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In contrast to well-known bivariate statistical approach, which is known to properly forecast extreme response levels for two-dimensional systems, the research validates innovative structural reliability method, which is particularly appropriate for multi-dimensional structural responses. The disadvantage of dealing with large system dimensionality and cross-correlation across multiple dimensions is not a benefit of traditional dependability approaches that deal with time series. Since offshore constructions are built to handle extremely high wind and wave loads, understanding these severe stresses is essential, e.g. wind turbines should be built and operated with the least amount of inconvenience. In the first scenario, the blade root flapwise bending moment is examined, whereas in the second, the tower bottom fore-aft bending moment is examined. The FAST simulation program was utilized to generate the empirical bending moments for this investigation with the load instances activated at under-rated, rated, and above-rated speeds. The novel reliability approach, in contrast to conventional reliability methods, does not call for the study of a multi-dimensional reliability function in the case of numerical simulation. As demonstrated in this work, it is now possible to assess multi-degree-of-freedom nonlinear system failure probability, in the case when only limited system measurements are available.

Green seaweed, as the most abundant species of macroseaweeds, is an important marine biological resource. It is a rich source of several amino acids, fatty acids, and dietary fibers, as well as polysaccharides, polyphenols, pigments, and other active substances, which have crucial roles in various biological processes such as antioxidant activity, immunoregulation, and anti-inflammatory response. In recent years, attention to marine resources has accelerated the exploration and utilization of green seaweeds for greater economic value. This paper elaborates on the main nutrients and active substances present in different green seaweeds and provides a review of their biological activities and their applications for high-value utilization. Graphical abstract

This research presents two unique techniques for engineering system reliability analysis of multi-dimensional non-linear dynamic structures. First, the structural reliability technique works best for multi-dimensional structural responses that have been either numerically simulated or measured over a long enough length to produce an ergodic time series. Second, a novel extreme value prediction method that can be used in various engineering applications is proposed. In contrast to those currently used in engineering reliability methodologies, the novel method is easy to use, and even a limited amount of data can still be used to obtain robust system failure estimates. As demonstrated in this work, proposed methods also provide accurate confidence bands for system failure levels in the case of real-life measured structural response. Additionally, traditional reliability approaches that deal with time series do not have the benefit of being able to handle a system's high dimensionality and cross-correlation across several dimensions readily. Container ship that experiences significant deck panel pressures and high roll angles when travelling in bad weather was selected as the example for this study. The main concern for ship transportation is the potential loss of cargo owing to violent movements. Simulating such a situation is difficult since waves and ship motions are non-stationary and complicatedly non-linear. Extreme movements greatly enhance the role of nonlinearities, activating effects of second and higher order. Furthermore, laboratory testing may also be called into doubt due to the scale and the choice of the sea state. Therefore, data collected from actual ships during difficult weather journeys offer a unique perspective on the statistics of ship movements. This work aims to benchmark state-of-the-art methods, making it possible to extract necessary information about the extreme response from available on-board measured time histories. Both suggested methods can be used in combination, making them attractive and ready to use for engineers. Methods proposed in this paper open up possibilities to predict simply yet efficiently system failure probability for non-linear multi-dimensional dynamic structure.

Polysaccharides are usually composed of more than ten monosaccharide units, which are connected by linear or branched glycosidic bonds. The immunomodulatory effect of natural polysaccharides is one of the most important bioactive function. In this review, molecular weight, monosaccharide (including galactose, mannose, rhamnogalacturonan-I arabinogalactan and uronic acid), functional groups (namely sulfate, selenium, and acetyl groups), types of glycoside bond connection (including 13-1,3-D-glucosyl, alpha-1,4-D-glucosyl, 13-1,4-D-glucosyl, alpha-1,6-D-glucosyl, 13-1,4-D-mannosyl, and 13-1,4-D-Xylopyranosyl), conformation and the branching degrees are systematically identified as their contribution to the immunostimulatory activity of polysaccharides. At present, studies on the structure-activity relationships of polysaccharides are limited due to their low purity and high heterogeneity. However, it is an important step in providing useful guidance for dietary supplements with polysaccharides. The chemical structures and the process of immune responses induced are necessary to be discussed. Polysaccharides may bind with the cell surface receptors to modulate immune responses. This review mainly discusses the structure-activity relationship of dietary polysaccharides.

