Fabrication and Microhardness Analysis of MWCNT / MnO 2 Nanocomposite

Recent research has shown that carbon nanotube (CNT) acts as a model reinforcement material for fabricating nanocomposites. The addition of CNT as a reinforcing material into the matrix improves the mechanical, thermal, tribological, and electrical properties. In this research paper multiwalled carbon nanotube (MWCNT), with different weight percentage (5%, 10%, and 15%), was reinforced into manganese dioxide (MnO 2 ) matrix using solution method. The different weight % of MWCNT/MnO 2 nanocomposite powders was compacted and then sintered. The phase analysis, morphology, and chemical composition of the nanocomposites were examined by X-ray diffractometer, Field Emission Scanning Electron Microscope (FESEM), and Energy Dispersive X-Ray (EDX), respectively. The XRD analysis indicates the formation of MWCNT/MnO 2 nanocomposites. The FESEM surface morphology analysis shows that MnO 2 nanotube is densely grown on the surface of MWCNT. Further, microhardness of MWCNT/MnO 2 nanocomposite was measured and it was found that 10wt% has higher microhardness in comparison to 5 and 15wt%. The microhardness of the composites is influenced by mass density, nanotube weight fraction, arrangement of tubes, and dispersion of MWCNT in H 2 SO 4 (aq) solution.


Introduction
In the last decade, nanoscience and nanotechnology have invited much interest of the researchers and industrial practitioners as enhancement in the property of the composite is noticed as the atomic dimension of reinforced material reduces to nanometer scale.
The need for advanced composite materials having enhanced functional properties and performance characteristics is ever increasing in industrial sectors.Theoretical and experimental studies have shown that CNT exhibits extremely high tensile modulus ∼1 TPa and strength ∼150 GPa.CNT also exhibits high flexibility, low density ∼1.3-1.4 g/cm 3 , and large aspect ratios ∼1000 [1].Experimental studies have also shown that MWCNT synthesized by chemical vapor deposition (CVD) exhibits Young's modulus of about 0.45 TPa and tensile strength of about 3.6 GPa during scanning electron microscope tension test [2].The superior mechanical properties of CNT are one of the primary reasons for considering the use of CNT as a filler material for fabricating metal, ceramic, or polymer matrix nanocomposite [3].
MnO 2 is considered as an electrode material for supercapacitors because of its low cost, environmental friendly nature, nontoxicity, and excellent capacitive performance in aqueous electrolytes [4].Many researchers have used different preparation methods of CNT/MnO 2 nanocomposites, namely, solution method, hydrothermal method, soft template method, facile synthesis method, electrodeposition method, and so forth.In comparison to simple mechanical mixing, a remarkable increase in capacitance as well as cycling performance for the nanosolution mixing of -MnO 2 2 Journal of Materials with MWCNT has been noticed [5].Using a simple in situ hydrothermal method single crystalline MnO 2 nanoparticles were uniformly grown on the surface of MWCNT [6].MnO 2 /CNT nanocomposite was successfully synthesized by soft template method and the products had poor crystalline characteristics, and the adjacent nanorods were fused to each other irregularly to form spherical-like agglomerations and MnO 2 nanorods were absorbed on CNTs in the CNT/MnO 2 composite [7].MWCNT/MnO 2 composite with different contents of birnessite-type MnO 2 coated on carbon nanotubes through a facile, simple, and green method of the redox reaction between KMnO 4 and carbon nanotubes without any other oxidant or reductant addition [8].Using a simple hydrothermal treatment, MWCNT/MnO 2 nanocomposite with a unique nanoarchitecture was successfully synthesized [9].The nanoporous MnO 2 sheath was composed of interconnected MnO 2 nanoflakes directly grown from the surface of the MWCNT.
A mesoporous MnO 2 /CNT composite was synthesized by the reduction of KMnO 4 with alcohol using a soft template method and mesoporous MnO 2 was coated uniformly on the CNTs, providing a high surface area [10].A low-cost aqueous solution method was developed to fabricate MWCNT/MnO 2 tubular nanocomposite at 80 ∘ C with interconnected MnO 2 nanoflakes coated on MWCNT [11].
Fabrication of electrodes for electrochemical supercapacitors has been done on the electrodes for electrochemical supercapacitors by cathodic electrodeposition of MnO 2 on CNTs, which were grown by CVD on stainless steel meshes [12].A combination of simple vacuum infiltration and CVD technique was used to prepare MnO 2 /CNT hybrid coaxial nanotube arrays using porous alumina templates [13].Exceptional change of nanocomposite properties can be achieved by introducing foreign materials into the inner cylinder of carbon nanotubes and by filling of MWCNT with MnO 2 using wet chemical method [14].
With electrochemical applications already being developed for MWCNT/MnO 2 nanocomposite, continuous study of the mechanical properties of MWCNT/MnO 2 nanocomposite can lead to useful multifunctional materials with simultaneous mechanical and electrochemical applications.10 wt% MWCNT/MnO 2 composite was prepared and investigated for its capacitance performance [5], which motivated us to find out the hardness of the nanocomposite.
The principal purpose of the hardness test is to determine the suitability of a material for a given application or the particular treatment to which the material has been subjected [15].Microhardness is an important parameter that could be used to define the mechanical properties in relation to its microstructure.Focus has been made on the relationship of hardness with grain size for commercially pure titanium and ultrafine grained titanium [16].Vickers and Knoop indentations of ultrafine grained titanium at different loads were ∼2.5 times harder than those of pure titanium.
Indentation microhardness of the -SiAlON ceramic using the Knoop and Vickers indenters has been measured and the results show that the measured Vickers and Knoop microindentation hardness values of the samples depend on load [17].The variation of Vickers hardness value follows the reverse indentation size effect trend, that is, an increase in Vickers hardness value with respect to load in the low-load region beyond where it becomes relatively constant.It has been reported that the hardness of composite material depends on the nature of the material, geometry of the penetrator, and how to perform test [18].Bulk Albased nanocomposites with nanosized SiC by the ultrasonic cavitation-based casting method were fabricated and it was found that 2.0 wt% SiC nanoparticles achieved approximately 20% improvement of hardness [19].

