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Sample records for alh3 vliyanie dobavok

  1. AlH 3 between 65 and 110 GPa: Implications of electronic band and phonon structures

    NASA Astrophysics Data System (ADS)

    Islam, A. K. M. A.; Ali, M. M.; Ali, M. L.

    2010-04-01

    A first-principles density-functional-theory method has been used to reinvestigate the mechanical and dynamical stability of the metallic phase of AlH 3 between 65 and 110 GPa. The electronic properties and phonon dynamics as a function of pressure are also explored. We find electron-phonon superconductivity in the cubic Pm-3 n structure with critical temperature T c = 37 K at 70 GPa which decreases rapidly with the increase of pressure. Further unlike a previously calculated T c-value of 24 K at 110 GPa, we do not find any superconductivity of significance at this pressure which is consistent with experimental observation.

  2. Elastic, superconducting, and thermodynamic properties of the cubic metallic phase of AlH3 via first-principles calculations

    NASA Astrophysics Data System (ADS)

    Wei, Yong-Kai; Ge, Ni-Na; Ji, Guang-Fu; Chen, Xiang-Rong; Cai, Ling-Cang; Zhou, Su-Qin; Wei, Dong-Qing

    2013-09-01

    The lattice dynamic, elastic, superconducting, and thermodynamic properties of the high-pressure cubic metallic phase AlH3 are studied within density function theory. The calculated elastic modulus and phonon dispersion curves at various pressures indicate that the cubic phase is both mechanically and dynamically stable above 73 GPa. The superconducting transition temperature was calculated using Allen-Dynes modification of the McMillan formula based on the Bardeen-Cooper-Schrieffer theory. It is found that Tc approaches a linear decrease in the low pressure range at the rate dTC/dP ≈-0.22 K/GPa but gradually decreases exponentially at higher pressure, and then it becomes 0 K upon further compression. The calculations indicate that Tc is about 2.042 K at 110 GPa, in agreement with experimental results. The soft phonon modes, especially the lowest acoustic mode, contribute almost 79% to the total electron-phonon coupling parameter sλ for cubic AlH3 at 73 GPa. However, they disappear gradually with increasing pressure, showing a responsibility for the variation of Tc. The thermodynamic properties of cubic AlH3, such as the dependence of thermal expansion coefficient αV on pressure and temperature, the specific heat capacity CP, as well as the electronic specific heat coefficient Cel, were also investigated by the quasi-harmonic approximation theory.

  3. Formation of Al2H7- anions--indirect evidence of volatile AlH3 on sodium alanate using solid-state NMR spectroscopy.

    PubMed

    Felderhoff, Michael; Zibrowius, Bodo

    2011-10-14

    After more than a decade of intense research on NaAlH(4) doped with transition metals as hydrogen storage material, the actual mechanism of the decomposition and rehydrogenation reaction is still unclear. Early on, monomeric AlH(3) was named as a possible transport shuttle for aluminium, but never observed experimentally. Here we report for the first time the trapping of volatile AlH(3) produced during the decomposition of undoped NaAlH(4) by an adduct of sodium alanate and crown ether. The resulting Al(2)H(7)(-) anion was identified by solid-state (27)Al NMR spectroscopy. Based on this indirect evidence of volatile alane, we present a simple description of the processes occurring during the reversible dehydrogenation of NaAlH(4). PMID:21879065

  4. Reaction kinetics for the solid state synthesis of the AlH3/MgCl2 nano-composite by mechanical milling.

    PubMed

    Duan, C W; Hu, L X; Sun, Y; Zhou, H P; Yu, H

    2015-09-14

    The process of mechanical milling has been proved to be a cost-effective way to synthesize the AlH3/MgCl2 nano-composite by using MgH2 and AlCl3 as reagents. However, so far there is no comprehensive knowledge of the kinetics of this process. In an effort to predict the reaction progress and optimize the milling parameters, the kinetics of the synthesis of the AlH3/MgCl2 nano-composite by mechanical milling of MgH2 and AlCl3 is experimentally investigated in the present work. The reaction progress or the transformation fraction upon milling for different times is evaluated using the isothermal hydrogen desorption test of the as-milled samples at 220 °C, which is much lower than the threshold temperature for the de-hydriding of the reagent MgH2 but enough for the de-hydriding of the as-synthesized nano-sized AlH3. The effects of milling parameters on the reaction kinetics as well as the underlying mechanism are discussed by referring to the mechanical energy input intensity, the vial temperature and the Gibbs free energy change for the reaction. Furthermore, it is found that the Johnson-Mehl-Avrami (JMA) model can well describe the kinetics theoretically. By fitting the experimental data with the JMA expression, the theoretical kinetics expressions, the equation parameters, and the activation energy are obtained. PMID:26256935

  5. A parallel vectorized implementation of triple excitations in CCSD(T) - Application to the binding energies of the AlH3, AlH2F, AlHF2 and AlF3 dimers

    NASA Technical Reports Server (NTRS)

    Rendell, Alistair P.; Lee, Timothy J.; Komornicki, Andrew

    1991-01-01

    An efficient method for various noniterative estimates of connected triple excitations in coupled-cluster theory is outlined and related to a similar expression occurring in Moller-Plesset perturbation theory. The method is highly vectorized and capable of utilizing multiple processors on a shared-memory machine, leading to computational rates in excess of one billion floating-point operations per second on four processors of a CRAY Y-MP. Using the new procedure, the binding energies of the D(2h) diborane-type dimers of AlH3, AlH2F, AlHF2, and AlF3 have been determined to be 32, 40, 20, and 47 kcal/mol, respectively. For Al2F6, the correlation procedure includes 232 molecular orbitals and over 1.5 x 10 to the 6th single and double coupled-cluster amplitudes, effectively accounting for over 2 x 10 to the 9th connected triple excitations.

  6. Regeneration of AlH3 studied with Raman and Infrared Spectroscopy

    NASA Astrophysics Data System (ADS)

    Lacina, David; Wegrzyn, J.; Reilly, J. J.; Graetz, Jason

    2010-03-01

    Aluminum hydride compounds are known to exhibit a 10% by weight hydrogen storage capacity that makes them suited for technologies that require hydrogen as a fuel. The current challenge associated with this material is how to regenerate the hydride from the spent fuel and H2 gas. We employ a two-step process to regenerate the hydride compound which first requires the formation of a stable aluminum hydride adduct using a tertiary amine. This is followed by a second step consisting of adduct separation and hydride recovery, involving transamination to create a less stable adduct. We present results which show that alane-amines can be formed by hydrogenation of catalyzed aluminum in a solvent at low pressures using one of several tertiary amines. Raman and infrared spectroscopy was performed on the products of these reactions to better understand the structure of the alane amines that are formed, as well as the hydrogenation reactions that take place. A vibrational analysis of the regeneration products performed with Raman and infrared spectroscopy is presented and will help clarify the molecular and vibrational structures of these alane amine adducts.

