The microstructure of Ce-TZP ceramics processed by a coating technique, compared to co-precipitated powder-based Ce-TZP, was analysed using a combination of XRD, SEM, TEM, etc. The results showed that CeO2-coated powder-based zirconia ceramics exhibited higher fracture toughness than co-precipitated powder-based zirconia ceramics in spite of a little decreasing hardness. Besides, a small amount of alumina addition can significantly improve the fracture toughness of CeO2-coated powder-based zirconia ceramics. In contrast with homogeneous grain size and stabilizer content distribution of co-precipitated powder-based Ce-TZP ceramics, SEM and EPMA analysis revealed that coating process resulted in wider grain size and stabilizer content distribution in this Ce-TZP ceramics. The superior fracture toughness of CeO2-coated Ce-TZP ceramics was mainly attributed to a fraction of highly transformable tetragonal ZrO2 grains with low ceria content. The residual stress existing between anisotropic monoclinic and tetragonal grains, and the thin glass films containing alumina and silica at grain boundary observed by TEM, were believed to benefit the preservation more tetragonal grains during sintering, contributing to the stress-induced martensitic phase transformation.
By replacing of the carbon atoms with boron and nitrogen in the crystalline phase of fcc diamond, new candidates for ultra-hard materials have been proposed in the BC2N stoichiometry. Their characterisation is here performed using methods based on density functional theory in its local density approximation. Full geometry relaxation of the substituted diamond gives the possibility to discover metastable systems which are isoelectronic with diamond and cubic boron nitride (c-BN). Such a kind of alloys are expected to be thermally and chemically (i.e. versus oxidation) more stable than diamond and harder than c-BN. That is, ternary B–C–N compounds could likely supersede the expensive diamond in various applications. In the present work we also employ first-principles methods to predict the mechanical properties of the achieved three-dimensional BC2N phases. Calculations of the cohesive properties, bulk and elastic moduli are presented. Considering the correlation between the hardness and the shear modulus and the values of the bulk moduli, we predict that the phases I and II of BC2N could be harder than c-BN. Electronic properties were studied in detail by means of density of states and band structure analysis. The calculation of the C, N and B K ELNES spectra is also shown for the presented phases.
The synthesis of monometallic hydrotalcite-like compounds (MHLC) type Co(II)–Co(III) materials was achieved by precipitation of a solution of cobalt nitrate(II) followed by microwave irradiation during the hydrothermal treatment step. The effects of irradiation time, precipitant agent and atmosphere were studied in order to establish the preparation conditions and the stability of these materials. As expected, these materials required careful preparation conditions, mainly a very slow addition of the precursors. NH4OH had to be used as precipitant agent in an air atmosphere. Short microwave irradiation times during the hydrothermal treatment provided a better crystallization of MHLC. The low stability of these compounds (∼200°C) was explained by the presence of several Co-complexes between the layers of the laminar structure. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), differential thermal analysis and thermogravimetric analysis (DTA/TGA), and thermoprogrammed oxidation (TPO) were used as characterization techniques.
Investigations on the series of manganese-substituted lithium ferrites reveal the destructive effect of manganese ions on the ordering of Li+ cations in the spinel-type crystal lattice. A spectacular order–disorder transition with increasing substitution of Mn3+ ions has been observed with infrared spectroscopy, in the region of lattice vibrations. The preference of manganese ions for the octahedral coordination, appears to be associated with the migration of Li+ from octahedral to tetrahedral positions. Results for the LiFe5−yMnyO8 solid solution have been confronted with an order–disorder transition in the pure lithium ferrite.
Cd2[MII(CN)6]·8H2O, where M=Fe, Ru and Os, form a family of isomorphous compounds which crystallize in the monoclinic P21/n space group. MII cations present their usual octahedral coordination, while the cadmium atom coordinates to three nitrogen atoms from the cyanide groups and three water molecules forming a distorted octahedron. The rest of the water molecules are located in small channels in the structure and are hydrogen bonded to the coordinated ones. Adjacent cadmium octahedra share one edge to form the moiety: Cd2N6(H2O)4, in such a way that two metal sequences are observed: MII–Cd–MII and MII–Cd–Cd–MII, forming a tridimensional network. Coordinated and zeolitic water molecules can be removed preserving the M–CN–Cd framework.
Monetite and hydroxyapatite have been synthesized by hydrothermal treatment of monocalcium phosphate monohydrate (MCPM) suspension at 160°C and 200°C. The monetite formed whiskers typically having length, diameter and aspect ratio in the range of 26–40 μm, 1–2 μm and 20–26, respectively and the hydroxyapatite shaped needle like crystals typically being in the range of 0.2–0.5 μm in length and 0.02–0.04 μm in diameter. The morphology of the crystals seems to be controlled mainly by the solubility of the reactant species. The phase of the hydrothermal product is determined by the combination of the pH and the Ca/P ratio of the aqueous solution. The whiskers obtained can be used as a bioactive reinforcement for composites.
