Full text of DOI:10.1557/JMR.2000.0028

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Ultrafine MnFe O powder preparation by combusting the
2
4
2+
2+
coprecipitate with and without Mg or Zn additives
a)
Hsuan-Fu Yu and Wen-Bing Zhong
Department of Chemical Engineering, Tamkang University,
Taipei Hsien, Taiwan 25137, Republic of China
(Received 14 June 1999; accepted 11 October 1999)
Ultrafine MnFe O powder with its crystallites less than 100 nm was prepared using a
2
4
2+
combustion process. The coprecipitates containing the stoichiometric amount of Mn
and Fe to form MnFe O were prepared by dissolving the required metallic nitrates
3+
2
4
in de-ionized water and adding NH OH to adjust the pH of the solutions to 9. The
4
collected dried precipitates were then heated up to predetermined temperatures and
then quickly contacted with the acetone spray. Upon contacting with the heated
precipitates, the acetone spray was ignited. The combustion of acetone caused the
precipitates to form crystalline MnFe O without chemical segregation. The
2
4
crystallinity of MnFe O powder so obtained depended on the ignition temperature of
2
4
acetone spray. MnFe O powder obtained at acetone ignition temperature of 773 K had
2
4
higher crystallinity than that obtained at acetone ignition temperature of 523 K. The
2+
2+
presence of a small amount of Mg or Zn in the composition of the coprecipitates
promoted the mobility of constituent ions of the combusted powder and resulted in
bigger MnFe O crystallites at a lower acetone ignition temperature.
2
4
2
+
I. INTRODUCTION
powder with Mn , special care must be taken to avoid
the oxidation of divalent manganese ions during prepa-
Ferrites with an inverse spinel molecular structure
have soft magnetic properties and are important compo-
nents in the electronic industries. The quality and perfor-
mance of ferrite products are greatly influenced by their
microstructures. To have a better-controlled microstruc-
ture, the ferrite powder used in production should have
the characteristics of high chemical homogeneity, ultra-
fine sizes with narrow size distribution, spherical-like
shapes, and high sinterability. Several nonconventional
routes for high-quality ferrite powder preparation have
been used or are under development, such as the sol-gel
2
+
ration because Mn is unstable in air. The oxidation of
Mn would result in phase separation and make the
formation of Mn-ferrites difficult. Chaudhuri and Roy
prepared manganese ferrite by firing the corresponding
coprecipitates up to 1573 K and then cooling down the
2+
12
1
2
method3, the hydrothermal method, the aerosol
–5
6–11
method,
and the coprecipitation method.
Among
them, the coprecipitation technique, because of its easy
operation and because it is ready for mass production, is
the most common one adopted in the production of fer-
rite powder. The coprecipitation technique used in
preparing ferrite powder involves preparation of homo-
geneous solution that contains the required ratio of cat-
ions, precipitation of insoluble precursors of the ferrites
by adding precipitating agents, separation of the solid
precipitates from the liquid solution, removal of the
soluble residuals by washing and drying, and formation
of the ferrite powder by pyrolysis. In preparing ferrite
FIG. 1. XRD patterns for the combusted specimens (without any addi-
tives) obtained by igniting the acetone spray at (a) 523 K and (b) 773 K.
a)
Address all correspondence to this author.
170
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2+
2+
H-F. Yu et al.: Ultrafine MnFe O4 powder preparation by combusting the coprecipitate with and without Mg or Zn additives
2
FIG. 2. Photomicrographs for the combusted specimens (without any additives) obtained by igniting the acetone spray at 523 K: (a) SEM and
(b) TEM, and by igniting the acetone spray at 773 K: (c) SEM and (d) TEM.
treatment on the corresponding coprecipitates was intro-
duced to provide an alternative way to prepare ultrafine
2
+
MnFe O powder. Moreover, the effects of Mg and
2
4
2+
Zn additives on the crystallinity of resultant MnFe O
2
4
particles were also investigated.
