arXiv subjects

Jun Lu

Publications and source records attributed to Jun Lu.

At least 109 records · Page 6Linked to original sources

arXiv2019

Influence of Si doping and O2-flow on arc deposited (Al,Cr)2O3 coatings

(Al,Cr)2O3 coatings with Al/(Al+Cr) = 0.5 or Al = 70 at.%, doped with 0, 5 or 10 at.% Si, were deposited on hard metal and Si(100) substrates to elucidate the influence of Si on the resulting coatings. The chemical analysis of the coatings showed between 3.3 and 7.4 at.% metal fraction Si incorporated into all studied coatings depending on cathode Si-composition. The incorporated Si content does not change significantly with different oxygen flow covering a wide range of deposition conditions from low to high O2 flow during growth. The addition of Si promotes the metastable B1-like cubic structure over the thermodynamically stable corundum structure. The hardness determined by nanoindentation of the as-deposited coatings is slightly reduced upon Si-incorporation as well as upon increased Al-content. Si is found enriched in droplets but can also be found at a lower content, evenly spread, without visible segregation on the ~5 nm scale, in the actual oxide coating. The positive effect of improved cathode erosion upon Si-incorporation has to be balanced against the promotion of the metastable B1-like structure, having lower room temperature hardness and inferior thermal stability compared to the corundum structure.

cond-mat.mtrl-sci↗

arXiv2019

A general Lewis acidic etching route for preparing MXenes with enhanced electrochemical performance in non-aqueous electrolyte

Two-dimensional carbides and nitrides of transition metals, known as MXenes, are a fast-growing family of 2D materials that draw attention as energy storage materials. So far, MXenes are mainly prepared from Al-containing MAX phases (where A = Al) by Al dissolution in F-containing solution, but most other MAX phases have not been explored. Here, a redox-controlled A-site-etching of MAX phases in Lewis acidic melts is proposed and validated by the synthesis of various MXenes from unconventional MAX phase precursors with A elements Si, Zn, and Ga. A negative electrode of Ti3C2 MXene material obtained through this molten salt synthesis method delivers a Li+ storage capacity up to 738 C g-1 (205 mAh g-1) with high-rate performance and pseudocapacitive-like electrochemical signature in 1M LiPF6 carbonate-based electrolyte. MXene prepared from this molten salt synthesis route offer opportunities as high-rate negative electrode material for electrochemical energy storage applications.

physics.app-ph↗

arXiv2019

Multielemental single-atom-thick A layers in nanolaminated V2(Sn, A)C (A=Fe, Co, Ni, Mn) for tailoring magnetic properties

Tailoring of individual single-atom-thick layers in nanolaminated materials offers atomic-level control over material properties. Nonetheless, multielement alloying in individual atomic layers in nanolaminates is largely unexplored. Here, we report a series of inherently nanolaminated V2(A'xSn1-x)C (A'=Fe, Co, Ni and Mn, and combinations thereof, with x=1/3) synthesized by an alloy-guided reaction. The simultaneous occupancy of the four magnetic elements and Sn, the individual single-atom-thick A layers in the compound constitute high-entropy-alloy analogues, two-dimensional in the sense that the alloying exclusively occurs in the A layers. V2(A'xSn1-x)C exhibit distinct ferromagnetic behavior that can be compositionally tailored from the multielement A-layer alloying. This two-dimensional alloying provides a structural-design route with expanded chemical space for discovering materials and exploit properties.

cond-mat.mtrl-sci↗

arXiv2019

Synthesis of MAX Phases Nb2CuC and Ti2(Al0.1Cu0.9)N by A-site Replacement Reaction in Molten Salts

New MAX phases Ti2(AlxCu1-x)N and Nb2CuC were synthesized by A-site replacement by reacting Ti2AlN and Nb2AlC, respectively, with CuCl2 or CuI molten salt. X-ray diffraction, scanning electron microscopy, and atomically-resolved scanning transmission electron microscopy showed complete A-site replacement in Nb2AlC, which lead to formation of Nb2CuC. However, the replacement of Al in Ti2AlN phase was only close to complete at Ti2(Al0.1Cu0.9)N. Density-functional theory calculations corroborated the structural stability of Nb2CuC and Ti2CuN phases. Moreover, the calculated cleavage energy in these Cu-containing MAX phases are weaker than in their Al-containing counterparts, indicating that they are precursor candidates for MXene derivation.

