Thermal behavior of aluminum powder and potassium perchlorate mixtures by DTA and TG

In this work the thermal decomposition characteristics of micron sized aluminum powder + potassium perchlorate pyrotechnic systems were studied with thermal analytical techniques. The results show that the reactivity of aluminum powder in air increases as the particle size decreases. Pure aluminum with 5 μm particle size has a fusion temperature about 647 °C, but this temperature for 18 μm powder is 660 °C. Pure potassium perchlorate has an endothermic peak at 300 °C corresponding to a rhombic-cubic transition, a fusion temperature around 590 °C and decomposes at 592 °C. DTA curves for Al₅/KClO₄ (30:70) mixture show a maximum peak temperature for thermal decomposition at 400 °C. Increasing the particle size of aluminum powder increases the ignition temperature of the mixture. The oxidation temperature increased by enhance in the aluminum content of the mixture.     

Thermal decomposition of potassium chlorate in presence of chromium(III) oxide and nickel(II) chromite(III)

A study of thermal behaviour of intimate mixtures of different molar ratios of potassium chlorate and chromium(III) oxide, and potassium chlorate and nickel(II) chromite(III) was made by employing thermogravimetry, differential thermal analysis, chemical analysis, infrared spectroscopy and X-ray powder diffraction analysis. Potassium chlorate in presence of Cr(III), starts decomposing around 200°C which is much below the decomposition temperature of pure KClO₃. Each mole of Cr(III) takes up 8/3 moles of KClO₃ to become oxidized into potassium dichromate.     

Improving safety performance of lactose-fueled binary pyrotechnic systems of smoke dyes

The lactose/KClO₃ is a widely used pyrotechnic mixture to vaporize organic materials, such as smoke dyes. However, because of low ignition temperature of this mixture, serious precaution should be taken into account to prevent its accidental self-ignition. In order to find a safe and efficient alternative of this conventional mixture, KClO₃ has been replaced by common oxidizing agents including KMnO₄, KNO₃, KClO₄, Ba(NO₃)₂, PbO₂ and NH₄ClO₄. TG and DTA analysis have been used to obtain thermal characteristic of the mixtures. Based on ignition temperature of the pyrotechnic mixtures we can divide them into four categories as follows: (1) the mixture igniting at low temperature, i.e., at about 200 °C. (2) Moderate temperature igniting mixture, in which ignition occurs at 300–400 °C. (3) High temperature igniting mixture with ignition temperature higher than 400 °C .(4) Not igniting mixtures. Also, the apparent activation energy (E), ΔG ^#, ΔH ^#, ΔS ^# and critical ignition temperature (T _b ) of the ignition processes of low and moderate temperature igniting mixtures were obtained from the DSC experiments. Finally, among the investigated mixtures, lactose/KNO₃ can be considered as a safe and efficient pyrotechnic composition for vaporization of organic materials, such as smoke dyes, due to its moderate safe ignition temperature.     

Differential thermal analysis of ignition temperatures in a self-propagating high-temperature synthesis reaction

Differential thermal analysis was carried out on the self-propagating high-temperature synthesis reaction 3TiO₂+4Al+3C→3TiC+2Al₂O₃. The results allow the ignition temperature of the reaction to be estimated and the reaction mechanism to be identified. The ignition temperature was 900°C and the results suggest that the reaction proceeds by an initial reaction between titania and aluminium (3TiO₂+4Al→3Ti+2Al₂O₃) and the titanium formed reacts with the carbon (Ti+C→TiC).     

DTA study of the chlorination of fly ash and bauxite in the presence of carbon

The chlorination processes of fly ash and bauxite in the presence of carbon were studied by means of a gas-flow type DTA, X-ray analysis and SEM observation, and the reactivity of Al-compounds as their constituents was compared. In the case of fly ash, the exothermic peak due to the formation of AlCl₃ (mainly) and FeCl₃ appeared at about 790–920°C. The reactivity of Al estimated from the DTA peak temperature depended on the particle size, carbon content and preparation temperature of fly ash, and was much lower than that of bauxite. Fractional conversion of Al was about 60–70%, when fly ash (?300 mesh) was heated up to 900°C in Cl₂ at 5°C min^?1 of heating rate. In the case of bauxite, two exothermic peaks due to the chlorination of Fe and Al appeared at about 270 and 490°C, respectively. The chlorination of Al was completed at 550°C under the above conditions.     

