The dielectric properties of ceramics in the Ti0,-rich region of the BaO-TiO, system were investigated. In the composition range between BaTi,O, and TiO,, another compound, Ba,Ti,O,,, can be obtained when calcining and sintering conditions are controlled carefully. When dense and single-phase, this ceramic has excellent dielectric and physical properties. At 4 GHz, the dielectric K = 39.8, Q = 8000, and T~ (tern erature coefficient of dielectric constant) = -2 4 k 2 p p m k I . Introduction IELECTRIC microwave resonators offer the possibility of D high-quality integrated filters. The primary obstacle to the study and application of filters utilizing dielectric resonators has been the lack of a suitable material. A dielectric resonator is a piece of unmetalized ceramic with a high dielectric constant in which the electromagnetic fields are confined to the dielectric region and its immediate vicinity by reflections at the dielectric-air interface. The dielectric properties required for practical microwave filters are high dielectric Q (>3000) and a low temperature coefficient of dielectric constant (~~= O f 4 0 ppm/"C). In addition, a high dielectric constant (K>35 to 40) is desirable.The Ti-rich region of the BaO-TiO, system has seemed promising for locating a composition which exhibits temperature compensation while retaining suitable K and loss properties. Several compositions with temperature-independent dielectric constants at radio frequencies have been reported. Bunting and co-workers,',* who surveyed the dielectric properties of the Ba0-Sr0-Ti0, system at 1 MHz, observed temperature compensation with low loss and K = 37 for the binary composition containing 83.3 mol% TiO,. In a study of the ternary systems BaO-SnO,-TiO, and BaO-ZrO,-TiO,, Jonker and KwestrooJ found a promising combination of dielectric properties at 1 MHz for ternary compositions containing 18.5 mol% BaO and =4mol% SnO, or ZrO,. On theother hand, Schwarzbach and Plocek4 reported temperature compensation in the binary at 80 mol% TiO, (BaTi,O,) and an increasingly negative7 as the amount of BaO was decreased. The work of Naumann et u L .~ agrees well with Ref. 4. There has been less investigation at microwave frequencies. Masse and c o -~o r k e r s~~~ reported that temperature compensation is best for BaTi,O,, for which K=39, Q=2500, and ~~= -4 9 X 10-fi/cC. Table I summarizes the results of these studies for the 75 to 100 mol% TiO, region of the BaO-TiO, system.The data on phase relations in this system are also confusing. In an apparently thorough investigation, Rase and RoyR found only 3 binary compounds, BaTi,O,, BaTi,O,, and TiO,, for the region 75 to 100 mo19 TiO,. On the other hand, Jonker and Kwestroo? using similar experimental conditions, identified Ba2Ti90,, within the Ti0,-rich binary region. Schwarzbach and Plocek4 also reported the existence of a Ba,Ti,O,, phase in the binary system, but their structure data did not agree with those of Jonker and Kwestroo. T i l l m a n n~~~'~ has reported the existence of BaTi,Ol, and BaTi,O,...
Phase relations in the system BaO-TiO, from 67 to 100 mol% TiO, were investigated at 1200" to 1450°C in 0,. Data were obtained by microstructural, X-ray, and thermal analyses. The existence of the stable compounds Ba,Ti,,O,,, Ba4TiI3O3,,, BaTi40,, and Ba2Ti9OZ0 was confirmed. The compound BaTi205 is unstable and either forms as a reaction intermediate below the solidus or crystallizes from the melt. The compounds Ba6Ti,,040 and Ba,Ti,,O,, decompose in peritectic reactions, and BaTiO, and Ba,Til,04, react to form a eutectic. Special conditions are required for the formation of BaZTi,O,,, which decomposes in a peritectoid reaction at 1420°C. The new phase diagram is presented.
Microwave measurements of Ba2Ti902,, show that this ceramic is uniquely suited for dielectric resonators. (Suitable ceramics should have a high dielectric constant K , a low dielectric loss (high Q), and a low temperature coefficient of resonant frequency, T~. ) At 4 GHz, Ba2Ti9OZ0 resonators have Q >SOW, K =39.8, and r f = 2 ppm/"C. Measurements of Q and T~ were made on unmetallized ceramic resonator disks positioned in a waveguide; K was measured using a dielectric post resonator technique. From 4 to 10 GHz, Q approaches that for a copper waveguide cavity, whereas the temperature coefficient is typically 8 times lower.
The electrical resistivity of well sintered BaPbOs ceramics was measured from -50" to 100°C (Fig. 3). The electrical resistivity at 25°C was 8.3 X ohm-cm, and the temperature coefficient of resistivity was +O. 15%',l0C indicating typical metallic conduction. IV. ConclusionThe solid-state reaction of BaC03 and PbO to form Ba-Pb03 was studied by DTA, TGA, and X-ray analysis. Formation of BaPbOs started at 600°C which WAS in agreement with previous analyses. The formation of BaPb03 9 A 7 \ X6 2.8 3.0 3.2 3.6 3.8 4.0 4.2 +Yl@ 2 The thermal decompositions of BaTiO (czO4)z .-4Hz0, BaTiO (OH)zCz04.2Hp0, SrTiO ( C~0 4 ) 2 . -4H20, and SrTiO(OH)YCz04.Hz0 were investigated using TGA, DTA, and effluent gas analysis. The stoichiometry of the decompositions is discussed and it is proposed that a reduced state of titanium is formed as an intermediate.
The heterogeneous phase distribution found in Ba2Ti9OZ0 ceramic resonators results from a temperature-dependent phase boundary and slow reaction kinetics. When sintered at 1350°C or higher in oxygen the BaZTi9OZ0 phase becomes slightly reduced and barium-rich. Thus a stoichiometric composition forms rutile and "BazTi902:' phase. On slow cooling the excess barium diffuses to the oxygen-rich surface where it reacts to form an envelope of rutile-free material surrounding a core containing a small amount of rutile.
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