Nanostructured State Research Articles

Amorphous Silk Fibroin Nanofiber Hydrogels with Enhanced Mechanical Properties.

Silk fibroin (SF) hydrogels have been engineered as universal substrates for various tissue regenerations and drug delivery. Although different physical and chemical crosslinking strategies are developed to form SF hydrogels with suitable performances, a significant gap remains to match specific requirements of various tissues. Here, amorphous SF nanofibers with more tyrosine residues outside the surfaces are used to replace traditional SF. Under the same crosslinking conditions, the use of amorphous SF nanofibers results in tougher properties, four times higher stiffness than that from traditional SF solutions. Unlike previous SF hydrogels, the SF nanofiber hydrogels show high tunability in wide modulus range of 0.6-160 kPa under low SF concentrations (below 5 wt%), showing improved mechanical match with various soft tissues. Better stability and cytocompatibility are also achieved, further confirming the superiority of the hydrogels as the tissue substrates. Therefore, a feasible strategy is developed to optimize the performances of SF hydrogel via tuning the nano-structural state in aqueous solutions, which will enrich SF-based hydrogel family in future.

Interfacial phenomena in natural nanostructured materials based on kaolinite and calcite in blends with nanosilica and neem leaf powder

To improve structural characteristics of clays and neem leaf powder (NLP)/clay composites, nanosilica was used as an addition upon mechanical treatment. This results in composites with natural nanostructured materials (NNM) possessing improved structural characteristics and bioactivity. The blends based on NNM with crystalline kaolinite alone and kaolinite/α-quartz or calcite/α-quartz + clay and amorphous admixtures and additions with nanosilica or nanosilica/NLP were treated in a knife mill. The materials were characterized using TEM, SEM, XRD, 1H MAS and 29Si CP/MAS NMR, FTIR spectroscopy, elemental analysis, nitrogen adsorption, quantum chemistry, release of bioactive components and bactericidal effects against various bacteria. There are regularities in the characteristics depending on a type of NNM, particulate morphology and textural features of solids. The morphological and structural characteristics of components slightly changed in the blends because the treatment was performed using relatively low mechanical loading. Release of chlorophyll and anthocyanins is greater for NLP/A―300/NNM composites (especially characterized by minimal confined space effects) than for NLP alone. Some clays and composites demonstrate bactericidal activity tested against four types of bacteria. The morphological, textural, and structural characteristics of the NNM/A―300 and NNM/A―300/NLP systems, as well NLP transformation into a nanostructured state, allow one to expect high activity of the composites as bioactive systems interesting for practical applications in medicine and cosmetics.

Thermal Stability of Nanostructured Internally Oxidized Vanadium Alloy with Combined Dispersion and Substructural Hardening

The paper studies the effect of the annealing temperature on microstructural transformation and micro-hardness variation in an internally oxidized vanadium alloy V—Cr—ZrO2 deformed by high-pressure torsion in Bridgman anvils. It is shown that the development of large plastic strains and subsequent annealing of the particle-reinforced V—Cr—ZrO2 alloy lead to the formation of a fine-grained structure (with about 1 µm grain size) with a high density of high-angle boundaries pinned by ZrO2-based nanoparticles. Such high-defect states are characterized by a more than twofold increase in microhardness with the major contribution of grain boundary hardening. The conducted research has revealed the main relaxation features of V—Cr—ZrO2 alloy deformed by high-pressure torsion at room temperature. The heat treatment of the studied material at 800°C is shown to activate recovery and polygonization. Primary recrystallization is observed upon temperature increase to S00°C. A further increase in temperature in the interval 950–1050°C intensifies collective recrystallization, due to which the fraction of equi-axed grains increases significantly. Secondary recrystallization is activated at 1200°C and, as a result, individual grains grow in size. These processes are accompanied by a decrease in the V—Cr—ZrO2 alloy microhardness from 3500 to 2000 MPa. Dispersion and substructural hardening are analyzed, and their contribution to the strength is studied. It is shown that the high thermal stability of the nanostructural and fine-grained states is ensured by the high density of uniformly distributed ZrO2-based nanoparticles (of size 3–10 nm) that pin the high-angle grain boundaries.

