1. Author: E. Pascual San José
Directed by: I.G. Loscertales
March 13, 2015
Summary of the final project: ‘Design and development of a pilot system to produce
conductive micro-layers for EMI shielding of insulating thermoplastic materials
manufactured by electro-hydrodynamic techniques’
With telecommunication systems booming, it is essential to develop novel shielding
materials to shield electromagnetic waves. These materials are known as electromagnetic
interference shielding (EMI-S) materials. The use of EMI-S in electronic equipment
reduces undesirable effects such as electromagnetic interferences or electromagnetic
coupling in equipment partially or fully sharing the electromagnetic spectrum. In the
current project, we propose innovative solutions to classic shielding approaches like using
a metallic mesh or cage, developing new solutions based on thin conductive films. In
particular, conductive thin films applied on flat plastic sheets (PEEK, high performance
polymer) are developed and designed here. New composite materials are developed to
manufacture EMI-S boxes for electronic devices. They are primarily intended for use in
aircraft cabins, although they could be used in other industrial applications.
Within the project framework a deposition pilot system of conductive paintings is
developed. For that purpose, electrospray is the deposition technique selected to grow
thin films. Electrospray is a technique which employs an electric field to generate droplets
from a conductive liquid. Its main characteristics are a high control of generated droplets
up to a nanometric scale, simplicity and low-cost set-up, scaling-up potential, precision
and low power consumption. As-deposited thin films are doped with electrically
conductive carbon nanoparticles, which create a percolated network, providing a metallic
superficial character to the plastic sheet and thus enabling EMI-S.
Before depositing thin layers, conductive paintings were designed and developed,
different solvents, carbon nanoparticles and binders that allow generating stable carbon
nanoparticle suspension and exhibiting good nanoparticle adhesion to the substrate were
tested. After the conductive ink design, several deposition tests were performed analyzing
electrical conductivities of the sample, both EMI-S and electrical conductivity are related.
Carbon nanotubes (CNTs) were selected as the best nanoparticles due to its appropriate
quality/price ratio, epoxy resin (already accepted in aircraft industry) was selected as the
binder agent of the CNTs. CNTs were dispersed effectively in the polar solvent N-N
dimethylformamide (DMF). DMF was chosen due to its good properties to be sprayed with
the electrospray technique as well as to be compatible with the epoxy resin. Finally,
specimens were dried by assisted ventilation followed by post annealing in a vacuum
oven.
The first step of the scaling up was carried out to increase throughput x10. The main
atomization electrospray parameters were investigated to achieve a continuous and dense
film. As the investigation progressed, electrical conductivity of the samples increases by 8
orders of magnitude (and thus improving EMI-S) by increasing CNTs load, film thickness
as well as using complementary techniques to electrospray such as traditional air-driven
spray.
Finally, testing plan was designed and conducted to manufacture EMI-S micro films
deposited onto PEEK substrate with the optimized paintings. As-deposited specimens
were characterized by scanning electron microscopy (SEM). Furthermore, both electrical
conductivity and EMI-S were tested yielding 1 S/m (initially epoxy resin is dielectric) and
2 dB, respectively.