• Volume 33, Issue 4

August 2010,   pages  339-517

• Modelling of flame temperature of solution combustion synthesis of nanocrystalline calcium hydroxyapatite material and its parametric optimization

Hydroxyapatite (HAp), an important bio-ceramic was successfully synthesized by combustion in the aqueous system containing calcium nitrate-di-ammonium hydrogen orthophosphate-urea. The combustion flame temperature of solution combustion reaction depends on various process parameters, and it plays a significant role in the phase formation, phase stability and physical characteristics of calcium hydroxyapatite powder. In this work, an attempt has been made to evaluate the influence of each selected process parameters on the flame temperature as well as physical characteristics of powder, and to select an optimal parameters setting using Taguchi method. A regression model has also been developed to correlate the input parameters, viz. batch size, diluents, fuel to oxidizer ratio and initial furnace temperature, with flame temperature of the solution combustion reaction. The adequacy of the developed model has been checked using analysis of variance technique.

• Novel fabrication of silica nanotubes using multi-walled carbon nanotubes as template

Silica nanotubes were synthesized using multi-walled carbon nanotubes (MWCNTs) as template. The as-obtained samples were characterized by infrared spectroscopy (FTIR), X-ray diffraction (XRD), transmission electron microscopy (TEM), field emission scanning electron microscope (FE–SEM) and photoluminescent (PL) spectroscopy. The results indicate that the thickness of the outer walls is about 10 nm and the inner diameter is completely dependent on the size of MWCNTs. The as-fabricated silica nanotubes emit a strong violet light under excitation of 250 nm.

• Temperature dependent growth and optical properties of SnO2 nanowires and nanobelts

SnO2 nanowires and nanobelts have been grown by the thermal evaporation of Sn powders. The growth of nanowires and nanobelts has been investigated at different temperatures (750–1000°C). The field emission scanning electron microscopic and transmission electron microscopic studies revealed the growth of nanowires and nano-belts at different growth temperatures. The growth mechanisms of the formation of the nanostructures have also been discussed. X-ray diffraction patterns showed that the nanowires and nanobelts are highly crystalline with tetragonal rutile phase. UV-visible absorption spectrum showed the bulk bandgap value (∼ 3.6 eV) of SnO2. Photoluminescence spectra demonstrated a Stokes-shifted emission in the wavelength range 558–588 nm. The Raman and Fourier transform infrared spectra revealed the formation of stoichiometric SnO2 at different growth temperatures.

• Photoinduced properties of nanocrystalline TiO2 sol–gel derived thin films

In this paper, nanostructure TiO2 thin films were deposited on glass substrates by sol–gel dip coating technique. X-ray diffraction and Fourier transform infrared spectroscopy were used to determine film behaviour. The super-hydrophilicity was assessed by contact angle measurement. Photocatalytic properties of these films were evaluated by degradation of methylene blue under UV irradiation. The XRD pattern of TiO2 powder samples confirmed the presence of polycrystalline anatase phase with a crystal size of 17 nm. The results indicated that UV light irradiation had significant effect on super-hydrophilic and photocatalytic properties of TiO2 thin films.

• Synthesis of nano-particle and highly porous conducting perovskites from simple in situ sol–gel derived carbon templating process

Nano-sized La0.6Sr0.4Co0.2Fe0.8O$_{3–\delta}$ (LSCF) and La0.8Sr0.2MnO$_{3–\delta}$ (LSM) oxides were synthesized by a simple in situ sol–gel derived carbon templating process. Nano-sized LSCF–carbon and LSM–carbon composites were first obtained with a grain size of 20–30 nm. Further calcination of the obtained composites under air resulted in the nano-sized pure-phase perovskites with crystalline size of as small as 14 nm. Such a decrease in crystalline size of perovskite via the indirect calcination process was ascribed to the suppressing effect of carbon in the grain growth of perovskite. Furthermore, when the in situ created carbon was applied as a template for pore forming, a highly porous perovskite sintering body packing from the nano-sized perovskite oxide was obtained.

