Corundum Crystals Research Articles

In situ boro-mullite phase formation in Glass–Ceramic glazes: Anatase–rutile phase transformations and microstructure properties

‘‘Boro-mullite’’ is the name for mullites produced through the synthesis of boron-rich material and playing a critical role in producing high-performance materials. Also, the stability of the anatase phase in the glaze and against temperature has been addressed and improved by many studies. In situ boro-mullite crystals generated using various glass-ceramic glaze compositions that contain TiO2 and their impact on the transformation of anatase into a rutile phase were examined in this study. Additionally, the samples' water absorption, bulk density, apparent porosity and shrinkage, Vickers hardness, and optical properties were evaluated at various Al2O3 and B2O3 amounts at 700 °C, 800 °C, 900 °C, 1000 °C and 1100 °C. X-ray diffraction analysis revealed that as the temperature and Al2O3 ratio rose, it triggered the boro-mullite phase formation. On the other hand, increasing the Al2O3 ratio led to a rise in the samples' sintering temperature and delayed the anatase to rutile phase transformation. Furthermore, the presence of corundum crystals retarded the anatase crystal size growth during the remodelling process of the conversion of anatase to rutile. While the increase in the B2O3 ratio sharpened the peaks of albite crystals, the formation of rutile crystals accelerated with the increase in temperature and caused inhibition of albite formation. The albite phase's presence facilitated the creation of the liquid phase and achieved the 0.010 % water absorption and 0 % apparent porosity values as well the highest bulk density and microhardness with 2.476 g/cm3 and 550 H V. The yellowness (+b∗) values were found to be the highest at 1100 °C where the rutile crystals reached their maximum size. The anatase-rutile conversion is postponed and the increase in the yellowness value is prevented by the glaze's formation of boro-mullite. FESEM-EDS analysis showed that the temperature was the driving force in the transformation of acicular boro-mullite crystal into highly acicular prismatic needle crystal and spherical anatase crystals into the rod shape rutile.

Micro-Inclusions in Corundum from Puesto La Peña Complex Ouachitites, Mendoza, Argentina: A Window to the Mantle

Abstract Corundum xenocrysts from ouachititic dikes related to the ultrapotassic Puesto La Peña Complex, Mendoza, Argentina, contain inclusions of oxides, silicides, and silicates. Oxides identified are hibonite, carmeltazite, and as yet unnamed minerals consisting of ZrTiO3, (Al,Ti,Zr) oxide, and (Mn,REE) oxide. An unnamed Ti2(Mn,Fe)4Si5 phase was also recognized. Silicates include wodegongjieite and grossmanite. All minerals are enriched in rare earth elements. The formation of these mineral inclusions under extreme P-T conditions would have occurred in the primeval Earth, preserved in the lithospheric mantle and protected from oxidation by crystallization of corundum through the introduction of mantle-derived magma-transported fluids. The corundum crystals would have been transported by a rising plume in a back-arc extensional setting during the Miocene and incorporated as xenocrysts in an alkaline ouachititic magma, conferring its peraluminous signature.

Prospects of corundum crystals application as Cherenkov radiators

The work investigates the spectral transmittance of corundum (Al2O3) crystal plates before and after irradiation by an electron beam. It reveals the fact that even with a significant irradiation time, the spectral transparency of the plates changes slightly. Moreover, it demonstrates that for a wavelengths of ∼300 nm, the transparency of the sample remains almost stable, while for shorter wavelengths brightening of the corundum plate is obtained. It is concluded that the use of corundum crystals as Cherenkov radiators for detecting of charged particles beams is prospective.

