Enhanced Property of W–Cu Composites by Minor Addition of Ag
2019 Volume 60 Issue 9 Pages 1908-1913
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2019 Volume 60 Issue 9 Pages 1908-1913
Nano-scale W–20Cu (mass%) and W–18Cu–2Ag (mass%) composite powders were obtained by mechano-thermochemical process, followed by liquid phase sintering process from 1200 to 1300°C. The results indicate that the relative density and electrical conductivity of W–18Cu–2Ag composite were much superior to the W–20Cu composite at all temperatures. Microstructural investigation reveals that minor addition of Ag is beneficial to the densification of W–Cu composite. This can be ascribed to the stronger the wettability and capillary force of liquid Cu–Ag to W compared to that of pure Cu. Furthermore, during cooling Ag can precipitate into the voids between the W particles in W-rich area where is hard to be infiltrated by liquid Cu at high temperature, which is also beneficial to electrical conductivity.
Schematic representation of the densification process of W–Cu and W-18Cu–2Ag composites during sintering and subsequent cooling process.
Tungsten–copper (W–Cu) composites combine the advantages of both Cu and W, i.e. high heat-resistance, ablation-resistance and low thermal expansion coefficient from W and high thermal and high electrical conductivity from Cu.1–3) With these properties, W–Cu alloys have been widely used in a variety of environments4,5) such as electric resistance welding, electro-discharge machining electrodes, electrical contacts, and also for devices in electronic packaging,6,7) such as heat sink or heat base in microwave communication systems, lead frame of central processing system (CPU) and large-scale integrated circuit.
As we all know, residual pores are the most harmful factor influencing the thermal and electrical properties of materials. Therefore, high densification and homogeneous dispersion of ingredients are the precondition of all other properties. However, because the mutual insolubility or negligible solubility of W–Cu system at both low and high temperature, W–Cu powder compacts show poor sinterability even at high temperature. W–Cu alloy is a pseudo-alloy, which is generally characterized by distinct particles of one metal dispersed in a matrix of the other one even after densification at high temperatures.8)
Infiltration is a traditional method for fabrication of W–Cu alloy through pushing the molten Cu into the W skeleton by capillary force.1,9,10) This process generally gives rise to relatively high density, ablation-resistance, high thermal and electrical conductivity. However, it is difficult to fabricate the W–Cu alloys with complex geometry and high Cu contents exceeding 20%.11,12) In addition, microstructure of W–Cu alloys by this process is not homogenous due to the significantly varied capillary force along the sample depth. Powder metallurgy is another process to produce W–Cu composites. However, it is also difficult to obtain fully dense microstructure due to mutual insolubility of W and Cu13) and low wettability of liquid Cu to W.14) Other methods such as mechanical alloying (MA),15–18) sol-gel process19) and homogeneous precipitation have been exploited to synthesize homogeneous composite powders with ultra-fine particle size, which have been proved effective on the densification of W–Cu system.20) However, the contaminations introduced during the long-term milling, as well as the environmental problems have attracted much consideration by researchers. Compare with above methods, active sintering can lower the sintering temperature and enhance the sintering ability of W–Cu composite by adding transition elements. There are many researches have been done to prepared W–Cu composite closed to theoretical density by adding minor of active elements such as Fe, Co, Ni, Pd to increase the wettability of copper and tungsten.21,22) However, owing to the presence of activated elements, the thermal and electrical conductivity of W–Cu alloys decrease significantly.23) Silver possess the excellent conductivity among all metals. Silver has been used as additives in the fabrication of Mo–Cu composites by liquid phase sintering to achieve densification without reducing the electrical and thermal conductivities.24) G. Taghavi et al.8) have compared the density property of W–Cu/Ag and W–Ag composites. The results show that adding small amount of Ag will enhance the densification due to the higher wettability of Ag and W than that of Cu and W. However, there are few researches have done on the impact of Ag on the properties of W–Cu composites at present.25,26)
Mechano–thermochemical process is a technique that combined ball mill and chemical synthesis to produce nano-scaled composite oxide powders.27) Subsequently, co-reduction of the oxide powder under reducing atmosphere generally gives rise to sub-micro W–Cu composite powder with higher sinterablilty compared to the above-mentioned processes.28)
In order to enhance the sintering ability and electrical conductivity of W–Cu composites, a small amount (2 mass%) of silver was added as an activated element, as the solution silver does not have significant influence on both the electrical and thermo conductivity of Cu.29) In the current research, the Mechano–thermochemical process was used to fabricate W–Cu composite. Firstly, the precursor WO3, CuO and Ag powders calcined in air atmosphere, which is followed by higher energy milling, co-reduction and sintering process, sequentially. Effective of Ag contents on microstructure, relative density (RD) and electrical conductivity was investigated at different sintering temperature. The enhancement mechanism of sintering ability of W–Cu powder by the small addition of Ag was discussed in details.
