Microwave assisted Processing for the Realization of [631230]

Microwave assisted Processing for the Realization of
Nano -Ceramics Susceptors from SiC

Hathazi Francisc – Ioan
University of Oradea , Faculty of Electrical Engineering and Information Technology
Oradea , Universit ății st., no.1 , Rom ânia
[anonimizat]

Abstract —The Interest in the manufacture of nanostructured
ceramic devices has increased lately, due to miniaturized
requirements, multifunctionality and improved reliability. The
main obstacl e to achieving the full potential of susceptible nano –
ceramics is to prevent unwanted growths, while high
densification is achieved during conventional high – temperature
processing. In these experiments, detailed studies have been
carried out on the man ufacture of microwave assisted ceramic
susceptors.
Keywords — high frequency electromagnetic field; nano –
ceramic susceptors
I. INTRODUCTION
The susceptible nano – ceramic industry is driven by
technological and innovation challenges. Susceptible ceramics
provide the basic components to support a variety of products,
being used both as active components for temperature control
or as passive components. Consequently, the individual size of
susceptible ceramic equipment has declined steadily, despite
the manu facturing difficulty, which makes materials typically
subject to classical physics, the modern industry is driven by
miniaturization. This requires reducing the size of all active
and passive components to micrometric scale without
compromising performance and reliability. One of the best
possible ways to achieve component size reduction is the use of
multiple layers of ceramic in one block. Multi -layered ceramics
offer more advantages in terms of performance, reliability, cost
compared to cast -ceramic in a single block. Susceptible
ceramics are thermal energy storage devices in the vast
majority of applications in which are used . Therefore, the way
to increase storage capacity is either by improving the
dielectric permittivity of the ceramic material, or by reducing
the thickness of the dielectric ceramic layers. This requires the
development of new high permittivity materials and processing
techniques in order to obtain thinner layers of ceramics without
damaging the performance of the assembly.
II. FUNDAMANTAL CONSIDETRATIONS
Gases, liquids and solids can interact with microwaves and
can be heated, under certain conditions the gases can be
excited by microwaves to form plasma which is also useful for
processing [1]. Microwaves can be reflected, absorbed and / o r transmitted through materials. Reflection and absorption
requires microwave interaction with the material, while
transmission is the result of partial reflection and incomplete
absorption. During the interaction, heat energy is generated in
the material mainly by absorption. Microwaves can interact
with materials either by driving processes or by polarization
[2].
Conventional heating is typically done with electric
furnaces that allow temperatures up to 2. 700 ° C to be reached.
The main variables of the process are the combustion program
used (temperature vs. time) and the atmosphere [3]. The most
commonly used furnace for conventional heating is the
electric resistance oven, where the resistor is the heater
element of the furnace supplying the heat. The maximum
furnace and atmosphere temperature is limited by the heating
elements used [4]. Under conventional heating, heating takes
place due to thermal radiation and depends on the thermal
conductivity of the processed materials. Since metals have a
high th ermal conductivity, heating is easier and more uniform,
while ceramic, due to low thermal conductivity, takes longer
to obtain uniform heating [5]. In order to obtain uniform
heating in ceramics in an resonable time, the microwave
heating will be used wher e the heating is independent of the
conductivity of the material.

Fig. 1. Conventional heating vs microwave heating

The experiments and studies have shown significant
improvements in the microwave heating process, this
including very fast heating rates, much lower sintering
temperature, accelerated and much improved diffusion rates,
high production yields. Microwave heating is different from

the conventional one [6]; in microwave technology, heat is
internally generated through the interaction between th e high
frequency electromagnetic field and the material, however,
microwave heating control is more complicated than
conventional heating. Heating depends not only on the
operational parameters of the magnetron, but also on the
electrical and thermal prope rties of the material, one of which
is that some of these parameters change with temperature [14].
Microwaves are subject to optical laws and may be
transmitted, absorbed or reflected by type of material as
shown in Figure 2. When materials are heated usin g high
frequency electromagnetic waves, heating is usually produced
by the interaction between the components and the particles
loaded into the material. Therefore, steering and polarization
mechanisms are responsible for heating. Interaction between
micro waves and materials and the ability to produce heat in
some materials is primarily governed by the electrical and
magnetic field of the components, the amplitude, the phase
angle and the wave propagation capacity [15].

