Influences of Fe2O3 additives on the dielectric properties of BiNbO4 ceramics under different...
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Ceramics International 30 (2004) 1341–1345 Influences of Fe 2 O 3 additives on the dielectric properties of BiNbO 4 ceramics under different sintering atmosphere Yong Yang ∗ , Shihua Ding, Xi Yao Functional Materials Research Laboratory, Tongji University, Shanghai 200092, China Received 4 December 2003; received in revised form 18 December 2003; accepted 22 December 2003 Available online 6 May 2004 Abstract BiNbO 4 ceramic is one of the potential low-firing microwave dielectric materials for microwave applications. In this work, Fe oxides were doped into BiNbO 4 ceramics. The samples were sintered in air and flowing N 2 , respectively. A comparative study of the sintering performance, the crystalline morphology and the dielectric properties (at both low frequency and microwave frequency) of samples sintered under both atmosphere were performed by means of SEM, LCR meter, network analyzer, and thermal environmental test chamber. Material’s dielectric performance under microwave frequency is especially considered. Then, the influences of Fe 2 O 3 additives on the dielectric properties of BiNbO 4 ceramics is investigated. At the same time, the defect influences on the dielectric properties were also discussed. © 2004 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: B. Defect; BiNbO 4 ceramics; Sintering atmosphere 1. Introduction The development of dielectric materials for microwave ap- plications has been focused on reducing the size and weight of RF-front end components in mobile communications. Re- cently, the development of low temperature co-firing ceram- ics (LTCCs) has attracted much interest because of their ap- plication in multilayer integrated circuits (MLICs) such as LC filters [1]. In order to satisfy the devices requirements, the microwave dielectric ceramics with low sintering tem- perature are needed to co-fire with low-loss conductors such as Ag or Cu. So, finding dielectric ceramic materials with low sintering temperature and excellent dielectric properties has become a major focus. Bismuth-based dielectric ceramics have been proposed as candidate dielectrics for LTCC due to their low sintering temperature and excellent dielectric properties. BiNbO 4 ceramic with practical dielectric properties at microwave frequency was first reported by Kagata et al. [2]. Many researchers have made strong efforts to modify the dielec- tric properties and the temperature coefficient [3,4]. In this work, we doped Fe 3+ (the valence may shift to 2+) into ∗ Corresponding author. Fax: +86-21-65985179. E-mail address: [email protected] (Y. Yang). BiNbO 4 ceramics, and sintered the samples under differ- ent atmosphere. By observing the corresponding dielectric properties, the influences of defect on the dielectric proper- ties of BiNbO 4 ceramics were discussed. 2. Experimental procedures Specimen were prepared by conventional solid-state reac- tion technique. The starting materials to form BiNbO 4 com- pound were high-purity (>99.9%) Bi 2 O 3 , and Nb 2 O 5 pow- ders. The powders were mixed according to the stoichiomet- ric ratio of BiNbO 4 and the Fe 2 O 3 doping amount of x wt.% (x = 0.3, 0.5, 0.8, and 1.0). All mixtures were ball-milled for 24 h with deionized water. After dried, the reagent was calcined at 800 ◦ C for 2 h. Then the powders were uniaxially pressed into pellets in a steel die. Those pellets were sin- tered in air and flowing N 2 (0.2 l/min) for 2 h, respectively. The sintering temperature ranged from 950 to 980 ◦ C. By using the Archimedes method, the bulk densities of the sintered ceramics were measured. Crystal structure of these Fe 2 O 3 doped BiNbO 4 ceramics were investigated by means of the X-ray diffraction (XRD) patterns (BRUKER, D8 advanced). The micrograph observation of the ceramics was performed by a SEM (JEOL, JSM-5510). An HP8753E 0272-8842/$30.00 © 2004 Elsevier Ltd and Techna Group S.r.l. All rights reserved. doi:10.1016/j.ceramint.2003.12.110
Transcript of Influences of Fe2O3 additives on the dielectric properties of BiNbO4 ceramics under different...