Investigation of the effect of BaO-Al2O3 variations for BAS glass-ceramic glaze: Insights into thermal, phase, microstructural and surface features

This study aims to investigate the phase transformation, thermal behavior, microstructure evaluation and surface feature of the barium aluminosilicate (BAS) glass-ceramic glaze by changing barium oxide (BaO) and aluminium oxide (Al2O3) amounts. Differential thermal analysis (DTA) analysis showed that the first crystallization occurred at 800 °C and referred to the celsian formation, and crystallization temperature increased as Al2O3 content increased. Characteristic temperature and high thermal viscosity behavior was examined by hot-stage microscope analysis and the Vogel–Fulcher–Tammann (VFT) equation. X-ray diffraction (XRD) analysis showed that replacing Al2O3 with BaO resulted in an enhanced celsian (Ba0.8Al1.6Si2.4O8) phase. On the other hand, increasing Al2O3 content promoted the corundum (α-Al2O3) phase. The glassy and crystalline phase amount was determined by using Rietveld method. Field Emission Scanning Electron Microscope (FESEM) observations showed rectangular, strip-like celsian crystals, unshaped corundum crystals and porous microstructure. Surface roughness and the height distribution of surfaces were supported by 3D topographic mapping and determined according to ISO 4287. The highest BaO-contained R1 exhibits lower peaks with 190 µm and valleys with 80 µm, on the contrary, the highest Al2O3-contained R5 has high peaks with 252.7 µm, and deep valleys with 150 µm. Finer celsian crystals and lowest surface roughness ensured the cleanability of the surface and resulted in Class 5 according to ISO 10545-14. When Al2O3 content increased by 30 wt% and above, cleanability began to weaken, resulting in Class 1 and Class 2. The surface has become opaque with an increase in the Al2O3 content.

Microstructure and thermal shock resistance of ZrO2–Al2O3–A3S2 composites for casting filter materials

ZrO2–Al2O3-A3S2(3Al2O3·2SiO2) composites are ideal Casting filter materials with excellent mechanical properties and thermal shock resistance. In this paper, 3Y–ZrO2 (3 mol% Y2O3-stabilized ZrO2) was added to kaolin and α-Al2O3 to prepare ZrO2–Al2O3-A3S2 composites by pressureless sintering. The results showed that when the addition amount of 3Y–ZrO2 is 15 wt%, the bending strength and fracture toughness of the composites sintered at 1600 °C are 152.51 MPa and 3.52 MPa m1/2, respectively. The bending strength increased to 161.83 MPa after 30 thermal shocks (1100 °C to room temperature), with an increase rate of 6.11 %. The ZrO2 crystals are evenly distributed between corundum and mullite crystals, effectively preventing abnormal growth of corundum and mullite crystals, and a small number of pores exist in the samples, which could act as a “buffer zone” to prevent the expansion of cracks in the thermal shock, resulting in high mechanical strength and excellent thermal shock resistance for composites.

Vibrational properties of OH groups associated with divalent cations in corundum (α-Al2O3)

Abstract. The infrared spectra of synthetic corundum (α-Al2O3) samples either doped directly with divalent cations (Mg2+) or containing divalent cations formed by reduction of trivalent cations in H2 gas (Co2+, Ni2+) may display broad OH stretching bands at ∼3000 cm−1 due to the structural incorporation of trace amounts of hydrogen. Experimental spectra recorded from some natural sapphires display a similar absorption band associated with a dominant absorption at 3161 cm−1, and some beryllium-diffused corundum crystals show a band at 3060 cm−1. All of these also display smaller and generally narrower bands between 1900 and 2700 cm−1, whose natures are poorly defined. In this work, the atomic-scale structure, relative stability and infrared spectroscopic properties of a series of OH defects in corundum (α-Al2O3) are theoretically investigated at the density-functional-theory level. The investigated defects consist of interstitial H+ ions forming OH groups and compensating for the charge imbalance related to the presence of divalent cations (Be2+, Mg2+, Cr2+, Mn2+, Fe2+, Co2+, Ni2+) substituted for Al3+ at nearby octahedral sites. Bands occurring at ∼3000 cm−1 in experimental spectra are assigned to the OH stretching modes of some of these defects, with bands observed around 1900 and 2700 cm−1 being assigned to overtones of corresponding OH bending modes. The results also support the assignment of the so-called “3161 cm−1 series”, observed in experimental spectra of some rubies and yellow sapphires, to structural OH groups in association with Fe2+ ions, rather than Si4+, as has been previously proposed. These inferences are also supported by analysis of correlations between band areas in experimental infrared spectra extracted from a database of corundum gemstones. A qualitative explanation relating the anomalous intensity and the polarisation properties of the OH bending overtone bands to the electrical anharmonicity of OH groups involved in medium-strength H bonds is proposed.

