Three-dimensional computer modeling of grain growth and pore shrinkage during sintering

This paper deals with modeling of grain growth and pore shrinkage during sintering. The simulation takes as the theoretical basis the curvature of the grain boundaries and the pore surfaces and their mobilities. The movement of the grain boundaries is calculated for each grain separately using the flux equation for grain growth j=c_k(1/r₂−1/r₁), c_k being a rate constant, r₁ and r₂ effective radii for each grain boundary. The modification of these boundaries in a micro structure is calculated in two and three-dimensional space starting with a Voronoy mosaic at the beginning of sintering. Two kinds of pores are distinguished: sphere like pores situated between two grains and non spherical pores between at least three grains. The results of several computer runs are shown and discussed.     

Sorption of 60Co from aqueous solutions on layered double hydroxides of Mg, Al, and Nd

Sorption of ⁶⁰Co from aqueous solutions of various compositions on layered double hydroxides (LDHs) of Mg, Al, and Nd in carbonate and hydroxide forms and on layered double oxides (LDOs) of Mg and Al was studied. ⁶⁰Co is poorly sorbed from aqueous nitrate solutions onto LDH-Mg-Al-OH. The ⁶⁰Co distribution coefficient K _d does not exceed 50 ml g^?1 at a phase contact time of 15 min and V/m = 50 ml g^?1. At the same time, ⁶⁰Co is efficiently sorbed from 10^?3?C10^?5 M aqueous Co(NO₃)₂ solutions onto LDH-Mg-Al and LDH-Mg-Nd with CO ₃ ^2? in the interlayer space. At a phase contact time of 15 min and V/m = 50 ml g^?1, K _d exceeds 2 × 10⁴ ml g^?1 for LDH-Mg-Nd and does not exceed 5 × 10³ ml g^?1 for LDH-Mg-Al. The ⁶⁰Co desorption from LDH-Mg-Nd-CO₃ into 0.05?C0.2 M solutions of Na₂CO₃, NaNO₃, (NH₄)₂C₂O₄, and Na₂H₂EDTA and into distilled water was studied. Na₂H₂EDTA is the most efficient desorbing agent. After 15-min contact of LDHMg(⁶⁰Co)-Nd-CO₃ with 0.1 and 0.05 M Na₂H₂EDTA solutions, the degree of desorption of ⁶⁰Co is ??100 and ??99%, respectively.     

Effects of Co doping on structure and optical properties of TiO2 thin films prepared by sol-gel method

Co-doped TiO₂ thin films were synthesized on quartz substrates by sol-gel method. Atomic force microscopy results indicate that the surfaces of the Ti_1 − xCo_xO₂ (0 ≤ x ≤ 0.10) films become smooth and compact with increasing Co content. X-ray diffraction results show that all the films are rutile phase structure and Co doping leads to lattice contraction. X-ray photoelectron spectroscopy results reveal that the predominant oxidation state of Co is divalent. Peak positions of Raman-active modes (B_2g, A_1g and E_g) shift to lower frequency with increasing Co content. The refractive index n at 670 nm from transmittance spectra increases with increasing Co content. The OBG varies between 3.10 and 3.26 eV. Note that optical band gap (OBG) first increases and then decreases with increasing Co content, reaching its maximum value when x is 0.03. These results suggest that the increasing mechanism of OBG is related to the decrease of grain size, compressive stress, and reduction of rutile TiO₂, and the decreasing mechanism of OBG is involved with defect and impurity. The competition of the two mechanisms leads to the strange change of OBG.     

