Development of composite material aluminum-carbon nanotubes with high hardness and controlled thermal conductivity

Introduction

Currently, carbon nanotubes (CNTs) are one of the most popular nano-additives in aluminum composites production. The main reasons for their popularity as nano-additives are excellent properties of CNTs, for example, high strength properties ¹ and thermal conductivity (3000 W/(m×K)) ² while relatively low cost due to large-scale production. Moreover, CNTs (like other carbon nanomaterials) are significantly changing properties of the basic material while adding in relatively small amounts up to several wt.%. ³

Research scientists are typically trying to improve the aluminum strength properties using CNTs while maintaining low density and cost. However, solving these tasks requires homogeneous distribution of CNTs into the aluminum matrix and obtaining of well-compacted material.

The hot pressing is widely used for the compaction of composite material. This method allows manufacturing composites with almost theoretical density while forming the good interaction between CNT and the aluminum matrix. Also, the advantages of this method include the ability to precisely control of power and duration of the temperature impact, which allows researchers to monitor the chemical processes in the composite material (especially formation of Al3C4).

Mixing in a ball mill is one of the most popular techniques for mixing aluminum and CNTs. ⁴ For this purpose CNT in powder form are used, which consists of a different size CNTs' aggregates. Many authors have obtained high strength aluminum–CNTs composite materials by mixing in a ball mill followed by hot compaction and extrusion.^5–7

However, mixing in a ball mill is a long process (one to tens of hours^8–10) and may cause mechanical damages of CNTs. Researchers use other methods such as plasma spraying^11,12 and catalyst chemical vapor deposition (CCVD), which involves CNTs synthesis on the aluminum particles surface through a metal-catalyst for eliminating disadvantages of mixing using a ball mill. CCVD ensures a very high homogeneity by coating each aluminum particle with CNTs while avoiding mechanical damages of CNTs. Authors of some works^13,14 for example, have increased the hardness of pure aluminum 4.3 times and the tensile strength 2.8 times by synthesizing 5 wt.% CNTs. In Kol'tsova et al., ¹⁵ the authors increased the hardness of composite containing 1 wt.% CNT compared to pure aluminum by two times using the CCVD method for CNT growth.

Besides the mixing technique, the interface between aluminum and CNT strongly influences the final properties of metal composites. In Pillari et al., ¹⁶ the authors used high-energy ball milling and vacuum hot pressing methods that managed to increase the Brinell hardness of aluminum alloy (AA2219) by 6% using 0.5 mass % MWCNT. Guo et al. ¹⁷ used high-energy ball milling, spark plasma sintering and hot extrusion methods, and the authors were able to increase ultimate tensile strength and 0.2% yield strength by 3.2 times and 3.1 times, respectively, using 1 vol.% CNT. In Guo et al., ¹⁸ the authors were able to modify the surface of a CNT and improve the interface between aluminum and CNT using various oxidizing agents, which led to an increase in mechanical properties. Using 1 vol.% CNT, the authors were able to achieve the ultimate tensile strength, yield strength, and elongation growth rates of 50, 240, and 300% respectively, of the aluminum composite with oxidized CNT, compared to the aluminum composite with original CNT.

It should be noted that various industries use aluminum as a constructional material, but depending on the specific area, requirements to industries can be different. Also, if the requirement of high mechanical strength and low density for aluminum composites is, as a rule, universal, then requirements to thermalphysic properties can be different.

For example, aluminum should have a high thermal conductivity when used as a heat sink, and a low thermal conductivity when used as a heat insulator. It is not infrequent that aluminum with different thermal conductivity is required within one product, for example, for various electrical appliances such as smartphones.

This study demonstrates the production of aluminum-CNTs composite material with controlled thermal conductivity and a density comparable to that of aluminum but with high hardness (by two times higher than that of pure aluminum). The article presents the results of the study of the properties of the composite, depending on the parameters of hot pressing.

Materials

At the first stage, CNTs were synthesized on the PA-4 aluminum powder particles (particle size less than 63 µm, purity of 99.5 wt.%) using the CCVD method. Acetylene has been used as a source of carbon and nickel – like a metal catalyst, the synthesis temperature was 550℃. The CNTs content was 1 wt.%. The synthesis procedure has presented in detail in our work. ¹³ The resulting composite aluminum-CNTs powders (from now on ALT) were sintered in an industrial press DA 0040 at a pressure of 2–5 GPa, a temperature of 400–1600℃ and a holding time of 15–600 s.

