Study on stability of formation for gravity field exploration subjected to disturbance based on prescribed performance control.

Double satellite formation for gravity field exploration is a complex space virtual instrument system with high-precision, whose normal manner is threatened by the space debris environment. The normal manner of the formation based on control with prescribed performance is studied. Based on the random impact of space debris, nonlinear dynamical equations with 20 variables are established considering the relative attitude of the double satellite. The interferential characteristics and expected stability under certain disturbance conditions by the space debris in low Earth orbit are analyzed. To simplify the relative motion of the formation and the motion of the test mass (TM) relative to the cage, a prescribed performance function is introduced to constrain the relative attitude errors of transient and steady states. An adaptive attitude control design method based on a prescribed performance function is proposed. Finally, the analysis is carried out. The results show that the probability of normal manner of the formation is about 78.45% in the first year and about 45.59% in the first three years. The normal manner of the double satellite formation for gravity field exploration can be effectively simulated and analyzed based on the prescribed performance control methods.

[Comparison of two methods for detection of Chlamydia trachomatis and Ureaplasma urealyticum in male reproductive tract].

OBJECTIVE: To investigate the value of clinical application of simultaneous amplification and testing of RNA (SAT-RNA) for detecting Chlamydia trachomatis (CT) and Ureaplasma urealyticum (UU) by comparing with the polymerase chain reaction testing of DNA (PCR-DNA) method. METHODS: Specimens from both urethra swab and the first avoid urine which should be at least one hour after the previous urination were collected from 163 men who were scheduled for in vitro fertilization and embryo transfer (IVF-ET) treatment due to female factors at Center for Reproductive Medicine, Shengjing Hospital of China Medical University during the period of April 2016 to April 2017. Among the 163 men, 109 simultaneously provided semen that was collected after 3-7 days of sexual abstinence for the testing. Urine and semen specimens were detected for CT and UU with SAT-RNA, while urethra swab specimens were detected for CT and UU with standard PCR-DNA. Detection results of the SAT-RNA were compared with those of the PCR-DNA method. RESULTS: The positive rate of UU in the urethra swab detected with PCR-DNA and that of UU in the urine with SAT-RNA were 47.24% and 47.85%, respectively, and the coincidence rate was 93.25%. In addition, the positive and negative coincidence rates were 93.51% and 93.02%, respectively, and the concordance between the two methods was very good (Kappa=0.865). On the other hand, the positive rate of CT in the swab specimen tested with PCR-DNA was 3.07% and that of CT in urine with SAT-RNA was 4.29%, and the coincidence rate was 97.55%. Moreover, the positive and negative coincidence rates were 80.00% and 98.10%, respectively, and the concordance between the two methods was good (Kappa=0.654). Regarding SAT-RNA detection of UU in the urine and semen specimen of the 109 patients, the positive rates of UU in the urine and semen specimens were 50.46% and 44.95%, respectively; and the coincidence rate between the two specimens was 88.99%. In addition, the positive coincidence rate and the negative coincidence rate was 93.88% and 85.00%, respectively, and the concordance between the two specimens was good (Kappa=0.780). Similarly, SAT-RNA detection of CT in the urine and semen specimens showed the positive rate was 5.50% and 3.67%, respectively; and the two specimens showed 98.17% coincidence rate. The positive and negative coincidence rates were 100.00% and 98.10%, respectively, and the concordance was also good (Kappa=0.791). CONCLUSION: SAT-RNA detection of CT and UU in the urine specimen showed good concordance with the PCR-DNA detection of CT and UU in the urethra swab specimen. In addition, the concordance was also good between the urine and semen specimens detected with SAT-RNA. These results indicate that, as a less invasive and equally accurate procedure, SAT-RNA may be more suitable for clinical application.

Flatband in a three-dimensional tungsten nitride compound.

Herein, the flatband of a W1N2 crystal is theoretically investigated. It is revealed that the flatband can be well-described by a tight-binding model of the N12 skeleton, where the dispersion of the flatband is governed by both the ppσ bonding strength (Vppσ) and the ppπ bonding strength (Vppπ) between the nearest-neighbor N atoms. It is also found that the proper strength of the ppπ bonding between neighboring N atoms plays a prime role in the formation of the flatband. In addition, when the compound is doped with holes, the electrons at the flatband are fully polarized, showing a ferromagnetic character. This behavior has a weak correlation with the on-site Coulomb interaction U. Moreover, several three-dimensional compounds possessing flatbands in the whole k space are predicted.

A machine learning based deep potential for seeking the low-lying candidates of Al clusters.

