Al Substitution Research Articles

Mn ions‐activated Gd3(Al,Ga)5O12 garnet solid‐solution ceramics: Cation substitution for dual wavelength red‐emission

AbstractRed‐emitting color‐convertors have attracted considerable attention for promising applications in solid‐state lighting (SSL) to improve color rendition. However, the current nitride and fluoride phosphor powders have encountered several challenges, such as high cost, narrow emission bands, and insufficient stability during operation, which limit the development of high‐power full‐spectrum SSL. In this study, thermally robust Gd3(Al,Ga)5O12:Mn (GAGG:Mn) solid‐solution ceramics (SSCs) with dual wavelength red‐emission bands were prepared via an oxygen solid‐state sintering reaction. The doped Mn ions occupied octahedral Al3+ and Ga3+ sites to generate Mn4+ luminescent centers with pronounced deep‐red emissions peaking at 698 nm (2E → 4A2), and Mn2+ luminescent centers with broad red emissions at 628 nm (4T1 → 6A1). Because the cationic radius matching effect induced the regulation of valence state of Mn, the photoluminescence of the GAGG:Mn SSCs can be tailored by the substitution of Al3+ with Ga3+. Moreover, the Mn3+ also existed in the GAGG lattice host, and their concentration decreased with increasing Ga3+ contents owing to the mismatch of ionic radius between Mn3+ and Ga3+ ions. With the optimization of Al/Ga ratio and concentration of Mn ions, a broad emission band ranging from 550 to 800 nm (bandwidth = 250 nm) was achieved from Gd3Al3Ga2O12:0.3%Mn SSCs upon 465‐nm excitation. Moreover, the GAGG:Mn SSC has over 17‐fold enhanced thermal conductivity compared with the corresponding phosphor powder. This paper opens a door of regulating the valence state of luminescence centers with cation substitution and the application of oxide red‐emitting color‐convertors.

New insights into intrinsic point defects of delafossite CuAlO2 as optoelectronic materials

AbstractDelafossite CuAlO2 has great potential in optoelectronics applications. The intrinsic point defects play the decisive roles in the optoelectronic properties and application of CuAlO2. However, there are still disputes about their relevant underlying physical mechanisms at the atomic scale. Here, the formation of intrinsic point defects of CuAlO2 and their effects on optical absorption spectra and electronic structure have been systematically investigated using first‐principles calculations. In oxygen‐rich and oxygen‐poor chemical environments, acceptor‐type defects such as Cu vacancy and Cu substitution on the Al site can spontaneously form and produce p‐type conductivity at room temperature. This is the important physical basis for CuAlO2 to be widely used as transparent conductive oxide films. The shallow acceptor‐type levels produced by Cu substitution on Al site and O vacancy, and the shallow donor‐type levels produced by interstitial Cu, are favorable for the separation of photo‐generated electron–hole pairs, which can be suitable for applications in photocatalysis and photovoltaics. The Cu or Al vacancy can produce delocalized energy band at the top of valence band and pinning Fermi level at the same time, which may make CuAlO2 a degenerate semiconductor and produce local surface plasmon resonance effect. Al substitution on Cu site and interstitial Al or O form complex deep defect energy bands in the forbidden band and produce significant multiband optical absorption in the visible–NIR region. They are expected to become alternative materials for photoluminescence and photoelectric detection. This work demonstrates the importance of intrinsic defect engineering for optimizing optoelectronic properties and thus promoting the discovery of novel optoelectronic materials.

