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Electrical properties of some S-n-alkylisothiouronium compounds
Materials Letters 16 (1993) North-Holland
33-38
Electrical properties of some S-n-alkylisothiouronium
compounds
M.Sh. Ramadan a, M.M. El-Banna b a Chemistry Department, Faculty of Science, Alexandria University, Alexandria, Egypt b Chemistry Department, Faculty of Education, Alexandria University, Alexandria, Egypt
A.E. Hamed
and M.E. Kassem
Physics Department, Faculty of Science, Alexandria University, Alexandria, Egypt Received
16 September
1992; in final form 26 November
1992
Dielectric constant e’ and electrical conductivity tz are measured for pressed samples of S-n-butylisothiouronium Cl, Br, I and Pi (picrate) compounds at room temperature using an impedance meter in the frequency range 0.005-500 kHz. The conduction of the samples is found to decrease in the order Cl- > Br- > I- z Pi-. The anionic size parameter is found to be the predominant factor to discuss the data. The study of electrical properties is extended to show the temperature depenence of the S-n-heptylisothiouronium picrate sample. The interpretation of the data is only carried out at 13.4 kHz.
1. Introduction
In some solids, dipolar molecules have been found to possess sufficient orientational freedom to give dielectric constants comparable to those in the liquid state. Analysis of dielectric properties as a function of frequency has generally been called impedance spectroscopy, dielectric relaxation spectroscopy or more appropriately ac spectroscopy. This reflects the collective response of microscopic polarization processes under an external field. It is a useful and important tool to study defects, microstructure, surface property and electrical conductivity [ 1 ] for materials including dielectric, ionic conductors, aqueous solutions and absorbate-absorbent interfaces [ 2-41. In an ideal dielectric material there would be no free ion conduction, but in actual materials, drift of electrons or free ions may be produced in the applied field. It is desirable to examine the relation between the dielectric constant and the apparent conductivity. In this work, the frequency dependence of the ac impedance in wide range of frequencies (0.005-500 kHz) at room temperature is used to investigate the electrical properties such as dielectric constant and 0167-577x/93/$
06.00 0 1993 Elsevier Science Publishers
conductivity for pressed samples of S-n-butylisothiouronium CH,- ( CH2) 3-S-C (NH,): chloride, bromide, iodide and picrate (trinitrophenolate) compounds. Moreover, the electrical properties and their relation to the impedance frequency for pressed sample of S-n-heptylisothiouronium picrate CHJ(CH,),-S-C(NH,): Pi- are studied at 20, 23, 30 and 40°C. The interpretation is given only at constant frequency 13.4 kHz.
2. Experimental 2. I. Sample preparation S-n-alkylisothiouronium compounds R-SC(NH2):X-, where R=butyl CH3-(CH,)J or heptyle CH,-(CH,), and X=Cl-, Br-, I-, and Pi- were synthesised by mixing equimolecular amounts of thiourea and respective alkyl halide (RX) in suitable amount of acetone in case of Cl-, Br-, and Icompounds [ 5 1. The addition of RX was carried out through the condenser in 15 min, then the mixtures were allowed to reflux for about 5-6 h. In case of Picompounds [ 6 1, equimolecular amounts of thiourea and RX were mixed in a suitable amount of ethyl
B.V. All rights reserved.
33
Volume 16, number 1
MATERIALS LETTERS
alcohol and refluxed for 20 min. by the addition of an equimolecular acid to the mixture, and refluxed In this method the preparation to the following simple addition tion [7]: I&N
This was followed amount of picric again for 20 min. will be according bimolecular reac-
\ , C-S+RX
February 1993
3. Results and discussion Many dielectric functions have been used to describe the frequency-dependent properties of material. Among them the most important ones are the complex impedance (Z*), complex dielectric constant (e*) and complex condutivity (a*). These functions may be expressed in terms of E* as [ 111 E*(v)=c’(v)+ic”(v),
I&N
Z*( v)=Z’(
v)+iZ”(
v)=
_
1
JWCO
6*
and a*( v) = eo/z*c, II
111
where X=Cl, Br, I and Pi. The addition product has been proved to possess ionic structure. This was done by either the quantitative determination of the halide ion or by conductometric method [ 7-91. Crystallization of the samples was carried out from absolute ethyl alcohol; dried under vacuum using an Abderhalden apparatus [ lo]. A quartz mortar was used to obtain powdered samples. 2.2. Apparatus and impedance
measurements
The purified material was pressed under a pressure of 9.8 x 10’ Pa to form discs of a diameter of 10 mm and thickness of 1 mm. Contacts were made using silver electrodes. The sample was placed in a holder designed to minimize stray capacitance. The dieletric properties were measured using a Tesla BM 507 impedancemeter. The measurements were performed in the frequency range 0.005-500 kHz. The error of the impedance measurement (Z) was about -+ 5% of the value read off on the meter. The temperature was measured for S-n-alkylisothiouronium picrate samples using a copper-constantan thermocouple mounted in close proximity to the specimen of interest. The accuracy of the temperature measurement was within & 0.1 “C.
