Kinetics of thermal decomposition of niobium hydride

High-purity niobium powders can be obtained from the well-known hydride-dehydride (HDH) process. The aim of this work was the investigation of the structural phase transition of the niobium hydride to niobium metal as function of temperature, heating rate and time. The niobium powder used in this work was obtained by high-temperature hydriding of niobium machining chips followed by conventional ball milling and sieving. X-ray diffraction measurements were carried out in vacuum using a high-temperature chamber coupled to an X-ray diffractometer. During the dehydriding process, it is possible to follow the phase transition from niobium hydride to niobium metal starting at about 380 °C for a heating rate of 20 °C/min. The heating rate was found to be an important parameter, since complete dehydriding was obtained at 490 °C for a heating rate of 20 °C/min. The higher dehydriding rate was found at 500 °C. Results contribute to a better understanding of the kinetics of thermal decomposition of niobium hydride to niobium metal.     

Production of niobium powder by magnesiothermic reduction of niobium oxides in a cyclone reactor

In the last years, a variety of processes respectively process steps have been investigated for the production of niobium powder. This is due to the fact that niobium capacitors could be a viable alternative to tantalum capacitors from a performance, availability, and price point of view. The reduction of niobium pentoxide by magnesium results in fine powders with high specific surface area but has the disadvantages of a very exothermic nature and the formation of magnesium niobate. It is shown in this work that the application of a continuously operating cyclone reactor and the use of niobium(IV) oxide as raw material solve these problems. A good control of the highly exothermic reaction within the cyclone reactor was achieved in the cyclone reactor by the ratio between gas flow rate and powder flow rate as well as by a proper preheating of the gas.     

Friction welding of niobium and tungsten pseudoalloy joints

The friction welding of niobium and tungsten pseudoalloy D18 was investigated. The friction welding process was found to proceed differently than in the case of niobium-tungsten joints. Stable Nb-Fe-Ni-W intermetallic forms would occur in the joint zone. The use of copper as the intermediate layer prevented the intermetallic phases from forming and joints with a strength of 317 MPa were produced.     

Synthesis of niobium aluminides by electro-deoxidation of oxides

Niobium aluminides were produced by the direct reduction of mixtures of niobium and aluminium oxides. Pellets of the mixed oxides were made the cathode in molten CaCl₂ or a CaCl₂–XCl melt, where X was either Na or Li. The oxygen in the pellets ionised, dissolved in the melt, and was discharged at the anode. The cathode reduced to a mixture of Nb₃Al and Nb₂Al which corresponded to the ratio of Nb to Al in the original oxide pellets.     

Investigation of niobium nitride and oxy-nitride films grown by MOCVD

Niobium nitride (NbN) and niobium oxy-nitride (NbO_xN_y) thin films were grown by metalorganic chemical vapor deposition (MOCVD) on Si(100) and Si(111) substrates using [Nb(N^tBu)(NMe₂){C(NⁱPr)₂(NMe₂)}₂] [NB; ^tBu = (CH₃)₃C; Me = CH₃; ⁱPr = (CH₃)₂CH] as a simultaneous Nb and N precursor. While NbN films were synthesized under a pure N₂ atmosphere, NbO_xN_y films were synthesized under N₂-O₂ flow (N₂:O₂ = 1-5) in the temperature range 400-600 °C, as well as by NbN deposition followed by ex-situ thermal treatments under flowing O₂ at 400-600 °C. The samples were subjected to a multi-technique characterization in order to elucidate the interplay between their structure, morphology and composition and the adopted processing parameters. Particular attention was devoted to the presence of Nb-N and Nb-O-N phases and their distribution in the films, as well as to surface oxidation phenomena. For the first time, niobium oxy-nitride coatings were obtained by CVD starting from the above precursor compound, with growth rates up to 270 Å/min on Si(111) at 600 °C. The films were characterized by a columnar-like/globular morphology when supported on Si(100)/Si(111) and revealed a higher crystallinity on the latter substrate. Surface and in-depth compositional analyses evidenced a limited carbon contamination and the Co-existence of niobium nitride, NbON and Nb₂O₅. In particular, the presence of the latter in the outermost sample layers was explained by oxidation phenomena occurring upon contact with the outer atmosphere.     

Nb521铌合金电子束焊工艺研究

对Nb521铌合金进行了电子束焊工艺试验研究,优化了工艺参数,分析了焊缝表面成形及焊接接头组织、常温和高温力学性能.结果表明:Nb521具有良好的电子束焊接性能,焊缝常温拉伸强度、屈服强度均能达到母材的95％以上,延伸率达到母材的86.7％,在1600℃以下焊缝性能良好.     

