Narrow Results

MnSi_1.73 and MnSi samples were grown by spark plasma sintering (SPS) from different Si/Mn ratio powders, at different sintering temperatures, and for different sintering times. X-ray diffraction (XRD) measurements showed samples containing MnSi and MnSi_1.73 and Si phases, depending on the initial stoichiometries. Measurements of the Seebeck coefficient revealed p-type conductance for all samples. The Seebeck coefficients of the samples with MnSi pure phase were very low (about 10 μ V/K) and changed little at the temperature range measured. The Seebeck coefficients of the samples with MnSi_1.73, MnSi and Si phases were similar to that of the sample with near-pure MnSi_1.73 phase, which were larger than those of the samples with MnSi_1.73 and MnSi phases, but a little smaller than those of the samples with MnSi_1.73 and Si phases. It seems that, for the samples with the same phases, larger ratio of the strongest intensity peak of MnSi_1.73 to , MnSi_1.73 to or both lead to larger Seebeck coeffiecients.

N-type polycrystalline higher manganese silicide (MnSi_1.7) films are prepared on thermally oxidized silicon substrates by magnetron sputtering. MnSi_1.85, Si, and carbon targets are used in the experiments. By co-sputtering of the MnSi_1.85 and Si targets, n-type MnSi_1.7 films are directly obtained. By increasing the Si content to the deposited films, both the Seebeck coefficient and electrical resistivity increase to high values. A Si intermediate layer between the MnSi_1.7 film and substrate plays an important role on the electrical properties of the films. Without the interlayer, the Seebeck coefficient is not stable and the electrical resistivity is higher. For preparation of MnSi_1.7 films by solid phase reaction, a sandwich structure Si/MnSi_x/Si (x < 1.7) and thermal annealing are used. A carbon cap layer is used as a doping source. With the carbon doping, the electrical resistivity of the MnSi_1.7 film decreases, while the Seebeck coefficient increases slightly. For reactive deposition, the MnSi_x (x < 1.7) film is directly deposited on the heated substrate with a Si intermediate layer. By using a Si cap layer, a MnSi_1.7 film with a Seebeck coefficient of -292 μV/K and electrical resistivity of 23 × 10^-3 Ω-cm at room temperature is obtained. The power factor reaches 1636 μW/mK² at 483 K. With such a high power factor, the n-type MnSi_1.7 material may be superior to p-type MnSi_1.7 material for the development of thermoelectric generators. Several smaller (0.036 - 0.099 eV) and intermediate (0.10 - 0.28 eV) activation energies are observed from the curves of logarithm of the resistivity versus reciprocal temperature. The larger activation energies (0.35 - 1.1 eV) are consistent with the reported energy band gaps for higher manganese silicides.

MnSi_1.7 films with different thicknesses (16–242 nm) are prepared by magnetron sputtering and electron beam evaporation. When the MnSi_1.7 film thickness is about 40 nm or above, MnSi_1.7 films are p-type in the whole temperature range (300–700 K) in agreement with reports in literature. By co-sputtering of MnSi_1.85 and silicon targets or deposition of Si/Mn multi-layers with a larger thickness ratio, silicon is added to the films and the Seebeck coefficients transform from positive to negative with increasing temperature. The Seebeck coefficients at room temperature and 633 K are +0.098 mV/K and -0.358 mV/K, respectively. By reducing the MnSi_1.7 film thickness to 27 nm, the transition of Seebeck coefficient from positive to negative is also observed although silicon is not added intentionally. When an ultra-thin aluminum layer is deposited between MnSi_x(x < 1.7) and Si layers to enhance silicon diffusion, the p- to n-type transition temperature decreases about 100 K. The silicon-added MnSi_1.7 films usually have higher electrical resistivity.

