Preparation of Ti1− zNbzN ceramics by spark plasma sintering Ti1− zNbzOxNy nanopowders

Ti_1−zNb_zN ceramics were fabricated by sintering nanocrystalline titanium-niobium oxynitride (Ti_1−zNb_zO_xN_y) powders using spark plasma sintering (SPS) technique at 1060 °C for 3 min in an N₂ atmosphere. The phase composition and microstructure were characterized by XRD, SEM, TEM and EDS. The results showed that Ti_1−zNb_zN ceramics remained the cubic structures of Ti_1−zNb_zO_xN_y powders. There were XRD peak shifts in the cubic phases between Ti_1−zNb_zN ceramics and corresponding Ti_1−zNb_zO_xN_y powders. During the sintering process, oxygen separated from Ti_1−zNb_zO_xN_y to form titanium-niobium oxides. Ti_1−zNb_zN (0 < z < 1) had a more compact structure than TiO_xN_y and NbO_xN_y. Ti_0.5Nb_0.5N ceramic had the biggest grain size in the series of Ti_1−zNb_zN.     

Calcium zinc manganites, Ca4Mn7Zn3O21 − δ (0.5 < δ < 2.5) with beta-alumina or magnetoplumbite-type structure and their nonlinear electrical transport and magnetic properties

Phase-pure calcium zinc manganite, Ca₄Mn₇Zn₃O_21 − δ has been synthesized by the solid-state reaction between CaMnO₃, ZnO and Mn₃O₄ at ∼ 1300 °C. Conversion of Ca₄Mn₇Zn₃O_21 − δ to Ca₄Mn₇Zn₃O_19 ± δ occurs at 1330-1350 °C which is influenced by the oxygen nonstoichiometry associated with increasing Mn³⁺/Mn⁴⁺ ratio. The Ca₄Mn₇Zn₃O_21 − δ phase is rhombohedral (space group = R3¯m) and has the β″-alumina (BA)-type structure consisting dominantly of Mn⁴⁺ (3d³) ions. The Ca₄Mn₇Zn₃O_19 ± δ is hexagonal (space group = P6₃/mmc) having the magnetoplumbite (MP)-type structure containing dominantly Mn³⁺ (3d⁴) ions. The electrical transport properties of Ca₄Mn₇Zn₃O_21 − δ and Ca₄Mn₇Zn₃O_19 ± δ are indicative of the activated-type small polaron hopping conductivity below 366 and 392 K respectively. The anisotropic charge transport is influenced only in a limited way by the mixed-valency of Mn prevailing in the manganite spinel blocks. Highly nonlinear current-voltage (I-V) curves with nonlinearity coefficient, α, up to 64 was achieved at the turn-on field strength E_t ∼ 110-160 V/mm. Magnetic properties of Ca₄Mn₇Zn₃O_21 − δ and Ca₄Mn₇Zn₃O_19 ± δ have been investigated which showed the antiferromagnetic-insulating (AFI) behavior with T_N = 103 and 80 K and with an AF-ordered effective magnetic moment, μ_eff = 3.54 and 4.28 μ_B/Mn respectively. The Ca₄Mn₇Zn₃O_19 ± δ phase showed the ferrimagnetic interaction below T_N (at ∼ 20 K).     

Stability of the perovskite phase LaBO3 (B = V,Cr, Mn,Fe, Co,Ni) in reducing atmosphere I. Experimental results

The chemical stability of perovskites LaBO₃ where B = V, Cr, Mn, Fe, Co, Ni was studied by thermogravimetry at 1000°C in gas mixtures of $\text{CO}{}_2\text{H}{}_2$, $\text{O}{}_2\text{CO}{}_2$ and $\text{O}{}_2\text{Ar}$ at 1 bar.The stability limits of the perovskite phases expressed in terms of -log Po₂★ (Po₂★ = critical oxygen partial pressure in bar) were for LaCrO₃ and LaVO₃ (greater than 21.1), LaFeO₃ (16.95), LaMnO₃ (15.05), LaCoO₃ (7.0) and for LaNiO₃ (～0.6). The changes in standard enthalpy ΔH° and entropy ΔS° of the following reactions were obtained.$\text{LaVO}{}_4\text{=}\text{LaVO}{}_3\text{+}\text{1}\text{20}{}_2\text{δ=328}\text{kJ}\text{mol}\text{°=135}\text{J}\text{mol·deg.}$,$\text{LaMnO}{}_3\text{=}\text{1}\text{2}\text{La}{}_2\text{O}{}_3\text{+}\text{MnO}$     

