Effect of heating atmosphere composition and content on phase and morphology distribution of copper oxide layer

doi:10.21203/rs.3.rs-3326982/v1

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Effect of heating atmosphere composition and content on phase and morphology distribution of copper oxide layer

https://doi.org/10.21203/rs.3.rs-3326982/v1

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published 13 Mar, 2024

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When copper billet is heated in the rolling reheating furnace, some oxides which affect the surface quality may remain on the substrate. This study investigates the effects of different heating atmosphere compositions and contents (N₂, O₂, CO₂ and H₂O) on the micro-morphology and phase evolution of the exfoliated and residual oxides on copper billet surface by high-temperature oxidation experiments. The results show that the main phase of the exfoliated oxides is CuO attached to loose Cu₂O, and the residual nodular oxides are also CuO which directly adhere to copper matrix; water vapour can increase the number and size of Cu₂O particles on the interface between exfoliated oxide and copper matrix, and effectively reduces the number of residual nodular oxides; the vapour and its decomposition products may effectively eliminate the pores within the oxide layer and at the oxide-matrix interface, while CO₂ may increase the pore size inside the oxide layer.

copper billet

high-temperature oxidation

water vapour

carbon dioxide

cupric oxide

cuprous oxide

Copper is one of the most widely used non-ferrous metals, which is an indispensable basic material for economic development and technological progress. Due to the rapid development of electronic information, the consumption of high-precision copper strip is increasing in recent years^[1]. However, the nodular oxides may form on the copper surface in the heating atmosphere during rolling reheating furnace, resulting in quality decline on the product surface^[2].

Copper oxidation at high temperature is generally summarized as oxygen first adsorbed on copper substrate to generate Cu₂O, and then Cu₂O continues to react with oxygen to form CuO^[3–5]. From the perspective of oxidation thermodynamics and kinetics, the oxidation essence of copper is mutual diffusion of Cu and O. The thickness growth of oxide layer mainly relies on the diffusion of Cu ions to the outside^[6]. However, the actual oxidation is quite complicated due to many influencing factors. Among them, one important factor is metallic properties such as microstructure and purity ^[7]. Moreover, the oxides properties play an important role in its further oxidation. For example, the stress of oxide layer may increase due to the different thermal expansion coefficients of CuO and Cu₂O. This may cause the oxide layer to crack or flake easily when its stress exceeds the bonding strength between oxides and substrate. Then, the direct contact of reaction atmosphere with the copper matrix may accelerate oxidation rate, sometimes resulting in residual nodular oxides on the copper surface ^[8]. The fundamental factor affecting copper oxidation is the atmosphere environment which it is exposed. Feng^[9] investigated the oxidation kinetics of copper in pure oxygen and air using thermogravimetric analysis (TGA), who suggested that the oxidation rate of copper in moist air is higher than that in dry air, but the rate in pure oxygen is almost the same as that in dry air. Bridges^[10] et al. proposed that the weight gain of copper is parabolic with respect to time when oxidized in oxygen (0.026–20.4 atm) at 600–1000 ℃. Yongfu Zhu^[11] et al. further investigated the mechanism of copper oxidation in oxygen (0.1 MPa) at 350–1050 ℃, who found that the restrictive step of oxidation rate is the outward diffusion of Cu⁺ in Cu₂O.

Above studies mainly focused on the copper oxidation in oxygen or air, in which the growth mechanism of oxide layer is mostly investigated. However, the heating atmosphere is a multi-component mixture [N₂, O₂, CO₂ and water vapour(H₂O)] in rolling reheating furnace. Each atmosphere composition would play an effect on the copper oxidation, which make part of the oxide layer peel off. Some of the oxides remain on the copper surface and form nodules, which are significantly thicker than the substrate around it. In the subsequent rolling process, the nodules would be rolled into the copper matrix, which increases the milling thickness and thus improves the production cost. This study investigated the micro-morphology and phase evolution of the exfoliated and residual oxides and analyzed the formation mechanism of nodular oxides by controlling the composition and content of atmosphere in the reheating furnace (N₂, O₂, CO₂ and water vapor characterized in terms of dew-point temperature).

2.1 Materials

The experimental samples were copper billet (Cu content of 99.999%) before entering the rolling reheating furnace, which were cut into block with the size 50 mm×30 mm×5 mm.

