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An F-atom with ∼1 eV translational energy was aimed at a line of fluorocarbon adsorbates on Cu(110). Sequential ‘knock-on’ of F-atom products was observed by STM to propagate along the 1D fluorocarbon line. Hot F-atoms travelling along the line in six successive ‘to-and-fro’ cycles paralleled the rocking of a macroscopic Newton's cradle.

We report the observation of electron-induced unidirectional planar molecular rotation of para-chlorostyrene on Si(100), studied by scanning tunneling microscopy (STM) at room temperature and by ab initio theory. This bifunctional molecule is shown to be favorable to the electron-induced rotation since the phenyl group acts as a pivot and the vinyl as a lever arm. In the initial configuration, both phenyl and vinyl are attached to silicon dimers along the same row of the substrate. The first electron from the STM tip is observed to induce a lateral shift of the vinyl to “state 1” in which the vinyl is bound asymmetrically to one side of a silicon dimer. The second electron is found to give rise to a ∼60° rotation to “state 2”, a configuration in which the vinyl has swung around the phenyl to an adjacent dimer row. The impulsive two-state (I2S) model was employed to explain the conversion of the initial state to state 1 and the conversion of state 1 to state 2. These two successive impulses were computed b...

Crossed molecular beams of gases have provided definitive information concerning the dynamics of chemical reactions. The results have, however, of necessity been averaged over collisions with impact parameters ranging from zero to infinity, thus obscuring the effect of this important variable. Here we employ a method through which impact parameter averaging is suppressed in a surface reaction. We aim a highly collimated reactive 'projectile' molecule along a surface at a stationary adsorbed 'target' molecule, with both the projectile and target being observed by Scanning Tunnelling Microscopy (STM). The projectile was CF2 recoiling from electron-induced bond-breaking in chemisorbed CF3 on Cu(110) at 4.6 K. The collimation of the resulting CF2 'surface-molecular-beam' restricted it to a lateral spread of ±1° as a consequence of its interaction with the Cu rows below. This collimation was modelled by molecular dynamics simulations. In the experiments the recoiling CF2 projectile was aimed, successively, at impact parameters of b = 0 and +3.6 Å at a chemisorbed second CF2, b = +1.8 Å at a chemisorbed I-atom, or b = -4.0, b = -0.4 and b = +3.2 Å at a chemisorbed vinyl radical. The pattern of reactive and non-reactive scattering was determined by STM. These collimated surface-molecular-beams have the potential to aim molecular projectiles with selected impact parameters at the many target species identifiable at a surface by STM.

By inducing chemical reactions at chosen collision miss-distances, we introduce a new measurable in surface reaction dynamics., Collision geometry is central to reaction dynamics. An important variable in collision geometry is the miss-distance between molecules, known as the “impact parameter.” This is averaged in gas-phase molecular beam studies. By aligning molecules on a surface prior to electron-induced dissociation, we select impact parameters in subsequent inelastic collisions. Surface-collimated “projectile” molecules, difluorocarbene (CF2), were aimed at stationary “target” molecules characterized by scanning tunneling microscopy (STM), with the observed scattering interpreted by computational molecular dynamics. Selection of impact parameters showed that head-on collisions favored bimolecular reaction, whereas glancing collisions led only to momentum transfer. These collimated projectiles could be aimed at the wide variety of adsorbed targets identifiable by STM, with the selected impact parameter assisting in the identification of the collision geometry required for reaction.

