Corrosion Behavior of an Mg2Sn Alloy

In the present work, the corrosion behavior of the Mg₂Sn alloy (Mg_66.7Sn_33.3, concentration in at.%) has been studied. The alloy was prepared from high purity Sn and Mg lumps by induction melting in argon. The alloy was composed of intermetallic Mg₂Sn with a small amount of Mg₂Sn + (Sn) eutectic. The corrosion behavior was studied by hydrogen evolution, immersion, and potentiodynamic experiments. Three aqueous solutions of NaCl (3.5 wt.%), NaOH (0.1 wt.%) and HCl (0.1 wt.%) were chosen as corrosion media. The alloy was found to be cathodic with respect to metallic Mg and anodic with respect to Sn. The corrosion potentials of the Mg₂Sn alloy were −1380, −1498 and −1361 mV vs. sat. Ag/AgCl in HCl, NaCl and NaOH solutions, respectively. The highest corrosion rate of the alloy, 92 mmpy, was found in aqueous HCl. The high corrosion rate was accompanied by massive hydrogen evolution on the alloy’s surface. The corrosion rate was found to decrease sharply with increasing pH of the electrolyte. In the NaOH electrolyte, a passivation of the alloy was observed. The corrosion of the alloy involved a simultaneous oxidation of Mg and Sn. The main corrosion products on the alloy surface were MgSn(OH)₆ and Mg(OH)₂. The corrosion mechanism is discussed and implications for practical applications of the alloy are provided.     

Facilitated Synthesis of Mg2Ni Based Composites with Attractive Hydrogen Sorption Properties

Composites based on Mg₂Ni with 5% activated carbon from apricot stones (ACAP) have been prepared by ball milling and subsequent annealing in hydrogen atmosphere. The purpose of the primary metal (Mg, Ni, and V) milling was to reduce the particle size and achieve a good contact between them, without forming intermetallic compounds. During hydriding/dehydriding at 300 °C the amount of the Mg₂Ni phase progressively increased, and after 10 cycles about 50% Mg₂(Ni,V) was achieved. The hydrogenation produced mainly Mg₂NiH₄, but small amounts of MgH₂ and VH_x were also detected in the powder mixture. Relatively high hydrogen storage capacity and fast hydriding/dehydriding kinetics of the Mg_2.1Ni_0.7V_0.3—5 wt.% ACAP composite were determined both from hydrogen gas phase and electrochemically.     

Calculation Based on the Formation of Mg2Si and Its Effect on the Microstructure and Properties of Al–Si Alloys

In order to investigate the effect of Mg₂Si formation on the microstructure and properties of an Al−Si alloy, the critical point of a hypereutectic Al−17Si−4Cu−Mg alloy was calculated by Pandat software. The calculation results of the equilibrium phase diagram show that the critical point for Mg₂Si phase formation for the alloy was obtained when the Mg content was 2.2%. The contents of 0.5 wt.% Mg and 2.5 wt.% Mg were selected as the research object. The content of Mg increased from 0.5 wt.% to 2.5 wt.%, the eutectic Si in the matrix was reduced, and the Chinese character-like Mg₂Si phase appeared in the microstructure. In the peak ageing state, in addition to θ″ and Q′ phases that were mainly precipitated, there was also needle-like β″ precipitation in the 2.5 wt.% Mg content alloy. Larger precipitates were found in 2.5 wt.% content alloys, mainly due to the promotion of the solid solution having the aggregation and segregation of more solute elements in the matrix. The tensile strength, elongation, and hardness of hypereutectic Al−17Si−4Cu−0.5Mg alloy under peak ageing were 331 MPa, 3.11%, and 152.1 HB, respectively. The tensile strength and the elongation decreased while the hardness increased with the 2.5 wt.% Mg content, which is due to the formation of hard and brittle Mg₂Si and Al₈FeMg₃Si, which has a splitting effect on the matrix.     

