Nonlinearity in regulating the metal to insulator transition of ReNiO3 towards low temperature range

Among the existing material family of the correlated oxides, the rare earth nickelates (ReNiO₃) exhibit broadly adjustable metal to insulator transition (MIT) properties that enables correlated electronic applications, such as thermistors, thermochromics, and logical devices. Nevertheless, how to accurately control the critical temperature (T_MIT) of ReNiO₃ via the co-occupation of the rare-earth elements is yet worthy to be further explored. Herein, we demonstrate the non-linearity in adjusting the T_MIT of ReNiO₃ towards lower temperatures via introducing Pr co-occupation within ReNiO₃ (e.g., Pr_xNd_1-xNiO₃ and Pr_xSm_1-xNiO₃) as synthesized by KCl molten-salt assisted high oxygen pressure reaction approach. Although the T_MIT is effectively reduced via Pr substitution, it does not strictly follow a linear relationship, in particular, when there is large difference in the ionic radius of the co-occupation rare-earth elements. Furthermore, the most significant deviation in T_MIT from the expected linear relationship appears at an equal co-occupation ratio of the two different rare-earth elements, while the abruption in the variation of resistivity across T_MIT is also reduced. The present work highlights the importance to use adjacent rare-earth elements with co-occupation ratio away from 1:1 for achieving more linear adjustment in designing the metal to insulator transition properties for ReNiO₃.     

Ignition temperatures of dust layers: Flaming and non-flaming

Combustible dusts, based on tests conducted in the Setchkin Furnace on five representative dusts, are shown to have two distinct layer minimum ignition temperatures, designated MIT_non-flaming and MIT_flaming. The MIT_non-flaming is the one usually reported in the literature; the MIT_flaming is determined by starting at a much higher temperature and working downwards. MIT_flaming can yield a different ranking of dust flammability than MIT_non-flaming. The MIT_flaming appears to be more closely correlated to the MIT of dust clouds and to the test prescribed for flammable solids in the US Code of Federal Regulations than the MIT_non-flaming.     

Corrosion behavior of high-temperature superconductor YBa2Cu3-yAgyO7-x in the presence of water

Because of the presence of water vapor, YBa₂Cu₃O_7-x degrades easily in the atmosphere and thus very difficult to put it to practical use. When Cu is partially substituted with Ag, however, there is not much a decrease in the critical temperature (T_c) : T_c is 89 K compared with 91 K without substitution. Moreover, partial substitution of Ag increases density, hardness, and superconducting particle size and above all improves considerably its stability in water. In this study YBa₂Cu₃O_7-x and YBa₂Cu_3-yAg_yO_7-x as a result of a partial substitution of Ag for Cu were synthesized by pyrophoric method. We investigated their stabilities in water by XRD, SEM, and EPMA after immersing samples in distilled water for 3 hours. We can see that YBa₂Cu_2.94Ag_0.06O_7-x was the largest particle size and anticorrosion behavior.     

CaTiO3 induced ferroelectric phase coexistence and low temperature dielectric relaxation in BaTiO3–BaZrO3 ceramics

The series of 0.86BaTiO₃–(0.14?x)BaZrO₃–xCaTiO₃ (abbreviated as BT–BZ–xCT) ceramics with 0.03 ≤ x ≤ 0.11 were studied to obtain high piezoelectric properties. Rietveld refinement analysis indicated that the BT–BZ–CT compositions follow a gradual rhombohedral (R) → orthorhombic (O) + R → O + tetragonal (T) → T phase transformation with increasing x. Clear evidence of the series of ferroelectric phase transitions was also found in the dielectric results. The R‐O and O‐T transition temperature shifted close to ambient temperature, while the Curie temperature slightly increased with increasing x. In addition to the dielectric loss peaks associated with the structural phase transitions, a broad low‐temperature dielectric loss peak was detected in the R phase at T = 90‐150 K. This dielectric relaxation was attributed to the domain wall freezing and fits well to the Vogel‐Fulcher model with activation energy E_a ≈ 60‐300 meV and freezing temperature T_VF ≈ 75‐140 K. High piezoelectric strain coefficient (d₃₃*) of about 1030 pm/V at 10 kV was achieved at x = 0.07, and a high Curie temperature (T_C) was maintained at about 375 K.     

