and Gregory S. Girolami

Transcription

1 High Energy Nanomaterials: Aluminum and Boron Ralph G. Nuzzo and Gregory S. Girolami Dept. of Chemistry University of Illinois Urbana-Champaign

2 Boron is an attractive fuel for propellants Boron has the third highest gravimetric heat of combustion (14.1 kcal/g), after H 2 and Be. Boron has the highest volumetric heat of combustion (33.4 kcal/cm 3 ). Energy content 3 times higher on volume basis and 1.4 times higher on weight basis than that of hydrocarbon fuels. Challenges Combustion pathways often lead to HOBO (with less energy release) as opposed to the desired B 2 O 3 Oxide layer on boron particles inhibits surface vaporization and results in ignition delays

3 10-50 nm boron nanoparticles Prepared by gas phase pyrolysis of B 10 H 14 at ~800 C B 10 H ºC/ 1atm Ar carrier gas 2hrs 10 B + 7 H 2 Semicrystalline (25 Å domain) α-rhombohedral boron Spherical shapes, weakly aggregated No surface oxide coating Tube Furnace

4 TGA studies of oxidation of boron nanoparticles We eight (%) ºC Combustion of boron nanoparticles Temperature ºC Oxidations were performed in pure oxygen at a heating rate of 10 ºC / min No significant oxidation up to ~500 ºC; rapid oxidation begins at ~570 ºC. Heat release 17.2 kj/g; mass gain is ~100 % Corresponds to conversion of ~50% of particle mass to B 2 O 3.

5 TEM and STEM studies of oxidation of boron nanoparticles Combustion of the boron nanoparticles at <900 C oxidizes the outside of the particles, but the inside remains unreacted boron. STEM-EELS analysis confirms ~50 % oxidation to B 2 O 3 Would surface derivatization change the results? 10 nm TEM image of a boron nanoparticle after oxidation. Surface derivatization of boron nanoparticles has never previously been demonstrated STEM-EELS data showing the oxygen is located on the outside of the boron nanoparticle.

6 Synthesis of Br and F capped boron nanoparticles Boron nanoparticles treated with excess Br 2 in benzene yields boron nanoparticles capped with bromine. Br rwith excess Br2 in benzene Br Br Br EA reveals that particles are >90% boron and >9% bromine (by mass). B+XeF 2 5 mol % Benzene 72 hrs/rm. Temp. F F F +Xe Similar il reactions of boron nanoparticles with XeF 2 leads to samples with >95% boron and ~3% fluorine. F Sputtering experiments reveal halogen is on the surface of the particles. Combustion of surface derivatized boron nanoparticles.

7 DSC studies of coated nanoparticles The bromine and fluorine coated particles are passivated: the onset temperatures es for combustion o are C C higher than for the as-synthesized boron nanoparticles. The heats released appear similar, but...

8 TGA studies of bromine coated nanoparticles Oxidations in pure oxygen with a heating rate of 10 ºC / min. The particles are resistant to oxidation with no significant oxidation up to ~610 ºC. Weig ght (%) Rapid oxidation begins around 620 ºC. After the reaction is complete the total t amount of boron consumed to yield B 2 O 3 is 51% ºC Temperature ºC

9 TGA studies of fluorine coated nanoparticles Oxidations were performed in pure oxygen with a heating rate of 10 ºC / min. No significant oxidation up to ~520 ºC, at which point sustained oxidation commences Weight (%) After the reaction is 175 complete the total amount of 155 boron consumed to yield B 2 O 3 is 73% 135 A 46% increase in combustion yield compared to boron nanoparticles or Br coated nanoparticles ºC Temperature ºC coated nanoparticles Temperature ºC

10 F-coated boron nanoparticles show more complete combustion Boron Co onsumed (%) Catalytic effect! Recall that F content is only 3% by mass 20 Involvement of boron oxyfluoride species? 0 B B(Br) B(F)

11 Boron nanoparticles suspended in polyfluoroethers Sonocation is a convenient method for the synthesis of nanoparticles. Apparatus for sonocation of decaborane. Extremely high temperatures are achieved in a safe environment. Need a nonvolatile & inert solvent. Krytox oil (a polyfluoroether) is an excellent choice. Light scattering experiments reveal the particle size to be ~30 nm with a narrow distribution. Stable suspension of boron nanoparticles in Krytox oil.