Higher manganese silicide film (HMS, MnSi_x, x = 1.73–1.75) with addition of Si:B has been prepared on quartz substrate (SiO₂) by magnetron sputtering of MnSi₂ and Si:B (1 at.% B content) targets. It is found that the Si:B-added HMS film has a much lower electrical resistivity (R) but maintains its high Seebeck coefficient (S). As a result, the thermoelectric power factor, P_F = S²/R, is greatly enhanced. It is also found that the metal In together with Ag-paste can be used as ohmic contact materials for measuring the electrical properties of the HMS film. The thermoelectric power factor can reach 1255 μW/m-K² at 733 K for the Si:B-added HMS film, which is about two times higher than that of the pure HMS film.

The samples of Cu_0.9Mg_0.1R_yFe_2-yO₄, where y = 0.01 and R = Sm, Dy, Ho and Hf, were prepared by standard ceramic method. All investigated samples were sintered at 1150°C with a heating rate of 4°C/min and sintering time of 8 h. X-ray diffraction study of the compositions revealed the formation of cubic spinel structure with the appearance of small peaks indicating the presence of secondary phases. Seebeck coefficient was obtained from thermo-electromotive force (emf) measurements. The alternation of the Seebeck coefficient sign between (+)ve and (-)ve means that the two conduction mechanisms take place simultaneously. The dielectric parameters such as dielectric constant, quality factor were determined as a function of temperature and at different frequencies. The decrease in Fe³⁺ ions on the octahedral site decreased the polarization of the system, through the dielectric transition point.

In this paper, we report a large enhancement in the thermoelectric power factor in CrSi₂ film via Si:B (1 at.% B content) addition. The Si:B-enriched CrSi₂ films are prepared by co-sputtering CrSi₂ and heavily B-doped Si targets. Both X-ray diffraction patterns and Raman spectra confirm the formation of the crystalline phase CrSi₂. Raman spectra also indicate the crystallization of the added Si:B. With the addition of Si:B, the electrical resistivity $(R)$ decreases especially at low temperatures while the Seebeck coefficient $(S)$ increases above 533 K. As a result, the thermoelectric power factor, $\mathrm{\text{PF}}=S^2/R$, is greatly enhanced and can reach $716\times 10^{-6}\mathrm{\text{W/m}}\cdot {\mathrm{\text{K}}}^2$ at 583 K, which is much larger than that of the pure CrSi₂ film.

The FP-LAPW method is utilized to investigate the elastic, optoelectronic and thermoelectric properties of ${\mathrm{\text{XTiO}}}_3$$(\mathrm{\text{X}}=\mathrm{\text{Ca, Sr}}$ and $\mathrm{\text{Ba}})$ within the GGA. The calculated lattice constants and bulk modulus are found in agreement with previous studies. The present oxide–perovskite compounds are characterized as elastically stable and anisotropic. ${\mathrm{\text{CaTiO}}}_3$ and ${\mathrm{\text{SrTiO}}}_3$ are categorized as ductile compounds, whereas the ${\mathrm{\text{BaTiO}}}_3$ compound is in the critical region between ductile and brittle. The DOS and the band structure calculations reveal indirect $(\mathrm{\text{M}}$–$\mathrm{\Gamma })$ energy bandgap for the present compounds. The hydrostatic pressure increases the energy bandgap and the width of the valence band. The character of the band structure does not change due to this pressure. The optical parameters are calculated in different radiation regions. Beneficial optics applications are predicted as revealed from the optical spectra. The transport properties are applied as a function of the variable temperatures or carrier concentration. It is found that the compounds under study are classified as a p-type semiconductor. The majority charge carriers responsible for conduction in these calculated compounds are holes rather than electrons.

The carrier effective masses as well as thermoelectric and electronic properties of ternary chalcogenides compounds of the form Ag₃AuX₂(X = Se, Te) have been studied using first-principles method based on density functional theory. For the treatment of exchange-correlation energy, we have used the generalized gradient approximation (GGA) by Perdew, Burke and Ernzerhof (PBE-GGA) and Wu–Cohen (WU-GGA) schemes. Both the compounds (Ag₃AuSe₂ and Ag₃AuTe₂) are direct bandgap semiconductors. The p-type nature of these compounds is confirmed by the effective mass calculations. In thermoelectric measurements, different parameters (electrical conductivity, carrier concentration, Seebeck coefficient, thermal conductivity and thermoelectric power) are calculated. It is found that all these parameters increase with the increase in temperature for both the compounds. The obtained results for these compounds such as the direct bandgap nature and their high value of the thermoelectric power make them valuable candidates for different device applications.

