Optical absorption and EPR studies of borate glasses with PbCrO4 and Pb2CrO5 microcrystals

The optical absorption (in the 400–1000 nm region) and electron paramagnetic resonance (EPR) spectra of 50PbO-20Cr₂O₃-(30-y)B₂O₃-yAl₂O₃ glasses and those crystallized by suitable thermal treatments have been studied. The results are applied to account for the crystallization process, which revealed the crystalline products due to PbCrO₄ and Pb₂CrO₅, and crystallization yield was found to be drastically enhanced (by as much as 85 vol%) with Al₂O₃ additives. The addition of Al₂O₃ in the present glasses and/or the thermal treatments induce the optical absorption intensity in the visible region at the expense of the near infrared region and reflect a gradual decrease in the EPR linewidth, H, at g2. The chromium in these cases most likely exists in a thermodynamic equilibrium between Cr³⁺ and Cr⁶⁺ oxidation states and the Al₂O₃ favours the Cr³⁺Cr⁶⁺ nucleation transformation to accord with the above observations. In addition, the crystalline PbCrO₄ and Pb₂CrO₅ comprise CrO ₄ ^2– groups with the Cr⁶⁺ oxidation state of chromium. This presents a phenomenological correlation for a local symmetry similarity of the CrO ₄ ^2– groups in glass and the microcrystalline products and observation of an enhanced crystallization yield due to Pb₂CrO₅ crystallites in the glasses using Al₂O₃ additives. The chromium occupying the tetragonal CrO ₄ ^2– anion sites migrates rather easily from the glass (subject to the thermal treatment) to the growing nucleation sites of PbCrO₄/Pb₂CrO₅, whereas those Cr³⁺ usually occupying the octahedral sites, which are associated with characteristically large stabilization energy, hardly play a direct role in the crystallization.     

Corrosion in H2/H2S mixtures of a Ni77Cr16Fe7 alloy previously covered with an external Cr2O3 layer

A thermodynamic and kinetic study of sulphidization of Ni₇₇Cr₁₆Fe₇ alloys — previously covered with a Cr₂O₃3 layer — was made in a H₂/H₂S mixture, under a wide range of sulphur pressures (10^–4 atm <p(H₂S) < 10^–1 atm) and at temperatures between 700 and 1000° C. The mechanism for sulphur penetration through the external chromium oxide and the competition between Cr₂O₃ reduction and CrS formation were studied with the help of thermogravimetry and analytical techniques. The essential role of chromium carbides appears to be to modify the distribution of sulphur in the structure and to lower the critical energy for sulphide formation. Two growth laws were found to apply for external layer formation, one for the industrial form of the alloy and the other for a purer synthetic form. The alteration in mechanical properties of the external sulphidized zone were used to show the initial softening effect of chromium depletion, then the hardening effect of sulphide precipitation.     

Synthesis, structure and exchange bias in Cr2O3/CrO2/Cr2O5 particles

We report on the synthesis, structure and magnetic properties of a novel exchange bias system with Cr₂O₃/CrO₂/Cr₂O₅ interfaces. Chromium oxide particles with mixed chromium valences were prepared by sintering CrO₃ in air. X-ray diffraction patterns show that CrO₃ lost its oxygen gradually with increasing temperature and time through Cr₃O₈, Cr₂O₅, CrO₂, and finally Cr₂O₃ at temperatures above 760 K. X-ray photoelectron spectra indicate a low CrO₂ content and a binding energy of 579.3 eV for Cr 2p_3/2 photoelectrons in Cr₂O₅. Chromium dioxide was found to stably coexist with Cr₂O₃ and Cr₂O₅ in the particles. Magnetic measurements show hysteresis loop shifts in the sample, indicating an exchange bias induced by antiferromagnetic Cr₂O₃/Cr₂O₅ in ferromagnetic CrO₂. An exchange bias of 9 mT at 5 K and a coercivity of 26.3 mT were observed in the chromium oxide particles containing CrO₂.     

