Showing papers on "Thermogravimetric analysis published in 1991"

Synthesis and characterization of polymers produced by horseradish peroxidase in dioxane

Abstract: Polymers were synthesized from substituted phenolic and aromatic amine compounds with hydrogen peroxide as the source of an oxidizing agent and horseradish peroxidase enzyme as the catalyst. The polymerization reaction was carried out in a monophasic organic solvent with small amounts of water at room temperature. Conditions for the synthesis of polymers with respect to reaction time and yield were studied with a number of monomers at different concentrations and in solvents with different buffers with pH range of 5.0–7.5. Physical and chemical properties of these homo-and copolymers were determined with respect to melting point, solubility, elemental analysis, molecular weight distribution, infrared absorption (including FTIR), solid-state 13C nuclear magnetic resonance, thermal gravimetric analysis, and differential scanning calorimetry. The enzyme catalyzed reactions produced polymers of molecular weight greater than 400,000 which were further fractionated by differential solubility in solvent mixtures and the molecular weight distribution of the polymer fractions were determined. In general, the polymers synthesized have low solubilities, high melting points, and some degree of branching.

Conversion mechanisms of a polycarbosilane precursor into an SiC-based ceramic material

Abstract: The pyrolysis of a PCS precursor has been studied up to 1600 °C through the analysis of the gas phase and the characterization of the solid residue by thermogravimetric analysis, extended X-ray absorption fine structure, electron spectrocopy for chemical analysis, transmission electron microscopy, X-ray diffraction, Raman and Auger electron spectroscopy microanalyses, as well as electrical conductivity measurements. The pyrolysis mechanism involves three main steps: (1) an organometallic mineral transition (550 < T p < 800 °C) leading to an amorphous hydrogenated solid built on tetrahedral SiC, Si02 and silicon oxycarbide entities, (2) a nucleation of SiC (1000 < T p < 1200 °C) resulting in SiC nuclei (less than 3 nm in size) surrounded with aromatic carbon layers, and (3) a SiC grain-size coarsening (T p > 1400 °C) consuming the residual amorphous phases and giving rise simultaneously to a probable evolution of SiO and CO. The formation of free carbon results in a sharp insulator-quasimetal transition with a percolation effect.

KINETIC ANALYSIS OF THERMOGRAVIMETRIC DATA XXVIII Thermal decomposition of some metal and ammonium salts of hexabromoplatinic acid

Abstract: The potassium, ammonium and thirteen amine salts of hexabromoplatinie acid were obtained and characterized by chemical analysis. The thermal decompositions of these complex salts were studied by derivatograph and differential scanning calorimetry. The natures of the pyrolysis processes are discussed and compared with those obtained for the analogous ehlorocomplexes. From the TG curves, kinetic parameters were derived for different stages of the thermal decomposition.

Problems in DSA® coating deposition by thermal decomposition

Abstract: Several DSA® coatings were prepared by thermal decomposition of metallic chloride salts on titanium base. It was shown that the painting composition does not necessarily correspond to the final coating composition. Thermogravimetric and fluorescent X-ray measurements were used to identify and characterize the material losses. It was found that IrO2, Ta2O5 and RuO2 compounds can be deposited with almost 100% yield, while SnO2 and Pt precursors give high material losses. The influence of parameters affecting the deposition yield are discussed.

Hydration/dehydration characteristics of struvite and dittmarite pertaining to magnesium ammonium phosphate cement systems

Abstract: Struvite, an important reaction product in magnesium ammonium phosphate cement systems, was synthesized in the laboratory. The elevated-temperature dehydration and then roomtemperature hydration characteristics of the dehydrated products were studied by thermogravimetric analyses and X-ray diffraction techniques. From isothermal experiments, struvite is found to be thermally unstable in air at temperatures above 50 ‡C. Struvite can lose part or all of its ammonia and water molecules depending on the time and temperature of heat treatment, ultimately forming magnesium hydrogen phosphate. This decomposed product is X-ray amorphous and upon room-temperature rehydration can form struvite, unknown hydrates or newberyite, alone or in combination with each other, depending on the amount of ammonia left in the structure. However, when struvite is heated in excess water, it only loses its water of crystallization to form the monohydrate, dittmarite. Dittmarite is thermally more stable than struvite and like struvite also forms magnesium hydrogen phosphate on decomposition. At room temperature and in the presence of excess water, dittmarite can slowly transform to the hexahydrate, struvite. The consequence of structural similarities between struvite and dittmarite and conditions under which they may be present in cured cements are described.

