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Theoretical Analysis of Microwave Sintering of Ceramics M. Zamrun Firihu
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Percepatan Reaksi Kimia dengan Pemanasan Mikrowave I. N. Sudiana dan M. Zamrun Firihu
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Ponderomotive Force Generated by Microwave During Sintering M. Zamrun Firihu dan I. N. Sudiana
44-48
JURNAL APLIKASI FISIKA
VOLUME 11
NOMOR 2
AGUSTUS 2015
Ponderomotive Force Generated by Microwaves During Sintering M. Zamrun Firihu dan I.N. Sudiana Jurusan Fisika, Fakultas Matematika dan Ilmu Pengetahuan Alam, Universitas Halu Oleo, Kendari, Sulawesi Tenggara, 93231 E-mail :
[email protected]
ABSTRAK Dalam beberapa tahun terakhir, banyak peneliti telah melaporkan pengamatan dari "efek microwave" di berbagai proses material seperti pada sintering, annealing, sintesis, Istilah efek microwave tidak didefinisikan secara ketat. Sering digunakan untuk menandai perbedaan hasil ketika pemanasan microwave digunakan dibandingkan dengan pemanasan biasa dengan pemanas listrik atau gas. Efek microwave dapat dijelaskan dalam dua kategori yakni efek termal dan efek non termal. Banyak hasil eksperimen menunjukkan adanya efek nonthermal dari microwave yang meningkatkan laju difusi atom di zat padat. Dalam teori sintering, atom berpindah karna adanya gaya pemicu (driving force) sebagai penyebabnya misalnya karna perbedaan tekanan permukaan, suhu, gravitasi, dll. Para ahli meyakini bahwa selama pemanasan dengan microwave ada gaya tambahan yang menyebabkan atom berdifusi lebih cepat akibat kehadiran medan listrik yang dinamai ponderomotive force. Dalam tulisan ini dibahas gaya ini terkait data ekperimen yang telah diperoleh beberapa tahun terakhir. Keywords: efek mikrowave, efek nonthermal, poderomotive force, difusi atom I.
deposition which provides, in particular,
INTRODUCTION
a
Many researchers have reported
possibility
of
faster and
more
controllable temperature ramp-up and
observations of "microwave effects" in a
its
variety of material processes [1-3].
selectivity
which
can
provide
concentration of energy deposition in
Microwave effect is sometime used to
the desired region result in precision
mark the enhancement of the processing
heating. All the listed peculiarities of
rate when microwave heating is utilized
microwave heating can be treated as the
such as enhancement of densification,
thermal action of the electromagnetic
atomic diffusion, chemical reaction rate
field on matter. All microwave effects
[3]. In principle, the potential benefits of
can be explained specifically to thermal
microwave heating, caused by the
and nonthermal effect.
volumetric nature of microwave energy
44
Ponderomotive Force……………………………………………………………………………(Zamrun,dkk)
Numerous experimental results suggest
existence
of
a
processes in ionic crystals subjected to a
specific
microwave field [4]. The basic idea was
nonthermal action of microwaves on
comparatively simple, based on the
mass transport in crystalline solids [4].
effect of rectification of the oscillatory
The characteristics of porosity under
vacancy drift near grain boundaries and
microwave heating were found to differ
interfaces. The additional driving force
considerably from those observed in a
is known as ponderomotive force.
conventional sintering process under the same conditions. A direct influence of the microwave field on the transport phenomena in crystals was found in investigations of atomic diffusion [1], as well as in a study of the observation of a quasi-stationary electric current induced in a dielectric subjected to pulsed microwave irradiation [5]. However, the physical mechanisms of the effect are not completely clear. Some researchers proposed the additional driving force exist during microwave processing. Especially,
Rybakov
and
Semenov
proposed the first model being able to explain quantitatively the existence of an additional driving force for transport
II. RESULTS OF MICROWAVE PROCESSING OF MATERIAL Microwave enhanced sintering has been reported by several scholars [1-3,6-7 ]. It has received much attention because of the observed substantial decrease in sintering temperature and fast heating rates. Tian, et al.[6], have performed sintering of alumina at 1700 oC for 12 minutes. They reported that the sintered samples achieved 99.8% of theoretically density, with fine grain of 1.9 m. The effect of microwave is not only in ceramic
processing.
Microwave
enhanced drying have been also reported [8-9]. Some microwave enhancement results are shown in Fig 1 to 3.
