Investigation of thermoelectric properties of Cu2GaxSn1 − xSe3 diamond-like compounds by hot pressing and spark plasma sintering

P-type compounds Cu₂Ga_xSn_1 ? xSe₃ (x = 0.025, 0.05, 0.075) with a diamond-like structure were consolidated using hot pressing sintering (HP) and spark plasma sintering (SPS) techniques. High-temperature thermoelectric properties as well as low-temperature Hall data are reported. Microstructural analysis shows that the distribution of Ga is homogeneous in the samples sintered by HP but inhomogeneous in the samples sintered by SPS, even with an electrically isolating and thermally conducting BN layer during the sintering. The Seebeck coefficients of the samples sintered by HP and SPS show similar dependence on the carrier concentration and are insensitive to the composition inhomogeneity. In contrast, the composition inhomogeneity results in lower carrier mobility and thus lower electrical conductivity in the samples sintered by SPS than those sintered by HP. Lattice thermal conductivity is further reduced through Ga doping; however, this effect is weakened by the inhomogeneous distribution of Ga. Due to their larger carrier mobility and lower lattice thermal conductivity, the samples sintered by HP exhibit 15–35% higher thermoelectric figure of merits (ZT) than those SPS samples with a high Ga doping level and without the coated BN layer, in which the composition homogeneity is worse. A ZT value of 0.43 is obtained for the HP Cu₂Ga_0.075Sn_0.925Se₃ sample at 700 K.     

Extrinsic and intrinsic magnetic properties of Co1−x Fex Sb3

We report magnetic properties of iron in Co_1−x Fe_x Sb₃ for x in the range 0<x<0.2, since x=0.2 is found to be the limit of solubility of iron in the skutterudite lattice. The magnetic ions diluted in the matrix carry a small magnetic moment reduced to that of the spin-only S=1/2 value of the Fe³⁺ in the low spin d⁵ configuration in presence of a strong crystal field that screens the orbital momentum. The magnetic properties give evidence that a small fraction of iron is spin-frozen in magnetite ferrimagnetic clusters, and antiferromagnetic FeO clusters. Because both types of clusters represent only very minor phases, their detection by the usual analytical means such as X-rays is not possible. The remaining part is diluted in the matrix to form a semimagnetic semiconductor characterized by a Fe–Fe nearest-neighbor exchange interaction J that is antiferromagnetic, with |J|/k_B19.6  K.     

Effects of point defects on properties of B2 NiAl: A first-principles study

The basic properties, heats of formation, energies of formation, equilibrium concentration of point defects, elastic properties, and electronic structures for point defect structures of B2-NiAl crystal are detailed analyzed by the density functional theory. Compared with B2-NiAl and other B2 intermetallic compounds, purity NiAl has the better ductility and bonding strength. According to the calculated heats of formation, energies of formation, and equilibrium concentration of point defects, the Ni antisite and Ni vacancy are primary defects in B2-NiAl crystal. The calculated G/B0 and Cauchy pressure parameters C12–C44 values confirm that Ni vacancy, Ni antisite, and Al antisite can promote the brittleness of B2-NiAl, and in which Ni vacancy is the primary defect, while Al vacancy with a low concentration can improve ductility of B2-NiAl. The density of state confirms that B2-NiAl intermetallic compounds are conductors, and point defects can promote the stability of the system expect Ni antisite defect.     

Mechanical spectroscopy of quasicrystals

Quasicrystals are solids with long-range quasiperiodic order and orientational order with disallowed crystallographic symmetry (e.g. 5-fold). In addition to the point defects known from crystals (vacancies, interstitials, or antistructure atoms) quasicrystals contain phasonic defects (phasons), which represent violations of the matching rules. Phasons allow displacement of atoms without contribution of vacancies. In icosahedral AlPdMn single quasicrystals two internal friction peaks are observed. Peak A at 370 K (f=3 Hz) has an activation enthalpy of 0.98 eV and a frequency factor close to the Debye frequency (2×10¹⁵ s⁻¹) and is therefore attributed to point defects, i.e. atomic jumps enabled by phason flips or vacancies. Peak B at 870 K (f=3 Hz) is determined by an activation enthalpy of 4 eV and an abnormal high frequency factor (3×10²⁴ s⁻¹), is much broader than a simple Debye-peak, and highly asymmetric. The temperature variation of the relaxation time exhibits significant deviation from ideal Arrhenius behaviour for higher temperatures (T>920 K) indicating a sort of phase transition. Peak B can be attributed to a relaxation process in which dislocation movement is controlled by creation and movement of phason defects.     

