**9. Results analysis**

356 Heat Treatment – Conventional and Novel Applications

**R1 (x10+5) [**Ω**]** 

**Q01 (x10-7) [**Ω**-1m-2sn]**

315 0,18 1,20 1,00 310 0,90 3,76 0,83 305 3,57 1,21 1,00

**n1**

300 10,50 1,34 1,00 0,14 7,60 0,73 295 27,93 1,11 1,00 0,20 9,66 0,69 290 39,00 1,18 1,00 0,30 5,12 0,72 285 0,48 4,40 0,73 280 0,71 3,43 0,74 275 1,08 2,44 0,76

315 0,29 10,08 0,74 0,15 6,77 0,71 310 2,01 6,24 0,79 0,22 5,12 0,73 305 4,86 4,76 0,81 0,34 5,70 0,72 300 11,21 3,91 0,87 0,47 3,89 0,74 295 23,12 3,86 0,80 0,85 3,51 0,73 290 37,10 3,17 0,93 1,06 1,81 0,78 285 86,23 3,28 0,90 2,01 3,23 0,72 280 3,07 1,48 0,78 275 4,84 2,43 0,72

315 0,11 13,64 0,84 0,05 6,75 0,75 310 0,75 14,05 0,82 0,09 5,21 0,77 305 2,34 9,66 0,79 0,13 3,70 0,77 300 3,93 7,09 0,83 0,17 6,54 0,73 295 30,86 4,07 0,94 0,40 1,94 0,78 290 49,56 3,16 0,95 0,69 1,76 0,78 285 1,09 1,48 0,78 280 1,99 1,11 0,80 275 3,13 0,95 0,80

315 0,65 5,78 0,68 1,80 3,38 0,72 310 3,88 2,60 0,84 2,85 2,53 0,74 305 7,05 2,25 0,88 4,24 2,29 0,73 300 21,02 2,19 0,85 7,13 1,73 0,75 295 32,91 1,81 0,89 11,05 1,25 0,77 290 55,64 1,63 0,87 16,78 1,07 0,78 285 25,29 0,95 0,78 280 37,81 0,86 0,78 275 60,92 0,70 0,78

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

**R2 (x10+4) [**Ω**]** 

**Q02 (x10-8) [**Ω**-1m-2sn]** 

**n2**

**Sample Temp.** 

600

650

700

800

measuring temperatures.

**(K)** 

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 treatment at 120 °C, which are also transparent.


Lithium Niobiosilicate Glasses Thermally Treated 359

The XRD patterns of the 88Si sample TT at 600 ºC and of the 92Si sample TT at 650 ºC, both translucent, showed that the LiNbO3 crystalline phase is already formed. In the SEM micrographs of those samples it is possible to observe particles with a size that does not exceeds 500 nm, which are associated to those crystallites. The observation of particles in the 88Si sample, treated at temperatures where in the 92Si samples no particles were observed, is justified to the fact that the amount of lithium and niobium ions present in the 88Si sample is greater. As it is known, Li+ and Nb5+ ions, which due to their high field strength, can promote phase separation in the vitreous network [34]. When treated at 800 °C the 88Si sample became opaque. The SEM micrographs of this sample show particles with a maximum size of 200 nm, but by XRD it was detect not only the LiNbO3 phase but also the SiO2, and Li2Si2O5 and Li3NbO4 phases. In the samples of the 92Si composition, TT at 800 °C, which are translucent, the LiNbO3 and SiO2 crystal phases are also present. Thus, the opacity of the 88Si sample, TT800, can be attributed to the presence of particles associated with the

The Li3NbO4 crystalline phase, present in the 88Si sample TT at 800 °C, compared with the LiNbO3 phase, is richer in Li+ ions. Thus, taking into account that the molar amount of Li+ and Nb5+ ions present in the sample is equal ([Nb] / [Li] = 1), it can be confirmed the existence, in this sample, of a greater quantity of Nb5+ ions inserted structurally in glass matrix, than Li+ ions. The formation of the Li2Si2O5 crystalline phase contributes to the decrease of the number of these Li+ ions. The absence in the Raman spectra of all the 92Si and 88Si samples, of a band at 800-850 cm-1 related to the vibration of the Nb-O-Si bonds, indicates that Nb5+ ions are probably embedded in the matrix as network modifiers. The decreasing of the number of lithium and niobium ions, structurally inserted in the glass matrix, with the increase of the TT temperature, is responsible for the decrease of the dc

The 88Si sample, TT at 800 ºC, shows the minimum value of σdc, of all samples of this composition, indicating that this is the one that should have the smallest number of ions structurally inserted in the glass network. However, the σdc is also affected by the presence of particles, particularly those of LiNbO3, which are characterized by a high resistivity (~ 1021

The particles observed by SEM in the sample TT800 exhibit a morphology and size similar to that observed in the samples TT at 600 and 700 °C. The detection, by Raman spectroscopy, in the samples TT at temperatures above 500 ºC, of vibrations assigned to the LiNbO3 crystal structure (mainly the ones centered at 240 and 680 cm-1) leads us to consider that the particles observed by SEM are LiNbO3 crystallites. Knowing that the particle size is higher in the sample TT 650 than in the sample TT at 700 ºC, and that the intensity of the XRD peaks associated with the LiNbO3 phase is also higher in the sample TT650, it can be assumed that the increase of the TT temperature from 650 to 700 °C promotes a dissolution of particles. Thus, the volume ratio between LiNbO3 particles and the matrix glass is higher in the sample TT650 and the sample TT700 will have a larger number of Li+ and Nb5+ ions structurally inserted in the glass matrix. Thus, it can be justified the observed increase of σdc

crystalline phases of Li2Si2O5 and Li3NbO4.

conductivity.