Electrochemical wall mass transfer in liquid particulate systems

Ionic mass transfer coefficients between the wall of a 2.081 inch tube and liquid fluidized beds of lead glass, soda glass and lucite spheres have been measured using the diffusion-controlled reduction of ferricyanide ion at a nickel cathode for porosities 0.90 to 0.45 and Schmidt numbers 580 to 2100. The developed fluidization mass transfer coefficient for 41 < D_T/d_p < 105 were correlated by i_D E = 0.274 Re_H^?0.38 for 10 < Re_H <1600 and by J_D E = 0.455 Re_H^?0.44 for 16.7 < D_T/d_p < 27 and 50 < Re_H < 3500. Re_H is the hydraulic Reynolds number = d_H up/μ_E and d_H is D_T E/[1 + (3/2) ((1–E)) (D_T/d_p)). The distinct effect of D_T/d_p ratio is attributed to wall effects and the non-particulate behaviour of the fluidized bed for D_T/d_p < 27. Measurements in the open pipe and packed bed agreed very well with literature values. The packed bed gives highest mass transfer coefficients at given Re_H.     

Synthesis, crystal structures, and electronic spectra of (1,8-naphthyridine)Re(CO)3Cl and [(1,8-naphthyridine)Cu(DPEPhos)]PF6

Two 1,8-naphthyridine (nap) metal complexes (nap)Re^I(CO)₃Cl (1) and [(nap)Cu^I(DPEPhos)]PF₆ (2) were synthesized and characterized by NMR-, emission, and absorption spectroscopy, elemental analysis, mass spectrometry, and X-ray structural analysis. In both complexes, the nap ligand coordinates with both N atoms to the metal centre in a bidentate manner. 1 and 2 exhibit a broad phosphorescence in solid state at T = 300 K, which is completely quenched in solution at r.t. In addition, the gas-phase structures of both complexes were optimized at the B3LYP/6-31G(d,p) level of theory.     

Liquid‐Solid Mass Transfer Behavior of a Vertical Array of Closely Spaced Horizontal Tubes in Relation to Catalytic and Electrochemical Reactor Design

The rates of mass transfer at a vertical array of closely spaced horizontal tubes were measured by the limiting‐current technique under single‐phase flow, gas sparging and two‐phase flow. The single‐phase flow data were correlated by the equation: Sh = 0.75 Sc^0.33 Re^0.59. The gas sparging data with no net solution flow were correlated by the equation: J = 0.31(Re_g.Fr)^–0.22. For two‐phase flow, the gas flow was found to enhance the rate of array mass transfer by a factor ranging from 1.25 to 5.25, depending on Re_g and Re. The enhancement ratio increases with decreasing Re and increasing Re_g. For Re ≥ 2500, the rate of mass transfer approaches the value of single‐phase flow, regardless of the value of Re_g, which ranged from 7 to 41. The importance of the present geometry in building electrochemical and catalytic reactors, where exothermic liquid‐solid diffusion‐controlled reactions take place, is highlighted. The present geometry offers the advantage that the outer surface acts as a turbulence promoter while the inner surface acts as a heat exchanger.     

The ratio of terminal velocity to minimum fluidising velocity for spherical particles

The analogy between the states of a particle falling at its terminal velocity in a fluid and that of a particle in a bed, at incipient fluidization by the same fluid, suggests the possibility of a correlating minimum fluidizing and terminal velocities and of predicting the minimum fluidizing velocity. A semi-theoretical curve has been obtained, relating (Ret/Re_mf) to Fn = gp_F (rH_S – rH_F) d³/μ² and it has been compared with new experimental data collected for this purpose in the range 10³<Fn<10⁸. Analytical expressions for (Re_t/Re_mf) are proposed.     

In Situ Measurements on Solid Oxide Fuel Cell Cathodes – Simultaneous X‐Ray Absorption and AC Impedance Spectroscopy on Symmetrical Cells

A solid oxide fuel cell in operando is a complex multiphasic entity under electrical polarization and operating at high temperatures. In this work, we reproduce these conditions while studying transition metal redox chemistry in situ at the cathode. This was achieved by building a furnace that allowed for X‐ray absorption near‐edge structure and AC impedance spectroscopy data to be obtained simultaneously on symmetrical cells while at operating temperatures. The cell electrodes consisted of phases from the Ruddlesden–Popper family; La₂NiO_4+δ, La₄Ni₃O_10–δ, and composites thereof. The redox chemistry of nickel in these cathodes was probed in situ through investigation of changes in the position of the X‐ray absorption K‐edge. An oxidation state reduction (Ni³⁺ to Ni²⁺) was observed on heating the cells; this was correlated to changing concentrations of ionic charge carriers in the electrode. Polarizing the cells resulted in dramatic changes to their electrical performance but not to the bulk redox chemistry of the electrode. The implications of this with respect to explaining the polarization behavior are discussed.     

