Thermal decomposition of zinc hydroxy azide

The isothermal decomposition of zinc hydroxy azides, Zn(OH)_2-x(N₃)_x: follows deceleratory kinetics throughout the temperature range studied. The initial part of the decomposition fits into unimolecular decay law, log (1-a) = -kt. The contracting volume law satisfactorily describes the data at higher degrees of decomposition. The maximum value ofa upto which the slow decomposition could be recorded was 0.75. The aged form of zinc hydroxy azide decomposes with much lower rates and slightly different topochemical characteristics. The decrease in the rate of decomposition on ageing has been attributed to the formation of carbonate on the surface of the compound during storage. The change in topochemical behaviour is traced to the reported layer structure of zinc hydroxy azide. The effect of pre-heating on subsequent thermal decomposition has also been discussed.     

Multiferroicity in La1/2Nd1/2FeO3 nanoparticles

Nano-sized La_1/2Nd_1/2FeO₃ (LNF) powder is synthesized by the sol–gel citrate method. The Rietveld refinement of the X-ray diffraction profile of the sample at room temperature (303 K) shows the orthorhombic phase with Pbnm symmetry. The particle size is obtained by transmission electron microscope. The antiferromagnetic nature of the sample is explained using zero field cooled and field cooled magnetisation and the corresponding hysteresis loop. A signature of weak ferromagnetic phase is observed in LNF at low temperature which is explained on the basis of spin glass like behaviour of surface spins. The dielectric relaxation of the sample has been investigated using impedance spectroscopy in the frequency range from 42 Hz to 1 MHz and in the temperature range from 303 K to 513 K. The Cole–Cole model is used to analyse the dielectric relaxation of LNF. The frequency dependent conductivity spectra follow the power law. The magneto capacitance measurement of the sample confirms its multiferroic behaviour.     

Polytypic transformations during the thermal decomposition of cobalt hydroxide and cobalt hydroxynitrate

The isothermal decomposition of cobalt hydroxide and cobalt hydroxynitrate at different intervals of temperature leads to the formation of Co₃O₄. The phase evolution during the decomposition process was monitored using powder X-ray diffraction. The transformation of cobalt hydroxide to cobalt oxide occurs via three phase mixture while cobalt hydroxynitrate to cobalt oxide occurs through a two phase mixture. The nature of the sample and its preparation method controls the decomposition mechanism. The comparison of topotactical relationship between the precursors to the decomposed product has been reported in relation to polytypism.     

Kolmogorov–Johnson–Mehl–Avrami kinetics for non-isothermal phase transformations ruled by diffusional growth

We report on the functional form of the rate of the transformed volume fraction in non-isothermal phase transitions occurring by nucleation and diffusional growth. The microscopic growth rate is computed by solving the diffusion problem for time-dependent diffusion coefficient. The growth law is further employed in the Kolmogorov–Johnson–Mehl–Avrami (KJMA) theory for describing the time dependence of the transformed volume at constant heating rate. It is demonstrated that the transformation rate separates in the product of volume fraction and actual temperature functions. In the framework of the KJMA approach this factorization is exact. It is also shown that for real systems (due to the high values of the reduced activation energies for nucleation and growth), the kinetics is in excellent agreement with the stretched exponential function appropriate for isothermal transformations.     

Catalytic synthesis of nanosized feathery carbon structures via the carbide cycle mechanism

The morphology of carbon nanostructures obtained by 1,2-dichloroethane decomposition on the 90% Ni/Al₂O₃ catalyst under different reaction conditions was studied by high-resolution transmission electron microscopy. A new carbon product was discovered, which received the name of feathery carbon. The product has an extremely loose disordered structure consisting of separate fragments of a graphite-like phase. The structural disordering is assumed to be caused by the variation of chlorohydrocarbon decomposition conditions on the frontal face of the metal particle. This changes the character of carbon atom diffusion from the frontal face to the backside face of the nickel particles and finally results in a feathery morphology of the carbon phase. The specific surface area of feathery carbon is 300–400 m²/g.     

