Effects of Crystal Structure on Microwave Dielectric Properties of Ceramics

Transcription

1 Journal of the Korean Ceramic Society Vol. 5 No. 5 pp. 5~ Review Effects of Crystal Structure on Microwave Dielectric Properties of Ceramics Eung Soo Kim Chang Jun Jeon Sung Joo Kim and Su Jung Kim Department of Materials Engineering Kyonggi University Suwon -70 Korea (Received April 008; Accepted May 8 008) ABSTRACT Microwave dielectric properties of Ti were investigated based on the structural characteristics. The dielectric constant (K) was dependent on the dielectric polarizabilities of the specimens and the deviation of the observed dielectric polarizabilities (α ) from the theoretical dielectric polarizabilities (α ) were decreased with increasing of -site bond valence. Quality factors (Qf) were affected by the sharing type of and B octahedra. Temperature coefficient of resonant frequency (TCF) was decreased with increasing of average. Key words : Microwave dielectric properties ctahedral distortion Dielectric polarizabilities Bond valence W. Introduction ith the rapid growth of the wireless communication industry tailoring the microwave dielectric properties of ceramics has been widely studied for high-performance materials suitable to wireless applications. However most studies in this field have been focused on improving the microwave dielectric properties through various experiments. Therefore the dependence of microwave dielectric properties on composition and crystal structure should be studied to predict and control effectively the microwave dielectric properties of ceramics. Microwave dielectric properties are largely dependent on the structural characteristics of materials. In particular the temperature coefficient of resoanant frequency (TCF ) is closely related to the distortion of oxygen octahedra resulting from the changes of bond length between the octahedral cation and oxygen ion ) while the quality factor (Qf ) is affected by the sharing type of oxygen octahedra. ) The dielectric constant (K) is dependent not only on the dielectric polarizabilities of the composing ions of the compound but also on the rattling or compression effect of the octahedral cation resulting from the changes of bond valence between the octahedral cation and oxygen ion. ) In general -based ceramics such as Ti have - and B-site oxygen octahedra composed of the octahedral cation with six oxygen ions. Even though the difference of the B-site ionic size (Ti + = 0.05 Å W + = 0. Å Nb 5+ = 0. Å Ta 5+ = 0. Å) is not much at the coordination number of ) the crystal Corresponding author : Eung Soo Kim eskim@kyonggi.ac.kr Tel : Fax : structure of these -based ceramics was distinguished from the sharing type of and B octahedra unit-cell volume and bond valence between octahedral cation and oxygen ion. Therefore the microwave dielectric properties of Ti ceramics were investigated with regard to the crystallographic considerations of oxygen octahedra.. Experimental Procedure xide powders with purity above 99.9% were used as starting materials. The powders were weighed according to the compositions of Ti and and then milled with Zr balls in ethanol for h. To obtain a single phase of each composition was calcined at 700 o C for h while Ti and were calcined at 00 o C for h respectively. These calcined powders were re-milled for h and pressed into pellets isostatically under the pressure of 500 kg/cm. These pellets were sintered from 00 o C to 00 o C for h in air. Powder X-ray diffraction analysis (D/Max-C RIGAKU Japan) was used to determine the crystalline phases. From the Rietveld refinements of X-ray diffraction (XRD) patterns the atomic positions lattice parameters and unit cell volumes of the sintered specimens were obtained using a Rietan-000 program. 5) The micro- structure of the sintered specimens was observed via scanning electron microscope (SEM JSM-700F JEL Japan). The dielectric constant (K) and unloaded Q value at 9 ~ GHz were measured by the Hakki and Coleman method. ) The temperature coefficient of the resonant frequency (TCF ) was measured by the cavity method 7) in the temperature range from 0 o C to 80 o C. 5

2 5 Journal of the Korean Ceramic Society - Eung Soo Kim et al. Vol. 5 No. 5 Table. Sintering Temperature and Density of Ti Nb and Ta Sintering Apparent temperature ( o C) density (g/cm ) Relative density (%) Ti Nb Ta Results and Discussion The optimal sintering temperature of Ti ceramics (-based ceramics) changed with the type of B-site cations used (B = Ti + W + Nb 5+ Ta 5+ ) as summarized in Table. To reduce the effects of density on the microwave dielectric properties the relative density was above 95% of theoretical density of each composition. As shown in Fig. -based ceramics showed the single phase with their own crystal structures: trigonal ilmenite for Ti monoclinic wolframite for orthorhombic columbite for and tetragoral tri-rutile for respectively. The refined atomic positions and lattice parameters of the sintered specimens obtained from the Rietveld refinement are shown in Table. For the -based ceramics the coordination number of - and B-site cations is six and octahedra are connected to B octahedra with their own sharing type (corner edge face) of octahedra in the unit cell. Although the difference of the B-site ionic size (Ti + = 0.05 Å W + = 0. Å Nb 5+ = 0. Å Ta 5+ =0. Å) ) is not much the unit cell volume changed remarkably with the type of B-site cations used due to the changes of crystal Fig.. X-ray diffraction patterns of (a) (b) Ti (c) Nb and (d) Ta sintered specimens. structure. These results could be attributed to the differences of the bond strength resulting from the sharing type of and B octahedra. The bond strength between an octahedral cation and an oxygen ion was closely related with the bond valence which is a function of the bond length and bond valence parameter. 8) Therefore the bond valence could be applied to the estimation of bond strength. The bond valences between a + cation and an oxygen ion v - were calculated from eq. (). 9) Table. Atomic Positions and Lattice Parameters of Ti Nb and Ta Ti Nb Ta Space group R-H P/c Pbcn P /mnm Atom Wyckoff site x y z ccupation c Ti c f f W e g g c Nb 8d d d d a Ta e f j Lattice parameters a b c

