Solid Deposit-Induced High Temperature Oxidation

The present study is aimed at investigating the high temperature oxidation induced by ash deposition from use of alternative fuels. The alloys and coatings being studied are typical of those used in current power generating gas turbines, as well as those that may be used in advanced systems. To achieve this objective, the alloys Rene„S N5, GTD 111, and IN 738 as well as these alloys coated with platinum aluminide and CoNiCrAlY were exposed to conditions relevant to corrosion induced by using alternative fuels. The test conditions representative of deposits from use of alternative fuels were selected based upon initial experiments that involved testing the alloy Rene„S N5 with a platinum aluminide coating at 750„aC, 950„aC, and 1150„aC in a variety of environments with deposits of CaO, CaSO4, and Na2SO4. Based upon the results from such tests, a temperature (950oC) and a deposit (CaO) were selected for the further experiments to compare the corrosion characteristics of all of the alloys and coatings. At 950oC with deposits of CaO, which are the selected experimental conditions obtained from the preliminary tests, accelerated cyclic oxidation experiments were performed with all uncoated and coated superalloys in extra dry air and wet (PH2O=0.1atm) air to compare corrosion characteristics of each with one another. Experimental details will be described followed by the presentation of experimental results and discussion. Additionally, uncoated GTD 111 specimens were exposed to different contaminants and moisture level environments to study the effect of contaminant level and water vapor pressure on CaO-induced degradation. Then, CaO deposits were coated on thermal barrier coatings (TBCs) and specimens with TBCs were exposed to the cyclic oxidation environments. The effects of deposits other than CaO, such as Fe2O3 and SiO2, on the oxidation characteristics of the specimens were also investigated. Finally, a mechanism for high temperature oxidation induced by CaO deposits was developed. It turns out that CaO directly reacts with protective oxides, such as Al2O3 and/or Cr2O3, to form non-protective ternary Ca compounds. Cracks are initiated and propagate along the weak interface between Ca compounds and underlying oxide layers resulting in spallation of Ca compound layers

Sparked Reduced Graphene Oxide for Low-Temperature Sodium Beta Alumina Batteries

Wetting Na metal on the solid electrolyte of a liquid Na battery determines the operating temperature and performance of the battery. At low temperatures below 200 degrees C, liquid Na wets poorly on a solid electrolyte near its melting temperature (T-m = 98 degrees C), limiting its suitability for use in low-temperature batteries used for large-scale energy-storage systems. Herein, we propose the use of sparked reduced graphene oxide (rGO) that can improve the Na wetting in sodium-beta alumina batteries (NBBs), allowing operation at lower temperatures. Experimental and computational studies indicated rGO layers with nanogaps exhibited complete liquid Na wetting regardless of the surface energy between the liquid Na and the graphene oxide, which originated from the capillary force in the gap. Employing sparked rGO significantly enhanced the cell performance at 175 degrees C; the cell retained almost 100% Coulombic efficiency after the initial cycle, which is a substantial improvement over cells without sparked rGO. These results suggest that coating sparked rGO is a promising but simple strategy for the development of low-temperature NBBs. © 2019 American Chemical Society11sciescopu

Formation of Interfacial Reaction Layers in Al₂O₃/SS 430 Brazed Joints Using Cu-7Al-3.5Zr Alloys

The formation of interfacial reaction layers was investigated in an α-Al2O3/430 stainless steel (SS430) joint brazed using a Cu-7Al-3.5Zr active brazing alloy. Brazing was conducted at above its eutectic temperature of 945 °C and below liquidus 1045 °C, where liquid and solid phases of the brazing alloys coexists. At 1000 °C, the liquid phase of the brazing alloy was wet onto the α-Al2O3 surface. Zr in the liquid phase reduced α-Al2O3 to form a continuous ZrO2 layer. As the dwell time increased, Zr in the liquid phases near α-Al2O3 interface was used up to thicken the reaction layers. The growth kinetics of the layer obeys the parabolic rate law with a rate constant of 9.25 × 10−6 cm·s−1/2. It was observed that a number of low yield strength Cu-rich particles were dispersed over the reaction layer, which can release the residual stress of the joint resulting in reduction of crack occurrence

