1

Macroscale Characterization of Reactive & Mechanical Behaviors of Nano-Structured Energetic Materials

Kenneth K. KuoKenneth K. KuoThe Pennsylvania State UniversityThe Pennsylvania State University

University Park, PA 16802University Park, PA 16802

Presented at Presented at

NanoNano--Engineered Energetic Materials (NEEM) Engineered Energetic Materials (NEEM) MURI Kickoff MeetingMURI Kickoff Meeting

University Park, PAUniversity Park, PASeptember 14, 2004September 14, 2004

2

Characterization of Important Burning Behavior of a Solid Propellant

SYMBOL DESCRIPTIONa (Ti) Pre-exponential factor of the Saint Robert’s burning rate law ( ) nb ir a T P=

for propellant at an initial temperature of Tin Pressure exponent in the above burning rate lawao Pre-exponential factor of the Saint Robert’s burning rate law for propellant

at an initial reference temperature, Ti,ref i.e., ,( ) exp ( )i o P i i refa T a T Tσ = −

σP Temperature sensitivity, 1 b

Pb i P

rr T

σ ∂

≡ ∂ A Arrhenius pre-exponential factor [ ]exp /b a u sr A E R T= −Ea Activation energyQs Net heat release from the burning surface reactionλP Thermal conductivity of the propellantb Coefficient in the following surface temperature vs. pressure relationship:

,( ) ( )m

s s ref refT T b P P− = −m Exponent in the above surface temperature vs. pressure relationship

3

Schematic Diagram of Solid Propellant Strand Burner (SPSB)

• Up to 9,500 psi capability

• -60oC

4

Solid Propellant Strand Burner (SPSB)

• SPSB capabilities/features

Optically accessibleUp to 9,500 psi capabilityTemperature control –60oC < T < 80oCActive pressure controlConstant N2 purge entrains and cools exhaust productsSurge tank limits pressure excursions

Setra Pressure

Transducer

Surge Tank

Window for

optical access

SPSB

N2 inlet

To exhaust system

5

Ultra High Pressure Strand Burner (UHPSB)

Up to 30,000 psi capabilityConstant temperature control Not optically accessible

TV set

Power

ONOFF

FIRE

TC/BW Continuity

Compressor Exhaust

Exhaust

Air

To lab

Compressor Building

SV1

HV8

OFFRemoteManual

(P1)

HV7

HV6

HV5

HV2

HV3

HV1

Continuity (Pmax=30,000 psi)

(Pmax=30,000 psi)

HV9

ON

OFF

ON

OFF

(Test Cell)High Pressure

Combustion Lab(Control Room)

Water In

Water Out

High PressureCompressor

Cylinder A:Test Chamber

Cylinder B:Storage Tank

SolenoidValve Test Sample

Large N2 mass limits pressure excursionRemote controlsNo purge flow

6

Propellant Sample Preparation

BW #2

BW #1

Igniter

Curved CutThermocouple

• Propellant samples can be prepared with embedded breakwires and thermocouples

• Breakwires in a propellant strand are used in determining burn rate of a given propellant

• Micro-thermocouples are used to monitor propellant strand initial temperature and the thermal profile adjacent to the burning surface

7

Micro-thermocouple for Temperature Measurements

• Thermocouples down to a few microns in thickness can be embedded in solid propellant strands, using the cone and cup configuration.

• The R- or S-type high-temperature thermocouples can measure temperature up to ~2000 K.

