B-site Ions Research Articles

Effects of cerium incorporation on the catalytic oxidation of benzene over flame-made perovskite La1−xCexMnO3 catalysts

Perovskite-type La1−xCexMnO3 (x=0–10%) catalysts were prepared by flame spray pyrolysis and their activities during the catalytic oxidation of benzene were examined over the temperature range of 100–450°C. The structural properties and reducibility of these materials were also characterized by X-ray diffraction (XRD), N2 adsorption/desorption, H2 temperature-programmed reduction (H2-TPR) and X-ray photoelectron spectroscopy (XPS). The incorporation of Ce was found to improve the benzene oxidation activity, and the perovskite in which x was 0.1 exhibited the highest activity. Phase composition and surface elemental analyses indicated that non-stoichiometric compounds were present. The incorporation of Ce had a negligible effect on the specific surface area of the perovskites and hence this factor has little impact on the catalytic activity. Introduction of Ce4+ resulted in modification of the chemical states of both B-site ions and oxygen species and facilitated the reducibility of the perovskite. The surface Mn4+/Mn3+ ratio was increased as a result of Ce4+ substitution, while a decrease in the surface-adsorbed O/lattice O (Oads/Olatt) ratio was observed. The relationship between the surface elemental ratios and catalytic activity was established to allow a better understanding of the process by which benzene is oxidized over perovskites.

Synthesis of homo- and heterobimetallic NiII–MII (M = Fe, Co, Ni, Zn) complexes based on an unsymmetric ligand framework: Structures, spectroscopic features, and redox properties

Several homo- and heterobimetallic NiII–MII complexes (MII=Fe, Co, Ni, Zn) supported by an unsymmetric polydentate ligand (L13−) are reported (L13− is the trianion of 2-[bis(2-hydroxy-3,5-tert-butylphenyl)aminomethyl]-4-methyl-6-[(2-pyridylmethyl)iminomethyl]phenol). The L13− chelate provides two distinct coordination environments: a planar tridentate {N2O} site (A) and a tetradentate {NO3} site (B). Reaction of L13− with equimolar amounts of NiII and MII salts provides bimetallic complexes in which the NiII ion exclusively occupies the tetragonal A-site and the MII ion is found in the tripodal B-site. X-ray crystal structures revealed that the two metal centers are bridged by the central phenolate donor of L13− and an anionic X-ligand, where X=μ-1,1-acetate, hydroxide, or methoxide. The metal ions are separated by 3.0–3.1Å in the MAMBX structures, where MA and MB indicate the ion located in the A and B sites, respectively, and X represents the second bridging ligand. Analysis of magnetic data and UV–Vis–NIR spectra indicate that, in all cases, the two metal ions adopt high-spin states in solution. The NiAII centers undergo one-electron reduction at −1.17 V vs. SCE, while the NiII and CoII ions in the phenolate-rich B-site are reduced at lower potentials. Significantly, the NiAII center possesses three open or labile coordination sites in a meridional geometry, which are generally occupied by solvent-derived ligands in the crystal structures. The NiMBX complexes serve as structural mimics of heterometallic Ni-containing sites in biology, such as the C-cluster of carbon monoxide dehydrogenase (CODH).

Effect of A-site and B-site ion replaced with small ions on the intermediate phase in PbZrO3 ceramic

The solid solution of (1 − x)PbZrO3–xNaNbO3 ceramics, where x = 0.0–0.08, was synthesized by solid state reaction. The basic characterizations were performed using X-ray diffraction (XRD), dielectric spectroscopy, hysteresis measurement and differential scanning calorimetry (DSC) techniques. The results indicated that the crystal structure of the solid solution, (1 − x)PZ–xNN, where x = 0.00–0.08, is of orthorhombic symmetry. It was found that the effect of NN being replaced with small ions at the A-site and B-site can induce an AFE-like phase in PZ. The FE intermediate phase of PZ cannot be induced, although Zr4+ ions were substituted by small Nb5+ ions. This is due to the decreasing average rate of radii in the A-site (0.1 Å mol−1) being higher than that in the B-site (0.08 Å mol−1).

