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The microwave dielectric properties of xTiO2(1−x)CeO2 ceramics
January 2002
Materials Letters 52 Ž2002. 240–243 www.elsevier.comrlocatermatlet
The microwave dielectric properties of x TiO 2 ž1 y x / CeO 2 ceramics Duck-Hwan Kim ) , Sang-Kyu Lim, Chul An Electronics Department, Sogang UniÕersity, Seoul 121-742, South Korea Received 14 May 2001; accepted 24 May 2001
Abstract The x TiO 2Ž1 y x .CeO 2 system was investigated and its dielectric properties were measured. In order to achieve a temperature-stable material, we studied a method of combining a positive temperature coefficient material with a negative one. CeO 2 was measured to have dielectric constant ´ r s 24, Q ) f s 57,000 and t f s y104 ppmr8C. TiO 2 has ´ r s 93, Q ) f s 10,000 and t f s 350 ppmr8C. x TiO 2 Ž1 y x .CeO 2 system has the dielectric properties as follows: 32 - ´ r - 74, 25,000 - Q ) f - 43,000 and y100- t f - 300. It is the temperature-stable material that is achieved. A series of new x TiO 2 Ž1 y x .CeO 2 dielectric materials were synthesized. Its dielectric characteristicsŽ ´r , Q ) f and t f . are intermediate between TiO 2 and CeO 2 . It is found that the ceramic in the x TiO 2 Ž1 y x .CeO 2 system has excellent dielectric characteristics at microwave frequencies. q 2002 Elsevier Science B.V. All rights reserved. Keywords: TiO 2 ; CeO 2 ; x TiO 2 Ž1 y x .CeO 2 ceramics; Dielectric constant; Temperature coefficient of resonant frequency; Q ) f ; Mixed phases; Temperature-stable material
1. Introduction Ceramic compositions are extensively used in the manufacture of electronic components. Especially microwave dielectric ceramics are increasingly used for filters, resonator and mixers in systems for wireless communication w1x. In microwave telecommunications, the usefulness of dielectric ceramics for microwave resonators has been recognized because they permit miniaturization of microwave devices. The materials used for dielectric resonators are required to have such dielectric properties as follows. Ž1. Dielectric constant Ž ´ r . ) 20. The dielectric con) Corresponding author. Tel.: q82-2-706-3401; fax: q82-2706-3401. E-mail address: [email protected] ŽD.-H. Kim..
stant is an important factor for making microwave components smaller. Ž2. Q ) f value ) 30,000 GHz. It contributes to ensure lower dielectric loss. Ž3. Temperature coefficient of resonant frequency Žt f . - "10 ppmr8C w2x. In practical use, it is important to adjust the t f of the dielectric resonator to zero. Conventionally, there are two approaches to develop an excellent ceramic material as follows. One is to make a new dielectric ceramic material. The other is to combine more than two materials for characteristic compensation. The most popular method of achieving this goal involves mixing two or more compositions with different dielectric properties. In other words, to adjust the t f to zero, two or more compounds having negative and positive t f values are employed to form a solid solution or mixed phases w3x.
00167-577Xr02r$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 1 . 0 0 4 3 4 - 7
D.-H. Kim et al.r Materials Letters 52 (2002) 240–243
In order to achieve a temperature-stable material, we examined that a method of combining a positive temperature coefficient material with a negative one. Among simple oxide compounds, TiO 2 exhibits the good dielectric properties as follows; high ´ r Žs 104., low loss Ž Q ) f s 40,000 GHz. and large positive t f Žs 460 ppmr8C. w4x. TiO 2 ceramic has been researched to modify its dielectric properties and to synthesize binary oxides system with other oxides w5,6x. Until recently, some researchers have studied also ceramic compositions containing titanate w7–9x. In these studies, TiO 2 has been chosen an additive material to tailor dielectric properties of other ceramic compositions. Also, CeO 2 is a good dielectric ceramics with low ´r Žs 24., high Q ) f Žs 58,000 GHz. and large negative t f Žs y106 ppmr8C.. By following experimental procedures, we anticipated that the characteristics of the composite would be compensated. In this study, the microwave dielectric material of x TiO 2 Ž1 y x .CeO 2 system, with temperature compensating characteristics was prepared using a conventional mixed-oxide method. We discuss the microwave dielectric properties of x TiO 2 Ž1 y x .CeO 2 ceramics where x is a molar fraction.
