K.V. Ramanujachary

Hydrothermal synthesis and low temperature conduction properties of substituted ceria ceramics

Abstract Rare earth or alkaline earth oxide-substituted ceria (CeO 2 ) powders were successfully prepared via hydrothermal synthetic route. Using these powders after freeze-drying, highly dense ceramics of Ce 1− x M x O 2 with M = Ca 2+ , Sr 2+ , Sm 3+ , Gd 3+ and Y 3+ and x = 0.2 were prepared by sintering at much lower temperatures than 1700 °C, common for CeO 2 preparation by conventional ceramic techniques. For example, 20 mol% samaria-substituted ceria with 98% of the theoretical density was sintered at 1450 °C. The conductivities determined by an ac impedance method were approximately 100 times higher than that of yttria-stabilized zirconia (YSZ). The conductivity was optimal with Sm 3+ substitution ( σ (210 °C ) ~10 −6 S/cm). The analysis of the impedance spectra indicate that the observed impedance at the relatively low temperatures of measurement (300–600 °C) is predominantly dependent on grain boundary effects.

Investigations on the structural, electrical, and magnetic properties of Nd2−xSrxNiO4+δ

Abstract Solid-solutions of the type Nd 2− x Sr x NiO 4+ δ (0 ≤ x ≤ 1) were prepared by codecomposition of the metal nitrates and were characterized by means of powder X-ray diffraction, TGA, electrical resistivity, and magnetic susceptibility measurements. Nd 2 NiO 4+δ possesses orthorhombic symmetry at ambient temperatures and exhibits a phase transition to pseudo-tetragonal symmetry upon doping with Sr 2+ ( x ≈ 0.2). The tetragonality ratio c a t measured as a function of x shows an anomalous increase at x ≈ 0.2 and a maximum near x = 0.6. Considerable deviation from oxygen stoichiometries was observed in the orthorhombically distorted members, while for the samples with tetragonal geometry the oxygen contents were near the ideal value. All the samples, except NdSrNiO 4 , were semiconducting from room temperature down to 10 K. NdSrNiO 4 showed a metal-semiconductor transition at ∼ 190 K. The magnetic susceptibility shows Curie-Weiss behavior with an onset of long range antiferromagnetic ordering ( T N = 17 K) in some of the intermediate members of the solid-solution series. The effect of Sr 2+ substitution on the structural and electronic properties of Nd 2− x Sr x NiO 4+δ are discussed in terms of the mixed valence character of Ni 2+ Ni 3+ , distortions in the local coordination of Niue5f8O octahedra, and correlation effects of d x 2 − y 2 bands in the Niue5f8O basal plane.

Crystal growth of alkali metal molybdenum bronzes by a temperature gradient flux technique

Abstract Single crystals of alkali metal molybdenum bronzes of the type: A 0.9 Mo 6 O 17 with A = Na, K and A 0.3 MoO 3 where A = K, Rb, Cs have been grown by the temperature gradient flux technique. Impure polycrystalline specimens of K 0.33MoO 3 and MoO 3 like phases which may be sodium and potassium intercalates respectively, have also been observed. Optimal growth conditions determined for each of the phases are reported. Na 0.9Mo 6 O 17 is monoclinic and a quasi two-dimensional metallic conductor at room temperature, similar to Li 0.9 Mo 6 O 17 and K 0.9 Mo 6 O 17 . A mechanism for the formation of bronze phases is proposed based on X-ray powder diffraction and DTA results.

Synthesis, structure and properties of the oxynitrides SrWO2N and SrMoO2.5N0.5

Two oxynitrides, SrWO2N and SrMoO2.5N0.5, were synthesized by the ammonolysis of SrMO4 (M = W, Mo) at 900 and 800 °C, respectively. Both oxynitrides adopt the cubic perovskite structure (space group Pm3m) with a = 3.97951(9) A for SrWO2N and a = 3.9773(1) A for SrMoO2.5N0.5. The Mue5f8(O,N) and Srue5f8(O,N) bond lengths, determined from the Rietveld refinement of the powder X-ray diffraction data, are in good agreement with those reported for other oxynitrides with the perovskite structure. Nearly temperature independent electrical resistivities were observed for both compounds. The magnetic susceptibility measurements indicated Pauli-paramagnetic behavior in both cases. A sharp anomaly observed in the magnetization data of SrMoO2.5N0.5 at ~50 K has been attributed to a spin-glass-like transition.

