Dielectric properties and electrical conductivity of ...

0 downloads 0 Views 984KB Size Report
100 °C between 24 and 48 hours in order to remove water and solvents. Finally, the solids are ... mm thick with a hydraulic press under 8 ton. ..... I.F. Mironyuk , V.M. Gun'ko , M.O. Povazhnyak , V.I. Zarko ,V.M. Chelyadin , R. Leboda , J.
Dielectric properties and electrical conductivity of MgO synthesized by chemical precipitation and sol-gel method

Rabeh Mbarki*, Ahmed Hichem Hamzaoui, Adel M’nif

Laboratoire de Valorisation des Matériaux Utiles (LVMU), Centre National des Recherches en Sciences des Matériaux, Technopole Borj Cedria BP 73, 8027 Soliman, Tunisia *corresponding author: Phone: + 21679325250; e-mail: [email protected]

1 This provisional PDF is the accepted version. The article should be cited as: Eur. Phys. J. Appl. Phys. , doi: 10.1051/epjap/2014130287

Abstract. MgO Powders were synthesized via simple chemical precipitation (SPC) and sol-gel process (SG). The electrical behavior of these powders was determined by complex impedance spectroscopy using an alternative current conductivity at various temperatures and frequencies. For MgO elaborated by SG, the activation energy is 1.49 eV while MgO prepared by SPC, this energy is equal to 0.39 and 4.13 eV. The structural properties of MgO powders were analyzed by X-ray diffraction and FT-IR spectroscopy. The results show that the cristallites size was 28,4 nm for MgO SPC and 42,5 nm for MgO SG. Others methods such DTA, TGA, BET and SEM were used to characterized MgO materials.

Keywords: MgO, sol-gel, chemical precipitation, dielectric, electrical conductivity, impedance spectroscopy.

1 Introduction In the past decades, scientific researchers were deeply interested by nanostructures materials because of their unique properties. In fact, this importance is due to their interesting physico-chemical properties and the wide range of potential applications in nanodevices. MgO is one of the most interesting nanostructure materials; it has remarkable electronic, magnetic, and mechanical properties because of its structural characteristics [1]. MgO was widely used in catalysis [2, 3], remediation against toxic waste [4, 5] antibacterial materials [6, 7] refractory materials, paints and superconductors products [8, 9]. Where many methods were used for MgO nanoscale and nanostructure preparation, such as nanorods and porous nanowires [10]. MgO nanoparticles with high specific surface area were studied in various applications such as catalyst, catalyst support, adsorbent and degradation materials for a large number of pollutants [11, 12]. Also,

2

there are extensive researches to mitigate air pollutants, including volatile organic compounds and acid gases [13, 14]. Therefore, low-cost methods to produce magnesium oxide with a small distribution size and large surface area were required. Simple chemical precipitation (SPC) and sol-gel (SG) synthesis followed by drying and thermal treatment are the most common technique to prepare this material. These methods allow a good chemical homogeneity and homogeneous distribution of sizes. Other advantages of these methods reside in the simplicity of the preparation and cheap raw materials. Synthesis of MgO powders by chemical precipitation and sol-gel processes is described hereafter. Ammonia solution is used to prepare MgO as precipitate or as gel. In both cases polyethylene glycols (PEG 300) act as co-solvent dispersant. However it is important to notice that MgO gel is prepared under 50 °C and Nitrogen atmosphere. The present work deals with synthesis of MgO by chemical precipitation and solgel method. Activation energies, structural, thermal and mass loss of these materials were investigated. XRD, FT-IR, DTA, TGA, SEM and BET were used to gain a deeper insight of synthesis process and phase transformation occurring in the thermal treatment procedure. The dielectric properties of synthetic materials were performed using impedance spectroscopy.

