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BULETINUL INSTITUTULUI POLITEHNIC DIN IAŞI
Tomul LV (LIX), fasc. 1-2, 2009

Secţia
ŞTIINŢA ŞI INGINERIA MATERIALELOR
USING IONIC TRIODE IN A PLASMA STUDY FACILITY IN TERMS OF

TERMIC AND MASS TRANSFER
BY
MIHAI AXINTE, CARMEN NEJNERU, MANUELA PERJU, Ion HOPULELE
Abstract: Discharging process can produce an undesirable phenomena such as arc discharge or the double
cathode effect. Active screen plasma nitriding (ASPN) is an emerging surface engineering technology. By
placing the ionic triode is amended both polarized electric field distribution between electrodes and the
characteristic volt-amperes curve configuration of electrical discharge. We can change a classical plasma
nitriding facility very easy into ionic triode terms by placing a screen between anode and cathode with a power
source to the active screen. A simplified scheme for a fizico-mathematical model for stability study of electrical
discharge inside the thermo-chemical treatment is presented. Analysis of the test facility has resulted in a better
stability in terms of discharge for polarization of the screen and possibly an increase in heating efficiency.
Keywords: active screen plasma nitriding, ionic triode..
1. Introduction
Heating installations currently operating in the heat treatments sectors have a

low thermal efficiency, both due to constructive solutions, and inefficient methods of
chemical energy or electrical energy transformation required in heating premises by
thermo-chemical treatment. Using modern heating fluidized layer in the flow of
electrons in vacuum, plasma and laser are effective methods to increase yield and
quality of plants and treated parts.
A classical plasma nitridings should create necessary conditions for
achieving an electrical luminescence in an atmosphere of nitrogen and hydrogen at a
pressure as low as (2-12) MBA. where the nitriding parts should be the discharging
cathode, voltage electrical 400 - 1000V.
Physically, the ionic nitriding facility form ionic diode cathode where the
cathode is the nitrided part subject and the discharge atmosphere is formed from a
mixture of nitrogen and hydrogen at low pressure.
Physical discharging process produce an undesirable phenomena such as arc
discharge or the double cathode effect, also part of the accumulated heat in the piece is
lost in the environment by radiation.
To avoid these 2 phenomena several measures are taken to prevent, so to
prevent arc discharge there are used specialized electronic circuits and to avoid double
cathode is amended in convenient way pressure and supply voltage. Reduced heat loss
is achieved through the use of thermal screens.
In case of ionic triode the screen between anode and cathode is electrically
polarized, so it changes the configuration of electric field between anode and cathode.
64 SORIN POPESCU et al.
Changing the electrical field between anode and cathode changes both
degeneration conditions of the discharge in electric arc and the double cathode forming
by adjusting electric potential of the screen.
Due to changing electric field that surround the part in the thermo-chemical
treatment process the forming difused layer parameters will change
For the analysis of nitriding process in ionic triode conditions was
manufactured a laboratory facility that allows to track the nitriding technology and
physical parameters of discharge in the tenuous gas environment.
2. Facility for analysis the nitriding process in conditions of using an ionic

triode.system type
The physical principle of a triode ion consists in introducing a polarized screen

