A Novel Method for Preparation of High Strength Hollow Spherical. Silicon Dioxide By Plasma Technology

Transcription

1 A Novel Method for Preparation of High Strength Hollow Spherical Silicon Dioxide By Plasma Technology Junmei Fan, Chen Lu, Fangli Yuan, Peichen Zhao State Key Laboratory of Multi-phase Complex System, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, China ABSTRACT Spray drying and DC thermal plasma had been introduced to prepare hollow silica beads (HSB). The products were characterized by XRD, FESEM and laser particle size analyzer et al. Results revealed that HSB with particle size from 20 μm to 120 μm could be obtained, the average true density of HSB could range from 0.40 g/cm 3 to 0.96 g/cm 3, and the corresponding compressive strength ranged from 60 MPa to 95 MPa. The preparation method was fast and continuous, the production capability in the laboratory could reach 4kg/h. Keywords: Hollow silica beads, Plasma, Low density, High strength 1. INTRODUCTION Due to the characteristics of low specific gravity, satisfactory heat resistance, heat insulating properties, pressure resistance and impact resistance, hollow silica particles are used in a variety of applications ranging from heat insulating, sound absorption, optical and lightweight composite materials etc, where they may serve as modifiers, enhancers, rigidifiers, and/or fillers,[1-7] but they have only limited utility when very high strength are required, for example, when they are used as filler for polymeric parts made by injection molding operations, used as filler for drilling fluids etc. Until now, hollow silica beads (HSB) can be produced via various methods, such as soft-template methods[8-10], hard-template methods[11], spray pyrolysis[12] and spray drying[13]. However, the mechanical properties of HSB prepared via these methods are either not reported or not satisfactory. It has been reported that hollow zirconia spheres have been prepared via plasma treatment, and have been further used to thermal spraying coatings[14], which could prove that these spheres had good mechanical properties. Thermal plasma processing provides unique advantages of high temperature, rapid quenching rate, high purity, and high yield rate. The dominating condition to form a hollow particle in plasma flow is sufficient gas content in feedstock powder material. This can be obtained by application of porous particles, binders in agglomerates which can evaporate at high temperature, powders with high specific surface area providing gas adsorption, etc. This work is aimed at producing micrometer-sized HSB with desirable mechanical strength. Here we present a preparation way by plasma sintering spray dried silica particles. We have prepared large amount of HSB which possess different compressive strength. The preparation method is rapid and continuous, and the 1

2 obtained HSB have promising applications. 2.1 Materials and Preparation. 2. EXPERIMENTAL Silica slurry was first prepared by mixing nano-silica (An Hui Jing Ye Nano Technology CO., LTD., China) and sodium silicate in water and ball milling for a period of 2h using a high speed agitated beads mill (Wuxi Xinguang Powder Processing Craft CO., Ltd., China). No dispersant or blowing agent was used. The nano-silica content was varied from 10wt.% to 20wt.%, while the weight ratio of sodium silicate to nano-silica kept at The next step was making spray-dried agglomerated particles (SDAP). The dryer we used was an industrial centrifugal spray dryer. The slurry was delivered by a peristaltic pump into a rotary atomizer, where the slurry was atomized by centrifugal energy and friction force. In the meantime, heated air was blown into the drying chamber through a ceiling air disperser, then evaporated the water of the droplets to make dry particles, which were collected at the bottom of the drying chamber. During the drying process the speed of the rotary atomizer was set at RPM, the feedrate of slurry was kept at 100 ml/min, and the inlet and outlet gas temperature were 190 oc and 110oC, respectively. The last step in the process of making HSB was to use our plasma sintering system to make the SDAP into a fully homogenized hollow structure. The plasma sintering system which was set up by our laboratory consisted of a DC plasma generator, a water-cooled reactor and a powder-collecting filter. The SDAP which were delivered by our homemade feeder were injected into the plasma flame along the radius into the region under the plasma-gun nozzle exit. In practical experiment, the primary gas and secondary gas of the plasma generator were both nitrogen; the flow for primary gas was 4.0 m3/h and for secondary gas was 2.0 m3/h; the power level might vary from 30 to 50 kw by adjusting the voltage and the current of the plasma generator, and the feedrate of the SDAP might vary from 70 to 150 g/min. After sintering, water was introduced to pick out the floating product as final HSB. 2.2 Characterization The morphology and structure of the samples were observed by scanning electron microscope (JEOL, JSM-6700F). The phase of the samples were determined by X-ray diffraction (PANalytical, X Pert PRO MRD). Particle size distribution was determined by a laser particle size analyzer (Beckman Coulter, LS ). The bulk density was measured by Hall flowmeter. The average true density of HSB was measured by pycnometer (Micromeritics, AccuPyc 1340). The compressive strength of HSB was measured by a hydrostatic pressing method: the HSB were dispersed in glycerol and the strength value reported was the hydrostatic pressure at which 10 percent by volume of the HSB collapsed. 3. RESULTS AND DISCUSSION Figure 1 shows the general morphology of SDAP. We can see from Figure 1a that 2

