La–Si–O–N glasses: Part I. Extension of the glass forming region

The nitrogen-rich part of the glass forming region in the La–Si–O–N system has been the subject of a comprehensive study. Glasses were prepared by heating powder mixtures of La metal, Si₃N₄ and SiO₂ in a nitrogen atmosphere at 1650–1800 °C. By this new synthesis route, glasses containing up to 68 e/o of N and 62 e/o of La were prepared, showing that the glass forming region is significantly larger than previously reported. The glasses were characterized by elemental analysis, differential thermal analysis, X-ray powder diffraction, and scanning electron microscopy. They were found to be X-ray amorphous and homogenous, with the majority of them containing small amounts of crystalline La silicides and elemental Si. Glass transition temperatures (T_g) were found to vary between 900 and 1100 °C and crystallization to occur typically 120 °C above T_g. The forming of the glasses was investigated by characterizing samples taken out at various steps of the heating cycle. The results indicate that the glass formation is strongly dependent on reaction kinetics. A strong exothermal reaction occurs at temperatures 900–1100 °C, leading to the formation of assemblies of amorphous and crystalline (oxy)nitride phases that melt upon further heating at 1650–1800 °C.     

The Thermal Conductivity of Polymer‐Derived Amorphous Si–O–C Compounds and Nano‐Composites

Silicon oxycarbide glasses can be produced over a range of Si–O–C compositions by the controlled pyrolysis of polymer precursors. We present measurements of the thermal conductivity of a silicon oxycarbide glass after two different heat treatments and two Si–O–C nano‐composites, hot‐pressed at 1600°C, up to 1000°C and compare them to fused silica, amorphous carbon, and SiC. The temperature dependence of their thermal conductivities is similar to other amorphous materials. The presence of low volume fractions of nanoparticles of hafnia (4.5 v/o) or zirconia (7.4 v/o) dispersed within the amorphous matrix only modifies the conductivity slightly, consistent with a simple Maxwell model, and does not affect the temperature dependence of the thermal conductivity above room temperature.     

Salts of Tetrazolone – Synthesis and Properties of Insensitive Energetic Materials

Tetrazolone (5‐oxotetrazole, 1 ) is formed by diazotization of 5‐aminotetrazole in the presence of CuSO₄. Nitrogen‐rich salts such as guanidinium ( 2 ), 1‐aminoguanidinium ( 3 ), 1,3‐diamino‐guanidinium ( 4 ), 1,3,5‐triamino‐guanidinium ( 5 ), ammonium ( 6 ), hydrazinium ( 7 ) and the hydroxylammonium ( 8 ) salts of tetrazolone were prepared by facile deprotonation or metathesis reactions. All compounds were characterized by single‐crystal X‐ray diffraction, vibrational spectroscopy (IR and Raman), multinuclear NMR spectroscopy, elemental analysis and DSC measurements. The heats of formation of 2–8 were calculated using the atomization method based on CBS‐4M enthalpies. With these values and the experimental (X‐ray) densities several detonation parameters such as the detonation pressure, velocity, energy and temperature were computed using the EXPLO5 code (V.5.04). In addition, the sensitivities towards impact, friction and electrical discharge were tested using the BAM drop hammer and friction tester as well as a small scale electrical discharge device.     

Identification of the Zn Substitution Sites in La–Zn Substituted SrAl12O19 from 27Al Solid‐State NMR Studies

The La–Zn substituted hexagonal strontium aluminate, Sr_1?xLa_xAl_12?xZn_xO₁₉, with the magnetoplumbite structure and having five different coordination environments for Al with different symmetries, is investigated using ²⁷Al solid‐state NMR to get detailed information on the sites of substitution of Zn and the associated changes in the local coordination environments of Al. The objective of the study was to get information on the local structural variations in the isostructural La–Co substituted strontium ferrite, Sr_1?xLa_xFe_12?xCo_xO₁₉, showing enhanced magnetic performance on substitution. The NMR studies on the aluminate give direct evidence for the sites of substitution and the changes in the local coordination environments. It is found that Zn is substituted at the 2a and 4f₂ AlO₆ octahedral sites. However, an interesting observation from the NMR studies is the stabilization of the Al site occupancy at the penta‐coordinated 2b site over the distorted tetrahedral 4eAl site, without any substitution at these sites. Large changes in the quadrupolar coupling constant of the 2a and 4e sites are observed between x = 0.2 and 0.3, corresponding to the compositional region showing higher performance in the case of Sr_1?xLa_xFe_12?xCo_xO₁₉, indicating the role of distortion of local coordination environments on suitable substitution in controlling the performance parameters.     

