Synthesis of a Novel Polyethoxysilsesquiazane and Thermal Conversion into Ternary Silicon Oxynitride Ceramics with Enhanced Thermal Stability

Abstract

A novel polyethoxysilsesquiazane ([EtOSi(NH)1.5]n, EtOSZ) was synthesized by ammonolysis at −78 °C of ethoxytrichlorosilane (EtOSiCl3), which was isolated by distillation as a reaction product of SiCl4 and EtOH. Attenuated total reflection-infra red (ATR-IR), 13C-, and 29Si-nuclear magnetic resonance (NMR) spectroscopic analyses of the ammonolysis product resulted in the detection of Si–NH–Si linkage and EtO group. The simultaneous thermogravimetric and mass spectrometry analyses of the EtOSZ under helium revealed cleavage of oxygen-carbon bond of the EtO group to evolve ethylene as a main gaseous species formed in-situ, which lead to the formation at 800 °C of quaternary amorphous Si–C–N with an extremely low carbon content (1.1 wt %) when compared to the theoretical EtOSZ (25.1 wt %). Subsequent heat treatment up to 1400 °C in N2 lead to the formation of X-ray amorphous ternary Si–O–N. Further heating to 1600 °C in N2 promoted crystallization and phase partitioning to afford Si2N2O nanocrystallites identified by the XRD and TEM analyses. The thermal stability up to 1400 °C of the amorphous state achieved for the ternary Si-O-N was further studied by chemical composition analysis, as well as X-ray photoelectron spectroscopy (XPS) and 29Si-NMR spectroscopic analyses, and the results were discussed aiming to develop a novel polymeric precursor for ternary amorphous Si–O–N ceramics with an enhanced thermal stability.

Microporosity and CO2 Capture Properties of Amorphous Silicon Oxynitride Derived from Novel Polyalkoxysilsesquiazanes

“…As previously reported [24], the gaseous species, m / z ratios at 45 and 16 detected for EtOSZ (Figure 6a) were assigned to SiNH 3 + and NH 2 + , respectively. These fragment ions could be due to the partial decomposition of the silsesquiazane linkage.…”

“…Figure 15 shows the 29 Si-NMR spectrum of 800 °C-pyrolyzed EtOSZ measured in our previous study [24]. One broad peak at around −100 ppm was deconvoluted to three peaks centered at −110, −100, and −90 ppm that were assigned to SiO 4 , HO–SiO 3 (Q3), and SiO 3 N, respectively.…”

“…The weak signals at −53.6 and −61.4 ppm were thought to be attributed to the small amount of by-products (below 5%) that could not be removed by the distillation after the alkoxylation of SiCl 4 (Equation (1)). The signals at −53.6 and −61.4 ppm were assigned to (EtO) 2 –Si–(NH) 2 (linear or cyclic) and (EtO) 3 –Si–NH, respectively [24]. …”

“…The purity of gaseous ammonia used in this study was >99.9% (Sumitomo Seika Chemicals, Osaka, Japan). Eight kinds of [ROSi(NH) 1.5 ] n analogues were synthesized by varying alcohol species through the common synthetic route (Figure 1) in the same manner and under the same conditions as previously reported for the synthesis of EtOSZ [24]. After the first reaction, the ROSiCl 3 was isolated by distillation.…”

“…Recently, we reported a novel single source precursor for a silicon oxynitride (Si–O–N) system, ethoxysilsesquiazane ([EtOSi(NH) 1.5 ] n , EtOSZ) [24]. Under inert atmosphere up to 800 °C, this polymer exhibited a unique thermal decomposition behavior: a cleavage of the oxygen–carbon bond of the EtO group to evolve ethylene as a main gaseous species formed in-situ, which led to the formation of amorphous Si–O–C–N with an extremely low carbon content (1.1 wt %) compared to the theoretical EtOSZ (25.1 wt %).…”