Kinetics of and preparation of nickel by Ni3S2−NiO reaction under reduced pressure

The reaction between Ni₃S₂ (liquid) and NiO (solid) resulting in the formation of Ni and SO₂ was investigated in the temperature range 800° to 1200°C under a reduced pressure of <0.1 mm Hg. From the kinetic studies in the temperature range 950° to 1150°C, the reaction was found to proceed in three stages: i) Up to about 25 pct reduction, the rate of reaction was high and followed approximately a cubic rate law. During this stage, the reaction is thought to be under mixed control. Activation energy for the first 10 pct reduction was found to be approximately 45 kcal per mole. ii) From about 25 to 90 pct reduction, it obeyed the parabolic rate law, with an activation energy of 86±6 kcal per mole. This value is in agreement with the activation energy reported in the literature for the diffusion of sulfur in nickel. iii) Beyond 90 pct reduction, the reaction was very sluggish owing to the poor availability of the reactants. Optimum conditions for preparing nickel sponge by the above reaction and its processing into thin strips have been standardized. Some of the properties of the metal thus produced have also been incorporated.     

Effect of metallic vacancies on the electronic structure of niobium nitride

The effect of metallic vacancies on the density of states in niobium nitride is examined. Calculations were carried out using the recursion and coherent-potential methods for NbN and Nb_0.75N. The recursion density of states in nonstoichiometric niobium nitride is obtained for parallel and antiparallel arrangement of the metallic vacancies. It is found that the formation of such vacancies leads to the formation of an electronic spectrum of additional (in comparison with the complete compound) states in the high-energy part of the hybrid metal-nonmetal region. The changes in the density of states of Nb_0.75N in relation to the degree of ordering in the metallic sublattice are also examined. Institute of Materials Science, National Academy of Sciences of Ukraine, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 7–8, pp. 86–89, July–August, 1997.     

A PRELIMINARY STUDY OF THE REACTIONS BETWEEN TUNGSTEN CARBIDE AND TUNGSTEN CARBIDE - 6% COBALT POWDERS AND WET AND DRY HYDROGEN

Abstract

The apparent activation energy for the following reactions has been determined by measuring the amount of carbon removed from samples of powder heated to various temperatures in an atmosphere of either dry or wet hydrogen:

Dry Hydrogen

“As-carburized” tungsten carbide: no reaction up to 900°C.

Water–milled tungsten carbide: 8·38 kcal/mole over the range 600–900°C.

Water–milled tungsten carbide+ 6% cobalt: complex reaction over the range 600–900°C.

Wet Hydrogen

“As-carburized” tungsten carbide: 58·1 kcal/mole over the range 800–900°C.

Water–milled tungsten carbide: 34·8 kcal/mole over the range 700–900°C.

Water–milled tungsten carbide+ 6% cobalt: 38·4 kcal/mole over the range 825–900°C; 7·38 kcal/mole over the range 600–800°C.

It has been shown that ball-milling in water changes both the physical and chemical properties of tungsten carbide powder. These changes are discussed in relation to the marked differences in reaction rates. A number of possible reasons for these differences are given.     

Kinetics of chlorination and carbochlorination of pure tantalum and niobium pentoxides

Kinetics of chlorination and carbochlorination of pure Nb₂O₅ and Ta₂O₅ were studied by thermogravimetric analysis between 385 °C and 1000 °C using Cl₂-N₂ and Cl₂-CO-N₂ gas mixtures. Standard free energy changes of the reactions and phase stability diagrams of Nb-O-Cl and Ta-O-Cl systems were calculated. The chlorination reaction order, for both oxides, with respect to Cl₂ in the Cl₂-N₂ gas mixture was 0.82. The apparent activation energies (E _a ) for Nb₂O₅ chlorination were 208 and 86 kJ/mole for temperatures lower and higher than 850 °C, respectively. The experimental data could be described by a shrinking sphere model between 700 °C and 1000 °C. The chlorination mechanism, between 700 °C and 850 °C, was likely controlled by the chemical reaction. For T > 850 °C, the overall Nb₂O₅ chlorination rate was affected by the allotropic transformation of the Nb₂O₅ T form to M form. Between 925 °C and 1000 °C, E _a for Ta₂O₅ chlorination was 246 kJ/mole. In this case, the most appropriate model was also that of shrinking sphere suggesting that the chlorination of Ta₂O₅ was controlled by the chemical reaction. For both oxides, the carbochlorination reaction order with respect to Cl₂+CO partial pressure, in the gas mixture, was about 2. The mathematical analysis of carbochlorination data indicates that Nb₂O₅ and Ta₂O₅ reactions could be described by shrinking sphere or cylinder, respectively. Below 600 °C, the E _a values of Nb₂O₅ and Ta₂O₅ carbochlorination were 74 and 110 kJ/mole, respectively. Chemical reaction was probably the rate controlling step in both cases. An anomaly characterized by a decrease of the reaction rates occurs in the Arrhenius plots between 600 °C and 800 °C. This anomaly could be attributed to the thermal decomposition of COCl₂ formed in situ during the carbochlorination.     

