S.V. Narasimhan

Optical absorption and photoluminescence spectroscopy of the growth of silver nanoparticles

Results obtained from the optical absorption and photoluminescence (PL) spectroscopy experiments have shown the formation of excitons in the silver-exchanged glass samples. These findings are reported here for the first time. Further, we investigate the dramatic changes in the photoemission properties of the silver-exchanged glass samples as a function of postannealing temperature. Observed changes are thought to be due to the structural rearrangements of silver and oxygen bonding during the heat treatments of the glass matrix. In fact, photoelectron spectroscopy does reveal these chemical transformations of silver-exchanged soda glass samples caused by the thermal effects of annealing in a high vacuum atmosphere. An important correlation between temperature-induced changes of the PL intensity and thermal growth of the silver nanoparticles has been established in this Letter through precise spectroscopic studies.

Formation of zinc ferrite by solid-state reaction and its characterization by XRD and XPS

A dry mixture of ZnO and α-Fe2O3was annealed at 1200°C; the progress of the formation of the ferrite was monitored by XRD and XPS analyses at different time intervals. The presence of octahedral zinc cation was observed along with the regular tetrahedral Zn in the sample that had undergone 30 minute heat treatment. After three hours of heating, pure normal zinc ferrite was formed. The Zn 2p3/2peak binding energy, intensity and line shape were analyzed extensively to show the diffusion of ZnO and the growth of ferrite at different stages of heat treatment. Analysis of the Fe 2p3/2line-shape supported the substitution of Fe2+by zinc cations during ferrite formation. The binding energy values of the Zn 2p levels for stoichiometric and non-stoichiometric ferrites were also determined and surface segregation of the zinc was observed by XPS in the non-stoichiometric ferrites.

Oxidation state of uranium: an XPS study of alkali and alkaline earth uranates

X-ray photoelectron spectroscopy (XPS) studies are performed on some mixed oxides of uranium with alkali and alkaline earth metals (Na, Li, Rb, Tl, Sr, Ba) to investigate the presence of multiple valence states of uranium in these compounds. Photoelectron peak positions, peak widths and satellite positions were measured to identify the chemical states. The analysis of the shake up satellites and the deconvolution of the principal peaks are used to quantify the chemical states. The domination of U(V) chemical state is inferred in Na2U3O9 and Sr2U3O10.

Electrochemical passivation of iron alloys and the film characterisation by XPS

The initial stages of oxide growth and composition of passive films developed electrochemically on iron alloys viz. AISI 304, 316 and incoloy 800 were studied by X-ray photoelectron spectroscopy. The results showed an enrichment of chromium in the passive films and the thickness of the films were of the same order on all the alloys. The hydroxide species were found to dominate at the oxide solution interface. The air oxidised film showed the presence of both chromium and iron in the oxidised state unlike the electrochemically developed passive film where chromium was found to be the dominant oxidised species.

Biofouling and microbial corrosion problem in the thermo-fluid heat exchanger and cooling water system of a nuclear test reactor

This article discusses aspects of biofouling and corrosion in the thermo-fluid heat exchanger (TFHX) and in the cooling water system of a nuclear test reactor. During inspection, it was observed that >90% of the TFHX tube bundle was clogged with thick fouling deposits. Both X-ray diffraction and Mössbauer analyses of the fouling deposit demonstrated iron corrosion products. The exterior of the tubercle showed the presence of a calcium and magnesium carbonate mixture along with iron oxides. Raman spectroscopy analysis confirmed the presence of calcium carbonate scale in the calcite phase. The interior of the tubercle contained significant iron sulphide, magnetite and iron-oxy-hydroxide. A microbiological assay showed a considerable population of iron oxidizing bacteria and sulphate reducing bacteria (105 to 106 cfu g−1 of deposit). As the temperature of the TFHX is in the range of 45–50°C, the microbiota isolated/assayed from the fouling deposit are designated as thermo-tolerant bacteria. The mean corrosion rate of the CS coupons exposed online was ∼2.0 mpy and the microbial counts of various corrosion causing bacteria were in the range 103 to 105 cfu ml−1 in the cooling water and 106 to 108 cfu ml−1 in the biofilm.

