Niobium Silicides Research Articles

Effect of cyclic mechanical treatment on the final properties of Nb + 2Si powder mixtures prepared for SHS

The mechanical pretreatment of a powder mixture of niobium and silicon was studied experimentally and theoretically using the new macrokinetic theory of mechanochemical synthesis. The method of organizing the operation mode of the ball mill affects the degree of grinding and activation of the powder composition, as well as the rate of formation of intermediate phases. The effect of cycling on the temperature inside the mill drum, particle size of the powder mixture, formation of mechanically synthesized niobium silicides, and structure of the initial components has been studied. Varying the cycling can lead to safe and emergency operation of the mill. The final mixtures' characteristics may vary significantly depending on the number of cycles of discontinuous mechanical treatment, even if the total operating time of the ball mill remains the same. The safe conditions for cyclic operation of the mill were determined. The effects of different mechanical treatment modes on the grinding, activation, and phase formation of a powder mixture were investigated. Various parameters were analyzed, including thermophysical, physico-chemical, and kinetic characteristics of the cyclic operation of the AGO-3 ball mill, the kinetics of grinding and activation of Nb and Si particles, and the rate of mechanochemical reaction in the mechanically activated mixture of Nb + 2Si.

Investigating the Replacement of Nickel-based Superalloys with Niobium-Based Superalloys in the New Generation of Turbine Blades in the Aero-space Industry

To improve the thermal properties of superalloys or replacing them with new materials is necessary to increase engine efficiency. When considerable progress in superalloys is unlikely, researchers started to turn their attention to alternative materials. Metallic silicide alloys are cheaper choices for increasing engine efficiency. The melting point for molybdenum and niobium silicide is much higher than the melting point of the current alloys. This shows these alloys have the potential to increase temperature in gas turbine engines. These alloys have poor oxidation resistance. Improving oxidation resistance of each alloy system is a significant step toward a new material for turbine blades. To improve oxidation resistance in these alloys, many experiments have been done in which alloy elements such as Ti, Cr, Al, Zr, Ta, and Sn have been used. According to these experiments, it can be concluded that Zr is practically ineffective and it cannot improve oxidation resistance in alloys. In some temperatures, Ta is even harmful for oxidation resistance. Sn has the best function to increase oxidation resistance by forming a rich layer of Sn, oxygen penetration into depth is prevented. Despite these alloy elements, they are not enough resistant to oxidation to function in higher temperatures. To be replaced by nickel-based superalloys in gas turbine blades in the aerospace industry they need to have higher oxidation resistance.

Synthesis of highly disperse NbSi2–Si3N4 and Si3N4–NbN composite powders

AbstractThe behavior of reaction powder mixtures of stoichiometric compositions calculated for the synthesis of higher niobium nitride and silicide in the temperature range 800–1400°С in vacuum according to the reactions 4Nb + Si3N4 and 3Nb + 2Si3N4, respectively, has been investigated. The decomposition of Si3N4 is accompanied by the formation of lower nitride and lower silicide phases of niobium. The content of the silicide phase increases with heat treatment temperature, and, at 1400°С, NbSi2 forms. The nitride phase remains practically unchanged up to 1300°С and disappears at 1400°С. On the basis of the established regularities, we have developed synthesis methods for Si3N4–NbN and Si3N4–NbSi2 powders, which include the introduction of excess Si3N4 with respect to the stoichiometric composition. The combined two‐stage technological cycle of synthesis of Si3N4–NbN involves vacuum annealing at 1000°С and subsequent nitriding at 1300°С. The synthesis method for NbSi2–Si3N4 assumes the in situ successive formation of niobium silicide phases up to 1400°С. The synthesized Si3N4–15NbN and NbSi2–14Si3N4 powders consist of multiphase agglomerates with a mean size of 5.5 and 8.8 µm, respectively, and a nanodisperse fraction with a mean particle size of 200 nm.

