Thermoelectric properties of Ca<Subscript>3</Subscript>Co<Subscript>4</Subscript>O<Subscript>9+δ</Subscript> with Lu substitution

A series of Ca_3−xLu_xCo₄O_9+δ (x = 0, 0.1, 0.2, and 0.4) has been prepared by sol–gel method. The effects of Lutetium substitution on thermoelectric properties of Ca₃Co₄O_9+δ have been systematically investigated from 4 to 335 K. With the partial substitution of Lu³⁺ for Ca²⁺, the resistivity and thermopower for Lu-doped samples increase, while their thermal conductivity decreases. The dimensionless figure of merit for Ca_2.8Lu_0.2Co₄O_9+δ material (ZT = 0.032) is about five times better than that for Ca₃Co₄O_9+δ (ZT = 0.007) at 335 K. Strikingly, for Lu-doped samples, the thermopower exhibits a steeper upturn at low temperatures.     

Effect of Gd-doping on thermoelectric properties of Ca<Subscript>3</Subscript>Co<Subscript>4</Subscript>O<Subscript>9+δ</Subscript> ceramics

A series of Ca_3−x Gd _x Co₄O_9+δ precursor powders were synthesized by the polyacrylamide gel method, and their ceramics were obtained by the Spark Plasma Sintering (SPS). There were lots of defects in the sheet-like grains from SEM and TEM observations. The electrical and the thermal transport properties were obviously affected by the material microstructure. The small polaron hopping conduction mechanism was determined above 600 K, and the hopping activation energy increased with the increase of doping contents. It was found that the Seebeck coefficient and the resistivity of doped samples were markly enhanced due to the impurity compensation effect, and their thermal conductivities were decreased due to the defects scattering. The maximum figure of merit of ZT = 0.24 at 973 K was obtained for Ca_2.7Gd_0.3Co₄O_9+δ.     

Thermoelectric properties of transition metals-doped Ca<Subscript>3</Subscript>Co<Subscript>3.8</Subscript>M<Subscript>0.2</Subscript>O<Subscript>9+δ</Subscript> (M = Co,Cr, Fe,Ni, Cu and Zn)

The thermoelectric properties of the Ca₃Co₄O_9+δ and the transition metals-doped Ca₃Co_3.8M_0.2O_9+δ (where M = Cr, Fe, Ni, Cu and Zn) ceramics were reported. Ca₃Co₄O_9+δ single phase was checked by using X-ray diffraction analysis performed for the Ca₃Co_3.8M_0.2O_9+δ samples. The scanning electron micrographs showed some degrees of grains alignment in the compacted direction. The resistivity of the samples measured from 100 up to 700 °C varies in magnitude for different transition metals substitution. The variation of resistivity was explained by a change of carrier concentration induced by the doped ions. The thermopower increased with increasing temperature but showed no obvious change for any transition metals doping. The thermal conductivities changed for the doped samples but were relatively independent of temperature. The ZT was calculated to be the highest for the Fe substitution for the whole measurement temperature with the maximum value of 0.12 at 700 °C.     

Thermodynamic and structural properties of PrBaCo<Subscript>2</Subscript>O<Subscript>5 + δ</Subscript>

The oxygen content of the double cobaltite PrBaCo₂O_{5 + δ} was determined by coulometric titration in broad temperature and oxygen pressure ranges, and the partial thermodynamic functions of oxygen were evaluated. The results on the nonstoichiometry and structural properties of PrBaCo₂O_{5 + δ} demonstrate that, at 5 + δ < 5.3, this cobaltite has a high-temperature phase with lattice parameters a = 11.774 Å, b = 11.932 Å, and c = 7.609 Å. The composition dependences of \(\Delta \bar H(O)\) and \(\Delta \bar S(O)\) indicate that the 3a_p × 3a_p × 2a_p orthorhombic structure persists in the range 5.15 ≤ 5 + δ ≤ 5.30. Some of the Co³⁺ ions are shown to disproportionate according to the scheme 2Co³⁺=Co²⁺+Co⁴⁺.     

