Chemistry Reference
In-Depth Information
1
Introduction
CONTENTS
1.1 PlasmaChemistry ............................................................... 1
1.2 HistoricalNotes.................................................................. 1
1.3 ThermalandNonthermalPlasmaChemistry................................... 2
1.4
Specifics of Nonthermal Plasma Chemistry.................................... 3
1.1 PLASMA CHEMISTRY
The plasma state of matter (also named fourth state) is characterized by the exis-
tence of free electrons, positive and negative ions, as well as excited species and
radicals in mixture with the other neutrals of gaseous systems (atoms, molecules) or
liquids. Well-known examples for natural plasmas are the interstellar matter, stars,
the ionosphere, lightnings, and flames.
Plasmas are generated by the input of thermal energy, electric field energy, radi-
ation, or beams (lasers, UV photons, electrons, protons), respectively. The principles
of plasma generation are summarized in Figure 1.1.
The focus of the topic is on
technical plasmas in gaseous systems
that are mostly initiated by applied electric
fields. Examples of such electrical gas discharges are the corona and barrier dis-
charge, sparks, arcs, and plasma torches, operating usually at atmospheric pressure.
Technicallyimportant low-pressureplasmas arethedifferent kinds of glowdischarges
driven by dc and rf voltages or microwaves.
Gas heating by thermal energy and/or collisions of neutrals with free electrons
and photons of sufficiently high energy initiate the production of free radicals and
further charged species. The generated particles are the source for various chemical
reactions in the volume as well as on the plasma interfaces, forming new compounds,
depositing layers, and modifying the properties of materials.
The science and application of chemical conversions in plasmas, including
reactive processes at interfaces, is the subject of plasma chemistry.
1.2 HISTORICAL NOTES
The existence of chemical reactions in plasmas, initiated by the input of electrical
energy in gaseous atmosphere, is a very old experience of mankind, with even Homer
describing the
smell of sulfur
∗
in the
Iliad
in air after lightning.
∗
Sulfur was later identified as nitrogen oxides.
1