A general approach to monodisperse perovskite microspheres

BaTiO3, PbTiO3, SrTiO3, and Pb(Zr,Ti)O3 microspheres with uniform size and narrow size distribution have been successfully synthesized by a novel hydrothermal and annealing approach. In this approach, the chemical reaction and crystallization process of the ABO3 perovskite oxides were separated in two steps. Spherical particles containing the B-site ions were obtained first via a controlled hydrolysis and aging process. Then, during hydrothermal treatment, the A-site ions were incorporated in situ into the microspheres to form amorphous perovskite microspheres. The particles were further crystallized with preserved spherical morphology under subsequent annealing treatment. The BET surface areas of the TiO2 gel particles, the amorphous PbTiO3 and the as-annealed PbTiO3 microspheres were 245.7m2/g, 41.67m2/g and 4.53m2/g, respectively, showing a significant change of the surface feature in the preparation process. This approach also allowed the microspheres diameters to be manipulated from 100nm to 1500nm in a controlled manner. Most of the microspheres were composed by closely packed nano-sized particles. Furthermore, the Pb(Zr,Ti)O3 microspheres with an average diameter of 200nm exhibited single crystal features, indicating highly oriented growth in the crystallization process. The microspheres were very stable, and still maintained spherical shape after higher temperature calcination.

Characterization and microwave dielectric properties of TiO2-modified 0.65CaTiO3–0.35Sm0.9Nd0.1AlO3 ceramics

Using various amounts of TiO2, the 0.65CaTiO3–0.35Sm0.9Nd0.1AlO3 ceramics were prepared via the conventional solid-state reaction method. The relationships between the crystalline structure and microstructure, as well as the microwave dielectric properties, were investigated. The sintered ceramics formed a solid solution, and short-range ordering at B-site ions was detected by Raman spectroscopy. A slight amount of Al-rich secondary phase was observed along the grain boundary, which increased with the TiO2 content. The additional TiO2 promoted the densification of ceramics and positively affected the dielectric constant, ɛr and the temperature coefficient of resonant frequency, τf. Nevertheless, the excess Al-rich secondary phase destroyed the short-range ordering of the B-site ions and deteriorated the Q×f value. Considering all three parameters of the microwave dielectric properties, the 0.25wt% TiO2-modified 0.65CaTiO3–0.35Sm0.9Nd0.1AlO3 ceramic exhibited the best performance of ɛr=40.43, Q×f=51531GHz and τf=−3.7ppm/°C sintered at 1445°C for 3h.

Electrical Conductivity and Defect Structure of LaNi1-X Fe x O3-δ

The effect of cation composition in LaNi1-x Fe x O3-δ on oxygen nonstoichiometry has been investigated with iodometric titration. For LaNi0.6Fe0.4O3-δ , temperature and oxygen partial pressure dependence of oxygen nonstoichiometry has been determined by equilibrium thermogravimetry. The P(O2) and temperature dependence of the electrical conductivity of LaNi0.6Fe0.4O3-δ has also been measured. It has been clarified that δ generates with Ni substitution and maximum δ was observed for LaNi0.6Fe0.4O3-δ . Temperature and P(O2) dependence of oxygen nonstoichiometry and electrical conductivity is small for LaNi0.6Fe0.4O3-δ , probably because variation of average valence of B-site ion by temperature and P(O2) is small since charge compensation between Ni and Fe is easy to occur. The easy occurrence of the charge compensation corresponds to hopping conduction with small, temperature and P(O2) independent activation energy. By combination of oxygen nonstoichiometry and electrical conductivity, it has been clarified that LaNi0.6Fe0.4O3-δ is p-type conductor with mobility of 0.257 cm2V-1s-1, which is independent on temperature and P(O2). Hole concentration at temperatures between 300 °C and 800 °C under -4 ≤ log{P(O2)/bar}≤0 is 3~4 x1021 cm-3.