Atomic positions and displacements in piezoelectric materials Ca3TaGa3Si2O14 and Ca3TaGa1.5Al1.5Si2O14 investigated by Ta-Lα X-ray fluorescence holography

The atomic positions and displacements of atoms around the Ta atom in piezoelectric materials Ca3TaGa3Si2O7 (CTGS) and Ca3TaGa1.5Al1.5Si2O7 (CTGAS) were investigated at 100 K by Ta-Lα X-ray fluorescence holography (XFH). The experimental atomic images were compared with the simulated ones using the crystal structures of CTGS and CTGAS, which were determined by single crystal X-ray diffractometry (SC-XRD). The atomic positions agreed between XFH and SC-XRD experiments. With the help of XFH simulation, the displacements of Ta, Ca, Si, and Ga atoms relative to the Ta atom were qualitatively analyzed using experimental atomic image intensities. The relative displacement of the Ca atom was increased by Al substitution, while those of the Ta and Ga atoms were decreased. The first principles calculation based on density-functional theory (DFT) was performed to understand bonding character between constituents. The evaluation of the crystal orbital Hamilton population (COHP) clarified that the Ca-O bond has strong ionic character different with the other bonds, suggesting that the positional shift of the Ca atom is responsible for the piezoelectricity in CTGS. The effect of Al substitution on piezoelectricity was also considered based on the change in the Ca-O bond.

Distribution of multiple Al substitution in HY zeolite and Brønsted acid strength - A periodic DFT study

In petrochemical industry, zeolites as solid acid catalysts play an important role and Y zeolites with high surface area and large pore openings are mainly used for catalytic cracking. Aiming at the problem that the specific positions of Al atoms in zeolites cannot be accurately distinguished experimentally and the acid strength can only be studied qualitatively, we used computational model and method to study the position of Al substitution and the resulted acid strength of Y zeolite at atomic and molecular level. The position of Al substitution with gradually decreasing Si/Al ratio (Si/Al = 47–2.4) and the corresponding Brønsted acid strength of the resulting multiple acid sites of HY zeolites are systematically investigated based on periodic DFT calculation including dispersion correction (GGA-PBE-D3). The thermodynamically more stable structures prefer Al distribution having the next-nearest neighbor of aluminum atom pairs (Al–O–Si–O–Al), and based on these stable configurations, the adsorption of ammonia (NH3) and pyridine (C5H5N) has been computed to estimate the Brønsted acid strength. It is found that the adsorption of NH3 and C5H5N prefers the formation of ammonium (NH4+) and pyridinium (C5H5NH+) ions at each acid sites and the adsorption enthalpy does not correlate with the Bader charge of the acid proton. Most importantly, the adsorption enthalpy decreases with the decrease of Si/Al ratio, indicating that the more the Al substitution the weaker the Brønsted acid strength. It is noted that the adsorption enthalpy of all individual acid sites at a given Si/Al ratio can differ strongly, despite the formation of NH4+ and C5H5NH + ions, indicating their mutual interaction and local confinement effect. The calculated adsorption enthalpies agree fully with the experimentally measured results (Si/Al = 8.6–2.4). Hyroxyl nest, defect sites formed from dealumination, has much weaker acid strength than the Brønsted acid sites. Our results provide a rational basis for further studies on the enhanced catalytic activity of HY zeolites with different Si/Al ratios.

Improvement of hydrogenation sorption properties of Ti0.90V0.30Mn1.00Ni0.80 for ultra-high pressure metal-hydride compressor

Metal-hydride compressors are considered as an alternative to mechanical compressors in Hydrogen Refueling Stations. In this paper, several Ti-based AB2 compounds were synthesized and their hydrogen sorption properties were evaluated under ultra-high pressure up to 40 MPa. All the studied compounds exhibit absorption plateau pressure close to or higher than 10 MPa at 30 °C. The substitution by a small amount of Al was shown to drastically decrease the hysteresis. All the compounds herein studied follow the trend ln(P)∝−V0, except for the Nb-containing compound. This compound has a different behavior towards hydrogenation and exhibits sorption pressure much higher than expected from the above-mentioned relationship. It was confirmed that the Al and Nb substitution in AB2 compounds plays a great role on reducing hysteresis and increasing the sorption pressure, respectively.

