Composition Fluctuations Research Articles

Optically thick GaInAs/GaAsP strain-balanced quantum-well tandem solar cells with 29.2% efficiency under the AM0 space spectrum

GaAs is often used as a multijunction subcell due to its high material quality on GaAs substrates, despite having a non-optimal bandgap. The bandgap can be beneficially reduced using many layers of thin, strain-balanced GaInAs in a superlattice or quantum well device, but achieving excellent carrier collection without increased recombination has proven challenging. Here, we develop and demonstrate high performance, optically thick GaInAs/GaAsP strain-balanced solar cells. Excellent material quality is achieved in thick superlattices by using growth conditions that limit progressive thickness and composition fluctuations. Bandgap-voltage offsets as low as 0.31 V are shown in superlattice cells using thin, highly strained GaP barriers. Optically thick superlattice cells with over 2500 nm of total GaInAs in the depletion region are developed, enabling 3.8 mA/cm2 of extra photocurrent beyond the GaAs band edge under the AM0 space spectrum. Optimized superlattice solar cells are incorporated into two-junction devices that achieve 29.2% efficiency under the AM0 space spectrum due to their improved bandgap combination and high subcell voltages.

Atom-Scale Chemistry in Chalcopyrite-Based Photovoltaic Materials Visualized by Atom Probe Tomography.

Chalcopyrite-based materials for photovoltaic devices tend to exhibit complex structural imperfections originating from their polycrystalline nature; nevertheless, properly controlled devices are surprisingly irrelevant to them in terms of resulting device performances. The present work uses atom probe tomography to characterize co-evaporated high-quality Cu(In,Ga)Se2 (CIGS) films on flexible polyimide substrates either with or without doping with Na or doping with Na followed by K via a post-deposition treatment. The intent is to elucidate the unique characteristics of the grain boundaries (GBs) in CIGS, in particular the correlations/anti-correlations between matrix elements and the alkali dopants. Various compositional fluctuations are identified at GBs irrespective of the presence of alkali elements. However, [Cu-poor and Se/In,Ga-rich] GBs are significantly more common than [Cu-rich and Se/In,Ga-poor] ones. In addition, the anti-correlations between Cu and the other matrix elements are found to show not only regular trends among themselves but also the association with the degree of alkali segregation at GBs. The Na and K concentrations exhibited a correlation at the GBs but not in the intragrain regions. Density functional theory calculations are used to explain the compositional fluctuations and alkali segregation at the GBs. Our experimental and theoretical findings not only reveal the benign or beneficial characteristics of the GBs of CIGS but also provide a fundamental understanding of the GB chemistry in CIGS-based materials.

Fault detection of rotating machinery based on marine predator algorithm optimized resonance-based sparse signal decomposition and refined composite multiscale fluctuation dispersion entropy.

Feature extraction is the key to the fault detection of rotating machinery based on vibration signals, and the quality of the features influences the reliability of the detection. This paper develops a fault feature extraction method of rotating machinery based on optimized resonance-based sparse signal decomposition and refined composite multiscale fluctuation dispersion entropy. First, resonance-based sparse signal decomposition is used to decompose the vibration signals adaptively. In order to obtain the resonance-based sparse signal decomposition algorithm with optimal performance, the marine predator algorithm is used for the parameters optimization with correlation kurtosis as the fitness function. Subsequently, based on the refined composite coarse-grained process and fluctuation dispersion entropy, a refined composite multiscale fluctuation dispersion entropy is developed, enabling a more accurate and comprehensive measure of the complexity of time series. Then, all feature matrices are input to the support matrix machine for fault identification. Experiments are conducted using two typical rotating machinery datasets for the validity of the proposed method, and comparisons are made with other methods. The results show that the proposed scheme outperforms other comparative methods regarding classification accuracy and stability. In addition, the proposed scheme can obtain relatively reliable classification results even when the data volume is small and the background noise is significant, demonstrating the scheme's potential for application in practical engineering.