Based on the electronic structure, the physical properties of ${\mathrm{\text{Ca}}}_{1-x}{\mathrm{\text{Yb}}}_x{\mathrm{\text{Zn}}}_2{\mathrm{\text{Sb}}}_2$ ($x=0$, 0.25, 0.5, 0.75, 1) Zintl compounds are studied. The transport properties can be significantly changed by varying the composition $x$. The materials under study are more metallic with increasing $x$ and behaves like a semiconductor when $x$ decreases. It is found that ${\mathrm{\text{CaZn}}}_2{\mathrm{\text{Sb}}}_2$ exhibits a larger thermopower magnitude ($S=241\mu \mathrm{\text{V/K}}$ at $T=700\mathrm{\text{K}})$ and the Seebeck coefficient decreases as $x$ increases. The calculated figure of merit factor of ${\mathrm{\text{YbZn}}}_2{\mathrm{\text{Sb}}}_2$ is found to be low, this is explained by the fact that its structure is very compact and its bandgap is small which lead to high electrical and thermal conductivity due to high carrier concentration ($n=1.25\cdot 10^{20}{\mathrm{\text{cm}}}^{-3}$ at $T=300\mathrm{\text{K}}$). On other hand a narrow-gap (0.46 eV for ${\mathrm{\text{CaZn}}}_2{\mathrm{\text{Sb}}}_2$), provides a balance between a high Seebeck coefficient and low electronic thermal conductivity, with a slight increase in the carrier concentration when the temperature increases ($3.87\cdot 10^{19}{\mathrm{\text{cm}}}^{-3}$ at 600 K). As a consequence, ${\mathrm{\text{CaZn}}}_2{\mathrm{\text{Sb}}}_2$ compound is predicted to have good performance for thermoelectric applications. The electrical $(\sigma )$ and the thermal $(K)$ conductivity for ${\mathrm{\text{CaZn}}}_2{\mathrm{\text{Sb}}}_2$ compound in both directions (along $x$ and $z$-axes) are calculated. It is obtained that $({\sigma }_{xx})$ is 120% of $({\sigma }_{zz})$ at high-temperature, whereas $S_{zz}$ Seebeck coefficient was higher than $S_{xx}$ especially at $T=300\mathrm{\text{K}}$ ($S_{zz}=246\mu \mathrm{\text{V/K}},S_{xx}=213\mu \mathrm{\text{V/K}})$. The large value of $S_{zz}$ showed that the transport is dominated by zz-axis.

The work theoretically calculated the electronic structure and electrical transport properties of two configurations of single-walled MoS₂ nanotubes: armchair nanotubes (ANTs) and zigzag nanotubes (ZNTs) based on the density functional theory and Boltzmann transport method. ANTs have an indirect one. while ZNTs have a direct bandgap structure. The Seebeck coefficient ($S$), electrical conductivity ($\sigma )$ and power factor ($S^2\sigma )$ were calculated as a function of carrier concentration, chemical potential and temperature using the Boltzmann transport method. The calculated power factor ($S^2\sigma$) indicates that the most promising electronic properties were exhibited by $p$-type ANTs and $n$-type ZNTs. The $S^2\sigma$ of narrow bandgap (6, 6) (7, 7) (8, 8) semiconductors reached $4.0\times 10^{-4}$, $6.3\times 10^{-4}$ and $6.3\times 10^{-4}\mu$WK${}^{-2}$m${}^{-1}$ at room-temperature, respectively. (7, 7) nanotube have a maximum power factor of $3.44\ast 10^{-3}\mu$WK${}^{-2}$m${}^{-1}$ at 950 K, and the maximum power factor of ANTs is almost twice that of ZNTs.

