Effect of fiber content on the thermoelectric behavior of cement

The effect of discontinuous stainless steel fibers (diameter 60 m) as an admixture in cement paste on the thermoelectric behavior (the Seebeck effect) was systematically studied as a function of fiber volume fraction from 0 to 0.50 vol%. Without fibers, cement paste has an absolute thermoelectric power of +3 V/°C. A fiber content of up to 0.20 vol% makes the absolute thermoelectric power more negative (down to –63 V/°C), whereas a fiber content of 0.20–0.50 vol% makes the absolute thermoelectric power more positive (up to +31 V/°C)—even more positive than the positive value for the steel fiber by itself (+8 V/°C). The value is zero at a steel fiber content of 0.27 vol%. The effects are probably due to carrier scattering rather than conduction.     

Crack-tip damaged zones in rubber-toughened amorphous polymers: a micromechanical model

The various stages of crack propagation in rubber-toughened amorphous polymers (onset and arrest, stable and unstable growth) are governed by the rate of energy dissipation in the cracktip damaged zone; hence the relationship between the applied stress intensity factorK ₁ and the damaged zone size is of utmost importance. The size of the crack-tip damaged zone has been related toK ₁ via a parameter which is characteristic of the material in given conditions: this factor is proportional to the threshold stress for damage initiation in a triaxial stress field, and has been denoted by ^*. Theoretical values of ^* have been calculated by means of a micromechanical model involving the derivation of the stresses near the particles and the application of damage initiation criteria. The morphology, average size and volume fraction of the rubbery particles have been taken into account together with the nature of the matrix. The calculated values of ^* have been successfully compared with the experimental ones, for a wide set of high-impact polystyrenes (HIPS) and rubber-toughened poly(methyl methacrylate) (RTPMMA).Nomenclature PS; HIPS polystyrene; high-impact polystyrene - PMMA; RTPMMA poly(methyl methacrylate); rubber-toughened PMMA - MI; CS/H; CS/R particle morphologies (multiple inclusion; hard core - rubber shell; rubber core - rigid shell) - K _r;K _g bulk moduli of rubber and glassy materials - G _r;G _g shear moduli of the same materials - v _p particle volume fraction - L mean centre-to-centre distance between neighbouring particles - B; H; W standard names for the dimensions of the compact tension specimen - R _y size of the crack-tip plastic zone in a homogeneous material - h half thickness of the crack-tip damaged zone - r; polar coordinates around the crack tip (Fig. 1) - r;r _p distance from particle centre; particle radius - p normalized distance from the particle (Equation 5) - K ₁;K _1c;K _1p stress intensity factor; critical values ofK ₁ at the onset of and during crack growth - G _1c plane strain energy release rate - _y yield stress in uniaxial tension - _th macroscopic threshold stress for the onset of local damage initiation in a composite material - ^* characteristic parameter (Equation 3) - ⁰; ₁ ⁰ ; ₂ ⁰ ; ₃ ⁰ applied stress tensor and its three principal stresses - ⁰ uniaxial applied stress - ; ₁; ₂; ₃ local stress tensor and its three principal stresses - A tensor which elements are the ratios of those of over those of ⁰ (Equation 4) - v Poisson's coefficient of the matrix - g triaxiality factor of the crack-tip stress field - _e; _p Mises equivalent stress; dilatational stress (negative pressure) - I ₁;I ₂ invariants of the stress tensor - U ₁;U ₂ material parameters for argon and Hannoosh's craze initiation criterion (Equation 12)     

Viscosity of the Grain-Boundary Phase and Sintering of TiB2-Modified Alumina

A model is proposed for the sintering of powders via the sliding of particles facilitated by a grain-boundary phase. Data on the initial-stage sintering kinetics of TiB₂-modified alumina are used to assess the temperature-dependent viscosity of the grain-boundary phase, formed by Al₂O₃, oxidation products of TiB₂, and impurities present on the surface of alumina particles. In the range 1570–1620 K, the temperature variation of viscosity is fitted well by the expression 25.9exp(143308/RT).     