Influence of thin platinum layer on the magnetic properties of multiple layers of CVD cobalt thin films

This paper presents the results obtained on the multiple layers of cobalt (Co)/platinum (Pt) and cobalt (Co)/platinum (Pt)/cobalt (Co) on the oxidised silicon substrate. The cobalt layers were deposited by metal-organic chemical vapour deposition on oxidised-silicon substrates at 450?°C, in H₂ ambient with 2-torr processing pressure. The platinum layers were deposited by E-beam evaporation in a separate vacuum system. The magnetic properties of Co/Pt/Co and Co/Pt multilayer were compared with the single cobalt layers of similar thicknesses on the oxidised silicon substrate. From the hysteresis loops it was observed that thin cobalt layers on oxidised-silicon substrate shows hard magnetic property with coercivity H _c values of 360 Oe and 500 Oe respectively for the 30- and 15-nm cobalt layers. The multiple layers of Co/Pt/Co and Co/Pt of cobalt thickness 15- and 30-nm with platinum 1.5-nm spacer-layer show significant change in magnetic properties (i.e. coercivity H _c and magnetisation M _s ) and, gave soft magnetic properties with H _c values 51 and 49 Oe respectively, which are significantly less than the H _c values of single cobalt layers on oxidised silicon. Also, single and multiple layers cobalt with platinum were annealed and compared with the as-deposited layer structures. From the microstructure analysis by SEM, and AFM it was found that the single and multilayer had similar roughness. Magnetic images were observed by MFM and analyzed in terms of domain structure.     

Growth mechanism of epitaxial CoSi2 on Si and reactive deposition epitaxy of double heteroepitaxial Si/CoSi2/Si

The growth mechanism of epitaxial CoSi₂ was studied using Co/Ti/Si multilayer solid phase reaction. Results showed that phase formation was controlled by diffusion of Co through the growing CoSi_x, although at the early stage of CoSi₂ growth the diffusion of Co could be controlled by a Ti layer. A reactive deposition technique was also evaluated by using a conventional magnetron sputtering system. Results showed that an epitaxial CoSi₂ layer was formed by controlling the Co sputtering rate not to exceed the Co diffusion rate through CoSi_x. However, the surface of CoSi₂ became rough when the deposition rate was much slower than the Co diffusion rate through CoSi_x. The roughness was caused by the formation of CoSi₂ (111) facets at the interface between CoSi₂ and the Si substrate. Si/CoSi₂/Si double heteroepitaxial structures were fabricated when Si and Co were sequentially sputter-deposited on a Si (100) substrate.     

Synthesis of mesoporous Co/Ce-SBA-15 materials and their catalytic performance in the catalytic oxidation of benzene

Cerium-containing SBA-15 mesoporous materials, with different Ce/Si molar ratios, were synthesized by a direct hydrothermal synthesis method and further modified by impregnation with 10, 15 and 20 wt.% Co. Characterizations by powder X-ray diffraction (XRD), N₂ sorption, inductively coupled plasma (ICP) and UV-vis spectroscopy were carried out. The small-angle XRD and N₂ sorption characterizations showed that these Co supported materials have less-ordered mesoporous structures with partial blockage of pores, and their specific surface area, pore volume and pore size were relatively lower than those of unsupported cerium-containing SBA-15. Spinel Co₃O₄ constituted the predominant cobalt phase in the prepared catalysts, and CeO₂ was also detected. All the Co supported catalysts exhibited high catalytic activity in the oxidation of benzene.     

NMR Study of 3He Adsorbed in Zeolite Pores

The NMR properties of ³He adsorbed in the pores of MCM-41 zeolite have been studied in the temperature range 1.4 to 15 K, at pulsed NMR frequencies of 1.66 and 3.26 MHz. At a coverage x=0.84 monolayer, the linewidth 1/T ^* ₂ scales approximately linearly with the magnitude of the static magnetic field, and T ^* ₂ increases linearly with increasing temperature with an extrapolated low temperature limit of order 80 µs. However T ₂ is significantly longer, increasing from 0.7 µs at the lowest temperatures investigated in a way suggesting thermally activated motion. We attribute T ^* ₂ to static field variations between pores arising from paramagnetism of the zeolite and the random orientation of the pores. On the other hand T ₁ increases monotonically with decreasing temperature to around 80 ms at 1.6 K, indicating that the correlation time of the local magnetic field fluctuations τ _c>ω ^?1 ₀. A detailed temperature dependence of T ₁ and T ₂ has been carried out at a coverage of 0.32 monolayer. Here a minimum in T ₁ is observed at 11 K for a Larmor frequency of 3.26 MHz corresponding to a correlation time τ _c of 5×10^?8 s. T ₁ increases by around three orders of magnitude on cooling to 1.8 K. At this temperature T ₁ decreases significantly with increasing coverage while T ₂ shows a very much weaker coverage dependence.     

