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Mechanically induced chemical and structural changes in materials

Castricum, H.L.

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Citation for published version (APA):Castricum, H. L. (2001). Mechanically induced chemical and structural changes in materials.

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Download date: 12 Sep 2020

Chapter 1

Introduction

1.1 Historical perspective

From prehistoric times, mechanical milling (MM) has been known as a technique for refinement and processing of materials. Around 1966 AD, the first advanced application was developed in which the disordering characteristics were used by John Benjamin and co-workers [1] at the International Nickel Company (INCO) Research Laboratories. By means of MM, coating of oxide particles by a Ni layer was attempted in order to make normally unwettable oxide particles wettable by a Ni-Cr alloy. Other techniques had proven unsuccessful. The superalloy could indeed be produced by means of the Ni-coated oxide particles. Later, oxide-dispersion-strengthened (ODS) alloys for high-temperature applications were produced on an industrial scale [2]. Amorphisation by MM of a Co-Y intermetallic compound [3] and by mechanical alloying (MA) of elemental Nb and Ni powders [4] was carried out in the early 1980s. These were the first experiments in which MM was applied as a non-equilibrium processing technique. Subsequently, a variety of mixtures has been mechanically milled, leading to the formation of supersaturated solid solutions, nanocrystalline, amorphous and quasicrystalline phases.

Mechanical activation of powders may induce chemical reactions at much lower tem­peratures than usually required. These mechanochemical reactions may lead to the for­mation of pure metals and nanocomposites. An example of a typical (redox-)reaction is the formation of Fe from Ca and FeO. Although the first applications of the technique are also very old (e.g. for procuring fire), mechanochemistry as such has originated in the 1940-50s on grinding and rolling of elastomers and other polymers [5], and was only later applied to inorganic materials [6]. Especially in Russia (and the former Soviet Union), much work has been carried out, but unfortunately many of the reports are currently unavailable. Some reviews in English do exist however, e.g. ref. [7].

A number of special conferences on MM and mechanical alloying have been organised, including the annual International Symposium on Mechanically Alloyed, Metastable and Nanocrystalline Materials (ISMANAM). The topic has also become an integral part of the triennial international conferences on Rapidly Quenched and Metastable Materials (RQ). In the triennial International Conferences on Mechanochemistry (INCOMe) on the other hand, the focus of the discussion is more on the mechanochemistry of inorganic materials and organic polymers, as well as on mechanical activation.

MM has been thoroughly reviewed in a number of overview papers [9, 10]. The review

CHAPTER 1

paper of Bakker [9] is based on a large volume of work on binary intermetallic alloys that has been carried out at the University of Amsterdam in a period of approximately 10 years [11]. The work describes the disordering of binary intermetallics and attempts to predict the resulting structure by means of a semi-empirical model proposed by Miedema and co-workers [12, 13]. Extensive use has been made of magnetic measurements for assessing the nature of the disorder. Currently, for the majority of the binary systems the structure after deformation is well understood, and this type of work has now been largely completed [14].

1.2 Scope of the thesis

While disordering of binary intermetallics was well understood, interest was invoked in other types of systems. Exploration of the effects of MM on different classes of materials was the motivation to initiate the present research. Examination of a binary intermetal­lic compound consisting of a non-transition element with a high atomic number and a transition metal, Bi2Pd5, was carried out to test the validity of Miedema's model in this (unconventional) type of system. Moreover, a comparison could be made with other disor­dering techniques, notably vapour deposition and ion irradiation. In order to investigate the influences of fracture and cold welding, changes in the size and morphology of pure metals by milling together with a non-metallic medium were also briefly explored. Al­though some (mainly Russian) work had been carried out on mechanochemical reactions, a great potential field of research was recognised here, and a second focus was directed to mechanochemistry. For polymers, the chemical and physical structure are determined by covalent binding between the atoms. For most of these materials, mechanochemical reac­tions appeared to play only a minor role. However, changes in the structure of a number of polymers were found to deserve attention. Mechanochemical reactions were found more prominent for metals and oxides that were mechanically milled in various atmospheres. Mixtures of Cu and Zn oxides could be prepared that are active as catalysts in the synthe­sis of methanol. Reactions that occur in this system have been investigated extensively, both during MM and during successive heat treatments. Investigation of this system may-serve as an example for specific applications of MM and, explicitly, mechanochemistry. The various systems are further introduced in the respective chapters.

