Search

In this paper, we examine the cleavage fracture anisotropy in tantalum carbides, namely, TaC, α‐Ta2C, and ζ‐Ta4C3 − x, using density functional theory (DFT) calculations. Our investigation identifies the presence of multiple low‐energy cleavage planes indicating multiple potential pathways for crack propagation in these ceramics, even the low symmetry compounds. The anisotropy characteristics of cleavage fractures exhibited by α‐Ta2C and ζ‐Ta4C3 − x closely align with the intrinsic fracture anisotropy observed in TaC. Notably, there exist at least three pyramidal planes in ζ‐Ta4C3 − x whose cleavage energies are less than those of the carbon‐depleted basal planes, previously reported to have the lowest cleavage energy. The observed preference in experiments for cleavage along carbon‐depleted basal planes, exclusive of other identified low‐energy planes, points to factors beyond cleavage energy influencing cleavage plane preference. [ABSTRACT FROM AUTHOR]

A novel aerosol activation scheme was developed and implemented into the WRF Thompson–Eidhammer Aerosol‐Aware microphysical module. While in most of the numerical weather prediction (NWP) models the activation is based on updraft velocity, even in the fog, the new parameterization considers the local rate of cooling and water vapor flux in the evaluation of the aerosol activation rate. In addition, diagnostic variables are added to evaluate the visibility reduction due to formation of haze droplets. The impact of changing the activation scheme is demonstrated by a case study of a well‐observed fog event. Data observed during the fog event campaign at Budapest and observed at standard meteorological stations are compared with the output of the numerical model. The results are summarized as follows: (i) the inhomogeneous spatial and temporal distribution of the number concentration of droplets is an inherent characteristic of the new parameterization scheme. (ii) Compared to the prior parameterization based on updraft velocity, the new parameterization scheme increases the number concentration of the droplets significantly, especially at the top of the fog. As a consequence, it reduces the downward short‐wave radiation flux prolonging the lifetime of the fog by about 30–60 min. (iii) Analyses reveal that earlier dissipation of the fog comparing to the observed data cannot be explained only by overestimation of the downward short‐wave radiation flux. (iv) The new method is able to evaluate the reduction of visibility due to haze. [ABSTRACT FROM AUTHOR]

The hafnium‐rich portion of the of the hafnium‐nitrogen phase diagram is dominated by a substoichiometric rocksalt HfN1‐x, the ζ‐Hf4N3−x, the η‐Hf3N2−x, and the elemental Hf phase. The zeta and eta nitride phases have a close packed metal atom stacking sequence but their nitrogen atom ordering has yet to be concretely identified. With respect to the composition of these phases, recent computational studies of their phase stability using density functional theory (DFT) are not in agreement with reported experimental observations. In this work, we re‐examine the phase stability of the zeta and eta phases using DFT combined with enumerated searches using the known metal atom stacking sequences of these phases but with variable carbon concentration and ordering. We have found new structures for the zeta and eta phases that are now in better agreement with experimental findings. Furthermore, we report a new eta phase, η‐Hf12N7, which lies on the convex hull and has a nitrogen atom ordering that is substantially different from the zeta phase. This work also demonstrates the importance of configurational entropy in dictating the finite temperature phase diagrams in this system. [ABSTRACT FROM AUTHOR]

Solute segregation was examined in zirconium diboride and zirconium‐tantalum diboride solid solution ceramics that were produced by reactive hot pressing. Microstructural analysis demonstrated that the ZrB2 and (Zr,Ta)B2 ceramics reached nearly full relative density and were nominally phase‐pure. X‐ray diffraction was consistent with full incorporation of Ta into solid solution within the ZrB2 structure, and energy‐dispersive spectroscopy demonstrated that tantalum was well distributed throughout the bulk of the Ta‐doped specimens. The weak characteristic X‐rays for B led to inaccurate results for total atom concentrations in boride ceramics by energy‐dispersive spectroscopy. Atom probe tomography was used to analyze the amount and spatial distribution of Ta species. No obvious Ta segregation was observed in grains or grain boundaries. However, nitrogen strongly segregated to a grain boundary. This study demonstrated that atom probe tomography is an accurate method for characterizing the amount and spatial distribution of metallic and nonmetallic species in ZrB2 ceramics. [ABSTRACT FROM AUTHOR]