Phase relations and diffusion layer formation in the systems Cu-Nb-Sn and Cu-Nb-Ge

Condensed phase relations were determined for part of the system Cu−Nb−Sn at 1100 and 1000°C, and for part of the system Cu−Nb−Ge at 1100°C. Diffusion experiments in both systems were conducted at 1100 and ∼800°C. The two types of experiments were compared in order to undertand the diffusion synthesis of superconducting A15 structure compounds from Cu−Ge or Cu−Sn bronzes. Whether an A15 layer of Nb₃Ge or Nb₃Sn can form by diffusion depends on the orientation of two-phase tielines in the appropriate system. The tielines existing between bronze and Nb₃Sn in the Cu−Nb−Sn system are closely followed by the diffusion path inbrozen-Nb diffusion couples so that a super conducting Nb₃Sn layer forms. Bronze-Nb₃Ge tielines are precluded in the Cu−Nb−Ge system by a two-phase field, Nb−Nb₅Ge(Cu) solid solution. Diffusion paths from bronze to Nb do not enter the Nb₃Ge phase field and no Nb₃Ge diffusion layer forms. The possibility exists that the addition of other chemical components might favorably modify the phase relations such that bronze diffusion synthesis can succeed for Nb₃Ge.     

The surface tension of liquid silver alloys: Part II. Ag−O alloys

The surface tensions of liquid Ag-O alloys have been determined by the sessile drop method. The surface activity of oxygen, as measured by ?(dσ/dX _O)X_O→0_j, where σ is the surface tension of the metal andX _O the mole fraction of oxygen, is quite large and equals 3.80×10⁵ dyne per cm at 980°C and 1.35×10⁵ dyne per cm at 1108°C. The heat of adsorption of oxygen is estimated to be of the order of 30 kcal per mole. Application of the monolayer approximation shows that liquid silver becomes saturated with oxygen when each adsorbed oxygen atom occupies an area of 33±5Ų. Small additions of platinum to silver do not change the characteristics of the adsorption of oxygen appreciably. An analysis of the data is consistent with the conclusion that saturation of the surface of liquid silver with oxygen results from the formation of an ionic two-dimensional compound at the surface. This hypothesis is tested in the case of several other systems and yields satisfactory results. The structure of these compounds is discussed. In the case of the Ag-O system, it appears to correspond to the stoichiometry Ag₃O.     

Kinetics of the thermal dissociation of Co4S3 under reduced pressure

Kinetics of the thermal dissociation of Co₄S₃ were investigated under reduced pressures of 5 × 10^-2 and 1.5 × 10^-4 mm Hg in the temperature range 1120° to 1405°C. The initial 10 pct dissociation was found to take place in accordance with a linear rate law, with activation energies of 20 and 17 kcal per mole under low and high vacuums, respectively. Subsequent dissociation up to about 35 pct was observed to be parabolic in nature, with activation energies of 38 and 40 kcal per mole under the respective vacuums. Further dissociation up to about 90 pct was found to fit into another linear rate law, with activation energies of 47 and 49 kcal per mole under the reduced pressures of 5 × 10^-2 and 1.5 × 10^-4 mm Hg, respectively. Beyond 90 pct, the dissociation was found to be very sluggish. These results have been interpreted with the help of the Co-S phase diagram. It has also been possible to achieve more than 99 pct dissociation of Co₄S₃ in 2 hr at 1405°C under low vacuum or 1355°C under high vacuum.     

Kinetics of diffusion in the Nb-Al system

Diffusion kinetics were studied by varying the time and temperature (800° to 1300°C) of dipping solid niobium into molten aluminum. It was found by X-ray diffraction analysis that only the Al₃Nb phase forms at the surface of the niobium specimen and that the thickness of this layer for a given dip temperature varies parabolically with time. The activation energy and the preexponential factor of the diffusion parameter, (δD) were found to be 36.5 ∓ 0.45 kcal per mole and 2.0 ∓ 0.34 sq cm per sec, respectively.     