34
)
(1)
where i = and C, is for a disc thickness
fi, w is the angular frequency ( = 2n: v) the geometrical capacitance which is given by the vacuum permittivity ear area A and t by the relation
C,=ci,A/t.
(2)
The measured impedances were plotted in the complex planes for (S-n-buis)+Cl-, Br-, I- and Pisamples. Typical spectra related to the frequency of the applied sinusoidal voltage are given in fig. 1. The diagrams are known as semicircular impedance or the so-called Cole-Cole diagram [ 121. The classical semicircular form can be represented by a simple parallel R-C circuit [ 13 1. The capacitance C=lim[-2nvZ”(v)]-’ U-m
(3)
is that for the measured sample whereas the resistance R=limZ(v) V-+0
is the grain surface resistance. The grain surface resistance R could be figured out from the intersection of the low-frequency semicircles with the Z’ axis. The calculated value could be employed to shed light on the variation of the dc conductivity a& of samples. The calculated values of the grain surface resistance increase by increasing the anionic size of the compound (fig. I ). The values for the for Cl-, Br-,
February 1993
MATERIALS LETTERS
Volume 16, number 1
3.1. Dielectric constants (t} and loss tangent (tan 6” In ionic solids, the dielectric constant still arises only from the shift of charge which is more important than the electronic shift [ 14, p. 1351. For an ideal condenser ( 14, p. 54) of geometrical capacitance C,, the charging current Eoe’C, is 90” out of phase with the alternating potential (E is energy field). In a condenser in which absorptive polarization occurs, the current also has a component Eo&‘C’, in phase with the potential. The loss tangent is given by
Pi
/
d I
tan 6--Y/e’
0
Rdc
4
8
lZRdc
16
if x l&xl
Fig. I. Variation of Z” with Z’ for (S-n-buis)+Xtemperature.
at room
,
(4)
where E’ is the measured dielectric constant of the dielectric material in the condenser and en is the loss factor. As the frequency approaches zero, E’ approaches the static dielectric constant co; as the frequency approaches infinity, E’ approaches E,, the optical dielectric constant where the permanent dipoles are unable to contribute. Fig. 2 shows the effect of varying t’ with lnffor CH,-(CH,),-S-C(NH,):Iwhere t, is equal to 2.15. 3.2. Electrical conductivity (a)
I- and Pi- are 1.7~ 106, 1.8X 106, 12.8X lo6 and 72.0 x 1O6 R respectively. The semicircular diagram of the picrate sample is found to be greater than those of halide samples. This indicates that the free ion electrons inside the phenol ring of picrate ion suffer from high resistivity during the impedance process causing the transference of a small charge. This highest resistivity attributed to the highest delocalisation of electron attracting nitro groups will produce a good delocalisation effect as shown in the following scheme:
It can be deduced from the addition products II and III (see section 2) for CH3-(CH1)3-SC(NH,)$ X- that the number of electrons attracted by X- was reduced by increasing its anionic size at a given temperature. This fact is attributed to the weakness of ionic bond character from Cl- to I-. In
i
2.25 -
N+” 10 Em
I
2.101 7
9
8
10
Inf
Fig. 2. Variation of 6’with lnfvalues for (S-n-buis) +I- at room temperature.