Controllable synthesis and luminescence of YPO<Subscript>4</Subscript>:Ln<Superscript>3+</Superscript> (Ln = Eu and Sm) nanotubes

YPO₄:Ln³⁺ (Ln = Eu and Sm) nanotubes were synthesized by a precipitation process in the presence of SDS. The XRD results showed that all samples have a xenotime type tetragonal structure, indicating that doped rare earth ions have no influence on the phase. The SEM and TEM images showed that all samples are nanotubes. On basis of the morphology of samples and the properties of SDS, the possible formation mechanism was speculated. YPO₄:Eu³⁺ and YPO₄:Sm³⁺ nanotubes showed characteristic emission bands of Eu³⁺ and Sm³⁺ ions, respectively. For YPO₄:Eu³⁺/Sm³⁺ nanotubes, the codoping Sm³⁺ ions can enhance the emission intensity of Eu³⁺ ions.     

Synthesis and properties of solid electrolyte Ce<Subscript>0.9</Subscript>Gd<Subscript>0.1</Subscript>O<Subscript>2–δ</Subscript> with Co,Cu, Mn,Zn doping

The influence of small additions (1, 3, 5 mol %) of transition metal (Co, Cu, Mn, Zn) oxides on the properties of solid electrolyte Ce_0.9Gd_0.1O_2–δ (GDC) have been investigated. It has been shown that the addition of dopants results in intensification of GDC sintering and reduction of the shrinkage end temperature by 300–400°C, which decreases in the sequence Zn–Mn–Co–Cu. The ultimate dopant concentration above which the further activation of GDC sintering does not occur is about 3 mol % for Co, Cu, and Mn and about 1 mol % for Zn. It has been shown that Co and Cu increase the total conductivity of GDC, while Mn and Zn decrease it.     

Synthesis and study of structural,dielectric, magnetic and magnetoelectric properties of K<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>NbO<Subscript>3</Subscript>–CoMn<Subscript>0.2</Subscript>Fe<Subscript>1.8</Subscript>O<Subscript>4</Subscript> composites

Magnetoelectric (ME) composites consisting of K_0.5Na_0.5NbO₃ (KNN) as ferroelectric phase and CoMn_0.2Fe_1.8O₄ (CMFO) as ferrite phase with general formula (x) CoMn_0.2Fe_1.8O₄–(1???x) K_0.5Na_0.5NbO₃ (x?=?10, 20, 30, 40 and 50 wt%) were synthesized using solid state reaction method. X-ray diffraction analysis asserts the existence of component phases including spinel phase of CMFO and orthorhombic phase of KNN. Field emission scanning electron microscopy has been used for studying the morphology and calculation of average grain size. The temperature dependent dielectric properties including dielectric constant (\(\varepsilon ^{\prime}\)) and dielectric loss (tan δ) at different frequencies has been studied and both are found to increase with incorporation of CMFO. Magnetic hysteresis loops have been measured at temperatures of 300 and 5 K. Variation of magnetization versus temperature has been studied in field cooled and zero field cooled modes. Polarization versus electric field (P–E) hysteresis loops are obtained at room temperature indicating presence of ferroelectric ordering in the composites at room temperature. The remnant polarization (2P_r) and coercive field (2E_c) are found to decrease linearly with incorporation of CMFO. ME voltage coefficient (α_ME) has been measured. The maximum value of α_ME is found to be 5.941 mV/cm-Oe for 10% CMFO–90% KNN bulk composite.     

Ln<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript>:Mn<Superscript>4+</Superscript> (Ln = Y and Lu): non-rare-earth doped red phosphor for improving light properties of white light emitting diodes

We fabricated a series of Y₃Al₅O₁₂:Mn⁴⁺ and Y_1?yLu_yAl₅O₁₂:Mn⁴⁺ phosphors by a solid state reaction. The phase and the optical properties of the synthesized phosphors were investigated. Under the excitation at 465 nm, Y₃Al₅O₁₂:Mn⁴⁺ phosphors show emission bands locating at deep red regions, which is induced by the spin- and parity-forbidden ²E_g → ⁴A_1g transitions of Mn⁴⁺. The substitution of Y³⁺ by Lu³⁺ decreases the lattice parameter and thus strengthens the crystal field strength, which gives rise to the blue shift of emission band for Y_1?yLu_yAl₅O₁₂:Mn⁴⁺ phosphors. Due to the compensation of red light by Y₃Al₅O₁₂:Mn⁴⁺ or Y_1?yLu_yAl₅O₁₂:Mn⁴⁺ phosphor, the values of correlated-color-temperature for fabricated LEDs are decreased, which leads to the suitable application for them in indoor illumination.     