Coercive force and microstructure in a Zr-permalloy

An alloy containing 80.0 pct Ni, 12.65 pct Fe, 6.74 pct Mo, 0.36 pct Zr, and 0.25 pct Mn by weight was cast, homogenized, and successively cold rolled into thin strips with area reductions of 0, 50, 75, and 90 pct. Annealed samples were studied by optical and electron microscopy, electron diffraction, and magnetic testing to determine the effects of cold work and annealing upon the microstructure and magnetic properties of the alloy. Cold work produced a high initial hardness together with high coercive force. Recrystallization of the cold worked structures occurred upon annealing at 600°C (873 K) and above and caused significant and parallel decreases in hardness and coercive force. The activation energy for recrystallization was found to be 80.5 kcal/g mole (337.0 kJ/g mole) for the 50, 75, and 90 pct cold worked specimens. After annealing at 600°C (873 K), a small number of spherical Ni₄Mo particles were observed, but the particles produced little change in magnetic properties apparently because of their relatively coarse size and large spacing. Beginning at 700°C (973 K) ribbon-shaped particles of a Ni₅Zr intermetallic compound also precipitated out of solid solution. Both the Ni₄Mo and Ni₅Zr precipitates were the result of a homogeneous continuous precipitation reaction within the grains. A peak in coercive force at 800°C (1073 K) is attributed to domain wall pinning associated with the fine distribution of rodlike Ni₅Zr particles. Cold working 90 pct and aging at 800°C (1073 K) was found to increase coercive force by almost 60 pct from the minimum produced by complete recrystallization. Annealing, however, decreased hysteresis and improved squareness.     

Solid-gas reactions driven by mechanical alloying of niobium and tantalum in nitrogen

Solid-gas reactions of niobium and tantalum with molecular nitrogen driven by mechanical alloying (MA) have been investigated by X-ray diffraction, transmission electron microscopy, and differential thermal analysis. It was found that the phase transition followed a sequence of Nb₂N → Nb₃N₄ → NbN when Nb was milled with N₂. However, Ta₂N and an amorphous phase were formed when Ta was milled with N₂. The chemosorption of nitrogen onto the clean metal surfaces created by ball milling is believed to be the fundamental process governing solid-gas reactions, and the defects generated during MA can promote the diffusion of adsorbed nitrogen, and consequently the formation of metal nitrides. The difference in phase transition between the two systems is discussed.     

Correlation Between Experimental and Calculated Phase Fractions in Aged 20Cr32Ni1Nb Austenitic Stainless Steels Containing Nitrogen

A centrifugally cast 20Cr32Ni1Nb stainless steel manifold in service for 16 years at temperatures ranging from 1073 K to 1123 K (800 °C to 850 °C) has been characterized using scanning electron microscopy (SEM), electron probe micro-analysis (EPMA), auger electron spectroscopy (AES), and X-ray diffraction (XRD). Nb(C,N), M₂₃C₆, and the silicide G-phases (Ni₁₆Nb₆Si₇) were all identified in a conventional SEM, while the nitride Z-phase (CrNbN) was observed only in AES. M₂₃C₆, Z-phase and G-phase were characterized in XRD. Thermodynamic equilibrium calculations using ThermoCalc Version S, with the TCS Steel and Fe-alloys Database (TCFE6), and Thermotech Ni-based Superalloys Database (TTNI8) were validated by comparing experimental phase fraction results obtained from both EPMA and AES. A computational study looking at variations in the chemical composition of the alloy, and how they affect phase equilibria, was investigated. Increasing the nitrogen concentration is shown to decrease G-phase formation, where it is replaced by other intermetallic phases such as Z-phase and π-phase that do not experience liquation during pre-weld annealing treatments. Suppressing G-phase formation was ultimately determined to be a function of minimizing silicon content, and understabilizing the Nb/(C + 6/7N) ratio.     

