Dielectric properties of zeolite based metal oxide nanocomposites

Dielectric properties of zeolite based metal oxide nanocomposites

Nano-Structures & Nano-Objects 17 (2019) 248–258 Contents lists available at ScienceDirect Nano-Structures & Nano-Objects journal homepage: www.else...

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Nano-Structures & Nano-Objects 17 (2019) 248–258

Contents lists available at ScienceDirect

Nano-Structures & Nano-Objects journal homepage: www.elsevier.com/locate/nanoso

Dielectric properties of zeolite based metal oxide nanocomposites Madhuri Lakhane b , Kashinath Bogle a , Rajendra Khairnar a , Shailendra Dahiwale c , ∗ Ramphal Sharma d , Vinod Mokale d , Megha Mahabole a , a

School of Physical Sciences, S.R.T.M. University, Nanded, India St. John College of Engineering and Management, Palghar, India Department of Physics, S.P. Pune University, Pune, India d Department of Nanotechnology, Dr. B.A.M. University, Aurangabad, India b c

highlights

graphical abstract

• Novel metal oxide nanoparticles reinforced composites have been developed. • Incorporation of nanoparticles leads to augmentation of the dielectric properties which in turn are found to be the function of metal oxide loading. • ZnO and TiO2 nanoparticles blended ZSM-5 composites exhibit high dielectric permittivities. • CuO/ZSM-5 composites possess giant dielectric permittivity values and low dielectric dissipation factors. Hence, it can be used as high-k material for energy storage applications.

article

info

Article history: Received 28 September 2018 Received in revised form 15 December 2018 Accepted 15 January 2019

a b s t r a c t Metal oxide/ZSM-5 composites are developed and dielectric properties are studied. The dielectric properties of composites depend on type and concentration of metal oxide nanoparticles. All composites show higher dielectric permittivities than pure ZSM-5 zeolite. The composites show a trend of gradual decrease in dielectric constant with increase in metal oxide content. The highest dielectric constant values are observed for 25% ZnO, TiO2 and CuO metal oxide fillers. The dissipation factors are also found to exhibit low values. The ac conductivity remains constant in low frequency region and increases linearly at high frequencies. CuO blended composites depict extremely high dielectric values. © 2019 Published by Elsevier B.V.

1. Introduction Composite material is a combination of two or more materials with noticeably different, superior and unique physico-chemical properties. The materials, with different structure and composition are combined in such a way that they act coherently though remaining separate and distinct at some level. The individual constituent materials retain their identities and properties because they do not fully merge into one another and actual bonds may ∗ Corresponding author. E-mail address: [email protected] (M. Mahabole). https://doi.org/10.1016/j.nanoso.2019.01.008 2352-507X/© 2019 Published by Elsevier B.V.

or may not present. Generally, there are two main types of components; matrix and reinforcing material. The matrix material supports and binds together the reinforcing material and the reinforcing component imparts its special properties to augment the matrix properties. Appropriate combination of matrix and reinforcement material leads to the development of a new material that may exactly meet the requirements of a particular application. The various extraordinary properties like non-corrosive and nonconductive nature, flexibility, light weight, low maintenance, long life, design flexibility, high hardness, high melting point, low density, high thermal conductivity, chemical stability and enhanced performance, exhibited by the composites depending on type and

