P.K. Ajikumar

Nanocrystalline and metastable phase formation in vacuum thermal decomposition of calcium carbonate

Abstract Well characterised, polycrystalline powders of commercially procured CaCO 3 were thermally decomposed in the vacuum as well as in the flowing gas atmosphere for the purpose of studying solid state transformations. The characterisation of the end product CaO, obtained from the thermal decomposition, revealed contrasting features in the powder X-ray diffractograms. While the flowing gas method, conducted inside a thermogravimetric analyser (TGA), indicated formation of stable microcrystalline calcia, the decomposition under dynamic vacuum revealed formation of metastable-nanocrystalline calcia. The latter study was carried out in an evolved gas analysis-mass spectrometry (EGA-MS) facility. Experiments were also conducted inside the high temperature XRD (HTXRD) machine. The paper attempts to bring out possible mechanisms responsible for formation of these end products with such glaring structural contrast. Non-equilibrium conditions prevalent under dynamic vacuum condition as well as misfit strain energy available from CaCO 3 /CaO interface are presumed to be the reason behind such metastable transformations. Kinetic analysis of the transformation revealed prevalence of nucleation and growth phenomena. Corresponding Arrhenius factors were also calculated.

Thermogravimetry-evolved gas analysis-mass spectrometry system for materials research

Thermal analysis is a widely used analytical technique for materials research. However, thermal analysis with simultaneous evolved gas analysis describes the thermal event more precisely and completely. Among various gas analytical techniques, mass spectrometry has many advantages. Hence, an ultra high vacuum (UHV) compatible mass spectrometry based evolved gas analysis (EGA-MS) system has been developed. This system consists of a measurement chamber housing a mass spectrometer, spinning rotor gauge and vacuum gauges coupled to a high vacuum, high temperature reaction chamber. A commercial thermogravimetric analyser (TGA: TG + DTA) is interfaced to it. Additional mass flow based gas/vapour delivery system and calibration gas inlets have been added to make it a versatile TGA-EGA-MS facility. This system which gives complete information on weight change, heat change, nature and content of evolved gases is being used for (i) temperature programmed decomposition (TPD), (ii) synthesis of nanocrystalline materials, (iii) gas-solid interactions and (iv) analysis of gas mixtures. The TPD of various inorganic oxyanion solids are studied and reaction intermediates/products are analysed off-line. The dynamic operating conditions are found to yield nanocrystalline products in many cases. This paper essentially describes design features involved in coupling the existing EGA-MS system to TGA, associated fluid handling systems, the system calibration procedures and results on temperature programmed decomposition. In addition, synthesis of a few nanocrystalline oxides by vacuum thermal decomposition, gas analysis and potential use of this facility as controlled atmosphere exposure facility for studying gas-solid interactions are also described.

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.

Evolved Gas Analysis by Mass Spectrometry

Abstract A quadrupole mass spectrometer based evolved gas analyser has been built in our laboratory with necessary UHV hardware, computer interface and software for conducting real-time multiple ion detection mass spectrometry over a wide dynamic pressure regime. Thermal decomposition behaviour of model salts CaCO3, CuSO4.5H2O, Pb(NO3)2 and AlNH4(SO4)2.12H2O has been studied to standardise this system. Reaction parameters for the dehydration and decomposition of CuSO4 5H2O are computed and found to be in agreement with the reported literature values.

Non-isothermal kinetics of decomposition of AlNH4(SO4)2.12H2O by EGA-MS

Abstract Real-time multiple-ion detection trend analysis mass spectrometry has been employed to study the temperature-programmed decomposition of AlNH 4 (SO 4 ) 2 · 12H 2 O in the temperature range 300–1200 K. Significant correlations are established with certain non-isothermal solid state kinetic rate expressions through the use of fraction release plots obtained from Evolved Gas Analysis Mass Spectra (EGA-MS). The EGA mass spectra clearly resolve the dehydration stage and various other stages associated with the thermal decomposition. The dehydration step is concomitant with stage I of a three-stage ammonia release followed by the final decomposition of Al 2 (SO 4 ) 3 . These stages are found to comply with models based on random nucleation and diffusion approaches. A change in rate-governing mechanism was noticed with increase in the heating rate for the dehydration step. Relevant Arrhenius parameters such as the activation energy and pre-exponential factor were determined for all the decomposition stages. The ultimate product resulting from the decomposition was confirmed as γ-alumina by X-ray diffraction studies.