8

Typical Temperature Profile Measuredby Using Micro-thermocouples

• This figure shows a typical thermocouple trace of a propellant

• As seen in the figure, the surface temperature is 751 K and the thermal wave thickness is approximately 60 micrometers

300

400

500

600

700

800900

1000

-120 -100 -80 -60 -40 -20 0 20 40Distance From Surface [ µm]

Solid Phase

GasPhase

Tsurface

= 751.1 K

P = 640 psiarb= 1.52 cm/s

2000

9

Video images for Burning Rate Determination

• Propellant burning at 8,000 psi

Time [ms]: 0 140 270 450 630

1 mm

10

SEM Micrographs of Two RDX-based Composite Propellants with Same Formulation but Different Particle Sizes

• Advanced Moderate Energy (AME) Propellant with micron-sized RDX particles (2000X)

• Advanced Moderate Energy (AME) Propellant with nano-sized RDX particles (2000X)

11

Measured and Correlated Burning Rate and rb Ratio for AHE and AME Propellants

• Burning rate of the AME Propellant with nano-sized RDX is similar to that with micron- sized RDX; however, the shock and impact sensitivities are expected to be lower for the propellant with nano-sized RDX particles

• High burning rate ratio was achieved using the AHE and AME propellants

0.1

1

10

100

0.8

1.2

1.6

2

2.4

2.8

3.2

1000 104 105

AME - RDX/BBA/Alex Nano-RDX/BBA/Alex AHE - HNF/RDX/BBA/AL

AHE/AMEAHE/Nano-RDX

Burn

Rat

e

Burning R

ate Ratio

Pressure [psig]

12

Burning Rate vs. Initial Temperature

• The temperature sensitivities of this propellant are determined from the slopes of these plots for two different pressures.

13

Temperature Sensitivity versus Pressure for Two Selected Propellants

14

Temperature Sensitivity of a Gun Propellant

0.0015

0.002

0.0025

0.003

0.0035

500 1000 1500 2000

4 6 8 10 12

Pressure [psia]

Pressure [MPa]• The temperature sensitivity

is defined using the following equation:

• This parameter can be used to find the Novozhilov κ-stability factor

ln bp

i p

rT

σ ∂

= ∂

( )- p s iκ σ≡ Τ Τ

15

Arrhenius Burning Rate Law

16

Novozhilov Stability Parameters

• The two Novozhilov stability parameters are the κ-stability factorand the γ-stability parameter and are defined below

• Stability using the Novozhilov parameters is defined as:

К < 1 or К > 1 and γ > γ* where

• By using the surface temperature measurements, these two parameters can be used to determine the stability of a propellant

( ) ss ii

, Tp p

κ σ γ ∂

≡ − ≡ ∂

TT T

( )( )1

1*2

+κ−κ

=γ

17

Stability Plot for MURI #1a Propellant

• Small unstable combustion regime was identified

18

Characterization of New Propellants and Nano Energetic Materials

• Physical PropertiesDensity, particle size distribution, etc.

• Micro and Macroscopic Imaging Particle shape, size, and their distribution in propellantsSurface features of original and partially-burned propellants

• Thermal AnalysisAmount of unreacted (“active”) material present based on mass gained during oxidationReactivity and stored energy based on exotherm and onset temperature

• Other Physical & Chemical Analysis• Combustion Analysis

Compare and correlate particle and propellant properties to combustion results and performance

19

Particle Characterization Capability Using PSU’s PCL Facilities

• Characteristics such as particle size, surface area,particle density, etc. can be measured and quantified. The particle characteristics of a material greatly influence many other chemical and physical characteristics including: stability, chemical reactivity, and material strength.

Helium Autopycnometer calculates particle densitiesBET can perform a multipoint surface area analysis Microtrac/Ultra Fine Particle Analyzer - particle size measurements can be made ranging from 0.003 to 6.5 microns.Horiba Particle Size Distribution Analyzer - particle size measurements can be made ranging from 0.01 µm to 300 µm.Laser Diffraction Particle Size Analyzer - provides particle size data ranging from 0.03 µm to 280 µm.Malvern Mastersizer - utilizing the wet technique, particle sizes throughout a dynamic size range of 0.05 µm to 900 µm can be measured, while the dry technique allows the user to quickly measure dry powders ranging from 0.5 µm to 900 µm.

20

Various Types of Electron Microscopy

• Scanning Electron Microscopy (SEM) uses a focused electron beam to scan small areas of solid samples. Spatial resolution of 4 nm is typical.

• Environmental Scanning Electron Microscopy (ESEM) has the capability for using different gases under controlled temperature or controlled rate of temperature increase.