In this study, molecular dynamics simulation is adopted to investigate the friction and wear mechanism of Fe-based amorphous composited coatings. It was found that the ceramic phase can improve plasticity of the composite coatings to reduce wear. In addition, when the Al2O3 content is 15 wt%, due to the stress concentration between ceramic phase and amorphous phase, the viscosity flow in the amorphous phase results in plastic deformation. Through this, the best wear resistance has been obtained. In addition, with the increase of compression depth, the number of atoms accumulated constantly increases, and the degree of plastic deformation becomes higher in the composite coatings. Finally, a high sliding speed results in the reduction of the wear resistance of coatings.

This article provides two unique methodologies that may be coupled to study the dependability of multidimensional nonlinear dynamic systems. First, the structural reliability approach is well suited for multidimensional environmental and structural reactions and is either measured or numerically simulated over sufficient time, yielding lengthy ergodic time series. Second, a unique approach to predicting extreme values has technical and environmental implications. In the event of measurable environmental loads, it is also feasible to calculate the probability of system failure, as shown in this research. In addition, traditional probability approaches for time series cannot cope effectively with the system's high dimensionality and cross-correlation across dimensions. It is common knowledge that wind speeds represent a complex, nonlinear, multidimensional, and cross-correlated dynamic environmental system that is always difficult to analyze. Additionally, global warming is a significant element influencing ocean waves throughout time. This section aims to demonstrate the efficacy of the previously mentioned technique by applying a novel method to the Norwegian offshore data set for the greatest daily wind cast speeds in the vicinity of the Landvik wind station. This study aims to evaluate the state-of-the-art approach for extracting essential information about the extreme reaction from observed time histories. The approach provided in this research enables the simple and efficient prediction of failure probability for the whole nonlinear multidimensional dynamic system.

The definition of atomic-scale contact is a very ambiguous issue owing to the discrete atomic arrangement, which hinders the development of contact theory and nano-tribological techniques. In this work, we studied the atomic-scale contact area and their correlations with friction force based on three distinct contact definitions (interatomic distance, force, and interfacial chemical bonds) by performing large-scale atomistic simulations on a typical ball-on-disk contact model. In the simulations, the measured contact areas defined by interatomic distance, force, and interfacial chemical bonds (referred as to

The traditional single-shot multiBox detector (SSD) for the recognition process in sea cucumbers has problems, such as an insufficient expression of features, heavy computation, and difficulty in application to embedded platforms. To solve these problems, we proposed an improved algorithm for sea cucumber detection based on the traditional SSD algorithm. MobileNetv1 is selected as the backbone of the SSD algorithm. We increase the feature receptive field by receptive field block (RFB) to increase feature details and location information of small targets. Combined with the attention mechanism, features at different depths are strengthened and irrelevant features are suppressed. The experimental results show that the improved algorithm has better performance than the traditional SSD algorithm. The average precision of the improved algorithm is increased by 5.1%. The improved algorithm is also more robust. Compared with YOLOv4 and the Faster R-CNN algorithm, the performance of this algorithm on the P-R curve is better, indicating that the performance of this algorithm is better. Thus, the improved algorithm can stably detect sea cucumbers in real time and provide reliable feedback information.

