Self Assembled Al Fe2o3 Nanoenergetics Biology Essay

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In recent years, there has been great interest in nanostructured materials that have offered attractive potential for significant performance gains compared to conventional bulk materials. Nanoenergetics are defined as reactive composite materials containing nano scale components and possess properties unobtainable by bulk energetic materials due to their higher surface area per volume. Energetic materials are generally defined as substances or mixtures with high amount of stored chemical energy that can be released for their intended application. Ultrafine nanoenergetic mixtures have been shown to exhibit factors of improvements in reaction rates compared to conventional grain size mixtures.

Some of the commonly used energetic materials such as TNT, RDX etc. are based on monomolecular compounds. These materials having both fuel and oxidizer in a single compound enable reaction to occur at a high release rate but with relatively low energy density. Composite energetic materials on the other hand, produce energy release density values much higher than the monomolecular ones but at a lower release rate. This main drawback is governed by mass transport between the reactant components, which is inherent in composite systems resulting in slower release rate. To reduce mass transport limitations between the reactants and utilizing on the advantage of high energy release densities, nano-scale components are used to create a superior class of energetic materials.

The type of nanoenergetics examined here will be a subgroup of composite energetic materials comprising of a metal fuel (aluminium) which will be closely mixed with metal oxide (iron oxide) particles.This type of material is frequently named as superthermites or metastable intermolecular composites (MIC) . A thermite system typically consists of a fuel and oxidizer, and reaction between the two produces a substantial exothermic heat release. The reaction temperature and heat of combustion can be extremely high and are commonly used for ordinance applications. Such reactions involve oxidation/reduction reactions that can provide their own oxygen needed for combustion and as such, are self sustaining and difficult to stop. The Fe2O3 /Al mixture is a classical thermite system which has been widely used for the many applications such as additives to propellants and explosives, airbag ignition materials, hardware destruction devices, welding torches etc. due to their high adiabatic temperature of 3135 K and energy density of 3.71kJ/g.

The energy released in redox reactions of thermite energetic materials depends mainly on the arrangement of the oxidizer and fuel molecules. This reaction is a solid-state diffusion driven process whereby the interfacial contact area between the oxidizer and the fuel is an important factor. Hence, an increase in this interfacial contact area will subsequently result in an enhanced rate of energy release. The efforts made so far in energetic thermite system usually involves physical random mixing of oxidizer and fuel leading to inhomogeneous distribution of the oxidizer and fuel, and thus lesser interfacial area of contact. In addition, physical mechanical mixing often introduces unplanned stressed induced reactions which become an inherent dangerous process.

A single synthesis process may yield nanoparticles of varying size, structure, shape and chemistry. All of these factors can greatly impact the energetic properties of such nanoenergetics. Taking into account that thermite reaction is a surface diffusion driven process, the shape and size of the reactant components will inevitably differ in its specific surface area and the way they are arranged, ultimately influencing the reaction rate. Knowledge in the materials properties based on size and in particular, shape of the reactant components is scarcely reported in the energetic materials community and needs to be addressed. Such effects play important roles in effective optimization of energetic formulation and will determine the diffusion and respective reaction rates.

The work presented is concerned with the synthesis and characterization of nanoenergetic superthermites (primarily the Al-Fe2O3 system) with the goal of attaining distinctive ordered morphologies using self assembling technique, therefore resulting in enhanced exothermic energy release rate. This study also aims to understand the different parameters (eg, oxidizers' morphologies, reactants' size, equivalence ratio and methods of synthesis) affecting the overall reaction kinetics of such assembled system.

RESEARCH MOTIVATION

The efforts made so far usually involve physical random mixing of oxidizer and fuel leading to inhomogeneous distribution with inherent problems inhibiting the full potential of reactive thermite systems. Reaction initiation mechanisms that capture the effects of reactants' morphologies, reactant size, fuel to oxidizer ratio and modes of initiation in energetic materials are still not well understood.

In light of past developments, approaches to produce nanostructured superthermites (Al and Fe2O3) by directed self-assembly methods have been developed. By doing so, particle to particle level self assembly whereby the fuel nanoparticles are arranged in an orderly and controlled manner around the oxidizer can be achieved. The ordered composition results in maximum interfacial contact area between the fuel and oxidizer, and hence attaining higher energy released at a very fast rate. In addition, the appeal of synthesizing composite energetic materials from the bottom-up approach offers the possibility of precisely control the morphology, particle size and composition which will also be the main focus of this thesis.

