Grafted SiC nanocrystals: For enhanced optical, electrical and mechanical properties of polyvinyl alcohol

Accepted Manuscript Grafted SiC nanocrystals: For enhanced optical, electrical and mechanical properties of polyvinyl alcohol Isha Saini, Annu Sharma, Rajnish Dhiman, Sanjeev Aggarwal, Sita Ram, Pawan K. Sharma PII:

S0925-8388(17)31382-8

DOI:

10.1016/j.jallcom.2017.04.183

Reference:

JALCOM 41587

To appear in:

Journal of Alloys and Compounds

Received Date: 16 April 2016 Revised Date:

13 April 2017

Accepted Date: 18 April 2017

Please cite this article as: I. Saini, A. Sharma, R. Dhiman, S. Aggarwal, S. Ram, P.K. Sharma, Grafted SiC nanocrystals: For enhanced optical, electrical and mechanical properties of polyvinyl alcohol, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.04.183. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Grafted SiC nanocrystals: For enhanced optical, electrical and mechanical properties of polyvinyl alcohol a,*

, Rajnish Dhiman b, Sanjeev Aggarwal a, SitaRam c, Pawan K.

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Isha Saini a, Annu Sharma Sharma c a

Department of Physics, Kurukshetra University, Kurukshetra 136 119, India Department of Physics and Chemistry, University of Southern Denmark, Campusvej 55, DK5230 Odense M, Denmark. c Department of Chemistry, Kurukshetra University, Kurukshetra 136 119, India b

corresponding author: [email protected]

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*

5.6

4.0 3.2 2.4

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4.8

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KHN (Kgf/mm 2)

6.4

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1.6

0

0.015

Concentration (wt%) Concentration (wt%)

1

3.2 3.0 2.8 2.6 2.4 2.2 2.0 1.8 1.6 1.4

Penetration Depth (µ m)

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Tel: 01744-238410 Ext 2130 Fax: 01744-238277

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Grafted SiC nanocrystals: For enhanced optical, electrical and mechanical properties of polyvinyl alcohol

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Isha Saini a, Annu Sharma a*, Rajnish Dhiman b, Sanjeev Aggarwal a, SitaRam c, Pawan K. Sharma c a

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*corresponding author: [email protected] Tel: 01744-238410 Ext 2130 Fax: 01744-238277

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Department of Physics, Kurukshetra University, Kurukshetra 136 119, India Department of Physics and Chemistry, University of Southern Denmark, Campusvej 55, DK5230 Odense M, Denmark. c Department of Chemistry, Kurukshetra University, Kurukshetra 136 119, India b

Abstract: Polyvinyl alcohol (PVA) grafted SiC (PVA-g-SiC)/PVA nanocomposite was synthesized by incorporating PVA grafted silicon carbide (SiC) nanocrystals inside PVA matrix.

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In-depth structural characterization of resulting nanocomposite was carried out using fourier transform infrared spectroscopy (FTIR), raman spectroscopy and X-ray diffraction (XRD) measurements. UV-Visible absorption spectroscopy was used to study optical properties such as

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optical energy gap (Eg), Urbach’s energy (Eu), refractive index (n), real (ε1) and imaginary (ε2)

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parts of dielectric constant of PVA as well as PVA-g-SiC/PVA nanocomposite film. Refractive index of PVA increased from 1.5 to 2.0 for PVA-g-SiC/PVA nanocomposite film containing 0.015 wt% of PVA-g-SiC nanocrystals at 550 nm wavelength. Dispersion of refractive index was analyzed using the Wemple–DiDomenico single oscillator model and dispersion parameters (E0 and Ed) were determined. Microhardness measurements performed at an applied load of 9.8 mN showed an increase in the Knoop microhardness number (KHN) of PVA containing 0.015 wt% PVA-g-SiC nanocrystals. Detailed analysis of current-voltage data indicates that the conduction

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mechanism responsible for increase in conductivity of PVA-g-SiC/PVA nanocomposite film is voltage dependent and Schottky mechanism is the dominant conduction mechanism at medium and high voltage regions.

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Keywords: Nanocomposites; Carbides; Mechanical properties; Hardness; Charge transport. 1. Introduction

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Materials with innovative properties are the need of the hour for diverse applications. Persistent efforts have been made in the last few decades by making use of novel nanoscience and

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nanotechnological interventions to design the so called advanced materials [1-3]. Nanocomposites comprise an important class of such materials which are increasingly attracting the attention of researchers all around the world due to the interesting properties exhibited by them making them materials of choice for their widespread applications in diverse disciplines

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such as photovoltaics [4], optoelectronics [5], energy storage [6], super capacitors [7], flame retardants [8], sensors [9] etc.