Carburization and decarburization kinetics of iron in CH4 — H2 mixtures between 1000 – 1100 °C

In this study the rate constants of the methane decomposition reaction on iron surfaces were determined in the 1000–1100°C temperature range, by grav? metric methods. Earlier works showed that the reaction velocity was given by The results indicate that the constant values vary from 2.72 × 10^?6 to $\text{16.74 × 10}{}^{\mathrm{?6}}\text{mol C/cm}{}^2\text{/sec/atm}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ for k and 2.61 × 10^?8 to $\text{8.62 × 10}{}^{\mathrm{?8}}\text{mol C/cm}{}^2\text{/sec/atm}{}^{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ for k′ between 1000 and 1100°C.     

Preparation,phase structure and microwave dielectric properties of a new low cost MgLi2/3Ti4/3O4 compound

A novel Li-based spinel compound with the composition of MgLi_2/3Ti_4/3O₄ was synthesized by the conventional solid-state reaction method. The phase structure was studied by X-ray diffraction (XRD) technique. When the calcination temperature was over 1050 °C, a single phase compound which has a cubic structure [Fd-3m (227)] with cell parameters of a = 8.4057 Å, V = 593.91 Å³, ρ = 3.51 g cm⁻³ and Z = 8 was obtained. MgLi_2/3Ti_4/3O₄ ceramic could be well densified after sintering above 1125 °C. The microwave dielectric properties were measured using a microwave vector network analyzer in the frequency range of 7–9 GHz MgLi_2/3Ti_4/3O₄ ceramic sintered at 1125 °C for 2 h showed microwave dielectric properties of ?_r = 20.2, Q × f = 62,300 GHz, and τ_f = −27.1 ppm °C⁻¹. Furthermore, 0.95MgLi_2/3Ti_4/3O₄–0.05CaTiO₃ ceramic sintered at 1200 °C for 2 h exhibited good properties of ?_r = 22.6, Q × f = 48,000 GHz, and τ_f = −2.3 ppm °C⁻¹.     

Electronic Structures of Rare-earth Half-metallic Ferromagnet CoLa<Subscript>2</Subscript>O<Subscript>4</Subscript>

The half-metallic ferromagnet CoLa₂O₄ was predicted based on density function theory. Its magnetic and electric properties were studied systematically. Results show that CoLa₂O₄ has half-metallicity and its molecular magnetic moment is 3.00 μ_B, which is lower than 4.0 μ_B of Fe₃O₄. CoLa₂O₄ is more sensitive than Fe₃O₄ in the same applied field. La-ions have little spin polarization. The electronic structures of a Co-ion and a La-ion are a_1g ¹↑a_1g ¹↓t_1u ³↑t_1u ³↓e_g ²↑e_g ²↓t_2g ³↑ and a_1g ¹↑a_1g ¹↓t_1u ³↑t_1u ³↓ t_2g ³↑t_2g ³↓e_g ²↑e_g ²↓e_g ^2*↑e_g ^2*↓, respectively.     

