Study of the structural and dielectric properties of ceramic obtained from residual electrocoagulation

Abstract

This work aims to develop an eco-friendly ceramic obtained from the sludge generated in the textile industry treatment by electrocoagulation to demonstrate a possibility of natural raw materials’ extraction decrease. The sludge generated by the electrocoagulation treatment was treated by some sintering processes. The presence of two crystalline phases was confirmed through the Rietveld refinement: Fe₂O₃ and NiCr₂O₄. In the microwave range it is possible to confirm the relative stability of dielectric permittivity over the wide frequency range. The Temperature Coefficient of Capacitance results demonstrate that the samples can be classified as a class 4 ceramic capacitor. The activation energy measurement profile is similar for all samples. The results showed that samples obtained from the sludge present low dielectric permittivity, two thermo-activated processes one to water loss and another to conduction with activation energy of approximately 0.6 eV for all analysed samples. The manufactured samples can be applied as a barrier layer capacitor and dielectric resonator antennas.

Keywords

Eco-friendly ceramic sludge textile industry treatment electrocoagulation dielectric properties

Introduction

Normally, technical ceramics use natural and pre-treated raw materials, but some efforts have been made to reuse industrial wastes, not only as minor additives but also as major components [1]. According to Mymrin et al. [2] waste utilisation is a valuable source of components for new materials’ production; thus it is environmentally and economically the best way for their management.

The textile industry produces one of the worst environmental contaminants due to the disposal of industrial residue without proper treatment. The solid residue generated, especially the textile sludge, is a semi-solid material that contains organic and inorganic matter. Since textile sludge is produced in large quantities and disposal in landfills is high, the reuse of textile sludge can contribute to reducing the environmental impact caused by the disposal of this residue in the environment [3 -5]. Electrocoagulation (EC) has recently received attention because it appears as a simple, versatile technology and is environmentally compatible, with great potential for effective removal of various pollutants in textile industrial wastewater [6,7]. EC discharges flocs of metallic hydroxides into the effluent to be cleaned, by electrodissolution of soluble anodes [7 9]. The metal ions released from the anode, at an appropriate pH, remove the dissolved contaminants [7] by sedimentation or flotation [9,10]. During the treatment of the textile industrial wastewater by EC Electrocoagulated Metal Hydroxide Sludge (EMHS) is produced, which needs to be deposited in controlled landfills [11,12].

The aim of this study was to study the sludge generated in the textile industry treatment by electrocoagulation and to make feasible its use as a barrier layer capacitor and as a dielectric resonator antenna (DRA) [13 -18].

Experimental procedure

Sample preparation

The effluents were obtained from an electrocoagulation treatment of textile industrial wastewater of the industry that manufactures hammocks. The samples were named A and B. Filters (100% cellulose) were used for separating the solid part of the sludge from the liquid. The preparation procedure of the pellets for further measures can be seen in Figure 1. The calcination was made in a furnace at 600 °C per 4 h and a porcelain mortar was used for the homogenisation.

To remove chloride and other salts for subsequent calcination, the samples were washed with distilled water. The solution was manually mixed in a beaker with a glass rod for 10 min. Double volume of water was used in sample B, because this sample had not undergone any previous electrocoagulation treatment and thus contained a higher amount of salts.

The samples were pressed into a metallic mould using a uniaxial press for 5 min under a constant pressure of 8.1 MPa. The sintering process of the green bodies was carried out in a furnace at 800°C, 900°C and 1000°C per 4 h. After the sintering process, the ceramics (pellets and cylinders) were polished. The nomenclature of the samples is shown in Table 1.

Table 1

Nomenclature of the samples.

Sample	Sinterisation
A8, B8	800°C/4 h
A9, B9	900°C/4 h
A10, B10	1000°C/4 h

Sample characterisation

Thermal analysis and X-ray Fluorescence (XRF) were performed on the calcined sample. The sintered powders were examined at room temperature by X-ray diffraction (model X'Pert Pro MPD – PANalytical) using CoKα radiation (λ = 1.788965 Å), to identify the phases formed. The dielectric properties of the samples were analysed through the Complex Impedance Spectroscopy (CIS) and Hakki-Coleman method. Impedance spectroscopic examination at room temperature was performed on impedance analysers: Agilent 4294A (40 Hz–110 MHz) and Agilent E4991A (1 MHz–3 GHz). For impedance spectroscopic examination with temperature variation, the Solartron impedance analyser 1260A was used. The Hakki-Coleman method was employed using a Vector Network Analyser (Agilent – model N5230A) at room temperature.

