Solubility Of Nb Research Articles

Thermodynamic modelling of the M 6C carbide in cemented carbides and high-speed steel

Newly published experimental information on phase equilibria in cemented carbides and high-speed steel has shown that the solubility of Co, Nb, Si, Ta and V in the M 6C carbide is higher than predicted by thermodynamic calculations. To account for this new information the model for M 6C was extended. A thermodynamic description of the M 6C carbide in cemented carbides and high-speed steels is presented. Several experimental studies have shown that there is a solubility of Si in M 6C in high-speed steels, and the new model describes this solubility. A significant solubility of Co and Nb has also been observed, and was accounted for. The solubility of Nb, Ta and V in M 6C in cemented carbide systems is described. The models are discussed, and the calculated equilibria are compared with the new experimental information.

1373K isothermal section of phase diagram for Nb–W–Zr ternary system

The isothermal section of the Nb–W–Zr system at 1373K has been successfully measured by means of electron probe microanalysis (EPMA) on diffusion couples. This isothermal section includes two single-phase regions, ``α'' and (W, Nb)2Zr and one two-phase region ``α'' + (W, Nb)2Zr. The maximum solid solubility of Nb in (W, Nb)2Zr is 12.23at.% Nb.

Phase equilibria of the Cu–Nb–Ti system at 850 °C

The phase equilibria of the Cu–Nb–Ti system at 850 °C were investigated by a high-efficiency diffusion couple approach. The samples were studied by means of optical microscopy, scanning electron microscopy and electron probe microanalysis. At 850 °C, the phase TiCu 2, the stability range of which is controversial in the literature, was observed. The solubilities of Nb in Ti 2Cu, TiCu and Ti 3Cu 4 were determined to be 3.5, 3.1 and 4.6 at.%, respectively. The solubility of Nb in TiCu 4 was negligibly small. The following three-phase equilibria were observed: Ti 2Cu + TiCu + (Nb,Ti), TiCu + Ti 3Cu 4 + (Nb,Ti) and Ti 3Cu 4 + TiCu 4 + (Nb,Ti).

Phase equilibria of the Fe–Nb–Ti system at 900 °C

The isothermal section at 900 °C of the Fe–Nb–Ti system was determined using the diffusion couple approach with the aid of equilibrated alloys. The specimens were examined by means of optical microscopy, scanning electron microscopy, and electron probe microanalysis. No ternary compound was found. The isothermal section consists of five single-phase regions, six two-phase regions and two three-phase regions. The solubility of Nb in TiFe were determined to be 6 at.%, and the solubility of Ti in μ-Nb 19Fe 21 to be 23.6 at.%. NbFe 2 and TiFe 2 form a continuous solid solution (Nb, Ti)Fe 2.

Formation of the microstructure in Cu-Nb alloys

In order to increase the mechanical strength of copper while retaining its high electrical conductivity the copper matrix is strengthened by Nb particles. For thermodynamical reasons the mutual solubility of Nb in copper is negligible. Therefore, a Cu-Nb alloy is prepared by mechanically alloying and subsequent hot pressing. The formation of a solid solution in a first step and of precipitates in a Cu-10 at.%Nb alloy in the second one as well as the effect of these precipitates on the mentioned properties is discussed. The influence of different temperatures during hot pressing on density, mechanical and electrical properties, etc. is investigated.

Dielectric properties of 0.7BaO·0.3SrO·(1 − y)TiO 2· yNb 2O 5 composite ceramics

The 0.7BaO·0.3SrO·(1 − y)TiO 2· yNb 2O 5 (BSTN) composite ceramics with both the perovskite and the tungsten bronze phases were successfully prepared by controlling some excessive contents of components and traditional ceramic process where 0.1 < y < 0.7. The phase composition and dielectric properties of the composite ceramics were investigated by XRD and LCZ Meter. According to the results, the dielectric properties of the BSTN have both characteristics of the perovskite phase and the tungsten bronze phase. The ɛ r and tan δ of the BSTN depend on the relative content of the perovskite phase as well as the solubility of Nb 5+ ions in it. The samples in which two phases coexist clearly have higher dielectric constant than the samples that only contain one solid solution, but the tan δ of those two phases coexisting is also higher. The ɛ r and tan δ decrease with the increase of applied frequency for same composition. In the BSTN composite ceramics, the phase transition temperature of the tungsten bronze phase depends on the solubility of Ti 4+ ions in the phase and the relative content of the perovskite phase. The lowest transition temperature is around 200 °C and it is about 90 °C lower than the pure tungsten bronze phase Sr 0.3Ba 0.7Nb 2O 6.

