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Current Applied Physics 7 (2007) 186–192

Effect of powder loading on the excitation temperatureof a plasma jet in DC thermal plasma spray torch

G. Shanmugavelayutham a,*, V. Selvarajan a, P.V.A. Padmanabhan b,K.P. Sreekumar b, N.K. Joshi b

a Plasma Physics Laboratory, Department of Physics, Bharathiar University, Coimbatore 641 046, Indiab L&PTD, BARC, Mumbai 400 085, India

Received 20 December 2005; accepted 17 March 2006Available online 21 June 2006

Abstract

A DC non-transferred mode plasma spray torch was fabricated for plasma spheroidization. The effect of powder-carrier gas and pow-der loading on the temperature of the plasma jet generated by the torch has been studied. The experiment was done at different inputpower levels; the temperature of the jet was within 5000–7000 K argon was used as plasma gas and powder-carrier gas. Nickel powderparticles in the size range from 40 to 100 lm were processed. The temperature of the jet was estimated after flowing powder-carrier gasonly into the plasma jet and with powder-carrier gas feeding powder into the flame. On introduction of powder-carrier gas and powderloading the temperature of the jet was found to decrease appreciably down to 11%. The temperature of the plasma jet was estimatedusing the Atomic Boltzmann plot method.� 2006 Elsevier B.V. All rights reserved.

PACS: 52.70.Kz; 52.70.Nc; 52.77.Fv

Keywords: DC plasma jet; Plasma spheroidization; Temperature of the plasma jet; Plasma diagnostics

1. Introduction

The use of plasma torches as clean and efficient heatsources in industry has been common for many years. How-ever many of the applications, such as materials processingand plasma spray deposition would benefit from a betterunderstanding of the physical processes governing the inter-action of the plasma and the injected material. The temper-ature and velocity fields in the arc plume as it exits from thenozzle are of special interest as the interaction generallyoccurs within this region [1].

The plasma spray process is widely used to produceceramic and metallic coatings, functionally graded coatings

1567-1739/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.cap.2006.03.003

* Corresponding author. Present address: Joining and Welding ResearchInstitute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka 567 0047,Japan. Fax: +91 422 2422 387.

E-mail address: velu1976@yahoo.co.in (G. Shanmugavelayutham).

and can also be adapted to produce novel materials andultra fine particles [2]. In order to obtain the high qualityproducts, it is necessary to develop the plasma spray torchwhich has excellent performance, i.e. to generate a stableand clean plasma jet over a wide range of processing con-ditions and the plasma parameters like plasma tempera-ture, gas flow rate and input power.

In plasma processing applications actual temperature ofthe plasma jet is a significant factor. Hence it is interestedto study the effect of powder-carrier gas and powder load-ing into the jet. In this paper temperature of argon plasmajet was determined and the effect of powder-carrier gas(argon) at different input power levels and the effect offeeding of nickel powder into the plasma jet were experi-mentally studied. Also temperature of the jet was theoreti-cally calculated using polynomial fit method.

A DC non-transferred plasma spray torch has been fab-ricated for spheroidisation of particles [3–6]. To improve

Fig. 1. Schematic of the electrodes: (a) cathode, (b) anode.

G. Shanmugavelayutham et al. / Current Applied Physics 7 (2007) 186–192 187

the performance characteristics of the torch (Fig. 1), thenozzle as an extended type and make a hole perpendicu-larly at the end of nozzle axis for powder flow were fabri-cated [7,8]. In the present work, the effect of powderloading, in the materials processing process, on the temper-ature and stability of the plasma jet generated in a non-transferred mode DC plasma spray torch have beenstudied [9,10]. It was found that powder injection into theplasma jet reduced its temperature considerably.

