Acta Chini. Slov. 2001, 48, 87-99. 87 CaTiOs-BASED CERAMICS: MICROSTRUCTURAL DEVELOPMENT AND DIELECTRIC PROPERTIES f Danilo Suvorov, Matjaž Valant, Boštjan Jančar and Srečo D. Škapin "Jožef Stefan" Institute, Jamova 39, SI-1001 Ljubljana, Slovenia Tel: +386 1477 3 235, Fax.: +386 1 426 3 126 t This paper is dedicated to the memory of Professor Dr. Drago Kolar Received 23-01-2001 Abstract Ceramics based on CaTi03-NdA103 solid solutions were synthesized in order to study their dielectric microwave properties. Microstructural analysis was performed with scanning electron microscopy (SEM) and transmission electron microscopy (TEM) using different analytical methods such as energy-dispersive X-ray spectroscopy (EDXS). It was observed that the heating conditions during sintering and subsequent cooling strongly affect the microstructural development of the CaTi03-NdA103-based ceramics. Various types and concentrations of structural defects were identified, for example, dislocations, twins and/or antiphase boundaries. All such defects resulted in a degradation of the dielectric microwave properties, in particular the quality factor Q. The dielectric properties of CaTi03-NdA103-based ceramics can be improved by an appropriate thermal treatment. Some of the microwave dielectric properties can also be improved by using iso- or alio-valent dopants. Introduction Commercial wireless communication applications emerged in the late 1970s, evolved in the 1980s, and are expanding rapidly through the 1990s. Numerous systems are rapidly filling the 400 MHz - 20 GHz band. At the start of the last decade cellular telephone (400 MHz - 1GHz), Television Receive Only (TVRO, 2 - 5GHz), Direct Broadcasting (DBS, 11 - 13GHz), and especially satellite communications become deployed worldwide. Today, the most important applications are wireless cable; high definition and interactive TV; collision avoidance; global positioning; cellular satellite; and personal communication systems (PCS) of many types. Among several factors, industrial growth has been spurred by the development of special ceramics and their commercialization as high-volume, low-cost products. These materials are easily D. Suvorov, M. Valant, B. Jančar, S. D. Škapin: CaTi03-Based Ceramics: Microstructural... 88 Acta Chini. Slov. 2001, 48, 87-99. integrated into rf7microwave circuits. They function as frequency filters, capacitors, inductors, and signal-distributing elements. Microwave dielectric ceramics for wireless and global communications must exhibit low dielectric losses (high Q-values), high relative permittivities (s'r) and temperature-stable dielectric properties. There is a constant demand for electronic devices with improved properties that will enable further miniaturization and increase the reliability of microwave components. Although several manufacturers may produce similar components for the same application, there are subtle differences in circuit design, construction and packaging. Since the frequency drift of a device is a consequence of the overall thermal expansion of its unique combination of construction materials, each design requires a slightly different xf for temperature compensation. To fulfil the requirements for further miniaturization of the components, today's materials with s'r < 35 and Qxfr ~ 50.000 need to be replaced by materials with higher relative permittivity values (s'r > 45) which maintain high quality factors (Qxfr ~ 45.000). It has already been demonstrated that ceramics based on Ljfc/sTiOs-LaAlOs solid solutions can fulfil some of these demands.1 Another promising example are ceramics based on CaTiC^-NdAlOs solid solutions.2'3 CaTi03 is an orthorhombic perovskite with s'r ~ 170, Qxfr ~ 3.500 and a high positive temperature coefficient of resonant frequency Tf (xf = + 800 ppm/K).4'5 At 1300 °C it