5
Microstructure and Some Microwave Properties of Ba 2 Ti 9 O 20 Doped with ZrO 2 Koldayeva, Yu. 1 ; Nono, M. C. A. 1 ; Castro, P. J. 2 ; and Fonseca, S. T. 1 Laboratório Associado de Sensores e Materiais - LAS 2 Laboratório Associado de Plasma - LAP Instituto Nacional de Pesquisas Espaciais - INPE C.P. 515 - 12201-970 - São José dos Campos, SP Keywords: ZrO 2 - Ba 2 Ti 9 O 20 ceramics, microstructure, dielectric resonators, microwave dielectric properties Abstract The effects of different concentrations of ZrO 2 on the microstructure and dielectric properties of barium nanotitanate (Ba 2 Ti 9 O 20 ) dielectric resonators (DRs) for microwave applications are reported. High dielectric constant, small dielectric loss as well as frequency high stability are very important for use in microwave telecommunication, particularly, in a local oscillator of an INPE Communication Satellite. Three different samples of Ba 2 Ti 9 O 20 with different percentages of ZrO 2 were prepared: with 2.0, 2.46 and 3.0 mol% dopant content. The dielectric constant and unloaded quality factor were determined for each different sample. Introduction Various kinds of dielectric ceramic for microwave telecommunications, satellite-broadcasting systems and great number of personal communication systems have been developed during last decades [1]. Those materials should fulfill the requirements of the high dielectric constant as well as the quality factor high values and low temperature coefficient to be used as dielectric resonators (DRs) [1-7]. Several ceramic compositions (BaTi 4 O 9 , BaTi 5 O 11 , and Ba 2 Ti 9 O 20 ) based on barium and titanium compounds [1-5] have presented good dielectric properties. The Ba 2 Ti 9 O 20 ceramic attracts more attention due to its temperature coefficient which is generally lower than other ones [8-10]. The difficulty for obtaining pure Ba 2 Ti 9 O 20 phase formation can be attributed to the high potential energy barrier and curvature of the equilibrium phase boundary. Metallic cations, as Sn 4+ , Zr 4+ , Nb 5+ that can form substitutional solid solutions, are usually used for the Ba 2 Ti 9 O 20 stabilization [8-12]. It has been demonstrated that the ceramic porosity affects the dielectric constant (ε r ), the quality factor (Q 0 ) and the thermal coefficient (τ f ) of the DR, as is shown in Fig. 1. Microstructures, dielectric constant (ε r ), and unloaded quality factor (Q o ) of Ba 2 Ti 9 O 20 ceramics doped with zirconia (ZrO 2 ) were investigated in this work. These ceramics are developed for application as microwave dielectric resonators particularly in a local oscillator of an INPE Communication Satellite. Experimental Procedure Different samples were prepared: Ba 2 Ti 9 O 20 specimen and others with zirconia content. They were mixed compliantly with the chemical compound stoichiometry: 18.2 mol% of BaO and 81.8 mol% of TiO 2 for preparation of undoped Ba 2 Ti 9 O 20 ceramics and others with equimolar substitutions of TiO 2 for ZrO 2 (2 mol%, 2.46 mol%, 3 mol%) for doped ones. The powder mixture was compacted by a uniaxial (40 MPa) and isostatic (300 MPa) pressings. So, cylindrical test bodies were produced with preselected ratio H/D (where H is the height and D,

Microstructure and Some Microwave Properties of …Microstructure and Some Microwave Properties of Ba2Ti9O20 Doped with ZrO2 Koldayeva, Yu.1; ... Ba2Ti9O20 ceramic attracts more attention

  • Upload
    others

  • View
    29

  • Download
    0

Embed Size (px)

Citation preview

Page 1: Microstructure and Some Microwave Properties of …Microstructure and Some Microwave Properties of Ba2Ti9O20 Doped with ZrO2 Koldayeva, Yu.1; ... Ba2Ti9O20 ceramic attracts more attention

Microstructure and Some Microwave Properties of Ba2Ti9O20 Doped with ZrO2

Koldayeva, Yu.1; Nono, M. C. A.1; Castro, P. J.2; and Fonseca, S. T.

1Laboratório Associado de Sensores e Materiais - LAS

2Laboratório Associado de Plasma - LAP Instituto Nacional de Pesquisas Espaciais - INPE C.P. 515 - 12201-970 - São José dos Campos, SP

Keywords: ZrO2 - Ba2Ti9O20 ceramics, microstructure, dielectric resonators, microwave dielectric properties

Abstract The effects of different concentrations of ZrO2 on the microstructure and dielectric properties of barium nanotitanate (Ba2Ti9O20) dielectric resonators (DRs) for microwave applications are reported. High dielectric constant, small dielectric loss as well as frequency high stability are very important for use in microwave telecommunication, particularly, in a local oscillator of an INPE Communication Satellite. Three different samples of Ba2Ti9O20 with different percentages of ZrO2 were prepared: with 2.0, 2.46 and 3.0 mol% dopant content. The dielectric constant and unloaded quality factor were determined for each different sample. Introduction