35
Volume 16, number
1
MATERIALS
E”W
E"f
(5)
where 0.9 x 10 I2 is the ratio of the farad to the electrostatic unit of capacitance. From eq. ( 5 ), electrical conductivity is shown to vary directly with the frequencyf: The maximum a,,, value can be obtained from the elimination of charge due to the dissipation of the energy field as heat. Different rr values were obtained related to f values for CH3-( CH2)3-SC(NH,)$ Cl-, Br-, I- and Pi- pressed samples to equal to 5.88X lo-‘, 5.55X lo-‘, give a,, 0.785x10-’ and 0.14X10-’ Q-’ cm-’ respec-
R-S-C(NH,):Cl->R-S-C(NH,):Br>R-S-C(NH,)$I->R-S-C(NH,):Pi-
9 \
\
\
36
+Pi-
.
Nassar et al. [ 15 ] suggested that the organic compounds phenyl-2,4_dinitrophenyl ethers and 2,4-di-
\
Fig. 3. Plots of 2” versus 2 for (S-n-heptis)
1993
tively. The shift of maximum location for the above compounds is essentially due to the decrease in both the electronegativity and ionic size of halide parts. In case of Pi-, it represents a separation class with a comparitively lower conduction than halides due to the decrease of charge transfer from the substituted donor ring containing the electron attracting nitro groups which reduce the ability of electrons to eliminate from the phenolate ring. The transition of electron inside the phenolate ring towards unoccupied antibonding n* orbital of the acceptor three NO2 groups [ 15 ] shows high inhibition of the charge to transfer from the picrate moiety. In general, the electric conduction was found to decrease in the order
addition, the charge density on the ion surface becomes small in the order Cl- > Br- > I-. If a potential difference is established between the parallel condenser plates, a charge per unit area will develop on each plate and polarization will be created in the dielectric. The conductivity (T(I;z- ’ cm- ’ ) due to the dissipation of the field energy can be given by the following equation [ 14, p. 701:
o= 47ro.9x 10’2 = 1.8x 10’2 ’
February
LETTERS
at different
\
\
\
\
\
\\ \ I
temperatures.
(C is the centre of the circuit. )
Volume 16, number 1
February 1993
MATERIALS LETTERS
nitrophenol have faint semiconducting behaviour originated from an intramolecular charge transfer from the substituted phenoxy ring to the 2,4-dinitrophenyl moiety. Higazy et al. [ 161 show that zeolite reveals semiconducting features based predominantly on an ionic mechanism. They also show that the zeolite sites are the active centers responsible for the ionic conduction. The situation of semiconductance phenomena is opposite to that in the solutions of these ionic compounds separately in a given solvent, e.g. in ethanol [ 6 1, where the solvation factor can play the predominant role in controlling the mobility of the ions due to ion-dipole interaction. The limiting ionic conductances for these ions have reverse trend to that of the solid state.
-11
-12
-13
-14 6 5 15
-16
-17
3.3. The effect of temperature The variation of the real impedance part Z’ with the imaginary part Z” for the CH,-(CH*),-SC(NH,):Pisample at different temperatures is shown in fig. 3A at 20 and 23°C and fig. 3B at 30 and 40°C. The calculated values of Rdc from fig. 3 were found to be 85x106, 50x106, 1.8~10~ and 1.5~ lo6 LI at 20, 23, 30 and 40°C respectively. It is interesting to observe that Rdc dropped by 35 x 1O6 fi when raising temperature from 20 to 23, this indicates that most charge carriers are sensitive to this small change of temperature. The variation of the dielectric constant E’with increasing temperature at constant frequency ( 13.4 kHz) is restudied for CH3- (CH, )6-SC(NH,):Pi-. It is found that at lower temperatures, the molecules are rigid and have less oriented forces which give small dielectric constant. By raising the temperature, the number of molecules capable to rotate about their long axes increases [ 17 ] giving higher t’ values. The electric conduction a, for S-n-heptylisothiouronium picrate at a given frequency is estimated at different temperatures. When plotting a, against reciprocal of temperature, a straight line is obtained using the linear regression method according to the equation In+=--25942.6(1/T)+71.19,
16 3\0
3.1
3.2
3.3
3.6
3.5
l/T x103
Fig. 4. Plot of In cversus l/Tfor
(S-n-heptis)+Pi-.
where the correlation factor (r) is equal to -0.955. Higazy et al. [ 16 ] analyzed the ac conductivity of the decomposed zeolite form H ZSM-5 in the 300700 K temperature range. They showed that as the temperature drops to 373 K, sorbed water seems to participate in the conduction as a vehicle assisting the proton mobility. The dependence of electric conduction on the field energy (E) is found to obey the equation [ 181 u,=cr,
exp( -E/kT)
,
(7)
where a0 is the pre-exponential factor and k is the Boltzmann constant. The calculated E value of the heptyl compound at 13.4 kHz is found to be 2.24 eV as estimated from the slope of the curve in fig. 4. This high value could be due to the increase of tunneling conduction by temperature.
