Thermal decomposition of ammonium uranates precipitated from uranyl nitrate solution with ammonium hydroxide

Thermal decomposition of ammonium uranates precipitated from uranyl nitrate solutions on addition of aqueous ammonium hydroxide under various conditions has been examined by thermogravimetry (TG), differential thermal analysis (DTA), infrared spectroscopy and X-ray diffraction study. The TG curves of all precipitates show the weight-loss corresponding to the calculated value as UO₃·NH₃·H₂O. The DTA curves of the precipitates give the endotherms at about 130, 210 and 590 °C and the exotherms at 340–420 °C. As a result, it is found that ammonium uranates thermally decompose to amorphous UO₃ at about 400 °C, and transform to U₃O₈ via β-UO₃.     

Sequential determination of urea and HMTA in the process solution of sol-gel route of advanced nuclear fuel fabrication

Determinations of hexamethylene tetramine (HMTA) and urea in the process solutions are required to optimize their concentrations for obtaining high quality ceramic oxide microspheres, for monitoring the washing procedure and for their subsequent recovery, recycling or waste disposal. Determination of urea in the feed solution by conventional procedures is difficult as it contains HMTA. It is more so in the effluent as it contains hydrolytic products like formaldehyde, methylol derivatives of urea, ammonium nitrate and ammonium hydroxide used for washing the gel microspheres. This work describes a derivative potentiometric method using a microprocessor-based autotitrator. Peaks on the first derivative of the titration plot corresponded to constituents of different basicities. Urea was selectively hydrolyzed at room temperature by the catalytic action of urease enzyme leaving HMTA unaffected. Ammonium hydroxide and ammonium bicarbonate produced from urea and HMTA were sequentially titrated for the analysis of the feed solution to obtain the three corresponding peaks respectively. Two separate titrations were required for the analysis of the effluent solution, which contained free ammonia also. One aliquot was first titrated directly without adding urease (for free ammonia and HMTA) and another aliquot was titrated after treatment with urease. The end points due to the ammonia used for washing and that from urea hydrolysis merged resulting in the appearance of three peaks again. Using this sequential method the relative standard deviations were found to be 0.81% and 1.38% for urea and HMTA, respectively, in eight determinations when the aliquots contained 50 to 75 mg of urea and 75 to 125 mg of HMTA. Feed and effluent solutions of the process stream were analyzed.     

Preparation and thermal decomposition of indium hydroxide

Summary Indium hydroxides were prepared by the mixing of aqueous indium nitrate solution with sodium or ammonium hydroxide solutions under various conditions. The thermal decomposition of the resulting materials was examined by thermogravimetry, differential thermal analysis, X-ray diffraction study and infrared spectroscopy. It has been found that sodium hydroxide solution is more suitable than the addition of ammonium hydroxide solution to prepare indium hydroxide in well crystallization; the thermal decomposition of indium hydroxide, in which the composition is In(OH)₃·xH₂O where x￡2, proceeds according to the following process: In(OH)₃·xH₂O?cubic In(OH)₃?cubic In₂O₃     

Thermal decomposition of ammonium cerous carbonate

The thermal decomposition of ammonium ceryl(III) carbonate (ACeC) [NH₄CeO(CO₃)] was investigated by thermogravimetry, differential thermal analysis and X-ray diffraction. The results showed three endothermic stages of decomposition, each involving a loss in weight. The first stage, at 65.5 °C, is characteristic of the removal of adsorbed water, the second stage, at 214.8 °C, is associated with ammonia release, and the third stage, at 263.6 °C, relates to the removal of carbon dioxide.     

Preparation of ammonium uranate in the recovery treatment of non-fissioned uranium

A number of ammonium uranates have been obtained, all of them different in their X-ray diffraction patterns. This contribution attempts to demonstrate that different series of compounds can be produced depending on the preparation method. The ammonium uranates were precipitated from solutions of uranyl nitrate using gaseous ammonium to determine the effect of precipitation pH on their composition and morphology.     

