PLASMA PROCESSING UPDATE
A newsletter from
the
Facilitation
Centre for Industrial Plasma Technologies,
Institute for
Plasma Research
Issue 49 May - December 2005
Editor's Note
¨
Plasma Torch Igniter
for Scramjets
¨
Surface
Characterization of Plasma Modified 316 Stainless Steel
¨
Power Balance at
Cathode in Glow Discharges
¨
Non-Exclusive
Nitriding Technology Transferred to M/s Milman Thin Films Pvt. Ltd.
¨
IPR in India R & D 2005
Reliable
high altitude ignition of fuel-air mixture in main combustor and after burner
is still a problem for major aircraft gas turbine engines. Development of a
pilot burner for combustion assistance and toxicity reduction from take-off to
landing has been started. The idea to use plasma torch and active products of
rich plasma-fuel mixture reactions for ignition and flame stabilization has
been a solution to these problems. Many researchers have obtained positive
results in the generation of active radicals, reduction of ignition time delay,
extension of flammability limits, and flame stabilization at various fuel-air
mixture flows and fuel compositions using plasmas. FCIPT has recently developed
a plasma torch igniter for DRDL,
There has been an increasing
demand for applying surface analytical techniques to the characterization of
materials that have been surface modified by methods such as self-assembled
monolayers, chemical adsorption reactions, electroless deposition, plasma
treatment and many others. Surface analysis provides important information on
the surface composition of a material, which may be different from the nominal
bulk composition like, the chemical state of the elements on the surface,
contaminants in thin films, the nature of chemical interactions and changes in
the surface properties. X-ray photoelectron spectroscopy (XPS) has become a
very well-established analytical tool for the investigation and
characterization of material surfaces. Dr. P. M. Raole explains the use of XPS
to study the surface of SS 316 modified by plasma immersion ion implantation
technique.
When an electrode is biased negative (~ few hundred
volts) with respect to a metallic chamber, maintained at a sub-atmospheric
pressure (~ few mbar), glow discharge plasma is formed around the electrode
(cathode). The plasma supplies ions and other species to the cathode, along
with other events at the cathode, as a result its
temperature rises. Various modes of power input to the cathode were estimated
from the discharge parameters. The contribution of ions and neutrals to total
input power was obtained theoretically from respective velocity distribution at
the cathode considering charge exchange as the dominant collision mechanism
inside the sheath region. A remarkable observation was made by Mr. Suraj, where
he found that the major contribution to the energy input at the cathode comes
from energetic neutrals generated by the charge exchange collision inside the collisional
ion sheath.
Alphonsa Joseph
Editorial Assistance: A. Satyaprasad
The
Institute for Plasma Research (IPR) is exclusively devoted to research in
plasma science, technology and applications.
The
Facilitation Centre for Industrial Plasma Technologies (FCIPT) links the
Institute with the Indian industries and commercially exploits the IPR
knowledgebase. FCIPT interacts closely with entrepreneurs through the phases of
development, incubation, demonstration and delivery of technologies. Complete
package of a broad spectrum of plasma-based industrial technologies and
facilitation services is offered.
Some of
the recent achievements of FCIPT are: plasma nitriding of industrial components
to increase wear resistance and hardness, coating of quartz-like films on
brassware to inhibit oxidation and tarnishing, ceramic synthesis and
processing, plasma ion implantation and ion plating for surface engineering,
thermal plasma technologies for smelting of minerals and waste treatment etc.
The
Centre has process development laboratories, jobshops and material
characterisation facilities. The process development laboratory exploits the
areas of expertise in plasma and other allied fields of the institute in
developing new plasma based technologies for the industry. The jobshop executes
job work for surface and material treatment on an industrial scale to promote
the acceptance of plasma based technologies and to generate techno-commercial
data relevant to entrepreneurs. The advanced instruments in the material
characterisation facility are open to users from industry, research
establishments and universities.
This
newsletter is designed to help you keep abreast with the developments in the
important field of plasma assisted manufacturing and to look for new industrial
opportunities. We would be very happy to have you write to us on ways of
improving this service or visit us for further discussions.
