The role of temperature on dielectric relaxation and conductivity mechanism of dark conglomerate liquid crystal phase

Article abstract of DOI:10.1016/j.physb.2015.12.050

In this study, dielectric properties and ac conductivity mechanism of the bent-core liquid crystal 3′-{4-[4-(3,7-Dimethyloctyloxy)benzoyloxy]benzoyloxy}-4-{4-[4-[6-(1,1,3,3,5,5,5-heptamethyltrisiloxan-1yl)hex-1-yloxy]benzoyloxy]benzoyloxy}biphenyl (DBB) have been analyzed by impedance spectroscopy measurements at different temperatures. According to the polarizing microscopy results, DBB liquid crystal compound exhibits a dark conglomerate mesophase (DC^[?] phase) which can be identified by the occurrence of a conglomerate of domains with opposite chirality. The chiral domains of this low-birefringent mesophase become more visible by rotating the polarizer. The variation of the real (ε′) and imaginary (ε″) parts of dielectric constant with angular frequency and Cole-Cole curves of DBB have been analyzed. The fitting results for dispersion curves at different temperatures revealed that DBB system exhibits nearly Debye-type relaxation except for 125 °C. Moreover, it has been determined that while the relaxation frequencies shift to higher frequencies as the temperature increases from 25 °C to 125 °C, the peak intensities remarkably decrease with increasing temperature. According to Cole-Cole plot and phase angle versus frequency curve, it has been determined that DBB LC may have a possibility of utilizing as a super-capacitor at room temperature. Furthermore, it has been found that the conductivity mechanism of the DBB alters from Correlated Barrier Hoping (CBH) model to Quantum Tunneling Model (QMT) with in increasing temperature at high frequency region. In terms of CBH model, optical band gaps at 25 °C and 75 °C temperatures have also been calculated. Finally, activation energies for some selected angular frequencies have also been calculated.

Full text of DOI:10.1016/j.physb.2015.12.050

Physica B 485 (2016) 21–28
Contents lists available at ScienceDirect
Physica B
journal homepage: www.elsevier.com/locate/physb
The role of temperature on dielectric relaxation and conductivity
mechanism of dark conglomerate liquid crystal phase
b
c
Alptekin Yildiz a,b, Nimet Yilmaz Canli b, , Zeynep Güven Özdemir , Hale Ocak ,
n
Belkız Bilgin Eran c, Mustafa Okutan b
a Istanbul Technical University, Department of Physics Engineering, 34469 Maslak, Istanbul, Turkey
b Yildiz Technical University, Department of Physics, 34210 Esenler, Istanbul, Turkey
c Yildiz Technical University, Department of Chemistry, 34210 Esenler, Istanbul, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
In this study, dielectric properties and ac conductivity mechanism of the bent-core liquid crystal 3′-{4-[4-
(3,7-Dimethyloctyloxy)benzoyloxy]benzoyloxy}-4-{4-[4-[6-(1,1,3,3,5,5,5-heptamethyltrisiloxan-1yl)hex-1-
yloxy]benzoyloxy]benzoyloxy}biphenyl (DBB) have been analyzed by impedance spectroscopy measure-
ments at different temperatures. According to the polarizing microscopy results, DBB liquid crystal
compound exhibits a dark conglomerate mesophase (DC[*] phase) which can be identified by the oc-
currence of a conglomerate of domains with opposite chirality. The chiral domains of this low-bi-
Received 17 November 2015
Received in revised form
29 December 2015
Accepted 30 December 2015
Available online 4 January 2016
Keywords:
refringent mesophase become more visible by rotating the polarizer. The variation of the real (
ε′) and
Bent-core liquid crystals
Dark conglomerate phase
Dielectric studies
Conductivity mechanisms
Activation energy
imaginary ( ″) parts of dielectric constant with angular frequency and Cole–Cole curves of DBB have
ε
been analyzed. The fitting results for dispersion curves at different temperatures revealed that DBB
system exhibits nearly Debye-type relaxation except for 125 °C. Moreover, it has been determined that
while the relaxation frequencies shift to higher frequencies as the temperature increases from 25 °C to
125 °C, the peak intensities remarkably decrease with increasing temperature. According to Cole–Cole
plot and phase angle versus frequency curve, it has been determined that DBB LC may have a possibility
of utilizing as a super-capacitor at room temperature. Furthermore, it has been found that the con-
ductivity mechanism of the DBB alters from Correlated Barrier Hoping (CBH) model to Quantum Tun-
neling Model (QMT) with in increasing temperature at high frequency region. In terms of CBH model,
optical band gaps at 25 °C and 75 °C temperatures have also been calculated. Finally, activation energies
for some selected angular frequencies have also been calculated.