Effect of Nano-Y2O3 on the Microstructure and Properties of WC-Reinforced Ni-Based Composite Surfacing Layer

In this study, a WC-reinforced Ni-based surfacing layer was prepared on Q235 steel plate by plasma arc welding. The effects of nano-Y₂O₃ with different contents (0 wt.%, 0.4 wt.%, 0.8 wt.%, 1.2 wt.%, and 1.6 wt.%) on the microstructure, phase composition, microhardness, and wear resistance of the surfacing layer were studied by scanning electron microscope (SEM), energy dispersive spectrometer (EDS), X-ray diffraction (XRD), microhardness test, and pin-on-disk test. The results show that the phase composition of the surfacing layer was γ-Ni, FeNi₃ solid solution, WC, W₂C, M₂₃C6, M₆C, Cr₇C₃, and other carbides. When the addition of nano-Y₂O₃ was 1.2 wt.%, it has a good improvement on microstructure grain refinement and carbide hard-phase increase. Compared with other contents, 1.2 wt.% nano-Y₂O₃ surfacing layer has the highest microhardness and the lowest friction coefficient and wear loss. At this time, the wear mechanism is abrasive wear accompanied by slight adhesive wear.     

Influence of Aging on Corrosion Behaviour of the 6061 Cast Aluminium Alloy

The influence of AlFeSi and Mg₂Si phases on corrosion behaviour of the cast 6061 aluminium alloy was investigated. Scanning Kelvin probe force microscopy (SKPFM), electron probe microanalysis (EPMA), and in situ observations by confocal laser scanning microscopy (CLSM) were used. It was found that Mg₂Si phases were anodic relative to the matrix and dissolved preferentially without significantly affecting corrosion propagation. The AlFeSi phases’ influence on 6061 aluminium alloy local corrosion was greater than that of the Mg₂Si phases. The corroded region width reached five times that of the AlFeSi phase, and the accelerating effect was terminated as the AlFeSi dissolved.     

Effect of Al2Ca Addition and Heat Treatment on the Microstructure Modification and Tensile Properties of Hypo-Eutectic Al–Mg–Si Alloys

The current study investigated the microstructure modification in Al–6Mg–5Si–0.15Ti alloy (in mass %) through the minor addition of Ca using Mg + Al₂Ca master alloy and heat treatment to see their impact on mechanical properties. The microstructure of unmodified alloy (without Ca) consisted of primary Al, primary Mg₂Si, binary eutectic Al–Mg₂Si, ternary eutectic Al–Mg₂Si–Si, and iron-bearing phases. The addition of 0.05 wt% Ca resulted in significant microstructure refinement. In addition to refinement, lamellar to fibrous-type modification of binary eutectic Al–Mg₂Si phases was also achieved in Ca-added (modified) alloy. This modification was related to increasing Ca-based intermetallics/compounds in the modified alloy that acted as nucleation sites for binary eutectic Al–Mg₂Si phases. The dendritic refinement with Ca addition was related to the fact that it improves the efficacy of Ti-based particles (TiAl₃ and TiB₂) in the melt to act as nucleation sites. In contrast, the occupation of oxide bifilms by Ca-based phases is expected to force the iron-bearing phases (as iron-bearing phases nucleate at oxide films) to solidify at lower temperatures, thus reducing their size. The as-cast microstructure of these alloys was further modified by subjecting them to solution treatment at 540 °C for 6 h, which broke the eutectic structure and redistributed Mg₂Si and Si phases in Al-matrix. Subsequent aging treatment caused a dramatic increase in the tensile strength of these alloys, and tensile strength of 291 MPa (with El% of 0.45%) and 327 MPa (with El% of 0.76%) was achieved for the unmodified alloy and modified alloy, respectively. Higher tensile strength and elongation of the modified alloy than unmodified alloy was attributed to refined dendritic structure and modified second phases.     

