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aluminum composites
NEW CARBON MATERIALS Volume 34, Issue 3, Jun 2019 Online English edition of the Chinese language journal Cite this article as: New Carbon Materials, 2019, 34(3): 275-285
RESEARCH PAPER
Effect of the graphene content on the microstructures and properties of graphene/aluminum composites Jian Wang1,2, Li-na Guo1, Wan-ming Lin1, Jin Chen1,*, Chun-lian Liu1, Shao-da Chen1, Shuai Zhang1, Tian-tian Zhen1 1
College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China;
2
Taiyuan Iron and Steel(Group) CO., LTD., Taiyuan 030003, China
Abstract:
A graphite mold was filled with a mixed powder of aluminum and graphene. It was evacuated to 2×10-1 Pa, pre-pressed under 10
MPa, heated to 450 °C where it was kept for 5 min, pressed again under 40 MPa, heated to 600 °C where it was kept for another 5 min to prepare graphene/aluminum (G/Al) composites. The effect of the graphene content on the microstructure, and thermal, electrical, mechanical and anticorrosive properties of the composites was investigated. Results indicate that when the graphene content is 0.5 wt.%, it is uniformly dispersed at grain boundaries in the Al matrix. Compared to pure Al the resulting composite has thermal and electrical conductivities that are respectively 7.1 % and 4% higher, tensile strength and hardness that are 30.6 % and 44% higher, and a corrosion resistance that is 31% higher. When the graphene content exceeds 0.5 wt.%, it agglomerates at grain boundaries in the Al matrix, leading to a decrease of all the above-mentioned properties. Key Words:
Graphene powder; Aluminum matrix composites; Microstructure and properties
1 Introduction
for brake disc and piston, and has become a new key material for lightweight automobiles and high-speed trains [5].
Aluminum matrix composite is one of the most representative metal matrix composites. As an excellent lightweight structural and functional material, aluminum matrix composite has design flexibility in its properties. Its hardness, strength, density, thermal properties and electrical properties can be adapted to different performance requirements by optimizing the matrix material, the reinforcing material, the volume fraction and shape of the reinforcing material, the distribution of the reinforcing material and the preparation method [1-2].
In this paper, graphene/aluminum (G/Al) composites A1 (pure aluminum), A2 (0.5wt.% G/Al), A3 (1wt.% G/Al), A4 (1.5 wt.% G/Al) were prepared by Field Activated and Pressure Assisted Synthesis (FAPAS). The effect of the graphene powder content on the hardness, electrical conductivity, thermal conductivity and corrosion resistance of G/Al composites was investigated.
The current research on aluminum matrix composites focuses on two aspects [3]. One is the composite material with high strength enhanced by continuous fibers, which is widely used in the aerospace field. Another is the composite material with outstanding performance enhanced by discontinuous reinforcements, which has a wide range of applications. Among them, the particle-reinforced aluminum matrix composite has the greatest development potential owing to its high specific strength and specific modulus, good wear resistance, damping and thermal conductivity, and small thermal expansion coefficient [4]. Graphite/aluminum composite has the characteristics of light weight, good thermal conductivity and wear resistance. It is a new type of material
The graphene powder, prepared by a mechanical peeling method, was supplied by Shanxi Hengyi Tianjia Nano Material Technology Co., Ltd., and aluminum powder was used as the matrix material. The aluminum powder and graphene were mixed by using a high-speed vibrating ball mill (QM-3B), and the entire preparation process was carried out in an argon atmosphere.
2 2.1
2.2
Experimental Materials
Preparation of G/Al composites
The mixed powder of aluminum and graphene was filled into a graphite mold with a diameter of φ20, which was evacuated to 2×10-1 Pa, pre-pressed under 10 MPa, heated to 450 oC with a heating rate of 10 oC /min, kept for 5 min,
Received date: 30 May 2019; Revised date: 15 Jun. 2019 *Corresponding author. E-mail: [email protected] Copyright©2019, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. DOI: 10.1016/S1872-5805(19)60016-8
Jian Wang et al. / New Carbon Materials, 2019, 34(3): 275-285
pressed again under 40 MPa, heated to 600 oC and kept for 5 min for sintering. 2.3
Characterization
Scanning electron microscopy SEM (TESCANMIRA3LMH/LMU), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM) and HRTEM (JEM-2100F) were used to observe the microscopic morphology of graphene and aluminum in the composites and to estimate the layer number of graphene. Corrosion resistance, mechanical properties, thermal conductivity and conductivity are tested by an electrochemical workstation (HC1660E), a Vickers hardness tester (MCRO-586), a microcomputer control electronic universal
testing machine (DNS200)，a thermal conductivity detector (NETZSCH-LFA447) and an electrical conductivity detector (FD120).
3 3.1
Results and discussion Microstructure of the graphene and G/Al composites
Fig. 1(a) is the Raman spectrum of graphene. In the Raman spectrum, the D peak, the G peak and the 2D peak are located near 1 351, 1 580 and 2 696 cm-1 respectively. The peak intensity ratio of the G peak to the 2D peak is related to the number of graphene layers, the 3rd-4th layers of graphene IG/I2D is about 0.5, and the 5th layers is about 1[6-7]. The IG/I2D of 1.17 is about 5-10 graphene layers.
Fig. 1 (a) the Raman spectrum of graphene, (b) SEM image of graphene and (c) TEM image and HRTEM image of prepared graphene.
Fig. 1(b) is the SEM morphology view of graphene. Fig. 1(c) is the TEM and HRTEM image of graphene. TEM is used to observe the microscopic morphology and size of graphene, and to characterize the number of graphene layers. The surface of the graphene is wrinkled, exhibiting a folded layered structure and a surface morphology rich in wavy folds. The HRTEM image is used to calculate the number of graphene layers, and the number of graphene layers is 5 as shown below. The relative density of pure Al and G/Al composites is shown in Fig. 2(a). The relative density of composites is important to determine the performance of a material. The relative densities of pure Al and G/Al composites are 99.1%, 98.7%, 98.1%, and 97.6% for A1, A2, A3 and A4, respectively. The relative density of A1 is as high as 99.1%, which indicates that pure Al is densest and has a lowest porosity. However, after the addition of graphene, the relative densities of the G/Al composites are all lowered with increasing graphene content. With increasing the graphene content, the relative density of the composites decreases significantly. This is mainly due to the presence of pores in the composites during the hot press sintering process, and the graphene sheets with a high porosity are agglomerated at the grain boundary of the Al matrix, which affects the density of the composites. Fig. 2(b) is the SEM image of A1, and Fig. 2(c), 2(d) and 2(e) are the SEM and EDS diagrams of the G/Al composites (A2, A3 and A4), respectively. The distribution of graphene
and Al on the composite surface has been observed by SEM and EDS, and the composition of the two phases on the surface is analyzed. As shown in Fig. 2(c), the surface of A2 has no obvious defects such as impurities and pores, and the distribution of the two phases on the surface of the A2 composite is relatively uniform, indicating that a small amount of graphene has a limited influence on the microstructure of the composite. With increasing the graphene content, there is a certain degree of agglomeration found on the surface of A3 in Fig. 2(d). Due to the large specific surface area of graphene, the surface tension is very high, which makes graphene unstable. Only spontaneous agglomeration can reduce its surface energy, which can achieve a stable thermodynamics state. Therefore, with a further increase of the graphene content, the graphene agglomeration phenomenon is obvious on the surface of A4 as shown in Fig. 2(e), and defects such as pores and cracks appear on the surface of A4, which is an important cause of deterioration of composite properties and is consistent with the change in the relative density of the composites in Fig. 2(a). The TEM morphology of A2 is shown in Fig. 3(a). Due to the good wetting properties of the matrix Al and graphene, the interface between the Al matrix and graphene is tight, and no defects such as holes and cracks are found. At the same time, the diffraction pattern of selected area (SADP) is performed on the interface between the Al matrix and the graphene in the annular region as shown in Fig. 3(a).
