Advances in Ma terials Physics and Che mist ry, 2012, 2, 28-30
doi:10.4236/ampc.2012.24B008 Published Online December 2012 (htt p://www.SciRP.org/journal/ampc)
The Reaction Sequence and Dielectric Properties of
BaAl2Ti5O14 Ceramics
Xiaogang Yao, Wei Chen, Lan Luo
Shanghai Institute of Ceramics, Chines e Academy of S cience, Shanghai, China
Received 2012
ABSTRACT
To investigate the correct react io n s equ en ce of BaO-Al2O3-5TiO2 s yste m, powder s calci ned at d iffe ren t temperat ur es are an al yzed b y
x-ray diffraction. The results show that the source phase BaCO3 decomposes below 800°C, TiO2 and Al2O3 start to consume at 900
and 1100°C, respectivel y. BaTi4O9 ph ase appears at 1000°C whil e BaAl2Ti5O14 phase starts t o reveal at 1200 °C. As the temperature
increases, the den sity, dielect ric const ant and quality facto r of the BaAl2Ti5O14 ceramic in crease and keep unchan ged at 1350°C. The
dielectric pr operties of BaAl2Ti5O14 ceramic sintered at 1350°C for 3h are: εr=35.8, Q×f=5130GHz, τf=-6.8ppm/°C.
Keywords: R eaction Sequence; BaAl2Ti5O14; Ceramics; Dielectric Properties
1. Introduction
The rapid progress in mobile communication has created a
tremendous demand for the microwave dielectric materials with
high dielectric constant, low dielectric loss and near-zero
temperature coefficient of resonator frequency [1,2]. As a
typical high permittivity system, Ba6-3xLn8+2xTi18O54 (Ln = La,
Sm, Nd) has attracted plenty of attention for the high dielectric
constant over 80 [3,4]. However, the shortcoming of BLT
system is its relatively high dielectric loss (low quality factor)
which h as restricted its commercial ap plication.
Much work was done to lower the dielectric loss of the BLT
system. Zhu improved the Q × f value of Ba4.2Nd9.2Ti18O54
ceramic by doping with LnAlO3 (Ln = La, Nd, Sm) [5]. TiO2
was added into Ba6-3xSm8+2xTi18O54 by Ohsato and excellent
dielectric properties were obtained [6]. Our previous work has
revealed that the crucial point to lower the dielectric loss of
BLT system is preventing the reduction of Ti4+ at high sintering
temperatures [7]. Al2O3 or MgO was used as an acceptor to
suppress the reduction of Ti4+ in Ba4.2Sm9.2Ti 18O54 cer amic and
was very effective to improve the Q×f value of the BST
ceramic. In our present work, Al2O3 was added into Ba-Sm-Ti
and Ba-Nd-Ti systems. The dielectric loss has been reduced
effectively. The common results are closely related to a new
phase BaAl2Ti5O14 (BAT) which is observed in both systems.
No rel ev a nt inf ormation about the ne w BAT phase wa s repor te d.
In this paper, BaAl2Ti5O14 cera mic was pr epared b y the solid
state reaction. X-ray diffraction was used to identify the crys-
tallin e phas e at each calcin in g temperatur e fro m 800 to 1350 °C.
The reaction sequence of BaO-Al2O3-5TiO2 system was deter-
mined and the microwave dielectric properties of BaAl2Ti5O14
ceramic were measur ed.
2. Experiment
The BAT ceramic powders were prepared according to the
desired stoichiometry of BaAl2Ti5O14 by mixing the chemical
grade starting materials BaCO3 (99.9%), Al2O3 (99.9%) and
TiO2 (99.9%). After ground in deionized water with ZrO2 balls
for 24h, the mixture was dried and then calcined at different
temperatures from 800 to 1350°C in air for 1h. The optimally
calcined BAT powders were milled for 24h, dried at 120°C and
granulated with polyvinyl alcohol (PVA). The granules were
preformed and then sintered at 1275~1375°C in air for 3h with
a heating rate of 5°C/min.
The bul k densi ties o f th e BAT ceramic were measured by the
Archimedes method. The crystalline phases of the calcined
BAT powders and sintered BAT ceramic were analyzed by a
Rigaku D/max 2 550 V X-ray diffracto meter with a con vention al
Cu-Kα radiation in the range of 10~70° with a step size of
0.02°. The microstructure of the BAT ceramic was examined
by a Hitachi S-4800 field emission scanning electron micro-
scope. The dielectric properties of the polished BAT samples
were tested the TE011 mode of an Agilent E8363A PNA series
network analyzer with a frequency ranges from 3 to 4GHz. τf
was tested in the temperature ranges from 20 to 80°C and cal-
culated by noting the change in resonant frequency as:
( )
60
f 211
=f -ff τ
(1)
Here, f1 and f2 represent the resonant frequencies at 20 and
80°C, r espectively.
3. Results and Discussion
3.1. Reaction Sequenc e of BaO-Al2O3-5TiO2 ststem
Figure 1 shows the XRD patterns of the BAT powders calcined
at different temperatures from 800 to 1350°C for 1h. Here, the
phases identified by X-ray diffraction at each calcining
temperature are listed in Table 1. Six phases are observed as
various temperatures are used. BaCO3, Al2O3 and TiO2 phases
are observed at 800 °C with a new phase BaTiO3 accompanied.