Improved microwave dielectric properties of (Mg0.5Ti0.5)3+ Co-substituted Mg2Al4Si5O18 cordierite ceramics

In this work, the Mg2Al4-x(Mg0.5Ti0.5)xSi5O18 (0 ≤ x ≤ 0.20) ceramics were successfully synthesized via the solid-state reaction. The main phase was Mg2Al4Si5O18 with an orthorhombic structure, and the secondary phases (SiO2, TiO2 and Al2SiO5) were formed during the synthesis of Mg2Al4-x(Mg0.5Ti0.5)xSi5O18 ceramics. The substitution of Al3+ by (Mg0.5Ti0.5)3+ occurs complex chemical reaction: Mg ions prefer to occupy the Mg-site deficiency in Mg2Al4Si5O18; Ti ions substitute Si-site at x < 0.12, whereas Ti ions also occupy Al-site and remaining to form TiO2 phase at x ≥ 0.12. The dielectric constant (εr), quality factor (Qf) and temperature coefficient of resonance frequency (τf) are mainly dominated by the ionic polarizability, relative density, secondary phases and bond valence of Mg-site. Additionally, the bond ionicity (fi) of Mg–O, lattice energy (U) of Si–O and bond energy (E) of Al–O also influence εr, Qf and τf, respectively. The moderate (Mg0.5Ti0.5)3+ ions substitution not only effectively improve Qf while maintaining low εr, but also reduce densification temperature. The excellent microwave dielectric properties with εr = 4.45, Qf = 33,995 GHz and τf = −39 ppm/°C were obtained in Mg2Al4-x(Mg0.5Ti0.5)xSi5O18 (x = 0.08) ceramic sintered at 1375 °C, which is potential for millimeter-wave communication.

A highly porous MgAl2O4 spinel-supported Mn3O4 as a reusable catalyst for glycolysis of postconsumer PET waste

A porous MgAl2O4 spinel-supported Mn3O4 catalyst, which is an efficient and robust catalyst for polyethylene terephthalate (PET) glycolysis, was synthesized from MgAl-layered double hydroxide (LDH) as a template via thermal and chemical treatment, followed by Mn oxide loading. A mesoporous structure in the MgAl2O4 spinel provided a high surface area (278 m2/g) and defects in which Mn3O4 was highly dispersed and strongly stabilized by the substitution of Al3+ and incorporation of Mn cation into the defects. The strong interaction of Mn oxide and the porous MgAl2O4 spinel support led to a dramatic increase of moderate acidic and basic properties, and to high resistance for Mn leaching during glycolysis. Using the Mn3O4/p-spMgAl800 catalyst, a yield of bis(2-hydroxyethyl) terephthalate > 95 % was achieved within 3 h at 190 °C. Despite eleven recycling times with regeneration, the catalytic activity remained without significant deactivation and was completely recovered after thermal regeneration at 500 °C.

Improved catalytic performance of Pd-Cu/NaY zeolite by tunning Al distribution for the synthesis of dimethyl carbonate

Dimethyl carbonate (DMC) is a green chemical that is widely used in the synthesis of polycarbonate and the electrolyte of batteries. The Coal to DMC is a cost-effective pathway and the carbonylation of methyl nitrite (MN) is a critical step. The Pd-based catalysts have been extensively investigated for this reaction. However, the effect of the catalyst support (NaY zeolite) is rarely studied and unclear. Here, the mechanisms are disclosed by combining experiments and theoretical simulations. We find in experiments that the activity of the catalysts increases when the Si/Al molar ratio of NaY zeolite increases from 1.37 to 1.75, and then decrease when the Si/Al ratio reaches 2.33. Density functional theory (DFT) simulations results indicate that the CO adsorption strength on the catalysts is affected by not only the Si/Al ratio but also the distribution pattern of Al substitutions. The CO adsorption strength is moderated by the population of the Al-O-Si-O-Si-O-Al and Al-O-Si-O-Al linkages in Pd-NaY. The theoretical results provide useful information to clarify the effects of Al distribution for the catalysts with NaY zeolite as support.