in the electric field between anode and cathode.
This screen is intended to change the configuration of electric field between
anode and cathode, so that the discharge to change in a convenient way. Polarized
screen conditions are good for both of discharge starting and the discharge stability.
A classical plasma nitridings facility is very easy to adjust in the ionic triode
conditions by an electrical power source to the active screen.
In order to establish the electric discharge operation conditions in the heating
process and the diffused layer effects we designed a laboratory facility that allows the
ionic triode presence parameters measurement.
Operational scheme plant is shown in Figure 1.
Fig.1. Block scheme of the discharge in the tenuous gas in terms of ionic triode.
1 - Transformer - rectifier, 2 - Anodic current regulator, 3 - Device for measuring the anodic current,
4 - Current regulator for polarization display, 5 - Device for measuring the current screen, 6 – The
installation support, 7 -Vacuum chamber, 8 - Metal anode, 9 - Polarized screen, 10 - Under heating
tubes, 11 - Insulated sheath metal, 12 –Thermocouple, 13 - Temperature measuring and controlling
installation, 14 - Regulating gas flow system , 15 - Gas tank , 16 - Vacuum pump, 17 - Vacuum
regulating system.
Bul. Inst. Polit. Iaşi, t. LV (LIX), f. 1-2, 2009 65
After chamber washing and a vacuum of 10-1 Torr connects anodic current and
regulates the discharging current. After a while the power is connected to the screen
and adjust the desired value. It shall then reduce the degree of vacuum by introducing
the gas then adjust anodic current to the desired value. Due to changing discharge
conditions the screen current is changing, which requires appropriate correction.
When the temperature reaches near the prescribed temperature the anodic
current discharge is automatically reduced.
A photo of the plant is shown in Figure 2 where is observed both power supply,
adjustment and measurement desk and the precinct work.
Fig.2 Photo- installation in terms of ionic triode:

1 - desk power adjustment and control, 2 - Vacuum bell, 3 - Anode, 4 - Polarized screen, 5-Under
heating tubes
Analysis of the test facility has resulted in a better stability in terms of

discharge for polarization of the screen and possibly an increase in heating efficiency.
3. Electric discharge stability analysis in tenuous gas
In the technological aspects (for the thermo-chemical treatment), the discharge

in the tenuous gas must be stable and efficient in terms of heat and mass transfer in the
superficial layer. In electrical discharge between two electrodes in a tenuous gas there
are a range of physical phenomena that lead to a distribution of electric field (Fig.3).
Fig. 3 - Distribution of electric field in the discharge

1 - Tenuous gas space, 2 - Anode, 3 - Cathode, 4 - Distribution of electric field, U-voltage power
between anode and cathode, d-distance between anode and cathode
From electric field distribution analysis, it follows that the voltage drop occurs
near the cathode, leading to a process of heating it. This phenomenon is even more
intense as the current is higher and the vacuum is lower (1-10 Torr).
To have a picture of the phenomenon must be analyzed the characteristic curve
volt-amps of discharge into the environment of tenuous gas, an example is provided in
Figure 4.
Fig. 4. Characteristic volt-amperes curve for neon at a pressure of 1 Torr in a tube length of
50 cm and eletrodes section of 10 cm2
To achieve thermo-chemical treatment is used the discharge conditions of FG
part, which gives both a good stability of discharge and a power strong enough to
ensure that the process of heating the cathode.
An objective analysis of the technological process of thermo-chemical

treatment by discharge into the environment of tenuous gas can be achieved by using a
physico-mathematical model so as to reveal the stability of the process taking into
account characteristic diagram volt-amperes.
To ensure stability of the process must take into account both the source of
electric power and consumer characteristics (the discharge gas in tenuous gas.
A simplified scheme for a fizico-mathematical model for stability study of
electrical discharge inside the thermo-chemical treatment is presented in figure 5.
Fig. 5 - The source-consumer circuit for discharge into the environment of tenuous gas
. Us-voltage power source,
U-voltage discharge between electrodes,
R-resistance on the circuit power,
I-current intensity.
Under the scheme of figure 5 can be written following equation (physico-

mathematical model):
Us = U + IxR (1)
Product I x R represents the drop of resistance voltage circuit.Electrical voltage

U (of the discharge process) is in accordance with characteristic curve volt-amperes
(presented in Figure 4) the portion is F-G:
U=Φ(I) (2)
If in the download process appears a disturbance, meaning a variation of current

(an instability), then the voltage will change according to changes in current.
(3)
Power supply will respond with a power of selfinduction, knowing that the
power network is currently via a high voltage transformer. Changing supply voltage
Us will value:
(4)
L inductance value depends on the electrical transformer and network features.

