Amorphous Cr Research Articles

Improved wear and corrosion resistance of chromium(III) plating by oxynitrocarburising and steam oxidation

Postsurface treatments, such as oxynitrocarburising and steam oxidation, were applied to amorphous Cr plating on SM45C mild carbon steel obtained from a trivalent (III) chromium bath to improve wear and corrosion properties. Higher wear properties of hardness (HV 1450), wear rate (1.9×10 −6 mm 3/rev) and friction coefficient (0.4) were found for both the oxynitrocarburised and the steam-oxidized Cr platings due to the formation of chromium carbide, Cr 7C 3, during the treatments. Excellent corrosion resistance was also observed for the posttreated Cr platings due to the crack healing effect as a result of the formation of iron oxides (Fe 3O 4, Fe 2O 3) and/or nitride (Fe 4N), suggesting that these treatments are highly effective in enhancing wear and corrosion resistance of the chromium platings.

Distribution of chromium species in catalysts supported on ZrO 2/Al 2O 3 and performance in dehydrogenation

Chromium catalysts (2–3 wt% Cr) supported on Al 2O 3 and ZrO 2/Al 2O 3, prepared with Cr(NO 3) 3·9H 2O or K 2CrO 4, were characterized and studied in the isobutane dehydrogenation. Depending on the chromium salt and the support, CrO 3, Cr 2O 3, and K 2CrO 4 species were observed in the calcined samples, and amorphous Cr 2O 3 remained after reduction with H 2. The preparation of ZrO 2/Al 2O 3 support yields zirconia dispersed on the surface of alumina. In the Cr/ZrO 2/Al 2O 3 calcined catalysts, the chromium sites were distributed on the ZrO 2 and Al 2O 3 surfaces. The O 2 chemisorption and the activity and selectivity to isobutene were also affected by the chromium species distribution. The addition of potassium decreased the O 2 chemisorption. In the isobutane dehydrogenation, the turnover number and selectivity to isobutene increased for the catalysts prepared with K 2CrO 4.

Studies on thermal dehydration of hydrated chromic oxide

Hydrated chromic oxide was prepared by the reduction of potassium dichromate in an autoclave using sucrose. The thermal dehydration of the parent gel in different atmospheres was studied comparatively using the techniques of thermal gravimetry, differential thermal analysis, X-ray diffraction analysis, FT-IR spectroscopy, and gas adsorption. The results of thermogravimetric and differential thermal analysis show that the temperature at which the glow phenomenon occurs is dependent on the gaseous atmosphere in the heat-treatment process and increases in the following order: air, nitrogen, and carbon dioxide. When heated in CO 2, the parent gel is transformed into crystalline α-Cr 2O 3 going through the phase of amorphous Cr 2O 3. In air, however, it is decomposed directly into crystalline α-Cr 2O 3 without going through the amorphous Cr 2O 3 phase. It is also found that microcrystallites of α-Cr 2O 3 have been formed before the glow phenomenon occurs at 390 °C in air and at 590 °C in CO 2, respectively. The mechanism for thermal dehydration in air and CO 2 is discussed.

The crystallization of Cr–Si–Ni–Al amorphous films—nucleation and growth of intermetallic phase Cr(Al,Si) 2

When sputtered amorphous Cr–Si–Ni–Al films with an atomic ratio [Si]/[Cr]>2 were heated up to temperature of 600 °C, they were found to crystallize into two phases: the intermetallic phase Cr(Al,Si) 2 and the pure silicon phase; the growth rate of the crystalline phases at the initial stage of reaction was faster than that at the final stage, resulting from the combined redistribution of Cr, Si, Al and Ni elements between the initial crystalline phases and the parent amorphous phase. The formation of the crystalline phases resulted in a significant increase of both the electrical resistivity and the temperature coefficient of resistance (TCR) for the annealed Cr–Si–Ni–Al films. In particular, the intermetallic Cr(Al,Si) 2 particles, which were embedded in the amorphous matrix, were responsible for achieving a zero TCR. Moreover, the deviation between the TCR 25 125 (TCR averaged between 25 °C and 125 °C) and the TCR −55 25 (TCR averaged between −55 °C and 25 °C) in the annealed Cr–Si–Ni–Al films with small TCR was diminished.