Please visit our website: http://www.plasmaindia.com
Plasma Torch Igniter for
Scramjets
A scramjet is a type of engine which is designed to
operate at high speeds, normally associated with rocket propulsion. It is
different than a rocket because it uses air collected from the atmosphere to
burn its fuel, rather than carrying oxidiser in tanks. Normal jet engines and
ramjet engines also use air collected from the atmosphere in this way. The
problem is that collecting air from the atmosphere causes drag, which increases
quickly as the speed increases. Also, at high speed, the air collected becomes
so hot that the fuel doesn't burn properly any more.
It was dicovered by NASA during its subscale scramjet
engine tests that there exists two flight regimes where a source of ignition or
combustion enhancement is required with hydrogen fuel. At low flight mach
numbers, the total temperature is below the autoignition temperatue for
hydrogen and an igniter is required.
Whereas at Mach 7 simulated flight conditions, a source of combustion
enhancement is required to augment the combustion and compensate for the high
velocities and short residence times.
The concept of using a plasma
torch as an igniter and flameholder has been extensively investigated. The
effectiveness of the plasma torch is thought to be related to the ability of
hydrogen atoms to reduce the ignition delay time of a hydrogen-air mixture. The
reults indicate that 0.1% hydrogen atoms are as effective as 20% silane in
reducing ignition delay times at the lower tempratures. The plasma torch is an
externl source of hydrogen atoms and should be an effective pilot for
supersonic combustion.
FCIPT has built a plasma torch
igniter for scramjets and has successfully demonstrated the torch with nitrogen
gas and supplied to DRDL,
Pencil
plasma torch was used for dissociation of hydrocarbons like butadiene, kerosene
etc.. Kerosene is a complex mixture of
hydrocarbons and its composition includes C9 to C16 hydrocarbons with a boiling
range of about 300-550 degrees F. The
hydrocarbons were analyzed using flame ionization detector and inorganic gases
were analyzed using thermal conductivity detector in gas chromatograph. FCIPT has investigated the reforming of
kerosene using single and multiple plasma torches. It was observed that
kerosene gets dissociated into carbon-monoxide, hydrogen, methane etc. Multiple
plasma torches placed down stream may be exploited to provide ionized and
excited hydrogen molecule to aid the combustion in a scramjet engine.
Description of the plasma torch:
FCIPT has developed a small and
light weight plasmatron for fuel reforming which does not involve water cooling.
Plasmatron is an electrical discharge device that takes advantage of the finite
conductivity of gases at elevated temperatures. Plasmatron fuel converter
provides electrical discharges in flowing gases of hydrocarbon fuels and air.
This generates reactive species in the flowing gases and reformation of
hydrocarbon fuels into hydrogen rich gas takes place. The plasmatron consists
of central tungsten wire of 2 mm diameter as powered arc electrode and an
annular copper cup as grounded electrode Fig.-1. The flow of the gas is
tangential to arc electrode. The capacitor is connected in series with the
electrodes of the torch module for the purpose of ballasting the electric
discharge. Between the gaps a particular gas or mixture of gases is flowing at
a specific pressure, which limits the diameter of the plasma jet. The type and
composition of the feedstock, the flow pressure at the electrode and arc gap
determines the required voltage differential between the electrodes to initiate
an electric arc.
Fig. 1:
Experimental set up for plasma pyrolysis of kerosene
Fig.2 shows the carbon
monoxide (CO) formation with the applied voltage. Kerosene vapor is fed in ppm
level with nitrogen as a carrier gas using gas mixing unit in the torch. The
formation of CO is less when 20 lpm nitrogen plus 658 ppm kerosene flows
through the torch in comparison to 15 lpm nitrogen plus 878 ppm kerosene.
Fig. 2:
Plot of Carbon monoxide gas with change in voltage
Fig.3 shows the graph
between NOx (ppm) and applied voltage. From the graph it was observed that at
15lpm N2 plus 878 ppm kerosene flow through the torch produced NOx
which decreased with voltage. Similarly at 20lpm N2 plus 658 ppm
kerosene flow the NOx formation increases with applied voltage.