  7. Materials Data on AlH3 (SG:167) by Materials Project

    SciTech Connect

    Kristin Persson

    2014-11-02

    Computed materials data using density functional theory calculations. These calculations determine the electronic structure of bulk materials by solving approximations to the Schrodinger equation. For more information, see https://materialsproject.org/docs/calculations

  8. Hydrazine bisalane is a potential compound for chemical hydrogen storage. A theoretical study.

    PubMed

    Nguyen, Vinh Son; Swinnen, Saartje; Leszczynski, Jerzy; Nguyen, Minh Tho

    2011-09-14

    Electronic structure calculations suggest that hydrazine bisalane (AlH(3)NH(2)NH(2)AlH(3), alhyzal) is a promising compound for chemical hydrogen storage (CHS). Calculations are carried out using the coupled-cluster theory CCSD(T) with the aug-cc-pVTZ basis set. Potential energy surfaces are constructed to probe the formation of, and hydrogen release from, hydrazine bisalane which is initially formed from the reaction of hydrazine with dialane. Molecular and electronic characteristics of both gauche and trans alhyzal are determined for the first time. The gauche hydrazine bisalane is formed from starting reactants hydrazine + dialane following a movement of an AlH(3) group from AlH(3)AlH(3)NH(2)NH(2) rather than by a direct attachment of a separate AlH(3) group, generated by predissociation of dialane, to AlH(3)NH(2)NH(2). The energy barriers for dehydrogenation processes from gauche and transalhyzal are in the range of 21-28 kcal mol(-1), which are substantially smaller than those of ca. 40 kcal mol(-1) previously determined for the isovalent hydrazine bisborane (bhyzb) system. H(2) release from hydrazine bisalane is thus more favored over that from hydrazine bisborane, making the Al derivative an alternative candidate for CHS. PMID:21776513

  9. Aluminium hydride: a reversible material for hydrogen storage.

    PubMed

    Zidan, Ragaiy; Garcia-Diaz, Brenda L; Fewox, Christopher S; Stowe, Ashley C; Gray, Joshua R; Harter, Andrew G

    2009-07-01

    Aluminium hydride has been synthesized electrochemically, providing a synthetic route which closes a reversible cycle for regeneration of the material and bypasses expensive thermodynamic costs which have precluded AlH(3) from being considered as a H(2) storage material. PMID:19557259

  10. Solid State NMR Studies of the Aluminum Hydride Phases

    NASA Technical Reports Server (NTRS)

    Hwang, Son-Jong; Bowman, R. C., Jr.; Graetz, Jason; Reilly, J. J.

    2006-01-01

    Several solid state NMR techniques including magic-angle-spinning (MAS) and multiple-quantum (MQ) MAS experiments have been used to characterize various AlH3 samples. MAS-NMR spectra for the 1H and 27Al nuclei have been obtained on a variety of AlH3 samples that include the (beta)- and (gamma)- phases as well as the most stable (alpha)-phase. While the dominant components in these NMR spectra correspond to the aluminum hydride phases, other species were found that include Al metal, molecular hydrogen (H2), as well as peaks that can be assigned to Al-O species in different configurations. The occurrence and concentration of these extraneous components are dependent upon the initial AlH3 phase composition and preparation procedures. Both the (beta)-AlH3 and (gamma)-AlH3 phases were found to generate substantial amounts of Al metal when the materials were stored at room temperature while the (alpha)-phase materials do not exhibit these changes.

  11. Towards direct synthesis of alane: A predicted defect-mediated pathway confirmed experimentally

    DOE PAGESBeta

    Wang, Lin -Lin; Herwadkar, Aditi; Reich, Jason M.; Johnson, Duane D.; House, Stephen D.; Pena-Martin, Pamela; Rockett, Angus A.; Robertson, Ian M.; Gupta, Shalabh; Pecharsky, Vitalij K.

    2016-08-18

    Here, alane (AlH3) is a unique energetic material that has not found a broad practical use for over 70 years because it is difficult to synthesize directly from its elements. Using density functional theory, we examine the defect-mediated formation of alane monomers on Al(111) in a two-step process: (1) dissociative adsorption of H2 and (2) alane formation, which are both endothermic on a clean surface. Only with Ti dopant to facilitate H2 dissociation and vacancies to provide Al adatoms, both processes become exothermic. In agreement, in situ scanning tunneling microscopy showed that during H2 exposure, alane monomers and clusters formmore » primarily in the vicinity of Al vacancies and Ti atoms. Moreover, ball milling of the Al samples with Ti (providing necessary defects) showed a 10 % conversion of Al into AlH3 or closely related species at 344 bar H2, indicating that the predicted pathway may lead to the direct synthesis of alane from elements at pressures much lower than the 104 bar expected from bulk thermodynamics.« less

  12. Novel methods for synthesizing halide-free alane without the formation of adducts

    NASA Astrophysics Data System (ADS)

    Dinh, Long V.; Knight, Douglas A.; Paskevicius, Mark; Buckley, Craig E.; Zidan, Ragaiy

    2012-04-01

    Many of the current synthesis methods for aluminum hydride (alane—AlH3) involve reacting AlCl3 and LiAlH4 in solvents. The reaction requires the formation of an alane adduct such as AlH3ṡ[(C2H5)2O] prior to obtaining crystallized stable α-AlH3. This process requires several hours of pumping in a vacuum system to remove the ether and convert the alane etherate into stable α-alane. This crystallization process is both costly and hazardous because a large amount of highly flammable material (e.g. ether) is removed by vacuum pumps over several hours. Conversely, the work presented herein describes novel methods to synthesize adduct-free alane. It is demonstrated here that AlH3 can form by mixing AlCl3 and LiAlH4 in the solid state and heating to 75∘C; only α-AlH3 was obtained. The α-AlH3 product can be washed with minimal solvents leading to zero formation of alane adducts. In addition, the unwanted LiCl by-product is also removed during the solvent wash, resulting in halide-free α-alane. Although simply mixing and heating the reactants led to a 40% yield of alane, having the reactants compacted and mechanically pressed while heating increases the yield to 60% crystalline α-AlH3.

  13. Towards Direct Synthesis of Alane: A Predicted Defect-Mediated Pathway Confirmed Experimentally.

    PubMed

    Wang, Lin-Lin; Herwadkar, Aditi; Reich, Jason M; Johnson, Duane D; House, Stephen D; Peña-Martin, Pamela; Rockett, Angus A; Robertson, Ian M; Gupta, Shalabh; Pecharsky, Vitalij K

    2016-09-01

    Alane (AlH3 ) is a unique energetic material that has not found a broad practical use for over 70 years because it is difficult to synthesize directly from its elements. Using density functional theory, we examine the defect-mediated formation of alane monomers on Al(111) in a two-step process: (1) dissociative adsorption of H2 and (2) alane formation, which are both endothermic on a clean surface. Only with Ti dopant to facilitate H2 dissociation and vacancies to provide Al adatoms, both processes become exothermic. In agreement, in situ scanning tunneling microscopy showed that during H2 exposure, alane monomers and clusters form primarily in the vicinity of Al vacancies and Ti atoms. Moreover, ball milling of the Al samples with Ti (providing necessary defects) showed a 10 % conversion of Al into AlH3 or closely related species at 344 bar H2 , indicating that the predicted pathway may lead to the direct synthesis of alane from elements at pressures much lower than the 10(4)  bar expected from bulk thermodynamics. PMID:27535100

