The characterization of NbAl2O3 and NbMgO interfaces in MBE grown NbMgONbAl2O3 multilayers

NbMgONbAl₂O₃ multilayer stacks were prepared by sequental deposition of Nb and MgO films on the (0112) (R) plane of sapphire, using Molecular Beam Epitaxy (MBE). Cross-sectional specimens of this multilayer were prepared and various interfaces investigated by high resolution transmission electron microscopy (HREM). Orientation relationships between Nb and Al₂O₃ and between Nb and MgO were determined and misfit dislocations, steps and facets at the NbAl₂O₃ and NbMgO interfaces were analyzed. The HREM observations show that, under electron beam irradiation, a modified layer forms at the NbAl₂O₃ interface by a solid state reaction. Electron beam-induced grain growth in the Nb film also takes place.     

The internal friction of the pearlitic,bainitic and martensitic transformations in FeNiC alloys

The low frequency internal friction of the pearlitic, bainitic and martensitic transformations in three FeNiC alloys has been studied. The results show that the internal friction of the martensitic transformation is in correspondence with the model based on the hysteresis loss mechanisms associated with stress-induced motion of the interface dislocations, while that of the pearlitic and bainitic transformations obeys Postnikov's model. At a constant frequency, the internal friction peaks of the pearlitic and bainitic transformations increase as the cooling rate increases, but the temperature of the internal friction peaks decreases, and the Q_max⁻¹ is directly proportional to the T/f_mT_m. When the frequency increases, the Q_max⁻¹ of the pearlitic and bainitic transformations lowers apparently, and the temperature of the internal friction peaks increases. The internal friction of the isothermal bainitic transformation in a FeNiC alloy has also been studied, which proves that there appears a damping peak in the incubation period, and which may be attributed to the nucleation of bainite. Consequently it infers that the mechanism of the bainite reaction seems similar to that of pearlite.     

Effects of cerium doping position on physicochemical properties and catalytic performance in methanol total oxidation

The physicochemical properties of Pd and PdPt catalysts which possess different Ce doping position were investigated by techniques of TEM, XRD, N₂ adsorption-desorption, XPS and FT-IR. The catalytic performance for methanol total oxidation was examined to study the effects of Ce adding position. CeO₂Al₂O₃TiO₂ (CAT) catalysts that Ce is directly introduced into support show higher reactivity and CO₂ selectivity than CeO₂/Al₂O₃TiO₂ (Ce/AT) samples in which Ce is loaded by impregnation method. The characterization results reveal that the Ce doping position does not cause obvious otherness of basic crystalline phase and mesoporous structure of support. However, the Ce doping position affects the pore shapes of support and then influences the pore diameter. CAT catalysts possess more abundant adsorbed oxygen and more Ti³⁺ can transform the more gaseous oxygen into the active oxygen species on the catalyst surface, which is beneficial to the reaction. The AlOTi bridges in CAT facilitate the cooperation of Al and Ti species, which further speeds up the reaction rate.     

Designing structural defects to releive thermal stress

Role of structural defects such as misfit dislocation and interfacial ledge in diffusion bonded metal-ceramic interface in relieving thermal stress both at high and intermediate temperatures was examined in NbAl₂O₃, NiSi₃N₄, AgBi₂Sr₂CaCu₂O₈ and other metal-ceramic systems. It is shown that chemical reaction plays important roles both in shaping the interface structures and making the structural defects to act in relieving thermal stress.     

