Morphological: Optical, and Mechanical Characterizations of Non-Activated and Activated Nanocomposites of SG and MWCNTs

Abstract

Nanocomposites of silica gel (SG) and multiwalled carbon nanotubes (MWCNTs) of relatively low concentrations (0.25, 0.50, and 0.75 wt%) were characterized before and after annealing. Adsorption is a surface phenomenon, and based on this, the morphology of the composites was investigated by scanning electron microscopy (SEM). The produced images show that the MWCNTs were embedded into the silica gel base material. Fourier transform infrared (FTIR) transmittance spectroscopy showed that MWCNTs were not functionalized within the matrix of silica gel and MWCNT composites. However, after annealing the composites at 400 °C for 4 h in air, evidence of activation was observed in the FTIR spectrum. The effects of the embedding of MWCNTs on porosity, specific surface area, and pore size distribution were studied using Raman spectroscopy. The Raman spectra of the prepared composites were mainly dominated by characteristic sharp scattering peaks of the silica gel at 480, 780, and 990 cm⁻¹ and a broad band centered at 2100 cm⁻¹. The scattering peaks of MWCNTs were not well pronounced, as the homogeneity of the composite is always questionable. Nanosizer analysis showed that at 0.25 wt%, the distribution of MWCNTs within the silica gel was optimal. Vickers hardness measurements showed that the hardness increased with the increasing weight percent of MWCNTs within the composite matrix, while annealing enhanced the mechanical properties of the composites. Further studies are required to investigate the pore structure of silica gel within the matrix of MWCNTs to be deployed for efficient cooling and water purification applications.

1. Introduction

The identity of the discoverers of carbon nanotubes (CNT) is a subject of some controversy. For years, scientists assumed that Sumio Iijima had discovered CNTs in 1991 [1], however, researchers had repeatedly reported their observation of CNTs and Multi Walled Carbon Nanotubes (MWCNTs) for decades, going all the way back to 1952 [2,3]. CNTs were first synthesized by the members of the Institute of Physical chemistry and Electrochemistry of Russian Academy of Sciences, Radushkevich and Lukyanovich [2]. The contribution of Iijima was significant, as the main properties of CNT were systematically investigated by his research group, where they observed MWCNTs as a by product of fullerene synthesis and investigated their characteristics using various techniques, including TEM [3].

CNTs have played a crucial role and have been widely utilized in a range of scientific disciplines, including chemistry, materials science, electrical engineering, and physics. The potential applications of CNTs have been investigated extensively, as they are appealing for a variety of technological and scientific areas [4,5]. Multiwalled carbon nanotubes (MWCNTs) are now becoming increasingly attractive from a practical point of view, due to their greater diameter and reduced strain in addition to their economic value. Moreover, they have superior mechanical properties, electrical conductivity [4], and high heat transfer performance [6]; based on these properties, they have been deemed appropriate for use as reinforcing fillers for high-performance polymer nanocomposites [7,8].

Silica gel is one of the most common porous materials used in commercial adsorption chillers because it has a porous structure and is nontoxic, nonpolluting, and abundant [6,7]. It has been observed that introducing MWCNTs, with their high surface area along with other physical properties, to the porous structure of SiO₂ results in the enhanced adsorbing performance of cooling chillers [8]. Additionally, it is worth mentioning that nanosilica (nano-SiO₂) has outstanding qualities such as its tiny particle diameter, higher surface area, and consequently higher activity and superior mechanical properties [9,10].

It is of great interest to study the morphology of MWCNT/SiO₂ composites, and several studies have presented their observations on the physical structures of prepared composites [11,12] using SEM. SEM micrographs showed that MWCNT/SiO₂ composites accumulate spherical structures with an average diameter of 226 nm. Energy-dispersive X-ray spectroscopy (EDX) was used to determine the elemental proportions of the composites, whereby MWCNT/SiO₂ nanocomposites were found to be composed of oxygen (51.38 wt%), silicon (40.74 wt%), and carbon (8.18 wt%). Fourier transform infrared transmittance spectroscopy (FTIR) was used to investigate the structural properties of MWCNTs and MWCNT/SiO₂. More detailed SEM investigations [12,13,14] have reported that the majority of MWCNTs tend to interweave with one another; however, the surface characteristics of SiO₂/MWCNTs are distinct. Moreover, multiple SiO₂ nanoparticles are densely and consistently dispersed on the surface of MWCNTs, significantly improving the roughness of the surface.