undergoes a high-temperature phase transition to form a cubic structure. NdA103 is a rhombohedral perovskite with s'r ~ 22, Qxfr ~ 58.000 and a negative temperature coefficient of resonant frequency (xf = - 33 ppm/K).6 Therefore, depending on the chemical composition of the CaTi03-NdA103 solid solution, relative permittivities (s'r) higher than 45 can be obtained accompanied by high quality factors (Qxfr > 40.000) and a temperature-stable coefficient of resonant frequency xf. However, the resulting dielectric microwave properties depend strongly on the firing conditions, especially on the sintering temperature and the cooling rate, indicating the possibility that the structural development during sintering and cooling plays a key role. D. Suvorov, M. Valant, B. Jančar, S. D. Škapin: CaTi03-Based Ceramics: Microstructural... Acta Chini. Slov. 2001, 48, 87-99. 89 In the present work, results on the microstructural characterization of CaTiO?-NdA103-based ceramics are reported. Such ceramics show very promising dieleclric properties in the microwave frequency range, which can be improved by iso- and/or alio-valent doping. Experimental Samples were synthesized by a solid-state-reaction method. Starting powders CaC03 (Johnson Matthey 99,99%), Ti02 (Johnson Matthey >99 %), Nd203 (Johnson Matthey 99,99%) and A1203 (Johnson Matthey 99,99%) were mixed in alcohol, dried, pressed into pellets and calcined with intermediate crushing for 40 hours in the temperature range 1350 °C to 1450 °C until equilibrium was achieved. For the purpose of microwave-property measurements and microstructural analysis, calcined powders were isostatically pressed into discs and sintered at temperatures between 1350 °C and 1500 °C for 10 hours. The progress of the reaction was monitored by X-ray diffraction analyses (XRD) with a Philips PW 1710 X-ray powder diffractometer using CuK„ radiation. Microstructural analyses of the samples were performed with a JEOL JXA-840A scanning electron microscope (SEM) equipped with a Tracor-Northern energy-dispersive X-ray spectrometer (EDXS) and with a JEOL JEM-2000FX transmission electron microscope (TEM) with attached Link AN 10000 EDXS system. The microwave dielectric properties were measured by a Network Analyzer (HP, Model HP 8719C, Santa Rosa, CA). We applied the closed resonant-cavity method using TEois mode. In order to determine the unloaded Q-value, the reflection coefficients (Su) were analysed as proposed by Kajfez and Hwan.7 The permittivity was calculated using the variation improvement of the Itoh-Rudokas model.8 Systematic errors in the microwave property measurements was minimized by the use of an improved approach to the measurements, described elsewhere.9 The temperature coefficient of the resonant frequency xf were measured in the temperature range from 20 to 60 °C. D. Suvorov, M. Valant, B. Jančar, S. D. Škapin: CaTi03-Based Ceramics: Microstructural... 90 Acta Chini. Slov. 2001, 48, 87-99. Results and discussion The typical microstructure of a ceramic with a composition 70 mol % CaTiQ3-30 mol % NdAlOs after sintering at 1450 °C for 10 hours is shown in Fig. 1. The microstructure is dense, with an average grain size of approximately 10 urn. Fig. 1: Microstructure of 70 mol % CaTiO3-30 mol. % NdAl03-based ceramics after sintering at 1450 °Cfor 10 hours (cooling rate 5 °C/minute). In Fig. 2 a TEM micrograph of an area near the grain boundary (GB) in aCaTi03-NdA103 sample is shown. The grains are composed of many smaller, irregularly shaped domains (D). Besides domains, planar loop-forming features were observed inside the grains (Fig. 3a and 3b). Due to their characteristic shape, and based on bright-field (BF) - dark-field (DF) experiments (two-beam case condition), and the contrast and symmetry of the fringes, we determined that these planar defects (boundaries) are of the n type, so we were able to conclude that they are antiphase boundaries (APB). D. Suvorov, M. Valant, B. Jančar, S. D. Škapin: CaTi03-Based Ceramics: Microstructural... Acta Chini. Slov. 2001, 48, 87-99. 