Influence of Niobium Pentoxide and Sintering Temperature on Mechanical and Electrical Properties of Glass-ceramics Obtained from Recycled Automotive Windshields

Abstract

Automotive windshield represents a large volume of solid waste due to limited recycling. Glass-ceramics were produced from discarded windshields with Nb₂O₅ as a nucleating agent at concentrations of 0, 5, 10, and 15 wt% at 700 and 800 °C sintering temperatures. Mechanical and electrical characterizations of glass-ceramics were performed. Equibiaxial flexural strength was performed and related to porosity. At 700 °C, Nb₂O₅ favors the monoclinic NaNbO₃ phase with perovskite structure. At 800 °C, high Nb₂O₅ content formed crystalline phases of perovskite and quartz. Dielectric constant and electrical conductivity increased with the addition of Nb₂O₅, reaching 64 and 4.3 μS/m for 15 wt% Nb₂O₅ at 700^oC. At 800^oC, niobium pentoxide reduces the biaxial flexural strength from 28.7 to 14.4 MPa; while increasing the electrical conductivity from 0.10 to 0.33 µS/m for samples with 0 and 15 wt%.

Keywords:
Recycled windshield; niobium pentoxide; flexural strength; electrical properties

1. Introduction

Environmental problems like the increasing number of landfills and depletion of natural resources are becoming more critical. Glass recycling is a well-known process; however, glass is one of the primary solid wastes in landfills. Moreover, the world car production reaches around 70 million units per year¹, and every car uses ca. 13 kg of glass², especially in the windshields. Therefore, the reuse and recycling of automotive windshields^3,4 must be incentivized to reduce the environmental impacts. The development of new materials from solid waste can reduce the consumption of raw materials and energy^4,5,6.

Waste glass can be heat treated to produce glass-ceramics, which are materials with excellent mechanical properties, durability, biocompatibility, and chemical stability, presenting better properties than previous glass⁷. However, the properties are strongly influenced by the chemical composition of raw material and the processing routes, such as heat treatment^7,8. Thus, the applications are diverse, for example, porous glass-ceramics⁹ that absorb sound¹⁰, dense glass-ceramics for tiles of civil construction,^11,12 or even biomedical ceramics applied to prostheses and teeth¹³.

The production of glass-ceramics from solid waste is an alternative to reduce the environmental impacts and the withdrawal of natural resources. Therefore, several residues have been used to obtain glass-ceramics with different characteristics and applications, as shown in the following cited works. Si and Ding¹⁴ produced wollastonite glass-ceramics from window glass powder. Yan et al¹⁰ have produced porous glass-ceramics from high titanium blast furnace slag, and waste glass was applied to sound absorption. Similarly, Cao et al.¹⁵ produced porous glass-ceramics with metallurgical slag and glass residues. Yio et al.¹⁶ used furnace bottom ash and glass. Zhang and Liu¹⁷ produced glass-ceramics from soda-lime glass waste with suitable mechanical properties for civil structures. Kang et al.¹⁸ and Lu et al.⁵ used granite waste to produce glass-ceramics, and they evaluated the nucleating agent effects. Thus, the reuse of solid waste and the production of glass-ceramics has been investigated aiming at diverse applications, in a growing concern about the effects on climate change.

In the case of recycled glass to produce glass-ceramics, the raw materials can come from objects like bottles, soda-lime glass, photovoltaic glass, and liquid crystal display (LCD) glass¹⁹. In addition to these sources, automotive laminated glass has also been reused to produce new materials, despite being more challenging to recycle due to its mixture with polymer PVB - polyvinyl butyrol^2,20. Souza-Dal Bó et al.²¹ used laminated glass waste to manufacture ceramic frits used in floor and wall coatings. Waste produced by the automotive industry improves the mechanical properties of white ceramics²². Therefore, automotive laminated glass can be reused efficiently and economically, benefiting the environment.

Nucleating agents ease the crystallization of amorphous materials. Lu et al.²³ obtained an increase in crystallinity with higher concentrations of MgO in glass-ceramics from waste materials. Similarly, Lu et al.⁵ also obtained increased crystallinity in their glass-ceramics with different boehmite contents. Moreover, the nucleating agents significantly alter the glass-ceramic behavior after the sintering process, affecting the physical, chemical, and mechanical properties^24,25. For instance, Fan et al.¹⁹ analyzed the effects of magnesium oxide on the dielectric properties and flexural strength of glass-ceramics obtained from the glass of liquid crystal display (LCD).

