Preparation Of Low-density Hollow Carbon Microspheres And Resulting Composite Materials

Hollow carbon microspheres (HCMs) have many unqiue properties including low bulk density, low thermal conductivity, good chemical/thermal stability and the ablity to absorb electromagnetic wave, making them great potential in the development of low-weight functional composites for the aerospace. In this thesis, HCMs with low broken particle ratio and high strength were prepared through carbonization of hollow phenolic microspheres (HPMs) Process conditions including carbonization temperature, acid washing and oxidation process were studied to improve the mechanical strength and thermal property of HCMs. The optimized HCMs were employed as low-density filler for phenolic-resin based composites. In addition, nickel-coating HCMs were prepared through a chemical plating process for the low-density microwave-absorbing materials. The major conclusions can be summaried as follows:1) Preparation of low-density HCMs and resulting mechanical property. It was found that hollow carbon microspheres obtained by direct carbonization of HPMs had very high broken particle ratio and low mechnical strength, making them impossible directly use for practical application. Pretreatment by acid washing of HPMs could effectively remove the inorganic ashes, and thus significantly improve the mechanical properties of HCMs. A low broken particle ratio of 18.30% and low isostatic pressing broken particle ratio of 30.64% can be achieved. Mild oxidation of acid-treated HPMs could further improve the properties of HCMs, with the low broken particle ratio up to 10.03% and low isostatic pressing broken particle ratio as low as 17.34%.2) Preparation of phenolic resin/hollow microspheres and mechanical properties of the composite material. These hollow microspheres were used as fillers to prepare low-density phenolic-based composites. Results demonstrated that the composites had lower density and higher mechanical properties than those without optimization by the acid-treatment and mild oxidation. Compared to the composites using HCMs prepared by dicreact carbonization, the compressive stress and thermal conductivity of the composites using optimized HCMs could be improved from 41.41 MPa to 46.02 MPa and from 0.126 W·m-1·K-1 to 0.115 W·m-1·K-1, respectively.3) Chemical nickel plating of low-density HCMs and absorbing properties. Chemical plating has been employed to coat metal nickel onto the surface of HCMs. The best formula for electroless nickel plating solution was determined as:nickel sulfate 32 g/L,36 g/L sodium phosphite,19 g/L Lactic acid,14 g/L sodium acetate. With the increase of nickel content, dielectric properties and microwave absorbing properties of the microspheres could be significantly improved, e.g.10 dB within 5～10 GHz, and with a maximum absorption up to 20 db.