Influence of thermal and thermoelectric treatments on structure and electric properties of B2O3–Li2O–Nb2O5 glasses

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Journal of Non-Crystalline Solids 354 (2008) 901–908 www.elsevier.com/locate/jnoncrysol

Influence of thermal and thermoelectric treatments on structure and electric properties of B2O3–Li2O–Nb2O5 glasses M.P.F. Grac¸a b

a,*

, M.G. Ferreira da Silva b, M.A. Valente

a

a Physics Department (I3N), Aveiro University, 3800-193 Aveiro, Portugal Glass and Ceramic Engineering Department (CICECO), Aveiro University, 3800-193 Aveiro, Portugal

Received 20 January 2007; received in revised form 27 July 2007 Available online 27 September 2007

Abstract A transparent glass with the composition 60B2O3–30Li2O–10Nb2O5 (mol%) was prepared by the melt quenching technique. The glass was heat-treated with and without the application of an external electric field. The as-prepared sample was heat-treated (HT) at 450, 500 and 550 C and thermoelectric treated (TET) at 500 C. The following electric fields were used: 50 kV/m and 100 kV/m. Differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman, dc and ac conductivity, as a function of temperature, were used to investigate the glass and glass-ceramics properties. LiNbO3 crystals were detected, by XRD, in the 500 C HT, 550 C HT and 500 C TET samples. The presence of an external electric field, during the heat-treatment process, improves the formation of LiNbO3 nanocrystals at lower temperatures. However, in the 550 C HT and in the TET samples, Li2B4O7 was also detected. The value of the rdc decreases with the rise of the applied field, during the heat-treatment. This behavior can indicate an increase in the fraction of the LiNbO3 crystallites present in these glass samples. The dc and ac conduction processes show dependence on the number of the ions inserted in the glass as network modifiers.The Raman analysis suggests that the niobium ions are, probably, inserted in the glass matrix as network formers.These results reflect the decisive effect of temperature and electric field applied during the thermoelectric treatment in the structure and electric properties of glass-ceramics.  2007 Elsevier B.V. All rights reserved. PACS: 63.50; 72.20; 77.22; 77.84 Keywords: Oxidation reduction; Glass-ceramics; Raman scattering; X-ray diffraction; Conductivity; Dielectric properties, relaxation, electric modulus; Scanning electron microscopy

1. Introduction In recent years there has been a considerable amount of interest in the preparation, and study of the physical properties of nanoparticles with ferroelectric properties (BaTiO3, PbTiO3, LiNbO3, LiTaO3, etc.) embedded in a glass matrix [1,2]. There are advantages in the choosing of a glass matrix, as a host of the ferroelectric crystallites, because the glass nanocomposites have a lower level of porosity. It is also possible to engineer their microstructure

*

Corresponding author. E-mail address: [email protected] (M.P.F. Grac¸a).

0022-3093/$ - see front matter  2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2007.08.016

using heat-treatment processes, controlling the crystalline sizes, the number and the volume fraction of the ferroelectric material. Another advantage is the interesting dielectric, electro-optic and non-linear optical properties that they exhibit [1]. Lithium niobate (LiNbO3) is an important ferroelectric material due to its excellent pyroelectrical, piezoelectrical and photorefractive properties [3,4] and it is used for the fabrication of active waveguides, modulators, and switches for application in integrated optical circuits [5]. The LiNbO3 crystals have a high Curie temperature, Tc = 1210 C [6]. The usual preparation of LiNbO3 crystals, by the Czochralski method, is time consuming, very expensive and the obtained crystals exhibit Li deficiency [3,4]. Glass-ceramics with the ferroelectric crystal

