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Polymeric heat exchangers (PHXs) have been used in applications involving weight restrictions, chemical compatibility, and fouling issues. Additive manufacturing (AM) or 3D printing provide new solutions to previously inaccessible combinations of properties and geometries. There are some advancements in the PHXs by AM; however, the process and the properties of materials still need further investigation to improve the overall performance. In this study, additively manufactured polyethylene terephthalate glycol (PETG) composites reinforced with graphite and pitch-based carbon fibers were evaluated for their potential application as PHXs. The thermal conductivity, volumetric heat capacity, coefficient of thermal expansion, creep behavior, and long-term performance were studied in detail. Our results reveal that the composites have an anisotropic thermal conductivity. The thermal conductivity along the printing direction is higher than the layer building direction due to the shear-induced alignment of the fillers. The printed composites achieve good thermal stability with 80% lower CTE at room temperature than neat PETG. Creep tests suggest the creep and creep recovery were highly temperature-dependent, and the deformation can be recovered when the temperature is below glass transition. These results suggest additive manufactured composites be potentially used for heat exchanger applications at low temperatures. [ABSTRACT FROM AUTHOR]

A metal matrix composite powder of molybdenum (Mo) + TiC was produced by mechanical alloying (MA) and used in additive manufacturing by electron beam powder bed fusion along with pure Mo powder to form sandwich structures. The Mo + TiC solid layers formed mixed structures of Mo with discrete TiC particles, eutectic Mo + TiC, and Mo dendrites. Thermodynamic modeling showed that the system contained an invariant eutectic reaction in the composition range used and indicated that the system was highly sensitive to changes in composition and temperature. [ABSTRACT FROM AUTHOR]

The thermal expansion of a thermally sprayed coating material is an important design-relevant property from both processing and application service points of view. High thermal stresses and residual strains can arise during deposition, and thermal exposure in service. The linkage to process conditions which can impact both physical and chemical attributes of the microstructure and consequently its expansion behavior is of great importance. The observed dilation response of various coatings is discussed based on the processing effects that induce differences in microstructural features and the impact of residual strain and annealing of defects on the dilation behavior during first thermal exposure. The coefficient of thermal expansion (CTE) of coatings was found to approach the bulk when the material is not sensitive to oxidation in-flight. For those materials sensitive to oxidation, mathematical models used to describe effective properties of composite materials were adopted to describe the CTE property. A large irreversible dilation change was found in the first heating cycle due to defect annealing (referred to as thermal shakedown). Several reasons are hypothesized for the magnitude of such dilation change. Together, the paper presents a comprehensive analysis of thermal expansion behavior of a diversity of thermal spray processed materials. [ABSTRACT FROM AUTHOR]

We report on the influence of sintering gas atmosphere on the thermoelectric (TE) performance of Ca3Co4O9 + δ ceramics made from precursor powders synthesized using chemical sol-gel route. Two sets of polycrystalline pellets were sintered in air and oxygen atmosphere, respectively. Sintering gas atmosphere and sintering temperature were found to have great influence on the Ca3Co4O9 + δ crystal orientation and formation of the Co3O4 secondary phase. All samples, regardless of sintering atmosphere, had similar Seebeck coefficients S that increased with measurement temperature. By contrast, the electrical resistivity ρ changed dramatically as the sintering temperature and atmosphere changed. For samples sintered in air, when the sintering temperature is above 1193 K, ρ increased as the sintering temperature increased. For samples sintering in oxygen, ρ decreased monotonically as the sintering temperature increased up to 1253 K. Benefited from the reduced ρ, the electrical power factor of the sample sintered in the oxygen at 1253 K is 0.539 mW m−1 K−2, and 40% greater than that from the sample sintered at 1193 K in air. Dimensionless figure-of-merit ZT of the sample sintered at 1253 K in oxygen is 0.28 at 1073 K, which is 20% greater than that of samples sintered in air at 1193 K. Overall, the highest TE performance was obtained for Ca3Co4O9 + δceramics sintered in oxygen at 1253 K, which is significantly higher than the Ca3Co4O9 + δ decomposition temperature of 1199 K in air. [ABSTRACT FROM AUTHOR]