The crystal structure and composition of the zeta phase in the group VB transition metal carbides are not completely understood despite decades of experimental studies. As such, the phase rarely appears on phase diagrams of the group VB transition metal carbides. There is currently renewed interest in this phase, as tantalum carbide composites exhibit high fracture toughness in the presence of this phase. This work extends the initial computational study using density functional theory of the phase stability of the zeta phase in the tantalum carbide system, where the tantalum carbide zeta‐phase crystal structure and stability were determined, to the niobium and vanadium carbides. It is shown that the zeta phases in the three systems share the same crystal structure and it is an equilibrium phase at low temperatures. The carbon atom ordering in the three different phases is explored and it is demonstrated that the zeta phase in the tantalum carbides prefers to order carbon atoms differently than in the niobium and vanadium carbide zeta phases. Finally, the properties of this crystal are computed, including elastic constants, electronic densities of states and phonon dispersion curves, to illustrate that this crystal structure is similar to other transition metal carbides. [ABSTRACT FROM AUTHOR]

A systematic electronic structure density functional theory search for the zeta phase in the tantalum carbon system has been conducted, demonstrating that the zeta phase does indeed lie on the convex hull, establishing low‐temperature thermodynamic stability, and is structurally stable. Even though the zeta phase is most oft reported as a R3¯m ζ‐Ta4C3‐x structure, these new computational results reveal that the perfectly ordered zeta phase occurs at the Ta3C2 composition and belongs to the C2/m space group. Furthermore, these results suggest that the R3¯m space group may be a result of an order‐disorder transition and potential deviations from ideal stoichiometry at higher temperatures. The structure of this Ta3C2 compound can be described as ordered strings of carbon vacancies on a single carbon plane, which is also the preferred plane where additional carbon vacancies or interstitials form. These defects have modest formation energies, which provides further explanation for the small homogeneity range in zeta phase. [ABSTRACT FROM AUTHOR]

In this second part of a cloud microphysics scheme intercomparison study, we focus on biases and variabilities of stratiform precipitation properties for a midlatitude squall line event simulated with a cloud‐resolving model implemented with eight cloud microphysics schemes. Most of the microphysics schemes underestimate total stratiform precipitation, mainly due to underestimation of stratiform precipitation area. All schemes underestimate the frequency of moderate stratiform rain rates (2–6 mm/hr), which may result from low‐biased ice number and mass concentrations for 0.2–2‐mm diameter particles in the stratiform ice region. Most simulations overestimate ice water content (IWC) at altitudes above 7 km for temperatures colder than −20 °C but produce a decrease of IWC approaching the melting level, which is opposite to the trend shown by in situ observations. This leads to general underestimations of stratiform IWC below 5‐km altitude and rainwater content above 1‐km altitude for a given rain rate. Stratiform precipitation area positively correlates with the convective condensate detrainment flux but is modulated by hydrometeor type, size, and fall speed. Stratiform precipitation area also changes by up to 17%–25% through alterations of the lateral boundary condition forcing frequency. Stratiform precipitation, rain rate, and area across the simulations vary by a factor of 1.5. This large variability is primarily a result of variability in the stratiform downward ice mass flux, which is highly correlated with convective condensate horizontal detrainment strength. The variability of simulated local microphysical processes in the stratiform region plays a secondary role in explaining variability in simulated stratiform rainfall properties. Plain Language Summary: This is a unique model intercomparison study about different microphysics parametrizations commonly used, with the purposes of examining model biases and variability as well as identifying major factors/processes leading to bias and variability. The study simulated a well‐observed squall line MCS from MC3E field campaign, and focused on the stratiform precipitation, following on our part 1 study focusing on convective part. We employed a more constrained approach compared with past intercomparison studies to better identify processes contributing to the differences. Another unique part is our comprehensive model evaluation, that is, we identify stratiform columns and evaluate vertical evolution of cloud properties including size distribution. We find that most of the microphysics schemes underestimate total stratiform precipitation, mainly due to underestimation of stratiform precipitation area. Moderate stratiform rain rates are underestimated, mainly due to incorrect vertical evolution of ice particles. Stratiform precipitation properties across the simulations vary by a factor of 1.5, primarily a result of variability in detrained condensate amount. In addition, we find that stratiform precipitation area correlates well with detrainment amount and is modulated by the detrained hydrometeor properties. So convective microphysics plays a key role in determining stratiform properties. Key Points: Underestimation of total stratiform precipitation in most simulations is mainly due to underestimation of stratiform precipitation areaLow‐biased rainwater content is caused by low‐biased stratiform ice water content below 5‐km altitude for a given rain rateSimulated stratiform precipitation variability is correlated with variability in the convective condensate being detrained [ABSTRACT FROM AUTHOR]

