Nanothermites

Eric Lafontaine, Marc Comet

• Publisher: Wiley-ISTE
• Year: 2016
• Print ISBN: 9781848218376
• eBook ISBN: 9781119330202
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

The recent introduction of the "nano" dimension to pyrotechnics has made it possible to develop a new family of highly reactive substances: nanothermites. These have a chemical composition that is comparable to that of thermites at submillimeter or micrometric granulometry, but with a morphology having a much increased degree of homogeneity. This book discusses the methods of preparation of these energetic nanomaterials, their specific properties, and the different safety aspects inherent in their manipulation.

1
Elaboration of Nanoparticles

Many methods have been developed for the preparation of metal powders depending on their desired basic characteristics, such as size, size distribution, morphology and specific surface area, to which other properties can be added, such as chemical reactivity and electrostatic stability. These methods for producing powders can be grouped in three large families. The first group is based on solid-phase methods, which consist essentially of mechanical milling and mechanosynthesis or reactive milling.

The second one is based on methods by which powders are elaborated in the liquid phase:

1. – sonochemical synthesis;
2. – microemulsion synthesis;
3. – solvothermal synthesis;
4. – sol-gel synthesis.

The third group includes methods for elaborating powders in the gas phase:

1. – inert gas condensation;
2. – explosion of metal wires;
3. – thermal plasmas;
4. – laser ablation;
5. – pyrotechnic synthesis.

This chapter does not claim to provide an exhaustive approach to all of the nanoparticle synthesis methods, as it more specifically focuses on various implementations for obtaining nanosized metal or metal oxide particles that may present an interest for the mixture-based formulation of nanothermites.

1.1. Solid-phase elaboration

1.1.1. Mechanical milling

For thousands of years, man has resorted to various milling methods in order to reduce the particle size of materials. However, in the 1960s, Benjamin developed a new method, high energy milling, which allowed for obtaining materials with nanoscale organization [BEN 70, BEN 74].

Starting from the 1980s, this technique was rapidly developed, as it allowed for obtaining structural states that are difficult to obtain by other synthesis methods, and even impossible to obtain by classical methods, such as melting solidification.

As an example, the preparation of the amorphous alloy type of compounds [WEE 88] can be mentioned. The works of Gaffet et al. showed that injected mechanical power was the parameter controlling the crystalline to amorphous transition in nickel alloys. This is obtained in a planetary mill by independently varying the rotation speeds of the disc and satellites [GAF 91]. A further example is to obtain supersaturated solid solutions from immiscible elements at thermodynamic equilibrium [YAV 92] or metastable crystalline phases [KOC 96, KOC 93].

Two terms are commonly employed in the literature to refer to high energy milling. The term mechanosynthesis is used when powders of different nature are milled together to finally obtain materials presenting new alloy compositions and/or new structures, or when chemical reactions are activated by the mechanical energy transferred during the phenomena triggered by milling [SUR 01, GLU 08].

The term mechanical milling or mechanomilling is employed when high energy milling is used not only to reduce the size of powder grains but also to modify the structure and/or microstructure of powders.

1.1.1.1. Principle

High energy milling is a method that permits us to obtain in solid phase ultrafine and homogeneous powders by exerting mechanical stress on a material. This consists of introducing one or several materials in a sealed chamber that contains one or several impactors, which are generally spherical, and shake everything in a more or less forceful manner. Under the effect of the mechanical energy transferred during collisions, the powder grains are subjected to very strong plastic deformations and go through a sequence of fractures and cold welding.

The plastic deformation rate increases enormously under milling [DEL 97], which leads to a significant increase in the hardness of the material with milling duration [KIM 95].

Nevertheless, a material’s toughness cannot increase indefinitely with decreasing grain size because the reinforcement mechanism relies on the pile-up of dislocations at the level of obstacle...

Table of contents

• Cover
• Table of Contents
• Title
• Copyright
• Introduction
• 1 Elaboration of Nanoparticles
• 2 Methods for Preparing Nanothermites
• 3 The Experimental Study of Nanothermites
• 4 Nanothermites and Safety
• Conclusion
• Bibliography
• Index
• End User License Agreement