Showing papers on "Thermogravimetric analysis published in 1994"

Oxidation Mechanisms and Kinetics of 1D-SiC/C/SiC Composite Materials: I, An Experimental Approach

Abstract: The oxidation of unidirectional SiC/C/SiC model composites has been investigated through thermogravimetric analysis, optical/electron microscopy, and electrical measurements. The influence of temperature and carbon interphase thickness on the oxidation of the composites is discussed. The oxidation involves three phenomena: (i) reaction of oxygen with the carbon interphase resulting in pores around the fibers, (ii) diffusion of oxygen and carbon oxides along the pores, and (iii) reaction of oxygen with the pore walls leading to the growth of silica layers on both the fibers and matrix. In composites with a thin carbon interphase (e.g., 0.1 μm) treated at T > 1000°C the pores are rapidly scaled by silica. Under such conditions, the oxidation damages are limited to the vicinity of the external surface and the materials exhibit a self-healing character. Conversely, long exposures (300 h) at 900°C give rise to the formation of microcracks in the matrix related to mechanical stresses arising from the in situ SiC/SiO2 conversion, fly, the self-healing character is not observed in composites with a thick interphase (e.g., 1 μm) since carbon is totally consumed before silica can seal the pores.

Kinetics and Mechanisms of Oxidation of 2D Woven C/SiC Composites: I, Experimental Approach

Abstract: The oxidation behavior of a 2D woven C/SiC composite partly protected with a SiC seal coating and heat-treated (stabilized) at 1,600 C in inert gas has been investigated through an experimental approach based on thermogravimetric analyses and optical/electron microscopy. Results of the tests, performed under flowing oxygen, have shown that the oxidation behavior of the composite material in terms of oxidation kinetics and morphological evolutions is related to the presence of thermal microcracks in the seal coating as well as in the matrix. Three different temperature domains exist. At low temperatures ( 1,100 C), such diffusion mechanisms are limited by sealing of the microcracks by silica; therefore, the degradation of the composite remains superficial. The study of the oxidation behavior of (i) the heat-treated composite in a lower oxygen content environment (dry air) and (ii) themore » as-processed (unstabilized) composite in dry oxygen confirms the different mechanisms proposed to explain the oxidation behavior of the composite material.« less

The thermal stability of the polymorphs of hexanitrohexaazaisowurtzitane, Part I

Abstract: Phase transitions in the α-, β-, γ-, and ϵ-polymorphs of 2,4,6,8,10,12-hexanitrohexaazaisowurtzitane (HNIW) have been studied as a function of temperature. Described are the results of high temperature equilibrium solvation studies coupled with Fourier transform infrared spectroscopy (FTIR) for the identification of polymorphic conversion. These results are augmented by data in Part II from differential scanning calorimetry (DSC), differential thermal analysis/thermogravimetric analysis (DTA/TGA), and optical hot stage microscopy(6). The thermodynamic stability order of ϵ > γ > α-hydrate > β is shown, with the epsilon polymorph the most thermodynamically stable phase of HNTW at room temperature. The existence of multiple α-hydrate phases is shown to complicate the determination of the equilibrium P-T phase diagram of HNIW.

Structural Characterization of Sol-Gel Derived Oxycarbide Glasses. 1. Study of the Pyrolysis Process

Abstract: A gel resulting from the cohydrolysis of dimethyldiethoxysilane, a silane precursor, and tetraethoxysilane, a silica precursor, has been pyrolyzed to a silicon oxycarbide network. This conversion has been carefully characterized using various structural techniques: thermogravimetric analysis coupled with mass spectrometry, multinuclear solid-state magic angle spinning nuclear magnetic resonance ( 29 Si, 1 H, and 13 C), infrared spectroscopy, and electronic spin resonance. The pyrolysis process can be divided in two main stages. In the 400-650 °C temperature range, redistribution reactions between Si-O and Si-C bonds occur with evolution of low molecular weight siloxanes. New Si sites are created, but the methyl groups remain intact. From 650 to 1000 °C, the conversion of the polymeric network into an inorganic material takes place. Cleavage of C-H, Si-C, but also Si-O bonds occurs with evolution of methane, hydrogen and water. Aromatic carbon units are formed. Heat treatment at 1000 °C in argon flow leads to the formation of an amorphous silicon oxycarbide phase, characterized by SiC x O 4-x units. The presence of a free carbon phase has also been shown.

Decomposition Behavior of Polyurethanes via Mathematical Simulation

Abstract: A series of polyurethane elastomers with different hard-segment contents of 24% (M24), 34% (M34), 50% (M50), and 100% (M100) have been investigated through thermogravimetric analysis (TGA) measurement and mathematical simulation. The fourth-order Runge–Kutta method was used to solve the differential equations, and the solutions were obtained by running a written computer program. The activation energy and the frequency factor of polyurethane decomposition can be obtained by only one nonisothermal measurement of TGA. The same values of the activation energy and the frequency factor were obtained using the different constant heating rates under air for the elastomer with a hard-segment content of 34%. The activation energy decreases as the hard-segment content of polyurethanes increases. The polyurethane materials can be used at room temperature for several millennia failure through lifetime calculation. Comparison of the values of t1/2, the heat stability of polyurethanes decreases in the following order: M24 > M34 > M50 > M100; this phenomenon can be explained due to the heat instability of the urethane group. The highest concentration of the urethane group in the M100 structure results in the worst heat stability. © 1994 John Wiley & Sons, Inc.

