the sequence of solid-phase transitions in aluminium oxide and a decrease of cw-Al,O, formation temperature. Keywords:
Solid-phase
Materiuls:
a-, x-, F-, c, O-Al,O,
transformations;
Alumina;
Mechanical
activation
1. Introduction Aluminium oxides are widely used in industry as the initial raw material for the preparation of ceramic and composite materials, and as the carriers and catalysts in chemical synthesis. Different modifications of Al,O, are used, so the question of a series of factors influencing phase transitions in Al,O, is important for the technologies of ceramic and catalyst production. Existing methods of regulating the transformations of different modifications of aluminium oxide involve various additives, as well as the use of fine powder. Essentially, a different approach to the
*Corresponding author. Present address: Dept. of Total and Bioorganic Chemistry, Novosibirsk Medical Institute, Krasny Prospekt 52, Novosibirsk, Russia. Fax: +383 2 209405; e-mail: [email protected] 0167.2738/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SO167-2738(97)00379-2
solution of this problem is based on mechanical activation of the initial materials [l-4]. The goal of the present work was to study the influence of mechanical treatment in energy-strained planetary mills on the mechanism of phase transitions and on the changes of microstructure of y- and X-Al,O, powder.
2. Experimental Pseudobemite-derived y-A&O, and X-Al,O, obtained by annealing the corresponding hydroxides were used as an initial material in the present work. Mechanical activation was carried out in energystrained planetary mills with and without elimination of heat in steel drums with steel balls. Samples l-l .5 g in mass were thermally treated in a muffle furnace, the sample being placed in the furnace only when the
O.V. Andtyyushkova
648
et al.
I Solid
State
given annealing temperature was achieved. X-ray phase analysis was carried out with the help of a diffractometer HZG-4 using monochromatized Cu Kcx emission. Phase composition was determined according to state concisely the method [5]. Specific surface of the material was controlled according to the BET method by thermodesotption of argon.
Since it is known [6] that mechanical activation with and without elimination of heat leads to essentially different processes in the powder under activation, experiments were carried out with the oxides under investigation using mills both with and without aquatic cooling. The fragments of the diffraction pattern of x- and y-Al,O, before and after mechanical treatment are shown in Fig. 1 and Fig. 2. It should be stressed that the position of diffraction maxima characteristic of these phases [7] is not changed after MA. The intensity of the diffraction maximum in the region d/n = 2.12 A (20 = 42.5”) was found to be increased after MA of the oxides under investigation, both with and without elimination of heat. The maxima in the range d/n = 1.95 A (20 =46”) and d/n = 1.40 A (20 = 67”) were
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found to get flattened in both cases. After the activation of X-Al,O, for 15 min without elimination of heat, the X-ray pattern in the region 20 = 40-50” is somehow similar to the diffraction pattern characteristic of the product of thermal decomposition of gibbsite. Supposing that the change of the relation between depicts the process of the intensities N = H,,lH,,,, amorphization, we studied the dependence of this factor on the time of MA. We suppose that the change of the line d/n = 1.40 A directly depicts the changes in the structure of the oxide. Since the height of the background is also changed after MA, it is more correct to characterise the change of the line dln = 1.40 A with the relation F = H6,1HT2 involving the intensity (height) of the line in view of the background level to the background intensity. The change of the factor under consideration (F) with increasing the time of MA is complex by character and correlates with the changes of specific surface of the samples (Fig. 3) which is decreased by a factor of 5-6 during MA. This fact points to the interrelation between the intensity of the line d/n = 1.40 A and the deformation of the initial particles which results from the formation of dense aggregates. With increasing the time of MA, X-ray pattern of
3. Results
70
Ionics
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Fig. 1. Diffraction pattern of the samples of initial X-AI,O, (1) and products of thermal decomposition treatment: (2) 30 min with heat elimination; (4) 15 min without heat elimination.
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of gibbsite
2e” (3) and mechanical
0.V.
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Fig. 2. Diffraction pattern of the samples of initial y-Al,O, elimination: (3) 30 min with heat elimination.