Niobium Silicide Diffusion Coatings: The Impact of Cu and of Ti on the Microstructure and High-Temperature Behavior

Abstract Silicide diffusion coatings on niobium offer attractive properties for applications at temperatures above 1,000°C because of the ability to form a protective oxide, silicon dioxide. The combination of a double layer of silicides, NbSi2 and Nb5Si3, provides a gradient of properties to the coating, mitigating the formation and propagation of cracks that otherwise allow oxygen to interact with the substrate. However, a temperature above 1,200°C is required to process double-layer silicide coatings, increasing the challenge of controlling the process and incrementing the cost of the process. The need to enhance silicide coatings without compromising their competitiveness motivates the study of the individual effects of copper and of titanium. This study processed silicide coatings by double pack cementation adding copper or titanium, respectively, to the pack mixture. Three pack mixture compositions were used: with 15 % wt. silicon, with 15 % wt. silicon + 7 % wt. copper, and 15 % wt. silicon + 7 % wt. titanium. Processing was carried out at 1,000°C for 6 hours. The coatings processed with silicon resulted on a single NbSi2 layer, but the pack mixture containing silicon and copper led to thick double-layer coatings of NbSi2 and a continuous layer of Nb5Si3, in spite of the low processing temperature. In contrast, the pack mixture containing silicon and titanium led to thinner coatings composed of a layer of NbSi2 and the discontinuous presence of Nb5Si3. Results suggest that copper and titanium impact the gaseous diffusion during processing and also the vacancy concentration in the ordered structure of NbSi2. Exposure to temperature reveals a change on the oxidation mechanisms after processing with titanium and a more significant mass gain of coatings processed with copper.

Synthesis in Nb + 2Si Mixtures Produced in Different Modes of Discrete Mechanical Activation

An experimental study is performed of the two-stage mechanochemical synthesis of niobium silicide. Theoretical estimates are made on the basis of the resulting macrokinetic model. The initial Nb + 2Si mixture is mechanically activated in the first stage, and the NbSi2 product is synthesized in the second. The effect the period of mechanical treatment has on the activated mixture’s morphology, temperature, and rate of burning is studied, along with the phase composition of the final reaction product. Kinetic constants are calculated that describe the dynamics of the two-stage mechanochemical synthesis of niobium silicide.

Two-Stage Mechanochemical Synthesis of Niobium Silicides during Layer-by-Layer Combustion

Two-Stage Mechanochemical Synthesis of Niobium Silicides during Layer-by-Layer Combustion

Two-Stage Mechanochemical Synthesis of Niobium Silicides during Layer-by-Layer Combustion

Two-Stage Mechanochemical Synthesis of Niobium Silicides during Layer-by-Layer Combustion

Stability, metallicity, and magnetism in niobium silicide nanofilms

Modern superconducting qubits based on two-dimensional (2D) transmons typically involve the growth of Nb thin films on high-resistivity Si substrates. Since imperfections at the Nb-Si heterointerface have been implicated as a source of two-level systems that limit quantum coherence times, detailed characterization and understanding of niobium silicide interfacial layers are critical to improving superconducting qubit technology. While bulk binary intermetallic niobium silicide phases are well understood, the thermodynamic phase stability and properties of ultrathin niobium silicides, such as those found at the Nb-Si heterointerface in 2D transmons, have not yet been explored. Here, we report finite-sized effects for ultrathin niobium silicide films using density functional theory calculations and predict nanoscale stabilization of ${\mathrm{Nb}}_{6}{\mathrm{Si}}_{5}$ over the bulk $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Nb}}_{5}{\mathrm{Si}}_{3}$ phase. This result is consistent with our experimental observations of a niobium silicide interfacial layer between a sputtered Nb thin film and the underlying Si substrate. Furthermore, our calculations show that ${\mathrm{Nb}}_{6}{\mathrm{Si}}_{5}$ nanofilms are nonmagnetic, making them superior to nanofilms of $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Nb}}_{5}{\mathrm{Si}}_{3}$ that exhibit antiferromagnetic correlations detrimental to long coherence times in superconducting qubits. By providing atomic-scale insight into niobium silicide nanofilms, this paper can help guide ongoing efforts to optimize Nb-Si heterointerfaces for long coherence times in superconducting qubits.

