Synthesis, characterization, and microwave dielectric properties of Sr 2-xLa2Mg1+xW2O12 (x=0, 1) ceramics

Article abstract of DOI:10.1111/j.1551-2916.2010.03809.x

SrLa₂Mg₂W₂O₁₂ and Sr ₂La₂MgW₂O₁₂ ceramics were prepared by the conventional solid-state ceramic route and their dielectric properties were investigated in the radio an

Full text of DOI:10.1111/j.1551-2916.2010.03809.x

J. Am. Ceram. Soc., 93 [9] 2467–2469 (2010)
DOI: 10.1111/j.1551-2916.2010.03809.x
r 2010 The American Ceramic Society
ournal
J
Synthesis, Characterization, and Microwave Dielectric Properties of
Sr2 La Mg W O (x 5 0, 1) Ceramics
ꢀx
2
11x
2
12
z
y
z
z
Vineeth Venugopal, Prabhakaran Sreekumari Anjana, Om Parkash, Devendra Kumar, and
Mailadil Thomas Sebastian
w,y
z
Department of Ceramic Engineering, Institute of Technology, Banaras Hindu University, Varanasi 221005, India
yNational Institute for Interdisciplinary Science and Technology (CSIR), Trivandrum 685019, India
61
W
SrLa Mg W O and Sr La MgW O ceramics were pre-
2
show NaCl-type ordering in the octahedral sites while the
2
2
12
2
2
2
12
pared by the conventional solid-state ceramic route and their
dielectric properties were investigated in the radio and micro-
wave frequency regions. SrLa Mg W O sintered at 15001C
A site vacancies show an ordering on the c plane. These A site
vacancies appear to be randomly distributed only to half of the
A sites in alternate c-planes. The X-ray diffraction (XRD) pat-
2
2
2
12
5
has e 5 25.2, Q ꢁ f 5 15 900 GHz (at 4.9 GHz), and s 5 0
tern of Sr La MgW O is similar to that of Ba (CoRE )O
12
r
u
f
2
2
2
12
4
2
ppm/1C. Sr La MgW O sintered at 15251C has e 5 24.7,
except for the relative intensity and systematic shift in 2y. The
2
2
2
12
r
˚
Q ꢁ f 5 35 000 GHz (at 4.7 GHz), and s 5 ꢀ83 ppm/1C. The
reflections correspond to a hexagonal cell with a 5 5.5943 A and
u
f
21
˚
c 5 26.606 A. The Mg ions occupy all the corner-sharing oct-
ahedra and the W ions occupy three-face-shared octahedral,
which interpose a vacant octahedron.
dielectric properties of these ceramics are reported for the first
time.
61
I. Introduction
II. Experimental Procedure
HE current revolution in mobile communication technology
is largely due to the development of temperature-stable di-
The Sr2 La Mg W O (x 5 0.1) ceramics were prepared
2
ꢀx
2
11x
12
T
using the conventional high-temperature solid-state synthesis
route using high-purity powders of SrCO WO
Mg O (499%; Sigma Aldrich, St. Louis, MO),
14 ꢂ 5H
and La O (99%, IRE). Stoichiometric proportions of the chem-
electric materials having a high dielectric constant and quality
factor. These materials have proved to be cost effective, com-
pact, and easily integrable in microwave circuits. They find ap-
plications in microwave subsystems such as oscillators, filters,
and antennas. The important characteristics required for sub-
strate applications are low-relative permittivity, high quality
factor 42000 (at 10 GHz), and low temperature coefficient of
resonant frequency in the range of ꢀ10 to 110 ppm/1C. The
medium permittivities in the range 20–50 are used for satellite
3
,
3
,
C H
4 2
5
O
2
2
3
icals were weighed and ball milled for 24 h using zirconia balls in
distilled water media. The slurry was dried and then calcined
twice at 12001C for 6 h. The calcined powders were finely
ground, mixed with 4 wt% poly vinyl alcohol (3 wt%; BDH
laboratory, Poole, U.K., molecular weight ꢃ22000, degree of
hydrolysis 498%), and dried. They were then pressed into disk-
