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HfCxN1-x ceramic nanocomposites
Materials Chemistry and Physics 260 (2021) 124122
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Low frequency dielectric behavior and AC conductivity of polymer derived SiC(O)/HfCxN1-x ceramic nanocomposites Eranezhuth Wasan Awin a, Soumya Sridar a, 1, Adhimoolam Bakthavachalam Kousaalya a, 2, S. S. Lokesh Vendra a, Ekaterina Koroleva b, Alexey Filimonov c, Sergey Vakhrushev b, Ravi Kumar a, * a
Laboratory for High Performance Ceramics, Department of Metallurgical and Materials Engineering, Indian Institute of Technology-Madras (IIT Madras), Chennai, 600036, India Neutron Research Laboratory, Ioffe Institute, St.-Petersburg, 194021, Russia c Institute of Physics, Nanotechnology and Telecommunications, Peter the Great St. Petersburg Polytechnic University, St.-Petersburg, 195251, Russia b
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
• In situ crystallization of size controlled SiC(O)/HfCxN1-x nanocrystals through PDC route. • Core-shell structure and presence of free carbon enhances the permittivity values. • Colossal high permittivity values due to the space charge polarization mechanism. • AC conductivity followed Jonscher’s law implying hopping type mechanism.
A R T I C L E I N F O
A B S T R A C T
Keywords: Dielectric properties AC conductivity Spark plasma sintering Nanocomposites
The dielectric behavior of spark plasma sintered SiC(O)/HfCxN1-x nanocomposites synthesized through polymer derived ceramic route was investigated in the frequency range of 1 kHz to 1 MHz at room temperature. The nanostructural features revealed HfCxN1-x nanocrystals encapsulated in a nanometric thin layer of carbon dispersed uniformly in a SiC(O) matrix with segregated free carbon. The nanocomposites exhibited colossal permittivity values in the order of 103 at 1 kHz which reduced to 646 at 1 MHz. The interfacial polarization mechanism existing between complex nanostructural interfaces and the percolation of HfCxN1-x nanocrystals are believed to be responsible for the high permittivity values observed in the measured frequency range. The AC conductivity exemplified a frequency independent behavior at lower frequencies while at higher frequencies, the conductivity exhibited frequency dependence, indicating the existence of hopping type mechanism.
* Corresponding author. E-mail address: [email protected] (R. Kumar). 1 Present address: Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, PA, 15261, USA. 2 Present address: Department of Chemical Engineering, Rowan University, 201 Mullica Hill Road, Glassboro, NJ – 08028, USA. https://doi.org/10.1016/j.matchemphys.2020.124122 Received 6 September 2020; Received in revised form 25 November 2020; Accepted 30 November 2020 Available online 2 December 2020 0254-0584/© 2020 Elsevier B.V. All rights reserved.
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1. Introduction
2. Experimental details
Precursor derived ceramics (PDC) synthesized by the pyrolysis of preceramic polymeric precursors allow enough flexibility for materials design at atomistic and microstructural scales enabling tunable func tionality. The excellent high temperature oxidation resistance [1], chemical/mechanical stability [2,3] including resistance to deformation at elevated temperatures [4] make them suitable for use at extremely harsh operating conditions. Structural applications apart, PDCs also have been considered for various functional applications due to the tailorable magnetic, optical and dielectric properties [5–7]. Materials with high dielectric constants that exemplify stable per formance over a wide range of temperatures are well suited for the electronics and semiconductor industries [8]. However, the operating temperatures of most of the commercial dielectrics based on BaTiO3 ◦ including lead containing ceramics are limited to 200–250 C [9], while concerns on the usage of lead still prevails. The possibility to control the chemistry as well as the crystallization of desired phases which can control the dielectric behaviour/electrical conductivity makes precursor approach towards designing dielectrics all the more attractive. Also, PDCs by virtue of their ability to withstand temperatures in the order of ◦ 800–1000 C free of any phase transitions provide immense potential for the realization of the next generation advanced dielectric materials. The investigation of dielectric behaviour of PDCs so far is predomi nantly restricted to silicon based materials [10–14]. Most of the dielectric studies on PDCs explored till date focused on their use for electromagnetic wave absorption in the X-band frequency. For instance, SiC/HfCxN1-x nanocomposites have been explored for its microwave absorption in the X-band frequency range at room temperature and shown to be promising [15,16]. The design of core-shell microstructures by tailoring the polymeric precursor facilitated the tuning of electrical conductivity and dielectric properties. The high electrical conductivity values (~104 S/cm) possessed by hafnium carbonitrides (HfCxN1-x) and the unique microstructural features present in the system has been attributed to the enhancement in the dielectric properties. In addition to this, the dielectric properties of SiC/HfCxN1-x nanocomposites were also found to be tunable by varying the amount of hafnium precursor content during the synthesis process. In order to further extend the application of PDCs in the field of microelectromechanical systems (MEMS) and sen sors, a basic understanding of the low frequency dielectric behaviour and electrical properties (AC conductivity) are essential. For instance, amorphous PDCs exhibit colossal dielectric values in the order of 105 at 0.03 Hz [12,17,18]. The interfacial polarization existing between the amorphous matrix and free carbon was attributed to the enhancement in dielectric values. However, quantitative estimates and scientific un derstanding of the AC conductivity behaviour of amorphous PDCs are rather limited [10,19,20]. In addition, reports on the dielectric behav iour of crystalline PDCs at low frequency range and over a wide range of temperatures are rather scarce. Transition metal carbides/carbonitrides owing to their high me chanical and chemical stability and transport properties (electrical and thermal conductivity) under extreme conditions can be potential alter natives [21–23]. Although the inherent properties of transition metal carbonitrides serves the objectives, its application at higher tempera tures can still be restricted due to its poor oxidation resistance [24] which possibly can be resolved by the in situ crystallization of such carbonitrides in a stable matrix. In this work, therefore, we report the dielectric properties of highly dense spark plasma sintered SiC(O)/HfCxN1-x nanocomposites processed through precursor route. X-ray diffractograms (XRD) were used to un derstand the phase evolution and the presence of free carbon was confirmed using Raman spectroscopy. The nanocomposites exhibited colossal permittivity in the low frequency range and the mechanisms responsible for the enhanced dielectric values are elucidated. The AC conductivity behavior is discussed in the context of the results obtained from dielectric measurements.