• Dielectric and thermal studies on gel grown strontium tartrate pentahydrate crystals

Results of dielectric and thermal studies on strontium tartrate pentahydrate crystals are described. The value of dielectric constant is shown to be independent of temperature till 360 K at all the frequencies (110–700 kHz) of the applied a.c. field. It increases abruptly achieving a peak value of 25.5 at 100 kHz; the peak value being strongly dependent on frequency. In the temperature range, 87 &lt; 𝑇 &lt; 117°C, the value of 𝜀' falls suggesting a transition at around 100°C or so. The dielectric constant, 𝜀', of the material is shown to be frequency dependent but temperature independent in the pre- or post-𝑇c range 87 &lt; 𝑇 &lt; 117°C, suggesting that the contribution towards polarization may be due to ionic or space charge polarization which gets eliminated at higher frequencies. The ferroelectric transition is supported by the results of thermoanalytical studies. It is explained that crystallographic change due to polymorphic phase transition may be occurring in the material, besides the change due to loss of water molecules, which leads to the dielectric anomaly at around 100°C. Coats–Redfern approximation method is applied for obtaining non-isothermal kinetic parameters leading to calculation of activation energies corresponding to three decomposition stages of material in the temperature ranging from 379–1113 K.

• Temperature dependence of electromechanical properties of PLZT 𝑥/57/43 ceramics

The compositions of lead lanthanum zirconate titanate PLZT [Pb(Zr0.57Ti0.43)O3 + 𝑥 at% of La, where 𝑥 = 3, 5, 6, 10 and 12] have been synthesized using mixed oxide route. The temperature dependent electromechanical parameters have been determined using vector impedance spectroscopy (VIS). The charge constant 𝑑31 and elastic compliance $s^{E}_{11}$ show a peak in all the samples at a temperature 𝑇mt much below the ferroelectric – paraelectric transition temperature, whereas the series resonance frequency 𝑓s shows a dip at these temperatures. The Poisson’s ratio 𝜎𝐸 increases with temperature 𝑇 showing a broad peak at a temperature higher than 𝑇mt. The voltage constant 𝑔31 decreases and the planar coupling coefficient 𝐾p remains constant up to half of the 𝑇mt and then falls sharply with 𝑇. Half of the 𝑇mt can, therefore, be used for specifying the working temperature limit of the piezoceramics for the device applications.

• Optical properties of lead–tellurite glasses doped with samarium trioxide

The optical properties of a new family of 𝑥Sm2O3–(40–𝑥)PbO–60TeO2 glasses are investigated. The optical absorption spectra were recorded at room temperature in the UV-visible region. From the absorption edge studies, the values of optical bandgap energies have been evaluated. The refractive index, molar refraction and polarizability of oxide ions have been calculated by using Lorentz–Lorentz relations. The non-linear variations of the above optical parameters are discussed with respect to samarium concentration.

• Optical and X-ray photoelectron spectroscopy of PbGeO3 and Pb5Ge3O11 single crystals

Pb5Ge3O11 crystals are found to exhibit pale yellow colouration while PbGeO3 are colourless. X-ray photoelectron spectroscopy (XPS) measurements show lead deficiency in both the crystals. The results also reveal a stronger ionic character for PbGeO3 as compared to Pb5Ge3O11 crystal. The binding energy of Ge3𝑑 core level in the case of Pb5Ge3O11 crystal is found to be smaller than the binding energy of germanium oxide, thereby indicating the incomplete oxidation of Ge ions in the crystal lattice. On gamma ray irradiation, the transmission of both the crystals is observed to deteriorate uniformly over the entire wavelength range, which has been attributed to the oxidation of some of the lattice Pb ions. On gamma irradiation the changes observed in O1𝑠 core level energies for both the crystals are seen to be consistent with the changes noted in the Pb4𝑓7/2 and Ge3𝑑 spectra. Interestingly, the results reveal oxidation of surface Ge atoms with atmospheric oxygen under gamma irradiation.