Effect of Dy-substitution on structural and magnetic properties of MnZn ferrite nanoparticles

This paper describes the structural and magnetic properties of Dy-substituted Mn-Zn ferrite nanoparticles. Mn-Zn-Dy ferrite nanoparticles of the composition Mn_(0.5)Zn_(0.5)Dy_xFe_(2-x)O_4(x=0.05, 0.1,0.15 and 0.2) were synthesized by a facile chemical co-precipitation method. The samples were characterized through X-ray diffraction(XRD), transmission electron microscopy(TEM), Fourier transform infrared spectroscopy(FTIR), vibrating sample magnetometer(VSM) and Curie temperature. The XRD patterns confirm the synthesis of single crystalline phase of Mn-Zn-Dy ferrite nanoparticles. Lattice parameter increases with increase in Dy-substitution which confirms the replacement of Fe~(3+) ions by Dy~(3+) ions.Crystallite size is of the order of 6-8 nm for all these samples. The particle sizes observed from TEM analysis are in good agreement with the XRD values. The magnetic measurements show superparamagnetic nature of the samples. The saturation magnetization decreases with increase in Dyconcentration and can be correlated to modifications in the A-B exchange interactions as a result of the structural modifications due to Dy-substitution. The Curie temperature for Mn_(0.5)Zn_(0.5)Fe_2 O_4 nanoparticles is 124 ℃ and decreases up to 84 ℃ with the increase in the Dy-concentration. The decrease in Curie temperature can be attributed to the weakening of the superexchange interaction between A-site and B-site as a result of Dy-substitution. The low value of Curie temperature and higher value of thermomagnetic coefficient k_T shown by these samples makes them suitable for the preparation of temperature sensitive ferrofluid for heat transfer applications.     

Thermodynamic analysis of aluminate formation at Fe/Al2O3 and Cu/Al2O3 interfaces

Subsolidus ternary phase relationships in the systems FeAlO and CuAlO are reviewed in order to provide a basis for thermodynamic analysis of aluminate formation at Fe/α-Al₂O₃ and Cu/α-Al₂O₃ interfaces. Expressions for the critical oxygen activity in the Fe-rich and Cu-rich solid solutions necessary to stabilize the equilibrium aluminate phases (FeAl₂O₄ and CuAlO₂, respectively) in the presence of α-Al₂O₃ are derived using data available in the literature. As in previous work in the NiαAl₂O₃ system, aluminate formation by solid state reaction of Fe and Cu with α-Al₂O₃ was calculated to require a threshold concentration of oxygen in the metal, which is of the order of 1/5 the solubility limit. The sults are presented in stability diagrams and compared with previous results on the Ni/α-Al₂O₃ system in terms of the free energies of formation of the aluminates and the relative free energes of solution of oxygen in the metals. The results are also compared with available experimental observations on Fe/α-Al₂O₃ and Cu/α-Al₂O₃ interfaces.     

Reactions and phase relations in the TiAlO system

Reactions between titanium and alumina were studied experimentally for Al₂O₃ substrates with a titanium-based coating and for planar TiAl₂O₃ diffusion couples in the temperature range between 800 and 1100°C. Isothermal sections through the phase diagram were determined by using these results as well as by investigating equilibrated alloys. These experimental sections agree with those calculated from thermodynamic data. The morphology and layer thickness of the observed reactions zones have been explained on the basis of these phase relations, making use of diffusion data from binary systems TiO and TiAl. The important role of the initial Ti thickness on the type of reaction products has been demonstrated.     

Structure of AlAl and AlSi3N4 interfaces bonded at room temperature by means of the surface activation method

AlAl joints and AlSi₃N₄ joints were manufactured at room temperature by means of the surface activation method. In this procedure, the surfaces to be bonded are sputter-cleaned and activated by argon fast-atom-beam (FAB) irradiation and then brought into contact with each other under a slight pressure. The primary concept of the method is based on the idea that clean metal surfaces are inherently active and react with other elements such as oxygen, nitrogen and carbon, and thus form a strong bonding with ceramics even at room temperature. Therefore it was expected that the method will be effective only in an ultra high vacuum so that the sputtered surfaces remain clean until joining. It was found, however, that such a clean environment is not necessary for joining. Surfaces activated in a vacuum containing some residual gas can be bonded very strongly and do not lose the capacity of adhering even after exposure to the residual gases after sputt-cleaning. It is found by transmission electron microscopy that the interface structure depends remarkably upon the vacuum condition in which the surfaces to be bonded are sputter-cleaned. An intermediate layer consisting of a partially amorphous phase was found in AlAl joints bonded under a vacuum pressure of 10⁻⁵Pa, while an AlAl joint without an intermediate layer was formed by ultra high vacuum bonding. It is assumed that oxygen or water in the residual gas of the vacuum was absorbed on the Al surface and dissolved into metal by the argon irradiation. Thus, an amorphous AlO solid solution or a kind of aluminium oxide is formed. AlSi₃N₄ joints manufactured in a high vacuum from surface activated materials showed also an intermediate layer which was, however, entirely amorphous over the whole interface and was considered to have been formed predominantly on the Si₃N₄ surface.     