Ω. cm, at 300 K [34])

**Table 5.** Relaxation time (τZ) and the CCPE capacitors, dielectric constant (ε´) and dielectric loss (tan δ), at 1kHz and 300 K, of the 88 Si samples.

The XRD patterns of the 88Si sample TT at 600 ºC and of the 92Si sample TT at 650 ºC, both translucent, showed that the LiNbO3 crystalline phase is already formed. In the SEM micrographs of those samples it is possible to observe particles with a size that does not exceeds 500 nm, which are associated to those crystallites. The observation of particles in the 88Si sample, treated at temperatures where in the 92Si samples no particles were observed, is justified to the fact that the amount of lithium and niobium ions present in the 88Si sample is greater. As it is known, Li+ and Nb5+ ions, which due to their high field strength, can promote phase separation in the vitreous network [34]. When treated at 800 °C the 88Si sample became opaque. The SEM micrographs of this sample show particles with a maximum size of 200 nm, but by XRD it was detect not only the LiNbO3 phase but also the SiO2, and Li2Si2O5 and Li3NbO4 phases. In the samples of the 92Si composition, TT at 800 °C, which are translucent, the LiNbO3 and SiO2 crystal phases are also present. Thus, the opacity of the 88Si sample, TT800, can be attributed to the presence of particles associated with the crystalline phases of Li2Si2O5 and Li3NbO4.

358 Heat Treatment – Conventional and Novel Applications

τ**z1 (x10-3) [s]** 

**CCPE1 (x10-7) [F]** 

τ**z2 (x10-6) [s]** 

315 0,35 1,20 1873,33 0,47 310 2,65 1,32 1722,50 0,48 305 6,92 1,21 1585,02 0,54 300 26,53 1,06 0,59 1,52 1330,68 0,68 295 51,34 1,11 0,72 1,13 1178,92 0,88 290 75,79 1,18 0,86 0,99 1013,83 1,06 285 1,45 1,06 859,40 1,33 280 2,10 1,06 722,08 1,61 275 3,10 1,08 575,66 1,84

315 0,50 1,27 0,36 0,82 2265,03 0,89 310 11,37 2,25 0,63 1,02 1921,36 1,86 305 26,53 2,27 1,06 1,05 1561,59 2,06 300 63,66 2,67 1,34 1,01 1266,14 2,36 295 132,63 2,42 2,50 1,03 858,17 2,62 290 198,94 2,83 2,69 1,00 635,53 2,79 285 454,73 3,01 6,51 1,09 384,23 2,70 280 8,18 1,05 231,64 2,38 275 13,84 0,98 155,36 1,93

315 1,01 4,33 0,19 1,31 4792,50 0,97 310 9,95 5,82 0,35 1,52 3866,72 1,97 305 26,53 4,28 0,42 1,25 3363,29 2,59 300 33,86 3,84 0,57 1,19 3052,55 3,07 295 227,36 3,73 0,90 0,89 1668,21 3,11 290 265,26 2,93 1,56 0,89 1047,35 3,62 285 2,35 0,86 666,41 3,83 280 4,08 0,84 329,39 3,61 275 6,39 0,85 188,13 2,91

315 1,30 0,62 5,45 1,03 355,43 1,97 310 10,61 1,24 8,72 1,09 254,31 2,10 305 22,74 1,39 12,56 1,04 164,21 2,47 300 61,21 1,42 20,85 1,07 92,67 2,75 295 88,42 1,37 29,98 1,04 62,13 2,75 290 159,15 1,26 45,80 1,07 46,20 2,53 285 67,78 1,05 36,99 2,20 280 100,35 1,04 31,24 1,84 275 147,91 0,96 26,36 1,47

**Table 5.** Relaxation time (τZ) and the CCPE capacitors, dielectric constant (ε´) and dielectric loss (tan δ), at

**CCPE2 (x10-9) [F]** 

ε**´ tan** δ

**(K)** 

**Sample Temp.** 

600

650

700

800

1kHz and 300 K, of the 88 Si samples.

The Li3NbO4 crystalline phase, present in the 88Si sample TT at 800 °C, compared with the LiNbO3 phase, is richer in Li+ ions. Thus, taking into account that the molar amount of Li+ and Nb5+ ions present in the sample is equal ([Nb] / [Li] = 1), it can be confirmed the existence, in this sample, of a greater quantity of Nb5+ ions inserted structurally in glass matrix, than Li+ ions. The formation of the Li2Si2O5 crystalline phase contributes to the decrease of the number of these Li+ ions. The absence in the Raman spectra of all the 92Si and 88Si samples, of a band at 800-850 cm-1 related to the vibration of the Nb-O-Si bonds, indicates that Nb5+ ions are probably embedded in the matrix as network modifiers. The decreasing of the number of lithium and niobium ions, structurally inserted in the glass matrix, with the increase of the TT temperature, is responsible for the decrease of the dc conductivity.