The influence of hydrothermal synthesis conditions on gyrolite texture and specific surface area

Influence of hydrothermal synthesis conditions on the gyrolite specific surface area, dominant pore size and their differential distribution by the radius were determined. The synthesis of gyrolite has been carried out in unstirred suspensions within 32, 48, 72, 120, 168 h at 200°C temperature from a stoichiometric composition (the molar ratio of CaO/SiO₂ was equal to 0.66 where water/solid ratio of the suspension was equal to 10.0) of the initial CaO and SiO₂·nH₂O mixture. It was found that the structure of gyrolite and the shape of dominated pores (from pores between parallel plates to cylindrical pores) changes prolonging the duration of hydrothermal synthesis. The stable gyrolite crystal lattice was formed only after 120 h of isothermal curing. Its specific surface area S _BET = 38.28 m²/g, the radius of dominant plate pores r _p = 30–40 Å, the cumulative pore volume ΣV _p = 0.08 cm³/g. It was determined that the pores with 4.0–5.0 nm radius were dominated in gyrolite structure after 168 h of synthesis. It was estimated that the ion exchange between gyrolite with less orderly structure in Zn(NO₃)₂ + NH₄OH alkaline solution ( ${c_{{{\text{Zn}}^{2+}}}}$ —0.3 g/dm³) proceeds more faster and effectively.     

Characterizations of ferromagnetic Zn1−xCoxO thin films grown on Al2O3 (0001) by reactive radio-frequency magnetron sputtering coupled with post-growth annealing

High-quality ferromagnetic Zn_1−xCo_xO thin films were deposited on a sapphire (0001) substrate at 600 °C by using reactive radio-frequency magnetron sputtering coupled with post-annealing treatment for 1 h at 580 °C under an Ar atmosphere. High resolution X-ray diffraction patterns show that hexagonal wurzite crystal structures of undoped ZnO film were maintained even after Co doping up to 4.5 at.% without forming Co clusters or oxides. X-ray photoelectron spectroscopy spectra represent the energy difference of 15.42 eV between Co2p_3/2 and Co2p_1/2, which is different from 15.05 eV of Co clusters. The characteristic absorption bands near 658, 616, and 568 nm wavelengths out of UV-VIS-IR spectroscopy spectra are correlated with the d-d transitions of tetrahedrally coordinated Co²⁺ ions. The low temperature photoluminescence spectrum for undoped ZnO shows a strong near-band edge (NBE) emission peak of 3.42 eV without deep level emission peaks. But, Co content increases in Zn_1−xCo_xO film, the NBE emission peak intensity decreases and another emission peak at 3.37 eV as well as a broad green emission peak at around 2.5 eV starts to appear with larger intensity due to the more actively creating oxygen vacancies. The emission peak at 3.37 eV proves the interaction between Co ions and the hydrogenic electrons in the impurity band and also supports the typical ferromagnetic hysteresis curves obtained by superconducting quantum interface device magnetometry at 300 K for Zn_1−xCo_xO films. High insulator characteristics are observed for as-grown Zn_1−xCo_xO films whereas it exhibits n-type characteristics with the increased carrier concentration, mobility, and resistivity after post-growth annealing. The spintronic devices could be fabricated with the utilization of Zn_1−xCo_xO films grown by the economically feasible reactive radio-frequency magnetron sputtering coupled with the post annealing treatment.     