Thermally induced phase transformations of 12-tungstophosphoric acid 29-hydrate : synthesis and characterization of PW8O26-type bronzes

Abstract: The phase transformations of 12-tungstophosphoric H3PW12O40-29H2O (29-WPA) acid in the temperature range from ambient temperature to 1150°C were investigated and characterized by differential thermal analysis (DTA), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), X-ray powder diffraction (XRPD), scanning electron microscopy (SEM), infrared (ir) and Raman spectroscopies From room temperature to 550°C, 29-WPA passes through a dehydration process, which characterizes the formation of different crystallohydrates, in anhydrous form as well as “denuded” Keggin's anions, the D-phase (PW12O38) During these processes, Keggin's anions are not disturbed too much and they are preserved up to about 550°C The “D” phase is transformed by solid-solid recrystallization at about 600°C in a new monophosphate bronze type compound PW8O26 Unit cell dimensions were calculated from XRPD data (a o=07515 nm) With the temperature increasing up to 1150°C, novel synthesized cubic bronze passed through three polymorphous phase transitions According to a general formula for monophosphate tungsten bronzes (WO3)2m (PO4)4 all four polymorphs have m=16

Ion-exchange properties of lithium aluminium layered double hydroxides

Abstract: The synthesis of layered lithium aluminium hydrotalcite-like materials is described along with different anion exchange procedures for the preparation of materials intercalated with chloride, nitrate and vanadate. The products have been characterised using elemental chemical analysis, powder X-ray diffraction, Fourier-transform infrared spectroscopy and thermogravimetric analysis. The matrices are found to be reasonably stable to acid treatment at pH 4.5 for periods of up to 72 h, with anion exchange taking place. Results indicate that total exchange of interlayer carbonate for chloride, nitrate and vanadate may be accomplished. The thermal properties of the materials have been studied: they demonstrate interesting differences in thermal behaviour compared with hydrotalcite.

Conversion mechanism of perhydropolysilazane into silicon nitride-based ceramics

Abstract: The pyrolysis of perhydropolysilazane in anhydrous ammonia has been studied up to 1000 °C through the analysis of the gas phase and the characterization of the solid residue by thermogravimetric analysis, Fourier transform-infrared analysis. X-ray photoelectron spectroscopy, X-ray diffraction and 29Si cross-polarization magic angle spinning-nuclear magnetic resonance. The pyrolysis mechanism involves three main steps: (1) below 400 °C, evaporation of residual solvent; (2) from 400–600 °C, reaction with ammonia leading to an increase of nitrogen content and formation of preceramic polymer-ceramics intermediate solid with a three-dimensional network; (3) from 600–1000 °C, completion of the formation of an amorphous hydrogenated solid with composition close to silicon nitride.

Thermally reversible polymeric sorbents for acid gases: CO2, SO2, and NOx

Abstract: Weakly acidic gases such as CO2, SO2, NO2, and NO discharged directly into the atmosphere have been suggested to contribute to the formation of acid rain and to global climate changes. In an effort to develop an effective process by which to remove these pollutants from gas streams, we synthesized polymeric sorbents that bind acid gases. In an earlier work, we reported that polymers carrying pendant amine groups are capable of absorbing carbon dioxide to form thermally reversible amine/CO2 addition products. In this study, we investigated the effects of amine structure on acid gas sorption and the thermal reversibility of the sorption–desorption reactions of various acid gases including CO2, SO2, NO, and NO2 with both linear and cross-linked polymers using thermogravimetric analysis. © 1994 John Wiley & Sons, Inc.

Development of radiation‐grafted FEP‐g‐polystyrene membranes: Some property–structure correlations

Abstract: Property–structure correlation in proton exchange membranes, prepared by simultaneous radiation grafting of styrene into FEP films and their subsequent sulfonation, was evaluated. The distribution of ionic sites across the membrane matrix was determined by microprobe measurements. The properties of these membranes, such as ion exchange capacity, swelling and ionic resistivity as a function of the degree of grafting, were studied. The thermal stability of membranes was studied using thermogravimetric analysis and ion exchange capacity measurements. Membranes undergo considerable structural changes in terms of the increase in the ionic content, enhanced hydrophilicity and decrease in crystallinity with the increasing degree of grafting. A correlation between some physical properties and stuctural changes occurring during the membrane preparation was established.

