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  • Author or Editor: A. Ortega x
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Application of the appropriate N fertilizer rate for wheat production is needed to improve and sustain productivity. Different methods have been developed over time to estimate these needs. The objective of this work was to evaluate the relationship basal N rate at planting — NDVI (normalized difference vegetative index) by means of a spline regression to estimate further N needs of spring wheat. Experiments were established in two planting systems; permanent beds and conventional in solid stands. Three flat N rates (25, 50, and 75 kg N ha −1 , and 30, 60 and 90 kg Nha −1 for permanent beds and conventional planting, respectively) plus an unfertilized check plot were applied according to three N timing treatments (whole rate at planting or end of tillering, and split at planting and at the end of tillering). Before the application of N treatments at the end of tillering, plots were divided into two halves to apply variable N rates according to the first segment of the spline model. Results indicated that parameter estimates from the spline regression vary within each planting system. However, variable N rates estimated for each year and location were lower than flat N rates. In spite of those differential fertilizer rates, grain yield resulting for the application of variable N rates were similar to flat N rates. Pooled data analysis suggests that NDVI readings greater than 0.56 and 0.65 for permanent beds and conventional planting, respectively, the application of N fertilizer at the end of tillering can be excluded as grain yield will not be modified.

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The errors in the activation energies of solid-state reactions determined with the Piloyan method are more larger than those previously assumed in the literature. On the other hand, the errors in the kinetic parameters are strongly dependent on the kinetic law obeyed by the reaction. A theoretical explanation of this behaviour is given.

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Abstract  

It has been shown the ability of the Sample Controlled Reaction Temperature (SCRT) method for both discriminate the kinetic law and calculate the activation energy of the reaction. This thermal decomposition is best described by a Johnson–Mehl–Avrami kinetic model (with n = 2) with an activation energy of nuclei growth which fall in the range 52–59 kJ mol−1. The process is not a single-step because the initial rate of decomposition is likely to be limited by nucleation. The results reported here constitute the first attempt to use the new SCRT method to study the kinetic of the thermal decomposition of cobalt nitrate.

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Abstract  

The cyclic and Controlled Rate Thermal Analysis method (CRTA) has been used. The two rates automatically selected in the cyclic curve are small enough to allow the two states of the sample to be compared have nearly the same reacted fraction. Thus, the activation energy can be calculated without previous knowledge of the actual reaction mechanism. Provided that the activation energy,E, is known, a procedure has been developed for determining the kinetic law obeyed by the reaction by means of master curves that represent the values of the reacted fraction, α, as a function of−E/R(1/T-1/T 0.5),T 0.5 being the temperature at which α=0.5. This procedure has been tested by studying the thermal decomposition reaction of BaCO3.

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Abstract  

It has been demonstrated that a single plot of the values of Δlnα1/2/Δln(1-α) (taken from a single α−T curve obtained under a controlled linear increase of the reaction rate) as a function of the corresponding values of Δ(1/T)/Δln(1−α) permits the simultaneous determination of both the activation energy and the kinetic model in accordance with a solid state reaction.

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Abstract  

It has been demonstrated that the kinetic data on solid-state reactions show a good fitting to the expressiong(α)=kt, regardless of the nature of theg(α) function previously assumed for performance of the calculations. Moreover, the activation energy value obtained from the Arrhenius law is quite independent of the kinetic function assumed.

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Abstract  

The results obtained in this work point out that both SIA and CRTA traces, simulated assuming the same values of the reaction rate, are identical provided that either 'n order’ or diffusion controlled kinetics are concerned. The α-T CRTA plots of solid state reactions fitting an Avrami-Erofeev kinetic model show that the temperature decreases with increasing α until reaching a minimum at a value of the reacted fraction α = αmin characteristic of the Avrami-Erofeev exponent. The shapes of the corresponding SIA diagrams are quite different and a very long isothermal step is obtained. Moreover, the reaction rate is not maintained constant during the isothermal period but it shows a maximum at a value of α depending on the Avrami-Erofeev coefficient. It is important to point out that this αmax value agrees with the corresponding αmin calculated from a CRTA curve by assuming the same Avrami-Erofeev exponent.

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