Microstructure, electrical properties, and dc aging characteristics of Tb4O7-doped ZnO-based varistors

The microstructure, electrical properties, and dc-accelerated aging characteristics of Tb₄O₇-doped ZnO-based varistors were investigated for different Tb₄O₇ amounts and sintering temperatures. The sintered density increased with increasing Tb₄O₇ amount and sintering temperature. The average grain size decreased with increasing Tb₄O₇ amount and increased with increasing sintering temperature. The varistor voltage and nonlinear coefficient increased with increasing Tb₄O₇ amount and decreased with increasing sintering temperature. The stability was worse with increasing Tb₄O₇ amount for the varistors sintered at 1,300 °C. The 0.5 mol% Tb₄O₇-doped varistors sintered at 1,350 °C exhibited a good stability for dc-accelerated aging stress of 0.95 V _{1 mA}/150 °C/24 h.     

Electrochemical studies on cathode blends of LiMn2O4 and Li[Li1/15Ni1/5Co2/5Mn1/3O2]

Composite cathodes were prepared by blending LiMn₂O₄ spinel and Li[Li_1/15Ni_1/5Co_2/5Mn_1/3O₂] layer by simple mixing/ball milling followed by calcination at 800 °C. The prepared blend materials were subjected to XRD and charge–discharge studies. The cycling results revealed that the discharge capacity and cycleability of LiMn₂O₄ can be considerably increased upon blending the material with layered Li[Li_1/15Ni_1/5Co_2/5Mn_1/3O₂].     

Nonlinear electrical properties and aging characteristics of (NiO, MgO, Cr2O3)-doped Zn-Pr-Co-R (R = Y, Er) oxide-based varistors

The electrical properties and stability of the varistors, which composed of (NiO, MgO, Cr₂O₃)-doped Zn-Pr-Co-R (Y, Er) oxide-based ceramics, were investigated for different additives. The breakdown voltage of the varistors increased in order of NiO→undoped→MgO→Cr₂O₃: 1200→1551→1691→1959 V/cm for ZPCY system and undoped→NiO→MgO→Cr₂O₃: 1024→1041→1500→1668 V/cm for ZPCE system, respectively. The nonlinear coefficient value increased in order of undoped→NiO→MgO→Cr₂O₃: 21→25→38→50 in ZPCY system and NiO→undoped→MgO→Cr₂O₃: 27→32→35→38 in ZPCE system, respectively. In ZPCY and ZPCE systems, the Cr₂O₃-additives most greatly improved the nonlinear properties. In Cr₂O₃-doped system, ZPCY system exhibited higher nonlinear properties than that of ZPCE system. The stability against d.c. accelerated aging stress was higher in Cr₂O₃-additives than in NiO- and MgO-additives for ZPCY system and was higher in NiO-additives than in MgO- and Cr₂O₃-additives for ZPCE system.     

Synthesis and Microwave Dielectric Properties of MgSiO3 Ceramics

MgSiO₃ ceramics were synthesized and their microwave dielectric properties were investigated. The Mg₂SiO₄ phase was formed at temperatures lower than 1200°C, while the orthorhombic MgSiO₃ phase started to form by the reaction of SiO₂ and Mg₂SiO₄ in the specimen fired at 1200°C. The structure of the MgSiO₃ ceramics was transformed from orthorhombic to monoclinic when the sintering temperature exceeded 1400°C. A dense microstructure was developed for the specimens sintered at above 1350°C. The excellent microwave dielectric properties of ɛ_r=6.7, Q × f =121 200 GHz, and τ_f=−17 ppm/°C were obtained from the MgSiO₃ ceramics sintered at 1380°C for 13 h.     

Morphological evolution of PbTiO3 microstructures synthesized by topochemical microcrystal conversion

PbTiO₃ (PT) microstructures were fabricated by topochemical microcrystal conversion (TMC) and their morphological evolution was investigated with variation of synthesis temperature and relative molar ratio of reactants. Using molten salt synthesis, Aurivillius phase of PbBi₄Ti₄O₁₅ (PBiT) synthesized at 1050°C was prepared as precursor. Large plate-like PT microstructures with lengths of hundreds of micrometer were optimally obtained when the relative ratio (x) of (PbCO₃)₂·Pb(OH)₂ to PBiT in reactants was 6. In addition, relatively homogeneous PT platelets were achieved at the annealing temperature of 1050°C by TMC; these materials are probably suitable for use in templated grain growth. However, a Pb-deficient secondary phase of PbTi₃O₇ formed in cases of low x values of less than 4 and low annealing temperatures of less than 1050°C. The porous PT pellets, orthogonally assembled by numerous PT platelets, were achieved by increasing temperature to 1100°C. Further increase in temperature to 1150°C could lead to partial formation of denser PT blocks.     