Large Electromechanical Response in Lead‐Free La‐Doped BNKT–BST Piezoelectric Ceramics

Piezoceramics 0.99[(Bi_0.5Na_0.4K_0.1)_1?xLa_xTiO₃]?0.01[Ba_0.7Sr_0.3TiO₃] (BNKT–BST–La_x, x = 0–0.030) were synthesized using a conventional solid‐state reaction method. X‐ray diffraction revealed a phase transition from a tetragonal to cubic phase at x ≥ 0.005. The maximum dielectric constant as well as the depolarization temperature (T_d) decreased with increasing La content. La addition interrupted the polarization and strain hysteresis loops and demonstrates that the ferroelectric order of the BNKT–BST ceramics lead to a reduction in the remnant polarization and coercive field. However, the destabilization of the ferroelectric order is accompanied by a significant increase in the unipolar strain which is highest at x = 0.020 with a value of ~0.39% and corresponding normalized strain, d^*₃₃ (= S_max/E_max) of 650 pm/V. It was observed that the unipolar strain of x = 0.020 is very temperature insensitive up to 125°C, even at 125°C the d^*₃₃ is as high as ~431 pm/V. Moreover, an electric‐field‐dependent XRD was conducted to identify the main source of the high strain and a recoverable transformation from cubic to a rhombohedral–tetragonal mixed phase was observed. The recoverable field‐induced phase transformation is suggested to be the main cause for the obtained large strain at x = 0.020 in the BNKT–BST–La_x ceramics.     

Laser‐induced Deflagration for the Characterization of Energetic Materials

Standard propellant and detonation tests typically performed to characterize the performance of energetic materials require large quantities of material (at least tens of grams) and can be expensive and time‐consuming. This work introduces a method for characterizing the deflagration of energetic materials in a laboratory setting, using only 15–20 mg of energetic material. Temperature, energy release and emission signatures were measured and analyzed for the laser‐induced deflagration of 8 different conventional military explosives. Graphite nanoparticles and micron‐sized aluminum powder were added to the explosive compositions to investigate their effect on the emission signatures. A high‐speed color camera recorded the deflagration events and was utilized as a full‐color pyrometer to calculate the average temperatures and image hotspots; the temperatures maps were compared to those measured by conventional two‐color pyrometry. The laboratory‐scale method presented here can be applied to novel energetic materials under development that may be available only in limited quantities to evaluate their potential as propellants or reduced emission signature explosives prior to scale‐up.     

Optical Property of Dy3+‐and Ce3+‐Doped Si–B–Na–Sr Glasses

Novel Dy³⁺ and Ce³⁺ doped Si–B–Na–Sr (SBNS) glasses were synthesized by melt‐quenching technique. Excited by 327 nm, the 0.5Dy³⁺‐and 0.5Ce³⁺‐doped SBNS exhibits white emission with Commission Internationale de L'Eclairage coordinates of (0.308, 0.280). Basic optical characterizations have been performed by measuring the absorption and emission spectra and calculating Judd–Ofelt intensity parameters, radiative probability, luminescence branching ratio, cross sections, and effective bandwidth. The Judd–Ofelt parameters Ω₂, Ω₄, and Ω₆ indicate a high asymmetrical environment and covalent environment in the optical glass. The emission color of Ce³⁺ and Dy³⁺ codoped transparent glass can be tuned from blue to white through energy transfer from Ce³⁺ to Dy³⁺ ions. The resulting glass may have potential application in white‐light‐emitting source.     

