Properties Of Higher Manganese Silicides Research Articles

Enhancing thermoelectric properties of higher manganese silicide through facilitated non-equilibrium reactions via the introduction of additional Carbon nano-dots

Higher manganese silicide (HMS) has gained significant attention as a compelling choice for thermoelectric (TE) applications at medium temperatures due to its abundance, environmental friendliness, and impact resistance properties. Recent achievement in HMS involves leveraging the synergistic effect of energy filtering, modulation doping, and boundary scattering to enhance the TE performance. However, excellent TE properties on HMS usually demands extreme preparation processes such as shock compression, melt spinning technique, or chemical vapor transport approach, as well as the use of rare and expensive elements or nano-dots (NDs), such as Re, Ge, Ag/Pt alloy, or CsPbBr3 quantum dots. In this study, a facile wet ball milling combing with spark plasma sintering is used to streamline the synthesis processes. Relatively green, cost-effective, and accessible C NDs are used to facilitated non-equilibrium reactions and enhance TE properties of HMS. Due to the C-doped effect, the PF increased from 1.42 to 1.62 mW m−1 K−2 at 723 K, representing an approximately 14 % improvement compared to the pristine HMS. The introduction of C, derived SiC and Mn NDs result in the enhancing phonon scattering, and leading to a reduction in lattice thermal conductivity from 2.21 to 1.81 W m−1 K−1. Consequently, the figure of merit (zT) increases from 0.42 to 0.55 at 823 K, representing an enhancement of approximately ∼ 31 %. Predictably, the incorporation of C NDs into semiconductors stands as one of the effective strategies for enhancing the TE properties.

Anisotropic correlations in higher manganese silicides

Anisotropic correlations are vital for understanding the thermoelectric transport properties of higher manganese silicides (HMSs). In this work, we establish the implications of anisotropy on thermoelectric performance by examining the orientational variants in melt-grown single crystal and directional configurations of spark plasma sintered polycrystalline specimens of HMS. Single crystal specimen aligned perpendicular to the c-axis (i.e., along the cleavage ab-plane) exhibited high electrical and thermal conductivity, while the maximum Seebeck coefficient was measured along the c-axis direction. Similarly, polycrystalline specimens exhibited a weakened anisotropy with an enhanced weighted mobility and reduced thermal conductivity than their monocrystal counterpart resulting in a relatively higher thermoelectric figure-of-merit (zT). The characteristic MnSi striations were found to be predominantly grown perpendicular to the c-axis in melt-grown single crystals while peritectic decomposition during sintering is commonly observed in sintered polycrystals thereby emanating an inherently disordered and modulated lattice structure in the synthesized HMS crystals. It was observed that the zT for single and polycrystalline HMSs could be erroneously estimated if charge and phonon transport properties are measured in different directions. This study offers newer insights into the structure-property relationships of HMSs and demonstrates the need for more careful considerations in addressing the anisotropic behavior and microstructural evolutions that prevails in HMS crystals.

Thermoelectric properties of higher manganese silicide consolidated by flash spark plasma sintering technique

We investigated thermoelectric properties of higher manganese silicide obtained by the simultaneous use of ball milling and flash spark plasma sintering (FSPS) technique, and the results were compared those of the samples prepared with conventional spark plasma sintering (SPS). Despite the very short sintering process less than 50 s, FSPS samples reproduced well the crystallographic and thermoelectric properties of SPS sample made with long sintering time with more than 3500 s. Notably, the grain growth was significantly suppressed in the FSPS samples, although it has had minor effects on the transport properties.

Thermoelectric Properties of Higher Manganese Silicide Consolidated by Flash-Sintering Technique

Thermoelectric Properties of Higher Manganese Silicide Consolidated by Flash-Sintering Technique

Thermoelectric Properties of Higher Manganese Silicide Consolidated by Flash-Sintering Technique

Thermoelectric Properties of Higher Manganese Silicide Consolidated by Flash-Sintering Technique

Incorporating element doping and quantum dot embedding effects to enhance the thermoelectric properties of higher manganese silicides

