Myriam H. Aguirre

Hierarchical adaptive nanostructured PVD coatings for extreme tribological applications: the quest for nonequilibrium states and emergent behavior

Abstract Adaptive wear-resistant coatings produced by physical vapor deposition (PVD) are a relatively new generation of coatings which are attracting attention in the development of nanostructured materials for extreme tribological applications. An excellent example of such extreme operating conditions is high performance machining of hard-to-cut materials. The adaptive characteristics of such coatings develop fully during interaction with the severe environment. Modern adaptive coatings could be regarded as hierarchical surface-engineered nanostructural materials. They exhibit dynamic hierarchy on two major structural scales: (a) nanoscale surface layers of protective tribofilms generated during friction and (b) an underlying nano/microscaled layer. The tribofilms are responsible for some critical nanoscale effects that strongly impact the wear resistance of adaptive coatings. A new direction in nanomaterial research is discussed: compositional and microstructural optimization of the dynamically regenerating nanoscaled tribofilms on the surface of the adaptive coatings during friction. In this review we demonstrate the correlation between the microstructure, physical, chemical and micromechanical properties of hard coatings in their dynamic interaction (adaptation) with environment and the involvement of complex natural processes associated with self-organization during friction. Major physical, chemical and mechanical characteristics of the adaptive coating, which play a significant role in its operating properties, such as enhanced mass transfer, and the ability of the layer to provide dissipation and accumulation of frictional energy during operation are presented as well. Strategies for adaptive nanostructural coating design that enhance beneficial natural processes are outlined. The coatings exhibit emergent behavior during operation when their improved features work as a whole. In this way, as higher-ordered systems, they achieve multifunctionality and high wear resistance under extreme tribological conditions.

Observation of the spin Seebeck effect in epitaxial Fe3O4 thin films

We report the experimental observation of the spin Seebeck effect in magnetite thin films. The signal observed at temperatures above the Verwey transition is a contribution from both the anomalous Nernst (ANE) and spin Seebeck (SSE) effects. The contribution from the ANE of the Fe3O4 layer to the SSE is found to be negligible due to the resistivity difference between Fe3O4 and Pt layers. Below the Verwey transition, the SSE is free from the ANE of the ferromagnetic layer and it is also found to dominate over the ANE due to magnetic proximity effect on the Pt layer.

Electrochemical topotactic oxidation of nonstoichiometric perovskites at ambient temperature

Different approaches to describe nonstoichiometry in perovskite-related oxides, oxygen diffusion models and kinetic data on electrochemical intercalation of oxygen in perovskite-related oxides are considered in the present study. It is demonstrated that the formation of microheterogeneous systems is characteristic of nonstoichiometric perovskite-related oxides. The adequate models should be considered for the proper characterization of the low temperature oxygen transport in oxides. These models should account for the high concentration of the high diffusivity paths in the sample. For microdomain textured Ca0.5Sr0.5FeO2.5 perovskite as example, the kinetic analysis of electrochemical oxygen intercalation is carried out.  2002 Editions scientifiques et medicales Elsevier SAS. All rights reserved.

Measurements of current-voltage-induced heating in the Al/SrTiO3−xNy/Al memristor during electroformation and resistance switching

Heating of the Al/SrTiO3−xNy/Al memristor is characterized during electroformation and switching of the resistances. The electrode with the higher voltage potential is heated to higher temperatures than the electrode with the lower potential, suggesting a reversible (nonstable) displacement of the anions in a low voltage region (|V|<±3 V). Application of a threshold voltage appropriate for resistance switching (|V|≥±3 V) facilitates migration of anions to the anode interface and increases the local anode temperature to a maximum of 285 °C. The hysteretic I-V curves are discussed taking into account tunnel barrier formation/break and inhomogeneous Schottky barrier modification at the anode interface.

Unconventional scaling and significant enhancement of the spin Seebeck effect in multilayers

Thermal spin pumping constitutes a novel mechanism for generation of spin currents; however their weak intensity constitutes a major roadblock for its usefulness. We report a phenomenon that produces a huge spin current in the central region of a multilayer system, resulting in a giant spin Seebeck effect in a structure formed by repetition of ferromagnet/metal bilayers. The result is a consequence of the interconversion of magnon and electron spin currents at the multiple interfaces. This work opens the possibility to design thin film heterostructures that may boost the application of thermal spin currents in spintronics.

