KINETICS OF CALCIUM OXIDE CHLORINATION AND PORE CLOSURE MODELS

The chlorination of cylindrical lime pellets was studied using thermogravimetric analysis by varying the chlorine concentration, pellet size and temperature. It was determined that the reaction is first order with respect to Cl₂. The intrinsic activation energy, E₁, is estimated to be 8.09 kcal/gmol in the temperature range of 333°C to 472°C.

A two-dimensional expanding grain model that accounts for bulk flow was developed and successfully applied to the lime chlorination system. The effect of varying the chlorine concentration, pellet size, and temperature can be explained in terms of the model. In addition, the modeling results seem to imply that even for a system with solid product expansion, the surface porosity remains finite after the maximum conversion is obtained.

     

KINETICS OF CALCIUM OXIDE CHLORINATION AND PORE CLOSURE MODELS

The chlorination of cylindrical lime pellets was studied using thermogravimetric analysis by varying the chlorine concentration, pellet size and temperature. It was determined that the reaction is first order with respect to Cl₂. The intrinsic activation energy, E₁, is estimated to be 8.09 kcal/gmol in the temperature range of 333°C to 472°C.

A two-dimensional expanding grain model that accounts for bulk flow was developed and successfully applied to the lime chlorination system. The effect of varying the chlorine concentration, pellet size, and temperature can be explained in terms of the model. In addition, the modeling results seem to imply that even for a system with solid product expansion, the surface porosity remains finite after the maximum conversion is obtained.     

Ta2O5与Nb2O5对TiO2基压敏陶瓷电性能的影响

研究了Ta2O5与Nb2O5对TiO2基压敏陶瓷电性能的影响.按照配方TiO2+0.003(按摩尔计)(SrO+Bi2O3+SiO2)+x Ta2O5+yNb2O5配制3种组分的圆片试样,各组分摩尔量分别为x=0.000 75,y=0;x=0.000 375,y=0.000 375及x=0,y=0.000 75.采用电流-电压、电容测量、阻抗分析和势垒高度测量等实验手段,分析了Ta2O5和Nb2O5的作用机理.结果发现,x=0.000 75的样品显示出最低的压敏电压梯度(E10 mA=7.9 V/mm)和最大的表观介电常数(εra=5.88×104),y=0.000 75的样品显示出最高的压敏电压梯度(E10mA=48.9 V/mm)和最小的表观介电常数(εra=1.39×104).这表明,掺Ta2O5可有效地降低压敏电压,提高介电常数.     

合成三聚氯氰的氯化和聚合反应宏观动力学研究

利用非等温固定床反应器，研究了三聚氯氰合成的氯化和聚合反应宏观动力学，实验表明氢氰酸与氯气的氯化反应为气相均相反应，而氯氰的聚合反应为吸附在活性炭表面上的氯氰表面反应为控制步骤的气固催化反应。实验测得氯化反应的宏观反应动力学方程为对氢氰酸的一级反应，即（－ｒＡ）１＝２．４５×１０７ｅｘｐ（－２．９０×１０４／ＲＴ）ｃＡ测得聚合反应的宏观动力学方程为（－ｒＡ）２＝ｋ［ｐ３Ａ－ｐＲ／Ｋｐ］／［１＋ＫＡｐＡ＋ＫＲｐＲ］３ｋ＝４．０×１０７ｅｘｐ（－４．３５×１０４／ＲＴ）Ｋｐ＝６．７６×１０－２３ｅｘｐ（３．５２×１０５／ＲＴ）ＫＡ＝４．０×１０３ｅｘｐ（－３．３９×１０４／ＲＴ）ＫＲ＝１．０５×１０３ｅｘｐ（－５．０２×１０４／ＲＴ     

Nb2O5掺杂对BaTiO3陶瓷介电常数的影响

BaTiO3陶瓷以其优异的铁电、压电和绝缘性能广泛应用于体积小、容量大的电子器件材料,但其在一定的工作温度区间介电常数却呈现不稳定变化。对其常用的改性方法进行了分类和阐述,同时对Nb2O5掺杂改性对BaTiO3陶瓷介电性能的影响作了简单介绍,最后讨论了Nb2O5在多层陶瓷电容器制备领域的应用和重要性。     

