**5. History and present state of the mechanochemistry and its relation to tribochemistry**

### **5.1. Summary of historical background based on Takacs's work**

The feasibility that chemical reactions in solids can be initiated by mechanical deformation and/or tribological contact, had been considered for almost 120 years back. At that time M. Carey Lea [1] wrote *'Mechanical force can bring about reactions which require expenditure of energy, which energy is supplied by mechanical force precisely in the same way that light, heat, and electricity supply energy in the endothermic changes which they bring about'*.

Typical historical reviews of mechanochemistry by Takacs [2-3] have presented Matthew Carey Lea as the first systematic researcher on the chemical effects of mechanical action. Mechanical energy triggering chemical reactions had been of particular significance in development of mechanochemistry, known for over two centuries.

Early twentieth century (1919) Ostwald considered mechanical energy influence on chemical reactions and coined the term 'mechanochemistry'. However, at that time Ostwald was not able to say anything on the independent character and the importance of this field of chemistry. Another detailed review paper [6] on mechanochemistry deals also with its historical development. Superb experimental historical achievement of Carey Lea relates to the cinnabar (HgS) decomposition by trituration in a copper mortar with a copper pestle to produce Hg element and thereby to combine mechanochemistry with tribochemistry, due to the fact that trituration relates to friction 'tribos'. This clear description of the mechanochemical action on initiation of chemical processes is described in Takac's papers [3] as follows 'Native cinnabar (mercuric sulfide, HgS) was rubbed with vinegar in a copper mortar with a copper pestle yielding the liquid metal.' According to Tackacs [3] the source of such information is assigned to "Theophrastus' History of Stones" [65], a 1774 English translation of Theophrastus' Greek original written at the end of the 4th century B.C. It is the earliest preserved text on any subject related to chemistry or metallurgy. Thereafter, the sentence on the preparation of mercury is very probably the earliest reference to any mechanochemical reaction, extending the documented history of the process by two and half millennia [3].

General Approach to Mechanochemistry and Its Relation to Tribochemistry 227

The intermediate interacting with copper positively charged sites can produce CuS and Hg. It is assumed that during the trituration in a copper mortar with a copper pestle, electron (eΘ) is emitted and positively charged copper atom (**Cu**) is produced. Thus, the NIRAM

Please note that here we have to take into account not only CuS but also 2Cu2S:

**5.3. Comparison of the NIRAM approach with the generation process of** 

The NIRAM concept is based on the hypothesis that low energy electrons (1 to 4 eV), emitted from rubbing surfaces, can be the key factor in some tribochemical reactions [11,29,40,51].The concept of the MARs generation process in polymers is based on the polymer mechanical degradation via two types C – C bond cleavage: (i) homogeneous and (ii) heterogeneous. Generation process of polymer mechano-anions is combined with the polymer triboelectricity phenomenon caused by mechanical scission of the polymer main

It is generally known that the friction between dielectrics results in the buildup of electric charge. The energy of electrons emitted from polymers amounts to scores of keV, with the emission itself being a lengthy, slowly decaying process; such electrons are called mechanoelectrons. Most recent paper [67] reports (see also references in [67]) the mechano-emission arises as a result of the ionization of surface traps at the expense of the energy which is released in the annihilation of the defects which are formed during cleavage; the slow electrons are accelerated in the field of negatively charged segment of the freshly cleaved surface. Slow electrons appear upon the ionization of surface traps. The energy of the electrons was evaluated from the deviation of the electron beam in a magnetic field and by

At this point the following question is asked. Is here the only mechanism involved that electron transfers from the mechano-anion (R**Θ**) produced via heterogeneous bond cleavage as suggested by Sakaguchi et al. [68]? The electron transfer from R**Θ** was documented by the following reaction in dark with tetracyanoethylene (TCNE) electron scavenger as follows:

other considered compounds might include Cu2S and CuS2.

**mechano-anion-radicals (MARs) in polymers** 

based reactions take place:

chain on a friction surface.

measuring their passage through obstacles.

