Research on the activity of high-energy ball milled
AlO Powders using the dissolving method 23
Xi shengqi Liu Xinkuan Li Pengliang Zhou Jingen
The School of Materials Science and Engineering, The state key lab of metallic materials strength
Xi’an Jiaotong University, Xi’an China 710049
The activity of high-energy ball milled AlOpowders was researched by using the dissolving 23
method. The results showed that the activity of the whole Al2O3 particles was enhanced by high-energy ball milling. Before been milled, the activation energy of these particles was 18kJ/mol when they dissolved in dilute hydrochloric acid, and it decreased to 4kJ/mol after milling. In the test of dissolving in the acid-deficient EDTA[CHN，CHCOOH！], it showed that, as milling time 222
prolonged, the dissolved amount of milled Al2O3 particles is increased. Milled for long time, though the specific surface area was decreased as the particles congregated, the dissolving amount of AlOwas increased greatly. This indicated that the thick of activate layer on the milled AlO23 23
surface increased as milling time increased.
It is a useful technique to prepare an active solid using ball milling. The refinement of this technique is widely researched. But there ware few researches on the mechanochemical effects. Since the idea of mechanical alloying was put out, there are plenty of researches on the high-energy ball milling. Most researches put the main aim onto the systems in which the alloying or the phase transformation and/or the chemical reaction could occurred during high energy ball milling. There were a few researches on the activity of high-energy ball milling [2-5]. But the main attentions of these researches were put on the active effect on the sintering of ball milled powders. As a new technology, the active process during the high-energy ball milling is fundament for synthesizing material. The active process is also very important for us to understand the mechanism of mechanical active synthesizing new materials by high-energy ball milling. So, it is very necessary to study the active process of high-energy ball milling. In this paper, we reported the activity of high-energy ball milled AlO powders using the dissolving method. 23
The raw materials used in this experiment are the pure AlO powders. The characteristics of these 23
powders were show in table 1. The powders were milled using a high-energy stirred ball mill under argon atmosphere. The canister of the mill was cooled using circulation water during milling. The speed of the drive shaft is 600 rpm and the ball to powder weight ratio is 20:1. After predetermined milling time, the milled powders were removed from the vial and analyzing.
Table 1 the characteristics of pure Al2O3 powder
Impurity ( wt% ) Material Granularity (；m) Fe sulfate chloride Pb
75 0.005 0.03 0.01 0.005 O powder Al23
The specific surface area of milled powders was tested using the nitrogen adsorption, i.e. the BET method. The apparatus of ASAP2000 type was used in this test. The particle size, or the calculating particle diameter d, could be conversed by the following formulation : s
d = 6 /？?s s
Here, ？ is the density of materials and s is specific surface area.
The hydrochloric acid with the 10% concentration was used in the dissolving method for determining the activity of milled Al2O3 particles. Dissolved milled particles in these solution for
，3different time, the upper solution was took out to analysis the content of Al in this solution, and
，3the amount of dissolved Al2O3 was calculated. The content of Alwas tested using the atom
absorb spectrum method and a atom absorb spectrophotometer of WFX-1F type was used. The test conditions of milled Al2O3 particles dissolved in the acid-deficient EDTA[CHN2；3，CHCOOH！] solution were as following. The concentration of EDTA is 10M and the ph is 7. 22
The EDTA solution is 1L. The milled Al2O3 particles dissolving in the solution is 5g for each test. and the dissolved time is 50 hours. The test was completed at room temperature. As same as in the test of dissolved in 10% HCI solution, after dissolved for enough time, the upper solution was
，3took out to analysis the content of Al in this solution, and the amount of dissolved Al2O3 was
，3calculated. The content of Alwas tested using the atom absorb spectrum method and a atom
absorb spectrophotometer of WFX-1F type was used.
The results and analysis
1 the variation of specific surface area during high-energy ball milling
Fig.1 showed the variation of specific surface area of Al2O3 particles with time during high-energy ball milling。When milling time was not very long(as milling time increased(the
specific surface area of Al2O3 increased, but when milling time exceed 10 hours(it decreased in
some extent and the particle size increased slightly。This was due to the particles reunite as excess
Fig.1 the variation of specific surface area and particle size with the milling time
2 dissolved in dilute hydrochloric acid
The Al2O3 with the perfect crystalline could only dissolved in dilute acid rarely. When it was activated by ball milling, the amount of Al2O3 powder dissolved in acid increased obviously. The activity of solid could be denoted by using the dissolving method. The heat released when the CaO was dissolved in water was used to evaluate the activity of this solid.