Niobium pentoxide has attracted interest because it affects relevant properties of glasses and glass-ceramics; in these materials, niobium oxide can change density, dielectric constant²⁶, crystallinity^27,28, chemical stability¹³, and biocompatibility¹³. It is used as a nucleating agent^27,29 to promote crystallization that influences the optical and dielectric properties of glass-ceramics³⁰. Moreover, improved chemical stability and biocompatibility by applying niobium oxide as a nucleating agent have been reported²⁹. Hence, great attention has been raised considering adopting of the niobium oxide as a nucleating agent for glass-ceramics in electronic and biomedical applications³⁰. Thus, it has been used in the products such as batteries, solar cells, supercapacitors, prostheses, and even as dental material^31,32,33. Nevertheless, as Zhao et al.²⁶ reported, the effects of this nucleating agent on the microstructure and properties of glass-ceramics are not yet clearly understood. In their work²⁶, the addition of niobium pentoxide increased the dielectric constant of glass-ceramics. This effect was related to the formation of the calcium pyroniobate phase, which has a dielectric constant higher than diopside that was the predominant phase in glass-ceramics produced without niobium oxide as nucleating agent. The authors reported that the enhanced crystallization caused by niobium pentoxide, at least for diopside glass-ceramics, has a mechanism different than those of traditionally nucleating agents such as Fe₂O₃, Cr₂O₃, and TiO₂. In previous work, Gualberto et al.³⁴ identified the formation of sodium niobate with the use of niobium pentoxide. The alkali niobates (K,Na)NbO₃, named KNN, have extremely good piezoelectric properties, although their synthesis is difficult and the crystalline structure complex²⁹.

In this work, we sought to investigate the production of glass-ceramic from automotive windshield glass, with the selection of niobium pentoxide as a nucleating agent. The characterization was carried out to evaluate the influence of the nucleating agent concentration and sintering temperature on the mechanical and electrical properties of the glass-ceramics produced from recycled windshields.

2. Materials and Methods

2.1. Production of glass-ceramics

The used windshield contained two layers of glass intercalated with a polymer layer of polyvinyl butyral (PVB). The glasses were obtained from scrapped cars (model Gol, carmaker Volkswagen, Brazil). The chemical composition of the windshield glass is given in Table 1 ³⁴, and it was evaluated by quantitative x-ray fluorescence. This composition is compatible with float glass produced with silica-soda-lime, that are used in the windshields of different car brands³⁵. According to Saint-Gobain publication³⁶, the commercial original and spare windshields made from float glass have stringent specifications.

Table 1
Glass composition of windshield.

Composition	SiO2	Na2O	CaO	MgO	Al2O3
Wt. (%)	80.10	8.86	7.97	2.34	0.73

The used windshield glass was cut into smaller pieces and placed in the ball mill container along with the alumina grinding bodies. A ball mill with a diameter of 170 mm and a length of 230 mm was used. Three types of spheric bodies were used. They had diameters of 40, 35, and 25 mm, and the quantity of each body type was 15, 30, and 90 balls, respectively. A mass of 1.0 kg of the dry windshield was used. Dry grinding was carried out for 24 h with a 100-rpm rotation.

After this process, the larger pieces of glass and PVB were manually separated and removed from the produced glass powder. The glass powder had its particles classified through a 65-mesh sieve. The specimens were molded with four concentrations of Nb₂O₅ (CBMM, Brazil, purity higher than 98.5%) added to the glass powder. The final composition of specimens had 0, 5, 10, and 15 wt% of niobium pentoxide.

During the forming process, pure water was added (10 wt%) to facilitate the compaction of samples. Samples of 3.0 g were compacted in a circular metal die, with a diameter of 24.0 mm, applying a uniaxial pressure of 25.8 MPa for one minute (P10 ST, 98 kN hydraulic press, Bovenau, Brazil). The shaped specimens were dried in an oven (400/3ND, New Ethics, Brazil) at 110 °C for 1.0 h. Subsequently, the samples were sintered in a muffle furnace (N1100, Fornitec, Brazil) with a heating rate of 5.0 °C/min at the final temperatures of 700 or 800 °C for 1.0 h. Based on works in which glass-ceramics were produced from raw materials similar to the windshield glass^{5,15,19,23,26}, the adopted sintering temperatures were 700 and 800 °C. These temperatures were probably higher than glass transition temperature (Tg) and close to the crystallization temperature (Tc). Therefore, a lower heating rate was used to allow good densification before the crystallization onset. The chosen 1.0 h sintering time is based on past experiences³⁷, comparing glass-ceramics obtained from 1.0 h to 3.0 h sintering times.

