Thermoelectric Power Measurements Research Articles

Phase quantification in a duplex stainless steel welds using NDT techniques

ABSTRACT Duplex stainless steels are composed of ferrite and austenite in equal volume fractions of these phases, which provides an excellent combination of mechanical properties and corrosion resistance, especially when compared with conventional stainless steel grades. When these steels are welded, there are significant changes in the austenite-to-ferrite ratio therefore in their final properties. Nowadays, the ferritoscope is the unique non-destructive technique used to quantify these two phases content in situ. This tester instrument is limited to the evaluation of large areas, is not suitable for the heat affected zone and is also very sensitive to the surface finish, being considered an intermediate precision technique. So, it is necessary to develop more reliable field techniques. In this research, thermoelectric power measurements and induced current techniques are proposed to evaluate the phase quantification in S32205 welds. According to the results, both techniques are reliable and highly sensitive to phase changes in the weld bead, heat affected zone and base metal. For the thermoelectric power technique, a probe with nickel tip should be used to obtain the highest sensitivity and for the induced current technique, a probe type-pencil with a central frequency of 6 MHz should be used.

Physical and Thermoelectric Properties of Ternary Semiconducting NiO–V2O5–TeO2 Glasses

Five samples of ternary semiconducting 65V2O5 –xNiO–(35–x) TeO2 glasses, with 5 ≤ x ≤ 25 (x in mol%), were prepared using a press-quenching method from a glass melt. The thermoelectric Power (TEP), density, oxygen molar volume (Vm) and x-ray diffraction (XRD) were analyzed. TEP measurements were conducted within the temperature range of 300 – 506 K for the mentioned glass compositions. Information on the creation of polarons and the disorder energy arising from random fields was gathered. By applying Heikes’ equation and the small polaron model theory to the TEP, the results obtained from experimental data were effectively explained. The study revealed that an increase in the NiO content in the glass led to a rise in density and a consistent decrease in molar volume. All glasses exhibited a singular phase structure.

Tempering of Dual Phase steels: Microstructural evolutions and mechanical properties

Modelling the microstructural evolutions and mechanical properties during the tempering of Dual Phase steels is a key objective for industrials as this phenomenon has a strong impact on the final properties. After having determined the tempering kinetics of fully martensitic steels between 100°C and 550°C using thermoelectric Power and hardness measurements, time–temperature equivalences were applied to determine the activation energies of the mechanisms controlling martensite tempering. Two tempering stages were clearly identified and thanks to the use of TEM, SEM and tomography techniques, they could be attributed to cementite precipitation, its spheroidization and recovery phenomena. The second stage was found to be retarded with increasing the manganese content of the steel contrary to the first stage. From these studies, a JMAK model was developed to predict the microstructurals evolutions during tempering. Then, an extension of the Hybrid-mean Field Composite model developed in a previous paper to predict the tensile curves of fresh martensite was proposed to take into account the microstructural evolutions of martensite during tempering. The model was tested for a wide range of physical parameters of the microstructure (phase fraction and chemical composition) and for different tempering heat treatments and gave good agreement with experimental tensile curves.

Effects of Pb and carbonate substitutions on the superconducting properties of TlSr4Cu2OzCrO4

We have investigated the effects of Pb and carbonate substitutions on the phase formation and the superconducting properties of the intergrowth superconductor TlSr4Cu2OzCrO4. X-ray diffraction data showed that nearly single-phase samples with tetragonal structure were obtained in the compositions (Tl1-xPbx)Sr4Cu2OzCrO4 (0 ≤ x ≤ 0.5) and (Tl0.5Pb0.5)Sr4Cu2Oz(CrO4)1-y(CO3)y (0 ≤ y ≤ 1.0). Resistivity measurements indicated that the transition temperature decreased significantly as the Pb doping content x increased to x = 0.5. In contrast to the Pb doping effects, the carbonate substitution in the (Tl0.5Pb0.5)Sr4Cu2Oz(CrO4)1-y(CO3)y system resulted in an improvement of the superconducting transition temperature up to y = 0.9. The y = 1.0 sample showed a reduced transition temperature, compared to the x = 0.9 sample. The experimental results are discussed in conjunction with the change in hole carrier concentration due to the Pb and carbonate substitutions, based on the data from room temperature thermoelectric power measurements.

