Effect of CeO2 Nanoparticles on Interface of Cu/Al2O3 Ceramic Clad Composites

Cu/Al2O3 ceramic clad composites are widely used in electronic packaging and electrical contacts. However, the conductivity and strength of the interfacial layer are not fit for the demands. So CeO2 nanoparticles 24.3 nm in size, coated on Al2O3 ceramic, promote a novel CeO2–Cu2O–Cu system to improve the interfacial bonded strength. Results show that the atom content of O is increased to approximately 30% with the addition of CeO2 nanoparticles compared with the atom content without CeO2 in the interfacial layer of Cu/Al2O3 ceramic clad composites. CeO2 nanoparticles coated on the surface of Al2O3 ceramics can easily diffuse into the metallic Cu layer. CeO2 nanoparticles can accelerate to form the eutectic liquid of Cu2O–Cu as they have strong functions of storing and releasing O at an Ar pressure of 0.12 MPa. The addition of CeO2 nanoparticles is beneficial for promoting the bonded strength of the Cu/Al2O3 ceramic clad composites. The bonded strength of the interface coated with nanoparticles of CeO2 is increased to 20.8% compared with that without CeO2; moreover, the electric conductivity on the side of metallic Cu is 95% IACS. The study is of great significance for improving properties of Cu/Al2O3 ceramic clad composites.


Introduction
Cu/Al 2 O 3 ceramic clad composites have anti-wear, anti-corrosion, and anti-high temperature characteristics of ceramics and maintain the high conductivity and machinability of copper. They have been widely used in rail transit, electronic packaging, and electrical contacts [1,2]. Although Cu/Al 2 O 3 ceramic clad composites have the advantages of a ceramic and a copper, ceramic is brittle and difficult to process. Assembly and connection structures of ceramic and metal are often used. To obtain a stable and reliable Cu/Al 2 O 3 ceramic clad structure, the wettability between metals and ceramics and the formation of brittle compounds at the interface must be addressed. These problem are of great research significance [3,4].
Many researchers have carried out the research. At present, brazing and diffusion bonding are the main methods to achieve connections between ceramics and metals. Breslin et al. [5] have suggested that the key problem was the interfacial wettability between the ceramics and metals, so a method of co-continuous ceramic composites was proposed [6,7]. The surface of ceramics coated a layer of Mo-Mn could improve wettability [8]. Active metals, such as Ag, Ti, Zr and V, have been added to study effects. However, general oxygen content was lower than 1 Pa to avoid oxidizing [9][10][11]. Burgess et al. [12,13] have used a Cu-Cu 2 O eutectic liquid system to bond Cu layers with ceramics for the first time. However, Fan Jinglian et al. [14] have discovered that wettability between Cu and Al 2 O 3 was still not significantly improved when the temperature was increased from 1200 to 1400 • C.
Results [15,16] have shown that the contact angle between the molten Cu and Al 2 O 3 ceramic was 158 • -170 • under oxygen-free conditions at 1100-1300 • C, so they were non-wetting each other. Diemer et al. [17] have found that by controlling the oxygen partial pressure (p O2 ) and oxygen content in the copper simultaneously, contact angle could be varied between 125 • and 22 • . Evaluation of the Gibbs adsorption equation for the liquid/solid interface at 1300 • C suggests that adsorption of a Cu-O complex at that interface plays a key role in promoting wetting. Formation of CuAlO 2 and dissolution of Al 2 O 3 in the melt also influence the contact angle, especially in the range of p O2 > 1 Pa. When the content of O was higher than 2 at.%, Cu began wetting the Al 2 O 3 . Huang [18] and Chatterjee [19] have found that the addition of oxides to metal solders could improve the wettability between Al 2 O 3 ceramics and Cu layers. However, there are few reports on improving bonded strength between Cu and Al 2 O 3 ceramics by the addition of rare earth oxides and reducing the bonded temperature. Thus, new methods are needed to solve these problems.
In this study, CeO 2 nanoparticles coat the interface between Al 2 O 3 ceramics and Cu to form a new CeO 2 -Cu 2 O-Cu system to increase interfacial strength. Under a special gas pressure and temperature, the poor strengths of the ceramic/copper composites will be improved. The new phases and elements diffusing at low temperatures are studied. A new Cu/Al 2 O 3 ceramic clad composite with the addition of CeO 2 is fabricated.

