Material Characterization - Exploration of Testing Methods for Silver Powder Specific Surface Area

Apparent properties of spherical silver powder (sphericity, tap density, specific surface area, particle size distribution) and sintering characteristics significantly affect the conductivity and photoelectric conversion efficiency of the positive electrode. The sintering shrinkage of silver powder is correlated with specific surface area; the larger the specific surface area, the easier the silver powder is to sinter. Ultrafine silver powder is typically mixed with binders, solvents, and additives through mechanical blending to form conductive silver paste. The apparent properties of silver powder, including particle size, specific surface area, and morphology, impact important parameters such as contact resistance, adhesion, and density after the paste is sintered into a film.

Spherical silver powder is a primary component of conductive silver paste, and its conductive characteristics rely predominantly on the silver powder. The content of silver powder in the paste directly influences its conductivity. In a sense, enhancing its conductivity is beneficial, but when its content exceeds the critical volume concentration, its conductivity does not necessarily improve. The suitable silver content in silver paste is typically in the range of 60% to 70%.

This article aims to investigate the influence of different pretreatment conditions on the results of silver powder specific surface area.

Testing Principle

Static capacity method: The system is evacuated to a low pressure, and then the sample tube is immersed in a Dewar flask containing liquid nitrogen. A certain amount of non-adsorbed gas (usually helium) is introduced into the sample tube, which has been evacuated to a stable low pressure, to determine the dead volume. Carefully remove the gas used to determine the dead volume, then introduce a certain amount of adsorbate into the sample tube and measure the adsorption of the adsorbate by the sample at a certain P/P0 value (Va). For multi-point measurements, within the range of P/P0 values 0.05-0.30, the adsorption of the adsorbate in the sample is measured at at least three different P/P0 values, starting from low to high. The BET equation is plotted, and the specific surface area is calculated.

Schematic Diagram of Static Capacitance Method

The impact of different pretreatment conditions on the surface area

In order to remove impurities such as gases and moisture from the surface of the sample and obtain a clean and dry sample surface, pre-treatment is usually performed before testing the specific surface area. Pre-treatment conditions include: pre-treatment temperature, pre-treatment time, and pre-treatment methods (inert gas purging and vacuum pumping).

The principles for selecting the pre-treatment temperature are: under the premise of not altering the sample characteristics, choose a temperature as high as possible to quickly remove impurities from the sample surface. For samples with lower moisture content, vacuum pumping is more efficient than inert gas purging.

For silver powder samples with low moisture content, the vacuum pumping method is chosen for pre-treatment, and heating is not required.

Select samples from three silver powder manufacturing plants and test the BET under different pre-treatment conditions. The comparative results are shown in Table 2.

Select samples from three silver powder manufacturing plants and test the BET under different pre-treatment conditions. The comparative results are shown in Table 2.

As seen in Table 2, while ensuring the integrity of the measured area, the three types of silver powder samples from Manufacturing Plant B, under the condition of "no heating, vacuum pumping for 1 hour," exhibit varying linearity factors. For sample 1#, the linearity factor is 0.9, for sample 2#, it is 0.99, and for sample 3#, it meets the criterion of four nines (0.9999). When the pre-treatment condition is "heating at 70°C and vacuum pumping for 2 hours," the specific surface area of the silver powder samples decreases slightly, and the linearity factor increases.

For samples from Manufacturing Plant C, the results under "heating at 100°C and vacuum pumping for 1 hour" are smaller than those under "no heating, vacuum pumping for 1 hour," and there is a deterioration in linearity.

The silver powder samples tested in this study were all treated with dispersants, meaning that the sample surfaces are coated with dispersants. Dispersants have different temperature tolerances depending on their types. In conclusion, it is advisable to choose an appropriate pre-treatment temperature for degassing based on the temperature tolerance of the dispersant. This ensures that the linearity coefficient of the BET is equal to or greater than 0.9999.

Summary:

Silver powder (such as ultrafine silver powder, electronic silver paste, and solar cell slurry) is widely used in the fields of photovoltaic cells and microelectronics industries. The surface properties of silver powder can impact the conductivity and photoelectric conversion efficiency of the front electrode in solar cells. It also plays a crucial role in the contact resistance, adhesion, and densification of the conductive paste after sintering.

The specific surface area influences the sintering shrinkage of silver powder; the larger the specific surface area, the easier the silver powder is to sinter. Pre-treatment conditions are a major factor affecting the accuracy of the specific surface area of silver powder. It is essential to choose an appropriate pre-treatment temperature based on the sample's temperature tolerance (while ensuring that the measured area of the sample is not less than 5 m²). This ensures reliable results and provides a solid foundation for subsequent analyses and research.