The generalized lewis acid-base titration of palladium and niobium

The high thermodynamic stability of alloys composed of platinum group metals and group IVB and VB metals has been explained by an electronic interaction analogous to the Lewis acid-base concept for nontransition elements. The analogy is further demonstrated by the titration of palladium by addition of niobium. The activity of niobium in solid palladium was measured as a function of concentration by solid-state galvanic cells and study of the ternary oxide phase diagram. The galvanic cells were of the type Pt/NbO₂,Nb₂O_4.8/YDTJNbO_y,Nb_pd/Pt where the solid electrolyte is yttria-doped thoria (YDT). Ternary phase diagrams for the Pd-Nb-0 and Rh-Nb-0 systems were obtained by characterizing samples equilibrated at 1000 °C. The phase relationships found in the ternary diagrams were also used to derive thermochemical data for the alloys. Thermochemical quantities for other acid-base stabilized alloys such as Nb-Rh, Ti-Pd, and Ti-Rh were also measured. The excess partial molar ΔG^xs/R of niobium at infinite dilution was determined to be -31 kilo-Kelvin at 1000 °C, and theAG°JR of formation of a mole of NbPd_3.55 is —21 kilo-Kelvin. These results and those for the other systems are used to assess the importance of valence electron configuration, nuclear charge, and crystal field effects in the context of generalized Lewis acid-base theory. It is concluded that both the nuclear charge of the atom and crystal field splitting of the valence orbitals significantly affect the basicity of the platinum group metals.     

Nonequilibrium synthesis of nbai3 and Nb-Ai-V

In this study, the isothermal oxidation behavior of laser-clad NbAl₃ has been investigated in the temperature range between 800 °C and 1400 °C in air. The effect on oxidation of vanadium microalloying, used to increase the ductility of the otherwise brittle NbAl₃ and discussed in Part I, [1] has also been considered. Bulk and surface oxide chemistry has been investigated using X-ray diffraction and X-ray photoelectron spectroscopy (XPS), respectively. Oxidation kinetics have been determined from weight gain data. The XPS and X-ray diffraction data show that NbAl₃ does not exclusively form a protective A1₂O₃ layer when oxidized in air. The oxidation products at 800 °C are a mixture of Nb₂O₅ and A1₂O₃, while at 1200 °C, a mixture of NbAlO₄, Nb₂O₅, and Al₂O₃ is formed. At 1400 °C, a mixture of NbAlO₄, A1₂O₃, NbO₂, NbO_2.432, and Nb₂O₅ forms. Upon addition of vanadium, the oxidation rate is found to dramatically increase and may be related to the formation of (Nb, V)₂O₅ and VO₂, which grows in favor of protective A1₂O₃. Consequently, although vanadium may be a good additive in terms of its potential for improving ductility in NbAl₃, it is not in terms of its deleterious effects on oxidation.     

Embrittlement of titanium alloys by long time,high temperature exposure

α stabilized titanium alloys are known to exhibit embrittlement after long-time exposures above ∼800°F. The time-temperature dependency of this embrittlement phenomenon in the Ti-6Al-2Sn-4Zr-2Mo and Ti-5Al-6Sn-2Zr-lMo-0.25Si alloys was observed using a substandard fracture mechanics test. Room temperature slow-bend tests of fatigue precracked Charpy specimens were used to monitor toughness degradation after unstressed thermal exposures in the temperature range of 800° to 1100°F for times to 5000 hr. The activation energy for the embrittlement process was found to be ∼25 to 28 kcal per g mole, which approximates that for diffusion of aluminum or tin in α-Ti. The embrittlement is attributed to the Ti₃X (X = Al, Sn) phase with the rate controlling step that of diffusion controlled growth of the Ti₃X phase domains. The embrittlement process is reversible by heat treatment at temperatures above the α + Ti₃X two phase region.     

Electrotransport of carbon in molybdenum and uranium

The electrotransport velocity of carbon in molybdenum and uranium was measured over a temperature range slightly below their melting points. Carbon was found to have a positive effective valence of 2.26 to 1.74 in molybdenum over the temperature range of 1890 to 2320°C and a negative value of - 5.0 in γ-uranium between 850 and 1000°C. The effective valences of nitrogen and oxygen were also observed to be positive in molybdenum and negative in uranium but their magnitudes were not determined. The diffusion coefficients for carbon in both metals were determined over the same temperature ranges.¹⁴Carbon was used as a tracer in the molybdenum work. The diffusion coefficient for carbon in molybdenum is described by the equationD = D ₀ exp (-†H/RT) whereD ₀ and †H are 0.033 cm²/s and 153 kJ/mole (36.60 kcal/mole), respectively. The values forD ₀ and †H for carbon in γ-uranium were determined as 0.218 cm₂/s and 123 kJ/mole (29.40 kcal/mole), respectively. Electrotransport was shown to be an effective method of purifying a small amount of each metal with regard to carbon as indicated by resistance ratio measurements and chemical analysis. A correlation is also presented showing the relationship between the atomic size of the solvent metal and the sign of the effective charge of the migrating solute.     