The commercial tungsten trioxide powders (WO3 > 99.9% purity), copper oxide powders (CuO > 99.9% purity) and silver powders (Ag > 99.9% purity) were used in present study. To prepare composite powders, WO3 and CuO powders were premixed in a cross-rotary mixer with and without 2 mass% Ag powders for 24 hours, which were preparing for fabricating W–20 mass%Cu (xCu = 0.420) and W–18 mass%Cu–2 mass%Ag (xCu = 0.383, xAg = 0.025) alloys, respectively. The mixed powders were then calcined at 800°C in air atmosphere in a muffle furnace for 1 hour to obtain oxide precursors. Then these oxide precursors were ball milled and co-reduced to obtain nano W–Cu and W–Cu–Ag powders. In ball milling procedure, a cemented carbide container of 45 ml and cemented carbide balls of 5 mm were used as the milling media. The rotational speed was 700 rpm. Considering both particle size and impurities of oxide powder, the milling time 4 hours with the ball to powder ratio of 1:1 was used. In co-reduction procedure, the fine oxide powders were reduced in flowing hydrogen atmosphere at 800°C for 1 hour, with a hydrogen flow velocity of 1 L/min. The fine reduced W–Cu and W–Cu–Ag powders were then sintered at 1200°C, 1250°C and 1300°C for 2 hours, respectively. The heating rate of both activated sintering process was 10°C/min. Hydrogen is chosen as sintering atmosphere due to which is able to remove the residual oxygen of compacts and promote sintering densification.
The phase constituents of baked powders were determined by Philips X’Pert X-ray diffractometer (XRD), with Cu Kα of 1.542 Å radiation source in Bragg-Brentano geometry and diffraction angle 2θ between 20 and 70°. The relative density of the sintered samples was measured by Archimedes’ principle. Eddy Current Conductivity Meter was used to measure electrical conductivity. Scanning electron microscope (SEM) and electron-probe microanalyzer (EPMA) were used to observe the microstructure and analysis the components of the final W–20Cu and W–18Cu–2Ag composites.
XRD patterns of the Ag-free and Ag-containing oxide powder after baking at 800°C for 1 h in air atmosphere are shown in Fig. 1. The results indicate that the W–Cu oxide composite powder was changed into copper tungstate (CuWO4−x) and residual WO3 in the case of Ag-free oxide powder, which was caused by ratio of constituents. At the same time, content of residual CuO is almost zero. Ag addition is observed to decrease the peak intensity of WO3 in the baked powder, since Ag2W4O13 was generated from WO3 and Ag after baking. SEM micrographs of W–20Cu and W–18Cu–2Ag calcined powders are shown in Fig. 2, demonstrating two kinds of calcined powders have been slightly agglomerate or aggregated, the powders ranging from several micrometers to about 70 µm in size together with some fine oxide particles. The sintering behavior of the sufficiently oxidized WO3 and CuO powders are performed at a high temperature, so the oxide composite powder is able to maintain the powder state instead of being sintered into blocks.
XRD patterns of W–20Cu and W–18Cu–2Ag oxide precursors calcined at 800°C for 1 h in air atmosphere.
SEM micrographs of (a) W–20Cu and (b) W–18Cu–2Ag oxide precursors calcined at 800°C for 1 h in air atmosphere.