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phase seems to be an interesting alternative due to its relatively low preparation time and costs [3,4,7]. The main goal of the present work is the preparation of the 60B2O3–30Li2O–10Nb2O5 (mol%) glass composition, by the melt-quenching method, and glass-ceramics with LiNbO3 particles through a heat-treatment process. The study of the effect of the application of an external electric field, during the treatment cycle is another purpose of this study. In the text HT indicates heat-treatment and TET the heat-treatment with an applied electric field (thermoelectric treatment). The structural and electric changes of the samples, due to the treatment conditions, were studied. The use of B2O3, a typical glass former, is justified by its known properties and applications as nonlinear optical material and as laser host [8]. 2. Experimental 2.1. Glass and glass-ceramic preparation A glass with the molar composition 60B2O3–30Li2O– 10Nb2O5 was prepared by melt-quenching. This composition enables the formation of a transparent glass suitable for optical applications. The glass was prepared from reagent grade boron oxide (B2O3 – Merck), lithium carbonate (Li2CO3 – Merck) and niobium oxide (Nb2O5 – Merck). The reagents, in the appropriate amounts, were mixed for 1 h, in an agate ball-milling planetary system. The mixture was heated in a platinum crucible at 700 C, for 2 h (to remove the CO2 from the Li2CO3) and melted at 1100 C for 30 min. The molten material was quenched by pouring on a stainless steel plate and pressing by a second plate, both at room temperature, to obtain flat samples with thickness of 103 m, approximately. The result was a colorless and transparent glass. This glass was annealed at 300 C (during 1 h) to eliminate internal stress and slowly cooled to room temperature (as-prepared sample). To obtain the glass-ceramics, the as-prepared sample was heat-treated (HT) in air, in a horizontal tubular furnace, at 450, 500, 550 and 600 C (during 4 h) with a heating rate of 75 C/h. These temperatures were chosen in agreement with the result of the differential thermal analysis of the as-prepared sample. The thermoelectric treatments (TET) were performed in a vertical furnace where the as-prepared sample is placed between two platinum electrodes, which are connected to a high dc-voltage supply (PS325 – Stanford Research System), working between 25 V and 2500 V. The samples were TET at 500 C, during 4 h with the same heating rate used in the HT process, with an applied electric field of 50 kV/m (500B sample) and 100 kV/m (500C sample). The applied field was shut off during the cooling process of the sample. The values of the applied voltage were chosen in agreement with the samples thickness and the desirable value of the electric field (50 kV/m or 100 kV/m). The 100 kV/m was the highest electric field that could be applied without the appearance of dark zones in the sample. For electric fields larger than

100 kV/m the samples present dark zones beginning at the cathode and growing until the anode with the increasing of the treatment time. The HT and TET of the as-prepared sample were repeated, for checking the reproducibility. 2.2. Structural measurements The differential thermal analysis (DTA) was performed in a Lynseis Apparatus typ L92/095, in the temperature range of 20–800 C, with a heating rate of 5 C/min using Al2O3 as reference. The X-ray diffraction patterns were obtained at room temperature, using bulk samples, in a Philips X’Pert sys˚ ) at 40 kV, and tem, with a Ka radiation (k = 1.54056 A 30 mA, with a step of 0.05 and a time per step of 1 s. The Raman spectroscopy, of bulk samples, was performed in a back-scattering geometry, between 100 and 2000 cm1, in a T64000, Jobin Yvon SPEX spectrometer. A microscope objective (·50) focused the exciting light (Ar laser, k = 514.5 nm) onto the sample (spot diameter < 0.8 lm). The scanning electron microscopy (SEM) was performed in a Philips XL 30 system on the surface and fracture surface of the samples (covered with gold before microscopic observation). 2.3. Electrical measurements For the electrical measurements the opposite sides of the samples were painted with silver paste. The electrical dc conductivity (rdc) measurements were made using a Keithley 617 programmable electrometer, as a function of the temperature, between 200 and 370 K, in steps of 5 K. The accuracy of the temperature control and measurement is less than 0.1 K over the whole range. After the temperature become stable a dc voltage of 100 V was applied and 10 s later, the dc current measurement begins with 10 consecutive readings, in steps of 2 s. During these measurements the samples were in a helium atmosphere, to improve the heat transfer and eliminate the moisture. The ac conductivity (rac) was measured using a Solartron SI 1260 Impedance/gain-phase analyzer, in the temperature range of 270–315 K, measuring the real and the imaginary part of the sample impedance (Z* = Z 0  jZ00 ) in the frequency range of 10 mHz to 32 MHz. The analytical background, used in the electrical data analysis, was as follows: (a) The Arrhenius expression (Eq. (1)) has been used to fit the temperature dependence of the rdc [9–14]:   EaðdcÞ rdc ¼ r0 exp  ; ð1Þ kBT where r0 is a pre-exponential factor, Ea(dc) the activation energy, kB the Boltzmann constant and T the temperature. (b) The rac was calculated using rac ¼ xe0 e00 [11,12], where x represents the angular frequency, e0 the