Temperature programmed decomposition of uranyl nitrate hexahydrate

Abstract Temperature programmed decomposition (TPD) of uranyl nitrate hexahydrate has been studied using evolved gas analysis mass spectrometry (EGA-MS) in the temperature range 300–1400 K. Thermogravimetric (TGA) investigations were performed in the temperature range 300–1100 K. An attempt has been made to resolve the complexity of decomposition behaviour through suitable comparison of TGA and EGA-MS data. Kinetic control regimes for various decomposition stages could be deduced from EGA-MS data. The corresponding activation energies and frequency factors were also evaluated. Kinetics based on random nucleation and diffusion was found to be rate controlling. The residue left over after each decomposition stage was analysed by XRD and XPS to determine structure and composition. The ultimate product was found to be a mixture of UO 3 H 1.17 and U 3 O 8 : the former being a topotactic hydrogen spill over compound of UO 3 . Complete conversion of this residue to U 3 O 8 was noticed during ion beam exposure of the residue which was performed in the course of XPS investigations.

Synthesis and characterization of nanocrystalline thoria obtained from thermally decomposed thorium carbonate

Abstract Nanocrystalline thoria was synthesized by temperature programmed decomposition of Th(CO 3 ) 2 in an evolved gas analysis mass spectrometer set-up. The structural and stoichiometric changes encountered in the decomposition pathway were studied by off-line thermogravimetry (TGA), X-ray diffraction and X-ray photoelectron spectroscopy (XPS). Accurate conversion temperature for transformation of Th(CO 3 ) 2 →ThO 2 was arrived from the XPS measurements. Fourier transform infrared (FT-IR) measurements were used to compare vibrational activities of nano and bulk polycrystalline thoria. Raman spectroscopic studies indicated optical phonon confinement effects in nanocrystalline thoria. High resolution transmission electron microscopic examination on starting material, intermediates and nanocrystalline final product were carried out for studying the microstructure in the nanometer scale.

XPS and SIMS analysis of gold silicide grown on a bromine passivated Si(111) substrate

Abstract When a thin film of Au (∼100 nm) deposited under high vacuum conditions on a chemically prepared Br-passivated Si(111) substrate was annealed around 363°C, epitaxial layer-plus-island mode growth of gold silicide was observed along with some unreacted gold in stringy patterns. This unreacted gold was removed by etching the sample in aqua regia. X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS) measurements were carried out on these samples. SIMS results reveal that the height of the islands is about 1.2 μm and the silicide/Si interface is abrupt. XPS measurements were made after sputtering the sample surface at constant intervals of time. Si 2 p , Au 4 f , C 1 s and O 1 s photoelectrons were detected. XPS spectra of Si 2 p are resolved into three peaks corresponding to bulk Si, Si in silicide and Si in oxide. The Au 4 f 7/2 peak in the silicide is shifted by 1–1.2 eV towards higher binding energy compared to metallic Au. The shift of Si 2 p towards the higher binding energy in the silicide is understood from the higher electronegativity of Au, while the shift of Au 4 f 7/2 peak towards higher binding energy is known to be due to d-electron depletion to form an sd hybrid. The XPS peak intensity profile with sputtering time indicates that the thin uniform layer (∼5.5 nm) of gold silicide is sandwiched between a thin (∼2.8 nm) SiO 2 layer and the Si(111) substrate.

Dissolution behaviour of magnetite film formed over carbon steel in dilute organic acid media

Magnetite is the major corrosion product formed over the carbon steel in the primary heat transport system of the pressurized heavy water reactor (PHWR). This magnetite usually accumulates radioactivity during reactor operation. The dissolution of the host magnetite is achieved by chemical formulations in order to get rid of the radioactivity trapped in the oxide; the underlying base metal also participates in the process by contributing electron to reduce the ferric ion or by undergoing corrosion. In the present study, the role of base metal in the dissolution of magnetite in various chelating agents has been investigated. The liberated hydrogen was measured by using an amperometric hydrogen sensor. The magnetite dissolution rate and the corrosion rate of carbon steel in the formulations were calculated. The effect of temperature, pH and concentration of the chelating agents on the magnetite film dissolution was studied in detail. The mechanism of base metal aided magnetite dissolution is discussed.

Application of polymer electrolyte based hydrogen sensor to study corrosion of carbon steel in acid medium

Hydrogen sensors find extensive application in hydrogen leak detection. These sensors can also be utilized in process monitoring. A fuel cell based room temperature amperometric hydrogen sensor has been developed in our laboratory. This sensor is used to study corrosion of carbon steel (CS) with EAC formulation, which is a mixture of EDTA, ascorbic acid and citric acid. EAC formulation is a recommended chemical formulation to dissolve magnetite formed by corrosion of CS. The paper describes a new method using hydrogen sensor to obtain the rate of corrosion of CS by EAC as a function of time.