Non-linear anelasticity due to dislocation–solute atom interactions in solid solutions

A theory for the non-linear, strain-amplitude-dependent internal friction (ADIF) in solid solutions due to dislocation–solute atom interactions has been developed. The suggested model accounts for different modes of dislocation–solute atom interactions:

• solute atoms, distributed in the dislocation glide plane, represent short-range obstacles for the dislocation motion;

• solute atoms, situated away from the dislocation glide plane, create diffuse weak long-range elastic stress fields, also impeding dislocation motion.

Dislocations overcome localised obstacles under the combined action of the applied stress and thermal energy, whereas diffuse long-range obstacles can be surmounted only athermally. Numerical calculations of the ADIF, strain-amplitude-dependent modulus defect, their ratio, and of the fraction of the athermal ADIF component have been performed. The model predicts a complicated multistage behaviour of the parameters of the non-linear anelasticity in the strain amplitude–temperature–solute concentration domain which is in excellent agreement with recent experimental data.     

Internal friction and Young’s modulus of V–Ti–Cr alloy before, during and after proton irradiation

Using a composite oscillator, the Young’s modulus and the internal friction of annealed, plastically bent and bent–straightened V–4Ti–4Cr alloy specimens have been measured, in a wide amplitude range, before, during and after proton irradiation. The proton energy and flux were 8 MeV and 10¹² p cm⁻² s⁻¹. The in situ dependencies on the proton beam current and temperature have been obtained. Softening and overheating (which looks like softening–strenghtening of the alloy) effects, which arise due to radiation damage, have been revealed. The influence of beam heating and defect accumulation–annihilation on the acoustic properties of the annealed and pre-strained samples, during and after proton irradiation, are briefly discussed.     

Internal friction of the glassy tektites

Tektites are naturally occurring glasses, perhaps of meteoritic origin. They occur all over the world. They range in size from μm to decimeter. They are based on the compounds silica and alumina, with smaller concentrations of alkali metal and transition metal oxides. Their origin is in vigorous dispute; some scientists think they are of terrestrial earth origin, others believe they come from interplanetary space, as is true of meteorites. We have secured seven specimens from various regions of the earth and have measured the internal friction spectrum of several in the kilocycle frequency range above room temperature, where internal friction peaks of synthetic glasses have been observed. A well defined loss peak is observed around 500 K in a number of specimens. We have also measured the loss spectrum of obsidian and of blast furnace slag. From comparison of these measurements, we conclude that tektites are probably of volcanic origin, the loss peaks being associated with the reorientation of Fe–oxygen complexes.     

Amplitude dependent damping in HgI2 wide band gap semiconductor

Amplitude dependent damping in crystals is considered as one of the methods of mechanical spectroscopy for investigating different mechanisms of dislocation microplasticity. A short review of the amplitude dependent damping studies in semiconductors is presented. A new acousto–optic experimental technique for studying a dislocation point defect interaction developed over the last several years is described. Experimental data of photoacoustic investigations of HgI₂ single crystals obtained in a wide amplitude range at frequencies of longitudinal vibrations of about 100 kHz are reviewed. More recent results are presented as well. Besides, acousto–electric and acousto–optic (for a band of photoluminescence spectrum) effects which result from a high amplitude ultrasonic treatment (HAUT) with the amplitudes from the amplitude dependent damping range illustrate the possibilities of the technique. A model of an electron or hole trapping center which transforms in a dislocation pinning point during the sample illumination is assumed to be the most convenient to explain the experimental data. Under the influence of HAUT, one can destroy (due to dislocation depinning) the center which consists of the dislocation, point defect (it is, surely, a lattice, iodine or mercury, vacancy or interstitial) and the trapped electron or hole. A binding energy between the dislocation and the photosensitive pinning point defect has been estimated by displacement under the HAUT of specific points in the luminescence excitation spectrum. A possible practical use of HAUT for a restoration of performance of γ-ray HgI₂ detectors is briefly discussed.     

Anelastic relaxation due to interstitial solute atoms in face-centred cubic metals

The behaviour of light interstitial elements dissolved in transition metals of the face-centred cubic structure has been studied by measurements of mechanical relaxation. Those interstitial solute atoms such as C and H give rise to anelastic relaxation effects as they form complexes, such as close pairs. Detailed characterization of such complexes is possible by examining the effects of crystal orientation, solute concentration and temperature on the relaxation effect. Recent experimental results on Ni–C, Ni–C–H and Pd–C systems are presented.     

New p- and n-type skutterudites with ZT > 1 and nearly identical thermal expansion and mechanical properties

Using thermoelectricity to directly convert (waste) heat energy into useful electricity faces a number of challenges. Not only are optimised thermal and electrical transport properties required resulting in a high figure of merit ZT and a high thermal–electric conversion efficiency η over a wide temperature range, thermoelectric (TE) materials must have sufficient mechanical integrity to survive continuous heating–cooling cycles. Thermal expansion of the material as well as the mechanical properties play an important role, i.e. their values should be as similar as possible for p- and n-type alloys to avoid stresses when used in a TE device. In this paper multiple filled p- and n-type skutterudites (Ba,Sr,DD,Yb)_y(Fe_1–xNi_x)₄Sb₁₂ with a ZT > 1 and η ≈ 13% are presented, for the first time showing, in contrast to hitherto investigated skutterudites, nearly identical thermal expansion coefficients and elastic moduli. The ZT values of these skutterudites could be further enhanced by more than 20% after severe plastic deformation via high-pressure torsion.     

