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Active vacuum brazing of CNT films to metal substrates for superior electron field emission performance

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ABSTRACT

The joining of macroscopic films of vertically aligned multiwalled carbon nanotubes (CNTs) to titanium substrates is demonstrated by active vacuum brazing at 820 °C with a Ag–Cu–Ti alloy and at 880 °C with a Cu–Sn–Ti–Zr alloy. The brazing methodology was elaborated in order to enable the production of highly electrically and thermally conductive CNT/metal substrate contacts. The interfacial electrical resistances of the joints were measured to be as low as 0.35 Ω. The improved interfacial transport properties in the brazed films lead to superior electron field-emission properties when compared to the as-grown films. An emission current of 150 μA was drawn from the brazed nanotubes at an applied electric field of 0.6 V μm−1. The improvement in electron field-emission is mainly attributed to the reduction of the contact resistance between the nanotubes and the substrate. The joints have high re-melting temperatures up to the solidus temperatures of the alloys; far greater than what is achievable with standard solders, thus expanding the application potential of CNT films to high-current and high-power applications where substantial frictional or resistive heating is expected.

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(a) SEM image of a CNT film brazed to Ti with the Ag–Cu–Ti alloy. (b) SEM image of the fillet showing the metal matrix composite region, the diffusion zone and the aligned CNTs. (c) SEM top view image after removal of the top CNT layer. (d) High magnification HeIM image of the top of a metal matrix composite bundle showing metal-sheathed nanotubes protruding from the matrix. (e) SEM image of the fillet when brazing CNTs on Ti/Ni with the Ag–Cu–Ti alloy. (f) SEM image of the diffusion zone and coated bundles.
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Figure 5: (a) SEM image of a CNT film brazed to Ti with the Ag–Cu–Ti alloy. (b) SEM image of the fillet showing the metal matrix composite region, the diffusion zone and the aligned CNTs. (c) SEM top view image after removal of the top CNT layer. (d) High magnification HeIM image of the top of a metal matrix composite bundle showing metal-sheathed nanotubes protruding from the matrix. (e) SEM image of the fillet when brazing CNTs on Ti/Ni with the Ag–Cu–Ti alloy. (f) SEM image of the diffusion zone and coated bundles.

Mentions: A second alloy, Ag–Cu–Ti, containing only 1.75 wt% of Ti was used to join CNT films to Ti and Ti/Ni substrates at 820 °C, that is, above the liquidus temperature of this alloy. A typical CNT film brazed to Ti after silicon lift-off is shown in figure 5(a). A fillet is seen on the edge of the CNT film similarly to what was observed for the Cu–Sn–Ti–Zr braze, however the metal matrix composite region is now separated from the top CNT region by a thin diffusion zone as shown in figure 5(b). Cu and Ag especially are known to be highly mobile on graphene. Again, the bare CNTs in region 1 were removed mechanically and revealed extensive bundling leading to a porosity of ∼48% as shown in figure 5(c). A high magnification HeIM image of the top of one of the metal matrix bundles reveals individual metal-sheathed CNTs protruding from the matrix (figure 5(d)). Evidently, the CNTs were not fully converted to TiC here. This is due to the reduced Ti content and lower brazing temperature. Slight microstructural differences are observed when brazing CNTs on Ti/Ni. The fillet height is reduced and bundling is less pronounced with the metalized substrate (figure 5(e)). Furthermore, a region a few micrometers in length with metal-coated bundles is now seen below the diffusion zone (figure 5(f)). Additional EDX elemental mappings led to very similar results as in the case of brazing with the Cu–Sn–Ti–Zr alloy.


Active vacuum brazing of CNT films to metal substrates for superior electron field emission performance
(a) SEM image of a CNT film brazed to Ti with the Ag–Cu–Ti alloy. (b) SEM image of the fillet showing the metal matrix composite region, the diffusion zone and the aligned CNTs. (c) SEM top view image after removal of the top CNT layer. (d) High magnification HeIM image of the top of a metal matrix composite bundle showing metal-sheathed nanotubes protruding from the matrix. (e) SEM image of the fillet when brazing CNTs on Ti/Ni with the Ag–Cu–Ti alloy. (f) SEM image of the diffusion zone and coated bundles.
© Copyright Policy - open-access
Related In: Results  -  Collection

License 1 - License 2
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getmorefigures.php?uid=PMC5036490&req=5

Figure 5: (a) SEM image of a CNT film brazed to Ti with the Ag–Cu–Ti alloy. (b) SEM image of the fillet showing the metal matrix composite region, the diffusion zone and the aligned CNTs. (c) SEM top view image after removal of the top CNT layer. (d) High magnification HeIM image of the top of a metal matrix composite bundle showing metal-sheathed nanotubes protruding from the matrix. (e) SEM image of the fillet when brazing CNTs on Ti/Ni with the Ag–Cu–Ti alloy. (f) SEM image of the diffusion zone and coated bundles.
Mentions: A second alloy, Ag–Cu–Ti, containing only 1.75 wt% of Ti was used to join CNT films to Ti and Ti/Ni substrates at 820 °C, that is, above the liquidus temperature of this alloy. A typical CNT film brazed to Ti after silicon lift-off is shown in figure 5(a). A fillet is seen on the edge of the CNT film similarly to what was observed for the Cu–Sn–Ti–Zr braze, however the metal matrix composite region is now separated from the top CNT region by a thin diffusion zone as shown in figure 5(b). Cu and Ag especially are known to be highly mobile on graphene. Again, the bare CNTs in region 1 were removed mechanically and revealed extensive bundling leading to a porosity of ∼48% as shown in figure 5(c). A high magnification HeIM image of the top of one of the metal matrix bundles reveals individual metal-sheathed CNTs protruding from the matrix (figure 5(d)). Evidently, the CNTs were not fully converted to TiC here. This is due to the reduced Ti content and lower brazing temperature. Slight microstructural differences are observed when brazing CNTs on Ti/Ni. The fillet height is reduced and bundling is less pronounced with the metalized substrate (figure 5(e)). Furthermore, a region a few micrometers in length with metal-coated bundles is now seen below the diffusion zone (figure 5(f)). Additional EDX elemental mappings led to very similar results as in the case of brazing with the Cu–Sn–Ti–Zr alloy.

View Article: PubMed Central - PubMed

ABSTRACT

The joining of macroscopic films of vertically aligned multiwalled carbon nanotubes (CNTs) to titanium substrates is demonstrated by active vacuum brazing at 820 °C with a Ag–Cu–Ti alloy and at 880 °C with a Cu–Sn–Ti–Zr alloy. The brazing methodology was elaborated in order to enable the production of highly electrically and thermally conductive CNT/metal substrate contacts. The interfacial electrical resistances of the joints were measured to be as low as 0.35 Ω. The improved interfacial transport properties in the brazed films lead to superior electron field-emission properties when compared to the as-grown films. An emission current of 150 μA was drawn from the brazed nanotubes at an applied electric field of 0.6 V μm−1. The improvement in electron field-emission is mainly attributed to the reduction of the contact resistance between the nanotubes and the substrate. The joints have high re-melting temperatures up to the solidus temperatures of the alloys; far greater than what is achievable with standard solders, thus expanding the application potential of CNT films to high-current and high-power applications where substantial frictional or resistive heating is expected.

No MeSH data available.


Related in: MedlinePlus