2. Principle: emission spectroscopy method

Temperature measurements in non-transferred plasmaarc systems have been made using both probes [11] andspectroscopic diagnostics [12–15]. The probe methods havepoor spatial resolution, restricted measurement range andinduce flow turbulence. The most widely used spectroscopicdiagnostic in such plasmas is the absolute intensity method.This method requires the validity of local thermodynamicequilibrium (LTE) assumptions and a standard source forcalibration. In this paper the atomic Boltzmann plotmethod has been used to determine the excitation temper-ature of a plasma in a non-transferred arc jet. This methoddoes not require a standard source for calibration. Theintensity of several optically thin lines of Ar–I in the wave-length region 400–452 nm were recorded along a given lineof sight chord in a plasma source. However, application ofthe atomic Boltzmann method in the case of spatially inte-grated measurements cannot give the central axis tempera-ture without application of the Abel inversion technique. Amethod [16] to obtain the temperature at the axis of theplasma without performing the calculations of an Abelinversion has been suggested. It is applicable to DC plasmatorches where the plasma column has a high-gradientGaussian temperature profile.

The excitation temperature of the plasma jet was esti-mated from the intensity of atomic emission lines from the

jet using atomic Boltzmann method [17–21] which gives theaverage temperature along the line of sight of measurement.

The temperature is determined using the equation:

logðIk=gkAkÞ ¼ C � ð625Ek=T Þ ð1Þwhere

C =ln(lhcp0/4p),Ak line frequency,gk degeneracy of level k,Ek energy of the excited level,k wavelength,I intensity of the line,p0 particle density can be found from this intercept, if

the path l is known.

A plot between log(Ik/gkAk) and Ek yields a straight lineby best fit method. The inverse of the slope of the line givesthe plasma jet temperature. The relative error in tempera-ture measurement by the Boltzmann plot method is between3% and 4% [22].

Temperature has also been estimated by modifiedatomic Boltzmann method which yields the axial tempera-ture using the following equation [16–19]:

logðIEkk=gkAkÞ ¼ C � ð625Ek=T Þ ð2ÞA plot between log(IEkk/gkAk) and Ek yields straight

line by best fit method. The inverse of the slope of the linefit gives the central axis temperature of the plasma jet.

We used this modified atomic Boltzmann technique todetermine the central axis temperature of a plasma column.In this technique, a plot of log(IEkk/gkAk) as a function ofEk gives the central axis temperature; I is the spatially inte-grated line intensity, k is the wavelength and Ek and gk arethe excitation energy and statistical weight of atomic statek respectively. Optically thin lines were chosen in the wave-length region 400–452 nm their emissivity increases expo-nentially with rising temperature as required for thevalidity of this method [16]. The method used here is validfor the case of T < Tm, where Tm is the temperature at whichthe line emissivity reaches its maximum. No Ar–II lines werepresent in this wavelength region under the operatingparameters of the spray torch. The technique presented heremay be used to determine the centre axis temperature ofindustrial plasma devices and thus laborious data process-ing for Abel inversions may be avoided.

3. Experimental setup and methodology

3.1. Experimental setup

The plasma torch used in these experiments consists of awater cooled thoriated tungsten cathode and copper anodenozzle operating at atmospheric pressure. A schematic ofthe complete system is given in Fig. 2. The arc was vortexstabilized and argon was used as the plasma gas. The torchwas operated at four different power levels – 6.5, 7.9, 11.4

Fig. 2. Schematic diagram of plasma processing setup.

Table 1Typical operating parameters

Arc current 240–400 AArc voltage 27–31 VElectrode gap 12 mmPlasma gas flow rate 20 lpmPowder-carrier gas flow rate 3 lpmAnode cooling water flow rate 11.75 lpmCathode cooling water flow rate 10.15 lpmPowder feed rate (nickel) 9 g/minQuenching medium Air

188 G. Shanmugavelayutham et al. / Current Applied Physics 7 (2007) 186–192

and 12.1 kW input power with an argon flow rate of 20 lpmand powder-carrier gas flow rate of 3 lpm. Typical operat-ing parameters are given in Table 1.

For spheroidisation experiments, the torch was housedvertically inside a cylindrical reaction chamber. In thearrangement, the tip of the nozzle and the plasma jet is

Pre-amp

PMT

Plasma torch

LensInput slit

FlameAperture

Fig. 3. Block diagram of

inside the chamber. There is provision on the wall of the cyl-inder to view the jet for diagnostics.

