In-situ Process Monitoring for Plasma Synthesis of Alumina Nanoparticles

J-W Shin1, H Miyazoe1, M Leparoux1, S Siegmann1, J-L Dorier2, and C Hollenstein2

1 Swiss Federal Institute for Materials Science and Technology EMPA Feuerwerkerstrasse 39, CH-3602 Thun, Switzerland 2 Centre de Recherches en Physique des Plasmas, Ecole Polytechnique Fédérale de Lausanne EPFL PPB Station 13, CH-1015 Lausanne, Switzerland

ABSTRACT

The process parameters of an inductively coupled thermal plasma used for nanopowder synthesis are experimentally investigated using various plasma diagnostics and in-situ powder monitoring methods. An enthalpy probe technique is applied to characterize the plasma properties under particle-free conditions. The evaporation of microscale alumina precursors is monitored in-situ by optical emission spectroscopy and laser light extinction measurements. Hereby, the influence of two significant process parameters, flow rates of plasma central gas and feed rates of precursor powder are extensively investigated. The emission line intensity of aluminum metal vapor is used as an indicator for alumina precursor evaporation. The number fraction of plasma-treated precursors is calculated from the precursor number density obtained from laser light extinction. The synthesized powders are collected in a sampling unit and characterized ex-situ by particle size analysis, X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) measurement, and scanning electron microscopy (SEM). At low flow rates (LCG) of the torch central gas, higher plasma enthalpy, a more laminar powder flow, a faster cooling, a higher quenching efficiency, and a smaller particle size of the synthesized nanoparticles have been observed in comparison to high amounts of the central gas (HCG).

KEYWORDS: Alumina Nanoparticle, Thermal Plasma, Process Monitoring

I. INTRODUCTION Among the gas phase nanoparticle synthesis routes, the inductively coupled thermal plasma (ICP) process presents the advantage of a high production rate, a controlled atmosphere leading to high purity products and a high flexibility concerning the materials to be synthesized [1-3]. Typically solid precursors are fed into the plasma where they are evaporated and, by a controlled condensation with cold gas, engineered nanoparticles are formed [4]. The final powder properties like the particle size and size distribution, the phase and chemical composition, as well as the morphology are determined by process factors involving plasma properties and powder trajectories, and interactions between powders and plasma. Therefore it is an important task for process control and optimization to monitor the on-going process steps, in order to understand plasma-powder interactions and to find a correlation between process parameters and synthesized powder properties. In this investigation, the influence of the swirling central gas flow of an inductively coupled Ar-H2 thermal plasma used for nanopowder synthesis is experimentally investigated using various plasma diagnostic and powder monitoring methods. Enthalpy probe technique is applied to characterize the plasma properties such as specific enthalpy, temperature and velocity for different central gas flow rates under powder-free condition. Furthermore, enthalpy probe measurements are also carried out under quenching conditions. Quenching is a significant process step for controlling the nanoparticle properties such as particle size and crystallographic phases. The evaporation of micro-scaled alumina precursors injected axially into the plasma is monitored on-line simultaneously by optical emission spectroscopy (OES) and laser extinction (LE) measurements. Finally, the synthesized powders are collected in a sampling unit and ex-situ characterised by various techniques to validate the in-situ diagnostic measurements.