Heat Treatment-Induced Microstructure and Property Evolution of Mg/Al Intermetallic Compound Coatings Prepared by Al Electrodeposition on Mg Alloy from Molten Salt Electrolytes

A method of forming an Mg/Al intermetallic compound coating enriched with Mg₁₇Al₁₂ and Mg₂Al₃ was developed by heat treatment of electrodeposition Al coatings on Mg alloy at 350 °C. The composition of the Mg/Al intermetallic compounds could be tuned by changing the thickness of the Zn immersion layer. The morphology and composition of the Mg/Al intermetallic compound coatings were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), and electron backscattered diffraction (EBSD). Nanomechanical properties were investigated via nano-hardness (nHV) and the elastic modulus (EIT), and the corrosion behavior was studied through hydrogen evolution and potentiodynamic (PD) polarization. The compact and uniform Al coating was electrodeposited on the Zn-immersed AZ91D substrate. After heat treatment, Mg₂Al₃ and Mg₁₇Al₁₂ phases formed, and as the thickness of the Zn layer increased from 0.2 to 1.8 μm, the ratio of Mg₂Al₃ and Mg₁₇Al₁₂ varied from 1:1 to 4:1. The nano-hardness increased to 2.4 ± 0.5 GPa and further improved to 3.5 ± 0.1 GPa. The Mg/Al intermetallic compound coating exhibited excellent corrosion resistance and had a prominent effect on the protection of the Mg alloy matrix. The control over the ratio of intermetallic compounds by varying the thickness of the Zn immersion layer can be an effective approach to achieve the optimal comprehensive performance. As the Zn immersion time was 4 min, the obtained intermetallic compounds had relatively excellent comprehensive properties.     

The Effect of Sn Addition on Zn-Al-Mg Alloy; Part I: Microstructure and Phase Composition

In this study, the addition of Sn on the microstructure of Zn 1.6 wt.% Al 1.6 wt.% Mg alloy was studied. Currently, the addition of Sn into Zn-Al-Mg based systems has not been investigated in detail. Both as-cast and annealed states were investigated. Phase transformation temperatures and phase composition was investigated via DSC, SEM and XRD techniques. The main phases identified in the studied alloys were η(Zn) and α(Al) solid solutions as well as Mg₂Zn₁₁, MgZn₂ and Mg₂Sn intermetallic phases. Addition of Sn enabled the formation of Mg₂Sn phase at the expense of Mg_xZn_y phases, while the overall volume content of intermetallic phases is decreasing. Annealing did not change the phase composition in a significant way, but higher Sn content allowed more effective spheroidization and agglomeration of individual phase particles.     

Analysis of the Crystallization Kinetics and Thermal Stability of the Amorphous Mg72Zn24Ca4 Alloy

The aim of this study was to analyze the crystallization of the Mg₇₂Zn₂₄Ca₄ metallic glass alloy. The crystallization process of metallic glass Mg₇₂Zn₂₄Ca₄ was investigated by means of the differential scanning calorimetry. The glass-forming ability and crystallization are both strongly dependent on the heating rate. The crystallization kinetics, during the isothermal annealing, were modelled by the Johnson–Mehl–Avrami equation. Avrami exponents were from 2.7 to 3.51, which indicates diffusion-controlled grain growth. Local exponents of the Johnson–Mehl–Avrami equation were also calculated. In addition, the Mg phase—being the isothermal crystallization product—was found, and the diagram of the time–temperature phase transformation was developed. This diagram enables the reading of the start and end times of the crystallization process, occurring in amorphous ribbons of the Mg₇₂Zn₂₄Ca₄ alloy on the isothermal annealing temperature. The research showed high stability of the amorphous structure of Mg₇₂Zn₂₄Ca₄ alloy at human body temperature.     