• Energy Dispersive Spectroscopy (EDS) is a standard procedure for identifying and quantifying elemental composition of sample areas as small as a few cubic micrometers. Characteristic X-rays are produced when a material is bombarded with electrons in a SEM. Detection of these x-rays can be accomplished by an energy dispersive spectrometer to yield 2 dimensional elemental mapping of micron-sized features.

• Transmission electron microscopy (TEM) - A high voltage electron beam is passed through a thin (50-200 nm) solid sample. Contrast is derived by electrons scattering from atoms in the material. Atomic scale imaging is possible in MCL’s field emission instrument. In addition to the shape and size of the particles, phase identification and separation, chemical analysis at the sub-10 nm level , and characterization of microstructures, defects and chemical compositions can be made.

21

SEM Images of Aluminum Powder

• For Alex® aluminum powder:Extremely spherical particlesSize ranges from 50 to 250 nm

500 nm2 µm

20,000 times magnification 100,000 times magnification

• Other particles:Varying degrees of uniform spheres and agglomerationAverage diameter from 30 to 150 nm

22

Geometry of Aluminum Flakes

• Approximate dimensions of flake:4 to 50 µm in width50 to 200 nanometers in thickness

(Photos courtesy of Dr. May Chan of NSWC-China Lake)

23

TEM of Aluminum Nano-Rods

(image courtesy of NanoMat, Inc., North Huntingdon, PA)

24

Thermal Analysis Capability UsingPSU’s TAL Facilities

• The Thermal Analysis Laboratory (TAL) contains several instruments designed to characterize materials based on their behavior under various heat conditions.

Differential Scanning Calorimetry (DSC) measures the temperatures and heat flows associated with transitions in materials to provide information about physical and chemical changes that involve endothermic and exothermic processes.Thermogravimetric Analysis (TGA) measures changes in weight of a sample with increasing temperature. Differential Thermal Analysis (DTA) measures the difference in temperature between a sample and a thermally inert reference as the temperature is raised, providing information on exothermic and endothermic reactions taking place in the sample.

• Typical applicationsComposition of multicomponent systems Thermal and oxidative stability Effect of reactive atmospheres on materials Melting point and phase transitionsHeat capacity

25

DSC/TGA Curves of Silberline Flakes

• Weight gain due to oxidation from 97 to 174% of original mass• Two-stage energy release—a portion below Tmelt

-15

-10

-5

0

5

10

15

20

100

110

120

130

140

150

160

170

180

0 200 400 600 800 1000 1200

DSC Analysis

TGA Analysis

Hea

t Flo

w [W

/g]

Weight %

Temperature [oC]

TMELT

= 660 oC (aluminum)

HR = 5oC/min in O2 Environment

97%

174%

26

DSC/TGA Curves of NTECH-50

• Weight gain due to oxidation from 97 to 151% of original mass• Two-stage energy release—most below Tmelt

-20

0

20

40

60

90

100

110

120

130

140

150

160

0 200 400 600 800 1000 1200

DSC Analysis

TGA AnalysisH

eat F

low

[W/g

]W

eight %

Temperature [oC]

TMELT

= 660 oC (aluminum)

HR = 5oC/min in O2 Environment

97%

151%

27

Other Available Analysis Tools

• Field emission Auger electron spectroscopy - a finely focused electron beam ionizes atoms in the near surface by the production of a core hole. The ion loses energy by filling this core hole with an electron from a shallower level combined with ejection of an electron. The energy of this Auger electron is characteristic of the atom from which it was emitted and the number of electrons is proportional to the concentration of that element in the sample. The relatively low energy makes the technique inherently surface sensitive with the majority of the Auger electrons in a given sample originating from the outer 5-10 nm. In certain elements (Al, Mg, Si, In, Cu) the energy is a function of the local environment of the atom yielding a chemical (or oxidation) state sensitive tool.

