| Over the past decade, hollow microspheres of micrometer to nanometer have been used in widespread areas such as catalysis, drug delivery, waste removal, artifical-cells and buoyancy materials, due to their unique properties. Hollow glass microspheres (HGM) with low density and high specific strength have been used to prepared composites in the field of deep-sea exploration. For example, hollow glass microspheres/resin-matrix composites are commonly known as syntactic foam. Their mechanical properties are mainly determined by basic materials and their structure, such as the mechanical properties of HGM and HGM/matrix interface.Many methods have been developed for the synthesis of hollow inorganic structures, such as nozzle reactor approaches, emulsion/phase extraction techniques, and self-assembly processes including organic template method. Compared with known methods of preparing hollow glass microspheres, template method has more advantages in tailoring the hollow structure. Composites materials with core-shell structure were built by assembly of the shell material onto the surface of templates, followed by removal of the templates via selective dissolution of the core using appropriate solvents or via thermal treatment. Through the method, the interior diameter and the shell thickness can be controlled, via employing templates of different size and varying the concentration of inorganic oxide powder. Meanwhile, hollow microspheres of different kinds of inorganic oxide powder can be prepared.To improve the mechanical properties of hollow glass microspheres, glass-ceramics in the system MgO-Al2O3-SiO2 was prepared as shell material, due to its excellent mechanical properties of higher mechanical strength up to 450 MPa, a hardness up to 13 GPa, and Young's moduli up to 140 GPa. Upon cooling below the glass transition temperature, the large shrinkage of low-quartz solid solution combined with the much smaller shrinkage of the residual glass matrix developed high stress, due to its high thermal expansion coefficient. Those frozen-in stresses have been reported to be associated with the high mechanical strength. Meanwhile, the shell of the as-prepared hollow microspheres is not as smooth as the shell of traditional HGM, so the interface of HGCM/matrix may possess higher adhesive strength, this will be demonstrated in later work.In this study, Hollow glass-ceramics microspheres (HGCM) are successfully fabricated by a simple technique using polyacrylamide microspheres (PAM) as template. The corresponding HGCM are obtained by a thermal treatment of the core-shell microspheres, which are synthesized with organic template method. The effects of the concentration of glass powder, the Hydrophile-Lipophile Balance (HLB) value, the size of glass powder, and the amount of pre-adsorbed water on the morphologies of the core-shell microspheres and HGCM have been investigated. The size, morphology, the thickness of shell, core-shell structure and crystalline phase of HGCM and core-shell microspheres were determined via XRD, SEM and TGA. Meanwhile, the influences of initiator and surfactant on the monodispersity and average diameter of PAM were investigated via SEM. The results showed the average diameter and monodispersity of PAM can be controlled by varying the amount of initiator and surfactant. With the increase of the amount of APS, the average diameter of products increases but the monodispersity decreases. Meanwhile, the increase of the amount of APS will decrease the relative molecular mass of polyacrylamide to increase its solubility and decrease the amount of secondary particles, which lead to the increase of average diameter and the decrease of monodispersity. Conversely, the average diameter decreases and the monodispersity increases with the increase of Span-80. In the synthesis, Span-80 was used as surfactant, due to its steric hindrance effect. As the Span-80 increased, it would hinder the colliding between the monomer droplets and more secondary particles were obtained, which led to the decrease of the average diameter and the increase of monodispersity. In addition, the agglomeration of core-shell microspheres and HGCM can be reduced by using PAM with pre-adsorbed water and adjusting the HLB value. In addition, the amount of solid beads decreases obviously by reducing the concentration of glass powder and adjusting the HLB value. As PAM with pre-adsorbed water are used as template, there is still part of AM and water left on the surface of template due to approaching the maximum water holding capacity of template. As the polymerization continues, all monomers and cross-linkers are reacted and a cross-linked PAM network is formed on the interface with the glass powder tightly connected with template. Nevertheless, when PAM without pre-adsorbed water are used as template, the AM and water in the slurry is completely adsorbed into the PAM and the cross-linked PAM network is not formed on the surface of template. For this reason, the glass powder can not be restricted on the template surface and has the tendency of aggregation. In addition, the core-shell microspheres have the tendency of agglomeration due to the affinity between PAM. It has been well-known that the HLB value at 6 was the range of forming W/O solution and the aqueous phase has the tendency of agglomeration in the organic phase. Intact HGCM with fine spherical morphology were obtained with the HLB value at 7.5 and the surface is not smooth. In addition, the amount of solid beads increases with HLB value up to 12.5 with seldom intact HGCM formed, which is the range of forming O/W solution. In this range, the aqueous slurry can not be well dispersed in the organic phase and the glass powder has the tendency of aggregation to form solid beads. Thus, the HLB value at 7.5 is appropriate, at which the organic phase can wet the surface of glass powder and the glass powder is well dispersed, and the coating process is only depending on hydrophilicity of PAM. The thickness of shell can be controlled by varying the concentration of glass powder and the ratios of pre-adsorbed water and the water in the slurry. Increasing the sintering temperature led to smooth and compact surface.Herein, we first report the synthesis of HGCM with spherical morphology, diameter of ca.10-60μm and shell thickness less than ca.2μm through template method. In our experiments, PAM was used as template and glass-ceramics in the system MgO-Al2O3-SiO2 was prepared as shell material. Meanwhile, through varying the amount of initiator and surfactant, PAM with diameter up to 104μm and fine monodispersity are synthesized. |
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