Dielectric properties of (1-x)CaTiO3-xCa(Zn1/3Nb 2/3)O3 ceramic system at microwave frequency

Article abstract of DOI:10.1111/j.1551-2916.2005.00064.x

The microwave dielectric properties of the (1-x)CaTiO₃- xCa(Zn_1/3Nb_2/3)O₃ ceramic system have been investigated. The ceramic samples sintered at 1300°-1450°C for 4 h in air exhibit orthorhombic pervoskite and form a complete solid solution for different x value. When the x value increased from 0.2 to 0.8, the permittivity ε_r decreased from 115 to 42, the unloaded quality factor Q × f increased from 5030 to 13030 GHz, and the temperature coefficient τ_f decreased from 336 to -28 ppm/°C. When x = 0.7, the best combination of dielectric properties, a near zero temperature coefficient of resonant frequency of τ_f～ -6 ppm/°C, Q × f 10 860 GHz and ε_r ～ 51 is obtained.

Full text of DOI:10.1111/j.1551-2916.2005.00064.x

J. Am. Ceram. Soc., 88 [2] 453–455 (2005)
DOI: 10.1111/j.1551-2916.2005.00064.x
ournal
J
Dielectric Properties of (1ꢀx)CaTiO3–xCa(Zn1/3Nb2/3)O3 Ceramic
System at Microwave Frequency
Hanxing Liu, Hongtao Yu,w Zhongqing Tian, Zhenghua Meng, Zhaohui Wu,
and Shixi Ouyang
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of
Technology, Wuhan 430070, China
The microwave dielectric properties of the (1ꢀx)CaTiO3–
xCa(Zn1/3Nb2/3)O3 ceramic system have been investigated.
The ceramic samples sintered at 13001–14501C for 4 h in air
exhibit orthorhombic pervoskite and form a complete solid so-
lution for different x value. When the x value increased from 0.2
to 0.8, the permittivity er decreased from 115 to 42, the unloaded
quality factor Q ꢁ f increased from 5030 to 13030 GHz, and the
temperature coefficient sf decreased from 336 to ꢀ28 ppm/1C.
When x 5 0.7, the best combination of dielectric properties, a
near zero temperature coefficient of resonant frequency of
sfBꢀ6 ppm/1C, Q ꢁ fB10860 GHz and erB51 is obtained.
(3) Nonmonotonic mixture-like behavior: Solid solution be-
tween A21B41O3 and A21(B211B251)O3 or A21(B131B251)O3 such
as CaTiO3–Ca(Mg1/3Nb2/3)O37 and CaTiO3–Pb(Fe1/2Nb1/2)O3.8
This paper investigates two ceramics in the third category.
The perovskite CaTiO3 ceramics exhibit dielectric properties
of erB170, Q ꢁ f valueB3600 GHz, and a large positive
tfB800 ppm/1C,6 and the Ca(Zn1/3Nb2/3)O3 ceramics possess
dielectric properties of erB35, Q ꢁ f valueB16 000 GHz,
and a negative tfBꢀ43 ppm/1C.9 The purpose of the present
study is to develop a new dielectric compound, which has
high dielectric, high quality factor and near to zero tf by incor-
porating Ca(Zn1/3Nb2/3)O3 into CaTiO3, and to study the
variation of microwave dielectric properties of the ceramic
system (1ꢀx)CaTiO3–xCa(Zn1/3Nb2/3)O3 as a function of com-
position (x).
I. Introduction
HE proliferation of commercial wireless technologies, such
as cellular phones, global positioning systems, and satellite
T
broadcasting, has placed increasing demands on the perform-
ance of dielectric resonators in the microwave frequency range.
These microwave dielectric ceramics must combine a high rela-
tive permittivity (er425) with a low dielectric loss, which means
a high-quality factor (Q ꢁ f45000 GHz) and a near zero tem-
perature coefficient of resonant frequency (tf ꢂ 0 ppm/1C).
These three parameters concerning the properties of microwave
