Properties of Bi2Te3 single crystals doped with Sn

The effect of doping with Sn on the properties of Czochralski-grown Bi₂Te₃ crystals was studied. The effective segregation coefficient for Sn was determined to be 0.6. The thermoelectric power, electrical conductivity, and Hall coefficient of the doped crystals were measured at room temperature. Doping with low Sn concentrations (0.2-0.5 at. %) was found to have only a weak effect on the electrical properties ofp- type Bi₂Te₃. Doping with 0.7-1 at. % Sn reduces the thermoelectric power and increases the electrical conductivity and hole concentration. Lattice thermal conductivity is a nonmonotonic function of Sn concentration. The thermoelectric figure of merit of Bi₂Te₃ doped with less than 0.6 at. % Sn exceeds that of undoped Bi₂Te₃.     

Interfacial reaction and properties of Sn/Cu solder reinforced with graphene nanosheets during solid–liquid diffusion and reflowing

In this paper, different mass fractions (0, 0.01, 0.03, 0.05, 0.07, and 0.09 wt.%) of graphene nanosheets (GNSs) were selected as a strengthening phase to promote the performances of Sn/Cu solder joint. The wettability and shear performance of Sn-xGNSs/Cu solder and the growth behavior of intermetallic compound (IMC) during solid–liquid diffusion at 250 °C and under multiple reflows (1, 2, 4, and 8 times) were systematically discussed. Results exhibited that the wettability of GNSs doped solder improved as GNSs added and 0.05% GNSs addition would promote the spreading area of the composite solder effectively. The scalloped Cu₆Sn₅ IMC layer was formed at the interfacial Sn-xGNSs/Cu. Based on the adsorption theory, GNSs doping was conducive to prohibiting the interfacial IMC growth of Sn solder reinforced with GNSs during soldering process and reflowing process. Meanwhile, the IMC overtly became thinner in Sn-0.05GNSs solder with comparison to plain solder. What’s more, GNSs addition brought about an enhancement in the mechanical performance of GNS-containing solder. The fracture surface of solder joint after doping GNSs transformed from a brittle pattern to a ductile pattern.

     

Thermoelectric power factor of Ti<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>V<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>NiSn alloys

Ti_{1 − x} V _x NiSn (x = 0–0.10) substitutional solid solutions have been prepared by doping the intermetallic semiconductor n-TiNiSn (half-Heusler phase) with vanadium, a donor impurity, and their resistivity and thermopower have been measured at temperatures from 80 to 380 K. The results demonstrate that, when doping of TiNiSn causes no type inversion, the thermoelectric power factor of the solid solution markedly exceeds that of the undoped ternary compound.     

X-Ray Diffraction Studies on LaBa2Cu3Ox Cuprates with Iron Substitution

LaBa₂Cu_3–y Fe _y O _x ceramic samples with y = 0.00–1.50 are synthesized by the solid-state reaction technique. Rietveld analysis for X-ray diffraction is performed on these iron-doped samples. A BaCuO₂ impurity phase and a ceramic cuprate phase coexist in each sample. An orthorhombic-to-tetragonal (OT) phase transition occurs in the doping range of 0.03y0.06, and a tetragonal-to-orthorhombic (TO) one occurs in the doping level of 0.10y0.25. There is a jump in the structural parameters due to the iron doping. The occupancy of oxygen at the O(4) site, which is in the La plane at z = 1/2, increases with increase in iron content. These results may relate to the iron preferential occupancy for the Cu(1) site at the lower doping level, and for Cu(2) sites at the higher doping level.     

Modulation of Vacancy Defects and Texture for High Performance n-Type Bi2Te3 via High Energy Refinement

The carrier concentration in n-type layered Bi₂Te₃-based thermoelectric (TE) material is significantly impacted by the donor-like effect, which would be further intensified by the nonbasal slip during grain refinement of crushing, milling, and deformation, inducing a big challenge to improve its TE performance and mechanical property simultaneously. In this work, high-energy refinement and hot-pressing are used to stabilize the carrier concentration due to the facilitated recovery of cation and anion vacancies. Based on this, combined with SbI₃ doping and hot deformation, the optimized carrier concentration and high texture degree are simultaneously realized. As a result, a peak figure of merit (zT) of 1.14 at 323 K for Bi₂Te_2.7Se_0.3 + 0.05 wt.% SbI₃ sample with the high bending strength of 100 Mpa is obtained. Furthermore, a 31-couple thermoelectric cooling device consisted of n-type Bi₂Te_2.7Se_0.3 + 0.05 wt.% SbI₃ and commercial p-type Bi_0.5Sb_1.5Te₃ legs is fabricated, which generates the large maximum temperature difference (ΔT_max) of 85 K at a hot-side temperature of 343 K. Thus, the discovery of recovery effect in high energy refinement and hot-pressing has significant implications for improving TE performance and mechanical strength of n-type Bi₂Te₃, thereby promoting its applications in harsh conditions.     

