We investigate plane-dependent microstructures in an SRR99 single-crystal superalloy in the as-cast condition. Using optical and scanning electron microscopy, we examine sections parallel to the (001), (110), and (111) planes. Dendrite traces display "+" symmetry on (001), asymmetric "+" symmetry on (110), and an "×" pattern on (111). The γ' precipitates appear as irregular cube-like, triangular-prism-like, and triangular-pyramid-like shapes on (001), (110), and (111), respectively, whereas eutectic pools and carbides show no marked plane-dependent differences under the present conditions. These plane-specific observations provide a concise basis – limited to the as-cast state – for discussing anisotropic microstructural features that may be relevant to hot-cracking susceptibility and for clarifying how sectioning orientation influences apparent morphology in single-crystal superalloys.

Active repeating Fast Radio Bursts (FRBs), with their large number of bursts, burst energy distribution, and their potential energy evolution, offer critical insights into the FRBs emission mechanisms. Traditional pipelines search for bursts through conducting dedispersion trials and looking for signals above certain fluence thresholds, both of which could result in missing weak and narrow-band bursts. In order to improve the completeness of the burst set, we develop an End-to-end DedispersE-agnostic Nonparametric AI model (EDEN), which directly detect bursts from dynamic spectrum and is the first detection pipeline that operates without attempting dedispersion. We apply EDEN to archival FAST L-band observations during the extreme active phase of the repeating source FRB 20121102A, resulting in the largest burst set for any FRB to date, which contains 5,927 individual bursts, tripling the original burst set. The much enhanced completeness enables a refined analysis of the temporal behavior of energy distribution, revealing that the bimodal energy distribution remains stable over time. It is rather an intrinsic feature of the emission mechanisms than a consequence of co-evolving with burst rate.

Two-dimensional (2D) ferroelectric materials have important application potential in device miniaturization due to their characteristics of only being a few atomic layers thick and non-volatility. How to design high-performance ferroelectric memory devices based on 2D ferroelectric materials has attracted extensive attention. In this work, based on the 2D organic ferroelectric material semi-hydroxylized graphane (SHLGA), which has in-plane ferroelectric polarization along three different directions, we construct a 2D organic ferroelectric tunnel junction (FTJ). By means of density functional theory (DFT) and the non-equilibrium Green's function (NEGF) method, we calculate the transport properties of the FTJ under different polarizations and obtain a giant tunnel electroresistance (TER) ratio of 7.55 × 104%. We find that the mechanism behind the TER effect in the organic SHLGA is based on the unique built-in electric field. That is, among the three ferroelectric polarization directions, any two directions have an angle of 120°. As a result, the built-in electric fields along the transport direction of the FTJ under different ferroelectric polarization directions are different. Moreover, our study shows that the giant TER effect can also be achieved by utilizing the asymmetry of the polarization along the transport direction of the ferroelectric material itself, which provides another route for the design of 2D FTJs.

随着定向凝固技术与工艺的不断发展,工程技术人员对枝晶生长的控制也越来越精准.为使枝晶生长朝着人们设定的方向发展,进而获得理想的组织结构,凝固过程的控制成为关键因素.定向凝固技术制备的合金虽能提高其综合力学性能,但受到枝晶生长等多方因素影响,因此有必要对枝晶生长方面的研究工作进行评述.综述了定向凝固工艺参数、合金成分和晶体取向对枝晶生长的影响,尤其是对一次枝晶间距的影响.通过建立物理模型来模拟枝晶的生长过程,一方面来验证理论模型的有效性,另一方面对干扰因素进行预测和控制.此外,论文还分析与讨论了一次枝晶间距的极端范围与边界条件.最后,对影响定向凝固控制枝晶生长的因素进行了总结和展望.

