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MgO system
Solid State Ionics 170 (2004) 33 – 42 www.elsevier.com/locate/ssi
Electrical properties of pure and Li2O-doped NiO/MgO system A.M. Salem a, M. Mokhtar b, G.A. El-Shobaky b,* a
b
Physics Division, National Research Centre, Dokki, Cairo, Egypt Laboratory of Surface Chemical Catalysis, Department of Physical Chemistry, National Research Centre, Dokki, Cairo, Egypt Received 14 October 2002; received in revised form 13 June 2003; accepted 26 January 2004
Abstract NiO/MgO solid solution was prepared by heating a fixed amount of magnesium basic carbonate with increasing amounts of nickel nitrate followed by calcination at 1073 K. The mol fraction (MF) of NiO was varied between 0.053 and 0.333. The sample containing 0.25 MF NiO was doped with 2.5 and 5 mol% Li2O followed by calcination at 1073 K. The XRD patterns of various investigated solids were measured and different electric properties were investigated. These properties include: r, e , tan d and the activation energy of electrical conductivity. The results showed that pure and variously doped solids consist of MgO phase while all the diffraction lines of NiO disappeared completely indicating the formation of NiO/MgO solid solution. The dissolution of 0.25 MF NiO resulted in a measurable increase in the lattice constant, a of MgO which decreased by increasing the amount of NiO above this limit, falling to values smaller than that measured for pure MgO. The ac and dc values of r were found to increase progressively by increasing the mol fraction of NiO and also by increasing the amount of Li2O added. The values of e and tan d were found to decrease progressively by increasing the NiO-content and also by increasing the frequency of ac current. However, the activation energy of r increased by increasing both the mol fraction and the amount of Li2O, which did not run parallel to the observed increase in the r values due to these treatments. This discrepancy had been attributed to a compensation effect of the pre-exponential factor of the electrical conductivity, ro. The increase in r value due to increasing the MF value of NiO present was attributed to a significant increase in the concentration of the charge carriers without affecting their energetic nature (Ni2 +, Ni3 +). While the induced increase in r value of the investigated system due doping with Li2O resulted mainly from an effective increase in the mobility of the charge carriers present. D 2004 Elsevier B.V. All rights reserved. q
q
Keywords: NiO/MgO system; Electrical properties; Li2O
1. Introduction Transition metal oxides supported on finely divided support material are successfully used in catalyzing different reactions [1 – 8]. These supported solids can also find different applications including closed cycle carbon dioxide lasers and air purification devices besides their applications as semiconducting materials [9]. MgO can dissolve different amounts of transition metal oxides such as CuO, NiO, Co3O4 and MnO forming solid solutions [6]. The amounts of transition metal oxide dissolved in MgO lattice depend mainly on the nature of these oxides, the calcinations conditions and doping with certain foreign cations [9 – 15]. The surface, catalytic and electrical properties of these solid solutions have been studied by several authors using differ-
* Corresponding author. E-mail address: [email protected] (G.A. El-Shobaky). 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2004.01.034
ent techniques [16 – 23]. The effect of Li2O-doping on electrical properties of CoO/MgO solid solution made the object of the recent work done by one of the authors [9].The present work reports a study on the effects of Li2O-doping and amount of NiO in the electrical properties of NiO/MgO solid solution preheated at 800 jC.
2. Experimental 2.1. Materials Pure and NiO/MgO mixed solid samples were prepared by wet impregnation of finely powdered magnesium basic carbonate solid with calculated amount of nickel nitrate dissolved in the least amount of distilled water sufficient to make pastes. The pastes were dried at 383 K then calcined at 1073 K for 6 h. The molecular formula of the calcined solid samples were 0.05, 0.15, 0.20, and 0.25 NiO/MgO. Two
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A.M. Salem et al. / Solid State Ionics 170 (2004) 33–42
Li2O-doped sample were prepared by treating NiO/MgO sample having the formula 0.25 NiO/MgO with calculated amount of lithium nitrate followed by drying then calcination at 1073 K for 6 h. The amounts of Li2O dopant were 2.5 and 5 mol%. The entire chemicals employed were of analytical grade and supplied by Merck. 2.2. Techniques An X-ray investigation of pure and doped mixed solids precalcined at 1073 K was conducted using a Philips diffractometer (type PW1390). The patterns were run with ˚ ) at 30 kV and 10 iron filtered cobalt radiation (k = 1.7889 A mA with scanning speeds of 2j in 2h min 1 and 0.5j 2h min 1 for phase identification and line broadening profile analysis , respectively. The lattice parameter a of MgO was calculated from the diffractions (220), (311) and (222) faces using the expression a=(h2 + k2 + l2)0.5 (k/2 sin h) and the crystallite size d was estimated using Scherrer equation: d ¼ Bk=ðb1=2 coshÞ where B is Scherrer constant (0.89), b1/2 is full width half maximum (FWHM) of the diffraction peak in radian , k the wave length of the X-ray beam and h is the diffraction angle. The ac electrical conductivity, rac, real dielectric constant, e , and dielectric loss, tand were measured in the frequency range 102 – 106 Hz using an RLC meter (type Hioki 3531 Z Hitester). The electric capacity, C, dissipation factor, tan d and the resistance, R were obtained directly from the bridge from which e , and rac were calculated. The q
q
Fig. 2. X-ray diffraction pattern for pure and 2.5, 5.0 Li2O-doped NiO/MgO sample with (MF = 0.25).
electric measurements were carried out on samples in the form of discs having a radius of 0.6 cm and thickness of 0.25 cm obtained by compressing a fixed weight of powdered samples under hydraulic pressure of 5 tons/cm2. Different electrical measurements were carried out on these disks after being coated with a thin layer of silver to avoid any fluctuation in the values of electric and dielectric parameters.
3. Results and discussions 3.1. XRD investigation of pure and doped solids X-ray diffractograms of pure and doped solids precalcined at 1073 K were determined. The diffractograms of the investigated solids are illustrated in Figs. 1 and 2 for pure and doped solids. An additional sample having the formula 0.25 NiO/MgO precalcined at 673 K was also investigated. Table 1 Variation of the lattice parameter a and crystallite size d of MgO as a function of NiO (MF) and amount of Li2O added
Fig. 1. X-ray diffraction patterns of pure NiO/MgO having different MF values.
NiO (MF)
Lattice parameter ˚] a of MgO [A
Crystallite size ˚] d of MgO [A
0.000 0.053 0.176 0.250 0.333 0.250 pure + 2.5 mol% Li2O + 5.0 mol% Li2O
4.2122 F 5 10 4 4.2207 F 5 10 4 4.2280 F 5 10 4 4.2072 F 5 10 4 4.2004 F 5 10 4 4.2072 F 5 10 4 4.2107 F 5 10 4 4.2219 F 5 10 4
135 130 115 105 96 105 128 160
A.M. Salem et al. / Solid State Ionics 170 (2004) 33–42
Fig. 3. The DC electrical conductivity values, rdc measured at 307 K (a) and 410 K (b), respectively, versus the MF values for pure solid solutions.
