P. N. Kotru

Microhardness of flux grown pure doped and mixed rare earth aluminates and orthochromites

The results of microhardness measurements on flux-grown crystals of (i) single (pure) rare earth aluminates RAlO3 (R = Eu, Gd, Dy, Er) and rare earth orthochromites RCrO3 (R = Y, Gd, Yb), (ii) rare earth aluminates doped with neodymium, erbium, ytterbium and holmium, and (iii) mixed rare earth aluminate crystals of the type (La1−x) Pr(x)AlO3 (x=0, 0.25, 0.75 and 1.00) are presented. The variations in the microhardness value with load are non-linear in all cases. Kicks law fails to explain the observed variations. Instead, they are best explained by the application of the idea of materials resistance pressure in the modified law proposed by Hays and Kendall. The results indicate that the doping does not increase the hardness value of crystals in all cases. The hardness instead depends on the composition of the parent material as well as the dopant entering into the crystal lattice. Mixed rare earth aluminate crystals are shown to be harder than those of single rare earth aluminates.

Microhardness measurements on single crystals of flux-grown rare earth perovskites (orthoferrites, orthochromites and aluminates)

The results of indentation-induced microhardness testing studies of flux-grown single crystals of rare earth orthoferrites, RFeO3 (R=Gd to Er and Yb), rare earth orthochromites RCrO3 (R=La, Eu and Dy), and rare earth aluminates RAlO3 (R=La, Sm, Gd, Eu and Ho) are presented. The variation in the value of microhardness with load is observed to be non-linear in the case of all these materials. It is found that the results are not in accordance with Kicks law. The results have been analysed and the applicability of the idea of materials resistance pressure in the modified law as proposed by Hays and Kendall [Metallography6 (1973) 275] is discussed.

Load and directional effects on microhardness and estimation of toughness and brittleness for flux-grown LaBO3 crystals

Results of microhardness measurements on (100) and (110) planes of flux-grown LaBO3 crystals, in the applied load range of 10–100g, are presented. The microhardness was found to decrease with increasing load in a non-linear manner. By applying Hays and Kendalls law, the materials resistance pressure and other constants of the equation could be calculated. Hardness anisotropy, showing periodic variation of Hv with the maxima and minima repeating at every 15° change in orientation of the indentor, is described and discussed. Hmax/Hmin are estimated as 1.14 and 1.06 for (100) and (110) planes, respectively. The fracture toughness values, Kc, determined from measurements of crack lengths, are estimated to be 1.6, 1.7 MN m−3/2 (for (100) planes) and 1.2, 1.5 MN m−3/2 (for (110) planes) at 90 and 100g loads, respectively. The brittleness index, Bi, is estimated as 4.6, 4.0 μm−1/2 (for (100) planes) 6.0, 4.6 μm−1/2 (for (110) planes) at 90 and 100g, loads respectively.

Growth of lanthanum tartrate crystals in silica gel

The spherulitic, dendritic and single-crystal growth of hydrated lanthanum tartrate by controlled diffusion in silica gels is reported. The influence of growth parameters, e.g. reactant concentrations, gel pH, gel ageing, on the size and nucleation density of crystals has been studied. Operative mechanisms of crystallization, results of growth kinetics and morphology of crystals are discussed. The adsorption property of the gel is found to play a vital role during the crystallization of lanthanum tartrate crystals, Parabolic kinetics, characteristic of a one-dimensional diffusion-controlled process, for single crystals is observed to be obeyed in case of variation of upper reactant concentration.

Characterization and thermal behaviour of gel grown mixed rare-earth (Didymium) tartrate crystals

Sherulites, crystal aggregates and platelets of Di2 (C4H4O6)3 · 5H2O mixed crystals were grown in gel using the single tube diffusion method. The material was characterized by using different techniques such as chemical analysis, EDAX, X-ray and electron diffraction, infrared and mass spectroscopy. The thermal behaviour of the material was studied using differential thermal analysis, DTG, thermogravimetric analysis and differential scanning calorimetry. The material is thermally unstable and starts decomposing at 50‡ C. Thermal analysis results indicate the application of the contracting cylindrical kinetic model for the solid state reactions involved in the decomposition process. The values of kinetic parameters, e.g. order of reaction, activation energy and frequency factor are worked out. The implications are discussed.