Peculiarities of the Phase Formation during Electroconsolidation of Al2O3–SiO2–ZrO2 Powders Mixtures

This paper is devoted to the sintering process of Al₂O₃–SiO₂–ZrO₂ ceramics. The studied method was electroconsolidation with directly applied electric current. This method provides substantial improvements to the mechanical properties of the sintered samples compared to the traditional sintering in the air. The research covered elemental and phase analysis of the samples, which revealed phase transition of high-alumina solid solutions into mullite and corundum. Zirconia was represented mainly by tetragonal phase, but monoclinic phase was present, too. Electroconsolidation enabled samples to reach a density of 3.0 g/cm³ at 1300 °C, while the sample prepared by traditional sintering method obtained it only at 1700 °C. For the composite Al₂O₃—20 wt.% SiO₂—10 wt.% ZrO₂ fabricated by electroconsolidation, it was demonstrated that fracture toughness was higher by 20–30%, and hardness was higher by 15–20% compared to that of samples sintered traditionally. Similarly, the samples fabricated by electroconsolidation exhibited elastic modulus E higher by 15–20%. The hypothesis was proposed that the difference in mechanical and physical properties could be attributed to the peculiarities of phase formation processes during electroconsolidation.     

Investigation of the Structure and Corrosion Resistance of Novel High-Entropy Alloys for Potential Biomedical Applications

High-entropy alloys are a new generation of materials that have attracted the interest of numerous scientists because of their unusual properties. It seems interesting to use these alloys in biomedical applications. However, for this purpose, the basic condition of corrosion resistance must be fulfilled. In this article, selected corrosion properties of self-composed high-entropy alloys are investigated and compared with conventional biomedical alloys, that is titanium alloys and stainless steels. Corrosive parameters were determined using the potentiodynamic method. X-ray diffraction studies were performed to characterize the crystal structures. Microstructures of the prepared materials were examined using a scanning electron microscope, and surface hardness was measured by the Vickers method. The results show that investigated high-entropy alloys are characterized by simple structures. Three out of four tested high-entropy alloys had better corrosion properties than conventional implant alloys used in medicine. The Al_0.7CoCrFeNi alloy was characterized by a corrosion potential of −224 mV and a corrosion current density of 0.9 μA/cm²; CoCrFeNiCu by −210 mV and 1.1 μA/cm²; TiAlFeCoNi by −435 mV and 4.6 μA/cm²; and Mn_0.5TiCuAlCr by −253 mV and 1.3 μA/cm², respectively. Therefore, the proposed high-entropy alloys can be considered as potential materials for biomedical applications, but this requires more studies to confirm their biocompatibility.     

Synergistic Effects of Pr6O11 and Co3O4 on Electrical and Microstructure Features of ZnO-BaTiO3 Varistor Ceramics

This paper investigated the effects of Pr₆O₁₁ and Co₃O₄ on the electrical properties of ZnO-BaTiO₃ varistor ceramics. The Pr₆O₁₁ doping has a notable influence on the characteristics of the nonlinear coefficient, varistor voltage, and leakage current where the values varied from 2.29 to 2.69, 12.36 to 68.36 V/mm and 599.33 to 548.16 µA/cm², respectively. The nonlinear varistor coefficient of 5.50 to 7.15 and the varistor voltage of 7.38 to 8.10 V/mm was also influenced by the use of Co₃O₄ as a dopant. When the amount of Co₃O₄ was above 0.5 wt.%, the leakage current increased from 202.41 to 302.71 μA/cm². The varistor ceramics with 1.5 wt.% Pr₆O₁₁ shows good nonlinear electrical performance at higher breakdown voltage and reduced the leakage current of the ceramic materials. Besides, the varistor sample that was doped with 0.5 wt.% Co₃O₄ was able to enhance the nonlinear electrical properties at low breakdown voltage with a smaller value of leakage current.     