The diffractogram of this particular sample (not given) consists of all diffraction lines of MgO and NiO phases. This finding clearly indicates the absence of any solid –solid interaction between NiO and MgO preheated a 673 K. On the other hand, the diffractograms of pure and doped solids subjected to heat treatment at 1073 K (cf. Figs. 1 and 2)
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consisted only of all diffraction lines of MgO phase indicating thus a complete dissolution of NiO in MgO lattice forming homogeneous solid solutions [9,17,18]. The formula of these solid solutions are given by; Ni0.05Mg0.95O, NiO0.15Mg0.85O, NiO0.20Mg0.80O, NiO0.25Mg0.75O. The values of mol fraction (MF) of NiO present in these solid solutions are; 0.053, 0.176, 0.25, 0.333, respectively. The formation of such solid solutions requires thus a heat treatment at 1073 K. The effect of the amount of NiO present and amount of Li2O added on the lattice constant of MgO phase was investigated and the results obtained are given in Table 1. Inspection of Table 1 revealed that: (i) The dissolution of increasing amounts of NiO in MgO lattice resulted in a progressive increase in the lattice constant of MgO reaching to a maximum limit at MF = 0.176. (ii) The increase in the amount of NiO dissolved in MgO matrix above this limit led to a measurable decrease in the lattice parameters of MgO falling to a value smaller than that measured for pure MgO lattice. (iii) The dissolution of Li2O in the investigated system led to a limited increase in the lattice parameter of the MgO phase. (iv) The increase in the amount of NiO led to a progressive decrease in the crystallite size of MgO while Li2O doping of NiO /MgO that having MF = 0.25 increased the crystallite size of MgO. This finding shows that Li2O enhances the particle adhesion of MgO crystallites leading to big sized MgO particles. The dissolution of NiO in MgO lattice yielding NiO – MgO solid solution should be normally accompanied by an increase in the lattice parameter of MgO solvent. This conclusion comes from the fact that the ionic radius of Ni2 + is bigger than that of Mg2 +. In fact the ionic radii of divalent magnesium ion and divalent nickel ion are 0.65 and ˚ , respectively [24]. So , the observed measurable 0.78 A increase in the lattice parameter of MgO can be attributed to the dissolution of NiO in MgO lattice via substitution of some of Mg2 + host cations by Ni2 + ions. However, the observed decrease in the lattice parameter of MgO by increasing the amount of NiO above MF = 0.176 could be
Fig. 4. Variation of Log rdc versus 1/T for the investigated NiO/MgO with different MF values.
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Table 2 Activation energies (DE and DE*) of electrical conductivity and preexponential factor, Log ro for NiO/MgO containing different MF values MF
0.053 0.176 0.250 0.333
Log ro [V 1 cm 1] 4.355 3.016 3.650 2.650
DEr (eV)
DEr* (eV)
Reg. I
Reg. II
0.307 0.372 0.359 0.440
0.773 0.995 0.794 0.861
0.307 0.282 0.266 0.250
between NiO and MgO took place leading to an increase in the binding energy of (Ni 2 p 3/2) from 854.5 eV in the case of NiO to 856 eV in the case of NiO – MgO solid solution. This finding suggested an effective transformation of some of Ni2 + to Ni3 + ions. In other words, the increase in the amount of NiO dissolved in NiO matrix above certain limit (MF = 0.25 and 0.333 ) might be accompanied by an increase the oxidation state of nickel oxide present. 3.2. dc Conductivity measurements
attributed to an effective transformation of some of Ni2 + ions into Ni3 + ions (the ionic radius of Ni3 + ions being 0.64 ˚ ) [24]. Ruckenstein and Hang Hu [32] have reported that A nickel magnesium mixed oxides containing 37 mol% NiO prepared by the same method followed in the present work and precaclcined at 1073 K consisted of NiO – MgO solid solution. These authors claimed also that an electron transfer
The dc electrical conductivity was determined for pure and Li2O-doped NiO/MgO solid solutions precalcined at 1073 K. Fig. 3 depicts the variation of Log r values as a function of MF for pure solid solutions at 307 and 410 K respectively. It is clearly shown from the figures that r increases progressively as a function of MF, the increase
Fig. 5. Variation of Log rdc at 307 K (a) and 410 K (b), respectively, versus amounts of Li2O-doped NiO/MgO sample with MF = 0.25.
A.M. Salem et al. / Solid State Ionics 170 (2004) 33–42
was however more pronounced for r measured at 307 K. The presence of 0.333 mol NiO per mol MgO leads to increase in the value r307K of about 102-fold. MgO is a refractory material and behaves almost as an electric insulator. On the other hand, NiO behaves as a p-type semiconductors [23] and the electric current is carried only by a hoping mechanism via an electron transfer between Ni3 + and Ni2 + ions. So, the observed increase in the electrical conductivity of MgO solid due to dissolution of increasing amounts of NiO can be attributed to creation of increasing amounts of charge carriers and the greater the amount of NiO dissolved, the greater the concentration of the charge carriers. Fig. 4 depicts the variation of Log r as a function of 1/T for different pure solid solutions. It is clearly shown from Fig. 4 that the curves relating Log r versus 1/T consists of two parts at two different temperature ranges, the first extends between 307 and 410 K and the second between 410 and 617 K indicating two values of activation energy of electrical conductivity Er for each sample. The different values of activation energy (Er) were calculated for different pure solid solutions and the data obtained are given in Table 2. Inspection of Table 2 reveals the following: (i) Er value for each solid solution in region I is almost 1/2 of that for region II. (ii) Er measured for both temperature regions increases progressively by increasing the MF of NiO present. These results did not run parallel to the observed measurable increase in the electrical conductivity of the investigated system due to treatment with increasing amounts of NiO. This treatment should lead to a decrease in Er value and not an increase. This discrepancy can be resolved by considering the observed change in the values of the pre-exponential factor, ro in the Arrhenius equation due to the dissolution of increasing amounts of NiO in MgO lattice. The values of ro for different investigated solids were calculated and the data obtained are given in second
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column of Table 2. The values of the activation energy of the electrical conductivity was recalculated adopting Log ro of the sample containing the minimum value of MF (0.053) to the other solids. The computed Er* values are given in the last column of Table 2. It is clearly shown from Table 2 that Er* decreases progressively by increasing the MF values. So, the observed increase in the electric conductivity of the investigated system, which attains 102-fold, was accompanied by a corresponding significant decrease of the values of the activation energy. Fig. 5 shows the variation of Log r measured at 307 and 410 K, respectively, as a function of the amount of Li2O added to the sample containing MF of NiO equals to 0.25. It is shown from Fig. 5 that r increases progressively by increasing the amount of lithia added. The presence of 5 mol% Li2O led to an increase of more than 102-fold. Fig. 6 shows the dc electrical conductivity for pure and Li2Odoped sample. It is clear from the figure that one electrical conduction mechanism dominates for the heavily doped sample (5 mol% Li2O) while two conduction mechanisms persist in the case of pure and the sample doped with 2.5 mol% Li2O. It can also be seen from Fig. 6 that r increases by increasing the amount of Li2O added. The addition of 5 mol% Li2O to the investigated system followed by calcination at 1073 K resulted in an increase of 102-fold for r measured at room temperature. The observed significant increase in r value due to Li2O-doping of the investigated system might reflect an effective increase in the mobility of the charge carriers. In fact, it has been reported that Li2O acts as a reflux material via increasing the mobility of different cations involved in a variety of mixed solids [25]. Furthermore, it has been reported by one of the authors that Li2O-doped NiO conducted at moderate temperatures under a reduced pressure of 10 6 Torr much increases its electric conductivity and decrease Er [26]. These modifica-
Fig. 6. Variation of Log rdc versus 1/T for pure and Li2O-doped NiO/MgO samples with MF = 0.25.