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nature of matrix, reinforcing material and application areas, engender their widespread industrial applications [1,2]. Hence, composites are preferred in various applications like solar cells, building and bridge construction, sporting equipment, wind energy, sensors, optoelectronic device elements, construction of laser diodes & LEDs, catalysis, car industry, spacecraft, aircraft, naval industry, electronic packaging and energy storage [3–8]. Moreover, it is reported in the literature that the dielectric properties of composites can be tailored using variety of materials having easily discernible dielectric permittivities and can be changed by using fillers of variable shape, size and conductivity [9–16]. Zeolites are microporous crystalline aluminosilicates consisting of tetrahedral framework, mobile cations, and adsorbed molecules. The utility of zeolites in diverse fields is due to availability of the number of distinct zeolite framework structures and the fact that different materials with the same framework structure can exhibit widely different chemical properties. The framework contains channels or interconnected voids with well-defined pore diameter. The fascinating and unusual properties of these materials are large surface area, uniform structural framework with cavities and cages, high adsorption capacity, excellent ion exchange capability, good thermal stability and unique molecular sieve nature [17–19]. It is well known that for a specific application, functionalities as well as the properties of zeolites can be improved not only by ion exchange process, doping or incorporating certain transition metal ions in frame structure but also by formation of composites [20– 31]. In our earlier work, the dielectric properties of nano ZSM-5 zeolite have been reported. Moreover, there are very few reports on the effect of various metal oxides nanoparticles on the dielectric constant of zeolite matrix. Hence, the present work deals with the modification of primary phase of ZSM-5 zeolite in order to improve its dielectric properties. Semiconducting metal oxides, at the nanometre scale, is a class of promising materials with distinctive properties and structures. The use of metal oxide nanoparticles is suitable in many applications such as biomedical, catalysis, pharmaceutical products, surface coatings, semiconductors, gas sensor the medical and especially in dielectric field. Zinc oxide (ZnO) is n-type semiconductor which belongs to a wurtzite structure family. It possesses a dielectric behaviour at low temperature when its energy barrier is large, and its diffusion coefficient is low (ε ≈ 7–11). Besides this, ZnO has built in polarization [32–36]. Titanium oxide (TiO2 ) is also a wide energy gap (3 eV) n-type semiconductor and exhibits stable dielectric properties characterized by a high relative dielectric constant (40–100) and low dielectric loss [37–40]. Copper oxide (CuO) is one of the important p-type semiconducting materials characterized by a narrow band gap of 1.2 eV and dielectric constant near to 18 [41–43]. Being nanosized, it is expected to have comprehensive interaction region for these nanocomposites which may affect their electrical behaviour. Since, these metal oxides exhibit high dielectric constants; they are employed as the secondary phases with the main phase to have nano-particle reinforced composites for enhanced dielectric behaviour.Such composites find wide applicability as high dielectric materials in high charge storage capacitors [41–43]. Therefore, main aim of the present work is to focus on the search of composites, with specific combinations, with optimal dielectric properties. Hence, the study deals with preparation of metal oxide reinforced zeolite composites and their characterization of by XRD, FTIR, SEM and BET techniques. Room temperature dielectric responses of Metal oxide/Zeolite binary system have also been reported.

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2. Experimental 2.1. Preparation of nano ZSM-5 Microwave hydrothermal technique is used for synthesis of nano ZSM-5 without organic templates as per Cheng et al. [44]. In brief, sodium aluminate and silica sol are used as silica and alumina sources respectively. Initially, sodium source is prepared by dissolving sodium hydroxide and sodium aluminate in appropriate amount of double distilled water. Silica sol is added, dropwise, to sodium source under constant stirring situation. The mixture is continuously stirred further for about 5 h in order to have homogeneous gel. The obtained homogeneous gel is exposed to microwave radiations for 5 min prior to hydrothermal treatment and treated hydrothermally at 453 K for 18 h under autogenously pressure. Synthesized nano ZSM-5 zeolite sample is then finally sintered at 500 ◦ C for 2 h and employed as the main matrix for preparation of composites [20]. 2.2. Preparation of composites ZSM-5 zeolite, synthesized in our laboratory, is employed as a main matrix and commercially available Merck grade metal oxide nanoparticles namely ZnO, TiO2 & CuO are used without further purification as fillers for the preparation of particle reinforced nano-composites. In this study, series of composites having 25, 50 and 75 wt % of each metal oxide have been fabricated under similar conditions. In a typical procedure, ZSM-5 zeolite and metal oxide nanoparticles, weighed for a specific combination, are thoroughly dry mixed for 10 h to obtain homogeneous mixture. 2.3. Characterization The composites, thus prepared, are then characterized by XRD technique for crystal structure determination. X-ray diffraction patterns of ZSM-5, metal oxides and composites are obtained with the help of Rigaku X-ray diffractometer using CuKα radiation (λ = 0.154056 nm) in a scanning range of 5◦ –60◦ (2θ ). The presence of various functional groups in zeolite, metal oxides and composite samples are identified by FTIR spectrophotometer (Shimadzu make) with a scan range of 4000–400 cm−1 with a resolution of 4 cm−1 . Microstructures of these samples are visualized using JEOL-JSM-5 spectrometer. BET analysis is also carried out by using Quantachrome BET analyser. 2.4. Measurements of dielectric properties All composite samples are compacted into the pellets of 13 mm diameter having 2 mm thickness using polyvinyl alcohol (PVA) as a binder material. The pellets are sintered at 500 ◦ C in PID controlled air furnace for 2 h. The flat surfaces of the pellet are then coated with silver conducting paint to ensure good electrical contacts. These pellets are used as samples for dielectric measurements. The capacitances (Cp), real and imaginary parts of impedance are measured with a parallel-plate capacitor arrangement using QuadTech 7600 LCR meter. The measurements are carried out at room temperature, in the frequency range 10 Hz–2 MHz. Dielectric constants (ε ′ ) are calculated by using the formula: ε ′ = [Cp × t]/[ε0 ×A], where Cp is the capacitance of the sample, t is the thickness of the sample, ε0 is the permittivity of the vacuum, and A is the area of cross-section of the sample pellet. AC conductivity is calculated using real and imaginary parts of complex impedance with the aid of following formula:

