1.7. Discharges in porous materials and capillaries
Atmospheric pressure discharges typically form
thin filamentary plasma channel. For this reason they are
frequently called microdischarges.
The microdischarges produce a high density of
energetic electrons and free radicals at relatively low
energy consumption and so they represent a potential
method for flue gas treatment.
The efficiency of the treatment effect can be enhanced when the discharge
plasma is combined with a catalyst.
Because of the typical structure of catalysts (pellets, honeycomb
monoliths or porous ceramics), it is necessary to study the generation and propagation
of microdischarges inside narrow
cavities, capillaries, and porous dielectric materials.
Discharges in porous materials
We investigated the generation of microdischarges inside porous ceramics by AC high voltage (Fig. 1).
The physical properties of the microdischarges were diagnosed by electrical measurements,
photographic visualization, and optical emission spectroscopy. The effects of pore size, discharge
power, gas mixture composition, humidity and temperature
on the discharge properties and its development were
The optimum generation and distribution of
the microdischarges was observed with pore sizes of 50 μm
and 80 μm. The emission spectra of the microdischarges
indicated their strongly non-equilibrium character. A
theoretical analysis of the microdischarge mechanism
was elaborated, accounting for the pore size and the elementary
processes such as ionization, recombination, and
Selectivity and energy efficiency of plasma-induced chemical processes can be significantly improved when
combined with a catalyst. The generation of stable discharge plasma inside the narrow capillaries of an
automobile catalytic honeycomb is, however, quite difficult. We developed a novel method which
uses the combination of packed-bed or barrier discharge connected in series with the honeycomb
capillaries (Fig. 2). By the application of DC voltage across the capillaries, streamers are extended from the
packed-bed discharge into the capillaries (Fig. 3). With this method, ionization can be made inside fine channels
of honeycomb catalyst made of insulating materials. This discharge is designated as "honeycomb
discharge". Electrical and optical characteristics of the honeycomb discharge were also measured.
Fig. 2. Schematic of the "honeycomb
Fig. 3. Images of honeycomb discharge for various ac and dc voltages in 3 cm capillaries
[ISO 400, f/4, and 1/4 s].
More details in:
Hensel K., Martišovitš V., Machala Z., Janda M., Leštinský M., Tardiveau P., Mizuno A.: Electrical and Optical Properties of AC Microdischarges in Porous Ceramics, Plasma Process. Polym. 4 (7-8), 682-693 (2007). abstractdownloadcitations: 34