Transport,magnetic and structural properties of Mott insulator MnV2O4 at the boundary between localized and itinerant electron limit

The effect of Zn and Cr doping on the transport and magnetic properties of MnV₂O₄ have been investigated using resistivity, thermoelectric power (TEP), magnetization, neutron diffraction and X-ray diffraction techniques. It is observed, that with increase in Zn substitution the non-collinear orientation of Mn spins with the V spins decreases which effectively leads to the decrease of structural transition temperature more rapidly than Curie temperature. Investigations also show that with Zn doping both the Curie temperature (T _C) and structural transition temperature (T _S) decrease, while the gap between them increases rapidly. On the other hand, with Cr doping on the V site the T _C remains almost constant but T _S decreases rapidly. Moreover, with Zn doping both resistivity and TEP decrease, whereas with 10 % Cr doping the TEP decreases and a change of sign occurs indicating an increase in the band gap. This leads to the decrease of the mobility of the polaronic holes than the mobility of the electronic polarons at low temperature.     

Cd-Doped Cu0.5Tl0.5Ba2Ca2Cu3?yCdyO10?δ (y=0,0.5,1.0,1.5,2.0) Superconductors

Superconducting properties of cadmium doped Cu_0.5Tl_0.5Ba₂Ca₂Cu_3−y Cd _y O_10−δ (y=0,0.5,1.0,1.5,2.0) samples have been studied using X-ray diffraction, resistivity, ac-susceptibility and FTIR absorption measurements. In X-ray diffraction studies these samples have shown to have tetragonal structure. The zero resistivity critical temperature and magnitude of diamagnetism are suppressed with the increased incorporation of Cd in the final compound. A change in the shape of FTIR absorption spectra, after doping, has shown the incorporation of Cd in the unit cell. A systematic hardening of the apical oxygen modes and softening of the CuO₂ planar modes of vibration with increased Cd doping have shown that it is incorporated in the unit cell of Cu_0.5Tl_0.5Ba₂Ca₂Cu_3−y Cd _y O_10−δ (y=0,0.5,1.0,1.5,2.0) superconductors. The FTIR absorption measurements of these samples have shown that hardening of the apical oxygen modes of types Cu(1)–O(2)–Tl and Cu(1)–O(2)–Cu(2)/Cd _y (y=0,0.5,1.0,1.5,2.0) increases with the increase of Cd doping in the samples. A softening of the CuO₂ planar oxygen mode Cu(2)–O–Cu(2) is also observed with the increased Cd doping in the final compound. It is most likely that hardening of the apical oxygen modes and the softening of the planar modes of vibration are associated damped harmonic oscillations produced by heavier Cd atoms in the CuO₂ planes, which suppress the phonon population from a desired level, reducing the magnitude of superconductivity in the final compound.     

Microstructures and mechanical properties of (AlCoCrFeMn)100 − xCux high-entropy alloys

A series of (AlCoCrFeMn)_100???xCu_x high-entropy alloys (0, 4, 8, 12, 16 at.-%) were prepared by vacuum arc furnace melting and their phase composition, microstructure and mechanical properties were systematically investigated. The results show that Cu can induce phase transformation from the orthorhombic phase to the Laves phase in (AlCoCrFeMn)_100???xCu_x high-entropy alloys and the volume fractions of Laves phase increases from 0 to 70% with increasing Cu content. The compression fracture strain increases from 5 to 16% and the compression fracture strength also increases from 1380 to 2140?MPa with Cu content increases. The increased volume fraction of the Laves phase is the main factor for the ductility increases.     