The major diagnostic tool in the experiment is the mono-chromator (Thermo Oriel, 1/4M) and accessories to esti-mate the plasma jet temperature. It consists of an opticalarrangement, monochromator, photomultiplier tube, powersupplies, oscilloscope, X–T recorder and computer to oper-ate the grating rotation and run the scanning parameters.The block diagram for the monochromator setup is illus-trated in Fig. 3. The radiation from the plasma jet is focusedby convex lenses on the monochromator input slit. Themonochromator input slit width is 50 lm and monochroma-tor resolution @500 nm is 0.25 nm. The PMT has a photo-cathode of dia 25 mm and wavelength range 185–850 nm.The PMT output signal is preamplified and then fed intothe X–T recorder/oscilloscope. The spectral line intensitieswere recorded in arbitrary scale.

Thermo Oriel Monochromator

lifier X-T Recorder/ Oscilloscope

Computer

the diagnostics setup.

G. Shanmugavelayutham et al. / Current Applied Physics 7 (2007) 186–192 189

3.2. Methodology

3.2.1. Calibration

The monochromator is calibrated using a standardwavelength source. Both mercury and sodium sources wereused for calibration.

The torch is put in operation and a stable plasma jet isobtained. The jet is viewed through the window of the

Fig. 4. Atomic spectra of the plasma jet: (a) plasma gas; (b) plasma gas withloading.

plasma reaction chamber. Two convex lenses of mediumfocal length were used to collect the radiation from theplasma jet and focused on the input slit. The light is col-lected from the side of the plasma jet by optical arrange-ment system such that the monochromator input slit isabout a meter away from the plasma jet and at the samehorizontal level. Radiation from the slit falls on the gratingand mirrors and is received by the PMT.

powder-carrier gas and (c) atomic spectra of the plasma jet with powder

6800

Plasma gas Plasma gas+Powder-carrier gas Plasma gas+Powder-carrier gas+Nickel powder

190 G. Shanmugavelayutham et al. / Current Applied Physics 7 (2007) 186–192

Different spectral lines are scanned from 400 nm to452 nm in steps 0.2 nm with 1 ms dwell time per step byusing monoutility program which was created by ThermoOriel. The radiation collected for temperature measure-ment is at 5 mm down from the nozzle exit. From the pow-der-carrier gas port, it is 7 mm down. The powder-carriergas after introduction gets heated by the hot plasma andas such after attaining thermal equilibrium, the tempera-ture should satisfy the simple relation

F 1T 1 ¼ F 2T 2

where F1 and F2 are total gas flow rates, for plasma gas andfor plasma with powder-carrier gas and T1 and T2 are thecorresponding temperatures. The spectral lines are viewedin the oscilloscope.

Spectral intensities were recorded for the plasma: (i) jetonly (argon is plasma forming gas), (ii) plasma jet togetherwith powder-carrier gas, and (iii) plasma jet with powderthrough powder-carrier gas.

6 7 8 9 10 11 12 134400

4600

4800

5000

5200

5400

5600

5800

6000

6200

6400

6600E

xcita

tion

tem

pera

ture

, T (

K)

Input power, P (kW)

Fig. 5. Variation of excitation temperature at different input power levels.

6 7 8 9 10 11 12 13

4400

4600

4800

5000

5200

5400

5600

5800

6000

6200

6400

6600

6800

Tem

pera

ture

of t

he p

lasm

a je

t,T (

K)

Input power, P (kW)

Experimental theoretical

Fig. 6. Comparison of plasma jet temperature by experimental andpolynomial fit method as a function of input power levels.

4. Results and discussion

The excitation spectra recorded for argon plasma, argonplasma with powder-carrier gas (argon) and argon withcarrier including powder loading are shown in Fig. 4(a)–(c). Fig. 4(a) shows the spectral lines of the plasma jet withplasma gas (argon) only.

