150 kilometers deep, hard rock has already partially melted under high temperature and pressure. The temperature here is more than 1000 degrees Celsius, and the pressure is 50000 times that of the surface. In an environment like a furnace, the most wonderful transformation in nature is going on. After hundreds of millions of years, the plain hexagonal graphite crystals have gradually transformed into crystal clear natural diamonds (diamonds), which are not only the witness of countless happy marriages, but also the hardest substance in nature.
Diamond is extremely hard because of its molecular structure. In diamond, every carbon atom is hybridized by SP3 hybridization. That is to say, the valence electrons distributed in these four hybrid orbitals will combine with the valence electrons of the other four carbon atoms to form covalent bonds, forming a regular tetrahedron. It is this solid and compact three-dimensional structure that endows diamond with extremely high hardness. At the same time, all the valence electrons in diamond participate in the formation of covalent bonds, and there are no free electrons. This special crystal structure makes diamond non-conductive.
The left picture shows the SP2 hybridized carbon atoms and their planar molecular structures; The right picture shows the carbon atoms hybridized with SP3 and the molecular structure of diamond (Photo: 10.3844/ajeassp.2018.766.782).
Although the hardness of diamond is invincible in nature, if you smash your girlfriend's diamond ring to the ground, you may see the cracks of the diamond, or even be crushed to pieces. This is because diamond is hard but brittle, and it is easy to break when it is hit by a hard object. In fact, for superhard crystalline materials, hardness and toughness are often incompatible. This is mainly attributed to the atomic crystal structure of diamond: the diamond crystal is composed of periodically repeated structural units, and it is this ordering that makes the structure of the crystal different in different orientations, and the hardness of the crystal is also anisotropic with the change of the crystal orientation, and those "softer" crystal faces become the "soft ribs" of diamond.
In the field of materials, a concept corresponding to "crystal" is "glass". In contrast to an ordered crystal, a glassy state, that is, an amorphous material, has a relatively disordered structure with short-range order in a small region of a few atoms. What is the unexpected performance of this chaotic structure with a certain order?
Recently, in a study published in the National Science Review (NSR), a team of researchers from Yanshan University developed a new glass material that is not only harder than diamond, but also has toughness and semiconductor properties that diamond does not have. Academician Tian Yongjun, who led the research on
the strongest and hardest glass
, has been deeply engaged in the field of superhard materials. For example, as early as 2013, he led a team to synthesize a nano-twin cubic boron nitride that is harder than diamond. This breakthrough was also published in the Journal Nature. In the latest study, Tian Yongjun's team used fullerene (C60) as the raw material. The carbon atoms of fullerenes are all SP2 hybrid, with regular structure and high symmetry. Therefore, at 800 ℃, a pressure of 5 GPa is enough to destroy the highly symmetric structure of fullerene.
Fullerene C60 (Photo: 10.1021/ja076761k)
The research team is taking advantage of this property, and they hope to break up the crystal structure of fullerene under the right conditions of high temperature and pressure. The SP2 hybridized carbon in the original structure is converted to SP3 hybridization to a greater extent. The purpose of deconstructing it is to reconstruct it again in order to obtain a disordered and imperfect glassy state. For this purpose, they chose to continuously increase the temperature at a high pressure of 25 GPa. With the increase of temperature, the regular crystal structure gradually disintegrates, and the crystal structure can be completely transformed into a glassy state at 800 ℃.
After that, with the further increase of temperature, unexpected changes appeared. At 1000 ℃, the material no longer shows the structural characteristic peak similar to graphite in the X-ray diffraction spectrum, but appears a broad diffraction peak corresponding to the diamond crystal plane. This is totally different from the glassy carbon materials synthesized in the past, which have been reported to show diffraction peaks similar to graphite structure, that is to say, the main hybridization mode of carbon atoms is still SP2. In the latest study, the SP2 hybrid carbon of fullerenes is gradually transformed into SP3 hybrid, and at 1000 ℃, the regular tetrahedral structure of SP3 hybrid takes the lead and occupies the dominant position.
For the research team, 1000 degrees Celsius is just the beginning. As they continued to increase the reaction temperature, the proportion of SP3 hybridization in carbon atoms increased-electron energy loss spectroscopy confirmed that the degree of SP3 hybridization was about 69%, 77% and 94% at 1000 C, 1100 C and 1200 C, respectively. The higher the degree of SP3 hybridization, the greater the density of the material. Under the high-resolution transmission electron microscope, the average "particle size" is also getting smaller and smaller, and the distribution tends to be uniform. For the glassy state, this is a measure of order in the overall chaotic structure, implying a gradual decrease in disorder and a subsequent increase in order. The research team named the new "glass" at 1000 C, 1100 C and 1200 C as AM-I, AM-II and AM-III, respectively. (AM is amorphous, indicating a glassy state.)
Among them, AM-III with the highest degree of SP3 hybridization and the highest density formed at 1200 ℃ is of particular concern. The mechanical properties of AM-III show that its Vickers hardness (HV) is as high as ~ 113 GPa, which can be used to carve the crystal plane of single crystal diamond with Vickers hardness of 103 GPa. In addition to its ultra-high hardness, AM-III's strength is comparable to that of diamond: the surface of the material withstands pressures of up to ~ 70 GPa without cracking. This is the hardest and strongest glassy carbon ever found. The super-hard glass made by
Tian Yongjun's team can scratch diamonds (photo source: original paper)
. In addition, the high hardness of AM-III is basically the same in all directions inside the material, that is, it is isotropic. Compared with diamond, which has "soft ribs" due to anisotropy, AM-III, as a new type of glass, perfectly solves the problem of insufficient toughness of superhard crystals.
In addition to its ultra-hard and ultra-strong mechanical properties, AM-III is also a semiconductor with a band gap (the energy difference between the lowest point of the conduction band and the highest point of the valence band) ranging from 1.5 to 2.2 eV, which is comparable to the band gap of the most commonly used semiconductor amorphous silicon thin film. Therefore, this new "glass" with superior mechanical properties and semiconductor properties is expected to play an important role in the field of photovoltaics (converting solar energy into electricity).
This is not the first time that the team has made such an innovative attempt in the field of superhard materials. Now, new experiments have revealed the possibility that disordered glasses can rival ordered crystals. The crystal structure is deconstructed step by step, and new chemical bonds are formed, and finally a disordered and imperfect glassy state is obtained. These chaotic structures, which essentially have a unique order, can bring surprises, even beyond the perfect crystal of order. It allows scientists to see that the characteristics of "glass" can be brought into full
浙公网安备33010802003254号