Showing papers on "Thermal decomposition published in 1987"

Electrochemical surface properties of Co3O4 electrodes

Abstract: The electrochemical behaviour of Co3O4 layers deposited by thermal decomposition of Co(NO3)2 at 200–500°C on titanium supports with and without an interlayer of RuO2 has been studied by cyclic voltammetry, chronopotentiometry and potential step experiments in alkaline solutions. Such variables as the calcination temperature, the solution pH, the potential sweep rate and the oxide loading have been investigated in detail to determine their influence on voltammetric peaks and voltammetric charge. Insight has been gained into the relevance of the latter to surface area determination and to proton diffusion into the oxide layer. The role of the support-active layer interface and especially that of the RuO2 interlayer has been scrutinized. The importance of surface studies for the understanding of the electrocatalytic behaviour of Co3O4 electrodes has been analysed.

Kinetics of high-temperature thermal decomposition of SiO2 on Si(100)

Abstract: The decomposition of oxide films of 50 to 500 A on Si(100) during ultrahigh vacuum anneal has been studied in a scanning Auger microscope. The decomposition of the oxide occurs locally, in that voids form in the oxide and grow laterally with time and temperature, leaving the oxide areas in between unperturbed. Void growth kinetics data are derived from in situ absorbed current and secondary electron imaging during vacuum anneal. The growth of the void diameter is found to be linear in time with an activation energy of 2.0±0.25 eV. This implies that oxide decomposition rates after the void nucleation phase are dominated by chemical reactions and/or diffusion processes near the circumference, not by the nature of the defect which nucleated the void.

Phase equilibria in the Al-Si-C system

Abstract: The Al-C, Si-C, and Al-Si-C systems were investigated by metallography, X-ray diffraction, electron microprobe, and thermal analysis. Internally sealed liquidus crucibles were used to determine the solubility of C in Al and Si in the range 1700° to 2150 °C and isothermal sections for the ternary system at 2000° and 2150 °C. The isopleth Al4C3-SiC, determined in the range 1900° to 2300 °C, was found to contain the ternary intermediate phases 2A14C3 · SiC and A14C3 · SiC, which decompose incongruently at 2085° and 2080 °C, respectively. The incongruent decomposition temperature of A14C3 (2156 °C) was confirmed. A phase reported by others with the stoichiometry Al4C3-2SiC was not confirmed. A partial liquidus surface was mapped from the carbon solubility and thermal analysis measurements.

Synthesis and Thermal Stability of Aluminum Titanate Solid Solutions

Abstract: Aluminum titanate solid solutions with empirical formulas of Al2Ti1-xZrxO5, Al6(2-x)(6+x)Si6x/(6+x)□6x/(6+x)TiO5, and Al2(1-x)MgxTi1+xO5 were synthesized by reaction sintering and annealed at 900° to 1300°C in air to evaluate the thermal stability. Substitution of Al in Al2TiO5 by Si and 2Al by Mg and Ti ions to form solid solutions such as AI6(2-x)/(6+x)l-Si6x/(6+x)□6x/(6+x)TiO5, and Al2(1-x)MgxTi1+xO5 was effective in controlling the thermal decomposition, but substitution of Ti by Zr had little effect.

Structural Transformations in the Decomposition of Mg(OH)2 and MgCO3

Abstract: A transmission electron microscopy study has been carried out to understand the structural transformation mechanisms of decomposition of Mg(OH)2 and MgCO3 under vacuum. Both Mg(OH)2 and MgCO3 decompose topotactically to yield porous pseudomorphic MgO of normal structure with definite orientation relationships. The decomposition of Mg(OH)2 yields MgO with a single orientation relationship, but that of MgCO3 leads to one major and two minor orientation relationships with two, six, and three variants, respectively. The major orientation relationships in the formation of 2- to 3-nm cubic MgO particles are discussed in terms of a correlation between oxygen octahedra in the reactant solids. The small cubic MgO particles may aggregate spontaneously to reduce the excess surface energy and result in minute cracks.

