Structural Characterization and Dielectric Properties of the Effect of Carbon Nanotubes and Silicon on AA2024 Metal Matrix Composites


 This research work focus on the formation of AA2024-carbon nanotubes-silicon hybrid metal matrixcomposites. Structure morphology, structural characterization, elemental identification and dielectric properties of AA 2024 in the presence of carbon nanotubes, silicon andits combinations at various proportions was evaluated using SEM, XRD, EDX and Hioki 3532-50 LCR Hi-Tester. A two-stage stir casting method was used for the fabrication of AA2024 hybrid metal matrix composites. It was observed that the size of the AA 2024 + 4% CNT + 2% Si composite wasfound to be 23.6 nm, this shows enhanced results than other composites prepared. Dielectric properties of composites were characterized as a function of composition and frequency. It was found that the dielectric constant, dielectric loss and dissipation factor decreases smoothly with an increase of reinforcements and also frequency.


Introduction
MMCs lled with nanoparticles are pro cient materials, appropriate for an entire host purpose. These composites contain metal as a matrix crammed with nanoparticles marking many properties extremely dissimilar from those of the base material. The nanoparticles can advance the base material in terms of physical, chemical and mechanical properties [1].
Aluminium / Aluminium 2024 alloy has a high strength to weight ratio and excellent fatigue resistance. Because of this, Aluminium/Aluminium 2024 alloy is extensively employed in the aircraft industry. Tribological properties are the main drawbacks of the composites. For this reason, the engineering society has got to build up a totally unique material with superior tribological properties [2][3][4][5].
The discovery of carbon nanotubes (CNT) opened new perspectives for the event of composite materials. Carbon nanotubes have outstanding properties [6] and that they have been involved within the fabrication of composites as reinforcements. CNTs have a rarity and outstanding electrical and mechanical properties [7]. Additionally to good chemical and thermal stability, CNT demonstrates high yield strength and modulus of elasticity values [8].
Silicon (Si) is the most copious electropositive element within the Earth's crust. It is a metalloid with an evident metallic luster and extremely brittle. Pure silicon is termed as an intrinsic semiconductor, although the concentration of its semiconduction is highly increased by adding a tiny low amount of impurities. Silicon produces different series of hydrides, different halides and plenty of series of oxygen-containing compounds. Silicon is additionally a major element of some steels and also the main element in bricks. Elemental silicon gives more resistance to materials like aluminium, magnesium and copper [9].
Analysis of dielectric property ends up in the state changes from initial unspoiled condition to the ultimate breakdown condition due to the application of an assorted applied eld. Dielectric properties are generally used for identifying the strongest and weakest nature of the material prior to the applying load. Dielectric properties could even be wont to portray mechanical properties like strength, life, and sturdiness of the material [10].
The present work has been focused on the fabrication of hybrid metal matrix composites by the utilization of carbon nanotubes and silicon into the AA 2024 matrix by stir casting method [11]. The prepared composites were subjected to structural, elemental, and dielectric properties analysis.

Method of preparing composites
High energy ball milling was used for the synthesis of silicon nanoparticles [12] and stir casting unit was used for the synthesis of composite materials.
In this work, AA2024-CNT, AA2024-silicon, AA2024-CNT-silicon at various proportions was prepared. Pure AA 2024 was melted up to 750C and the molten material was stirred between 10 to 15 min at an impeller speed of 325 rpm. At this stage, carbon nanotubes, silicon and their mixtures are added. The carbon nanotubes and silicon nanoparticles and their various proportions were heated up to 300C for 3 hours to require away the dampness. The resultant composite material was transferred into the eternal metallic pattern. The liquid material was allowed to solidify within the pattern.

Method of Preparing Composites for SEM, EDX, XRD and dielectric studies
The composites produced were examined by using SEM, EDX, XRD and also dielectric studies. A piece of composite material of 1 cm 2 area and 2 mm thickness ( Fig. 1) is used for SEM and EDX, 2 cm length 1 cm width and 2 mm thickness ( Fig. 2) is used for XRD and 1 cm 2 area and 1.5 mm thickness (Fig 3) was used for the dielectric properties. The sample is belt grinded, polished with emery papers and washed out, these samples are shown in Fig. 1, Fig. 2 and Fig. 3.

Scanning Electron Microscope and EDX analysis
SEM gives thorough high-resolution images of the samples. This can be done by scanning a focused electron beam across the surface of the samples and also detecting the secondary electron beam. Quantitative elemental information of the prepared samples was identi ed with the help of an Energy Dispersive X-Ray Analyzer (EDX). JEOL Model JSM-6390 LV scanning electron microscope (SEM) equipped with an energy dispersive X-ray (EDX) detector of the Oxford data reference system was used in this study.

