Search

The hygroscopicity of expanded vermiculite is one of major influence factors for thermal insulating performance. To explore the water absorption properties of expanded vermiculite prepared via electrical heating method, heating temperature effect on the hygroscopicity of expanded vermiculite was investigated under various humidity circumstances. The influencing mechanisms were analyzed by means of Thermogravimetric Analysis-Differential Scanning Calorimeter technique (TG-DSC), Fourier-Transform Infrared Spectroscopy (FTIR), X-Ray Diffraction (XRD), and the Brunauer–Emmett–Teller technique (BET) for the measurement of the specific surface area. The results of water absorption experiments indicated that the hygroscopicity of vermiculite showed an inversely proportional relationship to heating temperature and the water absorption content of EV-900 was only 6.206 mg/g with a dramatic decline of 91.5% in comparison with that of RV. This decline of water absorption capacity was mainly attributed to the decrease of interlamellar spacing and specific surface area. In addition, environment humidity had a significant effect on water absorption capacity and velocity. The water absorption content of EV-400 sample increased dramatically from 48.414 mg/g at the humidity of 55%–72.929 mg/g at the humidity of 95%, and the equilibrium time reduced from 3 h to 0.5 h. This study will provide a new perspective and theoretical guidance for the development of vermiculite based thermal insulating materials.

The thermal insulating performance of expanded vermiculite is closely related to expansibility. A novel method of Mg2+ modification was proposed to improve the vermiculite expansion ratio. The modification process was characterised by energy dispersive X-Ray spectroscopy, X-ray diffraction, Fourier-transform infrared spectroscopy, thermogravimetric-differential scanning calorimeter and scanning electron microscopy. Moreover, the thermal expansion experiment was performed to investigate the expansion behaviours of Raw-V and Mg–V samples at various temperatures. The result indicated that the introduction of Mg2+ could increase the vermiculite interlayer water content through hydration interaction, and the expansion ratio increased 13.7% under the heating temperature of 900 °C after modification. To explain the interaction of magnesium ion and water molecules, a molecular dynamics model was developed, and the simulation results were validated using the binding energy calculation. The results of this study can provide a new perspective and theoretical guidance for the commercial process of vermiculite expansion.

Aimed at improving vermiculite's thermal expansibility, a novel method of Na+ modification has been proposed. The modification effects were characterized via X-ray fluorescence spectroscopy, X-ray diffraction, and thermogravimetric-differential thermal analysis. The result indicated that sodium ions entered vermiculite interlayers through the exchange of interlamellar calcium ions. The effects of the heating time on the expansion ratio of Raw-V and Na-V samples were investigated at the temperature range of 400–700 °C. The result indicated that the maximum increment in the expansion ratio could reach up to 26.5% after Na+ modification. The influencing mechanism of Na+ modification on the thermal expansibility of vermiculite was explored via molecular dynamics simulation and the binding energy and dehydration enthalpy change calculation. The simulation and calculation results showed a good agreement with the expansion experiment result. This study provides a novel method for the preparation of high-performance expanded vermiculite.

Bentonite is an important clay for preparation of adsorbents for potential treatment of industrial effluents. However, due to its natural occurrence as fine particles and swelling after moisture absorption, bentonite is difficult to be separated from the liquid for regeneration purpose, which limits its application in effluent treatment. In this study, two different bentonite-polypropylene composites were synthesized, i.e., with and without pore-enlarging treatment. They both showed strong ability to resist hydraulic agitation damage with a small breakage rate in water. The obtained results from SEM and XRD characterization revealed that the compounding of bentonite with the polypropylene was a physical process, and the structural characteristics of bentonite particles were maintained in the new products. The granular composite adsorbents were effective in removing Pb2+ in aqueous solution. The suggested pore-enlarging treatment was verified as a successful operation to increase greatly the adsorption rate of contaminants on the granular bentonite composite.

Fumed silica based thermal insulating composite is a novel nanoporous material with excellent thermal insulating performance. However, shrinkage at higher temperatures (>800 °C) is a great challenge in application. In this work, fumed silica based thermal insulating composite was prepared by a dry molding method and fumed alumina was introduced to improve high-temperature thermal stability of the composite. The result shows volume shrinkage can decrease from 18.49% to 2.61% at 1000 °C when 20 wt% fumed alumina is added as an inhibitor. Thermal shrinkage inhibition mechanism was investigated by molecular dynamics simulation. The models of fumed silica and fumed alumina super cells were built and mean square displacement (MSD) was calculated. The result indicates MSD of fumed silica is 0.198 A2 at 1000 °C while that of fumed alumina is only 0.0151 A2. In addition, thermal insulating property was comprehensively studied and the effect of fumed alumina on thermal conductivity was analyzed. The result shows this composite has excellent thermal insulating performance with thermal conductivity of 0.052 W/m·K at 800 °C and the introduction of fumed alumina has almost no effect on thermal insulating property. This work can provide theoretical guidelines for the choice of shrinkage inhibitor.

Nanoporous thermal insulating composites of fumed silica, opacifier, and fiber have been prepared by a dry molding method. SiC is adopted as an opacifier to improve thermal insulating performance at higher temperatures where radiation heat transfer plays an important role in entire heat transfer system. Radiative heat attenuation mechanisms have been illuminated by calculating thermal radiation optical parameters including scattering, absorption, and attenuation factors. The result shows fumed silica has low attenuation factor in infrared wavelength range, only 0.00188 for infrared radiation with a wavelength of 3.14 μm. The introduction of SiC can remarkably restrain radiative heat transfer, and the maximum attenuation factor reaches up to 5.848 for infrared radiation with a wavelength of 8.646 μm. The effect of SiC particle size on thermal radiation optical parameters is also studied. The choice of SiC particle size should match up with serving temperature of this insulation. In addition, thermal conductivity measurement has been employed to verify the accuracy of calculation results. Thermal conductivity can significantly decrease from 0.119 to 0.041 W/(m K) at 773 K when 25 wt% SiC (0.877 μm) is added into the composites.

Developing nanoporous thermal insulation materials is an effective method for solving energy consumption problems. In this study, fumed silica based thermal insulating composite was prepared using a novel dry molding method. Thermal conductivity was measured using a guarded hot plate method in the steady state. The results indicated that this composite exhibited excellent thermal insulating performance, achieving an ultra-low thermal conductivity of 0.0205 W/mK at 100 °C. By analyzing the microstructure using field emission scanning electron microscopy (FESEM), we found that the fiber surface was modified by a 12 μm-thick layer of fumed silica particles that reduced solid heat transfer. However, at high temperatures, thermal insulating performance decreased rapidly, and thermal conductivity reached up to 0.119 W/mK at 500 °C. Based on this problem, a heat transfer model was constructed to explain micro-nano scale heat transfer mechanisms of this composite. The results indicated that gas-solid coupled thermal conductivity occupied a large proportion of 78.3% at 100 °C, but the value was only 0.015 W/mK. As the serving temperature increased, radiative heat transfer gradually occupied the dominant position in total heat transfer. Radiative thermal conductivity reached up to 0.101 W/mK at 500 °C, which was approximately 93.3% of total thermal conductivity. Therefore, it was apparent that the increase in radiative heat transfer was the main cause of the decline in thermal insulating performance at high temperatures. Additionally, the effect of the fiber mass ratio on thermal conductivity was investigated at various temperatures. Empirical and simulated data were compared, and the results revealed that the heat transfer model was basically consistent with experimental findings, achieving a mean error of only 4.78%.