S. Fiameni

RFX-mod wall conditioning by lithium pellet injection

Plasma?wall interaction is one of the most important issues that present magnetic confinement devices have to face. In the RFX-mod reversed field pinch experiment plasma?wall interaction has become a hard point increasing plasma current up to the RFX-mod maximum design value of 2?MA, since in this case local power deposition can be as high as 10?MW?m?2. Since the first wall of RFX-mod is entirely covered by graphite tiles different techniques have been tested to control hydrogen wall influx: He glow discharges cleaning, He discharges at high plasma currents, wall boronization and baking. With the best results obtained by boronization, at high plasma currents all such techniques improve the situation but do not allow a complete and stationary hydrogen influx reduction. Furthermore, in the presence of localized high power load the wall still responds providing very high influxes. In order to improve this situation wall conditioning by lithium has been tested. As a first lithization method to deposit a controllable amount of lithium on the wall, a room temperature pellet injector has been used (maximum pellet diameter of 1.8?mm and maximum length of 5?mm). Lithium coatings with a theoretical thickness of about 10?nm have been applied both to clean graphite tiles and over boronized ones. Lithization demonstrated to be effective in lowering hydrogen wall recycling to a value smaller than that of boronized graphite, with the effect lasting 20?30% more than in the boronized case. Compared with boronization, lithization slightly improves (by about 30%) particle confinement time and also clearly affects edge particle transport providing a lower edge density and more peaked density profiles. Lithization also reduces carbon content by about 10% over boronization but still no clear improvement has been observed in terms of energy confinement. Similar results have been obtained performing lithization over boronized graphite.

Structural, compositional and functional properties of Sb-doped Mg2Si synthesized in Al2O3-crucibles

Magnesium silicide (Mg2Si) is a promising candidate for thermoelectric energy conversion due to its low toxicity, the abundance of its raw constituents and its low density, allowing manufacturing of light, sustainable and relatively cheap devices. Mg2Si needs to be doped in order to increase its efficiency, making this material competitive among materials operating in the intermediate temperature range. In this work, a synthesis procedure based on melting of the raw elements in easily available and cheap Al2O3 crucibles was developed to obtain polycrystalline Sb doped Mg2Si materials in a wide range of compositions. Powders from the crushed lumps were consolidated via spark plasma sintering and then thermally annealed to obtain dense pellets of Sb:Mg2Si with Sb = 0.0, 0.1, 0.3, 0.5, 0.7, 1.0 and 1.5 at%. The effects of Sb doping and of the synthesis and sintering technique on composition, morphology and stability of n-type Mg2Si are discussed. Transport properties (Seebeck coefficient, electrical and thermal conductivity, charge carrier density) were evaluated in order to elucidate the composition–property relationship within this material system and find the optimal doping amount to optimize its thermoelectric properties.

Silica-Based Materials for Thermoelectric-Legs Embedding

Sol-gel chemistry was used to synthesize low-density SiO2 aerogel for matrix embedding of silicide-based (Mg2Si and MnSi(2−x)) thermoelectric legs. In thermoelectric (TE) modules, the heat conduction in air and the convective and radiative contribution to the heat transfer play an important role in the reduction of the efficiency of the module. Silica aerogels are known for the lowest thermal conductivity of any non-evacuated solid. With this in mind, silica-based aerogel materials were employed to fill the void spaces between the thermoelectric legs of a module. In order to do this, different synthesis procedures were taken into account to produce suitable silica materials. It is important that the silica can be easily cast into place, avoiding mechanical cracks of the matrix. Silica aerogel typically requires a supercritical drying step to remove the pore fluid from the SiO2 gel, avoiding the collapse of the pores. This procedure is not practical for TE-legs embedding and it is dangerous, expensive, and time-consuming. It is known that replacing the –OH groups with organic hydrophobic substituents in the SiO2 pores prevents the pore-shrinkage and the sintering of the matrix during solvent evaporation step. This allows synthesizing relatively light materials at low temperature and ambient pressure, with no need of supercritical drying of the gel. The obtained aerogels were characterized by thermogravimetric analysis and differential scanning calorimetry to evaluate the stability of the material and the chemical modification with increasing temperature. The thermal expansion of the silica was evaluated by means of dilatometry. Finally, the thermal diffusivity was measured with the laser flash method.

Finite Element Approach for the Evaluation and Optimization of Silicide-Based TEG

Numerical modelling represents an effective tool for designing and evaluating the performances of thermoelectric power generators (TEG). In particular, the finite element (FE) method allows performing multiphysics simulation, that is coupling different physical phenomena, such as heat transfer, thermoelectric effects, and Joule heating. In this work, FE modeling is at first used to reproduce the results of the open circuit voltage and output power measurements on an undoped Mg2Si TE-chip under large temperature differences. Furthermore, the conversion efficiency of a 16-chip TEG module has been calculated with different ratios of the cross sections of the n-type (Bi-doped Mg2Si) and the p-type (higher manganese silicide, HMS) legs. In both analyses, the thermal and electrical conductivities and Seebeck coefficient are given, as input, in function of temperature. The effects of thermal and electrical contact resistances were taken into account, by introducing thin thermally/electrically resistive layers in the numerical model.