2 Materials and methods 2.1 Synthesis The elaboration of MgO powders by simple chemical precipitation (SPC) is as follow: Magnesium Chloride Hexahydrate (MgCl2.6H2O, (10 g) scharlau 98.9%), was mixed with (5 mL) polyethylene glycol (PEG300) and ethanol (Aldrich 99%) to make a transparent solution. A mixture of ammonia and ethanol (5 mL) is added

3

drop wise to the above solution at room temperature. For sol-gel (SG) method, the MgO is synthesized by refluxing 10 g of MgCl2.6H2O with 5 mL of polyethylene glycol and 50 mL of ethanol. The synthesis reaction is carried out under nitrogen atmosphere-controlled at 50 °C. A mixture of ammonia hydroxide and anhydrous ethanol is added drop wise to the above solution. The obtained material is dried at 100 °C between 24 and 48 hours in order to remove water and solvents. Finally, the solids are calcined at 500 °C for 12 hours. The final materials are named MgO SPC and MgO SG. 2.2 Measurements X-ray diffraction spectra are recorded by a diffractometer with radiation Philips X'pert PRO Cu-Ka (λ = 1.5406 Å). Where The acceleration voltage, the emission current are respectively equal to 40 KV, 40 mA whereas the 2 range was between 5 and 90° and the scanning speed was fixed at 0.02°/s. Data acquisition is performed by a control unit and diffraction processing spectra is performed using the software "X'pert High score" containing the database JCPDS cards. Samples are prepared as pellets with KBr to be analyzed by using Fourier transform spectrometer (Fourier Perkin - Elmer 1600 series FT-TR) in the range 400 to 4000 cm-1. TGA and DTA measurements are performed simultaneously by the same appareillous SETARAM SETSYS 1750. Temperature variation is performed in the range 25 to 1000 ° C with a step of 10 °C .mn-1. Scanning electron microscopy (SEM) (Philips Fei Quanta 200 scanning electron microscope) and Brunauer, Emett and Teller (BET) (Quantachrome Autosorb-1 instrument using N2 gas) are used to observe the surface morphology of the materials. Measurements of electrical conductivity are performed by means of an electrode configuration. The samples were pressed into pellets of 13 mm diameter and 0.5

4

mm thick with a hydraulic press under 8 ton. Measurements were performed using an impedance type Hewlett-Packard 4192 with temperature range 100-800 °C and frequency 5 Hz-13 MHz. To ensure good electrical contact between the sample and the electrical connections, a silver layer was deposited on each circular face surface of the pellet. The resistance value is determined at each temperature using Zview program.

3 Results and discussion 3.1 X-ray diffraction XRD spectra are identical for both samples synthesized by the SPC and SG methods (Fig. 1). All peaks can be attributed to a pure phase of MgO in agreement with JCPDS PDF (No. 45-946). It is noted that the diffraction peaks of MgO (SG) are the finest, showing a better crystallization. The crystallite size of MgO (SPC) and MgO(SG) calculated using Scherrer formula [15] are respectively equal to 28,4 nm and 42,5 nm respectively.  D 

kλ β * cosθ

3.2 Infrared spectroscopy The FTIR spectra of MgO (SPC) and MgO (SG) powders are shown in Figure 2. The spectra exhibit SPC MgO and SG MgO two absorption bands vibration, characteristic of hydroxyl ion Mg(OH)2 with different peak intensities and position [1630 and 1450 cm-1] which may be caused by different neighbors . The presence of hydroxyl ions in both MgO is probably due to the humidity of the environment, due to the high sensitivity of MgO to water [16]. An others hydroxyl group

5

absorption peaks in the region of 4000 to 3000 cm-1 were observed in the same spectra. To confirm this result, thermal analyses DTA-TGA were performed as shown in (Fig. 3a). On can observe that a mass loss of about 8% at 420 °C correspond to dehydration of Mg(OH)2 and formation of MgO for the material prepared by SPC as shown in the flowing equation 1. Mg(OH)2

MgO + H2O

(1)

Such phenomenon is not observed in case of MgO (SG) sample, as illustrated by absence of weight loss only endothermic phenomenon in the DTA/TGA spectra of this material (Fig. 3b) obtained. It was reported that, water amount removed from Mg(OH)2 product is about 30% [17] which is higher than that the ration found in our work for the SPC MgO this may be explain the partial hydration of MgO (SPC) by atmospheric humidity during its storage or handling.

