La3+ and Ce3+ Doping of Hard-Magnetic Ferrites

Rare-earth-doped ferrites with the general formula M_{1 – x} R _x · nFe₂O₃ (M = Ba, Sr, Pb; R = La, Ce; x = 0–0.1; n = 4–6) are prepared by solid-state combustion synthesis. The effects of the doping procedure (before or after combustion synthesis), dopant content, and heat-treatment conditions on the magnetic and mechanical properties of the ferrites are examined. The results indicate that doped materials can be used to fabricate permanent magnets with enhanced density, remanence, and energy product. In addition, rare-earth doping improves the strength of the ferrite materials, eliminating their main drawback—inherent brittleness.     

Thermodynamic Approach to Optimization of SrTiO3 Chemical Vapor Deposition from Volatile Metalorganic Precursors

Thermodynamic analysis is used to examine the possibility of producing SrTiO₃ films in two precursor systems containing different volatile Ti compounds: TiO(dpm)₂–Sr(dpm)₂–N₂O–Ar (I) and Ti(OPr ⁱ )₂(dpm)₂–Sr(dpm)₂–N₂O–Ar (II). The results demonstrate that, at an initial Ti : Sr atomic ratio of 1 : 1 and a ratio of flow rates /f _Ar = 1, system I contains no region of single-phase SrTiO₃ deposition. Raising the N₂O concentration in the vapor phase makes it possible to deposit SrTiO₃. System II contains a broad temperature range of SrTiO₃ deposition (400–1300 K) over the entire pressure range examined, p _total = 1–100 Pa. These conclusions are verified by depositing films in systems I and II (in particular, in system I in the presence of N₂O activated in an rf discharge). The films grown in system I are found to consist of crystalline SrTiO₃ and an amorphous phase containing residual organics, in particular in the form of CH _x groups with x = 1–3. The films produced in system II consist of nanocrystalline SrTiO₃. In this system, the equilibrium phase SrTiO₃ is formed on the substrate surface via solid-state reaction between TiO₂ and SrO intermediates. The rf activation of N₂O makes it possible to grow crystalline SrTiO₃ at much lower temperatures.     

Fine yellow α-SiAlON:Eu phosphors for white LEDs prepared by the gas-reduction–nitridation method

Yellow-emitting α-SiAlON:Eu²⁺ phosphors were synthesized by the gas reduction and nitridation of a homogeneous oxide precursor in a CaO–Al₂O₃–SiO₂–Eu₂O₃ system at 1400–1450 °C using an NH₃–CH₄ mixture gas as a reduction–nitridation agent. The precursor was prepared by a sol–gel process using a low-cost nitrate, tetraethyl orthosilicate and citric acid as the starting materials. The effects of reaction parameters such as heating rate, temperature, holding time and CH₄ content on the composition, microstructure and photoluminescence of the prepared powders were investigated. Nearly single-phase α-SiAlON was successfully synthesized by the one-step gas reduction and nitridation without the need for post-annealing at a higher temperature. The prepared powders consisted of relatively well-dispersed and uniform crystals with a hexagonal shape. The photoluminescence spectra of Eu-doped Ca-α-SiAlON phosphors excited by near-ultraviolet or blue light showed a broad, yellow emission band at 500–700 nm, which agrees well with that obtained from phosphors prepared by the conventional solid-state reaction.     

Hardness and elastic properties of Bi2O3-based glasses

The hardness and elastic properties of 20PbO · xBi₂O₃ · (80 – x)B₂O₃ glasses with x = 20–60 were evaluated through usual Vickers indentation and nanoindentation tests. The glass transition temperature (T _g = 295–421°C), Vickers hardness (H _v = 2.9–4.5 GPa), true hardness (H = 1.5–3.8 GPa) and Young's modulus (E = 24.4–72.6 GPa) decreased monotonously with increasing Bi₂O₃ content. This compositional trend demonstrates that the strength of Bi–O chemical bonds in these glasses is considerably weak compared with B–O bonds and plastic deformations under indentation loading occur easily. The elastic recovery after unloading was about 45% for the glasses with x = 20–50, and the Poisson's ratio was 0.27 for the glass with x = 20. The fracture toughness was evaluated to be 0.37–0.88 MPam^1/2 from the values of H _v and E, and it was proposed that not only weak Bi–O bonds but also boron coordination polyhedra (BO₃ or BO₄) and their arrangements affect on crack formation. From the temperature dependence of Vickers hardness up to the glass transition region, it was suggested that the glasses with high Bi₂O₃ contents belong to the category to fragile glass-forming liquids.     

