NEEM MURINEEM MURI

High Energy Nanomaterials:High Energy Nanomaterials:Aluminum and BoronAluminum and BoronAluminum and BoronAluminum and Boron

Ralph G. Nuzzo Ralph G. Nuzzo and Gregory S. Girolami

Dept. of ChemistryUniversity of IllinoisUrbana-Champaign

NEEM MURINEEM MURI Boron is an attractive fuel for propellants

• Boron has the third highest ggravimetric heat of combustion (14.1 kcal/g), after H2 and Be.

• Boron has the highestBoron has the highest volumetric heat of combustion (33.4 kcal/cm3).

• Energy content 3 times higher• 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 B2O33– Oxide layer on boron particles inhibits surface vaporization and results

in ignition delays

NEEM MURINEEM MURI10-50 nm boron nanoparticles

• Prepared by gas phase pyrolysis of B10H14 at ~800 °CB10H14 700-800ºC/ 1atm Ar carrier gas

2 hrs10 B + 7 H2

• Semicrystalline (25 Å domain) α-rhombohedral boron• Spherical shapes, weakly aggregated • No surface oxide coating

Tube Furnace

NEEM MURINEEM MURI TGA studies of oxidation ofboron nanoparticles

195

215

155

175

195

571 ºCeigh

t (%

)

95

115

135571 ºC

We

C b ti f b 9520 220 420 620 820

Temperature ºC

Combustion of boron nanoparticles

• 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 B2O3.

NEEM MURINEEM MURI TEM and STEM studies of oxidation of boron nanoparticles

C b ti f th b• Combustion of the boron nanoparticles at <900 °C oxidizes the outside of the particles, but the inside

TEM image of a boron nanoparticle

after oxidation.p ,remains unreacted boron.

• STEM-EELS analysis confirms ~50 % oxidation

10 nm

confirms 50 % oxidation to B2O3

• Would surface derivatization change thederivatization change the results?

• Surface derivatization of boron nanoparticles hasboron nanoparticles has never previously been demonstrated

STEM-EELS data showing the oxygen is located on the outside of

the boron nanoparticle.

NEEM MURINEEM MURI Synthesis of Br and F capped boron nanoparticles

• Boron nanoparticles treated with excess Br2 in benzene

Br

rwith excess Br2 in benzene yields boron nanoparticles capped with bromine.

• EA reveals that particles

Br

Br

Br

F

• EA reveals that particles are >90% boron and >9% bromine (by mass).

Si il ti f b

B + XeF2 + XeBenzene72 hrs/Rm. Temp.

F

F

F

5 mol %

• Similar reactions of boron nanoparticles with XeF2leads to samples with >95% boron and ~3% fluorine.

• Sputtering experiments reveal halogen is on the surface of the particles.

Combustion of surface derivatized boron nanoparticles.

surface of the particles.

NEEM MURINEEM MURI DSC studies of coated nanoparticles

The bromine and fluorine coated particles are passivated: the onset temperatures for combustion are 15-70 °C higher t e o set te pe atu es o co bust o a e 5 0 C g ethan for the as-synthesized boron nanoparticles. The heats released appear similar, but...

NEEM MURINEEM MURI TGA studies of bromine coated nanoparticles

• Oxidations in pure oxygen with a heating rate of 10 ºC / min.

• The particles are 185

205

resistant to oxidation with no significant oxidation up to ~610 ºC.

R id id ti b i 145

165

185

ght

(%)

• Rapid oxidation begins around 620 ºC.

• After the reaction is l t th t t l 105

125

145627 ºC

Wei

g

complete the total amount of boron consumed to yield B2O3is 51%.

85

105

20 220 420 620 820Temperature ºCp

NEEM MURINEEM MURI TGA studies of fluorine coated nanoparticles

• Oxidations were performed in pure oxygen with ain pure oxygen with a heating rate of 10 ºC / min.