Atomistic understanding of incipient plasticity in BCC refractory high entropy alloys

Understanding the incipient plastic behavior of refractory high-entropy alloys (RHEAs) is crucial for their high-temperature applications. In this study, the initial dislocation nucleation and motion mechanisms in the TaTiZrV RHEA and their dependence on temperature were investigated upon nanoindentation by molecular dynamics (MD) simulations. Compared with the high stress-driven homogeneous nucleation criterion in pure BCC metals, the Zr-V short-range orders in the RHEA facilitate preferential inhomogeneous nucleation at low stress. The local compositional fluctuation not only causes intermittent slipping of screw dislocations by the trapping-detrapping mechanism but also severely reduces the moving rate of edge dislocations. In particular, the initial dislocation nucleation in the RHEA becomes more difficult with the increasing temperature. Based on the competitive mechanism between multiple-element-induced lattice distortion and point-defect-induced lattice distortion, the reason for their excellent retained mechanical properties at high temperatures was revealed. This study would provide theoretical support for the development of RHEAs in high-temperature technological applications.

Component fluctuation and lattice distortion-controlled oxidation behavior of refractory complex concentrated alloys

The influence of compositional fluctuation and lattice distortion on the oxidation behavior of Ti2ZrTax refractory complex concentrated alloys (RCCAs) were revealed. Chemical complexity of Ti2ZrTax results in the formation of various oxides, including saturated and unsaturated oxides, which reduces the protection of oxidized layer and leads to the high oxidation rate. When metastable Ti2ZrTax alloys were oxidated and cooled in air, spinodal decomposition occurred in the matrix. The wavelength of spinodal decomposition and lattice distortion are dependent on the stability of alloys. Component fluctuation and lattice distortion make O atoms preferentially diffuse along the Zr-enriched region.

Influence of stress resistance on technological and productive qualities of cows

The paper presents the research results on the level of stress resistance influence on technological indicators of udder, component structure of milk and reproductive ability of the Ukrainian Black-and-White dairy cows. The relevance and practical significance of the research is in fact that only constitutionally strong and stress-resistant animals can be healthy and high-producing. The scientific novelty is resulted in the study of energy characteristics of cows and fluctuations in the component composition of milk of different stress-resistant cows at different times of the day.
 The method of dividing cows into groups was used taking into account the minutely milk flow during milking cows by "unfamiliar milkmaid". The level of inhibition of the lactation reflex caused by sudden changes of milking conditions was determined, in particular, pre-milking udder preparation and milking cows by an "unfamiliar milkmaid", i.e. an experimenter, which can cause retardation of the lactation reflex. Due to the formation of experimental groups from half-sibling cows, the variability of indicators based on the influence of genetic factors is minimized.
 It is established that stress-resistant cattle are more technological and productive. Factorial analysis revealed the effect of stress resistance on the functional parameters of the udder of cows. It was found that the impact factor is 27.60% on the duration of milking (P> 0.99), 55.66% on milking during the first minute (P> 0.999), 25.53% on milking during the first three minutes (P> 0.99), 40.40% on the average milking rate (P> 0.999), 32.52% on the maximum milking rate (P> 0.999), and 69.57% on the intensity factor of milk flow inhibition (P> 0.999). The qualitative composition of milk under the influence of technological stress has changed in both experimental groups. However, acute fluctuations were observed in stress-sensitive cows in terms of fat milk yield by 0.11%, protein milk yield by 0.08% and dry matter content by 0.11%, compared to stress-resistant peers. Stress-resistant animals released more energy with milk by 4.03 MJ per day (12.6%; P> 0.999); they had a higher energy index by 3.26% (P> 0.99), spent less energy on synthesis of milk with energy value of 1 MJ by 0.19 MJ (8.3%; P> 0.95) and released more energy with milk per 1 kg of metabolic mass by 0.038 MJ (12.1%; P> 0.99).

