Crop Reference Evapotranspiration: A Discussion of the Concept,Analysis of the Process and Validation

The study at first recalls the concept of “potential evapotranspiration” (PET), originally considered equal to the evaporation climatic demand; then, it reminds the steps of its progressive evolution toward the concept of “reference crop evapotranspiration” (ET₀) determined on irrigated grass. A physical analysis conducted on the evaporation process is subsequently reported to help clarifying the links between ET₀ and evaporation climatic demand. This analysis clearly demonstrates that the equivalence of ET₀ to evaporation climatic demand is not correct, although still common assumption in recent scientific literature, particularly in hydrology. The study also identifies two processes acting in opposite directions in the dynamics of ET₀: (1) the climatic variables determining the evaporation demand, and (2) the canopy resistance which slows down the response of irrigated grass to such demand. The analysis of the respective impact of these two processes on ET₀ dynamics shows that the available energy is the dominant process. This variable takes into account the 60–70% of the variation of ET₀, both at hourly and daily scales, while canopy resistance only explains 10–20% of ET₀ variation of irrigated grass. The study regards different climatic situations. Possible effects on practical applications were also discussed in the conclusions, together with comments on the correct canopy resistance modelling.     

Effect of temperature on thermo-mechanical properties of Macro-Defect-Freecement-polymer composite

In this work thermo-mechanical properties of MDF cements have been analyzed under complex thermal hystories. In particular the temperature sensitivity of the material has been investigated using dynamical-mechanical analysis. Several tests have been performed to investigate the behaviour of the material from room temperature to 250 °C at a fixed frequency (1 Hz). The viscoelastic behaviour of the composite was then analyzed at different frequencies: it was observed that the material exhibits two main transitions, around 60 °C and 160 °C. For comparison with differential scanning calorimeter analysis the first peak was attributed to the glass transition of the PVAAc while the second one was associated to the melting of the crystalline part of the polymer. In addition, dynamical-mechanical analysis that included complex thermal hystories, were performed both on pure polymer films, and on the MDF cements, confirming that the drop of mechanical properties with temperature is essentially due to the nature of the viscoelastic component. Results obtained are in agreement with the picture of a composite in which a polymeric thermoplastic matrix surrounds partially hydrated cement particles.     

Fundamentals of MRAM Technology

Developments in portable communication and computing systems are creating a growing demand for nonvolatile random access memory that is dense and fast. None of the existing solid-state memories can provide all the needed attributes in a single memory solution. Therefore, to achieve the required multiple functionality requirements, a number of different memories are being used while compromising performance and adding cost to the system. Magnetoresistive Random Access Memory (MRAM) has the potential to replace these memories in various systems with a single, universal memory solution. Key attributes of MRAM technology are nonvolatility and unlimited read and write endurance. In addition, MRAM can operate at high-speed and is expected to have competitive densities. In this paper we describe several fundamental technical and scientific aspects of MRAM with emphasis on recent accomplishments that enabled our successful demonstration of a 256 kbit memory chip.     

Phase transformations during the cure of unsaturated polyester resins

The phase transformations occurring during the cure of an unsaturated polyester resin have been investigated by different techniques, including high frequency dynamic–mechanical analysis by ultrasonic wave propagation. The increase of longitudinal sound velocity can be attributed to the increase of longitudinal modulus L′ while irreversible viscous losses are responsible for the increase of sound attenuation. A correspondence between the changes in the velocity and attenuation and the phase transformations (gelation and vitrification) can be observed. The ultrasonic properties have been compared with the gel time values obtained from parallel-plate rheological measurements. Finally, a DSC analysis has been carried out to compare the evolution of degree of reaction with that of the ultrasonic modulus.     

Symmetric GTO and snubber component characterization in PWMcurrent-source inverters

The pulsewidth-modulated (PWM) current-source inverter (CSI) used in AC motor drive applications can be implemented with symmetric gate turn-off thyristors (GTOs). One of the major difficulties in the optimization of the GTO switch and the snubber components of the inverter is the variation in different switching conditions encountered during normal operation. Past work has concentrated on the GTO and snubber components in voltage-source applications, where commutation of the GTO device is an independent process and does not affect the operation of the other inverter devices. This paper proposes the characterization of the GTO and the snubber components by formulation of the CSI equivalent circuit during the device commutation period. From the equivalent circuit, the state equations are derived, thereby obtaining accurate voltage and current waveforms of the GTO and associated snubbers. From the analysis, the component power loss can be calculated and optimization performed. Simulation results are verified by using both a laboratory prototype and medium-voltage drive system