Approximately, 55% of the energy produced from conventional vehicle resources is lost due to heat losses. An efficient waste heat recovery process will lead to improved fuel efficiency and greenhouse gas emissions. Thermoelectric generators (TEGs) are heat recovery devices that are being widely studied by a range of energy-intensive industries. Efficient solid-state thermoelectric devices are good candidates to reduce fuel consumption in an automobile. Thermoelectric materials have had limited automotive applications due to the automotive waste heat recovery temperature range, the rarity and toxicity of some materials, and the limited ability to mass manufacture thermoelectric devices from expensive TE materials. However, skutterudite is one class of material that has demonstrated significant promise in the transportation waste heat recovery temperature domain. Durability and reliability of the TEGs are the most significant concerns in the product development process. Cracking of the materials at hot-side interface is found to be a major failure mechanism of TEGs under thermal loading. Cracking affects not only the structural integrity but also the energy conversion and overall performance of the system. In this paper, cracking of thermoelectric material as observed in performance testing is analyzed using numerical simulations and analytic experiments. This paper shows, with the help of finite element analysis (FEA), the detailed distribution of stress, strain, and temperature is obtained for each design. Finite element (FE)-based simulations show the tensile stresses as the primary factor causing radial and circumferential cracks in the skutterudite. For a TEG design, loading conditions and closed-form analytical solutions of stress/strain distributions are derived. Scenarios with minimum tensile stresses are sought. These approaches yield the minimum of stress/strain fields which produce cracks. Finally, based on these analyses and computational fluid dynamics (CFD) studies, strategies in tensile stress reduction and failure prevention are proposed.

This investigation reports the output parameters of thermoelectric modules on based on metal/LAST(T) hot side contacts.

Results on fabrication and characterization of metal hot side electrical contacts to nanostructured AgPbmSbTe2 + m and Ag(PbSn)mSbTe2 + m materials by using SnTe diffusion bonding are presented.

Room temperature Young's modulus, shear modulus and Poisson's ratio were measured by Resonant Ultrasound Spectroscopy (RUS) for a total of 29 skutterudite specimens cut from four hot pressed billets: (i) two p-type (Ce 0.9 Fe 3.5 Co 0.5 Sb 12 ) billets, (ii) one n-type (Co 0.95 Pd 0.05 Te 0.05 Sb 3 ) billet with 0.1 atomic% Ce addition and (iii) one n-type billet without Ce addition. Within a given hot pressed billet, the elastic moduli were very uniform from specimen to specimen. Also, Ce-doping of the n-type specimens did not result in a significant shift in the elastic modulus. For n-type specimens with 98% relative density, the mean Young's modulus, shear modulus and Poisson's ratio were 140.6 ± 0.2 GPa, 57.34 ± 0.05 and 0.226 ± 0.001 GPa, respectively. For the p-type specimens of similar relative density, the mean Young's and shear moduli were approximately 7% lower than the n-type, while there was no significant difference between the mean Poisson's ratio values of the n-type and p-type 98% dense specimens. The measured elastic moduli agreed relatively well with the literature values for other Sb-based skutterudite compositions. In addition, nanoindentation measurements on three mutually orthogonal specimen faces showed no appreciable anisotropy in Young's modulus.

Two-phase PbTe–PbS materials, in which PbS is a nanostructured phase, are promising thermoelectric materials for the direct conversion of heat energy into electricity. In this study, a Vickers indentation mean hardness of 1.18 ± 0.09 GPa was measured for hot pressed specimens Pb 0.95 Sn 0.05 Te–PbS 8% while the mean hardness of cast specimens was 0.68 ± 0.07 GPa. The mean fracture toughness of the not pressed specimens was estimated as 0.35 ± 0.04 MPa m 1/2 via Vickers indentation. Resonant Ultrasound Spectroscopy (RUS) measurements on hot pressed specimens gave mean values of Young's modulus, shear modulus and Poisson's ratio of 53.1 GPa, 21.4 GPa and 0.245, respectively while for the cast specimens the Young's and shear moduli were about 10% lower than for the hot pressed, with a mean value of Poisson's ratio of 0.245. The differences between the hardness and elastic moduli values for the cast and hot pressed specimens are discussed.