Ignition limits of hydrogen–methane–air mixtures over metallic Rh at a pressure of 1–2 atm

The ignition temperatures and effective activation energies of the ignition limits of mixtures (40–70% H₂ + 60–30% CH₄)_stoich + air over Rh were experimentally determined at a pressure of 1 atm in the temperature range 20–300 °C. Over an ignition-treated Rh surface, the ignition temperature of a mixture of 70% H₂ + 30% methane + air is 62 °C. This indicates the potential of using Rh to markedly lower the ignition temperature of fuels based on hydrogen–methane mixtures.     

Effect of Bismuth Oxide Particles Size on the Thermal Excitation and Combustion Properties of Thermite Systems

The influence of Bi₂O₃ particles size at the sub-micron scale on the thermal excitation threshold and combustion performance of nano-thermite systems was investigated. Three formulas were designed and prepared, Al(100 nm)/Bi₂O₃(170 nm), Al(100 nm)/Bi₂O₃(370 nm) and Al(100 nm)/Bi₂O₃(740 nm). The samples were characterized and tested by SEM, XRD, and DSC techniques. Electrical ignition and combustion experiments were performed. The results showed that with the increase of the particle size of Bi₂O₃, in the case of slow linear heating, the exothermic heat decreased (1051.2 J g⁻¹, 527.3 J g⁻¹ and 243.6 J g⁻¹) and the thermal excitation threshold temperature increased (564.52 °C, 658.1 °C and 810.9 °C). Simultaneously, the state of the thermite reaction correspondingly changed to solid-solid, liquid-solid and liquid-liquid thermite reaction. In the case of rapid heating , the increase in particle size increased the excitation current (0.561A, 0.710A and 0.837A). During the combustion process, the thermite system with the smallest Bi₂O₃ particle size showed the largest combustion rate, and that with the largest particle size had the longest combustion duration.     

Studies on ignition and afterburning processes of KClO4/Mg pyrotechnics heated in air

Thermal behavior of KClO₄/Mg pyrotechnic mixtures heated in air was investigated by thermal analysis. Effects of oxygen balance and heating rates on the TG?CDSC curves of mixtures were examined. Results showed that DSC curves of the mixtures had two exothermic processes when heated from room temperature to 700?°C, and TG curve exhibited a slight mass gain followed by a two-stage mass fall and then a significant mass increase. The exothermic peak at lower temperature and higher temperature corresponded to the ignition process and afterburning process, respectively. Under the heating rate of 10?°C?min^?1, the peak temperatures for ignition and afterburning process of stoichiometric KClO₄/Mg (58.8/41.2) was 543 and 615?°C, respectively. When Mg content increased to 50%, the peak ignition temperature decreased to 530?°C, but the second exothermic peak changed little. Reaction kinetics of the two exothermic processes for the stoichiometric mixture was calculated using Kissinger method. Apparent activation energies for ignition and afterburning process were 153.6 and 289.5?kJ?mol^?1, respectively. A five-step reaction pathway was proposed for the ignition process in air, and activation energies for each step were also calculated. These results should provide reference for formula design and safety storage of KClO₄/Mg-containing pyrotechnics.     

The influence of alumina passivation on nano-Al/Teflon reactions

The reaction kinetics of aluminum (Al) and polytetrafluoroethylene (PTFE or Teflon) were recently examined using nanoparticles of both Al and Teflon. Results showed a unique pre-ignition reaction (PIR) associated with the nano-Al/Teflon mixture that was not significant in the micron-Al/Teflon mixture. The PIR is caused by fluorination of the alumina (Al₂O₃) shell passivating the Al particles and reduces the onset temperature of Al ignition for nano compared with micron particle mixtures. Because the alumina shell was found to play a key role in the reaction mechanism, this communication extends our understanding of the interaction between alumina and Teflon by examining the influence of alumina particle size, and therefore surface area, on the fluorination reaction with Teflon. Differential scanning calorimetry analysis show that reaction kinetics vary dramatically as the alumina particle size is reduced from 50 to 15 nm diameter. Specifically, for 15 nm diameter alumina, the first exotherm (corresponding to the PIR) exhibits three times more heat of reaction than for the 30, 40, or 50 nm alumina particles. These results show how particle size and specific surface area affect the Al–Teflon reaction mechanism.     