The Effect of Ca Dopant on the Electrical and Dielectric Properties of BaTi4O9 Sintered Ceramics

The current research examines the impact of Ca²⁺ substitution on the phase and electrical properties of (Ba_1−xCa_x)Ti₄O₉, (x = 0.0, 0.3, 0.6, and 0.9) sintered pellets synthesized by solid-state reaction method. The as-synthesized samples were analyzed using X-ray diffraction (XRD) and impedance spectroscopy. The emergence of orthorhombic phase fit into space group Pnmm was revealed by XRD, and the addition of Ca resulted in a considerable shift in grain size. Dielectric properties were determined using an impedance spectroscopy in a wide frequency range from 1MHz to 3 GHz. The dielectric properties i.e., dielectric constant (ε_r) and dielectric loss (tanσ), were measured at 3 GHz frequency. The frequency-dependent parameters such as conductivity, dielectric constant, and dielectric loss indicated that the relaxation process is a Maxwell–Wagner type of interfacial polarization. The improved dielectric properties and low energy loss have made (Ba_1−xCa_x)Ti₄O₉ a prominent energy storage material. This study provides the possibility to improve its dielectric properties and reduce energy loss, making it an excellent energy storage material.     

High-Temperature Chemical Stability of Cr(III) Oxide Refractories in the Presence of Calcium Aluminate Cement

Al₂O₃-CaO-Cr₂O₃ castables are used in various furnaces due to excellent corrosion resistance and sufficient early strength, but toxic Cr(VI) generation during service remains a concern. Here, we investigated the relative reactivity of analogous Cr(III) phases such as Cr₂O₃, (Al_1−xCr_x)₂O₃ and in situ Cr(III) solid solution with the calcium aluminate cement under an oxidizing atmosphere at various temperatures. The aim is to comprehend the relative Cr(VI) generation in the low-cement castables (Al₂O₃-CaO-Cr₂O₃-O₂ system) and achieve an environment-friendly application. The solid-state reactions and Cr(VI) formation were investigated using powder XRD, SEM, and leaching tests. Compared to Cr₂O₃, the stability of (Al_1−xCr_x)₂O₃ against CAC was much higher, which improved gradually with the concentration of Al₂O₃ in (Al_1−xCr_x)₂O₃. The substitution of Cr₂O₃ with (Al_1−xCr_x)₂O₃ in the Al₂O₃-CaO-Cr₂O₃ castables could completely inhibit the formation of Cr(VI) compound CaCrO₄ at 500–1100 °C and could drastically suppress Ca₄Al₆CrO₁₆ generation at 900 to 1300 °C. The Cr(VI) reduction amounting up to 98.1% could be achieved by replacing Cr₂O₃ with (Al_1−xCr_x)₂O₃ solid solution. However, in situ stabilized Cr(III) phases as a mixture of (Al_1−xCr_x)₂O₃ and Ca(Al_12−xCr_x)O₁₉ solid solution hardly reveal any reoxidation. Moreover, the CA₆ was much more stable than CA and CA₂, and it did not participate in any chemical reaction with (Al_1−xCr_x)₂O₃ solid solution.     

Influence of Co Content and Chemical Nature of the Co Binder on the Corrosion Resistance of Nanostructured WC-Co Hardmetals in Acidic Solution

