Sharp Exothermic Peak Research Articles

DTA study of RuO 2 formation from the thermal decomposition of ruthenium(III) hydrate

Ruthenium(III) hydrate has been precipitated in aqueous solution from the soluble hydrated RuCl 3. The thermal behavior of ruthenium(III) hydrate has been investigated by DTA and TG in air and in nitrogen. The compound undergoes a disproportionation reaction between 100 and 150°C yielding Ru(metal)+RuO 2· xH 2O. In nitrogen, the metal crystallizes beyond 450°C whereas it is fully oxidized in air in the temperature range 150–300°C. RuO 2 crystallizes on a sharp exothermic peak, the temperature of which depends on the residual water content. An estimate of the enthalpy of crystallization is given, and the oxidation mechanism is discussed.

Thermal analysis of lanthanide polyphosphates

A study of thermal analysis of some lanthanide polyphosphates has been reported. The DTA curves of Ce, Pr and Sm polyphosphates were of similar nature. They exhibited a sharp exothermic peak indicating crystallization temperature (T c) around 520 °C. However, no peak indicatingT m was observed upto 800 °C. TG curves of Ce, La, Pr, Sm, Y polyphosphates indicated weight loss occurring in three stages. The integral procedural decomposition thermal analysis (ipdt) indicated that these materials had refractory nature.

Thermal decomposition of aluminum hydroxide prepared by NH3 gas-AlCl3 solution reaction.

A DTA curve for the powder prepared by the reaction of NH3 gas and AlCl3 solution (G-L method) had a sharp exothermic peak at 360°C and a following endothermic peak which could not be observed on the curve for the powder prepared by the reaction of AlCl3 solution with NH4OH solution (L-L method). The exothermic peak corresponds to the heat of crystallization of NH4Cl included as an impurity and the endothermic peak does to a sublimation of NH4Cl. The remarkable weight loss on a TG curve was observed corresponding to the endothermic peak.Very small peaks at 710° and 800°C on the DTA curve for the powder by the G-L method could not observed on the curve from L-L method.The X-ray diffraction peaks for the powder prepared by the G-L method and heated at 800°C were broader than those prepared by L-L method. These broadnesses are due to the smaller particle size.

Thermal analysis of copolyphosphates

DTA and TG were carried out on a number of sodium-containing copolyphosphates. The DTA curves were characteristic of long-chain polyphosphates and were similar in nature. All of them showed a sharp exothermic peak indicating crystallization temperature (Tc) followed by a sharp endothermic peak giving the value of melting temperature (Tm). TG curves showed weight loss occurring in two stages, the first one beginning at 50°C (loss of absorbed moisture) and the second one in the range 300-500°C (corresponding toTc). The values ofTm andTc appeared to decrease with increase in molecular weight. This could be due to the presence of crystallizable comonomer units in the copolyphosphate glasses.Tc andTm also appeared to increase with increase in the atomic weight of the cation other than sodium.

Conformational transitions of poly(β-benzyl L-aspartate)

Conditions that affect the conformation of poly(β-benzyl L-aspartate) in films have been studied. A slow transition from the left-handed to the right-handed α-helical form is observed under certain conditions in half-dried film cast from chloroform solutions and after exposure of amorphous films of the left-handed β-helical form to chloroform vapor. This transition is caused by adsorption of moisture. Thermal transitions of these films were examined by differential scanning calorimetry and infrared spectroscopy. The right-handed α-helices change to left-handed ω-helices around 140°C, and sharp exothermic peak is observed in this region for some films.

Crystallization of amorphous Si films formed by chemical vapor deposition

The crystallization process of chemically vapor deposited (CVD) amorphous Si films was studied by using a differential scanning calorimeter. A sharp exothermic peak due to amorphous–polycrystalline transition was obtained in the 665–700 °C range. The crystallization temperature was determined to be 665 °C. Transmission-electron diffraction and transmission-electron microscopic examinations revealed that, when amorphous Si film is heated at a slightly higher temperature than the crystallization temperature, the Si dendrite grains nucleate, and grow in size with heating time.