Dielectric and electromechanical properties of Mn-doped 0.2875 Pb(Mg1/3Nb2/3)O3–0.2875 Pb(Yb1/2Nb1/2)O3–0.425 PbTiO3 ceramics

In this study, 0.2875 Pb(Mg1/3Nb2/3)O3–0.2875 Pb(Yb1/2Nb1/2)O3–0.425 PbTiO3 (0.2875PMN–0.2875PYbN–0.425PT) ternary ceramic composition was doped with 1 mol% MnCO3 in order to induce hard character for potential high-power applications. Dense 0.2875PMN–0.2875PYbN–0.425PT ceramics with 1 mol% MnCO3 addition were fabricated after sintering at 1100 °C. e r = 1728, tanδ = 0.35 %, d 33 = 320 pC/N, d 31 = −103 pC/N, Q m = 467, k p = 0.40, k 31 = 0.24, k 33 = 0.49, and T c = 280 °C were measured for Mn-doped ceramics. However, undoped ceramics had e r = 2380, tanδ = 1.95 %, d 33 = 433 pC/N, d 31 = −145 pC/N, Q m = 60, k p = 0.43, k 31 = 0.27, k 33 = 0.48, and T c = 285 °C. Acceptor Mn2+/Mn3+ ions presumably substituted B-site ions in the perovskite structure and formed defect dipole pairs. The electrically “hard” character was induced as a result of the domain wall pinning due to the existing defect pairs. Particularly, increasing Q m from 60 to 467 and decreasing tanδ from 1.95 to 0.35 % after Mn doping showed that Mn-doped 0.2875PMN–0.2875PYbN–0.425PT ceramics with “hard” character are potential candidates for high-power projector and transducer applications.

Improved magnetostrictive properties of Co–Mn ferrites for automobile torque sensor applications

Strain derivative of local anisotropy, which depends on the magnetostriction, is the figure of merit for the magnetic materials used in automobile torque sensor applications. In order to improve the strain derivative, a series of samples with chemical formulae CoMnxFe2−xO4 (x=0.00–0.60) was prepared by a conventional ceramic technique. X-ray diffraction patterns of the samples confirm cubic spinel crystal structures. The magnetic properties, such as saturation magnetization and coercivity, were measured and the Curie temperature was estimated. Magnetostrictive properties were studied by the tensile strain gauge method. Manganese substituted cobalt ferrites have shown improved strain derivative values as compared to the pure cobalt ferrite. The strain derivative has been observed to increase for larger Mn substituent concentrations in CoMnxFe2−xO4. This behavior has been ascribed to the net decreased anisotropic contribution due to the increased presence of Mn3+ ions in B-sites and a corresponding migration of Co2+ ions to A-sites, and also to the microstructural grain size of the samples. The coercivity and magnetostriction measurements are in support of the strain derivative variations. The results are discussed on the basis of cationic site occupancies, variations in grain size and reduced anisotropy of the system.

Mn3+ substituted Co–Cd ferrites, CoCd0.4MnxFe1.6−xO4 (0.1 ⩽ x ⩽ 0.6): Cation distribution, structural, magnetic and electrical properties

The substitution of Mn3+ ions in cobalt–cadmium ferrites (synthesized via sol–gel auto-combustion method) is found to remarkably influence their structural, magnetic and electrical properties. The structural evolutions of the nanophase, investigated using powder X-ray diffraction (XRD) studies reveal the formation of single-phased cubic spinel structures, with Fd-3m space group. As the Mn3+ concentration is increased, the values of the structural parameters like lattice constant, crystallite size, X-ray density, bulk density and porosity undergo significant changes. The value of lattice constant is found to be ∼8.40Å and the average crystallite size is in the range of 41–45nm. The porosity in the entire ferrite samples is low (4.0–10.2%). The magnetic properties, like saturation magnetization and coercivity, show a decreasing trend with increase in Mn3+ concentration. Using the values of saturation magnetization, the cation distribution has been proposed. The proposed cation distribution is further correlated with the electrical properties. It is observed that the D.C. resistivity increases with Mn3+ concentration, suggesting occupancy of Mn3+ ions in the octahedral B-sites which leads to the dilution of Fe2+–Fe3+ conduction. The activation energy calculated from the D.C. resistivity vs temperature curves is found to be ∼0.50eV.

