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1.
This is the first of three papers in which the kinetics of formation of chemically bonded phosphate ceramics is discussed. A literature survey indicates that the formation of such ceramics is a three-step process. First, oxides dissolve in a phosphoric acid or an acid phosphate solution and metal ions are released into the solution. The aquoions formed from these cations then react with phosphate anions and form a gel of metal hydrophosphates. In the last step, the saturated gel crystallizes into a ceramic. In this paper, we have proposed that the dissolution is the controlling step and developed a general dissolution model of the kinetics of formation of these ceramics. As an example, the model is used to discuss the kinetics of formation of magnesium phosphate ceramics in detail. In the second and third papers, the model has been used to develop processes to form ceramics of alumina and iron oxides. 相似文献
2.
Arun S. Wagh Susan Grover Seung Y. Jeong 《Journal of the American Ceramic Society》2003,86(11):1845-1849
This is the second of three papers on a dissolution model that describes the formation of chemically bonded phosphate ceramics. In this paper, we discuss the kinetics of formation of aluminum phosphate ceramics between 100° and 150°C. Using basic thermodynamic formulations, we calculated the temperatures of maximum solubility of alumina and its hydrated phases and predicted the temperatures of formation of ceramics. Differential thermal and X-ray diffraction analyses on samples made in the laboratory confirm these temperatures. The resulting ceramics of alumina bonded with aluminum phosphate (berlinite) show a high compressive strength of 16 000 psi. We have concluded that rapid evaporation of excess water in the slurry generates porosity in the ceramics, and that better processing methods are needed. A consolidation model is presented that describes the microstructure of the ceramic. It predicts that a very small amount of alumina must be converted to form the bonding phase; hence, the product is mostly alumina with a thin coating of berlinite on the surface of alumina particles. 相似文献
3.
在1023 K温度下用热重法结合XRD和EDX分析研究了70%H2-30%CH4混合气体还原铁矿石制备碳化铁的过程,测定了原矿和还原中间产物的微孔分布. 结果表明,相同反应条件下微观结构不同的铁矿石其还原速率和碳化速率有较大差别,巴西块矿、澳大利亚块矿及南非球团矿在50 min内都可以全部还原,此后生成碳化铁;而南非块矿则需90 min才能基本还原,此后没有碳化铁生成. 疏松多孔的铁矿石较致密铁矿石的还原速率快,生成的还原铁结构疏松,有利于碳化反应的进行. 铁矿石还原后,0.5~3.5 nm微孔孔容积率约提高2倍,分布规律与原矿石相似,3.5~13 nm的微孔孔容积提高显著,这些对还原速率和碳化铁的生成反应有较大影响. 相似文献
4.
Nonradiative Energy Losses and Radiation Trapping in Neodymium-Doped Phosphate Laser Glasses 总被引:4,自引:0,他引:4
Fluorescence radiation trapping and nonradiative energy losses from the Nd3+ 4 F3/2 state are reported for two widely used commercial phosphate laser glasses (LHG-8 and LG-770). The effects of hydroxyl-group, transition-metal (Cu, Fe, V, Co, Ni, Cr, Mn, and Pt), and rare-earth (Dy, Pr, Sm, and Ce) impurities on the 4 F3/2 nonradiative decay rate in these glasses are quantified. Nd concentration quenching effects are reported for doping levels ranging from about 0.5 × 1020 to 8.0 × 1020 ions/cm3 . The results are analyzed using the Förster–Dexter theory for dipolar energy transfer. Quenching rates for transition-metal ions correlate with the magnitude of spectral overlap for Nd emission (donor) and the metal ion absorption (acceptor). The nonradiative decay rates due to hydroxyl groups follow Förster–Dexter theory except at low Nd-doping levels (≲2 × 1020 ions/cm3 ) where the quenching rate becomes independent of the Nd concentration. The data suggest a possible correlation of OH sites with Nd ions in this doping region. The effects of radiation trapping on the fluorescence decay are reported as a function of sample size, shape, and doping level. The results agree well with the theory except for samples with small doping-length products; in these cases, multiple internal reflections from the sample surfaces enhance the trapping effect. 相似文献
5.