(6) 37
Volume 16, number 1
MATERIALS LETTERS
References
February 1993
[ 111 H. Frohlich, Theory of dielectrics (Oxford Univ. Press, Oxford, 1949) p. 10.
[ 1 ] L.L. Hench, in: Proceedings of the 14th Symposium Phys. Sot. Aerosp. Proc. (UK) l-2-2 (1968). [2] I.D. Raistrick, Ann. Rev. Mater. Sci. 16 (1986) 343. [ 3 ] J.R. MacDonald, J. Electroanal. Chem. 25 ( 1987) 223. [4] G. Jones, Dielectric and related molecular processes, Vol. 3 (The Chemical Society, London, 1977 ) p. 176. [5] B. Huynes, Qualitative organic analysis, 2nd Ed. (Macmillan, London, 1967) p. 148. [ 61 A.M. Hafez, H. Sadek and M.Sh. Ramadan, Electrochem. Acta 24 (1979) 957. [ 71 H. Goldschmidt, Z. Electrochem. 19 ( 19 13) 226. [8] E.L. Brown and N. Camphell, J. Chem. Sot. ( 1937) 1699. [9] L. Schotte and S. Viebel, Acta Chem. Stand. 7 (1953) 1357. [ lo] I. Vogel, Practical organic chemistry (Longmans, London, 1966) p. 139.
38
[ 12 ] K.S. Cole and R.H. Cole, J. Chem. Phys. 9 ( 1941) 34 1. [ 131 J.L. Carpentier, A. Lebrun and F. Perdu, J. Phys. Chem. Solid 50 (1989) 145.
[ 141 C.P. Smyth, Dielectric behaviour and structure (McGrawHill, New York, 1955).
[ 151 A.M.G. Nassar, N.A. Ibrahim, M.E. Kassem and M.M. ElBanna, Egypt J. Solids 7 (1985).
[ 161 A.A. Higazy, M.E. Kassem and M.B. Sayed, J. Phys. Chem. Solids, to be published.
[ 171 J.O. Williams, Advances in physical organic chemistry, Vol. I6 (Academic Press, New York, 1978).
[ 181 H. Kollmann and M. Silver, eds., Symposium on electrical conductivity in organic solids (Interscience Publishers, New York, 1961) p. 137.
33-38
Electrical properties of some S-n-alkylisothiouronium
compounds
M.Sh. Ramadan a, M.M. El-Banna b a Chemistry Department, Faculty of Science, Alexandria University, Alexandria, Egypt b Chemistry Department, Faculty of Education, Alexandria University, Alexandria, Egypt
A.E. Hamed
and M.E. Kassem
Physics Department, Faculty of Science, Alexandria University, Alexandria, Egypt Received
16 September
1992; in final form 26 November
1992
Dielectric constant e’ and electrical conductivity tz are measured for pressed samples of S-n-butylisothiouronium Cl, Br, I and Pi (picrate) compounds at room temperature using an impedance meter in the frequency range 0.005-500 kHz. The conduction of the samples is found to decrease in the order Cl- > Br- > I- z Pi-. The anionic size parameter is found to be the predominant factor to discuss the data. The study of electrical properties is extended to show the temperature depenence of the S-n-heptylisothiouronium picrate sample. The interpretation of the data is only carried out at 13.4 kHz.
1. Introduction
In some solids, dipolar molecules have been found to possess sufficient orientational freedom to give dielectric constants comparable to those in the liquid state. Analysis of dielectric properties as a function of frequency has generally been called impedance spectroscopy, dielectric relaxation spectroscopy or more appropriately ac spectroscopy. This reflects the collective response of microscopic polarization processes under an external field. It is a useful and important tool to study defects, microstructure, surface property and electrical conductivity [ 1 ] for materials including dielectric, ionic conductors, aqueous solutions and absorbate-absorbent interfaces [ 2-41. In an ideal dielectric material there would be no free ion conduction, but in actual materials, drift of electrons or free ions may be produced in the applied field. It is desirable to examine the relation between the dielectric constant and the apparent conductivity. In this work, the frequency dependence of the ac impedance in wide range of frequencies (0.005-500 kHz) at room temperature is used to investigate the electrical properties such as dielectric constant and 0167-577x/93/$
06.00 0 1993 Elsevier Science Publishers
conductivity for pressed samples of S-n-butylisothiouronium CH,- ( CH2) 3-S-C (NH,): chloride, bromide, iodide and picrate (trinitrophenolate) compounds. Moreover, the electrical properties and their relation to the impedance frequency for pressed sample of S-n-heptylisothiouronium picrate CHJ(CH,),-S-C(NH,): Pi- are studied at 20, 23, 30 and 40°C. The interpretation is given only at constant frequency 13.4 kHz.