Fig. 2. Interaction of microwaves with matter
Microwaves penetrate metals only in thin layers of the
order of 1 μm. Continuous metals can be considered to be
opaque to microwaves or to be good reflectors of microwaves,
most electrical (or dielectric) ceramics such as Al2O3, MgO,
SiO2, and glasses are transparent in the microwave, however,
if they are heated above a certain critical temperature Tc, they
begin to absorb high -frequency electromagnetic radiation.
Other ceramics, such as Fe 2O3, Cr 2O3 and SiC, absorb high
frequency electromagneti c radiation with high efficiency [7].
The important properties of an material for interactin are
permitivity,  (for a dielectric material) and permeability, 
(for magnetic material). Typically, the relative permittivity is
used r =  / 0 and relative permeability r =  / 0 [8].
. When microwaves penetrate the material, the high
frequency electromagnetic field induces translational
movement in free, bound and dipole loads. The induced
motion is strong because it causes the system to move away
from the natural balance, this resistance, due to the friction
forces, elastic and inertial, leads to the dissipation of energy
[9]. As a result, the electric field associated with microwave
radiation is attenuated and heats the material. The loss tangent
(tanδ) is used to represent the losses generated by these
mechanisms.
(1)
Average power dissipated per unit volume of material is
given by:
(2)
According to equation 2, the power absorbed by the
material depends on:
– frequency and the square of the electric field
amplitude;
– dielectric permitivity and material loss tangent;
– E depends on the size, geometry and location of
the material in the microwave o ven cavity and
the design and volume of the cavity.
As the electric field is attenuated, the penetration depth (D)
of the microwaves is an important and useful parameter, the
penetration depth at which incident power is reduced by half.
(3)
Silica carbide, SiC, is the only stable chemical composition
of silicon and carbon, for convention, the term carbide is
applied because Si possesses a lower or carbon -like
electronegativity. Both the silicon atoms and the carbon atoms
are tetravalent and since the electronegativity difference
between Si and C is reduced, the bond is essentially covalent.
As evidence of strong covalent bonding, silicon carbide has
hardness and high melting point, SiC samples have a very high
binding energy. Due to its high dispersio n point, SiC is
considered as refractory carbide [10].
The SiC susceptible ceramic is an extremely important
material in the industry and has been exploited for unique
chemical and mechanical properties at high temperatures. The
measured properties tend to show a large spread in the
reported values, [11] strongly depending on how the SiC
sample is synthesized and processed, its purity, politope and
its morphology. However, silicon carbide ceramics presents
superior mechanical properties such as high hardnes s and
strength, especially at high temperatures and extreme wear and
tear resistance, with the only disadvantage of being brittle.
III. EXPERIMENTS AND RESULTS
Several experimental approaches have been adopted, taking
into account the behavior of the reagent, a nd have been
improved using a trial and error approach. Heating of the single
pellet reactant was unsuccessful and the addition of susceptors
such as carbon was generally inefficient if silicon flour was not
used to surround the reaction vessel and to prov ide thermal
insulation. The synthesis was attempted in the microwave oven
having a multimodal cavity. The furnace where the
experiments were carried out comprise a variable power
magnetron so the experiments were performed at different
powers and the mater ials exposed to the experiments were
tested at different times and temperatures [12].
The stoichiometric amount of silicon and active carbon
(0.300 g sample weight) were ground in a ball mill. The
selected samples were subjected to the same procedure, othe r

than the active carbon, these being replaced with graphite, the
powders milled were mixed with distilled water and cold
pressed (5 tons, for 10 minutes) in a 15 mm mold. The pressed
samples were embedded in graphite (acting as microwave
susceptor) in a 1 7mm silicon open tube, which was surrounded
with low dielectric loss silicon flour, Figure 3.