Thermal and stability study of tetracene using differential scanning calorimetry

A thermal analysis study was made of tetracene using differential scanning calorimetry (DSC). The effect of different scan speeds was investigated. At scan speeds of 0.625 to 10°C min −1 two large rounded exothermic peaks were produced. The peaks occurred at an increasingly high temperature as the scan speed increased (for example, the peaks occurred at 128 and 130°C at a scan speed of 0.625°C min −1 and at 148 and 150°C at a scan speed of 10°C min −1. When tetracene was heated at a scan speed of 80°C min −1 only one large sharp exothermic peak was produced. It is believed that the two peaks obtained at scan speeds of 0.625 to 10°C min −1 represent decomposition of the tetracene in two successive stages, while the one peak obtained at 80°C min −1 represents an explosion. A stability test for tetracene is proposed that involves heating of the tetracene in aluminum pans from the DSC apparatus in ovens at 100, 75, and 60°C, removing the pans and samples at intervals of 30 min, 24 h, and 7 days, respectively, subjecting the samples to DSC at 1.25°C min −1, and noting the time interval in the oven that produces a DSC curve that shows obliteration of the second peak. Two lots of tetracene made by different processes showed marked differences in stability characteristics.

Curing of Novolak with Hexamine

The reactions of phenol, novolak, and dihydroxydiphenylmethane with hexamine have been examined by differential scanning calorimetric analysis (DSC), thermogravimetric analysis (TGA), infrared (IR) spectroscopy, and nuclear magnetic resonance (NMR) spectroscopy.The results may be summarized as follews:(1) Free phenol in novolak decreases the melting point of novolak and accelerates the gelation reation (Fig.1 Table 1).(2) Free phenol reacts with hexamine to form the complexes (Reaction(1)). The decomposition and volatilization of a part of the complexes take place by heating to 120 C (Fig.2 Table 2).(3) The reaction of novolak with hexamine was similar to that of phenol with hexamine, and these reactions lead to the dibenzylamine-type compounds via the above complexes mainly.The methylene-crosslinked structure is partly formed in the region of 130-170 C, accompanied with both endothermic peaks at around 130 C and a sharp exothermic peak in the DSC curves (Reaction(2) Fig.1).(4) Rate constants of the above reactions were calculated by assuming first-order reactions. The apparent activation energy was estimated to be about 20 kcal per mole by using the methods of Kissinger and Fava (Figs.4, 5 and 6 Table 4).

Thermal Properties and Crystallization of Magnesium Highpolyphosphate Glass

A study has been made for obtaining informations on thermal properties and crystallization process of magnesium phosphate glass with the molar composition MgO⋅P2O5. The thermal properties such as linear expansion, DTA figure and specific volume change by heat-treatment of the glass were tested. The crystallization temperature was determined from the DTA, and the crystal growth in the glass specimen heat-treated was observed through a microscope.From the thermal expansion curve, transformation temperature (glass transition point) (Tg) and softening temperature of the phosphate glass were determined to be 563°C and 595°C, respectively. Linear coefficients of the thermal expansion below Tg(αl) and above Tg(αh) were 98×10-7 and 99×10-6cm/cm⋅°C, respectively. A wide difference between αl and αh can be interpreted in connection with the characteristics of this glass having an open structure.The DTA curve of the fine powdered glass showed that the crystallization started at 630°C and finished at 698±5°C, which points were determined from the sharp exothermic peak. In the case of the bulky glass sample, it was indicated that the crystallization initiated at about 730°C and completed above 800°C, which temperatures were pointed out by deviation from the basic line in the DTA chart and by the broad exothermic peak, respectively.The crystallization of the phosphate glass specimen began at its surface. The crystals formed at the surface were observed to be fine granular particles and to be smaller than several microns in size. The size of the crystallite was affected by keeping conditions of the sample, i.e., temperature and time. During the crystallization process at higher temperatures than about 660°C, after the whole surface of the specimen had been filled with fine crystal particles, the crystal growth proceeded inside of the specimen. The X-ray diffraction patterns of both the surface and the inside crystals coincided with that of Mg(PO3)2 which is mentioned in ASTM X-ray powder data. The crystals observed inside of the specimen were oriented vertically to the surface, and they were rod-and/or needlelike polycrystals.The rate of crystal growth proceeding from the surface of the specimen toward its inside in a steady state increased linearly with increasing temperature within the range of 600°C to 700°C. The activation energy of the crystal growth was determined to be 80kcal/mol in the above temperature range.

