Investigation of the stability of Co-doped apatite ionic conductors in NH3

Hydrogen powered solid oxide fuel cells (SOFCs) are of enormous interest as devices for the efficient and clean production of electrical energy. However, a number of problems linked to hydrogen production, storage and transportation are slowing down the larger scale use of SOFCs. Identifying alternative fuel sources to act as intermediate during the transition to the full use of hydrogen is, therefore, of importance. One excellent alternative is ammonia, which is produced on a large scale, is relatively cheap and has the infrastructure for storage and transportation already in place. However, considering that SOFCs operate at temperatures higher than 500 °C, a potential problem is the interaction of gaseous ammonia with the materials in the cathode, anode and solid electrolyte. In this paper, we extend earlier work on high temperature reactions of apatite electrolytes with NH₃ to the transition metal (Co) doped systems, La_9.67Si₅CoO₂₆ and La₁₀(Si/Ge)₅CoO_26.5. A combination of PXRD, TGA and XAFS spectroscopy data showed a better structural stability for the silicate systems. Apatite silicates and germanates not containing transition metals tend to substitute nitride anions for their interstitial oxide anions, when reacted with NH₃ at high temperature and, consequentially, lower the interstitial oxide content. In La_9.67Si₅CoO₂₆ and La₁₀(Si/Ge)₅CoO_26.5 reduction of Co occurs as a competing process, favouring lower levels of nitride-oxide substitution.     

The reaction of spiro[indoline‐naphthopyrans] with nitrogen oxides. Not a case of biradical double trapping

The reactions of six differently substituted photochromic spiro[indoline‐naphtopyrans] with ·NO or ·NO₂ under normal daylight conditions have been investigated by means of EPR spectroscopy along with those of three structurally related spiro[indoline‐benzopyrans]. The spectra due to cyclic oxynitroxides originating from double trapping of biradicals by ·NO were observed with the three latter derivatives, this finding being in agreement with previous results. Similar signals were also observed with the six former compounds, but in this case they were responsible for just a minor component of the spectra, the main spectral signals being due to hitherto unreported paramagnetic species that on the basis of their spectral parameters are identified as iminoxy radicals. DFT calculations at the B3LYP/6‐31G* level carried out on a variety of radicals support this assignment. Copyright © 2009 John Wiley & Sons, Ltd.     

Effect of the Sol-Gel Matrix on the Performance of Ammonia Fluorosensors Based on Energy Transfer

We report on the effect of organomodified sol-gel materials on fluorosensors for ammonia. The fluorosensors are based on ion pairs composed of an inert fluorophore and a pH-sensitive absorber dye and are embedded in sol-gel glass. Upon exposure to ammonia, deprotonation of the pH-sensitive dye bromophenolblue occurrs, and consequently, energy is transferred from the fluorophore rhodamine B or tetramethylrhodamine ethyl ester to the absorber. The response of the fluorosensors using different ratios of precursors, such as tetramethoxysilane and phenyltrimethoxysilane, is investigated. Detection limits for sol-gel layers composed of 50% tetramethoxysilane and 50% phenyltrimethoxysilane are as low as 0.1 mg/L of aqueous ammonia. Response times are of the order of 3 to 6 min for forward response. The reversibility of the sensor is related to the composition of the organically modified sol-gel glasses and is fastest for sensor layers composed of pure phenyltrimethoxysilane. Conditioning, regeneration, and storage of the layers are shown to be of vital importance for the performance of the sensor layers.     

Studies on azotetrazolate based high nitrogen content high energy materials potential additives for rocket propellants

This paper reports the synthesis, characterisation and thermolysis studies of a series of azotetrazolate salts, viz., ammonium/guanidinium/triaminoguanidinium [azotetrazolate]. TG-DTA and DSC results of these compounds exhibited their thermal stability up to 180°C. DSC indicated the highest heat release (2804 J g^–1) for guanidinium azotetrazolate salt during exothermic decomposition. FTIR of the decomposition products of azotetrazolate salts showed bands at 3264 and 2358 cm^–1 which may be attributed to gaseous species such as NH₃ and HCN or NH₂CN. The sensitivity data suggests low vulnerability of ammonium and guanidinium salts. In cyclic voltammetric studies all the salts showed similar response in reduction reactions. Triamino guanidinium azotetrazolate (TAGAZ) was incorporated into solid propellant formulations in order to establish the compatibility of this class of compounds. DSC results revealed that it does not have adverse effect on thermal stability of double base matrix. The burning rate data obtained indicated that TAGAZ acts as an efficient energetic additive in composite modified double base (CMDB) propellant formulations in high-pressure region.The authors are grateful to Dr. Haridwar Singh, Outstanding Scientist and Director, HEMRL for constant encouragement to carry out this work. Authors also thankful to Dr. R. S. Satpute, Dr. A. N. Nazare and Dr. C. N. Divekar for their assistance in propellant processing.