A Machine-Learning based Deep Potential (DP) model for Al clusters is developed through training with an extended database including ab initio data of both bulk and several clusters in only 6 CPU/h. This DP model has good performance in accurately predicting the low-lying candidates of Al clusters in a broad size range. Based on our developed DP model, the low-lying structures of 101 different sized Al clusters are extensively searched, among which the lowest-energy candidates of 69 sized clusters are updated. Our calculations demonstrate that machine-learning is indeed powerful in generating potentials to describe the interaction of atoms in complex materials.

Linear scaling algorithm for tight-binding molecular dynamics simulations.

The linear scaling or O(N) methods, which exhibit linear scaling with respect to the size of system, are a powerful tool for theoretically treating a huge system containing many atoms. We present a new linear scaling algorithm for large-scale tight-binding molecular dynamics simulations based on the divide-and-conquer approach, in which a system is divided into subsystems and each subsystem is calculated separately. Different from the common realization of the divide-and-conquer approach, our proposed method avoids building the density matrix or electronic density and gives a new strategy to access the physical properties of a large system. We apply this method to the tungsten metallic system and show that this method very effectively yields the same results including the atomic structures, the melting point, the formation energy of defects, and the electronic properties as those obtained from the exact diagonalization of tight-binding Hamiltonian matrix of a whole system. This method has the advantages of linear scaling complexity, less memory consumption, and high parallel efficiency, which make it to be used for the large-scale simulations.

First-principles study of intrinsic point defects in MgSiAs2.

MgSiAs2 is a potential infrared-nonlinear optical (IR-NLO) material, due to its fine second harmonic generation (SHG) performance and high laser damage threshold (LDT). In this study we systematically investigate its native point defects including vacancies, interstitials, and antisites using first-principles calculations with the Heyd-Scuseria-Ernzerhof hybrid functional. The defect formation energies and transition levels at the dilute limit are evaluated. Of the thirteen different point defects studied, nine kinds of defects show deep transition levels, which might contribute to the limiting of the transparency spectrum of MgSiAs2 compounds. The defects with the highest concentration at equilibrium are found to be cation antisites, MgSi and SiMg, serving as acceptors and donors respectively. Because of their significantly low formation energies, they lead to Fermi level pinning in the band gap and constrain the carrier concentration doping. Our calculations show that a change in growth conditions affects modestly the formation energy of defects or the concentration of charge carriers.

The formation of H bubbles at small-angle tilt grain boundaries in W films.

The accumulation of H at the small-angle tilt grain boundary (GB) in the W(001) surface is investigated, on the basis of the first-principles calculations. By exploring the solution and diffusion behaviors of H at the GB, we find that the small-angle GB can capture the H atoms nearby, serving as a nucleation site of H bubbles. With the increasing number of trapped H atoms, the GB expands gradually, and the GBs can be unripped with an areal density of H up to 5.0 × 1015 H atoms per cm2, leading to the formation of H bubbles. Moreover, H2 molecules are observed, when the areal density of H atoms in GB is over 6.6 × 1015 atoms per cm2. According to our calculations, we propose a possible formation mechanism of H bubbles observed in the experiment, which is valuable for improving the service performance of W as a plasma-facing material in nuclear fusion reactors.

New structures of bilayer germanium nanosheets predicted by a particle swarm optimization method.

A global search for the stable structures of bilayer Ge (BLG) is performed, and the most stable and meta-stable BLG structures are predicted for the first time. Phonon-spectrum calculations and ab initio molecular dynamics simulations confirm their dynamical and thermal stability. The computed electronic structures suggest that the most stable structure is metal while the meta-stable structure of BLG is a semiconductor with an indirect band gap (0.32 eV at the level of PBE functional and 0.81 eV at the level of HSE06). By straining the layer plane of the meta-stable BLG, we observe a phase transition from semiconductor to metal. Furthermore, the adsorption of gas molecules of CO, CO2, NH3, NO and NO2 on the meta-stable structure is also studied. Our results show that the predicted meta-stable BLG also possesses a good feature in gas sensors.

Energetic and kinetic dataset on interaction of the vacancy and self-interstitial atom with the grain boundary in α-iron.

We provide the dataset of the vacancy (interstitial) formation energy, segregation energy, diffusion barrier, vacancy-interstitial annihilation barrier near the grain boundary (GB) in bcc-iron and also the corresponding interactive range. The vacancy-interstitial annihilation mechanisms in the bulk, near the GB and at the GB at across scales were given.

The influence of liquid Pb-Bi on the anti-corrosion behavior of Fe3O4: a first-principles study.