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Table 3 Activation energies (DEr and DEr*) of electrical conductivity and preexponential factor, Log ro for pure and Li2O NiO/MgO sample containing MF = 0.25 Li2O %
Log ro [V 1 cm 1]
DEr (eV) Reg. I
Reg. II
0 2.5 5.0
3.650 2.650 2.860
0.359 0.481 0.450
0.794 0.798 0.450
DEr* (eV) 0.359 0.292 0.230
tions of the electric properties had been attributed to an effective increase in the mobility of the charge carriers (Ni3 + +1e ! Ni2 +). So, the observed increase in r due to doping with Li2O of the investigated system can express an effective increase in the mobility of the charge carriers. The DEr values for pure and Li2O-doped solids were calculated and the results obtained are given in Table 3. Table 3 shows that DEr increases by increasing the amount of Li2O added to the investigated system. This finding did not express the observed measurable increase in r values due to such treatment. This discrepancy was also resolved by considering the induced changes in Log ro value of NiO/MgO system due to Li2O-doping. The values of Log ro were calculated for pure and doped solids and data obtained are given in second column of Table 3. The values of DEr were re-calculated adopting Log ro for the undoped sample to the other doped ones, DEr* was obtained and given in the last column of Table 3. It can be seen from Table 3 that Er* decreases progressively by increasing the amount of Li2O added. This decrease in the value of DEr*, which expresses the observed increase in r value could be attributed to an effective increase in the mobility of the charge carriers.
and dielectric loss, tan d for the investigated NiO/MgO solid solutions with different MF values are illustrated in Figs. 7, 9 and 10), respectively. The effect of the angular frequency, x, on the ac electrical conductivity, rac, is represented in Fig. 7. The figure shows that the conductivity increases linearly at low frequency range (0.1 – 80 kHz). On increasing the frequency beyond this frequency range, the conductivity has a slight tendency to increase indicating a normal behaviour for ac electrical conductivity. The rac conductivity variation with frequency as a whole (Fig. 7) can be attributed to the heterogeneities of the material (dielectric structure) consists of two layered capacitors based on the idea that the well conducting grains are separated by grain boundaries of lower conductivity. The ac electrical conductivity in the low frequency range is related to the well conductive grains. However, at higher frequency range, the ac electrical conductivity is due to the conductive grains boundaries which are formed during the sintering process of the samples by the superficial reduction or oxidation of small crystallites as a result of their direct contact with the firing atmosphere [27,28]. The ac electrical conductivity results can be explained considering the real part of ac conductivity according to [29]: rac ¼ r1 ðT Þ þ r2 ðxÞ
The temperature dependent of first term r1(T) is related to drift of electric charge carriers, independent of frequency and follows an Arrhenius relation. The 2nd term, r2(x) is related to the dielectric relaxation caused by bound charge carriers and frequency dependent. The terms can be written in the power law form [29,30] as: r2 ðxÞ ¼ Bxn
3.3. ac Electrical conductivity The room temperature frequency dependent of ac electrical conductivity, rac, real dielectric constant, e q
ð1Þ
ð2Þ
where x = 2pf is the angular frequency, B and n are composition and temperature-dependent parameters, the
Fig. 7. Variation of the room temperature AC electrical conductivity versus the angular for the investigated NiO/MgO with different MF values.
A.M. Salem et al. / Solid State Ionics 170 (2004) 33–42
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Fig. 8. (a) Variation of DC electrical conductivity, rdc, at 307 K and ac electrical conductivity, rac, at selected frequencies versus the MF values for the investigated pure solid solutions. (b) Variation of the parameters n and B with the values of MF for pure solid solutions.
exponent n is dimensionless while B has conductivity units (V 1 cm 1). Fig. 8a shows the effect of increasing MF values on the room temperature dc conductivity, rdc
and ac conductivity, rac at selected different frequencies; 10, 80 kHz, and 1 MHz respectively. The figure shows that the dc electrical conductivity is less than the real ac
Fig. 9. Effect of angular frequency, x, on the real dielectric constant for the investigated NiO/MgO with different MF values.
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A.M. Salem et al. / Solid State Ionics 170 (2004) 33–42
Fig. 10. Effect of angular frequency, x, on the dielectric loss for the investigated NiO/MgO with different MF values.
electrical conductivity. This could be related the fact that the rdc(T) is apart of rac according to Eq. (1). Analysis of the obtained ac electrical conductivity data, one can find that our results agree well with Eq. (2) at low frequency range. The values of n and B were determined from the linear parts from Fig. 7 for different MF values. For n = 0, the electrical conduction is frequency independent, or dc dependent and frequency dependent for 1>n>0 [29]. The values of n in the present work lie between 0.09 and 0.16 or in the frequency-dependent range. Since B follows the character of conductivity with composition, i.e. it increases on increasing value of MF and n determines the degree of frequency dependence of the AC conductivity, at certain frequency, x, the conductivity increases as the MF value
increases as a result, n must increase, too. Fig. 8b confirms the expected behaviour for both B and n, where both B and n increases with increasing the MF value for the investigated solid solution. The effect of angular frequency, x, on the real dielectric constant, e and dielectric loss, tan d, at room temperature for the investigated NiO/MgO solid solution with different MF values is shown in Figs. 9 and 10, respectively. The figures depict that both e (Fig. 9) and tan d (Fig. 10) decrease on increasing frequency. It is clear that tan d shows a dielectric relaxation peak at a certain frequency (6 kHz) for solid solutions of different MF values. The dielectric relaxation (peak) takes place when the jumping frequency of the dielectric charge carriers becomes approximately equal q
q
Fig. 11. Variation of the room temperature ac electrical conductivity versus the angular frequency, x, for pure and Li2O-doped NiO/MgO samples with MF = 0.25.
