thermal characterization and optimization of a plasma downstream reactor for particle surface...

Full Paper

S566

Thermal Characterization and Optimization ofa Plasma Downstream Reactor for ParticleSurface Modification

Christian Roth, Adrian Spillmann, Axel Sonnenfeld, Philipp Rudolfvon Rohr*

A plasma downstream reactor for the continuous treatment of dry, cohesive, and temperaturesensitive powders is investigated by means of fiber optic temperature measurements. Amethod to determine the different heat flux terms (plasma heating, radiation, and convection)is introduced and applied to calculate the inner wall and neutral gas temperature at severalpositions within the reactor. By this means undesirable discharge regions downstream theplasma zone are found. Consequently, the thermal characterization of the reactor allowsoptimization of the reactor design to confine the plasma to the downer tube and to avoidundesirable temperature hot spots.

Introduction

Many products and intermediate substances in the

chemical, pharmaceutical or food industry exist in the

form of powders. The plasma downstream reactor (PDR)

concept for the effective modification of particle surfaces

either by surface activation or nanoparticle depositionwas

shown earlier.[1,2] For instance, the surface free energy of

polymer powders such as HDPE (high-density polyethy-

lene) is increased leading to improved wettability of the

powder.[1] This enables a good dispersion of the powder in

polar liquids such as water without using tensides.

Furthermore, nanoparticles can be formed and attached

on the substrate particle surface by plasma enhanced

chemical vapor deposition (PECVD) in one single proces-

sing step resulting in an increased powder flowability.[2]

This is since the deposited nanostructures act as spacers

between the substrate particles, and thereby decrease the

attractive van der Waals force between the cohesive

particles.

C. Roth, A. Spillmann, A. Sonnenfeld, P. Rudolf von RohrETH Zurich, Institute of Process Engineering, 8092 Zurich,SwitzerlandFax: (þ41) 44 632 13 25; E-mail: [email protected]

Plasma Process. Polym. 2009, 6, S566–S570

� 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

To the best of our knowledge, plasma surface treatment

of fine powders in large quantities is only rendered

possible in an efficientmanner in the PDR. The particles are

uniformly dispersed in a rarefied gas and instantaneously

mixed with the active gas species created in the plasma

zone. This makes the surface modification in the PDR a

very fast process, as the mean residence time of the

particles in the discharge is less than 0.1 s. This is superior

to conventional processes achieving the same result.[3]

In principal, non-thermal plasmas are ideal for the

treatment of temperature sensitive materials. But still, the

particles absorb energy in the discharge and gain rapidly in

temperature due to their comparably small mass. Hence a

thermal characterization of the reactor is necessary to

guarantee suitable process conditions for the powder

treatment. In this study, a measurement technique is

presented and applied to determine the temperature and

the different heat flux terms at several positions in the

reactor.

Experimental Part

The PDR setup consists basically of a dosage andmixing system for

the powder and the process gases, the downer tube where the

DOI: 10.1002/ppap.200931402

Plasma Downstream Reactor

plasma treatment takes place, a cyclone and filter unit for solid–

gas separation after the powder treatment and a double-stage

vacuum pump. The entire setup has already been described by

Spillmann et al.[4]

Figure 1 shows the schematic diagram of the downer tube. The

gas enters from the top through a metallic flange and streams

downwards through a glass pipewith an inner diameter of 40mm

and a length of 500 mm (Plane joint tube, Buchi Glas Uster AG,

Switzerland). The power is capacitively coupled into the reactor

over two half-shell-shaped copper electrodes of 300 mm length

and 40mmwidth. The powered electrode is indicated on the right-

hand side in the scheme, whereas the second electrode and the

two flanges are connected to the ground.

Temperature measurements have been performed at four

different heights at four radial positions in the reactor and four

peripheral positions outside the reactor (indicated by spheres in

Figure 1) with a fiber optic system (Polytec, FOTEMP-Multi-

channel). The used sensor type (TS2/3) has a probe tip diameter of

1.0 mm, where a gallium-arsenic chip with a temperature

dependent band edge displacement of 0.4 nm�K�1 is attached at

the top of the fiber optic cable. The crystal is excited with white

Figure 1. Schematic of the original downer tube and the con-nected flanges with the measurement points indicated byspheres.



light and the displacement of the band edge is measured with a

spectrometer which allows the instantaneous calculation of the

corresponding temperature. The complete thermal probe is built

out of nonconductive materials and thus suited for application in

plasma environments. The overall system inaccuracy is �2 8C.The measured temperature represents the probe temperature

which does not correspond to the mean thermodynamic

temperature of the ionized gas in a non-equilibrium low pressure

plasma. However, the probe temperature can be used to determine

the specific heat flux terms towards the probe surface. Various

authors[5–8] applied different techniques to measure the heat flux

in technical plasmas for material processing. Depending on the

process, appropriate assumptions are necessary.

