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visible in Figure 2. Indeed, the optimal values were 2.5 cm for ri and 0.05 L/min for QZn, i.e., 0.1 L/min as the total flowrate. These two parameters strongly influenced the fluid dynamic conditions established in the rotating liquid film, which was generated onto the surface of the disk [46]. The residence time τ [s] on the disk can be calculated as [37]: Nanomaterials 2020, 10, 1321 81 / / / 10 of 15 = 16 − (2) where Q is the total inlet flowrate [m3/s], ν is the kinematic viscosity [m2/s], assumed equal to that of where vr [m/s] is the average radial velocity calculated according to a simplified centrifugal model [35]. water being the diluted solution, and rd [m] is the disk radius. Figure 5 summarizes the influence of Figure 6 displays the influence of ε (a) and of τmix (b) on the nZnO particles mean diameter. ri on residence time. Figure 5. ri influence on residence time. Figure 5. ri influence on residence time. The mean particle’s diameter trend with respect to ε and τmix was, of course, analogous, as the The increase of ri and Q caused a decrease in the reagents residence time on the rotating disk latter parameter depends on the former, and showed that the same minimum occurred for ri = 2.5 cm, surface; therefore, τ being too low, or Q too large, may hinder completion of the reaction. Usually, at all Q values. The order of magnitude of ε and τmix were in line with those obtained in a previous study, these kind of precipitation reactions performed by the SDR are very rapid [38], with induction times working at similar operative conditions (Q = 0.1–0.2 L/min) [34]. It has already been demonstrated lower than 1–1.5 ms. Therefore, the influence of the analyzed parameters on the reaction yield was that when the mixing time of the SDR is in the order of 0.1–1 ms, the obtained particles can reach quite limited within the investigated range. Indeed, at fixed ri, an increase in Q caused a yield dimensions lower than 100 nm, as obtained in the present study [35]. reduction of approximately 1–3% (see Table 2), whereas when maintaining Q as constant, an increase The subsequent runs were performed by fixing the Q to 0.1 L/min and the ri to 2.5 cm, according in ri led to a yield reduction of 1–2%. These parameters also influenced the average dimension of the to the lower mean diameter obtained. The influence of the Zn(II) precursor molar concentration on obtained nanoparticles, mainly due to the different specific power values dispersed in the rotating dwasliquitdeflilimiεte[Wd,/kcog]mapndarmedixiwngitthimtheaτtmioxf[sω],a(seaelreFaidgyuroebs3e)r.veTdhinspcarenviboeusexwpolrakisn[e1d0,2b4y]:considering that when the SDR is used, the average size of the produced nanoparticles is mainly influenced by ε ε= 1ωr+v −ωr+v (3) and of τ (i.e., by ω and fluid dynamic conditions), whereas the initial reagents concentration can mix 2τ influence d only to some extent, as reported in previousυwo.rks, where classical synthesis pathway and equipment were also used. τ = 12 ε (4) The results obtained by the SDR are well comparable with those reported in literature by Liu and where vr [m/s] is the average radial velocity calculated according to a simplified centrifugal model Zeng[4375].,FGigauoret6adl.i[s4p8la]yasntdheWinirflunemncoenogfkεo(la)etanadl.[o4f9τ]m.ixI(nb)thoinstwheornkZ,nthOepmaretiacnlehsymderaonddyinaammetiecrd.iameter of the obtained particles was always in the range of 50–80 nm, with a lower OH−/Zn(II) molar ratio in comparison with those reported in the aforementioned studies, where the most commonly used ratio value was 10/1. In detail, in the first study, ZnO nanorods of an effective diameter of approximately 50 nm, were obtained by means of hydrothermal synthesis at 180 ◦C for 20 h, using ethylenediamine (EDA) at an EDA/Zn(II) molar ratio of 50/1, in addition to the adoption of a Zn(II)/OH− molar ratio of 20/1. Therefore, the synthesis required a higher energy demand and a larger quantity of reagents to obtain a homogeneous rod-like structure for the product, if compared with the SDR equipment. Regarding the second mentioned study [45], the authors adopted a OH−/Zn(II) molar ratio equal to 10/1 and produced ZnO nanorods with an average diameter of approximately 100 nm, assembled into sphere-like structures, by means of hydrothermal synthesis at 95 ◦C for 5 h, using hexamethylenetetramine (HMT) at a HMT/Zn(II) molar ratio equal to 1/1. Finally, the last study [46] reported ZnO nano-rods assembled in flower-structures, similar to those reported in the present study (Figure 4a,b), with a mean diameter in the range of 30–80 nm, using hydrothermal synthesis (60 ◦C for 6 h) and adopting a OH−/Zn(II) molar ratio equal to 10/1. The main influence of the OH− concentration was on the ZnO nanoparticles morphology variation: at a lower OH−/Zn(II) molar ratio (2/1), they were of spherical shape, and at a higher molar ratio (10/1–20/1), they became rod/wire-shaped, when the classical hydrothermalPDF Image | Spinning Disk Reactor Nano Production Intensification
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