A.M. Salem et al. / Solid State Ionics 170 (2004) 33–42
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Fig. 12. Effect of angular frequency, x, on the real dielectric constant for pure and Li2O-doped NiO/MgO samples with MF = 0.25.
to the external applied ac electric field. The increasing in the electrical conductivity, rac (Fig. 7), and decreasing in e (Fig. 9) and tan d (Fig. 10) with the angular frequency, x, are in agreement with the theoretical relationship [31] which predicts that rac is directly proportional to the x, while e and tan d are inversely proportional to x; q
q
xe tand 4p q
rðxÞ ¼
ð3Þ
The fact that the curves relating to tan d versus angular frequency for different solid solutions showed maxima located at the same frequency clearly indicates that the relaxation time is always the same for those solids with different MF values. In other words, the mobility of charge carriers in these solids was not affected by the MF of NiO present. So, the observed increase in r value by increasing
MF of NiO present should be attributed to an effective increase in the concentration of charge carriers as being mentioned before in the present work. Fig. 11 shows the room temperature ac electrical conductivity, rac, with frequency for pure NiO/MgO solid solution (MF = 0.25) and that doped with 2.5 and 5 mol% Li2O, respectively, in the investigated frequency range. It can also be seen that the same trend shown in Fig. 7 was observed for Li2O-doped samples. However, there is a progressive increase in the electrical conductivity values for heavily doped sample (5 mol% Li2O). Figs. 12 and 13 show the variation in the real dielectric constant, e , and tan d for pure and Li2O-doped samples. The figures depicts that both dielectric parameters decrease continuously with increasing the angular frequency, x. The values of e , and tan d decrease progressively with increasing the amount of
Fig. 13. Effect of angular frequency, x, on the dielectric loss for pure and Li2O-doped NiO/MgO samples with MF = 0.25.
q
q
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Li2O, reaching to a minimum value for heavily doped sample (5 mol% Li2O).
4. Conclusions The following are the main conclusions drawn from the results: 1. MgO dissolves increasing amounts of NiO forming NiO/ MgO solid solution heaving the formula NixMg1 xO by heating the mixed oxides at 1073 K. The maximum amount of NiO dissolved attained 0.333 mol per mol MgO. 2. The dissolution process was accompanied by a measurable decrease in the lattice parameter of MgO phase reaching to the maximum limit in presence of 0.2 mol NiO per mol MgO. 3. The dc and ac electrical conductivity (rdc, rac) of the investigated solid solution increase progressively by increasing the amount of NiO present. An increase of about 102-fold as found for the sample rich in NiO (0.333 mol NiO). 4. Li2O-doping of NiO/MgO heaving 0.2 mol NiO per mol MgO followed by calcination at 1073 K resulted in significant increase in the value of rdc and rac. 5. The increase in the amount of NiO present in different samples led to a significant decrease in DEr due to an effective increase in the concentration of charge carriers (Ni3 + + e ! Ni2 +). 6. The doping process led to a measurable decrease in the value of DEr due to an effective increase in the mobility of the charge carriers present. 7. rac for pure and various doped solids preheated at 1073 K was found to increase almost linearly by increasing the frequency of the applied AC electric field reaching a maximum limit at 106 Hz. 8. The relative permittivity, e and dielectric loss, tan d for pure and various doped solids were found decrease on increasing the frequency of the applied AC electric field. 9. The relaxation time in pure NiO/MgO solid solution exhibited almost the same value irrespective of the amount of NiO present. This might indicate that the increase in the amount of NiO added did not change the mobility of the charge carriers but increase their concentration. q
References [1] G.A. El-Shobaky, G.A. Fagal, M. Mokhtar, Appl. Catal., A Gen. 155 (1997) 167.
[2] T. Osaki, T. Horichi, T. Sugiama, K. Suzuki, T. Mori, Catal. Lett. 52 (1998) 171. [3] E. Bekyarova, P. Fornasiero, J. Kaspar, M. Graziani, Catal. Today 45 (1998) 179. [4] F.J.R. Van Neer, B. Van der Linden, A. Bliek, Catal. Today 38 (1997) 115. [5] T. Turek, D.L.M. Trimm, D.Stc. Black, N.W. Cant, Appl. Catal., A Gen. 116 (1994) 137. [6] E. Ruckenstein, Y. Hang Hu, Appl. Catal., A Gen. 183 (1999) 85. [7] S. Wang, G.Q. Max Lu, Energy Fuels 12 (1998) 248. [8] G.A. El-Shobaky, N.H. Amin, N.M. Deraz, S. El-Molla, Adsorp. Sci. Technol. 19 (1) (2001) 45. [9] G.A. EL-Shobaky, N.R. Radwan, G.M. Turky, Solid State Ionics 156 (2003) 337. [10] K. Inamura, T. Takyu, J. Phys. Chem. 45 (1991) 1310. [11] G. Ghiotti, F. Boccuzzi, J. Chem. Soc., Faraday Trans. I 79 (1983) 1843. [12] F. Arena, F. Frusteri, L. Plyasova, A. Parmaliana, J. Chem. Soc., Faraday Trans. I 94 (1998) 3385. [13] Z. Sojka, E. Giamello, M. Che, A. Zecchina, K. Dyrek, J. Phys. Chem. 92 (1988) 1541. [14] V. Indovina, D. Cordischi, S. Febraro, M. Occhiuzzi, J. Chem. Soc., Faraday Trans. I 81 (1985) 37. [15] K. Faih, B. Marsan, Can. J. Chem. 75 (1997) 1597. [16] G.A. El-Shobaky, N.M. Deraz, Mater. Lett. 47 (2001) 231. [17] G.A. El-Shobaky, M.N. Hamed, F.F. Abdalla, S.M. El-Molla, Colloids Surf. 207 (2002) 293. [18] G.A. El-Shobaky, F.F. Abdalla, M.N. Hamed, S.M. El-Molla, Colloids Surf. 207 (2002) 302. [19] H.G. El-Shobaky, W.M. Shaheen, M. Mokhtar, Adsorp. Sci. Technol. 19 (2001) 621. [20] L.M. Levinson, H.R. Philipp, J. Appl. Phys. 47 (1976) 1117. [21] M. Abd El-Alim, J. Am. Ceram. Soc. 72 (1989) 28. [22] A.K. Jonscher, J. Non-Cryst. Solids 8 – 10 (1972) 293. [23] P. Kostad, Non-Stoichiometry, Diffusion and Electrical Conductivity in Binary Metal Oxides, Wiley-Interscience, New York, 1972. [24] N.N. Greenwood, Ionic Crystals Lattice Defects and Non-Stoichiometry, Butterworth, London, 1964, pp. 37 – 41. [25] G.A. El-Shobaky, G.A. Fagal, A. Abd El-Aal, A.M. Ghouzza, Thermochim. Acta 256 (1995) 429; G.A. El-Shobaky, G.A. Fagal, A. Abd El-Aal, A.M. Ghouzza, Thermochim. Acta 275 (1996) 235. [26] P.C. Gravelle, G.A. El-Shobaky, S.J. Teichner, in: K. Hauffe, Th. Volkestein (Eds.), Symp. Electr. Phen. Chem. Catal., Water de Gruter and Co., Berlin, 1969, p. 124. [27] J. Maxwell, in: Electricity and Magnetism, vol. 1, Oxford Univ. Press, London, 1873, p. 328. [28] K. Wanger, Ann. Phys. (Leipz.) 40 (1913) 817. [29] A. Jonscher, Dielectric Relaxation in Solids, Chelsea Dielectric Press, London, 1983. [30] Y. Yamazaki, M. Satou, J. Appl. Phys. 12 (1973) 988. [31] J. Smit, H.P. Wijn, Ferrites, Cleaver-Hume Press, London, 1959, Chap. XII. [32] E. Ruckenstien, Y.-H. Hu, Appl. Catal., A Gen. 154 (1997) 185; E. Ruckenstien, Y.-H. Hu, Appl. Catal., A Gen. 183 (1999) 85.