σ =(

Z′ Z ′2

− Z ′′2



t A

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confirming the wurtzite type hexagonal structure of ZnO [32–36]. Fig. 1B(b–d) reveals the presence of typical anatase TiO2 peaks near 2θ values of 25.28◦ , 37.9◦ , 48.2◦ , 53.8◦ and 55.2◦ along with the major peaks of host ZSM-5 matrix. Fig. 1C(b–d) corresponds to composites, wherein three peaks belonging to CuO show their appearance along with ZSM-5 peaks [37–40]. Phase analysis, carried out by indexing of data for metal oxides and zeolites, shows that their crystal structures are retained in composite form as well. Existence of XRD peaks corresponding to particular metal oxide and zeolite in each composite reveal that every composite is just a physical mixture of its constituents and exhibits a two-phase structure. These profiles also confirm dispersion of metal oxide in zeolite matrix. No new phase structure has been detected assuring that there is no chemical interaction between the constituents, main matrix (zeolite) and reinforcing material (metal oxide). A typical trend of a systematic decrease in relative intensity for all ZSM-5 peaks and increase in intensities of most of the metal oxide peaks with increase in its concentration is exhibited by all composites. Further, sharp diffraction peaks apparent in all XRD profiles show good crystallinity of grown nanoZSM-5 zeolite, metal oxides as well as composites samples. 3.2. FTIR analysis

Fig. 1. (A–C) The XRD patterns of metal oxide/ZSM-5 composites showing existence of ZSM-5 and metal oxides peaks; [a] ZSM-5, [b] 25% metal oxide/ZSM-5, [c] 50% metal oxide/ZSM-5, [d] 75% metal oxide/ZSM-5, and [e] bare metal oxide.

where Z′ and Z′′ are the real and imaginary parts of the impedance, A is the area of sample and t is the thickness of the sample. The Cole–Cole lots are obtained by plotting real part of impedance on x-axis and imaginary part of impedance on y-axis. 3. Results and discussion 3.1. XRD analysis Fig. 1[A–C] presents the XRD profiles corresponding to ZnO/ ZSM-5, TiO2/ZSM-5 and CuO/ZSM-5 composites. The XRD plots for nano ZSM-5 and pure metal oxide are included for reference. The blending of ZnO with ZSM-5 host matrix can be visualized from Fig. 1A(b–d) by the presence characteristic peaks of ZnO corresponding to (100), (002), (101), (102) and (110) hkl planes

The FTIR spectra for ZnO/ZSM-5, TiO2/ZSM-5 & CuO/ZSM-5 are displayed in Fig. 2(A,B & C) respectively and are compared with that of virgin ZSM-5 and respective metal oxides. Fig. 2A(a–e) shows FTIR spectra of ZnO/ZSM-5 composites as a function of ZnO concentration. The spectrum of ZSM-5 is shown in Fig. 2A(a). The absorption band appearing at 482 cm−1 is due to bending vibrations of Si-O-Si bonds and presence of this band confirms the characteristic of five membered ring pentasil structure of ZSM-5. A broad band near 701 cm−1 can be assigned to external symmetric stretching vibrations of Si-O group. The absorption bands present at 898 cm−1 and 1111 cm−1 are attributed to external symmetric and internal asymmetric stretching vibrations of Si-O-Si bonds [20]. The structure sensitive absorption band (a small shoulder peak), attributed to asymmetric vibration of T-O bond corresponding to external linkages between TO4 tetrahedra, is observed to be present at 1220 cm−1 . This band is found to be disappearing with increasing ZnO concentration (75%). Presence of a weak and broad band near 1590 cm−1 corresponds to bending vibrations of adsorbed water molecules. The peak, which is assigned to vibrations of hydroxyl group, is also present at 3683 cm−1 . FTIR spectrum of ZnO (Fig. 2A(e)) includes peculiar ZnO bands at 470, 670 and 1111 cm−1 . The IR spectra of ZnO/ZSM-5 composites are shown in Fig. 2A(b–d). In case of composites, the ZSM-5 band assigned to hydroxyl group disappears. The existence of characteristics ZSM-5 bands corresponding to bending mode of water molecules (at 1590 cm−1 ) and T-O bond vibrations (shoulder peak at 1220 cm−1 ) up to 50% ZnO concentration can be visualized from Fig. 2A(b–c). These bands vanish for still higher ZnO amount. It is observed that the intensities of absorption bands at 1111, 898, 482 cm−1 are observed to be decreasing with increase in ZnO concentration. The characteristic feature of these spectra, compared with the spectra of ZSM-5 and ZnO, is the splitting of a broad band at 701 cm−1 into doublet peaks. A new band is appeared at 670 cm−1 which can be attributed to stretching vibrations of Zn-O bond and band at 701 cm−1 shifts at higher energy side [32–36]. Moreover, intensities of these bands also decrease with increase in ZnO concentration for a composite, while broadening the bands. The band at 482 cm−1 shifts towards lower wave number (470 cm−1 ) with ZnO amount for all the composites. Fig. 2B(a–e) displays the FTIR spectra for ZSM-5, TiO2 /ZSM-5 composites and TiO2 . The spectra of ZSM-5 and TiO2 reveal the presence of typical absorption bands assigned to them [20,37– 40]. The spectra for composites show similar trend of decrease in