EPR and optical absorption of Cr3+ in binary Na2O-B2O3 glasses

The optical absorption and EPR spectra of Cr³⁺ in binary sodium borate glasses have been studied as functions of chromium concentration and Na₂O/B₂O₃ ratio in the glass; the ligand field and EPR parameters have been calculated and were found to be independent of Cr₃₊ concentration in any particular glass. In low-alkali borate glasses (Na₂O = 11 or 14 mol %) a single symmetrical EPR line was observed with g _¦=g =1.984±0.001 corresponding to perfect octahedral symmetry of the Cr³⁺ ion in these glasses. With increasing Na₂O content of the glass, the EPR line becomes more asymmetric (characteristic two-peaked pattern); this has been explained as being due to axial elongation of the six co-ordinated Cr³⁺-complex in these glasses.     

Phase relationships in the system Cr-W-O and thermodynamic properties of CrWO4 and Cr2WO6

The phase diagram of the Cr-W-O system at 1000° C was established by metallographic and X-ray identification of the phases present after equilibration in evacuated silica capsules. Two ternary oxide phases, CrWO₄ and Cr₂WO₆ were detected. The oxygen potential over the three-phase mixtures, W+Cr₂O₃ s+CrWO₄, WO_2.90+CrWO₄+Cr₂WO₆ and Cr₂O₃+CrWO₄+Cr₂WO₆, were measured by solid state cells incorporating Y₂O₃ stabilized ZrO₂ electrolyte and Ni+NiO reference electrode. The Gibbs' energies of formation of the two ternary phases can be represented by the following equations $$\begin{gathered} W(s) + \tfrac{1}{2} Cr_2 O_3 (s) + \tfrac{5}{4} O_2 (g) \to CrWO_4 (s) \hfill \\ \Delta G^0 = - 172 047 + 48.725T ( \pm 230) cal mol^{ - 1} \hfill \\ Cr_2 O_3 (s) + WO_3 (s) \to Cr_2 WO_6 (s) \hfill \\ \Delta G^0 = - 3 835 + 0.235{\rm T} ( \pm 500) cal mol^{ - 1} \hfill \\ \end{gathered}$$      

Role of chromium ion valence states in ZnO-As2O3-Sb2O3 glass system by means of spectroscopic and dielectric studies

ZnO-Sb₂O₃-As₂O₃ transparent glasses containing small concentrations of chromium ions (introduced as Cr₂O₃) ranging from 0 to 0.2 mol% is prepared. A number of studies viz., XRD, SEM, DTA, optical absorption, FT-IR, Raman, ESR spectra, magnetic susceptibility and dielectric properties (constant ?′, loss tan δ, ac. conductivity σ_ac over a wide range of frequency and temperature as well as dielectric breakdown strength at room temperature) of these glasses have been carried out as a function of chromium ion concentration. The results have been analysed in the light of different oxidation states of chromium ions. The analyses indicates that when the concentration of chromium ions is low, these ions mostly exist in Cr⁶⁺ and Cr⁵⁺ states, occupy network forming positions with CrO₄²⁻ and CrO₄³⁻ structural units respectively and increase the rigidity of the glass network. When the concentration of chromium ions is gradually increased, these ions seem to be existing mostly in Cr³⁺ state.     

Solid-state thermal transformations in mixtures of platinum(II) tetraammine with ammonium chromate and ammonium dichromate

Thermal transformations of 3[Pt(NH₃)₄]Cl₂–(NH₄)₂CrO₄ (I) and 3[Pt(NH₃)₄]Cl₂–2(NH₄)₂Cr₂O₇ (II) mixtures in a solid state have been studied by thermal analysis and mass spectrometry in an inert (Ar) atmosphere and air at temperatures from 40 to 550°C. The chemical and phase compositions of the thermolysis products obtained in argon have been determined: Pt and Cr₂O₃ in mixture I and Pt, a platinum-based Pt_{1 – y} Cr _y solid solution, and Cr₂O₃ in mixture II. The thermal transformation products of mixtures I and II in air consist of two phases: Pt and Cr₂O₃. It has been shown that interactions in the mixtures proceed through the formation of a metallic chromium phase. The materials obtained in this study can find application as catalysts for low-temperature fuel cells, as well as for organic synthesis processes.     

