Preparation of nanocrystalline nickel oxide from nickel hydroxide using spark plasma sintering and inverse Hall-Petch related densification

Nanocrystalline nickel oxide (NiO) was prepared from nickel hydroxide by Spark plasma sintering (SPS) and the mechanisms involved in the densification of NiO were studied. Reverse precipitated nickel hydroxide powders were SPS processed at 400, 600 and 700?°C with 70?MPa pressure. Pure NiO with 12?nm crystallite size formed after 400?°C sintering process. However NiO grains had grown to 18 and 38?nm after 600 and 700?°C sintering respectively. NiO pellets prepared using 600 and 700?°C SPS sintering schedules had relative densities of 83% and 94% respectively. Two displacement rate regimes were observed during densification of NiO in both 600 and 700?°C sintering processes. Decomposition of nickel hydroxide and particle sliding of NiO led to first displacement rate maximum while inverse Hall-Petch based plastic deformation facilitated densification during the constant second displacement rate regime. No densification occurred during sintering holding times indicating the limited role that diffusion played during densification.     

Thermal post buckling behavior of rectangular antisymmetric cross-ply composite plates

Summary The thermal post buckling behavior of simply-supported, antisymmetric cross-ply plates with immovable edges is investigated in this paper. For this purpose, a one term approximation for the inplane and transverse displacements is assumed and Rayleigh-Ritz method is used to obtain the equations of equilibrium. The question, whether the true thermal buckling (bifurcation buckling under thermal loads) exists for such plates is also addressed to. The conditions under which the thermal buckling in the classical sense occurs are derived. It is shown that nonlinear bending stiffness for such plates is direction dependent, because of an extra quadratic nonlinear term in addition to the usual cubic nonlinear term in the governing equilibrium equation. Results are presented for various plate configurations.     

Ductile-phase toughening and Fatigue-Crack Growth in Nb-Reinforced Molybdenum Disilicide Intermetallic Composites

A study has been made of the role of ductile-phase toughening on the ambient temperature fracture toughness and fatigue-crack propagation behavior of a molybdenum disilicide intermetallicmatrix composite reinforced with 20 vol pct niobium spheres. Using disk-shaped compact DC(T) samples, only moderate improvements (∼24 pct) in fracture toughnessK _lcvalues were found for the composite compared to the unreinforced MoSi₂ matrix material. Moreover, (cyclic) fatigue- crack propagation was seen at stress intensities as low as 75 to 90 pct ofK _Ic, with growth rates displaying a high dependency (∼14) on the applied stress-intensity range. The lack of significant toughening due to the incorporation of ductile Nb particles is associated with an absence of crack/particle interactions. This is attributed to the formation of a weak reaction-layer interface and elastic mismatch stresses at the crack tip between the Nb and MoSi₂, both factors which favor interfacial debonding; moreover, the spherical morphology of Nb phase stabilizes cracking around the particle. Results suggest that increasing the aspect ratio of the distributed Nb rein- forcement phase with attendant interfacial debonding and eliminating possible Nb-phase em- brittlement due to interstitial impurity contamination are critical factors for the successful development of tougher Nb/MoSi₂ structural composites. Formerly with McDonnell Formerly with McDonnell     

Post-buckling behaviour of moderately thick cylindrically orthotropic annular plates

A finite element formulation including the effects of shear deformation and cylindrically orthotropic material properties is described for studying the post-buckling behaviour of annular plates. Numerical results for the buckling load parameter and ratios of nonlinear load parameter to buckling load parameter for various values of orthotropic properties, thicknesses and radii ratios of the plates are presented.     

A new chip-thickness model for performance assessment of silicon carbide grinding

The chip-thickness models, used to assess the performance of grinding processes, play a major role in predicting the surface quality. In the present paper, an attempt has been made to develop a new chip-thickness model for the performance assessment of silicon carbide grinding by incorporating the modulus of elasticities of the grinding wheel and the workpiece in the existing basic chip-thickness model to account for elastic deformation. The new model has been validated by conducting experiments, taking the surface roughness as a parameter of evaluation .     

Membrane degumming of crude rice bran oil: Pilot plant study

The procedure for the classical chemical refining of vegetable oils consists of degumming, alkali neutralization, bleaching, and deodorization. Conventional refining of rice bran oil using alkali gives oil of acceptable quality, but the refining losses are very high. A critical work has been carried out to study the application of membrane technology in the pretreatment of crude rice bran oil. Oils intended for physical refining should have a low phosphorus content, and this is not readily achievable by the conventional acid/water degumming process. The application of membrane technology for the pretreatment of rice bran oil has been investigated. The process has already been successfully applied to other vegetable oils. Ceramic membranes, which are important from the commercial point of view, were examined for this purpose. The results showed that the membrane‐filtered oils met the requirements of physical refining, with a substantial reduction in color. It was observed that most of the waxy material was also rejected. Experiments were carried out to establish the relationship between permeate flux and rejection with membrane pore size, trans‐membrane pressure and micellar solute concentration.     