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permittivity of the empty space and e00 de imaginary part of the sample permittivity (e*= e 0 + je00 ). The imaginary part of the complex permittivity (e00 ), was obtained using the complex impedance formalism, Z* = 1/(le*) (where l = jxC0, x is the angular frequency, C0 the capacitance of a vacuum (air) capacitor with plates identical to those formed by silver paint electrodes covering pffiffiffiffiffiffiffithe flat-parallel surfaces of the samples and j ¼ 1 [10–14]). In order to normalize the impedance data a Z rel was calculated by Z rel ¼ Z  ðA=dÞ, where A is the electrode area and d the sample thickness. The ac activation energy (Ea(ac)) was calculated using an Arrhenius expression similar to Eq. (1). 3. Results The DTA, of the as-prepared sample, shows two exothermic effects at 529 and 565 C and the glass transition temperature (Tg) is approximately 470 C (Fig. 1). The thermal treatments were made in agreement with DTA results. The photographs (Fig. 2) reveal the samples physical aspect, at the macroscopic level. The colorless and transparent as-prepared sample becomes translucent when treated at

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450 and 500 C (HT sample). The 500B, 500C and 550 HT samples are white and opaque (Fig. 2). In the 500 C TET sample, subject to an electric field of 250 kV/m, the presence of dark zones was observed. The anode and the cathode sides of the TET samples have similar aspect. The LiNbO3 crystalline phase was detected in the 500 C and 550 C HT samples and in the TET samples (500B, 500C) by XRD (Fig. 3). However, in the 550 HT, 500B and 500C samples, some low intensity diffraction peaks are assigned to lithium-diborate (Li2B4O7) phase. It should be point out that, for a better visualization, the XRD spectra (Fig. 3) has different multiplicative factors. The XRD patterns of the anode and cathode sides of the TET samples are very similar. The XRD patterns of as-prepared and 450 HT samples reveal the amorphous nature of these samples. The sample treated at 600 C shows the presence of the LiNbO3, Li2B4O7, LiNb3O8 crystalline phases and, also, some unidentified diffraction peaks. The Raman spectra (Fig. 4) of the as-prepared, 450 and 500 HT samples are similar and show the presence of the 870, 660 and 240 cm1 bands. The 240 cm1 band is also visible in the 550 HT, 500B and 500C samples. In these samples bands at 640, 438, 370–373, 333–336, 280 and 170 cm1 were detected. In the 550 HT sample spectrum (Fig. 4) a band at 955 cm1 and a shoulder at 710 cm1

0 -19

-5

-20

-21

-10 -22

ΔV (μV)

470 ºC

-15

-23 400

420

440

460

480

500

520

540

-20

-25 Exo -30

565 ºC

529 ºC

-35 100

200

300

400

500

600

700

800

Temperature (ºC) Fig. 1. DTA of the as-prepared sample.

Fig. 2. Optical photographs of the samples: (a) as-prepared; (b) 500 HT; (c) 500B; (d) 500C. The smaller scale is 1 mm.

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x

(this shoulder is also present in the 500C sample) were detected. This shoulder was observed at 690 cm1 in the 500B sample Raman spectrum. The SEM analysis was used to verify the microstructure changes of the as-prepared sample with the treatment conditions (Fig. 5). It can be observed an increase in the particles size and a decrease in the number with the increasing of the HT temperature. The 450 HT sample has the highest number of particles with maximum dimensions of 500 nm (Fig. 5(b)). In the 500 HT sample the particle size is 1 lm, approximately. An increase in the particle size and a decrease in the number of particles can be observed

x o

x x

ox

x

o

500C

x o

500B

Intensity (arb.units)

x

ox

x x

x

x

x

x

x

x

x x

x

x

550

x

o

o

o

oo o x

500

450

10

20

30

40

50

60

70

2 Theta

710

240

640

Fig. 3. XRD patterns collected from surface of the treated samples (peaks marked: · LiNbO3; s Li2B4O7).