Results and discussion

Thermal analysis and X-ray fluorescence

Thermal Gravimetry (TG) and Differential Scattering Calorimetry were used for the study of the mass loss and the energy change with the increase of temperature due to variation in the sample (water losses, organic material combustion, volatile material losses, reaction between components in the sample, etc.). In general, the processes that can be occur with temperature are:

The first process is associated with dehydration of the sample (i.e. the humidity, interstitial water or hydroxides present in the sample). The second is related to the combustion reaction of organic materials and the third is related to the volatile materials (metallic oxides with low fusion temperature or gas decomposition of halides).

The thermogravimetry and the differential scanning calorimetry (DSC) in Figure 2 show three processes marked in the graphic, of which, first, the dehydration associated with incomplete drying of sludge occurs, followed by, second, organic degradation by organic material from electrocoagulation, third, the metallic oxides’ formation and finally halide decomposition. The analysis of TG and DSC enables the choice of temperature fabrication in which the electroceramics are more stable with removal of organic matter and halides [19 -25].

The XRF was performed in the sludge for identification and quantification of elements present in the sample. Table 2 presents the results for two samples. The high concentration of iron in the samples is due to corrosion of the steel electrode in both electrocoagulation processes. The high concentration of chlorates reinforces the necessity for the sample washing process to remove them as shown in Figure 1. The pretreatment of sludge with distilled water removes most of the chlorates, avoiding the sample break with the liberation of chlorine gas in calcination and sintering of electroceramic fabrication.

Table 2

Identification and quantisation of elements present in samples A and B.

	Sample
	A		B
Element	Sludge (% element)	Treated sludge (% element)	Treated sludge (% element)
Fe	49.01	74.05	75.60
Cl	19.27	−	−
Ca	12.05	−	1.21
Cr	11.26	16.95	11.12
Ni	7.20	8.04	1.21
Sr	1.22	−	−
Mn	−	0.96	1.04
Al	−	−	2.20

X-ray diffraction

The ICSD (Inorganic Crystal Structure Database) was used for a pre-identification process. The presence of two crystalline phases was confirmed through Rietveld refinement by DBWS9807 associated with graphical interface DBWStools [26 -28]: Fe₂O₃ (ICSD: 201098, Rhombohedral, space group R3ch) and NiCr₂O₄ (ICSD:84376, Cubic, space group Fd3mz) as shown Figure 3. It is not possible to identify the elements Mn and Al in the refinement because they are in low concentrations. These elements (Mn and Al) can be in the crystalline lattice of the iron oxide or nickel chromate. The refinement showed good concordance.

The R_wp (R-factor weighted profile) values (see Table 3) were in the range between 10% and 20%, which are acceptable values for X-ray Powder Diffraction [29]. The values of R_Braag were below 9 for almost all the phases refined, indicating that the refined crystal structure model is in good agreement with the observed data [29]. The R_Bragg values in sample B8 for nickel chromate were higher than 9, which was due to the inadequate crystallographic information file (CIF) utilised in this Rietveld refinement, where the same CIF was used for all samples in the refinement process, but it is in accordance with literature [29] and the concentrations of iron oxide and nickel chromate were concordant with other B samples. The analyses showed that removal of chlorates was effective. The presence of iron, manganese, chromium and nickel in sludge is explained by the composition of 304 stainless steel.

Table 3

Rietveld parameters and sample composition obtained by X-ray diffraction.