2. Experimental 2. I. Sample preparation S-n-alkylisothiouronium compounds R-SC(NH2):X-, where R=butyl CH3-(CH,)J or heptyle CH,-(CH,), and X=Cl-, Br-, I-, and Pi- were synthesised by mixing equimolecular amounts of thiourea and respective alkyl halide (RX) in suitable amount of acetone in case of Cl-, Br-, and Icompounds [ 5 1. The addition of RX was carried out through the condenser in 15 min, then the mixtures were allowed to reflux for about 5-6 h. In case of Picompounds [ 6 1, equimolecular amounts of thiourea and RX were mixed in a suitable amount of ethyl
B.V. All rights reserved.
33
Volume 16, number 1
MATERIALS LETTERS
alcohol and refluxed for 20 min. by the addition of an equimolecular acid to the mixture, and refluxed In this method the preparation to the following simple addition tion [7]: I&N
This was followed amount of picric again for 20 min. will be according bimolecular reac-
\ , C-S+RX
February 1993
3. Results and discussion Many dielectric functions have been used to describe the frequency-dependent properties of material. Among them the most important ones are the complex impedance (Z*), complex dielectric constant (e*) and complex condutivity (a*). These functions may be expressed in terms of E* as [ 111 E*(v)=c’(v)+ic”(v),
I&N
Z*( v)=Z’(
v)+iZ”(
v)=
_
1
JWCO
6*
and a*( v) = eo/z*c, II
111
where X=Cl, Br, I and Pi. The addition product has been proved to possess ionic structure. This was done by either the quantitative determination of the halide ion or by conductometric method [ 7-91. Crystallization of the samples was carried out from absolute ethyl alcohol; dried under vacuum using an Abderhalden apparatus [ lo]. A quartz mortar was used to obtain powdered samples. 2.2. Apparatus and impedance
measurements
The purified material was pressed under a pressure of 9.8 x 10’ Pa to form discs of a diameter of 10 mm and thickness of 1 mm. Contacts were made using silver electrodes. The sample was placed in a holder designed to minimize stray capacitance. The dieletric properties were measured using a Tesla BM 507 impedancemeter. The measurements were performed in the frequency range 0.005-500 kHz. The error of the impedance measurement (Z) was about -+ 5% of the value read off on the meter. The temperature was measured for S-n-alkylisothiouronium picrate samples using a copper-constantan thermocouple mounted in close proximity to the specimen of interest. The accuracy of the temperature measurement was within & 0.1 “C.
34
)
(1)
where i = and C, is for a disc thickness
fi, w is the angular frequency ( = 2n: v) the geometrical capacitance which is given by the vacuum permittivity ear area A and t by the relation
C,=ci,A/t.
(2)
The measured impedances were plotted in the complex planes for (S-n-buis)+Cl-, Br-, I- and Pisamples. Typical spectra related to the frequency of the applied sinusoidal voltage are given in fig. 1. The diagrams are known as semicircular impedance or the so-called Cole-Cole diagram [ 121. The classical semicircular form can be represented by a simple parallel R-C circuit [ 13 1. The capacitance C=lim[-2nvZ”(v)]-’ U-m
(3)
is that for the measured sample whereas the resistance R=limZ(v) V-+0
is the grain surface resistance. The grain surface resistance R could be figured out from the intersection of the low-frequency semicircles with the Z’ axis. The calculated value could be employed to shed light on the variation of the dc conductivity a& of samples. The calculated values of the grain surface resistance increase by increasing the anionic size of the compound (fig. I ). The values for the for Cl-, Br-,
February 1993
MATERIALS LETTERS
Volume 16, number 1
3.1. Dielectric constants (t} and loss tangent (tan 6” In ionic solids, the dielectric constant still arises only from the shift of charge which is more important than the electronic shift [ 14, p. 1351. For an ideal condenser ( 14, p. 54) of geometrical capacitance C,, the charging current Eoe’C, is 90” out of phase with the alternating potential (E is energy field). In a condenser in which absorptive polarization occurs, the current also has a component Eo&‘C’, in phase with the potential. The loss tangent is given by
Pi
/
d I
tan 6--Y/e’
0
Rdc
4
8
lZRdc
16
if x l&xl
Fig. I. Variation of Z” with Z’ for (S-n-buis)+Xtemperature.