The 88Si sample, TT at 800 ºC, shows the minimum value of σdc, of all samples of this composition, indicating that this is the one that should have the smallest number of ions structurally inserted in the glass network. However, the σdc is also affected by the presence of particles, particularly those of LiNbO3, which are characterized by a high resistivity (~ 1021 Ω. cm, at 300 K [34])

The particles observed by SEM in the sample TT800 exhibit a morphology and size similar to that observed in the samples TT at 600 and 700 °C. The detection, by Raman spectroscopy, in the samples TT at temperatures above 500 ºC, of vibrations assigned to the LiNbO3 crystal structure (mainly the ones centered at 240 and 680 cm-1) leads us to consider that the particles observed by SEM are LiNbO3 crystallites. Knowing that the particle size is higher in the sample TT 650 than in the sample TT at 700 ºC, and that the intensity of the XRD peaks associated with the LiNbO3 phase is also higher in the sample TT650, it can be assumed that the increase of the TT temperature from 650 to 700 °C promotes a dissolution of particles. Thus, the volume ratio between LiNbO3 particles and the matrix glass is higher in the sample TT650 and the sample TT700 will have a larger number of Li+ and Nb5+ ions structurally inserted in the glass matrix. Thus, it can be justified the observed increase of σdc

#### 360 Heat Treatment – Conventional and Novel Applications

from the sample TT650 to the sample TT700. The decrease of σdc from the as-prepared glass (TT500) to the sample TT650 and from the TT700 sample to the TT800 sample is justify by a decrease in the number of charge carriers.

Lithium Niobiosilicate Glasses Thermally Treated 361

´ 4.0 *SiO*

≈ ), it is

ε

decrease of the σac, from the sample TT700 to the sample TT800, is justified by the decrease

The existence of two zones with different morphology (surface and bulk), in the 92Si composition samples TET, indicate that the dielectric response can be associated to an

**Figure 42.** Equivalent electric circuit model: i) sample (a – surface; b – bulk); ii) electric model where Ra and Ca represent the resistance and the capacity related with the sample surface characteristics, Rb and

In the dielectric analysis, the electric circuit comes down to a combination of three capacitors in series: two related with the sample surfaces and the third with the bulk characteristics. Knowing that the thickness of the samples are about 1.0 mm, the thickness of the surface part where the particles are embedded is below 3 μm (sample 700A), the dielectric constant (ε ') of LiNbO3 is higher than 103 at 300 K and 1 kHz [29], being much higher than the ε ' of

reasonable to consider that the dielectric behavior can be controlled by the characteristics of the bulk. Briefly, the analysis of the electrical circuit consisting of three capacitors in series

. 2

*C C <sup>C</sup> C C* <sup>=</sup> <sup>+</sup>

If we consider that Ca >> Cb than Ceq ≈ Cb. Therefore the increase of ε' in the samples series treated at 750 ° C, with the increase of the external electric field amplitude may be associated with an increase in the number of dipoles present in the bulk region of the samples. In these samples it was observed a decrease in the LiNbO3 particle size, with the increase of the applied electric field amplitude. This reduction is also associated with the decrease of intensity of the Raman band centered at 630 cm-1 related to the presence of LiNbO3 particles [24], indicating a decrease in the number of dipoles associated with those particles. By increasing the electric field amplitude applied during the heat treatment, the σdc increases. This behavior must be related to the presence of a higher number of Li+ and Nb5+ ions inserted structurally in the glass matrix. Thus it can be assumed an increase in the number of electric dipoles in the bulk zone with the increase of the amplitude of the electric field.

*eq*

*a b*

*a b*

Cb the resistance and capacity related to the bulk characteristics; iii) approximate model.

the bulk sample zone, assumed to consist mainly of glass matrix ( <sup>2</sup>

shows that the equivalent capacity is

of lithium and niobium ions, since both samples have similar Ea(ac) values.

equivalent electric circuit model as shown in figure 42.

The 92Si samples present a σdc much lower than that observed in the 88Si samples. This difference is explained by the existence of a larger number of Li+ and Nb5+ ions, structurally inserted in the glass matrix, in the 88Si samples network.

In the 92Si samples TET at 650 °C, with amplitude lower than 500 kV/m, it were observed particles in the sample surface by SEM but not detected by XRD. This phenomenon is probably due to the fact that these particles may have a amorphous crystallinity nature. The detection by XRD, in the 650C sample and in the samples TT at temperatures above 650 °C, of LiNbO3 crystal phase, indicates that the particles observed in the 650C sample surface must be associated with LiNbO3.

In both compositions, and in the temperature range of the conductivity measurements, two zones with different activation energies were observed. This behavior was adjusted with an Arrhenius equation and the activation energy calculated. The only exception was the 88Si sample TT at 500 ºC. This atypical behavior can be explained considering that this sample has a microstructure with a high porosity [2]. Kincs et al. [35] found that the non-Arrhenius behavior in glasses disappears with the densification of the glass. Thus, increasing the TT temperature in the 88Si glass composition a structural densification is activated.

In all 88Si and 92Si samples, the σdc increases with the increase of the measurement temperature. This behavior, typical of a thermally stimulated process, can be attributed to the increase of the charge carriers energy, with the increase of the temperature. Assuming the ionic conductivity model, where conduction is done by the "jumps" of the charge carriers through the potential barriers [25;26;27;28], the increase of the energy of those charge carriers make their movements easier, thus increasing the σdc.