Amédée Djemai Georges Calas Jean Pierre Muller 《Journal of the American Ceramic Society》2001,84(7):1627-1631
Six mullite samples, derived from heat-treated natural kaolinites with various iron content, were investigated and compared to synthetic monophasic mullite. They were analyzed by X-ray diffraction, diffuse reflectance spectroscopy, and electron paramagnetic resonance. To quantify mullite coloration, the CIE colorimetric system was used. In contrast to synthetic mullite, samples showed charge transfer bands involving O2− and Fe3+ ions as well as ferric crystal-field transitions due to Fe3+ ions in iron oxide nanoparticles. Absorption edges showed red shifts. The resulting yellowness, saturation of which increased with the content of iron oxide nanoparticles, is direct evidence for the coloring effect of Fe3+ ions. 相似文献
6.
Guorong Chen ¶ Stefania Baccaro Martin Nikl Angelica Cecilia Yunxia Yang Du Eva Mihokova 《Journal of the American Ceramic Society》2004,87(7):1378-1380
The ultraviolet absorption and emission spectra of Ce3+ doped phosphate glasses based on gadolinium and sodium phosphates are studied in the present work. Compared with the cerium-free phosphate glass matrixes, CePO4 doping shifts the ultraviolet absorption edge of glasses into the longer wavelength because of the 4f–5d absorption transition of Ce3+ . The emission spectra correlate such red-shift with glass compositions. Explanations are based on variation in the optical basicity of glasses, which corresponds not only to the alkali content but also to the O/P ratio. The latter is proportional to the number of nonbridging oxygen in the oxyanion unit (PO4 ), and most likely plays an overwhelming role in the overall optical basicity of glasses. 相似文献
7.
Curing Behavior and Structure of Magnesium Phosphate Chemically Bonded Ceramics with Different MgO to KH2PO4 Ratios 下载免费PDF全文
Ai‐juan Wang Xiao‐jie Fan Jun‐ming Li Ding Chen 《International Journal of Applied Ceramic Technology》2015,12(6):1124-1130
Magnesium phosphate chemically bonded ceramics attracted much attention because of its good biocompatibility. In this work, curing behavior and microstructure of magnesium phosphate chemically bonded ceramics with different MgO to KH2PO4 ratios were investigated. The results showed that the setting time decreased and the maximum temperature during the solidifying process increased as the increase of MgO to KH2PO4 ratio. Phase compositions were different for those samples fabricated with different MgO to KH2PO4 ratios. KH2PO4, MgO, and hydrated product existed in the sample with lower MgO to KH2PO4 ratio because of the incomplete reaction, and numerous acicular polycrystals were found. There were only MgO and hydrated product existed with the higher MgO to KH2PO4 ratio. Besides, a gelatinous structure with lots of cracks was also found. Clearly, the properties of magnesium potassium phosphate chemically bonded ceramics could be controlled by changing the MgO to KH2PO4 ratio. It is better to fabricate magnesium phosphate chemically bonded ceramics with optimal structure and properties suitable for biomedical applications in range of MgO to KH2PO4 ratio from 3:1 to 4:1 in this preparating conditions. 相似文献
8.
One-Step Synthesis and Consolidation of Nanophase Iron Aluminide 总被引:1,自引:0,他引:1
Frédéric Bernard Frédéric Charlot Eric Gaffet Zuhair A. Munir 《Journal of the American Ceramic Society》2001,84(5):910-914
The simultaneous synthesis and densification of nanophase iron aluminide is investigated. Elemental nanophase reactants produced by mechanical activation were reacted by field activation with the simultaneous application of uniaxial pressure. The process was demonstrated in this work by the synthesis of dense nanometric FeAl. Iron and aluminum powders were co-milled in a specially designed planetary mill to obtain nanometric reactants and to avoid formation of any product phases. These powders were then subjected to high AC currents (1250–1500 A) and pressures in the range of 70–106 MPa. Under these conditions, a reaction was initiated and completed within a short period of time (2.5–3.5 min). XRD and energy dispersive analyses showed the product to be Fe0.515 Al0.485 . The relative density of the product ranged from 98.7% to 99.7%. The crystallite size of the FeAl intermetallic was determined by XRD line-broadening analysis using the Williamson–Hall method. The size ranged from 32 to 89 nm and showed a dependence on lattice direction. 相似文献
9.
