Mechanical chemistry

An historical perspective

Mechanochemical effects have been used by man since prehistoric times, says Laszlo Takacs of the University of Maryland, US, explains. At the end of the 4th century BC, the  Greek Theophrastus wrote History of Stones, the earliest preserved text on any subject related to chemistry or metallurgy. In it he described how cinnabar – the mineral mercuric sulfide – was rubbed with vinegar in a copper mortar with a copper pestle to extract liquid mercury. This is a very clear description of a mechanochemical process, according to Takacs, making this probably the earliest reference to any mechanochemical reaction.

Ancient history deserves careful study from a mechanochemist’s point of view, believes Francesco Delogu of the University of Sardinia, Cagliari. ‘For example, the metallurgical process leading to the fabrication of swords in Japan, and later in Europe, is substantially a mechanochemical process. The metal was made extremely hot and then deformed in the presence of reactive substances such as oxygen, nitrogen and carbon. The dissolution of these elements in the metallic matrix was induced by the repeated deformation of the metal under the hits of the blacksmith’s hammer. That is mechanochemistry.’

Modern terminology

In more modern times, however, defining mechanochemistry has become less straightforward. ‘I believe it is best to define mechanochemistry broadly, as the branch of chemistry dealing with the chemical effects of mechanical action,’ says Takacs. ‘In many cases, especially in inorganic and metallurgical systems, the most important direct effect is “activation”, meaning the creation of fresh surfaces and lattice defects.

‘The main reaction follows and it is thermochemical, but activation increases its rate. Some people like to exclude this [effect] from consideration, although many bonds are broken mechanically during the activation process. Most potential applications are along these lines.’

Hauke Clausen-Schaumann of the University of Munich and Martin Beyer of the Institute for Physical Chemistry at Christian-Albrechts-University in Kiel, Germany, define mechanochemistry as lowering the activation energy of chemical reactions by a mechanical potential.¹ Clausen-Schaumann explains: ‘When the potential energy surface of a chemical reaction is altered by a mechanical potential in such a way that the transition state is affected – for example, the activation energy is lowered – it’s mechanochemistry. On the other hand, if the mechanical energy is, for example, merely converted into heat, it’s thermal activation rather than mechanochemistry.’

Some researchers would like a tighter definition. Gerd Kaupp of the University of Oldenburg, Germany, argues that ‘the term mechanochemistry must be restricted to breaking of intramolecular bonds directly by an external mechanical action, which is followed by further chemical reactions’.

Kaupp is not the only one with this restrictive opinion, says Takacs, but this is certainly not the broadly accepted view. ‘I do not think being restrictive serves any good purpose, especially considering that the overwhelming majority of mechanochemical investigations and potential applications would be excluded that way. I prefer a broad definition and – as much as possible – clearly defined subfields, such as mechanochemical synthesis and mechanical activation.’

Delogu also favours a general definition. ‘My personal point of view is that every chemical process activated by, or somehow connected with, the collision or mechanical action must be included among the mechanochemical ones. It is not easy to rule out that electron transfers, for example, play a role in mechanochemical processes.’

A number of processes take place when a solid is placed under a mechanical stress, such as milling, grinding, shearing, kneading, stirring, pulling or cutting, Delogu continues. During milling, for example, the solid is severely deformed, and defects form. New surfaces are formed by fracture, while new interfaces are created by welding. The result is enhanced chemical reactivity. ‘Mechanical energy is deposited in the solid, which [reaches] very high local temperatures. These can be high enough to induce local melting and vaporisation processes. All of these phenomena can be accompanied by local electronic excitations, charge separation processes, generation of electric fields, and then emission of electromagnetic radiation.’

Applications

In one example of mechanochemistry – mechanical alloying (MA) – repeated welding, fracturing and rewelding of powder particles takes place in a ball mill. The reaction powders are placed inside a reactor together with the milling balls and shaken.

MA was originally developed to produce oxide-dispersion strengthened (ODS) nickel- and iron-based superalloys for the aerospace industry. This is where fine particles of Y₂O₃ or ThO₂ are uniformly dispersed in a nickel- or iron-based superalloy.

Challapalli Suryanarayana, now at the University of Central Florida, US, explains that  mechanically alloyed ODS materials can operate at higher temperatures and with increased load-bearing capacity than alloys without an oxide dispersion.² The stable oxide particles inhibit motion in the metal matrix and increase the alloy’s resistance to high temperature and creep deformation. The homogeneous distribution of elements achieved by MA also confers extra stability at high temperatures. In addition, these materials have excellent oxidation and corrosion resistance.

Inco Alloys International of Huntington, West Virginia, US, now a subsidiary of Special Metals, US, produces mechanically alloyed materials that have a host of applications. Nickel-base superalloys are suitable for gas turbine vanes, turbine blades, and sheets for use in oxidising or corrosive atmospheres. Iron-base alloys find use in gas turbine combustion chambers while the aerospace industry employs aluminium-based alloys for airframes.