Materials and Methods
The specimens were prepared in a vacuum tube furnace (Boyun Tong company, Nanjing, China), which was vacuumed to 0.01 MPa and then filled Ar gas at a pressure of 0.  After preparation, the samples were etched in a solution containing 3 g of FeCl 3 , 2 mL of HCl, and 96 mL of C 2 H 5 OH. Their metallurgical structures and microstructures were examined by scanning electron microscopy (SEM S-4800, Hitachi, Tokyo, Japan), and backscattered electron imaging (BSE, Hitachi, Tokyo, Japan) under a control voltage of 20 kV. The electric conductivity was measured at 60 kHz using a digital portable eddy current tester (FD-102, Xiamen xinrui instrument Ltd., Xiamen, China). The bonded strength was measured with a nanomechanical test for Nano Test 600 (Micro Mmaterials Ltd., Wrexham, UK). XRD patterns were obtained using a Bruker D8 Advance (Bruker Ltd., Karlsruhe, Germany) with Cu Kα radiation. The wettability of the two ceramics was poor, so there are many cracks in the bonded interface at 1300 • C, as shown in Figure 2a. The measurements are taken at room temperature. However, a closely bonded interface is formed between the Cu and Al 2 O 3 by addition of CeO 2 nanoparticles, as shown in Figure 2b. There are no cracks in the bonded interface at 1300 • C. The conductivity of the Al 2 O 3 ceramic is 0% IACS (international annealed copper standard), whereas the conductivity of the side of metallic Cu is 95% IACS, so the conductivity of Cu in the clad composites is reserved. Figure 3 shows the bonded strengths of the interfacial layer of Cu/Al 2 O 3 clad composites with CeO 2 and without CeO 2 at the bonded temperature 1300 • C. The bonded strength of Cu/Al 2 O 3 interfacial layer with nanoparticles of CeO 2 is 990.3 MPa; however, that without CeO 2 is 820.1 MPa. The bonded strength of the interfacial layer coated with nanoparticles of CeO 2 increases 20.8% compared with that without CeO 2 . Therefore, nanoparticles of CeO 2 can improve the bonded strength.