Phase change materials are identified for their ability to rapidly alternate between the amorphous and crystalline phases and have large contrast in the optical/electrical properties of the respective phases. The materials are not only primarily used in memory storage applications, but also recently they have been identified as potential thermoelectric materials [D. Lencer et al., Adv. Mater.23, 2030–2058 (2011)]. Many of the phase change materials studied today can be found on the pseudo-binary (GeTe)1−x(Sb2Te3)x tie-line. While many compounds on this tie-line have been recognized as thermoelectric materials, here we focus on Ge4SbTe5, a single phase compound just off of the (GeTe)1−x(Sb2Te3)x tie-line, which forms in a stable rocksalt crystal structure at room temperature. We find that stoichiometric and undoped Ge4SbTe5 exhibits a thermal conductivity of ∼1.2 W/m K at high temperature and a large Seebeck coefficient of ∼250 μV/K. The resistivity decreases dramatically at 623 K due to a structural phase transition which leads to a large enhancement in both thermoelectric power factor and thermoelectric figure of merit at 823 K. In a more general sense, the work presents evidence that phase change materials can potentially provide a new route to highly efficient thermoelectric materials for power generation at high temperature. [ABSTRACT FROM PUBLISHER]

In waste heat recovery applications, thermoelectric (TE) generators are subjected to thermal gradients and thermal transients, creating mechanical stresses in the TE legs. Such stresses are functions of the elastic moduli of the TE material. For SnTe1±x matrices (where x = 0.0 or 0.016) composite specimens with 0-4 vol% SiC nanoparticle (SiCNP) additions, the elastic moduli (Young's modulus, shear modulus, and Poisson's ratio) were measured by resonant ultrasound spectroscopy from room temperature (RT) to 663 K. The effects of matrix composition and the SiCNP additions on the RT intercepts and the slopes of the elastic modulus as a function of temperature are also discussed. [ABSTRACT FROM AUTHOR]

The coefficient of thermal expansion (CTE) is a key design parameter for thermoelectric (TE) materials, especially in energy harvesting applications since stresses generated by CTE mismatch, thermal gradients, and thermal transients scale with the CTE of the TE material. For the PbTe-PbS-based TE material (PbSnTe)(PbS)-0.055 % PbI over the temperature ranges of 293-543 and 293-773 K, a CTE, α, of 21.4 ± 0.3 × 10 K was measured using (1) dilatometry and (2) high-temperature X-ray diffraction (HT-XRD) for powder and bulk specimens. The CTE values measured via dilatometry and HT-XRD are similar to the literature values for other Pb-based chalcogenides. However, the processing technique was found to impact the thermal expansion such that bloating (which leads to a hysteresis in thermal expansion) occurred for hot pressed billets heated to temperatures >603 K while specimens fabricated by pulsed electric current sintering and as-cast specimens did not show a bloating-modified thermal expansion even for temperatures up to 663 K. The relationship of bloating to the processing techniques is discussed, along with a possible mechanism for inhibiting bloating in powder processed specimens. [ABSTRACT FROM AUTHOR]

The microstructural evolution in two potential solid oxide fuel cell (SOFC) sealing glass materials exposed to air and a gas mixture of steam + H + N at 800 °C up to 10000 h was determined. The glass exposures were performed on common SOFC substrates like alumina and zirconia. Characterization of the crystalline phases and pore size distribution was performed for the specimens with various exposure conditions. Comparison of the microstructural and chemical stability of the two glasses was performed based on known trends related to glass chemistry. It was observed that multicomponent glasses followed few rules for chemical and microstructural stability reported in the literature for glasses with fewer components. The two glasses examined in this study displayed adequate resistance to devitrification but marginal resistance to porosity changes in the SOFC environment exposure. The implications of the results for the design and long-term performance of SOFC seals are discussed. [ABSTRACT FROM AUTHOR]

