The effect of isovalent substitutions and dopants of 3d-metals on the properties of ferroelectricssemiconductors
Electrophysical properties and microstructure of PTCR ceramics of the system (Ba,Ca,Sr,Y)TiO3 + y%Mn have been investigated. It has been shown that manganese ions increase the potential barrier at grain boundaries and form a high-resistance outer layer in (Ba,Ca,Sr,Y)TiO₃ ceramics. The resistance...
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Інститут фізики конденсованих систем НАН України
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irk-123456789-1207012017-06-13T03:06:05Z The effect of isovalent substitutions and dopants of 3d-metals on the properties of ferroelectricssemiconductors V'yunov, O.I. Kovalenko, L.L. Belous, A.G. Electrophysical properties and microstructure of PTCR ceramics of the system (Ba,Ca,Sr,Y)TiO3 + y%Mn have been investigated. It has been shown that manganese ions increase the potential barrier at grain boundaries and form a high-resistance outer layer in (Ba,Ca,Sr,Y)TiO₃ ceramics. The resistance of grains, outer layers and grain boundaries, the values of temperature coefficient of resistance as well as the varistor effect as a function of manganese content of PTCR materials have been investigated. Метою даної роботи було вивчення впливу йонів мангану на властивості областей ПТКО кераміки на основі (Ba,Ca,Sr,Y)TiO₃, що відрізняються за електричними властивостями. Було знайдено, що ріст вмісту мангану в кераміці на основі титанату барію збільшує опір границь і зовнішніх шарів зерен, але практично не змiнює опору зерен; при цьому потенціальний бар’єр на границях зерен зростає. Проведені дослідження ПТКО кераміки на основі титанату барію в широкому частотному і температурному інтервалах дозволяють стверджувати, що йони мангану знаходяться переважно на границях зерен і слабо впливають на опір зерен. Такий розподіл домішки мангану суттєво покращує властивості ПТКО матеріалів. 2003 Article The effect of isovalent substitutions and dopants of 3d-metals on the properties of ferroelectricssemiconductors / O.I. V'yunov, L.L. Kovalenko, A.G. Belous // Condensed Matter Physics. — 2003. — Т. 6, № 2(34). — С. 213-220. — Бібліогр.: 14 назв. — англ. 1607-324X PACS: 61.66.Fn, 77.80.Bh, 78.40.Fy DOI:10.5488/CMP.6.2.213 http://dspace.nbuv.gov.ua/handle/123456789/120701 en Condensed Matter Physics Інститут фізики конденсованих систем НАН України |
| institution |
Digital Library of Periodicals of National Academy of Sciences of Ukraine |
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DSpace DC |
| language |
English |
| description |
Electrophysical properties and microstructure of PTCR ceramics of the system
(Ba,Ca,Sr,Y)TiO3 + y%Mn have been investigated. It has been shown
that manganese ions increase the potential barrier at grain boundaries and
form a high-resistance outer layer in (Ba,Ca,Sr,Y)TiO₃ ceramics. The resistance
of grains, outer layers and grain boundaries, the values of temperature
coefficient of resistance as well as the varistor effect as a function of
manganese content of PTCR materials have been investigated. |
| format |
Article |
| author |
V'yunov, O.I. Kovalenko, L.L. Belous, A.G. |
| spellingShingle |
V'yunov, O.I. Kovalenko, L.L. Belous, A.G. The effect of isovalent substitutions and dopants of 3d-metals on the properties of ferroelectricssemiconductors Condensed Matter Physics |
| author_facet |
V'yunov, O.I. Kovalenko, L.L. Belous, A.G. |
| author_sort |
V'yunov, O.I. |
| title |
The effect of isovalent substitutions and dopants of 3d-metals on the properties of ferroelectricssemiconductors |
| title_short |
The effect of isovalent substitutions and dopants of 3d-metals on the properties of ferroelectricssemiconductors |
| title_full |
The effect of isovalent substitutions and dopants of 3d-metals on the properties of ferroelectricssemiconductors |
| title_fullStr |
The effect of isovalent substitutions and dopants of 3d-metals on the properties of ferroelectricssemiconductors |
| title_full_unstemmed |
The effect of isovalent substitutions and dopants of 3d-metals on the properties of ferroelectricssemiconductors |
| title_sort |
effect of isovalent substitutions and dopants of 3d-metals on the properties of ferroelectricssemiconductors |
| publisher |
Інститут фізики конденсованих систем НАН України |
| publishDate |
2003 |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/120701 |
| citation_txt |
The effect of isovalent substitutions
and dopants of 3d-metals on the