Crystal structure, thermal expansion and electrical properties of a new compound Al0.33DyGe2

A new ternary compound Al_0.33DyGe₂ has been synthesized and studied from 298 K to773 K by means of X-ray powder diffraction technique. The crystal structural refinement of Al_0.33DyGe₂ has been performed by using the Rietveld method. The ternary compound Al_0.33DyGe₂ crystallizes in the orthorhombic of the defect CeNiSi₂-type structure (space group Cmcm, a = 0.41018(2)nm, b = 1.62323(6)nm, c = 0.39463(1)nm, Z = 4 and D_calc = 8.004 g/cm³). The average thermal expansion coefficients α_a, α_b and α_c of Al_0.33DyGe₂ are 1.96 × 10^− 5 K^− 1, 0.93 × 10^− 5 K^− 1 and 1.42 × 10^− 5 K^− 1, respectively. The bulk thermal expansion coefficient α_V is 4.31 × 10^− 5 K^− 1. The resistivity is observed to fall from 387 to 308 µΩ cm between room temperature and 25 K.     

Thermal expansion of corundum structure Rh2O3

The directional thermal expansion coefficients of the corundum structure form of Rh₂O₃ were determined from room temperature to 850°C by x-ray diffraction methods. Rh₂O₃ has a lower thermal expansion and is less anisotropic in thermal expansion than alumina. The directional thermal expansion coefficients of Rh₂O₃ expressed in second degree polynominal form are: $\text{“α}{}_{\mathrm{<mtext>a</mtext>}}\text{” = 5.350 ×10}{}^{\mathrm{?6}}\text{+ 1.281 ×10}{}^{\mathrm{?9}}\text{T}\text{? 1.133 ×10}{}^{\mathrm{?14}}\text{T}{}^2\text{/°}\text{C}$ and $\text{“α}{}_{\mathrm{<mtext>c</mtext>}}\text{” = 5.246 ×10}{}^{\mathrm{?6}}\text{+ 6.369 ×10}{}^{\mathrm{?9}}\text{T}\text{? 7.480 ×10}{}^{\mathrm{?14}}\text{T}{}^2\text{/°}\text{C}$.     

Effect of loading history on stress corrosion cracking in high strength steel

The effect of preloading on crack nucleation time was examined with compact tension specimens having various notch radius in 0.1N-H²SO⁴ aqueous solution for 200°C tempered AISI 4340 steel. Crack nucleation time t_n increases by preloading for a given apparent stress intensity factor K_p2. The curve K_?2 vs. t_n deviates upward from the curve for the non preloading case. A linear relationship between the crack nucleation time and parameter is seen in semi-log diagram, where is taken as the value at t_n=α due to preloading. The apparent threshold stress intensity factor increases with K_?2 which is the apparent stress intensity factor of preloading. A detached crack is nucleated at some distance from the notch root and extends in a form of circle. This distance increases with increasing K_?2. The effect of load reduction during crack growth was examined. When the K-value was reduced from K₁ to K₂, an incubation time was observed before the crack started growing under the K₂-value. The incubation time t_m tends to increase with increasing ΔK = K₁-K₂. The threshold stress intensity factor was also found to increase for high load reduction.In order to explain these experimental results, a new dislocation model is proposed on the basis of stress induced diffusion of hydrogen in high stress region ahead of the notch root or a crack. This model suggests that the change in the crack nucleation time and the increase of the incubation time due to preloading or load reduction are caused by reducing the hydrostatic pressure and by spreading the hydrogen saturated region which requires more time for the hydrogen accumulation due to preloading or load reduction. The theory predicts the experimentally observed relations between and t_n and between log t_in and ΔK.     

Specific heat of hole-doped vanadates: Y1−xCaxVO3, Pr1-xCaxVO3 and Nd1−xSrxVO3

We have investigated the specific heat of hole-doped vanadium oxides Y_1−xCa_xVO₃ (0 ≤ x ≤ 0.11), Pr_1−xCa_xVO₃ (0 ≤ x ≤ 0.3) and Nd_1−xSr_xVO₃ (0 ≤ x ≤ 0.2) probably for the first time by applying the Modified Rigid ion model (MRIM). The results obtained on temperature dependent (1 K ≤ T ≤ 300 K) specific heat are in reasonably good agreement with the experimental data. The impact of lattice distortions on the elastic and thermal properties of the present Mott insulators is portrayed by an atomistic approach. The scope of further improvement in the present model has also been discussed.