In this work, the influence of Pb and Bi atoms on the anti-corrosion behavior of the oxide film (Fe3O4) formed on steel surface is investigated based on first-principles calculations. Through calculations of the formation energies, we find that Pb and Bi atoms can promote the formation of point defects, such as interstitial atoms and vacancies in Fe3O4. Besides, the effects of the concentration of Pb (or Bi) and pressure on the formation of these defects are also studied. Our results depict that a high density of Pb (or Bi) and compression pressure can promote the formation of defects in Fe3O4 significantly. Furthermore, the energy barriers for Pb and Bi atom migration in Fe3O4 are also estimated using the climbing image nudge elastic band (CI-NEB) method, which implies that Pb and Bi can diffuse more easily in Fe3O4 compared to Fe. Our results reveal the underlying mechanism of how Pb and Bi influence the anti-corrosion ability of oxide films in an accelerate driven system (ADS). It is instructive for improving the corrosion resistance of the oxide films in the ADS.

Effect of irradiation defects on the corrosion behaviors of steels exposed to lead bismuth eutectic in ADS: a first-principles study.

In accelerator driven systems (ADSs), steels will suffer not only from the irradiation damage produced by protons or neutrons, but also from the dissolution corrosion induced by the liquid lead-bismuth eutectic (LBE). In this work we investigate the interactions between LBE atoms (Pb, Bi) and the irradiation induced defects X (X is helium, vacancy or divacancy) in α-Fe based on first-principles calculations. It is found that LBE atoms repulse each other without irradiation defects, while they aggregate easily with the defects to form X-Pbn and X-Bin complexes. This indicates that the irradiation defects could promote the aggregation of LBE atoms in iron, especially Bi atoms. The total binding energies of the X-Pbn and X-Bin complexes increase with the number of Pb and Bi atoms, respectively. The origin of the total binding energies of the complexes is further discussed via the electronic structures and the distortion of the crystalline lattice. Finally, the concentration evolutions of the Vac-(Bi)n complexes and unbound vacancies with temperature are predicted by the mass action analysis. This work provides important information for the synergistic effect of irradiation and LBE corrosion on the steels in the ADSs, which can be used as basic parameters for further study.

Boron-tuning transition temperature of vanadium dioxide from rutile to monoclinic phase.

The effect of the doped boron on the phase transition temperature between the monoclinic phase and the rutile phase of VO2 has been studied by performing first-principles calculations. It is found that the phase transition temperature decreases linearly with increasing the doping level of B in each system, no matter where the B atom is in the crystal. More importantly, the descent of the transition temperature is predicted to be as large as 83 K/at. % B, indicating that the boron concentration of only 0.5% can cause the phase transition at room temperature. These findings provide a new routine of modulating the phase transition of VO2 and pave a way for the practicality of VO2 as an energy-efficient green material.

Strain-induced phase transition of a C58 solid.

Four kinds of C58-based carbon solids and some of their physical properties are predicted by the first principles calculations. These carbon solids display different electronic structures, due to their different space symmetries and connecting patterns. Among these solids, the most stable one with the P3m1 space group displays semiconducting properties, with a direct band gap of about 0.12 eV, being consistent with solid C58 films achieved in the experiment. Interestingly, this carbon solid exhibits a semiconducting-metallic phase transition under the applied isotropic strain. The mechanism underlying such a phase transition is addressed.

An energetic evaluation of dissolution corrosion capabilities of liquid metals on iron surface.

Using first principles calculations, dissolution corrosion of liquid metals on iron surfaces has been investigated by calculating adsorption energies of metal atoms in the liquid phase on the surface and escape energies of surface Fe atoms. The adsorption energies, characterizing the stability of the adsorbed atoms on the investigated surfaces, show that Bi is more stable than Pb and Au. The escape energies, representing the energy required for an Fe atom to escape from the surface, show that adsorbed Pb makes surface Fe atoms escape more easily than Bi and Au. The combination of adsorption energy and escape energy indicates that the corrosion capabilities of liquid metals decrease in the order Bi > Pb > Au. This is further proved by the investigation of surface properties, such as inter-layer distance, magnetic momentum and charge density difference. The results are consistent with experimental results that Fe can be corroded more severely in Bi than in Pb. In the case of liquid alloys, chemical proportions of compositions are incorporated to evaluate the corrosion capabilities of Pb-Bi eutectic (LBE) and Pb-Au eutectic (LGE). It is found that LBE has more severe corrosion capability than LGE. The energetic calculation is further developed in evaluating the effect of alloying elements in popular steels on the dissolution corrosion. The results indicate that Si, V, Nb and Mo may mitigate the dissolution corrosion of martensite steels in liquid Pb, Bi and Au.

