Application of Gas Adsorption Technology in Conductive Paste Industry

Conductive paste is a special functional material with both conductive and bonding properties, widely used in new energy batteries, photovoltaic, electronics, chemical industry, printing, military and aviation and other fields. Conductive paste mainly includes conductive phase, bonding phase and organic carrier, of which the conductive phase is the key material of conductive paste, determining the electrical properties of the paste and the mechanical properties after film formation.

The commonly used materials of conductive phase include metal, metal oxide, carbon materials and conductive polymer materials, etc. It is found that the physical parameters such as specific surface area, pore size and true density of conductive phase materials have an important influence on the conductivity and mechanical properties of the slurry. Therefore, it is particularly important to accurately characterise physical parameters such as specific surface area, pore size distribution and true density of conductive phase materials based on gas adsorption technology. In addition, precise tuning of these parameters can optimise the conductivity of the pastes to meet the requirements of different applications.

01 Conductive paste introduction

According to the actual application of different types of conductive paste is not the same, usually according to the different types of conductive phase, can be divided into conductive paste: inorganic conductive paste, organic conductive paste and composite conductive paste. Inorganic conductive paste is divided into metal powder and non-metallic two kinds of metal powder mainly gold, silver, copper, tin and aluminium, etc., non-metallic conductive phase is mainly carbon materials. Organic conductive paste in the conductive phase is mainly conductive polymer materials, which has a smaller density, higher corrosion resistance, better film-forming properties and in a certain range of conductivity adjustable and so on. Composite system conductive paste is currently an important direction of conductive paste research, the purpose is to combine the advantages of inorganic and organic conductive paste, the inorganic conductive phase and organic material support body organic combination, give full play to the advantages of both.

Conductive phase as the main functional phase in the conductive paste, to provide electrical pathway, to achieve electrical properties, its specific surface area, pore size and true density and other physical parameters have a greater impact on its conductive properties.

Specific surface area: the size of the specific surface area is the key factor affecting the conductivity, within a certain range, a larger specific surface area provides more electronic conduction pathways, reducing the resistance, making the conductive paste more conductive. High conductivity is critical in many applications, such as in electronic devices to ensure efficient conduction of circuits.

Pore size: The choice of pore size has a significant impact on both electron conduction and ion diffusion. Conductive phases with smaller pore sizes can reduce the rate of ion diffusion, which can be advantageous in some battery applications, allowing for higher charging and discharging rates. However, too small a pore size may also result in hindering electron conduction. Therefore, the aperture size needs to be carefully selected based on the specific application requirements.

True Density: True density reflects how close together the atoms or molecules of the conducting phase are. Higher true densities usually indicate a more compact structure, which facilitates electron conduction. Higher true density materials such as metals or metal oxides are often used in applications requiring high electrical conductivity.

Therefore, during the R&D process, the above physical parameters are accurately characterised to ensure that the prepared conductive pastes have the required electronic conductivity, mechanical properties and stability. The following is a detailed description of the case study on the characterisation of the adsorption properties of pastes with different conductive phases.

02 Metal conductive paste adsorption performance characterisation

Metal conductive pastes include precious metals Au, Ag, Pd, Pt, etc., non-precious metals Cu, Ni, Al, etc., Au conductive pastes have excellent performance, but expensive, in order to reduce the cost of the general use of silver powder, silver on the ceramic surface has a strong adhesion, can be formed on the surface of the ceramic continuous dense uniform thin layer of silver electrodes have a greater capacitance than the other electrode materials, but the silver in the action of the electric field will Produce electron migration, reducing the conductivity and thus affecting the life. Compared with other metal-based conductive pastes, copper powder is inexpensive and has superior conductive properties, but the defect is that copper is chemically active and oxidises easily, resulting in an increase in resistivity.

Copper powder and silver powder as a common and important conductive paste, its sintered film resistance, adhesion and densification and other important parameters to a certain extent depends on its particle morphology, dispersion, particle size and specific surface area properties. Professor Lv Ming found that the smaller the particle size, the larger the specific surface area and thus the larger the specific surface energy, and the lower the melting point, which is conducive to the solidification of nanosilver powders in silver pastes at lower sintering temperatures, and can be used in certain temperature-sensitive scenarios. CIQTEK's EASY-V series of specific surface area testers were used to determine the specific surface area of copper and silver powders, and the results were 2.71m²/g and 1.59m²/g, respectively (Figs. 1 and 2), with the P/P0 selection points ranging from 0.05 to 0.30, the linear fit＞0.999, and the intercepts were all positive, which indicated that the test results were accurate and reliable, and that the instrument was highly automated, simple and convenient to operate, and had a high testing efficiency. It is easy and convenient to operate, and the testing efficiency is high.