As a result of these disruptions, relationship 1 acquires the following form:
Us+ΔUs=U+ΔU+R(I+ΔI) (5)
Introducing the 3,4 values in the relationship 5 resulting:
(6)
Using 1 and simplifying the relationship bearing in mind that the Us-U-IR=0
resulting:
(7)
This relationship can be written as:
(8)
By integrating resulting:
(9)
Conditions for the constant (C) are subject to the disturbance triggering:
For t = 0 corresponds
(10)
Thus the relationship (9) can be written as:
(11)
For the perturbation ( ) to be reduced as quickly as possible it needs to have

an electrical resistance (R) on electrical supply circuit of the discharge as high and a
slope of the curve characteristic volt-amperes more pronounced (F-G portion ).
Increasing resistance from the electrical supply leads to lower efficiency plant
which requires the minimum possible limiting value of this resistance.
Regarding discharge characteristic curve inclination may be altered by adjusting
gas pressure in the chamber and by changing the work current.
By placing the thermal screen is amended both polarized electric field

distribution between electrodes and the characteristic volt-amperes curve configuration
of electrical discharge in the environment of tenuous gas.
For this purpose facility can be used to analyze the thermo-chemical
technological treatment.process stability.
4. Conclusions:
1.-Ionic trioda has positive efect to the nitrided item surface quality, ionic
sputtering do not affect the item roughness.
2.-From the aproached matematical model we observe that facility energetic
efficiency grows.
3.-Ionic triode gets all the unwanted discharges avoiding the arc and the
double cathode.
Received August 11, 2009 Technical University “Gh. Asachi” Iaşi
REFERENCES:
1- Găluşcă D.G. - Plasma nitriding, Sedcom publishing house, Iaşi, 1997

2- Popa, Gh.- Technological applications of plasma, Al. I. Cuza University publishing house, Iaşi,
1998
3- Ştefan. M. - Considerations About the Thermal and thermochemical Treatment. Equipments
Utilisation using Vacuum Atmosphere. 15th Heat Treatment National Conference Dunaujvaros,
Hungaria, 19-21 oct. 1993
4- Vermesan G. - Plasma nitriding technological bases, Sibiu University publishing house, Sibiu,
1992,
5- Axinte M., Carmen Nejneru, Hopulele I., Stefan M ., Facility for study heating and diffusion
processes in terms of ionic triode plasma, Suceava, The 15-th International conference Tehnomus
2009 pag. 223.

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BULETINUL INSTITUTULUI POLITEHNIC DIN IAŞI

Tomul LV (LIX), fasc. 1-2, 2009

USING IONIC TRIODE IN A PLASMA STUDY FACILITY IN TERMS OF

MIHAI AXINTE, CARMEN NEJNERU, MANUELA PERJU, Ion HOPULELE

Keywords: active screen plasma nitriding, ionic triode..

Heating installations currently operating in the heat treatments sectors have a

2. Facility for analysis the nitriding process in conditions of using an ionic

The physical principle of a triode ion consists in introducing a polarized screen

Fig.2 Photo- installation in terms of ionic triode:

Analysis of the test facility has resulted in a better stability in terms of

3. Electric discharge stability analysis in tenuous gas

In the technological aspects (for the thermo-chemical treatment), the discharge

Fig. 3 - Distribution of electric field in the discharge

An objective analysis of the technological process of thermo-chemical

Under the scheme of figure 5 can be written following equation (physico-

Product I x R represents the drop of resistance voltage circuit.Electrical voltage

If in the download process appears a disturbance, meaning a variation of current

L inductance value depends on the electrical transformer and network features.

Introducing the 3,4 values in the relationship 5 resulting:

This relationship can be written as:

Thus the relationship (9) can be written as:

For the perturbation ( ) to be reduced as quickly as possible it needs to have

By placing the thermal screen is amended both polarized electric field

1- Găluşcă D.G. - Plasma nitriding, Sedcom publishing house, Iaşi, 1997

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