Mechanism of adaptability for the nano-structured TiAlCrSiYN-based hard physical vapor deposition coatings under extreme frictional conditions

Recently, a family of hard mono- and multilayer TiAlCrSiYN-based coatings have been introduced that exhibit adaptive behavior under extreme tribological conditions (in particular during dry ultrahigh speed machining of hardened tool steels). The major feature of these coatings is the formation of the tribo-films on the friction surface which possess high protective ability under operating temperatures of 1000 °C and above. These tribo-films are generated as a result of a self-organization process during friction. But the mechanism how these films affect adaptability of the hard coating is still an open question. The major mechanism proposed in this paper is associated with a strong gradient of temperatures within the layer of nano-scaled tribo-films. This trend was outlined by the performed thermodynamic analysis of friction phenomena combined with the developing of a numerical model of heat transfer within cutting zone based on the finite element method. The results of the theoretical studies show that t...

In situ reduction of (100) SrTiO3

Abstract Low energy electron diffraction (LEED) and photoelectron spectroscopy (PES) have been employed to study the (100) SrTiO3 surface annealed in ultra high vacuum. The annealing was performed up to 830°C. After annealing at 830°C, we have observed from PES spectra the appearance of the conduction band and from LEED patterns the ( 5 × 5 )R 26.6° superstructure. Both features seem to have the same origin: the ordering of oxygen vacancies.

One-Step Dry Method for the Synthesis of Supported Single-Crystalline Organic Nanowires Formed by π-Conjugated Molecules

We present for the first time a general vacuum process for the growth of supported organic nanowires formed by pi-conjugated molecules, including metalloporphyrins, metallophthalocyanines, and perylenes. This methodology consists on a one-step physical vapor deposition of the pi-conjugated molecules. The synthesis is carried out at controlled temperature on substrates with tailor morphology which allows the growth of organic nanowires in the form of squared nanofibers and nanobelts. The study of the nanowires by electron diffraction and HRTEM combining with the results of a theoretical analysis of the possible arrangement of the pi-conjugated molecules along the nanowires reveals that the nanowires show a columnar structure along the fiber axis consisting of pi-stacked molecules having a herringbone-like arrangement. The formation of these nanowires on different substrates demonstrates that the growth mechanism is independent of the substrate chemical composition. An in-depth phenomenological study of the formation of the nanowires drives us to propose a growth mechanism based on a crystallization process. Furthermore, the growth method allows the fabrication of two particular 1D heterostructures: binary and open core@shell organic nanofibers.

Synthesis and transport properties of SrTiO3−xNy/SrTiO3−δ layered structures produced by microwave-induced plasma nitridation

SrTiO3?xNy/SrTiO3?? layered structures were synthesized from single crystals of SrTiO3 (1?0?0) by ammonolysis using a microwave-induced plasma (MIP). Samples prepared using a low ammonia flow rate (50?100?mL?min?1) gave nanocrystals of TiN at the surface, while the samples prepared in an ammonia flow in the range 125?175?mL?min?1 did not show impurity phases. Electrical resistance measurements of SrTiO3?xNy/SrTiO3?? single crystals show metallic-like behaviour in the temperature range 2?300?K, with a residual resistance ratio R(300?K)/R(4.2?K) of ca 50. The Hall mobility follows the power law ?H ? T?2.3 in the temperature range 150?300?K, which is attributed to n-type charge carriers.

Thermoelectric performance of spin Seebeck effect in Fe3O4/Pt-based thin film heterostructures

We report a systematic study on the thermoelectric performance of spin Seebeck devices based on Fe3O4/Pt junction systems. We explore two types of device geometries: a spin Hall thermopile and spin Seebeck multilayer structures. The spin Hall thermopile increases the sensitivity of the spin Seebeck effect, while the increase in the sample internal resistance has a detrimental effect on the output power. We found that the spin Seebeck multilayers can overcome this limitation since the multilayers exhibit the enhancement of the thermoelectric voltage and the reduction of the internal resistance simultaneously, therefore resulting in significant power enhancement. This result demonstrates that the multilayer structures are useful for improving the thermoelectric performance of the spin Seebeck effect.