分析了焦炉煤气生产直接还原铁没有发展起来的原因,介绍了焦炉煤气生产直接还原铁的4种工艺流程及国内外用焦炉煤气生产直接还原铁试验与研究进展情况等,认为山西省具备了用焦炉煤气大规模生产直接还原铁的基本条件,应积极争取条件,大规模发展焦炉煤气生产直接还原铁。 相似文献
10.
Nibambin Soro Laurent Aldon Josette Olivier-Fourcade Jean Claude Jumas Jean Paul Laval Philippe Blanchart 《Journal of the American Ceramic Society》2003,86(1):129-134
This paper examines the role of iron in mullite nucleation and growth from kaolins. We chose two typical raw kaolins containing a reduced impurity level and characterized by very different degrees of crystallinity of the kaolinite phase. Both the structural iron in kaolinite and also some iron deposited onto phyllosilicate layers by a chemical route were considered. After firing in the 900–1100°C temperature range, the Fe environment was determined by Mössbauer spectroscopy. From X-ray spectra of samples fired at 1250°C, mullite stoichiometries were obtained by Rietveld refinements. It was shown that iron contributes to the structural reorganization stage of the material, when mullite is nucleated. Fe atoms are essentially in octahedral sites, which favors an increase of the c parameter of the orthorhombic cell. The iron quantity attains a saturation level for an Fe-to-Al ratio between 0.3 and 0.4, depending on the raw kaolinite crystallinity. Besides mullite, the excess iron associates with titanium to form a pseudobrookite phase and hematite. 相似文献
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13.
Dhirendra Mohan Ram Prasad Srikumar Banerjee 《Journal of the American Ceramic Society》2001,84(9):2126-2128
Lead magnesium niobate (PMN) and lead iron niobate (PFN) were prepared by the semiwet hydroxide route, and their dielectric properties were measured in temperature ranges around their peak dielectric constants. The dielectric constant of PFN was much larger as compared with that of PMN sintered at the same temperature. The dielectric properties of PMN and PFN are compared and explained on the basis of their structure development. 相似文献
14.
Iron dihydride complexes often play important roles in catalytic reactions that employ reducing agents such as H2, boranes, and silanes. Development of more efficient and selective iron-based catalysts for these processes requires effective synthetic strategies and a deeper understanding of the reactivity of the dihydride complexes. The purpose of this review is to provide the readers with an overview of different ligand systems that have been used to support iron dihydride complexes. 相似文献
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纳米零价铁(nZVI)作为一种治理重金属、核素污染物的环保材料而被广泛关注,而黏土矿物作为常见的重金属吸附材料虽成本低廉、来源广泛,但去除性能又普遍有限。通过液相还原法制备nZVI/高岭石复合材料来强化高岭石去除水中Cr(VI)的性能,利用X射线衍射仪(XRD)、扫描电子显微镜(SEM)和透射电子显微镜(TEM)技术对其结构进行表征,考察了Cr(VI)初始浓度、nZVI/高岭石投加量、温度等条件对Cr(VI)去除率的影响,并探讨了其对水中Cr(VI)的去除能力及机制。结果表明,nZVI/高岭石中的nZVI结晶度低,粒径在50~100 nm,nZVI/高岭石在60 min时对Cr(VI)的去除率达91.7%,分别比nZVI和高岭石提高了2.7倍和18.5倍。nZVI/高岭石对Cr(VI)的去除动力学符合准二级动力学模型,表观反应活化能为27.97 kJ/mol,去除是吸附、还原和共沉淀共同作用的结果。通过nZVI强化可提升高岭土在水处理和环境修复中的应用前景。 相似文献
17.
