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Effects of mechanical activation and two-step sintering on the structure and electrical properties of cordierite-based ceramics
Ceramics International 42 (2016) 13909–13918
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Effects of mechanical activation and two-step sintering on the structure and electrical properties of cordierite-based ceramics Nina Obradović a,n, Suzana Filipović a, Nataša Đorđević b, Darko Kosanović a, Smilja Marković a, Vladimir Pavlović a, Dragan Olćan c, Antonije Djordjević c,d, Martin Kachlik e, Karel Maca e a
Institute of Technical Sciences of SASA, 11000 Belgrade, Serbia Institute for Technology of Nuclear and other Raw Mineral Materials, 11000 Belgrade, Serbia c School of Electrical Engineering, University of Belgrade, 11000 Belgrade, Serbia d Serbian Academy of Sciences and Arts, 11000 Belgrade, Serbia e CEITEC BUT, Brno University of Technology, 61600 Brno, Czech Republic b
art ic l e i nf o
a b s t r a c t
Article history: Received 9 May 2016 Received in revised form 30 May 2016 Accepted 30 May 2016 Available online 31 May 2016
The mechanical activation can help reduce production costs and modify cordierite-based materials. In this study, the structure and electrical properties of the mechanically activated MgO-Al2O3-SiO2 system have been analyzed. The mixtures of MgO-Al2O3-SiO2 powders were mechanically activated in a planetary ball mill for time periods ranging from 0 to 160 min. Structural investigations were performed on the obtained powders. The effects of activation and a two-step sintering process on the microstructure and crystal structure were investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The thermal stability was analyzed using DTA. Electrical measurements showed variations of the dielectric constant (εr) and loss tangent (tan δ) as a function of the time of mechanical treatment. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: A. Milling C. Dielectric properties D. Cordierite Sintering
1. Introduction Cordierite-based ceramics are promising materials in different branches of industry, because of their low dielectric constant (εr ¼5–6), low thermal expansion coefficient (α ¼ 1–2 10 6 °C 1), high resistivity (ρ 41012 Ω cm) and chemical stability [1]. Cordierite is one of the most important phases of the MgO-SiO2-Al2O3 system [2]. The properties of cordierite ceramics strongly depend on the composition and nature of raw starting materials, the presence of impurities or additives, as well as fabrication techniques. There are many methods for synthesizing cordierite-based ceramics, including the solid-state reaction, sol-gel, and crystallization of glass. Due to its simplicity and low-cost production, among other methods for cordierite-based materials preparation, the sintering of oxide powders by solid-state reactions has gained the highest interest [3–6]. It is well known that in this type of materials, electrical properties (especially tan δ and εr) strongly depend on the microstructure of sintered samples [7,8]. The phase composition and microstructure of the final material n
Corresponding author. E-mail address: [email protected] (N. Obradović).
http://dx.doi.org/10.1016/j.ceramint.2016.05.201 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
also depend on the starting components and their reactivity. Mechanical activation procedures have been successfully used to enhance the reactivity of materials by particle size reduction and amorphization. This procedure overcomes the kinetic constraints and the demand for high processing temperatures, which are common in conventional solid-state synthesis methods. Moreover, the mechanical activation of powders promotes the rate of diffusion of reactants across the phase boundaries and is a way to increase the kinetics of solid-state reactions [9]. Many researchers have also emphasized the efficiency of mechanical milling of precursors in the densification and the final amount of relevant phases in sintered bodies [10–13]. Also, milling media have a great impact on the final morphology of treated powders. One of the significant disadvantages of milling in air is the formation of agglomerates, which further affects densification and sinterability, and the use of water or alcohol as a milling medium is a convenient way to eliminate this drawback [8,14]. Several sintering processes are capable of controlling the microstructural characteristics of ceramics, such as rapid-rate sintering [15], rate-controlled sintering [16], two-step sintering (TSS), etc. [17]. Particularly noteworthy is two-step sintering, which has been applied to many materials owing to its ability to effectively control the grain growth (the first step, S1) and densification of
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cordierite-based materials along with the influence of TSS conditions on the final properties of powder mixtures prepared using these procedures. Therefore, the main objective of this study is to investigate the effects of the mechanical activation and two-step sintering of cordierite-based material compositions on the crystallization, densification, microstructure, and electrical properties of the final product.
2. Experimental procedure
Fig. 1. XRD patterns of non-activated and activated powder mixtures.
Table 1. Microstructure parameters determined by Scherrer's equations.
ρD 1010 (cm 12)
Act. time (min)
Powder (hkl)
Dhkl (nm)
0
MgO (220) SiO2 (100) SiO2 (112)
31.82 110.76 72.17
39.51 3.26 7.68
0.2108 0.1732 0.1134
10
MgO SiO2 SiO2 MgSiO3 (221)
27.28 66.46 60.14 18.17
53.75 9.06 11.06 12.11
0.2492 0.2884 0.1361 0.7331
40
MgO SiO2 SiO2 MgSiO3
27.28 66.46 60.14 19.22
53.75 9.06 11.06 10.83
0.2492 0.2884 0.1361 0.6927
80
MgO SiO2 SiO2 MgSiO3
23.86 66.46 51.55 18.99
70.26 9.06 15.05 11.08
0.2813 0.2884 0.1588 0.6998
160
MgO SiO2 SiO2 MgSiO3
10.04 47.47 17.18 20.13
396.82 17.75 135.52 9.87
0.6695 0.4041 0.4745 0.6611
ehkl
ceramics (the second step, S2) [18]. The most important advantage of this approach compared to the conventional pressureless sintering is that the densification of samples continues with a significantly slower grain growth [19]. As Chen and Wang postulate, there is an interval of temperatures where densification occurs, while the migration of grain boundaries has not yet started [20]. This temperature interval, named “kinetic window”, and the sintering that occurs in it, is the key to densification without grain growth. Two-step sintering (TSS) was mainly used in the case of nano-sized materials to avoid the final grain growth, for a large number of ceramic materials, such as barium titanate, strontium titanate, alumina, etc. [21–23]. It has been shown that the efficiency of TSS is different for different groups of materials [24,25]. Finally, TSS is a very convenient way to obtain a microstructure with a well defined grain distribution. According to our best knowledge, there have been no attempts to investigate the effect of mechanical activation in wet media for
