Diamond can be amorphous: from scenario to reality - Dr. Zhidan Zeng

AUGUST 22, 2017

A team led by HPSTAR scientist, Dr. Zhidan Zeng synthesizes a new form of carbon—“amorphous diamond”—under high pressure and temperature (HPHT). This bulk amorphous diamond obtained under HPHT can be maintained to ambient conditions for potential applications, realizing possible the hardest amorphous (glass) material ever discovered. This work is recently published as an article by Nature Communications (Synthesis of quenchable amorphous diamond, doi:10.1038/s41467-017-00395-w).

Diamond is a famous allotrope of carbon with many unparalleled properties. In particular, diamond has the highest hardness and thermal conductivity of any ever known bulk materials, resulting in widespread industrial applications. Most of its unique properties originate from its extremely strong sp3 C-C bonding. The search for other bulk carbon allotrope with the same sp3 C-C bonding has been a long-sought after yet elusive goal. In principle, any substance can be formed in an amorphous solid state. Is a fully sp3 C-C bonded amorphous diamond also possible? By combining high pressure and high temperature, Z.D. Zeng and her colleagues successfully synthesized a completely sp3-bonded tetrahedral amorphous carbon-- “amorphous diamond”, and recovered it to ambient conditions, for the first time.

Despite of the highest hardness, diamond as a crystalline material, is anisotropic and contains “weak planes” (cleavage planes) which make it easily to break along certain directions. “In contrast to diamond, the ‘amorphous diamond’ has a disordered three dimensional structure, therefore it is ideally isotropic and does not contain weak planes.” said Zhidan Zeng, “This structure with the strong three dimensional C-C bond network make amorphous diamond a super strong material. Our experiments confirm it has an ultrahigh bulk modulus exceeding any other known amorphous materials. And we expect that amorphous diamond may have many other unique properties complementary to or even beyond diamond, which need further exploration.”

The three important Group-14 elements C, Si and Ge all have sp3-bonded crystalline phases with the same diamond-cubic structure. Si and Ge also have well-known sp3-bonded tetrahedral amorphous forms which have broad applications. However, the amorphous counterpart in C, an amorphous form of diamond, was conspicuously missing until now. This research finds this missing member of the carbon allotrope family, and demonstrates that it can be maintained at ambient conditions.

“Carbon-carbon bonding is very flexible, such as sp1-, sp2- and sp3-hybridized bonds, resulting in numerous carbon allotropes. Among them, diamond is unique because it was the only bulk form of carbon with fully sp3 C-C bonds. People have been looking for other bulk sp3-bonded carbon materials over decades, but besides diamond, fully sp3 bonded carbon allotrope seems impossible.” Said Qiaoshi Zeng, “We combined high temperature and high pressure and carefully mapped the two dimensional temperature-pressure parameter space. Finally we obtained fully tetrahedrally bonded carbon in an amorphous form. These results enhance our fundamental understanding of carbon materials and will trigger intense follow-up study on the mechanical, thermal, optical, electrical, and chemical properties of amorphous diamond for potential applications.”

Other co-authors in this team include HPSTAR’s Liuxiang Yang, Hongbo Lou, Hongwei Sheng, Wenge Yang, and Ho-kwang Mao.

Caption: Atomic structure of diamond (left) compared with amorphous diamond (right). Diamond is crystalline and anisotropic, the single crystalline diamond shown in the left picture contains lots of facets. In contrast, amorphous diamond is isotropic like glass, and it may be cut to any shape including ideal sphere.

媒体报道：

Carnegie Science: https://carnegiescience.edu/news/amorphous-diamond-synthesized

Phys.Org: https://phys.org/news/2017-08-amorphous-diamond.html

科技日报： http://digitalpaper.stdaily.com/http_www.kjrb.com/kjrb/html/2017-09/04/content_377306.htm?div=-1

金刚石是天然存在的硬度最高的材料，同时还具有最高的弹性模量（体模量），最高的原子密度、最高的热导率等优异性质。这些优异性能和其特殊结构有关。本研究采用玻璃碳（glassy carbon）作为起始材料，利用高压原位激光加温技术首次成功合成了块体状的100% sp³ 共价键的新型非晶态碳材料。通过同步辐射x 射线衍射、高分辨电子显微镜及电子能量损失谱等多种实验手段，一致证明这种新型碳材料具有典型非晶态结构，且材料内部所有碳原子间的共价键都是sp³ 键，因而是真正的“非晶态金刚石”。非晶态金刚石的合成说明金刚石并不是唯一的全部碳原子都以sp³ 键结合的碳材料，改变了我们对碳材料的传统认知。“非晶态金刚石”由于其无序的原子结构而具备非晶材料各向同性的特点，且材料内部不存在晶界、位错等传统晶体缺陷，又因高强度sp³ 共价键的存在而很可能具备接近甚至超越单晶金刚石的优异性能（高压原位同步辐射x 射线衍射实验已经证实其体模量高于金刚石），作为一种新型的超硬材料，可能在众多科学技术领域取得重要应用。