JOURNAL OF MATERIALS SCIENCE 32 (1997) 459—464
Crystallization of a Bi–Pb–Fe–Cd–Oglass
under magnetic annealing
YI HU
Department of Materials Engineering, Tatung Institute of Technology, Taipei, Taiwan
Magnetic annealing with a tunable solenoid magnetic field from 0—240 G, was conducted on
aBi—Pb—Fe—Cd—O glass containing 20% Fe
2
O
3
, which was prepared by the melt-quenching
process. The crystalline phases of the annealed samples were identified as -Bi
2
O
3
and
BiFeO
3
. Evidence of the formation of the crystalline BiFeO
3
which was strongly magnetically
enhanced at the surface of the samples, was obtained from X-ray diffraction patterns and
EPR spectra. Based on the structure transition of Fe
3>
ions, a crystallization mechanism for
the BiFeO
3
crystals under magnetic annealing has been proposed.
1. Introduction
Glass-ceramics made by controlled crystallization of
special glasses have become one of the new types of
engineering materials developed during the past few
decades. This kind of glass-ceramic exhibits high den-
sity with good mechanical properties. Its production
process also provides a good means of combining
a wide range of unusual properties of glasses and
ceramics. Therefore, a controlled crystallization pro-
cess of glass has been developed into an independent
new field of technology and is now commonly em-
ployed in heat treatment [1], and additions of nuclea-
tion agents [2, 3] have been successfully adapted for
the crystallization process, producing many high-
quality glass-ceramic materials. However, these
techniques are limited by our understanding of the
mechanisms of crystallization that are involved
in the formation of crystal nuclei and their sub-
sequent growth. It is therefore necessary to exploit
a different process to obtain more information on the
crystallization mechanisms. In this work, magnetic
annealing was used to study the crystallization of
a glass.
The application of a magnetic field during heat
treatment is termed ‘‘magnetic annealing’’ and is fre-
quently applied to many alloy systems [4—6]. Yet, in
the literature, the effects of magnetic annealing on the
crystallization of glasses has not been reported. When
certain alloys are magnetically annealed, they may
develop a permanent magnetic anisotropy. These in-
duced anisotropies are of considerable interest to
study the basic magnetic phenomena and to design
magnetic materials for applications. However, mate-
rials for magnetic annealing should have high mag-
netic susceptibility [7—9]. In this study, we chose the
iron-containing magnetic glass for the magnetic
crystallization experiment. The aim of the present
investigation was to examine the effect of mag-
netic annealing on the crystallization of a magnetic
glass.
2. Experimental procedure
2.1. Preparation of glass samples
Bismuth oxide and lead oxide glasses have been
studied for their good infrared transmission [10—12].
However, systems containing only bismuth oxide and
lead oxides are difficult to make into glass. In order to
produce glass from these oxides, some modifying ox-
ides have been used to increase the glass-forming
range. Additions of ferric and cadmium oxides, which
act as glass-forming stabilizers, resulted in a large
region of glass formation [12]. Therefore, the high
solubility of Fe
O
in bismuth—lead glasses is suitable
for the present study, to produce magnetic glasses.
A glass with the composition 40Bi
O
—25PbO—
20Fe
O
—15CdO in molar ratio was prepared by the
melting and quenching process. Reagent-grade bis-
muth carbonate (BiCO
, '99%), lead oxide (PbO,
'99%), iron oxide (Fe
O
, '99%), and cadmium
oxide (CdO, '99%) were completely mixed, pre-
heated at 800
°
C for 1 h, and then melted at 1200
°
C
for 30 min in air in alumina crucibles. Glass sheets
were formed by pressing the melt between two copper
plates. These glass sheets were subsequently annealed
at 200
°
C for 12 h to release the internal stress. The
as-quenched glass sheets have a thickness of about
1.2 mm and were double-sided polished to about
0.5 mm thickness and cleaned with alcohol in an ultra-
sonic bath at room temperature.
2.2. DSC thermal analysis
A differential scanning calorimetry (DSC, Perkin—El-
mer DSC-4) was used to study the crystallization
process of glasses and to predetermine the heat-treat-
ment temperature [13]. In the present study, DSC was
conducted with heating rates from 5
°
C min
\
in air
and a typical DSC curve of the glasses is shown in
Fig. 1. An endothermic peak due to the glass
transition (¹
:610 K) and an exothermic peak due to
crystallization (¹
:690 K) are clearly observed.
0022—2461 1997 Chapman & Hall
459