cond-mat.mtrl-sci↗

arXiv2019

Element Replacement Approach by Reaction with Lewis Acidic Molten Salts to Synthesize Nanolaminated MAX Phases and MXenes

Nanolaminated materials are important because of their exceptional properties and wide range of applications. Here, we demonstrate a general approach to synthesize a series of Zn-based MAX phases and Cl-terminated MXenes originating from the replacement reaction between the MAX phase and the late transition metal halides. The approach is a top-down route that enables the late transitional element atom (Zn in the present case) to occupy the A site in the pre-existing MAX phase structure. Using this replacement reaction between Zn element from molten ZnCl2 and Al element in MAX phase precursors (Ti3AlC2, Ti2AlC, Ti2AlN, and V2AlC), novel MAX phases Ti3ZnC2, Ti2ZnC, Ti2ZnN, and V2ZnC were synthesized. When employing excess ZnCl2, Cl terminated MXenes (such as Ti3C2Cl2 and Ti2CCl2) were derived by a subsequent exfoliation of Ti3ZnC2 and Ti2ZnC due to the strong Lewis acidity of molten ZnCl2. These results indicate that A-site element replacement in traditional MAX phases by late transition metal halides opens the door to explore MAX phases that are not thermodynamically stable at high temperature and would be difficult to synthesize through the commonly employed powder metallurgy approach. In addition, this is the first time that exclusively Cl-terminated MXenes were obtained, and the etching effect of Lewis acid in molten salts provides a green and viable route to prepare MXenes through an HF-free chemical approach.

cond-mat.mtrl-sci↗

arXiv2019

Compositional dependence of epitaxial Tin+1SiCn MAX-phase thin films grown from a Ti3SiC2 compound target

We investigate sputtering of a Ti3SiC2 compound target at temperatures ranging from RT (no applied external heating) to 970 oC as well as the influence of the sputtering power at 850 oC for the deposition of Ti3SiC2 films on Al2O3(0001) substrates. Elemental composition obtained from time-of-flight energy elastic recoil detection analysis shows an excess of carbon in all films, which is explained by differences in angular distribution between C, Si and Ti, where C scatters the least during sputtering. The oxygen content is 2.6 at.% in the film deposited at RT and decreases with increasing deposition temperature, showing that higher temperatures favor high purity films. Chemical bonding analysis by X-ray photoelectron spectroscopy shows C-Ti and Si-C bonding in the Ti3SiC2 films and Si-Si bonding in the Ti3SiC2 compound target. X-ray diffraction reveals that the phases Ti3SiC2, Ti4SiC3, and Ti7Si2C5 can be deposited from a Ti3SiC2 compound target at substrate temperatures above 850 oC and with growth of TiC and the Nowotny phase Ti5Si3Cx at lower temperatures. High-resolution scanning transmission electron microscopy shows epitaxial growth of Ti3SiC2, Ti4SiC3, and Ti7Si2C5 on TiC at 970 oC. Four-point probe resistivity measurements give values in the range 120 to 450 micro-Ohm-cm and with the lowest values obtained for films containing Ti3SiC2, Ti4SiC3, and Ti7Si2C5.

cond-mat.mtrl-sci↗

arXiv2018

Effect of ion-implantation-induced defects and Mg dopants on thermoelectric properties of ScN

For applications in energy harvesting, environmentally friendly cooling, and as power sources in remote or portable applications, it is desired to enhance the efficiency of thermoelectric materials. One strategy consists of reducing the thermal conductivity while increasing or retaining the thermoelectric power factor. An approach to achieve this is doping to enhance the Seebeck coefficient and electrical conductivity, while simultaneously introducing defects in the materials to increase phonon scattering. Here, we use Mg ion implantation to induce defects in epitaxial ScN (111) films. The films were implanted with Mg+ ions with different concentration profiles along the thickness of the film, incorporating 0.35 to 2.2 at.% of Mg in ScN. Implantation at high temperature (600 C), with few defects due to the temperature, does not substantially affect the thermal conductivity compared to a reference ScN. Samples implanted at room temperature, in contrast, exhibited a reduction of the thermal conductivity by a factor of three. The sample doped with 2.2 at.% Mg also showed an increased power factor after implantation. This study thus shows the effect of ion-induced defects on thermal conductivity of ScN films. High-temperature implantation allows the defects to be annealed out during implantation, while the defects are retained for room-temperature implanted samples, allowing for a drastic reduction in thermal conductivity.

