Fig. 1 shows the phase compositions and microstructures of raw powders(Ti3AlC2 and Ti3C2Tx). Ti3AlC2 was characterized by the granular morphology with smooth surfaces(Fig. 1a), and Ti3C2Tx exhibited a multilayered morphology with the layer thickness of 0.15~0.37 μm(Fig. 1b). Fig. 1b obviously shows that the(002) diffraction peak of Ti3C2Tx is tilted towards low angle, which also confirms the expansion of Ti3C2Tx layer space.
Fig.2 shows the different microstructural characteristics of Ti3AlC2 and Ti3C2Tx. Ti3AlC2 particle displays a smooth margin at the bright field TEM images(Fig. 2a), but the margin of Ti3C2Tx particle after etching takes flakiness morphology(Fig.2c). Fig. 2b displays the high resolution transmission electron microscope(HRTEM) image of Ti3AlC2 corresponding to the  crystal band axis direction. There is a layer of Al atoms between every three layers of Ti atoms, and the distance between the two Al layers is 1.115 nm. HRTEM image indicates that the Al layer is almost completely etched out of Ti3AlC2(Fig. 2d), and the distance of three Ti layers increase to 1.651 nm. The TEM results are consistence with those of XRD and SEM results, and further confirms the structural difference between Ti3AlC2 and its derivative Ti3C2Tx.
The microstructures and element distributions of Ag/Ti3AlC2 and Ag/Ti3C2Tx composites are displayed in Fig. 3. As shown in Fig. 3a and Fig. 3b, both reinforcements(Ti3AlC2 and Ti3C2Tx) uniformly distribute in Ag matrices, Ti3AlC2 retains the granular morphology while Ti3C2Tx takes the stripe-shaped morphology. Fig. 3(c-j) displays the element distributions of Ag, Ti and Al in composites, which further confirms that Ti3AlC2 and Ti3C2Tx take different shapes in Ag matrices. Moreover, slight diffusion of Al with Ag is observed in Ag/Ti3AlC2(Fig. 3f), while a few Al element is detected in Ag/Ti3C2Tx (Fig. 1l), which is consistent with the XRD and TEM results.
Contact materials are usually manufactured into various shapes, necessitating excellent machinability. A typical negative case is SnO2 whose high hardness leads to the poor machinability of Ag/SnO2, which hinders its substitute to CdO. Fig. 4 shows the Vickers hardness of Ag/Ti3C2Tx, Ag/Ti3AlC2, and pure Ag(for reference). Ag/Ti3C2Tx possesses intermediate hardness(64 HV), and can be cut into different shapes, including rod, rivet, disc and square(the insert in Fig. 4). The good machinability originates from the 2D structure of Ti3C2Tx, in which weak van der Waals interaction exists between layers. In addition, contacts usually carry high current density in service, thus a low resistivity is a prerequisite to potential electrical contact materials. As shown in Fig. 4, the Ag/Ti3C2Tx and Ag/Ti3AlC2 composites own low resistivity(16×10-3 μΩ·m of Ag). In particular, the resistivity of Ag/Ti3C2Tx(30×10-3 μΩ·m) is 29% lower than that of Ag/Ti3AlC2(42×10-3 μΩ·m), which is very meaningful for the practice application.
The improved conductivity of Ag/Ti3C2Tx can be explained from three aspects: higher conductivity of Ti3C2Tx than that of Ti3AlC2, enhanced interface bonding between Ag and Ti3C2Tx, deformability of the stripe-shaped Ti3C2Tx in Ag matrix.