Niobium Silicide Multilayers Processed by In Situ Synthesis During Deposition of Powder Mixtures

Niobium Silicide Multilayers Processed by In Situ Synthesis During Deposition of Powder Mixtures

Multi-cycle of AISI 5135 steel modification by irradiation of the “film (Si (0.2 μm) + Nb (0.2 μm))/(AISI 5135 steel) substrate” system with an intense pulsed electron beam

Steel AISI 5135 surface layer modification carried out by high-cycle high-speed melting of the “film (Si + Nb)/(steel AISI 5135) substrate” system with an intense pulsed electron beam with an impact area of several square centimeters, have been implemented in a single vacuum cycle on the “COMPLEX” setup. The regime of the system “film (Si (0.2 μm) + Nb (0.2 μm))/(steel AISI 5135) substrate” irradiation with an intense pulsed electron beam (20 J/cm2, 200 μs, 3 pulses, 3 cycles) which makes it possible to form a surface layer with high thermal stability have been revealed. This layer is characterized by high hardness, more than 3 times higher than the hardness of AISI 5135 steel in the original (ferrite-pearlite structure) and wear resistance, more than 90 times higher than the wear resistance of the initial AISI 5135 steel. It is shown that the high strength and tribological properties of steel are due to the formation of the hardening phase particles (niobium silicide of Nb5Si3 composition).

Regularities of structure and properties modification of the steel surface layer as a result of high-cycle high-speed melting of the “film (Si + Nb) / (steel) substrate” system by an intense pulsed electron beam

Surface alloying is a promising method for improving the service characteristics of machine parts and mechanisms. The aim of this work is to analyze the patterns of evolution of the elemental and phase composition, the state of the defect substructure of the surface layer of carbon steel subjected to multi-cycle treatment (the number of cycles was varied from 1 to 5), which combines the formation of a film / substrate system and irradiation with an intense pulsed electron beam. Silicon and niobium are used as alloying elements. It has been established that irradiation of the system “film (Si + Nb) / (steel 5135) substrate” at an electron beam energy density of 20 J / cm2 leads to the formation of a multiphase high-speed cellular crystallization structure in the surface layer. The size of the crystallization cells varies from 180 nm to 520 nm, reaching the maximum size after 3 cycles of deposition-irradiation. The surface layer of steel is quenched during processing to form a martensitic structure. It was found that the Nb5Si3 is the hardening phase of the modified layer, the maximum amount of what (up to 9 wt%) was revealed after three processing cycles. It is shown that the maximum hardness of the surface layer of modified steel 5135, equal to 9300 MPa (3.2 times higher than the hardness of steel 5135 in the initial state), is achieved after 3 cycles of deposition / irradiation. The wear resistance of steel at this processing mode increases more than 90 times. It has been suggested that the hardening of steel surface layer is due to the formation of a quenched structure and the release of niobium silicide particles.

Nb silicide coatings processed by double pack cementation: Formation mechanisms and stability

Niobium silicide coatings were processed on Nb metal using double pack cementation procedures at temperatures in the range 1000 °C–1200 °C for up to 6 h. Coating thickness and hardness were affected by processing parameters. Lower processing temperatures resulted in single-layer NbSi2 coatings while the highest processing temperature (1200 °C) produced coatings with two layers, NbSi2 and Nb5Si3, the latter at the NbSi2/Nb interface. Analysis of the results shows that Nb5Si3 forms by decomposition of NbSi2 and interdiffusion of Si and Nb. The influence of processing parameters on vacancy density and of vacancy density on the coating formation mechanisms and properties is discussed. Silicide-coated Nb had superior oxidation resistance to pure Nb. However, under the conditions tested a two-layer scale of Nb2O5 and SiO2 was formed. Analysis of the data indicates that Nb2O5 forms rapidly on the surface of the coatings at lower temperatures whereas SiO2 forms at the top of the oxide scale. It is concluded that pre-oxidation treatment is essential to ensure formation of an SiO2 scale, which is associated with enhanced oxidation performance.