The adopted sintering time resulted in a denser glass-ceramic than those sintered for 3.0 h, being 1.0 h preferred in this study. Four compositions and two sintering temperatures were used; thus, eight groups were produced. The L group is related to samples sintered at 700 °C and the H to those sintered at 800 °C. The numbers 0, 5, 10, and 15 wt% are related to the amount of niobium pentoxide added. For each test, at least 10 samples were prepared and tested.

2.2. Characterization

Differential thermal analysis (DTA) of the green bodies was performed in the Simultaneous Thermal Analyzer STA 6000 (PerkinElmer, USA). The tests were carried out in an air atmosphere, with a heating rate of 5.0 °C/min, the same as the sintering process, starting from 30 to 900 °C.

X-ray diffraction (XRD) was performed in a milled sample of each group after sintering. The powder method was selected, with CuKα radiation, at 40 kV/40 mA, goniometer speed of 0.02° per step, with a count time of 0.5 s per step, being 2θ collected from 5 to 75°, with position-sensitive detector (LynxEye, England). Qualitative interpretations of the spectra were performed by comparison with standards of the PDF02 database (The ICDDs Powder Diffraction File^™, PDF^®).

The bulk specific mass, the apparent porosity, and the water absorption were obtained according to Archimedes’ method. Ten specimens were used for each experimental condition studied.

The biaxial flexural strength test was conducted on a universal testing machine AGX-Plus (Shimadzu, Japan), following the standard test method C1499-15 for monotonic equibiaxial flexural strength of advanced ceramics at ambient temperature³⁸. Following the standard, ten specimens were tested to obtain the average of each group. This test is ring-on-ring type, where the uniaxial load is applied by a ring in the center of the specimen, and another ring supports the specimens. Masking tape was used to keep the fragments of the specimen together after the fracture. The breaking biaxial stress supported by the specimen, ${\sigma }_f$, is obtained by Equation (1).

${\sigma }_f=\frac{3F}{2\pi h^2}\left[\left(1-v\right)\frac{D_S^2-D_L^2}{D^2}+\left(1-v\right)ln\left(\frac{D_S}{D_L}\right)\right]$ (1)

The parameter $D_S$ is the mean diameter of the outer ring that supports the specimen. $D_L$ is the mean diameter of the inner ring that loads the specimen. D is the average of three measurements of the diameters, and h is the average of three thickness measurements. F is the breaking load supported by the specimens. The parameter $v$ is the glass-ceramic Poisson coefficient (0.20)³⁹. The samples break within 10-15 s under an average displacement rate of 0.50 mm/min. $D_S$ and $D_L$ diameters have values of 12.0 mm and 5.0 mm, respectively. Ten replicas were used to estimate the mechanical behavior for each condition. The reliability of the biaxial flexural strength results was determined by the Weibull method⁴⁰. Moreover, Tukey tests was applied to the flexural strength with 0.05 confidence level to reveal the influence of niobium pentoxide content.

The scanning electron microscopy (SEM) analysis was carried out on the fracture surface of the specimens. The microscope JSM-6510LV (JEOL, Japan) was used.

The dielectric properties were determined by applying a voltage of 1.0 V, varying the frequency from 20 Hz to 100 kHz; however, the parameters were taken at 1.0 kHz. The electrical measurement was performed with Wayne Kerr 3245 inductance analyzer (Wayne Kerr Electronics, UK). Additionally, the dissipation factor ($k$), which allows evaluating the electrical conductivity of the material (${\sigma }_{el}$), was measured according to Equation (2).

${\sigma }_{el}=k\omega C\frac{L}{A}$ (2)

where C is the capacitance of electrode, $\omega$ is the angular frequency, L is the length, and A is the cross-sectional area⁴¹. Five specimens from each group were tested. This test used adhesive electrodes for electrostimulation (Carcitrode, Brazil) cut in the sample size. The electrodes were assembled as a sandwich, and the measurement of glass-ceramic properties was performed. A four-wire configuration was used to improve the accuracy by reducing the effects of Ohmic resistances of contacts and wires. The average cross-section area and the length were 380 mm² and 3.9 mm, respectively.