Ferroelectric tunnel junctions (FTJs) are very promising as a new type of nonvolatile memory devices due to the tunneling electroresistance (TER) effect. In recent years, with the rise of two-dimensional (2D) materials, 2D ferroelectrics and their application in FTJs have attracted intensive attention, with the advantage of greatly reducing the FTJ based memory device sizes, as demanded by the ongoing device minituriazation in modern electronic circuits. However, all present schemes for realizing giant TER ratio with 2D FTJs are based on the polarization reversal of the whole ferroelectric layer upon an electrical field. In this work, we explore the quantum transport properties of the 2D FTJs with the partial reversal of polarization, namely, the formation of domain walls (DWs) by constructing two kinds of FTJs. One is in a uniform-polarization state and the other one is a state with domain walls. Structural relaxation confirms the stability of the domain-wall state. By quantum transport calculation, we obtain a TER ratio as high as 2.75 x 104%. Further analysis of the electronic structure shows that there is charge accumulation or charge depletion at the two DWs. Such asymmetric interface polarization charges result in a built-in electrical field and thus affect the distribution of the effective potential along the transport direction. This leads to partial metal-insulator transition around the DWs and finally the giant TER ratio. Our results indicate that DWs may greatly affect the quantum transport and provide a new mechanism for realizing giant TER effect in 2D FTJs.

The dominant factor of competitive grain growth was studied by trinary-crystal seeds during directional solidification. It was found that the grain overgrowth rate was weakly dependent on the temperature gradient when the misorientation angle of the misoriented grain was limited (£15°), which was inconsistent with the classical theoretical assumption that the grain overgrowth rate was determined by the difference of the tip undercooling between the competing grains. In contrast, the grain overgrowth rate was sensitive to the alloy composition. These phenomena were attributed to the mechanisms of solute interaction and sidebranching events, and the solute field was the dominant factor to govern the overgrowth behavior of the competing grains.

Hydrostatic pressure on superconductor FeSe increases the Wyckoff position, z(Se), and decreases lattice constants. However, previously only the increasing of z(Se) was emphasized to be important in determining the electron-phonon coupling. We explicitly study each of the two factors individually to understand its influence on the electron-phonon coupling, band structure and Raman frequencies. We find that the increasing of z(Se) enhances the states around the Fermi level more, while the decreasing of the lattice constants enhances the phonon frequencies more, which together increase the electron-phonon coupling under pressure. Based on the above facts, we predict and prove that the in-plane biaxial strain on FeSe increases the electron-phonon coupling due to the increasing z(Se) value and the decreasing in-plane lattice constant. Our results uncover the factors affecting the increase of the electron-phonon coupling and provide information on how to enhance the electron-phonon coupling in FeSe.

To accurately predict the microstructure evolution of competitive columnar grain growth, the influence of the potential dominant factors of competition growth such as thermal field, solute field and flow field on the overgrowth behavior of trinary-crystal samples during directional solidification was systematically investigated, with the preferred orientation of the experimental grains orienting parallel and at a limited misorientation angle with respect to the temperature gradient direction. It was found that the grain overgrowth rate was weakly dependent on the temperature gradient, which was inconsistent with the classical theoretical assumption that the grain overgrowth rate was determined by the difference of the tip undercooling between the competing grains. In contrast, the grain overgrowth rate was sensitive to the solute field around the dendrite tips. Additionally, with increasing the natural convection, the grain overgrowth rate tended to be promoted at low withdraw speed. These phenomena were attributed to the mechanisms of solute interaction in the converging case and sidebranching events in the diverging case, while the solute field was the dominant factor to govern the overgrowth behavior of the competing grains. Moreover, a new model of competitive grain growth based on the solute field was proposed to predict the microstructure evolution of the Nickel-based superalloys during the directional solidification process. In this new model, the primary spacing of the well-oriented grain in the converging case was smaller than that in the diverging case due to the sidebranching events and dendrite lateral motion, which highlighted a new mechanism of primary spacing evolution in the directional columnar solidification structure.

Directional solidification (DS) has been widely used to produce aero-engine and gas turbine blades of nickel-based superalloys. The preferred crystallographic orientation of nickel-based superalloys is [001], so the [001] columnar-grain structure can form after DS. Due to the low Young's modulus and the elimination of transverse grain boundaries, the [001] columnar-grain structure has beneficial mechanical behavior. The competitive grain growth dominates the production of columnar grains. There are two views about competitive grain growth, which are consistent for diverging grains but not consistent for converging grains. In the case of convergence of the first view, the grain boundary (GB) was parallel to the favorably aligned dendrites, which indicates that the favorably aligned grain cannot be eliminated. For converging grains of the second view, not only the favorably aligned dendrites could block unfavorably aligned ones, but also the unfavorably aligned dendrites could block favorably aligned ones. Thus, the converging grain boundary moved from unfavorably aligned grain to favorably aligned grain. Finally, the favorably aligned grain may be eliminated. The study about the two views was carried out in the case of the same secondary orientation but did not taken into account the secondary orientation. Up to now, the literatures about the effect of secondary dendrite orientation on competitive growth is rarely and their views contradict with each other. In this work, the bi-crystal and ter-crystal plates with different secondary orientations were produced to study the influence of secondary orientation on competitive grain growth. For the bi-crystal with the same primary orientation, as the secondary GB angle increased, the GB was nearly at the middle of the plate sample, which indicated that the competitive grain growth was weak and could be neglected. For the ter-crystal with different primary orientations, not the secondary orientation but the primary orientation could obviously affect competitive grain growth. In the case of converging grains, the change of secondary dendrite orientation had no effect on the competitive growth behavior and grain growth rate; the favorably and unfavorably aligned dendrites could block each other, which disagreed with Walton-Chalmers model and in good agreement with the results of Zhou. In the case of diverging grains, the result agreed with Walton-Chalmers model and Zhou's result.