Thermal analysis of ammonium,mono-, di- and triethanolammonium alginates

Thermal behaviour of ammonium (NH₄alg), mono- (MEAalg), di- (DEAalg) and triethanolammonium (TEAalg) salts of alginic acid (Halg) was investigated by thermogravimetry (TG) and differential scanning calorimetry (DSC). Salts were prepared by the direct reaction of alginic acid with the ammonium hydroxide and with the respective ethanolamines. After preparation the compounds were lyophilized during 24 h and characterized by FTIR spectroscopy and elemental analysis (C, H and N). Under air the compounds exhibited three successive thermal decomposition steps: dehydration, decomposition of the polymeric matrix and finally, burning of carbonaceous residue. Under nitrogen two steps (dehydration and decomposition) were observed. The stability order of this series of compounds was: TEAalg this series of compounds was: TEAalg<DEAalg<NH₄alg<Halg≈MEAalg. DSC curves between –50 and 150°C did not show any thermal events suggesting that after lyophilization probably non-freezing type water is present in the system.     

Development of a reconversion method for uranyl nitrate to oxide in the reconversion step of reprocessing of irradiated fuel

Ammonium uranyl carbonate (AUC) precipitation is developed for the conversion of uranyl nitrate to oxide in the uranium reconversion step of reprocessing of irradiated fuel by the addition of ammonium carbonate salt. Different precipitation conditions of AUC are studied. The solubility of AUC as a function of uranium concentration in the feed at different temperatures using ammonium carbonate salt as precipitant is studied. This study indicates that 95-99.8% of uranium is recovered as AUC by precipitating 5-125 g/l of uranium with loss of uranium (250-10 ppm) in the filtrate by adding ammonium carbonate salt. It is also observed that the solubility of AUC increased as the concentration of uranium decreased. Thermal decomposition is carried out by thermogravimetry/differential thermal analysis (TG/DTA) and evolved gas analysis-mass spectrometry (EGA-MS) to find out AUC decomposition and gases evolved during decomposition. Studies are also carried out to characterize AUC by using X-ray diffraction (XRD). The data show that AUC obtained by the above conditions is very much consistent with published information.     

两步沉淀法制备耐高温和优异还原性能CeO2材料

分别采用两种沉淀方法制备了CeO2:以传统的氨水为沉淀剂,在氨水沉淀法前引入碳铵沉淀步骤(两步沉淀法)。采用热重-差热(TG-DTA)、傅里叶变换红外(FTIR)、X光电子能谱(XPS)等手段对沉淀及其分解过程进行了研究。结果表明,在两步沉淀法中的第一步,碳酸物种为主要沉淀物种,而在第二步中被氢氧根取代。X射线衍射(XRD)和透射电子显微镜(TEM)结果表明,两步沉淀法生成的沉淀颗粒粒径更大。通过两步沉淀法制备的CeO2与氨水沉淀相比具有更好的抗高温老化性能和还原性能。经过900℃焙烧3 h后,仍然具有25 m2.g-1和0.11 cm3.g-1的比表面和孔容。     

The influence of porosity of ammonium perchlorate (AP) on the thermomechanical and thermal properties of the AP/polyvinylchloride (PVC) composite propellants

The influence of porous ammonium perchlorate (P_OAP) on the thermomechanical and combustion behavior of solid rocket propellants based on polyvinylchloride binder has been investigated. Differential scanning calorimetry, differential thermogravimetry, dynamic mechanical thermal analysis, and scanning electronic microscopy measurements were used for thermomechanical and thermal decomposition properties assessment. The results obtained indicate that lower glass transitions of the propellants and catalytic effect of combustion are obtained with P_OAP.     

Homogeneous precipitation of mullite precursors

The synthesis of mullite powders or gels from an aqueous precursor solution has been studied. The starting solution is made by hydrolyzing TEOS into an aqueous solution of aluminium nitrate. When this solution is sprayed into a solution of ammonia in isopropanol, a nearly monophasic mullite precursor is obtained, whereas if this precipitation is made into an aqueous solution of ammonia, or ammonium carbonate, a diphasic colloidal precipitate is formed. By a slow and homogeneous precipitation in aqueous solution, induced by in-situ generation of ammonia by thermal hydrolysis of urea, a monophasic gel is also obtained. The samples have been studied by DTA, DSC, TGA, XRD and dilatometry.     

Study of some volatile compounds evolved from the thermal decomposition of atenolol

Simultaneous thermogravimetry–differential scanning calorimetry (TG–DSC) and coupled thermogravimetry–infrared spectroscopy (TG–FTIR) analyses were used to study the antihypertensive drug atenolol. The TG–DSC curves provided information concerning the thermal stability and decomposition profiles of the compound. From the TG–FTIR coupled techniques, it was possible to identify ammonia and isopropylamine as possible volatile compounds released during the thermal decomposition of the drug.     

