Coherency strain induced hardening in bulk polycrystalline ceramics: A case study with MgO-based “ceramic alloys”

The role of coherency strain at the matrix/precipitate interface toward hardening of bulk polycrystalline “ceramic alloys” has been established here. Formation of “near ideal” bulk polycrystalline ceramic microstructure characterized by the presence of uniformly distributed coherent “ultra-fine” MgCr₂O₄ particles (size: ~25 nm) within matrix (MgO) grains was achieved via solid-state precipitation during aging treatment of bulk supersaturated MgO–Cr₂O₃ solid solutions (formed during pressureless sintering in air, followed by fast cooling). The as-aged MgO–MgCr₂O₄ “ceramic alloys” exhibited hardness increment by ~73% over that of phase pure bulk MgO upon aging for just 10 hours at 1000°C in air. Evidences toward the presence of significant coherency strains across the MgO/MgCr₂O₄ coherent interfaces were obtained with transmission electron microscopy. Analysis based on hardening mechanisms and comparisons with MgO–MgFe₂O₄ system, having lesser hardening due to lower misfit strain at MgO/MgFe₂O₄ coherent interfaces (despite greater content of second-phase particles), confirm the dominant role of coherency strains toward hardness enhancement in “ceramic alloys.”     

The reaction between the magnesia–chrome brick and the molten slag of MgO–Al2O3–SiO2–CaO–FetO and the resulting microstructure

The reaction and microstructure at the interface of MgO–Cr₂O₃ brick and the molten slag of MgO–Al₂O₃–SiO₂–CaO–Fe_tO after static slag corrosion at 1823–1923 K for various times and the resulting microstructure were investigated and characterized. After the static slag corrosion at 1923 K for 4 h, the XRD results show the major phases of periclase MgO, MgCr₂O₄ spinel, and CaMgSiO₄ as the minor phase. MgCr₂O₄ phase causes MgO to form a discontinuous phase in MgO–Cr₂O₄ brick. After static slag corrosion at 1923 K for 4 h, SEM micrographs show that brick interior cracks, MgO and dissolved MgO. MgO dissolved due to the molten plag penetrated into the brick interior and reaction with it, leading to a localized dissolution of brick slag. TEM micrographs and ED patterns demonstrate that the minor phase of (Mg, Fe)(Al, Cr)₂O₄ precipitates in the MgCr₂O₄ matrix.     

Enhanced thermal conductivity and flexural strength of sintered reaction-bonded silicon nitride with addition of (Y0.96Eu0.04)2O3

Sintered reaction-bonded silicon nitride (SRBSN) with high thermal conductivity was obtained using (Y_0.96Eu_0.04)₂O₃ and MgO as sintering additives. Green compacts were nitrided at 1400°C for 4 h. Post-sintering was carried out at 1850 and 1900°C for 4 h, respectively. In reaction-bonded silicon nitride (RBSN) doped with Y₂O₃ and MgO, the β-Si₃N₄ content and nitridation degree were 51.1% and 93.8%, respectively. However, the β-Si₃N₄ content and nitridation degree were 72.6% and 96.7% in a nitrided compact doped with (Y_0.96Eu_0.04)₂O₃ and MgO. After post-sintering, the phase composition, microstructure, mechanical properties, and thermal conductivity were investigated. After sintering at 1900°C for 4 h, the thermal conductivity of SRBSN doped with (Y_0.96Eu_0.04)₂O₃ and MgO was increased by 16.5% compared to that of the samples doped with Y₂O₃ and MgO. The highest hardness of 1639 HV and the good flexural strength of 776.4 MPa were also achieved in the sample doped with 2-mol.% (Y_0.96Eu_0.04)₂O₃ and 5-mol.% MgO.     

A study of masses based on chemically synthesized materials in the system MgO-Cr2O3

Conclusions Additions in the form of TiO₂ or ZrO₂ are recommended in order to obtain effective sintering of the chemically synthesized composite MgO + MgCr₂O₄, obtained from the thermal decomposition of magnesium chromate. The articles prepared in accordance with the new process based on dense-sintered clinker have excellent physicochemical properties. The experimental tap hole blocks made using the clinker process showed 1.5 times greater resistance than that of fused periclase blocks when tested at the Nizhnetagilsk Metallurgical Combine.Translated from Ogneupory, No. 2, pp. 35–38, February, 1984.     