Investigation on the reaction of iron powder mixture as a portable heat source for thermoelectric power generators

This paper reports our investigation on the thermal behavior and ignition characteristics of iron powder and mixtures of iron with other materials such as activated carbon and sodium chloride in which iron is the main ingredient used as fuel. Thermal analysis techniques such as differential scanning calorimetry (DSC) and thermogravimetric analysis were used to characterize the materials and for further understanding of reaction kinetics of the pyrophoric iron mixtures. The experimental results demonstrated that iron micron particles react exothermically to the oxygen in atmosphere and produced iron oxide with ignition temperature of 427.87 °C and heat generation of 4,844 J g^?1. However, in this study, the pyrophoric iron mixture acts as a heat source for the thermoelectric power generators, the final mixture composition is determined to compose of iron powder, activated carbon, and sodium chloride with the mass ratio of approximately 5/1/1. The mixture generated two exothermic peaks DSC curves that showed ignition temperature of 431.53 and 554.85 °C and with a higher heat generation of 9,366 J g^?1 at higher temperature. The effects of test pan materials and heating rate on the ignition were also examined by DSC method. Kinetic data such as the activation energy (E _a), the entropy of activation (ΔS ^# ), enthalpy of activation (ΔH ^# ), and Gibbs energy of activation (ΔG ^# ) on the ignition processes was also derived from the DSC analysis. From the ignition temperature, heat generation, and kinetics test data, the mass ratio of 5/1/1 proved to generate the most amount of heat with high temperatures for the standalone thermoelectric power generators.     

Characterization of metal oxide-doped lutetium orthoferrite powders from the pigmentary point of view

This thesis deals with the preparation and characterization of inorganic pigments based on perovskite structure of metal oxide-doped LuFeO₃. Powder samples were prepared by the conventional ceramic method, i.e., solid-state reaction. Heating temperature was chosen according to results of TG/DTA. Prepared pigments were incorporated into an organic binder system, and their color properties were evaluated by measuring the reflectance in the visible region of light. The most interesting color properties were obtained by preparation of sample Lu_0.98Ca_0.02FeO_3?δ with mineralizer LiF at the temperature 900 °C. Mean size of its particles is 4 μm. X-ray diffraction analysis confirmed a single-phase orthorhombic structure with lattice parameters a = 0.521310 nm, b = 0.55535 nm, and c = 0.75626 nm. Thermal stability of the sample is limited by the temperature of 1,150 °C. Further, the effectiveness of other metal oxide (CoO-, ZnO-, Bi₂O₃-, and Sb₂O₃)-doped Lu_0.98Ca_0.02FeO_3?δ system was evaluated with respect to their phase composition, thermal stability, particle size distribution, and color properties. The conclusions of the research showed that a sample containing antimony oxide is the mixture with the best pigmentary-application properties. The powder has a clear orange color, high thermal stability up to 1,340 °C, and mean particle size 4 μm.     

Synthesis and properties of a solid solution formed in the CrVMoO7 - AlVMoO7 system

It has been shown by the methods of X-ray powder diffraction (XRD), differential thermal analysis (DTA) and infrared spectroscopy (IR) that solid solutions of a formula Cr_1−xAl_xVMoO₇, where xε (0−0.65), are formed in the system CrVMoO₇-AlVMoO₇. The obtained research results have proven that the ions Al3+ are incorporated into the crystal lattice of CrVMoO₇ instead of Cr³⁺, which causes a contraction of the lattice and a shift of IR absorption bands towards higher values of wavenumbers. The phases Cr_1−xAl_xVMoO₇ melt incongruently in the temperature range from 710°C (for x=0.65) to ∼820°C in the case of x close to zero This revised version was published online in July 2006 with corrections to the Cover Date.     