Low-temperature selective catalytic reduction of NO with NH3 using perovskite-type oxides as the novel catalysts

In order to investigate the activities of perovskite-type mixed oxides for selective catalytic reduction (SCR) of NO with NH3, a series of LaBO3 or La2BO4 perovskite-based catalysts with different B-site ions (B=Cu, Co, Mn, and Fe) was prepared by citrate complexation procedure and characterized by XRD, BET, TPD of NO+O2 and NH3. It was found that Mn-based perovskite exhibited the best catalytic activity, yielding 78% NO conversion at 250°C. The NH3 adsorption ability of the perovskites is the key factor for the SCR performance. In situ DRIFTS investigation was further carried out over LaMnO3 to explore the reaction mechanism. It is revealed that the SCR reaction starts with NH3 chemical adsorption, being thought as the rate-determining step. The mechanism essentially involves a Langmuir–Hinshelwood mechanism mainly depending on an interaction between NH4+ ionic species and nitrite species in the low-temperature range.

Performance of LaBO<sub>3</sub>/HZSM-5 Catalysts for Eliminating Soot from Diesel Exhaust

The LaBO3/HZSM-5 samples were prepared by the impregnation method. The structure of catalysts was examined by XRD. The catalytic activity for the combustion of soot particulate was evaluated by a technique of the temperature-programmed reaction. In the LaBO3/HZSM-5 catalyst, the Mn as B-site ion, the catalyst was the good candidate catalyst for the soot catalytic combustion. In the LaMnO3/HZSM-5 catalyst, the catalytic activity was tunable by changing the metal components of the perovskite-type oxide at B-site, the Fe partial substitution for Mn of LaMnO3/HZSM-5 enhanced the catalytic activity, and the combustion temperature of soot particle was lower than LaMnO3/HZSM-5 sample without substitution, corresponding Tm of 401oC and the selectivity of CO2 77.0%.

열차폐코팅용 (La1-xGdx)2Zr2O7산화물의 상형성과 열전도도

With increase in operating temperature of gas turbine for higher efficiency, it is necessary to find new materials of TBC for replacement of YSZ. Among candidate materials for future TBCs, zirconate-based oxides with pyrochlore and fluorite are prevailing ones. In this study, phase structure and thermal conductivities of <TEX>$(La_{1-x}Gd_x)_2Zr_2O_7$</TEX> oxide system are investigated. <TEX>$(La_{1-x}Gd_x)_2Zr_2O_7$</TEX> system are comprised by selecting <TEX>$La^{3+}/Gd^{3+}$</TEX> as A-site ions and <TEX>$Zr^{4+}$</TEX> as B-site ion in <TEX>$A_2B_2O_7$</TEX> pyrochlore structures. With powder mixture from each oxide, <TEX>$(La_{1-x}Gd_x)_2Zr_2O_7$</TEX> oxides are fabricated via solid-state reaction at <TEX>$1600^{\circ}C$</TEX>. Either pyrochlore or fluorite or mixture of both appears after heat treatment. For the developed phases along <TEX>$(La_{1-x}Gd_x)_2Zr_2O_7$</TEX> compositions, thermal conductivities are examined, with which the potential of <TEX>$(La_{1-x}Gd_x)_2Zr_2O_7$</TEX> compositions for TBC application is also discussed.

Investigation of the Dielectric Properties of Antimony Doped Potassium Sodium Niobate Single Crystal (K&lt;sub&gt;0.5&lt;/sub&gt;Na&lt;sub&gt;0.5&lt;/sub&gt;) NbO&lt;sub&gt;3&lt;/sub&gt; Grown by Flux Method

Single crystals of K0.5Na0.5NbO3 (KNN) and 5wt%, 10wt%, 15wt% of Sb doped KNN crystals were grown by flux method. The formation of crystalline structure, microstructure, domain structure and the dielectric properties were investigated for both pure and Sb doped KNN single crystals. X-ray diffraction (XRD) pattern shows that pure and doped KNN single crystals have orthorhombic perovskite structure. The doped crystals have slight shrinkage in the unit cell volume. The partial substitution of the B-site ion Nb5+ by the Sb5+ ion in the KNN single crystal results in decreasing phase transition temperatures TO-T and Curie temperatures TC of the doped crystals with increasing amount of Sb. The dielectric properties of the doped crystals show significant improvement with doping concentration. The peaks slightly shift towards lower frequencies with increasing dopant concentration.

