2. Experimental procedure
Analytical grade (PA ACS) MgðNO3Þ2:6H2O from Vetec Química
Fina and polyvinyl alcohol from Isofar were used as received without
further purification. An appropriate amount of metal nitrate
and PVA were first dissolved in deionized water. The PVA solution
(10 wt.%) was heated to 60 5 C for 0:5 h. The nitrate solution
was then slowly added to the PVA solution with constant stirring,
at a molar ratio of 1 : 2 nitrate to monomer unit. The final mixed
solution was concentrated at 70 5 C until it became a viscous liquid
with a slight yellow color liquid, and then it was dried at
80 5 C to form the gel. The gel precursor was then burned under
controlled conditions to produce nanoscale MgO powder.
To investigate the grain formation and the growth mechanisms
of the nanocrystalline MgO, the combustion of the gel was investigated
using two different experiments. The influence of the heat
treatment time was investigated at 600 C under combustion in
air, for 15; 30; 60; 120 and 240 min. The sample was placed inside
the furnace which was heated at a rate of 5 C/min until the selected
temperature was reached. After heating for the appropriate
amount of time, the sample was removed from the furnace and
cooled to room temperature. The influence of combustion temperature
was investigated using a constant heat treatment time of
60 min and temperatures from 500 to 900 C. The heating process
was the same as previously described, but the sample was cooled
inside the furnace.
The powders were characterized using scanning electron
microscopy (SEM), with a Jeol JSM-6390LV, transmission electron
microscopy (TEM), with a Jeol JEM-2010, and X-ray diffraction
(XRD), with a Bruker D-5000. X-ray diffraction data were collected
in Bragg–Bretano geometry using a Siemens D5000 X-ray diffractometer
that was equipped with Cu tube and a graphite monochromator.
The scanning range was 10–90ð2HÞ with a step
size = 0.02ð2HÞ and a counting time per step of 2 s. The diffraction
data were analyzed using the Rietveld method as implemented in
the TOPAS 4.2 software package [Bruker AXS, 2009] with fundamental
parameters [16,17] to properly describe the instrumental
broadening. The crystallite size and microstrain were evaluated
using the double-Voigt function approach with integral breath, proposed
by Balzar [18]. The specific surface areas were calculated
using the BET method (Brunauer–Emmett–Teller) and the pore-size
distribution were calculated using the BJH (Barett–Joyner–Halenda)
method [19]. The analyses were performed at 77.30 K using a
Micromeritics ASAP 2020 and nitrogen gas as the adsorbent.
The powders obtained at different combustion temperatures
were also investigated using infrared (IR) absorption spectroscopy.
The IR spectra were recorded at r.t. using a PerkinElmer Spectrum
400 FT-IR/ FT-FIR spectrometer with the KBr pellet on the polyethylene
substrate technique. Spectra were recorded in the ranges
from 4000–450 cm1 (medium) and 700–30 cm1 (far-IR region).