X-Ray Diffraction Studies on LaBa2Cu3O x Cuprates with Iron Substitution

LaBa₂Cu_3?y Fe _y O _x ceramic samples with y = 0.00?1.50 are synthesized by the solid-state reaction technique. Rietveld analysis for X-ray diffraction is performed on these iron-doped samples. A BaCuO₂ impurity phase and a ceramic cuprate phase coexist in each sample. An orthorhombic-to-tetragonal (OT) phase transition occurs in the doping range of 0.03≤y≤0.06, and a tetragonal-to-orthorhombic (TO) one occurs in the doping level of 0.10≤y≤0.25. There is a jump in the structural parameters due to the iron doping. The occupancy of oxygen at the O(4) site, which is in the La plane at z = 1/2, increases with increase in iron content. These results may relate to the iron preferential occupancy for the Cu(1) site at the lower doping level, and for Cu(2) sites at the higher doping level.     

High performance functionally graded and segmented Bi2Te3-based materials for thermoelectric power generation

Bi₂Te₃-based materials possess a figure of merit maximum over a narrow temperature range. When used in a generating mode over a large temperature difference the material operates at a substantially lower overall figure of merit than its maximum value. The conversion efficiency of a thermoelectric generator for low temperature waste heat recovery can be increased by employing functionally graded or segmented materials. In this work functionally graded p-type Bi₂Te₃-based thermoelectric materials have been prepared from melt by the Bridgman method using double doping technique. Segmented n-type thermoelement has been fabricated by joining two Bi₂Te₃-based materials with figure of merit maximum at 270 K and 380 K. The thermoelectric properties of the materials and a thermocouple comprised of p-type functionally graded and n-type segmented materials have been measured over a temperature range 200 K–450 K. The material efficiency of the thermocouple over the temperature gradient 223 K–423 K is estimated to be 10% compared with 8.8% for a standard Bi₂Te₃-based materials.     

Transport Properties and Raman Scattering Studies of Superconducting Ferromagnet Ru1−x Al x Sr2GdCu2O8

Transport properties and Raman spectra of Ru_1–x AlxSr₂GdCu₂O₈ (Al-doped Ru-1212) with x=0, 0.05, 0.1 and 0.15 have been investigated. The superconducting transition temperature of the Al-doped Ru1212 increases with increasing Al content up to 10%, and then decreases with increasing Al content. The observed result seems to indicate that excess holes induced by Al doping could transfer from RuO₂ plane to CuO₂ plane through apical oxygen for 0<x<0.1. Whereas for x>0.1, the holes might be trapped or localized in the RuO₂ plane due to a local disorder induced by Al doping. The temperature dependence of the critical field can be fitted as H _c2(T)=34(1–T/Tc)^4.8, suggesting that the vortex dynamics of Ru-1212 might be intrinsically different from that of the other high-T _c cuprates. The largest magnetoresistance (MR) of the Al-doped Ru-1212 occurs around magnetic ordering temperature, indicating that the exchange interaction between the carriers in CuO₂ planes and the spins in RuO₂ planes dominates the magneto-transport properties of the Ru-1212 system. The peak position of the 150 cm^–1 involving Cu vibration along the c-axis hardly changes as x increases. It reveals that Al doesn't occupy Cu site. A new Raman active mode around 550 cm^–1 observed in the Al-doped compounds might be attributed to doping-induced A_1g -like apical oxygen mode.     

Microstructure,mechanical properties and shape memory effect of Cu–Hf–Al–Ni alloys

The influence of hafnium element’s incorporation on a Cu–xHf–13.0Al–4.0Ni (wt-%) (x?=?0.5, 1.0 and 2.0) high-temperature shape memory alloy was investigated systematically. The results show that the matrix of Cu–xHf–13.0Al–4.0Ni (x?=?0.5, 1.0 and 2.0) alloys is 18R martensite, and an orthorhombic-structured Cu₈Hf₃ phase is formed and distributed at the grain boundaries. The grain size is significantly reduced with increasing Hf content. The mechanical properties of Cu–xHf–13.0Al–4.0Ni (x?=?0.5, 1.0 and 2.0) alloys are improved by Hf doping due to the combination of refinement strengthening, solid solution strengthening and second phase strengthening. After heating under pre-strain of 10%, the shape memory effect of the Cu–1.0Hf–13.0Al–4.0Ni alloy reaches 5.6%, which is obviously higher than that of the Cu–13.0Al–4.0Ni alloy.     

