W. Lee Perry

Plasma torch production of macroscopic carbon nanotube structures

Abstract Analysis of the many studies of carbon nanotube formation in high-temperature ovens clearly indicates the key requirements of nanotube formation are an ‘atomic’ carbon source and a source of nanometal particles. We adapted this formulation to the high temperature (>3000 K) environment found in a low-power (

A novel technique to quantify film--tablet interfacial thickness.

The purpose of the project was to develop an x-ray photoelectron spectroscopic technique to quantify film–tablet interfacial thickness. Tablets containing 99% dibasic calcium phosphate were coated with an acrylic polymer containing chlorine (Eudragit® RL 30 D). The film–tablet interface was defined as the region where phosphorous from the tablet and chlorine from the film coating were detected simultaneously. Five-minute intermittent ion-sputtering was used in conjunction with the conventional x-ray photoelectron spectroscopy to achieve depth profiling of the sample. The slow ion-sputtering rate of approximately 56 nm/min was overcome through sample preparation and analysis from the tablet side of the sample. This process eliminated the need to sputter through the entire film coating to reach the area of interest. Initially, only phosphorous from the tablet was detected. As the depth profiling proceeded, the chlorine signal increased and the relative intensity of the phosphorous decreased. By plotting the relative intensities of each element against the ion-sputtering time and using linear regression, the point at which the two lines intersected (tmid) was determined. By doubling tmid and multiplying by the estimated ion-sputtering rate, the thickness of the film–tablet interface was estimated to be approximately 34 μm. Further studies are needed to correlate the film–tablet interfacial thickness with the adhesive properties of coated solids.

Electromagnetically induced localized ignition in secondary high explosives: Experiments and numerical verification

In order to more fully understand the factors that influence the thermal “hot-spot” initiation of high explosives, we have chosen a model system for study that uses the internal and localized dissipation of long wavelength electromagnetic energy (microwaves). High purity organic crystals generally interact weakly with microwave energy. Therefore, the addition of electromagnetically absorbing inclusions of silicon carbide provides a tractable system for the study of hot-spot ignition phenomena. Previously, we developed a simple analytic model that illuminates the important physical factors. In this work, we verify the relationships of the analytic model using a series of experiments and a numerical model.

Electromagnetically induced localized ignition in secondary high explosives

A model for electromagnetically induced hot spots is developed from the well-established theories of dielectric mixtures, microwave absorption, heat transfer, and thermal ignition. This mathematical model is used to elucidate the interplay among these theories for a microwave heated system of secondary high explosive within which isolated electromagnetically lossy spheres are randomly distributed. Results are shown in this article for the specific case of pentaerythritol tetranitrate with embedded spheres of varying diameter and conductivity illuminated by a uniform time harmonic electromagnetic field of 1, 8, and 15 GHz. It is shown that for a given frequency and electric field strength internal to a particle embedded in a secondary high explosive, there exists a range of values for the particle’s diameter and conductivity for which electromagnetically induced hot-spot ignition of the high explosive is possible. By providing an accurate estimate for the range of necessary geometric and electrical propert...

Energy release characteristics of the nanoscale aluminum-tungsten oxide hydrate metastable intermolecular composite

Tungsten oxides are of interest as an oxidant for metals in metastable intermolecular composites (MICs), a reactive nanoscale powder useful for such applications as electric matches and gun primers. Smaller particles typically lead to fast reaction rates in this class of energetic material, and we have synthesized nanoscale WO3∙H2O using wet chemistry. Analysis by electron microscopy and small angle x-ray scattering revealed an approximately 100-nm-wide by7-nm-thick platelet morphology. X-ray diffraction verified the orthorhombic structure and composition of the hydrate. A MIC material was formulated using 44nm Al as the fuel. Performance was measured using a pressure cell where total enthalpy change and energy release rate was measured. This report includes the thermodynamic analysis of the pressure cell (calorimetry) that allows the determination of these metrics. Accuracy of the technique is discussed. Performance of the hydrate was found to significantly exceed that of MIC formulated with dehydrated t...