  14. Aluminium Diphosphamethanides: Hidden Frustrated Lewis Pairs.

    PubMed

    Styra, Steffen; Radius, Michael; Moos, Eric; Bihlmeier, Angela; Breher, Frank

    2016-07-01

    The synthesis and characterisation of two aluminium diphosphamethanide complexes, [Al(tBu)2 {κ(2) P,P'-Mes*PCHPMes*}] (3) and [Al(C6 F5 )2 {κ(2) P,P'-Mes*PCHPMes*}] (4), and the silylated analogue, Mes*PCHP(SiMe3 )Mes* (5), are reported. The aluminium complexes feature four-membered PCPAl core structures consisting of diphosphaallyl ligands. The silylated phosphine 5 was found to be a valuable precursor for the synthesis of 4 as it cleanly reacts with the diaryl aluminium chloride [(C6 F5 )2 AlCl]2 . The aluminium complex 3 reacts with molecular dihydrogen at room temperature under formation of the acyclic σ(2) λ(3) ,σ(3) λ(3) -diphosphine Mes*PCHP(H)Mes* and the corresponding dialkyl aluminium hydride [tBu2 AlH]3 . Thus, 3 belongs to the family of so-called hidden frustrated Lewis pairs. PMID:27271936

  15. Advances in the electrochemical regeneration of aluminum hydride

    NASA Astrophysics Data System (ADS)

    Martínez-Rodríguez, Michael J.; García-Díaz, Brenda L.; Teprovich, Joseph A.; Knight, Douglas A.; Zidan, Ragaiy

    2012-03-01

    In previous work, a reversible cycle that uses electrolysis and catalytic hydrogenation of spent Al(s) for the regeneration of alane (AlH3) was reported. In this study, the electrochemical synthesis of alane is improved. Advances in the electrochemical regeneration of alane have been achieved via the use of lithium aluminum hydride (LiAlH4) and lithium chloride (LiCl). Lithium chloride reacts in a cyclic process and functions as an electro-catalytic additive that enhances the electrochemical process by increasing the cell efficiency and the alane production. Electrochemical techniques are used to show that the increased rate of alane generation is due to the electro-catalytic effect of lithium chloride, rather than an electrolyte enhanced effect.

  16. Calculation of the 13C NMR shieldings of the C0 2 complexes of aluminosilicates

    NASA Astrophysics Data System (ADS)

    Tossell, J. A.

    1995-04-01

    13C NMR shieldings have been calculated using the random-phase-approximation, localized-orbital local-origins version of ab initio coupled Hartree-Fuck perturbation theory for CO 2 and and for several complexes formed by the reaction of CO 2 with molecular models for aluminosilicate glasses, H 3TOT'H3 3-n, T,T' = Si,Al. Two isomeric forms of the CO 2-aluminosilicate complexes have been considered: (1) "CO 2-like" complexes, in which the CO 2 group is bound through carbon to a bridging oxygen and (2) "CO 3-like" complexes, in which two oxygens of a central CO 3 group form bridging bonds to the two TH 3 groups. The CO 2-like isomer of CO 2-H 3SiOSiH 3 is quite weakly bonded and its 13C isotropic NMR shielding is almost identical to that in free CO 2. As Si is progressively replaced by Al in the - H terminated aluminosilicate model, the CO 2-like isomers show increasing distortion from the free CO 2 geometry and their 13C NMR shieldings decrease uniformly. The calculated 13C shielding value for H 3AlO(CO 2)AlH 3-2 is only about 6 ppm larger than that calculated for point charge stabilized CO 3-2. However, for a geometry of H 3SiO(CO 2) AlH 3-1, in which the bridging oxygen to C bond length has been artificially increased to that found in the - OH terminated cluster (OH) 3SiO(CO 2)Al(OH) 3-1, the calculated 13C shielding is almost identical to that for free CO 2. The CO 3-like isomers of the CO 2-aluminosili-cate complexes show carbonate like geometries and 13C NMR shieldings about 4-9 ppm larger than those of carbonate for all T,T' pairs. For the Si,Si tetrahedral atom pair the CO 2-like isomer is more stable energetically, while for the Si,Al and Al,Al cases the CO 3-like isomer is more stable. Addition of Na + ions to the CO 3-2 or H 3AlO(CO 2)AlH 3-2 complexes reduces the 13C NMR shieldings by about 10 ppm. Complexation with either Na + or CO 2 also reduces the 29Si NMR shieldings of the aluminosilicate models, while the changes in 27Al shielding with Na + or CO 2 complexation are much smaller. Complexation with CO 2 greatly increases the electric field gradient at the bridging oxygen of H 3AlOAlH 3-2, raising it to a value similar to that found for SiOSi linkages. Comparison of these results with the experimental 13C NMR spectra support the formation of CO 2-like complexes at SiOSi bridges in albite glasses and CO 3-like complexes at SiOAl and AlOAl bridges in albite and nepheline glasses. Changes in the calculated shieldings as Na + ions are added to the complexes suggest that some of the observed complexes may be similar in their CO 2-aluminosilicate interactions, but different with respect to the positions of the charge-compensating Na + ions.

  17. Complexation of trivalent cations (Al(III), Cr(III), Fe(III)) with two phosphonic acids in the pH range of fresh waters.

    PubMed

    Lacour, S; Deluchat, V; Bollinger, J C; Bernard Serpaud

    1998-08-01

    The complex formation constants of two phosphonic acids, HEDP and ATMP, with three trivalent metallic cations, Al(III), Cr(III) and Fe(III), have been determined by acid-base titration at 25 degrees C and constant ionic strength (0.1 mol l(-1), KNO(3)), using Martell and Motekaitis' computer programs. Species distribution curves showed that all three cations are in complex form in the pH range of fresh waters (5-9). The study of different cation/ligand ratios proved that both ligands mainly form anionic soluble complexes for systems having an excess of ligand-as protonated and unprotonated forms and especially ternary complexes with HEDP. For higher metal concentrations (excess of cation), weakly soluble species of HEDP and ATMP were formed with Al(III) and Cr(III). Two insoluble complexes with ATMP have been identified by SEM/EDAX as AlH(3)X((s)) and Cr(2)X((s)). Regarding Fe(III) species, Fe(OH)(3(s)) precipitate seems to predominate in solution. PMID:18967224

  18. Ultrafast Bulk Diffusion of AlHxin High-Entropy Dehydrogenation Intermediates of NaAlH4

    SciTech Connect

    Zhang, Feng; Wood, Brandon C; Wang, Yan; Wang, Cai-Zhuang; Ho, Kai-Ming; Chou, Mei-Yin

    2014-08-14

    Using first-principles molecular dynamics (FPMD) and total-energy calculations, we demonstrate low-barrier bulk diffusion of Al-bearing species in γ-NaAlH4, a recently proposed high-entropy polymorph of NaAlH4. For charged AlH4– and neutral AlH3 vacancies, the computed barriers for diffusion are <0.1 eV, and we directly observe the predicted diffusive pathways in FPMD simulations at picosecond time scales. In contrast, such diffusion in the α phase is inaccessible to FPMD, consistent with much higher barriers. The transport behavior of γ-NaAlH4, in addition to key dynamical and structural signatures, is consistent with experimental observations of high-mobility species, strongly supporting the idea that an intermediate transition from the α phase to a high-entropy polymorph facilitates the hydrogen-releasing decomposition of NaAlH4. Our results provide an answer to longstanding questions regarding the responsible agent for the experimentally observed efficient Al transport during dehydrogenation and suggest that mass transport and phase transformation kinetics are coupled. Implications for understanding the (de)hydrogenation of undoped and catalyzed NaAlH4 are discussed.