Influences of aging on thermal decomposition mechanism of high performance oxidizer ammonium dinitramide

Ammonium dinitramide (ADN) is one of the several promising new solid propellant oxidizers. ADN is of interest because its oxygen balance and energy content are high, and it also halogen-free. One of the most important characteristics of a propellant oxidizer, however, is stability and ADN is known to degrade to ammonium nitrate (AN) during storage, which will affect its performance. This study focused on the effects of aging on the thermal decomposition mechanism of ADN. The thermal behaviors of ADN and ADN/AN mixtures were studied, as were the gases evolved during their decomposition, using differential scanning calorimetry (DSC), thermogravimetry–differential thermal analysis-infrared spectrometry (TG–DTA-IR), and thermogravimetry–differential thermal analysis-mass spectrometry (TG–DTA-MS). The results of these analyses demonstrated that the decomposition of ADN occurs via a series of distinct stages in the condensed phase. The gases evolved from ADN decomposition were N₂O, NO₂, N₂, and H₂O. In contrast, ADN mixed with AN (to simulate aging) did not exhibit the same initial reaction. We conclude that aging inhibits early stage, low temperature decomposition reactions of ADN. Two possible reasons were proposed, these being either a decrease in the acidity of the material due to the presence of AN, or inhibition of the acidic dissociation of dinitramic acid by NO ₃ ^? .     

Thermal behavior of glutamic acid and its sodium,lithium and ammonium salts

Glutamic acid (H2_glu) and its lithium, sodium and ammonium monosalts were submitted to thermal analysis using thermogravimetry (TG) and differential thermal analysis (DTA). The main goal of these studies was to compare the relative thermal stability and to evaluate the effect of the counter ion in the thermal decomposition pathways. Salts were obtained by direct neutralization of the purified acid with LiOH, NaOH or NH₄OH and were characterized by elemental analysis (C, H and N) and IR spectroscopy. Decomposition occurred after conversion to the pyroglutamic acid or the respective pyroglutamates and ammonium salt loosing NH₃ being converted to H₂glu before decomposition.     

Thermal decomposition kinetics of potassium iodate

The thermal decomposition of potassium iodate (KIO₃) has been studied by both non-isothermal and isothermal thermogravimetry (TG). The non-isothermal simultaneous TG–differential thermal analysis (DTA) of the thermal decomposition of KIO₃ was carried out in nitrogen atmosphere at different heating rates. The isothermal decomposition of KIO₃ was studied using TG at different temperatures in the range 790–805 K in nitrogen atmosphere. The theoretical and experimental mass loss data are in good agreement for the thermal decomposition of KIO₃. The non-isothermal decomposition of KIO₃ was subjected to kinetic analyses by model-free approach, which is based on the isoconversional principle. The isothermal decomposition of KIO₃ was subjected to both conventional (model fitting) and model-free (isoconversional) methods. It has been observed that the activation energy values obtained from all these methods agree well. Isothermal model fitting analysis shows that the thermal decomposition kinetics of KIO₃ can be best described by the contracting cube equation.     

Study of nano‐Cu/SiO2 catalysts for highly selective hydrogenation of acetophenone

Hydrogenation of acetophenone over nano‐Cu/SiO₂ catalysts was investigated. The catalysts, prepared by a liquid precipitation method using various precipitating agents, were characterized using low‐temperature nitrogen adsorption, X‐ray diffraction, temperature‐programmed desorption of ammonia, hydrogen temperature‐programmed reduction, transmission electron microscopy and X‐ray photoelectron spectroscopy. It was found that the catalysts prepared by a homogeneous precipitation method had better activity and stability than those prepared by a co‐precipitation method. The catalyst prepared using urea as precipitating agent had well‐dispersed copper species, high surface area and abundant pore structure. The catalytic performance and mechanism of the Cu/SiO₂ catalysts were further studied. It was found that the activity and stability of the catalysts could be improved by adjusting the proportion of Cu⁺/(Cu⁺ + Cu⁰). The sample prepared using urea as precipitating agent presented higher activity and selectivity. Also, the catalyst prepared using urea maintained a high catalytic performance while being continuously used for 150 h under the optimal reaction conditions.     

