Search Results (13)

High-alkali coal can cause slagging and fouling and impact the operational lifespan of the boilers. Traditional single-indicator methods often yield inconsistent results when evaluating the slagging risk of high-alkali coal. In this study, six coal samples were selected and systematically analyzed for their slagging characteristics using scanning electron microscopy (SEM), X-ray fluorescence (XRF), X-ray diffraction (XRD), and ash morphology analysis. Furthermore, a comprehensive evaluation model was constructed by integrating the technique for order preference by similarity to ideal solution (TOPSIS) with the entropy weight method. Additionally, based on images of ash morphology, the fractal dimension (D) was introduced as a quantitative indicator to predict slagging tendency through crack characteristics. The results show that TF, ZD, and KB samples, which are rich in alkaline oxides (CaO, Fe₂O₃, Na₂O, K₂O), form low-melting-point eutectic silicates during combustion, resulting in significant melting and agglomeration with wide cracks between aggregates, indicating a strong slagging tendency. Their fractal dimensions (D) range from 1.81 to 1.92. In contrast, HM and WQ samples, dominated by SiO₂ and Al₂O₃, form high-melting-point mullite and quartz, showing loose ash morphology with uniformly distributed cracks and a weak slagging tendency, with D values of 1.68 and 1.75, respectively. A significant negative correlation was observed between D and the E-TOPSIS model (y = 3.54 − 1.72x). Therefore, fractal analysis allows for rapid assessment of slagging risk without the need for complex chemical testing. This study provides valuable insights for predicting the slagging tendency of high-alkali coal during combustion. Full article

The solidification processes of two compositions, hypereutectic (21.0 mol% Y₂O₃–79.0 mol% Al₂O₃) and eutectic (18.5 mol% Y₂O₃–81.5 mol% Al₂O₃), were used via the horizontal directional solidification (HDS) method to produce two ingots with dimensions of 317 × 220 × 35 mm and 210 × 180 × 35 mm, respectively. The first ingot was heterogeneous and characterized by a two-layer structure with an expressed horizontal boundary, which is parallel to the solidification direction (an experimental fact observed for the first time), separating eutectic-type ceramics in the upper layer from the lower one containing the YAG dendrites. Considering the heat transfer feature characteristic of the HDS method and its action during the solidification of materials scattering thermal radiation, an explanation of the occurrence of such structure has been proposed. On this basis, the solidification parameters of the second ingot, providing its homogeneous structure, were selected. Characterization of the crystallographic texture and microstructure of both ingots revealed the advantage of the second solidification processing conditions. Full article

Eutectic high-entropy alloy (EHEA) has an outstanding combination of high strength and excellent ductility due to its dual-phase structure. In this study, AlCoCrFeNi_2.1 coatings on stainless steel substrates and bulk AlCoCrFeNi_2.1 alloys were prepared by mechanical alloying (MA). The results show that EHEA coatings with a dual-phase microstructure of BCC-AlNi/FCC-CoCrFe were obtained. A long milling duration promotes a uniform microstructure and homogeneous distribution of composition in the EHEA coatings, increasing hardness. Furthermore, annealing treatment encourages the formation of Cr-rich phases and Al-rich oxides, which affect the mechanical properties of coatings and bulk materials. Additionally, the presence of oxide dispersoids can cause the embrittlement of materials due to the formation of Al-Y-O oxide pores, resulting in the degradation of the mechanical properties of materials. Full article

Seven FeCrAlY-Al₂O₃ nanoceramic composite coatings are deposited on F/M steel via plasma spraying and laser remelting. A systematic investigation is conducted to examine the dependence of microstructure, mechanical properties, and lead–bismuth eutectic (LBE) corrosion resistance on the nano-Al₂O₃ addition and different Cr and Al contents. With the increase in Al content in FeCrAlY, gradual refinement of the coating grains occurs. The addition of nano-Al₂O₃ promotes the elemental segregation and precipitation of the second phase. The nano-Al₂O₃ notably enhances the mechanical properties of the coatings that are primarily attributed to second-phase and fine-grain strengthening. After LBE corrosion tests, intergranular corrosion morphology could be observed, where the contents of Cr and Al significantly influence the corrosion behavior of the coatings at varying temperatures. Full article

