Int. Journal of Refractory Metals and Hard Materials 31 (2012) 147–151

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Int. Journal of Refractory Metals and Hard Materials

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In situ synthesis and electrical properties of CuW–La2O3 composites

Kun Qian, Shuhua Liang ⁎, Peng Xiao, Xianhui WangSchool of Materials Science and Engineering, Xi'an University of Technology, Jinhua Road 5, Xi'an, Shaanxi 710048, PR China

⁎ Corresponding author. Tel.: +86 29 82312185; fax:E-mail address: liangxaut@yahoo.cn (S. Liang).

0263-4368/$ – see front matter © 2011 Elsevier Ltd. Alldoi:10.1016/j.ijrmhm.2011.10.005

a b s t r a c t

a r t i c l e i n f o

Article history:Received 28 February 2011Accepted 16 October 2011

Keywords:CuW alloyPowder metallurgyMicrostructureErosion

CuW–La2O3 composites were fabricated using an in-situ synthesis. The breakdown voltage in vacuum, mov-ing trajectory of cathode spots, electrical conductivity, and hardness of CuW–La2O3 composites were carefullyexamined. The microstructures were characterized by a scanning electron microscope (SEM). The resultsshow that CuW–La2O3 composites have the maximum hardness of 220HB and the electrical conductivity of45% IACS when the content of La2O3 is 0.75 wt.%. In comparison with CuW alloy, the dielectric strength, arclife and the arc mobility of the CuW–La2O3 composites increased by 36.9%, 9.7% and 46.6%, respectively. Asa result, the addition of La2O3 is useful to improve the properties of CuW alloys and the in situ synthesizedCuW–La2O3 composites should have excellent arc erosion resistance.

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1. Introduction

Electrical contact materials made of CuW alloy are widely used inhigh-voltage circuit breakers because of its high strength, excellentarc erosion resistance and conductivity [1–4]. These contact materialsnot only transport current, but also experience high-temperature arcerosion over time. With the increasing load of network power, the re-quirement for contact materials becomes stringent. Such demandposes a challenge for researchers to develop new electrical contactmaterials with improved arc erosion resistance [1]. A large numberof studies show that the mechanical properties of CuW alloys can beimproved by activated sintering and other methods [5–8]. Usingnanometer-sized tungsten powders as starting materials can improvethe mechanical properties of CuW alloys [9]. Injection molding andmechanical–thermal chemical processing can be used to refine cop-per phase, and optimize the spatial distribution of copper and tung-sten [2,10]. Most of these studies, however, are concentrated on theimprovement of the intrinsic properties of CuW alloys, little workhas been done so far on the effective control of arc erosion by improv-ing the microstructure of CuW alloys. It is generally believed that thefailure of CuW contact materials is caused by the concentrated arcerosion [11,12]. Ding et al. [11,13] show that arc breakdown occurspreferably on the selective phase with low work function in CuCr al-loys. It may be a good way to improve the arc erosion resistance byadding some components with low work function in CuW alloy. Inthis context, La2O3 which has low work function, high melting pointand good chemical stability, is a promising material to improve thearc erosion resistance. In this study, La2O3 was doped into tungstenmatrix by liquid–solid doping method, followed by the fabrication

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of CuW–La2O3 composites by sintering and infiltration. The effectsof La2O3 addition on the microstructure, hardness, electrical conduc-tivity, breakdown strength, chopping current and arc life were inves-tigated. The motions of cathode spots of CuW–La2O3 compositesduring arc discharge were studied using a high speed digital camera.

2. Experimental procedures

WO3 (purity>99%) powders and La2O3, Cu2O powders with a di-ameter of 3–5 μm and 30–50 μm, respectively, were used as the start-ing materials. The La2O3 powders were firstly dissolved in nitric acidaqueous solution (65% concentration), then followed by the additionof WO3 powders while stirring. This mixture was dried in a cruciblefurnace at 200 °C for 2 h, and pyrolysed at 870 °C for 3 h. The pyro-lysed powders were subsequently mixed with Cu2O powders for60 h. These mixed powders were reduced in hydrogen gas at 600 °Cfor 30 min and 900 °C for 90 min. After the treatment, the mixedpowders were compressed into a disk with a diameter of 20 mmand a thickness of 8 mm, followed by sintering at 1000 °C for 2 hand by infiltrating molten copper in hydrogen atmosphere. The con-tent of Cu in all CuW and CuW–La2O3 samples was 30 wt.%. The elec-trical conductivity was tested by a 7501-type eddy-currentconductivity instrument, and hardness was studied using a HB-3000Brinell hardness tester.