Electrical properties of pure and Li2O-doped NiO/MgO system A.M. Salem a, M. Mokhtar b, G.A. El-Shobaky b,* a
b
Physics Division, National Research Centre, Dokki, Cairo, Egypt Laboratory of Surface Chemical Catalysis, Department of Physical Chemistry, National Research Centre, Dokki, Cairo, Egypt Received 14 October 2002; received in revised form 13 June 2003; accepted 26 January 2004
Abstract NiO/MgO solid solution was prepared by heating a fixed amount of magnesium basic carbonate with increasing amounts of nickel nitrate followed by calcination at 1073 K. The mol fraction (MF) of NiO was varied between 0.053 and 0.333. The sample containing 0.25 MF NiO was doped with 2.5 and 5 mol% Li2O followed by calcination at 1073 K. The XRD patterns of various investigated solids were measured and different electric properties were investigated. These properties include: r, e , tan d and the activation energy of electrical conductivity. The results showed that pure and variously doped solids consist of MgO phase while all the diffraction lines of NiO disappeared completely indicating the formation of NiO/MgO solid solution. The dissolution of 0.25 MF NiO resulted in a measurable increase in the lattice constant, a of MgO which decreased by increasing the amount of NiO above this limit, falling to values smaller than that measured for pure MgO. The ac and dc values of r were found to increase progressively by increasing the mol fraction of NiO and also by increasing the amount of Li2O added. The values of e and tan d were found to decrease progressively by increasing the NiO-content and also by increasing the frequency of ac current. However, the activation energy of r increased by increasing both the mol fraction and the amount of Li2O, which did not run parallel to the observed increase in the r values due to these treatments. This discrepancy had been attributed to a compensation effect of the pre-exponential factor of the electrical conductivity, ro. The increase in r value due to increasing the MF value of NiO present was attributed to a significant increase in the concentration of the charge carriers without affecting their energetic nature (Ni2 +, Ni3 +). While the induced increase in r value of the investigated system due doping with Li2O resulted mainly from an effective increase in the mobility of the charge carriers present. D 2004 Elsevier B.V. All rights reserved. q
q
Keywords: NiO/MgO system; Electrical properties; Li2O
1. Introduction Transition metal oxides supported on finely divided support material are successfully used in catalyzing different reactions [1 – 8]. These supported solids can also find different applications including closed cycle carbon dioxide lasers and air purification devices besides their applications as semiconducting materials [9]. MgO can dissolve different amounts of transition metal oxides such as CuO, NiO, Co3O4 and MnO forming solid solutions [6]. The amounts of transition metal oxide dissolved in MgO lattice depend mainly on the nature of these oxides, the calcinations conditions and doping with certain foreign cations [9 – 15]. The surface, catalytic and electrical properties of these solid solutions have been studied by several authors using differ-
* Corresponding author. E-mail address: [email protected] (G.A. El-Shobaky). 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2004.01.034
ent techniques [16 – 23]. The effect of Li2O-doping on electrical properties of CoO/MgO solid solution made the object of the recent work done by one of the authors [9].The present work reports a study on the effects of Li2O-doping and amount of NiO in the electrical properties of NiO/MgO solid solution preheated at 800 jC.
2. Experimental 2.1. Materials Pure and NiO/MgO mixed solid samples were prepared by wet impregnation of finely powdered magnesium basic carbonate solid with calculated amount of nickel nitrate dissolved in the least amount of distilled water sufficient to make pastes. The pastes were dried at 383 K then calcined at 1073 K for 6 h. The molecular formula of the calcined solid samples were 0.05, 0.15, 0.20, and 0.25 NiO/MgO. Two
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A.M. Salem et al. / Solid State Ionics 170 (2004) 33–42
Li2O-doped sample were prepared by treating NiO/MgO sample having the formula 0.25 NiO/MgO with calculated amount of lithium nitrate followed by drying then calcination at 1073 K for 6 h. The amounts of Li2O dopant were 2.5 and 5 mol%. The entire chemicals employed were of analytical grade and supplied by Merck. 2.2. Techniques An X-ray investigation of pure and doped mixed solids precalcined at 1073 K was conducted using a Philips diffractometer (type PW1390). The patterns were run with ˚ ) at 30 kV and 10 iron filtered cobalt radiation (k = 1.7889 A mA with scanning speeds of 2j in 2h min 1 and 0.5j 2h min 1 for phase identification and line broadening profile analysis , respectively. The lattice parameter a of MgO was calculated from the diffractions (220), (311) and (222) faces using the expression a=(h2 + k2 + l2)0.5 (k/2 sin h) and the crystallite size d was estimated using Scherrer equation: d ¼ Bk=ðb1=2 coshÞ where B is Scherrer constant (0.89), b1/2 is full width half maximum (FWHM) of the diffraction peak in radian , k the wave length of the X-ray beam and h is the diffraction angle. The ac electrical conductivity, rac, real dielectric constant, e , and dielectric loss, tand were measured in the frequency range 102 – 106 Hz using an RLC meter (type Hioki 3531 Z Hitester). The electric capacity, C, dissipation factor, tan d and the resistance, R were obtained directly from the bridge from which e , and rac were calculated. The q
q
Fig. 2. X-ray diffraction pattern for pure and 2.5, 5.0 Li2O-doped NiO/MgO sample with (MF = 0.25).
electric measurements were carried out on samples in the form of discs having a radius of 0.6 cm and thickness of 0.25 cm obtained by compressing a fixed weight of powdered samples under hydraulic pressure of 5 tons/cm2. Different electrical measurements were carried out on these disks after being coated with a thin layer of silver to avoid any fluctuation in the values of electric and dielectric parameters.
3. Results and discussions 3.1. XRD investigation of pure and doped solids X-ray diffractograms of pure and doped solids precalcined at 1073 K were determined. The diffractograms of the investigated solids are illustrated in Figs. 1 and 2 for pure and doped solids. An additional sample having the formula 0.25 NiO/MgO precalcined at 673 K was also investigated. Table 1 Variation of the lattice parameter a and crystallite size d of MgO as a function of NiO (MF) and amount of Li2O added
Fig. 1. X-ray diffraction patterns of pure NiO/MgO having different MF values.
NiO (MF)
Lattice parameter ˚] a of MgO [A
Crystallite size ˚] d of MgO [A
0.000 0.053 0.176 0.250 0.333 0.250 pure + 2.5 mol% Li2O + 5.0 mol% Li2O
4.2122 F 5 10 4 4.2207 F 5 10 4 4.2280 F 5 10 4 4.2072 F 5 10 4 4.2004 F 5 10 4 4.2072 F 5 10 4 4.2107 F 5 10 4 4.2219 F 5 10 4
135 130 115 105 96 105 128 160
A.M. Salem et al. / Solid State Ionics 170 (2004) 33–42
Fig. 3. The DC electrical conductivity values, rdc measured at 307 K (a) and 410 K (b), respectively, versus the MF values for pure solid solutions.