Polysilazane (Clariant, now known as Merck, Germany) and hafnium tetra n-butoxide (Sigma-Aldrich, India) were used as the starting pre cursors to produce Si-Hf-C-N(O) nanocomposites. The modification of polysilazane was done by adding 20 vol% of hafnium tetra n-butoxide and stirring for 1 h. The mixed precursor solution was subsequently ◦ ◦ pyrolyzed at 1000 C for 2 h at a heating rate of 2 C/min under argon atmosphere. The pyrolyzed product was then ground to fine powder and spark plasma sintered (SPS, Sumitomo Coal Mining Co. Ltd, Japan) at ◦ ◦ 1400 C, maintaining a heating rate of 100 C/min in vacuum with an applied load of 50 MPa and held for 30 min as shown in Fig. 1. The ◦ sintering temperature was restricted to 1400 C to avoid the crystalli zation of silica phase. X-ray diffraction (XRD) was performed to understand the phase evolution of the sintered sample using D8 Discover Bruker X-ray diffractometer (USA) with Cu Kα as the radiation source at a scan rate 1s/step in the 2θ range between 20 and 90◦ . The crystallite size and volume fraction of the crystalline phases were calculated using Scher rer’s equation and Rietveld refinement (FullProf Suite) respectively. The pseudo-Voigt function which combines both Gaussian and Lorentzian contributions was used to refine the parameters. Raman spectroscopy (Labram HR 800, Horiba, Japan) was done to determine the presence of free carbon. The micro- and nanostructural features were imaged using scanning electron microscopy (SEM, FEI Quanta 400, USA) and high resolution transmission electron microscopy (HRTEM, JEOL 3010, Japan) respectively. The elemental mapping was done using energy dispersive X-ray spectroscopy (EDS, Bruker XFlash® 6 | 10, USA). The dielectric properties of the sintered samples were investigated using dielectric resonance spectrometer (Novocontrol Alpha-N high resolution dielectric analyser, Germany) at varied tem ◦ ◦ peratures (30 C–250 C) and frequency ranges (1 kHz–1 MHz). The samples were coated with silver paint on both sides to obtain good electrical contact. 3. Results and discussion 3.1. Phase evolution and structure of free carbon The pyrolysis of hafnium modified polysilazane was found to be amorphous till 1100 ◦ C and subsequently at devitrification temperatures undergoes phase separation resulting in the formation of crystalline SiC, HfCxN1-x with segregated free carbon. The XRD of the nanocomposite revealed crystallization of SiC and HfCxN1-x as shown in Fig. 2a. The HfCxN1-x phase was observed to have a shift in 2θ angles when compared to stoichiometric hafnium carbide (HfC) indicating the phase to be a solid solution of HfC and hafnium nitride (HfN). The formation of solid solution between cubic HfN and HfC is possible since both of them possess the same crystal structure as well as similar atomic radii. Hence, the peak at 2θ = 34.2◦ , 39.5◦ and 57◦ corresponds to the (111), (002) and (022) planes of HfCxN1-x respectively. The calculated crystallite size of HfCxN1-x nanocrystals was found to be 21 nm. The volume fraction and lattice parameter of HfCxN1-x determined using Rietveld refinement was found to be 15 vol% and 4.607 Å respectively. The FTIR spectrum (Fig. 2b) shows the bonding characteristics of the nanocomposite. The absorption bands of Hf-C-N at (930–950) cm− 1 was found to be overlapping with the Si–C bands at 835 cm− 1 15. Further more, the band observed at 1047 cm− 1 resulting from Si–O–Si bonds [25] indicates the presence of oxygen in the system. The Raman spectrum was used to understand the graphitization of free carbon in SiC/HfCxN1-x as exemplified in Fig. 2c. The spectrum exhibited peaks at 1370 cm− 1 and 1570 cm− 1 corresponding to graphitic sp [3] and sp [2] carbon respectively, indicating that the segregated carbon is in a state of high disorderness [15,26]. This could be ascribed to the fact that free carbon is still in completely disordered sp [2] amorphous state. 2
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Fig. 1. Generalized schematic depicting the synthesis procedure and the sintering process.
Fig. 2. (a) XRD of sintered sample revealing the crystallization of SiC and HfCxN1-x provided with the reference patterns of HfN and HfC (b) FTIR spectrum exhibiting the bonding characteristics and (c) Raman spectra implying the presence of free carbon.
3.2. Micro/nano structural features
carbon in the nanocomposite. Even after prolonged exposure during the ◦ SPS process (for 30 min) at 1400 C, the growth of HfCxN1-x nanocrystals were rather limited which could be possibly due to the diffusion barrier created by the presence of carbon layer. The HR-TEM images of HfCxN1-x nanocomposites depicted in Fig. 4b reveals lattice fringe spacing of 0.25 nm and 0.26 nm corresponding to SiC and HfCxN1-x nanocrystals. The EDS spectrum confirmed the presence of Si, Hf, C, N and O at sub nanometric level (Fig. 4c).
Fig. 3a shows the SEM images of sintered SiC(O)/HfCxN1-x nano composite in backscattered mode. The bright regions observed in the micrograph correspond to hafnium enriched regions whereas the dark regions are ascribed to the areas rich in silicon. The EDS area scan and mapping was done to confirm the presence/distribution of silicon, hafnium, carbon, nitrogen and oxygen in the matrix (Fig. 3b and c). The TEM micrographs clearly revealed the distribution of HfCxN1-x nanocrystals distributed in a SiC matrix (not shown). The average size of the HfCxN1-x nanocrystals was found to be in the range of 18–27 nm which was in close agreement with the size calculated from Scherrer’s equation. The inset in Fig. 4a illustrates the presence of carbon layer (thickness ~3 nm) around the HfCxN1-x nanocrystals and the Fast Fourier-Transform (FFT) pattern confirmed the carbon layer to be amorphous in nature due to the absence of spots in the FFT spectrum. The Raman spectrum also substantiates the presence of disordered
3.3. Dielectric properties The real (ε′ ) and imaginary (ε") part of dielectric constant can be determined according to the Debye theory ′
ε = ε∞ +
3
εs − ε∞ 1 + ω2 τ2
(1)
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Materials Chemistry and Physics 260 (2021) 124122
Fig. 3. (a) Backscattered electron micrograph of SiC(O)/HfCxN1-x nanocomposite revealing hafnium enriched regions (bright) (b) Area scan EDS confirming the presence of Si, Hf, C, N and O and (c) EDS elemental mapping showing the distribution of elements.
