Influence of Graphene and Silver Addition on Aluminum’s Thermal Conductivity and Mechanical Properties Produced by the Powder Metallurgy Technique

Abstract

The high heat dissipation of high-power electronic equipment has become a major cause of damage, especially the central processing units (CPUs) of computers and other electronic devices. Accordingly, this research aims to improve the thermal conductivity as well as the mechanical properties of aluminum (Al) by mono and hybrid reinforcements of silver (Ag) and graphene (G) so that they can be used for heat dissipation. The structures of the prepared powders were investigated using the X-ray diffraction (XRD) technique. Furthermore, the sintered composites’ microstructure, density, thermal conductivity, mechanical properties, and electrical conductivity were investigated. The results showed that adding Ag percentages led to forming the Ag₂Al phase while adding graphene decreased the crystallite of the milled powder. The SEM results showed that the samples had high densification, which was slightly reduced with increasing percentages of reinforcements. Importantly, Al’s thermal conductivity and mechanical properties were significantly improved due to the addition of Ag and G reinforcements with a slight decrease in electrical conductivity. The highest thermal conductivity was observed a 278.86 W/mK in the sample containing 5 vol.% of Ag and 2.5 vol.% of G, which was improved by about 20.6%. In contrast, the highest microhardness and Young’s modulus were 39.19 HV and 71.67 GPa, which resulted in an improvement of about 30.7 and 17.8% for the sample containing 2.5 vol.% of Ag and 5 vol.% of G when compared to the Al matrix. Based on these promising findings, it is possible to infer that the objective of this study was effectively attained and that the created composites are appropriate for such applications.

1. Introduction

The field of materials science and engineering has seen significant advancements in recent years, particularly with the development of new composite materials. One such composite is produced by the powder metallurgy technique and consists of aluminum with additions of graphene and silver. This composite has garnered significant interest due to its potential for improved thermal conductivity and mechanical properties [1]. Graphene’s exceptional thermal conductivity and mechanical properties make it a strong candidate for improving the performance of composite materials such as aluminum. Furthermore, silver has been shown to enhance thermal conductivity while also improving the mechanical properties of composites [2,3]. Therefore, the addition of graphene and silver to aluminum using powder metallurgy offers a promising strategy for achieving enhanced thermal conductivity and improved mechanical properties [4].

This composite material could have significant implications in various fields, including electronics and aerospace industries where thermal performance is critical for successful operation. The potential to improve the thermal conductivity and mechanical properties of aluminum is significant as it can contribute to the development of materials that are more efficient, durable, and reliable. Moreover, the powder metallurgy technique has several advantages over other manufacturing methods, including the ability to achieve complex shapes and near-net shape components with little or no waste material [5].

It has been observed recently that high-power electronic equipment is developing rapidly, which results in extremely compact component sizes and higher heat flux densities in integrated circuits. Accordingly, heat removal has become a critical challenge for the development of many industrial fields, especially electronic devices, and circuits, and the development of the next generation of heat exchangers will be impossible without the discovery of materials with high thermal conductivity in addition to having high mechanical properties and inexpensive manufacturing costs [6,7]. The performance of heat-dissipating components can be improved using one of two methods. The first method is to try to improve the material’s thermal conductivity, while the second method is to improve the component structure to enhance the heat dissipation capabilities of the component [8]. Aluminum (Al) is considered one of the most promising materials for heat dissipation due to its low cost and lightweight; while its acceptable thermal conductivity (second only to copper (Cu)) and its poor mechanical properties, and therefore, it is necessary to improve these properties together without improving one at the expense of the other in order to be used on a large scale in many applications, especially heat dissipation [9,10,11].

Silver (Ag) is a metal with high thermal and electrical conductivity; thus, when added to Al, it is expected to improve its thermal conductivity and not collapse electrical conductivity. In addition, a new phase, i.e., Ag₂Al, will be formed due to the reaction between Al and Ag, improving Al’s mechanical properties. Graphene (G) is another interesting reinforcement, thanks to its outstanding thermal conductivity, electrical conductivity, and superior mechanical properties. Therefore, G can be widely used in improving not only the mechanical properties but also the thermal conductivity of Al [4,7].

Despite the reported expected benefits of adding these Al reinforcements, the limited wettability of the ceramic particles with the Al melt matrix limits the manufacture of these composites, which is a major hindrance in synthesizing these materials by conventional casting procedures [12,13]. By controlling the particle size, the powder metallurgy (PM) technique provides a homogeneous dispersion of the reinforcement particles in the matrix.