Preparation of (110)-oriented SrTiO3 film on quartz glass by laser chemical vapor deposition

SrTiO₃ (STO) film was prepared on quartz glass by laser chemical vapor deposition at a deposition temperature (T_dep) ranged from 760 to 1104 K. Effect of the T_dep on the orientation, crystallinity, texture, and microstructure of the STO film was investigated. As the T_dep was increased, the preferred orientation of the STO film tended to be (110)-orientated with corresponding texture coefficient (TC) on the (110) reflection enhanced from 2.3 to 6; meanwhile, the full width at half maximum of the ω-scan on the (110) reflection decreased from 0.85° to 0.59°. The (110)-oriented grains were in wedge shape about 60 × 150 nm in size, which tended to be flat at an elevated T_dep of 1104 K.     

Study on the Anti-Coking Nature of Ni/SrTiO3 Catalysts by the CH4 Pyrolysis

A solid phase crystallization (spc) method was applied for the preparation of SrTiO₃-supported Ni catalysts and compared to the impregnation (imp) method. spc-Ni_0.2/SrTiO₃ has highly dispersed and stable Ni metal particles resulting in higher activity and higher sustainability against coking than imp-Ni_0.2/SrTiO₃ in the partial oxidation of CH₄. Both catalysts were tested for the CH₄ pyrolysis in order to elucidate the catalytic nature against coking of spc-Ni_0.2/SrTiO₃. The amount of carbon and the rate of H₂ formation were similar over both catalysts at both 773 and 1073 K. On both catalysts, CH₄ continuously decomposed at 773 K, while the rate of CH₄ pyrolysis quickly decreased at 1073 K. Fibrous carbons grew up with a Ni metal particle on the tip of the fiber at 773 K, while carbon balls and short carbon fibers with a Ni metal particle encapsulated inside formed and no sufficient growth of the fiber was observed at 1073 K. The carbon species formed at 773 K was hydrogenated completely to CH₄ around 873 K, while the hydrogenation of that formed at 1073 K needed higher temperature around 1073 K. However, the carbon species formed on both the catalysts at either 773 or 1073 K was completely oxidized around 773 K. Thus, judging from the anti-coking nature, the behaviors in the CH₄ pyrolysis are similar over both catalysts, nonetheless spc-Ni_0.2/SrTiO₃ was far superior to imp-Ni_0.2/SrTiO₃ in the CH₄ oxidation. It is likely that the high sustainability against coking of spc-Ni_0.2/SrTiO₃ is not due to its intrinsic nature suppressing the coking but due to its high activity of reforming which can quickly eliminate the carbon formed on the catalyst surface.     

Effect of Yb,Gd, and Nd Substitution at the Sr Site on the Metal–Insulator Transition of the (Bi,Pb)-2212 System

Resistivity measurements in the temperature range (64–300 K) were used to study the metal–insulator transition (MIT) driven by a change in the carrier concentration of the Bi_1.7Pb_0.4Sr₂Ca_1.1Cu_2.1O_y [(Bi,Pb)-2212] system. The carrier-concentration is changed by substituting rare-earth (RE) elements (Nd, Gd, and Yb) at the Sr site of (Bi,Pb)-2212. Results show that at higher levels of RE substitution, MIT occurs in (Bi,Pb)-2212, during which the resistivity becomes minimum at a particular temperature(T_min) for a particular doping level. Below this temperature, resistivity increases with a decrease in the temperature and vice versa, showing an insulating and a metallic nature, respectively. This T_min and the carrier concentration at which MIT occurs depend on the substituted RE.     