Various kinds of dielectric ceramic for microwave telecommunications, satellite-broadcasting systems and great number of personal communication systems have been developed during last decades [1]. Those materials should fulfill the requirements of the high dielectric constant as well as the quality factor high values and low temperature coefficient to be used as dielectric resonators (DRs) [1-7]. Several ceramic compositions (BaTi4O9, BaTi5O11, and Ba2Ti9O20) based on barium and titanium compounds [1-5] have presented good dielectric properties. The Ba2Ti9O20 ceramic attracts more attention due to its temperature coefficient which is generally lower than other ones [8-10]. The difficulty for obtaining pure Ba2Ti9O20 phase formation can be attributed to the high potential energy barrier and curvature of the equilibrium phase boundary. Metallic cations, as Sn4+, Zr4+, Nb5+ that can form substitutional solid solutions, are usually used for the Ba2Ti9O20 stabilization [8-12].

It has been demonstrated that the ceramic porosity affects the dielectric constant (εr), the quality factor (Q0) and the thermal coefficient (τf) of the DR, as is shown in Fig. 1. Microstructures, dielectric constant (εr), and unloaded quality factor (Qo) of Ba2Ti9O20 ceramics doped with zirconia (ZrO2) were investigated in this work. These ceramics are developed for application as microwave dielectric resonators particularly in a local oscillator of an INPE Communication Satellite.

Experimental Procedure

Different samples were prepared: Ba2Ti9O20 specimen and others with zirconia content. They were mixed compliantly with the chemical compound stoichiometry: 18.2 mol% of BaO and 81.8 mol% of TiO2 for preparation of undoped Ba2Ti9O20 ceramics and others with equimolar substitutions of TiO2 for ZrO2 (2 mol%, 2.46 mol%, 3 mol%) for doped ones. The powder mixture was compacted by a uniaxial (40 MPa) and isostatic (300 MPa) pressings. So, cylindrical test bodies were produced with preselected ratio H/D (where H is the height and D,

Page 2: Microstructure and Some Microwave Properties of …Microstructure and Some Microwave Properties of Ba2Ti9O20 Doped with ZrO2 Koldayeva, Yu.1; ... Ba2Ti9O20 ceramic attracts more attention

the diameter), so that the final dimensions of sintered ceramics should be suitable for obtaining the desired resonant frequency range. The samples were synthesized and sintered at 1300°C for 3 hours. Afterwards, the ceramic crystallographic phase composition were evaluated from the X-rays diffraction (Philips X-rays diffratometer, Model PW3710) patterns of powders. The fracture surfaces were observed by scanning electron microscopy (SEM, JEOL, Model JSM-5310). Microwave characteristics were determined utilizing the test setup shown in Fig. 2.

3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.80

10

20

30

40

50

Porosity

(ppm

/ o C)

τ f

0.818 mol of TiO2+ 0.182 mol of BaO4 - 6 GHzDtheoretical = 4.8 g/cm3

-100

-50

Q

5000

3000

1000

εr

( g/cm3)Density

τf

Q

εr

Figure. 1 The influence of porosity on dielectric characteristics of Ba2Ti9O20 ceramics.

Figure.2 Experimental setup for measuring the microwave parameters [6,7].

It is important for applications to know accurately the permittivity (dielectric constant) of the

material in order to determine the dimensions of the DR for a given frequency. This parameter is measured using a test box made from golden copper, were the cylindrical sample with radius a and height H, is placed between two parallel conducting plates [3, 4]. This configuration when the metal plates make contact with the DR allows that it can operate at the electromagnetic mode TE011, that owns the largest energy portion. Among all the frequency modes, this is the only one

Page 3: Microstructure and Some Microwave Properties of …Microstructure and Some Microwave Properties of Ba2Ti9O20 Doped with ZrO2 Koldayeva, Yu.1; ... Ba2Ti9O20 ceramic attracts more attention

the frequency of which decreases as the metal plate moves away from the resonator. Knowing the experimental value of the resonant frequency we can determine the dielectric constant through a field equation relating resonant frequency, dielectric constant and resonator dimensions. The calculation was made running the software “Mathematica” in a microcomputer [3, 4].

As for the quality factor, providing the test box dimensions are at least three times the resonator size and inserting the DR between low-loss teflon spacers, then the metallic losses can be ignored.

The microwave measurements were performed according to the test setup shown in Figure 3. The DR was excited by means of an electric probe with optimum coupling. Another electric probe is used as a receiving device to detect the sign radiated by the resonator. Results In a previous work [13] those all doped ceramics have presented the barium nanotitanate as the major phase. The analyses both of the samples with and without Zr4+ substitutions (doped and undoped samples, correspondently) show that the Ba2Ti9O20 and BaTi4O9 phases and the small quantities of TiO2 formed in all samples, except for the ceramic doped with 3 mol% ZrO2 where any BaTi4O9 phase was not detected.