Iodine Pentoxide Nano‐rods for High Density Energetic Materials

The iodine pentoxide is one of the most advanced oxidizers for nanostructured energetic formulations among the thermites due to the highest energy release per volume 25.7 kJ cm⁻³. The size and shape of iodine pentoxide particles have a strong impact on the pressurization rates during the reaction. Although micro‐sized iodine pentoxide particles are commercially available, nano‐sized particles, which are desired for various nano‐energetic applications, are not available on the market. Conventional wet chemical methods are unable to produce iodine pentoxide nanoparticles due to high solubility in water. In this study, we demonstrate fabrication of iodine pentoxide nano‐rods by high energy mechanical treatment of micro‐sized I₂O₅ particles. Tuning the energy dose in high‐energy ball milling is allowing to produce I₂O₅ nano‐rods with diameter of 50–100 nm and length of 300–600 nm. The produced nano‐rods exhibited 10 % smaller decomposition energy compared to the precursor of micro particles. The experiments showed that the nano‐energetic materials prepared with nano‐sized I₂O₅ rods have pressure discharge value of 43.4 MPa g⁻¹ which is two times higher than using commercial iodine pentoxide particles.     

Near‐Infrared and Visible Absorption Spectroscopy of Nano‐Energetic Materials Containing Aluminum and Boron

Near‐infrared and visible absorption spectra are measured for nano‐energetic materials consisting of thirteen different types of Al or B nano‐particles in the 30 nm to 1 μm size range embedded in nitrocellulose or Teflon oxidizers, using an integrating sphere absorption spectrometer. The Al nano‐particle absorption is generally similar to the spectra of bulk Al, but the absorption strength is size‐dependent and up to twelve times more intense. Absorption coefficients are determined as a function of nano‐particle loading. For Al particles where the size is known, per particle cross sections are also determined. The ratio of cross section to volume decreases with increasing particle size. The implications of these measurements for nano‐sized energetic material analysis and for laser‐ignition experiments are briefly discussed.     

Properties of Ca–(Y)–Si–Al–O–N–F Glasses: Independent and Additive Effects of Fluorine and Nitrogen

Thirty glasses of composition (in equivalent percent) 20‐xCa:xY:50Si:30Al:(100‐y‐z)O:yN:zF, with x = 0, 10; y = 0, 10, 20, and z = 0, 1, 3, 5, 7 were prepared by melting and casting. All glasses were X‐ray amorphous. Glass molar volumes (MV) decreased with nitrogen substitution for oxygen for all fluorine contents and, correspondingly, glass fractional compactness increased. Fluorine substitution of oxygen had virtually no effect on molar volume or fractional glass compactness for the three nitrogen contents tested. Young's modulus and microhardness were virtually unaffected by fluorine substitution for oxygen while nitrogen substitution for oxygen caused increases in these two properties. Glass‐transition temperature and dilatometric‐softening point values all decreased with increasing fluorine substitution levels, while increasing nitrogen substitution caused values for these thermal properties to increase. Correspondingly, the thermal expansion coefficient increased with fluorine and decreased with nitrogen substitution levels. Using property value differences between glasses containing fluorine and the corresponding glass containing 0 eq.% F enabled 24 data points to be used to determine the effect of fluorine on T_g,dil and T_DS. The trends were linear with a gradient for both properties of the order of ?22°C (eq.% F)^?1. For the nitrogen effect, 20 data points were analyzed for trend effects. As expected from earlier work, all trends had good linearity. Gradients were for T_g,dil and T_DS +2.5°C (eq.% N)^?1, which are fairly similar to previous results in oxynitride systems. All of the data collected and its analysis clearly shows that the substitution effects of fluorine for oxygen and nitrogen for oxygen are independent and additive with the fluorine substitution. The property trends of the glasses are discussed in terms of their implications for glass structure.     