Dielectric properties and phase transition of samarium-doped BaSn0.1Ti0.9O3 ceramics

The dielectric properties and phase transition of Sm3+-doped BaSn0.1Ti0.9O3 (BTS10) ceramics were investigated. The amount of Sm2O3 greatly affected the dielectric properties of BTS10. A dielectric constant of 5500 with 0.6 mol% Sm2O3 and a dissipation factor of only 0.005 with 0.1 mol% Sm2O3 were assessed. The variation in the dielectric constant indicated that Sm3+ ion is mainly substituted for the A-site ions when x≤0.6 mol%, whereas it is mainly substituted for B-site ions when x exceeded 0.6 mol%. Meanwhile, the variation in the dissipation factor showed that a very small amount of Sm3+ occupied the B site when x≤0.1 mol%. The materials showed relaxor-like behaviour with a diffuse-type ferroelectric phase transition. The diffusivity increased when the amount of Sm2O3 is increased, which also implied that there should be a very small amount of Sm3+ occupying the B site when x≤0.1 mol%.

The structure and low-energy phonons of the nonferroelectric mixed perovskite: BaMg1/3Ta2/3O3

The structure of BaMg1/3Ta2/3O3 (BMT) has been studied using x-ray scattering. The phonons have been measured and the results are similar to those of other materials with a perovskite structure such as PbMg1/3Nb2/3O3 (PMN). The acoustic and lowest energy optic branches were measured but it was not possible to measure the branches of higher energy, possibly this is because they largely consist of oxygen motions. High-resolution inelastic measurements also showed that the diffuse scattering was strictly elastic and not directly related to the phonon spectra. Diffuse scattering was observed in BMT near the (H ± 1/2, K ± 1/2, L ± 1/2) points in the Brillouin zone and these had a characteristic cube shape. This arises from ordering of the B-site ions in BMT. Additional experiments revealed the diffuse scattering in BMT similar in shape to Bragg reflections at wavevectors of the form (H ± 1/3, K ± 1/3, L ± 1/3). Such reflections were also observed by Lufaso (2004 Chem. Mater. 16 2148) from powders and suggest that this structure of BMT consists of four differently oriented domains of a trigonal structure and results from a different ordering of the B-site ions from that responsible for the scattering at the (H ± 1/2, K ± 1/2, L ± 1/2) points. The results lead us to suggest that for BMT single crystals the bulk has the properties of a cubic perovskite, whereas the surface may have quite different structure from that of the bulk. This difference resembles the behaviour of cubic relaxors like PMN and PMN doped by PbTiO3, where significant surface effects have been reported.

Non-Stoichiometric Effect in (K,Na)NbO3-Based Perovskite Ceramics

We prepared Li0.04(K0.5Na0.5)x(NbySb0.06)O3 ceramics using the conventional solid-state sintering method and investigated the non-stoichiometric effect on sintering behavior and electrical properties in these material systems. The deficiency of B-site ions gave rise to poor densities, while the densification was improved by adding excess Nb2O5. As the vacancy concentration in the A-site of the perovskite structure increased, the diffuse phase transition was enhanced and the transition temperature (To–t) was lowered. Furthermore, high deficiency in the A-site with x=0.90 and y=0.98 promoted a second phase formation. An increase of alkali cation vacancies led to large remnant polarization and enhanced a pinched hysteresis loop, while the remnant polarization decreased due to a second phase formation. The electro-mechanical coupling factors were significantly influenced by density and second phase formation, not non-stoichiometry, while dielectric constants were influenced by non-stoichiometry, not second phase formation.

