Growth of hydroxyapatite on silica gels in the presence of organic additives: kinetics and mechanism

Synthetic hydroxyapatite was grown on surfaces of silica gels by immersing silica monoliths in a simulated body fluid at 37 °C. The gels were prepared by the sol-gel method and the drying process was controlled by using different additives (mono- and di-ethylene glycol, formamide and glycerin) to obtain large monoliths. The additives affect the growth of the apatite by changing the kinetic constants of chemical reactions on the silica surface. Surface areas and pore size distributions were determined, energy dispersion and FTIR spectra obtained, and scanning electron microscopy performed. The formation of the hydroxyapatite competes with the formation of crystalline calcium carbonate, but the results allow optimization of conditions for the growth of the former as a function of the type and concentration of the additive. For these optimal conditions, the mechanism and the order of the reaction were determined. Received: 15 February 2000 / Reviewed and accepted: 28 June 2000     

磁流变胶的沉降和力学性能研究

研制了一种新型的磁流变胶（magnetorheological gels），它具有良好的沉降稳定性和优良的磁流变效应。在2500r/min的离心实验中，离心20min之后才完全沉降：其剪切应力与剪切速率的关系可用Herschel-Bukley模型进行很好的描述；剪切屈服应力在小磁场下剪切屈服应力与磁场成线性关系，当磁场为70kA/m时，剪切屈服应力达到了53．9kPa。     

Direct computation of instability points for contact problems

 The extended system is known as a reliable algorithm for the direct computation of instability points on the equilibrium path of mechanical structures. This article describes the application of the extended system as critical point computation method to mechanical contact problems. In this type of problems inequality constraints have to be considered. Moreover a prediction method based on the extended system algorithm is presented which allows the detection of favorable starting values for a critical point computation on the equilibrium path. Dedicated to the memory of Prof. Mike Crisfield, for his cheerfulness and cooperation as a colleague and friend over many years.     

Time stepping schemes for coupled displacement and pore pressurē analysis

 The paper investigates the performance of various time stepping schemes for coupled displacement and pore pressure analysis. A number of alternative forms of the automatic time stepping method proposed by Sloan and Abbo (1999a) are also presented. These alternative schemes use different updates for the displacements and pore pressures and also adopt different starting conditions for the iterations. The automatic schemes are compared with an implicit θ-method, as well as an explicit method, through analysis of a variety of problems involving undrained loading, drained loading, and consolidation for Mohr-Coulomb and critical state models. As expected, the numerical results confirm that the explicit scheme is neither accurate nor robust. Although the implicit θ-method is accurate and fast, it fails to give a solution in a number of cases where the time step is large. The automatic schemes are shown to be accurate, fast and generally robust. Two of the automatic schemes proposed never fail to furnish a solution for the cases considered. In addition, all the automatic schemes are able to constrain the time-stepping (temporal integration) error in the displacements and pore pressures to lie near a prescribed tolerance, provided the iteration error tolerance is properly chosen. For complex soil models, it is important that the latter is set sufficiently small in order for the schemes to be able to constrain the time-stepping error to lie within a prescribed tolerance. Dedicated to the memory of Prof. Mike Crisfield, for his cheerfulness and cooperation as a colleague and friend over many years.     

Influence of fiber length on the mechanical properties of wood-fiber/polypropylene prepreg sheets

Natural fiber based composites have the potential to improve the mechanical properties of plastics while reducing the cost and weight. This study shows a practical method of blending natural-fiber with polypropylene to form a mat and then consolidated into a sheet by hot pressing. The natural fibers assessed were Pinus radiata and Eucalyptus regnan high temperature thermomechanical pulps and sisal (Agave sisalana) fibers. The tensile strength was shown to decrease with an increase in fiber content, while the tensile modulus was shown to increase. Tensile and flexural modulus were positively influenced by fiber length. The water performance tests of the sheets generally showed approximately 20% weight gain and approximately 3% thickness swell at 30% fiber content. The natural fiber surface chemical composition was determined by X-ray photoelectron spectroscopy and shown to be primarily covered with hydrophobic material such as lignin and extractives, while polypropylene was shown to be partially oxidized. Received: 18 September 2000 / Reviewed and accepted: 20 September 2000     

Desorption and pervaporation properties of zeolite-filled Poly(dimethylsiloxane) membranes