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Fig. 3. (A–C) SEM micrographs of composites revealing the microstructure; [A] 25% ZnO/ZSM-5, [B] 25% TiO2/ZSM-5 and [C] 25% CuO/ZSM-5. Fig. 2. (A–C) FTIR spectra for metal oxide/ZSM-5 composites revealing the occurrence of characteristic peaks due to metal oxide and ZSM-5; [a] ZSM-5, [b] 25% metal oxide/ZSM-5, [c] 50% metal oxide/ZSM-5, [d] 75% metal oxide/ZSM-5, and [e] bare metal oxide.

intensities with increase in TiO2 concentrations for ZSM-5 bands at 3683, 1590 and 898 cm−1 . A shoulder peak with weak intensity (1220 cm−1 ) disappears for higher TiO2 concentration. However, intensities of bands existing at 1111 cm−1 and 482 cm−1 appear to remain unchanged and this may be due to the overlapping of TiO2 and ZSM-5 bands. The inclusion of TiO2 can be confirmed by the appearance of a new shoulder peak at 800 cm−1 with a diminishing intensity. A broad peak present at 701 cm−1 for pure ZSM-5 does not appear in the FTIR spectra of composites. The comparative FTIR plots for ZSM-5, CuO/ZSM-5 composites, and CuO are shown in Fig. 2C(a–e). The peaks for ZSM-5 structure are present in Fig. 2C(a). Similarly, the absorption band spectrum for CuO is displayed in Fig. 2C(e). The plot shows the characteristic bands at 945, 605, 532, and 484 cm−1 which are attributed to CuO vibrations along with the band for atmospheric CO2 vibrations [40– 43]. Fig. 2C(b–d) gives information about CuO/ZSM-5 composites

as a function of CuO content. It is observed that absorption bands assigned to stretching and bending motion of H2 O, existing at 3683 cm−1 and 1590 cm−1 for ZSM-5 do not appear at all in composites. The peculiar ZSM-5 peak present at 898 cm−1 also vanishes for composites. In case of composites, twin bands are observed near 1111 cm−1 & 945 cm−1 , which may be due to merging of CuO and ZSM-5 peaks. 3.3. SEM analysis The surface morphology of the composites is investigated by means of Scanning Electron Microscopy (SEM). The microstructure of composites with 25% loading is displayed in Fig. 3(A–C). Microstructure of ZnO/ZSM-5 & TiO2/ZSM-5 reveals the samples are composed of grains of variable sizes (< 0.2 µm) with the pores embedded somewhere in the structure. The surface is found to be fully covered by irregular shaped particles /cubic structures/ platelet with some rod like structures. However, the SEM micrograph of CuO/ZSM-5 depicts uniform structure.

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Fig. 4. (A–D) BET adsorption isotherms for; [A] ZnO/ZSM-5 composite, [B] TiO2/ZSM-5 composite, [C] CuO/ZSM-5 composite and [D] ZSM-5.

3.4. BET analysis The BET adsorption isotherms for composites with 25% metal oxide loading are displayed in Fig. 4(A–C). The porosity of the all the samples is investigated using N2 physisorption. The BET surface area (SBET ), as well as total pore volume and average pore radius are reported in Table 1. The BET surface area increases upon metal oxide loading. It is found to be highest for CuO/ZSM-5 composite. The total pore volume and average pore radius are also large for CuO blended ZSM-5 composite. 3.5. Dielectric behaviour analysis 3.5.1. Dielectric permittivity 3.5.1.1. Frequency dependence. The change in effective permittivity as a function of frequency at room temperature for ZSM-5 composites with ZnO, TiO2 & CuO nanoparticles and at variable filler concentrations are shown in Fig. 5(A–C). It is evidenced from these figures that all the composites show typical nature of decrease in dielectric constant with increase in frequency of applied field.

