Showing papers on "Thermal decomposition published in 1980"

Thermal decomposition of human tooth enamel

Abstract: Further insight into human tooth enamel, dense fraction (TE), has been obtained by following the change and loss of CO3 2−, OH−, structurally incorporated H2O, Cl−, and, indirectly, HPO4 2− after TE had been heated in N2 or vacuum in the range 25–1000°C. Quantitative infrared spectroscopic, lattice parameter, and thermogravimetric measures were used. Loss of the CO3 2− components begins at much lower temperature (e.g., 100°C) than previously recognized, which has implications for treatments in vitro and possibly in vivo. CO3 2− in B sites is lost continuously from the outset; the amount in A sites first decreases and then increases above 200° to a maximum at ∼800°C (>10% of the possible A sites filled), where it is responsible for an increase ina lattice parameter. A substantial fraction of the CO3 2− in B sites moves to A sites before being evolved, apparently via a CO2 intermediary. This implies an interconnectedness of the A and B sites which may be significant in vivo. No loss of Cl− was observed at temperatures below 700–800°C. Structural OH− content increases ∼70% to a maximum near 400°C. Structurally incorporated water is lost continuously up to ∼800°C with a sharp loss at 250–300°C. The “sudden”a lattice parameter contraction, ∼0.014A, occurs at a kinetics-dependent temperature in the 250–300°C range and is accompanied by reordering and the “sharp” loss of ∼1/3 of the structurally incorporated H2O. The hypothesis that structurally incorporated H2O is the principal cause of the enlargement of thea lattice parameter of TE compared to hydroxyapatite (9.44 vs 9.42A) is thus allowed by these experimental results.

Thermal behavior of hydrotalcite and of anion-exchanged forms of hydrotalcite

Abstract: Chloride-, sulfate-, and perchlorate-exchanged forms of hydrotalcite have been prepared and their layer spacings determined after equilibration in hydrous atmospheres and after heat-treatments up to the temperature of decomposition in the range 300–400°C. The initial carbonate form of hydrotalcite and also brucite, for purposes of comparison, have been similarly studied. Only chloride-hydrotalcite and brucke exhibit a single phase stable to the decomposition temperature. The other anionic forms exhibit various phases with different layer spacings which are interpreted in terms of the size, orientation, and stability of the anions and, in some cases, the presence of additional water. A regularly interstratified form of sulfate-hydrotalcite (layer spacing = 19.80 A) is obtained at room temperature and relative humidity <50%; at higher humidities, a fully hydrated phase is obtained, and at 50°C a collapsed form is obtained. In the preparation of perchlorate-hydrotalcite, an interstratified phase (layer spacing = 17.0 A) was recognized with alternating carbonate and Perchlorate layers, although the evidence is less certain.

X‐ray photoelectron spectroscopy and paramagnetic resonance evidence for shock‐induced intramolecular bond breaking in some energetic solids

Abstract: Solid samples of 1,3,5, trinitro 1,3,5, triazacyclohexane (RDX), trinitrotoluene (TNT), and ammonium nitrate were subjected to shock pulses of strength and duration less than the threshold to cause detonation. The recovered shocked samples were studied by x‐ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR). The results of these measurements indicate that the shock pulse either broke or altered the internal bonds of the molecules of the solid. The results of the shock decomposition are compared with measurements of the uv and slow thermal decomposition of these materials using the same experimental techniques.

Micropore formation due to thermal decomposition of acicular microcrystals of α-FeOOH

Abstract: The thermal decomposition of synthetic acicular microcrystals of α-FeOOH has been investigated by nitrogen adsorption isotherms, powder X-ray diffraction patterns, and electron microscopic observation. The surface area of the microcrystals determined from the nitrogen isotherms increases remarkably with the progress of decomposition. The highest surface area is obtained on a sample decomposed in vacuo at 300°C for 4 hr. On samples calcined at temperatures higher than 300°C, their surface areas decrease significantly. It has been found from de Boer's t plot that the marked increase in surface area can be attributed to the formation of approximately uniform slitshaped micropores, their width being estimated to be 0.82 nm. The presence of such micropores has been confirmed by electron microscopic observation. The marked decrease in surface area accompanies the disappearance of micropores and the appearance of macropores. The nonuniform broadening of the X-ray diffraction peaks for the sample decomposed at 300°C can be explained on the basis of the slit-shaped micropores developed along the c axis of the α-FeOOH microcrystals.

