Papers by Keyword: Polysiloxane

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Authors: G.N. Arjun, T.L. Lincy, T.S. Sajitha, S. Bhuvaneshwari, Thomas Deepthi, Deepa Devapal
Abstract: Polysiloxane resin copolymer was synthesized through acid catalyzed hydrolysis of methyl triethoxysilane (MTEOS) and diethoxytetramethyldisiloxane (DEOTMDS). The effect of reaction time on the properties of the polymer was studied and this copolymer was characterized by GPC, 29Si NMR, IR, TGA, viscosity, refractive index, specific gravity and solid content. 29Si NMR and IR showed characteristic signals of Si-O-Si linkage which confirmed the formation of the polymer. GPC and solid content analysis showed an increasing trend in molecular weight with reaction time. Thermo gravimetric analysis showed that the polymer was thermally stable upto ≈ 260°C and all the polymers gave a ceramic residue in the range of 77-80% at 900°C. Siloxane prepared inhouse and methyl phenyl silsequioxane (control) were used as coating materials and atomic oxygen (AO) resistance was evaluated on Al-Kapton, carbon polyimide composite and glass polyimide composite. The mass loss and surface morphology of the coated samples were measured at different time intervals. It is observed that mass loss of polysiloxane coated samples was very less, compared to coated control samples. The morphology of all the samples were studied using FESEM. Erosion kinetics and surface morphology investigation indicate that the polysiloxane coating possesses excellent AO resistance, and displays better cracking resistance on AO exposure.
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Authors: Marco Antônio Schiavon, I.V.P. Yoshida, A.C. Silva, Wilson Acchar
Abstract: Ceramic matrix composites (CMC) were prepared by the active-filler-controlled polymer pyrolysis process (AFCOP) using a polysilsesquioxane resin filled with metallic niobium and alumina powders. Samples containing 60 wt% of polysilsesquioxane and 40 wt% of metallic niobium and alumina powders mixtures were homogenized, uniaxially pressed and pyrolysed in an alumina tube furnace up to 1400 °C, under argon flow. The ceramic products were characterized by X-ray diffraction (XRD), thermogravimetry (TGA), differential thermal analysis (DTA), Fourier transform infrared (FTIR) and energy-dispersive (EDS) spectroscopies. XRD analysis of the products showed the presence of crystalline phases such as NbC, Nb3Si, Nb5Si3, SiC, crystoballite and mullite. Thermogravimetry data of the composites presented low weight losses at 1000 °C. DTA curves showed an endothermic peak at 1350 °C, which was associated to the beginning of carbothermic reduction and/or the formation of silicon oxide and carbide. In addition, an exothermic peak at 1400 °C was associated to the formation of the mullite phase.
369
Authors: Marco Antônio Schiavon, I.V.P. Yoshida, José Carlos Bressiani, Wilson Acchar
Abstract: In this work, ceramic matrix composites (CMC) were prepared by AFCOP process, using a polysiloxane network filled with metallic niobium and aluminum powders as active fillers. The liquid polysiloxane precursor was loaded with a suitable polymer/filler ratio in relation to stoichiometric Nb : C and Al : O molar ratios. Changing Al for a-Al2O3, which acted as an inert filler, non-stoichiometric conditions were obtained. The mixtures were blended, uniaxially warm pressed, and pyrolysed in flowing argon at 800, 1000 and 1200 °C. Thermogravimetry was used to follow the weight changes during the pyrolysis process. X-ray diffraction was used to identify the formation of new crystalline phases, such as Al2O3, NbC, Nb2C and Al3Nb in the composites. Sintered specimens were also characterized by SEM and EDS. The results indicated good potential for this system to obtain multiphasic composite material in the Al-Nb system at lower temperatures.