Hovhannisyan, M. A., Summary of the Results of Research and Selection of Corundum Crystalsfor Development and Creation of UV and VUV Detectors

The paper summarizes the results of our research conducted on the study of corundum crystals with various activating agents for development and creation of UV and VUV detectors. Corundum crystals with certain parameters were chosen as the optimal converter of UV and VUV radiant energy into the visible region of radiation for creating UV and VUV detectors with high spatial and temporal characteristics, which also have high radiation and thermal resistance.

Cherenkov Radiators Based on Diamond and Corundum Crystals

Cherenkov Radiators Based on Diamond and Corundum Crystals

Film casting technology for aluminium oxide ceramic substrates

The paper presents the technology of aluminum oxide ceramic substrates formation by film casting using "KEKO" line (Slovenia). Components of binder (dispersion phase) including polymeric bonding agent, plasticizer, dispersant, solvent are presented. The following binder components were used: polymeric binder - polyvinyl butyral, plasticizer - dibutyl phthalate, dispersant - fish oil and solvent - mixture of toluene and ethanol. The preparation regimes for the slurry, casting and lamination of the ceramic strip are given. According to the thermal analysis (DSC and TG) of raw corundum substrate the main mass of the removed binder occurs in the temperature range 240-450oC, the observed exo-effects in this temperature range are caused by successive processes of decomposition and removal of binder components. Based on the results of thermal analysis the regime of organic component removal from the "raw blanks-substrates" which provides for heating to a temperature of 500oC with a given rate of heating and cooling is determined. The technology of film-casting provides obtaining raw billet-substrate with a given density and the necessary technological strength. We experimentally set the firing mode of corundum substrates in vacuum (0.1-1.0 Pa) at a speed of temperature 100оС/h to 1450оС with an exposure time of 2 hours, then at a speed of 60оС/h to a maximum temperature of 1620-1660оС with an exposure time of 4 hours. The substrates were cooled at a rate of 100oC/h. The structure of the annealed substrates is represented by corundum crystals with sizes predominantly not more than 10 microns, intergrain porosity not more than 0.5%. Sintered products have high density and minimum number of defects.

Controls of crystal shape on degassing mechanisms in crystal-rich magmas with rhyolitic groundmass melts

Understanding the mechanical effects of crystals on degassing kinetics and permeability development in silicic magmas is important for modeling eruptions and examining first order controls on eruption style. We conducted high-pressure-high-temperature isothermal decompression experiments to investigate the role of crystal shape on permeability development and pore pathway geometry. Experiments were performed on hydrous rhyolitic glass (76.3 wt.% SiO2) seeded with variable amounts of equant (aspect ratio ∼1.8 ± 0.6) corundum crystals and elongate (aspect ratio ∼10 ± 5.5) wollastonite crystals to approximate natural phenocryst and microphenocryst/microlite populations, respectively. We measured total porosity, connected pore volume and permeability directly from the experimental charges by applying Archimedes' principle to determine bulk density, helium-pycnometry to measure connected porosity, and a custom-made permeameter to measure permeability. The experimental samples developed permeability at a critical melt porosity (ϕc-melt, above which degassing is enhanced due to bubble coalescence) of ∼55 vol.% vesicles for the corundum experiments and ∼48 vol.% for the wollastonite-bearing experiments; these values are considerably lower than the ϕc-melt>63 vol.% for prior crystal-free experiments (Lindoo et al., 2016; deGraffenried et al., 2019). Critical porosity is reduced when crystals comprise at least ∼20 vol.%, regardless of shape. Connected porosity increases and average bubble size decreases with increasing abundance of elongate wollastonite crystals, explained by the onset of yield strength behavior induced by loosely touching crystal frameworks that form at decreasing crystallinities with increasing elongation of crystals comprising the network. When the population of high-aspect-ratio crystals reaches random loose packing, the resulting reduction in interstitial melt available for unimpeded bubble expansion forces the bubbles to connect at lower total vesicularity. It therefore seems likely that crystal-bearing intermediate magmas experience an abrupt increase in degassing efficiency when crystallinity attains random loose packing. In hydrous magmas, the efficiency of decompression-driven degassing and resulting formation of anisotropic groundmass crystals is controlled by magma composition and decompression rate. Enhanced gas loss in slowly ascending (and crystallizing) magma may aid the development of dense conduit plugs, thus increasing the possibility of violent Vulcanian explosions.