The dependence of dielectric permittivity on frequency can be divided into two regions: a low frequency region (10 Hz to 1 kHz) wherein ε ′ decreases prominently with increasing frequency up to 1 kHz indicating a strong dependence of this parameter on the frequency at the low frequency region. A high frequency region above 1 kHz where the dielectric permittivity remains constant. It may be termed as plateau region wherein composites show frequency independent behaviour. It is known that permittivity is a frequency dependent parameter for ZSM-5 zeolite [20]. Zeolites exhibit very peculiar structures. Their fully cross-linked framework structures enclose regular internal cavities and channels of discrete sizes and shapes depending on their chemical composition and crystal structure. These channels and voids are normally occupied by water molecules, and the cations; mainly alkali or alkaline earth metal ions, so as to balance the negative charge of the framework. These extra framework cations, present in the channels, are mobile. Therefore, the total polarization is ionic in nature. It is well known that the ions, being heavier, give less rapid response to applied alternating field. The observed decrease in dielectric constant is due to the fact that

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Table 1 Various BET parameters. Sample

Surface area as per MBET (m2 /g)

Total pore volume cc/g

Average pore radius (Å)

ZSM-5 25%ZnO/ZSM-5 composite 25% TiO2/ZSM-5 composite 25% CuO/ZSM-5 composite

0.778 1.543 7.883 31.083

9.409 × 10−04 cc/g for pores smaller than 208.0 Å (radius) at P/Po = 0.95154 1.507× 10−3 for pores with radius less than 205.88 Å at P/Po.= 0.951032 1.275× 10−2 for pores smaller than 194.15 Å (radius) at P/Po = 0.947952 5.055 × 10−2 for pores with radius less than 194.04 Å at P/Po = 0.947797

24.18 19.53 32.36 32.52

is responsible for high dielectric constant, space charge polarization arising due to charge accumulation at the grain boundary is the main contributor. In our case, ZnO, TiO2 and CuO, being ceramic, exhibit ionic polarization and the intrinsic permittivity in ZnO, TiO2 & CuO nanoparticles also decreases with increasing frequencies of the applied field [32–43]. The dielectric constant at lower frequency can be attributed to space charge contribution. As the frequency increases, electronic and ionic contribution became dominant with gradual decrease in the space charge contribution. Hence, dielectric constant for all metal oxides decreases with the increase in frequency and becomes relatively constant at higher frequency. Therefore, it may be a combined decreasing effect of the permittivity with frequency for both (ZSM-5) and the filler nanoparticles (metal oxides) which gives a decrease in the effective permittivity of composites. Hence, the dielectric constant for all composites decreases with increase in frequency.

Fig. 5. (A–C) Dielectric constant as a function of frequency of applied ac field; [A] ZnO/ZSM-5 composites, [B] TiO2 /ZSM-5 composites, [C] CuO/ZSM-5 composites.

with the increase in frequency, the ionic displacement begins to lag the field reversals and contributing less to the dielectric constant. As a result, charges accumulate in the space charge region at the interface/ grain boundary due to the net polarization effect. At still higher frequencies the field reversals are very fast, and the ions displacements cannot cope up with the field alterations. Therefore, no excess ions can accumulate in the electric field direction and ionic polarization decreases. Hence, the dielectric constant decreases with increase in frequency of applied ac field [19]. Generally, the electric response in ceramic materials is very complex and composed of different types of polarization mechanisms like electronic, ionic, orientation and space-charge polarization. Though at low frequencies, each polarization phenomenon