Phase Equilibria and Lattice Parameters of Fe2Nb Laves Phase in Fe-Ni-Nb Ternary System at Elevated Temperatures

ABSTRACTPhase equilibria in Fe-Ni-Nb ternary system at elevated temperatures have been examined, in order to identify the two-phase region of γ-Fe (austenite) and ε-Fe2Nb (C14). The ε single phase region exists in the range of 27.5 to 35.5 at.% Nb in the Fe-Nb binary system, and it extends toward the equi-niobium concentration direction up to 44 at.% Ni in the ternary system at 1473 K, indicating that more than half of the Fe atoms in Fe2Nb can be replaced with Ni. Thus, the γ+ε two-phase region exists extensively, and the solubility of Nb in γ phase increases from 1.5 to 6.0 at.% with increase in Ni content. The lattice parameters of a and c in the C14 Laves phase decrease with increasing Ni content. The change in a axis is in good agreement with calculation based on Vegard's law, whereas that of c axis is much larger than the calculated value. The result suggests that atomic size effect is responsible for a-axis change and the binding energy is dominant factor for the c-axis change. To extend these findings to development of new class of austenitic steels strengthened by Laves phase, an attempt has been made to control the c/a ratio by alloying. The addition of Cr is effective to make the c/a ratio close to the cubic symmetry value (1.633).

Solubility of Niobium(V) under Cementitious Conditions: Importance of Ca-niobate

AbstractThe solubility of niobium was investigated for Ca and pH conditions relevant for cementequilibrated solutions. For the pH range considered (9.5-13.2), the dissolved Nb concentration decreases with increasing pH. Overall, experiments lead to Nb concentrations between 2·10-5 M and 2·10-9 M. For all pH values, the dissolved Nb concentration also decreases systematically with increasing Ca concentration. X-ray diffraction measurements of selected experiments confirmed the presence of a solid Ca-Nb-oxide phase, with CaNb4O11·8H2O (hochelagaite) being the most likely composition. On the basis of these findings an empirical regression model for the prediction of Nb solubility data as a function of pH and Ca concentration was derived. This empirical relation is consistent with the presence of a solubility limiting Ca-Nb solid phase and permits to predict aqueous Nb solubility values in cementitious environments over a relatively wide range of conditions. Predicted values are in good agreement with independent experimental results.

Mapping of the Nb–Ti–Si phase diagram using diffusion multiples

A high-efficiency diffusion-multiple approach was employed to map the phase diagram of the Nb–Ti–Si ternary system, which is critical for the design of niobium silicide-based composites. These composites have high potential as a replacement for Ni-base superalloys for jet engine applications. Titanium is one of the most important elements for oxidation resistance and fracture toughness enhancement. Three isothermal sections of Nb–Ti–Si at 1000, 1150 and 1200 °C were constructed from the results obtained from diffusion multiples using scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and electron backscatter diffraction (EBSD). Extremely high solubility of Nb in Ti5Si4 (up to ∼46 at.% Nb at 1200 °C, substituting for Ti) was observed.

Methane conversion over Nb-doped ceria

Methane steam reforming and dry methane conversion over ceria, and ceria doped with 1.4 and 5% Nb cation has been investigated at 900 °C (a typical solid oxide fuel cell temperature). The influence of the calcination atmosphere on the Nb-doped ceria has also been studied. Use of reducing conditions leads to significantly lower crystallite size, higher specific surface area and greater Nb solubility. Nb-doping lowers activity mainly as a consequence of strong segregation of Nb to the ceria surface. Kinetics of steam reforming are interpreted in terms of a redox mechanism.