• Preparation of Ti3AlC2 by mechanically activated sintering of 3Ti/Al/2C/0.2Sn

The mechanically activated sintering process was adapted to synthesize titanium aluminum carbide (Ti3AlC2) at low temperature. A mechanically induced self-propagation reaction occurred by mechanical alloying of 3Ti/Al/2C powder mixtures. In addition to powder products, a large amount of rigor granules with a size of 0.5 ∼ 10 mm were produced. Fine powders containing Ti3AlC2, Ti2AlC and TiC were obtained. The granules composed of Ti3AlC2, Ti2AlC and TiC. Adding Sn may remove Ti2AlC and enhance the synthesis of Ti3AlC2. After Sn was added, the products only contained Ti3AlC2 and TiC. The Ti3AlC2 content of the powders and granules were 75 wt% and 88 wt%, respectively. The mechanically alloyed products were pressureless sintered at 900–1300°C for 2 h. Sintering of these products at 900 ∼ 1200°C yields samples containing over 95 wt% Ti3AlC2. The sintered powder compacts with high purity Ti3AlC2 had a fine organization. The lath Ti3AlC2 of the granules had a length of 10–20 𝜇m.

• Thermal expansion of doped lanthanum gallates

Thermal expansion of several compositions of Sr and Mg-doped LaGaO3 including an 𝐴-site deficient composition (La0.9Sr0.1)0.98(Ga0.8Mg0.2)O2.821 were measured in the temperature range from 298 to 1273 K. The effect of doping on thermal expansion was studied by varying the composition at one site of the perovskite structure (either 𝐴 or 𝐵), while keeping the composition at the other site invariant. Thermal expansion varied nonlinearly with temperature and exhibited an inflexion between 550 and 620 K, probably related to the change in crystal structure from orthorhombic to rhombohedral. The dependence of average thermal expansion coefficient (𝛼av) on the dopant concentration on either 𝐴 or 𝐵 site of the perovskite structure was found to be linear, when the composition at the other site was kept constant. Mg doping on the 𝐵-site had a greater effect on the average thermal expansion coefficient than Sr doping on the 𝐴-site. Cation deficiency at the 𝐴-site decreases thermal expansion when compositions at both sites are held constant.

• Properties of calcium fluoride up to 95 kbar: A theoretical study

We have studied the high-pressure and high-temperature behaviour of calcium fluoride (CaF2) using molecular dynamics simulations with the Born–Mayer–Huggins potentials. The thermal (pressure-volume) equation of state, radial distribution functions (RDFs) just nearby the melting temperature, isothermal bulk modulus, and volume thermal expansion coefficients were calculated from the isobaric and isothermal ensemble. Structural and thermodynamical properties at ambient temperature were well reproduced for the cubic fluorite-type phase of CaF2. At an extended pressure and temperature ranges, the thermal expansivity and bulk modulus of CaF2 have also been predicted. The RDFs indicated that the melting temperature is bracketed between 1600 and 1700 K, and the thermodynamic properties of CaF2 with fluorite-type cubic structure are investigated in the pressure range 0–95 kbar and temperature up to 1600 K.

• Phase transition study in a [Cu2HgI4 : 0.𝑥AgI] mixed composite system

A novel composite superionic system, [Cu2HgI4 : 0.𝑥AgI], (𝑥 = 0.2, 0.4, 0.6 mol wt.%), was prepared. A [Cu2HgI4] system was used as the host. Electrical conductivity was measured to study the transition behaviour at frequencies of 100 Hz, 120 Hz, 1 kHz and 10 kHz in the temperature range 90–170°C using a Gen Rad 1659 RLC Digibridge. Conductivity increased sharply during the 𝛽–𝛼 phase transition. Upon increasing the dopant-to-host ratio, the conductivity of the superionic system exhibited Arrhenius (thermally activated)-type behaviour. DTA, DTG, TGA and X-ray powder diffraction were performed to confirm doping effect and transition in the host. The phase transition temperature increased with an increase in the dopant concentration. Activation energies in eV for pre- and post-transition phase behaviour are also reported. Due to an interaction between [Cu2HgI4] and AgI, the addition of AgI to [Cu2HgI4] shifted the phase transition of the host [Cu2HgI4].