3.5.1.2. Effect of filler concentration and filler type. Remarkable differences are found in dielectric constant (ε ′ ) of main zeolite matrix and composite materials. Although almost all composites reveal increased dielectric constant in comparison with the bare ZSM5, the addition of ZnO, TiO2 and CuO with different concentrations exhibit different dielectric behaviours for the corresponding composites as depicted in inset of Fig. 5(A–C). As anticipated, the incorporation of metal oxides in zeolite structure modifies the dielectric constant. It is observed that ZSM-5 zeolite in pristine form possesses a dielectric constant of the order of 130 at 10 Hz. Dielectric constants for ZnO/ZSM-5 composites are higher than that for pristine ZSM5 zeolite. A ZnO/ZSM-5 composite having 25% ZnO contribution exhibits maximum value of dielectric constant (800) at 10 Hz compared to composites with higher ZnO concentrations as shown in Fig. 5(A). A systematic decrease in dielectric constant with increase in ZnO amount is also observed for these composites. The dielectric plots for other two composites namely TiO2 /ZSM5 and CuO/ZSM-5, which are also analogous to that for ZnO/ZSM5, are presented in Fig. 5(B) and (C) respectively. These composites also show similar trend of gradual decrease in dielectric constant with increase in metal oxide contents. In case of TiO2 /ZSM-5 composites, the value of the highest dielectric constant is found to be 825 at 10 Hz for 25% TiO2 concentration. However, for the same 25% CuO concentration in a CuO/ZSM-5 composite enormous hike in dielectric constant is observed as depicted in Fig. 5(C). Thus, it can be concluded that dielectric constant increases considerably with the impregnation of metal oxides in the zeolite matrix. The dielectric properties of materials are mainly decided by their polarizability at a given frequency. In this case, the nanocomposites can be treated as a two phase dispersion system consisting of zeolite as continuous matrix and nanoparticles of metal oxides as fillers. It is well known that uniqueness of zeolite structure lies in the fact that the aluminosilicate framework of silicon, oxygen and aluminium is formed by sharing the corner oxygen atoms by SiO4 and AlO4 tetrahedra, primary building units. Framework contains cages, channels, voids/empty space leading to porosity. As per XRD and FTIR results of zeolite composites, crystal structures of ZSM-5 zeolite matrix and reinforcing metal oxides are retained in composite form. Hence, it is expected that semiconducting metal oxides are embedded in empty spaces. It may be working as if

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many semiconducting nano-clusters isolated by nano-zeolite dielectric material. For such a two components system, free charge carriers migrate through the material, space charges build up at the interfaces of the constituents acting as many capacitors. This interfacial polarization process, also known as Maxwell–Wagner– Sillar effect, between two nano-clusters may be responsible for enhancement in the charge storage capacity and dielectric constant. In other words, the charged surface layers that arise at the interface between dissimilar media when free charges move within the phases of the composite under the action of an applied electric field is main contributor to significant augmentation of dielectric constant [35,39,42]. This may also be due to the higher dielectric constants of reinforcing metal oxides and primary zeolite matrix. Generally, the dielectric relaxation processes in a composite can be attributed to the relaxation processes existing in each component. As a result of mixing of two different dielectrics, it is possible to have a new kind of dielectric relaxation process such as the Maxwell–Wagner effect. Furthermore, new dielectric relaxation processes in composites are greatly affected by distribution of the reinforcing particles. It is well known that uniform distribution of filler particles within a matrix leads to the strongest contribution to the dielectric properties of the composites. The values of the dielectric constants of CuO nanoparticles incorporated ZSM-5 composites are much higher than that for ZnO & TiO2 filled ZSM-5. Higher values of dielectric permittivities for CuO reinforced ZSM-5 composites are due to an internal barrier layer capacitance effect (IBLC) encountered as a result of hopping of charge carriers between Cu2+ and Cu3+ ions present in copper oxide and the interfacial polarization (Maxwell–Wagner polarization) at the grain boundary [38,39]. Thus, IBLC and the interfacial polarization of the CuO nanoparticles play a major role in the dielectric properties. In our case, SEM micrograph of 25%CuO/ZSM-5 composite, Fig. 3(C), reveals uniform microstructure whereas the microstructure of others composites show mixture of particles with variable shape & sizes. Thus, the microstructure uniformity of a composite may also be responsible for the highest enhancement in dielectric constant of the composite. 3.5.2. Dissipation factor The behaviour of dissipation factor as a function of frequency and metal oxide loading is presented in Fig. 6(A–C). The dissipation factor shows maxima at about 1 kHz for pure ZSM-5. This relaxation phenomenon can be connected with dissipation of energy effect of electrode. Dissipation factor for ZnO/ZSM-5 composite with 25% and 75% ZnO loading is found to be higher compared with pure ZSM-5 at lower frequencies and goes on decreasing with increase in frequency as depicted in Fig. 6(A). This trend in dissipation factor, at lower frequencies, may be due to space charge polarization. In case of TiO2 blended ZSM-5 composites, presented in Fig. 6(B), dissipation factors are observed to be lower than that for pristine ZSM-5. Each composite shows same trend of decrease of dissipation factor with increase in frequency. The effect of CuO on dissipation factor of composites can be visualized from Fig. 6(C). The value of dissipation factor for 25% CuO loading possesses a value slightly higher that bare ZSM-5. Two humps at very low frequency (≈ 30 Hz) and at frequency higher than 1 kHz. It decreases thereafter. These relaxation phenomena can be correlated with energy dissipation on the space charge polarization & grain boundaries effect. For other concentration (50%), the loss maxima appear in slightly higher frequency range (≈ 50 Hz). Composite with still higher concentration, do not exhibit any like hump nature rather goes on decreasing with frequency.