Low metal–insulator transition temperature of Ni-doped vanadium oxide films

Elemental doping is the main means to regulate the phase transition of vanadium oxide (VO₂); however, the effects of low valence elemental (<4+) doping on the phase transition of VO₂ are still controversial. In the present work, Ni-doped VO₂ films were prepared on quartz glass by direct current reactive magnetron sputtering and subsequent annealing. With the increase of the Ni doping content, the phase transition temperature of heating (T_H) of the VO₂ films decreased from 73.4 °C to 52.4 °C. The temperature required for the occurrence of phase transition (T_b) was lower than T_MIT. Different from the undoped VO₂ film, the Ni-doped VO₂ films had a T_b of around 30 °C. XRD and Raman results revealed that some rutile VO₂ microcrystals appeared in the vanadium oxide films because of the lattice distortion by incorporated Ni. Hence, rutile VO₂ micro-crystallinities significantly facilitated the phase transition of monoclinic VO₂ to rutile one.     

Pd-doped LaCoO3 regenerative catalyst for automotive emissions control

The effect of partial substitution of Co by Pd in LaCoO₃ perovskite structure (i.e., LaCo_0.95Pd_0.05O₃) and the reductive diffusion of Pd from the bulk of perovskite to its surface, thus forming Pd nanoparticles, on CO and C₃H₈ oxidation present in air (simulated exhaust gas) are reported. X-ray powder diffraction (XRD) analyses confirm the perovskite structure for the catalysts. Scanning electron microscopy (SEM) and BET surface area measurements show that partial substitution of Co by Pd decreases the crystallite size of the perovskite and therefore increases its surface area. H₂-temperature programmed reduction (TPR) experiments reveal that Pd reduces at 135 °C and facilitates the reduction of Co in the perovskite structure. By partial reduction of the Pd containing catalyst at 180 °C for 30 min, the complete oxidation temperatures of CO and C₃H₈ decrease by about 70 and 50 °C, respectively.The reduction duration of the Pd containing catalyst strongly affects the T₅₀ and T₉₀ temperatures (temperatures at which 50 and 90% conversion occurs, respectively) and has an optimum, where it decreases by increasing the reduction temperature of the catalyst.     

Electrochemical study of mass transfer in decaying annular swirl flow Part II: Correlation of mass transfer data

This paper presents correlations of local mass transfer at the inner rod and the outer wall in annular decaying swirl flow generated by axial vane swirl generators. Four swirl generators with vane angles in the range 15–60° to the duct axis were used and experiments were carried out in a Reynolds number range 3300–50000 and at a Schmidt number of 1650. The results were correlated in the general form Sh _x = 0.0204 Re _x ^0.86 (1 + tan _i )^0.53 Sc ^1/3, for the inner rod, and Sh _x = 0.0224 Re _x ^0.86 (1 + tan _o)^0.55 Sc ^1/3, for the outer pipe. Comparison is made with heat transfer data for work with a similar entry configuration.     

Temperature-sensing characteristics of NaGd(MoO4)2: Sm3+, Tb3+ phosphors

In this study, Sm³⁺/Tb³⁺-co-doped NaGd(MoO₄)₂ phosphors were prepared via the hydrothermal method, with sodium citrate used as a chelator. X-ray diffraction confirmed the structure of the samples, and the test outcomes showed that the phosphors exhibited a body-centered tetragonal structure. Field-emission scanning electron microscopy results showed that the specimen morphology changed with the change in the Cit^3?/Re³⁺ molar ratio. Moreover, the measured temperature-dependent emission spectra showed that Sm³⁺ and Tb³⁺ had different quenching trends; thus, the fluorescence intensity ratio can be used to represent temperature. In addition, the outcome of this experiment revealed that the temperature-sensing sensitivity of the phosphors gradually increased with the increasing Cit^3?/Re³⁺ ratio, and the highest sensitivity value was 0.346 K^?1 (at 503 K, Cit^3?/Re³⁺ = 2). When the temperature was 298–369 K, the temperature-sensing relative sensitivity increased with increasing Cit^3?/Re³⁺, but in the range 374–503 K, the relative sensitivity decreased with increasing Cit^3?/Re³⁺. The highest relative sensitivity value of the sample was 2.7% K^?1 (404 K, Cit^3?/Re³⁺ = 0). Additionally, the Commission International del’Eclairage chromaticity coordinates displayed that the luminous colors of Sm³⁺/Tb³⁺-co-doped specimens continuously changed from green to red as the temperature changed.     