Synthesis and investigation of magnesium chromium spinel

Magnesium chromium spinel nanopowders have been synthesized through cocrystallization. The formation and structural perfection of the MgCr₂O₄ crystalline phase have been investigated in the temperature range 500–1400°C. The average size of crystals of the MgCr₂O₄ phase has been determined at different temperatures. The influence of the temperature, porosity, and reducing medium on the electrical conductivity of the MgCr₂O₄ phase has been studied.     

Fabrication of reaction-bonded Cr2O3 ceramics

Reaction-bonding to form Cr₂O₃ can be achieved by gaseous oxidation of a Cr phase. The reaction-bonding process is best conducted by complete oxidation before sintering. Below ≈800 °C, the activation energy for oxidation is 220 kJ mol⁻¹, indicating the predominance of Cr³⁺ outward diffusion along high diffusivity paths, e.g., grain boundaries and dislocations. At higher temperatures, the activation energy is reduced to 52 kJ mol⁻¹ as a result of oxygen transport along lower-energy paths, e.g., along microcracks, and the internal and external surfaces. In spite of the decrease in activation energy, the access of oxygen to the inside of the powder compact is hindered by the progressive densification of the oxidizing powder compacts. Maximum densification is achieved for fully oxidized Cr/Cr₂O₃ compacts when the oxygen partial pressure is close to that of the Cr-Cr₂O₃ equilibrium. 0.1 wt% MgO addition increases the density and reduces the grain size of the reaction-bonded Cr₂O₃ samples due to the possible formation of the spinel phase MgCr₂O₄. ZrO₂ and MgO additions improve the fracture strength and toughness of conventionally sintered Cr₂O₃ and change its fracture mode from intergranular to intragranular. For reaction-bonded Cr₂O₃ samples with or without MgO addition, their fracture strength and toughness data are roughly the same as those of sintered Cr₂O₃ doped with ZrO₂ and MgO and their fracture surfaces are predominantly intragranular.     

Surface modification of sintered magnesium oxide (MgO) with chromium oxide (Cr2O3) by pulsed laser irradiation in air and liquids

Surface modification of ceramic materials by laser irradiation is widely used as a non-contact, fast and thermally activated process to generate micro and nanostructures. The effects of liquids while surface modification by laser irradiation of ceramic materials under liquid environment are least explored so far. This study reports the effects of pulsed laser irradiation in air and liquids on the microstructure and morphologies of ceramic materials. Chromium oxide (Cr₂O₃) was mixed in different concentrations (3, 5 and 7% in weight) into magnesium oxide (MgO) matrix and was sintered at 1650 °C. The structure and morphology of the sintered ceramic pellets were characterized using X-ray diffraction and scanning electron microscopy. Presence of the spinel magnesium chromium oxide (MgCr₂O₄) was identified in these samples. For surface modification of these samples, laser irradiation is carried out in air and liquids (methanol, isopropyl alcohol and acetone) using 2 ns pulsed lasers (532 nm) of different pulse repetition rates and energies. The microstructure and morphologies of the samples after irradiation was analyzed and their crystalline structure and composition were maintained after laser irradiation. It was observed that the surface morphologies of the ceramic pellets were modified by laser irradiation as a combined effect of the medium (air/liquids), energy fluence and the concentration of the Cr₂O₃ in MgO. Our results show that pulsed laser irradiation especially in liquids is an effective technique for modifying surface morphology of ceramic materials.     

The effect of alumina-based sintering aid on the microstructure,selected mechanical properties,and coefficient of friction of Cf/SiC composite prepared via spark plasma sintering (SPS) method