Sample	R _wp	Phases	R _Bragg	wt-%
A8	16.33	Iron oxide	5.31	70.10
A8	16.33	Nickel chromate	8.28	29.90
A9	16.47	Iron oxide	4.87	70.78
A9	16.47	Nickel chromate	7.29	29.22
A10	16.63	Iron oxide	4.70	71.74
A10	16.63	Nickel chromate	6.83	28.26
B8	16.08	Iron oxide	5.41	79.49
B8	16.08	Nickel chromate	11.35	20.51
B9	16.26	Iron oxide	3.97	75.29
B9	16.26	Nickel chromate	7.29	24.71
B10	17.29	Iron oxide	4.73	75.78
B10	17.29	Nickel chromate	8.19	24.22

Dielectric properties

The dielectric properties (dielectric permittivity, dielectric loss, conductivity and temperature coefficients) are directly related to the crystalline structure [17]. Other parameters that can alter the dielectric properties are the synthesis (reaction to oxide formation) and the processing of the electroceramic (conformation of ceramic in pellets, cylinder, cube, etc.) [30,31].

The CIS method was used for measurements in the RF range at room temperature. The measures were carried out by the capacitance measurement of pellets with sides metallised to form a parallel plate capacitor. The dielectric loss was obtained directly by dissipation factor (D) and the dielectric permittivity values were obtained through the capacitance (C_p) from Equation (1): (1) where ε_r is the relative dielectric permittivity, C_p is the capacitance (Farad), h is the height of pellets (m), A is the circular area of electrodes (m²) and ε₀ is the dielectric permittivity in the vacuum (Farad m^-1).

Figure 4 shows ε_r and dielectric loss (tan δ) for the samples from 100 Hz to 100 MHz. All samples presented high ε_r at low frequency (100 Hz) with values reaching 1000. The ε_r decreases with frequency due to the dielectric relaxation [32,33]. It is possible to see two relaxation processes, where the first occurs in low frequency and the second above 1 kHz (see Figure 4(b)). The samples sintered at lower temperatures present the higher values in minor frequencies as a result of the electrode effect, which occurs in lower frequencies. The irregularities on the surface of the sample (lower densification) amplify this effect. The dielectric permittivity values converge to values close to each other above 1 MHz. Thus, the first relaxation process observed in the dielectric spectra is the electrode effect or presence of water in the ceramic, corresponding to the hygroscopic characteristic. The second relaxation process refers to phase: Fe₂O₃+ NiCr₂O₄.

In the dielectric loss measurement (Figure 4(a)) the loss tangent (tan δ) presents two peaks relative to the relaxation processes similarly in the dielectric permittivity spectra. There is one peak well defined at 1 MHz and another below 100 Hz. The samples present a similar chemical composition and geometric parameters (radius and height) and so the relaxation process in around 1∼3 MHz is characteristic of the ceramic, while the relaxation process at <100 Hz is due to electrode effect or humidity.

The results of dielectric properties in microwave range are shown in Figure 5 and possibly confirm the relative stability of dielectric permittivity over wide frequency range.

These results are in agreement with the measurements obtained in the Agilent 4294A, but in the frequency range analysed it was observed that the temperature was a factor determinant in dielectric properties of samples B, once the dielectric permittivity increasing of temperature probably due improvement of density of samples [34], but for samples A, it is not observed significant changes.

The temperature effect on dielectric properties of electroceramic is an important study for a wide range of applications such as capacitors, resonant circuit applications, filters, barrier layer capacitors, etc. [35 -37]. To verify the extent to which electroceramics are insulating activation energy was used (energy necessary to charge transport in electroceramic). Moreover, it is feasible to measure the temperature coefficient of capacitance, a very important parameter in the electronic industry. Pellets painted with a conductor ink were measured from 30 to 300°C for analysing the temperature effect.

The dielectric analysis shows an interesting behaviour of the samples with temperature variation. Figure 6 shows the dielectric analysis of dielectric permittivity with two distinct profiles. The first profile extends from room temperature to around 120°C (P1) and the second from 120 to 300°C (P2). The normalised dielectric permittivity and real part of impedance were used to facilitate viewing of variation properties with temperature. For P1 the dielectric permittivity in the samples presents a decreasing of values as a result of water loss by temperature rise.

Since the chloride loss begins close to 800°C, the maximum temperature of the water loss is affected by sintering temperatures, whereas for A8 and B8 it is close to 150°C, for A9 and B9 it is 120°C and for A10 and B10 it is around 100°C. The samples A8 and B8 present a strong interaction of chloride and water due to higher concentration of chloride. At a higher temperature, it is expected that chloride concentration gradually decreases, as observed for samples A10 and B10 where the water loss with temperature is close to 100°C.

