The Effect of Sm2O3 Content on Crystal Structure and Microwave
Dielectric Characteristics of BiNbO4 Ceramics
Wen-Cheng Tzou1, Cheng-Fu Yang2*, Kai-Huang Chen3,
Yuan-Tai Hsieh4, Chien-Min Cheng4
1
Department of Electro-Optical Engineering, Southern Taiwan University, Tainan, Taiwan, R.O.C.
2
*Department of Chemical and Materials Engineering, National University of Kaohsiung,
Kaohsiung, Taiwan, R.O.C. Corresponding Author: cfyang@nuk.edu.tw
3
Department of Electronics Engineering and Computer Science, Tung-Fang Institute of
Technology, Kaohsiung, Taiwan, R.O.C.
4
Department of Electronic Engineering, Southern Taiwan University, Tainan, Taiwan, R.O.C.
Keywords: X-ray diffraction; crystal growth; microwave dielectric characteristics
Abstract. With the addition of 0.5wt% V2O5, the (Bi1-xSmx)NbO4 (x= 0.05, 0.10, and 0.15)
ceramics can be densified around 920oC. As the sintering temperatures increase from 920oC to
1020oC, the crystal phases of (Bi1-xSmx)NbO4 ceramics change from the α(orthorhombic)-phase to
the β(triclinic)-phase. As x increases from 0.05 to 0.15, the temperature for the α-(Bi1-xSmx)NbO4
phase to disappear is shifted from 1000oC to 980oC and the temperature for the β-(Bi1-xSmx)NbO4
phase to appear is shifted from 960oC to 940oC. The saturated dielectric constants (r) of
(Bi1-xSmx)NbO4 ceramics show no apparent change with the increase of sintering temperatures.
The saturated quality values (QXf) first increase, reach a maximum at 1000oC~ 1020oC, then
slightly decrease with the increase of sintering temperatures. And we will show that the sintering
temperatures also have large influence on temperature coefficients of resonant frequency (τf).
Introduction
Bismuth-based dielectric ceramics were known as low-fire materials and had been studied for
multilayer ceramic capacitors or multi-layer microwave filter [1]. However, BiNbO4 ceramics with
practical dielectric properties at microwave frequency were developed by H.Kagata et al [2]. These
BiNbO4-based ceramics had a high dielectric constant (~44), a high quality value (Qxf), and a
higher temperature coefficient of resonant frequency (τf) [3,4]. In the BiNbO4-based ceramic
system, the ceramic compositions were the most important parameter in tailoring of its microwave
dielectric properties. For the BiNbO4-based ceramics, Bi2O3 was substituted by Nd2O3 or Nb2O5 by
Ta2O5 to improve the microwave dielectric characteristics. The reasons to cause the change in the
microwave dielectric properties were the substitution of Bi2O3 by Nd2O3 and Nb2O5 by Ta2O5. All
(Bi1-xNdx)NbO4 and Bi(Nb1-xTax)O4 ceramics investigated had the similar crystalline phase with
that of BiNbO4 ceramic. However, we speculated that the relative amounts of crystalline phase
might play a significant role on the characteristics of BiNbO4-based ceramics. For that, we were
interested to study the effects of Sm2O3 addition on the sintering behavior and microwave
dielectric characteristics of (Bi,Sm)NbO4 ceramics. In this study, the Bi2O3 was substituted by
Sm2O3 to form the (Bi1-xSmx)NbO4 compositions, where x=0.05, 0.1, and 0.15. The BiNbO4-based
ceramics was hard to be densified below 1100oC without the addition of sintering aid [3]. For that
0.5 wt% V2O5 was used as the sintering aid of (Bi1-xSmx)NbO4 ceramics. In this study, the aim of
this work was to contribute to a better understanding of different compositions on the crystal phase
variation of V2O5-doped (Bi1-xSmx)NbO4 ceramics.