Experimental insights toward carrier localization in in-rich InGaAs/InP as candidate for SWIR detection: Microstructural analysis combined with optical investigation

Hyperspectral imaging has been flourished thanks to the huge investigation of the infrared spectrum from NIR to LWIR bands. The ternary InGaAs has been investigated herein in the context of studying the structural dependences of localization phenomenon by X-ray diffraction (XRD), scanning electron microscopy-energy dispersive X-ray (SEM-EDX), Raman, ultraviolet–visible (UV–vis), and photoluminescence (PL) techniques. Using metal-organic vapor phase epitaxy (MOVPE), we succeed to grow the InGaAs directly on InP substrate at 560 °C as an active layer with indium concentration exceeding the “golden” value (53%) to enlarge its cutoff absorption wavelength. X-ray diffraction proved a good crystallinity of the heterostructure with a sharp peak related to the thick substrate and another peak attributed to the thin layer of InGaAs. Moreover, an interfacial layer appeared at the logarithmic scale of XRD patterns and was confirmed by Raman analysis. The SEM-EDX revealed an average indium concentration (62%), almost the growth concentration. However, a cross-section compositional profile over the heterostructure showed an inhomogeneous distribution of the indium. This is predictable from the composition fluctuation in the indium-containing alloys and the volatility (surface segregation) of As (In). On the other side, the optical investigation of InGaAs demonstrated an anomalous behavior of luminescence versus temperature, manifested by the S-shape feature. This trend stems from the potential fluctuation induced by the non-uniform distribution of indium. A numerical simulation was developed based on the localized state ensemble (LSE) model to well-reproduce this anomaly by giving the best fitting parameters and comparing them with those calculated using the semi-empirical models (Viña and Pässler). The results reported here will help in optimizing the epitaxy design of future InGaAs/InP and further studying its surface morphology and device performance.

Research on deformation resistance of strip steel and rolling force prediction during cold rolling based on the hot rolling parameters

ABSTRACT In view of the influence of the fluctuation of incoming material composition, finishing temperature and coiling temperature on the deformation resistance in the cold rolling process during the hot rolling process, the optimization objective function is established by combining the analytic method and big data regression. Further, the relevant characteristic coefficients are calculated through a large number of on-site actual data, and the deformation resistance model of hot-rolled finished strip and the rolling pressure prediction model of cold rolling process are obtained. Through the field test, the deviation between the predicted rolling force and the actual rolling force is less than 2%, and the accuracy of the model can meet the engineering requirements. More importantly, after using the research results described in this paper, the quality of cold rolled strip shape has been significantly improved, which can greatly improve the economic efficiency and market competitiveness of enterprises.

Phase-field model of grain boundary diffusion in nanocrystalline solids: Anisotropic fluctuations, anomalous diffusion, and precipitation

The anisotropic phase-filed model of grain boundary diffusion and precipitation of solute in nanocrystalline solids has been developed. In this model, the Cahn–Hilliard equation is generalized for the anisotropic phase-field diffusion of solute and anisotropic compositional fluctuations. It is found that dynamics of solute concentration profile demonstrates the anomalous diffusion behavior with scaling parameters depending on the mobility ratio and microstructure of a solid solution. It is noteworthy that the increase in source concentration can slow down the concentration front propagation due to uphill diffusion or formation of a new phase. Parameters of grain boundary diffusion control the precipitation dynamics. In particular, a decrease in transverse diffusion coefficient is responsible for longer incubation time, and lower rates of nucleation and nuclei growth in comparison with the case of isotropic solute transport near grain boundaries. Transport properties of boundary and bulk are responsible for the formation of the bimodal size distribution function of second phase particles and specific kinetics of average radius and number density.

Disorder-induced topological phase transition in HgCdTe crystals

Using the self-consistent Born approximation, we study a topological phase transition appearing in bulk HgCdTe crystals induced \emph{uncorrelated} disorder due to both randomly distributed impurities and fluctuations in Cd composition. By following the density-of-states evolution, we clearly demonstrate the topological phase transition, which can be understood in terms of the disorder-renormalized mass of Kane fermions. We find that the presence of heavy-hole band in HgCdTe crystals leads to the topological phase transition at much lower disorder strength than it is expected for conventional 3D topological insulators. Our theoretical results can be also applied to other narrow-gap zinc-blende semiconductors such as InAs, InSb and their ternary alloys InAsSb.