Thermomechanical characterization is important to material evaluation and device design in the development of thermoelectric technology. In this study, we utilize the resonant ultrasound spectroscopy (RUS) technique to examine the elastic behavior of a cast LAST (Pb–Sb–Ag–Te) material with a composition of Ag0.86Pb19Sb1.0Te20 between room temperature and 823 K. The temperature-dependent Young’s modulus exhibits a monotonically decreasing trend with increasing temperature. However, an abnormal slope change in the Young’s modulus–temperature curve around 500 K is observed. In addition, hysteresis between heating and cooling data in the temperature range of 450–550 K is observed, which appears to be dependent on the heating/cooling rate during the RUS experiments such that the hysteresis disappears when the heating/cooling rate was decreased from 5 to 2 K min–1. In this study we propose an order–disorder transition model for the anomalous temperature-dependent Young’s modulus behavior observed in this study.

Porosity in thermoelectric materials affects the thermal, electrical and mechanical properties of the materials. In this study, the resonant ultrasound spectroscopy technique was used to determine the Young's modulus, E, shear modulus, G, and Poisson's ratio, ν, of 12 hot pressed and five cast polycrystalline specimens of the thermoelectric material LAST (lead–antimony–silver–tellurium) as a function of volume fraction porosity, P, for P ranging from 0.01 to 0.14. A least-squares fit of the Young's and shear moduli data to the relationships E = ED(1 − bPEP) and G = GD(1 − bPGP), respectively yielded ED = 58.4 ± 0.6 GPa and GD = 23.0 ± 0.2 GPa, where ED and GD are the estimated Young's modulus and shear modulus at room temperature for theoretically dense specimens, respectively. The unitless, material-dependent constants bPE and bPG were bPE = 3.5 ± 0.2 and bPG = 3.5 ± 0.2.

The thermal expansion for two compositions of cast and hot-pressed LAST (Pb–Sb–Ag–Te) n-type thermoelectric materials has been measured between room temperature and 673 K via thermomechanical analysis (TMA). In addition, using high-temperature X-ray diffraction (HT-XRD), the thermal expansion for both cast and hot-pressed LAST materials was determined from the temperature-dependent lattice parameters measured between room temperature and 623 K. The TMA and HT-XRD determined values of the coefficient of thermal expansion (CTE) for the LAST compositions ranged between 20 × 10−6 K−1 and 24 × 10−6 K−1, which is comparable to the CTE values for other thermoelectric materials including PbTe and Bi2Te3. The CTE of the LAST specimens with a higher Ag content (Ag0.86Pb19Sb1.0Te20) exhibited a higher CTE value than that of the LAST material with a lower Ag content (Ag0.43Pb18Sb1.2Te20). In addition, a peak in the temperature-dependent CTE was observed between room temperature and approximately 450 K for both the ca...

In this study, we focus on the room-temperature mechanical properties of LAST (Pb–Sb–Ag–Te) materials fabricated using casting with slow-cool temperature profiles (cooling rate 5°C h−1). The higher biaxial flexural strength is potentially beneficial for the mechanical integrity of thermoelectric devices.

In the open literature, reports of mechanical properties are limited for semiconducting thermoelectric materials, including the temperature dependence of elastic moduli. In this study, for both cast ingots and hot-pressed billets of Ag-, Sb-, Sn- and S-doped PbTe thermoelectric materials, resonant ultrasound spectroscopy (RUS) was utilized to determine the temperature dependence of elastic moduli, including Young's modulus, shear modulus and Poisson's ratio. This study is the first to determine the temperature-dependent elastic moduli for these PbTe-based thermoelectrics, and among the few determinations of elasticity of any thermoelectric material for temperatures above 300 K. The Young's modulus and Poisson's ratio, measured from room temperature to 773 K during heating and cooling, agreed well. Also, the observed Young's modulus, E, versus temperature, T, relationship, E(T) = E 0(1–bT), is consistent with predictions for materials in the range well above the Debye temperature. A nanoindentation study o...

