Single-Calcination Synthesis Mechanism of Pure-Perovskite Pb(Ni1/3Nb2/3)O3-PbTiO3 Using a Coating Method

Pure-perovskite 0.64Pb(Ni_1/3Nb_2/3)O₃–0.36PbTiO₃ (PNN–PT) powder has been successfully synthesized by only one-step calcination using a coating method. SEM photograph shows that PNN–PT powder with the size of 2–4 μm is cubic and well dispersed. Based on X-ray diffraction analysis, solid-state reactions in the process of calcination in PbO–Nb₂O₅–TiO₂–NiO system are investigated. In comparison with conventional solid state method, the single-calcination synthesis mechanism of pyrochlore-free lead-based niobate ferroelectrics using a coating method is tentatively proposed. A typical coating structure of Ni precipitate-coated Nb₂O₅ powder facilitates the formation of perovskite PNN–PT phase at a relatively low calcination temperature, resulting in the successful synthesis of pyrochlore-free PNN–PT powder in one-step calcination at 900°C.     

Synthesis of MgAl2O4 Spinel Powder by Combination of Sol–Gel and Precipitation Processes

MgAl₂O₄ spinel precursor was prepared by a novel method combining a sol–gel process with the "traditional" precipitation process. The thermal decomposition and phase development of the precursor were analyzed, and the degree of agglomeration of the calcined powder was assessed by determining its particle size and crystal size. The optimum calcination temperature was determined based on the variation of specific surface areas, crystal size, and particle size. Completely crystallized ultrafine spinel powder ( d ₅₀=600 nm, specific surface area=105 m²/g) was obtained after calcination at 900°C.     

ZnO/EP复合材料的制备及其导热性能

采用溶胶-凝胶法制备纤锌矿型氧化锌(zinc oxide,ZnO)粉体,考察了煅烧温度对ZnO粉体质量的影响;将不同煅烧温度获得的ZnO填充于环氧树脂(epoxy resin,EP)得到系列ZnO/EP复合材料,采用红外光谱仪(Fourier transform infrared spectroscopy,FTIR)和场发射扫描电子显微镜(field emission scanning electron microscope,FESEM)对ZnO/EP复合材料进行结构和形貌表征,研究了ZnO粉体粒径及填充量对ZnO/EP复合材料导热性能的影响。结果表明,ZnO粉体粒径随煅烧温度升高而增大,其中700 ℃下制得的ZnO粉体粒径最大且纯度高。当ZnO填充量一定时,ZnO粉体粒径越大,越有利于提升ZnO/EP复合材料的导热性能;随ZnO填充量的增加,ZnO/EP复合材料的热导率不断提高,当ZnO体积分数为30.05%时,复合材料热导率达到0.54 W/(m·K),较纯环氧树脂提高了184%,且保持良好的力学性能。     

Influence of calcination conditions on carrying capacity of CaO-based sorbent in CO2 looping cycles

This study examines the loss of sorbent activity caused by sintering under realistic CO₂ capture cycle conditions. The samples tested here included two limestones: Havelock limestone from Canada (New Brunswick) and a Polish (Upper Silesia) limestone (Katowice). Samples were prepared both in a thermogravimetric analyzer (TGA) and a tube furnace (TF). Two calcination conditions were employed: in N₂ at lower temperature; and in CO₂ at high temperature. The samples obtained were observed with a scanning electron microscope (SEM) and surface compositions of the resulting materials were analyzed by the energy dispersive X-ray (EDX) method. The quantitative influence of calcination conditions was examined by nitrogen adsorption/desorption tests, gas displacement pycnometry and powder displacement pycnometry; BET surface areas, BJH pore volume distributions, skeletal densities and envelope densities were determined. The SEM images showed noticeably larger CaO sub-grains were produced by calcination in CO₂ during numerous cycles than those seen with calcination in nitrogen. The EDX elemental analyses showed a strong influence of impurities on local melting at the sorbent particle surface, which became more pronounced at higher temperature. Results of BET/BJH testing clearly support these findings on the effect of calcination/cycling conditions on sorbent morphology. Envelope density measurements showed that particles displayed densification upon cycling and that particles calcined under CO₂ showed greater densification than those calcined under N₂. Interestingly, the Katowice limestone calcined/cycled at higher temperature in CO₂ showed an increase of activity for cycles involving calcination under N₂ in the TGA. These results clearly demonstrate that, in future development of CaO-based CO₂ looping cycle technology, more attention should be paid to loss of sorbent activity caused by realistic calcination conditions and the presence of impurities originating from fuel ash and/or limestone.     

