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Nanothermites

Wiley-ISTE (Verlag)
Erschienen am 14. Juli 2016
344 Seiten
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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.
Introduction ix
Chapter 1. Elaboration of Nanoparticles 1
1.1. Solid-phase elaboration 2
1.1.1. Mechanical milling 2
1.2. Liquid-phase elaboration 19
1.2.1. Sonochemistry 19
1.2.2. Microemulsion synthesis 23
1.2.3. Solvothermal syntheses 28
1.2.4. Sol-gel syntheses 39
1.3. Gas-phase elaboration 43
1.3.1. Condensation in inert gas 43
1.3.2. Explosion of metal wires 46
1.3.3. Thermal plasma synthesis 48
1.3.4. Laser ablation 60
1.3.5. Pyrotechnic synthesis 73
Chapter 2. Methods for Preparing Nanothermites 87
2.1. Introduction 87
2.2. Physical mixing 89
2.2.1. Mixing in hexane 89
2.2.2. Mixing in isopropanol 92
2.2.3. Mixing in water 95
2.2.4. Mixing in other solvents 96
2.2.5. Dry mixing 97
2.2.6. Aerosol synthesis of the "building blocks" for physical mixing 98
2.3. Coating 100
2.3.1. Coating oxide with fuel 101
2.3.2. Coating fuel with oxide 102
2.3.3. Coating of fuel with a metal layer 104
2.4. Sol-gel method 104
2.4.1. Formation of oxide around the metal particles 105
2.4.2. Preparation of oxides subsequently mixed with metal 108
2.5. Impregnating porous solids 111
2.6. Assembly 114
2.6.1. Chemical methods 114
2.6.2. Biological methods 117
2.6.3. Electrical methods 118
2.7. Structuring at the surface of substrates 121
2.8. Conclusions and perspectives 125
Chapter 3. The Experimental Study of Nanothermites 127
3.1. Introduction 127
3.2. Study and properties of main fuels 128
3.2.1. Nanometric aluminum 129
3.2.2. Other fuels of interest 157
3.3. Oxidizers of interest for nanothermites 166
3.3.1. Metal or metalloid oxides 167
3.3.2. Oxidizing salts 189
3.4. Methods for the characterization of nanothermites 195
3.4.1. Reactive characterization 196
3.4.2. Morphological characterizations 219
3.5. Conclusion: performance of nanothermites and their enhancement 225
Chapter 4. Nanothermites and Safety 229
4.1. Introduction 229
4.2. Pyrotechnic safety 229
4.2.1. Definition and measurement of sensitivity 230
4.2.2. Techniques used for the desensitization of nanothermites 232
4.2.3. Pyrotechnic risk assessment 236
4.2.4. Regulatory aspects 239
4.3. Neutralization of nanothermites 239
4.4. Toxicological risk 243
4.4.1. Toxicity of the constituents and reaction products of nanothermites 244
4.4.2. Analyses of specific risks and good practices 250
4.5. Conclusions and perspectives 257
Conclusion 259
Bibliography 263
Index 323

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 obstacles, such as grain boundaries. Therefore, Hall-Petch law is valid as long as the grain size of the nanocrystalline material can sustain the dislocation pile-up.

[1.1]

where sc is the yield strength, d is the size of crystal grains and s0 and k are material-dependent constants.

Nieh and Wadsworth [NIE 91] propose a mechanism that describes the dislocation pile-up at grain boundaries and carry forward a relation that allows us to estimate the critical distance between two dislocations.

[1.2]

where G is the shear modulus, ? is Poisson's ratio, b is the Burgers vector, h is the material's hardness and Lc is the critical distance between two dislocations.

Nevertheless, the decrease in grain size is also limited by the rate of dislocation recombination during milling. Fetch et al. [FEC 90] have shown that dislocations induced by mechanical milling combine and annihilate starting from a certain level of constraint, which results in a decrease in dislocation density. This effect is all the more important for the materials with low melting points, such as aluminum. Thus, for these materials, dislocation density is instead controlled by the recombination rate rather than by the deformation energy as a result of milling. The opposite is observed for materials with high melting point. Grain size is instead controlled by plastic deformation. When equilibrium is reached, the new deformations that can occur are grain boundary slidings that do not influence nanostructure [KIM 95, ZHA 01].

Eckert et al. have shown that the minimal grain size induced is inversely proportional with the melting temperature for the group of metals with face-centered cubic crystal structure. This seems quite clear for the four metals with the lowest melting points (Al, Ag, Cu and Ni) [ECK 92]. The other metals having a face-centered cubic crystal structure and higher melting points, which were studied (Pd, Rh, Ir) along with the metals with centered cubic and hexagonal close packed crystal structure [FEC 90], seem to have a grain size that is constant with the melting temperature.

Figure 1.1. Grain size as a function of melting temperature according to data from [ECK, 92, FEC 90, OLE 96]

Ball milling and associated methods provide effective means for producing particles from bulk starting materials, and in the case of brittle materials it is possible to obtain a size reduction in a range below 100 nm.

Works conducted by Svrcek et al. aimed at the fragmentation of crystalline silicon by means of a planetary mill have allowed us to obtain particles of sizes between 2 and 6 nm, as well as silicon clusters of around 16 nm. The latter are disassembled by adding several drops of 30% ammonia, the size of particles then being brought to around 4 nm [SVR 05]. Russo et al. have obtained nanoparticles with a mean diameter of 55 nm, but also in a small quantity [RUS 11].

In the case of surfactant-assisted direct milling of metal particles, Chakka et al. have milled iron and cobalt for 50 h in the presence of around 10% by weight mixture of oleic acid and heptane, thus obtaining more or less spherical particles with sizes between 3 and 9 nm [CHA 06]. In slightly different milling conditions (milling duration of 20 h, same surfactant, but with a concentration of 50% by weight), Poudyal et al. have obtained mainly nanoplatelets with a diameter ranging between 5 and 30 µm and a thickness ranging from 20 to 200 nm, but also a small fraction of nanoparticles of the order of 6 nm [POU 11].

This approach may prove ineffective for ductile and malleable materials because the particles are not easily fractured and are cold welded. However, by using a surfactant (oleic acid) diluted at 3 or 5% in a polar solvent (acetonitrile), during milling under argon atmosphere in a planetary ball mill (Retsch PM 400) for 3 h, McMahon et al. obtained nanoparticles of aluminum, iron and copper with sizes indicated in Table 1.1 [MCM 14].

Table 1.1. Size of nanoparticles obtained by mechanical milling of ductile metals

Metal Type of balls Size of particles Al Aluminum balls
8 mm diameter bimodal
5-10 nm (24% by weight)
20-50 nm Fe Mild steel balls
3 mm diameter 10-20 nm Cu Copper cylinder
6.35 mm diameter,
6 mm length Bimodal
250 nm (7% by weight)
500-900 nm

1.1.1.2. The main types of mills

In order to decrease grain size by mechanical milling or mechanosynthesis, various types of mills can be employed, such as:

  1. - vibratory mill;
  2. - attritor;
  3. - ring mill;
  4. - planetary mill.
1.1.1.2.1. Vibratory mill

Vibratory mills consist of a vial that is set in vibration motion at high frequency. Among them, we can distinguish a type of mill with only one vibration axis and only one milling ball and another type of mill with high-frequency vibration along the three axes. There are many models, which are marketed by several companies such as Fritsch (Pulverisette 0 or 23, for example) or Spex, with the 8000 M shaker mill model.

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