3.3 Structural studies SEM photographs of MgO (SPC) and MgO (SG) are depicted in Figure 4a. It is obvious that the SPC methods the MgO are formed under aggregates about 10 um. Whereas the MgO elaborated by SG method do it slower formation in the annealing time lading to small aggregates (Fig.4b); as it well be confirmed by the specific surface study using BET methods. We have obtained that the SBET (MgO SPC) = 16.55 m2 g-1 whereas the SBET (MgO SG) = 8.30 m2 g-1 which did not reflect the reality of the anemometrical MgO prepared with both methods as obtained by XRD measurements. 3.4 Conductivity study

6

The complex impedance spectroscopy study was performed on MgO (SPC) and MgO (SG) to clarify the nature and mode of conduction of the materials. Impedance measurements were performed in the temperature range 392-572 °C for MgO (SPC) and 558-672 °C for MgO (SG). Results are shown as Nyquist diagram in Figures 5 and 6. Complex impedance diagrams reflecting the variation of the imaginary part Z'' as function of the real part Z' show that the experimental points are situated on arcs of circles which do not pass through the origin, and centered below the real axis when temperature over 414 °C. It can be seen clearly that resistances decreases with temperature increases. This indicates a thermally activated conduction [18, 19]. The equivalent circuit of these materials could be constituted by a series connection of a resistor R1, together with a resistor R2 and a capacitor C in parallel as shown in Figure 5b. It should be noted that the value of R1 becomes zero when the temperature T is lower than 414 °C in case of MgO (SPC). Whereas for MgO (SG), the equivalent circuit may be described as a series connection of a resistor R1 together with a capacitor C in parallel with resistor R2. Furthermore, the graphical representation of conductivity variation v.s 1000/T (Fig. 7) confirms the evolution of the electric character with temperature as shown in eq (2). Ln (σT) = A exp(-Ea/KT)

(2)

Arrhenius behavior σ T = A exp(-Ea/KT) is observed for both samples MgO(SPC) and MgO(SG) in the ranges temperature study. Moreover with regard to MgO (SPC) the change in slope, observed between 426 and 451 °C, which corresponds to activation energies respectively which are equal to 4.14 and 0.39 eV, it may be explained from the endothermic dehydration as mentioned in eq (1). Furthermore, when T more than 451 °C, the ionic conduction

7

of MgO is probably assured by the O2- ion and the activation energy in this case is Ea ≈ 0.39 eV. In the case of MgO (SG), the graph (Ln (σT) vs. 1000/T is characterized by a single straight line. This result is confirmed by thermal analysis where no chemical reaction in the studied interval is observed. The activation energy is equal to 1.49 eV which is lower than of MgO (SPC). This result could be explained by the presence of carbon formed during the incomplete combustion. Moreover, the conductivity (σ) of MgO (SPC) increases from 2.810-3 S m-1 to 9.510-1 S m-1 (34 times) when the temperature rises from 393 to 546 °C. In case of MgO (SG), σ increased from 4.210-5 S m-1 to 4.310-3 S m-1 (~ 100 times) when the temperature goes from 514 to 753 °C. 3.5 Dielectric study To determine the conduction mechanism of these materials, the modulus complex formalism was used. M*=1/e*=1/ (M’ + iM”) (3) where M’=ɛ’/ (ɛ’2 + ɛ”2) (4) M”=ɛ”/ (ɛ’2 + ɛ”2) (5)

' 

 ''

1 Z' L Z'  2 2 2 C0 Z '  Z ' '   0tl Z '  Z ' '2

1 Z'' L Z"  2 2 2 C0 Z '  Z ' '  0tl Z '  Z "2

(6)

(7)