Formation of carbonitride precipitates in hardfacing alloy with niobium addition

Niobium, as the most effective second-phase forming element, was added in the Fe-Cr13-C-N hard-facing alloy to get carbonitride precipitates. Morphology and composition of carbonitride in the hardfacing alloy were studied by optical microscopy, scanning electron microscopy, and electron probe microanalyzer. The ther-modynamics and the effect on the matrix of the formation of carbonitride were also discussed. It was found that niobium carbonitrides are complex Nb(C, N) precipitate distributed on grain boundary and matrix of the hardfacing alloy. Under as-welded condition, primary carbonitride particles were readily precipitated from the hardfacing alloy with large size and morphology as they were formed already during solidification. Under heat treatment condition, a large number of secondary carbonitrides can pre-cipitate out with very fine size and make a great secondary hardening effect on the matrix. As a result, addition of niobium in the hardfacing alloy can prevent the formation of chromium-rich phase on grain boundaries and inter-granular chromium depletion.     

Nb元素对低合金耐磨钢组织性能的影响

对含Nb和不含Nb两种成分低合金耐磨钢板NM400热轧和热处理态的组织性能进行了研究,并对比分析了微量Nb元素对其组织性能的影响规律。研究结果表明:在低合金耐磨钢中添加质量分数为0.02%的Nb,在相同的控轧控冷和离线热处理工艺条件下,钢板强度和硬度增加,低温冲击韧性提高。在相同的工艺条件下,微量Nb元素的添加对钢板组织中原始奥氏体晶粒的细化是其低温韧性提高和硬度增加的主要原因。     

The influence of the magnetic field configuration on plasma parameters and microstructure of niobium nitride films

Niobium nitride (NbN) coatings have a variety of interesting properties such as high chemical inertness, excellent mechanical properties, high electrical conductivity, high melting point, and a superconducting transition temperature around 16 K. We have investigated the effects of magnetic field configuration on the plasma characteristic (electron temperature, plasma density, the ion-to-metal flux ratio J_i/J_a and energy parameter E_p) and the microstructure of NbN films grown with a variable magnetron system. The coatings were deposited under identical deposition conditions but with varying the configuration of the magnetic field in the magnetron. The plasma characteristics were determined by planar and cylindrical Langmuir probes for the different magnetic field configurations. The film microstructure and composition were analyzed by X-ray diffraction, scanning electron microscopy and X-ray photoelectron spectroscopy, respectively. The film hardness and Young's Modulus were measured by Nanoindentation.The variation of the magnetic field with respect to the unbalance state showed that the field changed from a minimum of 3.6 to a maximum of 4.6 mT at the substrate position (5 cm away from target) while in the target center the corresponding values were 49.0 to 98.0 mT, respectively. The lower magnetic field at the target resulted in higher J_i/J_a ratios, plasma densities and potentials. These characteristics resulted in changes in the value of E_p and as this increased the preferred crystalline orientation changed from [200] to [111] and the hardness and Young Modulus increased to 40 GPa and 430 GPa, respectively.     

Effect of niobium in medium alloyed ductile cast irons

The present work studies the effect of niobium addition on the mechanical properties and microstructure of an as-cast 4·3%Ni alloyed ductile iron. The experimental iron was made from high purity raw materials in a laboratory induction furnace. Eight castings were produced with niobium content in the range 0–0·8%. Silicon and carbon content were fixed at 2·4 and 3·1% respectively for all the castings. A complete micro-structural characterisation was undertaken for all the as-cast alloys. Niobium was observed to be directly related to the formation of polygonal niobium carbides of the type NbC. Such carbides increased in size and amount as niobium content increased in the alloys. The amounts of pearlite and ferrite phases were not affected by the niobium content added in the present study. Neither nodule count nor nodularity was affected by such niobium addition. Mechanical properties such as hardness, tensile strength, yield strength and ductility were measured for the alloy; a small strengthening of the alloys was observed as the niobium amount increased. Such strengthening is explained in terms of NbC formation during solidification. IJCMR/372     

The effects of niobium and nickel on the corrosion resistance of the zinc phosphate layers

In this investigation the viability of nickel substitution by niobium in zinc phosphate (PZn) baths has been studied. Samples of carbon steel (SAE 1010) were phosphated in two baths, one containing nickel (PZn + Ni) and the other with niobium substituting nickel (PZn + Nb). Potentiodynamic polarization curves (anodic and cathodic, separately) and electrochemical impedance spectroscopy (EIS) were used to evaluate the corrosion resistance of the phosphated carbon steels in a 0.5 mol L^− 1 NaCl electrolyte. The phosphate layers obtained were analysed by X-ray diffraction and it was found that they are composed of Zn₃(PO₄)₂.4H₂O (hopeite) and Zn₂Fe(PO₄)₂.4H₂O (phosphophylite). Surface observation by scanning electron microscopy (SEM) showed that the PZn + Ni layer is deposited as needle-like crystals, whereas the PZn + Nb layer shows a granular morphology. The electrochemical results showed that the PZn + Nb coating was more effective in the corrosion protection of the carbon steel substrate than the PZn + Ni layer. The results also suggested that nickel can be replaced by niobium in zinc phosphate baths with advantageous corrosion properties of the layer formed.     