physics.app-ph↗

arXiv2018

CompNet: Neural networks growing via the compact network morphism

It is often the case that the performance of a neural network can be improved by adding layers. In real-world practices, we always train dozens of neural network architectures in parallel which is a wasteful process. We explored $CompNet$, in which case we morph a well-trained neural network to a deeper one where network function can be preserved and the added layer is compact. The work of the paper makes two contributions: a). The modified network can converge fast and keep the same functionality so that we do not need to train from scratch again; b). The layer size of the added layer in the neural network is controlled by removing the redundant parameters with sparse optimization. This differs from previous network morphism approaches which tend to add more neurons or channels beyond the actual requirements and result in redundance of the model. The method is illustrated using several neural network structures on different data sets including MNIST and CIFAR10.

cs.NE↗

arXiv2018

Reducing over-clustering via the powered Chinese restaurant process

Dirichlet process mixture (DPM) models tend to produce many small clusters regardless of whether they are needed to accurately characterize the data - this is particularly true for large data sets. However, interpretability, parsimony, data storage and communication costs all are hampered by having overly many clusters. We propose a powered Chinese restaurant process to limit this kind of problem and penalize over clustering. The method is illustrated using some simulation examples and data with large and small sample size including MNIST and the Old Faithful Geyser data.

cs.LG↗

arXiv2018

Machine learning modeling for time series problem: Predicting flight ticket prices

Machine learning has been used in all kinds of fields. In this article, we introduce how machine learning can be applied into time series problem. Especially, we use the airline ticket prediction problem as our specific problem. Airline companies use many different variables to determine the flight ticket prices: indicator whether the travel is during the holidays, the number of free seats in the plane etc. Some of the variables are observed, but some of them are hidden. Based on the data over a 103 day period, we trained our models, getting the best model - which is AdaBoost-Decision Tree Classification. This algorithm has best performance over the observed 8 routes which has 61.35$\%$ better performance than the random purchase strategy, and relatively small variance over these routes. And we also considered the situation that we cannot get too much historical datas for some routes (for example the route is new and does not have historical data) or we do not want to train historical data to predict to buy or wait quickly, in which problem, we used HMM Sequence Classification based AdaBoost-Decision Tree Classification to perform our prediction on 12 new routes. Finally, we got 31.71$\%$ better performance than the random purchase strategy.

cs.LG↗

arXiv2017

An Equivalence of Fully Connected Layer and Convolutional Layer

This article demonstrates that convolutional operation can be converted to matrix multiplication, which has the same calculation way with fully connected layer. The article is helpful for the beginners of the neural network to understand how fully connected layer and the convolutional layer work in the backend. To be concise and to make the article more readable, we only consider the linear case. It can be extended to the non-linear case easily through plugging in a non-linear encapsulation to the values like this $σ(x)$ denoted as $x^{\prime}$.

cs.LG↗

arXiv2017

Group-Average and Convex Clustering for Partially Heterogeneous Linear Regression

In this paper, a subgroup least squares and a convex clustering are introduced for inferring a partially heterogenous linear regression that has potential application in the areas of precision marketing and precision medicine. The homogenous parameter and the subgroup-average of the heterogenous parameters can be consistently estimated by the subgroup least squares, without need of the sparsity assumption on the heterogenous parameters. The heterogenous parameters can be consistently clustered via the convex clustering. Unlike the existing methods for regression clustering, our clustering procedure is a standard mean clustering, although the model under study is a type of regression, and the corresponding algorithm only involves low dimensional parameters. Thus, it is simple and stable even if the sample size is large. The advantage of the method is further illustrated via simulation studies and the analysis of car sales data.

stat.ME↗

arXiv2017

Hyperprior on symmetric Dirichlet distribution

In this article we introduce how to put vague hyperprior on Dirichlet distribution, and we update the parameter of it by adaptive rejection sampling (ARS). Finally we analyze this hyperprior in an over-fitted mixture model by some synthetic experiments.

