Effects of Ti thickness on microstructure and mechanical properties of alumina–Kovar joints brazed with Ag–Pd/Ti filler

Alumina, metalized with Ti by magnetron sputtering, has been successfully bonded to a Kovar alloy using Ag–5 wt% Pd filler. The effects of Ti content on the interfacial microstructure and mechanical properties were investigated. The results showed that a reaction layer was formed at the alumina/filler interface. Microanalysis indentified the reaction products to be titanium oxide and Pd–Ti compounds. The thickness of the reaction layer and the residual filler layer played determinable roles on the joint strength. The maximum four-point flexural strength of the brazed joints at room temperature reached as high as 177.3 MPa when the thickness of Ti layer was 3 μm.     

Gas absorption with or without chemical reaction in laminar non-newtonian falling liquid films

An analytical solution is presented for gas absorption with or without a first-order or zero-order chemical reaction in a laminar non-Newtonian power-l model falling liquid film. For physical absorption, the first ten eigenvalues, series coefficients and related quantities are computed accurately by a quasinumerical method which shows considerable improvement over previous investigations. The range of applicability of the penetration theory solution is also established to indicate in what regions will the finite film thickness and complete velocity profile be important in determining the absorption rate. It is found that the range of dimensionless axial contact length X* in which the penetration theory is valid diminishes rapidly with increasi values of the power-law index n. For chemical absorption, the solution can be obtained by a linear superposition principle in terms of a “transie part” in which the effect of hydrodynamics within the liquid film is of importance and a “steady part” in which the reaction rate is controlling. In the “transient part” solution, the first ten eigenvalues and related quantities are reported for a variety of values of n and the dimensionl reaction rate parameter k_l* or k₀*. Certain asymptotic solutions from the penetration theory are also given and their range of applicab estimated. For any given n, it is estimated that only when k₁* or k₀* is less than approximately 10 will the finite film thickness and velocity profile have any effect on the absorption rate as compared to that calculated from the penetration theory with chemical reaction. The non-Newt character of the liquid film also has a significant influence on the absorption rate. At a fixed X*, the absorption enhancement due to reaction is when n = ∞ and is smallest when n = 0. The solutions obtained in this work are useful either for predicting absorption rates or for deter molecular diffusivity (and reaction rate constant) of gases in non-Newtonian falling liquid films.     

Microstructural evolution in MgOp/AlN composites

MgO_p/AlN composite has been fabricated by directed melt nitridation of pure Al block covered with a powder mixture of 0.5–1 mm magnesia particles and 0.075–0.15 mm chemically pure magnesium powder in flowing N₂ in the range of 900–1200 °C. The extent of Al nitridation and the depth of Al penetration into the MgO particles increases with temperature. The phase composition in the matrix from metal rich to ceramic rich can be adjusted by controlling processing temperature. A multilayer microstructure of MgO/MgAl₂O₄/AlN surrounds the MgO particles due to the interface reaction. The thickness of each layer in this structure varies with processing parameters such as the temperature and local Mg concentration, depending on the influence of these processing parameters on the interface reaction of MgAl₂O₄ formation and Al nitridation.     

Morphological studies of randomized dispersion magnetite nanoclusters coated with silica

In this study, we report a simple way to produce randomized dispersion magnetite nanoclusters coated with silica (RDMNS) via Stöber process with minor modifications. The morphology of silica coated magnetite nanoclusters was emphasized by studying various reaction parameters including alcohols with different polarities as co-solvents, concentration of alcohol-water, concentration of alkaline catalyst (ammonia), and concentration of TEOS monomer. The results of transmission electron microscope (TEM) showed that the sizes and morphological behaviour of the magnetite nanoclusters vary accordingly with the different reaction parameters investigated. The results showed that ethanol would be the best candidate as co-solvent in the preparation of randomized dispersion magnetite nanoclusters. Besides, the optimum alcohol-water ratio has been determined to be 70-30% v/v as this concentration range could render desired shape of randomized dispersion magnetite nanoclusters. The volume of ammonia (NH₃) catalyst in the reaction media also strongly governs the formation of silica coated magnetite nanoclusters in a desired shape. Apart from that, the addition of TEOS monomer into the reaction media has to be well-controlled as the excess amount of monomer added might affect the thickness of the silica layer that is coated on the magnetite nanoparticles. Prior to silica coating, the bare magnetite nanoparticles were first treated with trisodium citrate (0.5 M) to enhance the particles’ dispersibility. Improvement in the size distribution and dispersibility of the magnetite nanoparticles after the citrate treatment has been examined using TEM. The XRD results show that the magnetite samples retained good crystallinity although they have been surface-modified with citrate group and silica. The Fourier transform infrared (FT-IR) and thermogravimetric analysis (TGA) prove that the magnetite nanoparticles have been successfully coated with citrate and silica. The superparamagnetic behaviour of the magnetite samples was confirmed by VSM. The produced silica coated magnetite nanoclusters possess great potential to be applied in bio-medical research and clinical diagnosis application.     

