Journal of Alloys and Compounds 468 (2009) 522–527
Effects of Nd/Sm ratio and glass addition on the microwave dielectric
properties of Ba
4.5
(Sm
(0.8-x)
Nd
x
Bi
0.2
)
9
Ti
18
O
54
ceramics
Yu-Chuan Wu, Sea-Fue Wang
∗
, Yuh-Ruey Wang, Wen-Jung Wu
Department of Materials and Mineral Resources Engineering, National Taipei University of Technology, 1, Sec. 3,
Chung-Hsiao E. Road, Taipei 10608, Taiwan, ROC
Received 3 August 2007; received in revised form 5 January 2008; accepted 9 January 2008
Available online 2 May 2008
Abstract
In this study, Ba
4.5
(Sm
(0.8-x)
Nd
x
Bi
0.2
)
9
Ti
18
O
54
ceramics with various amounts of glass were prepared. The effects of Nd/Sm ratio and glass
content on the microstructure evolution and microwave dielectric properties were evaluated. SEM micrographs revealed that the microstructures
of Ba
4.5
(Sm
(0.8-x)
Nd
x
Bi
0.2
)
9
Ti
18
O
54
ceramics with 4 wt.% glass sintered at 1150
◦
C are uniform and their grain sizes range from 1 to 3 m. The
dielectric constant of Ba
4.5
(Sm
(0.8-x)
Nd
x
Bi
0.2
)
9
Ti
18
O
54
ceramics increased with increasing sintering temperature or the Nd content, and the τ
f
value
also increases with increasing Nd content. With 6–10 wt.% glass added, Ba
4.5
(Sm
0.24
Nd
0.56
Bi
0.2
)
9
Ti
18
O
54
ceramics obtain maximum densities at
1100
◦
C. No second phase was detected in the XRD patterns. Grains are columnar-like and grain size increases significantly with increasing glass
content and sintering temperature. The results showed that Ba
4.5
(Sm
0.24
Nd
0.56
Bi
0.2
)
9
Ti
18
O
54
ceramics with 2 wt.% glass sintered at 1150
◦
C had a
sintering density of 5.6 g/cm
3
, dielectric constant of 87.9, Q·f of 2523 GHz, and τ
f
of 22.8 ppm/
◦
C.
© 2008 Published by Elsevier B.V.
Keywords: Ceramics; Sintering; Dielectric response; Scanning electron microscopy; X-ray diffraction
1. Introduction
Microwave dielectric materials should have a high dielectric
constant (ε
r
), high unloaded Q and the temperature coeffi-
cient of resonant frequency (τ
f
) close to 0 ppm/
◦
C, in order
to reduce the size of resonators and achieve prominent fre-
quency selectivity and stability [1]. Tungsten–bronze type
Ba
6-3x
R
8+2x
Ti
18
O
54
(R = rare earth) ceramics were widely stud-
ied for their microstructures and microwave dielectric properties
with various x values, due to their high dielectric constants and
low dielectric losses [2,3]. These systems require sintering tem-
peratures up to 1300
◦
C [2,3].
The new tungsten bronze-type Ba
6-3x
R
8+2x
Ti
18
O
54
(R = rare
earth) structure with 2 × 2 perovskite unit cells has a three-
dimensional framework of [TiO
6
]
8-
octahedra connected at
all spices. The crystal structure is orthorhombic Pbnm (No.
62), with a = 1.213 nm, b = 2.227 nm, c = 0.764 nm, and Z =2
[4,5]. Different isovalent cations for Ba
2+
and R
3+
can main-
∗
Corresponding author. Fax: +886 2 2731 7185.
E-mail address: sfwang@ntut.edu.tw (S.-F. Wang).
tain electrostatic stability. The atomic ratio of Ti and O is
precisely 1:3 on the framework, which is the same as that
for perovskite-type structure [4]. The solid solution has the
basic structural formula as [S
10
]
A1
[S
4
]
A2
[V
4
]
C
B
18
X
54
. The S
10
and S
4
indicate that there are ten and four cations occupy-
ing A1 rhombic and A2 pentagonal sites, respectively, and
the C site is trigonal which is usually vacant [6]. B and
X are cation and anion, which correspond to Ti
4+
and O
2-
ions, respectively. Ohsato [5,6] reported that the substitution
mode is [Ba
2-3y
R
8+2y
V
y
]
A1
[Ba
4
]
A2
[V
4
]
C
Ti
18
O
54
in the range
of 0 ≤ y ≤ 2/3. Three Ba
2+
ions are replaced by two R
3+
ions
and a vacancy V for A1 site arises. A part of the Ba
2+
ions
occupying the smaller A1 rhombic sites lead to the generation
of internal strain. In the y = 2/3 composition, the substitution
mode is [R
9.33
V
0.67
]
A1
[Ba
4
]
A2
Ti
18
O
54
, the R ions and Ba ions
are ordered in the rhombic and pentagonal sites, respectively,
which results in the lowest strain, and thus the highest quality
factor. The distribution of the R ions with a smaller size might
reduce the internal strain or fluctuation of d-spacing, and result in
the non-linear variation of quality factor [5]. Ideal coordination
numbers (CN) in the tungstenbronze-type Ba
6-3x
R
8+2x
Ti
18
O
54
crystal structure are 12-CN in A1 sites and 15-CN in A2 sites,
0925-8388/$ – see front matter © 2008 Published by Elsevier B.V.
doi:10.1016/j.jallcom.2008.01.099