It is well-accepted that the mechanical alloying (MA) process is one of the most advantageous PM methods for producing Al matrix nanocomposites. In order to produce Al matrix nanocomposites by the MA method, various steps are necessary. These steps include blending/milling, compacting and sintering in vacuum/inert gas. One could say that this method outperforms the others in terms of speed, reinforcement, and lower processing temperatures, as well as superb microstructure control, including particle size, morphology, and weight percentage of metal matrix [14,15].

In addition, it allows the dissolution of reinforcement assemblies and the diffusion-based production of alloys starting from pure metals, as well as the production of preforms by reacting with reinforcements on-site [16,17,18]. Previous research has been conducted to improve the mechanical properties of aluminum by adding ceramics, and other research has been performed to improve the thermal conductivity of aluminum by adding copper, graphite, or rare earth elements (RE). However, the novel aspect of our research is that we are adding hybrid reinforcements made of silver and graphite to aluminum in order to obtain composites that have both high mechanical properties and high thermal conductivity. Through the use of the PM process, Al alloy matrix composites were created by adding 5 volume percent of silver and/or 0.5 volume percent of gold. It was determined how the sintered samples were compared in terms of their physical, mechanical, thermal, and electrical characteristics.

2. Experimental Procedure

2.1. Preparation and Characterization of Powders

The as-supplied Al and Ag powders have average particles size of 45 μm and 90 nm which ≈99.5 and 99.9% purity, respectively; it was provided by Sigma Aldrich, while the graphene sheet was with a purity of 99.9%, a sheet thickness of 3–10 nm and provided by Alfa Aesar. The Al matrix composites reinforced with various volume percentages of Ag and G particles are listed in

Table 1. Batch design of the investigated Al/Ag/G samples.

Sample	Composition (vol.%)
	Al	G	Ag
A	100	-----	-----
AG	99.5	0.5	-----
AA	95	-----	5
AGG1	97.25	0.25	2.5
AGG2	97	0.5	2.5
AGG3	94.75	0.25	5

. Additionally, the chemical analysis was performed for Alas seen in

Table 2. Chemical analysis of aluminum.

Element	Mg	Si	Fe	Cu	Zn	Mn	Ti	Cr	Al
(wt.%)	0.08	0.04	0.04	0.06	0.05	0.01	0.02	0.01	balance

. The powders were milled for 25 h in a planetary ball mill with a speed of 400 rpm. The microstructure of Ag and G powder was investigated by transmission electron microscopy (TEM, type JEOL JEM-1230, Tokyo, Japan). The phases of the milled powders were detected using the X-ray diffraction (XRD; Philips PW) analysis. X-ray line broadening (B) for the principle planes was used to determine the crystalline size, lattice strain, and dislocation density using the formulae described in Refs. [17,18].

2.2. Sintering and Physical Properties

The prepared powders were compacted and sintered at 570 °C in argon for 1 h with a heating rate of 5 °C/min. Notably, the obtained discs had a diameter of 15 mm and a thickness of 5 mm. Archimedes’ method measured all samples’ bulk density and apparent porosity.

2.3. Microstructure of the Sintered Samples

The microstructure of the samples was examined by field emission scanning electron microscopy coupled with energy dispersive spectroscopy (FESEM-EDS; Quanta FEG25, Quanta, Tokyo, Japan) after coating with a thin gold film.

2.4. Mechanical Properties

As shown in Ref. [19], the microhardness was measured using (a Vickers tester) according to the ASTM: B933-09 with an applied load of 1.9 N for 10 s and calculated using the equation as follows:

$$Hv=1.854\frac{p}{d^2}$$

(1)

where p is the applied indentation load and d is the measured indentation diagonal.

The compressive strength of the samples was determined at room temperature. The group of elastic moduli was measured using the pulse-echo technique and bulk density of samples. The longitudinal and shear velocities, i.e., V_L and vs. were measured and the Lame’s constants (i.e., λ and μ) were calculated using V_L, vs. and bulk density (ρ) of the sintered specimens according to the formula [20,21]:

$$\mathsf{\lambda }=\mathsf{\rho }\left({\mathrm{V}}_{\mathrm{L}}^2-2{\mathrm{V}}_{\mathrm{S}}^2\right)$$

(2)

$$\mu =\mathsf{\rho }{\mathrm{V}}_{\mathrm{S}}^2$$

(3)