ε′′ =
εs − ε∞ σ + = ε′′p + ε′′c ωε0 1 + ω2 τ2
(i) Existence of interfacial polarization
(2)
The interfaces developed between the carbon encapsulated HfCxN1-x and SiC due to the differences in the electrical conductivity values could result in interfacial polarization. In addition to this, the grain boundaries existing between SiC nanocrystals are also believed to enhance the dielectric values in the investigated frequency range. It is also speculated that the charge accumulation at the interfaces between HfCxN1-x and carbon layer for the enhanced dielectric permittivity values.
where ε∞ is the dielectric permittivity at high frequency, εs is the static relative permittivity, τ is the relaxation time, σ is the electrical con ductivity and ε0 is the permittivity in vacuum. From Eqns. (1) and (2), it is understood that the real part of permittivity could be related to the polarization whereas the imaginary part could be associated with po larization and electrical conductivity. 3.3.1. Real part of permittivity The high values of ε′ at lower frequencies and low magnitude values at higher frequencies implies accumulation of charges at the interfaces. Under the excitement of electromagnetic wave (EMW), the charge car riers accumulate at the interfaces, thereby leading to interfacial polari zation. However, since certain portion of charge carriers cannot cope up with the rapid changes of EMW, resulting in the decrement of real values of permittivity at higher frequencies [27]. Fig. 5a shows the real part of dielectric permittivity as a function of frequency. The sample showed an order of magnitude drop in dielectric constant values (6.4 × 103 to 646) with increase in frequency range from 1 kHz to 1 MHz. Interestingly, it could be observed that the nanocomposites exhibit colossal dielectric value with respect to amorphous SiCN (13 at 1 kHz & 6 at 1 MHz) [17] at the investigated frequency range. Since, neither atomic nor electronic polarization mechanisms are plausible in the chosen frequency range, the only possible mechanism that could explain the enhanced dielectric constant is space charge polarization. The dielectric permittivity measured at X-band frequencies of SiC/HfCxN1-x/C ceramic nano composites shows values to be in the range of 5–10 [28]. The enhancement in dielectric constant of SiC/HfCxN1-x nano composite with respect to amorphous SiCN [17] could be attributed to the following possibilities:
(ii) Presence of conductive phases The HfCxN1-x nanocrystalline phases (electrical conductivity in the order of 104 Scm− 1) [29] could form a percolation system when the volume fraction of these phases approaches percolation threshold. The percolation threshold of spherical particles of size less than 50 nm is expected to be less than 10 vol% [30]. In this study, the average crys tallite size of HfCxN1-x nanocrystals were found to be in the range of 20 nm and the amount of HfCxN1-x nanocrystals determined using Rietveld refinement was calculated to be 15 vol%. Hence, the colossal value of ε′ could be attributed to the percolation effect of the conductive HfCxN1-x nanocrystals. (iii) Presence of free carbon The presence of free carbon as revealed from the Raman spectra, could enhance the permittivity values. In accordance to the Maxwell Garnett rule [31], due to the difference in electrical conductivity values between the free carbon phase and other phases results in the motion and accumulation of charge carriers at the interfaces under applied frequencies. This could lead to the enhancement in dielectric permit tivity. The enhancement of dielectric permittivity and loss in polymer derived SiCN ceramics has been attributed to the presence of free carbon 4
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Fig. 4. (a) HR-TEM micrograph revealing the presence of carbon layer around HfCxN1-x nanocrystals (Inset: FFT pattern of the carbon layer) (b) Lattice fringes of SiC and HfCxN1-x nanocrystals and (c) EDS spectrum confirming the presence of Si, Hf, C, N and O at subnanometric level.
and SiC [13,14]. The high values of ε′ at lower frequencies and low magnitude values at higher frequencies imply accumulation of charges at the interfaces. Under the excitement of electromagnetic wave (EMW), the charge car riers accumulate at the interfaces, thereby leading to interfacial polari zation. However, since certain portion of charge carriers cannot cope up with the rapid changes of EMW, results in the decrement of real values of permittivity at higher frequencies [27]. 3.3.2. Imaginary part of permittivity and dielectric losses The ε", with increase in frequency showed a linear decrease in the log-log plot as shown in Fig. 5b. The ε" value at 1 MHz was 1.3 × 104 and was three orders of magnitude higher at 1 kHz, raising up to 1.1 × 107. Due to the presence of highly conductive phases in the nanocomposite, Eqn. (2) could be approximated to Eqn. (3) based on the free electron theory. i.e.,
ε′′ =
σ ωε0
(3)
Hence, it could be concluded that ε" values depends directly on the σ values of the phases present in the nanocomposite. In the present nanocomposite, the σ values are determined by the core HfCxN1-x nanocrystals and free carbon.
Fig. 5a. Influence of ε′ over frequency in SiC(O)/HfCxN1-x nanocomposites.
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Fig. 5b. Dependence of ε" over frequency in SiC(O)/HfCxN1-x nanocomposites.
The tanδ values gradually increased with increase in frequency (25 at 1 MHz to 3.3 × 103 at 1 kHz) as shown in Fig. 5c. At higher frequencies, the long range movement of electrons decreases thereby reducing fre quency leakage. The loss peak observed at around 1–2 kHz range cor responds to the interfacial polarization mechanism. The high loss values reported in this study further substantiates the percolation theory and the role of conductive phases in governing the dielectric properties. 3.3.3. Dielectric permittivity and losses at higher temperatures The dependence of dielectric permittivity values on temperature were understood by analyzing Fig. 6a. The dielectric permittivity values were found to be gradually increasing with increase in temperature initially, with a rapid increase in values at approximately 160 ◦ C for 1 kHz, 10 kHz, 100 kHz and 1 MHz. The broad peak of dielectric loss was also observed to shift to higher temperatures with frequency (Fig. 6b). These observations corroborate the fact that the interfacial - charge polarization is the dominant mechanism. In order to accumulate suffi cient charge carriers at the interfaces, the charge carrier relaxation time should match with the frequency of the applied electric field. It could be inferred that the interfacial - charge polarization is thermally activated
Fig. 6. (a) Dielectric permittivity and (b) loss measured over a wide range of frequencies at higher temperatures.
due to the rapid increase in dielectric permittivity values after a particular frequency and the shift in loss peaks to higher temperatures with frequencies. The schematic representation of the possible dielectric mechanism in the nanocomposites is shown in Fig. 7a. The TEM micrograph shown in Fig. 4a clearly suggests that the carbon encapsulated HfCxN1-x nano crystals are distributed uniformly with in the SiC matrix. The space charge polarization existing between the grain boundaries of SiC poly crystals, SiC and HfCxN1-x phases could account for colossal permittivity values. Concomitantly, accumulation of charge carriers at the interface between free carbon and the matrix (Fig. 7b) could result in space charge polarization. Fig. 5c. tanδ as a function of frequency in SiC(O)/HfCxN1-x nanocomposites. 6
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Fig. 7. (a) Schematic representation of the polarization mechanism in SiC(O)/HfCxN1-x nanocomposites and (b) HR-TEM micrograph indicating the presence of free carbon.
