Preparation and characterization of Mo-15 Cu superfine powders by a gelatification-reduction process

Journal of Alloys and Compounds 476 (2009) 226–230

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Preparation and characterization of Mo-15 Cu superfine powders by a gelatification-reduction process Peng Song, Jigui Cheng ∗ , Lei Wan, Jingsong Zhao, Yifang Wang, Yanbo Cai School of Materials Science and Engineering, Hefei University of technology, Hefei 230009, PR China

a r t i c l e

i n f o

Article history: Received 20 July 2008 Received in revised form 11 September 2008 Accepted 17 September 2008 Available online 6 November 2008 Keywords: Gelatification reduction Mo–Cu powders Sinterability

a b s t r a c t Mo-15 wt.%Cu composite powders were synthesized by a gelatification-reduction process, in which precursor gelcasts were obtained by adding an initiator into a suspension containing ammonium heptamolybdate, copper oxide, acrylamide organic monomer and some additives. The gelcasts were then calcined and hydrogen-reduced to convert into Mo–Cu powder. Phase constitution and morphology of the gelcasts, the calcined powder, as well as the resulting Mo–Cu powder were characterized. Density and microstructure of the sintered Mo–Cu parts were checked to investigate sinterability of the Mo–Cu powder. It was shown that the gelatification-reduction process produces superfine Mo–Cu powder with particle size of about 100 nm. The Mo–Cu powder shows good sinterability. Relative density of 99.59% of the theoretical is achieved for Mo–Cu powder compacts sintered at 1150 ◦ C in H2 atmosphere. Furthermore, the sintered Mo–Cu parts exhibit excellent physical and mechanical properties. Maximum electrical conductivity is 41.75%IACS. Bending strength and Vickers hardness for Mo–Cu compacts sintered at 1150 ◦ C were 833.65 MPa and 300.15 MPa, respectively. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Powder metallurgical Mo–Cu alloys exhibit excellent physical property, high thermal and electrical conductivity, low and alterable thermal expansion coefficient. Despite relatively low thermal conductivity, density difference between Mo and Cu is smaller than that between W and Cu, and Mo has lower melting point and hardness than that of W. Therefore, Mo–Cu alloys are also easy to sinter and to process than W–Cu material. This makes Mo–Cu alloy to be widely used for electronic packing devices, heat sink materials and vacuum applications and be of great interest in aeronautics, portable apparatus and some other advanced fields [1–3]. In most of the applications, high-dense Mo–Cu materials with homogeneous microstructure are required for high performance. Mo–Cu parts are commonly fabricated by liquid phase sintering of Mo–Cu powder compacts or by copper infiltration sintering [4]. For Mo–Cu alloys with relatively low Cu content, however, full-densification is difficult to achieve by the liquid phase sintering or infiltration process. Moreover, copper infiltration process does not result in a homogeneous microstructure, which is detrimental to mechanical property of the materials [5,6].

∗ Corresponding author. Fax: +86 551 2901793. E-mail address: [email protected] (J. Cheng). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.09.097

It has been shown that using of homogeneous and ultra-fine composite powder can effectively improve sinterability of a powder compact, especially in a liquid phase sintering system such as Mo–Cu system, in which the dominant sintering mechanism is known to be a particle rearrangement [7,8]. Therefore, attempts have been made to prepare ultra-fine and well-dispersed Mo–Cu powder to facilitate the rearrangement of Mo particles during liquid phase sintering to increase sintering densification and to improve microstructural homogeneity of Mo–Cu materials [7]. It has been reported that ultra-fine Mo–Cu composite powders can be prepared by mechanical alloying (MA) [9,10]. But it would introduce impurity into the powders from the tank or medium which decreases physical property of the resulting Mo–Cu materials. Recently, some chemical methods such as co-precipitation and sol–gel processes have been tried to synthesize ultra-fine Mo–Cu powders with high purity and excellent sinterability [7,11,12]. Gelcasting is a new near-net shape forming process of advanced ceramic and other materials, in which raw material powders with low solubility and organic monomer are first mixed in a solvent to form a well-dispersed suspension. Then an initiator is added into the suspension and gelatification occurrs by polymerizing the monomer to create a macropolymeric network. Therefore, the well-dispersed solid particles in the slurry are immobilized homogeneously in the gelcasts. The idea of gelatification has recently been applied to prepare ceramic powder precursors and the whole process is simple to perform [13]. Compared to traditional mechanical-mixing of raw material powders, the homogeneity of