Conducting polymer composites

Abstract: Electrically conducting polypyrrole coatings on inorganic cores were obtained by deposition of pyrrole on catalytically active particles. The finely dispersed core materials were hematite (polyhedral and spindle-type), silica coated with hematite, and cerium(IV) oxide. The degree of polymer coverage could be controlled by varying the aging time and the properties of the carrier particles. The so prepared coated powders were examined by transmission electron microscopy and further characterized by elemental and thermogravimetric analyses, as well as by electrophoresis, X-ray diffraction, and conductivity measurements. The conductivities were comparable to those reported for pure polypyrrole and they increased with pressure.

Characterization and long-range reactivity of zinc ferrite in high-temperature desulfurization processes

Abstract: The chemical reactivity of zinc ferrire was studied experimentally to demonstrate the potential use of zinc ferrite as a sorbent in high-temperature desulfurization of coal gases. Fifty cycles of H 2 S absorption from simulated coal gas and regeneration under 4.5% oxygen were conducted in a laboratory-scale, packed-bed reactor system simulating gas compositions of a fixed-bed, air-blown gasifier and a regeneration scheme typical of a moving-bed process. Approximately 70% of the theoretical fractional conversion, as determined by thermogravimetric analysis (TGA), was maintained by the sorbent. Less than 1% residual total sulfur and total carbon were measured in the sorbent. Undesired solid phases, (e.g., metal carbides, sulfates, and elemental iron) were absent in the samples as determined by powder X-ray diffraction

Thermogravimetric study of the desorption of cyclohexylamine and pyridine from an acid-treated wyoming bentonite

Abstract: Four 15 g samples of an unsedimented Wyoming'bentonite were treated with 200 cm 3 0(0.025, 0-050, 0-100 and 0.250 mol dm -3 H2SO 4 for 1 h at room temperature (samples I-IV, respectively). Three further 15 g samples were treated with 200 cm 3 of 50% (v/v) H2SO 4 for 1 h at 20~ (sample V), and 1 and 2 h under reflux (samples VI and VII, respectively). X-ray fluorescence and diffraction studies revealed that only samples VI and VII suffered any substantial structural attack. The resulting acidity of the clays, determined by cyclohexylamine desorption, indicated that sample V contained the largest number of protons at 0-59 mmo( H + (g clay)- 1. Sample V was also the most efficient catalyst for the dehydration and etherification of hexan-l-ol, giving a combined product yield of 17-0% after 2 h reflux in neat reactant. The parent bentonite and samples I and 11 showed no discernible catalytic activity despite measured acidities of 0-1, 0-24 and 0.34 mmol H + (g clay)-k In contrast samples III and IV gave combined product yields of 4-5 and 11-0%, respectively, which correlated well with the measured acidities of 0.38 and 0.48 mmol H § (g clay) -1. Samples VI and VII, prepared by reflux in acid, contained 0.3 and 0-1 mmol H § (g clay) -1, respectively, and gave combined product yields of 13.0 and 6.0%.

Kinetics of thermal decomposition of metal acetates

Abstract: The acetates of magnesium, nickel, copper, manganese, sodium and barium were subjected to thermal decomposition by means of thermogravimetric techniques (TG) under a constant flow of nitrogen. The decompositions occurred in steps and the kinetics of every set of reactions was determined by the Coats and Redfern method. These results were analysed to establish the decomposition kinetics and hence to calculate activation energies. The activation energies were also determined by applying the Horowitz-Hugh method, which yielded similar results.

Synthesis and characterization of a new copper(II) phthalocyaninate substituted with four 15-membered tetraazamacrocycles and its water-soluble pentanuclear complexes

Abstract: The synthesis of a new copper(II) phthalocyaninate substituted with four 15-membered tetraazamacrocycles from 15,16-dibromo-2,5,9,12-tetra(toluene-p-sulphonyl)-2,3,4,5,6,7,8,9,10,11,12,13-dodecahydro-1H-2,5,9,12-benzotetraazacyclopentadecine is described. Quaternization of the aza groups leads to water-soluble products. The tetraaza macrocyclic substituents bind transition-metal ions (CoII, NiII, CuII and ZnII) to give pentanuclear complexes whose electronic spectra indicate the presence of monomeric units in solution. The thermal stabilities of the compounds were determined by thermogravimetric analysis.