• No significant oxidation up to ~520 ºC, at which point

235to 520 C, at which point sustained oxidation commences

• After the reaction is 175

195

215

t (%

)

complete the total amount of boron consumed to yield B2O3 is 73% 135

155

590 ºC

Wei

ght

• A 46% increase in combustion yield compared to boron nanoparticles or Br coated nanoparticles

95

115

20 220 420 620 820

590 C

Temperature ºCcoated nanoparticles Temperature ºC

NEEM MURINEEM MURI F-coated boron nanoparticles show more complete combustion

70

60

(%)

40

50

onsu

med

(

Catalytic effect!

30

Bor

on C

o

Recall that F content is only 3% by mass

10

20 Involvement of boron oxyfluoride species?

0B B(Br) B(F)

NEEM MURINEEM MURI Boron nanoparticles suspended in polyfluoroethers

• Sonocation is a convenient method for the synthesis of Apparatus for method for the synthesis of nanoparticles.

• Extremely high temperatures hi d i f

Apparatus for sonocation of decaborane.

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

Stable suspension of boron nanoparticles 30 nm with a narrow

distribution. boron nanoparticles

in Krytox ® oil.

NEEM MURINEEM MURI 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 CH2Cl2 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.

NEEM MURINEEM MURI Properties of boron nanoparticle/ Krytox mixture

• The as prepared boron nanoparticles stay suspended in Krytox® oil for >3 months.

• Combustion results in a highly exothermic reaction.

Q tit ti d t il f b ti• Quantitative details of combustion under investigation (Yetter)

Combustion of boron ti l i K t ®nanoparticles in Krytox ®.

NEEM MURINEEM MURI Recent results on aluminum nanoparticles

Al nanoparticles are generated by reductive elimination of H2from aluminum hydridesfrom aluminum hydrides

AlH3 . NEt3heptane

nanoAl[Ti(iPrO)4]

cappingreagent

[ ( )4]

passivated aluminum particles

• Above scheme gives aluminum nanoparticles ~30 nm in size

Elemental anal sis sho s particles are >90% Al• Elemental analysis shows particles are >90% Al

NEEM MURINEEM MURI As-synthesized aluminum nanoparticles are air sensitive

Δ H=7.4 kcal/gram

• Aluminum nanoparticles oxidize readily and exothermically in air.

• While smaller nanoparticles burn faster as fuels, they also oxidize upon storageupon storage.

TEM Image of Crystalline Aluminum Nanoparticles

NEEM MURINEEM MURI Transition metals as passivants for aluminum nanoparticles

• Aluminum nanoparticles coatedAluminum nanoparticles coated with transition metals in a core-shell fashion have the potential to be less susceptible to

id ti th l i l

Pure Aluminum Core

oxidation than aluminum alone.

• Higa et al. have reported that after an oxide layer is formed

Alon Ni coated Al particles, oxidation occurs slower. Pd, Pt Ag, and Au seem to have no effecteffect.

• First row transition metals are preferable due to their smaller mass

Protective Transition Metal Coating

mass.

Higa et al., Chem Mater, 2005, 17, 4086.

NEEM MURINEEM MURI Depositing transition metals on aluminum nanoparticles

acac =

• Cu(acac)2, Ag(acac), AuCl(SMe2), 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 % MMetal Precursor

Relative % Al Relative % M

Ni(acac)2 88 % Al 12 % NiRu(acac) 82 % Al 18 % RuRu(acac)3 82 % Al 18 % Ru

Table comparing the percentage of Al to the percentage of each metal. Data obtained via XPS.

NEEM MURINEEM MURI Aluminum nanoparticles coated with nickel

• Freshly coated aluminumFreshly coated aluminum nanoparticles are pyrophoric.

• Nickel nanoparticles coat the f f th l isurface of the aluminum

nanoparticles.

• Nickel does not completelyNi Al

Nickel does not completely cover the aluminum nanoparticle allowing for the aluminum to be further oxidizedoxidized. .

