**8. 88Si samples composition results**

346 Heat Treatment – Conventional and Novel Applications

**Figure 24.** ε´ versus frequency, at 300 K, for the 92Si samples treated at 650 ºC.

**Figure 25.** ε´ versus frequency, at 300 K, for the 92Si samples treated at 700 ºC.

Figure 27 shows the macroscopic aspect of the 88Si sample composition, TT at 500, 600, 650, 700 and 800 ºC, during 4 hours. It can be seen that the as-prepared glass (TT at 500 ºC) is translucent and for TT above 700 ºC it becomes opaque.

**Figure 27.** Photographs of the 88 Si samples TT at temperatures between 500 and 800 ºC (the minor scale division = 1mm).

Figure 28 shows the XRD patterns of the samples TT. This spectrum shows the presence of LiNbO3 crystalline phases and cristobalite (SiO2), in the samples treated at temperatures above 650 °C. With the increase of the TT temperature up to 700 °C it was detected also the lithium silicate (Li2Si2O5) crystalline phase. In order to confirm the indexing of some diffraction peaks observed in the sample TT at 800 °C, the XRD was carried out in a new sample, TT at 800 °C but during 8h (sample 800-8h). Analyzing the pattern of this sample it is suggested the presence, in the samples treated at 800 °C, of the Li3NbO4 crystalline phase.

Lithium Niobiosilicate Glasses Thermally Treated 349

In figure 29 it is shown the Raman spectra of the surface of all 88Si TT samples. The Raman spectrum of the samples treated at temperatures below 600 °C shows the presence of broad bands centered at ~ 680 and ~ 280 cm-1. With the increase of the thermal treatment temperature, less wide bands but with higher intensity are detected. The bands centered at 630-680 and 240 cm-1 observed in samples TT at 500, 600 and 650 °C are due to vibrations of NbO6 octahedrons [16;17]. The bands at 440, 360, 334 and 280 cm-1, detected in the sample TT at 800 °C, are assigned to the vibration of NbO6 octahedrons, which are associated with the LiNbO3 crystal structure [16;17]. It must be noted the non-detection of Raman bands associated with vibrations of the type Si-O-Si or Nb-O-Si. The non-detection of vibrations associated to Nb-Si-O bonds, which according to Lipovski et al. [24] should be present between 800 and 850 cm-1, suggests that the niobium ions are introduced into the glass matrix only as network modifier. Micrographs of the sample surface of the 88Si samples TT at 500ºC, 600, 650, 700 and 800 °C are shown in figure 30. The SEM micrographs show, in the surface of the sample heattreated at 600 °C, particles (Fig. 30b). The size of those particles, also observed in samples TT at 700 and 800 ºC is similar (150-200 nm). However, increasing the temperature of TT leads to an increase in the number of particles. The sample TT at 650 °C has a particle size of around 2 μm (Fig. 30c). The micrograph of this sample also present particle agglomerates

The dependence of the dc conductivity with the temperature of measurement, for all TT samples, is shown in figure 31. The Arrhenius model was used to adjust the ln(σdc) with the inverse of the measurement temperature, allowing the calculation of the activation energy (Ea(dc)) through the Arrhenius equation. From figure 31 it is verified the existence of two temperature zones, with different activation energies. The first zone (A) is between 230 and 300 K and second (B) is between 310 and 370 K. The as-prepared sample (TT at 500 °C) presents, for measuring temperatures above 300 K, a behavior that is non-adjustable

through the Arrhenius equation. The calculated values of Ea(dc) are shown in Table 3.