Fig. 3. – Schematic representation of the reaction
Sample Reactive
Carbon Power of
the
magnetron
[W] Microwave
irradiation
[min.] Volume
of added
water
[ml.]
1 granular
active
carbon 250 5 0,25
2 granular
active
carbon 250 15 0,25
3 granular
active
carbon 250 30 0,25
4 granular
active
carbon 500 5 0,45
5 granular
active
carbon 500 15 0,45
6 granular
active
carbon 500 30 0,45
7 granular
active
carbon 750 5 0,55
8 granular
active
carbon 750 15 0,55
9 granular
active
carbon 750 30 0,55
10 graphite 1.000 5 0
11 graphite 1.000 15 0
12 graphite 1.000 30 0

The samples pressed from the initial reactions were visibly
cracked, the color of the samples chang ed from dark gray to
gray / green, the active carbon with the least added water as binder is dominated by the reflections and the cubic
modification of the silicon carbide. It should also be noted that
graphite reflections are almost certainly from suscept ible dust
that has not been successfully removed from pressed samples
[13]. If the volume of water added as a binder to the mixture is
increased, it can be seen from the models that the conversion
into silicon carbide takes longer. While in small water vol ume
sets of β -SiC are visible for reaction times of five minutes, in
this case there are no traces of silicon carbide under five
minutes. Beyond this reaction time, it is possible to obtain
monophosphate β -SiC and, if the power is maintained for 30
minutes , the product does not oxidize or change with respect
to phase composition or crystallinity.
With graphite as a carbon source, dried pressed powders
have proven to be the only way to obtain parallelepipedal
shapes that have maintained their integrity durin g pressing,
adding any volume of water in these cases (even <0.10 ml) has
produced suspensions which could not be properly embedded
in the mold. Even dry samples, however, were difficult to
process, when graphite replaces granular carbon as a carbon
source , it is possible to obtain silicon carbide after 5 minutes.
After 30 minutes for β -SiC is clear that the evolution in the
SiC phase fraction over time is more predictable using
granular activated carbon and the β -SiC mono -phase phase
can be produced using activated charcoal for irradiation times
of 5 minutes and more much [16]. The better yield of β -SiC
from the reactions starting from the granular activated carbon
could be due to the contributing factors such as the better
dielectric properties of the gran ular activated carbon compared
to the graphite and the added influence of the added water to
the reaction mixture.

Fig. 4. Carbon images captured with a metalograph
microscope

IV. CONCLUSIONS
Selection of Si and C sources, use of water as a binder and
choice of source of high frequency electromagnetic radiation
(power, cavity) are variables that can be successfully used to
control the purity of silicon carbides and processing time. In
particular, SiC synthesis was obtained in a time period o f 5 ÷
30min, by designing synthesis experiments that do not require
reactive environments. Furthermore, for the first time, no
further purification steps were required, as the reactions can
lead to SiC near phase.
Significantly, the mode and hence the micr owave heating
rate can be exploited to dictate the morphology of carbons
from nano -wires to large crystals. Using more sophisticated
higher microwave power methods, beyond improving product
purity, reducing processing complexity and reducing
processing tim es.
In order to obtain complete control over phase formation,
crystal growth, and to gain a more in -depth understanding of
the reaction mechanism, it will be necessary to develop
cutting -edge measurement techniques. The fact that these
reactions can be car ried out in the ambient it is important in
designing convenient and relatively cheap procedures.
ACKNOWLEDGMENT
This work was co financed from the European Regional
Development Fund through Sectoral Operational Program
“Increase of the Economic Competitiven ess” – POS-CCE
2007 -2013, project number POS -CCE 1843/48800, "Increasing
the capacity of research – development of the interdisciplinary
laboratories for the technologies in electrical engineering",
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