C_f-SiC air brake discs are being developed due to their high-temperature oxidation resistance compared to conventional C_f/C discs. The C_f-SiC air brake discs should have a coefficient of friction (COF) close to 0.4, a low wear rate, a density higher than 95% of the theoretical density, and flexural strength of more than 200 MPa. To reach the properties of C_f-SiC composite to the required characteristics of the air brake disc, different amounts of alumina-based sintering aid were used. For this purpose, first silicon carbide nanoparticles, sintering aids Al₂O₃–MgO, MgAl₂O₄, Al₂O₃–Y₂O₃, Al₂O₃–SiO₂–MgO, and carbon fiber (20 wt%) with a 5-mm length were prepared. Next, the final composite bulk was created via the SPS method at 1900 °C under a pressure of 50 MPa. The density of the sample sintered with the Al₂O₃–SiO₂–MgO sintering aid was higher than that of other sintering aids. The density value was obtained at 98% and 100% at 8 wt% and 4 wt% respectively. It was also found that the use of 4 wt% of Al₂O₃–SiO₂–MgO offered better mechanical properties compared to 8 wt%, due to the absence of Al₈Si₄O₂₀ phase at 4 wt%. The examination of mechanical properties showed that the hardness (3564 Vickers) and flexural strength (479 MPa) of the sample with the Al₂O₃–SiO₂–MgO sintering aid were higher than those of other sintering aids. The samples with the Al₂O₃–SiO₂–MgO sintering aid with 4 wt% revealed a COF of 0.41, showing the closest feature to the desired indices of aircraft brake discs.     

Effect of raw material composition on microstructures and properties of microporous MgO–Mg(Al,Fe)2O4 refractory aggregates

Microporous MgO–Mg(Al, Fe)₂O₄ refractory aggregates were prepared by the in-situ decomposition synthesis method using the magnesite, Al(OH)₃ and Fe₂O₃ as raw materials. The effect of raw material composition (theoretical Mg(Al, Fe)₂O₄ contents were 0–55 wt %) on their microstructure and strengths was investigated. When the theoretical Mg(Al, Fe)₂O₄ contents were relatively low (0–5.5 wt %), the number of neck connections between the particles in the microporous MgO–Mg(Al, Fe)₂O₄ refractory aggregates was small. As the theoretical Mg(Al, Fe)₂O₄ contents increased to be 11–22 wt %, the number of neck connections increased and the compressive strengths were enhanced. When the theoretical Mg(Al, Fe)₂O₄ contents increased to be excessive (33–55 wt %), the inter-particle pore size further increased due to the increase of volume expansion caused by the formation of more spinel, resulting in a decrease of compressive strength. Overall, when the theoretical Mg(Al, Fe)₂O₄ contents were 11–22 wt %, the microporous MgO–Mg(Al, Fe)₂O₄ refractory aggregates showed the excellent performances with the median pore sizes of 17.37–25.46 μm, the apparent porosities of 23.4–28.1%, the bulk densities of 2.57–2.79 g/cm³ as well as the compressive strengths of 41.2–75.8 MPa.     

Synergistic effect of bialkali metal chlorides promoted magnesia on oxidative coupling of methane

Upon promoting MgO (prepared via a sol-gel process) with any binary mixture of the alkali metal chlorides, catalytic systems are obtained which are more active, selective and much more stable with time-on-stream than the respective monoalkali promoted MgO in the oxidative coupling of methane (OCM) to C₂ hydrocarbons. The best catalytic performance is obtained over (5 mol% NaCl+5 mol% CsCl)/MgO, which exhibits a C₂ yield of 19.7% compared to 5.9 and 4.1% over 10 mol% NaCl/MgO and 10 mol% CsCl/MgO, respectively, at atmospheric pressure, a temperature of 750 °C, a space velocity of 15000 cm³ g^–1 h^–1, = 608 Torr and CH₄/O₂ = 4. A series of different combinations among the five alkali chlorides were made and the afore-mentioned synergistic effect was always observed. The basicity and base strength distribution of the bialkali chloride systems (measured by the gaseous acid adsorption or benzoic acid titration methods) are significantly higher than those of the respective monoalkali halide systems. The relationship between the catalytic performance and basicity/base strength distribution is explored.     

