STUDY OF THE BEHAVIOR OF CERAMIC MATERIALS EXPOSED TO MICROWAVES RELATED TO THEIRLATTICE

Idalia Gómez, Juan Aguilar

Universidad Autónoma de Nuevo León, Apartado Postal 076 F, San Nicolás de losGarza, Nuevo León 66450, México

ABSTRACT

This is a summary of a study about the behavior of two kinds of ceramicmaterials, 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 torelate the behavior of the exposed materials to their lattice.

INTRODUCTION

There has been many discussions about the mechanism that governs themicrowave energy absorption, although dipole losses, ion jump relaxation andohmic effects [Binner, 1993] have benn proposed, the real contribution of eachmechanism is still unknown. In this study we compared CaZrO3 against BaTiO3(perovskites), and MgAl2O4 against ZnAl2O4 (spinels). The results show that therole of the species themselves are more important that the lattice that theyconform.

Perovskite

Some ionic ceramics are complex, they are based on a cubic system with more thantwo types of ions within the unitary cell. Both, the octahedral and tetrahedralsites are either partially or fully occupied by ions. The lattice of the calciumzirconate is a perovskite, in which the ions Ca2+ and O2- are combined forconforming a compact cubic lattice with the Zr4+ located over the octahedralsites. Each oxygen is surrounded by four calcium ions, at the center of theunitary face centered cubic cell the zirconium is coordinated with six oxygenions.

Spinel

The spinel structure is based over a super lattice made of oxygen ions in a facecentered cubic cell with just one fraction of the octahedral and tetrahedralsites occupied. The compounds with an stoichiometry AB2O4 in which A and Bcations are 2+ and 3+ valence respectively are part of the spinel structure. Theunitary spinel contains eight face centered cubic sublattices of oxygen. Half ofthe octahedral sites and one eighth of the tetrahedral ones are occupied. Theregular spinel (AB2O4), such as MgAl2O4, the Al

3+ cations are in the octahedralsites and the Mg2+ cations are in the tetrahedral ones.

EXPERIMENTAL PROCEDURE

The diagnosis experimental design was 2f type, the chosen variables were compactload (none and 1500 Kg/cm2), applied power (1000 and 2000 W) and exposed mass (8and 16 gr.)For obtaining the magnesia-alumina spinel the samples were compacted by a load of1500 Kg/cm2, the power levels were 1500 and 2000 W and the masses were 7 and 14gr. The mixture ratios were stoichiometry 1:1 with a graphite bed of 0.5 gr.

because it is necessary to take the reactives to a temperature where they becomemicrowave absorbents. The materials employed and their characteristics arepresented in the Table I. The samples had a disc shape with the dimensions thatare presented in Table II.

Table I. Particle size of theemployed materials

Material Particle size(µµµµm)

ZnO ≅3BaCO3 ≅1TiO2 ≅3ZrO2 ≅10CaO ≅1MgO ≅1Al2O3 ≅1

Table II. Dimensions of the samplesto be processed

Test and mass(gr.)

diameter(cm)

height(cm)

CaZrO3: 8 2.9 0.45CaZrO3: 16 2.9 0.89MgAl2O4: 7 2.9 0.63

The perovskite BaTiO3 and the spinel ZnAl2O4 were produced at a single power of1500 W with masses of 9 and 16 gr. respectively from the materials that weredescribed in Table I.After an analysis of the interaction of a high alumina crucible with the sampleit was decided to keep it as a container for the graphite and the sample, thewhole system was insulated with ceramic fiber for decreasing the heat losses. Theaverage characteristics of the crucible were: Weight: 115 gr.; volume of thecrucible: 41.8 cc; density: 2.75 gr./cc; capacity: 26.9 cc.The generator was a 3 KW variable power magnetron at 2.45 GHz, the waveguide wasa WR284 type and the cavity was a 25.4 cm X 25.4 cm X 25.4 cm made of stainlesssteel.Several data such as temperature, forward and reflected power were recordedduring the test. Taking the temperature under an electromagnetic field is rathercomplicated because thermocouples can disturb the microwave pattern and theachieved temperatures were too high (around 2000 °C) so it was decided to useoptical pyrometry, due to the unknown emisivity factor there is an error involvedin using such technique, however we were adjusting this value against a referencetemperature. Other aspect is that the cavity has a screen as a door, which isactually one of the walls of the cavity and is interfering with the temperaturemeasurement. Further tests with and without the screen during cooling (withoutthe microwave field) allow to determine that the real temperature (comparedagainst thermocouples) the temperature that was read during the test actually isabout 200°C above the one given by the optical pyrometer. An analysis of thecavity shows where the maximum are located (this was also confirmed by moving thesample and by burning a polymer inside the cavity [A. García, 1997] so that thesample was sat in these places. The tuning was achieved with a four-screw tunerin such a way that the reflected power was minimum, having in this case themaximum absorption. The samples were analyzed by means of x rays diffraction andelectronic microscopy. The absorbed energy was estimated from both, the forwardand reflected power.

RESULTS AND DISCUSSION

Calcium zirconate

Although one of the most important results is the achieved reaction degree. TableIII shows the maximum achieved temperature in the experiments for determiningwhich one of the variables had the greatest influence. The graphic of normalprobability is presented in figure 1, although it seems obvious that the power isthe most important factor it is convenient to review the effect of the rest ofthe variables, and the possible interaction between them. Further analysis shows

that the compactatation variable does not have a significant effect over theachieved temperature.

Table III. Results of the experiments conducted for obtaining CaZrO3.Experiment A B C Tmax(

0C) Time forachievingTmax(

0C)A1 - - - 440 10'A2 + - - 392 10'A3 - + - 2204 4'30"A4 + + - 2010 4'30"A5 - - + 403 9'A6 + - + 223 9'A7 - + + 972 4'20"A8 + + + 1280 6'40"

where:

Variable\Level Low- High+A:Compact(Ton/cm2) 0 1.5

B:Power(KW) 1 2C:Mass(gr.) 8 16

Figure 5.7 MEB images of CaZrO3 obtained by means of microwaves at 1500 W.

Magnesia-alumina spinel

Table III. Results of the experiments conducted for obtaining MgAl2O4

Exp. Power (W) mass(gr.)

comp.@

1500Kg/cm2

Exposition timeand power

Tmax(°C) Time (minutes) forachieving Tmax (°C.)

B1 2000 6 no 3' @ 2000W3' @ 800W2' @ 1500W

2114 2:56

B2 2000 6.8 yes 2' 55" 1196 2:29B3 1500 13.7 no 14' 2127 5:07

BC2 2000 7 yes 5' 30" 2142 4:41BC3 2000 7.38 yes 8' 1931 7:09

10µm 10µm

REFERENCES

Aguilar J., González M., Gómez I. 1997. Microwaves as an energy source forproducing magnesia-alumina spinel. J. Microwave Power and Electrom Energy 2 (32),74-79

Binner J. The potential of microwave processing of ceramics, Materials World 1,(3), 152

García A. Irradiación de termoplásticos mediante microondas y su efecto sobre laadhesión en materiales compuestos. Thesis of Master of Science, UniversidadAutónoma de Nuevo León, 1997.