In this work, we investigated the stabilities of a newly developed symmetric flat-tube solid oxide fuel cells with double-sided cathodes (DSC cells) for long-term discharge and redox cycling operations. The DSC cell was discharged at 750°Cusing pure H2 fuel for ca. 2030 h and the degradation rate was ca. 10% per one thousand hours. In addition, H2 and air were alternatively supplied to the anode of DSC cells for tens of cycles in order to investigate the redox-stability. The duration time of oxidation process was found to be dominant for the stability of the cell. The microstructure of the DSC cells before and after these tests were investigated in order to clarify the degradation mechanisms. A great number of Ni particles with a diameter of ca. 2 μm was exsolved in the large pores of anode support layer after the redox cycling test with long duration time of oxidation. Such phenomenon induced by redox cycling has not been reported in the literatures. Based on the experimental data, the mechanisms were discussed.

We reveal the generation of the “Graphene Arch-Bridge” on a diamond (111) surface by Si doping via first-principles calculations. The “Graphene Arch-Bridge” is different from a simple graphene stru...

We investigated the effect of a cyclic chain structure on the wear resistance of polymer brushes via coarse-grained molecular dynamics simulations. Friction simulations were performed for cyclic an...

Because silicon-based ceramics show superlow friction in aqueous environments, these materials have attracted much attention for the development of water lubrication systems. The superlow friction ...

One of the key challenges for ammonia-fed anion exchange membrane fuel cells is to the ammonia electro-oxidation reaction (AEOR) at anode, which has sluggish kinetics and generates atomic nitrogen (Nads) poisoning the Pt catalyst. In this study, a comparative study on Pt/Ta3N5, Pt/Ta2O5, Pt/carbon black, and Pt plate are conducted in order to clarify the promoting effect of the support materials for Pt catalysts. X-ray photoelectron spectroscopy analysis and density functional theory calculations reveal that the support materials significantly affect the electron condition of the Pt, resulting in the tuned adsorption energy of Nads on Pt surface. The electrochemical analysis demonstrates that the weakened adsorption of Nads lowers the coverage of Nads on Pt surface, resulting in the enhanced performance and stability of Pt catalysts for AEOR. In particular, Pt/Ta3N5 catalyst exhibits a current density of 5.92 mA cm−2 of AEOR at −0.34 V vs. SCE, which is higher than that of Pt/Ta2O5 (2.56 mA cm−2, −0.35 V vs. SCE) and Pt/C (4.45 mA cm−2, −0.26 V vs. SCE). The achievements in this study demonstrate the importance of controlling the type of supports for the development of an active electrocatalyst for continuous AEOR.

Fe45Cr16Mo16C18B5 amorphous coatings were deposited on the mild steel substrate (0.45 wt.% C) by plasma spraying, and their corrosion resistance in 3.5 wt.% NaCl solution was compared with those of the substrate and 316 stainless steel. Potentiodynamic tests revealed that the as-prepared coatings had higher resistance to local corrosion than 316 stainless steel due to their wide passivation range. Electrochemical impedance spectroscopy (EIS) measurements further explained the long-term corrosion process of the coatings and indicated their excellent corrosion resistance. Based on this analysis, a schematic diagram of the corrosion mechanism of the coatings was proposed. The obtained results imply that Fe45Cr16Mo16C18B5 amorphous coatings have a promising prospect for industrial applications.

The performance of carbon- and silicon-based solid lubricants is strongly affected by the tribochemical reactions, especially with hydrogen and oxygen. Even though understanding and unveiling these...

A mechanism to achieve superlow friction in water lubrication is still in the debate because friction is accompanied by complex mechanochemical processes at sliding interfaces. Here, we experimenta...

It is important to understand the mechanical properties of diamond-like carbon (DLC) for use not only in frictionand wear-resistant coatings, but also in vibration reduction and damping increase at the layer interfaces. However, the mechanical properties of DLC are influenced by the working temperature and its density, and the applications of DLC as coatings are limited. In this work, we systematically studied the deformation behaviors of DLC under different temperatures and densities using compression and tensile testing of DLC by molecular dynamics (MD) methods. In our simulation results, the values of tensile stress and compressive stress decreased and tensile strain and compressive strain increased as the temperature increased from 300 K to 900 K during both tensile and compressive processes, indicating that the tensile stress and tensile strain depend on the temperature. During the tensile simulation, Young’s modulus of DLC models with different densities had a different sensitivity to the increase in temperature, and the DLC model with a high density was more sensitive than that with a low density, which was not seen in the compression process. We conclude that the Csp3-Csp2 transition leads to tensile deformation, while the Csp2-Csp3 transition and relative slip dominate compressive deformation.