Numerical studies of natural convection phenomena for a vertical cylinder with multiple lateral baffles in triangular and hexagonal enclosures

Numerical case studies for natural convection were conducted for triangular and hexagonal enclosures as a follow-up to a previous study of natural convection for a baffled cylinder in a square enclosure. For a Prandtl number of 0.71, a range of the length ratio corresponding to the spatial ratio was varied from 0.4 to 0.9, and Rayleigh numbers from 2.5 × 103 to 5.0 × 107 was considered. To model the buoyancy force, the Boussinesq approximation was adopted. The boundary condition of constant wall temperature was applied to the cylinder and enclosure surfaces. Overall trends of the heat transfer performance were separated into conduction- and convection-dominated regimes, similar to the previous study. Because of the wider space of the triangular enclosure, however, an advantage of the convection effect was observed in the triangular enclosure. Due to the short distance between the cylinder and enclosure, the hexagonal enclosure had a more conduction-dominated characteristic than that of the triangular enclosure. A strong correlation was identified between the location of the high-energy vortex and the Nusselt number distribution. Using the calculation results, an improved average Nusselt number correlation is proposed to include triangular, square and hexagonal enclosures

Improving ionic conductivity of Nasicon (Na3Zr2Si2PO12) at intermediate temperatures by modifying phase transition behavior

Molten sodium (Na) anode high temperature batteries, such as Na-NiCl2 and Na-S, draw attentions to be used in stationary electricity storage applications. Recent efforts are exerted to lower their operating temperatures down to below 200 degrees C in order to adopt ultra-low cost cell production, establish easier maintenance, pursue enhanced safety, and more. One of main challenges in lowering the operation temperature is radical decrease in ionic conductivity of their solid electrolytes. Na3Zr2Si2PO12 (Nasicon) is considered as a solid electrolyte for the lower temperature operation. Here we report Na ionic conductivity of Nasicon at 150 degrees C increases by adding Ge element. The ionic conductivity of Ge-added sample (Na-3[Zr2-delta Ge delta]Si2PO12, delta = 0.1, 0.2) is measured as high as 1.4 x 10(-2) S cm(-1) at 150 degrees C which is about two times higher than those of the bare Nasicon. The phase transition temperature of the Ge-added samples is lowered, thereby the volume fraction of the rhombohedral phase, which is stable at higher temperatures and exhibits higher Na ion conductivities, increases. This finding provides a useful guideline to further increase the ionic conductivity of Nasicon solid electrolytes, which can advance materialization of lower temperature operating Na batteries.11Nsciescopu

Effect of Bonding Temperature on Crack Occurrences in Al2O3/SS 430 Joints Using Cu-Based Brazing Alloys

The effect of bonding temperature on crack occurrences in α-Al2O3/SS 430 joints using Cu-based brazing alloys was investigated with emphasis on the microstructural characterization, hardness, and analytical residual stresses of the joints. The brazing was conducted using Cu-7Al-xTi and Cu-7Al-xZr (x = 2.5, 3.5, and 4.5) alloys at 1000 °C and 1080 °C leading to solid–liquid and liquid-state bonding, respectively. Cracks occurred in the joints brazed at 1080 °C irrespective of the alloys, while crack-free joints were obtained at 1000 °C for joints with only Cu-7Al-xZr alloys. Increases in the bonding temperature or utilization of Cu-7Al-xTi alloys led to a formation of brittle Fe-containing intermetallic or Fe-Cr phases in the brazed seams due to the dissolution of Fe from SS 430, which deteriorated the mechanical properties of the brazed seam. Maximum residual stresses of the real brazed joint were obtained by combining the calculated yield strength and measured hardness of the brazed seams. Eventually, when the hardness of the brazed seam was less than 107 Hv, the yield strength was 124 MPa or less and the maximum residual stress generated in the joint corresponded to 624 MPa or less, leading to a crack-free joint