Raman investigations of NH3 adsorption on TiO2, Nb2O5, and Nb2O5/TiO2

A study has been carried out of the interactions of NH₃ with TiO₂, Nb₂O₅, and Nb₂O₅/TiO₂. Raman spectroscopy was used to characterize the structure of the adsorbed NH₃ and perturbations of species present at the surface of the adsorbent. On each oxide, NH₃ adsorbs predominantly at Lewis acid sites. Hydrogen-bonding occurs between the adsorbed NH₃ and OH groups present on the surface of TiO₂. A small concentration of NH ₄ ⁺ is observed, consistent with the relatively low concentration of Brønsted acid sites compared to Lewis acid sites on each of the samples investigated. Exposure of Nb₂O₅/TiO₂ to NH₃ at temperatures up to 500°C does not result in partial reduction of the supported niobia.     

低温烧结CuO-V_2O_5-Bi_2O_3掺杂Zn_3Nb_2O_8陶瓷的微波介电性能

研究了CuO–V2O5–Bi2O3作为烧结助剂对Zn3Nb2O8陶瓷的烧结特性、微观结构、相结构及微波介电性能的影响。CuO–V2O5–Bi2O3复合掺杂可以将Zn3Nb2O8陶瓷的烧结温度从1150℃降到900℃。在900℃烧结4h的Zn3Nb2O8–0.25%(质量分数,下同)CuO–1.5%V2O5–1.5%Bi2O3陶瓷的密度达到了理论密度的98.1%,相对介电常数为18.8,品质因数与谐振频率之积为39442GHz。该体系的介电性能和陶瓷的致密度与烧结助剂的含量及烧结温度密切相关,陶瓷的致密度和相对介电常数随CuO–V2O5–Bi2O3烧结助剂含量的增加而增加,同样陶瓷的致密度和相对介电常数也随烧结温度的升高而提高。     

Nb2O5薄膜的制备及性能研究

以NbCl5作为前驱物,用微波催化的溶胶-凝胶法制备了Nb2O5薄膜,并在不同温度下进行热处理。经透过率测试及用X射线衍射分析仪和原子力显微镜对薄膜的相结构和表面形貌进行测试分析,测试结果表明：制得的薄膜主晶相为假六方Nb2O5晶相（TT-Nb2O5晶相）。薄膜厚度具有良好的透光率、良好的空架结构。     

Nb2O5掺杂Co2Z 六角铁氧体的烧结行为和电磁特性

采用固相反应法制备Nb2O5掺杂Co2Z六角铁氧体[Ba3(Co0.4Zn0.6)2Fe24O41,Z-型].通过显微结构观察和电磁性能的测试分析,研究了不同Nb2O5掺量对Z-型铁氧体烧结特性和电磁特性的影响.结果表明:由于Nb5 固溶于铁氧体中,增加了空穴浓度,使体扩散加剧,进而促进材料的烧结.适当添加Nb2O5可提高样品的致密化程度,改善Z-型六角铁氧体的显微结构和电磁特性.在Nb2O5掺量为Co2Z一次预烧料质量(下同)的0～0.8%内,随Nb2O5掺量增加,在Z-型六角结构相生长的同时,伴生M-相六角结构.六角结构致密度增加,磁导率增大,铁磁共振频率点移向低频.掺量为0.8%Nb2O5的样品在1 260℃烧结6 h,得到材料的相对密度为95%左右,起始磁导率为33,高μQ积,低损耗角tgδ矫顽力Hcb.     

