Influence of coil current modulation on TiO2 nanoparticle synthesis using pulse-modulated induction thermal plasmas
Introduction
Inductively coupled thermal plasmas (ICTPs) operating at several tens of kilowatts are widely used for various material processes such as syntheses of diamond films [1], thermal barrier coatings [2], syntheses of fullerene [3], materials surface modifications [4], and nanoparticle syntheses [5], [6], [7]. For example, titanium dioxide (TiO2) nanoparticles have attracted much interest for various applications such as photocatalysts [8], photonic crystals [9], photovoltaic cells [10], and gas sensors [11]. The synthesis of TiO2 nanoparticles has therefore been investigated in many research efforts using a steady-state type of ICTP [6], [7].
We have developed some high-power modulated induction thermal plasma systems, known as PMITP and AMITP [12], [13]. The pulse-modulated induction thermal plasma (PMITP) system can modulate the coil current sustaining the induction thermal plasma into a rectangular waveform. It can change the temperature and radical densities as well as the gas flow field in the thermal plasma in a time domain [12]. Some studies have tried to adopt the PMITP for surface modification of materials using good controllability of the temperature and radical densities [14], [15], [16]. A high-power arbitrary-waveform-modulated induction thermal plasma (AMITP) system has also been developed for additional enhancement of the degrees of freedom in thermal plasma control [13]. The AMITP system can modulate the coil current not only into a rectangular waveform but also into an externally given waveform, which provides precise time-domain control of the temperature of thermal plasmas [13].
We have started studying nanoparticle synthesis using a modulated induction thermal plasma [17]. The two following main effects can be expected in nanoparticle syntheses using modulated thermal plasmas. One is rapid and complete evaporation of injected raw materials during high-power input time duration, with subsequent rapid cooling of the evaporated material during the low-power input period. In particular, the low-power input period in the modulation might provide rapid cooling of the evaporated material, which can be described as the temperature decay rate | ∂ T/∂ t|. This temperature decay rate | ∂ T/∂ t| is expected to be of 105–106 K/s order in the PMITP, in addition to the temperature history of the evaporated material by convection |u ⋅ ∇ T|, where T is the temperature, t is the time, and u is the gas flow vector. In other words, we anticipate that the temperature history of evaporated material |dT/dt| is | ∂ T/∂ t + u ⋅ ∇ T|. The rapid temperature history of 105–106 K/s for evaporated material is known to be useful to create nanoparticle synthesis [18], [19]. The other is the enhancement of the time-averaged temperature-gradient, which also causes cooling of evaporated material during transfer to the downstream chamber. It is related with |u ⋅ ∇ Tave|, where Tave is the average temperature. This enhancement of the time-averaged temperature gradient has been observed also in other applications of the PMITP in experiments and numerical simulations conducted in our previous works [15], [20]. Actually, our previous study related to nanoparticle synthesis using PMITP [17], revealed that the higher degree of the coil current modulation for the PMITP can provide smaller TiO2 nanoparticles at a fixed duty factor (DF) of 80%, and that the weight fraction of anatase-TiO2 nanoparticles to the synthesized particles were 80–90%, almost irrespective of the modulation degree. In addition, our estimation of the time-averaged temperature in the reaction chamber revealed that the time-averaged temperature was decreased by the modulation of the coil current even at the same input power of 20 kW used in our previous study. However, the effect of current modulation for different duty factors and the instantaneous temperature variation have been not yet investigated.
This paper describes the influence of modulation conditions such as the duty factor (DF) and the shimmer current level (SCL) on TiO2 nanoparticle synthesis using Ar–O2 PMITP at 20 kW. For this purpose, 15 combinations of DF and SCL were experimentally studied to assess their effects on the mean particle size and the weight fraction of synthesized nanoparticles. Furthermore, the time evolution in the temperature of the thermal plasma in the reaction chamber was determined using time resolution spectroscopic observation to investigate rapid changes in the temperature. Based on the temperature measurement results, the relation between the temperature decay rate during the low input-power duration and the mean diameter of synthesized nanoparticles was obtained to investigate instantaneous rapid cooling effects of evaporated material. The main contribution of this paper is the experimental confirmation of the pulse modulation effect on nanoparticle synthesis.
Section snippets
Experimental arrangements
Fig. 1 depicts the experimental setup and the details of this setup can be found in our previous paper [17]. We used a radio frequency (rf) power supply with a metal-oxide semiconductor field emission transistor (MOSFET). It can also modulate electric current of several hundreds of amperes for supplying the coil current. The amplitude of the coil current sustaining induction thermal plasmas was modulated into a rectangular waveform in this work. The modulated coil current has modulation
SEM images and mean diameters
Fig. 4 presents examples of FE-SEM micrographs of particles synthesized using induction thermal plasma with and without the coil current modulation. Panel (a) portrays the image for DF = 100%, i.e. a non-modulation condition, whereas panels (b)–(e) portray images for different DFs. For all images, the SCL was fixed at 80%. As might be readily apparent, reducing DF from 100% to 67% roughly increases the particle size. In contrast, reducing DF from 67% to 60% seems to increase the particle size
Conclusions
In summary, a 20 kW class of Ar–O2 pulse-modulated induction thermal plasma (PMITP) was used for synthesis of TiO2 nanoparticles for different modulation conditions including those of the shimmer current level and duty factor. Experimental results show that a modulation condition of 80%DF–65%SCL provides smaller nanoparticles among 15 combinations of DF and SCL, and that the weight fraction of anatase TiO2 in the synthesized particles was estimated as 80%–90%, almost independent of the
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2013, OptikCitation Excerpt :The chemical precipitation method is one of the attractive powder synthesis methods, because it is possible to produce nanoparticles with high specific surface area and improved crystallinity. TiO2 nanoparticles have been prepared by different methods such as, chemical precipitation method [19], chemical vapor deposition (CVD) [20], the sol–gel technique [21,22], sputtering [23], hydrolysis, microemulsion method [24], spray deposition [25], aerosol-assisted chemical vapour deposition method [26], thermal plasma [27], hydrothermal method [28] and microwave-assisted hydrothermal synthesis [29]. Among these methods, chemical precipitation method is a simple process to synthesis TiO2 nanoparticles.
Recent development of new inductively coupled thermal plasmas for materials processing
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