Papers by Keyword: Amorphous SiO₂

Abstract: The NO anneal has been shown to effectively remove 99% of defects in SiC based devices. However, the details of interactions of NO molecules with amorphous (a)-SiO₂ and SiC/SiO₂ interface are still poorly understood. We use DFT simulations to investigate the NO incorporation energies in a-SiO₂, and how these are affected by the steric environment. The results explain the ease with which NO molecules incorporate into a-SiO₂ and give an insight into the diffusion paths they take during annealing. We highlight the importance of exhaustive sampling for exploring NO diffusion pathways.

Abstract: The article is focused on study of the effect of filtering diatomite as a partial replacement of cement in an amount of 5-30%. Diatomite is used in civil engineering especially for its high content of an amorphous SiO₂, chemical resistance, high porosity, and good thermal insulating properties. Diatomite can be used as a pozzolanic material due to the high content of amorphous SiO₂.Cement was replaced by filtering diatomite in amount of 5, 10, 15, 20, 25 and 30% by weight. Diatomite was characterized by chemical composition, granulometry and pozzolanic activity.The specimens were tested in bulk density, bending strength and compressive strength, hydration process was investigated using thermal analysis, and microstructure was observed by scanning electron microscopy. All results were compared to reference sample.

Abstract: This paper presents selected results from a study concerning mechanical fracture parameters related to the formation of the solid structure of cement mortars. The tested mixtures were composed of an invariable quantity of cement (type CEM I 42.5 R) and an increasing amount of amorphous SiO₂ (marked with a P), or a variable quantity of cement replaced by varied quantities of amorphous SiO₂ (marked with an N). It was decided that the consistencies of the fresh mixtures should be identical: to ensure this, the water-cement ratio was increased as necessary. Quantification of the mechanical fracture parameters of the studied composites (fracture toughness, fracture energy, compressive strength, modulus of elasticity) was performed via the evaluation of fracture tests performed on beam specimens with an initial stress concentrator loaded in three-point bending. Subsequently, the results of the experiments in the form of load versus displacement diagrams were processed using the effective crack fracture model and the work of fracture method.

Abstract: Silicon-based anode is one of the most promising anode materials for next generation batteries due to its high theoretical capacity of about 4200 mAh/g. However, the hurdle of using such high capacity anode material is its large volumetric change during lithiation and delithiation causing capacity fading. In this study, in order to circumvent the large volumetric change, nanograined size SiO₂ incorporated with Fe having compositions of Fe_xSi_1-xO₂ (x=0.05, 0.10) have been synthesized using a modified sol-gel processing and fully characterized for its structure, morphology, and thermal properties. From the different characterization results, the samples synthesized at low processing temperatures (400 and 600 °C) suggest an amorphous structure with grain size of about 5 nm and Fe successfully incorporated into the amorphous nanograined SiO₂ matrix.

Abstract: Rice husk is the covering of rice seeds and a by-product of milling rice grain. This study is conducted to investigate the production of silica (SiO₂) formed from waste rice hull (RH) at different processing temperatures and study its structure, morphology, and thermal properties. Thermal analysis by thermogravimetric analysis (TGA) of the dried RH showed two mass-loss steps associated to the moisture desorption and thermal decomposition. Powder X-ray diffraction patterns of the rice hull calcined at 550 ^oC showed a purely amorphous SiO₂ structure while those calcined for 900 ^oC for 1 hour and for 3 hours showed a glass-ceramics and crystalline SiO₂ structure, respectively. This structural result is supported by the results obtained from the FTIR and Raman analyses of the samples. On the other hand, the Scanning electron microscopy (SEM) images showed the morphology of the samples revealing an increasing particle and grain size of the samples calcined at higher temperatures and longer heat treatment duration. In addition, Energy dispersive X-ray (EDX) spectra of both amorphous and crystalline SiO₂ samples confirm that the sample contains mostly silicon and oxygen. Thus, in this study, the desired form of either amorphous or crystalline SiO₂ from waste rice hull can be successfully obtained by controlled heat treatment.

Abstract: Rice husk ash (RHA), a waste product of the rice industry, is rich in SiO2. The large amount of SiO2 freely obtained from it provides an abundant and cheap alternative to metallurgical grade SiO2 for many industrial applications. The detailed analysis of the physical dimensions such as length, width and thickness of four varieties (HMT, Sonam, 1001 and JSR) was done. The FTIR analysis of the ash obtained by burning rice husk at different temperatures proves the formation of nano SiO2. The study of the isothermal thermal degradation of the RHA was done by TGA analysis .During TGA analysis it was observed that, mass trace suddenly goes backwards along the abscissa and then continues forward normally. This characteristic behavior of the TGA plot was attributed to the formation of nano SiO2 during incineration of the RHA. The minimum value of the percentage crystallinity of the SiO2 at 500°C proved the formation of amorphous SiO2.

Abstract: Quartz ceramic has a great potential use in engineering ceramic materials. However, due to the cristobalite transformation in sintering process, the mechanical properties are weakened. The influence of Al3+ additions on bending strength and open porosity of quartz ceramics was determined by adding Al2O3 (sample 1) and aluminum sulfate solution (sample 2). The content of Al3+ ranged from 1% to 7%. Both samples were sintered at 1200°C for 2h. The bulk density of samples was measured by Archimede’s displacement method and the flexural strength of the specimens was tested using a conventional three point flexural method. The addition of 3% Al3+ of aluminum sulfate leads to better properties (bending strength at 14Mpa and open porosity at 23%). Glass theory was applied to explain the mechanisms of inhibiting crystallization of Al3+ additions.

Abstract: Hybrid sol–gel process was used to fabricate the precursor particles of amorphous SiO₂ coating at the composites of SiC_p⁄Cu, and then formed by Vacuum hot-pressed method. Various methods were used to measure the sintered compacts of different SiC volume fraction. The results showed that the densities, bending strength and conductivity were decreased slightly with the increases in the amounts of SiC, and the vickers hardness were increased first and then decreased. The techniques of DTA–TG, SEM–EDS and XRD were used to characterize the phases and the morphologies of the composite particles and composites. It was inferred that the formation of SiO₂–Cu₂O eutectic mixture in this system and the viscous interaction of amorphous SiO₂ improved the interfacial modification of the composites of SiC_p⁄Cu.

Abstract: Normal thermal conductivity of amorphous and crystalline SiO₂ nano-films is calculated by nonequilibrium molecular dynamics (NEMD) simulations in the temperature range from 100 to 700K and thicknesses from 2 to 6nm. The calculated temperature and thickness dependences of thermal conductivity are in good agreement with previous literatures. In the same thickness, higher thermal conductivity is obtained for crystalline SiO₂ nano-films. And more importantly, for amorphous SiO₂ nano-films, thickness can be any direction of x, y, z-axis without effect on the normal thermal conductivity, for crystalline SiO₂ nano-films, the different thickness directions obtain different thermal conductivity results. The different results of amorphous and crystalline SiO₂ nano-films simply show that film thickness and grain morphology will cause different effects on thermal conductivity.

Abstract: The phosphoric acid-based geopolymers were synthesized using chemosynthetic Al2O3-2SiO2 and phosphoric acid at ambient temperature, curing for 28d at room temperature. The influence of the water from the geopolymer materials on the dielectric properties was investigated. The phase’s transformation was measured by XRD. The amorphous SiO2 from unreacted particles transformed to quartz crystal at 950 °C when exposure to 1350 °C, some mulite appeared, however, the dielectric properties change a little. After the geopolymer materials were heated at 350 °C, its dielectric loss could depress down to lower level of 10-3.