II. EXPERIMENTAL The nanopowder synthesis system used for this study consists of an ICP torch, a synthesis chamber, a filtration unit, and a powder feeder as schematically illustrated in figure 1. The inductively coupled plasma torch (PL-35, TEKNA Plasma System Inc., Canada) is mounted on the top of the synthesis chamber and is operated at 13.56 MHz with argon and hydrogen up to a maximum input power (IP) of 35 kW. The torch comprises a water cooled four-turn copper coil, a ceramic confinement tube with an inner diameter of 35 mm and an intermediate tube separating central gas from sheath gas. The central gas (CG) is tangentially introduced through a metallic gas distributor giving rise to a swirl along the powder injection probe, while the sheath gas (SG) flows axially along the ceramic wall. Argon is used as central gas and a mixture of argon and hydrogen as sheath gas. Two flow rate ranges for the central gas are taken into consideration in this work. A flow rate between 6 and 12 slpm is referred to as a low amount of central gas (LCG) and a flow rate of 21 to 27 slpm as a high amount of central gas (HCG). The flow rate of the sheath gas is kept constant at 80 slpm of Ar and 6 slpm of H2. The Ar quenching gas (QG) is injected by a ring with 8 holes which is mountable at different axial positions. The holes with a diameter of 2 mm are perpendicularly orientated toward plasma jet. A water cooled stainless steel powder injector with an opening diameter of 3 mm is axially mounted in the central position of the torch at the height of the first induction coil. This injection probe is connected to a commercial powder feeder (TWIN 10-2, Sulzer Metco, Switzerland). The precursor powder was introduced into the plasma using argon as carrier gas (CaG) with a flow rate of 10 slpm. As precursor, α-alumina powder (AA-5, Sumitomo Chemical Co., Ltd, Japan) with a mean

diameter of 4.7 μm was used. Three different feed rates (FR) (1.4, 2.4 and 4.7 g min-1) are applied. This alumina powder has a very narrow size distribution (-6 +4 μm) allowing a reliable calculation of powder number density from in-situ laser extinction measurements. The water cooled synthesis chamber (1500 mm long) has several view ports for easy access of in-situ process monitoring tools. These view ports enable a simultaneous process monitoring by at least two different techniques e.g. optical emission spectroscopy (OES) and laser extinction (LE) at various axial and radial positions as shown in figure 1 and 2. A chamber pressure (CP) of 40 kPa is maintained using a water ring pump and a diaphragm-valve. A specially designed filtration unit comprising a small bypass membrane filter and a large production filter made of several woven metallic filters is connected to the synthesis chamber [5]. In the present work, the bypass membrane filter is used for on-line sampling of the synthesized nano-sized powders. The sampled powders are characterized ex-situ by a particle size analyzer (LS230, Beckman Coulter Inc., USA), a transmission electron microscope (TEM) (CM20, Philips, Netherlands), and a high resolution scanning electron microscope (HR-SEM) (S4800, Hitachi, Japan).

z = 0 mm

+ z

z = 90 mm

10 mW

HeNe laser

optical fibre connected to OES system

bulk head

enthalpy

console

gas sampling

water

In/Out

mass

spectrometer

data

acquisition

purging

system

powder collector 2

on-line

sampling

filter

water ring

pump

powder collector 1

view ports

ICP Torch RF (35 kW)

power supply

matching

box

injection probe

powder

feeder

main filter

gas

exhaust

+ x

exhaust Fig. 1: Schematic of the experimental set-up for the nanopowder synthesis and enthalpy probe measurements.

The enthalpy probe technique is a reliable plasma diagnostic tool for high temperature thermal plasmas [6-7]. The enthalpy probe is a water cooled gas sampling and stagnation pressure probe used to determine the enthalpy, heat load, temperature and velocity of the high temperature plasma jet. More detailed information on the basics of the enthalpy probe technique for thermal plasmas is given, for example, by Rahmane et al [8]. The enthalpy is determined on the basis of a calorimetric two step energy balance measurement, and the plasma temperature under a given gas pressure can subsequently be calculated once the gas composition has been determined by mass spectrometer, assuming local thermodynamic equilibrium (LTE). The measurements were carried out at three different axial positions from the exit of the torch (z = 2, 22, 60 mm). The probe is also equipped with a displacement system so that the radial profiles can be measured. To avoid

probe damage due to the extremely high heat load close to the torch exit, a lower input rf power (20 kW) was used with respect to the power (25 to 35 kW) for particle synthesis.