1
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Ionics
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the samples treated for 5 min and longer with the activator with elimination of heat, demonstrate several lines in the positions correlating with the most intensive lines of a-Al,O,. This process is more clearly expressed in the activation of x-form. Similar changes were observed in the X-ray pattern of yAl,O, after the treatment with shock waves equivalent to the pressure of 40 kbar [8]. Its amount is then stabilised. It should be noted that the milling of iron in the samples of y- and X-Al,O, activated without cooling was 0.62 and 0.22% correspondingly, as analytical data show. This level of iron impurity is known [9] to have no influence on phase transitions in y-Al,O, but causes a slight elevation of a-AlzO, content after calcination at 1200°C [lo]. MA of X-Al,O, for several minutes results in a sharp decrease of the dimensions of oxide particles to 0.06-0.09 pm from 0.3-0.5 pm. Heating of the sample at 850” leads to the occurrence of single crystals of o-Al,O, as smooth and non-porous particles. It is known [ 111 that (Y- and X-Al,O, are similar in structure, being characterised by hexagonal packing of anions. The difference in lattice parameters is 12.5 and 3.34% for a, and cO, correspondingly. So, the conditions exist for the orientation nucleation to occur. From this point of view, the turn
649
activation:
(2)
2ec 15 min without heat
and shift of microcrystal blocks in the structure of particles of initial oxide, being the result of MA, must be accompanied by the changes of corresponding distances which, in turn, seems to promote the formation of wAl,O,. The results of X-ray diffraction studies of thermally treated samples are ranges according to the initial forms subjected to MA and to the intermediate high-temperature forms which appear as a result of thermal treatment. They are shown in Fig. 4. It is shown that X-Al,O, content within the temperature range of thermal treatment under consideration (800-1050°C) is determined mainly by the treatment temperature. ~-Form content is decreased with the duration of mechanical treatment nearly according to the exponential law, so the formation of K-form is quenched after 10 min MA in the whole range of thermal treatment. At the same time, its content grows with increasing the calcination temperature and reaches its maximum at 1000°C (Fig. 4b). a-Al,O, content is determined by the duration of mechanical treatment and the temperature of thermal treatment. For each temperature value, there is a characteristic time of MA, so that if the duration of MA exceeds this value, the correlation between 01Al,O, content and the duration of MA is practically
650
0.K Andryushkova et al. I Solid State Ionics 101-103
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observe the presence of 8-Al,O, in the studied samples. The change of o-oxide content is similar by character to that of the ‘x-row’ (Fig. 4f). After mechanochemical treatment for more than 10 min, the formation of o-form is observed at a temperature as low as 850°C. The character of operation during MA was changed as follows: time was packed by stages, treatment for 15 min, cooling of the rollers for 15 min, then again activation for 15 min. This caused the changes in the sequence of polymorphic transitions of y-Al,O,. It follows from the data given in Table 1 that the chosen type of time ‘packing’ has caused the change of MA influence on phase transitions. Coincidently obtained data on the specific area after milling and sintering, on the powder morphology and phase composition indicate that the formation of the low surface area aggregates consisted of arbitrarily oriented nanocrystallites proceeds to (Yformation.
4. Discussion
0
2b
‘MA’
’
30
Fig. 3. Dependence of the relation between diffraction line intensity and specific area on MA duration and mill type (TV=H,, / HJ2 5, F=H,,lH,2). (a) and (b) - activation, with heat elimination, correspondingly. (c) and (d) - activation of y- and X-AJO,, without heat elimination, y- and X-Al,O,, correspondingly.
linear. In other words, there is a definite ‘induction’ period of the sensitivity to the influence of MA duration on the phase transition to a-Al,O, (Fig. 4~). Change of &A&O, content is a complicated characteristic. It is determined by the calcinating temperature (Fig. 4e). At the temperature higher than 900°C the type of influence of mechanochemical treatment on S-form content is similar to that of k-form, but the formation of &form is completely quenched only at the mechanical treatment longer than 15 min. It should be emphasized that we did not
According to the results of the present work, mechanochemical treatment for 15 min (Table 2) is accompanied with the following sequences of formation and changes of temperature for transition forms of oxides during short-scale heating: x “‘2
x + (Y ‘“z’ cu-Al,O,
y “‘F
y + (Y“‘%c o-Al,O,.