690

640

438 438

336 373

280

170

371

500C

870

660

240

955

438

710

336 370

280

640

500B

170

Intensity (arb.units)

333

280

LiNbO3

550

500

100

200

300

400

500

600

700

800

Raman Shift (cm-1) Fig. 4. Raman spectra of all samples.

900

1000

Fig. 5. SEM micrographs of all samples: (a) as-prepared; (b) 450 HT; (c) 500C.

M.P.F. Grac¸a et al. / Journal of Non-Crystalline Solids 354 (2008) 901–908

with increasing the applied field during the heat-treatment. Thus, in the 500B sample the observed particles have dimensions of 2 lm and in the 500C sample the particle dimensions is 3 lm (Fig. 5(c)). The rise of the applied field promotes also an increase in the number of the parti-

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cles. These particles were observed in the samples free surface and in the fracture surface. The dc (rdc) and ac (rdc) conductivities, measured at room temperature (Table 1), decrease with the increase of the HT temperature and the applied electric field. The

Table 1 The dc conductivity (rdc) at 300 K, activation energy of the rdc (Ea(dc)), ac conductivity (rac) at 300 K and 1 kHz, activation energy of the rac (Ea(ac)) and dielectric constant (e 0 ) at 300 K and 1 kHz for all sample Sample

rdc (·108 Sm1)

Ea(dc) (kJ/mol)

rac (·107 Sm1)

Ea(ac) (kJ/mol)

e0

As-prepared 450 500 550 500B 500C

259.1 ± 3.5 171.9 ± 2.4 58.2 ± 0.7 1.17 ± 0.02 18.7 ± 0.3 10.6 ± 0.1

62.5 ± 0.7 62.4 ± 0.6 63.3 ± 0.6 66.1 ± 1.1 61.5 ± 0.9 67.2 ± 1.3

32.1 ± 1.1 22.9 ± 1.1 17.5 ± 5.7 0.10 ± 0.05 12.7 ± 0.4 10.8 ± 0.3

36.8 ± 1.1 30.9 ± 0.4 32.2 ± 1.2 – 30.0 ± 1.0 29.1 ± 0.5

18.8 ± 0.7 17.2 ± 0.8 17.9 ± 0.6 11.4 ± 0.5 16.9 ± 0.6 16.2 ± 0.4

-15

ln(σdc) [Sm-1]

-20

-25

-30

-35 2.5

3

3.5

4

4.5

1000/T [K-1]

Fig. 6. The ln (rdc) temperature dependence of all samples: j as-prepared; h 450;  500;  550; d 500B; s 500C. -14

ln(σac) [Sm-1]

-15

-16

-17

-18

-19 3.1

3.2

3.3

3.4

3.5

3.6

3.7

3.8

-1

1000/T [K ] Fig. 7. The ln (rac) as a function of temperature of all samples, at 1 kHz: j as-prepared; h 450;  500; m 550; d 500B; s 500C.

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3.14 kJ/mol

-9

1 kHz

-10

10 kHz

ln (σac)

-11

100 kHz

10.4 kJ/mol -12

1 MHz

-13 -14

31.3 kJ/mol -15

36.8 kJ/mol

-16 -17 3.1

3.2

3.3

3.4

3.5

3.6

3.7

3.8

-1

1000/T [k ] Fig. 8. The ln (rac) as a function of frequency of the as-prepared sample.

