Crystals

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22 pages, 6414 KiB

Open AccessArticle

Experimental Investigation and Machine Learning Modeling of Tribological Characteristics of AZ31/B₄C/GNPs Hybrid Composites

by Dhanunjay Kumar Ammisetti, Bharat Kumar Chigilipalli, Baburao Gaddala, Ravi Kumar Kottala, Radhamanohar Aepuru, T. Srinivasa Rao, Seepana Praveenkumar and Ravinder Kumar

Crystals 2024, 14(12), 1007; https://doi.org/10.3390/cryst14121007 - 21 Nov 2024

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Abstract

In this study, the AZ31 hybrid composites reinforced with boron carbide (B₄C) and graphene nano-platelets (GNPs) are prepared by the stir casting method. The main aim of the study is to study the effect of various wear parameters (reinforcement percentage (R), [...] Read more.

In this study, the AZ31 hybrid composites reinforced with boron carbide (B₄C) and graphene nano-platelets (GNPs) are prepared by the stir casting method. The main aim of the study is to study the effect of various wear parameters (reinforcement percentage (R), applied load (L), sliding distance (D), and velocity (V)) on the wear characteristics (wear rate (WR)) of the AZ91/B₄C/GNP composites. Experiments are designed using the Taguchi technique, and it was determined that load (L) is the most significant parameter affecting WR, followed by D, R, and V. The wear mechanisms under conditions of maximum and minimum wear rates are examined using SEM analysis of the worn-out surfaces of the specimens. From the result analysis on the WR, the ideal conditions for achieving the lowest WR are R = 4 wt.%, L = 15 N, V = 3 m/s, and D = 500 m. Machine learning (ML) models, including linear regression (LR), polynomial regression (PR), random forest (RF), and Gaussian process regression (GPR), are implemented to develop a reliable prediction model that forecasts output responses in accordance with input variables. A total of 90% of the experimental data points were used to train and 10% to evaluate the models. The PR model exceeded the accuracy of other models in predicting WR, with R² = 0.953, MSE = 0.011, RMSE = 0.103, and COF with R² = 0.937, MSE = 0.013, and RMSE = 0.114, respectively. Full article

(This article belongs to the Special Issue Feature Papers on "Hybrid and Composite Crystalline Materials" 2023–2024)

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12 pages, 2952 KiB

Open AccessArticle

3D Hierarchical Composites of Hydrotalcite-Coated Carbon Microspheres as Catalysts in Baeyer–Villiger Oxidation Reactions

by Marta Estrada-Ruiz, Daniel Cosano, Dolores Esquivel, Francisco J. Romero-Salguero and José Rafael Ruiz

Crystals 2024, 14(10), 878; https://doi.org/10.3390/cryst14100878 - 5 Oct 2024

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Abstract

The use of heterogeneous catalysts is fundamental in the search for sustainable chemical processes. Research on hierarchical materials is a growing field aimed at optimizing the synthesis of catalysts. In this work, layered materials with metals of different cationic ratios and three-dimensional hierarchical [...] Read more.

The use of heterogeneous catalysts is fundamental in the search for sustainable chemical processes. Research on hierarchical materials is a growing field aimed at optimizing the synthesis of catalysts. In this work, layered materials with metals of different cationic ratios and three-dimensional hierarchical structures have been synthesized in a simple and easy way using carbon spheres as support. All materials were characterized with various techniques such as XRF, elemental analysis XRD, FT-IR, SEM, and TEM to study their composition and structure. Finally, these materials were used in the Baeyer–Villiger reaction, which was carried out under optimized conditions. The results showed that the metal ratio was an important factor in the coating process, affecting the catalytic capacity of the materials. Full article

(This article belongs to the Special Issue Feature Papers on "Hybrid and Composite Crystalline Materials" 2023–2024)

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12 pages, 1877 KiB

Open AccessArticle

Characterization of Chromium Cations in CrAPO-5 Metal Aluminophosphate

by Sergei A. Zubkov, Elena D. Finashina, Valery N. Zakharov and Leonid Kustov

Crystals 2024, 14(9), 758; https://doi.org/10.3390/cryst14090758 - 26 Aug 2024

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Abstract

Chromium-substituted aluminophosphate (CrAPO-5) with the AFI crystal structure was prepared for the first time by using microwave conditions at the stage of gel formation and crystallization. This approach allowed the reduction of the time required for the synthesis of CrAPO-5 from 60–70 h [...] Read more.