Influence of γ-Irradiation on the Thermal Decomposition Of (BaC2O4+CuO) Mixture

Influence of γ-irradiation, 0.1–0.75 MGy on the thermal decomposition of (BaC₂O₄+CuO, 10 mol%) mixture has been studied between the temperature range 678-718 K. It is evident from ‘α-f’ plots that decomposition of both unirradiated and irradiated mixtures follows: (i) initial gas evolution (initial puff of gas), (ii) acceleratory and (iii) decay periods. Irradiation promotes the initial gas evolution (α₁), decreases the fractional decomposition (α), the effect being higher at lower doses, without altering the nature of the isotherms. But these effects increase with increasing radiation dose. Analysis of the kinetic data in the light of theories of various kinetic models are found to be well fitted to Prout-Tompkins, Avrami-Erofeyev and contracting square mechanisms. It is found that irradiation diminishes the rate of reaction of both the periods and decreases the energy of activation of the acceleratory stage without affecting the same in the decay period. Plausible mechanism of decomposition and the effect of γ-irradiation thereon has been discussed in detail.     

镍前驱体对非负载型镍催化剂上甲烷分解活性的影响

 分别以硝酸镍和乙酸镍为前驱体, 采用沉淀法制备了非负载型 Ni 催化剂, 运用 X 射线衍射、H2-程序升温还原及 CH4 程序升温表面反应对催化剂进行了表征, 并考察了 Ni 催化剂上 CH4 分解反应活性. 结果表明, 以乙酸镍为前驱体制得的 NiO 样品粒子尺寸较小, 较易被还原, 还原后得到的催化剂催化 CH4 分解活性和稳定性较高; 而以硝酸镍为前驱体制得的 NiO 样品粒子尺寸较大, 较难被还原, 还原后催化剂上 CH4 分解活性和稳定性较低. 制备过程中乙酸镍与溶剂乙二醇所形成的配合物是获得尺寸较小 NiO 样品的关键.     

Deep Oxidation of Methane over Nano-Sized Ferrites with Spinel Structures

Ferrites with spinel structures as catalysts for the deep oxidation of methane have been studied by X-ray phase analysis and temperature programmed reduction. The most active catalysts are the nano-sized ferrites of cobalt and nickel, prepared by the decomposition of polynuclear complexes, with Al₂O₃ as carrier and with addition of a surface active agent to increase the thermal stability of the catalysts. At low temperatures (up to 450 °C), the effect of the size factor appears with the increase in specific catalytic activity of cobalt and nickel ferrites with decreasing of their particles. A correlation of the catalytic activity with the quantity and mobility (reactivity) of oxygen in the ferrites has been established.     

A study of non-isothermal kinetics of limestone decomposition in air (O2/N2) and oxy-fuel (O2/CO2) atmospheres

The non-isothermal experiments of limestone decomposition at multi-heating rates in O₂/N₂ and O₂/CO₂ atmospheres were studied using thermogravimetry. The limestone decomposition kinetic model function, kinetic parameters of apparent activation energy (E), and pre-exponential factor (A) were evaluated by Bagchi and Malek method. The results shown that in 20 % O₂/80 % N₂ atmosphere, the limestone decomposed slowly following the contracting sphere volume model controlled by boundary reaction (spherical symmetry) in two stages, and the E increased by about 50 kJ mol^?1 in the second decomposition stage. But in 20 % O₂/80 % CO₂ atmosphere, the presence of high-concentration CO₂ significantly inhibited the limestone decomposition, and made the decomposition process occur at high temperature with a rapid rate; the decomposition kinetics was divided into three stages, the first stage was an accelerated decomposition process following the Mampel Power law model with the exponential law equation, the second stage followed the nth order chemical reaction model as an α–t deceleration process, and the third stage belonged to the random nucleation and nuclei growth model with the Avrami–Erofeev equation. And with the heating rate increasing, the reaction order n showed a slight rise tendency. The E was about 1,245 kJ mol^?1 in 20 % O₂/80 % CO₂ atmosphere, but was only about 175 kJ mol^?1 in 20 % O₂/80 % N₂ atmosphere. The E and A increased markedly in the O₂/CO₂ atmosphere.     