91 Fig 2: TEM mircographs of a CaTi03-NdAl03 sample: area near the grain boundary (GB) showing the presence of domains (D). Fig. 3: TEM micrographs of antiphase boundaries in a CaTi03-NdAl03 sample: a.) bright-field, b.) dark-field. Note symmetrical contrast in BF and inversely symmetrical contrast of fringes in CDF, characteristic ofAPB boundaries. D. Suvorov, M. Valant, B. Jančar, S. D. Škapin: CaTi03-Based Ceramics: Microstructural... 92 Acta Chini. Slov. 2001, 48, 87-99. The microwave dielectric property measurements on the samples (Table I.) revealed a high degree of correlation between the dielectric properties and the chemical composition of the CaTi03-NdA103 based ceramics. Table I.: Microwave dielectric properties of CaTi03-NdAl03-based ceramics vs. chemical composition. CT -NA (mol%) Calcination (°C/h) Sintering (oC/h) H\ Qxfr (GHz) Wf (ppm/K) 73-27 1250/10 + 1350/10 1450/10 44,99 31.047 -15 71-29 1400/10 + 1400/10 1450/10 45,11 38.439 + 6 70-30 1250/10+1350/10 1450/10 43,73 34.813 + 14 67-33 1350/10 + 1400/10 1450/10 41,98 42.921 +45 60-40 1250/10 + 1350/10 1450/10 37,18 40.776 +114 The results shown in Table I. clearly indicate a decrease of relative permittivity e'r with an increased concentration of NdAl(>3 in solid solution. At the same time the Qxfr-value and temperature coefficient of resonant frequency xf gradually increases with increasing amounts of NdA103. The dielectric properties of CaTi03-NdA103-based ceramics can be improved by an appropriate thermal treatment. However, it was found that the cooling rate shows a much stronger influence on the resulting microwave dielectric properties of the ceramics (especially on the Q-value) than the heating rate (Table IL). D. Suvorov, M. Valant, B. Jančar, S. D. Škapin: CaTi03-Based Ceramics: Microstructural... Acta Chini. Slov. 2001, 48, 87-99. 93 Table II: Microwave dielectric properties of 70 mol% CaTiO3-30 mol % NdAl03 based ceramics vs. cooling rate (Ts = 1450 °C/12 hours. Ts (°C) Cooling rate 1450 71 Oh 0.5 7min 1450 710h 5 7min 1450 710h 50 7min e'r 45 43 43 Qxfr 44.000 38.000 33.000 Tf(ppm/K) +3 +3 +3 A variety of suitable dopants can be used to improve the microwave dielectric properties of CaTi03-NdA103-based ceramics (Table III.). Table III.: Possible dopants for the improvement of the microwave dielectric properties of CaTiOs-NdAlOs-based ceramics. Element Coord. No. Ionic Radius (Â) Ionic Polarizability (Â3 ) Ca2+ 12 1.34 3.16 Ba2+ 12 1.61 6.40 Sr2+ 12 1.44 4.24 Mg2+ 8 0.89 1.32 Nd3+ 12 1.27 5.01 Sm3+ 12 1.24 4.74 La3+ 12 1.36 6.07 Ti4+ 6 0.605 2.93 A13+ 6 0.535 0.79 Ga3+ 6 0.620 1.50 D. Suvorov, M. Valant, B. Jančar, S. D. Škapin: CaTi03-Based Ceramics: Microstructural... 94 Acta Chini. Slov. 2001, 48, 87-99. Due to their high ionic polarizability we performed a set of experiments in which Nd3+ was substituted with Sm3+ and La3+ (in fact NdA103 was substituted with LaA103 or SmAlOs). Microstructural analysis confirmed the formation of monophase ceramics during sintering at 1450°C for 12 hours with a significant amount of closed porosity (Fig. 4a). Densification can be improved by inhibiting grain growth during sintering which was achieved by either increasing the concentration of LaA103 or by substituting 1 at % Sr2+ for Ca2+ (Fig. 4b). sintering at 1450 °Cfor 12 hours (cooling rate 5 °C/minute). Measurements of the microwave dielectric properties revealed that both, Sm3+ and La3+ increases the relative permittivity e'r of ceramics. The temperature coefficient of resonant frequency xf reached a value close to zero for compositions 69 mol % CaTiO?