First-principles study of short range order and instabilities in AuCu,AuAg and AuPd alloys

Interaction energies derived from first-principles total energy calculation are used together with the Cluster Variation Method in order to study phase diagrams, ordering instabilities and the Fourier spectrum of the Warren-Cowley short range order parameters for AuCu, AuAg and AuPd alloys. The present study is carried out using both the tetrahedron and the tetrahedron-octahedron approximations of the CVM. It is shown that the tetrahedron approximation predicts the wrong Fourier spectrum for the SRO parameters although it gives qualitatively correct phase diagrams. In particular, the tetrahedron approximation fails to reproduce the expected maxima in the fluctuation spectrum at the 〈1,0,0〉-type reciprocal space vectors, characteristics of L1₀ and L1₂ ordering systems. The correct features of the k-space fluctuation spectrum is obtained in the tetrahedron-octahedron approximation.     

The bonding charge density of β′NiAl

The (100), (110), (111) and (200) low-angle structure factors of β′NiAl have been accurately measured by high energy electron diffraction (HEED) and it was found that, except for the (200) reflection, these agreed closely with the values deduced by interpolation between the best pure element crystal atomic scattering (form) factors; the (200) structure factor was found to be about 2% lower than the best equivalent pure element value. The trend of these results is somewhat similar to those obtained from powder X-ray diffraction measurements made on β′NiAl over 25 years ago and indicates that there is no ionic component in the bonding scheme for this alloy. This was confirmed by the generation of deformation electron density distributions (DEDDS) from the structure factor data which showed that bonding is this alloy is achieved by the depletion of electrons from both the nickel and aluminum atom sites with extensive charge build-up at the mid-points between nearest-neighbor (n.n.) NiAl atoms (〈111〉 directions). This shows that bonding in this intermetallic compound is essentially covalent with some metallic character and no ionic component. The DEDDs also showed a large depletion of electrons between second n.n. AlAl atoms (〈100〉 directions) which also occurred, but to a lesser extent second n.n. NiNi atoms. This suggests that as well as the atomic size difference, there are electronic reasons why n.n. and second n.n. AlAl bonds are not favored in this B2 alloy type. This explains why β′FeAl, β′CoAl and β′NiAl are usually fully ordered and do not easily tolerate Al antistructure atoms on their transition metal sublattices. The build-up of electrons between n.n. atoms along 〈111〉 and depletion between second n.n. NiNi and AlAl atoms along 〈100〉 also gives indications of the electronic origins of the ω phase (collapse of atoms along 〈111〉 directions towards vacancy or antistructure atom defects) and the elastic softening associated with shear along [011] on (011), which occurs in β′NiAl containing more than about 53 at.% Ni.     