ABO3-δ (A = La, Sr, B = Fe, Co) perovskites are useful in a wide range of applications, including their recent exploration for application in high-temperature optical oxygen sensing for energy conversion devices such as solid oxide fuel cells. To elucidate the dependence of functional properties and oxygen vacancy formation on defect chemistry and composition, first principles calculations are presented. The obtained results show that oxygen vacancy (VO) formation energies are in the order of LaFeO3 > LaCoO3 > SrFeO3 > SrCoO3. Furthermore, the influence of VO on the electronic and optical properties is investigated for the high temperature stable phases (T = 1100 K). For the LaFeO3 insulator, the VO donated electrons are all localized on the down-spin d3z2-r2 orbitals of the nearest Fe ions. These defect states located in the band gap induce a drop in the energy onset of absorption as pristine bulk → V2+O → V1+O → V0O, and especially, an extra absorption peak appears between 0.5 and 1.5 eV due to V0O and V1+O formation. In the rest of the crystals that expressed a metallic feature, the VO donated electrons partially localize on the down-spin d3z2-r2 orbital and partially delocalize through the lattice, by which the absorption peaks (0.5-2.0 eV for LaCoO3, 0.0-0.5 eV for SrFeO3 and SrCoO3) from the electronic excitation near the Fermi level are enhanced. A high VO concentration of oxygen divacancy in SrFeO3 and SrCoO3 could enhance charge localization on down-spin d3z2-r2 orbitals, resulting in a remarkable increase of optical absorption at 1.5-3.0 eV.

Tungsten tetraboride (WB4) and its solid solutions represent one of the most promising candidates for superhard metals; however, the structural and bonding uncertainties regarding the fractionally occupied metal and boron sites have impeded an in-depth understanding of these compounds. Here, we examine the interstitial arrangements of boron atoms and polyhedral bonding in synthesized WB4 using W L-edge X-ray absorption spectroscopy and X-ray photoemission spectroscopy. We identify a nonrandom distribution of W vacancies and B3 trimers at the crystallographic W 2b site, instead of a full occupation. Furthermore, this peculiar structural arrangement is associated with two distinct sets of W and B binding states and a large value of density of states at the Fermi level (EF), which suggests an inhomogeneous charge transfer at different crystallographic W and B sites with a preferred metallic bonding. Theoretical calculations elucidate that, while the B3 trimers help form a three-dimensional covalent bonding network, the W vacancies are crucial to optimize the EF location and thus enhance the bonding strength. Our findings provide key insights into the hardening mechanism in WB4, which has broad implications for the rational design and synthesis of this class of materials.

