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Amorphous materials such as glass, polymer and amorphous alloy have broad applications ranging from daily life to extreme conditions due to their unique properties in elasticity, strength and electrical resistivity. A better understanding of atomic structure of amorphous materials will provide invaluable information for their further engineering and applications. However, experimentally determining the three-dimensional (3D) atomic structure of amorphous materials has been a long-standing problem. Due to the disordered atomic arrangement, amorphous materials do not have any translational and rotational symmetry at long-range scale. Conventional characterization methods, such as the scattering and the microscopy imaging, can only provide the statistic structural information which is averaged over the macroscopic region. The knowledge of the 3D atomic structure of amorphous materials is limited. Recently atomic resolution electron tomography (AET) has proven an increasingly powerful tool for atomic scale structural characterization without any crystalline assumptions, which opens a door to determine the 3D structure of various amorphous materials. In this review, we summarize the state-of-art characterization methods for the exploration of atomic structures of amorphous materials in the past few decades, including X-ray/neutron diffraction, nano-beam and angstrom-beam electron diffraction, fluctuation electron microscopy, high-resolution scanning/transmission electron microscopy, and atom probe tomography. From experimental data and theoretical descriptions, 3D structures of various amorphous materials have been built up. Particularly, we introduce the principles and recent progress of AET, and highlight the most recent groundbreaking feat accomplished by AET, i.e., the first experimental determination of all 3D atomic positions in a multi-component glass-forming alloy and the 3D atomic packing in amorphous solids. We also discuss the new opportunities and challenges for characterizing the chemical and structural defects in amorphous materials.

Deciphering the three-dimensional atomic structure of solid-solid interfaces in core-shell nanomaterials is the key to understand their catalytical, optical and electronic properties. Here, we probe the three-dimensional atomic structures of palladium-platinum core-shell nanoparticles at the single-atom level using atomic resolution electron tomography. We quantify the rich structural variety of core-shell nanoparticles with heteroepitaxy in 3D at atomic resolution. Instead of forming an atomically-sharp boundary, the core-shell interface is found to be atomically diffuse with an average thickness of 4.2 Å, irrespective of the particle's morphology or crystallographic texture. The high concentration of Pd in the diffusive interface is highly related to the free Pd atoms dissolved from the Pd seeds, which is confirmed by atomic images of Pd and Pt single atoms and sub-nanometer clusters using cryogenic electron microscopy. These results advance our understanding of core-shell structures at the fundamental level, providing potential strategies into precise nanomaterial manipulation and chemical property regulation.

Multiple twinning to form fivefold twinned nanoparticles in crystal growth is common and has attracted broad attention ranging from crystallography research to physical chemistry and materials science. Lattice-misfit strain and defects in multiple twinned nanoparticles (MTP) are key to understand and tailor their electronic properties. However, the structural defects and related strain distributions in MTPs are poorly understood in three dimensions (3D). Here, we show the 3D atomic misfit and strain relief mechanism in fivefold twinned icosahedral nanoparticles with amorphization and dislocations by using atomic resolution electron tomography. We discover a two-sided heterogeneity in variety of structural characteristics. A nearly ideal crystallographic fivefold face is always found opposite to a less ordered face, forming Janus-like icosahedral nanoparticles with two distinct hemispheres. The disordered amorphous domains release a large amount of strain. Molecular dynamics simulations further reveal the Janus-like icosahedral nanoparticles are prevalent in the MTPs formed in liquid-solid phase transition. This work provides insights on the atomistic models for the modelling of formation mechanisms of fivefold twinned structures and computational simulations of lattice distortions and defects. We anticipate it will inspire future studies on fundamental problems such as twin boundary migration and kinetics of structures in 3D at atomic level. The crystallographically forbidden fivefold symmetry in icosahedra remains a long-standing mystery. Here, the authors resolve the three-dimensional atomic structure of Janus-like icosahedral nanoparticles with atomic resolution by electron tomography. [ABSTRACT FROM AUTHOR]

Metal-oxide interfaces with poor coherency have specific properties comparing to bulk materials and offer broad applications in heterogeneous catalysis, battery, and electronics. However, current understanding of the three-dimensional (3D) atomic metal-oxide interfaces remains limited because of their inherent structural complexity and the limitations of conventional two-dimensional imaging techniques. Here, we determine the 3D atomic structure of metal-oxide interfaces in zirconium-zirconia nanoparticles using atomic-resolution electron tomography. We quantitatively analyze the atomic concentration and the degree of oxidation, and find the coherency and translational symmetry of the interfaces are broken. Atoms at the interface have low structural ordering, low coordination, and elongated bond length. Moreover, we observe porous structures such as Zr vacancies and nano-pores, and investigate their distribution. Our findings provide a clear 3D atomic picture of metal-oxide interface with direct experimental evidence. We anticipate this work could encourage future studies on fundamental problems of oxides, such as interfacial structures in semiconductor and atomic motion during oxidation process. A detailed understanding of metal-oxide interfaces is essential for uncovering their intrinsic properties. Here, the authors investigate the 3D atomic structure of metal-oxide interfaces in Zr-ZrO2 nanoparticles using atomic-resolution electron tomography. [ABSTRACT FROM AUTHOR]

In view of the damage to the durability of concrete structures caused by the coupled effects of loading and sulfate attack in coastal and saline soil environments, a numerical model for the sulfate ion diffusion in compressed concrete was proposed. Firstly, based on Fick's second law, a theoretical diffusion model of sulfate ions in concrete under loading was established by considering the relationship between stress and concrete porosity. Secondly, a three-phase mesoscopic convex polygonal stochastic aggregate model of concrete containing cement mortar, interfacial transition zone, and natural aggregate was established by a self-programmed program, which enabled the mesoscopic simulation of sulfate ions diffusion in compressed concrete. Finally, the validity of the theoretical and mesoscopic models was verified by comparative analysis with the experiment results of full immersion of compressed concrete in a sulfate solution. Then the ion diffusion and damage process of compressed concrete specimens with different water-cement ratios in sulfate solutions of different concentrations were numerically analyzed. The findings indicate that the sulfate ion concentration at the same depth gradually decreased as the compressive stress level increased. The effects of sulfate concentration and the water-cement ratio on sulfate ion diffusion were more evident than with compressive stress. The extent to which compressive stress inhibited ion diffusion was influenced more by the water-cement ratio than by the sulfate concentration. Reducing the water-to-cement ratio appropriately made the compressed concrete more resistant to the sulfate attack. [ABSTRACT FROM AUTHOR]

China's efforts to use biogas as an alternative fuel in the 1970s have matured to make biogas a reliable, inexpensive source of fuel for lighting, cooking and agriculture. Research into standard designs and training for farmers on biogas techniques have resulted in construction of nearly 5 million biogas digesters in rural areas. Digesters are being used for sewage treatment and supply 64,000 urban families with biogas. The residue from the digestion process makes excellent fertilizer.