Mechanism of Be-Thermodiffusion in Rutile Inclusions of Fancy Sapphires

The solid-state chemistry of inclusions in Al2O3 is of high relevance in several applications, spanning from photonics to materials engineering and gemology. Be-thermodiffusion is a common treatment of corundum crystals to improve their optical properties. The chemistry of such a high-temperature process is still unclear. Particularly, the interconversion between corundum host and guest inclusion species involves many chemical species and several steps, whose knowledge would guide the tuning of corundum chemical preparation. Herein, 43 thermodiffusion-treated sapphires are investigated, with a focus on 14 samples endowed with blue halos, via optical microscopy, scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDS) quantitative chemical analyses, Raman microspectroscopy, and X-ray powder diffraction. This study searches for the snapshots of the chemical path and then recomposes them to propose a novel chemical mechanism in which Be reduces TiO2 inclusion, which is then converted, mediated by iron ions, to several titanates. The chemical mechanism is discussed in terms of applications not limited to only gemology but to any derivative corundum crystal preparation.

Investigation and Selection of Corundum Crystals with Various Activated Agents for the Creation of UV Detectors of 50–400 nm Wavelengths

Investigation and Selection of Corundum Crystals with Various Activated Agents for the Creation of UV Detectors of 50–400 nm Wavelengths

Beryllium incorporation in corundum

AbstractThe structure of corundum is not amenable to accommodating beryllium, yet Be is commonly added to corundum for purposes of colour modification. Where the main sites in corundum are octahedrally coordinated, Be is tetrahedrally coordinated in all known Be minerals. To attempt to explain this discrepancy, the configurations of Be in the Be–Al–O±H system were determined using density functional theory (DFT). In the Be–Al–O system, the most likely Be configuration is in triangular coordination, replacing Al and bonded to three oxygen on the basal plane (large triangle) of the Al-vacant octahedral site. When H is added, OH groups form at the opposite face of the same octahedron, pointing either into or out of the site. Be–H (beryllium hydride) defects are also stable at the DFT level, with Be bonded to three O and one H. As some defects contain O–H or Be–H bonds, the DFT calculations can be corroborated by FTIR spectroscopy. Spectra recorded from 1000 corundum crystals with elevated Be confirm that bands centred at 3062 and 2490 cm–1correspond to Be-associated defects. Polarised spectra show that the band at 2490 cm–1resolves into two, one centred at 2491 cm–1, with a shoulder at 2523 cm–1, one of which is probably associated with a Be–H bond. The 3062 cm–1band resolves into at least three, including a major band at 3058 cm–1, a shoulder at 3140 cm–1and a small, sharp band at 3082 cm–1. Whilst there are caveats relating to curve fitting and baseline subtraction, the 3058 cm–1band can probably be attributed to the defect where H points into the Al-vacant octahedral site, and the 3140 cm–1band to the defect where the H points out. To our knowledge, this is the first occurrence of Be in triangular coordination proposed for a naturally occurring mineral. This substitution mechanism may also explain the high diffusivity of Be, as Be is effectively associated with its own Al vacancy.