On the creep behavior of the Ti3Al titanium aluminide Ti25Al10Nb3V1Mo

The temperature and stress dependence of the steady-state creep behavior of the Ti₃Al alloy Ti25Al10Nb3V1Mo (at.%) has been evaluated. Two microstructural conditions were evaluated as follows: As processed (rolled) consisting of the fine grained (approx. 6–10 μm) β plus ordered α₂ phase and beta heat treated consisting of coarse grained (approx. 150 μm) retained ordered B2 phase with a fine Widmanstatten structure within the grain interiors. The steady-state creep behavior of both microstructural conditions was studied over the temperature range of 650–815°C. The apparent creep activation energies and stress exponents were measured for both microstructural conditions. The temperature and stress dependence of the steady-state creep rate of both microstructures can be described well by the power law creep equation suggesting dislocation motion as the operative deformation mechanism. Over the temperature-stress regime of the present study, the creep deformation of the fine grained microstructure possibly breaks down into a low temperature (dislocation core diffusion controlled) regime and a high temperature (bulk diffusion controlled) regime within the power law creep region as indicated by the apparent creep activation energies measured. Upon β heat treatment, creep deformation is found to be governed by a single rate limiting process. At temperatures and stress levels where a direct comparison can be made, the steady-state creep rates of the β heat treated Ti-25-10-3-1 exhibit an order of magnitude decrease over those of the processed material. This suggests the possibility of some mechanism other than power law creep controlling within the regime corresponding to the low apparent activation energy of the fine grained microstructure.     

Ultra high vacuum diffusion bonded NbAl2O3 and CuAl2O3 joints—The role of welding temperature and sputter cleaning

Polycrystalline niobium and copper are welded in UHV (2 × 10⁻¹⁰mbar) to alumina (99.7 wt%) at various temperatures. Prior to joining the surfaces of the metal and ceramic to be welded are sputter-cleaned by argon ions at 3–5 keV with a maximum dose of 5 × 10¹⁹Ar⁺/cm². The amount of bonded area at the interface depend on welding temperature and welding time. After 1 h welding time the fraction of bonded area is 98% for NbAl₂O₃ joints at 0.65T_m (T_m = melting point of the metal in K). The amount of unbonded regions at the interface of CuAl₂O₃ joints decrease from 20% after 1 h welding time to 5% after 3 h welding time at 1010°C. Plastic flow and creep determine the pre closure mechanism at the metal-ceramic interface. Fracture energy G_c and the fracture resistance K_c of the UHV bonded metal-ceramic joints depend strongly on welding temperature and the amount of bonded area. For CuAl₂O₃ joints sputter-cleaning is a prerequisite for reliable measurements of the fracture energy. Without sputter-cleaning most of these joints did not withstand the cutting procedure during fabrication of four-point bend test beams. The fracture energy of NbAl₂O₃ joints manufactured without sputter-cleaning is low and can only be determined for joints welded above 1500°C. Impurities at the metal-ceramic interface are assumed to be responsible for the decrease in bond strength of joints manufactured without sputter-cleaning.     

The influence of Mn dispersoid content and stress state on ductile fracture of 2134 type Al alloys

Ductile fracture studies have been conducted on four high purity AlCuMgZr (2134 type) alloys containing 0, 0.31. 0.61 and 1.02 wt% Mn in the under and overaged conditions having similar yield strengths. The second phase particle content, i.e, Mn rich dispersoids and Mn containing large particles, increased with increasing Mn content. In both aging conditions maximum ductility and toughness were observed for the 0.31% Mn alloy and minimum values were observed for the 1.02% Mn alloy, The largest void content or damage accumulation due to void nucleation and growth at any strain level occurred in the 1.02% Mn alloy, consistent with ductility values. The 0.31% Mn alloy showed the highest ductility in both aging conditions, Although the void volume fractions for the 0.31% Mn alloy were similar to those of the 1.02% Mn alloy, accumulation occurred at higher strains. The void nucleation and growth data and microstructural analysis suggest that the 0.31% Mn additions provide sufficient submicrometer Mn-dispersoids to homogenize slip without producing large Mn-rich primary particles which decrease ductility. Ductility was observed to decrease with increasing triaxial constraint which increased void volume fraction and void growth rates. However, the degree of triaxiality had little or no effect on the nucleation rate of voids.     

