Gotowa bibliografia na temat „Electrochemical Systhesis”

A green approach is reported for the production of few layered graphenes (FLGs) via electrochemical route utilising the benefits of anodic exfoliation process, wherein electrochemical intercalation of nitrate ions into pyrolytic graphite resulted in electrochemical exfoliation of nitrate ions-intercalated graphite electrode. The role of applied potential in intercalation and concentrations of nitric acid are well defining factors in controlling the number of layers in FLGs. The success of this approach was confirmed by FTIR, wherein smaller particles of intercalated graphite led to broader peaks due to increased interaction with light wave. The SEM images showed several layers of graphene stacked together and slightly twisted at edges. An increased exfoliation in intercalated graphite was revealed by XRD patterns. Desirable conductive properties of the FLGs synthesised makes it a viable option for utility as conductive ink.

There is quiet revolution going on, and its name is nanotechnology. Without much fanfare, a host of innovations are coming our way. Use of electrochemistry, the solid/liquid interface science, in nanoscience and nanotechnology may range from nanosystems, to nanosynthesis, to nanocharacterisation. The characteristic reaction may be ion transfer reaction (ITR) or electron transfer reaction (ETR).The nanoscale electrochemistry covering from metallic and semiconductor based nanoparticles,nanoarrays,nanotubes,nanopits, to self assembled molecule monolayers i.e. bioelectrochemical systems with redox metalloprotein or DNA based molecules, has began to unravel the complexities of these systems. Electrochemistry is a suitable method for coupling particles activity to external circuitry. It has been successfully used in investigating the effects and kinetics of charge transfer at Q-dots using scanning electrochemical microscopy (SECM), by controlled transport reactions. Electrochemical processes i.e. reaction at solid/ liquid interface controlled by an externally applied voltage, are increasingly involved in nanostructured fabrication as a relatively inexpensive superior quality products, easy to handle and a reliable tool.

In recent years, fluids containing suspension of nanometer-sized particles (nano fluids) have been an active area of research due to their enhanced thermal conductivity over the base fluids. This makes them very attractive as heat transfer fluids in many applications such as coolants in the automobile and electronics industries, and manufacturing processes. Stable nano fluids are being investigated for numerous applications, including cooling, manufacturing, chemical and pharmaceutical processes, medical treatments, cosmetics, etc. In a better description, nano fluids are engineered colloidal suspensions of nano particles (<100 nm) in a base fluid. Common base fluids include de-ionized water and organic liquids. In this investigation, the two step method of synthesis of ultra fine Al-Cu alloy powder particles and stable dispersion in base fluid is done. Ultrafine powders were prepared by milling elemental Al and Cu powders for 50 hours in a planetary mill. Aiming at the dispersion of nano-Al-Cu is regarded as the guide of heat transfer enhancement, the stability of Al-Cu alloy particles in de-ionized water were studied under different pH values by using nano zeta meter. It is found from XRD that the crystallite size is around 7 nm and lattice strain value is around 1.4 % for Al-Cu. After 50 hours of milling, particles size has been reduced from 28 m to 300 nm. Transmission electron microscopy (TEM) shows that each particles consists of large number of crystallites of size around 10-15 nm. The stability of nanofluids was also studied by nano zeta meter at different pH of nanofluids for constant ultrasonication time and magnetic stirring. It has been found from Nano zeta meter that the suspension is best stable at pH value of 9.5 corresponding to zeta potential value of -90.60 for Al-Cu alloy with the presence of surfactant.