3. Results and Discussion

The differential thermal analysis (DTA) data are depicted in Figure1. There is an inflection point with low intensity in the curves in the 600-700 °C range. This event is likely associated with the glass transition temperature, Tg. Zhao et al.²⁶ reported the Tg was close to 635 °C for glass-ceramics produced with vitreous materials of similar composition with Nb₂O₅ addition. Nonetheless, they also did not observe a significant influence of Nb₂O₅ on Tg. As the nucleation in glasses generally starts around 50 °C above the Tg^26,42,43, the sintering temperatures adopted for the glass-ceramics in this paper were probably higher than the Tg.

Figure 1
Differential thermal analysis curves of green bodies with and without the addition of Nb₂O₅ at concentrations of 0, 5, 10, and 15 wt%.

Figure 2 depicts the diffractograms of sintered glass-ceramics for 1.0 h at 700 and 800 °C for different niobium pentoxide contents. Crystalline phase peaks are identified in all glass-ceramics, except at 700 °C without Nb₂O₅, although not detected by DTA analysis, likely due to the short time in high temperature of thermal analysis. Zhao et al.²⁶ also identified the reduction in the crystallization temperature of glass-ceramics with the addition of this nucleating agent. In the present work, temperatures of 700 and 800 °C were used to sinter glass-ceramics, both higher than Tg, to crystallize the material with Nb₂O₅. According to Morais et al.⁴³, crystalline phases were found for pure Nb₂O₅ above 550 ºC. Below 550 ºC, the samples remain amorphous.

Figure 2
Diffractograms of sintered glass-ceramics for 1.0 h, at 700 and 800 ºC for different niobium pentoxide contents.

Different crystallization of XRD spectra were observed with Nb₂O₅ presence in 5, 10, and 15 wt%. At 700 ºC, the formation of the NaNbO₃ monoclinic phase with perovskite (P) structure (JCPDS 01-074-2449 and 00-044-0060) was observed with characteristic peaks at 14.42, 17.18, 22.58, 28.35, 29.10, 29.59, 32.42, 36.58, and 46.25. In addition, peaks between 51.1 and 57.91 (2θ) were observed by Morais et al.⁴³ and Li et al.⁴⁴. The diffractograms for samples sintered at 800 ºC showed the monoclinic formation of NaNbO₃, the same phase observed in samples at 700 ºC; however, the crystalline peak was associated with quartz (Q) ^45,46.

The diffractograms of glass-ceramic produced at 800 °C reveal a peak at 21.74° related to the highest intensity peak of the crystalline structure of quartz (Q). Since the second-most intense peak is not identified at 35.67°, it suggests an imperfect structure. Furthermore, the peak at 21.74° is not observed at 700^oC. Thus, even without adding the nucleating agent, an incipient crystallization occurs at 800^oC but not at 700 °C.

Table2 shows the sintered glass-ceramic average specific mass, porosity, and water absorption for 1.0 h at 700 and 800 °C. There is an increase in the values of apparent porosity with the concentration of niobium pentoxide at both temperatures. When comparing the sintering temperatures, it is observed that the specimens at 800 °C absorb less water and have lower apparent porosity, even though they are less dense. In this case, this may be due to greater surface glazing at 800^oC, which prevents water absorption. This vitrification does not occur in the sample bulk, as evidenced by SEM (Figure3). This phenomenon hinders the water penetration in Archimedes’ tests, also observed in the previous work³⁴.

Table 2
Specific mass, porosity, and water absorption of sintered groups at 700 and 800 °C.

Group	Property	Mean ± SD
L0	Specific mass (g/cm3)	2.422±0.045
	Water absorption (%)	0.417±0.665
	Porosity (%)	0.998±0.562
L5	Specific mass (g/cm3)	2.393±0.015
	Water absorption (%)	0.642±0.113
	Porosity (%)	1.547±0.273
L10	Specific mass (g/cm3)	2.332±0.076
	Water absorption (%)	2.085±0.657
	Porosity (%)	4.945±1.467
L15	Specific mass (g/cm3)	2.205±0.019
	Water absorption (%)	6.789±0.443
	Porosity (%)	16.055±0.998
H0	Specific mass (g/cm3)	1.868±0.006
	Water absorption (%)	0.042±0.009
	Porosity (%)	0.079±0.017
H5	Specific mass (g/cm3)	1.900±0.009
	Water absorption (%)	0.146±0.030
	Porosity (%)	0.278±0.058
H10	Specific mass (g/cm3)	1.839±0.013
	Water absorption (%)	0.684±0.174
	Porosity (%)	1.267±0.322
H15	Specific mass (g/cm3)	2.062±0.016
	Water absorption (%)	3.119±0.309
	Porosity (%)	6.639±0.663

Figure 3
SEM photomicrographs of the glass-ceramic of the fracture surfaces of sintered samples at 700 ºC: (a) 0, (b) 5, (c) 10, and (d) 15 wt% of Nb₂O₅.