Phase Characterization of Lead Zirconate Titanate Obtained from Organic Solutions of Citrates

Lead zirconate titanate (PZT) with several Zr/Ti ratios was prepared by the organic solution of citrates of the respective cations. The Rietveld method was applied to perform quantitative analysis of the coexisting phases. The ratio of tetragonal/rhombohedral phases increased with calcination temperature. For the composition where Zr/Ti = 53/47, the tetragonal quantity varied from 25% to 35% for temperatures ranging from 600° to 800°C. For the powder with Zr/Ti = 57/43, only the rhombohedral phase was observed after calcination at 700°C. For the powder with Zr/Ti = 49/51, only the tetragonal phase was observed after calcination at 800°C. The amount of tetragonal phase was controlled in Zr/Ti = 55/45 by seeding the powder with the tetragonal phase of PZT.     

Synthesis and Structural Characterization of a Metastable Mullite-Like Alumina Phase

A solution combustion technique for the synthesis of different β-alumina compositions in the Na₂O· x Al₂O₃ system (where x =5, 6, or 7) is described along with the structural characterization of the materials prepared. The amorphous powder obtained after a combustion reaction between the nitrate salts of the cations and aminoacetic acid was calcined in air at different temperatures in the range from 600°C up to 1300°C. The phases were investigated by powder X-ray diffraction (XRD) and infrared spectroscopic measurements. A metastable mullite-like alumina phase was found to form as an intermediate at a minimum calcination temperature of 750°C and stable up to 1000°C, which transformed completely into β/β"-alumina phases beyond a temperature of 1100°C. The crystal structure of the mullite-like alumina phase was deduced by rietveld refinement of slow scan powder XRD data. A better understanding of the crystal structure of the mullite-like alumina was possible using other supplementary experimental evidences.     

Effect of sintering temperature on microstructure and transport properties of Li3xLa2/3−xTiO3 with different lithium contents

Li_3xLa_2/3−xTiO₃ (LLTO) powder with different lithium contents (nominal 3x = 0.03–0.75) was synthesized via a simple sol–gel route and then calcination of gel-derived precursor at 900 °C which was much below the calcination temperature required for synthesizing the LLTO powder via solid state reaction route. The LLTO powder of sub-micron sized particles, derived from such sol–gel method, showed almost no aggregation. Starting from the sol–gel-derived powder, the LLTO ceramics with different lithium contents were prepared at different sintering temperatures of 1250 and 1350 °C. It demonstrated that our sol–gel route is quite simple and convenient compared to the previous sol–gel method and requires lower temperature for the LLTO. Our results also illustrated that lithium content significantly affects the structure and ionic conductivity of the LLTO ceramics. The dependence of the ionic conductivity on the lithium content, lattice structure, microstructure and sintering temperature was investigated systematically.     

Fabrication of Transparent, Sintered Sc2O3 Ceramics

We report here the fabrication of transparent Sc₂O₃ ceramics via vacuum sintering. The starting Sc₂O₃ powders are pyrolyzed from a basic sulfate precursor (Sc(OH)_2.6(SO₄)_0.2·H₂O) precipitated from scandium sulfate solution with hexamethylenetetramine as the precipitant. Thermal decomposition behavior of the precursor is studied via differential thermal analysis/thermogravimetry, Fourier transform infrared spectroscopy, X-ray diffractometry, and elemental analysis. Sinterability of the Sc₂O₃ powders is studied via dilatometry. Microstructure evolution of the ceramic during sintering is investigated via field emission scanning electron microscopy. The best calcination temperature for the precursor is 1100°C, at which the resultant Sc₂O₃ powder is ultrafine (∼85 nm), well dispersed, and almost free from residual sulfur contamination. With this reactive powder, transparent Sc₂O₃ ceramics having an average grain size of ∼9 μm and showing a visible wavelength transmittance of ∼60–62% (∼76% of that of Sc₂O₃ single crystal) have been fabricated via vacuum sintering at a relatively low temperature of 1700°C for 4 h.     