cs.LG↗

arXiv2017

Equilibrium selection via Optimal transport

We propose a new dynamics for equilibrium selection of finite player discrete strategy games. The dynamics is motivated by optimal transportation, and models individual players' myopicity, greedy and uncertainty when making decisions. The stationary measure of the dynamics provides each pure Nash equilibrium a probability by which it is ranked. For potential games, its dynamical properties are characterized by entropy and Fisher information.

math.OC↗

arXiv2017

Population games and Discrete optimal transport

We propose a new evolutionary dynamics for population games with a discrete strategy set, inspired by the theory of optimal transport and Mean field games. The dynamics can be described as a Fokker-Planck equation on a discrete strategy set. The derived dynamics is the gradient flow of a free energy and the transition density equation of a Markov process. Such process provides models for the behavior of the individual players in population, which is myopic, greedy and irrational. The stability of the dynamics is governed by optimal transport metric, entropy and Fisher information.

math.OC↗

arXiv2017

Contact resistance between two REBCO tapes under load and load-cycles

No-insulation (NI) REBCO magnets have many advantages. They are self-protecting, therefore do not need quench detection and protection which can be very challenging in a high Tc superconducting magnet. Moreover, by removing insulation and allowing thinner copper stabilizer, NI REBCO magnets have significantly higher engineering current density and higher mechanical strength. On the other hand, NI REBCO magnets have drawbacks of long magnet charging time and high field-ramp-loss. In principle, these drawbacks can be mitigated by managing the turn-to-turn contact resistivity (Rc). Evidently the first step toward managing Rc is to establish a reliable method of accurate Rc measurement. In this paper, we present experimental Rc measurements of REBCO tapes as a function of mechanical load up to 144 MPa and load cycles up to 14 times. We found that Rc is in the range of 26-100 uOhm-cm2; it decreases with increasing pressure, and gradually increases with number of load cycles. The results are discussed in the framework of Holm's electric contact theory.

physics.ins-det↗

arXiv2015

Towards the Complete Relational Graph of Fundamental Circuit Elements

A complete and harmonized fundamental circuit relational graph with four linear and four memory elements is constructed based on newly defined elements, which provides a guide to developing novel circuit functionalities in the future. In addition to resistor, capacitor and inductor which are defined in terms of a linear relationship between the charge q, the current i, the voltage v, and the magnetic flux, Chua proposed in 1971 the fourth linear circuit element to directly relate magnetic flux and charge. A non-linear resistive device defined in memory i-v relation and dubbed memristor, was later attributed to such an element and has been realized in various material structures. Here we clarify that the memristor is not the true fourth fundamental circuit element but the memory extension to the concept of resistor, in analogy to the extension of memcapacitor to capacitor and meminductor to inductor. Instead, a two-terminal device employing the linear magnetoelectric effects, termed transtor, possesses the function of relating directly flux and charge and should take the position of the fourth linear element. Moreover, its memory extension, termed memtranstor, is proposed and analyzed here.

cond-mat.mes-hall↗

arXiv2015

Structure and bonding in amorphous iron carbide thin films

We investigate the amorphous structure, chemical bonding, and electrical properties of magnetron sputtered Fe1-xCx (0.21<x<0.72) thin films. X-ray, electron diffraction and transmission electron microscopy show that the Fe1-xCx films are amorphous nanocomposites, consisting of a two-phase domain structure with Fe-rich carbidic FeCy, and a carbon-rich matrix. Pair distribution function analysis indicates a close-range order similar to those of crystalline Fe3C carbides in all films with additional graphene-like structures at high carbon content (71.8 at% C). From X-ray photoelectron spectroscopy measurements, we find that the amorphous carbidic phase has a composition of 15-25 at% carbon that slightly increases with total carbon content. X-ray absorption spectra exhibit increasing number of unoccupied 3d states and decreasing number of C 2p states as a function of carbon content. These changes signify a systematic redistribution in orbital occupation due to charge-transfer effects at the domain-size dependent carbide/matrix interfaces. Four-point probe resistivity of the Fe1-xCx films increases exponentially with carbon content from 200 mu-Ohm-cm (x=0.21) to 1200 mu-Ohm-cm (x=0.72), and is found to depend on the total carbon content rather than the composition of the carbide. Our findings open new possibilities for modifying the resistivity of amorphous thin film coatings based on transition metal carbides by control of amorphous domain structures.

cond-mat.mtrl-sci↗