Zinc reduction of MoO 3 in a self-propagating high-temperature synthesis process
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S.K. KO, Student. and C.W. WON, S.S. CHO. and B.S. CHUN. Professors, are with the Department of Metallurgical Engineering, Chungnam National University, Taeion Ci.ty 305-764, Korea. Manuscript submitted/tree 29, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS B
this work, we intended to prepare high-purity molybdenum powders by reduction of MoO3 through SHS using zinc instead of hydrogen and examined the effects of various experimental factors on the reduction of MOO). "file precursors used in this work were powders of MoO~ (99+ pet, average particle size 10 to 20/zm) and Zn (99.5+ pet, average particle size 20 p.m). Predetermined amounts of the reactants were mixed in an alumina ball mill and pressed into pellets of 30-ram diameter and 20- to 25-ram height under various compaction pressures (40 to 170 MPa). The experimental variables were the molar ratio of the two reactants and compaction pressure. The chamber (Figure l) was purged and filled with argon gas to atmosphere pressure before the ignition. The combustion temperature on SHS reaction was measured using a data acquisition system and a personal computer. The product mixture, which was easily breakable in a mortar, was leached with HCI solution to remove ZnO under various temperatures and concentrations and for various lengths of time. The prepared molybdenum powder was analyzed by X-ray diffraction to examine its crystal structure and by scanning electron microscopy to examine its microstructure. Its chemical composition was determined by the inductively coupled plasma method. The adiabatic temperature (T~a) can be used as a general indication of the temperature at the combustion front. It can also be used in a semiquantitative way to ascertain whether the synthesis of a given material can be accomplished by a self-propagating method. It has been empirically suggested that combustion reactions will not become self-sustaining unless T~d -> 1800 K.t"~ The reaction of this experiment can be represented as follows: MoO3 + 3Zn --~ Mo + 3ZnO
[I]
And, the theoretical adiabatic temperature of this reaction can be calculated as follows:
,q, z5 I. Vacuum Pump 2. Ar Gas 4. Cooling System 5. Green Pellet 7. Tungsten Filament 8. Thermooouple 10. Computer ~ . Heat-resisting Glass
3. Gage 6. Power Su~pty g. Data Acquisition 12. Vent
Fig. 1--Schematic diagram of the SHS reactor. VOLUME 27B. APRIL 1996--315
Table !.
Final P r o d u c t s S y n t h e s i z e d by the SHS Process
MoO3:Zn Mole Ratio
Products
Compaction Pressure (MPa)
Before Leaching
After Leaching Mo, MoOs Mo, MOO., Mo Mo Mo
Zn mole ratio
1:2.0 1:3.0 1:3.5 1:4.0
40
Mo, ZnO, MoO> Mo, ZnO, MoO, Mo, ZnO, Zn Mo, ZnO, Zn
Compaction pressure (MPa)
1:3.5
40 110 170
Mo, ZnO, Zn
a)
9 tll
D
(110)
[] Mo
, (211}
b)
10
20
30
40
50
60
70
80
20
~ ~ 1 10
20
1 ;t(}
411
I
I
50
60
I "-'-" 70
80
Fig. 3--Effects of compaction pressure on the X-ray diffraction patterns of reaction products (MOO:: Zn = 1.0:3.5, after leaching): (a) 40 MPa, 110 MPa, and (c) 170 MPa.
(b)
20
Fig. 2--X
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