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Preparation of ultrafine molybdenum powder by vapour phase reaction of the MoO3-H2 system
Jourffal of the Less-Common
metals,
157 (1990)
L5 - LlO
L5
Letter
Preparation of ultrafine molybdenum powder by vapour phase reaction of the MOO,-H, system KOJI SHIBATA, KIYOSHI TSUCHIDA and AK10 KATO of Applied 812 (Japan)
~e~ar~rn~n~
Fukuoka
Chemists,
~acuf~y of engineering,
Kyushu
University,
(Received June 20,1989)
1. Introduction Ultrafine metal powders are finding application in many fields [l]. Fine molybdenum powders are used for the fabrication of conductive films in electronic devices; for this application, control of the powder characteristics is very important. In the present work, the preparation of ultrafine moly~en~ powders by vapour phase reaction of the Moos-H, system was studied, with the emphasis on the relation between particle size and preparation conditions. 2. Experimental de tails The apparatus used for the powder synthesis is shown in Fig. 1. The reaction tube is made of a-alumina (I.D. 22 mm). Moos powder (purity, 99.976, Ishizu Seiyaku Co. Ltd.), contained in a molybdenum boat, was placed between the pre-heating furnace and the main furnace. Moos vapour was carried into the reaction zone using a stream of nitrogen (400 ml min-“) which was mixed with a stream of an HZ--N, gas mixture (150 ml min-I). The evaporation rate of Moos was calculated from the weight change of the MOO, source. In the present work, the mixing tempe~ture Tnn of the
Fig_ 1. Apparatus for powder synthesis: A, pre-heating furnace; B, main furnace; C, reaction tube (alumina, I.D. 22 mm); D, chromel-alumei thermocouple; E, I%-Rh(6-30) thermocouple; I?, molybdenum boat for Moos source; G, molybdenum plate and nozzle; H, fiask. @ Elsevier Sequoia/Printed
in The Netherlands
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MOO,-N2 and Hz-N2 streams and the reaction temperature
TR at the centre of the main furnace were varied. A molybdenum plate was placed at the mixing point in the reaction tube to prevent back-flow of the Hz-N, gas mixture. As-synthesized powder was trapped in flasks or by a filter (Microtex-F, Kyowa Filter Co. Ltd., Japan) and identified by X-ray diffraction (XRD), using Cu Ko radiation. The average particle size was calculated from transmission electron micrographs. The equilibrium constant for the present reaction is given below [Z].
MoO3(g) + 3&(g) log
MO(S) + 3H,O(g)
rr, = 9.0(1100 “c),
(1)
log KP = 6.0(1400 “C)
3. Results and discussion The preparation conditions examined in the present work are the concentrations of hydrogen and Mo03, the mixing temperature TM and the reaction temperature TR . The results are summarized in Table 1. The powder yield was calculated from the amount of powder which was trapped in the flasks or by the filter. The remainder of the reaction product was deposited in the reaction tube. A small amount of the powder product passed through the traps when flasks were used. TABLE 1 Preparation conditions and properties of synthesized powder Run
TR (“C)
TM (“C)
TV * [Moo31 b [H2Jb (“C) (vol.%) (vol.%)
Average particle size (nm)c
Product phase
1 2 3 4 5
1300 1300 1300 1300 1300
1100 1100 1100 1100 1100
827 827 827 827 827
1.3 1.4 1.0 1.0 1.1
1.8 4.5 9.1 18.0 27.0
50 51 56 59
Moo2 MO, MoOa MO 35 MO 39 MO 39
Flask Filter Filter Filter Fiiter
6 7 8 9
1500 1500 1500 1500
1000 1100 1200 1400
827 827 827 827
1.8 1.9 2.1 2.3
9.1 9.1 9.1 9.1
77 58 56 46
MO MO MO MO
6 40 30 25
Flask Filter Flask Flask
10 11
1200 1400
1100 1100
827 827
0.8 1.7
9.1 9.1
52
MO MO
36 52
Filter Filter
12 13 14
1400 1400 1400
1100 1100 1100
810 850 900
1.0 2.6 5.4
9.1 9.1 27.0
38 64 69
MO MO MO
9 27 28
Flask Flask Flask
aMelting point of MoOa is 795 “C. bpercentage for the total reacting gas mixture MoOs + Ha + Na. Walculated by volume base.
