Fabrication of Fe-Ni Ultrafine Particles Epoxy Composite Film Using the LbL Assisted Composite Plating Method
Authors: Fujita Naoyuki , Takeuchi Makoto , Fujiwara Kouta , Muto Hiroyuki 
Published: May 2020

Magnetic ultrafine particles-insulator composite films including Fe-Ni ultrafine particles and insulating epoxy films used for electromagnetic wave absorbing films were synthesized by a newly developed electrochemical method termed the “LbL assisted composite plating method”.
The reaction solution containing 40 ml / L water-solve epoxy resin prepared by NIPPON PAINT Co., LTD and 0-3 g / L Fe-Ni ultrafine particles 60 nm in diameter manufactured by the Nilaco corporation were used for film fabrication. Cu substrates and Ti / Pt plates were used as the cathode and anode, respectively.
Epoxy films were deposited by the following schemes:
O2 + 2H2O +4e- → 4OH-

(Resin)3NH+ + OH- → (Resin)3N + H2O
In LbL assisted composite plating, charged Fe-Ni ultrafine particles are attracted to the cathode by electrostatic force simultaneously with insulator epoxy film electrodeposition. LbL treatment was performed using 1 wt.% PSS (Poly (sodium 4-styrenesulfonate)) and 0.5 mM NaCl solution, and 1 wt.% PDDA (poly (diallyl dimethyl ammonium chloride)) and 0.5 mM NaCl solution. Fe-Ni ultrafine particles were alternately immersed in PSS solution and PDDA solution.
In water Fe-Ni ultrafine particles before LbL treatment have 20 mV zeta potential. After immersion in PSS solution, Fe-Ni ultrafine particles have -40 mV zeta potential, and after immersion in PDDA solution, Fe-Ni zeta potential increases to 60 mV. This increase of zeta potential means that the amount of charged electrons on the ultrafine particles surface were increased through a layer by layer structure of cationic and anionic polyelectrolytes formed on the particles. The Fe-Ni ultrafine particles not treated with LbL precipitated in the reaction solution within 30 minutes, while the particles subjected to the LbL treatment were stable in the reaction solution over 16 days. Figure 1 shows the particle size distribution of Fe-Ni ultrafine particles with LbL treatment and without LbL treatment in water. The peak diameters of the distribution curves with LbL treatment and without LbL treatment were 35 nm and 52 nm, respectively. LbL treatment using PSS and PDDA were effective for increasing the charging electrons on Fe-Ni ultrafine particles and for enhancing the dispersibility of ultrafine particles in the reaction solution.
At electrophoresis of only Fe-Ni ultrafine particles without epoxy film on Cu substrate, the 4.3-fold increase in the mass of deposited particles with LbL treatment was observed compared with no LbL treatment, as a result of the increase of charging electrons on Fe-Ni particles.
Electrodeposition of Fe-Ni ultrafine particle-epoxy composite films was conducted under a constant current density of 4 mA / cm2, and 5-minute deposition time using the LbL assisted composite plating method. The film thickness was measured from the cross-section SEM image of the films. The film thickness of the deposited film prepared from the reaction solution containing 3 g / L Fe-Ni fine particles without LbL treatment is about 30μm, whereas that prepared from the reaction solution containing 3 g / L Fe-Ni particles with LbL treatment is about 80 μm. From the EDX mapping image of the film prepared using Fe-Ni particles without LbL treatment, the agglomerates of Fe-Ni particles were observed. On the other hand, there are no agglomerates in the film prepared using Fe-Ni particles with LbL treatment.
These aggregates of Fe-Ni particles not treated with LbL were generated due to less electron charging on the particle surface than particles having LbL treatment, and act as the growth inhibiter of the epoxy film. Therefore, the film thickness of the films prepared using Fe-Ni particles not treated with LbL was less than that of the film prepared using Fe-Ni particles having LbL treatment.
Figure 2 shows the Fe-Ni content in the films dependence on the concentration of Fe-Ni ultrafine particles in the reaction solution. The Fe-Ni content in the films increased in proportion to the Fe-Ni concentration in the reaction solution. Using LbL treatment, the Fe-Ni content in the films increased 1.6 times compared to that without LbL treatment. Over 22 vol. % Fe-Ni ultrafine particles included in epoxy films were successfully obtained using LbL assisted composite plating from the 3 g / L Fe-Ni content reaction solution.
The experimental results clearly show the dispersibility in the reaction solution could be greatly improved using LbL treatment for Fe-Ni ultrafine particles, and Fe-Ni ultrafine particles uniformly dispersed in epoxy film could be fabricated using our newly developed electrochemical method.

Figure 1