Methods

Material studies were carried out using the Mira-3M (TESCAN, Czech Republic) scanning electron microscope (SEM) including an X-max energy dispersive X-ray spectrometer (EDX) (Oxford instruments, Great Britain) and transmission electron microscope (TEM) Zeiss Libra 200FE, 50 eV, Germany. X-ray diffraction studies were performed on the instrument XRD-7000 (Shimadzu, Japan) CuKα (λ = 0.154051 nm). Raman spectroscopy was performed on a Horiba Yobin-Yvon LabRam HR 800 device (532 nm laser, 1800 lines per mm diffraction grating, Japan). The true density of sintered samples was determined by hydrostatic weighing on the AND-GR-202 scale, Japan. X-ray photoelectron spectroscopy (XPS) studies were performed on a Thermo Scientific K-alpha device (USA). Brinell hardness of sintered samples was measured on a Zwick/Roell ZHU 250 universal hardness tester (Germany). The thermal conductivity of the sintered samples was measured using the laser flash method in the DXF-200 system (USA).

Results and discussion

Figure 1 shows the distribution of the metal-catalyst (nickel) on the aluminum particle. OPEN IN VIEWER

As can be seen in Figure 1, the metal catalyst (nickel) is uniformly distributed over the surface of the aluminum particles. Figure 2 shows the SEM and TEM image of composite aluminum-CNTs (ALT) after the CNTs synthesis. OPEN IN VIEWER

The resulting powder is an aluminum particles uniformly coated by CNTs ensuring a high homogeneity of the material (Figure 2).

Figure 3 presents the results of X-ray and Raman ALT studies. OPEN IN VIEWER

As can be seen in Figure 3(a), high-intensity peaks of aluminum are observed, but there is no signal from the CNT, which is associated with their low content in the material (1 wt.%). A similar diffractogram was observed in Pillari et al. ¹⁶ Figure 3(b) shows the typical Raman spectrum of our ALT samples. Only the typical Raman spectrum of multilayer CNTs can be seen. The intensity ratio I _D /I _G ∼ 0.72 indicates the presence of a large number of structural defects, which is characteristic of MWCNT obtained by CVD and CCVD methods. ¹⁹ As was shown in Okpalugo et al., ¹⁹ the intensity ratio I _D /I _G for MWCNT synthesized by such methods could reach values up to 1.7 times.

Sintered ALT tablets and a micrograph of the sample surface are shown in Figure 4. OPEN IN VIEWER

As can be seen from Figure 4(b), each aluminum particle is insulated with a CNT layer, which is a critical parameter affecting the properties of the composite. The density of the samples after sintering at a pressure of 2 GPa was 2.7 ± 0.05 g/cm³ (98% of the theoretical density). Practically, similar results were obtained for samples sintered at a pressure of 5 GPa.

Figures 5 and 6 show the hardness and thermal conductivity of ALT samples sintered at a pressure of 5 and 2 GPa for 60 s from the sintering temperature, respectively. OPEN IN VIEWER

OPEN IN VIEWER

Figure 6.

Dependence of the thermal conductivity of ALT samples sintered at 5 and 2 GPa for 60 s depending on the sintering temperature. The red curve (circles are experimental points) at 5 GPa, the black curve (squares are experimental points) at 2 GPa, the blue curve (horizontal line) is the hardness of pure aluminum. Dashed lines are the melting points of aluminum at a pressure of 2 and 5 GPa.

As can be seen from Figures 7 and 8, using CNTs, we managed to increase the sample hardness from 30 to 50–55 HB (almost two times), and also to obtain adjustable thermal conductivity in the range from 50 to 150 W/(m × K). OPEN IN VIEWER

OPEN IN VIEWER

Figure 8.

Dependence of the thermal conductivity and hardness of ALT on the holding time during sintering. (a) Dependence of the thermal conductivity of ALT on the holding time at a temperature of 1000℃ and a pressure of 2 GPa, the dashed line is the thermal conductivity of pure aluminum. (b) Hardness of ALT as a function of the holding time at a temperature of 1000℃ and a pressure of 2 GPa (squares are experimental points), the dashed line is the hardness of pure aluminum.