The hexagonal phase of $\mathrm{AgBiS}{\mathrm{e}}_{2}$ has been discovered as a promising thermoelectric material for room-temperature applications. However, its basic conduction type is still ambiguous, and its current ZT value is pretty low. To improve the thermoelectric performance of $\mathrm{AgBiS}{\mathrm{e}}_{2}$, we apply band engineering to modify its band structure by introducing defects to increase the band degeneracy. From the calculated intrinsic point defect formation energies of $\mathrm{AgBiS}{\mathrm{e}}_{2}$ at different growth conditions, we clarify that the conducting behavior of $\mathrm{AgBiS}{\mathrm{e}}_{2}$ is a $p$-type semiconductor, and the Ag vacancy is the dominated acceptor. Based on scrutinizing the band structure of $\mathrm{AgBiS}{\mathrm{e}}_{2}$, two kinds of methodologies can be used to modify its band structure to achieve high band degeneracy: (i) shifting the Fermi level into the valence band using intrinsic defects, and (ii) converging several valence-band maxima by introducing extrinsic defects. We find that the intrinsic Ag vacancy is helpful to significantly increase the power factor, leading to a large ZT for Ag vacancy-doped $\mathrm{AgBiS}{\mathrm{e}}_{2}$: the maximum ZT value is increased to 0.3--0.5 at near room temperature. Based on analyzing the bonding characters and atomic energy levels in the compound, we predict several extrinsic dopants (Cu, Rh, and Pd) that can be used to converge three valence-band maxima. Our work provides methodologies to improve the room-temperature thermoelectric applications of $\mathrm{AgBiS}{\mathrm{e}}_{2}$ by tuning its band structures using intrinsic or extrinsic defects.

Substitution of bismuth by rare earth (RE) ions is of great technological importance to develop room-temperature BiFeO3-based multiferroic materials. Despite this interest, many fundamental properties and the structure-property correlations of RE-doped BiFeO3 remain poorly understood. Here we report a systematical experimental and theoretical exploration on the structural phase transition in Bi1-x La x FeO3 (0  ⩽  x  ⩽  0.2) ceramics. By using x-ray absorption fine structure spectroscopy, we for the first time show that the La3+ dopants in fact substitute the Bi site of secondary nanosized particles with orthorhombic Pbam symmetry instead of the long-believed parental rhombohedral R3c phase at all La3+ doping concentrations (0.001  ⩽  x  ⩽  0.2). This homogeneously mixed two-phase compound cannot be detected by the x-ray diffraction until La content approaching x  =  0.1. The finding is further supported by complementary studies of transmission electron microscopy and thermodynamic preference, and it casts serious challenges on the prevailing assumption of La3+ substitution on the Bi3+ site in R3c structure when x  ⩽  0.1 as well as the previously proposed origin of enhanced functional properties based on morphotropic phase boundary. This new insight may ignite a revival on exploring the underlying multiferroic mechanisms in BiFeO3-based materials and facilitate the bottom-up design of novel multifunctional devices.

Cu2ZnSnS4 (CZTS) is a promising low cost thin-film solar cell material. However, the charge localized defects greatly hinder the improvement of the solar cell efficiency, thus the identification and knowledge of the possible charge localized defects in it are extremely important. Using hybrid functional calculation, we find that Cus, and Cuz, are the main charge localized defects in CZTS. In detail, our results show that Cusn is a deep level recombination center. Moreover, the growth condition of Sn determines the population of Cus because the stable chemical potential region of dys ([1.74, 0] eV) is larger than that of Apo, ([0.77, 0] eV). Thus Sn-rich growth condition is proposed to suppress the Cusn. As for Cu-zn antisites, part is deep acceptors which will be beneficial for the efficiency of solar cell, while part forms donor-acceptor pairs with Zn-cn. The Cu-zn + Zn-cu donor-acceptor pairs will lead to large potential fluctuation in CZTS, which is a disadvantageous factor. Fortunately, such pairs can be greatly suppressed by Cd doping due to two reasons: one is that the Cd atoms prefer to substitute the Zn atomic sites leading to the reduction of the Cu-zn, concentration, and the other is that the Cd dopant in the CZTS makes it difficult for its neighboring Zn atom be substituted by Cu atom.

In this study, based on first-principles calculations we report a possible mechanism of efficiency improvement of Sb-doped Cu2ZnSnS4 (CZTS) solar cells from the Sb-related defect point of view.

Using hybrid density functional theory, here, we have investigated the electronic, defect and migration properties of Na-related defects in the earth-abundant solar cell absorber material Cu2ZnSnSe4 (CZTSe). We find that among all the Na-related defects, NaZn acts the same way as VCu and CuZn; it is an acceptor and contributes to the p-type conductivity. NaSn is a deep level defect, but it is energetically unfavorable and can be suppressed by the growth conditions. Besides, through migration energy analysis, we prove that Na can easily move in CZTSe through interstitial Na and Cu vacancy mediated mechanisms.