Mixtures of MgO, Al2O3 and SiO2 (all 99% Sigma-Aldrich) were used in these experiments. Prior to the milling process, the precursor MgO powder was calcined at 700 °C for 2 h in order to remove Mg(OH)2. The mixtures of MgOþAl2O3 þ SiO2 with the 2:2:5 M ratio were mechanically activated in ethanol by grinding in a high-energy planetary ball mill (Fritsch Pulverisette 5). ZrO2 vessels and balls were used with the powder to balls mass ratio of 1:40. The milling process was performed in air for 10, 40, 80, and 160 min. Non-activated (manually mixed in mortar with ethanol) and mechanically activated powder mixtures were denoted as MAS-0, MAS-10, MAS-40, MAS-80, and MAS-160, according to the milling time. After mixing and milling, the powders were dried at 120 °C for 2 h and sieved. X-ray powder diffraction patterns were obtained using a Philips PW-1050 diffractometer with λCu-Kα radiation and a step/time scan mode of 0.05° s 1. The measurements were performed at room temperature in an air atmosphere. The average particle size and particle size distribution were determined by a laser light-scattering particle size analyzer (PSA). The used instrument was Mastersizer 2000 (Malvern Instruments Ltd., UK) particle size analyzer based on laser diffraction, covering the particle size range of 0.02–2000 mm. For the PSA measurements, the powders were dispersed in distilled water, in an ultrasonic bath (low-intensity ultrasound, at a frequency of 40 kHz and power of 50 W), for 5 min. The thermal behavior was determined by simultaneous TG-DTA (Setsys, SETARAM Instrumentation, Caluire, France) in the temperature range between RT and 1400 °C under the air flow of 20 ml min 1, in an Al2O3 pan. The experiments were done using a heating rate of 20 °C min 1. The morphology of the powder mixtures obtained after milling was characterized by scanning electron microscopy (JEOL JSM6390 LV). The sintered samples were polished and then thermally etched at 1250 °C for 5 min. The thermally etched samples were covered by a golden layer to increase the conductivity of the surface before observing by SEM (Philips XL30). The binder-free powders were compacted using the uniaxial double action pressing process in an 8-mm diameter tool (Hydraulic press RING, P-14, VEB THURINGER). The applied pressure was 392 MPa and an amount of 0.3 g of the powders was used for each pressed sample. The compacts were placed in an alumna boat and heated in a tube furnace (Lenton Thermal Design Typ 1600). The samples were two-step sintered: non-isothermal up to 1300 °C (heating rate 20 °C/min, cooling rate 10 °C/min), and then dwelling at 1200 °C for 4 h. The sintering process was performed in an air atmosphere. The samples that were subjected to the TSS process were denoted as MAS-0-S, MAS-10-S, MAS-40-S, MAS-80-S, and MAS-160S, according to the activation time and sintering. The density of all samples was established by the Archimedes method (EN 623-2). The measurements of the relative permittivity of the sintered samples, metalized on the top and bottom with silver, were performed using an Agilent E5061A network analyzer, in the frequency range between 1 MHz and 1 GHz. The samples were placed in a custom-made coaxial chamber. The reflection coefficient of the chamber was measured by the analyzer and the relative
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Fig. 2. a) SEM, b) EDS, and c) mapping for non-activated powder MAS-0.
complex permittivity of the samples was extracted using electromagnetic models.
The X-ray diffraction patterns for non-milled and milled powders are presented in Fig. 1. The initial powder mixture MAS-0 exhibits sharp peaks of SiO2 (JCPDS PDF 65-0466) and MgO (JCPDS PDF 87-0651), and broadened peaks of amorphous Al2O3 (JCPDS PDF 75-0782), while the intensities of all starting phases in the samples that were mechanically treated for 10 are lower. Lowering and broadening of diffraction peaks indicates a significant refinement in crystallite size and defect formation during the initial stage of the mechanochemical activation. The analysis of the broadening of Bragg reflections is a very convenient way to determine the mean size of crystalline domains from powder diffraction data. The size of crystallites was determined by applying Scherrer's equations and calculated by the approximation method [26]:
k⋅λ β⋅ cos θ
(1)
4 Dhkl2
(2)
β (3) 4⋅tan θ where: Dhkl – average crystallite size, k – Scherrer's constant, λ – wavelength, β – integral breadth, and θ – angular peak position. The microstructure parameters obtained from Scherrer's method: crystallite size (Dhkl), density of dislocations (ρD), and lattice microstrain (ehkl) are presented in Table 1. These calculations were performed for the most intensive reflections and reflections with no overlapping. The calculations of the values of the mean crystallite size and the relative lattice strain obtained from the broadening of the diffraction line of the non-activated samples and the samples activated for 10 min revealed that the mean crystallite size of MgO (220) and SiO2 (100) and (112) phases decreased from 31.82 to 27.28 nm, from 110.76 to 66.46 nm, and from 72.17 to 60.14 nm, respectively, while the relative lattice strain of these phases increased from 0.2108 to 0.2492, from 0.1732 to 0.2884, and from 0.1134 to 0.1361, respectively. As the activation time increased up to 10 min, the presence of magnesium silicate clinoenstatite, MgSiO3, besides the MgO, SiO2, and Al2O3 phases, was noticed, indicating the beginning of a mechanochemical reaction in the system (JCPDS PDF 84-0652 ehkl =
3. Results and discussion
Dhkl =
ρD =
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Fig. 3. a) SEM, b) EDS, and c) mapping for activated powder MAS-10. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Clinoenstatite MgSiO3). With the increase of the activation time, the intensity of MgO and SiO2 peaks became lower, while the MgSiO3 peaks became higher. The calculations of the values of the mean crystallite size and the relative lattice strain of MgO and SiO2 for the same (hkl) planes showed further decrease until they reached their minimum values of 10.04 nm for MgO, 47.47 nm, and 17.18 nm for SiO2, and the final increase of the relative lattice strain, 0.6695 for MgO, 0.4041, and 0.4745 for SiO2, respectively. The calculations of the values of the mean crystallite size and the relative lattice strain of MgSiO3 for the samples activated for 10, 40, 80, and 160 min revealed a slight increase of the mean crystallite size of MgSiO3 from 18.17 to 20.13 nm as the activation time increased, while the relative lattice strain decreased from 0.7331 to 0.6611. The constant collisions and rearrangement of the powder particles caused by mechanical impact, shock, shear, and deformation created opportunities for two or more MgSiO3 nuclei to meet at favorable positions leading to a growth in crystallite size. Thus, the growth of nanocrystallites is a result of constant collisions and rearrangement of the nanocrystalline nuclei and it proceeded as the activation time increased [27]. After the samples had been mechanically activated for 160 min, the traces of Al2O3 completely disappeared, while a new modification of MgSiO3 (JCPDS PDF 88-1924 Magnesium Silicate MgSiO3) was observed along with the peaks of clinoenstatite MgSiO3, MgO, and SiO2. The scanning electron micrograph for the non-activated
mixture MAS-0, along with the EDS image and mapping, is shown in Fig. 2. The micrograph indicates polygonal irregularly shaped particles. Powder particles are not connected and they are incoherent, which was expected due to the preparation of the initial non-activated mixture (only mixed, without milling). The EDS image proves the presence of the starting elements, Al, Mg, O and Si, while the elemental mapping shows clearly separated phases. The mechanical activation of powder mixtures that lasted for 10 min led to the attrition of powder particles, their homogenization, and better interconnections between them, resulting in the increase of their specific surface area and, therefore, the increased reactivity of the samples. Smaller particles of MgO and SiO2 were banked up on the agglomerates of Al2O3, which are clearly visible in the background of the SEM micrograph (Fig. 3). Furthermore, the mapping shows two distinct areas: one area obviously represents separated phases, while the other shows green particles that belong to Al2O3, covered with blue and red particles belonging to MgO and SiO2, respectively. Further milling led to the additional reduction of powder particles with no clearly defined Al2O3 agglomerates. The EDS image indicates the same distribution of elements, although the mapping suggests a more homogeneous distribution of MgO and SiO2 phases over the Al2O3 phase (Fig. 4.). The mechanical activation that lasted for 80 min yielded powder particles that had almost the same size as those in the
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Fig. 4. a) SEM, b) EDS, and c) mapping for activated powder MAS-40.