Fig. 3:
Plot of NOx formation with change in voltage
Optical emission spectroscopy
result shows that as the power to the torch increases the dissociation of
kerosene is more effective and the hydrogen generation is increased. Hence, pencil plasmatron, a low
current high voltage torch is good for dissociation of low concentration of
hydrocarbons. An increase in the power of torch increases the hydrogen
generation, also the formation of CO is nearly constant with power. Further FCIPT also conducted
ground experiments with this plasma torch and optimized the parameters for
igniting the scramjet at DRDL.
Surface Characterization of Plasma Modified 316
Stainless Steel
Surface
property changes such as improved mechanical, chemical, optical and other
properties using surface modifications play a significant role in materials
applications in the areas such as fusion research, nano-science and technology,
nuclear technology, space technology, semiconductor technology, biomedical
science, engineering & manufacturing etc. There has been continuing efforts
to understand the property changes and correlate these changes with the surface
modifications [1, 2]. However, deeper understanding of the surface modifications
at the nano-scale and atomistic level is required for the controlled processing
of material surfaces in specific applications.
Surface
modification of materials using low pressure plasma techniques such as plasma
ion implantation and plasma sputtering can lead to the surface changes
including the composition, bonding, formation of different phases near the
surface, surface morphology and topography. These changes are the manifestation
of the material character and its behavior during the process, and dependent on
plasma parameters as well as the environment in which the processing is done.
The modified
layer is typically of sub-micron to few micron deep.
The state of the implanted ion and its depth distribution is required to be
studied to understand modification status and mechanism. The
modified surface is characterized by different surface techniques to know
details of modification at the sub-micron to nanometer depths. XPS provides the best option for the analysis
of a modified surface by both these processes.
X-ray photoelectron spectroscopy (XPS), also referred
to as electron spectroscopy for chemical analysis (ESCA), is a sensitive
technique for investigating the elemental composition and the associated
chemical bonding states of the near-surface region of a sample. The
technique relies on the emission of secondary electrons from the surface after
the near surface atoms have been excited with X-rays. XPS provides the
energy resolution needed to detect elemental peak position energy shifts due to
chemical bond formation. Information is typically obtained for the region
within ~50 Ĺ of the outer surface. Angle resolved XPS can allow analysis
of regions even closer to the sample surface. Relative atomic
concentration percentages can be determined with a sensitivity of 0.1 to 1
atomic % for Li and heavier elements. Analysis is carried out in an
ultrahigh vacuum system (~10-9 Torr or better). It, also, can be used
effectively for the concentration depth profiling of thin layers while simultaneously
getting bonding information and electronic structure at each step. This is
explained by the following example.
The PIII of
nitrogen in austenitic stainless steel has been carried out in a vacuum chamber
with a base pressure of 10-5 mbar and working pressure of 10-3
mbar [3]. A higher fluence, by varying pulse repetition rate, was delivered to
keep the same treatment temperature of 400 °C for bias of -1 kV, whereas a lower fluence is
delivered for higher substrate bias of –20 kV.
XPS spectra
have been recorded for the samples with substrate bias of -1 kV and -20 kV, at
different depths, to understand the compositional status of the near surface
region and the phases present in that region. Table 1 gives binding energies
for Fe 2p, Cr 2p, Ni 2p and N1s.
Table 1: XPS binding energies for
substrate bias = -1 kV and –20 kV
|
Substrate bias (kV) |
XPS peak |
B.E. After 10 mins. sputtering (eV) |
B.E. After 20 mins. sputtering (eV) |
|
-1 |
Cr 2p |
574.3 |
574.3 |
|
-20 |
|
574.3, 575.8 |
574.3, 576.2 |
|
-1 |
Fe 2p |
706.9 |
706.9 |
|
-20 |
|
707.1 |
707.1 |
|
-1 |
N 1s |
396.6 |
396.6 |
|
-20 |
|
396.7, 397.1 |
397.1 , 397.5 |
|
-1 |
Ni 2p |
852.7 |
852.7 |
|
-20 |
|
852.9 |
852.9 |
XPS
depth profiles for nitrogen implanted samples with substrate bias of –1 and –20
kV have been shown in Fig. 1 and Fig. 2.
Figure 1: XPS elemental depth profile for nitrogen
implanted 316 SS at -1kV