Effects of Sintering Aids on the Transparency and Conversion Efficiency of Cr4+ Ions in Cr: YAG Transparent Ceramics

Tetravalent chromium‐doped Y₃Al₅O₁₂ ceramics were fabricated by solid‐state reactive sintering method using high‐purity Y₂O₃, α‐ Al₂O₃, and Cr₂O₃ powders as the starting materials. CaO and MgO were co‐doped as the sintering aids. The effects of TEOS and divalent dopants (CaO and MgO) on the optical qualities, the conversion efficiency of Cr⁴⁺ ions, and the microstructure evolutions of 0.1 at.% Cr⁴⁺: YAG ceramics were investigated. Fully dense, dark brown colored Cr⁴⁺: YAG ceramics with an average grain size of 3.1 μm were achieved. The in‐line transmittance of the as‐prepared ceramic at 2000 nm was 85.3% (4 mm thick), and the absorption coefficient at 1030 nm (the characteristic absorption peak of Cr⁴⁺ ions) was as high as 3.7 cm^?1, which was higher than that of corresponding single crystals fabricated by Czochralski method.     

Action of iron melts on spinel-periclase refractories

Conclusions Using methods of phase, chemical, x-ray, and petrographic analysis we investigated the reaction between melts of Fe₃O₄ and MgFe₂O₄ with certain compositions on the basis of the systems MgO-Al₂O₃-Cr₂O₃.The phase inversions with the action of the melt consist in the development of solid solutions between the spinel phase and the iron reagent.Destruction by the iron melt of the spinel phases in the system MgO-Al₂O₃-Cr₂O₃ is due to the laminating scaling of the solid solutions, expecially those enriched by MgCr₂O₄ which in a large degree is expressed in the MgCr₂O₄ itself. The cause of this is that owing to the sharp differences in the composition of the zones as a result of bad wetting of MgCr₂O₄ by the iron melt, apparently, we get substantial stresses which produce lamination of the scaly growths.In the system MgO-Al₂O₃-Cr₂O₃ the lower resistance to iron melts is possessed by compositions enriched with MgCr₂O₄.The free magnesia exerts a positive effect on the resistance of the spinels to the action of iron melts. However the worst properties, as previously, are shown by compositions enriched with MgCr₂O₄.The melt of MgFe₂O₄ gives rise to substantial volume changes in the reaction zone, and to a lower degree, the same destruction of the refractories as the magnetite melts.Our practical conclusions for the technology of refractories is to express a preference for the compositions with substantial preponderence of periclase over spinels, of which, in turn, the magnesia-alumina spinel should be preferred to compositions of the type periclase-chromite, when we are considering the thermal resistance of magnesia refractories working in conditions of iron-reagent action.     

Effect of titanium dioxide on sintering and stabilization of ZrO2 in zirconium-alumina and spinel-zirconium mixtures

Conclusions The addition of titanium dioxide considerably reduces the sintering point of alumina and zirconia-alumina mixtures. Titanium dioxide does not have this effect on the sintering of the ternary equimolecular mixture ZrO₂MgO Al₂O₃. But the zero porosity was not even attained during firing up to 1700°. When the three-component mixture ZrO₂ + MgO + Al₂O₃ was sintered, spinel formed first, after which, at a higher temperature, there formed a solid solution ZrO₂ and MgO. In the presence of titanium dioxide some of the magnesium oxide apparently forms magnesium titanate with the TiO₂, and this impairs the stabilization of the zirconium dioxide, in view of which it is partially detected in monoclinic form in the fired mixtures. The addition of 2% TiO₂ reduces the temperature of polymorphous transitions of ZrO₂ by approximately 200°.Specimens of the composition 90% Al₂O₃ + 10% ZrO₂ and ZrO₂MgOAl₂O₃=111 show better spalling resistance than those made of alumina and those made of zirconium dioxide stabilized with magnesium or calcium oxide.Pure pre-synthesized spinel does not react with zirconium dioxide when fired up to 1600°, nor with titanium dioxide up to 1500°.The coefficient of thermal linear expansion of the equimolecular mixtures ZrO₂-MgO-Al₂O₃ and ZrO₂-CaO-Al₂O₃ is considerably lower than that of the corresponding mixtures without alumina.When the three component equimolecular mixtures ZrO₂-CaO-Al₂O₃ is sintered, calcium aluminate and the solid solution ZrO₂-CaO are formed.The two-component compositions Al₂O₃-ZrO₂ and three-component MgO-Al₂O₃-ZrO₂ have high refractoriness, satisfactory spalltng resistance, good stability-underload at high temperatures, and can be used as super duty refractories.     

