Research

20 pages, 6220 KiB

Open AccessEditor’s ChoiceArticle

The Beneficial Impact of Mineral Content in Spent-Coffee-Ground-Derived Hard Carbon on Sodium-Ion Storage

by Sonya Harizanova, Ivan Uzunov, Lyubomir Aleksandrov, Maria Shipochka, Ivanka Spassova and Mariya Kalapsazova

Materials 2024, 17(5), 1016; https://doi.org/10.3390/ma17051016 - 22 Feb 2024

Viewed by 633

Abstract

The key technological implementation of sodium-ion batteries is converting biomass-derived hard carbons into effective anode materials. This becomes feasible if appropriate knowledge of the relations between the structure of carbonized biomass products, the mineral ash content in them, and Na storage properties is [...] Read more.

The key technological implementation of sodium-ion batteries is converting biomass-derived hard carbons into effective anode materials. This becomes feasible if appropriate knowledge of the relations between the structure of carbonized biomass products, the mineral ash content in them, and Na storage properties is gained. In this study, we examine the simultaneous impact of the ash phase composition and carbon structure on the Na storage properties of hard carbons derived from spent coffee grounds (SCGs). The carbon structure is modified using the pre-carbonization of SCGs at 750 °C, followed by annealing at 1100 °C in an Ar atmosphere. Two variants of the pre-carbonization procedure are adopted: the pre-carbonization of SCGs in a fixed bed and CO₂ flow. For the sake of comparison, the pre-carbonized products are chemically treated to remove the ash content. The Na storage performance of SCG-derived carbons is examined in model two and three Na-ion cells. It was found that ash-containing carbons outperformed the ash-free analogs with respect to cycling stability, Coulombic efficiency, and rate capability. The enhanced performance is explained in terms of the modification of the carbon surface by ash phases (mainly albite) and its interaction with the electrolyte, which is monitored by ex situ XPS. Full article

(This article belongs to the Special Issue Size-Dependent Effects in Materials for Environmental Protection and Energy Application)

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13 pages, 1875 KiB

Open AccessArticle

Water Splitting Reaction Mechanism on Transition Metal (Fe-Cu) Sulphide and Selenide Clusters—А DFT Study

by Ellie Uzunova, Ivelina Georgieva and Tsvetan Zahariev

Materials 2024, 17(1), 56; https://doi.org/10.3390/ma17010056 - 22 Dec 2023

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Abstract

The tetracarbonyl complexes of transition metal chalcogenides M₂X₂(CO)₄, where M = Fe, Co, Ni, Cu and X = S, Se, are examined by density functional theory (DFT). The M₂X₂ core is cyclic with either [...] Read more.

The tetracarbonyl complexes of transition metal chalcogenides M₂X₂(CO)₄, where M = Fe, Co, Ni, Cu and X = S, Se, are examined by density functional theory (DFT). The M₂X₂ core is cyclic with either planar or non-planar geometry. As a sulfide, it is present in natural enzymes and has a selective redox capacity. The reduced forms of the selenide and sulfide complexes are relevant to the hydrogen evolution reaction (HER) and they provide different positions of hydride ligand binding: (i) at a chalcogenide site, (ii) at a particular cation site and (iii) in a midway position forming equal bonds to both cation sites. The full pathway of water decomposition to molecular hydrogen and oxygen is traced by transition state theory. The iron and cobalt complexes, cobalt selenide, in particular, provide lower energy barriers in HER as compared to the nickel and copper complexes. In the oxygen evolution reaction (OER), cobalt and iron selenide tetracarbonyls provide a low energy barrier via OOH* intermediate. All of the intermediate species possess favorable excitation transitions in the visible light spectrum, as evidenced by TD-DFT calculations and they allow photoactivation. In conclusion, cobalt and iron selenide tetracarbonyl complexes emerge as promising photocatalysts in water splitting. Full article

(This article belongs to the Special Issue Size-Dependent Effects in Materials for Environmental Protection and Energy Application)

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17 pages, 6005 KiB

Open AccessArticle

Corrosion-Resistive ZrO₂ Barrier Films on Selected Zn-Based Alloys

by Irina Stambolova, Daniela Stoyanova, Maria Shipochka, Nelly Boshkova, Silviya Simeonova, Nikolay Grozev, Georgi Avdeev, Ognian Dimitrov and Nikolai Boshkov

Materials 2023, 16(24), 7673; https://doi.org/10.3390/ma16247673 - 15 Dec 2023

Cited by 1 | Viewed by 697

Abstract

This work presents the enhanced corrosion resistance of newly developed two-layer composite coatings deposited on low-carbon steel: electrodeposited zinc alloy coatings (Zn–Ni with 10 wt.% Ni (ZN) or Zn–Co with 3 wt.% Co (ZC), respectively) and a top ZrO₂ sol–gel layer. Surface [...] Read more.

