K.K. Bamzai

Preparation, and Structural and Magnetic Properties of Ca Substituted Magnesium Ferrite with Composition MgCaxFe2−xO4 ( = 0.00, 0.01, 0.03, 0.05, 0.07)

Calcium substituted magnesium ferrite with composition MgCaxFe2−xO4 (where = 0.00, 0.01, 0.03, 0.05, 0.07) was prepared by ceramic technique. These compositions were then subjected to detailed study for structural and magnetic properties. X-ray diffraction studies reveal the formation of single phase cubic spinel. The values of lattice constant increase with the increase in calcium concentration from = 0.00 to = 0.03 and then decrease. Scanning electron microscopic (SEM) technique was used to study the morphology of the grown materials. The grain size was calculated using average intercept line method. The elemental composition of pure and calcium substituted magnesium ferrite was obtained from energy dispersive X-ray analysis (EDAX) spectrum. The hysteresis loop confirms the magnetic behaviour of the prepared composition, which is then discussed on the basis of cation distribution. The parameters such as saturation magnetization, coericivity, and retentivity are calculated. The Curie temperature was found to decrease with increasing calcium content.

Dielectric, Ferroelectric and Thermal Studies of Gel Grown Yttrium Tartrate (YT) Crystals

Yttrium tartrate (YT) single crystals have been grown by gel encapsulation technique. The crystallinity of the grown crystals was confirmed by XRD technique. The dielectric constant, dielectric loss and ac conductivity was measured as a function of frequency in the range 1 kHz to 1 MHz and temperature range of 30°C to 250°C. Dielectric studies shows two transition peaks at temperature of 80°C and 210°C. These transition peaks are further supported by thermal studies. The nature of transitions can be explained by relation (1/ϵ′−1/ϵ ′ max ) = A (T − Tc)γ and is reported to be diffused transitions. The variation of ac conductivity with temperature and frequency has been reported and is represented by the relation σ (ω) = Aω s . The saturation polarization (P s ), remanent polarization (P r ) and coercive field (E c ) of the polarization-electric field hysteresis are reported for this material. These dielectric results are then correlated with thermal studies. The material (YT) is proposed to be ferroelectric.

Effect of Oxygen Content on Dielectric Properties of Float Zone Grown Titanium Dioxide (TiO2) Crystal

Dielectric studies have been carried out to see the effect of oxygen content on two different sets of rutile (TiO2) single crystals grown by float zone technique. One set is having pale yellow colour which is the correct characteristic colour of rutile crystals whereas another have dark blue colour which contains oxygen deficiency. The variation of dielectric constant (ϵ′), the dielectric loss (tan δ) and the electric conductivity (σ) at different temperature and frequency of the applied electric field was studied for both these crystals. There is significant change in the values of dielectric constant for blue sample as compared to yellow sample. The dielectric constant in most of the region for oxygen deficient crystal (blue colour) is large as compared to true stiochoimetric ratio (pale yellow) of the rutile crystal. Based on conductivity behaviour, it was found that the correlated barrier hopping (CBH) model is the most predominant hopping mechanism in the case of the yellow crystal, whereas in the case of the blue crystal, hopping may be due to overlap of large polarons.

Dielectric and Ferroelectric Studies on Gel Grown Samarium Tartrate (SmT) Crystals

Samarium tartrate (SmT) single crystals have been grown by gel encapsulation technique. The crystallinity of the grown crystals was confirmed by XRD technique. The dielectric constant, dielectric loss and ac conductivity was measured as a function of frequency in the range 5 kHz to 1 MHz and temperature range 20°C to 275°C. Dielectric studies show a transition peak at temperature 240°C. This transition peak is further supported by thermal and IR studies. The nature of the transition can be explained by the relation (1/ϵ ′ − 1/ϵ ′ max ) = A (T − T c )γ and is reported to be diffused. The variation of ac conductivity with temperature and frequency has been reported and is represented by the relation σ ac = Aω s . The saturation polarization (P s ), remanent polarization (P r ) and coercive field (E c ) of the polarization – electric field hysteresis are reported for the material. The material (SmT) is proposed to be ferroelectric.

Thermal characteristics of pure and substituted gel grown Gd-molybdate crystals

Polycrystalline spherulitic crystals of pure Gd-heptamolybdate and single and twinned crystals of substituted Gd-Ba-molybdate were grown by using gel encapsulation technique. The thermal behaviour of these crystals was studied using the thermoanalytical techniques, which included TG, DTA and DSC. Thermal analysis suggests decomposition of the materials in one or more than one stages. Results obtained on application of TG based models viz. Horowitz-Metzger, Coats-Redfern and Piloyan-Novikova, are reported. According to the results of the kinetics of thermal decomposition, the random nucleation model is shown to be the one that is relevant to the decomposition of single rare earth (Gd) containing material and contracting sphere to the decomposition of the substituted (Gd-Ba) one. The kinetic parameters viz. the order of reaction, frequency factor and energy of activation using above-mentioned models, are computed and shown to bear reasonably good agreement.