Consider the ALT thermal conductivity and hardening mechanism. As can be seen from Figure 8, the thermal conductivity of the samples is 1.5–4 times lower than the thermal conductivity of pure aluminum, but increases with sintering temperature. This unexpected result is explained by the chaotic orientation of CNTs grown on the surface of aluminum particles, which leads to the formation of a CNT layer. The CNT layer well isolates aluminum particles from each other and assumes the role of the heat-insulating layer. The increase in thermal conductivity with an increasing sintering temperature of a composite material, similar to the strength properties of ALT, is due to chemical reactions between aluminum and CNTs with the formation of aluminum carbide Al3C4, which was shown by the XPS method. The results of XPS for samples of a composite material sintered at different temperatures are shown in Figure 7.

The peak was observed in the region 284.4 eV, which indicates the sp-2 hybridization of carbon in the sample, typical for two-dimensional lattices of graphite, graphene, and nanotubes ²⁰ (see Figure 7(a)). The peak observed in the region of 281–282.5 eV indicates that the beginning of the formation of aluminum carbide^21,22 appears at a temperature of 720℃. An increase in the sintering temperature leads to further growth of aluminum carbide fraction in the sample, as shown in Figure 7(c) and (d). It should be noted that the broadening and increase in the asymmetry of the peak observed in the XPS spectrum of the sample sintered at 980℃ (Figure 7(c)), which indicates the destruction of carbon fibers and the formation of new bonds, an amorphous component sp ³ (285.5 eV), ²³ and a carboxyl groups (288.2 eV). ²⁰

An increase in the sintering temperature leads to gradual destruction of the thermal barrier layer of CNTs due to the formation of aluminum carbide. The pressure of hot pressing, as in the previous case, determines only the rate of chemical reaction between aluminum and CNTs.

However, when the sintering temperature rises, the formation of a new phase – Al₃C₄ begins, which primarily leads to growing of hardness by improving bonds between aluminum and CNT through a thin Al₃C₄ layer, as noted in Skvortsova et al. ²⁴ and Bakshi and Agarwal, ²⁵ followed by a decrease in hardness, when too many of CNTs are transformed into aluminum carbide.

When the pressure drops to 2 GPa, chemical reactions between aluminum and CNTs are accelerated at the same sintering temperatures, which ensures the same results under milder conditions.

Figure 8 shows the dependences of thermal conductivity on sintering temperature of the ALT samples sintered at pressure 5 GPa and 2 GPa for 60 s.

Studies of the dependence of the properties of the composite material on the holding time during sintering were carried out to confirm our hypothesis that the key parameter influencing the properties of ALT is the sintering temperature. This experiment allows us to determine the limit values of hardness and thermal conductivity for ALT, which can be achieved using the proposed technique. The results are presented in Figure 8.

As can be seen from Figure 8, by adjusting the synthesis time, we managed to obtain a composite with high hardness (50–55 HB) and adjustable thermal conductivity (50–150 W/(m×K)) as with a change in temperature.

The increase in the holding time during sintering of ALT leads to an increase in the thermal conductivity of composite materials, while the strength decreases due to the gradual conversion of CNTs to the aluminum carbide, Figure 8. It should be noted that with a holding time of 600 s (Figure 8(a)), the thermal conductivity of the composite ceases to grow and reaches a constant value, which may indicate the conversion of most of the CNTs fractions to aluminum carbide.

Conclusions

We obtained the aluminum-CNTs composite material with 1 wt.% of CNTs using the CCVD technique followed by hot pressing. The composite material has a density of 2.7 ± 0.05 g/cm³, which indicates a high degree of interaction between the aluminum matrix and CNTs.

It has been found that it is possible to produce a composite material aluminum-CNTs with a hardness of up to 60 HB (by two times higher than pure Al) and a variable thermal conductivity of 50–150 W/(m×K) by controlling temperature during hot pressing.

These results are achieved by controlling the temperature and holding time during hot pressing of the composite material. It is also necessary to control the formation of aluminum carbide Al₃C₄, which makes a key influence on the properties of the aluminum-CNTs composite material.

As a result, the developed method opens up broad prospects for the production of ALT finished products with high-performance properties.