Calcium-phosphate ceramic and glasses in combination with titanium and titanium alloys could be successfully used in clinical practice. In the present work the interaction between titanium and phosphate glasses from the system P2O5–CaO–ZnO is investigated. The precursors used are NH4H2PO4, CaCO3, ZnO and Ti. The initial glasses in the system P2O5–CaO–ZnO are synthesized by melting in corundum crucibles. The thermal treatment of samples is performed in the temperature range from 800 to 1250°C for 20 min prepared by several variants of contact between initial phases: (i) homogenized mixture of glassy powder and titanium powder; (ii) a monolithic glass in contact with titanium plate; (iii) packets from bulk titanium substrate and powdered phosphate glass mixed with Ti or Ca3(PO4)2. For the characterization of the specimens the following methods are used: X-ray diffraction (XRD), infrared (IR) spectroscopy and transmission electron microscopy (TEM). It is established that the obtained final products present glass-crystalline polyphase materials containing different crystal phosphates, titanium phosphide, TiO2, TiO and titanium. As a result of the solid state reaction between the metal and phosphate glass a transition region is found. The applied technique of preparation is an initial step towards the development of gradient materials with controlled properties.
Tantalum pentoxide (Ta2O5) is promising for coating and piezoelectric applications and has been considered as the dielectric gate material for the next generation of memory devices. In this work, TiO2-doped Ta2O5 crystals were prepared using the laser-heated pedestal growth technique, as grown specimens were found to crystallize in monoclinic phase at room temperature. The structural modifications in these crystals resulting from variation of TiO2 composition (0–11%) and temperature (−248–900°C) were studied using Raman spectroscopy. The low frequency external modes (v<100 cm−1) that originate from the interaction between TaO5−2nn/Ta6O+612 clusters/polyhedra exhibit a strong compositional and temperature dependence in terms of their intensity and frequency variations. The Raman spectral evolutions suggested a monoclinic to orthorhombic structural phase transition at about 327, 397, 487, and 577°C for 0, 5, 8, and 11% TiO2-doped Ta2O5, respectively.
Two new alkali tellurates Cs2SO4·Te(OH)6 (CsSTe) and Cs2SeO4·Te(OH)6 (CsSeTe), are obtained by action of telluric acid H6TeO6 with respectively, caesium sulfate Cs2SO4 and caesium carbonate Cs2CO3 and selenic acid H2SeO4. CsSTe is rhombohedric, R3, with a=b=7.4790(3) Å, c=16.6370(6) Å, Z=3, V=805.92(5) Å3 and ρmeas=3.656 g/cm3. Its atomic arrangement is described by some planes of pure octahedra TeO6 altering with planes of pure tetrahedra SO4 perpendicular to some directions. CsSeTe is monoclinic, P21/c, with a=12.817(6) Å, b=7.519(3) Å, c=12.432(5) Å, β=111.16(2)°, V=1117.3(8) Å3, Z=4 and ρmeas=3.795 g/cm3. Its structure exhibits infinite TeO66− and SeO42− planes where the alkali cations are intercalated. In both structures a network of strong hydrogen bonds connects the different components in the building of the crystal.
Traditional electrode materials for lithium-ion storage cells are based on materials which have both mixed electron and ion transport (for Li+). They are typically crystalline layered structures such as metal oxides that have high redox potentials, and act as positive electrodes; and graphitic carbons capable of reversible uptake of Li at low potentials which act as negative electrodes. Recently, however, nanostructured solid state materials, which are comprised of two or more compositional or structural phases, have been considered. This new area has been particularly exploited in the area of negative electrode design, where the intimate mix of components at the nanoscale permits and enhances Li reversibility. It also include cathode materials where materials that function on the basis of intergrowth structures (internal composites) have been found to be beneficial; and insulating materials where the limitations to electron transport must be overcome by judicious design of nanostructured composites. The research trends and future prospects are discussed.
Thin films of rare-earth nickelates RNiO3 (R=Pr, Nd, Sm, Gd) were prepared under a reduced oxygen pressure of <0.02 bar by MOCVD on perovskite substrates. The film–substrate lattice mismatch is critical for the epitaxial stabilization of RNiO3. Increase of the lattice mismatch or film thickness results in the deposition of rare-earth oxides and NiO instead of RNiO3. The transport properties of the films on LaAlO3 were similar to those of the bulk material of the same composition under an applied pressure of 9 kbar. The properties of RNiO3 films with a sharp metal-to-insulator transition can be effectively tuned by the lattice strain.
Fourier Transform Infrared Spectroscopic investigation has been carried out to study the structural changes and coordination of Al and Si in the mullite system as a function of temperature (60–1400°C). The FTIR spectral patterns of the gel and the gel calcined at 200°C show bands corresponding to Al–O–Al and Si–O–Si vibrations. At higher temperatures hydroxyl groups and organics present have been removed. The absence of fine structure in the region 400–900 cm−1 in the temperature range 400–1000°C is interpreted as due to the existence of nanostructured alumina, silica and alumina–silica agglomeration in the amorphous state. Analysis of the spectra indicates the presence of both octahedral and tetrahedral coordination at 1000°C, the temperature around which spinel is formed. The shifting of the asymmetric stretching frequencies of the Si–O–Si and Al–O–Si networks to higher wave numbers indicate the formation of mullite at 1200°C. Crystalline nature of the product is indicated by the sharpening of spectral peaks. The spectra also indicates the completion of mullite formation at 1250°C.
