Properties Of Higher Manganese Silicides Research Articles

Effects of (Al,Ge) double doping on the thermoelectric properties of higher manganese silicides

Experiments and analysis have been carried out to investigate the effects of Al and (Al,Ge) doping on the microstructure and thermoelectric properties of polycrystalline higher manganese silicide (HMS) samples, which were prepared by solid-state reaction, ball milling, and followed by spark plasma sintering. It has been found that Al doping effectively increases the hole concentration, which leads to an increase in the electrical conductivity and power factor. By introducing the second dopant Ge into Al-doped HMS, the electrical conductivity is increased, and the Seebeck coefficient is decreased as a result of further increased hole concentration. The peak power factor is found to occur at a hole concentration between 1.8 × 1021 and 2.2 × 1021 cm−3 measured at room temperature. The (Al,Ge)-doped HMS samples show lower power factors owing to their higher hole concentrations. The mobility of Mn(Al0.0035GeySi0.9965-y)1.8 with y = 0.035 varies approximately as T−3/2 above 200 K, suggesting acoustic phonon sca...

Thermoelectric Properties of Higher Manganese Silicides Prepared by Mechanical Alloying and Hot Pressing

Higher manganese silicides (HMS), MnSi1.75–δ, were synthesized by mechanical alloying and consolidated by hot pressing. The optimum condition of mechanical alloying was ball milling at 400 rpm for 6 h, and sound sintered compacts could be obtained by hot pressing at temperature higher than 1073 K. The phase fraction of HMS showed no significant difference with compositional (δ) variation, but the MnSi1.75 specimen had the lowest fraction of MnSi of approximately 3%. The lattice constants of HMS with compositional variation were similar to values reported in the literature. All specimens showed Nowotny phase with tetragonal structure, and exhibited i-type conduction at measuring temperatures between 323 K and 823 K. HMS behaved as degenerate semiconductors in that the absolute values of the Seebeck coefficient increased and the electrical conductivity slightly decreased with increasing temperature. MnSi1.73 showed the highest figure of merit of 0.28 at 823 K.

Influence of Addition of Alumina Nanoparticles on Thermoelectric Properties of Higher Manganese Silicide

ABSTRACTHigher manganese silicide (HMS) is a low-cost and eco-friendly thermoelectric material available for recovering waste heat of 500 to 900 K. In this research, we tried to uniformly disperse the alumina nanoparticles (ANPs) in the HMS matrix to reduce the thermal conductivity and to improve the thermoelectric performance. Influence of addition of ANPs on the thermoelectric properties was investigated. It was confirmed that ANPs were uniformly dispersed in the HMS grain boundary. The lattice thermal conductivity was reduced by adding ANPs. As a result, the maximum thermoelectric performance of ZT=0.58 was achieved at about 800 K by adding 1 vol% of ANPs. The performance of ANPs-added HMS was improved about 25 %.

Effects of Si content on phase composition and thermoelectric properties of higher manganese silicide

MnSi1.70+x(x=0, 0.05, 0.1, 0.15) compounds have been prepared by induction melting-annealing procedure combined with spark plasma sintering method. The phase composition and the thermoelectric properties of higher manganese silicide (HMS) with different Si contents are investigated. The results indicate that the samples with x＜0.1 include HMS phase and MnSi phase, and the relative content of MnSi phase decreases with x value increasing. The sample with x=0.1 is single phase HMS. The phase compositions of the sample with x＞0.1 are HMS and Si. As x value inereases the electrical conductivity of the sample gradually decreases, while the Seebeck coefficient increases because metallic MnSi phase decreases. The carrier concentration and the effective mass of the sample at room temperature decrease and the carrier mobility increases with x value increasing. In the sample with x=0.1, the impurity phase content is the least, which results in the lowest thermal conductivity and a minimum value of 2.25 W ·m-1K-1 at 800 K. The maximum ZT value of 0.45 is obtained at 850 K for MnSi1.80.

Doping Effects on Thermoelectric Properties of Higher Manganese Silicides (HMSs, MnSi1.74) and Characterization of Thermoelectric Generating Module using p-Type (Al, Ge and Mo)-doped HMSs and n-Type Mg2Si0.4Sn0.6 Legs

The effect of Al Doping on the thermoelectric properties of higher manganese silicides, MnSi1.74 (HMSs) parallel and perpendicular to the c-axis was investigated. It was found that Al doping increased electrical conductivity and decreased thermoelectric power parallel to the c-axis, because Al doping of the Si site increased carrier density according to the valence control rule. Al doping was also effective for lowering thermal conductivity and raising the figure of merit parallel to the c-axis. After optimizing the Al content (y=0.0035), the thermoelectric properties of (Al, Ge and Mo)-doped HMSs parallel and perpendicular to the c-axis were evaluated and compared to those of nondoped and Al-doped HMSs. Two types of thermoelectric module consisting of n-type Mg2Si0.4Sn0.6 and p-type (Al, Ge and Mo)-doped HMSs legs parallel (or perpendicular) to the c-axis were fabricated, and their thermoelectric performances were compared to each other. The discrepancy between the measured maximum output power and the value estimated from the thermoelectric data of constituent materials was well explained by the resistance of a porous Al electrode. The measured heat flux of the module was also compared to the estimated heat flux. The maximum energy conversion efficiency of the module parallel to the c-axis was clearly higher than that perpendicular to the c-axis, because of a higher output power and a lower heat flux, and was higher than 7% at ΔT=520 K (303–823 K).

Preparation and properties of MnSi 1.7 on Si(001)

The most rich silicon silicides of manganese, the group of higher manganese silicides (HMS), have the composition MnSi x with x in the range from 1.67 to 1.75. This material group with a tetragonal crystal structure shows semiconducting electronic properties, with promising application in thermoelectric devices. This paper reports the structural and morphological properties of HMS prepared by a reactive deposition process on Si(001) under UHV conditions. X-ray diffraction shows, for all samples grown at substrate temperatures ranging from 400 to 750°C, the growth of HMS only. Scanning electron microscopy and Rutherford backscattering spectrometry show a transition from film growth to island growth with increasing substrate temperature. A detailed analysis of the XRD spectra shows a change of the texture of HMS at the transition of the sample morphology. The results are discussed on the basis of anisotropic growth rates for differently oriented grains of HMS.

Photoelectric properties of higher manganese silicide (HMS)-SiMn-M structures

Photoelectric properties of (HMS)-SiMn-M structures, where HMS is higher manganese silicide formed in diffusion doping of silicon with manganese and M is metal contact, have been studied in a wide range of temperatures and light wavelengths. After illumination of these structures with h = 1.12 eV light at T = 77 K is terminated, the photocurrent decays back to the dark current in a complicated manner, following a curve with two regions. The duration of decay is about several seconds and more than 105 s in the first and second regions, respectively. Deep infrared (IR) and temperature quenching of photoconductivity in these structures have been studied. Under illumination with h1.12 eV light and at low temperatures these structures can be considered HMS-i-p-M structures, with the i-region playing essential role in the generation of space-charge-limited currents (SCLC). Breakdown of the SCLC mode in these structures by ionization of manganese levels with extra IR light (h = 0.42-0.6 eV) or by heating results in a sharp decrease of the photocurrent flowing through the structures (especially in the case of temperature quenching in a narrow temperature range T = 15-20 K).