The parameters of charge carriers, such as their frequency hopping and relaxation time could be determined by this modulus complex. The advantage of this formalism with respect to the complex impedance is to avoid problems related to grain boundary effects, polarization of electrodes and other inter-facial effects in solid electrolytes [20]. In order to verify the contribution of these phenomena in the conduction process, we considered the variation of the real part M,’ as seen above, of the complex modulus at different temperatures with in frequencies (Fig. 8). The graphical 8

representation shows that M’ tends to a constant limit value M’∞ = 1/ε'∞ for high frequencies in the temperature range study. In addition, for low frequencies, M’ goes to zero .These findings indicate that the above mentioned phenomena can be neglected when the electric data are analyzed by this formalism [21]. These spectra show asymmetrical relaxation peaks which were centered in the scattering region M˝/M˝max. The Extremum of these peaks is the fp frequency [22, 23], this extrumum shifts to the high frequency values when the temperature increases as shown in (Fig. 9). It should be noted that the left domain frequencies of the peak corresponds to long distance movements of mobile ions. However, the right one of the peak corresponds to the confined ions in their potential wells. The small area where the maximum peak relaxation is observed indicates a change in decreasing frequency corresponds to short-distance mobility into a long distance movement, of disordered cations [24]. The most probable relaxation time τσ is determined from the relationship ωτσ=1 witch ω = 2πfpmax [25].where fpmax is the frequency at maximum of the peak of M˝/M˝max. MgO (SPC) relaxation time τσ is lower than that of MgO (SG). In both cases relaxation time decreases when temperature increases. In order to determine the ionic conduction mechanism we plot the variation of Ln (σT) and Ln (fp,max) as a function of 1000/T as shown in Figure 10. It can be seen from the figure an important change in relaxation frequency fp= 1/2πτσ and electrical conductivity was remarked. The experimental data of fp are suitably fitted according Arrhenius low: fp max=fp0exp (-Ea(fp)/kT) (8) Where Ea (fp) is the activation energy of the relaxation process which is determined from the variation of Ln (fpmax) versus 1000/T. Figure 10 regroups the plot Arrhenius law for both MgO SPC and MgO SG. From the Fig. 10a, three linear regions in three different temperature domains for MgO

9

SPC are observed. These regions correspond to three activation energies: Ea1(fp)= 0.46eV, Ea2(fp) = 1.52 eV and Ea3(fp) = 0,054 eV corresponding to three temperature domains [380-402 °C], [402-421 °C] and [421-450 °C] respectively. For MgO SG, there are only two linear regions in two different temperature domains. So, the activation energies are Ea1(fp)= 0.03 eV and Ea2(fp)= 0.31 eV corresponding to [560630°C] and [650-760 °C] respectively, as shown in Fig. 10b. The difference between the values of activation energy deduced from conductivity  (Ea(σ)) and relaxation frequency (Ea(fp)) do not reveal the hopping mechanism [26] either for MgO SPC or MgO SG. Figure 11 show the plot of total electrical conductivity(σt) within frequencies at different temperatures. It is clear from theses plots that high and low frequencies regions can be separated by a change in slope at determined value of frequency f for each temperature. This frequency, as it is known, is the frequency relaxation. The dependency between conductivity and frequency for different temperatures, for both types of MgO, can be explained by the universal law of Jonscher (Universal Power Law UPL) [27] σt(wT) = σdc(T) + Aωs = σac + σdc (9) Where σac and σdc are the AC and DC conductivities, due to band conduction, σdc(T) is the conduction of the current system, A and s are constants dependent on temperature. Figure 11 presents the plot of Ln(σt) versus Lnf (KHz). The results show that the Ea(dc) is almost equal to Ea(ac) and σdc are lower than σac for the two elaboration methods of MgO. Furthermore, it is noted that σdc magnitude increases about 50 times when the temperature increases from 393 to 456 °C for MgO (SPC) and from 514 to 753 °C for MgO (SG). The exponent “s” is calculated from the slope of straight-line fit of experimental data at high frequency region as indicated in Fig. 12. When temperature increases, “s” values decrease from 4.2 to 2 for the

10

MgO SG and from 2 to 1.8 for MgO SPC. The exponent “s” has a range of values between 0.6 and 1 depending on the material. However there are exceptions with “s” having a value much lower than 0.6 or higher than 1 [28]. Moreover, when “s” becomes more than 1, a localized hopping of the species with a small hopping and/or reorientational motion without leaving the neighborhood [29, 30].