Optical band gap
& 2016 Elsevier B.V. All rights reserved.
1. Introduction
switching in LC phases exhibited by the achiral bent-core liquid
crystals [17], the liquid crystal studies comprising achiral bent-
Liquid crystals which exhibit the fluidity of liquids and the
anisotropy of crystals are the advanced technological materials
which have significant interest for applications in mobile data
processing and communication tools in the last decades [1–3].
Until 1996, liquid crystal research had been mainly focused on the
synthesis of rod like or disc like molecules because of their ap-
propriate LC phases such as nematic phase in display applications
[3–11]. Although some researchers synthesized several bent-core
liquid crystals and reported their some properties [12–15], their
importance was not been realized until 1996. It was known that
molecular chirality has been necessary to produce a chiral liquid-
crystalline phase [16]. After the discovery of electro-optical
core compounds had a great deal of interest due to their out-
standing physical properties which differ significantly from me-
sophases of calamitic and disc-like compounds. Numerous sys-
tematic studies have been done in order to obtain unique LC
phases of these achiral molecules showing macroscopic polar or-
der and superstructural chirality to date 1,2,3, [18–22]. The re-
markable behavior of this new class of thermotropic liquid crystals
is the presence of superstructural chirality which results from the
combination of tilt and polar order of the molecules in successive
layers. This leads to LC phases showing anti-ferroelectric and fer-
roelectric switching 1,18.
Bent-core liquid crystals are known to exhibit optically iso-
tropic phases, for example: the dark conglomerate (DC) phases
with a sponge-like structure [23,24], and B4-type helical filament
phases [25,26]. The DC phases have no birefringence or low-
n
Corresponding author.
E-mail address: niyilmaz@yahoo.com (N.Y. Canli).
http://dx.doi.org/10.1016/j.physb.2015.12.050
0921-4526/& 2016 Elsevier B.V. All rights reserved.

22
A. Yildiz et al. / Physica B 485 (2016) 21–28
birefringence. In the DC phases, layers are strongly deformed and
organized in a sponge-like structure. Due to the special organi-
zation of the layers, the mesophases appear optically isotropic
between crossed polarizers on cooling from the isotropic liquid.
Dark and bright domains can be observed by the rotation of the
polarizer by a small angle and the brightness of these chiral do-
mains exchange on rotation of the polarizer in the opposite di-
rection. The optical activity in the DC phase of achiral bent-core
mesogens is attributed to the layer chirality or to the coupling of
molecular conformational chirality to the layer chirality 1 [27,28].
Due to their attractive functional properties, DC phases based
materials are of significant interest for a variety of application such
as organic semiconductors, thin film transistors ands solar cells
[29,30], NLO materials [31,32] and thin film polarizers [33]. Here
we investigated the influence of temperature on dielectric prop-
erties and conductivity mechanisms of the dark conglomerate (DC)
phase formed by a silylated achiral bent-core molecule with a
branched alkyl chain.
Transition temperatures were measured using a Mettler FP-82
HT hot stage and control unit in conjuction with a Leica polarizing
microscope. The associated enthalpies were obtained from DSC-
thermograms which were recorded on a Perkin-Elmer DSC-7,
heating and cooling rate: 10 °C minꢀ1
.
Spectroscopic data and mesomorphic properties for the target
material DBB had already been given in Ocak et al. [34]. The phase
transition temperatures and chemical structure of DBB are shown
in Fig. 1.