Formation of porous glass via core/shell-structured poly(methyl methacrylate)/powder glass prepared by ultrasonic irradiation in liquid CO2

The formation of porous glass ceramic via core/shell-structured poly(methyl methacrylate)(PMMA)/powder glass was investigated. Core/shell structures were prepared via ultrasonic irradiation in high-pressure liquid carbon dioxide (CO₂) using PMMA microspheres as the core material and glass powder as the shell material. The mean particles sizes of PMMA template microspheres and glass powder were 9.8 μm and 0.9 μm, respectively. After removal of the PMMA template by calcination in air, porous glass was obtained. The products were characterized by scanning electronic microscopy (SEM) and thermogravimetric-differential thermal analysis (TG-DTA). The average pore diameter of porous glass was 4.3 μm. Compared with porous glass prepared by the other method, the porous glass prepared by ultrasonic irradiation of liquid CO₂ was achieved the narrow pore size distribution (CV = 35%) and the higher porosity (89%). The pores are not isolated and connected each other. Furthermore, the effects of experimental conditions, such as coating method, crosslink density of the template PMMA microspheres, ultrasonic intensity and calcination temperature, on the product morphology were investigated. The higher ultrasound intensity achieved the uniform coating of PMMA templates with powder glass. The calcination temperature and crosslinked density of PMMA template microspheres affect the pore structure.     

Mechanochemical Synthesis of 0.9[0.6Pb(Zn1/3Nb2/3)O3·0.4Pb(Mg1/3Nb2/3)O3]·0.1PbTiO3

Lead zinc niobate–lead magnesium niobate–lead titanate (PZN–PMN–PT) ceramic powders of perovskite structure have been prepared via a mechanochemical processing route. A single-phase perovskite powder of ultrafine particles in the nanometer range was successfully synthesized when a MZN powder (columbite precursor) was mechanically activated for 10 h together with mixed lead and titanium oxides. The following steps are involved when the ternary oxide mixture is subjected to an increasing degree of mechanical activation. First, the starting materials are significantly refined in particle size as a result of the continuous deformation, fragmentation and then partially amorphized at the initial stage of mechanical activation. This is followed by the formation of perovskite nuclei and subsequent growth of these nuclei in the activated oxide matrix with increasing activation time. When calcined at various temperatures in the range of 500–800°C, pyrochlore phase was not detected by XRD phase analysis in the mechanochemically synthesized powder. Only a minor amount (∼2%) of pyrochlore phase was observed when the calcination temperature was raised to 850°C. The PZN–PMN–PT derived from the mechanochemically synthesized powder can be sintered to ∼98% relative density at a sintering temperature of 950°C. The PZN–PMN–PT sintered at 1100°C for 1 h exhibits a dielectric constant of ∼18 600 and a dielectric loss of 0.015 at the Curie temperature of 112°C when measured at a frequency of 0.1 kHz, together with a d ₃₃ value of 323 ×10⁻¹² pC/N.     

Nanosized Zinc-Oxide Particles Derived from Mechanical Activation of Zn5(NO3)2(OH)8·2H2O in Sodium Chloride

Nanosized ZnO particles are successfully synthesized via mechanical activation of a zinc nitrate hydroxide hydrate (Zn₅(NO₃)₂(OH)₈·2H₂O) precursor in NaCl matrix for 15 h. The ZnO particles obtained are in the nanosize range of ∼20 nm, with a well-established hexagonal morphology. They compare favorably with those derived from conventional calcination of the precursor. The decomposition of Zn₅(NO₃)₂(OH)₈·2H₂O precursor and formation of nanocrystalline ZnO cannot be completed by mechanical activation in the absence of NaCl, which acts as both an effective dispersing matrix and drying agent although it remains chemically inert during mechanical activation. The powder derived from calcination at 400°C does not possess powder characteristics comparable to that of the powder derived from the mechanical activation in NaCl, because of the extensive particle coarsening and aggregation at the calcination temperature.     