An effort has been made to synthesize Al-based nanostructure by mechanical alloying (MA). Elemental powder of Al, Si and Ni were blended to obtain nominal composition of Al75Si15Ni10. Alloying was carried out in a high energy planetary ball mill using stainless steel grinding media at 300 r.p.m. up to 50 h. Toluene was used as the process control agent (PCA). The ball to powder weight ratio was maintained at 10:1. The phase evolution of the milled samples was studied by X-ray diffraction (XRD) analysis. The microstructural characterization of the milled powder was followed by scanning electron microscopy (SEM) and XRD. Dissolution of Si and Ni in Al was found to be 15% and 10% respectively along with the formation of some intermetallic phases. SEM micrographs showed that the powder morphology was changed from coarse layered structures obtained by very short period of milling to finer as the milling time increased. XRD and energy dispersive X-ray analysis (EDX) showed the formation of a homogeneous solid solution of the above said blends after milling for 50 h. The crystallite size, lattice strain (%) and lattice parameter were calculated from major XRD peaks. It shows that the crystal size decreased very rapidly up to 25 h of milling and then slowly became almost constant with further milling, whereas, lattice strain (%) increased gradually up to 25 h very rapidly and then very slowly became nearly constant with progress of milling. This suggests that major structural changes and dissolution of the alloying elements almost completed by 25 h, and further milling refined the product by MA. The lattice microstrain of the material increases exponentially. It increases rapidly up to 25 h and then increased slowly as the milling progresses further. The change of lattice parameter of Al-rich solid solution showed a rapid decrease throughout the process of MA. This is because of the entrance of Si and Ni atoms into the lattice of the Al which causes distortion in it. The change in the above mentioned parameters were determined up to 30 h of milling as on further milling Al peaks vanishes because of formation of partially amorphous structure along with some intermetallic phases.

The current work aims to improve the thermal conductivity of distilled water by dispersing electrolytic grade iron powder. Thermal conductivity of fluids is an important parameter in deciding their usability in various commercial applications. Nanoparticles dispersed in fluids generally show interesting properties with respect to thermal conductivity. Iron particles were dispersed in distilled water in different volume fractions (1, 2 and 3 percent respectively) and the resultant fluids were analysed in terms of their thermal conductivity. To study the effect of particle size, the as received iron powder was also ball milled and the same set of studies were repeated with the milled powder. All the conductivity measurements were carried out at room temperature the data were compared with the conductivity value of the pure distilled water. The effect of solid powder additions on distilled water results the increase in thermal conductivity with increase in concentration of iron powder. Effect of milled powder was also compared with that of un-milled powder

In this study, few-layer graphene nanosheets (FLGNSs) have been synthesized by electrochemical intercalation followed by exfoliation technique. Three different protic electrolytes such as aq. H2SO4, HClO4 and HNO3 have been used separately. The major intercalants are 2 4 SO  , 4 ClO and 3 NO anions of different sizes, where the rate of impact to the pyrolytic graphite sheet has been monitored by varying the concentration of the electrolytes to 0.5, 1.0, 1.5 and 2.0 M in each case. The effects of sizes of the intercalants and its rate of intercalations on the as-synthesized FLGNSs have been studied. From the in-situ analyses, the exfoliation rates have been significantly increased with the increase in size as well as the concentration of the intercalants. Various physicochemical analyses on the electrochemically exfoliated FLGNSs have been performed in the colloidal as well as solid state. From the colloidal state, the exfoliated FLGNSs dimensions as well as its conductivity have been measured. The thermal stability and yield of FLGNSs flakes have been measured by TGA. The structural properties like phase, lattice spacing, dis-orderness and crystallite sizes of the as-synthesized FLGNSs have been analyzed by XRD and Raman spectroscopy. The (002) and (001) lattice planes of graphene and graphene oxide has been observed at around 24.5° and 11° (2θ) from the XRD spectra respectively. Again, the characteristics peaks at around 1345, 1590 and 2700 cm-1 corresponds to D, G and 2D bands of the FLGNSs in the Raman spectra respectively. This shows the mixture of sp2 and sp3 contents in the electrochemically exfoliated FLGNSs. The qualitative as well as quantitative analyses of the functional endowment on the FLGNSs have been performed by FTIR, XPS and UV-visible spectroscopy. The FTIR analysis depicts the presence of various hydroxylation, carboxylation and aromatic carbon structures in the FLGNSs. The quantification of the functional groups and sp2 content in the FLGNSs has been analyzed by XPS. The UV-visible spectra show the electronic transitions of π-π* and n-π* due to the presence of C=C bond in the aromatic structure and C=O, carbonyl functional groups respectively. The optical band gaps also have been measured from the Tauc plots. The morphological as well as topographical analyses have been performed by FESEM, TEM and AFM. From the FESEM, the domain sizes, agglomerations, curliness at the edges and stratified nature of FLGNSs have been shown. From the TEM analyses, the number of layers in the graphene sheets measured from the lattice fringe analysis. Again the number of layers has been analyzed by the topographic analysis performed by AFM and it varies in between 3-8 layers. The functional application as supercapacitive performance of the as-synthesized FLGNSs have been performed by Swagelok type configured two electrode potentiostat. From the cyclic voltammetry (CV) and charge-discharge (CD) measurements, the FLGNSs synthesized from 1.5 M H2SO4 (S3), 2.0 M HClO4 (C4) and 1.0 M HNO3 (N2) electrolytic conditioned shows maximum capacitance in the respective categories. The Ragone plot shows maximum energy density of 12.35 Wh kg-1 and maximum power density of 3.01 kW kg-1 by S3 and N2 FLGNSs respectively. From the 5000 cycle CD test, it has been observed that the FLGNSs shows ~100 % stability in delivering the power performance. It attributes to the non-faradic EDLC reactions of the materials. The FLGNSs obtained from the extreme electrolytic conditions such as 2.0 M of H2SO4 (S4), HClO4 (C4), and HNO3 (N4) are used as nano-filler in the epoxy (EF) and glass fiber/epoxy (GEF) matrixed polymer composite structures. The nano-fillers have been used 0.1 and 0.3 wt.% in the composite structures. The functional groups present in the FLGNSs act as anchoring agent to the epoxy polymer for enhancement in the mechanical properties. It has been observed that the N4 FLGNSs nano-filler show the maximum enhancement of 42.6 and 28.2 % of flexural strength in EF and GEF polymer composite structures. Similarly, the modulus has been increased to 33.5 and 57.7 % in the EF and GEF polymer composite structures. The fact is attributed to the high extent of the carboxyl-functional endowment in the N4 FLGNSs than S4 and C4 FLGNSs. Again at high concentration of nano-filler to 0.3 wt.%, the mechanical properties of the composites have been drastically reduced. The fact depicts the agglomeration of the FLGNSs in the epoxy matrix, which inhibits the homogeneous bonding in the polymer structures.