Optical emission spectroscopy (OES) was used to observe the vaporized species resulting from interactions of the injected precursor with the plasma. The light emitted from the plasma was collected by an optical fiber and transferred to a spectrometer (S2000 Ocean Optics, Inc) equipped with a 1024 element linear CCD-array detector. The spectral sensitivity over the wavelengths used (from 340 nm to 880 nm) was calibrated by means of a tungsten lamp. For investigation of the evaporation of alumina precursor, the intensities of the spectral lines of the metallic aluminum vapor, which originates from the evaporating alumina, were measured under different conditions. The aluminum emission lines at 396.15 nm, 669.60 nm and 783.53 nm were investigated. In addition to optical emission spectroscopy, a laser extinction measurement system was adapted to the synthesis chamber. A 10 mW HeNe laser at a wavelength of λ = 632.8 nm was installed at the location where the maximum emission line intensity of aluminum vapor was observed (z = 90 mm). The intensity of the transmitted laser beam was detected by a silicon photodiode. The number density (NP) of the non plasma-treated precursor powder can be obtained from the ratio of the transmitted (IT) to incident (Io) laser beam intensities under the assumption that nano-scaled particles do not contribute to the extinction intensity and the measured particles are spherical and mono-dispersed. Moreover, particles or agglomerates formed from the gas phase are not considered for the determination of Np, since at this axial position, the high plasma temperature of over 4000 K estimated from the enthalpy probe measurement is enough high not to allow formation and growth of the large particles or agglomerates. Figure 2 shows a schematic top view of the experimental set-up for optical emission spectroscopy and laser extinction measurements.

synthesis

chamber: 320 mm

ICP torch: 35 mm

view ports

HeNe laser

λ = 632 nm

detector

DP

ND

BP

ND

lensoptical fibre

DP

to spectrometer Fig. 2: Schematic top view of the experimental arrangement for process monitoring (BP: band pass filter, ND: neutral density filter, DP: Diaphragm).

III. RESULTS AND DISCUSSION Influence of Central Gas Flow Rates Figures 3(a) and 3(b) show the light emitted from the alpha-phase alumina precursor axially injected into the plasma at a CG flow rate of 6 slpm (a) and 27 slpm (b), respectively. The interaction between the injected powder and the plasma jet in the low

central gas condition results in a broad, long and bright column, whereas at the high gas flow rate condition a widely-dispersed conical distribution is observed. These different behaviors result from the trajectory patterns of the injected precursor powder as well as from the plasma properties such as enthalpy and temperature distributions, basically affected by the vortex of the central gas flow. Therefore, it can be deduced from the visual observations that the low flow rate of CG would lead to a more laminar gas flow and a better confinement of the powder in comparison to the high flow rate of CG.

Fig. 3: Visualisation of alumina precursor injected into the plasma (FR: 2.4 g min-1), (a) 6 slpm (LCG), (b) 27 slpm (HCG).

Contour plots in figure 4 give different profiles of the specific enthalpies measured with the enthalpy probe at three axial positions for different central gas flows. The enthalpy of the plasma is one of the most important process parameters, since it plays a key role for precursor evaporation in the powder synthesis system. The relative error on the enthalpy was below 15%. The contour plots have been made by interpolation of the measured axial profiles. A polynomial fitting was performed for the measurements data. Since the contour images are built up on basis of the measured datasets, the enthalpy profiles represent asymmetric features of the plasma.

Fig. 4: Contour images of the specific enthalpy measured by enthalpy probe, (a) 12 slpm (LCG), (b) 27 slpm (HCG).