The diffractogram of this particular sample (not given) consists of all diffraction lines of MgO and NiO phases. This finding clearly indicates the absence of any solid –solid interaction between NiO and MgO preheated a 673 K. On the other hand, the diffractograms of pure and doped solids subjected to heat treatment at 1073 K (cf. Figs. 1 and 2)
35
consisted only of all diffraction lines of MgO phase indicating thus a complete dissolution of NiO in MgO lattice forming homogeneous solid solutions [9,17,18]. The formula of these solid solutions are given by; Ni0.05Mg0.95O, NiO0.15Mg0.85O, NiO0.20Mg0.80O, NiO0.25Mg0.75O. The values of mol fraction (MF) of NiO present in these solid solutions are; 0.053, 0.176, 0.25, 0.333, respectively. The formation of such solid solutions requires thus a heat treatment at 1073 K. The effect of the amount of NiO present and amount of Li2O added on the lattice constant of MgO phase was investigated and the results obtained are given in Table 1. Inspection of Table 1 revealed that: (i) The dissolution of increasing amounts of NiO in MgO lattice resulted in a progressive increase in the lattice constant of MgO reaching to a maximum limit at MF = 0.176. (ii) The increase in the amount of NiO dissolved in MgO matrix above this limit led to a measurable decrease in the lattice parameters of MgO falling to a value smaller than that measured for pure MgO lattice. (iii) The dissolution of Li2O in the investigated system led to a limited increase in the lattice parameter of the MgO phase. (iv) The increase in the amount of NiO led to a progressive decrease in the crystallite size of MgO while Li2O doping of NiO /MgO that having MF = 0.25 increased the crystallite size of MgO. This finding shows that Li2O enhances the particle adhesion of MgO crystallites leading to big sized MgO particles. The dissolution of NiO in MgO lattice yielding NiO – MgO solid solution should be normally accompanied by an increase in the lattice parameter of MgO solvent. This conclusion comes from the fact that the ionic radius of Ni2 + is bigger than that of Mg2 +. In fact the ionic radii of divalent magnesium ion and divalent nickel ion are 0.65 and ˚ , respectively [24]. So , the observed measurable 0.78 A increase in the lattice parameter of MgO can be attributed to the dissolution of NiO in MgO lattice via substitution of some of Mg2 + host cations by Ni2 + ions. However, the observed decrease in the lattice parameter of MgO by increasing the amount of NiO above MF = 0.176 could be
Fig. 4. Variation of Log rdc versus 1/T for the investigated NiO/MgO with different MF values.
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A.M. Salem et al. / Solid State Ionics 170 (2004) 33–42
Table 2 Activation energies (DE and DE*) of electrical conductivity and preexponential factor, Log ro for NiO/MgO containing different MF values MF
0.053 0.176 0.250 0.333
Log ro [V 1 cm 1] 4.355 3.016 3.650 2.650
DEr (eV)
DEr* (eV)
Reg. I
Reg. II
0.307 0.372 0.359 0.440
0.773 0.995 0.794 0.861
0.307 0.282 0.266 0.250
between NiO and MgO took place leading to an increase in the binding energy of (Ni 2 p 3/2) from 854.5 eV in the case of NiO to 856 eV in the case of NiO – MgO solid solution. This finding suggested an effective transformation of some of Ni2 + to Ni3 + ions. In other words, the increase in the amount of NiO dissolved in NiO matrix above certain limit (MF = 0.25 and 0.333 ) might be accompanied by an increase the oxidation state of nickel oxide present. 3.2. dc Conductivity measurements
attributed to an effective transformation of some of Ni2 + ions into Ni3 + ions (the ionic radius of Ni3 + ions being 0.64 ˚ ) [24]. Ruckenstein and Hang Hu [32] have reported that A nickel magnesium mixed oxides containing 37 mol% NiO prepared by the same method followed in the present work and precaclcined at 1073 K consisted of NiO – MgO solid solution. These authors claimed also that an electron transfer
The dc electrical conductivity was determined for pure and Li2O-doped NiO/MgO solid solutions precalcined at 1073 K. Fig. 3 depicts the variation of Log r values as a function of MF for pure solid solutions at 307 and 410 K respectively. It is clearly shown from the figures that r increases progressively as a function of MF, the increase
Fig. 5. Variation of Log rdc at 307 K (a) and 410 K (b), respectively, versus amounts of Li2O-doped NiO/MgO sample with MF = 0.25.
A.M. Salem et al. / Solid State Ionics 170 (2004) 33–42
was however more pronounced for r measured at 307 K. The presence of 0.333 mol NiO per mol MgO leads to increase in the value r307K of about 102-fold. MgO is a refractory material and behaves almost as an electric insulator. On the other hand, NiO behaves as a p-type semiconductors [23] and the electric current is carried only by a hoping mechanism via an electron transfer between Ni3 + and Ni2 + ions. So, the observed increase in the electrical conductivity of MgO solid due to dissolution of increasing amounts of NiO can be attributed to creation of increasing amounts of charge carriers and the greater the amount of NiO dissolved, the greater the concentration of the charge carriers. Fig. 4 depicts the variation of Log r as a function of 1/T for different pure solid solutions. It is clearly shown from Fig. 4 that the curves relating Log r versus 1/T consists of two parts at two different temperature ranges, the first extends between 307 and 410 K and the second between 410 and 617 K indicating two values of activation energy of electrical conductivity Er for each sample. The different values of activation energy (Er) were calculated for different pure solid solutions and the data obtained are given in Table 2. Inspection of Table 2 reveals the following: (i) Er value for each solid solution in region I is almost 1/2 of that for region II. (ii) Er measured for both temperature regions increases progressively by increasing the MF of NiO present. These results did not run parallel to the observed measurable increase in the electrical conductivity of the investigated system due to treatment with increasing amounts of NiO. This treatment should lead to a decrease in Er value and not an increase. This discrepancy can be resolved by considering the observed change in the values of the pre-exponential factor, ro in the Arrhenius equation due to the dissolution of increasing amounts of NiO in MgO lattice. The values of ro for different investigated solids were calculated and the data obtained are given in second
37
column of Table 2. The values of the activation energy of the electrical conductivity was recalculated adopting Log ro of the sample containing the minimum value of MF (0.053) to the other solids. The computed Er* values are given in the last column of Table 2. It is clearly shown from Table 2 that Er* decreases progressively by increasing the MF values. So, the observed increase in the electric conductivity of the investigated system, which attains 102-fold, was accompanied by a corresponding significant decrease of the values of the activation energy. Fig. 5 shows the variation of Log r measured at 307 and 410 K, respectively, as a function of the amount of Li2O added to the sample containing MF of NiO equals to 0.25. It is shown from Fig. 5 that r increases progressively by increasing the amount of lithia added. The presence of 5 mol% Li2O led to an increase of more than 102-fold. Fig. 6 shows the dc electrical conductivity for pure and Li2Odoped sample. It is clear from the figure that one electrical conduction mechanism dominates for the heavily doped sample (5 mol% Li2O) while two conduction mechanisms persist in the case of pure and the sample doped with 2.5 mol% Li2O. It can also be seen from Fig. 6 that r increases by increasing the amount of Li2O added. The addition of 5 mol% Li2O to the investigated system followed by calcination at 1073 K resulted in an increase of 102-fold for r measured at room temperature. The observed significant increase in r value due to Li2O-doping of the investigated system might reflect an effective increase in the mobility of the charge carriers. In fact, it has been reported that Li2O acts as a reflux material via increasing the mobility of different cations involved in a variety of mixed solids [25]. Furthermore, it has been reported by one of the authors that Li2O-doped NiO conducted at moderate temperatures under a reduced pressure of 10 6 Torr much increases its electric conductivity and decrease Er [26]. These modifica-
Fig. 6. Variation of Log rdc versus 1/T for pure and Li2O-doped NiO/MgO samples with MF = 0.25.