Thermodynamic equilibrium calculations of sulfur poisoning in Ce–O–S and La–O–S systems

Sulfur poisoning is still a problem in many application areas of exhaust gas catalysts despite the fact that the sulfur levels, e.g. in gasoline are being continuously reduced. The aim of this study was to calculate thermodynamic equilibrium compositions of sulfur in Ce–O–S and La–O–S systems in the presence of precious metals (Pt, Pd, and Rh), which all were considered as bulk materials, in order to understand the experimentally observed sulfur poisoning under the real applications of exhaust gas catalysts. Depending on temperature and oxygen partial pressure, sulfur can be present in the form of sulfates, sulfides and oxysulfides. It is thermodynamically favorable that cerium oxide reacts with SO₂ to form cerium sulfate at low temperatures and cerium oxysulfides at high temperatures. Lanthanum oxide reacts with SO₂ to form sulfate under oxidizing conditions and sulfides under reducing conditions.     

White Si–O–C Ceramic: Structure and Thermodynamic Stability

While pyrolysis of a polysiloxane precursor in argon typically produces a black amorphous Si–O–C ceramic containing “free” carbon (sp² carbon), pyrolyzing the same precursor in hydrogen leads to a white amorphous ceramic with a negligible amount of sp² carbon and a considerable hydrogen content. ²⁹Si magic‐angle‐spinning nuclear magnetic resonance (MAS NMR) spectroscopy confirms the existence of very similar bonding environments of Si atoms in the Si–O–C network for both samples. In addition, ¹H NMR spectroscopic measurements on both samples reveal that the hydrogen atoms are bonded mainly to carbon. For the thermodynamic analysis, the enthalpies of formation with respect to the most stable components (SiO₂, SiC, C) of the black‐and‐white Si–O–C samples obtained after the pyrolysis at 1100°C are determined using high‐temperature oxidative drop‐solution calorimetry in a molten oxide solvent. The white ceramic is 6 kJ/g‐atom more stable in enthalpy than the black one. Although the role of hydrogen in the thermodynamic stability of the white sample remains ambiguous, the thermodynamic findings and structural analysis suggest that the existence of sp²‐bonded carbon in the amorphous network of polymer derived Si–O–C ceramics does not provide additional thermodynamic stability to the ceramic.     

La–Ag–Co perovskites for the catalytic flameless combustion of methane

Ag represents an interesting dopant for the highly active LaCoO₃ perovskites used for the catalytic flameless combustion (CFC) of methane, due to its ability to adsorb and activate oxygen and to the possibility of incorporation into the framework as Ag⁺ or Ag²⁺, with formation of oxygen vacancies. In the present work we compared the catalytic activity and resistance to sulphur poisoning of a series of LaCoO₃, x%Ag/LaCoO₃, La_1−xAg_xCoO₃ samples (nominal composition), the latter two notations indicating post-synthesis Ag loading or direct incorporation during the synthesis, respectively. The samples were prepared by flame pyrolysis (FP) and by the sol–gel (SG) method, leading to different particle size and possibly to different incorporation degree of the dopant, quantified by Rietveld refinement of XRD patterns.Higher activity was observed, in general, with fresh catalysts synthesised by FP. The SG samples demonstrated a slightly better resistance to sulphur poisoning when considering the conversion decrease between the fresh and the poisoned samples, due to lower surface exposure. However, interesting data have been obtained with some of the Ag-doped poisoned FP samples, performing even better than the fresh SG-prepared ones.Ag addition led to a complex change of activity and resistance to poisoning. The activity of FP-prepared samples doped with a small amount of Ag (e.g. 5 mol%) was indeed lower than that of the undoped LaCoO₃. By contrast, a further increase of Ag concentration led to increasing catalytic activity, mainly when big extraframework Ag particles were present. By contrast, for SG samples a low Ag amount was beneficial for activity, due to an increased reducibility of Co³⁺.     

