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Temperature-Stable Barium Titanate Ceramics

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Temperature-Stable Barium Titanate Ceramics Electrocomponent Science and Technology (C)Gordon and Breach Science Publishers Ltd. 1976, Vol. 2, pp. 241-247 Printed in Great Britain TEMPERATURE-STABLE BARIUM TITANATE CERAMICS CONTAINING NIOBIUM PENTOXIDE IAN BURN Research and Development Laboratories, Sprague Electric Company, North Adams, Massachusetts U.S.A. (Received May 27, 19 75) Niobium is known to lower the Curie point and broaden the permittivity peak of barium titanate. However, the distribution of niobium in sintered barium titanate is often not homogeneous, being influenced by such preparation variables as large cation/small cation stoichiometry and firing conditions. In addition to these factors, it was found that, at least up to about 5 mol % Nb, the distribution of niobium in the barium titanate grains could be regulated by small amounts (<1 mol %) of oxides of Co, Mg, Ni or Mn. These oxides appear to form niobates that produce high permittivity solid solutions in the "skins" of the barium titanate grains and permit dielectrics to be obtained with permittivity K 2000,) stable to within +-10% over the temperature range -55C to 125C. INTRODUCTION principal impurities. A small amount of work was also done with a similar barium titanate $ with comparable The influence of niobium on the temperature impurity level but small BaO/TiO2 ratio (0.99). Up dependence of the permittivity of barium titanate has to about 5 mol % niobium oxide (NbO2.s) was added been described by several authors. -a Some to the barium titanate powder together with various inconsistencies in the earlier data 1,2 have been other oxides in amounts less than 1 mol %. explained by Kahn a who found that the addition of In most cases about 100 gms of powder were Nb to prereacted barium titanate resulted in a ball-milled for 12 h. in a suitable organic binder at homogeneous distribution only under conditions of roughly 77 wt % solids in a 200 cc capacity porcelain grain growth during sintering. When little grain mill containing alumina balls. This procedure resulted growth occurred, the permittivity vs. temperature in efficient mixing with negligible mill contamination curves indicated the presence of Nb-free barium but little comminution of the initial particle size, titanate in the grain cores.4 which was less than about 2/m. The slip was cast on As pointed out by Plessner and West and Rutt, 6 a glass plate after milling, using the doctor-blade if an additive that lowers the Curie temperature of technique, and when dry was cut into small squares barium titanate is distributed in a non-homogeneous approximately 10 x 10 x 0.5 mm. In a few instances but controlled manner, the material can be of the powder was spray dried and pressed into discs of practical use as a high permittivity (K 2000) similar thickness to the squares. The samples were temperature.stable dielectric. Dielectrics of this type fired in a tube furnace in air mainly at 1400C with a have been described that contain several mol % 5 h. soak. Electrodes of commercial silver paste were NaNbOa 7-9, as well as some in which the niobium then painted or screen-printed on both sides of the distribution was regulated by additions of Li or squares or discs and matured in the usual way FeB' o, or Mg. 8' 11 It was found in the present work (750C; h. cycle). that certain other additives also modify the niobium The electroded samples were allowed to age for distribution and an attempt has been made to explain one day and then the change in capacitance at kHz this effect. over the range -60C to 130C was measured. This was accomplished by first balancing an impedance comparator with the sample at 25 C, cooling the 2 EXPERIMENTAL PROCEDURE The barium titanate was a commercial material' with fCOF grade from TAM Division of NL Industries Inc., a reported BaO/TiO2 mole ratio of 1.015 and Niagara Falls, N.Y. containinl; 1.4 (mol) % Sr, 1.0% A1 and 0.4% Nb as $5018 grade also from TAM Division of NL Industries. 241 242 I. BURN / .,"/ \',., / .,*', / ".\ \\ 2.1 m/o / ./'1 \ \. -20 / .,'.