Abstract: This review summarizes the phase stability in the group IVB (Ti‐C; Zr‐C; Hf‐C) and group VB (V‐C; Nb‐C; Ta‐C) transition‐metal carbides. The order parameter functional (OPF) method and density functional theory (DFT) method have been used to predict phase equilibria in these systems. Extensive experimental investigations have attempted to determine both phase stability as a function of composition as well the crystal structures present using X‐ray diffraction, neutron diffraction, electron backscatter detection, and selected area electron diffraction. These investigations have demonstrated that the structures that form are based on the close‐packing of the metal atoms and the arrangement of the carbon atoms in the octahedral interstices. In general, the rocksalt B1 phase is stable for all of the transition‐metal carbides, with their substoichiometry tolerance increasing with temperature; vanadium carbide is the exception due to its negative vacancy formation energy. Vacancy‐ordered M6C5 phases have been predicted and experimentally confirmed in both groups of carbides; however, kinetic limitations often inhibit the formation of vacancy‐ordered phases, which has contributed to controversy in phase identification. The vacancy‐ordered M4C3 phase has been predicted for select carbides and has only been observed in zirconium carbide. In contrast, the stacking fault phase ζ‐M4C3−x has been readily reported in the group VB carbides (but not in the group IVB carbides). The vacancy‐ordered M3C2 has been predicted by DFT for the group IVB carbides but not in the group VB carbides, whereas OPF predicts its stability in both carbides. Vacancy‐ordered M3C2 phases have been experimentally observed in the Ti‐C and Hf‐C systems. Finally, the M2C phase has been predicted in both group carbides, except for hafnium carbide, with an order‐disorder transition with temperature. These factors result in phase diagrams that are similar among all the carbides, but each phase diagram is unique due to subtle differences in bonding that result in slight differences in thermodynamically stable phases. [ABSTRACT FROM AUTHOR]

The ACTFL Oral Proficiency Interview (OPI) and accompanying proficiency guidelines have become a mainstay in academic foreign language assessment. In 2006, an asynchronous electronic interface of the OPI was made available in which candidates are asked to record themselves responding to questions delivered aurally via computer, labeled the Oral Proficiency Interview-computer (OPIc). Thompson, Cox, and Knapp () found that both instruments produced similar results but that many students in their sample who scored at the Advanced Low level on the traditional OPI scored one sublevel higher on the OPIc. The primary objective of the present study is to analyze lexical features, i.e., lexical diversity and lexical density, and discursive features, i.e., temporal and repair fluency, of students' speech from a subset of OPI and OPIc exams to examine the relationship between the language elicited from each exam type, i.e., face-to-face and via computer. The research consists of two studies that focus on candidates who received (1) an Advanced Mid rating on both assessments, and (2) those who received Advanced Mid on the OPIc and Advanced Low on the OPI. While the two exams did not seem to elicit disparate levels of lexical density and diversity, significant differences surfaced in measures of fluency. [ABSTRACT FROM AUTHOR]

An intercomparison study of a midlatitude mesoscale squall line is performed using the Weather Research and Forecasting (WRF) model at 1 km horizontal grid spacing with eight different cloud microphysics schemes to investigate processes that contribute to the large variability in simulated cloud and precipitation properties. All simulations tend to produce a wider area of high radar reflectivity ( Z e > 45 dBZ) than observed but a much narrower stratiform area. The magnitude of the virtual potential temperature drop associated with the gust front passage is similar in simulations and observations, while the pressure rise and peak wind speed are smaller than observed, possibly suggesting that simulated cold pools are shallower than observed. Most of the microphysics schemes overestimate vertical velocity and Z e in convective updrafts as compared with observational retrievals. Simulated precipitation rates and updraft velocities have significant variability across the eight schemes, even in this strongly dynamically driven system. Differences in simulated updraft velocity correlate well with differences in simulated buoyancy and low-level vertical perturbation pressure gradient, which appears related to cold pool intensity that is controlled by the evaporation rate. Simulations with stronger updrafts have a more optimal convective state, with stronger cold pools, ambient low-level vertical wind shear, and rear-inflow jets. Updraft velocity variability between schemes is mainly controlled by differences in simulated ice-related processes, which impact the overall latent heating rate, whereas surface rainfall variability increases in no-ice simulations mainly because of scheme differences in collision-coalescence parameterizations. [ABSTRACT FROM AUTHOR]