4 Conclusions MgO SPC and MgO SG powders are synthesized respectively by chemical precipitation and sol-gel methods. The crystallites sizes of the materials are 28.4 nm and 42.5 nm for MgO SPC and MgO SG respectively. This analysis shows a partial hydration of MgO SPC and negligible weight loss for MgO SG. The dielectric study show that the equivalent circuit is generally composed of a resistor in series with a parallel assembly of resistor and capacity. The orders of conductivity magnitude are increased by 34 and 100 times for MgO SPC and MgO SG respectively, but with respect to the Arrhenius law. Complex impedance spectroscopy shows that conduction mechanism is not hopping mechanism for both regimes. Future work will be focused on the variation of “s” with temperature and its related phenomenon. References 1. L. Kumari , W. Z. Li , C.H. Vannoyb, R.M. Leblanc, D.Z. Wang, Cera. Inter. 35 3355 (2009) 2. B.M. Choudary, R.S. Mulukutla, K.J. Klabunde, J. Am. Chem. Soc. 125 2020 (2003) 3. T. Karasuda, K. Aika, J. Catal. 171, 439 (1997) 4. P. Jeevanandam, K.J. Klabunde, Langmuir 18, 5309 (2002) 11

5. R. Kakkar, P.N. Kapoor, J. Phys. Chem. B 108, 18140 (2004) 6. J.V. Stark, K.J. Klabunde, Chem. Mater. 8 1913 (1996) 7. J. Sawai, H. Kojima, H. Igarashi, A. Hashimoto, S. Shoiji, T. Sawaki, A. Hakoda, E. Kawada, T. Kokugan, M. Shimizu, World J. Micro. Biol. 16 187 (2000) 8. A.N. Copp, J. Am. Ceram, Soc. Bull. 74 135 (1995) 9. A. Bhargava, J. A. Alarco, I. D. Mackinnon, D. Page, A. Iiyushechkin, Mater. Lett, 34 133 (1998) 10. Q. Yang, J. Sha, L. Wang, X.Y. Ma, J. Wang, D.R. Yang, Nanotechnology 15 986 (2005) 11. L. Chen, X. Sun, Y. Liu, Y. Li, Appl. Catal. A 265, 123 (1999) 12. D. Gulkova, O. Solcova, M. Zdrazil, Micropor. Mater. 76 137 (2004) 13. A. Khaleel, R. S. Mulukutla, I. Mishakov, V. Chesnokov, A. Volodin, V. Zaikovski, N. Sun, K.J. Klabunde, Acta Mater. 11 459 (1999) 14.V. Stengl, S. Bakardjieva, M. Marıkova, P. Bezdicka, J. Subrt, Mat. Let. 57 3998 (2003) 15.Y. Ding, G. Zhang, H. Wu, B. Hai, L. Wang, Y. Qian, Chem. Mater. 13 435 (2001 ) 16.J.A. Wang, O. Novaro, X. Bokhimi M.E. Llanos, T. Lopez, E. LopezSalinas, R. Gomez, J. Navarrete. Mat. Let. 35 317 (1998) 17.I.F. Mironyuk , V.M. Gun’ko , M.O. Povazhnyak , V.I. Zarko ,V.M. Chelyadin , R. Leboda , J. Skubiszewska-Zie, W. Janusz, App. Sur. Sci. 252 4071 (2006) 18. I. Madhi, M. Saadoun, B. Bessais, Proc. Eng. 47 192 (2012) 19. A. Madani, A. Cheikh-Amdouni, A. Touati, M. Labidi, H. Boussetta, C. Monty, Sen. Act. B 8 109 (2005)