properties of ferroelectricssemiconductors / O.I. V'yunov, L.L. Kovalenko, A.G. Belous // Condensed Matter Physics. — 2003. — Т. 6, № 2(34). — С. 213-220. — Бібліогр.: 14 назв. — англ. |
| series |
Condensed Matter Physics |
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2025-07-08T18:25:57Z |
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2025-07-08T18:25:57Z |
| _version_ |
1837104270309064704 |
| fulltext |
Condensed Matter Physics, 2003, Vol. 6, No. 2(34), pp. 213–220
The effect of isovalent substitutions
and dopants of 3d-metals on the
properties of ferroelectrics-
semiconductors
O.I.V’yunov∗, L.L.Kovalenko, A.G.Belous
V.I.Vernadskii Institute of General and Inorganic Chemistry
32/34 Palladina Ave., 03680 Kyiv-142, Ukraine
Received September 2, 2002
Electrophysical properties and microstructure of PTCR ceramics of the sys-
tem (Ba,Ca,Sr,Y)TiO3 + y%Mn have been investigated. It has been shown
that manganese ions increase the potential barrier at grain boundaries and
form a high-resistance outer layer in (Ba,Ca,Sr,Y)TiO3 ceramics. The re-
sistance of grains, outer layers and grain boundaries, the values of temper-
ature coefficient of resistance as well as the varistor effect as a function of
manganese content of PTCR materials have been investigated.
Key words: PTCR, manganese dopant, varistor effect, microstructure,
potential barrier, complex impedance
PACS: 61.66.Fn, 77.80.Bh, 78.40.Fy
1. Introduction
Positive temperature coefficient of resistance (PTCR) occurs in ferroelectric
semiconducting ceramics based on doped barium titanate near Curie point due to
the formation of potential barriers at grain boundaries [1]. Therefore, PTCR ce-
ramics are synthesized in the conditions at which semiconducting grains and high-
resistance grain boundaries are formed. In particular, this is achieved when yttrium
ions are partially substituted for barium ions and grain boundaries oxidized dur-
ing sintering of ceramics in the air. Complex impedance (Z∗) and complex electric
modulus (M∗) analysis in a wide frequency range showed the presence of semicon-
ducting grains, high-resistance grain boundaries and outer layers between grains and
grain boundaries in PTCR materials. These areas of PTCR ceramics are electrically
non-uniform and can be represented by an equivalent circuit, which includes three
parallel RC-elements connected in series [2–4]. The low magnitude of resistivity
change in the PTCR region, viz. ratio of maximum (ρmax) to minimum (ρmin) resis-
tivity, and large varistor effect, viz. reduction of resistivity under external electric
∗E-mail: vyunov@ionc.kar.net
c© O.I.V’yunov, L.L.Kovalenko, A.G.Belous 213
O.I.V’yunov, L.L.Kovalenko, A.G.Belous
field, are the basic difficulties in the use of barium-titanate-based PTCR materials
in the devices working at high strengths of electric field. The magnitude of varistor
effect can be estimated from the dependence of normalized resistivity ρE/ρ0, where
ρE and ρ0 are resistivity of the sample at zero and nonzero voltage, on the electric
field strength (E). Earlier investigations of PTCR barium titanate [5] showed that
the magnitude of varistor effect correlates with the average grain size of ceramics,
namely, the varistor effect is smaller in fine-grained ceramics. Both average grain size
and the varistor effect decrease at partial substitution of calcium and strontium for
barium in PTCR barium titanate (Ba,Ca,Sr,Y)TiO3. The magnitude of resistivity
change in this case remains unchanged. Besides, the partial isovalent substitution in
the barium site of barium titanate shifts the Curie point towards low-temperature
range, and this expands the area of application of PTCR-materials. However, the
low magnitude of resistivity change in PTCR area does not permit to utilize the
above materials at high strengths of the electric field. The decrease in varistor effect
and the increase in the magnitude of resistivity change are also attributed to the
increase in the resistance of grain boundaries [6,7]. In particular, this occurs when
the synthesized materials are doped with acceptors (for example, manganese) [8].
High-resistance grain boundaries in manganese-doped PTCR materials are formed
due to redox transformations of manganese oxides in the same temperature range in
which redox processes, accompanied with the formation of trivalent titanium, pro-
ceed [9]. However, the information concerning the distribution of manganese dopant
in a polycrystalline material is scantily presented in the literature. This does not
permit to explain the formation mechanisms of PTCR effect, as well as to control
the properties of the ceramics based on barium titanate with manganese dopant.
