Nanocalorimetric analysis of the ferromagnetic transition in ultrathin films of nickel

We report on in situheat capacity measurements (370-800K) using quasiadiabatic ultrafast differential scanning nanocalorimetry in thin films(1-200nm) of Nigrown by electron beam evaporation. The heat capacity shows a broad peak with a rounded maximum that is attributed to the decrease of long-range interactions in the ferromagnetic to paramagnetic phase transition of Ni. The calorimetric data exhibit a reduction of the Curie temperature as the thickness of the films (or the average grain size) decreases. The magnitude of the jump in specific heat at TC scales with the number of surface or interface atoms

180 diffusion through amorphous SiOs and cristobalite

Secondary ion mass spectrometry was used to profile the diffusion of oxygen in polycrystalline β-cristobalite and vitreous SiO2. The tracer concentration profiles of cristobalite are consistent with a model based on two mechanisms: bulk and short-circuit diffusion. The profiles of partially crystallized samples containing vitreous SiO2 and β-cristobalite were fitted using the sum of two complementary error functions and taking account of some interstitial-network exchange. The bulk oxygen diffusivity, in the temperature range 1240-1500 °C, is about five times greater for vitreous silica than for β-cristobalite

Magnetic transitions in alpha-Fe_2O_3 nanowires

Magnetic transitions in single-crystal alpha-F_2O_3 (hematite) nanowires, grown by thermal oxidation of iron powder, have been studied in the range of 5-1023 K with a superconducting quantum interference device below room temperature and with a vibrating sample magnetometer at higher temperatures. The broad temperature range covered enables us to compare magnetic transitions in the nanowires with the transitions reported for bulk hematite. Morin temperatures (T-M) of the nanowires and of hematite bulk reference powder were found to be 123 and 263 K, respectively. Also the Neel temperature (T-N) of the nanowires, 852 K, was lower than the bulk T-N value. Measurements of the magnetization as a function of temperature show an enhanced signal in the nanowires, which suggests a decrease in the anti ferromagnetic coupling. A coercive field observed below T-M in the hysteresis loops of the nanowires is tentatively explained by the presence of a magnetic phase

Correlation between the surface chemistry and the atmospheric corrosion of AZ31, AZ80 and AZ91D magnesium alloys

X-ray photoelectron spectroscopy (XPS) was used in order to investigate the correlation between the surface chemistry and the atmospheric corrosion of AZ31, AZ80 and AZ91D magnesium alloys exposed to 98% relative humidity at 50 °C. Commercially pure magnesium, used as the reference material, revealed MgO, Mg(OH)2 and tracers of magnesium carbonate in the air-formed film. For the AZ80 and AZ91D alloys, the amount of magnesium carbonate formed on the surface reached similar values to those of MgO and Mg(OH)2. A linear relation between the amount of magnesium carbonate formed on the surface and the subsequent corrosion behaviour in the humid environment was found. The AZ80 alloy revealed the highest amount of magnesium carbonate in the air-formed film and the highest atmospheric corrosion resistance, even higher than the AZ91D alloy, indicating that aluminium distribution in the alloy microstructure influenced the amount of magnesium carbonate formedThanks to the MCYT for the financial support given to this work (Project MAT2006-13179-C02-01-02)Peer reviewe

Cathodoluminescence and photoluminescence of highly luminescent CdSe/ZnS quantum dot composites

We report room-temperature cathodoluminescence and photoluminescence spectra originating from ZnS overcoated CdSenanocrystals, 33 and 42 Å in diameter, embedded in a ZnS matrix. The thin-filmquantum dot composites were synthesized by electrospray organometallic chemical vapor deposition.Cathodoluminescence and photoluminescence are dominated by the sharp band-edge emission characteristic of the initial nanocrystals. The emission wavelength can be tuned in a broad window (470-650 nm) by varying the size of the dots. The cathodoluminescence intensity depends on the crystallinity of the ZnS matrix and the voltage and current density applied

Influence of layer microstructure on the double nucleation process in Cu/Mg multilayers