Fig. 6. (A–C) Variation in Dissipation factor with frequency of an applied ac field for metal oxide blended composites; [A] ZnO/ZSM-5, [B] TiO2/ZSM-5, [C] CuO/ZSM-5.

3.5.3. Cole–Cole plots Plots of the real and imaginary impedance components for ZSM-5 and metal oxide/ZSM-5 composites samples are depicted in Fig. 7(a–j). Cole–Cole profiles for composites (Fig. 4(b–d)) show that blending of ZnO with ZSM-5 results in change in semicircular behaviour for ZSM-5 to straight line for composites with 25% and 50% ZnO concentrations. A linear region at low frequency with large slopes demonstrates the diffusion of filler in main matrix. For the highest ZnO concentration, again an intermediate frequency semicircular arc is obtained. This perfect semicircle indicates only one mechanism of polarization. The position of maxima occurs near 10 kHz indicating low relaxation time compared to that of ZSM-5. Typical impedance diagram for TiO2 /ZSM-5 composites reveals high frequency arc with low frequency branch is obtained for low TiO2 concentration. Further increase in TiO2 concentration results in change in semi-circular nature to straight line nature. A

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Fig. 7. (a–j) Complex impedance plots of ZSM-5 and metal oxide/ZSM-5 composites of different compositions; (a) ZSM-5, (b–d) ZnO/ZSM-5, (e–g) TiO2 /ZSM-5, (h–j) CuO/ZSM-5.

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Fig. 8. (A–C) AC conductivity for pure and metal oxide/ZSM-5; [A] ZnO/ZSM-5 composites, [B] TiO2/ZSM-5 composites, [C] CuO/ZSM-5 composites.

CuO/ZSM-5 composite with 25% CuO content reveals the presence of high frequency arc with a tail on low frequency side. For 50% CuO contribution semi-circular nature changes and frequency corresponding to maximum shifts to low frequency value (61 kHz) indicating more conductive nature and for still higher CuO concentration, single semicircle is obtained, and low frequency branch found to be ceasing. Maxwell–Wagner interfacial polarization occurs in the heterogeneous systems where permittivity of the constituent phases is different. When the relation between ε ′ and ε ′′ is a symmetrical circle arc, it suggests a single relaxation process. It is caused by the Maxwell–Wagner effect, being attributed to the interfacial polarization mechanism. 3.5.4. AC conductivity The variation of ac conductivity with frequency is depicted in Fig. 8(A–C) for metal oxide/ZSM-5 composites respectively. Each member in each composite series exhibits the same trend. The composites show frequency independent behaviour in low frequency region. This is followed by upward bending of plateau region at about 105 Hz in almost all samples except for 75% CuO/ZSM5 composite wherein bending starts at about 104 Hz. However,

in high frequency region, called as hopping region, the ac conductivity deviates from the plateau region and the value of ac conductivity increases linearly with frequency in accordance with the universal power law. The increase in conductivity may be due to the hopping of charge carrier (ions). Such hopping phenomenon is more common in zeolite materials. Conclusions The dielectric properties of ZSM-5 can be improved by impregnating ZnO, TiO2 and CuO metal oxides without altering the other intrinsic properties. The dielectric properties of composites are found to be the function of type and concentration of metal oxide. All the composites show enormous increase in dielectric permittivity upon 25% metal oxide loading. The dielectric constant for CuO/ZSM-5 composites exhibit extraordinary dielectric constants. The ac conductivity of composites remains constant in low frequency region and increases linearly with frequency at high frequencies. The nanocomposites blended with CuO exhibit giant dielectric constants and low dielectric losses.