Temperature Coefficient of Capacitance (TCC) is an important electric parameter for development of electronic engineering circuits and it was calculated by the CIS study using Equation (2) [35]: (2)

The TCC values are shown in Table 4. Generally, a decreasing of capacitance with temperature occurs. However, as suggested by the International Electrotechnical Commission (IEC), Electronic Industries Alliance (EIA), IEC/EN 60384-1, IEC/EN 60384-8/9/21/22 and EIA RS-198, these values are necessary for application in materials such as ceramic capacitor, resonant circuit or barrier layer capacitor. In this work, the TCC results demonstrate that the samples can be classified as class 4 ceramic capacitor (can be applied as a barrier layer capacitor).

Table 4

Temperature coefficient of capacitance for the samples.

Sample	10 kHz	100 kHz	1 MHz
A8	−9940.18	−25521.07	−27399.62
A9	−7497.58	−12249.57	−15912.72
A10	−13204.30	−26220.54	−21733.57
B8	−5802.92	−19331.81	−13541.05
B9	−6456.27	−10129.57	−19225.90
B10	−20013.40	−25076.27	−15938.014

The electric thermo-activated process can be studied by CIS and some electric properties are useful for the analysis of this process: imaginary part of impedance, electric modulus, dielectric permittivity and real part of conductivity. Each peak in the graphic of the imaginary part with frequency shows one or more peaks associated with the relaxation process. The Arrhenius relation measures the magnitude of the thermo-activated process by Equation (3) [35,38]: (3) where A is according to the frequency of maximums of spectra of the imaginary part of the electric modulus (M′), impedance (Z′), dielectric permittivity (ε′) or by the conductivity, A₀ is a pre-exponential factor, k is the Boltzmann's constant, T is the absolute temperature and E_a is the energy necessary for the thermo-activated process to occur (generally called activation energy).

The Arrhenius relation from maximums frequency for M′, Z′ and also conductivity are shown in Figure 7 and these relations show two thermo-activated processes. The E_a measurement profile is similar for all samples; thus only the graphs for sample A10 are shown. The first thermo-activated process is associated with the conduction process in high temperature (>100°C); the second process is related to the water loss that occurs under 100°C.

The maximum frequency shifts for smaller frequencies can be explained by the loss of water or water/chloride when the sample is heated, see Figure 7(b). The values of activation energy measured for each parameter are shown in Table 5, where for samples sintered at 900°C it was not possible to measure each component (water loss and conduction) as in other samples, and this impossibility can be due to the high chloride concentration. The maximum decomposition of chlorides occurs around 850°C (see Figure 2), combined with humidity present in the sample. Only samples A9 and B9 did not present activation energy values for each process close to each other.

Table 5

Activation energies obtained for the samples by Arrhenius relation for conductivity, Z ″ and M″.

	Conductivity		Z″		M″
Sample	Ea (eV)	Ea (eV) Water loss	Ea (eV)	Ea (eV) Water loss	Ea (eV)	Ea (eV) Water loss
A8	0.63	−0.85	0.63	−0.84	0.63	−0.71
A9	1.22	−−	1.11	−	0.73	−
A10	0.59	−1.02	0.65	−0.87	0.6	−0.96
B8	0.6	−1.23	0.65	−1.16	0.61	−0.93
B9	1.44	−−	−	−	0.98	−0.66
B10	0.62	−1.1	0.71	−1.06	0.62	−0.92

The Nyquist plots were used to study the electric phenomenon present in polycrystalline ceramics and caused by grain, grain boundary and interfacial effect by CIS. The Nyquist plots are shown in Figure 8 for samples sintered at 800°C and 1000°C. The samples sintered at 900°C also presented arcs in Nyquist plots, but the magnitude of impedances did not allow the comparison of the results with other samples as seen in Table 6. The Nyquist plots demonstrate that samples with high sintering temperatures have lower impedance compared with other temperatures. Samples B, due to their high impedance, provide an electroceramic more resistive than samples A. The samples sintered at 900°C follow the same behaviour of other electric results, presenting electric characteristic very dispersive when compared to other temperatures and this can be associated with chloride formation in this sintering temperature as previously discussed.

Table 6

Parameter for elements utilised in equivalent circuit for best fitting experimental results.