(3)

(4)

(7)

(6)

(5)

#### **5.2. Action mechanism of the first mechanochemical discovery**

Another paper [66] reviews widely and precisely mechanochemistry of solids. Describing reaction of mechanochemical synthesis a kind of conclusion is clearly expressed *"One of the key problems to be solved in this area is what is the start, or trigger, of the self-propagating hightemperature synthesis process? Is the Joule heat or the formation of contacts between the particles, which would be sufficient for self-heating in the contact zone to transfer the process into the regime of self-propagating high-temperature synthesis".* 

This Section demonstrates how the NIRAM approach might be applied to account for the mechanism of cinnabar (HgS) decomposition to produce Hg element and thereby to interconnect mechanochemistry with tribochemistry. It is hypothesised that both typical mechanochemical and tribochemical reaction processes are mostly triggered by triboemitted negative particles. The proposed decomposition mechanism encompasses three major steps. Mechanical action emits low-energy electrons. The emitted electrons interact with Hg=S to produce negative-ion-radical (NIR) reactive species. NIR reacting with O2 produces unstable HgSO2 which decomposes to metallic Hg and SO2. In brief summary, the NIRAM approach can demonstrate the first feasible mechanism of the cinnabar mechanochemical decomposition to produce Hg element and thereby to interconnect mechanochemistry with tribochemistry.

Now we need to consider the mechanism of cinnabar (HgS) decomposition by trituration in a copper mortar with a copper pestle to produce Hg element and thereby to more interconnect mechanochemistry with tribochemistry, due to the fact that trituration relates to friction 'tribos'. In this case also the reaction of HgS with copper (Cu) is taken into account. The whole reaction is:

$$H\text{gS} + \text{Cu} \rightarrow H\text{g} + \text{CuS} \tag{2}$$

The reaction might proceed via the negative-ion-radical reactive intermediate

General Approach to Mechanochemistry and Its Relation to Tribochemistry 227

$$\mathbf{^\ast Hg} - \mathbf{S^\odot} \tag{3}$$

The intermediate interacting with copper positively charged sites can produce CuS and Hg. It is assumed that during the trituration in a copper mortar with a copper pestle, electron (eΘ) is emitted and positively charged copper atom (**Cu**) is produced. Thus, the NIRAM based reactions take place:

$$\text{"Hg} - \text{S}^{\ominus} + \text{Cu}^{\otimes} \longrightarrow \text{"Hg} - \text{S} - \text{Cu} \tag{4}$$

$$\text{"Hg} + \text{S} \cdot \text{Cu} + \text{"Hg} + \text{S} \cdot \text{Cu} \xrightarrow{\text{s}} \text{2Hg} + \text{2CuS} \tag{5}$$

Please note that here we have to take into account not only CuS but also 2Cu2S:

$$\mathbf{C}\mathbf{u} - \mathbf{S} - \mathbf{S} - \mathbf{C}\mathbf{u} \tag{6}$$

other considered compounds might include Cu2S and CuS2.

226 Tribology in Engineering

half millennia [3].

tribochemistry.

account. The whole reaction is:

*self-propagating high-temperature synthesis".* 

chemistry. Another detailed review paper [6] on mechanochemistry deals also with its historical development. Superb experimental historical achievement of Carey Lea relates to the cinnabar (HgS) decomposition by trituration in a copper mortar with a copper pestle to produce Hg element and thereby to combine mechanochemistry with tribochemistry, due to the fact that trituration relates to friction 'tribos'. This clear description of the mechanochemical action on initiation of chemical processes is described in Takac's papers [3] as follows 'Native cinnabar (mercuric sulfide, HgS) was rubbed with vinegar in a copper mortar with a copper pestle yielding the liquid metal.' According to Tackacs [3] the source of such information is assigned to "Theophrastus' History of Stones" [65], a 1774 English translation of Theophrastus' Greek original written at the end of the 4th century B.C. It is the earliest preserved text on any subject related to chemistry or metallurgy. Thereafter, the sentence on the preparation of mercury is very probably the earliest reference to any mechanochemical reaction, extending the documented history of the process by two and

