Grain growth in TiO2-added ZnO–Bi2O3–CoO–MnO ceramics prepared by chemical processing

The effect of TiO₂ on the grain growth of the ZnO–Bi₂O₃–CoO–MnO ceramic system prepared by chemical coprecipitation, was studied between 1150 and 1300 °C in air. Bi₂O₃ melts during firing, and then TiO₂ dissolves into Bi₂O₃-rich liquid. TiO₂ initially reacts with Bi₂O₃ to form Bi₄Ti₃O₁₂. Above ≈1050 °C, Bi₄Ti₃O₁₂ reacts with ZnO to form Zn₂TiO₄ spinel phase. The kinetic study of grain growth carried out using the expression Gⁿ–G_oⁿ=K_o·t·exp(−Q/RT) gave grain exponent (n) value as 6 and the apparent activation energy (Q) as 226.46 kJ/mol. 1.00 mol% TiO₂ addition increased the grain growth exponent value from 6 to 7 and apparent activation energy with 1.00 mol% TiO₂ addition was found to be 197.10 kJ/mol. The ZnO grain size gradually increases with increasing TiO₂ content. Addition of TiO₂ may increase the reactivity of the Bi₂O₃-rich liquid towards the ZnO grain, thus affecting the ZnO grain growth.     

Synthesis and properties of Bi0.5(Na1−x−yKxAgy)0.5TiO3 lead-free piezoelectric ceramics

Bi_0.5(Na_1−x−yK_xAg_y)_0.5TiO₃ piezoelectric ceramics were prepared by conventional ceramic processes. X-ray diffraction patterns show a pure perovskite structure, indicating that the K⁺ and Ag⁺ ions substitute for the Na⁺ ions in Bi_0.5Na_0.5TiO₃. The temperature dependence of the dielectric constant and dissipation factor shows all ceramics to experience two phase transitions: from ferroelectric to anti-ferroelectric and from anti-ferroelectric to paraelectric. The transition temperature from ferroelectric to anti-ferroelectric and the temperature at which the dielectric constant reaches its maximum value decrease with the increase of K⁺ amount. At room temperature, the ceramics containing 17.5–20 mol% K⁺ and 2 mol% Ag⁺ exhibit high piezoelectric constant (d₃₃ = 180 pC/N) and high electromechanical coupling factor (k_p = 35%).     

Processing and Dielectric Properties of Sillenite Compounds Bi12MO20−δ (M = Si, Ge, Ti, Pb, Mn, B1/2P1/2)

Six sillenite compounds Bi₁₂MO_20-δ (M = Si, Ge, Ti, Pb, Mn, B_1/2P_1/2) were synthesized, and the resulting single-phase powders were then sintered to obtain ∼97% dense ceramics. An analysis of their microwave dielectric properties, performed at ∼5.5 GHz, revealed a permittivity of ∼40 for all six compounds. The temperature coefficient of resonant frequency was the lowest for the Pb analogue (−84 ppm/K) and was found to increase with increasing ionic radius of the B-site ion to a value of −20 ppm/K for the Bi₁₂SiO₂₀ and Bi₁₂(B_1/2P_1/2)O₂₀ compounds. The Q × f value is a maximum for Bi₁₂SiO₂₀ and Bi₁₂GeO₂₀ with 8100 and 7800 GHz, respectively. The dielectric properties of the sillenites have been correlated with the structure of the oxygen network of the sillenite crystal lattice. As a result of its low sintering temperature (850°C), chemical compatibility with silver, low dielectric losses, and temperature-stable permittivity, the Bi₁₂SiO₂₀ compound is a suitable material for applications in low-temperature cofiring ceramic (LTCC) technology.     

Highly textured (Na1/2Bi1/2)0.94Ba0.06TiO3 ceramics prepared by the screen-printing multilayer grain growth technique

The screen-printing multilayer grain growth (MLGG) technique was successfully applied to perovskite-structured lead-free piezoelectric ceramics. Highly textured (Na_1/2Bi_1/2)_0.94Ba_0.06TiO₃ ceramics with (1 0 0) orientation were firstly fabricated by MLGG method with (or without) template particles. The MLGG approach using anisotropic Bi₄Ti₃O₁₂ templates resulted in >90% grain orientation, whereas the same approach without template particles resulted in high orientation degree. The grain orientation mechanism of MLGG using screen-printing was different form that of tape-casting and extrusion in templated grain growth (TGG) and reactive templated grain growth (RTGG) techniques. The interface between adjacent layers, which were formed by screen-printing, was the main mechanism for the texture development in MLGG technique. Compared with other grain orientation techniques, screen-printing was a simple, inexpensive and effective method to fabricate grain oriented lead-free piezoelectric ceramics.     