Phase Transformation from Cubic to Rhombohedral in Doped Bi2O3-CaO System

In the CaO-Bi 2 O 3 system, the rhombohedral phase was easily obtained. However, the cubic phase was hardly observed in this system. It is believed that the amount of oxygen vacancies and the size of doping ions play an important role in the phase transformation of doped Bi 2 O 3 system. In this study, the effect of Nb 2 O 5 addition on the structural stability and conductivity of CaO-Bi 2 O 3 solid electrolyte was investigated. With the addition of Nb 2 O 5 into Bi 1,4 Ca 0,6 O 2.4 , the rhombohedral phase gradually disappeared. In the sample of 6 mol% Nb 2 O 5 doped Bi 1.4 Ca 0.6 O 2.4 , all rhombohedral phase was not observed. Instead, a single cubic phase was found. The cubic structure of Bi 1.4 Co 0.6 O 2.4 was stabilized by the addition of Nb 2 O 5 . When 10 mol% Nb 2 O 5 was added into Bi 1.4 Ca 0.6 O 2.4 , a second phase, Bi 5 Nb 3 O 15 , appeared because the solubility of Nb in the cation sublattice was reached. The ionic conductivity of Bi 1.4 Ca 0.6 O 2.4 was also enhanced by the addition of Nb 2 O 5 . The conductivity of 6 mol% Nb 2 O 5 doped Bi 1.4 Ca 0.6 O 2.4 was as high as 0.72 S/cm at 800°C. The microstructure of Nb 2 O 5 doped Bi 1.4 Ca 0.6 O 2.4 shows good agreement with XRD result. A single and isotropic microstructure was seen from the 6 mol% Nb 2 O 5 doped sample. When the second phase appeared, the suppression of grain growth was observed in 10 mol% Nb 2 O 5 Bi 1.4 Ca 0.6 O 2.4 .

Dilution and microsegregation in dissimilar metal welds between super austenitic stainless steel and nickel base alloys

Super austenitic stainless steels are often welded using high Mo, Ni base filler metals to maintain the corrosion resistance of the weld. An important aspect of this processing is the weld metal dilution level, which will control the composition and resultant corrosion resistance of the weld. In addition, the distribution of alloying elements within the weld will also significantly affect the corrosion resistance. Dissimilar metal welds between a super austenitic stainless steel (AL-6XN) and two Ni base alloys (IN625 and IN622) were characterised with respect to their dilution levels and microsegregation patterns. Single pass welds were produced over the entire dilution range using the gas tungsten arc welding process. Microstructural characterisation of the welds was conducted using light optical microscopy, scanning electron microscopy, and quantitative image analysis. Bulk and local chemical compositions were obtained through electron probe microanalysis. The quantitative chemical information was used to determine the partition coefficients k of the elements in each dissimilar weld. The dilution level was found to decrease as the ratio of volumetric filler metal feedrate to net arc power increased. Reasons for this behaviour are discussed in terms of the distribution of power required to melt the filler metal and base metal. In addition, the segregation potential of Mo and Nb was observed to increase (i.e. their k values decreased) as the Fe content of the weld increased. This effect is attributed to the decreased solubility of Mo and Nb in austenite with increasing Fe additions. Since the Fe content of the weld is controlled by dilution, which in turn is controlled by the welding parameters, the welding parameters have an indirect influence on the segregation potential of Mo and Nb. The results of the present work provide practical insight for corrosion control of welds in super austenitic stainless steels.

Structural stability of mechanically alloyed (Mg+10Nb) and (MgH 2+10Nb) powder mixtures

In order to improve the hydrogen storage characteristics of magnesium, both chemical alloying by Nb and mechanical alloying (MA) of (Mg+10 wt.%Nb) and (MgH 2+10 wt.%Nb) powder mixtures were investigated, with particular attention paid to their structural stability. Extensive powder refinement was noted for both compositions within 20 h of milling at 250 rpm. Even nano-sized particles were generated in the hydride mixture. XRD and Rietveld analyses show the formation of a bcc phase in each case. The amount of the bcc phase increases with increasing milling time to the detriment of Nb. For the (Mg+10 wt.%Nb) mixture, it is confirmed that the newly formed phase is a bcc-(Nb,Mg) solid solution, with an extended solubility of Nb in Mg. However, for the (MgH 2+10 wt.%Nb) powder mixture, the new bcc phase can be a Nb hydride (NbH x , x<1.0), or a bcc-(Nb,Mg) solid solution, or a (Nb,Mg)H x solid solution, or even a mixture of the three.