Powder yield (%)
Trupping method
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3.1. Effect of hydrogen concentration The effect of hydrogen concentration on particle size was examined at Ta = 1300 “C and Thn= 1100 “C. The results are given in Table 1 (runs 1 - 5). The particle size of molybdenum increased a little with increasing hydrogen concentration. At the molar ratio H,:MoOs = 3.2 (run 2), the colour of the as-synthesized powder was dark blue, and a trace of MOO* was detected by XRD. At the molar ratio H2:Mo03 = 1.4 (run l), hexagonal plate-like particles of MOO, (Fig. 2(a)) were produced. The vapour phase reduction of such MoOz particles gave molybdenum particles showing a skeletal structure (Fig. 2(b)). Such skeletal particles may have a detrimental effect when used as raw materials for conducting films. 3.2. Effect of mixing and reaction temperatures The effect of mixing temperat~e TM was examined in runs 6 - 9. Transmission electron micrographs of as-synthesized powders are shown in Fig. 3. The as-synthesized powder consisted of uniform, spherical particles showing partial aggregation. Average particle sizes are plotted against TM in Fig. 4; the particle size increased with decreasing TM. The crystal sizes calculated from the peak width of the XRD patterns were almost the same as the particle size derived from transmission electron mi~ro~phs, indicating that the molybdenum particles are single crystalline. However, the effect of reaction temperature Z’n on the particle size is small, as seen in runs 3, 7 and 11. The slight increase in particle size at TR = 1500 “C may be due to the increase in MOO, concentration. As shown above, Tnn has a larger effect on the particle size of molybdenum than does TR. This fact indicates that the formation of molybdenum particles from Moos vapour occurs rapidly at TM where the Moo3 and HZ-N, streams are mixed. In other words, TNI is the true reaction temperature in the present reaction system.
Fig. 2. Transmission electron micrograph of (a) hexagonal plate-like particles of MOO,; (b) skeletal molybdenum particles produced from the MoOz particles.
400nm Fig. 3. Transmission electron micrographs of as-synthesized powder. (b) 1100 “C; (c) 1200 “C; (d) 1400 “C.
TM: (a) 1000 “C;
Miring temperature (TM) KZ)
Fig. 4. Effect of mixing temperature TM on average particle size of molybdenum.
The decrease in particle size, i.e. the increase in nucleation rate with increasing TM, may be due to the increase in the reduction rate of MOO,. Morooka et al. [33 reported the preparation of molybdenum powder by
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vapour phase reaction of the MoCl,-H, system. In their work, the particle size of molybdenum was increased by the coalescence of particles at TR > 1200 “C when the particle sizes were small (6 - 10 nm). However, the particle size of molybdenum was almost constant over the range TR = 1300 - 1500 “C in the present work. This difference between the two investigations may be due to the difference in particle size of the molybdenum: in the present work, the particle size of the molybdenum was almost ten times larger than that in the study by Morooka et al. For this reason, coalescence of particles was not observed, even above 1300 “C. 3.3. Effect of Moo3 concentration The effect of Moos concentration was examined at TNI = 1100 “C and TR = 1400 “C (runs 11 -14). The Moos concentration was varied by changing the evaporation temperature (TV) of MOO,. The result is shown in Fig. 5. In run 14 (TV = 900 “C), the concentration of hydrogen was increased to keep the molar ratio of H,:MoOs above 3. When the efficiency of the vapour phase reaction is loo%, the particle size D can be expressed as
Co is the concentration of Mo03, N is the number of particles produced per unit volume of the reacting gas mixture, M is the atomic weight of molybdenum and p is the density of molybdenum. In the present work the particle size of molybdenum was found to be proportional to [MoO~]~‘~. This means that the number of nucleation centres was constant over the concentration range of Moo3 examined.
[MoOalv3
(vol%l”’
Fig. 5. Effect of concentration of MoOa on average particle size of molybdenum (hydrogen concentration: 0, 9.1 vol.%; 0, 27 vol.%).
4. Conclusion In the present work, ultrafine molybdenum powder was prepared by vapour phase reaction of the MOO,-H, system. The results are summarized below. (1) Molybdenum powder consisting of uniform, spherical particles 40 - 70 nm in diameter was obtained at 1300 - 1500 “C.