Synthesis and thermoelectric properties of the quaternary type-I Si clathrates K8-δGaxAlySi46-x-y

The effects of substituting Ga atoms with Al on the thermoelectric properties of the ternary Si clathrate K₈Ga₈Si₃₈ were assessed over the temperature range of 10–700 K. For this purpose, the series of quaternary Si clathrates K_8-δGa_xAl_ySi_46-x-y (x + y ≈ 8; 0 ≤ y < 8) was synthesized by heating mixtures of K, Ga, Al and Si at temperatures between 1223 and 1353 K. The lattice constant of K_8-δGa_xSi_46-x was found to increase with the Al content, y, and Al substitution also significantly changed the transport properties of the clathrate, even though Al and Ga are isovalent. Electrical resistivity measurements showed that the conduction mechanism changed from variable range hopping conduction to metallic conduction with increasing y. The Seebeck coefficient, S, was negative for all of the samples, indicating that the dominant carriers were electrons, while the absolute value of S decreased with y. All specimens had approximately the same value of thermal conductivity, κ, on the order of 1.4 W/Km at 300 K, except for K_7.7Al_7.5Si_38.8. K_7.5Ga_4.8Al_3.0Si_38.7 exhibited the largest power factor, and therefore the highest thermoelectric dimensionless figure of merit, ZT, with a value of 0.066 at 550 K. This value is approximately six times that of the ternary Si clathrate K_7.4Ga_7.7Si_38.9.     

Sb掺杂Mg2-xZnxSi固溶体的热电性能

利用B2O3助熔剂法结合SPS技术制备了Mg2-xZnxSi0.99Sb0.01(0 ≤ x ≤ 0.1)固溶体。测量了300 K - 780 K温度区间内试样的电导率、塞贝克系数和热导率。发现晶格热导率随Zn取代量的增大而降低。而电导率随Zn取代量的增大而先降低后增大。讨论了影响电导率与晶格热导率的变化规律的具体内在机制。所有样品中x=0.075样品的功率影子最高,在780 K达到1.76 mWm-1K-2,比基体Mg2Si0.99Sb0.01高约18%。x=0.1样品具有最低的晶格热导率,在770 K达到2.86 Wm-1K-1。低晶格热导率使Mg1.9Zn0.1Si0.99Sb0.01具有最高热电优值,在780 K达到0.37。     

Preparation of pure Higher Manganese Silicides through wet ball milling and reactive sintering with enhanced thermoelectric properties

A simple and effective process is used to synthesize undoped Higher Manganese Silicides (HMS), involving ball milling under soft conditions to obtain homogeneous mixtures of constituting elements, and subsequent spark plasma sintering for a direct solid state reaction. For comparison purpose, the ball milling step is carried out under both dry and wet conditions using n-hexane as the liquid medium, Analysis of the granulometry demonstrates that the wet milling process in n-hexane results in finer particles, thus improving the reaction rate later on. According to X-ray diffraction and scanning microscopy, materials produced via wet milling and spark plasma sintering contain only HMS while dry milled samples contain MnSi impuritties. The Seebeck coefficient of the wet milling sample is 20% higher, while its electrical resistivity is 23% lower than those of the dry milling one over the whole temperature range. Moreover, the thermal conductivity was reduced up to 30% when using n-hexane as milling media. The maximum thermoelectric figure of merit obtained is 0.55 at 850 K, a high value for undoped HMS.     

Effect of Al-doping on suppression of thermal conductivity in Si dispersed β-FeSi2

Silicon dispersed β-FeSi₂ with different aluminium concentrations are synthesized using eutectoid decomposition of α-Fe₂Si_5−xAl_x (0 ≤ x ≤ 0.1). Phase fractions, microstructure and thermoelectric properties of the above compositions have been investigated. Al-doping in Si dispersed β-FeSi₂ results in increased hole-carrier concentration thereby enhancing the electrical conductivity without compromising the Seebeck coefficient. This results in maximum power factor value of 4.7 μWcm⁻¹ K⁻² at 773 K for the sample with x = 0.1 which is significantly higher than that of an undoped sample. The thermal conductivity of the samples was fitted with the Debye-Callaway model to understand the various scattering processes involved. The analysis shows that an increased point defect scattering of phonons with Al-doping in addition to scattering by Si/β-FeSi₂ interface lowers the thermal conductivity significantly.     