Charge Puddles Driven Complex Crossover of Magnetoresistance in Non-Topological Sulfur Doped Antimony Selenide Nanowires.

A race to achieve a crossover from positive to negative magnetoresistance is intense in the field of nanostructured materials to reduce the size of memory devices. Here, the unusual complex magnetoresistance in nonmagnetic sulfur-doped Sb2Se3nanowires is demonstrated. Intentionally, sulfur is doped in such a way to nearly achieve the charge neutrality point that is evident from switching of carrier type from p-type to n-type at 13 K as inferred from the low-temperature thermoelectric power measurements. A change from 3D variable range hopping (VRH) to power law transport with α = 0.18 in resistivity measurement signifies a Luttinger liquid transport with weak links through the nanowires. Interestingly, high magnetic field induced negative magnetoresistance (NMR) occurring in hole dominated temperature regimes can only be explained by invoking the concept of charge puddles. Spot energy dispersive spectroscopy (EDS), magnetic force microscopy (MFM) measurements, Tmottand Regel plot indicate an enhanced disorder in these sulfurized nanowires that are found to be the precursor for the formation of these charge puddles. Tunability of conducting states in these nanowires is investigated in the light of interplay of carrier type, magnetic field, temperature, and intricate intra-inter wire transport that makes this nanowires potential for large scale spintronic devices.

Thermoelectric power of overdoped Tl2201 crystals: charge density waves and T 1 and T 2 resistivities

We report measurements of the in-plane thermoelectric power (TEP) for an overdoped (OD) crystal of the single layer cuprate superconductor Tl2Ba2CuO (Tl2201) at several hole concentrations (p), from 300 or 400 K to below the superconducting transition temperature (Tc ). For p = 0.192 and 0.220, small upturns in the TEP below 150 K are attributed to the presence of charge density waves (CDW) detected by resonant inelastic x-ray scattering studies. This suggests that measurement of the TEP could provide a simple and effective guide to the presence of a CDW. Over a certain temperature range, often strongly restricted by the CDW, the TEP is consistent with the Nordheim-Gorter rule and the T 1 and T 2 terms in the in-plane resistivity of similar crystals observed below 160 K. Two scenarios in which the T 1 scattering term is uniform or non-uniform around the Fermi surface are discussed. As found previously by others, for uniform scattering the T 1 terms give scattering rates (τ −1) at lower p that are somewhat larger than the Planckian value and fall to zero for heavily OD crystals. Near 160 K, τ −1 from the T 2 terms corresponds to the Planckian value.

Non-destructive evaluation of aging in welded pipeline X60 and X65 by thermoelectric power means

In this investigation, the sensibility of the thermoelectric power (TEP) to the aging condition of API X60 and API X65 steel pipeline welded joints was evaluated. Specimens of both API welded joints were artificially aged at 300 °C for 3, 7, 9, 18, 30 and 45 h, the microstructural evolution was assessed via optical microscopy (OM) and scanning electron microscopy (SEM). Image analysis revealed no significant changes in the microstructure with respect to the aging time. Vickers micro-hardness evaluation showed that both welded joints reach a maximum hardness value after 3 h of artificial aging followed by a reduction in its value in the next aging conditions. TEP was measured using the hot tip technique applying different hot tip temperatures (55, 60, 65, 70 °C), where higher temperature values showed to increase the sensibility of the technique to detect microstructural changes that affect micro-hardness and where not possible to observe via OM and SEM. TEP and micro-hardness exhibit an opposite correlation, which was confirmed calculating the Pearson's correlation coefficient (r). The correlation coefficient decreases when higher hot tip temperatures where employed to measure TEP. The good correlation between TEP values and micro-hardness allows to use TEP measurement as a prediction tool of material micro-hardness.