Synthesis and Characterization of Hydrophobic TMES/TEOS Based Silica Aerogels

The experimental results on the effect of adding trimethylethoxysilane (TMES) as a co-precursor on the hydrophobicity and physical properties of tetraethoxysilane (TEOS) based silica aerogels, are reported. The molar ratio of TEOS, ethanol (EtOH), water (0.001 M oxalic acid catalyst) was kept constant at 1:5:7 respectively, while the molar ratio of TMES/TEOS (A) was varied from 0 to 0.6. It has been observed that as the A value increases, the gelation time increases. The hydrophobicity was tested by measuring the contact angle, and the surface chemical modification was confirmed by the FTIR spectroscopy studies. The thermal stability of the hydrophobic aerogels was studied in the temperature range from 25 to 800°C. The hydrophobic nature of the aerogel could be maintained up to a temperature of 287°C and above this temperature the aerogels become hydrophilic. The bulk density and the optical transmittance of the aerogels have been found to decrease with increase in A value. The aerogels have been characterized by Fourier transform infrared spectroscopy (FTIR), Optical transmittance, Scanning electron microscopy (SEM), Differential thermal analysis (DTA) and Thermogravimetric analysis (TGA), and Contact angle measurements.     

The pitting corrosion of sputtered Fe base alloy films

Predominantly single phase Fe base alloy films were prepared by a DC sputtering method. The compositions of the alloy films, as determined by Auger electron spectroscopy (AES) were Fe-7 at% Cr and Fe-18 at% Cr, Fe-11 at% W and Fe-19 at% W, Fe-9, −13 and −27 at% Ta. The pitting potential of the sputtered alloys determined in chloride solutions was found to be strongly dependent on the nature and the concentration of the alloying element. The pitting potential of sputtered Fe-18 at% Cr and Fe-27 at% Ta alloy films were approximately 0.9 V higher than that of bulk Fe and 0.6 V higher than that of Fe-19 at% W alloy film. A very good correlation was found between the pitting potential of Fe alloy films and the solubility of an alloying element oxide in acidic solutions with a pH corresponding to that within the stable pits of pure Fe (pH = 2). The pitting potential of the Fe alloy was found to increase with a decrease in the solubility of the alloying element oxide in acidic solutions. The pitting potentials of the sputtered Fe-7 at% Cr and Fe-18 at% Cr alloy films were were significantly higher than those of the bulk materials with the same composition. The higher pitting potentials of these alloy films are tentatively attributed to the presence of very fine particles of the oxide phase that are homogeneously distributed in the sputtered alloys.     

Fatigue-crack propagation behavior in monolithic and composite ceramics and intermetallics

We study microstructural mechanisms of fatigue crack growth in advanced monolithic and composite ceramics and intermetallics. Much attention is devoted to the contribution of cycling loading to the hindrance of mechanisms that lead to a considerable increase in toughness (crack-tip shielding) of these materials. For example, in intermetallics with a ductile phase, such as -TiNb-reinforced -TiAl or Nb-reinforced Nb₃Al, a significant increase in toughness caused by the presence of uncracked ductile phase inside a crack is retarded under cyclic loading because ductile particles immediately fail by fatigue. Similarly, in monolithic ceramics, e.g., in alumina (aluminum oxide) or silicon nitride, the significant plasticization appearing under monotonic loading is greatly diminished under cyclic loading due to gradual wear at the grain-matrix interface. In fact, the nature of fatigue in such low-plasticity materials differs essentially from the well-known mechanisms of fatigue in metals and is governed, first of all, by a decrease in shielding, which depends on the loading cycle and time. The susceptibility of intermetallics and ceramics to fatigue degradation under cyclic loading affects seriously the possibility of structural use of these materials in practice. In particular, in this case, it is difficult to apply strength calculation methods that take into account the presence of defects and to implement life-prediction procedures.Center for Advanced Materials, Lawrence Berkeley Laboratory, University of California, USA. Published in Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 30, No. 3, pp. 7–35, May–June, 1994.     

Characterization and evaluating the mechanical performance of halloysite nanotubes reinforced acrylonitrile butadiene styrene/polycarbonate composites exposed to aggressive environmental conditions

Developing light weight polymer based composites dispersed with novel reinforcements which can function well in the presence of aggressive environments is an active research field in the materials engineering. Hence, in the current work, halloysite nanotubes (1 %, 2 %, 4 %, 6 %, 8 % and 10 % by weight) were reinforced into acrylonitrile butadiene styrene/polycarbonate blend and the role of reinforcing phases on the mechanical performance under aggressive environmental conditions has been evaluated. Hardness was measured as gradually increased in the composites with the increased content of the reinforcements. Impact strength of the composites was observed as increased in the composites up to 4 % reinforcement and further decreased. Increased strength was measured for the composite up to 2 % reinforcement. Ductility of the composites was decreased as reflected form the decreased % of elongation with the higher fraction of reinforcements due to induced brittleness. The composites were exposed to diluted sulfuric acid for 3 h and 6 h at 60 °C and then subjected to tensile loading. With the increased time of exposure, composites with 1 % and 2 % reinforcement exhibited relatively better performance.