Fig. 1 Specific surface area test results of copper powder

Fig. 2 Specific surface area test results of silver powder

03 Characterisation of adsorption properties of carbon based conductive pastes

Carbon conductive paste is generally carbon black, graphene, carbon nanotubes, etc. It is mainly used as a conductive agent for positive and negative electrode materials in batteries, which is one of the key auxiliary materials for batteries. The conductive agent can allow electrons to travel freely between the positive and negative electrodes and the collector. Efficient conductive agent should be uniformly attached to the positive and negative electrode materials to form a three-dimensional network structure to ensure the smooth flow of current.

Carbon black is a point contact particle conductive agent, with a certain degree of adhesion but no directionality, it is not easy to form a network pathway, usually using carbon black with a large specific surface area, using carbon black particles of small size, more particles per unit volume, it is easy to contact each other to form a network pathway. Graphene is a sheet conductive agent with surface or line contact, graphene has a large specific surface area, and it is easy to form more SEI and consume lithium ions when added to the negative electrode (except for coated micron silicon), so it is generally added to the positive electrode to improve the multiplicity and low-temperature performance. Carbon nanotube conductive agents are fibrous and ductile in both length and width directions, making them easier to build into networks. Carbon nanotubes are used as a conductive agent in the battery industry, the advantage is that the amount of additive compared to the traditional conductive carbon black is greatly reduced, and at the same time can make the amount of bonding agent reduced to about 50% of the original, the disadvantage is that the dispersion is poor, and in order to improve the use of carbon nanotubes, the need to strictly control its specific surface area. Suitable specific surface area of carbon nanotubes structural stability, there is a certain dispersion can form a better conductive network, easy to powder, easy to fall off the silicon-based negative electrode material is the most suitable conductive agent.

The above analysis shows that the specific surface area is an important test index in carbon based conductive pastes. As shown in Figure 3, the conductive carbon black, graphene and carbon nanotubes used as conductive agents for positive and negative electrode materials in the battery industry were measured using the EASY-V series of specific surface area testers, with different specific surface area results of 58.40m²/g, 523.33m²/g, 308.41m²/g.

Figure 3-1 Conductive Carbon Black BET：58.40 m²/g

Figure 3-2 Graphene BET：523.33 m²/g

Figure 3-3 Carbon Nanotubes BET：308.41 m²/g

04 Characterisation of adsorption properties of composite conductive pastes

Composite conductive paste refers to the addition of many different types of conductive phases, such as carbon black-graphene, carbon black-carbon nanotubes, silver powder-graphene, and other composite conductive pastes, so as to make it have better performance performance. Compared with a single conductive phase, composite conductive paste has the advantages of more stable conductivity, wider range of use and more affordable. For example, LiCoO₂-SP-CNTS composite cathode plates are prepared from traditional carbon black and carbon nanotubes using a cold rolling method, which not only plays a role in the conductive network constructed by carbon nanotube fibrous materials, but also avoids the agglomeration of carbon nanotubes, and can show excellent conductivity through the combination of traditional conductive carbon black and activated material particles.

Because graphene and carbon nanotubes contain more holes so the specific surface area is larger, resulting in its surface energy is also higher, easy to cause agglomeration, thus need to analyse its pore size test. Using CIQTEK's own EASY-V series of specific surface and pore size analyser test results are shown below (Figure 4 and Figure 5), through the nitrogen adsorption and desorption isotherms can be seen, the two are mainly for the Ⅳ isotherms, the partial pressure ratio of P/P0 in 0.4 after the adsorption and detachment did not completely overlap, which means that hysteresis loops are generated, indicating that there is a certain degree of mesoporous structure. The NLDFT-pore analysis shows that both materials have relatively abundant pore size distributions at 1 nm and from 3 nm to 50 nm, and their total pore volumes are 1.43 cm³/g and 3.16 cm³/g, respectively. 

Figure 4-1 Nitrogen adsorption and desorption isotherms for carbon nanotube materials

Figure 4-2 NLDFT-pore size distribution of carbon nanotube materials

Figure 5-1 Nitrogen adsorption and desorption isotherms for graphene materials

Figure 5-2 NLDFT-pore size distribution of graphene materials

05 True density determination of conductive paste

Conductive phase of the physical indicators will affect the performance of the conductive paste, such as silver powder true density, sintering is not easy to form holes, high densities, can get dense conductive excellent conductive film layer. CIQTEK self-developed EASY-G series of true density tester for the determination of the true density of silver powder, as shown in Figure 6, the true density of 8.896 g / ml, many times the test value is only in the third decimal point fluctuations, high test accuracy, and the instrument is equipped with three different specifications of the sample cell to meet the needs of different shapes of the sample volume test.

Fig. 6 True density test results of silver powder material