Bulk samples of MgB 2 were prepared with 5, 10, and 15 wt % Y 2 O 3 nanoparticles, added using a simple solid-state reaction route. Transmission electron microscopy showed a fine nanostructure consisting of ϳ3-5 nm YB 4 nanoparticles embedded within MgB 2 grains of ϳ400 nm size. Compared to an undoped control sample, an improvement in the in-field critical current density J C was observed, most notably for 10% doping. At 4.2 K, the lower bound J C value was ϳ2 ϫ10 5 A cm Ϫ2 at 2 T. At 20 K, the corresponding value was ϳ8ϫ10 4 A cm Ϫ2 . Irreversibility fields were 11.5 T at 4.2 K and 5.5 T at 20 K. © 2002 American Institute of Physics. ͓DOI: 10.1063/1.1506184͔In slightly more than one year after the discovery of superconductivity in magnesium diboride, there is now a wide body of evidence indicating that MgB 2 does not contain intrinsic obstacles to current flow between grains, unlike the high-temperature superconducting cuprates. Evidence for strongly coupled grains has been found even in randomly aligned, porous, and impure samples, 1,2 suggesting that dense forms of MgB 2 will be attractive in high-current applications at 20-30 K and perhaps 4.2 K. So far, however, bulk samples have demonstrated modest values of the irreversibility field 0 H*(T) reaching about 4 T at 20 K and 8 T at 4.2 K.3 For comparison, established low-temperature superconductors, e.g., NbTi ͑10 T͒ and Nb 3 Sn ͑20 T͒, have significantly higher irreversibility fields at 4.2 K, while Bi 2 Sr 2 Ca 2 Cu 3 O 10 ͑3 T͒ is becoming established at 20 K. 4 MgB 2 tape results are somewhat more promising, with 0 H* values of above 5 at 20 K, 5-8 where partial orientation of crystallites parallel to the field is playing a role. Since the irreversibility field is the practical limit to magnet applications, it is desirable to make 0 H* values as high as possible.A central question is how to further increase the irreversibility field in addition to introducing crystallographic texture. Alloying additions, such as atomic substitution for Mg or B or added interstitial atoms, increase electron scattering and decrease the coherence length, producing higher upper critical and irreversibility fields.9,10 Adding nanometer-scale defects can produce similar effects. For example, proton irradiation studies showed that 0 H* increased significantly from ϳ3.5 to ϳ6 T at 20 K with only moderate damage, corresponding to atomic displacements of a few %, due to either vacancies or interstitials.11 Mechanical processing also produces structural defects, and similar increases in the irreversibility field have been reported. 6,8,12 These increases were steeper than the concomitant reductions in the critical temperature T c , suggesting it is viable to improve the accessible field range without sacrificing other superconducting properties too much.To explore more practical and scaleable routes to defect incorporation in bulk MgB 2 , the present study explores chemical and nanostructural changes via addition of nanoparticles. Coherently ordered Mg-B-O precipitates are known to ...

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The critical current density J c flowing in thin YBa 2 Cu 3 O 7−␦ ͑YBCO͒ films of various thicknesses d has been studied magnetometrically, both as a function of applied field H and temperature T, with a central objective to determine the dominant source of vortex pinning in these materials. The films, grown by a BaF 2 ex situ process and deposited on buffered rolling assisted biaxially textured substrates ͑"RABiTS"͒ substrates of Ni-5 % W, have thicknesses d ranging from 28 nm to 1.5 m. Isothermal magnetization loops M͑H ; T͒ and remanent magnetization M rem ͑T͒ in H = 0 were measured with H ʈ c-axis ͑i.e., normal to film plane͒. The resulting J c ͑d͒ values ͑obtained from a modified critical state model͒ increase with thickness d, peak near d ϳ 120 nm, and thereafter decrease as the films get thicker. For a wide range of temperatures and intermediate fields, we find J c ϰ H −␣ with ␣ ϳ͑0.56-0.69͒ for all materials. This feature can be attributed to pinning by large random defects, which theoretically has power-law exponent ␣ =5/8. Calculated values for the size and density of defects are comparable with those observed by TEM in the films. As a function of temperature, we find J c ͑T , sf͒ϳ͓1−͑T / T c ͒ 2 ͔ n with n ϳ 1.2-1.4. This points to "␦T c pinning" ͑pinning that suppresses T c locally͒ in these YBCO materials.