It is easy to d edu ce that BaTiO3 is the product of the reaction of
BaCO3 and TiO2. With increasing the temperature to 900°C, no

X. G. YAO ET AL.
29
BaCO3 is residual whi le the trace of BaTiO4 is detected. Fewer
BaTi O3 but a predominant BaTiO4 phase is found at 1000°C.
Not any change is observed until the calcining temperature is
increased to 1200°C. The diffraction peaks of TiO2 and BaTiO4
are gettin g weak while a new ph ase BaAl2Ti5O14 appears. Only
a single BaAl2Ti5O14 phase exists over 1300°C.
We can easil y write d own the rea ction seq uence of the BaO-
Al2O3-5TiO2 system with the increasing of calcining tempera-
ture from 800 to 1350°C.
Below 800°C:
32
BaCO BaO+CO→↑
(2)
800°C:
23
BaO+TiO BaTiO→
(3)
900~1000°C:
3 249
BaTiO +3TiOBaTi O→
(4)
1200°C:
492322 514
BaTi O+AlO+TiOBaAl TiO→
(5)
Figure 2 shows the DSC curve of the BaO-Al2O3-5TiO2
powder heated from room temperature to 1350°C. As shown in
Figure 2, thr ee exothermic peak s are observed at 825, 925 and
1300°C which correspond very well to the temperatures at
which the reactions (3)-(5) ha p pe n.
3.2. Dielectric Properties of BaAl2Ti5O14 Ceramic
From what has been discussed above, we can draw the conclu-
sion that the sintering temperature of the BaAl2Ti5O14 ceramic
is between 1300 and 1350°C. Thus, five temperature points
(1275, 1300, 1325, 1350, 1375°C) are used to study the effect
of temperat ure on the dielectric p ro perties.
Figure 1. XRD patterns of BaO-Al2O3-5TiO2 powders calcined at
differen t temper at u res from 800 t o 13 50°C for 1h .
Ta ble 1. Crystalline phases exist of not exist at each calcining tem-
perature.
Tc/°C BaCO3 TiO2 Al2O3 BaTiO3 BaTiO4 BaAl2Ti5O14
800 Y Y Y Y N N
900 N Y Y Y Y N
1000 N Y Y N Y N
1100 N Y Y N Y N
1200 N Y N N Y Y
1250 N Y N N N Y
1300 N N N N N Y
1350 N N N N N Y
Figure 3 shows the density of BAT ceramics at different sin-
tering temperature. With the increasing temperature from 1275
to 1375°C, the density of B AT ceramics increa ses fro m 4.02 to
the maximum value 4.17g/cm3 at 1350°C, and then decreases
slightly. We can conclude that the optimized sintering tem-
perature of the BAT ceramics is 1350°C.
Figure 4 shows th e SEM i mages of the B AT ce ramic s ampl es
sintered at 1350°C for 3h. As we can see from Figure 4(a), the
BAT cera mic h as a compact structure but a heterogeneous grain
size. The avera ge size is 10 μm, as shown in Figure 4(b). Irre-
gularly grown grains are seen in both images. The formation
mechanism of these huge grains is still inexplicit and needs
further research.
Figure 2. DSC curve of BAT powders.
Figure 3. Density of the BAT ceramics sintered at different tem-
peratures from 1275 to 1375°C.
Figure 4. SEM images of the BAT ceramic sintered at 1350°C for
3h: (a) ×500 ; (b) ×1000.

X. G. YAO ET AL.
30
Figure 5 sho ws the dielectr ic constan t of BAT cera mic s sin-
tered at d ifferent temperatures. It is n ot strange th at the change
in dielectric constant with temperature shows the same regular-
ity with th at o f den sit y, since a mo re co mpact stru ctu re mean s a
lower porosity. The dielectric constant of BAT ceramics reach
35.8 after sintering at 1350°C for 3h.
Figure 6 shows the Q × f value of the BAT ceramics sin tered
at different temperatures from 1275 to 1375°C for 3h. With the
increasing temperature, the Q × f value of BAT ceramics in-
creases from 4324GHz at 1275°C to the maximum value 5130
GHz at 1350°C. The heterogeneous grain size has very bad
effect on the Q × f value. Huge grains can increase the dielec-
tric loss significantly. Therefore, much work need be done to
obtain a more homogenous grain distribution so as to improve
the Q × f value of BAT ceramics.
Figure 7 shows the τf value of BAT ceramics sint ered at dif-
ferent temperatures for 3h. The τf value of the B AT cerami cs is
slightly affected by the sintering temperature. BAT ceramics
sintered at 1350°C for 3h has a negative τf value of -6.8ppm/°C.
Figure 5. Dielectric constant of the BAT ceramics sintered at dif-
ferent temperatures from 1275 to 1375°C.
Figure 6. Q×f value of the BAT ceramics sintered at different tem-
peratures from 1275 to 1375°C.
Figure 7. Resonator frequency of temperature coefficient of the
BAT ceramics sintered at different temperatures from 1275 to
1375°C.
4. Conclusion
BaAl2Ti5O14 ceramic is prepared by the conventional solid state
reaction. The reaction sequence of BaO-Al2O3-5TiO2 system
has been established. The result shows that the ideal calcining
temperature of BAT powder is 1200°C and the best sintering
temperature of BAT ceramic is 1350°C. The BAT ceramic has
a heterogeneous grain distribution which has very bad effect on
its dielectric properties especially for the Qf value. The dielec-
tric properties of BAT ceramic sintered at 1350°C for 3h are:
εr=35.8, Q×f= 5130GHz and τf=-6.8ppm/°C.
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