12

20.I.F. Mironyuk, V.M. Gun’ko, M.O. Povazhnyak, V.M. Chelyadin, R. Leboda, J. Skubiszewska-Zie˛ba, V.I. Zarko, W. Janusz, App. Sur. Sci. 252 4071 (2006) 21.H. Ambrus, C.T. Mohnihan, P.B. Macedo , J. Phys. Chem. 76 3287 (1972) 22.F. Mounir , H.N. Karima , B.S. Khaled , F. Mokhtar, Physica B. 407 2593 (2012) 23.B. Louati, K. Guidara, Mat. Sci. Eng. B. 177 838-843 (2012) 24.F.S. Howel, R.A. Bose, P.B. Macedo, C.T. Moynihan, J. Phys. Chem. 78 639 (1974) 25.H.K. Patel, S.W. Martin, Phys. Rev. B, 45 10292 (1992) 26.B.V.R. Chowdari R. Gopalakrishnan, So. Sta. Ion., 125 193 (1999) 27.R.M. Hill, A.K. Jonsche, Contemp. Phys. 24 75 (1983) 28. G. Raju Gorur, Dielectrics in electric fields (University of WindsorWindsor, Ontario, Canada, 2003), pp.161-169 29.H. Nefzi, F. Sediri, H. Hamzaoui, N. Gharbi, J. Solid State Chem. 190 150 (2012) 30.K. Funke, Prog. Solid State Chem. 22 111 (1993)

13

Figure captions Fig. 1. X-ray patterns of MgO SPC and MgO SG. Fig. 2. FT-IR spectra of MgO SPC and MgO SG. Fig. 3. TG (1) and DT (2) curves of heating of MgO SPC (a) and MgO SG (b). Fig. 4. SEM micrographs of (a) MgO SPC and (b) MgO SG. Fig. 5. Impedance spectra of the MgO SPC interface at different temperatures. Fig. 6. Impedance spectra of the MgO SG interface at different temperatures. Fig. 7. Arrhenius conductivity diagrams of MgO SPC and MgO SG. Fig. 8. Variation of log(M') versus log(f) at different temperatures of (a) MgO SPC and (b) MgO SG. Fig. 9. Plots of the modulus (M"/M" max) versus Log (f) of (a) MgO SPC and (b) MgO SG at various temperatures. Fig. 10. The relaxation frequency fp and the conductivity as a function of temperature of (a) MgO SPC and (b) MgO SG. Fig. 11. Variation of the total conductivity Log(f) at different temperatures of (a) MgO SPC and (b) MgO SG. Fig. 12. Variation of "s” versus temperature of MgO SPC and MgO SG.

14

Table captions Table 1. τσ of MgO SPC and MgO SG at different temperatures. Table 2. σdc of MgO SPC and MgO SG at different temperatures.

15

Figure captions

16000

60000

(200)

MgO (SPC)

14000

50000

MgO (SG)

Intensity (a.u.)

12000 40000

10000 8000

30000

(220)

6000

20000

4000

(222) (311)

(111) 2000

10000

0 0

10

20

30

40

50

2(degrees)

Fig. 1

16

60

70

80

90

0 100

Fig. 2

17

0

105

a

M g O (S P C )

9 8 .3 3 %

100

-2

-4

T D (V )

w eight loss(% )

(1 ) 95

-6

(2 )

9 0 .3 3 % 234°C

90

-8

8 7 .1 7 %

-1 0

85

420°C 80

100

200

300

400

-1 2

500

600

700

800

900

1000

T e m p e r a tu r e (°C )

0

100 98

b

9 9 ,8 1 %

- 0 ,7 0 %

96

M g O (S G )

9 9 ,1 3 % (1)

-5

-10

92

(2)

90

-15

88

338 °C

86

-20

84 82 80 100

200

300

400

500

600

T em p era tu re (°C )

Fig.3

18

700

800

900

-25 1000

T D (V )

w eight loss(% )

94

Fig. 4

19

7500

a

7000 6500

3 9 2 °C 3 9 4 °C 3 9 8 °C 4 0 2 °C 4 0 6 °C 4 1 0 °C 4 1 4 °C

M g O (S P C )

6000 5500 5000

   

4500 4000 3500 3000 2500 2000 1500 1000 500 0 0

650

2000

b

600 550

4000

6000

     

