Preparations Of Loading Structured Nano Composites And Their Microwave Absorbing Behaviors

Microwave absorbing materials have always been a research focus in the fields of national defense technology and functional materials. However, it is quite difficult to find an individual material to meet the requirement of impedance matching and strong absorption capability simultaneously in a broad frequency band. Therefore, a common thought of composite preparation for microwave absorption has been gradually formed in the international range. Among the absorbing composites, one kind of composites with the loading of one component on the surface of another component is more beneficial to improve the composite uniformity and stability between different components, so the loading structured composite has gradually become a focus for the design and preparation of absorbing composite. Three kinds of loading structured nano composites were prepared in this thesis and their controllable synthesis, electromagnetic and microwave absorbing behaviors were investigated. Based on the above research, the commonality of the morphology, structure, composition and the consistent rule of microwave absorption property of the as-prepared three kinds of loading structured composites were analyzed, and the multi-mechanism for microwave absorption of the loading structured nanoscale composites was developed.Nanoscaled magnetic particles were uniformly loaded on the surface of tetra-needle like ZnO whisker(abbr. T-ZnOw) and the composition of the particles was controlled by an in situ synthesis and heat treatment method. The loading structured product of M-ZnOw and its intermediate product (noted as Semis) were obtained. The study on the in situ synthesis mechanism showed that plolyethylene glycol(abbr. PEG) played the role of bridging T-ZnOw and nanoscaled magnetic particles through the hydrogen bond and coordinate combination between iron oxide particles and PEG molecule as well as the physical absorption between T-ZnOw and PEG molecule. As shown in the electromagnetic properties, T-ZnOw had no magnetic property and couldn’t attenuate the energy of microwave by magnetic loss; M-ZnOw with a magnetic component of Fe3O4and an iron content over than2wt%appeared ferromagnetic property and could absorb microwave through such magnetic loss mechanisms as magnetic hysteresis loss, natural magnetic resonance loss and exchange magnetic resonance loss. In addition, M-ZnOw appeared much better dielectric loss property than T-ZnOw because the double-complex medium of Fe3O4was loaded on the surface of T-ZnOw, and the dielectric loss tangent (tanδE) of M-ZnOw was obviously greater than that of T-ZnOw. Also, M-ZnOw appeared obviously stronger absorbing ability than T-ZnOw in the frequency band of2-18GHz. The absorbing behavior of M-ZnOw was influenced by the iron content and loading uniformity. M-ZnOw with an iron content of9wt%showed the best absorbing property in the researched iron content range of2-16wt%, and M-ZnOw with uniform loading composite effect showed superiorer absorbing capability to that with irregular loading effect. Furthermore, M-ZnOw appeared higher absorbing capability than the handcrafted composite which had the same composition and component contents as M-ZnOw. As far as the microwave absorbing mechanisms of M-ZnOw were concerned, it was concluded that besides the intrinsic electric and magnetic loss mechanisms resulting from the components in M-ZnOw, nano loss effect was improved and heterogenous interface loss effect was brought to M-ZnOw because of its loading composite structure.Nanoscaled Ni-P plating was coated on the surface of carbon nanotubes (CNTs) and the crystal structure of the plating was adjusted by an electroless plating and heat treatment (N2,450℃,4h) method. Thus, the loading structured products of P-CNTs (with amorphous Ni-P plating) and HT-CNTs (with polycrystalline Ni and Ni3P plating) were obtained. Moreover, the morphology, loading uniformity and plating thickness of P-CNTs were controlled by adjusting the process time of purification and electroless plating. Further study indicated that different HT-CNTs prepared through heat treatment of different P-CNTs showed a consistent difference on loading uniformity as their corresponding P-CNTs. The electromagnetic properties showed that P-CNTs appeared lower resistive loss than CNTs; the dielectric loss of P-CNTs was lower or higher than that of CNTs respectively in the relatively high or