Cr2O3 Solid Solutions Research Articles

Synthesis and reversible oxidation/reduction behavior of delafossite-type CuCr1−x Al x O2

The oxygen storage capacity (OSC) and oxidation–reduction properties of CuCr1−x Al x O2 with the delafossite-type structure were investigated. Solid solutions with the delafossite-type structure were synthesized by a solid-state reaction route at 1100–1150 °C in N2 atmosphere. The substitution of Al3+ for Cr3+ in CuCrO2 suppressed OSC and enhanced its stability to heat treatments under cyclic oxidative/reductive atmosphere. Under 5% H2/He atmosphere, the delafossite phase was reduced to a mixture containing Cu and a corundum-type Cr2O3 solid solution (x = 0–0.3). In the region with x = 0.5–0.9, metallic Cu and delafossite phase coexisted. Following repetitive reduction by H2-pulse injection, a transition alumina phase was detected. The transformation between delafossite- and spinel-type structures can occur without significant diffusion of atoms, and therefore, the cyclic oxidation/reduction of CuCr1−x Al x O2 delafossite is reversible. These results suggested that the reduction of CuCr1−x Al x O2 is the key step in the reversible oxygen storage/release process.

In situ (Al,Cr)2O3-Cr composite coating fabricated by reactive plasma spraying

In situ (Al,Cr)2O3-Cr composite coating was fabricated by reactive plasma spraying Al-Cr2O3 composite powder. Phase composition and microstructure of the as-sprayed coating was characterized by X-ray diffractometer, scanning electron microscope and transmission electron microscope. Indentation fracture behavior of the coating was investigated. It is found that the microstructure of the coating consists of [(Al,Cr)2O3+Cr] eutectic and Cr+[(Al,Cr)2O3+Cr] hypoeutectic. The eutectic microstructure of the coating shows the characteristic of nano structure, in which Cr particles with size less than 40nm distribute in (Al,Cr)2O3 solid solution matrix. The hypoeutectic microstructure of the coating consists of Cr particles with size larger than 1µm and [(Al,Cr)2O3+Cr] divorced eutectic. The toughness of the (Al,Cr)2O3-Cr composite coating is significantly improved compared with that of the conventional Al2O3 coating.

Cast ceramics by metallothermic SHS under elevated argon pressure

Cast ceramic composites were prepared by metallothermic SHS under elevated Ar pressure (5 MPa). Variation in green composition was used to affect the phase composition of combustion products. Under some optimized conditions, resultant ceramics can be obtained in the form of Al2O3–Cr2O3 solid solutions, Al2O3–Cr2O3 ∙ хZrO2 composites or single-phase Al2MgO4. Such materials seem promising for use in jewelry, process engineering (casting molds, cutting tools), and aerospace industry.

Ferroelectromagnetic solid solutions on the base piezoelectric ceramic materials for components of micromechatronics

In the presented work, a ferroelectromagnetic solid solutions based on PZT and ferrite powders have been obtained. The main aim of combination of ferroelectric and magnetic powders was to obtain material showing both electric and magnetic properties. Ferroelectric ceramic powder (in amount of 90%) was based on the doped PZT type solid solution while magnetic component was nickel–zinc ferrite Ni1−xZnxFe2O4 (in amount of 10%). The synthesis of components of ferroelectromagnetic solid solutions was performed using the solid phase sintering. Final densification of synthesized powder has been done using free sintering. The aim of the work was to obtain and examine in the first multicomponent PZT type ceramics admixed with chromium with the following chemical composition Pb0.94Sr0.06(Zr0.46Ti0.54)O3+0.25at% Cr2O3 and next ferroelectromagnetic solid solution based on a PZT type ferroelectric powder (Pb0.94Sr0.06(Zr0.46Ti0.54)O3+0.25at% Cr2O3) and nickel–zinc ferrite (Ni0.64Zn0.36Fe2O4), from the point of view of their mechanical and electric properties, such as: electric permittivity, ε; dielectric loss, tanδ; mechanical losses, Q−1; and Young modulus, E.

Hot corrosion behavior of Al2O3–Cr2O3 refractory by molten glass at 1200 °C under static condition

Hot corrosion behavior of Al2O3–Cr2O3 refractory by molten glass at 1200°C under static condition was monitored. Glass-refractory interface was investigated by XRD, SEM and EDX analysis. After 36h of testing, no visible change was observed, but SEM analysis revealed a corroded region of 315μm. Corrosion occurs via dissolution of Al2O3 followed by mass exchange and formation of Fe2O3–Cr2O3 solid solution, which acts as a barrier to the molten glass. However, the Cr2O3 fractions of the refractory remained uncorroded.