EDS of Interface
The element distributions of the interfaces of the composite materials are presented in this section. Figure 4 shows the energy-dispersive X-ray spectroscopy (EDS) of the Cu/Al 2 O 3 interface without CeO 2 . In the range of 0-40 µm, the contents of Al and O are high, whereas the content of Cu is the lowest, which indicate that 40 µm is the dividing line of the interface of the clad composites. However, in the range of 40-150 µm, the content of Cu is the highest, and a small amount of Al and O can diffuse to the copper layer. On the bonding surface coated with nanoparticles of CeO 2 , the contents of these elements are different from that without CeO 2 , as shown in Figure 5. In the range of 0-40 µm, the contents of Al and O are higher than Cu, whereas the content of Cu is the lowest in all. This indicates that 40 µm is the dividing line of the interface for the clad composites. The content of O is significantly increased to 30% with addition of CeO 2 compared with that without CeO 2. , which indicate that CeO 2 could raise the content of O in the interface. The atom content of O tested by EDS is 15.4% when CeO 2 nanoparticles are not coated in the surface of Al 2 O 3 ceramic; however, the atom content of O is increased to 20.4% when CeO 2 nanoparticles are coated. Again, the results prove that the atom content of O is 30% higher compared with that without CeO 2.
Moreover, the content of Ce arises from the range of 0-150 µm due to the addition of CeO 2 .   Figure 6 shows the XRD patterns of the nanoparticles of CeO 2 , the size of which are 24.3 nm for 28.5 • according to the Peak Search Report of XRD. Thus, rare earth oxide coated on the surface of Al 2 O 3 ceramics is CeO 2 . Figure 7 shows the SEM image of CeO 2 . Due to the presence of nanoparticles of CeO 2 , elements are active and easily diffused into the layers, which is beneficial for reducing the bonding temperature. Figure 8 shows the BSE images of the interface at bonded temperatures of 1300 and 1500 • C. At 1500 • C, Al and Cu easily diffuse into each other to form the compound of CuAlO 2 . Which is beneficial to the improvement of wettability [8]. At 1300 • C, the diffusion rates of the elements are decreased and oxidation cannot occur in time; however, the interface coated with CeO 2 shows that Al and Cu can diffuse into each other quickly in 5 min. Consequently, slags and cracks cannot form at low temperatures. The results show that the addition of nanoparticles CeO 2 is beneficial for reducing the bonding temperature.    [15,16]. Thus, the Cu and Al 2 O 3 are completely non-wetting when the Cu is molten. To achieve excellent properties of Cu/Al 2 O 3 clad composite, it is necessary to improve the wettability between them. A small amount of O could reduce the wetting angle between Cu and Al 2 O 3 [17]. When the content of O is higher than 2 at.%, Cu begins wetting the Al 2 O 3 . Moreover, Cu and Cu 2 O could form a eutectic liquid [20]. However, CuO is easily formed at this temperature, so its formation must be strictly prevented. Therefore, reducing the content of O to prevent the formation of CuO and promoting the formation of Cu-Cu 2 O are the key factors. In this study, CeO 2 can react with copper to form Cu 2 O instead of CuO, promoting the formation of the Cu-Cu 2 O eutectic solution. Yang, Y.M. [21] declared that in CeO 2 crystal structure Ce +4 was easily converted to Ce +3 , or vice versa, which makes the nanoparticles of CeO 2 have strong functions of storing and releasing oxygen, as shown in chemical Equation (1). Chemical Equation (2) shows that Cu 2 O can be formed under the condition of trace oxygen content. The chemical equations are as follows:

Mechanisms Discussion
The reduction of CeO 2 to Ce 2 O 3 has been reported in the high temperature sintering of fine CeO 2 particles by [22,23]. The authors in reference [24] have also proved the conclusion by High Resolution Transmission Electron Microscopy (HRTEM) imaging and Fast Fourier Transformation (FFT) diffraction. Thus, nanoparticles of CeO 2 can improve the bonded strength, as CeO 2 nanoparticles have strong functions of oxygen storage and release and, thus, the addition of CeO 2 nanoparticles play an important role for a novel system of CeO 2 -Cu 2 O-Cu at an Ar pressure of 0.12 MPa.
The authors in [17] reported that, due to the mutual diffusion and redistribution of chemicals in the melting process, these oxides react with Al 2 O 3 under certain conditions to form CuAlO 2 , which is beneficial to the improvement of wettability [8]. The corresponding Gibbs free energies (∆G 0 ) [25] are as follows: Equations (3) and (4) show that the Gibbs free energies of the above reactions are negative at 1300 • C, so CeO 2(1−X) can absorb a small amount of O 2 to prevent CuO formation, and CuAlO 2 can also be formed spontaneously.
The reaction temperature of 1300 • C is 65 • C higher than the melting point of Cu 2 O (1235 • C). Thus, Cu 2 O reacts rapidly with Al 2 O 3 to form compounds of CuAlO 2 at the interface in only 5 min. The formation of these low melting point copper oxides and interfacial compounds (CuAlO 2 ) is beneficial for the liquid phase copper wetting of the alumina ceramic (melting point of CuO is 1200 • C). Under certain conditions, CeO 2 is decomposed at 1000 • C and easily diffuses to the Cu layer, accelerating the formation of the CeO 2 -Cu 2 O-Cu eutectic, as shown in Figure 9. Figure 10 shows the SEM images and EDS of the interface. A strong structure, which is the key to strengthening the bonding interface, is formed due to the triangular type of the Cu 2 O, and Y type of CeO 2 and CuAlO 2 .

Conflicts of Interest:
The authors declare no conflict of interest.