Spatiotemporal patterns in a porous catalyst during light-off and quenching of carbon monoxide oxidation

A detailed numerical model was used to simulate the behavior of carbon monoxide oxidation within a porous platinum/alumina catalyst during temperature ramps. The model was validated in previous work by fitting step-response experiments which were performed over a range of temperatures and in which concentration gradients over the catalyst layer were directly measured. As a result of the low CO and O₂ concentrations used, the catalyst layer could be considered isothermal. The numerical experiments performed with the model in this work reveal complex spatial patterns of species and local reaction rate which change with time and temperature.As temperature is increased, CO desorbs and reaction rapidly increases, reacting adsorbed CO off the Pt surface and producing a peak in CO₂ production during catalyst light-off. Over a nonporous surface of the same material, the reaction rate would be an order-of-magnitude lower and no CO₂ peak would be produced. At steady state after reaction light-off has been obtained, reaction occurs in a narrow zone below the external face of the layer which is exposed to the constant feed gas composition. As temperature is then decreased, the CO₂ production rate decreases gradually as the front of the region covered with adsorbed CO penetrates further and pushes the reaction zone deeper into the catalyst layer. When the adsorbed CO front reaches the internal face, the CO₂ production rate drops abruptly as the reaction “quenches”.Catalyst layer thickness was changed over the range 0.06-1.0 mm at constant total Pt content. As the layer thickness was decreased, the steady-state CO₂ production rate after light-off increased, however the range of temperatures in which the catalyst was active decreased. Three qualitatively different sets of spatiotemporal patterns were obtained as the layer thickness was changed from relatively thin, to medium, to thick. Analysis of the patterns provides understanding of the temperature-dependent behavior of the catalyst and how this behavior varies with catalyst layer thickness.     

Low-temperature growth of lead magnesium niobate thick films by a hydrothermal process

Pyrochlore free PbMg_1/3Nb_2/3O₃ (PMN) thick films were prepared by using a hydrothermal process from oxide-based suspension precursors. PMN thick films have been grown on titanium substrates at 150 °C for 8 h. The effect of processing parameters such as reaction durations and mineralization conditions on perovskite phase formation on the microstructures is very critical. By controlling the operating parameters, high quality PMN thick films on titanium substrates were fabricated. The films with a single perovskite structure were smooth and the surfaces were free of micrometer scale cracks. The thickness of the film was about 20 μm. Dielectric responses of the PMN films were characterized in detail. The samples showed excellent reproducibility in the measurement of dielectric responses.     

Oxidation and defect control of CVD SiC coating on three-dimensional C/SiC composites

Two kinds of multi-layer CVD SiC coatings were prepared on a three-dimensional C/SiC composite. Oxidation behavior of the coating and the composite were studied and the effect of defects in the coating on its oxidation protection property were investigated. Above 1200 °C, thickness of the oxide film formed on the coating was related to oxidation time by the Fick’s first law X(t)²=Bt, the diffusion rate constant increased with oxidation temperature according to the Arrhenius’ relation ln B=−32?483/T+1.4048. Morphology of the interface between the CVD SiC and its oxide film was different after oxidation at temperatures from 1200 to 1500 °C. It was interpreted by consideration of the interfacial stress produced by thermal expansion mismatch and the CO gas pressure produced by interfacial reaction.     

