Papers by Keyword: Non-Stoichiometry

Abstract: Theoretical ab initio calculations of the electronic structure were performed for the non-stoichiometric Mn_1.75Co_1.25Al Heusler alloy and compared with the electronic structure of the stoichiometric Mn₂CoAl full Heusler alloy. Both compounds are assumed to have the L2₁-type crystal structure in the calculations, the non-stoichiometry is taken into account as a substitution of a Mn atom in a supercell. The calculation for the non-stoichiometric composition of Mn_1.75Co_1.25Al showed that taking non-stoichiometry into account leads to a decrease of the total magnetic moment. In comparison with the inverse type of Mn₂CoAl, in both Mn₂CoAl and Mn_1.75Co_1.25Al, the metallic type of the total density of states at the Fermi level was obtained in our calculations. In Mn_1.75Co_1.25Al, the total density of electronic states is found to be close to the one of the stoichiometric Mn₂CoAl alloy in the majority spin projection, and in the minority spin projection spin polarization leads to the formation of the more intense peaks due to the appearance of an additional non-stoichiometric cobalt with a significant magnetic moment, as well as an increase in the magnetic moments of the other magnetic ions.

Abstract: This article reviews the studies of diffusion and defect phenomena induced by high-concentration zinc diffusion in the single-crystal III-V compound semiconductors GaAs, GaP, GaSb and InP by methods of transmission electron microscopy and their consequences for numerical modelling of Zn (and Cd) diffusion concentration profiles. Zinc diffusion from the vapour phase into single-crystal wafers has been chosen as a model case for interstitial-substitutional dopant diffusion in these studies. The characteristics of the formation of diffusion-induced extended defects and of the temporal evolution of the defect microstructure correlate with the experimentally determined Zn profiles whose shapes depend on the chosen diffusion conditions. General phenomena observed for all semiconductors are the formation of dislocation loops, precipitates, voids, and dislocations and of Zn-rich precipitates in the diffusion regions. The formation of extended defects near the diffusion front can be explained as result of point defect supersaturations generated by interstitial-substitutional zinc exchange via the kick-out mechanism. The defects may act as sinks for dopants and as sources and sinks for point defects during the continuing diffusion process, thereby providing a path to establishing defect-mediated local point defect equilibria. The investigations established a consistent picture of the formation and temporal evolution of defects and the mechanisms of zinc diffusion in these semiconductors for diffusion conditions leading to high-concentration Zn concentrations. Based on these results, numerical modelling of anomalously shaped dopant concentration profiles leads to satisfactory quantitative results and yields information on type and charge states of the point defect species involved, also for near-surface Zn concentration profiles and the absence of extended defects.

Abstract: The oxygen chemical diffusion coefficient in (U, Pu)O_2-x was determined by thermo-gravimetry as functions of the Pu content, oxygen-to-metal ratio and temperature. The surface reaction was considered in the diffusion coefficient determination. The activation energy for the chemical diffusion coefficient was 60 kJ/mol and 65 kJ/mol, respectively, in (U_0.8Pu_0.2)O_2-x and (U_0.7Pu_0.3)O_2-x.

Abstract: Two-step sintering with controlled oxygen partial pressure by using metallic buffer was applied to obtain dense non-stoichiometric CeO_2-x. The samples prepared with single step sintering were cracked. The two-step sintering promoted almost full densification ≈ 98% in relative density without any cracks. Relative density of CeO_2-x reached more than 98% of the theoretical density. The value of x in CeO_2-x was obtained in the range of 1.68 in Mn/MnO to 1.98 in Co/CoO.

Abstract: Lead-free (1-x)[(Na_0.515K_0.485)_0.94Li_0.06(Nb_0.99Ta_0.01)O₃]-xBiAlO₃ (NKLNT-BA; x = 0, 0.005, 0.010, 0.015, and 0.020) ceramics were fabricated by a conventional mixed-oxide method. The effects of BiAlO₃ addition on the phase structure, microstructure and electrical properties of ceramic were then studied. The result indicated that grain size decreased with increasing of BiAlO₃ content. In the composition range studied, the perovskite phase with the coexistence of the orthorhombic and tetragonal phases was identified at approximately x 0.005 by the X-ray diffraction analysis and dielectric spectroscopy, which led to a significant enhancement of the piezoelectric properties. The tetragonality increased with further increasing x. The temperature dependence of dielectric properties showed that the addition of BiAlO₃ slightly decreased the ferroelectric tetragonal-paraelectric cubic phase transition temperature (T_C), but greatly shifted the polymorphic phase transition from the ferroelectric orthorhombic to the ferroelectric tetragonal phase (T_OT) to lower room temperature. The dielectric and piezoelectric properties are enhanced for the composition near the orthorhombic-tetragonal polymorphic phase boundary. The unmodified-(Na_0.515K_0.485)_0.94Li_0.06(Nb_0.99Ta_0.01)O₃ ceramics exhibit optimum electrical properties (d₃₃ = 225 pC/N and T_C = 418°C).