Novel carbon-free niobium silicide/oxide nanocomposites for lithium-ion battery anodes

Carbon-free niobium silicide (NbSi2)-based composites, starting from silicon monoxide and niobium metal elements, are synthesized by a simple top-down approach with a high-energy mechanical milling process facilitating the manufacture of nanosized materials, and are employed as anode materials for lithium-ion batteries. The final product has three phases comprising Nb, NbSi2, and Nb2O5, whose structures and morphologies are thoroughly examined using advanced characterization techniques. Although the as-prepared Nb/NbSi2@Nb2O5 composites do not contain carbon, the volume expansion is mitigated during the (de)alloying process with Li ions and the composites possess good electrical conductivity. The carbon-free Nb/NbSi2@Nb2O5 composites, specifically the Nb/NbSi2@Nb2O5 (2:8 & 3:7) electrodes, exhibit stable electrochemical performance (430 & 400 mAh g−1 after 300 cycles) as well as remarkable rate performance with discharge capacities of 699 & 491 mAh g−1 at a high current density of 10 A g−1 with 86 & 80% of normalized capacity retentions. This noticeable improvement can be due to the presence of Nb in the NbSi2 phase and the amorphous Nb2O5 matrix in the composites, which provide high electrical conductivity (for Nb) in addition to forming stable structural stability (for Nb and Nb2O5). Therefore, the novel Nb/NbSi2@Nb2O5 composites presented here provide potential for viable application in high-performance anodes for lithium-ion cells.

Study of Ti-Zr-Nb-Be filler metal interaction with silicon carbide based ceramics during brazing process

This work is devoted to an analysis of the composition of a silicon carbide based ceramic brazed seam as a result of its interaction with rapidly quenched titanium-zirconium-niobium-beryllium filler metal. Structural-phase studies based on EDX and EBSD analysis, mechanical shear tests, and microhardness measurements of brazed joints were carried out. It was shown that titanium, zirconium, and niobium silicides as well as particles of titanium carbosilicides and silicon carbides in the silicon matrix are formed in the brazed seam, probably because of the presence of free silicon in the base material, which leads to increasing joint microhardness and unstable shear strength results.

The study of pressing mechanically activated niobium and silicon powder mixtures

The paper presents the results obtained in the study of the pressing process of mechanically activated powder mixtures of the niobium – silicon system. Pressing often precedes the process of self-propagating high-temperature synthesis and the initial density of the compressed samples can have a significant impact on the regime of subsequent high-temperature combustion, on the composition and properties of the final product (niobium silicide). The obtained dependences of porosity on the time of mechanical activation showed the features of pressing, which are associated with the formation of layered agglomerates in the process of mechanical activation. It was found that the compression curves of mechanoactivated powders differ qualitatively from the standard compression curves of fine powders. The paper considers the influence of mechanical activation time on the density of the compressed samples obtained by varying the pressing pressure and the time spent under the press. The relationship between the density of compresses and layered agglomerates, which are formed during mechanical activation, is considered.

Mechanically activated SHS of Nb5Si3 and Nb5Si3/Nb composites

Nb5Si3 and Nb are the main components of niobium silicide-based composites, which are promising materials for high-temperature structural applications. Self-propagating high-temperature synthesis (SHS) is an attractive method for fabricating advanced materials from powders via exothermic reactions between the mixture constituents. However, to overcome kinetic difficulties of Nb–Si reaction, SHS of niobium silicides usually requires additional heat input. The present work focuses on a technique that addresses this problem by adding a mechanical activation step before SHS. Specifically, short-term ball milling was used for mechanical activation of Nb/Si mixtures with the goal of conducting SHS of Nb5Si3 and Nb5Si3/Nb composites with no preheating. The milled mixtures were compacted and ignited in an argon environment. It has been shown that a planetary mill is an effective tool for mechanical activation of Nb/Si mixtures. Milling for 10 min enabled SHS of the product that consisted of α-Nb5Si3 and γ-Nb5Si3 phases with traces of Nb3Si. For the first time, Nb5Si3/Nb composites were obtained by mechanically activated SHS with no preheating. Combustion took place at an Nb/Si mole ratio as high as 3, i.e. at 25 at% Si in the mixture. At this mole ratio, a spinning propagation of the combustion wave was observed, and the content of the γ phase, which has poor mechanical properties, was dramatically decreased.

Physical and chemical properties of composite (Mo1-xNbx)Si2

In this study, the effect of NbSi2 in the range from 10 to 90 wt% was investigated. on the properties of composites (Mo1-xNbx)Si2 obtained by hot pressing a solid-phase mixture of molybdenum and niobium disilicides at a temperature of 1700°C. Graphs of the dependence of density, bending strength, electrical resistivity, CLTE and microhardness of the content of NbSi2 were obtained. It was found, that with an increasing level of NbSi2 in the content, the flexural strength drops to 130 MPa. The electrical resistivity depending on the content of niobium silicide is extreme with a maximum for the composition (Mo0,5Nb0,5)Si2. In the concentration range of NbSi2 20-100 wt.% CLTE and microhardness of the composite within the error does not change.

