No abstract

No abstract

No abstract

Changes in the dielectric properties during the ferroelectric transition of commercial BaTiO 3 powders were determined in situ, by analyzing the low-energy-loss region of electron energy loss spectroscopy ͑EELS͒ spectra in a transmission electron microscope, at room temperature ͑ferroelectric phase͒ and 150°C ͑paraelectric phase͒. A comparison of experimental EELS spectra and ab initio density-functional theory calculations ͑WIEN2K code͒ within the generalized gradient approximation ͑GGA͒ is presented. A characteristic peak around 11 eV appears in the imaginary part of the dielectric function in paraelectric phase, which is absent in ferroelectric phase. The origin of the characteristic peak is analyzed by means of energy band structure calculations.

Mechanical alloying technique, such high energy ball milling process, is suitable for producing composite metal powders with a fine controlled microstructure. This method is crucial for obtaining homogeneous distribution of nano-sized dispersoids in a more ductile matrix (e.g. aluminium-or copper based alloys). The reinforcing particles Al4C3 have become an interesting reinforcing material because their high level of physical and mechanical properties, e.g. high temperature strength, thermal cyclic resistance, wear resistance and low linear expansion coefficient. Therefore, the reinforcement of the aluminum using Al 4 C 3 has recently become the subject of many studies and widely used for products and structures [1,2].The Al-based composites were produced by mixing Al powder (99.5 % purity) with 1 and 2 wt. % of Al 4 C 3 nanoparticle powder (previously synthetized by mechanical milling and subsequent heat treatment), each Al-Al 4 C 3 mixtures powder were mechanically milled in a high energy Simoloyer mill during 8 h. Argon was used as the milling atmosphere and ~4 ml methanol as a process-control agent. The device and milling media used were made from hardened steel. The milling ball to powder weight ratio was set to 50:1. Consolidated samples were obtained by pressing the powder mixtures during 20 s at 350 MPa in uniaxial load. The consolidate sample was sintered during 2h at 650°C. The Al4C3 reinforcing phase dispersed into the aluminum matrix also was observed by using electron microscopy analyses. Fig. 1a shows a STEM bright-field image of Al-20 sample (2 wt. % of Al 4 C 3 and not sintered), the image shows a graphite nanoparticle of about 10 nm long which is close to an Al4C3 nanoparticle that shows an irregular shape present in the aluminum matrix. Fig. 1b shows the simulated image that shows the interplanar distance of the A selected area of a Al 4 C 3 nanoparticle and the interplanar distance of the B selected area of a graphite nanoparticle. The Fig. 2a shows a STEM bright-field representative image of a rod-shaped aluminum carbide nanoparticle of about 100 nm long and 10 nm wide in the Al matrix of Al-22 sample (2 wt. % of Al 4 C 3 and sintered during 2 h). The HRTEM image of Figure 2b shows the interplanar distances of a rod shaped particle which correspond to Al 4 C 3 compound. At seems the Al 4 C 3 irregular shape nanoparticles dispersed into the Al matrix during the mechanical milling change to a regular rod-shaped after the sintering process.

The dielectric properties of the intermetallic cubic Laves phase compound YFe2 were determined by analyzing the low loss region of the EELS spectrum in a transmission electron microscope. From these data, the optical joint density of states (OJDS) was obtained by Kramers-Kronig analysis. Since maxima observed in the OJDS spectra are assigned to interband transitions; these spectra can be interpreted on the basis of numerical calculations performed with the Wien2k code, using the Fully-Linearized-Augmented-Plane wave (FLAPW) method within the Local-Spin-Density Approximation. Comparison between experimental results and theory shows good agreement.

No abstract

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.