Formation of Composite Powders and Ceramics from β-SiC-Cr2O3-C Mixtures

The phase-formation process in the disperse -SiC + SiO₂ + Cr₂O₃ + C system treated in vacuum at 1273, 1473, 1673, and 1873 K was studied by X-ray phase analysis, IR spectroscopy, EPR, and other methods. It was found that at treatment temperatures of 1273, 1473, and 1673 K, the major process is carbothermal reduction of Cr₂O₃ and SiO₂ with the formation of chromium carbosilicide and chromium carbide (Cr₃C₂). At T = 1873 K, on the surface of the SiC particles, metastable chromium silicate forms on the base of SiO₂ and Cr₂O₃. The reduction of Cr₂SiO₄ by silicon carbide is accompanied by the formation of CrSi₂, Cr₅Si₃, and chromium carbosilicide. The phase composition of the products of interaction and their distribution over the volume of the composite powder and ceramic material are determined by the dispersion composition of the starting SiC powder and by the degree of development of reduction processes in local volumes of the disperse and compacted material.     

Rapidly quenched sodium tungsten bronzes

Rapid quenching was applied to the sodium tungsten bronzes, Na _x WO₃, of differentx values. The crystallization processes of the coloured flakes obtained were analysed by differential thermal analysis and X-ray powder diffraction. It was found that an amorphous bronze phase is present in the quenched Na_0.71WO₃, whereas quenched Na_0.84WO₃ was composed of tungsten crystal and Na₂O-WO₃ glass. This difference is explained by the following equilibrium present in the melt of Na _x WO₃: 3xNa₂WO₄+(6–4x)WO₃+xW6Na _x WO₃ This equilibrium is shifted to the right whenx is small (x<0.7), while it is shifted to the left whenx is large (x>0.8).     

Insights into structure and high-temperature oxidation behavior of plasma electrolytic oxidation ceramic coatings formed in NaAlO<Subscript>2</Subscript>–Na<Subscript>2</Subscript>CrO<Subscript>4</Subscript> electrolyte

Plasma electrolytic oxidation (PEO) ceramic coatings were formed on Ti₂AlNb alloy in various NaAlO₂ electrolytes containing 2 g L^?1, 4 g L^?1, and 6 g ^?1 Na₂CrO₄ additive, respectively. The influence of Na₂CrO₄ additive in NaAlO₂ electrolyte on structure and high-temperature oxidation behavior at 800 °C was investigated. The Na₂CrO₄ additive in the NaAlO₂ electrolyte not only promotes the formation of γ-Al₂O₃ phase and densification of ceramic coatings, but also participates directly in the growth of ceramic coating to form new Cr₃O and (Al_0.948Cr_0.052)₂O₃ phases. The PEO ceramic coatings formed in NaAlO₂ electrolytes with 2 g L^?1 and 4 g L^?1 Na₂CrO₄ additive show better oxidation resistance than those PEO coatings formed in a NaAlO₂ basic electrolyte based on isothermal oxidation tests at 800 °C up to 150 h. A thin and uniform isothermally oxidized layer is formed in the interlayer adjacent the substrate, which protects the substrate from the inward diffusion of oxygen and the outward diffusion of metallic elements. The PEO ceramic coatings formed in NaAlO₂ electrolyte with 4 g L^?1 Na₂CrO₄ additive exhibit the least mass gain among all the specimens, which is only a half of the ceramic coating formed in a NaAlO₂ basic electrolyte without any Na₂CrO₄ additive.     

Liquidus Relations in the Na2WO4–LiPO3–WO3System

The liquidus relations in the Na₂WO₄–LiPO₃–WO₃system (diagonal section of the quaternary mutual system Li,Na||PO₃,WO₄,WO₃) were studied by thermal analysis at WO₃contents of 60 mol %. The results demonstrate that, in the composition region studied, the liquidus surface comprises the primary-crystallization fields of Na₂WO₄, LiPO₃, and the congruently melting compounds Na₂WO₄· WO₃, 3LiPO₃· WO₃, Na₂WO₄· NaPO₃, and 2Li₂WO₄· LiPO₃. The low-melting compositions revealed in the system studied are of interest for the preparation of Li _x Na _y WO₃bronzes.     

