Showing papers on "Thermogravimetric analysis published in 1987"

Curing studies of a polyimide precursor

Abstract: Complexes between diphenylcarbamido-dicarboxybenzene and 1-methyl-2-pyrrolidinone, 1/4 and 1/2 M, are obtained by reacting pyromellitic dianhydride with aniline in the solvent 1-methyl-2-pyrrolidinone Decomplexation and imidization of this diamic acid are studied by thermogravimetric analysis, differential scanning calorimetry, Fourier transform infrared spectrometry, polarizing microscopy, and gas chromatography/mass spectrometry Side reactions competing with the imide formation are discussed The presence of solvent is found to markedly influence imidization

Kinetics and initial stages of oxidation of aluminum nitride: Thermogravimetric analysis and x-ray photoelectron spectroscopy study

Abstract: The resistance of AlN powder to oxidation upon heat treatment in air is investigated using thermogravimetric analysis (TGA) and x‐ray photoelectron spectroscopy (XPS). The results reveal that oxidation is initiated at temperatures above 700 °C. For temperatures between 750 and 900 °C, an intermediate oxidation state, which involves the formation of oxynitrides, is detected by XPS and only a linear increase in weight with time is observed with TGA. Oxynitride formation precedes the formation of aluminum oxide and the oxynitride layer persists forming an interfacial layer between oxide and nitride. At temperatures above 900 °C XPS indicated the formation of a thick oxide layer while TGA measurements showed a linear followed by a nonlinear increase in weight with time. These results suggest that the initial stage of oxidation is controlled by surface reactions followed by diffusion controlled oxidation. The activation energy for surface reaction and diffusion oxidation are 36 and 56 kcal/mol, respectively.

Thermogravimetric and High-Temperature X-Ray Studies on the Orthorhombic-to-Tetragonal Transition of YBa2Cu3Oy

Abstract: Thermogravimetric experiment was performed to determine oxygen content in the high Tc superconductor YBa2Cu3Oy. The y varies from 5.8 to 6.5 depending on temperature and oxygen fugacity, indicating that the average valence of copper ions is between 2 and 1. The oxygen content is strongly affected by quenching process of a sample. High-temperature X-ray diffraction study showed orthorhombic-to-tetragonal transition (second or possibly higher order) at around 900 K. Combining this fact with the thermogravimetric data, we concluded that the compound having y less than ~6.0 has a tetragonal symmetry.

Tin oxide surfaces. Part 17.—An infrared and thermogravimetric analysis of the thermal dehydration of tin(IV) oxide gel

Abstract: The thermal dehydration of high-surface-area (ca. 180 m2 g–1) tin(IV) oxide gel has been studied by thermogravimetric analysis and infrared spectroscopy in the temperature range 293–673 K. The thermogravimetricanalysis data show that two types of physisorbed water are present, one which is largely removed by evacuation at ambient temperature and a more strongly held type which is removed by evacuation at 373–423 K. Further mass loss occurring at higher temperatures is due to the condensation of hydroxyl groups on the surface of the oxide particles. The amount of physisorbed molecular water present on the freshly prepared gel corresponds to a ca. three-monolayer coverage, reducing to ca. 0.74 of a monolayer after evacuation at 295 K, and is totally removed after evacuation at 423 K. The surface concentration of hydroxyl groups is determined to be 19.8 nm–2 at 423–473 K, falling to 16.2 nm–2 at 573 K and 13.0 nm–2 at 673 K. Infrared spectra confirm that physisorbed molecular water [v(OH) 3800–2000 cm–1, δ(OH) 1640 cm–1] is removed by 423 K. Isolated and hydrogen-bonded surface hydroxyl groups give rise to stretching modes at 3640 cm–1(sharp) and 3500 cm–1(very broad), respectively. Surface hydroxyl deformation bands are found at 870, 940, 1175 and 1245 cm–1. The bands at 870 and 950 cm–1 are removed on evacuation by 473 and 523 K, respectively. The deformation bands at 1175 and 1245 cm–1 which remain at 523 K are assigned to isolated surface hydroxyl groups. The band at 770 cm–1, which increases in intensity during dehydration, is assigned to surface Sn—O—Sn linkages formed by the dehydroxylation process. The results are interpreted in terms of hydroxylated exposed [100], [101](both of which contain clusters of three hydroxyl groups attached to each surface tin atom) and [110](which contains both isolated and geminal pairs of hydroxyl groups attached to surface tin atoms) crystal planes of the rutile structure, the former predominating.