Microwave dielectric properties of MgAl2O4–CoAl2O4 spinel compounds prepared by reaction-sintering process

MgAl₂O₄ spinel exhibits fascinating microwave dielectric properties, but the synthesis of dense MgAl₂O₄ ceramics requires high firing temperatures. In this study, Co is introduced into MgAl₂O₄ ceramics to improve their sinterability and microwave dielectric properties. An Mg_1−xCo_xAl₂O₄ solid solution of a spinel structure was observed in the MgAl₂O₄–CoAl₂O₄ system, and dense Mg_1−xCo_xAl₂O₄ ceramics were obtained by sintering at 1475–1500 °C in air for 2–6 h. Co addition is effective in lowering the sintering temperature to 1475 °C. Q × f of Mg_1−xCo_xAl₂O₄ ceramics was increased to 49,300 GHz with an increase in Co content to 0.2, but degraded with a further increase in Co content. The temperature coefficient of resonant frequency of Mg_1−xCo_xAl₂O₄ ceramics was sustained at between −73 and −23 ppm/°C to the variation of Co content.     

Synthesis and electrochemical characterization of LixCoO2 for lithium-ion batteries

The cathode material Li_xCoO₂ was synthesized by preliminary mechanochemical activation of precursor oxide mixture powders, followed by thermal treatment at 800 °C for 5 h. The effects of the molar ratio of Li/Co on the electrochemical behavior were examined. The Li_xCoO₂ at Li/Co=1.07 showed superior cycling stability to the Li/Co=1.0 sample. This is attributed to the disappearance of a phase transition related to monoclinic distortion and the relatively lowered transport resistance in Li/Co=1.07 sample.     

Electrical properties of exchange-enhanced systems: Co in (Pd95Rh5)

The electrical resistivity of several (Pd₉₅Rh₅)-based alloys containing between 1.1 and 2.6 at % Co has been measured up to 300K. In alloys containing between 1.4 and 2.6 at % Co the incremental resistivity δ?(T) is found to contain a Tsu2 term at low temperatures, as previously found for comparable amounts of Fe in the same host, and from which it is inferred that the restoration or removal of wave vector conservation for electron-magnon scattering in this type of system is not a mean-free-path effect. Estimates of the acoustic spin-wave stiffness are derived from the measured Tsu2 terms. These resistivity data also enable the magnetic ordering temperatures T _c and the s-electron- and d-electron-local moment exchange couplings J _s-local and J _d-local to be estimated. Differences in the behavior of the incremental resistivity δ?(T) in the paramagnetic regime of (Pd₉₅Rh₅)Fe and (Pd₉₅Rh₅)Co systems are tentatively discussed in terms of the possibility of localized spin fluctuations at the Co sites.     

Structural, Electrical, and Magnetic Properties of the Co-Substituted Bi-2212 System Textured by Laser Floating Zone Technique

In this work, Bi₂Sr₂CaCu_2?x Co _x O _y (x=0.0, 0.05, 0.10, and 0.25) textured superconductors were prepared by a LFZ melting technique. In all cases, the powder X-ray diffraction patterns of samples show that the Bi-2212 phase is the major one. All samples have good oriented structure, which is a typical picture for superconductors prepared by the LFZ method. Magnetization hysteresis loops, made for all samples at two different temperatures, showed that the loops become narrower with increasing temperature and doping levels. In addition, the effect of Co doping on the critical current density, J _c, of Bi₂Sr₂CaCu_2?x Co _x O _y has been estimated from hysteresis loop measurement by using Bean’s model. The increase of the Co amount in the Bi₂Sr₂CaCu_2?x Co _x O _y structure significantly decreases the critical current density, showing worse connectivity of the grains. All the results indicate that Co substitution for Cu produces the deterioration on the superconducting properties, compared with the undoped samples.     