Therefore, the aim of this work was to study the distribution of manganese ions
in (Ba,Ca,Sr,Y)TiO3 ceramics and its effect on the properties of grains, outer grain
layers and grain boundaries of PTCR ceramics.
2. Experimental
Extra-pure BaCO3, CaCO3, SrCO3, TiO2, Y2O3, SiO2, MnSO4 and water solu-
tion of ammonia were used as starting reagents. Powders were ball-milled in agate
mortar. In order to reduce the pollution of mixed powders during the milling, the
working surfaces of crushing cylinders were covered with vacuum rubber. Uniform
distribution of manganese dopant was ensured by its precipitation from solutions.
Electrophysical properties of samples sintered at 1340–1360 ◦C have been investi-
gated. The grain sizes in the ceramics were determined using X-ray microanalyser
JCXA Superprobe 733 (JEOL, Japan). Aluminium electrodes were fabricated by
burning in Al paste. Electrical properties of the ceramics were studied at direct and
alternating current. Impedance analyzer PGSTAT–30 (Solartron) was used for mea-
surements in the frequency range 100 Hz – 1 MHz, and BM–560 Q-meter was used
for measurements in the frequency range 50 kHz – 35 MHz. Equivalent circuit and
values of its components were determined using Frequency Response Analyzer 4.7
PC program.
214
The effect of isovalent substitutions and dopants of 3d-metals
(a) (b) (c) (d)
Figure 1. Microstructure of PTCR ceramics in (Ba,Ca,Sr,Y)TiO3 + y mol.% Mn
system: y = 0 (a), 0.002 (b), 0.006 (c), 0.01 (d), ×1000.
3. Results and discussion
The average grain size of PTCR ceramics of the system (Ba,Ca,Sr,Y)TiO3 does
not change with manganese content (figure 1). The temperature dependence of re-
sistivity of PTCR ceramics can be schematically divided into 3 ranges (figure 2).
Range I extends from room to phase transition temperature and is characterized by
relatively low resistivity which decreases with temperature. Range II lies above the
phase transition temperature where a rapid growth of resistivity is observed (PTCR
effect). Range III exists at a high temperature and is characterized by high resistivity
which decreases with temperature. When the ceramics are doped with manganese,
the magnitude of resistivity change in PTCR area increases (see figure 2), and the
varistor effect essentially decreases (figure 3). On the basis of the above fact it may
be assumed that the potential barrier at grain boundaries increases with manganese
content.
Figure 2. Resistivity of PTCR ceram-
ics of (Ba,Ca,Sr,Y)TiO3 +y mol.% Mn
system versus temperature; y = 0 (1),
0.002 (2), 0.006 (3), 0.01 (4), 0.02 (5),
0.03 (6).
Figure 3. Normalized resistivity
(lg ρE/ρ0) of PTCR ceramics of
(Ba,Ca,Sr,Y)TiO3 + y mol.% Mn
system versus external electric field;
y = 0 (1), 0.002 (2), 0.006 (3), 0.01 (4),
Tmeas. = 20 ◦C.
215
O.I.V’yunov, L.L.Kovalenko, A.G.Belous
Figure 4. Imaginary components of complex impedance Z ′′ and electric modulus
M ′′ of PTCR ceramics of (Ba,Ca,Sr,Y)TiO3 (a, b) and (Ba,Ca,Sr,Y)TiO3 +
0.01 mol.% Mn systems (c, d) versus frequency at various temperatures.
The results of frequency investigations of the PTCR ceramics can be analyzed as
four types of dependences: of complex impedance (Z∗), complex admittance (Y ∗),
complex permittivity (ε∗) and complex electric modulus (M ∗). The complex quan-
tities are interrelated: M ∗ = 1/ε∗ = jωCoZ
∗ = jωCo(1/Y ∗) (where j =
√
−1) For
an analysis, the results of investigations were presented as frequency dependences of
the imaginary components of complex impedance Z ′′ and complex electric modulus
M ′′, which for a parallel RC element are described by the equations [2–4, 10, 11]:
Z ′′ = R · ωRC
1 + (ωRC)2
, M ′′ =
ε0
C
· ωRC
1 + (ωRC)2
, (1)
where ω = 2πf is angular frequency (f denote frequency in Hz) and ε0 is the
permittivity of free space (8.854 · 10−14 F · cm−1).