Materials play not only an important role in academic research but also a rather important role in daily life. The discovery of a new material can make a significant contribution to both scientific research and society development. For example, the discovery of graphene in 2004 has attracted enormous attention in academic circle and graphene has shown promising applications in electronic and optical devices. However, the period of discovering a new material is too long to satisfy the demand of society. To speed up the process, Advanced Manufacturing Partnership was proposed in 2011 by US President Obama. This plan has a key part: Materials Genome Initiative (MGI). MGI is made up with three part, (1) construction of high-throughput computation platforms, (2) construction of high-throughput experimental platforms and (3) construction of databases. The high-throughput computation and experimental platforms have promising applications in investigating known materials as well as discovering new materials with high efficiency since they can produce a large amount of data in a short term. Databases can provide convenience for data analysis and machine learning that is a prevailing way to design new materials. In this paper, we present our new designed High-throughput Computational Platform for Random Alloys. The platform is built by using Fortran and Shell languages and its main function is to calculate mechanical and thermodynamic properties of random alloys in large quantities automatically by employing the Vienna ab - initio Simulation Package (VASP). There are two main problems that are needed to be solved in the construction of High-throughput Computational Platform for Random Alloys. The first one is the creation of structures for random binary alloys. Another is to set the comparable computational parameters for all calculations automatically. To create structures for binary random alloys, we employ Alloy Theoretic Automated Toolkit to obtain a set of special quasi- random structure templates. Then, we collect all structure information for elementary substance from literature or by performing the first principles calculations. Finally, we design the platform to create special quasi-random structures (POSCAR) for all kinds of binary random alloys by using the templates and structure information of elementary substance combine with the Neumann-Kopp rule. In order to set comparable computational parameters for the VASP calculations, we write a code to read the structure information from POSCAR and use the information to set a comparable k-mesh for all calculations. The INCAR file is created by using the information from POSCAR and POTCAR. After all input files are generated, the platform employs VASP to perform the first-principles calculations. The platform is also designed to extract and analyze calculation results as well as print a calculation report and save all the calculation data into a database. The platform also has some useful functions such as error information-outputting system and error-skipping system. The function is rather important since convergence problem is inevitable in the first-principle calculations especially in high throughput calculations. To test the efficiency of the platform, elastic constants of 47 pure metals are investigated which indicates a computer with eight cores can finish all calculations in one day. And all calculated results match those in the literature well. By using the platform, the mechanical properties of zirconium based binary alloys are investigated and we find the elastic constants and elastic modulus can be well described by a parabolic function.

The magnetic and ferroelectric properties of multiferroic AgCrS2 are investigated by the full potential linearized augmented plane wave method. For the low temperature crystal structure, the ground state is found to be in the collinear double ferromagnetic striped state with a [Formula: see text] spin structure, which is in good agreement with the neutron scattering result. The interlayer magnetic interactions are comparable to the intralayer ones, indicating that AgCrS2 is actually a three-dimensional frustrated antiferromagnet. Moreover, an analysis of Born effective charges and electric polarization reveals that the partial ferroelectricity in AgCrS2 belongs rather to the 'geometric ferroelectric' class, in which exchange striction and/or lattice distortion play an important role in stabilizing the polar structure.

We study the abnormal ferromagnetism in α-K2AgF4, which is very similar to high-TC parent material La2CuO4 in structure. We find out that the electron correlation is very important in determining the insulating property of α-K2AgF4. The Ag(II) 4d9 in the octahedron crystal field has the t2g6eg3 electron occupation with eg x2-y2 orbital fully occupied and 3z2-r2 orbital partially occupied. The two eg orbitals are very extended indicating both of them are active in superexchange. Using the Hubbard model combined with Nth-order muffin-tin orbital (NMTO) downfolding technique, it is concluded that the exchange interaction between eg 3z2-r2 and x2-y2 from the first nearest neighbor Ag ions leads to the anomalous ferromagnetism in α-K2AgF4.

The electronic structures of the possible charge states of vacancies in the earth-abundant solar cell absorber material Cu2ZnSnS4 (CZTS) are investigated using screened-exchange hybrid density functional theory. We find that all the charge states of anionic S vacancies (VS) are not stable because VS rehybridizes with the nearest neighbor Sn atom, reducing the valence of the Sn atom. Therefore, there is no charge transition level for VS. Instead, all the charge states of cation vacancies are stable. The copper and zinc vacancies show delocalized features which create shallow charge transition levels, while the tin vacancy shows localized feature which creates deep charge transition levels within the band gap and acts as a recombination center in CZTS.