The particle size distribution of Ag-free oxide precursors after ball milling process is presented in Fig. 3 indicates that the mean particle size of milled oxide precursors decreased to 154 nm and the particle size is mainly in the range of 40–400 nm. The particle size distribution of Ag-containing oxide precursors after ball milling process is not given here, since the content of Ag in the composite powders was low and it has little effect on the powder refinement process. Content of principal impurity iron (Fe) in milled powder is less than 0.004% (mass%), which means the powder has high purity after ball milling process. The high pure oxide powder is a precondition to obtain W–20Cu and W–18Cu–2Ag composites with high electrical conductivity.
Particle size distribution of W–20Cu after milling process and co-reduction process.
The XRD patterns for W–20Cu and W–18Cu–2Ag powders reduced at 800°C for 2 h in hydrogen atmosphere are shown in Fig. 4. It includes W and Cu metallic phases for both W–20Cu and W–18Cu–2Ag composite powders, indicating that all oxide phases were reduced to metallic composite powders. The Ag element was not detected by XRD in W–Cu–Ag co-reduced powder due to its low content.
XRD patterns of W–20Cu and W–18Cu–2Ag powder co-reduced at 800°C for 1 h in hydrogen atmosphere.
Figure 5 shows the SEM micrographs of W–20Cu and W–18Cu–2Ag reduced powders. The spherical or ellipsoid morphology and partially agglomerated and aggraded can be observed in these images. The particle size of both reduced powders is about 100–300 nm as shown in Fig. 3. The residual oxygen content of powder reduced at 800°C for 1 hour is 0.303% (mass%).
SEM of (a) W–20Cu and (b) W–18Cu–2Ag powder co-reduced at 800°C for 1 hour in hydrogen atmosphere.
Since the sintering process of W–18Cu–2Ag composites is liquid phase sintering, sintering temperature increasing improved the diffusivity ratio of W, Cu and Ag particles within a certain range and result in increasing of homogenization and densification.8,30) However, the high sintering temperature can cause the volatilization of liquid copper and segregation of material produced, which will result in decrease of relative density.12) Hence, the choice of sintering temperature is the key to determine the sintering process. The relationship between relative density and sintering temperature of W–20Cu and W–18Cu–2Ag composites is given in Table 1. The result reveals that the relative density was enhanced by increasing in the sintering temperature in both W–Cu and W–18Cu–2Ag composites, due to high temperature is beneficial for the bonding and diffusion properties of the powders. The relative densities are 88.1%, 93.2% and 99.5% for W–20Cu composite samples sintered at 1200°C, 1250°C and 1300°C, respectively. Owing to a fine and homogeneous distribution of the constituent phases, the synthesized composite powders show high sinterability. Moreover, as seen in the results, the relative density of W–18Cu–2Ag specimens are 92.4%, 97.1% and 99.7% for W–Cu–Ag composites respectively, which are higher than W–Cu specimens in the same sintering condition.
Figure 6 shows the microstructure morphology of W–20Cu samples sintered at 1300°C for 2 h in hydrogen atmosphere. Figure 6(a) shows a pretty dense structure. As it is observed, W particles have a homogeneous distribution in the Cu matrix, forming a network without aggregation. However, a few of voids can still be observed clearly. The oxide precursor of W–20Cu composite has a compound with a mixture of CuWO4−x + WO3. The residual WO3 formed W-rich powder after co-reduction process and W-rich area in the composite after liquid phase sintering. During liquid phase sintering, the W-rich area in the composite only solid-state sintering between W particles was performed and liquid Cu was not able to infiltrate into the pores which resulted in voids and low relative density. Furthermore, the light particles are W and dark area is Cu according to EDS results (Fig. 6(b)).
SEM of W–20Cu composite stringed at 1300°C for 2 hours in hydrogen atmosphere.