low frequency band. P-CNTs was paramagnetic materials and had magnetization loss property to a certain extent. As P-CNTs was heat treated and transformed to HT-CNTs, the tan δE was higher than that of CNTs in the whole frequency band of2-18GHz, and the magnetic loss tangent (tan δM) was further increased compared with P-CNTs, and magnetic hysteresis loss effect was also obtained. Further study showed that the absorbing property of CNTs could be improved to a certain extent through loading structured composite, and better uniformity of loading composite was more beneficial to the increase of heterogenous interface loss effect and nano loss effect, which finally leaded the P-CNTs and HT-CNTs products to appear much stronger absorption in much wider frequency band.Amorphous polypyrrole (PPy) doped with SO42-was synthesized by a chemical oxidation method and PEG/Fe3O4water-based ferrofluid was prepared through an in situ synthesis method. Furthermore, core-shell structured Fe3O4/PPy composite was prepared by a soft-template synthesis method esatbolished in this thesis. In this special loading structured composite, PPy was still amorphous and Fe3O4was crystalline, and coordinate combination was formed between some N atoms of PPy and some Fe atoms of Fe3O4. the morphology, iron content and conductivity of Fe3O4/PPy composite was controlled by adding acetonitrile into the reaction mixture and gradually rising the content of acetonitrile. When the volume ratio of acetonitrile to water (abbr. VRAW) was in the range of0:10-3:7, the prepared Fe3O4/PPy composites were in the morphology of particles with uniform diameter. However, when VRAW was further increased to4:6-5:5, the obtained Fe3O4/PPy composites were in the morphology of two kinds of particles which had obviously different diameter (the large particles were core-shell structured Fe3O4/PPy in diameter of about500nm, and the small particles were intrinsic PPy in diameter of about70nm). As the gradual increase of VRAW, the iron content and conductivity of the as-prepared Fe3O4/PPy composites were gradually increased and decreased, respectively. The synthesis mechanism of Fe3O4/PPy composites based on the combination of pyrrole’s polymerization at the surface and surface layer of PEG/Fe3O4colloidal particles and in the acetonitrile micells was put forward. Moreover, the controllable mechanism of the macro and micro parameters of the as-prepared Fe3O4/PPy composites was theoretically clarified by the proposed synthesis mechanism. As the further study shown, although the dielectric and magnetic loss tangent of SO were not superior to that of PPy, the core-shell structured Fe3O4/VPPy product of SO appeared obviously better absorbing property than pure PPy, which was mainly attributed to the core-shell structure of SO redounded to improve matching and complementary effects of electric and magnetic loss which were resulted from PPy and Fe3O4, respectively. Among the prepared Fe3O4/PPy products, SO (with an iron content of5.59wt%) showed the highest tanδE and the comparable tanδM to S5which had the highest iron content (14.69wt%) because SO had the best core-shell composite effect, a high doping degree of PPy molecular and the largest conductivity. SO also appeared the best microwave absorbing behavior among the as prepared Fe3O4/PPy composites in2-18GHz, which was attributed to its best matching and complementary effects of electric and magnetic loss as well as the improvement of its best core-shell composite effect and dispersion property of nano particles to the heterogenous interface absorbing effect and nano loss effect.Multi-mechanism for microwave absorption of loading structured nano composite were analyzed and brought forward through the comparison and investigation on the commonality of micro structure and composition as well as the consistent rule of the as-prepared three kinds of loading structured composites (M-ZnOw, HT-CNTs and Fe3O4/PPy), which were concluded as the combination of intrinsic electric and magnetic loss effects resulted from the composite components with the improved or newly induced absorption mechanisms of nano loss effect and heterogenous interface loss effect. Heterogeneous interface absorbing mechanism was a peculiar absorbing mechanism that loading composite absorber differed from single component absorber and handcrafted composite absorber, which could be embodied to a certain extent through enhancing electric and magnetic loss of the loading composite absorber.