The observed quantitative relations between the transition forms of two different rows (Fig. 4) show that the type of dependence of the decrease of low-temperature oxide forms (y and x) and the increase of high-temperature form, Al,O,, on MA time and heating temperature is similar. Differences are essential for the transition forms 6 and K. These differences are likely to be linked with the mode of interconnection between the structures of the transition forms.
0.V Andryushkova
et al. I Solid State tonics 101-103
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b
io
i
i5
min
Fig. 4. Dependence of phase content [X (a), K (b), (Y (c), y (d), 6 (e), (Y (f)] on MA duration 8OO”C, (2) 85O”C, (3) 9OO”C, (4) 950°C (5) lOOO”C, (6) 1050°C.
Table 1 The effect of step time packing annealing on y-Al,O,. Treatment
conditions T(“C)
15+ 15
850 900
y-Al&L y-AlzO, +u-AIZO,
Table 2 The effect of mechanical Sample
X-Al,O,
T(“C)
0 1120 1170 1220 1270
Y-A~~O,
0 1120 1170 1220 1270
treatment
and
Phase content
MA (min)
15+ 15
of mechanical
treatment
0 min s
x
Y
y,6 y.6 f3 0
time and annealing
10 min
PC
s
220 94 80 43 31
X
180 53 58 39 18
310 160
Y
64 69
x> K(lO), a(121 X, K(20), o( 19) K, o(44) K. o(68)
Y. 604) Y. N20) 0, o(l) 0, o(5)
210 170 120 86 68
PC X
X x* o(7) X, a(37). o(31) K, o(59) Y
Y. 603) y. 6(46) Y. %44) 0, o(2)
of the further roasting:
(1)
In the case of ‘x-Al,O,’ row, consequent transitions occur between the forms characterised by hexagonal packing of the anion shell (x, K, a) [ 11,121. In the case of ‘y-A&O, row, the structures of transition forms are different: the lattice of yoxide is cubic, that of &form is practically cubic, while cr-Al,O, possesses hexagonal packing of the anion sub-lattice [ 111. The influence of MA is in this
on phase content (PC, weight percentage)
5 min
PC
X X K K
(less than 1%)
and temperature
and specific surface area (S, m’/g)
15 min S
30 min
PC 95 65 56 34 17
200 110 90 100 53
S
x x. o(1) x. o(5) X. ~(34). ~(27) a, o(59) y Y. %28), ~3 %16), Y. %20), Y. 6(29),
o(l) a(l) o(6) (~(5 1)
PC 90 63 61 32 18
260 100 80 86 36
X
X? o(5) x, o(l1) x. o(51) x. (~(81) Y
Y, W28). o( 1) y, %26), o(9) Y, %26), o(6) _
S 80 63 40 22 16 240 67 54 40 _
652
0.V Andryushkova
et al. I Solid State lonics 101-103
case more complicated by character, depending both on the duration of MA and, which is more important, on the sintering temperature. In order to interpret the reasons of the observed significant influence of MA (Table 3) on phase transitions of aluminium oxides, it is necessary to create a notion of the mechanism of these transitions. The analysis of literature data reveals that the mechanisms of solid-phase transitions of low-temperature y- and X-Al,O, into a-Al,O, are poorly studied. It is a common supposition that the formation of a-form proceeds via either two inseparable stages, namely, agglomeration accompanied by synchroshift, or the diffusion mechanism of transformation including the stages of nucleation and growth of the particles of o-form. Some authors [9,13,14] propose that the first mechanism is preferable, and synchroshift occurs when the agglomerated clusters (aggregates) achieve critical size. Complete transformation of y- and x-A&O, into cu-AI,O, includes not only structural transformation but also the removal of OH-groups which stabilise the lattice of low-temperature aluminium oxide. If low-temperature dehydration is supposed to occur under the action of MA, this would result in the formation of new Al,O, clusters containing no OH-groups. These clusters can substantially differ in structure from 6-, 8-, K-forms due to the low probability (especially at low temperature) of the diffusion of Al”+ cations, which is needed for the transitions in the chains y-+6+0 and X+K to take place. This may be the observed amorphous and vitreous phase. These clus-