Ceramics International 30 (2004) 1341–1345
Influences of Fe2O3 additives on the dielectric properties of BiNbO4
ceramics under different sintering atmosphere
Yong Yang∗, Shihua Ding, Xi Yao
Functional Materials Research Laboratory, Tongji University, Shanghai 200092, China
Received 4 December 2003; received in revised form 18 December 2003; accepted 22 December 2003
Available online 6 May 2004
Abstract
BiNbO4 ceramic is one of the potential low-firing microwave dielectric materials for microwave applications. In this work, Fe oxides weredoped into BiNbO4 ceramics. The samples were sintered in air and flowing N2, respectively. A comparative study of the sintering performance,the crystalline morphology and the dielectric properties (at both low frequency and microwave frequency) of samples sintered under bothatmosphere were performed by means of SEM, LCR meter, network analyzer, and thermal environmental test chamber. Material’s dielectricperformance under microwave frequency is especially considered. Then, the influences of Fe2O3 additives on the dielectric properties ofBiNbO4 ceramics is investigated. At the same time, the defect influences on the dielectric properties were also discussed.© 2004 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: B. Defect; BiNbO4 ceramics; Sintering atmosphere
1. Introduction
The development of dielectric materials for microwave ap-plications has been focused on reducing the size and weightof RF-front end components in mobile communications. Re-cently, the development of low temperature co-firing ceram-ics (LTCCs) has attracted much interest because of their ap-plication in multilayer integrated circuits (MLICs) such asLC filters [1]. In order to satisfy the devices requirements,the microwave dielectric ceramics with low sintering tem-perature are needed to co-fire with low-loss conductors suchas Ag or Cu. So, finding dielectric ceramic materials withlow sintering temperature and excellent dielectric propertieshas become a major focus.
Bismuth-based dielectric ceramics have been proposedas candidate dielectrics for LTCC due to their low sinteringtemperature and excellent dielectric properties. BiNbO4ceramic with practical dielectric properties at microwavefrequency was first reported by Kagata et al.[2]. Manyresearchers have made strong efforts to modify the dielec-tric properties and the temperature coefficient[3,4]. In thiswork, we doped Fe3+ (the valence may shift to 2+) into
∗ Corresponding author. Fax:+86-21-65985179.E-mail address: [email protected] (Y. Yang).
BiNbO4 ceramics, and sintered the samples under differ-ent atmosphere. By observing the corresponding dielectricproperties, the influences of defect on the dielectric proper-ties of BiNbO4 ceramics were discussed.
2. Experimental procedures
Specimen were prepared by conventional solid-state reac-tion technique. The starting materials to form BiNbO4 com-pound were high-purity (>99.9%) Bi2O3, and Nb2O5 pow-ders. The powders were mixed according to the stoichiomet-ric ratio of BiNbO4 and the Fe2O3 doping amount ofx wt.%(x = 0.3, 0.5, 0.8, and 1.0). All mixtures were ball-milledfor 24 h with deionized water. After dried, the reagent wascalcined at 800◦C for 2 h. Then the powders were uniaxiallypressed into pellets in a steel die. Those pellets were sin-tered in air and flowing N2 (0.2 l/min) for 2 h, respectively.The sintering temperature ranged from 950 to 980◦C.
By using the Archimedes method, the bulk densities ofthe sintered ceramics were measured. Crystal structure ofthese Fe2O3 doped BiNbO4 ceramics were investigated bymeans of the X-ray diffraction (XRD) patterns (BRUKER,D8 advanced). The micrograph observation of the ceramicswas performed by a SEM (JEOL, JSM-5510). An HP8753E
0272-8842/$30.00 © 2004 Elsevier Ltd and Techna Group S.r.l. All rights reserved.doi:10.1016/j.ceramint.2003.12.110
1342 Y. Yang et al. / Ceramics International 30 (2004) 1341–1345
950 960 970 9806.4
6.5
6.6
6.7
6.8
6.9
7.0
7.1
7.2
7.3
App
aren
t den
sity
(g/
cm3 )
x=0.6 airx=0.8 airx=1.0 airx=1.2 airx=0.6 N2x=0.8 N2x=1.0 N2x=1.2 N2
Sintering temperature (OC)
Fig. 1. Bulk densities of Fe2O3 doped BiNbO4 ceramics at differentsintering atmosphere.