previous mixture, but the presence of agglomerates was evident (Fig. 5). This is usual, as the introduction of high energy into the system with prolonged milling time always leads to a drastically increased reactivity of powder particles [28]. At the beginning of the milling process, the powder particles become reactive, capable of formation of new phases, and then, with additional energy transfer, practically forced to make segregations. Those segregations are a serious disadvantage during the sintering process. The particles within the MAS-160 powder are regularly shaped and around 0.2 mm in size. With prolonged milling times, no agglomerates were detected. The applied milling time had a great influence on the fragmentation of agglomerates. The obtained microstructure should provide a better sinterability, greater densification with no grain growth, and, finally, improved properties [29,30]. EDS confirms the presence of all elements, but the mapping indicates the most homogeneous distribution of phases (Fig. 6.). The results of particle size analysis, presented in Fig. 6, are in accordance with the SEM analysis. The average particle size d(0.5) for the non-activated mixture (MAS-0) is around 8.5 mm, and the size decreases with increased activation times. Also, a bimodal distribution is noticed, owing to the non-homogeneous mixture and the presence of smaller particles (belonging to SiO2 and MgO phases) and agglomerates (belonging to Al2O3 phase). A more uniform and narrower distribution was more pronounced with prolonged milling times, and no agglomerates were detected (Fig. 7).
The influence of thermal treatment on the mechanically activated mixtures was analyzed by DTA-TG techniques. The milling process led to significant changes in the DTA-TG curves. The curves of non-activated mixtures and those activated for 10–160 min, with a heating rate of 20 °C min 1, are shown in Fig. 8a and b. All samples have well-defined DTA peaks. The first one is a strong endothermic peak at around 100 °C, which belongs to humidity evaporation. The second, weak endothermic peak at around 400 °C corresponds to the dehydroxylation of Al2O3-based compounds [31]. The second important endothermic peak is observed in MAS0, centered at 570 °C, corresponding to the structural α-β phase transition in crystalline SiO2 [32]. This peak has a tendency to decrease its area when milling. In MAS-160, it becomes reduced to a small hump. Within the range 1200–1400 °C, a series of effects were detected in the activated samples. De Aza et al. [33] found two peaks at 1275 and 1357 °C, using highly sensitive equipment, both corresponding to the formation of cordierite. In our case, the first endothermic peak can be detected in the MAS-10 sample and it is centered at 1260 °C. This peak gradually shifts to lower temperatures with the increase of grinding time. It reaches its minimum value of 1210 °C when milled for 160 min and has a tendency to decrease the intensity, which corresponds to μ-cordierite formation. This tendency is due to the reduction of energy necessary to destroy the crystalline structure of the system. The energy supplied by the planetary mill causes a structural disorder, through the distortion or breakage of
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Fig. 5. a) SEM, b) EDS, and c) mapping for activated powder MAS-80.
the crystalline structure. This is evident from the reduction in the intensity of XRD peaks. In the activated samples, the exothermic peak for MAS-10 at 1282 °C shifts to temperatures of about 1243 °C, and it is assigned to the formation of α-cordierite [34]. The analysis of the TG curves confirms the behaviors observed by DTA since dehydroxylation is accompanied with the corresponding mass loss. The samples show two mass loss steps that are not so well separated at 200 and 400 °C, which are in accordance with the DTA peaks described, corresponding to water evaporation, and the dehydroxylation process, respectively [35]. The mass loss in the MAS-0 sample is 3% and it gradually increases to 10% for MAS-160, owing to greater water absorption during the preparation and the milling process. Based on the characteristic temperatures defined by DTA, we established the optimal parameters for TSS. The values of the obtained densities and open and closed porosity are given in Table 2. The results of the sample density measurements were in agreement with the microstructural examination. The average sample densities after two-step sintering ranged from 57.8% to 65.4% of the theoretical densities. The samples used in this study exhibited relatively low densities, which may have considerably adverse impact on dielectric properties. The scanning electron micrographs of the activated and sintered samples are shown in Fig. 9. The non-activated sample was destroyed during sample polishing. The insufficient bonding strength between grains led to the disintegration of the sample. A
significant level of porosity is present in the MAS-10-S sample, around 36%. Grain boundaries are less pronounced in the sample MAS-40-S, although a microstructure with small grains remains, the level of densification is higher, while the open porosity is slightly decreased. The prolonged mechanical activation followed by two-step sintering did not resulted in significant changes in the microstructure; grains retain their shape and size around one micron or less, decreasing the porosity negligibly, no grain growth was observed, while the sample MAS-160-S has highest relative density and lowest relative open porosity, as expected. The exception from this trend is the sample MAS-80-S, which has the highest value of relative open porosity, around 40%, and the lowest density, owing to the presence of agglomerates in the starting mixture and their sintering. The phase composition of the non-activated and activated samples after the two-step sintering process is presented in Fig. 10. The XRD pattern of the initial non-activated sintered sample indicates that several phases – Al2O3, SiO2, MgO, spinel MgAl2O4, MgSiO3, and Mg2SiO4 – are present within the MAS-0-S sintered mixture. The formation of cordierite began during the sintering of the mixture activated for 10 min, although significant amounts of the starting SiO2 phase and the spinel MgAl2O4 phase are still present. The diffraction pattern of MAS-40-S, among well crystallized sharp peaks of cordierite and MgAl2O4, shows small amounts of ZrSiO4 and ZrO2, as a consequence of high-energy milling in a ZrO2 jar and balls. The phase composition of the
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Fig. 6. a) SEM, b) EDS, and c) mapping for activated powder MAS-160.