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A.M. Salem et al. / Solid State Ionics 170 (2004) 33–42
Table 3 Activation energies (DEr and DEr*) of electrical conductivity and preexponential factor, Log ro for pure and Li2O NiO/MgO sample containing MF = 0.25 Li2O %
Log ro [V 1 cm 1]
DEr (eV) Reg. I
Reg. II
0 2.5 5.0
3.650 2.650 2.860
0.359 0.481 0.450
0.794 0.798 0.450
DEr* (eV) 0.359 0.292 0.230
tions of the electric properties had been attributed to an effective increase in the mobility of the charge carriers (Ni3 + +1e ! Ni2 +). So, the observed increase in r due to doping with Li2O of the investigated system can express an effective increase in the mobility of the charge carriers. The DEr values for pure and Li2O-doped solids were calculated and the results obtained are given in Table 3. Table 3 shows that DEr increases by increasing the amount of Li2O added to the investigated system. This finding did not express the observed measurable increase in r values due to such treatment. This discrepancy was also resolved by considering the induced changes in Log ro value of NiO/MgO system due to Li2O-doping. The values of Log ro were calculated for pure and doped solids and data obtained are given in second column of Table 3. The values of DEr were re-calculated adopting Log ro for the undoped sample to the other doped ones, DEr* was obtained and given in the last column of Table 3. It can be seen from Table 3 that Er* decreases progressively by increasing the amount of Li2O added. This decrease in the value of DEr*, which expresses the observed increase in r value could be attributed to an effective increase in the mobility of the charge carriers.
and dielectric loss, tan d for the investigated NiO/MgO solid solutions with different MF values are illustrated in Figs. 7, 9 and 10), respectively. The effect of the angular frequency, x, on the ac electrical conductivity, rac, is represented in Fig. 7. The figure shows that the conductivity increases linearly at low frequency range (0.1 – 80 kHz). On increasing the frequency beyond this frequency range, the conductivity has a slight tendency to increase indicating a normal behaviour for ac electrical conductivity. The rac conductivity variation with frequency as a whole (Fig. 7) can be attributed to the heterogeneities of the material (dielectric structure) consists of two layered capacitors based on the idea that the well conducting grains are separated by grain boundaries of lower conductivity. The ac electrical conductivity in the low frequency range is related to the well conductive grains. However, at higher frequency range, the ac electrical conductivity is due to the conductive grains boundaries which are formed during the sintering process of the samples by the superficial reduction or oxidation of small crystallites as a result of their direct contact with the firing atmosphere [27,28]. The ac electrical conductivity results can be explained considering the real part of ac conductivity according to [29]: rac ¼ r1 ðT Þ þ r2 ðxÞ
The temperature dependent of first term r1(T) is related to drift of electric charge carriers, independent of frequency and follows an Arrhenius relation. The 2nd term, r2(x) is related to the dielectric relaxation caused by bound charge carriers and frequency dependent. The terms can be written in the power law form [29,30] as: r2 ðxÞ ¼ Bxn
3.3. ac Electrical conductivity The room temperature frequency dependent of ac electrical conductivity, rac, real dielectric constant, e q
ð1Þ
ð2Þ
where x = 2pf is the angular frequency, B and n are composition and temperature-dependent parameters, the
Fig. 7. Variation of the room temperature AC electrical conductivity versus the angular for the investigated NiO/MgO with different MF values.
A.M. Salem et al. / Solid State Ionics 170 (2004) 33–42
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Fig. 8. (a) Variation of DC electrical conductivity, rdc, at 307 K and ac electrical conductivity, rac, at selected frequencies versus the MF values for the investigated pure solid solutions. (b) Variation of the parameters n and B with the values of MF for pure solid solutions.
exponent n is dimensionless while B has conductivity units (V 1 cm 1). Fig. 8a shows the effect of increasing MF values on the room temperature dc conductivity, rdc
and ac conductivity, rac at selected different frequencies; 10, 80 kHz, and 1 MHz respectively. The figure shows that the dc electrical conductivity is less than the real ac
Fig. 9. Effect of angular frequency, x, on the real dielectric constant for the investigated NiO/MgO with different MF values.
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A.M. Salem et al. / Solid State Ionics 170 (2004) 33–42
Fig. 10. Effect of angular frequency, x, on the dielectric loss for the investigated NiO/MgO with different MF values.
electrical conductivity. This could be related the fact that the rdc(T) is apart of rac according to Eq. (1). Analysis of the obtained ac electrical conductivity data, one can find that our results agree well with Eq. (2) at low frequency range. The values of n and B were determined from the linear parts from Fig. 7 for different MF values. For n = 0, the electrical conduction is frequency independent, or dc dependent and frequency dependent for 1>n>0 [29]. The values of n in the present work lie between 0.09 and 0.16 or in the frequency-dependent range. Since B follows the character of conductivity with composition, i.e. it increases on increasing value of MF and n determines the degree of frequency dependence of the AC conductivity, at certain frequency, x, the conductivity increases as the MF value
increases as a result, n must increase, too. Fig. 8b confirms the expected behaviour for both B and n, where both B and n increases with increasing the MF value for the investigated solid solution. The effect of angular frequency, x, on the real dielectric constant, e and dielectric loss, tan d, at room temperature for the investigated NiO/MgO solid solution with different MF values is shown in Figs. 9 and 10, respectively. The figures depict that both e (Fig. 9) and tan d (Fig. 10) decrease on increasing frequency. It is clear that tan d shows a dielectric relaxation peak at a certain frequency (6 kHz) for solid solutions of different MF values. The dielectric relaxation (peak) takes place when the jumping frequency of the dielectric charge carriers becomes approximately equal q
q
Fig. 11. Variation of the room temperature ac electrical conductivity versus the angular frequency, x, for pure and Li2O-doped NiO/MgO samples with MF = 0.25.
