Conferences out of México
(refeered)
VALDEZ (Z.), GUILLEMET (S.), AGUILAR (J.), DURAND (B.), HINOJOSA (M.).-Conductivity
measurements of a spinel manganite ceramic in a microwave field. 9th Conference
AMPERE,
Loughborough, Inglaterra, Sept. 2003.
Abstract
Several models have been proposed to describe the mechanism of microwave
heating: dipole rotation, ionic jump, skin depth conduction. However, all
these models depend on the information provided by dielectrical, thermal
and indirect measurements. The results obtained from these measurement
techniques have not permitted to fully understand nor to explain the wide
variety of phenomena occuring when heating a material using microwave energy.
At present time, the study of physical properties in the microwave field
is better performed for measuring the complex part of permittivity.
This work presents and analyses an experimental technique, which allows
measuring the electrical conductivity of a ceramic material while exposed
to a microwave field.
The ceramic material used for this study is a mixed valence nickel-iron
manganite that is known to be an electrical semi-conductor. This ceramic
was placed in a wave-guide and a standing microwave field was applied to
allow the introduction of the electrodes in the wave-guide. By means of
an ohmeter, its electrical conductivity was measured at different temperatures.
This technique could help bringing light to some questions regarding
the mechanisms of microwave heating, especially when a Joule effect due
to ohmic conduction could be related to such mechanisms.
KHARISSOVA (O.), NIETO (I.), AGUILAR (J.), ORTIZ (U.), HINOJOSA (M.).-Condensation
of carbon vapour in the microwave oven. Aceptado para los Procedings de
MRS, Fall Meeting 2002. Nanomaterials for structural applications, 2003.
AGUILAR (J.), ORTIZ (U.), RODRIGUEZ (J.).-Characterization fo SiC produced
by means of microwaves. 8th Conference Ampere,
Bayreuth, Alemania, Sept. 2001.
Abstract
The purpose of this work is to present the characterization of silicon
carbide that was produced in our laboratory by means of microwaves as an
energy source, discussing process parameters such as power and achieved
temperature. This material was processed from a silicon oxide and graphite
mixture that was placed into an aluminum oxide crucible. The set was in
turn placed into a multimode cavity in a location chosen according to the
best heating rate. The power source was a magnetron operating at 2.45 GHz
and at output power up to 2000 W for times as long as 1500 seconds. The
mixture reached temperatures in the order of 2000°C, which is high
enough for having an appreciable reaction degree. The lattice was determined
by means of X-rays diffraction and the structure was observed by means
of scanning electron microscopy. Most of the product was B-SiC, which was
expected because the temperature was close to this polytype growth regime.
One aspect that is pointed out is regarding with the variation of the structure
related to the heating rate, despite the homogeneity of the lattice of
the formed product, the exhibited structure went from coarse grains to
whiskers needle shaped. Finally a comparison against SiC from an electrical
resistance and commercial SiC for fired products is shown.
More text.
VALDEZ (Z.), AGUILAR (J.).-Influence of Al2O3
on the production of MgAl2O4 with microwaves. 35th
Microwave Power Symposium, International
Microwave Power Institute, Montreal, Canadá. Jul. 2000, pp.
72-74
Abstract
A summary of a study about the influence of Al2O3
content in Al2O3-MgO mixtures on the amount of produced
MgAl2O4 is presented. It was considered that Al2O3
and MgAl2O4 improve their absorption capacity to
microwaves as temperature increases, therefore it was assumed that more
Al2O3 in the mixture would promote MgAl2O4
production when it is processed with microwaves. In order to quantify this
influence, comparisons between materials produced by microwave and conventional
heating at 1200°C were carried out. A discussion about this comparison
is included.
Más texto.
KHARISSOVA (O.), AGUILAR (J.), ORTIZ (U.).-Effect of Fe on sintering
of Al2O3-MgO-Fe2O3 spinel.
MRS,
Nucleation and Growth Processes in Materials, Vol. 580, pp. 105-110.
