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Argon, ethylene, and acetylene are supplied to the gas manifold. The flow rates can be adjusted individually using electronically operated mass flow controllers (58xx series; Brooks Instruments, Hatfield, PA, USA). The gas feed ends in a Swagelok quick connect, a special valve that is automatically sealed on both ends upon disconnec- tion (see inset in Fig. 2.3). The residual volume for this type of quick connect is just 0.1 ml. The exhaust side of the tube has a low pressure one-way check valve attached, preventing backflow of exhaust gas or ambient air into the tube even when it is not pressurized or installed in the CVD setup. The entire quartz tube can thus be hermet- ically sealed and removed from the furnace by disconnecting it at the quick connect and behind the check valve. Coating considerations. In order to obtain optimal coating results, it is important to quantify the amount of carbon deposited during each run. Lacking other practical and reliable means, this is done by weighing the sample before and after deposition. Assuming no material loss, spherical particles, and uniform coating, the thickness of the carbon shell can be calculated from the carbon weight and the particle’s geometry using the following relation (see also Fig. 2.4): mcarbon mshell ρC · 4/3π rs3hell − rc3ore = =43 433.(2.1) mtotal mcore + mshell ρLi2S · /3πrcore + ρC · /3π rshell − rcore For particles with a diameter of 1 μm, adding 2 % carbon by weight would already result in a coating of ∼ 300 nm thickness. In order to obtain reasonably thin coatings, the amount of carbon deposited should stay below 1.0 % for all but the smallest parti- cles. Since the above assumptions are typically not fulfilled, this calculation can only serve as a first approximation and further measurements are required to determine the thickness and uniformity of the coating more precisely. Coating procedure. Before starting the actual coating, the following preparatory steps need to be completed: The gas manifold is assembled and flushed with Ar- gon for several minutes. Next, the quartz tube is removed at the quick connects so that the inlet side of the manifold is now closed, but under pressure. The caps are removed from the quartz tube, which is then cleaned, dried and transferred into the glovebox using its large antechamber. Inside the glovebox, the previously ball-milled Li2S is put into a ceramic boat and weighed (Fig. 2.5); typically 500–2000mg go in a large boat so that its floor is only thinly covered. The boat is placed in the quartz tube and aligned centered before the caps are attached and tightened. Next, the loaded quartz tube is transferred out of the glovebox and installed in the gas manifold (outlet side first). The pressurized Argon in the gas manifold immediately starts to flush the tube. A mechanical stand is used to stabilize the tube during the following CVD run to 21PDF Image | Lithium-Sulfur Battery: Design, Characterization, and Physically-based Modeling
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