Synthesis and characterization of ferromagnetic cobalt-doped tin dioxide thin films

Co-doped SnO₂ thin films are grown on sapphire (0001) substrates at 600 °C by the technique of dual-beam pulsed laser deposition. The prepared films show preferred orientation in the [100] direction of the rutile structure of SnO₂. Nonequilibrium film growth process results in doping Co into SnO₂ much above the thermal equilibrium limit. A Film with 3% of Co is ferromagnetic at room temperature with a remanent magnetization of ∼ 26% and a coercivity of ∼ 9.0 mT. As Co doping content x increases, the optical band gap absorption edge (E₀) of the Co-doped SnO₂ thin films initially shows a redshift at low x up to x = 0.12 and then increases at the higher x, which are attributed to the sp-d exchange interactions and alloying effects, respectively.     

First-Principles Study of Structural,Electronic, Magnetic and Half-Metallic Properties of the Heusler Alloys Ti<Subscript>2</Subscript>ZAl (Z = Co,Fe, Mn)

Using the first-principles calculations based on density functional theory within the generalized gradient approximation (GGA), we investigate the structural, electronic and magnetic properties of the Ti₂ZAl (Z = Co, Fe, Mn) alloys with the CuHg₂Ti-type structure. The optimized equilibrium lattice constants were found to be 6.08 Å for Ti₂CoAl, 6.07 Å for Ti₂FeAl and 6.16 Å for Ti₂MnAl. The Ti₂ZAl (Z = Co, Fe, Mn) alloys are found to be half-metallic ferromagnets. The total magnetic moment of Ti₂ZAl (Z = Co, Fe, Mn) is 2, 1 and 0 µ _B, respectively, which is in agreement with the Slater–Pauling rule M _tot=Z _tot- 18. The Ti₂ZAl (Z = Co, Fe, Mn) have a band gap of 0.64745, 0.57795 and 0.39327 eV, respectively.     

A novel route for the preparation of CoCr<Subscript>2</Subscript>O<Subscript>4</Subscript>/SiO<Subscript>2</Subscript> nanocomposite starting from Co(II)–Cr(III) carboxylate complex combinations

This study reports the preparation and characterization of the spinel CoCr₂O₄. In order to obtain 20% CoCr₂O₄/80% SiO₂ and 50% CoCr₂O₄/50% SiO₂ (mol%) nanocomposites, we have used a versatile pathway based on the thermal decomposition of some particular precursors, Co(II) and Co(III) carboxylate-type complex combinations, inside the SiO₂ matrix. The ligands of these coordination compounds result in the redox reaction between Co(II) and Cr(III) nitrates and 1,3-propylene glycol by heating at 150 °C of the gels (tetraethylorthosilicate–metal nitrates–1,3-propylene glycol). The as-obtained precursors, embedded in silica gels, were characterized by FT-IR spectrometry and thermal analysis. Both precursors decompose up to 350 °C, leading to the corresponding metal oxides inside the silica matrix. X-ray diffraction of the powders annealed at different temperatures has evidenced the formation of CoCr₂O₄ starting with 400 °C for 20% CoCr₂O₄/SiO₂ and 300 °C for 50% CoCr₂O₄/SiO₂. This behaviour can be explained by the fact that, by thermal decomposition of the chromium carboxylates, a nonstoichiometric chromium oxide Cr₂O_3+x is formed. At ~ 400 °C, Cr₂O_3+x turns to α-Cr₂O₃, which interacts with CoO leading to cobalt chromite nuclei inside the pores of the silica matrix. CoCr₂O₄ has been obtained as nanocrystallites homogenously dispersed within the silica matrix as resulted from XRD, TEM and EDX mapping, with mean particle size in the range 5–20 nm.     