Oxygen storage–release behavior of delafossite-type CuCr1−x M x O2 (M = Fe, Ga)

The oxygen storage capacity (OSC) and oxidation–reduction properties of delafossite-type CuCr1−xMxO2 (M = Fe and Ga) were investigated. The solid solutions having 3R-type delafossite-type structure were synthesized by a solid-state reaction method at 860–1100 °C in N2 atmosphere. Substitution of Fe3+ and Ga3+ into Cr3+ site of CuCrO2 improved their OSC and stability under cyclic oxidative/reductive atmosphere. CuCr0.5Fe0.5O2 and CuCr0.3Ga0.7O2 exhibited the largest OSC for M = Fe and Ga, respectively. The thermogravimetry in an oxidative atmosphere and XRD analyses revealed that CuCr1−xMxO2 oxidized to Cu(Cr1−xMx)2O4 spinel and CuO. Under 5 % H2/He atmosphere, CuCr1−xFexO2 was reduced to Cu and (Cr, Fe)3O4 spinel phase, which was eventually reduced to Fe and Cr2O3. In the M = Ga system, the delafossite phase was reduced to Cu + Cr2O3 solid solution (x = 0–0.1) or Cu + Ga2O3 solid solution (x = 0.9–1.0). In the region x = 0.3–0.7, metallic Cu was formed, coexisting with the delafossite phase, without the formation of Cr and Ga oxides. These results suggest that oxygen release behaviors were attributed to reductive decomposition for CuCr1−xFexO2 and deposition of Cu from the delafossite phase maintaining the structure for CuCr1−xGaxO2 (x = 0.3–0.7).

Thermo-mechanical behavior of Al2O3–Cr2O3 refractories: Effect of TiO2

Dense Al2O3–Cr2O3 refractories having equimolar ratio of Al2O3 and Cr2O3 were developed by reaction sintering of calcined alumina and chromium(III) oxide in the temperature range 1600–1700°C under reducing atmosphere. Effects of TiO2 on the Al2O3–Cr2O3 refractories were investigated with respect to densification, thermal expansion, microstructure, flexural strength at different temperatures and thermal shock resistance behavior. XRD studies revealed the presence of only one phase (Al2O3–Cr2O3 solid solution) in all the sintered samples. Addition of TiO2 imparts better densification and grain growth. Samples having 1wt% TiO2 exhibit optimum flexural strength at room temperature as well as at elevated temperatures. Thermal shock resistance investigation from 1200°C to room temperature revealed that samples having 1wt% TiO2 show a diminutive increase in flexural strength after 6th cycle of thermal shock. The retained flexural strength of samples having 1wt% TiO2 after 12th cycle of thermal shock is much higher (101MPa) than that of TiO2 free samples (56MPa).

Formation of cubic structured (Al1 − xCrx)2 + δO3 and its dynamic transition to corundum phase during cathodic arc evaporation

Mixed aluminum–chromium oxide coatings in the form of (Al,Cr)2O3 solid solutions have attracted extensive research interest during the past years due to their successful use for challenging wear applications and the comparative ease of their moderate-temperature deposition by physical vapor deposition techniques. During our research into the reactive cathodic arc deposition of this type of coating, we found a previously unobserved transition between two crystalline aluminum–chromium oxide structures. During the early growth stage, films arc-deposited from Al0.55Cr0.45 targets form a first zone, that was found to contain exclusively the metastable cubic fcc-(Al1−xCrx)2+δO3 phase. This kinetically favored phase is reproducibly followed by the growth of a second zone made of the initially expected corundum phase, α-(Al1−xCrx)2+δO3, as observed by TEM. This dynamic transition has a significant effect on the film properties. XPS studies and structural data show that the formation of fcc-(Al1−xCrx)2+δO3 with a (200) preferred orientation arises from the initial presence of a metastable monoxide (M1−xO) film, which is stabilized by the incorporation of metal vacancies (31%) in the B1 structure. However, as the thickness of coating increases, the thermodynamic aspect becomes more important as compared to kinetics and leads to a loss of structural stability in the cubic layer, which is a kinetically favored phase. As a result, the system will transform into the metastable corundum α-(Al1−xCrx)2+δO3, which is thermodynamically more stable than the cubic phase. In this paper, formation of fcc-(Al1−xCrx)2+δO3 and its transformation to corundum phase are discussed in detail with respect to the structural and electronic properties of the different phases.