12 Separation of boron nanoparticles from Krytox oil Addition of an equivalent volume of Vertrel induces separation of the boron nanoparticles from the solvent. Separation time is 1-6 hours. The particles can be isolated by filtration, followed by washing with CH 2 Cl 2 and pentane. 2 2 XPS analysis shows particles are ~80% boron and ~10% fluorine (atomic %). Sputtering experiments show the particles are homogenous. Separation of boron nanoparticles from krytox using Vertrel. Vertrel = mixture of 2,3-dihydrodecafluoropentane, trans-1,2-dichloroethylene and cyclopentane.

13 Properties of boron nanoparticle/ Krytox mixture Combustion of boron nanoparticles in Krytox. The as prepared boron nanoparticles stay suspended in Krytox oil for >3 months. Combustion results in a highly exothermic reaction. Quantitative details of combustion under investigation (Yetter)

14 Recent results on aluminum nanoparticles Al nanoparticles are generated by reductive elimination of H 2 from aluminum hydrides AlH 3.NEt 3 heptane [Ti( i PrO) 4 ] nanoal capping reagent passivated aluminum particles Above scheme gives aluminum nanoparticles ~30 nm in size Elemental analysis shows s particles are >90% Al

15 As-synthesized aluminum nanoparticles are air sensitive Δ H=7.4 kcal/gram Aluminum nanoparticles oxidize readily and exothermically in air. TEM Image of Crystalline Aluminum Nanoparticles While smaller nanoparticles burn faster as fuels, they also oxidize upon storage.

16 Transition metals as passivants for aluminum nanoparticles Aluminum nanoparticles coated with transition metals in a coreshell fashion have the potential to be less susceptible to oxidation than aluminum alone. Higa et al. have reported that after an oxide layer is formed on Ni coated Al particles, oxidation occurs slower. Pd, Pt Ag, and Au seem to have no effect. First row transition metals are preferable due to their smaller mass. Pure Aluminum Core Al Protective Transition Metal Coating Higa et al., Chem Mater, 2005, 17, 4086.

17 Depositing transition metals on aluminum nanoparticles acac = Cu(acac) 2, Ag(acac), AuCl(SMe 2 ), Ni(acac) 2, Pd(acac) 2, Pt(acac) 2, Ru(acac) 3 The transition metal acetylacetonate (acac) complex is reduced by the aluminum metal and leaves behind a coating of transition metal. Metal Relative % Al Relative % M Precursor Ni(acac) 2 88 % Al 12 % Ni Ru(acac) 3 82 % Al 18 % Ru Table comparing the percentage of Al to the percentage of each metal. Data obtained via XPS.

18 Aluminum nanoparticles coated with nickel Freshly coated aluminum nanoparticles are pyrophoric. Nickel nanoparticles coat the surface of the aluminum nanoparticles. Nickel does not completely cover the aluminum nanoparticle allowing for the aluminum to be further oxidized.. Ni Al Results confirmed by XPS and STEM-EDS. 20 nm TEM i f Al ti l TEM image of a Al nanoparticle coated with nickel nanoparticles

19 Aluminum nanoparticles coated with transition metals Ni Al Ag Al 2 nm 2 nm Al(Ni) Al(Ag)

20 (S)TEM Images of Cu on Al (Left) C- s -STEM micrograph of Al nanoparticles treated with Cu(acac) 2. (Middle) Expanded d view of boxed region in left image. (Right) EDS spectra of spots 1 and 2 in left image. - Spot 1 gives peaks due to Al and O. -Spot 2 gives peaks due to Cu at 1.022, 8.04, and 8.98 kev due to the L 1 -M 2,3, K-L 2,3, and K-M 2,3,4,5 excitations. - The sample grid was constructed of Mo.

21 (S)TEM images of Ag on Al (Left) C s -STEM micrograph of Al nanoparticles treated with Ag(acac). (Middle) Expanded view of boxed region in left image. (Right) EDS spectra of spots 1 and 2 in left image. - Spot 1 gives only Al and O peaks. - Spot 2 gives Ag peaks at kev due to L 1,2,3 excitations to the M and Nl levels. l - The sample grid was constructed of Mo.