The relationship of oxygen content, structural phase transition and Tc in Tl2Ba2CuO6 ± δ

Abstract Polycrystalline samples of Tl 2 Ba 2 CuO 6 ± δ were prepared by solid state reaction technique, with different heat treatments. Samples that were quenched from 860°C in air were pseudo-tetragonal and exhibited superconducting onsets at ∼ 84 K. Samples slow cooled from 860°C to room temperature in air showed a clear orthorhombic distortion and exhibited onsets at ∼ 20 K. No evidence of superconductivity was observed for samples prepared in sealed tubes at 860°C which were orthorhombic. Post annealing the 20 K and non-superconducting samples in argon atmosphere at 400–450°C for 1–2 h resulted in superconductivity at ∼ 50 K. A reversible structural phase transition from pseudo-tetragonal to orthorhombic symmetry accompanied by oxygen uptake/loss was observed in controlled oxygen atmosphere at ∼ 300°C as evidenced by DTA, TGA and high temperature X-ray diffraction studies. Variations in the oxygen content, although very small, have dramatic effects on the superconducting properties of Tl 2 Ba 2 CuO 6 ± δ . The dependence of superconductivity on structure, oxygen content/ordering is discussed.

GROWTH AND PROPERTIES OF LA2-XSRXCUO4 FILMS

Single‐crystalline films of La2−xSrxCuO4 have been grown epitaxially on SrTiO3, NdGaO3, and LaSrAlO4 substrates by laser ablation. We show that record values of the superconducting transition temperature may be achieved by high‐pressure annealing in oxygen. The films exhibit values of Tco above 38 K, as well as a linear variation of the resistivity with T and excellent crystal quality.

New metal-oxide-type pH sensors

Abstract Two new types of metal-oxide pH sensors with a direct solid-state contact have been developed and tested. Their performance was demonstrated near ambient temperatures with a samarium substituted ceria ceramic membrane and single crystals of several molybdenum oxide bronzes. The pH sensors with Na-molybdenum-oxide bronzes show near-ideal Nernstian behaviour in the pH range 3–9. The response was not affected by the direction of the pH change. In contrast, the pH sensor with the Na-tungsten bronze sensitive electrode (SE) shows a deviation from linearity, as well as non-Nernstian behaviour with a slope − 76.3 mV/pH at 25 °C. The response time of most molybdenum bronze pH sensors is less than 5 s for 90% response. The pH sensors with stabilized ceria membrane show close to ambient temperature non-Nernstian behaviour in the pH range 3–10 with a slope − 23.8 mV/pH at 25 °C. With increasing temperatures the behaviour of ceria membrane pH sensors approaches more and more a Nernstian response with slope − 39.9 mV/pH at 60 °C. Both pH sensors also can be used to follow acid-base titrations.

Magnetic, electrical, and microstructural properties of YBa 2 Cu 3 O 7 : A comparison of sol-gel, co-precipitated, and solid state processing routes

Samples of YBa{sub 2}Cu{sub 3}O{sub 7} were prepared by sol-gel, co-precipitation, and solid state processes. Sol-gel samples were prepared from a solution of yttrium, barium, and copper nitrates dissolved in ethylene glycol, co-precipitated samples were made by the amorphous citrate method, and solid state samples were prepared by conventional high temperature reaction of the appropriate metal oxides and carbonates. The sol-gel process was shown to yield superconducting samples of superior Meissner effect, critical current, and critical field. The co-precipitated samples contain impurities that affect the critical properties. The sol-gel and co-precipitated processes yield materials with well-formed, plate-like particles with a fairly uniform size of about 10 {mu}m. The grains in the solid state sample are smaller but have a much wider distribution of sizes than the samples prepared by solution methods.

Molybdenum oxide bronzes as pH sensors

Abstract New metal-oxide-type pH sensors have been developed and tested with a direct solid state contact method. Performance was demonstrated at ambient temperature with single crystals of molybdenum oxide bronzes. The pH sensors responded reproducibly to the acidity changes of the analyte in the range of pH 4–9. The response time is less than 5 s for 90% response. pH sensors with single crystals of molybdenum oxide bronzes can be used to follow pH titrations.

Quasi-two-dimensional electronic properties of the sodium molybdenum bronze, Na0.9Mo6O17

Four probe electrical resistivity measurements between 1.5 and 300 K were made on single crystals of the violet-red bronze Na /SUB 0.9/ Mo/sub 6/O/sub 17/ grown by a temperature gradient flux technique. The temperature variation of the resistivity shows metallic conductivity and highly anisotropic behavior similar t K /SUB 0.9/ Mo/sub 6/O/sub 17/ and Li /SUB 0.9/ Mo/sub 6/O/sub 17/. The room-temperature resistivity, measured in the direction parallel to the plate axis, is 3.0 X 10/sup -3/ ..cap omega.. cm and perpendicular to that axis it is 0.21 ..cap omega.. cm. A transition observed at about 88 K is possibly related to the onset of a charge density wave. The temperature variation of the susceptibility show Pauli paramagnetic behavior at high temperature, and highly anomalous behavior in the vicinity of the transition at low temperatures.