This work presents the enhanced corrosion resistance of newly developed two-layer composite coatings deposited on low-carbon steel: electrodeposited zinc alloy coatings (Zn–Ni with 10 wt.% Ni (ZN) or Zn–Co with 3 wt.% Co (ZC), respectively) and a top ZrO₂ sol–gel layer. Surface morphology peculiarities and anti-corrosion characteristics were examined using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDX), atomic force microscopy (AFM), water contact angle (WCA) measurements, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) analyses, potentiodynamic polarization (PDP) curves, corrosion potential (Ecorr), polarization resistance (Rp) measurements (for a prolonged period of 25 days) and open-circuit potential (OCP). The results were compared with the corrosion peculiarities of usual zinc coating. The zirconia top coatings in both systems were amorphous and dense, possessing hydrophobic nature. The experimental data revealed an increased corrosion resistance and protective ability of the ZC system in comparison to that of ZN due to its smooth, homogeneous surface and the presence of poorly crystallized oxides (ZnO and Co₃O₄), both later playing the role of a barrier for corrosive agents. Full article

(This article belongs to the Special Issue Size-Dependent Effects in Materials for Environmental Protection and Energy Application)

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0 pages, 2566 KiB

Open AccessArticle

Naphthalene Monoimides with Peri-Annulated Disulfide Bridge—Synthesis and Electrochemical Redox Activity

by Monika Mutovska, Natali Simeonova, Stanimir Stoyanov, Yulian Zagranyarski, Silva Stanchovska and Delyana Marinova

Materials 2023, 16(23), 7471; https://doi.org/10.3390/ma16237471 - 01 Dec 2023

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Abstract

Nowadays, organosulfur compounds provide new options in the development of full organic ion batteries. However, many drawbacks (such as kinetics limitations during the reversible oxidation of disulfides with cleavage of S–S bond, as well as solubility in non-aqueous electrolytes) make their commercialization difficult. [...] Read more.

Nowadays, organosulfur compounds provide new options in the development of full organic ion batteries. However, many drawbacks (such as kinetics limitations during the reversible oxidation of disulfides with cleavage of S–S bond, as well as solubility in non-aqueous electrolytes) make their commercialization difficult. Herein, a new concept for the design of organosulfur compounds with regulated redox properties and limited solubility is proposed. As a proof-of-concept, we designed peri-disulfo-substituted 1,8-naphthalimide derivatives, in which the alkyl chain length and halogen substituents (Cl or Br) at positions 3 and 6 are varied. The compounds were synthesized by an originally developed procedure starting from tetrahalonaphthalic anhydride via nucleophilic substitution at both peri-positions in the respective imide. Using ionic liquid electrolyte, it was found that the new peri-dithiolo-1,8-naphthalimides can participate in n- and p-type redox reactions at about 2.0 V and above 4.0 V vs. Li/Li⁺, respectively. The redox potentials are sensitive mainly to whether Cl or Br substituents are available in the molecule architecture, while the alkyl chain length determines the kinetics of the redox reactions. Among all compounds, the chloro-substituted compound with the shorter alkyl chain displays the best kinetics for both low- and high-voltage redox reactions. Full article

(This article belongs to the Special Issue Size-Dependent Effects in Materials for Environmental Protection and Energy Application)

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16 pages, 7242 KiB

Open AccessEditor’s ChoiceArticle

Graphene-Based Composites for Thermoelectric Applications at Room Temperature

by Sonya Harizanova, Vassil Vulchev and Radostina Stoyanova

Materials 2023, 16(23), 7262; https://doi.org/10.3390/ma16237262 - 21 Nov 2023

Cited by 1 | Viewed by 757

Abstract

The thermoelectric materials that operate at room temperature represent a scientific challenge in finding chemical compositions with three optimized, independent parameters, namely electrical and thermal conductivity and the Seebeck coefficient. Here, we explore the concept of the formation of hybrid composites between carbon-based [...] Read more.

The thermoelectric materials that operate at room temperature represent a scientific challenge in finding chemical compositions with three optimized, independent parameters, namely electrical and thermal conductivity and the Seebeck coefficient. Here, we explore the concept of the formation of hybrid composites between carbon-based materials and oxides, with the aim of modifying their thermoelectric performance at room temperature. Two types of commercially available graphene-based materials are selected: N-containing reduced graphene oxide (NrGO) and expanded graphite (ExGr). Although the NrGO displays the lowest thermal conductivity at room temperature, the ExGr is characterized by the lowest electrical resistivity and a negative Seebeck coefficient. As oxides, we choose two perspective thermoelectric materials: p-type Ca₃Co₄O₉ and n-type Zn_0.995Al_0.005O. The hybrid composites were prepared by mechanical milling, followed by a pelleting. The thermoelectric efficiency was evaluated on the basis of its measured electrical resistivity, Seebeck coefficient and thermal conductivity at room temperature. It was found that that 2 wt.% of ExGr or NrGO leads to an enhancement of the thermoelectric activity of Ca₃Co₄O₉, while, for Zn_0.995Al_0.005O, the amount of ExGr varies between 5 and 20 wt.%. The effect of the composites’ morphology on the thermoelectric properties is discussed on the basis of SEM/EDS experiments. Full article

(This article belongs to the Special Issue Size-Dependent Effects in Materials for Environmental Protection and Energy Application)

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11 pages, 4880 KiB

Open AccessArticle

Synthesis and Luminescent Properties of Barium Molybdate Nanoparticles

by Maria Gancheva, Reni Iordanova, Iovka Koseva, Georgi Avdeev, Gergana Burdina and Petar Ivanov

Materials 2023, 16(21), 7025; https://doi.org/10.3390/ma16217025 - 03 Nov 2023

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Abstract

BaMoO₄ was obtained via facile mechanochemical synthesis at room temperature and a solid-state reaction. An evaluation of the phase composition and structural and optical properties of BaMoO₄ was conducted. The influence of different milling speeds on the preparation of BaMoO₄ [...] Read more.