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(2) The mixing temperature of the MoOsN, and HZ-N, streams had a significant effect on the particle size. The particle size could be controlled by varying the Moo3 concentration. Acknowledgment This work was supported by a Grant-in-Aid for Scientific Research on Priority Areas (No. 62603023) from The Ministry of Education, Science and Culture, Japan. References 1 T. Hayashi and Y. Saito, Kagaku, 39 (1984) 667. 2 I. Barin and 0. Knacke, Thermochemical Properties of Inorganic Substances, Springer, Berlin, 1973. 3 S. Morooka, A. Kobata, T. Yasutake, K. Ikemizu and Y. Kato, Kagaku Kogaku Ronbunsyu, 13 (1987) 485.
metals,
157 (1990)
L5 - LlO
L5
Letter
Preparation of ultrafine molybdenum powder by vapour phase reaction of the MOO,-H, system KOJI SHIBATA, KIYOSHI TSUCHIDA and AK10 KATO of Applied 812 (Japan)
~e~ar~rn~n~
Fukuoka
Chemists,
~acuf~y of engineering,
Kyushu
University,
(Received June 20,1989)
1. Introduction Ultrafine metal powders are finding application in many fields [l]. Fine molybdenum powders are used for the fabrication of conductive films in electronic devices; for this application, control of the powder characteristics is very important. In the present work, the preparation of ultrafine moly~en~ powders by vapour phase reaction of the Moos-H, system was studied, with the emphasis on the relation between particle size and preparation conditions. 2. Experimental de tails The apparatus used for the powder synthesis is shown in Fig. 1. The reaction tube is made of a-alumina (I.D. 22 mm). Moos powder (purity, 99.976, Ishizu Seiyaku Co. Ltd.), contained in a molybdenum boat, was placed between the pre-heating furnace and the main furnace. Moos vapour was carried into the reaction zone using a stream of nitrogen (400 ml min-“) which was mixed with a stream of an HZ--N, gas mixture (150 ml min-I). The evaporation rate of Moos was calculated from the weight change of the MOO, source. In the present work, the mixing tempe~ture Tnn of the
Fig_ 1. Apparatus for powder synthesis: A, pre-heating furnace; B, main furnace; C, reaction tube (alumina, I.D. 22 mm); D, chromel-alumei thermocouple; E, I%-Rh(6-30) thermocouple; I?, molybdenum boat for Moos source; G, molybdenum plate and nozzle; H, fiask. @ Elsevier Sequoia/Printed
in The Netherlands
L6
MOO,-N2 and Hz-N2 streams and the reaction temperature
TR at the centre of the main furnace were varied. A molybdenum plate was placed at the mixing point in the reaction tube to prevent back-flow of the Hz-N, gas mixture. As-synthesized powder was trapped in flasks or by a filter (Microtex-F, Kyowa Filter Co. Ltd., Japan) and identified by X-ray diffraction (XRD), using Cu Ko radiation. The average particle size was calculated from transmission electron micrographs. The equilibrium constant for the present reaction is given below [Z].
MoO3(g) + 3&(g) log
MO(S) + 3H,O(g)
rr, = 9.0(1100 “c),
(1)
log KP = 6.0(1400 “C)
3. Results and discussion The preparation conditions examined in the present work are the concentrations of hydrogen and Mo03, the mixing temperature TM and the reaction temperature TR . The results are summarized in Table 1. The powder yield was calculated from the amount of powder which was trapped in the flasks or by the filter. The remainder of the reaction product was deposited in the reaction tube. A small amount of the powder product passed through the traps when flasks were used. TABLE 1 Preparation conditions and properties of synthesized powder Run
TR (“C)
TM (“C)
TV * [Moo31 b [H2Jb (“C) (vol.%) (vol.%)
Average particle size (nm)c
Product phase
1 2 3 4 5
1300 1300 1300 1300 1300
1100 1100 1100 1100 1100
827 827 827 827 827
1.3 1.4 1.0 1.0 1.1
1.8 4.5 9.1 18.0 27.0
50 51 56 59
Moo2 MO, MoOa MO 35 MO 39 MO 39
Flask Filter Filter Filter Fiiter
6 7 8 9
1500 1500 1500 1500
1000 1100 1200 1400
827 827 827 827
1.8 1.9 2.1 2.3
9.1 9.1 9.1 9.1
77 58 56 46
MO MO MO MO
6 40 30 25
Flask Filter Flask Flask
10 11
1200 1400
1100 1100
827 827
0.8 1.7
9.1 9.1
52
MO MO
36 52
Filter Filter
12 13 14
1400 1400 1400
1100 1100 1100
810 850 900
1.0 2.6 5.4
9.1 9.1 27.0
38 64 69
MO MO MO
9 27 28
Flask Flask Flask
aMelting point of MoOa is 795 “C. bpercentage for the total reacting gas mixture MoOs + Ha + Na. Walculated by volume base.