The electrochemical corrosion resistance of nanostructured hardmetals with grain sizes d_WC < 200 nm was researched concerning Co content and the chemical nature of the Co binder. Fully dense nanostructured hardmetals with the addition of grain growth inhibitors GGIs, VC and Cr₃C₂, and 5 wt.%Co, 10 wt.%Co, and 15 wt.%Co were developed by a one cycle sinter-HIP process. The samples were detailly characterized in terms of microstructural characteristics and researched in the solution of H₂SO₄ + CO₂ by direct and alternative current techniques, including electrochemical impedance spectroscopy. Performed analysis revealed a homogeneous microstructure of equal and uniform grain size for different Co contents. The importance of GGIs content adjustment was established as a key factor of obtaining a homogeneous microstructure with WC grain size retained at the same values as in starting mixtures of different Co binder content. From the conducted research, Co content has shown to be the dominant influential factor governing electrochemical corrosion resistance of nanostructured hardmetals compared to the chemical composition of the Co binder and WC grain size. Negative values of E_corr measured for 30 min in 96% H₂SO₄ + CO₂ were obtained for all samples indicating material dissolution and instability in acidic solution. Higher values of R_p and lower values of i_corr and v_corr were obtained for samples with lower Co content. In contrast, the anodic Tafel slope increases with increasing Co content which could be attributed to more pronounced oxidation of the higher Co content samples. Previously researched samples with the same composition but different chemical composition of the binder were introduced in the analysis. The chemical composition of the Co binder showed an influence; samples with lower relative magnetic saturation related to lower C content added to the starting mixtures and more W dissolved in the Co binder during the sintering process showed better corrosion resistance. WC-5Co sample with significantly lower magnetic saturation value showed approximately 30% lower corrosion rate. WC-10Co sample with slightly lower relative magnetic saturation value and showed approximately 10% lower corrosion rate. Higher content of Cr₃C₂ dissolved in the binder contributed to a lower corrosion rate. Slight VC increase did not contribute to corrosion resistance. Superior corrosion resistance is attributed to W and C dissolved in the Co binder, lower magnetic saturation, or WC grain size of the sintered sample.     

Pulse Plasma Sintering of NiAl-Al2O3 Composite Powder Produced by Mechanical Alloying with Contribution of Nanometric Al2O3 Powder

NiAl-Al₂O₃ composites, fabricated from the prepared composite powders by mechanical alloying and then consolidated by pulse plasma sintering, were presented. The use of nanometric alumina powder for reinforcement of a synthetized intermetallic matrix was the innovative concept of this work. Moreover, this is the first reported attempt to use the Pulse Plasma Sintering (PPS) method to consolidate composite powder with the contribution of nanometric alumina powder. The composite powders consisting of the intermetallic phase NiAl and Al₂O₃ were prepared by mechanical alloying from powder mixtures containing Ni-50at.%Al with the contribution of 10 wt.% or 20 wt.% nanometric aluminum oxide. A nanocrystalline NiAl matrix was formed, with uniformly distributed Al₂O₃ inclusions as reinforcement. The PPS method successfully consolidated NiAl-Al₂O₃ composite powders with limited grain growth in the NiAl matrix. The appropriate sintering temperature for composite powder was selected based on analysis of the grain growth and hardness of Al₂O₃ subjected to PPS consolidation at various temperatures. As a result of these tests, sintering of the NiAl-Al₂O₃ powders was carried out at temperatures of 1200 °C, 1300 °C, and 1400 °C. The microstructure and properties of the initial powders, composite powders, and consolidated bulk composite materials were characterized by SEM, EDS, XRD, density, and hardness measurements. The hardness of the ultrafine-grained NiAl-Al₂O₃ composites obtained via PPS depends on the Al₂O₃ content in the composite, as well as the sintering temperature applied. The highest values of the hardness of the composites were obtained after sintering at the lowest temperature (1200 °C), reaching 7.2 ± 0.29 GPa and 8.4 ± 0.07 GPa for 10 wt.% Al₂O₃ and 20 wt.% Al₂O₃, respectively, and exceeding the hardness values reported in the literature. From a technological point of view, the possibility to use sintering temperatures as low as 1200 °C is crucial for the production of fully dense, ultrafine-grained composites with high hardness.     