Cation Ordering and Dielectric Characteristics in Barium Zinc Niobate

Barium zinc niobate (Ba(Zn_1/3Nb_2/3)O₃) (BZN) complex perovskite has been reported to have special microwave dielectric properties with close relation of its ordered structure. This study investigated the effect of calcination on the evolution of ordered structure and on quality factor with Raman spectroscopy, X-ray diffractometry, and transmission electron microscopy. The results revealed that single calcination at a lower temperature inhibited the growth of the ordered domain during sintering. In contrast, the 1:2-ordered domain in double-calcined BZN powder grew significantly with a higher sintering temperature and a longer soaking time. It is attributed that double calcination caused a higher degree of 1:2 ordering and better homogeneity. At the same time, the quality factor of the sintered ceramic body was highly promoted when using double-calcined powder. A close relation of the quality factor with the size of ordered domain, the degree of 1:2 ordering, and the relative density of BZN ceramics was presented.     

氧化镧粉体的制备及其对晶粒粒度的影响

以硝酸镧、聚乙二醇和柠檬酸为原料,采用溶胶-凝胶法制备了氧化镧粉体。研究了在制备过程中分散剂种类（聚乙二醇与柠檬酸）、焙烧温度、焙烧时间等因素对氧化镧粉体晶粒尺寸的影响及氧化镧的吸水性能。结果表明：以聚乙二醇为分散剂制备的氧化镧粉体晶粒粒度比柠檬酸为分散剂制备的样品晶粒粒度小;干凝胶在780 ℃下煅烧1 h,可以完全分解为氧化镧。焙烧温度越高,时间越长,晶粒尺寸就越大;氧化镧的吸水性很强,吸水后形成氢氧化镧。该研究可为控制粉体晶粒度提供参考。     

Comparison of the morphology and structure of WO3 nanomaterials synthesized by a sol-gel method followed by calcination or hydrothermal treatment

Tungsten (VI) oxide (WO₃) nanomaterials were synthesized by a sol-gel method using WCl₆ and C₂H₅OH as precursors followed by calcination or hydrothermal treatment. X-Ray diffraction (XRD), scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM) equipped with energy dispersive X-ray spectroscopy (EDX) were used to characterize the structure and morphology of the materials. There were significant differences between the WO₃ materials that were calcinated and those that were subjected to a hydrothermal process. The XRD results revealed that calcination temperatures of 300°C and 400°C gave hexagonal structures and temperatures of 500°C and 600°C gave monoclinic structures. The SEM images showed that an increase in calcination temperature led to a decrease in the WO₃ powder particle size. The TEM analysis showed that several nanoparticles agglomerated to form bigger clusters. The hydrothermal process produced hexagonal structures for holding times of 12, 16, and 20 h and monoclinic structures for a holding time of 24 h. The SEM results showed transparent rectangular particles which according to the TEM results originated from the aggregation of several nanotubes.     

溶胶-凝胶制备TiO2空气净化特性研究

利用XRD等手段对以钛酸四丁酯为原料采用溶胶-凝胶法制备的TiO₂纳米粒子的表面特性及其对空气环境中乙醛的光催化降解性能进行了测定。结果表明，乙醇与钛酸四丁酯体积比、溶胶pH、加水量和焙烧温度等对催化剂降解乙醛的活性有显著影响。在实验条件范围内，较短胶凝时间制备出的纳米粉末晶粒尺寸较小，活性较高。焙烧温度不但影响TiO₂粉末的粒径，而且对晶型的组成也有影响。锐钛矿晶型的光催化活性明显高于金红石晶型，锐钛矿型向金红石型转变的临界温度在（500～600） ℃。实验得到TiO₂最佳制备条件为：V(乙醇)∶V(钛酸四丁酯)=4、溶胶pH=3.0、加水量4 mL和焙烧温度(400～500) ℃。     

Yb-La/TiO_2光催化剂的制备工艺研究

采用溶胶-凝胶法制备镱镧共掺杂二氧化钛光催化剂Yb-La/TiO_2,在紫外光照射下,以罗丹明B作为目标降解染料,考察催化剂的光催化性能。结果表明,制备Yb-La/TiO_2光催化剂的最佳条件为:n(La)∶n(Yb)∶n(Ti)=1.2∶1.0∶100,钛酸丁酯用量10 m L,无水乙醇用量50 m L,聚乙二醇用量0.4 g,冰醋酸用量2 m L,pH=2.3,陈化4天,150℃干燥24 h,550℃焙烧2 h。此条件下制备的Yb-La/TiO_2光催化剂对罗丹明B的降解率达93.37%,重复使用3次后,降解率仍高于80%。     