Experimental Procedure
For the preparation of (Bi1-xSmx)NbO4 compositions by conventional solid-state reaction, the
stoichiometry must be precisely controlled, since there are various stable satellite phases existed in
the vicinity of the (Bi1-xSmx)NbO4 composition. Proportionate amounts of reagent-grade starting
materials of Bi2O3, Sm2O3 and Nb2O5 were mixed in according to the composition (Bi1-xSmx)NbO4
(x= 0.05, 0.1, 0.15) and ball-milling with deionized water for 5h. After drying and grinding, then
the powders were calcined at 800C for 2h. After calcination and grinding, the (Bi1-xSmx)NbO4
powders were analyzed by the X-ray diffraction (XRD) patterns, except the α (orthorhombic)phase (Bi,Sm)NbO4, the satellite (Bi,Sm)8Nb18O57, (Bi,Sm)5Nb3O15 and (Bi,Sm)2Nb10O28 phases
were also found in the calcined (Bi1-xSmx)NbO4 powders. After calcining, the 0.5wt% V2O5 was
added as the sintering aid. After mixing with 0.5wt% V2O5, the mixed powders were uniaxially
pressed into pellets in a steel die. Sintering of these pellets was carried out at temperatures between
920oC and 1040oC for 4h in air. The crystal structures of (Bi1-xSmx)NbO4 ceramics were analyzed
by using an X-ray powder diffractometer. The bulk densities of sintered (Bi1-xSmx)NbO4 specimens
were measured Archimedes method. Microwave dielectric characteristics were measured by Hakki
and Coleman’s dielectric resonator method [5], which was improved by Courtney [6]. The
microwave dielectric constants could be accurately determined by measuring the resonant
frequency of the TE011 resonant mode and verified by the TE01δ resonant mode using the HP8720B
network analyzer. The temperature coefficients of resonant frequency (τf) were defined as follows:
τf = (f85 - f20) / f20*65
(1)
Where f20 and f85 were the resonant frequency at 20oC and 85oC, respectively.
Results and Discussion
From the XRD patterns, the satellite phase are not found in the sintered (Bi1-xSmx)NbO4
ceramics, and the Sm2O3 content and sintering temperatures have large effects on the crystal
structures of (Bi1-xSmx)NbO4 ceramics. The X-ray diffraction (XRD) patterns of V2O5-doped
(Bi0.95Sm0.05)NbO4 ceramics are shown in Fig.1. For the (Bi0.95Sm0.05)NbO4 ceramic sintered
below 940oC, only the α(orthorhombic)-phase (Bi,Sm)NbO4 is observed and the satellite phases
existed in the calcined powders are not observed. As the sintering temperatures increase, the
crystal intensities of α-phase (Bi,Sm)NbO4 first increase, reach a maximum at about 960oC, and
then decrease and disappear at 1020oC-sintered (Bi0.95Sm0.05)NbO4 ceramic. The high temperature
β(triclinic)-phase (Bi,Sm)NbO4 starts to reveal in the 960oC-sintered (Bi0.95Sm0.05)NbO4 ceramics.
The crystal intensities of β-phase (Bi,Sm)NbO4 increase with the increase of sintering temperatures
and saturate at 1020oC-sintered (Bi0.95Sm0.05)NbO4 ceramics.
x
intensity
o
x
x
x x
1000oC
1020oC
o
xxx x
o
o
oo
o
x
xx x xx x
o
x
x x
x xx x
x x xx x xxx x
x x
oo
o
o o o
o
intensity
1020oC
(b)
(a)
x
o
980oC
960oC
x
x
x
1000oC
o
980 C
x
o
x
x
x
x
x
x x x x x x xx x x x x x x x x x
x x xx xx x
oo
o
o
o
o
960oC
940oC
940oC
x x
xx
x
x
920oC
oo
0
o
o
o
oo
oo
oo
20
o
o
o
o
o
o
40
oo
oo
o
o
o
o
o o
60
920 C
o o
0
o
o
o
o
oo
oo o
20
o
o
o
o
40
o
o
o
o
o ooo
ooo o
60
2 value
2 value
Fig.1 The X-ray patterns of V2O5-doped (Bi1-xSmx)NbO4 ceramic, as a function of sintering
temperatures. (o:α-phase, x:β-phase). (a) x=0.05 and (b) x=0.15.
The X-ray diffraction patterns of V2O5-doped (Bi0.85Sm0.15)NbO4 ceramics are shown in Fig.2.