Preparation of yttrium, lanthanum, cerium, and neodymium basic carbonate particles by homogeneous precipitation

Abstract: Uniform yttrium, lanthanum, cerium, and neodymium basic carbonate particles were prepared by homogeneous precipitation. Powders were characterized with respect to size, shape, crystal structure, and thermal decomposition behavior. Yttria precursor particles were spherical, monosized (0.4 {mu}m), and amorphous; whereas lanthana, neodymia, and ceria precursors were prismatic (ranging from 1 to 6 {mu}m in size) and crystalline. Crystal structure was found to be ancylite-type orthorhombic symmetry in all three cases. Upon heating in air, yttrium, lanthanum, and neodymium precursors underwent two-step decomposition to first form oxycarbonate and then oxide. Cerium hydroxycarbonate decomposed in a single step to form the oxide.

Preparation of activated carbon by thermal decomposition of used automotive tires

Abstract: Etude de la pyrolyse avec adjonction permanente d'eau. Etude des composes formes ainsi que de l'aire superficielle du charbon actif

Ordering and decomposition in the high-temperature superconducting compound YBa2Cu3Ox.

Abstract: The method of concentration waves is applied to structural transformations in superconducting Y-Ba-Cu oxide. The method predicts an ordering reaction that results in a tetragonal-to-orthorhombic phase transformation, and also yields the correct structure of the Y${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ orthorhombic ordered phase. The results suggest that at low temperature the off-stoichiometric orthorhombic phase is thermodynamically unstable with respect to secondary decomposition into a mixture of ordered orthorhombic and disordered tetragonal phases, whose microstructure can be anticipated.

Novel solid solution precursor method for the preparation of ultrafine Ni-Zn ferrites

Abstract: Ultrafine Ni-Zn ferrites, $Ni_xZn_{1-x}Fe_20_4$, have been prepared by the thermal decomposition/combustion of novel solid solution precursors of the type, $(N_2H_5)_3Ni_xZn_{1-x}Fe_2(N_2H_3COO)_9.3H_20$ where x = 0.2 to 0.8. The novelty of the precursors being their low temperature, exothermic, gas producing self-sustained decomposition. Fine particle nature of the ferrites has been indicated by X-ray powder diffraction, transmission electron microscopy and surface area measurements. Fine particle Ni-Zn ferrites sinter at $1000^0C$, 24 h to achieve almost 99% theoretical density.

Bonding and thermal decomposition of propylene, propadiene, and methylacetylene on the Rh(111) single-crystal surface

Abstract: The reactions of propylene, propadiene, and methylacetylene with Rh(111) crystal faces in ultrahigh vacuum (UHV) from 80 to 800 K have been studied by low-energy electron diffraction (LEED), high-resolution electron energy loss spectroscopy (HREELS), and thermal desorption spectroscopy (TDS). All three hydrocarbons adsorb intact at 80 K. At this temperature, methylacetylene forms a p(2 x 2) LEED pattern while propylene and propadiene adsorb as disordered monolayers. Above 200 K, propylene and propadiene also form p(2 x 2) LEED patterns which are stable up to 270 K. The propylene fragment in this temperature range is propylidyne (CCH/sub 2/CH/sub 3/). By room temperature all the C/sub 3/ hydrocarbons have decomposed to ethylidyne (CCH/sub 3/) and polymerized C/sub x/H fragments. Surface hydrogen is instrumental in the C-C bond breaking, and partial deuteriation studies were performed to determine which C-C bonds break first. The ethylidyne species are stable up to 400 K, above which they decompose to C/sub x/H fragments. The bonding and the thermal decomposition pathways for propylene on Rh(111) are compared to Pt(111) surface chemistry.