BaMoO₄ was obtained via facile mechanochemical synthesis at room temperature and a solid-state reaction. An evaluation of the phase composition and structural and optical properties of BaMoO₄ was conducted. The influence of different milling speeds on the preparation of BaMoO₄ was explored. A shorter reaction time for the phase formation of BaMoO₄ was achieved using a milling speed of 850 rpm. A milling speed of 500 rpm led to partial amorphization of the initial reagents and to prolongation of the synthesis time of up to 3 h of milling time. Solid-state synthesis was performed via heat treatment at 900 °C for 15 h. X-ray diffraction analysis (XRD), infrared (IR) and UV diffuse reflectance (UV-Vis) and photoluminescence (PL) spectroscopy were carried out to characterize the samples. Independently of the method of preparation, the obtained samples had tetragonal symmetry. The average crystallite sizes of all samples, calculated using Scherrer’s formula, were in the range of 240 to 1540 Å. IR spectroscopy showed that more distorted structural MoO₄ units were formed when the compound was synthesized via a solid-state reaction. The optical band gap energy of the obtained materials was found to decrease from 4.50 to 4.30 eV with increasing crystallite sizes. Green- and blue-light emissions were observed for BaMoO₄ phases under excitation wavelengths of 330 and 488 nm. It was established that the intensity of the PL peaks depends on two factors: the symmetry of MoO₄ units and the crystallite sizes. Full article

(This article belongs to the Special Issue Size-Dependent Effects in Materials for Environmental Protection and Energy Application)

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12 pages, 3887 KiB

Open AccessArticle

Preparation, Characterization, and Antibacterial Properties of Cu-Fibreboards

by Lyubomir Aleksandrov, Nadezhda Rangelova, Nevena Lazarova-Zdravkova, Nelly Georgieva, Mirela Dragnevska and Sanchi Nenkova

Materials 2023, 16(21), 6936; https://doi.org/10.3390/ma16216936 - 28 Oct 2023

Cited by 1 | Viewed by 942

Abstract

In the present study, copper modified fibreboards were prepared and their existing phase, morphology, and antibacterial behaviour were investigated. The copper content and the physical and mechanical properties of fibreboards (thickness, bending strength, and swelling) were determined. X-ray diffraction analysis (XRD) showing diffraction [...] Read more.

In the present study, copper modified fibreboards were prepared and their existing phase, morphology, and antibacterial behaviour were investigated. The copper content and the physical and mechanical properties of fibreboards (thickness, bending strength, and swelling) were determined. X-ray diffraction analysis (XRD) showing diffraction peaks typical for cellulose, Cu₂S, and Na₂SO₄, depended on the preparation conditions. The average size of the Cu₂S crystals varied between 20 and 50 nm. The morphology of the obtained fibreboards, as well as the size and shape of copper particles, were observed by scanning electron microscopy (SEM) and transition electron microscopy (TEM). The antibacterial activity was tested against Gram-positive (Bacillus subtilis 3562) and Gram-negative (Escherichia coli K12 407) bacteria. The tests showed that the materials had higher antibacterial activity against E. coli, which depended on their preparation conditions. Based on these results, the obtained copper fibreboards can be used as antibacterial agents in the packaging and building industry. Full article

(This article belongs to the Special Issue Size-Dependent Effects in Materials for Environmental Protection and Energy Application)

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10 pages, 2527 KiB

Open AccessArticle

Hydrogen Storage Properties of Ball Milled MgH₂ with Additives- Ni, V and Activated Carbons Obtained from Different By-Products

by Eli Grigorova, Pavel Markov, Boyko Tsyntsarski, Peter Tzvetkov and Ivanka Stoycheva

Materials 2023, 16(20), 6823; https://doi.org/10.3390/ma16206823 - 23 Oct 2023

Viewed by 873

Abstract

The hydrogen sorption of materials based on 80 wt.% MgH₂ with the addition of 15 wt.% Ni or V and 5 wt.% activated carbons synthesized from polyolefin wax, a waste product from polyethylene production (POW), walnut shells (CAN), and peach stones (CPS) [...] Read more.

The hydrogen sorption of materials based on 80 wt.% MgH₂ with the addition of 15 wt.% Ni or V and 5 wt.% activated carbons synthesized from polyolefin wax, a waste product from polyethylene production (POW), walnut shells (CAN), and peach stones (CPS) prepared by milling under an inert Ar atmosphere for a period of 1 h, is investigated. All precursors are submitted to pyrolysis followed by steam activation in order to obtain the activated carbons. The hydrogen sorption evaluations are carried out for absorption at 473 and 573 K with pressure of 1 MPa and for desorption at 623 and 573 K with pressure of 0.15 MPa. The composition of the samples after milling and hydrogenation is monitored by X-ray diffraction analyses. The 80 wt.% MgH₂–15 wt. %Ni–5 wt.% POW or CAN after absorption–desorption cycling and in a hydrogenated state at 573 K and 1 MPa are analyzed by TEM. Full article

(This article belongs to the Special Issue Size-Dependent Effects in Materials for Environmental Protection and Energy Application)

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23 pages, 9703 KiB

Open AccessArticle

Effect of TiO₂ on Pd/La₂O₃-CeO₂-Al₂O₃ Systems during Catalytic Oxidation of Methane in the Presence of H₂O and SO₂

by Ralitsa Velinova, Silviya Todorova, Daniela Kovacheva, Hristo Kolev, Yordanka Karakirova, Pavel Markov, Katerina Tumbalova, Georgi Ivanov and Anton Naydenov

Materials 2023, 16(20), 6784; https://doi.org/10.3390/ma16206784 - 20 Oct 2023

Viewed by 1617

Abstract

New results on the effect of TiO₂ on Pd/La₂O₃-CeO₂-Al₂O₃ systems for catalytic oxidation of methane in the presence of H₂O and SO₂ have been received. Low-temperature N₂-adsorption, XRD, [...] Read more.