-31 mol % SmAlOs and 66 mol % CaTi03-34 mol % LaA103. However, only the addition of SmA103 also improved Qxfr-value (Table IV.). D. Suvorov, M. Valant, B. Jančar, S. D. Škapin: CaTi03-Based Ceramics: Microstructural... Acta Chini. Slov. 2001, 48, 87-99. 95 Table IV.: Microwave dielectric properties of CaTi03-LaAl03 and CaTi03-SmAl03 ceramics sintered at 1450 °Cfor 12 hours. Composition (mol %) H\ Qxfr(GHz) Wf(ppm/K) CaTi03 SmAlOs 75/25 51 31.000 +31 70/30 45 42.000 +1 65/35 41 42.000 -18 Ca Ti03 -LaAlOs 47 36.000 +13 44 30.000 -3 41 33.000 -17 Ga3+ seems to be a very suitable replacement for Al3+. Ga3+ possesses a much higher ionic polarizability than Al3+, the ionic radii are very similar for both (0.535 Â for Al3+ and 0.620 Â for Ga3+) and both have a coordination number of 6. However, microstructural analysis showed the development of a nonhomogenous microstructure after sintering the CaTi03-NdGa03 ceramics for 12 hours at 1450 °C (Fig. 5). Fig. 5: Microstructure of 70 mol % CaTiO3-30 mol % NdGa03-based ceramics after sintering at 1450 °C for 12 hours (cooling rate 5 °C/minute): nonhomogenous microstructure with LaGa03-based precipitates, formation of core-shell structure in solid-solution grains. D. Suvorov, M. Valant, B. Jančar, S. D. Škapin: CaTi03-Based Ceramics: Microstructural... 96 Acta Chini. Slov. 2001, 48, 87-99. Formation of a multiphase microstructure in the CaTi03-NdGa03-based ceramics did not affect relative permittivity and xf significantly, but in general decreased the Qxfr value (Table V.). Table V.: Microwave dielectric properties of CaTi03-NdGa03 ceramics, sintered at 1450 °Cfor 12 hours. Composition (mol.%) CaTiOs -NdGaOs 70/30 65/35 60/40 CaTiOs -LaGaOs 70/30 65/35 60/40 CaTiOs -SmGaOs 70/30 65/35 60/40 Partial substitution of Ba2+, Sr2+ and Mg2+ for Ca2+ in CaTi03 strongly inhibited grain growth during sintering. Very high relative permittivities were achieved when 10 mol% Ca2+ was replaced with Sr2+ (s'r > 62). When Mg2+ was used a spinel-based second phase formed during heat treatment causing nonhomogenous microstructural development and thus worsening the microwave dielectric properties (Fig. 6). D. Suvorov, M. Valant, B. Jančar, S. D. Škapin: CaTi03-Based Ceramics: Microstructural... H'r 49 45 44 52 48 45 51 45 42 Qxfr (GHz) 32.000 38.000 30.000 27.000 32.000 34.000 18.000 34.000 35.000 Wf(ppm/K) +35 +1 -18 +40 +2 -20 +4I +1 -11 Acta Chini. Slov. 2001, 48, 87-99. 97 Fig. 6: Microstructure of 70 mol % (Ca0.9Mgo.i)Ti03-30 mol % SmAl03-based ceramics after sintering at 1450 °C for 12 hours (cooling rate 5 ^C/minute): nonhomogenous microstructure with spinel-based precipitates. Conclusions Ceramics based on CaTi03-NdA103 solid solutions show very promising microwave dielectric properties: s'r higher than 45, Qxfr > 40.000 and a temperature-stable coefficient of resonant frequency xf. These properties can be significantly improved by iso-valent doping and by processing parameters such as cooling rate. Microstructural analyses confirmed the presence of various types of defects in such ceramics which strongly influence the resulting microwave properties. Depending on the sintering temperature and cooling rate, dislocations, twins and antiphase boundaries were found. The concentration of the defects is high, which leads us to the conclusion that the resulting dielectric microwave properties of such ceramics can be even further improved by a proper thermal treatment resulting in a decrease in the concentration of the identified microstructural defects. Experimental D. Suvorov, M. Valant, B. Jančar, S. D. Škapin: CaTi03-Based Ceramics: Microstructural... 