Chemical control on the size and properties of nano NiFe2O4 synthesized by sol–gel autocombustion method

Nickel ferrite nanoparticles have been synthesized by sol–gel auto combustion route. The significant role played by nitric acid added to the precursor solution in controlling the reaction rate phase purity, crystallinity, crystallite size, thermal and magnetic properties of nanoparticles was explored and reported. Also, the influence of annealing on the properties were studied. Samples of average crystallite size ranging from 10 nm to 40 nm have been obtained by controlling the HNO₃ concentration and by increasing the annealing temperature. The size-dependent structural, thermal and magnetic properties were investigated and reported. The Hopkinson peak was observed for all the crystalline samples near the Curie temperature. The highest value 47.3 emu/g of saturation magnetization was obtained for the sample prepared with higher concentration (6 mol/L) of HNO₃.     

Electrochemical corrosion and materials properties of reactively sputtered TiN/TiAlN multilayer coatings

TiN/TiAlN multilayers of 2 μm thickness were successfully prepared by reactive DC magnetron sputtering method. XRD pattern showed the (1 1 1) preferential orientation for both TiN and TiAlN layers. XPS characterization showed the presence of different phases like TiN, TiO₂, TiON, AlN and Al₂O₃. Cross sectional TEM indicated the columnar growth of the coatings. The average RMS roughness value of 4.8 nm was observed from AFM analysis. TiN/TiAlN coating showed lower friction coefficient and lower wear rate than single layer coatings. The results of electrochemical experiments indicated that a TiN/TiAlN multilayer coating has superior corrosion resistance in 3.5% NaCl solution.     

Studies on slip casting behavior of lanthanum strontium manganite

Dispersion conditions for slip (slurry) formulation of a powder mixture of lanthanum strontium manganite (La_0.84Sr_0.16MnO₃ - LSM) and carbon (pore former) in water was studied through detailed zeta-potential and rheological measurements. The zeta potential variation with pH for the aqueous suspensions of only LSM or carbon exhibited a maximum value in alkaline medium (−40 mV to −50 mV at a pH of 10-11), establishing the pH window for their co-dispersion for slurry formulation. A study of the viscosity variation with shear rate for the slurries with varying solid content (in the range of 45-65 wt.%) exhibited pseudo-plastic flow behavior, indicating presence of flocculates in them. The yield stress values obtained from the Casson equation reduced with decreasing solid content, indicating reduction in the flocculate strength. The slip with solid content of 50 wt.% exhibited optimum flow characteristics to form long tubes with uniform wall thickness (wall thickness 2-4 mm and length of 150-200 mm). The tubular specimens formed after controlled carbon burn out and sintering at 1400 °C for 1 h possessed about 35% open porosity. The porosity remained the same upon further sintering at 1400 °C for 8 h.     

Preparation of a titanium carbide coating on carbon fibre using a molten salt method

A method for preparing protective titanium carbide (TiC) coatings on carbon fibres has been developed using a molten salt synthesis method. The TiC coatings were formed on the surface of carbon fibres in a reaction medium consisting of Ti powder in a mixture of molten LiCl-KCl-KF salts under an argon atmosphere at 900 and 950 °C. The structure and morphology of the TiC coatings were characterized by XRD, SEM and energy dispersive X-ray (EDX) analyses. The coatings consisted of homogeneous single phase cubic TiC with thicknesses in the range of 60-800 nm. Variation of the synthesis time and reaction mixture was found to significantly affect the thickness and integrity of the TiC coating although variation of the reaction temperature had little effect. The coating thickness was closely related to the composition of the molten salts and to the molar ratio between the carbon fibre and titanium.     

Structural characterization of bulk ZrTiO4 and its potential for thermal shock applications

Zirconium titanate, ZrTiO₄, is a well known compound in the field of electroceramics. Furthermore, it shows a large potential as structural material for thermal shock resistance applications, since it presents crystallographic anisotropy in thermal expansion. However, there is no information in the current literature about its thermomechanical behaviour. In this work, single phase zirconium titanate bulk materials have been prepared from well dispersed ZrO₂ and TiO₂ mixed suspensions, combining reaction and conventional sintering processes. The crystal structures of ZrTiO₄ have been studied by the Rietveld method for bulk samples. The structural evolution upon the cooling rate has been unravel, as the b-axis strongly decreases for slow cooled samples when compared to quenched materials. For the first time apparent Young's modulus (≈130 GPa) and Vickers hardness (≈8 GPa) values of a fully dense single phase zirconium titanate material have been evaluated and its potential for thermal shock applications has been analysed in comparison with other thermal shock resistant materials.     