The EDX data of various MWCNTs demonstrate that, when they are coated with a nano-SiO₂ layer, the C content is dramatically reduced, but the O and Si content is increased, as predicted. Raman spectroscopy was used to better characterize the surface properties of MWCNTs and SiO₂/MWCNTs. The characteristic scattering Raman bands of carbonaceous materials, namely the D-band and G-band, were observed at approximately 1343 and 1580 cm⁻¹, respectively. All of the results indicated that the modification method has a reasonably benign effect on the ordered crystal structure of MWCNTs and is advantageous for the preservation of their superior mechanical capabilities [15]. Functionalizing MWCNTs has been part of SEM studies [16], which have shown a uniform coating of amorphous SiO₂ [17] with a thickness of 10 nm. Additionally, elemental mapping of the composites using EDX showed signals of C, Si, and O, which demonstrates that SiO₂ is uniformly deposited onto MWCNTs. It has been observed that MWCNT/SiO₂ disperses well in water, whereas pristine MWCNTs settle to the bottom [16].

The current study was conducted as part of an ongoing effort to characterize silica gel composites, where the lowest studied concentration of MWCNTs previously studied was 1 wt%. However, this study looks closely at the effect of relatively low concentrations of the nano-additives of MWCNTs (0.25, 0.5, and 0.75 wt%), thereby avoiding the accumulation of MWCNTs observed in previous studies. The morphology and optical properties, along with the effect of annealing on the mechanical properties, were determined to investigate the possible functioning of the composites. The study will allow us to determine how to further employ the current composites or, otherwise, develop an efficient cooling system and other related applications.

2. Chemicals and Methodology

The average width was about 100 nm, and more than 75% of the particles were around 10 to 20 µm with an aspect ratio of more than three, 12–16 nm in diameter and the carbon purity of approximately 90%. The nanotube compounds are generated by merging MWCNTs in a silica gel matrix at three weight percentages (0.25, 0.50, and 0.75 wt%). This produces translucent nanotube compounds following gelation. When treated at 7.7 GPa pressure and 25 °C, this results in compounds that are thick and tough, with no crack formation. Such an approach was earlier described in some detail by de Andrade et al. [18]. Here, the materials were characterized using scanning electron microscopy (SEM) along with energy-dispersive X-ray spectroscopy (EDX), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy (RS), zeta potential (ZP) analysis, and microhardness testing. Annealing was carried out at 400 °C for 4 h in an air environment in a Carbolite annealing furnace (Chamber Furnace CWF 1200, Cambridge, UK), whereby the rate of temperature increase was 20 °C/min, and the cooling rate was sensibly slow at less than 2 °C/min.

2.1. Materials: Silica Gel (SG) and Multiwalled Carbon Nanotubes (MWCNTs)

Granules of Fuji RD silica gel were purchased from Fuji Silysia Chemical Ltd. (Kasugai Aichi, Japan) and had the specifications described in

Table 1. Silica gel specifications.

Items		Unit	Spec	Test Method
Adsorption Capacity	20% RH	%	10 ↑	KS T 1084
	50% RH		25 ↑	KS T 1084
	90% RH		35 ↑	KS T 1084
Bulk Density		g/cc	670–750	KS T 1084
pH Value		-	4.0–6.0	KS T 1084
Specific Resistance		Ω·cm	3000 ↑	KS T 1084
Moisture Content		%	2 ↓	KS T 1084
Surface Area		m2/g	670–770	BET
Pore Volume		mL/g	0.35–0.55	BET
Pore Volume per Gram		A	20–28	-
Particle Size
Size	20 mesh over	%	5.0 ↓	KS T 1084
	40 mesh under	%	5.0 ↓	KS T 1084

. Multiwalled carbon nanotubes (MWCNTs) were obtained from (SkySpring Nanomaterials Inc., Leeds, UK) (product #0553CA, Lot #0553–090916). The purity of the MWCNTs was above 95%, with average diameters ranging between 10 and 20 nm.