-/ \ / ,..'" \ "o,. / i" \ \. -o /+/" -..._._.., ',, -40 -,.;; ".,, -55 0 2,5 c 85 125 FIGURE Influence of niobium oxide on the permittivity of barium titanate sintered 5 h. at 1400C. sample to -60C, and heating at roughly 3C/min. optimum temperature stability in the case of the while continuously recording the change in 5018 grade barium titanate. Judging by the high capacitance. temperature permittivity peak, the latter material appeared to have a more uniform crystallinity and a higher purity than the COF barium titanate in the 3 RESULTS cores of the grains. Dielectric constant at room temperature was 1800 _+ 50 in the case of the Figure shows how the permittivity varied with COF-based material and 2050 -+ 50 for that using the temperature for increasing additions of niobium to the 5018 barium titanate. The slightly lower capacitance (COF) barium titanate. When the niobium was less at -55C obtained for pressed discs compared with than about 3 mol %, some connected porosity was the cast squares (Figures 3 and 5) was perhaps due to apparent in the ceramic for the firing conditions used, and the presence of some unreacted barium titanate was evident from the secondary capacitance peak. Although a dense ceramic was obtained for 3.8% Nb, there was no evidence of any significant grain growth during sintering (Figure 2), and some unreacted barium titanate could still be detected. Figure 3 illustrates how small additions of cobalt oxide could modify the permittivity-temperature characteristics for 3.8 mol % Nb. As the cobalt was increased, the presence of unreacted barium titanate became more evident, and as seen from Figure 4 the amount of porosity increased. Figure 5 compares data (obtained from pressed discs) for the different grades of barium titanate and shows that the effect of the cobalt addition was qualitatively similar in both cases. FIGURE 2 Surface microstructure of barium titanate with However, a smaller amount of Co was needed for 3.8 m/o Nb sintered 5 h. at 1400C. BARIUM TITANATE CERAMICS 243 \ +10 f /// \'\. ' "%",, -I0 x 3 / ', "x. 0.75 m/o / ', -, ....... / ,o o C FIGURE 3 Influence of cobalt oxide on the distribution of Nb (S.8 m/o) in barium titanate sitered 5 h. at 1400C. a difference in packing density in the green state. following firing at 1400C for periods of less than When the cobalt was added together with a similar 5 h. (Figure 8). These were essentially similar to those amount of TiO2, densification was improved and the described by Prokopowiczs and by Rutt. 11 degree of reaction between the barium titanate and Nickel oxide produced similar results to those for the niobium was slightly diminished (Figure 6). Co and Mg, as did manganese oxide, though the latter As shown in Figure 7, virtually identical was slightly less effective in modifying the niobium permittivity characteristics were obtained when the reaction with the barium titanate (Figure 9). It can same molar amounts of magnesium oxide were used also be seen from Figure 9 that equivalent amounts of instead of cobalt oxide. When the amount of niobium iron had little influence on the permittivity- was increased to approximately 5 mol %, dense temperature characteristics, and this was also true for temperature-stable dielectrics could be obtained Cr, as well as for Ba or Ca. (b) (a) FIGURE 4 Increase in porosity with cobalt oxide additions to barium titanate with 3.8 m/o Nb sintered 5 h. at 1400C. (a) 0 m/o Co, (b) 0.9 m/o Co. Fractured surfaces. 244 I. BURN w z w z 5'5 FIGURE 5 Temperature-stable dielectrics obtained from different types of barium titanate containing 3.8 m/o Nb with additions of cobalt oxide and sintered 5 h. at 1400C: (A) COF barium titanate with 0.9 m/o Co, and (B) 5018 grade barium titanate with 0.6 m/o Co. 4 DISCUSSION and (2) Why did additives such as Co, Mg, Ni or Mn modify solid solution of the niobium and retard The experimental data pose two main questions" densification, while equivalent amounts of Fe, Cr, Ca (1) Why was densification of the barium titanate or Ba, for example, apparently did not? Additions of made easier as the amount of niobium was increased; TiO2, and ions that substitute for Ti4 in the titanate +10 w .z -r 0 w z -55 0 2 85 125 C FIGURE 6 Influence of TiO2 additions on the permittivity of barium titanate with 3.8 m/o Nb and 0.9 m/o Co sintered 5 h. at 1400C. BARIUM TITANATE CERAMICS 245 +10 I \\ / I I 0.9 m/o FIGURE 7 Influence of magnesium oxide on the distribution of Nb (3.8 m/o) in barium titanate sintered 5 h. at 1400C. lattice, generally improve the sinterability of barium sintefing (-1350C). 2 Conversely, additions of BaO titanate that is closely stoichiometric in Ba and Ti. or additives that substitute for Ba (e.g. Ca) generally This is because formation of a liquid titanate phase is retard sintering. In the present case, it seems probable encouraged at the temperatures commonly used for that a liquid niobate phase, as well as a liquid titanate +10 /"\ u.l < O- ..,..,'-'-., / //, o . "\. 4.6 m/o ND ,, / () -I0- 5.4 m/o Nb <>C FIGURE 8 Temperature-stable dielectrics with higher Nb content (as shown) and 0.8 m/o Mg sintered at 1400C for (A) 2 h, (B) 1.5 h. 246 I. BURN // ' z ,' K.
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