Graphene is a single layer of pure carbon atom that are bonded together in hexagonal or honeycomb lattice structure. The gradual destruction of metal surface by chemical or electrochemical reaction with their environment forms corrosion. The graphene layer has been used as protective thin film on metal surface by electorphoretic depositon (EPD). In the present work, graphene has been synthesized by electrochemical intercalation and exfoliation of pyrolytic graphite sheet by various electrolytes (H2SO4, HNO3, and HCLO4) in varying concentration. The prepared dispersed graphene oxide (GO) solutions were deposited on copper surface with working area of 2cm2 by electrophoretic deposition technique at various (0.1, 0.5, 1 wt %) concentration of graphene. The sodium dodecyl sulfate (SDS) anionic surfactant has been used as binder to increase the thickness of coating. The graphene oxide coatings were used with the aim that it will act as a protective layer for corrosion of Cu substrate. The thickness of the graphene coated thin film was characterized by surface profiler and atomic force microscopy. Morphology of graphene nano sheets and coated graphene was analyzed by FESEM, which has showed clearly microstructure of GNS layer. Topography of graphene coated specimen characterized by AFM and crystal structure, crystalline planes and phases of graphene sheet were characterized by X-ray diffraction. (0 0 2) and (1 0 0) planes showed the graphene sheet has been confirmed by X-ray diffraction. The electrochemical corrosion behavior of graphene coating on Cu in 0.1M NaCl solution has been investigated by potentiodynamic linear sweep voltametry technique. However, the tendency (corrosion potential) is nobler for bare copper substrate. The study needs further experiment and optimization to reach to an affirmative conclusion. The application of GO layer has improved the corrosion resistance property of coated Cu due to better barrierquality of GO layer.

The thin adherent oxide layer formed on the metal surface keeps the surface of metal protective from corrosion i.e. known as passivation.This oxide layer may break due to different types of environmental effects leading to rapid corrosion.Hence the study on breakdown of passivity should permit one for the interpretation of process parameter and environmental restriction for long time of serviceable of the metal.The 304 stainless steel is having the higher percentage of chromium making it corrosion proof literally. In this thesis the study is on the passivity breakdown of 304 stainless steel in deaerated borate buffer solution as a function of pH and Cl ions.The pitting corrosion studies of 304 SS is carried out in borate buffer solution at varying pH(8.3,9.3,10.3)in the presence of varying chloride ions (0.1M, 0.5M, 1M).Corrosion studies were performed by potentiodynamic scans,microscopy techniques and point defect model.The study of the passivity and pitting can be evaluated through a point defect model.As increasing the pH of borate buffer solution, the breakdown potential increases i.e. the pitting tendency decreases whereas it got decreased with increasing chloride ion concentration. The value of breakdown potential was also found to increase with the potential scan rate.After obtaining the potentiodynamic curve pitting studies of the surface is done by optical microscope.The microstructure is shown in above figure at 1M chloride ion and at varying pH.The probability distribution in breakdown potential is the confirmation with the analytical prediction of the breakdown potential distribution obtained from the point defect model.

