Phase stability and mechanical properties of niobium dihydride

First-principles calculation shows that the NbH_x phases (1 ≤ x ≤ 2) with face-centered cubic (FCC), orthorhombic (FCO), and tetragonal (FCT) modifications are all energetically favorable with negative heats of formation, while FCC NbH₁ and NbH_1.25 could not be formed due to their mechanical unstableness. It is also revealed that FCT and FCO could coexist in NbH₁ and NbH_1.25, FCC and FCT coexist in NbH_1.5 and NbH_1.75, while only FCC in NbH₂. Calculations also indicate that the magnitude of the elastic moduli of NbH_x phases at each H concentration is as follows: E > B > G, and the G, E, and G/B values of NbH_x phases reach a minimum when x = 1.5. Moreover, electronic structures are discussed to provide a deep understanding of various properties, and the derived results are in good agreement with similar experimental evidence in the literature.     

Microalloyed steels

This review considers the compositions, the main process routes, microstructure and structural properties of microalloyed steels. The background and brief history are followed by sections dealing with aspects of precipitation, which control grain size and dispersion strengthening in ferrite–pearlite steels, the approaches to modelling thermomechanical processing and the influence of multiple additions of transition metals on properties. High strength acicular ferrite/bainite steels used for linepipe are included and lead to super bainite steels. Around 12% of the world strip production is processed by the thin slab direct charging route, which is considered in some detail. The weldability of microalloyed steels now embraces joining using friction stir welding, which is discussed. Over the years, many approaches have been developed to predict the structural properties of these steels. They comprise several quantifiable microstructural features including possible atom clusters, relatively recently identified through atom probe tomography. A comprehensive collection of references is provided.     

Characterisation of the surface films formed on Nb during electrodissolution in aqueous alkali

Anodic polarisation of Nb in warm and concentrated aqueous alkali results in sustained electrodissolution. The process is investigated in NaOH by voltammetric, steady-state and impedance techniques. The j-E steady-state curve is characterised by a current peak and a subsequent plateau extending up to the explored positive limit (8 V_SCE). Upon addition of tartrate to the electrolyte the dissolution current increases markedly. In some NaOH+Na tartrate electrolytes a dissolution-precipitation mechanism sets in, with formation of a surface layer which causes a gradual current decrease; this layer is identified by XRD and elemental analysis as a mixed Na and Nb oxide hydrate. Larger currents may be sustained in KOH+K tartrate solutions without any precipitation process. Impedance diagrams recorded in the plateau region are analysed on the basis of the equivalent circuit resulting from the surface charge model; the oxide formation ratio and the main model parameters are estimated and their temperature dependence is discussed.     

CeO2颗粒对铌合金表面镀渗复合涂层抗氧化性能的影响

为了提高铌合金的高温抗氧化性,采用化学镀结合包埋渗技术在铌合金表面制备了含有CeO_2颗粒的复合涂层,研究了复合涂层的微观结构和高温抗氧化性能。结果表明,不含CeO_2的Al/Ni涂层以NiAl相为主,Al/Ni-CeO_2涂层则含NiAl、NiAl_3、Al3Nb和CeO_2等相。经1000℃氧化测试,Al/Ni复合涂层氧化50 h后增重为8.0 mg/cm~2,表面主要生成Al_2O_3、AlNbO_4相;Al/Ni-CeO_2复合涂层50 h后氧化增重为4.0 mg/cm~2,表面以Al_2O_3、CeO_2、NiAl、NiAl_3、Al3Nb、AlNbO_4相为主。高温氧化后,2种涂层样品表面均生成连续致密的Al_2O_3膜,涂层与基体结合良好;含CeO_2的涂层,其稀土氧化物主要在Ni膜拖拽力作用下富集于涂层互扩散区。稀土氧化物颗粒的添加细化涂层组织,降低涂层中Al元素的消耗,填补涂层中的孔洞,增强了氧化膜与涂层的粘附力,有效提高了涂层的抗氧化性。     

铌对高性能球墨铸铁微观组织的影响

利用图像分析软件定量分析了铌对球墨铸铁石墨球形态和铸态组织的影响。研究结果表明,添加铌能够使石墨化程度和球化率降低,圆度和石墨球大小增加,石墨球数量显著降低,但少量铌对球墨铸铁的石墨化、球化率、圆度、石墨球大小等影响不大。少量铌能显著增加珠光体数量。     

超导材料用铌表面清洗工艺

采用乳化剂溶液在超声波作用下对金属铌表面进行去油去灰清洗,并用硝酸和氢氟酸混合酸液进行酸洗,通过扫描电镜(SEM)、光电子能谱(XPS)及腐蚀深度对清洗表面进行分析。结果表明:采用乳化剂溶液在超声波作用下清洗对去油去灰有很好效果,可以获得均一的酸洗初始腐蚀界面,其中乳化剂温度为40～50℃,超声波频率为20～25kHz,采用混合酸液浓度为35%分析纯硝酸和10%分析纯氢氟酸的清洗工艺能达到较好的清洗效果和均一的表面状态。合理控制硝酸和氢氟酸浓度对于保证清洗质量和防止材料吸氢脆化很关键,通过腐蚀可以实现表面以Nb2O5状态存在转变为以单质铌存在,实现氧化物的溶解,改善表面结构;铌清洗后在环境放置稳定性强,不容易吸水、氧化和变质;清洗液特征可以通过游离氟离子、铌离子、酸度进行表征。     

