AFOSR-TN-57-391 ASTIA Document No. AD 132 466 ENGINEERING RESEARCH INSTITUTE THE UNIVERSITY OF MICHIGAN ANN ARBOR ADDITIONS OF Fe205 TO BaTiO3 - SrTiO3 FERROELECTRICS Technical Report No. 3 Solid State Devices Laboratory Department of Electrical Engineering H. Diamond V. Chang Project 2495 AIR FORCE OFFICE OF SCIENTIFIC RESEARCH AIR RESEARCH AND DEVELOPMENT COMMAND SOLID STATE SCIENCE DIVISION CONTRACT NO. AF 18(603)-8, DIVISION FILE NO. 40-24 December 1957

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The University of Michigan ~ Engineering Research Institute LIST OF ILLUSTRATIONS Fig. No. Page 1 Dielectric constant at 1000 cps versus temperature for small additions of Fe203 to BaTiO3 ceramics. 5 2 Shift and splitting of the Curie temperature with increasing additions of Fe203 to a BaTiO3-SrTiO3 ceramic. 6 3 Ternary diagram with respect to the solubility of Fe203 in BaTiO3 - SrTiO3. 7 4 Sintering Fe205 with BaO and TiO2 in varying stoichiometric proportions. 8 ii

The University of Michigan ~ Engineering Research Institute ABSTRACT If small percentages of Fe203 are added to polycrystalline BaTiO3 and fired at l1400~ as a ceramic, the Curie Temperature of the ceramic remains constant while the peak permittivity is lowered, However, if the Fe203 is added to BaO and TiO2, the iron goes into the lattice predominately in the titania sites and decreases the Curie Temperature as well as the peak permittivity. Additions of Fe2O3 to mixed BaTiO3-SrTiO3 compositions result both in a lowering of the Curie Temperature and a depression of the peak permittivity of the single phase system. No changes in lattice dimensions with the additions were observed when the Ba to Sr ratio was held fixed. iii

The University of Michigan ~ Engineering Research Institute ADDITIONS OF Fe203 TO BaTiO3 - SrTiO3 FERROELECTRICS Several investigators,1,2 working with ferroelectric single crystals, have observed that BaTiO3 single crystals containing small percentages of Fe203 have lowered Curie temperatures. However, experiments in our laboratory on polycrystalline ceramic ferroelectric compositions, both of the pure BaTiO3 type and the BaxSrl.xTiO3 compositions, have yielded some rather interesting results with respect to the addition of Fe203. Sintering the pure BaTiO3 with the Fe203 additives did not, as is the case for the single crystals, have any affect in shifting the cubic to tetragonal ferroelectric transition to lower temperatures. What is observed is a gradual suppression of the peak permittivity of the material as more and more of the Fe203 was added to the BaTiO3. This was found to hold when as little as.2 mol percent or as much as 6 mol percent of the Fe203 was added, and under the rather severe firing schedule of 1400~C for two hours. Figure 1 shows this behavior where the incremental permittivity for BaTiO3 compositions containing varying quantities of the Fe203 additive are plotted against temperature. It is clear that the Curie peak remains at about the same temperature regardless of the amount of Fe203 present. On the other hand, when a "sufficient" amount of strontium ion is present in a composition of the form BaxSrlxTiO3 and the composition is sintered with small quantities of the Fe203 additive, the effect of the iron ion in shifting the Curie temperature is much more marked than the same effect due to the strontium, It was found, however, that for a given amount of strontium present, only a certain maximum amount of iron could enter the lattice before a separate phase makes its appearance. This is seen in the dielectric data of 1 1

The University of Michigan ~ Engineering Research Institute Figure 2 for a typical composition of Ba 91Sr o09TiO3 fired with increasing amounts of the Fe203. The barium to strontium ratio is held constant so that the effect of the lowering of the Curie temperature is due to the iron entering the perovskite lattice. It is observed that the Curie temperature is gradually lowered and the peak permittivity is suppressed as more and more of the iron enters into the lattice. A point is then reached where the downward shift in Curie temperature stops and apparently a new phase appears as is evidenced by the occurrence of a new peak in permittivity at around 125~C —the Curie temperature of the pure BaTi03o This effect is first noticed for.5 mol percent of Fe203 additive in the composition described above. Dielectric measurement techniques were used to determine the ferroelectric cubic to tetragonal crystallographic transition that is indicated by the Curie peaks in the perovskite type ferroelectrics. The smooth shifting of the Curie temperature upon variation of the percentages of one iron or another in the ternary indicates a homogeneous ferroelectric composition. On the other hand, as is corroborated by x-ray data, if a second Curie peak suddenly makes its appearance it can be said that the composition is no longer homogeneous but that it contains a separate ferroelectric phase. It has been therefore possible to construct a phase boundary line in a ternary diagram of BaTiO3 - SrTiO3 - Fe203, as seen in Figure 3, for small percentage-wise additions of the Fe203, Compositions in the narrow region to the left of the phase boundary form a solid solution and the compositions indicated to the right of this line all show the additional characteristic small peak at about 125~C. Along the boundary, the ratio of mol percent of SrTiO3 to Fe203 is about eight to one. X-ray powder photographs were made with a series of compositions of constant barium- to - strontium ratios in order to study the influence of the 2 -