Accelerating the Low-Temperature Catalytic Oxidation of Acetone over Al-Substituted Mn–Al Oxides by Rate-Limiting Step Modulation

In order to enhance the catalytic activity and improve the stability of Mn-Al oxides in acetone oxidation, it is interesting to have found that modulating and accelerating the rate-limiting step by Al substitution rather than just mixing of Mn and Al is crucial for hydrocarbon efficient catalytic destruction. Here, a series of Mn-Al oxides with different Al substitution ratios were prepared by a scalable and facile hydrothermal-redox strategy. The reaction rate, selectivity, and stability of the representative α-MnO2 catalyst in acetone oxidation can be remarkably promoted by simple replacing of the partial framework Mn with Al, which changes the rate-limiting step from acetic acid dissociation to ethanol decomposition accelerated by H2O molecules. Among them, MnAl0.5 displays the best catalytic performance with 90% of acetone converted at just 165 °C and a remarkable CO2 yield. DFT results suggest that the py and px orbitals of the O element take part in the formation of the carbonyl group when the intermediate of removing H* from ethanol reacts with the hydroxyl group of H2O. The dxz orbital of Mn with p-electron of Al plays a vital role in the rate-limiting step. The work provides new insights into engineering catalysts for industrial VOC efficient and economical mineralization.

Promotional effects of calcination temperature and H2O on the catalytic activity of Al-substituted MnAlO catalysts for low-temperature acetone oxidation.

In order to enhance the role of Al in the materials, Al-substituted MnAlO catalysts were synthesized via the hydrothermal-redox method at different calcination temperatures for acetone oxidation. There were Al-substituted α-MnO2 and amorphous aluminum oxide existed with homogeneous dispersion of elements in the catalysts. The surface property, reaction rate, CO2 yield and water resistance of MnAlO catalysts were greatly affected by calcination temperatures. MnAlO-450 catalyst exhibited the best catalytic performance (acetone conversion of 90% at 165°C) with CO2 yield higher than 99.7%, which was mainly related to the weaker Mn-O bond strength, lower temperature reducibility and abundant Lewis acid sites. The acetone conversion of MnAlO-450 increased by as much as 16% in the presence of 1vol% H2O compared to that in the absence of H2O at T50 (the temperature for 50% conversion of acetone). The acceleration consumption of ethanol as the main by-product by H2O improved the catalytic performance. This work would shed light on the Al substitution based catalysts for OVOC oxidation with highly efficient and water resistance.

Synthesis and luminescence properties of red‐emitting AlN:Mn2+ spherical phosphors

AbstractIn this study, we successfully prepared red‐emitting AlN:Mn2+ spherical phosphors by using nitrogen pressure–assisted carbothermal reduction–nitridation method. The optimum preparation parameters were explored by changing nitrogen pressure, CaF2 content, reaction temperature, and the amount of MnCO3. It was confirmed that uniform spherical shape, large particle size, less impurity content, and suitable Mn2+ concentrations could effectively enhance the luminous intensity of AlN:Mn2+ phosphors. The structural compositions of the AlN:Mn2+ phosphors were characterized by X‐ray diffraction and X‐ray photoelectron spectroscopy, indicating that Mn2+ ions were successfully doped into AlN lattice by substitution of Al3+ ions, thus providing the red‐emitting luminescence centers. Based on the experimental results, we further proposed the underlying growth mechanism of spherical AlN phosphors. It was suggested that an adequate amount of liquid Ca‐aluminates was essential to achieve uniform spherical morphology and high fluorescence efficiency.

