Amorphous Nanomaterials. Lin Guo
of quinine (the main component of antimalarial drugs), quinine obtained from quinoline tincture mainly exists in the form of amorphous [11]. It does not affect its medical value but can greatly enhance the natural quinoline. In 1862, G. Gore pointed out in On the Properties of Electro-Deposited Antimony that when antimony was electrodeposited, different deposition conditions would lead to two kinds of antimony monomers with different structures, i.e. crystalline antimony and amorphous antimony [12]. He also reported that amorphous antimony showed different physical and chemical properties.
Before the twentieth century, the research on amorphous materials was still in the enlightenment stage. The amorphous materials were mainly found in the preparation of traditional crystal materials. The morphology and properties of this “novel” material were fully compared with traditional crystal materials. Because X-ray has not been discovered and modern crystallography has not been developed, the essential characteristics of the disordered structure have not been discovered. However, these studies are very important for the development of amorphous and crystal. For example, in the study of amorphous sulfur, Liebig argued that the softness of amorphous sulfur proved a remarkable fact that the smallest particles that make up a solid are movable to some extent and not perfectly connected. Modern X-ray crystallography verified that the structure of amorphous sulfur should be a spiral chain structure changed from the S8 ring of crystal sulfur, endowed it with the same elasticity as rubber. There is no doubt that, the recognition at that time has a great significance in the research of solid science.
The amorphous state of phosphorus is the same as that of sulfur. Similar to the octahedral ring structural unit of sulfur crystal, the structural unit of phosphorus crystal is a regular tetrahedral structure composed of phosphorus atoms (P4). When the crystal of white phosphorus is melted at a high temperature, P4 tetrahedron transforms into a chain-connected structure. Then, the amorphous red phosphorus could be obtained by quenching, maintaining its chain-like structure. Recently, amorphous phosphorus has showed great applications in lithium-ion batteries and sodium-ion batteries because of their superhigh theoretical capacity. Similarly, in some polymer materials, when the asymmetry of the atoms connected changes irregularly, the polymer will form a random stereomer, which will behave as an amorphous state. Because of the complexity of the molecular structure of ultralong chains, the atomic arrangement modes of amorphous nonmetallic elements such as amorphous sulfur, red phosphorus, and amorphous polymers are not clear yet. However, they all have glass transition temperatures similar to those of glass, so they belong to the category of amorphous.
1.3.4 Modern Amorphous Materials 2-Metallic Glass
Metallic glass is the most abundant and widely used material in modern amorphous scientific research. Because its main synthetic process and property are similar to traditional glass, it is usually just named as glass.
Learn from the quenching technology in glass and smelt, the earliest attempts were made to condense high-temperature metal vapors (Bi, Ga, Sn, etc.) on ultra-low-temperature (2–4 K) substrates and use large instantaneous temperature differences to stabilize disorder structure. In 1934, German scientist Krammer used vapor deposition to obtain the first systematic preparation of amorphous alloys. Subsequently, a variety of metals, including semiconductors such as As and Te, were produced by gas-phase quenching [13]. However, these reports have not attract much attention to amorphous materials because the preparation process is more limited and cannot be extended to other materials.
For metals and alloys, the traditional manufacturing process has long been melting and colling. Because the difference of local structure and density between metal liquids and their solids is very small, it has been thought that metal liquids cannot be overcooled. However, in 1952, Turnbull David discovered that metals can be supercooled to 20% of their melting temperature, breaking this conclusion and providing a theoretical basis for the development of amorphous alloys [14]. In 1960, Pol Duwez et al. [15] reported in Nature that a band-shaped amorphous Au–Si alloy was synthesized by quenching for the first time. He improved the rapid quenching process with a quenching rate of 106 K s−1, made the disordered atoms in the metal melt unable to rearrange to crystal, and obtain amorphous material. Since then, metallic glass has quickly attracted widespread attention, and quenching has also become a basic method for synthesizing amorphous materials.
Especially since 1988, Inoue et al. [16] summarized three experimental rules for obtaining bulk amorphous alloys:
1 (1) The alloy should be composed of more than three alloy elements;
2 (2) There should be more than 12% atomic size difference between the main elements;
3 (3) The mixing heat between the elements should be negative.
The preparation of metallic glass were then promoted from low-dimensional materials to bulk amorphous materials. Many excellent characteristics are fully utilized, so it has become a research field with important application prospects.
Metallic glass showes a unique disordered structure, without defects such as dislocations and grain boundaries in the crystal, endowing them with many unique superior properties. For example, in terms of mechanical properties, metallic glasses exhibit high strength, high hardness, high wear resistance and corrosion resistance, high fatigue resistance, low elastic modulus, large elastic strain limit, etc. Thus, metallic glass possesses broad potential applications in the fields of engineering mechanics, biological sciences, and aerospace. For example, the amorphous alloys in almost every alloy system have achieved several times higher strength than the crystalline material. In 2011, Zhang Tao et al. [17] developed a CoTaB ternary alloy with a compressive strength of 6.0 GPa and a specific strength of 650 Nm g−1, which reached the highest record for the strength of metal materials.
At the same time, the introduction of micro/nanoscale heterogeneous structures or the second phase in bulk amorphous materials could significantly improve the toughness of amorphous materials. In 2007, according to Poisson’s ratio criterion, Wang Weihua et al. [18] adjusted the composition of the Zr–Cu–Ni–Al metallic alloy and prepared an amorphous alloy system with a multilevel microscale heterogeneous structure, which showed high strength (1.7 GPa) and very large compressive plasticity (strain > 150%). These amorphous alloys can even be bent to 90° at room temperature (Figure 1.6a–d). In 2008, WL Johnson et al. [19] improved the composition of the amorphous alloy and controlled the content of each component to synthesize the Zr–Ti–Nb–Cu–Be metallic alloy with a micron-scale precipitated second phase. For the first time, the fracture deformation has been increased to more than 10%, and up to 14%. At the same time, the fracture toughness of the amorphous alloy reached 170 MPa m0.5, indicating an excellent toughness (Figure 1.6e–g).
In addition to the mechanical properties, another key point for amorphous materials attracting widespread attention is the application in catalysis. Amorphous materials exhibit high activity surface with unsaturated coordination sites and unique local environment with uniform chemical states and atomic structures. Whether in theoretical research as a model catalyst or performance exploration as a practical catalyst, the study of amorphous materials is significant. For example, in 1925, Constable systematically discussed the difference between amorphous materials and crystalline materials during the process of catalytic decomposition by calculating the active center and pointed out that amorphous materials may have higher performance [20]. In 1980, at the Seventh International Conference on Catalysis, Gerard V. Smith of the University of Southern Illinois at Carbondale [21] firstly demonstrated amorphous alloys as a new system for catalytic reactions. In 1983, Brower et al. reported in Nature that Pd-based metallic glass has a higher selectivity than crystalline Pd in hydrogenation reactions [22], which gradually opened the prelude to the research of amorphous alloys in the field of catalysis.