Nanocrystalline Transition Metal Oxides as Catalysts in the Thermal Decomposition of Ammonium Perchlorate

Nanocrystalline transition metal oxides (NTMOs) have been successfully prepared by three different methods: novel quick precipitation method (Cr₂O₃ and Fe₂O₃); surfactant mediated method (CuO), and reduction of metal complexes with hydrazine as reducing agent (Mn₂O₃). The nano particles have been characterized by X‐ray diffraction (XRD) which shows an average particle diameter of 35–54 nm. Their catalytic activity was measured in the thermal decomposition of ammonium perchlorate (AP). AP decomposition undergoes a two step process where the addition of metal oxide nanocrystals led to a shifting of the high temperature decomposition peak toward lower temperature. The kinetics of the thermal decomposition of AP and catalyzed AP has also been evaluated using model fitting and isoconversional method.     

Synthesis of Nanocrystalline TiO2 Particles by Hydrolysis of Titanyl Organic Compounds at Low Temperature

Fourier transform infrared analysis, nuclear magnetic resonance, and thermogravimetric analysis show that most of the solid product prepared from the reaction of Ti(OC₄H₉)₄ and excess (CH₃CO)₂O is a mixture of titanyl organic compounds. Nanocrystalline TiO₂ particles, which include anatase TiO₂, rutile TiO₂, and a mixture of anatase and rutile, can be obtained from hydrolysis of the titanyl organic compounds under normal pressure at 60°C. The particle size, shape, and formation process of the crystals have been studied using X-ray diffraction and transmission electron microscopy. The specific-surface-area data for a rutile TiO₂ sample and the powders obtained after calcination at different temperatures have been measured by the Brunauer–Emmett–Teller method.     

Hydrothermal Synthesis of Weakly Agglomerated Nanocrystalline Scandia-Stabilized Zirconia

A pure cubic phase of weakly agglomerated, nanocrystalline, 8-mol%-Sc₂O₃-stabilized zirconia (8ScSZ) was synthesized by a two-step hydrothermal treatment in the presence of urea: a stock solution of metal nitrates and urea was heated at 80°C for 48 h and then at 180°C for 72 h. The omission of the first low-temperature treatment resulted in more monoclinic phase. The effects of urea concentration and calcining temperature on the crystallization of the as-synthesized nanocrystalline 8ScSZ are also discussed.     

Dielectric and Ferroelectric Properties of Nanocrystalline Bi2Ti2O7 Prepared by a Metallorganic Decomposition Method

Nanopowders of Bi₂Ti₂O₇ were synthesized by a metallorganic decomposition (MOD) technique. Pure Bi₂Ti₂O₇ nanocrystals formed after annealing at 550°C for 5 min. X-ray patterns show that Bi₂₀TiO₃₂ is a metastable phase during Bi₂Ti₂O₇ formation. It was found that there were two peaks in the curves of the dielectric response as a function of temperature for pressed nanocrystalline Bi₂Ti₂O₇ samples. The Curie temperature decreases with decrease of grain size whereas the ferroelectric-ferroelectric phase transition temperature increases. The hysteresis loops observed also suggest that Bi₂Ti₂O₇ might belong to a ferroelectric material.     

Tetragonal Nanocrystalline Barium Titanate Powder: Preparation, Characterization, and Dielectric Properties

Nanocrystalline tetragonal-BaTiO₃ powder was prepared using a hydrothermal method, under moderate conditions, and with a high precursor concentration. Characterization via X-ray diffractometry and differential scanning calorimetry confirmed that the average particle size and tetragonal content of the prepared powder were 70 nm and 80%, respectively. The sintered sample made from the prepared powder had a room-temperature dielectric constant of 6900, which was high for BaTiO₃.     

Nanocrystalline Zirconia Surface-Doped with Alumina: Chemical Vapor Synthesis, Characterization, and Properties

Nanocrystalline zirconia with a grain size of ∼5 nm was surface-doped with 3 and 30 mol% alumina by chemical vapor synthesis using two sequential hot-wall reactors. The powders were characterized by high-resolution transmission electron microscopy, X-ray diffractometry (XRD), and nitrogen adsorption. Aqueous dispersions were studied by zeta-potential measurements and photocorrelation spectroscopy. Green and sintered ceramic bodies were investigated by high-resolution scanning electron microscopy, XRD, and nitrogen adsorption. Zirconia surface-doped with 3 mol% of alumina displays substantial changes in dispersability and sinterability compared with pure nanocrystalline zirconia.     