La-, K- and Nb-doped 0.90(Bi0.5Na0.5TiO3)–0.10PbTiO3

Lanthanum, Potassium and Niobium have been selected as cation dopants to modify the relaxor characteristics of 0.90(Bi_0.5Na_0.5TiO₃)–0.10PbTiO₃. The experimental results show that La lowers the phase transition temperatures and decreases the grain size. In contrast, the grain size of K-doped composition tends to increase. Furthermore, the maximum of dielectric permittivity and the Curie temperature increase as compared to those of La-doped material. La can improve the broadness of dielectric permittivity of 0.90(Bi_0.5Na_0.5TiO₃)–0.10PbTiO₃. However, Nb is a better promising dopant for enhancing the relaxor behavior for this composition.     

Sintering behavior of Al2TiO5 base ceramics and their thermal properties

Sintering behavior of Al₂TiO₅ without and with various additives and the thermal properties of the sintered material—thermal expansion and decomposition—were investigated. The precursors of Al₂TiO₅ powders were prepared by homogeneous precipitation and coprecipitation. Sintering of pure Al₂TiO₅ gave a fine grained-structure at 1300°C, but resulted in large-grained and cracked microstructures at 1400 and 1500°C. Addition of ZrO₂ or BaO gave fine-grained microstructures with a small increase in thermal expansion. Addition of ZrO₂, BaO or ZrSiO₄, especially ZrSiO₄, was effective in suppressing the thermal decomposition of Al₂TiO₅ at 1100°C. ©     

Sintering behavior and microwave dielectric properties of Bi2O3–ZnO–Nb2O5-based ceramics sintered under air and N2 atmosphere

The sintering behavior and dielectric properties of the monoclinic zirconolite-like structure compound Bi₂(Zn_1/3Nb_2/3)₂O₇ (BZN) and Bi₂(Zn_1/3Nb_2/3−xV_x)₂O₇ (BZNV, x = 0.001) sintered under air and N₂ atmosphere were investigated. The pure phase were obtained between 810 and 990 °C both for BZN and BZNV ceramics. The substitution of V₂O₅ and N₂ atmosphere accelerated the densification of ceramics slightly. The influences on microwave dielectric properties from different atmosphere were discussed in this work. The best microwave properties of BZN ceramics were obtained at 900 °C under N₂ atmosphere with _r = 76.1, Q = 850 and Q_f = 3260 GHz while the best properties of BZNV ceramics were got at 930 °C under air atmosphere with _r = 76.7, Q = 890 and Q_f = 3580 GHz. The temperature coefficient of resonant frequency τ_f was not obviously influenced by the different atmospheres. For BZN ceramics the τ_f was −79.8 ppm/°C while τ_f is −87.5 ppm/°C for BZNV ceramics.     

Evaluation of the role played by bismuth molybdates in Bi2Sn2O7–MoO3 catalysts used for partial oxidation of isobutene to methacrolein

The catalytic behaviour of multiphasic catalysts based on -bismuth pyrostannate, Bi₂Sn₂O₇, was investigated in the selective oxidation of isobutene into methacrolein. When -Bi₂Sn₂O₇ is mixed with MoO₃, strong cooperation effects on the yield and selectivity in methacrolein occur. However, XRD analyses performed on samples after test revealed the formation of a low quantity of -bismuth molybdate, -Bi₂Mo₃O₁₂, when the reaction temperature exceeded 673 K. Additional experiments were therefore carried out on the “Bi–Sn–Mo–O” catalysts in order to shed light on the role of Bi₂Mo₃O₁₂ in the synergetic effects observed in the Bi₂Sn₂O₇–MoO₃ system. The experimental results are discussed in terms of several hypotheses. First, the intrinsic activity of Bi₂Mo₃ O₁₂ is probably the simplest explanation for the synergetic effects, although experiments have shown that this phase present in a low quantity is only poorly active. Second, catalytic tests made on Bi₂Sn₂O₇–Bi₂Mo₃O₁₂ mechanical mixtures have evidenced a cooperation between these two ternary oxides, particularly when Bi₂Sn₂O₇ was the major component of the mixture. Consequently, it is likely that a synergy between Bi₂Sn₂O₇ and the in situ generated Bi₂Mo₃O₁₂ might play a role in the synergy observed in the Bi₂Sn₂O₇–MoO₃ association. Third, as bismuth pyrostannate was previously shown to behave as an oxygen donor phase with respect to WO₃, a remote control mechanism could therefore occur between Bi₂Sn₂O₇ and MoO₃, independently from the formation of -Bi₂Mo₃O₁₂.     