			Sample
	Element		A8	A9	A10	B8	B9	B10
30°C	R_g		1.47 × 10⁵	2.64 × 10⁸	1.16 × 10⁵	2.20 × 10⁵	1.04 × 10⁷	2.92 × 10⁵
	C_g		7.55 × 10⁻¹¹	5.88 × 10⁻¹¹	5.35 × 10⁻¹¹	6.95 × 10⁻¹¹	2.15 × 10⁻¹¹	5.19 × 10⁻¹¹
	R_gb		2.18 × 10⁵	2.37 × 10⁸	6.03 × 10⁴	2.27 × 10⁵	1.00 × 10⁸	2.55 × 10⁵
	CPE_gb	P	1.13 × 10⁻⁶	3.22 × 10⁻¹¹	2.72 × 10⁻⁶	4.14 × 10⁻⁷	6.31 × 10⁻⁸	1.41 × 10⁻⁶
		n	0.357	0.982	0.255	0.405	0.386	0.249
260°C	R_g		2.45 × 10⁴	8.49 × 10⁶	9.73 × 10⁴	8.92 × 10⁵	8.62 × 10⁶	6.95 × 10⁴
	C_g		4.09 × 10⁻¹¹	2.12 × 10⁻¹¹	7.41 × 10⁻¹¹	5.98 × 10⁻¹¹	1.24 × 10⁻¹¹	4.04 × 10⁻¹¹
	R_gb		4.06 × 10⁵	1.76 × 10⁷	2.06 × 10⁴	8.97 × 10⁵	8.95 × 10⁶	1.84 × 10⁵
	C_gb		4.76 × 10⁻¹¹	2.92 × 10⁻¹¹	4.55 × 10⁻¹¹	1.19 × 10⁻¹⁰	4.66 × 10⁻¹¹	8.07 × 10⁻¹¹
280°C	R_g		1.71 × 10⁴	4.33 × 10⁶	4.95 × 10⁴	7.92 × 10⁵	4.70 × 10⁶	1.22 × 10⁵
	C_g		5.06 × 10⁻¹¹	8.50 × 10⁻¹¹	6.69 × 10⁻¹¹	6.81 × 10⁻¹¹	5.65 × 10⁻¹¹	9.38 × 10⁻¹¹
	R_gb		2.45 × 10⁵	7.86 × 10⁶	4.14 × 10³	1.77 × 10⁵	5.68 × 10⁶	3.01 × 10⁴
	C_gb		4.86 × 10⁻¹¹	1.49 × 10⁻¹¹	4.55 × 10⁻¹¹	7.67 × 10⁻¹¹	1.17 × 10⁻¹¹	4.44 × 10⁻¹¹
300°C	R_g		1.24 × 10⁴	2.20 × 10⁶	3.20 × 10⁴	6.47 × 10⁵	1.01 × 10⁶	6.65 × 10⁴
	C_g		5.46 × 10⁻¹¹	7.13 × 10⁻¹¹	6.58 × 10⁻¹⁰	5.65 × 10⁻¹¹	2.15 × 10⁻¹⁰	1.11 × 10⁻¹⁰
	R_gb		1.51 × 10⁵	3.33 × 10⁶	4.88 × 10³	5.28 × 10⁴	2.61 × 10⁶	2.09 × 10⁴
	C_gb		4.99 × 10⁻¹¹	1.53 × 10⁻¹¹	4.60 × 10⁻¹¹	6.87 × 10⁻¹¹	1.35 × 10⁻¹¹	1.11 × 10⁻¹¹

If the frequency range is adequate and the relaxation time (τ = RC) for each electric phenomenon is very different from each other (τ_g>>τ_gb>>τ_e) it tis possible see three contributions (three arcs). The Nyquist plots are fitted by electric response of equivalent electric circuit formed by parallel associations of capacitors and resistors, so each contribution was calculated. All samples showed arc asymmetric for all Nyquist plots created. For samples measured at 30°C it was necessary to use constant phase element (CPE) to fit satisfactory results. Only one RC parallel association was tested, but the adjustment has not converged to the experimental results. The only arc in the Nyquist plot is due to an association of resistors and capacitors in parallel; however, the curve fitting was only possible with two R-CPE associations and these two processes have similar relaxation time.

