Chemistry Reference
In-Depth Information
Another typical PECVD process is the deposition of TiC. Widely used is the
reaction of titanium tetrachloride with methane in a hydrogen atmosphere
H
2
−→
(
(
TiCl
4
)
gas
+
(
CH
4
)
gas
TiC
)
solid
+
4
(
HCl
)
gas
.
(8.77)
Besides methane, other carbon sources like propane, heptane, and toluene are
applied. The pressure varies between 1 mbar and normal pressure. Typical process
times are 2-3 h. Morphology, structure, and chemical composition are estimated by
the substrate, the deposition temperature, the pressure, and the precursor for C. Two
independent chemical reactions take place. TiCl
4
will be reduced to Ti by hydrogen
and reacts with C to form TiC. The reactions will be limited by the supply of reactive
C species. The partial pressure ratio of the precursors CH
4
/TiCl
4
estimates the amount
of C in the gas atmosphere. It should be about one for substrates that do not contain Fe,
Co, and Ni. Since these elements act as catalysts for the decomposition of methane,
the ration can be increased to 3-8 if the substrate contains these elements [403]. The
substrate temperature is between 850
◦
C and 1050
◦
C in normal CVD technique and
can be reduced to about 500
◦
C in PECVD processes [402].
8.3 PULSED ELECTRICAL DISCHARGES IN LIQUIDS
The topic of (pulsed) electrical discharges generated in water, aqueous solutions,
and liquids is very broad, and it is impossible to treat this topic in its full depth
and full breadth adequately in a single chapter. Therefore, this chapter makes no
attempt to provide a comprehensive coverage of all aspects relating to the physics
and chemistry of pulsed electrical discharges in liquids. Instead, we will give a brief
overview of the basic mechanisms involved in the generation of pulsed electrical
discharges in a liquid. Subsequently, we will describe some specific experiments
aimed at elucidating the mechanisms that lead to the generation of reactive species
(OH, O
3
,O,H
2
O
2
) by a pulsed discharge in water [404-411]. As an example of the
technological applications of pulsed electrical discharges in liquids, we will discuss
the disinfection and decontamination of the water using pulsed electrical discharges,
a process that has the advantage that it does not require the transfer or disposal of
chemicals, which is a necessity in conventional disinfection processes using O
3
or
Cl
2
. An extended review of nonthermal plasmas in liquids and in contact with liquids
is given by Bruggeman and Leys [412].
8.3.1 Background
The generation of a discharge in water requires electric field strengths of about
2 MV/cm (depending on the conductivity of the water) and proceeds through a reduc-
tion in the local density by field or heat-related mechanisms (i.e., by forming gaseous
bubbles in the liquid phase), thus creating conditions favorable for the development
of electron and ionization avalanches [404,413]. This process consumes a significant
amount of power and converts a large fraction of the input energy into heat. Producing
electrical discharges in water in the presence of externally introduced gas bubbles
improves the energy balance with more energy used to produce chemically active