2.2 Experimental Methodology

The oxidation experiment of copper was carried out in a vacuum tube atmosphere furnace ( Fig. 1). According to the laboratory equipment capacity and combined with the actual production process (average heat temperature is 940 ℃ and holding time is 3.5 h), the copper heating process curve (shown in Fig. 2) was designed. In addition, the corresponding experimental atmosphere was designed based on the composition of the atmosphere in the rolling reheating furnace of copper billets, as shown in Table 1. In this experiment, the total flow rate of the atmosphere was 2 L/min, in which the contents of N₂, O₂ and CO₂ were controlled by the corresponding mass flow meters, and the water vapor was regulated by controlling the dew point temperature^[12].

Table 1

Experiment atmosphere composition for high-temperature oxidation (volume fraction, %)
Serial number	N₂	O₂	CO₂	H₂O	Dew point/℃	P_O2(T = 25 ℃)/atm	P_O2(T = 940 ℃)/atm
1	69.724	0.441	9.411	20.424	60.700	4.411×10^− 3	4.411×10^− 3
2	90.589	0	9.411	0	\	2.452×10^− 9	4.722×10^− 7
3	79.576	0	0	20.424	60.700	3.523×10^− 8	8.476×10^− 7
4	70.165	0	9.411	20.424	60.700	3.768×10^− 8	1.320×10^− 6
5	99.559	0.441	0	0	\	4.410×10^− 3	4.410×10^− 3
6	90.148	0.441	9.411	0	\	4.410×10^− 3	4.410×10^− 3
7	79.135	0.441	0	20.424	60.700	4.410×10^− 3	4.411×10^− 3

The exfoliated oxide layer and residual oxides of oxidized samples were detected by a small angle diffraction with the X-ray diffractometer (XRD). The surface morphology of residual oxides (SE), the cross-sectional morphology of exfoliated oxide layer (OM), and the cross-sectional morphology of the residual oxide (BSE) were observed using optical microscopy and field emission scanning electron microscopy (Nova 400 Nano SEM), respectively. In addition, the elemental composition and distribution of the residual oxides on the copper surface were observed by an electron probe microanalyzer (EPMA 8050G).

3.1 Macroscopic morphology of residual oxides on copper surface with different heating atmosphere

The macroscopic morphology of the residual oxides on the copper surface with different heating atmospheres is shown in Fig. 3. There is no external oxide layer is formed on the copper surface when the oxidizing composition in heating atmosphere is CO₂, H₂O and CO₂ + H₂O mixture. However, many small oxides liked white "spots" are generated in above reaction condition, meanwhile the order of its number is CO₂ + H₂O༞CO₂༞H₂O [Fig. 3(d), (b), (c)]. When the oxidizing composition is O₂, O₂ + CO₂ mixture and O₂ + CO₂ + H₂O mixture, the external oxide layer is formed on the copper surface, and part of them peels off. When the oxidizing composition is O₂ + H₂O, an external oxide layer is formed on the copper surface and it completely peels off [Fig. 3(g)]. The macroscopic morphology of exfoliated oxides is shown in Fig. 3(h), in which the black oxides formed at the atmosphere-copper interface, and the dark-red oxides formed at the interface between substrate and external oxide layer, as shown in Fig. 4(a).

3.2 Phase composition and microscopic morphology of residual oxides on copper surface with different heating atmosphere

3.2.1 Phase and element distribution of residual oxides

The phase composition of oxides on copper surface with different heating atmospheres is shown in Fig. 4. When the oxidizing composition is CO₂ + H₂O mixture, the residual oxide on copper surface is only Cu₂O. When the oxidizing composition is O₂, the residual nodular oxides are the mixture of CuO and Cu₂O, in which CuO accounts for a very high proportion. When the oxidizing composition is O₂ + H₂O mixture, both the exfoliated and residual oxides on the copper surface are mixture of CuO and Cu₂O, in which the exfoliated oxide is almost entirely CuO, and the residual oxide is mostly Cu₂O.