3.0? 3 0 ? 6 0 2.5 4 5 ? 4 5 ?50 / % 6 0 ? 3 0 ? / %2.0量 3量 3 O 1.5240 O Al 2 Al 1.0解 解 溶0.5溶30
0.0024601234t / ht / h
a球磨后溶解 b 原始氧化铝溶解
Fig. 2(a) showed the amount of Al2O3 powder milled for 50 hours dissolved in 10% HCI solution at different dissolved time. The unit used in this fig is the percentage of the dissolved powder to total powder immerged in the solution. As dissolved time prolonged, the amount of dissolved Al2O3 increased. The higher this test system temperature was, the larger the amount of dissolved Al2O3.were. Compared to the primal Al2O3 (fig 2 b)which was not milled, the amount of the milled Al2O3 dissolved in acid enhanced one magnitude. According to the research on the kinetics of oxide dissolved in acid, it is found that, for short dissolved time, the kinetics of oxide dissolved in acid is suited to the type of phase boundary reaction controlling. If the dissolved amount marked as ;, then the dissolved kinetics could be as following:
1/3 k t = 1 - ( 1 - ; )
Fig.3 was the kinetic curves of Al2O3 dissolved in dilute HCI transformed from Fig2 by the above formulation. It could be found that, although the dissolved amount of the milled Al2O3 was larger than that of the original Al2O3, they all obey the same dissolved kinetics rule. The relation
1/3between the 1 - (1 - ; ) and the dissolved time t is linearity, the slope of the curve is the k.
The activation energy could be calculated according to the dissolved kinetic curves at different temperature. Plot the ln k to 1 / T, as showed in fig.4, the activation energy of original Al2O3 was
ln k0.8ln k0.250.7
0.50.153.03.13.23.33.03.13.23.3 -1 -1 -1 -1T×1000 / KT ×1000 / K
a For milled Al2O3 b For original Al2O3
Fig.3 the kinetic curves of Al2O3 dissolved in dilute HCI
calculated 18KJ/mol and that of milled Al2O3 was only 4kJ/mol.
The process of Al2O3 dissolved in diluted HCI is in practice the process of reaction between Al2O3 and HCI solution. The reaction formula is as following:
，，3O ， 6 H？ 2Al + 3 HO Al232
The enhance of solid Al2O3 activity represents the energy of Al2O3 powder increased, it was in favor of this reaction, so that, the activation energy of this reaction decreased. 3 dissolved in the acid-deficient EDTA
When a solid dissolved in the acid-deficient such as EDTA, unlike in the strong acid, the atoms in crystalline lattice could not be corroded; only the active atoms in the surface could be corroded and dissolved in solution. As been milled, the crystalline of the solid were fined and its size decreased to nano scale, the ratio of the active atoms in the surface to the total atoms is enhanced largely, and in the milled particles, the thick of the active player in surface increased, so that, the corroded and dissolved atoms of milled solid in acid-deficient would be increased obviously. The method dissolving in EDTA was used in the complex oxides to measure the surface-active substance of solid.  Papirer had measured the surface-active layer of milled Al2O3 using this method .The method dissolving in EDTA could be used to determinate the thick of the surface-active layer.
Fig.4 showed the amount of Al2O3 dissolved in EDTA when it milled for different time. As the milling time increased, the amount of Al2O3
dissolved in EDTA increased. As the
dissolving in EDTA was only take place on 600
the surface of milled particle, so that, the -1450dissolved amount of milled powder in EDTA
should increased as the specific surface area / m g L300 3+of Al2O3 particle was increased. In Fig.4, A l
解although the specific surface area of Al2O3 150溶particle was increased dramatically in the
0early milling stages (see fig.1), the amount of 0.1110100
t / hAl2O3 dissolved in EDTA increased slightly. Otherwise, when the milling time prolonged for long time, though the specific surface area 图 2.29 在EDTA中溶解量 of Al2O3 particle was decreased slightly (also 与球磨时间的关系 see Fig.1), the amount of Al2O3 dissolved in
EDTA increased obviously. This indicated that the thick of activate layer on the milled AlO23
surface increased dramatically when milling time is very long. The milling process of Al2O3 during high energy ball milling could be divided into two active stages. The first stage was the particle refinement and the second stage was the surface active layer increasing.
The activity of high-energy ball milled AlOpowders was researched by using the dissolving 23
method. The following conclusions could be drawn out:
1 the activity of the whole Al2O3 particles was enhanced by high-energy ball milling. When they dissolved in dilute hydrochloric acid, the activation energy of these particles decreased from 18kJ/mol before milled to 4kJ/mol after milled for 50 hours.
2 By the method dissolving in EDTA, it could be found that the thick of activate layer on the
Osurface increased dramatically when milling time is very long. The milling process of milled Al23
Al2O3 during high energy ball milling could be divided into two active stages.
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