Atomic layer deposition on porous powders with in situ gravimetric monitoring in a modular fixed bed reactor setup

Abstract

A modular setup for Atomic Layer Deposition (ALD) on high-surface powder substrates in fixed bed reactors with a gravimetric in situ monitoring was developed. The design and operation are described in detail. An integrated magnetically suspended balance records mass changes during ALD. The highly versatile setup consists of three modular main units: a dosing unit, a reactor unit, and a downstream unit. The reactor unit includes the balance, a large fixed bed reactor, and a quartz crystal microbalance. The dosing unit is equipped with a complex manifold to deliver gases and gaseous reagents including three different ALD precursors, five oxidizing or reducing agents, and two purge gas lines. The system employs reactor temperatures and pressures in the range of 25-600 °C and 10 to 1 bar, respectively. Typically, powder batches between 100 mg and 50 g can be coated. The capabilities of the setup are demonstrated by coating mesoporous SiO powder with a thin AlO (submono) layer using three cycles with trimethylaluminium and HO. The self-limiting nature of the deposition has been verified with the in situ gravimetric monitoring and full saturation curves are presented. The process parameters were used for a scale-up in a large fixed bed reactor. The samples were analyzed with established analytics such as X-ray diffraction, N adsorption, transmission electron microscopy, and inductively coupled plasma optical emission spectrometry.

Investigating the Trimethylaluminium/Water ALD Process on Mesoporous Silica by In Situ Gravimetric Monitoring

“…A detailed setup description and experimental procedure is given elsewhere [15]. The amount of initial substrate varied between 100–600 mg.…”

“…To achieve such a low loading, the highest number of ALD cycles is three. The mesoporous SiO 2 starting material (Davisil, grade 636, 506 m 2 g −1 ) is a widely used support material for heterogeneous catalysts and therefore of particular interest for surface modifications [14,15]. …”

“…The studies are conducted in a self-built versatile ALD setup using fixed particle beds, presented elsewhere [15]. The fixed-bed offers an excellent gas-solid contact.…”

Atomic Layer Deposition on Porous Materials: Problems with Conventional Approaches to Catalyst and Fuel Cell Electrode Preparation

“…In one study, precursor exposure times of almost 2 h were required to saturate just 0.2 grams of SiO 2 powder with a specific surface area of 506 m 2 /g in a fixed-bed reactor. The entire ALD cycle (consisting of the precursor exposure step, the first purge step, the oxidant exposure step, and the second purge step) lasted 8 h [56].…”

“…As mentioned earlier, the practicality of using a static system for large samples has been questioned due to the possible requirement for long exposures to saturate the surface with the precursor. However, since only a few ALD cycles are required to reach substantial weight loadings (see Section 1.4), long exposure and purge times (on the order of 10's of minutes or perhaps even hours) may be acceptable, as demonstrated by the fixed bed static reactor design developed by researchers at BASF [56]. It should be noted that exposure times can be shortened by using agitated or rotating bed reactors [44,66,67], using thin pelletized samples (Figure 3) [70][71][72][73], and increasing precursor partial pressures [56,57].…”