Element doping and nano-inclusion embedding are effective approaches to enhance the electrical conductivities and decrease the lattice thermal conductivities of thermoelectric (TE) materials, respectively. However, the intrinsic low electrical thermal conductivities and high electrical properties are severely sacrificed, and the final figure of merit (ZT) is usually restricted. In this study, Ag doping and Pt quantum dot (QD) embedding were synchronously achieved via embedding Ag/Pt alloy QDs into the higher manganese silicides to avoid the conventional single-element doping strategy. The power factor (at 823 K) was enhanced from 1.57 × 10−3 W m−1 K−2 to 1.82 × 10−3 W m−1 K−2 (∼16%) due to the ∼18% increase in carrier concentration that was derived from the Ag doping effect. Simultaneously, the lattice thermal conductivity (at 823 K) decreased from 2.65 W m−1 K−1–1.92 W m−1 K−1 (∼28%) because of the broadband phonon scattering effect that resulted from the residual Pt QDs inclusions. Synthetically, the optimal ZT value increased by ∼52% from 0.42 to 0.64 at 823 K. This study demonstrated that incorporating metastable alloy QDs to obtain element doping and nano-inclusion embedding effects is a novel and feasible means to enhance the ZT value of HMS. This method is also possibly applicable to other alloy QD/TE composites.

Structure and thermoelectric properties of higher manganese silicides synthesized by pack cementation

Higher manganese silicides (HMS) are promising alternative materials for middle to high temperature thermoelectric applications as a low-cost, non-toxic and highly stable p-type leg. Many of the preparation methods that have been reported previously require long-time and energy consuming processes, as well as expensive equipment, and often do not result in a material of sufficient quality. In this study, the simple, cost-effective and eco-friendly technique of pack cementation is applied. HMS powders synthesized at different experimental conditions are studied and compared considering their structure, composition, short-term thermal stability in air and thermoelectric properties. X-ray diffraction analysis, X-ray photoelectron spectroscopy, scanning electron microscopy, thermogravimetry and thermoelectric measurements (in terms of Seebeck coefficient, electrical and thermal conductivity) were employed for the characterization of the material and evaluation of its performance. All samples were identified as HMS and only some negligible traces of MnSi were detected. They moderately oxidize when heated non-isothermally under air atmosphere up to 1473 K, while the presence of HMS remains dominant even at such high temperatures. Their thermoelectric properties were remarkable for an undoped material, with a maximum figure of merit (ZT) of 0.47 at 777 K. Pack cementation appeared to have a great potential as the synthesis route of high-efficiency HMS.

Microstructure and thermoelectric properties of higher manganese silicides fabricated via gas atomization and spark plasma sintering

Higher manganese silicides (HMS) powders were successfully prepared by a gas atomization technique. The gas atomized (GAed) powders showed spherical morphologies and a wide particle size distribution. No MnSi and Si phases were detected by XRD analyses, indicating that the high cooling rate during rapid solidification is effective in inhibiting the formation of second phases. When the GAed powders were consolidated by spark plasma sintering (SPS), the densified samples revealed almost the same phase composition as the powders and an isotropic feature. The contrast differences between different grains in backscattered electron images are believed to be associated with orientation differences. The HMS samples SPSed at different temperatures exhibited similar values of the Seebeck coefficient, but larger electrical conductivity and power factor were obtained in the sample SPSed at 1000 °C. Moreover, the HMS samples showed higher fracture toughness compared to those fabricated by a solid-state reaction and SPS process.

Synchronized enhancement of thermoelectric properties of higher manganese silicide by introducing Fe and Co nanoparticles

Abstract Introduction of nanophases is known to be effective in improving thermoelectric performance as it allows the simultaneous engineering of electronic and thermal transports. In this study, we synthesized Fe and Co nanoparticle-embedded MnSi1.787Al0.01 nanocomposites through a simple nanometal-decoration technique and spark plasma sintering. The nanoparticles introduced in the matrix caused energy band bending at the matrix–nanoparticle interface, which induced charge transfer and energy filtering effects. These two seemingly opposing effects were combined so as to increase the power factor of the nanocomposites by enhancing the electronic transport. Moreover, we found that the lattice thermal conductivity decreased owing to intensified phonon scattering. Hence, a maximum ZT of 0.53 (at 773 K) was achieved in 0.6 vol% Fe nanoparticle-embedded nanocomposites, which is 25% higher than that of the pristine sample.