As the thermal conduction of the used glass fibers is very low

and no material deposition takes place during the temperature

measurements the three main heat transfer mechanisms are

supposed to be radiation, convection and heating due to the

plasma. Convection always takes place if a fluid comes into

contact with a solid surface and a temperature gradient between

the surface and the bulk fluid is present. Especially at high

temperatures, radiation heat transfer accounts for a significant

fraction of the total energy exchange between the walls and the

sensor. The thermal probe in the plasma zone charges itself

negatively due to the high mobility of electrons and a small

sheath is built around the sensor. This negative net charge

attracts the positive ions and the resulting ion bombardment

and ion-electron recombination on the surface leads to a

significant heating of the probe. The heat balance over the

sensor is then given by the following equation:

dTSdt

mScp;S ¼ AS_Q00plasma � _Q00

convection � _Q00radiation

� �(1)

where TS(t) is the sensor temperature, mS is the mass of the

sensor, cp,S is the heat capacity of the sensor and AS is the surface

area of the sensor. The heat flux due to convection _Q00convection is

described by the product of the convective heat transfer coefficient

between the sensor and the gas ac,S, and the temperature

difference TS(t)-Tgas(t) between the sensor and neutral gas. The

radiation heat transfer _Q00radiation can be calculated according to the

Stefan-Boltzmann law as the difference between the sensor and

inner wall temperature Tw,in(t) to the power of four multiplied

with the Stefan-Boltzmann constant ss and the emissivity of the

sensor eS which is estimated to be equal to 0.9 for the polymer

coverage of the sensor tip. The parameter _Q00plasma accounts for the

heat flux originating from the electron and ion bombardment and

ion-electron recombination on the sensor surface.

In our approach, the thermal probe is used to determine these

three different heat fluxes and the heat balance for the sensor

reads

dTSdt

mS cp;SAS

¼ _Q00plasma � ac;S TSðtÞ � TgasðtÞ

� �� ss � "S TSðtÞ4 � Tw;inðtÞ4

� � (2)

The fraction mS cp;S�AS is determined in a cooling experiment

under ambient conditions and is equal to 322�12 J�m�2�K�1. The

www.plasma-polymers.org S567

C. Roth, A. Spillmann, A. Sonnenfeld, P. Rudolf von Rohr

S568

transient inner wall temperature is calculated with a numerical

approach. The downer tube is discretized into many cylinder

shells, and the measured temperatures of the outer wall and the

surrounding gas act as a boundary condition assuming natural

convection outside the glass cylinder. Therefore the transient

temperature of each cylinder shell can be calculated step by step

by solving the heat equation in cylindrical coordinates until the

shell of the inner wall is reached.

In order to determine all the other unknown parameters, 300 s

of the heating phase and 300 s of the subsequent cooling phase

under the same conditions but without the plasma state are

recorded and analyzed separately. The cooling phase is used to

determine the convective heat transfer coefficient ac,S. The term_Q00plasmais then equal to zero and the heat balance (2) is rewritten as

TS

Plasma

� 2009

ðtÞ ¼ Tgas � 1

ac;S

� dTSdt

mS cp;SAS

þ ss"S TSðtÞ4 � Tw;inðtÞ4� �� (3)

The gas temperature Tgas(t) drops very slowly compared to the

sensor temperature TS(t) and its derivative, as it correlates strongly

with the reactor wall temperature. Thus Tgas(t) is assumed to be a

constant (Tgas) for a short period of the cooling phasewhich is used

to determine ac,S. By plotting TS(t) as a function of the heat flux in

the brackets of Equation (3) for every time step t, a linear

correlation is found. The reciprocal of the slope of the correspond-

ing linear regression is then equal to ac,S as exemplarily illustrated

in Figure 2b.

