Effects of Calcination on the Physical and Photocatalytic Properties of TiO<Subscript>2</Subscript> Powders Prepared by Sol–Gel Template Method

TiO₂ powders were prepared by sol–gel template method and calcined under different conditions. XRD, BET and TEM were used to characterize the TiO₂ powders obtained. The photocatalytic activity of TiO₂ was investigated by the degradation of methyl orange. It was found that TiO₂ powder has the highest photocatalytic activity at a calcination temperature of 673 K. The effects of physical properties such as surface area, crystallinity and crystal phase on the photocatalytic activity of TiO₂ were discussed.     

Sol-gel modified Pechini method for obtaining nanocrystalline KRE(WO4)2 (RE = Gd and Yb)

KRE(WO₄)₂ (RE = Gd and Yb) nanocrystalline powder was obtained by the modified sol-gel Pechini method. The precursor powder was calcined between 923 and 1023 K for a maximum of 6 h at air atmosphere. DTA-TG of the precursor powder shows that the temperature for total calcination is around 800–850 K. Molar ratio between the complexing agent and the metal ions in the first step of the method and molar ratio between the complexing agent and the ethylene glycol in the second step of the method were studied to optimize the preparation process. X-ray diffraction and IR spectroscopy were used to study the transformation from precursor powder into a crystalline monoclinic phase. Raman spectroscopy was used to study the vibrational structure of the nanoparticles. The Scherrer formula was used to confirm the grain sizes visualized by SEM and TEM techniques. Small nanoparticles in the range of 20–50 nm of monoclinic KREW have been successfully obtained by this methodology.     

Influence of grain size on the oxygen permeation performance of perovskite-type (Ba0.5Sr0.5)(Fe0.8Zn0.2)O3−δ membranes

The effect of grain size distribution in perovskite-type (Ba_0.5Sr_0.5)(Fe_0.8Zn_0.2)O_3−δ (BSFZ) ceramics on their oxygen permeation behaviour has been investigated by variation of calcination temperature in powder production and sintering time for the ceramics. The membranes were examined via scanning electron microscopy (SEM), transmission electron microscopy (TEM) and oxygen permeation experiments. We found that the dwell time during sintering has an important influence on the microstructure of the ceramic. The longer the dwell time, the further proceeds the grain coarsening, which affects the oxygen permeation in a positive way and leads to an enhanced permeation. Supplementary, decreasing calcination temperature in perovskite powder synthesis delivers fine powders with grain sizes less than one micrometer and thus smaller grains in the ceramic. Unfortunately, the grain size distribution in sintered membranes is not constant through membrane cross-sections since grains in the bulk are smaller compared to those at the surface which is not favorable for the oxygen permeation of the ceramics. The activation energy was determined to be in the range of 51–53 kJ/mol and its variation does not exhibit a dependence of grain size changes. High-resolution transmission electron microscopy proved that grain boundaries are atomically thin without any interfacial phases. We come to the conclusion that the transport rate of the oxygen permeation is limited predominantly by bulk diffusion and due to the fact that grain boundaries in BSFZ act as barriers for bulk diffusion, this material is a high mobility material.     

BaBi2Nb2O9 powder and their ceramics prepared by aqueous solution–gel method

BaBi₂Nb₂O₉ powders and their ceramics were prepared by aqueous solution?Cgel method. The phase evolution and microstructure of samples including powders and ceramics were determined by X-rays, SEM. An homogenous BaBi₂Nb₂O₉ powder was observed after thermal treatment at the temperature as low as 400?°C. The texture phenomena has been observed in the ceramics during the process of sintering, the size and area of oriented grains increased with increase the sintering temperature, and the grain growth occurs in preferential direction which is parallel to the plane of the ceramic plate. The density of this ceramics has been determined, the study revealed that the density increases with sintering temperature and reached a maximum at 1,000?°C and then decreases gradually. The characteristic diffuse phase transition of the BaBi₂Nb₂O₉ ceramics was observed at about 200?°C.     