Advanced Na-NiCl₂ Battery Using Nickel-Coated Graphite with Core–Shell Microarchitecture

Stationary electric energy storage devices (rechargeable batteries) have gained increasing prominence due to great market needs, such as smoothing the fluctuation of renewable energy resources and supporting the reliability of the electric grid. With regard to raw materials availability, sodium-based batteries are better positioned than lithium batteries due to the abundant resource of sodium in Earth’s crust. However, the sodium–nickel chloride (Na-NiCl₂) battery, one of the most attractive stationary battery technologies, is hindered from further market penetration by its high material cost (Ni cost) and fast material degradation at its high operating temperature. Here, we demonstrate the design of a core–shell microarchitecture, nickel-coated graphite, with a graphite core to maintain electrochemically active surface area and structural integrity of the electron percolation pathway while using 40% less Ni than conventional Na-NiCl₂ batteries. An initial energy density of 133 Wh/kg (at ∼C/4) and energy efficiency of 94% are achieved at an intermediate temperature of 190 °C

Sodium Ion Diffusion in Nasicon (Na₃Zr₂Si₂PO₁₂) Solid Electrolytes: Effects of Excess Sodium

The Na superionic conductor (aka Nasicon, Na_1+<i>x</i>Zr₂Si_<i>x</i>P_3–<i>x</i>O₁₂, where 0 ≤ <i>x</i> ≤ 3) is one of the promising solid electrolyte materials used in advanced molten Na-based secondary batteries that typically operate at high temperature (over ∼270 °C). Nasicon provides a 3D diffusion network allowing the transport of the active Na-ion species (i.e., ionic conductor) while blocking the conduction of electrons (i.e., electronic insulator) between the anode and cathode compartments of cells. In this work, the standard Nasicon (Na₃Zr₂Si₂PO₁₂, bare sample) and 10 at% Na-excess Nasicon (Na_3.3Zr₂Si₂PO₁₂, Na-excess sample) solid electrolytes were synthesized using a solid-state sintering technique to elucidate the Na diffusion mechanism (i.e., grain diffusion or grain boundary diffusion) and the impacts of adding excess Na at relatively low and high temperatures. The structural, thermal, and ionic transport characterizations were conducted using various experimental tools including X-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and electrochemical impedance spectroscopy (EIS). In addition, an ab initio atomistic modeling study was carried out to computationally examine the detailed microstructures of Nasicon materials, as well as to support the experimental observations. Through this combination work comprising experimental and computational investigations, we show that the predominant mechanisms of Na-ion transport in the Nasicon structure are the grain boundary and the grain diffusion at low and high temperatures, respectively. Also, it was found that adding 10 at% excess Na could give rise to a substantial increase in the total conductivity (e.g., ∼1.2 × 10^–1 S/cm at 300 °C) of Nasicon electrolytes resulting from the enlargement of the bottleneck areas in the Na diffusion channels of polycrystalline grains

Enhancing Li Ion Battery Performance by Mechanical Resonance

© 2021 American Chemical Society.The quest for safe and high-performance Li ion batteries (LIBs) motivates intense efforts seeking a high-energy but reliable anode, cathode, and nonflammable electrolyte. For any of these, exploring new electrochemistry methods that enhance safety and performance by employing well-designed electrodes and electrolytes are required. Electrolyte wetting, governed by thermodynamics, is another critical issue in increasing Li ion transport through the separator. Herein, we report an approach to enhancing LIB performance by applying mechanical resonant vibration to increase electrolyte wettability on the separator. Wetting is activated at a resonant frequency with a capillary wave along the surface of the electrolyte, allowing the electrolyte to infiltrate into the porous separator by inertia force. This mechanical resonance, rather than electrochemistry, leads to the high specific capacity, rate capability, and cycling stability of LIBs. The concept of the mechanical approach is a promising yet simple strategy for the development of safer LIBs using liquid electrolytes.11Nsciescopu