Samples of the 88Si composition show a decrease in the ac conductivity, with the increase of the TT temperature, indicating that the TT affects the structure of the glass in such way that the number of units responsible for this conduction mechanism (dipoles and/or ions) decreases or their movement, in response to the applied ac field, becomes more difficult. The Ea(ac), calculated using the Arrhenius equation, is similar for the samples TT500, TT700 and TT800 (~ 45 kJ/mol). The remaining samples showed different Ea(ac) values (TT600 ~ 32 kJ/mol and TT650 ~ 52 kJ/mol). The decrease of the Ea(ac) from the sample TT500 to the sample TT600, indicates a decrease in the height of the potential barriers associated with this conduction process, suggesting that the decrease of σac is due to a decrease in the number of dipoles, associated with lithium and niobium ions structurally inserted in the glass matrix. Considering that the particles observed in the sample TT600 are LiNbO3 particles, the decreased of σac can be explained by the formation of dipoles related with these crystals, which are difficult to depolarization [29;36] in this measuring temperatures. The sample TT650 has the highest value of Ea(ac). This is related to the presence of LiNbO3 particles agglomerates, which increases the difficulty of the associated dipoles movements. The decrease of the σac, from the sample TT700 to the sample TT800, is justified by the decrease of lithium and niobium ions, since both samples have similar Ea(ac) values.

360 Heat Treatment – Conventional and Novel Applications

decrease in the number of charge carriers.

must be associated with LiNbO3.

inserted in the glass matrix, in the 88Si samples network.

from the sample TT650 to the sample TT700. The decrease of σdc from the as-prepared glass (TT500) to the sample TT650 and from the TT700 sample to the TT800 sample is justify by a

The 92Si samples present a σdc much lower than that observed in the 88Si samples. This difference is explained by the existence of a larger number of Li+ and Nb5+ ions, structurally

In the 92Si samples TET at 650 °C, with amplitude lower than 500 kV/m, it were observed particles in the sample surface by SEM but not detected by XRD. This phenomenon is probably due to the fact that these particles may have a amorphous crystallinity nature. The detection by XRD, in the 650C sample and in the samples TT at temperatures above 650 °C, of LiNbO3 crystal phase, indicates that the particles observed in the 650C sample surface

In both compositions, and in the temperature range of the conductivity measurements, two zones with different activation energies were observed. This behavior was adjusted with an Arrhenius equation and the activation energy calculated. The only exception was the 88Si sample TT at 500 ºC. This atypical behavior can be explained considering that this sample has a microstructure with a high porosity [2]. Kincs et al. [35] found that the non-Arrhenius behavior in glasses disappears with the densification of the glass. Thus, increasing the TT

In all 88Si and 92Si samples, the σdc increases with the increase of the measurement temperature. This behavior, typical of a thermally stimulated process, can be attributed to the increase of the charge carriers energy, with the increase of the temperature. Assuming the ionic conductivity model, where conduction is done by the "jumps" of the charge carriers through the potential barriers [25;26;27;28], the increase of the energy of those charge

Samples of the 88Si composition show a decrease in the ac conductivity, with the increase of the TT temperature, indicating that the TT affects the structure of the glass in such way that the number of units responsible for this conduction mechanism (dipoles and/or ions) decreases or their movement, in response to the applied ac field, becomes more difficult. The Ea(ac), calculated using the Arrhenius equation, is similar for the samples TT500, TT700 and TT800 (~ 45 kJ/mol). The remaining samples showed different Ea(ac) values (TT600 ~ 32 kJ/mol and TT650 ~ 52 kJ/mol). The decrease of the Ea(ac) from the sample TT500 to the sample TT600, indicates a decrease in the height of the potential barriers associated with this conduction process, suggesting that the decrease of σac is due to a decrease in the number of dipoles, associated with lithium and niobium ions structurally inserted in the glass matrix. Considering that the particles observed in the sample TT600 are LiNbO3 particles, the decreased of σac can be explained by the formation of dipoles related with these crystals, which are difficult to depolarization [29;36] in this measuring temperatures. The sample TT650 has the highest value of Ea(ac). This is related to the presence of LiNbO3 particles agglomerates, which increases the difficulty of the associated dipoles movements. The

temperature in the 88Si glass composition a structural densification is activated.

carriers make their movements easier, thus increasing the σdc.

The existence of two zones with different morphology (surface and bulk), in the 92Si composition samples TET, indicate that the dielectric response can be associated to an equivalent electric circuit model as shown in figure 42.

**Figure 42.** Equivalent electric circuit model: i) sample (a – surface; b – bulk); ii) electric model where Ra and Ca represent the resistance and the capacity related with the sample surface characteristics, Rb and Cb the resistance and capacity related to the bulk characteristics; iii) approximate model.