2.2. Preparation of SG with MWCNTs Using Mechanical Dispersion

Silica gel/MWCNT composites were prepared by adding MWCNTs to silica gel using the mechanical dispersion method [19]. The samples were prepared with different fractions of nanoparticles, corresponding to 0.25% (SG-1CNT), 0.5% (SG-2CNT), and 0.75% (SG-3CNT) by mass (

Table 2. Concentrations of various prepared composites of silica gel and MWCNTs and the amounts used in preparation.

No	Silica Gel	MWCNTs
1	4.9875 g (99.75 w/w %)	0.0125 g (0.25 w/w %)
2	4.975 g (99.5 w/w %)	0.025 g (0.5 w/w %)
3	4.9625 g (99.25 w/w %)	0.0375 g (0.75 w/w %)

). Different proportions of MWCNTs were weighed and added to deionized water. Magnetic stirring was applied for 1 h, and the nanofluid was then added directly to the silica gel, followed by sonication for 3 h (Figure 1).

3. Characterization Tools, Results, and Discussion

3.1. Scanning Electron Microscopy (SEM)

A scanning electron microscope uses electrons accelerated by tens to hundreds of kilovolts as the imaging component. A high resolution arises from the shorter wavelength ${\lambda }_e$of electrons, which is dependent on the electron acceleration voltage $V_0$, given by the following equation:

$${\lambda }_e\left(A\right)=\sqrt{\frac{150}{V_0\left(\mathrm{kV}\right)}}$$

(1)

The magnification of the microscope $M$ can be obtained from the following ratio:

$$M=\raisebox{1ex}{$I$}\!\left/ \!\raisebox{-1ex}{$S$}\right.$$

(2)

where $I$ is the trace length on the display image and $S$ is its length on the sample. Primary electrons interact with the sample, producing several signals. One possible interaction is with the electrons within the shell of the target atoms to gain energy from the collision, break away, and then be detected. These are called secondary electrons, which have low energy.

$$R=0.0552V_0^{1.67}/\rho$$

(3)

where $R$ is the average electron range in µm, $V_0$ is the applied voltage in kV, and $\rho$ is the density of the material in g/cm³. Because secondary electrons have low energy, only those very close to the surface are able to reach the detector, which is held at a positive voltage to draw them away from the sample. The intensity variations of the secondary electrons are the basis for the contrast in images of various features in the sample [20].

The samples were mounted on a flat surface with a conducting material (adhesive tape). This establishes a good electrical path to the ground, which is a requirement to prevent the buildup of negative charges delivered by the electron beam. On the other hand, electrically nonconductive specimens require evaporating treatment in general so as to make them conductive. By directing the focused beam of electrons across the sample surface and identifying secondary or backscattered electron signals, SEM produces detailed, high-resolution pictures of the sample. Additionally, an Energy-Dispersive X-Ray Analyzer (EDX or EDA) is utilized to determine the elements and their quantitative composition.

EDX examines the distribution of energy and intensity of X-rays created by electron beam excitation on the sample surface, from which the composition of elements throughout the specified area covered by the electron beams can be estimated with a high degree of accuracy [21]. As a result, this approach to compositional characterization is extremely effective and advantageous [22].

Figure 2 presents an SEM preview of MWCNTs at a 1 μm scale, with close-up images that allow us to estimate that their diameter is no less than 12 nm. The tangled appearance does not allow us to measure their length. SEM images of the various composites of MWCNTs of 0.25, 0.5, and 0.75 wt% show a randomly oriented network of nanotubes. The surfaces of the MWCNTs support the intermittent distribution of silica gel particles.