22 (S)TEM images of Au on Al (Left) C s -STEM micrograph of Al nanoparticles treated with AuCl(SMe 2 ). (Middle) Expanded view of boxed region in left image. (Right) EDS spectra of spots 1 and 2 in left image. - Spot 1 gives Au peaks at and kev for the L 3 -M 5 and L 2 -M 1 transitions, and a broad band enveloping the , , and kev for the L 1,2,3 transitions to the M and N levels. - Spot 2 gives only Al and O peaks. - The sample grid was constructed of Mo.

23 (S)TEM images of Ni on Al (Left) C s -STEM micrograph of Al nanoparticles treated with Ni(acac) 2. (Middle) Expanded view of boxed region in left image. (Right) EDS spectra of spots 1 and 2 in left image. - Spot 1 gives peaks due to Ni at kev for K-L 1,2,3 excitations and at kev for K-M 1,2,3,4,5 excitations. - Spot 2 gives peaks due to Al and O only. - The sample grid was constructed of Mo.

24 (S)TEM images of Pd on Al (Left) C s -STEM micrograph of Al nanoparticles treated with Pd(acac) 2. (Middle) Expanded view of boxed region in left image. (Right) EDS spectra of spots 1 and 2 in left image. - Spot 1 gives peaks due to Al and O only. - Spot 2 gives peaks due to Pd between kev for L 1,2,3 excitations to the M and N levels. - The sample grid was constructed of Mo.

25 (S)TEM images of Pt on Al (Left) C s -STEM micrograph of Al nanoparticles treated with Pt(acac) 2. (Middle) Expanded view of boxed region in left image. (Right) EDS spectra of spots 1 and 2 in left image. - Spot 1 gives peaks due to Al and O only. - Spot 2 gives peaks for Pt at (L 3 -M 1 ), (L 2 -N 5 ), and kev (L 1 -M 1 ), and broader bands spanning (L 3 -M 4,5 ), (L 1,2,3 -M 4,5 and N 2,3,4 ), and kev (L 1 -M 2,3 ). - The sample grid was constructed of Mo.

26 Aluminum nanoparticles coated with transition metals 20 nm 20 nm 20 nm Al(Cu) Al(Ag) Al(Au) 50 nm 20 nm 50 nm 50 nm Al(Ni) Al(Pd) Al(Pt) Al(Ru)

27 Lemons into lemonade: not coated nanoparticles, but nanocomposites The transition metal nanoparticles are catalysts for the reduction of the metal acac complex The mechanism involves chemistry at two places: reduction at transition metal (M n+ M) and oxidation at aluminum surface (Al Al 3+ ) If it were possible to poison the transition metal catalysis selectively, but leave the Al surface electrochemically active, uniformly coated particles might result. Thiols? This chemistry gives nanocomposites of Al nanoparticles and very small transition metal particles (including nickel). Are these nanocomposites highly energetic materials?

28 Achievements under MURI grant Boron nanoparticles >97 % pure can be prepared by pyrolysis of decaborane (B 10 H 14 ) at C; particle sizes are nm in diameter; crystalline domains are ~25 Å in size Combustion of boron particles under O 2 up to 900 C produces 17.2 kj/g, and is self-limiting owing to formation of an oxide coating First surface derivatization of B nanoparticles achieved; F-coated B nanoparticles show sustaining combustion reaction and higher ΔH Mixtures of B nanoparticles and polyfluoroethers show promise as high- energy materials Aluminum nanoparticles have been prepared and characterized by TEM, (S)TEM, EELS, EDS, XRD, XPS, and TGA/DSC. Attempts to passivate the surface with transition metals does not result in a uniform coating, but instead generates a hierarchically organized nanocomposite of Al nanoparticles and very small transition metal clusters Research greatly aided by collaborations with Yetter on thermal properties Research greatly aided by collaborations with Yetter on thermal properties and combustion behavior, and with Dlott on behavior under laser irradiation

29 Forward from here? A new class of nanostructured reactive materials (RMs) that combine the high energy release characteristic of thermites or intermetallic formation the large brisance of gas-evolving organic EM. From new structures come new properties.

30 Acknowledgments NEEM-MURI MURI program Prof. Richard Yetter and Mr. Michael Weismiller (DSC and TGA) Prof. Dana Dlott (shock wave studies of Al clusters) Prof. Kenneth K. Suslick (sonocation apparatus) Andrew Sealey, Wontae Noh, Charles W. Spicer, Brian J. Bellott and Sergio Sanchez Staff of the Center for Microanalysis of Materials, U. Illinois Staff of the School of Chemical Sciences, U. Illinois