We investigated two series of samples of YBa2Cu3−zSczO7−δ prepared at the sintering temperature 967 and 980°C and the two series of YBa2−ySryCu3O7−δ–Ag2O systems differing in silver doping with 2 and 10 wt% Ag2O by X-ray diffraction, resistive and detailed magnetization measurements. Doping levels of Sc, Sr and Ag2O range from z=0 to 1.0, y=0.0 to 1.0 and Ag2O (2 and 10 wt%), respectively. For all doping levels y, z and silver oxide the superconducting phases at 77.3 K were observed. For the higher Sc content z, a deterioration of the critical temperature (Tc), the density, the volume fraction of superconducting phase, as well as the volume magnetization (magnitude) M, the volume magnetic susceptibility, the first magnetic penetration field (Hp1) and the slope of magnetization loops of the samples were observed in relation to sintering temperature. Increasing Sr content results in gradual decreasing of superconducting properties: Tc, Hp1, and M, the slope of magnetization loops and magnetization hysteresis, they all decrease with increasing Sr content at higher Sr levels.
In this work we present preliminary results on the sensitivity to methane gas of zinc oxide thin films deposited by spray pyrolysis. It was found that using highly resistive (above 104 Ω cm) thin films and by performing the measurements at 200°C a sensitivity better than one order of magnitude was found to detect 2000 ppm of methane. A linear dependence on the sensitivity between 100 and 2000 ppm of methane was also obtained.
A new approach to the growth of complex oxide films free of any secondary phases using the volatile component was developed. The technique consists in a self-tuning of the oxide film composition during the growth due to the evaporation of the excess PbO. It was successfully used for the MOCVD growth of very smooth single phase La1−xPbxMnO3 (x=0.1–0.6) films on perovskite substrates and MgO at a deposition rate of ∼1 μmh−1. High quality of the films was explained by the presence of quasi-liquid PbO surface layer activating the surface diffusion during the growth.
Effect of different crystallographic structure of various substrate cuts on diffusion processes into optical crystals is demonstrated for lithium niobate and sapphire. After the low or moderate temperature diffusion processes the X-cuts, which are parallel to the C3 axis, always contained much more dopants comparing with the Z-cuts, which are oriented perpendicularly to the C3. This has been proved for annealed proton exchange (APE) technology for fabrication of optical waveguides in lithium niobate and for the moderate temperature incorporation of erbium ions into both materials. Thus the possibility of localized doping by Er3+ diffusion at moderate (lower then 500°C) temperature is for the first time demonstrated for sapphire single crystal wafers. The strong anisotropy of the doping is explained on the bases of suitable orientation of the cleavage planes to the substrates surfaces in the X-cuts that allows for better penetration of the diffusing particles into the material.
Electronic and magnetic properties of the ferromagnets UCu2Ge2 (122), UGe2 (122), and of the newly found U3Cu4Ge4 (344) intermetallic system are self-consistently calculated within the local spin density functional theory using the augmented spherical wave (ASW) method. The influence of hybridisation on the chemical bonding and magnetic behaviour is discussed from the densities of states (DOS) as well as from the crystal orbital overlap population (COOP) leading to suggest comparative influences between electronic effects of the hybridisation as with respect to the structural changes when the constituents of (122) and (12) are built within U3Cu4Ge4. From the spin-only magnetic results the important contribution of the orbital effects is analysed.
The aim of this study was to find an optimum silicon carbide-based dielectric material for high-temperature electronic applications. The crystal–chemical criteria adopted in the selection of appropriate material were: a nanocrystalline structure, Eg≫3.5 eV, α∼3.5×10−6 1/K. These criteria are fulfilled by aluminium nitride. Thin nano-aluminium nitride layers were deposited on single-crystal silicon carbide plates (n-type) by impulse plasma assisted CVD. The structure of the aluminium nitride layers and the electro–physical properties of the AlN–SiC junction were examined.
Effect of enhanced (up to 1.2 GPa) hydrostatic argon pressure on creation of oxygen-related micro-defects (such as oxygen clusters, OC’s, and precipitates, OP’s) in Cz–Si during high temperature–high pressure treatment, HT–HP, at up to 1400 K, was investigated by chemical selective etching, FTIR, and photoluminescence, PL, methods. Cz–Si contained interstitial oxygen, Oi, with a concentration, c0, up to 1.1×1018 cm3; prior to the HT–HP treatment some samples were pre-annealed at (720, 830) K–105 Pa to create nucleation centres for Oi precipitation. The HT–HP treatment results in decrease of c0, revealing defect-related PL bands and enhanced (in comparison to effect of annealing at 105 Pa) creation of oxygen-related micro-defects.