2. Experimental procedure Specimens were prepared by the conventional ceramic method. The starting materials ŽTiO 2 , CeO 2 . were weighed according to the ratio of x TiO 2 Ž1 y x .CeO 2 , where x s 0, 0.2, 0.4, 0.6, 0.8 and 1, respectively. x TiO 2 Ž1 y x .CeO 2 powders were wet ball-milled in deionized ŽD.I.. water for 20 h. After drying, they were calcined at 800 8C for 3 h in air. The calcined powders were milled again in D.I. water for 20 h. After being reground with ethanol, the powders were mixed with organic binder of 1 wt.% polyvinyl alcohol ŽPVA.. Pellets of 10 mm in diameter and 8–9 mm in thickness were uinaxially pressed at 2 tons. Sintering was conducted at 1230– 1450 8C for 2 h. Dielectric characteristics at microwave frequency such as permittivities and dielectric losses were measured at 4–8 GHz by Hakki–Coleman w10x dielectric resonator method. The value of temperature coefficient of resonant frequency was determined in the temperature range
241
of 0–80 8C. The density of the ceramic was measured by ASTM ŽC20-87..
3. Result and discussion Fig. 1 plots the dielectric constant of x TiO 2 Ž1 y x .CeO 2 Žwhere x s 0, 0.2, 0.4, 0.6, 0.8 and 1. compounds and these are presented as a function of the sintering temperatures. The dielectric constant rose, with increasing amount of TiO 2 . The dielectric constants firstly increased with the sintering temperature. The dielectric constants reached a maximum value at a certain sintering temperature. In the sintering temperature range between 1250 and 1300 8C, dielectric constants are the highest value. The maximum values were shifted to higher temperatures as the amount of TiO 2 increased. It is related with its density. The density of x TiO 2 Ž1 y x .CeO 2 system is shown in Fig. 2. And it is presented as a function of the sintering temperatures. The density fell with increasing amount of TiO 2 . In the sintering temperature range between 1250 and 1300 8C, densities are the highest value. It is similar with dielectric constants. In general, the higher density, the higher dielectric constant of one composition. Fig. 3 shows quality factors as a function of the sintering temperatures. Quality factors roughly decease with increasing content of TiO 2 . Also in the sintering temperature range between 1250 and 1300 8C, Q ) f values are the highest. Maximum Q ) f
Fig. 1. Dielectric constant of x TiO 2 Ž1y x .CeO 2 system.
242
D.-H. Kim et al.r Materials Letters 52 (2002) 240–243
Fig. 4. Temperature coefficient of x TiO 2 Ž1y x .CeO 2 system. Fig. 2. Density of x TiO 2 Ž1y x .CeO 2 system.
values were 43,000, 26,000, 30,000 and 25,000 GHz at x s 0.2, 0.4, 0.6 and 0.8, respectively. In Fig. 4, the temperature coefficients are shown. The temperature coefficients increase with increasing amount of TiO 2 . But it decreases roughly with increasing sintering temperature. Materials in this range are quasi-stable in the range of x factor 0.2–0.4, in the range of sintering temperature of 1250–1300 8C. Temperature coefficient of resonant frequency is y14.9 and y17.7 ppmr8C at x s 0.2 and 1250 8C and x s 0.4 and 1270 8C. We expect to create dielectric ceramics with near-zero temperature coefficient in x TiO 2 Ž1 y x .CeO 2 system. From above results, its dielectric characteristics Ž ´ r , density, Q ) f and t f . are intermediate between TiO 2 and CeO 2 . Such findings are typical, when two compounds form mixed phases and there is less
Fig. 3. Q ) f values of x TiO 2 Ž1y x .CeO 2 system.
effects from secondary phases w2x. Also this indicates that its dielectric characteristics could be predicted by the logarithmic-mixing rule w3x. By proper selection of two compounds having positive and negative dielectric temperature coefficients, it is possible to achieve predictable temperature compensation.