Interplay of explosive thermal reaction dynamics and structural confinement

Explosives play a significant role in human affairs; however, their behavior in circumstances other than intentional detonation is poorly understood. Accidents may have catastrophic consequences, especially if additional hazardous materials are involved. Abnormal ignition stimuli, such as impact, spark, friction, and heat may lead to a very violent outcome, potentially including detonation. An important factor influencing the behavior subsequent to abnormal ignition is the strength and inertia of the vessel confining the explosive, i.e., the near-field structural/mechanical environment, also known as confinement (inertial or mechanical). However, a comprehensive and quantified understanding of how confinement affects reaction violence does not yet exist. In the research discussed here, we have investigated a wide range of confinement conditions and related the explosive response to the fundamentals of the combustion process in the explosive. In our experiments, a charge of an octahydrotetranitrotetrazine-...

Metallization of poly(4-methyl-1-pentene) microcellular foam

Low density microcellular foam from poly(4‐methyl‐1‐pentene) (PMP) has been metallized by laser‐induced chemical vapor deposition (LICVD). KrF excimer laser radiation (248 nm) was used to photolytically decompose molybdenum hexacarbonyl with and without a buffer gas. Metal oxycarbide deposits 100 μm in diameter were formed. The microstructure of the PMP foam was unaltered. The deposition was confirmed to be 100 μm using scanning electron microscopy and energy dispersive x‐ray spectroscopy. X‐ray fluorescence was used to determine that approximately 80 ng of material was deposited.

Impact-induced friction ignition of an explosive: Infrared observations and modeling

A contaminant (grit) trapped between an explosive and an impacted surface can significantly reduce the impact energy required to initiate a secondary high explosive. Several severe accidents have occurred when an explosive charge was dropped from a height insufficient to cause ignition by heating due only to plastic deformation; the frictional heating from embedded grit has been implicated. Here, we describe an idealization of this situation where a small sample of a polymer-bonded cyclotetramethylenetetranitramine explosive (HMX-PBX 9501), with a 400 μm diameter sphere of silica embedded in the surface, was impacted between instrumented transparent anvils and infrared images were recorded. The instrumentation provided temperature and the work done by the friction between the grit and the anvil surface for the impact process, up to ignition. All experiments were conducted under impact conditions insufficient to cause ignition without grit. Ignition occurred at approximately 500 μs, a grit temperature of 1...

Observation of localized charge transport in isolated microscopic mats of single-wall carbon nanotubes

We have formulated a composite consisting of a low dielectric permittivity, low loss granular organic crystal, and 0.1% single-walled carbon nanotubes (SWNTs). The composite morphology was electrically isolated “mats” of SWNTs distributed on the surface of individual crystals having a typical particle size of 20μm. The real part of the composite permittivity (relative) was not significantly different from that of the matrix material (∼2.0), while the dielectric loss increased from 0.0035 to 0.3 at 2.45GHz. We used Maxwell-Garnett mixing theory, which accounts for interfacial polarization, to calculate complex permittivity and conductivity of the SWNT mats. The results indicated weak electrical contact between SWNTs within the mats, implying that localized electron transport was the primary dielectric loss mechanism.

Quantification of reaction violence and combustion enthalpy of plastic bonded explosive 9501 under strong confinement

The confinement experienced by an explosive during thermal self-initiation can substantially affect performance in terms of deflagration–to–detonation characteristics and explosion/detonation violence. To this end, we have developed an experiment to quantitatively observe enthalpy change and reaction violence in thermally initiated plastic bonded explosive (PBX) 9501. Traditionally, researchers attempt to quantify violence using terminal observations of fragment size, fragment velocity, and through subjective observations. In the work presented here, the explosive was loaded into a heated gun assembly where we subjected a 300 mg charge to a cook-off schedule and a range of static and inertial confinements. Static confinement was controlled using rupture disks calibrated at 34.5 and 138 MPa. The use of 3.15 and 6.3 g projectile masses provided a variation in inertial confinement. This was a regime of strong confinement; a significant fraction of the explosive energy was required to rupture the disk, and th...