The adsorption and thermal decomposition of water on clean and oxygen‐predosed Al(111)

Abstract: The adsorption of water on both clean and oxygen‐predosed Al(111) has been studied by vibrational spectroscopy using electron energy loss spectroscopy (EELS). At 130 K, adsorption on either surface is competitively associative and dissociative. The dominant dissociation product is a hydroxyl species. On the clean surface, adsorption is predominantly molecular, while in the presence of oxygen, adsorption is predominantly dissociative. In contrast to the low temperature behavior, adsorption of water on clean Al(111) at 300 K is completely dissociative, resulting in oxygen adsorption and surface oxidation. Adsorbed hydroxyl species can be produced at 300 K by prolonged water exposure. Upon heating a low‐temperature water layer adsorbed on either surface, molecular water desorption and further decomposition both occur. The production of adsorbed hydroxyl species from water reaches a maximum at 250 K on the clean surface and at 350 K on the oxygen‐predosed surface. The hydroxyl species decompose above these te...

High-resolution electron energy loss spectroscopy and thermal programmed desorption studies of the chemisorption and thermal decomposition of ethylene and acetylene on nickel(100) single-crystal surfaces

Abstract: The chemisorption and thermal decomposition of acetylene and ethylene on Ni(100) surfaces were studied by using thermal programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). Acetylene chemisorbs molecularly at 90 K forming a rehybridized (sp/sup 3/) acetylenic moiety on the surface. This fragment is stable up to 270 K, where dehydrogenation takes place to form CCH residues. Heating further, above 400 K, leaves only carbon on the nickel surface. Ethylene also adsorbs molecularly but with little rehybridization, H bonding to the metal. Stepwise dehydrogenation takes place as the crystal temperature is increased. Vinyl is formed as a product of a unimolecular reaction at 170 K for low coverages of ethylene (theta less than or equal to 0.4), but this process is inhibited at higher coverages and only takes place around 200 K. A strong isotope effect was also observed, and C/sub 2/D/sub 4/ decomposition only occurred at temperatures about 40 K higher. Vinyl decomposes further to form an acetylenic moiety at about 230 K. Further heating is followed by stepwise dehydrogenation similar to that observed when acetylene is adsorbed. No H-D scrambling is observed during the thermal treatment for partially or fully deuterated molecules.

Metalorganic deposition process for preparing superconducting oxide films

Abstract: A metalorganic deposition method is disclosed for manufacturing a superconducting oxide film on a substrate, in which a mixed metalorganic precursor is coated and heated to its thermal decomposition temperature to create an amorphous mixed metal oxide layer. The amorphous layer is then converted to a crystalline coating by further heating followed by cooling in the presence of O 2 atmosphere.

Pyrolysis of acetylene behind reflected shock waves

Abstract: The thermal decomposition of acetylene has been studied in the temperature and pressure regimes of 1900–2500 K and 0.3–0.55 atm using a shock tube coupled to a time-of-flight mass spectrometer. A series of mixtures varying from 1.0–6.2% C2H2 diluted in a Ne-Ar mixture yielded a carbon atom density range of 0.24–2.0 × 1017 atoms cm−3 in the reflected shock zone. Concentration profiles for C2H2, C4H2, and C6H2 were constructed during typical observation times of 750 μs. C8H2 and trace amounts of C4H3 were found in relatively low concentrations at the high-temperature end of this study. A mechanism for acetylene pyrolysis is proposed, which successfully models this work and the results obtained by several other groups employing a variety of analytical techniques. Two values of the heat of formation for C2H(134 ± 2 and 127 ± 1 kcal/mol) were employed in the modeling process; superior fits to the data were attained using the latter value. The initial step of acetylene decomposition involves competition between two channels. In mixtures ( 200 ppm, the dominant initial step is second order. The rate constant for the second-order reaction is described by the equation Benzene concentrations predicted by the model are below the TOF detectability limit. C4H3 was observed in the 6.2% C2H2 mixture in accordance with the proposed mechanism.