Oxidation of Cr2AlC coatings in the temperature range of 1230 to 1410 °C

The isothermal oxidation behavior of Cr2AlC coatings on alumina substrates was investigated in the temperature range of 1230 to 1410°C. The structure, surface morphology, microstructure evolution and chemistry of the reaction products have been investigated. In the investigated temperature range, the Cr2AlC films form a dense continuous oxide scale consisting of α-Al2O3 on Cr carbides. The oxidation rates determined by thermo gravimetric analysis (TGA) were parabolic, indicating that diffusion through the scale is the rate limiting mechanism. The activation energy for oxidation was determined to be 348kJmol−1 and the parabolic rate constant at 1230°C was 7.1×10−10kg2m−4s−1. Hence, the oxidation behavior is comparable to NiAl in the temperature range and time intervals investigated. With increasing oxidation time voids form at the interface between oxide and Cr carbides and the amount of Cr7C3 increases at the expense of Cr3C2. Based on our thermodynamic calculations the oxygen partial pressure below the oxide scale increases as Al is depleted and Cr carbides oxidize, resulting in CO gas- and Cr2O3-formation. The formation of gas may together with the depletion of Al and Cr lead to the significant void formation observed in the Cr carbide interlayer. Observation of both Cr carbide precipitates and the formation of (Al,Cr)2O3 solid solution support this notion. For comparison bulk Cr2AlC was oxidized. It is argued that the absence of pores in oxidized bulk Cr2AlC is due to the considerably larger amount of Al available.

A theoretical investigation of α-Fe2O3–Cr2O3solid solutions

We have examined the thermodynamic stability of alpha-Fe(2)O(3)-Cr(2)O(3) solid solutions as a function of temperature and composition, using a combination of statistical mechanics with atomistic simulation techniques based on classical interatomic potentials, and the addition of a model magnetic interaction Hamiltonian. Our calculations show that the segregation of the Fe and Cr cations is marginally favourable in energy compared to any other cation distribution, and in fact the energy of any cation configuration of the mixed system is always slightly higher than the combined energies of equivalent amounts of the pure oxides separately. However, the positive enthalpy of mixing is small enough to allow the stabilisation of highly disordered solid solutions at temperatures of approximately 400 K or higher. We have investigated the degree of cation disorder and the effective cell parameters of the mixed oxide as functions of temperature and composition, and we discuss the effect of magnetic interactions and lattice vibrations on the stability of the solid solution.

Intercluster porous nanocomposites from combination of [AlO4Al12(OH)24(H2O)12]7+ cation and Anderson-type [Al1−x Cr x Mo6O24H6]3− anion: XRD, TG-DTA, SEM-EDAX and EPR studies

The synthesis of intercluster porous nanocomposites obtained from polyoxometallate compounds such as the [AlO4Al12(OH)24(H2O)12]7+ cation (named Al13) and the Anderson-type [Al1−x Cr x Mo6O24H6]3− anion (named Al1−x Cr x Mo6) has been performed in order to study the interaction between the two cluster ions, the stability of the XMo6 planar configuration, the products obtained after thermal treatment, the structure and the local symmetry of the Cr3+ species. Chemical, thermal, structural and spectroscopic characterizations of the original and thermally treated phases have been followed by different techniques such as TG-DTA, XRD, SEM-EDAX, and mainly by EPR. All the results have shown that the structure of the intercluster nanocomposites (Al13)(Al1−x Cr x Mo6)2 precursors is similar to that reported by Son et al. for the chromium-free (Al13)(AlMo6)2 intercluster nanocomposite [Son et al., J. Am. Chem. Soc. 122 (2000) 7432]. After thermal treatment in air at several temperatures of the (Al13)(Al1−x Cr x Mo6)2 nanocomposites the following phases have been observed and characterised: (i) at 400 °C an amorphous phase containing dispersed Cr3+ ions; (ii) at 700 °C a crystalline phase corresponding to Cr2(MoO4)3/Al2(MoO4)3 solid solutions; (iii) at 950 °C α-Al2O3/Cr2O3 solid solutions with a random dispersion of the Cr3+ ions.

Ultrafine Cr-Doped Al2O3 Prepared by Detonation Synthesis

Data are presented on the phase composition, particle size distribution, EPR, and luminescence of ultrafine chromium-doped alumina powders prepared by detonation synthesis. The largest particles in the powders are shown to consist mainly of a Cr2O3 solid solution in α-Al2O3. The luminescence spectrum of fine particles shows, in addition to lines characteristic of ruby, extra lines which are tentatively attributed to Cr3+ ions incorporated substitutionally into θ-Al2O3.