New results on the effect of TiO₂ on Pd/La₂O₃-CeO₂-Al₂O₃ systems for catalytic oxidation of methane in the presence of H₂O and SO₂ have been received. Low-temperature N₂-adsorption, XRD, SEM, HRTEM, XPS, EPR and FTIR techniques were used to characterize the catalyst. The presence of Ce³⁺ on the catalytic surface and in the volume near the lantana was revealed by EPR and XPS. After aging, the following changes are observed: (i) agglomeration of the Pd-clusters (from 8 nm to 12 nm); (ii) transformation of part of the TiO₂ from anatase to larger particles of rutile; and (iii)—the increase in PdO/Pd—ratio above its optimum. The modification by Ti of the La₂O₃-CeO₂-Al₂O₃ system leads to higher resistance towards the presence of SO₂ most likely due to the prevailing formation of unstable surface sulfites instead of thermally stable sulfates. Based on kinetic model calculations, the reaction pathway over the Pd/La₂O₃-CeO₂-TiO₂-Al₂O₃ catalyst follows the Mars–van Krevelen mechanism. For evaluation of the possible practical application of the obtained material, a sample of Pd/La₂O₃-CeO₂-TiO₂-Al₂O₃, supported on rolled aluminum-containing stainless steel (Aluchrom VDM^®), was prepared and tested. Methane oxidation in an industrial-scale monolithic reactor was simulated using a two-dimensional heterogeneous reactor model. Full article

(This article belongs to the Special Issue Size-Dependent Effects in Materials for Environmental Protection and Energy Application)

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16 pages, 7283 KiB

Open AccessEditor’s ChoiceArticle

Network Structure and Luminescent Properties of ZnO–B₂O₃–Bi₂O₃–WO₃:Eu³⁺ Glasses

by Aneliya Yordanova, Margarita Milanova, Reni Iordanova, Margit Fabian, Lyubomir Aleksandrov and Petia Petrova

Materials 2023, 16(20), 6779; https://doi.org/10.3390/ma16206779 - 20 Oct 2023

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Abstract

In this study, we investigated the influence of Bi₂O₃ and WO₃ on both structure and optical properties of 50ZnO:(49 − x)B₂O₃:1Bi₂O₃:xWO₃; x = 1, 5, 10 glasses doped with [...] Read more.

In this study, we investigated the influence of Bi₂O₃ and WO₃ on both structure and optical properties of 50ZnO:(49 − x)B₂O₃:1Bi₂O₃:xWO₃; x = 1, 5, 10 glasses doped with 0.5 mol% Eu₂O₃. IR spectroscopy revealed the presence of trigonal BØ₃ units connecting superstructural groups, [BØ₂O]⁻ metaborate groups, tetrahedral BØ₄⁻ units in superstructural groupings (Ø = bridging oxygen atom), borate triangles with nonbridging oxygen atoms, [WO₄]²⁻ tetrahedral, and octahedral WO₆ species. Neutron diffraction experimental data were simulated by reverse Monte Carlo modeling. The atomic distances and coordination numbers were established, confirming the short-range order found by IR spectra. The synthesized glasses were characterized by red emission at 612 nm. All findings suggest that Eu³⁺ doped zinc borate glasses containing both WO₃ and Bi₂O₃ have the potential to serve as a substitute for red phosphor with high color purity. Full article

(This article belongs to the Special Issue Size-Dependent Effects in Materials for Environmental Protection and Energy Application)

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14 pages, 2510 KiB

Open AccessEditor’s ChoiceArticle

Machine Learning Prediction of the Redox Activity of Quinones

by Ilia Kichev, Lyuben Borislavov, Alia Tadjer and Radostina Stoyanova

Materials 2023, 16(20), 6687; https://doi.org/10.3390/ma16206687 - 14 Oct 2023

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Abstract

The redox properties of quinones underlie their unique characteristics as organic battery components that outperform the conventional inorganic ones. Furthermore, these redox properties could be precisely tuned by using different substituent groups. Machine learning and statistics, on the other hand, have proven to [...] Read more.

The redox properties of quinones underlie their unique characteristics as organic battery components that outperform the conventional inorganic ones. Furthermore, these redox properties could be precisely tuned by using different substituent groups. Machine learning and statistics, on the other hand, have proven to be very powerful approaches for the efficient in silico design of novel materials. Herein, we demonstrated the machine learning approach for the prediction of the redox activity of quinones that potentially can serve as organic battery components. For the needs of the present study, a database of small quinone-derived molecules was created. A large number of quantum chemical and chemometric descriptors were generated for each molecule and, subsequently, different statistical approaches were applied to select the descriptors that most prominently characterized the relationship between the structure and the redox potential. Various machine learning methods for the screening of prospective organic battery electrode materials were deployed to select the most trustworthy strategy for the machine learning-aided design of organic redox materials. It was found that Ridge regression models perform better than Regression decision trees and Decision tree-based ensemble algorithms. Full article

(This article belongs to the Special Issue Size-Dependent Effects in Materials for Environmental Protection and Energy Application)

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0 pages, 8562 KiB

Open AccessArticle

Polycarboxy/Sulfo Betaine—Calcium Phosphate Hybrid Materials with a Remineralization Potential

by Diana Rabadjieva, Rumiana Gergulova, Konstans Ruseva, Alexander Bonchev, Pavletta Shestakova, Marin Simeonov, Radosveta Vasileva, Dragomir Tatchev, Rositsa Titorenkova and Elena Vassileva

Materials 2023, 16(20), 6640; https://doi.org/10.3390/ma16206640 - 11 Oct 2023

Cited by 1 | Viewed by 1000

Abstract

Biomacromolecules control mineral formation during the biomineralization process, but the effects of the organic components’ functionality on the type of mineral phase is still unclear. The biomimetic precipitation of calcium phosphates in a physiological medium containing either polycarboxybetaine (PCB) or polysulfobetaine (PSB) was [...] Read more.