The ac conductivity (σac), measured at 300 K and 1 kHz, decreases with the increase of the TT temperature (Table 3). The activation energy, Ea(ac), was calculated using the Arrhenius formalism. The lines in figure 31 represent the result of that calculation and the obtained

Figures 34 to 41 show the dependence of Z'' with the frequency, at various measurement temperatures, for all TT samples. It was observed the existence of two dielectric relaxation mechanisms. The first, in the low frequency region (< 100 Hz) and the second in the high frequency region (> 1 kHz). For frequencies below 1 Hz, a high dispersion of the Z\* values is observed and assigned to the sensitivity of the measuring instrument in this low frequency region. The impedance spectra were adjusted to the electrical equivalent circuit model, shown in figure 33, using the CNLLS algorithm. Thus, in the spectra shown in figures 34 to 41, the lines represent the adjustment obtained with this fitting process. It is observed that with the increase of the temperature of measurement, there is a tendency for a better definition of the relaxation curves. Thus, and for the lower measuring temperatures it was found that the adjustment process diverged, not been possible to fit the experimental data

(Fig. 30d), not detected in any other sample.

values are in Table 3.

with this theoretical model.

**Figure 28.** XRD spectra of the 88Si samples TT at temperatures between 500 and 800 ºC (x LiNbO3; O Li2Si2O5; \* SiO2 (cristobalite); + Li3NbO4).

In figure 29 it is shown the Raman spectra of the surface of all 88Si TT samples. The Raman spectrum of the samples treated at temperatures below 600 °C shows the presence of broad bands centered at ~ 680 and ~ 280 cm-1. With the increase of the thermal treatment temperature, less wide bands but with higher intensity are detected. The bands centered at 630-680 and 240 cm-1 observed in samples TT at 500, 600 and 650 °C are due to vibrations of NbO6 octahedrons [16;17]. The bands at 440, 360, 334 and 280 cm-1, detected in the sample TT at 800 °C, are assigned to the vibration of NbO6 octahedrons, which are associated with the LiNbO3 crystal structure [16;17]. It must be noted the non-detection of Raman bands associated with vibrations of the type Si-O-Si or Nb-O-Si. The non-detection of vibrations associated to Nb-Si-O bonds, which according to Lipovski et al. [24] should be present between 800 and 850 cm-1, suggests that the niobium ions are introduced into the glass matrix only as network modifier.

348 Heat Treatment – Conventional and Novel Applications

**Figure 28.** XRD spectra of the 88Si samples TT at temperatures between 500 and 800 ºC (x LiNbO3; O

Li2Si2O5; \* SiO2 (cristobalite); + Li3NbO4).

Micrographs of the sample surface of the 88Si samples TT at 500ºC, 600, 650, 700 and 800 °C are shown in figure 30. The SEM micrographs show, in the surface of the sample heattreated at 600 °C, particles (Fig. 30b). The size of those particles, also observed in samples TT at 700 and 800 ºC is similar (150-200 nm). However, increasing the temperature of TT leads to an increase in the number of particles. The sample TT at 650 °C has a particle size of around 2 μm (Fig. 30c). The micrograph of this sample also present particle agglomerates (Fig. 30d), not detected in any other sample.

The dependence of the dc conductivity with the temperature of measurement, for all TT samples, is shown in figure 31. The Arrhenius model was used to adjust the ln(σdc) with the inverse of the measurement temperature, allowing the calculation of the activation energy (Ea(dc)) through the Arrhenius equation. From figure 31 it is verified the existence of two temperature zones, with different activation energies. The first zone (A) is between 230 and 300 K and second (B) is between 310 and 370 K. The as-prepared sample (TT at 500 °C) presents, for measuring temperatures above 300 K, a behavior that is non-adjustable through the Arrhenius equation. The calculated values of Ea(dc) are shown in Table 3.

The ac conductivity (σac), measured at 300 K and 1 kHz, decreases with the increase of the TT temperature (Table 3). The activation energy, Ea(ac), was calculated using the Arrhenius formalism. The lines in figure 31 represent the result of that calculation and the obtained values are in Table 3.