Kinetic analysis of thermal dehydration and hydrolysis of MgCl2·6H2O by DTA and TG

Abstract: The thermal decomposition of MgCl2·6H2O (non-dried and partly dried) and the kinetics of the process were studied by DTA, TG, DTG, IR, X-ray diffraction and chemical analysis of Mg and Cl. The reactions which occurred in the course of the thermal analysis were identified as dehydration (in steps), thermal hydrolysis of MgCl2·H2O and dehydrochloridization of magnesium hydroxy chlorides. Melting of the phases MgCl2·6H2O, MgCl2·4H2O and MgCl2 was also identified in the thermal curves. Thermal weight loss continued up to 800 °C in flowing air or nitrogen, but only up to 700 °C in static air. MgO was the end-product of thermal treatment in both cases.

Catalytic growth rate enhancement of electron beam deposited iron films

Abstract: Submicron (<0.25 μm) wide lines of iron have been deposited by low‐energy (0.5–3.0 keV) electron beam induced decomposition of iron pentacarbonyl. Selective area thermal decomposition of iron pentacarbonyl has also been demonstrated. It was found that under certain conditions, the thermal decomposition only occurred on areas where a thin iron film had been previously deposited by electron stimulated decomposition. At 125 °C, the measured thermal decomposition probability was roughly 900 times greater on the deposited iron than previously reported data show for thermal decomposition on bare silicon. Anomalously high deposition yields of 15 to 50 iron atoms per electron were measured during electron stimulated decomposition.

The degradation of guayule rubber and the effect of resin components on degradation at high temperature

Abstract: Degradation of pure guayule rubber and rubber in the presence of stearic, oleic, linoleic, and linolenic acid, has been studied at high temperatures (from 150°C to 600°C) using thermogravimetric analysis (TGA). On-line mass spectrometric analysis of the products of decomposition has also been done to understand the mechanism of degradation. Degradation of rubber starts around 230°C in air and 330°C in nitrogen. Presence of acids changes the onset of degradation, because of low decomposition temperature of the acids. In the derivative curve, there is one Tmax for degradation in nitrogen; two for rubber; and three for rubber containing acids are observed for degradation in air. The activation energy of degradation, as observed by isothermal kinetics, in the 1–10% weight loss region, is 225 kJ/mol in nitrogen and 167 kJ/mol in air. In the 10–100% region, however, the activation energy of degradation measured by the Freeman and Carroll method using dynamic thermogravimetry, is 239 kJ/mol in both nitrogen and air atmosphere. The main products of pyrolysis in inert atmosphere are propylene, propane, isobutylene, methyl butene, isoprene, 2,3-dimethyl cyclopentene, octene, 2,4-dimethylcyclohexene, dipentene, etc. Isoprene is found to be the most abundant. Fragments having m/e values of 136, 121, 107, 93, 79, and 53 are also produced in large quantities. The ratio of concentration of dipentene to isoprene increases marginally with temperature. The concentration of other fragments however increases drastically with temperature. The additives have no effect on the nature of products obtained. The conversion to different fragments depends upon the temperature of degradation and the stability of intermediate products. All smaller molecules are obtained from either dipentene or isoprene. A mechanism of formation of these products has been suggested.

The thermal decomposition of cerium(III) nitrate

Abstract: The thermal decomposition of anhydrous Ce(NO3)3 has been studied. The thermal decomposition reaction is described by the second order kinetic equation, [1/(1−α)]−1=kt. The apparent activation energy was determined asEa=104 kJ mol−1 while the enthalpy of the reaction was estimated asδHr=111.1 kJ mol−1. The decomposition reaction differs from that observed for Nd(NO3)3.

Thiophene chemisorption and thermal decomposition on nickel(100) single-crystal surfaces

Abstract: Etude par thermodesorption et par spectrometrie de perte d'energie electronique a haute resolution. La liaison C-S se rompt en dessous de 90 K et un metallocycle se forme a la surface. Ce metallocycle semble etre le compose stoechiometrique C 4 H 3 qui s'adsorbe a un angle de la surface metallique