The SEM images of the pristine separate MWCNTs (Figure 2) and silica gel (Figure 3) and of the composites (Figure 4, Figure 5 and Figure 6) show the topography of the composites in relation to their original topography. The SEM micrographs of the composites show that at relatively low concentrations, the MWCNTs connect the isolated particles of silica gel (see Figure 4, Figure 5 and Figure 6). Silica gel was found to be fully attached to the MWCNTs, and in some cases it fully surrounded them, particularly at the higher concentration (0.75 wt%; Figure 6) [23,24]. As can be seen in Figure 3, with silica gel particles with an average size above 500 μm, as the concentration of MWCNTs increased, the incorporation of silica gel particles within the lengthy MWCNTs increased (see Figure 4, Figure 5 and Figure 6). These observations agree with the detailed SEM investigations undertaken recently by Li et al. [12]. It has been reported that most MWCNTs tend to interweave with one another; however, the surface characteristics of SiO₂/MWCNTs are distinct. EDX analysis of silica gel showed that there were two main elements, oxygen (O) and silicon (Si). The concentrations of these elements were 58 to 65% and 30 to 35%, respectively, in addition to minor concentrations of nitrogen (N), boron (B), and aluminum (Al).

3.2. FTIR Measurements

The samples were analyzed using Fourier transform infrared spectroscopy (FTIR) with a PerkinElmer Spectrum GX device (Hopkinton, MA, USA), which has a spectral resolution greater than 0.15 cm⁻¹. FTIR analysis is a technique that utilizes infrared light to examine test samples and determine their chemical properties.

The FTIR instrument delivers infrared radiation with a wavelength of around 10,000 to 100 cm⁻¹ through the material, with some of the radiation being absorbed and some passing through. The sample molecules convert the absorbed radiation to rotational and/or vibrational energy. The resulting signal at the detector is a spectrum, typically between 4000 and 400 cm⁻¹, which represents the sample’s molecular fingerprint. Because each molecule or chemical structure generates a unique spectral fingerprint, FTIR analysis is an excellent tool for chemical characterization. The absorbance intensity is proportional to the amount of functionality present in the material [25].

FTIR measurements, as shown in Figure 7, confirmed that the MWCNTs were not activated [26] within the matrix of silica gel composite [27]. For pristine silica gel, MWCNTs, and MWCNT/SiO₂ composites, the FTIR analysis clearly reveals the structure of the latter. Previous studies examined the transmittance spectrum of the composites, which also confirmed that the MWCNT/SiO₂ composite structures could be successfully synthesized using a similar method [11]. Moreover, the functionalization of these MWCNTs enhanced the transmittance FTIR peaks [28].

FTIR of the annealed composites of various concentrations showed various absorption peaks and shoulders at wavenumbers between 500 and 1300 cm⁻¹, confirming the functionalization of the composites at various concentrations of SG occurred physically through annealing. Comparing the FTIR transmittance spectra of the composites before and after annealing (Figure 7 and Figure 8, respectively), the presence of the multiple peaks and shoulders in the region of wavenumber between 500 to 1750 cm⁻¹ confirms functionalizing MWCNTs [29,30].

3.3. Raman Spectroscopy

The micro-Raman spectrometer offers a critical means for investigating the fundamental properties of flaws in composite systems. The regulated use of flaws to modify the chemical and physical characteristics of graphene, comparable to the silicon industry’s use of doping, ought to be expanded upon [29].

In brief, a Raman microscope (RM2000) from Renishaw UK Sales Ltd. (New Mills, Gloucestershire, UK) consists of a 25 mW air-cooled laser source and a mirror that reflects the laser beam onto the notch filter and into an optical microscope to deliver the beam to the sample and then transmit the Raman signal to the spectrometer. The notch filter blocks the scattered laser radiation but allows the Raman signals to go through to the spectrometer. The measurements were performed in backscattering configuration, which has become standard for Raman spectroscopy measurements.