Jian Wang et al. / New Carbon Materials, 2019, 34(3): 275-285
Fig. 2
(a) Relative densities of pure Al and G/Al composites, (b) the SEM image of pure Al and (c)-(e) the SEM and EDS images of G/Al composites.
As shown in Fig. 3(d), no aluminum carbide is found. In order to further characterize the interfacial microstructure of Al matrix and graphene, the interface morphology of A2 is characterized by HTEM. And it is shown in Fig. 3(b), the FFT (Fast-Fourier Transform) analysis of the interface structure between the Al matrix and graphene is performed. The results
are consistent with SADP, and no Al-C compound is detected at the interface between the Al matrix and graphene. 3.2 Thermal conductivities and electrical conductivities of pure Al and G/Al composites The thermal conductivities of pure Al and G/Al composites are shown in Fig. 4(a), A1 (154 W/mK), A2 (165
Jian Wang et al. / New Carbon Materials, 2019, 34(3): 275-285
Fig. 3 (a) The TEM and (b) HTEM images of the A2, (c) the diffraction pattern of the square area in (b) and (d) the diffraction pattern of the selected area of (a).
Fig. 4 (a) Thermal conductivities of pure Al and G/Al composites and (b) the electrical conductivities of pure Al and G/Al composites.
W/mK), A3 (157 W/mK), and A4 (148 W/mK). Compared with A1, the thermal conductivities of A2 and A3 increase by 7.1% and 1.9%, respectively, and the thermal conductivity of A4 decreases by 3.9%. The thermal conduction mechanism of metal materials is mainly achieved by the lattice wave vibration and the movement of free electrons. If a particle in a crystal lattice is at a higher temperature state, its thermal vibration is stronger, while the adjacent particle is at a lower temperature state and the thermal vibration is weaker. Due to the interaction force between the particles, with the effect of stronger particles on the weaker particles, the vibration is intensified, and the energy of the thermal vibration increases, which increases the heat transfer and transmission. While, graphene has a high thermal conductivity. Since graphene and Al matrix has good
wettability and interfacial bonding effect, graphene dispersed at the grain boundary of Al matrix can effectively compensate for the defects at the grain boundary of Al matrix. In the heat transfer process, the movement of phonons and free electrons in the Al matrix is facilitated, the thermal resistance at the interface of the matrix material is reduced, and the influence of defects such as pores generated during the sintering process on the thermal conductivity is enhanced. With increasing the graphene content, graphene reaches a thermodynamically stable state by spontaneous agglomeration. A large amount of graphene is deposited at the grain boundary of Al matrix, which reduces the interface bonding strength of G/Al composites, which leads to the increase of defect concentration at the interface of G/Al composites. The mean free path of phonons is reduced during the heat transfer
Jian Wang et al. / New Carbon Materials, 2019, 34(3): 275-285
process, and the collision between phonons exacerbates the nonlinear thermal vibration between the lattices, resulting in a decrease in the thermal conductivity of the G/Al composites. The electrical conductivities of pure Al and G/Al composites are shown in Fig. 4(b), with A1 (50% IACS), A2 (52% IACS), A3 (48% IACS) and A4. (46% IACS). The electrical conductivity of A2 increases by 4% and that of A3 and A4 decreases by 4% and 8%, respectively as compared with A1. A crystal lattice of aluminum atoms forms a uniform electric field. Due to the manufacturing defects, the collision of free electrons in the crystal lattice produces electrical resistance that reduces the electrical conductivity of the Al metallic material. Because of the excellent electrical conductivity of graphene and the good wettability between graphene and Al matrix, the addition of graphene can not only improve the preparation defects of the matrix material, but also repair the defects of the graphene edge and reduce defect concentration on the edge of graphene. The graphene dispersed in the Al matrix can form a conductive network on the surface of the matrix material, effectively improve the defect concentration at the grain boundary of the Al matrix, facilitate the movement of electrons in the matrix material, and improve the migration rate of electrons and the mean free path in the composite material, reduce the scattering ability of
electrons in the matrix material, thereby improving the electrical conductivity of the G/Al composites. The conduction mechanism of G/Al composites is similar to that of thermal conduction. With increasing the graphene content further, the interfacial bonding energy of G/Al composites decreases. The segregation of graphene at the grain boundaries of the matrix material results in an increase in the interface defect concentration of the matrix material. The effective electron number and band structure of the composite material are changed, the locality of the electron is enhanced, and the lattice distortion greatly increases the dislocation density of the matrix material, thereby reducing the mean free path of the electron. At the same time, the scattering ability of electrons in the matrix material is aggravated, resulting in a decrease in the electrical conductivity of the composites. 3.3 Mechanical properties of pure Al and G/Al composites The X-ray diffraction analysis of pure Al and G/Al composites is shown in Fig. 5(a). Due to the good wettability between graphene and Al, there is a possibility of interfacial reaction between graphene and Al matrix. The interfacial reaction reduces the interfacial energy of graphene and Al matrix to some extent, and contributes to the chemical bonding of graphene and Al matrix.
Fig. 5 (a) XRD patterns of Al and G/Al composites, (b) Vicker hardness of Al and G/Al composites.