All values fitted used in equivalent circuit for capacitors, resistors and CPE are listed in Table 6. The results confirm some aspects aborted previously as the thermal-activated process for water losses due to the humidity present in these samples. The adjustment was only possible by use of CPE, as can be seen through the equivalent circuit. In the measurements above 150°C, the use of CPE is not necessary, indicating that water or humidity does not interfere with the measurement. The resistance of grain and grain boundary presents a decreasing of value with temperature and this is supported by the thermo-activated process calculated previously.

The dielectric characterisation in the microwave range was made by the Hakki-Coleman method [30,31], where the dielectric permittivity and dielectric loss were obtained from analysis of TE₀₁₁ mode of the dielectric resonator which is more commonly used for easy identification [39,40]. Just the samples sintered at 1000°C were analysed (more densification). The dielectric properties obtained are listed in Table 7. Sample B presents higher dielectric permittivity and dielectric loss similar tot sample A as seen inhe CIS study.

Table 7

Microwave measurements obtained from the Hakki-Coleman method for the samples.

Sample	ε _r	D (mm)	H (mm)	tan δ (10⁻³)	f₀ (GHz)
A10	6.98	12.524	6.842	3.16	11.892625
B10	7.19	11.870	6.374	9.63	12.489250

Dielectric resonator antenna

The samples A10 and B10 were tested as DRA in the microwave range. The study of the samples as antenna in order to validate the experimental values and to obtain far fields parameter (gain, efficiency, radiation diagram, etc.) was performed. The cylindrical sample was placed on a ground plane and was laterally excited by a coaxial probe mounted across the conducting ground plane as shown in Figure 9. The fundamental HE_11δ mode of the cylindrical DRAs was used due to the lateral fed-probe [41,42]. The operational frequency of DRA fabricated is estimated by Equation (4) in the HE_11δ mode: (4) where ε is the dielectric permittivity and h is the height of the sample.

The experimental results were fitted from numerical simulations using the High Frequency Structure Simulation (HFSS^®) software; the model used follows the setup shown in Figure 9. Some air gaps were added in the model for better adjustment of the simulated data. The main features of these air gaps are the precise localisation of DRA in relation to the probe and to offset imperfections of DRA in your faces, since the surface of DRA is not a perfect plane.

The high frequency calculated compared to the experimental frequency is explained by the natural behaviour of ε, where a decreasing of values with frequency is expected. The ε calculated showed values higher than that calculated by the Hakki-Coleman method, but it is because the measurement frequencies in Hakki-Coleman were 11.89 and 12.48 GHz while the antenna measurement was around 7.2 GHz, see Figure 10(a).

Sample A10 was fabricated in triplicate to evaluate the reproducibility (Figure 10(b)), which showed good reproducibility. The numerical simulation of results of sample B10 was performed in HFSS to obtain far field parameters; as samples A10 and B10 are dielectric equivalent in the microwave range, only one numerical simulation was necessary. The results are shown in Figure 10(c). The good agreement between experimental and simulated data demonstrates good reliability in far field parameters calculated by HFSS software. The diagram of sample B10 presented in the radiation diagram is typical of cylindrical DRA with good agreement of simulation data and experimental data. The far field parameters calculated show gain of 5.5 dBi with antenna efficiency of 95% and 5.9 dBi; these results demonstrate good results compared to commercial products, e.g. the router's antenna presents gain of 3 dBi [24,43 -45].

Conclusions

The fabrication of electroceramics from sludge obtained by electrocoagulation process is viable. The samples present two crystalline phases (Fe₂O₃ and NiCr₂O₄). The CIS studies showed low dielectric permittivity with two thermo-activated processes (water loss and conduction). The pretreatment of sludge with distilled water removed the chlorates. The electric response was fitted by an equivalent circuit with two RC parallel associations. The samples sintered at 900°C presented different electric and dielectric properties compared with other sintering temperatures. The samples can be applied as a barrier layer capacitor, due to TCC and ε_r values, and as a DRA. In addition, the operating frequency can be tuned by changing the dimensions of the ceramic cylinder to a frequency of interest.

Footnotes

Disclosure statement

No potential conflict of interest was reported by the authors.

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