Characterization and thermal behaviour of gel-grown gadolinium tartrate crystals

Gadolinium tartrate crystals in the form of spheru!itoe were synthesized by using a controlled diffusion system in sillica gel. Characterization of the material was performed by utilizing the techniques of chemical analysis, X-ray and electron diffraction, infrared and mass spectroscopy, and by As thermal behaviour. The material turns out to be a dihvdrate and the chemical composition vvoethus established as Gd2(C4H4O6)3 · 2H2O. The data obtained from the thermal analysis show the tendency of the material to decompose, and this is further confirmed by mass spectroscopy The decomposition process is completed in four steps until gadolinium oxide is obtained at 840* C. The energetics of the reactions at each stage of decomposition have been examined and mechanisms for the decomposition reactions are proposed.

Dielectric and piezoelectric characteristics of samarium modified lead titanate ceramics

Abstract Dielectric and piezoelectric properties of samarium (Sm) modified lead titanate ceramics (Pb1−xSmxTi1−2yWyFeyO3 with x=0.02, 0.05, 0.10, 0.20 and y=0.02), prepared by high temperature solid state reaction method, have been studied. Variation of dielectric constant and conductivity at room temperature shows frequency dependence and is expressed by Kramers Kronig relation (e′(ω)−eα=A1ωs−1) and σac(ω)=A2ωs, respectively. All compositions show phase transition and the temperature of phase transition is found to decrease with increase in Sm substitution. Nature of transition studied by the relation (1/e′)−(1/emax)=C(T−Tc)γ is found to be diffused. A nonlinear increase in the value of d33 is observed with the increase in Sm content.

Optical microscopic studies on grown and etched surfaces of flux grown LaAlO3 crystals

Surface structures on as-obtained flux-grown crystals of LaAlO3 have been investigated. Strictly oriented square, circular and rhombus shaped pointed, as well as flat-bottomed etch pits are observed. Etch pits along lineage boundaries, intersecting low-angle tilt boundaries and helical dislocations are illustrated and described. Different orientation of etch pits reveal twinning in LaAlO3 crystals. Microdisc patterns and flux inclusions are also observed. The etch patterns on the as-obtained LaAlO3 crystals are explained to be as a result of the flux cleaning operation of crystals in HNO3. Experiments on etching established HNO3 to be a dislocation etchant for LaAlO3 crystals. Dislocation etching kinetics of the HNO3-LaAlO3 surface system are investigated for the freshly identified sites as well as for sites having a previous history of etching. Data obtained on the effects of etching time, etchant concentration and temperature on the dislocation etch rates, are analysed. The results obtained are presented and discussed.

Dielectric studies of lanthanum heptamolybdate crystals grown from gels

Abstract Results of dielectric measurements conducted on lanthanum heptamolybdate grown by a gel technique employing the system La(NO 3 ) 3 MoO 3 NH 4 OHHNO 3 Na 2 SiO 3 are presented. The variations of the dielectric constant e′, dielectric loss tan δ and conductivity σ with the temperature and frequency are critically analyzed. The dielectric constant increases with temperature at lower frequencies (10 3 Hz), reaching a maximum value of 320 at 125 °C, after which it abruptly decreases. The dielectric loss tan δ and conductivity σ show both a temperature as well as a frequency dependence. The sharp increase in e′ with temperature is attributed to total polarization resulting from a space charge effect. The functional relationship between the dielectric constant, conductivity and temperature is determined by fitting the observed data with the respective polymeric expressions of the type ϵ ′ = a 0 + a 1 T 2 - lnσ = a + b ( T - T 0 ) 2 The non-linearities and anomalous behaviour of the dielectric constant near the transition temperature, supported by hysteresis studies, indicate the possibility of these materials ferroelectric.

Crystal growth of yttrium and samarium tartrates from silica gels

Abstract The results on the growth of hydrated crystals of rare earth tartrates R 2 (C 4 H 4 O 6 ) 3 , where RY, Sm from silica gels, in two different temperature ranges (25–30°C and 35–40°C), employing single-and double- diffusion techniques, are presented. The effects of various parameters such as gel pH, gel aging and concentration of reactants have been investigated for each system. We report a new kind of periodic zone, generally known as Liesegang rings, for yttrium tartrate crystals when grown in the temperature range 35–40°C. The periodicity of nucleation zones, however, is broken for samarium tartrate crystals. An attempt has been made to improve the size of spherulites by using the method of seeded growth.