Fig. 4(a) contains standard argon lines. Fig. 4(b) is thespectrum of the plasma jet when powder-carrier gas isflown into the jet. As is seen no new lines are observed.However, the intensity levels of the lines are seen to be dif-ferent from those in Fig. 4(a).

The spectrum of the plasma jet when nickel powder inthe size range 30–90 lm was fed into the jet through thepowder-carrier gas, is shown in Fig. 4(c). Ar–I lines (namely404.442 nm, 415.859 nm, 418.188 nm, 420.067 nm, 425.936nm, 426.629 nm, 427.217 nm, 430.010 nm, 433.356 nm,434.517 nm and 451.073 nm) were used for calculating thetemperature. The other observed line 419.073 nm/419.103nm was not well resolved and thus was not considered.The values of Ik were measured and values of gk, Ak andEk at various wavelengths of Ar–I lines were taken fromthe literature [23].

The spectrum contains additional lines and also thespectral intensities are different from those of Fig. 4(a)and (b). These additional lines could be identified as thoseof copper. Nickel does not contain copper. Hence the ori-gin of the lines may be due to the anode nozzle. The linesmay be attributed to the emission from eroded copper(due to the impact of the powder particles from the nozzle)and melts and partially vaporizes.

Actually after powder processing experiment the dam-age occurred on the inner surface of the nozzle near the exithas been observed in similar manner as in [24]. Such dam-age has not been seen when the torch was operated withoutthe powder.

4.1. Effect of input power

It is seen in Fig. 5 that the excitation temperature of theplasma jet increases with increase in input power to thetorch. The curves before and after introduction of pow-der-carrier gas exhibit (near) linearity upto a power levelof 11.4 kW. However beyond 11.4 kW, for 12.1 kW, allthe four curves deviate from linearity showing a rapidincrease in temperature.

The linearity between input power and temperatureshows that the energy loss to the electrodes and walls bythe arc plasma remains proportional to the input power.However beyond 11.4 kW heat energy transmitted to theelectrodes and walls is smaller and has more energy investedinto the plasma jet, which results in rapid increase in temper-

G. Shanmugavelayutham et al. / Current Applied Physics 7 (2007) 186–192 191

ature. This is true for all the four cases studied. It appearsthat the excitation temperature is high beyond 6000 K andhence more energy is invested into the plasma resulting ina rapid increase of temperature.

Using the polynomial fit method [25] temperature hasbeen calculated and compared with experimentally deter-mined values are show in Fig. 6. It may be seen that agree-ment between experimental and polynomial fit methodvalues are good with a maximum difference is 20% at about6.5 kW input power level.

4.2. Effect of powder-carrier gas

The effect of powder-carrier gas on jet temperature isshown in Fig. 5 and Table 2. The temperature decreaseson passing of powder-carrier gas into the plasma jet. Underthe Boltzmann method, the decrease is from 11% to 3%when the initial plasma jet temperature is increased from5140 to 6587 K and under the modified Boltzmann methodthe decrease is from 11% to 3% when the temperature isincreased from 5297 to 6840 K.

The table shows that at the point of measurement, theequilibrium temperature has not been reached. Thereforethe temperature reduction corresponds to the amount ofheat gained by the powder-carrier gas atoms. At higher ini-tial plasma temperatures, the rate of heat transfer to argonas powder-carrier gas (introduced at room temperature) ishigher. Hence the temperature difference with and withoutpowder-carrier gas becomes lower when initial plasma tem-perature becomes higher. As seen in Table 2, the results arequalitatively the same when modified Boltzmann method isused for estimation of temperature.