Biomacromolecules control mineral formation during the biomineralization process, but the effects of the organic components’ functionality on the type of mineral phase is still unclear. The biomimetic precipitation of calcium phosphates in a physiological medium containing either polycarboxybetaine (PCB) or polysulfobetaine (PSB) was investigated in this study. Amorphous calcium phosphate (ACP) or a mixture of octacalcium phosphate (OCP) and dicalcium phosphate dihydrate (DCPD) in different ratios were identified depending on the sequence of initial solution mixing and on the type of the negative functional group of the polymer used. The more acidic character of the sulfo group in PSB than the carboxy one in PCB determines the dominance of the acidic solid phases, namely, an acidic amorphous phase or DCPD. In the presence of PCB, the formation of ACP with acicular particles arranged in bundles with the same orientation was observed. A preliminary study on the remineralization potential of the hybrid material with the participation of PSB and a mixture of OCP and DCPD did not show an increase in enamel density, contrary to the materials based on PCB and ACP. Moreover, the latter showed the creation of a newly formed crystal layer similar to that of the underlying enamel. This defines PCB/ACP as a promising material for enamel remineralization. Full article

(This article belongs to the Special Issue Size-Dependent Effects in Materials for Environmental Protection and Energy Application)

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16 pages, 3821 KiB

Open AccessEditor’s ChoiceArticle

Mg, Zn Substituted Calcium Phosphates—Thermodynamic Modeling, Biomimetic Synthesis in the Presence of Low-Weight Amino Acids and High Temperature Properties

by Diana Rabadjieva, Rumiana Gergulova, Kostadinka Sezanova, Daniela Kovacheva and Rositsa Titorenkova

Materials 2023, 16(20), 6638; https://doi.org/10.3390/ma16206638 - 11 Oct 2023

Cited by 1 | Viewed by 973

Abstract

The preparation of specially doped calcium phosphates (CaPs) is receiving a great deal of attention from researchers due to CaPs’ enhanced capabilities for application in medicine. Complexation and precipitation in a complicated electrolyte system including simulated body fluids that are enriched with Mg [...] Read more.

The preparation of specially doped calcium phosphates (CaPs) is receiving a great deal of attention from researchers due to CaPs’ enhanced capabilities for application in medicine. Complexation and precipitation in a complicated electrolyte system including simulated body fluids that are enriched with Mg²⁺ and Zn²⁺ ions and modified with glycine, alanine and valine were first evaluated using a thermodynamic equilibrium model. The influence of the type and concentration of amino acid on the incorporation degree of Mg and Zn into the solid phases was predicted. Experimental studies, designed on the basis of thermodynamic calculations, confirmed the predictions. Amorphous calcium phosphates double-doped with Mg and Zn were biomimetically precipitated and transformed into Mg, Zn-β—tricalcium phosphates (TCP) upon calcination. The Rietveld refinement confirmed that Mg²⁺ and Zn²⁺ substituted Ca²⁺ only at the octahedral sites of β-TCP, and in some cases, fully displacing the Ca²⁺ from them. The resulting Mg, Zn-β–TCP can serve as a reservoir for Mg and Zn ions when included in the formulation of a biomaterial for bone remodeling. The research conducted reveals the effect of combining mathematical models with experimental studies to pre-evaluate the influence of various additives in the design of materials with predetermined properties. Full article

(This article belongs to the Special Issue Size-Dependent Effects in Materials for Environmental Protection and Energy Application)

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16 pages, 4532 KiB

Open AccessArticle

Ultra-Short Laser-Assisted Micro-Structure Formations on Mg/Zn Double-Doped Calcium Phosphate Ceramics for Enhanced Antimicrobial Activity

by Albena Daskalova, Kostadinka Sezanova, Liliya Angelova, Tsvetelina Paunova-Krasteva, Rumiana Gergulova, Daniela Kovacheva and Diana Rabadjieva

Materials 2023, 16(20), 6626; https://doi.org/10.3390/ma16206626 - 10 Oct 2023

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Abstract

Bacterial infection is one of the most common and harmful medical issues following the implantation of materials and devices in the body leading to antibiotic resistance of diverse bacterial strains. In this work, a novel approach is presented combining adopted laser-based patterning method [...] Read more.

Bacterial infection is one of the most common and harmful medical issues following the implantation of materials and devices in the body leading to antibiotic resistance of diverse bacterial strains. In this work, a novel approach is presented combining adopted laser-based patterning method in addition to doping (Mg and Zn) metal ions to prepare calcium phosphate ceramic substrate, applicable in medicine, with enhanced surface antimicrobial characteristics. The preparation of tablets containing Mg (8.44 mol%) and Zn (2.63 mol%) β-tricalcium phosphate involved biomimetic precipitation of amorphous calcium phosphate in media of simulated body fluid enriched with Mg²⁺ and Zn²⁺ ions as well as the presence of valine as an organic additive, followed by step-wise calcination up to 1000 °C. The results from laser processing showed formation of deeper patterns with increased surface roughness (from 4.9 µm to 9.4 µm) as laser power and velocity increase, keeping constant the hatch sizes of 50 µm. The textured surfaces consist of peaks and valleys arrangement that change the morphology of Escherichia coli cells and decrease of cell viability. Our study reveals the possibilities of the application of ultra-short laser radiation as a potential alternative therapy for controlling the antimicrobial effect of the ceramic surface. Full article