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227

Fig. 1. Flow chart for preparing Mo–Cu composite powders.

the mixture is improved by wet-mixing and gelatification process. This is beneficial to the diffusion and phase transformation and to avoiding component aliquation in the later calcination and reduction processes [14,15]. But up to now, there have been few reports on using this process to prepare Mo–Cu and other metallic powder precursors. In this work, a gelatification process was designed to prepare Mo–Cu powder precursors, in which gelcasts were obtained from ammonium heptamolybdate, copper oxide. Mo–Cu powders were finally obtained by calcining the gelcasts and reducing the calcined powders. The present work has shown that the gelatification-reduction process produces ultra-fine and homogenous Mo-15 wt.%Cu powder, which is beneficial to obtain high-density Mo–Cu materials with excellent physical and mechanical properties. 2. Experimental

Fig. 2. DTA-TG curves of the dried gelcasts.

at 1050–1150 ◦ C with a heating rate of 10 ◦ C/min in H2 atmosphere for 2 h. Fig. 1 schematically shows the process of preparing Mo–Cu composite powders. 2.2. Characterization Simultaneous differential thermal analysis and thermogravimetry (DTA-TG) were carried out on the dried gelcasts. The samples were heated from room temperature to 700 ◦ C at a heating rate of 10 K /min under a dynamic airflow. The calcined gelcasts and the resulting Mo–Cu powders were characterized by X-ray diffraction (XRD). Particle size and morphology of the calcined gelcasts and the resulting Mo–Cu powders were examined by transmission electron microscopy (TEM). Density of the sintered Mo–Cu parts was measured by the Archimedes method and was expressed as relative to the theoretical value of 9.99 g/cm3 . Linear sintering shrinkage was calculated by measuring the dimension of the green and the sintered Mo–Cu compacts. Microstructure of the sintered Mo–Cu parts was observed by scanning electron microscopy (SEM). Electrical conductivity of the Mo–Cu specimens was determined by a DC four-probe method, and mechanical properties of the specimens were also investigated.

2.1. Sample preparation Stoichiometric ammonium heptamolybdate (NH4 )6 Mo7 O24 ·4H2 O), copper oxide (CuO) powders with a mean particle size of 7 ␮m were added into a aqueous solution containing acrylamide (AM) and N,N -methylenebisacrylamide (MBAM) organic monomer (AM:MBAM = 20:1) to form a suspension by ball-milling the solution. Then ammonium persulfate [(NH4 )2 S2 O8 ] initiator was added into the suspension and gelatification occurred and wet gelcasts formed. After drying and calcination, the gelcasts were reduced in H2 atmosphere at 700 ◦ C for 90 min to convert into Mo-15 wt.% powders. Dew point and flow rate of the H2 atmosphere are negative 50 ◦ C and 100 ml/min, respectively. The obtained Mo–Cu powder was pressed at a pressure of 200 MPa in an 18 mm diameter, cylindrical rigid die, and a 5 mm × 50 mm square die. Relative density of the green compact is about 60% of theoretical density. The pressed Mo–Cu powder compacts were then sintered

3. Results and discussion 3.1. Characterization of the gelcasts Fig. 2 shows DTA-TG results of the dried gelcasts. The gelcasts are composed of ammonium heptamolybdate, copper oxide, organic monomer and some other additives. In the temperature ranging 200–500 ◦ C, a mass loss of about 11% is accompanied by an exothermic DTA peak at 470 ◦ C, which indicates the thermal decomposition of the dried gels and the removal of the remnant water occluded

Fig. 3. XRD patterns of the gelcasts calcined at different temperatures.