Another paper [66] reviews widely and precisely mechanochemistry of solids. Describing reaction of mechanochemical synthesis a kind of conclusion is clearly expressed *"One of the key problems to be solved in this area is what is the start, or trigger, of the self-propagating hightemperature synthesis process? Is the Joule heat or the formation of contacts between the particles, which would be sufficient for self-heating in the contact zone to transfer the process into the regime of* 

This Section demonstrates how the NIRAM approach might be applied to account for the mechanism of cinnabar (HgS) decomposition to produce Hg element and thereby to interconnect mechanochemistry with tribochemistry. It is hypothesised that both typical mechanochemical and tribochemical reaction processes are mostly triggered by triboemitted negative particles. The proposed decomposition mechanism encompasses three major steps. Mechanical action emits low-energy electrons. The emitted electrons interact with Hg=S to produce negative-ion-radical (NIR) reactive species. NIR reacting with O2 produces unstable HgSO2 which decomposes to metallic Hg and SO2. In brief summary, the NIRAM approach can demonstrate the first feasible mechanism of the cinnabar mechanochemical decomposition to produce Hg element and thereby to interconnect mechanochemistry with

Now we need to consider the mechanism of cinnabar (HgS) decomposition by trituration in a copper mortar with a copper pestle to produce Hg element and thereby to more interconnect mechanochemistry with tribochemistry, due to the fact that trituration relates to friction 'tribos'. In this case also the reaction of HgS with copper (Cu) is taken into

The reaction might proceed via the negative-ion-radical reactive intermediate

*HgS Cu Hg CuS* (2)

**5.2. Action mechanism of the first mechanochemical discovery** 

#### **5.3. Comparison of the NIRAM approach with the generation process of mechano-anion-radicals (MARs) in polymers**

The NIRAM concept is based on the hypothesis that low energy electrons (1 to 4 eV), emitted from rubbing surfaces, can be the key factor in some tribochemical reactions [11,29,40,51].The concept of the MARs generation process in polymers is based on the polymer mechanical degradation via two types C – C bond cleavage: (i) homogeneous and (ii) heterogeneous. Generation process of polymer mechano-anions is combined with the polymer triboelectricity phenomenon caused by mechanical scission of the polymer main chain on a friction surface.

It is generally known that the friction between dielectrics results in the buildup of electric charge. The energy of electrons emitted from polymers amounts to scores of keV, with the emission itself being a lengthy, slowly decaying process; such electrons are called mechanoelectrons. Most recent paper [67] reports (see also references in [67]) the mechano-emission arises as a result of the ionization of surface traps at the expense of the energy which is released in the annihilation of the defects which are formed during cleavage; the slow electrons are accelerated in the field of negatively charged segment of the freshly cleaved surface. Slow electrons appear upon the ionization of surface traps. The energy of the electrons was evaluated from the deviation of the electron beam in a magnetic field and by measuring their passage through obstacles.

At this point the following question is asked. Is here the only mechanism involved that electron transfers from the mechano-anion (R**Θ**) produced via heterogeneous bond cleavage as suggested by Sakaguchi et al. [68]? The electron transfer from R**Θ** was documented by the following reaction in dark with tetracyanoethylene (TCNE) electron scavenger as follows:

$$\text{R'} + \text{TCNE (mixing in the dark)} \longrightarrow \text{R'} + \text{TCNE''} \tag{7}$$

Figure 18 summarizes all the process steps, where ●R1 and ●R2 relate to free radicals of homogeneous C–C bond cleavage. R and R+ relate to anion and positive ion species of the heterogeneous polymer bond scission. Radical R● concerns electron transfer from the negative ion R- (anion) to TCNE either during mixing or photo irradiation.

