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Properties, Microstructure and Composition of Vermiculite and Insulation Composites

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Tutor: GuHuaZhi
School: Wuhan University of Science and Technology
Course: Materials Science
Keywords: Vermiculite,Insulation composites,In-situ modification bygelation,Modification b
Type: PhD thesis
Year:  2012
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Energy shortage is an important problem faced by all countries in the world, and the mosteffective way to solve the energy shortage is energy saving. The most effective way to save energyis development and application of insulation materials. Expanded vermiculite itself has low thermalconductivity and bulk density, moreover, the scale structure of its granule surface has the ability toreflect thermal radiation. It is a potential insulation refractory material with high application value.In this work, the insulation material was prepared from expanded vermiculite and forsterite.Based on analysis of grey relational degree, the preparation process of expandedvermiculite-forsterite composites was optimized. Prepared by the optimized preparation process, theinsulation material can endure rupture strength of9.33MPa and compression strength of15.74MPa.At the temperature of200oC, its thermal conductivity is about0.19W/m¡¤K, at300oC, about0.22W/m¡¤K£¬at600oC, about0.26W/m¡¤K.Based on the polarization theory and curing mechanism of bond system, the bond system ofcomposites was optimized, and the relationship between the properties of composites and manyfactors were studied, such as the concentration and dosage of aluminum dihydrogen phosphate, thetypes of magnesia and MgO/P2O5molar ratio. Based on the synergistic effect theory, we try toimprove the thermal insulation propertiy of composites by way of selecting appropriate admixtureto improve the intensity and thermal insulation of composites material. The addictive amount ofK2Ti6O13whisker is2wt%, Titanium dioxide2wt%, BiOCl8wt%, which can produce the bestheat-resistance of expanded vermiculite and forsterite composites material. At the temperature of300oC, its thermal conductivity is0.117W/m¡¤K, at600oC0.169W/m¡¤K, at800oC0.184W/m¡¤K, at1000oC0.190W/m¡¤K.In order to improve the microstructure of composites material, furthermore to enhance themechanical and thermal insulation properties of composites, we adopted method of gel in situ tomodify expanded vermiculite and research the conditions for preparation process of modifiedexpanded vermiculite. Through modification of gel-in situ, the pore structure of expandedvermiculite varied from micrometers to nanometer. So the microstructure of composites can beimproved, and the mechanical strength and thermal insulation properties of composites increase. Theoptimized preparation process of gel in situ modification is: n(epoxy propane):n(A1):n(methanamide)):n(alcohol)=5.5:1:0.8:30, ambient drying. As is shown in SEM of expanded vermiculite after being optimizedby aluminum gel in situ, the framework of aluminum gel in situ is made up of sphere-like Al2O3particles, the particle size is fairly regular, the mean particle diameter is40nm, the pore size amongparticles is45nm, and the distribution of pore size is fairly regular. After being calcined under the temperature of900oC and1000oC for4hours, it can be still kept in good multi-porous network,without any distinct signs of conglobation or porous structure collapse.By means of in-situ modification to expanded vermiculite, thermal conductivity of compositesmaterial has been remarkably reduced. The higher proportion the modified expanded vermiculitetakes up in total weight of vermiculite, the lower the thermal conductivity of composites material is,and the better the thermal insulation. When modified expanded vermiculite takes up50wt%of totalamount of vermiculite, the thermal conductivity of composites material is the lowest. At thetemperature of300oC, its coefficient of thermal conductivity is0.13W/m¡¤K, at600oC0.157W/m¡¤K,at800oC0.169W/m¡¤K, at900oC0.168W/m¡¤K. Compared with non-modified expanded vermiculitecomposites material, the modified expanded vermiculite composites material can reduce thermalconductivity by20-30%at300oC,30-40%at600oC,35-45%at800oC or900oC.In order to expand the application of vermiculite and increase its added value, we adopted ionexchange method to modify vermiculite by inorganic salt, insert polynuclear Al into the layers ofvermiculite, and take advantage of the micro holes and oxidized pillars in inter-layers of inserting ionsafter calcination. In order to furthermore improve microstructure of vermiculite, we also discuss thepreparation process of pillared vermiculite, and research the dynamics of ion exchange reaction. Theanalysis of XRD indicates that after being pillared, the interlayer spacing of A1pillared vermiculitecrystal face is18.42, the interlayer spacing of calcinated Al pillared vermiculite is17.26.Through inserting layer of polynuclear Al, vermiculite gains perpetual8.82interlayer free space.From the characterization of vermiculite microstructures before and after pillarization by means ofTEM, TG-DSC, XRD, FT-IR and nitrogen sorption isotherms, we find out that polynuclear Alpillared vermiculite is much better than raw vermiculite in thermal stability and interlayermicrostructure. During the ion exchange of vermiculite and keggin-Al7+13, intra-particle diffusionprocess is controlling step, the apparent activation energy of its reaction is26.79kJ/mol, and itskinetic equation can be denoted as follows:(?)
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