LAST (lead–antimony–silver–tellurium) compounds comprise a family of semiconducting materials with good thermoelectric properties. However, the as-cast form of LAST exhibits large grain size and hence low mechanical strength. Powder processing can produce a fine powder particle size that enhances fracture strength, however the powders tend to agglomerate if the individual powder diameters are less than a few microns across. Dry milling or wet milling (hexane additions of 0 cm3 and 10 cm3) produced hard agglomerates roughly 40 μm in diameter while wet milling with hexane additions of 25 cm3, 30 cm3 or 50 cm3 resulted in small, porous agglomerates roughly 20 μm in diameter. Thus, by adjusting the amount of milling liquid used while milling LAST powders, one can shift from hard to soft agglomerates, where the literature shows that soft agglomerates are less harmful to the final, sintered product. Also, in agreement with the results from the literature on other materials, wet milling of LAST powders produced smaller particle sizes but required longer times to reach the grindability limit.

Compositional changes (alloying) in metals, ceramics and semiconductors lead to changes in microstructure and mechanical properties that range from subtle to dramatic. In particular, hardness measurements have been widely used to track changes in mechanical properties as a function of composition. In this study, composition dependence of the Vickers indentation hardness, H, is examined for 17 specimens that include 15 different n-type compositions within the quaternary chalcogenide family of LAST (lead-antimony-silver-tellurium) high-temperature thermoelectric materials. In addition, nanoindentation measurements were made on six of the 17 specimens that were indented via Vickers indentation. The H values obtained independently by the microindentation (Vickers indentation) and nanoindentation techniques agreed very well. In this study, relationships from the literature for the parabolic dependence of H on the single compositional variable x for pseudo binaries AB1−xCx is successfully extended to describe the composition dependence of H for a true quaternary system. Possible implications of hardness change on fabrication and in-service reliability are also discussed.

For 17 cast lead–antimony–silver–telluride (LAST) thermoelectric specimens (representing 14 different chemical compositions), a combination of Vickers and Knoop microindentation techniques were used to determine the composition-dependent Young's modulus, E, which ranged from 24 to 68 GPa. Following microindentation, independent nanoindentation measurements were also performed on 10 of the 17 specimens. In the literature, for pseudobinary joins in ternary or quaternary compounds (with the compositions A x B1– x C or A x B1– x CD, respectively), changes in the Young's modulus have been expressed as quadratic functions of the compositional parameter x. In this study, we extend the quadratic functional form to a paraboloid in four composition variables to describe composition-dependent changes in E for the LAST compounds. Also, the composition-dependent changes in LAST are compared to the trends observed in the literature for E and bulk modulus for systems described by a single compositional variable.

Although thermoelectric (TE) materials are often fabricated as cast ingots, there has been recent interest in the powder processing of these materials. Cast TE materials typically have grain sizes as large as several hundred microns, but powder processing (grinding, milling and then sintering) can produce dense specimens with a reduced grain size, an improved mechanical integrity and enhanced TE properties. In the TE literature when powder processing is employed, little or no description is provided of the powder processing techniques and the powder processing parameters are either not characterized or only a mean particle size is given. In this study, powders milled from solid cast ingots of the TE compound LAST (lead–antimony–silver–tellurium) were characterized via Coulter Counter, to measure the mean powder particle size and the size distribution, scanning electron microscopy (SEM), to examine the powder particle morphology, and X-ray diffraction (XRD), to detect possible phase changes or amorphizatio...

For thermoelectric applications, the best materials have high electrical conductivity and thermopower and, simultaneously, low thermal conductivity. Such a combination of properties is usually found in heavily doped semiconductors. Renewed interest in this topic has followed recent theoretical predictions that significant increases in performance are possible for nanostructured materials, and this has been experimentally verified. During exploratory synthetic studies of chalcogenide-based bulk thermoelectric materials it was discovered that several compounds spontaneously formed endotaxially embedded nanostructures. These compounds have some of the best known properties for bulk thermoelectric materials in the 500–800 K temperature range. Here we report our continued efforts to better understand the role of the nanostructures while concurrently furthering the development of these new materials (for example n-type lead–antimony–silver–tellurium, and p-type lead–antimony–silver–tin–tellurium) into thermoelectric power-generation devices.