Spectroscopic study of Zn1−xCoxO thin films showing intrinsic ferromagnetism

Dilute magnetic semiconductors are widely studied due to their potential applications in spin-resolved electronics. We report the direct evidences of intrinsic ferromagnetism in the primarily ferromagnetic ZnO:Co thin films using near-edge X-ray absorption fine structure (NEXAFS) and soft X-ray magnetic circular dichroism (XMCD). The single phase Zn_1−xCo_xO thin films with nominal compositions (0.00 ≤ x ≤ 0.15) were synthesized by a spray pyrolysis technique, which exhibit room temperature ferromagnetism as revealed by alternating gradient force magnetometer (AGFM) measurements. The spectroscopic measurements indicate that most of Co dopants have substituted for Zn sites in ZnO matrix and they are present in divalent Co²⁺ (d⁷) state with tetrahedral symmetry according to the atomic multiplet calculations. The O 1s NEXAFS spectra suggest strong hybridization between O 2p and Co 3d electrons within ZnO matrix. The Co 2p XMCD measurements rule out the magnetism due to the presence of Co clusters, and show that Co–O–Co bonding provides localized magnetic moments leading to ferromagnetism.     

The enhanced magneto-optical Kerr effect in Co/TiO2 multilayer films

Spectral dependences of the polarization plane rotation angle (??_k) in the polar Kerr effect in Co/TiO₂ multilayer nanocomposite films have been studied in the 400?C1000 nm wavelength range. It is established that the sign, magnitude, and shape of the magneto-optical spectrum depend on the dielectric spacer thickness and the number of layers in the structure. The Kerr rotation angle in Co/TiO₂ multilayers is significantly greater than that in homogeneous Co films of the same thickness. The angle of rotation of the polarization plane reaches a record high value of 2??_k = 7.3° in the Co(5 nm)/TiO₂(17 nm) multilayer structure with number of layers n = 8 at a wavelength of 540 nm.     

Structure, Phonon Vibration, and Spin Correlation of LaBa2Cu3−x Co x O y

Polycrystalline samples LaBa₂Cu_3?xCo _x O _y (0 ≤ x ≤ 1.0) were synthesized by solid state reaction method. The structure, phonon vibration, conduction, and spin correlation were investigated by means of X-ray diffraction, infrared spectra, resistivity, and electron spin resonance. It is found that there are orthorhombic–tetragonal and tetragonal–orthorhombic structural transitions with Co doping, and the conduction behavior changes from metallic to semiconducting. With the increase of Co content, the Cu(1)—O(1) phonon mode around 531 cm^?1 softens, the Cu(2)—O(2) phonon mode around 657 cm^?1 hardens, and the Cu(1)—O(4) mode around 583 cm^?1 is nearly unchanged. The Cu²⁺ spins tend to localize with Co doping. The changes in structure, phonon vibration, and spin correlation with Co doping are analyzed and discussed.     

Influence on the Tc of Co and F Co-doping into SmFeAsO at Fe and O Site

Series of superconductive samples in which Co and F co-doping into SmFeAsO were prepared by mechanical alloying (MA) and sequent sintering method to investigate the influence on Tc of the charge carrier introduced into SmO layers and FeAs layers simultaneously. For the SmFe_{1 ?} xCo _x AsO with optimum Co doping content of x= 0.1, F doping can still enhance the Tc until up to 23 K when doping content is 15 % (from our previous works, the highest Tc of SmFe_{1 ?} xCo _x AsO is 12.5 K). The charge carrier density of SmFe_0.9Co_0.1AsO_0.85 F _0.15 is larger than that of SmFe_0.9Co_0.1AsO, characterized by Hall coefficient measurement. On the other hand, for optimum F-doping SmFeAsO_0.8 F _0.2, simultaneous Co doping decreases the Tc dramatically. For the under-doping SmFeAsO_0.9 F _0.1, the Tc rises with the increasing Co doping content to 10 % and then falls down with more Co doping into the compounds. The results can offer some experimental evidence of the competition and restriction among the factors influencing the Tc of the SmFeAsO by doping Co and F simultaneously.