A comparison of the effect of nanostructured MgCr2O4 and FeCr2O4 additions on the microstructure and mechanical properties of direct-bonded magnesia-chrome refractories

The effect on the microstructure and mechanical properties of direct-bonded magnesia-chrome refractories of additions of nanostructured MgCr₂O₄ and FeCr₂O₄ is reported. The nanostructured additives, synthesized by the citrate-nitrate route and calcined at several different temperatures, were characterized by XRD, BET and TEM. Additions of 0.5 and 1 wt % of these nanostructured oxides were made to magnesia-chrome refractories and calcined at 1650 ^OC in a shuttle kiln. Their microstructures were analyzed by SEM/EDX and their physical and mechanical properties (permanent linear change (PLC), bulk density, apparent porosity, cold crushing strength (CCS) and hot modulus of rupture (HMOR) were determined according to the respective DIN standards. The addition of the nanostructured oxides to the magnesia-chrome refractories facilitated the formation of secondary spinels, influencing the physical and mechanical properties. FeCr₂O₄ additions increased the size of the secondary spinel due to liquid phase formation in the presence of magnetite impurities in the FeCr₂O₄ nano-powder. The addition of nano-sized MgCr₂O₄ and FeCr₂O₄ to the base formulation of the refractory increased the CCS from 67.4 MPa to 82.8 MPa and 81.0 MPa respectively, while nano-sized MgCr₂O₄ increased the HMOR value from 5.48 MPa to 5.91 MPa and nano-sized FeCr₂O₄ increased the HMOR from 5.48 MPa to 5.72 MPa. This smaller increase than that obtained with FeCr₂O₄ additions is attributed to liquid phase formation in the presence of magnetite, as observed by XRD.     

The Structure of Fusion-Cast High-Chrome Oxide Refractories in the Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> - MgO - Al<Subscript>2</Subscript>O<Subscript>3</Subscript> System

The phase composition and structure of fusion-cast refractories composed of 57.0 – 84.2% Cr₂O₃, 4.3 – 36.1% MgO, 2.0 – 9.7% Al₂O₃, and 2.4 – 6.9% SiO₂ have been studied by petrographic and x-ray spectral microprobe analysis methods. Refractories high in MgO with modulus M = (Cr₂O₃ +Al₂O₃)/MgO = 1.64 – 3.1 are shown to consist of spinel phase Mg(Cr, Al)₂O₄ and silicate glass. Refractory materials (80.8 – 84.2% Cr₂O₃, 4.3 – 4.7% MgO, 2.0 – 9.7% Al₂O₃, and 2.7 – 6.9% SiO₂ with M = 18.7 – 20.2) are three-phase systems composed of spinel, escolaite, and glass phase. These materials, owing to their high corrosion resistance, have promising potentiality for practical applications.__________Translated from Novye Ogneupory, No. 12, pp. 69 – 74, December, 2004.     

Selective synthesis of alcohols from syngas and hydroformylation of ethylene over supported cluster-derived cobalt catalysts

Hydroformylation of ethylene and CO hydrogenation were studied over cobalt-based catalysts derived from reaction of Co₂(CO)₈ with ZnO, MgO and La₂O₃ supports. At 433 K a similar activity sequence was reached for both reactions: Co/ ZnO > Co/La₂O₃ > Co/MgO. This confirms the deep analogy between hydroformylation and CO hydrogenation into alcohols. In the CO hydrogenation the selectivity towards alcohol mixture (C₁-C₃) was found to be near 100% at 433 K for a conversion of 6% over the Co/ZnO catalyst; this catalyst showed oxo selectivity higher than 98% in the hydroformylation of ethylene. Magnetic experiments showed that no metallic cobalt particles were formed at 433 K. It is suggested that the active site for the step that is common to both reactions is related to the surface homonuclear Co²⁺/[Co(CO)₄]^– ion-pairing species.     