Dislocation etchants for flux grown YFeO3 single crystals

Abstract Etching studies have been carried out on two different crystallographic planes, i.e., 110 and 001, of flux grown YFeO3 single crystals. We report two dislocation etchants: (i) H3PO4 and (ii) H3PO4 in combination with HNO3. The etch pit density is estimated to be in the range of (4–5) x 104 cm-2.

Irradiation Effects on Dielectric Behaviour of M—Type Hexaferrites

Pure Strontium hexaferrite grown by flux method were used in present investigation. Dielectric analysis was carried out using fully automated HP4192A impedance analyzer. The samples were irradiated using Lithium beam of energy 50 MeV with fluence rate 1 × 1013 ions/cm2 at NSC, New Delhi. Dielectric studies of both irradiated and unirradiated samples have been carried out and the effect of irradiation is studied. The variation of dielectric constant ϵ′ for unirradiated and irradiated SrFe12O19 were studied as a function of frequency of a.c. field in the frequency range 1 KHz to 10 MHz from room temperature to 200°C. Dielectric constant is found to decrease with frequency, however, ϵ′ increases with temperature for both virgin as well as irradiated crystals. The crystals show increase in dielectric constant after irradiation. The variation of tanδ with frequency shows cusps for all samples. The variation has been explained on the basis of Maxwell-Wagner Interfacial polarization.

Growth, characterization and thermal behaviour of gel grown mixed Gd-Sr-molybdate crystals

Abstract The growth of mixed Gd–Sr–molybdate crystals, bearing stoichiometric composition Gd 2 Sr 7 Mo 9 O 37 ·14H 2 O, is achieved by gel encapsulation technique, using the system RCl 3 (NH 4 ) 6 Mo 7 O 24 NH 4 NO 3 HNO 3 Na 2 SiO 3 (where R=Gd:Sr). Addition of Sr in the lattice of pure Gd-heptamolybdate (reported elsewhere) influences the morphological development of grown crystals. While pure Gd-heptamolybdate assumes spherulitic morphology under all conditions of growth, addition of Sr modifies the morphology of mixed crystals drastically and develops them as agglomerations of stacked hexagonal crystal platelets. The mixed crystals carry water of hydration and are thermally unstable. Various physico-chemical techniques, which include optical and scanning electron microscopy/XRD/energy dispersive analysis of X-rays and FT-IR were used to characterize the grown crystals. Thermal behaviour of these crystals using the thermoanalytical techniques, including TG/DTA and DSC is discussed. Thermal analysis suggests decomposition of the material in different temperature intervals. Results obtained on application of TG based models viz., Horowitz–Metzger, Coats–Redfern and Piloyan–Novikova are reported. The results of the kinetics of thermal decomposition suggest contracting sphere model as the one that is relevant to the decomposition of mixed Gd–Sr–molybdate. The kinetic parameters, viz. the order of reaction, frequency factor and energy of activation using above-mentioned models, are computed.

Hardness and etching studies on flux grown YFeO/sub 3/ crystals

Results of Vickers microhardness (H/sub v/), fracture toughness (K/sub c/), brittleness (B/sub i/), and dislocation etching characteristics on [110] and [001] planes of flux grown yttrium orthoferrite crystals are reported. The values of H/sub v/, K/sub c/, B/sub i/ are estimated to be 1235.08 Kg/mm/sup 2/, 0.1912 gm/ mu m/sup 3/2/, and 8.1955 mu m/sup -1/2/, respectively, for [110] plane, whereas for [001] plane the values of H/sub v/, K/sub c/, and B/sub i/ are 1027.12 kg/mm/sup 2/, 0.5975 gm/ mu m/sup 3/2/, and 1.875 mu m/sup -1/2/, respectively. The variation o H/sub v/ with applied load is explained to follow from the Hays and Kendalls law. Etching kinetics of YFeO/sub 3/ surfaces in H/sub 3/PO/sub 4/ at different temperatures leads to activation energy values for etch pits; the activation energies for dissolution parallel to [110] and [001] planes (i.e., lateral dissolution E/sub L/) are 0.41 eV and 0.34 eV, respectively. The activation energies for dissolution perpendicular to [110] and [001] planes (i.e., vertical dissolution E/sub D/) are 0.20 eV and 0.21 eV, respectively. >

Delineation of defect structures in flux-grown GdFeO3 crystals by etching

Abstract Etch patterns on (110) and (001) faces of as-obtained GdFeO3 crystals are illustrated and described. Rectangular etch pits oriented with their longer sides in 〈100〉 direction and shorter sides in 〈010〉 direction are exhibited by (001) faces, whereas almost circular etch pits are exhibited by (110) faces of as-obtained crystals. Hexagonal etch pits and triangular etch pits produced as a result of etching in boiling HNO3 on (001) and (110) faces are respectively illustrated. Evidence in support of twinning in some crystals is offered. The etch pattern on as-grown crystals are attributed to some fault in their growth conditions. Edge nucleated growth pyramids on a (110) face and conical hillocks on (001) faces exhibiting pits at the centres of their initiation are illustrated. The conical hillocks are suggested to be as a result of preferential growth at the point of emergence of screw dislocations.