ABSTRACT: In recent years, high performance electrolytic copper foil has been widely used in electronic industry and lithium battery industry. In the process of preparation and production of copper foil, additives have a great influence on the microstructure and properties of copper foil. Because of the problems of common additives, it is difficult to meet the demand of new products. In order to improve the products, it is necessary to develop better performance additives. In this paper, the influence of common additives on the structure and properties of electrolytic copper foils is summarized. There are inorganic chloride ions, organic polyether compounds, sulfur compounds and gelatin, etc. . On this basis, the optimization of the microstructure and properties of the electrolytic copper foils with the new additives is discussed: first, 6.0 mg/l cerium sulfate can refine the grains uniformly and improve the mechanical properties of the copper foils. Secondly, the composite additive of polydithio-sodium Dipropane sulfonate-hydroxyethyl cellulose-sodium tungstate can make the copper crystal face to (220) preferred orientation and promote the uniform densification of the coating. After the third and dimercaptobenzimidazole treatment, the grain size and the number of grain boundaries of the copper foil become larger, and the strength and Surface roughness of the copper foil decrease. 

Key words: Electrolytic Copper Foil; additive; texture; mechanical properties; microstructure introduction electrolytic copper foil is an important raw material in electronic industry, mainly used in circuit board and lithium-ion battery. 

The electrolytic copper foil is electrodeposited by direct current, the insoluble metal is the anode and the titanium shaft is the Cathode, the two are immersed into the copper sulfate electrolyte for electrolysis, the copper ion is deposited on the surface of the titanium shaft to form the copper foil, then the copper foil is peeled off, washed and dried, after winding, the copper foil is formed, and then the copper foil is obtained by surface treatment process.

1. The microstructure and properties of electrolytic copper foil and its influence on the microstructure and properties of electrolytic copper foil mainly refer to the growth of grain and crystal plane of copper foil, Surface roughness and texture of copper foil. The macroscopical growth of crystal grain and crystal face is the effect on the appearance and roughness of smooth surface and rough surface of copper foil, because the growth of crystal face of copper grain is the precipitation of copper atom in the process of Electrolysis, from liquid phase copper ion to solid phase copper crystal, the process of changing from one solid phase to another. It is found that the surface of copper foil deposited with large grains is often uneven, and even the top of the foil is aggregated and the copper spines are formed. The grain size is not uniform, and the surface of copper foil deposited with large grains is often uneven, the copper foil formed at this time has a larger Surface roughness. The reverse is true when the copper foil is formed by the deposition of small copper grains. The texture of copper foil refers to the preferred orientation of crystal plane and Crystal Plane Growth. The essence of texture formation is that copper atoms are deposited according to various mechanisms to form corresponding crystal planes, the growth speed of each crystal plane is different, some crystal planes will grow preferentially, and the crystal planes which grow preferentially will form texture, the preferred orientation of these grains is irregular. In different environment, its stress and energy will change gradually. The effect of texture on the microstructure and properties of electrolytic copper foil is the largest, which is mainly reflected in its internal stress and mechanical properties. This is due to the existence of texture, affect the performance of copper foil. In general, the macroscopic properties of copper foil depend on the microstructure such as texture.

2. The effect of different additives on the microstructure and properties of the electrolytic copper foil, the texture of the electrolytic copper foil has the greatest effect on the microstructure and properties of the copper foil, which is mainly reflected in the internal stress and mechanical properties. Therefore, changing the texture of copper foil is the key to optimize its structure and properties. How does the additive affect the texture? After adding additive [2,4] into the raw foil system, the migration ability of copper ions is relatively low. After adding additive, the copper ions around the nucleation center will be adsorbed and the concentration of copper ions around the nucleation center will be reduced, and then the growth of crystal plane will be inhibited, to induce the growth of a new crystal plane (220) . The additives can promote the formation of new nucleation centers and cover other grain faces. The grain growth state will be greatly affected. The additive can make the grain of copper foil become finer and the shrinkage tensile stress become larger. Because of the poor surface activity of copper ion, the electron density of the additive is higher than that of the surrounding particles, and the surface compressive stress is higher. 2.1 effect of general additives on the microstructure and properties of electrolytic copper foils. At present, general additives [5] mainly include inorganic chloride ions, organic polyether compounds and sulfur compounds, gelatin, etc. . 

2.2 the effect of chloride ion on the microstructure and properties of electrolytic copper foil in acidic copper plating, chloride ion [6,7] has leveling effect. The structure of the cathode deposit was improved by adding hydrochloric acid and other additives. In the electrodeposition of copper, chloride ion can increase the concentration of copper on the electrode surface, decrease the activation polarization and promote the growth of the crystal nucleus. When the concentration of chloride ion is low, the (100) cone-shaped single crystal is parallel deposited on the surface of titanium. When the concentration of chloride ion is high, the formation of (111)-sided cubic crystal promotes the cathode polarization, and when the concentration is lower than the critical value, the dendritic striations will occur. 