Effect of alumina incorporation on restricting grain growth of nanocrystalline tin(IV) oxide

In this project, nanocrystalline SnO₂ powders were successfully prepared by (a) citrate sol-gel and (b) direct precipitation methods. Powders were characterized using thermal analysis techniques (DTA-TG-DSC), X-ray powder Diffraction (XRD), surface area (BET) and electrical conductivity measurements. XRD patterns showed the presence of the cassiterite structure. SnO₂ particles, prepared through sol-gel method exhibit crystallite sizes in the range from 3.1 to 22.3 nm when the gel is heat treated at different temperatures up to 900°C. SnO₂ nanocrystallites prepared by the precipitation method are comparatively larger in size. The higher specific surface area was obtained for the powder prepared using sol-gel method and the obtained average grain size (d) is relatively large compared with that of the average crystallite size. The powders show a semiconducting behavior with increasing temperature. The higher conductivity obtained for SnO₂ prepared by sol-gel method can be attributed to their smaller average crystallite size. XRD of alumina doped powder exhibits finer particles than pure SnO₂. TEM images showed that the particles are spherical in shape and consist of a core of SnO₂ surrounded by a coating of alumina. The calculated surface area was found to decrease with temperature increases. Due to the effective role of Al₂O₃ additive as a grain growth inhibitor for the matrix grains, the observed surface area for the coated materials are predominantly higher than for the uncoated materials.      

Anisotropic thermoelectric properties of Bi1.9Lu0.1Te2.7Se0.3 textured via spark plasma sintering

Spark plasma sintering method was applied to prepare bulk n-type Bi_1.9Lu_0.1Te_2.7Se_0.3 samples highly textured along the 001 direction parallel to the pressing direction. The texture development is confirmed by X-ray diffraction analysis and scanning electron microscopy. The grains in the textured samples form ordered lamellar structure and lamellar sheets lie in plane perpendicular to the pressing direction. The average grain size measured along the pressing direction is much less as compared to the average grain size measured in the perpendicular direction (∼50 nm against ∼400 nm). A strong anisotropy in the transport properties measured along directions parallel and perpendicular to the pressing direction within the 290 ÷ 650 K interval was found. The specific electrical resistivity increases and the thermal conductivity decreases for the parallel orientation as compared to these properties for the perpendicular orientation. The Seebeck coefficient for both orientations is almost equal. Increase of the electrical resistivity is stronger than decrease of the thermal conductivity resulting in almost three-fold enhancement of the thermoelectric figure-of-merit coefficient for the perpendicular orientation (∼0.68 against ∼0.24 at ∼420 K). The texturing effect can be attributed to (i) recovery of crystal structure anisotropy typical for the single crystal Bi₂Te₃-based alloys and (ii) grain boundary scattering of electrons and phonons. An onset of intrinsic conductivity observed above T_d ≈ 410 K results in appearance of maxima in the temperature dependences of the specific electrical resistivity, the Seebeck coefficient and the thermoelectric figure-of-merit coefficient and minimum in the temperature dependence of the total thermal conductivity. The intrinsic conductivity is harmful for the thermoelectric efficiency enhancement since thermal excitation of the electron-hole pairs reduces the Seebeck coefficient and increases the thermal conductivity.     

Characterization and preparation of nanocrystalline MgCuZn ferrite powders synthesized by sol–gel auto-combustion method