  19. Development and evaluation of portable and wearable fuel cells for soldier use

    NASA Astrophysics Data System (ADS)

    Thampan, T.; Shah, D.; Cook, C.; Novoa, J.; Shah, S.

    2014-08-01

    A number of fuel cell systems have been recently developed to meet the U.S. Army's soldier power requirements. The operation and performance of these systems are discussed based on laboratory results and limited soldier evaluation. The systems reviewed are primarily intended for soldier use in an austere environment with minimum access to resupply and vehicular transportation. These applications require high power and energy density sources that are portable (300 W) and wearable (20 W) to minimize the soldier's load burden. Based on soldier field evaluations of portable fuel cell systems, improvements in power density and compatibility with logistical fuels are required to be successfully deployed. For soldier worn applications, a novel chemical hydride system has shown significant advances in power and energy density while maintaining a small form factor. The use of a high energy dense fuel cartridge (800 Wh kg-1) based on AlH3 (Alane) thermolysis, allows a power density of (28 W kg-1) which offers promising weight savings compared to the standard military batteries.

  20. Thermochemistry of Alane Complexes for Hydrogen Storage: A Theoretical and Experimental Investigation

    PubMed Central

    2011-01-01

    Knowledge of the relative stabilities of alane (AlH3) complexes with electron donors is essential for identifying hydrogen storage materials for vehicular applications that can be regenerated by off-board methods; however, almost no thermodynamic data are available to make this assessment. To fill this gap, we employed the G4(MP2) method to determine heats of formation, entropies, and Gibbs free energies of formation for 38 alane complexes with NH3−nRn (R = Me, Et; n = 0−3), pyridine, pyrazine, triethylenediamine (TEDA), quinuclidine, OH2−nRn (R = Me, Et; n = 0−2), dioxane, and tetrahydrofuran (THF). Monomer, bis, and selected dimer complex geometries were considered. Using these data, we computed the thermodynamics of the key formation and dehydrogenation reactions that would occur during hydrogen delivery and alane regeneration, from which trends in complex stability were identified. These predictions were tested by synthesizing six amine−alane complexes involving trimethylamine, triethylamine, dimethylethylamine, TEDA, quinuclidine, and hexamine and obtaining upper limits of ΔG° for their formation from metallic aluminum. Combining these computational and experimental results, we establish a criterion for complex stability relevant to hydrogen storage that can be used to assess potential ligands prior to attempting synthesis of the alane complex. On the basis of this, we conclude that only a subset of the tertiary amine complexes considered and none of the ether complexes can be successfully formed by direct reaction with aluminum and regenerated in an alane-based hydrogen storage system. PMID:22962624

  1. Syntheses, structures, and surface aromaticity of the new carbaalane [(AlH)(6)(AlNMe(3))(2)(CCH(2)R)(6)] (R = Ph, CH(2)SiMe(3)) and a stepwise functionalization of the inner and outer sphere of the cluster.

    PubMed

    Stasch, Andreas; Ferbinteanu, Marilena; Prust, Jörg; Zheng, Wenjun; Cimpoesu, Fanica; Roesky, Herbert W; Magull, Jörg; Schmidt, Hans-Georg; Noltemeyer, Mathias

    2002-05-15

    The reaction of the acetylene RC triple bond CH (R = Ph, CH(2)SiMe(3)) with an excess of AlH(3).NMe(3) in boiling toluene leads to the carbaalane [(AlH)(6)(AlNMe(3))(2)(CCH(2)R)(6)] (R = Ph 1, CH(2)SiMe(3) 2) in good yield. Treatment of 2 with BCl(3) under varying conditions gives the chlorinated products [(AlCl)(6)(AlNMe(3))(2)(CCH(2)CH(2)SiMe(3))(6)] 3 and [(AlCl)(6)(AlNMe(3))(2)(CCH(2)CH(2)SiMe(2)Cl)(6)] 4, respectively. The latter clearly demonstrates that the cluster can be stepwise functionalized within the inner and outer sphere. The X-ray single-crystal structures of 1, 2, and 4 have been determined. All compounds have in common that the central core consists of a cluster having eight aluminum and six carbon atoms. The bonding properties in this cluster are described as a new manifestation of three-dimensional surface aromaticity. Each Al(4)C fragment of the cube is formed by four bonds with three electron pairs, thus leading to a strong delocalization of the electrons. A phenomenological modeling using a three-dimensional Hückel scheme with fitted parameters to reproduce the energies from ab initio calculations revealed that the orbital scheme localized at one Al(4)C fragment possesses an orbital sextet with a large HOMO-LUMO gap. This is in line with the criteria of aromaticity. The idea of aromaticity was sustained also by qualitative valence bond reasons enumerating the different resonance structures by means of graph theoretical methods. PMID:11996585

  2. Site and chirality selective chemical modifications of boron nitride nanotubes (BNNTs) via Lewis acid-base interactions.

    PubMed

    Sundaram, Rajashabala; Scheiner, Steve; Roy, Ajit K; Kar, Tapas

    2015-02-01

    The pristine BNNTs contain both Lewis acid (boron) and Lewis base (nitrogen) centers at their surface. Interactions of ammonia and borane molecules, representatives of Lewis base and acid as adsorbates respectively, with matching sites at the surface of BNNTs, have been explored in the present DFT study. Adsorption energies suggest stronger chemisorption (about 15-20 kcal mol(-1)) of borane than ammonia (about 5-10 kcal mol(-1)) in both armchair (4,4) and zigzag (8,0) variants of the tube. NH3 favors (8,0) over the (4,4) tube, whereas BH3 exhibits the opposite preference, indicating some chirality dependence on acid-base interactions. A new feature of bonding is found in BH3/AlH3-BNNTs (at the edge site) complexes, where one hydrogen of the guest molecule is involved in three-center two-electron bonding, in addition to dative covalent bond (N: → B). This interaction causes a reversal of electron flow from borane/alane to BNNT, making the tube an electron acceptor, suggesting tailoring of electronic properties could be possible by varying strength of incoming Lewis acids. On the contrary, BNNTs always behave as electron acceptor in ammonia complexes. IR, XPS and NMR spectra show some characteristic features of complexes and can help experimentalists to identify not only structures of such complexes but also the location of the guest molecules and design second functionalizations. Interaction with several other neutral BF3, BCl3, BH2CH3 and ionic CH3(+) acids as well as amino group (CH3NH2 and NH2COOH) were also studied. The strongest interaction (>100 kcal mol(-1)) is found in BNNT-CH3(+) complexes and H-bonds are the only source of stability of NH2COOH-BNNT complexes. PMID:25559141

  3. Understanding the role of Ti in reversible hydrogen storage as sodium alanate: a combined experimental and density functional theoretical approach.