shaped pucks of 18 mm diameter and 9 mm height at a pressure
of about 200 MPa using a tungsten carbide die. The green com-
pacts were fired at a rate of 51C/min up to 6001C and soaked at
this temperature for half an hour to expel the binder. The pellets
were sintered in air at temperatures in the range 13501–16001C
for 4 h. The cooling rate was 31C/min up to 8001C. They were
polished to remove surface irregularities and subsequently char-
acterized to determine the dielectric properties. The phase purity
and crystallinity were determined using an X-ray diffractometer
1
communications and in cell phone base stations. Complex per-
ovskite systems such as BMT and BZT have been extensively
1
studied in the literature. The dielectric properties of many tung-
4
state materials such as AWO (A5 Mg, Zn, Ni, and Co) and
RE(Ti0.5 (RE 5 Pr, Nd, Sm, Gd, Tb, and Dy) are re-
W0.5)O
4
2
,3
ported in the microwave frequency range. In this communi-
cation, we report the radio frequency (MHz) as well as
the microwave dielectric properties of two compositions:
SrLa Mg W O and Sr La MgW O . To the best of our
2
2
2
12
2
2
2
12
knowledge, the microwave dielectric properties of these compo-
sitions have not been reported to date. Although the ceramics
appear similar at the first glance, they differ significantly in their
crystal structure and electronic properties. However, both the
ceramics are suitable candidates for applications in microwave
circuitry; particularly as substrates in monolithic microwave in-
tegrated circuits.
(
Philips Corp., Almelo, the Netherlands). The bulk density was
measured using the Archimedes principle. The dielectric con-
stant and the dielectric loss were measured in the radio fre-
quency region using an LCR meter (HIOKI 3532-50 LCR Hi
Tester, Nagano, Japan). The dielectric properties Q
u
, e
r
, and t
f
of the materials were measured in the microwave frequency
range (2–6 GHz) using a vector network analyzer (Model 8753
ET, Agilent Tech., Palo Alto, CA) with a copper invar resonant
cavity and the TE01d mode of resonance. The interior of the
cavity is coated with silver and a low-loss quartz spacer is used
for supporting the ceramic. To measure the coefficient of
thermal variation of resonant frequency, the variation of the
resonant frequency of the TE01d mode was studied in the range
of 201–801C. The sintered samples were thermally etched by
heating SrLa Mg W O to 14501C and Sr La MgW O to
4
Torri reported for the first time the preparation, defect struc-
ture, and characterization of Sr2ꢀxLa
ceramics. The powder pattern for SrLa
2
Mg11x
W
2
O
12 (x 5 0, 1)
Mg W
2 2
2
O
12, in general
features, has been shown to be very similar to that of
La2.67(Mg W )O with an orthorhombic cell with parameters
5
12
2
2
2
1
˚
˚
˚
a 5 7.841 A, b 5 7.858 A, and c 5 7.893 A. The ions Mg and
R. Ubic—contributing editor
2
2
2
12
2
2
2
12
14751C at a rate of 101C/min. The samples were kept at these
temperatures for 30 min and then cooled to 10001C at 101C/min
and then allowed to cool to room temperature by natural cool-
ing. The surface morphology of the etched samples was studied
Manuscript No. 27350. Received January 8, 2010; approved March 21, 2010.
Author to whom correspondence should be addressed. e-mail: mailadils@yahoo.com
w
2
467

2
468
Rapid Communications of the American Ceramic Society
Vol. 93, No. 9
with sintering temperature as shown in Table I. The dielectric
constant of SrLa Mg 12 is corrected for porosity using the
following equation derived by Penn et al.