Experimental
In this paper, we report the microhardness of 5, 10, and 15 wt% of MWCNT in MWCNT/MnO 2 nanocomposites.Also, we applied a novel technique called solution method [5] with some modification to fabricate MWCNT/MnO 2 nanocomposites powder.The solution method is simple and cost effective for preparing MWCNT/MnO 2 nanocomposite powder.
The crystallographic information of the nanocomposites was investigated by X-ray diffraction (MiniFlex 2 goniometer, X-Ray 30 kV/15 mA Div.Slit 1.25 ∘ with a scan rate of 2 ∘ /min) with a Cu K  target.Morphologies of the MWCNT/MnO 2 nanocomposites were characterized by FESEM (Hitachi's semi-in-lens technology, model SU-70, accelerating voltage = 500 V to 30 kV, magnification: ×800,000) attached to Schottky Gun which enables dealing with EDX analysis.The sintering process was done in muffle furnace (make: Wild Barfield; model: HT25; max.temp.: 1200 ∘ C) for densification of powder particle.Also, the microhardness was measured using Vickers microhardness tester (VLPAK2000; make: Mitutoyo, Japan; model: MicroWizHard microhardness tester).

Materials.
The reinforcing material used was MWCNT having average diameter of 20 nm, length of 50 m, specific surface area of 60∼260 m 2 /gm, and 95% purity and purchased from Redox Nano Lab, Uttar Pradesh, India, and the oxidizing agent used was potassium permanganate (KMnO 4 ) of laboratory grade.The solvent used during composite preparation was sulphuric acid (2 M) solution.