Siloxane substituted bent-core compound DBB exhibits dark
conglomerate phase with ferroelectric switching 34. On cooling
from the isotropic liquid, dark and bright domains have been ob-
served by rotating the polarizer by a small angle. The brightness of
these domains is exchanged on rotation of the polarizer in the
opposite direction. If the sample was rotated between the slightly
decrossed polarizers, the brightness of the chiral domains was not
changed. This indicates a conglomerate of domains with opposite
chirality as typical for dark conglomerate phases. Polarized light
optical photomicrographs on cooling are shown in Fig. 2.
2. Experimental
2.2. Preparation of liquid crystal cell
2.1. Mesomorphic properties of DBB liquid crystal
Empty cell has been prepared by indium–tin-oxide (ITO) coated
glass plates purchased by Instec Colorada Inc. The thickness of the
The synthesis of bent-core liquid crystal molecule, 3′-{4-[4-
(3,7-Dimethyloctyloxy) benzoyloxy] benzoyloxy} -4-{4-[4-[6-
(1,1,3,3,5,5,5- heptamethyltrisiloxan-1yl) hex-1-yloxy] benzoyloxy]
benzoyloxy} biphenyl (DBB) has been carried out by the procedure
in our previous work [34]. Firstly, 4-[4-(3,7-Dimethyloctyloxy)ben-
zoyloxy]benzoic acid has been attached to 4′-benzyloxybiphenyl-3-
ol [35] by esterification using dicyclohexylcarbodiimide (DCC) and
4-dimethylaminopyridine (DMAP) as catalysts to yield 4'-Benzy-
loxy-3-[4-(3,7-Dimethyloctyloxy)benzoyloxy]benzoyloxybiphenyl.
After hydrogenolytic debenzylation (H2, 10% Pd/C in THF) of the
benzylated intermediate and DCC esterification with 4-[4-(5-hex-
enyloxy)benzoyloxy]benzoic acid [36], olefine terminated bent-core
compound, 3′-{4-[4-(3,7-Dimethyloctyloxy)benzoyloxy]benzoyloxy}-
4-{4-[4-(5-hexenyloxy)benzoyloxy]benzoyloxy}biphenyl, has been
obtained. In the final step, 1,1,1,3,3,5,5-heptamethyltrisiloxane has
been attached to the olefinic precursor by a hydrosilylation reac-
tion using Karstedt's catalyst to yield the target siloxane sub-
stituted compound DBB [34] (Scheme 1).
empty cell was set at d¼970.2
μ
m by Mylar spacer. The ITO cell
has been filled with DBB by capillary action.
3. Results and discussions
3.1. Analysis of temperature effect on frequency dependent dielectric
parameters
The dielectric properties of DBB Liquid crystal have been ana-
lyzed by HP 4192A Impedance Analyzer within the frequency in-
terval of 5 Hz–15 MHz at various temperatures varying from 25 °C
to 150 °C.
The variations of the real part of the dielectric constant with
angular frequency at these temperatures are shown in Fig. 3.
As shown in Fig. 3, regardless of the operating temperature, the
real component of dielectric constant has a characteristic satura-
tion region and sharp decrease behavior as the angular frequency
increases from 104 to 108 rad/s. It has been observed that as the
Compound DBB was characterized 1H-, 13C-NMR and 29Si-NMR
(Varian Unity 500 and Varian Unity 400 spectrometers, in CDCl3
solutions, with tetramethylsilane as internal standard). Micro-
temperature increases, the maximum value of
decreases.