Nanocrystalline Maghemite (γ-Fe2O3) in Silica by Mechanical Activation of Precursors

γ-Fe₂O₃ nanocrystallites dispersed in an amorphous silica matrix have been successfully prepared for the first time by mechanical activation of a chemistry-derived precursor at room temperature. The initial 10 h of mechanical activation triggered the formation of nanocrystallites of Fe₃O₄ in a highly activated matrix. Increasing the mechanical-activation time led to a phase transformation from Fe₃O₄ to γ-Fe₂O₃. The γ-Fe₂O₃ phase was well established after mechanical activation of the precursor for 30 h. Further increasing the mechanical-activation time to 40 h induced the formation of α-Fe₂O₃. The mechanical-activation-grown γ-Fe₂O₃ nanocrystallites were ∼10–12 nm in size and well dispersed in the silica matrix, as observed using TEM. They demonstrated superparamagnetic behavior at room temperature when measured using a Mössbauer spectrometer and a vibrating sample magnetometer (VSM). In addition, the γ-Fe₂O₃ derived from 30 h of mechanical activation exhibited a value of saturation magnetization as high as 62.6 emu/g.     

Effect of LiCl on the Rate of Calcite Decomposition

The rate of decomposition of calcite powder in vacuum was measured by the Langmuir and Knudsen methods. Measured pressures in effusion cells in the absence of additives approach the equilibrium value. The Langmuir experiments gave an apparent equilibrium CO₂ pressure of =3×10⁻⁴ times the known pressure for the reaction. It is suggested that the CO₂ is in equilibrium with the calcite but the CaO is not. In Langmuir experiments, 10 wt% LiCl does not increase the initial decomposition rate. In the Knudsen cells, 10 wt% LiCl can increase the initial rate by >2 times. LiCl reduces the surface area of the product CaO under either condition from ≥30 to ∼1 m²/g. Probably an LiCl-CaO eutectic provides a solution path for CaO recrystallization and, at high CO₂ pressures, also for CaCO₃ decomposition.     

Raman Spectroscopy of Nanocrystalline Ceria and Zirconia Thin Films

The results of Raman-scattering studies of nanocrystalline CeO₂ and ZrO₂:16% Y (YSZ) thin films are presented. The relationship between the lattice disorder and the form of the Raman spectra is discussed and correlated with the microstructure. It is shown that the Raman line shape results from phonon confinement and spatial correlation effects and yields information about the material nonstoichiometry level.     

In Situ Formation of Metal Nanoparticles on Ceramic Surface

A new preparation method for nanocomposites is proposed in which nanometer-scale nickel particles are precipitated homogeneously on the magnesia surface. Our approach is based on a conventional method that uses a reaction of difference in oxide stability in reductive atmosphere. We found that aluminum doping of the material precursor enabled preferential precipitation on the oxide substrate. We also have applied the preparation method for the nickel-copper/magnesia system. An addition of copper was effective for reducing the particle diameter. We found that morphology created by the method could be controlled by varying precursor composition.     

Thermal Decomposition of Energetic Materials 86. Cryogel Synthesis of Nanocrystalline CL‐20 Coated with Cured Nitrocellulose

An unusual energetic composite, in which spherical nano‐dimensional particles of CL‐20 were uniformly coated with HDI‐cross‐linked nitrocellulose, was synthesized by the sol‐gel to cryogel method. Up to 90% solid loading was achieved. The particle size of CL‐20 was determined to be in the range of 20–200 nm by transmission electron microscopy, atomic force microscopy, and X‐ray powder diffraction. The decomposition characteristics of the composite were investigated by DSC and T‐jump/FTIR spectroscopy. The decomposition properties were controlled mostly by nitrocellulose until the percentage of CL‐20 was well above 50%. The drop weight impact sensitivity of the cryogels was essentially independent of the composition.     

纳米碳酸钙及其在塑料高性能化改性中的应用

介绍了不同晶形的纳米碳酸钙的特性、制备方法及其表面处理技术，简述了各种纳米碳酸钙改性塑料的制备方法，对纳米碳酸钙在聚氯乙烯、聚乙烯、聚丙烯等塑料的高性能化改性中的应用进行了综述，本文还指出纳米碳酸钙应用于塑料改性中存在的问题。     

纳米碳酸钙在橡胶高性能化改性中的应用

概述了纳米碳酸钙的特点及制备方法，介绍了它在橡胶中的分散工艺、补强填充机理，以及其在橡胶高性能化改性中的应用。     

Single-Source Sol-Gel Synthesis of Nanocrystalline ZnAl2O4: Structural and Optical Properties

Nanometer-sized zinc aluminate (ZnAl₂O₄) particles were synthesized from heterometal alkoxides, [ZnAl₂(OR)₈], possessing an ideal cation stoichiometry for the ZnAl₂O₄ spinel. ZnAl₂O₄ is formed at 400°C, which is the lowest temperature reported for the formation of monophasic ZnAl₂O₄. ²⁷Al magic-angle spinning nuclear magnetic resonance spectroscopy revealed that ZnAl₂O₄ possesses an inverse structure at <900°C, while the normal spinel phase is observed at higher temperatures. The homogeneity of the in-depth composition and Zn:Al stoichiometry (1:2) was confirmed by electron spectroscopy for chemical analysis. Evaluation of the valence-band spectra of ZnAl₂O₄ and ZnS suggested that the hybridization of O 2 p and Zn 3 d orbitals is responsible for lowering the bandgap in the latter. The average crystallite size showed an exponential relationship to the calcination temperature (X-ray diffractometry and transmission electron microscopy data). The optical spectra of different spinel powders (average particle sizes, 20–250 nm) showed that the absorption edge exhibits a blue shift as particle size decreases.     