Adaptive attenuation of hierarchical composition fluctuations augments the plastic strain of a high entropy steel

A body-centred cubic (BCC) high entropy steel with a spinodal-like nanopattern and atomic-scale local chemical fluctuations exhibits controlled attenuation of its chemical complexity with deformation. Changes in the chemical composition of the spinodal structure measured using energy dispersive X-ray spectroscopy reveal that the average composition peak-to-peak amplitude decreases by 67% from 4.9 at.% to 1.6 at.% with increasing strain. On the other hand, the short-range chemical fluctuations, assessed with atomic strain mapping, displays a 48% decrease in the average strain peak-to-peak amplitude from 3.03 at.% to 1.59 at.% under mechanical loading. The reduction in local strain brought about by increased chemical homogeneity at both levels enables more uniform, steady deformation leading to extended ductility (13.7±1.9%), all the while maintaining ultrahigh strength (2.92±0.36 GPa, placing it among the highest values reported). The interactions between dislocations and concentration waves are identified and found to be responsible for this compelling effect on the newly created steel.

Distribution and Ecology of Phlebotomine Sand Flies (Diptera: Psychodidae) in Endemic and Nonendemic Area of Leishmaniasis in Northern Morocco.

Northern Morocco is endemic for cutaneous and visceral leishmaniasis. Our entomological investigations aim to evaluate the risk of Leishmania transmission by determining the species composition, the density, and seasonal fluctuation of sand fly populations in endemic and nonendemic areas of leishmaniasis in Tetouan province (North-Western Morocco). Using Sticky-paper traps, 8,370 specimens were collected between May and November 2015 in two localities: peri-urban area of Tetouan city, where leishmaniasis is endemic and that of the Oued Laou village where no cases of leishmaniasis have been recorded. Six sand fly species were identified. The genus Phebotomus was represented by five species: Phlebotomus ariasi, Phlebotomus. longicuspis, Phlebotomus perniciosus, Phlebotomus kazeruni, and Phlebotomus sergenti, while the genus Sergentomyia was represented by only one species Sergentomyia minuta. Phlebotomus perniciosus was dominant in the nonendemic area (47%) while Phlebotomus sergenti was dominant in the endemic area (51%). The spatio-temporal distribution of sand fly populations is discussed according to biotic and abiotic variables. Seasonal fluctuation in sand fly density showed a bimodal pattern for the subgenus Larroussius and a unimodal pattern for the subgenus Paraphlebotomus in Tetouan city. But, in Oued Laou village, a unimodal density distribution for species of the Larroussius subgenus and a bimodal seasonal distribution for species of the subgenus Paraphlebotomus were identified. We affirm the coexistence, in the study area, of vectors of both cutaneous and visceral leishmaniasis, namely P. ariasi, P. longicuspis and P. perniciosus vectors of Leishmania infantum and P. sergenti vector of L. tropica. However, the geographic distribution, the specific abundance, and the activity reveal significant differences between endemic and nonendemic areas in the region.

Pseudo-4D view of the growth and form of locked eutectic colonies

We investigate solidification of an Al-Cu alloy as a model system to understand the emergence of patterns (such as lamellar, rod and maze-like) within eutectic colonies. To uncover the morphological transitions in situ and in 3D, we introduce here a new synchrotron-based procedure termed pseudo-4D X-ray imaging. Our method simultaneously maximizes the temporal (200 ms) and spatial resolution (0.692μm2/pixel) over that of traditional imaging approaches. The wealth of information obtained from this procedure enables us to visualize the development of a crystallographically ‘locked’ eutectic microstructure in the presence of thermosolutal convection. This data provides direct insight into the mechanism of the lamella-to-rod transition as the eutectic accommodates fluctuations in interfacial composition and growth velocity. We offer evidence to show that this transition is diffusive. It is brought about by impurity-driven forces acting on the solid-solid-liquid trijunction that must overcome the stiffness of the solid-solid interfaces. Our pseudo-4D imaging strategy holds broad appeal to the solidification science community, as it can overcome the space-time trade-off in conventional in situ X-ray microtomography.