Growth and structure of Mg(Fe<Subscript>0.8</Subscript>Ga<Subscript>0.2</Subscript>)<Subscript>2</Subscript>O<Subscript>4 − δ</Subscript> films

Films 150–200 nm in thickness, with the nominal composition Mg(Fe_0.8Ga_0.2)₂O_{4 − δ} have been grown on (100) single-crystal silicon substrates by ion-beam sputtering in vacuum. The effect of growth and annealing conditions on the crystal structure and morphology of the films has been studied, and the thermal conditions for the growth of spinel-structure films have been optimized.     

Phase relations in the K<Subscript>2</Subscript>MoO<Subscript>4</Subscript>–Ln<Subscript>2</Subscript>(MoO<Subscript>4</Subscript>)<Subscript>3</Subscript>–Zr(MoO<Subscript>4</Subscript>)<Subscript>2</Subscript> (Ln = La–Lu,Y) systems

We have studied phase relations in the K₂MoO₄–Ln₂(MoO₄)₃–Zr(MoO₄)₂ (Ln = La–Lu, Y) systems by the method of “intersecting cuts,” identified pseudobinary joins in their composition triangles, and constructed their phase compatibility diagrams. The systems have been shown to contain new ternary molybdates with the general formula K₅LnZr(MoO₄)₆ (Ln = Dy–Lu and Y). The thermal characteristics of the synthesized compounds have been studied by differential scanning calorimetry in the temperature range 25–700°C. The new ternary molybdates crystallize in a trigonal structure (sp. gr. R\(\bar 3\)c, Z = 6).     

Effect of the synthesis technique on the physicochemical properties of Ce<Subscript>0.8</Subscript>(Sm<Subscript>0.75</Subscript>Sr<Subscript>0.2</Subscript>Ba<Subscript>0.05</Subscript>)<Subscript>0.2</Subscript>O<Subscript>2 − δ</Subscript>

A multicomponent solid electrolyte of composition Ce_0.8(Sm_0.75Sr_0.2Ba_0.05)_0.2O_{2 − δ} has been synthesized by three different techniques: solid-state reaction, laser evaporation, and the glycine nitrate process. Its microstructure, sintering kinetics, and electrical properties have been studied in relation to the synthesis technique. Ceramics produced using laser evaporation consisted of submicron (0.2 μm) grains and offered the highest electrical conductivity: 27 × 10⁻³ S/cm at 873 K.     

Microstructure and properties of the Ba<Subscript>0.75−x</Subscript>Ca<Subscript>x</Subscript>La<Subscript>0.25</Subscript>Fe<Subscript>11.6</Subscript>Co<Subscript>0.25</Subscript>Ti<Subscript>0.15</Subscript>O<Subscript>19</Subscript> hexaferrites

In the case of Ti⁴⁺ remain unchanged, the Ca²⁺ substituted Ba_0.75?xCa_xLa_0.25Fe_11.6Co_0.25Ti_0.15O₁₉ (0?≤?x?≤?0.05) were prepared by conventional solid-state reaction method at temperature of 1280 °C. A ball-to-power weight ratio of 10:1. Their crystal structure and magnetic properties were mainly investigated. The results show that the single magnetoplumbite phase structure transformed into the multiphase structure. Meanwhile, the small amount of α-Fe₂O₃ phase existed in M-type phase. The micrographs were observed by a field emission scanning electron microscopy (SEM). Vibrating sample magnetometer (VSM) was used to analyze the magnetic properties. The saturation magnetization (M _s ) first increases then decreases when x from 0 to 0.03. But, when x from 0.03 to 0.05, the saturation magnetization (M _s ) first increases then decreases too. The maximum value is at x?=?0.04 (M _s ?=?70.73 emu/g). The value of coercivity (H _c ) first increases then decreases when x from 0 to 0.04. But, the value increased when x from 0.04 to 0.05. The maximum value is at x?=?0.02 (H _c ?=?1691 Oe).     

