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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.

No MeSH data available.


(a) Applied voltage versus anode-CNT distance and (b) field-emission current versus applied electric field for the brazed CNT film on Ti/Ni and for the CNT film grown on Si. (b) The ideal emitter behavior is described by the FN model according to: I(E) = fFN(E) (dashed line). The contact resistances can be obtained from the resistor-limited FN fits according to: I(E) = fFN(E − IR) (solid lines). (c) Literature comparison of emission current density versus applied electric field with the results obtained in this work.
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Figure 7: (a) Applied voltage versus anode-CNT distance and (b) field-emission current versus applied electric field for the brazed CNT film on Ti/Ni and for the CNT film grown on Si. (b) The ideal emitter behavior is described by the FN model according to: I(E) = fFN(E) (dashed line). The contact resistances can be obtained from the resistor-limited FN fits according to: I(E) = fFN(E − IR) (solid lines). (c) Literature comparison of emission current density versus applied electric field with the results obtained in this work.

Mentions: The field-emission behavior of the brazed CNT films on Ti/Ni was measured with a SAFEM and compared to the emission of a CNT film grown on Si. The instrument allows an accurate determination of the CNT apex height by means of the voltage versus anode-CNT distance plots which are shown in figure 7(a). From the resulting linear plot, the location of the emitter apex can be extrapolated as the height for V = 0. This is a very important aspect, since the real anode-CNT apex distance can be accurately determined for every measurement, obtaining a direct measurement of the applied electric field. In addition to the CNT height determination, the slope of the curve gives information related with the so called field enhancement factor (β) caused by the accumulation of the electric field lines at the CNT apex due to their high aspect ratio (see inset in figure 7(b)). The β value for an individual CNT is uniquely related with the geometry of the emitter and can be calculated in first approximation (i.e. floating sphere model) from the equation β = h/r, with h and r the height and radius of the CNT, respectively. However, dense CNT forest samples present drastically reduced β values due to the screening from neighbor tubes (inset in figure 7(b)) which emission is usually limited by randomly distributed ones that stick out from the sample. The determination of β can be calculated from the slope of the voltage versus anode-CNT distance curves assuming that the electric field needed at the CNT apex to achieve an emission current of 50 nA is around 4000 V μm−1 [31]. It is remarkable that the slopes obtained from the V versus anode-CNT distance are very low (between 0.38 and 0.2 V μm−1) giving rise to extremely high β values ranging from around 10 000 to 20 000. Such high values are obtained for both brazed and as-grown CNT with a radius of around 10 nm as determined by SEM images in figure 2. The calculated β indicates that the height of tubes which stick out from the forest surface is around 100–200 μm, which is in good agreement with the SEM images.


Active vacuum brazing of CNT films to metal substrates for superior electron field emission performance
(a) Applied voltage versus anode-CNT distance and (b) field-emission current versus applied electric field for the brazed CNT film on Ti/Ni and for the CNT film grown on Si. (b) The ideal emitter behavior is described by the FN model according to: I(E) = fFN(E) (dashed line). The contact resistances can be obtained from the resistor-limited FN fits according to: I(E) = fFN(E − IR) (solid lines). (c) Literature comparison of emission current density versus applied electric field with the results obtained in this work.
© Copyright Policy - open-access
Related In: Results  -  Collection

License 1 - License 2
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Figure 7: (a) Applied voltage versus anode-CNT distance and (b) field-emission current versus applied electric field for the brazed CNT film on Ti/Ni and for the CNT film grown on Si. (b) The ideal emitter behavior is described by the FN model according to: I(E) = fFN(E) (dashed line). The contact resistances can be obtained from the resistor-limited FN fits according to: I(E) = fFN(E − IR) (solid lines). (c) Literature comparison of emission current density versus applied electric field with the results obtained in this work.
Mentions: The field-emission behavior of the brazed CNT films on Ti/Ni was measured with a SAFEM and compared to the emission of a CNT film grown on Si. The instrument allows an accurate determination of the CNT apex height by means of the voltage versus anode-CNT distance plots which are shown in figure 7(a). From the resulting linear plot, the location of the emitter apex can be extrapolated as the height for V = 0. This is a very important aspect, since the real anode-CNT apex distance can be accurately determined for every measurement, obtaining a direct measurement of the applied electric field. In addition to the CNT height determination, the slope of the curve gives information related with the so called field enhancement factor (β) caused by the accumulation of the electric field lines at the CNT apex due to their high aspect ratio (see inset in figure 7(b)). The β value for an individual CNT is uniquely related with the geometry of the emitter and can be calculated in first approximation (i.e. floating sphere model) from the equation β = h/r, with h and r the height and radius of the CNT, respectively. However, dense CNT forest samples present drastically reduced β values due to the screening from neighbor tubes (inset in figure 7(b)) which emission is usually limited by randomly distributed ones that stick out from the sample. The determination of β can be calculated from the slope of the voltage versus anode-CNT distance curves assuming that the electric field needed at the CNT apex to achieve an emission current of 50 nA is around 4000 V μm−1 [31]. It is remarkable that the slopes obtained from the V versus anode-CNT distance are very low (between 0.38 and 0.2 V μm−1) giving rise to extremely high β values ranging from around 10 000 to 20 000. Such high values are obtained for both brazed and as-grown CNT with a radius of around 10 nm as determined by SEM images in figure 2. The calculated β indicates that the height of tubes which stick out from the forest surface is around 100–200 μm, which is in good agreement with the SEM images.

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.