Thermal degradation of polyacrylamide and poly(acrylamide‐co‐acrylate)

Abstract: The thermal degradation behavior of polyacrylamide and poly(acrylamide-co-acrylate) was studied by differential scanning calorimetry, thermogravimetric analysis, gas chromatography/mass spectrometry, and carbon-13 solid state nuclear magnetic resonance. The degradation products over the temperature range of 115–450°C were characterized. Mechanisms are proposed for the degradation processes involved.

Curing studies of a polyimide precursor. II. Polyamic acid

Abstract: The cure of pyromellitic dianhydride–oxydianiline polyamic acid is compared with that of a model compound, diphenylcarbamido–dicarboxy–benzene, both having been synthesized in 1-methyl-2-pyrrolidinone (NMP). Laser interferometry and thermogravimetric analysis showed that, analog to the diamic acid model compound, the polyamic acid forms complexes with NMP (repeat unit/NMP: 1/4 and 1/2). Thermal stability of these complexes was found to be similar to the ones of the diamic acid model compound as revealed by comparison of decomplexation temperature–heating rate curves. Activation energies of the decomplexation processes for the diamic acid are given. Anhydride formation, a reaction found in the cure of the model compound to compete with imidization, takes place also during the polyamic acid cure at fairly low temperatures. The molecular weight drop due to chain scission resulting from anhydride formation was followed by viscometry.

Preparation of High-Tc Superconducting Oxides by the Amorphous Citrate Process

Abstract: The high-Tc superconducting oxide Yba2Cu3O7–y has been prepared by the amorphous citrate process. A citrate-nitrate solution was prepared at room temperature and dehydrated at around 80°C to yield a solid precursor material. The precursor was fired to high temperature to form the desired compound. The decomposition mechanism was studied with thermogravimetric analysis and X-ray diffraction. Intermediate products were formed during the decomposition reactions. Nearly phase-pure Yba2Cu3O7-y powders were obtained by firing the precursor in air to 900°Cfor2 h. Sintered samples were typically over 90% dense and exhibited good superconducting properties.

Study of char gasification by carbon dioxide. 1. Kinetic study by thermogravimetric analysis

Abstract: Coal char gasification with carbon dioxide was performed in a thermogravimetric apparatus. In the derivation of the kinetic rate equation by use of the thermogravimetric apparatus, the temperature investigated ranged from 1158 to 1253 K and the carbon dioxide partial pressure from 30.3 to 70.9 kPa. The reaction rate was independent of the particle size from 44 to 710 ..mu..m under these conditions. The volume reaction model and the grain model were examined to interpret the experimentally obtained conversion-time curves. The experimental results could be well expressed by the volume reaction model.

Preparation of single crystals of MoB2 by the aluminium-flux technique and some of their properties

Abstract: Crystal growth of MoB2 (R-3m;a hex. = 0.30136(6) nm,c hex. = 2.0961 (4) nm) from high-temperature aluminium solutions (1000 to 1500°C) has been investigated, and the optimum conditions for obtaining single crystals of this compound were examined. Starting with about 33 to 53 g of a mixture of molybdenum, boron and aluminium, in which the atomic ratio is 1 : 2.5 : 53.3 to 88.9, MoB2 single crystals of about 1.5 mm in size were grown without being accompanied by any other crystal phases. The stoichiometry and the crystal structure were corroborated by chemical analysis and single-crystal X-ray diffractometry. The Vickers microhardness measured in the (001) plane is in the range 2170 to 2470 kg mm−2. The oxidation of the crystal in air is examined by thermogravimetric and differential thermal analyses up to 1200°C.

Ion-exchange and surface properties of titania gels from Ti(III) solutions

Abstract: The present investigation deals with the surface properties, thermal behavior, and ion-exchange characteristics of titania gels prepared from titanous chloride under different conditions of pH and temperature. The surface properties, exchange characteristics, and structures of these gels were studied with the aid of thermogravimetric analysis, nitrogen adsorption, visible spectrophotometry, and X-ray diffraction. Studies of the exchange behavior of four different cations, Cu2+, Ni2+, Co2+, and Cr3+, do not suggest a clearcut relationship between the extent of surface area and the ion-exchange properties. It is suggested that the “bridged OHs” on the titania surface are responsible for the cation-exchange properties of the gels. A high pH of preparation would lead to a large number of available exchangeable OH groups, whereas a low pH of preparation would lead to the formation of gels with low exchange capacities because of minimal exchangeable sites. In the case of the trivalent Cr3+ ion and its large hydrated radius the size of the pores seems to play a crucial role in terms of selectivity. Thermogravimetric studies reveal the presence of two types of water (interstitial and rigidly bound water in the form of hydroxyl ions) in samples prepared at a high pH. An additional weight loss appears in the case of samples prepared at a low pH and seems to arise from the loss of coordinately bound water ligands.