Influence of MXene (Ti3C2) Phase Addition on the Microstructure and Mechanical Properties of Silicon Nitride Ceramics

This paper discusses the influence of Ti₃C₂ (MXene) addition on silicon nitride and its impact on the microstructure and mechanical properties of the latter. Composites were prepared through powder processing and sintered using the spark plasma sintering (SPS) technic. Relative density, hardness and fracture toughness, were analyzed. The highest fracture toughness at 5.3 MPa·m^1/2 and the highest hardness at HV5 2217 were achieved for 0.7 and 2 wt.% Ti₃C₂, respectively. Moreover, the formation of the Si₂N₂O phase was observed as a result of both the MXene addition and the preservation of the α-Si₃N₄→β-Si₃N₄ phase transformation during the sintering process.     

The Formation Process and Strengthening Mechanism of SiC Nanowires in a Carbon-Coated Porous BN/Si3N4 Ceramic Joint

Porous BN/Si₃N₄ ceramics carbon-coated by carbon coating were joined with SiCo38 (wt. %) filler. The formation process and strengthening mechanism of silicon carbide nanowires to the joint were analyzed in detail. The outcome manifests that there is no distinct phase change in the porous BN/Si₃N₄ ceramic without carbon-coated joint. The highest joint strength was obtained at 1320 °C (~38 MPa). However, a larger number of silicon carbide nanowires were generated in the carbon-coated joints. The highest joint strength of the carbon-coated joint was ~89 MPa at 1340 °C. Specifically, silicon carbide nanowires were formed by the reaction of the carbon coated on the porous BN/Si₃N₄ ceramic with the SiCo38 filler via the Vapor-Liquid-Solid (VLS) method and established a bridge in the joint. It grows on the β-SiC (111) crystal plane and the interplanar spacing is 0.254 nm. It has a bamboo-like shape with a resemblance to alloy balls on the ends, and its surface is coated with SiO₂. The improved carbon-coated porous BN/Si₃N₄ joint strength is possibly ascribed to the bridging of nanowires in the joint.     

Microgalvanic Corrosion of Mg–Ca and Mg–Al–Ca Alloys in NaCl and Na2SO4 Solutions

As a kind of potential biomedical material, Mg–Ca alloy has attracted much attention. However, the role of Ca-containing intermetallics in microgalvanic corrosion is still controversial. In 0.6 mol/L NaCl and Na₂SO₄ solutions, the microgalvanic corrosion behavior of the second phase and Mg matrix of Mg–Ca and Mg–Al–Ca alloys was examined. It was confirmed that the Mg₂Ca phase acts as a microanode in microgalvanic corrosion in both NaCl and Na₂SO₄ solutions, with the Mg matrix acting as the cathode and the Al₂Ca phase acting as the microcathode to accelerate corrosion of the adjacent Mg matrix. It was also found that Cl⁻ and SO₄²⁻ have different sensibilities to microgalvanic corrosion.     

Mg2+ Doping Effects on the Structural and Dielectric Properties of CaCu3Ti4O12 Ceramics Obtained by Mechanochemical Synthesis

In this study, ceramic CaCu₃Ti₄O₁₂ (CCTO) and CaCu_3−xMg_xTi₄O₁₂ solid solutions in which 0.1 ≤ x ≤ 0.5 were prepared by the mechanochemical method, realized by a high-energy ball milling technique. The effects of the Mg²⁺ ion concentration and sintering time on the dielectric response in the prepared ceramics were investigated and discussed. It was demonstrated that, by the use of a sufficiently high energy of mechanochemical treatment, it is possible to produce a crystalline product after only 2 h of milling the mixture of the oxide substrates. Both the addition of magnesium ions and the longer sintering time of the mechanochemically-produced ceramics cause excessive grain growth and significantly affect the dielectric properties of the materials. The X-ray diffraction (XRD) analysis showed that all of the as-prepared solid solutions, CaCu_3−xMg_xTi₄O₁₂ (0.0 ≤ x ≤ 0.5), regardless of the sintering time, exhibit a cubic perovskite single phase. The dielectric study showed two major contributions associated with the grains and the grain boundaries. The analysis of the electric modules of these ceramics confirmed the occurrence of Maxwell–Wagner type relaxation, which is dependent on the temperature.