Deposition rates of titanium nitride plates prepared by chemical vapour deposition of TiCl4+NH3 system

Titanium nitride plates (TiN_x,x = 0.74–1.0, about 2 mm thick maximum) were prepared by chemical vapour deposition (CVD) using TiCI₄, NH₃ and H₂ as source gases. The effects of CVD conditions, i.e. gas molar ratio (m _N/Ti = NH₃/TiCI₄) and deposition temperature (T_dep), on deposition rates and surface morphology were examined, and the deposition mechanism of the CVD-TiN_x plates was discussed. The relationship between m_N/Ti and deposition rates showed a maximum peak at certainm _N/Ti, and this maximum peak shifted to lowerm _N/Ti with increasing_{T dep}. The activation energy for the formation of CVD-TiN_x plates was about 80 kJ mol^–1 in the lower temperature range. The decomposition reaction of NH₃ gas could be associated with the rate-controlling step. At higher temperatures, the diffusion process may be the rate-controlling step, and a large amount of powder (mainly NH₄Cl) was formed in the gas phase. The highest deposition rate obtained in the present work was 1.06×10^–7 ms^–1 (0.38 mmh^–1) atT _dep = 1773 K andm _N/Ti = 0.87.     

Electrical transport properties of amorphous Se78−x Te22Bi x films

D.C. Conductivity measurements on the thin films of a-Se_78–x Te₂₂Bi _x system (where x = 0, 0.5, 2 and 4) are reported in the temperature range 213–390 K and the density of states (DOS) near the Fermi level is calculated using dc conductivity data. It is found that the conduction in all the samples takes place in the tails of localized states. The conduction in the high temperature region 296–390 K is due to thermally assisted tunneling of electrons in the localized states at the conduction band edge. In the low temperature region 213–296 K conduction takes place through variable range hopping in the localized states near the Fermi level.     

Structural investigations of phosphate glasses: a detailed infrared study of the x(PbO)-(1−x) P2O5 vitreous system

The results and detailed discussion of an extensive experimental study of infrared spectra of the x (PbO)-(1–x)P₂O₅ vitreous system (x=0.3–0.75) together with a brief review of infrared spectra of phosphate compounds, are presented. Theoretical models employed in the interpretation of infrared spectra of glasses have been reviewed. The frequency ranges of various infrared bands belonging to PO ₄ ^3– and P₂O ₇ ^4– , observed in different phosphate compounds, are discussed. The glassy and quenched samples were prepared from PbO and NH₄H₂PO₄ by the rapid quenching technique. The infrared spectra of the constituents of the system, PbO and P₂O₅, in their polycrystalline and glassy forms, have been discussed. The intensity and wavenumbers of the infrared bands around 1600 and 3300 cm^–1, assigned to the bending and stretching modes in H₂O trapped by the hygroscopic glasses, have been followed for different compositions with x<0.5. The changes observed in these infrared bands established the role of water as an additional glass modifier. The intensity and frequency variations of the infrared bands have been followed through all the compositions for characteristic phosphate group frequencies including P=O, P-O-P stretching and bending modes and P-O bending mode. The results clearly suggest that the x(PbO)-(1–x)P₂O₅ system undergoes gradual structural changes from metaphosphate (x=0.5), to pyrophosphate (x=0.66) and to orthophosphate (x=0.75). The continuing presence of the infrared band, in varying intensity, in the region 1200–1280 cm^–1 attributed to P=O, suggests that the glass-forming ability of the binary system is extendable at least up to x=0.66 composition, and that no complete rupture of P=O bond by Pb²⁺ takes place. The ionic character of the phosphate groups, P-O^(–), PO ₄ ^3– is well revealed by significant changes with the PbO content in the spectral features of the infrared bands around 1120 and 980 cm^–1 respectively. The maximum intensity of the P-O^(–) band at 1120 cm^–1 for 55 mol% PbO suggests a partial breakdown of the covalent vitreous network of the phosphates and formation of a crystalline phase consisting of ionic groups PO ₄ ^3– , P₂O ₆ ^2– and P₂O ₇ ^4– for PbO greater than 55 mol%. The observed pattern of variation in the intensity of the infrared bands in the 940–1080 cm^–1 region attributed to the v₃-mode in PO ₄ ^3– , suggests a gradual transformation of PO ₄ ^3– units to PO ₃ ^– groups in lead meta-phosphate glass and then their restoration to PO ₄ ^3– groups of pyro- and ortho-phosphate quenched samples. The results indicate a gradual decrease in the number of bridging oxygens and increase in the resonance behaviour of non-bridging oxygens as the mole percentage of metal oxide (PbO) increases in the glass. The infrared spectra of several binary phosphate glasses have been reviewed in the context of the study of effect of the cation on the infrared spectra. It is found that the influence of the cation on the infrared spectra of phosphate glasses does not show any striking regularity. Theoretical calculations of these band frequencies were found to agree well only in the case of pure stretching (P=O and O-H) vibrations and pure bending (P-O-P and O-H) vibrations. The disagreement in the case of P-O^(–), P-O-H and other modes of P-O-P groups, has been attributed to the mixed nature of modes occurring in glasses. The changes in the positions of the characteristic bands and their relative intensities are strongly dependent on the structural units and PbO content in the phosphate glasses and the results emphasize the role of PbO as a network modifier.     