OBECTIVES

This overall goal of this work aims to understand the reaction kinetics of self assembled Al - Fe2O3 nanoenergetics in promoting enhanced energetic capabilities presented in the flow diagram Fig. 1. The project objectives are namely 1) to study the effects of oxidisers' morphologies and size, fuel to oxidiser ratio, different self assembly methods and 2) reaction kinetic studies of self-assembled nanothermites based on ignition wire tests, dynamic pressure measurements, thermal analysis (DSC) and lastly, 3) correlate the different effects in the overall reaction mechanism.

Fig. 1.1. Flow diagram illustrating the research rationale and focus of study.

CHAPTER 2. BACKGROUND

2.1 OVERVIEW

This chapter will discuss prior and current research of reactive thermite systems. Section 2.2 will relate an introduction to reactive thermite systems, in particular nano-scaled superthermites. Section 2.3 will describe the current efforts made from synthesis methods to the current problems faced in optimizing the combustion kinetics performance of such systems. Section 2.4 describes the concept in self-assembly of binary system of particles into ordered arrays and discusses the relevance and advantages of self-assembly to the field of energetic materials.

2.2 REACTIVE THERMITE SYSTEM

The oxidation-reduction thermite reaction was first described by Goldschmidt in 1908. These extremely exothermic reactions involve a transfer of electrons between one reactant to another, where, in essence, one reactant is oxidized, while the other is reduced. This reaction is highly reactive and produces energy in the form of heat. This energy can be of use in several applications such as reactive fill to enhance warhead projectiles and primers for conventional ammunition. In recent years, thermites have also being used in environmental protection research for the treatment of zinc hydrometallurgical wastes. Interestingly, nano-sized superthermite system has even been employed in biomedical treatment of cancerous cells. Such nanothermite composite leads to an accelerated rate of combustion, generating high energy shock waves comparatively to Mach numbers up to 3, literally blasting off the cancerous cells leaving nearby cells unharmed.

The scheme of a thermite reaction is shown in Eq. 1 involving a solid-state reduction-oxidation reaction. A summary of several thermite reactions and their associated theoretical exothermic heats of reaction are shown in Table 2.1. Note that all of these systems possess large theoretical exothermic heats of reaction.

M(1)O (s) + M(2) (s) à M (1) (s) + M(2)O (s) + ∆H (1)

Table 2.1 Summary of theoretical exothermic heats of reaction of selected thermite reactions (adapted from )

Thermite mixture

ρTMD/ g/cm3

Adiabatic reaction temperature/ K

ΔHR/ kJ/g

without

phase changes

with phase changes

Fe2O3+ 2Al

4.175

4382

3135

-3.96

3Cu2O + 2Al

5.280

4132

2843

-2.41

4Al+ 3TiO2

3.59

1955

1752

-1.53

Fe2O3 + 2B

4.661

2646

2065

-2.47

2B + 3CuO

5.665

4748

2843

-3.09

5Fe2O3+ 6Ta

9.185

2383

2138

-0.98

The reaction is self-sustaining upon initiation with the oxidizer acting as an oxygen source that constantly supplies oxygen to promote combustion even in the absence of surrounding oxygen supply. However, the reactants must be physically mixed and therefore, the mass transfer process remains as the main rate determining step inhibiting the rate of energy release. This solid-state diffusion driven process depends mainly on the interfacial contact area between the oxidizer and the fuel. An increase in this interfacial area will result in an enhanced rate of energy release. Thus, it is expected that the arrangement of oxidizer and fuel plays an important role in the combustion kinetics of the overall thermite system.

Research has been intensively focused on the development of nanoenergetic composite materials. As previously mentioned, traditional micron-scale energetic composites, due to mass transport issues inhibit high energy release rates. It is known that the most effective method is to reduce the scale of the fuel and oxidizer components to increase the number of contact points between the fuel and oxidizer increases with decreasing particle size, thus achieving higher mass transport. The increase in specific surface area at nano-scale allows for more reaction locations and enhances the heat transfer rates. Moreover, physical properties such as melting point and boiling point decrease with decreasing particle sizes which improves the ignition characteristics. Finally, the total time for complete combustion of a particle also decreases significantly as the particle size decreases stated by the D2 law of combustion that the burning time of a spherical particle varies linearly with the square of the particle diameter. This implies that for a reduction in particle diameter by a factor of 10 will result in a 100 fold decrease in burning time. This will, in turn, significantly increase the energy release rate for a given mass of material.