New simultaneous thermogravimetry and modulated molecular beam mass spectrometry apparatus for quantitative thermal decomposition studies

Abstract: A new type of instrument has been designed and constructed to measure quantitatively the gas phase species evolving during thermal decompositions. These measurements can be used for understanding the kinetics of thermal decomposition, determining the heats of formation and vaporization of high‐temperature materials, and analyzing sample contaminants. The new design allows measurements to be made on the same time scale as the rates of the reactions being studied, provides a universal detection technique to study a wide range of compounds, gives quantitative measurements of decomposition products, and minimizes interference from the instrument on the measurements. The instrument design is based on a unique combination of thermogravimetric analysis (TGA), differential thermal analysis (DTA), and modulated beam mass spectroscopy (MBMS) which are brought together into a symbiotic relationship through the use of differentially pumped vacuum systems, modulated molecular beam techniques, and computer control and ...

Catalytic effect of heavy metal oxides on crude oil combustion

Abstract: Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were applied to crude oil combustion in the presence and absence of metal oxides. It was found that the effect of titanium oxide was similar to that of silica and alumina. Vanadium, nickel, and ferric oxides behaved similarly in enhancing the endothermic reactions. In the presence of a large surface area such as with silica, the surface reactions are predominant and unaffected by the small amount of metal oxide present. Kinetic analysis of the DSC curves revealed that the activation energies and the frequency factors of the hydrocarbon and the coke combustion reactions, estimated for all the metal oxide additives including silica and alumina, followed the same normal compensation trend. The crude oil mixture of 90% ferric oxide was the only exception to this trend.

Validity of using methanol for studying the microstructure of cement paste

Abstract: Experiments were conducted to assess the extent of interaction of methanol with calcium hydroxide at 22°C. The following techniques were used: X-ray diffraction, infrared spectroscopy, surface-area determination, length change, calorimetry, differential thermal analysis and thermogravimetric analysis. It was concluded that either calcium methoxide or a methylated complex is formed when calcium hydroxide is mixed with methanol.

Thermal stability of substituted phenol-formaldehyde resins

Abstract: A variety of crosslinked phenol-, or derivatized phenol-formaldehyde polymers have been synthesized. The phenol derivatives included o- and p-cresol, p-nonylphenol, m-pentadecylphenol, and raw and distilled cashew nutshell liquid. The derivatives were copolymerized with phenol and formaldehyde using sulfuric acid as catalyst to yield novolak-type prepolymers, which were then cured with hexamethylenetetramine. Thermogravimetric analysis was used to evaluate the thermal stability of the cured resins. It was found that the thermal stability of the resins decreased with increasing amounts of cashew nutshell liquid, with distilled cashew resins being slightly more thermally stable than raw cashew resins. The thermal stability of resins containing substituted phenols with alkyl chains was observed to increase from n-pentadecylphenol to cresol to phenol. The trend in thermal stability of the resins may be explained on the basis of alkyl groups being less thermally stable than aromatic rings. The amount of hexamethylenetetramine used to cure the prepolymers also influences the thermal stability of the resins. It appears that a level of 10 to 15% hexamethylenetetramine maximizes the thermal stability of the resin.

Phase transformations of gel-derived magnesia partially stabilized zirconias

Abstract: Both chloride-free (amorphous, group A) and chloride-containing (crystalline, group B) ZrO2-8.1 mol % MgO powders produced by means of gelatinized coprecipitation have been calcined at 450 to 1450° C and characterized by differential thermal analysis/thermal gravimetric analysis, X-ray diffraction, electron microscopy, and infrared spectroscopy. For both groups A and B, the powder density, chloride content, crystallite size, crystallization temperature, and the initial temperature for metastable tetragonal-to-monoclinic (Tm → M) transformation decrease with increasing concentration of NH4OH with which samples were prepared. High-temperature tetragonal-to-cubic transformation of group B also revealed the same relationship, but the inverse for group A. An explanation based on the nature of the Zr-O bond and internal strain have been proposed for both transitions. In addition, it is also found that silica can inhibit both transitions and raise the critical crystallite size for Tm → M transitions, but water vapour and chloride have the inverse effect on the Tm → M transition.