II. EXPERIMENTAL PROCEDURE
All chemicals used in this study were reagent purity
and were used without further purification.
2+
2+
Mn(NO ) ⅐ 6H O, Fe(NO ) ⅐ 9H O, and Mg or Zn -
3
2
2
3 3
2
nitrate hexahydrate (the additives) in molar ratios of
:2:x (where x ס 0, 0.001, 0.005, 0.02, and 0.05) were
dissolved in de-ionized water to form aqueous solutions
of 0.15 M. NH OH (concentration: 29.8 wt%) was
1
FIG. 3. DSC curves for the combusted specimens (without any addi-
tives) obtained at two different acetone ignition temperatures (heating
rate 5 K/min).
4
(aq)
then dropped into the solution, with continuous stirring,
until the pH of the solution reached 9. Adding ammo-
nium hydroxide caused the formation and precipitation
of insoluble solids (the corresponding metallic hydrox-
ides) and adjusting the solution to basic conditions was to
assure the completion of precipitation. The collected pre-
cipitate was washed with de-ionized water, filtered with
0.2-␮m membranes, and then dried at 353 K. After
grinding, the dried specimen was then thermally treated
in an oven at 523 K or 773 K for 1 h, followed by a
13
specimen in a stream of nitrogen. Bakare et al. and
14
Sankarshana Murthy et al. used stabilized MnO to pre-
pare the required Mn-ferrites. On the other hand, Tang
15
et al. treated the coprecipitates with a digestion process
to produce fine Mn Fe O (0.2 < x < 0.7) powder. To
x
3−x 4
2
+
avoid the oxidation of Mn during preparation and to
shorten the production time, in this work a combustion
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2+
2+
H-F. Yu et al.: Ultrafine MnFe O4 powder preparation by combusting the coprecipitate with and without Mg or Zn additives
2
combustion process. It is to be noted that setting the oven
temperature to 523 K or higher is to assure the decom-
position of precipitates to form the corresponding oxides.
The combustion process was carried out by quickly re-
moving the heated specimen from the oven and contact-
ing the specimen with the acetone spray. The elevated
temperatures of the heated specimens ignited the acetone
spray and caused the specimens to undergo vigorous
burning. For the sake of comparison, another set of dried
specimens was heated in the oven at the temperatures
mentioned above and then naturally cooled down in air to
room temperature. In the following discussion, the fired
specimen is used to represent the powder that was ob-
tained from an air heating and cooling procedure and the
combusted specimen is then used to stand for those ob-
tained by contacting the heated specimen with the ac-
etone spray. The specimens obtained from these two
processes were analyzed using differential scanning calo-
rimetry (DSC; DSC 200, Netzsch), x-ray diffraction
electron microscopy (TEM; H-7000, Hitachi), to deter-
mine the characteristics of specimens. The effects of
2+
2+
Mg and Zn additives on the resultant MnFe O par-
2
4
ticles were also examined by comparing the characteris-
2+
2+
tics of MnFe O powder with and without Mg or Zn
2
4
additions. The resulting results are discussed.
III. RESULTS AND DISCUSSION
A. Air heating and cooling process
Regardless of compositions of the starting solutions
and temperatures of the heat treatment, the fired speci-
mens obtained by naturally cooling the heated specimens
in air did not contain any crystalline phase of MnFe O .
2
4
XRD examination showed that heated up to 773 K, the
2+
2+
fired specimens with no Mg or Zn additions were
(
XRD; M03X-HF, Mac Science), scanning electron
microscopy (SEM; S-800, Hitachi), and transmission
FIG. 5. TEM photomicrographs for the combusted specimens obtained
at an acetone ignition temperature of 523 K. Molar ratios for the speci-
2
+
2+
3+
2+
2+
3+
2+
FIG. 4. XRD patterns for the combusted specimens with Mg addi-
tion, obtained by igniting the acetone spray at 523 K.
mens (a) Mn :Fe :Mg ס 1:2:0.001 and (b) Mn :Fe :Mg
ס
1:2:0.05.