(This article belongs to the Special Issue Size-Dependent Effects in Materials for Environmental Protection and Energy Application)

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16 pages, 6647 KiB

Open AccessEditor’s ChoiceArticle

Carbon-Based Composites with Mixed Phosphate-Pyrophosphates with Improved Electrochemical Performance at Elevated Temperature

by Sonya Harizanova, Trajche Tushev, Violeta Koleva and Radostina Stoyanova

Materials 2023, 16(19), 6546; https://doi.org/10.3390/ma16196546 - 04 Oct 2023

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Abstract

Sodium iron phosphate-pyrophosphate, Na₄Fe₃(PO₄)₂P₂O₇ (NFPP) emerges as an excellent cathode material for sodium-ion batteries. Because of lower electronic conductivity, its electrochemical performance depends drastically on the synthesis method. Herein, we provide a [...] Read more.

Sodium iron phosphate-pyrophosphate, Na₄Fe₃(PO₄)₂P₂O₇ (NFPP) emerges as an excellent cathode material for sodium-ion batteries. Because of lower electronic conductivity, its electrochemical performance depends drastically on the synthesis method. Herein, we provide a simple and unified method for synthesis of composites between NFPP and reduced graphene oxide (rGO) and standard carbon black, designed as electrode materials for both sodium- and lithium-ion batteries. The carbon additives affect only the morphology and textural properties of the composites. The performance of composites in sodium and lithium cells is evaluated at elevated temperatures. It is found that NFPP/rGO outperforms NFPP/C in both Na and Li storage due to its hybrid mechanism of energy storage. In sodium half-cells, NFPP/rGO delivers a reversible capacity of 95 mAh/g at 20 °C and 115 mAh/g at 40 °C with a cycling stability of 95% and 88% at a rate of C/2. In lithium half-cells, the capacity reaches a value of 120 mAh/g at 20 and 40 °C, but the cycling stability becomes worse, especially at 40 °C. The electrochemical performance is discussed on the basis of ex situ XRD and microscopic studies. The good Na storage performance of NFPP/rGO at an elevated temperature represents a first step towards its commercialization. Full article

(This article belongs to the Special Issue Size-Dependent Effects in Materials for Environmental Protection and Energy Application)

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21 pages, 3651 KiB

Open AccessArticle

Influence of the Chemical Composition of Ceria Conversion Coatings, Sealed in Solution of NaH₂PO₄ and Ca(NO₃)₂, on the Corrosion Behavior of Aluminum

by Aleksandar Tsanev, Reni Andreeva and Dimitar Stoychev

Materials 2023, 16(19), 6499; https://doi.org/10.3390/ma16196499 - 29 Sep 2023

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Abstract

The corrosion-protective influence of eco-friendly ceria conversion coatings deposited on Al-1050 alloy, additionally treated in mixed NaH₂PO₄ and Ca(NO₃)₂ solution, was studied. The main aim of this work was to investigate how the obtained mixed systems of [...] Read more.

The corrosion-protective influence of eco-friendly ceria conversion coatings deposited on Al-1050 alloy, additionally treated in mixed NaH₂PO₄ and Ca(NO₃)₂ solution, was studied. The main aim of this work was to investigate how the obtained mixed systems of coatings eliminates the negative role of cracks and pores on the surface formed after deposition only of ceria coating. For this purpose, the growth structure, main components and corrosion resistance of the so formed protective systems were investigated by SEM, EDS, XRD, XPS and electrochemical (PDP, Rp, etc.) methods. The results obtained show that the basic components of the conversion layers (before and after exposure in model corrosion media) are characterized by Al₂O₃, Al(OH)₃, CePO₄ and Ca₅(PO₄)₃(OH). Based on these results, the optimal conditions of immersion treatment(s) of Al substrate are established. At these regimes, the relationship of co-deposited Ce³⁺, PO₄³⁺ and Ca²⁺-containing components of the conversion layers determine the maximum values of their polarization resistance—a basic criterion for corrosion protection of Al. This effect is related to the formation of fill out of the defects of the conversion coatings and additional Ca₅(PO₄)₃(OH), CePO₄ AlPO₄ and Al(OH)₃ deposits, leading to the decrease of the corrosion rate. Full article

(This article belongs to the Special Issue Size-Dependent Effects in Materials for Environmental Protection and Energy Application)

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13 pages, 18765 KiB

Open AccessArticle

Formation of Oriented Nanowires from Mixed Metal Oxides

by Anna Dikovska, Genoveva Atanasova, Rumen Nikov, Georgi Avdeev, Zara Cherkezova-Zheleva, Daniela Paneva and Nikolay Nedyalkov

Materials 2023, 16(19), 6446; https://doi.org/10.3390/ma16196446 - 27 Sep 2023

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Abstract

In this study, we present a physical method for the fabrication of oriented nanowires composed of mixed metal oxides. Pulsed laser deposition carried out in the air under atmospheric pressure was used for the production of samples. Two sets of experiments were performed [...] Read more.