Structural and optical characterization of BaTiO3 thin films prepared by metal-organic deposition from barium 2-ethylhexanoate and titanium dimethoxy dineodecanoate

Abstract: Single phase BaTi03 thin films were prepared by metal-organic deposition (MOD) using barium 2-ethylhexanoate and titanium dimethoxy dineodecanoate as the metal-organic precursors. A series of experiments was conducted on the metal-organic spin-coated films and their correspondingly annealed samples by employing experimental techniques ranging from thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), x-ray diffraction (XRD), and scanning electron microscopy (SEM), to various optical property characterization methods. A better understanding has been achieved regarding the metal-organic decomposition mechanism, the solid-state BaTi03 film formation and crystallization process, as well as the relationship between the structure and the optical properties of the prepared films. The conclusions of our experiments are as follows: Upon annealing the barium 2-ethylhexanoate spin-coated films, barium carbonate (BaC03) is formed at a relatively low temperature of 300 °C, and at an annealing temperature around 700 °C forms the barium peroxymonocarbonate (BaCO4). Upon annealing the titanium dimethoxy dineodecanoate spin-coated films, anatase is first formed at a low annealing temperature about 400 °C and transforms to rutile phase around an annealing temperature of 800 °C. Upon annealing the spin-coated films from the equimolar mixture of barium 2-ethylhexanoate and titanium dimethoxy dineodecanoate formulations, BaTi03 is formed around an annealing temperature of 600 °C via solid-state reaction between BaCO3 and TiO2 (anatase). The structure of MOD prepared BaTiO3 films has several specific features: instead of the columnar structure in physical vapor deposited (PVD) films, the crystal grains in granular shape are characteristic of MOD films, and the crystallites are much larger near the surface of the film than near the substrates. Optical properties of the prepared BaTi03 films have been reported. Optical characterization shows that the scattering losses contribute dominantly to the total optical losses. The relationship between the structure and the optical properties of the prepared films has also been discussed.

Effect of copper oxides on the thermal oxidative degradation of the epoxy resin

Abstract: The effect of copper oxides on the thermal oxidative degradation of a brominated epoxy resin–dicyandiamide system was studied using Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA). The addition of small amounts of Cu2O or CuO fillers to the epoxy resins affected the relative amounts of highly reacted cyclic species formed during thermal aging and induced catalytic degradation of the epoxy resins. The overall and initial activation energies of the degradation process were found to decrease, and the order in the degradation kinetics of the epoxy resin changed from a near zero order to negative domain (autocatalytic nature) in the presence of copper oxides. © 1994 John Wiley & Sons, Inc.

Grinding effect on kaolinite-pyrophyllite-illite natural mixtures and its influence on mullite formation

Abstract: The effect of dry grinding (mechanochemical activation) on a natural mixture of aluminium silicates composed of kaolinite, pyrophyllite and potassium mica (illite) was studied. The evolution of the ground material was followed by X-ray diffraction, differential thermal analysis, thermogravimetric analysis, scanning electron microscopy and nitrogen adsorption methods. Grinding produces a strong structural alteration of the silicates that constitute the mixture, increasing the surface area and decreasing particle size, with progressive amorphization and formation of agglomerates as detected by X-ray diffraction and scanning electron microscopy. Marked changes were observed in the thermogravimetric analysis and differential thermal analysis curves as influenced by grinding, with weight losses at lower temperatures and the appearance of an exothermic differential thermal analysis effect at 985‡C, which increases in intensity and sharpness as grinding time proceeds. After grinding using the pure silicates, kaolinite, pyrophyllite and illite, it was found that the increase in intensity and sharpness of this exothermic effect was only associated with the presence of pyrophyllite in the natural mixture irrespective of the presence of kaolinite and/or illite, at least up to 325 min grinding. This effect is produced in the ground mixture by mullite formation mainly from pyrophyllite by combined mechanical (by grinding) and thermal treatments.

Thermogravimetric analysis of poly(alkyl methacrylates) and poly(methylmethacrylate‐g‐dimethyl siloxane) graft copolymers

Abstract: The thermal stabilities of various poly(alkyl methacrylate) homopolymers and poly(methyl methacrylate-g-dimethyl siloxane) (PMMA-g-PSX) graft copolymers have been determined by TGA As expected, the thermal stabilities of poly(alkyl methacrylates) were a function of the ester alkyl group, and polymerization mechanism In particular, thermally labile linkages, which result from termination during free radical or nonliving polymerization mechanisms, decrease the ultimate thermal stabilities of the polymers However, graft copolymers, which were prepared by the macromonomer technique with free radical initiators, exhibited enhanced thermal stability compared to homopolymer controls

Gases emitted during thermal decomposition of a polypropylene film and a polyurethane adhesive

Abstract: The thermal decomposition of a polypropylene film, a polyurethane adhesive, as well as a polypropylene coated with a polyurethane adhesive have been studied by a thermogravimetric (TG) analysis, at a heating rate of 5°C a minute with air flow. During the thermal analysis different decomposition steps which correspond to different weight loss rates were obtained on thermal curves. Gases were collected between the different decomposition steps and then analyzed. The results reported deal with the conversion of carbon into oxides and aldehydes—ketones, and also with that of nitrogen into hydrogen cyanide and isocyanates. The findings lead to a better understanding of the hazards and risks of toxic emissions that may be generated by the gas formation following heating of these materials.