3 the obtained dry products are composed of nearly spherical particles with size ranging from 10 μm to 100 μm, there is a small dimple on the surface of each particle which can be attribute to the explosion of interior water vapor and shrinkage of the half-dried particle s surface film during drying progress. The almost regular shape and smooth surface of the particles could be further confirmed from the magnified SEM image shown in Figure 1b, and some broken particles observed from this SEM image confirm the existence of a porous structure in the particles. Figure 1. (a) low magnification (b) high magnification SEM images of SDAP. The morphology and structure of obtained HSB were investigated by SEM and XRD analyses as shown in Figure 2. Figure 2a gives the general SEM image of HSB, which exhibits that the final products are spherical particles. The regular shape and compact surface of the particles can be further confirmed from the magnified SEM image shown in Figure 2b, and it can be seen that in contrast to SDAP, the surface of plasma sintered HSB is relatively rough, on which many nano-sized spheres are distributed. This is because that melting-spheroidization and evaporation-condensation often coexist during plasma sintering process. Melting-spheroidization results in microspheres while evaporation-condensation leads to spherical nano-particles. The detailed inner structure of a single particle is shown in Figure 2c, which indicates that a single large void lies in the center of the spheres; the shell of the spheres is solid, the thickness is homogeneous and is about 2 μm. When SDAP enter a plasma flame melting starts at their surface forming a liquid film which catches residual gas. Further heating of a liquid spheres leads to expansion and consolidation of gas cavities and surface tension provides spherical shape of inner void. After the liquid hollow sphere leaves plasma flame and solidifies, a cavity is fixed and generally it is in the center of the particle, providing uniform shell thickness. The phase structure of HSB were investigated by XRD analysis shown in Figure 6d, and from the pattern, it is clearly that only one broad diffraction peak can be observed, obviously demonstrating that HSB have amorphous structure, which can be attributed to the large supercooling degree during plasma sintering. 3

4 Figure 2. (a) low magnification (b) high magnification SEM images; (c) enlarged cross-section SEM image of a single particle; and (d) XRD pattern of HSB. In order to prepare HSB that have different mechanical properties, we prepared slurry that had different silica content of 10wt.%, 15wt.% and 20wt.%, the corresponding spray-dried products were named as SDAP-10, SDAP-15 and SDAP-20 respectively. The properties of these three kinds of SDAP are shown in table 1, from which we can see that the mean diameter and bulk density of SDAP increase as solid content goes up, and the bulk density is much smaller than the real density of silica, suggesting high porosity within SDAP. Next, we used optimized plasma sintering technology to sinter these SDAP into HSB with floating rate higher than 40% by weight of the SDAP. The corresponding properties of HSB are shown in table 2 which presents us that higher silica content results in smaller mean diameter, bigger average true density and thus higher compressive strength of HSB. The obtained series of HSB possessing satisfactory low true density and high compressive strength will have promising applications as fillers or lightening agent etc in many fields. 4

5 Table 1. Properties of SDAP Sample D 10 D 50 D 90 Mean diameter Bulk density (g/cm 3 ) SDAP SDAP SDAP Sample Mean diameter Table 2. Properties of HSB Bulk density (g/ cm 3 ) Average true density (g/ cm 3 ) Compressive strength (MPa) HSB HSB HSB CONCLUSION We have successive prepared hollow silica beads via a spray drying-plasma sintering process. The obtained HSB have low true density (from 0.67 to 0.96 g/cm 3 ) and high compressive strength (from 60 to 95 MPa). The preparation method was fast and continuous, the production capability in the laboratory could reach 4kg/h. The HSB have promising applications in many fields. REFERENCES 1. Liang, J. Z., Mechanical properties of hollow glass bead-filled ABS composites. Journal of Thermoplastic Composite Materials 2005, 18 (5), Hu, X.; Xu, H. S.; Li, Z. M., Morphology and properties of poly(l-lactide) (PLLA) filled with hollow glass beads. Macromolecular Materials and Engineering 2007, 292 (5), Liang, J. Z., Tensile properties of hollow glass bead-filled polypropylene composites. J. Appl. Polym. Sci. 2007, 104 (3), Liang, J. Z.; Li, F. H., Simulation of heat transfer in hollow-glass-bead-filled polypropylene composites by finite element method. Polymer Testing 2007, 26 (3), Liu, J. H.; Wei, J.; Li, S. M., Preparation and characteristics of Fe3O4 magnetic thin films plated on hollow glass spheres. Mater Lett 2007, 61 (7), Wei, J.; Liu, J. H.; Li, S. M., Electromagnetic and microwave absorption properties of Fe3O4 magnetic films plated on hollow glass spheres. Journal of Magnetism and Magnetic Materials 2007, 312 (2), Ahluwalia, B. S.; Lovhaugen, P.; Helleso, O. G., Waveguide trapping of hollow glass spheres. Optics Letters 2011, 36 (17), Zhang, H.; Wu, J.; Zhou, L.; Zhang, D.; Qi, L., Facile Synthesis of Monodisperse Microspheres and Gigantic Hollow Shells of Mesoporous Silica in Mixed Water Ethanol Solvents. Langmuir 2006, 23 (3),

6 9. Liu, S.; Wei, M.; Sui, X.; Cheng, X.; Cool, P.; Van Tendeloo, G., A scanning electron microscopy study on hollow silica microspheres: defects and influences of the synthesis composition. Journal of Sol-Gel Science and Technology 2009, 49 (3), Wu, X. F.; Lu, H. R.; Wang, Z. Q.; Xu, X. H., Preparation of hollow silica beads via soft template calcinating route. Journal of Sol-Gel Science and Technology 2010, 54 (2), Chen, M.; Wu, L.; Zhou, S.; You, B., A Method for the Fabrication of Monodisperse Hollow Silica Spheres. Adv. Mater. 2006, 18 (6), Kim, K. D.; Choi, K. Y.; Yang, J. W., Formation of spherical hollow silica particles from sodium silicate solution by ultrasonic spray pyrolysis method. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2005, 254 (1-3), Cheow, W. S.; Li, S.; Hadinoto, K., Spray drying formulation of hollow spherical aggregates of silica nanoparticles by experimental design. Chemical Engineering Research and Design 2010, 88 (5-6), Longo, F. N.; Bader, N. F.; Dorfman, M. R. Hollow sphere ceramic particles for abradable coatings. EP86938-A; JP A; US A; CA A; EP86938-B; DE G,