Higher enthalpies are measured for the LCG (fig. 5a), in particular at the centre of the plasma, where the precursor powder trajectory is expected. For the LCG, the enthalpy drastically increases with decreasing axial distance from the torch exit, while it does not increase much for the HCG. Furthermore, as the measurements are performed closer to the torch, larger enthalpy differences between LCG and HCG conditions are observed. The plasma in LCG case (fig. 4a) shows a faster cooling over this axial distance in the centre area (an enthalpy reduction of 2615 kJ kg-1 over 58 mm) than in HCG case (1265 kJ kg-1 over 58 mm). The low gradient of the enthalpy on radial and axial directions of the HCG could be explained by a higher amount of vortex central gas flow which enhances the gas mixing and the thermal diffusion. However, this radially flat profile of enthalpy on radial axis is not the preferred condition for the synthesis of the nanoparticles, because the high enthalpy or temperature at the plasma boundary region would not contribute to melting and evaporation of the powders in the central stream. On the contrary, there is an energy loss by radiation [9]. The high enthalpies for LCG as shown in figure 4(a) are more advantageous to evaporate the injected powder. The plasma-particle interactions were investigated with optical emission spectroscopy (OES) as shown in figure 5. Micro-scaled alumina powders were injected axially into the Ar-H2 plasmas studied previously. The emission line intensity of the aluminum metal vapor at 669.60 nm was chosen for investigating the dependence on central gas flow rate. Aluminum is the main species appearing after evaporation of the alumina powder at high temperatures [10]. At 35 kW, the line intensities are drastically reduced when the flow rate of CG increases from 6 slpm to 18 slpm, whereas the intensity changes between 18 slpm and 27 slpm are small. At a lower input power, the similar central gas dependency has been found. This central gas dependency can be explained by the results of the enthalpy probe measurement. The higher enthalpy of LCG (fig. 4) allows increased evaporation of the alumina powder.

Fig. 5: Intensities of aluminium emission spectral line at λ = 669.60 nm vs. flow rate of the central gas (SG: 80 slpm Ar + 6 slpm H2, CaG: 10 slpm Ar, CP: 40 kPa, FR: 2.4 g min-1). Figure 6 shows the particle size distributions of the collected powders synthesized without application of quenching gas. For all three central gas flows investigated, multimodal particle distributions were obtained. The measured particle size of the large particles (L) lies between 4 and 5 μm, which corresponds well to the mean size of the alumina precursor powder. The presence of large particles (L) implies that a part of the injected precursor has not been plasma-treated probably due to the fact that they did not flow through the plasma plume, but passed outside the plasma. However, a decrease in the

volume fraction as well as in the mean size of these particles is observed with decreasing central gas flow as illustrated by the line shift from X to X* in figure 6. Besides the non plasma-treated particles, particles in the submicron ranges are observed. The particle size distribution shows two maxima after plasma treatment of the injected precursor powder [11-12]. One peak is around 100 nm marked as S and the other is around 400 - 800 nm marked as M. The particles with size M are considered to be large hard-agglomerates of the fine particles. These hard-agglomerates cannot be separated from each other by external shear forces such as ultrasonic treatment performed before particle size analysis. However, the fraction of M particles is expected to decrease if quenching gas is properly applied.

μ

XX*

L

Y

Y* S

M

Fig. 6: Particle size distribution of the alumina powder collected in the sampling filter (SG: 80 slpm Ar + 6 slpm H2, CaG: 10 slpm Ar, CP: 40 kPa, FR: 2.4 g min-1).

The particles with size S are expected to include the synthesized primary nanoparticles and small agglomerates. The low flow rate of CG features a drastic increase of S sized particles volume fraction (Y to Y*) in comparison to the higher flow rates of central gas. This results from the more efficient precursor evaporation and the higher plasma enthalpy gradient as evidenced by the OES and the enthalpy probe measurements. This explanation is supported by scanning electron microscopy (SEM) as presented in figure 7. The images are taken from the same particle samples analyzed in figure 6.

Fig. 7: High resolution SEM images of the synthesized alumina nanoparticles (SG: 80 slpm Ar + 6 slpm H2, CaG: 10 slpm Ar, CP: 40 kPa, FR: 2.4 g min-1); scale bar on images: 50 nm, (a) 27 slpm, (b) 6 slpm.