Chromium-substituted aluminophosphate (CrAPO-5) with the AFI crystal structure was prepared for the first time by using microwave conditions at the stage of gel formation and crystallization. This approach allowed the reduction of the time required for the synthesis of CrAPO-5 from 60–70 h to 6 h. CrAPO-5 metal aluminophosphate prepared by microwave-assisted synthesis is studied by UV-visible and IR spectroscopy and X-ray photoelectron spectroscopy. The material is characterized by the presence of Cr³⁺ and Cr²⁺ ions in the framework with predominating Cr³⁺ ions introduced via isomorphous substitution, as well as some minor amounts of extra-framework Cr³⁺ species, which are present presumably in the state of α-Cr₂O₃. The latter species can be partially reduced to Cr²⁺ species in the presence of CO or H₂. XPS study of CrAPO-5 revealed the presence of Cr³⁺ ions in the framework. A TPR experiment showed that the reduction of chromium starts at about 450–500 °C. Full article

(This article belongs to the Special Issue Feature Papers on "Hybrid and Composite Crystalline Materials" 2023–2024)

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12 pages, 3922 KiB

Open AccessArticle

A New Type of Acidic OH-Groups in the LTL Zeolite

by Alessandro Contini, Martin Jendrlin and Vladimir Zholobenko

Crystals 2024, 14(8), 745; https://doi.org/10.3390/cryst14080745 - 21 Aug 2024

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Abstract

Acidic properties of ion-exchanged LTL zeolites have been studied using FTIR spectroscopy, complemented by X-ray powder diffraction, SEM-EDX, XRF and N₂ physisorption. Infrared spectra of the ion-exchanged zeolites show the presence of two intense bands of the bridging OH-groups: a narrow band [...] Read more.

Acidic properties of ion-exchanged LTL zeolites have been studied using FTIR spectroscopy, complemented by X-ray powder diffraction, SEM-EDX, XRF and N₂ physisorption. Infrared spectra of the ion-exchanged zeolites show the presence of two intense bands of the bridging OH-groups: a narrow band at ~3640 cm⁻¹ that is attributed to Si(OH)Al groups freely vibrating in 12 MR and a broad, intense band at ~3250 cm⁻¹ that is assigned to bridging OH groups forming hydrogen bond with neighbouring oxygen atoms, e.g., in six-membered rings. The former can be selectively removed by caesium or rubidium cations with up to 3 Cs⁺ or Rb⁺ per unit cell readily ion-exchanged into the LTL zeolite, replacing an equivalent number of acidic OH-groups or K⁺ cations within the structure. The cation migration of the larger cation, evaluated by the Rietveld refinement method, occurs mostly via the main 12 MR channels. By contrast, less than 1 Li⁺ or Na⁺ cation per unit cell can be introduced under similar conditions. Accordingly, the concentration of Si(OH)Al groups in back-exchanged NH₄-K-LTL with smaller cations (Li⁺, Na⁺) does not differ considerably from the concentration of Brønsted acid sites in the original NH₄-K-LTL. Lower concentrations of acid sites have been detected in the samples back-exchanged with Cs⁺, Rb⁺ and K⁺. In addition, the acidic properties of NH₄-LTL samples have been compared with a structurally related NH₄-MAZ zeolite. Full article

(This article belongs to the Special Issue Feature Papers on "Hybrid and Composite Crystalline Materials" 2023–2024)

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13 pages, 4519 KiB

Open AccessArticle

Mechanical, Electrical, and Glass Transition Behavior of Copper–PMMA Composites

by Victor H. Poblete and Mariela P. Alvarez

Crystals 2023, 13(3), 368; https://doi.org/10.3390/cryst13030368 - 21 Feb 2023

Cited by 3 | Viewed by 1711

Abstract

The mechanical, electrical, and glass transition behaviors (Tg) of polymethylmethacrylate (PMMA)–metal systems have been studied. Considering both the particle size and the metal filler concentration, the electrical conductivity showed a clear dependence on the sample thickness to reach percolation. An increase of up [...] Read more.