network analyzer was used to measure the dielectric constant(εr) and the quality factor valuesQ at microwave frequencyby Hakki and Coleman’s dielectric resonator method[5,6].The temperature coefficient of resonant frequencyτf wasdetermined as follows:
τf = f80 − f20
60× f20× 106 (ppm/◦C)
Simultaneously, the temperature depended dielectric proper-ties at low frequencies (1, 10, and 100 kHz) were also mea-
Fig. 3. SEM micrographs of Fe2O3 doped BiNbO4 ceramics: sintered at 970◦C: (a) x = 0.3, in air; (b) x = 0.3, in N2; (c) x = 1.0, in air; and (d)x = 1.0, in N2..
20 25 30 35 40 45 50 55 60
* orthorhombic+ triclinic
+
***** *** ****
*
*
** * *
2θ
c
ount
(a)
(b)
(c)
(d)
Fig. 2. XRD patterns of Fe2O3 doped BiNbO4 ceramics sintered at 980◦C:(a) x = 0.3, in air; (b)x = 1.0 in air; (c) x = 03, in N2; and (d)x = 1.0,in N2.
sured by means of a HP4284 precision LCR meter and athermal environmental test chamber.
3. Results and discussion
The bulk density values of Fe2O3 doped BiNbO4 ceramicswith different Fe2O3 content versus sintering temperatureswere showed inFig. 1. As a result, the bulk densities of thesamples sintered in air increased steadily as the sintering
Y. Yang et al. / Ceramics International 30 (2004) 1341–1345 1343
temperature increased from 950 to 980◦C. It can also beseen that the density values of samples sintered in N2 variedfaster than those sintered in air whenx value ranged from0.3 to 1.0.
Fig. 2 showed the XRD patterns of the samples sinteredat 980◦C under both air and N2. It could be seen that allsamples exhibited the low temperature phase (orthorhombicBiNbO4) as the main crystalline phase. However, triclinicBiNbO4 phase appeared at 2θ = 29.3 for samples sinteredin 980◦C. The amount of the triclinic BiNbO4 phase in-creased with the the increasing of Fe content. The amountof triclinic phase in the samples sintered in N2 was less thanthose sintered in air. However, no triclinic phase was foundin samples sintered at 970◦C or lower temperature. The re-sults suggested that Fe doping lowered the phase transitiontemperature, and low oxygen partial pressure atmospheremay hold back the phase transition procedure.
950 960 970 98036
38
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Die
lect
ric c
onst
ant
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0.3 wt% 0.5 wt% 0.8 wt% 1.0 wt%
950 960 970 98040
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Die
lect
ric c
onst
ant
0.3 wt% 0.5 wt% 0.8 wt% 1.0 wt%
Sintering temperature (˚C)
Fig. 4. Dielectric constants of BiNbO4 ceramics withx = 0.3–1.0 as afunction of sintering temperature.
Fig. 3 shows the SEM micrograph of the surface of thesintered samples. It can be seen that as thex value increased,the grain size became smaller, and the ceramics sintered inN2 were apparently not as dense as those sintered in air.It can also be found that very small pores existed in grainboundary and triple point when the Fe content increased to1.0 wt.%.
The variation of the microwave dielectric constant (εr) andQ value at microwave frequency as a function of sinteringtemperature is shown inFigs. 4 and 5. Generally, dielectricconstant increased when sintering temperature ranged from950 to 980◦C. To the samples sintered in air,εr values in-creased with the increasing Fe content. But the rules seemedto be reversed to the samples sintered in N2. This could becorrelated with the valency change of Fe. TheQ values ofthe samples sintered in air firstly increased with the sinter-ing temperature and reached a maximum value at 960◦Cand then decreased.Q values of the samples sintered in N2showed a similar trend, but the maximum value appeared at
950 960 970 9800
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sintered in air
Q v
alue
0.3 wt%0.5 wt%0.8 wt%1.0 wt%
950 960 970 9800
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2000sintered in N
2
Q v
alue
0.3 wt% 0.5 wt% 0.8 wt% 1.0 wt%
Sintering temperature (˚C)
Sintering temperature (˚C)
Fig. 5. Q values of BiNbO4 ceramics withx = 0.3–1.0 as a function ofsintering temperature.