Thermal decomposition of cobalt(II) acetate tetrahydrate studied with time-resolved neutron diffraction and thermogravimetric analysis

Abstract: The thermal decomposition of cobalt acetate tetrahydrate has been studied using time-resolved powder neutron diffraction. By using selectively deuterated samples, the loss of water or the breakdown of the acetate group can be identified by following the decrease in the incoherent background of the diffraction pattern as the hydrogen atoms are lost. The results suggest that by 150 °C dehydration is complete and a glass-like phase is formed. Crystallization of this anhydrous acetate occurs at 200 °C. Further heating initiates a two-stage decomposition of the anhydrous acetate terminated by the formation between 275–310 °C of a tetrahedrally co-ordinated cubic zinc blende form of CoO. This transforms at 310 °C to a rock-salt structure. The neutron diffraction data have been complemented by thermogravimetric and chemical analyses from which we have been able to propose some possible intermediate decomposition products and suggest an explanation for the formation of the unusual zinc blende form of CoO.

Glycidyl methacrylate and N‐vinyl‐2‐pyrrolidone copolymers: Synthesis, characterization, and reactivity ratios

Abstract: Free radical copolymerization of glycidyl methacrylate (GMA) and N-vinyl-2-pyrrolidone (VPD) was carried out using 2,2′-azobisisobutyronitrile (AIBN), in chloroform at 60°C. The polymers were characterized by IR and 13C-NMR spectroscopic methods. The percentage composition of the copolymers were established by microelemental analysis and by 13C-NMR spectroscopy. The copolymerization reactivity ratios were computed using the Fineman-Ross method, the Kelen-Tudos method, and the extended Kelen-Tudos method. The molecular weights of the copolymers were obtained by GPC measurements. Thermal properties of the homo- and copolymers were also studied by thermogravimetric analysis. The copolymers were tested as adhesives for leather-to-leather and leather-torubber bonding.

A thermogravimetric study of the oxidative growth of Al2O3/Al alloy composites

Abstract: The oxidation of liquid Al–Mg–Si alloys at 900–1400 °C was studied by thermogravimetric analysis (TGA). The development of a semi-protective surface layer of MgO/MgAl2O4 allows the continuous formation of an Al2O3-matrix composite containing an interpenetrating network of metal microchannels at 1000–1350 °C. An initial incubation period precedes bulk oxidation, wherein Al2O3 grows from a near-surface alloy layer by reaction of oxygen supplied by the dissolution of the surface oxides and Al supplied from a bulk alloy reservoir through the microchannel network. The typical oxidation rate during bulk growth displays an initial acceleration followed by a parabolic deceleration in a regime apparently limited by Al transport to the near-surface layer. Both regimes may be influenced by the Si content in this layer, which rises due to preferential Al and Mg oxidation. The growth rates increase with temperature to a maximum at ∼1300 °C, with a nominal activation energy of 270 kJ/mole for an Al−2.85 wt. % Mg−5.4 wt. % Si alloy in O2 at furnace temperatures of 1000–1300 °C. An oscillatory rate regime observed at 1000–1075 °C resulted in a banded structure of varying Al2O3-to-metal volume fraction.

Kinetics and mechanism of thermal decomposition of insitu generated calcium carbonate

Abstract: Thermogravimetric studies on two varieties of calcium carbonate viz., analytical reagentgrade and insitu generated from calcium oxalate monohydrate, were carried out. The kinetics and mechanism of their solid-state thermal decomposition reaction were evaluated from the TG data using integral methods and the effect of procedural factors such as heating rate, sample mass and method of computation on them were also studied. The procedural variables in the range studied had no marked influence on the results; however the kinetic parameters were marginally higher for the insitu generated calcium carbonate. This trend is explained by the presence of more micropores in the insitu generated calcium carbonate as well as the mechanism of its decomposition following phase boundary reaction with cylindrical symmetry.

Study of adsorption desorption of contaminants on single soil particles using the electrodynamic thermogravimetric analyzer

Abstract: The isothermal adsorption and desorption of organic vapors on a single soil particle was studied with the aid of the electrodynamic thermogravimetric analyzer (EDTGA). Toluene and carbon tetrachloride were tested at room temperature during their adsorption on Spherocarb, montmorillonite, and Carbopack particles. The maximum amount of either organic compound adsorbed was comparable to that required for one monolayer coverage of the particle surface area. Significant differences among various pairs of solid-organic vapor examined were identified and correlated to differences in solid pore structure and chemical affinity between the organic compound and the solid.