Key Engineering Materials Vol. 617

Abstract: Geopolymers are inorganic materials having interesting properties for many using areas. It is well known that geopolymers present various properties depending on the aluminosilicate sources used but lack of understanding are effective. This study consists on the comprehension of the formation of different networks in geopolymers by modify the reactivity of siliceous species in the reactive mixture with ammonium molybdate. These experiments evidenced the existence of various polycondensation reactions between materials leading to the formation of different networks. Moreover the use of ammonium molybdate addition in the mixture induces a slowdown of the reaction.

Abstract: Cement is widely produced and used. However, cement production significantly consumes energy and natural resources and globally accounts for approximately 5% of man-made CO₂ emissions.

Abstract: This work focuses on the development and study of nanostructured magnetic materials by a " bottom-up " strategy allowing the modulation of their magnetic behavior (soft to - hard). The main goal of this work is to tune the magnetic bulk coercivity properties of new nanostructured bulk materials based on magnetic nanowires assembly. Spark-Plasma-Sintering (SPS) process allows an organization of magnetic nanowires inside the sintered material and preserving at the same time the nanoscale character of the nanowires in order to take advantage of the confinement peculiar magnetic properties. We show here that changing the temperature and magnetic field parameters during this new sintering process is at the origin of a controlled anisotropy and coercitivity in the as-obtained bulk systems.

Abstract: Pulse electric current sintering (PECS) is applied to synthesis M₅Si₃ type Si-Ti-Zr system intermetallic compounds, Si₃Ti_xZr_(5-x) x=0-5_, directly from raw powders of silicon, titanium and zirconium. Almost full conversion to Si₃Ti_xZr_(5-x) are achieved by PECS method with self-propagating high temperature synthesis (SHS) reaction. The adiabatic temperatures are susceptible to sintering behaviors such as vacuum pressure in chamber and displacement of pressure axis on the PECS process. The large quantities of gases are released in Zr-rich samples. The bulk shapes with almost dense samples are obtained at Ti-rich samples.

Abstract: Possibility of a use of SiC as SPS die was examined. Although SiC has good strength even at high temperature, electrical conductivity is too low as the SPS die. Maximum output voltage of typical SPS machine is 10 V. Joule heat of SiC by application of 10 V is too small to increase its temperature. One idea to solve this problem in this study is to apply higher voltage to the SiC die. Using prototype high voltage SPS equipment, the temperature of SiC die was successfully elevated. Another idea is to heat the SiC die in advance. SiC has a semiconducting behavior, so that the electrical conductivity at high temperature becomes sufficient for the ordinary SPS equipment. Using SPS machine which has hot-wall, heaters surrounding the die, the temperature of the SiC die was successfully increased by application of low voltage. This SPS had similar sintering effects as the ordinary ones. Partly transparent alumina was successfully prepared. Initial compositional fluctuation of Pb (Zr,Ti)O₃ was maintained after it was densified by this SiC-SPS.

Abstract: Highly-crystallized fine LiMn₂O₄ powders have been synthesized by a combination of the mechanochemical (MC) and solvothermal (ST) treatments of LiOH and MnO₂ in Acetone. The MC product is amorphous, but, shows the formation of Mn-O-Li bond. The ST post treatment promotes the nucleation of LiMn₂O₄ by the chemical reduction of Mn⁴⁺ to Mn³⁺. The estimated chemical compositions are LiMn (III)_0.11Mn (IV)_1.89O_4.45 (MC) and LiMn (III)_0.76Mn (IV)_1.12O_3.88 (MC/ST). The MC/ST powder crystallized at 800°C indicates the cell capacity of 119mA.h.g^-1. The dilution of Acetone with Tetrahydrofuran is effective to reduce the particle size. The MC/ST product using the diluent shows the fine particle size of 150nm and the excellent cell capacity of 134mA.h.g^-1.