8000

10000

12000

418°C 422°C 426°C 430°C 435°C 440°C 445°C 450°C 450°C 460°C 470°C 480°C 490°C 500°C 510°C 520°C

M gO ( SPC )

500 450

|z" (   |

400 350 300 250 200 150 100 50 0

0

50

100

150

200

250

300

350

z '( 

Fig. 5

20

400

450

500

550

600

650

500000

a

450000

5 5 8 °C 5 6 3 °C 5 6 7 °C 5 7 0 °C 5 7 5 °C 5 8 4 °C 5 8 8 °C 5 9 3 °C 5 9 8 °C

M g O (S G )

400000 350000

 Z " ( )

300000 250000 200000 150000 100000 50000 0 0

200000

50000

b

175000

100000 150000 200000 250000 300000 350000 400000 450000 500000

Z '( )

6 0 0 °C 6 0 3 °C 6 0 8 °C 6 1 2 °C 6 1 7 °C 6 2 2 °C 6 2 7 °C 6 3 2 °C 6 3 7 °C 6 4 0 °C 6 4 7 °C 6 5 2 °C 6 5 7 °C 6 6 2 °C 6 6 7 °C 6 7 2 °C

M g O (S G )

150000

 Z " ( 

125000 100000 75000 50000 25000 0 0

25000

50000

75000

100000

Z '( 

Fig. 6

21

125000

150000

175000

200000

0,0

T=426°C

-0,5

Ea(=0.39eV

-1,0 -1,5 -2,0 -2,5 -3,0

Ea()=4.13eV

Ln( T )

-3,5 -4,0 -4,5

Ea()=1,49eV

-5,0 -5,5

MgO (SPC) MgO (SG)

-6,0 -6,5 -7,0 -7,5 -8,0 -8,5 0,9

1,0

1,1

1,2

1000/T( K-1)

Fig. 7

22

1,3

1,4

1,5

-1 0

a

-1 2

3 8 8 °C 3 9 0 °C 4 1 0 °C 4 5 0 °C 5 0 0 °C 5 5 0 °C 5 9 0 °C

M gO ( SPC )

-1 4 -1 6

L n (M ')

-1 8 -2 0 -2 2 -2 4 -2 6 -2 8 -3 0

-0 ,5

0 ,0

0 ,5

1 ,0

1 ,5

2 ,0

2 ,5

3 ,0

3 ,5

4 ,0

4 ,5

5 ,0

L n f (K H z ) -9 -1 0 -1 1

b

5 6 7 °C 6 0 0 °C 6 2 2 °C 6 8 2 °C 7 0 2 °C 7 5 7 °C

M g O (S G )

-1 2 -1 3

L n (M ')

-1 4 -1 5 -1 6 -1 7 -1 8 -1 9 -2 0 -2 1 -2 2 0 ,0

0 ,5

1 ,0

1 ,5

2 ,0

2 ,5

L n f (K H z )

Fig. 8

23

3 ,0

3 ,5

4 ,0

4 ,5

1 ,1

a

1 ,0 0 ,9

4 1 2 °C 4 1 4 °C 4 1 8 °C 4 2 0 °C 4 2 2 °C 4 2 4 °C 4 2 6 °C 4 3 0 °C

M gO ( SPC )

0 ,8 0 ,7

M " /M "

m ax

0 ,6 0 ,5 0 ,4 0 ,3 0 ,2 0 ,1 0 ,0

-0 ,1 -0 ,5

0 ,0

0 ,5

1 ,0

1 ,5

2 ,0

2 ,5

3 ,0

3 ,5

4 ,0

4 ,5

L n f(K H z) 1 ,0

b

5 6 7 °C 6 0 0 °C 6 2 2 °C 6 8 2 °C 7 0 2 °C 7 5 7 °C

M g O (S G )

M "/M " m ax

0 ,8

0 ,6

0 ,4

0 ,2

0 ,0 0 ,0

0 ,5

1 ,0

1 ,5

L n f(K H z )

Fig. 9

24

2 ,0

2 ,5

3 ,0

3 ,5

0 ,5

-1 ,0

L n ( T )

