Reaction‐Bonded Boron Carbide/Magnesium–Silicon Composites

Reaction‐bonded boron carbide was manufactured by infiltrating porous boron carbide preforms at 1273 K with a Mg‐Si eutectic alloy. The resulting composite material consists, in addition to the original B₄C, of SiC, Mg₂Si, and a Mg‐rich complex boride/carbide Mg_x(Al,Si)_y(B,C)_z phase. The composites display high hardness (1700 HV), Young's modulus (356 MPa) and a moderate bending strength (230 MPa). The ballistic efficiency (of about 6.7), as determined by the depth of penetration method, is much higher than that of alumina and similar to that of silicon‐infiltrated reaction‐bonded composites.     

Combined method for studying and calculating the multicomponent diffusion in a mixture with an inert gas

A combined method for calculating the diffusion for a mixture with an inert gas in which the molar flux of the latter is equal to zero is considered. The method is based on using only one of three Stefan-Maxwell equations, the equation for the inert gas. This equation is reduced to a linear ordinary differential equation of the first order, which has a simple solution. The equations for the inert gas are closed using the modified Fick’s first law. The method makes it possible to radically simplify the study and calculation of multicomponent diffusion. Comparison with the experimental data taken from the literature showed that the deviation of the calculated curves from experimental points for a ternary mixture is within the experimental error.     

The effect of aluminum on the microstructure and phase composition of boron carbide infiltrated with silicon

Reaction-bonded boron carbide is prepared by pressureless infiltration of boron carbide preforms with molten silicon in a graphite furnace under vacuum. The presence of Al₂O₃ parts in the heated zone, even though not in contact with the boron carbide preform, causes aluminum to appear in the liquid silicon. The formation of aluminum sub-oxide (Al₂O) stands behind the transport of aluminum into the composite. The presence of aluminum in the boron carbide–silicon system accelerates the transformation of the initial boron carbide particles into B_x(C,Si,Al)_y and Al_1.36B₂₄C₄, newly formed carbide phases_. It also leads during cooling to the formation of some Si–Al solid solution particles. The effect of Al on the microstructural evolution is well accounted for by the calculated isothermal section of the quaternary Al–B–C–Si phase diagram, according to which the solubility of boron in liquid silicon increases with increasing aluminum content. This feature is a key factor in the evolution of the microstructure of the infiltrated composites.     

Effect of supplementation with chromium picolinate on antibody titers to 5-hydroxymethyl uracil

This study was designed to determine the effect of acetabular abduction on the polyethylene wear rates of the acetabular component. The hypothesis of this study is that acetabular placement, in particular abduction, effects contact forces and therefore polyethylene wear. A total of 364 total hip arthroplasties that were performed between 1974 and 1976 were included in this study. The cemented all-polyethylene acetabular components were the same for each case and came from a single supplier. The polyethylene wear rates were calculated by measuring the shortest radius from the center of the prosthetic femoral head to a point on the outer surface of the acetabular cup. An immediate postoperative radiograph was compared with a follow-up radiograph at least 9.5 years later. A standardized radiograph was used to prevent differences due to magnification. The abduction or inclination of the acetabular cups was measured in all of the patients. The amount of acetabular cup abduction was measured relative to the ischial tuberosity line. The mean abduction was 44.1 degrees with a standard deviation of 9.2 degrees and a median of 44 degrees. The range of abduction was from 0 degrees to 85 degrees. This analysis failed to show a correlation between the amount of acetabular abduction and polyethylene wear rates (Pearson's correlation coefficient = 0.0679; P = .20). With a sample size of 364, there is over a 95% change (statistical power) of detecting an underlying true correlation between wear and abduction that is greater than or equal to 0.20. We were unable to demonstrate a difference in wear that would correlate with the differing degrees of abduction of the acetabular socket. We felt that the radiographic measurements of wear were quite accurate. This method of determining acetabular abduction has also been documented and supported in previous literature and has allowed us to accurately determine acetabular abduction. The results of our study demonstrate that within the normal ranges described by our study, polyethylene wear did not significantly increase with increased abduction of the acetabular component.     

The effect of Fe addition on processing and mechanical properties of reaction infiltrated boron carbide-based composites

Dense metal-ceramic composites based on boron carbide were fabricated using boron carbide and Fe powders as starting materials. The addition of 3.5–5.5 vol% of Fe leads to enhanced sintering due to the formation of a liquid phase at high temperature. Preforms, with about 20 vol% porosity were obtained by sintering at 2,050 °C even from an initial boron carbide powder with very low sinterability. Successful infiltration of the preforms was carried out under vacuum (10⁻⁴ torr) at 1,480 °C. The infiltrated composite consists of four phases: B₁₂(C, Si, B)₃, SiC, FeSi₂ and residual Si. The decrease of residual Si is due to formation of the FeSi₂ phase and leads to improved mechanical properties of the composites. The hardness value, the Young modulus and the bending strength of the composites fabricated form a powder mixture containing 3.5 vol% Fe are 2,400 HV, 410 GPa and 390 MPa, while these values for the composites prepared form iron free B₄C powder are 1,900 HV, 320 GPa and 300 MPa, respectively. The specific density of the composite was about 2.75 g/cm³. The experimental results regarding the sintering behavior and chemical interaction between B₄C and Fe are well accounted for by a thermodynamic analysis of the Fe–B–C system.

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An imperceptible semi-blind image watermarking scheme in DWT-SVD domain using a zigzag embedding technique

Multimedia Tools and Applications - This research work presents a semi-blind image watermarking scheme based on transforms domain DWT-SVD using an efficient embedding technique. This scheme...     

Efficient reactive monitoring

Networks are monitored in order to ensure that the system operates within desirable parameters. The increasing complexity of networks and services provided by them increases this need for monitoring. Monitoring consists of measuring properties of the network, and of inferring an aggregate predicate from these measurements. Conducting such monitoring introduces traffic overhead that may reduce the overall effective throughput. This paper studies ways to minimize the monitoring communication overhead in IP networks. We develop and analyze several monitoring algorithms that achieve significant reduction in the management overhead while maintaining the functionality. The main idea is to combine global polling with local event driven reporting. The amount of traffic saving depends on the statistical characterization of the monitored data. We indicate the specific statistical factors that affect the saving and show how to choose the right algorithm for the type, of monitored data. In particular, our results show that for Internet traffic our algorithms can save more than 90% of the monitoring traffic     

Boron carbide/magnesium composites: Processing,microstructure and properties

Composites with a high fraction of the ceramic phase were fabricated by infiltration of 80% dense preforms with liquid Mg or Mg alloys, under an Mg vapor atmosphere. The infiltration was performed at 1123 K in a semi-hermetically closed container, from which gas had been evacuated. The Mg vapor atmosphere was achieved by heating subsequently to the relatively high temperature of the Mg infiltration process. The specific weight of the composites is about 2.44 g/cm³. The microstructure of the composite consists of the newly formed MgB₂ and ternary carbide MgB₂C₂ phases that connect the initial B₄C particles and some residual metal. A thermodynamic analysis of the interaction between B₄C and liquid Mg was conducted and its results are in good agreement with the experimental observations. The mechanical properties of the composites were investigated and discussed.