The size of the focal plane spot is determined by excitatory laser wavelength λ and the objective’s numerical aperture $NA$; the diameter of spot $D$ is given by the following equation [20]:

$$D=\frac{1.22\lambda }{NA}$$

(4)

Raman spectroscopy was used to investigate possible changes in the scattering spectra of pristine silica gel and pristine MWCNTs and their composites. Further investigation of the possible structural phase transformations within Vickers residual indentations made in silica gel and MWCNT composites is suggested. A DPSS laser beam was used, and a diode-pumped solid-state laser was employed to excite the sample. The laser power delivered at the sample point in all experiments was 9 mW, as measured with a calibrated power meter (FieldMaster GS, Coherent, Inc., Santa Clara, CA, USA). The specifications were as follows: diameter of the laser spot on specimen surface: ~2.1 µm; aperture: 25 µm pinhole; grating: 900 lines/mm; estimated resolution 2.7–4.2 cm⁻¹; range limit for wavelength: 50–3500 cm⁻¹. The peak positions were measured to 1 cm⁻¹ accuracy. The acquisition time varied from a few seconds to a few minutes depending on the neutral density filters used [30].

The observed Raman scattering peaks of silica gel (see Figure S1 in Supplementary Materials) are in accordance with those in previous works [31,32]. The Raman bands of silica were observed at 495 and 605 cm⁻¹ and showed Si–H rocking vibrations.

When Raman analysis of CNTs is conducted, three sharp peaks are routinely observed: the tangential stretching G mode (1500–1600 cm⁻¹), the D mode (1350 cm⁻¹), and the radial breathing modes (RBMs) (100–400 cm⁻¹). These characteristic scattering peaks were assigned by Rao et al. [33] and later summarized by Akbar et al. [34]. The ratio of the intensities of the D and G bands can be used to evaluate the disorder density of the nanotube walls. The Raman spectrum of silica gel shown in Figure 9 exhibits sharp peaks at 480, 780, and 990 cm⁻¹ and a broad band centered at 2100 cm⁻¹ in addition to two superimposed peaks at 3250 and 3400 cm⁻¹. The Raman spectrum of the as-received MWCNTs was fitted using two Lorentzian peaks at 1250 cm⁻¹ (D band) and 1750 cm⁻¹ (G band) and a Gaussian peak at 2700 cm⁻¹ (D0 band), superimposed on a broad band. Figures S2–S4 show scattering peaks characterizing the silica gel, where there is no clear presence of scattering peaks of MWCNTs, particularly at 0.25 wt%. The Raman scattering spectra of 0.5 and 0.75 wt% show traces of the broad band at 2000 cm⁻¹, one of the characteristics of the amorphous bands of MWCNTs [2], or it might from amorphous SG as adding MWCNTs could reduce crystallization of SG as it has been reported in other polymers [32,33]. Figure 9 summarizes Raman spectra of the previous samples under consideration, pure SG, pure MWCNTs, the composites of various MWCNTs weight percents.

3.4. Particle Size Distribution

The electrophoretic mobility measurements were performed using a Zetasizer Nano ZS (Malvern Panalytical Ltd., Malvern, Worcestershire, UK) working in the particle size range from 0.6 nm to 6 μm and using a 4 mW He–Ne (633 nm) laser [35]. We obtained true measurements of the size distribution, particle concentration, particle charge, and charge distribution. We carried out complex analyses of heterogeneous samples and performed real-time measurements of particle properties to assess subtle changes over time with high precision for quality control and the assessment of product stability. The electrophoretic mobility measurements were performed using a dynamic light scattering (DLS) Zetasizer Nano ZS instrument (Model ZEN3600, Malvern Instruments Ltd., Cambridge, UK), working in the particle size range from 0.6 nm to 6 μm and a 4 mW He–Ne (633 nm) laser with a scattering angle of 173°. Measurements were performed at room temperature (300 K). The distribution of MWCNTs within the silica gel matrix and MWCNT composites were characterized with the Zetasizer, which provided information about the MWCNTs’ nanoparticle size distribution, dispersion properties, stability, and propensity. Particle size analysis is a key element because many properties of nanomaterials are size-dependent. These data of aggregation are important for designing and tailoring new nanomaterials for specific applications. The distribution of MWCNTs in 0.25 wt% concentration is centered, approximately symmetrically, around 250 nm. A higher intensity of distribution among the various MWCNT concentrations was seen at 0.25 wt%. Improved dispersion and homogeneity on the fractured surface at 0.25 wt% were observed with a higher tendency for reproducibility. Generally, MWCNTs remain agglomerated, number or intensity is a beneficial parameter when MWCNTs are in dispersed form.