A well-known method for the formation of nanocomposite is the addition of miniscule amount of

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nanofiller [10] to a host polymer matrix resulting in eliminating or improving upon some of the

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inherent disadvantages of polymers such as low hardness and low electrical conductivity. The homogenous dispersion of inorganic nanoparticles in organic polymers is a key parameter as well as a challenge in realization of polymer nanocomposites with enhanced mechanical, electrical and optical properties. Integration of silicon carbide (SiC) nanocrystals as reinforcing filler is a promising way to overcome the limitations of conventional polymers. SiC nanocrystals exhibit high hardness and strength, high resistance against corrosion, low thermal expansion coefficient, high thermal conductivity etc [11, 12]. Such interesting properties of SiC

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nanocrystals prompt researchers to expect that polymer nanocomposites impregnated with SiC nanocrystals will exhibit new functionalities that originate from the cooperative effect of the distinctive properties of the two constituents. However, it is not possible to obtain the full

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potential of SiC nanocrystals as the reinforcing material because of the tendency of SiC nanocrystals to form agglomerates due to Van der Waal forces and due to its poor interfacial adhesion with polymer matrices [13]. Functionalization of the surface of SiC nanocrystals is an

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established method to improve its dispersion and interaction with the host matrices and to enhance the resulting device performance [14-16]. Fradetal et al. have shown that localization of

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the functionalization in a small area around the SiC nanostructures leads to high performances sensors [17]. Hody et al. developed a method to improve the interaction of SiC nanopowders with the matrix by functionalizing its surface with carboxylic groups using an r.f. (13.56 MHz) low pressure plasma reactor [18]. Schoell et al. successfully demonstrated the functionalization

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of n-type 3C-SiC surfaces with organosilanes [19]. Although a significant research focus lies on the fabrication of SiC based nanocomposites, yet there is still a need to devote more attention to develop methods to achieve proper production techniques to obtain high performance

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nanocomposites for more demanding applications, such as in microelectronics.

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An attempt has been made in this direction to obtain nanocomposite of SiC nanocrystals dispersed in PVA matrix by first grafting the surface of SiC nanocrystals with PVA (PVA-g-SiC) to improve its dispersion in PVA matrix. The main aim of the present work is to provide an indepth analysis of how PVA grafted SiC nanocrystals help in enhancement of optical, electrical and mechanical properties of PVA. PVA matrix was chosen as a host material as it is water soluble and is one of the most promising polymer due to its unique characteristics such as easy processability, high hydrophilicity, biocompatibility and non-toxicity, making it an ideal matrix

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for electronic and optoelectronic applications [20, 21]. Its bioinertness tend to make it a material of choice in the biomedical field. Furthermore, it can readily be grown into thin films by solution casting technique which is an excellent property to obtain flexible devices. Hence it is of utmost

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importance to analyze the modifications induced in various properties of PVA by incorporation of nanocrystals for various practical applications. Very few reports are available in literature studying the electrical conductivity of SiC nanoparticles dispersed in PVA matrix. Tkaczyk [22]

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has observed a substantial increase in current with addition of 5 wt% SiC nanoparticles in polybutadiene matrix and have concluded that hopping and Poole-Frenkel conduction

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mechanisms are chiefly responsible for increased conductivity of polybutadiene. We have earlier shown the considerable enhancement in conductivity and microhardness of PVA after incorporation of functionalized SiC nanocrystals in PVA matrix [16]. In the present work, the effect of PVA-g-SiC nanocrystals on optical, mechanical and electrical properties of PVA has

2.1. Materials

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2. Experimental section

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been investigated in detail.

Analytical reagent grade PVA (molecular weight = 1,25,000 g/mol) obtained from Ranbaxy was

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used as received. SiC nanocrystals of size 50-150 nm were synthesized by the solid phase reaction using nanoporous carbon black (Vulcan® XC-72) as the carbon source. In this reaction carbon was reacted with molten silicon. Detailed mechanism of SiC nanocrystals formation has been explained in Dhiman et al [23]. Nitric acid, sulphuric acid and ethylene glycol used for the preparation of nanocomposites were procured from Rankem, India. All the solutions were obtained using deionized water.

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2.2. PVA grafting Prior to grafting, 500 mg of SiC nanocrystals were suspended in 100 ml mixture of concentrated sulfuric acid and concentrated nitric acid (1:1) for 6 hr with the help of an ultrasonicator.

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Subsequently functionalized SiC nanocrystals were separated by centrifugation and washed with deionized water several times till pH reaches in the range of 5–7. Later they were dried at 50°C in oven for 4 hr.

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For grafting procedure, 1 g PVA was dissolved in 80 ml ethylene glycol in a beaker and heated to 140°C while stirring vigorously on a magnetic stirrer. A transparent and viscous solution was

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obtained after stirring the solution for 4 hr. The functionalized SiC nanocrystals were then mixed with PVA solution at 90°C. The resulting solution was homogenized by stirring followed by ultrasonication for 3-4 hr which resulted in a black solution. The precipitates of nanocrystals were allowed to settle down and washed several times with deionized water to neutralize the pH.

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Finally the PVA-g-SiC nanocrystals were dried in the oven at 50°C overnight. The schematic of this reaction could be expressed as given in Fig. 1.