Al-based thermite system is typically used since they exhibit appropriate reaction kinetics and high density. Aluminum on its own has a relatively high heat of combustion (31kJ/g). However, the reactivity of pure aluminum is such that the surface metal reacts with oxygen to form a metal oxide layer which is of no value as a fuel, and therefore reduces the efficiency. The oxide layer creates a diffusion barrier layer for oxidizers and hinders the ignition of the aluminum. Micron sized aluminum typically ignites in an oxidizing atmosphere only after heating up to temperatures close to the melting point of alumina at 2300K. Nanometer aluminum however, has been shown to be reactive in its solid state at much lower temperature. Dikici et al. reported the reaction mechanism for the micron Al undergoes a diffusion mechanism with the rate of diffusion of Al towards oxygen determine the reaction rate. Nano Al on the other hand, undergoes a melt dispersion mechanism whereby upon melting of the Al core, the high pressure built up causes the oxide shell to spallate and reaction is not limited by diffusion through the oxide layer. It has also been shown that the phase change in the alumina layer is an important ignition criterion as the particle sizes decrease. At temperatures lower than melting temperature of Al2O3, the oxide layer changes to a denser γ-Al2O3 and exposes pure aluminium to the oxidizer. The surface area of pure aluminum exposed from a phase change is shown to increase by 8000 times when particle sizes decreases from micron scale to the nanoscale. In the presence of an oxidizer, (e.g. Fe2O3) a thermite reaction with the exposed aluminum can occur to produce large exothermic heat at > 3000K. The huge thermal stress resulting from such reaction is sufficient to cause the alumina shell to crack or melt, and expose the fresh Al underneath for further combustion. The combustion rate can be thus be further exploited by using nano-size to improve the ignition kinetics.

2.3 SYNTHESIS OF THERMITE SYSTEMS

The traditional method of producing thermite system is by conventional mechanical mixing of powders of metal oxide and fuel. Ultrasonic mixing is one of the most common methods for combining the reactants and is relatively inexpensive. Large quantities of material can also be prepared in a short time. However, this is an inherently dangerous process due to safety issues associated with unplanned reactions if mixing is to initiate the stress-induced reaction. A derivative of mechanical mixing using arrested reactive milling (ARM) was also extensively studied by Dreizin's group. It mainly involves arresting/ceasing the milling process prior to the spontaneous reaction of the thermite mixture in the miller to obtain a highly reactive metastable composite. ARM is a "top-down" process, whereby the composites are produced by continuous refinement of micron-scale starting materials using a shaker mill. The high relative velocity of the grinding media will repeatedly deform and fracture powder particles and storing kinetic energy in the metal powders. It is able to raise the metals powders to an activated state where the energy needed to overcome the energy barrier is reduced as shown in Fig. 2.1. Thus, a faster ignition and combustion kinetics can be achieved.

Fig. 2.1. Free energy path diagram showing the activation energy required to overcome prior to reaction.

One of the problems faced using ARM is the unstructured nature of the milled composite that causes large and almost uncontrollable variability in the combustion behavior. Morphology of the final thermite composite is also unpredictable due to the random collision process between the milling balls and the particles. Moreover, preparation of such materials requires the time of spontaneous initiation to be known as accurately as possible. Current theoretical treatment of the milling process is not sufficiently advance to predict initiation precisely and parametric studies are often needed to determine that. This simply means that this approach is time consuming and costly. Safety issues such as damage to the milling vial because of the high local temperatures and pressures caused by the reaction may need to be considered too.

Another attempt by researchers from the Lawrence Livermore National Laboratory (LLNL) used a sol-gel chemistry approach to prepare nanostructured energetic materials. This method allows the fuel to residue within the pores of the solid matrix and reduces the mass transport distance between the fuel and the oxidizer thus, increasing the overall efficiency. Fig. 2.2 shows the schematic diagram of such a material produced by sol-gel synthesis. Nanometer-sized particles are first formed upon addition of chemicals in a solution forming a "sol". The sols are then linked to form the "gel" with the remaining solution residing within the pores.