dielectrics are correlated to the resonator size, frequency selec-
tivity, and temperature stability of the system, respectively. Each
dielectric property requires precise control to satisfy the de-
mands of microwave circuit designs. Various compounds such
as Ba2Ti9O20, (Zr, Sn)TiO4, Ba(Mg1/3Ta2/3)O3, and BaO–
Ln2O3–TiO2(where Ln is a lanthanum) have been researched
and have found application in the microwave communica-
tions.1–4 Another practicable way of obtaining a suitable ceram-
ic is to mix two compounds, one with a positive and the other
with a negative temperature coefficient, to form a solid solution
or mixture phases to obtain a zero temperature coefficient of
resonant frequency. Mixed ceramic approaches can be catego-
rized into three groups:
II. Experimental Procedure
(1ꢀx)CaTiO3–xCa(Zn1/3Nb2/3)O3 powders with compositions
of x 5 0.2, 0.4, 0.6, 0.7, and 0.8 were prepared using the con-
ventional ceramics processes by a two-stage calcinations proce-
dure. As starting materials, highly pure CaCO3, TiO2 (99%),
ZnO (99.5%), and Nb2O5 (99.99%) were used. First, the pow-
ders of CaTiO3 and Ca(Zn1/3Nb2/3)O3 were individually pre-
pared according to the appropriate molar ratio, and ground in
ethanol for 12 h in a ball mill with agate balls in ethanol. The
prepared powders were dried and calcined in an alumina cruci-
ble at 12001C for 4 h in air, respectively. Second, the calcined
powders were mixed for 10 h according to the molar fraction
(1ꢀx)CaTiO3–xCa(Zn1/3Nb2/3)O3 (x 5 0.2, 0.4, 0.6, 0.7, 0.8) in a
ball mill with 5% PVA as a binder. These powders were pressed
into pellets of 12 mm diameter and 6 mm thickness under a
uniaxial pressure of 200 MPa. After debinding, these pellets
were sintered in alumina crucibles at 13001–14501C for 4 h in air.
The sintering temperatures were chosen to obtain the highest
bulk density for each composition.
The crystalline phases of the prepared samples were investi-
gated by powder X-ray diffraction (XRD, Rigaku D/Max-YB)
and the surfaces of the sintered samples were observed by a
scanning electron microscope (SEM, Akashi Seisakusho Jsm-
5610LV). The bulk densities of the sintered pellets were mea-
sured by the Archimedes method. The relative permittivity (er)
and the quality factor (Q ꢁ f ) at microwave frequencies were
measured using an HP8722ET network analyzer. The tempera-
ture coefficient of resonant frequency was obtained by measur-
ing the resonant frequency of the TE01d mode at 201C ( f20) and
801C ( f80).
(1) Anomalous behavior: Solid solution between complex
perovskites in which the dielectric properties increase or de-
crease to outside the range exhibited by the end members such
as Ba(Ni1/3Nb2/3)O3–Ba(Zn1/3Nb2/3)O3.5
(2) Linear change: Solid solution between simple perov-
skites in which the microwave dielectric properties vary linearly
with compositions such as SrZrO3–SrTiO3 and CaZrO3–
CaTiO3.6
H. M. O’Bryan, Jr.—contributing editor
Manuscript No. 00064. Received January 12, 2004; approved April 12, 2004.
Supported by Key Project of Ministry of Education of People’s Republic of China, and
973-Project under 2002 CB613303.
III. Results and Discussion
The (1ꢀx)CaTiO3–xCa(Zn1/3Nb2/3)O3 system is observed to
wAuthor to whom correspondence should be addressed. e-mail: yuhongtao2001@163.
com
exhibit an orthorhombic perovskite structure like the CaTiO3-
453