Ce³⁺ doped Al₂O₃-YAG eutectics were successfully grown by the horizontal directional crystallization method. The crystallization rate of eutectic growth was changed in the 1–7.5 mm/h range at a growth temperature of 1835 ℃. The microstructure of eutectic samples was investigated using scanning electron microscopy and X-ray microtomography. The intrinsic morphology of eutectic represents the stripe-like channel structure with a random distribution of the garnet Y₃Al₅O₁₂ (YAG) and Al₂O₃ (sapphire) phases. The content of these phases in the stripes changes in the 52.9–55.3% and 46.1–47.1% ratios, respectively, depending on the growth rate of the crystallization of the eutectic samples. The luminescent properties of the eutectic demonstrated the dominant Ce³⁺ luminescence in the garnet phase. The luminescence of the Ce³⁺ ions in Al₂O₃ has also been observed and the effective energy transfer processes between Ce³⁺ ions in the Al₂O₃ and YAG garnet phases were revealed under high-energy excitation and excitation in the UV Ce³⁺ absorption bands of sapphire. The phosphor conversion properties and the color characteristics (Al₂O₃-YAG):Ce eutectic with different thicknesses were investigated under excitation by a blue LED. We have also tested the prototypes of white LEDs, prepared using a blue 450 nm LED chip and (Al₂O₃-YAG):Ce eutectic photoconverters with 0.15 to 1 mm thicknesses. The results of the tests are promising and can be used for the creation of photoconverters for high-power white LEDs. Full article

The effect of Y₂O₃ addition on the microstructure and properties of the laser cladded Al-Si alloy coating on the surface of AZ91D magnesium alloy was investigated in this study. The experimental results showed that the Al-Si + Y₂O₃ cladding layers contained α-Mg, Mg₂Si, Al₄MgY and a small amount of Al₁₂Mg₁₇ phases. The coarse dendrites, reticulated eutectic structures and massive phases in the coatings tended to be refined and gradually uniformly distributed with the increased amount of Y₂O₃. The introduction of Y₂O₃ into the cladding layer favored the improvement of microhardness and wear resistance due to the grain refinement strengthening and dispersion strengthening. The addition of Y₂O₃ also promoted the reduction of localized corrosion sites and made the corrosion surface smoother, implying that the corrosion resistance of the Y₂O₃-modified coatings was better than that of the unmodified cladding layer. Full article

In order to prevent the lead-bismuth eutectic (LBE) corrosion of stainless-steel components used in nuclear reactors, the FeCrAlTiC-xY₂O₃ coatings were prepared on 304 stainless steel (304SS) by laser cladding. After adding Y₂O₃, Y₂TiO₅ and Y₂Ti₂O₇ formed, which have a combined strengthening effect on improving hardness. The 0.2 wt.% Y₂O₃ coating showed the highest hardness as ~489 HV. In the 400 °C wear test, the weight loss of coating samples was less than ~5.2 mg, while the weight loss of 304SS samples was ~35.5 mg. The 0 wt.% Y₂O₃ coating showed the highest wear resistance, indicating that adding Y₂O₃ could result in the decrease of wear resistance. The LBE corrosion behaviors of coatings at 500 °C were investigated. The results showed that a uniform and dense oxide scale with a low growth rate was obtained on the coating surface, and no penetration of LBE into the coating was observed. After 1000 h of corrosion, the oxide scale of coatings grew to merely a ~0.3 μm thickness. The corrosion resistance mechanism of the coating in oxygen-saturated LBE at 500 °C was proposed based on experimental results along with a thermodynamic and kinetic analysis. Full article

