33 research outputs found
Foamed Nanocomposites for EMI Shielding Applications
INTRODUCTION : The addition of nanoparticles having specific properties inside a matrix with different
properties creates a novel material that exhibits hybrid and even new properties. The
nanocomposites presented in this paper combine the properties of foamed polymers
(inexpensive, lightweight, easy to mould into any desired shape, etc.) with those of carbon
nanotubes (CNTs). The addition of any conductive nanoparticles to an otherwise insulating
matrix leads to a significant increase of the electrical conductivity. But CNTs have a very
high aspect ratio; a much lower content of CNTs is therefore required to get the same
conductivity increase as the one obtained with more compact nanoparticles.
This is especially interesting for EMI shielding materials since, as will be explained in
further details in this chapter, it is desirable for such materials to have a high conductivity
but a low dielectric constant, in order to minimize the electromagnetic power outside the
shield casing but also to minimize the power reflected back inside the casing, as is explained
in section 2. In particular, two parameters of interest when comparing shielding materials
are detailed and discussed.
The polymer/CNTs nanocomposites were fabricated and characterized using a two-step
diagnostic method. They were first characterized in their solid form, i.e. before the foaming
process and the most interesting polymer matrices (with embedded CNTs) could be
selected. This way, only the promising blends were foamed, therefore avoiding the
unnecessary fabrication of a number of foams. These selected blends were foamed and then
characterized. The samples, both solid and foamed, are described and their fabrication
processes are briefly explained in section 3 while the characterization methods are shown in
section 4.
A simple electrical model is given and explained in section 5 and an optimized topology for
the foams is also proposed in the second part of the same section.
The measurement results for the solids and for the mono-layered and multi-layered foams
are summarized and discussed in section 6. They are then compared to results obtained
using the electrical model presented in the previous section and they are also correlated to
rheological characterizations
Wideband electrical characterisation of carbon-based nanocomposites
Nanocomposite materials are composed of two or more different materials having different physical properties forming distinct ‘phases’. At least one of these constituents has dimensions of the order of the nanometre and the properties of these nanoparticles lead to the creation of a unique composite material. In particular, nanocomposites based on high aspect ratio particles like carbon nanotubes should exhibit outstanding mechanical, thermal and electrical properties at very small particle content. Carbon-based nanocomposites are a rapidly growing field of investigation. The range of available shapes, types and properties of these composites is constantly increasing and that of their possible applications is wider still.
In this thesis, the wideband electrical properties of these nanocomposite materials are investigated. First a thorough examination of the literature is presented. Different characterisation techniques are developed and their adequacy with the nanocomposite characteristics is discussed. A wideband, from DC to 40 GHz characterisation of polymer-CNT composites is presented in details and used to extract information on the material microstructure, as well as the influence of the nanoparticle content on the electrical properties. Specifically designed nanocomposites for particular applications are presented; from composite foams for EMI shielding applications to dispersed solutions of carbon particles exhibiting a non-ohmic behaviour. Finally, nanostructured membranes for fuel cell applications are also shown.
The particular aim of this thesis is to find the best adequacy between characterisation method, material properties and required characteristics for a given application.(FSA 3) -- UCL, 201
Some features of bulk melt-textured high-temperature superconductors subjected to alternating magnetic fields
peer reviewedMonolithic, large grain, (RE)Ba2Cu3O7 high-temperature superconductors (where RE denotes a rare-earth ion) are known to be able to trap fields in excess of several teslas and represent thus an extremely promising competing technology for permanent magnet in several applications, e.g. in motors and generators. In any rotating machine, however, the superconducting permanent magnet is subjected to variable (transient, or alternating) parasitic magnetic fields. These magnetic fields interact with the superconductor, which yields a reduction of the remnant magnetization. In the present work we quantify these effects by analysing selected experimental data on bulk melt-textured superconductors subjected to AC fields. Our results indicate that the non-uniformity of superconducting properties in rather large samples might lead to unusual features and need to be taken into account to analyse the experimental data. We also investigate the evolution of the DC remnant magnetization of the bulk sample when it is subjected to a large number of AC magnetic field cycles, and investigate the experimental errors that result from a misorientation of the sample or a mispositioning of the Hall probe. The time-dependence of the remnant magnetization over 100000 cycles of the AC field is shown to display distinct regimes which all differ strongly from the usual decay due to magnetic relaxation
Foamed nanocomposites for EMI shielding applications
INTRODUCTION : The addition of nanoparticles having specific properties inside a matrix with different
properties creates a novel material that exhibits hybrid and even new properties. The
nanocomposites presented in this paper combine the properties of foamed polymers
(inexpensive, lightweight, easy to mould into any desired shape, etc.) with those of carbon
nanotubes (CNTs). The addition of any conductive nanoparticles to an otherwise insulating
matrix leads to a significant increase of the electrical conductivity. But CNTs have a very
high aspect ratio; a much lower content of CNTs is therefore required to get the same
conductivity increase as the one obtained with more compact nanoparticles.
This is especially interesting for EMI shielding materials since, as will be explained in
further details in this chapter, it is desirable for such materials to have a high conductivity
but a low dielectric constant, in order to minimize the electromagnetic power outside the
shield casing but also to minimize the power reflected back inside the casing, as is explained
in section 2. In particular, two parameters of interest when comparing shielding materials
are detailed and discussed.
The polymer/CNTs nanocomposites were fabricated and characterized using a two-step
diagnostic method. They were first characterized in their solid form, i.e. before the foaming
process and the most interesting polymer matrices (with embedded CNTs) could be
selected. This way, only the promising blends were foamed, therefore avoiding the
unnecessary fabrication of a number of foams. These selected blends were foamed and then
characterized. The samples, both solid and foamed, are described and their fabrication
processes are briefly explained in section 3 while the characterization methods are shown in
section 4.
A simple electrical model is given and explained in section 5 and an optimized topology for
the foams is also proposed in the second part of the same section.
The measurement results for the solids and for the mono-layered and multi-layered foams
are summarized and discussed in section 6. They are then compared to results obtained
using the electrical model presented in the previous section and they are also correlated to
rheological characterizations
High frequency characterization of polymer membranes with embedded carbon nanotubes for fuel cell applications
Investigation of Ionic Conductivity in Track-etched Nanoporous Polyimide Membranes using a Microwave Technique
The in situ characterization of membranes for fuel cell
applications is a rather complex and time-consuming procedure. It is
therefore interesting to develop simple, short, and reproducible
characterization tests and to limit in situ measurement only to the most
promising materials. In this paper, a new characterization method is
proposed to extract the DC ionic conductivity of track-etched
nanoporous polyimide membranes from microwave measurements