Spectroscopic ellipsometry studies of amorphous PZT thin films with various Zr/Ti stoichiometries

Amorphous stoichiometric Pb(Zr _x Ti_{1 – x)}O₃) (PZT) thin films with various values of x were deposited on Si(100) substrates by the sol-gel technique. The influence of Ti content on the optical properties was studied by spectroscopic ellipsometry (SE) in the UV-visible region. Using a four-phase fitting model, the refractive index n and extinction coefficient k was obtained by analyzing the SE spectra. The optical band gap energies E _g for these films were reported under the assumption of a direct band-to-band transition. It has been found that the refractive index, extinction coefficient and band gap energy of the films were functions of the film compositions. The refractive index of the PZT films increases linearly with increasing Ti content. On the other hand, the optical band gap energy of the PZT films decreases with increasing Ti content.     

Preparation and properties of ferroelectric Pb1 − xCaxTiO3 thin films produced by the polymeric precursor method

Pb_{1 – x} Ca _x TiO₃ thin films with x = 0.24 composition were prepared by the polymeric precursor method on Pt/Ti/SiO₂/Si substrates. The surface morphology and crystal structure, and the ferroelectric and dielectric properties of the films were investigated. X-ray diffraction patterns of the films revealed their polycrystalline nature. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) analyses showed the surface of these thin films to be smooth, dense and crack-free with low surface roughness. The multilayer Pb_{1 – x} Ca _x TO₃ thin films were granular in structure with a grain size of approximately 60–70 nm. The dielectric constant and dissipation factor were, respectively, 174 and 0.04 at a 1 kHz frequency. The 600-nm thick film showed a current density leakage in the order of 10^–7 A/cm² in an electric field of about 51 kV/cm. The C-V characteristics of perovskite thin films showed normal ferroelectric behavior. The remanent polarization and coercive field for the deposited films were 15 C/cm² and 150 kV/cm, respectively.     

Optical and other physical characteristics of Ge–Se–Cd thin films

Amorphous Ge₂₀Se_80−xCd_x thin films with different compositions (x = 0, 2.5, 5, 7.5 and 10 at.%) were deposited onto glass substrates by thermal evaporation. The reflection spectra, R(λ), of the films at normal incidence were obtained in the spectral region from 400 to 2500 nm. Based on the use of the maxima and minima of the interference fringes, a straightforward analysis proposed by Minkov has been applied to derive the optical constants and the film thickness for the Ge₂₀Se_80−xCd_x thin films. The dispersion of the refractive index is discussed in terms of the single-oscillator Wemple and DiDomenico model. Tauc relation for the allowed non-direct transition describes the optical transition in the studied films. With increasing cadmium content the refractive index increases while the optical band gap decreases. The optical band gap decreases from 2 to 1.5 eV with increasing cadmium content from 0 to 10 at.%. The chemical-bond approach has been applied successfully to obtain the excess of Se–Se homopolar bonds and the cohesive energy of the Ge₂₀Se_80−xCd_x system.     

Machinable and mechanical properties of sintered Al2O3-Ti3SiC2 composites

Two-phase composites consisting of (1 – x) Al₂O₃ and xTi₃SiC₂ (x = 0–1) were prepared by spark plasma sintering (SPS). Sintered densities larger than 98% of theoretical density were achieved when the specimens were sintered at 1300°C for 5 min (in vacuum, at pressure 30 MPa). When content of Ti₃SiC₂ increased up to 30 wt%, composites were found to be machinable—they could be drilled easily using conventional Fe-Mo-W drills or gravers. The mechanical properties of the (1 – x) Al₂O₃–xTi₃SiC₂ composites were evaluated. The bending strength, Vickers hardness of the specimens had the following ranges: 428 ± 10.2 (x = 0) to 673 ± 15.4 Mpa (x = 1) (bending strength at room temperature); 19.9 (x = 0) to 4.0 GPa (x = 1) (Vickers hardness).     