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2.3. Nanocomposite synthesis

1g PVA was dissolved in 20 ml distilled water at room temperature. A known amount of PVA-g-

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SiC nanocrystals was dissolved in distilled water at room temperature under ultrasonication for 1 hr for homogenous dissolution of PVA-g-SiC nanocrystals. Solution having 0.015 wt% PVA-gSiC nanocrystals and PVA were mixed together using a magnetic stirrer followed by ultrasonication. The resulting solution was casted in petri dish and then dried in air at room temperature for about 7 days until the solvent was completely evaporated. The resulting nanocomposite film of ~15 µm thickness was then peeled off for further characterization. For

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comparison purpose, thin film of pristine PVA was also prepared in the same manner. 2.4. Characterization Interaction between PVA and PVA-g-SiC nanocrystals and changes induced in PVA matrix after

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incorporation of PVA-g-SiC nanocrystals were studied using ABB Horizon (mb 3000) FTIR spectrophotometer in the range of 4000–400 cm−1 and a Dilor raman spectrometer using 532 nm wavelength laser with resolution 3-5 nm. Surface morphologies of PVA-g-SiC nanocrystals

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inside the PVA matrix were examined using JEOL scanning electron microscope operated at

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20kV. Dispersion of the nanocrystals was examined using TECNAI high resolution transmission electron microscope operated at 200kV. X-ray diffraction measurements were carried out on a Mini Flex II Rigaku diffractometer using Cu Kα radiation (λ= 0.154 nm) in the wide angle region from 20° to 80°. Absorption spectra of PVA and PVA-g-SiC/PVA nanocomposite film were obtained using a shimadzu double beam double monochromator spectrophotometer (UV-2550)

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equipped with an integrating sphere assembly ISR-240A in the wavelength range of 190 nm to 900 nm with a resolution of 1 nm. The absorption spectra of PVA and PVA-g-SiC/PVA nanocomposite film were recorded by keeping air as reference whereas for recording diffuse

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reflection spectra, BaSO4 powder was taken as the reference material. The absorption data so

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obtained has been utilized to determine various optical parameters such as optical energy gap, Urbach’s energy, refractive index and dielectric constant. Surface hardness measurements were carried out on a UHL microhardness tester using a Knoop indenter. Test load of 9.8 mN was applied for a dwell time of 30 sec. The current-voltage characteristics of PVA and PVA-gSiC/PVA nanocomposite film were studied with Keithley (6517A) digital electrometer using 8009 resistivity text fixture with computer interface by the four probe method at room temperature. The measurements were carried out by measuring the current (I) as a function of

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applied voltage (V) in the range 0–100V in multiple steps of 5V. 3. Results and discussion

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3.1. Structural characterization FTIR spectroscopy was used to analyze the changes induced in the structure of PVA after addition of PVA-g-SiC nanocrystals. Fig. 2 shows the FTIR spectra of PVA (curve a) and PVA-

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g-SiC/PVA nanocomposite film (curve b). The spectrum of PVA displays peaks at around 3504 cm−1 and at 2961 cm−1 which are attributed to stretching vibrations of O–H and C–H bonds,

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respectively. The peaks at 1730 cm−1 and 1255 cm−1 are ascribed to the stretching vibrations of C=O and C–O respectively which are due to the residual acetyl group present in PVA. The vibrational peaks at 1434 cm−1 and 859 cm-1 are assigned to C–H bending and stretching mode of PVA respectively while peak at 1097 cm–1 is assigned to C–O stretching vibrations in PVA.

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Presence of these vibrational peaks confirms the monomer structure of PVA. The assignments of various peaks made in this study are in reasonable agreement with those reported in literature [24, 25]. From Fig. 2 (curve b), it can be clearly seen that the peak due to O–H stretching

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vibration has shifted from 3504 cm−1 in PVA to the lower wavenumber 3261 cm−1 in nanocomposite film and the peak due to C–H stretching vibration has shifted from 2961 cm−1 in

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PVA to 2917 cm−1 in the nanocomposite film. Moreover, in the FTIR spectra of nanocomposite film, stretching vibrations of the Si–C bonds at 781 cm−1 has also been observed [26]. Hence, the presence of vibrations corresponding to PVA as well as SiC nanocrystals is an indication of the strong molecular interactions between PVA and PVA-g-SiC nanocrystals. Fig. 3 presents the raman spectra of PVA and PVA-g-SiC/PVA nanocomposite film in which the structural changes introduced in PVA after incorporation of PVA-g-SiC nanocrystals can be

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clearly seen. Fig. 3 (curve a) shows all major peaks associated with various functional groups of PVA. The vibrations in the raman spectra of PVA at 854 cm−1 and 918 cm−1 have been ascribed to the stretching of C–C bonds while peak at 1117 cm−1 corresponds to the stretching vibrations

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of C–O bond. The peaks at 1362 cm−1 and 1462 cm−1 has been ascribed to the bending vibrations of C–H and O–H bonds and the peak at 2920 cm-1 is due to the stretching vibrations of C–H bond. The assignments of various peaks made in this study are in reasonable agreement with

