USE OF EPMA AND SEM IN THE STUDY OF HIGH TEMPERATURE REACTIONS IN OXIDE SYSTEMS

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USE OF EPMA AND SEM IN THE STUDY OF HIGH TEMPERATURE REACTIONS IN OXIDE SYSTEMS

M. Trontelj, D. Kolar, M. Kosec

To cite this version:

M. Trontelj, D. Kolar, M. Kosec. USE OF EPMA AND SEM IN THE STUDY OF HIGH TEM- PERATURE REACTIONS IN OXIDE SYSTEMS. Journal de Physique Colloques, 1984, 45 (C2), pp.C2-747-C2-750. �10.1051/jphyscol:19842172�. �jpa-00223846�

USE OF EPMA AND SEM I N THE STUDY OF H I G H TEMPERATURE REACTIONS I N OXIDE SYSTEMS

M. Trontelj, D . Kolar and M. Kosec

"JoHef Stefan" I n s t i t u t e , "E. KardeZj" University of LjubZjana, LjubZ jana, YugosZavia

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Abstract

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I - I t a m D E M O N

In m y cases, reactive diffusion leads to the formation of new phases with structu- ral and Mysical praperties different fran those of the starting material. When the partial diffusion coefficients of the individual mqonents in the reaction product are not the m, p h e n m a analogous to the Kirkendall effect take place. In such conditions, the f o m t i o n of a new phase takes place mainly or practically exclusi- vely a t one of the phase bomdaries only. 'Ikis is equivalent to the formation of pores and cracks a t the opposite phase bouncky. In oomplex heterqeneous systenrs several reaction products rray be fonred. Identification of the ccmpmds by X-ray diffraction is usually done after &librating the system and thus the c a p n m d s are the equilibrium ones. For identifying metastable phases different methods are essential. Metastable phases temporarily formed i n the system influence sintering and developmt of the microstructure of the material, and thus its properties.

Interdiffusion @en- can be mveniently elucidated by WlA and SEM.

h t h i s paper, such techniques are applied to the analysis of reactions which take place during formtion of canplex ferroelectric ceramics in the systems BaTiO3- SrTi03, BTi03-BaZr03 and PtJTi03-PbZr03.

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Diffusion i n the BaTiO3-SW3 and BaTi03-BaZr03 systems were studied i n a model axxisting of a sghere sintered onto a plate. (Fig. 1)

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19842172

C2-748 JOURNAL DE PHYSIQUE

m the analysis of PbTi03-PbZr03, a small p e l l e t of PbTi03 was pressed i n t o PwLr03 powder and sintered, covered with PbZr03 t o diminish evaporation of PW.

For model w r i n m t s the powder of one c m p n e n t was sintered and crushed. Fracti- ons of 200-125 urn were spheroidized in a H2/02 flarne a t 18030-2000O~. Spheres were fractioned by sieving and sintered t o dense, polished plates of another c v e n t . Samples were cut through spheres perpedicular t o t h e plate, polished and subjected to EPM and Sl analysis.

I11 - RESULTS BaTiO3-SrTi03

X-ray microprobe analysis of the neck forrned during sinterin9 of a p l y c r y s t a l l i n e BaTi03 sphere t o a polished SrTi03 plate a t 13& showed t h a t Ba diffused along the surface of SrTiOg and into its bulk. To maintain e l e c t r i c a l neutrality, it is assu- n ~ d t h a t 02- ions acccanpany the diffusion of Ba2+ ions o r t h a t oxygen is transported via the vapour phase. Penetration of Sr ions i n t o the BaTi03 sphere w a s much slower.

It may be further observed that the neck area b e m rich in titanium. Quantitative analysis of t h i s area revealed a BaO:Ti@ r a t i o of approximately 1:2.5.

Microstructural examination of contact regions formed between BaTi03 spheres and BaZr03 plates indicated the presence of liquid phase a t a f i r i n g temperature where t h i s was not expected. EPMA demmstrated preferential diffusion of ~ iions into ~ + BaZr03. Kirkendall type porosity i s also visible i n BaTi03 (Fia. 2).

PbTi03-PbZr03

Analysis of the PT/P*e diffusion couple shmed preferential diffusion of ~ i ~ + . Line profile analysis revealed a T i concentration gradient i n PbZr03, while Z r in PbTi03 was not detected.

IV

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In a l l systems investigated preferential diffusion of one of the w p e n t s w a s found. This leads t o porosity in the particles of one cmpnent, and t o the f o m t i o n of metastable phases which form transient liquid phase a t the sintering temperature.

A consequence of the formation of Ba-ply- and orthotitmates i n the BaTiO3-SrTi03 system is a low t e q e r a t u r e eutectic m e l t £ram which (Ba,Sr)Ti03 solid solution grains grow. DistwAance i n neck g e e t r y in the early sintering stage and Kirkendall type porosity brouqht about by preferential diffusion in t h i s system oppose densi- fication during the f i r i n g of green ompacts (1). In the microstructure, exaggerated grain growth i s observed which i s a consequence of liquid phase f o m t i o n a t BaTi03- SrTiO3 contacts. The effect is demonstrated in Fig. 3.

Fig. 3: Microstructure of sintered BaTi03-SfliO3 powders. 13800C, 2 h

The results for BaTi03-BaZr03 indicate similar p h e n m a =.in the BaTi03-Sfli03 system. Preferential diffusion of ~ iions causes f o m t i o n of a ~ + low melting point polytitanate

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solid solution, which is molten a t the firing temperature, accelerated Ba(Ti,Zr)03ss formation by a dissolution- precipitation mechanism and is esentidlly C!Cxx%m. l'ke process results i n a final homqenous Ba(Ti,Zr)03 solid solution.

A t room temperature, tetragonal PbTi03 and orthomtbic PbZr03 form a solid solutim Pb(Ti,Zr)03 over the whole canpsition range. Malysis of solid solution forration of PZT in F?FPZ pawder mixtms a t elevated temperatures ( 9 0 0 - 1 2 m ) indicated pre- ferential diffusion of ~ i(2). ~ +

mm of PZ/PT diffusion couple confindi the preferential diffusion of ~ i ~ + . The cansequences of preferential diffusion, like i n other system in this investiga- tion,are local and transient changes in the A/B ratio of the AE03 capmnd. The excess PbO which remains in ~ ideficient PbTi03 influenaes the reaction kinetics ~ + and microstructure of the Pb(Ti,Zr)03 solid solution ceramic, either by forration of a low melting point eutectic or by v a p u r phase transport. Similar p h e n m are observed on firin5 PT-PZ couples in a PbO-containing protective atnosphere to diminish the evaporation of WO.

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Different diffusivities of various e l m t s in reaction mixtures my cause transient forration of nmequilibriun phases, which can greatly influence the reaction rates and properties of materials made by reactive sintering. Of particular inportance is the forration of transient liquid phase a t temperatuns well belaw the temperature which is expscted on the basis of the equilibrim phase diagram. These p h e n m a are clearly demnstrated in the BaTi03-SrTi03, BaTi03-BaZr03 and PbTi03-PbZrn3 system.

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This work was supported by the Research Council of Slovenia.

V I I - FETE-

1. Kolar D., Trontelj M., Stadler Z., J. Am. Ceram. Soc., 65 (1982) 470 2. N m a S., J. Phys. Soc. Japan,

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