The evaluation of the heating phase leads to the remaining

parameters _Q00plasma and Tgas(t). For this, the basic Equation (2) is

first rewritten one more time as given below such that the whole

right-hand side term can be plotted for every time step t:

_Q00plasma þ ac;S TgasðtÞ ¼

dTSdt

mScp;SAS

þ ss "S TSðtÞ4 � Tw;inðtÞ4� �

þ acTSðtÞ(4)

Figure 2. (a) Example for the evaluation of a typical temperaturemeasurement by analyzing (b) the cooling phase and (c) theheating phase separately.

Table 1. Standard experimental conditions.

Plasma forward power 75 W

Pressure 2 mbar

Argon flow rate 500 sccm

Oxygen flow rate 500 sccm

It can be assumed that the heat flux originating from the

plasma is a constant value but the gas temperature increases with

the ongoing heating of the whole plasma reactor starting from the

initial gas temperature before the plasma is ignited. Plotting the

right-hand side of Equation (4) as a function of the time, a step

function in the beginning and a continuous increase afterwards

can be noticed (as shown in Figure 2c), where the step corresponds

to the value of _Q00plasma and the steady increase divided by the

convective heat transfer coefficient is related to the neutral gas

temperature.

This methodology is used to determine the three types of heat

flux: radiation, convection, and plasma heating, and thereby the

transient neutral gas temperature inside the plasma reactor at

various positions. For the sake of simplification, in this

preliminary study, no powder or monomer is introduced into

the system. The experimental conditions are summarized in

Table 1.

Process. Polym. 2009, 6, S566–S570

WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Results

Figure 3 shows the spatial distribution of the plasma heat

flux in the reactor. The heat flux is higher in the middle of

the reactor than near the walls as many charge carriers are

DOI: 10.1002/ppap.200931402

Plasma Downstream Reactor

Figure 3. Spatial distribution of the plasma heat flux in theoriginal reactor at standard experimental conditions. Figure 4. Temperature increase as a function of the time and

radial position at the height of the electrode center in the originalreactor, measured at standard experimental conditions. The glasswalls and the electrodes are indicated by hatched areas.

lost over the walls and thus the ion density is higher in the

plasma bulk compared to the sheath region. It is also

noticeable that the calculated heat flux is elevated on the

side of the powered electrode.

The gas enters from the top and gets ionized in

between the two electrodes. Hence the plasma heat flux

is similar at the positions P2, P3, and P4 while the value

at P1 (electrode top edge) is much smaller. At the position

P1 the gas has not yet reached the same degree of

ionization as further down at the other measurement

positions. Compared to this the plasma heat flux is still

very high at the position P4 which means that the

discharge continues until the lower flange. This unfavor-

able fraction of the discharge could be one reason for the

often occurring powder accumulation at the reactor walls

and clogging problems in this part of the reactor. For

reactor optimization, this means that the geometry or the

experimental conditions should be changed to avoid a

discharge in the lower flange. The spatial distribution of

the plasma heat flux shows that this parameter correlates

strongly with the local charge carrier density in the

reactor.

The measured outer wall temperatures and the calcu-

lated inner wall and gas temperatures are shown in

Figure 4 for the position P2 in the middle of the electrodes.

Before the plasma is ignited the temperature of the gas is

nearly equal to the wall temperature. During the dis-

charge, the reactor walls are heated by the ion bombard-

ment and surface recombination of charged species at the

inner wall. A part of this energy is conducted through the

glass wall and emitted to the surrounding. Another part is

transferred to the rarefied gas within the reactor. The

initial temperature of the gas when it enters the downer

tube is 23� 2 8C as it was measured before the discharge.

After a 5 min operation, the walls in the plasma reach

temperatures between 35 and 40 8C. The heat transfer

from the hot walls to the colder gas occurs mainly by



convection resulting in a temperature profile with the

highest gas temperature near the walls.

Even though the gas molecules spend less than 0.05 s

between the upper flange and the electrode center while

flowing through the reactor, the gas temperatures in the

middle of the reactor increase by more than 10 8C in the

case of wall temperatures between 35 and 40 8C. This fastheat transfer from the hot walls into the center of the

downer tube is enabled by the high mean free path of gas

molecules in rarefied surroundings.

Regarding reactor optimization, two main conclusions

can be drawn: the plasma zone is not limited in between

the two electrodes but enlarged towards the lower flange,

which means that the powder is still treated in the region

of the lower flange. The second finding is the fast heat

transfer from the wall to the rarefied gas. A wall cooling

unit would therefore be suitable to keep the gas

temperaturewithin the reactor at a constant and low level.

