Edgar Lara-Curzio

Carbon/Carbon Composite Bipolar Plate for Proton Exchange Membrane Fuel Cells

Carbon/carbon‐composite bipolar plates for proton exchange membrane fuel cells (PEMFC) have been fabricated by slurry molding a chopped‐fiber preform followed by sealing with chemically vapor‐infiltrated carbon. The resulting component is hermetic with respect to through‐thickness leakage and has a high electronic conductivity (200–300 S/cm) as a result of the deposited graphitic carbon. The material has a low density due to retained porosity resulting in a low‐weight component. Biaxial flexure strength was measured to be 175 ± 26 MPa. Cell testing of a active area, single‐sided plate indicated very low cell resistance and high efficiency, but with a somewhat steep drop‐off in voltage with current at high values. Corrosion testing indicated minimal corrosion in fuel cell environments.

Understanding the Degradation of Silicon Electrodes for Lithium-Ion Batteries Using Acoustic Emission

Silicon is a promising anode material for lithium ion battery application due to its high specific capacity, low cost, and abundance. However, when silicon is lithiated at room temperature it can undergo a volume expansion in excess of 280% which leads to extensive fracturing. This is thought to be a primary cause of the rapid decay in cell capacity routinely observed. Acoustic emission (AE) was employed to monitor activity in composite silicon electrodes while cycling in lithium ion half-cells using a constant current-constant voltage procedure. The major source of AE was identified as the brittle fracture of silicon particles resulting from the alloying reaction that gives rise to LixSi phases. The largest number of emissions occurred on the first lithiation corresponding to surface fracture of the silicon particles, followed by distinct emission bursts on subsequent charge and discharge steps. Furthermore, a difference in the average parameters describing emission during charge and discharge steps was observed. Potential diagnostic and materials development applications of the presented AE techniques are discussed.

Analysis of oxidation-assisted stress-rupture of continuous fiber-reinforced ceramic matrix composites at intermediate temperatures

Abstract A model is presented to estimate the reliability and time-to-failure of an unidirectional continuous fiber-reinforced ceramic composite when subjected to stresses beyond the matrix cracking stress. The particular case of oxidation-assisted stress-rupture at intermediate temperatures is considered. The effects of stress and temperature on the reliability of the model composite are examined. Model predictions are presented for the specific case of CG-Nicalon™/SiC CFCCs with carbonaceous fiber coatings.

Low-temperature heat capacity and localized vibrational modes in natural and synthetic tetrahedrites

The heat capacity of natural (Cu12−x (Fe, Zn, Ag)x(Sb, As)4S13) and synthetic (Cu12−xZnxSb4S13 with x = 0, 1, 2) tetrahedrite compounds was measured between 2 K and 380 K. It was found that the temperature dependence of the heat capacity can be described using a Debye term and three Einstein oscillators with characteristic temperatures that correspond to energies of ∼1.0 meV, ∼2.8 meV, and ∼8.4 meV. The existence of localized vibrational modes, which are assigned to the displacements of the trigonally coordinated Cu atoms in the structure, is discussed in the context of anharmonicity and its effect on the low lattice thermal conductivity exhibited by these compounds.

N site ordering effect on partially ordered Fe16N2

Partially ordered Fe16N2 thin films have been fabricated on Fe (001)-buffered GaAs (001) single-crystal substrates by a facing target sputtering process. The saturation magnetization has been systematically investigated as a function of N site ordering in partially ordered Fe16N2 thin films, which is found to be increased monotonically with the increase in the N site ordering parameter, reaching up to 2.68 T at high ordering case. A model discussion is provided based on the partial localization of 3d electron states in this material system, which successfully rationalizes the formation of the giant saturation magnetization in chemically ordered Fe16N2. We further demonstrate that the average magnetic moment of partially ordered Fe16N2 sensitively depends on the special arrangement of Fe6N clusters, which is the key to realize high magnetic moment in this material system.