The University of Michigan ~ Engineering Research Institute iron on the perovskite lattice, It was found that the mean lattice constant did not change within the limits of +.01 A over a range of Fe203 content of from zero to ten mol percent, even though over this same range the Curie tenperature shifted from approximately room temperature down to -700C. This, plus the fact that the atomic radius of iron is very close to that of titanium, leads one to believe that the iron enters the titanium sites in the lattice. That is, the lattice dimensions are more or less set by the comparatively large barium and strontium ions, whereas the Curie temperature is changed by the modification of the binding forces upon the addition of the iron, The favoring of the titanium site for the iron additive was also shown by the following. Instead of sintering the Fe203 with a pre-reacted BaTi03, BaO and TiO2 in varying stoichiometric proportions were reacted with Fe203, Of these compositions, one was stoichiometrically balanced, the second was deficient in the TiO2, and the third deficient in the BaO. The amount of added Fe203 for all the samples was the same and amounted to 2 mol percent. Figure 4 shows that for these compositions, the iron goes into solution and that the Curie point is shifted to lower temperatures. There is no evidence of a sepa-. rate BaTiO3 phase as would be observed by the occurrence of a peak at 125~C. The lowest Curie temperature is found for the sample deficient in TiO2 and the highest (except for the case of the pure BaTiO3, used as a control sample) for the sample with the excess of TiO2. From this it is evident that the iron favors the titanium sites, although results of paramagnetic resonance experiments3 have indicated a divalent form for iron in BaTiO3. We wish to express our appreciation to Professor Lars Thomassen for his helpful discussions and valuable suggestions.

The University of Michigan * Engineering Research Institute REFERENCES 1, J. P. Remeika, J.A.C.S. 76, 940-941 (1954). 2. A. Nishioka, K. Sekikawa, N. Owaki, J. Phys. Soc. Japan 11, 180-191 (1956), 3. A, W. Hornig, E. T. Jaynes, H. E. Weaver, Phys. Rev, 96, 1703 (1954).

7000 -I 6000 Pure BaTiO3.9 Yb 5000 0 04 05% Fe2O3 \J1 3000.06% Fe203 2000.2% Fe203 1000 Y.9 0~~~~~~~~~~~~~~~~~~~~~~ -31 -3 22 46 69 94 113 133 153 Temperature, 0C Fig. 1. Dielectric constant at 1000 cps versus temperature for small additions of Fe205 to BaTi03 ceramics.

5000 S 5000 4000 ~~~~Ba9 jSr09TiO3 4000 t- +.5% Feoa'3 23000 + 1.50/ FeZO-3_ 2.000 ( +2% Feo,0 -3 1000Vbm -36 -9 17 41 65 87 108 129 149 Temperature, 00 C Fig. 2. Shift anda splitting of the curie temperature with increaSifg FaditiOg5 of Fe2Oa to a BaTiOy3srTiO5 ceramic.

The University of Michigan * Engineering Research Institute 100% BaTiO3 95 90 85 080 75 25% SrTi03 25% Fe203 Fig. 3. Ternary diagram with respect to the solubility of Fe203 in BaTiO3 - SrTiO3.

3000 I 1 B BaO-TiO + Fe2O,3 2500 + Excess TiO2 2000 E 1500 BaOOTiO2+ Fe203 1000 500 ~- - BaO-TiO2 + Fe203 1 Excess BaO 0~~~~~~~~~~~~~~~~~~~~~~~ -33 -6 20 44 67 90. 110 132 152 Temperature, 00 Fig. 4. Sintering Fe203 with BaO and TiO2 in varying stoichiometric proportions.

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