This is the first study applying Weibull statistics to the strength distribution of a thermoelectric (TE) material and to determine the fracture strength of a member of the LAST-T (lead–antimony–silver–tellurium–tin) family of high-temperature TE compounds. The p-type TE material Ag0.9Pb9Sn9Sb0.6Te20 was cast from the melt and specimens cut from the resulting ingot were fractured in a ball-on-ring biaxial fracture test. The Weibull parameters (characteristic strength and Weibull modulus) were obtained from the fracture data. Implications of the Weibull analysis on the fabrication of TE devices are discussed.

Thermo-mechanical fatigue of nickel-base superalloys has been carried out for more than twenty years, but a unified standard for such tests has yet to be fully formulated. As a consequence of this, even excluding differences in testing procedure from laboratory to laboratory, the range of testing conditions in use is very large. In this paper, the authors consider the effect of the different parameters in a TMF test such as hold-time and temperature-mechanical phase angle on the thermo-mechanical response of test samples. This is carried out in terms of both stress response and sample life, in order to identify the common trends in the sample damage during the evolution of the mechanical response. To date, no definitive test program systematically covering the different variables has been published in the scientific literature. As, even after a long span of time, the literature data for many alloys is sparse, the work presented here concentrates on analyzing TMF response data from test programs carried out by the authors on the single crystal nickel-base alloy SRR99. [S0094-4289(00)01003-3]

When mechanical harvesting of tart cherries first began in the late 1950s, methods were developed to safely harvest and handle cherries so quality would be maintained for processing. In recent years, however, fruit quality maintenance has become an industry-wide problem. Cushion materials installed on mechanical harvesters in the 1960s through the 1970s to prevent splitting and bruising of fruit have been replaced by smooth hard surfaces. Split, bruised and soft fruit results in the loss of income and can cause problems in the pitting process. Production practices and characteristics of orchard sites were evaluated in relation to fruit firmness and susceptibility to softening and mechanical damage. Fruit firmness before mechanical harvesting at 31 orchard sites averaged 650 N/m (44.8 lbf/ft). After harvest, average firmness dropped 35% to 421 N/m (29.0 lbf/ft). Mechanical damage for fruit collected off various harvesters averaged 12%. Drop tests evaluated cherry firmness sensitivity and identified cushion materials that can reduce firmness loss during mechanical harvesting. Firmness loss averaged 28% for a 0.9 m (3 ft) drop onto a hard surface compared to 6 to 10% for four cushion materials. Cherries dropped 4 m (13 ft) on a tightly stretched tarp without cushion material resulted in a loss of firmness of 35%, but only 14 to 28% with cushion material. To minimize mechanical damage and maintain firmness, all hard harvester surfaces that cherries impact must be covered with adequate cushioning.

PbTe-PbS materials are promising for thermoelectric power generation applications. For the composition of (Pb0.95Sn0.05Te)0.92(PbS)0.08 nanostructuring from nucleation and growth and spinodal decomposition has been reported along with thermal conductivity of approximately 1.1 W/m·K at 650 K [1]. Based on temperature-dependent measurements of electrical conductivity, thermopower, and thermal conductivity, the thermoelectric figure of merit, ZT, are ~1.5 at 650 K for cast ingots.To develop larger quantities of material for device fabrication, advancement in the synthesis, processing and production of (Pb0.95Sn0.05Te)0.92(PbS)0.08 is necessary. Powder processing of samples is a well-known technique for increasing sample strength, and uniformity. In this presentation, we show sample fabrication and processing details of pulsed electric current sintering (PECS) processed (Pb0.95Sn0.05Te)0.92(PbS)0.08 materials and their thermoelectric properties along with the latest advancements in the preparation of these materials.