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those reported in literature [27, 28]. It can be discerned from the raman spectra of PVA-gSiC/PVA nanocomposite film that most of the peaks corresponding to PVA vibrations have been

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lost and the peaks at 1335 cm-1 and 1586 cm-1, which correspond to the disorder D band and graphite like G band, are observed. The D-band corresponds to the breathing mode of k-point phonons of A1g symmetry while the G-band represents first-order scattering process of sp2 carbons in E2g stretching mode [29, 30]. Moreover, the peak observed at 2920 cm-1

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corresponding to C–H stretching vibrations are of considerably reduced intensity in the raman spectra of PVA-g-SiC/PVA nanocomposite film as compared to peak observed in PVA, which suggests that molecular structure of PVA has been significantly modified after the addition of

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PVA-g-SiC nanocrystals. This result might be indicative of the formation of intermolecular complexes between PVA-g-SiC nanocrystals and PVA matrix. Hence, FTIR and raman

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spectroscopy results clearly provide the evidence of the presence of strong molecular interaction between PVA-g-SiC nanocrystals and PVA matrix. In order to observe the morphology and distribution of PVA-g-SiC nanocrystals in PVA matrix SEM was carried out. Fig. 4 (a) and (b) presents the SEM images of PVA and PVA-g-SiC/PVA nanocomposite film containing 0.015 wt% PVA-g-SiC nanocrystals respectively. Fig. 4 (b) depicts almost uniform distribution of nanocrystals inside the PVA matrix. The agglomeration

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observed in the SEM image of PVA-g-SiC/PVA nanocomposite film (Fig. 4 (b)) is quite minute in comparison to that observed in nanocomposite films containing as received SiC nanocrystals [13]. Fabrication of nanocomposite films containing small quantity of as received SiC

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nanocrystals was difficult as they tended to agglomerate and settle down at the bottom when nanocomposite films were drawn. As a result, higher concentrations of as received SiC nanocrystals were utilized to synthesize nanocomposites. On the other hand, a small quantity of

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PVA-g-SiC nanocrystals could be dispersed easily in the solvent using ultrasonicator and did not settle down at the bottom when nanocomposite films were drawn. Hence, it can be inferred that

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grafting has improved the dispersion of SiC nanocrystals.

Fig. 5 presents the HRTEM image of PVA-g-SiC/PVA nanocomposite film which further shows the homogenous dispersion of nanocrystals inside the PVA matrix. HRTEM image clearly reveals that the agglomeration among the nanocrystals has been reduced and they are uniformly

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distributed due to increased interaction with the surrounding matrix i.e PVA. Fig. 6 presents the XRD pattern of PVA (curve a) and PVA-g-SiC/PVA nanocomposite film

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(curve b) containing 0.015 wt% PVA-g-SiC nanocrystals. It can be discerned from Fig. 6 that the XRD pattern of PVA does not show any prominent peak. However, the XRD pattern of

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nanocomposite film has peaks at 2θ values 35.2°, 41.03°, 59.7°, 71.6° and 75.1° corresponding to SiC nanocrystals. These diffraction peaks matched perfectly with the standard data available for SiC in joint committee of powder diffraction standards (JCPDS) file no. 29- 1129 [31, 32]. From these observations it could be implied that the crystalline geometry of SiC nanocrystals has not changed after the fabrication of nanocomposite film. XRD pattern was further used to calculate the interplanar distance (d) between the crystal planes using the formula [33]:

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where n is the order of reflection, which may be any integer (1, 2, 3, . . . ). The value of

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interplanar spacing at major peaks corresponding to the (111), (200), (220), (311) and (222) planes comes out to be 2.52 Å, 2.18 Å, 1.54 Å, 1.31 Å and 1.25 Å respectively.

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3.2. Optical characterization

UV-Vis absorption spectroscopy has been utilized to ascertain various optical parameters of

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PVA-g-SiC/PVA nanocomposite film formed after dispersing PVA-g-SiC nanocrystals in PVA matrix. The optical behaviour of nanocomposites attains special attention because of their applicability in various optical and opto-electronic devices. Various factors such as transmission, reflection, absorption and scattering of the photons incident on the nanocomposites are the key

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key parameters which decide their optical properties at the macroscopic level. These factors help in determining various optical parameters such as optical energy gap (Eg), Urbach’s energy (Eu), refractive index (n). Hence, in order to evaluate the potential applications of nanocomposites as

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optical materials, it is very crucial to determine their absorption characteristics. To determine the optical energy gap also called the HOMO-LUMO (Highest Occupied Molecular

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Orbital–Lowest Unoccupied Molecular Orbital) gap of the films, the absorption coefficient, α, was

determined using the following formula [34]:

where A is the absorbance and d is the film thickness. At high absorption coefficient levels where α > 104 cm-1, the absorption coefficient for amorphous materials can be related to the energy of the incident photon [35] by the relation:

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where hν is the photon energy, α is the absorption coefficient, Eg is the optical energy gap, B

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depends on transition probability and can be assumed constant within the optical frequency range and the constant n has discrete values ½, 3/2, 2 or 3 depending on whether the transition is direct allowed, direct forbidden, indirect allowed or indirect forbidden transitions respectively.