Fuel in matrix pores

Oxidizer

Fig. 2.2. Schematic diagram of thermite system produced using the sol-gel method.

The sol-gel approach to energetic materials offers better control over the composition of the solid at the nanometer scale which is difficult to achieve by physical mixing techniques. However, one key disadvantage of using such method is the necessary dilution of the thermite mixture with inert oxide precursors such as Al2O3 or SiO2 (from dissolved AlCl3 salt and silicon alkoxide respectively). This leads to a pyrotechnic material with reduced energy content compared to a pure iron (III) oxide-Al mixture due to the dead weight. Sol- gel reactants also often have organic impurities that make up about 10% of the sample mass, which results in reduced energy release. Furthermore, this approach relies on random mixing process rather than directed assembly, and thus, the full reaction kinetics potential is not fully realized.

A modified sol-gel method that uses electrostatic forces existing between charged aerosol particles is recently developed to enhance the interaction between the oppositely charged fuel and oxidizer particles. The sol-gel precursors were atomized by an atomizer to produce aerosol- particles and subsequently charged by diffusion charging of gas ions. Nevertheless, aero-sol-gel method still possesses similar inherent problem of incomplete conversion from sol-gel chemistry to form the iron oxide. This greatly reduces the total heat of reaction due to the significant amount of "dead weight" from sol-gel impurities.

Foils of energetic Al- Fe2O3, Al-CuO, CuO-Al and Al-Ni composites prepared by depositing the reactants onto a substrate is another approach considered to increase the interaction between the fuel and oxidizer. The thickness of the deposited film can be tailored-controlled so that the amount and diffusion distance between reactants can be optimized in the combustion process. This allows better uniformity between reactants to be achieved. However, this is at the expense of small-scale production. The procedure can be time-consuming and costly producing only small amounts of material at a single time. Foil-like shape of the final material also restricts further processing into formulations that require certain shape and geometry, which is less appealing to the more versatile powdered form.

Table 2.2 summaries the efforts made so far to prepare different energetic thermite systems using various approaches, which mainly involving random mixing of oxidizer and fuel. Attempts to increase the interfacial interaction have been done. However, many inherent problems still exist. A kinetically controlled thermite reaction requires a well-homogenized mix of the two components. Unfortunately, current available methods are still not the most desirable approach.

Research Group

Method

Types of thermite systems

Advantages

Disadvantages

Properties Studied and Results

M.L. Pantoya et. al.

(TexasTech University)

Ultrasonic Mixing

Al-MoO3

Al- Fe2O3

Easy, cheap and can mix large batches of thermite powders in a short time.

Dangerous process due to unplanned reactions during mixing may initiate stress-induced reaction.

Varies using different sizes of Al (nano to micron) composites.

Combustion velocity increased with reduced Al size from 5-50m/s.

Ignition delay time reduced from 10000ms to 10ms with reduced Al size.

M.L. Pantoya et. al.

(TexasTech University)

Chemical Sol Gel Method

Al- Fe2O3

Enable uniformly disperse solid fuels within a nanoscale oxidizer

Prevents gradients in properties of the final material due to settling.

Large amount of impurities retarded wave speed.

Large improvements in combustion wave velocities compared to ultrasonic mixing thermites.

Significant improvements in annealed sol-gel oxidizers compared to as-synthesized ones in terms of combustion wave velocities.

E.L. Dreizin. et. al.

(New Jersey Institute of Technology)

Mechanical Ball milling

Al- Fe2O3

Al- MoO3

Al- CuO

Able to obtain highly metastable milled composites.

Make use of arrested milling to produce the thermite prior to spontaneous reaction of the mixture.

Unstructured nature of the milled composite causes large and almost uncontrollable variability in the combustion behavior

Activation energies of ignition were determined to be 152 ± 19 and 170 ± 25 kJ/mol for the Al-MoO3 and Al-Fe2O3 nano-composites.

Ignition temperatures for Al- Fe2O3and Al-MoO3 are 1249K and 1104K at 3000K/s heating rate respectively.

R.L. Simpson et. al.