Nanocrystalline Mg–Cu–Zn ferrite powders were successfully synthesized through nitrate–citrate gel auto-combustion method. Characterization of the nitrate–citrate gel, as-burnt powder and calcined powders at different calcination conditions were investigated by using XRD, DTA/TG, IR spectra, EDX, VSM, SEM and TEM techniques. IR spectra and DTA/TGA studies revealed that the combustion process is an oxidation–reduction reaction in which the NO₃ ⁻ ion is oxidant and the carboxyl group is reductant. The results of XRD show that the decomposition of the gel indicated a gradual transition from an amorphous material to a crystalline phase. In addition, increasing the calcination temperature resulted in increasing the crystallite size of Mg–Cu–Zn ferrite powders. VSM measurement also indicated that the maximum saturation magnetization (64.1 emu/g) appears for sample calcined at 800 °C while there is not much further increase in M _s at higher calcination temperature. The value of coercivity field (H _c) presents a maximum value of 182.7 Oe at calcination temperature 700 °C. TEM micrograph of the sample calcined at 800 °C showed spherical nanocrystalline ferrite powders with mean size of 36 nm. The toroidal sample sintered at 900 °C for 4 h presents the initial permeability (μ _i) of 405 at 1 MHz and electrical resistivity (ρ) of 1.02 × 10⁸ Ω cm.     

Monolayer V2O5/TiO2–ZrO2 catalysts for selective oxidation of o-xylene: preparation and characterization

A series of TiO₂?CZrO₂ supported V₂O₅ catalysts with vanadia loadings ranging from 4 to 12 wt% were synthesized by a wet impregnation technique and subjected to various thermal treatments at temperatures ranging from 773 to 1,073?K to understand the dispersion and thermal stability of the catalysts. The prepared catalysts were characterized by X-ray powder diffraction (XRD), BET surface area, oxygen uptake, and X-ray photoelectron spectroscopy (XPS) techniques. XRD results of 773?K calcined samples conferred an amorphous nature of the mixed oxide support and a highly dispersed form of vanadium oxide. Oxygen uptake measurements supported the formation of a monolayer of vanadium oxide over the thermally stable TiO₂?CZrO₂ support. The O 1s, Ti 2p, Zr 3d, and V 2p core level photoelectron peaks of TiO₂?CZrO₂ and V₂O₅/TiO₂?CZrO₂ catalysts are sensitive to the calcination temperature. No significant changes in the oxidation states of Ti⁴⁺ and Zr⁴⁺ were noted with increasing thermal treatments. Vanadium oxide stabilized as V⁴⁺ at lower temperatures, and the presence of V⁵⁺ is observed at 1,073?K. The synthesized catalysts were evaluated for selective oxidation of o-xylene under normal atmospheric pressure in the temperature range of 600?C708?K. The TiO₂?CZrO₂ support exhibits very less conversion of o-xylene, while 12 wt% V₂O₅ loaded sample exhibited a good conversion and a high product selectivity towards the desired product, phthalic anhydride.     

焙烧温度对CuO/Fe2O3/ZrO2甲醇水蒸气重整制氢催化剂性能的影响

采用XRD、TPR和EXAFS等手段，考察了焙烧温度对CuO/Fe2O3/ZrO2物化性能和甲醇水蒸气重整制氢活性及其选择性的影响。结果表明，催化剂中氧化铜的晶粒随着焙烧温度的提高而增大，铜的配位环境发生变化。在焙烧温度623K－723K范围内，对甲醇水蒸气重整反应的甲醇转化率和氢选择性影响较小，其结构参数变化值较小。当焙烧温度提高到923K时，催化剂的活性因ZrO2晶化和铜组分的聚焦而显著降低。结果铁的加入使ZrO2的相变温度向后推移，并且有效地阻止了CuO颗粒的聚集。     

纳米掺铁二氧化钛的sol-gel法制备与表征 (Ⅱ)纳米掺铁二氧化钛的微晶特性与晶体生长

本文结合透电镜分析研究了sol-gel方法制备的纯二氧化钛和掺铁二氧化钛干凝胶的晶化过程,计算了在不同的煅烧温度下二氧化钛微晶的晶胸参数,晶粒度,畸变等参数的变化关系,应用非晶物质晶化晶核生长速率议程计算的昌粒生长活化能表明晶粒生长分为两个阶段,临界点大约为相变温度,纯的和掺铁的二氧化钛在两个阶段的晶粒生长活化能分别约为20．8kJ.mol^-1,70.9kJ.mol^-1和26．6kJ.mol^-1,78.8kJ.mol^-1.这个差别可能是由于相主过程首先发生在小晶粒上,导致小晶粒生长较为困难所致。     