Catalytic combustion of soot over alkali doped SrTiO3

The effect of introduction of alkalies (Me = Li, K, Cs) into SrTiO₃ on the physico-chemical properties of resulted materials and their catalytic activity in soot combustion was studied. Two groups of SrTiO₃ based perovskites were prepared: substituted in A-position of the structure (Sr_1 − xMe_xTiO₃, x = 0.05–0.2) and impregnated with the same amount of alkali metals. Prepared materials exhibit low specific surface area and perovskite structure, only these ones impregnated with the highest amount of Cs (K) show weak XRD signals of Me₂O. TPD-O₂ experiments show bimodal profiles of O₂ desorption curves with maximums corresponding to individual step of alkali nitrates thermal decomposition. It is supposed that second peak of O₂ desorption from impregnated SrTiO₃ can be related to reversible decomposition of MeNO₃. XPS shows that surface of SrTiO₃ substituted with K (Cs) is much richer in these elements than the surface of impregnated one. Prepared materials lower the temperature of soot ignition from 530 (inert) to 470 °C for SrTiO₃ and to 302–303 °C for Sr_0.8K_0.2TiO₃ and Sr_0.8K_0.2TiO₃, respectively. Substituted materials are more active in soot combustion than impregnated ones. A mechanism explaining effect of alkali metals nitrate addition to SrTiO₃ on its catalytic activity in soot combustion is proposed.     

Oxidative coupling of methane over oxide catalysts with layered structure

The catalytic properties of complex oxides with a layered structure Bi₂GeO₅, Bi₂SiO₅, Bi₄Ti₃O₁₂, Bi₂CaSrCu₃O₈+x and YBa₂Cu₃O₆+δ in oxidative coupling of methane (OCM) have been studied. Bi₂EO₅ metastable compounds have been found to possess the high activity and C₂-selectivity 53–70%. It has been assumed that the intergrowth boundaries for the Bi₂EO₅ decomposition products, on which active catalytic sites may be located, play a specific role on the catalytic performance of the oxides. Bi₄Ti₃O₁₂ and Bi₂CaSrCu₃O₈+x have close catalytic activity values but Bi₂CaSrCu₃O₈+x as well as YBa₂Cu₃O₆+δ are not selective in OCM reaction.     

Synthesis and Characterization of a New Sillenite Compound—Bi12(B0.5P0.5)O20

A new sillenite compound, Bi₁₂(B_0.5P_0.5)O₂₀, was synthesized using a solid-state reaction method. The stoichiometry was confirmed by XRD analyses, microstructural investigations, and quantitative elemental analysis. An investigation of the dielectric properties at frequencies from 100 Hz to 1 MHz revealed a broad, highly frequency-dispersive, relaxor-like dielectric anomaly, which appeared in the temperature range of −80°–100°C. The permittivity, Q × f value, and temperature coefficient of the resonant frequency, measured at ∼5.5 GHz, were determined to be 37.4, 850 GHz, and −19 ppm/K, respectively.     

Photocatalytic activity of Cu2O/TiO2, Bi2O3/TiO2 and ZnMn2O4/TiO2 heterojunctions

Cu₂O/TiO₂, Bi₂O₃/TiO₂ and ZnMn₂O₄/TiO₂ heterojunctions were studied for potential applications in water decontamination technology and their capacity to induce an oxidation process under VIS light. UV–vis spectroscopy analysis showed that the junctions-based Cu₂O, Bi₂O₃ and ZnMn₂O₄ are able to absorb a large part of visible light (respectively, up to 650, 460 and 1000 nm). This fact was confirmed in the case of Cu₂O/TiO₂ and Bi₂O₃/TiO₂ by photocatalytic experiments performed under visible light. A part of the charge recombination that can take place when both semiconductors are excited was observed when a photocatalytic experiment was performed under UV–vis illumination. Orange II, 4-hydroxybenzoic and benzamide were used as pollutants in the experiment. Photoactivity of the junctions was found to be strongly dependent on the substrate. The different phenomena that were observed in each case are discussed.     