Nb2O5薄膜的溶胶—凝胶法制备

以金属醇盐为前驱体获得涂膜液，采用浸渍－提拉法制备Ｎｂ２Ｏ５薄膜，研究了不同催化环境对涂膜液溶胶的影响，不同涂膜次数样品的反射透过情况。并由此推算了薄膜的厚度及折射率。ＸＲＤ结果表明：５６０℃热处理１ｈ后得到正交晶相的Ｎ２Ｏ５薄膜；Ｎｂ２Ｏ５薄着色为淡黄色。     

Phase evolution in plasma sprayed Nb2O5 coatings

Nb₂O₅ polymorphism and defect chemistry depend on the temperature, pressure, atmosphere composition and the initial crystallography. Plasma spray of Nb₂O₅ is a pathway to form coatings with in-situ metastable and nonstoichiometric phases, however so far unexplored. This study aimed to understand the phase evolution of plasma sprayed Nb₂O₅ coatings, and its effect on their morphology and properties. Phase evolution from H-Nb₂O₅ in the feedstock, to T-Nb₂O₅, TT-Nb₂O₅, N-Nb₂O₅, H-Nb₂O₅, Nb₁₂O₂₉ and NbO₂ in the coatings depends on the plasma Ar/H₂ ratio and its related enthalpy. The microstructure shows a layered distribution of nonstoichiometric phases at the splat boundaries and splat cores composed of T-Nb₂O₅ or TT-Nb₂O₅. The presence and distribution of these phases are related to the thermomechanical and electrical properties. The mechanisms driving the formation of these coatings are based on the Nb₂O₅ incongruent vaporization which promote retention of nonstoichiometric phases and the rapid solidification of metastable phases.     

Effect of the electronic properties of Mo sulfide phase on the hydrotreating activity of catalysts supported on Al2O3, Nb2O5 and Nb2O5/Al2O3

Activity in thiophene hydrodesulfurization (HDS) and in the three routes of 2,6-dimethylaniline (DMA) decomposition was examined on Mo sulfide catalysts supported on Al₂O₃, Nb₂O₅ and Nb₂O₅–Al₂O₃. Catalysts activity is enhanced when Mo phase is deposited on niobium-containing support. For HDS and for the hydrogenation route of DMA decomposition, the niobium-containing support strongly contributes to the catalyst activity whereas the activity of the Mo phase per Mo atom decreases with the increase of niobium amount in the support. By contrast, as for the DMA route, which leads to xylene formation (XYL), the activity of the Mo sulfide phase per Mo atom is strongly enhanced. The electronic properties of the MoS₂ phase were studied by means of IR spectroscopy of CO adsorption. Comparison of ν(CO/Mo) wavenumbers reveals an upward shift when Mo sulfide phase is deposited on Nb-containing support. The modification of the electronic properties of the sulfide phase is related to an interaction Mo–Nb either through the formation of a mixed Mo–Nb sulfide phase, or through the interaction MoS₂ slabs – support whose strength depends on the support acidity. Hence, the beneficial effect for xylene formation route is attributed to a decrease of the electron density of the Mo sulfide phase that should strengthen the DMA adsorption on the sulfide phase.     

Photocatalytic Nb2O5-doped TiO2 nanoparticles for glazed ceramic tiles

TiO₂ nanoparticles are currently used as coating for self-cleaning building products. In order to achieve high self-cleaning efficiency for outdoor applications, it is important that titania is present as anatase phase. Moreover, it is desirable that the particle sizes are in nano-range, so that a large enough surface area is available for enhanced catalytic performance. In this work, TiO₂ nanoparticles doped with 0–5 mol% Nb₂O₅ were synthesized by co-precipitation. Nb₂O₅ postponed the anatase to rutile transformation of TiO₂ by about 200 °C, such that after calcination at 700 °C, no rutile was detected for 5 mol% Nb₂O₅-doped TiO₂, while undoped TiO₂ presented 90 wt% of the rutile phase. A systematic decreasing on crystallite size and increasing on specific surface area of TiO₂ were observed with higher concentration of Nb₂O₅ dopant. Photocatalytic activity of anatase polymorph was measured by the decomposition rate of methylene blue under ultraviolet and daylight illumination and compared to commercial standard catalyst (P25). The results showed enhanced catalysis under UV and visible light for Nb₂O₅-doped TiO₂ as compared to pure TiO₂. In addition, 5 mol% Nb₂O₅-doped TiO₂ presented higher photocatalytic activity than P25 under visible light. The enhanced performance was attributed to surface chemistry change associated with a slight shift in the band gap.     

