January 2021
Lin Guo
Beihang University, Beijing
1 Introduction
1.1 Introduction of Amorphous Materials
Amorphous material is a type of substance in which basic units do not exhibit long-range order (LRO) in space but maintain some ordered characteristics only in the range of several atomic scope. Different from the traditional crystalline materials with regularly arranged basic units, amorphous materials are characterized by atomic short-range (<1 nm) order and long-range (>1 nm) disorder [1]. Compared with anisotropic crystalline materials, amorphous materials exhibit macroscopic homogeneity and isotropy. Based on this internal structure, amorphous materials do not have crystal defects such as dislocations, grain boundaries, etc., which endows the amorphous materials with some excellent mechanical properties such as high strength, high hardness, high wear resistance, high fatigue resistance, etc. At the same time, amorphous surface exhibits a high degree of unsaturated atomic sites or dangling bonds, which made it desirable as a model catalyst or a practical catalyst. Thus, the special disordered surface structure can provide more active sites than traditional crystal nanomaterials and further improve the catalytic performance. Apart from that, the special atomic arrangement of amorphous structures can effectively regulate the electronic state of the material, leading to optimized transmission of electrons or ions in catalysis. Therefore, the intensive study of amorphous materials has important implications for theoretical and practical exploration of solid materials.
In the order of aggregates, modern physics classifies conventional materials as solid, liquid, gaseous, and plasma states (Figure 1.1), which are considered to be the basic material forms that make up our surroundings. In addition to the three states of solid, liquid, and gas, which are common in our perception, the plasma state consists of gaseous molecules after ionization (thermal ionization, photoionization, impact ionization, etc.), and its behavior is mainly controlled by the Coulomb interaction between ions and electrons. The four basic states are stable under some certain conditions, and the transformation of physical states from one to another is due to the change of thermodynamic states such as temperature, volume, enthalpy, and entropy.
Compared to the stable basic states, the amorphous materials are considered to be a metastable intermediate state of liquid-to-solid transition. Its precursor is a viscous liquid (supercooled liquid) that has begun to undergo agglomeration transformation. In addition, the next stage of an amorphous structure is a crystal with a stable periodic arrangement of the constituent elements. Amorphous materials have a phase similar to that of solid, which is embodied as a solid shape. Meanwhile, it has a disordered atomic arrangement similar to that of liquid. Without the fixed atomic pattern as crystal, or the dynamic equilibrium as liquid and gas, amorphous materials are intervened formatted solid with unstable state.
Figure 1.1 The research scope of amorphous materials. T, V, H, and S are the abbreviation of temperature, volume, enthalpy, and entropy, respectively.
Few materials researches in the past hundred years discussed the amorphous system, most of the studies have used crystal models to construct their perceptions about the formation of materials. A sufficiently sound theoretical system has been established for crystals owing to its accurately and constantly regular atomic arrangements. Compared with these traditional crystalline materials, amorphous materials only have short-range order (SRO) in 1–2 atomic scope, without translational symmetry or rotational symmetry (Figure 1.2). This disordered state makes the amorphous material extremely difficult to be studied both experimentally and theoretically. For example, even though the spherical aberration correction technique and cryo-electron microscopy technique have achieved great development today, the most basic structural or the atomic arrangement of amorphous materials has not been effectively solved.
In both experimental observation and theoretical reasoning, the systematical arrangement and the regular morphology are the basis of scientific research, which can greatly simplify people’s cognitive process. The establishment of crystallography relies heavily on a theoretical system based on mathematics. However, among all the substances that make up the world, the ones with the perfect and regular arrangement are the only very special cases. The chaotic and disordered state is the real cornerstone of the whole condensed matter. From the water on which life depends, to the life itself, to the vast universe, to clusters of atoms tens of thousands of times smaller than bacteria, they are not arranged in regular form. Meanwhile, even though scientists have a complete and systematic understanding on crystal materials, researches are necessary to be carried out with the amorphous counterparts, such as glass, plastics and rubber, to further explore their irreplaceable role in many key areas.
Figure 1.2 (a) The unit cell of PdS from different views. (b) Amorphous structure of eight cells of PdS with a quench cooling in dynamic simulation.
Therefore, increasing attention has been received to the study of amorphous materials. In 1995, Science published a special issue: Through the Glass Lightly. It invited dozens of top scientists to put forward the ideas for the future of science in the twenty-first century. Philip Warren Anderson of Princeton University, who won the 1977 Nobel Prize in Physics for his fundamental theoretical research on the electronic structure of magnetic and disordered systems, believed that the deepest and most interesting unsolved problem in solid states theory is probably the theory of the nature of glass and glass transition. This can be the next breakthrough in the coming decade [2].
Ten years later in 2005, at the commemoration of the 125th anniversary of Science, another special issue named What Don’t We Know invited many of the most influential scientists in various fields to raise 125 scientific problems that need to be solved urgently in this new century. Amorphous material was still listed among them: What is the nature of the glassy state? [3] Molecules in a glass are arranged much like those in liquids but are more tightly packed. Where and why does liquid end and glass begin?
At the 9th International Conference on bulk amorphous alloys, held in Xiamen University in 2012, Takeshi Egami from Oak Ridge National Laboratory and Tennessee State University, one of the most famous scientists in the field of amorphous materials and physics, concluded the conference by saying, the amorphous field is an area without textbooks, and aspiring young people should actively engage in research areas where textbooks are not yet available.
1.2 Structural Differences between Amorphous Materials and Crystals
According to the arrangement of atoms, modern science classifies solid materials into three categories: crystals, quasicrystal and amorphous materials (Figure 1.3).
1.2.1 Crystals and Quasicrystals
For crystal materials, the atomic arrangement has both translational