Therefore, in order to improve the mechanical properties of G/Al composites and to inhibit the formation of carbides, according to the Al-C phase diagram, the sintering temperature of the G/Al composites is controlled below 660 °C to avoid the formation of the compound Al 4C3, which affects the performance of G/Al composites. The crystalline phase of the G/Al composites has been examined using an X-ray diffractometer, and no formation of the compound Al4C3 is detected. The Vickers hardness of pure Al and G/Al composites is shown in Fig. 5(b), with A1 (37.9 HV), A2 (49.5 HV), A3 (48.2 HV), and A4 (47.5 HV). Compared with A1, the hardness of G/Al composites increased by 30.6%, 27.1% and 25.3% for A2, A3 and A4, respectively. The strengthening mechanism of G/Al composites include dislocation strengthening, fine grain strengthening and Orowan strengthening mechanism. The relationship between the grain
sizes of the G/Al composites and the filler graphene can be calculated by the equation (1)[21]：
dc d p
1
3
fv
1
(1)
Where dc is the grain size of the composites, dp is the diameter of the filler material graphene, and fv is the volume fraction of the filler material graphene. It can be known from the equation (1) that the grain size of the filler determines the grain size of the composite. Since the particle size of graphene is very small, the addition of graphene can suppress grain growth, refine the crystal grains of the matrix material, and have the effect on fine-grained G/Al composites. There is a certain thermal expansion mismatch between graphene and Al matrix. On the one hand, the thermal expansion mismatch strain acts on the matrix in the form of prestress, and on the other hand, it may release the dislocation loop and eliminate thermal stress. Therefore, the addition of graphene causes the
Jian Wang et al. / New Carbon Materials, 2019, 34(3): 275-285
matrix material to have a higher dislocation density, which serves as a dislocation strengthening effect on Al matrix, as shown in the equation (2)[22]：
d
1
MGb 2
(2)
Where M is the average orientation factor, α is a constant, G is the shear modulus, b is the Burgers vector, and ρ is the dislocation density. As shown in the equation (2), the addition of graphene causes the thermal expansion mismatch strain of the Al matrix to form dislocations at the interface between the Al matrix and the graphene. The increase in dislocation density also increases the yield strength of the G/Al composites. At the same time, the graphene dispersed at the grain boundary of the Al matrix can effectively prevent the diffusion of atoms through the grain boundary, enhance the effect of the grain boundary to prevent dislocation movement, and diffuse and strengthen the Al matrix, which improves the hardness of the G/Al composites.
The fractured surface morphology of pure Al and G/Al composites is shown in Fig. 6. Graphene is distributed at the edge of the dimple of the G/Al composites. Fig. 6(a) is the fracture profile of A1, and the fracture is a typical plastic fracture morphology. Fig. 6(b) is the fracture morphology of A2. The addition of graphene makes the dimple size and depth of the G/Al composites changed, and a certain layered structure appears. This is an important indicator of the deterioration of composite toughness. Some graphene is distributed at the edge of the dimple, which is caused by the crack propagation of the A2 composite along the interface during the fracture process. Fig. 6(c) and Fig. 6(d) displays the fracture morphology of A3 and A4, respectively. When the mass fraction of graphene in the matrix increases, the matrix material exhibits a thin layer structure and a granular structure for A3 and A4, respectively. The interfacial bonding strength and load transfer between the Al matrix and graphene are weakened, resulting in a decrease in the mechanical properties of the composites.
Fig. 6 The SEM fractured surface morphology of G/Al composites with different graphene contents: (a) A1; (b) A2; (c) A3; (d) A4.
The stress-strain curves of pure Al and G/Al composites are shown in Fig. 7(a). In order to further analyze the change trend of mechanical properties of GNPs/Al composites with the graphene content, the ultimate tensile strength (UTS) and fracture elongation of A1 and G/Al composites has been calculated, as shown in Fig. 7(b). The UTS and fracture elongation of A1 are 91 MPa and 60%, respectively. The UTS and fracture elongation of G/Al composites are 131 MPa and 42% for A2, 115 MPa and 34% for A3, and 104 MPa and 24% for A4, respectively. Compared with A1, the UTS of G/Al composites increases by 44%, 26% and 14%, and the fracture
elongation decreases by 30%, 43% and 60% for A2, A3 and A4, respectively. As the reinforcing phase of the Al matrix, graphene is dispersed inside the grains of the matrix material, which can effectively hinder the movement of dislocations, form dislocation loops and cause dislocation proliferation, and improve the ability of G/Al composites to resist deformation. Orowan strengthens G/Cu composites. The Orowan strengthening mechanism of G/Al composites is shown in the equation (3)[21-22]：
Jian Wang et al. / New Carbon Materials, 2019, 34(3): 275-285
Orowan
2Gb
2 1 v
1 2
1
lnD b
(3)
Where G is the shear modulus of the matrix material, b is the Burgers vector, v is the Poisson ratio, λ is the average spacing of the filler, and D is the average particle size of the filler. As shown in the equation (3), the addition of graphene
helps to increase the yield strength of G/Al composites. However, with increasing the graphene content, graphene agglomeration occurs at the interface of G/Al composites. The physical interface between Al matrix and graphene increases, the binding energy of interface decreases, and the mechanical properties of G/Al composites deteriorate.
Fig. 7 Tensile properties of pure Al and G/Al composites with different graphene contents.(a) stress-strain curves and (b) relationship of ultimate tensile strength and fracture elongation
3.4 Electrochemical corrosion behavior of G/Al composites Tafel curves and electrochemical impedance spectra (EIS) of pure Al and G/Al composites were obtained with an electrochemical workstation. Fig. 8(a) is a polarization curve of pure Al and G/Al composites. Compared with pure Al, the polarization curves of G/Al composites move toward the negative direction as a whole. The essence of this electrochemical polarization
Fig. 8
phenomenon is that the electron migration rate is faster than the electrode reaction rate. At the time of the anode reaction, the addition of graphene enables the metal ions in the matrix material to be transferred to the solution at a higher rate than the electrons flow from the cathode into the external circuit. The accumulation of excess negative charge on the cathode causes the cathode potential to move in the negative direction.
Pure Al and G/Al composites, (a) Tafel curves, (b) Nyquist plots, (c) Bode plots and (d) Equivalent electrical circuits model.