Fe2O3-AC吸附剂对水中对硝基苯酚的吸附动力学研究

对Fe改性活性炭制得Fe2O3-AC吸附剂,并用于对硝基苯酚废水处理;通过静态试验方法研究所制备吸附剂对水溶液中对硝基苯酚的吸附等温和吸附动力学过程。结果表明,Fe2O3-AC吸附剂对废水中对硝基苯酚的吸附过程符合Langmuir等温方程,对硝基苯酚最大吸附量可达286.4 mg/g,内扩散不是吸附过程唯一的控制步骤,准2级动力学模型能更好的反映Fe2O3-AC吸附剂对对硝基苯酚的吸附机理。     

Synthesis and characterization of Nb2O5@C core-shell nanorods and Nb2O5 nanorods by reacting Nb(OEt)5 via RAPET (reaction under autogenic pressure at elevated temperatures) technique

The reaction of pentaethoxy niobate, Nb(OEt)₅, at elevated temperature (800 °C) under autogenic pressure provides a chemical route to niobium oxide nanorods coated with amorphous carbon. This synthetic approach yielded nanocrystalline particles of Nb₂O₅@C. As prepared Nb₂O₅@C core-shell nanorods is annealed under air at 500 °C for 3 h (removing the carbon coating) results in neat Nb₂O₅ nanorods. According to the TEM measurements, the Nb₂O₅ crystals exhibit particle sizes between 25 nm and 100 nm, and the Nb₂O₅ crystals display rod-like shapes without any indication of an amorphous character. The optical band gap of the Nb₂O₅ nanorods was determined by diffuse reflectance spectroscopy (DRS) and was found to be 3.8 eV.     

Sb2O5和Bi2O3添加剂对Ag(Nb1-xTax)O3陶瓷结构、形貌和性能的影响

研究了Sb2O5和Bi2O3添加剂对Ag(Nb1-xTax)O3陶瓷材料的结构、形貌和介电性能的影响.结果表明:当Sb2O5和Bi2O3的质量分数较少(＜2.5%)时,不会影响Ag(Nb1-xTax)O3的钙钛矿结构,但能促进其烧结,使所得陶瓷样品更均匀致密.添加适量的Sb2O5和Bi2O3均可使Ag(Nb1-xTax)O3的介电常数(ε)增大,介电损耗(tgδ)减小,介电性能的温度稳定性得到改善.Bi2O3较Sb2O5对降低Ag(Nb1-xTax)O3陶瓷损耗及改善温度稳定性的效果更佳.     

Selective catalytic reduction of NO with NH3 over Nb2O5-promoted V2O5/TiO2 catalysts

In contrast to previous claims, the addition of niobia to catalysts containing vanadia supported on titania resulted in much enhanced activity for low-temperature SCR of NO with NH₃ only at low vanadia loadings. Niobia promoted catalysts could also be demonstrated to show higher selectivities to N2, especially at high temperatures and low vanadia loading. This enhancement of the activity cannot be explained only on the basis of the observation that niobia stabilized the surface area of the catalyst: calculations of the activation energy suggest that a different mechanism of the reaction may be at work at low vanadia loadings.     

Preparation and morphology of anisometric KSr2Nb5O15 particles

Anisometric KSr₂Nb₅O₁₅ (KSN) particles were synthesized by conventional mixed-oxide (CMO) method and molten salt synthesis (MSS) method, respectively. In MSS method, preparation of KSN was carried out in both SrCO₃–Nb₂O₅–K₂CO₃–KCl system (system I) and SrNb₂O₆–Nb₂O₅–KCl system (system II). The results revealed that the single-phase KSN obtained by CMO method was clumped and without obvious acicular morphology, making it difficult to use in texturing processes. In system I, though acicular KSN particles were synthesized by controlling the amount of KCl salt, they were easily contaminated by the presence of blade-like Sr₂Nb₂O₇. In contrast, pure KSN particles with high aspect ratio and uniform size were successfully synthesized by controlling the ratio of SrNb₂O₆/Nb₂O₅ in the new system (system II). The obtained KSN particles which are 5–30 μm in length and 2–4 μm in diameter are ideal templates for fabricating textured ceramics with tungsten bronze structure. Growth mechanisms in MSS method were also proposed in this work.