S. Vaucher

Structural changes of silicon upon high-energy milling investigated by Raman spectroscopy

This study showed pronounced changes in the Raman scattering of silicon powder during high-energy ball milling. The powders were milled for 1–18 h in a steel ball mill in argon. The approximate pressure imposed on particles was 2 GPa. The spectra of the as-milled powders were compared with the initial silicon. It was found from the Raman peak position shifts that milling generated strains in the silicon lattice, bringing about a transformation of cubic silicon to tetragonal silicon and amorphization. The relative amount of new phases was determined from the area under the measured Raman peaks. (Some figures in this article are in colour only in the electronic version)

Applicability Study of Classical and Contemporary Models for Effective Complex Permittivity of Metal Powders

Abstract Microwave thermal processing of metal powders has recently been a topic of a substantial interest; however, experimental data on the physical properties of mixtures involving metal particles are often unavailable. In this paper, we perform a systematic analysis of classical and contemporary models of complex permittivity of mixtures and discuss the use of these models for determining effective permittivity of dielectric matrices with metal inclusions. Results from various mixture and core-shell mixture models are compared to experimental data for a titanium/stearic acid mixture and a boron nitride/graphite mixture (both obtained through the original measurements), and for a tungsten/Teflon mixture (from literature). We find that for certain experiments, the average error in determining the effective complex permittivity using Lichtenecker’s, Maxwell Garnett’s, Bruggeman’s, Buchelnikov’s, and Ignatenko’s models is about 10%. This suggests that, for multiphysics computer models describing the processing of metal powder in the full temperature range, input data on effective complex permittivity obtained from direct measurement has, up to now, no substitute.

Mechanism and kinetics of the reduction of magnetite to iron during heating in a microwave E-field maximum

The time-resolved x-ray diffraction method was applied in situ to investigate the carbothermal reduction of magnetite by carbon black in a 2.45 GHz microwave field. The kinetics of the phase transformation sequence is analyzed within the Kolmogorov–Johnson–Mehl–Avrami formalism. The first reduction stage is the solid-state transition of magnetite to wustite, followed by the partial conversion of nearly stoichiometric wustite to iron. The kinetics of the initial reduction of Fe3O4 to primary wustite is phase boundary controlled. Behind the reaction front, primary wustite rapidly evolves toward stoichiometric FeO. Cation vacancies and the dynamic clustering of structural defects influence the kinetics of the reduction of the nearly stoichiometric secondary wustite to porous iron.

Phase selectivity of microwave heating evidenced by Raman spectroscopy

Phase-selective temperature evolution of silicon-diamond powder mixtures under 2.45GHz microwave irradiation is followed using a Raman microscope fitted onto a microwave cavity. This spectroscopic method is shown to enable the collection of thermal information for both the silicon and the diamond phase selectively. An initial difference up to 100°C is found between the thermal trajectories extracted from silicon and diamond. A thermal equilibration is observed, subsequently suggesting that heat is flowing from hot silicon towards the colder diamonds. The existence of microscopic thermal gradients reflects the specific interaction of each of the two components with the applied electromagnetic field, governed here by the free carrier concentration. In addition to phase-specific temperature measurements, monitoring of chemical changes during microwave heating is also illustrated for magnetite powder. Raman spectroscopy turns out to be a powerful contact-free, noninterfering tool to follow and analyze on-line...

The Effects of Different Architectures on Thermal Fatigue in Particle Reinforced MMC for Heat Sink Applications

Particle reinforced metal matrix composites are developed for heat sink applications. For power electronic devices like IGBT modules (Insulated Gate Bipolar Transistor) a baseplate material with high thermal conductivity combined with a low coefficient of thermal expansion is needed. Commonly AlSiC MMC are used with a high volume content of SiC particles (~ 70 vol.%). To improve the performance of these electronic modules particle reinforced materials with a higher thermal conductivity are developed for an advanced thermal management. For this purpose highly conducting diamond particles (TC ~ 1000 W/mK) are embedded in an Al matrix. These new diamond reinforced MMC were investigated concerning their thermal fatigue mechanisms compared to the common AlSiC MMC. Differences in reinforcement architecture and their effects on thermal fatigue damage were studied by in situ synchrotron tomography during thermal cycling.