Samples sintered at 800 °C exhibited a lower specific mass than those sintered at 700 °C, even with less apparent porosity and water absorption. Specific mass is influenced by the flow viscosity of the sintering materials. Viscous flow and crystallization increase at high temperatures; however, an intense crystallization before sintering produces a porous material^8,19. Thus, a higher temperature favors crystallization, hinders the sintering, and produces a lower specific mass. This fact is consistent with the higher porosity observed in the fracture surface of samples sintered at 800 °C (Figure4).

Figure 4
SEM photomicrographs of the glass-ceramic of the fracture surfaces of sintered samples at 800 ºC: (a) 0, (b) 5, (c) 10, (d) 15 wt% of Nb₂O₅.

At elevated temperatures, Zhang and Liu¹⁷ observed higher surface densification than in bulk. Moreover, at higher temperatures, the viscous flow resistance of glass is lower at the surface than in bulk. Thus, it is noticed that specimens sintered at 800 °C densified more intensely on the surface, resulting in lower water absorption and apparent porosity.

Figures 3 and 4 show the SEM images of glass-ceramics sintered at 700 and 800 °C, respectively. The fracture surface of samples with 0, 5, 10, and 15 wt% of Nb₂O₅ are depicted. Higher sintering temperature (group H) increases the number of pores and, in minor effect, its size. The morphology of the fracture surface (Figure3– 700^oC and Figure4– 800^oC) shows more pores at high temperatures. However, the specific mass is higher for the materials processed at 700^oC than 800^oC, indicating the vitrification of the surface processed at 800^oC that blocks water permeation.

The average biaxial flexural strengths are shown in Table3. It is observed a reduction of the strength with increasing concentration of Nb₂O₅ at both temperatures. Moreover, the standard deviation reduces with the nucleating agent. This effect may be related to the intense crystallization of the specimens, with high contents of nucleating agents that produce more homogeneous materials. Furthermore, the Weibull modulus (m) of most groups is greater than 3. For fragile materials, the m parameter is considered acceptable in the range of 3-15, with the largest being the most reliable^19,24. The exception is the L15 group that presents a borderline value of 2.96 modulus. Although it is slightly lower than 3, this value is very close to the acceptable value. The determination factors R², depicted in Table3, are all greater than 0.88. According to work carried out by Defez et al.⁴⁷, R² values for flexural strength of ceramics greater than 0.80 are considered satisfactory. The equibiaxial mechanical test used here has advantages concerning the uniaxial test. Multiaxial stress states are required to evaluate failure theories applicable to component design and to assess samples with anisotropic flaw distributions³⁸. Moreover, equibiaxial tests reduce the effects of specimen edge preparation as compared to uniaxial tests.

Table 3
Biaxial flexural strength, Weibull parameters, and Tukey analysis. Tukey test was applied to the averages within each temperature with a 0.05 confidence level. The superscripts with the same letter mean no statistical difference.

Group	Max stress (MPa)	Weibull modulus m	R2
L0	34.83±10.32a	3.25	0.96
L5	21.04±5.63b	3.63	0.92
L10	14.67±3.27c	4.44	0.92
L15	11.22±3.35c	2.96	0.96
H0	28.66±7.59d	3.63	0.88
H5	16.66±3.29e	4.92	0.93
H10	13.80±2.27e	6.14	0.98
H15	14.36±1.82e	7.66	0.97

• Tukey test was applied to the averages within each temperature with a 0.05 confidence level. The superscripts with the same letter mean no statistical difference.

The Tukey test was used to compare post hoc all pairwise. An equal letter was termed for conditions without statistical significance (p>0.05); otherwise, different letter shows means statistically significant (p<0.05). At 700^oC, the strengths for L10 and L15 are similar (letter c). Without a nucleating agent (L0, letter a), this nucleating content produces a significantly higher mechanical strength than other pentoxide contents. L5 presents a significant strength value for a 0.05 confidence level. At a high temperature, 800^oC, the 5-15% content range does not produce a difference in the values, using the letter e. In other words, the addition of niobium pentoxide affects the mechanical behavior compared to without and with niobium pentoxide, but the higher temperature affects more than the nucleating amounts.