The role played by zeolite fillers in PDMS membranes was studied by desorption and pervaporation. The data on the desorption of different solvents from the membranes made of poly(dimethylsiloxane) (PDMS) filled with silica and different zeolites show that the solvent molecules are extracted in two stages. The first stage, which occurs at a low temperature (ca.50 °C), would correspond to the extraction of the solvent molecules in the ’bulk-like state’ in PDMS, while the last stage, at high temperature, would correspond to the extraction of the solvent molecules in the ’bound state’. There is a clear contribution of the zeolites to the membrane performances in pervaporation, even at a low zeolite content in PDMS (ca. 20 wt. %). The hydrophobic zeolites enhance significantly the organic flux, leading to an improvement of both the flux and the selectivity of PDMS. The ZSM-5 zeolite imparts to PDMS a higher selectivity but lower flux compared with SY-2 zeolite. The silica filler exhibits a crosslinking effect on PDMS, i.e. a slightly higher selectivity and lower flux than the pure PDMS membrane. Received: 11 June 2001 / Accepted: 11 June 2001     

Unstable interstitial-vacancy pairs and paradoxes of high temperature properties of metals

There are some quite general and quite unexpected features of the high temperature behaviour of metals being reviewed by Ya. Kraftmakher. These are the discrepancy between the estimated number of point defects, determined by equilibrium and quenching experiments, the discrepancy between a number of point defects, estimated by high temperature dilatometric measurements and heat capacity ones, as well as the peculiarities in changing the self-diffusivity mechanisms at high temperatures. The mechanisms of these phenomena had been analysed, but some new qualitative and semi-quantitative approaches are here suggested for explaining the named paradoxes as the basis of a model of unstable pairs vacancy-atom at interstice, being the third type of equilibrium point defects in solids, in addition to Frenkel’s and Shottky’s defects. Received: 11 March 1999 / Reviewed and accepted: 7 April 1999     

Gelation theory for solutions of polymers with loci of strong interaction

This paper presents a statistical mechanical analysis of gelation in certain solutions ofA/B copolymers. Repeat unitsA, the major units, must constitute about 90 mole% or more of the copolymer, and are compatible with the solvent. Repeat unitsB, the minor units, must be totally incompatible with the solvent and represent loci of strong interaction between the macromolecular chains. If these interactions are sufficiently strong they can lead to gelation of the mixture. The following simplifications of the model afford results that can be expressed in simple, closed form: (1) TheB units are uniformly distributed along the chain backbone. (2) Polymer concentration must be greater then that for chain overlap. (3)B–B interactions lead to dimerization ofB units only. Examples include: (1) Any aqueous polymer solution where theA repeat units are hydrophilic and theB repeat units are hydrophobic;B–B interactions are thus by hydrophobic bonding. (2) Neutralized acrylic acid/acrylamide copolymers where theB are acrylamide repeat units which associate by hydrogen bonds. (3) Ionomer solutions whereB–B bonding is by dipole-dipole interaction. The results show that there is a critical copolymer concentration for gel formation, C _c . It turns out that C _c decreases withB content for low dimerization energies and increases withB content for higher dimerization energies. At intermediate dimerization energies, C _c is insensitive to theB content. The two extreme regimes of behavior are similar to contradictory predictions of previously published theories. Received: 18 September 2000 / Reviewed and accepted: 20 September 2000     

The microstructure and its properties of Ni3Al based composite

 Composites of B-doped ductile Ni₃Al alloy matrix with no-oxide WC ceramic powders were produced by mechanical alloying, half-sintering and build-up welding. WC powders form non-continuous hardening phases, which are distributed in Ni₃Al matrix, wetting well with the matrix. The hardness and the structure stability are retained to temperatures of at least 850°C. After build-up welding, most of the NiAl phase left after sintering was changed into other phases and some graphite was precipitated in the matrix. The sand-laden water wear test showed expected results. Received: 12 October 1998/Accepted: 2 November 1998     

Thermal and dielectric properties of thermal and microwave cured thermoset polymers

 We compare microwave and thermal curing of DGEBA thermosets hardened with BDMA catalyzed HHPA. The glass transition temperature and (complex) dielectric constant are monitored throughout the process, for variable hardener contents and curing times. Received: 5 December 1997 / Accepted: 2 January 1998     