The mechanical, electrical, and glass transition behaviors (Tg) of polymethylmethacrylate (PMMA)–metal systems have been studied. Considering both the particle size and the metal filler concentration, the electrical conductivity showed a clear dependence on the sample thickness to reach percolation. An increase of up to 400% of strain-to-failure for the 2% v/v of nanometric filler composites in the mechanical test was observed. Tg analysis showed a decrease in the glass transition temperature when the increase of nanometric metallic filler reached the limit of 2% v/v. Over this concentration, the Tg values showed a tendency to reach the original value of the polymeric matrix without conductive filler. For the 20% v/v micrometric filler composites, the strain-to-failure increased up to 58%, but in the Tg analysis of this composite, no relevant changes were observed when the micrometric metallic filler was increased. Full article

(This article belongs to the Special Issue Feature Papers on "Hybrid and Composite Crystalline Materials" 2023–2024)

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13 pages, 4071 KiB

Open AccessArticle

Ion-Beam Synthesis of Structure-Oriented Iron Nanoparticles in Single-Crystalline Rutile TiO₂

by Iskander R. Vakhitov, Nikolay M. Lyadov, Vladimir I. Vdovin, Anton K. Gutakovskii, Vladimir I. Nuzhdin, Lenar R. Tagirov and Rustam I. Khaibullin

Crystals 2023, 13(2), 355; https://doi.org/10.3390/cryst13020355 - 18 Feb 2023

Viewed by 1677

Abstract

Magnetic nanoparticles embedded into semiconductors have current perspectives for use in semiconducting spintronics. In this work, 40 keV Fe⁺ ions were implanted in high fluences of (0.5 ÷ 1.5) × 10¹⁷ ion/cm² into an oxide semiconductor and single-crystalline TiO₂ [...] Read more.

Magnetic nanoparticles embedded into semiconductors have current perspectives for use in semiconducting spintronics. In this work, 40 keV Fe⁺ ions were implanted in high fluences of (0.5 ÷ 1.5) × 10¹⁷ ion/cm² into an oxide semiconductor and single-crystalline TiO₂ plates of rutile structure with (100) or (001) face orientations. Microstructure, elemental-phase composition, and magnetic properties of the Fe-ion-implanted TiO₂ were studied by scanning and transmission electron microscopies (SEM and TEM), X-ray photoelectron (XPS) and Rutherford backscattering (RBS) spectroscopies, as well as vibrating-sample magnetometry (VSM). The high-fluence ion implantation results in the formation of magnetic nanoparticles of metallic iron beneath the irradiated surface of rutile. The induced ferromagnetism and observed two- or four-fold magnetic anisotropy are associated with the endotaxial growth of Fe nanoparticles oriented along the crystallographic axes of TiO₂. Full article

(This article belongs to the Special Issue Feature Papers on "Hybrid and Composite Crystalline Materials" 2023–2024)

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12 pages, 3362 KiB

Open AccessArticle

The Role of Intermediate Phases in the Crystallization of Aluminophosphate Sieves on Examples of AlPO-11 and AlPO-41

by Marat R. Agliullin, Aidar V. Fayzullin, Zulfiya R. Fayzullina and Boris I. Kutepov

Crystals 2023, 13(2), 227; https://doi.org/10.3390/cryst13020227 - 27 Jan 2023

Cited by 1 | Viewed by 1767

Abstract

The formation of intermediate phases during aging of the reaction gel composition 1.0Al₂O₃•1.0P₂O₅•1.5DPA•40H₂O and its subsequent crystallization into molecular sieves AlPO-11 and AlPO-41 was studied in this work. The initial gels and crystallization [...] Read more.

The formation of intermediate phases during aging of the reaction gel composition 1.0Al₂O₃•1.0P₂O₅•1.5DPA•40H₂O and its subsequent crystallization into molecular sieves AlPO-11 and AlPO-41 was studied in this work. The initial gels and crystallization products were characterized by XRD; MAS NMR ²⁷Al and ³¹P; scanning electron microscopy (SEM); transmission electron microscopy (TEM); and N₂-physical adsorption. It has been found that the nature of the aluminum source used to prepare the gel has a significant effect on the properties of the resulting intermediate phases. It is shown that by changing the chemical and phase composition of the intermediate aluminophosphate, it is possible to control the morphology and characteristics of the secondary porous structure of the AlPO-11 molecular sieve. The formation of the intermediate phases with a layered structure opens the possibility to synthesize high-phase purity AlPO-41 at the di-n-propylamine/Al₂O₃ ratio = 1.5. The formation mechanisms of AlPO-11 and AlPO-41 are proposed depending on the phase composition of the intermediate phases. Full article

