This study is concerned with the "up-cycling" postconsumer wastes into the value-added products of multiwall carbon nanotubes (CNTs). It investigated combinations of methods for pretreating different types of stainless-steel catalysts that increase the yield of CNTs form common waste plastics. The polymers were pyrolyzed at 800 °C, in nitrogen, and their gaseous pyrolyzates were passed over fixed catalyst substrates preheated to 800 °C. The substrates consisted of SS-304, SS-316 and SS-316L fine stainless-steel wire-cloths. Therein synthesis of CNTs took place by chemical vapor deposition (CVD). The wire-cloths were used either as-received or chemically etched by acid wash and/or heat-treated in air, nitrogen or helium at 800 °C, and then rapidly air-quenched. Results showed that the catalyst type, composition and pretreatment method, as well as the type of feedstock, are all influential on the yields and physical characteristics of the synthesized CNTs.
Multiwall carbon nanotubes (MWCNTs) were generated from recycled lowdensity polyethylene (LDPE). The polymer was pyrolyzed at 800 °C in inert atmospheres, and the gaseous pyrolyzateslight hydrocarbons and hydrogenserved as carbon donors for the MWCNTs, which were catalytically grown by chemical vapor deposition (CVD) at 800 °C. This investigation examined the influences of carrier gas type and flow rate on the synthesis of MWCNTs on stainless steel wire cloth substrates. The influences of the carrier gases (nitrogen, helium, argon, or carbon dioxide gases) and their flow rates (0.1, 1, and 2 L/ min) were both found to have significance on both the yields and the characteristics of the produced MWCNTs. The obtained yields were the highest and the tubes were the longest in nitrogen, due to an apparent, yet unexpected, rate-boosting effect of this nominally inert gas. The MWCNTs' purity in the obtained yields appeared to be enhanced at the highest examined flow rate (2 L/min).
The performance of a magnetic-field-assisted finishing (MAF) process, an advanced surface finishing process, is severely affected by the rheological properties of an MAF brush. The yield stress and viscosity of the MAF brush, comprising iron particles and abrasives mixed in a liquid carrier medium, change depending on the brush’s constituents and the applied magnetic field, which in turn affect the material removal mechanism and the corresponding final surface roughness after the MAF. A series of experiments was conducted to delineate the effect of MAF processing conditions on the yield stress of the MAF brush. The experimental data were fitted into commonly used rheology models. The Herschel–Bulkley (HB) model was found to be the most suitable fit (lowest sum of square errors (SSE)) for the shear stress–shear rate data obtained from the rheology tests and used to calculate the yield stress of the MAF brush. Processing parameters, such as magnetic flux density, weight ratio of iron and abrasives, and abrasive (black ceramic in this study) size, with p-values of 0.031, 0.001 and 0.037, respectively, (each of them lower than the significance level of 0.05), were all found to be statistically significant parameters that affected the yield stress of the MAF brush. Yield stress increased with magnetic flux density and the weight ratio of iron to abrasives in MAF brush and decreased with abrasive size. A new process model, a rheology-integrated model (RM), was formulated using the yield stress data from HB model to determine the indentation depth of individual abrasives in the workpiece during the MAF process. The calculated indentation depth enabled us to predict the material removal rate (MRR) and the instantaneous surface roughness. The predicted MRR and surface roughness from the RM model were found to be a better fit with the experimental data than the pre-existing contact mechanics model (CMM) and wear model (WM) with a R2 of 0.91 for RM as compared to 0.76 and 0.78 for CMM and WM. Finally, the RM, under parametric variations, showed that MRR increases and roughness decreases as magnetic flux density, rotational speed, weight ratio of iron to abrasive particles in MAF brush, and initial roughness increase, and abrasive size decreases.
Magnetic-Field Assisted Finishing (MAF) is a polishing process that utilizes a slurry mixture made of ferrous and abrasive particles in a liquid medium, known as a brush. The brush attached to a magnetic tool directly interacts with the surface of a workpiece and removes any imperfections and defects in the surface giving a smooth and nice surface finish. In this study, two distinct MAF setups were applied to the surface of chromium alloyed low carbon steel sheets to achieve the surface finish. The preliminary studies were conducted on one setup to understand the polishing behavior of the sheets and the other setup was designed to polish larger areas of the sheets to mimic the practical sheet producing environment. The effect of processing conditions such as types and sizes of abrasives, brush composition, and finishing time to attain the final surface roughness of the sheets was studied. The brush with the weight composition of 4:1:1.5 (iron: 3 μm black ceramic: silicone) was found to be the optimal condition for polishing the sheet metal samples. The optimal conditions obtained were applied to the larger scaled experimental setup. The final surface roughness of 38 nm and 220 nm were achieved in these experimental setups, respectively.
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