When the heating atmosphere is N₂ + O₂, the microscopic morphology of residual oxides at the oxide-matrix interface is shown in Fig. 5. The black nodule oxide [Fig. 5(a) 1 position] is dense which macroscopic morphology is showed in Fig. 5(b) (1 position). The dark-red oxide [Fig. 5(a) 2 position] distributed near the nodular oxide is a few small particles, which macroscopic morphology is showed in Fig. 5(b) (2 position).

This experiment analyzes the elements concentration of O and Cu at oxide-matrix interface by EPMA in order to determine the physical phase composition of the nodular oxides and its their nearby small particles. Figure 6(a) shows the EPMA mapping of the interface position, in which region A and B are not exactly in the same plane due to the presence of nodular oxide. For this reason, the focusing position selected in this experiment is in region B. Based on this, the oxygen concentration in region A could be observed [Fig. 6(a)], which value is extremely low because the Popper peak of O is broader. However, the copper concentration in region A could be not observed [Fig. 6(a)] because the Popper peak of Cu is narrower. Therefore, the mapping of O and Cu elements in region A and region B at position 1,2,3 are locally enlarged. The nodular oxides in region A are dense with large grain size, which phase is mainly CuO as shown in Fig. 6(b). It is in agreement with the results of Sneha Samal^[13]. Figure 6(c) shows that the phase of oxides are exfoliated and the residual is mainly Cu at position 1 in region B. Figure 6(d) shows that the phase exposed after oxides peeling off in the left zone is Cu, and the phase of residual small particles in the right zone is mainly Cu₂O at position 2 in region B ^[14].

Figure 6(e) shows that at position 3 in region B, there are small particles of Cu₂O on the left side and a bare copper matrix on the right side. The above results show that the residual nodular oxides on the copper billet surface are mainly dense CuO with large grain size; the dark red area near the residual nodular oxides is composed of a mixture of Cu₂O and Cu, in which the grain size of Cu₂O is small and the distribution is uneven.

3.2.2 Surface microscopic morphology of residual oxides

The microscopic morphology of the residual oxides on copper surface with different heating atmospheres is shown in Fig. 7. When the oxidizing composition is CO₂, H₂O and CO₂ + H₂O mixture, there is no large-sized oxide grains observed on the surface. Meanwhile combined with the XRD results under the condition of CO₂ + H₂O (Fig. 4), it can be seen that the surface oxides are all Cu₂O, and their grain sizes are in the order of H₂O༞CO₂ + H₂O༞CO₂ [Fig. 7(c), (d), (b)]. When the oxidizing composition is O₂ + H₂O, the residual oxide on the surface is known to be a mixture of CuO and Cu₂O from its XRD results. But the large-sized grains of CuO are not observed on the sample surface, which is replaced by small particles of Cu₂O that are uniformly distributed [Fig. 7(g)]. When the oxidizing composition is O₂, O₂ + CO₂ and O₂ + CO₂ + H₂O, it can be seen from its XRD and EPMA results that the residual oxide on the surface is a mixture of CuO and Cu₂O. At this time, there are both large-sized grains of CuO and small particles of Cu₂O on sample surface. As seen from Fig. 3, the oxide surface is dark-red when nodular oxide is thin [Fig. 3(a)]. At this time the amount of CuO remaining on the substrate is small, which is cross-distributed with Cu₂O [Fig. 7(a)]. The oxide surface is black when nodular oxide is thicker [Fig. 3(e), (f)], in which a large area of CuO remains on the sample surface [Fig. 7(e), (f)].

3.3 Microscopic morphology of oxide layer on copper surface with different heating atmosphere

3.3.1 Microscopic morphology of peeled oxide layer

Figure 8 shows the cross-sectional microcosmic morphology of exfoliated oxide layer when the oxidizing composition is O₂ + CO₂ + H₂O, O₂, O₂ + CO₂ and O₂ + H₂O. As seen from the figure, the thickness of exfoliated oxide layers are all around 200 µm, which main phase is CuO combined with the XRD results.