《分离科学与技术》2012,47(5):760-766
The carbothermic reduction of high alumina iron ore in the absence/presence of sodium carbonate (Na2CO3) was carried out for alumina-iron separation by wet magnetic separation. Sodium carbonate is found to be capable of improving the separation of alumina and iron, as well as increasing the particle size of metallic iron significantly. When the high alumina ore briquettes were reduced at 1050°C for 80 min, the average particle size of metallic iron was approximately 100 μm in the presence of sodium carbonate, which is bigger than the size of 50 μm in the absence of sodium carbonate. Compared with the absence of sodium carbonate, the Al2O3 content of iron concentrate decreased from 4.33% to 1.29%, while the Al2O3 removal rate increased from 43.70% to 83.37% with the addition of 9% sodium carbonate. Experimental evidence showed that Na2CO3 reacted with Al2O3 and SiO2 to form sodium silicate, aluminum silicate, and sodium aluminosilicate, and decreased the content of Fe in the slags, which improved the separation between the alumina and iron during the magnetic separation. 相似文献
18.
FeO(OH)/C催化水合肼还原芳香族硝基化合物制备芳胺 总被引:6,自引:3,他引:3
采用浸渍法制备了FeO(OH)/C催化剂。用X-射线粉末衍射法对FeO(OH)/C进行了表征,结果表明FeO(OH)在活性炭中高度分散。催化剂中FeO(OH)含量的增加能提高其催化活性,但过多的FeO(OH)无法全部负载,催化剂中FeO(OH)的质量分数以10%为宜。催化剂具有较好重复使用性,重复使用8次活性未明显下降。在5mL乙醇中以0.02g10%FeO(OH)/C为催化剂,1.0mmol芳香族硝基化合物和2.0mmol水合肼于75℃反应一定的时间,芳胺的收率为96.3%~99.5%。 相似文献
19.
Philip P. Rodenbough Chengjunyi Zheng Yuxuan Liu Chenyuan Hui Yuxuan Xia Ziying Ran Yanjun Hu Siu‐Wai Chan 《Journal of the American Ceramic Society》2017,100(1):384-392
Uniform sets of mono‐crystalline nanoparticles ranging from 6 nm to over 100 nm were prepared for the MgO, Co3O4, and Fe3O4 oxide systems. The nanoparticles were characterized by transmission electron microscopy (TEM) and x‐ray diffraction (XRD). A careful analysis shows increased lattice parameter for smaller nanoparticles of each oxide system: 0.47% expansion from bulk for 7 nm MgO crystallites, 0.15% expansion from bulk for 9 nm Co3O4 crystallites, and 0.13% expansion from bulk for 6 nm Fe3O4 crystallites. The compressive surface stresses and expansion energies against hydrostatic pressure for each oxide system were calculated, respectively, to be 4.13 N/m and 1.8 meV/formula unit for MgO, 3.09 N/m and 0.87 meV/formula unit for Co3O4, and 1.26 N/m and 0.67 meV/formula unit for Fe3O4. The fundamental understanding of oxide nanoparticle mechanics as presented here will facilitate integration of these materials into technological applications in a rationally designed manner. 相似文献
20.
采用直接还原?磁选工艺从湖南某地高磷铁矿含碳团块中提铁,研究了还原温度、还原时间、团块碱度、还原剂焦粉用量、添加剂Na2SO4用量和废塑料替代焦粉量等因素对磁选精矿指标的影响. 结果表明,在还原温度1150℃、还原时间40 min、团块碱度0.8、碳氧摩尔比0.9的条件下,添加4% Na2SO4同时添加废塑料替代25%焦粉,可得到金属化率为88.77%的焙烧矿,磁选后可得到铁品位91.99%、金属化率92.26%及P含量0.20%的金属铁粉,铁回收率达86.74%. 相似文献