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Fig. 4. TEM image of the gelcasts calcined at different temperatures.

in the organic monomer networks and the crystal water in the ammonium heptamolybdate. No apparent mass loss occurs at temperature above 500 ◦ C, indicating the formation of oxides of copper and molybdenum. Fig. 3 shows XRD patterns of the calcined gelcasts. There exist CuO, MoO3 , CuMoO4 , MoO2.75 , MoO2.89 and other molybdenum oxides in the powder calcined at 550 ◦ C (Fig. 3a). However, only MoO3 , CuO and CuMoO4 (Fig. 3b) were found in the powder calcined at 600 ◦ C. The intensity of XRD peaks increases with increasing calcination temperature in Fig. 3, which indicates the growth of crystallite size of the resulting powders. Fig. 4 shows TEM micrographs of powders calcined at different temperature. The mean particle size is about 100 nm for powder calcined at 600 ◦ C. However, particle size grows to 200 nm as the calcination temperature rises to 700 ◦ C, and the powder becomes aggregated.

at T > 200 ◦ C in H2 atmosphere. However, molybdenum oxides were reduced according to following Eq. [16]. MoO3 (s) + H2 (g) = MoO2 (s) + H2 O(g)

(1)

0.5MoO2 (s) + H2 (g) = Mo(s) + H2 O(g)

(2)

The standard Gibbs energy of reaction (2) (G2 ) can be calculated as following: Mo(s) + O2 (g) = MoO2 (s), G3 = [−578200 + 166.5T ](J · mol−1 )

(3)

Mo(s) + 1.5O2 (g) = MoO3 (s), G4 = [−740150 + 246.73T ](J · mol−1 )

(4)

3.2. Hydrogen reduction process of the calcined powders The calcined powders (i.e. mixes of copper and molybdenum oxides) are reduced in H2 atmosphere to convert into Mo–Cu powders. It is well known that copper oxides are easy to reduce into Cu

H2 (g) + 0.5O2 (g) = H2 O(g), G5 = [−247500 + 55.86T ](J mol−1 )

(5)

G1 = G3 + G5 − G4 = [4800 − 17.73T ](J mol−1 ) G2 = G5 − 0.5 G3 = [41600 − 27.39 T ](J mol−1 )

 G2 =

G2

+ RT ln

PH2 O



PH2

Mo powders can be prepared if the reaction (2) occurs spontaneously. In this case there exists the following equation:



G2 = G2 + RT ln

Fig. 5. XRD patterns of the Mo–Cu powders by reducing the calcined gelcasts at different temperature.

PH2 O PH2



<0

G2 depends on the reduction temperature (T) and the PH2 O /PH2 in the reduction atmosphere. The PH2 O /PH2 value is related to flow rate of H2 atmosphere. PH2 O /PH2 value is measured to be about 0.13–0.15 in a  50 mm × 1000 mm laboratorial tubular furnace [17]. The reduction temperature of reaction (2), therefore, should be above 650 ◦ C to reduce MoO3 into Mo completely.

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Fig. 6. TEM images of the Mo–Cu powders reduced at different temperature.

