The Microstructure and Properties of Copper with Ceria Nanoparticles Addition

Copper-based composites strengthened by ceria nanoparticles were processed by conventional powder metallurgy: mixing (30 min and 46 rpm), compaction (cold, uniaxial, 1080 MPa for 10 s) and sintering (800˚C for 6 h in vacuum atmosphere of 10 −5 torr). It was studied the microstructure (optical mi-croscopy, scanning electron microscopy), X-ray diffraction with Rietveld refinement and some properties (electrical conductivity, Vickers hardness and fracture analysis) of the compositions 92 wt% Cu - 8 wt% CeO 2 and 80 wt% Cu - 20 wt% CeO 2 . The results showed uniform phase distribution, low porosity and ceria disperse inside copper grain. In despite of properties, the composites had electrical conductivity of 38% IACS and 15% IACS and hardness of 69 and 88 HV 5 , respectively. The results of 92 wt% Cu - 8 wt% CeO 2 composites were promising, and they are in according with actual literature.


Introduction
Copper is widely used in electrical and electronic applications due to its high electrical conductivity; even so, its low mechanical resistance is a barrier for some applications who requires high mechanical stress.
Metal matrix composites (MMCs) consist in adding ceramic particles in a soft metal with the aim to improve its mechanical resistance; however, this reinforcing phase also increases the scattering of electrons on the material and, consequently, decreases its electrical conductivity. The main challenge of MMCs re-TiO 2 have been used as reinforcement phase. Ceria is a rare earth oxide that has outstanding properties like high coefficient of thermal expansion, low thermal conductivity, good corrosion resistance, high melting point and chemical and thermal stability [4] [5]. Its high coefficient of thermal expansion is a special asset for MMCs processed by powder metallurgy, because of the mismatch between the coefficient of thermal expansion of the metallic matrix and ceramic reinforcement usually causes thermal stresses during sintering stage.
Composite 88 wt% Cu -12.2 wt% CeO 2 prepared by electrodeposition exhibited increased microhardness and wear resistance than the pure copper coating, reaching the maximum value of 280 HV 0.05 , this hardness improvement comes from crystallite size refinement in the copper matrix structure, produced by ceria dispersion [4]. In according to this, another study observed that ceria may increase the hardness of the copper as long its particles has been finely dispersed in the matrix, the presence of agglomerates in the composite microstructure could decrease the hardness. The authors observed that it is more difficult to avoid the occurrence of this agglomerates as ceria contents increases [5] [6] [7] [8]. Composite W-Cu with 1 wt% and 2 wt% of ceria processed by powder metallurgy exhibit, respectively, electrical conductivity of 47% IACS and 40% IACS, they also show some porosity, aggregated particle and lower density than sample without ceria addition [7].
There are just a few studies concerning the microstructure and the properties of composites strengthened by CeO 2 . In this scenario, the aim of this work was to process copper-based MMCs by powder metallurgy and study the microstructure and some properties (electrical conductivity, hardness and fracture) of the composites 92 wt% Cu -8 wt% CeO 2 and 80 wt% Cu -20 wt% CeO 2 .

Experimental
Copper (given by Metallic Materials Laboratory of the Nuclear and Energy Research Institute) and Ceria (Sigma-Aldrich, average size < 25 nm) powders were mixed in a cylindrical mill for 30 min and 46 rpm, followed by cold compaction into cylindrical pellets (diameter: 10 mm) using uniaxial hydraulic press with applied pressure of 1080 MPa for 10 s. The green samples were then sintered and solubilized in a tubular furnace (800˚C, 6 h, and vacuum atmosphere 10 −5 torr) at a heating rate of 10˚C/min. After these steps, it was obtained cylindrical samples with diameter 10 mm, high 1.5 mm and mass 1 g. It was also processed a standard sample of 100 wt% Cu under the same conditions of composite samples.
The as received copper and ceria powders were characterized by particle size analyzer (only copper powder, CILAS 1064), Scanning Electron Microscope

Powder Characterization
The SEM and TEM images of the powders are shown in Figure 1. The powders