The constitution of the Ni-Al-Ru system

The constitution of the Ni-Al-Ru system has been investigated in the range 0 to 50 at% Al. Isothermal sections at 1523 and 1273 K have been determined using microstructural observations, electron probe microanalysis and X-ray diffraction. The phases present were: nickel-based solid solution (); (based on Ni₃Al); solid solutions based on NiAl and RuAl, respectively (designated ₁ and ₂), and ruthenium-based solid solution (Ru). The maximum solubility of Ru in was 5 at%. ₁, and ₂ show extensive range of solubilities, namely up to 20at% Ru in ₁ and up to 25 to 35 at% Ni in ₂. Three-phase equilibrium between , ₂ and (Ru) existed at 1523 and 1273 K. Also at 1523 K, three-phase equilibria existed between , and ₁ and ,₁ and ₂, while at 1273 K, the equilibria were between , ₁,₂ and , , ₂ indicating the occurrence of a reaction +₁, +₂ at a temperature between 1523 and 1273 K. Liquidus features have been deduced from data on as-solidified structures. Lattice parameter data and hardnesses are also reported.     

Effect of antimony doping on magnetic properties of Ni-Zn ferrites

The influence of Sb⁵⁺ doping on magnetic initial permeability () and magnetic loss factor () in Ni-Zn ferrites over a frequency range from 10 kHz to 1 GHz at room temperature is studied. The Curie temperature (T _c), saturation magnetization (M _s), lattice constant (a) and mean grain diameter (D _m) of the pure and doped ferrites have also been evaluated. Domain wall relaxation has been observed in all the samples. Using the existing theories the magneto-crystalline anisotropy constant (K) and certain domain wall constants like wall damping parameters (), domain wall energy (_w), the wall mobility () and the wall mass (m _w) have been evaluated and the results are compared and discussed with the similar data available on other ferrimagnetic oxides.     

Microstructure and hardness of as-cast in situ TiB short fibre reinforced Ti-6Al matrix composites

XD^TM technique has been successfully used to prepare TiB short fibre reinforced Ti-6Al matrix composites. Macrostructure and microstructure have been observed by optical microscopy and SEM in order to study the influence of cooling rate on the morphology, size and distribution of TiB. Due to the cooling rate, there exist three kinds of macrostructure: fine grain zone, columnar grain zone and coarse equiaxed grain zone, corresponding to the cooling rate of 100–500 K/s, 20–50 K/s and less than 10 K/s respectively. In the fine grain zone, TiB distributes randomly in matrix with main rod morphology with 3 m in width and 50 m in length. In the columnar and coarse grain zone, a colony structure was observed in which TiB distributes with a special orientation direction with matrix. A lamellar TiB with up to 50 m width and 200 m length was also formed. It was indicated that the decreasing of the cooling rate changes the morphology of TiB from rod to lamellar shape, and markedly increase the length and aspect ration of TiB, from 50 m to 200 m and from about 15 to 200, respectively. TEM results show that the rod TiB has a hexagonal cross section. Vickers hardness testing shows a little reinforcement geometry dependence, but the average hardness of 484 MPa is much higher than that of unreinforced matrix alloy.     

Question of determining the composition of composites by a tomographic method

One possible method of determining the volume content of composite components by using x-ray calculational tomography is examined.Notation , _T, _fi, P_b, _b, _w, _im material densities, theoretical, fiber, binder, pore, water, impregnating fluid - _fi, _b, _p fiber, binder, pore volume contents - f_fi, f_b, f_p, f_di, f_di ^*, f_cij, f_im linear coefficients of attenuation of the fiber, binder, air of the i-th layer of the dry specimen - n quantity of cells in the i-th layer, m_fi, m₁ m₂, weight of the dry and impregnated specimens in air and in water - C_cl closed porosity - S_f rms deviation of the linear attenuation coefficients - volume closed pore content Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 56, No. 5, pp. 738–745, May, 1989.     

High Tc: Top Down or Bottom Up?

We present and discuss recent tunneling results, which shed some light on the two contradicting approaches to the high-T _c problem. The top-down approach starts from the ansatz that the anomalous properties of the cuprates in their normal state above T _c need to be understood first, before the problem of superconductivity can be tackled successfully. On the other hand, in the bottom-up approach, both high T _c and the anomalous normal-state properties come about because of the vicinity of a quantum critical point. Our new tunneling results indicate that in the superconducting state, at some critical doping, the order parameter goes from a pure d-wave on the underdoped side to a complex one on the overdoped one. This symmetry breaking lends support to the hypothesis that there does exist a quantum critical point and, therefore, favors the bottom-up approach.     

