Seamless joining of silicon carbide ceramics through an sacrificial interlayer of Dy3Si2C2

A ternary carbide Dy₃Si₂C₂ coating was fabricated on the surface of SiC through a molten salt technique. Using the Dy₃Si₂C₂ coating as the joining interlayer, seamless joining of SiC ceramic was achieved at temperature as low as 1500 °C. Phase diagram calculation indicates that seamless joining was achieved by the formation of liquid phase at the interface between Dy₃Si₂C₂ and SiC, which was squeezed out under pressure and continuously consumed by the joining interlayer. This work implies the great potential of the family of ternary rare-earth metal carbide Re₃Si₂C₂ (Re = Y, La-Nd) as the sacrificial interlayer for high-quality SiC joining.     

Design of Fe-based nanocrystalline alloys with superior magnetization and manufacturability

Fe-based alloys with a nanocrystalline-amorphous nanostructure exhibit superior soft-magnetic performances; however they generally suffer from the low magnetization because of their heavy doping for an acceptable manufacturability. In this study, we proposed a revolutionary nanostructure-construction concept based on preforming dense nuclei in the melt-quenching process with a critical cooling-rate and refining the nano-structure via transient metalloid-rich interfaces. A novel alloy composition of Fe_85.5B₁₀Si₂P₂C_0.5 was developed via our multi-metalloid stabilization and critical formability strategies by using a total of only 4.6 wt. % light metalloids. This unique alloy design effort leads to unprecedented magnetic properties with the super-high B_s of 1.87 T and µ_e of 1.0–2.5 × 10⁴, which outperform all commercial counterparts and have a high potential to substitute for commercial bulk Si-steels currently used for soft-magnetic applications. This hetero-structuring and lean-alloying strategy provides a paradigm for the next-generation of magnetic materials.     

Influence of carbon-fiber felts on the development of carbon–carbon composites

Oxidized PAN-fiber felt was carbonized to 600, 1000, and 1800 °C, respectively. Different carbon/carbon composites (C/C composites) were prepared from oxidized PAN-fiber felt, the carbonized felts, and resol-type phenol–formaldehyde resin. These composites were then carbonized and graphized at temperatures of between 600 and 2400 °C. The C/C composite made with oxidized PAN-fiber felt showed a strong fiber/matrix bonding, and those developed from the carbonized felt (heat-treatment of 1800 °C) showed a poor fiber/matrix bonding. The graphitized composites reinforced with the oxidized PAN-fiber felt resulted in having a high flexural strength (325 MPa), and the graphitized composites reinforced with the carbonized felt (carbonized at 1800 °C) had a low flexural strength (9 MPa). It was found that the stress-orientation promoted the formation of the anisotropic texture around the fibers as well as between the fibers. This felt may very well be able to provide a low-cost route for producing multidimensional C/C composites.     

Modeling and cycling control of carbonate fuel cell power plants

A mathematical process model for an internal reforming molten carbonate fuel cell power plant is discussed in this paper. The dominant thermal and chemical dynamic processes are modeled for the cell stack array and balance-of-plant, including cathode gas preparation, heat recovery, and fuel processing. Physical data is obtained from a 2 MW system design that was a precursor to a demonstration plant operated at the City of Santa Clara, CA, USA. Steady state validation for several load points is provided for the cell stack array and a load cycling control system is described and tested under ramping operation between load points.     

Ultrafast high-temperature sintering of lanthanum-chromite-based ceramics

Conventional sintering of lanthanum-chromite-based ceramics typically requires a long isothermal duration, which leads to severe loss of volatile components. In this study, we prepared dense LaCrO₃ (LCO), La_0.8Ca_0.2CrO₃ (LCCO), and La_0.8(Mg_0.05Ca_0.05Sr_0.05Ba_0.05)CrO₃ (4LCO) ceramics with stable single-phase structures through ultrafast high-temperature sintering (UHS), and the total sintering period was shorter than 8 min. An investigation of the effects of sintering parameters and alkaline earth (AE) metal dopants on the density showed that doping with AE metal promoted the densification of LaCrO₃ through the liquid-phase-assisted sintering mechanism. The hardness and conductivity of the ceramics were in the order of LCCO>4LCO>LCO because of the effect of lattice distortion and the relative densities of the pellets. This work presents a compositional-design-based method to obtain high-performance perovskite-type oxides, and it is expected to broaden the use of UHS for the densification of novel ceramics.     

Ultrafast low-temperature near-seamless joining of Cf/SiC using a sacrificial Pr3Si2C2 filler via electric current field-assisted sintering technique

A novel layered structure material, Pr₃Si₂C₂, was synthesized at a low temperature of 850 °C using a molten salt approach for the first time, and subsequently used as the joining filler for carbon fibers reinforced SiC composites (C_f/SiC). A robust near-seamless C_f/SiC joint was successfully obtained at 1509 °C (T_i) for 30 s, while an ultrafast heating rate of 6000 °C/min was applied via electric field-assisted sintering technology. The near-seamless joining process was attributed to the newly precipitated SiC grains, which were densified well with the C_f/SiC matrix by liquid-assisted sintering. The liquid phase was in-situ formed by the eutectic reaction between Pr₃Si₂C₂ and SiC. The shear strength of the near-seamless joint obtained at 1509 °C for 30 s was 17.6 ± 3.0 MPa. The failure occurred in the C_f/SiC matrix. The formation of near-seamless C_f/SiC joints dismisses the issues related to thermal mismatch between C_f/SiC matrices and traditional joining fillers.     

The effect of external stress on the discontinuous precipitation in an AlZn alloy at high and low temperatures

The effect of external tensile stress on continuous precipitation (DP) has been investigated in an Al21.8 at.% Zn alloy at high (215°C) and low (75 and 50°C) temperatures. The ratio of the macroscopic lattice diffusivity, D, to the DP boundary velocity, v, (D/v) is estimated to be larger than the interatomic spacing, λ, at the high temperature, and smaller than λ at the low temperatures. Under tensile stresses, the DP rates are enhanced at the grain boundary segments oriented transverse to the stress direction and suppressed at those oriented parallel to it at both high and low temperatures. Furthermore, Yi and Park show the DP rate changing continuously with temperature over the range where D/v increased from values much smaller than λ to those much larger. These results show that the diffusional coherency strain is the major driving force for DP even at low temperatures where, with D/v < λ, no solute diffusion is usually assumed to occur in front of the moving DP boundaries.     

Electrochemical behavior and electrolytic preparation of lead in eutectic NaCl−KCl melts

A novel molten salt extraction process consisting of chlorination roasting and molten salt electrolysis was proposed to develop a more efficient and environmental friendly technology for recovering lead from spent lead acid batteries (LABs). The feasibility of this process was firstly assessed based on thermodynamics fundamentals. The electrochemical behavior of Pb(II) on a tungsten electrode in the eutectic NaCl−KCl melts at 700 °C was then investigated in detail by transient electrochemical techniques. The results indicated that the reduction reaction of Pb(II) in NaCl−KCl melts was a one-step process exchanging two electrons, and it was determined to be a quasi-reversible diffusion-controlled process. Finally, potentiostatic electrolysis was carried out at −0.6 V (vs Ag/AgCl) in the NaCl−KCl−PbCl₂ melts, and the obtained cathodic product was identified as pure Pb by X-ray diffraction analysis. This investigation demonstrated that it is practically feasible to produce pure Pb metal by electrochemical reduction of PbCl₂ in eutectic NaCl−KCl melts, and has provided important fundamental for the further study on lead recovery from spent LABs via molten salt extraction process.