Denis P. Opra

Functional coatings formed on the titanium and magnesium alloys as implant materials by plasma electrolytic oxidation technology: fundamental principles and synthesis conditions

Abstract Metallic implants have been successfully used in medicine for the past 60–70 years. Historically, implants were designed only as mechanical devices, whereas the biological aspects of their application were beyond the researchers’ interest. The improvement of living conditions and the increase of the average life span have changed the situation. The clinical requirements for medical implants rise up substantially. Presently, it seems impossible to imagine the use of metallic implants in the human body without preliminary surface modification to modulate the interaction between the surrounding biological environment and the implant. The review highlights the most recent advances in the field of functional coatings formed on implants by the plasma electrolytic oxidation technology. Special attention is dedicated to the principles of surface modification of the commercially pure titanium, titanium nickelide, and Mg-Mn-Ce magnesium alloy. The advantages and disadvantages of the method and the characteristics of these materials are discussed from this point of view. Some aspects of this review are aimed at corrosion protection of implants with application of polymer materials.

Electrochemical performance of Klason lignin as a low-cost cathode-active material for primary lithium battery

Abstract A few classes of organic compounds are promising electrode-active materials due to their high power and energy densities, low cost, environmental friendliness, and functionality. In the present work, the possibility of using Klason lignin extracted from buckwheat husks as a cathode-active material for a primary lithium battery has been investigated for the first time. The reaction mechanism in the lithium/lignin electrochemical cell was suggested based on the deep galvanostatic discharge (up to 0.005 V) data and cyclic voltammetry results. The dependence of the electrochemical behavior of the Klason lignin on the milling degree was evaluated. The maximum specific capacity of the lignin is equal to 600 mAh·g −1 at a discharge current density of 75 μA·cm −2 . Beneficial effect of the thermal treatment of the Klason lignin cathode at 250 °C on the cell performance was established. It was found that the discharge capacity of the cell increased by 30% in the range from 3.3 to 0.9 V for the treated cathode material. These results demonstrate the prospects of using Klason lignin-based electrochemical cells as low-rate primary power sources.

Fabrication of Battery Cathode Material Based on Hydrolytic Lignin

The relationship between the discharge performance of a lithium/hydrolytic lignin battery and the parameters of the cathode material fabrication process has been revealed. Electrochemical characterization was carried out using a galvanostatic discharge experiments. It was found that the specific capacity of hydrolytic lignin was found to be 600 mAh/g at a discharge current density of 45 μA/cm2. The results demonstrate the prospects of hydrolysis lignin-based batteries application as low-rate power sources.

Nanostructured microtubes based on TiO2 doped by Zr and Hf oxides with the anatase structure

The nanostructured microtubes based on TiO2 have been prepared on the carbon fiber template using the sol-gel method. The microtubes consist of nanoparticles of metal oxides: TiO2/ZrO2 and TiO2/HfO2. The dependence of microtubes morphology and nanoparticles structure on the synthesis conditions has been studied using the methods of SEM, SAXS, and Raman spectroscopy. It has been demonstrated that at the stoichiometric ratio of up to 0.04 for Zr/Ti and up to 0.06 for Hf/Ti microtubes consist of uniform nanoparticles with the anatase structure. Along with further increase of the dopants content in the microtubes composition, nanoparticles acquire the core-shell structure. It has been suggested that nanoparticles have a core composed of the solid solutions Ti1-xZrxO2 or Ti1-xHfxO2 and a shell consisting of zirconium or hafnium titanate. The fabricated Zr- and Hf-doped TiO2 materials were investigated in view of their possible use as anode materials for Li-ion batteries. Charge- discharge measurements showed that the doped samples manifested significantly higher reversibility in comparison with the undoped TiO2. The method opens new prospects in synthesis of nanostructured materials for Li-ion batteries application.

Nanostructured TiO2-TiOF2 composite synthesized by the original method of pulsed high-voltage discharge as anode material for Li-ion battery

It has been demonstrated for the first time that an original method of pulsed high-voltage discharge is efficient for the preparing of nanostructures promising for application in Li-ion batteries. In particular, a nanostructured TiO2-TiOF2 composite is synthesized as a result of destructing Ti electrodes and polytetrafluoroethylene in plasma. It is established that TiO2-TiOF2 is a porous structure composed of TiO2 and TiOF2 nanocrystallites 40–200 nm in size. The diameter of pores varies from 3 to 5 nm. The discharge capacity of a Li/TiO2-TiOF2 half-cell during a first cycle at a current density of 20 mA/g in voltage range from 3 to 0.005 V amounted to 1370 mA h/g, which exceeds (due to the presence of TiO2) the theoretical capacity of TiOF2. The cycling of Li/TiO2-TiOF2 characterizes the stability of the capacity about 205 mA h/g after the 20th cycle.