Another useful application of MA is in the production of supercorroding magnesium alloys. These operate as short-circuited galvanic cells to corrode rapidly and predictably with an electrolyte, such as seawater, to produce heat and hydrogen gas. Such an alloy system can be used as a heat source for warming deep-sea divers, as a gas generator to provide gas for buoyancy, or as a fuel in hydrogen engines or fuel cells.

A company in Western Australia has taken conventional ball milling processes a step further with its patented mechanochemical processing (MCP) technology to produce nano-sized powders. Antaria, originally a spin-off company from the University of Western Australia, claims its technology lends itself to the synthesis of particles with unique morphology. For example, it produces plate-like particles of aluminium oxide that can be used as refractory die coatings, abrasion resistant coatings, high temperature lubricants, pearlescent pigments, advanced ceramics, composites and transparent coatings. Antaria also makes powders and dispersions of zinc oxide and cerium oxide with ‘enhanced’ UV absorption, abrasion resistance, conductivity, infra red reflection, and antibacterial properties. 

Meanwhile, the Germany company TriboChem is testing mechanical processing to remove toxic polyhalogenated pollutants, like PCBs and dioxins, from the environment. Volker Birke of the University of Applied Sciences in Suderburg set up the company to commercialise his dehalogenation technology. This involves placing polyhalogenated compounds together with a base metal –  aluminium, magnesium or sodium – and a hydrogen donor (an alcohol) in vibrating ball mills.

The mills mechanochemically activate and promote the reductive dehalogenation reaction, claims Birke. The organic bound halogen is removed from the molecule entirely, leaving a harmless inorganic chloride. Feasibility studies are underway to test removing pure PCBs or PCBs in contaminated soil, filter dust and transformer oil.

Future directions

Research is focusing on investigating mechanochemistry to design polymers with improved properties. Jeffrey Moore’s group at the University of Illinois, US, for example, is working with ‘mechanophores’, stress- or strain-activated molecular units that can be inserted into a polymeric material to transform its properties in response to a local mechanical force.^{3, 4}

Such materials could be used in damage-sensing probes to detect a composite material’s mechanical failure threshold before a catastrophic failure occurs. Or they could be used to activate catalysis.

Traditionally, homogeneous catalysis is turned on by activating pre-catalysts. However, a mechanophore-based catalyst could be ‘turned on’ with the right amount of mechanical stimulus and produce activation. 

Moore’s team is also developing mechano-responsive materials. Applying mechanical energy would elicit a bulk response across a polymer, such that a fundamental property would change. This would mean that materials could ‘rebuild’ themselves under stress or ‘self-heal’ after damage.

Others are working on self-healing and other smart materials that can ‘sense’ forces. For example, Stephen Craig’s group at Duke University, US, is investigating how to create material architectures that can channel macroscopic forces to the right molecules, and to the right atoms and bonds.⁵ Their aim is to understand how mechanical forces can produce specific reaction mechanisms.

Another area under investigation is organic synthesis, where mechanochemistry offers several advantages. The process is solvent-free and allows highly selective product formation. In some cases, ball milling may offer an easier manufacturing option, too.  Ball milling, as well as microwave heating and ultrasound irradiation can be used to support conventional laboratory techniques, says Carsten Bolm of RWTH Aachen University in Germany.^6,7 A number of organocatalytic processes could be improved and superior results have been achieved compared with reactions done under standard conditions.

Meanwhile, researchers are still working toward understanding bulk mechanochemical processes. What is really happening when particles collide within those ball mills?

Francesco Delogu and Gabriele Mulas of the University of Sardinia, Sassari, are studying the events taking place during, and immediately after, ‘the collision’ during milling. Delogu is looking at the regions of trapped powders where there is mechanical energy, such as surface defects. ‘It is in such regions that the most interesting, and extreme, phenomena can take place. For example, numerical simulations suggest that local transient temperatures and pressures as high as 10,000K and 20GPa can be reached by local excited states exhibiting a lifetime of about 1ps and involving 2000 to 4000 atoms. These findings indicate that local excitations at the surface of deforming solids can even result in the formation of plasma.’

Delogu is convinced that mechanochemistry has great potential. ‘The key evidence is that the mechanochemical processes occurring at collision are extremely effective, with yields and rates orders of magnitude higher than the ones of thermally activated reactions. In principle, this evidence should assure a bright future to industrial applications of mechanochemistry.’

References

1. M. K. Beyer and H. Clausen-Schaumann, Chem. Rev.,2005, 105, 2921.
2. Challapalli Suryanarayana, Prog. Mater. Sci., 2001, 46, (1-2),1.
3. Jeffrey L. Moore et al, Nature, 2007, 446, 423.
4. Jeffrey L. Moore et al, Chem. Rev., 2009, 109, 5755.
5. Stephen Craig et al, Science, 2010, 329, 1057.
6. Carsten Bolm et al, Green Chem., 2008, 10, 1131.
7. Carsten Bolm et al, Adv. Synth. & Catal., 2007, 349, 2213.