Thermoelectric power measurements on graphite pencil lead and traces

Thermoelectric power measurements on graphite pencil lead and traces

Thermoelectric Power (TEP) of Semiconducting Cobalt–Tellurite Glasses

The TEP, or Seebeck coefficient (S), for five bulk glasses in different compositions of binary semiconducting CoO–TeO2 glasses has been measured in the temperature range 304–490K. The glasses were prepared by melting dry mixtures of analytical reagent grads of CoO and TeO2 at a temperature of 1000 °C for 1h using quenching form the melt technique. TEP measurements have been performed utilizing specifically designed sample holders on annealed samples. The glasses were found to be n–type semiconductors with S in the range from –293 to –460 μV/K. S was found to be independent of temperature for all glass compositions, and Heikes’ formula adequately satisfied the TEP experimental results. The analyses provide evidence on the polaron formation and the disorder energy due to random fields.

Microchemistry evolution during industrial DC casting and subsequent homogenization of aluminum alloy AlMn0.5Mg0.5 (AA 3105)

The properties of non-heat treatable Al wrought alloys are influenced by the materials microchemistry in terms of the portion of alloying elements either in solid solution or in precipitated form. In the present study we analyze the phase selection during solidification as well as the changes in solute level and particle state during subsequent homogenization of the Al-Mn alloy AA 3105 (ISO AlMn0.5Mg0.5). Characterization of the microchemistry reactions is performed by a combination of optical and scanning electron microscopy with differential scanning calorimetry and measurements of thermoelectric power as well as specific electrical resistivity. The results are underpinned by CALPHAD simulations of the second-phase particles in the as-cast state, while the changes in solute level as well as volume, size and type of second-phase particles during homogenization are tracked with a statistical microchemistry simulation tool named ClaNG. Owing to the high Si content of the present alloy the as-cast state is dominated by α-Al(Fe,Mn)Si phase constituent particles together with a few Al6(Mn,Fe) and Mg2Si phases. Homogenization is characterized by the formation and redissolution of Mg2Si and, at higher temperatures, dispersoids of the α-Al(Fe,Mn)Si phase.

Influence of Er/Fe Substitution on Mg-Zn Nanoparticles’ Electromagnetic Properties and Applications

Mg0.8Zn0.2ErxFe2-xO4 (x = 0.00, 0.005, 0.01, 0.015, 0.02, and 0.025) nanoparticles made using the citrate gel autocombustion process were examined for their structure, morphology, and behavior. The single-phase cubic spinel structure was formed according to the diffraction pattern. Observations revealed that as the Er focus went from to 0.0 to 0.025, the average crystallite size (D) increased from 12.4 to 18.6 nm. The findings of the SEM and EDAX analyses reveal that the particles are uniform, with just a little amount of agglomeration and no impurity pickup. Nanoparticles from transmission electron microscopes (TEM) were present, the range from 12 to 19 nm. In IR spectroscopy using the Fourier transform (FTIR), nearly all the spinel ferrites presented generate two absorption bands that have wavelengths of around 400 cm and 600 cm. The BET surface area of the Er3+ ion doping Zn-Mg ferrites rises from 23.860 m2/gm to 29.845 m2/gm. The TG–DTA analysis of the prepared samples confirms the thermal stability of the samples; the temperature ranges from 100 to 750 °C.The transition temperature (Seebeck coefficient) of the samples was studied using thermoelectric power (TEP) measurement studies, and it was found that all the samples showed N-type semiconductor behavior. With an increase in erbium concentration, DC conductivity decreases. At room temperature, the magnetic characteristics of hysteresis loops, squareness ratio (SQR), anisotropy constant (K), magnetic moment (), coercivity (Hc), saturation magnetization (Ms), and retentivity (Mr) were examined. When erbium concentration rises, the magnetic moment (B) increases. The saturation magnetization values were 288.4615 and 244.5266 emu/g, and squareness ratio values from 0.01554 to 0.03303 were observed. These materials are converted from hard permanent magnet materials to soft magnet materials.