Some applications of thermomagnetometry to the study of chromindur alloys

Thermomagnetic experiments are shown to be a useful and rapid technique for studying phase changes involving magnetic materials. Particular examples are chosen from Chromindur II (Fe28Cr10.5Co) alloys having different thermal histories. The Curie temperature of the single phase alloy is found to be ～650°C and the metastable spinodal decomposition boundary is at essentially the same temperature. The Cr-rich near equilibrium decomposition product is less magnetic and has a lower T_c, while the Cr-deficient product is more magnetic, with a T_c ? 720°C.Comparisons are made at different heating and cooling rates between temperatures measured using magnetic standards and those measured by a thermocouple in close proximity to the sample.     

Solid-state decomposition studies on fluoroperoxo species of transition metals. Part VI. Kinetics of isothermal decomposition of M2Zr2(O2)2F6 · 2 H2O (M = Rb+, or Cs+)

The decomposition of solid fluoroperoxozirconates of alkali metals, M₂Zr₂(O₂)₂F₆ · 2 H₂O (M = Rb⁺, Cs⁺), is carried out in vacuum under isothermal conditions. The stoichiometry of the reaction may be represented by the equation, M₂Zr₂(O₂)₂F₆ · 2 H₂O(S) — M₂Zr₂O₂F₆(s) + O₂(g) + 2 H₂ O(g) (condensed). The fractional decomposition α is determined by measuring the pressure of oxygen evolved during pyrolysis with a McLeod gauge. The α values range from 0.06 to 0.70 for the rubidium and from 0.06 to 0.79 for the caesium species in the temperature ranges 107–202°C and 101–219°C, respectively. The α—time data for both compounds show that the kinetics are deceleratory throughout the course of the decomposition reaction. In both compounds, the initial stages of decomposition are described by a unimolecular decay law, while the later stages obey a contracting volume equation at all temperatures. The activation energies from Arrhenius plots are 14.0 and 10.9 kcal mole^?1 for the rubidium and 12.9 and 11.2 kcal mole^?1 for the caesium compound.     

Multiphoton tea CO2 laser excitation of the triplet state of propynal

Multiple infrared photon excitation of propynal triplet molecules gives rise to a strongly perturbed phosphorescence. Following absorption of a few IR photons per molecule the phosphorescence spectrum extends to higher energy, the intensity increases, the decay — deviating from the original exponential decay — accelerates and the emission quantum yield drops dramatically. These findings are explained in terms of temperature sensitive radiative (T₁ → S₀) and radiationless (T₁ → S₀) processes with the vibrational temperature as the determining factor. During the perturbed triplet decay, the IR excitation initially confined to the vibrational degrees of freedom becomes distributed among all degrees of freedom which results in a decrease in the vibrational temperature and thus a complex phosphorescence decay.     

Variable-temperature (295—15 K) raman spectra of dodecacarbonyl-triangulo-ruthenium(O) and -osmium(O), M3(CO)12 (M = Ru,Os)

Laser Raman spectra of solid dodecacarbonyl-triangulo-ruthenium(0) and -osmium(0), M₃(CO)₁₂ (M = Ru, Os), have been recorded at 295, 200, 100 and 15 K using a surface scanning technique to avoid sample decomposition. The data indicate that neither compound undergoes a phase change throughout the temperature range investigated. Contrary to earlier measurements at room temperature only, site-symmetry and correlation effects are more widespread than hitherto suspected.     

A comparative study of the effects of decomposition rate control and mechanical grinding on the thermal decomposition of aluminum hydroxide

Summary Two different processes of the thermal decomposition of synthetic bayerite, i.e., the non-isothermal decomposition of mechanically ground sample in flowing N₂ and the controlled rate thermal decomposition of crystalline bayerite under vacuum, were investigated comparatively. In comparison with the conventional non-isothermal decomposition of crystalline bayerite in flowing N₂, the reaction temperature of the thermal decomposition was lowered by the individual effects of mechanical grinding of the sample and the reaction rate control. These decomposition processes indicated similar behavior characterized by the restricted changes of the specific surface area during the course of decomposition reaction and the formation of an amorphous alumina as the decomposition product. Different thermal behaviors were observed for those amorphous Al₂O₃ produced by the respective decomposition processes.     