Detection of hidden oxygen interstitials in neutron-irradiated corundum crystals

An α-Al2O3 (corundum) possesses unique optical, electrical and mechanical properties, demonstrates high tolerance to heavy irradiation and, in particular, is in a short list of candidates for optical/diagnostics windows in advanced fission and forthcoming fusion (DEMO) reactors. However, material functionality is strongly affected by structural defects induced by radiation of different types. Optical and magnetic characteristics of Frenkel defects (interstitial-vacancy pairs) as well as the processes of their thermal annealing (radiation damage recovery) have been investigated by means of optical absorption and the EPR methods in α-Al2O3 single crystals exposed to fast neutrons with fluence of 6.9 × 1018n/cm2. For the first time in metal oxides, a single oxygen interstitial, which is not adjacent to any other imperfection, has been detected. The analysis of the EPR spectra/parameters testifies that this interstitial forms with a regular oxygen ion a superoxide ion O2−, stabilized by a trapped hole. The interstitial becomes mobile above 500 K and recombines with a complementary oxygen vacancy that is a part of electronic F+ or F centers. The thermal annealing kinetics of the F and F+ centers has been theoretically analyzed in terms of interrelated diffusion-controlled recombination reactions of radiation defects. The analysis indicates that both, negatively charged and neutral oxygen interstitials (relevant absorption bands at 5.6 and 6.5 eV, respectively) co-exist in similar concentrations.

Effect of manganese doping of corundum on the content and diffusion of water and other gases

The content of volatile gas-liquid impurities, degassing and diffusion characteristics of Mn-doped corundum synthesized in a supercritical water fluid were determined using the method of kinetic thermodesorption mass spectrometry. Water is the main volatile impurity in the crystals of Mn-doped corundum. An increase in the concentration of manganese in corundum increases the water content in the low-temperature form and affects the high-temperature water release by the diffusion mechanism in different ways: for MnCl2-precursor, the content of the high-temperature form increases, and for KMnO4 – decreases. The introduction of manganese into corundum significantly lowers the water content in the structure compared to undoped corundum, which is a favorable factor in the production of transparent ceramics. Carbonaceous impurities (CO2, CO, hydrocarbons CxHy) are the second largest group of volatile gas-liquid impurities. With an increase in the manganese content in corundum (for both Mn precursors), the release of hydrogen decreases while the release of oxygen increases. The reduction of hydroxyls to hydrogen and the oxidation of manganese ions are most effective when the concentration of hydroxyls in the corundum structure is significantly higher than the concentration of manganese ions. The diffusion coefficients of water in corundum with various manganese concentrations and with different Mn precursors were determined. The obtained values of water diffusion coefficients and their temperature dependencies make it possible to calculate the degassing time of the material to a certain degree at any temperature.

Characteristics and formation of corundum within syenite in the Yushishan rare metal deposits in the northeastern Tibetan Plateau

Abstract Corundum is rarely found in situ within alkali syenites. A corundum-bearing syenite was found in the Yushishan rare metal deposits of the eastern section of the Altyn Tagh fault in the northeastern Tibetan Plateau, but the characteristics and formation of corundum remain unknown. We describe a corundum-bearing syenite dike emplaced in biotite plagioclase gneiss that suffered overprinted deformation with characteristics of mylonitization. The corundum crystals have variable grain sizes, and the largest ones are megacrystic with growth zoning. The corundum crystals contain a variety of mineral inclusions that are divided into primary and secondary. The primary mineral inclusions within the corundum include variable contents of Fe-Ti oxide needles, ilmenite, zircon, monazite-(Ce), potassium feldspar, pyrochlore, columbite-(Fe), magnetite, samarskite-(Y), and pyrite that indicate corundum crystallized in peraluminous Zr-rich and Si-poor alkali rock with variable TiO2 contents. Secondary mineral inclusions include Zn-rich hercynite, ilmenite, magnetite, annite, fluorapatite, and intergrowths of ilmenite with columbite-(Fe) and goethite that reveal late-stage influx of Zn-, Ti-, Fe-, and F-bearing fluids into corundum that caused metasomatism and element migration and mineral precipitation. The trace element analysis of corundum shows high-Fe and -Ga contents and low-Mg and -Cr contents that are consistent with the characteristics of corundum of magmatic origin. The trace element characteristics and the oxygen isotopes (6.2–8.2‰) results indicate that corundum crystallized in melts with the involvement of Al-rich and Si-poor crustal material.