Oxidation of magnesium in the systems NaClO4-Mg-Metal oxide (peroxide)

Oxidation of magnesium in mixtures NaClO₄ + Mg + metal oxide or peroxide has been investigated using differential thermal analysis (DTA). In the systems with peroxides Na₂O₂, Li₂O₂, BaO₂, CaO₂ or ZnO, magnesium oxidizes simultaneously with decomposition of NaClO₄ in the region 380–520°C, which is 100–200°C below the oxidation temperature of magnesium in air. In the ternary systems with transition-metal oxides NiO, CuO, FeO, and Fe₂O₃, magnesium transforms into oxide at above 600°C after sodium perchlorate had been decomposed completely. The low-temperature oxidation of magnesium occurs in the systems in which sodium chlorate is accumulated during the catalytic decomposition of NaClO₄.     

Studies on surface molecular motion of oligomeric polystyrene by differential scanning calorimetry

There are many studies on the surface molecular motion of polymer films [ 1 ], but no report on surface thermal properties of polymer because of experimental difficulties. The thermal property of oligomeric polystyrene (PS) was investigated by differential scanning calorimetry (DSC) in the present study. In order to increase the ratio of surface area to volume of PS particles, the DSC samples were prepared by mechanically grinding mixtures of PS and Al₂O₃ powders. The grinding mixtures of these powders with low particle size showed a transition at a low temperature of 14–17 °C (much lower than the bulk glass transition temperature, T_g), and this low‐temperature transition was dependent on the size of PS particles. This transition seems to result from the surface molecular motion of the activated surface layer of PS. Copyright © 2000 John Wiley & Sons, Ltd.     

The Hydrothermal Synthesis of Sulfate Cancrinite (SO4‐CAN): Relations between Si‐Al Sources and Crystal Quality

Sulfate cancrinite (SO₄‐CAN) Na₈[AlSiO₄]₆(SO₄)(H₂O)_n (2.6 < n < 3.2) was synthesized under hydrothermal conditions at 200 °C and 48 hours. Three different Si‐Al sources were inserted: (a) kaolinite (K), (b) a gel of sodium‐waterglass and sodium aluminate (G), and (c) an oxide mixture of cristobalite and corundum (CK). The products were characterized by X‐ray powder diffraction (XRD), ²⁹Si and ²⁷Al MAS NMR spectroscopy, scanning electron microscopy (SEM), energy dispersive X‐ray spectroscopy (EDXS) and BET surface area analysis as well as simultaneous thermoanalysis (TG‐DTA). Mostly nanocrystals of platelet habit and dimensions smaller than 100 nm (beside a very few larger crystals) were observed from K. In contrast the gel (G) yielded to cancrinites with typical rod‐like morphology and dimensions of 100 × 500 nm. Large prismatic cancrinites up to 5 μm width and 10 μm length were observed from the oxide mixture CK. Furthermore the transition kinetics of the cancrinites into nosean was investigated at a temperature of 1000 °C under open conditions. This process was already complete after 1 hour for the nanocrystals from K but needed 10 hours for the nanocrystals from G and the microcrystals from CK. The relations between the individual Si‐Al source and the crystal properties like size, habit, and thermal transition kinetics were discussed.     

Thermochemical studies on the formation and constitution of the zinc oxide—aluminium oxide system

This study is based on thermogravimetric (TG), differential thermal analysis (DTA) and chemical analysis of the ZnOAl₂O₃ system. The coprecipitation from mixed nitrate salt solutions of zinc and aluminium results in the formation of zinc basic carbonate and aluminium hydroxide, and is also a precursor to aluminate spinel (2ZnO·3Al₂O₃) only in the samples in which aluminium is present in near or above stoichiometric quantities. Grinding of the mixtures of individual precipitates maintains the similarity with coprecipitates in forming a “precursor”, but to a lesser extent. The endothermic peak in DTA at 275°C in some coprecipitated and mixed samples hints at the formation of a precursor since the individual precipitate does not have a peak at this temperature. The “precursor” to spinel obtained in the precipitation stage in some coprecipitated samples is freely soluble in 1 M HCl, and that obtained at 450°C is partially soluble which cannot be detected by the usual X-ray technique due to its highly disordered structure in amorphous state. The “precursor” is converted around 800°C to an actual spinel structure, which is almost insoluble in M HCl and is detectable by X-rays.     