3.4. AC conductivity behavior Fig. 8a illustrates the dependence of AC conductivity with respect to frequency measured at different temperatures. The quick transfer of electrons with increase in frequency resulted in an increase in conduc tivity values of the nanocomposites. It could be clearly seen that the AC conductivity is frequency independent at lower frequencies whereas shows a frequency dependent behavior at higher frequencies. The AC conductivity (σac) values fitted using Jonscher’s law [32] (Eqn. (4)) clearly implies the existence of hopping type mechanism in SiC (O)/HfCxN1-x nanocomposites.
σac = σdc + A(2πf)s
(4)
where σdc is the direct conductivity at lower frequencies, A is a char acteristic parameter, f is the frequency and s is an exponential function which depends on temperature. The s values were found to be decreasing with increase in tempera ture as shown in Fig. 8b suggesting the possibility of hopping
Fig. 8b. Dependence of exponent s with temperature in SiC(O)/HfCxN1-x nanocomposites.
mechanism. In order to further comprehend the conductive mechanism at lower frequencies, AC conductivity values at 1 kHz was considered to be equivalent to DC conductivity values. Fig. 9 depicts the frequency in dependent conductivity following the exponential dependence of tem perature as shown in Eqn. 5 [ ] σ = σo exp − (To /T)1/β (5) where σo is the prefactor and To is the characteristic temperature. The values of β, a constant, determines the conductivity mechanism. It could be observed that a near to linear fit was clearly attained when β = 4, implying that conduction at low frequencies was driven by hopping mechanism.
Fig. 8a. σac as a function of frequency measured at different temperatures. 7
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Fig. 9. Low frequency conductivity as a function of measuring temperatures fitted for β = 4.
4. Conclusions
Acknowledgments
The low frequency dependence dielectric behavior of SiC(O)/ HfCxN1-x nanocomposite at room temperature has been unveiled based on the spectroscopic and nanostructural features presented in this study. The colossal high permittivity values of the nanocomposite is ascer tained to the existence of space charge polarization existing between SiC polycrystals, SiC and HfCxN1-x phases, presence of free carbon and the percolation of HfCxN1-x nanocrystals. The occurrence of highly conductive free carbon and the core-shell structure exhibited by HfCxN1x – C phase could be attributed to the high values of imaginary dielectric permittivity and losses. The AC conductivity increased with increase in frequency with T− 1/4 dependence revealing the hopping conductive mechanism.
The funding received from the Indo-Russian Joint Project (DSTRFBR) is gratefully acknowledged. The work was funded by the grant received from the Department of Science and Technology, Ministry of Science and Technology, Government of India (project no. DST/INT/ RFBR/IDIR/P-12/2016). The authors would also like to thank the DST Unit of Nanoscience, Department of Chemistry and Central Electron Microscopy Facility of IITMadras for availing access to HR-TEM and SEM respectively. References [1] Y. Wang, Y. Fan, L. Zhang, W. Zhang, L. An, Polymer-derived SiAlCN ceramics resist oxidation at 1400 ◦ C, Scripta Mater. 55 (4) (2006) 295–297, https://doi.org/ 10.1016/j.scriptamat.2006.05.004. [2] R. Riedel, A. Kienzle, W. Dressler, L. Ruwisch, J. Bill, F. Aldinger, A silicoboron carbonitride ceramic stable to 2,000◦ C, Nature 382 (6594) (1996) 796–798, https://doi.org/10.1038/382796a0. [3] Y. Wang, W. Fei, Y. Fan, L. Zhang, W. Zhang, L. An, Silicoaluminum carbonitride ceramic resist to oxidation/corrosion in water vapor, J. Mater. Res. 21 (7) (2006) 1625–1628, https://doi.org/10.1557/jmr.2006.0210. [4] R. Riedel, L.M. Ruswisch, L. An, R. Raj, Amorphous silicoboron carbonitride ceramic with very high viscosity at temperatures above 1500◦ C, J. Am. Ceram. Soc. 81 (12) (1998) 3341–3344, https://doi.org/10.1111/j.1151-2916.1998.tb02780.x. [5] S. Sarkar, Z. Gan, L. An, L. Zhai, Structural evolution of polymer-derived amorphous SiBCN ceramics at high temperature, J. Phys. Chem. C 115 (50) (2011) 24993–25000, https://doi.org/10.1021/jp203287h. [6] Y. Wang, K. Wang, L. Zhang, L. An, Structure and optical property of polymerderived amorphous silicon oxycarbides obtained at different temperatures, J. Am. Ceram. Soc. 94 (10) (2011) 3359–3363, https://doi.org/10.1111/j.15512916.2011.04491.x. [7] A. Saha, S.R. Shah, R. Raj, S.E. Russek, Polymer-derived SiCN composites with magnetic properties, J. Mater. Res. 18 (11) (2003) 2549–2551, https://doi.org/ 10.1557/JMR.2003.0356. [8] Z.M. Dang, J.K. Yuan, S.H. Yao, R.J. Liao, Flexible nanodielectric materials with high permittivity for power energy storage, Adv. Mater. 25 (44) (2013) 6334–6365, https://doi.org/10.1002/adma.201301752. [9] A. Zeb, S.J. Milne, High temperature dielectric ceramics: a review of temperaturestable high-permittivity perovskites, J. Mater. Sci. Mater. Electron. 26 (12) (2015) 9243–9255, https://doi.org/10.1007/s10854-015-3707-7. [10] B. Ma, Y. Wang, K. Wang, X. Li, J. Liu, L. An, Frequency-dependent conductive behavior of polymer-derived amorphous silicon carbonitride, Acta Mater. 89 (2015) 215–224, https://doi.org/10.1016/j.actamat.2015.02.020.