ln (rdc) – temperature dependence is presented in Fig. 6. The dc activation energy (Ea(dc)) of the as-prepared, 450 and 500 HT samples (Table 1) have similar values but, in the 550 HT sample, the dc activation energy increases. The 500B sample presents an Ea(dc) similar to that of the 500 HT sample. Nevertheless, the Ea(dc) increases with the increase of the applied electric field (Table 1). The ln (rac) temperature dependence, at 1 kHz, is shown in Fig. 7 and the ac activation energy (Ea(ac)) is registered in Table 1. The as-prepared sample presents the highest Ea(ac). In the other samples the Ea(ac) can be considered constant. In the case of the 550 HT sample the ln (rac) temperature dependence does not presents an Arrhenius behavior and the Ea(ac) could not be estimated. The temperature dependence of the rac, at several frequencies, for the as-prepared sample, is shown in Fig. 8. The increase of the frequency leads to an increase in the ac conductivity value and a decrease in the activation energy. This behavior is observed for all samples, with the exception of the 550 HT sample where the Ea(ac) could not be calculated (Fig. 7). The dielectric constant value, measured at 300 K and 1 kHz, for all samples is in Table 1. The increase of the heat treatment temperature leads to a decrease of e 0 . The TET samples present a e 0 value close to that of 500 HT sample.

550 C HT sample. The identification, in the XRD pattern of the 500 C HT sample (Fig. 3), of a peak that coincides with that of the main diffraction peak of the LiNbO3 crystal, suggests that the particles, with an average size of 1 lm (observed by SEM (Fig. 5(b))) are LiNbO3 crystallites. Comparing the XRD patterns of the 500 C HT sample and that of the TET samples (500B and 500C) (Fig. 3), where the LiNbO3 and Li2B4O7 phases were detected, it becomes reasonable to admit that the presence of an external electric field, during the heat treatment process, promotes the crystallization at lower temperatures. It is visible, at the macroscopic level (Fig. 2), the effect of a high applied electric field. Dark zones are present in the 500 C sample treated with an applied electric field of 250 kV/m, as already observed in the SiO2–Li2O–Nb2O5 glass system [15]. This phenomenon, in accordance with the Kusz [16] and Zeng [17] results, suggest the existence of an oxidation–reduction reaction in the samples, activated by the electric field. Thus, the oxidation–reduction reaction can be summarized in the following equations:

4. Discussion

Anodic equation : O2 ðglass networkÞ 1 ! O2 ðglass–anode interfaceÞ þ 2e 2 Cathodic equation : Liþ ðglass networkÞ þ e ! Li ðglass–cathode interfaceÞ

The 60B2O3–30Li2O–10Nb2O5 (mol%) composition, prepared by the melt-quenching technique, gives rise to a colorless and transparent glass. The as-prepared sample was submitted to thermal treatments with and without the presence of an external electric field. The presence of LiNbO3 and Li2B4O7 crystallites was detected by XRD (Fig. 3), by Raman spectroscopy (Fig. 4) and by SEM (Fig. 5), in the

However, considering the glass color change after the TET, other processes can be possible. For example, the reduction of Nb5+ to a lower oxidation state may occur [17,18]. The Raman spectroscopy results (Fig. 4) show the presence in the as-prepared, 450 and 500 HT samples, of the 870 cm1 band, that can be, in agreement with Alekseeva

