Автоматический выпуск конструкторской документации в PDM-системе

BaFe12O19 powders were prepared by coprecipitation and by glass crystallization. Electron microscopic investigations revealed that the particles of the glass crystallized powder exhibit excellent uniform shapes and narrow size distribution in comparison to the coprecipitated one consisting of irregularly shaped, polycrystalline particles with a broad and inhomogeneous size distribution. The crystallites of the coprecipitated powder particles, however, are smaller (mean diameter D = 0.11 μm) and the size distribution is narrower than that of the particles of the glass crystallized powder with a mean diameter of D = 0.42 μm. The lattice perfection of the particles and the crystallites of both ensembles is quite good. In order to determine the magnetic properties - above all the effective anisotropy field - we used six different methods: transverse susceptibility χt, singular point detection (SPD), ferromagnetic resonance (FMR), torsion pendulum (TP), rotational hysteresis losses (RHL), remanence curves (RC). It has been found that differences in coercivity values HcJ are not dramatic (about ±3%). Effective anisotropy fields Ha measured with “switching” methods (RHL, RC) are smaller than those measured with “stiffness” ones (FMR, TP, SPD, χt), by about 1700 Oe for the coprecipitated powder and about 2500 Oe for the glass crystallized one. The glass crystallized particle assemblies used for the present investigations do not exhibit pure Stoner-Wohlfarth behaviour, because of their relatively large particle diameters. The ratio HcJ/Ha ≈ 0.39 of the coprecipitated powder is relatively near the ideal value of 0.48, although the particles are considerably mechanically aggregated. © 1992

Abnormal Distracter Processing in Adults with Attention-Deficit-Hyperactivity Disorder

Background: Subjects with Attention-Deficit Hyperactivity Disorder (ADHD) are overdistractible by stimuli out of the intended focus of attention. This control deficit could be due to primarily reduced attentional capacities or, e. g., to overshooting orienting to unexpected events. Here, we aimed at identifying disease-related abnormalities of novelty processing and, therefore, studied event-related potentials (ERP) to respective stimuli in adult ADHD patients compared to healthy subjects. Methods: Fifteen unmedicated subjects with ADHD and fifteen matched controls engaged in a visual oddball task (OT) under simultaneous EEG recordings. A target stimulus, upon which a motor response was required, and non-target stimuli, which did not demand a specific reaction, were presented in random order. Target and most non-target stimuli were presented repeatedly, but some non-target stimuli occurred only once (‘novels’). These unique stimuli were either ‘relative novels ’ with which a meaning could be associated, or ‘complete novels’, if no association was available. Results: In frontal recordings, a positive component with a peak latency of some 400 ms became maximal after novels. In healthy subjects, this novelty-P3 (or ‘orienting response’) was of higher magnitude after complete than after relative novels, in contrast to the patients with an undifferentially high frontal responsivity. Instead, ADHD patients tended to smaller centro-parietal P3 responses after target signals and, on a behavioural level, responded slower than controls

Structural and Magnetic Properties of Ba-ferrite Fine Particles Grown by Glass Crystallization

The dependence of the specific saturation magnetization on the particle size of pure and Co-Ti-substituted Ba-ferrites was interpreted in terms of a nonmagnetic surface layer parallel to the c-planes of 1.3 nm and 0.7 nm, respectively, in thickness. Electron microscope investigations of the particles demonstrate that this effect arises neither from a sufficient fraction of superparamagnetic particles in the ensemble nor from the Co-Ti substitution or from surface defects such as steps and stacking disorder of the building blocks. Extrapolating this surface effect to small sizes yields a critical size, with the particles losing their ferrimagnetic behaviour. This critical size agrees surprisingly well with the volume calculated for the transition of the single domain state to the superparamagnetic one. © 1991 Springer Verlag