Polarimetric upgrades to the U.S. radar network have allowed new insight into the precipitation processes of tropical cyclones. Previous work by the authors compared the reflectivity at horizontal polarization and differential reflectivity observations from two hurricanes to simulated radar observations from the WRF model, and found that the aerosol-aware Thompson-Eidhammer microphysical scheme performed the best of several commonly used bulk microphysical parameterizations. Here we expand our investigation of the Thompson-Eidhammer scheme, and find that though it provided the most accurate forecast in terms of wind speed and simulated radar signatures, the scheme produces areas in which the differential reflectivity was much higher than observed. We conclude that the Thompson-Eidhammer scheme produces drop size distributions that have a larger median drop size than observed in regions of light stratiform precipitation. Examination of the vertical structure of simulated differential reflectivity indicates that the source of the discrepancy between the model and radar observations likely originates within the melting layer. The treatment of number production of rain drops from melting snow in the microphysical scheme is shown to be the ultimate source of the enhancement of differential reflectivity. A modification to the scheme is shown to result in better fidelity of the radar variables with the observations without degrading the short-term intensity forecast. Additional tests with an idealized squall line simulation are consistent with the hurricane results, suggesting the modification is generally applicable. The modifications to the Thompson- Eidhammer scheme shown here have been incorporated into updates of the WRF model starting with version 3.8.1. [ABSTRACT FROM AUTHOR]

Using a data set exclusively prepared for and licensed to them by the College Board, the authors examined the growth of the Advanced Placement program in foreign languages and overall trends in regard to the number of candidates who sat for the foreign language exams, their gender, their scores, and their ethnicity over the 36-year period covering 1979 to 2014. Results indicated that while the number of candidates for all exams grew, the Spanish Language and Culture exam demonstrated exponential growth unparalleled by any other exam. While scores have varied based on the iteration of the exam, they have continued to improve overall. Females continued to make up the majority of foreign language test takers in every state and outperformed males on average. Data also revealed that white candidates were losing ground to ethnic and racial minority test takers overall, but particularly in the case of the Spanish, Chinese, and Japanese examinees. [ABSTRACT FROM AUTHOR]

While studies have been done to rate the validity and reliability of the Oral Proficiency Interview (OPI) and Oral Proficiency Interview-Computer (OPIc) independently, a limited amount of research has analyzed the interexam reliability of these tests, and studies have yet to be conducted comparing the results of Spanish language learners who take both exams. For this study, 154 Spanish language learners of various proficiency levels were divided into two groups and administered both the OPI and OPIc within a 2-week period using a counterbalanced design. In addition, study participants took both a pre- and postsurvey that gathered data about their language learning background, familiarity with the OPI and OPIc, preparation and test-taking strategies, and evaluations of each exam. The researchers found that 54.5% of the participants received the same rating on the OPI and OPIc, with 13.6% of examinees scoring higher on the OPI and 31.8% scoring higher on the OPIc. While the results found that students scored significantly better on the OPIc, the overall effect size was quite small. The authors also found that the overwhelming majority of the participants preferred the OPI to the OPIc. This research begins to fill important gaps and provides empirical data to examine the comparability of the Spanish OPI and OPIc. [ABSTRACT FROM AUTHOR]

Objective: To examine the validity of a tablet computer-based audiometer for measuring hearing thresholds in a moderately noisy environment.Study Design: A prospective single-cohort repeated measures study.Setting: Tertiary care institution providing hearing health care to a diverse population.Subjects and Methods: Subjects included 49 participants (44 adults, 5 children) with all degrees of hearing sensitivity. Potential participants were excluded if they were <4 years old or had cognitive deficits or ear drainage. Participants were tested with established audiometric tests and a tablet audiometer. Threshold values were measured for both ears using various stimuli (500, 1000, 2000, and 4000 Hz). Testing with the tablet audiometer was conducted in a non-sound-treated room; room properties were characterized by ambient noise level and reverberation testing. Calibration assessment of the tablet audiometer was performed on 3 occasions.Results: Within the test thresholds for hearing level, results from the tablet device were within 10 dB of those determined by conventional audiometry for 164 of 172 hearing levels and did not show proportional bias over the testing range. Calibration assessment showed accurate proximity between output and target values, though crosstalk and linearity failed initial assessments.Conclusion: The tablet-based automated audiometer presents a new method for threshold hearing assessment outside conventional sound booths. [ABSTRACT FROM AUTHOR]

Labeling theory has long held a rather significant place in sociology generally, and in symbolic interaction more specifically. Yet, in its long history, labeling theorists have seldom considered how interactional contexts mediate the effective application of labels. Similarly, labeling theory, with its focus on deviance, has largely neglected positive instances of labeling. In this article, I consider an instance of labeling in a tutoring session and show how the local interactional context of the application of a label is accomplished such that the label 'smarter than you think' is made to stick to the student. In doing so, I demonstrate how labeling theory can be productively extended to consider positive labeling as well as the interactional contexts that mediate these labeling processes. In closing, I propose that this approach could help develop labeling theory into a complex and nuanced theory of the social constitution of human behavior. [ABSTRACT FROM AUTHOR]