Nanocrystallization of amorphous alloys using microwaves: in situ time-resolved synchrotron radiation studies

Important energy and time savings can be achieved with the thermal treatment of materials by replacing conventional heating methods with microwave heating. The nano- crystallization of Co-Fe-W-B amorphous alloy powders under microwave irradiation was followed for the first time by in situ time-resolved synchrotron radiation powder diffraction. It is shown that even a very short exposure to the electromagnetic field (single pulse microwave application) typically of the order of a few seconds is sufficient to obtain the bulk nano- crystalline state. A metastable high-temperature Co-W-B orthorhombic phase forms during the microwave heating, which gradually transforms to the tetragonal Co2B stable phase.

Temperature Measurement by Microwave Radiometry: Application to Microwave Sintering

Temperature is a key parameter in industrial manufacturing, and its control is very often directly related to the quality of the products. Microwave-assisted processing has gained worldwide acceptance in powder technologies, in particular for the sintering of ceramic parts. High-energy efficiency, fast heating rate, and new and improved properties of the materials are typically observed. For example, fully dense bodies could be produced with improved mechanical properties due to the finer grain size. In fast-processing conditions, the system is mostly out of thermal equilibrium. A complex temperature-distribution pattern develops inside the heated parts, which can lead to localized melting or detrimental distortions if it is not under control. Today, none of the available thermometric methods (thermocouples, optical fiber, infrared, etc.) gives access to this volumetric information. We propose the use of microwave radiometry to noninvasively measure and control the temperature during the microwave sintering processes.

Temperature Measurement by Microwave Radiometry

Temperature is an important parameter in the industrial world. For example, temperature control is of a greater concern in food processing industry for safety reasons as well as for product quality. Recently, microwave processing of powder metallurgical bodies has been shown to be very promising. Fully dense bodies with improved mechanical properties could be produced due to finer grain size. In the field of the sintering of materials, a control of the temperature inside heated parts is necessary to avoid local melting or distortions. The medical field is another sector particularly interested in non invasive techniques for the measurement of human temperature. In fact, the corporal temperature is proved to be a pertinent parameter in the scope of diagnosis, monitoring of many pathologies and for the posology of some medicines. None of the current methods for measuring temperature (thermocouples, optic fibers, infrared, etc...) give by a non-invasive way continuous temperature information. So, microwave radiometry brings a neat solution to measure and control non-invasively temperature inside a dissipative material. This paper is concerning the design and the realization of specific radiometric sensors in order to measure and control temperatures from a non-invasive way by microwave radiometry either in industrial or in medical applications.

Rapid synthesis and densification of single-phase Al–Cu–Fe quasicrystals by spark plasma sintering or microwave heating

Quasicrystalline (QC) phases are often stable only within narrow composition domains. For this reason, the synthesis of larger amounts of single-phase quasicrystalline powders is difficult. Powder metallurgical approaches, based on mechanical milling followed by conventional heating, have been explored in the recent past. The manufacturing process for single-phase quasicrystals – either in the form of powders or as bulk parts – can be accelerated by orders of magnitude using rapid heating methods that involve pulsed electric currents and/or high-frequency electromagnetic fields. Prior knowledge of the phase transformation sequence and transformation kinetics, as revealed by in situ time-resolved synchrotron radiation experiments, is crucial in obtaining single-phase quasicrystals. We report on the simultaneous synthesis and densification of bulk single-phase Al–Cu–Fe QCs by spark plasma sintering (SPS) within minutes and on the ultrafast synthesis of single-phase Al–Cu–Fe quasicrystalline powders by microwave heating within seconds. The effect of electric current application in the rapid processing of pre-alloyed powders is discussed in relation to the faster diffusion and enhanced phase transformation kinetics.

Microwave energy absorption driven by dynamic structural and magnetization states in Fe85B15 metallic glass ribbons

The kinetics of the microwave crystallization of Fe85B15 metallic glasses was investigated in situ using the time-resolved x-ray diffraction method. It is shown that the recorded thermal profile during the microwave exposure of the ribbons bears a close relationship with the dynamic magnetization state during the decomposition of the amorphous phase into nanocrystalline α-Fe and Fe3B phases.