Abstract
The influence of Fe on the microstructure of Al2O3-MgO-Fe2O3
spinel is described in this work. Heating a mixture of initial oxides in
an electric furnace at 1400º C for 15 hrs produced the spinel. A ternary
diagram for concentration ranges: Al2O3 (5-75 mol%),
MgO (10-80 mol%), and Fe2O3 (3-70 mol%) summarize
the constitution of this spinel. Amount, composition and weight ratios
of each phase were determined by X-rays powder diffraction.
Microstructure of specimens was characterized by scanning electronic
microscopy and X-ray spectrometry. Distribution, size, shape of phases
and pores were examined. Hardness measurements of spinel completed this
study.
More text.
GOMEZ (I.), AGUILAR (J.).-Study of the behavior of ceramic materials
exposed to microwaves related to their lattice. 34th Microwave Power Symposium,
International
Microwave Power Institute, Washington, DC. Jul. 1999, pp. 5-8.
Abstract
This is a summary of a study about the behavior of two kinds of ceramic
materials, perovskite (CaZrO3)and BaTiO3)and spinel
(MgAl2O4 and ZnAl2O4), processed
by means of microwaves at about 2000°C. The chosen variables were;
mass, compact degree of the reactives and the applied power. The aim was
to relate the behavior of the exposed materials to their lattice.
More text.
AGUILAR (J.), PEARCE (J.).-Study of the thermal behavior of materials
exposed to microwaves achieving temperatures over 650°C, International
Microwave Power Institute, Chicago, ILL. Jul. 1998, pp. 47-50
Summary
In this work we present the result of the experiments conducted on
aluminum oxide and graphite, which are materials often encountered in many
ceramics, where we were able to take temperatures during the tests by means
of thermocouples, we also present a discussion about such technique and
the non- transparency of the container.
One application of the microwaves that has became popular is the heating
of materials, thus an important aspect is the thermal behavior of the materials
that are being exposed to a microwave field. The techniques for taking
temperatures directly are limited to a low range (maximum 250 °C),
very appropriate for water based materials, such as food, and recently
some polymers. The case of the ceramic materials is totally different because
the necessary temperature for conducting any process usually is far above
650°C. The ordinary techniques for taking temperatures at these conditions
are limited to optical pyrometry and some indirect evidence rather from
the processed material than during the processing itself. In this work
two materials, widely encountered in the refractory industry, were selected;
aluminum oxide and the graphite.
Más texto.
AGUILAR (J.), GOMEZ (I.), GONZALEZ (M.), MORALES (J.).- Description
of the heating behavior of some ceramic materials in a microwave field.
32nd Microwave Power Symposium, International
Microwave Power Institute, Ottawa, Canadá. Jul. 1997, pp. 38-41
Summary
This work is focused on the behavior of different ceramics exposed
to microwaves in contact with a metallic substrate. The idea is to evaluate
the influence of this metal on the heating, aiming to the production of
ceramic coatings "in-situ". We used a thermocouple shield as the metal
into the reagents. Achieved temperatures in this particular experimental
conditions suggest that reaction over a metal are feasible, even for materials
that are not to good for absorbing microwaves.
More text.
GOMEZ (I.), AGUILAR (J.), GOMEZ (I.).-Fabrication of calcium zirconium
oxide by microwave energy. 31st Microwave Power Symposium, International
Microwave Power Institute, Boston MA. Jul. 1996, pp. 100-103 Idem.
pp. 104-107
Summary
Calcium zirconium oxide, CaZrO3, was made by microwave processing.
The mixture of ZrO2 + CaCO3 was heated in the cavity
of a microwave oven (2.45 GHz, 800 watts). X-ray diffraction shown that
crystalline calcium zirconium oxide was obtained , after 20 - 60 minutes
of microwave heating. The results of this study demonstrated that calcium
zirconium oxide ceramics can be obtained with microwave radiation.
More text.
