An In Situ Silicon Nanoparticle Size Distribution Measurement Scheme for Silane Plasma Synthesis Process in Vacuum Environments

In this study, an in-situ silicon (Si) nanoparticle size distribution measurement scheme for silane plasma synthesis processes is proposed. In recent years, there have been significant interests in utilizing silicon nanoparticles as anode material for Lithium-Ion Batteries (LIB). Despite its superior energy capacity compared with other materials, Si had not been known as a favorable anode material because of its high volume fluctuations during multiple charging/discharging cycles [1]. However, researchers have succeeded in solving this drawback by depositing the material in the form of nanoparticles with diameters smaller than 150 nm [2].

One of key interests regarding the use of Si nanoparticles for LIB anodes is developing efficient and reliable manufacturing processes for practical synthesis. Among various methods, synthesizing Si nanoparticles in the gas phase using silane plasma has emerged as a promising method because high-purity nanoparticles of tens-of-nanometer size with good monodispersity can be produced continuously over long periods [3]. However, to our knowledge, there is no in-situ scheme for measuring the size distribution of the nanoparticles during the synthesis process due to their size ranges and vacuum conditions. Many of the particle size analysis in this research field relies on ex-situ methods (e.g., SEM, TEM). In order to achieve greater reliability for the nanoparticle synthesis, in-situ nanoparticle size measurement schemes are needed for the process.

In this study, an in-situ nanoparticle size distribution measurement scheme, which consists of a charge neutralizer, a multipole electrode-based particle guiding device, and a quadrupole mass analyzer (QMA) is proposed. After installing the scheme in the middle of the Si nanoparticle transfer lines, charge status of sampled nanoparticles is neutralized to achieve Boltzmann equilibrium charge distributions. Among them, singly charged nanoparticles are guided into the QMA through the guiding device. The QMA only transfers nanoparticles which have a certain mass-to-charge ratio, by using AC and DC electric source parameter modulations. Therefore, the number density for each nanoparticle size can be obtained by collecting nanoparticles at the end of the QMA.

Particle trajectory simulations are conducted with the assumption that the charge status of nanoparticles has already been neutralized. Results show that with the proposed scheme, singly charged and target-sized Si nanoparticles with diameters ranging from 5 to 500 nm, sampled via a 10 mm diameter input aperture, can be collected at the end of the QMS with a 21.9% transmission efficiency. And, a diameter classification resolution of ±0.9% is obtained under the same particle conditions. The results are achieved using AC electric sources with frequencies up to 140 kHz and amplitudes up to 100 V, and DC electric sources up to 15 V, which are feasible values for hardware implementation.

[1] Obrovac, M. N., et al. "Reversible cycling of crystalline silicon powder." Journal of The Electrochemical Society 154.2 (2006): A103.
[2] Liu, X. H., et al. "Size-dependent fracture of silicon nanoparticles during lithiation." ACS nano 6.2 (2012): 1522-1531.
[3] Kunze, F., et al. "Validation of the silicon nanoparticle production on the pilot plant scale via long-term gas-phase synthesis using a microwave plasma reactor." Applications in Energy and Combustion Science 15 (2023): 100195.