Students' first and target language are often used by both teachers and students during instruction in the foreign language classroom (Levine, 2011). In this study, the frequency of and reasons for students' and teachers' use of English or Spanish were analyzed using video recordings of 40 class sessions taught by eight randomly selected Spanish 102 teachers and by eight randomly selected Spanish 202 teachers. All of the videos were transcribed, and a word count was made to determine the overall use of Spanish and English by the students and teachers. The relationship between the number of code-switches by teachers and students and the overall use of Spanish and English in the classroom were analyzed. Finally, the code-switches between languages were categorized and counted to determine if students or teachers initiated the switch and under what circumstances, as well as the influence of code-switching on the interlocutor's subsequent language choice. The results indicate that teacher-initiated code-switches had the most influence on students' subsequent language choice and that teachers code-switched more often, even though students used a higher overall percentage of the first language. In addition, there was a strong positive relationship between the number of code-switches and the overall use of Spanish and English during instruction. [ABSTRACT FROM AUTHOR]

Legal and social sanctions on overt discrimination in educational institutions were implemented to redress the inequalities associated with years of segregation and unequal treatment of groups on the basis of social identity characteristics such as race, gender, and nationality. In this article, we draw on research and theory demonstrating that, despite sanctions on overt discrimination, biases and cues of threat persist in educational institutions and are often present in subtle and ambiguous forms. These threats burden members of historically stigmatized groups (e.g., women, racial minority group members) with stress in the form of negative achievement expectations that ultimately undermine their psychological well-being and academic achievement and success outcomes. To capture the process by which these threats impact students from historically stigmatized groups, we present a critical review and analysis of research and theory from the identity-based Rejection Sensitivity literature. This literature demonstrates the interplay between the individual's appraisals and coping mechanisms with the threatening messages present in the environment. Our analysis applies a stress and coping framework to integrate the literatures on social identity threat and identity-based Rejection Sensitivity, and to ground education policy recommendations. [ABSTRACT FROM AUTHOR]

A wide variety of microstructures have been obtained by vacuum plasma spraying ( VPS) 39Ta: 61C atomic percent feedstock powders. During processing, the powder feed was fed through a high energy VPS plasma plume, where altering nozzle angle changed the overall retained carbon concentration in the deposited material. The samples were subsequently sintered and hot isostatic pressed to homogenize and consolidate the microstructure. The microstructures consisted of grains that were either equiaxed or acicular. In the samples with less carbon loss, the equiaxed grains were either the TaC phase or a TaC matrix that encased fine laths of Ta4C3. In the sample with the most carbon loss, acicular grains were found containing layered and parallel TaC, Ta2C, and Ta4C3 laths along the major-axis of the grains. The phases of the compounds have been determined by using complimentary X-ray diffraction and electron diffraction techniques. Focused ion beam serial sectioning and transmission electron microscopy tilt series tomography were performed to generate three-dimensional reconstructions of the microstructure morphologies. This article addresses how tantalum carbide microstructures are controlled by the overall concentration and phase fraction content in each of these samples. [ABSTRACT FROM AUTHOR]

The oxide inclusion and porosity defect structures in a tantalum carbide specimen fabricated from vacuum plasma spraying with postspraying sintering and hot-isostatic pressing has been characterized. The tantalum carbide powders were obtained using a carbothermal reduction process of tantalum oxide precursors. During its fabrication, oxide-based inclusions formed from intrinsic impurities in the powder. Using serial sectioning and three-dimensional reconstruction techniques, interconnected globular oxide inclusions were revealed to be within the matrix phase and in the grain boundaries. The oxide phase was identified to be z-Ta2O5 through selected area electron diffraction. The two- and three-dimensional porosity size distribution was compared and accounted for ~2% of the volume. [ABSTRACT FROM AUTHOR]

The oxide inclusion and porosity defect structures in a tantalum carbide specimen fabricated from vacuum plasma spraying with postspraying sintering and hot‐isostatic pressing has been characterized. The tantalum carbide powders were obtained using a carbothermal reduction process of tantalum oxide precursors. During its fabrication, oxide‐based inclusions formed from intrinsic impurities in the powder. Using serial sectioning and three‐dimensional reconstruction techniques, interconnected globular oxide inclusions were revealed to be within the matrix phase and in the grain boundaries. The oxide phase was identified to be z‐Ta2O5 through selected area electron diffraction. The two‐ and three‐dimensional porosity size distribution was compared and accounted for ~2% of the volume. [ABSTRACT FROM AUTHOR]