From equations (1) it follows that:
ωmax =
1
RC
, Z ′′
max
=
R
2
, M ′′
max
=
ε0
2C
. (2)
Equations (2) show that the shift of the peaks Z ′′
max
and M ′′
max
in frequency (ωmax)
is associated with a change in the values of both capacity and resistance in the
corresponding RC element of the equivalent circuit. The Z ′′
max value is sensitive to
the change in the resistance, and the M ′′
max
value is affected by the capacity.
Figure 4 shows frequency dependences of Z ′′ and M ′′ of samples of the systems
(Ba,Ca,Sr,Y)TiO3 and (Ba,Ca,Sr,Y)TiO3 + y mol.% Mn, investigated at various
216
The effect of isovalent substitutions and dopants of 3d-metals
Figure 5. Resistance of grain (1), outer layer (2) and grain boundary (3) of PTCR
ceramics of the (Ba,Ca,Sr,Y)TiO3 (a) and (Ba,Ca,Sr,Y)TiO3 + 0.01 mol.% Mn
systems (b) versus temperature.
temperatures. The plot of Z ′′(f ) exhibits one peak, and the plot of M ′′(f ) exhibits
two peaks: one in the medium-frequency range (104 – 105 Hz) and the other in the
high-frequency ( > 108 Hz) range. The positions of Z ′′
max
and M ′′
max
do not coincide in
frequency. This may be accounted for by the fact that the behavior of these maxima
is affected by different electroactive ceramic regions. The change in the value and
position of the maximum in the plot of Z ′′(f ) is associated with the change in the
electrophysical properties of the grain boundary, and that of M ′′(f ) at 104 – 105 Hz
and at > 108 Hz is associated with the change in the electrophysical properties
of the grain outer layer and the grain, respectively [2–4]. Figure 4 shows that the
frequency of Z ′′
max in (Ba,Ca,Sr,Y)TiO3 ceramics decreases and the value of Z ′′
max
increases with the increase of temperature. This is due, according to equations (2),
to an increase in the resistance of the grain boundary. The positions of M ′′
max
in
the medium-frequency range slightly shifts with the increase of temperature, and
the value of M ′′
max
essentially increases. This is due, according to equation (2), to a
decrease in the capacitance and an increase in the resistance of the outer layer of the
grain. A calculation carried out on the basis of our experimental data corroborated
the above conclusion.
Temperature dependences of the resistance of electrically different areas of the
PTCR ceramics (Ba,Ca,Sr,Y)TiO3 and (Ba,Ca,Sr,Y)TiO3 + 0.01 mol.% Mn are
shown in figure 5. The variation of the resistance of the outer layer with temperature
is similar to that of the grain boundary. Hence, the PTCR effect in (Ba,Ca,Sr,Y)TiO3
ceramics without manganese dopant occurs due to a change in electrophysical prop-
erties of the grain boundaries and the outer layers.
Temperature dependences of capacitances of (Ba,Ca,Sr,Y)TiO3 PTCR ceramics
are shown in figure 6. The capacitance of the grain boundary of the PTCR ceramic
slightly changes with temperature, and the capacitance of the outer layer varies
with temperature by the Curie-Weiss law (figure 6a). The capacitance of the grain
boundary and the outer layer decreases in manganese-doped ceramics (figure 6b).
The results of investigations of PTCR (Ba,Ca,Sr,Y)TiO3 ceramics properties at
room temperature as a function of manganese content are shown in figure 7. As is
217
O.I.V’yunov, L.L.Kovalenko, A.G.Belous
Figure 6. Inverse capacitance of grain boundary (1) and outer layer (2) of the
systems (Ba,Ca,Sr,Y)TiO3 (a) and (Ba,Ca,Sr,Y)TiO3+0.01 mol.% Mn (b) versus
temperature.
evident from the data presented, the grain boundary resistance increases, whereas
the grain resistance remains practically unchanged with the increasing manganese
content. This is due to the manganese being not incorporated into the whole bulk
of PTCR barium titanate grain in the concentration range under study.
To ascertain the reason of the increase in the multiplicity of resistance change
in the PTCR region of ceramics as a function of manganese content, the magnitude
of the potential barrier at the grain boundary has been calculated. The variation
of the resistance in the temperature range ca. 17–50 ◦C (see figure 2, range I) and
ca. 300–500 ◦C (see figure 2, range III) is described by the equations [1, 12]:
ρS = ρI
0 · e
E
I
a
kT , ρd = ρIII
0 · e
E
III
a
kT , (3)
where ρ0 is a constant for material [13]; Ea is the activation energy of conductivity;
k is Boltzmann constant (1.38 · 10−23 J/K = 8.62 · 10−5 eV/K).