Enhancing Thermoelectric Properties of Higher Manganese Silicide (HMS) by Partial Ta Substitution

In this work, the thermoelectric properties of the p-type higher manganese silicide (HMS) were enhanced by partially substituting Mn with Ta. The ingots of the compound Mn36.4−xTaxSi63.6, where x = 0.00, 0.25, 0.50. 0.75 and 1.00, were synthesized via arc melting which were then cast into a ribbon shape by a melt spinning process. After that, the crushed ribbons were consolidated to form a bulk sample by spark plasma sintering. The x-ray diffraction analysis showed that the single phase of HMS existed for x up to 0.50. Above that, the evidence of secondary phases was found, confirmed by scanning electron microscope imaging with elemental mapping. For thermoelectric properties measurement, the Seebeck coefficient and electrical conductivity were insignificantly different in the pure-phase samples. On the other hand, the samples with secondary phases showed increased electrical conductivities but slightly decreased Seebeck coefficients. The thermal conductivity was suppressed in all Ta-substituted samples. The lowest lattice thermal conductivity was found in the sample with the impurity phase (TaSi2) due to the enhanced phonon scattering. Consequently, the ZT of the Ta-substituted HMS was enhanced with the peak ZT of 0.37 at 813 K, which is about 28% higher than that of the pristine HMS.

Preparation and optical properties of higher manganese silicide, (Mn,Fe)Siγ, thin films

In this article, the optical properties of thin films of higher manganese silicide (HMS) systems, MnSiγ and (Mn,Fe)Siγ, were investigated. Band structure calculations were performed using the Mn11Si19 and (Mn31/44Fe13/44)11Si19 crystal structure models of HMS to predict the conduction types and band gaps of MnSiγ and Mn0.7Fe0.3Siγ, respectively. Using a pulsed laser deposition method, p-type MnSiγ and n-type Mn0.7Fe0.3Siγ thin films with a-axis orientation were grown on R-sapphire substrates. The measured direct band gaps were 0.81(1) eV for the MnSiγ thin film and 0.83(2) eV for the Mn0.7Fe0.3Siγ thin film. These results demonstrate the potential of (Mn,Fe)Siγ-based near-infrared absorption solar cells.

Failure analysis and mechanical reliability of cast higher manganese silicide

This work focuses on the correlation between microstructure and mechanical properties of higher manganese silicide (HMS). A single crystal HMS ingot was cast from elemental Mn and Si with a nominal composition of Mn15Si26. The microstructure of the ingot was characterized, and its fracture strength was evaluated following ASTM C1161 bending test. The correlations between elastic modulus, fracture strain, fracture stress and toughness were studied and Weibull distribution analysis was applied to fracture strength data. Statistical results show a wide distribution in the mechanical prosperities. Fractography reveals the profound effect of the minority MnSi phase on the failure mechanism of HMS. The negative effect of the minority second phase MnSi particles on the mechanical reliability of HMS is highlighted.

Improved microstructure and thermoelectric properties of higher manganese silicide processed by reactive spark plasma sintering

In this paper, the crystal structure, microstructure, thermoelectric properties and figure of merit (zT) of highly pure higher manganese silicide (HMS) alloys are reported and discussed without the bias generally introduced by impurities in published results. The alloys were produced by both solid-state reaction diffusion assisted by spark plasma sintering and conventional arc melting in order to evaluate the effect of the process on the microstructure and on the resulting properties of HMS. The effect of Ge addition is also explored. Properties diagram for thermoelectric materials is displayed to assess the performance of un-doped and Ge-doped HMS alloys in comparison with the state of the art. Electrical conductivity and zT at 500 °C of the HMS alloys studied here exceed published properties achieved with similar alloys, providing new process options for reliable, affordable and efficient thermoelectric applications.

Influence of Al and Mg Addition on Thermoelectric Properties of Higher Manganese Silicides Obtained by Reactive Sintering

Higher manganese silicides (HMS), represented by MnSix (x = 1.71-1.75), are promising p-type candidates for thermoelectric (TE) energy harvesting systems at intermediate temperature range. The mate ...

Phase Formation and Thermoelectric Properties of Doped Higher Manganese Silicides (Mn15Si26)

We herein report substitutional doping effects on the electronic and thermal transport properties of higher manganese silicides (HMS) Mn15Si26. Polycrystalline bulks of Mn0.972A0.028Si1.80 and MnSi1.75B0.028 (A = V, Cr, Mo/B = Al, Ge) were fabricated by a solid-state reaction combined with the spark plasma sintering technique, and their thermoelectric properties were evaluated. We found that thermoelectric performance of Mn15Si26 was significantly enhanced due to the simultaneous improvement in electronic transport and phonon scattering via partial substitution of foreign atoms at Mn- and/or Si-sites. Through the small amount of Cr doping at the Mn-site and Al and Ge doping at the Si-site, the power factor was improved due to enhancement in density of the state's effective mass. Thermal transport properties could be also manipulated due to the point defect phonon scattering effect, and reduced lattice thermal conductivity was obtained with Ge-doped HMS. As a consequence, the maximum dimensionless figure␣of merit ZT of 0.64 at 773 K (increased 50% compared to undoped Mn15Si26) was obtained in Ge-doped Mn15Si26.