CRediT authorship contribution statement Eranezhuth Wasan Awin: Conceptualization, Methodology, Vali dation, Investigation, Formal analysis, Visualization, Writing - original draft. Soumya Sridar: Conceptualization, Methodology, Investigation, Formal analysis. Adhimoolam Bakthavachalam Kousaalya: Concep tualization, Investigation. S.S. Lokesh Vendra: Formal analysis, Investigation. Ekaterina Koroleva: Supervision, Writing - review & editing. Alexey Filimonov: Supervision, Writing - review & editing. Sergey Vakhrushev: Supervision, Writing - review & editing. Ravi Kumar: Conceptualization, Supervision, Formal analysis, Funding acquisition, Writing - review & editing. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Low frequency dielectric behavior and AC conductivity of polymer derived SiC(O)/HfCxN1-x ceramic nanocomposites Eranezhuth Wasan Awin a, Soumya Sridar a, 1, Adhimoolam Bakthavachalam Kousaalya a, 2, S. S. Lokesh Vendra a, Ekaterina Koroleva b, Alexey Filimonov c, Sergey Vakhrushev b, Ravi Kumar a, * a
Laboratory for High Performance Ceramics, Department of Metallurgical and Materials Engineering, Indian Institute of Technology-Madras (IIT Madras), Chennai, 600036, India Neutron Research Laboratory, Ioffe Institute, St.-Petersburg, 194021, Russia c Institute of Physics, Nanotechnology and Telecommunications, Peter the Great St. Petersburg Polytechnic University, St.-Petersburg, 195251, Russia b
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
• In situ crystallization of size controlled SiC(O)/HfCxN1-x nanocrystals through PDC route. • Core-shell structure and presence of free carbon enhances the permittivity values. • Colossal high permittivity values due to the space charge polarization mechanism. • AC conductivity followed Jonscher’s law implying hopping type mechanism.
A R T I C L E I N F O
A B S T R A C T
Keywords: Dielectric properties AC conductivity Spark plasma sintering Nanocomposites
The dielectric behavior of spark plasma sintered SiC(O)/HfCxN1-x nanocomposites synthesized through polymer derived ceramic route was investigated in the frequency range of 1 kHz to 1 MHz at room temperature. The nanostructural features revealed HfCxN1-x nanocrystals encapsulated in a nanometric thin layer of carbon dispersed uniformly in a SiC(O) matrix with segregated free carbon. The nanocomposites exhibited colossal permittivity values in the order of 103 at 1 kHz which reduced to 646 at 1 MHz. The interfacial polarization mechanism existing between complex nanostructural interfaces and the percolation of HfCxN1-x nanocrystals are believed to be responsible for the high permittivity values observed in the measured frequency range. The AC conductivity exemplified a frequency independent behavior at lower frequencies while at higher frequencies, the conductivity exhibited frequency dependence, indicating the existence of hopping type mechanism.
* Corresponding author. E-mail address: [email protected] (R. Kumar). 1 Present address: Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, PA, 15261, USA. 2 Present address: Department of Chemical Engineering, Rowan University, 201 Mullica Hill Road, Glassboro, NJ – 08028, USA. https://doi.org/10.1016/j.matchemphys.2020.124122 Received 6 September 2020; Received in revised form 25 November 2020; Accepted 30 November 2020 Available online 2 December 2020 0254-0584/© 2020 Elsevier B.V. All rights reserved.
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1. Introduction
2. Experimental details
Precursor derived ceramics (PDC) synthesized by the pyrolysis of preceramic polymeric precursors allow enough flexibility for materials design at atomistic and microstructural scales enabling tunable func tionality. The excellent high temperature oxidation resistance [1], chemical/mechanical stability [2,3] including resistance to deformation at elevated temperatures [4] make them suitable for use at extremely harsh operating conditions. Structural applications apart, PDCs also have been considered for various functional applications due to the tailorable magnetic, optical and dielectric properties [5–7]. Materials with high dielectric constants that exemplify stable per formance over a wide range of temperatures are well suited for the electronics and semiconductor industries [8]. However, the operating temperatures of most of the commercial dielectrics based on BaTiO3 ◦ including lead containing ceramics are limited to 200–250 C [9], while concerns on the usage of lead still prevails. The possibility to control the chemistry as well as the crystallization of desired phases which can control the dielectric behaviour/electrical conductivity makes precursor approach towards designing dielectrics all the more attractive. Also, PDCs by virtue of their ability to withstand temperatures in the order of ◦ 800–1000 C free of any phase transitions provide immense potential for the realization of the next generation advanced dielectric materials. The investigation of dielectric behaviour of PDCs so far is predomi nantly restricted to silicon based materials [10–14]. Most of the dielectric studies on PDCs explored till date focused on their use for electromagnetic wave absorption in the X-band frequency. For instance, SiC/HfCxN1-x nanocomposites have been explored for its microwave absorption in the X-band frequency range at room temperature and shown to be promising [15,16]. The design of core-shell microstructures by tailoring the polymeric precursor facilitated the tuning of electrical conductivity and dielectric properties. The high electrical conductivity values (~104 S/cm) possessed by hafnium carbonitrides (HfCxN1-x) and the unique microstructural features present in the system has been attributed to the enhancement in the dielectric properties. In addition to this, the dielectric properties of SiC/HfCxN1-x nanocomposites were also found to be tunable by varying the amount of hafnium precursor content during the synthesis process. In order to further extend the application of PDCs in the field of microelectromechanical systems (MEMS) and sen sors, a basic understanding of the low frequency dielectric behaviour and electrical properties (AC conductivity) are essential. For instance, amorphous PDCs exhibit colossal dielectric values in the order of 105 at 0.03 Hz [12,17,18]. The interfacial polarization existing between the amorphous matrix and free carbon was attributed to the enhancement in dielectric values. However, quantitative estimates and scientific un derstanding of the AC conductivity behaviour of amorphous PDCs are rather limited [10,19,20]. In addition, reports on the dielectric behav iour of crystalline PDCs at low frequency range and over a wide range of temperatures are rather scarce. Transition metal carbides/carbonitrides owing to their high me chanical and chemical stability and transport properties (electrical and thermal conductivity) under extreme conditions can be potential alter natives [21–23]. Although the inherent properties of transition metal carbonitrides serves the objectives, its application at higher tempera tures can still be restricted due to its poor oxidation resistance [24] which possibly can be resolved by the in situ crystallization of such carbonitrides in a stable matrix. In this work, therefore, we report the dielectric properties of highly dense spark plasma sintered SiC(O)/HfCxN1-x nanocomposites processed through precursor route. X-ray diffractograms (XRD) were used to un derstand the phase evolution and the presence of free carbon was confirmed using Raman spectroscopy. The nanocomposites exhibited colossal permittivity in the low frequency range and the mechanisms responsible for the enhanced dielectric values are elucidated. The AC conductivity behavior is discussed in the context of the results obtained from dielectric measurements.