Mesh quality is a major factor affecting the structure of computational fluid dynamics (CFD) calculations. Traditional mesh quality evaluation is based on the geometric factors of the mesh cells and does not effectively take into account the defects caused by the integrity of the mesh. Ensuring the generated meshes are of sufficient quality for numerical simulation requires considerable intervention by CFD professionals. In this paper, a Transformer-based network for automatic mesh quality evaluation (Gridformer), which translates the mesh quality evaluation into an image classification problem, is proposed. By comparing different mesh features, we selected the three features that highly influence mesh quality, providing reliability and interpretability for feature extraction work. To validate the effectiveness of Gridformer, we conduct experiments on the NACA-Market dataset. The experimental results demonstrate that Gridformer can automatically identify mesh integrity quality defects and has advantages in computational efficiency and prediction accuracy compared to widely used neural networks. Furthermore, a complete workflow for automatic generation of high-quality meshes based on Gridformer was established to facilitate automated mesh generation. This workflow can produce a high-quality mesh with a low-quality mesh input through automatic evaluation and optimization cycles. The preliminary implementation of automated mesh generation proves the versatility of Gridformer.

In this paper, the corrosion resistances of Mg-Nd binary alloys with various contents of the neodymium (Nd) element in sulfate-reducing bacteria (SRB) were studied. In the SRB medium, the results of weight loss experiments showed that the increase in the Mg12Nd phase in the alloy increased the galvanic corrosion and the corrosion rate. However, when the continuous network distribution of the second phase formed, the corrosion resistance of the alloy improved. The biofilm was formed by the adhesion of the SRB. Meanwhile, the protection from the corrosion improved due to the corrosion products, which prevent the penetration of corrosive ions. On the other hand, the products of biological metabolism accelerated the corrosion of the matrix.

Efficient nitrogen reduction reaction (NRR) catalysis relies on the active sites of the electrocatalyst being capable of adsorption of hydrogen ions and nitrogen molecules. As has been well reported, nitrogen-doped carbons are efficient electrocatalysts for the hydrogen evolution reaction (HER) due to their ability to adsorb hydrogen ions, to combine them and to release H2. The introduction of other heteroatoms to disturb the successive HER active sites in nitrogen-doped carbon, and to offer binding sites for N2, is beneficial for efficiently catalyzing the NRR. Here, we rationally synthesize boron, nitrogen and fluorine ternary-doped carbon (BNFC) from the carbonization of cigarette butts saturated with the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate (C8H15N2BF4). Nitrogen atoms in BNFC are capable of adsorption of hydrogen ions, and the adjacent B or F heteroatoms offer Lewis acid sites for chemisorption of N2; in the case of this synergistic catalysis, a boosted electrocatalytic activity toward the NRR with an NH3 yield rate of 41 μgNH3 h−1 mgcat.−1 and a faradaic efficiency as high as 14% at −0.4 V vs. reversible hydrogen electrode (RHE) are recorded for BNFC in 0.05 M H2SO4 electrolyte. Moreover, a comparably high NH3 yield rate (39 μgNH3 h−1 mgcat.−1) and faradaic efficiency (16%) at −0.4 V vs. RHE are also recorded in 0.1 M NaOH electrolyte due to the capability of water dissociation and adsorption of generated hydrogen ions by the nitrogen atoms in BNFC. In addition, due to the modulated electronic structure of the carbon atoms by means of heteroatom doping, BNFC also exhibits a Pt-like oxygen reduction reaction (ORR) activity with a half-wave potential of 836 mV vs. RHE and superior oxygen evolution reaction (OER) demanding a low overpotential of 313 mV to achieve 10 mA cm−2. As a consequence, the assembled rechargeable zinc–air batteries (ZABs) perform at a 1.8-fold higher power density than commercial Pt/C–IrO2 and sustain this for 2300 cycles with minimal degradation in performance. The battery performance of all-solid-sate ZABs also outperforms that of commercial Pt/C–IrO2 systems and can power the OER–NRR coupled device to electrochemically generate NH3.

