H.T. Wu et al. / Journal of Alloys and Compounds 509 (2011) 2232–2237
2233
sol–gel process were reported in the present literature. The goal of
this research was to explore the capabilities of sol–gel method, for
reducing the sintering temperature. The influence of sintering tem-
perature on the crystalline phase, microstructure, and microwave
dielectric properties of MN ceramics was investigated. The results
showed that the aqueous sol–gel process was the most effective and
least expensive technique used for the preparation of MN ceramics
with low sintering temperature.
2.
Experimental
Samples with composition of MN ceramics were prepared through the sol–gel
process, with high-purity (99.9%) Mg(NO3)2·6H2O and Nb2O5 as raw materials.
Firstly, Nb2O5 were weighed accurately according to the mass ratio and dissolved in
the appropriate amount of HF after heating at hot water bath for 20 h and then the
ammonia water was added into the above solutions in order to form chemical precip-
itate of niobic acid (Nb(OH)5). Subsequently, the Nb(OH)5 precipitate was filtered off
and washed with distilled water for six times to remove the H+ and F+ ions and then
dissolved completely in citric acid water solution by continuous magnetic stirring
at 300 rpm for 15 min. Meanwhile, a stoichiometric amount of Mg(NO3)·6H2O was
added to the above solution and then the solution was stirred for another 30 min.
Finally, the ethyl alcohol (20–40 ml) was added to the as-prepared mixed solution in
drops and stirred for 1 h to form a transparent and stable sol. pH of the solution was
Fig. 1. TG–DTA curves of the MN gel in oxygen atmosphere.
maintained in the range of 3.5–5 by adding buffering agents. The sol was heated at
curves, indicating the minimum firing temperature to synthesize
the magnesium niobate phases. By comparison, the DTA results
mentioned by other solid-state methods [2,3] meant different MN
◦
8
0–90 C for about 1 h to obtain a xerogel. The xerogel was decomposed at various
◦
◦
temperatures ranging from 550 C to 850 C in a muffle furnace for crystallization.
The as-prepared powers were ball milled in a polyethylene jar for 4 h using ZrO2
balls in ethanol medium to reduce the conglobation phenomena. The powders were
then mixed with polyvinyl alcohol as a binder, granulated and pressed into cylindri-
cal disks of 10 mm diameter and about 5 mm height at a pressure of about 200 MPa.
◦
formation temperatures, all of which were above 600 C.
Fig. 2 showed the XRD patterns of MN xerogel calcined in oxy-
◦
◦
◦
gen atmosphere ranging from 550 C to 850 C for 30 min. At 550 C
the crystallization of MN took place and this phenomenon was rec-
ognized from the exothermic peak of DTA and the corresponding
◦
These pellets were preheated at 600 C for 4 h to expel the binder and then sintered
at selected temperatures for 4 h in air at a heating rate of 5 C/min.
◦
In order to analyze the phase formation of MN ceramics, the as-formed MN xero-
gel was characterized using thermogravimetry (TG) and differential thermal analysis
◦
TG profile as described above. The gel fired at 550 C consisted of
the predominant phases of Mg Nb O , which were matched with
(
DTA) to study its thermal properties. Phase analysis of the MN powder and ceramic
4
2
9
was conducted with the help of a Rigaku diffractometer (Model D/Max-B, Rigaku Co.,
Japan) using Ni filtered Cu K␣ radiation (ꢁ = 0.1542 nm) at 40 kV and 40 mA settings.
Based on the XRD analysis, the raw MN powder was examined for its morphol-
ogy and particle size using a field emission scanning electron microscopy (Model
Jeol JSM-7600F, FEI Co., Japan). The bulk densities of the sintered ceramics were
measured by the Archimedes method. An HP8720ES network analyzer (Hewlett-
Packard, Santa Rosa, CA) was used for the measurement of microwave dielectric
properties. The dielectric constants were measured using the Hakki–Coleman post-
resonator method by exciting the TE011 resonant mode of the dielectric resonator
using the electric probe of an antenna as suggested by Hakki and Coleman and
Courtney [20]. The unloaded quality factors were measured using the TE01d mode
in the cavity method [21]. All the measurements were made at room temperature
and in the frequency range 9–10 GHz. The temperature coefficients of the resonant
JCPDS file number 38-1459. A minor phase of Mg5Nb O with
4
15
orthorhombic symmetry was developed accompanying Mg Nb O
4
2
9
◦
◦
as separated phases just at 30.16 and 31.30 .As the temperature
◦
increasing to 850 C, the intensity of the Mg Nb O peaks was grad-
4
2
9
ually enhanced and the Mg5Nb O phase had been found to nearly
4
15
disappear. Using the sol–gel method, we found that the calcination
◦
temperature was remarkably decreased to 550 C, in comparison
to these of the conventional mixed oxide route reported earlier
[
2–4,7,8] and other chemical methods [9–13] as shown in Table 1.
In Fig. 2 the influence of calcination temperature on X-ray
◦
diffraction of the samples was observed in the variations of inten-
sity and full width at half maximum (FWHM) of the diffraction
peaks. With increasing the firing temperature, the characteristic
frequency were measured at the temperature range of 25–85 C.
3
. Results and discussion
3.1. Phase formation and characterization of MN powders
Fig. 1 showed the TG–DTA curves of the MN xerogel in pure
◦
oxygen atmosphere at a heating rate of 10 C/min. The results indi-
◦
cated that the obvious weight losses began at 80 C and all chemical
reactions involving weight losses, such as decomposition of the
organic polymeric network with evolution of CO2 and H O, were
2
◦
completed below 531.6 C. The total weight loss was about 80%,
which occurred in three steps: (i) initial weight loss (about 10%)
◦
below 250 C, resulting from the evaporation of residual solvent
and water, with wide and weak endothermic peaks from about
◦
1
00 C, (ii) a second significant weight loss (about 60% caused by
the decomposition of the organic polymeric network with evolu-
◦
tion of CO and H O, with endothermic peaks ranging from 250 C
2
2
◦
to 400 C, and (iii) a third weight loss (about 10–20%) in the TG
curves, combined with an exothermal peak at the temperature
region of 400–531.6 C, which was attributed to the oxidation of
◦
metal–organic groups. The similar TG results of MN xerogel were
also reported in other microwave dielectric ceramics with citrate
sol–gel method [14–17]. No further significant weight loss and
◦
◦
◦
Fig. 2. X-ray diffraction patterns of the MN gel calcined at 550 C, 650 C, 750 C,
◦
thermometric peaks were observed above 531.6 C in the TG–DTA
◦
850 C for 30 min.