Thermoelectric detection of the magnetic field by fluxgate gradiometer on subsurface tin inclusions embedded in a copper bar

A comparison between published analytical results with measured experimental results of the magnetic field produced by thermoelectric currents of subsurface semi-spherical pure tin inclusions embedded in a copper bar under the influence of an external heating and cooling into the specimen is presented. The measurements were taken from a 12.7 mm-thick copper bar with different subsurface semi-spherical pure tin inclusion diameters and lift-off distances between the sensor tip and specimen surface varied from 12.7 to 3.18 mm and from 1 to 8 mm, respectively. The experimental magnetic flux density measurements show a reasonable agreement as compared to analytical data as a function of the lift-off distances with the different inclusion diameters with the exception of the intrinsic material background magnetic signal that affected deeply the detectability of subtle imperfections in noncontacting thermoelectric measurements.     

On the application of magneto-optic films in NDE

Some applications of magneto-optic (MO) films in the field of nondestructive evaluation (NDE) are presented. The method for imaging and measuring of magnetic fields on micrometer scale is based on the analysis of changes of the magnetic domain structure caused by the detected field. A process of MO film evaluation and calibration is described. The existence of the leakage field around a fatigue crack in nonmagnetic austenitic material is detected by a highly sensitive MO film. An experimentally determined distribution of the excitation field of an eddy current probe is compared with results obtained by the finite element method (FEM) numerical procedure.     

Current deflection NDE for the inspection and monitoring of pipes

Routine inspection of oil and gas pipes for time dependent degradation is essential. Pipelines are most commonly inspected using In-Line Inspection (ILI), however restrictions from pipe geometry, features or flow rate can prevent its use. Facility pipework rarely facilitates ILI, and external inspection often warrants the undesirable removal of the pipe insulation and cladding. This work investigates the applicability of a current deflection non-destructive evaluation technique for both the detection and growth monitoring of defects, particularly focusing on corrosion. Magnetic sensors are used to monitor variations in the spatial distribution of the induced magnetic flux density outside a pipe that arise from deflection of an injected electric current around inner or outer wall defects. An array of orthogonal magnetoresistive sensors has been used to measure the magnetic flux density surrounding six-inch schedule 40 seamless and welded carbon steel and austenitic steel pipes. The measurements were stable and repeatable to the order of 100 pT which suggests that the defect detection or growth monitoring of corrosion-type defects may be possible with a few amps of injected current when measurements are taken at around 50 mm lift-off. The sensitivity of the technique is dependent on factors including defect geometry, sensor lift-off, bends, variations in nominal pipe geometry or material properties, and the presence of ferromagnetic objects, each of which were investigated using either experiment or a validated finite element model.     

Automatic flaw detection in NDE signals using a panel of neural networks

The magnetic flux leakage (MFL) method is widely used for the assessment of the condition of off-shore cables, gas pipes and aerial tramway cables. At Empa, a measuring device was developed to use the physical effect of MFL for large diameter steel cables as used in civil engineering structures.It is considered that the automatization of the flaw detection task would offer great advantages, both from an economic point of view as well as in terms of reliability. In this work the application of a feature extraction model based on an analytical approximation of the MFL intensity is presented. Parameters that are closely related to the size and position of a defect are used as fitting variables in the solution of the inverse problem that links the recorded MFL intensity to the flaw that generates it.Using these significant parameters a committee of neural networks is trained in order to obtain a robust flaw detection algorithm.     

NDE技术在FGH95粉末材料中的应用

采用超声无损评价技术即NDE技术,对不同的粉末实验件进行了超声检测研究。结果表明,粉末材料中的声速和声衰减是表征材料显微组织的两个重要超声波参量,其中声速与材料的致密性有关,声衰减与材料的晶粒度有关。     

Study of molybdenum precipitation in steels using thermoelectric power measurement

The precipitation of molybdenum in iron has been investigated using thermoelectric power measurement. We found an increase of the thermoelectric power of iron with the molybdenum content and a coefficient of its influence has been determined. A correlation between secondary hardening due to the precipitation of Mo₂C carbides and thermoelectric power variations has been established.     