In this study, we present a physical method for the fabrication of oriented nanowires composed of mixed metal oxides. Pulsed laser deposition carried out in the air under atmospheric pressure was used for the production of samples. Two sets of experiments were performed by applying nanosecond and picosecond laser ablation, respectively. The depositions were performed using the laser ablation of mixed targets from iron oxide and zinc oxide as the initial materials in different ratios. The experiments were carried out in a magnetic field, which allowed us to control the morphology of nanostructures. The structure, microstructure, morphology, and composition of the structures obtained were studied in relation to the sample composition and laser ablation regime applied. The morphological analysis revealed that the structure of the samples consisted mainly of nanowire-like features reaching tens of micrometers in length. These nanowires were composed of nanoparticles and oriented predominantly in parallel to magnetic field lines. Nanoparticles produced using ps ablation were, on average, smaller than those obtained by ns ablation of the same target. Using ablation with ps laser pulses, we were able to produce new composite materials or materials containing unstable phases. Full article

(This article belongs to the Special Issue Size-Dependent Effects in Materials for Environmental Protection and Energy Application)

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16 pages, 3283 KiB

Open AccessEditor’s ChoiceArticle

Effect of B₂O₃ on the Structure, Properties and Antibacterial Abilities of Sol-Gel-Derived TiO₂/TeO₂/B₂O₃ Powders

by Albena Bachvarova-Nedelcheva, Reni Iordanova, Angelina Stoyanova, Nelly Georgieva, Veronica Nemska and Tsvetelina Foteva

Materials 2023, 16(19), 6400; https://doi.org/10.3390/ma16196400 - 25 Sep 2023

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Abstract

This paper studies the influence of B₂O₃ on the structure, properties and antibacterial abilities of sol-gel-derived TiO₂/TeO₂/B₂O₃ powders. Titanium(IV) butoxide, telluric(VI) acid and boric acid were used as precursors. Differences were observed in [...] Read more.

This paper studies the influence of B₂O₃ on the structure, properties and antibacterial abilities of sol-gel-derived TiO₂/TeO₂/B₂O₃ powders. Titanium(IV) butoxide, telluric(VI) acid and boric acid were used as precursors. Differences were observed in the degree of decomposition of Ti butoxide in the presence of H₃BO₃ and H₆TeO₆ acids. The phase transformations of the obtained gels in the temperature range of 200–700 °C were investigated by XRD. Composite materials containing an amorphous phase and different crystalline phases (metallic Te, α-TeO₂, anatase, rutile and TiTe₃O₈) were prepared. Heating at 400 °C indicated a crystalline-to-amorphous-phase ratio of approximately 3:1. The scanning electron microscopy (SEM) analysis showed the preparation of plate-like TiO₂ nanoparticles. The IR results showed that the short-range order of the amorphous phases that are part of the composite materials consists of TiO₆, BO₃, BO₄ and TeO₄ structural units. Free B₂O₃ was not detected in the investigated compositions which could be related to the better connectivity between the building units as compared to binary TiO₂/B₂O₃ compositions. The UV-Vis spectra of the investigated gels exhibited a red shift of the cut-off due to the presence of boron and tellurium units. The binary sample achieved the maximum photodegradation efficiency (94%) toward Malachite green dye under UV irradiation, whereas the ternary sample photoactivity was very low. The compositions exhibited promising antibacterial activity against E. coli NBIMCC K12 407. Full article

(This article belongs to the Special Issue Size-Dependent Effects in Materials for Environmental Protection and Energy Application)

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14 pages, 29175 KiB

Open AccessArticle

Picosecond Pulsed Laser Deposition of Metals and Metal Oxides

by Anna Dikovska, Genoveva Atanasova, Tina Dilova, Aleksandra Baeva, Georgi Avdeev, Petar Atanasov and Nikolay Nedyalkov

Materials 2023, 16(19), 6364; https://doi.org/10.3390/ma16196364 - 22 Sep 2023

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Abstract

In this work, we present the fabrication of thin films/nanostructures of metals and metal oxides using picosecond laser ablation. Two sets of experiments were performed: the depositions were carried out in vacuum and in air at atmospheric pressure. The subjects of investigation were [...] Read more.

In this work, we present the fabrication of thin films/nanostructures of metals and metal oxides using picosecond laser ablation. Two sets of experiments were performed: the depositions were carried out in vacuum and in air at atmospheric pressure. The subjects of investigation were the noble metals Au and Pt and the metal oxides ZnO and TiO₂. We studied and compared the phase composition, microstructure, morphology, and physicochemical state of the as-deposited samples’ surfaces in vacuum and in air. It was found that picosecond laser ablation performed in vacuum led to the fabrication of thin films with embedded and differently sized nanoparticles. The implementation of the same process in air at atmospheric pressure resulted in the fabrication of porous nanostructures composed of nanoparticles. The ablation of pure Pt metal in air led to the production of nanoparticles with an oxide shell. In addition, more defects were formed on the metal oxide surface when the samples were deposited in vacuum. Furthermore, the laser ablation process of pure Au metal in a picosecond regime in vacuum and in air was theoretically investigated using molecular dynamics simulation. Full article

(This article belongs to the Special Issue Size-Dependent Effects in Materials for Environmental Protection and Energy Application)

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18 pages, 3901 KiB

Open AccessEditor’s ChoiceArticle

Study on the Dye Removal from Aqueous Solutions by Graphene-Based Adsorbents

by Paunka Vassileva, Vencislav Tumbalev, Diana Kichukova, Dimitrinka Voykova, Daniela Kovacheva and Ivanka Spassova

Materials 2023, 16(17), 5754; https://doi.org/10.3390/ma16175754 - 22 Aug 2023

Cited by 1 | Viewed by 879

Abstract

In the current investigation, the removal efficiency regarding a cationic dye, methylene blue (MB), from three graphene-based materials was investigated. The materials’ characterization process involved instrumental methods such as XRD, XPS, SEM, TEM, FTIR, and nitrogen adsorption at 77 K. The survey examined [...] Read more.