Porosity significantly affects the mechanical resistance, as shown in Figure5. The porosity behavior is roughly the opposite of that observed for the resistance for each group. Pores have null mechanical resistance; thus, the biaxial strength reduces with porosity. Moreover, adding a nucleating agent increases porosity and reduces the resistance, regardless of the temperature. Even though the biaxial flexural strength values are not high, they are comparable with results of glass-ceramic composed of apatite and mullite crystals, glass-ceramic obtained from solid waste¹⁶, calcium phosphate cement, used in dentistry⁴⁸, as well as those of dental porcelain layers⁴⁹. The maximum strength is obtained for 0% niobium pentoxide at 700^oC, with low porosity and a mean porous diameter of ca. 5 µm.

Figure 5
Biaxial flexural strength and porosity at 700 (L) and 800 ºC (H) for 0, 5, 10, and 15 wt% Nb₂O₅.

The electrical properties measured at the frequency of 1.0 kHz of glass-ceramic are shown in Table4. The niobium pentoxide content and the sintering temperature affect the measured electrical properties in different ways. The most significant dielectric constants occurred for those glass-ceramics produced with 15 wt% of Nb₂O₅ content. The higher Nb₂O₅ concentration favors the increase of the dielectric constant due to the formation of the perovskite structure, NaNbO₃. Niobium pentoxide promotes a solid-state reaction with glass powder, which contains sodium in its composition, forming sodium niobate above 550 °C⁴³. At 700^oC, the higher Nb₂O₅ concentration contents increase the dielectric constant, likely caused by the crystals of NaNbO₃ with only a perovskite structure. At 800 °C, the increase in the dielectric constant is smaller, probably because of the quartz phase. Thus, as observed by X-ray diffractograms, the produced glass-ceramics have two phases: perovskite sodium niobate and quartz. Due to the shape irregularities of some samples, the standard deviation of electrical parameters was relatively high concerning the mean, as for L5. This adverse effect is not found in very homogenous materials, such as metallic alloys. For ceramics, on the other hand, even with replicas, this dispersion can occur.

Table 4
Electrical properties measured at 1.0 kHz.

Group	Dielectric constant	Dissipation factor (%)	Electrical conductivity (µS/m)	Capacitance (pF)
L0	9.4±0.5	14±9	0.1±0.09	8.3±0.5
L5	9.8±0.2	20±26	0.09±0.14	9.0±0.1
L10	11.8±0.6	10±2	0.07±0.02	11.4±0.4
L15	63.6±3.8	121±7	4.3±0.2	64±4
H0	7.8±0.5	25±12	0.10±0.07	5.2±0.5
H5	8.2±0.3	14±9	0.07±0.04	6.3±0.2
H10	8.9±0.4	17±9	0.09±0.05	7.9±0.3
H15	17.9±0.61	33±2	0.33±0.03	18.0±0.8

Niobates exhibit high dielectric constant, high polarization, and piezoelectric properties¹⁶. Thus, the monoclinic phase of NaNbO₃ with a perovskite structure was formed in samples with the addition of niobium pentoxide. This phase supports the increase in dielectric constant for higher oxide concentration.

According to Yongsiri et al.⁵⁰, glasses may have a dielectric constant from 4.5 to 10. Without the addition of niobium pentoxide, the dielectric constant of glass-ceramic did not differ significantly from commercial glasses. However, with a small addition of niobium pentoxide, an increase in the dielectric constant is observed, reaching the highest value in the concentration of 15 wt%.

The formation of the crystalline perovskite structure reduces the ion content in the glass structure, decreasing the number of mobile charge carriers. Therefore, the electric conductivity reduces up to 10 wt%. Nonetheless, for 15 wt% at 700^oC, the electrical conductivity is higher and likely is related to the high dielectric constant. Shanker et al. (2009)⁵¹ reported a constant dielectric of 209 for pure NaNbO₃ measured at room temperature and 1.0 kHz. For condition H15 (15% Nb₂O₅ and 800^oC), the dielectric constant is approximately 18, and for L15 (15% Nb₂O₅ and 700^oC) is approximately 64. At 800^oC, the quartz is present in a high amount, as shown in the XRD peaks of Figure2. Quartz has a low dielectric constant, around 3.8, which lowers the glass-ceramic constant. Dissipation factor represents the relative energy expenditure to storage a certain amount of energy, such as the d.c. Ohmic conductivity or a dissipation due to rotation of dipoles⁵². Although the relative quantity of phases, neither their morphology, were determined, these aspects likely reduce the efficiency of capacitors and increase the electrical conductivity.