    PubMed

    Chaudhuri, Santanu; Graetz, Jason; Ignatov, Alex; Reilly, James J; Muckerman, James T

    2006-09-01

    We report the results of an experimental and theoretical study of hydrogen storage in sodium alanate (NaAlH(4)). Reversible hydrogen storage in this material is dependent on the presence of 2-4% Ti dopant. Our combined study shows that the role of Ti may be linked entirely to Ti-containing active catalytic sites in the metallic Al phase present in the dehydrogenated NaAlH(4). The EXAFS data presented here show that dehydrogenated samples contain a highly disordered distribution of Ti-Al distances with no long-range order beyond the second coordination sphere. We have used density functional theory techniques to calculate the chemical potential of possible Ti arrangements on an Al(001) surface for Ti coverages ranging from 0.125 to 0.5 monolayer (ML) and have identified those that can chemisorb molecular hydrogen via spontaneous or only moderately activated pathways. The chemisorption process exhibits a characteristic nodal symmetry property for the low-barrier sites: the incipient doped surface-H(2) adduct's highest occupied molecular orbital (HOMO) incorporates the sigma antibonding molecular orbital of hydrogen, allowing the transfer of charge density from the surface to dissociate the molecular hydrogen. This work also proposes a plausible mechanism for the transport of an aluminum hydride species back into the NaH lattice that is supported by Car-Parrinello molecular dynamics (CPMD) simulations of the stability and mobility of aluminum clusters (alanes) on Al(001). As an experimental validation of the proposed role of titanium and the subsequent diffusion of alanes, we demonstrate experimentally that AlH(3) reacts with NaH to form NaAlH(4) without any requirement of a catalyst or hydrogen overpressure. PMID:16939263

  4. Structural Characterization of Metal Hydrides for Energy Applications

    NASA Astrophysics Data System (ADS)

    George, Lyci

    Hydrogen can be an unlimited source of clean energy for future because of its very high energy density compared to the conventional fuels like gasoline. An efficient and safer way of storing hydrogen is in metals and alloys as hydrides. Light metal hydrides, alanates and borohydrides have very good hydrogen storage capacity, but high operation temperatures hinder their application. Improvement of thermodynamic properties of these hydrides is important for their commercial use as a source of energy. Application of pressure on materials can have influence on their properties favoring hydrogen storage. Hydrogen desorption in many complex hydrides occurs above the transition temperature. Therefore, it is important to study the physical properties of the hydride compounds at ambient and high pressure and/or high temperature conditions, which can assist in the design of suitable storage materials with desired thermodynamic properties. The high pressure-temperature phase diagram, thermal expansion and compressibility have only been evaluated for a limited number of hydrides so far. This situation serves as a main motivation for studying such properties of a number of technologically important hydrides. Focus of this dissertation was on X-ray diffraction and Raman spectroscopy studies of Mg2FeH6, Ca(BH4) 2, Mg(BH4)2, NaBH4, NaAlH4, LiAlH4, LiNH2BH3 and mixture of MgH 2 with AlH3 or Si, at different conditions of pressure and temperature, to obtain their bulk modulus and thermal expansion coefficient. These data are potential source of information regarding inter-atomic forces and also serve as a basis for developing theoretical models. Some high pressure phases were identified for the complex hydrides in this study which may have better hydrogen storage properties than the ambient phase. The results showed that the highly compressible B-H or Al-H bonds and the associated bond disordering under pressure is responsible for phase transitions observed in brorohydrides or alanates. Complex hydrides exhibited very high compressibility suggesting possibility to destabilize them with pressure. With high capacity and favorable thermodynamics, complex hydrides are suitable for reversible storage. Further studies are required to overcome the kinetic barriers in complex hydrides by catalytic addition. A comparative study of the hydride properties with that of the constituting metal, and their inter relationships were carried out with many interesting features.

  5. Aryl(silyl)amino group stabilized hydridosilanediols: synthesis and characterization and use for preparation of alumino(hydrido)siloxanes.

    PubMed

    Wang, Xiaoping; Li, Jiancheng; Chen, Shimin; Liu, Weiping; Ye, Qingsong; Zhu, Hongping

    2016-04-21

    Aryl(silyl)amino group stabilized hydridosilanediols RSiH(OH)2 (R = N(SiMe2Ph)-2,6-iPr2C6H3 (), N(SiMe3)-2,6-iPr2C6H3 (), and N(SiMe2Ph)-2,4,6-Me3C6H2 ()) were prepared from the controlled hydrolysis of the related RSiHCl2 () each in the presence of aniline as the HCl acceptor. Reactions of with AlMe3, AliBu3, AlH(iBu)2, and AlH3·NMe3, respectively, yielded alumino(hydrido)siloxanes [2,6-iPr2C6H3N(SiMe2Ph)Si(H)OAlMe(THF)]2 (), [2,6-iPr2C6H3N(SiMe2Ph)Si(H)OAliBu(THF)]2 (), [2,6-iPr2C6H3N(SiMe2Ph)Si(H)O2]3[Al(THF)]2 (), and [2,6-iPr2C6H3N(SiMe2Ph)Si(H)OAlH(THF)]2 (). The reaction of with AlMe3 gave [2,6-iPr2C6H3N(SiMe3)Si(H)OAlMe(THF)]2 (), a compound similar to . Compounds are characterized by NMR ((1)H, (13)C, and (29)Si) and IR spectroscopy and CHN elemental analysis, of which and are further studied by X-ray crystallography. Compounds and feature cyclic structures all with the skeleton core of Si2O4Al2 while compound exhibits a bicyclic structure having a core of Si3O6Al2. Melting point measurements indicated that are thermally stable bearing the geminal SiH and SiOH groups. Compounds and are thermally stable as well with the O atom-bridged SiH and AlR (R = Me, iBu, or H) groups. PMID:26975000

  6. Computational study of pristine and titanium-doped sodium alanates for hydrogen storage applications

    NASA Astrophysics Data System (ADS)