Figure 8(b) shows the exfoliated oxide layer is loose when the oxidizing composition is O₂, in which there is a row of pores parallel to the oxide-matrix interface. When the oxidizing composition is O₂ + CO₂, the "parallel pores" move down to the external oxide-matrix interface [Fig. 8(c)]. Compared with that in Fig. 8(b), the number and size of pores at this interface are reduced. When the oxidizing atmosphere is O₂ + H₂O, the external oxide-matrix interface is almost poreless. When the oxidizing composition is O₂ + CO₂ + H₂O [Fig. 8(a)], there are no "parallel pores" in the external oxide layer. Meanwhile the size and number of pores at oxide-matrix interface are in between compared with Fig. 8(b) and Fig. 8(d). In addition, the number of pores inside the exfoliated oxide layer with each atmosphere is in the order of O₂ + CO₂ > O₂ > O₂+CO₂ + H₂O, whereas there are almost no pores inside the external oxide layer under the atmosphere of O₂ + H₂O.

3.3.2 Sectional microscopic morphology of residual oxides

As shown in Fig. 9, the thickness of residual oxides on copper surface is about 35 µm, 60 µm, 250 µm, and 30 µm, respectively when the oxidizing composition is O₂ + CO₂ + H₂O, O₂, O₂ + CO₂ and O₂ + H₂O. When nodular oxides (O₂ + CO₂ + H₂O, O₂, O₂ + CO₂) are formed on the copper surface, its oxide thickness is significantly greater than the residual oxide thickness when the oxidizing composition is O₂ + H₂O. Meanwhile, the nodular oxides are tightly connected to the matrix, with only small-sized pores remaining at the connection surface. Comparison with Fig. 8 shows that when the number and size of pores at the external oxide-matrix interface are more and larger [Fig. 8(a)-(c)], the nodular oxides are more likely to remain on the substrate [Fig. 9(a)-(c)]. It is speculated that compared with the oxidizing composition of O₂ + H₂O, the pores formed in external oxide layer are unevenly distributed when the oxidizing composition is O₂ + CO₂ + H₂O, O₂ and O₂ + CO₂. During the peeling of the oxide layer, the loose and porous parts at the interfaces are all peeled off from the substrate, while the external oxide layer tightly connect with matix may form nodular oxides. Combining the XRD and EMPA results, it can be seen that the main phase of the nodular oxide is CuO, which is accompanied by a small amount of Cu₂O around it.

4.1 Thermodynamic calculations of copper oxides

The high-temperature oxidation of Cu mainly involves the following four reactions Eqs. 1–4 It can be seen from their Gibbs free-energy-variation curves (Fig. 10), the largest tendency is the formation of Cu₂O from the reaction of Cu with O₂. Although the most thermodynamically stable product of copper is Cu₂O, the Cu⁺ in Cu₂O is in an intermediate valence state in terms of chemical activity and would be continuously oxidized by oxygen to a stable highest valence state. These two steady states are distinct and can be affected by temperature and oxygen partial pressure ^[15]. As shown in Fig. 11, Cu can react with an oxidizing composition to form Cu₂O when the partial pressure of ambient oxygen is greater than the oxygen partial pressure of Cu₂O formation at some temperature. The oxygen partial pressures required for the formation of CuO within the temperature range of 0-1000 ℃ as shown in Fig. 12 and Fig. 13 .

The equivalent partial pressures of oxygen are all in the range of 2.4×10^− 9-1.4×10^− 6 atm when the oxidizing composition is CO₂, H₂O and CO₂ + H₂O (Table 1).The maximum value of them is only larger than that of the partial pressure of oxygen formed by the reaction of Cu and O₂ to generate Cu₂O. Therefore, only Cu₂O can be formed but CuO cannot under the above three atmospheres, which is consistent with the macroscopic (Figs. 3) and microscopic (Figs. 7) morphology of residual oxides on copper surface under these conditions. The equivalent partial pressure of oxygen are all about 4.41×10^− 3 atm when the oxidizing composition is O₂, O₂ + CO₂, O₂ + H₂O and O₂ + CO₂ + H₂O (Table 1), which is greater than the partial pressure of oxygen formed by the reaction of Cu and O₂ to generate Cu₂O and CuO. Therefore, both Cu₂O and CuO are generated on copper surface, which is consistent with the results in Fig. 3. However, when the temperature is greater than 850 ℃, the reaction of Cu₂O with O₂ to form CuO will not occur because its partial pressure of oxygen formed for this reaction (Fig. 13) has exceeded the equivalent partial pressure of oxygen for the heated atmosphere (4.41×10^− 3 atm). As can be seen from Fig. 2, the heating stage from room temperature to 850 ℃ only accounts for about 20% in the whole heating stage. Therefore, the large-scale external oxide (CuO) on copper surface is mainly generated by the reaction between Cu and O₂ when the heating temperature is greater than 850 ℃.