Thermal Decomposition of Aminonitrobenzodifuroxan

In this study, the thermal decomposition properties of aminonitrobenzodifuroxan are studied using a differential scanning calorimeter (DSC), a thermogravimeter (TG), an X‐ray diffractometer, a mass spectrometer (MS), and a Fourier transform infrared spectrometer (FTIR). The results demonstrate that aminonitrobenzodifuroxan undergoes thermal decomposition in the solid state. Under elevated temperatures, the decomposition primarily involves two steps: separation of nitro group and ring‐scission of the furoxan circles at 198.1 °C, and decomposition of the relatively stable residues (benzofuroxan circle) at 199.1 °C. Moreover, it is found that among the products, nitrogen dioxide undergoes oxidation and catalysis on the host molecule during the whole decomposition. Based on Kissinger and Ozawa functions, we deduce that the activation energies of these two reactions are 167.68 and 204.55 kJ mol⁻¹, respectively. The released energy (ΔH) of CL‐18 is −1781.8 J g⁻¹.     

Nonisothermal Synthesis of Yttria-Stabilized Zirconia Nanopowder through Oxalate Processing: I, Characteristics of Y-Zr Oxalate Synthesis and Its Decomposition

To obtain powder with a composition of 3 mol% Y₂O₃–97 mol% ZrO₂, a process of Y-Zr oxalate powder production has been optimized, to produce an oxalate with minimal particle size. The methodology of the nonisothermal decomposition of Y-Zr oxalate has been explained. Characteristics of the nonisothermal decomposition of different oxalates have been studied. Nanocrystalline Y₂O₃-stabilized ZrO₂ (YSZ) powder with a narrow size distribution of primary particles and aggregates was produced. The zirconia powder that was obtained from the smallest oxalate powder via nonisothermal decomposition had a particle size of 8–10 nm. The YSZ powder was weakly aggregated, with a narrow aggregate-size distribution of 70–90 nm.     

Chemical Kinetics of the Thermal Decomposition of NTO

The kinetics of thermal decomposition of 3‐nitro‐2,4‐dihydro‐3H‐1,2,4‐triazol‐5‐one (NTO) in the temperature interval from 200 °C to 260 °C was investigated using a glass Bourdon gauge. The overall decomposition reaction includes two distinct stages: the fast first‐order decomposition and the subsequent autocatalytic reaction. The importance of the first stage increases with increasing decomposition temperature and decreasing loading density of the Bourdon gauge (m/V). A period of preliminary heating, at a lower temperature, strongly influences the autocatalytic stage when the decomposition is carried out at a higher temperature. In the temperature domain 200–220 °C, the Arrhenius constants of the decomposition reaction are found to be close to the values usually observed for nitrocompounds: E=173 kJ/mol and log₁₀ k≈12.5 (s⁻¹). It is shown that a simple model of NTO decomposition based on an autocatalytic reaction of the m‐th order can describe the course of the decomposition at high temperature but the m number appears to be excessively high, up to 4. A new model of the decomposition is developed, including an initial monomolecular reaction, decomposition of the crystalline substance, and an autocatalytic reaction of NTO dissolved in liquid decomposition products. This model gives the common order of autocatalysis, m=1.     

Thermal Decomposition of Pentaerythritol Tetranitrate

A chemical kinetic model for the thermal decomposition of the solid high explosive pentaerythritol tetranitrate (PETN) is developed for prediction of times to thermal explosion using the Chemical TOPAZ heat transfer computer code. The model is based on times to thermal explosion measured in a new One Dimensional Time to Explosion (ODTX) apparatus. ODTX experiments are reported for pure PETN and for Semtex 1A. The pure PETN results are accurately modeled using a four reaction decomposition process in which an autocatalytic process produces intermediate reaction product gases, which subsequently react in a second order gas phase process to produce the final reaction products. Semtex 1A exhibits longer times to explosion than PETN at low temperatures, indicating that its endothermic binder decomposition absorbs heat produced by PETN decomposition. This binder reaction is modeled as a first order endothermic process. Three experiments on 5.08 cm diameter unconfined cylinders of PETN ramp heated to explosion at different rates are reported. The PETN model accurately predicts the thermocouple records and explosion times for these unconfined experiments in which only intermediate gaseous products can form.