In the present experiment, the visible flame length isapproximately 25 mm at 6.5 kW power level as determined

Table 2Temperature of the plasma jet at different power levels (Boltzmann plot meth

Input power (kW) Excitation temperature (K) Difference intemperature (K(3) = (1) � (2)

Plasma gas(argon)(1)

Plasma gas +powder-carriergas (argon)(2)

6.5 5140 4576 5647.9 5546 5134 412

11.4 6200 6001 19912.1 6587 6397 190

Table 3Temperature of the plasma jet at different power levels (modified Boltzmann p

Input power (kW) Excitation temperature (K) Difference intemperature(3) = (1) � (

Plasmagas (argon)(1)

Plasma gas + powder-carrier gas (argon)(2)

6.5 5297 4700 5977.9 5732 5298 434

11.4 6440 6217 22312.1 6840 6661 179

by a simple optical measurement. When the power level isincreased upto 12.1 kW, the flame length slightly increasesto 29 mm and when the powder-carrier gas (3 lpm argon)is introduced into the plasma jet, its length is decreased by4 mm. This finding can be explained by the fact that thepowder-carrier gas is fed perpendicular to plasma gas veloc-ity. When the powder-carrier gas mixes with the plasma gas,the velocity of the plasma gas is reduced due to momentumexchange. As flame length is proportional to velocity of thejet gas, the introduction of powder-carrier gas is expectedto reduce the flame length.

On introduction of powder-carrier gas the plasma flamelength decreases which will result in smaller residence timefor the particles which are processed and also the tempera-ture is decreased.

4.3. Effect of powder loading

When the powder is loaded, it is observed that for 6.5 kWinput power the difference is marginal and is not significantwhen we note that the relative error in temperature measure-ment by the Boltzmann plot method is between 3% and 4%[22]. For input power 7.9, 11.4 and 12.1 kW there is clearindication that the temperature falls due to the introductionof nickel powder. When the temperature is high, the reduc-tion in temperature is very significant in the results obtainedfrom Atomic Boltzmann method as well from modifiedAtomic Boltzmann method (Fig. 4(c), Tables 2 and 3).

The amount of heat transferred to a powder particledepends on the initial plasma temperature and the in-flighttime of the particle. The temperature difference with andwithout powder loading does not depend substantiallyon the initial plasma temperature. For example, for initialplasma jet temperature 6397 K the reduction in temperature

od)

)Estimatedequilibriumtemperature (K)(4)

Excitationtemperature (K)with nickel powderloading(5)

Difference intemperature (K)(6) = (2) � (5)

4470 4599 234823 5060 745391 5706 2955728 6109 288

lot method)

(K)2)

Estimatedequilibriumtemperature (K)(4)

Excitation temperature(K) with nickelpowder loading(5)

Difference intemperature (K)(6) = (2) � (5)

4606 4728 284984 5211 875600 5907 3105948 6343 318

192 G. Shanmugavelayutham et al. / Current Applied Physics 7 (2007) 186–192

(288 K) is not much different from the case for initialtemperature 6001 K (reduction 295 K). It may be notedthat feedstock powders have anything but regular shapes.We cannot expect uniform flow rate into jet or heat transferproportional to the quantity of particles fed into the jet,because of sharp edges and surface to mass ratio of particles.

The initial temperature in modified Boltzmann is higherthan that due to Boltzmann method (Table 3). Therefore inmodified Boltzmann method the reduction in temperatureis also higher. For example, 12.1 kW input power, thetemperature reduction in Boltzmann method is 4.6% andthat in modified Boltzmann method it is 5.1%.

5. Conclusions

The performance characteristics of a newly designedplasma torch were examined during processing, especiallythe effect of powder loading on the plasma jet temperature.The axial temperature variation of an argon plasma jet dis-charging into surrounding air has been measured for aplasma spray torch at 6.5, 7.9, 11.4 and 12.1 kW inputpower levels. It has been shown that by using a simpleexperimental setup for recording the spatially integratedspectral intensities emitted by an inhomogeneous, axisym-metric, optically thin plasma column, it is possible toobtain the temperature at the central axis of the plasmaby a modified atomic Boltzmann plot method without theneed to perform an Abel inversion. The main conclusionsare the following:

• This proposed new spray torch sustained a stable andclean plasma jet operation during powder loading.

• Injections of the powder materials into the arc resultedin decrease of the jet temperature. However, the extentof the jet temperature remained over 6000 K.

