1,843 research outputs found

    A computational investigation of the phase and microstructural stability in transition metal carbides and nitrides

    Get PDF
    The group IVB and VB transition metal carbides and nitrides represent one of the major classes of ultrahigh temperature ceramics (UHTCs). Here, we investigate the stability of these compound at low temperature for a wide range of stoichiometries using electronic structure density functional theory (DFT). This, combined with intelligent search algorithms, have been able to suggest potential stable phases in these materials. The results of which have highlight a competition between vacancy ordering in the carbon/nitrogen depleted rocksalt matrix with other stacking fault derived structures, such as the nanolamellar zeta phase (M4C3 or M4N3). Using this DFT phase stability information, a model has been constructed that provides direct insight into how phase stability controls microstructure. Through this model, we have found that a barrier-free state can exist for the nucleation of the stacking fault phases, which now can describe the propensity of faulting in specific types of carbides and nitrides. Even more intriguing is the consequence in microstructure formation between the meta-stable and stable versions of the zeta phase in the carbides and nitrides respectively. Through these computational tools and models, we are able to elucidate the underlying physics that gives rise to phase and microstructure stability for this particular class of UHTCs. Please click Additional Files below to see the full abstract

    The zeta phase in the transition metal carbides and nitrides: Structure, microstructure and properties

    Get PDF
    Please click Additional Files below to see the full abstract

    Age as a risk factor for acute mountain sickness upon rapid ascent to 3,700 m among young adult Chinese men.

    Get PDF
    BackgroundThe aim of this study was to explore the relationship between age and acute mountain sickness (AMS) when subjects are exposed suddenly to high altitude.MethodsA total of 856 young adult men were recruited. Before and after acute altitude exposure, the Athens Insomnia Scale score (AISS) was used to evaluate the subjective sleep quality of subjects. AMS was assessed using the Lake Louise scoring system. Heart rate (HR) and arterial oxygen saturation (SaO2) were measured.ResultsResults showed that, at 500 m, AISS and insomnia prevalence were higher in older individuals. After acute exposure to altitude, the HR, AISS, and insomnia prevalence increased sharply, and the increase in older individuals was more marked. The opposite trend was observed for SaO2. At 3,700 m, the prevalence of AMS increased with age, as did severe AMS, and AMS symptoms (except gastrointestinal symptoms). Multivariate logistic regression analysis showed that age was a risk factor for AMS (adjusted odds ratio [OR] 1.07, 95% confidence interval [CI] 1.01-1.13, P<0.05), as well as AISS (adjusted OR 1.39, 95% CI 1.28-1.51, P<0.001).ConclusionThe present study is the first to demonstrate that older age is an independent risk factor for AMS upon rapid ascent to high altitude among young adult Chinese men, and pre-existing poor subjective sleep quality may be a contributor to increased AMS prevalence in older subjects

    One-Bit Covariance Reconstruction with Non-zero Thresholds: Algorithm and Performance Analysis

    Full text link
    Covariance matrix reconstruction is a topic of great significance in the field of one-bit signal processing and has numerous practical applications. Despite its importance, the conventional arcsine law with zero threshold is incapable of recovering the diagonal elements of the covariance matrix. To address this limitation, recent studies have proposed the use of non-zero clipping thresholds. However, the relationship between the estimation error and the sampling threshold is not yet known. In this paper, we undertake an analysis of the mean squared error by computing the Fisher information matrix for a given threshold. Our results reveal that the optimal threshold can vary considerably, depending on the variances and correlation coefficients. As a result, it is inappropriate to use a constant threshold to encompass parameters that vary widely. To mitigate this issue, we present a recovery scheme that incorporates time-varying thresholds. Our approach differs from existing methods in that it utilizes the exact values of the threshold, rather than its statistical properties, to enhance the estimation performance. Our simulations, including the direction-of-arrival estimation problem, demonstrate the efficacy of the developed scheme, especially in complex scenarios where the covariance elements are widely separated

    One-bit target detection in collocated MIMO Radar and performance degradation analysis

    Get PDF
    Target detection is an important problem in multipleinput multiple-output (MIMO) radar. Many existing target detection algorithms were proposed without taking into consideration the quantization error caused by analog-to-digital converters (ADCs). This paper addresses the problem of target detection for MIMO radar with one-bit ADCs and derives a Rao's test-based detector. The proposed method has several appealing features: 1) it is a closed-form detector; 2) it allows us to handle sign measurements straightforwardly; 3) there are closed-form approximations of the detector's distributions, which allow us to theoretically evaluate its performance. Moreover, the closed-form distributions allow us to study the performance degradation due to the one-bit ADCs, yielding an approximate 2 dB loss in the low-signal-to-noise-ratio (SNR) regime compared to 34-bit ADCs. Simulation results are included to showcase the advantage of the proposed detector and validate the accuracy of the theoretical results.The work of David Ramírez was supported in part by the Ministerio de Ciencia e Innovación, jointly with the European Commission (ERDF) under Grant PID2021-123182OB-I00 (EPiCENTER) and in part by the Comunidad de Madrid under Grant Y2018/TCS-4705 (PRACTICO-CM). The work of Lei Huang was supported in part by the National Science Fund for Distinguished Young Scholars under Grant 61925108 and in part by the Joint fund of the National Natural Science Foundation of China and Robot Fundamental Research Center of Shenzhen Government under Grant U1913203

    Covariance matrix recovery from one-bit data with non-zero quantization thresholds: Algorithm and performance analysis

    Get PDF
    Covariance matrix recovery is a topic of great significance in the field of one-bit signal processing and has numerous practical applications. Despite its importance, the conventional arcsine law with zero threshold is incapable of recovering the diagonal elements of the covariance matrix. To address this limitation, recent studies have proposed the use of non-zero clipping thresholds. However, the relationship between the estimation error and the sampling threshold is not yet known. In this article, we undertake an analysis of the mean squared error by computing the Fisher information matrix for a given threshold. Our results reveal that the optimal threshold can vary considerably, depending on the variances and correlation coefficients. As a result, it is inappropriate to adopt a constant threshold to encompass parameters that vary widely. To mitigate this issue, we present a recovery scheme that incorporates time-varying thresholds. Our approach differs from existing methods in that it utilizes the exact values of the threshold, rather than its statistical properties, to increase the estimation accuracy. Simulation results, including those of the direction-of-arrival estimation problem, demonstrate the efficacy of the developed scheme, especially in complex scenarios where the covariance elements are widely separated.The work of Yu-Hang Xiao was supported in part by the National Natural Science Foundation of China under Grant 62201359. The work of Lei Huang was supported in part by the National Science Fund for Distinguished Young Scholars under Grant 61925108, and in part by the National Natural Science Foundation of China under Grant U1913221. The work of David Ramírez was supported in part by MCIN/AEI/10.13039/501100011033/FEDER, UE, under Grant PID2021-123182OB-I00 (EPiCENTER), and in part by the Office of Naval Research (ONR) Global under Contract N62909-23-1-2002.Publicad
    corecore