纳米掺铁二氧化钛的sol-gel法制备与表征 (Ⅱ)纳米掺铁二氧化钛的微晶特性与晶体生长

本文结合透射电镜分析研究了 sol-gel方法制备的纯二氧化钛和掺铁二氧化钛干凝胶的晶化过程。计算了在不同的煅烧温度下二氧化钛微晶的晶胞参数,晶粒度,畸变等参数的变化关系。应用非晶物质晶化晶核生长速率方程计算的晶粒生长活化能表明晶粒生长分为两个阶段,临界点大约为相变温度。纯的和掺铁的二氧化钛在两个阶段的晶粒生长活化能分别约为 20.8kJ· mol-1,70.9kJ· mol-1和 26.6kJ· mol-1,78.8kJ· mol-1。这个差别可能是由于相变过程首先发生在小晶粒上,导致小晶粒生长较为困难所致。     

Production of hydrogen by oxidative steam reforming of methanol over Cu/SiO2 catalysts

Selective production of hydrogen by oxidative steam reforming of methanol (OSRM) was studied over Cu/SiO₂ catalyst using fixed bed flow reactor. Textural and structural properties of the catalyst were analyzed by various instrumental methods. TPR analysis illustrates that the reduction temperature peak was observed between 510?K and 532?K at various copper loadings and calcination temperatures and the peaks shifted to higher temperature with increasing copper loading and calcination temperature. The XRD and XPS analysis demonstrates that the copper existed in different oxidation states at different conditions: Cu₂O, Cu⁰, CuO and Cu(OH)₂ in uncalcined sample; CuO in calcined sample: Cu₂O and metallic Cu after reduction at 600?K and Cu⁰ and CuO after catalytic test. TEM analysis reveals that at various copper loadings, the copper particle size is in the range between 3.0?nm and 3.8?nm. The Cu particle size after catalytic test increased from 3.6 to 4.8?nm, which is due to the formation of oxides of copper as evidenced from XRD and XPS analysis. The catalytic performance at various Cu loadings shows that with increasing Cu loading from 4.7 to 17.3?wt%, the activity increases and thereafter it decreases. Effect of calcination shows that the sample calcined at 673?K exhibited high activity. The O₂/CH₃OH and H₂O/CH₃OH molar ratios play important role in reaction rate and product distribution. The optimum molar ratios of O₂/CH₃OH and H₂O/CH₃OH are 0.25 and 0.1, respectively. When the reaction temperature varied from 473 to 548?K, the methanol conversion and H₂ production rate are in the range of 21.9–97.5% and 1.2–300.9?mmol?kg^?1?s^?1, respectively. The CO selectivity is negligible at these temperatures. Under the optimum conditions (17.3?wt%, Cu/SiO₂; calcination temperature 673?K; 0.25 O₂/CH₃OH molar ratio, 0.5 H₂O/CH₃OH molar ratio and reaction temperature 548?K), the maximum hydrogen yield obtained was 2.45?mol of hydrogen per mole of methanol. The time on stream stability test showed that the Cu/SiO₂ catalyst is quite stable for 48?h.     

Diffusional study of the reaction of sulfur dioxide with reactive porous matrices

A single-pellet high-temperature diffusion cell reactor is used to study the sulfation of calcined Greer limestone pellets. The effective diffusivities of gases through the reactive pellets, during the calcination and sulfation are determined. The experimentally determined effective diffusivity of sulfur dioxide through the pores of the product shell is used in the modified expanding grain model to obtain the diffusivity of sulfur dioxide through the product shell of the grains as a function of the conversion and the reaction temperature. The activation energy for the initial diffusivity of sulfur dioxide through the product shell of grains, is found to be 34.13 kcal mol^?1; the diffusivity values decreased with increasing conversion. Additionally it is found that the ratio of the tortuosity of reacting shell of pellet to the initial pellet tortuosity before any sulfation was increased with increasing conversion.     