On the preparation of nanosized Al2(WO4)3 by a precipitation method

Nanosized aluminum tungstate, Al₂(WO₄)₃, is prepared by a precipitation reaction between Na₂WO₄ and Al(NO₃)₃. The structure of the precipitated composition is determined by powder XRD analysis, IR and ²⁷Al MAS NMR spectroscopy. The thermal properties are examined by DSC, DTA and TG analyses combined with gas evolved analysis. Particle sizes and morphology are examined by TEM analysis. Precipitation reaction leads to the formation of an amorphous composition, which consists of dimer and trimer aluminum hydroxide species and WO₄^2? groups. Finely dispersed particles with dimensions of about 25 nm are formed. The precipitated composition is decomposed to amorphous Al₂(WO₄)₃ immediately after H₂O release. At 630 °C, amorphous Al₂(WO₄)₃ crystallizes in an orthorhombic modification of Al₂(WO₄)₃, the enthalpy of crystallization being 58 kJ/mol. The nanosized particles remain intact after the crystallization of amorphous Al₂(WO₄)₃. A significant particle growth take places when nanosized Al₂(WO₄)₃ is heated from 600 to 800 °C.     

Thermal stability of aluminium hydroxycarbonates with monovalent cations

Dawsonite-type compounds of formula MAl(OH)₂CO₃ (M = Na. K, NH₄) as well as a laminar hydrotalcite-type hydroxycarbonate of composition [Al₂Li(OH)₆]₂CO₃·4H₂O. have been hydrothermally synthesized The thermal decomposition of these compounds was monitored by DTA and TG, and the resulting products have been studied by X-ray and IR techniques. Sodium and potassium dawsonites are destroyed at 335°C. yielding a poorly crystalline compound in which part of the overall carbonate is present; the remaining carbonate is lost between 600 and 700°C, yielding NaAlO₂ and KAlO₂, respectively Ammonium dawsonite and lithium hydrotalcite are less stable, their thermal decomposition occurring at about 240°C. The ammonium dawsonite heated at 680°C shows the presence of A1₂O₃ with a poorly ordered structure, while lithium hydrotalcite yields poorly crystalline γ-Al₂O₃ at 500°C and a mixture of γ-LiAlO₂ and LiAl₅O₈ when the compound is heated at higher temperatures ( ~ 1000°C).     

Use of Methods of Thermal Analysis in Studying Ceramic Materials on the Basis of Al2O3, ZrO2, Si3N4, SiC and Inorganic Binder

By thermoanalytical methods TG, DTG, DTA there have been investigated the processes occurring during the formation of ceramic materials on the basis of Al₂O₃, ZrO₂, Si₃N₄, SiC,and inorganic binder. IR spectroscopy has been an additional research method. It's been determined that with the use of H₃PO₄ as the binder for ceramic materials, the mechanisms of thermal decomposition are connected with the following processes: 1. removal of weakly tied and crystallized water in the temperature range of120–230°C, the removal being characterized by the endothermic effect, 2. interaction of the initial powder components of the ceramic materials with orthophosphoric acid conditioned by a strong exothermic effect on the DTA curve in the range of 230–530°C, 3. overlapping of endo- and exo-effects, testifying to a complex mechanism of thermal transformations, 4.oxidizing of the non-reacted silicon at the temperature of 720(760)°C, an increase of mass is observed on the TG curve as a result of the formation of SiO₂ – crystoballite. This revised version was published online in July 2006 with corrections to the Cover Date.     

Synthesis,characterisation and thermal decomposition of double sulfate

Upon evaporation at room temperature of an aqueous mixture containing Al(III) sulfate and trishydroxymethyl-ammoniummethane sulfate in a molar ratio 1:2, double sulfate as crystalline product was obtained. The stoichiometry of the obtained compound was determined by means of elemental and TG analysis. For identification, IR-spectra and X-ray powder diffraction patterns were done. It was found that the general formula of the obtained compound is Al(HOCH₂)₃CNH₃(SO₄)₂·₆H₂O. as revealed by TG, DTG and DTA analysis, the dehydration of the AL-compound takes place in one step which points out that the six water molecules are bonded in the same way. The thermal decomposition of the anhydrous compound starts at about 260°C and is very complex. This process takes place in many steps which are not well resolved. The pathway of the thermal decomposition is also supposed. This revised version was published online in July 2006 with corrections to the Cover Date.