As seen in x-ray diffraction, the addition of a nucleating agent promoted the formation of the crystalline structure of sodium niobate. In Singh et al.⁵³ paper, this structure showed a dielectric constant of 20.7, measured at the same frequency as the present work. The glass-ceramic specimens produced with only glass powder or low agent content exhibit characteristics of insulating material. However, the specimens with higher niobium oxide content have electrical conductivity features that approach the semiconductor range (${10}^0<{\sigma }_{el}<{10}^{10}$µS.m^-1)⁵⁴, with the sample L15 within this extensive range covering ten orders of magnitude typical for semiconductors. Furthermore, the dielectric constant augments strongly with the addition of 15 wt% of Nb₂O₅, likely caused by the greater quantity of crystals with perovskite structure. Thus, a higher dielectric constant occurs in specimens with 15 wt% of Nb₂O₅ than those that do not form sodium niobates or have a negligible amount.

Alkaline niobates, which include sodium niobate, have perovskite structures and semiconductive features. The Nb⁵⁺ ion induces ferroelectric behavior, which presents spontaneous polarization. The polarization promotes charge separation, created by the electronic transition from the valence band to the conduction band^55-57. Alkaline sodium and potassium niobates, KNN, are a promising alternative to replace lead zirconate titanate, PZT, which has high lead content, highly harmful to the environment⁵⁸. However, although these alkaline niobates have excellent piezoelectric properties, their application is still limited by difficulties in their synthesis. One of the main challenges is to control the property reproducibility, usually associated with synthesis methods and sintering processes³³. Thus, it becomes clear that materials containing alkaline niobates exhibit technological interest, and the production route with recycled glass powder and niobium pentoxide presented here may be a promising route.

Higher sintering temperatures did not increase the dielectric constant, despite facilitating crystallization. This behavior is affected by the greater porosity⁵⁹ or by the formation of two crystalline phases, decreasing the amount of the sodium niobate crystals. The higher electrical conductivities are also related to a higher percentage of niobium pentoxide. The highest electrical conductivity, 4.3 μS/m, is observed at 700 °C with 15 wt% of Nb₂O₅. Some glass-ceramics produced have electrical conductivity similar to semiconductor materials widely used for photocatalysis^33,54. For example, silicon carbide, a semiconductor, can have electrical conductivity in the range of 5-10 μS/m⁶⁰, comparable to the obtained results.

4. Conclusion

The obtained results show that using recycled glass powder from automotive windshields is possible to produce glass-ceramic. This glass-ceramic could be applied in electric devices, besides the reduction use of mineral resources from nature. The conclusions of the paper are:

i) The addition of Nb₂O₅ allowed crystallization to occur at 700 °C, which does not occur with pure windshield glass powder. Increasing the sintering temperature to 800 °C promotes crystallization regardless of the addition of a nucleating agent.

ii) A higher concentration of niobium oxide produces the monoclinic NaNbO₃ crystals at 700 °C; however, when sintered at 800 °C, two crystalline phases (monoclinic sodium niobite and quartz) are formed.

iii) The sintering temperature and the Nb₂O₅ contents influence significatively the electric parameters but do not affect the behavior uniformly.

iv) Equibiaxial flexural test is adequate for ceramics with flaw distribution and revealed that Nb₂O₅ addition in the range 0-15 wt% reduces the strength. At 800^oC, Tukey statistical test reveals that the mechanical behavior can be classified simply as without or with niobium pentoxide. Moreover, the strength reduced with the porosity, which increased with the addition of niobium oxide. The maximum strength is obtained for 0% niobium pentoxide at 700^oC, with low porosity and a mean porous diameter of ca. 5 μm.

5. Acknowledgements

This work was supported by FAPERJ and CNPq. This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) – Finance Code 001.

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Publication Dates

• Publication in this collection
  30 May 2022
• Date of issue
  2022

History

• Received
  04 Nov 2021
• Reviewed
  25 Apr 2022
• Accepted
  11 May 2022