Facile ultrasonic-assisted synthesis of micro–nanosheet structure Bi4Ti3O12/g-C3N4 composites with enhanced photocatalytic activity on organic pollutants

The Bi₄Ti₃O₁₂/g-C₃N₄ composites with microsheet and nanosheet structure were prepared through facile ultrasonic-assisted method. The SEM and TEM results suggested that the nanosheets g-C₃N₄ were stacked on the surface of regular Bi₄Ti₃O₁₂ sheets. Comparing with pure Bi₄Ti₃O₁₂ and g-C₃N₄, the Bi₄Ti₃O₁₂/g-C₃N₄ composites showed significant enhancement in photocatalytic efficiency for the degradation of RhB in solution. With the mass ratio of g-C₃N₄ increasing to 10 wt%, the Bi₄Ti₃O₁₂/g-C₃N₄-10% presented the best photocatalytic activity. Its photocatalysis reaction constant was approximately 2 times higher than the single component Bi₄Ti₃O₁₂ or g-C₃N₄. Meanwhile, good stability and durability for the Bi₄Ti₃O₁₂/g-C₃N₄-10% were confirmed by the recycling experiment and FT-IR analysis. The possible mechanism for the improvements was the matched band positions and the effective separation of photo-excited electrons (e^-) and holes (h⁺). Furthermore, based on the results of active species trapping, photo-generated holes (h⁺) and superoxide radical (·O₂^-) could be the main radicals in reaction.     

NO decomposition and reduction by CH4 over Sr/La2O3

Both NO decomposition and NO reduction by CH₄ over 4%Sr/La₂O₃ in the absence and presence of O₂ were examined between 773 and 973 K, and N₂O decomposition was also studied. The presence of CH₄ greatly increased the conversion of NO to N₂ and this activity was further enhanced by co-fed O₂. For example, at 773 K and 15 Torr NO the specific activities of NO decomposition, reduction by CH₄ in the absence of O₂, and reduction with 1% O₂ in the feed were 8.3·10⁻⁴, 4.6·10⁻³, and 1.3·10⁻² μmol N₂/s m², respectively. This oxygen-enhanced activity for NO reduction is attributed to the formation of methyl (and/or methylene) species on the oxide surface. NO decomposition on this catalyst occurred with an activation energy of 28 kcal/mol and the reaction order at 923 K with respect to NO was 1.1. The rate of N₂ formation by decomposition was inhibited by O₂ in the feed even though the reaction order in NO remained the same. The rate of NO reduction by CH₄ continuously increased with temperature to 973 K with no bend-over in either the absence or the presence of O₂ with equal activation energies of 26 kcal/mol. The addition of O₂ increased the reaction order in CH₄ at 923 K from 0.19 to 0.87, while it decreased the reaction order in NO from 0.73 to 0.55. The reaction order in O₂ was 0.26 up to 0.5% O₂ during which time the CH₄ concentration was not decreased significantly. N₂O decomposition occurs rapidly on this catalyst with a specific activity of 1.6·10⁻⁴ μmol N₂/s m² at 623 K and 1220 ppm N₂O and an activation energy of 24 kcal/mol. The addition of CH₄ inhibits this decomposition reaction. Finally, the use of either CO or H₂ as the reductant (no O₂) produced specific activities at 773 K that were almost 5 times greater than that with CH₄ and gave activation energies of 21–26 kcal/mol, thus demonstrating the potential of using CO/H₂ to reduce NO to N₂ over these REO catalysts.     

Morphology selective construction of β-cyclodextrin functionalized Fe3O4-Bi2WO6 nanocomposite with superior adsorptivity and visible-light-driven catalytic activity

Controlled growth of Bi₂WO₆ nanorods with exposed [0 0 1] facets and the fabrication of an Fe₃O₄-Bi₂WO₆ magnetic composite by a microwave-assisted polyol process, were achieved in this study. The adsorptivity and photocatalytic performance of the composite toward sunset yellow dye degradation were greatly enhanced by the β-cyclodextrin cavities on its surface, firmly anchored through a cetyltrimethylammonium bromide linkage. A series of examinations and characterizations were carried out to determine the influence of various factors on the morphological modulation-photocatalytic behavior of the pure Bi₂WO₆ prior to final functionalization. Changing the pH of the precursor solution impacted the formation of 0D, 2D, and 3D structures; however, the presence of hexamethylenetetramine surfactant induced the development of 1D nanorod structure. A reasonable crystal growth mechanism was proposed to elucidate the formation process. Conversely, the mechanism of the activity enhancement of β-cyclodextrin functionalized Fe₃O₄-Bi₂WO₆, compared to that of the non-functionalized samples, could be realized with the assistance of chemical trapping experiments on sunset yellow, and was confirmed on the colorless antibiotic (sulfamethoxazole). The high performance and durability of this composite can be attributed to the facet-dependent activity, large adsorption capacity due to inclusion interactions, enhanced visible light absorption, and efficient charge separation.     