    Dathar, Gopi Krishna Phani

    The emphasis of this research is to study and elucidate the underlying mechanisms of reversible hydrogen storage in pristine and Ti-doped sodium aluminum hydrides using molecular modeling techniques. An early breakthrough in using complex metal hydrides as hydrogen storage materials is from the research on sodium alanates by Bogdanovic et al., in 1997 reporting reversible hydrogen storage is possible at moderate temperatures and pressures in transition metal doped sodium alanates. Anton reported titanium salts as the best catalysts compared to all other transition metal salts from his further research on transition metal doped sodium alanates. However, a few questions remained unanswered regarding the role of Ti in reversible hydrogen storage of sodium alanates with improved thermodynamics and kinetics of hydrogen desorption. The first question is about the position of transition metal dopants in the sodium aluminum hydride lattice. The position is investigated by identifying the possible sites for titanium dopants in NaAlH4 lattice and studying the structure and dynamics of possible compounds resulting from titanium doping in sodium alanates. The second question is the role of titanium dopants in improved thermodynamics of hydrogen desorption in Ti-doped NaAlH4. Though it is accepted in the literature that formation of TiAl alloys (Ti-Al and TiAl3) is favorable, reaction pathways are not clearly established. Furthermore, the source of aluminum for Ti-Al alloy formation is not clearly understood. The third question in this area is the role of titanium dopants in improved kinetics of hydrogen absorption and desorption in Ti-doped sodium alanates. This study is directed towards addressing the three longstanding questions in this area. Thermodynamic and kinetic pathways for hydrogen desorption in pristine NaAlH4 and formation of Ti-Al alloys in Ti-doped NaAlH 4, are elucidated to understand the underlying mechanisms of hydrogen desorption. Density functional theory formalism as implemented in CASTEP (Cambridge Serial Total Energy Package) is used to study the structure and energetics of pristine and Ti-doped sodium alanates. From investigations of various models of sodium alanates with Ti dopants, it is shown that the difference between the energy required for Ti→SNa (Ti-substituted Na at the lattice site on the surface) and Ti→TI (Ti placed on top of the surface interstitial SI site) is 0.003 eV atom-1, and is minimal compared to other models. Since less energy is required for Ti→S Na and Ti→TI, these two sites (SNa and T I) would be preferred by the Ti dopants. In Ti→SNa model, Ti is coordinated to two aluminum and seven hydrogen atoms resulting in the possible formation of a TiAl2H7 complex. At elevated temperatures (423 and 448 K), the number of aluminum atoms coordinating with titanium in the complex increase from two (at distances in the 2.6-2.7 A range) to five (at distances in the 2.6-2.7 A range). Besides the formation of a Ti-Al-H complex, Al-Al association (with a 2.97 A bond length) is also seen from the DFT-MD results. In the case of Ti→TI, Ti is coordinated to two aluminum and two hydrogen atoms resulting in the possible formation of a TiAl2H2 complex. TiAl2 H2 complex becomes TiAl3H6 and TiAl 3H7 at elevated temperatures of 423 and 448 K, respectively. The investigation of thermodynamics pathways in Ti-doped sodium alanates illustrates a three step reaction pathway to the formation of TiAl3 (Ti and AlH3 after the first reaction, TiAl after the second and finally TiAl3). This investigation also suggests aluminum in its +3 oxidation state present in aluminum hydride species is responsible in the formation of Ti-Al alloys. From kinetics studies, the proposed mechanism is related to transition from AlH4- to AlH6 3-. The rate limiting step is determined to be associated with hydrogen evolution from association of AlH3 species nucleating aluminum phase. This step is 15 kJ/mol higher than the nearest highest barrier in the reaction path related to transition from AlH52- to AlH63-. From the DFT-MD simulations, it is observed that the titanium dopants are present on the surface during the entire simulation time and exhibit the role in catalytic splitting of hydrogen from surrounding AlH4 groups. Besides the catalytic role, Ti dopants also form bonds with Al, and we also see that the AlH4 groups on the surface and that are present in the sub-surface layers are drawn towards the Ti dopants. This association of Al around titanium indicates the initiation of Al nucleation site facilitated by Ti dopants residing on the surface.

  7. Metal hydride and pyrophoric fuel additives for dicyclopentadiene based hybrid propellants

    NASA Astrophysics Data System (ADS)

    Shark, Steven C.

    The purpose of this study is to investigate the use of reactive energetic fuel additives that have the potential to increase the combustion performance of hybrid rocket propellants in terms of solid fuel regression rate and combustion efficiency. Additives that can augment the combustion flame zone in a hybrid rocket motor by means of increased energy feedback to the fuel grain surface are of great interest. Metal hydrides have large volumetric hydrogen densities, which gives these materials high performance potential as fuel additives in terms of specifc impulse. The excess hydrogen and corresponding base metal may also cause an increase in the hybrid rocket solid fuel regression rate. Pyrophoric additives also have potential to increase the solid fuel regression rate by reacting more readily near the burning fuel surface providing rapid energy feedback. An experimental performance evaluation of metal hydride fuel additives for hybrid rocket motor propulsion systems is examined in this study. Hypergolic ignition droplet tests and an accelerated aging study revealed the protection capabilities of Dicyclopentadiene (DCPD) as a fuel binder, and the ability for unaided ignition. Static hybrid rocket motor experiments were conducted using DCPD as the fuel. Sodium borohydride (NabH4) and aluminum hydride (AlH3) were examined as fuel additives. Ninety percent rocket grade hydrogen peroxide (RGHP) was used as the oxidizer. In this study, the sensitivity of solid fuel regression rate and characteristic velocity (C*) efficiency to total fuel grain port mass flux and particle loading is examined. These results were compared to HTPB combustion performance as a baseline. Chamber pressure histories revealed steady motor operation in most tests, with reduced ignition delays when using NabH4 as a fuel additive. The addition of NabH4 and AlH3 produced up to a 47% and 85% increase in regression rate over neat DCPD, respectively. For all test conditions examined C* efficiency ranges between 80% and 90%. The regression rate and C* efficiency mass flux dependence indicate a shift towards a more diffusion controlled system with metal hydride particle addition. Although these types of energetic particles have potential as high performing fuel additives, they can be in low supply and expensive. An opposed flow burner was investigated as a means to screen and characterize hybrid rocket fuels prior to full scale rocket motor testing. Although this type of configuration has been investigated in the past, no comparison has been made to hybrid rocket motor operation in terms of mass flux. Polymeric fuels and low melt temperature fuels with and without additives were investigated via an opposed flow burner. The effects of laminar and turbulent flow regimes on the convective heat transfer in the opposed flow system was depicted in the regression rate trends of these fuels. Regression rate trends similar to hybrid rocket motor operation were depicted, including the entrainment mechanism for paran fuel. However, there was a shift in overall magnitude of these results. A decrease in regression rate occurred for HTPB loaded with passivated nano-aluminum, due to low resonance time in the reaction zone. Previous results have shown that pyrophoric additives can cause an increase in regression rate in the opposed flow burner configuration. It is proposed that the opposed burner is useful as a screening and characterization tool for some propellant combinations. Gaseous oxygen (GOX) was investigated as an oxidizer for similar fuels evaluated with RGHP. Specifically, combustion performance sensitivity to mass flux and MH particle size was investigated. Similar results to the RGHP experiments were observed for the regression rate tends of HTPB, DPCD, and NabH 4 addition. Kinetically limited regression rate dependence on mass flux was observed at the higher mass flux levels. No major increase in C* efficiency was observed for MH addition. The C* efficiency varied with equivalence ratio by approximately 10 percentage points, which was not observed in the RGHP experiments. A 10 percentage point decrease in C* efficiency was observed with increasing mass flux in the system. This was most likely due to poorly mixed fuel and oxidizer in center of the combustion chamber at the higher mass flux levels. Detailed measurements of the hybrid rocket combustion zone is useful for understanding the mechanisms governing performance, but can be difficult to obtain. Traditional slab burner configurations have proven useful but are operationally limited in pressure and mass flux ranges. A new optical cylindrical combustor (OCC) design is presented that allows surface and flame zone imaging and tracking during hybrid rocket motor operation at appreciable mass flux and pressure levels, > 100 kg/s/m2 and > 0.69 MPa. The flame height and regression rate sensitivity to mass flux and chamber pressure was examined for the same fuels examined in the GOX hybrid rocket motor, with the addition of DCPD fuel loaded with Al and unpassivated mechanically activated Al-PTFE. The regression rate trends were on the same order of magnitude of traditional hybrid rocket motor results. A flame height decrease was observed for increased mass flux. The flame height increased with NabH 4 addition, which is most likely a function of increased blowing at the surface. There was no appreciable flame height sensitivity to NabH4 particle size. There was no relative change in flame height or regression rate between the Al and AL-PTFE addition. The OCC allowed visualization of the hybrid rocket fuel flame zone at mass flux and pressure levels that are not known to be report for traditional slab burner configurations in literature. The OCC proved to be a new useful tool for investigated hybrid rocket propellant combustion characteristics.