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et al. [19], related to the vibrations of the NbO4 tetrahedra [20–22]. Gao et al. [19] refer the existence of a certain amount of this structural unit in the amorphous matrix of niobiosilicate glasses with low Nb2O5 content (<15 mol%) and with SiO2 (>56.7 mol%). For higher Nb2O5 content it has been shown that only NbO6 octahedra exist in the amorphous network [19]. This suggests that the niobium ions are, probably, inserted in the glass structure as network formers. When the treatment temperature increases, or an electric field is applied, the 870 cm1 band disappears (Fig. 4). Thus, the niobium ions that were initially inserted structurally in the glass network become part of the LiNbO3 crystalline phase. The vibration bands of this crystalline structure were observed in the 550 HT, 500B and 500C samples. The 640, 438, 370–373, 333–336, 280, 240 and 170 cm1 bands are assigned to the NbO6 octahedrons vibrations proceeding from LiNbO3 in crystalline form [21,22–26]. It was observed that with the increase of the applied field, during the thermal treatment, the width of these Raman bands decreases and the intensity increases (Fig. 4), suggesting that the presence of the electric field promotes the crystallization of LiNbO3. Moreover, comparing the XRD patterns of the 500 HT and 500TET samples, the intensity of the LiNbO3 main peak increases with the increase of the applied field and also the others diffraction peaks related to the LiNbO3 phase. In the samples HT at a temperature below 550 C, the presence of the Raman band around 660 cm1 (Fig. 5) is an indication of pentaborate groups presence in the glass network [26,27]. In the samples where the XRD detected the presence of lithium borate crystallites (550 HT, 500B and 500 C – Fig. 3) the 660 cm1 band disappears and new bands, at 690–710 cm1, are detected, indicating the presence of the chain type metaborate groups in the glass network [28]. It is known that vitreous B2O3 present a structure of linked triangles forming planar rings (boroxol rings) and the addition of network modifiers, to the B2O3 matrix, can result in tetrahedral boron formation [29]. The absence of bands centred at 770 cm1, 930 cm1 and 808 cm1 show that tetraborate, triborate, and boroxol rings groups are not detected in the present structures [30]. The increase of the treatment temperature and of the applied field amplitude leads to a decrease in the dc and ac conductivity (Table 1), indicating a decrease in the number of the network modifier ions and thus a decrease in the number of charge carriers. However, these electrical conduction processes also depends on the charge carriers mobility, which can be studied by the height of the free energy barriers [11]. The activation energy of the dc conductivity (Ea(dc)) is associated with the height of the free energy barriers. The Ea(dc) of the as-prepared, 450 and 500 C HT samples is very similar (Table 1). Thus, in these samples the rdc decreases, with the rise of the treatment temperature, must be assigned, essentially, to the decrease of concentration of the charge carriers. The in concentration of the charge carriers can be associated with the free Li+ ions, because the Raman spectroscopy indicates that

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some niobium ions, mainly in the as-prepared, 450 and 500 HT samples, are inserted in the network as glass matrix formers. The formation of Li2B4O7 crystals, which present a low conductivity (at room temperature r < 1010 X1 m1 [30]), and the presence of high resistivity LiNbO3 crystallites [3], that can be considered the main crystalline phase in these glass-ceramics (Fig. 3), will contribute to the decrease of the glass-ceramics conductivity. The observed increase of Ea(dc) in the 550 HT sample and in the TET samples (Table 1), that shows a decrease in the charge carriers mobility through the matrix network, and the decrease in the number of free ions justify the lower rdc value. It should be pointed out that for the used measurement temperature interval and for the 550 HT, 500A and 500B samples, the linearization of the rdc(T) data, using the Arrhenius formalism, shows low regression coefficients (r2 > 0.98) but better than those observed when using the Mott’s formalism [31]. The increase of rac, with the increase of the measurement temperature (Fig. 7), shows that the electric units mobility increases. With the increase of the treatment temperature, the rac decreases (Table 1), which can be related to the decrease of the free ions number and with the increase of the volume ratio between LiNbO3 particles and the glass phase. LiNbO3 crystals are, typically, difficult to depolarize at room temperature and thus their contribution to the rac decrease. This should be the justification for the rac behavior of the 550 HT sample (Fig. 7). The decrease of the e 0 value (Table 1), with the increase of the heat treatment temperature, suggests that the LiNbO3 crystals, that possess a e 0 > 1000 at room temperature and 1 kHz [32], are distributed in the glass network with a random orientation and thus their contribution to the dipolar moment is low. The TET samples (treated at 500 C) present a e 0 similar to that of the 500 HT sample indicating that, in this glass composition, the presence of an external electric field does not favor the LiNbO3 crystallization with a preferred orientation [9,15].

5. Conclusions The glass composition 60B2O3–30Li2O–10Nb2O5 (mol%) was prepared by the melt-quenching method and a colorless glass was obtained. LiNbO3 crystallites were precipitated in the glass matrix for heat-treatments above 500 C. The formation of lithium diborate phase (Li2B4O7) also occurred. The Raman spectroscopy shows that in the transition glass to glass-ceramic the glass network suffers a transformation. Thus, pentaborate groups are present in the glass structure and metaborate groups in the glass-ceramic structures. The presence of electric field, during the heat-treatment process, promotes crystallization at lower temperatures. The number of network modifier ions and the presence of LiNbO3 crystallites are the dominants factors in the electrical conduction process.

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