3.3. Characterization of the Mo–Cu Powders Fig. 5 shows XRD patterns of the Mo–Cu powders obtained by reducing the calcined gelcasts at different temperature. Mo, Cu, MoO2.75 and Mo8 O23 are found in the powders reduced at 600 ◦ C. However, only diffraction peaks of Mo and Cu elements are observed when reduction temperature increases to 700 ◦ C. This is consistent with the thermodynamics analysis. The degree of peak broadening decreases with increasing reduction temperature, which indicates the increase in the crystallite size of the Mo and Cu components in the Mo–Cu powders. Fig. 6 shows TEM images of the Mo–Cu powders reduced at different temperature. The Mo–Cu powders reduced at 700 ◦ C exhibit regular particle shape and narrow particle size distribution (Fig. 6a). The mean particle size is about 100 nm. However, mean particle size increases to about 200 nm as reduction temperature increases to 800 ◦ C (Fig. 6b), and the particles have irregular shape and a broad particle size distribution. 3.4. Sintered density and microstructure of the Mo–Cu samples Table 1 and Table 2 list sintered densities and sintering shrinkage of the Mo–Cu powder compacts, respectively. It can be seen that sintering densification of the Mo–Cu compacts depends greatly on sintering temperature. Sintered densities and sintering shrinkage of Mo–Cu compacts increase with increasing sintering temperature from 1050 to 1150 ◦ C. Mo–Cu compacts sintered at 1050 ◦ C have a relative density of about 92%. But this value increases to more than 98% for compacts sintered at temperature higher than 1083 ◦ C (the melting point of copper), and a maximum rel-

Table 2 Linear shrinkage rate of the Mo–Cu compacts sintered at different temperature.

Table 1 Density of the Mo–Cu compacts sintered at different temperature. Density

Sintered density (g/cm3 ) Relative density (%)

ative density of 99.59% is achieved at 1150 ◦ C. This indicates that Mo–Cu powder prepared by gelatification-reduction process possesses excellent sinterability. It is well known that the powder has undergone radial (side) pressure and axial pressure in a singleaction pressing, and the radial pressure is always smaller than the axial pressure. The difference between the radial (side) pressure and axial pressure results in the high green density in the pressing direction and the relatively low density in the cross section. This also results in the heterogeneous shrinkage during sintering. Therefore, it was seen from Table 2 that radial shrinkage of the compacts is larger than axial shrinkage after sintering. When Mo–Cu compacts are sintered at 1050 ◦ C, a temperature lower than melting point of copper, sintering densification of the compacts is related to the solid-state sintering. The intensity of the solid-state densification mainly depends on the mean size of the particles [18,19]. Ultra-fine and well-dispersed Mo–Cu powders are obtained in this work, which is beneficial to sintering densification. Therefore, relative density of 92.01% is obtained for Mo–Cu compacts sintered at 1050 ◦ C. When sintering temperature increases to 1100 and 1150 ◦ C, the densification is related to liquid-phase sintering, which is considered as a dominant sintering mechanism for the Mo–Cu alloy. In this case, Cu melts and flows through the structure, the movement of liquid phase provides a capillary force which causes rearrangement of the Mo particles and induces mass transport. Furthermore, because of the superfine particle size, liquid Cu is easier to fill into the pores between Mo particles to further facilitate the densification. Therefore, Mo–Cu samples reach 98.27% densification after sintering at 1100 ◦ C and a maximum relative density of 99.59% of the theoretical is obtained

Sintering temperature (◦ C) 1050

1100

1150

9.19 92.01

9.82 98.27

9.95 99.59

Sintering temperature (◦ C)

Radial shrinkage rate (%)

Axial shrinkage rate (%)

1050 1100 1150

17.9 23.3 24.1

17.2 18.9 20.1

230

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Fig. 7. SEM micrographs of the Mo–Cu samples sintered at 1150 ◦ C.

Table 3 Bending strength and Vickers hardness of the Mo–Cu compacts sintered at different temperature. T (◦ C)

Bending strength (MPa)

Vickers hardness (MPa)

1050 1100 1150

622.57 726.92 833.65

195.60 279.73 300.15

Table 4 Electrical conductivity of the Mo–Cu compacts sintered at different temperature. T (◦ C)

Electrical conductivity (IACS* ) (%)

1050 1100 1150

35.04 39.09 41.75

*

International Annealed Copper Standard.