• Synthesis and characterization of layered oriented hydrogen titanate micro-tube films

Continuous and uniform films of oriented hydrogen titanate tubes with diameters, c.a. 200 nm, were prepared directly via porous anodic aluminum oxides (AAO) templates without any further removal of templates. Simple impregnation method was applied with aqueous solution titanium tetrafluoride (TiF4) as the precursor to form the titania sol–gel. Scanning electron microscope (SEM) results show that the films are uniform, tubes highly oriented and even in length, diameter and morphology. Hydrogen trititanate (H2Ti3O7) phase is proved by X-ray diffraction (XRD) patterns. Layered films of oriented tubes are fabricated when impregnation time is well controlled. The formation mechanism is drawn with the help of energy dispersion spectra (EDS). Due to the existence of 𝐹, the hydrolysis of Ti4+ is relatively slow along inner channel walls of AAO, which eventually forms compact tube arrays. The solubility among different parts of AAO template is of big difference in acidic surrounding. Layered films of oriented tubes are, therefore, fabricated when impregnation time is well controlled. The channel size of the AAO template takes control in deciding the pore size of the titania micro-tubes and reaction time, the length of the tubes.

• Crystallization and spectroscopic studies of manganese malonate

The preparation of manganese malonate crystals by gel method and its spectroscopic studies are reported. X-ray diffraction (XRD) pattern reveals the crystalline nature. The FTIR and FT Raman spectra of the crystals are recorded and the vibrational assignments are given with possible explanations. Diffuse reflectance spectroscopy (DRS) is used to measure the bandgap (𝐸g) of the material.

• Microstructural and densification study of natural Indian magnesite in presence of zirconia additive

The sintering and microstructural evaluation of Indian magnesite was carried out in presence of zirconia. Zirconia in monoclinic form was added in the range 3–6 wt% with respect to raw magnesite and the sintering temperature selected were 1500–1600°C for 2 h. The main impurities present in the magnesite were Fe2O3, CaO, SiO2. The natural crystalline magnesite could be sintered with bulk density of 3.38 g/cc (A.P. 1.54%) at 1550°C/2 h. But the higher bulk density (3.50 g/cc) and minimum apparent porosity (A.P. 0.25%) was attained at 1550°C/2 h with the 3 wt% zirconia additive. On firing magnesite with zirconia as additive, a crystalline phase, magnesio-zirconate, was identified at the triple point regions of the direct bonded periclase grains. The morphology of the periclase grains were changed from subrounded/rounded to angular shaped in presence of zirconia as additive.

• Micrometer size grains of hot isostatically pressed alumina and its characterization

Alumina samples were prepared from two different particle size powders. Finer particle compacts when heated along with coarser particle compacts at same processing temperatures produce bigger grain microstructures due to higher grain growth. An unconventional method of etching by molten V2O5 was adopted to look at the microstructure for accuracy in reported data. On an average starting with finer particles give microstructure with a grain size of 5.5 𝜇m and starting with coarser particles, give microstructure with 2.2 𝜇m average grain size. The flexural strength is around 400 MPa for alumina samples prepared from finer powder in comparison with about 550 MPa for alumina samples prepared from coarser powder. The Vickers hardness in 5.5 𝜇m grain microstructure is around 20 GPa in comparison to about 18 GPa in microstructure with smaller grains of 2.2 𝜇m size.

• Effect of agglomeration during coprecipitation: Delayed spinellization of magnesium aluminate hydrate

Precipitation of magnesium aluminate hydrate with faster addition of ammonia at desired pH causes agglomeration. Agglomerated powder, without any further treatment, on calcination forms intermediate compounds at low temperatures (≤ 900°C). The intermediate compounds on further heat treatment (≥ 1000°C) decompose into MgO, MgAl2O4 and 𝛼-Al2O3. Effect of agglomeration and absorption of foreign ions such as Cl, SO$^{2-}_{4}$, and NH$^{+}_{4}$ in complex compounds probably cause loss of Al3+ and Mg2+ ions during heat treatment, and stoichiometry changes. Powders prepared by continuous method with better control of process parameters than batch process yields better spinellization.