For the material (Pb0.95Sn0.05Te)1-x(PbS)x nanostructuring from nucleation and growth and spinodal decomposition were reported to enhance the thermoelectric figure of merit over bulk PbTe, producing ZT of 1.1 - 1.4 at 650 K for x = 0.08[1]. Thermoelectric modules made from (Pb0.95Sn0.05Te)1-x(PbS)x materials with various hot-side metal electrodes were fabricated and tested. Short circuit current was measured on unicouples of Pb0.95Sn0.05Te – PbS 8% (n-type) legs and Ag0.9Pb9Sn9Sb0.6Te20 (p-type) legs over 10 (A) for a hot side temperature of 870K, and a cold side of 300K. Hot pressed (Pb0.95Sn0.05Te)1-x(PbS)x materials were also investigated for module fabrication. Investigations of the electrical properties of hot-pressed (Pb0.95Sn0.05Te)1-x(PbS)x materials are presented along with the latest advancements in the fabrication and characteristics of modules based on the processing of these materials.

Low electrical contact resistance is essential for the fabrication of high efficiency thermoelectric generators in order to convert heat to electricity. These contacts must be stable to high temperatures and through thermal cycling. A ratio of the contact resistance to the leg resistance below 0.1 is the goal for fabrication of a high efficiency thermoelectric power generation device. Here we present the fabrication procedures and characterization of contacts of metal alloys to Pb-Sb-Ag-Te (LAST) and Pb-Sb-Ag-Sn-Te (LASTT) compounds. Contacts were fabricated and measured for both ingot and hot pressed materials. Stainless steel 316 has shown a low resistance contact to these thermoelectric materials when the proper bonding conditions are used. Different time-temperature-pressure conditions for bonding to n-type and to p-type legs are presented. Contact resistances below 10μΩcm2 have been measured. In addition, break tests have shown bond strengths exceeding the semiconductor fracture strength. One of the considerations used in selecting iron alloys for electrical interconnects is the similarity in the coefficient of thermal expansion to the LAST and LASTT materials which is 18 ppm/°C and relatively temperature insensitive. Contacts to the thermoelectric materials were accomplished by diffusion bonding in a furnace developed in our lab at Michigan State University. The furnace is capable of reaching temperatures of up to 1000°C with a controlled atmosphere of a reducing gas. Fabrication procedures and contact data are presented in this paper.

PbTe-based thermoelectric (TE) materials exhibit promising thermoelectric properties and have potential applications in waste heat recovery from sources such as truck engines and shipboard engines. TE components designed for these applications will be subject to mechanical/thermal loading and vibration as a result from in-service conditions, including mechanical vibration, mechanical and/or thermal cycling, and thermal shock.In the current study, we present and discuss the mechanical properties of several PbTe-based compositions with different dopants and processing methods, including n-type and p-type specimens fabricated both by casting and by powder processing. Room temperature hardness and Young's modulus are studied by Vickers indentation and nanoindentation while fracture strength is obtained by biaxial flexure testing. Temperature dependent Young's modulus, shear modulus, and Poisson's ratio are studied via resonant ultrasound spectroscopy (RUS).

Thermoelectric modules are of great interest for power generation applications where temperature gradients of approximately 500K exist, and hot side temperatures near 800K. The fabrication of such modules requires optimization of the material compositions, low contact resistivities, and low thermal loss.AgPbmSbTe2+m (LAST) and Ag(Pb1-xSnx)m SbTe2+m (LASTT) compounds are among the best known materials appropriate for this temperature range. Various measurement systems have been developed and used to characterize bulk samples in the LAST and LASTT systems within this operating temperature range. From the characterized data, modeling of modules based on these materials and segmented legs using LAST(T) with Bi2Te3 have been used to identify the optimal geometry for the individual legs, and the length of the Bi2Te3 segments. We have segmented LAST(T) with Bi2Te3 and achieved contact resistivities of less than 10 μΩ•cm2.Here we give a detailed presentation on the procedures used in the fabrication of thermoelectric generators based on LAST, LASTT, and segmented with Bi2Te3 materials. We also present the output data on these generators.

Lead-Antimony-Silver-Tellurium (L-A-S-T) materials, synthesized at Michigan State University, show promising thermoelectric properties at high temperatures for use in power generation applications. Recent scaled-up quantities of L-A-S-T show a ZT=1.4 at 700 K approaching the figure of merit for samples made in small quantities. These materials are of great interest for power generation applications with hot side temperatures in the range of 600-800 K. Developing these materials into working devices requires minimization of the thermal and electrical parasitic contact resistances, so various fabrication methods are under investigation. To examine each method, a new measurement system has been developed to characterize these devices under various load and temperature gradients. An introduction to the system will be presented, as well as results for devices made of the L-A-S-T materials.