Water-absorbing properties of hydrogels based on polymeric complexes

Colloidal and chemical properties of poly-meric complexes based on polyacrylamide and polyvinyl alcohol have been investigated. Several factors affecting water absorption by polymeric complexes have been studied: concentration of initial aqueous polymer solutions; components ratio in the mixture; temperature and duration; sorbed aqueous solutions properties (pH; kind of ions). Macromolecular globules of polyacrylamide in aqueous solutions upon addition of polyvinyl alcohol exhibit interactions of hydroxyl and amide groups. The heat treatment of polycomplexes aqueous solutions leads to formation of cross-linked products (hydrogels) which are not soluble in water and show superabsorbents properties. Variation of conditions of polycomplex formation allows to regulate the properties of the resulting products such as hydrogel strength and the water absorption in wide ranges. This gives us possibilities for optimal use of the hydrogels in function of their specific applications. Received: 17 October 2000 / Reviewed and accepted: 18 October 2000     

Thermal properties and phase structure of PP + oligopinene blends

Blends of polypropylene (PP) and β-oligopinene, a synthetic low molar mass resin, were prepared by melt mixing in a Haake Rheocord at 473 K, 30 rpm during 600 s. The influence of the terpenic resin on the thermal properties, phase structure and crystallization of PP was evaluated. The DSC results indicated the presence of phase separation. The SEM analysis shows that the oligopinene was extracted along the whole surface and it was observed entities as cut cylinder atributted to low molar mass PP. The isothermal crystallization of blends was conducted at 396, 398 and 403 K. The mixtures did not show phase separation in molten, during crystallization and after solidification. The spherulite radial growth rate of PP decreased and was dependent on the temperature and composition. The results indicated that the system is an immiscible blend with at least three phases: a fine dispersion of the oligopinene in the amorphous PP, some boundary solubility of PP in oligopinene and a crystalline PP. Received: 13 November 2000 / Reviewed and accepted: 14 November 2000     

Thermal and mechanical properties of porous Y-PSZ/zircon composites

 The thermal and mechanical properties of sintered porous composites of yttria-partially-stabilized-zirconia (Y-PSZ) and zircon (ZrSiO₄) were investigated for a broad range of compositions. Fracture strengths of these composites were significantly improved with the zircon addition (0 to 50wt%). The addition of zircon also improved the thermal shock resistance. Specimens sintered at 1500^oC for 6 h with 15–20% porosity were shown to have superior strength and thermal shock resistance. These findings have been used in the manufacturing of ceramic permanent molds for brass casting. Received: 5 December 1997 / Accepted: 13 December 1997     

Numerical algorithm for triphasic model of charged and hydrated soft tissues

 This paper devises an efficient numerical algorithm for solving a two-dimensional triphasic model of charged and hydrated soft tissue by using the radial basis functions. The proposed numerical method is applied directly as a simple meshless collocation algorithm to approximate the solution of the governing system of continuity, momentum, and constitutive equations for the triphasic model. Since there is no requirement on meshing, the method can easily be applied to solve problems under complicated geometry. For verification, numerical simulations of stress, strain, and fluid flow patterns for a plane strain and an axisymmetric mechano-electrochemical coupling model with real synovial joint are given respectively. Classical domain decomposition technique is also combined successfully with the proposed method for solving large scale problems with numerical verification given in solving the axisymmetric case. Received 20 November 2001 / Accepted 28 November 2001     

Study on the propagation of inlet flow distortion in axial compressor using an integral method

 An integral method is investigated and developed in the current work. The effects of the parameters of inlet distortions on the trend of downstream flow feature in compressor are simulated. Other than the drag-to-lift ratio of the blade and the inlet incidence angle, it is found that the distorted inlet velocity is another essential parameter to control the distortion in propagation. Based on this study, a novel critical distortion line and corresponding critical distortion factor are proposed to express the effect of the two essential inlet parameters on the propagation of distortion, namely, the inlet incidence angle and the distorted inlet velocity. From the viewpoint of compressor efficiency, the propagation of inlet flow distortion is further described by a compressor critical performance and its critical characteristic. The results present a useful physical insight to an axial flow compressor behavior and asymptotic behavior of the propagation of inlet distortion, and confirm the active role of compressor in determining the velocity distribution when compressor responds to an inlet flow distortion. Received: 20 December 2001 / Accepted: 21 August 2002 The authors would like to thank HQ RSAF for permission to publish this work, their financial support and encouragement. The first author wants to acknowledge Prof. Frank Marble of California Institute of Technology, for bringing the problem to the author's attention and for his helpful discussion.     