Firstly, based on the first-principle band structure calculations, in Ti3AlC2, Ti 3d state contributes the majority of the total densities of states(DOS) at Fermi level; removal of the Al layers from Ti3AlC2 results in the redistribution of Ti 3d states from broken Ti-Al bonds into delocalized Ti-Ti metallic-like bonding states, leading to the increase of local DOS maximums at Fermi level. Thus, in MXene(Ti3C2Tx), the electron density of states near Fermi level(N(Ef)) is 1.9~3.2 times higher than that in the corresponding MAX(Ti3AlC2). Secondly, EDS mapping indicates the existence of O and F elements, which may come from the functional groups(-F, -OH) of Ti3C2Tx surface(Fig. 5a). Generally, the hydrophilicity of -F/-OH functional groups is beneficial to the bonding between Ti3C2Tx and metal matrix, which avoids the similar phenomenon of poor interface bonding between carbon nanotubes, fibers and metal matrix. In addition, the SEM observation also displays the tight bonding between Ti3C2Tx and Ag matrix without cracks and holes, as shown in Fig. 3b and Fig. 3g. Hence, the uniform microstructure and good bonding of Ag/Ti3C2Tx improved the conductivity. Thirdly, as shown in Fig. 3c, 3g, the microstructure of stripe-shaped Ti3C2Tx is obviously different from that of granular Ti3AlC2 in Ag matrix. The 2D layered structure of Ti3C2Tx facilitates its deformability during preparation. The Ti3C2Tx were cold compacted into stripe-like Ti3C2Tx(average thickness of ~3 microns), whereas Ti3AlC2 retains its original shape(average diameter of ~10 microns). Fig. 5b and 5c schematically illustrate the relationship between the electron transmission and the shape of reinforcements in composites. In contrast with granular Ti3AlC2, stripe-shaped Ti3C2Tx has smaller cross-sectional area perpendicular to the current direction, minimizing the scattering section for electrons and the resistance to the electron transmission. In summary, the excellent machinability and electrical conductivity makes Ag/Ti3C2Tx a promising substitute to Ag/CdO.
However, as shown in Fig. 6, the maximum tensile strength of Ag/Ti3C2Tx composite(32.77 MPa) is far less than that of Ag/Ti3AlC2 composite(145.52 MPa). The superior tensile strength of Ag/Ti3AlC2 composite derives from the interdiffusion between active Al atomic layer with Ag matrix. On the contrary, the absence of Al layer leads to the weaker interface bonding strength between Ti3C2Tx and Ag matrix, which finally deteriorates the mechanical property of entire Ag/Ti3C2Tx composite.
In order to further investigate the property of Ag/Ti3C2Tx, electrical arc discharging experiment were carried out on this contact surface under a harsh condition(AC-3, 100A, 400V, GB14048.4-2010). The Ag/Ti3C2Tx contact failed to make and break after 1233 arc discharging. The optical image shows that the shape of contact remains well with some dents and protuberances(Fig. 7a, 7b). The surface morphologies of Ag/Ti3C2Tx contact after arc erosion are subsequently exhibited in Fig. 7c, complete edge and flat surface were further confirmed by SEM. Some Ag spheres, solidified Ag blocks, and small cracks were observed at high-magnification SEM image(Fig. 7d). Fig. 7e-7h exhibit the microstructure and chemical composition of Ag/Ti3C2Tx contact surface. There are many irregular dark block surrounded by little bright particles(Fig. 7e). As shown in Fig. 7f, area 1(dark block) contains large amount of Ti, O, F with trace of Ag and Al, which may be attributed to the Ti-O-F mixture produced by electrical arc erosion to Ti3C2Tx. Area 2(bright particles) mainly composes of Ag, F, and O. It is deduced that the Ag-O-F mixture were produced by the absorption of O2 in liquid Ag and reaction with -F function group of Ti3C2Tx. Fig. 7h displays two types of spheroid particles at magnified SEM image. EDS analysis results show that both the particles contained vast N element, which indicated that these two particles compose of Ag-O-F-N.
The relative mass loss of Ag/Ti3C2Tx(54% after 1233 times arc discharging) is considerably more than that of Ag/Ti3AlC2(0.82% after 3000 times arc discharging), which is also attributed to the absence of Al layer in Ti3AlC2. As analyzed previously, the lack of Al-Ag interdiffusion leads to the weak bonding strength of Ti3C2Tx with Ag, and thus decrease the mechanical property of composite, which accordingly impairs the resistance to electrical arc impact damage. In addition, the absence of Al-Ag interdiffusion also results in the poor wettability of Ti3C2Tx with Ag during electrical arc discharging and consequently decrease viscosity of molten pool, finally deteriorating the resistance to the material transfer of Ti3C2Tx and Ag under electrical arc high-temperature. Nonetheless, there is still space for the further improvement of the arc erosion resistance of Ag/Ti3C2Tx with superior electrical conductivity by the composition design, structure optimization, technique promotion in the following work.