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For lower absorption coefficient levels α < 104 cm-1, the absorption coefficient α is described by

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the Urbach’s formula [36]:

where h is the Plank's constant, α is the absorption coefficient of the optical absorption near the

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band edge, Eu is the Urbach’s energy and is an indicator of the defect levels. Optical energy gap of PVA and PVA-g-SiC/PVA nanocomposite film was calculated using equation (3). A graph has been plotted between (αhν)1/2 calculated from UV-Visible absorption

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data and photon energy, hν (Fig. 7). Optical energy gap Eg was determined by extrapolation of best fit line between (αhν)1/2 and hν to intercept the hν axis such that α = 0. Table 1 presents the

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values of optical energy gap so obtained. It can be discerned from table 1 that the value of Eg for PVA came out to be 4.1 eV which decreases to 3.4 eV for PVA-g-SiC/PVA nanocomposite film. Incorporation of nanocrystals in the PVA matrix have reduced the gap between HOMO and LUMO levels by formation of charge transfer complexes as localized states within the gap [37, 38]. Presence of these localized states within the HOMO-LUMO gap of PVA lowers the potential barrier between them and hence facilitates the low energy transitions. Moreover, a decreased energy gap leads to an increased disorder within the nanocomposite film. Urbach’s

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energy, Eu, which is a measure of disorder was obtained by taking the reciprocal of slope of best fit line between ln(α) and photon energy, hν (Fig. 8). From table 1, a sharp increase in the value of Urbach’s energy from 0.55 eV for PVA to 1.01 eV for nanocomposite film has been observed.

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Thus, it can be interpreted that disorder in the form of localized states within the HOMO-LUMO gap of PVA matrix has thus been formed after the incorporation of PVA-g-SiC nanocrystals.

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Fig. 9 shows the variation of refractive index of PVA and PVA-g-SiC/PVA nanocomposite film containing 0.015 wt% PVA-g-SiC nanocrystals with wavelength. In the case of normal

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incidence, the reflectivity (R) may be expressed in terms of the real refractive index (n) and extinction coefficient (k) by the following equation [39]:

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where k is the extinction coefficient and is given by:

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where λ is the wavelength of the light used. From Fig. 9 it is clear that the refractive index increases from 1.5 for PVA to 2.0 for PVA-g-SiC/PVA nanocomposite film at 550 nm

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wavelength. The evaluation of refractive indices of nanocomposites is also of considerable importance for applications in integrated optical devices such as switches, filters, modulators etc. where the refractive index of a material is the key parameter in the design of a device. The increase in refractive index indicates increased density within the nanocomposite film. This increased density leads to a reduction in the inter-atomic spacing due to the intermolecular hydrogen bonding between nanocrystals and the OH groups of PVA [40, 41].

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The optical properties of any solid material are characterized by the complex refractive index (n*=n+ik) [42]. The real part of the refractive index (n) gives the dispersion and the imaginary part of the refractive index (k) gives the absorption of the electromagnetic wave. Refractive

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index dispersion plays an important role in the research for optical materials because it is a significant factor which determines the potential of a solid to be used in fabrication of devices for spectral dispersion and optical communication. From Fig. 9 it can be seen that for PVA

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refractive index decreases with increasing wavelength showing normal dispersion but in case of nanocomposite film, it decreases in the wavelength region 500 nm to 650 nm displaying normal

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dispersion however it increases in the region from 300 nm to 500 nm displaying an anomalous behaviour i.e it increases with increasing wavelength.

Wemple and DiDomenico developed single oscillator model to analyze the refractive index in the normal dispersion region (the transparent region) [43]. Refractive index (n) at frequency ν is

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related to the energy parameter Ed, which is a measure of the strength of interband optical

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transition and single oscillator energy E0 [44] as:

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The values of Ed and E0 can be obtained from the intercept and slope of the linear fitted lines by plotting (n2−1)−1 against (hν)2 as shown in Fig. 10. Values of Ed and E0 obtained using equation (6) are tabulated in table 2. Further, to yield the long wavelength refractive index (n∞), average interband oscillator wavelength (λo) together with the average oscillator strength (So) for the PVA and PVA-g-SiC/PVA nanocomposite film, the single term oscillator model equation [45] is considered which is as follows:

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The values of n∞ and λo were determined from the slope and intercept of linear fitted line of the plots of (n2 − 1)−1 versus λ−2 as illustrated in Fig. 10 and are tabulated in table 2.