Двухуровневые наноструктурные состояния в металлических материалах c ОЦК-решеткой после деформации кручением на наковальнях Бриджмена

Двухуровневые наноструктурные состояния в металлических материалах c ОЦК-решеткой после деформации кручением на наковальнях Бриджмена

Addressing Reversibility of R-NHC Coupling on Palladium: Is Nano-to-Molecular Transition Possible for the Pd/NHC System?

It has recently been shown that palladium-catalyzed reactions with N-heterocyclic carbene (NHC) ligands involve R-NHC coupling accompanied by transformation of the molecular catalytic system into the nanoscale catalytic system. An important question appeared in this regard is whether such a change in the catalytic system is irreversible. More specifically, is the reverse nano-to-molecular transformation possible? In view of the paramount significance of this question to the area of catalyst design, we studied the capability of 2-substituted azolium salts to undergo the breakage of C-C bond and exchange substituents on the carbene carbon with corresponding aryl halides in the presence of Pd nanoparticles. The study provides important experimental evidence of possibility of the reversible R-NHC coupling. The observed behavior indicates that the nanosized metal species are capable of reverse transition to molecular species. Such an option, known for phosphine ligands, was previously unexplored for NHC ligands. The present study for the first time demonstrates bidirectional dynamic transitions between the molecular and nanostructured states in Pd/NHC systems. As a unique feature, surprisingly small activation barriers (<18 kcal/mol) and noticeable thermodynamic driving force (-5 to -7 kcal/mol) were calculated for C-C bond oxidative addition to Pd(0) centers in the studied system. The first example of NHC-mediated Pd leaching from metal nanoparticles to solution was observed and formation of Pd/NHC complex in solution was detected by ESI-MS.

Stability of ternary nanocrystalline alloys in the Pt–Pd–Au system

Binary alloys have frequently been found to exhibit nanostructured states that are stabilized against grain growth when the minority alloying element strongly segregates to grain boundaries, thereby reducing the grain boundary excess energy. This concept has been formalized in several theoretical frameworks that are now used to design stable nanocrystalline binary alloys. In this work, we consider the extension of such a framework to ternary alloy systems. By considering pairwise interactions between atoms in ternary systems, we identify an experimental system, Pt–Au–Pd, and produce combinatorial alloy films in this system to evaluate their stability against grain growth. In several of these nanostructures stability against grain growth up to 400 °C is observed due to the co-segregation of Au and Pd solutes to the grain boundaries. This result is in line with calculations of a grain boundary energy-based estimation of stability after extending the analysis to account for the nontrivial ternary interaction.

Peculiarities of nanostructured state formation in heat-treatable Al–Mg alloys

The effect of long-time homogenization and aging on the formation of disperse β′ phase particles and β phase particles has been analyzed. Alloys solidified with the cooling rate of ∼10− 1 °C/s (Al–10%Mg and Al–10%Mg–0.1%Sc) and alloys melted by centrifugal casting with the cooling rate of ∼ 103 °C/s (Al–10%Mg) were chosen for comparative analysis. А significant heterogeneity of the distribution of the strengthening β′-phase after aging is observed in Al–10%Mg alloys obtained by the ingot technology. The greatest hardening up to 100% is achieved after using the centrifugal casting due to the formation of the β′-phase during aging.

Effect of thermomechanical treatment on deformational behavior of ferromagnetic Fe–Ni–Co–Ti alloy under uniaxial tension

To improve the functional properties of a ferromagnetic shape memory alloy, a thermomechanical treatment that includes drawing followed by quenching and high-temperature annealing was proposed, as a result of which a nanostructured state is formed. Thermomechanical treatment (TMT) increases the alloy thermoelasticity, although it is accompanied by a large temperature hysteresis of martensitic transformation and results in a significant hardening of the matrix, which, in its turn, enlarges the effects of shape memory and superelasticity (or pseudoelasticity). It has been established that TMT of Fe–Ni–Co–Ti shape memory alloy contributes to an increase in reversible superelastic deformation up to ~ 3%. In a superelastic cycle with wide mechanical hysteresis, a large dissipation energy per loading–unloading cycle was gained, that favors the use of this alloy as a damper of mechanical oscillations.