Preparation and Characterization of Hexaferrite Powders

A survey is given of the preparation and characterization of hexaferrite powders. Regarding the technical applications of such powders the main emphasis is put on M-type barium ferrite ones. Of the different preparation techniques the glass crystallization method is concluded to be convenient to generate excellent powders for magnetic recording media. Therefore this outstanding method is described in more detail and results about the growth kinetics of M-type particles are presented. The characterization is focused on the chemical and structural as well as magnetic properties of the powder particles

Properties of Barium Hexaferrite Powders for Magnetic Recording

Substituted barium hexaferrite powders prepared by the glass crystallization method are investigated geometrically and magnetically. Evaluating a special kind of remanence curves allows the anisotropy field distribution of the powders to be determined. In order to obtain the intrinsic anisotropy field distribution the influence of thermal fluctuations must be separated from the data. The magnetic properties of powders of different composition are discussed. © 1993

Structural and Magnetic Properties of BaFe₁₂₋₂ₓCoₓTiₓO₁₉ Powders Prepared by the Glass Crystallization Method

The glass crystallization method is shown to be suitable for preparing BaFe12-2xCox TixO19 powders with a nearly perfect crystallographic structure and a narrow particle size distribution. The dependences of the specific magnetization, the coercivity, and the particle size distribution on both the annealing temperature and the Co-Ti content correlate in a characteristic way. Furthermore, the lattice parameters, the specific surfaces and the thickness of the effective nonmagnetic surface layer were determined. The coercivity of the hexaferrite magnetic tapes is more than 8 × 103 A/m higher than for the corresponding powders

Improved correspondence of fMRI visual field localizer data after cortex-based macroanatomical alignment

Studying the visual system with fMRI often requires using localizer paradigms to define regions of interest (ROIs). However, the considerable interindividual variability of the cerebral cortex represents a crucial confound for group-level analyses. Cortex-based alignment (CBA) techniques reliably reduce interindividual macroanatomical variability. Yet, their utility has not been assessed for visual field localizer paradigms, which map specific parts of the visual field within retinotopically organized visual areas. We evaluated CBA for an attention-enhanced visual field localizer, mapping homologous parts of each visual quadrant in 50 participants. We compared CBA with volume-based alignment and a surface-based analysis, which did not include macroanatomical alignment. CBA led to the strongest increase in the probability of activation overlap (up to 86%). At the group level, CBA led to the most consistent increase in ROI size while preserving vertical ROI symmetry. Overall, our results indicate that in addition to the increased signal-to-noise ratio of a surface-based analysis, macroanatomical alignment considerably improves statistical power. These findings confirm and extend the utility of CBA for the study of the visual system in the context of group analyses. CBA should be particularly relevant when studying neuropsychiatric disorders with abnormally increased interindividual macroanatomical variability

Improved correspondence of fMRI visual field localizer data after cortex-based macroanatomical alignment

Studying the visual system with fMRI often requires using localizer paradigms to define regions of interest (ROIs). However, the considerable interindividual variability of the cerebral cortex represents a crucial confound for group-level analyses. Cortex-based alignment (CBA) techniques reliably reduce interindividual macroanatomical variability. Yet, their utility has not been assessed for visual field localizer paradigms, which map specific parts of the visual field within retinotopically organized visual areas. We evaluated CBA for an attention-enhanced visual field localizer, mapping homologous parts of each visual quadrant in 50 participants. We compared CBA with volume-based alignment and a surface-based analysis, which did not include macroanatomical alignment. CBA led to the strongest increase in the probability of activation overlap (up to 86%). At the group level, CBA led to the most consistent increase in ROI size while preserving vertical ROI symmetry. Overall, our results indicate that in addition to the increased signal-to-noise ratio of a surface-based analysis, macroanatomical alignment considerably improves statistical power. These findings confirm and extend the utility of CBA for the study of the visual system in the context of group analyses. CBA should be particularly relevant when studying neuropsychiatric disorders with abnormally increased interindividual macroanatomical variability