In figure 7(a) it is difficult to distinguish individual particles. The particles are irregularly shaped and mostly attached to each other, and no boundary around the particles can be found, which is called hard-agglomeration. The particles seem to have grown together in a liquid phase such as grain coalescence in the sintering process. In contrast, figure 7(b) clearly shows the spherical single primary nanoparticles, which seem to be weakly bound to each other without sinter necks. These soft-agglomerates could be separated by an external shear force. Therefore, a remarkably high volume fraction of the S sized particles could be obtained for low CG flow rates. Fig. 8(a) shows the influence of the quenching gas flow on the plasma enthalpy in terms of the different central gas conditions. The enthalpies without quenching gas flow were measured at an axial position of z = 60 mm, while those with quenching gas flow were measured at z = 75 mm whereby the quenching gas was injected at z = 60 mm. Even though the measurements for the quenching cases were carried out at a slightly lower input power, the tendency of the enthalpy reduction depending on the central gas flow is clearly observed. For the LCG condition, the enthalpy with quenching gas flow is much more reduced in comparison to the HCG condition. The lower quenching effect of HCG on the enthalpy reduction can be assumed to result from the strong swirl flow of the HCG which hinders the penetration of the quenching gas.

(a)

α δθ

Θ a

(b)

Fig. 8: (a) Radial profiles of the plasma enthalpies; w/o: without, w: with, QF: quenching gas flow (56 slpm Ar), LCG: 12 slpm Ar CG, HCG: 27 slpm CG, solid and dashed lines: poly-nomial fittings for LCG and HCG, respectively. (b) XRD analysis for phase transformation of the alumina nanoparticles produced under two different CG conditions. XRD analysis in figure 8(b) shows an occurrence of the different crystallographic phases of the synthesized alumina nanoparticles depending on the flow rates of central gas under quenching conditions. Hereby an Ar quenching gas with a flow rate of 28 slpm was injected at the axial position of 95 mm. A mixed phase consisting of α-alumina and θ-alumina was observed for the as-received precursor powder. Under HCG condition, the vaporized particles are condensed completely to θ-alumina. Unlike the HCG case, the major peaks of δ-alumina were observed for the LCG case with some minor peaks of θ-alumina. According to other work dealing with alumina condensation which can be found in literature [13-14], the occurrence of the transition aluminas are predominantly determined by the difference of the cooling rates. The transformation sequence of the transition

aluminas illustrated as follows [15] shows that the δ-alumina is more stable than the θ-alumina at lower temperature range.

γ⎯⎯⎯ →⎯δ⎯⎯⎯ →⎯θ⎯⎯⎯ →⎯α K1023K1173K1473 Therefore, the rise of δ-phases and disappearance of several peaks of the θ-alumina at the LCG gives a consistent explanation with the higher quenching effects of the LCG conditions on enthalpy reduction as shown in figure 8(a). Influence of Precursor Feed Rate The intensity of the aluminum metal vapor measured by OES was used as an indicator for alumina precursor evaporation and is plotted in figure 9 vs. the powder feed rate as a function of the input rf powers. The figure shows that a maximum in the aluminum emission line intensity exists up to 30 kW. At an input power of 25 kW, the aluminum emission line intensity increases first with increasing powder loading and reaches a maximum Imax. Further increase of the precursor feed rate reduces the aluminum emission line intensity. However, the decrease in the aluminum line intensity at high feed rates depends on the input rf power. At 35 kW, the aluminum line intensity at the higher feeding rate of 4.7 g min-1 is considerably raised. Furthermore, the small difference in the aluminum line intensities at a feed rate of 2.4 g min-1 between 30 kW and 35 kW means that at 30 kW, most of the injected precursor within the plasma could have been evaporated. Hence, the increased line intensity at the feed rate of 4.7 g m-1 for 35 kW indicates that at 35 kW, the additional power of 5 kW is used then for more evaporation of the precursor. Therefore, it can be concluded that the maximum in the aluminum emission intensity (Imax) would be shifted towards higher feed rates at higher input rf power.