Figure 7. Resistance of grain (1) and
grain boundary (2) of PTCR ceramics
of (Ba,Ca,Sr,Y)TiO3 + y mol.% Mn
system versus Mn content; Tmeas. =
20 ◦C.
Figure 8. Potential barrier at grain
boundaries (Φ0) of PTCR ceramics of
(Ba,Ca,Sr,Y)TiO3 + y mol.% Mn sys-
tem versus temperature; y = 0 (1),
0.01 (2).
218
The effect of isovalent substitutions and dopants of 3d-metals
Table 1. The effect of manganese content on the characteristics of temperature
dependence of the resistance of PTCR ceramics (Ba,Ca,Sr,Y)TiO3 .
Manganese Range I Range II Range III
content, mol.% RI
0
, Ω EI
a
, eV nD · b2, cm−1 RIII
0
, Ω EIII
a
, eV
0 4.2 0.04 3.2·108 2000 0.13
0.002 4.7 0.04 3.6·108 800 0.22
0.006 4.8 0.04 4.2·108 120 0.36
0.01 5.2 0.04 5.4·108 36 0.44
0.02 5.8 0.04 5.5·108 0.7 0.69
0.03 8.2 0.04 5.8·108 0.5 0.72
The variation of the resistance in the temperature range ca. 100–300 ◦C, where a
PTCR effect manifests itself, is usually described in terms of the Heywang model [1]:
ρ = α · ρS · e
Φ0(T )
kT , (4)
where α is the geometric factor; Φ0(T) is the height of the potential barrier at the
grain boundary:
Φ0(T ) =
e2 · nD · b2
2 · εi(T ) · ε0
, (5)
e is the electron charge; nD is the electron volume concentration; b is the potential
barrier thickness (2b = nS/nD, where nS is surface concentration of acceptor states);
εi(T ) is grain permittivity which varies in ferroelectrics by the Curie-Weiss law:
εi(T ) = C/T − Θ (where C is Curie constant and Θ is Curie temperature).
From equations (3) and (4) we can get:
ρ = α · ρ0 · e
E
I
0
kT · exp
e2 · nD · b2(T − Θ)
2 · ε0 · C · kT
. (6)
The results of calculations for (Ba,Ca,Sr,Y)TiO3 ceramics according to equations
(3) and (6) show that in the temperature range I the resistance remains unchanged
and conductivity activation energy EI
a
decreases with the increasing manganese con-
tent, whereas in the temperature range III the resistance decreases and conductivity
activation energy increases (table 1). The magnitude of the potential barrier at the
grain boundaries of PTCR barium titanate, which accounts for the increase in the
multiplicity of resistance change in the PTCR region, were calculated using equa-
tion 5 (figure 8) and agreed with the literature data [14].
Thus, the investigations of the manganese-doped PTCR ceramics based on
(Ba,Ca,Sr,Y)TiO3 carried out by us over a wide frequency and temperature range
showed that the manganese content slightly affects the grain resistance. Manganese
ions are mainly at the grain boundaries and in the grain outer layer and act as
acceptors. This greatly improves the properties of PTCR materials: the multiplic-
ity of the resistance change in the PTCR region increases, and the varistor effect
decreases.
219
O.I.V’yunov, L.L.Kovalenko, A.G.Belous
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Вплив ізовалентних заміщень і домішок 3d-металів
на властивості сегнетоелектриків-напівпровідників
О.І.В’юнов, Л.Л.Коваленко, А.Г.Білоус
Інститут загальної та неорганічної хімії ім. В.І.Вернадського
03680 Київ-142, просп. Палладіна, 32/34
Отримано 2 вересня 2002 р.
Метою даної роботи було вивчення впливу йонів мангану на власти-
вості областей ПТКО кераміки на основі (Ba,Ca,Sr,Y)TiO3, що відріз-
няються за електричними властивостями. Було знайдено, що ріст
вмісту мангану в кераміці на основі титанату барію збільшує опір
границь і зовнішніх шарів зерен, але практично не змiнює опору
зерен; при цьому потенціальний бар’єр на границях зерен зрос-
тає. Проведені дослідження ПТКО кераміки на основі титанату барію
в широкому частотному і температурному інтервалах дозволяють
стверджувати, що йони мангану знаходяться переважно на грани-
цях зерен і слабо впливають на опір зерен. Такий розподіл домішки
мангану суттєво покращує властивості ПТКО матеріалів.
Ключові слова: ПТКО, домішка мангану, варисторний ефект,
мікроструктура, потенціальний бар’єр, комплексний імпеданс
PACS: 61.66.Fn, 77.80.Bh, 78.40.Fy
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