Polysilazane (Clariant, now known as Merck, Germany) and hafnium tetra n-butoxide (Sigma-Aldrich, India) were used as the starting pre cursors to produce Si-Hf-C-N(O) nanocomposites. The modification of polysilazane was done by adding 20 vol% of hafnium tetra n-butoxide and stirring for 1 h. The mixed precursor solution was subsequently ◦ ◦ pyrolyzed at 1000 C for 2 h at a heating rate of 2 C/min under argon atmosphere. The pyrolyzed product was then ground to fine powder and spark plasma sintered (SPS, Sumitomo Coal Mining Co. Ltd, Japan) at ◦ ◦ 1400 C, maintaining a heating rate of 100 C/min in vacuum with an applied load of 50 MPa and held for 30 min as shown in Fig. 1. The ◦ sintering temperature was restricted to 1400 C to avoid the crystalli zation of silica phase. X-ray diffraction (XRD) was performed to understand the phase evolution of the sintered sample using D8 Discover Bruker X-ray diffractometer (USA) with Cu Kα as the radiation source at a scan rate 1s/step in the 2θ range between 20 and 90◦ . The crystallite size and volume fraction of the crystalline phases were calculated using Scher rer’s equation and Rietveld refinement (FullProf Suite) respectively. The pseudo-Voigt function which combines both Gaussian and Lorentzian contributions was used to refine the parameters. Raman spectroscopy (Labram HR 800, Horiba, Japan) was done to determine the presence of free carbon. The micro- and nanostructural features were imaged using scanning electron microscopy (SEM, FEI Quanta 400, USA) and high resolution transmission electron microscopy (HRTEM, JEOL 3010, Japan) respectively. The elemental mapping was done using energy dispersive X-ray spectroscopy (EDS, Bruker XFlash® 6 | 10, USA). The dielectric properties of the sintered samples were investigated using dielectric resonance spectrometer (Novocontrol Alpha-N high resolution dielectric analyser, Germany) at varied tem ◦ ◦ peratures (30 C–250 C) and frequency ranges (1 kHz–1 MHz). The samples were coated with silver paint on both sides to obtain good electrical contact. 3. Results and discussion 3.1. Phase evolution and structure of free carbon The pyrolysis of hafnium modified polysilazane was found to be amorphous till 1100 ◦ C and subsequently at devitrification temperatures undergoes phase separation resulting in the formation of crystalline SiC, HfCxN1-x with segregated free carbon. The XRD of the nanocomposite revealed crystallization of SiC and HfCxN1-x as shown in Fig. 2a. The HfCxN1-x phase was observed to have a shift in 2θ angles when compared to stoichiometric hafnium carbide (HfC) indicating the phase to be a solid solution of HfC and hafnium nitride (HfN). The formation of solid solution between cubic HfN and HfC is possible since both of them possess the same crystal structure as well as similar atomic radii. Hence, the peak at 2θ = 34.2◦ , 39.5◦ and 57◦ corresponds to the (111), (002) and (022) planes of HfCxN1-x respectively. The calculated crystallite size of HfCxN1-x nanocrystals was found to be 21 nm. The volume fraction and lattice parameter of HfCxN1-x determined using Rietveld refinement was found to be 15 vol% and 4.607 Å respectively. The FTIR spectrum (Fig. 2b) shows the bonding characteristics of the nanocomposite. The absorption bands of Hf-C-N at (930–950) cm− 1 was found to be overlapping with the Si–C bands at 835 cm− 1 15. Further more, the band observed at 1047 cm− 1 resulting from Si–O–Si bonds [25] indicates the presence of oxygen in the system. The Raman spectrum was used to understand the graphitization of free carbon in SiC/HfCxN1-x as exemplified in Fig. 2c. The spectrum exhibited peaks at 1370 cm− 1 and 1570 cm− 1 corresponding to graphitic sp [3] and sp [2] carbon respectively, indicating that the segregated carbon is in a state of high disorderness [15,26]. This could be ascribed to the fact that free carbon is still in completely disordered sp [2] amorphous state. 2
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Fig. 1. Generalized schematic depicting the synthesis procedure and the sintering process.
Fig. 2. (a) XRD of sintered sample revealing the crystallization of SiC and HfCxN1-x provided with the reference patterns of HfN and HfC (b) FTIR spectrum exhibiting the bonding characteristics and (c) Raman spectra implying the presence of free carbon.
3.2. Micro/nano structural features
carbon in the nanocomposite. Even after prolonged exposure during the ◦ SPS process (for 30 min) at 1400 C, the growth of HfCxN1-x nanocrystals were rather limited which could be possibly due to the diffusion barrier created by the presence of carbon layer. The HR-TEM images of HfCxN1-x nanocomposites depicted in Fig. 4b reveals lattice fringe spacing of 0.25 nm and 0.26 nm corresponding to SiC and HfCxN1-x nanocrystals. The EDS spectrum confirmed the presence of Si, Hf, C, N and O at sub nanometric level (Fig. 4c).
Fig. 3a shows the SEM images of sintered SiC(O)/HfCxN1-x nano composite in backscattered mode. The bright regions observed in the micrograph correspond to hafnium enriched regions whereas the dark regions are ascribed to the areas rich in silicon. The EDS area scan and mapping was done to confirm the presence/distribution of silicon, hafnium, carbon, nitrogen and oxygen in the matrix (Fig. 3b and c). The TEM micrographs clearly revealed the distribution of HfCxN1-x nanocrystals distributed in a SiC matrix (not shown). The average size of the HfCxN1-x nanocrystals was found to be in the range of 18–27 nm which was in close agreement with the size calculated from Scherrer’s equation. The inset in Fig. 4a illustrates the presence of carbon layer (thickness ~3 nm) around the HfCxN1-x nanocrystals and the Fast Fourier-Transform (FFT) pattern confirmed the carbon layer to be amorphous in nature due to the absence of spots in the FFT spectrum. The Raman spectrum also substantiates the presence of disordered
3.3. Dielectric properties The real (ε′ ) and imaginary (ε") part of dielectric constant can be determined according to the Debye theory ′
ε = ε∞ +
3
εs − ε∞ 1 + ω2 τ2
(1)
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Fig. 3. (a) Backscattered electron micrograph of SiC(O)/HfCxN1-x nanocomposite revealing hafnium enriched regions (bright) (b) Area scan EDS confirming the presence of Si, Hf, C, N and O and (c) EDS elemental mapping showing the distribution of elements.