Figures 34 to 41 show the dependence of Z'' with the frequency, at various measurement temperatures, for all TT samples. It was observed the existence of two dielectric relaxation mechanisms. The first, in the low frequency region (< 100 Hz) and the second in the high frequency region (> 1 kHz). For frequencies below 1 Hz, a high dispersion of the Z\* values is observed and assigned to the sensitivity of the measuring instrument in this low frequency region. The impedance spectra were adjusted to the electrical equivalent circuit model, shown in figure 33, using the CNLLS algorithm. Thus, in the spectra shown in figures 34 to 41, the lines represent the adjustment obtained with this fitting process. It is observed that with the increase of the temperature of measurement, there is a tendency for a better definition of the relaxation curves. Thus, and for the lower measuring temperatures it was found that the adjustment process diverged, not been possible to fit the experimental data with this theoretical model.

Lithium Niobiosilicate Glasses Thermally Treated 351

(a) (b)

(c) (d)

**Figure 30.** SEM micrographs of the 88Si samples: a) as-prepared (TT at 500 ºC); b) TT600; c) TT650; d)

500 379,70 ± 9,86 50,95 ± 1,78 -- 20,10 ± 0,02 45,79 ± 0,53 600 163,71 ± 2,41 57,66 ± 1,03 31,66 ± 1,71 15,02 ± 0,62 31,71 ± 0,31 650 55,42 ± 0,74 66,24 ± 0,40 44,00 ± 0,94 2,49 ± 0,09 52,17 ± 2,27 700 120,82 ± 1,66 64,35 ± 0,43 32,76 ± 0,77 2,80 ± 0,12 45,60 ± 2,70 800 3,47 ± 0,05 61,02 ± 0,89 49,96 ± 0,72 0,60 ± 0,02 43,83 ± 2,33

**Table 3.** dc conductivity (σdc), at 300 K, dc activation energy (Ea(dc) ) for the: A - low temperature region

**Ea(dc) (B) [kJ/mol]**  σ**ac (x10-6) [**Ω**-1m-1]** 

(e) (f)

**Ea(ac) [kJ/mol]**

**Ea(dc) (A) [kJ/mol]** 

TT650; e) TT700; f) TT800.

**Sample** σ**dc (x10-8)**

**[**Ω**-1m-1]** 

(230-300 K); B – high temperature region (310-370 K).

**Figure 29.** Raman spectra of the 88Si samples TT at 120, 500, 600, 650, 700 and 800 ºC.

350 Heat Treatment – Conventional and Novel Applications

**Figure 29.** Raman spectra of the 88Si samples TT at 120, 500, 600, 650, 700 and 800 ºC.

**Figure 30.** SEM micrographs of the 88Si samples: a) as-prepared (TT at 500 ºC); b) TT600; c) TT650; d) TT650; e) TT700; f) TT800.


**Table 3.** dc conductivity (σdc), at 300 K, dc activation energy (Ea(dc) ) for the: A - low temperature region (230-300 K); B – high temperature region (310-370 K).

Lithium Niobiosilicate Glasses Thermally Treated 353

**Figure 33.** Equivalent electric circuit model used to adjust the impedance data of the 88Si samples

**Figure 34.** Z´´ versus frequency for the 88Si sample TT at 600 ºC (low frequency region). The lines

**Figure 35.** Z´´ versus frequency for the 88Si sample TT at 600 ºC (high frequency region). The lines

([R0(R1CPE1)(R2CPE2)]).

represent the theoretical adjust.

represent the theoretical adjust.

**Figure 31.** ln(σdc) versus 1000/T for all 88Si samples TT between 500 and 800 ºC.

**Figure 32.** ln(σac) versus 1000/T for the samples 88Si TT between 500 and 800 ºC.

Table 4 presents the parameter values of the electrical equivalent circuit for all analyzed samples. Note that the value of the parameter R0, representing the value of Z' when ω → ∞ was considered equal to 0 Ω. The parameters R1 and CPE1 (CPE represents a constant phase element [18]) are associated with the dielectric relaxation mechanism found at low frequencies (< 100 Hz) and the parameters R2 and CPE2 with the relaxation mechanism detected in the high frequency region.