The longitudinal modulus, Young’s modulus, shear modulus, bulk modulus and Poisson’s ratio (i.e., L, E, B, G and ν, respectively) of the sintered specimens were calculated according to the formula [22,23]:

$$\mathrm{L}=\mathsf{\lambda }+2\mu$$

(4)

$$\mathrm{G}=\mu$$

(5)

$$\mathrm{E}=\mu \frac{3\mathsf{\lambda }+2\mu }{\mathsf{\lambda }+\mu }$$

(6)

$$\mathrm{B}=\mathsf{\lambda }+\frac{2}{3}\mu$$

(7)

2.5. Thermal Conductivity

The thermal conductivity tester Hongjin (Dongguan, China) was used to measure the investigated samples’ thermal conductivity at room temperature. This device is based on the transient plane source (TPS) technique, which involves applying a small amount of heat to one side of a sample and monitoring the transient temperature rise on the opposite side using a thin-film thermocouple sensor. This method allows for rapid and accurate measurement of thermal conductivity, with the added advantage of being able to measure samples of various shapes and sizes. Furthermore, owing to its non-destructive testing nature and simple operation, the TPS technique is particularly suitable for use in quality control during material production

2.6. Electrical Conductivity

The electrical conductivity (σ) of the sintered nanocomposites was measured at 30 °C, using the Keithley 6517B system according to the formula [24]:

$$\sigma =\frac{h}{RA}$$

(8)

where R, h and A are the electrical resistance, the diameter of the specimen and the surface area of the specimen, respectively.

3. Results and Discussion

3.1. Microstructure of the Reinforcements Used

Figure 1a,b shows the TEM photos of Ag and G as starting materials. TEM is used to give better insight into the reinforcements’ morphology and particle size. In Figure 1a, the Ag particles show a high degree of agglomeration and a recorded particle size of about 89.2 nm, while the G appears as a thin particle layer (Figure 1b).

3.2. Phase Evolution

The XRD patterns, Figure 2, are correlated with Al, Ag, and G powders in the range of 2θ equal to 20–80°. According to the standard patterns (JCPDS 89-4037, 87-0717, and 75-2078), the crystalline peaks in the XRD patterns obviously indicate the existence of Al and Ag, respectively. On the other hand, G demonstrates no discernible lattice periodicity-related diffraction peaks, demonstrating its amorphous nature. Moreover, the Al and Ag powders exhibit a cubic crystal structure, while G exhibits a rhombohedral crystal structure.

Figure 3 shows the XRD patterns of A, AG, AA, AGG1, AGG2, and AGG3 samples after 25 h of milling. It is evident from the figure that the evolution of the characteristic peaks of the Ag₂Al phase was identified according to (JCPDS 87-0712). It should be noted that this phase appears only in Ag-containing samples due to the interaction of the added Ag and Al. Noteworthy, there are no peaks for the graphene phase because the amounts of added graphene are so low that they are difficult to detect by the XRD technique. It can be said that when G and Ag are added to the base, the peaks lose a bit of their intensity and gain a bit of width. This is because the lattice deformation increases and the grain size gets smaller. For more clarification of the difference in the effect of adding Ag and graphene on the crystal size and lattice strain of Al, the following mechanism can be explained as follows: During milling, the crystal size and lattice strain of powders were changed as a consequence of the repeated deformation, fracturing and welding processes. In the case of adding Ag to Al, it means the formation of a ductile-ductile system at the beginning of milling. In this system, the welding process predominates and particles are rather flattened because of the strong plastic deformation occurring in the early stage of the milling process. Then, the crystal size decreases and lattice strain increases since the fracturing predominates in the milling process and particles appear to be rather uniform in size. In contrast to the addition of Ag, the addition of graphene sheet means the creation of a ductile–brittle system that is characterized by a uniform dispersion for graphene sheet (a brittle phase) in the Al matrix (ductile phase). This system includes three stages. First, a deformation occurs for ductile particles and fragmentation for brittle particles. Second, as the ductile particles (i.e., Al) start to weld, the brittle particles (i.e., graphene sheet) come between two or more ductile particles at the instant of ball collision. Finally, the composite particles are obtained when the graphene sheet situate at the interfacial boundaries of the welded Al particles [25,26,27].

Due to these noteworthy changes, Al’s crystal size and lattice strain were estimated and listed in

Table 3. The crystal size and lattice strain of the milled samples.

Sample Code	Crystal Size (nm) ± 0.1	Lattice Strain (%) ± 0.001
A	18.04	0.4813
AG	16.62	0.5157
AA	12.86	0.6450
AGG1	15.28	0.5544
AGG2	15.06	0.5611
AGG3	12.58	0.6619

after adding various Ag and G reinforcements volume ratios.