mixture MAS-80-S remains as in the previous case, although a decrease in the SiO2 amount is noticed. On the contrary, the XRD pattern of the sample activated for 160 min and sintered indicates a cordierite phase, with MgAl2O4, zirconia compounds, without the presence of the starting phases. The increase in the milling time followed by two-step sintering led to a better recrystallization and an increase in peak intensities, with an increased amount of the cordierite phase, although the unwanted zirconia compounds remained. The identification of the obtained reflections was carried out using the JCPDS PDF cards (078-2427 for Al2O3, 065-0466 for SiO2, 077-2364 for MgO, 075-1796 for MgAl2O4, 0740816 for MgSiO3, 076-0561 for Mg2SiO4, 083-1383 for ZrSiO4, 0741201 for ZrO2, and 082-1884 for Mg2(Al4Si5O18) cordierite, indialite modification). In order to measure the relative permittivity of the sintered samples, each sample was metalized on the top and bottom surfaces with silver. Parallel-plate capacitors were obtained whose diameter was around 8 mm and the height was around 3 mm. A custom-made test fixture was used in the form of a coaxial chamber. This test fixture enabled measurements up to several GHz. From the measured reflection coefficient of the chamber, the relative complex permittivity of the samples ( εr = εr′ − j⋅εr″) is extracted using electromagnetic models. For frequencies up to 1 GHz, a quasi-static model suffices [36]. For higher frequencies, a fullwave solver, like WIPL-D [37], is appropriate. For our samples, the reflection coefficient of the chamber was
measured in the frequency range between 1 MHz and 1 GHz using an Agilent E5061A network analyzer. The error of the results for εr is estimated to be less than 5%. The real part of the relative complex permittivity ( ε′r ) of all samples was found to be practically independent of frequency in the whole frequency range. However, it varied from sample to sample. The dielectric constants and average densities of the samples are shown in Fig. 11 as a function of the activation time. It has been widely found and accepted that higher densities result in the enhancement of dielectric constants [38]. The relative permittivity was higher for denser samples (i.e. for samples which had lower porosity). The obtained results are in agreement with the obtained microstructures, presented in Fig. 9, where MAS-160-S has the highest density and relative permittivity ( ρ = 2.06 g cm−3, ε′r = 4.25). Knowing that air has a very low dielectric permittivity (approximately 1), it therefore follows that samples containing high level of pores or air would be inclined to have a lower dielectric constant [39]. The highest value of the relative open porosity present in sample MAS-80-S, 40%, as a consequence of agglomerates within the starting powder (Fig. 5a), resulted in the lowest value of the relative permittivity. The applied measurement technique was broadband, and the shape and size of the samples were not optimized for dielectric measurements. Hence, the imaginary part of the relative permittivity ( ε″ r ) was partly obscured by the measurement noise. Nevertheless, from ε′r and ε″ r , the loss tangent ( tan δ =
εr″ ) εr′
could be
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Fig. 7. Particle size analysis of non-activated and activated powder mixtures.
estimated to be below 0.02, which was within an acceptable range for practical applications.
4. Conclusions In the present study, cordierite-based ceramic materials were prepared using mechanical activation followed by the two-step sintering technique. The phase compositions, microstructures, and electric properties of the bulk materials prepared by TSS were characterized systematically. The following conclusions are drawn from the investigations:
Sample
Theor. dens. Abs. dens. Rel. dens. Rel. open (g cm 3) (g cm 3) (%) por. (%)
Rel. clos. por. (%)
MAS-0-S MAS-10-S MAS-40-S MAS-80-S MAS-160-S
3.20 2.95 2.90 3.05 3.15
3.00 3.70 3.10 3.30 3.30
1.85 1.79 1.84 1.74 2.06
57.80 60.70 63.50 57.10 65.40
39.20 35.60 33.40 39.70 31.30
(1) An intensive mechanical activation in ethanol led to a better homogeneity of powder mixtures, particle size reduction, an increase in powder reactivity through the increase in specific surface area, along with the formation of a new phase – MgSiO3, after 10 min of activation, as confirmed by XRD. For the MgO and SiO2 powders with prolonged mechanical activation, the crystallite size, obtained by an approximation method, drastically decreased, down to nano-sized dimensions (10.04 nm for MgO and 17.18 and 47.47 nm for SiO2). Also, an increase in the dislocation density and lattice microstrain is observed. A slight increase in the crystallite size of the newly formed phase, MgSiO3 (18.17–20.13 nm), indicates a process of recrystallization during an intensive mechanochemical activation. (2) Five characteristic temperatures are detected in the system using DTA and they correspond to humidity evaporation, the dehydroxylation of Al2O3-based compounds, the structural α-β phase transition in crystalline SiO2, and μ-cordierite and α-cordierite formation. The tendency of the activated powders to shift their characteristic temperatures to lower values is due to the reduction of the energy necessary to destroy the crystalline structure of the system. The energy supplied by the planetary mill causes a structural disorder, through the distortion or breakage of the crystalline structure, evident from the reduction of the intensities of XRD peaks. (3) The two-step sintering of the activated powders leads to cordierite formation. As little as 10 min of intensive milling accompanied by TSS is sufficient for the reaction. The densities obtained after TSS increased from 1.79 g cm 3 for MAS-10-S to 2.06 g cm 3 for MAS-160-S. The microstructures indicate porous ceramics with grains smaller than 1 mm, without grain growth with prolonged milling time. The density has the dominant influence on the dielectric permittivity (3.39 for MAS-10-S, increasing to 4.25 for MAS-160-S), with a loss tangent below 0.02.
Fig. 8. a) DTA curves of mixtures with 20 °C/min heating rate and b) TG curves of mixtures with 20 °C/min heating rate.
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Fig. 9. SEM micrographs of two-step sintered powder mixtures: a) MAS-10-S, b) MAS-40-S, c) MAS-80-S, and d) MAS-160-S.
Fig. 11. Dielectric constants and absolute densities of two-step sintered samples as the function of time of mechanical activation.
2020 (LQ1601). The authors would like to thank to Dr. Miodrag Mitrić for the XRD measurements. Fig. 10. XRD patterns of two-step sintered powder mixtures.
Acknowledgement This research was performed within the project OI 172057 funded by the Ministry of Education, Science and Technological Development of the Republic of Serbia and project no. 15-06390S funded by the Grant agency of the Czech Republic. The research has been also financially supported by the Ministry of Education, Youth and Sports of the Czech Republic under the project CEITEC
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[4] N.N. Sampathkumar, A.M. Umarji, B.K. Chandrasekhar, Synthesis of α–cordierite (Indialite) from fly ash, Mater. Res. Bull. 30 (9) (1995) 1107–1114. [5] S. Kurama, E. Ozel, N. Ay, Synthesis and sintering of cordierite at low temperature from kaolin and magnesium hydroxide, Key Eng. Mater. 264–268 (2004) 925–928. [6] Z. Acimovic, L. Pavlovic, L. Trumbulovic, L. Andric, M. Stamatovic, Synthesis and characterization of the cordierite ceramics from non standard raw materials for application in foundry, Mater. Lett. 57 (2003) 2651–2656. [7] H. Zhu, X.-J. Kuang, C.-H. Wang, H.-T. Xia, L. Li, P. Hu, X.-P. Jing, Z.-X. Tang, F. Zhao, Z.-X. Yue, Annealing effects on conductivity and microwave dielectric loss of MgTiO3 ceramics, Jpn. J. Appl. Phys. 50 (2011) 065806-065806-5. [8] X. Kuang, X. Jing, Z. Tang, Dielectric loss spectrum of ceramic MgTiO3 investigated by AC impedance and microwave resonator measurements, J. Am. Ceram. Soc. 89 (2006) 241–246. [9] I. Lin, S. Nadiv, Review of the phase-transformation and synthesis of inorganic solids obtained by mechanical treatment (mechanochemical reactions), Mater. Sci. Eng. 39 (2) (1979) 193–209. [10] N. Đorđević, P. Jovanić, Influence of mechanical activation on electrical properties of cordierite ceramics, Sci. Sinter. 40 (2008) 47–53. [11] J.R. Neto, R. Moreno, Effect of mechanical activation on the rheology and casting performance of kaolin/talc/alumina suspensions for manufacturing dense cordierite bodies, Appl. Clay Sci. 38 (3–4) (2008) 209–218. [12] T. Ebadzadeh, Effect of mechanical activation and microwave heating on synthesis and sintering of nano-structured mullite, J. Alloy. Compd. 489 (2010) 125–129. [13] B. Fotoohi, S. Blackburn, Effects of mechanochemical processing and doping of functional oxides on phase development in synthesis of cordierite, J. Eur. Ceram. Soc. 32 (2) (2012) 267–2272. [14] A. Kassas, J. Bernard, C. Lelievre, D. Houivet, T. Hamieh, Sintering study of ceramic material based on magnesium titanate in presence of lithium fluoride additives, J. Mater. Sci. Eng. A 2 (7) (2012) 550–559. [15] M.J. Mayo, Processing of nanocrystalline ceramics from ultrafine particles, Int. Mater. Rev. 41 (1996) 85–115. [16] H. Palmour, M.L. Huckabee, Process for Sintering finely derived Particulates and resulting Ceramics products,U. S. Patent 3, 900 (1975) 542. [17] X.H. Wang, X.Y. Deng, H.L. Bai, H.Z. Zhou, W.G. Qu, L.T. Li, Two-step sintering of ceramics with constant grain-size, ІІ: BaTiO3 and Ni–Cu–Zn ferrite, J. Am. Ceram. Soc. 89 (2006) 438–443. [18] I.W. Chen, X.H. Wang, Sintering dense nanocrystalline ceramics without finalstage grain growth, Nature 404 (2000) 168–171. [19] G.B.P. Ferreira, J.F. da Silva Jr, R.M. do Nascimento, U.U. Gomes, A.E. Martinelli, Two-Step Sintering Applied to Ceramics, Sintering of Ceramics - New Emerging Techniques, Dr. Arunachalam Lakshmanan (Ed.), ISBN: 978–953-51–00171, InTech. (2012). [20] K. Bodišova, P. Šajgalik, D. Galusek, P. Švančarek, Two-stage sintering of alumina with submicrometer grain size, J. Am. Ceram. Soc. 90 (2007) 330–332.