In the dielectric analysis, the electric circuit comes down to a combination of three capacitors in series: two related with the sample surfaces and the third with the bulk characteristics. Knowing that the thickness of the samples are about 1.0 mm, the thickness of the surface part where the particles are embedded is below 3 μm (sample 700A), the dielectric constant (ε ') of LiNbO3 is higher than 103 at 300 K and 1 kHz [29], being much higher than the ε ' of the bulk sample zone, assumed to consist mainly of glass matrix ( <sup>2</sup> ´ 4.0 *SiO* ε ≈ ), it is reasonable to consider that the dielectric behavior can be controlled by the characteristics of the bulk. Briefly, the analysis of the electrical circuit consisting of three capacitors in series shows that the equivalent capacity is

$$\mathcal{C}\_{eq} = \frac{\mathcal{C}\_a \mathcal{C}\_b}{\mathcal{C}\_a + \mathcal{D} \mathcal{C}\_b}$$

If we consider that Ca >> Cb than Ceq ≈ Cb. Therefore the increase of ε' in the samples series treated at 750 ° C, with the increase of the external electric field amplitude may be associated with an increase in the number of dipoles present in the bulk region of the samples. In these samples it was observed a decrease in the LiNbO3 particle size, with the increase of the applied electric field amplitude. This reduction is also associated with the decrease of intensity of the Raman band centered at 630 cm-1 related to the presence of LiNbO3 particles [24], indicating a decrease in the number of dipoles associated with those particles. By increasing the electric field amplitude applied during the heat treatment, the σdc increases. This behavior must be related to the presence of a higher number of Li+ and Nb5+ ions inserted structurally in the glass matrix. Thus it can be assumed an increase in the number of electric dipoles in the bulk zone with the increase of the amplitude of the electric field.

#### 362 Heat Treatment – Conventional and Novel Applications

In the samples series TET at 700 °C, the behavior of ε', with the increase of the external electrical field amplitude, is opposite to the one observed in the samples TET at 750 °C. The increase of the particle size in the samples TET at 700 °C, with the increase of the external electrical field amplitude, was observed by SEM and by Raman spectroscopy (increase of the intensity of the band centered at 630 cm-1). This fact will cause a reduction in the number of dipoles in the sample bulk zone, thereby reducing the ε' value. The same occurs in the sample TET at 650 °C. Thus, these results suggest that in the samples TET at 650 °C and 700 ºC the Nb5+ and Li+ ions migrate from the bulk zone to the surface, contributing to the increase of the particles size.

Lithium Niobiosilicate Glasses Thermally Treated 363

In the 92Si composition, the Z\* behavior as a function of frequency, for the TET samples treated at 700 ºC is opposite to that observed in the samples TET at 750 ºC, with the increase of the external electric field amplitude (the series TET at 650 and 700 ºC have a similar behavior). In the samples TET at 750 ºC, the increase of the amplitude of the external electrical field, promote an increase in the values of the R and Y0 parameters. This behavior should be associated with the increase of the number of particles in the sample bulk zone. In the samples TET at 650 and 700 °C, the number of electrical units within the sample (bulk region) decreases with the increase of the amplitude of the external electrical field, and therefore, increases the number of surface particles, justifying

The 92Si composition presents dielectric constant (ε ') and dielectric loss (tan δ = ε'' / ε' ) values, much lower than those observed in the 88Si composition samples. The justification for this value disparity is the larger number of electric dipoles existing in the 88Si composition sample. In this composition, the decrease of ε', from the sample TT at 600 ºC to the sample TT at 650 ºC, measured at room temperature and 1 kHz, is due to the presence, in the sample TT650, of particle agglomeration and also to high size particles, which promotes the decrease of the number of dipoles associated with lithium and niobium ions structurally inserted in the glass matrix. Furthermore it is likely that the crystal orientation of the particles and agglomerates, do not present a preferential grow direction contributing to the

The parameter tan δ (92Si composition) increases in the samples TET at 750 °C, with the increase of the amplitude of the external electrical field, mainly due to the increase of the ε'' component. This behavior will cause an increase in the conductivity, which is corroborated by the R parameter behavior. The decrease in the value of τZ, in the samples TET at 750 °C with the increase of the amplitude of the electric field, suggests that the electrical units, present in these samples, follow the ac electric field more easily. In the TET samples treated at 650 °C, the behavior of tan δ (with increasing the amplitude of the external electric field) is opposite to that observed in the samples TET at 750 °C. This can be attributed to a significant increase in the ac resistivity. Moreover ε' decreases due to the increase of the surface particles amount and thus reducing the amount of ions in the glass bulk zone.

The presence in samples of the 88Si composition of two relaxation peaks, in the Z\* spectrum, similar to those already observed in other glass containing LiNbO3 crystals [38;39;40] suggests that the dc conductivity at low temperature and the dielectric relaxation at highfrequency can be assigned to the ion conduction and ionic polarization, respectively. The σdc behavior at temperatures higher than 300 K and the relaxation process in the low frequency

1. The sol-gel method allows the preparation of glasses and glass ceramics with compositions that by the melt-quenching method are extremely difficult to prepare.

range should be assigned to interfacial electrode-sample polarization [38;39;40].

the Z\*(ω) opposite behavior.

decrease of the dipole moment [36;37].

**10. Main conclusions** 

The dielectric response, using the impedance formalism (Z\*), as a function of frequency and temperature for the 92Si composition samples was adjusted to the physical model consisting on the equivalent circuit shown in figure 43 [R1(RCPE1)].

**Figure 43.** Equivalent electric circuit model.

It is noted that this model fits the experimental data reasonably, showing for all the samples a value of the parameter n very close to 1 (> 0.9), indicating that CPE element behaves very close to a capacitor. The value of R1 ~0 was considered in all samples.