Table 3 The effect of initial composition transition aluminas and MA conditions (I -- 10 W/g; 2 - 25 W/g) on specific surface area (S, m’/g) and o-phase content (cu, weight percentage)
I
Sample
2
s
cy
s
(Y
X K
64 50 37 20 23
7 0 59 58 _
_ _ _ _
_ _ _ _
K(0.2)f8(0.8)
36
II
x(0.2)
45
_
35
36
47
_
50
36
;(0.8) + X(0.2) %0.2)+X(0.8)
0
+ O(O.8)
6.2
11 81
(1997) 647-653
ters can serve as the nuclei of the direct c-w-Al,O, formation avoiding intermediate forms. However, dehydration of y- and x-forms with simultaneous formation of amorphous aluminium oxide is not the only reason for o-A120, occurrence. Mechanical loading of porous materials similar in structure to the aggregates of y- and X-Al,O, causes simultaneous occurrence of compressing, stretching and shift tensions. Therefore, the most favourable conditions are created in the present case for the realization of diffusion creep and dislocation slipping. Moreover, the shift of 02- layers can occur if the applied shift tension reaches sufficiently high values. The following conclusions can be made from the consideration of the observed peculiarities of thermal behaviour of mechanically treated aluminium oxides: the formation of a-Al,O, in mechanically activated oxides takes place in the temperature range 850900°C characterised by corundum agglomeration according to the mechanism of surface diffusion. This points to the possibility of synthesis of porous corundum on the basis of mechanochemical technique, without using toxic mineralizers; it is principally possible to obtain, on the basis of mechanochemical technologies, low-temperature corundum with developed specific area.
References [I] F.W. Dynys, J.W. Halloran, J. Am. Ceram. Sot. 65(9) (1982) 442. [2] SM. Paramzin, B.P. Zolotovsky. O.P. Krivoruchko et al.. Izv. SO AN SSSR. Ser. Khim. Nauk I (1989) 33. [3] I.J. Lin, S. Nadiv, P. Bar-On, Thermochim. Acta 148 (1989) 301. [4] B.B. Bokhonov, I.G. Konstanchuk, VV Boldyrev. Mater. Res. Bull. 30(10) (1995) 1277. 151 R.A. Shkrabina, Yu.K. Vorob’ev, E.M. Moroz et al.. Kinetika i Kataliz 22 (1981) 1080. [6] L.T. Menzheres, VP. Isupov, N.P. Kotsupalo, Izv. SO AN SSSR. Ser. Khim. Nauk 3 (1988) 53. [7] VA. Ushakov, E.M. Moroz, React. Kinet. Catal. Lett. 24( I2) (1984) 113. [8] B.C. Adamenko, P.O. Pashkov, L.N. Tambovtseva. Poroshkovaya Metallurgiya 10 (1978) 93. [9] G.C. Bye, G.T. Simpkin, J. Am. Ceram. Sot. 57(8) (1974) 367. [ IO] VI. Vereshchagin, VYu. Zelinsky. T.A. Khabas et al., ZhPKh 55(9) (1982) 1946.
O.V. Andryushkova
et al. I Solid State Ionics 101-103
[I l] B.K. Lippens, Y.Y. Steggerda, Stroenie i Svoystva Adsorbentov i Katalizatorov, Mir, Moscow, 1973. [12] N.A. Toropov et al. (Eds.), Diagrammy Sostoyaniya Silikatnykh Sistem: Spravochnik, Issue 2, Nauka, Leningrad, 1970.
(1997) 647-653
653
[13] V.A. Gagarina, V.N. Kuklina, L.G. Khomyakova et al., Kinetika i Kataliz 13 (1972) 174. 1141 S.V. Raman, R.H. Doremus, R.M. German, Mater, Sci. Res. 16 (1984) 253.