Thermal stability of the system BaO-BaO2

The thermal stability of barium peroxide obtained via the oxidation of BaO was studied. The investigations were carried out with a Q1500D MOM derivatograph. The flow rate of air or oxygen-argon mixtures was 25–30 l/h. It was found that the thermal properties of the BaO-BaO₂ system and of BaO₂ depend on the partial pressures of oxygen and water vapour during the process of oxidation of BaO. The most stable BaO₂ can be produced from high-purity BaO with a special crystal structure.

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Zusammenfassung Es wurde die thermische Stabilität von Bariumperoxid, erhalten durch die Oxidation von BaO, untersucht. Die Untersuchungen wurden mittels eines MOM-Derivatographen Q-1500D ausgeführt. Der Durchsatzstrom der Luft bzw. der Sauerstoff/Argon-Gemische betrug 25–30 1/h. Es zeigte sich, daß die thermischen Eigenschaften des Systemes BaO-BaO₂ bzw. von BaO₂ vom partiellen Druck des Sauerstoffes bzw. vom Wasserdampfdruck abhängen, der bei der Oxida'tion von BaO vorherrscht. Das stabilste BaO₂ konnte aus hochreinem BaO mit einer speziellen Kristallstruktur gewonnen werden.

     

Thermal stability of the amorphous system Ge20As14(SexS1−x)52I14

Results of thermal investigations of the amorphous five-component chalcogenide system Ge₂₀As₁₄(Se_xS_1?x)₅₂I₁₄ are presented. Differential thermal analysis (DTA), derivative differential thermal analysis (DDTA), and dilatometry were employed to determine the temperatures of softening and partial crystallization of the samples. Thermal treatment of the samples at 1000°C and recording of the corresponding thermogravimetric (TG) and derivative thermogravimetric (DTG) curves allowed an elucidation of the full mechanism of their decomposition, which proceeds via seven characteristic phase transitions.     

Thermal analysis and longtime stability of amorphous Co100−xBx alloys

Crystallization behaviour of amorphous Co_100?xB_x alloys (17 <x ≤ 40) has been investigated by differential thermal analysis (DTA) and dynamic temperature X-ray diffraction (DTXD) methods in the freshly prepared state and at a period of about eight years after preparation. The crystallization temperatures lie in the range 670 K–760 K. An average decrease of about 1.25 K/year over a period of eight years has been observed to take place in the crystallization temperatures of these materials. The value of heat of crystallization (ΔH _cr) and activation energy lie in the range 2.3 kJ/g-at – 5.9 kJ/g-at and 2.1 eV – 2.4 eV, respectively. The phases obtained at crystallization temperatures during DTXD analysis have been discussed.     

Thermal stability of the crystallisation nucleant

The 2,2',4,4',6,6'hexanitrostilbene, HNS, nucleant, used in the crystallisation of 2,4,6,trinitrotoluene, TNT, was precipitated from molten TNT and examined by differential scanning calorimetry, DSC, at several stages during purification by vacuum sublimation. During purification a broad endotherm, associated with nucleant decomposition, which could be resolved into two endotherms, depending on the sublimation temperature, was observed. Pure nucleant prepared at 70C showed a similar behaviour during thermal annealing for extended periods of time at >85C. Thus TNT, retained in the recrystallised HNS nucleant, may be migrating during the purification process or may occupy a range of lattice sites, which exhibit different activation energies for migration to the surface of the solid during thermal decomposition of the nucleant. Loss of TNT from the nucleant, during purification, could produce some free HNS. The activation energy for nucleant decomposition, which may be a two-stage processes with the initial mobility of the TNT being the limiting reaction, was estimated to be 210 kJ mol^–. The lattice sites available for the TNT in the host HNS nucleant require elucidation and are the subject of further studies to be published at a later date.     

Thermal stability of the silica-aminopropylsilane-polyimide interface

Thermal degradation of the silica–aminopropylsilane–amic acid/imide interface was studied by modifying a high-surface-area, neutral silica gel with a number of substituted aminopropylsilanes (APS). These substrates were reacted further with phthalic anhydride or aromatic amic acid monomers and the thermal decomposition of the adsorbed/reacted materials was monitored by thermogravimetric analysis (TGA) and infrared (IR) spectroscopy. The 3-aminopropyltriethoxysilane/poly[N,N′-(p,p′-oxydiphenylene)pyromellitimide] interface was also evaluated by this method. Comparison clearly distinguishes the thermal decomposition of surface-bound APS from surface-bound alkylphthalimides, the adhesion product of alkylamines and aromatic amic acids. Alkylamine imidization with the elimination of aromatic amine (analogous to polymer scission) and the decomposition of the surface-bound imide are shown in the amic acid TGA profiles. This imidization and the accompanying aniline elimination begin at about 130°C, under nitrogen, to form the surface alkyl imide which slowly decomposes at 400°C. TGA analysis indicates that the surface-bound imide undergoes minimal degradation under nitrogen at 370 ± 10°C; temperatures above this threshold range produce changes in the APS–imide interface.     