Study of nanostructural and non-adiabatic small polaron hopping conduction in nanostructured Bi2O3 - Fe2O3 – BaTiO3

The nanostructured samples of BFBT were synthesized by mixing a stoichiometric ratio of the reactants (Bi2O3 - Fe2O3 – BaTiO3) in a high-energy planetary ball mill system. The mixture was mechanically milled for 1, 2, 5, and 10 h. Structural characterization of the mechanosynthesized powder for 5h followed by heat-treatment at 673, 873, and 1073K for 5 h has been investigated through the X-ray diffraction technique. After 5 h of mechanical milling and heat-treatment at 873K for 5 h, HRTEM confirmed the nanostructure of BFBT two samples with an average particle size of 14.79 and 49.77 nm, respectively. The effect of the heat-treatment temperature of nanostructured BFBT samples on their conductivity is studied in conformity with Mott’s theory. The measurements of electrical conductivity and thermoelectric power show the semiconductor behavior of the prepared samples. The increase in crystal size of the heat-treated nanostructured BFBT samples decreases the grain boundaries, leading to the conductivity enhancement. The conduction was predominantly caused by electronic transport between Fe ions, which revealed the non-adiabatic SPH. The hopping carrier mobility was found to be the primary factor for determining the conductivity in the nanostructured BFBT system. The Greaves model for variable range hopping (VRH) was investigated for the prepared samples.

Impurity metallic conduction below the critical concentration of metal-insulator transition in FeCo x Si

Analysis on very detailed measurements of resistivity (ρ) and thermoelectric power (S) of magnetic impurity (Co) substituted iron silicide (FeSi) has been presented in this report. The impurity valence electrons of Co dominate the whole physical properties at low temperatures below 35 K, below the critical concentration x c (≈0.02). The negative thermopower and the positive slope in the resistivity at low temperatures are exciting and show that the system is not entirely insulator below the critical concentration of metal–insulator transition (x c ). So, due to the external impurity electrons, the system’s magnetic ground state could change considerably compared to the parent compound FeSi. This report may help unveil the interesting low-temperature transport properties between x = 0 and x = 0.04 (FeCo x Si). Two band model and variable range hopping model were employed to explain the low-temperature electrical and thermal transport properties.

Antiferromagnetism of CeCd0.67As2 existing deep inside the narrow gap semiconducting state

Single crystals of $R$Cd$_{0.67}$As$_2$ ($R$ = La and Ce) have been synthesized by high temperature ternary melt and their physical properties have been explored by means of magnetization, specific heat, electrical resistivity, Hall coefficient, and thermoelectric power measurements. $R$Cd$_{0.67}$As$_2$ compounds indicate a (structural) phase transition at high temperatures, accompanied by a remarkable increase of the electrical resistivity with an extremely low carrier concentration. CeCd$_{0.67}$As$_2$ exhibits a large magnetic anisotropy and an antiferromagnetic (AFM) order below $T_{N} = 4$~K. Magnetic susceptibility curves, together with magnetization isotherms and specific heat, are analyzed by the point charge model of crystalline electric field (CEF). In the paramagnetic state, the observed magnetic properties can be well explained by the CEF effects, implying that the 4$f$ moments remain localized. Electrical resistivity measurements, together with Hall resistivity and thermoelectric power, also suggest highly localized 4$f$ electrons, where Kondo contributions are negligible. The low temperature physical properties manifest strong magnetic field dependencies. For $H \perp c$, $T_{N}$ shifts to lower temperature as magnetic field increases, and eventually disappears at $H_{c} \sim 60$~ kOe. Inside the AFM state, three metamagnetic transitions are clearly evidenced from the magnetization isotherms. The RKKY interaction may be responsible for the AFM ordering in CeCd$_{0.67}$As$_2$, however it would have to be mediated by extremely low charge carriers. Although the AFM ordering temperature in CeCd$_{0.67}$As$_2$ can be continuously suppressed to zero, no AFM quantum phase transition is expected due to the lack of conduction electron clouds to screen the 4$f$ moments.