10 20 30 40 50 60 70 80

A - BaTi4O9B - Ba2Ti9O20C - TiO2 3 mol% ZrO2

2.46 mol% ZrO2

2 mol% ZrO2

0 mol% ZrO2

CCC

CC

BBBBBBBBBBBB

BBB

B

BB

C

BB

AC

CC

CC

CC

C

BBBBB

BBB

B

BA

AAAA

CC C CC BB B B B B BBBBB

BB

B

B A A AA A A

C CC

BB

B AAAA AAAAAAA

AA

AA

AAA

Inte

nsity

(u.a

.)

2θ (degrees)

Figure. 2 X-rays diffraction patterns for the investigated ceramics

Page 4: Microstructure and Some Microwave Properties of …Microstructure and Some Microwave Properties of Ba2Ti9O20 Doped with ZrO2 Koldayeva, Yu.1; ... Ba2Ti9O20 ceramic attracts more attention

The scanning electronic microscopy analyses of the ceramic fracture surfaces showed a high

densification degree of the microstructure and the presence of few pores (Fig.3). These defects appear due to the packing powder flaws occurred during the compaction step. The defects can be minimized by optimizing the grain size distribution of these powder mixtures that in turn can be obtained by a suitable procedure in the grinding process.

a b

c d

Figure. 4 SEM images for fracture surface microstructure of the investigated ceramics: (a)

without ZrO2, (b) 2 mol % of ZrO2, (c) 2.46 mol % of ZrO2, and (d) 3 mol % of ZrO2

It is very important to obtain very dense microstructure, ideally without any porosity, so that

this characteristic influences on dielectric performance of ceramics, as reported in Fig.1. Despite the fact there are some pores in these ceramics, even so they have presented high dielectric constants which increase with zirconia amount (Fig. 4a). The unloaded quality factor values (Fig. 4b) depend strongly on forming phases. According to the XRD results, the highest Q0 value ceramics without zirconia can have been influenced by the major BaTi4O9 phase content, because it has the lowest dielectric loss among titanates [9]. In case of the doped ones the influence of ZrO2 dopant on quality factor values do not showed a linear variation, due to the phase distribution to be unhomogeneous (Fig. 4b).

Page 5: Microstructure and Some Microwave Properties of …Microstructure and Some Microwave Properties of Ba2Ti9O20 Doped with ZrO2 Koldayeva, Yu.1; ... Ba2Ti9O20 ceramic attracts more attention

0.0 0.5 1.0 1.5 2.0 2.5 3.035

36

37

38

39

40

ε r

ZrO2 (mol %)

a

0.0 0.5 1.0 1.5 2.0 2.5 3.02000

3000

4000

5000

6000

Q o

ZrO2 (mol %)

b

Figure. 4 Dielectric properties: a) dielectric constant and b) unloaded quality factor.

Conclusions The results obtained in the present investigation are very promising for the complete Ba2Ti9O20 phase stabilization of dielectric resonators, including those with the zirconium addition. The ceramics showed a high density state with few pores. These microstructural flaws can be minimized by using a suitable grinding step of the initial powders. Moreover, zirconia addition influenced the increase of dielectric constant and quality factor values. References [1] D. Hennings; P. Schnabel, Philips J. Res., Vol.38, no. 6 (1983), p. 295. [2] J.M. Wu; H.W. Wang,, J. Amer. Cer. Soc., Vol. 71, no. 10 (1988), p. 869. [3] P. Tandon; J. Goyette; Y.K.Bose, J. Mater. Sci. Let., Vol. 14 (1995), p. 1373. [4] H.M. O’Bryan Jr; J. Thomson Jr, J. Amer. Cer. Soc., Vol. 66, no. 12 (1983), p. 66 [5] M.C.A. Nono; P.J. Castro, 36th Brazilian Ceramic Meeting, (1992), p. 609. [6] P.J. Castro; M.C.A. Nono, J. Microwave and Optoeletronics, Vol.4, n.1 (1999), p. 27. [7] P.J. Castro. Doctor Thesis, POLI/USP, Brazil, 1989. [8] W.-Y. Lin; R.A. Gerhardt; R.F. Speyer, J. Mater. Sci., Vol. 34 (1999), p. 3021 – 3025. [9] W.-Y. Lin; R.F. Speyer, J. Am. Ceram. Soc., Vol. 82, no. 5 (1999), p. 1207–1211. [10] W.-Y. Lin; R.F. Speyer, J. Am. Ceram. Soc., Vol. 82, no. 2 (1999), p. 325–330. [11] M.C.A. Nono; P.J. Castro, J. Mater. Sci. Forum,, Vols. 416-418 (2003), p. 11-16. [12] S.-F. Wang, T. C. K. Yang, C.-C.Chiang, S. H. Y. Tsai, Cer. Inter., Vol. 29 (2003), p. 77-81.