Fabrication of MWCNT/MnO 2 Nanocomposite.
The powder metallurgy route has been applied to make the pellet shape of MWCNT/MnO 2 nanocomposites.The method consists of different steps for preparation of nanocomposites and shape to products based on the heat-activated process of binding individual particles of powders.
The powder metallurgy steps are as follows.
(1) Synthesis of 5 wt% MWCNT/MnO 2 Nanocomposite.0.10 gram of MWCNT was immersed in 250 mL of 2 M boiling H 2 SO 4 (aq.) and continuously stirred using magnetic stirrer for three hours, in the beaker of 500 mL capacity, to disperse MWCNT.The acid treatment in boiling H 2 SO 4 results in introduction of many oxygen-containing functional groups, such as carboxylic, carbonyl, and hydroxyl groups on the MWCNT surface [5].After three hours 4 grams of KMnO 4 was added at 80 ∘ C-90 ∘ C. The precipitate was observed as the color of the solution changed from purple to brown.The setup is shown in Figure 1.After completion of reaction, the system was then allowed to cool down to room temperature.The obtained precipitate black in color was filtered using crucible filter and washed with deionized water and acetone to remove the remaining ions and impurities and then dried at 60 ∘ C in laboratory oven for one day.Approximately about 1.96 grams of yield was obtained.
Similarly, 10% and 15 wt% MWCNT/MnO 2 nanocomposites were synthesized.Slowly cooling down to room temperature after 3 hours of dispersion in H 2 SO 4 (aq) solution, the MWCNT settled down at base of beaker.This shows poor dispersion of MWCNT in the composite.
(2) Blending.The nanocomposites prepared with various weight fractions of MWCNT powders (5, 10, and 15 wt%) for uniform distribution were blended by hand blending method using mortar pestle.
(3) Compaction.The blended nanocomposites were compacted in a hydraulic press machine to produce a disc shape of solid green sample at room temperature.The pressure limiting valve was set for a maximum oil pressure of 150 kgf/cm 2 .A compaction pressure of 50 kgf/cm 2 was also used to compact the test samples.The die and punch assembly used during bulk composites preparation, having maximum diameter of die of 15 mm, is shown in Figure 2.
(4) Sintering.Thermogravimetric-differential thermal analysis shows that MnO 2 decomposes rapidly to form Mn 2 O 3 at temperature above 500 ∘ C [7].Therefore sintering temperature should be below 500 ∘ C. The samples were heated up from room temperature to 450 ∘ C at a heating rate of 15 ∘ C/min for 30 minutes and this temperature was held constant for 1 hour.All samples were then allowed to attain room temperature

Surface Morphology Analysis Using Field Emission Scanning Electron Microscope (FESEM)
. FESEM analysis of 5 wt% MWCNT/MnO 2 composite shows that the large amount of MnO 2 nanotubes of average diameter ∼25 nm is densely grown on the surface of multiwalled carbon nanotubes [5] as shown in Figure 5(a).This shows that the MnO 2 nanotubes have been synthesized using simple solution method.It can also be seen that the tubes have agglomerated which may be due to nonhomogeneous distribution of MWCNT during composite powder preparation.Similarly the surface  The potassium may come from KMnO 4 during composite powder preparation and may be located within the tunnel of -MnO 2 [5].Other impurities, silicon and aluminium, may come during powder metallurgy steps.The EDX analysis also indicates absence of platinum as impurity, which was found in XRD analysis of MWCNT.During nanocomposite powder preparation, the obtained precipitate black in color was filtered and washed with deionized water and acetone to remove the remaining ions and impurities which may cause removal of platinum from nanocomposite.Table 1 shows the chemical composition of 5 wt% MWCNT/MnO 2 nanocomposite.It is found that 5 wt% contains 4.55 wt% of MWCNT.
Similarly, the 10 wt% MWCNT/MnO 2 composite also shows the presence of some impurities as sulphur (S) and potassium (K) and 15 wt% MWCNT/MnO 2 composite also contains small amount of potassium (K) as impurity.