2.3 the effect of polyether organics on the microstructure and properties of electrolytic copper foils is a Polyethylene glycol compound, which can be oriented and adsorbed at the interface between Cathode and solution, thus reducing the interfacial tension of the surface, to enhance the wetting ability of the interface between electrolyte and Cathode, to improve the flatness and wettability of the deposit, to increase the dispersion ability of copper ion and to eliminate the Pinhole of the deposit, the grains of the coating are uniform, fine and compact. The results show that the texture of copper (220) increases with the increase of Polyethylene glycol content, and the texture of copper (111) increases first and then decreases. It has little effect on the tensile strength of copper foil, but it can reduce the elongation of copper foil. 2.4 The influence of Gelatin on the microstructure and properties of electrolytic copper foils the molecular weight of gelatin will affect the surface morphology of copper foils. When the adsorption area on the cathode surface is large, the hair surface of copper foils will be in a hilly shape, and the adsorption area is small when the molecular weight is small, the rough surface of the copper foil is sharp and tapered. Gelatin can improve the tensile strength and elongation of copper foil at room temperature, but reduce the tensile strength and elongation of copper foil at high temperature. Because it can promote the (200) , (111) texture growth, the gelatin micelle in acid solution is positively charged (the Amino group in the structure) , the CATION electrophoresis moves to the place with the high current density adsorption and carries on the complex with the Copper Ion [10] , makes the copper ion difficult to obtain the electron, therefore, the rapid deposition of copper ion on the Cathode Roller is hindered, so that the growth point of copper on the cathode roller is increased and the crystal nucleus is more uniform, so gelatin has leveling effect.

3. The effect of cerium sulfate on the microstructure and properties of Copper Foil Rare Earth metals contain cerium, vanadium and copper, among which yttrium and cerium are the most used. The 4F orbitals in the inner layer of rare earth metals are not filled with electrons, and their vacancies have a strong attraction to electrons and can be chemically nonmetal with many metals to form alloy coatings. It was found that proper amount of cerous sulfate can refine the grain and make it even densified. When 6.0 mg/l of cerous sulfate salt was added, the effect of grain refinement was the best, and the mechanical properties of copper foil were improved. When cerous sulfate is not added, the cone-shaped grains in the microstructure of copper foil are larger and different in size. The reason is that during electrolytic deposition of copper, the over-potential is low, the nucleation is less, and the nucleation grains grow rapidly into cone-shaped grains, the growth of subsequent nucleation grains was inhibited. The reason why cerium sulfate can refine the crystal grain is that cerium cation can improve the cathode polarization and the current efficiency, and cerium cation can easily adsorb on the active point of crystal growth, inhibit the crystal grain growth and refine the crystal grain of copper foil, but too much cerium will weaken its characteristic adsorption and lose the effect of grain refinement. 3.2 the effect of composite additives on the microstructure and properties of electrolytic copper foils was investigated. Sodium Dithiodipropane sulfonate-hydroxyethyl cellulose-sodium tungstate is a composite additive. The additive can obviously enhance the polarization ability of the electrode, which indicates that the composite additive can inhibit the electrodeposition of copper. With the increase of electrodeposition time, the number of Micron copper grains deposited on the surface of copper foil increases and the quality of copper foil increases. The composite additive [11] can promote the uniform and compact coating and improve the effect of deep plating. A uniform layer of micron-sized grains was deposited on the surface of copper foil. The effect of deep plating was good, but the grains were too small and the enhancement of peel strength was very limited. The shape of the apex tumor was improved obviously, from sharp to smooth and round. Inhibition of dendrite grain formation at the top of copper peak. The reason is that in the same electrodeposition parameters, the complexing effect of the composite additive improves the effect of deep plating. Hydroxyethyl cellulose (HEC) is a kind of nonionic active agent, which can improve the elongation and tensile strength of copper foil. Sodium tungstate can adjust the growth rate of crystal and promote the preferred orientation of copper foil from (111) , (200) to (220) . 3.3 effect of dimercaptobenzimidazole on microstructure and properties of electrolytic copper foils. Adding dimercaptobenzimidazole can make the grain size of copper foils increase, the number of grain boundaries decrease and the hindrance to dislocations decrease, this reduces the mechanical strength and Surface roughness of the copper foil. The additive can also inhibit the nucleation of electrodeposition, because the surface of copper foil bulge high potential, inhibit electrodeposition, then copper ion migration to low potential concave deposition, so that the surface of copper foil becomes more smooth. Therefore, dimercaptobenzimidazole has a leveling effect. With the increase of the concentration of the additive, the preferential degree of (220) crystal plane decreases, because the additive can inhibit the deposition and nucleation of copper ions, the nucleation rate of (111) and (200) crystal plane is faster, the density of crystal plane is higher, and the preferential degree of crystal plane increases, the degree of (220) crystal plane preference is reduced. When the concentration of dimercaptobenzimidazole was 8.0 mg/l, the crystal particles on the surface of copper foil became smaller, and when the concentration was 12.0 mg/l, the abnormal particles were produced, which affected the surface properties of copper foil. In the process of electrolytic deposition of copper ions, because the deposition rates of copper ions in vertical and parallel directions are not consistent, the deposition layer of electrolytic copper foil produces concave and convex points, and the potential difference is high at the convex points, where copper ions are deposited first, there will be pointed particles on the surface.

4. Conclusion chlorine ion has leveling effect in common additives, which can increase the concentration of copper on the electrode surface, reduce the activation polarization and promote the growth of crystal nucleus. The organic Polyethylene glycol can improve the dispersion of copper ions and eliminate pinholes, so that the grains of the coating are uniform and compact and the elongation is reduced. Gelatin can improve the tensile strength and elongation of copper foil at room temperature, but reduce its tensile strength and elongation at high temperature. The optimization of the structure and properties of copper foil with the new additive was further discussed: adding 6.0 mg/l cerium sulfate could refine the grains and improve the mechanical properties of copper foil. The composite additive of polydithio-sodium Dipropane sulfonate-hydroxyethyl cellulose-sodium tungstate can make the coating even and compact and improve the effect of deep plating. Sodium tungstate can adjust the growth rate of crystal and promote the preferred orientation of copper foil from (111) and (200) to (220) . When the concentration of dimercaptobenzimidazole was 8.0 mg/l, the particle size on the surface of copper foil became fine, and the preferential degree of (111) and (200) crystal planes increased, and the preferential degree of (220) crystal planes decreased. 

Source: Chinanews.com, by Yao Guohuan

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