Detection of surface and subsurface heterogeneities by the hot tip thermoelectric power method

This paper is dedicated to the possibility of using hot tip thermoelectric power (HTTEP) measurement for the detection of surface and subsurface metallic heterogeneities. Depending on the size of the tip, metallic heterogeneities of different sizes and depths can be detected. A numerical simulation based on the resolution of transport equations has been developed in order to test the potentialities of this technique. Simulations and HTTEP measurement obtained on artificial metallic defects in samples are compared. The influence of the nature, size and depth of these heterogeneities on the measured thermoelectric power (TEP) is discussed.     

On thermoelectric figure-of-merit of Pb0.78Sn0.22TeGe solid solution

The investigation results of temperature dependences of thermoelectric parameters in the 140–440 K temperature range are reported for Pb_0.78Sn_0.22TeGe(0.5 at.%) solid solution, having p-type conductivity. The aim is to determine its perspectives as a promising material for thermoelements. Investigations have shown that the temperature dependences of thermoelectric parameters have some interesting features caused by phase transition observed at temperature 290 K.     

Stray flux effects on the magnetic hysteresis parameters in NDE of low carbon steel

The magnetic minor hysteresis loops and parameters of low carbon steel plate samples annealed at different temperatures have been measured and compared with the results of ring samples. The influences of stray flux in the fringe areas of the plate on the magnetic properties were discussed with the help of a finite element approach. It is found that the coercivity H_C and the hysteresis loss W_H of all of the plate samples sharply deviate from the values of the corresponding ring samples with the same applied field amplitude H_a larger than 3000 A/m. It is proposed that the fringe areas of the plate sample are magnetized by the stray flux to an irreversible state at large Ha, and the remanent fields act against the applied field in the inverse direction, resulting in the extreme increase of H_C and W_H. Although these parameters measured at high-applied field can reflect the microstructure changes, low field results are preferable for the practical use.     

Magnetic monitoring of the fatigue process of the rim material of railway wheel sets

Materials diagnostic by NDT is an important element of manufacture and service. The classic methods of technical diagnostic do not provide extensive knowledge of the technical condition of all of the elements. Constructional elements built of iron alloy can be characterised by magnetic properties, hence it may be inspected by magnetic methods and the recognition of material technical condition becomes more comprehensive. The paper presents the influence of fatigue processes on the magnetic parameters of the material. The results of the monitoring procedures of the wheel set tread surface in practise are also given.     

Changes in the thermoelectric power of a Ni base superalloy induced by elastic and plastic strain

Induced changes in the thermoelectric power (TEP) of the Inconel 718 by elastic and plastic strain have been previously reported by López Cuéllar et al. [1]. Now, in this work, the TEP of a nickel-based superalloy Waspaloy with different heat treatments has been followed during stressing tests. In all cases plastic deformation has been attained. TEP measurement variations of ∼160 nV/°C have been attained for a certain heat treatment that produces the desirable γ′ phase, and low changes of TEP are observed for the treatment that does not produce γ′. Like for the Inconel 718, results indicate that TEP of the Wasploy is clearly affected by the elastic and plastic strain induced during tests. The previously proposed model to describe the change in TEP induced by the strain in specimens of Inconel 718 [1], is validated with the Waspaloy superalloy in this work. Thus, these results confirm that the TEP technique is a powerful tool to detect non-desirable states and levels of strain in alloys containing γ′ phase like the Waspaloy and Inconel superalloys.     

Cu effects on radiation embrittlement of Ni-containing complex model alloys and the related potentials of the thermoelectric method

One of the major damage contributions to radiation embrittlement of steels is due to copper precipitation. Precipitation mechanism continues until saturation depending on available amount of precipitants, copper concentration in particular.At the JRC-IE measurement of thermoelectric voltage using a specially developed non-destructive method to assess the embrittlement state of materials are regularly performed. The results analysis, from the test of complex sets of nickel containing alloys with different copper content levels, for both fresh and irradiated conditions, are discussed in this paper with the aim of identifying, if copper precipitation to saturation can be followed by non-destructive measurements. The findings are promising showing that the STEAM non-destructive method has very high potential for studying the copper kinetics part in the radiation embrittlement phenomenon.     

Extrinsic properties affecting the thermoelectric power output of few layer graphene/polyvinylidene fluoride composite thin films

Graphene containing thin film polymer composites possess favorable thermoelectric properties that allow them to be considered for use in low powered applications where durability and weight are important considerations. The use of these materials will be subject to the properties of the specific application, making it necessary to quantify the dependence of the thermoelectric power output on these extrinsic properties. In this study, we show that the power output for a 60 wt% few layer graphene/polyvinylidene composite is dependent on the absolute temperature, temperature difference, and dimensions of the heat source and magnitude of the load resistance. Additionally, the peak power output is achieved when the load resistance matches the internal resistance of the composite. These results demonstrate that it is necessary to optimize the extrinsic properties of the specific application when designing a thermoelectric module to maximize the power output.     