In addition to the above oxidation reactions, when the oxidizing composition is H₂O, O₂ + H₂O and O₂ + CO₂ + H₂O,, H₂ decomposed from H₂O is also able to undergo a reduction reaction with copper oxides at temperature exceeding 784°C because it’s higher than the critical temperature for water vapor decomposition ^[16]. From the Gibbs free energy changes for each reaction (Table 2), it is clear that the reduction of CuO to Cu₂O is the easiest and the reduction of Cu₂O to Cu is the most difficult. When the oxidizing composition is CO₂, O₂ + CO₂ and O₂ + CO₂ + H₂O, the decomposition of CO₂ in these atmospheres is more difficult than the decomposition of H₂O mentioned above. It has been documented that CO₂ decomposes by only 1.8% at 2000°C^[17]. It can also be obtained from Table 1 that the equivalent partial pressure of oxygen in the oxidizing composition of H₂O at 940°C is slightly larger than that in CO₂. While the grain sizes of the oxides on copper surface formed in H₂O are much larger than those formed in CO₂ [Fig. 7(b), (c)]. This further indicates that the amount of CO₂ decomposition is extremely small than that of H₂O at same temperature. Although CO₂ is difficult to decompose, it can undergo adsorption-desorption reactions with copper oxides (Table 3). The thermodynamic values show that CO₂ adsorbs on the CuO surface and reacts with it to form CuCO₃, which in turn decomposes rapidly into CuO and CO₂. However,CO₂ is very difficult to react with Cu₂O.

Table 2 Gibbs free energy of the reduction reaction between H₂ and copper oxide at 940℃

Reaction		ΔG/(J/mol)
(5)	2CuO(s)+H₂(g)=Cu₂O(s)+H₂O(g)	-1.64×10⁵
(6)	CuO(s)+H₂(g)=Cu(s)+H₂O(g)	-1.32×10⁵
(7)	Cu₂O(s)+H₂(g)=2Cu(s)+H₂O(g)	-1.01×10⁵

Table 3 Thermodynamics of adsorption-desorption reactions involving CO₂ at various temperature

Reaction		ΔH/(kJ mol^-1)	ΔS/ (J mol^-1K^-1)	ΔG/(kJ mol^-1)	T/(K)	Reaction possibility
(8)	Cu₂O(s)+CO₂(g)→2CuO(s)+CO(g) (adsorption)	+141.0	-23.0	+147.9	298.0	Not favourable
(9)	CuO(s)+CO₂(g)→CuCO₃(s) (adsorption)	-45.5	-169	+4.9	298.0	Favourable
(10)	CuCO₃(s) →CuO(s)+CO₂(g)(desorption)	+45.5	+169	-4.9	298.0	Favourable (very low temperature)
(11)	2CuCO₃(s)→Cu₂O(s) + 2CO₂(g) + 1/2O₂(g) (desorption)	+233.0	+447.6	-4.2	530.0	Favourable (high temperature)

4.2 Formation process of exfoliated and residual oxides

When the oxidizing composition is O₂ + CO₂ + H₂O, O₂, O₂ + CO₂ and O₂ + H₂O, it can be seen from the previous analysis that the main physical phases of the exfoliated oxide layer and the residual nodular oxide are all CuO. In the macroscopic morphology, there is dark-red Cu₂O on the copper billet surface after the external oxide layer is peeled off (Fig. 3), while there is no Cu₂O at the interface between the residual nodular oxide and the substrate (Fig. 4). In terms of microscopic morphology, the external oxide layer is more prone to spalling when there is a greater amount of Cu₂O at the interface with the substrate (Fig. 7). EPMA results in Fig. 6 confirm that the position where nodular oxide partially peels off is copper substrate rather than the Cu₂O. The exfoliated oxide layer is characterized by the presence of continuous large-sized pores at the exfoliation interface (Fig. 8), whereas small discontinuous pores are present at the nodular oxide-matrix interface (Fig. 9). Based on the above phenomena, it is speculated that the cause of the nodular oxide is: (1) CuO is formed directly on the copper surface without Cu₂O between them. (2) Small discontinuous pores are present at the CuO-matrix interface. On the contrary, an easily exfoliated external oxide layer is formed when loose and porous Cu₂O exists at the CuO-matrix interface; meanwhile, continuous pores are formed at the exfoliated interface when the Cu₂O particles are small in size and large in number.

