The centimeter-long strands of MSCNTS used in our experiments are synthesized by the catalytic chemical vapor deposition (CVD) method. The CNTs in the MSCNTS are mainly multi-walled, with diameters ranging from 5 nm to 30 nm and lengths of more than 4 millimeters.

The specific fabrication steps of the MSCNTS-HWFS include:

1. A thin MSCNTS with diameters of about tens of micrometers are pulled out from a thick as-grown MSCNTS using precise tweezers;
2. Then the MSCNTS is suspended straightly between the two 4.4 mm-spacing metal prongs;
3. A drop of conducting adhesive is dripped on each contact area and is dried under 120 °C heating conditions for 2 hours for fixing the two ends of the strand and improving the contact characteristics.

Figure 1a is a schematic diagram of the MSCNTS flow-rate sensor and Figure 1b is the enlarged view of the MSCNTS which shows the MSCNTS is composed of many individual CNTs (the MSCNTS was cut off for showing its cross-section).

Aluminum oxide (Al₂O₃) is known to have good thermal conductivity and insulating property. To reduce the disturbance by the surrounding environment, the surface of the MSCNTS is deposited with a layer of Al₂O₃ via the magnetron sputtering. The thickness of the Al₂O₃ layer can be controlled by sputtering time. Figure 2 shows the MSCNTS’s scanning electron microscope (SEM) images where 50 nm Al₂O₃ is uniformly sputtered on the MSCNTS surface. Inset in Figure 2b is the EDS (Energy dispersive x-ray spectroscopy) analysis of the MSCNTS coated with a layer of Al₂O₃, which demonstrates that the Al₂O₃ has been coated on the MSCNTS’s surface.

We firstly tested the steady-state performance of the MSCNTS-HWFS in a wind tunnel where maximum air velocity was about 15 m/s, which was calibrated by a commercial HWFS to guarantee measurement accuracy. The fabricated MSCNTS-HWFS was operated in constant-current (CC) mode: a constant-current source applies a constant current on the MSCNTS, and the voltage drop of the MSCNTS-HWFS was recorded at different air velocity conditions. Although the resistance values of each MSCNTS-HWFS and Pt100 (reference commercial HWFS) were different, they retained the same power (0.01 W) in the comparable measurement. Figure 3a shows the steady-state response curves of the MSCNTS-HWFS at different air velocities wherein the CNT surface is bare.

Figure 3b shows the steady-state response curves of Pt100 at different air velocities. In CC mode, the sensitivity S of the HWFS can be defined as follows:

This study also measured the response curves of the MSCNTS-HWFS coated with a layer of Al₂O₃. Figure 4 reveals an improvement in the curves’ repeatability at the covered MSCNTS-HWFS. The hysterisis error is reduced to 1.56%. The sensitivity of the covered MSCNTS-HWFS improves slightly (the value is 4.24%) based on the calculations.

We further tested the dynamic response of the coated MSCNTS-HWFS. The time responses under 8.5 m/s flow velocity were recorded as shown in Figure 5 by the withdrawing baffle plate method for producing the air flow step (both MSCNTS-HWFS and Pt100 work at a power of 0.01 W). As shown in Figure 5 the response time of MSCNTS-HWFS is about tens of microseconds, which is faster than that of the Pt100 (about several seconds).

We also measured the sensitivity of another MSCNTS-HWFS (sample 2#) where the MWCNTs in the strand have larger average diameter (about 50 nm) than that in the MWCNTS (sample 1#), shown in Figure 2. Figure 6 shows the SEM images of sample 2# which show that sample 2# has a looser structure than that of sample 1#. The sensitivity measurement results show that sample 2# has lower sensitivity than that of the sample 1#, which proves sample 1# has a larger S/V ratio than that of sample 2# due to the smaller average diameter of MWCNT in sample 1#. The experimental results and the SEM images also demonstrate the loose structure in the MSCNTS has better heat transfer than that of the compact structure in the MSCNTS.

The theoretical analysis and experiments prove that MSCNTS-HWFS has higher sensitivity than conventional Pt HWFS due to the large S/V ratio of the MSCNTS. It is also found that it effectively improves repeatability and reduces hysterisis error via the coating of the Al₂O₃ layer on the CNT surface. The reason may be that the covered CNTs are not easily disturbed by the surrounding environment and the heat could be transferred through the covered layer with high thermal conductivity. Because circumjacent humidity and gas don’t influence the resistance of the MSCNTS coated with Al₂O₃, the stability of the covered MSCNTS-HWFS is improved. The experimental curves also show that the MSCNTS-HWFS has higher sensitivity in low air speed range, which suits for low-speed precision measurement. The coated Al₂O₃ layer on the CNT surface is very thin, so MSCNTS-HWFS also has fast dynamic response.