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Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Effects of mechanical activation and two-step sintering on the structure and electrical properties of cordierite-based ceramics Nina Obradović a,n, Suzana Filipović a, Nataša Đorđević b, Darko Kosanović a, Smilja Marković a, Vladimir Pavlović a, Dragan Olćan c, Antonije Djordjević c,d, Martin Kachlik e, Karel Maca e a
Institute of Technical Sciences of SASA, 11000 Belgrade, Serbia Institute for Technology of Nuclear and other Raw Mineral Materials, 11000 Belgrade, Serbia c School of Electrical Engineering, University of Belgrade, 11000 Belgrade, Serbia d Serbian Academy of Sciences and Arts, 11000 Belgrade, Serbia e CEITEC BUT, Brno University of Technology, 61600 Brno, Czech Republic b
art ic l e i nf o
a b s t r a c t
Article history: Received 9 May 2016 Received in revised form 30 May 2016 Accepted 30 May 2016 Available online 31 May 2016
The mechanical activation can help reduce production costs and modify cordierite-based materials. In this study, the structure and electrical properties of the mechanically activated MgO-Al2O3-SiO2 system have been analyzed. The mixtures of MgO-Al2O3-SiO2 powders were mechanically activated in a planetary ball mill for time periods ranging from 0 to 160 min. Structural investigations were performed on the obtained powders. The effects of activation and a two-step sintering process on the microstructure and crystal structure were investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The thermal stability was analyzed using DTA. Electrical measurements showed variations of the dielectric constant (εr) and loss tangent (tan δ) as a function of the time of mechanical treatment. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: A. Milling C. Dielectric properties D. Cordierite Sintering
1. Introduction Cordierite-based ceramics are promising materials in different branches of industry, because of their low dielectric constant (εr ¼5–6), low thermal expansion coefficient (α ¼ 1–2 10 6 °C 1), high resistivity (ρ 41012 Ω cm) and chemical stability [1]. Cordierite is one of the most important phases of the MgO-SiO2-Al2O3 system [2]. The properties of cordierite ceramics strongly depend on the composition and nature of raw starting materials, the presence of impurities or additives, as well as fabrication techniques. There are many methods for synthesizing cordierite-based ceramics, including the solid-state reaction, sol-gel, and crystallization of glass. Due to its simplicity and low-cost production, among other methods for cordierite-based materials preparation, the sintering of oxide powders by solid-state reactions has gained the highest interest [3–6]. It is well known that in this type of materials, electrical properties (especially tan δ and εr) strongly depend on the microstructure of sintered samples [7,8]. The phase composition and microstructure of the final material n
Corresponding author. E-mail address: [email protected] (N. Obradović).
http://dx.doi.org/10.1016/j.ceramint.2016.05.201 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
also depend on the starting components and their reactivity. Mechanical activation procedures have been successfully used to enhance the reactivity of materials by particle size reduction and amorphization. This procedure overcomes the kinetic constraints and the demand for high processing temperatures, which are common in conventional solid-state synthesis methods. Moreover, the mechanical activation of powders promotes the rate of diffusion of reactants across the phase boundaries and is a way to increase the kinetics of solid-state reactions [9]. Many researchers have also emphasized the efficiency of mechanical milling of precursors in the densification and the final amount of relevant phases in sintered bodies [10–13]. Also, milling media have a great impact on the final morphology of treated powders. One of the significant disadvantages of milling in air is the formation of agglomerates, which further affects densification and sinterability, and the use of water or alcohol as a milling medium is a convenient way to eliminate this drawback [8,14]. Several sintering processes are capable of controlling the microstructural characteristics of ceramics, such as rapid-rate sintering [15], rate-controlled sintering [16], two-step sintering (TSS), etc. [17]. Particularly noteworthy is two-step sintering, which has been applied to many materials owing to its ability to effectively control the grain growth (the first step, S1) and densification of
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cordierite-based materials along with the influence of TSS conditions on the final properties of powder mixtures prepared using these procedures. Therefore, the main objective of this study is to investigate the effects of the mechanical activation and two-step sintering of cordierite-based material compositions on the crystallization, densification, microstructure, and electrical properties of the final product.
2. Experimental procedure
Fig. 1. XRD patterns of non-activated and activated powder mixtures.
Table 1. Microstructure parameters determined by Scherrer's equations.