电流密度对铌表面熔盐脉冲电沉积渗硅的影响

采用熔盐脉冲电沉积法在纯铌表面渗硅,利用辉光放电光谱仪（GDOES）、扫描电镜（SEM）和能谱仪（EDS）、X射线衍射仪（XRD）分析不同电流密度对渗硅层厚度、成分、组织及相结构的影响,并对渗硅层的抗氧化性进行了研究。结果表明,渗硅层厚度随电流密度的增大而增加,超过100 mA/cm2厚度增加缓慢。渗硅层晶粒随电流密度的增加由粗大变得细小。渗层与基体结合紧密,渗层组织较均匀整齐,致密无孔洞。渗硅层由单相NbSi2组成,在（110）和（200）晶面上择优生长。NbSi2渗层提高了纯铌的抗氧化性。     

Influence of siliconizing on the oxidation behavior of plasma sprayed MoSi2 coating for niobium based alloy

Plasma spraying combined with halide activated pack cementation (HAPC) was used to deposit silicide coating on Nb-based alloy. X-ray diffraction (XRD) and energy disperse spectrum (EDS) indicate the formation of the siliconized NbSi₂ transition layer and the sprayed MoSi₂ outer layer. NbSi₂ layer prepared with HAPC exhibits relatively uneven surface which could promote the deposition of the sprayed MoSi₂. The coating specimen with 5 h siliconizing presented the best oxidation resistance with only 0.18% mass gain after 25 h oxidation at 1200 °C in air. The synergistic protection effect, depending on the continuous silica layer formed on the coating surface and the dispersal silica within the coating and interface, is responsible for the excellent oxidation resistance of the coating.     

Ti-Mg复合脱氧对低碳钢中夹杂物及组织的影响

在高温钼丝炉内对实验钢采用Ti-Mg单独及复合脱氧处理,利用带有能谱仪的扫描电镜和光学显微镜研究脱氧产物的成分、大小分布及铸态微观组织的变化,分析夹杂物对晶内铁素体形核以及实验钢奥氏体晶粒大小的影响.结果表明,Ti-Mg复合脱氧后的产物可以作为TiN和MnS的析出核心,形成复合夹杂物;复合夹杂物在冷却过程中可以有效的促进晶内铁素体形核,同时抑制加热过程中奥氏体晶粒的粗化.     

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.     

Mass transfer modeling on the separation of tantalum and niobium from dilute hydrofluoric media through a hollow fiber supported liquid membrane

The separation of a mixture of tantalum and niobium in dilute hydrofluoric media via hollow fiber supported liquid membrane (HFSLM) was examined. Quaternary ammonium salt (Aliquat336) diluted in kerosene was used as a carrier. The various effects on the transport and separation of tantalum and niobium were studied: concentration of hydrofluoric acid in the feed solution, concentration of the carrier (Aliquat336) in the membrane phase, types of stripping solutions (NaClO₄, thiourea and HCl) and their concentration. The extraction of tantalum in the membrane phase from 0.3 M hydrofluoric acid (HF) by 3% (v/v) Aliquat336 was achieved by leaving niobium in the feed solution. Quantitative recovery of tantalum was achieved by 0.2 M NaClO₄. Furthermore, a mathematical model focusing on the extraction side of the liquid membrane system was presented in order to predict the concentration of tantalum at different times. The mass transfer coefficients of the aqueous feed (k_i) and the organic membrane phase (k_m) were estimated as 1.19 × 10⁻⁵ and 1.39 × 10⁻⁷ cm/s, respectively. Therefore, the mass transfer limiting step is the diffusion of tantalum-Aliquat336 through the liquid membrane. Moreover, mass transfer modeling was performed and the validity of the developed model evaluated. Experimental data and theoretical values were found to be in good agreement when the concentration of Aliquat336 in the membrane phase was below 4% (v/v).     