From the obtained values of n∞ and λo, the values of average oscillator strength So were

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calculated using the relation:

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where λo is the average interband oscillator wavelength and

is the average oscillator strength and is listed in table 2. Fig. 11 (A) and (B) show the variations of real (ε1) and imaginary (ε2) parts of the dielectric

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constant with energy for PVA and PVA-g-SiC/PVA nanocomposite film. The real part of the dielectric constant is a measure of charge storage capacity of the matrix and the imaginary part is

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a measure of dielectric losses. Together, the real and imaginary parts of dielectric constant describe the interaction of the material with the electromagnetic field. Real and imaginary parts of the dielectric constant are related to the n and k values as [45]:

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From Fig. 11 (A) and (B) it is clear that values of both real (ε1) as well as imaginary (ε2) parts of the dielectric constant has higher values with respect to PVA and the values of real part of

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dielectric constant are higher than imaginary part. Higher value of real part of dielectric constant indicates high energy storage capacity of prepared nanocomposite [46].

Table 2 shows that the oscillator energy Eo and the dispersion energy Ed of nanocomposite

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increases with respect to PVA. These variations in the value of optical constants may be attributed to the formation of charge transport complexes in the form of localized states between

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HOMO-LUMO gap in the PVA matrix. These charge transfer complexes make the lower energy transitions feasible and lead to the observed changes in optical parameters. 3.3. Microhardness studies

Fig. 12 depicts the variation in Knoop microhardness and penetration depth of PVA and PVA-g-

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SiC/PVA nanocomposite film. Indentations were produced using a Knoop indenter. Load of 9.8 mN was applied using a 30 sec loading cycle. The length of the longest imprinted diagonal was measured within 30 sec following indentation, using a digital eyepiece. The microhardness (Hk)

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values were calculated using the formula [47]:

where Hk is expressed in Kgf/mm2, the applied force P is in mN, and the diagonal d is in µm. 10 imprints were made on PVA as well as on nanocomposite film under each load and the average value was calculated. The penetration depth was calculated using the relation:

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It can be discerned from Fig. 12 that the microhardness of PVA calculated using equation (12) is 2.4

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Kgf/mm2 which increases to a value of 4.9 Kgf/mm2 for PVA containing 0.015 wt% PVA-g-SiC nanocrystals. Moreover, Fig. 12 also shows the decreased penetration depth (calculated using equation (13)) with addition of PVA-g-SiC nanocrystals inside PVA matrix. Decreased

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penetration depth is also an indication of increased microhardness of the nanocomposites as compared to virgin PVA. Microhardness is highly dependent upon the uniform distribution and

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interfacial interaction of nanoparticles with the polymeric matrix [48, 49] and the nanoparticles enhances the hardness if they are strongly bonded with the polymer [50]. Grafting of SiC nanocrystals with PVA has introduced various functional groups on its surface which might have acted as a source of bonding with the matrix, thereby, improving its dispersion

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(verified using TEM and SEM) within the PVA matrix. This improved dispersion has contributed towards the enhancement in microhardness of the PVA upon addition of PVA-g-SiC nanocrystals. The increase in hardness of PVA-g-SiC/PVA nanocomposite film in comparison to

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PVA can be attributed to the high hardness of SiC nanocrystals and also to the grafting of PVA

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on hard SiC nanocrystals which led to strong molecular bonding of PVA-g-SiC nanocrystals with PVA matrix.

3.4. Electrical characterization Fig. 13 shows the dependence of current I on applied voltage V for PVA (curve a) and PVA-gSiC/PVA nanocomposite film (curve b). It is clear from Fig. 13 that the conductivity of PVA increases after incorporation of PVA-g-SiC nanocrystals in PVA matrix. Furthermore, the I-V

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curves are nonlinear in nature. Nonohmic charge transport processes such as, Schottky emission (SE), Poole Frenkel effect (PF), space charge limited conduction (SCLC), etc are responsible for such nonlinear characteristics.

The exact conduction mechanism responsible for nonlinear

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variation of current with voltage was determined by plotting I-V data on a log-log scale for low as well as high voltage regions. It did not yield the slope n> 2 as expected for SCLC to occur [51] so this mechanism for conduction was ruled out. Therefore the conduction mechanism may

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be either Poole-Frenkel or Schottky.

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For the Schottky effect the current–voltage expression is given by [52]:

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and for the Poole-Frenkel effect current–voltage expression is given by:

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In these expressions A (=1.2 x 106 Am-2) is the Richardson constant, T is the absolute

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temperature, (φs) is the Schottky barrier height at the injecting electrode interface, k is Boltzmann’s constant. βs and βPF are, respectively, the Schottky and Poole-Frenkel field lowering coefficients.

Since the coefficient β is an essential factor that determines the magnitude of the coulombic field, the βs and βPF values were calculated theoretically using relation [52, 53]:

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where e is the electronic charge, εr is the relative permittivity and ε0 is the permittivity of free space.

The linear fittings of the log (I) vs V1/2 graph are used to determine whether the controlling

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conduction mechanism is Schottky emission or the Poole-Frenkel emission [54, 55]. Fig. 14 shows a plot of log (I) vs V1/2 for PVA and PVA-g-SiC/PVA nanocomposite film. It is quite

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clear from Fig. 14 that the entire voltage region can be divided into three different voltage regions with altered slopes for which the plot of log (I) vs V1/2 has an approximate linear relationship. These regions are classified as low voltage region (0 < V < 10 V), medium voltage region (10 < V < 30 V) and high voltage region (30 < V < 100 V). The values for βs and βPF were

as the high voltage regions.