By investigation of the different systems, we attempt to unravel the various effects of MM and try to assess which effects are most prominent for the chemical or structural changes in the specific systems under consideration. MM results in the formation of var­ious defects and may induce fracture and cold welding of particles, resulting in enhanced mixing. Moreover, the materials may become chemically activated by milling. Chemical activation can be attributed to fresh surfaces that are created, which may spontaneously react with other solid or gaseous species present in the milling vial. In addition, activa­tion may also imply factual supply of mechanical energy. MM may also give rise to an increase of the local temperature. By investigation of the various effects, different levels of interaction (or energies) can be distinguished for chemical and structural disordering. Except the solid phase (both crystalline and amorphous), liquid and gaseous phases are recognised to be involved in the disordering reactions as well.

Preparation of materials by means of MM is compared to other preparation tech-

Introduction

niques Oxidic mixtures can be prepared both by coprecipitation and by MM of oxides while mechanical alloying of elemental powders may lead to formation of a (disordered) intermetallic compound, which is conventionally prepared by melting. The effectiveness of MM is also balanced against other disordering techniques, including ion irradiation

and thermal quenching. . The prospects of MM in terms of applications of mechanically milled materials should

not be underestimated. The creation of novel materials that cannot be produced by means of more conventional methods may generate new insights into various specific systems and can thus disclose new directions and fields of research. Moreover, actual offshoots of MM can be found in the development of new products, of which several are actually being manufactured on an industrial scale.

1.3 Outline In chapter 2, the experimental techniques that are applied throughout the thesis are briefly introduced. MM of the Bi2Pd5 intermetallic compound is compared to vapour de­position and ion irradiation m chapter 3, and structural changes are interpreted m terms of Miedema's model. Chemical and structural changes by MM of polymers are considered in chapter 4 as well as relaxation behaviour of the disordered materials. In chapter 5, structural changes of metals in mixtures with non-metallic compounds are discussed. The influence of the milling temperature and the medium on the size and morphology of metal particles is examined by means of microscopic and calorimetric methods. Mechanochem-ical reactions of Cu and Zn oxides take place in various atmospheres and are described in chapter 6. The findings on these (separately milled) materials are used for discussion of their mixtures in chapter 7. Except description of the mechanochenucal reactions that take place in the Cu/ZnO system during milling, the potential of the mechanochemically prepared mixtures as active catalysts is investigated. The system is subjected to various thermal treatments, and the activity of the various mixtures in the synthesis of methanol is examined. A summary and discussion on the effects of mechanical milling is finally given in chapter 8. Because of the variety of systems investigated and techniques applied, a list of acronyms is included at the end of the thesis.

References

[1] J.S. Benjamin, Mater. Sei. Forum 88-89 (1992) 1

[2] J.S. Benjamin, Metall. Trans. A 1 (1970) 294

[3] A.E. Ermakov, E.E. Yurchikov and V.A. Barinov, Phys. Met. Metallogr. 52 (1981) 50

[4] C.C. Koch, O.B. Cavin, CG. McKamey and J.O. Scarbrough, Appl. Phys. Lett. 43 (1983)

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[5] N.K. Baramboim, Mechanochemistry of Polymers, Maclaren Sons Ltd., London (1964)

[61 G. Heinicke, Tribochemistry, Akademie Verlag, Berlin (1984)

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[7] P.Yu. Butyagin, Sov. Sei. Rev. B. Chem. 14 (1989) 1

[8] P.Yu. Butyagin and A.M. Dubinskaya (eds.), Advances in Mechanochemistry: Physical and Chemical Processes under Deformation, Chemistry Rev. 23,2 (1998)

[9] H. Bakker, G.F. Zhou and H. Yang, Progr. Mater. Sei. 39 (1995) 159

[10] C. Suryanarayana, Progr. Mater. Sei. 46 (2001) 1

[11] A.W. Weeber (1988), P.I. Loeff (1990), D.M.R. Lo Cascio (1992), L. Di (1992), G.F. Zhou (1994) and H. Yang (1996), Ph.D. Theses, Universiteit van Amsterdam

[12] A.R. Miedema, Philips Tech. Rev. 36 (1976) 217

[13] F.R. de Boer, R. Boom, W.C.M. Mattens, A.R. Miedema and A.K. Niessen, in: F.R. de Boer and D.G. Pettifor (eds.), Cohesion in Metals, Elsevier, North-Holland, Amsterdam (1988)

[14] I.W. Modder, Ph.D. Thesis, Universiteit van Amsterdam (1998)