四元Mg2Si0.27Ge0.05Sn0.65Sb0.03固溶体的热电性能

通过氧化硼助熔剂法和放电等离子烧结技术制备了Mg_2(1+x)Si_0.27Ge_0.05Sn_0.65Sb_0.03 (x = 0.05, 0.08)四元固溶体热电材料。测量了在300 K - 800 K 的温度区间内测试了所有四元固溶体试样的塞贝克系数、电导率和热导率。研究结果表明随着温度的升高电导率单调降低而塞贝克系数单调升高,所有样品的晶格热导率明显高于通过Abeles模型计算所得到的理论值。最高无量纲热电优值出现在x=0.08样品中,在800 K时达到最高值1.0.     

Low temperature relaxations associated with quantum tunnelling of H in Sc and Y

Acoustic spectroscopy and electrical resistivity measurements have been employed to investigate the two relaxation processes occurring at liquid He temperature in very dilute Y–H and Sc–H alloys, and ascribed to H tunnelling systems near single or paired O traps. At very low H concentration, the peak due to the O–H tunnelling system becomes extremely narrow and is describable in terms of incoherent tunnelling with phonon assisted transitions.     

Effects of noble metal addition on microstructure and thermoelectric properties of NaxCo2O4

The synthesis of Na_xCo₂O₄/Ag and Na_xCo₂O₄/Au composites was tried by mechanical milling and subsequent sintering. Ag and Au particles were added to the Na_xCo₂O₄ powder prior to the mechanical milling. The microstructure and thermoelectric properties of the Na_xCo₂O₄/Au composite were compared to those of the Na_xCo₂O₄/Ag composite and the Na_xCo₂O₄ single phase, and the effects of the Ag and Au addition on the thermoelectric performance of Na_xCo₂O₄ were discussed. Au particles around 2 μm or smaller in size, which were significantly smaller than Ag particles around 10 μm in size, were dispersed in the Na_xCo₂O₄ matrix. The Seebeck coefficient and the electrical resistivity of Na_xCo₂O₄ were slightly enhanced and significantly reduced by these noble metals addition, resulting in the large power factor of these composites. On the other hand, the Na_xCo₂O₄/Au composite showed the electrical resistivity larger than that of the Na_xCo₂O₄/Ag composite. Ag and Au addition markedly increased the thermal conductivity, and the dimensionless figure of merit of Na_xCo₂O₄ could not be improved by these noble metals addition.     

Microstructure and thermoelectric properties of NaxCo2O4/Ag composite synthesized by the polymerized complex method

The synthesis of a thermoelectric Na_xCo₂O₄/Ag composite was attempted by the polymerized complex (PC) process using AgNO₃ as an Ag source and subsequent sintering at 1153 K for 72 ks. The effects of the PC process and Ag addition to Na_xCo₂O₄ on microstructure and thermoelectric properties of the Na_xCo₂O₄/Ag composite were investigated. Ag was hardly substituted for Na and Co sites, and the sintered sample was composed of the Na_xCo₂O₄ and Ag phases. The electrical resistivity of the composite was smaller than that of the Na_xCo₂O₄ single phase and the Seebeck coefficient was slightly enhanced by Ag addition, resulting in the significantly large power factor. However, most of precipitated Ag particles in the Na_xCo₂O₄ matrix were coarse, 5–8 μm in size, and the thermal conductivity of the composite was high as compared to the Na_xCo₂O₄ single phase. From these results, the dimensionless figure of merit of the composite was almost the same as that of the Na_xCo₂O₄ single phase.     

Upconversion dynamics in Er doped nanocrystalline YAlO3

We report the luminescence and upconversion spectra of nanocrystalline YAlO₃ doped with trivalent erbium at concentrations of 5.0, 1.0 and 0.1 mol.%. The powder samples were prepared using a solution combustion reaction method, and the resulting YAlO₃ nanocrystals show, under wide-angle X-ray diffraction, a size in the range 20–40 nm. Efficient green and red emissions are observed at room temperature under continuous-wave pumping at 980 nm. A weak emission can also be detected in the blue at 410 nm. The upconversion dynamics were studied measuring the decay times and the pump-power dependence of the transitions to the ⁴I_15/2 ground state starting from the ²H_11/2, ⁴S_3/2 and ⁴F_9/2 excited states. Excited-state absorption (ESA) is found to be responsible for the higher energy (²H_11/2, ⁴S_3/2→⁴I_15/2) green transitions. On the other hand, for the ⁴F_9/2→⁴I_15/2 red transition a competing energy-transfer upconversion (ETU) mechanism is found, which accounts for the more than 100-fold increase in intensity of the red emission on passing from the lowest (0.1 mol.%) to the highest (5 mol.%) erbium concentration.