Factors influencing the formation of haloforms in the chlorination of humic materials

Abstract: Examined are the reactions of chlorine with humic and fulvic fractions extracted from the River Thames and tested under controlled conditions of pH and temperature. Also studied is the relationship between residual haloforms and total haloform formation from these acid fractions under conditions generally encountered in water treatment. Monitoring of chloroform production indicates that total chloroform is composed of two components: one produced by thermal decomposition of unidentified chlorinated intermediates and the other dissolved chloroform. The dominant factor influencing both the production of total chloroform and the relative proportions of these two components is pH. (2 drawings, 5 graphs, 15 references, 2 tables)

Iron pentacarbonyl decomposition over grafoil. Production of small metallic iron particles

Abstract: Mossbauer spectroscopy was the principal technique used to study the adsorption and thermal decomposition of iron pentacarbonyl on a high surface area, oriented graphite (Grafoil). The iron carbonyl physisorbs on the graphite at temperatures less than ca. 300 K, whereas decomposition of the carbonyl occurs upon heating to 378 K. The Mossbauer spectra of the decomposition product are sextuplets indicative of metallic iron. However, the room-temperature magnetic hyperfine splitting (ca. 300 kOe) is smaller and the peak widths (ca. 1.5 mm s-/sup 1/) are larger than those corresponding spectral parameters for bulk metallic iron (330 kOe and 0.28 mm s-/sup 1/, respectively). Computer simulations suggest that these differences are due to magnetic relaxation and to a distribution of magnetic hyperfine fields. For consistency with the Mossbauer spectra, it is proposed that the iron particles are present as thin rafts on the graphite surface. Transmission electron microscopy shows that these rafts are preferentially located along edges and steps on the graphite. Heating to 450 K leads to significant sintering of the iron particles.

Mechanism of thermal degradation of polyurethanes investigated by direct pyrolysis in the mass spectrometer

Abstract: Direct pyrolysis in the mass spectrometer (MS) yielded unequivocal evidence regarding the mechanism of thermal decomposition of N-monosubstituted and N-disubstituted polyurethanes. It was ascertained that direct pyrolysis in the MS detects the primary thermal fragments that originate from polyurethane pyrolysis. This is particularly useful when, as in the thermal decomposition illustrated in eq. (1), it is necessary to distinguish between primary and secondary thermal fragments in order to assess the thermal degradation mechanism. Our results indicate that N-monosubstituted polyurethane V undergoes a quantitative depolycondensation process. Instead, the thermal decomposition of the N-disubstituted polyurethane VI which occurs selectively in eq. (1) is demonstrated by the detection of thermal fragments that contain secondary amine and olefinic end groups. Finally, polyurethane VI shows a higher thermal stability with respect to polymer V because of the absence of the depolycondensation process, which accounts for the thermal degradation of the N-monosubstituted polyurethane V.

Thermal Decomposition Process of HMX

Abstract: The thermal decomposition of HMX has been investigated using thermoanalytical techniques and infrared spectroscopic study at both above and below its melting point. The weight loss phenomenon that occurs as the temperature is elevated at a constant heating rate has been clearly separated into four elementary processes which are induction period, sublimation, first order solid phase reaction, and highly exothermic liquid phase reaction by plotting them against the logarithm of the heating rate versus the reciprocal temperature. Hydroxymethyl formamide has been shown to be a major product of the liquid phase decomposition, which suggests that the decomposition of HMX in the liquid phase should be initiated by the N-N bond scission but not by the C-N bond scission.