ε′ remarkably
analysis was performed using
analyzer.
a
Leco CHNS-932 elemental
In order to define the effect of temperature on dielectric re-
laxation type of DBB, the dispersion curves for each temperature
have been fitted by Origin Lab 8.5. The equation of fitting function
has been given in Eq. (1)
Compound DBB [34]: 1H-NMR:
δ
(ppm)¼8.29 (d; JE8.7 Hz;
2 Ar–H), 8.28 (d; JE8.7 Hz; 2 Ar–H), 8.15 (d; JE8.9 Hz; 2 Ar–H),
8.14 (d; JE8.9 Hz; 2 Ar–H), 7.66 (d; JE8.7 Hz; 2 Ar–H), 7.51–7.50
(m; 2 Ar–H), 7.45 (broad s; 1 Ar–H), 7.37 (d; JE8.7 Hz; 2 Ar–H),
7.36 (d; JE8.7 Hz; 2 Ar–H), 7.30 (d; JE8.7 Hz; 2 Ar–H), 7.23–7.19
(m, 1 Ar–H), 6.98 (d; JE8.9 Hz; 2 Ar–H), 6.97 (d; JE8.9 Hz; 2 Ar–
H), 4.11–4.08 (m; 2H OCH2), 4.04 (t; JE6.5 Hz; 2 H, OCH2), 1.85–
1.79 (m; 3H, CH2, CH), 1.66–1.47, 1.39–1.15 (2m; 15H, CH, 7 CH2),
0.95 (d; JE6.4 Hz; 3H, CH3), 0.87 (d; JE6.6 Hz; 6H, 2 CH3), 0.57–
0.53 (m; 2H, SiCH2), 0.08 [s; 9H, Si–(CH3)3], 0.06 [s; 6H, Si–(CH3)2],
⎡
1
sin απ
2
⎤
⎥
⎥
1−α
1 + (ωτ)
⎢
ε′(ω) = ε′
+ (ε′
− ε′
)
high freq.
low freq.
high freq.
⎢
1
1−α
2(1−α)
1 + 2(ωτ)
sin απ + (ωτ)
2
⎢
⎣
⎥
⎦
(1)
where
known, absorption coefficient parameter,
zero and one (0o r1). When equals to zero, it corresponds to
Debye type relaxation. The non-Debye type occurs when the value
of absorption coefficient varies between; 0o o1 region [37].
ε′high
Dielectric relaxation parameters ε′low freq. , f and
α
and
τ
are absorption coefficient and relaxation time. As is
α
takes values between
α
α
0.02 [s; 6H, Si–(CH3)2]. 13C-NMR:
δ (ppm)¼164.45, 164.43, 164.31,
α
,
, α, τ
freq.
164.30 (CO), 164.16, 163.69, 155.34, 151.26, 150.58, 141.99, 137.95,
126.86, 126.82, 121.99, 120.97 (Ar–C), 132.32, 131.74, 129.77, 128.24,
124.60, 122.08, 122.03, 120.62, 120.38, 114.42, 114.41 (Ar–CH),
68.45, 66.81 (OCH2), 39.31 (CH2), 37.35, 36.10 (CH), 33.15, 29.96,
29.13, 28.07, 25.80, 24.75, 23.26 (CH2), 22.78, 22.69, 19.76 (CH3),
c
dielectric strength, Δεs are given in Table 1. Since absorption
coefficient values for 25 °C, 75 °C and 150 °C temperatures are very
close to zero, the relaxation type has been considered as nearly-
Debye. On the other hand, since
α has been determined as far from
18.35 (SiCH2), 1.95, 1.41, 0.34 (CH3). 29Si-NMR:
δ
(ppm)¼7.37, 7.09,
zero for 125 °C temperature, the relaxation mechanism has been
determined as non-Debye type. The effect of temperature on ab-
sorption coefficient are also shown in Fig. 4.
ꢀ20.97. C63H78O12Si3 (1111.56); Anal. Calc.: C, 68.07; H, 7.07.
Found: C, 68.36; H, 6.76%.

A. Yildiz et al. / Physica B 485 (2016) 21–28
23
O
OH
HO
O
+
BnO
O
O
1. DCC, DMAP, dry CH2Cl2, r.t.
2. H2, Pd/C, THF, 40 °C, 18 h
O
O
O
HO
O
O
O
O
OH
DCC, DMAP, dry CH2Cl2, r.t.