1
72
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2+
2+
H-F. Yu et al.: Ultrafine MnFe O4 powder preparation by combusting the coprecipitate with and without Mg or Zn additives
2
2
+
2+
noncrystalline materials; however, adding Mg or Zn
to the compositions caused the formation of crystalline
–Fe O at 773 K. It is to be noted that when the heating
were composed of MnFe O crystallites. By examining
2 4
the broadness of XRD peaks, the combusted specimen
obtained by igniting the acetone spray at 523 K contained
very tiny MnFe O crystallites. Increasing the ignition
␣
2
3
temperature was increased to 973 K, crystalline
–Mn O phase, in addition to ␣–Fe O , was detected in
2
4
␣
temperatures caused MnFe O crystallites to grow big-
2
3
2
3
2 4
the fired specimens for all compositions. Although the
ger. All combusted specimens exhibited magnetic char-
acteristics, which was observed by placing the powders
in a magnetic field. Figure 2 gives the SEM and TEM
photomicrographs of combusted specimens with ignition
temperatures at 523 and 773 K, respectively. All com-
busted powders consisted of submicron agglomerated
particles [Figs. 2(a) and 2(c)] and the sizes of crystallites
were much less than 100 nm [Figs. 2(b) and 2(d)]. The
TEM analysis gave the same conclusion as that obtained
from the XRD analysis: The crystallites obtained at an
ignition temperature of 773 K were much larger than
2+
2+
presence of small amount of Mg or Zn in the com-
positions could promote the crystallinity of the corre-
sponding metallic oxide, it is difficult to prevent the
oxidation of divalent manganese ions in an air heating
and cooling process. To reduce manganese ions from
3+
2+
Mn to Mn and/or to obtain the required MnFe O4
2
phase, the combustion process mentioned previously was
used in this study to produce ultrafine crystalline
MnFe O powder.
2
4
B. Combustion process
Figure 1 shows the XRD patterns of combusted speci-
2+
2+
mens, containing no additives of Mg or Zn , obtained
by igniting the acetone spray at 523 and 773 K, respec-
tively. It was evident that all these combusted powders
FIG. 7. TEM photomicrographs for the combusted specimens ob-
tained at an acetone ignition temperature of 523 K. Molar ratios for the
2
+
2+
3+
2+
2+
3+
2+
FIG. 6. XRD patterns for the combusted specimens with Zn addi-
tion, obtained by igniting the acetone spray at 523 K.
specimens (a) Mn :Fe :Zn ס 1:2:0.001 and (b) Mn :Fe :Zn
ס
1:2:0.05.
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2+
2+
H-F. Yu et al.: Ultrafine MnFe O4 powder preparation by combusting the coprecipitate with and without Mg or Zn additives
2
2
+
3+
2+
FIG. 8. TEM photomicrographs for the combusted specimens with a molar composition of: (a) Mn :Fe :Mg ס 1:2:0.001, (b)
2
+
3+
2+
2+
3+
2+
2+
3+
2+
Mn :Fe :Mg ס 1:2:0.05, (c) Mn :Fe :Zn ס 1:2:0.001, (d) Mn :Fe :Zn ס 1:2:0.05, obtained by igniting the acetone spray at 773 K.
those at 523 K. By comparing the results for these com-
busted specimens with those for the fired specimens, it is
suggested that the combustion of acetone, upon contact
with the heated specimens, provides a high enough tem-
perature environment for reducing manganese ions from
To promote the crystallinity of MnFe O in the com-
2 4
2+
2+
busted specimen, Mg or Zn were introduced into the
composition of coprecipitates. Figure 4 is the XRD pat-
terns for the combusted specimens, containing various
2
+
amounts of Mg additives, obtained by igniting the
3+
2+
2+
Mn to Mn and/or to form manganese ferrite. Acetone
spray ignited at higher temperatures instantly gives off
more energy and results in the formation of larger
MnFe O crystallites.
acetone spray at 523 K. Adding Mg at amounts of 2%
or less (with respect to the moles of manganese ions) into
the compositions seemed to enhance the mobility of con-
stituent ions and results in an increase of crystallinity of
MnFe O . However, the combusted specimen with 5%
2
4
Figure 3 gives the DSC curves of these combusted
specimens (containing no additives) and reveals their
thermal behavior. During heating in an air atmosphere,
the combusted specimen experienced exothermic
changes at temperatures between 398 and 598 K for the
one igniting the acetone spray at 523 K and between 448
and 723 K for that igniting the acetone spray at 773 K.