The Influence of Structural Factors on Strength and Electric Conductivity of Bulk Nanostructured Copper Alloys

The structural parameters ensuring high strength and low electrical resistance have been analyzed in Cu–Cr alloys exposed to severe plastic deformation (SPD) and aging. Specific resistance and strength characteristics have been modeled for a Cu–0.5 wt % Cr alloy with a nanostructure state caused by SPD. A comparison of modeling data with experimental values reveals the validity of the approach chosen for describing the above states. The formation of nanostructure states with high strength and low electrical resistance is shown to be due to simultaneous effects depending on the treatment conditions of materials.

Влияние структурных факторов на прочность и электропроводность объемных наноструктурных медных сплавов

AbstractThe structural parameters ensuring high strength and low electrical resistance have been analyzed in Cu–Cr alloys exposed to severe plastic deformation (SPD) and aging. Specific resistance and strength characteristics have been modeled for a Cu–0.5 wt % Cr alloy with a nanostructure state caused by SPD. A comparison of modeling data with experimental values reveals the validity of the approach chosen for describing the above states. The formation of nanostructure states with high strength and low electrical resistance is shown to be due to simultaneous effects depending on the treatment conditions of materials.

Phase transformations in Cd–Ni nanostructured system at elevated temperatures

Using X-ray diffraction (both in vacuum and in air), derivatomass-spectrometry and thermo-programmed oxidation, phase transformations in nanostructured Cd–Ni system occurring while heating up 720 °C were described. These metal powders were obtained by joint reduction from aqueous solutions of salts with hydrazine. When study their composition, earlier unknown phases were detected (intermetallide CdNi3 and solid solution CdxNi1–x with face centered cubic structure) and among the known intermetallides (Cd5Ni and CdNi), only Cd5Ni was found. As a consequence, phase transformations in the obtained nanostructured powders were remarkably different from those present on the Cd–Ni phase diagram. The intermediates and FCC phase decompose independently with formation of individual initial components (Cd and Ni) without intermediate product. At relatively high Cd concentration in the system, intermediate CdNi3 phase formation from Cd5Ni and CdxNi1–x phases, which are polar opposites in composition, is possible. The decomposition temperatures of Cd5Ni and melting point of Cd in the nanostructured state were lower than those specified by the phase diagram by 170 and 70 °C, respectively, which well corresponded to the concept of effective (high) temperatures attributed to the nanostructured systems due to the energy saturation of the con stituent nanocrystallites.

Ultra-fine grained β-type TNZT ELI alloy with high strength and low elastic modulus

This work deals with the improvement of mechanical properties of medical alloy Ti-34Nb-7Zr-4.6Ta ELI (TNZT) by severe plastic deformation (SPD) techniques at temperatures below the temperature of the onset of phase transformations in order to maintain a low modulus of elasticity. For the formation of a nanostructured state, high pressure torsion (HPT) at room temperature and equal channel angular pressing (ECAP) at 200 °C, that is, below the onset of diffusion ω-transformation, were used. It is shown that HPT leads to the formation of the UFG structure in the alloy with the grain size of about 110 nm without changing in the alloy phase composition. The ECAP processing allowed forming a subgrain structure with an average size of grains/subgrains of 500 nm and an ultimate tensile strength of 745 MPa, which exceeded the ultimate tensile strength after the standard aging in the (α+β) area (686 MPa). The issues of the effect of SPD on changes in the structure, phase transformations in the alloy and ways of maintaining a low modulus of elasticity in this alloy are discussed.