3.5. Vickers Hardness Test

High pressure occurs at the centers of planets and in explosions. High pressure can also be applied to laboratory samples in a controlled way using high-pressure instruments, such as a diamond anvil cell or an indentation test. The Vickers hardness test was conducted using a four-sided pyramidal diamond indenter with an angle of 136° between two opposite faces, as shown in Figure 10.

Vickers hardness $H_{\nu }$ is defined as load $P$ divided by surface area $A$ of the indentation. From the geometry of the shape (see Figure 10), we derived surface area $A$ of an indentation as follows:

$$A=\frac{a^2}{sin\left(\frac{136}{2}\right)}$$

(5)

$$\because d^2=2a^2$$

(6)

$$\therefore A=\frac{d^2}{2sin\left(\frac{136}{2}\right)}$$

(7)

$$H_v=1854.4\times \frac{p}{d^2}$$

(8)

These composites of silica gel and MWCNTs were characterized to evaluate the effect of MWCNTs on the composites’ mechanical characteristics [36,37,38]. Indentation tests (see Figure 10) were performed on these composites to obtain the variation in hardness by varying the composition of MWCNTs in the silica gel. The hardness of silica gel and MWCNTs is illustrated in Figure 11, along with the effect of adding various weights of MWCNTs on the overall hardness when compared to pure silica gel base substance. Vickers hardness measurements were undertaken using the Reichert Microhardness tester. There are three measurements were made on each sample at the specified load, the diameters of residual indentations were measured. The corresponding hardness was calculated then the average was taken. The results of the Vickers hardness analysis indicate that when silica gel was supported by 1CNT, 2CNT, and 3CNT, the hardness increased as the concentration of MWCNTs increased [38].

As can been seen in

Table 3. Vickers hardness values (Hv), at 200 gf load and 15 s loading time, of various composites of SG/MWCNTs (CN1, CN2, and CN3) with different concentrations of MWCNTs (0.25, 0.50, and 0.75 wt%) as is (before annealing), and after annealing at 400 °C for 4 h.

Composite	Vickers Hardness Value (Hv) in GPa
	Before Annealing	After Annealing
SG/MWCNTs (0.25 wt%)	12.04	17.57
SG/MWCNTs (0.50 wt%)	21.95	24.47
SG/MWCNTs (0.75 wt%)	29.26	31.02

, annealing increases composite hardness, which can possibly be attributed to the developed cross-linked structures and good dispersion of CNTs in the composite matrix [39]. Annealing might suppress the formation of voids and consequently improve the hardness or mechanical strength and lower MWCNT aggregation [40].

In summary,

Table 4. Summary of characterizations of nanocomposites including MWCNTs in the current study and in previous work.