2
2 2
W O
7
ꢀ
ꢁ
3
Pðer ꢀ 1Þ
ðerÞ ¼ er 1 ꢀ
(2)
0
2
er þ 1
where (e ) is the measured dielectric constant of the compound,
r 0
r
which contains a fractional porosity P and e is the actual di-
electric constant of the dielectric. The dielectric constant is max-
imum at 15001C with e 5 25.2 (microwave frequency). The
lowest value of the loss tangent is also shown at this sintering
r
ꢀ
4
temperature with tand 5 2.1 ꢁ 10 . The relationship between
dielectric constant and sintering temperature of SrLa Mg
W O shows a similar trend as between density and sintering
2
2
2
12
temperature. The SrLa Mg
2
15900 GHz (4.98 GHz) at 15001C and t
makes them very attractive materials for practical applications.
2
W
2
O
12 shows a maximum Q
u
ꢁ f of
f
of 0 ppm/1C, which
With a further increase in temperature, the sign of t changes
f
from positive to negative.
The dielectric properties and the variation of density with
sintering temperature of Sr La MgW O are given in Table II.
Fig. 1. Powder X-ray diffraction pattern of (a) SrLa
(b) Sr La MgW
2 2 2
Mg W O12 and
2
2
2
12
2
2
2 12
O .
A maximum densification of 92.4% is obtained when sintered at
5251C. The density of the composites is calculated using the
equation
1
using a scanning electron microscope (SEM, Model S-2400,
Hitachi, Tokyo).
rmixture ¼ V1r þ V2r2
(3)
1
III. Results and Discussion
1 2
where V and V are the volume fraction, rmixture is the calcu-
The room-temperature powder X-ray diffraction patterns of
SrLa Mg W O and Sr La MgW O are given in Fig. 1. The
powder pattern of SrLa Mg W O is fully indexed as a single-
lated theoretical density and r1 and r2 are the densities of
2
2
2
12
2
2
2
12
Sr La MgW O and SrWO , respectively. The density in-
2
2
2
12
4
2
2
2
12
creased continuously from 14001 to 15001C and becomes al-
most constant above 15001C. The presence of a small amount of
phase orthorhombic crystal (JCPDS file no: 35-0259), while that
of Sr La MgW 12 is indexed as a hexagonal cell (JCPDS file
no: 34-1327). The diffractogram for Sr La MgW O shows the
2
2
2
O
low-density secondary phase SrWO (theoretical density: 6.35)
in Sr La MgW O decreases the overall density of the matrix.
4
2
2
2
12
2
2
2
12
presence of SrWO
4
as a secondary phase. The two phases being
, the obtained diffraction pattern is
then utilized to determine the volume percentage using the equa-
The volume fraction of SrWO in the ceramic matrix has been
4
Sr La MgW 12 and SrWO
2
2
2
O
4
calculated to be about 4% by the intensity analysis of the XRD.
The dielectric constant of Sr La MgW O is calculated from
the mixture rule of ceramic composites using the equation
2
2
2
12
6
8
tion,
I
Að107Þ
þ IBð112Þ
e ¼ V e þ V e þ V e
(4)
r
1
r1
2
r2
3 r3
Sr2La2MgW O12ðvol%Þ ¼
ꢁ 100
(1)
2
I
Að107Þ
where V
3
is the volume fraction of porosity, er1, er2, and er3 are
La MgW 12, SrWO4, and air
is the dielectric constant of the
where I
A
and I
B
are the linear intensities of the main peaks of
Sr La MgW O (107)A and SrWO (112) , respectively. The
the dielectric constant of Sr
2
2
O
2
(er3 5 1), respectively, and e
r
2
2
2
12
4
B
intensity calculations indicate the presence of about 4 vol%
SrWO in the Sr La MgW 12 phase. The scanning electron
images of the two systems are presented in Fig. 2. Dense
grain formation is visible in both the systems with a nearly uni-
form grain size distribution.