Composition ranges of Ln<Subscript>2 − <Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>3 ± δ</Subscript> (Ln = Y,Ho, Er) solid solutions between 900 and 1400°C in air

The metal stoichiometry ranges of the Ln_{2 − x} Mn _x O_{3 ± δ} (Ln = Y, Ho, Er) manganites have been determined using X-ray diffraction analysis of ceramic samples prepared by reacting oxide mixtures in air at temperatures from 900 to 1400°C. The results are represented as partial phase diagrams of the Ln-Mn-O systems in air. Comparison of the phase diagrams demonstrates that the phase boundaries of the manganites are determined not only by the effective cation radius of the rare-earth metal. The solubilities of the binary oxides Y₂O₃, Ho₂O₃, and Mn₃O₄ in yttrium and holmium manganites increase with temperature, with significant quantitative distinctions. The Er₂O₃ and Mn₃O₄ solubilities in erbium manganite remain unchanged over the entire temperature range studied. The LnMn₂O₅ solubility in LnMnO_{3 ± δ} is an intricate function of temperature. We analyze the possible causes of the Ln₂O₃, Mn₃O₄, and LnMn₂O₅ solubility in the LnMnO_{3 ± δ} (Ln = Y, Ho, Er) manganites.     

Sol–gel derived Co<Subscript>3</Subscript>O<Subscript>4</Subscript> thin films: effect of annealing on structural,morphological and optoelectronic properties

Nanocrystalline Co₃O₄ thin films were prepared on glass substrates by using sol–gel spin coating technique. The effect of annealing temperature (400–700 °C) on structural, morphological, electrical and optical properties of Co₃O₄ thin films were studied by X-ray diffraction (XRD), Scanning Electron Microscopy, Electrical conductivity and UV–visible Spectroscopy. XRD measurements show that all the films are nanocrystallized in the cubic spinel structure and present a random orientation. The crystallite size increases with increasing annealing temperature (53–69 nm). These modifications influence the optical properties. The morphology of the sol–gel derived Co₃O₄ shows nanocrystalline grains with some overgrown clusters and it varies with annealing temperature. The optical band gap has been determined from the absorption coefficient. We found that the optical band gap energy decreases from 2.58 to 2.07 eV with increasing annealing temperature between 400 and 700 °C. These mean that the optical quality of Co₃O₄ films is improved by annealing. The dc electrical conductivity of Co₃O₄ thin films were increased from 10⁻⁴ to 10⁻² (Ω cm)⁻¹ with increase in annealing temperature. The electron carrier concentration (n) and mobility (μ) of Co₃O₄ films annealed at 400–700 °C were estimated to be of the order of 2.4–4.5 × 10¹⁹ cm⁻³ and 5.2–7.0 × 10⁻⁵ cm² V⁻¹ s⁻¹ respectively. It is observed that Co₃O₄ thin film annealing at 700 °C after deposition provide a smooth and flat texture suited for optoelectronic applications.     

Synthesis and properties of LnBaFeCoO5 + δ (Ln = Nd, Sm, Gd)

LnBaFeCoO_{5 + δ} (Ln = Nd, Sm, Gd) layered oxides have been synthesized and their crystal structure, thermal stability, thermal expansion, electrical conductivity, thermoelectric power, and magnetic susceptibility have been studied. The oxides have a tetragonal structure (sp. gr. P4/mmm) with unit-cell parameters a = 0.3909(2) nm and c = 0.7695(6) nm for Ln = Nd (δ = 0.65), a = 0.3908(3) nm and c = 0.7662(6) nm for Ln = Sm (δ = 0.37), and a = 0.3908(2) nm and c = 0.7613(6) nm for Ln = Gd (δ = 0.37). The LnBaFeCoO_{5 + δ} compounds are antiferromagnetic p-type semiconductors. With decreasing Ln³⁺ ionic radius, their electrical conductivity and linear thermal expansion coefficient decrease and their thermoelectric power and antiferromagnetic ordering temperature increase. Near 518–653 K, the linear thermal expansion coefficient of the LnBaFeCoO_{5 + δ} oxides increases from (12.9–16.6) × 10^?6 to (19.3–26.5) × 10^?6 K^?1, which is due to the release of weakly bound oxygen from the oxides. We have determined parameters of charge transport in the [Fe(Co)O₂] layers in the crystal structure of the LnBaFeCoO_{5 + δ} phases.     