CM186, a Hf containing nickel-base superalloy is an attractive alloy, in terms of castability and thus manufacturing costs, for aero engine and land based gas turbine applications. Mechanical property and coating characterisation studies of this alloy are sparse, whilst thermo-mechanical fatigue (TMF) data do not exist in the literature. In service however, engine components experience severe cyclic thermal gradients and mechanical loads and as a consequence, TMF is a major lifing factor. This study investigates the TMF behaviour of single crystal and directionally solidified CM186 alloy and the effect of a silicon modified al&de coating, Sermalloy 15 15, on the TMF lives. Strain controlled TMF tests on coated and uncoated samples are carried out with an out-of-phase 180’ difference between mechanical strain and temperature at 3 different strain ranges. In-situ monitoring of microcrack initiation and propagation and post-mortem fiactographic analysis are implemented to understand the damage mechanisms and coating performance.

Thermomechanical fatigue experiments were conducted under strain-control with two different strain-temperature loops, simulating the loading situation at critical locations of aero-engine turbine blades. These cycles (-135°lag cycles with Re-ratios equal to zero and to -∞, Tmin= 300°C, Tmax=1050°C; and an in-phase cycle with Re=0 and Tmin= 300°C, Tmax=850°C) were applied to two different single crystal nickel base alloys (SRR99 and CMSX6). Crack initiation and propagation were monitored in-situ by means of a computer vision system throughout the test at predefined numbers of cycles. It was shown that the specimen life under the influence of the -135°lag cycle is mainly controlled by crack propagation. Scanning electron microscopic investigations of the side and fracture surface of the specimens reveal multiple crack initiation at sites of preferential oxidation. The fracture surface indicates that crack propagation is mainly in a penny-shaped configuration. The in-phase cycle causes crack initiation from internal porosity; therefore an analysis based on surface observations is not possible. A detailed description of the TMF testing is given in [16]. To analyze the crack propagation under TMF loading a number of isothermal crack growth tests were conducted to study the influence of time, temperature and R-ratio on the crack growth rate.

When the FeNiCr alloy Alloy 800 is exposed to a mixture of H2S, CO and H2O (as well as to a CO free gas of this composition at 800°C and total pressure of 1.5 bar), internal as well as external corrosion takes place and sulfides, oxides and carbides are formed. Corrosion results in a decrease of creep strength under high load while it remains practically unchanged or is even improved under medium and low load. The unexpected absence of nickel sulfide formation in the above gas mixture is attributed to the comparatively low nickel activity in the alloy so that sulfide formation would occur only at sulphur partial pressures exceeding the pressures used in this investigation. The increase rate of crack propagation under load can be interpreted in terms of the cracking of the oxide surface layer on the metal, the cracks allowing the aggressive gas to freely penetrate to the metal surface. The tests with the CO free gas demonstrate that the strength loss is principally caused by sulfidation and oxidation while carburisation which leeds to a strengthening results in somewhat slower destrengthening rate.

High efficiency thermoelectric modules are of great interest for power generation applications where hot side temperatures of approximately 800K exist. The fabrication of such modules requires a multidisciplinary effort for the optimization of the material compositions, the engineering of the module systems, modeling and fabrication of the devices, and constant feedback from characterization. Pb-Sb-Ag-Te (LAST) and Pb-Sb-Ag-Sn-Te (LASTT) compounds are among the best known materials for this temperature range. Modeling of these materials and possible cascaded structures shows efficiencies of 14% can be achieved for low resistance contacts. Using antimony we have achieved contact resistivities less than 20 µΩ·cm2. Here we give a detailed presentation on the procedures used in the fabrication of thermoelectric generators based on these new materials. We also present the characterization systems and measurements on these generators.