Synthesis and Properties of BiFeO3–DyMnO3 Solid Solutions

(1 – x)BiFeO₃–xDyMnO₃ solid solutions with x= 0.03–0.30 are synthesized for the first time, and their dielectric and magnetic properties are studied. The solid solutions with x 0.08 have a rhombohedrally distorted perovskite structure, and those with x = 0.10–0.30 have an orthorhombically distorted structure. The 1-kHz electrical conductivity of the solid solutions exhibits semiconducting behavior and rises exponentially with temperature. The dielectric properties of the solid solutions are investigated in the microwave range (0.5–2.5 GHz) between room temperature and 250°C. The real part of their room-temperature dielectric permittivity reaches a maximum at x= 0.20, while the largest tan is observed at x = 0.03. The solid solutions with x= 0.03–0.08 exhibit high dielectric losses and are, therefore, candidate microwave absorbing materials. The magnetic and electrical ordering temperatures in the composition range studied exceed room temperature; that is, these solid solutions are new high-temperature seignettomagnetic materials.     

Structural,electrical and optical properties of copper selenide thin films deposited by chemical bath deposition technique

A low cost chemical bath deposition (CBD) technique has been used for the preparation of Cu_2–xSe thin films on glass substrates. Structural, electrical and optical properties of these films were investigated. X-ray diffraction (XRD) study of the Cu_2–xSe films annealed at 523 K suggests a cubic structure with a lattice constant of 5.697 Å. Chemical composition was investigated by X-ray photoelectron spectroscopy (XPS). It reveals that absorbed oxygen in the film decreases remarkably on annealing above 423 K. The Cu/Se ratio was observed to be the same in as-deposited and annealed films. Both as-deposited and annealed films show very low resistivity in the range of (0.04–0.15) × 10^–5 -m. Transmittance and Reflectance were found in the range of 5–50% and 2–20% respectively. Optical absorption of the films results from free carrier absorption in the near infrared region with absorption coefficient of 10⁸ m^–1. The band gap for direct transition, E_g.dir varies in the range of 2.0–2.3 eV and that for indirect transition E_g.indir is in the range of 1.25–1.5 eV.     

Luminescence of Eu3+-Activated Potassium Scandium and Potassium Yttrium Phosphate Vanadates

The photoluminescence spectra of potassium rare-earth phosphate vanadates K₃R_{1 – y} Eu _y (PO₄) _x (VO₄)_{2 – x} (R = Sc, Y) were measured in the range 450–800 nm under excitation at 337 nm. The energies of the Stark components of the ⁷ F _j(j= 0, 1, 2) multiplet were determined, and the crystal-field parameters were evaluated. The effects of the Eu³⁺concentration and PO₄ ^3–/VO₄ ^3–ratio on the luminescence of the materials studied and the concentration quenching of luminescence were analyzed. The materials with high Eu³⁺concentrations are shown to be potentially attractive as photo- and cathodoluminophors.     

Effect of Nonstoichiometry on the Structure and Dielectric Properties of PbMg1/3Nb2/3O3–PbTiO3 Ceramics

(1 – x)PbMg_1/3Nb_2/3O₃–xPbTiO₃ ceramics with x = 0.20–0.36 were prepared and characterized by x-ray diffraction, microstructural analysis, and dielectric measurements at different temperatures and frequencies. The phase composition, structural parameters, and ferroelectric transition temperatures of the ceramics were determined. The unit-cell volume and transition temperature were shown to decrease with increasing Pb vacancy concentration.     

Magnetic-Field-Induced Level Crossing and the Spin Dynamics of Excitons in Zn1?xMnxTe/ZnTe Quantum Wells

Magnetic-field-induced level crossing and the spin dynamics of excitons in a Zn_1–x Mn _x Te/ZnTe single quantum well are studied. The circularly-polarized photoluminescence (PL) shows that the down spin branch of the Zn_1–x Mn _x Te exciton overlaps with both the up and down spin branches of the ZnTe exciton at a crossing field (H _c) of 4 T, due to the giant Zeeman shift of Zn_1–x Mn _x Te. The PL intensities and lifetimes in each layer become gradually equal toward H _c, which shows the mixing of wavefunctions of the excitons generated in each layer. Above H _c, each branch of the spin-polarized exciton separates again. The lifetimes of the spin-polarized exciton PL reflect the spin-flip relaxation in ZnTe and the spin mixing between Zn_1–x Mn _x Te and ZnTe layers.     