Radiation-Induced Phenomena in Wide-Gap Laser Materials Used in High-Radiation Areas

The analysis of the surface states in experimental reflection spectra is carried out peculiarities of the formation of defects of the electronic structure of irradiation corundum crystals is considered. It is shown that change of crystals reflectional ability is related to radiative growth forced by aluminum ions. The processes of radiative growth depend on the type and energy of irradiating high-energy particles. Kinetic of that growth is presented, which relates the intensity of zones with free aluminum ions to the concentration of such zones in near-surface layer.

Effects of sintering temperature on microstructure and properties of low‐grade bauxite‐based ceramic proppant

AbstractLow density and high strength ceramic proppant was prepared by sintering high aluminium type low‐grade bauxite and high iron type low‐grade bauxite at the temperature range from 1300℃ to 1360℃. The phase composition and micromorphology of ceramic proppant were, respectively, characterized by X‐ray diffraction (XRD) and scanning electron microscope (SEM). The bulk density, apparent density, and breakage ratio of ceramic proppant were tested using standard methods. The results show that mullite crystal and corundum crystal are fully developed, the spatial network structure of rod‐shaped mullite is formed, and corundum crystal is evenly distributed in the sample when the sintering temperature is 1340℃. At this temperature, the bulk density of the sample is 1.42 g/cm3, the apparent density is 2.67 g/cm3, and the breakage ratio is 5.1% under the closure pressure of 52 MPa. Then, the growth mechanism of mullite crystal in ceramic proppant was explored. The results reveal that the mullite crystal in ceramic proppant conforms to the layer growth theory. The secondary crystal nucleus is formed in the growth process and shows a step growth mechanism. With the increase in sintering temperature, the preferred orientation growth of mullite crystal finally forms needle‐like morphology.

Intriguing minerals: corundum in the world of rubies and sapphires with special attention to Macedonian rubies

This lecture text presents features of the fascinating mineral corundum and its gem varieties, ruby and sapphire. The geologic occurrences of its varieties and the best-known world localities are presented in detail. The mineralogy and crystallography of corundum, as well as its occurrence forms, are discussed. The origin of their colour varieties and their usage are described, in addition to their important physical characteristics. Applications in the watch and laser industries and as an abrasive material, among others, are included. The ruby variety from dolomite marble in North Macedonia exhibits the unique optical phenomenon known as diasporescence–the inclusion of diaspore crystals in the corundum matrix. This phenomenon affects the parting of corundum crystals and influences their colour, density and hardness. Therefore, special attention is paid here to morphological and physical characteristics of the Macedonian gem corundum.

Study on the high-efficiency separation of Fe in extracted vanadium residue by sulfuric acid roasting and the solidification behavior of V and Cr

The large accumulation of extracted vanadium residue not only causes waste of resources but also causes environmental pollution. In this work, 96.8% Fe in extracted vanadium residue is efficiently and selectively leached by sulfuric acid roasting process at a lower temperature. Petroleum proppant is prepared from iron removal waste slag and low-grade bauxite, and V and Cr in extracted vanadium residue are solidified. Specifically, under the conditions of reaction temperature of 95 °C, pH of 2.5, and time of 6 h, 86.8% of Fe in the iron-rich leaching solution is converted into flower-like ammonium jarosite. After the thermal decomposition of ammonium jarosite, the product Fe2O3 still has a flower-like morphology. The microstructure of the petroleum proppant is that corundum crystals are filled in the gaps of the network structure of the mullite columnar crystals to form a staggered skeleton structure, which combines the advantages of two-phase materials. The total recovery rates of V and Cr in extracted vanadium residue are 93.74% V and 98.18% Cr, respectively. This study provided a cleaner production approach to recycle industrial waste, reduce the hazards of V and Cr, and reduce the cost of preparing α-Al2O3-3Al2O3·2SiO2 dual-phase structure petroleum proppants.