Electronic structure and d-band center control engineering over M-doped CoP (M = Ni, Mn, Fe) hollow polyhedron frames for boosting hydrogen production

The practical application of hydrogen evolution reaction (HER) through water splitting depends on the development of low cost and efficient non-noble-metal catalysts. As a potential electrocatalyst, the improvement of HER performance catalyzed by nanostructured transition metal phosphides still remains a great challenge. Tuning the novel nanostructure, morphology, and electronic state from nanoscale is of great important to achieve highly efficient HER electrocatalysts. Herein, we first developed an electronic structure and d-band center control engineering for accelerating the HER process in both acid and alkaline media over M-doped CoP (M = Ni, Mn, Fe) hollow polyhedron frames (HPFs), which were synthesized by a self-templating transformation (STT) strategy. Impressively, the HER electrocatalytic activity can be maximumly promoted and maintained at least 21 h for Ni-CoP/HPFs catalyst. Synchrotron-based X-ray absorption near-edge structure, X-ray photoelectron spectroscopy, auger electron spectroscopy, ultraviolet photoemission spectroscopy and density functional theory calculations consistently reveal the improved performance is attributed to the changes of the electronic structure and the downshift of d-band center after metal doping. The Ni-CoP/HPFs catalyst also indicates excellent activity with a cell voltage of 1.43 V to achieve the current density of 10 mA cm−2 and superior stability when it was employed as a cathode for HER and an anode for urea oxidation in 1 M KOH with 0.5 M urea. The success modulation of HER performance in current STT strategy will provide a promising pathway for designing various transition metal-doped compounds for energy-related catalysis processes.

Effect of deformation nanostructuring on thermal expansion and phase composition of Fe - 36% Ni alloy

The influence of deformation nanostructuring and subsequent annealing on the temperature dependence of the thermal expansion coefficient of the Fe-36% Ni Invar alloy is studied. The nanostructured state was obtained by severe plastic deformation via high pressure torsion in Bridgman anvils. An “anomalous” contraction of the nanostructured Fe-36% Ni alloy was detected during heating in certain temperature ranges. The phase composition of the nanostructured Invar was studied to explain the behavior of the thermal expansion coefficient. The revealed anomalies of the coefficients are explained by the formation of a bcc phase not observed in the Fe-36% Ni Invar alloy in the coarse-grained state. The formation of this phase becomes possible due to a significant increase in the diffusion coefficient as a result of nanostructuring.

Effect of Irradiation with a Continuous Beam of Accelerated Electrons on the Texture and Nanostructure of Carbon Black: a Study by Adsorption and High-Resolution Transmission Electron Microscopy

Textural, structural, and morphological changes occurring in globular carbon black of various particle size fractions under continuous irradiation with a continuous beam of 2.5-MeV accelerated electrons were studied by the methods of adsorption and high-resolution transmission electron microscopy. The electron irradiation leads to pronounced transformation of the solid globular framework of carbon black mainly into graphite-like nanocapsules of 10–50 nm size with the spacing between the graphene layers of the order of 0.355 nm. The observed effect leads to a decrease in the porosity (by a factor of approximately 1.65) and in the specific surface area (by a factor of 3–4) of the irradiated samples. This may be due to the transformation of the turbostratic (practically amorphous) form of carbon black into the nanostructured state with denser packing of carbon particles. The physicochemical properties of such carbon should be primarily determined by the structure of the solid framework.

Atomic-Force Microscopic Study of the Surface Morphology of the Nd2Fe14B Alloys Prepared by Various Techniques

Atomic force microscopy is used to study the nanostructured state of a number of Nd2Fe14B alloys. The starting alloys are synthesized in an argon atmosphere using an induction furnace. The nanostructured state of the Nd2Fe14B alloys is reached by two techniques, namely, severe plastic deformation and melt quenching; combined treatment, which includes severe plastic deformation of melt-quenched ribbons and subsequent annealing, is also used. The surface morphology of all studied samples is studied. These are used to interpret the magnetic hysteretic properties of the samples prepared by the different techniques.