System and method for thermogravimetric analysis

Abstract: An instrument for the chemical analysis of coal and other hydrocarbons combines TGA and FT-IR principles, and utilizes helium to carry the volatiles evolved by pyrolysis of the sample into the optical cell of the FT-IR spectrometer. The connection between the TGA furnace and the optical cell is substantially direct and non-impeding to fluid flow, and the carrier gas is preheated to the same temperature as the sample, preferably using a common heating element.

Thermal decomposition kinetics of Schiff base complexes of copper(II) and palladium(II)

Abstract: Thermogravimetric (TG), derivative thermogravimetric (DTG) and differential thermal analysis (DTA) curves of CuL2 and Pd(LH)2Cl2 (LH=salicylidene-2-aminofluorene and 2-hydroxy-1-naphthalidene-2-aminofluorene) in air are studied. Mass loss considerations at the main decomposition stages indicate conversion of the complex to oxides. Mathematical analysis of TG data shows that first order kinetics are applicable in all cases. Kinetic parameters (energy and entropy of activation and preexponential factor) are reported.

Influence of synthesis chemistry on alumina-zirconia powder characteristics

Abstract: 80Al2O3 · 20ZrO2 (wt%) powders were synthesized by three different chemical processes: (1) chemical polymerization, (2) destabilization of mixed sols, and (3) coprecipitation. The elementary particles produced by each of these processes were of the order of 1.5 to 3.0 nm and were amorphous to electron diffraction. The powders woe evaluated in terms of (a) weight loss (thermal gravimetric analysis, TGA) and thermal (differential thermal analysis, DTA) characteristics, (b) surface area (BET), pore size distribution, and density at different temperatures; and (c) the crystallization behaviour (XRD method) which showed that the physiochemical characteristics and crystallization behaviour of these chemically derived powders are significantly influenced by the chemistry of powder synthesis. Moreover, the powders produced by the chemical polymerization and the colloidal processes retained the tetragonal phase of zirconia completely during cooling from 1600° C, but significant transformation of tetragonal to monoclinic zirconia occurred in the coprecipitated powder under identical conditions. Furthermore, the 1550° C co!cined powder derived by the chemical polymerization process retained a substantial amount of tetragonal zirconia after annealing at 1000° C for 72h. The average alumina and zirconia crystal sizes of the calcined powders, calculated by the Scherrer formula, were in the 20 to 80 nm range.

Characterization of cured polysulfide polymers by thermal degradation: Pyrolysis‐GC and thermogravimetric studies

Abstract: Polymers prepared by curing thiol-terminated liquid polysulfide polymer [HS(RS2)nRSH, R = CH2CH2OCH2OCH2CH2] with p-quinonedioxime, PbO2, and MnO2 were studied by pyrolysis-GC and thermogravimetry. Characteristic differences were observed in the composition of the pyrolyzates under flash pyrolysis at 420 and 470°C. The concentration of the cyclic monomer, 1,3-dioxa-6,7-dithionane, in the pyrolyzates was found to vary considerably from polymer to polymer. The change in mechanism with the extent of degradation and the corresponding overall activation energies were evaluated by thermogravimetry. The pyrolysis-GC and thermogravimetric data were shown to provide partial characterization of the substrates. Mn appears to be in a coordinated complex in the MnO2-cured polymer.

Nonstoichiometry and electrical resistivity in two mixed metal oxides, La2NiO4-x and LaSrNiO4-x.

Abstract: Two mixed oxides, La2NiO4−x and LaSrNiO4−x, had the K2NiF4 structure in the oxygen-pressure range from 58000 down to 130 Pa at 1173 K. The oxygen-vacancy concentration, x, in both La2NiO4−x and in LaSrNiO4−x at 1173 K was determined by thermogravimetric analysis in the oxygen-pressure range from 58000 to 1.3 Pa. The vacancy concentration of LaSrNiO4−x was greater than that of La2NiO4−x at the same oxygen pressure at 1173 K. The electrical resistivities of La2NiO4−x and LaSrNiO4−x were measured at 473–1173 K under an oxygen atmosphere at 98–58000 Pa. The density of the state and the drift mobility of oxygen stoichiometric La2NiO4 and LaSrNiO4 were estimated from the resistivity data and from the results on the thermogravimetric analysis.