1344 Y. Yang et al. / Ceramics International 30 (2004) 1341–1345
970◦C. According to the XRD and the density result, thedecrease in theQ values may be caused by the amount varia-tion of the triclinic phase. With the Fe content increase from0.3 to 1.0 wt.%, theQ values decreased to a very low value.It could be explained as: when the ceramics were sinteredin N2, much more Fe ions changed their valence from 3+ to2+. When the Fe2+ substituted the Bi3+ at A site of the lat-tice, oxygen vacancies appeared to keep electrical neutrality.The amount of these oxygen vacancies increased with thexvalue. This could make theQ value decreased. The dielec-tric constants and dielectric loss at low frequencies (1, 10,and 100 kHz) versus testing temperature were measured bymeans of HP4284 LCR meter and the thermal test cham-ber. The curves are shown inFig. 6 (x = 0.3, sample sin-tered at 960◦C). As expected, the dielectric constants andloss increased to very high values as the result of thermalionic polarization when temperature was above 100◦C. Thedetails between 60 and 250◦C were magnified in the small
0 100 200 300 400 5000
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sintered in N2
1k10k100k
Die
lect
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onst
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lect
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1k 10k 100k
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-80
-40
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40
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120
Fig. 6. Dielectric properties of BiNbO4 ceramics with 0.6 wt.% ZnOaddition sintered at 970◦C as a function of temperature.
0.2 0.4 0.6 0.8 1.0-20
-10
0
10
20
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τ f (p
pm/ O
c)
X value(wt%)
sintered in air sintered in N
2
Fig. 7. τf values of Fe2O3 doped BiNbO4 ceramics sintered at 970◦Cas a function ofx value.
graphs in the figure. It can be seen that there were some dif-ferences between the curves of the samples sintered underdifferent atmosphere. When the temperature ranged from 60to 250◦C, relaxation phenomenon could be recognized onthe curves of those samples sintered in N2. But such phe-nomenon did not appeared on those samples sintered in air.It is known that low oxygen partial pressure could createlarge amount of oxygen vacancies in the lattice. The relax-ation phenomenon could be regarded as a proof of the exis-tence of the vacancies. Oxygen vacancies and doping cationat A site of the lattice (Bi3+ substituted by Fe2+) could beassociated by Coulomb force and form defect dipoles, thatcaused the relaxation phenomenon at about 120◦C. It couldbe decided that the oxygen vacancies were the main causeof the relaxation of the samples sintered in N2.
The microwave temperature coefficients (τf ) of Fe2O3doped BiNbO4 ceramics sintered at 970◦C under both atmo-sphere are shown inFig. 7. As a result,τf values of the sam-ples sintered in air decreased from about 20 to –10 ppm/◦Cas thex value increased.τf of those samples sintered inN2 varied between−5 and 35 ppm/◦C, reached a maximumvalue whenx = 0.8.
4. Conclusion
Fe2O3 addition made the microwave properties ofBiNbO4 ceramics poor (Q value< 2000). The Fe additionlowered the phase transition temperature. As the Fe amountincreased, theQ values decreased dramatically. The sinter-ing atmosphere also had a remarkable effect on the dielectricproperties. The oxygen vacancies created by low oxygenpartial pressure atmosphere could decrease theQ value, andthe vacancies also could cause the relaxation phenomenonat about 120◦C at low frequencies (1, 10, and 100 kHz).
Y. Yang et al. / Ceramics International 30 (2004) 1341–1345 1345
Acknowledgements
This work was supported by the Ministry of Sciencesand Technology of China through 863-project under grant2001AA325110, 973-project under grant 2002CB613302and the University Key Studies Project of Shanghai.
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