Figure 7 shows the EPMA elemental maps of W–18Cu–2Ag composites sintered at 1300°C for 2 h in hydrogen atmosphere. As the obtained result shows, W particles have a fine and homogenous distribution in the Cu and Ag matrix. Additionally, its porosity level is negligible, so the measured density near to the theoretical density is confirmed. It is able to observe that the Ag is distributed in the narrow gaps between W particles where the liquid Cu was not able to infiltrate at high temperature as shown in the circle. The wettability of liquid copper to tungsten is lower than liquid silver and liquid copper–silver phase according to the previous study.31) The energy of lateral capillary interaction between the two particles is obtained in the form:32–34)
| \begin{equation} \Delta W \approx 2\pi \sigma Q_{1}Q_{2}K_{0}(qL) \end{equation} | (1) |
| \begin{equation} Q_{k} = r_{k}\,\mathit{sin}\,\varPsi_{k}\quad k = 1,2 \end{equation} | (2) |
| \begin{equation} F \approx 2\pi \sigma Q_{1}Q_{2}qK_{1}(qL) \end{equation} | (3) |
| \begin{equation} q = \left(\frac{\rho g}{\sigma}\right)^{1/2} \end{equation} | (4) |
EPMA of W–18Cu–2Ag composite fabricated at 1300°C for 2 hours in hydrogen atmosphere.
The distribution of Ag in W–Cu composite is considered to be related to dissolution-precipitation behavior of Ag during sintering and cooling process. As seen in the schematic diagram (Fig. 8), Ag is able to dissolve completely into Cu matrix at high temperature while most of Ag will precipitate from Cu matrix during the subsequent cooling, according to Cu–Ag Phase Diagram36) and our previous research.37) During liquid sintering process, Ag dissolved in liquid Cu, which is considered to enhance the wettability between liquid Cu and W. The higher capillary force caused by higher wettability and higher wetting speed of liquid Cu–Ag phase to W than liquid Cu resulted in densification of the composites. During cooling process, Ag was preferentially precipitated from the Cu and uniformly distributed in the crystal boundaries and locations of voids with the temperature decreasing, which caused further densification of the composites.
Schematic representation of densification process of W–20Cu and W–18Cu–2Ag composites during cooling.
The value of electrical conductivity (EC) of W–20Cu and W–18Cu–2Ag composites sintered at 1200°C, 1250°C and 1300°C are listed in Table 1. It can be seen that the EC of W–18Cu–2Ag composites sinter at 1200°C, 1250°C and 1300°C are 32%IACS, 39%IACS and 42.5%IACS, respectively. The EC of W–20Cu composites sintered at 1250°C and 1300°C are 30%IACS and 41%IACS, respectively, which are lower than W–18Cu–2Ag composites at a same sintering temperature. Generally, there are two factors which have effect on electrical conductivity. One is relative density of composite. The porosity increases with relative density decreasing and the porosity has negative effect on electrical conductivity of composite.38) Another is that the Cu or Cu–Ag formed continued net structure or not. This structure provides electron conduction path for W–Cu–(Ag) composites and result in the increasing of electrical conductivity. The W–Cu composites fabricated by infiltration are not able to form copper of net structure due to that the W skeleton pre-sintered has negative effect on the forming of Cu net, which result in low electrical conductivity.39) The W–Cu composites fabricated by activated sintering process have low electron and conductivity due to that the activated elements go into solution with Cu and decrease electrical conductivity significantly.40,41) However, the Ag was separated out from the Cu–Ag solid solution and formed a new phase with single Ag element when cooling process in this study, which is coincide with the reports of Li et al.37) The Ag distributed in boundaries and filled the pores in the W-rich area forming a continuous Cu–Ag network along W-boundaries improved the interface and caused densification of the composites, which is the reason that the electrical conductivity of W–18Cu–2Ag is higher than W–20Ag composite.
In this work, mechano-thermochemical process was used to prepare W–20Cu and W–18Cu–2Ag powders. The relative density and electrical conductivity of W–20Cu and W–18Cu–2Ag sintered at different temperature are analyzed in detail. The main conclusions derived from the detailed analysis are summarized as follows:
The authors gratefully acknowledge the financial support from project of The Science Fund for Distinguished Young Scholars of Hunan Province, China (2016JJ1016), the project of Innovation and Entrepreneur Team Introduced by Guangdong Province (201301G0105337290) and the Special Funds for Future Industrial Development of Shenzhen (No. HKHTZD20140702020004).