P- and n-type higher manganese silicide (MnSi_1.7) films are characterized by Auger electron spectroscopy (AES). The relationship between Auger chemical shift and electrical property of the film has been established. Compared with pure Mn, the peak positions of Mn [MVV] Auger spectra in p- and n-type MnSi_1.7 films move to higher energy regions with +2.0 and +7.0 eV, respectively. New peaks around 50 eV in the Mn [MVV] Auger spectra, and 600, 654, and 705 eV in the Mn [LMM] Auger spectra appear in MnSi_1.7 films prepared by magnetron sputtering. These new peaks are considered to arise from iron impurities which are unintentionally introduced from the Mn–Si alloy target and during the magnetron sputtering process. The intensities of these new peaks are much stronger for the n-type MnSi_1.7 film. Compared with pure Si, the peak positions of Si [LVV] Auger spectra move to higher energy regions with +1.0 eV for both p- and n-type MnSi_1.7 films. However, the peak positions of Si [KLL] Auger spectra in p- and n-type MnSi_1.7 films move to lower energy regions with energy shifts between -1.0 and -3.0 eV.

Polycrystalline higher manganese silicide (MnSi_1.7, HMS) films with addition of aluminum and carbon are prepared on thermally-oxidized silicon substrates by electron beam evaporation and magnetron sputtering, respectively. An aluminum intermediate layer and a carbon cap layer are used as the doping sources. It is found that both the Seebeck coefficient and electrical resistivity are dependent on the amount of aluminum and carbon added to the films. The Seebeck coefficient changes a little in the temperature range 300 to 433 K and decreases considerably above 433 K when aluminum is added to the film. When carbon is added to the film, however, the Seebeck coefficient increases slightly. With addition of aluminum and carbon, the resistivity decreases. As a result, the thermoelectric power factor increases, especially for films with carbon addition. Several activation energies (0.022–0.20 eV) are observed from the curves of logarithm of resistivity versus reciprocal temperature. The larger activation energies of 0.35 and 0.51 eV are consistent with the energy band gaps for higher manganese silicides.

Nano-scale MnSi_1.7 films are prepared by thermal annealing of three-layer Si/MnSi_x/Si or bi-layer Si/MnSi_x (x < 1.7) structures at 923 K for 20–65 minutes. These layers are deposited on thermally oxidized silicon substrates at about 393 K by electron beam evaporation. It is found that the oxygen content in the MnSi_1.7 film can be reduced from about 10 at.% to 6 at.% by using the bi-layer structure MnSi_x/Si with the MnSi_x layer on top. With the reduction of oxygen content in the MnSi_1.7 film, the transition temperature from p-type to n-type decreases from 508 K to 463 K or less.

The influence of an AlO_x oxide or Si interlayer on the thermoelectric power factor of the higher manganese silicide (HMS, MnSi_y, y = 1.73–1.75) film deposited on quartz substrate is investigated. The HMS film and the interlayer are prepared on quartz substrate by magnetron sputtering of MnSi₂, Al, Si and Si:B (1 at.% B content) targets. It is found that the metallic phase MnSi is present in the semiconducting HMS film without an interlayer, resulting in a lower Seebeck coefficient, 0.160 mV/K, but not a lower electrical resistivity, 0.021 Ω ⋅cm at 683 K. The thermoelectric power factor is only 122 × 10^-6 W/mK² at 683 K. On the other hand, the metallic phase MnSi disappears and the Seebeck coefficient restores to its high value after using the AlO_x oxide or Si interlayer. Besides, the electrical resistivity decreases by using the AlO_x oxide or Si:B interlayer. The HMS film with an Si:B interlayer has the highest Seebeck coefficient, 0.247 mV/K, and the lowest electrical resistivity, 0.011 Ω ⋅cm, at 683 K. Thus, the thermoelectric power factor is enhanced and can reach 555 × 10^-6 W/mK² at 683 K.