a b s t r a c tA field measurement of ground vibration was performed on the Beijing À Shanghai high-speed railway in China. In this paper, the experimental results of vertical ground vibration accelerations induced by very high speed trains running over a non-ballasted track on embankment with speeds from 300 to 410 km/h are reported and analyzed in detail for the first time. Characteristics of ground vibration accelerations in both time and frequency domains are analyzed based on the test data. It is shown that the periodic exciting action of high-speed train bogies can be identified in time histories of vertical accelerations of the ground within the range of 50 m from the track centerline. The first dominant sensitive frequency of the ground vibration acceleration results from the wheelbase of the bogie, and the center distance of two neighboring cars plays an important role in the significant frequencies of the ground vibration acceleration. Variations of time-response peak value and frequency-weighted vertical acceleration level of ground vibration in relation with train speed as well as the distance from the track centerline are also investigated. Results show that the time-domain peak value of ground vibration acceleration exhibits an approximately linear upward tendency with the increase of train speed. With the increasing distance from the track centerline, the frequency-weighted vertical acceleration level of the ground vibration attenuates more slowly than the time-domain peak value of the ground vibration acceleration does. Severe impact of high-speed railway ground vibration on human body comfort on the ground occurs at the speed of 380-400 km/h. The results given in the paper are also valuable for validating the numerical prediction of train induced ground vibrations.

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The development of analysis on train-induced ground vibration is briefl y summarized. A train-trackground integrated dynamic model is introduced in the paper to predict the ground vibration induced by high-speed trains. Representative dynamic responses of the train-track-ground system predicted by the model are presented. Some major results measured from two fi eld tests on the ground vibration induced by two high-speed trains are reported. Numerical prediction with the proposed train-track-ground model is validated by the high-speed train running experiments. Research results show that the wheel/rail dynamic interaction caused by track irregularities has a signifi cant infl uence on the ground acceleration and little infl uence on the ground displacement. The main frequencies of the ground vibration induced by high-speed trains are usually below 80 Hz. Compared with the ballasted track, the ballastless track structure can produce much larger train-induced ground vibration at frequencies above 40 Hz. The vertical ground vibration is much larger than the lateral and longitudinal components.

Recently, the study of emotion recognition has received increasing attentions by the rapid development of noninvasive sensor technologies, machine learning algorithms and compute capability of computers. Compared with single modal emotion recognition, the multimodal paradigm introduces complementary information for emotion recognition. Hence, in this work, we presented a decision level fusion framework for detecting emotions continuously by fusing the Electroencephalography (EEG) and facial expressions. Three types of movie clips (positive, negative, and neutral) were utilized to elicit specific emotions of subjects, the EEG and facial expression signals were recorded simultaneously. The power spectrum density (PSD) features of EEG were extracted by time-frequency analysis, and then EEG features were selected for regression. For the facial expression, the facial geometric features were calculated by facial landmark localization. Long short-term memory networks (LSTM) were utilized to accomplish the decision level fusion and captured temporal dynamics of emotions. The results have shown that the proposed method achieved outstanding performance for continuous emotion recognition, and it yields 0.625±0.029 of concordance correlation coefficient (CCC). From the results, the fusion of two modalities outperformed EEG and facial expression separately. Furthermore, different numbers of time-steps of LSTM was applied to analyze the temporal dynamic capturing.INDEX TERMS Continuous emotion recognition, EEG, facial expressions, signal processing, decision level fusion, temporal dynamics.