The (Bi0.9Sm0.1)NbO4 (not shown here) and (Bi0.85Sm0.15)NbO4 ceramics reveal the similar results
with the (Bi0.95Sm0.05)NbO4 ceramics, except the difference in the temperatures to cause the
variation of the crystalline structure in the XRD patterns. As the results revealed in Figs.1-2 are
compared, the Sm2O3 content has the apparent influence on transition temperatures of crystalline
phases. The temperatures for (Bi0.95Sm0.05)NbO4 ceramics to reveal the β-phase (Bi1-xSmx)NbO4 is
960oC, and the temperatures for (Bi0.9Sm0.1)NbO4 and (Bi0.85Sm0.15)NbO4 ceramics to reveal
β-phase (Bi1-xSmx)NbO4 are 940oC. The temperature for (Bi0.95Sm0.05)NbO4 ceramics to
completely transform from the α-phase to the β-phase is 1020oC, and the temperatures for
(Bi0.9Sm0.1)NbO4 and (Bi0.85Sm0.15)NbO4 ceramics to completely transform are 1000oC.
From the X-ray patterns of (Bi1-xSmx)NbO4 ceramics, the SmNbO4 phase is not observed. This
result suggests that Sm2O3 will completely substitute the Bi2O3 site in the (Bi1-xSmx)NbO4
compositions of 0.5≦x≦0.15, and the substitution of Bi2O3 by Sm2O3 may influence the lattice
constants of (Bi1-xSmx)NbO4 ceramics. Compared the results shown in Figs.1-2, the XRD patterns
have no apparent shift as the Sm2O3 content increases. The lattice constants of (Bi1-xSmx)NbO4
ceramics are plotted in Fig.3 for (Bi1-xSmx)NbO4 ceramics sintered at 920oC (α-phase) and 1020oC
(β-phase). The lattice parameters of 920oC-sintered (Bi0.95Sm0.05)NbO4 ceramics are calculated to
give the following values: a=0.4997nm, b=1.1749nm and c=0.5699nm, and 1020oC-sintered
(Bi0.95Sm0.05)NbO4 ceramics are a=0.7476nm, b=0.5556nm and c= 0.77612nm. Even the Sm+3
(radius = 0.0964nm) owns smaller radius than Bi+3 (radius = 0.102nm) has, the lattice parameters
of (Bi1-xSmx)NbO4 ceramics have no apparent variation with the increase of Sm2O3 content.
7.3
density (g/cm3)
7
c
a
1020oC
b
5
12
b
940oC
8
4
c
a
0.05
7.1
6.9
6.7
* : x=0.05
+ : x=0.1
o : x=0.15
6.5
0.10
0.15
Sm2O3 content
Fig.2 The lattice constants of V2O5-doped
(Bi1-xSmx)NbO4 ceramics.
920
950
980
1010
1040
sintering temperature (oC)
Fig.3 The bulk densities of V2O5-doped
(Bi1-xSmx)NbO4 ceramic.
The bulk densities of (Bi1-xSmx)NbO4 ceramics are shown in Fig.3. At the range of 920C~
960C, the bulk densities increase may cause by the decrease of pores and the increase in grain
growth. Even the 960oC-sintered (Bi1-xSmx)NbO4 ceramics reveal a densified structure and normal
grain growth, the bulk densities still increase as the sintering temperatures are higher than 960oC.
The theoretical bulk density of the monoclinic-phase SmNbO4, α-phase and the β-phase BiNbO4
ceramics are 6.606g/cm3, 7.345g/cm3 and 7.5g/cm3, respectively [7]. Except the grain growth, the
bulk densities of (Bi1-xSmx)NbO4 ceramics will be relative to the Sm2O3 content and the crystalline
phase. As the sintering temperatures are higher than 960oC, the transition of (Bi1-xSmx)NbO4
ceramics from α-phase to β-phase would be the reason to cause the increase of bulk densities. Fig.3
also shows that the saturated bulk densities of (Bi1-xSmx)NbO4 ceramics slightly decrease with the
increase of Sm2O3 content. The theoretical bulk densities, calculated from XRD data, of β-phase
(Bi1-xSmx)NbO4 ceramics for x=0.05, 0.1 and 0.15 are 7.4353g/cm3, 7.4096g/cm3 and 7.3589g/cm3.
The measured bulk densities of 1020oC-sintered (Bi1-xSmx)NbO4 ceramics for x=0.05, 0.1 and 0.15
are 7.27g/cm3, 7.22g/cm3, and 7.19g/cm3. The results prove that the Sm2O3 has apparent influence
on the bulk densities of (Bi1-xSmx)NbO4 ceramics, because the lattice parameters have no apparent
change as the Bi2O3 is substituted by Sm2O3. Because the heavier Bi atom is substituted by lighter
Sm atom, the bulk densities of (Bi1-xSmx)NbO4 ceramics will decrease with increase of x value.