Mechanochemical Phenomena of Magnesium Hydroxide by Grinding

Studies were made to investigate fine dividing and structural change of Mg(OH)2 crystal during grinding as an example of mechanochemical dehydration of hydrate crystals. The mortar of super hard alloy (WC-Co system) was especially used instead of agate mortar to avoid the reaction with silica for grinding.In the case of grinding in air, Mg(OH)2 was rapidly converted into amorphous phase indicated. by X-ray and its weight increased considerably with time under the influence of H2O and CO2. The differential thermal analysis (DTA) curve of the ground material after 48 hours was quite similar to that of basic magnesium carbohate reported as 4 MgCO3 Mg(OH)2.4 H2O. In the case of grinding in the air removed H2O and CO2, Mg(OH)2 also becam. e amorphous phase but no mechanochemical dehydration was found from the weight change. In the DTA curve of amorphous Mg(OH)2, a sharp characteristic exothermic peak appeared ' immediately after decomposition of the hydroxide due to recrystallization of MgO. In both cases, specific surface area of the ground materials approached to a maximum (60 m2/g) but gradually decreased after 24 hours by the formation of secondary particle.In the case of grinding in vacuum (10-2 mmHg), MgO crystal was clearly found after 24 hours - as the results of mechanochemical dehydration with rapid conversion of the hydroxide into amorphous phase and the amount rised up to 55 % after 70 hours. With approaching to the limit of size reduction by grinding, the distorted and expanded crystal of MgO (lattice con: stant 4.2368A, crystallite size 100 A) should be mechanochemically equilibrated with amorphous Mg(OH)2. In this case, the specific surface area became much larger than that of air grinding (200 m2/g after 70 hours). Surface activity of ground materials were discussed from the number of basic points.

Mechanochemical Phenomena of Dimagnesium Phosphate Trihydrate by Grindingt

As particularities of the mechanochemical dehydration of MgHPO43H2O, it was shown that the salt converted rapidly into amorphous phase without forming any crystals of anhydrate and pyrophosphate by grinding, and the ground salt had a sharp exothermic peak due to recrystallization of pyrophosphate in differential thermal analysis curves.As the case of CaHPO42H2O, the escape of water molecules and the destruction of hydrogen bonds progressed in parallel during grinding.

Coexistence States of Iron in Synthesized Iron-bearing Allophane (Al2O3-SiO2-Fe2O3-H2O System)

Abstract A sharp exothermic peak at 980°C, which is found characteristic of allophane in its differential thermal analysis, shifted to the lower-temperature side and broadened as the iron content in the iron-coprecipitated samples increased. In the case of the iron-mixed samples, however, this exothermic peak did not shift, and a new broad exothermic peak, which was ascribed to hydrated ferric oxide, appeared. The crystallization processes in the heat-treated samples were found to depend on the coexistence state and the quantity of the iron. The dimensions of a unit cell of the mullite, crystallized at 1200°C from the iron-coprecipitated samples, varied with the iron content. The iron-mixed samples, however, showed little change in their cell dimensions. It is concluded that there are two states of the coexistence of iron in the allophanes. In one of them, iron ions form solid solutions of the allophane, i.e., as an iron-allophane. In the other, iron is in the mixture as the hydrated ferric oxide. The maximum amount of iron in the solid solution was estimated to be 5 wt% in the iron-allophane.