4. Conclusions The x TiO 2 Ž1 y x .CeO 2 system was investigated and its dielectric properties were measured. In order to achieve a temperature-stable material, we studied a method of combining a positive temperature coefficient material with a negative one. And we anticipated that the characteristics of the composite would be compensated. x TiO 2 Ž1 y x .CeO 2 system has the dielectric properties as follows: 32 - ´ r - 74, 25,000 - Q ) f - 43,000 and y100 - t f - 300. It is the temperature-stable material that is achieved in the range of x factor of 0.2–0.4 and in the range of sintering temperature 1200–1300 8C. A series of new x TiO 2 Ž1 y x .CeO 2 dielectric materials were synthesized. It is found that the ceramic in the x TiO 2 Ž1 y x .CeO 2 system has excellent dielectric characteristics at microwave frequencies. However, there are needed some further studies as follows. Scanning electron microscopy and powder X-ray diffraction data ought to be examined and collected to find a correlation between dielectric properties and both the crystal structure and the microstructure. In additions, the effect of second phases on the dielectric properties should be also considered. The equation representing the mixing relation for dielectric properties needs to be discussed.
D.-H. Kim et al.r Materials Letters 52 (2002) 240–243
Acknowledgements This work was supported by the Brain Korea 21 Project and Institute for Applied Science and Technology of Sogang University.
References w1x N. Setter, R. Waser, Acta Mater. 48 Ž2000. 151. w2x M. Takata, K. Kageyama, J. Am. Ceram. Soc. 72 Ž1989. 1955.
243
w3x D.H. Kim, S.K. Lim, C. An, J. Mater. Sci.: Mater. Electron. 10 Ž1999. 673. w4x A.J. Moulson, J.M. Herbert, Electroceramics: Materials Properties and Applications. Chapman & Hall, London, 1990, p. 225. w5x K. Fukuda, R. Kitoh, I. Awai, Jpn. J. Appl. Phys. 32 Ž1993. 4584. w6x J. Takahashi, K. Kageyama, T. Hayashi, Jpn. J. Appl. Phys. 30 Ž1991. 2354. w7x A.E. Paladino, J. Am. Ceram. Soc. 54 Ž1989. 168. w8x D.G. Lim, B.H. Kim, T.G. Kim, H.J. Jung, Mater. Res. Bull. 34 Ž1999. 1577. w9x Z.Y. Xu, X.M. Chen, Mater. Lett. 39 Ž1999. 18. w10x B.W. Hakki, P.D. Coleman, IRE Trans. Microwave Theory Tech. 8 Ž7. Ž1960. 402.
Materials Letters 52 Ž2002. 240–243 www.elsevier.comrlocatermatlet
The microwave dielectric properties of x TiO 2 ž1 y x / CeO 2 ceramics Duck-Hwan Kim ) , Sang-Kyu Lim, Chul An Electronics Department, Sogang UniÕersity, Seoul 121-742, South Korea Received 14 May 2001; accepted 24 May 2001
Abstract The x TiO 2Ž1 y x .CeO 2 system was investigated and its dielectric properties were measured. In order to achieve a temperature-stable material, we studied a method of combining a positive temperature coefficient material with a negative one. CeO 2 was measured to have dielectric constant ´ r s 24, Q ) f s 57,000 and t f s y104 ppmr8C. TiO 2 has ´ r s 93, Q ) f s 10,000 and t f s 350 ppmr8C. x TiO 2 Ž1 y x .CeO 2 system has the dielectric properties as follows: 32 - ´ r - 74, 25,000 - Q ) f - 43,000 and y100- t f - 300. It is the temperature-stable material that is achieved. A series of new x TiO 2 Ž1 y x .CeO 2 dielectric materials were synthesized. Its dielectric characteristicsŽ ´r , Q ) f and t f . are intermediate between TiO 2 and CeO 2 . It is found that the ceramic in the x TiO 2 Ž1 y x .CeO 2 system has excellent dielectric characteristics at microwave frequencies. q 2002 Elsevier Science B.V. All rights reserved. Keywords: TiO 2 ; CeO 2 ; x TiO 2 Ž1 y x .CeO 2 ceramics; Dielectric constant; Temperature coefficient of resonant frequency; Q ) f ; Mixed phases; Temperature-stable material
1. Introduction Ceramic compositions are extensively used in the manufacture of electronic components. Especially microwave dielectric ceramics are increasingly used for filters, resonator and mixers in systems for wireless communication w1x. In microwave telecommunications, the usefulness of dielectric ceramics for microwave resonators has been recognized because they permit miniaturization of microwave devices. The materials used for dielectric resonators are required to have such dielectric properties as follows. Ž1. Dielectric constant Ž ´ r . ) 20. The dielectric con) Corresponding author. Tel.: q82-2-706-3401; fax: q82-2706-3401. E-mail address: [email protected] ŽD.-H. Kim..