  8. Complex Hydride Compounds with Enhanced Hydrogen Storage Capacity

    SciTech Connect

    Mosher, Daniel A.; Opalka, Susanne M.; Tang, Xia; Laube, Bruce L.; Brown, Ronald J.; Vanderspurt, Thomas H.; Arsenault, Sarah; Wu, Robert; Strickler, Jamie; Anton, Donald L.; Zidan, Ragaiy; Berseth, Polly

    2008-02-18

    The United Technologies Research Center (UTRC), in collaboration with major partners Albemarle Corporation (Albemarle) and the Savannah River National Laboratory (SRNL), conducted research to discover new hydride materials for the storage of hydrogen having on-board reversibility and a target gravimetric capacity of ≥ 7.5 weight percent (wt %). When integrated into a system with a reasonable efficiency of 60% (mass of hydride / total mass), this target material would produce a system gravimetric capacity of ≥ 4.5 wt %, consistent with the DOE 2007 target. The approach established for the project combined first principles modeling (FPM - UTRC) with multiple synthesis methods: Solid State Processing (SSP - UTRC), Solution Based Processing (SBP - Albemarle) and Molten State Processing (MSP - SRNL). In the search for novel compounds, each of these methods has advantages and disadvantages; by combining them, the potential for success was increased. During the project, UTRC refined its FPM framework which includes ground state (0 Kelvin) structural determinations, elevated temperature thermodynamic predictions and thermodynamic / phase diagram calculations. This modeling was used both to precede synthesis in a virtual search for new compounds and after initial synthesis to examine reaction details and options for modifications including co-reactant additions. The SSP synthesis method involved high energy ball milling which was simple, efficient for small batches and has proven effective for other storage material compositions. The SBP method produced very homogeneous chemical reactions, some of which cannot be performed via solid state routes, and would be the preferred approach for large scale production. The MSP technique is similar to the SSP method, but involves higher temperature and hydrogen pressure conditions to achieve greater species mobility. During the initial phases of the project, the focus was on higher order alanate complexes in the phase space between alkaline metal hydrides (AmH), Alkaline earth metal hydrides (AeH2), alane (AlH3), transition metal (Tm) hydrides (TmHz, where z=1-3) and molecular hydrogen (H2). The effort started first with variations of known alanates and subsequently extended the search to unknown compounds. In this stage, the FPM techniques were developed and validated on known alanate materials such as NaAlH4 and Na2LiAlH6. The coupled predictive methodologies were used to survey over 200 proposed phases in six quaternary spaces, formed from various combinations of Na, Li Mg and/or Ti with Al and H. A wide range of alanate compounds was examined using SSP having additions of Ti, Cr, Co, Ni and Fe. A number of compositions and reaction paths were identified having H weight fractions up to 5.6 wt %, but none meeting the 7.5 wt%H reversible goal. Similarly, MSP of alanates produced a number of interesting compounds and general conclusions regarding reaction behavior of mixtures during processing, but no alanate based candidates meeting the 7.5 wt% goal. A novel alanate, LiMg(AlH4)3, was synthesized using SBP that demonstrated a 7.0 wt% capacity with a desorption temperature of 150°C. The deuteride form was synthesized and characterized by the Institute for Energy (IFE) in Norway to determine its crystalline structure for related FPM studies. However, the reaction exhibited exothermicity and therefore was not reversible under acceptable hydrogen gas pressures for on-board recharging. After the extensive studies of alanates, the material class of emphasis was shifted to borohydrides. Through SBP, several ligand-stabilized Mg(BH4)2 complexes were synthesized. The Mg(BH4)2*2NH3 complex was found to change behavior with slightly different synthesis conditions and/or aging. One of the two mechanisms was an amine-borane (NH3BH3) like dissociation reaction which released up to 16 wt %H and more conservatively 9 wt%H when not including H2 released from the NH3. From FPM, the stability of the Mg(BH4)2*2NH3 compound was found to increase with the inclusion of NH3 groups in the inner-Mg coordination

  9. The development of reactive fuel grains for pyrophoric relight of in-space hybrid rocket thrusters

    NASA Astrophysics Data System (ADS)