Thermogravimetric and differential thermal analysis of cellulose, hemicellulose, and lignin

AbstractThe thermal degradation of samples of cellulose, hemicellulose, and lignin have been investigated using the techniques of thermogravimetric analysis (TGA) and differential thermal analysis (DTA) between room temperature and 600°C. The results calculated from static and dynamic TGA indicated that the activation energyEfor thermal degradation for different cellulosic, hemicellulose, and lignin samples is in the range 36–60, 15–26, and 13–19 kcal/mole, respectively. DTA of all the wood components studied showed an endothermic tendency around 100°C in an atmosphere of flowing nitrogen and stationary air. However, in the presence of flowing oxygen this endothermic effect was absent. In the active pyrolysis temperature range in flowing nitrogen and stationary air atmospheres, thermal degradation of Avicel cellulose occurred via a sharp endothermic and a sharp exothermic process, the endothermic nadir and exothermic peak being at 320° and 360°C, respectively. In the presence of oxygen, combustion of Avicel cellulose occurred via two sharp exothermic processes. DTA studies of different cellulose samples in the presence of air showed that the shape of the curve depends on the sources from which the samples were prepared as well as on the presence of noncellulosic impurities. Potassium xylan recorded a sharp exothermic peak at 290°C in a nitrogen atmosphere, and in a stationary air atmosphere it yielded an additional peak at 410°C, while in the presence of oxygen the curve showed two sharp exothermic peaks. DTA traces of periodate lignin in flowing nitrogen and air were the same and showed two exothermic peaks at 320° and 410°C, while in the presence of oxygen there were two exothermic peaks in the temperature range 200°–500°C.

Oxidation of Fine Powders of Co-precipitated Mn-Zn Ferrites

Oxidation of fine powders of Mn-Zn Ferrites prepared by the ‘co-precipitation’ method has been presented.Differential thermal analysis showed a sharp exothermic peak corresponding to ‘hematite’ precipitation. A temperature of the peak, Tp, depends upon a heating rate φ and a crystallite size, ε, expressed by a simple relation:φ⋅ε∝exp(-Q/kTp), where Q=2.1±0.2eV. The relation indicates that the specific reaction rate is inversely proportional to crystallite size and that its temperature dependence is expressed by an activation process of 2.1eV. The size dependence could be explained by the oxidation of the total surface area and the temperature dependence by cation diffusion in Fe-poor Mn-Zn ferrite. Isothermal oxidation processes were studied by X-ray diffraction analysis and explained qualitatively by the same mechanism as the above relation.

The Accelerating Action of Calcium Thiosulfate on the Hydration of Portland Cement and Comparison with Other Inorganic Salts