at room
,
(4)
where E’ is the measured dielectric constant of the dielectric material in the condenser and en is the loss factor. As the frequency approaches zero, E’ approaches the static dielectric constant co; as the frequency approaches infinity, E’ approaches E,, the optical dielectric constant where the permanent dipoles are unable to contribute. Fig. 2 shows the effect of varying t’ with lnffor CH,-(CH,),-S-C(NH,):Iwhere t, is equal to 2.15. 3.2. Electrical conductivity (a)
I- and Pi- are 1.7~ 106, 1.8X 106, 12.8X lo6 and 72.0 x 1O6 R respectively. The semicircular diagram of the picrate sample is found to be greater than those of halide samples. This indicates that the free ion electrons inside the phenol ring of picrate ion suffer from high resistivity during the impedance process causing the transference of a small charge. This highest resistivity attributed to the highest delocalisation of electron attracting nitro groups will produce a good delocalisation effect as shown in the following scheme:
It can be deduced from the addition products II and III (see section 2) for CH3-(CH1)3-SC(NH,)$ X- that the number of electrons attracted by X- was reduced by increasing its anionic size at a given temperature. This fact is attributed to the weakness of ionic bond character from Cl- to I-. In
i
2.25 -
N+” 10 Em
I
2.101 7
9
8
10
Inf
Fig. 2. Variation of 6’with lnfvalues for (S-n-buis) +I- at room temperature.
35
Volume 16, number
1
MATERIALS
E”W
E"f
(5)
where 0.9 x 10 I2 is the ratio of the farad to the electrostatic unit of capacitance. From eq. ( 5 ), electrical conductivity is shown to vary directly with the frequencyf: The maximum a,,, value can be obtained from the elimination of charge due to the dissipation of the energy field as heat. Different rr values were obtained related to f values for CH3-( CH2)3-SC(NH,)$ Cl-, Br-, I- and Pi- pressed samples to equal to 5.88X lo-‘, 5.55X lo-‘, give a,, 0.785x10-’ and 0.14X10-’ Q-’ cm-’ respec-
R-S-C(NH,):Cl->R-S-C(NH,):Br>R-S-C(NH,)$I->R-S-C(NH,):Pi-
9 \
\
\
36
+Pi-
.
Nassar et al. [ 15 ] suggested that the organic compounds phenyl-2,4_dinitrophenyl ethers and 2,4-di-
\
Fig. 3. Plots of 2” versus 2 for (S-n-heptis)
1993
tively. The shift of maximum location for the above compounds is essentially due to the decrease in both the electronegativity and ionic size of halide parts. In case of Pi-, it represents a separation class with a comparitively lower conduction than halides due to the decrease of charge transfer from the substituted donor ring containing the electron attracting nitro groups which reduce the ability of electrons to eliminate from the phenolate ring. The transition of electron inside the phenolate ring towards unoccupied antibonding n* orbital of the acceptor three NO2 groups [ 15 ] shows high inhibition of the charge to transfer from the picrate moiety. In general, the electric conduction was found to decrease in the order
addition, the charge density on the ion surface becomes small in the order Cl- > Br- > I-. If a potential difference is established between the parallel condenser plates, a charge per unit area will develop on each plate and polarization will be created in the dielectric. The conductivity (T(I;z- ’ cm- ’ ) due to the dissipation of the field energy can be given by the following equation [ 14, p. 701:
o= 47ro.9x 10’2 = 1.8x 10’2 ’
February
LETTERS
at different
\
\
\
\
\
\\ \ I
temperatures.