ε′′ =
εs − ε∞ σ + = ε′′p + ε′′c ωε0 1 + ω2 τ2
(i) Existence of interfacial polarization
(2)
The interfaces developed between the carbon encapsulated HfCxN1-x and SiC due to the differences in the electrical conductivity values could result in interfacial polarization. In addition to this, the grain boundaries existing between SiC nanocrystals are also believed to enhance the dielectric values in the investigated frequency range. It is also speculated that the charge accumulation at the interfaces between HfCxN1-x and carbon layer for the enhanced dielectric permittivity values.
where ε∞ is the dielectric permittivity at high frequency, εs is the static relative permittivity, τ is the relaxation time, σ is the electrical con ductivity and ε0 is the permittivity in vacuum. From Eqns. (1) and (2), it is understood that the real part of permittivity could be related to the polarization whereas the imaginary part could be associated with po larization and electrical conductivity. 3.3.1. Real part of permittivity The high values of ε′ at lower frequencies and low magnitude values at higher frequencies implies accumulation of charges at the interfaces. Under the excitement of electromagnetic wave (EMW), the charge car riers accumulate at the interfaces, thereby leading to interfacial polari zation. However, since certain portion of charge carriers cannot cope up with the rapid changes of EMW, resulting in the decrement of real values of permittivity at higher frequencies [27]. Fig. 5a shows the real part of dielectric permittivity as a function of frequency. The sample showed an order of magnitude drop in dielectric constant values (6.4 × 103 to 646) with increase in frequency range from 1 kHz to 1 MHz. Interestingly, it could be observed that the nanocomposites exhibit colossal dielectric value with respect to amorphous SiCN (13 at 1 kHz & 6 at 1 MHz) [17] at the investigated frequency range. Since, neither atomic nor electronic polarization mechanisms are plausible in the chosen frequency range, the only possible mechanism that could explain the enhanced dielectric constant is space charge polarization. The dielectric permittivity measured at X-band frequencies of SiC/HfCxN1-x/C ceramic nano composites shows values to be in the range of 5–10 [28]. The enhancement in dielectric constant of SiC/HfCxN1-x nano composite with respect to amorphous SiCN [17] could be attributed to the following possibilities:
(ii) Presence of conductive phases The HfCxN1-x nanocrystalline phases (electrical conductivity in the order of 104 Scm− 1) [29] could form a percolation system when the volume fraction of these phases approaches percolation threshold. The percolation threshold of spherical particles of size less than 50 nm is expected to be less than 10 vol% [30]. In this study, the average crys tallite size of HfCxN1-x nanocrystals were found to be in the range of 20 nm and the amount of HfCxN1-x nanocrystals determined using Rietveld refinement was calculated to be 15 vol%. Hence, the colossal value of ε′ could be attributed to the percolation effect of the conductive HfCxN1-x nanocrystals. (iii) Presence of free carbon The presence of free carbon as revealed from the Raman spectra, could enhance the permittivity values. In accordance to the Maxwell Garnett rule [31], due to the difference in electrical conductivity values between the free carbon phase and other phases results in the motion and accumulation of charge carriers at the interfaces under applied frequencies. This could lead to the enhancement in dielectric permit tivity. The enhancement of dielectric permittivity and loss in polymer derived SiCN ceramics has been attributed to the presence of free carbon 4
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Fig. 4. (a) HR-TEM micrograph revealing the presence of carbon layer around HfCxN1-x nanocrystals (Inset: FFT pattern of the carbon layer) (b) Lattice fringes of SiC and HfCxN1-x nanocrystals and (c) EDS spectrum confirming the presence of Si, Hf, C, N and O at subnanometric level.
and SiC [13,14]. The high values of ε′ at lower frequencies and low magnitude values at higher frequencies imply accumulation of charges at the interfaces. Under the excitement of electromagnetic wave (EMW), the charge car riers accumulate at the interfaces, thereby leading to interfacial polari zation. However, since certain portion of charge carriers cannot cope up with the rapid changes of EMW, results in the decrement of real values of permittivity at higher frequencies [27]. 3.3.2. Imaginary part of permittivity and dielectric losses The ε", with increase in frequency showed a linear decrease in the log-log plot as shown in Fig. 5b. The ε" value at 1 MHz was 1.3 × 104 and was three orders of magnitude higher at 1 kHz, raising up to 1.1 × 107. Due to the presence of highly conductive phases in the nanocomposite, Eqn. (2) could be approximated to Eqn. (3) based on the free electron theory. i.e.,
ε′′ =
σ ωε0
(3)
Hence, it could be concluded that ε" values depends directly on the σ values of the phases present in the nanocomposite. In the present nanocomposite, the σ values are determined by the core HfCxN1-x nanocrystals and free carbon.
Fig. 5a. Influence of ε′ over frequency in SiC(O)/HfCxN1-x nanocomposites.
5
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Fig. 5b. Dependence of ε" over frequency in SiC(O)/HfCxN1-x nanocomposites.
The tanδ values gradually increased with increase in frequency (25 at 1 MHz to 3.3 × 103 at 1 kHz) as shown in Fig. 5c. At higher frequencies, the long range movement of electrons decreases thereby reducing fre quency leakage. The loss peak observed at around 1–2 kHz range cor responds to the interfacial polarization mechanism. The high loss values reported in this study further substantiates the percolation theory and the role of conductive phases in governing the dielectric properties. 3.3.3. Dielectric permittivity and losses at higher temperatures The dependence of dielectric permittivity values on temperature were understood by analyzing Fig. 6a. The dielectric permittivity values were found to be gradually increasing with increase in temperature initially, with a rapid increase in values at approximately 160 ◦ C for 1 kHz, 10 kHz, 100 kHz and 1 MHz. The broad peak of dielectric loss was also observed to shift to higher temperatures with frequency (Fig. 6b). These observations corroborate the fact that the interfacial - charge polarization is the dominant mechanism. In order to accumulate suffi cient charge carriers at the interfaces, the charge carrier relaxation time should match with the frequency of the applied electric field. It could be inferred that the interfacial - charge polarization is thermally activated
Fig. 6. (a) Dielectric permittivity and (b) loss measured over a wide range of frequencies at higher temperatures.
due to the rapid increase in dielectric permittivity values after a particular frequency and the shift in loss peaks to higher temperatures with frequencies. The schematic representation of the possible dielectric mechanism in the nanocomposites is shown in Fig. 7a. The TEM micrograph shown in Fig. 4a clearly suggests that the carbon encapsulated HfCxN1-x nano crystals are distributed uniformly with in the SiC matrix. The space charge polarization existing between the grain boundaries of SiC poly crystals, SiC and HfCxN1-x phases could account for colossal permittivity values. Concomitantly, accumulation of charge carriers at the interface between free carbon and the matrix (Fig. 7b) could result in space charge polarization. Fig. 5c. tanδ as a function of frequency in SiC(O)/HfCxN1-x nanocomposites. 6
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Fig. 7. (a) Schematic representation of the polarization mechanism in SiC(O)/HfCxN1-x nanocomposites and (b) HR-TEM micrograph indicating the presence of free carbon.