A.M. Salem et al. / Solid State Ionics 170 (2004) 33–42
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Fig. 12. Effect of angular frequency, x, on the real dielectric constant for pure and Li2O-doped NiO/MgO samples with MF = 0.25.
to the external applied ac electric field. The increasing in the electrical conductivity, rac (Fig. 7), and decreasing in e (Fig. 9) and tan d (Fig. 10) with the angular frequency, x, are in agreement with the theoretical relationship [31] which predicts that rac is directly proportional to the x, while e and tan d are inversely proportional to x; q
q
xe tand 4p q
rðxÞ ¼
ð3Þ
The fact that the curves relating to tan d versus angular frequency for different solid solutions showed maxima located at the same frequency clearly indicates that the relaxation time is always the same for those solids with different MF values. In other words, the mobility of charge carriers in these solids was not affected by the MF of NiO present. So, the observed increase in r value by increasing
MF of NiO present should be attributed to an effective increase in the concentration of charge carriers as being mentioned before in the present work. Fig. 11 shows the room temperature ac electrical conductivity, rac, with frequency for pure NiO/MgO solid solution (MF = 0.25) and that doped with 2.5 and 5 mol% Li2O, respectively, in the investigated frequency range. It can also be seen that the same trend shown in Fig. 7 was observed for Li2O-doped samples. However, there is a progressive increase in the electrical conductivity values for heavily doped sample (5 mol% Li2O). Figs. 12 and 13 show the variation in the real dielectric constant, e , and tan d for pure and Li2O-doped samples. The figures depicts that both dielectric parameters decrease continuously with increasing the angular frequency, x. The values of e , and tan d decrease progressively with increasing the amount of
Fig. 13. Effect of angular frequency, x, on the dielectric loss for pure and Li2O-doped NiO/MgO samples with MF = 0.25.
q
q
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A.M. Salem et al. / Solid State Ionics 170 (2004) 33–42
Li2O, reaching to a minimum value for heavily doped sample (5 mol% Li2O).
4. Conclusions The following are the main conclusions drawn from the results: 1. MgO dissolves increasing amounts of NiO forming NiO/ MgO solid solution heaving the formula NixMg1 xO by heating the mixed oxides at 1073 K. The maximum amount of NiO dissolved attained 0.333 mol per mol MgO. 2. The dissolution process was accompanied by a measurable decrease in the lattice parameter of MgO phase reaching to the maximum limit in presence of 0.2 mol NiO per mol MgO. 3. The dc and ac electrical conductivity (rdc, rac) of the investigated solid solution increase progressively by increasing the amount of NiO present. An increase of about 102-fold as found for the sample rich in NiO (0.333 mol NiO). 4. Li2O-doping of NiO/MgO heaving 0.2 mol NiO per mol MgO followed by calcination at 1073 K resulted in significant increase in the value of rdc and rac. 5. The increase in the amount of NiO present in different samples led to a significant decrease in DEr due to an effective increase in the concentration of charge carriers (Ni3 + + e ! Ni2 +). 6. The doping process led to a measurable decrease in the value of DEr due to an effective increase in the mobility of the charge carriers present. 7. rac for pure and various doped solids preheated at 1073 K was found to increase almost linearly by increasing the frequency of the applied AC electric field reaching a maximum limit at 106 Hz. 8. The relative permittivity, e and dielectric loss, tan d for pure and various doped solids were found decrease on increasing the frequency of the applied AC electric field. 9. The relaxation time in pure NiO/MgO solid solution exhibited almost the same value irrespective of the amount of NiO present. This might indicate that the increase in the amount of NiO added did not change the mobility of the charge carriers but increase their concentration. q
References [1] G.A. El-Shobaky, G.A. Fagal, M. Mokhtar, Appl. Catal., A Gen. 155 (1997) 167.
[2] T. Osaki, T. Horichi, T. Sugiama, K. Suzuki, T. Mori, Catal. Lett. 52 (1998) 171. [3] E. Bekyarova, P. Fornasiero, J. Kaspar, M. Graziani, Catal. Today 45 (1998) 179. [4] F.J.R. Van Neer, B. Van der Linden, A. Bliek, Catal. Today 38 (1997) 115. [5] T. Turek, D.L.M. Trimm, D.Stc. Black, N.W. Cant, Appl. Catal., A Gen. 116 (1994) 137. [6] E. Ruckenstein, Y. Hang Hu, Appl. Catal., A Gen. 183 (1999) 85. [7] S. Wang, G.Q. Max Lu, Energy Fuels 12 (1998) 248. [8] G.A. El-Shobaky, N.H. Amin, N.M. Deraz, S. El-Molla, Adsorp. Sci. Technol. 19 (1) (2001) 45. [9] G.A. EL-Shobaky, N.R. Radwan, G.M. Turky, Solid State Ionics 156 (2003) 337. [10] K. Inamura, T. Takyu, J. Phys. Chem. 45 (1991) 1310. [11] G. Ghiotti, F. Boccuzzi, J. Chem. Soc., Faraday Trans. I 79 (1983) 1843. [12] F. Arena, F. Frusteri, L. Plyasova, A. Parmaliana, J. Chem. Soc., Faraday Trans. I 94 (1998) 3385. [13] Z. Sojka, E. Giamello, M. Che, A. Zecchina, K. Dyrek, J. Phys. Chem. 92 (1988) 1541. [14] V. Indovina, D. Cordischi, S. Febraro, M. Occhiuzzi, J. Chem. Soc., Faraday Trans. I 81 (1985) 37. [15] K. Faih, B. Marsan, Can. J. Chem. 75 (1997) 1597. [16] G.A. El-Shobaky, N.M. Deraz, Mater. Lett. 47 (2001) 231. [17] G.A. El-Shobaky, M.N. Hamed, F.F. Abdalla, S.M. El-Molla, Colloids Surf. 207 (2002) 293. [18] G.A. El-Shobaky, F.F. Abdalla, M.N. Hamed, S.M. El-Molla, Colloids Surf. 207 (2002) 302. [19] H.G. El-Shobaky, W.M. Shaheen, M. Mokhtar, Adsorp. Sci. Technol. 19 (2001) 621. [20] L.M. Levinson, H.R. Philipp, J. Appl. Phys. 47 (1976) 1117. [21] M. Abd El-Alim, J. Am. Ceram. Soc. 72 (1989) 28. [22] A.K. Jonscher, J. Non-Cryst. Solids 8 – 10 (1972) 293. [23] P. Kostad, Non-Stoichiometry, Diffusion and Electrical Conductivity in Binary Metal Oxides, Wiley-Interscience, New York, 1972. [24] N.N. Greenwood, Ionic Crystals Lattice Defects and Non-Stoichiometry, Butterworth, London, 1964, pp. 37 – 41. [25] G.A. El-Shobaky, G.A. Fagal, A. Abd El-Aal, A.M. Ghouzza, Thermochim. Acta 256 (1995) 429; G.A. El-Shobaky, G.A. Fagal, A. Abd El-Aal, A.M. Ghouzza, Thermochim. Acta 275 (1996) 235. [26] P.C. Gravelle, G.A. El-Shobaky, S.J. Teichner, in: K. Hauffe, Th. Volkestein (Eds.), Symp. Electr. Phen. Chem. Catal., Water de Gruter and Co., Berlin, 1969, p. 124. [27] J. Maxwell, in: Electricity and Magnetism, vol. 1, Oxford Univ. Press, London, 1873, p. 328. [28] K. Wanger, Ann. Phys. (Leipz.) 40 (1913) 817. [29] A. Jonscher, Dielectric Relaxation in Solids, Chelsea Dielectric Press, London, 1983. [30] Y. Yamazaki, M. Satou, J. Appl. Phys. 12 (1973) 988. [31] J. Smit, H.P. Wijn, Ferrites, Cleaver-Hume Press, London, 1959, Chap. XII. [32] E. Ruckenstien, Y.-H. Hu, Appl. Catal., A Gen. 154 (1997) 185; E. Ruckenstien, Y.-H. Hu, Appl. Catal., A Gen. 183 (1999) 85.