Structural and Morphological Characterization of the Chromium-doped β-Spodumene to Ceramic Pigment

The structural and morphological properties of the novel chromium-doped β-spodumene based ceramic pigment were evaluated aiming to study its chemical and thermal stability in industrial applications. The pigment samples were synthesized by the proteic sol-gel method using gelatin as a ligand. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were performed. Rietveld refinement of experimental diffractograms and average crystallites size by the Scherrer’s method analysis were carried out. XRD results confirmed that the crystal structure of lattice corresponded to β-spodumene and the Rietveld refinement indicated the absence of structural changes with doping up to 3% of chromium. Scherrer’s method indicates that there are no trends changes in the average size of crystallites in the analyzed doping contents. SEM analysis indicated that there are no morphological changes in the chromium-doped particles. The present study concluded that structural or morphological alterations are not noticed for β-spodumene samples produced with chromium doping up to 3% by weight, and that this pigment possibly preserves the chemical and thermal stability of pure β-spodumene. The absence of changes in these properties by doping with low chromium contents were attributed to possible substitutions of Al3+ by Cr3+ ions in the β-spodumene crystal lattice.

Tailoring the electrical and thermal transport properties of LaCoO3 ceramic by band engineering and Fermi energy optimization via isovalent Al-substitution for thermoelectric application

Recognizing high performance thermoelectric (TE) ceramics is challenging due to high thermal conductivity and interdependent electrical and thermal transport properties. Herein we report the strategy of isovalent Al substitution in LaCoO3, which resulted in the enhancement of electrical conductivity by band engineering and increased charge carrier mobility via effective mass and Fermi energy optimization. The Al substitution in LaCoO3 not only enhances the electrical transport properties but also decrease the lattice thermal conductivity through enhanced phonon scattering originated from the lattice strain induced by huge mass fluctuation of Co and substituted Al atom. The results indicate that the electrical conductivity increase with increasing the Al substitution and the maximum value of 642 S cm−1 was observed at 753 K and the maximum power factor (73.3 µW m−1K−2) was achieved at 703 K for the sample LaCo0.97Al0.03O3. The Al-substitution enhanced the charge carrier mobility from 0.21 cm2 V−1 s−1 to 51.6 cm2 V−1 s−1 by decreasing the effective mass from 28.76*me to 12.76*me. The decreased carrier concentration with Al substitution is due to the upward shift of Fermi energy towards the conduction band. The lowest thermal conductivity value (0.87 W m−1K−1) was obtained at 303 K for the sample LaCo0.95Al0.05O3. The lattice thermal conductivity of LaCo0.95Al0.05O3 (1.259 W m−1K−1) was reduced about 48% when compared with pure LaCoO3 (2.437 W m−1K−1) at 753 K. The present work reveals the importance of decoupling the electrical and thermal transport properties in achieving high performance TE ceramics.