Simultaneous Removal of NO&lt;sub&gt;x&lt;/sub&gt; and PM over La&lt;sub&gt;&lt;i&gt;1-x&lt;/i&gt;&lt;/sub&gt;K&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;Co&lt;sub&gt;&lt;i&gt;1-y&lt;/i&gt;&lt;/sub&gt;Mn&lt;sub&gt;&lt;i&gt;y&lt;/i&gt;&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; Perovskite-Type Catalysts

To study the effect of K and Mn substitution on simultaneous removal of NOx and particulate materials (PM), a series of La1-xKxCo1-yMnyO3 (x, y=0.1, 0.2, 0.3, 0.4, 0.5) perovskite-type catalysts were synthesized by complex-combustion method. The catalytic performances of the catalysts were evaluated by temperature-programmed oxidation reaction (TPO). And the catalysts were characterized by XRD, FT-IR, H2-TPR techniques. After introducing K and Mn to LaCoO3 simultaneously, all the samples possess perovskite-type structures and the B-site ions form higher average oxidation state enhancing the catalytic performance obviously. La0.8K0.2Co0.7Mn0.3O3 catalyst exhibits not only the lowest T i(297 °C) temperature but also the lowest Tm (343 °C) temperature. Meanwhile, the NO reduction percentage over this catalyst can reach 97%.

Sr 함량이 Cu-doped LSM(La1-xSrxMn0.8Cu0.2O3)의 구조적변화와 전기전도도에 미치는 영향

<TEX>$La_{1-x}Sr_xMn_{0.8}Cu_{0.2}O_{3-{\delta}}$</TEX>(LSMCu)에서 Sr 함량에 따른 구조적 변화와 전기전도도에 대해 연구, 고찰하였다. EDTA citric complexing process(ECCP)로 페로브스카이트 구조를 갖는 <TEX>$La_{0.8}Sr_{0.2}MnO_3$</TEX>(LSM)와 <TEX>$La_{1-x}Sr_xMn_{0.8}Cu_{0.2}O_3$</TEX>(<TEX>$0.1{\leq}x{\leq}0.4$</TEX>)을 제조하였다. Sr 함량이 증가할수록 격자상수와 격자부피는 감소하는 경향을 나타내었으며, 이는 Sr 함량이 증가함에 따라 B-site에서 증가하는 <TEX>$Mn^{4+}$</TEX> 이온과 <TEX>$Cu^{3+}$</TEX> 이온의 영향인 것으로 판단하였다. <TEX>$0.1{\leq}x{\leq}0.3$</TEX> 범위의 조성에서 Sr 함량이 증가할수록 <TEX>$500{\sim}1000^{\circ}C$</TEX>에서 측정된 전기전도도는 증가하였고, x = 0.3 조성에서는 <TEX>$750^{\circ}C$</TEX>와 <TEX>$950^{\circ}C$</TEX>에서 각각 172.6 S/cm와 177.7 S/cm (최고값)를 나타내었다. 반면, x = 0.4 조성에서는 전기전도도가 감소하였는데 이는 입계에 발생한 산화물에 의한 영향으로 판단하였다. Sr 함량이 증가함에 따라 B-site에 존재하는 <TEX>$Mn^{4+}$</TEX> 이온과 <TEX>$Cu^{3+}$</TEX> 이온의 증가로 인해 격자수축이 발생하고, hopping mechanism에 관여하는 charge carrier들이 늘어나 전기전도도가 증가한 것으로 판단하였다. The structural change and the electrical conductivity with Sr content in <TEX>$La_{1-x}Sr_xMn_{0.8}Cu_{0.2}O_3$</TEX> (LSMCu) were studied. <TEX>$La_{0.8}Sr_{0.2}MnO_3$</TEX> (LSM) and <TEX>$La_{1-x}Sr_xMn_{0.8}Cu_{0.2}O_3$</TEX> (<TEX>$0.1{\leq}x{\leq}0.4$</TEX>) were synthesized by EDTA citric complexing process (ECCP). A decrease in the lattice parameters and lattice volumes was observed with increase of Sr content, and these results were attributed to the increasing <TEX>$Mn^{4+}$</TEX> ions and <TEX>$Cu^{3+}$</TEX> ions in B-site. The electrical conductivity measured from <TEX>$500^{\circ}C$</TEX> to <TEX>$1000^{\circ}C$</TEX> was increased with increase of Sr content in the <TEX>$0.1{\leq}x{\leq}0.3$</TEX> composition range, and it was 172.6 S/cm (at <TEX>$750^{\circ}C$</TEX>) and 177.7 S/cm (at <TEX>$950^{\circ}C$</TEX>, the maximum value) in x = 0.3. The electrical conductivity was decreased in x = 0.4 because of the presence of the second phase in the grain boundaries. The lattice volume was contracted by increase of <TEX>$Mn^{4+}$</TEX> ions and <TEX>$Cu^{3+}$</TEX> ions in B-site according to increase of Sr content and the electrical conductivity was increased with increase of charge carriers which were involved in the hopping mechanism.