(C is the centre of the circuit. )
Volume 16, number 1
February 1993
MATERIALS LETTERS
nitrophenol have faint semiconducting behaviour originated from an intramolecular charge transfer from the substituted phenoxy ring to the 2,4-dinitrophenyl moiety. Higazy et al. [ 161 show that zeolite reveals semiconducting features based predominantly on an ionic mechanism. They also show that the zeolite sites are the active centers responsible for the ionic conduction. The situation of semiconductance phenomena is opposite to that in the solutions of these ionic compounds separately in a given solvent, e.g. in ethanol [ 6 1, where the solvation factor can play the predominant role in controlling the mobility of the ions due to ion-dipole interaction. The limiting ionic conductances for these ions have reverse trend to that of the solid state.
-11
-12
-13
-14 6 5 15
-16
-17
3.3. The effect of temperature The variation of the real impedance part Z’ with the imaginary part Z” for the CH,-(CH*),-SC(NH,):Pisample at different temperatures is shown in fig. 3A at 20 and 23°C and fig. 3B at 30 and 40°C. The calculated values of Rdc from fig. 3 were found to be 85x106, 50x106, 1.8~10~ and 1.5~ lo6 LI at 20, 23, 30 and 40°C respectively. It is interesting to observe that Rdc dropped by 35 x 1O6 fi when raising temperature from 20 to 23, this indicates that most charge carriers are sensitive to this small change of temperature. The variation of the dielectric constant E’with increasing temperature at constant frequency ( 13.4 kHz) is restudied for CH3- (CH, )6-SC(NH,):Pi-. It is found that at lower temperatures, the molecules are rigid and have less oriented forces which give small dielectric constant. By raising the temperature, the number of molecules capable to rotate about their long axes increases [ 17 ] giving higher t’ values. The electric conduction a, for S-n-heptylisothiouronium picrate at a given frequency is estimated at different temperatures. When plotting a, against reciprocal of temperature, a straight line is obtained using the linear regression method according to the equation In+=--25942.6(1/T)+71.19,
16 3\0
3.1
3.2
3.3
3.6
3.5
l/T x103
Fig. 4. Plot of In cversus l/Tfor
(S-n-heptis)+Pi-.
where the correlation factor (r) is equal to -0.955. Higazy et al. [ 16 ] analyzed the ac conductivity of the decomposed zeolite form H ZSM-5 in the 300700 K temperature range. They showed that as the temperature drops to 373 K, sorbed water seems to participate in the conduction as a vehicle assisting the proton mobility. The dependence of electric conduction on the field energy (E) is found to obey the equation [ 181 u,=cr,
exp( -E/kT)
,
(7)
where a0 is the pre-exponential factor and k is the Boltzmann constant. The calculated E value of the heptyl compound at 13.4 kHz is found to be 2.24 eV as estimated from the slope of the curve in fig. 4. This high value could be due to the increase of tunneling conduction by temperature.
(6) 37
Volume 16, number 1
MATERIALS LETTERS
References
February 1993
[ 111 H. Frohlich, Theory of dielectrics (Oxford Univ. Press, Oxford, 1949) p. 10.
[ 1 ] L.L. Hench, in: Proceedings of the 14th Symposium Phys. Sot. Aerosp. Proc. (UK) l-2-2 (1968). [2] I.D. Raistrick, Ann. Rev. Mater. Sci. 16 (1986) 343. [ 3 ] J.R. MacDonald, J. Electroanal. Chem. 25 ( 1987) 223. [4] G. Jones, Dielectric and related molecular processes, Vol. 3 (The Chemical Society, London, 1977 ) p. 176. [5] B. Huynes, Qualitative organic analysis, 2nd Ed. (Macmillan, London, 1967) p. 148. [ 61 A.M. Hafez, H. Sadek and M.Sh. Ramadan, Electrochem. Acta 24 (1979) 957. [ 71 H. Goldschmidt, Z. Electrochem. 19 ( 19 13) 226. [8] E.L. Brown and N. Camphell, J. Chem. Sot. ( 1937) 1699. [9] L. Schotte and S. Viebel, Acta Chem. Stand. 7 (1953) 1357. [ lo] I. Vogel, Practical organic chemistry (Longmans, London, 1966) p. 139.
38
[ 12 ] K.S. Cole and R.H. Cole, J. Chem. Phys. 9 ( 1941) 34 1. [ 131 J.L. Carpentier, A. Lebrun and F. Perdu, J. Phys. Chem. Solid 50 (1989) 145.
[ 141 C.P. Smyth, Dielectric behaviour and structure (McGrawHill, New York, 1955).
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