(Invited, Digital Presentation) Pushing the Limits of High Nickel NMC Cathodes

Lithium-ion batteries now dominate electrochemical energy storage for vehicle propulsion and grid storage in addition to portable electronics. In 2022, they celebrate their 50th anniversary, and yet still achieve only 25% of their theoretical energy density. The dominant anode and cathode today are graphitic carbon and the layered NMC oxides, LI[NiMnCoAl]O2. Both need improving. Most of the carbon in the anode must go, and the NMCs need pushing to their limit and at the same time eliminating cobalt. I will today discuss the challenges faced as we push the limits of the NMCs to their limits, > 80% lithium cycling, and Ni>0.8 and Co <<0.1. The higher the Ni content the lower the charging voltage needed for a given cycling capacity, but the greater its surface and bulk reactivity. The issues discussed will include 1stcycle loss [1], high rate capability [2], cathode stability [3], and surface/bulk reactivity and the impact of surface/bulk modification [4,5] and the micron-size single crystals vs meatballs on these [6]. This work is being supported by DOE-EERE-BMR-Battery500 consortium.[1]. Hui Zhou, Fengxia Xin, Ben Pei, M. Stanley Whittingham, “What Limits the Capacity of Layered Oxide Cathodes in Lithium Batteries?”, ACS Energy Lett. 2019, 4: 1902−1906.[2]. Chunmei Ban, Zheng Li, Zhuangchun Wu, Melanie J. Kirkham, Le Chen, Yoon Seok Jung, E. Andrew Payzant, Yanfa Yan, M. Stanley Whittingham, and Anne C. Dillon “Extremely Durable High-Rate Capability of a LiNi0.4Mn0.4Co0.2O2 Cathode Enabled by Single–Wall Carbon Nanotubes”, Advanced Energy Materials, 2011, 1: 58-62.[3]. Hyung-Joo Noh, Sungjune Youn, Chong Seung Yoon, Yang-Kook Sun “Comparison of the structural and electrochemical properties of layered Li[NixCoyMnz]O2 (x = ¼ 1/3, 0.5, 0.6, 0.7, 0.8 and 0.85) cathode material for lithium-ion batteries”, J. Power Sources, 2013, 233: 121-130.[4]. Fengxia Xin, Hui Zhou, Yanxu Zong, Mateusz Zuba, Yan Chen, Natasha A. Chernova, Jianming Bai, Ben Pei, Anshika Goel, Jatinkumar Rana, Feng Wang, Ke An, Louis F. J. Piper, Guangwen Zhou, and M. Stanley Whittingham, “What is the Role of Nb in Nickel-Rich Layered Oxide Cathodes for Lithium-Ion Batteries?”, ACS Energy Letters, 2021, 6: 1377-1382.[5]. Ben Pei, Hui Zhou, Anshika Goel, Mateusz Zuba, Hao Liu, Fengxia Xin, and M. Stanley Whittingham, “Al Substitution for Mn during Co-Precipitation Boosts the Electrochemical Performance of LiNi0.8Mn0.1Co0.1O2”, J. Electrochemical Society, 2021, 68: 050532.[6]. Yujing Bi, Jinhui Tao, Yuqin Wu, Linze Li, Yaobin Xu, Enyuan Hu, Bingbin Wu, Jiangtao Hu, Chongmin Wang, JiGuang Zhang, Yue Qi and Jie Xiao, “Reversible planar gliding and microcracking in a single-crystalline Ni-rich cathode”, Science, 2020, 370: 1313-1317.

Efficient Nitrate Adsorption from Groundwater by Biochar-Supported Al-Substituted Goethite

Groundwater nitrate contamination is challenging and requires efficient solutions for nitrate removal. This study aims to investigate nitrate removal using a novel adsorbent, biochar-supported aluminum-substituted goethite (BAG). The results showed that an increase in the initial Al/(Al + Fe) atomic ratio for BAGs from 0 to 20% decreased the specific surface area from 115.2 to 75.7 m2/g, but enhanced the surface charge density from 0.0180 to 0.0843 C/m2. By comparison, 10% of Al/(Al + Fe) led to the optimal adsorbent for nitrate removal. The adsorbent’s adsorption capacity was effective with a wide pH range (4–8), and decreased with increasing ionic strength. The descending order of nitrate adsorption inhibition by co-existing anions was SO42−, HCO3−, PO43−, and Cl−. The adsorption kinetics and isotherms agreed well with the pseudo-first-order equation and Langmuir model, respectively. The theoretical maximum adsorption capacity was 96.1469 mg/g. Thermodynamic analysis showed that the nitrate adsorption was spontaneous and endothermic. After 10-cycle regeneration, the BAG still kept 92.6% of its original adsorption capacity for synthetic nitrate-contaminated groundwater. Moreover, the main adsorption mechanism was attributed to electrostatic attraction due to the enhancement of surface charge density by Al substitution. Accordingly, the BAG adsorbent is a potential solution to remove nitrate from groundwater.