Jian Wang et al. / New Carbon Materials, 2019, 34(3): 275-285
Table 1 shows the data obtained by fitting the polarization curves of pure Al and G/Al composites. The current densities of A1, A2, A3, and A4 are 4.72×10-7, 3.27×10-7, 3.45×10-6 and 3.57×10-6 μA/cm2, respectively. The addition of graphene has reduced the current density of A2, indicating that the addition of graphene contributes to alleviating the corrosion of the substrate by the corrosion solution. The protection efficiency of G/Al composites can be calculated by the equation (4):
P i %
' 1 icorr / icorr 100
(4)
Where icorr is the current density of the G/Al composites and i’corr is the current density of A1. Compared with A1, the protective efficiency Pi (protective efficiency) of A2 increases by 31%, and A3 and A4 decreases by 633% and 656%, respectively. The graphene distributed at the grain boundary of the Al matrix can effectively compensate for the defects at the interface of the matrix material, and form a protective film on the surface of the matrix material, which can effectively slow down and block the corrosion of the grain boundary of the matrix material by the etching solution, and reduce corrosion of the potential difference between the two poles of the battery, decrease the corrosion current density, thereby reducing the corrosion rate of the A2 composite. However, with increasing the graphene content, a large number of defects are generated at the interface between the matrix material and graphene, which deteriorates the bonding strength between the matrix material and graphene. The graphene is deposited at the grain boundary of the matrix material, which increases the potential difference between the two poles of the corrosion cell, which leads to a significant increase in the current density of the G/Al composites, A3 and A4, and the protection efficiency of the G/Al composites A3 and A4 decreases significantly. Table 1 Parameters from polarization curves obtained using Tafel tests for pure Al and G/Al composites in a 3.5 wt.% NaCl electrolyte solution. Samples
Ecorr (V)
A1 A2 A3 A4
-0.724 -0.737 -0.241 -0.266
Icorr
Protective 2
(μA/cm )
Efficiency(%)
4.72×10
-7
—
3.27×10
-7
31%
3.45×10
-6
-631%
3.57×10
-6
-656%
Fig. 8(b) is a Nyquist plot of pure Al and G/Al composites. Both pure Al and G/Al composites consist of a circular arc with a capacitive arc resistance in the high frequency region and no inductive arc in the low frequency region. The capacitive reactance arc of the material is caused by the resistance-capacitance relaxation process generated by the charge transfer resistance (Rt) and the double-layer interface capacitance (Qdl) under the action of Cl-. As shown in the figure, the radius of the capacitive reactance arc of A2 is
larger than that of A1, A3, and A4 in the high frequency region, indicating that A2 has a high charge transfer resistance (Rt) and is highly resistant to corrosion. The Bode diagrams of pure Al and G/Al composites are shown in Fig. 8(c). After adding graphene, the impedance modulus of A2 in the low frequency region is higher than that of A1, A3 and A4, and the corrosion resistance is improved to some extent. The phase peak of A2 and A3 and A4 move to the low frequency direction. The surface of the G/Al composite A2 forms a dense and flat film during electrochemical etching. The high quality of the surface layer of A2 helps to reduce the influence of the dispersion effect on the material and control the corrosion rate of the corrosion solution on the surface of A2. Therefore, the addition of graphene can improve the corrosion resistance of A2. Fig. 8(d) is an equivalent circuit model. The Rs of the solution resistance, Qdl of the double interface layer capacitance, and Rt of the charge transfer resistance were obtained by fitting data of electrochemical impedance spectroscopy (EIS) of pure Al and G/Al composites with the model. Table 2 shows the parameters from fitting the EIS data with the equivalent circuit model. The charge transfer resistance (Rt) value of A2 is 12% higher than that of A1, and the double layer interface capacitance (Qdl) of A2 is 0.5% lower than that of A1. Table 2 EIS parameters obtained using EEC for pure Al and G/Al composites in a 3.5wt.% NaCl electrolyte solution Sample
Rs(Ω·cm2)
Qdl(F·cm2)
Rt(Ω·cm2)
A1
5.72
2.12×10-5
1.163×104
A2
5.91
2.108×10-5
1.307×104
A3
6.03
2.763×10-5
5901
A4
6.09
2.971×10-5
5646
This indicates that the addition of graphene can reduce the interfacial capacitance of the matrix material, promote the formation of a dense and flat film layer on the surface of the substrate material, improve the charge transfer resistance of the composite material, reduce the dispersion effect of the material surface during electrochemical corrosion, and improve the corrosion resistance of the composite A2. The charge transfer resistance values of A3 and A4 are 49% and 51% lower than that of A1, and the double layer interface capacitance (Qdl) are increased by 30% and 40%, respectively. With increasing the graphene content, the interfacial capacitance of the composites increases. For one thing, the interface of the composites produces dielectric loss, and for another thing, the graphene agglomerated at the grain boundary of the matrix material makes the surface of the composite non-uniform, resulting in a large difference in the electrochemical activation energy of the composite surface. The change of the crystal structure of the Al matrix makes the
Jian Wang et al. / New Carbon Materials, 2019, 34(3): 275-285
rate constants at the grain boundary and the crystal plane different, which reduces the charge transfer resistance between the points on the surface of the composites, resulting in uneven surface corrosion of the composites, which affects the corrosion resistance of the composites. Therefore, excess graphene deteriorates the corrosion resistance. Fig. 9 is SEM images of electrochemical corroded pure Al and G/Al composites. After corrosion test by the 3.5 wt % NaCl solution, significant corrosion pits appeared on the surface of A1. The surface film structure of A2 is dense and flat, and the film structure formed on the surface of A3 and A4
Fig. 9
4
is partially dense and flat. This indicates that the addition of graphene can slow the corrosion rate of the corrosion solution on the surface of the A2 composite. With increasing the graphene content, a large amount of agglomerated graphene forms a potential difference on the surface of the electrode, resulting in uneven surface corrosion of the composites, A3 and A4. Cracks and voids appear on the surface of A3 and A4, which reduces their corrosion resistance. Therefore, the addition of 0.5 wt.% of graphene can effectively improve the corrosion resistance of the composite.
SEM images of electrochemical corroded pure Al and G/Al composites:(a) A1, (b) A2, (c) A3 and (d) A4.
Conclusion
Pure Al and G/Al composites have been prepared by mechanical alloying combined with field-activated and pressure-assisted synthesis (FAPAS). By optimizing the sintering process, the formation of the compound Al4C3 in the G/Al composites is effectively inhibited, and the interfacial bonding strength between the graphene and the Al matrix is improved. When the amount of graphene is 0.5wt.%, it can be uniformly dispersed at the grain boundary of Al matrix, promote the movement of phonons in the matrix material, reduce the thermal resistance of the interface, form a conductive network on the surface of G/Al composite (A2), increase electron mobility and mean free path, which helps to improve the thermal conductivity and electrical conductivity of G/Al composite (A2) by 7.1% and 4%, respectively. The addition of graphene can change the crystal structure of the matrix material and form a stress field of lattice distortion around the graphene. The stress field interacts with the dislocation stress field, hindering the dislocation motion, and the strength and hardness of the G/Al composite (A2) are increased by 30.6% and 44%, respectively. Graphene can reduce the dielectric loss of the interface capacitance of the matrix material, form a dense and flat film on the surface of the Al substrate, improve the charge transfer resistance of the G/Al composite (A2), and reduce the dispersion effect of the surface of the material in the electrochemical corrosion process, therefore the corrosion resistance of G/Al composite (A2) is improved by 31%. However, when the graphene content exceeds 0.5 wt.%, the graphene agglomerated at the grain boundary of the matrix reduces the interfacial bonding strength between the Al matrix and graphene, narrows the energy band width in the conduction band of the G/Al composites, and the localized enhancement of electrons leads
to a decrease in the performance of G/Al composites. So, the optimum addition amount of graphene in the G/Al composites is 0.5 wt.%.