Properties of Sn-doped Bi2Te3 − x Sex single crystals

The effect of Sn doping (0.2 and 0.4 at %) on the properties of Czochralski-grown single crystals of n-type Bi₂Te_2.85Se_0.15 solid solutions is studied. Thermoelectric power, electrical conductivity, thermal conductivity, and Hall effect measurements in the range 77–400 K demonstrate that Sn doping has a significant effect on the transport properties of the solid solutions. Between 300 and 370 K, the thermoelectric figure of merit of Bi_1.996Sn_0.004Te_2.85Se_0.15 single crystals is higher than that of the Sn-free solid solution. In addition, hot-microprobe thermoelectric power measurements, highly sensitive to variations in carrier concentration, indicate that the Sn-doped single crystals are very uniform in electrical properties, both along the growth direction and radially.     

Ferromagnetic properties of Cu- and Fe-codoped nanocrystalline CdO powders: Annealing in hydrogen promote long-range ferromagnetic order

A mixture of cadmium acetate dihydrate, bis(acetylacetonato)copper, and tris(acetylacetonato)iron complexes is thermally co-decomposition to synthesizes Cu and Fe-codoped CdO (CdO:Cu:Fe) powders. The aim of this study is to examine the effect Fe doping on the appearance and development of room temperature ferromagnetism (RT-FM) in the presence of Cu ions as catalyst. X-ray diffraction (XRD) and X-ray fluorescence (XRF) measurements were carried out to study powders purity and crystalline structure. The XRD results show that Cu and Fe ions are incorporated in the ZnO crystal lattice by occupying Zn sites thereby forming ZnO:Cu:Fe solid solution (SS), whereas XRF confirms the purity of the obtained SS. The magnetic properties of the as-prepared and H-annealed (CdO:Cu:Fe–H) SS powders was investigated as a function of %Fe doping level. It was observed that the magnetic behaviour of pure as-synthesised CdO is mainly diamagnetic with slight FM phase. However, its behaviour with Cu and Fe codoping transforms to the mostly paramagnetic overlapped with small FM behaviour. More importantly, it was found that that annealing under H₂ atmosphere induces the reduction of CdO to form pure Cd metal, resulting in total transformation to FM behaviour with strength increases consistently with doping. Furthermore, with hydrogenation a tremendous enhancement of FM parameters takes place so that the M_s increases by about 1230% reaching a very high value of 1.43 emu/g for 1.8%Fe doping level of CdO.     

Fine-grained polycrystalline MoTe2 with enhanced thermoelectric properties through iodine doping

Consisting of heavy elements and favorable electronic structure, MoTe₂ has great potential as a good thermoelectric material for heat-to-electricity conversion. While some experimental work has been performed on the p-type version, n-type MoTe₂ is theoretically predicted to have a great conversion efficiency and is crucial for eventual device functionality, yet has not been explored. Here, the preparation and thermoelectric properties of n-type iodine-doped nano-polycrystalline MoTe₂ are currently reported. Nano-polycrystalline MoTe_2???xI_x is obtained by ball milling and spark plasma sintering techniques. The composition, morphology and crystal structure of the prepared materials were analyzed by XRD and FESEM, which indicated a homogeneous single phase. The measured transport properties over the temperature range of 298–823 K indicate that iodine doping greatly enhances the carrier concentration and corresponding power factor, and drastically reducing the thermal conductivity. The ECR (Electrical conductivity ratios) carrier scattering analysis demonstrates that dislocation scattering is the main mechanism throughout the experimental temperature range. With the temperature and doping increasing, the thermal conductivity was reduced rapidly, and the minimum value was 1.19 Wm^??1 K^??1 at 673 K. The maximum value of the figure merit ZT?~?0.16 over 673–750 K, which is much higher than other reported values. These excellent properties imply that MoTe₂ will be an efficient candidate for thermoelectric applications.

     

Realizing zT of 2.3 in Ge1−x−ySbxInyTe via Reducing the Phase‐Transition Temperature and Introducing Resonant Energy Doping

GeTe with rhombohedral‐to‐cubic phase transition is a promising lead‐free thermoelectric candidate. Herein, theoretical studies reveal that cubic GeTe has superior thermoelectric behavior, which is linked to (1) the two valence bands to enhance the electronic transport coefficients and (2) stronger enharmonic phonon–phonon interactions to ensure a lower intrinsic thermal conductivity. Experimentally, based on Ge_1?xSb_xTe with optimized carrier concentration, a record‐high figure‐of‐merit of 2.3 is achieved via further doping with In, which induces the distortion of the density of states near the Fermi level. Moreover, Sb and In codoping reduces the phase‐transition temperature to extend the better thermoelectric behavior of cubic GeTe to low temperature. Additionally, electronic microscopy characterization demonstrates grain boundaries, a high‐density of stacking faults, and nanoscale precipitates, which together with the inevitable point defects result in a dramatically decreased thermal conductivity. The fundamental investigation and experimental demonstration provide an important direction for the development of high‐performance Pb‐free thermoelectric materials.     