Chemical interaction of InAs,InSb, GaAs,and GaSb with (NH<Subscript>4</Subscript>)<Subscript>2</Subscript>Cr<Subscript>2</Subscript>O<Subscript>7</Subscript>–HBr–C<Subscript>4</Subscript>H<Subscript>6</Subscript>O<Subscript>6</Subscript> etching solutions

This paper presents results on the kinetics and mechanism of the physicochemical interaction of InAs, InSb, GaAs, and GaSb semiconductor surfaces with (NH₄)₂Cr₂O₇–HBr–C₄H₆O₆ etching solutions under reproducible hydrodynamic conditions in the case of laminar etchant flow over a substrate. We have identified regions of polishing and nonpolishing solutions and evaluated the apparent activation energy of the process. The surface morphology of the crystals has been examined by microstructural analysis after chemical etching. The results demonstrate that the presence of C₄H₆O₆ in etchants helps to reduce the overall reaction rate and extend the region of polishing solutions.     

Effect of refractory nitride nanoadditives on the properties of (Bi,Pb)<Subscript>2</Subscript>Sr<Subscript>2</Subscript>Ca<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>10+δ</Subscript>

We have investigated the interaction between (Bi,Pb)₂Sr₂Ca₂Cu₃O_10+δ (Bi-2223) and small additions (0.05–0.3 wt %) of nitride powders (TaN, AlN, HfN, NbN, Si₃N₄, TiN, and ZrN) with a particle size from 0.02 to above 0.5 μm and the effect of these nitrides on the microstructure, phase composition, distribution, and morphology of the resulting second-phase inclusions. The concentration and particle size of the nitrides and sintering conditions are shown to influence the superconducting transition temperature T _c, critical current density j _c, irreversible remanent magnetization, bulk density, and mechanical properties of the Bi-2223/nitride composites.     

Influence of Co<Superscript>++</Superscript> ion concentration on magnetic and microwave properties of Ba<Subscript>(4−X)</Subscript>Co<Subscript>(2+X)</Subscript>Fe<Subscript>36</Subscript>O<Subscript>60</Subscript> (Ba-M-Y) hexaferrite

The present paper reports the influence of cobalt content on the structural, electrical, magnetic and microwave properties of barium hexaferrite synthesized via chemical co-precipitation method. The samples were characterized for their structural, electrical, magnetic and microwave characterizations using XRD, SEM, TEM, VSM etc. The transmission electron microscopy results showed that stacking of nanoparticles of size?~?50 nm. In addition, the highest saturation magnetization of 29.82 emu/g was observed for composition x?=?0.2. The microwave permittivity and permeability decreases with frequency and it varies with the cobalt concentration. Cobalt concentration strongly affects the microwave and magnetic properties of hexaferrite.     

Synthesis of thorn-like Ca<Subscript>2</Subscript>B<Subscript>2</Subscript>O<Subscript>5</Subscript>·H<Subscript>2</Subscript>O by hydrothermal method

Thorn-like polycrystalline Ca₂B₂O₅·H₂O microspheres with nano-sized slices were synthesized using boric acid and calcium hydroxide as reactants by a facile catalyst-free hydrothermal method at low temperature. The products were characterized by means of X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). The XRD pattern reveals that the Ca₂B₂O₅·H₂O is a monoclinic phase polycrystalline with cell parameters a = 0·6702, b = 0·5419 and c = 0·3558 nm. SEM also reveals that the monoclinic phase polycrystalline are thorn-like microspheres composed of many flakes with an average thickness of <100 nm. Possible reaction and growth mechanism were also discussed.