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RESEARCH PAPER
Effect of the graphene content on the microstructures and properties of graphene/aluminum composites Jian Wang1,2, Li-na Guo1, Wan-ming Lin1, Jin Chen1,*, Chun-lian Liu1, Shao-da Chen1, Shuai Zhang1, Tian-tian Zhen1 1
College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China;
2
Taiyuan Iron and Steel(Group) CO., LTD., Taiyuan 030003, China
Abstract:
A graphite mold was filled with a mixed powder of aluminum and graphene. It was evacuated to 2×10-1 Pa, pre-pressed under 10
MPa, heated to 450 °C where it was kept for 5 min, pressed again under 40 MPa, heated to 600 °C where it was kept for another 5 min to prepare graphene/aluminum (G/Al) composites. The effect of the graphene content on the microstructure, and thermal, electrical, mechanical and anticorrosive properties of the composites was investigated. Results indicate that when the graphene content is 0.5 wt.%, it is uniformly dispersed at grain boundaries in the Al matrix. Compared to pure Al the resulting composite has thermal and electrical conductivities that are respectively 7.1 % and 4% higher, tensile strength and hardness that are 30.6 % and 44% higher, and a corrosion resistance that is 31% higher. When the graphene content exceeds 0.5 wt.%, it agglomerates at grain boundaries in the Al matrix, leading to a decrease of all the above-mentioned properties. Key Words:
Graphene powder; Aluminum matrix composites; Microstructure and properties
1 Introduction
for brake disc and piston, and has become a new key material for lightweight automobiles and high-speed trains [5].
Aluminum matrix composite is one of the most representative metal matrix composites. As an excellent lightweight structural and functional material, aluminum matrix composite has design flexibility in its properties. Its hardness, strength, density, thermal properties and electrical properties can be adapted to different performance requirements by optimizing the matrix material, the reinforcing material, the volume fraction and shape of the reinforcing material, the distribution of the reinforcing material and the preparation method [1-2].
In this paper, graphene/aluminum (G/Al) composites A1 (pure aluminum), A2 (0.5wt.% G/Al), A3 (1wt.% G/Al), A4 (1.5 wt.% G/Al) were prepared by Field Activated and Pressure Assisted Synthesis (FAPAS). The effect of the graphene powder content on the hardness, electrical conductivity, thermal conductivity and corrosion resistance of G/Al composites was investigated.
The current research on aluminum matrix composites focuses on two aspects [3]. One is the composite material with high strength enhanced by continuous fibers, which is widely used in the aerospace field. Another is the composite material with outstanding performance enhanced by discontinuous reinforcements, which has a wide range of applications. Among them, the particle-reinforced aluminum matrix composite has the greatest development potential owing to its high specific strength and specific modulus, good wear resistance, damping and thermal conductivity, and small thermal expansion coefficient [4]. Graphite/aluminum composite has the characteristics of light weight, good thermal conductivity and wear resistance. It is a new type of material
The graphene powder, prepared by a mechanical peeling method, was supplied by Shanxi Hengyi Tianjia Nano Material Technology Co., Ltd., and aluminum powder was used as the matrix material. The aluminum powder and graphene were mixed by using a high-speed vibrating ball mill (QM-3B), and the entire preparation process was carried out in an argon atmosphere.
2 2.1
2.2
Experimental Materials
Preparation of G/Al composites
The mixed powder of aluminum and graphene was filled into a graphite mold with a diameter of φ20, which was evacuated to 2×10-1 Pa, pre-pressed under 10 MPa, heated to 450 oC with a heating rate of 10 oC /min, kept for 5 min,
Received date: 30 May 2019; Revised date: 15 Jun. 2019 *Corresponding author. E-mail: [email protected] Copyright©2019, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. DOI: 10.1016/S1872-5805(19)60016-8
Jian Wang et al. / New Carbon Materials, 2019, 34(3): 275-285
pressed again under 40 MPa, heated to 600 oC and kept for 5 min for sintering. 2.3
Characterization
Scanning electron microscopy SEM (TESCANMIRA3LMH/LMU), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM) and HRTEM (JEM-2100F) were used to observe the microscopic morphology of graphene and aluminum in the composites and to estimate the layer number of graphene. Corrosion resistance, mechanical properties, thermal conductivity and conductivity are tested by an electrochemical workstation (HC1660E), a Vickers hardness tester (MCRO-586), a microcomputer control electronic universal
testing machine (DNS200)，a thermal conductivity detector (NETZSCH-LFA447) and an electrical conductivity detector (FD120).
3 3.1
Results and discussion Microstructure of the graphene and G/Al composites
Fig. 1(a) is the Raman spectrum of graphene. In the Raman spectrum, the D peak, the G peak and the 2D peak are located near 1 351, 1 580 and 2 696 cm-1 respectively. The peak intensity ratio of the G peak to the 2D peak is related to the number of graphene layers, the 3rd-4th layers of graphene IG/I2D is about 0.5, and the 5th layers is about 1[6-7]. The IG/I2D of 1.17 is about 5-10 graphene layers.
Fig. 1 (a) the Raman spectrum of graphene, (b) SEM image of graphene and (c) TEM image and HRTEM image of prepared graphene.
Fig. 1(b) is the SEM morphology view of graphene. Fig. 1(c) is the TEM and HRTEM image of graphene. TEM is used to observe the microscopic morphology and size of graphene, and to characterize the number of graphene layers. The surface of the graphene is wrinkled, exhibiting a folded layered structure and a surface morphology rich in wavy folds. The HRTEM image is used to calculate the number of graphene layers, and the number of graphene layers is 5 as shown below. The relative density of pure Al and G/Al composites is shown in Fig. 2(a). The relative density of composites is important to determine the performance of a material. The relative densities of pure Al and G/Al composites are 99.1%, 98.7%, 98.1%, and 97.6% for A1, A2, A3 and A4, respectively. The relative density of A1 is as high as 99.1%, which indicates that pure Al is densest and has a lowest porosity. However, after the addition of graphene, the relative densities of the G/Al composites are all lowered with increasing graphene content. With increasing the graphene content, the relative density of the composites decreases significantly. This is mainly due to the presence of pores in the composites during the hot press sintering process, and the graphene sheets with a high porosity are agglomerated at the grain boundary of the Al matrix, which affects the density of the composites. Fig. 2(b) is the SEM image of A1, and Fig. 2(c), 2(d) and 2(e) are the SEM and EDS diagrams of the G/Al composites (A2, A3 and A4), respectively. The distribution of graphene
and Al on the composite surface has been observed by SEM and EDS, and the composition of the two phases on the surface is analyzed. As shown in Fig. 2(c), the surface of A2 has no obvious defects such as impurities and pores, and the distribution of the two phases on the surface of the A2 composite is relatively uniform, indicating that a small amount of graphene has a limited influence on the microstructure of the composite. With increasing the graphene content, there is a certain degree of agglomeration found on the surface of A3 in Fig. 2(d). Due to the large specific surface area of graphene, the surface tension is very high, which makes graphene unstable. Only spontaneous agglomeration can reduce its surface energy, which can achieve a stable thermodynamics state. Therefore, with a further increase of the graphene content, the graphene agglomeration phenomenon is obvious on the surface of A4 as shown in Fig. 2(e), and defects such as pores and cracks appear on the surface of A4, which is an important cause of deterioration of composite properties and is consistent with the change in the relative density of the composites in Fig. 2(a). The TEM morphology of A2 is shown in Fig. 3(a). Due to the good wetting properties of the matrix Al and graphene, the interface between the Al matrix and graphene is tight, and no defects such as holes and cracks are found. At the same time, the diffraction pattern of selected area (SADP) is performed on the interface between the Al matrix and the graphene in the annular region as shown in Fig. 3(a).