g-C3N4/Bi2MoO6复合光催化剂的制备及性能研究

结合热缩聚法和水热法制备了g-C₃N₄/Bi₂MoO₆复合光催化剂,利用X射线衍射（XRD）、傅里叶红外光谱（FT-IR）、扫描电子显微镜（SEM）、透射电子显微镜（TEM）、氮气吸附-脱附曲线、紫外-可见漫反射光谱（UV-Vis DRS）、光致发光光谱（PL）等分析测试技术对材料的结构和性能进行了表征,研究了材料光催化降解罗丹明B（RhB）的效果。结果表明,与纯Bi₂MoO₆相比,g-C₃N₄/Bi₂MoO₆复合材料提高了对可见光的吸收能力,减小了带隙宽度,在可见光激发下提高了降解RhB的光催化活性。其中,5% g-C₃N₄/Bi₂MoO₆复合材料对RhB的降解率最高,在可见光照射180 min对RhB的降解率为93%;而同样条件下Bi₂MoO₆对RhB的降解率为58%。重复性实验表明,复合材料在RhB光降解过程中是稳定的,具有较好的应用潜力。     

Rapid preparation of Bi2WO6 photocatalyst with nanosheet morphology via microwave-assisted solvothermal synthesis

Nanocrystalline Bi₂WO₆ photocatalyst with nanosheet morphology was successfully synthesized by a microwave-solvothermal process. The prepared samples were characterized by X-ray diffraction technique (XRD), BET surface area analysis, Uv–vis diffuse reflectance spectrum (Uv–vis DRS), field emission scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). In comparison with a conventional hydrothermal process, the microwave-solvothermal process presented many advantages in a shorter reaction time, higher surface area and more oxygen vacancies for the preparation of Bi₂WO₆ samples. The absorption edge of the samples is at ca. 445 nm, corresponding to band gap energy of about 2.8 eV. The morphology of nanosheets with nanocrystals was also observed. For the decomposition of Rhodamine B (RhB) under visible light irradiation (λ > 420 nm), nanocrystalline Bi₂WO₆ samples obtained by microwave-solvothermal process showed higher photocatalytic activity than that of the sample obtained by conventional hydrothermal process.     

g-C3N4/Bi/Bi2WO6光催化材料的协同改性研究

以五水合硝酸铋[Bi（NO₃）₃·5H₂O]为铋源、二水合钨酸钠（Na₂WO₄·2H₂O）为钨源通过水热法制备出多孔钨酸铋（Bi₂WO₆）,并以纳米板条堆叠形成椭球结构的类石墨相氮化碳（g-C₃N₄）为基底通过溶剂热法在原位还原金属铋（Bi）的同时制备出具有Z型异质结构的g-C₃N₄/Bi/Bi₂WO₆（CN/B/BWO）复合光催化材料。采用X射线衍射（XRD）、扫描电镜（SEM）、透射电镜（TEM）、氮气吸附-脱附等温线（BET）、紫外-可见吸收光谱（UV-Vis）和光致发光（PL）光谱等检测手段对制备的样品进行了表征。结果表明,金属铋可以作为类石墨相氮化碳和钨酸铋之间的电荷转移媒介,其产生的表面等离子体共振（SPR）效应可协同增强光生电子-空穴对的分离效率和载流子的迁移率,从而提升样品的光催化活性。采用350 W氙灯照射30 min,样品CN/B/BWO-0.7对盐酸四环素（TC-H）的降解率达到99.94%,并对其降解机理进行了探讨。     

硅酸铋(Bi4Si3O12)粉体制备的研究进展

Bi₄Si₃O₁₂晶体作为一种性能优异的新型闪烁体,在各个方面有着重要的作用,粉体制备有着重要的意义。本文综述了硅酸铋粉体的制备技术,有固相法、高能球磨法、溶胶-凝胶法、水热法和化学溶液分解法等,同时讨论了各个方法的优缺点,最后展望了硅酸铋粉体制备的未来发展趋势。     

Structure and microwave dielectric properties of Nd(2−x)/3LixTiO3

The structure evolution, and microwave dielectric properties of Nd_(2−x)/3Li_xTiO₃ ceramics (0 ≤ x ≤ 0.5) were investigated in this paper. X-ray diffraction (XRD) and scanning electron microscopy (SEM) results show that samples with x = 0.2–0.4 exhibit single phase. Multi-phases of Nd₂Ti₂O₇, Nd_2/3TiO₃ and Nd₂Ti₄O₁₁ were observed when x = 0 and 0.1. The concentration and ordering degree of A-site decrease with the increase of x value. The dielectric constant increases up to x = 0.2 and then decreases with the further increase of x value. The Qf value decreases with the increase of x value. The temperature coefficient of resonant frequency exhibits negative value and the absolute value decreases greatly with the decrease of x value.