The microwave dielectric constants (εr values) of (Bi1-xSmx)NbO4 ceramics are investigated,
and the results are shown in Fig.4. As the sintering temperatures are below 960oC, the εr values of
(Bi1-xSmx)NbO4 ceramics increase with the increase of sintering temperatures, that will be caused
by the increase in grain growth and the decrease in pores. As shown in Fig.4, the saturated εr values
are slightly decreased as the Sm2O3 content increases from x=0.05 to x=0.15. In according to the
reporter by Shannon [8], the molar polarizabilities of (Bi1-xSmx)NbO4 ceramics can be estimated
and the microwave dielectric constant will be predicted by Eq.(2) :
r value
Q x f (x104)
f value (ppm/oC)
α(A’B”O4) = α(A’1.5+) + α(B”2.5+) + 4 α(O2-)
(2)
Where A represents Bi and Sm, B represents Nb, and O represents oxygen. α(A’B”O4)
represents the molecular polarizability, α(A’1.5+), α(B”2.5+), and α(O2-) represents the ionic
polarizability of Bi3+ (or Sm3+), Nb5+, and O2- ions in Eq.(2). The α values of Bi3+, Sm3+, Nb5+, and
O2+ are 6.12, 4.74, 3.97, and 2.01Å, respectively. From Eq.(3), the α(Sm3+) is smaller than α(Bi3+)
and the molecular polarizabilities α(A’B”O4) values of (Bi1-xSmx)NbO4 ceramics will linearly
decrease with the increase of Sm3+ ion content. This is the reason that the measured microwave
dielectric constants of (Bi1-xSmx)NbO4 ceramics will decrease with the increase of Sm2O3 content.
The quality values (Qxf) of (Bi1-xSmx)NbO4 ceramics
0
are also investigated in Fig.4. As the sintering temperatures
-60
increase from 940oC to 980oC, the Qxf values increase
-120
apparently. The decrease in pores and increase in grain
-180
growth will cause this result. The maximum Qxf values are
about 57900, 69500, and 63800 for x=0.05, 0.1, and 0.15,
6
respectively. The temperature coefficients of resonant
4
frequency (f values) are also shown in Fig.4, the sintering
temperatures have large effect on the f values of
2
41
(Bi1-xSmx)NbO4 ceramics. As the sintering temperatures
38
increase from 980oC~1020oC, the f values of
* : x=0.05
+ : x=0.1
35
(Bi1-xSmx)NbO4 ceramics linearly change from small
o : x=0.15
o
o
negative values (-5.4 ppm/ C, -1.1 ppm/ C, and -8.9
32
920
960
1000
1040
ppm/oC for x=0.05, 0.1, and 0.15, respectively) to large
sintering temperature (oC)
o
o
negative values (-165 ppm/ C, -178 ppm/ C, and -130
Fig.4 The microwave dielectric
ppm/oC for x=0.05, 0.1, and 0.15, respectively). The characteristics
of
V2O5-doped
crystal phase of (Bi1-xSmx)NbO4 ceramics transforms from (Bi1-xSmx)NbO4 ceramics.
the α-phase to the β-phase will cause this result.
Conclusions
The crystal structure and the microwave dielectric characteristics of (Bi1-xSmx)NbO4 ceramics
have been developed in this study, and several conclusions are deduced. The V2O5-doped
(Bi1-xSmx)NbO4 ceramics can be densified at about 960oC. Though the bulk densities of
V2O5-doped (Bi1-xSmx)NbO4 ceramics increase as the sintering temperatures are higher than 960oC,
the crystal phase of (Bi1-xSmx)NbO4 ceramics changes from the α-phase to the β-phase would be
the reason. The microwave dielectric constants of (Bi1-xSmx)NbO4 ceramics increase with the
increase of sintering temperatures and saturate at about 980oC. The microwave dielectric constants
of (Bi1-xSmx)NbO4 ceramics slightly decrease as the β-phase (Bi1-xSmx)NbO4 phase is revealed.
The saturated Qxf values are independent on the Sm2O3 content. The β-phase (Bi1-xSmx)NbO4
ceramics have the larger f values than the α-phase (Bi1-xSmx)NbO4 ceramics do.
References
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[4] C. F. Yang: J.Mater.Sci.Let. 18 (1999) 805.
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[6] W.E.Courtney., IEEE Trans.on M.T.T. 18 (1970) 476.
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