Electrotransport and diffusion of Co,Fe, and Ag in γ-Cerium

Electrotransport mobilities and diffusion coefficients were obtained for radiotracer impurities of Fe, Co, and Ag in Ce. The iron and cobalt moved toward the anode with mobilities of ～10^?3 cm²/v-s in the range of 550° to 650°C. The silver moved to the cathode with mobilities of ～10^?5 cm²/v-s in the range of 600° to 700°C. The, diffusion coefficients obtained fit an Arrhenius equationD=D _o e ^?ΔH/RT with the following parameters: Fe:D _o=3.3×10^?4, ΔH=4.6 kcal/mole Co:D _o=10^?2, ΔH=11 kcal/mole Ag:D _o=1.4, ΔH=28 kcal/mole The results are compared with other rare-earth diffusion data, and the possibility of a substitutional-interstitial diffusion mechanism, is considered.     

High-temperature interactions of refractory metal matrices with selected ceramic reinforcements

Interdiffusion and reactions occurring at high temperatures between refractory metals (Nb and Ta) and ceramic materials (SiC and A1₂O₃) have been investigated. Diffusion couples were fabricated by depositing Nb and Ta films of ~l-μm thickness onto polished ceramic substrates. These diffusion couples were vacuum annealed at high temperatures for various times. Interfacial reactions were evaluated using optical metallography, Auger electron spectroscopy (AES), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Kinetic studies in the 800 °C to 1200 °C temperature range for the Nb/SiC system indicated that Nb₂C initially forms, followed by the more stable NbC_xSi_y phase. In some instances, layered structures containing the phases NbC, Nb₂C, and NbC_xSi_y, were observed. The activation energies of formation for the NbC_x and NbC_xSi_y, phases were determined from these measurements. Results from the Ta/SiC system were found to be similar to those from the Nb/SiC system. In both Nb/Al₂O₃ and Ta/Al₂O₃ diffusion couples, annealing for up to 4 hours in the 1100 °C to 1200 °C range did not result in any significant reactions. These results suggest that A1₂O₃ may be a promising diffusion barrier between Nb and Ta metal matrices and SiC ceramic reinforcements. formerly with Lockheed Research and Development Division, is Senior Member of Technical Staff, Sandia National Laboratories, Albuquerque, NM 87185     

Thermodynamics of the System NaF-AlF3: Part V. Solid Solution in Cryolite

Electrical resistivity measurements have been used to demonstrate the existence of solid solutions of AlF₃ in the high-temperature form of cryolite above the transition at 565°C. An expression was derived for the resistivity of single-phase mixtures as a function of composition and temperature, and it was then used to deduce the composition at the solid-solid phase boundaries. At the NaF-cryolite eutectic temperature (880°C) the boundary lies at 74.8 mole pct NaF, and at the chiolite peritectic temperature (737°C) the other boundary lies at 74.4 mole pct NaF. The variation of composition with activity of NaF is reasonably consistent with the hypothesis that solid solution leads to formation of AlF ₄ ¹ ions and cation vacancies. On this basis extrapolation is made for the solid-liquid boundaries, and the solid solution region is found to touch the liquidus at 74.0 mole pct NaF. On the above hypothesis the anomalous heat content of cryolite between 888°C and the melting point can be accounted for.     

Kinetics of Domain Growth in Ordered Ni4Mo

The kinetics of domain growth in Ni4Mo in the temperature range of 600 to 850 °C were investigated using transmission electron microscopy. It was found that domain growth in Ni₄Mo is analogous to metallurgical grain growth and can be described by the expressionD ⁿ =kt, whereD is the average domain size, t is the aging time, k is a constant, and the exponent n is the reciprocal of the slope of the log D vs log t plot. The value of n changed with temperature from 2.0 at 850 and 800 °C to 2.9 at 700 and 600 °C. This change was explained in terms of relative domain orientation effects. The activation energy for domain growth was obtained as 69 Kcal/mole (2.9 × 105 Joules/mole) in the temperature range of 800 to 850 °C and as 92 Kcal/mole (3.85 x 105 Joules/mole) in the temperature range of 600 to 700 °C, which on comparison with available diffusion data established that the growth process was interface-controlled at the higher temperatures and bulk diffusion-controlled at the lower temperatures.