    Steiner, Matthew Wellington

    This study presents and investigates a novel hybrid fuel grain that reacts pyrophorically with gaseous oxidizer to achieve restart of a hybrid rocket motor propulsion system while reducing cost and handling concerns. This reactive fuel grain (RFG) relies on the pyrophoric nature of finely divided metal particles dispersed in a solid dicyclopentadiene (DCPD) binder, which has been shown to encapsulate air-sensitive additives until they are exposed to combustion gases. An RFG is thus effectively inert in open air in the absence of an ignition source, though the particles encapsulated within remain pyrophoric. In practice, this means that an RFG that is ignited in the vacuum of space and then extinguished will expose unoxidized pyrophoric particles, which can be used to generate sufficient heat to relight the propellant when oxidizer is flowed. The experiments outlined in this work aim to develop a suitable pyrophoric material for use in an RFG, demonstrate pyrophoric relight, and characterize performance under conditions relevant to a hybrid rocket thruster. Magnesium, lithium, calcium, and an alloy of titanium, chromium, and manganese (TiCrMn) were investigated to determine suitability of pure metals as RFG additives. Additionally, aluminum hydride (AlH3), lithium aluminum hydride (LiAlH4), lithium borohydride (LiBH4), and magnesium hydride (MgH2) were investigated to determine suitability of metals hydrides as RFG additives or as precursors for pure-metal RFG additives. Pyrophoric metals have been previously investigated as additives for increasing the regression rate of hybrid fuels, but to the author's knowledge, these materials have not been specifically investigated for their ability to ignite a propellant pyrophorically. Commercial research-grade metals were obtained as coarse powders, then ball-milled to attempt to reduce particle size below a critical diameter needed for pyrophoricity. Magnesium hydride was ball-milled and then cycled in a hydride cycling apparatus to attempt to fracture the particles through hydrogen sorption and thermal stresses. These powders were then tested for pyrophoricity with atmospheric and pure concentrations of oxygen. The TiCrMn powder was chosen as the material for evaluation of propellant performance, and was mixed with DCPD in various weight ratios to determine the required additive loading needed for pyrophoricity of the bulk propellant. Weight percentages of 10, 20, 30, and 50 wt.% TiCrMn were used to evaluate relight capability and propellant performance, and weight loadings of 50, 70, and 90 wt.% TiCrMn were used to evaluate approximate maximum loading possible without rendering the propellant structurally unsound. Propellant tests were conducted in an opposed flow burner apparatus for sub-scale regression rate and relight experiments, and an optically accessible cylindrical combustion chamber (OCC) that allows high speed cameras to record the regressing propellant surface during combustion. Gaseous oxygen (GOX) was used as an oxidizer for all tests due to its ready availability and common use as a hybrid rocket oxidizer. Opposed flow burner experiments are an inexpensive means of rapidly testing various propellant formulations at different conditions, whereas OCC tests are useful for obtaining realistic data on how an RFG would likely operate as part of a propulsion system. Relight in the opposed flow burner was attempted by cycling oxygen and nitrogen flows with carefully timed solenoid valves to initiate and extinguish combustion, and to control the slow diffusion of oxygen to the surface of the propellant, which would render the TiCrMn non-pyrophoric. The opposed flow burner experiments did not conclusively demonstrate the pyrophoric relight capability of the RFG propellant due in part to the persistence of hot spots between oxygen and purge nitrogen cycles, as determined by high-speed imaging in the near infrared range. An opposed flow burner apparatus was then constructed within a vacuum chamber assembly thus preventing atmospheric oxygen from diffusing to the propellant surface, but these tests did not demonstrate pyrophoric relight. Future work is proposed to evaluate the effect of pyrophoric particle size in order to determine the role ignition delay of each particle has in the relight capability of RFGs. OCC experiments were conducted at a low and high GOX mass flux of approximately 150 and 300 kg/s/m2, respectively, at a nominal chamber pressure of 150 psia. Four strand compositions were used: pure DCPD, 30 wt.% pyrophoric TiCrMn powder with average particle diameters of approximately 1-10 microns, 30 wt.% oxidized TiCrMn powder with average particle diameters of approximately 1-10 microns, and 30 wt.% TiCrMn powder with average particle diameters of approximately 1-4 mm. Regression rate was measure by weight loss, average web thickness change at three axial locations on the strand, and through time-resolved tracking of the regressing propellant surface via high speed video. While visual observations suggest that the addition of TiCrMn significantly increases regression rate, initial data do not show a significant trend. Additionally, it is observed that the oxidized TiCrMn strands regress at the same rate as those loaded with pyrophoric TiCrMn, suggesting that erosive burning and heat addition of the added metal may be the cause of the observed increase in regression rate. The data are too sparse to make conclusions about the effect of particle size on regression rate, so further tests are recommended to develop a significant data set for the effect of pyrophoricity and particle size on regression rate. The test article was damaged at the end of the regression rate experimental campaign, which precluded the collection of relight data that was planned for strands loaded with 50 wt.% TiCrMn particles with an average diameter of approximately 1-4 mm. Though further tests are needed to demonstrate pyrophoric relight of an RFG, the current work establishes a baseline for RFG performance and suggests that pyrophoric relight is possible by tailoring the particle size of the pyrophoric metal additive to control heat release and ignition delay.

  10. Search for nitrates on Mars by the Sample Analysis at Mars (SAM) Instrument

    NASA Astrophysics Data System (ADS)

    Navarro-Gonzalez, Rafael

    One of the main goals of the Mars Science Laboratory is to determine whether the planet ever had environmental conditions capable of supporting microbial life. Nitrogen is a fundamental element for life, and is present in structural (e.g., proteins), catalytic (e.g., enzymes and ribozymes), energy transfer (e.g., ATP) and information storage (RNA and DNA) bio-molecules. Planetary models suggest that nitrogen was abundant in the early Martian atmosphere as dinitrogen (N _{2}). However, a fraction of N _{2} has been lost to space by sputtering and photochemical processes [1, 2], impact erosion [3], and chemical oxidation to nitrates [4, 5]. Nitrates produced early in Mars’ history by photochemistry may later decompose back into N _{2} by the current impact flux [6]. It is estimated that the Martian surface could contain soil nitrates at levels of 0.3 wt.% N, if mixed homogenously [6], or a layer of pure NaNO _{3} of about 3 m thickness [5] distributed globally. Nitrates are a fundamental source for nitrogen for terrestrial microorganisms. Therefore, the detection of soil nitrates is important to assess habitability in the Martian environment. The only previous attempt to search for soil nitrates was by TEGA and the MECA WCL on the Phoenix mission but no evolved N-containing species were detected [7]. Nitrates have been tentatively identified in two Martian meteorites: Nakhla [8] and EETA79001 [9]. SAM is capable of detecting nitrates by their thermal decomposition into nitric oxide, NO. SAM analyzed samples from Rocknest soil and two drill holes located at John Klein (JK) and Cumberland (CB) mudstones in the Sheepbed member of the Yellowknife Bay formation in Gale Crater. There appear to be several peaks associated with the release of m/z 30 in the temperature range from 150(°) °C to 600(°) °C. M/z 30 can be attributed to nitric oxide; however, other possible chemical interferences may be present, such as ethane (C _{2}H _{6}), formaldehyde (HCHO), diazene (N _{2}H _{2}), aluminum trihydride (AlH _{3}), and silylene (SiH _{2}), and they are assessed. The origin of nitric oxide is discussed and its thermal evolution is compared with analog studies of mixtures of nitrates and perchlorates [10]. [1] Luhmann, J.G., Johnson E. And Zhang, M.H.G.: 1992, Evolutionary impact of sputtering of the Martian atmosphere by O (+) pickup ions. Geophys. Res. Lett. 19, 2151-2154. [2] Jakosky, B.M. Pepin, R.O., Johnsom, R.E. and Fox, J.L: 1994, Mars atmospheric loss and isotopic fractionation by solar-wind-induced sputtering and photochemical escape. Icarus 111, 271-288. [3] Melosh, H.J. and Vickery, A.M.: 1989, Impact erosion of the primordial atmosphere of Mars. Nature 338, 487-489. [4] Mancinelli, R.L. and McKay, C.P. :1988, The evolution of nitrogen cycling. Origins Life 18, 311-325. [5] Manning, C.V., McKay, C.P., and Zahnle, K.J.: 2008, The nitrogen cycle on Mars: Impact decomposition of near-surface nitrates as a source for a nitrogen steady state. Icarus 197, 60-64. [6] Smith, M.L., Claire, M.W., Catling, D.C., and Zahnle, K.J.: 2014, The formation of sulfate, nitrate and perchlorate salts in the martian atmosphere. Icarus 231, 51-64. [7] Hecht, M. H., Kounaves, S.P., Quinn, R.C., West, S.J., Young, S.M.M., Ming, D.W.,Catling, D.C., Clark, B.C., Boynton, W.V.,Hoffman, J., DeFlores, L.P., Gospodinova, K., Kapit, J., and Smith,P.H.: 2009, Detection of Perchlorate and the Soluble Chemistry of Martian Soil at the Phoenix Lander Site. Science, 325, 64-67. [8] Grady, M.M., Wright, I. P., and Pillinger C. T.: 1995, Search for nitrates in Martian meteorite. J. Geophys. Res. 100, 5449. [9] Kounaves, S.P., Carrier, B.L., O’Neil, G.D., Stroble, S.T., Claire, M.W.: 2013, Evidence of martian perchlorate, chlorate, and nitrate in Mars meteorite EETA79001: implications for oxidants and organics, Icarus 229, 206. [10] Support from the following grants is acknowledged: IN106013 and CONACYT 98466.