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

Abstract

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.

1. Introduction

Cu/Al₂O₃ 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/Al₂O₃ 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/Al₂O₃ 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-Cu₂O 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 Al₂O₃ 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 Al₂O₃ 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 CuAlO₂ and dissolution of Al₂O₃ 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 Al₂O₃. Huang [18] and Chatterjee [19] have found that the addition of oxides to metal solders could improve the wettability between Al₂O₃ ceramics and Cu layers. However, there are few reports on improving bonded strength between Cu and Al₂O₃ 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, CeO₂ nanoparticles coat the interface between Al₂O₃ ceramics and Cu to form a new CeO₂–Cu₂O–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/Al₂O₃ ceramic clad composite with the addition of CeO₂ is fabricated.

2. 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 0.12 MPa. The Cu cubes (Zhejiang wanteng metal materials firm, Ningbo, China) were 99.90 wt.% Cu. The ceramic cubes (Shenzhen beilong electronic material factory, Shenzhen, China) were 99.9 wt.% Al₂O₃. The fabrication procedures were as follows: nanoparticles of CeO₂ coated the surface of Al₂O₃ ceramics → in situ Cu₂O formed in the Cu interface in the 40 °C air → the melting of Cu and Al₂O₃ ceramic clad composites at 1300 °C for 5 min in a furnace at an Ar pressure of 0.12 MPa → a cube with the dimensions 40 × 40 × 30 mm was formed, as shown in Figure 1.

After preparation, the samples were etched in a solution containing 3 g of FeCl₃, 2 mL of HCl, and 96 mL of C₂H₅OH. 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.

3. Results and Discussion

3.1. Structure and Hardness of Cu/Al₂O₃ Clad Composites

Figure 2 shows the SEM images of Cu/Al₂O₃ composites at bonded temperature of 1300 °C. When nanosized CeO₂ is not added, the metal Cu and Al₂O₃ ceramic could not form a new eutectic solution. 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 Al₂O₃ by addition of CeO₂ nanoparticles, as shown in Figure 2b. There are no cracks in the bonded interface at 1300 °C. The conductivity of the Al₂O₃ 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/Al₂O₃ clad composites with CeO₂ and without CeO₂ at the bonded temperature 1300 °C. The bonded strength of Cu/Al₂O₃ interfacial layer with nanoparticles of CeO₂ is 990.3 MPa; however, that without CeO₂ is 820.1 MPa. The bonded strength of the interfacial layer coated with nanoparticles of CeO₂ increases 20.8% compared with that without CeO₂. Therefore, nanoparticles of CeO₂ can improve the bonded strength.

3.2. 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/Al₂O₃ interface without CeO₂. 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 CeO₂, the contents of these elements are different from that without CeO₂, 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 CeO₂ compared with that without CeO_2., which indicate that CeO₂ could raise the content of O in the interface. The atom content of O tested by EDS is 15.4% when CeO₂ nanoparticles are not coated in the surface of Al₂O₃ ceramic; however, the atom content of O is increased to 20.4% when CeO₂ 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 CeO₂. Figure 5b shows that the CeO₂ coated on the surface of the alumina ceramics can easily diffuse into the metallic Cu layer, but not to the Al₂O₃ ceramics. O and Cu easily form a eutectic liquid of Cu₂O–Cu under certain conditions [12,13], so CeO₂ can accelerate to form the eutectic liquid of Cu₂O–Cu by storing or releasing O at Ar pressure of 0.12 MPa. However, in the range of 40–150 μm, the content of Cu is the highest, and the contents of O and Al are reduced. The eutectic liquid of Cu₂O–Cu is the key factor in improving the wettabilities of the Cu and Al₂O₃ layers.

3.3. Mechanisms Discussion

Figure 6 shows the XRD patterns of the nanoparticles of CeO₂, 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 Al₂O₃ ceramics is CeO₂. Figure 7 shows the SEM image of CeO₂. Due to the presence of nanoparticles of CeO₂, 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 CuAlO₂. 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 CeO₂ 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 CeO₂ is beneficial for reducing the bonding temperature.

Al₂O₃ and Cu usually do not wet each other. The contact angle between the molten Cu and Al₂O₃ ceramic is 158°–170° under oxygen-free conditions at 1100–1300 °C [15,16]. Thus, the Cu and Al₂O₃ are completely non-wetting when the Cu is molten. To achieve excellent properties of Cu/Al₂O₃ clad composite, it is necessary to improve the wettability between them. A small amount of O could reduce the wetting angle between Cu and Al₂O₃ [17]. When the content of O is higher than 2 at.%, Cu begins wetting the Al₂O₃. Moreover, Cu and Cu₂O 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–Cu₂O are the key factors. In this study, CeO₂ can react with copper to form Cu₂O instead of CuO, promoting the formation of the Cu–Cu₂O eutectic solution. Yang, Y.M. [21] declared that in CeO₂ crystal structure Ce⁺⁴ was easily converted to Ce⁺³, or vice versa, which makes the nanoparticles of CeO₂ have strong functions of storing and releasing oxygen, as shown in chemical Equation (1). Chemical Equation (2) shows that Cu₂O can be formed under the condition of trace oxygen content. The chemical equations are as follows:

CeO₂ ↔ CeO_2(1−X) + xO₂ (0 ≤ x ≤ 0.25)

(1)

Cu+O₂ → Cu₂O

(2)

The reduction of CeO₂ to Ce₂O₃ has been reported in the high temperature sintering of fine CeO₂ 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 CeO₂ can improve the bonded strength, as CeO₂ nanoparticles have strong functions of oxygen storage and release and, thus, the addition of CeO₂ nanoparticles play an important role for a novel system of CeO₂–Cu₂O–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 Al₂O₃ under certain conditions to form CuAlO₂, which is beneficial to the improvement of wettability [8]. The corresponding Gibbs free energies (ΔG⁰) [25] are as follows:

2Cu₂O + O₂ → 4CuO     ΔG⁰ = −62354 + 44.89 T

(3)

2Cu₂O + O₂ → 4CuO     ΔG⁰ = −62354 + 44.89 T

(4)

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 O₂ to prevent CuO formation, and CuAlO₂ can also be formed spontaneously.

The reaction temperature of 1300 °C is 65 °C higher than the melting point of Cu₂O (1235 °C). Thus, Cu₂O reacts rapidly with Al₂O₃ to form compounds of CuAlO₂ at the interface in only 5 min. The formation of these low melting point copper oxides and interfacial compounds (CuAlO₂) is beneficial for the liquid phase copper wetting of the alumina ceramic (melting point of CuO is 1200 °C). Under certain conditions, CeO₂ is decomposed at 1000 °C and easily diffuses to the Cu layer, accelerating the formation of the CeO₂–Cu₂O–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 Cu₂O, and Y type of CeO₂ and CuAlO₂.

4. Conclusions

(1)

The atom content of O is increased to approximately 30% with addition of CeO₂ nanoparticles 24.3 nm in size compared with the atom content without CeO₂ nanoparticles in the interfacial layer of the Cu/Al₂O₃ ceramic clad composites, so the addition of CeO₂ could raise the atom content of O;

(2)

CeO₂ nanoparticles coated on the surface of the Al₂O₃ ceramics can easily diffuse into the metallic Cu layer, but they do not in Al₂O₃ ceramics. 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;

(3)

The addition of CeO₂ nanoparticles is beneficial for promoting the bonded strength of Cu/Al₂O₃ ceramic clad composites. The bonded strength of the interface coated with nanoparticles of CeO₂ is 20.8% higher than that without CeO₂; however, the electric conductivity of metallic Cu is 95% IACS.