Efficient electrocatalysts for the hydrogen evolution reaction (HER) in both acidic and alkaline media are prominently important to advance the widespread application of industrial water splitting technology. In this work, we have synthesized ultrasmall rhodium–rhodium phosphide hetero-nanoparticles (3.4 nm, Rh–Rh2P@C) by partial phosphorization of Rh@C at 300 °C together with Na2HPO2. The constructed Rh–Rh2P@C requires only 24 mV and 37 mV overpotentials to reach 10 mA m−2 in 0.5 M H2SO4 and 1 M KOH electrolytes, which are lower than those of Rh@C and Rh2P@C due to the formation of heterointerfaces inducing modulation of local electronic structures and more active sites; as a result, a more moderate hydrogen adsorption–desorption strength is achieved for Rh–Rh2P@C as proved by theoretical calculation. Moreover, the calculated mass activity of Rh–Rh2P@C is 4 orders of magnitude higher than that of the benchmark Pt/C electrocatalyst. Besides, exceptional stability is recorded for Rh–Rh2P@C in both acidic and alkaline electrolytes due to the strong electronic interaction between Rh and Rh2P. This study offers a new strategy to manufacture efficient electrocatalysts for various applications.

The industrial electrochemical generation of hydrogen predominantly relies on the polymer electrolyte membrane water electrolyzer (PEMWE). An efficient and stable anodic electrocatalyst is extraordinarily important since the harsh acidic environment is corrosive to the metal-based catalyst, especially at high voltage. In this work, we deposited ultrasmall AuIr alloyed nanoparticles (1.7 nm) on carbon nanotubes (AuIr@CNT) as a bifunctional electrocatalyst for acidic water splitting. Due to the incorporated Au atoms, the neighboring Ir atom is positively charged, resulting in the electronic delocalization of Ir, which is favorable for reducing the energy barrier of the rate-limiting step (O* → OOH*) in the acidic oxygen evolution reaction (OER); thus, the overpotential for 10 mA cm−2 is 257 mV for AuIr@CNT in 0.5 M H2SO4 electrolyte, which is comparatively lower than that of Ir@CNT (279 mV) and commercial IrO2 (332 mV). Moreover, the mass activity of AuIr@CNT is 70 orders of magnitude higher than that of benchmark IrO2 at a potential of 1.6 V vs. RHE. More importantly, the formed AuIr@CNT electrocatalyst exhibits promising stability compared to commercial IrO2 and Ir@CNT, which is ascribed to the strong electronic interaction between Au and Ir. Subsequently, AuIr@CNT shows superior hydrogen evolution reaction (HER) catalytic activity, which is 6.3 times better than that of the benchmark Pt/C due to the well-constructed AuIr alloy structure at the atomic level triggering a more moderate hydrogen adsorption–desorption strength. The assembled water splitting device requires only 1.51 V to attain an electrocatalytic current density of 10 mA cm−2 for AuIr@CNT, which is lower than that of the Pt/C–IrO2 system (1.6 V); in addition, an excellent stability is also recorded.