stant is an important factor for making microwave components smaller. Ž2. Q ) f value ) 30,000 GHz. It contributes to ensure lower dielectric loss. Ž3. Temperature coefficient of resonant frequency Žt f . - "10 ppmr8C w2x. In practical use, it is important to adjust the t f of the dielectric resonator to zero. Conventionally, there are two approaches to develop an excellent ceramic material as follows. One is to make a new dielectric ceramic material. The other is to combine more than two materials for characteristic compensation. The most popular method of achieving this goal involves mixing two or more compositions with different dielectric properties. In other words, to adjust the t f to zero, two or more compounds having negative and positive t f values are employed to form a solid solution or mixed phases w3x.
00167-577Xr02r$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 1 . 0 0 4 3 4 - 7
D.-H. Kim et al.r Materials Letters 52 (2002) 240–243
In order to achieve a temperature-stable material, we examined that a method of combining a positive temperature coefficient material with a negative one. Among simple oxide compounds, TiO 2 exhibits the good dielectric properties as follows; high ´ r Žs 104., low loss Ž Q ) f s 40,000 GHz. and large positive t f Žs 460 ppmr8C. w4x. TiO 2 ceramic has been researched to modify its dielectric properties and to synthesize binary oxides system with other oxides w5,6x. Until recently, some researchers have studied also ceramic compositions containing titanate w7–9x. In these studies, TiO 2 has been chosen an additive material to tailor dielectric properties of other ceramic compositions. Also, CeO 2 is a good dielectric ceramics with low ´r Žs 24., high Q ) f Žs 58,000 GHz. and large negative t f Žs y106 ppmr8C.. By following experimental procedures, we anticipated that the characteristics of the composite would be compensated. In this study, the microwave dielectric material of x TiO 2 Ž1 y x .CeO 2 system, with temperature compensating characteristics was prepared using a conventional mixed-oxide method. We discuss the microwave dielectric properties of x TiO 2 Ž1 y x .CeO 2 ceramics where x is a molar fraction.
2. Experimental procedure Specimens were prepared by the conventional ceramic method. The starting materials ŽTiO 2 , CeO 2 . were weighed according to the ratio of x TiO 2 Ž1 y x .CeO 2 , where x s 0, 0.2, 0.4, 0.6, 0.8 and 1, respectively. x TiO 2 Ž1 y x .CeO 2 powders were wet ball-milled in deionized ŽD.I.. water for 20 h. After drying, they were calcined at 800 8C for 3 h in air. The calcined powders were milled again in D.I. water for 20 h. After being reground with ethanol, the powders were mixed with organic binder of 1 wt.% polyvinyl alcohol ŽPVA.. Pellets of 10 mm in diameter and 8–9 mm in thickness were uinaxially pressed at 2 tons. Sintering was conducted at 1230– 1450 8C for 2 h. Dielectric characteristics at microwave frequency such as permittivities and dielectric losses were measured at 4–8 GHz by Hakki–Coleman w10x dielectric resonator method. The value of temperature coefficient of resonant frequency was determined in the temperature range
241
of 0–80 8C. The density of the ceramic was measured by ASTM ŽC20-87..
3. Result and discussion Fig. 1 plots the dielectric constant of x TiO 2 Ž1 y x .CeO 2 Žwhere x s 0, 0.2, 0.4, 0.6, 0.8 and 1. compounds and these are presented as a function of the sintering temperatures. The dielectric constant rose, with increasing amount of TiO 2 . The dielectric constants firstly increased with the sintering temperature. The dielectric constants reached a maximum value at a certain sintering temperature. In the sintering temperature range between 1250 and 1300 8C, dielectric constants are the highest value. The maximum values were shifted to higher temperatures as the amount of TiO 2 increased. It is related with its density. The density of x TiO 2 Ž1 y x .CeO 2 system is shown in Fig. 2. And it is presented as a function of the sintering temperatures. The density fell with increasing amount of TiO 2 . In the sintering temperature range between 1250 and 1300 8C, densities are the highest value. It is similar with dielectric constants. In general, the higher density, the higher dielectric constant of one composition. Fig. 3 shows quality factors as a function of the sintering temperatures. Quality factors roughly decease with increasing content of TiO 2 . Also in the sintering temperature range between 1250 and 1300 8C, Q ) f values are the highest. Maximum Q ) f
Fig. 1. Dielectric constant of x TiO 2 Ž1y x .CeO 2 system.