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separately derived from the slope of a linear fit on plots of log (I) vs V1/2 at low, medium as well

By taking the high frequency dielectric constant as 1.5, the calculated values of βs and βPF are 3.1

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x 10-5 and 6.2 x 10-5 eV V-1/2 m1/2, respectively. The experimental value of β (as shown in table

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3) determined from Fig. 14 is found to be near to βPF values for lower voltage region. Hence, it is suggested that the dominant conduction mechanism is Poole-Frenkel in low voltage region. However, at the medium as well as higher voltage region experimental value of β becomes closer to βs. Hence, the dominant conduction mechanism at medium and higher voltages is Schottky conduction mechanism. There is a transition of the conduction mechanism from Poole-Frenkel to Schottky in the higher voltage regions. In many multi-component systems, similar transitions of electrical conduction mechanism have been reported. Moosvi et al [56] have reported that charge

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transport mechanism changes from Ohm’s law at lower voltages to space charge limited emission at higher voltages in PTP/[Fe(CN)3(dien)]·H2O nanocomposite. While investigating the conduction mechanism in PVC-stabilized with dibutylin laurate-maleate, Mahrous et al [57] have

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reported current due to thermally generated carriers at low voltage and identified Poole-Frenkel mechanism at higher voltages.

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4. Conclusions

PVA-g-SiC nanocrystals were successfully utilized to modify the structural, electrical and

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mechanical properties of polyvinyl alcohol matrix. FTIR and raman spectroscopy validates the presence of strong molecular interaction between PVA and PVA-g-SiC nanocrystals. Homogenous distribution of PVA-g-SiC nanocrystals inside the PVA matrix was confirmed using SEM and HRTEM. Optical energy gap decreased from 4.1 eV for PVA to 3.4 eV for PVA-

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g-SiC/PVA nanocomposite film and Urbach’s energy increased from 0.55 eV for PVA to 1.01 eV for PVA-g-SiC/PVA nanocomposite film. The refractive index increased from 1.5 for PVA to 2.0 for PVA-g-SiC/PVA nanocomposite film. Real and imaginary parts of dielectric constant

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increased with respect to PVA. Enhancement in the microhardness of PVA-g-SiC/PVA nanocomposite film as compared to PVA is an indication of presence of strong molecular

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interaction of PVA-g-SiC nanocrystals with PVA matrix. The analysis of I-V curves shows that the charge transport in the nanocomposite films consists of a combination of transport mechanism. Schottky mechanism was the foremost conduction mechanism at medium and high voltage regions whereas Poole-Frenkel was the chief conduction mechanism at low voltage regions. Acknowledgements

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One of the authors (IS) is thankful to UGC for providing UGC-BSR fellowship to carry out the proposed research work. Authors acknowledge the support from AIIMS, New Delhi for HRTEM

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Fig. 1. Schematic representation of grafting process. Fig. 2. FTIR spectra of (a) PVA and (b) PVA-g-SiC/PVA nanocomposite film containing 0.015 wt% PVA-g-SiC nanocrystals.

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Fig. 3. Raman spectra of (a) PVA and (b) PVA-g-SiC/PVA nanocomposite film containing 0.015 wt% PVA-g-SiC nanocrystals.

Fig. 4. SEM image of (a) PVA and (b) PVA-g-SiC/PVA nanocomposite film containing 0.015

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wt% PVA-g-SiC nanocrystals.

SiC nanocrystals.

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Fig. 5. HRTEM image of PVA-g-SiC/PVA nanocomposite film containing 0.015 wt% PVA-g-

Fig. 6. XRD spectra of (a) PVA and (b) PVA-g-SiC/PVA nanocomposite film containing 0.015 wt% PVA-g-SiC nanocrystals.

Fig. 7. Plots of (αhν)1/2 versus hν to determine optical energy gap in (a) PVA and (b) PVA-g-

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SiC/PVA nanocomposite film containing 0.015 wt% PVA-g-SiC nanoparticles. Fig. 8. Plot of ln(α) versus hν used to determine the Urbach’s energy in (a) PVA and (b) PVA-gSiC/PVA nanocomposite film containing 0.015 wt% PVA-g-SiC nanocrystals.

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Fig. 9. Refractive index variations of (a) PVA and (b) PVA-g-SiC/PVA nanocomposite film containing 0.015 wt% PVA-g-SiC nanocrystals.

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Fig. 10. Plot of (n2−1)−1 versus (hν)2 and (λ)−2 for (a) PVA and (b) PVA-g-SiC/PVA nanocomposite film containing 0.015 wt% PVA-g-SiC nanocrystals. Fig. 11. Variation of (A) real (ε1) and (B) imaginary (ε2) part of dielectric constant with energy for (a) PVA and (b) PVA-g-SiC/PVA nanocomposite film containing 0.015 wt% PVA-g-SiC nanocrystals. Fig. 12. Variation of hardness and penetration depth of PVA and PVA-g-SiC/PVA

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nanocomposite film at 9.8 mN load. Fig. 13. I-V characteristics of (a) PVA and (b) PVA-g-SiC/PVA nanocomposite film containing 0.015 wt% PVA-g-SiC nanocrystals.