Synthesizing the previous analyses and experimental results, and combining them with the current studies on the kinetics of copper oxidation^{[3–5,18−19]}, the high-temperature oxidation of copper in the oxidizing compositions of O₂, O₂ + H₂O and O₂ + CO₂ could be crudely speculated, which is schematically shown in Fig. 14.

The formation of oxide layer on copper surface is shown in Fig. 14(a) When the oxidizing composition is O₂. Firstly, O₂ is adsorbed on the copper substrate (stage ①). Then Cu react with O₂ to form Cu₂O and CuO respectively. Since the partial pressure of oxygen for the Cu₂O formation is much lower, Cu₂O is generated in most of the zone on the copper surface and CuO in the remaining zone (stage ②). Because both Cu₂O and CuO are P-type semiconductor oxides, copper ions have to migrate continuously to the oxide/heating atmosphere interface, which leads to the formation of metal vacancies at the oxide-matrix interface, resulting in a large number of pores. It takes two copper ions to produce one Cu₂O, while it takes only one copper ion to produce a CuO. Therefore, the metal vacancies underneath the Cu₂O are more than those underneath the CuO. This is also observed by Fig. 7, the CuO grain size is larger and the oxide layer in CuO zone is dense, meanwhile the Cu₂O grain size is smaller and the number of pores in this region is significantly higher. As the heating temperature increases (< 850 ℃), Cu₂O would react with O₂ to form CuO resulting in the thickness of oxide layer increases (stage ③). It takes one Cu₂O to produce two CuO, therefore the growth pores also appear at the Cu₂O-CuO interface. When the heating temperature is greater than 850 ℃ to 940 ℃ insulation stage, Cu₂O no longer reacts with O₂ to generate CuO. Then the copper ions that continue to migrate to the oxide layer-heating atmosphere interface react with the adsorbed oxygen to form CuO, resulting in the thickness of oxide layer continues to increase (stage ④). The rapidly growing CuO also grows laterally to form a "mushroom-shaped" oxide layer, which in turn forms pores inside the CuO layer parallel to the oxide-copper interface^[20]. In addition, the oxide layer releases growth stresses through plastic deformation as it thickens, resulting in the pores appear within it as well. During the cooling of copper sample (stage ⑤), since the more porosity at the Cu₂O-Cu interface and the greater difference in the thermal expansion coefficients of Cu₂O-Cu (> CuO-Cu₂O and CuO-Cu), the Cu₂O-Cu interface is more susceptible to cracking^[21], which cause the external oxide layer to spall off. Compared with the above, the fewer pores exists at CuO-Cu interface, so that the binding force is stronger. This makes it difficult for external oxide layer to peel off and form " nodular" oxides on the copper substrate. Due to the formation of the above mentioned "parallel pores" during the growth of CuO, part of the "nodular" oxide may also fracture at the "parallel" pore position. Ultimately only part of the nodular oxides remains on the substrate.

When the oxidizing composition is O₂ + H₂O [Fig. 14(b)], O₂ and H₂O are first adsorbed on the substrate (Stage ①). Then Cu reacts with O₂ to form Cu₂O and CuO, respectively (Stage ②), which likewise forms pores at the oxide-substrate interface and inside the oxide layer. As the heating temperature increases (< 850 ℃), Cu₂O reacts with O₂ to form CuO. Meanwhile, the CuO is reduced to Cu₂O by a small amount of H₂ that is decomposed by H₂O (stage ③). At this point, the small-sized H₂ which is incorporated into the oxide layer increases the mobility of dislocations and the plasticity of oxide layer. Therefore, the sufficient plastic flow effectively eliminates the vacancies generated by the outward migration of metal ions^[22]. When the heating temperature is greater than 850 ℃ to 940 ℃ insulation stage, the thickness of oxide layer continues to increase. Meanwhile H₂O decomposes more H₂ at the same time to reduce the CuO at CuO- Cu₂O interface to Cu₂O (stage ④). At this point, the pores in the oxide layer are also annihilated due to strong plasticity. During the cooling of copper (stage ⑤), almost all zones at the CuO-matrix interface are interspersed with loose and porous Cu₂O, which results in a complete exfoliation of the external oxide layer, with virtually no "nodular" oxides remaining.