A complex impedance analysis of the ionic conductivity of Na1+xZr2SixP3−xO12 ceramics

The ionic conductivity of pressed pellets of Na_1+xZr₂Si_xP_3?xO₁₂ (1.6<x<2.4) has been determined by the measurement of their complex impedance in the range of 5 Hz?108 MHz. The respective contributions of the grains and intercrystalline boundaries of the ceramics have been established and their activation energies for conduction (0.36 eV and 0.41 eV) calculated between 224 and 348 K. In most cases the intergrain resistance was as high or higher than the grain resistance. Samples totally free of impurities, mainly zirconium dioxide, were never prepared. The large intergrain resistance was attributed to these impurities, and the relatively low packing density of the grains.     

Preparation and characterization of scandia-doped tungsten powders prepared by a spray-drying method

Scandium oxide-doped tungsten powders were prepared by a new method of spray drying combined with two-step hydrogen reduction. The particle size of doped tungsten, powder morphology, and distribution of doped scandium oxide were characterized by scanning electron microscopy, X-ray diffraction, and laser diffraction particle size analysis. Experimental results indicated that the predecessor powders prepared by spray drying were spherical in shape. Two compounds, WO₃ and Sc₂W₃O₁₂, in the raw powder calcined at 600???C were transformed into metallic tungsten and scandia after reduction at 850???C by hydrogen for 1?h. The scandia-doped tungsten powders obtained had an average size of 950?nm and scandium oxide was distributed evenly throughout the tungsten powder. The mechanism of reduction of the doped tungsten oxide is discussed in this paper.     

Sintering properties of sol–gel derived lithium disilicate glass ceramics

Lithium disilicate (Li₂Si₂O₅) glass-ceramics were fabricated through two sol–gel methods: the nitrate route and the alkoxide route. Thermal analysis revealed different crystallization processing of two gel-derived powders. Li₂Si₂O₅ powders were obtained after heat treatment at 800?°C. These powders were pressed and pressureless sintered under 900–1030?°C. Microstructure of sintered samples revealed the grain size and morphology of Li₂Si₂O₅ ceramics. Although grain size in both samples increased with increasing sintering temperature, samples from the alkoxide route derived powders had more uniform grain size and pore distribution. In addition, open porosity decreased in both samples with increasing sintering temperature. Unlike familiar nucleation that resulted in grain growth mechanism, the Li₂Si₂O₅ particles developed into irregular large size grains at first, and then grew into rod-shaped grains.

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Structural and Physical Properties of La2/3Ca1/3MnO3 Prepared via a Modified Sol-Gel Method

High purity powder of manganese perovskite, La_2/3Ca_1/3MnO₃, is prepared using a modified sol-gel synthesis based on the dissolution and homogenisation of metal salts in ethanol-acetic acid mixture without using any complexing aid (e.g., polyol or polyhydroxy acid, etc.), which is essentially used in the polymeric precursor routes. This modification minimises the organic contamination in the resulting ceramic that is formed as single perovskite at 650°C, after short calcination time periods. The formation of a monophasic material and absence of second phases or phase segregation was confirmed by powder X-ray diffraction, energy dispersive X-ray and electron microscopy of the ceramic calcined at higher temperatures (800–1400°C). The calcined samples are nanocrystalline up to 1000°C (average particle size, 44 nm) however, significant particle growth is observed at higher temperatures with micron-sized grains present in the sample sintered at 1400°C. The sample exhibits the characteristic colossal magnetoresistance behaviour. Owing to the high chemical and structural purity of the obtained ceramic, the intrinsic bulk features like metal-insulator transition and ferromagnet-antiferromagnet behaviour of the polycrystalline sample are comparable to those observed in the single crystal La_2/3Ca_1/3MnO₃ specimen, used as a reference. The single-crystal-like properties are also corroborated by the observation of a sharp metallic fermi edge in the UPS measurements. The variable temperature photoemission spectra reveal a temperature dependent redistribution of spectral weight close to the fermi level corroborating the temperature dependent resistance and magnetoresistance of the sample. The material shows a homogeneous grain size and a high sinterability as shown by TEM and SEM studies, respectively. XPS study indicates a charge carrier hopping between Mn³⁺(3d⁴) and Mn⁴⁺(3d³) sites.     

