An Electrically Renewable Air Filter With Integrated Three-dimensional Nanowire Networks

Particulate matter?PM?is a major pollutant in air and it is harmful to human cardiovascular system and respiratory system.Reducing PM exhausts at the source and filtration on-site are two effective approaches to remedy PM pollution.Source control is a relatively long-term process while on-site filtration is more efficient and timesaving.The filtration materials previously developed include carbon-based materials,polymer nanofiber materials and other types of materials,but most of these materials cannot be renewed after being spent.This research develops a filter integrated with nanowire networks to remove PM particles and at the same time this filter shows a potential of being renewable.The initial idea of this work is to synthesize a porous ceramic with interconnected pores decorated with nanowires,but the experiments do not produce promising results.The experiments find that iron element can catalyze the growth of nanowires from the thermal decomposition products of a silicone polymer.Based on this concept,carbon nanowires are directly grown as three-dimensional networks on a 304 stainless steel mesh to form a robust and high efficiency air filter.The growth of the carbon nanowires is initiated by exposing the stain-less steel substrate to the vapor generated from the thermal decomposition of a silicone.The stainless steel is attacked by the carbon supersaturated vapor and releases metal nanoparticles,which catalyze the growth of carbon nanowires.When the heat treatment is carried out at 1100°C,a dense network of nanowires grows and the length of the nanowires can completely cover the openings of the stainless steel mesh.TEM shows that the average diameter of the nanowires is about 120 nm.The core region is a graphite phase while the outer layer is amorphous,and there are metal particles at the tips of the nanowires.XPS test shows that the content of carbon in the nanowire is about 90%,and the surface of the nanowire has polar groups such as C=O and C-O.The growth mechanism of the nanowires is further invesitgated.The gaseous hydrocarbons produced by the decomposition of silicone cause carbon supersaturation and metal dusting corrosion of the stainless steel.The bulk alloy near the surface is gradually pulverized to release metal nanoparticles,which catalyze the deposition of carbon on the surface and the continuous growth of carbon nanowires.The stainless-steel mesh with nanowire networks is assembled into a PM particulate filter and the performance is evaluated.The filter demonstrates a high initial PM_2.5 removal efficiency of 96.1%and a low pressure drop of 158 Pa.FTIR and XPS analyses show that the PM particles contain groups such as C=O,C-O,C-N,etc.,so it can be inferred that adsorption of the PM particles to the nanowires is facilitated by the interactions between these surface groups.Since the stainless steel mesh and the carbon nanowires both have good heat resistance and electrical conductivity,regeneration of the filter by electrical resistive heating is possible.TGA analysis shows that the decomposition temperature of carbon nanowires is above 580°C while the decomposition temperature range of PM particles is 180-550°C.TGA also find that the PM particles can be mostly burned off at450°C for 1.5 hours.The renewal capability of the filter is evaluated using multiple filtration-renewal cycles.The saturated filter is electrically heated to 450°C by a DC power supply for regeneration.In the first three filtration-renewal cycles,the renewed filter recovers the low pressure drop?²00 Pa?while still maintaining a high PM removal efficiency?>90%?,which confirms the feasibility of electrical renewal of the filter.However,the filtration efficiency of the carbon nanowires continues to decrease after the fourth filtration-renewal cycle.Therefore,the cycling performance of the filter still needs to be further improved in future research.