(Lawrenece Livermore National Lab)

Chemical Sol Gel Method

Al-Fe2O3-SiO2

Able to obtain nanostructured thermite

Fuel residues within the pores of the solid matrix reducing the mass transport distance.

High % "dead weight" (up to 10wt %) from organic precursors and impurities.

Random mixing process.

Combustion velocity varies from 10-100 m/s with different wt% SiO2.

Combustion velocity decreases with increasing SiO2content.

Table 2.2 Summary of the various groups' synthesis methods and their reported results.

Research Group

Method

Types of thermite systems

Advantages

Disadvantages

Properties Studied and Results

K.B. Ram et. al.

(Texas Tech University)

Electro deposition on Alumina nanoporous templates

Al- Fe2O3

Able to obain nanowire-array-based thin film of Al-Fe2O3thermite system.

Thickness of film can be controlled to reduce the diffusion distance and optimize combustion process.

Time-consuming, expensive.

Only small samples can be produced at one time

Need a substrate which is of no value in improving combustion kinetics.

Ignition temperature was found to be from410 °C to 700°C with different Aluminum deposition thickness

Flame temperature was also found to be independent of ignition temperature and is of the order of 4000 °C

R. Zachariah et. al. (University of Maryland)

Aerosol-based self assembly

Al- Fe2O3

Able to obtain charged aerosol particles with electrostatic interaction between the fuel and oxidizer.

Still require sol-gel chemistry as precursors and lead to large amount of dead weight.

Increasing particle charges will increase the combustion velocity.

Rate of exotherm from DSC was observed to increase by factor of 10 for charged thermite nanocomposites compared to uncharged ones.

Table 2.2 (continued)

2.4 NANOPARTICLE BINARY SELF ASSEMBLY

Binary self assembly of particles into ordered arrays has created many macroscale structures with interesting mechanical [37], optical [38], and electrical properties [39, 40]. Particle self assembly was first motivated when researchers found organized SiO2 nanoparticles in the opal gem which leads to its unusual optical properties. [41, 42] These organized structures were found to be driven by entropy, thus allowing only certain lattice structures to form under precise conditions [43, 44] as shown in Fig. 2.3.

Fig. 2.3 Self-assembled SiO2 particles of two different sizes formed naturally in the

opal gem [41]

More recent work has focused on binary systems of particles that can assemble due to other forces such as electrostatics, steric and dipole forces and Van Der Wals attraction [45, 46]. The conditions for these systems to self-assemble are less restrictive than entropy driven ones. Thus, a wide diversity of structures can be attained.

Using combinations of various nanoparticle building blocks, binary nanoparticle superlattice can be produced as shown in Fig. 2.4.

Fig. 2.4. TEM images and calculated unit cells of binary superlattices self-assembled from triangular nanoplates and spherical nanoparticles. a, b, self-assembled from LaF3 triangular and Au nanoparticles; c, self-assembled from LaF3 triangular nanoplates and PbSe nanocrystals. [46]

Self-assembly approach to produce superthermites could be of great significance since the relative positions of the fuel and oxidizer determine the burning characteristics of the particular material. If the fuel and oxidizer are able to be specifically positioned within the material, burning characteristics could be controlled to allow tailored-made properties for specific applications. This offers great advantages such as higher energy densities, faster rate of energy release, greater stability, and greater safety (sensitivity to unwanted initiation). This is particularly important in applications such as designing microelectromechanical systems (MEMS) which require nanothermites to specifically provide a directional microthrust as a source of onboard energy to generate power. [47] Adopting techniques in self-assembly to prepare heterogeneous energetic materials using components at nanoscale not only combines the advantages that nanoparticles can offer, but also allows a highly ordered structure to be achieved and manipulated to meet the desired application. Complete control can be obtained by varying the stoichiometry and particle size throughout the material to make an attractive bottom-up approach in the field of energetic materials.

Effect of ratio

In composite systems, desired energy properties can be attained by readily varied ratios of fuels and oxidizers. This may in turn enable a complete balance between the two to maximize the energy densities required for their intended application.

Rietveld ananlysis provide a good tool for revealing the concentration of phases in mixtures prior to a fair comparison. Applying Rietveld analysis allows the different phases to be identified on the basis of the crystal data. Fig. XXX shows a plot of calculated and measured XRD patterns, where the agreement identify the phase and calculate the individual components in %mass.

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