Comparison of different soft chemical routes synthesis of nanocrystalline LiMn2O4 and their influence on its physicochemical properties

A comparative study of nanocrystalline spinel LiMn₂O₄ powders prepared by two different soft chemical routes such as solution and sol-gel methods using lithium and manganese acetates are the precursors under different calcination temperatures. The dependence of the physicochemical properties of the spinel LiMn₂O₄ powder has been extensively investigated by using thermal analysis (TGA/DTA), FTIR, X-ray diffraction studies, SEM, specific surface area (BET) and electrical conductivity measurements. The results show that pure LiMn₂O₄ can be prepared from acetate precursors as starting materials at a low temperature of 600°C from solution route and 500°C from sol-gel method. The charge-discharge characteristics and the cycling behavior of Li/1M LiBF₄-EC/DEC electrolyte / LiMn₂O₄cells revealed that LiMn₂O₄ calcined at higher temperatures showed a high initial capacity, while the LiMn₂O₄calcined at lower temperatures exhibited a good cycling behavior.     

Effect of hydrogen reduction temperature on the microstructure and magnetic properties of Fe–Ni powders

The effect of hydrogen reduction temperature on the properties of Fe–Ni powders was described. The mixed powders of Fe-oxide and NiO were prepared by chemical solution mixing of nitrates powders and calcination at 350 °C for 2 h in air. The calcined powders formed small agglomeration with an average particle size of 100 nm. The microstructure and magnetic properties were investigated by using X-ray diffractometry, thermogravimetry, differential thermal analyzer, and vibrating sample magnetometer. Microstructure and thermal analysis revealed that the Fe-oxide and NiO phase were changed to FeNi₃ phase in the temperature range of 245–310 °C, and by heat-up to 690 °C the FeNi₃ phase was transformed to γ-FeNi phase. The reduced powder at 350 °C showed saturation magnetization of 76.3 emu/g and coercivity of 205.5 Oe, while the reduced powders at 690 °C exhibited saturation magnetization of 84.0 emu/g and coercivity of 14.0 Oe. The change of magnetic properties was discussed by the observed microstructural features.

     

The role of MgO in the thermal behavior of MgO–silica fume pastes

The compounds of MgO–silica fume (SF) pastes constitute magnesium silicate hydrate (M–S–H) in a new generation of basic castables. However, Mg(OH)₂ is a common reaction product with the formation of M–S–H. This study aims to reduce the formation of Mg(OH)₂ in MgO–SF pastes. In this study, MgO powders were prepared by calcining magnesite at different temperatures and then mixed with SF and water to prepare MgO–SF pastes. The properties of MgO powders were characterized, and the pH values in the pore solutions of MgO–SF pastes were measured. The MgO–SF pastes cured for 90 days were calcined at 500, 700, 900 and 1200 °C, and the microstructure was characterized afterward. The results showed that both the reactivity of MgO powders and the pH value of the pore solution of MgO–SF pastes were diverse, which essentially depended on the grain sizes and the crystalline degree of MgO. Increasing the calcination temperature of MgO was beneficial to reduce the formation of Mg(OH)₂ or even stop it when using MgO calcined at 1450 °C. Enstatite and forsterite formed for all MgO–SF pastes after calcination. However, the microstructure of MgO–SF paste with MgO calcined at 1450 °C was denser than others. MgO–SF pastes were suitable for the new-generation refractory castables. Notably, using MgO calcined at 1450 °C is more appropriate.