• Volume 25, Issue 4

August 2002,   pages  259-358

• Synthesis of Ba3ZnNb2O9–Sr3ZnNb2O9 solid solution and their dielectric properties

Oxides of the type, Ba3–𝑥Sr𝑥ZnNb2O9 (0 ≤ 𝑥 ≤ 3), were synthesized by the solid state route. Oxides calcined at 1000°C show single cubic phase for all the compositions. The cubic lattice parameter (a) decreases with increase in Sr concentration from 4.0938(2) for 𝑥 = 0 to 4.0067(2) for 𝑥 = 3. Scanning electron micrographs show maximum grain size for the 𝑥 = 1 composition (∼ 2 𝜇m) at 1200°C. Disks sintered at 1200°C show dielectric constant variation between 28 and 40 (at 500 kHz) for different values of 𝑥 with the maximum dielectric constant at 𝑥 = 1.

• Self-propagating high temperature synthesis and magnetic properties of Ni0.35Zn0.65Fe2O4 powders

Ni–Zn ferrite powders were synthesized by self-propagating high temperature synthesis (SHS) method. X-ray diffraction, TEM and vibrating sample magnetometry (VSM) were used to characterize the phase composition, microstructure and magnetic properties of the combustion products. The effect of the combustion temperature ($T_c$), the major parameter of the SHS process, on particle size, phase composition and magnetic properties of the products was also studied. The results showed that particle size grew with the increasing combustion temperature. The maximum saturation magnetization, $M_s$, increased with combustion temperature indicating the growth of grain size and high degree of ferritization, while residual magnetization, $M_r$, and coercive force, $H_c$, decreased. Compared with other methods, Ni0.35Zn0.65Fe2O4 ferrite powders with improved magnetic properties can be obtained by SHS at 1000°C.

• Crystal structure transformation of TiO2 in presence of Fe2O3 and NiO in air atmosphere

TiO2 is produced and marketed in two main grades viz. anatase and rutile. Both anatase and rutile have their own pigmentry properties and hence cannot be substituted by each other. Pure anatase on heating at higher temperatures undergoes crystallographic rearrangement to form rutile. This transformation in presence of NiO and Fe2O3 under air atmosphere was studied using XRD and SEM. The transformation temperature was found to be reduced much in presence of NiO and Fe2O3 and the extent of lowering was higher for NiO than Fe2O3. The activation energy for this transformation was also calculated. The method of preparation had major influence on the transformation.

• Structure determination at room temperature and phase transition studies above $T_c$ in ABi4Ti4O15 (A = Ba, Sr or Pb)

The room temperature structure of three compounds belonging to the Aurivillius family (𝑛 = 4), ABi4Ti4O15 (A = Ba, Sr or Pb) has been analysed. BaBi4Ti4O15 crystallizes in a tetragonal 𝐼4/𝑚𝑚𝑚 space group whereas SrBi4Ti4O15 and PbBi4Ti4O15 crystallize in the orthorhombic space group $A2_1am$. The starting model for the Sr and Pb analogues was derived from ab initio methods and refined using the Rietveld method. The cations Ba and Sr are disordered over the Bi sites while the Pb cation is found exclusively in the [Bi2O2]2+ layers. The TiO6 octahedra are tilted with the Ti–O bonds forming zigzag chains along the 𝑐’ axis. The displacement of Bi atoms along the 𝑎’ axis might be responsible for ferroelectricity in these compounds. The high temperature X-ray data above $T_c$ indicate no structural transition for A = Ba and Pb while A = Sr transforms to the tetragonal structure.

• Catalytic characterization of bi-functional catalysts derived from Pd–Mg–Al layered double hydroxides

Hydrotalcite like precursors containing PdII–MgII–AlIII with varying molar ratios, (Pd + Mg)/Al ≈ 3 and Mg/Pd ≈ 750 to 35, were prepared by coprecipitation of metal nitrates at constant pH. Characterization of samples as synthesized and their calcined products by elemental analyses, powder XRD, TG–DTA, FT–IR spectroscopy, TPR and N2 physisorption indicated a well crystalline hydrotalcite like structure with incorporation of Pd2+ in the brucite layers. Thermal decomposition of hydrotalcite precursors at intermediate temperatures led to amorphous mixed oxides, Pd/MgAl(O), which on reduction yielded bi-functional catalyst, Pd°/MgAl(O). The resultant catalysts with acid, base and hydrogenating sites, were highly active and selective for one-step synthesis of methyl isobutyl ketone (MIBK) from acetone and hydrogen. The results showed an optimal balance between acid-base and metallic sites were required to increase the selectivity of MIBK and stability of the catalysts.

