The aim of the submitted project is to gain fundamental knowledge acquisition about chemical mechanisms, phase transition phenomena as well as nucleation and growth processes of metal oxide nanoparticles by using miniaturized sensors in combination with (further developed) simulation methods in reactive flow systems. In order to achieve this goal, it is necessary to realize a microreactor which has been optimized regarding sensor technology, flow control and dwell time for the synthesis of metal oxide nanoparticles in near / supercritical water. For this purpose, the microreaction system has to be optimized in to the effect that the system is equipped with numerous sensors, so that electrochemical impedance spectroscopy (EIS) can be used to monitor place- and time-resolved: a) the mixing process, b) the reaction process in (multi-phase) flows, and c) particle formation and growth with spatial and temporal resolution.

The implementation of the sensors into the reactor, the integration into the test station as well as the data analysis are carried out by WGs Türk und Hanemann. The simulation of the thermo- and fluid dynamic properties regarding the mixing and the calculation of the expected sensor signals is carried out in WG Greiner. The close interlocking of the different aspects "particle synthesis, microreactor design (MR), sensor integration, simulation of particle formation (PB) and particle growth (PW)" is illustrated in the following sketch:

One focus of the previous work was the conception, construction, commissioning and optimization of a test plant for the continuous hydrothermal synthesis (CHTS) of metal oxide nanoparticles under near- and supercritical conditions. In this work package, the CHTS plant was constructed with a functioning measurement, control and data acquisition system taking into account the special and necessary precautionary and safety measures required for the operation of plants under high temperature and high pressure (T_max ≤ 723 K and p_max ≤ 40 MPa). Special attention was paid to the following aspects:

1. Low-pulsation operation of the plant under all process conditions, especially at high total mass flows of up to 80 g/min.

2. Simple and flexible combination of different mixer and reactor geometries. This enables the mixing and additive of different educt streams at various positions before and after the actual reaction zone for the work planned for the second advancement period.

3. Possibility of integrating the EIS measurement technology, based on the educt solution at all relevant positions for determination of the salt concentration and therefore the local (at various points in the plant) and final conversion (in the product solution).

Fig. 1 shows a schematic representation of the particle synthesis plant by CHTS set up in WG Türk. This test plant enables examinations under the following process conditions: 300 K ≤ T ≤ 723 K, 0.1 MPa ≤ p ≤ 40 MPa and 25 g/min ≤ ṁ₃ ≤ 80 g/min. The mixing ratio (= cold metal salt flow / hot water flow) is in the range between 0.05 and 0.4 whereas the salt concentration, c_Salt, is between 0.00125 and 0.15 mol/dm³, depending on the type of salt.

In close coordination with the project partners, a reactor design was developed which allows the necessary requirements regarding nanoparticle synthesis, fluid dynamics, but also the manufacturability of the corresponding negative structure for shaping using mechanical microfabrication respectively various 3D printing procedures. In conclusion, the mould insert must allow the moulding of defect-free components using ceramic powder injection moulding. In addition to the actual mixer structure (T-structure), the mould insert also contains supplementary structures to enable the integration of electrodes for electrochemical impedance spectroscopy (EIS) for tracking nanoparticle synthesis. The CAD drawings of the reactor structure and the corresponding mould insert structures can be seen exemplarily (Fig. 3). When constructing the mould inserts, however, it should be considered that all dimensions on the mould insert have to be larger than the final dimensions in the reactor in order to compensate for the sinter shrinkage of the ceramic moulding compound. This mainly depends on the ceramic filling level of the inserted moulding compound. In the present case this concretely means that the moulded component must have a width of 1.8 mm in order to achieve the desired channel widths of 1.6 mm in the sintered component.

Reactor fabrication

Powder injection moulding has been an established process for many years to produce ceramic or metallic components of different sizes close to the final contour cost-effectively and in large quantities and over the last 10 years, it has been further developed in the direction of ever smaller components, significantly increased precision and achievable minimum structure sizes well below 1 mm. Currently, lateral dimensions smaller than 10 µm, smallest structural details < 3 µm and dimensional accuracy ± 0.3% can be achieved in research operations. However, this is only possible if extensive research work is carried out at all individual steps in the process chain of moulding compound production, replication, debinding and sintering. The moulding compound production establishes the fundamentals for successful moulding and thermal post-processing. When selecting the moulding compound components (powder, binder, additives), attention has to be paid to ensure that no phase separation occurs under high shear loading. This could also be recreated using simulation calculations by WG Greiner in the past. In order to reduce the production time, plastic mould inserts were first produced for design studies using 3D printing (Fused Deposition Modeling, FDM respectively 3D InkJet/PolyJet process) in ABS (acrylic butadiene styrene) respectively in a thermosetting plastic and test moulding in powder injection moulding with a moulding compound containing 55 vol% aluminium oxide. After successful validation, brass forming tools were manufactured micromechanically (Fig. 4).

After moulding, following debinding and sintering, the reactor halves were sintered with the aid of glass solder and prepared for installation into the CHTS plant using an adapter (Fig. 5).

Sensor structure

For the construction of the sensor, electrodes had to be attached in pairs to the side; this was carried out after sintering the two reactor halves using a silver-containing epoxy 2-component-adhesive. A schematic cross section through the reactor is shown in Fig. 6, on the left. The positioning of the electrodes is a compromise between the requirements of the measurement technology and the operating conditions of the reactor: From the point of view of EIS, the electrodes have to be brought as close as possible to the reaction channel; from the point of view of the planned reaction conditions (pressure up to 35 MPa, T up to 750 K), the walls must be as solid as possible so that the reactor is pressure-resistant. A completely contacted ceramic reactor is shown in Fig. 6, on the right.

In order to verify the fundamental suitability of EIS for the detection of small salt concentrations, the change in impedance with sodium chloride concentration in water was determined using an electrically and fluid-contacted 3D-printed microreactor made of polyethylene terephalate (PET) (Fig. 7).

WG Türk