In the current investigation, the removal efficiency regarding a cationic dye, methylene blue (MB), from three graphene-based materials was investigated. The materials’ characterization process involved instrumental methods such as XRD, XPS, SEM, TEM, FTIR, and nitrogen adsorption at 77 K. The survey examined how various process factors influenced the ability of the studied materials to adsorb cationic dyes. These parameters encompassed contact time, initial dye concentrations, solution pH, and temperature. The adsorption procedure was effectively explained through the application of pseudo-second-order and Langmuir models. The maximum adsorption capacity for the best adsorbent at 293 K was found to be 49.4 mg g⁻¹. In addition, the study also determined the entropy, enthalpy, and Gibbs free energy values associated with the removal of MB and showed that the adsorption of MB is endothermic, feasible, and spontaneous. The results also revealed that the studied materials are suitable adsorbents for the removal of cationic dyes. Full article

(This article belongs to the Special Issue Size-Dependent Effects in Materials for Environmental Protection and Energy Application)

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16 pages, 16702 KiB

Open AccessEditor’s ChoiceArticle

Effect of Cu as a Minority Alloying Element on the Corrosion Behaviour of Amorphous and Crystalline Al-Ni-Si Alloy

by Vanya Dyakova, Yoanna Kostova, Boriana Tzaneva, Hristina Spasova and Daniela Kovacheva

Materials 2023, 16(15), 5446; https://doi.org/10.3390/ma16155446 - 03 Aug 2023

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Abstract

The effect of copper as a minority alloying element on the corrosion behaviour of amorphous and crystalline Al₇₄Ni₁₆Si₁₀ and Al₇₄Ni₁₅Si₉Cu₂ alloys was investigated in this study. Amorphous alloys were produced as [...] Read more.

The effect of copper as a minority alloying element on the corrosion behaviour of amorphous and crystalline Al₇₄Ni₁₆Si₁₀ and Al₇₄Ni₁₅Si₉Cu₂ alloys was investigated in this study. Amorphous alloys were produced as rapidly solidified ribbons using the Chill Block Melt Spinning (CBMS) method and subsequently annealed to complete crystallisation. The corrosion rate of alloys was obtained through continuous immersion tests in 3.5% NaCl at 25 °C and 50 °C for 360 h. The electrochemical parameters corrosion current density (J_corr) and corrosion potential (E_corr) were determined via the potentiodynamic polarisation test. The results showed better corrosion characteristics of amorphous alloys. The addition of 2 at.% copper to the Al₇₄Ni₁₆Si₁₀ alloy improved pitting corrosion resistance without significant effect on the corrosion current and potential. In immersion tests at 25 °C, the presence of copper resulted in an increase in the corrosion rate of about 300% for both amorphous and crystalline alloys. At a temperature of 50 °C, this increase is on average 130%. The apparent difference between the results of the two test methods is discussed in terms of the imperfections on the surface of rapidly solidified ribbons. The results of this study will contribute to a more complex understanding of the nature of amorphous alloys and their application. Full article

(This article belongs to the Special Issue Size-Dependent Effects in Materials for Environmental Protection and Energy Application)

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14 pages, 3258 KiB

Open AccessArticle

Impact of Beam Deflection Geometry on the Surface Architecture and Mechanical Properties of Electron-Beam-Modified TC4 Titanium Alloy

by Maria Ormanova, Borislav Stoyanov, Nikolay Nedyalkov and Stefan Valkov

Materials 2023, 16(15), 5237; https://doi.org/10.3390/ma16155237 - 26 Jul 2023

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Abstract

This paper aims to investigate the impact of beam deflection geometry on the structure, surface architecture, and friction coefficient of electron-beam-modified TC4 titanium alloys. During the experiments, the electron beam was deflected in the form of different scanning geometries, namely linear, circular, and [...] Read more.

This paper aims to investigate the impact of beam deflection geometry on the structure, surface architecture, and friction coefficient of electron-beam-modified TC4 titanium alloys. During the experiments, the electron beam was deflected in the form of different scanning geometries, namely linear, circular, and matrix. The structure of the treated specimens was investigated in terms of their phase composition by employing X-ray diffraction experiments. The microstructure was studied by scanning electron microscopy (SEM). The surface architecture was examined by atomic force microscopy (AFM). The friction coefficient was studied by a mechanical wear test. It was found that the linear and circular deflection geometries lead to a transformation of the phase composition, from double-phase α + β to α’ martensitic structure. The application of a linear manner of scanning leads to a residual amount of beta phase. The use of a matrix does not tend to structural changes on the surface of the TC4 alloy. In the case of linear geometry, the thickness of the modified zone is more than 800 μm while, in the case of EBSM using circular scanning, the thickness is about 160 μm. The electron-beam surface modification leads to a decrease in the surface roughness to about 27 nm in EBSM with linear deflection geometry and 31 nm in circular deflection geometry, compared to that of the pure TC4 substrate (about 160 nm). The electron-beam surface modification of the TC4 alloy leads to a decrease in the coefficient of friction (COF), with the lowest COF values obtained in the case of linear deflection geometry (0.32). The results obtained in this study show that beam deflection geometry has a significant effect on the surface roughness and friction coefficient of the treated surfaces. It was found that the application of a linear manner of scanning leads to the formation of a surface with the lowest roughness and friction coefficient. Full article

(This article belongs to the Special Issue Size-Dependent Effects in Materials for Environmental Protection and Energy Application)

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