• Metadynamic recrystallization in C steels

Metadynamic recrystallization has been investigated in three plain carbon steels (ENIA, EN2 and EN24) through the use of hot interrupted compression tests on a wedge plastometer. Holding time was 0.5 s between passes. Strain rates of 0.05 and 0.12/s and small strain increments of 3, 5 and 7% were employed. Test temperatures were varied between 800 and 1100°C.

Various incremental and continuous stress strain curves were highlighted at different temperatures and strain rates for 3 steels, ENIA, EN2 and EN24, resulting in varying flow stresses and strains. Highest peak stress was 180 MPa for EN24 at peak strain of 0.25 and 900°C, with a strain rate 0.12/s. Peak strain values for all steels at 1100°C was 0.133 at a strain rate of 0.05/s and 0.15 at a strain rate of 0.12/s.

Strain accumulation resulted in dynamic and metadynamic recrystallization with refinement to about 15 𝜇m for dynamic and 22 𝜇m for metadynamic recrystallization. Fractional softening, 𝑋, decreased from 0.27 to 0.12 as recrystallization times in metadynamic recrystallization increased from 0.9 s to 1.5 s at 1100°C. Time for 50% metadynamic recrystallization was also reduced as temperature increased.

For ENIA, a drop from 10000 s to 20 s, as temperature increased from 800 to 1100 °C was observed. For EN24 and EN2 steels, a drop from 4000 s to 6 s for similar temperature rise was observed.

Metadynamic recrystallization (at strains higher than critical strain) is observed to be a strong function of strain rate and a very weak function of temperature and strain. It significantly refined the austenite grain size prior to transformation.

• Fatigue crack growth retardation in spot heated mild steel sheet

A fatigue crack can be effectively retarded by heating a spot near the crack tip under nil remote stress condition. The subcritical spot heating at a proper position modifies the crack growth behaviour in a way, more or less, similar to specimen subjected to overload spike. It is observed that the extent of crack growth retardation increases with increasing level of overload as well as with increasing spot temperature. It is also observed that modification in crack growth behaviour is a function of location of heating spot and maximum retardation is observed at + 5 position.

• Hyperfine field distributions in disordered Mn2CoSn and Mn2NiSn Heusler alloys

Heusler alloys, Mn2CoSn and Mn2NiSn, were prepared and characterized by X-ray studies. Mössbauer studies using Sn-119 were carried out to investigate the hyperfine fields present at the Sn site in these alloys. The hyperfine field distribution in these alloys as well as X-ray studies point to the chemical disorder present in both alloys. Co-existence of a paramagnetic portion along with the magnetic hyperfine part was observed in Mn2CoSn even at low temperatures, while this was not found in Mn2NiSn spectra. Hyperfine fields at Sn site were calculated using Blandin and Campbell model and compared with the experimental results.

• Effect of alloying with zinc on SFE of aluminium by study of lattice imperfections in cold worked Al–Zn alloys

A detailed X-ray Fourier line shape analysis has been performed on three compositions of Al–Zn alloys viz. Al–3 55 wt% Zn, Al–14 7 wt% Zn and Al–19 3 wt% Zn in fcc phase. It has been found that deformation stacking faults, both intrinsic 𝛼' and extrinsic 𝛼'' are absent in the cold worked state and twin fault 𝛽 is found to be slightly present in the deformed lattice of the two initial compositions of the alloys. Similar to the effect of solute germanium and copper, respectively in Al–Ge and Al–Cu systems, hexagonal zinc also fails to impart faulting in fcc Al–Zn system. This corroborates the fact that aluminium has high stacking fault energy.

• Impact toughness of high strength low alloy TMT reinforcement ribbed bar

Charpy V-notch impact toughness of 600 MPa yield stress TMT rebars alloyed with copper, phosphorus, chromium and molybdenum has been evaluated. Subsize Charpy specimens were machined from the rebar keeping the tempered martensite rim intact. The copper–phosphorus rebar showed toughness of 35 J at room temperature. The toughness of copper–molybdenum and copper–chromium rebars was 52 J. The lower toughness of phosphorus steel is attributed to solid solution strengthening and segregation of phosphorus to grain boundaries. Due to superior corrosion resistance, copper–phosphorus TMT rebar is a candidate material in the construction sector.