Corrosion-resistant composite coatings on biodegradable magnesium alloys: In vitro studies

A composite coating was formed on MA8, MA14, and MA12 magnesium alloys by plasma electrolytic oxidation with subsequent immersion of samples into a superdispersed polytetrafluoroethylene suspension. In vitro volumetry determined that using this coating significantly reduces the magnesium alloy dissolution rate. It was shown that superdispersed polytetrafluoroethylene seals pores of the coating, thus reducing the corrosion rate in an artificial medium that mimics human blood by ionic composition. However, the surface of the calcium phosphate coating (Ca: P = 1.61) containing hydroxyapatite remains open for contact with the environment. The obtained data suggested that the proposed method for surface treatment of MA8, MA14, and MA12 alloys is promising for producing biodegradable protective coatings on magnesium medical implants.

Effect of Al(OH)3 in Enhancing PbSnF4 Anode Performances for Rechargeable Lithium-Ion Battery

Two-phase Al(OH)3–PbSnF4 composites (concentrations of aluminum hydroxide are equal to 5 wt.%, 15 wt.% and 30 wt.%) has been prepared by high-energy ball-milling method. The materials were employed as anodes in Li-ion batteries. It was established that PbSnF4-based systems yield high initial capacity of 800–1100 mAh g–1. The reversible specific capacity of Al(OH)3–PbSnF4 (aluminum hydroxide – 15 wt.%) after 10-fold charge–discharge cycling in the range of 2.5–0.005 V attains 120 mAh g–1, while the specific capacity of pure PbSnF4 is equal only to 20 mAh g–1. It has been shown that the deviation from 15 wt.% concentration of Al (OH)3 decreases cycling stability of lead fluorostannate (II).

Nanostructured Composite FeOF-FeF3 as Anode Material for Li-Ion Battery: The Original Method of Pulsed High-Voltage Discharge

The prospects of an original method of pulsed high-voltage discharge in synthesis of nanostructured FeOF–FeF3 composite anode for Li-ion battery has been demonstrated. Scanning electron microscopy investigation shows as-synthesized FeOF–FeF3 consists of nanocrystallites ranging in sizes from 10 to 300 nm. Galvanostatic discharge–charge cycling of the Li/FeOF–FeF3 half-cell at current density of 10 mA g–1 in the range from 3.0 to 0.005 V yields 210 mAh g–1 after 10 cycles. The electrochemical reaction mechanism within the Li/FeOF–FeF3 has been investigated by the cyclic voltammetry method.

α-MoO3 Nanostructure Synthesized in Plasma by an Original Method of Pulsed High-Voltage Discharge as Highly Reversible Anode for Secondary Lithium-Ion Battery

A new concept for synthesis of the nanostructured transition metal oxides had been proposed. In particular, the method of pulsed high-voltage discharge was adopted for synthesis of α-MoO3 nanostructure with orthorhombic crystal lattice. The as-prepared α-MoO3 was investigated as anode for Li-ion battery. The 30-fold charge–discharge cycling has shown that material specific capacity (approximately 90 mAh g–1) is not high, however excellent reversibility was achieved (the Coulombic efficiency equals to 99.9%). Thus the method opens new ways for the synthesis of nanomaterials with stable reversible capacities for Li-ion batteries.

Effect of Hf-doping on electrochemical performance of anatase TiO2 as an anode material for lithium storage

Hafnium-doped titania (Hf/Ti = 0.01; 0.03; 0.05) had been facilely synthesized via a template sol–gel method on carbon fibre. Physico-chemical properties of the as-synthesized materials were characterized by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, energy-dispersive X-ray analysis, scanning transmission electron microscopy, X-ray photoelectron spectroscopy, thermogravimetry analysis and Brunauer–Emmett–Teller measurements. It was confirmed that Hf4+ substitute in the Ti4+ sites, forming Ti1–xHfxO2 (x = 0.01; 0.03; 0.05) solid solutions with an anatase crystal structure. The Ti1–xHfxO2 materials are hollow microtubes (length of 10–100 µm, outer diameter of 1–5 µm) composed of nanoparticles (average size of 15–20 nm) with a surface area of 80–90 m2 g–1 and pore volume of 0.294–0.372 cm3 g–1. The effect of Hf ion incorporation on the electrochemical behaviour of anatase TiO2 in the Li-ion battery anode was investigated by galvanostatic charge/discharge and electrochemical impedance spectroscopy. It was established that Ti0.95Hf0.05O2 shows significantly higher reversibility (154.2 mAh g–1) after 35-fold cycling at a C/10 rate in comparison with undoped titania (55.9 mAh g–1). The better performance offered by Hf4+ substitution of the Ti4+ into anatase TiO2 mainly results from a more open crystal structure, which has been achieved via the difference in ionic radius values of Ti4+ (0.604 Å) and Hf4+ (0.71 Å). The obtained results are in good accord with those for anatase TiO2 doped with Zr4+ (0.72 Å), published earlier. Furthermore, improved electrical conductivity of Hf-doped anatase TiO2 materials owing to charge redistribution in the lattice and enhanced interfacial lithium storage owing to increased surface area directly depending on the Hf/Ti atomic ratio have a beneficial effect on electrochemical properties.