Dielectric relaxator behaviour of the system Sr1−xLaxTi1−xCoxO3 (x ⩽ 0.40)

The dielectric behaviour of the valence-compensated solid solution Sr_1–x La _x Ti_1–x Co_xO₃ (x=0.05, 0.10, 0.20, 0.30 and 0.40) has been studied as a function of temperature and frequency. Compositions withx=0.20, 0.30 and 0.40 exhibit high values of dielectric constant. This high dielectric constant is due partly to the presence of interfacial polarization, and partly to the formation of grain-boundary barrier layers in these materials. The presence of barrier layers is shown by complex plane impedance analysis.     

Thermal stability of amorphous phases in vapour-deposited binary alloy thin films of palladium with vanadium,niobium and tantalum

Binary alloys of the systems V-Pd, Nb-Pd and Ta-Pd were vapour deposited and investigated by transmission electron microscopy. The atomic fraction,x, was varied in steps of 0.1 from one pure element to the other. The range over which an amorphous phase is observed is found to increase in width going from 3d to 5d alloying element in palladium: the compositions where amorphous phases are found are V_1–x Pd _x (x = 0.5), Nb_1–x Pd _x (x = 0.4 to 0.6) and Ta_1–x Pd _x (x = 0.2 to 0.6). The composition range over which a crystalline phase is found correlates well with the single-phase solid solution region close to the melting temperature in the phase diagram. Crystallization of the V_0.5Pd_0.5 alloy takes place at 550 K. The amorphous Nb_1–x Pd _x (x = 0.4, 0.5) films crystallize at 850 K, whereas the amorphous Ta-Pd films crystallize between 850 and 1050K, depending on the composition. For Nb_1–x Pd _x (x = 0.4 to 0.5) and Ta_1–tx Pd _x (x = 0.2 to 0.6) primary crystallization takes place into an f c c phase. The second crystallization step leads to a phase with a complex structure. The result is a two-phase system. V_0.5Pd_0.5 and Nb_0.4Pd_0.6 crystallize polymorphically to an f c c solid solution. The crystallization temperatures for the compositions which display primary crystallization are higher than for the compositions which crystallize by a polymorphic reaction.     

Surface chemistry and film growth during TiN atomic layer deposition using TDMAT and NH3

Surface chemistry and film growth were examined during titanium nitride (TiN) atomic layer deposition (ALD) using sequential exposures of tetrakis-dimethylamino titanium (TDMAT) and NH₃. This ALD system is shown to be far from ideal and illustrates many potential problems that may affect ALD processing. These studies were performed using in situ Fourier transform infrared (FTIR) techniques and quartz crystal microbalance (QCM) measurements. Ex situ measurements also analyzed the properties of the TiN ALD films. The FTIR studies revealed that TDMAT reacts with NH_x* species on the TiN surface following NH₃ exposures to deposit new Ti(N(CH₃)₂)_x* species. Subsequent NH₃ exposure consumes the dimethylamino species and regenerates the NH_x* species. These observations are consistent with transamination exchange reactions during the TDMAT and NH₃ exposures. QCM studies determined that the TDMAT and NH₃ reactions are nearly self-limiting. However, slow continual growth occurs with long TDMAT exposures. In addition, the TiN ALD growth rate increases progressively with growth temperature. The resistivities of the TiN ALD films were ?10⁴ μΩ cm and the densities were ?3 g/cm³ corresponding to a porosity of ∼40%. The high porosity allows facile oxidation of the TiN films and lowers the film resistivities. These high film porosities will seriously impair the use of these TiN ALD films as diffusion barriers.     

Synthesis, Structure, and Properties of Ammonium Polyvanadomolybdate Xerogels

Powders and films of (NH₄)_{2 – x} H _x V_{12 – y} Mo _y O_{31 ±} · nH₂O (0 x 2, 0 y 3) xerogels with a layered structure were prepared for the first time. According to IR spectroscopy data, the V–O–Mo layers in the xerogels are similar to those in the delta vanadium bronze (NH₄)_0.5V₂O₅. X-ray photoelectron spectroscopy shows that the vanadium and molybdenum in the xerogels are mainly in the oxidation states 5+ and 6+, respectively. The materials also contain small amounts of V⁴⁺and OH^–. The thermal stability of the materials increases with Mo content. The electrical conductivity of the films depends on air humidity and reaches a maximum at the composition (NH₄)_1.5H_0.5V₉Mo₃O_{31 +} · nH₂O. At temperatures in the range 20–70°C and air humidity between 12 and 59%, the activation energy of conduction is very low.