Robust optimal operation of continuous catalytic reforming process under feedstock uncertainty

The continuous catalytic regenerative (CCR) reforming process is one of the most significant sources of hydrogen production in the petroleum refining process. However, the fluctuations in feedstock composition and flow rate could significantly affect both product distribution and energy consumption. In this study, a robust deviation criterion based multi-objective optimization approach is proposed to perform the optimal operation of CCR reformer under feedstock uncertainty, with simultaneous maximization of product yields and minimization of energy consumption. Minimax approach is adopted to handle these uncertain objectives, and the Latin hypercube sampling method is then used to calculate these robust deviation criteria. Multi-objective surrogate-based optimization methods are next introduced to effectively solve the robust operational problem with high computational cost. The level diagram method is finally utilized to assist in multi-criteria decision-making. Two robust operational optimization problems with different objectives are solved to demonstrate the effectiveness of the proposed method for robust optimal operation of the CCR reforming process under feedstock uncertainty.

Microscale deformation controlled by compositional fluctuations in equiatomic Nb–Mo–Ta–W alloys

Local fluctuations in the chemical composition of high-entropy alloys (HEA) are known to have a significant influence on their mechanical performance. In this work, we conclusively establish a direct link between the strength of micron-sized specimens and deviations from equiatomic stoichiometry in body-centered-cubic (bcc) refractory Nb–Mo–Ta–W HEA. We perform a detailed electron dispersive spectroscopy (EDS) line profile analysis of the compressed micropillars after in-situ scanning electron microscopy microcompression tests at room-temperature and strain rates between 10−4 and 10−1 s−1. We find that compositional fluctuations near the micropillar tip is the best quantitative predictor of yield strength over strain rate and grain orientation. In micropillars tip rich in W, we measure a yield strength of 1500 MPa, while for those with a Ta and Nb-rich oscillations, the yield strength is 1000 and 700 MPa, respectively. Finally, our high-resolution TEM analysis reveals the presence of numerous edge dislocations, suggesting that deformation is controlled by nucleation due to the absence of sufficient internal sources.

Effect of compositional fluctuation on the survival of bet-hedging species

Understanding the coexistence of diverse species in a changing environment is an important problem in community ecology. Bet-hedging is a strategy that helps species survive in such changing environments. However, studies of bet-hedging have often focused on the expected long-term growth rate of the species by itself, neglecting competition with other coexisting species. Here we study the extinction risk of a bet-hedging species in competition with others. We show that there are three contributions to the extinction risk. The first is the usual demographic fluctuation due to stochastic reproduction and selection processes in finite populations. The second, due to the fluctuation of population growth rate caused by environmental changes, may actually reduce the extinction risk for small populations. Besides those two, we reveal a third contribution, which is unique to bet-hedging species that diversify into multiple phenotypes: The phenotype composition of the population will fluctuate over time, resulting in increased extinction risk. We compare such compositional fluctuation to the demographic and environmental contributions, showing how they have different effects on the extinction risk depending on the population size, generation overlap, and environmental correlation.

Weekly fluctuation in phytoplankton macromolecular composition in response to environmental changes, in Eastern Harbour, Alexandria (Egypt)

Weekly fluctuation in phytoplankton macromolecular composition in response to environmental changes, in Eastern Harbour, Alexandria (Egypt)

A transfer predictive control method based on inter-domain mapping learning with application to industrial roasting process