The vacuum electrical breakdown test was performed in an arcextinguishing chamber modified by a TDR-40A furnace for crystalgrowth at a vacuum degree of 2×10−3 Pa. The polished sampleswere transferred into the cathode sample holder as a cathode. Apure tungsten rod with a tip radius of 1.5 mm was directly abovethe cathode and used as an anode. The operating voltage was set at8 kV. The cathode was moved towards anode at a speed of 0.2 mm/min until electrical breakdown occurred between the anode and cath-ode. The distance between two electrodes and voltage was recorded

Fig. 1. SEM images and XRD patterns (the insets) of mixed powders before and afterreduction (a) the mixture of WO3 and La2O3 powders obtained by pyrolysis, (b) Wpowders reduced fromWO3 by H2, and (c) the mixture of W and La2O3 after reduction.

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during the discharge and used to calculate the dielectric strength. Thecurrent–time curve was recorded using a TDS-2024 Tektronics oscil-lograph. The arcing behaviors between anode and cathode wererecorded by a Phantom V9 digital high-speed video camera. The res-olution of the image is 192×64 dpi and sampling rate was24,000 f/s. The surface morphologies of CuW alloys after the thirdelectrical breakdown were characterized by a JSM-6700 F scanningelectron microscopy (SEM).

3. Results and discussion

3.1. Effect of La2O3 on the size of tungsten powders

Fig. 1(a) shows the SEM image and XRD pattern (the inset) of WO3

and La2O3 mixture prepared by pyrolysing La(NO3)3 and WO3 at870 °C for 3 h. It could be seen that the size of resulting WO3 powdersbefore reduction was between 5 and 15 μm. Fig. 1(b) shows the SEMimage and XRD pattern (the inset) of pure WO3 powders after reduc-tion by H2. The XRD pattern indicated that WO3 powders werecompletely reduced into W powders. The size of tungsten powderswas between 1 and 4 μm and the size distribution became broad.Fig. 1(c) shows the SEM image and XRD pattern (the inset) of pyro-lysed mixture of WO3 and La2O3 after reduction by H2 gas. Theobtained W powders with La2O3 have a good spatial distributionalong with the relatively monodispersed particles with diameter ofabout 0.5 μm and without agglomeration and grain growth. This ob-servation can be explained by the growth mechanism of chemicalvapor migration [14]. In the reduction of pure WO3 by H2, WO3 iswith strong volatile, it is easy to form volatile hydrated tungsten ox-ides, the hydrated tungsten oxides in steamwere reduced to tungstenand deposited on the surface of previous reduced tungsten powders,resulting in W powders growing in each cycle. For WO3–La2O3 com-posites, it is sure that the presence of La2O3 refines the size of tung-sten particles, and makes the size distribution of tungsten particlesuniform. Since there are a lot of references revealed the effect ofLa2O3 on the size and distribution of Mo particles during reductionby H2[15–17], and both of the preparation process of La2O3 dopingmolybdenum and tungsten are very similar; Furthermore, tungstenand molybdenum also have near physical and chemical properties,therefore, La2O3 particles are considered to play the similar role dur-ing the reduction WO3, i.e., La2O3 particles could act as nucleationsites for vapor deposition of tungsten and disperse relatively evenlyon the surface of tungsten powders during reduction. Both effectshelp preventing the neighboring W powders from growth, resultingin uniform and fine W particle size in the final composites.

3.2. Effect of La2O3 on the microstructure and physical properties of CuWalloys

Fig. 2 shows the SEMmicrographs of CuW alloys without and withLa2O3 and the EDS line-scan across CuW–La2O3 composites. The mi-crostructure of CuW alloy without La2O3 was heterogeneous(Fig. 2a). When the content of La2O3 was 0.75 wt.%, the microstruc-ture of CuW–La2O3 composites became homogeneous, in which thenetwork copper phases (dark region) distributed evenly aroundtungsten skeleton (bright gray region). No obvious large accumula-tion of W or Cu was observed, and the size of W skeleton was im-proved slightly (Fig. 2b). The EDS result shows that those particlesbetween the bright and dark regions were La2O3 (Fig. 2c). However,when the content of La2O3 was increased to 1.5 wt.%, the agglomera-tion of La2O3 particles appeared, and resulting in some closed pores intungsten skeletons, as shown in Fig. 2d.