Determination of the thermophysical characteristics of alternatively freezing and thawing dispersed media by the method of solving inverse problems of heat conduction

The article explains an algorithm for determining the thermophysical characteristics of dispersed media with phase transitions based on the method of solving inverse problems of heat conduction.Notation r space coordinate - time - T temperature of the specimen - T₀ initial temperature - c_i, c_w, c_sk specific heat of ice, water, and of the organic-mineral skeleton, respectively - c_f, c_m, _f, _m specific heat and thermal conductivity in the frozen and melted zones, respectively - c effective heat capacity - thermal conductivity - p density - ₀, _sb bound and strongly bound moisture, respectively - (T) amount of nonfrozen water - R radius of the cylinder - q() heat flux - I functional - u₁(), U₂() measured temperatures of the specimen at the points r = 0 and r = R, respectively, at the instant - ₁, ₂ degree of confidence of the supplementary information - final instant of time - a, b, k, _s positive constants - L specific heat of melting - N number of grid nodes over space - n number of grid nodes over time - h grid step over space - grid step over time - solution of the conjugate system - s number of iteration Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 39, No. 2, pp. 292–297, August, 1980.     

Three-dimensional problems of strongly interacting arbitrarily located penny-shaped cracks

Three-dimensional elastic interactions of arbitrarily located and oriented penny-shaped cracks are considered, and solutions for the stress intensity factors (SIFs) are obtained for several crack geometries. The work is based on the method of analysis of many cracks problems developed by Kachanov [1]. Unlike most of the results available in literature, the obtained solutions remain accurate for closely spaced, strongly interacting cracks. The effects of stress shielding and amplification and the influence of various geometrical factors are examined. The simplest crack-microcrack interactions in three dimensions are considered. The approximations of weak interactions and of widely spaced cracks are discussed. The interactions that are weak in their impact on the effective elastic constants may be strong in their impact on SIFs; this indicates that no simple relation between the deterioration of elastic stiffness of a microcracking solid and its progression towards failure seems to exist.

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Résumé On considère les interactions élastiques sur trois dimensions de fissures en forme d'onglets disposées et orientées de façon arbitraire, et on obtient des solutions pour les facteurs d'intensité de contraintes pour diverses géométries de fissures. L'étude est basée sur la méthode d'analyse des problèmes de fissuration multiples, développées par Kachanov. A l'encontre de la plupart des résultats disponibles dans la littérature, les solutions obtenues demeurent exactes pour des fissures groupées et à interactions sévères. On examine les effets de bouclier ou d'amplification pour les contraintes, ainsi que l'influence de divers facteurs géométriques. On considère les cas les plus simples d'interactions en trois dimensions entre fissure et micro-fissure. On discute les approximations que constituent les notions de faibles interactions et de fissures largement espacées. Les interactions qui sont faibles au plan de leur impact sur les constantes élastiques effectives peuvent se révéler sévères par rapport au facteur d'intensité de contraintes. Ceci indique qu'il ne semble pas exister de relations simples entre la détérioration de la rigidité élastique d'un corps du fait d'une micro-fissuration, et son évolution vers la rupture.

     

Kinetic studies on β-SiC formation from homogeneous precursors

The kinetics of the carbothermic reduction of SiO₂ by carbon to produce -SiC from a homogeneous organic precursor has been investigated over the temperature range 1500 to 1800 °C in nitrogen by the use of a high-temperature thermobalance. The kinetic behaviour differed significantly from that of the heterogeneous reaction of SiO₂ and carbon particles. The weight-loss curves could be fitted well by the Avrami-Erofe'ev equation with an exponent of 1.5. The result was interpreted as showing instantaneous nucleation in a homogeneous matrix followed by the diffusion-controlled growth of -SiC. The obtained activation energy of 391 kJ mol^–1 was consistent with the assumption that the reaction is controlled by the diffusion of carbon through the amorphous matrix to the growing surface of -SiC.     

Separable functions in load separation for the ηpl and ηpl CMOD plastic factors estimation

The objective of this paper is to develop the load separation method for evaluating the _pl and _pl ^CMOD plastic factors used in the J estimation approach based on load versus displacement records. Appropriate forms for the geometry and deformation functions have been suggested from the EPRI Handbook solutions to produce the separable form for the load. The obtained functions are applied to evaluate the _pl and _pl ^CMOD plastic factors for center cracked tension specimen. The present load separation method gave results which are somewhat different from the estimated values of _pl given in the literature. For shallow cracks, the _pl and ^CMOD _pl plastic factors show considerable variation with crack size and the strain hardening exponent. For a deeply cracked CCT specimen, the ^CMOD _pl factor tends to the _pl factor and equals approximately unity. Abbreviations: CCT – center cracked specimen; CMOD – Crack Mouth Opening Displacement; EPRI – Electric Power Research Institute; FEM – Finite Element Method; LLD – Load Line Displacement.     