Formation of an intermetallic phase by crystallization in the Cr–Si–Ni–Al amorphous film

The microstructure of annealed Cr–Si–Ni–Al films was investigated by transmission electron microscopy (TEM). Sputtered amorphous Cr–Si–Ni–Al films crystallized into two phases: the intermetallic phase Cr(Al,Si) 2 and the pure silicon phase. The formation of the crystallization phases resulted in a significant increase in both the electrical resistivity and the temperature coefficient of resistance (TCR) for the annealed Cr–Si–Ni–Al films. Particularly, the intermetallic Cr(Al,Si) 2 particles, which were embedded in the amorphous matrix, were responsible for achieving a zero TCR. In addition, the growth rate of the crystallized phases at the initial stage of reaction was faster than that at the final stage, resulting from the combined redistribution of Cr, Si, Al and Ni elements between the initial crystallization phases and the parent amorphous phase.

The influence of zirconium on the crystallization behavior and electrical properties of Cr–Si–Al resistive films

Abstract The microstructure and electrical properties of annealed Cr–Si–Al and Cr–Si–Al–Zr films were investigated. When sputtered amorphous Cr–Si–Al and Cr–Si–Al–Zr films were heated up to a temperature of 700 °C, both were found to crystallize into two phases: the nanocrystalline Cr(Al,Si) 2 and Si phase. The addition of Zr into amorphous Cr–Si–Al films inhibited the nucleation and growth of the crystallization phase, resulting in the higher annealing temperatures for Cr–Si–Al–Zr films in comparison with Cr–Si–Al films to obtain a small temperature coefficient of resistance (TCR). As a result, the Cr–Si–Al–Zr films had higher electrical stability.

The role of aluminum in the crystallization of Cr–Si–Ni resistive films

The microstructure and electrical properties of both annealed Cr–Si–Ni and Cr–Si–Ni–Al films with an atomic ratio [Si]/[Cr]>2 were investigated. When sputtered amorphous Cr–Si–Ni and Cr–Si–Ni–Al films were heated up to temperature of 600 °C, they all were found to crystallize through two distinct stages. The first stage was due to the primary crystallization of nanocrystalline CrSi 2 and Si phase for Cr–Si–Ni films (or Cr(Si,Al) 2 and Si phase for Cr–Si–Ni–Al films), while the second stage was due to crystallization of the residual solute-enriched amorphous phase into CrSi 2 (or Cr(Si,Al) 2) and Si phase. The addition of Al into amorphous Cr–Si–Ni films caused the coarsening of the primary crystallization phase, resulting in a significant increase of both the electrical resistivity and the temperature coefficient of resistance (TCR) for the annealed films. Meanwhile, the subsequent crystallization of the residual solute-enriched amorphous phase was affected, resulting from the combined redistribution of Cr, Si, Al and Ni elements between the primary and parent amorphous phases.

Crystallization kinetics of electrodeposited amorphous Cr 84.8C 15.2 (CrC 0.18) films studied by differential scanning calorimetry

Crystallization kinetics of electrodeposited amorphous Cr 84.8C 15.2 (CrC 0.18) films based on differential scanning calorimetry measurements have been investigated. Two distinct exothermic peaks are observed at 655 and 816 K, which are due to the crystallization of b.c.c. α-Cr and h.c.p. Cr 7C 3, respectively, as identified by X-ray diffraction and transmission electron microscopy. The crystallization of dendritic α-Cr is a one-dimensional diffusion controlled growth with a decreasing nucleation rate. The growth direction of dendritic α-Cr is along the cube axes. The effective activation energy for the crystallization of α-Cr analyzed in terms of Avrami–Johnson–Mehl method is 188 kJ/mol, which is close to that obtained by Kissinger's method of 197.2 kJ/mol.