Potential high-performance magnet materials: Co- and Al-alloyedSm2Fe17

${\mathrm{Sm}}_{2}{\mathrm{Fe}}_{17}$ has long been known as a potential high-performance magnet whose deficiencies---planar anisotropy and lower-than-optimal ${T}_{c}$---can be remedied by nitrogen addition, but which presents synthesis difficulties. Herein we apply first-principles calculations to search for alternative low-cost, high-performance permanent magnets in this family, by exploring simultaneous Fe and Al substitution. Specifically, the goal is to improve properties of ${\mathrm{Sm}}_{2}{\mathrm{Fe}}_{14}{\mathrm{Al}}_{3}$ easy-plane magnet at the stoichiometric composition. Density functional theory calculations were executed for three series of compounds, i.e., ${\mathrm{Sm}}_{2}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x})}_{14}{\mathrm{Al}}_{3}$, ${\mathrm{Sm}}_{2}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x})}_{15}{\mathrm{Al}}_{2}$, and ${\mathrm{Sm}}_{2}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x})}_{16}\mathrm{Al}$. We find that substitution of Fe with 12--18 of Co in $%\phantom{\rule{4pt}{0ex}}{\mathrm{Sm}}_{2}{\mathrm{Fe}}_{14}{\mathrm{Al}}_{3}$ modifies the magnetic anisotropy type from easy plane to easy axis with a substantial anisotropy of 7.1 MJ/${\mathrm{m}}^{3}$. We also demonstrate that the largest part of magnetic anisotropy is introduced by $4f$ Sm atom electrons. Thus the rotation of magnetic moment orientation from $\ensuremath{\langle}1\overline{1}0\ensuremath{\rangle}$ to $\ensuremath{\langle}111\ensuremath{\rangle}$ is followed by an increase of the occupied $4f$ state number and, as a result, the orbital part of the magnetic moment of one of the Sm atoms. This increase of the occupied $4f$ state number at an energy $\ensuremath{\sim}\ensuremath{-}$4.3 eV results in a significant reduction of band structure energy. The substitution of Fe by Co does not significantly reduce the magnetization of the compound and keeps it slightly above 1 T. This combination of magnetic anisotropy and magnetization makes the compound a promising candidate for a permanent magnet.

Analysis of the electrical activation data in thermally annealed implanted Al/4H–SiC systems: A novel approach based on cooperativity

The dependence on the annealing temperature, Tann, of the Al asymptotic substitutional fraction φ∞ in implanted silicon carbide (4H–SiC), is addressed from a statistical mechanical point of view. Concepts relating to the cooperative nature of atomic motion in (extremely) viscous liquids or in plastic crystals are applied for the first time in describing Si ⇄ Al substitution and the tendency of the system towards a regularization of its crystalline structure during annealing. The worked out model, although inspired by Al/4H–SiC literature data, can be applied to similar systems with the due changes. The slope ΔEs of the Arrhenian lnφ∞ vs. 1/kBTann dependence encompasses both the energy that would characterize a single Si ⇄ Al ideal substitution in a regular lattice, and a negative contribution associated to the irregularity of the lattice at the substitution sites. The early evolution of the substitutional fraction φ(t;Tann) in implanted systems during annealing is then considered. The slope of lnφ vs. 1/kBTann at short annealing times, when the asymptotic state is not yet reached, provides information about the average energy barrier ΔEw characterizing on average the preliminary molecular rearrangements preceding the Si ⇄ Al substitution. Values of ΔEs in the order of 1 eV or a bit less are envisaged and worked out from φ∞-fittings, while ΔEw∼3 eV is found from the analysis of incomplete annealing. The theory applied in this work represents a reference for a more complete description of the structural evolution of implanted systems.