352 Heat Treatment – Conventional and Novel Applications






**ln (**σ**ac) [Sm-1]**








**ln(**σ**dc) [Sm-1]**





**Figure 31.** ln(σdc) versus 1000/T for all 88Si samples TT between 500 and 800 ºC.

2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 **1000/T [K-1]**

500

600

700

650

800

**Figure 32.** ln(σac) versus 1000/T for the samples 88Si TT between 500 and 800 ºC.

detected in the high frequency region.

Table 4 presents the parameter values of the electrical equivalent circuit for all analyzed samples. Note that the value of the parameter R0, representing the value of Z' when ω → ∞ was considered equal to 0 Ω. The parameters R1 and CPE1 (CPE represents a constant phase element [18]) are associated with the dielectric relaxation mechanism found at low frequencies (< 100 Hz) and the parameters R2 and CPE2 with the relaxation mechanism

2.6 2.8 3 3.2 3.4 3.6 3.8 4 4.2 4.4 **1000/T [K-1]**

**Figure 33.** Equivalent electric circuit model used to adjust the impedance data of the 88Si samples ([R0(R1CPE1)(R2CPE2)]).

**Figure 34.** Z´´ versus frequency for the 88Si sample TT at 600 ºC (low frequency region). The lines represent the theoretical adjust.

**Figure 35.** Z´´ versus frequency for the 88Si sample TT at 600 ºC (high frequency region). The lines represent the theoretical adjust.

Lithium Niobiosilicate Glasses Thermally Treated 355

**Figure 39.** Z´´ versus frequency for the 88Si sample TT at 700 ºC (high frequency region). The lines

**Figure 40.** Z´´ versus frequency for the 88Si sample TT at 800 ºC (low frequency region). The lines

**Figure 41.** Z´´ versus frequency for the 88Si sample TT at 800 ºC (high frequency region). The lines

represent the theoretical adjust.

represent the theoretical adjust.

represent the theoretical adjust.

**Figure 36.** Z´´ versus frequency for the 88Si sample TT at 650 ºC (low frequency region). The lines represent the theoretical adjust.

**Figure 37.** Z´´ versus frequency for the 88Si sample TT at 650 ºC (high frequency region). The lines represent the theoretical adjust.

**Figure 38.** Z´´ versus frequency for the 88Si sample TT at 700 ºC (low frequency region). The lines represent the theoretical adjust.

354 Heat Treatment – Conventional and Novel Applications

represent the theoretical adjust.

represent the theoretical adjust.

represent the theoretical adjust.

**Figure 36.** Z´´ versus frequency for the 88Si sample TT at 650 ºC (low frequency region). The lines

**Figure 37.** Z´´ versus frequency for the 88Si sample TT at 650 ºC (high frequency region). The lines

**Figure 38.** Z´´ versus frequency for the 88Si sample TT at 700 ºC (low frequency region). The lines

**Figure 39.** Z´´ versus frequency for the 88Si sample TT at 700 ºC (high frequency region). The lines represent the theoretical adjust.

**Figure 40.** Z´´ versus frequency for the 88Si sample TT at 800 ºC (low frequency region). The lines represent the theoretical adjust.

**Figure 41.** Z´´ versus frequency for the 88Si sample TT at 800 ºC (high frequency region). The lines represent the theoretical adjust.


Lithium Niobiosilicate Glasses Thermally Treated 357

The values obtained by fitting the experimental data using the model of the electrical equivalent circuit shown in figure 33 (Table 4), indicate that the parameters R1 and R2 have similar behavior, increasing with the increase of the measurement temperature. With the increase of the TT temperature, the parameters R1 and R2 have a similar behavior to that observed for the dc conductivity (Table 3). The values of the Q01 and Q02 parameter decrease with the increase of the measurement temperature. Q01, at room temperature, increases with the increase of the TT temperature up to 700 °C. Under these conditions, the parameter Q02 has an oscillatory behavior, decreasing from the sample TT600 to TT650, increasing to the sample TT700 and decreasing again to the sample TT800. The parameter n1 decreases with increasing the measurement temperature, except for the sample TT600 that shows a value of n1 = 1. At 300 K, n1 parameter value remains substantially constant, with the increase of the TT temperature (0.83< n1<0.87). The parameter n2 is almost constant for all samples,