Jian Wang et al. / New Carbon Materials, 2019, 34(3): 275-285
Fig. 2
(a) Relative densities of pure Al and G/Al composites, (b) the SEM image of pure Al and (c)-(e) the SEM and EDS images of G/Al composites.
As shown in Fig. 3(d), no aluminum carbide is found. In order to further characterize the interfacial microstructure of Al matrix and graphene, the interface morphology of A2 is characterized by HTEM. And it is shown in Fig. 3(b), the FFT (Fast-Fourier Transform) analysis of the interface structure between the Al matrix and graphene is performed. The results
are consistent with SADP, and no Al-C compound is detected at the interface between the Al matrix and graphene. 3.2 Thermal conductivities and electrical conductivities of pure Al and G/Al composites The thermal conductivities of pure Al and G/Al composites are shown in Fig. 4(a), A1 (154 W/mK), A2 (165
Jian Wang et al. / New Carbon Materials, 2019, 34(3): 275-285
Fig. 3 (a) The TEM and (b) HTEM images of the A2, (c) the diffraction pattern of the square area in (b) and (d) the diffraction pattern of the selected area of (a).
Fig. 4 (a) Thermal conductivities of pure Al and G/Al composites and (b) the electrical conductivities of pure Al and G/Al composites.
W/mK), A3 (157 W/mK), and A4 (148 W/mK). Compared with A1, the thermal conductivities of A2 and A3 increase by 7.1% and 1.9%, respectively, and the thermal conductivity of A4 decreases by 3.9%. The thermal conduction mechanism of metal materials is mainly achieved by the lattice wave vibration and the movement of free electrons. If a particle in a crystal lattice is at a higher temperature state, its thermal vibration is stronger, while the adjacent particle is at a lower temperature state and the thermal vibration is weaker. Due to the interaction force between the particles, with the effect of stronger particles on the weaker particles, the vibration is intensified, and the energy of the thermal vibration increases, which increases the heat transfer and transmission. While, graphene has a high thermal conductivity. Since graphene and Al matrix has good
wettability and interfacial bonding effect, graphene dispersed at the grain boundary of Al matrix can effectively compensate for the defects at the grain boundary of Al matrix. In the heat transfer process, the movement of phonons and free electrons in the Al matrix is facilitated, the thermal resistance at the interface of the matrix material is reduced, and the influence of defects such as pores generated during the sintering process on the thermal conductivity is enhanced. With increasing the graphene content, graphene reaches a thermodynamically stable state by spontaneous agglomeration. A large amount of graphene is deposited at the grain boundary of Al matrix, which reduces the interface bonding strength of G/Al composites, which leads to the increase of defect concentration at the interface of G/Al composites. The mean free path of phonons is reduced during the heat transfer
Jian Wang et al. / New Carbon Materials, 2019, 34(3): 275-285
process, and the collision between phonons exacerbates the nonlinear thermal vibration between the lattices, resulting in a decrease in the thermal conductivity of the G/Al composites. The electrical conductivities of pure Al and G/Al composites are shown in Fig. 4(b), with A1 (50% IACS), A2 (52% IACS), A3 (48% IACS) and A4. (46% IACS). The electrical conductivity of A2 increases by 4% and that of A3 and A4 decreases by 4% and 8%, respectively as compared with A1. A crystal lattice of aluminum atoms forms a uniform electric field. Due to the manufacturing defects, the collision of free electrons in the crystal lattice produces electrical resistance that reduces the electrical conductivity of the Al metallic material. Because of the excellent electrical conductivity of graphene and the good wettability between graphene and Al matrix, the addition of graphene can not only improve the preparation defects of the matrix material, but also repair the defects of the graphene edge and reduce defect concentration on the edge of graphene. The graphene dispersed in the Al matrix can form a conductive network on the surface of the matrix material, effectively improve the defect concentration at the grain boundary of the Al matrix, facilitate the movement of electrons in the matrix material, and improve the migration rate of electrons and the mean free path in the composite material, reduce the scattering ability of
electrons in the matrix material, thereby improving the electrical conductivity of the G/Al composites. The conduction mechanism of G/Al composites is similar to that of thermal conduction. With increasing the graphene content further, the interfacial bonding energy of G/Al composites decreases. The segregation of graphene at the grain boundaries of the matrix material results in an increase in the interface defect concentration of the matrix material. The effective electron number and band structure of the composite material are changed, the locality of the electron is enhanced, and the lattice distortion greatly increases the dislocation density of the matrix material, thereby reducing the mean free path of the electron. At the same time, the scattering ability of electrons in the matrix material is aggravated, resulting in a decrease in the electrical conductivity of the composites. 3.3 Mechanical properties of pure Al and G/Al composites The X-ray diffraction analysis of pure Al and G/Al composites is shown in Fig. 5(a). Due to the good wettability between graphene and Al, there is a possibility of interfacial reaction between graphene and Al matrix. The interfacial reaction reduces the interfacial energy of graphene and Al matrix to some extent, and contributes to the chemical bonding of graphene and Al matrix.
Fig. 5 (a) XRD patterns of Al and G/Al composites, (b) Vicker hardness of Al and G/Al composites.