3.3. Physical Properties

After being sintered at 575 degrees Celsius for one hour in argon, the impact of the amount of Ag and G present on the physical characteristics of aluminum and its composites is shown in Figure 4. According to the findings, the incorporation of Ag reinforcement had a beneficial impact on the bulk density, in contrast to the incorporation of G reinforcement, which had no such effect. In addition to this, the incorporation of reinforcements results in an increase in the apparent porosity of the composite material samples. This finding may be explained with the assistance of the XRD data, which revealed that the addition of Ag generates a higher density phase (Ag₂Al 7.56 g/cm³) in comparison to the density of Al (2.70 g/cm³), which resulted in an increase in the density of the composite based on the rule of the mixing. This outcome is in contrast to the addition of G since G has a lower density than Al does (around 2.26 g per cubic centimeter). In addition, the melting point of G is quite high, which decreases the rearrangement of particles that takes place during the sintering process. This results in an increase in porosity and a reduction in the density of the sintered samples [28]. When Ag is added, a phase known as Ag₂Al is produced. Because of this, the composites will now have an increased number of boundaries, which will result in a marginal rise in porosity.

3.4. Microstructure of the Sintered Samples

The findings of the research indicated that the incorporation of graphene and silver into aluminum led to the formation of a more refined microstructure, with grain sizes that were much smaller than those seen in the samples of pure aluminum. In addition to this, the addition of graphene and silver nanoparticles led to an increase in the density of the aluminum, which ultimately led to an improvement in the material’s mechanical characteristics. Specimens AGG2 and AGG3 were sintered for one hour at 570 degrees Celsius, and the resulting SEM-EDS images may be seen in Figure 5a,b. During densification, it was clearly seen that the nanocomposites under investigation reacted in an expected manner. In addition, Ag₂Al and G phases are located at the grain boundaries of the Al matrix. This is significant when taking into consideration that the sample that contains 2.5 vol.% Ag (AGG2) has a very uniform distribution of the Ag₂Al phase. However, it is important to note that this distribution lessens as the amount of Ag increases up to 5 vol.%. (AGG3). According to the results of the image mapping, the Ag₂Al and G phases were spread uniformly over the whole Al matrix, as shown in Figure 6. In addition to this, it was pointed out that the sample porosity seemed to be rather low based on the figure. Nevertheless, the sintering temperature of 570 degrees Celsius promotes diffusion throughout the heating process, which ultimately results in increased densification or the achievement of almost complete density.

3.5. Mechanical Properties

Figure 7, Figure 8 and Figure 9 display the effect of adding individual/combination of Ag and G reinforcements on the mechanical properties, including microhardness, ultimate strength, and elastic moduli of Al matrix composites. It is noticeable from the previous figures that the mechanical properties are improved by adding Ag and G. At the same time, it is clear that the effect of G is much greater than Ag in increasing the mechanical properties. For example, the microhardness of the Al matrix is 27.25 HV. After adding mono-reinforcement (AG and AA samples), the microhardness increased to 37.13 and 31.48 HV, respectively, while the AGG1, AGG2 and AGG3 samples enhanced by hybrid reinforcements recorded a hardness of 34.40, 38.19 and 35.61 HV , as shown in Figure 7.

It is worth noting that the slight improvement in the mechanical properties of Al by adding Ag is due to the formation of the solution strengthening (Ag₂Al) phase in addition to increasing the boundary inside the composite. On the other hand, the remarkable improvement in the mechanical properties of Al due to the addition of G can be attributed to several factors. First, G has a very high hardness compared to Al and its homogeneous distribution within the composites. Second, grain boundary pinning by G reinforcements is the primary cause of grain refining. The presence of G particles inhibits grain development during the sintering. Due to the finer grain sizes produced by the grain boundary pinning, there is a high volume of grain boundaries, which improves the composite’s strength because the dislocation motion is constrained across the Al matrix and graphene interface [29,30]. Figure 8 and Figure 9 are shows the improvement in the mechanical properties of Al by the addition of mono- or hybrid-reinforcements has been previously studied by Nourouzi et al. [31], Youness et al. [32], Abu Shanab et al. [33], Kumar et al. [34] and Ashok et al. [35]. In these studies, it is commonly emphasized that reinforcing particles such as G, aluminum oxide (Al₂O₃), zirconium dioxide (ZrO₂), titanium dioxide (TiO₂), silicon carbide (SiC), tantalum carbide-niobium carbide (TaC-NbC), and boron nitride-boron carbide (BN-B₄C) have a significant effect on improving the mechanical properties. Marioara et al. [36] and Guo et al. [37] reported an improvement in the mechanical properties of the Al-Mg-Si and Al-Mg alloys after the addition of traces of Ag particles.

