Search

We have analyzed the overgrowth problem arising in decelerated growth processes of spherical crystals in the frame of classical nucleation and growth theory developed by Kolmogorov, Johnson and Mehl, and Avrami (KJMA). To do that, simulations of decelerated growth transformations with a constant nucleation rate have been performed, changing the linear growth rate of spherically shaped nuclei from null (instantaneous growth rate) to constant (characteristic of interface controlled growth processes). We propose the determination of the actual kinetic parameters through the analysis of the inflection point of time evolution of transformed fraction. The correlations found between the effective kinetic parameters from direct KJMA analysis and the actual ones make it possible obtaining physically meaningful parameters. The proposed analysis has been applied to the nanocrystallization of amorphous FINEMET-type compositions.

Optimizing the performance of magnetocaloric materials is facilitated by understanding the thermomagnetic transitions they undergo, including the order of these transitions and their strength. Those exhibiting strong first-order phase transitions (FOPT) are accompanied by large heating and cooling responses but with relatively small cyclic responses, while materials with second-order (SOPT) character exhibit moderate heating and cooling responses. However, the lack of hysteresis could partially compensate for the lower magnitudes with a more cyclic response. One way to effectively maximize the cyclic response, combining the advantages of FOPT and SOPT, is to fine tune the transition towards the borderline of FOPT-SOPT, which can minimize hysteresis. For the well-known La(Fe,Si)13 family, it is challenging to identify and/or evaluate the critical point where FOPT crossovers to SOPT based on conventional techniques. To address these ambiguities, in this work, we apply the field dependence exponent n criteria to a series of lowly hysteretic and high-performance La(Fe,Mn,Si)13H magnetocaloric materials with compositions close to the critical one. Even if the sample with the lowest hysteresis resembles characteristics of SOPT, it is evidently identified as undergoing FOPT from the n criteria: (1) existence of n > 2 overshoot and (2) n at the transition temperature, ntransition, is 0.37. This proximity to the critical composition (ntransition=0.4) further explains the low hysteresis observed. This FOPT character of the series is confirmed by temperature-dependent 57Fe Mössbauer spectrometry studies, fitting the hyperfine field to the Bean-Rodbell model instead of the usual Brillouin function. As it is a zero-field method, the confirmation by Mössbauer spectrometry gives further strength to the n-criterion.

The onset of melting of standard samples, ascribed to surface melting, is generally used for calibration of calorimeters. However, in non-isothermal conditions, nucleation-driven volume melting, which is thermally activated, takes place. In this work, we propose an approximation in the frame of the classical nucleation and growth transformation kinetics to extend to non-isothermal regimes the analysis of processes governed by constant nucleation and interface controlled growth. The approximation allows both to observe the temperature dependence of nucleation activation energy with the overheating and to obtain the surface energy between the liquid nucleus and the surrounding solid phase for pure indium and lead (~ 10 mJ m−2) and for a Fe70B5C5Si3Al5Ga2P10 bulk metallic glass eutectic composition (~ 50 mJ m−2). These values are about 50% lower than the theoretical ones for homogeneous nucleation, which can be ascribed to the random heterogeneous nucleation occurring at the crystals boundaries.

The report contains the experimental and theoretical results obtained in a study to further develop an electronically controllable microwave window used as a single-pole, single-throw switch. Fabrication improvement and structure optimization resulted in advancing, by more than an order of magnitude, the state of the art in high-power broad-band semiconductor microwave switches at S- and X-band frequencies. A discussion is given of the microwave operating characteristics of the window which are basic for optimizing the window design. The recombination mechanisms present in the device are discussed. An account is given of the fabrication procedures used in the practical realization of the devices. The microwave mounts used for both S- and X-band windows are described. The results of the microwave tests carried out in the fabricated windows for determining their switching characteristics are presented. (Author)

The main objective of the research study was to further develop electronically controllable S- and X-band microwave windows. Results of the calculations carried out for determining the microwave transmission properties of X-band windows are presented. These show that the windows should be 4 to 7 mils thick to achieve switching from 0.5 db to 15 db with a resistivity change from 300 ohm-cm to 1 ohm-cm. The photolithographic mask and waveguide mount which was fabricated for the implementation of the X-band windows are described. Inductive strips in the injecting line structure reduce the insertion loss to 0.3 db. The X-band measurements carried out to test the broad band and low insertion loss capabilities of the windows are presented. (Author)

The main objective of this research study is to develop an electronically controllable S-band microwave silicon window in order to overcome some of the limitations of junction-type diodes in microwave switching applications. The modifications made on our high power S-band test facility, for achieving higher accuracy in small loss measurements, are described. The results of microwave tests performed on the previously designed windows of different resistivities, are presented. Graphs are given which show the spatial distribution of the injected excess carrier density in the window structure for several values of surface recombination velocity. The allowed depths of the p+ and n+ diffused layers for an efficient performance of the window are determined. An account is given of the laboratory procedure followed for the fabrication of the p-i-n silicon window structure.

The main objective of this research study is to develop an electronically controllable S-band microwave silicon window in order to overcome some of the limitations of junction-type diodes in microwave switching applications. In this report the facilities for carrying out S-band low and high power level tests on the microwave windows are described. The advantages of mounting the silicon window in a reduced height waveguide are indicated, and the details are given of the design of a S-band microwave transformer in order to match the reduced height waveguide to the standard height waveguide used in the microwave test set up. A description is given of the way the silicon window is mounted in the reduced height waveguide. The solution to the problem of determining the distribution of excess carriers is given and the results obtained are compared with the ones obtained in a previous report.

The main objective of this research study is to develop an electronically controllable S-band microwave silicon window in order to overcome some of the limitations of junction-type diodes in microwave switching applications. The initial problem has been the study of the microwave transmission properties of S-band silicon windows. The results are given of the study carried out for determining the insertion losses and the absorption coefficients of silicon windows as a function of the silicon resistivity. Some initial considerations in the design of the silicon window are presented. An account is given of the experimental program which has been started for the purpose of fabricating the necessary metallic overlays on the surface of the silicon window. (Author)