• Bandgap determination of P(VDF–TrFE) copolymer film by electron energy loss spectroscopy

The ferroelectric 𝛽 of poly(vinylidene fluoride trifluoroethylene), P(VDF–TrFE) is confirmed for 100 nm thickness spin coated copolymer film. The homogeneous coverage of the copolymer film is investigated by the help of X-ray photoelectron spectroscopy (XPS). Most importantly, the existing bandgap in the crystalline phase of the copolymer is determined directly from the electron energy loss spectroscopy (EELS).

• Preferential dissolution behaviour in Ni–Cr dental cast alloy

A Ni–Cr–Mo dental alloy was fabricated by three different casting methods, viz. centrifugal casting, high frequency induction casting and vacuum pressure casting. The dependence of cast microstructure on the electrochemical corrosion behaviour was investigated using potentiodynamic cyclic and potentiostatic polarization techniques, impedance spectroscopy and scanning electron microscopy. The experimental results were compared and discussed with those obtained for a Co–Cr–Mo counterpart. The results of the study showed that the variation in casting morphologies with casting methods has only marginal influence in the overall corrosion resistance of Ni–Cr and Co–Cr dental alloys. There was severe preferential dissolution of Ni rich, Cr and Mo depleted zones from the Ni–Cr–Mo alloy. The overall corrosion resistance property of the Co–Cr base alloy was better than that of the Ni–Cr base alloy.

• Effect of SiO$^{2-}_{3}$/OH on plasma electrolytic oxidation of Ti–5Mo–4V–3Al

Plasma electrolyte oxidation (PEO) was utilized to produce thick films on titanium and Ti–5Mo–4V–3Al alloys by immersing them in various solutions of Na2SiO3 and KOH with different concentrations to investigate the effect of SiO$^{2-}_{3}$/OH relations on the morphology and formed phases by utilizing SEM and XRD. Corrosion resistance is evaluated by open circuit potential (OCP) variation of samples in NaCl 3.5% and potentiodynamic polarization. The results show that the unstable film is formed by using more aggressive PEO electrolyte. By increasing this ratio, pore size varied from fine to coarse and the rate of corrosion decreased and OCP became more positive. The best protective film was formed in SiO$^{2-}_{3}$/OH ratio of 1.

• Behaviour of S 355JO steel subjected to uniaxial stress at lowered and elevated temperatures and creep

This paper considers main mechanical properties of structural-high strength low alloy (HSLA) S 355JO (ASTM A709 Gr50) steel subjected to uniaxial tensile tests at lowered and elevated temperatures. The engineering stress vs strain diagrams as well as curve’s dependence of ultimate and yield strengths vs both lowered and elevated temperatures are presented. The focus is also on specimen elongations vs temperature at elevated temperatures. Short-time creep tests for selected constant stresses at selected temperatures were curried out. Uniaxial creep behaviour for selected creep test was modeled by the rheological model. The creep curve determined by modeling procedure was compared with experimentally obtained one. Also, notch impact energy test, using Charpy pendulum impact machine was performed and according to the proposed formula, fracture toughness is calculated. All of experimental tests were performed using modern computer directed experimental systems.

• Erosive wear of a surface coated hydroturbine steel

In the present investigation, stellite-6, Cr3C2–NiCr and WC–Co–Cr coatings were deposited by DGun on a hot rolled 21Cr–4Ni–N steel meant for fabrication of hydro turbine underwater parts. The coatings have been characterized for microstructure, porosity, microhardness and crystalline nature. The erosion experiments were carried out using an air jet erosion test rig at a velocity of 120 ms-1 and impingement angles of 30° and 90°. Silicon carbide particles of size ranging between 500 and 700 𝜇m were used as erodent. Scanning electron microscopy (SEM) technique was used to analyse the nature and mechanism of erosion. Erosion behaviour is observed to be influenced largely by the nature and extent of porosity in the surface coatings.