3.6. Thermal Conductivity

The thermal conductivity of the Al/Ag-G nanocomposites with varying amounts of Ag and G can be shown in Figure 10. When compared to a pure aluminum matrix, the findings show that the thermal conductivity of the synthetic nanocomposite that is reinforced by mono and hybrid particles of Ag and G is much higher than that of the pure aluminum matrix. The thermal conductivity of the aluminum matrix is 231.15 W/m K, whereas the thermal conductivities of the prepared composites, namely AG, AA, AGG1, AGG2, and AGG3, are 247.39, 274.22, 258.67, 260.83, and 278.86 W/m K. These values represent an improvement of approximately 7.02, 18.63, 11.90, 12.84, and 20.64% respectively when compared to the thermal conductivity of the matrix. This boost in thermal conductivity of the nanocomposites by adding reinforcements is due to the thermal conductivity of the employed Ag and G reinforcements having a larger thermal conductivity than that of the Al matrix (239 W/m K) [38]. The thermal conductivity of the used Ag and G reinforcements was measured at 410 and 3000 W/m K, respectively. Producing Al-G composites allowed Saboori et al. [7], Pradhan et al. [4], and Wei et al. [39] to investigate the impact of G on the thermal conductivity of the Al matrix. These investigations have shown that the G reinforcement has a beneficial impact on thermal conductivity.

3.7. Electrical Conductivity

The electrical conductivity of Al matrix composites reinforced with mono- and hybrid-reinforcements are shown in Figure 11. The figure indicates that the Al matrix exhibits slightly lower electrical conductivity after reinforcement with Ag and/or G. These results agree, to a large extent, with previous work [40]. The electrical conductivity of the sintered samples AA, AG, AGA1, AGA2 and AGA3 are 29.10 × 10⁵, 28.93 × 10⁵, 28.99 × 10⁵, 28.54 × 10⁵ and 28.15 × 10⁵ Ω/cm², which decreases around 3.00, 4.28, 3.91, 6.26 and 8.72%, respectively, when compared with the conductivity of the unreinforced Al matrix which record 29.70 × 10⁵ Ω/cm². This decrease in the electrical conductivity is closely related to the value of the electrical conductivity of Ag and G (≈2.8 × 10⁵ and 8 × 10⁵ Ω/cm², respectively), which is much lower than that of Al (33.3 × 10⁵ Ω/cm²). Therefore, when they are added, they cause a slight decrease in the electrical conductivity of Al. Another reason for the decrease is that a metal’s electrical conductivity generally depends on its internal electrons’ movement. Therefore, the Ag₂Al phase and G increase the number of boundaries within the composites, and these boundaries cause electrons to scatter and thus lead to a decrease in electrical conductivity [41,42,43,44,45]. Taha et al. [33] investigated the effect of adding different contents of G particles up to 2 wt.% on the electrical conductivity of Al alloy. The results showed that adding the G particles causes a slight decrease in electrical conductivity. Khoshaim et al. [46] produced the Al7075-based hybrid composites reinforced with GNPs/BN/VC ceramics particles by a friction stir process. The result revealed a remarkable decrease in the electrical conductivity of the composites.

4. Conclusions

In this work, the production of aluminum (Al) based mono and hybrid composites with better thermal conductivity and mechanical properties using silver (Ag) and graphene (G) as reinforcements using the powder metallurgy (PM) method was shown. After milling, the Ag₂Al phase was formed by adding silver reinforcement particles, while graphene positively affects grain refinement. The results indicated that the addition of both Ag and G to the Al matrix had a positive effect on each of the mechanical properties and thermal conductivity and negatively on the electrical conductivity. The maximum improvement in thermal conductivity is 20.6% for the composite that contains 0.25 vol.% G and 5 vol.% Ag (AGG3). In comparison, the greatest improvement in microhardness is 43.8% for the composite containing 0.5 vol.% G and 2.5 vol.% Ag (AGG2) compared with the thermal conductivity and microhardness of the Al matrix. Moreover, the effect of adding Ag and G on Al’s electrical conductivity seems insignificant, as the maximum decrease in conductivity is 5.2% for the sample (AGG3).