• X-ray diffraction (XRD) - X-rays with wavelength on the order of lattice spacing are elastically scattered (i.e., diffracted) from the atomic planes in a crystalline material yielding diffraction peaks. Using Bragg’s Law, the resultant diffraction pattern can be used to identify crystalline phases, determine residual stresses, preferred orientation or grain size.

28

Additional Analysis Tools

• Atomic Force Microscopy (AFM) - provides 3-dimensional topographic information about a sample by probing its surface structure with a very sharp tip. The lateral resolution of the image can be as small as the tip radius (typically 5-15 nm), and the vertical resolution can be on the order of angstroms.

• Secondary Ion Mass Spectrometry (SIMS) - primary ions with moderate energy bombard the sample surface and remove material by sputtering. A fraction of the sputtered material consists of positive and negative ions. These secondary ions aredrawn into a mass spectrometer where they are analyzed according to their mass-to-charge ratio. Elements H-U (including isotopes) can be detected at sensitivity levels of 1-10 ppm (atomic) for most elements.

29

Active Content Analysis of Al Particles

• Chemical hydrolysis analysisSample placed into aqueous KOH solution where Al reacts with KOH to form Al(OH)-4 and hydrogen gas The quantity of hydrogen is measured and compared to the amount generated from a known standard (Valimet H-2 Al powder) to find active Al content Performed by Dr. Curtis E. Johnson of NAWC-China Lake

• Thermogravimetric analysis (TGA) Sample heated in an oxygen atmosphere until completely oxidizedActive Al content determined from measured mass gain and assuming all weight gain due to reaction to stable oxide compound

30

Active Content Results• In general, good

agreement between the two methods for Al

• Significant differences found among the different particles (unreacted content ranged from 33.5 to 88.8 % by weight) 0

20

40

60

80

100

Alex

®

WAR

P-1

SILB

AL

CLA

L

TEC

HA

L

Boro

n

C-A

lex®

IHD

-AR

IHD

-HE

NTE

CH

-80

NTE

CH

-50

AVA

L

C-B

oron

KOHTGA

Act

ive

Mat

eria

l Con

tent

[wt%

]

31

Derived Parameters of Interest for Various Nano-sized Particles

• Effective density (ρeffective) is useful for determining density enhancement of solid fuel/propellant formulations containing particles.

• Higher density is not necessarily better because oxide density greater than base material. 2.9

3.12.53.11.3

3.82.04.64.65.33.1

Average Oxide Thickness [nm]

assumed same as BoronC-Boron

3.0954NTECH-502.95102NTECH-803.2926IHD-HE

3.0245TECHAL

23

41assumed same as Alex®

82

flakesflakes

68160

Average Dia. [nm]

3.20WARP-1

3.43

2.92

2.37

3.002.83

2.84

ρeffective[g/cm3]

AVAL

IHD-ARCoated-AlexBoron

CLALSILBAL

Alex®

Particle Label

32

Evaluation of Combustion Enhancement by Nanoparticles Using Hybrid Rockets

• The hybrid rocket facility at the HPCL has been used to evaluate the propulsion performance of many energetic nano-sized particles

• All particles tested in same equipment under the same conditions

• Test motor is scaled-down version of flight motor so results and trends should be directly applicable to large-scale systems

• Hybrid rockets are very promising technologies for future space systems due to inherent safety and operability advantages over current systems

• One major disadvantage of hybrid rockets (low fuel regression rate) can be addressed by use of nanoparticles

33

LGCP Hybrid Motor Layout

Solid-fuel Grain

Phenolic Tube

Test StandGuide Support Blocks

Linear BearingLinear Guide Platform

Load Cell Support

Load Cell

Cradle Clamps

Motor Wall

End Plug RetainerInterchangeableGraphite Nozzle

Grain Length = 16 inchesGrain Diameter = 1.375 inchesCP Bore = 0.35 inches

ProtectiveGraphite Liner

34

Clean-Burning Plume Jet from a Hybrid Rocket

SF19 (5.65% Boron)

• Fuel Type: SF19 (5.65% boron)• Obtained clean burning plumes from solid fuels

containing nano-sized boron particles (with no visible particle streaks)