When the oxidizing composition is O₂ + CO₂ [Fig. 14(c)], less Cu₂O and CuO may generate on the substrate due to the large-sized CO₂ occupies part of O₂ (Stage ①). After CuO is generated, CO₂ is adsorbed on its surface and reacts to form CuCO₃. Then CuCO₃ is rapidly decomposed into CuO and CO₂, and a small portion of CO₂ is left inside the oxide layer which causes pores (Stage ②,③,④).Thus the oxidation process when the oxidizing composition is O₂ + CO₂ is generally similar to that in the oxidizing composition O₂. It is just that the size of CO₂ decomposed by CuCO₃ is so large, which cannot be fully incorporated into the oxide layer to increase the plasticity and eliminate the pores, but instead exacerbates the pores in the oxide layer^[23] .

The morphology observation and phase analysis of the copper samples after oxidation experiment were carried out by various material characterization methods, so as to find out some effect of heating atmosphere composition and content on phase and morphology distribution of copper oxide layer. The following results have been obtained:

The main phase of the exfoliated oxides and the residual nodular oxides on the copper surface are CuO. The reason for the difference in their morphology is the formation steps of CuO, which results in the difference in the bonding strength between external oxide layer and copper matrix. When CuO is generated by the reaction between Cu₂O and O₂, the loose and porous Cu₂O forms between external oxide layer and copper matrix. Then the external oxide layer is easy to peel off. When CuO is generated by the direct reaction between Cu and O₂, the bonding is strong, which may lead to part of the CuO finally remain on copper matrix to form nodular oxides.
Water vapour(H₂O) may attenuate the formation of nodular oxides on the copper surface. The reason for this may that H₂O promote the reaction of CuO at oxide layer-matrix interface to generate loose Cu₂O. When the number and size of Cu₂O particles at the connection interface are larger, the bonding strength between the external oxide layer and the substrate is weaker.
Water vapour(H₂O) and its decomposition products may effectively eliminate the pores within the oxide layer and at the oxide-matrix interface, while CO₂ may increase the pore size inside the oxide layer.

CRediT authorship contribution statement

Investigation, Conducting experiments, Formal analysis, Writing-original draft: Chufeng Lv; Conceptualisation, Data curation, Validation, Supervision, Writing-review and editing: Yue Guo; Methodology, Supervision: Jian Zhao; Review, Resources: Fangqin Dai; Review: Weidong Zeng; Supervision: Ming Liu.

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

Conflict of Interest: The authors declare that they have no conflict of interest.