These specimens after DSC analysis were subjected to
XRD analysis and the corresponding XRD patterns indi-
cated that the powders were all composed of tiny crys-
tallites of ␣–Mn O and ␥–Fe O . Accordingly, these
2
4
2+
Mg addition did not show any distinguishable differ-
ences from that of the combusted specimen without ad-
ditives [comparing Fig. 4(d) with Fig. 1(a)]. The TEM
photomicrographs of the specimens shows the effects of
2+
Mg additions on the crystallinity of combusted speci-
mens. As concluded from the XRD analysis, the com-
2+
busted specimen with 0.1% Mg additions [Fig. 5(a)]
was composed of larger MnFe O crystallites than that of
2
4
2+
the specimens with 5% Mg additions [Fig. 5(b)].
2+
The effects of Zn addition on the crystallinity of
2
3
2 3
2+
exothermic changes were mainly due to the oxidation of
divalent manganese ions and then the separation of
chemical phases. The differences in temperature ranges
of these exothermic changes were ascribed to the differ-
ences in crystallinity of the powders. MnFe O powder
MnFe O particles are shown in Fig. 6. More Zn ad-
2 4
ditions resulted in higher MnFe O crystallinity of the
2
4
combusted powder. Figure 7 shows the TEM photomi-
crographs for the combusted samples with 0.1% and 5%
2+
Zn additions. These TEM photomicrographs confirm
the conclusions drawn from the results of XRD analysis.
2
4
with higher crystallinity is more stable in air.
174
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2+
2+
H-F. Yu et al.: Ultrafine MnFe O4 powder preparation by combusting the coprecipitate with and without Mg or Zn additives
2
On the other hand, the combusted powders, regardless of
types and amounts of the additive, were all composed of
submicron primary particles with heavy agglomerations,
which were evident from their SEM photomicrographs.
Like those observed for the combusted specimens
bility of constituent ions of the combusted powder and
resulted in bigger MnFe O crystallites at a lower
acetone ignition temperature.
2
4
2
+
2+
without Mg or Zn additions, raising the ignition
temperatures of acetone spray increases the degree of
ACKNOWLEDGMENT
We gratefully acknowledge support of this research by
the National Science Council under Grant No. NSC89-
crystallinity and the size of crystallites of MnFe O4
2
in combusted specimens. The TEM photomicrographs
2214-E032-006.
2+
2+
of combusted specimens (with Mg or Zn additions of
0
7
.1% and 5%) obtained at an ignition temperature of
73 K are given in Fig. 8. By comparing Fig. 8 with
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perature of acetone spray increases to 773 K, the effects
of types and amounts of the additive are less pronounced.
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(1986).
Ultrafine MnFe O powder with the corresponding
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2
4
5
6
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2+
3+
the stoichiometric amount of Mn and Fe to form
MnFe O were heated up and then brought in contact
7
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2
4
8. D.W. Johnson, Jr., Ceram. Bull. 60, 221 (1981).
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crystalline MnFe O . Raising the acetone ignition tem-
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2
0, 1506 (1984).
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1
(
2
4
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perature increased the size of crystallites of the resultant
MnFe O . The thermal stability of MnFe O powder so
2
4
2 4
1
1
1
obtained in an air atmosphere depended on the crystal-
linity of MnFe O ; MnFe O powder with a higher crys-
2
4
2 4
tallinity showed a wider temperature range for thermal
2
+
2+
stability. The presence of small amount of Mg or Zn
in the composition of coprecipitates promoted the mo-
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