Effect of cooling rate on the thermoelectric and mechanical performance of Bi0.5Sb1.5Te3 prepared under a high magnetic field

Given the thermoelectric and mechanical performance of a given material is closely related to its microstructure, in this paper, the microstructure of p-type Bi_0.5Sb_1.5Te₃, fabricated by a high magnetic field assisted melting-solidification (HMAMS) process, is successfully tuned by regulating the cooling rate during the solidification process, and a systematic investigation has been carried out to the effect of the cooling rate on the crystal orientation, microstructure, thermoelectric and mechanical performance of the obtained materials. By this approach, the thermal conductivity is sharply reduced due to the intensive phonon scattering by the massive BST/Te and Te/BST_Ⅱ interfaces, while the power factor is less affected, and the flexural strength is enhanced owing to the narrowing of eutectic strip and spacing. Eventually, a highest ZT of 1.23 at 323 K with a maximal flexural strength 23.2 MPa has been obtained in the sample prepared under a 6 T magnetic field at a cooling rate of 16 K/min.     

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.     

Solubility of Ti in thermoelectric PbTe compound

For the first time, the temperature dependence of the solubility of Ti in PbTe is considered. The potential of Ti doping in PbTe was recently examined both theoretically and experimentally, in which a ∼9% efficiency enhancement was observed compared to an undoped PbTe. We present an experimental study of the system for different Ti concentrations indicating the solubility limit of Ti in PbTe and the effect of temperature on the morphology of the alloy. Exceeding the solubility limit leads to phase separation accompanied by formation of the TiTe₂ phase. At high temperatures, dissolution of the TiTe₂ secondary phase occurs for low Ti concentration, and the system transfers to the single-phase disordered solid solution. On the contrary, for the same temperatures, in the case of higher Ti concentrations the two-phase state is preserved. Theoretical explanation of the observed results is given using ab inito calculations combined with the statistical thermodynamic approach. The quasi-binary PbTe-TiTe₂ cross-section of the ternary Pb–Ti–Te phase diagram, indicating the Ti solubility in (Pb_1-cTi_c)Te is presented.     

Comparison of the evaluation of the carbon content in solid solution in extra-mild steels by thermoelectric power and by internal friction

The comparison of the evaluation of the carbon content in solid solution in extra-mild steels by thermoelectric power (TEP) and by internal friction (IF) showed the existence of a population of carbon atoms, which is not detected by IF but is visible by TEP. This population, which contributes to the strain ageing, could be constituted of carbon atoms in interaction with substitutional atoms.     

Synthesis and thermoelectric properties of the PbSe1−xTex alloys

The PbSe_1−xTe_x alloys with x = 0.2, 0.3, 0.5, 0.85 and 1.0 were prepared by induction melting, ball milling and spark plasma sintering techniques. The thermoelectric properties of the samples were investigated. The XRD analysis indicated that all samples are NaCl-type structure solid solutions Pb(Se,Te) containing nanograins. Increasing Te content resulted in increasing the lattice parameter a. The thermoelectric measurements show that all samples are n-type semiconductors in temperature range from 300 K to 673 K. The electrical resistivity of the doped sample is much smaller than that of pure PbSe, but comparable to that of PbTe. The absolute Seebeck coefficients for the doped sample PbSe_1−xTe_x with x = 0.2, 0.3 and 0.5 range from 150 μV/K at 300 K to 250 μV/K at 673 K, which is much larger than that of pure PbSe (66-138 μV/K), but smaller than that of PbTe (230-310 μV/K) in the same experimental conditions. The thermal conductivity for the doped sample PbSe_1−xTe_x with x = 0.2, 0.3 and 0.5 range from 0.95 to 0.66 W/m K, which is much smaller than that of pure PbSe (2.1-1.3 W/m K) or PbTe (1.4-1.1 W/m K). As a result, the figure of merit for the doped sample can be enhanced. The maximum dimensionless figure of merit ZT of 1.15 was obtained in the sample PbTe_0.5Se_0.5 at 573 K, more than 50% higher than that of pure PbTe prepared in the same condition.