Akademik Veri Yönetim Sistemi
3rd International Symposium on Materials for Energy Storage and Conversion , Belgrade, Sırbistan Ve Karadağ, 10 - 12 Eylül 2018, cilt.1, sa.1, ss.69
Carbon
coating of metallic particles are of considerable interest in a number of contexts.
One is related to the high reactivity of nanoparticles below the size of 100
nm. It is, therefore, necessary to protect or passivate them so as to prevent
their reaction with air[1]. Due to this high reactivity, nanoparticles also
tend to agglomerate and, in the process, tend to lose the functionality arising
from their small size. For this and other similar purposes, carbon is an ideal
material for encapsulation due to its high stability even at extreme
conditions. The high stability of carbon both in acidic and alkaline
environments, together with their high electrical conductivity, add further
functionality to the carbon coating. This is particularly useful in some of the
active materials used in electrochemical energy storage as they often have low
electrical conductivity. Silicon anodes in Lithium-ion batteries is a
well-known example
where carbon encapsulation help improve the conductivity. Perhaps the most
critical need for encapsulation in electrochemical energy storage is related to
large volume changes that often occur during cycling[2]. This leads to particle
fragmentation and to the loss of active material resulting in the capacity
decay.
Carbon encapsulation is not only useful to constrain this volume change, but by
keeping the fragmented particles together and conductive help provide a
solution to the capacity decay.
Particles can be carbon
coated with a variety of methods. Although recently solution based methods became
quite common, traditionally this has been achieved via gas-phase synthesis.
These include such methods as carbon arc discharge, spark discharge, chemical
vapor deposition and thermal plasma. Magnetic nanoparticles Fe, Ni, Co were
successfully synthesized using thermal plasma where the particles were
typically between 20 to 50 nm in size wrapped with several layers of graphite.
Similar encapsulations were achieved for these metals using arc discharge or
spark discharge processes. Since encapsulation, regardless of the method used,
brings C and the element in contact with each other at high temperature, many
of the elements are converted into carbides. A detailed account of the possible
routes leading to encapsulation was given by Seraphin et al.[3] and Elliot et
al.[4]. They mainly center on two
distinct possibilities in the formation of carbon shell. One is the expulsion
of carbon trapped within the metal or carbide to the surface during
crystallization of the core; this is the so called inside-out growth model. The
other is direct deposition of carbon shell onto the particle from external
carbon source.
Figure 1. Three
categories in the encapsulation of elemental particles [5].
The current study makes use of two methods; spark discharge
and thermal plasma in obtaining carbon encapsulated nanoparticles. The study
showed that elements W, V, Ti, and Si formed carbides which were encapsulated
successfully by graphitic layers forming a sound core-shell structure. Copper
formed a partially filled core-shell structure, attributed to its relatively
low condensation temperature, where considerable shrinkage seemed to have
occurred after the encapsulation. Mg could not be encapsulated in a core-shell
form but rather yielded an embedded structure where Mg is condensed onto
already condensed carbonaceous material. Analysis of current observations
coupled with those already reported data imply a simple mechanism for
encapsulation. Metals/compounds that are solid above the condensation
temperature of carbon give rise to a sound core-shell structure. Elements whose
condensation temperature is less than that of carbon could still produce
core-shell particles but they may be partially filled. It is estimated that the
process of graphitic encapsulation may be complete around 1900 K and partially
filled core-shell structure might develop depending on the volume shrinkage
upon cooling to room temperature. Elements/compounds whose condensation
temperature is below the encapsulation temperature fail to develop core-shell
structure. Instead they form embedded composite structure.
During the presentation we will
concentrate on two anode materials; one is silicon within the context of Li-ion
batteries and the other is Mg within the context of for NiMH batteries