Melt inclusions as indicators of the magmatic origin of carbonatite rare metal and rare earth minerals

Carbonatite melts, originating in the course of crystallization or liquid differentiation of mantle-carbonatized alkaline magmas, could contain primary enhanced quantities of alkalis, phosphorous, and incompatible trace elements (Nb, Ta, Zr, REE, Sr, Ba). This statement, based upon experimental data relative to fractionation of carbonatite-forming systems and solubility of Nb and La in synthetic carbonate liquids, allows us to suggest magmatic nature of rare metal (RMM) and rare earth (REM) minerals in carbonatites. Crystallization of RMM and REM from carbonatite melts is supported by the results of the present melt inclusion study in minerals of carbonatites. We observe the: (i) presence of RMM and REM in the form of solid inclusions in magmatic host-minerals; (ii) microprobe analyses of daughter rare metal and rare earth phases, entering into composition of crystallized melt inclusions; and (iii) conservation by rare metal and rare earth minerals of melt inclusions with high temperatures of homogenization.

Theoretical study on cracking behavior in two-phase alloys Cr–Cr 2X (X=Hf, Nb, Ta, Zr)

To examine the importance of thermal expansion mismatch on cracking in two-phase Cr–Cr 2X (X=Hf, Nb, Ta, Zr) alloys, we have calculated the coefficients of thermal expansion (CTE) of Cr and Laves-phase Cr 2X alloys by local-density-functional approach. A Debye model is used to approximate the phonon contribution through the elastic acoustic response. The sound velocities are determined by the calculated elastic constants and associated anharmonicity. The calculation shows that the CTE of Cr at high temperatures is notably larger than those of Cr 2X. If the difference in CTE between Cr and Cr 2X is a primary source of crack initiation, our results fail to explain the experimental observation that, among these four Cr–Cr 2X alloy systems, the ingot cracking is present mainly in Cr–Cr 2Nb. We suggest that for the cracking to occur,the presence of thermal mismatch stresses are retained by a hard and supersaturated Cr matrix (e.g. due to the relatively high solubility of Nb in Cr in the case of Cr–Cr 2Nb). On the other hand, the softer Cr matrix can accommodate thermal misfit dislocations plastically even though the CTE difference between Cr and Cr 2X is large.

Solubility and Raman spectra of Nb(V) in LiF–NaF–KF–Na 2O melts

The solubility of Nb(V) as a function of O 2− composition in the ternary eutectic LiF–NaF–KF (FLiNaK) melt at 700°C has been determined. Raman spectra at different oxide concentrations and temperatures were recorded. Based on chemical analysis of melt samples and bands in the spectra suggestions related to the Nb–O–F complex formation are given. The addition of Na 2O to FLiNaK containing 0.22 mole kg −1 K 2NbF 7, show that at: n O/ n Nb<2; there is no precipitation of Nb(V) compounds. The Raman spectra of the melt indicated the presence of monomeric NbF 7 2− at low oxide concentrations, which reacted with oxide to form monomeric NbOF 5 2− when r was increasing. Further additions of Na 2O formed the NbO 2F 4 3− species having a maximum concentration at n O/ n Nb=2; when 2< n O/ n Nb<3, a solid of the type AlkNbO 3 was formed. At n O/ n Nb=3, a minimum in the Nb(V) and O 2− solubilities were observed, but the structure of the species in the melt were uncertain. At higher oxide content, 3< n O/ n Nb<4, the AlkNbO 3(s) previously formed was dissolved, possibly with the formation of both corner and edge sharing distorted NbO 6 octahedra. The solubility of Nb 2O 5 in FLiNaK was measured as a function of temperature (550–800°C), and the dissolution mechanism seems to be given by the reaction: Alk ++Nb 2O 5(s)+4F −=NbO 2F 4 3−+AlkNbO 3(s). Δ H 0 sol for the reaction (based on concentrations in mole kg −1) was found to be ≈90 kJ mol −1 K −1.