Influence of crystalline electric field on the magnetic properties of CeCd3X3(X=P,As)

CeCd$_3$P$_3$ and CeCd$_3$As$_3$ compounds adopt the hexagonal ScAl$_3$C$_3$-type structure, where magnetic Ce ions on a triangular lattice order antiferromagnetically below $T_\text{N} \sim$0.42~K. Their crystalline electric field (CEF) level scheme has been determined by fitting magnetic susceptibility curves, magnetization isotherms, and Schottky anomalies in specific heat. The calculated results, incorporating the CEF excitation, Zeeman splitting, and molecular field, are in good agreement with the experimental data. The CEF model, with Ce$^{3+}$ ions in a trigonal symmetry, explains the strong easy-plane magnetic anisotropy that has been observed in this family of materials. A detailed examination of the CEF parameters suggests that the fourth order CEF parameter $B_{4}^{3}$ is responsible for the strong CEF induced magnetocrystalline anisotropy, with a large $ab$-plane moment and a small $c$-axis moment. The reliability of our CEF analysis is assessed by comparing the current study with earlier reports of CeCd$_{3}$As$_{3}$. For both CeCd$_{3}X_{3}$ ($X$ = P and As) compounds, less than 40 \% of $R\ln(2)$ magnetic entropy is recovered by $T_\text{N}$ and full $R\ln(2)$ entropy is achieved at the Weiss temperature $\theta_{p}$. Although the observed magnetic entropy is reminiscent of delocalized 4$f$-electron magnetism with significant Kondo screening, the electrical resistivity of these compounds follows a typical metallic behavior. Measurements of thermoelectric power further validate the absence of Kondo contribution in CeCd$_{3}X_{3}$.

Growth and characterization of InBi0.95Te0.05 and InBi0.90Te0.10 crystals

Indium bismuthide is a well-known III-V group semi metallic compound. The compositions used were InBi0.95Te0.05 and InBi0.90Te0.10 crystals. The zone melting method used to grow the crystals. The zone-melting furnace has shown a temperature gradient of 45 °C/cm, and it has yielded the good quality crystals obtained, having a growth rate of 0.3 cm/h. The characterization of crystals was carried out using powdered X-ray technique. Using the FTIR spectra, the direct type band gap has been evaluated. The thermoelectric power measurement was carried out for InBi0.95Te0.05 and InBi0.90Te0.10 crystals. The Van der Pauw method has been employed for the Hall measurements at room temperature. This work report the detailed results of the above mentioned study and conclusions drawn therein.

The Influence of Cryogenic Treatment on the Microstructure and Mechanical Characteristics of Aluminum Silicon Carbide Matrix Composites

Aluminum matrix composites have been widely used in aerospace and automotive fields due to their excellent physical properties. Cryogenic treatment was successfully adopted to improve the performance of aluminum alloy components, while its effect and mechanism on the aluminum matrix composite remained unclear. In this work, the effects of cryogenic treatment on the microstructure evolution and mechanical properties of 15%SiCp/2009 aluminum matrix composites were systematically investigated by means of Thermoelectric Power (TEP), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The results showed that TEP measurement can be an effective method for evaluating the precipitation characteristics of 15%SiCp/2009 aluminum matrix composites during aging. The addition of cryogenic treatment after solution and before aging treatment promoted the precipitation from the beginning stage of aging. Furthermore, the aging time for the maximum precipitation of the θ″ phase was about 4 h advanced, as the conduction of cryogenic treatment accelerates the aging kinetics. This was attributed to the great difference in the linear expansion coefficient between the aluminum alloy matrix and SiC-reinforced particles, which could induce high internal stress in their boundaries for precipitation. Moreover, the lattice contraction of the aluminum alloy matrix during cryogenic treatment led to the increase in dislocation density and micro defects near the boundaries, thus providing more nucleation sites for precipitation during the aging treatment. After undergoing artificial aging treatment for 20 h, the increase in dispersive, distributed precipitates after cryogenic treatment improved the hardness and yield strength by 4% and 16 MPa, respectively.