Diamond-like carbon (DLC) is one of the most promising solid lubricants for sliding against other materials, such as steel, alumina, and silicon carbide (SiC). During sliding, a DLC transfer film is usually formed on the counterpart surface, affording a low friction coefficient. It is well known that hydrogen in DLC strongly promotes the formation of the DLC transfer film. To further improve the lubricity of DLC, we investigate the formation mechanisms of the DLC transfer film on amorphous SiC and the influence of hydrogen on transfer film formation using reactive molecular dynamics simulations. In addition to the conventional transfer mechanism induced by surface adhesion, we herein propose the new transfer mechanism of “hydrocarbon-emission-induced transfer”. In the proposed transfer mechanism, hydrocarbon molecules are emitted from the DLC surface and subsequently adsorb on the counterpart surface during the continuous grinding of the sliding interface, ultimately generating the DLC transfer film. Furthermore, the addition of hydrogen atoms to DLC slightly increases the adhesion-induced transfer and greatly accelerates the “hydrocarbon-emission-induced transfer”, collaboratively contributing to substantial DLC transfer film formation. Thus, we suggest that the experimentally observed promotion of DLC transfer film formation by hydrogen is largely attributable to our proposed mechanism of “hydrocarbon-emission-induced transfer”.

Background: Novel coronavirus disease has been recently a concern for worldwide public health. To determine epidemic rate probability at any time in any region of interest, one needs efficient bio-system reliability approach, particularly suitable for multi-regional environmental and health systems, observed over a sufficient period of time, resulting in a reliable long-term forecast of novel coronavirus infection rate. Traditional statistical methods dealing with temporal observations of multi-regional processes do not have the multi-dimensionality advantage, that suggested methodology offers, namely dealing efficiently with multiple regions at the same time and accounting for cross-correlations between different regional observations. Methods: Modern multi-dimensional novel statistical method was directly applied to raw clinical data, able to deal with territorial mapping. Novel reliability method based on statistical extreme value theory has been suggested to deal with challenging epidemic forecast. Authors used MATLAB optimization software. Results: This paper described a novel bio-system reliability approach, particularly suitable for multi-country environmental and health systems, observed over a sufficient period of time, resulting in a reliable long-term forecast of extreme novel coronavirus death rate probability. Namely, accurate maximum recorded patient numbers are predicted for the years to come for the analyzed provinces. Conclusions: The suggested method performed well by supplying not only an estimate but 95% confidence interval as well. Note that suggested methodology is not limited to any specific epidemics or any specific terrain, namely its truly general. The only assumption and limitation is bio-system stationarity, alternatively trend analysis should be performed first. The suggested methodology can be used in various public health applications, based on their clinical survey data.

Ultraflat and damage-free single-crystal diamond is a promising material for use in electronic devices such as field-effect transistors. Diamond surfaces are conventionally prepared by the chemical mechanical polishing (CMP) method, although the CMP efficiency remains a critical issue owing to the extremely high hardness of diamond. Recently, OH radicals have been demonstrated to be potentially useful for improving the CMP efficiency for diamond; however, the underlying mechanisms are still elusive. In this work, we applied our previously developed CMP-specialized tight-binding quantum chemical molecular dynamics simulator to comprehensively elucidate the CMP mechanisms of diamond assisted by OH radicals. Our simulation results indicate that the diamond surface is oxidized by reactions with OH radicals and then a concomitant surface reconstruction takes place due to the distorted and unstable nature of the oxidized diamond surface structure. Furthermore, we interestingly reveal that the reconstruction of the diamond surface ultimately leads to two distinct removal mechanisms: (i) gradual atom-by-atom removal through the desorption of gaseous molecules (e.g., CO2 and H2CO3) and (ii) drastic sheet-by-sheet removal through the exfoliation of graphitic ring structures. Hence, we propose that promoting the oxidation-induced graphitization of the diamond surface may provide a route to further improving the CMP efficiency.