Post-hydrogenation of CVD deposited a-Si films

Abstract: Using plasma treatments at 400C we have performed hydrogenation (and deuteration) of amorphous silicon films grown by thermal decomposition of silane (CVD). Diffusion profiles have been measured on deuterium using SIMS and checked by nuclear reaction technique. At 400C hydrogen enters CVD a-Si with a diffusion coefficient of 6×10−14 cm2s−1. By comparing these profiles to ESR measurements we find that the ESR signal is eliminated at hydrogen concentration levels of the order of the dangling bonds density. The effect of the plasma treatment on conductivity of pure and phosphorus doped samples is discussed. The maximum value of σ in P doped samples is 0.7Ω −1 cm −1 .

Method for the preparation of melamine

Abstract: An improved method for the preparation of melamine by the conversion of urea and/or thermal decomposition products thereof. The urea and/or thermal decomposition products are converted to melamine in the presence of a gas mixture containing ammonia and carbon dioxide in a reaction zone containing a fluidized bed of catalytically active material. Melamine is desublimated from the melamine containing gas mixture in a desublimation zone by a dry-capture method leaving a desublimator off-gas mixture of ammonia, carbon dioxide and gaseous impurities. A major portion of this desublimator off-gas mixture is compressed and recirculated to the reaction zone as fluidizing gas for the bed of catalytically active material, without intervening treatment to remove gaseous impurities from the desublimator off-gas.

Kinetics of the thermal decomposition of pyridine in a flow system

Abstract: The kinetics of the pyrolysis of pyridine was studied in the temperature range of 875–1050°C, at 1 atm total pressure, in a Vycor stirred-flow reactor. Initial concentrations ranged from 0.25 to 2 mol % in helium and reaction times from 0.25 to 4 sec. It was found at the lower temperatures that the reaction was mixed first and second order. At the higher temperatures an autocatalytic term, zero order in the substrate and first order in the concentration of pyridine decomposed, was necessary in addition to the other two terms to describe the kinetic data. Most of the product was in the form of a primarily aromatic, nonvolatile tar; the major volatile products were HCN and C2H2. An increase in the surface-to-volume ratio of the reactor decreased the rate, indicating a radical chain or sequential reaction with radical consumption on the surface. Experiments with additives showed that C2H2 increased the rate. Using the kinetic and product data, some mechanistic considerations are discussed, and a mechanism is proposed to account for the first-order term.

Kinetics and mechanism of the germane decomposition

Abstract: The homogeneous gas-phase thermal decomposition kinetics of germane have been measured in a single-pulse shock tube between 950 and 1060 K at pressures around 4000 torr. The initial decomposition is GeH4 GeH2 + H2 in its pressure-dependent regime, with log k = 13.83 ± 0.78 – 50,750 ± 3570 cal/2.303RT. RRKM calculations suggest that the high-pressure Arrhenius parameters are log k GeH4(M ∞) = 15.5 – 54,300 cal/2.303RT. Extrapolations to static system pyrolysis conditions (T ∼ 600 K, P ∼ 200 torr) give homogeneous reaction rates which are much slower than those observed, hence the static system pyrolysis of germane must be predominantly heterogeneous. Shock-initiated pyrolysis reaction stoichiometry is 2 mol H2 per mole GeH4, suggesting that the subsequent decomposition of germylene is essentially quantitative. Investigations of the hydrogen product yields for pyrolysis of GeD4 in oCH3 further indicate that the germylene decomposition reaction is mainly GeH2 H2 + Ge, but that a small amount of reaction to H atoms may also occur.

Thermal Decomposition of Graphite Fluoride. I. Decomposition Products of Graphite Fluoride, (CF)n in a Vacuum

Abstract: The mechanism of thermal decomposition of graphite fluoride, (CF)n in a vacuum was studied by means of chemical and gas analyses, and the measurement of crystallinity. The crystallinity of (CF)n was slightly improved in the step of initial decomposition. The fluorine/carbon atomic ratio of initial gaseous product evolved by decomposition was about 2. The product is CF2 or C2F4, and then it gives several compounds by secondary reaction. Every second layers of (CF)n are decomposed, and the evolved molecules or radicals abstract fluorine atoms which are bonded to the neighboring two layers.