ρD 1010 (cm 12)
Act. time (min)
Powder (hkl)
Dhkl (nm)
0
MgO (220) SiO2 (100) SiO2 (112)
31.82 110.76 72.17
39.51 3.26 7.68
0.2108 0.1732 0.1134
10
MgO SiO2 SiO2 MgSiO3 (221)
27.28 66.46 60.14 18.17
53.75 9.06 11.06 12.11
0.2492 0.2884 0.1361 0.7331
40
MgO SiO2 SiO2 MgSiO3
27.28 66.46 60.14 19.22
53.75 9.06 11.06 10.83
0.2492 0.2884 0.1361 0.6927
80
MgO SiO2 SiO2 MgSiO3
23.86 66.46 51.55 18.99
70.26 9.06 15.05 11.08
0.2813 0.2884 0.1588 0.6998
160
MgO SiO2 SiO2 MgSiO3
10.04 47.47 17.18 20.13
396.82 17.75 135.52 9.87
0.6695 0.4041 0.4745 0.6611
ehkl
ceramics (the second step, S2) [18]. The most important advantage of this approach compared to the conventional pressureless sintering is that the densification of samples continues with a significantly slower grain growth [19]. As Chen and Wang postulate, there is an interval of temperatures where densification occurs, while the migration of grain boundaries has not yet started [20]. This temperature interval, named “kinetic window”, and the sintering that occurs in it, is the key to densification without grain growth. Two-step sintering (TSS) was mainly used in the case of nano-sized materials to avoid the final grain growth, for a large number of ceramic materials, such as barium titanate, strontium titanate, alumina, etc. [21–23]. It has been shown that the efficiency of TSS is different for different groups of materials [24,25]. Finally, TSS is a very convenient way to obtain a microstructure with a well defined grain distribution. According to our best knowledge, there have been no attempts to investigate the effect of mechanical activation in wet media for
Mixtures of MgO, Al2O3 and SiO2 (all 99% Sigma-Aldrich) were used in these experiments. Prior to the milling process, the precursor MgO powder was calcined at 700 °C for 2 h in order to remove Mg(OH)2. The mixtures of MgOþAl2O3 þ SiO2 with the 2:2:5 M ratio were mechanically activated in ethanol by grinding in a high-energy planetary ball mill (Fritsch Pulverisette 5). ZrO2 vessels and balls were used with the powder to balls mass ratio of 1:40. The milling process was performed in air for 10, 40, 80, and 160 min. Non-activated (manually mixed in mortar with ethanol) and mechanically activated powder mixtures were denoted as MAS-0, MAS-10, MAS-40, MAS-80, and MAS-160, according to the milling time. After mixing and milling, the powders were dried at 120 °C for 2 h and sieved. X-ray powder diffraction patterns were obtained using a Philips PW-1050 diffractometer with λCu-Kα radiation and a step/time scan mode of 0.05° s 1. The measurements were performed at room temperature in an air atmosphere. The average particle size and particle size distribution were determined by a laser light-scattering particle size analyzer (PSA). The used instrument was Mastersizer 2000 (Malvern Instruments Ltd., UK) particle size analyzer based on laser diffraction, covering the particle size range of 0.02–2000 mm. For the PSA measurements, the powders were dispersed in distilled water, in an ultrasonic bath (low-intensity ultrasound, at a frequency of 40 kHz and power of 50 W), for 5 min. The thermal behavior was determined by simultaneous TG-DTA (Setsys, SETARAM Instrumentation, Caluire, France) in the temperature range between RT and 1400 °C under the air flow of 20 ml min 1, in an Al2O3 pan. The experiments were done using a heating rate of 20 °C min 1. The morphology of the powder mixtures obtained after milling was characterized by scanning electron microscopy (JEOL JSM6390 LV). The sintered samples were polished and then thermally etched at 1250 °C for 5 min. The thermally etched samples were covered by a golden layer to increase the conductivity of the surface before observing by SEM (Philips XL30). The binder-free powders were compacted using the uniaxial double action pressing process in an 8-mm diameter tool (Hydraulic press RING, P-14, VEB THURINGER). The applied pressure was 392 MPa and an amount of 0.3 g of the powders was used for each pressed sample. The compacts were placed in an alumna boat and heated in a tube furnace (Lenton Thermal Design Typ 1600). The samples were two-step sintered: non-isothermal up to 1300 °C (heating rate 20 °C/min, cooling rate 10 °C/min), and then dwelling at 1200 °C for 4 h. The sintering process was performed in an air atmosphere. The samples that were subjected to the TSS process were denoted as MAS-0-S, MAS-10-S, MAS-40-S, MAS-80-S, and MAS-160S, according to the activation time and sintering. The density of all samples was established by the Archimedes method (EN 623-2). The measurements of the relative permittivity of the sintered samples, metalized on the top and bottom with silver, were performed using an Agilent E5061A network analyzer, in the frequency range between 1 MHz and 1 GHz. The samples were placed in a custom-made coaxial chamber. The reflection coefficient of the chamber was measured by the analyzer and the relative
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Fig. 2. a) SEM, b) EDS, and c) mapping for non-activated powder MAS-0.
complex permittivity of the samples was extracted using electromagnetic models.
The X-ray diffraction patterns for non-milled and milled powders are presented in Fig. 1. The initial powder mixture MAS-0 exhibits sharp peaks of SiO2 (JCPDS PDF 65-0466) and MgO (JCPDS PDF 87-0651), and broadened peaks of amorphous Al2O3 (JCPDS PDF 75-0782), while the intensities of all starting phases in the samples that were mechanically treated for 10 are lower. Lowering and broadening of diffraction peaks indicates a significant refinement in crystallite size and defect formation during the initial stage of the mechanochemical activation. The analysis of the broadening of Bragg reflections is a very convenient way to determine the mean size of crystalline domains from powder diffraction data. The size of crystallites was determined by applying Scherrer's equations and calculated by the approximation method [26]:
k⋅λ β⋅ cos θ
(1)
4 Dhkl2
(2)
β (3) 4⋅tan θ where: Dhkl – average crystallite size, k – Scherrer's constant, λ – wavelength, β – integral breadth, and θ – angular peak position. The microstructure parameters obtained from Scherrer's method: crystallite size (Dhkl), density of dislocations (ρD), and lattice microstrain (ehkl) are presented in Table 1. These calculations were performed for the most intensive reflections and reflections with no overlapping. The calculations of the values of the mean crystallite size and the relative lattice strain obtained from the broadening of the diffraction line of the non-activated samples and the samples activated for 10 min revealed that the mean crystallite size of MgO (220) and SiO2 (100) and (112) phases decreased from 31.82 to 27.28 nm, from 110.76 to 66.46 nm, and from 72.17 to 60.14 nm, respectively, while the relative lattice strain of these phases increased from 0.2108 to 0.2492, from 0.1732 to 0.2884, and from 0.1134 to 0.1361, respectively. As the activation time increased up to 10 min, the presence of magnesium silicate clinoenstatite, MgSiO3, besides the MgO, SiO2, and Al2O3 phases, was noticed, indicating the beginning of a mechanochemical reaction in the system (JCPDS PDF 84-0652 ehkl =
3. Results and discussion
Dhkl =
ρD =
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Fig. 3. a) SEM, b) EDS, and c) mapping for activated powder MAS-10. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Clinoenstatite MgSiO3). With the increase of the activation time, the intensity of MgO and SiO2 peaks became lower, while the MgSiO3 peaks became higher. The calculations of the values of the mean crystallite size and the relative lattice strain of MgO and SiO2 for the same (hkl) planes showed further decrease until they reached their minimum values of 10.04 nm for MgO, 47.47 nm, and 17.18 nm for SiO2, and the final increase of the relative lattice strain, 0.6695 for MgO, 0.4041, and 0.4745 for SiO2, respectively. The calculations of the values of the mean crystallite size and the relative lattice strain of MgSiO3 for the samples activated for 10, 40, 80, and 160 min revealed a slight increase of the mean crystallite size of MgSiO3 from 18.17 to 20.13 nm as the activation time increased, while the relative lattice strain decreased from 0.7331 to 0.6611. The constant collisions and rearrangement of the powder particles caused by mechanical impact, shock, shear, and deformation created opportunities for two or more MgSiO3 nuclei to meet at favorable positions leading to a growth in crystallite size. Thus, the growth of nanocrystallites is a result of constant collisions and rearrangement of the nanocrystalline nuclei and it proceeded as the activation time increased [27]. After the samples had been mechanically activated for 160 min, the traces of Al2O3 completely disappeared, while a new modification of MgSiO3 (JCPDS PDF 88-1924 Magnesium Silicate MgSiO3) was observed along with the peaks of clinoenstatite MgSiO3, MgO, and SiO2. The scanning electron micrograph for the non-activated
mixture MAS-0, along with the EDS image and mapping, is shown in Fig. 2. The micrograph indicates polygonal irregularly shaped particles. Powder particles are not connected and they are incoherent, which was expected due to the preparation of the initial non-activated mixture (only mixed, without milling). The EDS image proves the presence of the starting elements, Al, Mg, O and Si, while the elemental mapping shows clearly separated phases. The mechanical activation of powder mixtures that lasted for 10 min led to the attrition of powder particles, their homogenization, and better interconnections between them, resulting in the increase of their specific surface area and, therefore, the increased reactivity of the samples. Smaller particles of MgO and SiO2 were banked up on the agglomerates of Al2O3, which are clearly visible in the background of the SEM micrograph (Fig. 3). Furthermore, the mapping shows two distinct areas: one area obviously represents separated phases, while the other shows green particles that belong to Al2O3, covered with blue and red particles belonging to MgO and SiO2, respectively. Further milling led to the additional reduction of powder particles with no clearly defined Al2O3 agglomerates. The EDS image indicates the same distribution of elements, although the mapping suggests a more homogeneous distribution of MgO and SiO2 phases over the Al2O3 phase (Fig. 4.). The mechanical activation that lasted for 80 min yielded powder particles that had almost the same size as those in the
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Fig. 4. a) SEM, b) EDS, and c) mapping for activated powder MAS-40.