Optical properties of some laser crystals

 In the following work the results of temperature dependence refractive indices measurements, thermooptical coefficients and dispersion in a wide range of temperature are presented. For measurements of refractive indices the least deviation method was employed. The measurements were carried out in the temperature region of 20–800° C for BBO crystals and 20–600° C for LBO crystals. It is revealed that the character of the refractive indices change in these crystals is different although the refractive indices of both crystals decrease while temperature increases. Whereas the temperature dependence of refractive indices in Ba B₂O₄ is practically linear, the same temperature dependence in LiB₃O₅ has more complicated character. It contains some particular features of refractive indices dispersion and thermooptical coefficients. Received: 11 January 1999/Reviewed and accepted: 15 January 1999     

Microstructure and mechanical properties of Y2O3/SiC nanocomposites

 Y₂O₃-based nanocomposites were fabriacted by hot-press and the microstructures and mechanical properties were investigated. Transmission-electron-microscope observations revealed that the SiC particles were distributed both within Y₂O₃ matrix grains and at the grain boundaries. Significant mechanical properties improvements were identified particularly at high temperatures above 1000 oC both in air and inert atmospheres. Received: 2 January 1997 / Accepted: 27 March 1997     

Enhanced dielectric properties of modified Ta2O5 thin films

 Tantalum oxide (Ta₂O₅) is a promising high dielectric constant material for the DRAM applications because of its ease of integration compared to other complex oxide dielectrics. The dielectric constant and thermal stability characteristics of bulk Ta₂O₅ samples were reported to enhance significantly through small substitutions of Al₂O₃. However, this improvement in the dielectric constant of (1-x)Ta₂O₅-xAl₂O₃ is not clearly understood. The present research attempts to explain the higher dielectric constant of (1-x)Ta₂O₅-xAl₂O₃ by fabricating thin films with enhanced dielectric properties. A higher dielectric constant of 42.8 was obtained for 0.9Ta₂O₅–0.1Al₂O₃ thin films compared to that reported for pure Ta₂O₅ (25–30). This increase was shown to be closely related to a-axis orientation. Pure Ta₂O₅ thin films with similar a-axis orientation also exhibited a high dielectric constant of 51.7, thus confirming the orientation effect. The leakage current properties and the reliability characteristics were also found to be improved with Al₂O₃ addition. Received: 24 November 1998 / Reviewed and accepted: 7 December 1998     

Iterative solutions for implicit Time Discontinuous Galerkin methods applied to non-linear elastodynamics

 Time Discontinuous Galerkin methods require the factorization of a matrix larger than that exploited in standard implicit schemes. Therefore, they lend themselves to implementations based on predictor-multicorrector solution algorithms. In this paper, various convergent and computationally efficient iterative methods implemented in the unknown displacements for determining the solution of non linear systems are proposed. The iterative solutions presented here differ from those implemented in the unknown velocities in that they are computationally superior. The results of numerical simulations relevant to Duffing oscillators and to a stiff spring pendulum discretized with finite elements which are designed to evaluate the efficacy of these iterative methods with non-linear systems, show a low-computational expense when compared to earlier iterative schemes. Received: 27 May 2002 / Accepted: 28 January 2003 The financial support from the Italian Ministry for Education, Universities and Research (MIUR) is acknowledged. However, opinions expressed in this paper are those of the writers, and do not necessarily reflect the views of the sponsoring agency.     

Photochemical modification of some polynaphthalimides

A series of polynaphthalimdes from dinaphthalene dianhydride and aromatic diamines mainly with alkyl substituents at ortho position to amine groups along with the model compounds has been synthesized using one-step high temperature polycondensation. It was found that in the case of the model compounds and the polymers synthesized from diamines with alkyl groups having at α carbon two hydrogen atoms, cyclodehydration process can take place during heating or UV irradiation and formation of additional cycle of pyrrole structure was observed. The polynaphthalimides synthesized from aromatic diamines substituted with alkyl groups can be modified under heating or UV irradiation, towards additional ring formation of pyrrole type, which causes an increase of the polymers glass transition temperatures. Received: 25 August 2000 / Reviewed and accepted: 28 August 2000