Ceramics International 47 (2021) 14907–14912
Available online 9 July 2020
0272-8842/© 2020 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Sol-gel auto-combustion synthesis of Ba–Sr hexaferrite ceramic powders
Yaseen K.R. Shariff
a, b
, Harish K. Choudhary
a
, Vijay Khopkar
a
, Ankit Yadav
a
,
R. Madhusudhana
b
, Balaram Sahoo
a, *
a
Materials Research Centre, Indian Institute of Science, 560012, Bangalore, India
b
Centre for Nanotechnology, Department of Mechanical Engineering, The National Institute of Engineering, 570008, Mysuru, India
A R T I C L E INFO
Keywords:
Ba-Sr hexaferrites
Sol-gel auto-combustion synthesis
Thermal annealing
Structural phase-formations
ABSTRACT
We report the mechanism involved in sol-gel auto-combustion synthesis of Ba–Sr-hexaferrite (Ba
1-x
Sr
x
Fe
12
O
19
; x
= 0, 0.25, 0.5, 0.75 and 1, BSFO) ceramic powders through the analysis of the phases evolved during annealing
of the as-synthesized powders, along with their structure and morphological studies. The XRD patterns of the as-
synthesized samples indicate the formation of barium/strontium monoferrite ((Ba/Sr)Fe
2
O
4
) and maghemite
(γ-Fe
2
O
3
) phases along with a minute amount of hematite (α-Fe
2
O
3
) phase. Annealing of these samples facilitates
formation of BSFO phase through the solid state reaction between BaFe
2
O
4
and γ-Fe
2
O
3
phase. Interestingly, after
annealing the samples with x = 0, 0.5 and 1, at 1000
◦
C for 2 h, we observed that phase pure Ba–Sr hexaferrite
structure forms, but for samples with x = 0.25 and 0.75, high amount of hematite (α-Fe
2
O
3
) phase is observed,
especially for x = 0.75. The reason associated with this could be the large difference between the ionic radii of
Ba
2+
and Sr
2+
ions occupying the oxygen site. Furthermore, our study on annealing dependent phase evolution
confrms that, this difference in ionic radii forbids the formation of a single phase Ba–Sr hexaferrite. The growth
of clear hexagonal-shaped plate-like particles with varied particle sizes was observed for all the samples. The
particle size variation may be due to the infuence of the ionic radii difference on the sinterability of the samples.
Our study provides a better understanding of synthesis mechanism of Ba–Sr hexaferrite samples.
1. Introduction
The hexagonal ferrites, MFe
12
O
19
(M = Ba, Sr and Pb), are techno-
logically very useful class of materials due to their application in per-
manent magnets, high-density magnetic recording media and
microwave devices [1–9]. The large spontaneous magnetization with
strong magnetic anisotropy along the c-axis makes the hexagonal ferrites
suitable for their use as permanent magnetic materials [10]. Therefore,
these hard magnetic materials are widely investigated to fnd methods to
further improve their magnetic properties such as coercivity, ferro-
magnetic resonance behavior, electromagnetic radiation absorption
property and magneto-dielectric coupling [6,11–14]. Barium or stron-
tium hexaferrites, (Ba/Sr)Fe
12
O
19
, have magneto-plumbite (M)-type
hexagonal structure. The crystallographic space group associated with
this hexaferrites is P6
3
/mmc with each unit cell containing two mole-
cules (formula units) of the chemical composition (Ba/Sr)Fe
12
O
19
[15,
16]. Among these hexaferrites, BaFe
12
O
19
possesses relatively high
coercivity, high saturation magnetization and high magnetic anisotropy.
Along with excellent chemical stability and corrosion resistance
behavior [17–22]. These properties promotes these ferrites for their
wide use in traditional permanent magnets, high magneto-optic mag-
netoelectric/multiferroic, electronic components operating at Gigahertz
(GHz) frequencies and many microwave devices [2,5,23–28]. These
high technical importance of BaFe
12
O
19
have attracted extensive
attention during the last few decades. Simultaneously, strontium hex-
aferrite (SrFe
12
O
19
) is too one of the well-established hard ferrite mag-
netic material having wide range of applications in automotive, power
electronics and aerospace sectors etc. [29–31]. Pure strontium hex-
aferrite is isomorphous with the M type hexagonal structure of
BaFe
12
O
19
, but its dynamic and magnetic properties differ only slightly
from those of BaFe
12
O
19
.
Although Ba-hexaferrite and Sr-hexaferrite samples are individually
synthesized and reported in the literature [32–37], Ba and Sr based
mixed hexaferrites are diffcult to obtain as phase pure Ba–Sr hexaferrite
samples, especially, by low temperature synthesis methods [29,32,38].
To understand the origin of this diffculty in obtaining phase pure mixed
Ba–Sr hexaferrite is the aim of this work. Various synthesis methods
have been used to synthesize such M-type mixed hexaferrite samples
* Corresponding author.
E-mail address: bsahoo@iisc.ac.in (B. Sahoo).
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Ceramics International
journal homepage: www.elsevier.com/locate/ceramint
https://doi.org/10.1016/j.ceramint.2020.07.034
Received 16 March 2020; Received in revised form 3 July 2020; Accepted 4 July 2020