Electron beam irradiation and the self-consistent charge transport in bulk insulating samples are described by means of a new flight-drift model and an iterative computer simulation. Ballistic secondary electron and hole transport is followed by electron and hole drifts, their possible recombination and/or trapping in shallow and deep traps. The trap capture cross sections are the Poole-Frenkel-type temperature and field dependent. As a main result the spatial distributions of currents j(x,t), charges ρ(x,t), the field F(x,t), and the potential slope V(x,t) are obtained in a self-consistent procedure as well as the time-dependent secondary electron emission rate σ(t) and the surface potential V0(t). For bulk insulating samples the time-dependent distributions approach the final stationary state with j(x,t)=const=0 and σ=1. Especially for low electron beam energies E0<4keV the incorporation of mainly positive charges can be controlled by the potential VG of a vacuum grid in front of the target surface. For high beam energies E0=10, 20, and 30keV high negative surface potentials V0=−4, −14, and −24kV are obtained, respectively. Besides open nonconductive samples also positive ion-covered samples and targets with a conducting and grounded layer (metal or carbon) on the surface have been considered as used in environmental scanning electron microscopy and common SEM in order to prevent charging. Indeed, the potential distributions V(x) are considerably small in magnitude and do not affect the incident electron beam neither by retarding field effects in front of the surface nor within the bulk insulating sample. Thus the spatial scattering and excitation distributions are almost not affected.
The electron beam induced selfconsistent charge transport in layered insulators (here bulk alumina covered by a thin silica layer) is described by means of an electron-hole flightdrift model FDM and an iterative computer simulation. Ballistic secondary electrons and holes, their attenuation and drift, as well as their recombination, trapping, and detrapping are included. Thermal and field-enhanced detrapping are described by the Poole-Frenkel effect. Furthermore, an additional surface layer with a modified electric surface conductivity is included which describes the surface leakage currents and will lead to particular charge incorporation at the interface between the surface layer and the bulk substrate.As a main result the time dependent secondary electron emission rate σ(t) and the spatial distributions of currents j(x, t), charges ρ(x, t), field F (x, t), and potential V (x, t) are obtained. For bulk full insulating samples, the time-dependent distributions approach the final stationary state with j(x, t) = const = 0 and σ = 1. In case of a measurable surface leakage current the steady stationary state is reached for σ < 1. First measurements are extended to the sample current measurement including instationary components of charge incorporation and polarization as well as dc-components of leakage currents.(a)
High‐entropy alloys (HEAs) and multicomponent alloys are known for their promising properties and the almost infinite possibilities of the design of new compositions. The Al–Cr–Fe–Mn–Mo family of HEAs is chosen to promote the formation of a body‐centered cubic (bcc) structure that exhibits high hardness and yield strength. Powder metallurgy is preferred to the liquid route to avoid segregation problems. Two compositions of alloyed powder, equiatomic and optimized, are successfully prepared by mechanical alloying. X‐ray diffraction (XRD) indicates the formation of two major bcc phases: bcc#1 (a1 = 3.13–3.14 Å) and bcc#2 (a2 = 2.87–2.94 Å). Energy transferred during milling depends greatly on the milling device and process conditions. Scanning electron microscopy–energy‐dispersive X‐ray spectroscopy (SEM–EDX) images reveal that high‐energy shocks of balls enable to elaborate homogeneous powder with controlled Fe contamination coming from the grinding vial and balls, whereas low‐energy shocks enable to produce powder free of contamination by Fe, with heterogeneous particles. Finally, the milling process is optimized to obtain a homogeneous alloyed powder with as little contamination as possible.
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