The aim of this work is electrochemical exfoliation of pyrolytic graphite for mass production of few-layer graphene nano sheets. It is synthesized by intercalation of graphite sheets in the electrolyte of two different types of concentrations, one molar and two molar concentrations of nitric acid by application of positive bias. The voltage is gradually increased with an increment of 0.5V upto 8V and an interval of 3 minutes. The X-ray diffraction peaks corresponding to graphene sheet ((002) plane) were observed at 2θ positions of 26.35°. The morphology of as-synthesized FLGNSs is characterized by field emission scanning electron microscopy. The transparent layers of FLGNSs are observed in transmission electron microscopy. The number of layers in transparent graphene sheets is confirmed by the HRTEM. Through FTIR studies, the presence of functional groups of O-H and C-O has been identified. AFM topography revealed that the thickness of the single layer is in the range of 1 nm, and for few-layer graphene nano sheets are in the range of 5-6 nm only. However, FLGNSs could be readily distinguished through phase imaging of tapping-mode AFM, because of differences in hydrophobicity arising from their different oxygen contents. STM studies of graphene nanosheets revealed atomic scaled periodicity at very low tunneling currents (∼1 pA). Phase imaging showed distinct contrast difference between FLGNSs to the graphite substrate (HOPG), a result that was attributed to their extremely low conductivity. The atomically flatness of the graphene nano sheets and electronic properties were measured by scanning tunneling microscopy. Scanning probe spectroscopy revealed the electronic properties like the density of states (DOS) and Dirac point (DP) of graphene sheets. The synthesized material can be used as a base material for the future applications such as desalination of sea water, supercapacitors, sensors, solar cells, and coatings.

Nanostructured tungsten based alloys with nominal composition of W80Ni10Nb10,W79Ni10Nb10(Y2O3)1, W78Ni10Nb10(Y2O3)2, W72Ni10Nb15(Y2O3)3 (all in wt.%) are synthesized by mechanical alloying of elemental powders of tungsten (W), Nickel (Ni), Niobium (Nb) and Yittrium oxide (Y2O3) in high energy planetary ball milling machine followed by compaction at 500 MPa pressure for 5 mins and sintering at 1500o C for 2 h in Argonatmosphere. Investigation of phase and microstructure of milled powder and consolidatedsamples are carried out by X-ray diffraction (XRD), Scanning electron microscopy (SEM),Energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM).Minimum crystallite size of 20 nm is achieved in 20 h milled powder ofW72Ni10Nb15(Y2O3)3. The dislocation density for all the investigated alloys increases at 10 h of milling owing to hydrostatic pressure exerted by the nano-crystallites due to severe plastic deformation,however the rate of increase of dislocation density reduces after 10 h of milling due to formation of solid solution. The lattice parameter of W in W80Ni10Nb10 and W79Ni10Nb10(Y2O3)1, W78Ni10Nb10(Y2O3)2, W72Ni10Nb15(Y2O3)3 alloy expands at 10 h and 5h of milling and contracts thereafter respectively. The SEM micrograph reveals the presence of ultrafine particles at 20 h of milling for all alloys. Formation of hard, brittle NbNi intermetallic and Y2O3 disperoids is evident from XRD and SEM study of sintered alloys. Hardness, wear, oxidation, and compression test has been conducted to investigate the mechanical behaviour of oxide dispersion strengthened (ODS) and non-ODS sintered alloys. Increased Y2O3 content results in enhanced compressive strength, sinterability, oxidation resistance and wear resistance. Higher hardness and strength in Y2O3 dispersed alloys as compared to W80Ni10Nb10 can be attributed to dispersion strengthening mechanism by Y2O3.Maximum sinterability, hardness, of 93.38%, 6 GPa, has been achieved in W72Ni10Nb15(Y2O3)3 owing to the presence of high Y2O3 content and NbNi intermetallic.W79Ni10Nb10(Y2O3)1 shows superior oxidation resistance at 8001000°C as compared to restof the alloys.