previous mixture, but the presence of agglomerates was evident (Fig. 5). This is usual, as the introduction of high energy into the system with prolonged milling time always leads to a drastically increased reactivity of powder particles [28]. At the beginning of the milling process, the powder particles become reactive, capable of formation of new phases, and then, with additional energy transfer, practically forced to make segregations. Those segregations are a serious disadvantage during the sintering process. The particles within the MAS-160 powder are regularly shaped and around 0.2 mm in size. With prolonged milling times, no agglomerates were detected. The applied milling time had a great influence on the fragmentation of agglomerates. The obtained microstructure should provide a better sinterability, greater densification with no grain growth, and, finally, improved properties [29,30]. EDS confirms the presence of all elements, but the mapping indicates the most homogeneous distribution of phases (Fig. 6.). The results of particle size analysis, presented in Fig. 6, are in accordance with the SEM analysis. The average particle size d(0.5) for the non-activated mixture (MAS-0) is around 8.5 mm, and the size decreases with increased activation times. Also, a bimodal distribution is noticed, owing to the non-homogeneous mixture and the presence of smaller particles (belonging to SiO2 and MgO phases) and agglomerates (belonging to Al2O3 phase). A more uniform and narrower distribution was more pronounced with prolonged milling times, and no agglomerates were detected (Fig. 7).
The influence of thermal treatment on the mechanically activated mixtures was analyzed by DTA-TG techniques. The milling process led to significant changes in the DTA-TG curves. The curves of non-activated mixtures and those activated for 10–160 min, with a heating rate of 20 °C min 1, are shown in Fig. 8a and b. All samples have well-defined DTA peaks. The first one is a strong endothermic peak at around 100 °C, which belongs to humidity evaporation. The second, weak endothermic peak at around 400 °C corresponds to the dehydroxylation of Al2O3-based compounds [31]. The second important endothermic peak is observed in MAS0, centered at 570 °C, corresponding to the structural α-β phase transition in crystalline SiO2 [32]. This peak has a tendency to decrease its area when milling. In MAS-160, it becomes reduced to a small hump. Within the range 1200–1400 °C, a series of effects were detected in the activated samples. De Aza et al. [33] found two peaks at 1275 and 1357 °C, using highly sensitive equipment, both corresponding to the formation of cordierite. In our case, the first endothermic peak can be detected in the MAS-10 sample and it is centered at 1260 °C. This peak gradually shifts to lower temperatures with the increase of grinding time. It reaches its minimum value of 1210 °C when milled for 160 min and has a tendency to decrease the intensity, which corresponds to μ-cordierite formation. This tendency is due to the reduction of energy necessary to destroy the crystalline structure of the system. The energy supplied by the planetary mill causes a structural disorder, through the distortion or breakage of
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Fig. 5. a) SEM, b) EDS, and c) mapping for activated powder MAS-80.
the crystalline structure. This is evident from the reduction in the intensity of XRD peaks. In the activated samples, the exothermic peak for MAS-10 at 1282 °C shifts to temperatures of about 1243 °C, and it is assigned to the formation of α-cordierite [34]. The analysis of the TG curves confirms the behaviors observed by DTA since dehydroxylation is accompanied with the corresponding mass loss. The samples show two mass loss steps that are not so well separated at 200 and 400 °C, which are in accordance with the DTA peaks described, corresponding to water evaporation, and the dehydroxylation process, respectively [35]. The mass loss in the MAS-0 sample is 3% and it gradually increases to 10% for MAS-160, owing to greater water absorption during the preparation and the milling process. Based on the characteristic temperatures defined by DTA, we established the optimal parameters for TSS. The values of the obtained densities and open and closed porosity are given in Table 2. The results of the sample density measurements were in agreement with the microstructural examination. The average sample densities after two-step sintering ranged from 57.8% to 65.4% of the theoretical densities. The samples used in this study exhibited relatively low densities, which may have considerably adverse impact on dielectric properties. The scanning electron micrographs of the activated and sintered samples are shown in Fig. 9. The non-activated sample was destroyed during sample polishing. The insufficient bonding strength between grains led to the disintegration of the sample. A
significant level of porosity is present in the MAS-10-S sample, around 36%. Grain boundaries are less pronounced in the sample MAS-40-S, although a microstructure with small grains remains, the level of densification is higher, while the open porosity is slightly decreased. The prolonged mechanical activation followed by two-step sintering did not resulted in significant changes in the microstructure; grains retain their shape and size around one micron or less, decreasing the porosity negligibly, no grain growth was observed, while the sample MAS-160-S has highest relative density and lowest relative open porosity, as expected. The exception from this trend is the sample MAS-80-S, which has the highest value of relative open porosity, around 40%, and the lowest density, owing to the presence of agglomerates in the starting mixture and their sintering. The phase composition of the non-activated and activated samples after the two-step sintering process is presented in Fig. 10. The XRD pattern of the initial non-activated sintered sample indicates that several phases – Al2O3, SiO2, MgO, spinel MgAl2O4, MgSiO3, and Mg2SiO4 – are present within the MAS-0-S sintered mixture. The formation of cordierite began during the sintering of the mixture activated for 10 min, although significant amounts of the starting SiO2 phase and the spinel MgAl2O4 phase are still present. The diffraction pattern of MAS-40-S, among well crystallized sharp peaks of cordierite and MgAl2O4, shows small amounts of ZrSiO4 and ZrO2, as a consequence of high-energy milling in a ZrO2 jar and balls. The phase composition of the
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Fig. 6. a) SEM, b) EDS, and c) mapping for activated powder MAS-160.