Mass Density of MnO 2 /MWCNT Nanocomposite.
The mass density is determined using mass and volume of the nanocomposite.Table 2 shows mass density of the nanocomposite.The density analysis shows that the density of the nanocomposite increases with addition of MWCNT from 5 wt% to 10 wt% and then decreases from 10 wt% to 15 wt% as shown in Figure 7.The 15 wt% gives less density as the FESEM analysis shows that the tubes are disorderly arranged.For the sample, several trials of indentation with each load were made and the average values of the diagonal lengths of indentation marks were obtained.The Vickers microhardness number   at each load was calculated using the following equation [15,17]: where  is the applied test load in kgf,  is the average of two indentation diagonal lengths in mm, and 1.854 is a geometrical constant of the diamond pyramid.
(1) Microhardness of 5 wt% MWCNT/MnO 2 Nanocomposite.All the specimens were disc shaped of 15 mm diameter and 1.5 mm thickness.The surface finishes of nanocomposites are high and there was no need for specimen preparation  for microhardness test.A total of twenty-four points on the MWCNT/MnO 2 nanocomposite and hardness at each point were measured in order to get average reading.The microhardness data of 5 wt% MWCNT/MnO 2 nanocomposite specimen is shown in Table 3.The average microhardness of 5 wt% MWCNT/MnO 2 nanocomposite is   = 26.583kgf/mm 2 .Some of the indentation marked by Vickers

Conclusion
In the present study, 5, 10, and 15 wt% MWCNT/MnO 2 nanocomposites were successfully synthesized by using simple solution method.From the detailed investigation of the results obtained, the following conclusions can be drawn: (2) From surface morphology analysis by FESEM, it was confirmed that the MnO 2 nanorods were densely grown on the surface of MWCNT.The structure of MWCNT/MnO 2 nanocomposite is flake-like structure and 10 wt% gives orderly arrangement of tubes in comparison to 5 wt% and 15 wt% of MWCNT.
(3) The mass density analysis shows that the mass density of the nanocomposites increases with addition of MWCNT from 5 wt% to 10 wt% and then decreases from 10 wt% to 15 wt%.
(4) From microhardness analysis, it is clear that the addition of wt% of MWCNT into MnO 2 matrix from 5% to 10% increases the microhardness.By further addition of MWCNT up to 15%, the microhardness  decreases.The result of microhardness of these composites depends highly on their mass density; as the mass density increases, microhardness also increases.
The variation of microhardness at each position indicates nonhomogeneous distribution of MWCNT into MnO 2 matrix.All nanocomposite specimens were brittle and fractured.The adopted solution method results in poor dispersion of MWCNT as MWCNT settled at base slowly cooling to room temperature and causes lower microhardness.The structure of the nanocomposite is porous in nature [9].The porous structure of this nanocomposite gives easy exchange of ions and increases its electrochemical property.Consequently, it decreases the mechanical properties.
inside the furnace before exposure to the atmosphere.The sintered samples are as shown in Figures3(a), 3(b), and 3(c).
. The indentation mark by Vickers indenter at different positions of 10 wt% MWCNT/MnO 2 nanocomposite specimen is shown in Figure 11.The 10 wt% MWCNT/MnO 2 nanocomposite was brittle and fractured due to application of load as shown in Figure 12.Similarly, the average microhardness of 15 wt% MWCNT/MnO 2 nanocomposite is   = 18.93 kgf/mm 2 as shown in Table 5.The indentation mark by Vickers indenter at different positions of 15 wt% MWCNT/MnO 2 nanocomposite specimen is shown in Figure 13.The 15 wt% MWCNT/MnO 2 nanocomposite

Table 5 :
Microhardness data of 15 wt% MWCNT/MnO 2 nanocomposite.From the XRD patterns, tetragonal crystal structures of MnO 2 were confirmed and also the formation of MWCNT/-MnO 2 nanocomposites was confirmed.At different wt% of MWCNT/MnO 2 nanocomposite powder, the highest peaks were observed at 2-theta of 37.605 ∘ .