(This article belongs to the Special Issue Feature Papers on "Hybrid and Composite Crystalline Materials" 2023–2024)

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15 pages, 5539 KiB

Open AccessArticle

Phosphorus Recovery from Municipal Wastewater: Brucite from MgO Hydrothermal Treatment as Magnesium Source

by Panagiota D. Natsi, Konstantinos-Anastassios Goudas and Petros G. Koutsoukos

Crystals 2023, 13(2), 208; https://doi.org/10.3390/cryst13020208 - 23 Jan 2023

Cited by 4 | Viewed by 2539

Abstract

Crystallization of struvite (MgNH₄PO₄·6H₂O, MAPH) for both N and P recovery from municipal wastewater is an attractive alternative. The low magnesium concentration in municipal wastewater calls for low-cost magnesium sources. In the present work, the precipitation of [...] Read more.

Crystallization of struvite (MgNH₄PO₄·6H₂O, MAPH) for both N and P recovery from municipal wastewater is an attractive alternative. The low magnesium concentration in municipal wastewater calls for low-cost magnesium sources. In the present work, the precipitation of struvite from supersaturated solutions was investigated. The supersaturated solutions were prepared by the addition of either slurry of Mg(OH)₂ (brucite) obtained from the hydrothermal treatment of magnesium oxide (MgO) or using the aqueous solution equilibrated with brucite after separation from the solid. In both cases, in the supersaturated solutions prepared by the addition of ammonium dihydrogen phosphate, the Mg:NH₄:P ratio was 1:1:1, pH 7.0–11.0. The apparent order for the precipitation suggested that the dominant mechanism was surface diffusion according to the polynuclear model. The apparent rate constant for struvite precipitation was two orders of magnitude smaller for the slurry prepared supersaturated solution in comparison with the respective prepared from the brucite-equilibrated solution. Simulated wastewater (SWW), was supersaturated with respect to struvite, by inoculation with brucite. The apparent rate constant was two orders of magnitude higher than that corresponding to the supersaturated solutions prepared by the addition of solution equilibrated with brucite. The P recovery from SWW reached 70%. Full article

(This article belongs to the Special Issue Feature Papers on "Hybrid and Composite Crystalline Materials" 2023–2024)

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18 pages, 7663 KiB

Open AccessArticle

Embracing [XY₃]^m– and [XY₄]^m– Anions in Salts of [M(bpy)₃]^q⁺

by Edwin C. Constable and Catherine E. Housecroft

Crystals 2023, 13(1), 97; https://doi.org/10.3390/cryst13010097 - 5 Jan 2023

Cited by 2 | Viewed by 1841

Abstract

[M(bpy)₃]^q⁺ cations (bpy = 2,2′-bipyridine) are archetypical coordination entities containing chelating bidentate N,N′-donor ligands. Each propeller-shaped cation is chiral, existing as a Δ or Λ enantiomer. The supramolecular chemistry of [M(bpy)₃]^q⁺ cations [...] Read more.

[M(bpy)₃]^q⁺ cations (bpy = 2,2′-bipyridine) are archetypical coordination entities containing chelating bidentate N,N′-donor ligands. Each propeller-shaped cation is chiral, existing as a Δ or Λ enantiomer. The supramolecular chemistry of [M(bpy)₃]^q⁺ cations in the crystal is dominated by cation-anion interactions and, to a lesser extent, weaker non-covalent interactions. Analysis of the data for [M(bpy)₃]^q⁺ salts in the Cambridge Structural Database (CSD) reveals a ubiquitous motif in which homochiral sheets of cations generate cavities for the accommodation of anions. A series of related and common motifs in the solid-state structures of [M(bpy)₃]^q⁺ salts has been identified. One of the commonest motifs comprises a hexagon of six cations with anions either in the center or lying above and/or below the centroid. Full article

(This article belongs to the Special Issue Feature Papers on "Hybrid and Composite Crystalline Materials" 2023–2024)

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