Functionalization of nano-MgCr2O4 additives by silanol groups: a new approach to the development of magnesia-chrome refractories

This work describes a novel approach to improve the dispersion of a nanoparticle MgCr₂O₄ additive in the matrix of magnesia-chrome refractories by functionalizing the nanoparticle surfaces with silanol groups. The effect of the silanol groups on the nanoparticles within the refractory matrix was shown by FTIR spectroscopy, zeta potential measurements, UV–visible spectroscopy, Dynamic Light Scattering (DLS) and TEM to result in a decrease of the average particle size of the MgCr₂O₄ additive by about 50% due to de-agglomeration of the nano particles by the silanol groups. Silanol functionalization of the nano-additives in the magnesia-chrome refractories fired at 1600 °C resulted in a 65 Kgf/cm² increase in their compressive strength, while the hot modulus of rupture and corrosion resistance values of these refractories fired at 1400 °C were similar to those containing unfunctionalized nano-additives fired at 1600 °C. XRD and SEM results suggest the improvement in corrosion resistance is related to the formation of spinel phases in the refractories containing silanol-modified nano-additives.     

Silicothermal synthesis and densification of x-sialon in the presence of metal oxide additives

The effect is reported of seven inorganic oxide additives on both the formation mechanism and the densification of X-sialon prepared by a silicothermal process. The oxides were added to the starting mixture of halloysite clay, alumina and elemental silicon at a level of 1 wt% of the calculated final product, and fired in nitrogen at 1200–1500°C. The formation of X-sialon was monitored by thermal analysis, powder XRD and ²⁷Al and ²⁹Si solid state MAS NMR. The effects of the additives are temperature dependent, and influence the various stages of the reaction by differing degrees. The oxides which best promote the formation of crystalline X-sialon (Y₂O₃, CaO and MgO) are also those which facilitate the conversion of initially-formed Si₃N₄ to SiO₂N₂ and SiO₃N units, the latter being particularly enhanced by Y₂O₃, Fe₂O₃ enhances the initial nitridation of Si but suppresses X-sialon formation by stabilising the preceding mullite phase. Densification is most enhanced by Y₂O₃, CaO and CeO₂; MgO exerts its maximum effect on sintering at lower temperatures. The beneficial influence of MgO and Y₂O₃ on both X-sialon formation and sintering is due to the formation of liquid phases.     

Impact on aggregate/matrix bonding when a refractory contains zinc aluminate instead of spinel and magnesia-chrome

The high hot strength of MgO–Cr₂O₃ refractory is often ascribed to its intimate aggregate/matrix bonding. For a fundamental comparison with it, ∼2 mm aggregates of MgO and Al₂O₃ were separately embedded in ZnAl₂O₄ and MgAl₂O₄ matrices, sintered at 1600°C, and examined. It was found that similarity of thermal expansion coefficient (TEC) between the aggregate and the matrix is critical to achieve good bonding and this is more important than the extent of interdiffusion. The TEC mismatch of ≥5.7 × 10⁻⁶ K⁻¹ caused significant undesirable debonding in MgO aggregate/MgAl₂O₄ matrix sample and MgO/ZnAl₂O₄ despite >736 μm Zn²⁺ diffusion depth in the latter. Direct bonding, as inferred from a thicker interfacial reaction layer and a greater shift of the aggregate/matrix interface before and after firing, was better in MgAl₂O₄/ZnAl₂O₄ combination, followed by tabular Al₂O₃/ZnAl₂O₄ and Al₂O₃/MgAl₂O₄. Powder X-ray diffraction indicated that the volatilization of ZnAl₂O₄ at 1600°C in air was negligible compared to MgO–Cr₂O₃.     

Interaction of MgAl2O4 and MgCr2O4 spinels with calcoferruginous melts

Conclusions The interaction was studied between MgAl₂O₄ and MgCr₂O₄ spinels (and their solid solutions and compositions with periclase) with calcoferruginous melts, which are the essential constituents of various slag reagents.On exposure to a highly basic calcoferruginous melt, the magnesium-aluminum spinel forms magnesio-ferrite, brownmillerite, and some calcium aluminate.On interaction of magnesiochromite with calcium ferrite, chiefly high-temperature compounds form.With the action of highly ferruginous melts on magnesiochromite, the resulting solid solution increases in volume, and a distortion is observed at the boundary of the reaction and little-change zones of the samples.The addition of granular MgO to spinels or their solid solutions improves the chemical stability of the compositions against calcoferruginous melts [4].During use in contact with highly basic calcoferruginous melts, the addition of periclase to chromium spinellide additives is recommended; for use in contact with highly ferruginous melts, periclase and aluminum spinellide compositions are recommended.Translated from Ogneupory, No. 7, pp. 44–47, July, 1969.