Initial Design Concepts and 3D Modelling

To overcome the limitations of current cells previously discussed, a two new flow cell designs were considered for the use of in-situ SR-XRD whilst

facilitating X-ray measurements. The two distinctly different designs can be seen in the SolidWorks® model in Figure 6.2 (design 1) and Figure 6.3 (design 2).

In order to allow X-rays to penetrate into the working electrode within the cell design, an important design consideration is to implement a window made from a material that can transmit X-rays easily. Other important design considerations include the fluid flow profile through the cell and the positioning of the electrodes in order to complete the electrical circuit to monitor the electrochemistry on the working electrode (carbon steel) surface. These design features will be discussed in more detail later in this chapter. The first flow cell design is presented in Figure 6.2 and comprises two main components (the flow cell base (9) and a top plate (2)) that mount together with 10 x M3 countersunk screws (1) that thread into brass inserts (5) embedded within the bottom plate.

Figure 6.2 SolidWorks® _{model of flow cell design 1: a) Exploded view; b)} flow profile schematic.

The top plate design has a section to accommodate a 50 µm thick Kapton® window (3) to allow for X-ray transmittance. A 9 mm diameter cylindrical X65 grade carbon steel sample (6) fits into the base of the flow cell and flush mounted with the top surface of the base plate. Stainless steel Swagelok male tube connecters (8) will provide the inlet and outlet for the brine as seen in Figure 6.2 (b). A 1 mm thick (thickness can vary) Teflon gasket (4) which acts as a flow path has been designed with a parametric curve (see Figure 6.3 that creates and ‘eye’ shape profile to achieve a smooth and uniform flow over the steel sample.

Figure 6.3. Parametric curve design for the flow gasket in design 1. The second flow cell design is presented in Figure 6.4. Figure 6.4(a) shows an exploded view which entails two main components (the flow cell base (5) and a top plate (1)) that mount together with 8 x M5 bolts (7) that thread into the top plate into M5 x 0.8 mm helicoil threaded. The top plate design has a section to accommodate a 50 µm thick Kapton® _{window (3) to allow for X-ray} transmittance. A 9 mm diameter cylindrical X65 grade carbon steel sample (6) fits into the base of the flow cell and flush mounted with the top surface of the base plate. Stainless steel Swagelok male tube connecters (2) will allow the flow through the cell through the inlet and outlet as seen in Figure 6.4(b). A custom made O-ring made from Viton will be implemented within the bottom plate of the cell to prevent leakage and oxygen contamination.

Figure 6.4. SolidWorks® model of Flow cell design 2: a) Exploded view; b) Flow profile schematic.

An essential consideration for deciding which flow cell design to go forward with for manufacture is to determine the flow profile across the sample for both configurations.

Basic Computational Fluid Dynamics (CFD) analysis of the flow through both designs has been conducted using SolidWorks®. Figure 6.5 shows the flow path through design 1, whilst Figure 6.6 shows the flow through design 2. In order to attempt to replicate the conditions of pipelines and to allow the unimpeded flow patterns (no stagnant regions) and electrochemical measurements, conventional flow cells (that have been reviewed previously) are not suitable for this work. Similarly, this is also the case for design 1 taking into account the flow across the steel sample which is observed in Figure 6.5.

Figure 6.5. SolidWorks® CFD analysis of flow path at different flow rates for design 1: Left) Top view; Right) Side view cut through the centre plane. Focusing on the top view for each flow inlet velocity analysed (0.01, 0.1, and 1 m/s), the flow across the sample is non-uniform and stagnant regions are clearly present, more so at 0.1 and 1 m/s. This limits the flow cell to function at flow velocities of <0.01 m/s which is too small to be considered for this study. In comparison to design 2, the CFD analysis of the flow pattern across the sample (Figure 6.6 top view and cross section) shows a uniform and unimpeded flow across the sample at both 0.1 and 1 m/s and therefore based on these observations, design 2 was chosen for the in-situ SR-XRD work. A more complete analysis of the fluid flow through the flow cell was studied with

COMSOL Multiphysics and will be presented later in this chapter. The software provides the ability to simulate a more accurate representation of the flow characteristics through the flow cell and determine the flow efficiency based on experimentally measured input parameters.

Figure 6.6. SolidWorks® CFD analysis of flow path at different flow rates for design 2: Left) Top view; Right) Side view cut through the centre plane.

Another crucial factor limited the use for the first design for in-situ SR-XRD and instantly excluding the first design for the SR-XRD work was the geometry of the cell. Each flow cell design is intrinsically based on one of two specific designs, which correspond to two different measurement geometries (i.e. reflection and transmission modes of measurement as discussed in Chapter 3). In this case, based on the nature of the experiments, the only possible geometry is the reflection cell. However, design 1 mimics the geometry of a conventional reflection cell whereas design 2 has more of a thick film reflection geometry. Reflection cells generally comprise an X-ray transparent window (Kapton® _{in this set-up) that is situated parallel, or sometimes perpendicular} (as with thick-layer reflection cells), to the working-electrode interface, noting that this configuration is only utilised in surface analyses of substrates which is of interest in this work. This transmission window is not only responsible for transmitting X-rays but also containing the electrolyte whilst not severely attenuating the X-ray beam itself.

After carefully reviewing the geometry for both designs, design 2 was chosen for the in-situ SR-XRD work. This is based on the flow cell geometry in the first design masking out the majority of the reflected beam and regions of the detector due to the thickness of the top plate which is illustrated in Figure 6.7 (a). The second design will also mask portions of the reflected beam (Figure

6.7 (a)) but not quite as significant as the previous design and can provide enough data to radially integrate about the beam centre yielding 1D plots of intensity vs 2θ.

Figure 6.7. Flow cell configuration and X-ray reflection geometry: a) Design 1; b) Design 2.