Physical stability and relaxation of amorphous indomethacin

The alpha- and beta-relaxation processes in amorphous indomethacin have been studied by using differential scanning calorimetry. The beta-process has been detected as a small endothermic peak that emerges before the glass transition step when reheating samples previously annealed in the temperature region -20 to +5 degrees C. The activation energy of the beta-process is approximately 57 kJ mol(-1), and shows an increase with increasing temperature as it approaches the glass transition region. In the glass transition region, the effective activation energy of relaxation decreases with increasing temperature from 320 to 160 kJ mol(-1). Heat capacity measurements have allowed for the evaluation of the cooperatively rearranging region in terms of the linear size (3.4 nm) and the number of molecules (90). The beta-relaxation fades below -30 degrees C, which provides a practical estimate for the lower temperature limit of physical instability in indomethacin. It is demonstrated experimentally that nucleation of indomethacin takes place in the temperature region of the beta-relaxation.     

Thermal stability of ketoprofen

The purpose of this investigation is to calculate the kinetic parameters and the kinetic model for the studied process. The results are further used to predict the system’s behaviour in various circumstances. A kinetic study regarding the ketoprofen—involving active substance’s thermal decomposition—was performed under isothermal conditions and in a nitrogen atmosphere, for the temperature steps: 260; 265; 270; 275; and 280 °C. The thermogravimetry/derivative thermogravimetry data were processed by three differential methods: isothermal–isoconversional, Friedman’s isothermal–isoconversional, and isothermal model-fittings. The obtained results are in good accordance with those obtained under non-isothermal conditions of a previous study, and confirm the necessity for the kinetic parameters to be determined, under different thermal conditions, by the adequate calculation methods.     

Thermal stability of Peroxynitrates

Peroxynitrates are thermally unstable intermediates (at ambient temperatures) in the atmospheric degradation of hydrocarbons. In this work, thermal lifetimes of nine peroxynitrates have been measured as a function of temperature and, for two of them, also, as a function of total pressure. In the presence of excess NO, relative concentrations of the peroxynitrates were followed in a 420 I reaction chamber as a function of time by means of longpath IR absorption using a Fourier transform spectrometer. Original data on the unimolecular decomposition rate constants are presented for the peroxynitrates RO₂NO₂ with R = C₆H₁₁, CH₃C(O)CH₂, C₆H₅CH₂, CH₂I, CH₃C(O)OC(H)CH₃, C₆H₅OCH₂, (CH₃)₂NC(O), C₆H₅OC(O), and C₂H₅C(O). Thermal lifetimes at room temperature and atmospheric pressure are very short (in the order of seconds) for substituted methyl peroxynitrates (i.e., R'CH₂O₂NO₂) but rather long for substituted formyl peroxynitrates (i.e., R″C(O)O₂NO₂). Kinetic data from this and previous work from our laboratory are used to derive structure‐stability relationships which allow an estimate of the thermal lifetimes of peroxynitrates from readily available ¹³C n.m.r. shift data. ©1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 127–144, 1999     

Thermal stability of elastomers

Thermal transitions of elastomers are classified and problems concerning their stability are discussed. It is concluded that in the case of general-purpose elastomers no simple correlation exists between the energy of the bonds in the skeleton of a chain and their thermal stability. This also holds for the parameters of the physical structure of the chains. A high tendency to cross-linking, a high concentration of cross-links and their chemical structure give rise to a more perceptible effect.

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Zusammenfassung Thermische Umwandlungen von Elastomeren werden klassifiziert und mit der StabilitÄt von Elastomeren zusammenhÄngende Probleme diskutiert. Es wird gefolgert, da\ bei Elastomeren für allgemeine Verwendungszwecke keine einfache Korrelation zwischen der Energie der Bindungen im Kettengerüst und der thermischen StabilitÄt besteht. Das gilt auch für die Parameter der physikalischen Struktur der Ketten. Der Effekt einer starken Neigung zur Vernetzung, der Netwerkdichte und der chemischen Konstitution der Netzwerkbrücken ist dagegen augenscheinlicher.