3 May 008 Effects of Crystal Structure on Microwave Dielectric Properties of Ceramics 5 -site Bond Valence (V ) of Ti Nb and Ta d d d v v v R.9.7( ).05( ).09( ) 0.7( ) 0.78( ) 0.8( ) Ti.9.( ).0( ) 0.8( ) 0.88( ) Nb.9.080( ).( ) 0.5( ) 0.05( ) Ta.9.7( ).05( ) 0.8( ) 0.79( ) Table V Comparison of bserved and Theoretical Polarizabilities of Ti Nb and Ta bserved % Theoretical α )/ ( α α (Å ) K V (Å ) Z α (Å ) α Ti Nb Ta Table. unitcell ν = exp (R d ) b where R is the bond valence parameter d is the bond length between and oxygen ions and b is commonly taken to be a universal constant equal to 0.7 Å. The bond valence parameters followed the values in a previous report. The -site bond valence (V ) was defined as the sum of all the valences from a given cation and was calculated from eq. (). () ) 9) + 9) V = ν () ions used (B=W Ti Nb Ta ). At microwave frequencies the dielectric constant (K ) is not only dependent on the density and secondary phases but also on the crystal structure. Since the relative density was higher than 95% (Table ) and there was no secondary phase (Fig. ) in the given compositions the K was not significantly affected by the relative density or a secondary phase. Effects of crystal structure on the dielectric constant (K ) could be evaluated by the comparisons between the theoretical dielectric polarizabilities (a ) obtained from the additive rule and the observed dielectric polarizabilities (a ) obtained from the measured K using the Clauses - Mosotti equation. Table shows the comparisons of a and a of the based ceramics. Both a and a were dependent on the dielectric constant (K ) of each composition; however the deviations of a from a changed with the type of B-site ) ) Table summarizes the V of -based ceramics. The V of the specimens increased with the type of B-site cat ) Fig.. Dependence of polarizabilities deviations on -site bond valence (V ) of -based ceramics. Fig.. SEM micrographs of (a) (b) Ti (c) Nb and (d) Ta sintered specimens.

4 5 Journal of the Korean Ceramic Society - Eung Soo Kim et al. Vol. 5 No. 5 Table 5. Qf Value and Sharing Type of and B ctahedra of Ti and Qf (GHz) Crystal structure Sharing type of and B octahedra Ti 98 Ilmenite Face 0 Tri-rutile Edge 0888 Columbite Corner 08 Wolframite Corner cations (B =W + Ti + Nb 5+ Ta 5+ ). These results could be attributed to the -site bond valence (V ) shown in Table which resulted from the crystal structure of each composition. Fig. shows the dependence of polarizabilities deviations on V. With the increase of V the rattling of an Fig.. Dependence of TCF on the s of -based ceramics. octahedral cation ( + ) decreased; in turn the a decreased. Therefore K of the specimens was strongly dependent on the dielectric polarizabilities of composition. It has been reported ) that the Qf value depends on intrinsic factors such as crystal structure and lattice vibrations as well as on extrinsic factors such as density secondary phase and grain size. The effect of extrinsic factors on Qf value could be negligible due to a relative density of higher than 95% (Table ) and no secondary phase (Fig. ). As shown in Fig. the grain size of the specimen was larger than the Ti specimens. However the Qf value of was smaller than those of Ti and. Therefore the relationship between the crystal structure and Qf value of -based ceramics should be considered. The Qf value of Ti (ilmenite) was remarkably larger than those of (tri-rutile) (columbite) and (wolframite). The face sharing of and B octahedra has a larger Table. Average ctahedral Distortion and TCF of Ti Ti R -.7( ).( ).7( ).080( ) R -.05( ).0( ).05( ).( ) R -.09( ) Average R R B-.9( ).098( ).9( ).908 R B-.007( ).88( ).78( ).070 R B-.980( ).( ).959 R B-.7 R5 B-.080 R B-.79 Average R B site ( 0 ) B-site 0.080( ) ( ) ( B 0 ) Average ( 0 ) TCF (ppm/ o C)