mixture MAS-80-S remains as in the previous case, although a decrease in the SiO2 amount is noticed. On the contrary, the XRD pattern of the sample activated for 160 min and sintered indicates a cordierite phase, with MgAl2O4, zirconia compounds, without the presence of the starting phases. The increase in the milling time followed by two-step sintering led to a better recrystallization and an increase in peak intensities, with an increased amount of the cordierite phase, although the unwanted zirconia compounds remained. The identification of the obtained reflections was carried out using the JCPDS PDF cards (078-2427 for Al2O3, 065-0466 for SiO2, 077-2364 for MgO, 075-1796 for MgAl2O4, 0740816 for MgSiO3, 076-0561 for Mg2SiO4, 083-1383 for ZrSiO4, 0741201 for ZrO2, and 082-1884 for Mg2(Al4Si5O18) cordierite, indialite modification). In order to measure the relative permittivity of the sintered samples, each sample was metalized on the top and bottom surfaces with silver. Parallel-plate capacitors were obtained whose diameter was around 8 mm and the height was around 3 mm. A custom-made test fixture was used in the form of a coaxial chamber. This test fixture enabled measurements up to several GHz. From the measured reflection coefficient of the chamber, the relative complex permittivity of the samples ( εr = εr′ − j⋅εr″) is extracted using electromagnetic models. For frequencies up to 1 GHz, a quasi-static model suffices [36]. For higher frequencies, a fullwave solver, like WIPL-D [37], is appropriate. For our samples, the reflection coefficient of the chamber was
measured in the frequency range between 1 MHz and 1 GHz using an Agilent E5061A network analyzer. The error of the results for εr is estimated to be less than 5%. The real part of the relative complex permittivity ( ε′r ) of all samples was found to be practically independent of frequency in the whole frequency range. However, it varied from sample to sample. The dielectric constants and average densities of the samples are shown in Fig. 11 as a function of the activation time. It has been widely found and accepted that higher densities result in the enhancement of dielectric constants [38]. The relative permittivity was higher for denser samples (i.e. for samples which had lower porosity). The obtained results are in agreement with the obtained microstructures, presented in Fig. 9, where MAS-160-S has the highest density and relative permittivity ( ρ = 2.06 g cm−3, ε′r = 4.25). Knowing that air has a very low dielectric permittivity (approximately 1), it therefore follows that samples containing high level of pores or air would be inclined to have a lower dielectric constant [39]. The highest value of the relative open porosity present in sample MAS-80-S, 40%, as a consequence of agglomerates within the starting powder (Fig. 5a), resulted in the lowest value of the relative permittivity. The applied measurement technique was broadband, and the shape and size of the samples were not optimized for dielectric measurements. Hence, the imaginary part of the relative permittivity ( ε″ r ) was partly obscured by the measurement noise. Nevertheless, from ε′r and ε″ r , the loss tangent ( tan δ =
εr″ ) εr′
could be
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N. Obradović et al. / Ceramics International 42 (2016) 13909–13918 Table 2. Values of density and porosity of two-step sintered samples.
Fig. 7. Particle size analysis of non-activated and activated powder mixtures.
estimated to be below 0.02, which was within an acceptable range for practical applications.
4. Conclusions In the present study, cordierite-based ceramic materials were prepared using mechanical activation followed by the two-step sintering technique. The phase compositions, microstructures, and electric properties of the bulk materials prepared by TSS were characterized systematically. The following conclusions are drawn from the investigations:
Sample
Theor. dens. Abs. dens. Rel. dens. Rel. open (g cm 3) (g cm 3) (%) por. (%)
Rel. clos. por. (%)
MAS-0-S MAS-10-S MAS-40-S MAS-80-S MAS-160-S
3.20 2.95 2.90 3.05 3.15
3.00 3.70 3.10 3.30 3.30
1.85 1.79 1.84 1.74 2.06
57.80 60.70 63.50 57.10 65.40
39.20 35.60 33.40 39.70 31.30
(1) An intensive mechanical activation in ethanol led to a better homogeneity of powder mixtures, particle size reduction, an increase in powder reactivity through the increase in specific surface area, along with the formation of a new phase – MgSiO3, after 10 min of activation, as confirmed by XRD. For the MgO and SiO2 powders with prolonged mechanical activation, the crystallite size, obtained by an approximation method, drastically decreased, down to nano-sized dimensions (10.04 nm for MgO and 17.18 and 47.47 nm for SiO2). Also, an increase in the dislocation density and lattice microstrain is observed. A slight increase in the crystallite size of the newly formed phase, MgSiO3 (18.17–20.13 nm), indicates a process of recrystallization during an intensive mechanochemical activation. (2) Five characteristic temperatures are detected in the system using DTA and they correspond to humidity evaporation, the dehydroxylation of Al2O3-based compounds, the structural α-β phase transition in crystalline SiO2, and μ-cordierite and α-cordierite formation. The tendency of the activated powders to shift their characteristic temperatures to lower values is due to the reduction of the energy necessary to destroy the crystalline structure of the system. The energy supplied by the planetary mill causes a structural disorder, through the distortion or breakage of the crystalline structure, evident from the reduction of the intensities of XRD peaks. (3) The two-step sintering of the activated powders leads to cordierite formation. As little as 10 min of intensive milling accompanied by TSS is sufficient for the reaction. The densities obtained after TSS increased from 1.79 g cm 3 for MAS-10-S to 2.06 g cm 3 for MAS-160-S. The microstructures indicate porous ceramics with grains smaller than 1 mm, without grain growth with prolonged milling time. The density has the dominant influence on the dielectric permittivity (3.39 for MAS-10-S, increasing to 4.25 for MAS-160-S), with a loss tangent below 0.02.
Fig. 8. a) DTA curves of mixtures with 20 °C/min heating rate and b) TG curves of mixtures with 20 °C/min heating rate.
N. Obradović et al. / Ceramics International 42 (2016) 13909–13918
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Fig. 9. SEM micrographs of two-step sintered powder mixtures: a) MAS-10-S, b) MAS-40-S, c) MAS-80-S, and d) MAS-160-S.
Fig. 11. Dielectric constants and absolute densities of two-step sintered samples as the function of time of mechanical activation.
2020 (LQ1601). The authors would like to thank to Dr. Miodrag Mitrić for the XRD measurements. Fig. 10. XRD patterns of two-step sintered powder mixtures.
Acknowledgement This research was performed within the project OI 172057 funded by the Ministry of Education, Science and Technological Development of the Republic of Serbia and project no. 15-06390S funded by the Grant agency of the Czech Republic. The research has been also financially supported by the Ministry of Education, Youth and Sports of the Czech Republic under the project CEITEC
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