Low-voltage ionization of air with carbon-based materials

Abstract

Polycrystalline diamond and carbon nanotubes exhibit excellent vacuum field emission properties, characterized by low turn-on voltage and high current density. Their atmospheric field emission and ionization capabilities are reported in this paper. Highly graphitic polycrystalline diamond (HGPD) film was grown in a plasma-enhanced chemical vapor deposition process, and its ability to ionize atmospheric air was characterized and compared against carbon nanotubes.The HGPD sample was activated by applying a moderate voltage bias (340V) for an extended period across a 10 µm electrode gap. After activation, a turn-on voltage of 20 V and a sustainable current of 10 µA was observed with the same gap. Results also indicate that field emission helps to create a moderate ionization effect without catastrophic air breakdown. A hydrogen plasma treatment is shown to restore emission current back to or even exceeding the original level, which suggests an important role of surface termination in electron emission process. Carbon nanotubes were grown and tested but did not perform as well under similar conditions.

Planar microscale ionization devices in atmospheric air with diamond-based electrodes

“…However, the data are not perfectly linear especially at lower applied potentials (1/ > 0.05 V -1 ), suggesting that field emission likely does initiate below 20 V, and this very low-voltage current is due to random events common in a non-self-sustaining discharge. Figure 13 shows the variation of current with applied potential for three different HGPD ionization devices on Si for gaps of 10, 15, and 20 m, as well as data from Peterson et al [28]. As is evident, the performance of the present HGPD electrodes is comparable to potential-current performance of the HGPD thin film tested by Peterson et al [28] with maximum currents of approximately 1-5 A for the applied potential range tested.…”

“…Figure 13 shows the variation of current with applied potential for three different HGPD ionization devices on Si for gaps of 10, 15, and 20 m, as well as data from Peterson et al [28]. As is evident, the performance of the present HGPD electrodes is comparable to potential-current performance of the HGPD thin film tested by Peterson et al [28] with maximum currents of approximately 1-5 A for the applied potential range tested. Two separate ramps for the same 20 m device are also shown in Figure 13.…”

“…One of the probes was connected to a Keithley 6485 picoammeter and grounded, while the other probe was connected to a high-voltage power supply. (In this study, two different power supplies were used -a Spellman CZE1000 was used for voltages greater than 300 The results from HGPD devices were compared to point-to-plane measurements by Peterson et al [28] for HGPD thin-films on tungsten (W) and to new point-to-plane measurements for HGPD thin-films on Si. In this work, 'thin film' refers to a substrate that has complete HGPD coverage in contrast to the present ionization devices which consist of electrodes patterned from a thin film.…”

“…The basic growth procedure for HPGD thin films using bias-enhanced nucleation is documented in [28]. For the point-to-plane measurements, HGPD was grown on Si substrates with no SiO 2 insulating layer.…”

“…Prior work in the authors' group has considered various carbon-based materials, including highly graphitic polycrystalline diamond (HGPD) and carbon nanotubes (CNTs), for low-voltage ionization in air (Peterson et al [28]). Using a point-to-plane configuration, it was demonstrated that HGPD (as grown on a tungsten substrate) operated at appreciable currents (1-100 A) and low voltages (< 400 V) without breakdown occurring in the air gap.…”