2 ,4

E a ( f p ) = 0 ,0 9 e V

E a (  ) = 0 ,3 9 e V

-1 ,5

a

M g O (S P C )

E a ( fp ) = 0 ,0 5 e V

-2 ,0

2 ,3

E a ( f p ) = 0 ,4 4 e V

-2 ,5

E a () = 4 ,1 3 e V

-3 ,0

2 ,2

2 ,1

L n ( f p m ax )

0 ,0 -0 ,5

-3 ,5 -4 ,0

2 ,0

-4 ,5 -5 ,0

1 ,2 0

1 ,2 5

1 ,3 0

1 ,3 5

1 ,4 0

1 0 0 0 /T K -1

1 ,4 5

1 ,5 0

4 ,0

M g O (S G )

-3

b

E a ( fp ) = 0 ,0 3 e V

-4

1 ,9 1 ,6 0

1 ,5 5

3 ,8 3 ,6 3 ,4

E a (  ) = 1 ,4 9 e V

-6

L n(f p m ax )

L n(  T )

-5

3 ,2

-7

3 ,0

-8

2 ,8

E a ( f p ) = 0 ,3 0 e V

-9

2 ,6 0 ,9 6

0 ,9 8

1 ,0 0

1 ,0 2

1 ,0 4

1 ,0 6

1 ,0 8

1 ,1 0

1 ,1 2

1 0 0 0 /T ( K -1 )

Fig. 10

25

1 ,1 4

1 ,1 6

1 ,1 8

1 ,2 0

1 ,2 2

5

a

4 3

E

L n D C

2 1

dc

= 0 ,3 9 0 e V

-6

E

0

Ln (  t )

-1

3 8 8 °C 3 9 0 °C 4 1 0 °C 4 5 0 °C 5 0 0 °C 5 5 0 °C 5 9 0 °C

M g O (S P C )

-3

= 4 ,4 6 7 e V

dc

-9

-2

1 ,2

1 ,3

1 0 0 0 /T K

-3

-1

1 ,4

1 ,5

-4 -5 -6 -7 -8 -9

-0 ,5

0 ,0

0 ,5

1 ,0

1 ,5

2 ,0

2 ,5

3 ,0

3 ,5

4 ,0

4 ,5

5 ,0

L n f(K H z) 0

- 6 ,0

b

-1 -2

- 6 ,5

- 7 ,5

E a = 1 ,4 8 e V

L n d c

- 8 ,0

-3

5 6 7 °C 5 7 0 °C 6 0 0 °C 6 2 2 °C 6 8 2 °C 7 0 2 °C 7 5 7 °C

M g O (S G )

- 7 ,0

- 8 ,5 - 9 ,0 - 9 ,5

-4

- 1 0 ,0 - 1 0 ,5

L n (  t)

-5

- 1 1 ,0 0 ,9 5

1 ,0 0

1 ,0 5

1 ,1 0

1 ,1 5

1 ,2 0

1 ,2 5

-1

1 0 0 0 /T (k )

-6 -7 -8 -9 -1 0 -1 1 -0 ,5

0 ,0

0 ,5

1 ,0

1 ,5

2 ,0

2 ,5

L n f (K H z )

Fig. 11

26

3 ,0

3 ,5

4 ,0

4 ,5

5 ,0

2,2

5,0

MgO (SPC) MgO (SG)

2,1

4,5 4,0

S(T)

2,0

3,5

1,9

3,0 2,5

1,8

2,0 1,7

1,6

1,5 1,0 380 400 420 440 460 480 500 520 540 560 580 600 620 640 660 680 700 720 740 760 780

Temperature (°C)

Fig. 12

27

Table captions

Table 1.

Temperature (°C)

MgO SPC

τσ (s)

MgO SG

550

40 10-5

59 10-5

560

39 10-5

58.3 10-5

570

39 10-5

58.3 10-5

580

39 10-5

57.3 10-5

Table 2.

MgO SPC

MgO SG

Temperature (°C)

393

456

514

753

σdc (S m-1)

8 10-4

4 10-2

2.6 10-5

1.3 10-3

28