Corrosion behavior of Inconel 690 and 693 in an iron phosphate melt

The corrosion resistance of Inconel 690 and 693 coupons submerged in an iron phosphate melt has been investigated. After 155 days in an iron phosphate melt at 1050°C, which contained 30wt% of a simulated low activity waste (LAW at Hanford), the weight loss of Inconel 690 and 693 was 14% and 8%, respectively. The overall corrosion rate, calculated from the initial and final dimensions of each coupon, was 1.3 and 0.7μm/day for the Inconel 690 and 693, respectively. Scanning electron microscopy and X-ray diffraction of the submerged Inconel coupons after 155 days in the iron phosphate melt showed that an altered surface layer had formed which was depleted in nickel and consisted of a (Fe, Cr)2O3 solid solution. This altered layer appears to be chemically protective as indicated by the gradual reduction in weight loss which occurred with time in the iron phosphate melt. Inconel 693 appears to be a better candidate to use as an electrode in iron phosphate melts since its corrosion rate and weight loss was only about one half that of Inconel 690.

The influence of aliovalent impurities on the oxidation kinetics of nickel at high temperatures

The influence of chromium and sodium on the nickel oxidation kinetics has been studied as a function of temperature (1373–1673K) and oxygen activity (10−105Pa O2), using microthermogravimetric techniques. It has been shown that the oxidation of Ni–Cr and Ni–Na alloys, like that of pure nickel, follows strictly the parabolic rate law being thus diffusion controlled. In agreement with the defect model of Ni1−yO, it has been found that the oxidation rate of the Ni–Cr alloy is higher than that of pure nickel, the reaction rate is pressure independent and the activation energy of this process is lower. This implies that the concentration of double ionized cation vacancies in a Ni1−yO–Cr2O3 solid solution is fixed on a constant level by trivalent chromium ions, substitutionally incorporated into the cation sublattice of this oxide. In the case of the Ni–Na alloy, on the other hand, the oxidation rate is lower than that of pure nickel, the activation energy is higher and the oxidation rate increases more rapidly with oxygen pressure. These results can again be explained in terms of the doping effect, by assuming that univalent sodium ions dissolve substitutionally in the cation sublattice of nickel oxide.

Standard Gibbs Free Energy of Formation of MnO-saturated MnO Cr2O3 Solid Solution at 1873K.

The standard Gibbs free energy of formation of MnO-saturated MnO.Cr 2 O 3 solid solution (stoichiometric MnO.Cr 2 O 3 ), which is one of important thermodynamic properties of the MnO-SiO 2 -CrO x inclusion system to clarify the equilibrium relation among molten stainless steel and the inclusions formed during the stainless steel Si-deoxidation process, was determined at 1 873 K. By equilibrating molten iron or copper with MnO-saturated MnO.Cr 2 O 3 crucible under controlled oxygen partial pressure, P O2 , the standard Gibbs free energy changes of the following reactions at 1 873 K were determined to be: Mn (mass%, in molten Fe)+2Cr (mass%, in molten Fe)+2O 2 (g) =MnO.Cr 2 O 3 (MnO-satd.): ΔG°/kJ=-755±4 (P O2 =2×10 -6 -6×10 -5 Pa) Mn (mass%, in molten Cu)+2Cr (mass%, in molten Cu)+2O 2 (g) =MnO.Cr 2 O 3 (MnO-satd.): ΔG°/kJ=-807±7 (P O2 =3×10 -5 -1.5×10 -4 Pa) Mn (I)+2 Cr (s)+2 O 2 (g)=MnO.Cr 2 O 3 (MnO-satd.): ΔG o f ,MnO.Cr 2 O 3 /kJ.mol -1 =-958±8 (PO 2 =2×10 -6 -1.5×10 -4 Pa) The values of ΔG° and ΔG° f ,MnO.Cr 2 O are independent of the oxygen partial pressure within the P O range in the present study.

