DFT insights into the electronic structure, mechanical behaviour, lattice dynamics and defect processes in the first Sc-based MAX phase Sc2SnC

The ceramic and metallic properties of the MAX phases make them attractive for numerous technological applications. The very recent experimental synthesis of the first scandium (Sc) based MAX phase Sc2SnC is an important addition to the MAX phase family as it further expands the diversity of physical characteristics of this family. Here we employ density functional theory (DFT) calculations to investigate the structural, electronic, mechanical, lattice dynamical properties of Sc2SnC including defect processes to compare with those of existing M2SnC phases. The calculated structural properties are in good agreement with the experimental values. The new phase Sc2SnC is structurally, mechanically and dynamically stable. Sc2SnC is metallic with a mixture of covalent and ionic character. The covalency of Sc2SnC including M2SnC is mostly controlled by the effective valence. Sc2SnC in M2SnC family ranks second in the scale of deformability, softness and machinability. The elastic anisotropy level in Sc2SnC is moderate compared to the other M2SnC phases. Like other members of the M2SnC family, Sc2SnC has the potential to be etched into 2D MXenes and has the potential to be a thermal barrier coating (TBC) material. The hardness and melting point of Sc2SnC, including M2SnC, follows the trend of bulk modulus.

Effective-Field Theory for Model High-Tc Cuprates

Starting with a minimal model for the CuO2 planes with the on-site Hilbert space reduced to only three effective valence centers [CuO4]7−,6−,5− (nominally Cu1+,2+,3+) with different conventional spin and different orbital symmetry, we propose a unified non-BCS model that allows one to describe the main features of the phase diagrams of doped cuprates within the framework of a simple effective field theory. Unconventional bosonic superconducting phase related with a two-particle quantum transport is shown to compete with antiferromagnetic insulating phase, charge order, and metallic Fermi liquid via phase separation regime.

FORMATION OF POROUS SILICON ON A HIGHLY DOPED P-TYPE MONOCRYSTALLINE SILICON

Porous silicon layers were formed on a p-type silicon wafers by electrochemical anodisation. Dependencies of thickness and porosity of porous silicon layers as well as effective valence of silicon dissolution versus anodizing time and current density were obtained and analysed. A mathematical model for growth of layers of porous silicon was developed.

Фотоанодирование n-Si в присутствии перекиси водорода: зависимость от напряжения

The fundamental aspects of the electrochemical etching of lightly doped n -Si under backside illumination conditions are studied in a solution with a low HF concentration and a high concentration of hydrogen peroxide. The data obtained are compared with those for a control electrolyte containing no H_2O_2. The morphology of the self-organized macropores, their growth rate, porosity, effective valence, and the amount of dissolved silicon are examined in relation to the applied voltage. The anodization kinetics at low and high bias voltages is analyzed. It is found that, under the same illumination, the initial photocurrent in the peroxide electrolyte is approximately twice lower than in the aqueous electrolyte, which makes it possible to state that the quantum efficiency of the photocurrent is lower. As, however, the etching duration is made longer, the current in the peroxide electrolyte strongly increases to become higher than that in the control electrolyte based on H_2O. It is found that, in the presence of H_2O_2, the depthwise growth rate of the macropores increases by more than a factor of 2, and the porosity decreases. The vertical macropore channels have a diameter smaller than that for macropores formed in the aqueous electrolyte and their walls are poorly passivated, which causes branching and the formation of secondary mesopores, the number of which grows with increasing voltage. The effective valence of silicon dissolution in the presence of H_2O_2 decreases to less than 2. The results are interpreted in terms of the Gerischer and Kolasinski models.

Образование макропор в n-Si при анодировании в органическом электролите

The photoelectrochemical etching of solar-grade n -type silicon in a 4% solution of HF in dimethylformamide is experimentally studied. The pore morphology, porosity, effective valence, and etching rate are examined in relation to the applied voltage, illumination intensity of the sample’s backside, and process duration. It is found that the anodization of n -Si in an organic electrolyte substantially differs from that in aqueous solutions. This is manifested in that, at a voltage exceeding the threshold value, in the so-called breakdown mode, macropores with vertical walls begin to multiply and branch intensively due to the appearance of side pores. The appearance of secondary pores is accompanied by an increase in porosity, a decrease in the propagation velocity of the porous front deeper into the substrate, and rapid transition to the electropolishing mode. In the breakdown mode at a low illumination level, a fractal structure of macropores propagating along certain crystallographic directions is observed: 〈100〉 and along the previously unobserved 〈111〉. It is demonstrated that the morphology of macropores can be controlled in the course of anodization by passing from one mode to another upon changing the external parameters: voltage or illumination. It is shown that using an organic electrolyte makes it possible to obtain macroporous membranes with a porosity substantially exceeding that of macroporous membranes formed in aqueous electrolytes under the same conditions.

Chemical Bonds Properties and Spontaneous Polarization of Orthogonal SrBi2Ta2O9 Crystals

The bonds structure, atomic coordination situation and local cluster structure in SrBi2Ta2O9 were analyzed by means of the Atomic Environment Calculation (AEC), and then the SrBi2Ta2O9 crystal was decomposed into 20 pseudo-binary crystals with the crystal decomposition method. The chemical bonds properties, such as effective valence electron density and iconicity of the individual bond were calculated by the dielectric chemical bonds theory. And the correlation between chemical bonds properties and spontaneous polarization of the bismuth layered ferroelectrics was established. Finally, the spontaneous polarization in ferroelectric SrBi2Ta2O9 and other relevant ferroelectrics were calculated, which are in good agreement with the experimental values and other theoretical calculated values.

Microstructural impact on electromigration: A TCAD study

Current electromigration models used for simulation and analysis of interconnect reliability lack the appropriate description of metal microstructure and consequently have a very limited predictive capability. Therefore, the main objective of our work was obtaining more sophisticated electromigration models. The problem is addressed through a combination of different levels of atomistic modeling and already available continuum level macroscopic models. A novel method for an ab initio calculation of the effective valence for electromigration is presented and its application on the analysis of EM behavior is demonstrated. Additionally, a simple analytical model for the early electromigration lifetime is obtained. We have shown that its application gives a reasonable estimate for the early electromigration failures including the effect of microstructure.