Crystal Structure and Thermoelectric Properties of Lightly Vanadium-Substituted Higher Manganese Silicides (Mn1−x V x )Si γ )

To further enhance the thermoelectric (TE) properties of higher manganese silicides (HMSs), dissipation of layered precipitates of MnSi phase as well as optimization of hole carrier concentration are critical. We have prepared a lightly vanadium-substituted solid solution of HMS, (Mn1−xVx)Siγ, by a melt growth method. A 2% substitution of manganese with vanadium is found to dissipate MnSi precipitates effectively, resulting in a substantial increase in the electrical conductivity from 280 S/cm to 706 S/cm at 800 K. The resulting TE power factor reaches 2.4 mW/K2-m at 800 K, more than twice that of the V-free sample. The total thermal conductivity did not change significantly with increasing x owing to a reduction of the lattice contribution. As a consequence, the dimensionless figure of merit zT of the melt-grown samples increased from 0.26 ± 0.01 for x = 0 to 0.59 ± 0.01 for x = 0.02 at around 800 K.

Density functional theory study of the electronic structure and the thermoelectric properties of strained Mn4Si7

The strain effect on electronic structure and thermoelectric properties of Higher Manganese Silicides (HMSs) Mn4Si7 was studied using Density Functional Theory (DFT) and through solving Boltzman Transport Equation (BTE). We found that the tensile strain attempts to reduce the band gap while the compressive strain not much affect to band gap. The Seebeck coeficient was found to be increased with increasing temperature, which is very consistent to experiments. The electrical conductivity and power factor show highly degree of anisotropy, where in-plane direction is more dominant. The different behavior of electrical conductivity along in-plane and outof plane direction was explained due to the change of band dispersion in the valence band maximum (VBM).

Enhanced Thermoelectric Properties of W- and Fe-Substituted MnSi γ

We have investigated the effect of heavy-element (W) substitution on the thermoelectric properties of higher manganese silicide (HMS). Samples were prepared by arc melting followed by liquid quenching, where the latter assisted in achieving higher solubility for tungsten. We observed that Mn34.6W1.8Si63.6 was a p-type material, whereas simultaneous substitution of 12 at.% Fe made the higher manganese silicide an n-type material. The optimal carrier concentration was obtained by simultaneous substitution of Fe and W for Mn atoms. Although the samples were metastable, we successfully obtained bulk samples by a low-temperature (970 K), high-pressure (>100 MPa), long-duration sintering process. The lattice thermal conductivity was effectively reduced by W substitution, and the ZT value was improved to above 0.5 for both n- and p-type samples.

Cobalt and Iron Doping Effects on Thermoelectric Properties of Higher Manganese Silicides Prepared by Mechanical Milling and Pulse Discharge Sintering

Cobalt and Iron Doping Effects on Thermoelectric Properties of Higher Manganese Silicides Prepared by Mechanical Milling and Pulse Discharge Sintering

Effects of ball milling on microstructures and thermoelectric properties of higher manganese silicides

Bulk nanostructured higher manganese silicide (HMS) samples with different grain size are prepared by melting, subsequent ball milling (BM), and followed by spark plasma sintering (SPS). The effects of BM time on the microstructures and thermoelectric properties of these samples are investigated. It is found that BM effectively reduces the grain size to about 90nm in the sample after SPS, which leads to a decrease in both the thermal conductivity and electrical conductivity. By prolonging the BM time, MnSi and tungsten/carbon-rich impurity phases are formed due to the impact-induced decomposition of HMS and contamination from the tungsten carbide jar and balls during the BM, respectively. These impurities result in a reduced Seebeck coefficient and increased thermal conductivity above room temperature. The measured size-dependent lattice thermal conductivities agree qualitatively with the reported calculation results based on a combined phonon and diffuson model. The size effects are found to be increasingly significant as temperature decreases. Because of the formation of the impurity phases and a relatively large grain size, the ZT values are not improved in the ball-milled HMS samples. These findings suggest the need of alternative approaches for the synthesis of pure HMS with further reduced grain size and controlled impurity doping in order to enhance the thermoelectric figure-of-merit of HMS via nanostructuring.