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containing 0.015 wt% PVA-g-SiC nanocrystals.

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Fig. 14. Plot of log (I) versus V1/2 for (a) PVA and (b) PVA-g-SiC/PVA nanocomposite film

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Table 1. Optical parameters for PVA and PVA-g-SiC/PVA nanocomposite film. Urbach’s energy

Eg (eV)

Eu (eV)

PVA

4.1

0.55

Nanocomposite film

3.4

1.01

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Optical energy gap

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Table 2. Values of Optical parameters for PVA and PVA-g-SiC/PVA nanocomposite film. Ed (eV)

n∞

Λo (nm)

PVA

4.8

5.2

1.45

260

Nanocomposite film

9.7

28.9

1.99

127

So X 10-5 (nm-2)

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Eo (eV)

1.6

18.3

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Sample

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Table 3. Theoretical and experimental values of β. Theoretical

Experimental

βPF

βs

βexp

(eV V-1/2m1/2)

(eV V-1/2m1/2)

(X 10-5) (eV V-1/2m1/2) Ist

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Theoretical

2nd region

region 4.6

6 X 10-5

3 X 10-5

5.2

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Nanocomposite

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film

region

2.8

1.4

2.8

1.7

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PVA

3rd

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% Transmittance

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(b)

(b) PVA-g-SiC/PVA (a) PVA

(a)

3500

3000

2500

2000

1500

Wavenumber (cm

-1

1000

)

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Intensity (a.u.)

(b)

500

(b) PVA-g-SiC/PVA (a) PVA (a)

1000

1500

2000

2500

Wavenumber (cm

-1

3000

)

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4(B)

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4(A)

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Intensity (a.u.)

(b) PVA-g-SiC/PVA

(111)

(a) PVA

(200)

(b) (220) (311)

(222)

(a)

30

40

50

2

60

(degrees)

70

80

2.8

2.0

(b) PVA-g-SiC/PVA

(a) PVA

h )

(b) 1.6

(

1/2

2.4

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1.2 0.8 (a) 0.4 0.0 3.0

3.5

4.0

4.5

Energy (eV)

5.0

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0.3 0.0

(b) PVA-g-SiC/PVA

ln (

)

-0.3 (a) PVA

-0.6 -0.9

(a)

(b)

-1.2 -1.5 -1.8

0

1

2

3

4

Energy (eV)

5

6

Refractive index, n

2.2 2.1 2.0

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(b)

(b) PVA-g-SiC/PVA

1.9

(a) PVA

1.8 1.7

(a) 1.6 1.5 350

400

450

500

550

600

Wavelength (nm)

650

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-2

-6

2.6x10 0.8

2

(n -1)

-1

0.7

-6

2.8x10

(nm) -6

3.0x10

-2

-6

3.2x10

-6

3.4x10

(a)

(a) PVA

0.6

(b) PVA-g-SiC/PVA

0.5

0.4

(b)

0.3

0.2 4.0

4.2

4.4

(h

4.6

)

2

4.8

(eV)

2

5.0

5.2

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-7

8

2.5x10 11(A)

7

(b) PVA-g-SiC/PVA

(b) PVA-g-SiC/PVA

6

11(B)

-7

2.0x10

(a) PVA

(a) PVA

(b)

2

1

-7

1.5x10

5

(b)

-7

4

1.0x10

3

(a)

-8

5.0x10

(a)

2

0.0 2.0

2.5

h

3.0

(eV)

3.5

4.0

1.5

2.0

2.5

h

3.0

3.5

(eV)

4.0

4.5

2

KHN (Kgf/mm )

6.4 5.6

3.2 3.0 2.8

4.8

2.6 2.4

4.0

2.2

3.2

2.0

2.4

1.8 1.6 1.4 0

0.015

Concentration (wt%)

m)

1.6

Penetration Depth (

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Current (Amp.)

-6

2.0x10

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(b) PVA-g-SiC/PVA

-6

1.5x10

(b)

(a) PVA

-6

1.0x10

(a)

-7

5.0x10

0.0 0

20

40

60

Voltage (Volts)

80

100

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log (I)(Amp.)

-5.5

-6.0

(b)

-6.5 (a) -7.0 (b) PVA-g-SiC/PVA -7.5 (a) PVA -8.0

2

3

4

5 1/2

V

6

7

8

(Volts)

1/2

9

10

11

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Highlights
Synthesis of nanocomposite of PVA by embedding polymer grafted SiC nanocrystals.
Decrease in the optical energy gap upon grafted SiC nanocrystal addition.

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Enhancement of surface microhardness of PVA after incorporating grafted SiC nanocrystals.


Charge transport in the nanocomposite film consists of a combination of transport mechanism.

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Schottky mechanism responsible for conduction at higher voltage regions and Poole

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Frenkel at lower voltage regions.