The electrical conductivity and hardness of CuW–La2O3 compositesare presented in Fig. 3. The hardness increased first and then decreasedsignificantly with the increase of La2O3 content. The maximum hard-ness of 220HB was obtained for CuW–0.75 wt.%La2O3 composites. The

electrical conductivity fluctuated a bit in the composition range of0–0.75 wt.%La2O3, with increase of La2O3 content, the electrical conduc-tivity decreased gradually, the electrical conductivity of CuW–

0.75 wt.%La2O3 composites was about 45% IACS.The effect of additives on the final structure of composites can be

understood based on additive-grain interaction. Since they distribut-ed primarily adjacent to grain boundaries, the additives enhance theslip resistance, making it difficult for dislocation to cross the grain

Fig. 2. SEM micrographs of CuW alloys without and with La2O3 and the EDS line-scan across CuW–La2O3 composites. (a) SEM images of CuW alloys, (b) SEM images of CuW–

0.75La2O3 composites, (c) EDS pattern of CuW–0.75La2O3 composites. (d) EDS pattern of CuW–1.5La2O3 composites.

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boundaries, and then the dislocation piles up at the grain boundaries,resulting in improvement of the strength [3]. When La2O3 contentwas below 0.75 wt.%, this factor may have played a major role. As aconsequent, the hardness increased with the increase of La2O3 con-tent. However, excess La2O3 could lead to agglomeration of La2O3 par-ticles, which blocked pores of tungsten skeletons, reduced capillaryforce, and caused insufficient infiltration of molten Cu, thus decreas-ing the hardness.

The electrical conductivity of CuW–La2O3 composites decreasedslowly with increase of La2O3 content. This observation can beexplained as La2O3 has a poor electrical conductivity in comparison

Fig. 3. The electrical conductivity and hardness of CuW–La2O3 composites.

with Cu. Moreover, addition of La2O3 can inevitably cause lattice dis-tortion. According to the free electron theory [4], dislocation andpoint defects destroy the ideal crystal lattice, resulting in the in-creased scattering of free electron in these sites. On the other hand,La2O3 addition can improve the spatial distribution of Cu, and this mi-crostructural change helps to improve the electrical conductivity.These combined effects result in the less decreased electricalconductivity.

3.3. Electrical breakdown voltage in vacuum and movement of cathodespots of CuW alloy and CuW–La2O3 composites

The average of electrical properties of CuW alloys without andwith La2O3 after 50 times breakdown is given in Table 1. In compari-son with CuW alloy, the breakdown voltage of CuW–0.75La2O3 com-posites increased by 36.9% and arc life by 9.7%, while its choppingcurrent decreased by 15.3%. Fig. 4 shows the time-dependent dis-charge current curve for CuW alloy without and with La2O3. It is evi-dent that the current–time curve of the CuW–La2O3 composites

Table 1Electrical properties of CuW alloy and CuW–0.75%La2O3 composites after 50 testing cy-cles of breakdown in vacuum.

Materials Breakdown voltageintensity Eb (×107 V/m)

Choppingcurrent Ic (A)

Arc lifetimeτc (ms)

CuW 7.26 3.79 14.70CuW–0.75 wt.%La2O3 9.94 3.21 16.13

150 K. Qian et al. / Int. Journal of Refractory Metals and Hard Materials 31 (2012) 147–151

became smooth, and the discharge current showed much less fluctu-ation than that of the CuW alloy. This difference can be explained bytheir surface morphologies after the discharge and the characteristicsof cathode spots.

After excessive breakdown cycles, surface structure could no lon-ger represent the typical erosion morphology. Hence, the samplesafter third cycle of breakdown tests were chosen in this study. Fig. 5shows the surface erosion morphologies of CuW alloy and CuW–

La2O3 composites. The typical cathode spots recorded with an expo-sure period of 615 μs are shown as the insets. The erosion pits ofCuW alloy had the shape of volcano crater (Fig. 5a). The main erosionpits were large and deep. In addition, only a small amount of scat-tered erosion pits appeared around the main one, and there wassplashed Cu at the edge, they were in the form of whirlpool shape.To the contrary, no obvious large erosion pits were observed on thesurface of CuW–La2O3 composites, as shown in Fig. 5b. The erosionpits generated were relatively small and distributed evenly on thesurface. The pits were also quite shallow and with a little of spottyCu around the erosion pits. The overall surface of the cathode wassmooth and flat after the discharge tests.

Compared with CuW alloy, CuW–La2O3 composites had the splitcathode spots after the tests. This can be understood as follows. Forthe same material, the maximum and minimum currents that a singlecathode spot can carry are a fixed value [18,19]. During the arcing dis-charge, the density of the plasma cloud reduced [20]. The finely dis-persed La2O3 could act as the centers of the cathode spots whichwere generated simultaneously across the surface so as to reducethe load of current, resulting in the split of cathode spots.