Manipulating the Interfacial Band Bending For Enhancing the Thermoelectric Properties of 1T′-MoTe2/Bi2Te3 Superlattice Films

Interfacial charge effects, such as band bending, modulation doping, and energy filtering, are critical for improving electronic transport properties of superlattice films. However, effectively manipulating interfacial band bending has proven challenging in previous studies. In this study, (1T′-MoTe₂)_x(Bi₂Te₃)_y superlattice films with symmetry-mismatch were successfully fabricated via the molecular beam epitaxy. This enables to manipulate the interfacial band bending, thereby optimizing the corresponding thermoelectric performance. These results demonstrate that the increase of Te/Bi flux ratio (R) effectively tailored interfacial band bending, resulting in a reduction of the interfacial electric potential from ≈127 meV at R = 16 to ≈73 meV at R = 8. It is further verified that a smaller interfacial electric potential is more beneficial for optimizing the electronic transport properties of (1T′-MoTe₂)_x(Bi₂Te₃)_y. Especially, the (1T′-MoTe₂)₁(Bi₂Te₃)₁₂ superlattice film displays the highest thermoelectric power factor of 2.72 mW m⁻¹ K⁻² among all films, due to the synergy of modulation doping, energy filtering, and the manipulation of band bending. Moreover, the lattice thermal conductivity of the superlattice films is significantly reduced. This work provides valuable guidance to manipulate the interfacial band bending and further enhance the thermoelectric performances of superlattice films.     

Spatial Inhomogeneities in Single-Crystal HgBa2CuO4+δ from 63Cu NMR Spin and Quadrupole Shifts

⁶³Cu Nuclear Magnetic Resonance (NMR) measurements on a single crystal of the single-layer high-temperature superconductor HgBa₂CuO_4+δ are reported. From the analysis of the quadrupolar satellites and their anisotropic splitting a largely temperature-independent symmetric quadrupole tensor is deduced, despite substantial variation in the electrical field gradients at the Cu site. The strongly temperature-dependent magnetic shifts and linewidths of the ⁶³Cu central line for different field orientations also reveal substantial spin shift variations in the material. Linear dependences on the doping of both, the quadruple splitting as well as the spin shifts, explain over a large temperature range all the widths with a local doping variation corresponding to 2δ≈0.073.     

Oxygen-ion diffusion and electrical conduction of La2Mo2?2 x Fe x O9?δ systems

Based on the novel oxygen ion conductor La₂Mo₂O₉, a series of Fe-doped samples of La₂Mo_2−x Fe _x O_9−δ (x = 0, 0.025, 0.05, 0.1) was prepared by conventional solid-state reaction method. The structure, phase transition, oxygen ion diffusion and electrical conductivity were studied with X-ray diffraction (XRD), differential scanning calorimeter (DSC), direct current (dc) resistivity, and dielectric relaxation (DR) measurements. One DR peak associated with the short-distance diffusion of oxygen vacancies was observed in both temperature and frequency spectra. The activation energy for oxygen ion diffusion in Fe-doped La₂Mo₂O₉ samples was smaller than that in un-doped samples. Fe doping can increase the ionic conductivity of La₂Mo_2−x Fe _x O_9−δ samples as well as the ionic transference number in the temperature range from 680°C to 400°C in comparison with the un-doped samples, although the electronic conductivity slightly increases. It is found that because of the small solubility of Fe₂O₃ in La₂Mo₂O₉ (<5%), Fe doping cannot suppress the phase transition that occurred around 570°C, but 2.5% K doping at La site at the same time (e.g. in sample La_1.95K_0.05Mo_1.95Fe_0.05O_9−δ ) can completely suppress this phase transition and increase conductivity at lower temperatures.     

Thermoelectric Power of Al Doped La–Sr–Cu–O System

The normal-state transport properties of La_1.85–x Sr_0.15+x Cu_1–x Al _x O_y system have been investigated by means of resistivity and thermoelectric power. The structural analysis indicates that with the increase of x, both the lattice parameter a and c decrease. An MI transition appears with Al doping. Al doping partly perturb the periodic potential of the system, thus producing a weakly temperature dependent TEP. The different effect of doping between Al and other transition metal is discussed.