O
O
4
O
O
O
O
O
O
O
O
O
O
4
O
O
O
H
Karstedt's catalyst, toluene, r.t., 48 h
Si
Si
Si
O
O
O
O
O
O
O
O
O
O
DBB
Si
Si
Si
O
6
Scheme 1. Synthesis of the siloxane substituted compound DBB [34].
The dielectric strength, Δεs, has also been calculated by taking
the difference between the ε low freq. and the high freq. values. It
has been clearly observed that dielectric strength shows decreas-
ing behavior as the temperature increases from 25 °C to 150 °C
(see Fig. 4). The decrease in the value of Δεs can be interpreted as
the fact that the molecular alignment is getting easier as the
temperature increases from this temperature region.
Relaxation times and frequencies for each temperature have
also been calculated. It has been clearly observed that as the
temperature increases, the dielectric relaxation frequency shifts to
higher frequency (see Table 1). The significant increase in relaxa-
tion frequency with temperature can be originated from the
thermal activation of localized charge carriers which are re-
sponsible from dielectric polarization.
Moreover, as is clearly observed that at 125 °C, while absorp-
tion coefficient has its maximum value, relaxation time takes its
minimum value. From this point of view, one can deduce that
violation of crystal structure of the system.
′
ε
′
The variations of the imaginary part of the dielectric constant
with angular frequency at different temperatures are shown in Fig. 5.
As shown in Fig. 5, the imaginary component of dielectric
constant has a peak that correspond to dielectric relaxation. The
temperature dependence of relaxation frequencies, fc of DBB has
also been given in the inset of Fig. 5. It has been clearly observed
that while the relaxation frequencies (peak frequency) shift to
higher frequencies as the temperature increases from 25 °C to
125 °C, the peak intensities remarkably decrease with increasing
temperature. Since the magnitude of ε′′ is proportional to energy
dissipation, it has been understood that the LC material is more
stable at 125 °C. Moreover the decrease in relaxation frequency
above 125 °C may be associated with the phase change of the
material from Dark Conglaramate LC phase to isotropic.
Dielectric relaxation mechanism of DBB has also been analyzed
by Cole–Cole curves given in Fig. 6a. As shown in Fig. 6a, DBB exhibit
semicircle and straight line; but as the temperature increases the
shape of the semi-circles deforms. The variation of temperature on
these anomalies observed for
proaches to isotropic phase transition may be attributed to the
α
and
τ
when the temperature ap-

24
A. Yildiz et al. / Physica B 485 (2016) 21–28
O
O
O
O
O
O
O
O
O
O
O
DBB
Si
Si
Si
O
6
Compound
T/°C [ΔH kJ/mol]
Heating: DCFE[*] 140 [14.6] Iso
Cooling: Iso 141 [15.0] DCFE[*] 72 DC(g)
DBB [34]
[*]
Fig. 1. Phase transition temperatures T (°C) and enthalpies ΔH (kJ molꢀ1) of DBB (Perkin-Elmer DSC-7; heating/cooling rates 10 K minꢀ1 and abbreviations: DCFE ¼dark
[*]
[*]
conglomerate phase showing ferroelectric switching, DC(g) ¼non-switching glassy state of dark conglomerate mesophase; Iso¼ isotropic phase).
[*]
Fig. 2. Polarized light optical photomicrographs of DCFE mesophase at 109 °C of DBB between uncrossed polarizers (74°).
semi-circle above 25 °C, equivalent circuits at room temperature and
above 25 °C are different as given in Fig. 6b.
The room temperature Cole–Cole curve also indicates the sys-
tem may behave as a super-capacitor. This suggestion has also
been confirmed by the impedance measurement results. As is
known, vertical lines appears in the low frequency region of Cole–
Cole curve is accepted as an indicator of super-capacitive behavior.
Theoretically a typical Cole–Cole plot of super-capacitor includes
three characteristic regions: a semi-circle at high frequency region,
a Warburg line with the slope of ꢀ45° at intermediate frequency
region and a vertical line with the slope of ꢀ90° at low frequency
region [38,39]. In other words, the phase angle must be close to
ꢀ90° for ideal super-capacitors [40–44].
