Walid M. Hikal

Direct laser initiation and improved thermal stability of nitrocellulose/graphene oxide nanocomposites

We report on the enhancement and possible control of both laser ignition and burn rates of Nitrocellulose (NC) microfilms when doped with graphene oxide (GO). A Nd:YAG (1064 nm, 20 ns) laser is used to ignite GO-doped NC films at low temperatures. The effect of GO on the doping concentration of the activation energies of laser ignition and thermal stability of the NC films is studied. The activation energy of laser ignition decreases with increasing GO/NC weight ratio and attains a constant value with higher concentrations. This behavior is accompanied by an increase in the thermal stability.

Aerogel nanocomposites of ZnO–SnO2 as efficient photocatalysts for the degradation of rhodamine B

For the first time, aerogel nanocomposites containing ZnO and SnO2 were successfully prepared through a facile, sol–gel method without the use of a template or supporting matrix. These nanoparticles exhibit high potential for application as a photocatalyst for wastewater remediation.

Determination of sublimation rate of 2,4,6-trinitrotoluene (TNT) nano thin films using UV-absorbance spectroscopy

We report the in situ measurements of the sublimation rate and activation energy of continuous nanofilms of 2,4,6-trinitrotoluene (TNT) in air using UV absorbance spectroscopy. The films were prepared using acetone-dissolved TNT by simple spin coating deposition technique. Unlike traditional mass loss techniques, this new method is independent of the surface area of the sample which contributes to errors in determining physical parameters accurately in both bulk and thin films of materials. The calculated activation energy and temperature-dependent sublimation rates agree well with the reported values for TNT thin films. The results suggest that UV absorbance spectroscopy is an efficient tool in measuring thermodynamic properties in the nanometer scale for materials with absorbance in the UV region of the electromagnetic spectrum.

Simple method for determining the vapor pressure of materials using UV-absorbance spectroscopy.

Accurate thermodynamic parameters of thin films of materials are crucial in understanding their behavior in the nanometer scale. A new and simple method for determining the vapor pressure and thermodynamic properties of nanometer thick films of materials was developed based on UV-absorbance spectroscopy. Well-characterized benzoic acid was used to calibrate the spectrometer and the experimental conditions. The thermodynamic properties of pentaerythritol tetranitrate (PETN) were determined to validate the use of this new method. The estimated values of the thermodynamic parameters of PETN are in excellent agreement with the values reported using the most widely used Knudsen effusion method for determining vapor pressure lower than 1 pascal. The elegance of this method is its simplicity. The results indicate that UV-absorbance spectroscopy is a model-free and powerful technique in determining thermodynamic parameters in the nanoscale.

Thermo-optical determination of vapor pressures of TNT and RDX nanofilms

Accurate thermodynamic parameters of thin films of explosives are important for understanding their behavior in the nanometer scale as well as in standoff detection. Using UV-absorbance spectroscopy technique, accurate thermodynamic parameters such as activation energies of sublimation, sublimation rates, and vapor pressures of the explosives cyclotrimethylenetrinitramine (RDX) and 2,4,6-trinitrotoluene (TNT) were determined. The values of these parameters are in excellent agreement with those reported using traditional experiments based on gravimetry. In terms of the Clapeyron equation, the dependence of RDX and TNT vapor pressures on temperature can be described by the relations LnP (Pa)=39.6-15459/T (K) and LnP (Pa)=34.9-12058/T (K), respectively. Heats of sublimation of RDX and TNT were also determined to be 128kJ/mol and 100.2kJ/mol, respectively.

Rapid estimation of thermodynamic parameters and vapor pressures of volatile materials at nanoscale.

Non-isothermal measurements of thermodynamic parameters and vapor pressures of low-volatile materials are favored when time is a crucial factor to be considered, such as in the case of detection of hazardous materials. In this article, we demonstrate that optical absorbance spectroscopy can be used non-isothermally to estimate the thermodynamic properties and vapor pressures of volatile materials with good accuracy. This is the first method to determine such parameters in nanoscale in just minutes. Trinitrotoluene (TNT) is chosen because of its low melting temperature, which makes it impossible to determine its thermodynamic parameter by other rising-temperature techniques, such as thermogravimetric analysis (TGA). The well-characterized vapor pressure of benzoic acid is used to calibrate the spectrometer in order to determine the vapor pressure of low-volatile TNT. The estimated thermodynamic properties of both benzoic acid and TNT are in excellent agreement with the literature. The estimated vapor pressure of TNT is one order of magnitude larger than that determined isothermally using the same method. However, the values are still within the range reported in the literature. The data indicate the high potential for use of rising-temperature absorbance spectroscopy in determining vapor pressures of materials at nanometer scale in minutes instead of hours or days.