REGULARITIES OF FORMATION AND DEGRADATION OF THE MICROSTRUCTURE AND PROPERTIES OF NEW ULTRAFINE-GRAINED LOW-MODULUS Ti–Nb–Mo–Zr ALLOYS

Regularities of the formation of ultrafine-grained (UFG) and submicrocrystalline (SMC) structures in new nickel-free low-modulus Ti–Nb–Mo–Zr titanium β alloys under the action of plastic deformation were studied. Temperature-time ranges of the development of dynamic recrystallization processes under the simultaneous action of temperature and plastic deformation were determined. The recrystallization diagram of II type of the Ti–28Nb–8Mo–12Zr alloy was constructed and analyzed. It was shown using scanning electron microscopy and electron backscatter diffraction method that the UFG structure with an average grain size of no more than 7 μm and high fraction of high-angle grain boundaries is formed in the investigated alloys as a result of longitudinal rolling followed by annealing for quenching. It was found that the formation of the UFG structure leads to a significant increase in the strength and plastic characteristics of these alloys. The regularities of the formation of UFG and SMC structures in the titanium β alloys Ti–28Nb–8Mo–12Zr and VT30 widely used in industry under the action of plastic deformation by the helical rolling method were studied. It was shown that the helical rolling of the VT30 alloy leads to the formation of the homogeneous UFG state as opposite to the developed Ti–28Nb–8Mo–12Zr β alloy where this method causes structure softening with micropores and microcracks formed in the central region. It is possible to form a nanostructured state with an average grain size of about 100 nm in Ti–Nb–Mo–Zr titanium β alloys using high-pressure torsion method.

High-Entropy Coatings—Structure and Properties

Metal and nitride coatings of high-entropy alloys (HEA) having different phase compositions produced by different methods were investigated. It is shown that for the high-entropy coatings is characteristic the presence of the nanostructured state, which in parallel with the cluster structure provides hardness for metal coatings ~ 10–19 GPa; for nitride coatings are characteristic 50–60 GPa and the modulus of inelastic buckling more than 300 MPa. The relation of the lattice parameters, which are defined experimentally, to the parameter of the lattice of the most refractory metal in the HEA reflects the level of the elastic modulus relative to the theoretically possible both in the cast HEA and in metal coatings on their base.

Influence of Severe Plastic Deformation on the Structure and Properties of Al–Li–Cu–Mg–Zr–Sc–Zn Alloy

The structural and phase transformations in the Al–Li–Cu–Mg–Zr–Sc–Zn alloy have been studied by the electron microscopy after the aging for the maximum strength and in the nanostructured state after severe plastic deformation by high-pressure torsion. It has been shown that severe plastic deformation leads to the formation of a nanostructured state in the alloy, the nature of which is determined by the magnitude of deformation and the degree of completeness of the dynamic recrystallization. It has been established that deformation also causes a change in the phase composition of the alloy. The influence of the structural components of the severely deformed alloy on the level of mechanical properties, such as the hardness, plasticity, elastic modulus, and stiffness has been discussed.

Synthesis and catalytic activity of Ag nanoparticles dispersed on nitrogen-doped GOPx toward direct electrooxidation of formaldehyde

Silver nanoparticles (Ag NPs) were synthesized and dispersed on the electronically conductive and electrochemically accessible surface of a GOPx composite, which was then employed as a probe for formaldehyde (HCHO) oxidation. The surface nanostructure, crystallographic properties, and oxidation state of the metal present in the GOPx-Ag nanocomposite were investigated using high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. HCHO electrooxidation was evidenced by the appearance of a very well defined anodic peak with an onset potential of −0.783V, which is unique in the literature. The effect of various electroanalytical parameters like concentration, scan rate, and upper potential limit was investigated toward HCHO electrooxidation on GOPx-Ag catalyst. Amperometric study showed remarkable HCHO electrooxidation response with a wide linear range of 1μM–70mM, limit of detection (LOD) 0.167μM, limit of quantification (LOQ) 0.556μM and sensitivity of 35.74μAmM−1cm−2. The experimental results further indicated the long-term stability and practical feasibility of the GOPx-Ag catalyst, with excellent recoveries for HCHO oxidation. The remarkable catalytic efficiency and anti-poisoning properties were far superior to those of many previously reported HCHO sensors. The astonishing analytical performance of GOPx-Ag opens up a new path for fabricating promising HCHO sensors.