Abstract: Oxygen diffusivity in strontium titanate films with intentional non-stoichiometry, Sr_1+xTi_1+yO_δ with (1+x)/(1+y) = 0.71.5, was studied to reveal degradation behavior of Sr_1+xTi_1+yO_δ films. In order to utilize isotope tracer diffusion measurement, so-called gas/solid exchange method using ¹⁸O₂ gas, at relatively low temperature, very thin YSZ layer deposited on top of Sr_1+xTi_1+yO_δ film was used as catalyst for enhanced ¹⁸O[gas]/¹⁶O[solid] exchange at lower temperature. As a result, very high oxygen diffusivity at 673 K in Sr_1+xTi_1+yO_δ films with (1+x)/(1+y)>>1 was observed.

Abstract: The influence of cation inhomogeneity on magnetic and electron transport properties of (La_0.85Sr_0.15)_yMnO₃ (y=0.92-0.97) single crystals were studied. It was found that both the temperature of ferromagnetic ordering T_C and the temperature of orbital ordering T_OO are increased with the Mn content decreasing. For all six samples studied two peaks on magnetoresistance curves were observed: the first near T_C and the second close to T_OO. The magnetic field does not affect the position of the former maximum, while it moves the latter towards lower temperatures.

Abstract: Carbo-nitride synthesis was undertaken using a solar furnace at PSA in flowing N₂/Ar gas mixture under total pressure 1 atm and processing temperature T = 1600°C for some d-group transition elements (Ti; Zr, V, Nb, Mo, W) starting from 1.5G/M (graphite/metal powder mixture with mole ratio 1.5:1) compact to ensure co-presence of free carbon with the reaction product. Clear X-ray diffraction (XRD) evidence of formation of carbo-nitride was detected for Ti (IVa group metal) showing higher N content in the carbo-nitride synthesised in N₂ gas environment at partial pressure p(N₂) = 1 atm than that at p(N₂) = 0.5 atm. For M = V and Nb (Va group metals), formation of mono-carbide MC single-phase was detected in the N₂ environment showing no evidence of formation of carbo-nitride in spite of presence of N₂ in the environment. For M = Mo and W (VIa group metals), formation of higher carbide, among several options of carbide phases, appeared to be promoted in the N₂ gas environment although, like in cases with the Va group metals, no evidence of dissolution of N into the reaction product was detected. As such, at T = 1600°C in N₂ gas environment up to p(N₂) = 1 atm under concentrated solar beam, carbo-nitride formed from the 1.5G/M mixture only for IVa group metal (Ti) but not for Va and VIa group metals. Anyway, it seemed certain that N₂ gas affected somehow the reaction path between G and M to yield the carbide phase for M = V, Nb, Mo and W.

Abstract: The effect of nonstoichiometric Pb content on the properties of PZT has been studied. The samples of Pb_xBa_0.04(Li_1/4Sb_3/4)_0.02(Zr_0.52Ti_0.48)_0.98O₃ (0.94≤x≤0.98) were prepared by the solid state route and sintered at 1200
°C. The Pb content changed from 0.94 to 0.98. The result shows that when the Pb content deviates from the stoichiometric ratio (x=0.96), the grain growth and densification of ceramics are promoted. The phases analysis shows that ZrO₂ phase is detected when the amount of Pb is lower than 0.96, while when the Pb content is more than 0.96, the PbO phase is absent. The optimum composition of Pb_xBa_0.04(Li_1/4Sb_3/4)_0.02(Zr_0.52Ti_0.48)_0.98O₃ solid solution system are obtained when x is 0.95, and the ε^T33/ε0, d₃₃ and k_p of the PZT ceramics are: 2281, 516 pC/N and 0.80, respectively.

Abstract: Transparent Mg-Al spinel ceramics of four different nonstoichiometries were obtained utilizing two-stage sintering technique of vacuum sintering and post hot isostatic pressing using MgO·nAl2O3 powders with n of 1, 1.3, 1.5 and 1.8, respectively, as raw materials. The mechanical properties of the ceramics were measured and indentation shapes and radial crack propagations were observed. The results showed that change of molar ratio of Al2O3 to MgO leads to variation of indentation shapes and crack propagations of the ceramics, resulting in the change of mechanical properties of the ceramics. The flexural strengths of the ceramics increase first then decrease with increasing n. However, the hardness and fracture toughness of the ceramics increase with increasing n. As a result, an optimal n for high performance ceramic is suggested to be 1.5. In addition, mechanisms of toughening and strengthening of the transparent ceramics have also been discussed.