In the 88Si composition samples it was detected two relaxation mechanisms, which were fitted, using the CNLLS algorithm, to the equivalent circuit model shown in figure 33 ([R0(R1CPE1)(R2CPE2)]). From the obtained values, it should be noted that the parameters n1 and n2, associated to the elements and CPE1 and CPE2, respectively, are always higher than 0.73, which shows that also in this composition, the elements CPE tend to exhibit a behavior similar to a capacitive element. The relaxation mechanism, found in the high frequency region (R2CPE2) is assigned to the characteristics of the vitreous matrix ("bulk"), i.e., with the relaxation of dipoles associated with the ions structurally inserted in the glass matrix. The relaxation at the lowest frequencies was associated with surface features, namely those related with the dipoles related with the particles detected at the samples surfaces. The presence of those particles, related with the LiNbO3 crystalline phase, which is characterized by possessing dipoles which electrical depolarization is difficult, justifies the higher relaxation time. The fact that the relaxation time associated with the first depolarization mechanism, observed in the 88Si composition, is of the same order of magnitude to that observed in the 92Si samples composition, suggests that the electrical units responsible for both are the same, i.e., the dipoles associated with lithium and niobium ions inserted structurally in the glass matrix. In the 88Si composition, the relaxation time τZ1, related with the R1CPE1, and associated with the relaxation mechanism at the low frequency zone, is higher (~ 10-2 s) than the τZ2, which also indicates that the units responsible for this relaxation are more difficult to depolarize.

In the 92Si composition, the Z\* behavior as a function of frequency, for the TET samples treated at 700 ºC is opposite to that observed in the samples TET at 750 ºC, with the increase of the external electric field amplitude (the series TET at 650 and 700 ºC have a similar behavior). In the samples TET at 750 ºC, the increase of the amplitude of the external electrical field, promote an increase in the values of the R and Y0 parameters. This behavior should be associated with the increase of the number of particles in the sample bulk zone. In the samples TET at 650 and 700 °C, the number of electrical units within the sample (bulk region) decreases with the increase of the amplitude of the external electrical field, and therefore, increases the number of surface particles, justifying the Z\*(ω) opposite behavior.

The 92Si composition presents dielectric constant (ε ') and dielectric loss (tan δ = ε'' / ε' ) values, much lower than those observed in the 88Si composition samples. The justification for this value disparity is the larger number of electric dipoles existing in the 88Si composition sample. In this composition, the decrease of ε', from the sample TT at 600 ºC to the sample TT at 650 ºC, measured at room temperature and 1 kHz, is due to the presence, in the sample TT650, of particle agglomeration and also to high size particles, which promotes the decrease of the number of dipoles associated with lithium and niobium ions structurally inserted in the glass matrix. Furthermore it is likely that the crystal orientation of the particles and agglomerates, do not present a preferential grow direction contributing to the decrease of the dipole moment [36;37].

The parameter tan δ (92Si composition) increases in the samples TET at 750 °C, with the increase of the amplitude of the external electrical field, mainly due to the increase of the ε'' component. This behavior will cause an increase in the conductivity, which is corroborated by the R parameter behavior. The decrease in the value of τZ, in the samples TET at 750 °C with the increase of the amplitude of the electric field, suggests that the electrical units, present in these samples, follow the ac electric field more easily. In the TET samples treated at 650 °C, the behavior of tan δ (with increasing the amplitude of the external electric field) is opposite to that observed in the samples TET at 750 °C. This can be attributed to a significant increase in the ac resistivity. Moreover ε' decreases due to the increase of the surface particles amount and thus reducing the amount of ions in the glass bulk zone.

The presence in samples of the 88Si composition of two relaxation peaks, in the Z\* spectrum, similar to those already observed in other glass containing LiNbO3 crystals [38;39;40] suggests that the dc conductivity at low temperature and the dielectric relaxation at highfrequency can be assigned to the ion conduction and ionic polarization, respectively. The σdc behavior at temperatures higher than 300 K and the relaxation process in the low frequency range should be assigned to interfacial electrode-sample polarization [38;39;40].

## **10. Main conclusions**

362 Heat Treatment – Conventional and Novel Applications

on the equivalent circuit shown in figure 43 [R1(RCPE1)].

close to a capacitor. The value of R1 ~0 was considered in all samples.

increase of the particles size.

**Figure 43.** Equivalent electric circuit model.

are more difficult to depolarize.

In the samples series TET at 700 °C, the behavior of ε', with the increase of the external electrical field amplitude, is opposite to the one observed in the samples TET at 750 °C. The increase of the particle size in the samples TET at 700 °C, with the increase of the external electrical field amplitude, was observed by SEM and by Raman spectroscopy (increase of the intensity of the band centered at 630 cm-1). This fact will cause a reduction in the number of dipoles in the sample bulk zone, thereby reducing the ε' value. The same occurs in the sample TET at 650 °C. Thus, these results suggest that in the samples TET at 650 °C and 700 ºC the Nb5+ and Li+ ions migrate from the bulk zone to the surface, contributing to the

The dielectric response, using the impedance formalism (Z\*), as a function of frequency and temperature for the 92Si composition samples was adjusted to the physical model consisting

It is noted that this model fits the experimental data reasonably, showing for all the samples a value of the parameter n very close to 1 (> 0.9), indicating that CPE element behaves very