Stereospecific Effects of Oxygen‐to‐Sulfur Substitution in DNA Phosphate on Ion Pair Dynamics and Protein–DNA Affinity

Oxygen‐to‐sulfur substitutions in DNA phosphate often enhance affinity for DNA‐binding proteins. Our previous studies have suggested that this effect of sulfur substitution of both O_P1 and O_P2 atoms is due to an entropic gain associated with enhanced ion pair dynamics. In this work, we studied stereospecific effects of single sulfur substitution of either the O_P1 or O_P2 atom in DNA phosphate at the Lys57 interaction site of the Antennapedia homeodomain–DNA complex. Using crystallography, we obtained structural information on the R_P and S_P diastereomers of the phosphoromonothioate and their interaction with Lys57. Using fluorescence‐based assays, we found significant affinity enhancement upon sulfur substitution of the O_P2 atom. Using NMR spectroscopy, we found significant mobilization of the Lys57 side‐chain NH₃⁺ group upon sulfur substitution of the O_P2 atom. These data provide further mechanistic insights into the affinity enhancement by oxygen‐to‐sulfur substitution in DNA phosphate.     

Solubility Determination of Raw Energetic Materials in Molten 2,4‐Dinitroanisole

2,4‐Dinitroanisole (DNAN) is an ingredient used in several insensitive munition formulations that have recently been qualified by the US Army. A phenomenon known as irreversible growth is found to occur during conditioning cycles of insensitive munitions (IM) that contain DNAN. A possible cause of the irreversible growth maybe the potential solubility of energetic components formulated with melted DNAN. This report documents methods development and procedures used to determine the solubility of energetic constituents in molten DNAN at 100 °C. High performance liquid chromatography and ion chromatography were used for quantitation. Solubilities (given as g energetic per 100 g DNAN) of RDX, HMX, NTO, NQ, and AP were found as 13.7, 3.02, 0.222, 0.448, and 0.088, respectively.     

Kinetics of Corrosion of BN–ZrO2–SiC Ceramics in Contact with Si‐Killed Steel

The mechanism and kinetics of corrosion of BN–ZrO₂–SiC (MYCROSINT^®SO43) by molten Si‐killed steel was studied. Isothermal corrosion tests were performed for duration between 2 and 8 h. Refractory and steel composition and morphology changes were investigated. A kinetic model using process simulation software METSIM and thermochemical software FactSage was developed to understand refractory–steel interactions. The corrosion process showed a deviation from parabolic kinetics and was fitted by a combination of linear and parabolic terms. It was determined that corrosion of the BN–ZrO₂–SiC refractory was governed by dissolution of SiC and BN and removal of ZrO₂ as the other phases were eliminated.     

Effects of Nd Content on Structure and Chemical Durability of Zirconolite–Barium Borosilicate Glass‐Ceramics

The effects of Nd₂O₃ content (0–12 wt %) on crystalline phases, microstructure, and chemical durability of barium borosilicate glass‐ceramics belonging to SiO₂–B₂O₃–Na₂O–BaO–CaO–TiO₂–ZrO₂–Nd₂O₃ system were studied. The results show that the glass‐ceramics with 2–6 wt% of Nd₂O₃ possess mainly zirconolite and titanite phases along with a small amount baddeleyite phase in the bulk. Calcium titanate appears when the Nd₂O₃ content increases to 8 wt%, and the amount of quadrate calcium titanate crystals increases with further increasing content of Nd₂O₃. For the glass‐ceramics with 6 wt% Nd₂O₃ (Nd‐6), Nd elements homogeneously distribute in zirconolite, titanite, and residual glass phases. There is a strong enrichment of Nd in calcium titanate crystals for the sample with 10 wt% Nd₂O₃. The viscosity of Nd‐6 glass is about 49 dPa·s at 1150°C. Moreover, Nd‐6 glass‐ceramics show the lower normalized leaching rates of B (LR_B), Ca (LR_Ca), and Nd (LR_Nd) when compared to that of the sample with 8 wt% Nd₂O₃. After 42 days, LR_B, LR_Ca, and LR_Nd of the Nd‐6 glass‐ceramics are about 6.8 × 10^?3, 1.6 × 10^?3, and 4.4 × 10^?6 g·m^?2·d^?1, respectively.