Calcium chloride is now regarded as an excellent accelerator to the hydration of portland cement, however, its corrosive character to the steel in reinforced concrete is hardly ignored. Therefore, the accelerators without corrosive character are especially indispensable for concrete practice.In this paper, the accelerating action of calcium thiosulfate discovered or by the authors is investigated, in comparison with other inorganic soluble salts, by means of various methods.The accelerating ability of calcium thiosulfate on the setting of portland cement is very identical to that of calcium chloride. The early development of strength of portland cement increases in proportion to the amount of calcium thiosulfate added to portland cement. The early strength of mortar of portland blastfurnace slag cement (granulated blastfurnace slag 65%, portland cement 35%) is developed near that of ordinary portland cement, when calcium thiosulfate was used at the concentration of 3%. However, the accelerating action of calcium thiosulfate on the development of early strength of portland cement is slightly poorer than that of calcium chloride.The most characteristic of calcium thiosulfate as an accelerator will be that calcium thiosulfate does not give any corrosive action to the steel in concrete. The results of corrosion test showed that the test pieces of mild steel buried in mortar containing respectively 2% and 5% of calcium thiosulfate were scarcely corroded after the mortars were exposed under the atmosphere of outdoors for 6 months. Under a same condition, the test pieces of steel buried in the mortar containing 2% of calcium chloride was markedly corroded after only one month. Calcium nitrate did not show corrosive action. However, calcium nitrate can not be regarded as an accelerator.The heat liberation curves of the hydration of portland cement obtained by the use of a devised adiabatic calorimeter indicated that the accelerating action of inorganic salts on the hydration of portland cement is focused on the hydration of alite phase. The orders of effectiveness of anions and cations as accelerators of the hydration of portland cement are Cl->S2O3-->SO4-->NO3-, Cl->Br->I- and Ca++>Mg++>Na+.The hydration products of tricalcium silicate is CSH (II) when calcium chloride was present, is CSH (I) when gypsum was present and is not evidently identified when calcium thiosulfate was present. DTA curve of the 24 hours hydration product of C3S paste (H2O/C3S:50%) with 3% of calcium chloride showed a sharp exothermic peak at 640°C. The peak showed that the produced CSH (II) in hydration products was converted into β-C2S by heating. In the case of calcium thiosulfate, the continuous two exothermic peaks were obseved in DTA curve. The first peak could not be exactly assigned, however, the second was assigned as the oxidation of calcium sulfite which produced by the decomposition of calcium thiosulfate.

AMORPHOUS IRON OXIDES IN SOILS

SummaryThe identification of amorphous iron oxides in soils has often been based on a sharp exothermic peak at about 320°C. on D.T.A. records. This peak was given by several samples in the present study; in each case it was due to the organic matter present. This was only removed by prolonged treatment with concentrated H2O2, though the exothermic reaction was suppressed by using a nitrogen atmosphere in the furnace. The free iron oxides in these same soils are fully accounted for by the crystalline iron minerals present. It is suggested that previous analyses reporting amorphous iron oxide be viewed with caution.

Reactions in Silica‐Alumina Mixtures

Examination of mixtures of extremely pure silica and alumina shows that the greatest reactivity is not encountered with stoichiometric ratios to form mullite with the formula 3Al2O3. 2SiO2 but rather with the formula 2A12O3‐SiO2, and that reactivity also depends on the crystalline modification of alumina. A sharp exothermic differential thermal peak at 980°C. is attributed to three simultaneous reactions dependent on the silica‐alumina ratio of the mixture: (1) the crystallization of gamma alumina, (2) the crystallization of a hydrogen aluminum spinel (HAl5O5), and (3) the reaction of silica with the hydrogen aluminum spinel to form mullite.

Investigation of the coking properties of coal by vacuum differential thermal analysis

Vacuum differential thermal analysis has been used to study the coking properties of a number of bituminous coals. Samples of 200 mgs were run from room temperature to 600 degrees C at a heating rate of 10 degrees /min.The thermograms show two peaks that can be related to the plastic properties: (1) a strong exothermic peak beginning between 340-440 degrees C and reaching a maximum height between 440-520 degrees C; (2) a small, sharp exothermic peak superimposed on the limb of the larger peak and occurring between 430-500 degrees C. The beginning of No. 1 peak represents the softening temperature of the coal while its summit corresponds to the solidification temperature. No. 2 peak occurs in the temperature zone of maximum fluidity and maximum rate of swelling. The sharpness of this peak appears to be influenced by the fluidity of the coal.The changes that occur in the plastic properties of a coal through blending with other coals can be detected on the thermograms and in the small coke buttons formed in the thermocouple well. For example, by blending a high-volatile A coal with increasing amounts of a low-volatile coal, No. 2 peak occurs at progressively higher temperatures, diminishes in intensity and finally disappears. The coke buttons show changes in their cellular structure and degree of swelling.The thermograms and coke buttons can also be modified by the addition of diluents such as fusain, silica gel, quartz, clay, sulfur, charcoal, various inorganic salts and phenanthrene. The degree to which the thermal curves are affected depends on the type and amount of diluent, as well as the properties of the original coal.