Microstructural evolution and mechanical properties of niobium processed by equal channel angular extrusion up to 24 passes

We have systematically investigated the microstructural evolution of niobium (Nb) subjected to severe plastic deformation via equal channel angular extrusion (ECAE) up to 24 passes. The starting Nb billet material consists of a centimeter-scale grain size with a columnar structure. We have found that the grain size reduction of the Nb is almost saturated at ∼300 nm after eight passes of ECAE. However, the population of high-angle grain boundaries continues to increase with further ECAE, and no saturation appears to have been reached at 24 passes. We have evaluated the mechanical properties of the samples with different number of ECAE passes over a wide range of strain rates, from quasi-static to high strain rates. We have used strain-rate jump tests to examine the strain-rate sensitivity (SRS) of the processed samples and found that the SRS of the ECAE-processed Nb is ∼0.012, which is a factor of three smaller than that of the coarse-grained counterpart. The activation volume derived for plastic deformation indicates that the double-kink formation of screw dislocations is still the predominant deformation mechanism in the ECAE-processed Nb. Quasi-static true stress-strain curves exhibit elastic-nearly perfectly plastic behavior. The quasi-static yield strength is also nearly saturated after eight passes of ECAE. High-strain-rate compressive true stress-strain curves show uniform flow softening. However, the dynamic peak stress keeps rising with an increased number of ECAE passes, suggesting a strong grain boundary contribution to dynamic strengthening. Scanning electron microscopy of post-loaded surfaces displays a morphology of diffuse shear bands accompanying highly compressed grains. In our report, we demonstrate that grain boundaries of severely deformed metals play different roles at low, quasi-static vs. high-strain rates of mechanical loading. The difference is primarily determined by the strength of grain boundaries acting as dislocation barriers at different loading rates. This discovery is significant for the understanding of the effect of the microstructure as a function of the applied loading rate.     

Reactive pulsed laser deposition and characterization of niobium nitride thin films

We present a systematic study to explore the effect of important process variables on the composition and structure of niobium nitride thin films synthesized by Reactive Pulsed Laser Deposition (RPLD) technique through ablation of high purity niobium target in the presence of low pressure nitrogen gas. Secondary Ion Mass Spectrometry has been used in a unique way to study and fix gas pressure, substrate temperature and laser fluence, in order to obtain optimized conditions for one variable in single experimental run. The x-ray diffraction and electron microscopic characterization have been complemented by proton elastic backscattering spectroscopy and x-ray photoelectron spectroscopy to understand the incorporation of oxygen and associated non-stoichiometry in the metal to nitrogen ratio. The present study demonstrates that RPLD can be used for obtaining thin film architectures using non-equilibrium processing. Finally the optimized NbN thin films were characterized for their hardness using nano-indentation technique and found to be ~ 30 GPa at the deposition pressure of 8 Pa.     

Effects of electrolyte pH and temperature on dielectric properties of anodic oxide films formed on niobium

The effects of electrolyte pH and temperature on the structure and properties of anodic oxide films formed on niobium in phosphoric acid solution with the addition of NH₄OH for pH adjustment have been investigated. The film thickness formed at the same voltage slightly increased with increasing pH and significantly increased with increasing electrolyte temperature. The capacitance of the film was independent of electrolyte pH in an acid region, while it notably increased with increasing pH in an alkaline region. The relative permittivity of the film changed 43.7-80.5 when the electrolyte pH was increased from 1.6 to 10. The incorporation depth and content of phosphorus in the film were markedly suppressed at pH 10, and nitrogen was found to penetrate into a depth of 70%. Furthermore, the apparent transport number of Nb⁵⁺ ion decreased from 0.26 to 0.02 by a pH increase from 1.6 to 10. The notable changes in structure and dielectric properties of the anodic niobia film formed in the alkaline region would primarily be caused by the different incorporation behavior of electrolyte species such as phosphorous and nitrogen.     

On the sulphidation mechanism of niobium and some of Nb-alloys at high temperatures

The kinetics and mechanism of niobium sulphidation have been studied as a function of temperature (700-1000 °C) and sulphur pressure (10⁻⁴-10⁴ Pa) in pure sulphur vapour and H₂-H₂S gas mixtures, using microthermogravimetric technique. It has been found that in both different sulphidizing atmospheres the sulphidation process follows parabolic kinetics, being thus diffusion controlled. Marker experiments have shown that the slowest step of the overall reaction rate is the outward diffusion of cations. No influence of small amounts of impurities on the sulphidation rate has been observed in this study. Excellent agreement between calculated and experimentally determined parabolic rate constants has been obtained under the assumption, that the correct formula of the sulphide scale on niobium is Nb_2±yS₃ and not Nb_1+xS₂, as suggested by Gesmundo.It has been found that the rate of niobium sulphidation in H₂-H₂S gas mixtures is much higher than in pure sulphur vapour, strongly suggesting that the dissolution of hydrogen in the growing scale influences the defect structure in this sulphide.