Based on the values of the electrical equivalent circuit parameters (Table 4) it was calculated the relaxation time (τZ), associated with each relaxation mechanism, and the capacitor value which best approximates the CPE element (CCPE) [18]. The obtained values are in table 5. This table also presents the value of the dielectric constant and dielectric loss for all

It is verified that the τZ1 value is approximately three orders of magnitude higher than τZ2. However, both parameters decrease with the increase of the temperature of measurement for all samples, i.e., Z'' peak shifts to higher frequencies. The parameter CCPE1 presents a behavior similar to Q01 (Table 4), in function of the TT temperature, and is approximately

The glasses with the molar compositions 92SiO2-4Li2O-4Nb2O5 (92Si) and 88SiO2-6Li2O-6Nb2O5 (88Si) are, due to the high amount of SiO2, very difficult to prepare by conventional melt quenching method. However, the sol-gel method allows there preparation without major difficulties. The macroscopic aspect of the samples of these compositions depend on the presence, or absence, of inhomogeneities in the glass matrix, the particles size, quantity and refractive index [28]. In the case of the SiO2:LiNbO3 system, the refractive indices of SiO2 (~ 1.4 [4]) and LiNbO3 (~ 2.2 [29]) differ considerably. Although the translucent appearance of the as-prepared sample of the 88Si composition, the XRD pattern and SEM micrographs did not reveal the presence of heterogeneities of crystalline or amorphous nature type. The 92Si as-prepared sample is colorless and transparent. However, the results of Raman spectroscopy on the 88Si as-prepared sample showed the presence of bands centered at 240 and 680 cm-1, assigned to vibrations of NbO6 octahedrons associated with the LiNbO3 structure [30;31;32;33], indicating the probable presence of small LiNbO3 particles dispersed in the glass matrix. These bands are not present in the sample only subjected to the drying

measuring conditions and TT temperatures. Note that n1> n2, in all cases.

temperatures of measurement.

**9. Results analysis** 

two orders of magnitude higher than that CCPE2.

treatment at 120 °C, which are also transparent.

**Table 4.** Equivalent electric circuit parameters (R1, Q01, n1, R2, Q02 and n2) for all samples at several measuring temperatures.

The values obtained by fitting the experimental data using the model of the electrical equivalent circuit shown in figure 33 (Table 4), indicate that the parameters R1 and R2 have similar behavior, increasing with the increase of the measurement temperature. With the increase of the TT temperature, the parameters R1 and R2 have a similar behavior to that observed for the dc conductivity (Table 3). The values of the Q01 and Q02 parameter decrease with the increase of the measurement temperature. Q01, at room temperature, increases with the increase of the TT temperature up to 700 °C. Under these conditions, the parameter Q02 has an oscillatory behavior, decreasing from the sample TT600 to TT650, increasing to the sample TT700 and decreasing again to the sample TT800. The parameter n1 decreases with increasing the measurement temperature, except for the sample TT600 that shows a value of n1 = 1. At 300 K, n1 parameter value remains substantially constant, with the increase of the TT temperature (0.83< n1<0.87). The parameter n2 is almost constant for all samples, measuring conditions and TT temperatures. Note that n1> n2, in all cases.

Based on the values of the electrical equivalent circuit parameters (Table 4) it was calculated the relaxation time (τZ), associated with each relaxation mechanism, and the capacitor value which best approximates the CPE element (CCPE) [18]. The obtained values are in table 5. This table also presents the value of the dielectric constant and dielectric loss for all temperatures of measurement.

It is verified that the τZ1 value is approximately three orders of magnitude higher than τZ2. However, both parameters decrease with the increase of the temperature of measurement for all samples, i.e., Z'' peak shifts to higher frequencies. The parameter CCPE1 presents a behavior similar to Q01 (Table 4), in function of the TT temperature, and is approximately two orders of magnitude higher than that CCPE2.