Abstract: The process of obtaining a uniform dispersion of nanosized powders by applying two step process of mixing and adjusting viscosity was studied. The mixed paste was used as the electrode of direct ethanol fuel cells (DEFC). And power density of the fuel cells was measured. A high power density was realized by addition of acetone and propanol solvents to nanosized silica powder. However acetaldehyde concentration during the operation of these fuel cells did not decrease. An excess methanol was added to membrane electrode assembly composition of DEFC. The power density and current of DEFC were improved by adding methanol solvent. However, methanol addition over 55 % did not improve dispersion of nanosized powders. Transmission electron microscopy observation suggests that over 55% of methanol addition does not give good dispersion of nanosized powders.

Abstract: The n = 1 Ruddlesden-Popper (RP) oxides La_1.6Sr_0.4Ni_1-xCo_xO_4+δ (x = 0.6 and 0.8) have been prepared by the citrate-gel method and studied using thermal analysis and in situ neutron powder diffraction (NPD) under oxygen (x = 0.6) and subsequent 5%H₂ flow. On heating under O₂, the x = 0.6 sample loses oxygen from the interstitial site until it is emptied at 475 °C. Subsequent heating in 5%H₂ results in reduction which proceeds within two-steps; the first one occurs between ~170 and 300 °C and involves removal of the interstitial oxygen (O_int), the second step starting at ~ 400 °C and going on up to 600 °C (the highest temperature reached) involves oxygen deintercalation from the equatorial position (O_eq). After 20 min of isothermal heating at 600 °C, unless a thermodynamic equilibrium state was not reached there was no sign of impurity related to the decomposition of the sample and the composition refined to La_1.6Sr_0.4Ni_0.4Co_0.6O_3.85(3). The x = 0.8 sample exhibits similar behavior under hydrogen with slight shifts in temperature and the composition reached after the reducing cycle refined to La_1.6Sr_0.4Ni_0.2Co_0.8O_3.85(1). This study demonstrates that the Co-rich La_1.6Sr_0.4Ni_1-xCo_xO_4+δ oxides can withstand hydrogen reducing conditions with topotactic deintercalation of O_int and O_eq but are not stable enough at 600 °C in comparison to La_1.5Sr_0.5Ni_0.5Co_0.5O_4+δ owing to increased Sr/La ratio in the latter.

Abstract: The microcalorimeter detects the energy of incidentX-ray photon by the slight temperature rise of a device at a very low temperature, and the detector of a superconducting transition-edge sensor (TES) microcalorimeter has been developed for the measurement of energy dispersive X-ray photons of the energy of 10 to 20 keV with the energy resolution better than 100 eV of FWHM value by having the counting rate over 100 counts per second. A FE-SEM with a detector of TES microcalorimeter (SEM-TES) has already been developed for the extra-high resolution energy-dispersive X-ray spectrometry system to apply in electron-microprobe chemical analyses. Recently, we developed the sensitive correct function on the observed X-ray intensity data measured in the system. Then, the high energy resolution of revised SEM-TES system is precisely able to determine the element contents on the sample. In addition, the chemical bonding states can be clarified by the fluorescent X-ray intensities in the lower energy region derived with a low accelerating voltage of SEM-TES system. We applied the revised SEM-TES system to analyze chemical state on the electrode of nanosheets-derived H_x(Ni_1/3Co_1/3Mn_1/3)O₂ for newly developing electrochemical capacitors. We investigated the chemicalstate of cations by not the M-lines but the L-beta and L-alpha lines as the lower energy lines having relative large contribution on chemical bonding. As a result, we revealed a specific chemical state of the Mn different from that of Li(Ni_1/3Co_1/3Mn_1/3)O₂ electrode in LiB, that was supported by the result of TEM-EELS analysis. These results show that the SEM-TES system can be a useful analytical tool for the chemical bonding state and electronic structure determinations in a wide range of materials.

Abstract: Single phase La_2xSr_xMnO_4±δ (0.7 x 1.0) solid solutions were prepared via sol-gel synthesis. For all the compositions the structure turns out to be tetragonal I4/mmm. Samples were then heated for 12 h at different temperatures in O₂ to investigate the structural response to oxygen uptake. On increasing the La/Sr ratio the maximum amount of oxygen inserted (δ) reaches for x = 0.7 an unprecedented value of δ ~ 0.47 in K₂NiF₄-structure type oxides, as evidenced from iodometric titration and thermogravimetric analysis. For the x 0.8 compositions the oxidation resulted in an intermediate orthorhombic phase.