242
D.-H. Kim et al.r Materials Letters 52 (2002) 240–243
Fig. 4. Temperature coefficient of x TiO 2 Ž1y x .CeO 2 system. Fig. 2. Density of x TiO 2 Ž1y x .CeO 2 system.
values were 43,000, 26,000, 30,000 and 25,000 GHz at x s 0.2, 0.4, 0.6 and 0.8, respectively. In Fig. 4, the temperature coefficients are shown. The temperature coefficients increase with increasing amount of TiO 2 . But it decreases roughly with increasing sintering temperature. Materials in this range are quasi-stable in the range of x factor 0.2–0.4, in the range of sintering temperature of 1250–1300 8C. Temperature coefficient of resonant frequency is y14.9 and y17.7 ppmr8C at x s 0.2 and 1250 8C and x s 0.4 and 1270 8C. We expect to create dielectric ceramics with near-zero temperature coefficient in x TiO 2 Ž1 y x .CeO 2 system. From above results, its dielectric characteristics Ž ´ r , density, Q ) f and t f . are intermediate between TiO 2 and CeO 2 . Such findings are typical, when two compounds form mixed phases and there is less
Fig. 3. Q ) f values of x TiO 2 Ž1y x .CeO 2 system.
effects from secondary phases w2x. Also this indicates that its dielectric characteristics could be predicted by the logarithmic-mixing rule w3x. By proper selection of two compounds having positive and negative dielectric temperature coefficients, it is possible to achieve predictable temperature compensation.
4. Conclusions The x TiO 2 Ž1 y x .CeO 2 system was investigated and its dielectric properties were measured. In order to achieve a temperature-stable material, we studied a method of combining a positive temperature coefficient material with a negative one. And we anticipated that the characteristics of the composite would be compensated. x TiO 2 Ž1 y x .CeO 2 system has the dielectric properties as follows: 32 - ´ r - 74, 25,000 - Q ) f - 43,000 and y100 - t f - 300. It is the temperature-stable material that is achieved in the range of x factor of 0.2–0.4 and in the range of sintering temperature 1200–1300 8C. A series of new x TiO 2 Ž1 y x .CeO 2 dielectric materials were synthesized. It is found that the ceramic in the x TiO 2 Ž1 y x .CeO 2 system has excellent dielectric characteristics at microwave frequencies. However, there are needed some further studies as follows. Scanning electron microscopy and powder X-ray diffraction data ought to be examined and collected to find a correlation between dielectric properties and both the crystal structure and the microstructure. In additions, the effect of second phases on the dielectric properties should be also considered. The equation representing the mixing relation for dielectric properties needs to be discussed.
D.-H. Kim et al.r Materials Letters 52 (2002) 240–243
Acknowledgements This work was supported by the Brain Korea 21 Project and Institute for Applied Science and Technology of Sogang University.
References w1x N. Setter, R. Waser, Acta Mater. 48 Ž2000. 151. w2x M. Takata, K. Kageyama, J. Am. Ceram. Soc. 72 Ž1989. 1955.
243
w3x D.H. Kim, S.K. Lim, C. An, J. Mater. Sci.: Mater. Electron. 10 Ž1999. 673. w4x A.J. Moulson, J.M. Herbert, Electroceramics: Materials Properties and Applications. Chapman & Hall, London, 1990, p. 225. w5x K. Fukuda, R. Kitoh, I. Awai, Jpn. J. Appl. Phys. 32 Ž1993. 4584. w6x J. Takahashi, K. Kageyama, T. Hayashi, Jpn. J. Appl. Phys. 30 Ž1991. 2354. w7x A.E. Paladino, J. Am. Ceram. Soc. 54 Ž1989. 168. w8x D.G. Lim, B.H. Kim, T.G. Kim, H.J. Jung, Mater. Res. Bull. 34 Ž1999. 1577. w9x Z.Y. Xu, X.M. Chen, Mater. Lett. 39 Ž1999. 18. w10x B.W. Hakki, P.D. Coleman, IRE Trans. Microwave Theory Tech. 8 Ž7. Ž1960. 402.