简论硼砂生产过程中的矿石煅烧

在采用碳碱法制备硼砂时,必须先对硼矿石进行活化煅烧。煅烧的目的是改变矿石结构,将矿石中难以用碳解方式分解的硼镁石高温转化为易于分解的遂安石。介绍了矿石煅烧的几种方法、矿石煅烧的温度范围、硼铁矿石的煅烧。指出粉体闪速煅烧因单套装置生产能力大且机械化程度高、煅烧温度及生产状况易于控制、产品熟矿粉质量好且稳定等诸多优点而具有发展前景;硼镁石转化为遂安石的温度范围为620~800 ℃,硼镁石中含镁愈高煅烧温度区间愈宽且偏高,含铁愈高煅烧温度区间愈窄且偏低;对于单纯的硼铁矿石,目前的煅烧方式不能使其结构发生改变。     

MgO活性对新型水化硅酸镁水泥性能的影响

通过对原料轻烧氧化镁粉在不同温度下进行二次恒温煅烧1.5 h制备不同活性MgO,研究了不同活性MgO与硅灰(SF)和磷酸氢二钾(K₂HPO₄)所制备的新型水化硅酸镁水泥胶凝材料(又称水化磷硅酸镁水泥,MSPHC)的凝结时间、流动度、抗压强度、反应溶液pH值。结合X射线衍射(XRD)、热重分析(TG-DTG)和扫描电子显微镜(SEM)测试手段,分析其影响机理。结果表明:随着煅烧温度的升高,MgO衍射峰强度增大,MgO活性降低;活性越高的MgO制备的MSPHC净浆凝结时间越短且流动性越差,而活性适中MgO制备的MSPHC具有较好的力学性能。MSPHC最主要的水化产物是水化硅酸镁(M-S-H)凝胶,另外还有Mg(OH)₂和MgKPO₄·6H₂O(MKP)生成,原料轻烧氧化镁粉中的MgCO₃成分不参与体系反应。活性适中的MgO制备的MSPHC在28 d龄期内的水化产物M-S-H凝胶生成量最多,因此硬化体抗压强度最高。活性越高的MgO在MSPHC反应体系中溶解的速度越快,体系水化反应进程速度也越快。     

Preparation of 1-2-3 Superconductors from Hydroxide–Carbonate Coprecipitation

A batch coprecipitation of 1-2-3 superconductor precursor powder was accomplished by adding an aqueous solution of mixed nitrates rapidly to a mixture of sodium or potassium hydroxide and carbonate. The blue hydrogel, an intimate mixture of Y(OH)₃, BaCO₃, and Cu(OH)₂, was filtered and washed by decantation, using water sparged with CO₂ to control the pH. After a second filtration, drying and calcination, the product was a homogeneous powder, with an average particle size of about 1 μm, free of alkali, and with excellent superconducting properties. The process is reproducible, inexpensive, and easy to scale to higher production rates. Important process parameters are the following: the time and temperature of precipitation; alkali ratios; volume, temperature, and pH of the wash water; and time of calcination.     

Synthesis and Characterization of NiO–YSZ Anode Materials: Precipitation, Calcination, and the Effects on Sintering

Nickel oxide yttria-stabilized zirconia (NiO–YSZ) anode materials were synthesized via hydrolysis of the corresponding chloride solutions with NH₃, NH₃+NaOH, and NaOH as precipitation agents. Powder properties such as crystallite size, morphology, and sintering behavior of the final NiO–YSZ materials were also studied. The mechanism of the formation of NiO–YSZ was established for the different co-precipitation techniques by the direct observation of Ni(NH₃) _n ⁺² complexes, Ni(OH)₂ and NiO at different stages of the synthesis process. A direct relationship between the precipitation agent, the order of calcination from dry sample to final product, the final composition, the crystallite sizes and particle sizes of NiO, and the sinterability of the final products was established. A comparison of the powder and individual component properties indicate that the choice of precipitation agent greatly influences the final characteristics. Ni/YSZ materials prepared by NH₃+NaOH precipitation offer higher Ni dispersion and nanocrystallinity of both the Ni and YSZ phases. The conductivity of both prepared materials compares well with mixed-oxide materials of higher Ni content.