Bonding at metal-ceramic interfaces; AB Initio density-functional calculations for Ti and Ag on MgO

We have performed ab initio local density-functional calculations of the equilibrium geometries, force constants, interface energies and works of adhesion for lattice-matched interfaces between rocksalt-structured MgO and a f.c.c. transition or noble metal (M=Ti or Ag). The interfaces had (001)_M 6(001)_MgO and [100]_M 6 [100]_MgO. We used the full-potential LMTO method. Both Ti and Ag are found to bind on top of oxygen. The interface force constants are 3–4 times larger for Ti|MgO than for Ag|MgO. These, as well as the M-O distances (2.18A˚for TiO and 2.5A˚for AgO), indicate that the TiO bonding is predominantly covalent with the Ti oxidation state less than +1, and that the Ag-bonding is predominantly ionic, as in Ag₂O, with the Ag oxidation state close to +1. The calculated interface energies are both 0.8 eV/M|MgO and the interface adhesions are, rspectively, 1.2 eV/Ti|MgO and 0.9 eV/Ag|MgO. The calculated (strained) surface energies are, respectively, 1.1 eV/Ti|, 0.7 eV/Ag| and 1.0 eV/MgO|.     

Defects in Ni and Co silicide: Si interfaces

Silicide formation has been studied by deposition of Ni and Co on (001) and (111) Si substrates, followed by annealing. NiSi₂ and CoSi₂, exhibiting both the A (parallel lattices) and B (twin related lattices) type epitaxial relationships, and also CoSi (lattices rotated by 30° about a coincident [111] direction), have been investigated. Observations, using transmission electron microscopy, have been interpreted in the framework of a topological theory of interfacial discontinuities, and extensive agreement has been found between theory and experiment. In NiSi₂Si A-type specimens, crystal dislocations were observed to accomodate misfit, although this was not completely relieved in general. In addition, irregular arrangements of dislocations with b=1/4<111> were observed on (001) interfaces and at the intersection of obtuse pairs of {111} facets. (001) interfaces were frequently found to facet on {111} planes, sometimes on a fine scale with facet spacings as small as 6 nm. NiSi₂Si and CoSi₂Si (111) type B specimens exhibit arrays of interfacial dislocations with b=1/6<211>, partially accommodating misfit. On theoretical grounds, these dislocations are thought to exhibit interfacial steps at their cores. Direct confirmation of this was obtained in plan view ultra-thin specimens by using a contrast technique based on a method for revealing steps on crystal surfaces. Arrays of interfacial dislocations were also observed in CoSiSi (111) specimens. The components of their Burgers vector parallel to the interface were determined to be close to 1/3<110>, in agreement with theory. Possible evidence of disclination formation was also found in these samples.     

A comparative study of physicochemical and photocatalytic properties of visible light responsive Fe,Gd and P single and tri-doped TiO2 nanomaterials

High performance Fe-Gd-P tri-doped TiO_2 nanoparticles(1 at% for each dopant) were successfully synthesized by a modified sol-gel method. Various analytical and spectroscopic techniques were carried out to determine the physicochemical properties of the prepared samples, including XRD, EDX, FESEM,BET, FTIR, XPS, PL, EIS and UV-Vis diffuse reflectance spectroscopy. The photocatalytic activities of prepared samples were evaluated by photo degradation of methyl orange(MO) and 4-chlorophenol(4-CP) as model pollutants under visible light irradiation. Effects of each dopant on different properties of TiO_2 nanoparticles were investigated. Results show that Gd and P doping enhances TiO2 surface textural properties by forming Ti-O-Gd and Ti-O-P bonds. It is found that Gd plays a superior role in increasing oxygen vacancies and organic species on TiO_2 surface. Gd doping also facilitates transferring of the photo-induced charge carriers to the surface adsorbed species. The enhanced electronic band structure and visible light response, as well as high electron lifetime of Fe-Gd-P tri-doped sample is mainly attributed to Fe and Gd doping. The tri-doped TiO_2 with rate constant of k_(app)= 1.28 × 10~(-2) min~(-1) for MO and k_(app) = 0.94 × 10~(-2) min~(-1) for 4-CP, shows the highest photodegradation rate among all samples including undoped and single doped samples. The improved photocatalytic performance of Fe-Gd-P tridoped TiO_2 is due to the synergistic effect of enhanced surface chemistry and textural properties,increased number of surface adsorbed hydroxyl groups and organic species, improved visible light absorption, increased lifetime of the photo-induced electron/hole pairs and boosted interfacial charge transfer.     