In the future, it will become increasingly important to recover critical elements from waste materials. For many of these elements, purely mechanical processing is not efficient enough. An already established method is pyrometallurgical processing, with which many of the technologically important elements, such as Cu or Co, can be recovered in the metal phase. Ignoble elements, such as Li, are known to be found in the slag. Even relatively base or highly redox-sensitive elements, such as Zr, REEs, or Ta, can be expected to accumulate in the slag. In this manuscript, the methods for determining the phase formation and the incorporation of these elements were developed and optimized, and the obtained results are discussed. For this purpose, oxide slags were synthesized with Al, Si, Ca, and the additives, P and Ti. To this synthetic slag were added the elements, Zr and La (which can be considered proxies for the light REEs), as well as Ta. On the basis of the obtained results, it can be concluded that Ti or P can have strong influences on the phase formation. In the presence of Ti, La, and Ta, predominantly scavenged by perovskite (Ca_1−wLa_2/3wTi_1−(x+y+z)Al_4/3xZr_yTa_4/5zO₃), and Zr predominantly as zirconate (Ca_1−wLa_2/3wZr_4−(x+y+z)Al_4/3xTi_yTa_4/5zO₉), with the P having no effect on this behavior. Without Ti, the Zr and Ta are incorporated into the pyrochlore (La_2−xCa_3/2x−yZr_2+2/4y−zTa_4/5zO₇), regardless of the presence of phosphorus. In addition to pyrochlore, La accumulates primarily in britholite-type La oxy- or phosphosilicates. Without P and Ti, similar behavior is observed, except that the britholite-like La silicates do not contain P, and the scavenging of La is less efficient. Lithium, on the other hand, forms its own compounds, such as LiAlO₂(Si), LiAl₅O₈, eucryptite, and Li silicate. Additionally, in the presence of P, Li₃PO₄ is formed, and the eucryptite incorporates P, which indicates an additional P-rich eutectic melt. Full article

The phase relations in the Si-Al-Na-K-Li-F-H-O model granite system are studied experimentally at T = 800, 700 °C and P = 1 and 2 kbar, as well as at T = 600, 550, 500 and 400 °C and P = 1 kbar and different water content from 2 to 50 wt.%. The initial composition was set in such a way that the composition of the resulting silicate melt was close to the granite eutectic. It is shown that in the presence of Li, two immiscible melts are formed in the system—an aluminosilicate (L) and a salt alkali-aluminofluoride (LF). It is shown that at Т = 800 °С, Р = 1 kbar and 2 kbar and water content > 10 wt. %, three phases are equilibrium in the system: L, LF, and fluid (Fl). Cryolite (Crl), which does not contain REE, begins to crystallize from the salt melt at 700 °C. Quartz (Qtz) crystallizes from the silicate melt at 600 °C and the equilibrium phases are L, LF, Crl, Qtz. At T = 500 °C Qtz, Na and K aluminofluorides and polylithionite crystallize from the aluminosilicate melt. The joint crystallization of Crl and Qtz is observed. Large crystals of cryolite and elpasolite are formed in both the salt and silicate melts. At the same time, the residual salt melt enriched in Li and REE is partially preserved. LF is completely crystallized at 400 °C, and L is in a metastable state. It is established that REE, Sc, Y and Li accumulate in the salt melt up to 500 °C with partition coefficients >> 1. REE and Sc enter into composition of the crystal phases at T = 500 °C and 400 °C. Sc partially isomorphically replaces Al. REE most often forms its own fluoride phases of the LnF3 type. Full article

In order to improve the seawater corrosion resistance of Inconel 718 superalloy, a La₂Zr₂O₇/NiCoCrAlY thermal barrier coating corrosion resistant to 3.5 wt.% NaCl aqueous solution was prepared by laser cladding on Inconel 718 superalloy. X-ray diffraction (XRD), Scanning Electron Microscope (SEM), and electrochemical techniques were used to study the microstructure and the corrosion performance of the coating in 3.5 wt.% NaCl solution. The results show that the thermal barrier coating is mainly composed of primary La₂Zr₂O₇ phase and γ + laves/δ phase eutectic structure. The corrosion potential and corrosion current of the coating in 3.5 wt.% NaCl solution are higher and lower than that of the Inconel 718 substrate, respectively, indicating that the corrosion performance of the coating is better than that of the Inconel 718 substrate. The presence of La₂Zr₂O₇ phase in the thermal barrier coating is the main reason for its corrosion resistance to 3.5 wt.% NaCl solution. Full article