Understanding the origin of mobility enhancement in wedge-shaped c-GaN nanowall networks utilizing spectroscopic techniques

Recently, the electron mobility in wedge-shaped c-GaN nanowall networks has been estimated to cross the theoretical mobility limit for bulk GaN. Significant blue-shift of the bandgap has also been observed. Both the findings are explained in terms of two-dimensional electron gas (2DEG) formed at the central vertical plane of the walls due to the polarization charges at the two inclined faces. Carrier concentration and mobility have earlier been determined from thermoelectric power and conductivity measurements with the help of a statistical model. Due to the network nature of the system, direct measurements of these quantities from Hall experiments are not possible. Search for a better way to estimate mobility in this system thus becomes important. Since, strain can also lead to the blue-shift of the bandgap, it is also imperative to evaluate carefully the role of strain. Here, using Raman spectroscopy, we have estimated carrier concentration and mobility in these nanowall networks with varied average tip-widths. Depth distribution of strain and luminescence characteristics are also studied. The study reveals that strain has no role in the bandgap enhancement. Moreover, the electron mobility, which is determined from the lineshape analysis of the A1(LO)-plasmon coupled mode in Raman spectra, has been found to be significantly higher than the theoretical limit of mobility for bulk GaN for the same electron concentration. These results thus corroborate the picture of polarization induced vertical 2DEG formation in these walls as predicted theoretically.

Precipitation sequence in Al-Sc-Zr alloys revisited

Sc and Zr additions to Al provide significant strengthening through the formation of Al3(Sc,Zr) L12 dispersoids. Fe impurities are always present in commercial Al alloys and the effect of these impurities on precipitation mechanisms in Al-Sc-Zr alloys remain poorly understood. This is particularly relevant when considering recycled aluminium as recycling results in the build-up of impurities. We used a combination of resistivity and thermo-electric power (TEP) measurements to monitor the evolution of solid solution during ageing of an Al-Sc, Al-Zr and Al-Sc-Zr alloy. Sc first comes out of solution to form Al3Sc dispersoids, followed by Zr to form Al3Zr. A range of Al3Sc-core/Al3Zr-shell faceted morphologies were observed with high angular annular dark field scanning transmission electron microscopy (HAADF-STEM). The presence of a new class of Fe-rich plate-like precipitates in the {100}Al was revealed via HAADF-STEM. The ordered positions of the precipitate and matrix reflections in the selected area diffraction pattern and in high-resolution TEM images reveal that these precipitates display a multi-layered structure. The centre of the plates is pseudo-icosahedral with apparent five-fold symmetry, and is rich in Fe. The outside layer has a L12 structure and is rich in Sc and Zr. The layered structure allows complete coherency with the aluminium matrix. These precipitates are highly anisotropic with a thickness of 5-10 nm and a length up to ∼1000 nm. This is the first report of the formation of this type of coherent pseudo-icosahedral precipitates in isothermal conditions which is named the D-phase.

N-type SnS2 thin films spray-coated from transparent molecular ink as a non-toxic buffer layer for solar photovoltaics

This paper reports the effect of solvent evaporation temperature on spray-coated tin disulfide (SnS2) thin films from molecular ink. Thiourea and tin chloride were the key chemical reagents used for the synthesis of SnS2 transparent ink under atmospheric conditions. The structural and compositional properties of SnS2 thin films revealed formation of pristine hexagonal SnS2. The films are smooth, homogeneous resulting in band gaps ranging from 2 to 2.22 eV suited for a Cd-free alternative buffer layer for Cu-based multicomponent solar cells. Thermoelectric power measurement showed that tin disulfide films exhibit n-type conductivity. Activation energy estimated from temperature variation of electrical conductivity measurement varied from 40 to 90 mV. Our results suggest that ink-processed SnS2 can be used as a potential alternative for opto-electronic devices such as thin film solar cell and photodetector devices.