Preparation And Electromagnetic Characters Of Carbon Black, Polyaniline And Filled Composite Materials

The aim of this dissertation is to provide theoretical evidence and practical application for researching and developing new type electromagnetic wave absorbers. This investigation will show several disadvantages of narrow absorbing frequency bands and low microwave absorption in the traditional types of continuous conducting medium absorber whose absorbent is carbon black (CB) and develop new absorbent of doped polyaniline (PANi). In order to make substitutions for traditional types of microwave absorbers, a discrete conducting medium absorber is developed by isolating conductive medium in absorption matrix with pouring method, and new conductivity doped polyaniline is prepared by chemical oxidation in-situ polymerization process in high magnetic field. In addition, the "low-reflection layer - high shielding layer - low-reflection layer"sandwich shielding slab is put forward. The absorbents and composite slab are analysed by the advanced method of TEM, SEM, XRD, TGA, FT-IR, coaxial-waveguid and reflection loss.The analysis results indicate that the real and imaginary parts of N234 dielectric constant are high, which is helpful to improve microwave absorption. When CB is treated at 700℃, the electric conduction and the loss tangent are higher compared with that of untreated CB. The electric conduction of catalyzed CB by nano-Fe is improved obviously, on the contrary, the dielectric constant is lower and the loss tangent becomes low with increasing frequency which is contrary in comparison with the untreated CB.Polyaniline has better doped character and can be prepared for doped polyaniline with high electrical conductivity and electromagnetism performance by chemical redox polymerization in-situ. Doped polyaniline exhibits excellent electrical conductivity and electromagnetic shielding effectiveness (SE) by compounded with silicone rubber (SR). The result indicates that SE of PANi/SR composites increase and the volume resistivity decrease with increasing mass ratio loading of PANi in the SR. The measured SE of the composites are from -17 dB to -19.3 dB at 100 mass ratio loading of the PANi and the volume resistivity gets to 9.070x10°Ω-cm, which may been using for electronic packaging materials. In addition, doped PANi was firstly obtained by chemical oxidation in-situ polymerization process in high magnetic field (HMF) and the grain structure and shape were studied by TEM. The grain shape of in-situ polymerization PANi in 10T is open-ramified rod-like pattern with the diameter of 50nm and the volume resistivity of its composite increase 3 orders of magnitudeas compared with that of the composites in 0 T, but the grain shape of secondary doped PANi in 10T change to spherical structure about 20-30nm in diameter and the volume resistivity of its composite decrease 1 orders of magnitude, which are obvious different from irregular reef-like particle shape and electric conductivity of doped PANi in 0 T. It is thought that the reason for different grain shape is anisotropic diamagnetic susceptibility of doped PANi and orientations of the HMF.When CB mass fraction are 15-20 wt%, volume resistivity of CB/ABS continuous conducting medium drops sharply and when it are greater than 20 wt%, the drops tendency become slowly, and the volume resistivity with 30 wt% CB achieves the lowest value 103Qcm, and SE increases greatly with increasing CB content. Effective absorption frequency range of continuous conducting medium absorber is narrow and the absorption peaks are becoming high with increasing CB content. But its absorption peaks reach maximum when CB gets to 20 wt% and the volume resistivity is about 105Q-cm. Electromagnetic characters show different change tendency when Titanate coupling agent (TCA) is added into CB content. Titanate coupling agent (2% mass fraction of CB) can increase electrical conductivity and SE of CB/ABS composites, and on the contrary, 10% TCA can decrease its electrical conductivity and SE. When CB mass fraction is low, such as 5% and 10%, reflection loss of continuous conducting medium with 10% TCA is lower than that of the samples without the coupling agent;When CB mass fraction is high, such as 20% and 30%, absorption peak of the continuous conducting medium with 10% TCA is greater than that of the samples without coupling agent and effective absorption band also become broad. Microwave absorption of CB/ABS continuous conducting medium was calculated in according to formula of effective-medium theory and reflection loss and it is found that calculated values were close to experimental values, which could tell us that this calculating method was applied and could be used as valuable reference of absorbing slab design.The discrete conducting medium by isolating conducting medium in absorbing matrix exhibits the optimum absorptivity in 8~18GHz in comparison with that of the continuous conducting medium sample. With increasing CB content of the isolated conduting particles in the absorber, the reflection loss increases in frequency range of 8~18GHz. When mass fraction of CB gets to 30%, the reflection loss is above -lOdB in 8.5~18GHz and above -15dB in 15~18GHz. Especially, when the thickness of the discrete conducting medium absorber reaches 20 mm, the reflection loss is above -15 dB in 8—18 GHz. The shielding effectiveness of sandwich shielding slab is from -20dB to -40dB in frequency of 130-1500MHz.