• 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

NEEM MURINEEM MURI Aluminum nanoparticles coated with transition metals

AlNi Al

Ag

2 nm2 nm

Al(Ni) Al(Ag)

NEEM MURINEEM MURI(S)TEM Images of Cu on Al

• (Left) C-s-STEM micrograph of Al nanoparticles treated with Cu(acac)2. (Middl ) E d d i f b d i i l f i• (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.- Spot 2 gives peaks due to Cu at 1.022, 8.04, and 8.98 keV due to the p g p , ,

L1-M2,3, K-L2,3, and K-M2,3,4,5 excitations. - The sample grid was constructed of Mo.

NEEM MURINEEM MURI(S)TEM images of Ag on Al

• (Left) Cs-STEM micrograph of Al nanoparticles treated with Ag(acac). • (Middle) Expanded view of boxed region in left image(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 2.633-3.525 keV due to L1,2,3 excitations to the M and

N l lN levels. - The sample grid was constructed of Mo.

NEEM MURINEEM MURI(S)TEM images of Au on Al

• (Left) Cs-STEM micrograph of Al nanoparticles treated with AuCl(SMe2). • (Middle) Expanded view of boxed region in left image• (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 9.713 and 10.308 keV for the L3-M5 and L2-M1 transitions, and a broad band enveloping the 11.371, 12.147, and 13.3-14.2 keV for the L1,2,3 transitions to the M and N levels.

- Spot 2 gives only Al and O peaks. - The sample grid was constructed of Mo.

NEEM MURINEEM MURI(S)TEM images of Ni on Al

• (Left) Cs-STEM micrograph of Al nanoparticles treated with Ni(acac)2. • (Middle) Expanded view of boxed region in left image• (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 7.461-7.478 keV for K-L1,2,3 excitations and at 8.264-8.328 keV for K-M1,2,3,4,5 excitations. 1,2,3,4,5

- Spot 2 gives peaks due to Al and O only. - The sample grid was constructed of Mo.

NEEM MURINEEM MURI(S)TEM images of Pd on Al

• (Left) Cs-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• (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 2.833-3.533 keV for L1,2,3 excitations to

the M and N levels. - The sample grid was constructed of Mo.

NEEM MURINEEM MURI(S)TEM images of Pt on Al

• (Left) Cs-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(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 8.268 (L3-M1), 12.942 (L2-N5), and 13.156 keV

(L1-M1), and broader bands spanning 9.362-9.442 (L3-M4,5), 11.044-11.250 ( d ) d 13 2 2 13 361 k ( )(L1,2,3-M4,5 and N2,3,4), and 13.272-13.361 keV (L1-M2,3).

- The sample grid was constructed of Mo.

NEEM MURINEEM MURI Aluminum nanoparticles coated with transition metals

20 nm

Al(Ag)20 nm20 nm

Al(Cu)

20 nm

Al(Au)

50 nm

Al(Pt)

50 nm50 nm

Al(Ni)

50 nm50 nm

Al(Ru)

20 nm

Al(Pd)

NEEM MURINEEM MURI 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 (Mn+ → M) and oxidation at aluminum surface (Al → Al3+)

• 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?

NEEM MURINEEM MURI Achievements under MURI grant

• Boron nanoparticles >97 % pure can be prepared by pyrolysis of decaborane (B10H14) at 700 – 900 °C; particle sizes are 10 – 50 nm in diameter; crystalline domains are ~25 Å in sizedomains are 25 Å in size

• Combustion of boron particles under O2 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• 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 materialsenergy 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• 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 propertiesResearch greatly aided by collaborations with Yetter on thermal properties and combustion behavior, and with Dlott on behavior under laser irradiation

NEEM MURINEEM MURIForward from here…?

A new class of nanostructured reactive materials (RMs) that combineA new class of nanostructured reactive materials (RMs) that combine

• the high energy release characteristic of thermites or intermetallic formation

From new structures come new properties.

• the large brisance of gas-evolving organic EM.

From new structures come new properties.

NEEM MURINEEM MURIAcknowledgments

• NEEM-MURI programNEEM 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)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. Illinoisy ,• Staff of the School of Chemical Sciences, U. Illinois