According to the phase angle versus frequency curve given in
Fig. 6c, the phase angle equals to ꢀ90° for 3–32 kHz frequency
interval at room temperature. As the temperature is increased, the
phase angle of ꢀ90° has never been observed (see inset of Fig. 6c).
Hence, it has been revealed that DBB material media has a pro-
mising potential for utilizing as a super-capacitor at 25 °C for the
frequency interval of 3–32.6 kHz.
Fig. 3. The real component of dielectric constant of DBB at 25 °C, 75 °C, 125 °C and
150 °C temperatures.
3.2. Analysis of temperature effect on frequency dependent
conductivity
Cole–Cole characteristics also manifests itself as a noticeable decrease
in the diameter of the semi-circles. The effect of temperature on
equivalent circuit has also been investigated (see Fig. 6b). As is seen
from the Fig. 6a, since the straight lines tend to complete the second
To analyze the effect on temperature on electrical properties
of DBB, alternative current (ac) conductivity, sac, has been

A. Yildiz et al. / Physica B 485 (2016) 21–28
25
Table 1
Dielectric parameters calculated by fitting dispersion curves for 25 °C, 75 °C, 125 °C and 150 °C temperatures.
Temperature (°C)
Adj. R2
ε′
ε′high
α
Δεs
τ (s)
fc (MHz)
low freq.
freq.
25
75
125
150
0.99986
0.99929
0.99872
0.99946
43.740
32.447
18.642
17.645
0.369
0.357
1.133
1.550
0.0138
0.0168
0.4832
0.0260
43.371
32.090
17.509
16.095
9.956 ꢁ 10ꢀ8
7.068 ꢁ 10ꢀ8
3.157 ꢁ 10ꢀ8
3.632 ꢁ 10ꢀ8
1.59
2.25
5.04
4.38
In CBH model, developed by Elliot in 1987 [48,49], thermally
activated charge carriers hop between two localized sites over the
potential barrier separating them. The temperature dependence of
frequency exponent s is given by
s = 1 − 6k T/W
(
)
B
m
(3)
where kB is the Boltzmann constant (ev/K), T is the temperature
(K) and Wm is the optical band gap (eV) of the material.
In QMT model, developed by Pollak and Geballe in 1961, charge
carrier motion is provided by phonon assisted quantum mechan-
ical tunneling between localized states near the Fermi level
[50,51]. According to QMT model, there should be a linear re-
lationship between temperature and ac conductivity. Moreover,
the frequency exponent s must take values at around 0.8 and in-
dependent of temperature. The frequency dependency of s is also
predicted as
Fig. 4. Temperature dependence of absorption coefficient, α and dielectric strength,
Δεs.
s = 1 + [4/ ln (ωτ0)]
(4)
where τ0 is the relaxation time. With the progress of time, it
has been determined that if temperature and frequency range are
wider, “s” may also have values such as s¼1 and s41 known as
Nearly Constant Loss (NCL) and Super Linear Power Law (SLPL),
respectively [52–56]. Though SLPL is proposed to exist at very high
frequency but there have been some works for kHz and MHz
frequency range52 [57].
The frequency exponent values for each temperature have been
calculated at the low, intermediate and high frequency regions by
the slope of lnsac¼f (ln
ω) curves shown in Fig. 7. The results are
given in Table 2.
Since frequency exponent, s, is close to zero at the low fre-
quency region, the conductivity corresponds to nearly dc. In the
intermediate frequency region, the system displays SLPL behavior
regardless of the operating temperature. On the other hand, as the
frequency is increased to higher frequencies, the effect of tem-
perature has been clearly observed on the conductivity mechan-
ism. As is seen from Table 2, the conductivity mechanism changes
with temperatures from CBH (for 25 °C and 75 °C) to QMT (for
125 °C and 150 °C) model in the high frequency regions. According
to high frequency s parameter values given in Table 2, the optical
band gaps for 25 °C and 75 °C temperatures have been calculated
by Eq. (3) and determined as 0.423 eV and 0.343 eV, respectively.
