Basic Research On Kaolinite Based Conductive Mineral Materials

In the development and application of functional materials, mineral-based functional composites with various kinds of properties except for mechanical function have been successfully developed via compounding of a functional matrix and silicate minerals at the micro-nano level, using techniques such as outer surface loading, porous assembly and intercalation. These concepts and approaches take advantage of the structure and physicochemical properties of silicate minerals, such as the compositions, structures, morphologies and surface properties to prepare functional particles or spaces from natural minerals. In this thesis, according to the disadvantages the conductive materials, such as complex preparation process, high cost, deep color and poor thermal stability, we has realized the low-cost preparation and performance control of the light-colored conductive materials taking kaolinite-based conductive mineral material as object, on the basis of semiconductor energy levels and doping theory as well as mineral functionalized techniques. The structures and properties of the obtained products are characterized by X-ray diffraction (XRD), differential scanning calorimetry-thermogravimetry (DSC-TG), scanning electronic microscopy (SEM), transmission electron microscope (TEM), energy dispersive X-ray spectrometer (EDS), X-ray photoelectron spectrometer (XPS), Nitrogen gas adsorption-desorption isotherm, Fourier transform infrared spectroscopy (FTIR), ultraviolet-visible absorption spectrum (UV), photoluminescence spectrum (PL), particle size analysis, surface ζ-potential measurements, powder conductive performance testing and performance testing of the polymer composite techniques.The correlation between structural order degree, thermal stability, morphology and surface characteristics of kaolinite is established through studying the physicochemical properties of the four typical kinds of soft kaolins in China. The endothermic peak temperature shows a clear linear correlation with the crystallinity index (HI) of kaolinite. The relationship is Y=60.63X+457.45with a correlation coefficient of0.994, where Y is the temperature of the endothermic peak maximum and X is the crystallinity index (HI), which verifies the order degree of kaolinite. The formation mechanism of natural kaolinite sub-micron rods and the differences of surface characteristics between rod and layer kaolinite are discussed in great detail. It is proposed that the natural kaolinite sub-micron rods are made up of multilayer curved kaolinite unit, where the external surface is composed of siloxane (Si-O-Si) groups, and a gibbsite-like array of aluminol (Al-OH) groups exist in the internal surface. There are many narrow slit-shaped pores existing between the layers. The special arrangement endows kaolinite with abundant AlOH and SiOH groups, which could be helpful for putting forward the synthesis mechanism of mineral-based functional materials from kaolinite, and provides a theoretical foundation for clarifying the role of minerals in the synthesis control, structural and performance regulation of the functional materials.Al-doped ZnO (AZO) conductive material and AZO/kaolinite conductive mineral material with a flower-shaped morphology are prepared by aqueous phase precipitation method. The AZO is pale yellow and the minimum volume resistivity of two conductive materials are5.89×1044Ω·cm and1.14×105Ω·cm, respectively. The flower shaped conductive particles are composed of packed AZO micron chips with a loose structure. Kaolinite is completely coated by the AZO micron chips. A light grey conductive mineral material of kaolinite coated with antimony-doped tin oxide nanoparticles (ATO/kaolinite), is successfully prepared by chemical co-precipitation, which is rod and the volume resistivity is4.1Ω·cm. The greatest feature of the obtained material is the surface of kaolinite is well covered with a continuous, dense and uniform ATO layer which the thickness of about30～40nm and the particle size of less than10nm. The conductive property of the material could be well regulated by controlling the synthesis conditions. Nanosized ATO particles are coated on the surface of kaolinite to synthesize ATO/Kaolinite conductive material with a light grey color and flake-like morphology. The product has a volume resistivity of273.2Ω·cm with the same morphology as kaolinite. The coating is smooth and dense, which follows the SM growth mode. The effect of kaolinite heat treatment on the structure of conductive mineral material and the reasons for the differences in color and conductive performance of materials are mainly investigated. The results show that the ATO coating on the surface of1000℃calcined kaolinite is more dense and integrity than counterparts in other situations, and the product show better conductive performance when Sb existing as Sb5+mode.Taking kaolinite rods as representative matrix and nano-ATO as conductive function body, the formation process of the conductive mineral materials that based on the surface coating without adding any dispersing agent is simulated, relying on the experimental design and parameters control. The common construction mechanism of the coated conductive mineral materials is put forward through studying the change of morphology, structure, substance and performance of samples from different stage of coated structure formation process. It is proved that the pH value directly affects the formation rate and the particle size of hydrolyzated products, and then changes the surface ζ-potential of kaolinite whcih influencs the degree of kaolinite dispersion and the adsorption of hydrolyzated products, all of which are the key factors that relates to the coating state and particle size on the surface of matrix. There are three chemical states of Sn, Sn2+, Sn4+and Sn(-Cl) complex before heat treatment for conductive mineral material, and mainly as SnO2after calcination at700℃. For Sb ions, there are two chemical states as Sb5+and Sb3+in precursor. After calcinations, all Sb ions on the surfaces of the products transformed into Sb5+, which is crucial to obtaining conductive mineral materials with high performance. Besides, Sb is concentrated on the surfaces of the materials. The HRTEM images clearly exhibit the (001) lattice fringe of kaolinite and the (110) plane of SnO2in the precursor as well as the binding interface between Sn-Sb compounds and kaolinite rods. It is indicated that Si-O-Sn and Al-O-Sn chemical bonds are formed at the interface of Sb/Sn coated layers and kaolinite, in comparison of Al2p and Si2p peak positions for different samples.The light grey color rod (R-SSK) and flake (F-SSK) conductive mineral materials with well dispersion are successfully prepared in the pilot test. The volume resistivity of R-SSK and F-SSK is less than50Ω·cm and100Ω·cm, respectively, both of which show good thermal stability under 700℃. The surface resistivity of IPN coating that prepared by solution blend method is only104Ω, but shows excellent overall performance when the percentage of conductive powder reaches30wt%. The influences of morphology and content for conductive powder on surface resistivity for coating are discussed. The phenomenon that some free coupling agents are washed away from the surface of modified conductive material by the wet method is proved. The SSK/PP composite material is prepared using melt blending method. The effect of filling quantity, morphology and modification of conductive material on structure, mechanical and electrical properties of the composite are discussed. The volume resistivity of SSK/PP composite could reach to7.3×108Ω-cm when the percentage of R-SSK filler is40wt%, which is6-7order of magnitude lower than the pure PP material.Basing on the special morphology and surface characteristic of kaolinite, a light-colored conductive mineral material with uniform and compact surface coating is developed and systematically studied through the design of structure and property as well as surface modification technology. This work is centered on the goal that how to upgrade the technology of traditional mineral resources processing and achieve low-cost preparation and performance control of the light-colored conductive materials. The research line including materials development, design, synthesis, structure-property relationship and application, from the material laboratory synthesis, construction mechanism and surface microstructure evolution study to100kilogram grade production in the pilot test, finally the product application in the target fields, provides a research model for the functional mineral material that based on the surface coating. This work not only provides a new thought for the development of functional materials, but also shows basic fundamental and important application value for the refined processing of mineral resources and mineral materials.