Évolution thermique d'un hydroxyde de nickel lamellaire mal organisé avec des molécules d'eau en insertion entre les feuillets: comparaison avec l'hydroxyde de nickel cristallisé Ni(oH)2

The authors have studied the thermal decomposition of a turbostratic nickel hydroxide by means of DTA, TGA and isothermal analysis. The turbostratic compound has been described as a random stacking of parallel and equidistant nickel hydroxide layers with intercalary water layers.Well crystallized Ni(OH)₂ has been used as reference. The preparation and the main properties of both hydroxides have been indicated. It was found that the removal of intercalary water takes place simultaneously with the removal of hydroxyl groups at a temperature, not very different from the dehydration temperature of Ni(OH)₂. NO₃^? ions from the starting material and trapped water have been found on the dehydration product. The very fine nickel oxide shows a quite important increase of the lattice parameter (4.215 instead of 4.177 Å). A reversible variation of the inter-layer spacing from 8.5 to 7Å has been pointed out when the turbostratic hydroxide is heated up to 150°C.     

Cu-Co-Fe oxide catalysts supported on carbon nanotubes for the oxidation of CO

The catalytic activity and adsorption characteristics of the surface of catalysts in the form of carbon nanotubes produced on nickel and cobalt oxides with the Cu-Co-Fe oxide system as supported active phase were studied. At carbon nanotubes produced on nickel oxide with (10 + 10) wt.% of the catalytically active phase total conversion of CO to CO₂ is realized at 47 °C. This sample has high specific surface area and a large volume of mesopores. It was shown that the increase in catalytic activity correlates with the increase in the amount of the α₂ form of CO₂. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 42, No. 4, pp. 222–226, July–August, 2006.     

Short time dynamics in the isotropic phase of liquid crystals: the aspect ratio and the power law decay

Optical heterodyne detected optical Kerr effect (OHD-OKE) experiments are used to study the orientational dynamics of the liquid crystal 4^′-octyl-4-biphenylcarbonitrile (8CB) in the isotropic phase near the isotropic to nematic phase transition. The results are compared to those for three other liquid crystals. The 8CB data display a short time scale temperature independent power law decay and a long time scale exponential decay with a temperature dependence described by Landau–de Gennes theory. The power law exponent is −0.56. Combining this result with previous results for three other liquid crystals [J. Chem. Phys. 116 (2002) 6339; J. Chem. Phys. 116 (2002) 360], it is found that the power law exponent depends linearly on the aspect ratio of the liquid crystal.     

Studies on multistep thermal decomposition behavior of polytriazole polyethylene oxide‐tetrahydrofuran elastomer

Polytriazole polyethylene oxide‐tetrahydrofuran (PTPET) is an energetic propellant elastomer that is prepared using glycidyl azide polymer and trifunctional alkynyl‐terminated polyethylene oxide‐tetrahydrofuran. Its thermal decomposition, determined using thermogravimetic analysis, showed two mass‐loss peaks largely related to the decomposition of azide groups and the main chain. Flynn‐Wall‐Ozawa and Kissinger‐Akahira‐Sunose methods were deployed to obtain kinetic triplet parameters of PTPET thermal decomposition by the traditional model‐free method; the Coats‐Redfern approach was used as the model‐fitting method. Kinetics analysis indicated that the mechanism of the two‐step reactions were the primary‐reaction of first order and the power‐law phase reaction of the 2/3 order. The first decomposition stage of PTPET had an activation energy (E_a) of 113 to 116 kJ/mol while the second was 196 to 210 kJ/mol. The thermal decomposition of PTPET with different heating rates and mechanisms showed good kinetic compensation effects, the gas products being further studied with TG‐FTIR.     

Thermal decomposition of ammonium copper chromate: effect of the addition of barium

The influence of the addition of barium (as barium chromate) to ammonium copper chromate on its thermal decomposition has been investigated by TG, DTG and DTA in the temperature range 303–1273 K. The solid products formed in the decomposition at different temperatures were characterised by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy and also by measuring their specific surface area. The addition of BaCrO₄ at different concentrations (0–19.8%) was found not to change the general trend in the decomposition of ammonium copper chromate (which occurs in four distinct steps), except small changes in the temperature corresponding to DTG and DTA peaks. However, the sintering (or crystal growth) of the solid product is largely reduced by the presence of BaCrO₄.