Crystallization of Chromium Oxide Prepared by Sol-Gel Route

Chromia finds large applications as a catalyst base. The aim of the present investigation was to prepare chemically, mechanically and thermally stable porous Cr2O3 microspheres. Chromia microspheres (500–700 μm) were prepared by internal gelation technique of the sol-gel process. These microspheres were dried in air at various temperatures between room temperature and 300°C for further thermal studies in air/oxygen/argon. These gel microspheres were subjected to X-ray diffraction analysis (XRD) and thermal analysis. Crystallization of chromia takes place at 440°C in air and at 710°C in argon. It is reported that in air amorphous Cr(III) oxide partially gets oxidized to Cr(VI) below 350°C and reduces back to Cr(III) during crystallization. Maximum oxidation of about 30% could be achieved by heating chromia microspheres in pure oxygen at 300°C. The present study deals with a probable mechanism for the crystallization of chromia in presence of Cr(VI).

Synthesis of Cr 2O 3 nanoparticles by mechanochemical processing

The synthesis of chromium oxide (Cr 2O 3) nanoparticles by mechanochemical reaction and subsequent heat treatment was investigated. A solid-state displacement reaction Na 2Cr 2O 7+S→Cr 2O 3+Na 2SO 4 was induced during mechanical milling of Na 2Cr 2O 7+S powder mixture, forming amorphous Cr 2O 3 nanoparticles. Annealing the as-milled powder under vacuum above 520°C resulted in the formation of crystalline Cr 2O 3 nanoparticles separated from each other in a salt matrix. A simple washing process to remove the matrix yielded Cr 2O 3 particles of 10–80 nm in size, having a plate-like morphology.

Lithium secondary batteries using a lithium cobalt nitride, Li 2.6Co 0.4N, as the anode

Lithium cobalt nitride, Li 2.6Co 0.4N, has a high capacity of 900 mAh/g with good cycle performance in lithium deintercalation and intercalation. The good reversibility and low potential in comparison to lithium metal (0.7 V on average) suggests that this nitride is a good candidate for an anode in a lithium secondary battery. In order to use Li 2.6Co 0.4N as the anode for lithium secondary batteries, possible combinations with cathode materials were considered: (1) Li 2.6Co 0.4N/Li 0.64Mn 1.96O 4 cell. Lithium ions were initially extracted from Li 1.05Mn 1.96O 4 by using Br 2 as an oxidant; (2) Li 2.6Co 0.4N/a-Cr 3O 8 cell, using a Li-free material of amorphous Cr 3O 8, which has a high lithium insertion capacity of more than 300 mAh/g; (3) Li 1.37Co 0.4N/Li 1.10Mn 1.90O 4 cell. Li 1.37Co 0.4N was used as the anode with the lithium chemically extracted. These lithium secondary batteries showed very high capacity.

Surface-doping of Cr 2O 3 particles by the 119Sn Mössbauer probe : Influence of the gaseous environment on the dopant distribution

Chromium sesquioxide with 119Sn(II) dopant cations located just at the solid-gas interface (in the topmost cationic layer of the crystallites) has previously been synthesized by thermal treatment, under hydrogen flow, of Sn(IV) bulk-doped hydrated amorphous Cr(III) oxide. After exposure of the resulting Sn(II)/Cr 2O 3 material to various reactive gases, the probe environment was studied using Mössbauer spectroscopy. The present work reports synthesis of Sn(II)/Cr 2O 3 under argon atmosphere, instead of hydrogen, which could modify the probe behavior in reactivity experiments. In addition, it shows that migration of the dopant from the bulk towards the surface of the crystallites depends upon the ability of the synthesis atmosphere to act as an electron donor for the oxide matrix. After exposure of these Sn(II)/Cr 2O 3 samples to chlorine, the 119Sn probe environment is found to be the same as for Sn(II)/Cr 2O 3 prepared under hydrogen, which excludes HCl involvement in the surface rearrangement process evidenced by previous Mössbauer investigations.