Powder yield (%)
Trupping method
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3.1. Effect of hydrogen concentration The effect of hydrogen concentration on particle size was examined at Ta = 1300 “C and Thn= 1100 “C. The results are given in Table 1 (runs 1 - 5). The particle size of molybdenum increased a little with increasing hydrogen concentration. At the molar ratio H,:MoOs = 3.2 (run 2), the colour of the as-synthesized powder was dark blue, and a trace of MOO* was detected by XRD. At the molar ratio H2:Mo03 = 1.4 (run l), hexagonal plate-like particles of MOO, (Fig. 2(a)) were produced. The vapour phase reduction of such MoOz particles gave molybdenum particles showing a skeletal structure (Fig. 2(b)). Such skeletal particles may have a detrimental effect when used as raw materials for conducting films. 3.2. Effect of mixing and reaction temperatures The effect of mixing temperat~e TM was examined in runs 6 - 9. Transmission electron micrographs of as-synthesized powders are shown in Fig. 3. The as-synthesized powder consisted of uniform, spherical particles showing partial aggregation. Average particle sizes are plotted against TM in Fig. 4; the particle size increased with decreasing TM. The crystal sizes calculated from the peak width of the XRD patterns were almost the same as the particle size derived from transmission electron mi~ro~phs, indicating that the molybdenum particles are single crystalline. However, the effect of reaction temperature Z’n on the particle size is small, as seen in runs 3, 7 and 11. The slight increase in particle size at TR = 1500 “C may be due to the increase in MOO, concentration. As shown above, Tnn has a larger effect on the particle size of molybdenum than does TR. This fact indicates that the formation of molybdenum particles from Moos vapour occurs rapidly at TM where the Moo3 and HZ-N, streams are mixed. In other words, TNI is the true reaction temperature in the present reaction system.
Fig. 2. Transmission electron micrograph of (a) hexagonal plate-like particles of MOO,; (b) skeletal molybdenum particles produced from the MoOz particles.
400nm Fig. 3. Transmission electron micrographs of as-synthesized powder. (b) 1100 “C; (c) 1200 “C; (d) 1400 “C.
TM: (a) 1000 “C;
Miring temperature (TM) KZ)
Fig. 4. Effect of mixing temperature TM on average particle size of molybdenum.
The decrease in particle size, i.e. the increase in nucleation rate with increasing TM, may be due to the increase in the reduction rate of MOO,. Morooka et al. [33 reported the preparation of molybdenum powder by
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vapour phase reaction of the MoCl,-H, system. In their work, the particle size of molybdenum was increased by the coalescence of particles at TR > 1200 “C when the particle sizes were small (6 - 10 nm). However, the particle size of molybdenum was almost constant over the range TR = 1300 - 1500 “C in the present work. This difference between the two investigations may be due to the difference in particle size of the molybdenum: in the present work, the particle size of the molybdenum was almost ten times larger than that in the study by Morooka et al. For this reason, coalescence of particles was not observed, even above 1300 “C. 3.3. Effect of Moo3 concentration The effect of Moos concentration was examined at TNI = 1100 “C and TR = 1400 “C (runs 11 -14). The Moos concentration was varied by changing the evaporation temperature (TV) of MOO,. The result is shown in Fig. 5. In run 14 (TV = 900 “C), the concentration of hydrogen was increased to keep the molar ratio of H,:MoOs above 3. When the efficiency of the vapour phase reaction is loo%, the particle size D can be expressed as
Co is the concentration of Mo03, N is the number of particles produced per unit volume of the reacting gas mixture, M is the atomic weight of molybdenum and p is the density of molybdenum. In the present work the particle size of molybdenum was found to be proportional to [MoO~]~‘~. This means that the number of nucleation centres was constant over the concentration range of Moo3 examined.
[MoOalv3
(vol%l”’
Fig. 5. Effect of concentration of MoOa on average particle size of molybdenum (hydrogen concentration: 0, 9.1 vol.%; 0, 27 vol.%).
4. Conclusion In the present work, ultrafine molybdenum powder was prepared by vapour phase reaction of the MOO,-H, system. The results are summarized below. (1) Molybdenum powder consisting of uniform, spherical particles 40 - 70 nm in diameter was obtained at 1300 - 1500 “C.
LlO
(2) The mixing temperature of the MoOsN, and HZ-N, streams had a significant effect on the particle size. The particle size could be controlled by varying the Moo3 concentration. Acknowledgment This work was supported by a Grant-in-Aid for Scientific Research on Priority Areas (No. 62603023) from The Ministry of Education, Science and Culture, Japan. References 1 T. Hayashi and Y. Saito, Kagaku, 39 (1984) 667. 2 I. Barin and 0. Knacke, Thermochemical Properties of Inorganic Substances, Springer, Berlin, 1973. 3 S. Morooka, A. Kobata, T. Yasutake, K. Ikemizu and Y. Kato, Kagaku Kogaku Ronbunsyu, 13 (1987) 485.