Sublimation kinetics and diffusion coefficients of TNT, PETN, and RDX in air by thermogravimetry.

The diffusion coefficients of explosives are crucial in their trace detection and lifetime estimation. We report on the experimental values of diffusion coefficients of three of the most important explosives in both military and industry: TNT, PETN, and RDX. Thermogravimetric analysis (TGA) was used to determine the sublimation rates of TNT, PETN, and RDX powders in the form of cylindrical billets. The TGA was calibrated using ferrocene as a standard material of well-characterized sublimation rates and vapor pressures to determine the vapor pressures of TNT, PETN, and RDX. The determined sublimation rates and vapor pressures were used to indirectly determine the diffusion coefficients of TNT, PETN, and RDX for the first time. A linear log-log dependence of the diffusion coefficients on temperature is observed for the three materials. The diffusion coefficients of TNT, PETN, and RDX at 273 K were determined to be 5.76×10(-6)m(2)/sec, 4.94×10(-6)m(2)/s, and 5.89×10(-6)m(2)/s, respectively. Values are in excellent agreement with the theoretical values in literature.

In Situ Direct Measurement of Vapor Pressures and Thermodynamic Parameters of Volatile Organic Materials in the Vapor Phase: Benzoic Acid, Ferrocene, and Naphthalene

We report the direct determination of vapor pressures and optical and thermodynamic parameters of powders of low-volatile materials in their vapor phase using a commercial UV/Vis spectrometer. This methodology is based on the linear proportionality between the density of the saturated gas of the material and the absorbance of the gas at different temperatures. The vapor pressure values determined for benzoic acid and ferrocene are in good agreement with those reported in the literature with ∼2-7 % uncertainty. Thermodynamic parameters of benzoic acid, ferrocene, and naphthalene are determined in situ at temperatures below their melting points. The sublimation enthalpies of the investigated organic molecules are in excellent agreement with the ICTAC recommended values (less than 1 % difference). This method has been used to measure vapor pressures and thermodynamic parameters of organic volatile materials with vapor pressures of ∼0.5-355 Pa in the 50-100 °C temperature range.

Development of a carbon-supported Sn–SnO2 photocatalyst by a new hybridized sol–gel/dextran approach

Carbon-supported Sn–SnO2 photocatalysts have been prepared for the first time using the newly designed dextran-mediated, epoxide-assisted sol–gel method. By coupling dextran with the epoxide assisted sol–gel technique we have established an easily modulated one-pot synthesis that demonstrates tunable carbon content with concerted production of a biphasic system. In effect, this method targets efficient development of photocatalysts that are metal-incorporated and catalyst-supported, very few methods such as this exist in the literature. This method eliminates traditional treatment steps required to create porous xerogels thereby eliminating days of processing. The formation of nanostructured sol–gels by epoxide-driven polycondensation was achieved using propylene oxide or glycidol, and the effects of the epoxide and dextran were explored in the formation of SnO2 and Sn–SnO2 carbon composite nanomaterial. The photocatalytic activity of selected SnO2 and Sn–SnO2 carbon composites were tested for the degradation of Rhodamine B (RhB). The biphasic systems show higher photocatalytic activity than the pure SnO2 systems, a result of both increased heterojunction sites between metallic tin and tin oxide that lower recombination rates, in addition to the incorporation of a carbon network that displays increased dye adsorption. The original methods developed here should be further explored to access improved catalytic material.

Simultaneous determination of diffusion and sublimation kinetics at nanoscale: Pentaerythritol tetranitrate

Many theoretical studies have been proposed to understand the mechanism of pentaerythritol tetranitrate (PETN) coarsening. Up to date, no experimental observations of diffusion have been made. We present experimental evidence of diffusion of PETN at nanoscale, as observed by UV-absorbance spectroscopy. As a result of thermal gradient, non-isothermal heating of continuous PETN nanofilms results in an initial increased absorbance at ambient temperatures indicating thickness increase due to coarsening, followed by absorbance decease due to film sublimation at relatively higher temperatures. Diffusion kinetics of PETN is measured both isothermally and non-isothermally and the results are in very good agreement.