Effects of nanocrystalline heterogeneity on the corrosion behavior of sputter-deposited chromium–niobium alloys

The nanocrystalline heterogeneity has been introduced into sputter-deposited Cr–Nb alloys by heat treatment in order to clarify the effect of alloy heterogeneity on the corrosion resistance of the alloys. The as-deposited amorphous Cr–Nb alloys are spontaneously passive in 6 and 12 M HCl and show higher corrosion resistance than chromium and niobium. The heat treatment leads to the formation of two phase mixtures of bcc Cr and Cr 2Nb, Cr 2Nb and niobium-rich amorphous phase, Cr 2Nb and bcc Nb, and chromium-rich amorphous phase and bcc Nb, depending on the alloy composition and the heat treatment conditions. It has been found that the Cr 2Nb phase has high corrosion resistance, comparable to that of the amorphous alloys. The formation of nanocrystalline bcc Cr or bcc Nb phases results in the increase in the initial passive current density. However, the long time potentiostatic polarization has revealed that the steady state anodic current density of the heat-treated alloys is similar to that of the as-deposited alloys as a consequence of preferential dissolution of less corrosion-resistant phases. Accordingly, the high corrosion resistance of the amorphous Cr–Nb alloys is sustained even after the introduction of nanocrystalline heterogeneity.

Selective oxidation of H 2S to elemental sulfur over chromium oxide catalysts

CrO x and CrO x supported on SiO 2 have been found to be active for the selective oxidation of hydrogen sulfide to elemental sulfur. The catalysts show maximum sulfur yield at a stoichiometric ratio of O 2/H 2S, 0.5. Amorphous Cr 2O 3 exhibits higher yield of sulfur and has stronger resistance against water than supported Cr/SiO 2, especially at low temperatures. At high temperatures above 300°C, the sulfur yield over the supported catalyst becomes similar to amorphous Cr 2O 3 because the Claus reaction occurring on the silica support removes SO 2 to increase the sulfur yield. Active sites are the amorphous monochromate species that can be detected as a strong temperature programmed reduction (TPR) peak at 470°C. Catalytic activity can be correlated with the amount of labile lattice oxygen and the strength of Cr–O bonding. The reaction proceeds via the redox mechanism with participation of lattice oxygen.

Corrosion-resistant MnZrCr alloys in chloride-containing media

Abstract New series of nanocrystalline and amorphous single phase binary MnZr and MnCr and ternary MnZrCr manganese-base alloys are successfully prepared by DC magnetron sputtering. Electrochemical measurements in combination with XPS analysis are used for examining the effect of single and binary additions of zirconium and chromium metals on the passivity and breakdown of manganese-base alloys in borate–boric acid solutions of pH 8.4 containing 0.01–1 M NaCl. Significant improvement in the resistance to passivity breakdown is observed when chromium and zirconium are added simultaneously to manganese instead of single addition of zirconium or chromium metals. The beneficial effect of simultaneous addition of zirconium and chromium is based mainly on the synergistic interaction between chromium and zirconium cations in the homogeneous double oxyhydroxide passive films formed on the alloy by open circuit immersion.

Resistance fluctuations in amorphous Cr 74Fe 26 films

Magnetoresistance measurements Δ ρ/ ρ have been carried out on an amorphous Cr 74Fe 26 film in the temperature range of 4–120 K in a magnetic field up to 120 kOe. We observed that the isotropic component of the magnetoresitance at low temperatures first decreases with increasing field due to the decrease in the spin fluctuations (negative magnetoresistance), goes through a minimum at a certain field value, H c and then starts to increase (positive magnetoresistance) as the magnetic field is further increased. As the temperature increases, H c shifts to a higher field and the magnetoresistance on both sides of this field (negative and positive magnetoresistance) decreases. The magnetoresistance becomes almost zero at temperatures between 10–16 K. However, as the temperature is further increased, the magnetoresistance starts to increase again. This overall behaviour with the resistivity data have been interpreted successfully in terms of localization effects. We have also observed huge oscillations in the resistivity at low frequencies (7–70 mHz) in the temperature range where Δ ρ/ ρ becomes weaker. Starting with the temperature at about T=15 K, the amplitudes of these oscillations reduce with both increasing and decreasing temperatures. However, the frequencies of these oscillations increase with decreasing temperature. No detectable oscillation in the resistivity has been observed at temperatures below T=8 K and above T> T c (the Curie temperature, T c=43.8 K). We have no full explanation for this phenomenon, however, it is tempting to ascribe it to some kind of spin density wave.