454
Communications of the American Ceramic Society
Vol. 88, No. 2
120
Ca TiO
3
Ca (Zn Nb )0
1β 2β
β
100
80
60
x = 0.8
x = 0.7
x = 0.6
x = 0.4
x = 0.2
40
0.2
0.3
0.4
0.5
0.6
0.7
0.8
X values
10
20
30
40
50
60
70
2θ /Degree
Fig. 2. Dielectric constants (solid line: measured results; dotted line:
calculated results) of the (1ꢀx)CaTiO3ꢀxCa(Zn1/3Nb2/3)O3 system
(x 5 0.2 and 0.4: 14001C/4h, x 5 0.6, 0.7, and 0.8: 13501C/4 h).
Fig. 1. X-ray diffraction of the (1ꢀx)CaTiO3ꢀxCa(Zn1/3Nb2/3)O3 sys-
tem sintered at 14001C for 4 h.
500
450
400
350
300
250
200
150
100
50
14000
12000
10000
8000
Table I. Unit-Cell Volume (Vm) and Relative Density of the
(1ꢀx)CaTiO3ꢀxCa(Zn1/3Nb2/3)O3 System
Q × f
f
τ
X-ray
density
(g/cm3)
Measured
Relative
density
(%)
density
(g/cm3)
3
˚
Vm (A )
x values
Sintering conditions
0.2
0.4
0.6
0.7
0.8
14001C for 4 h 226.41 4.20
14001C for 4 h 230.11 4.34
13501C for 4 h 234.02 4.47
13501C for 4 h 235.84 4.54
13501C for 4 h 238.10 4.59
4.05
4.20
4.34
4.41
4.49
95.7
96.8
97.1
97.2
97.8
6000
0
4000
–50
0.2
0.3
0.4
0.5
X values
0.6
0.7
0.8
Table II. Dielectric Properties of the
(1ꢀx)CaTiO3ꢀxCa(Zn1/3Nb2/3)O3 System
Fig. 3. Qf values and tf of the (1ꢀx)CaTiO3ꢀxCa(Zn1/3Nb2/3)O3 sys-
tem (x 5 0.2 and 0.4: 14001C/4 h, x 50.6, 0.7, and 0.8: 13501C/4 h).
L
D
f0
Q ꢁ f
sf
x values Sintering conditions (mm) (mm) (GHz) er
(GHz) (ppm/1C)
0.2
0.4
0.6
0.7
0.8
14001C for 4 h 10.02 5.50 3.60 115 5030 336
practical application, a composition with suitable microwave
dielectric properties is obtained when x 5 0.7. For this compo-
sition the dielectric properties are erB51 and Q ꢁ fB10 860 GHz
with a near zero temperature coefficient of resonant fre-
quency of tfBꢀ6 ppm/1C. The details of dielectric properties
of the (1ꢀx)CaTiO3–xCa(Zn1/3Nb2/3)O3 ceramics are listed
in Table II.
14001C for 4 h 10.06 5.60 4.68 76 7560 164
13501C for 4 h 10.10 5.60 5.86 62 9540
13501C for 4 h 10.10 5.10 6.07 51 10860
25
ꢀ6
13501C for 4 h
9.98 5.30 6.94 42 13030 ꢀ28
type, and a single-phase structure is obtained for all com-
positions in Fig. 1. Because of the larger lattice parameter of
Ca(Zn1/3Nb2/3)O3, the unit-cell volume of all compounds in-
creases and the diffraction peaks shift to lower angles as x in-
creases. And it is well known that the porosity is closely related
to the permittivity and a low porosity is essential to obtain high
quality factor. As the x value increases, the measured density
increases and is larger than 95% of the theoretical density ob-
tained from XRD. The relative density of each compound is il-
lustrated in Table I, and all samples in the present study have
homogeneous grain growth.
IV. Conclusions
The microwave dielectric properties of single-phase ceramics
from the (1ꢀx)CaTiO3–xCa(Zn1/3Nb2/3)O3 system, prepared by
a solid-state reaction method, have been investigated. When the
x value increases from 0.2 to 0.8, the dielectric constant er de-
creases from 115 to 44, while the unloaded quality factor value
Q ꢁ f increases from 5030 to 13 030 GHz. The temperature
coefficient of resonant frequency tf varies in a wide range
from 336 to ꢀ28 ppm/1C. When x 5 0.7, the best combination
of dielectric properties, a near zero temperature coefficient of
resonant frequency of tfBꢀ6 ppm/1C, Q ꢁ fB10860 GHz, and
erB51, is obtained.
Figure 2 shows the measured results (solid line) and calculat-
ed results (dotted line) for the dielectric constants er of the re-
searched ceramic system. The er value of the well-sintered
ceramics decreases nonlinearly from 115 to 42 as the x value
increases from 0.2 to 0.8. Although the (1ꢀx)CaTiO3–xCa(Zn1/3
Nb2/3)O3 system forms a complete solid solution, as shown by
the XRD patterns in Fig. 1, its mixture-like behavior is similar
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