Lattice thermal conductivity of the Cu3SbSe4-Cu3SbS4 solid solution

The compositional dependence of the crystal structure and lattice thermal conductivity in the Cu3SbSe4-Cu3SbS4 system has been studied. The lattice parameters of the Cu3SbSe4-xSx compounds decrease linearly with x, and the tetragonal structure (space group 14−2m no. 121) of the end compounds is maintained at all compositions. The lattice thermal conductivity is much lower than that predicted by a simple rule of mixtures, which is typical for a solid solution. The Debye model produces a very reasonable fit to the experimental lattice thermal conductivity data when phonon scattering due to atomic mass and size differences between Se and S is taken into account. Compounds in this series are likely to improve upon the thermoelectric performance of Cu3SbSe4, which has shown ZT = 0.72 when optimized.

Temperature-dependent elastic moduli of lead telluride-based thermoelectric materials

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 Youngs modulus, shear modulus and Poissons 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 Youngs modulus and Poissons ratio, measured from room temperature to 773 K during heating and cooling, agreed well. Also, the observed Youngs 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 of Youngs modulus on the specimen faces showed that both the cast and hot-pressed specimens were approximately elastically isotropic.

Big Area Additive Manufacturing of High Performance Bonded NdFeB Magnets

Additive manufacturing allows for the production of complex parts with minimum material waste, offering an effective technique for fabricating permanent magnets which frequently involve critical rare earth elements. In this report, we demonstrate a novel method - Big Area Additive Manufacturing (BAAM) - to fabricate isotropic near-net-shape NdFeB bonded magnets with magnetic and mechanical properties comparable or better than those of traditional injection molded magnets. The starting polymer magnet composite pellets consist of 65 vol% isotropic NdFeB powder and 35 vol% polyamide (Nylon-12). The density of the final BAAM magnet product reached 4.8 g/cm3, and the room temperature magnetic properties are: intrinsic coercivity Hci = 688.4 kA/m, remanence Br = 0.51 T, and energy product (BH)max = 43.49 kJ/m3 (5.47 MGOe). In addition, tensile tests performed on four dog-bone shaped specimens yielded an average ultimate tensile strength of 6.60 MPa and an average failure strain of 4.18%. Scanning electron microscopy images of the fracture surfaces indicate that the failure is primarily related to the debonding of the magnetic particles from the polymer binder. The present method significantly simplifies manufacturing of near-net-shape bonded magnets, enables efficient use of rare earth elements thus contributing towards enriching the supply of critical materials.

Methodology for the determination of the interfacial properties of brittle matrix composites

The interfacial properties of a glass-ceramic matrix composite (SiC/CAS) were determined from single-fibre push-out tests using the interfacial test system. The coefficient of friction, μ, the residual clamping stress, σc, and fibre axial residual stress, σz, were extracted by fitting the experimental stress versus fibre-end displacement curves using the models of Hsueh, and Kerans and Parthasarathy. Using Hsuehs model, the intrinsic interfacial frictional stress (τ=μσc) was found to be 11.1±3.2 MPa, whereas by using Kerans-Parthasarathys model it was found to be 8.2±1.5 MPa. Comparisons between these models are included, together with a discussion of data analysis techniques.

Nanoindentation characterization of surface layers of electrical discharge machined WC/Co

Abstract This study applies nanoindentation and other analysis techniques to investigate the influence of wire electrical discharge machining (EDM) process on the structure and properties of machined surface layers of WC–Co composites. Multiple indents were conducted on the cross-section of the surface recast layer, sub-surface heat-affected zone, and bulk material. The energy disperse X-ray spectrometry and X-ray diffraction were used to analyze the material compositions in the heat-affected zone and recast layer and to study the electrical spark eroded surface. The indents were inspected by scanning electron microscopy to distinguish between regular and irregular indents in these three regions. Irregular indents were caused by the porosity, soft matrix material, separation of grain boundaries, and thermal cracks caused by EDM process. The hardness and modulus of elasticity obtained from regular indents in bulk material and heat-affected zone were comparable to those of WC. It was found that the recast layer had lower hardness and modulus of elasticity than the bulk material and heat-affected zone.