Since these energies correspond to mid-wavelength infrared
waves, one can deduce that DBB LC material absorbs mid-wave-
length infrared electromagnetic waves at the temperature interval
of 25–75 °C. On the other hand, the decrease in optical band gap
with increasing frequency can be attributed to the increase in
conductivity.
Fig. 5. The imaginary component of dielectric constant of DBB at 25 °C, 75 °C,
125 °C and 150 °C temperatures. The inset the temperature dependence of re-
laxation frequency.
calculated by Eq. (2)
σac = ωε0ε″
(2)
where
ω
,
ε0 and
ε
″ represent angular frequency, dielectric
constant of free space, and the imaginary part of dielectric con-
stant, respectively [45]. According to Jonscher's “Universal di-
electric response”, conductivity power law obeys s¼s0þΑωs re-
3.3. The relation between conductivity and temperature: Arrhenius
plot analysis
lation, s0 is the dc component and sac¼
Αωs is the ac component
The temperature dependence of conductivity is given by Eq. (5)
[46]. According to Jonscher, the value of frequency exponent “s”
was restricted to be between zero and one. While sE0 state
corresponds to dc conductivity [47], 0oso0.7 and 0.7rso1
conditions represent Correlated Barrier Hoping (CBH) Mechanism
and Quantum Mechanical Tunneling (QMT) model, respectively.
σ = σ0 exp E /k T
(
)
a
B
(5)
where s0 is a pre-exponential factor, Ea is the activation energy, kB
is the Boltzmann constant (ev/K), and T is the temperature (K). In

26
A. Yildiz et al. / Physica B 485 (2016) 21–28
Fig. 6. (a) Cole–Cole plots (b) equivalent circuits, and (c) phase angle versus frequency graphics of DBB LC at different temperatures.
Table 2
Frequency exponent values, s and related conductivity mechanisms for the low,
intermediate and high frequency regions at 25 °C, 75 °C, 125 °C and 150 °C
temperatures.
Temperature (°C) slow freq.
sintermediate
shigh
freq.
freq.
25
75
125
150
0.256 (CBH)
1.931 (SLPL)
0.078 (Nearly DC) 1.866 (SLPL)
0.122 (Nearly DC) 1.810 (SLPL)
0.090 (Nearly DC) 1.850 (SLPL)
0.369 (CBH model)
0.476 (CBH model)
0.857 (QMT model)
0.823 (QMT model)
Fig. 8, the variation of lnsac with (1000/T) are shown some selected
angular frequencies.
As shown in Fig. 8, conductivity has a tendency of increase with
increasing frequency. In addition, as the temperature decreases,
lnsac also linearly decreases. From this point of view, it can be
deduced that there is a thermally activated process for the charge
carriers' motion. According to fitting results, activation energies for
Fig. 7. The lnsac versus lnω graphics of DBB at various temperatures.
ω
¼1244 rad/s and ω¼11,083 rad/s have been found as 0.103 and
0.095 eV, respectively. The decrease in activation energy with in-
creasing frequency can be useful for the low energy and highly

A. Yildiz et al. / Physica B 485 (2016) 21–28
27
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4. Conclusions
In this work, the dielectric properties and ac conductivity me-
chanism of DBB LC with dark conglomerate phase have been in-
vestigated within the temperature interval of 25–150 °C. Accord-
ing to POM textures, the LC system exhibits isotropic phase above
140 °C. The effect of phase transition manifests itself as a sig-
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the sharp decrease in the value of dielectric strength when the
temperature increases may be accepted as a crucial indicator for
the fact that the increase in temperature makes easier the mole-
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of the LC compound in high temperatures, it is more stable ma-
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energy demand devices with this new liquid crystal material.
On the other hand, it has been observed that both the diameter
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semi-circles undergoes a deformation by increasing temperature.
The ac conductivity mechanism of the DBB has also been stu-
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temperature, the mechanism of charge carriers' motion between
localized sites changes from hopping mechanism to quantum
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Acknowledgments
This research has been supported by Yıldız Technical University
Scientific Research Projects Coordination Department with the
Project number 2014-01-GEP05.

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