Material	Functionalization	Characterization Techniques	Observations	Applications and Outcomes	References
SG/MWCNTs	Annealing (physical functionalization)	(1) SEM and EDX (2) FTIR (3) Raman spectroscopy (4) Micro-hardness test	(1) MWCNTs incorporated within SG matrix at low concentrations (2) Annealing functionalizing composites (3) Dominated mainly by MWCNTs (4) Hv increased with MWCNTs and annealing	Annealing at 400 °C for 4 h was sufficient for optimum results and improved optical and mechanical properties	Current study, 2021
SiO2-MgO coated MWCNTs	Chemical functionalization via sol–gel Physical functionalization, annealing in air at 400 °C for 4 h	(1) Raman spectroscopy (2) SEM	(1) Scattering peaks at 1345, 1574, and 2685 cm−1 corresponding to D, G, and G’ modes. (2) Incorporated in different types of polymer matrices, kept polymer parts together	Dispersion of MWCNTs increased in polymer matrices	Nemeth et al., 2019 [40]
CNTs/Se80Te16Cu4 glassy composites	CNTs incorporated into Se80Te16Cu4 glassy matrix via melt-quenched technique	(1) Vickers microhardness test (2) SEM	(1) Hardness increased with increased annealing temperature (2) Thermal annealing with successive increase in temperature could improve dispersion of CNTs	Greater mechanical strength	Upadhyay et al., 2018 [39]
Ti3AlC2-MWCNTs composites	Hot pressing method in Ar atmosphere at 1300 °C under a pressure of 20 MPa.	(1) Vickers Hardness (2) SEM	(1) Hardness increased with increased content of MWCNTs up to 2 wt% (2) MWCNTs enwrapped in Ti3AlC2 grains were observed in SEM micrographs	Mechanical properties of Ti3AlC2 ceramic were greatly enhanced by MWCNTs	Wang et al., 2016 [41]
MWCNTs/MIL-100(Fe) composite	Carboxylic acid-functionalized MWCNTs	(1) FTIR (2) SEM	(1) Chemical integrity of structure preserved upon incorporation of MWCNTs using proposed synthesis method (2) MWCNTs can also be seen implanted in surrounding MIL-100(Fe) matrix	Adsorption and cyclic stability	Qadir et al. 2016 [8]
SiO2/MWCNTs	SiO2/MWCNTs prepared by sol–gel method	(1) FTIR (2) SEM	(1) Absorbance peaks at 1065, 964, and 793 cm−1 observed (2) Better dispersion of MWCNTs in matrix	Optimized property needed in electronic packaging applications	Li et al., 2016 [42]

shows several studies on composites of base materials (including SG) and MWCNTs, where chemical and/or physical functionalization was undertaken along with several characterization techniques for possible application along with the current study. The hardness of the composites of SG and MWCNTs increased with increased contents of MWCNTs, in good agreement with previous work [39,41]. Previous studies also reported that as the temperature of annealing increases hardness increases [39]. Udah et al. [39] reported that the hardness of CNTs/Se₈₀Te₁₆Cu₄ glassy composites at 400 °C to be 35 GPa. Wang et al., [41] showed that the hardness of Ti₃AlC₂-MWCNTs composites at a high percent weight of MWCNTs (2 wt%) without annealing is 37.4 GPa.

4. Conclusions and Future Work

The study examined and characterized composites of silica gel and MWCNTs of 0.25, 0.50, and 0.75 wt%. Based on the morphological, optical, and mechanical characterizations and annealing, the following conclusions can be made:

(1)

SEM micrographs and EDX showed that silica gel and MWCNT composites were mixed, with the lengthy MWCNTs tangled and the silica gel particulates immersed between the nanotubes. Low concentrations of MWCNTs are sufficient to produce balanced composites.

(2)

Annealing at 400 °C confirmed the functionalization of the composites, as shown by the FTIR measurements.

(3)

Raman spectra were mainly dominated by the base materials, due to the presence of MWCNTs in very low concentrations.

(4)

The Nanosizer showed that the best distribution of MWCNTs in the composite matrix occurred at 0.25 wt%, with high reproducibility rates.

(5)

As the percent weight of MWCNTs increased within the matrix of silica gel composites, the Vickers hardness value increased. Annealing further improved the strength of the composites.

(6)

Further studies are required to investigate the pore structure of silica gel within the matrix of MWCNTs, as the enhancement of cooling and water purification is very much associated with these micro- and nanosized particulates.

Finally, these findings should encourage further studies of these composites of silica gel and MWCNTs to be deployed for various designed applications.