at 1150 ◦ C. Fig. 7 shows SEM micrographs of the sintered Mo–Cu samples. It can be seen that there exists a homogeneous distribution of Mo and Cu phase in the structure. The grain size is about 3–5 ␮m. 3.5. Physical and mechanical properties of the sintered Mo–Cu samples Bending strength and Vickers hardness of the Mo–Cu samples sintered at different temperature are tested. The results are listed in Table 3. Electrical conductivity of the Mo–Cu samples sintered at different temperature is listed in Table 4. The results show that the liquid-phase sintered Mo–Cu parts have better physical and mechanical properties than the solid-state sintered parts, which can be attributed to the higher density of the former. Maximum bending strength, Vickers hardness and electrical conductivity for the Mo–Cu specimens sintered at 1150 ◦ C are 833.65 MPa, 300.15 MPa and 41.75%IACS, respectively.

4. Conclusions Mo-15 wt.%Cu superfine powders were successfully synthesized by a gelatification-reduction process. The Mo–Cu powders exhibit polygonal particle shape and have particle size ranging 100–200 nm and a narrow particle size distribution. Furthermore, the Mo–Cu powders exhibit good sinterability. Relative density of more than 99% can be obtained for the Mo–Cu powder compacts sintered at 1150 ◦ C in H2 atmosphere for 2 h. Excellent physical and mechanical properties are also achieved for the sintered Mo–Cu parts. Typically the electrical conductivity, bending strength and Vickers hardness of the Mo–Cu specimen are 41.75%IACS, 833.65 MPa and 300.15 MPa, respectively. References [1] D.G. Kim, S.T. Oh, H. Jeon, C.H. Lee, Y.D. Kim, Journal of Alloys and Compounds 354 (2003) 239–243. [2] K.Q. Mou, Y.G. Kuang, Metallic Functional Materials 9 (3) (2002) 27–29. [3] K.S. Huang, H.S. Huang, Materials Chemistry and Physics 67 (2001) 92–100. [4] S.H. Hong, B.K. Kim, Materials Letters 57 (2003) 2761–2767. [5] D.D. Gu, Y.F. Shen, Materials Letters 62 (2008) 1765–1768. [6] D.Y. Zhang, X.H. Wu, China Molybdenum Industry 25 (4) (2001) 38–39. [7] J.G. Cheng, C.P. Lei, E.T. Xiong, Y. Jiang, Y.H. Xia, Journal of Alloys and Compounds 421 (2006) 146–150. [8] F.A. da Costa, F.A. Doria, Powder Technology 134 (2003) 123–127. [9] V. de, P. Martinez, C. Aguilar, J. Marin, F. Castro, Materials Letters 61 (2007) 929–933. [10] S.N. Alam, Materials Science and Engineering A 433 (2006) 161–168. [11] X.L. Shi, H. Yang, S. Wang, G.Q. Shao, X.L. Duan, Z. Xiong, T.G. Wang, Materials Chemistry and Physics 104 (2007) 235–239. [12] Z.Y. Hang, W.G. Chen, B.J. Ding, Rare Metal Material and Engineering 34 (6) (2005) 990–993. [13] T. Zhang, Z.Q. Zhang, J.X. Zhang, D.L. Jiang, Q.L. Lin, Materials Science and Engineering A 443 (2007) 257–261. [14] M. Potoczek, Ceramics International 32 (2006) 739–744. [15] K. Cai, Y. Huang, J.L. Yang, Journal of the European Ceramic Society 25 (2005) 1089–1093. [16] X.R. Zhao, C.Y. Bu, X.Y. Chen, China Molybdenum Industry 28 (8) (2004) 34–36. [17] J.G. Cheng, P. Song, Y.F. Gong, Y.B. Cai, Materials Science and Engineering A 488 (2007) 453–457. [18] G.S. Kim, S.T. Oh, Y.D. Kim, Materials Letters 58 (2004) 578–581. [19] M.H. Maneshian, A. Simchi, Journal of Alloys and Compounds 463 (2008) 153–159.