S. Battiston

Synthesis and Characterization of Al-Doped Mg2Si Thermoelectric Materials

Magnesium silicide (Mg2Si)-based alloys are promising candidates for thermoelectric (TE) energy conversion for the middle to high range of temperature. These materials are very attractive for TE research because of the abundance of their constituent elements in the Earth’s crust. Mg2Si could replace lead-based TE materials, due to its low cost, nontoxicity, and low density. In this work, the role of aluminum doping (Mg2Si:Alxa0=xa01:x for xxa0=xa00.005, 0.01, 0.02, and 0.04 molar ratio) in dense Mg2Si materials was investigated. The synthesis process was performed by planetary milling under inert atmosphere starting from commercial Mg2Si pieces and Al powder. After ball milling, the samples were sintered by means of spark plasma sintering to density >95%. The morphology, composition, and crystal structure of the samples were characterized by field-emission scanning electron microscopy, energy-dispersive spectroscopy, and x-ray diffraction analyses. Moreover, Seebeck coefficient analyses, as well as electrical and thermal conductivity measurements were performed for all samples up to 600°C. The resultant estimated ZT values are comparable to those reported in the literature for these materials. In particular, the maximum ZT achieved was 0.50 for the xxa0=xa00.01 Al-doped sample at 600°C.

Spark plasma sintering and thermoelectric evaluation of nanocrystalline magnesium silicide (Mg2Si)

Recently magnesium silicide (Mg2Si) has received great interest from thermoelectric (TE) society because of its non-toxicity, environmental friendliness, comparatively high abundance, and low production material cost as compared to other TE systems. It also exhibited promising transport properties, including high electrical conductivity and low thermal conductivity, which improved the overall TE performance (ZT). In this work, Mg2Si powder was obtained through high energy ball milling under inert atmosphere, starting from commercial magnesium silicide pieces (99.99xa0%, Alfa Aesar). To maintain fine microstructure of the powder, spark plasma sintering (SPS) process has been used for consolidation. The Mg2Si powder was filled in a graphite die to perform SPS and the influence of process parameters as temperature, heating rate, holding time and applied pressure on the microstructure, and densification of compacts were studied in detail. The aim of this study is to optimize SPS consolidation parameters for Mg2Si powder to achieve high density of compacts while maintaining the nanostructure. X-Ray diffraction (XRD) was utilized to investigate the crystalline phase of compacted samples and scanning and transmission electron microscopy (SEM & TEM) coupled with Energy-Dispersive X-ray Analysis (EDX) was used to evaluate the detailed microstructural and chemical composition, respectively. All sintered samples showed compaction density up to 98xa0%. Temperature dependent TE characteristics of SPS compacted Mg2Si as thermal conductivity, electrical resistivity, and Seebeck coefficient were measured over the temperature range of RT 600xa0°C for samples processed at 750xa0°C, reaching a final ZT of 0.14 at 600xa0°C.

Test Rig for High-Temperature Thermopower and Electrical Conductivity Measurements

A high-temperature test rig to simultaneously measure electrical conductivity and thermopower is described. The apparatus allows to perform measurements in a controlled atmosphere or vacuum to protect oxygen-sensitive materials. A spring-loaded mounting placed in the cold zone reduces the thermal contact resistance between the sample and two metallic blocks (the hot side and the heat sink) even at high temperatures. The hot-side metal block is periodically heated to obtain the thermopower from the slope of ΔV versus ΔT. Conductivity is measured before each thermopower measurement by a linear four-wire method. The automatic data acquisition and analysis are controlled by a LabView-based interface. Two interchangeable setups are possible. The first one uses silver blocks and K-type thermocouples and is suitable for temperatures from 300xa0K to about 1000xa0K. The second one uses W blocks and S-type thermocouples to allow higher-temperature measurements since all the hot-zone parts are made of Al2O3, Pt or W. The device was tested using PdAg alloy and Ni rods and, for the low-temperature range, the NIST standard reference material 3451 (bismuth telluride), strictly confirming the reference data.

Phase Content Influence on Thermoelectric Properties of Manganese Silicide-Based Materials for Middle-High Temperatures

The higher manganese silicides (HMS), represented by MnSix (xxa0=xa01.71 to 1.75), are promising p-type leg candidates for thermoelectric energy harvesting systems in the middle-high temperature range. They are very attractive as they could replace lead-based compounds due to their nontoxicity, low-cost starting materials, and high thermal and chemical stability. Dense pellets were obtained through direct reaction between Mn and Si powders during the spark plasma sintering process. The tetragonal HMS and cubic MnSi phase amounts and the functional properties of the material such as the Seebeck coefficient and electrical and thermal conductivity were evaluated as a function of the SPS processing conditions. The morphology, composition, and crystal structure of the samples were characterized by scanning electron microscopy, energy-dispersive x-ray spectroscopy, and x-ray diffraction analyses, respectively. Differential scanning calorimetry and thermogravimetric analysis were performed to evaluate the thermal stability of the final sintered material. A ZT value of 0.34 was obtained at 600°C for the sample sintered at 900°C and 90xa0MPa with 5xa0min holding time.

Effect of Synthesis and Sintering Conditions on the Thermoelectric Properties of n-Doped Mg2Si

Magnesium silicide (Mg2Si)-based alloys are promising candidates for thermoelectric (TE) energy conversion in the middle–high temperature range. The detrimental effect of the presence of MgO on the TE properties of Mg2Si based materials is widely known. For this reason, the conditions used for synthesis and sintering were optimized to limit oxygen contamination. The effect of Bi doping on the TE performance of dense Mg2Si materials was also investigated. Synthesis was performed by ball milling in an inert atmosphere starting from commercial Mg2Si powder and Bi powder. The samples were consolidated, by spark plasma sintering, to a density >95%. The morphology, and the composition and crystal structure of samples were characterized by field-emission scanning electronic microscopy and x-ray diffraction, respectively. Moreover, determination of Seebeck coefficients and measurement of electrical and thermal conductivity were performed for all the samples. Mg2Si with 0.1 mol% Bi doping had a ZT value of 0.81, indicative of the potential of this method for fabrication of n-type bulk material with good TE performance.

Introduction of Metal Oxides into Mg2Si Thermoelectric Materials by Spark Plasma Sintering

Oxide incorporation into thermoelectric Mg2Si-based materials was performed starting from commercial Mg2Si and commercial metal oxides by applying ball milling and spark plasma sintering (SPS) processing. The SPS conditions, such as sintering temperature, pressure, and holding time, were optimized with the aim of obtaining both full densification and oxide incorporation. Thermoelectric characterizations, such as Seebeck coefficient and electrical and thermal conductivity, were carried out and related to the pellet compositions. The morphology, composition, and crystallographic structure of the samples were characterized by field-emission scanning electron microscopy, energy-dispersive spectrometry, and x-ray diffraction analyses, respectively.

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.