In this study, the process optimization of sprayed powders and deposition performance for amorphous Al₂O₃–Y₃Al₅O₁₂ (YAG) coatings was investigated. The solid-state reaction mechanism of the nano-sized Al₂O₃ and Y₂O₃ powders with eutectic molar ratio was studied by multiple step-by-step heating cycle calcination processes. The calcination process could be adjusted according the dominant control factors like chemical reaction rate or ion diffusion rate. Finally, the actual deposition performance of the calcined powders (Al₂O₃/YAG) was examined by atmospheric plasma spraying (APS). XRD, SEM and DTA were used to characterize the powders and the as-sprayed coatings. The results showed that the reaction of Al₂O₃–Y₂O₃ system is a high temperature solid reaction dominated by Al³⁺ diffusion, the initial reaction process was usually belong to chemical kinetic range, then the reaction will be transformed from chemical kinetic range to diffusion dynamic range with the temperature increasing. The process optimization of powders was very effective, and the deposition effect of coating was effective. Full article

Sol–gel synthesis is an acknowledged method for obtaining fine inorganic powders of a different nature. Implementation of water-soluble polymers as gel-forming media makes this technique even more readily available, especially in cases where conventional gel formation is suppressed. In polymer–salt solutions, polymers serve as scaffolds for salt constituents’ bulk crystallization. When dried, solid salt particles are deposited on the polymer surface or in polymer matrix pores, which leads to higher grain size uniformity. The present work discusses the effect of drying conditions on phase composition and structure characteristics of complex oxide eutectics in ternary systems, CaO–Al₂O₃–Y₂O₃ (CAY) and MgO–Al₂O₃–Y₂O₃ (MAY), obtained from polymer–salt compositions based on polyvinyl alcohol (PVA), Na-salt of carboxymethylcellulose (Na-CMC) or polyvinylpyrrolidone (PVP). Microwave-assisted drying proved to be more efficient compared to convective process; however such technique requires careful selection of gel-forming polymer. Full article

An Nb-silicide based alloy will require some kind of coating system. Alumina and/or SiO₂ forming alloys that are chemically compatible with the substrate could be components of such systems. In this work, the microstructures, and isothermal oxidation at 800 °C and 1200 °C of the alloys (at.%) Si-23Fe-15Cr-15Ti-1Nb (OHC1) and Si-25Nb-5Al-5Cr-5Ti (OHC5) were studied. The cast microstructures consisted of the (TM)₆Si₅, FeSi₂Ti and (Fe,Cr)Si (OHC1), and the (Nb,Ti)(Si,Al)₂, (Nb,Cr,Ti)₆Si₅, (Cr,Ti,Nb)(Si,Al)₂ (Si)_ss and (Al)_ss (OHC5) phases. The same compounds were present in OHC1 at 1200 °C and the (Nb,Ti)(Si,Al)₂ and (Nb,Cr,Ti)₆Si₅ in OHC5 at 1400 °C. In OHC1 the (TM)₆Si₅ was the primary phase, and the FeSi and FeSi₂Ti formed a binary eutectic. In OHC5 the (Nb,Ti)(Si,Al)₂ was the primary phase. At 800 °C both alloys did not pest. The scale of OHC1 was composed of SiO₂, TiO₂ and (Cr,Fe)₂O₃. The OHC5 formed a very thin and adherent scale composed of Al₂O₃, SiO₂ and (Ti_(1−x−y),Cr_x,Nb_y)O₂. The scale on (Cr,Ti,Nb)(Si,Al)₂ had an outer layer of SiO₂ and Al₂O₃ and an inner layer of Al₂O₃. The scale on the (Nb,Cr,Ti)₆Si₅ was thin, and consisted of (Ti_(1−x−y),Cr_x,Nb_y)O₂ and SiO₂ and some Al₂O₃ near the edges. In (Nb,Ti)(Si,Al)₂ the critical Al concentration for the formation of Al₂O₃ scale was 3 at.%. For Al < 3 at.% there was internal oxidation. At 1200 °C the scale of OHC1 was composed of a SiO₂ inner layer and outer layers of Cr₂O₃ and TiO₂, and there was internal oxidation. It is most likely that a eutectic reaction had occurred in the scale. The scale of OHC5 was α-Al₂O₃. Both alloys exhibited good correlations with alumina forming Nb-Ti-Si-Al-Hf alloys and with non-pesting and oxidation resistant B containing Nb-silicide based alloys in maps of the parameters δ, Δχ and VEC. Full article