GONZALEZ (M.), GOMEZ (I.), AGUILAR (J.).-Microwave processing applied
to ceramic reactions. MRS Microwave Processing
of Materials V. Vol. 430, 1996, pp. 107-112
Summary
The energy necessary for ceramic and other materials processing usually
comes from fire or electricity. With fire we conduct the process at relative
low temperatures. In the case of the electric arc furnace, temperatures
are high enough to smelt the material and have a product. Processes such
as zirconium oxide stabilized with magnesium oxide and production of MgO
are good examples of the application of electricity. In any case, for conducting
a process it is necessary to supply energy, therefore we study how microwaves
can be employed as power source in the field of ceramics. There are extensive
researches on microwave sintering, but in this work, besides we use microwaves
for conducting reactions between oxides to produce ceramic materials, mainly
spinels. We are taking into account that nature of this process, interaction
wave-material, depends strongly on the kind of material. The objective
of this work is to show a summary of our studies about microwaves applied
to production of spinels. This work is part of a project related with microwave
processing of materials.
GOMEZ (I.), AGUILAR (J.), GONZALEZ (M.), MORALES (J.).-Kinetics of
reduction of iron oxides using microwaves as power source. MRS
Microwave Processing of Materials V. Vol. 430, 1996, pp. 423-428
Summary
This work deals with kinetic description of reduction of iron oxides
using microwaves as power source. Previous researches shown that it is
possible to conduct this kind of processes successfully, but real kinetic
comparisons have not been presented. We are looking for reactions which
do not take place when energy is supplied conventionally and that they
do when energy is supplied with microwaves even when temperature range
is about the same. Our research suggest that there is a microwave induced
transformation. This transformation is feasible from a thermodynamic view,
but normally it does not take place. We are finding out if these processes
are conducting when energy is applied by an energetic beam. The aim of
this work is to describe reduction kinetics, taking into account how the
iron oxide heats with microwaves compared with conventional heating and
to find out which reactions are taking place with microwaves and that do
not occur when conventional heating is employed.
AGUILAR (J.), GOMEZ (I.), MORALES (J.).-Microwaves for reduction of
iron ore. Beam Processing of Advanced Materials, TMS/ASM,
Cle. OH. Oct. 1995, pp.261-268
Summary
Results of reducibility tests of reduction conducted on iron ore pellets
with carbon are presented. Work done is centered on reduction kinetics
for a process where heat is supplied by microwaves. This work includes
a discussion related with conventional heating, and "cold centers" which
commonly are supposed avoided naturally with microwaves.
GOMEZ (I.), AGUILAR (J.).-Microwaves for reduction of iron ore pellet
by carbon. MRS Dynamic in Small Confining
Systems II, Vol. 366, 1995, pp. 347-352
Summary
Results of reducibility tests of reduction conducted on iron ore pellets
by carbon are presented in this work. Work done is centered on reduction
kinetics by carbon with conventional heat supply (heat transfer process)
compared with reduction kinetics where heat is supplied by microwaves to
whole volume. Ore used in this work is called "Alzada" which has reducibility
characteristics well known, and is often used as standard for comparison.
Importance of using actual pellet instead of dust is pointed. This work
is part of a project related with microwaves for reduction of metallic
oxides.
ORTIZ (U.), AGUILAR (J.), CAVAZOS (J.L.), VIRAMONTES (R.).-Carburization
of HP40 and AISI 304 Alloys by Reducing Gas Atmospheres. Low Carbon Steels
for the 90's,TMS/ASM
Pit. PA. 17-21 de Octubre de 1993. pp. 157-161.
Summary
This paper analyses the behavior of HP40+Nb steel (25% Cr, 35% Ni)
exposed to different corrosive atmospheres (oxi-carburizating) at high
temperatures (800°C and 900°C). Tests were carried out in a laboratory
reactor and a thermogravimetric analyzer (TGA). Thermodynamic conditions
for carburization and oxidation were calculated with a commercial software.
Weight increase as a function of time exhibits a parabolic shape (DW)2
= Kt, which agrees with other authors data, kinetic constant found to be
function of temperature and carbon activity. Parabolic shape at the beginning
of curves suggest that phenomena is controlled by diffusion. Stability
phase diagrams were thermodynamically calculated for Fe-Cr-Ni-C-O2
system for different temperatures. These diagrams show that, independently
of carbon activity, oxides are more stables than carbides. For high carbon
activity and low oxygen pressure the carbides are M7C3
and M4C. Further studies will permit to find kinetic boundaries,
taking into account actual species activity, instead of thermodynamic ones.
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