Coupling current loss reduction in cable-in-conduit superconductors by thick chromium oxide coating

It is known, that the coupling current losses in cable-in-conduit superconductors can be reduced by a Cr surface coating of the multifilament wires of the cable. Here we have measured for the first time, how a double layer of crystalline Cr and {\\em thick} amorphous Cr oxide reduces the coupling losses. Both layers were successively electrolytically deposited on the surface of the NbTi/Cu multifilament wires. We find that the loss reduction efficiency of this Cr/Cr-oxide double layer is superior to only a Cr layer or an oxidized Cu surface.

The effect of a Cr2O3-addition on the phase transformation and catalytic properties of γ-Al2O3 in treatment of lean-burn exhausts

The solid-state thermal behaviour of γ-Al2O3 doped with 10 mol% Cr (oxide) was studied with respect to phase-transition behaviour and the co-ordination of the dopant Cr cations. A series of transformations: γ-Al2O3 → δ-Al2O3 → θ-Al2O3 → α-Al2O3 was observed for Cr2O3-doped alumina samples between 500 °C and 1100 °C. Rapid grain growth occurred at temperatures close to 1100 °C. The electron spin resonance (ESR) spectra for the sample heat-treated at 500 °C corresponded to the resonance of β-Cr3+ in an amorphous Cr oxide impregnated onto the γ- and δ-alumina support. The change of ESR spectrum indicated the existence of Cr3+, suggesting the formation of a solid solution with the same structures as δ-Al2O3 and/or θ-Al2O3 at 800–1000 °C. The evaluation of catalytic activities for model exhaust was performed under lean-burn (an excess of oxygen) condition of air/fuel ratio A/F = 18 and space velocity SV = 100 000 h–1 . The modified Al2O3 catalyst heat-treated at 1000 °C in air showed removal conversion of 100% for hydrocarbon (C3H6), 92% for CO and 5% for NO at 550 °C. Present results suggest that Cr-modification to Al2O3 leads to catalytic improvement with good thermal durability.

X-ray and neutron small angle scattering from amorphous chromium–carbon–hydrogen-alloys

Amorphous Cr 80C 10H 10 and Cr 80C 10D 10 films were produced by electrodeposition and investigated by X-ray and neutron small angle diffraction. The surface scattering was separated by the variation of the thickness (X-rays) and by the immersion technique (neutrons). The amorphous specimen consists of a Cr-matrix which contains segregates in which the H atoms are replaced by C atoms or vice versa. The diameter of the segregates is 12 nm and the specimen consists of about 25 vol.% segregates. A phase separation in two amorphous phases, one consisting of a C-enriched chromium-carbide, the other of pure Chromium, occurs after annealing the alloys at 200°C for a few minutes. Longer annealing time causes a continuous crystallization of the amorphous Cr-phase. The correlation length for the amorphous Cr-phase is approximately 7 nm. The event of phase separation may explain the increase in hardness and corrosion resistance of the alloys observed in previous research.

Fabrication of grain-oriented Cr 2O 3 ceramics

CrO(OH) powders with thin hexagonal plate particles have been prepared by the hydrothermal treatment (380°C/2 h/15.2 MPa) of sol-gel-derived amorphous Cr 2O 3 powders. Densification has been performed by hot-pressing (1350°C/2 h/30 MPa), followed by pressureless sintering (1600°C/2 h) in carbon powders. Grain-oriented Cr 2O 3 ceramics (>98% of theoretical) are fabricated, in which the 〈0 0 1〉 axis is parallel to the hot pressing direction. They exhibit an anisotropy in electrical conductivity; the values measured at 700°C in the perpendicular and parallel directions for the 〈0 0 1〉 axis are 9.36 and 1.09 S m −1, respectively.