It is well known that aluminum (Al), boron (B) and copper (Cu) are acceptor impurities with shallow- and deep-energy levels in silicon (Si), respectively. Thus, Al and B impurities with shallow-energy levels in Si are essentially completely ionized at room temperature while Cu impurities with deep-energy levels in Si at higher temperature. In this paper, Al, B and Cu co-doped Si layer is used as a barrier layer while the higher manganese silicide layer (HMS) as a well layer. The Seebeck coefficient (S) of Al and Cu modulation doped film, HMS/Si:(Al + Cu), increases sharply above 583 K, reaches a peak value of 0.300 mV/K at 683 K, and then decreases with further increasing temperature. Concomitance with the great increase in Seebeck coefficient, however, the electrical resistivity (R) is still smaller than that of only Al modulation doped film, HMS/Si:Al. The Cu-induced Seebeck peak, S_max = 0.303 mV/K at 733 K, and reduction in electrical resistivity are also observed in (B + Al + Cu) modulation doped film, Si:(B + Al + Cu)/HMS/Si:(B + Al + Cu), where B is used to reduce the electrical resistivity further. As a result, the thermoelectric power factor (P_F = S²/R) is greatly enhanced and can reach 3.140 × 10^-3 W/m-K² at 733 K, which is larger than that of HMS bulk material.

A novel chemical alloying method of high-pressure and high-temperature (HPHT) has been used for the synthesis of bulk-skutterudite ${\mathrm{\text{In}}}_{0.5}{\mathrm{\text{Co}}}_4{\mathrm{\text{Sb}}}_{11.5}{\mathrm{\text{Te}}}_{0.5}$. Through HPHT method, the synthesis time has been shortened from a few days to 30min. The samples of ${\mathrm{\text{In}}}_{0.5}{\mathrm{\text{Co}}}_4{\mathrm{\text{Sb}}}_{11.5}{\mathrm{\text{Te}}}_{0.5}$ skutterudites were synthesized at 1.8–3.3GPa. We have studied the phase, the microstructure, and the temperature-dependent thermoelectric properties. The Seebeck coefficient, electrical conductivity and thermal conductivity were measured in the temperature range of 295–673K. As we expected, the thermal conductivity of sample ${\mathrm{\text{In}}}_{0.5}{\mathrm{\text{Co}}}_4{\mathrm{\text{Sb}}}_{11.5}{\mathrm{\text{Te}}}_{0.5}$ decreased with the increase of the synthetic pressure. A maximal ZT of 0.64 was achieved for the ${\mathrm{\text{In}}}_{0.5}{\mathrm{\text{Co}}}_4{\mathrm{\text{Sb}}}_{11.5}{\mathrm{\text{Te}}}_{0.5}$ synthesized at 1.8GPa at 673K. These results revealed that HPHT method may be helpful for optimizing electrical conductivity and thermal conductivity in a comparatively independent way.

The SOFC interconnect materials La${}_{0.75}$Sr${}_{0.25}$Cr${}_{1-x}$O${}_{3-\delta }$$(x=0$–$0.04)$ were prepared using an auto-ignition process. The influences of Cr deficiency on their sintering, thermal expansion and electrical properties were investigated. All the samples were pure perovskite phase after sintering at 1400${}^{\circ }$C for 4 h. The cell volume of La${}_{0.75}$Sr${}_{0.25}$Cr${}_{1-x}$O${}_{3-\delta }$ decreased with increasing Cr deficient content. The relative density of the sintered bulk samples increased from 93.2% $(x=0)$ to a maximum value of 94.7% $(x=0.02)$ and then decreased to 87.7% $(x=0.04)$. The thermal expansion coefficients of the sintered bulk samples were in the range of $10.60$–$10.98\times 10^{-6}{\mathrm{\text{K}}}^{-1}$ (30–1000${}^{\circ }$C), which are compatible with that of YSZ. Among the investigated samples, the sample with 0.02 Cr deficiency had a maximum conductivity of 40.4 Scm${}^{-1}$ and the lowest Seebeck coefficient of 154.8 $\mu$VK${}^{-1}$ at 850${}^{\circ }$C in pure He. The experimental results indicate that La${}_{0.75}$Sr${}_{0.25}$Cr${}_{0.98}$O${}_{3-\delta }$ has the best properties and is much suitable for SOFC interconnect material application.