9 páginas, 9 figuras, 3 tablas.-- et al.We have investigated by differential scanning calorimetry the thermal evolution of Cu/Mg multilayers with different modulation lengths, ranging from 7/28 to 30/120 nm. The Cu and Mg layers were grown by sequential evaporation in an electron beam deposition system. The phase identification and layer microstructure were determined by cross-section transmission electron microscopy, Rutherford backscattering, and scanning electron microscopy with focused ion beam for sample preparation. Upon heating, the intermetallic CuMg2 forms at the interfaces until coalescence is reached and thickens through a diffusion-limited process. Cross-section transmission electron microscopy observations show a distinct microstructure at the top and bottom of the as-prepared Mg layers, while no significant differences were seen in the Cu layers. We show that this effect is responsible for the observed asymmetry in the nucleation process between the Cu on Mg and the Mg on Cu interfaces. By modeling the calorimetric data we determine the role of both interfaces in the nucleation and lateral growth stages. We also show that vertical growth proceeds by grain development of the product phase, increasing significantly the roughness of the interfaces.The financial support by Project No. MAT2004-04761 granted by the Spanish Ministry of Education and 2005SGR00201 granted by the Direcció General de Recerca of the Generalitat of Catalonia is acknowledged. One of the authors (J.L.L.) acknowledges the financial support from the Hungarian National Science Fund (OTKA043437).Peer reviewe

Evaluating student acceptance level of e-learning systems

The e-learning information systems are widely used in universities to improve their courses, although this does not always achieve the objective of facilitating the teaching-learning process, there are several factors that avoid it. The aim of this study was to adapt a research model that describes the behaviour of use and the intended use of e-learning systems in Ecuador students, based on the Technology Acceptance Model (TAM) developed by Fred Davis. A self-report questionnaire was designed to examine the student's acceptance and attitude towards e-learning system. Data from 263 undergraduate and graduate students were collected in a University of Ecuador. The model is composed of eight constructs, namely, technical support, computer self-efficacy, subjective norms, satisfaction, perceived usefulness, perceived ease of use, intention to use the system and use the system. The model was estimated using structural equation modelling (SEM), for which the software was used R. It was found that the technical support and computer self-efficacy served as the two anchors of significant perception of fundamental constructs in TAM. Satisfaction became the sole determining factor in predicting use, while subjective norms were significant to the prediction of intention to use e-learning system. Overall, subjective norms, computer self-efficacy and perceived ease of use were able to explain the most variation observed in the intention of the students to use e-learning system.Postprint (published version

Formation of Pd2Si on single-crystalline Si (100) at ultrafast heating rates : an in-situ analysis by nanocalorimetry

The kinetics of intermediate phase formation between ultrathin films of Pd (12 nm) and single-crystalline Si (100) is monitored by in-situ nanocalorimetry at ultrafast heating rates. The heat capacity curves show an exothermic peak related to the formation of Pd2Si. A kinetic model which goes beyond the conventional linear-parabolic growth to consider independent nucleation and lateral growth of Pd2Si along the interface and vertical growth mechanisms is developed to fit the calorimetric curves. The model is used to extract the effective interfacial nucleation/growth and diffusion coefficients at the unusually high temperatures of silicide formation achieved at very fast heating rates

A new method to quantify and compare the multiple components of fitness-A study case with kelp niche partition by divergent microstage adaptations to Temperature

Point 1 Management of crops, commercialized or protected species, plagues or life-cycle evolution are subjects requiring comparisons among different demographic strategies. The simpler methods fail in relating changes in vital rates with changes in population viability whereas more complex methods lack accuracy by neglecting interactions among vital rates. Point 2 The difference between the fitness (evaluated by the population growth rate.) of two alternative demographies is decomposed into the contributions of the differences between the pair-wised vital rates and their interactions. This is achieved through a full Taylor expansion (i.e. remainder = 0) of the demographic model. The significance of each term is determined by permutation tests under the null hypothesis that all demographies come from the same pool. Point 3 An example is given with periodic demographic matrices of the microscopic haploid phase of two kelp cryptic species observed to partition their niche occupation along the Chilean coast. The method provided clear and synthetic results showing conditional differentiation of reproduction is an important driver for their differences in fitness along the latitudinal temperature gradient. But it also demonstrated that interactions among vital rates cannot be neglected as they compose a significant part of the differences between demographies. Point 4 This method allows researchers to access the effects of multiple effective changes in a life-cycle from only two experiments. Evolutionists can determine with confidence the effective causes for changes in fitness whereas population managers can determine best strategies from simpler experimental designs.CONICYT-FRENCH EMBASSADY Ph.D. gran