In the 88Si composition samples it was detected two relaxation mechanisms, which were fitted, using the CNLLS algorithm, to the equivalent circuit model shown in figure 33 ([R0(R1CPE1)(R2CPE2)]). From the obtained values, it should be noted that the parameters n1 and n2, associated to the elements and CPE1 and CPE2, respectively, are always higher than 0.73, which shows that also in this composition, the elements CPE tend to exhibit a behavior similar to a capacitive element. The relaxation mechanism, found in the high frequency region (R2CPE2) is assigned to the characteristics of the vitreous matrix ("bulk"), i.e., with the relaxation of dipoles associated with the ions structurally inserted in the glass matrix. The relaxation at the lowest frequencies was associated with surface features, namely those related with the dipoles related with the particles detected at the samples surfaces. The presence of those particles, related with the LiNbO3 crystalline phase, which is characterized by possessing dipoles which electrical depolarization is difficult, justifies the higher relaxation time. The fact that the relaxation time associated with the first depolarization mechanism, observed in the 88Si composition, is of the same order of magnitude to that observed in the 92Si samples composition, suggests that the electrical units responsible for both are the same, i.e., the dipoles associated with lithium and niobium ions inserted structurally in the glass matrix. In the 88Si composition, the relaxation time τZ1, related with the R1CPE1, and associated with the relaxation mechanism at the low frequency zone, is higher (~ 10-2 s) than the τZ2, which also indicates that the units responsible for this relaxation

1. The sol-gel method allows the preparation of glasses and glass ceramics with compositions that by the melt-quenching method are extremely difficult to prepare.

2. The drying process keeps the 92Si composition gel transparent and the 88Si translucent. The heat treatment with or without the presence of the external electrical field, makes the 92Si glass composition translucent. Samples of the 88Si composition, after TT at temperatures above 750 ºC, became opaque.

Lithium Niobiosilicate Glasses Thermally Treated 365

**Author details** 

**Acknowledgement** 

**11. References** 

(1986).

(1991).

(2005) 2951-2957.

(1) (2007) 1-8.

Manuel Pedro Fernandes Graça and Manuel Almeida Valente

[1] Mauritz, www.psrc.usm.edu/~mauritz/sol-gel.html (2002).

[6] F. Mehran, B.A. Scott, Solid State Communications, 11, 15-19 (1972).

protectores."- PhD thesis, Universidade de Aveiro, Portugal (1990).

[10] L.C. Klein, S. Ho, "Glasses for electronic applications", (1991) 221-233.

Klein, M. Greenblatt, J. Non-Cryst. Solids, 143, 21-30 (1992).

Materials", ISBN: 978-953-307-217-3, Intech publisher, 2011.

[13] K. Chou, J. Non-Cryst. Solids, 110 (1989) 122-124. [14] M. Prassas, "Silica Glass from Aerogels", in

http://www.solgel.com/articles/april01/aerog2.htm .

http://eande.lbl.gov/ECS/aerogels/saprep.htm .

The authors gratefully acknowledge the financial support from PEst-C/CTM/LA0025/2011 and from the Portuguese Science and Technology Foundation (FCT) - SFRH/BD/6314/2001.

[2] I. Schwartz, P. Anderson, H. de Lambilly, L.C. Klein, J. Non-Cryst. Solids, 83, 391-399

[3] E.O. Lawrence, Berkeley National Laboratory, "How silica aerogels are made", in

[4] J.M.F. Navarro, "El Vidrio" (CSIC-Fundación Centro Nacional del Vidrio, Madrid 1991). [5] S. Sakka, "Treatise on materials science and technology", vol 22, Academic Press, 1982.

[7] M. G. Ferreira da Silva, "Incorporação Estrutural de Iões de Transição em Vidros Preparados pelo Método Sol-Gel", PhD thesis, Universidade de Aveiro, Portugal (1990). [8] I.M.M. Salvado, "Preparação pelo processo "Sol-Gel" e caracterização de materiais dos sistemas SiO2-ZrO2, SiO2-TiO2, SiO2-Al2O3. Aplicação como revestimentos

[9] N. Venkatasubramanian, B. Wade, P. Desai, A S. Abhiraman, L.T. Gelbaum, 130, 144-156

[11] S.P. Szu, M. Greenblatt, L.C. Klein, Solid State Ionics, 46, 291-297 (1991); S.P. Szu, L.C.

[12] C. Sanchez, G.A.A. Soler-Illia, F. Ribot, D. Grosso, C.R. Chimie 6 (2003), 1131-1151.

[15] R. Ota, N. Asagi, J. Fukunaga, N. Yoshida, T. Fujii, J. Mat. Sci., 25, 4259-4265 (1990). [16] M.P.F. Graça, M.G.F. Silva, M.A. Valente , Journal of Non-Crystalline Solids, 351 (33-36)

[17] MPF Graça, MGF Silva and MA Valente, Journal of Sol-Gel Science and Technology, 42

[18] Manuel Pedro Fernandes Graça & Manuel Almeida Valente, Glass Ceramics with Para, Anti or Ferroelectric Active Phases – book chapter 11 (pp213-292) in: Ceramic

*University of Aveiro / I3N - Physics Department, Portugal* 


## **Author details**

364 Heat Treatment – Conventional and Novel Applications

crystalline phases.

at the glass surface.

behavior.

formation of LiNbO3 crystallites

preferential crystalline orientation.

glass matrix as network modifiers.

temperatures above 750 ºC, became opaque.

preferably in the sample surface or in the bulk zone.

