Nitride coatings are increasingly demanded in the cutting- and machining-tool industry owing to their hardness, thermal stability and resistance to corrosion. These properties derive from strongly ...covalent bonds; understanding the bonding is a requirement for the design of superhard materials with improved capabilities. Here, we report a pressure-induced cubic-to-orthorhombic transition at approximately 1 GPa in CrN. High-pressure X-ray diffraction and ab initio calculations show an unexpected reduction of the bulk modulus, K0, of about 25% in the high-pressure (lower volume) phase. Our combined theoretical and experimental approach shows that this effect is the result of a large exchange striction due to the approach of the localized Cr:t3 electrons to becoming molecular-orbital electrons in Cr-Cr bonds. The softening of CrN under pressure is a manifestation of a strong competition between different types of chemical bond that are found at a crossover from a localized to a molecular-orbital electronic transition.
Nitride coatings are increasingly demanded in the cutting- and machining-tool industry owing to their hardness, thermal stability and resistance to corrosion. These properties derive from strongly ...covalent bonds; understanding the bonding is a requirement for the design of superhard materials with improved capabilities. Here, we report a pressure-induced cubic-to-orthorhombic transition at -1GPa in CrN. High-pressure X-ray diffraction and ab initio calculations show an unexpected reduction of the bulk modulus, K(0), of about 25% in the high-pressure (lower volume) phase. Our combined theoretical and experimental approach shows that this effect is the result of a large exchange striction due to the approach of the localized Cr:t(3) electrons to becoming molecular-orbital electrons in Cr-Cr bonds. The softening of CrN under pressure is a manifestation of a strong competition between different types of chemical bond that are found at a crossover from a localized to a molecular-orbital electronic transition.
Zener relaxation in manganites? Salva, Horacio R.; Ghilarducci, Ada A.; Seiro, Silvia S. ...
Physica. B, Condensed matter,
10/2006, Letnik:
384, Številka:
1
Journal Article, Conference Proceeding
Recenzirano
We conducted internal friction (IF) and elastic modulus measurements on several manganites in the temperature range 100–500
K. The La
1−
x
Ca
x
MnO
3 series, with
x=0, 0.1, 0.2, 0.33 and 1, was ...studied showing the magnetic phase transitions and two additional IF peaks around 320 and 400
K, respectively. Both, peaks associated to relaxations, present activation enthalpies of 2–3 and 3–6
eV, whose intensity depends on
x. In this work we associate the 400
K IF peak to a Zener relaxation of the cations that occupied the same crystallographic site. We believe that the 320
K IF peak is due to different crystalline states of the samples.
This article studies the influence of the network structure on the activation energies of the α and β relaxations in vulcanized styrene butadiene rubber, SBR. A cure system based on sulphur and TBBS ...(N-t-butyl-2-benzothiazole sulfenamide) was used in the formulation of several compounds cured at 433 K. The activation energies were evaluated from internal friction (loss tangent) data of the compounds using an automated subresonant forced pendulum in a wide frequency range and between 80 K and 273 K. The internal friction data of the samples reveal two transitions, α and β, characterized by the temperatures Tα and Tβ, due to the glass transition and the phenyl group rotation of the copolymer, respectively. Although Tα increases at higher crosslink density, it shows also a dependence with the amount of polysulphide and monosulphide linkages present in the samples. The highest activation energy for this process is obtained for the samples with high crosslink density and 30% of monosulphides in this structure. In the case of the β-relaxation, there is a pronounced change in the activation energy between the uncured and the cured samples. The type of structure formed during vulcanization has an important effect in the activation energy of the segmental mode-process. In the case of the β-process, the cis-trans isomerization that takes place during vulcanization in the butadiene part of the SBR, might be the cause of conformational changes in the surrounding of the phenyl rings that affect the energy barrier associated to the phenyl rotation.
The hard magnetic properties of melt spun Nd60Fe30Al10 alloys are attributed to a major matrix nominally amorphous for X-ray diffraction, composed by two metastable nanosized (-5 nm) phases with ...different intrinsic magnetic properties. This composite system is investigated for the first time by mechanical spectroscopy techniques in the temperature range between 50 K and 450 K (1 kHz) where large annealing effects and two damping phenomena are detected. The as-cast microstructure irreversibly changes during annealing above 330 K, leading to a large modulus recovery accompanied by a reduction in the internal friction level. A relatively large relaxation effect is observed about 290 K, evidenced by a narrow internal friction peak with the corresponding step in the elastic modulus; this peak remains stable under thermal cycling between 200 K and 300 K but is affected by aging at 330 K and practically vanishes after heating to 450 K. Another internal friction peak is observed at about 250 K which has associated an anomalous modulus effect; in this temperature range, the internal friction and the elastic modulus exhibit heating/cooling hysteresis, which strongly depends on the extreme temperatures of the thermal cycle, a behavior frequently associated to first-order phase transformations.
The hard magnetic properties of melt spun Nd
60Fe
30Al
10 alloys are attributed to a major matrix nominally amorphous for X-ray diffraction, composed by two metastable nanosized (∼5
nm) phases with ...different intrinsic magnetic properties. This composite system is investigated for the first time by mechanical spectroscopy techniques in the temperature range between 50
K and 450
K (1
kHz) where large annealing effects and two damping phenomena are detected. The as-cast microstructure irreversibly changes during annealing above 330
K, leading to a large modulus recovery accompanied by a reduction in the internal friction level. A relatively large relaxation effect is observed about 290
K, evidenced by a narrow internal friction peak with the corresponding step in the elastic modulus; this peak remains stable under thermal cycling between 200
K and 300
K but is affected by aging at 330
K and practically vanishes after heating to 450
K. Another internal friction peak is observed at about 250
K which has associated an anomalous modulus effect; in this temperature range, the internal friction and the elastic modulus exhibit heating/cooling hysteresis, which strongly depends on the extreme temperatures of the thermal cycle, a behavior frequently associated to first-order phase transformations.
La técnica de Fricción Interna (FI) y módulo elástico como métodos para la caracterización de recubrimientos e intercaras y su potencialidad en la determinación de la adhesión de la película al ...sustrato son presentados. Se realizaron mediciones de fricción interna en un péndulo de torsión invertido en el modo de decaimiento libre de las oscilaciones, en aceros recubiertos con películas duras de TiN. Las películas se obtuvieron por deposición física de vapor (PVD) mediante Magnetron Sputtering.
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