Rare earth elements and other critical elements in the magnetic fraction of fly ash from several Polish power plants

Fly ash is a promising alternative source of many critical raw materials, including rare earth elements. According to the EU report, the demand for these metals is set to grow due to the economic growth of the developing countries, along with the development of new innovative technologies. A geochemical analysis showed that the overall content of critical and excessive elements in fly ash from several Polish power plants was higher than in the magnetic fraction. However, the outlook coefficient values for magnetic concentrates were higher than for ashes. Ashes and the magnetic fraction from brown coals seem to be the most promising source of REY recovery. The separation of fine-size grades from the analyzed ashes does not have a significant impact on REY concentration in the obtained products. A microanalysis showed that iron spinels were enriched in Co, Cu, Ni, Mn, Mo, and Zn. The concentration of these metals in magnetic concentrates insignificantly exceeded the world average value of fly ash. The sample with the highest heavy metal enrichment factor also had the highest Fe enrichment factor. The phase composition of the magnetic fraction was dominated by: magnetite, hematite, magnesioferrite, and other iron spinels with Mn and Al substitutions. The chemical composition of ferrospheres was determined using energy dispersive spectroscopy (EDS) together with the SEM technique, and was then linked to their morphology. It was concluded that the impact on the enrichment efficiency and thus the composition of the magnetic fraction had: the diversity of morphological structures of iron spinels, the presence of AlSi glass interlayers, the presence of substitutions in the magnetite crystal structure as well as the ash particle size distribution.

Electrochemical Hydrogenation Properties of New YNi&lt;sub&gt;3&lt;/sub&gt; and YNi&lt;sub&gt;4&lt;/sub&gt; Based Alloys

Seven multiphase YNi3 and YN4 based alloys have been prepared by arc-melting. Based on X-ray phase and structural analysis synthesized alloys depending on its composition contain intermetallics with PuNi3, Gd2Co7 and CaCu5 structure types. The influence of Y/La and Ni/Co/Mn/Al substitution on the discharge characteristics of the AB3 and AB4 electrodes was studied. Substitution Y by La and further Ni by Mn and Al lead to increasing discharge capacity from 40 to 341 mAh/g. Positive effect of Ni/Co/Al or Ni/Mn/Al substitution was also observed for the AB3 electrodes. The high discharge capacities of 318 mAh/g and 340 mAh/g were seen for the YNi2.65Co0.2Al0.15 and YNi2.65Mn0.2Al0.15 electrodes.

Unlocking New Redox Activity in Alluaudite Cathodes through Compositional Design

Sodium (Na)-ion conducting polyanionic structures are among the most promising cathode materials to enable more sustainable Na-based battery chemistries to replace current Li-based energy storage systems. Materials adopting the alluaudite structure have been found to reversibly intercalate Na, with phosphate and sulfate derivatives exhibiting moderate to high capacities when used as Na-ion cathodes. However, the development of alluaudite cathodes has been hampered by the fact that largely only Fe2+/3+ redox has been observed in this structure. Herein, we show that the Mn2+/3+ redox couple can be activated through compositional tuning of alluaudite compounds. Specifically, vanadate compounds were explored to increase the electrical conductivity as compared to the phosphates and sulfates. Al substitution was also employed to buffer the Jahn–Teller distortions of Mn(III)O6 octahedra, further facilitating electron transfer and redox processes. We report the synthesis of a series of new Na2Mn3–xAlx(VO4)3 alluaudite cathodes and employ synchrotron X-ray diffraction, solid-state nuclear magnetic resonance, scanning electron microscopy, density functional theory, X-ray absorption spectroscopy, and magnetometry to characterize the structure, morphology, electronic, and magnetic properties of the as-synthesized materials. Electrochemical Na de(intercalation) in Na2Mn3–xAlx(VO4)3 (x = 0, 0.05, 0.2) was probed through galvanostatic cycling, galvanostatic intermittent titration technique, and ex situ synchrotron X-ray diffraction. While these materials are all redox active, Al substitution results in a more than 2-fold increase in discharge capacity. The successful activation of a higher voltage Mn2+/3+ redox couple opens up a new compositional space for alluaudite-type Na-ion cathodes.