Direct Generation of a Voltage and Current by Gas Flow Over Carbon Nanotubes and Semiconductors

We report here a direct generation of measurable voltages and currents when a gas flows over a variety of solids even at the modest speed of a few meters per second. The underlying mechanism is an interesting interplay of Bernoulli’s principle and the Seebeck effect: Pressure differences along streamlines give rise to temperature differences across the sample; these in turn produce the measured voltage. The electrical signal is quadratically dependent on the Mach number $M$ and proportional to the Seebeck coefficient of the solids. Results are presented for doped Si and Ge , single wall and multiwall carbon nanotubes, and graphite. Our results show that gas flow sensors and energy conversion devices can be constructed based on direct generation of electrical signals.

Figure 1

(a) Schematic of the experimental setup. The flow rate at the exit point is deduced from the measured flow rate at the side port using the rotameter. (b) Sample: shaded portions mark the electrodes. The positive terminal of the voltmeter is connected to the right (R) of the sample of active length $d$ kept at an angle $\alpha =\pi /4$ with respect to the horizontal axis. (c) Typical response of n doped Ge for a flow of argon gas at $7\mathrm{m}/\sec \mathrm{}$.

Figure 2

Dependence of the signal V on the flow velocity of nitrogen for (bottom to top) n-Si (filled triangles), n-Ge (stars), graphite (open squares), Pt metal (plus) , SWNT (open circles), MWNT (open up triangles) and p-Si (filled down triangles).The solid lines are fit to $V=D{u}^2$, where $D$ is the fitting parameter given in Table . Inset shows the dependence of $V$ for n-Ge as a function of the active length $d$ for a fixed value of $u$. The solid line is a fit to the equation $V=a_1+a_2\{(1+d/x_1{)}^{2/3}-1\}$ where $a_1$ and $a_2$ are fitting parameters [14].

Figure 3

Plot of $V$ versus square of the Mach number $M$ for the flow of nitrogen gas . Data and symbols are same as in Fig. 2 . Solid lines are a fit to $V=A{M}^2$, where $A$ is a fitting parameter given in Table . The inset shows the dependence of $A$ on the seebeck coefficient $S$ and the line is a linear fit with the slope of 60 K.

Figure 4

$V$ versus $M^2$ for the flow of argon( filled squares ) and nitrogen (open circles). The lines show fit to linear equation. The inset shows the expanded plot of regime I.