98 Acta Chini. Slov. 2001, 48, 87-99. Samples were synthesized by a solid-state-reaction method. Starting powders CaC03 (Johnson Matthey 99.99%), Ti02 (Johnson Matthey >99 %), Nd203 (Johnson Matthey 99.99%) and A1203 (Johnson Matthey 99.99%) were mixed in alcohol, dried, pressed into pellets and calcined with intermediate crushing for 40 hours in the temperature range 1350 °C to 1450 °C until equilibrium was achieved. For the purpose of microwave-property measurements and microstructural analysis, calcined powders were isostatically pressed into discs and sintered at temperatures between 1350 °C and 1500 °C for 10 hours. The progress of the reaction was monitored by X-ray diffraction analyses (XRD) with a Philips PW 1710 X-ray powder diffractometer using CuK„ radiation. Microstructural analyses of the samples were performed with a JEOL JXA-840A scanning electron microscope (SEM) equipped with a Tracor-Northern energy-dispersive X-ray spectrometer (EDXS) and with a JEOL JEM-2000FX transmission electron microscope (TEM) with attached Link AN 10000 EDXS system. The microwave dielectric properties were measured by a Network Analyzer (HP, Model HP 8719C, Santa Rosa, CA). We applied the closed resonant-cavity method using TE0i5 mode. In order to determine the unloaded Q-value, the reflection coefficients (Su) were analysed as proposed by Kajfez and Hwan.7 The permittivity was calculated using the variation improvement of the Itoh-Rudokas model.8 Systematic errors in the microwave property measurements was minimized by the use of an improved approach to the measurements, described elsewhere.9 The temperatiure coefficient of the resonant frequency xf were measured in the temperature range from 20 to 60 °C. Acknowledgements The work was supported by the Ministry for Science and Technology of the Republic Slovenia (grant P-510). D. Suvorov, M. Valant, B. Jančar, S. D. Škapin: CaTi03-Based Ceramics: Microstructural... Acta Chini. Slov. 2001, 48, 87-99. 99 References and Notes 1. D. Suvorov, S. Škapin, M. Valant, D. Kolar, J. Mater. Sci. 1998, 55 (1), 85. 2. D. Suvorov, M. Valant, 101st Ann. Meet. AcerS, Indianapolis, May 1999, Book of Abstracts, 1999, 103. 3. B. Jančar, D. Suvorov, M. Valant, submitted for publication in J. Mater. Sci. Lett, 2000. 4. R. C. Kell, A. C. Greenham, G. C. E. Olds, J. Am. Ceram. Soc. 1973, 56(1), 352. 5. S. Kucheiko, J. W. Choi, H. J. Kim, H. J. Jung, J. Am. Ceram. Soc. 1996, 79(10), 2739. 6. S. Y. Cho, I. T. Kim, K. S.Hong, J. Mater. Res. 1999, 14(1), 114. 7. D. Kajfez and E. J. Hwan, IEEE Trans. 1977, MTT-332, 666. 8. T. Itoh and R. S. Rudokas, IEEE Trans. 1977, MTT-25, 52. 9. M. Valant, D. Suvorov and S. Maček, F erro electrics 1996, 176, 167. Povzetek Razvoj novih aplikacij za brezžične komunikacijske sisteme narekuje intenzivne raziskave takšnih keramičnih materialov, ki jih odlikujejo izboljšane dielektrične lastnosti v mikrovalovnem frekvenčnem območju (> 500 MHz). V delu poročamo o raziskavah dopiranih keramičnih trdnih raztopin na osnovi CaTi03 - NdA103. Značilno za navedeni binarni sistem je, da osnovni komponenti med tvorita trdno raztopino v celotnem koncentracijskem intervalu. Z višanjem vsebnosti NdA103 v trdni raztopini se temperatutni koeficient resonančne frekvenceTf znižuje ter pri sestavi 70 mol.% CaTi03 - 30 mol.% NdA103 doseže vrednost Tf = 0. Relativna dielektričnost e'r je višja od 43, faktor kvalitete Q pa preseže vrednost (Qxfr >34.000). Z izovalentnim dopiranjem takšne keramike njene mikrovalovne dielektrične lastnosti še izboljšamo. Tako delna zamenjava Nd3+ in/ali Al3+ z La3+, Sm3+ in Ga3+ povzroči zvišanje relativne dielektričnosti e'r ,vpliva pa tudi na faktor kvalitete Q. Na faktor kvalitete Q pomembno vpliva tudi režim termične obdelave keramike, še posebno faza hlajenja po sintranju. Med termično obdelavo keramike namreč v njej nastaja vrsta strukturnih napak (dislokacije, dvojčki in antifazne meje), ki odločilno vplivajo na rezultirajoče mikrovalovne dielektrične lastnosti. D. Suvorov, M. Valant, B. Jančar, S. D. Škapin: CaTi03-Based Ceramics: Microstructural...