In situ sampling uncovers the dynamics of particle genesis and growth in an aerosol tube reactor

Abstract

Making metal and ceramic powders using aerosol synthesis from vapour precursors, either in a flame or hot-wall tube reactor, is the basis for producing many thousands of tons of powder on an annual basis. To properly study this system, we have designed and built a model reactor with sampling points at evenly spaced axial positions. This allows us to take snapshots of the aerosol population at many points within the reactor. Nucleation followed by a surface reaction produces a solid phase extremely rapidly, within 0.01 s under typical conditions. This is followed by a transient state where nucleation, surface reaction and coagulation all interact to produce a strongly bimodal size distribution. After nucleation is extinguished, the size distribution approaches the selfpreserving limit as predicted for a coagulation-dominated process. The final structure is determined by the dominant sintering mechanism, which can be estimated from theory. The knowledge of this mechanism offers the possibility of selecting reactor conditions to produce powders with optimized properties. From Eq. 1, nickel carbonyl is favoured by lower temperature and higher pressure. The discovery of these compounds paved the way for industrial innovations in the areas of extractive metallurgy and chemical vapour deposition [3].Regarding extractive metallurgy, industrially significant processes for the low-cost purification of nickel by the carbonyl process have been in continuous operation since 1902. In these processes, metals that have the tendency to form carbonyls are readily separated from other metals by passing carbon monoxide through a bed of the finely divided feed, under conditions that favour the left-hand side of Eq. 1. Metals such as nickel will selectively form gaseous nickel carbonyl, leaving other metals such as copper, behind in the bed. The nickel carbonyl can then be deposited as pure nickel by raising the temperature.Regarding chemical vapour deposition (CVD), metal carbonyls can be been used as CVD precursors to make a variety of materials such as nickel-coated fibres, foam and powders [4]. In these processes, metals are deposited heterogeneously from the gas-phase onto a heated template, again using the reaction described in Eq. 1. The kinetics for the heterogeneous reaction

A new technique to derive particle size distributions and aerosol number concentrations directly from thermophoretic data

“…As the particles proceeded through the reactor, the fine particles began to coagulate, which had the effect of creating a temporary bimodal size distribution. Then as the metal carbonyl gas became depleted, the distribution approached the log-normal form as particle growth was mainly by coagulation alone (Wasmund and Coley, 2006). These dynamics were observed for all experiments, and were observed to speed up as the reactor temperature was increased.…”

A study of nucleation and the early stage of growth of nickel powder from a vapour nickel carbonyl precursor

“…The apparatus and experimental method used to obtain examples of particles formed by the decomposition of nickel carbonyl has been described elsewhere (Wasmund and Coley, 2006;Wasmund et al, 2007) and only a brief description will be given here. Nickel carbonyl vapour is carried in carbon monoxide and fed into the top of a hot-wall tube reactor, where it decomposes according to the reaction shown as Eq.…”

“…free-molecule regime (Dobbins and Megaridis, 1987;Arabi-Katbi et al, 2001;Pratsinis et al, 2000). More details about the experimental sampling technique are included elsewhere (Wasmund and Coley, 2006). The sampling time, measured geometric mean diameter, diameter of average volume, and estimated number concentration for the samples extracted by the sampler at the centre-line of the tube are included in Table 3.…”

Modeling of an aerosol reactor for optimizing product properties

“…Each of these phenomena can in principle occur simultaneously, which challenges our understanding of how to optimize the reactor design. NiðCOÞ 4 $ NiðsÞ þ 4 CO DG ðkJ=molÞ ¼ 155:20 À 0:3964 TðKÞ DH ðkJ=molÞ ¼ 140 (1) This reactor, its operating conditions, and the particle sampling technique, have been described in detail elsewhere 2,3 and will be briefly reviewed here, please refer to Figure 1 which illustrates the reactor in cross-section. The outside wall of the reactor is heated with five independently controlled 500 W resistance heating zones, which are evenly spaced along the reactor axis as shown in Figure 1.…”

“…For Case A, the reactor outside wall temperature is controlled between 575 and 6508C. The total inlet flow-rate is 3 standard liters per minute (slpm), consisting of 0.15 slpm of Ni(CO) 4 and 330 ppm of NH 3 , the balance being CO. The reactor configuration for Case A is as shown in Figure 1.…”