Microstructure of Composites
SEM ( Figure 2) and OM (Figure 3) images shows for both compositions a microstructure with good densification and with fine grains, the ceria phase was finely dispersed in the grain. In the composite with higher concentration of ceria (Figure 3(b)), it is possible to observe copper particles completely isolated by the ceria. Due to some particles agglomerated in the powder, it was observed larger phases of ceria from these agglomerates.
The composites presented homogeneity in the distribution of the phases and of the microstructure along the height and diameter with microstructure like the normal direction. The absence of segregation could indicate that the mixing process was efficient, mixing using cylindrical mill ensured uniform distribution of the particles in works of Venkateswaran et al. [9] and ASM International [10].
Among the samples, there was a maximum variation in height of 0.2 mm.
The composite with 20 wt% CeO 2 had internal cracks (indicated by the arrows in Figure 3(d)) in the horizontal direction (perpendicular to the compaction direction), this type of crack can occur during the beginning of compaction and during the removal of the compacted from the matrix.
As the pressure is released from the upper punch to the sample, the sample expands elastically and uniaxially because it tends to return to its original situation, due to sample stiffness or the friction of the matrix wall, this expansion does not occur at the edges, generating the cracks. In the withdrawal, the upper part is released from the compression forces of the matrix wall and undergoes lateral expansion while the lower part is still inside the matrix, in this transient stage shear stresses occur generating the cracks [11] [12] [13].   Figure 4 shows the XRD pattern of the composites obtained experimentally ("obs") accompanied by Rietveld calculated curve ("calc") and the difference among these curves ("diff"), it is also showed the identification of each peak (phase and planes). It was identified just the expected phases (Cu and CeO 2 ).
The oxidation of copper (Cu 2 O) at high temperature is a very common phenomenon to occur and has been reported by several authors, however, in the XRD patterns there is no indication of peak at the diffraction angles of this phase, which indicates that the control of the vacuum during sintering was efficient.
There is a shift in the (200) peak of the copper to positions 2θ smaller for all the composites, that can be observed by the format of the curve "diff" in the position of the 2θ -50˚ (Figure 4). The same occurs in the pure copper sample (processed under the same conditions), this type of displacement of the (200) peak occurs in CFC materials due to the increase in the probability of intrinsic stacking failure [14].
The crystalline structure of both, copper and ceria, is Face Centred Cubic where, in the case of ceria, the tetrahedral interstitial is filled by oxygen. Table 1 shows phase quantification, crystallite size and cell parameter for each phase of     [16].

Properties of Composites
The addition of the ceramic phase increased the hardness of copper, because ceria is inherently harder. Varying the percentage of the ceramic reinforcement from 8 to 20 wt% the hardness increased from 69 to 88 HV 5 . This behavior exposes good physical adhesion between the metal-ceramic phases [17] [18].
Part of the reason for the hardness not being so high may be due to sintering/homogenization time [19]. For example, Rajkovic et al. [15] observed that the hardness decreases from 1050 to 410 MPa after subjecting Cu-4%Al 2 O 3 composite to high temperatures (800˚C for 5h). Figure 5 shows the observation of fractographs of the composites using SEM.
It is observed a mixed fracture, fragile fracture regions (indicated by the arrows) suggest good adhesion in the matrix-reinforcement interface [20]. In the fractography of the composite 80 wt% Cu -20 wt% CeO 2 the presence of dimples is not clearly observed (at least with this magnification) and the fracture is predominantly fragile.

Conclusions
The copper-based composites reinforced with ceria were processed by powder metallurgy and had their microstructures and properties studied, the conclusions are as follows: • Copper and ceria powders had morphology, composition and particle sizes suitable for sample processing. • The composites 92 wt% Cu -8 wt% CeO 2 and 80 wt% Cu -20 wt% CeO 2 were processed by conventional powder metallurgy (mixing, compaction and sintering) and presented oxidation-free surface and relative density of 88% and 92%, respectively. • The microstructure shows good matrix densification, low porosity and good adhesion between phases with finely dispersed ceria in the copper grain boundary. The EDS microanalyses did not indicate any element other than expected. The XRD patterns did not indicate the formation of other phases beyond that expected and indicated. • The values of electrical conductivity obtained were 38% IACS and 15% IACS but reached 51% and 21% in relation to the pure copper sample processed under the same conditions. • The Vickers hardness values were 69 and 88 HV 5 , representing 230% and 293% of the hardness of the pure copper sample.
• The composites presented mixed fracture (ductile and brittle); the fractographs also indicated that the composite 92 wt% Cu -8 wt% CeO 2 is more ductile.
• Studies have suggested that with 20 wt% CeO 2 the electrical conductivity and fracture behavior of the composite are compromised, however, with 8 wt% CeO 2 promising results were obtained.