The work is devoted to the study of the thermoelectric properties of solid crystal Yb_xBi${}_{2-x}$Te₃. This work presents the results of studies of the thermoelectric properties of Yb_xBi${}_{2-x}$Te₃ ($x\le 0.10$) solid solutions before and after annealing in the temperature range 300–580K. Regularities of changes in the electrical conductivity, Seebeck coefficient, and total thermal conductivity of the samples depending on the content of ytterbium (Yb) are established. Dependences of electrical conductivity, Seebeck coefficient, and the thermal conductivity on temperature for the crystal under study are plotted. Temperature curves were recorded using a Termoscan-2 low-frequency temperature recorder at a heating rate of 283K/min. Studies of conductivity $\sigma$, thermoelectric power (S) were carried out by the four-probe method at direct current in the temperature range of 300–600K. Ohmic contacts were applied using alloys. It has been established that the optimal combination of these thermoelectric characteristics is achieved for the compositions $x=0.1$, which are characterized by the maximum thermoelectric index ($ZT=0.87$) of the figure of merit in the temperature range of 420–500K after annealing at 500K for $\tau =240$h. It was revealed that the Yb_xBi${}_{2-x}$Te₃ systems under study are n-type semiconductor thermoelectric materials in the temperature range of 300–600K.

Distinct types of materials are being explored for usage in thermoelectric (TE) and photovoltaic (PV) systems in order to alleviate the energy problem. In order to create nontoxic, more stable, and best-performance energy convergence devices, the state-of-the-art hybrid halide double perovskites (HHDPs) compound has been identified as a potential TE material and potential replacement for hazardous lead. We have proposed a new HHDP material (CH₃NH${}_3{)}_2$AgInCl₆ and performed its theoretical investigation via the full potential linear augmented plane wave (FP-LAPW) method. The computational results show that the studied compound exhibits a direct bandgap of 3.708eV and has exceptional PV characteristics in the ultraviolet (UV) region. The obtained thermodynamic (TD) characteristics confirm that the titled compound is thermally stable at various temperatures and pressures. At 300 K, the examined HHDP material has the highest ZT (=2.23) in the p-region. Materials with higher ZT_e values are substantially more important for generating electrical energy through waste heat. The ZT_e result validates the use of these materials in TE devices at ambient temperature. The parameters of this compound have been computed for the first time. This study identifies (CH₃NH${}_3{)}_2$AgInCl₆ as a novel HHDP compound. Through an in-depth exploration of its properties, this study offers valuable insight for further research with potential applications in clean energy systems.

Using density functional theory and Boltzmann equations, this study calculates and compares the electronic, optical, thermoelectric, and thermodynamic properties of bulk and single-layer germanium carbide structures. It has been shown that germanium carbide in the bulk structure has an indirect energy band gap of 1.61eV. In a single layer structure, it is a metal. Thermodynamic properties including specific heat at constant volume have been investigated using the quasi-harmonic approximation (QHA) package in QE. According to the studies conducted on optical properties, this material shows good reflection in the visible region. In addition, this structure has high electrical conductivity and low thermal conductivity, which is essential for thermoelectric materials. Thermoelectric calculations using semi-classical Boltzmann transfer theory show that the performance of two-dimensional compounds with graphene gives good thermoelectric results. At 100K, the highest figure of merit (ZT) for GeC is 1.00. Our findings evaluate the GeC compound as a suitable thermoelectric material in the temperature gradient from 100 to 900K.