Investigation of a High-Temperature Nb-Based Composite Material Mechanically Doped with Si

Abstract—The influence of Si and uncontrolled interstitial impurities (oxygen and carbon) in trace amounts on the strength characteristics of a high-temperature composite material (HCM) based on Nb obtained by the powder metallurgy technique has been studied. It has been established that the bending strength of specimens manufactured by mechanical homogenization and mechanical alloying exceeds the strength of a material obtained from an original niobium powder by a factor of 3–4.5, with the main contribution to the increase in strength being made by mechanical homogenization of the original niobium. Optimal HCM properties were obtained during alloying of the Nb powder in a planetary mill with 1 wt % Si and uncontrolled impurities (O and C). An assumption was put forward about the mechanisms of HCM hardening: when using homogenized Nb (without silicon), the hardening proceeds by two mechanisms, both by the cold-work hardening of niobium powder (dislocation mechanism) and by the uncontrolled impurities (O and C) that form ultrafine oxides and niobium carbides (dispersion hardening); after Si introduction, an additional dispersion hardening with niobium silicides takes place. The authors do not exclude the effect of porosity on the strength of the material.

Effects of Nb Content on the Mechanical Alloying Behavior and Sintered Microstructure of Mo-Nb-Si-B Alloys

Different from conventional Mo-Si-B-based alloys consisting of Moss, Mo3Si, and Mo5SiB2, Mo3Si-free Mo-Si-B-based alloys (Moss+Mo5Si3+Mo5SiB2 or Moss+Mo5SiB2) show great potentials for more excellent oxidation resistance and elevated temperature strength. In the present work, alloying element Nb was added to Mo-12Si-10B (at.%)-based alloy to suppress the formation of the Mo3Si phase. Mo-12Si-10B-xNb (x = 10, 20, 22, 24, 26, 28, 30, and 40) bulk alloys were fabricated using mechanical alloying followed by cold pressing and then sintering at 1773 K for 2 h. Effects of Nb content on the mechanical alloying behavior and then sintered microstructure were studied. The addition of Nb with an amount less than 30 at.% accelerated the mechanical alloying process, but 40 at.% Nb addition decreased the process due to excessive cold welding and high powder volume. For the sintered bulk alloy prepared from the mechanically alloyed powders milled for 30 h, a critical Nb content between 24 and 26 at.% was found to suppress Mo3Si production and γNb5Si3 phase formed in the alloys with the addition of Nb content more than 26 at.%. Prolongation of a prior milling process could facilitate the suppression of Mo3Si and delay the formation of niobium silicides.

Mechanochemical synthesis and characterisation of niobium silicide nanoparticles

Mechanochemical synthesis (MCS) investigations of niobium silicide powders in Nb2O5SiO2Mg powder mixtures were conducted via high-energy ball milling and leaching by means of process parameters such as the milling duration, initial Nb2O5 crystal structure, and excess Mg amount. The compositional, thermal, and microstructural properties of the products were characterised using X-ray diffractometry (XRD), Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC), particle size analysis (PSA), stereomicroscopy (SM), scanning electron microscopy (SEM), and transmission electron microscopy/energy-dispersive spectrometry (TEM/EDS). After the 5 h milling of the stoichiometric mixtures, an instantaneous ignition occurred and NbSi2, Nb5Si3, Nb, and MgO phases formed with some residual Mg. In addition, the use of excess Mg inhibited the formation of Nb5Si3. Afterwards, leaching treatments (in a 4 M HCl solution) completely removed the Mg-based phases from the synthesised products. The XRD calculations revealed that the resultant product consisted of 97 wt% NbSi2 and 3 wt% Nb5Si3 phases. The TEM analyses indicated that each Nb silicide nanoparticle was coated with a SiO2 layer. According to random measurements, the average particle size of the Nb silicide cores and average thickness of the SiO2 layer were approximately 61 and 7.5 nm, respectively.