Therefore, in order to improve the mechanical properties of G/Al composites and to inhibit the formation of carbides, according to the Al-C phase diagram, the sintering temperature of the G/Al composites is controlled below 660 °C to avoid the formation of the compound Al 4C3, which affects the performance of G/Al composites. The crystalline phase of the G/Al composites has been examined using an X-ray diffractometer, and no formation of the compound Al4C3 is detected. The Vickers hardness of pure Al and G/Al composites is shown in Fig. 5(b), with A1 (37.9 HV), A2 (49.5 HV), A3 (48.2 HV), and A4 (47.5 HV). Compared with A1, the hardness of G/Al composites increased by 30.6%, 27.1% and 25.3% for A2, A3 and A4, respectively. The strengthening mechanism of G/Al composites include dislocation strengthening, fine grain strengthening and Orowan strengthening mechanism. The relationship between the grain
sizes of the G/Al composites and the filler graphene can be calculated by the equation (1)[21]：
dc d p
1
3
fv
1
(1)
Where dc is the grain size of the composites, dp is the diameter of the filler material graphene, and fv is the volume fraction of the filler material graphene. It can be known from the equation (1) that the grain size of the filler determines the grain size of the composite. Since the particle size of graphene is very small, the addition of graphene can suppress grain growth, refine the crystal grains of the matrix material, and have the effect on fine-grained G/Al composites. There is a certain thermal expansion mismatch between graphene and Al matrix. On the one hand, the thermal expansion mismatch strain acts on the matrix in the form of prestress, and on the other hand, it may release the dislocation loop and eliminate thermal stress. Therefore, the addition of graphene causes the
Jian Wang et al. / New Carbon Materials, 2019, 34(3): 275-285
matrix material to have a higher dislocation density, which serves as a dislocation strengthening effect on Al matrix, as shown in the equation (2)[22]：
d
1
MGb 2
(2)
Where M is the average orientation factor, α is a constant, G is the shear modulus, b is the Burgers vector, and ρ is the dislocation density. As shown in the equation (2), the addition of graphene causes the thermal expansion mismatch strain of the Al matrix to form dislocations at the interface between the Al matrix and the graphene. The increase in dislocation density also increases the yield strength of the G/Al composites. At the same time, the graphene dispersed at the grain boundary of the Al matrix can effectively prevent the diffusion of atoms through the grain boundary, enhance the effect of the grain boundary to prevent dislocation movement, and diffuse and strengthen the Al matrix, which improves the hardness of the G/Al composites.
The fractured surface morphology of pure Al and G/Al composites is shown in Fig. 6. Graphene is distributed at the edge of the dimple of the G/Al composites. Fig. 6(a) is the fracture profile of A1, and the fracture is a typical plastic fracture morphology. Fig. 6(b) is the fracture morphology of A2. The addition of graphene makes the dimple size and depth of the G/Al composites changed, and a certain layered structure appears. This is an important indicator of the deterioration of composite toughness. Some graphene is distributed at the edge of the dimple, which is caused by the crack propagation of the A2 composite along the interface during the fracture process. Fig. 6(c) and Fig. 6(d) displays the fracture morphology of A3 and A4, respectively. When the mass fraction of graphene in the matrix increases, the matrix material exhibits a thin layer structure and a granular structure for A3 and A4, respectively. The interfacial bonding strength and load transfer between the Al matrix and graphene are weakened, resulting in a decrease in the mechanical properties of the composites.
Fig. 6 The SEM fractured surface morphology of G/Al composites with different graphene contents: (a) A1; (b) A2; (c) A3; (d) A4.
The stress-strain curves of pure Al and G/Al composites are shown in Fig. 7(a). In order to further analyze the change trend of mechanical properties of GNPs/Al composites with the graphene content, the ultimate tensile strength (UTS) and fracture elongation of A1 and G/Al composites has been calculated, as shown in Fig. 7(b). The UTS and fracture elongation of A1 are 91 MPa and 60%, respectively. The UTS and fracture elongation of G/Al composites are 131 MPa and 42% for A2, 115 MPa and 34% for A3, and 104 MPa and 24% for A4, respectively. Compared with A1, the UTS of G/Al composites increases by 44%, 26% and 14%, and the fracture
elongation decreases by 30%, 43% and 60% for A2, A3 and A4, respectively. As the reinforcing phase of the Al matrix, graphene is dispersed inside the grains of the matrix material, which can effectively hinder the movement of dislocations, form dislocation loops and cause dislocation proliferation, and improve the ability of G/Al composites to resist deformation. Orowan strengthens G/Cu composites. The Orowan strengthening mechanism of G/Al composites is shown in the equation (3)[21-22]：
Jian Wang et al. / New Carbon Materials, 2019, 34(3): 275-285
Orowan
2Gb
2 1 v
1 2
1
lnD b
(3)
Where G is the shear modulus of the matrix material, b is the Burgers vector, v is the Poisson ratio, λ is the average spacing of the filler, and D is the average particle size of the filler. As shown in the equation (3), the addition of graphene
helps to increase the yield strength of G/Al composites. However, with increasing the graphene content, graphene agglomeration occurs at the interface of G/Al composites. The physical interface between Al matrix and graphene increases, the binding energy of interface decreases, and the mechanical properties of G/Al composites deteriorate.
Fig. 7 Tensile properties of pure Al and G/Al composites with different graphene contents.(a) stress-strain curves and (b) relationship of ultimate tensile strength and fracture elongation
3.4 Electrochemical corrosion behavior of G/Al composites Tafel curves and electrochemical impedance spectra (EIS) of pure Al and G/Al composites were obtained with an electrochemical workstation. Fig. 8(a) is a polarization curve of pure Al and G/Al composites. Compared with pure Al, the polarization curves of G/Al composites move toward the negative direction as a whole. The essence of this electrochemical polarization
Fig. 8
phenomenon is that the electron migration rate is faster than the electrode reaction rate. At the time of the anode reaction, the addition of graphene enables the metal ions in the matrix material to be transferred to the solution at a higher rate than the electrons flow from the cathode into the external circuit. The accumulation of excess negative charge on the cathode causes the cathode potential to move in the negative direction.
Pure Al and G/Al composites, (a) Tafel curves, (b) Nyquist plots, (c) Bode plots and (d) Equivalent electrical circuits model.