Based on electron burst emission model and field-emission modelof asperity [18], the asperities have negative effects on the dielectricstrength during the process of breakdown because the electricalfield on the top of the asperity is larger than that on the surface ofnearby conductor and leads to a low work function on the launchingsite. As the CuW–La2O3 composites scattered erosion pits, a relatively

Fig. 4. Current–time curves of (a) CuW alloy and (b) CuW–0.75%La2O3 composites.

Fig. 5. Surface morphology of (a) CuW alloy and (b) CuW–0.75%La2O3 composites afterthe third cycle of breakdown test. The insets showed the corresponding cathode spots.

smooth surface and few asperities were created after the breakdowntest. Both these structural changes should result in improved break-down strength. Upon exposure to high temperature arc, the tungstenparticles in the skeleton were prone to grow. However, the dispersedLa2O3 particles suppressed this growth, thus enhanced the strength ofW matrix, reduced the arc erosion and increased the dielectricstrength [4].

The current of CuW alloy fluctuated violently during arc dis-charge, which could be understood based on the interaction be-tween current decay and metal vapor at the late stage of theprocess [21]. However, for the CuW–La2O3 composites, La2O3

Fig. 6. Velocity of cathode spots as a function of time for CuW alloy and CuW–

0.75%La2O3 composites.

151K. Qian et al. / Int. Journal of Refractory Metals and Hard Materials 31 (2012) 147–151

could give out electrons at low currents and generate heat toprovide metal vapor due to its low work function. Hence, its arcmaintained good stability (Fig. 4).

Besides the movement of cathode spots, the velocity of cath-ode spots is another important factor affecting the arc erosion.In order to get the relationship between the velocity and time,each cathode spot can be regarded as a separate point in the cal-culation. For CuW alloy, the velocity of the cathode spots fluctu-ated greatly with an average velocity of about 1.91 m/s (Fig. 6).The velocity of the cathode spots with CuW–La2O3 compositesvaried a limited range before 400 μs. After 400 μs, the velocityof the cathode spots showed the typical fluctuation of around3–5 m/s with an average value of 2.80 m/s, which was increasedby 46.6% comparing with that of CuW alloy.

The applied electrical field reduces the surface barrier of cathodeduring the electrical breakdown, and the reduced amount is designat-ed as Δφ. The relationship between Δφ and the applied electrical fieldintensity, E, can be written as [22]:

Δφ ¼ 1ffiffiffiffiffiffiffiffiffiffiffi4πε0

p e3=2E1=2 ð1Þ

where ε0 is the vacuum conductivity and e is the electron charge.From Eq. (1), the higher the external electrical field, the more the

emitting electrons are. As the electronic work function of La2O3 ismuch lower than that of Cu and W, and the difference betweenLa2O3 and Cu or W is much larger than that between Cu and W, theinternal electrical field at the interface of the three phases in CuW–

La2O3 composites increases. Thus, electrons are much easier to emitfrom La2O3 particles. During the initial stage of the breakdown, thebreakdown gap determines that the breakdown occurs beneath theanode due to the strong external electrical field (E=kQ/r2, where kis static electricity constant, Q is electric charge and r is distance).With decrease of applied electrical field intensity, the asperity formedat the initial stage became the dominated factor to affect the motiontrajectory of cathode spots. When the applied electric field intensitycould not sustain the continuous motion of cathode spots, the freeelectrons from La2O3 escaped from the cathode surface, resulting inthe stabilization of arc [23,24].

4. Conclusions

The uniformly dispersed W–La2O3 powders were synthesized insitu through pyrolysis and reduction by hydrogen gas. In comparisonwith W powders reduced from WO3, the particle size of W decreasessignificantly in the W–La2O3 composites. The electrical conductivitydecreases slightly with increasing amount of La2O3 up to 2.0 wt.%,while the hardness reaches the maximum of about 220HB at0.75 wt.% La2O3. CuW–0.75La2O3 composites have 36.9% higher di-electric strength, 15.3% lower chopping current and 9.7% longer arclife than those of the CuW alloy. In comparison with CuW alloy, theCuW–0.75La2O3 composites have better characteristic of split cathodespots and the movement of cathode spots is also about 46.6% faster.No apparent major erosion pit is formed on the surface of the elec-trode made of CuW–0.75La2O3 composites and the erosion pits aredistributed evenly after breakdown tests.

Acknowledgments

The authors would like to acknowledge the Key Program of Na-tional Natural Science Foundation of China (No. 50834003), the

National Natural Science Foundation of China (No. 50871085) andNational High-Tech Program (863) of China (No 2009AA03Z522) forproviding financial support for this research. This research is also par-tially supported by the Shaanxi Natural Science Foundation (No.2010JZ007) and Shaanxi Provincial Project of Special Foundation ofKey Disciplines.

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