Joining of mullite ceramics with yttrium aluminosilicate glass interlayers

Pellets of yttrium aluminosilicate glass (Y₂O₃:Al₂O₃:SiO₂ = 30:20:50 mol%) powder were used as the filler interlayers (0.4 mm thick) to join two mullite substrates. The glass interlayer partially melted at joining temperature to bond the substrates and then crystallized during cooling to have better bonding strength. The results showed that joining could be performed at 1390–1420 °C for 1–5 h with applied pressure of 0.02 MPa. After joining, the thickness of glass layers varied between 250 μm and 80 μm, depending upon the temperatures. The glass interlayer crystallized into cristobalite, mullite and Y₂Si₂O₇. When joining mullite/3 mol%yttria–zirconia substrates using the same glass pellet, a layer of zircon/mullite was formed at the interface, indicating that reaction occurred between glass and substrates. The formation of zircon usually accompanied with cracks in the substrates. These cracks deteriorated the strength. The achievable three-point bending strengths were 139 MPa for joined mullite and 76 MPa for joined mullite/3 mol%yttria–zirconia.     

Kinetics of Ceramic-Metal Composite Formation by Reactive Metal Penetration

The rate of composite formation via reactive metal penetration has been determined. The metal penetration depth (i.e., the reaction-layer thickness) was measured from cross sections of partially reacted samples. Samples were fabricated by immersing dense mullite preforms in a bath of molten aluminum at temperatures of 900°–1300°C and reacting the combination for up to 250 min. In general, the reaction-layer thickness increased linearly as the time increased. Penetration rates as high as 6.0 mm/h were measured; however, the aluminum penetration rate varied dramatically with time and temperature. The penetration rate increased when the reaction temperature was increased from 900°C to 1100°C, and the reaction-layer thickness increased linearly as the time increased in this temperature range. At temperatures of 1150°C and above, reaction-layer formation slowed or stopped after a relatively short period of rapid linear growth, because of an increase in silicon concentration near the reaction interface. The duration of the rapid linear growth period decreased from 25 min at 1150°C to <1 min at 1250°C. At temperatures of 1300°C and above, no reaction layer was detected by using optical microscopy. Kinetics data and transmission electron microscopy analysis suggest that the reaction was inhibited at higher reaction temperatures and longer times, because of silicon buildup and saturation at the reaction front. Calculations show that, as the reaction temperature increased, the silicon production increased faster than the silicon transport. The two rates were approximately equal at a temperature of 1100°C.     

MF包封DCPD自修复微胶囊的合成

自修复材料是目前材料领域的研究热点。它是由包封有活性物质的微胶囊、能使活性物质发生快速交联固化的催化剂和树脂组成。自修复用微胶囊的粒径与壁厚对微胶囊的力学性能有决定性影响,而微胶囊力学性能对自修复材料的应用起着非常重要的作用。本研究采用三聚氰胺-甲醛树脂(MF)为壳材料,双环戊二烯(DCPD)为芯材料,通过原位聚合法制备自修复材料用微胶囊。并使用马尔文粒径分析仪以及透射电镜等仪器研究酸值、反应时间以及乳化搅拌速率等条件对微胶囊粒径和壁厚的影响规律。研究结果表明,pH较低,粒径分布较差,pH对壁厚影响不大;反应时间越长,微胶囊壁厚越厚,反应时间对粒径没有明显影响;乳化搅拌速率越大,粒径分布越好,乳化搅拌速率对微胶囊壁厚没有明显影响。     

Characterization and creep properties of proton-conducting Yb-doped barium cerate

Polycrystals of Yb-doped barium cerate with composition BaCe_0.95Yb_0.05O_3−δ have been synthesized via a solid-state reaction route. The ceramic has a single orthorhombic perovskite phase, and the crystallographic unit cell is almost unperturbed with respect to undoped BaCeO₃. The compound exhibits a homogeneous distribution of equiaxed and submicron grains. The high-temperature mechanical properties have been studied for the first time. Mechanical tests were carried out in compression between 1100 and 1250 °C in air at constant initial strain rate. A gradual brittle-ductile transition was observed with increasing temperature and/or decreasing strain rate. Grain boundary sliding is the main deformation mechanism in the ductile region. In this regime, the true stress-true strain curves display an unusual behavior, with an initial strength drop followed by an extensive steady-state stage. This behavior is accompanied by the emergence of new, fine grains along the boundaries and triple junctions of the original grains during deformation.     