375
Authors: Ana Lúcia Exner Godoy, José Carlos Bressiani, Ana Helena Almeida Bressiani
Abstract: In this work alumina based composites ceramics with a polymeric precursor (polysiloxane) and metallic Ti with two different size particle distribution, Ti without milling (ATM) and 5h milling (ATM5), were characterized. PMS thermogravimetric curve indicates that the weight loss percentage was about 20%. The densities achieved were 3,50g/cm3 and 3,70g/cm3 for samples prepared with Ti as received and 5h milling, respectively. During decomposition of the polymer during pyrolysis and sintering, the particles of the matrix may react with carbon and Si from the polymer precursor, metallic Ti and gas atmosphere, nitrogen, to form different phases. The ATM samples presented Al2O3 and TiCN. The specimens prepared with 5h milling Ti (ATM5) is constituted by mullite besides Al2O3 and TiCN. The ATM5 samples have smaller densities than ATM but there is no significant difference in the hardness values.
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Authors: R.M. Rocha, Peter Greil, José Carlos Bressiani, Ana Helena A. Bressiani
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Authors: Li Qun Duan, Chen Chen Zhang, Qing Song Ma, Zhao Hui Chen
Abstract: Nanoporous carbonaceous materials derived from polysiloxane were first prepared by pyrolysis at 1300°C followed with hydrofluoric acid (HF) etching treatment. Their thermal stability of pore structure in inert condition was investigated in this paper by nitrogen adsorption technique in detail. The specific surface area (SSA) and pore volume (total pore volume, micropore volume, mesopore volume) decreased continually in the heat-treatment temperature range of 1000~1400°C. The average pore size almost kept the same with the raw sample. However, when the temperature exceeded 1400°C, the micropore interconnection began transforming to mesopore structure, which led to the decline of SSA and the increase of average pore size. Furthermore, the pore size distributions (PSDs) curves showed that heat-treatment had an advantage on the transition process of pore structure from disorder to regularity to some extent when heat-treated in the range 1000~1400°C for the most possible reason of relief of residue strain in the carbonaceous materials.
279
Authors: Johann Brandstetter, Petr Glogar, Dieter Loidl, Karl Kromp
Abstract: Unidirectionally reinforced ceramic matrix composites (CMC) with matrix derived by pyrolysis of polymethylsiloxane or polymethylphenylsiloxane resins and reinforced with Nicalon or Nextel fibres were investigated. In order to assess and mutually compare their mechanical properties and stability when exposed to hot air up to 1200°C their elasticity moduli were measured prior to and after this exposure. The oxidation - induced damage of the material was monitored also in terms of microstructural features. Moreover, an attempt was undertaken to assess the bonding level by monitoring the push - through of macroscopic bundles of fibres using a flat indentor.
340
Authors: Yue Zhi Cui, Xiao Liang Ren, Xue Ming Dong, Yong Tao Wang
Abstract: A functional polysiloxane monomer, with a double bond and a fluorophore pyrene at each end of the polysiloxane chain, respectively, was synthesized. Copolymerize this functional monomer with other acrylates in emulsion, we got a comb-like polymer in which pyrene fluorophore is at the end of each polysiloxane side-chain. This polymer shows great advantage in sensing performance for 2,4-dinitrotoluene (DNT) over its analog that was prepared just by mixing pyrene with pure polyacrate emulsion.
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Authors: Ana Lúcia Exner Godoy, José Carlos Bressiani, Ana Helena de Almeida Bressiani
Abstract: The effects of adding polymer precursor on the sintering behavior, microstructure and hardness of alumina ceramics have been studied. Polymer sintering aid polymethylhidrogensiloxane were used for alumina based ceramics sintered at 1650º C. The sintered materials were characterized using: (a) helium picnometry to determine apparent density; (b) x-ray diffraction analysis, scanning electron microscopy, transmission electron microscopy and energy dispersive x-ray spectrometry to determine the microstructure and composition; (c) Vickers indentation analysis to determine the hardness and fracture toughness. The addition of polymethylhidrogensiloxane yielded alumina/mullite composites. The preparation of ceramic composites using small amounts of polymer precursors is suited for alumina based ceramics. The processing route being simple, it has the potential to enable the manufacture of complex shaped pieces.
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