35

Aluminized HTPB Fuel Mass Burning Rates

Average Oxidizer Mass Flux [kg/m2-s]

80 100 120 140 160 180

Ave

rage

Mas

s B

urni

ng R

ate

[kg/

s]

0.007

0.01

0.02

0.03

0.04

13%ALEX (SF2)Pure HTPB (SF1)

6.5%WARP-1 (SF4) 13%SILBAL (SF7) 13%CLAL (SF8)13%WARP-1 (SF9)

13%TECHAL (SF11)13%Al325 (SF12) 13%C-ALEX (SF13) 13%AVAL (SF15) 13%IHD-AR (SF16)

13%NTECH50 (SF18) 13%NTECH80 (SF17)

36

Burn Rate of Two Different Aluminized Propellants

• This plot compares burning rates of two Army propellants at pressures between 500 and 2000 psia and initial temperatures of 0 °C, 25 °C, and 50 °C.

• Mench, Yeh, and Kuo (1998) showed substantial increase of burn rate using nano-sized aluminum particles.

2

3

4

5

6

789

10

200 400 600 800 1000

Ti = 50 °C

Ti = 25 °C

Ti = 0 °C

Ti = 25 °C

Ti = 0 °C

Ti = 50 °C

Average Pressure [psia]

Standard Aluminized Propellant

Alex®

Propellant

2000 30001

37

Energetic Materials Presser Capabilities

• Used to consolidate propellant and explosive powders into sample pellets or strands for combustion testing.

• Safety features to protect the operator include heavy steel plate construction, pressure release ports, shear pins, and remote computer-controlled operation.

• Interchangeable die system allows pellets of varying diameters to be produced (typically ¼”).

• Pressures of up to 40,000 psi have been achieved with RDX, producing a sample pellet with a density greater than 94 % of the theoretical maximum density.

38

Partially-Confined Hot Fragment Conductive Ignition (HFCI) Experiments

• This setup was specifically designed to study the spall fragment induced ignition of gun propellants.

• The HFCI ignition data are useful to guide the development of IM propulsion systems.

39

Go/No-Go Ignition Boundaries Under a Weakly Confined Environment

• The two propellants studied were M43 and XM39

40

Go/No-Go Ignition Boundaries Under a Highly Confined Enclosure

• The trend was reversed for these two propellants.

41

CO2 Laser Ignition Test Setup• Everlase CO2

laser with 300 to 800 W continuous wave power

• Maximum peak pulse power of 3500 W

• Minimum pulse length of 100 µs

• Maximum pulse rate of 2.5 kHz

• This laser system will be used for propellant ignition study

42

Laser Ignition Characteristics of New Propellants with Nanosized Energetic Particles

• Various ignition phenomena will be examined. The parameters of interest include the times for the onset of gas evolution, light emission, and self-sustained ignition as a function of heat flux. A high-powered CO2 laser will be employed, which may operate either in a pulsed wave mode with a maximum of 3,500 watts or a continuous wave mode with a maximum of 800 watts. Heat flux delivered to the propellant surface will range from 30 to 250 W/cm2

43

Double-Ended Strand Burner

• This special strand burner was designed for studying the gap-distance effect on propellant burning rate.

44

Burning Rate Measurement of Newly Processed Propellants

• Several windowed strand burners will be utilized to characterize the combustion behavior of solid propellants. The chamber can be operated at pressures up to 207 MPa and propellant initial temperature from –60 to +80 oC. Video data can be used to deduce the burning rate.

• In order to properly measure the burning rate as a function of pressure and web thickness, three techniques (acoustic methods, and real time x-ray radiography (RTR), and a windowed strand burner) appear to have the most promise. RTR offers the ability to monitor the instantaneous location of the propellant’s burning surface as a function of time.

45

Burning Surface Observation of Propellants with Energetic Nano-Particles

• A copper-vapor laser will be utilized to observe the burning surface phenomena, including particle ejection, average particle burning time in the gas-phase zone, particle agglomeration, etc.