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Thermal stability of alcohols

3,3-Dimethylbutanol-2 (3,3-DMB-ol-2) and 2,3-dimethylbutanol-2 (2,3-DMB-ol-2) have been decomposed in comparative-rate single-pulse shock-tube experiments. The mechanisms of the decompositions are The rate expressions are They lead to D(iC₃H₇? H) – D((CH₃)₂(OH) C? H) = 8.3 kJ and D(C₂H₅? H) – D(CH₃(OH) CH? H) = 24.2 kJ. These data, in conjunction with reasonable assumptions, give and The rate expressions for the decomposition of 2,3-DMB-1 and 3,3-DMB-1 are and      

Thermal stability of piroxicam

The application of thermal method is of great importance regarding the pharmaceutical problems such as the control of raw materials, the determination of purity, the qualitative and quantitative analysis of drug formulation, tests of thermal stability and compatibility and the determination of kinetic parameters etc. The purpose of a kinetic investigation is to calculate the kinetic parameters and the determination of the kinetic model for the studied process. The results are further used to predict the system’s behaviour in various circumstances. A kinetic study regarding the piroxicam—active substance’s thermal decomposition was performed under isothermal conditions and nitrogen atmosphere, for the temperature steps: 200, 205, 210, 215 and 220 °C. The TG/DTG data were processed by three differential methods: isothermal—isoconversional, Friedman’s isothermal isoconversional and isothermal model-fitting. The obtained results are in good accord between them, as well as with those obtained under non-isothermal conditions from a previous work and confirm the necessity of the kinetic parameters determining in different thermal conditions, by the adequate calculation methods.     

Thermal stability of polyurethanes

Summary The combined application of thermogravimetry reactiongas chromatography and gel-permeation chromatography permits to follow the heat degradation of polyurethane polymers in inert gas, air and water-saturated environment. The examinations give information on the rate of thermal degradation, the individual volatile degradation components, the critical points of the polymer chains and on the change of their molecular-weight distribution. Gas chromatographic examinations also permit the identification of the chain-extending components of different types of polyurethanes.     

Thermal stability and band gap study of heavy metal oxy-sulfide glass system

Novel oxy-sulfide glass system xPbS-(73 ? x) Bi₂O₃–27B₂O₃ with 6.06 ≤ x ≤ 36.35 named lead sulfide bismuth borate (LSBB) was prepared using normal melt and quench-casting technique. Phase transition temperatures t _g, t _x, t _pi, and t _l were noted from the DTA curves. Glass transition temperature t _g varied from 306 ± 2 to 336 ± 2 °C and the onset of crystallization temperatures t _x was between 331 ± 2 and 402 ± 2 °C. The glasses melted in the range 523 ± 2 to 597 ± 2 °C. The ratio t _g /t _l showed that compositions reported conform to the two-third law of glass formation. Hruby’s coefficient H _r witnessed the thermal stability of the system and that LSBB4 was most stable and glass formability factor k _gl showed that composition LSBB1–LSBB6 can easily corroborate vitreous state. The direct band gap energy varied from 1.56 to 3.07 eV, while indirect band gap energy for the fundamental absorption edge was 0.21–1.31 eV. Absorption edges obeyed Urbach rule. A broad range of band tailing was exhibited confirming amorphous state of the system.     

Thermal stability study of the protease inhibitors

A thermal and kinetic analysis of two protease inhibitors: nelfinavir mesylate and atazanavir sulfate, were carried out to find their thermal stability. DSC curves of both drugs showed exothermic transition. This observed process resulted in two steps. Obtained apparent activation energy pointed at low stability of studied protease inhibitors in water solutions.     

Thermal stability of transition-metal complexes

This review aims at justifying the relationship between the room-temperature structures of transition-metal complexes and their thermal stabilities. The different factors affecting the thermal stability were also clarified. The survey of a larger number of transition-metal complexes showed various correlations of thermal stability with metal ion, ligand character or counterion.     

Thermal oxidative stability of poly-p-xylylenes

The air oxidation of poly-p-xylylene films was studied at temperatures between 125 and 200°C. The oxidation kinetics were obtained from neutron activation (NA) oxygen analyses and infrared (IR) Spectroscopy. A correlation between the NA oxygen analyses and mechanical properties indicated that the amount of oxygen incorporated into these polymers before a significant degradation mechanical properties is about 1000 ppm for poly(dichloro-p-xylylene) and 5000 ppm for poly(monochloro-p-xylylene) or poly-p-xylylene. The activation energy for the oxidation of these polymers was about 30 kcal/mole. Long-term-use (100,000 hr) temperatures were also estimated for each of the poly-p-xylylenes studied. The 100,000-hr maximum continuous-use temperature is 112°C for poly(dichloro-p-xylylene), 72°C for poly(monochloro-p-xylylene), and 57°C for poly-p-xylylene.     

Thermal stability of fluorochloroindate glasses

New fluorindate glass compositions have been investigated in order to improve optical transmission as well as thermal properties. Chloride inclusion extends transmission of a fluoride matrix to longer wavelength in infrared region. In the present work thermal parameters of an IZnBS composition, based on InF₃, ZnF₂, BaF₂ and SrF₂, with various amounts of alkaline chlorides were investigated by differential scanning calorimetry. The chloride presence decreased all characteristic temperatures and increased both thermal stability and glass forming ability up to 10% of MCl content, where M=Li, Na, K and Rb. The presence of NaCl promoted glass phase separation. For samples containing same concentration of NaCl, this effect is accentuated for increasing the contents of SrF₂. This revised version was published online in August 2006 with corrections to the Cover Date.