3.4. AC conductivity behavior Fig. 8a illustrates the dependence of AC conductivity with respect to frequency measured at different temperatures. The quick transfer of electrons with increase in frequency resulted in an increase in conduc tivity values of the nanocomposites. It could be clearly seen that the AC conductivity is frequency independent at lower frequencies whereas shows a frequency dependent behavior at higher frequencies. The AC conductivity (σac) values fitted using Jonscher’s law [32] (Eqn. (4)) clearly implies the existence of hopping type mechanism in SiC (O)/HfCxN1-x nanocomposites.
σac = σdc + A(2πf)s
(4)
where σdc is the direct conductivity at lower frequencies, A is a char acteristic parameter, f is the frequency and s is an exponential function which depends on temperature. The s values were found to be decreasing with increase in tempera ture as shown in Fig. 8b suggesting the possibility of hopping
Fig. 8b. Dependence of exponent s with temperature in SiC(O)/HfCxN1-x nanocomposites.
mechanism. In order to further comprehend the conductive mechanism at lower frequencies, AC conductivity values at 1 kHz was considered to be equivalent to DC conductivity values. Fig. 9 depicts the frequency in dependent conductivity following the exponential dependence of tem perature as shown in Eqn. 5 [ ] σ = σo exp − (To /T)1/β (5) where σo is the prefactor and To is the characteristic temperature. The values of β, a constant, determines the conductivity mechanism. It could be observed that a near to linear fit was clearly attained when β = 4, implying that conduction at low frequencies was driven by hopping mechanism.
Fig. 8a. σac as a function of frequency measured at different temperatures. 7
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Fig. 9. Low frequency conductivity as a function of measuring temperatures fitted for β = 4.
4. Conclusions
Acknowledgments
The low frequency dependence dielectric behavior of SiC(O)/ HfCxN1-x nanocomposite at room temperature has been unveiled based on the spectroscopic and nanostructural features presented in this study. The colossal high permittivity values of the nanocomposite is ascer tained to the existence of space charge polarization existing between SiC polycrystals, SiC and HfCxN1-x phases, presence of free carbon and the percolation of HfCxN1-x nanocrystals. The occurrence of highly conductive free carbon and the core-shell structure exhibited by HfCxN1x – C phase could be attributed to the high values of imaginary dielectric permittivity and losses. The AC conductivity increased with increase in frequency with T− 1/4 dependence revealing the hopping conductive mechanism.
The funding received from the Indo-Russian Joint Project (DSTRFBR) is gratefully acknowledged. The work was funded by the grant received from the Department of Science and Technology, Ministry of Science and Technology, Government of India (project no. DST/INT/ RFBR/IDIR/P-12/2016). The authors would also like to thank the DST Unit of Nanoscience, Department of Chemistry and Central Electron Microscopy Facility of IITMadras for availing access to HR-TEM and SEM respectively. References [1] Y. Wang, Y. Fan, L. Zhang, W. Zhang, L. An, Polymer-derived SiAlCN ceramics resist oxidation at 1400 ◦ C, Scripta Mater. 55 (4) (2006) 295–297, https://doi.org/ 10.1016/j.scriptamat.2006.05.004. [2] R. Riedel, A. Kienzle, W. Dressler, L. Ruwisch, J. Bill, F. Aldinger, A silicoboron carbonitride ceramic stable to 2,000◦ C, Nature 382 (6594) (1996) 796–798, https://doi.org/10.1038/382796a0. [3] Y. Wang, W. Fei, Y. Fan, L. Zhang, W. Zhang, L. An, Silicoaluminum carbonitride ceramic resist to oxidation/corrosion in water vapor, J. Mater. Res. 21 (7) (2006) 1625–1628, https://doi.org/10.1557/jmr.2006.0210. [4] R. Riedel, L.M. Ruswisch, L. An, R. Raj, Amorphous silicoboron carbonitride ceramic with very high viscosity at temperatures above 1500◦ C, J. Am. Ceram. Soc. 81 (12) (1998) 3341–3344, https://doi.org/10.1111/j.1151-2916.1998.tb02780.x. [5] S. Sarkar, Z. Gan, L. An, L. Zhai, Structural evolution of polymer-derived amorphous SiBCN ceramics at high temperature, J. Phys. Chem. C 115 (50) (2011) 24993–25000, https://doi.org/10.1021/jp203287h. [6] Y. Wang, K. Wang, L. Zhang, L. An, Structure and optical property of polymerderived amorphous silicon oxycarbides obtained at different temperatures, J. Am. Ceram. Soc. 94 (10) (2011) 3359–3363, https://doi.org/10.1111/j.15512916.2011.04491.x. [7] A. Saha, S.R. Shah, R. Raj, S.E. Russek, Polymer-derived SiCN composites with magnetic properties, J. Mater. Res. 18 (11) (2003) 2549–2551, https://doi.org/ 10.1557/JMR.2003.0356. [8] Z.M. Dang, J.K. Yuan, S.H. Yao, R.J. Liao, Flexible nanodielectric materials with high permittivity for power energy storage, Adv. Mater. 25 (44) (2013) 6334–6365, https://doi.org/10.1002/adma.201301752. [9] A. Zeb, S.J. Milne, High temperature dielectric ceramics: a review of temperaturestable high-permittivity perovskites, J. Mater. Sci. Mater. Electron. 26 (12) (2015) 9243–9255, https://doi.org/10.1007/s10854-015-3707-7. [10] B. Ma, Y. Wang, K. Wang, X. Li, J. Liu, L. An, Frequency-dependent conductive behavior of polymer-derived amorphous silicon carbonitride, Acta Mater. 89 (2015) 215–224, https://doi.org/10.1016/j.actamat.2015.02.020.
CRediT authorship contribution statement Eranezhuth Wasan Awin: Conceptualization, Methodology, Vali dation, Investigation, Formal analysis, Visualization, Writing - original draft. Soumya Sridar: Conceptualization, Methodology, Investigation, Formal analysis. Adhimoolam Bakthavachalam Kousaalya: Concep tualization, Investigation. S.S. Lokesh Vendra: Formal analysis, Investigation. Ekaterina Koroleva: Supervision, Writing - review & editing. Alexey Filimonov: Supervision, Writing - review & editing. Sergey Vakhrushev: Supervision, Writing - review & editing. Ravi Kumar: Conceptualization, Supervision, Formal analysis, Funding acquisition, Writing - review & editing. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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