Pevzner MZ, Sergeev DG. Power Consumption, Control of Properties and of Continuous Annealing Process of Copper and Brass Rolled Products in Transverse Magnetic Field[J]. Metal Science and Heat Treatment, 2022,63(9–10).
Minich WR,Cazamias UJ,Kumar M, et al. Effect of Microstructural Length Scales on Spall Behavior of Copper[J]. Metallurgical and Materials Transactions A,2004,35(9).
Valerii V Belousov, A A Klimashin. High-temperature oxidation of copper[J]. Russian Chemical Reviews,2013,82(3).
Zhu YF, Mimura K, Isshiki M. Purity Effect on Oxidation Kinetics of Copper at 800–1050℃[J]. Journal of the Electrochemical Society,2004,Vol.151(1): 27–32
Singh R, Venkataraman BM. High Temperature Corrosion and Oxidation of Metals[J]. Metals,2019,9(9).
Mrowec S, Stokłosa A. Oxidation of copper at high temperatures[J]. Oxidation of Metals,1971,3(3).
Zhu Y, Mimura K, Hong HS, et al. Influence of Small Amounts of Impurities on Initial Oxidation of Copper at 400 and 800℃ in 0.1 MPa Oxygen Atmosphere[J]. Journal of The Electrochemical Society,2005,152(8).
Pryor MJ, Lin L. The mechanism of stress corrosion cracking of Cu-base alloys[J]. Corrosion Science,1990,30(2–3).
Feng Z, Marks CR, Barkatt A. Oxidation Rate Excursions During the Oxidation of Copper in Gaseous Environments at Moderate Temperatures[J].Oxidation of Metals,2003,60(5–6).
Bridges DW;Baur JP;Baur GS, et al. Oxidation of Copper to Cu₂O and CuO (600°-1000 ℃ and 0.026–20.4 atm Oxygen)[J]. Journal of The Electrochemical Society,2019,103(9).
Zhu YF, Mimura K, Lim JW, et al. Brief Review of Oxidation Kinetics of Copper at 350℃ to 1050 ℃: Physical Metallurgical and Materials Science[J].Metallurgical and Materials Transactions B,2006,Vol.37A: 1231–1237
Huin D, Flauder P, Leblond JB. Numerical simulation of internal oxidation of steels during annealing treatments[J]. Oxidation of Metals,2005,64(1):131–167.
Samal S, Mitra KS. Influence of Grain Shape, Size, and Grain Boundary Diffusion on High-Temperature Oxidation of Pure Metal Fe, Cu, and Zn[J]. Metallurgical and Materials Transactions,2015,46(8).
Zhao M, Shang F, Song Y, et al. Surface morphology, composition and wettability Cu₂O/CuO composite thin films prepared by a facile hydrothermal method[J]. Applied Physics A,2015,118(3).
Haugsrud R, Kofstad PK. On the Oxygen Pressure Dependence of High Temperature Oxidation of Copper[J]. Materials Science Forum,1997,343(251–254).
McCrea WH. The specific heat of water vapour and the theory of the dissociation of water vapour at high temperatures[J]. Mathematical Proceedings of the Cambridge Philosophical Society,1927,Vol.23(8): 942–950
Isahak WNRW, Ramli ZAC, Ismail MW, et al. Adsorption-desorption of CO₂ on different type of copper oxides surfaces: Physical and chemical attractions studies[J]. Journal of CO₂ Utilization,2013,Vol.2: 8–15
Zhu YF, Mimura K, Isshiki M. Oxidation mechanism of Cu₂O to CuO at 600–1050 ℃[J]. Oxidation of Metals,2004,Vol.62: 207–222
Haugsrud R. On the Influence of Non-Protective CuO on High-Temperature Oxidation of Cu-Rich Cu-Ni Based Alloys[J]. Oxidation of Metals,1999,52(5–6).
Chen YR, Yuen DYW. Effects of the Presence of Water Vapour on the Oxidation Behaviour of Low Carbon-Low Silicon Steel in 1%O₂–N₂ at 1173 and 1273 K[J]. Oxidation of Metals,2013,79(5–6).
Lee SJ, Jang YB, Kim SJ, et al. Comparison Study of Oxidation Behavior of Copper at Elevated Temperatures[J]. Advanced Materials Research,2014,3482(1025–1026).
Haugsrud R. The Influence of Water Vapor on the Oxidation of Copper at Intermediate Temperatures[J]. Journal of The Electrochemical Society,2002,149(1).
Koitaya T, Yamamoto S, Shiozawa Y, et al. Real-Time Observation of Reaction Processes of CO₂ on Cu(997) by Ambient-Pressure X-ray Photoelectron Spectroscopy[J]. Topics in Catalysis,2016,59(5–7).

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Effect of heating atmosphere composition and content on phase and morphology distribution of copper oxide layer

Status:

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Abstract

Figures

1. Introduction

2. Experimental Procedures

2.1 Materials

2.2 Experimental Methodology

3. Results

3.1 Macroscopic morphology of residual oxides on copper surface with different heating atmosphere

3.2 Phase composition and microscopic morphology of residual oxides on copper surface with different heating atmosphere

3.2.1 Phase and element distribution of residual oxides

3.2.2 Surface microscopic morphology of residual oxides

3.3 Microscopic morphology of oxide layer on copper surface with different heating atmosphere

3.3.1 Microscopic morphology of peeled oxide layer

3.3.2 Sectional microscopic morphology of residual oxides

4. Discussion

4.1 Thermodynamic calculations of copper oxides

4.2 Formation process of exfoliated and residual oxides

5. Conclusions

Declarations

References

Additional Declarations

Status:

Journal Publication

Version 1