5 May 008 Effects of Crystal Structure on Microwave Dielectric Properties of Ceramics 55 sharing area than the edge or corner sharing of and B octahedra. With the increase of sharing area of and B the Qf value of the specimens increased as shown in Table 5. Therefore the Qf value of the specimens was affected by the sharing type of and B octahedra. Even though and show the same corner sharing of and B octahedra the Qf value of was larger than that of. The reasons why showed a larger Qf value than that of may be related to the -site bond valence but are as yet unclear and under investigation. Table summarizes the average and the temperature coefficient of resonant frequency (TCF) of the -based ceramics. The individual bond lengths of oxygen octahedra were obtained from the atomic positions and lattice parameters as confirmed in Table. From the individual bond length of the oxygen octahedra the (D) of all the structures was calculated from eq. (). ) ( R () -- i R) = R where R i is an individual bond length and R is the average bond length of oxygen octahedra. With the type of B-site cations (B = Ta 5+ Ti + W + Nb 5+ ) the TCF of the specimens decreased due to the increase of average as shown in Fig.. A TCF of zero could be obtained by the control of the average of ceramics resulting from the substitution of ions with ionic sizes between 0.05 Å (Ti + ) and 0. Å (Ta 5+ ) for the B-site of -based ceramics.. Conclusions For Ti ceramics a single phase with trigonal ilmenite (B = Ti + ) monoclinic wolframite (B = W + ) orthorhombic columbite (B = Nb 5+ ) and tetragonal tri-rutile (B = Ta 5+ ) structures were confirmed respectively. The dielectric constant (K) was largely dependent on the dielectric polarizabilities of composition. The deviation of the observed dielectric polarizabilities (a ) from the theoretical dielectric polarizabilities (a ) decreased with increasing -site bond valence. The quality factor (Qf ) value of Ti was remarkably larger than those of and. These results could be attributed to the sharing area of and B octahedra. With the type of B-site cations (B = Ta 5+ Ti + W + Nb 5+ ) the temperature coefficient of resonant frequency (TCF) of the specimens decreased with increasing average octahedral distortion. Acknowledgements This work was supported by a Kyonggi University Research Grant 007. REFERENCES.E. S. Kim S. H. Kim and K. H. Yoon Dependence of Thermal Stability on ctahedral Distortion of (-x) (Ca 0. Li 0.9 Sm 0.7 )Ti -xlnal (Ln = Nd Sm) Ceramics J. Ceram. Soc. Jpn. 5-9 (00).. J. H. Shon Y. Inaguma S.. Yoon M. Itoh T. Nakamura S. J. Yoon and H. J. Kim Microwave Dielectric Characteristics of Ilmenite-Type Titanates with High Q Values Jpn. J. Appl. Phys (99).. E. S. Kim and K. H. Yoon Microwave Dielectric Properties of (-x)cati -xli / Sm / Ti Ceramics J. Euro. Ceram. Soc (00).. R. D. Shannon Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides Acta Cryst. A 75-7 (97). 5. F. Izumi and T. Ikeda A Rietveld-Analysis Programm RIETAN-98 and its Applications to Zeolites Mater. Sci. Forum 98-0 (000).. B. W. Hakki and P. D. Coleman A Dielectric Resonator Method of Measuring Inductive Capacities in the Millimeter Range IRE Trans. Microwave Theory Tech (90). 7. T. Nishikawa K. Wakino H. Tamura H. Tanaka and Y. Ishikawa Precise Measurement Method for Temperature Coefficient of Microwave Dielectric Resonator Material IEEE MTT-S Int. Microwave Symp. Dig (987). 8. H. S. Park K. H. Yoon and E. S. Kim Relationship between the Bond Valence and the Temperature Coefficient of the Resonant Frequency in the Complex Perovskite (Pb -x Ca x )[Fe 0.5 (Nb -y Ta y ) 0.5 ] J. Am. Ceram. Soc (00). 9. N. B. Brese and M. Keefe Bond-Valence Parameters for Solids Acta Cryst. B (99). 0. I. D. Brown and D. Altermatt Bond-Valence Parameters btained from a Systematic Analysis of the Inorganic Crystal Structure Database Acta Cryst. B -7 (985).. R. D. Shannon Dielectirc Polarizabilities of Ions in xides and Fluorides J. Appl. Phys (99).. D. M. Iddles A. J. Bell and A. J. Moulson Relationships between Dopants Microstructure and the Microwave Dielectric Properties of Zr -Ti -Sn Ceramics. J. Mater. Sci (99).. S. G. Mhaisalkar D. W. Readey S. A. Akbar P. K. Dutta M. J. Sumnar and R. Rokhlin Infrared Reflectance Spectra of Doped BaTi 9. J. Solid State Chem (99).