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Novel high dielectric constant materials have been developed by using metal oxide like ZnO, TiO2 , and CuO as reinforcing materials with ZSM-5 zeolite matrix. CuO blended ZSM-5 composites may be used as high-k dielectric materials. Acknowledgements Authors would like to thank Hon. Vice Chancellor, SRTM University, Nanded for his constant encouragement and support. This work is carried out under the major research project ‘SR/S2/CMP49/2009dt23/09/2010’ sanctioned by Department of Science and Technology, New Delhi. Conflict of 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. References [1] M.K.S. Sai, Review of composite materials and applications, Int. J. Latest Trends Eng. Technol. (IJLTET) 6 (3) (2016) 129–135, 10.21172. [2] M.N. Gururaja, A.N. Hari Rao, A review on recent applications and future prospectus of hybrid composites, Int. J. Soft Comput. Eng. (IJSCE) 1 (6) (2012) 352–355. [3] GV. Mahajan, Prof. V.S. Aher, Composite material: a review over current development and automotive application, Int. J. Sci. Res. Publ. 2 (1) (2012) 1–5. [4] G.M.N. Islam, B.S. Hassan, A.N.M.A. Haque, M.S. Mukhtar, S. Shaherbano, Embryonic phases of hard composites: a review, Adv. Res. Text Eng. 3 (2) (2018) 1026–1034. [5] Vishwesh Dikshit, Somen K. Bhudolia, Sunil C. Joshi, Multiscale polymer composites: a review of the interlaminar fracture toughness improvement, Fibers 5 (4) (2017) 38, http://dx.doi.org/10.3390/fib5040038. [6] Q. Nguyen, T. Ngo, P. Mendis, P. Tran, Composite materials for next generation building façade Systems, Civ. Eng. Archit. 1 (3) (2013) 88–95, http://dx.doi. org/10.13189/cea.2013.010305. [7] Chuanhao Li, Feng Wang, Jimmy C. Yu, Semiconductor/biomolecular composites for solar energy applications, Energy Environ. Sci. 4 (2011) 100–113, http://dx.doi.org/10.1039/C0EE00162G. [8] Jarkko Tolvanen, Jari Hannu, Jari Juuti, Heli Jantunen, Piezoelectric flexible LCP–PZT composites for sensor applications at elevated temperatures, Electron. Mater. Lett. 14 (2) (2018) 113–123, http://dx.doi.org/10.1007/s13391018-0027-0. [9] Qiwei Zhang, Jiwei Zhai, Lingbing Kong, Xi Yao, Percolative properties in ferroelectric-dielectric composite ceramics, Appl. Phys. Lett. 97 (2010) 182903, http://dx.doi.org/10.1063/1.3514246. [10] Lin Zhang, Z.-Y. Cheng, Development of polymer-based 0-3 composites with high dielectric constant, J. Adv. Dielectr. 1 (4) (2011) 389–406, http://dx.doi. org/10.1142/S2010135X11000574. [11] Wang Da-Wei, Jin Hai-Bo, Yuan Jie, Wen Bao-Li, Zhao Quan-Liang, Zhang De Qing, Cao Mao-Sheng, Mechanical reinforcement and piezoelectric properties of PZT ceramics embedded with nano-crystalline, Chin. Phys. Lett. 27 (4) (2010) 47701, http://dx.doi.org/10.1088/0256-307X/27/4/047701. [12] Zhang De-Qing, Wang Da-Wei, Yuan Jie, Zhao Quan-Liang, Wang Zhi-Ying, Cao Mao-Sheng, Structural and electrical properties of PZT/PVDF piezoelectric nanocomposites prepared by cold-press and hot-press routes, Chin. Phys. Lett. 25 (12) (2008) 4410–4413, http://dx.doi.org/10.1088/0256-307X/25/12/ 063. [13] Mao-Sheng Cao, Wei-Li Song, Wei Zhou, Da-Wei Wang, Simeon Agathopoulos, Dynamic compressive response and failure behavior of fiber polymer composites embedded with tetra-needle-like ZnO nanowhiskers, Compos. Struct. 92 (12) (2010) 2984–2991, http://dx.doi.org/10.1016/j.comstruct. 2010.05.010. [14] Da-Wei Wang, Mao-Sheng Cao, Jie Yuan, Ran Lu, De-Qing Zhang, Effect of sintering temperature and time on densification, microstructure and properties of the PZT/ZnO nanowhisker piezoelectric composites, J. Alloys Compd. 509 (24) (2011) 6980–6986, http://dx.doi.org/10.1016/j.jallcom.2011.03.186. [15] Lin Zhang, Xiaobing Shan, Patrick Bass, Yang Tong, Terry D. Rolin, Curtis W. Hill, Jeffrey C. Brewer, Dennis S. Tucker, Z. -Y. Cheng, Process and microstructure to achieve ultra-high dielectric constant in ceramic-polymer composites, Sci. Rep. 6 (2016) 35763, http://dx.doi.org/10.1038/srep35763. [16] M. Vadivel, R. Ramesh Babu, K. Ramamurthi b, M. Arivanandhan, Effect of PVP concentrations on the structural, morphological, dielectric and magnetic properties of CoFe2O4 magnetic nanoparticles, Nano-Struct. Nano-Objects 11 (2017) 112–123.

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