An atomistic insight into the corrosion of the oxide film in liquid lead-bismuth eutectic.

When used as a protective scale, the Fe3O4 layer covering the stainless steel surface in accelerator driven subcritical systems (ADS) is corroded by liquid lead-bismuth eutectics (LBE). By performing theoretical calculations, we reveal that both Pb and Bi at the interface between the LBE and the Fe3O4 scale, favorably adsorb onto the Fe3O4 surfaces, weakening the strength of Fe-O bonds nearby significantly. This facilitates the movement of iron atoms toward the deposited Pb(Bi) and away from the Fe3O4 surface, thus causing corrosion. Such corrosion behavior becomes severe if oxygen vacancies exist in the surface region.

Theoretical prediction of new carbon allotropes.

Novel carbon allotropes are predicted by optionally substituting carbon atoms in diamond with carbon tetrahedrons. All these allotropes exhibit semiconducting properties, with bandgaps ranging from 3.2 to 4.7 eV. The calculated cohesive energy, mass density, and the bulk modulus are essentially related with the component of the tetrahedron units in these allotropes. In particular, we reveal the evolution of mechanical and electronic properties with the component of tetrahedrons for this family of crystalline carbon allotropes. This sheds a new light of tuning electronic and elastic properties of new allotropes by controlling the composition of carbon tetrahedrons. In addition, the calculated Raman spectra for these allotropes exhibit different features, and Raman characteristic modes for the tetrahedron units are addressed, which are available for identifying these allotropes in experiment.

Tailoring the band gap of ZnO/MgZnO coaxial nanowires by the size and the component of Mg.

Based on first principles calculations, we investigate the changes of the band gaps of wurtzite ZnO/Mg(x)Zn(1-x)O coaxial nanowires by varying the size and Mg components. Our findings show that the band gaps of the concerned nanowires are tunable in different ways: the size of the shell (or the core) dominantly induces a band offset of the conduction band minimum, while the Mg concentration in the shell mainly causes a band offset of the valence band maximum. Statistic analysis reveals that such size- and Mg-component dependent band gaps are tightly correlated with enhanced hybridization between Zn and O induced by the Mg atoms in the shell.

Transferable tight-binding potential for germanium.

We report a new tight-binding potential model for Ge. In this model, the bonding environment around each bond is taken into account. The features of this model are extensively examined using various structures, such as the diamond bulk with point defects, reconstructed surfaces, nanowires and small and medium-sized clusters. The resulting configurations, energies and the phonon dispersion as well as the electronic structures of the tested systems are in agreement with those predicted at the level of density functional theory. This indicates that the new model is able to handle complex Ge-based materials.

Structures, stability, vibration entropy and IR spectra of hydrated calcium ion clusters [Ca(H(2)O)(n)](2+) (n = 1-20, 27): a systematic investigation by density functional theory.

The low-lying candidates of hydrated calcium ion clusters, [Ca(H(2)O)(n)](2+) with n = 1-20 and 27, have been extensively sought by using density functional theory (DFT) at BLYP/6-311+G(d,p) level. The results showed that the first hydration shell around the calcium ion was fully occupied by six water molecules, whereas the second hydration shell might be fully occupied with different numbers of water molecules. This just corresponds to different growth patterns of the hydrated calcium ion clusters. Furthermore, we revealed that the vibration entropy contributed to the free energy of an isomer significantly. As a result, the stability of some low-lying candidates at zero-temperature was not maintained at finite temperatures. Therefore, we suggested that, at finite temperatures, the realistic products of [Ca(H(2)O)(n)](2+) should be a mixture of the best candidate and some of metastable isomers for a given cluster size. For a cluster having second and/or third shell of water molecules, we found structural transitions between a low-lying structure and the lowest-energy structure undergoing much lower energy barriers. In addition, the IR spectra of the best candidates were predicted, in which the evolution of hydrogen-bond configurations with the cluster size was revealed.

"Compressing liquid": an efficient global minima search strategy for clusters.

In this paper we present a new global search strategy named as "compressing liquid" for atomic clusters. In this strategy, a random fragment of liquid structure is adopted as a starting geometry, followed by iterative operations of "compressing" and Monte Carlo adjustment of the atom positions plus structural optimization. It exhibits fair efficiency when it is applied to seeking the global minima of Lennard-Jones clusters. We also employed it to search the low-lying candidates of medium silicon clusters Si(n)(n=40-60), where the global search is absent. We found the best candidates for most sizes. More importantly, we obtained non-fullerene-based structures for some sized clusters, which were not found from the endohedral-fullerene strategy. These results indicate that the "compressing-liquid" method is highly efficient for global minima search of clusters.