Controllable synthesis of zinc-substituted alpha- and beta-nickel hydroxide nanostructures and their collective intrinsic properties

A new controllable homogeneous precipitation approach has been developed to synthesize zinc-substituted nickel hydroxide nanostructures with different Zn contents from a zinc nanostructured reactant. As typical layered double hydroxides (LDHs), zinc-substituted nickel hydroxide nanostructures can be formulated as NiZnx(Cl)y(OH)2(1+x)-y.z H2O (x=0.34-0.89, y=0-0.24, z=0-1.36). The structure and morphology of zinc-substituted nickel hydroxide nanostructures can be systematically controlled by adjustment of the zinc content. The effects of temperature and the amounts of ammonia and zinc nanostructured precursor on the reaction were systematically investigated. In our new method, although zinc-substituted alpha-and beta-nickel hydroxides have the typical 3D flowerlike architecture and stacks-of-pancakes nanostructures, respectively, their growth processes are different from those previously reported. A coordinative homogeneous precipitation mechanism is proposed to explain the formation process of zinc-substituted nickel hydroxide nanostructures. The zinc-substituted nickel hydroxide nanostructures exhibit some interesting intrinsic properties, and changing the zinc content can effectively tune their optical, magnetic, and electrical properties.     

Gravimetric analysis of tantalocolumbites by precipitation from homogeneous solution

An analysis of complex tantalocolumbites has been carried out by precipitation from homogeneous solutions. A homogeneous precipitation of tungsten, titanium, tantalum and niobium by thermal decomposition of the soluble peroxytungstates, described in previous papers, is used. Corrections for incomplete precipitation and coprecipitation phenomena are applied on the basis of the experimentally found values. Silicon and tin are separated by volatilisation as fluoride and iodide, respectively. Iron is extracted by means of isopropyl ether and the rare earth metals are precipitated homogeneously from an oxalate solution. Manganese is precipitated as the ammonium phosphate. The results are in good agreement with an independent method, the standard deviations being within 1 % for the major constituents.     

Chemistry and thermal decomposition of trinitropyrazoles

New energetic compounds-3,4,5-1H-trinitropyrazole (TNP), 1-methyl-3,4,5-1H-trinitropyrazole (MTNP) and ammonium 3,4,5-1H-TNP have been synthesized and characterized by thermal analysis. These new compounds can be considered as promising since the high heat of formation for them. To estimate the process of their thermal decomposition, the original technique for computer simulation was used. We generated the models for the mechanisms of thermal decay of synthesized compounds which allowed obtaining comprehensive spectrum of transformations of intermediates on the way to the final products of thermolysis. The preferred pathways were determined based on the results of activation energy (E _a) calculations (DFT 6-311⁺⁺G** method) of thermal decay reactions for each generated pathways. The thermal decomposition has been studied also experimentally by thermogravimetry (TG) and differential scanning calorimetry. Kinetic parameters of thermolysis were evaluated by model-free and -fitting methods using TG data. Model-free method has given not reliable data for TNP and MTNP compounds, whereas model-fitting yields kinetic equations with the good correlation with experimental TG data.     

Thermal decomposition of ammonium fluoromanganates(III)

The thermal decomposition of ammonium fluoromanganates (III) has been investigated in air and argon by simultaneous thermogravimetry and differential thermal analysis. Chemical analysis, X-ray powder data, and infrared spectra have been employed to characterise the intermediate and final products. The thermal decomposition can be described by the sequence (NH₄₃MnF₆ → (NH₄)₂MnF₅ → NH₄MnF₄ → MnF₂. Although penta- and tetra-fluoromanganates are well-defined compounds, the intermediate states could not be separated. In addition, a high temperature form of ammonium hexafluoromanganate has been observed.     

纳米阻燃氢氧化镁/聚氧化乙烯复合聚合物电解质

合成了纳米氢氧化镁作为聚氧化乙烯(PEO)基聚合物电解质的增塑剂和阻燃剂,并对其进行X射线衍射(XRD)、透射电子显微镜(TEM)和热重(TG)分析研究.制得的氢氧化镁为片状六方晶体,尺寸在50-80nm之间,纳米氢氧化镁在340℃时开始热分解.对纳米氢氧化镁/PEO复合聚合物电解质的电化学研究结果显示:纳米氢氧化镁/PEO复合聚合物电解质的离子电导率随着添加纳米氢氧化镁的质量分数的增加先增大后减小,其在5%-10%之间时,复合聚合物电解质的离子电导率达到最大值.纳米氢氧化镁的添加使复合聚合物电解质的阳极氧化电位有一定程度的提高,纳米氢氧化镁具有改善PEO阳极抗氧化能力的作用.