Anatase TiO 2 nanoparticles with average particle size ranging between 12 and 23 nm were synthesized by metalorganic chemical vapor deposition. The structure and particle size were determined by x-ray diffraction and transmission electron microscopy. The specific surface areas were measured by Brunauer-Emmett-Teller and ranged from 65 to 125 m 2 / g. The size effects on the stability of TiO 2 in the air were studied by x-ray diffraction and transmission electron diffraction for isochronally annealed samples in the temperature range of 700-800°C. Only anatase to rutile phase transformation occurred. With the decrease of initial particle size the onset transition temperature was decreased. An increased lattice compression of anatase with the raising of temperature was observed by the x-ray peak shifts. Larger distortions existed in samples with smaller particle size. The calculated activation energy for phase transformation decreased from 299 to 180 kJ/mol with the decrease of initial anatase particle size from 23 to 12 nm. The decreased thermal stability in finer nanoparticles was primarily due to the reduced activation energy as the size related surface enthalpy and stress energy increased.
Undoped and Nd3+-doped TiO2 nanoparticles were synthesized by chemical vapor deposition in order to tailor the band gap of TiO2. The doping reduced the band gap. The band gap was measured by ultraviolet-visible light absorption experiments and by near-edge x-ray absorption fine structure. The maximum band gap reduction was 0.55 eV for 1.5 at. % Nd-doped TiO2 nanoparticles. Density functional theory calculations using the generalized gradient approximation with the linearized augmented plane wave method were used to interpret the band gap narrowing. The band gap narrowing was primarily attributed to the substitutional Nd3+ ions which introduced electron states into the band gap of TiO2 to form the new lowest unoccupied molecular orbital.
Most of the research to date has focused on tailoring the interphase adhesion by controlling the degree of chemical bonding between fiber and resin. The interfacial shear strength (IFSS) has been increased as much as 40% by modified chemical surface bonding [1—3]. However, it is well known that increasing the interfacial strength of the fiber reinforced polymeric composite material often leads to a reduction in the fracture toughness and vice versa [4—12]. In this study, the effects of mechanical interlocking, in addition to chemical bonding on the strength and energy absorption of glass fiber/epoxy interphase, were studied by creating texture on the fiber surface through the phase separation of silane blends. A series of tetraethoxysilane (TEOS)/3-glycidoxypropyltrimethoxysilane (GPS) blends in solutions of ethanol and water was selected to treat the glass fiber surface. The fiber coated with different surface treatments shows the change in fiber surface morphology due to the addition of TEOS. X-ray photoelectron spectroscopy (XPS) analysis showed that the GPS preferentially migrates to the coating surface which suggests that phase separation induced by the silane blend was the primary mechanism for the texture formation. Atomic force microscopy (AFM) was used to scan the fiber surface after the coating and the fiber surface texture was quantified by the roughness values. In addition, a single-fiber Microdroplet shear test was conducted to assess the interfacial properties between the textured glass surface and an epoxy matrix. Traditionally, interfacial shear strength is the only quantity that was determined from the load vs. displacement curve after microdroplet test. In this study, a new data-reduction scheme was developed to determine the energy absorption due to different failure mechanisms by taking into consideration both machine compliance and fiber stretching in the energy calculation. The results show as much as a three-fold increase in specific sliding energy absorption without sacrificing interfacial shear strength. The examination of failure surfaces shows that failure mode propagates through the textured interphase in a more tortuous path, which results in greater degree of energy absorption during fiber—matrix pullout. This study shows the potential for using chemical bonding and mechanical interlocking effects to improve both strength and energy absorption in fiber reinforced composites.
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