Oxidation-induced degradation of strength of tungsten carbide-cobalt

Mixed aluminum–chromium oxide coatings in the form of (Al,Cr)2O3 solid solutions have attracted extensive research interest during the past years due to their successful use for challenging wear applications and the comparative ease of their moderate-temperature deposition by physical vapor deposition techniques. During our research into the reactive cathodic arc deposition of this type of coating, we found a previously unobserved transition between two crystalline aluminum–chromium oxide structures. During the early growth stage, films arc-deposited from Al0.55Cr0.45 targets form a first zone, that was found to contain exclusively the metastable cubic fcc-(Al1 − xCrx)2 + δO3 phase. This kinetically favored phase is reproducibly followed by the growth of a second zone made of the initially expected corundum phase, α-(Al1 − xCrx)2 + δO3, as observed by TEM. This dynamic transition has a significant effect on the film properties. XPS studies and structural data show that the formation of fcc-(Al1 − xCrx)2 + δO3 with a (200) preferred orientation arises from the initial presence of a metastable monoxide (M1 − xO) film, which is stabilized by the incorporation of metal vacancies (31%) in the B1 structure. However, as the thickness of coating increases, the thermodynamic aspect becomes more important as compared to kinetics and leads to a loss of structural stability in the cubic layer, which is a kinetically favored phase. As a result, the system will transform into the metastable corundum α-(Al1 − xCrx)2 + δO3, which is thermodynamically more stable than the cubic phase. In this paper, formation of fcc-(Al1 − xCrx)2 + δO3 and its transformation to corundum phase are discussed in detail with respect to the structural and electronic properties of the different phases.

Thermodynamics of Oxygen in Liquid Fe-Cr Alloy Saturated with FeO Cr2O3 Solid Solution.

The equilibrium relation between dissolved Cr and O in liquid high Cr steel has been assessed on the condition of pure solid Cr 2 O 3 saturation in our previous paper. Following this, the present work deals with the equilibrium between Cr and O in liquid iron saturated with FeO.Cr 2 O 3 solid solution and phase equilibria among liquid Fe-Cr alloy and Cr 2 O 3 containing oxides for the full understanding of the thermodynamic behavior of oxygen in liquid Fe-Cr alloy. The free energy of formation of FeO.Cr 2 O 3 and activities of the constituents in FeO.Cr 2 O 3 solid solution have also been measured by the chemical equilibrium technique at 1 823 to 1 923 K. The free energy of formation of FeO.Cr 2 O 3 was given by the following equation. Fe(s)+1/2 O 2 (g)+Cr 2 O 3 (s)=FeO.Cr 2 O 3 (s) ΔG=-307 600+66.82T (±6 800) (J/mol) (1 423 K<T<1 923 K) The activities of Fe t O(l) and Cr 2 O 3 (s) in FeO.Cr 2 O 3 solid solution exhibit negative deviation from ideality. The oxide phase in equilibrium with liquid Fe-Cr alloy is not Cr 2 O 3 but FeO.Cr 2 O 3 solid solution when Cr content in metal is less than the critical Cr content of approximately 7 mass%. The present experimental results are in good accord with the thermodynamic relation between dissolved Cr and O calculated by the parameters which have been proposed in our previous work, if appropriate correction for Cr 2 O 3 activity is applied.

The effect of Cr on the solubility of Al in orthopyroxene: experiments and thermodynamic modelling

The partitioning of chromium and aluminium between coexisting orthopyroxene and spinel in equilibrium with forsterite in the system MgO–Al2O3–SiO2–Cr2O3 (MAS–Cr) has been experimentally determined as a function of temperature, pressure and Cr/(Cr + Al) ratio. Experiments were conducted at temperatures between 1300 and 1500 °C and at pressures from 5 to 54 kbar. Previous experimental results on the (Al, Cr)2O3 and Mg(Al, Cr)2O4 solid solutions have been combined with the present results plus relevant data from the CMAS system to derive a thermodynamic model for Al–Cr-bearing orthopyroxenes, spinels and corundum–eskolaite solid solutions. The orthopyroxene solid solution can be modelled within the accuracy of all experimental constraints as a ternary solid solution involving the components Mg2Si2O6 ( E), MgAl2SiO6 (M) and MgCr2SiO6 (C), in which the activities are related to composition through the equations:

In situ formation of composite microstructures

SummaryMicrostructural evolution in two in situ reinforced composite systems, one produced by partial reduction and the other by grain growth, has been examined over a range of size scales using light microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Metal‐ceramic matrix composites were formed by the partial reduction of Al2O3–Cr2O3 solid solutions to form Cr metal particles in an Al2O3–Cr2O3 matrix which had a lower Cr2O3 concentration. In a second system, grain growth of Si3N4 during liquid phase sintering produced large, whisker‐like grains in a fine‐grained Si3N4 matrix, bonded by an oxynitride phase. The mechanisms controlling microstructural evolution in these two systems were examined.

Activity–Composition Relations in Refractory Oxide Solid Solutions at High Temperatures: The System Cr2O3–Al2O3

Activity–composition relations in Al2O3–Cr2O3 solid solutions at 1500° and 1600°C were determined by equilibrating members of this solid‐solution series with Mo–Cr alloys of known activity–composition relations and a gas phase of known oxygen potentials. The oxide solid solution shows considerable positive deviation from ideality.