Sol-Gel Processed TiO2-Based Nano-Sized Powders for Use in Thick-Film Gas Sensors for Atmospheric Pollutant Monitoring

Sol-gel routes were used to prepare pure and 5 at% and 10 at% Ta- or Nb-dope TiO2 nano-sized powders. The thermal decomposition behaviour of the precursors was studied using simultaneous thermogravimetric and differential thermal analysis (TG/DTA). X-ray diffraction (XRD) analysis showed that the powders heated to 400°C were crystalline in the anatase TiO2 structure. The pure TiO2 powder heated to 850°C showed the rutile structure. The addition of Ta and Nb inhibited the anatase-to-rutile phase transformation up to 950–1050°C. Ta was soluble in the titania lattice up to the concentration of 10 at%, while the solubility of Nb was 5 at%. Thick films were fabricated with these powders by screen printing technology and then fired for 1 h at different temperatures in the 650–1050°C range. Scanning electron microscopy (SEM) observations showed that the anatase-to-rutile phase transformation induces a grain growth of about one order of magnitude for pure TiO2. The addition of Ta and Nb is effective to keep the TiO2 grain size at a nanometric level even at 950°C, though grain growth was observed with increasing temperature. The gas-sensitive electrical response of the thick films were tested in laboratory, in environments with CO in dry and wet air. Conductance measurements showed a good gas response only for the nanostructured titania-based films. For field tests, the prototype sensors were placed beside a conventional station for atmospheric pollutant monitoring. The electrical response of the thick films was compared with the results of the analytical instruments. The same trend was observed for both systems, demonstrating the use of gas sensors for this aim.

Effect of oxygen activity on Nb solubility in BaTiO 3

This paper describes the effect of oxygen partial pressure on Nb solubility in polycrystalline BaTiO 3 at 1673 K. It was found that an increase of p(O 2) in the gas phase results in an increased Nb solubility in BaTiO 3. Annealing of polycrystalline BaTiO 3, covered with a thin layer of NbCl 5, in air at 1573 K results in Nb incorporation to the level of 0.1 at.% while annealing at the same temperature in nitrogen (involving 2% H 2) does not lead to Nb incorporation into BaTiO 3. The observed effect of p(O 2) on Nb solubility in BaTiO 3 is consistent with the defect disorder model of BaTiO 3.

Effects of Sn and Nb on massive hydriding kinetics of Zr–XSn–YNb alloy

Kinetic studies on the massive hydriding of Zr–0.4Nb– XSn ( X=0.5, 0.8, 1.5, 2.0) and Zr–0.8Sn– YNb ( Y=0.2, 0.4, 0.8, 1.0) ternary alloys are carried out at 400°C under atmospheric pressure by in situ weight gain measurements with thermo-gravimetric apparatus (TGA) and transmission electron microscope/energy dispersive X-ray spectrometer (TEM/EDX) analysis. It is confirmed that the hydriding kinetics follow a linear rate law after incubation time. It is found that the hydriding reaction rate decreases with increasing Sn content up to 1.5% and then sharply increases in the case of Zr–0.4Nb– XSn while it steadily increases with Nb content in the case of Zr–0.8Sn– YNb. The rate does not seem to be affected by the grain size in the XSn alloys, but is influenced in the YNb alloys. TEM/EDX analysis shows that there is no strong relationship between the intermetallic precipitates and the kinetic rate in the Zr– XSn– YNb alloy system. It is revealed in this study that the solubility limit of Sn in the Zr–0.4Nb– XSn ternary system becomes higher than that in the Zr–Sn binary system. On the other hand, the Nb solubility limit remains unchanged in the Zr–0.8Sn– YNb ternary system. Optimized compositions in the Zr– XSn– YNb ternary alloy are suggested to be about 1.5% Sn and as low Nb as possible in order to minimize hydrogen uptake.

Sol-Gel Nanosized Semiconducting Titania-Based Powders for Thick-Film Gas Sensors

Pure, 5 at%, and 10 at% Ta- or Nb-doped TiO2 nanosized powders were prepared by the sol-gel method. The powders heated to 400°C have the crystalline anatase structure. While the pure TiO2 powder heated to 850°C has the rutile structure, the addition of Ta and Nb inhibited the anatase-to-rutile phase transformation at this temperature. Ta was soluble in the titania lattice up to the concentration of 10 at%, while the solubility of Nb was 5 at%. Thick films were fabricated with these powders by screen printing technology and then fired at 650°C and 850°C for 1 h. SEM observations showed that the anatase-to-rutile phase transformation induces a grain growth of about one order of magnitude for pure TiO2. The addition of Ta and Nb is effective to keep the TiO2 grain size at the nanometric level even at 850°C. Conductance measurements showed that a good gas response is observed only for the nanostructured titania-based films. The CO response of these materials is only slightly affected by humidity.