• Experimental 𝑓'' of As at 170, 200, 250 and 300 K from the Bijvoet pairs of GaAs

Anomalous dispersion effects lead to the modification of the measured X-ray structure factors. In this work, we have determined the imaginary part of the anomalous dispersion correction terms (𝑓'' ) of arsenide atom (As), through the X-ray data collected using spherical single crystal of GaAs, at various temperatures, i.e. 170, 200, 250 and 300 K. It is stressed that more measurements of 𝑓'' of the elements are needed to confirm the theoretical calculations.

• On the symmetry of phosphorous doped ZnSe

The site symmetry of P doped ZnSe is analysed in detail here, as the recent experiments suggest two possible symmetries $T_d$ and $C_{3V}$. The reduction to $C_{3V}$ is attributed to the presence of natural impurity, Ga. Our calculations based on molecular model and Green’s functions suggest that the symmetry $C_{3V}$ is possible with ZnSe : P when Jahn Teller distortion of about ∼ 0.2 Å (towards one of Zn atom) is assumed. This has been supported by other experiments.

• Si diffusion in GaAs

Theoretical studies are carried out to ascertain the dominant mechanism of Si diffusion in GaAs. Lattice dynamical model calculations have shown that the most probable diffusion mechanism is through a single vacancy even though several experiments cannot fix the mechanism as substitutional, substitutional–interstitial pair or neutral defect pair.

• Thermally induced structural changes in Nomex fibres

Thermally aged Nomex fibres manifest several residual effects viz. reduction in X-ray crystallinity, weight loss and deterioration in tensile characteristics. Surface damages in the form of longitudinal openings, holes, material deposits etc have also been observed. Based on the data from thermally exposed fibres, the time needed for states of zero tensile strength and modulus have been predicted.

• Structural, dielectric and transport properties of Pb(Mn0.5W0.5)O3

Polycrystalline Pb(Mn0.5W0.5)O3, a ferroelectric oxide having perovskite structure, was prepared by high temperature solid state reaction technique. Preliminary X-ray diffraction analysis confirms single phase formation with the lattice parameters 𝑎 = 7.2501 Å, 𝑏 = 8.1276 Å and 𝑐 = 12.0232 Å. Room temperature dielectric constant (𝜀' ) and loss tangent (tan 𝛿) were scanned with respect to frequency in the range 100 Hz–1 MHz. Detailed study of dielectric constant and electrical conductivity reveals a phase change around 400 K, which is quite different from those in the other materials of the same type. Further, the seebeck coefficient (𝛼) is temperature independent. The conduction is interpreted as due to small polaron hopping.

• Infrared absorption spectroscopic study of Nd3+ substituted Zn–Mg ferrites

Compositions of polycrystalline Zn𝑥Mg1-𝑥Fe2–𝑦Nd𝑦O4 (𝑥 = 0.00, 0.20, 0.40, 0.60, 0.80 and 1.00; 𝑦 = 0.00, 0.05 and 0.10) ferrites were prepared by standard ceramic method and characterized by X-ray diffraction, scanning electron microscopy and infrared absorption spectroscopy. Far infrared absorption spectra show two significant absorption bands, first at about 600 cm–1 and second at about 425 cm–1 , which were respectively attributed to tetrahedral (A) and octahedral (B) sites of the spinel. The positions of bands are found to be composition dependent. The force constants, $K_T$ and $K_O$, were calculated and plotted against zinc concentration. Compositional dependence of force constants is explained on the basis of cation–oxygen bond distances of respective sites and cation distribution.

• Azomesogens with 1,2,4-trisubstituted benzene moiety

New mesogenic homologous series having an azo central linkage was synthesized by fixing a rigid 4-methyl phenyl azo group to resorcinol moiety. The terminal and lateral phenolic –OH groups were esterified, one by one, with 4-𝑛-alkoxybenzoyl and acetyl group, respectively. All the twelve homologues synthesized exhibit mesomorphism. The methoxy to 𝑛-decyloxy and 𝑛-hexadecyloxy derivatives exhibit monotropic nematic mesophase whereas, 𝑛-dodecyloxy and 𝑛-tetradecyloxy derivatives exhibit enantiotropic nematic mesophase. The mesogenic properties of the present series was compared with those of other structurally related mesogenic series.

• # Bulletin of Materials Science

Volume 43, 2020
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• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019