• Comparative corrosion study of Ag–Pd and Co–Cr alloys used in dental applications

The electrochemical behaviour of two Ag–Pd alloys (Unique White and Paliag) used in dental prosthetics construction for crowns and bridges and one Co–Cr alloy (Vitallium 2000) was studied in artificial saliva using the polarization curves and electrochemical impedance spectroscopy (EIS). The corrosion resistance was evaluated by means of the corrosion currents value and by coulometric analysis. The open circuit potential of Ag–Pd are attributed to dealloying followed by surface enrichment with Ag and the possible formation of an insoluble AgCl surface film on the respective alloy surfaces. Our results have shown that these alloys have a somewhat good corrosion resistance in artificial saliva. The corrosion current densities of Unique White and Vitallium 2000 alloys were very low (∼100 nA/cm2). For Ag–Pd alloys, when increasing the content of Cu, corrosion resistance decreases. The passivation of all samples occurred spontaneously at the open circuit potential. The electrochemical properties of the spontaneously passivated electrodes at the open circuit potential were studied by EIS. The polarization resistance (𝑅p) and the electrode capacitance (𝐶dl) were determined. The polarization resistance of all the samples increases with the immersion time. The polarization resistances are largest for Unique White (Ag–Pd) and Vitallium 2000 (Co–Cr) alloys. Because the electrochemical behaviour of the Co–Cr alloy was compared with that of Ag–Pd alloy, this type of alloy may be a suitable alternative for use in the manufacture of fixed dental prostheses. The present study, though limited, has shown that electrochemical characteristics can be used to identify such alloys. Knowledge of the in vitro corrosion behaviour of these alloys may lead to better understanding of any biologically adverse effects in vitro.

• Effect of carbon on corrosion resistance of powder-processed Fe–0.35%P alloys

The corrosion behaviour of phosphoric irons containing 0.35 wt % P, 2% copper, 2% nickel, 1% silicon, 0.5% molybdenum, with/without 0.15% carbon prepared by powder forging route were studied in different environments. The various environments chosen were acidic (0.25 M H2SO4 solution of pH 0.6), neutral/marine (3.5% NaCl solution of pH 6.8) and alkaline (0.5 M Na2CO3 + 1.0 M NaHCO3 solution of pH 9.4). The corrosion studies were conducted using Tafel extrapolation and linear polarization methods. The studies also compare Armco iron with phosphoric irons. It was observed that the addition of carbon improved the corrosion resistance of a Fe–0.35%P–2%Ni–2%Cu–1%Si–0.5%Mo alloy in all the environments. Corrosion rates were highest in acid medium, minimal in alkaline medium and low in neutral solution. SEM/EDAX was used to characterize the compositions.

• Pyrolysis of petroleum asphaltenes from different geological origins and use of methylnaphthalenes and methylphenanthrenes as maturity indicators for asphaltenes

Asphaltenes separated from two different crude oils from upper Assam, India, having different geological origins, viz. DK (Eocene) and JN (Oligocene–Miocene) were pyrolysed at 600°C and the products were analysed by gas chromatography–mass spectrometry (GC/MS) especially for the generated alkylnaphthalenes and alkylphenanthrenes. Both the asphaltenes produced aliphatic as well as aromatic compound classes. Alkylnaphthalenes and alkylphenanthrenes were identified by using reference chromatograms and literature data and the distributions were used to assess thermal maturity of the asphaltenes. The ratios of 𝛽-substituted to α-substituted isomers of both alkylnaphthalenes and alkylphenanthrenes revealed higher maturity of the JN asphaltenes than the DK asphaltenes. For both the asphaltenes the abundance of 1-methylphenanthrene dominated over that of 9-methylphenanthrene showing the terrestrial nature of the organic matter.

• Electron field emission from 𝑠𝑝2-induced insulating to metallic behaviour of amorphous carbon (𝑎-C) films

• # Bulletin of Materials Science

Current Issue
Volume 42 | Issue 6
December 2019

• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019