As a critical variable in the roasting process, the roasting temperature has a significant influence on operating conditions. Model predictive control (MPC) provides a path to stabilize the roasting temperature. However, process data collected at different periods usually follow different distributions due to the fluctuation of feed composition for the roasting process, result in a model mismatch on online control. For this reason, a transfer predictive control method based on inter-domain mapping learning (IDML-MPC) is proposed. The proposed method first treat historical and online data as two domains. Then, a distribution mapping function from one domain to another domain is learned to make the distribution of the historical data follow that of the online data. Finally, an accurate online prediction model is built, roasting temperature control is achieved by minimizing the cost function with respect to the predicted value and the control input. The effectiveness of the proposed method is demonstrated by comparative experiments based on a numerical example and a simulation platform of the roasting process. Experimental results compared with some state-of-the-art methods show that it is necessary to take into account the distribution differences between historical data and online data when production conditions change. The IDML-MPC improved the control performance for the roasting temperature with an average 56.98% reduction in the root mean square error.

Study on Mechanical Properties and Deformation Mechanism of TWIP Stainless Steel

In this study, based on the sensitivity of the chemical composition fluctuation to the thermodynamic parameter, which controls the level of the stacking fault energy (SFE), a series of high Cr–Mn–N twinning-induced plasticity (TWIP) stainless steels are designed by using a sublattice model, and their mechanical properties and micro deformation mechanism are analyzed The formation of mechanical twins (Mts) during the deformation makes the test steel show a perfect combination of strength and ductility after different solution treatments. Among them, after a solution treatment at 950 °C and 1050 °C, the 19Cr–0.7N and 19CrSi–0.7N samples have the maximum value with the product of the strength and plasticity reaching 60.7% and 64.6%, and 12Cr–CN has the maximum value after the solution treatment at 1200 °C, reaching 81.3%. The SFE values of the 19Cr–0.7N and 19CrSi–0.7N samples were relatively high, 48 mJ·m−2 and 45 mJ·m−2, respectively. The SFE of 12Cr–CN is 37 mJ·m−2, and the Mts grow rapidly during the deformation and maintain the highest twinning density under the same strain conditions. The characterization of the tensile samples occurs under different deformations by electron backscattered diffraction (EBSD) and transmission electron microscope (TEM). The results of the EBSD local misorientation difference angle analysis showed the Silicon element addition with a good Mts saturation rate. It is observed from the TEM that the nucleation process of the Mts with a high SFE is difficult, and the Mts emit and grow inward along the grain boundary during the tensile process and present a cross shape with the increase in strain. The contribution of the grain boundary strengthening (σ0), dislocation strengthening (σf), and twinning strengthening effect (σt) under dynamic micro-refinement to stress were calculated. It is known that under a certain amount of strain, the ratio of σt and σf changes with increasing, and when the contribution of the twinning deformation to the stress exceeds about 25%, the reinforcement of the plastic deformation is dominated by the plane of σf.

Evolution of Nanometer-Scale Microstructure within Grains and in the Intergranular Region in Thermoelectric Mg3(Sb, Bi)2 Alloys.

n-type Mg3Sb2-Mg3Bi2 alloys have been investigated as one of the most promising thermoelectric materials. To achieve high performance, a detailed understanding of the microstructure is required. Although Mg3Sb2-Mg3Bi2 is usually considered to be a complete solid solution, nanosized compositional fluctuations were observed within a matrix and in the vicinity of the grain boundary. As an inhomogeneous microstructure can be beneficial or detrimental to thermoelectric performance, it is important to investigate the evolution of compositional variations for the engineering and long-term use of these materials. Using scanning transmission electron microscopy and atom probe tomography, a Bi-rich phase and compositional fluctuations are observed in sintered and annealed samples. After annealing, the broad intergranular phase was sharpened, resulting in a greater compositional change in the intergranular region. Annealing considerably reduces the fluctuations of Bi and Mg content within the grain as observed in atom probe tomography. Weighted mobility and lattice thermal conductivity were both increased as a result of the homogenized matrix phase. The combined microstructure features of intragrain and grain boundary effects resulted in an increased thermoelectric figure-of-merit zT of Mg3Sb0.6Bi1.4. These findings imply that the optimization of thermal and electrical properties can be realized through microstructure tuning.