In order to confine the plasma within the glass tube and

to avoid a discharge development towards the lower

flange, a longer glass tube of 1.5 m length instead of 0.5 m

is installed. Compared to the sketch in Figure 1 the distance

between the upper flange and the electrode top edge is

enlarged to 325 mm instead of 75 mm and the distance

between the electrode bottom edge and the lower flange is

changed to 825 mm instead of 75 mm.

The heat flux originating from the plasma is calculated

again from temperature measurements in the long reactor

and given in Figure 5 for standard experimental condi-

tions. No temperature increase of the sensor is measured

150 mm above the electrodes during the discharge, and

therefore the value for the plasma heat flux is equal to zero.

At the twomeasurement positions between the electrodes

similar values as in the short reactor of approximately

www.plasma-polymers.org S569

C. Roth, A. Spillmann, A. Sonnenfeld, P. Rudolf von Rohr

Figure 5. Spatial distribution of the plasma heat flux in the 1.5 mlong reactor at standard experimental conditions.

S570

4000 W�m�2 are calculated. Already 300 mm below the

electrodes the plasma heat flux decreases to values below

400 W�m�2. Even further down in the lower flange the

calculated heat flux originating from the ion bombard-

ment and ion-electron recombination is nearly equal to

zero. This result shows that the discharge is confined to the

middle of the glass tube for the chosen standard conditions

in the enlarged downer tube. Powder treatment experi-

ments confirmed this assumption as the clogging of the

lower flange is no longer a problem due to reduced powder

adhesion and monomer deposition in this region.

The temperature calculations corroborate that the

discharge is confined to the region of the two electrodes.

Not only the measured wall temperatures but also the

calculated gas temperatures increase during the discharge

only at the two measurement positions between the

electrodes. No temperature change can be detected

150 mm above the electrode’s top edge, and the tem-

perature change at the measurement positions 300 mm

and further below the electrode’s lower edge lies within

the measurement inaccuracy of �2 8C and is therefore not

significant.



Conclusion

A fiber optic temperature measurement system has been

used to thermally characterize a plasma reactor for particle

treatment. A method to calculate the different heat fluxes

in a plasma reactor is presented and applied to determine

the radiation, convection, and plasma heating by an

adequate analysis of the heating phase during the

discharge and the cooling phase after the discharge.

The original reactor showed a high plasma heat flux not

only in between the main electrodes but also in the lower

flange, which indicates an ongoing discharge towards this

part of the reactor. The calculated neutral gas temperature

depends mostly on the inner wall temperature and

increases over time during the persisting discharge.

In a first optimization step, the reactor is enlarged and

according to the heat flux calculations the discharge is

confined to the middle of the reactor tube as the plasma

heat flux reaches only high values in between the main

electrodes. The enlarged reactor provides not only a better

defined discharge zone and longer operation times due to

reduced clogging of the lower flange, but also increases the

feasibility of the cooling unit as now only the downer tube

has to be cooled.

Received: September 19, 2008; Accepted: February 3, 2009; DOI:10.1002/ppap.200931402

Keywords: fiber optic temperature measurement; particles;plasma downstream reactor; surface modification; thermalcharacterization

[1] C. Arpagaus, A. Sonnenfeld, P. Rudolf von Rohr, Chem. Eng.Tech. 2005, 28, 87.

[2] A. Spillmann, A. Sonnenfeld, P. Rudolf von Rohr, Plasma Pro-cess. Polym. 2007, 4, 16.

[3] A. Spillmann, A. Sonnenfeld, P. Rudolf von Rohr, Plasma Pro-cess. Polym. 2008, 5, 753.

[4] CH WO2007036060-A1 (2007), ETH Zuerich, A. Spillmann, A.Sonnenfeld, P. Rudolf von Rohr.

[5] J. A. Thornton, Thin Solid Films 1978, 54, 23.[6] H. Kersten, D. Rohde, H. Deutsch, R. Hippler, W. W. Stoffels,

E. Stoffels, G. M. W. Kroesen, J. Berndt, Acta Physica Slovaca2000, 50, 439.

[7] G. Swinkels, H. Kersten, H. Deutsch, G. M. W. Kroesen, J. Appl.Phys. 2000, 88, 1747.

[8] J. E. Daugherty, D. B. Graves, J. Vac. Sci. Technol. A 1993, 11, 1126.

DOI: 10.1002/ppap.200931402

thermal characterization and optimization of a plasma downstream reactor for particle surface...

Documents