Influence of physicochemical treatments on spent palladium based catalyst for catalytic oxidation of VOCs

To recycle the spent catalyst for the removal of VOCs, the benzene, toluene, and xylene (BTX) complete oxidations were studied over pretreated palladium based spent catalyst in a fixed bed flow reactor system at atmospheric pressure. Two different pretreatment methods with gas (air and hydrogen) and acid aqueous solution (HCl, H(2)SO(4), HNO(3), H(3)PO(4) and CH(3)COOH) were used to investigate the catalytic activity of spent catalyst. The properties of the spent and pretreated Pd based catalyst were characterized by XRD, BET, TEM, ICP, and XPS. The results of light-off curves indicate that the catalytic activity of toluene oxidation for pretreated samples is in the order of hydrogen>air>HNO(3)>CH(3)COOH>H(2)SO(4)>H(3)PO(4)>HCl. In addition, the air and the acid aqueous pretreated catalyst activities were significantly decreased compared to that of the spent (or parent) catalyst. Moreover, hydrogen pretreated (or reduced) catalysts having mainly metallic form show the best performance in removing the toluene vapours compared to other pretreated samples. The reduction temperature made a significant difference in the catalytic performance of the spent catalyst pretreated with hydrogen. XPS results clearly supported that the palladium state of the spent catalysts pretreated at 300 degrees C was shifted more toward metallic form than other reduced catalysts. Furthermore, the results of a long-term test and catalytic activity of aromatic hydrocarbons also supported that the hydrogen pretreated spent catalyst was a good candidate for removing toxic compounds.     

Major effects on varistor properties and impulse aging behavior of Zn–V–Mn–Co–Dy ceramics with small sintering changes

The effect of sintering temperature on varistor properties and impulse aging behavior of ZnO–V₂O₅–MnO₂–CoO–Dy₂O₃ ceramics was systematically investigated. As the sintering temperature increased, the average grain size increased from 5.8 to 10.8 μm, whereas the sintered density decreased from 5.56 to 5.47 g/cm³. The breakdown field and the nonlinear coefficient increased from 1,812 to 3,379 V/cm and from 11 to 36, respectively, with an increase in the sintering temperature up to 925 °C. The varistor ceramics sintered at 950 °C exhibited the best clamp characteristics for the impulse-current of 1–50 A, with the clamp voltage ratio of K = 1.63–2.31. The varistor ceramics sintered at 925 °C exhibited the strongest stability against an impulse-current, with $ \% \,\Updelta {\text{E}}_{{ 1 {\text{ mA}}/{\text{cm}}^{2} }} = - 8. 1 \,\% ,\;\%\, \Updelta \alpha = - 4 4. 4\, \% ,{\text{ and}}\;\%\, \Updelta {\text{J}}_{\text{L}} = 3 8. 9\, \% $ % Δ E 1 mA / cm 2 = ? 8.1 % , % Δ α = ? 4 4.4 % , and % Δ J L = 3 8.9 % after applying the multi-impulse-current of 400 A.     

Mechanical degradation of API X65 pipeline steel by exposure to hydrogen gas

Hydrogen-induced degradations have generally been investigated through the ex situ testing of cathodically hydrogen-charged specimens. However, the cathodic charging cannot realize damage accumulation by gaseous hydrogen in transportation pipelines. Thus, we designed an ampule specimen which enables an in situ tensile test containing gaseous hydrogen. Ampule specimens made of API (American Petroleum Institute) X65 pipeline steel showed significant reductions of 3.4 % and 4.1 %, respectively, in their ultimate tensile strength and fracture strain after exposure to 20 MPa of hydrogen gas. The resulting fracture surface showed quasi-cleavage perpendicular to the loading direction and a fracture thickness close to the initial wall thickness, significantly different from the complex dimples and highly reduced fracture thickness by shear deformations in nitrogen gas and air environments.     

Crystal structure and microwave dielectric properties of ZnTi(Nb1−xTax)2O8 ceramics

The crystal structure and microwave dielectric properties of ZnTi(Nb_1−xTa_x)₂O₈ (0 ≤ x ≤ 1) ceramics sintered at 1200 °C for 2 h were investigated. For x < 0.5, solid solution phases with the α-PbO₂ structure, typical of ZnTiNb₂O₈, were obtained, whereas for 0.5 ≤ x < 1, mixtures of two solid solutions each respectively based on the α-PbO₂ structure and a trirutile structure, were obtained. The relative amount of the trirutile-structured phases increased as the Ta content increased in a given region, and the end member ZnTiTa₂O₈ formed a single phase with the trirutile structure. The microwave dielectric properties were closely related to the crystal structures. A material with a near zero temperature coefficient of resonant frequency could be obtained for x = 0.8, and its dielectric constant and quality factor (Q × f) were 40.5 and 41,000 GHz, respectively.     

Synthesis of [10Ῑ]‐Oriented (Na1−xKx)NbO3 Platelets by Using Hydrothermally Produced (K8−8xNa8x)Nb6O19·nH2O Precursor

Micrometer size [10] (N_1?xK_x)NbO₃ (NKN) platelets were synthesized by annealing (K_8?8xNa_8x)Nb₆O₁₉·nH₂O (KNNH) precursors at 500°C. The plate‐like KNNH precursors were produced from (1?y)NaOH‐yKOH + Nb₂O₅ specimens using the hydrothermal process at 160°C. The size of the NKN platelets was similar to that of the KNNH precursor, but the surfaces of the NKN platelets were rough while the KNNH precursor had a smooth surface. The formation of a rough surface is related to the vigorous evaporation of the H₂O from the KNNH platelets during the annealing process at high temperatures. NKN platelets with smooth surfaces could be synthesized using KNNH platelets, which were heated to 150°C to evaporate H₂O before annealing at 500°C. These NKN platelets can be used for the fabrication of textured NKN ceramics.