General Approach to Mechanochemistry and Its Relation to Tribochemistry 229

experiments showed that rubbing amber and wool caused the two materials to become oppositely charged. Our scientific understanding of contact electrification has not progressed too much and, it is still not known what species is being transferred between the wool and amber to generate the charge, and how rubbing influences the process [70]. That review paper concludes with a discussion that virtually all questions involving electrostatics are in fact open ones and the size of existing particles is of special importance. It was assumed and partly evidenced that small particles charge negatively and the larger particles should charge positively. Fig. 20 illustrates the effect of contact geometry for contact charging of bulk-scale

**Figure 20.** Contact between two material surfaces in a symmetric fashion resulting in a net positive charge on one surface and a net negative in an apparently random direction (*a*); contact between two materials' surfaces in an asymmetric fashion usually results in a net negative charge on the surface with the smaller contacting area and net positive charge on the surface with the larger contacting area (*b*).

Particularly important research and practical application of mechanochemistry is well reflected in INCOME (International Conference on Mechanochemistry) series of special meetings initiated in 1993 by International Mechanochemistry Association (IMA). IMA is the associate member of International Union of Pure and Applied Chemists (IUPAC). These conferences regularly serve as a common platform to bring together all stakeholders from academia, research and development organisations, along with industry to foster the growth of the discipline [71]. The first INCOME meeting was held in Slovakia (1993). Participants from 25 countries of 4 continents took part at the meeting. This international forum was preceded by eleven All-Union Symposia on Mechanochemistry and

**6. Present knowledge on mechanochemistry and tribochemistry** 

Figure taken from Reference [70].

**6.1. Brief introduction** 

Mechanoemission.

surfaces of identical insulator materials [70] and, some works referred in it.

**Figure 18.** Illustration and suggested evidence for heterogeneous C–C bond cleavage based on work [68].

The electron transfer reaction in the dark at 77K is promoted by physical mixing of fractured sample in the vibration glass ball mill and, after milling; the fractured sample was dropped into the ESR sample tube under vacuum in the dark at 77K and photo irradiated using an IR lamp with a glass filter corresponding to visible light [68]. Thus, everything is clear for the irradiation effect due to electron detachment. On the other hand, for reaction **(7)** here an alternative mechanism is proposed, as detailed in Fig. 19.

**Figure 19.** Application of the NIRAM approach to account for the mechano-anion-radical generation

Actually, generation of mechano-anion-radicals relates strictly to triboelectricity and is reviewed in reference [68]. It cites a wide range of papers which aimed at finding evidence for both heterogeneous C-C bond scission and polymer triboelectricity. The review strongly suggests production, eg. mechano-anions induced by mechanical fracture of PCV. EPR studies provide evidence TCNE radical-anion generation. However, the formulation of a satisfactory theory to account for the triboelectricity of polymers has yet to be established. The same is due to clear evidence for heterogeneous C **–** C bond scission. An extension of the NIRAM approach to better understand the mechano-anion-radicals allows considering tribochemistry as a branch of mechanochemistry.

Triboelectricity or contact electrification of materials is a very complex phenomenon. According to [69] the first studies on contact electrification were carried out over 2500 years ago, when experiments showed that rubbing amber and wool caused the two materials to become oppositely charged. Our scientific understanding of contact electrification has not progressed too much and, it is still not known what species is being transferred between the wool and amber to generate the charge, and how rubbing influences the process [70]. That review paper concludes with a discussion that virtually all questions involving electrostatics are in fact open ones and the size of existing particles is of special importance. It was assumed and partly evidenced that small particles charge negatively and the larger particles should charge positively. Fig. 20 illustrates the effect of contact geometry for contact charging of bulk-scale surfaces of identical insulator materials [70] and, some works referred in it.

**Figure 20.** Contact between two material surfaces in a symmetric fashion resulting in a net positive charge on one surface and a net negative in an apparently random direction (*a*); contact between two materials' surfaces in an asymmetric fashion usually results in a net negative charge on the surface with the smaller contacting area and net positive charge on the surface with the larger contacting area (*b*). Figure taken from Reference [70].