包埋渗结合激光熔覆制备复合涂层的高温抗氧化性

为了提高铌合金的抗氧化性能,采用铝硅共渗的方法在铌合金表面制备Al-Si涂层,结合激光熔覆技术于渗层上熔覆MOSi2涂层。探讨了Al-Si渗层的生长机制,研究了铌合金表面Al-Si渗层及MoSi2/Al-Si涂层的抗高温氧化性能。结果表明:Al-Si渗层的形成过程是源于Al、Si元素的先后沉积,优先形成了Al3Nb相。渗层厚度x与保温时间f遵循关系式:x=At1/2+7.4(1000℃:A=11.6,1050℃:A=16.2)。激光熔覆制备的MoSi2/Al-Si涂层均匀连续致密,与基体结合紧密,无裂纹孔洞等缺陷。主要相结构为MoSi2、Al3Nb、NbSi2、Nb5Si3和Mo(Si,Al)2。经1200℃氧化后,Al-Si渗层及MoSi2/Al-Si涂层都形成大量的SiO2保护膜,阻止了氧原子的进一步扩散。与Al-Si渗层相比,MoSi2层表面形成的连续致密混合氧化物有效避免了Al-Si渗层的快速消耗,MoSi2/Al-Si涂层的高温氧化优于Al-Si渗层。     

Niobium-based catalysts prepared by reactive radio-frequency magnetron sputtering and arc plasma methods as non-noble metal cathode catalysts for polymer electrolyte fuel cells

Two vacuum methods, reactive radio-frequency (RF) magnetron sputtering and arc plasma deposition, were used to prepare niobium-based catalysts for an oxygen reduction reaction (ORR) as non-noble metal cathodes for polymer electrode fuel cells (PEFCs). Thin films with various N and O contents, denoted as NbO_x and Nb-O-N, were prepared on glassy carbon plates by RF magnetron sputtering with controlled partial pressures of oxygen and nitrogen. Electrochemical measurements indicated that the introduction of the nitrogen species into the thin film resulted in improved ORR activity compared to the oxide-only film. Using an arc plasma method, niobium was deposited on highly oriented pyrolytic graphite (HOPG) substrates, and the sub-nanoscale surface morphology of the deposited particles was investigated using scanning tunneling microscopy (STM). To prepare practical cathode catalysts, niobium was deposited on carbon black (CB) powders by arc plasma method. STM and transmission electron microscopy observations of samples on HOPG and CB indicated that the prepared catalysts were highly dispersed at the atomic level. The onset potential of oxygen reduction on Nb-O-N/CB was 0.86 V vs. a reversible hydrogen electrode, and the apparent current density was drastically improved by the introduction of nitrogen.     

The fabrication of niobium powder by sodiothermic reduction process

Niobium powder was fabricated by sodiothermic reduction process using K₂NbF₇ as a raw materials, KCl and KF as the diluents and sodium as a reducing agent. The apparatus for the experiment was designed and built specifically for the present study. Varying properties of niobium powder depending on reaction temperature and excess of reducing agent were analyzed. The niobium particle size increased significantly as reduction temperature increased from 993 to 1093 K. The particle size was fairly uniform at given reaction temperature, varying from 0.2 μm to 50 nm depending on the reaction temperature. The yield of niobium powder increased from 58% to 83% with a increase in reaction temperature. The average particle size of niobium powder was improved from 70 nm to 0.2 μm with increase in the amount of sodium excess. In addition, the yield of niobium powder was 82% in the 5% sodium excess.     

Synthesis of Nb2O5 and Ta2O5 nanopowders by means of an endo-templating method

Finding efficient templates for the nanostructuring of materials is a key point. Here, the niobium (V) - and tantalum (V) oxide ceramics nanopowders have been synthesized by a hard-templating approach by using the tricalcium phosphate biomaterial (Ca₃(PO₄)₂.xH₂O) as template agent. The oxide ceramics were investigated by X-ray Diffraction (XRD), Fourier Transform Infrared (FTIR) spectroscopy, nitrogen physisorption and Scanning Electron Microscopy (SEM). It was observed that the surface properties (specific surface areas, pore volumes) of the Nb₂O₅ and Ta₂O₅ powders were strongly dependent of the amount of the Ca₃(PO₄)₂.xH₂O template previously used in the preparation of the [oxide ceramics/template] composites. For instance, with a Ca₃(PO₄)₂.xH₂O/ceramic salt weight ratio ranging from 0 to 1, the specific surface areas of Nb₂O₅ and Ta₂O₅ were gradually enhanced of 48–166 m²/g and 5–84 m²/g, respectively. The pore volumes were increased as well. The use of the eco-friendly tricalcium phosphate material (Ca₃(PO₄)₂.xH₂O) as template in the hard-templating approach may be suitable and efficient in the aim of synthesizing materials with enhanced surface properties.