mixed phase ceramic. The dielectric constant increases with the
sintering temperature, reaches a maximum of 25.4 at 1 MHz
when sintered at 15251C, and then starts decreasing. Very low
dielectric loss tangent of the order of 10 is also obtained at this
sintering temperature. The dielectric constant depends on the
density and phase constituents. Because of porosity and the
4
2
2
2
O
ꢀ4
2 2 2
The SrLa Mg W O12 showed a densification of 96.2% when
sintered at 15001C for 4 h. The dielectric properties such as the
dielectric constant and the dielectric loss are measured for
presence of SrWO
4
secondary phase with a very low dielectric
constant of 8.1, it is believed that the actual value of the di-
electric constant of phase-pure Sr La MgW 12 is higher than
9
2 2 2
SrLa Mg W O12 at 1 MHz. These properties vary substantially
2
2
2
O
2 2 2 2 2 2 12
Fig. 2. Scanning electron micrographs of (a) SrLa Mg W O12 and (b) Sr La MgW O .

September 2010
Rapid Communications of the American Ceramic Society
2469
Table I. The Dielectric Properties of SrLa Mg W O at Radio and Microwave Frequencies
2
2
2
12
1
MHz
Microwave frequency region
Sintering temperature (1C)
Densification (%)
(e )
r 0
e
r
tand
(e )
r 0
e
r
Q
u
ꢁ f (GHz)
t
f
(ppm/1C)
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
2
4
4
4
4
1425
1450
1475
1500
1525
93.2
94.5
94.6
96.2
95.5
22.6
22.8
22.9
24.0
23.6
25.0
24.7
23.8
24.5
23.9
4.4 ꢁ 10
9.4 ꢁ 10
6.4 ꢁ 10
2.1 ꢁ 10
9.7 ꢁ 10
19.8
21.6
22.1
23.7
23.3
21.9
23.4
23.9
25.2
24.9
11800
13300
14300
15900
14250
2.4
0.6
0.1
0.0
ꢀ1.0
Table II. The Dielectric Properties of Sr La MgW O at Radio and Microwave Frequencies
2
2
2
12
1
MHz
Microwave frequency region
Sintering temperature (1C)
Densification (%)
(e )
r 0
e
r
tand
(e )
r 0
e
r
Q
u
ꢁ f (GHz)
t
f
(ppm/1C)
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
2
3
3
4
4
1450
1475
1500
1525
1550
74.0
87.6
91.4
92.4
92.7
15.9
20.8
21.1
23.9
20.1
18.8
22.5
22.6
25.4
21.3
2.1 ꢁ 10
1.7 ꢁ 10
1.3 ꢁ 10
8.1 ꢁ 10
8.3 ꢁ 10
15.0
17.3
19.3
20.5
18.1
17.9
18.9
20.7
24.7
19.2
14760
20260
25230
35000
33200
ꢀ60.7
ꢀ66.9
ꢀ70.9
ꢀ83.2
ꢀ84.0
the observed value of 24.7 at microwave frequency.
Sr La MgW O sintered at 15251C shows a e of 24.7, Q ꢁ f
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2
2
12
r
u
1
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(
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f
of ꢀ55 ppm/1C.
4
2 2 2
Mg W O12 and
Sr
2
La
2
2
O
IV. Conclusion
5
The
Sr La MgW O ceramics have been studied both at low fre-
dielectric properties
of SrLa Mg W O
2 2 2 12
and
3
3
6
2
2
2
12
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7
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2
2 2 2
and 15251C, respectively. SrLa Mg W O12 has
r
e 5 25.2,
8
Q ꢁ f 5 15 900 GHz, and t 5 0 ppm/1C. Sr La MgW O has
u
f
2
2
2
12
e
r
5 24.7, Q
u
ꢁ f 5 35 000 GHz, and t 5 ꢀ83.2 ppm/1C. Both
f
Appl. Phys., 39, 2696–700 (2000).
9
the systems have excellent microwave properties, which make
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&