Objectives: Hepatocellular carcinoma (HCC) accounts for 70%-85% of liver cancer, and about 85% of HCC are hepatitis B virus-related hepatocellular carcinoma (HBV-HCC) in china. Most patients are already in the middle or late stages of the disease at the time of diagnosis, trans-hepatic arterial chemoembolization (TACE) combine with traditional Chinese medicine (TCM) have been reported as an effective treatment, and effective prognostic molecular markers are helpful to predict therapeutic efficacy. In this study, we aim to explore whether circulating cell-free mitochondrial DNA (ccf-mtDNA) content is associated with the outcome for HCC patients receiving TACE combine with TCM treatment. Method: Retrospective analysis was conducted in a cohort with 141 HBV-HCC patients. Univariate and multivariate analysis was conducted with Cox proportional risk regression model to explore the correlation between ccf-mtDNA content and patient prognosis. Kaplan-Meier method was used to draw the survival curve of ccf-mtDNA content and the survival prognosis of patients. Results: (1) high content of serum ccf-mtDNA is an independent risk factor for the prognosis of HBV-HCC patients treated with TACE combined with TCM adjuvant therapy (HR=4.010, 95%IC=1.252-12.844, P = 0.019). (2) K-M survival analysis showed that patients with high ccf-mtDNA content had poor prognosis (Log Rank P=0.027). Conclusions: Our findings suggest that ccf-mtDNA is a potential novel non-invasive biomarker for prognosis of HCC patients receiving TACE combine with TCM treatment.

Nowadays, the temperature gradient is considered as one of the most important parameters which impact the performance of the solid oxide fuel cell (SOFC). In this paper, a control strategy based on an input-output feedback linearization technology is derived for controlling the maximum temperature gradient within the anode fuel flow channel at the desired value. For the controller design, the temperature dynamic model is proposed and simplified to a control-oriented multi-input and multioutput nonlinear dynamic model. Then, this paper presents an input-output feedback linearization controller to realize the control objective by adjusting the cathode input air flow. Finally, the simulation results are given to demonstrate the accuracy of the proposed model in reflecting the temperature dynamic characteristics. Moreover, the compound controller is added for simulation as a comparison, which shows that the proposed controller is equipped with better effectiveness and efficiency in the presence of external disturbances.

Non-empirical law depicting how atomic-scale friction behaves is crucial to facilitate the practical design of tribosystems. However, progress in developing a practically usable friction law has stagnated because atomic-scale friction arises from the continuous forming and rupturing of interfacial chemical bonds and such interfacial chemical reactions are difficult to measure precisely in experiments. Here, we propose a usable friction law for atomic-scale contact by using atomistic simulations to correctly measure the interfacial chemical reactions of a realistic rough surface, and confirm its applicability to predicting how atomic-scale friction varies with temperature, sliding velocity, and load.

Chemical mechanical polishing (CMP) is a key manufacturing process for applying gallium nitride (GaN), especially the Ga-face GaN, to semiconductor devices such as laser diodes. However, the CMP efficiency for GaN is very low due to its high hardness and chemical stability. Experimentally, OH radicals appear able to improve the CMP efficiency of GaN polished by a SiO2 abrasive grain, whereas the mechanisms of the OH-radical-assisted CMP process remain unclear because experimental elucidation of the complex chemical reactions occurring among GaN substrate, abrasive grain, and OH radicals is difficult. In this work, we used our previously developed tight-binding quantum chemical molecular dynamics simulator to study the OH-radical-assisted CMP process of the widely employed Ga-face GaN substrate polished by an amorphous SiO2 abrasive grain in an effort to understand how OH radicals assist the CMP process and then aid the development of next-generation CMP techniques. Our simulations revealed that the OH-radical-assisted CMP process of GaN occurs via the following three basic reaction steps: (i) first, all hydrogen terminations on the GaN surface are replaced by OH terminations through continuous reactions with OH radicals; (ii) after the substrate is fully terminated by OH, the hydrogen atoms of these OH terminations are removed by reacting with newly added OH radicals, which forms H2O molecules and leaves energetic oxygen atoms with dangling bonds on the surface; and (iii) finally, these energetic oxygen atoms intrude inside the substrate with concomitant dissociation of Ga-N bonds and the generation of N2 and gallium hydroxide molecules, which accumulatively lead to the removal of N and Ga atoms from the substrate.