Effects of Ba Deficiency on Ion Ordering, Grain Growth, and Microwave Dielectric Properties of Ba1-xZn1/3Nb2/3O3 Ceramics

Ba1-xZn1/3Nb2/3O3 (0≤x≤0.02) ceramics were prepared by the solid-state reaction. The effects of Ba deficiency on the long-range order (LRO) of B-site ions, grain growth, and microwave dielectric properties of Ba1-xZn1/3Nb2/3O3 ceramics were investigated. Results show that Ba deficiency can effectively improve the sinterability of the ceramics and reduce the densification temperature by about 100 °C. The X-ray diffraction (XRD) and Raman spectra reveal that Ba-deficient samples exhibit a higher degree of LRO, and that the samples with x = 0.01 exhibit the highest degree of LRO. Furthermore, the degree of LRO increases with the decrease in sintering temperature in the range of 1400–1550 °C. Good microwave dielectric properties of εr = 39.8, Q ×f = 66532 GHz, and τf = +23.9 ppm/°C are obtained for the Ba-deficient samples with x = 0.01 when sintered at 1450 °C for 4 h. After a second sintering process at 1400 °C/10 h during cooling, the Q ×f values of Ba1-xZn1/3Nb2/3O3 ceramics are improved. An optimized Q ×f value of 72650 GHz is obtained for the Ba0.99Zn1/3Nb2/3O3 (x = 0.01) ceramics after two-step sintering.

Comparative study of nanometric Co-, Mn- and Fe-based perovskite-type complex oxide catalysts for the simultaneous elimination of soot and NOx from diesel engine exhaust

In this feature article, the effects of the intrinsic factors including structure, nanometric size effect, nature of B-site ions, A-site ion substitution, redox properties of three systems of nanometric La1−xMxCoO3, La1−xMxMnO3, La1−xKxFeO3 perovskite-type oxide catalysts, and the external factors containing contacting model of catalyst and soot, the content of O2 or NO, the ratio of catalyst and the gas flow rate on their catalytic performances for the simultaneous elimination of soot particles and NO were systematically investigated. The catalysts were characterized by means of XRD, FT-IR, SEM, XPS and H2-TPR, and the catalytic performances for the simultaneous removal of nitrogen oxides and diesel soot were evaluated by temperature-programmed oxidation reactions. All the prepared samples had the perovskite structure, possessed nano-sizes and gave high activities for the simultaneous elimination of soot particles and NO. On the one hand, the number of surface atoms is much greater than the normal bulk catalysts and the size of nanoparticle is small. These characters are favorable for the adsorption and activation of reactant molecules including the O2 or NO, even soot particle. On the other hand, the surface atoms of small nano-particles formed on the surface of the nanometric perovskite oxide samples possess high free energy, which improves the mobility of catalyst, and thus it favors the contact between the catalysts and soot. Besides the intrinsic factors, the external factors of reaction conditions containing contact model of soot and catalyst, the content of O2 or NO, the ratio of catalyst to soot and gas flow rate also play important roles in affecting the catalytic performances for the simultaneous removal of NO and soot. The reaction mechanisms for soot oxidation and the simultaneous removal of soot particles and NO over La1−xKxMnO3 were also briefly discussed.