Fig. 1. Drying curves of cocoa beans in a microwave and in an electric furnace [8] 45
JAF Vol 11 No. 2 (2015) 48-52
Figure 2. Effect of microwave frequency and green sample in microwave sintering alumina [10]
Fig. 3 Reduction of open and closed porosity of silica xerogel upon MMW as compared to conventional sintering [2] III.
free surfaces are replaced by lower
DISCUSSIONS
Because of all results are temperature
energy sites such as grain boundaries or
activation
be
crystalline regions. The formation of
associated to enhancement of diffusion.
these low-energy sites (neck region),
Sintering lowers the surface energy of
and subsequent reduction in surface
material by reducing surface area with
area. This reduction results in a decrease
concomitant formation of interparticle
in the overall surface energy and known
bonds. During sintering, high-energy
as driving force:
process,
they
can
(1)
46
JAF Vol 11 No. 2 (2015) 44-48
where G is free energy, s is specific
For drnsification, by simplifying of
surface energy, and A is surface area.
diffusion process using two particle
The stress associated with the curved
model the relation of densification to
surface as
material transport parameters can be expressed by following equation:
(2) where R1 and R2 are principal radii of curvature for this surface.
(3)
ziF rmscRT2πVmx2a33/2t3/2
Flux of atom, J, is product of inter
Where
atomic distance, reaction rate constant,
=
density,
L=
material
dimension, Q= activation energy, R is
and diffusing concentration.
the
universal
gas
constant,
T=temperature, Do is diffusion constant, s is specific surface energy, zi = charge Diffusion path of mass transport during
on the ion, F =Faraday constant = 9.65
sintering of crystalline materials can
kJ/ V, Erms = root mean square of the
occur by at least six mechanisms: vapor
electric field of microwave, A is the
transport
cross-sectional
(evaporation/condensation),
area
over
which
(volume)
diffusion occurs and Vm is the molar
diffusion, grain boundary diffusion, and
volume of the material being transferred,
plastic flow [25].
x is radius of neck, a is radius of
surface
diffusion,
lattice
particle, and t is time. Component is microwave contribution on atomic transport during sintering. It is driving force generated by electric field of microwaves. It should enhanced atomic transport during sintering. Application
of ponderomotive effect
was shown in experiment performed by Rybakov [11]. It was shown that a microwave field with the E-vector
Figure 4. Possible atomic diffusion ways during sintering [10] The enhancement shown in Figure 1-3
directed tangentially to a surface of a solid can develop a deformation-type
reported by researchers is indicated that
instability. This results in the formation
microwaves enhanced diffusion rate
of a corrugated profile on that surface
(mass transport flux) during processing.
with
47 45
JAF Vol 11 No. 2 (2015) 44-48
the spatial period on the order of 1 micrometer
controlled
adjusting
the
period
microwave
[3]
Zamrun, M F., I. N. Sudiana, Microwaves Enhanced Sintering Mechanisms in Alumina Ceramic Sintering Experiments, Cont. Eng. Sci, Vol. 9, 2016, 5, 237 – 247 [4] Rybakov, K.I. and V.E.Semenov, Physical Review B , 52[ 5], 3030 (1995). [5] S.A. Freeman, J.H. Booske, and R.F. Cooper, Phys. Rev. Lett., 74 [11], 2042 (1995). [6] Sudiana, I.N, Use of Microwave Energy for Material Processing in A Simple Laboratory, Jurnal Aplikasi Fisika, Vol. 10 No. 2, Oktober 2014, Hal. 77-81. [7] Y. L. Tian, D. L. Johnson, M. E. Brodwin, Ultrafine Microstructure of Al2O3 Produced by Microwave Sintering, Cer. Pow. Sci. II B. pp. 925932(1988). [8] Sudiana, I.N, S. Mitsudo, L. O. Ngkoimani, L. Aba, H. Aripin, I. Usman, Fast Drying of Cocoa Bean by Using Microwave, CICES2014, November 10-11, 2014, Kendari, Indonesia. [9] McMinn W. A. M., Khraisheh M. A. M., Magee T., R. A., Food Research International, 36, pp. 977-983 (2003). [10] M.F. Ashby,First report on Sintering Diagram, Acta. Met. 22, pp.275-289(1974). [11] K.I. Rybakov and V.E. Semenov, Materials Research Society Symposium Proceedings, Vol. 430, Pittsburgh, PA, 1996), p. 435.
by
power.
However no direct experiment can demonstrate a ponderomotive forcedriven mass transport up to now. One of important tasks in this field of research is now to demonstrate a ponderomotive force-driven mass transport in a direct experiment. IV. CONCLUSION A
nonthermal
action
of
microwaves which enhances diffusion in solids appears to be reasonable. As follows
from
the
theoretical
and
experimental results obtained for ionic crystalline solids and drying it can be viewed in terms of an additional driving force
(ponderomotive
experimental
results
on
force).
The
microwave
sintering indicate supports the theory. However no direct experiment can demonstrate a ponderomotive forcedriven mass transport up to now. REFERENSI [1]
[2]
Janney, M.A, H. Kimrey, W. Allen, J. Kiggans,1997. Enhanced diffusion in sapphire during microwave heating, J. Materials Science 32, 1347–1355. Sudiana, I.N., S. Mitsudo, T. Nishiwaki, P. E. Susilowati, L. Lestari, M. Z. Firihu, H. Aripin, Effect of Microwave Radiation on the Properties of Sintered Oxide Ceramics, Cont. Eng. Sci., Vol. 8 No. 34, 2015, pp. 1607-1615
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