Tunable negative thermal expansion and structural evolution in antiperovskite Mn3Ga1−xGexN (0 ≤ x ≤ 1.0)

The negative thermal expansion (NTE) and structural evolution of antiperovskite compounds Mn₃Ga_1?xGe_xN (0 ≤ x ≤ 1.0) were systematically investigated. Our results indicate the crystal structure of Mn₃Ga_1?xGe_xN changes from cubic (C) to tetragonal (T₄) with increasing Ge content by X‐ray diffraction (XRD).The negative thermal expansion from x = 0 (operation‐temperature range ?T = 20 K) to x = 0.4 (?T = 60 K) becomes broad and shifts to higher temperature, and then it became positive from x = 0.5 in Mn₃Ga_1?xGe_xN. Typically, Mn₃Ga_0.5Ge_0.5N shows low thermal expansion behavior between 300 and 450 K (?T = 150 K), and thermal expansion coefficient α is estimated to be 2 × 10^?6 K^?1. Furthermore, variable temperature XRD was measured to reveal the origin of NTE. The cubic I ‐ cubic II phases coexistence (x = 0.2) and cubic I ‐ tetragonal coexistence (x = 0.5, 0.6) was observed at low temperature. The tunable NTE is highly valuable for practical applications in precision devices.     

Catalytic Oxidation of CO over Nanocrystalline La1–xCexNiO3 Perovskite‐Type Oxides

Nanocrystalline La_1–xCe_xNiO₃ (x = 0.1, 0.3, 0.5, 0.7, 0.9) perovskite‐type oxide catalysts prepared by the Pechini method were employed in catalytic CO oxidation and the effect of substitution of La by Ce on CO conversion was evaluated. The results indicated the remarkable effect of La substitution with Ce on the catalytic performance at low temperatures. The reaction temperature had a significant influence on the stability of the catalysts. The La_0.1Ce_0.9NiO₃ sample exhibited the highest activity among the prepared catalysts in CO oxidation reaction. In addition, the influence of different parameters including pretreatment condition, feed ratio, and gas hourly space velocity (GHSV) on the catalytic performance was examined. The optimum catalyst proved high stability under severe reaction conditions in the presence of water vapor and CO₂ in the feed stream.     

Ultra‐Fast Fabrication of <110>‐Oriented β‐SiC Wafers by Halide CVD

Φ80 mm‐diameter, highly <110>‐oriented β‐SiC wafers were ultra‐fast fabricated via halide chemical vapor deposition (CVD) using tetrachlorosilane (SiCl₄) and methane (CH₄) as precursors. The effects of deposition temperature (T_dep) and total pressure (P_tot) on the orientations, microstructures, and deposition rate (R_dep) were investigated. R_dep dramatically increased with increasing T_dep where maximum R_dep was 930 μm/h at T_dep = 1823 K and P_tot = 4 kPa, leading to a maximum of 1.9 mm in thickness in 2 h deposition. The <110>‐oriented β‐SiC was obtained at T_dep > 1773 K and P_tot = 1–4 kPa. Growth mechanism of <110>‐oriented β‐SiC has also been discussed under consideration of crystallographic planes, surface energy, and surface morphology.     

Improved thermoelectric properties of layered Ti1−xNbxS2−ySey solid solutions

The thermoelectric properties of a series of the polycrystalline samples of titanium dichalcogenides with partial substitution of Ti for Nb and S for Se were investigated. It was found that sintering of the samples improved the thermoelectric efficiency (ZT), and the maximum ZT was achieved at sintering temperature of 600°C. A further increase in the sintering temperature (850°C and 950°C) led to the recrystallization of the samples, as a result, the Seebeck coefficient sharply decreased and electrical conductivity dramatically increased. The temperature dependences of electrical conductivity σ(T) in the temperature range from 4.2 to 300 K and Seebeck coefficient S(T) in the temperature range from 77 to 300 K were investigated in order to determine the nature of the observed improvements in thermoelectric properties due to double substitutions and sintering. Two-dimensionalization of electron transport properties of Ti_1−xNb_xS_2−ySe_y solid solutions was found. The Fermi energy E_F2D was estimated using the temperature dependences of Seebeck coefficient. The relationship between the Fermi energy E_F2D and figure of merit ZT was established. The effect of sintering on parameters σ(T), S(T), charge carrier concentration (n_2D), mobility (µ), and thermal conductivity (k) was found. The optimal value of Fermi energy E_F2D in terms of figure of merit ZT = 0.31 at room temperature (T = 300 K) was found for Ti_0.98Nb_0.02S_1.3Se_0.7 sample sintered at 600°C.