World Journal of Engineering
EXPANSION OF THE FERROELECTRIC PHASE TEMPERATURE INTERVAL
IN THE COMPOSITES (KNO3)1-x (BaTiO3)x AND (KNO3)1-x(PbTiO3)x
E.V. Stukova, S.V. Baryshnikov
Department of Engineering and Physics, Amur State University, Ignatievskoe shosse, 21, Blagoveschensk, Russia.
Department of Physics and Mathematics, Blagoveschensk State Pedagogical University, Lenina st., 104,
Blagoveschensk, Russia.
mediate phase III with the space group R3m can occur between about 124 °C and 110 °C instead of the direct inverse
transition I-II. Transitions between phase II and phases I or
III are reconstructive (do not follow the group-subgroup
relation). Phase III is ferroelectric. The occurrence of phase
III depends on thermal history of the sample and on pressure. At ambient pressure phase I transforms upon cooling
directly to phase II when warmed not higher than about 170
°C. The stability of phase III was found to improve for
KNO3 thin films [4] and particles confined within porous
glasses [5] and on doping bulk KNO3 with a small amount
of sodium ions (see [6] and references therein).
Three phase transitions, accompanied by changes in the
structure and properties, are observed in barium titanate
crystals. Phase transitions in barium titanate are transitions
of the displacement type. At temperatures above 120 °C
barium titanate has a cubic crystal structure of a perovskite
type. Below the temperature of 120 °C a phase transition
occurs, and BaTiO3 is a ferroelectric with a tetragonal
symmetry up to a temperature of 5 °C class P4mm. When
distortion of the cell abruptly occurs, spontaneous polarization appears, its value increases smoothly from Ps = 18
µC/cm2 near the Curie point up to about 26 µC/cm2 at room
temperature.
Lead titanate is a well-studied ferroelectric which has a
perovskite structure. At room temperature it has a tetragonal structure (P4mm). When heated to the Curie temperature equal to 493 °C, it undergoes a ferroelectric phase transition from tetragonal to cubic polar nonpolar phase.
Among ferroelectrics it is characterized by one of the highest values of spontaneous polarization at room temperature
– 75 µC/cm2.
Introduction
The solution of the problem, connected with understanding
the mechanisms of ordering dipole particles in ferroelectrics, has a long history. It dates back to the work of Debye
and Langevin, and it is still topical. Without taking into
account the ordering and interaction of dipole particles it is
not possible to describe any phenomena in ferroelectric
solid solutions, nor in dipole glasses, nor influence of dipole impurities on ferroelectrics properties. The nature of
cooperative phenomena in disordered systems raises great
interest. The mutual influence of ferroelectric particles in
different composites (see [1] and references) deserves special interest.
As ferroelectrics possess spontaneous polarization, and
small particles typically are typically single-domain, they
can be regarded as dipoles with a significant dipole moment, and interaction between them can not be neglected.
So in [2] it is shown that collective effects are observed for
ferroelectric powders KNO3 when all isolated powder particles experience a phase transition at the same time.
Composites based on ferroelectric materials may have a
very different structure. These may be polar particles in
weakly and strongly polarized matrices, polar particles in
the polar matrix, etc. Physical properties of small particles
in such composites are connected with the size and their
geometry. Besides, the volume ratio of the components and
the interaction of particles with a matrix and among themselves plays a significant role. All these factors, taken together, lead to the fact that the characteristics of structures
obtained in this way may differ significantly from the characteristics of initial materials.
This study examines the effect of BaTiO3 and PbTiO3 particles (5 – 30 microns) inclusion on the ferroelectric properties of KNO3 polycrystalline samples.
Apparatus and Procedures
We investigated the dielectric properties and the amplitude
of the third harmonic of the pressed samples
(KNO3)1-x(BaTiO3)x for x from 0.05 to 0.5 and
(KNO3)1-x(PbTiO3)x for x from 0.05 to 0.6. The samples
were in the form of tablets with a diameter of 12 mm and
thickness of about 1 mm, the tablets were pressed from the
mixture at the pressure of 6000 kg/cm2. As a reference we
used samples of polycrystalline potassium nitrate obtained
by the same procedure.
The dielectric permittivity and conductivity were measured
with an LCR-meter at a frequency of 1 MHz. The samples
were warmed up from room temperature to 180 °C and subsequently cooled back down. The temperature changes during the warming and cooling thermal cycles were not faster
Experiment
Materials
Potassium nitrate, KNO3, has long been used as an ingredient in explosives. The phase diagram for KNO3 is cited in
[3], for instance. At room temperature and pressure, KNO 3
crystallizes in an orthorhombic aragonite (Pmcn) phase.
This phase is usually referred to as phase II. Upon heating
to about 401 K it transforms to phase I, which has a disordered rhombohedral R 3 m (calcite-type) structure. When
phase I is cooling down at atmospheric pressure, an inter1055
World Journal of Engineering
than 1° min−1. The temperature was measured with accuracy of 0.1°C.
To measure the generation of the third harmonic, the sinusoidal electric field at a frequency of 2 kHz was applied to
the samples under study. The voltage amplitude was 50 V.
The output signal was Fourier transformed and the third
harmonic extracted by a digital spectrum analyzer assembled from an analog-digital converter connected to a computer.
80
Δ Temperature, oC
70
60
(KNO3)1-x(PbTiO3)x
50
40
30
20
10
Results and Discussion
(KNO3)1-x(BaTiO3)x
0
0
1.
2.
3.
x = 0.50
x = 0.55
4.
x = 0.60
γ
0,015
5.
0,01
0,005
0
6.
70
80
90
100
110
120
130
140
0,5
0,6
0,7
References
x = 0.30
60
0,4
The results can be explained within the framework of Landau-Ginzburg theory, taking into account the dipole-dipole
interaction [see [10] and references therein]. The electric
field inside the small particles is directly proportional to the
spontaneous polarization. The boundary conditions lead to
the fact, that near the surface of the particles the electric
field E = Ps(ε1/ε2), where ε1 is permittivity of the particle
inclusions, ε2 is the permittivity of KNO3. As we have
mentioned, the value of spontaneous polarization for the
lead titanate is twice Ps for the barium titanate, but the dielectric constant for BaTiO3 in the investigated temperature
range is one hundred times more than the corresponding
value for PbTiO3, which causes a larger internal field and
leads to a larger range of stabilization of the ferroelectric
phase.
x = 0.15
0,02
0,3
Conclusion
0,035
0,025
0,2
Fig. 3. The dependence of the ferroelectric phase of the
temperature interval on the sample composition
Fig 1. Temperature dependence of permittivity at heating
and cooling and the harmonics factor for polycrystalline
KNO3 at a frequency of 2 kHz.
Experimental data indicate that with increasing PbTiO3
content in the composite with x from 0.05 to 0.6 the temperature range of the ferroelectric phase existence increases
in comparison with pure potassium nitrate (Fig. 2).
0,03
0,1
x
In the ferroelectric phase at low frequencies, the nonlinearity is determined by the shape of the hysteresis loop D(E),
and the magnitude of the third harmonic is at least by one
order higher than in the paraelectric phase. Fig. 1 shows the
temperature dependences of the permittivity and the harmonic factor ( = U3/U) for poly-crystalline sample
KNO3.
150
o
Temperature ( C)
Fig. 2. Temperature dependence of the harmonics factor
( = U3/U) to (KNO3)1-x(PbTiO3)x at cooling.
7.
The inclusion of BaTiO3 small particles also lead to expansion of the existence of the ferroelectric phase, but
x  0.5 compared with lead titanate, this influence is greater
when x ferroelectric phase does not occur (Fig. 3).
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