A novel method to determine the fiber-matrix interfacial properties of ceramic matrix composites is proposed and evaluated; where micro-pillar samples containing inclined fiber/matrix interfaces were prepared from a SiC fiber-reinforced SiC matrix composites and then compression-tested using the nano-indentation technique. This new test method employs a simple geometry and mitigates the uncertainties associated with complex stress state in the conventional single-filament push-out method or tensile unloading-reloading hysteresis loop analysis method for the determination of interfacial properties. Based on the test results using samples with different interface orientations, the interfacial debond shear strength and the internal friction coefficient are explicitly determined and compared with values obtained by other test methods. SEM observation showed that micro compression caused an adhesive type of debonding between the fiber and the pyrolytic carbon interface. The results suggest that the debonding/failure behavior of the micro-pillars followed the Coulomb fracture criterion. The determined interfacial debond shear strength is ~100 MPa, which appears to be smaller than that determined from fiber push-out test for similar composite systems. The difference can be explained by the effect of normal stress (clamping stress) on the apparent interfacial debond shear strength. [ABSTRACT FROM AUTHOR]

We report on the effects of thermal processing on the microstructure and composition of a system with overall stoichiometry of 3Cu:1Sb:3Se with the aim of producing single-phase CuSbSe. It was found that slow cooling from the melt produced a multiphase material consisting of CuSe and CuSbSe, but devoid of CuSbSe. Cooling rapidly from the melt resulted in three-phase microstructures consisting of CuSe, CuSbSe, and CuSbSe. Subsequent annealing of the three-phase material between 325 and 400 °C shifted the composition toward nearly pure CuSbSe-the target compound of this work. The kinetics of the transformation into CuSbSe was successfully described using a modified Avrami model which suggests that diffusion is the rate-controlling step. Values of Young's modulus and hardness, obtained by nanoindentation, are reported for CuSe, CuSbSe, and CuSbSe. [ABSTRACT FROM AUTHOR]

The mechanical behavior of 430 stainless steel and pre-oxidized FeCrAlY open-cell foam materials of various densities was evaluated in compression at temperatures between 450°C and 600°C in an environment containing hydrogen sulfide and water vapor. Both materials showed negligible corrosion due to the gaseous atmosphere for up to 168 h. The monotonic stress-strain response of these materials was found to be dependent on both the strain rate and their density, and the 430 stainless steel foam materials exhibited less stress relaxation than the FeCrAlY for similar experimental conditions. Using the results from multiple hardening-relaxation and monotonic tests, an empirical constitutive equation was derived to predict the stress-strain behavior of FeCrAlY foams as a function of temperature, and strain rate. These results are discussed in the context of using these materials in a black liquor gasifier to accommodate the chemical expansion of the refractory liner resulting from its reaction with the soda in the black liquor. [ABSTRACT FROM AUTHOR]

We report the results of an investigation on the structural evolution of a potential new thermoelectric material, Cu3SbSe3, as a function of temperature from 25 to 390 °C. From high-temperature x-ray diffraction data, the refined lattice parameters were seen to change nonlinearly, but continuously, with temperature, with an increased rate of thermal expansion in the a and b lattice parameters from around 125 °C to 175 °C and negative thermal expansion in the c axis from around 100 °C to 175 °C. Crystallographic charge flipping analysis indicated an increase in the disorder of the copper cations with temperature. This reversible order/disorder phase transition in Cu3SbSe3 affects the transport properties, as evidenced by thermal conductivity measurements, which change from negative to positive slope at the transition temperature. This structural change in Cu3SbSe3 has implications for its potential use in thermoelectric generators. [ABSTRACT FROM AUTHOR]

High-temperature alloy design requires a concurrent consideration of multiple mechanisms at different length scales. We propose a workflow that couples highly relevant physics into machine learning (ML) to predict properties of complex high-temperature alloys with an example of the 9–12 wt% Cr steels yield strength. We have incorporated synthetic alloy features that capture microstructure and phase transformations into the dataset. Identified high impact features that affect yield strength of 9Cr from correlation analysis agree well with the generally accepted strengthening mechanism. As a part of the verification process, the consistency of sub-datasets has been extensively evaluated with respect to temperature and then refined for the boundary conditions of trained ML models. The predicted yield strength of 9Cr steels using the ML models is in excellent agreement with experiments. The current approach introduces physically meaningful constraints in interrogating the trained ML models to predict properties of hypothetical alloys when applied to data-driven materials. [ABSTRACT FROM AUTHOR]