Acknowledgements

The authors acknowledge the project funding by theBoard of Research in Nuclear Science (BRNS) and thankDr. N. Venkatramani, Director, Beam Technology Devel-opment Group, Bhabha Atomic Research Centre, Mum-bai, India for his interest and inspiration.

References

[1] D.A. Scott, P. Kovitya, G.N. Haddad, J. Appl. Phys. 66 (1) (1989)5232–5239.

[2] O. Chumak, V. Kopecky, V. Sember, M. Hrabovsky, Czechoslovak J.Phys. 52 (2002) 872–877.

[3] S. Saeki, K. Osaki, O. Fukumasa, in: Proceedings of the EighthInternational Symposium on Plasma Chemistry, vol. 4, Tokyo, Japan,1987, pp. 2259–2264.

[4] K. Osaki et al., in: Proceedings of the Fourth Symposium on BeamEngineering Advanced Material Syntheses, Tokyo, Japan, 1993, pp.11–14 (in Japanese).

[5] K. Osaki et al., in: Proceedings of the Fifth Symposium on BeamEngineering Advanced Materials Syntheses, Tokyo, Japan , 1994, pp.77–80 (in Japanese).

[6] K. Osaki et al., Mem. Fac. Eng. Yamaguchi Uni. 46 (1995) 111–117(in Japanese).

[7] K. Osaki et al., Mem. Fac. Eng. Yamaguchi Uni. 48 (1995) 101–106(in Japanese).

[8] K. Osaki et al, in: Proceedings of the ISAPS-97, Advanced AppliedPlasma Science, 1997, pp. 61–66.

[9] K. Osaki, Plasma Appl. Hybrid Funct. Mater 9 (2002) 43–44 (inJapanese).

[10] T. Sakuta, M.I. Boulos, Rev. Sci. Instrum. 59 (1998) 289–294.[11] P.Lj. Stefanovic, P.B. Pavlovic, M.M. Jankovic, S.N. Oka, J. Phys.

Coll. 51 (C5) (1990) 281–288.[12] A. Vardelle, J.M. Baronnet, M. Vardelle, P. Fauchais, IEEE Trans.

Plasma Sci. 8 (1980) 417–424.[13] D.A. Scott, P. Kovitya, G.N. Haddad, J. Appl. Phys. 66 (1989) 5232–

5239.[14] A.H. Dilawari, J. Szekely, J. Batdorf, R. Detering, C.B. Shaw, Plasma

Chem. Plasma Proc. 10 (1990) 321–332.[15] W.L.T. Chen, J. Heberlein, E. Pfender, Plasma Chem. Plasma Proc.

14 (1994) 317–332.[16] A. Marotta, J. Phys. D: Appl. Phys. 27 (1994) 268–272.[17] H.R. Griem, Plasma Spectroscopy, McGraw-Hill Book Company,

New York, 1964.[18] W.L. Weise, in: R.H. Huddlestone, S.L. Leonard (Eds.), Plasma

Diagnostics Techniques, Academic Press, New York, 1965.[19] K.S. Drellishak, Ph.D. Thesis, Northwestern Univ., Evanston, IL,

1963.[20] T.L. Eddy, IEEE Temp. Sci. 6 (2) (1976) 99.[21] R.H. Tourin, in: J.M. Beer (Ed.), Spectroscopic Gas Temperature

Measurement Pyrometry of Hot Gases and Plasmas, ElsevierPublishing Company, Amsterdam, New York, 1966.

[22] N.K. Joshi, S.N. Sahasrabudhe, K.P. Sreekumar, N. Venkatramani,Meas. Sci. Technol. 8 (1997) 1146–1150.

[23] W.L. Wiese, J.W. Brault, K. Danzmann, V. Helbig, M. Kock, Phys.Rev. A: Gen. Phys. 39 (1989) 2461–2471.

[24] K. Osaki, O. Fukumasa, S. Fujimoto, A. Kobayashi, Vacuum 65(2002) 305–309.

[25] G. Shanmugavelayutham, V. Selvarajan, Pramana-J. Phys. 61 (6)(2003) 1109–1119.