2. The drying process keeps the 92Si composition gel transparent and the 88Si translucent. The heat treatment with or without the presence of the external electrical field, makes the 92Si glass composition translucent. Samples of the 88Si composition, after TT at

3. The detection of LiNbO3 crystal phase is observed in the 92Si sample composition TT at temperatures above 650 °C and in the 88Si composition at temperatures above 600 °C. Increasing the annealing temperature it is promoted the appearance of secondary

4. SEM revealed that crystallization in both 92Si and 88Si compositions are predominantly

5. Samples of the 92Si composition, TET at temperatures below 650 ºC, promote the

6. Increasing the amplitude of the electric field in the 92Si sample series treated at 650 and 700 °C favors the increase in the particle size. The TET at 750 ºC presents the opposite

7. The decrease in the surface particle size associated with an increase in the number of dipoles within the sample, justifies the maximum value of ε' observed (9,44). Thus, the study of the dielectric constant enables to establish if the crystallization occurs,

8. The high values of ε', measured in the low frequency region (<1 kHz) in the 88Si samples, are due to interfacial polarization. The increase in the amount of surface particles promotes a decrease of ε', indicates that these particles grow without a

9. The 88Si composition sample treated at 650 °C shows particle agglomerates that are dissolved, with increasing the thermal treatment temperature. This structural change is substantiated by the SEM results and by the decreasing of the width and increasing of the intensity of the Raman bands. The results of σdc strengthen the hypothesis that the formation of secondary phases (Li2Si2O5 and Li3NbO4), in the 88Si samples composition

10. The results of the Raman spectroscopy indicate that the niobium ions are inserted in the

11. The samples of these two compositions present two different temperature zones with different activation energies indicating the presence of two different conduction mechanisms. In the samples of the 88Si composition it was also detected the presence of two dielectric relaxation mechanisms. The Ea(dc) associated with the region of low temperatures and the dielectric relaxation mechanism associated with the high frequency region are assigned to ionic conduction and ionic polarization, respectively. The relaxation mechanism observed in the low frequency region and the Ea(dc) associated with the region

12. The CNLLS algorithm associated with an electrical equivalent circuit model was used to adjust the dielectric behavior. The detection of two different relaxation mechanisms in the 88Si samples led to the use of the electrical equivalent circuit shown in figure 41. The results obtained by fitting the experimental data showed that this model may

comes from the dissolution of the LiNbO3 agglomerated particles.

of high temperatures, are due to interfacial polarization phenomena.

describe the dielectric behavior of these samples.

Manuel Pedro Fernandes Graça and Manuel Almeida Valente *University of Aveiro / I3N - Physics Department, Portugal* 

## **Acknowledgement**

The authors gratefully acknowledge the financial support from PEst-C/CTM/LA0025/2011 and from the Portuguese Science and Technology Foundation (FCT) - SFRH/BD/6314/2001.

## **11. References**

	- [19] MPF Graça, MGF Silva, ASB Sombra and MA Valente, Journal of Non-Crystalline Solids, 352 (42-49) (2006) 5199-5204.

**Chapter 15** 

© 2012 Huang et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons. org/licenses/by/3. 0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2012 Huang et al., licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**Cooling – As a "Heat Treatment"** 

**of the Bulk Metallic Glass Alloys** 

Metals play a significant role in human life since the Bronze Age. Metals' important advantages include higher toughness and predictable fracture behavior in all directions, which are fundamentally essential for engineering applications. In coarse-grain polycrystalline alloys, the plastic deformation is mediated by dislocations within the grains. Micromechanisms of dislocation-based plasticity have been well investigated. Taylor, Polanyi, and Orowan's speculative models and Hirsch and Whelan's experimental results clearly demonstrate that the existence of dislocations in the metals, like a double-edged sword, enhances the ductility, while reducing the theoretical strengths of most of the metallic crystalline systems [1]. However, the toughness depends on the integration of both strength and ductility. Hence, designing of advanced metallic materials to answer the challenging strength-ductility dilemma become an urgent call. There is natural limitation on the conventional polycrystalline metallic alloys. In practical uses, there are always some inherent defects in the crystalline phases, which degrade the alloys properties. Recently, the limitation of the crystalline-material strength was passed when the metallic alloys with amorphous structures were successfully synthesized in many material systems through advanced manufacturing methods[2]. Although most of the metallic elements exiting in the nature are present with crystalline structures which are the most stable structures with the lowest energy state, sometimes they can be made by various ways into metastable amorphous solid forms, such as rapid quenching techniques [3-5], mechanical alloying [6-8], accumulative roll bonding [9-12], and vapor condensation [13]. The characteristics of the mechanical, thermodynamic properties of such category of metallic materials are very similar to ceramic glasses, and thus they are also called as metallic glasses. Moreover, by introducing specific crystalline phases, such as crystalline dendrites, in an amorphous matrix, bulk metallic glass-

**for the Mechanical Behavior** 

E-Wen Huang, Yu-Chieh Lo and Junwei Qiao

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/51333

**1. Introduction** 