Ordering,ternary atom location and ageing in Ll2 trialuminide alloys

X-ray diffraction and high resolution electron microscope techniques have been used to examine the degree of order, atom site location, and precipitation in ternary titanium trialuminide alloys. For some of the alloys studied it is shown that precipitation of Al₂Ti occurs, essentially because of an excessively high Ti content. For all the alloys there are subtle changes occurring on annealing at temperatures of 600–800°C which affect both hardness and toughness of the materials. In an AlTiFe alloy there are signs of incipient, fine-scale precipitation, probably of Al₂Ti. In all these intermetallics there is partial chemical disorder, which is taken to suggest an inherent structural tendency to tetragonal atomic arrangement, even though the crystal retains the ordered Ll₂ cubic structure. The improved ductility of some of these intermetallics may follow from the reduced order obtained as a result of alloying.     

Reactions in the systems MoSi3N4 and NiSi3N4

The reaction products, formed during annealing of porous powder mixtures of Si₃N₄ with non-nitride forming metals like Ni or Mo, will depend on the partial pressure of N₂ in the atmosphere. In a diffusion couple, however, nitrogen has to be released at the Si₃N₄-interface during the formation of a metal silicide. It cannot escape easily and builds up a higher pressure of nitrogen at this interface. Therefore, the reaction products are different from those in porous pellets. This has been verified for NiSi₃N₄ and MoSi₃N₄ couples. The role of traces of oxygen on these reactions will be discussed.     

Structural properties and catalytic performance of the LaCuZn mixed oxides for CO2 hydrogenation to methanol

A series of La-Cu-Zn-O mixed oxide catalysts were synthesized by a co-precipitation method and calcined under different temperatures. The XRD, BET, TPR, N_2 O-adsorption, XPS, SEM and TPD techniques were carried out to measure the aimed catalysts. The results indicated that the chemical environment of lanthanum element changes with the increase of calcination temperature. The La_2 CuO_4 perovskite structure is obtained at the temperature higher than 823 K and the special copper species appear in the perovskites due to the special structure property. The catalysts with La_2 CuO_4 perovskite structure show higher methanol selectivity compared with the mixed copper catalyst. For the perovskite catalysts, the conversion of CO_2 changes with the same tendency of the copper species ratio((Cu~(α+)+Cu~0)/(Cu_(Total))%), which implied both Cu~(α+) and Cu~0 are important active sites in the perovskite catalyst for the reaction.     

Sputter deposited barrier coatings on SiC monofilaments for use in reactive metallic matrices—III. Microstructural stability in composites based on magnesium and titanium

An examination has been made of the chemical stability of SiC monofilaments, with and without sputtered coatings intended to produce diffusion barrier layers of Y₂O₃, in contact with matrices of MgLi alloy (up to 400°C) and Ti (up to 1000°C). Even very thin layers were found to offer some protection in the MgLi alloy, under conditions such that the uncoated fibres suffered catastrophic embrittlement by penetration of Li into the grain boundaries. Yttrium-coated fibres in a Ti matrix were found to exhibit only marginally improved stability when compared with uncoated fibres. The probable explanation for this has been identified as a tendency for Y to penetrate into the SiC fibre before a stable Y₂O₃ layer could form, although high hydrogen levels in the Ti matrix (absorbed during composite fabrication) may also have impaired the interfacial stability in much of the material examined. Fibre preoxidation prior to Y coating was found to inhibit this Y penetration into the fibre material, allowing a Y₂O₃ barrier layer to form in situ. This barrier layer has been shown to offer considerable fibre protection.