Jian Wang et al. / New Carbon Materials, 2019, 34(3): 275-285
Table 1 shows the data obtained by fitting the polarization curves of pure Al and G/Al composites. The current densities of A1, A2, A3, and A4 are 4.72×10-7, 3.27×10-7, 3.45×10-6 and 3.57×10-6 μA/cm2, respectively. The addition of graphene has reduced the current density of A2, indicating that the addition of graphene contributes to alleviating the corrosion of the substrate by the corrosion solution. The protection efficiency of G/Al composites can be calculated by the equation (4):
P i %
' 1 icorr / icorr 100
(4)
Where icorr is the current density of the G/Al composites and i’corr is the current density of A1. Compared with A1, the protective efficiency Pi (protective efficiency) of A2 increases by 31%, and A3 and A4 decreases by 633% and 656%, respectively. The graphene distributed at the grain boundary of the Al matrix can effectively compensate for the defects at the interface of the matrix material, and form a protective film on the surface of the matrix material, which can effectively slow down and block the corrosion of the grain boundary of the matrix material by the etching solution, and reduce corrosion of the potential difference between the two poles of the battery, decrease the corrosion current density, thereby reducing the corrosion rate of the A2 composite. However, with increasing the graphene content, a large number of defects are generated at the interface between the matrix material and graphene, which deteriorates the bonding strength between the matrix material and graphene. The graphene is deposited at the grain boundary of the matrix material, which increases the potential difference between the two poles of the corrosion cell, which leads to a significant increase in the current density of the G/Al composites, A3 and A4, and the protection efficiency of the G/Al composites A3 and A4 decreases significantly. Table 1 Parameters from polarization curves obtained using Tafel tests for pure Al and G/Al composites in a 3.5 wt.% NaCl electrolyte solution. Samples
Ecorr (V)
A1 A2 A3 A4
-0.724 -0.737 -0.241 -0.266
Icorr
Protective 2
(μA/cm )
Efficiency(%)
4.72×10
-7
—
3.27×10
-7
31%
3.45×10
-6
-631%
3.57×10
-6
-656%
Fig. 8(b) is a Nyquist plot of pure Al and G/Al composites. Both pure Al and G/Al composites consist of a circular arc with a capacitive arc resistance in the high frequency region and no inductive arc in the low frequency region. The capacitive reactance arc of the material is caused by the resistance-capacitance relaxation process generated by the charge transfer resistance (Rt) and the double-layer interface capacitance (Qdl) under the action of Cl-. As shown in the figure, the radius of the capacitive reactance arc of A2 is
larger than that of A1, A3, and A4 in the high frequency region, indicating that A2 has a high charge transfer resistance (Rt) and is highly resistant to corrosion. The Bode diagrams of pure Al and G/Al composites are shown in Fig. 8(c). After adding graphene, the impedance modulus of A2 in the low frequency region is higher than that of A1, A3 and A4, and the corrosion resistance is improved to some extent. The phase peak of A2 and A3 and A4 move to the low frequency direction. The surface of the G/Al composite A2 forms a dense and flat film during electrochemical etching. The high quality of the surface layer of A2 helps to reduce the influence of the dispersion effect on the material and control the corrosion rate of the corrosion solution on the surface of A2. Therefore, the addition of graphene can improve the corrosion resistance of A2. Fig. 8(d) is an equivalent circuit model. The Rs of the solution resistance, Qdl of the double interface layer capacitance, and Rt of the charge transfer resistance were obtained by fitting data of electrochemical impedance spectroscopy (EIS) of pure Al and G/Al composites with the model. Table 2 shows the parameters from fitting the EIS data with the equivalent circuit model. The charge transfer resistance (Rt) value of A2 is 12% higher than that of A1, and the double layer interface capacitance (Qdl) of A2 is 0.5% lower than that of A1. Table 2 EIS parameters obtained using EEC for pure Al and G/Al composites in a 3.5wt.% NaCl electrolyte solution Sample
Rs(Ω·cm2)
Qdl(F·cm2)
Rt(Ω·cm2)
A1
5.72
2.12×10-5
1.163×104
A2
5.91
2.108×10-5
1.307×104
A3
6.03
2.763×10-5
5901
A4
6.09
2.971×10-5
5646
This indicates that the addition of graphene can reduce the interfacial capacitance of the matrix material, promote the formation of a dense and flat film layer on the surface of the substrate material, improve the charge transfer resistance of the composite material, reduce the dispersion effect of the material surface during electrochemical corrosion, and improve the corrosion resistance of the composite A2. The charge transfer resistance values of A3 and A4 are 49% and 51% lower than that of A1, and the double layer interface capacitance (Qdl) are increased by 30% and 40%, respectively. With increasing the graphene content, the interfacial capacitance of the composites increases. For one thing, the interface of the composites produces dielectric loss, and for another thing, the graphene agglomerated at the grain boundary of the matrix material makes the surface of the composite non-uniform, resulting in a large difference in the electrochemical activation energy of the composite surface. The change of the crystal structure of the Al matrix makes the
Jian Wang et al. / New Carbon Materials, 2019, 34(3): 275-285
rate constants at the grain boundary and the crystal plane different, which reduces the charge transfer resistance between the points on the surface of the composites, resulting in uneven surface corrosion of the composites, which affects the corrosion resistance of the composites. Therefore, excess graphene deteriorates the corrosion resistance. Fig. 9 is SEM images of electrochemical corroded pure Al and G/Al composites. After corrosion test by the 3.5 wt % NaCl solution, significant corrosion pits appeared on the surface of A1. The surface film structure of A2 is dense and flat, and the film structure formed on the surface of A3 and A4
Fig. 9
4
is partially dense and flat. This indicates that the addition of graphene can slow the corrosion rate of the corrosion solution on the surface of the A2 composite. With increasing the graphene content, a large amount of agglomerated graphene forms a potential difference on the surface of the electrode, resulting in uneven surface corrosion of the composites, A3 and A4. Cracks and voids appear on the surface of A3 and A4, which reduces their corrosion resistance. Therefore, the addition of 0.5 wt.% of graphene can effectively improve the corrosion resistance of the composite.
SEM images of electrochemical corroded pure Al and G/Al composites:(a) A1, (b) A2, (c) A3 and (d) A4.
Conclusion
Pure Al and G/Al composites have been prepared by mechanical alloying combined with field-activated and pressure-assisted synthesis (FAPAS). By optimizing the sintering process, the formation of the compound Al4C3 in the G/Al composites is effectively inhibited, and the interfacial bonding strength between the graphene and the Al matrix is improved. When the amount of graphene is 0.5wt.%, it can be uniformly dispersed at the grain boundary of Al matrix, promote the movement of phonons in the matrix material, reduce the thermal resistance of the interface, form a conductive network on the surface of G/Al composite (A2), increase electron mobility and mean free path, which helps to improve the thermal conductivity and electrical conductivity of G/Al composite (A2) by 7.1% and 4%, respectively. The addition of graphene can change the crystal structure of the matrix material and form a stress field of lattice distortion around the graphene. The stress field interacts with the dislocation stress field, hindering the dislocation motion, and the strength and hardness of the G/Al composite (A2) are increased by 30.6% and 44%, respectively. Graphene can reduce the dielectric loss of the interface capacitance of the matrix material, form a dense and flat film on the surface of the Al substrate, improve the charge transfer resistance of the G/Al composite (A2), and reduce the dispersion effect of the surface of the material in the electrochemical corrosion process, therefore the corrosion resistance of G/Al composite (A2) is improved by 31%. However, when the graphene content exceeds 0.5 wt.%, the graphene agglomerated at the grain boundary of the matrix reduces the interfacial bonding strength between the Al matrix and graphene, narrows the energy band width in the conduction band of the G/Al composites, and the localized enhancement of electrons leads
to a decrease in the performance of G/Al composites. So, the optimum addition amount of graphene in the G/Al composites is 0.5 wt.%.
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