The effect of catalyst film thickness on the magnitude of the electrochemical promotion of catalytic reactions

The effect of catalyst film thickness on the magnitude of the effect of electrochemical promotion was investigated for the model catalytic reaction of C₂H₄ oxidation on porous Pt paste catalyst-electrodes deposited on YSZ. It was found that the catalytic rate enhancement ρ is up to 400 for thinner (0.2 μm) Pt films (40,000% rate enhancement) and gradually decreases to 50 for thicker (1 μm) films. The Faradaic efficiency Λ was found to increase moderately with increasing film thickness and to be described semiquantitatively by the ratio 2Fr _o/I ₀, where r _o is the unpromoted rate and I ₀ is the exchange current of the catalyst–electrolyte interface. The results are in good qualitative agreement with model predictions describing the diffusion and reaction of the backspillover O^2- species, which causes electrochemical promotion.     

Modeling of mass transfer and thermal cracking during the coking of Athabasca residues

The kinetics of thermal cracking of films of vacuum residue from Athabasca bitumen in the temperature range of was modelled with liquid-phase mass transfer, reaction-dependent fluid properties, and coke formation by reaction of cracked products in the liquid phase. Previous investigations on the thermal cracking of vacuum residue in thin films showed that at low film thickness the coke yield was insensitive to the temperature and heating rate for thin films of bitumen. The coke yield increased with the thickness of the initial film, in the range from 20 to . At the same time, the viscosity of the reacting liquid increased rapidly with time, which would slow down the diffusion of products inside the film. This coupling of transport and reaction would enhance the formation of coke by increasing the rate of recombination reactions. The concept of intrinsic coke is used in a new kinetic model to account for the minimum observed coke formation in thin films. With increasing film thickness, the increasing yield of extrinsic coke is modelled through the change in fluid properties as a function of extent of reaction, which reduces the rate of diffusion in the reacting liquid phase. The model was able to properly account for the insensitivity of coke yield in thin films to reaction temperature and the dependence of coke yield on the thickness of the liquid film.     

Amorphous carbon nanostructures from chlorination of ferrocene

The chlorination of ferrocene at different temperature conditions yields several carbon nanostructures, which were studied by means of transmission and scanning electron microscopies. Amorphous carbon nanotubes (α-CNTs) up to 10 μm long with thick walls and ∼15 nm of internal diameter were observed in a sample treated at 200 °C during 30 min. They consisted on ∼90% of carbon, while the remaining 10% consists on iron and chlorine. At this temperature, amorphous carbon bags and open-ended branches were also found. When chlorinating ferrocene at the same temperature but with longer reaction time (180 min), no α-CNTs were formed. At higher temperature (300 °C, 30 min), amorphous carbon bags were found, with lower content of residual chlorine and iron, and presenting thinner walls. In the sample treated at even higher temperature (900 °C, 30 min) the carbon nanobags (wall thickness ∼12 nm) were almost spherical and more graphitic, and without impurities.     

A study of film thickness dependence of the graphitizability of PMDA–ODA polyimide-derived carbon film

PMDA–ODA (pyromellitic dianhydride–4,4′-oxydianiline) polyimide (PI) films with thickness of 2–26 μm were synthesized and heat-treated at 2200–2500°C. The effect of film thickness on both the graphitizability of the PI-derived carbon film and the crystalline organization of the initial PI film was investigated by X-ray diffraction. The degree of graphitization of the resultant carbon film was observed to increase with decreasing initial PI film thickness, corresponding to the film thickness dependence of the crystalline ordering in the initial PI films. Structural factors related to PI film thickness are discussed for understanding the graphitizability of the resultant carbon.