X-Ray Diffraction Analysis
XRD pattern was recorded using Shimadzu XRD-6000 X-ray diffractometer that uses Cu Kα radiation (λ = 0.15406 nm) in the scan range 2θ = 10° to 90°. Shimadzu X'pert pro software was used to for the data collection. The peaks of the X-ray diffraction pattern observed are compared with the available standard JCPDS data to support the crystal structure.

Observation of dielectric property
Dielectric constant, dielectric loss and dissipation factor for the prepared samples were characterized as a function of reinforcements and frequency employing a Hioki 3532-50 LCR Hi-Tester. Hioki 3532-50 LCR Hi-Tester uses a touch panel as the user interface.
Examination frequency at high resolution can be set from 42 Hz to 5 MHz. Impedance |Z|, phase angle θ, L, C, and R, etc., (merely fourteen parameters) can be simultaneously displayed on the screen [13]. It shows that the carbon nanotubes have a smooth surface with bundles of tangled tubes [15]. The silicon has an irregular particle structure and a rough texture [16].

EDX Analysis
EDX spectra of the prepared samples are shown in Fig. 7 a-k and the results are summarized in table 1. Here, aluminium was found to be 91.96%, which is the major element in AA2024. In addition to that, copper (4.5%) and magnesium (1.64%) indicates that these elements play a signi cant role with aluminium to form an alloy. Manganese, iron, silicon, chromium and zinc were observed in the EDX spectrum of AA 2024, which are also supporting elements to form the AA2024 alloy. So, apart from aluminium other elements found in AA2024 are played some of the major roles in the formation of alloy.
The X-ray diffraction (XRD) analysis is employed to substantiate the structure of the materials which is obtained from used elements in the form of the prepared alloy composite samples. The consolidated results of XRD spectra for all samples are shown in Fig. 8 and the results are summarized in Table 2. The peaks were compared with the standard diffraction data to analyse the presence of various phases present in the composite materials.   (1 1 2), respectively [28]. Similarly, XRD image of silicon nanoparticles con rms the cubic structure according to JCPDS data (JCPDS le No. 80-0018) at 28.6°, 47.6°, 56.4°, 69.7° and 77.0º, which correspond to crystal planes of (1 1 1), (2 2 0), (3 1 1), (4 0 0) and (3 3 1), respectively [29].

Calculation of particle size
Using Debye-Scherrer formula [30, 31], the average particle size of the prepared composite materials were calculated and are summarized as shown in Table 3. It was observed that the particle size of AA 2024 was found to be 39.1 nm, and the particle size of AA 2024 + 4% CNT + 2% Si was found to be 23.6 nm. It is concluded that the average particle size decreases with an increase in carbon nanotubes and silicon nanoparticles. It is known that dielectric constant and also dielectric loss of material changes with frequency and also reinforcements. In view of this, a variation of dielectric constant and dielectric loss with an increase in frequency and reinforcements for the synthesized composites were studied and the results are pictorially represented as shown in Fig. 9 a-i & 10 a-i respectively, and the calculated values are tabulated ( Table 4).
The graph gives a clear signature of the compositional effect on the dielectric constant and dielectric loss of the materials. It is concluded from the results obtained that the dielectric constant and dielectric loss decreases smoothly with the addition of reinforcements and increase in frequency. Generally, as frequency increases, the net polarization of the material drops as each polarisation mechanism ceases to contribute, and hence its dielectric constant goes down. This mechanism is observed in the present study. Results indicate that the interrelation dipoles

Dissipation factor
A determination of energy lost through the turnaround of electric polarization is called the dissipation factor. It measures the ine ciency of insulating material. Variations of dissipation factor with frequency and reinforcements for the prepared composites are depicted in Fig. 11  The following conclusions have been made: SEM micrograph for the prepared samples reveals that the reinforcement particles are distributed evenly throughout the specimen.
EDX study discloses the identi cation of elements and their quantity. It was found that the quantity of aluminium was found to be 91.96%. This con rms that aluminium is the major element in AA2024. EDX spectra shows that all the major components of AA2024, carbon nanotubes and silicon.
The result of the XRD con rms the presence of various phases present in the composite materials. Crystal structure of various elements and particle size of the composites were also evaluated. For example, the diffraction peak (2θ) observed at 38.6º is corresponding to (1 1 1) plane of aluminium (Al) cubic phase which matches with the standard value (JCPDS le No. 04-0787).
Results obtained through EDX are in accordance with the results of XRD analysis.
In accordance with the results of the Hioki 3532-50 LCR Hi-Tester, the dielectric constant, dielectric loss and dissipation factor decreases with an increase in reinforcements and frequency, which has been attributed to interrelation dipoles. The test results obtained indicate that the dielectric properties offered by AA 2024 + 4% CNT + 2%Si composites were superior to those of other composites.