Imax

Fig. 9: Intensity of the Al(g) emission spectral line (λ = 669.60 nm) measured at z = 90 mm vs. feed rates for three different input power levels. Laser light extinction measurements are presented in figure 10(a). The laser light extinction at this axial position is assumed to take place by scattering and absorption of the incident laser beam only from the particles which are not treated by the plasma. The light extinction intensity of the particles below 0.1 μm is smaller by a factor of 104 in comparison to that of precursor particles according to the Lambert-Beer equation describing the

relationship between light extinction intensity and particle diameter. Therefore, the nano-sized particles do not contribute to the extinction even though they might exist. The large hard-agglomerates shown as M sized particles in figure 6 are also not believed to exert influence on the extinction intensity, because the agglomeration is expected to occur during a condensation process at lower axial positions. In figure 10(a), no difference in the extinction was measured for the powder feed rates below 2.4 g min-1, whereas a higher extinction is clearly observed at the highest feed rate. This phenomenon can be well explained by the result of emission spectroscopy shown in figure 9. The increase in the aluminum emission line intensity by changing the feed rates from 1.4 g min-1 to 2.4 g min-1 implies higher precursor evaporation. Hence, at the feed rate of 2.4 g min-1, no additional light extinction by coarse powders is observed. At the highest feed rate, the decrease of the aluminum line intensity indicates more presence of the coarse powders, which corresponds to the increase in the laser extinction.

(a)

(b)

Fig. 10: (a) Time-development of the normalized transmittance of the laser intensity for different feed rates at z = 90 mm (IP = 30 kW, CP = 40 kPa), (b) Estimation of number fraction of the plasma-treated precursor powder for different feed rates and plasma powers at z = 90 mm.

Figure 10(b) shows the number fraction of the precursor evaporation vs. the feed rate as a function of input rf power. This number fraction has been estimated from the number density (Np) of the non evaporated powders. For each feed rate, the calculated number densities are normalized by the Np measured without plasma. The obtained ratio is the number fraction of the remaining coarse powders which are not plasma-treated. With the assumption that the plasma-treated precursor powders do not contribute to extinction intensity, the number fraction of the evaporated powders can be obtained by subtraction from 100%. The behavior of the number fraction against the feed rate is similar to the one of the aluminum emission lines shown in figure 9. The OES is a direct measurement on the gas species vaporized from the precursor, while the laser extinction technique measures primarily the non-evaporated large powders and gives indirect but quantitative information on precursor evaporation. In this study both monitoring methods show well-correlated and consistent results. Depending on feed rate and plasma power there is a distinct maximum of precursor evaporation.

IV. CONCLUSIONS An increase in specific enthalpy and cooling rate in the plasma core is measured with decreasing central gas flow rate. A steeper gradient of plasma enthalpy over radial axis is observed for low flow rates of central gas (LCG) compared to high flow rates of central gas (HCG). A higher quenching efficiency is measured at LCG conditions. In-situ analysis of powder-plasma interaction made by OES shows much higher intensities of emission spectral lines of aluminum vapor resulting from vaporization of injected alumina powders for LCG, which can be caused by high plasma enthalpy. The laser extinction measurements confirm the favorable low central gas flow rate conditions for the precursor evaporation. The in-situ OES observations are validated by ex-situ particle size distribution analyses of the collected powders indicating that more precursor powders are plasma-treated and more nanoparticles are synthesized as central gas flow decreases. XRD analyses present a phase transition from alpha- to delta-alumina for LCG featuring higher quenching efficiency, while theta phased alumina is synthesized for HCG.

The investigated monitoring techniques have proved their potential for the synthesis of nanoparticles by thermal plasmas. These techniques could be suitable for the development of an evaporation on-line sensor in plasma environment aiming to guarantee an on-line process quality control for nanoparticle synthesis.

V. ACKNOWLEDGEMENTS This work was partially funded by CTI/TopNano 21 under project number 6078.2. The authors would like to acknowledge G. Buerki for the HR-SEM characterizations.

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