IR and Raman studies and effect of γ radiation on crystallization of some lead borate glasses containing Al2O3

IR and Raman spectra of glass ceramics based on PbO-Cr₂O₃-B₂O₃glass composition system have been studied. The bands characteristic of BO₃. and BO₄ functional groups are present in all the samples. An incorporation of Al₂O₃(up to 5 mol%) in the initial glass composition considerably changes the glass network structure and relative concentrations of BO₃ and BO₄ groups. The composition 50 PbO-20 Cr₂O₃-25 B₂O₃-5 Al₂O₃(in mol%) reveals a maximum fraction of boron in the BO₄ group. A sample of this composition heat treated at 850° C for 25 h shows a maximum crystallization fraction with Pb₂O · CrO₄ as a prominent crystalline phase. The glasses irradiated with rays inhibit the crystallization into the Pb₂O · CrO₄ phase. They also show relatively smaller thermal conductivity.     

Structural and physical characterization of the Li2O∶P2O5∶CrO1.5 ion-conducting glasses

Li₂OP₂O₅CrO_1.5 glasses containing 40, 50 and 60 mol% Li₂O and a varying P₂O₅-to-CrO_1.5 ratio have been synthesized. The glass-transition temperature and density increase as P₂O₅ is progressively replaced by CrO_1.5. The glasses, being green in colour and showing strong optical absorption at 450 nm and 660 nm, contain predominantly Cr^{3 +} ions and a relatively small amount of Cr^{6 +} ions. The proportions of phosphate species with one, two, and three non-bridging oxygen atoms are determined from Raman spectroscopy results. From X-ray photoelectron spectroscopy (XPS) binding energies of the Li 1s, P 2p, Cr 2p and O 1s core levels are determined. The O 1s spectrum is found to consist of two peaks which are attributed to the P=O, P-O-P, P-O, P-O-Cr, and Cr-O oxygen species. Conductivity of these glasses increases with increasing Li₂O content and with progressive replacement of P₂O₅ by CrO_1.5. The 0.60Li₂O O.32P₂O₅0.08 CrO_1.5 glass is found to have the highest conductivity, 3.2×10^–6–1 cm^–1 at 25 °C. Compositional dependence of the conductivity is discussed in relation to variations in the glass structure.     

Physico-chemical properties of chromium sesquisulphide

The range of stoichiometry and defect structure of chromium sesquisulphide, Cr₂S₃, have been determined for a range of temperature (1123 to 1273 K) and sulphur pressure (1.06×10^–1 to 1.77×10⁴ N m^–2) employing chemical analysis, marker and thermoelectric power measurement techniques. It has been shown that the upper limit of stoichiometry corresponds to CrS_1.54, while the lower limit extends to at least CrS_1.30. The conditions (temperature and sulphur pressure) under which the compound is stable at compositions intermediate to these values have been determined for S/Cr ratios at intervals of 0.02.It has been established that Cr₂S₃ is ann-type metal-excess compound within these composition limits, containing unassociated trivalent chromium interstitials and/or trivalent chromium interstitials associated with one quasi-free electron, as the predominant mobile defect species.     

Characterization and hot corrosion performance of LVPS and HVOF-CoNiCrAlYSi coatings

The CoNiCrAlYSi coatings were produced by using low vacuum plasma spray (LVPS) and high velocity oxy-fuel (HVOF) techniques on the Inconel-738. Hot corrosion behavior and microstructure characterization was investigated by exposing the sample to a molten film of Na₂SO₄-20? wt NaVO₃ at 880 °C for up to 560 h. The hot corrosion rate was determined by measuring the weight gain of the specimens at regular intervals for a duration of 20 h. The result of weight change measurements showed better hot corrosion resistance for HVOF-CoNiCrAlYSi coatings. This was attributed to the α-Al₂O₃ nucleation during the HVOF coating process and replacement of non-protective oxide due to a fluxing mechanism. It was also observed that the non-protective and porous oxides such as (Co,Ni)Al₂O₄ and (Co,Ni)Cr₂O₄ were formed on the both types of coatings due to large β-depleted zone after long exposure time of hot corrosion testing.     