Real time mass spectrometric study of temperature programmed decomposition of CuSO4·5H2O

A facility based on real time multiple ion detection trend analysis mass spectrometry has been set up in our laboratory for studying thermal decomposition behaviour of inorganic solids. The system has been used for studying decomposition of CuSO4· 5H2O. Non-isothermal kinetic rate expressions based on random nucleation and 3-d phase boundary migration (for dehydration stage), 3-d diffusion and 3-d phase boundary migration (for decomposition stage) were found to have significant correlations with Evolved Gas Analysis (EGA) data. Brief description of the experimental facility along with physical explanations behind compliance of EGA data to above models are discussed.

Temperature programmed decomposition of thorium nitrate pentahydrate

Abstract Temperature programmed decomposition (TPD) of thorium nitrate pentahydrate has been studied using evolved gas analysis–mass spectrometry (EGA-MS) in the temperature range 300–1200 K. A thermogravimetric (TGA) investigation was also carried out in the same temperature range. Complexity of the TGA decomposition profile was resolved through use of EGA-MS data. The activation energies and pre-exponential factors were determined for various gas release stages from the fractional extent of decomposition plots. Residues left over after each decomposition stage were analysed using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The XRD investigations revealed formation of a nanocrystalline thoria intermediate product, ultimately agglomerating to a microcrystalline phase. The XPS investigations indicated systematic alteration in the chemical environment around the thorium atom while the Th4+ oxidation state remained unchanged. This was further corroborated from the analysis of shake-up satellites of Th (4f5/2) spectra. The O/Th ratios for various intermediate products were also determined.

Sunlight active antibacterial nanostructured N-doped TiO2 thin films synthesized by an ultrasonic spray pyrolysis technique

Sunlight active antibacterial N-doped anatase TiO2 nanocrystalline thin films were synthesized on Si(100), quartz and glass substrates at 300–550 °C, in a single step by ultrasonic spray pyrolysis, using hexamine as the nitrogen source and characterized using field emission scanning electron microscopy, X-ray diffraction, Raman spectrometry, secondary ion mass spectrometry, X-ray photoelectron and UV-visible spectroscopy. The antibacterial activity under UV, sunlight and normal room lighting was studied and compared. The TiO2−x−3yN2y films showed enhanced antibacterial activity under room light and sunlight compared to the pristine TiO2 films. The enhanced activity with nitrogen doping is due to the photo-generated holes on the localized N 2p states above the O 2p valence band, which are mobile enough to participate in the surface redox reaction. The microstructure of the films varied from nanodot chains to triangular platelets to cuboids with increase in synthesis temperature, confirming control of morphology and size with synthesis temperature. The secondary ion mass spectrometric studies of the films revealed a uniform distribution of titanium, oxygen and nitrogen across the thickness of the film. The N-doping concentration was ∼3.4%, as confirmed by XPS. The films had a band gap of about 3.30 eV obtained from UV-Vis studies.

Reactive Pulsed Laser Deposition of Titanium Nitride Thin Films: Effect of Reactive Gas Pressure on the Structure, Composition, and Properties

Titanium nitride (TiN) thin films were deposited by reactive pulsed laser deposition (RPLD) technique. For the first time, the composition evaluated from proton elastic backscattering spectrometry, in a quantitative manner, revealed a dependence on the partial pressure of nitrogen from 1 to 10 Pa. Grazing incidence-XRD (GI-XRD) confirmed the formation of predominantly nanocrystalline TiN phase with a crystallite size of around 30 nm. The hardness showed maximum value of ~30 GPa when the composition is near stoichiometric and the friction coefficient was found to be as low as 0.3. In addition, a systematic optical response was observed as a function of deposition pressure from the surface of the TiN films using spectroscopic ellipsometry.