Alumina-chromium carbide composite through an internal synthesis method

In situ formation of chromium carbide particles, through a solid state reaction between Cr₂O₃ and SiC, for strengthening AI₂O₃ has been studied. Three kinds of chromium compound, Cr₃Si, Cr₃C₂ and Cr₇C₃ and mullite were formed in the alumina matrix. The reaction behaviour during hot pressing depends on heating parameters such as temperature and atmosphere. In a vacuum environment, the Cr₃Si particles formed first and was the dominant dispersed phase below 1550°C, while the Cr₇C₃ phase was only dominant above 1600°C. The Cr₃C₂ phase emerged briefly then diminished at temperature 1500°C. In an argon environment, however, the Cr₃C₂ phase was the main product component at temperatures ranging from 1450–1550 °C. The mullite phase formed concurrently through the diffusion of SiO₂ phase into the Al₂O₃ matrix, which is a by-product from the reaction between Cr₂O₃ and SiC. Incorporating chromium carbide or suicide particles into the Al₂O₃ matrix induces a strengthening effect. However, only when the content of dispersed phases is low and is mainly of Cr₃C₂ particles, is the strengthening effect significant. For instance, the composite, containing 5 vol% chromium carbide and hot-pressed at 1500°C in argon, gives a flexural strength and fracture toughness up to 600 MPa and 6.1 MPam^1/2, respectively.     

Breakdown of the protective oxide on 11 % Cr steel at high temperature in the presence of water vapor and oxygen,the influence of chromium vaporization

Abstract

We report on the effect of water vapor and oxygen on the oxidation of a ferritic/martensitic 11 % Cr steel (CrMoV11 1). The influence of pH₂O, exposure time, gas velocity and temperature was investigated. The samples were exposed to dry O₂, O₂+10 or 40 % H₂O for up to 336 hours. Total pressure was 1 atm (1.02 × 10⁵ Pa). The gas velocity was between 0.05 and 10 cm/s while temperature was in the range 450–700°C. The samples are investigated by thermogravimetry, GI-XRD, SEM/EDX, GDOES, FIB and TEM/EDX. Oxidation is strongly affected by the vaporization of CrO₂(OH)₂ in H₂O/O₂ environment. The mechanism of vaporization of CrO₂(OH)₂ from a Cr₂O₃ surface is modelled by DFT calculations. In the absence of chromium vaporization the alloy forms a protective oxide consisting of a corundum-type solid solution (Fe_1–xCr_x)₂O₃. The vaporization of chromium tends to deplete the oxide in chromium. In some cases the oxide remains protective in spite of chromium depletion while in other cases there is a transition to breakaway oxidation. In the latter case a thick layered scale forms, consisting of an outer hematite part and an inner iron-chromium spinel. Oxidation behavior in an O₂+H₂O environment is to a large extent determined by the ability of the metallic substrate to supply the oxide with chromium by diffusion in order to compensate for the losses by vaporization. The corrosivity of the environment increases with the concentration of water vapor and oxygen, with the gas velocity and with temperature.     

Electrical Properties of WO3-Based Ceramics

The electrical properties of WO₃ceramics with Na₂O, MnO₂, and Al₂O₃additions were investigated. The effects of postsintering heat treatment, electrode material, and NiO and Co₃O₄additions were examined.     

Chemical and structural properties of the system Fe2O3-Cr2O3

Chemical and structural properties of the mixed metal oxides (1–x)Fe₂O₃+xCr₂O₃ were studied by different techniques. X-ray powder diffraction showed the existence of solid solutions, (Fe_1–x Cr _x )₂O₃, over the whole concentration region, 0x1. The gradual replacement of Fe³⁺ with Cr³⁺ ions in samples prepared at 900°C caused changes in unit-cell parameters; most of these changes took place in the region fromx0.3–0.9. The samples having the fraction of Cr₂O₃ in the region from 0.7–0.8, contained two closely related phases, with slightly different compositions. After an additional heat treatment at 1100°C, these samples contained only one phase.⁵⁷Fe Mössbauer spectroscopy showed a gradual decrease of hyperfine magnetic field with increasing Cr₂O₃ content. The sample having the fraction of Cr₂O₃ of 0.7, and prepared at 900°C, exhibited two separated sextets at room temperature, in comparison with other compositions showing one sextet. It was shown that Fourier transform infrared (FT-IR) spectroscopy is a powerful method for the investigation of structural changes in these solid solutions. The increase in the Cr₂O₃ content resulted in shifts of the corresponding infrared bands. In addition, a gradual transition of the spectrum typical for -Fe₂O₃ to the spectrum typical for Cr₂O₃ was shown. The transition effects observed in the FT-IR spectra were correlated with the X-ray powder diffraction and⁵⁷Fe Mössbauer spectroscopic results.