Effect of porosity and alumina content on the mechanical properties of compocast aluminium alloy-alumina particulate composite

The effects of the size and volume fraction of alumina particles and of the porosity on the tensile strength of Al-4wt% Mg-alumina compocast particulate composite have been investigated. The contribution of porosity to the reduction in strength of the composite at various levels of alumina content is expressed as a linear function of porosity containing two experimentally determined parameters: ₀, the ultimate tensile strength at zero porosity level, and , a weakening factor. For a composite containing a lower level (<7 vol%) of alumina particles, a rapid decrease in the value of ₀ is observed with an increase in the volume fraction of alumina. The rate of reduction in strength slows down at higher alumina levels in the composite. An increase in particle size is found to reduce the value of ₀. The value of is found to decrease with an increase in the volume fraction of alumina in the composite. For a given alumina content the increase in average particle size from 22 to 115m is found to push the value of up a little, followed by a significant decrease with an increase in particle size to 195m. The role of porosity in the engineering fracture straine _f of the composite is found to increase almost linearly with the inverse of porosity above a critical level. For composites with a given level of porosity,e _f is found to increase with an increase in the alumina content of the composite.     

Investigations of electric dipole fields in type II superconductors during flux flow

Electric dipole fields in the mixed state of rectangular superconducting foils are investigated both theoretically and experimentally. These fields are generated by imposing an ac current to a contact pair during steady flux flow driven by a transport current and are measured using a second, movable contact pair. Theory predicts that the electric dipole field can be described by an anisotropic flux flow conductivity, whose components _x and _y are related by _x / _y =1–(j _c /j) cos² , where j is the dc current density, j _c is the dynamic critical current density, and is the field angle. Except for low j values at the threshold of flux flow, good agreement with experiment is found.This work was supported in part by U.S. Department of Energy, Contract No. W-7405-Eng-82, Division of Materials Sciences, budget code AG-01-02-03-3.     

High pressure consolidation of B6O-diamond mixtures

Sintered composites in the B₆O-xdiamond (x= 0–80 vol%) system were prepared under high pressure and high temperature conditions (3–5 GPa, 1400–1800°C) from the mixture of in-laboratory synthesized B₆O powder and commercially available diamond powder with various grain sizes (<0.25, 0.5–3, and 5–10 m). Relationship among the formed phases, microstructures, and mechanical properties of the sintered composites was investigated as a function of sintering conditions, added diamond content, and grain size of diamond. Sintered composites were obtained as the B₆O-diamond mixed phases when using diamond with grain sizes greater than 0.5 m, while the partial formation of the diamond-like carbon was observed when using diamond grain sizes less than 0.25 m. Microhardness of the sintered composite was found to increase with treatment temperature and pressure, and the fracture toughness slightly decreased. A maximum microhardness of H _v57 GPa was measured in the B₆O-60 vol% diamond (grain size < 0.25 m) sintered composite under the sintering conditions of 5 GPa, 1700°C and 20 min.     

Anisotropic Pair-breaking by Impurity Doping in High-Tc Cuprates: Modification of Interlayer Coupling

Impurity (Zn/Ni) doping effects on the superconducting coherence length and the penetration depth have been studied for the in-plane and the out-of-plane directions of YBa ₂ Cu ₃ O _y single crystals. We determined from diamagnetic susceptibility in the reversible region and from optical conductivity spectra. It was found that both in and the ac-anisotropy ratio is reduced with impurity doping. Both changes can be explained by assuming an increase in Josephson coupling strength due to impurity doping.     

Determination of rolling anisotropy by extensometry

A theory is presented for the determination of three coefficients characterizing anisotropic materials. These new coefficients of anisotropy advantageously replace Lankford's classic empirical coefficient,r, in the definition of the anisotropy of a rolled sheet. It is shown how these coefficients ₀, ₄₅ and ₉₀ may be deduced from the theory, provided certain assumptions are made concerning the rolled material. The validity of the theory was tested through experiments on rolled ultra high-purity iron sheet, ₀, ₄₅ and ₉₀ being determined by means of a special purpose-built extensometer.     

Measurements of Critical Current of Superfluid 4He through An Array of Submicron Apertures very near Tλ

Far from the lambda transition the critical flow of superfluid ⁴ He through a small orifice is determined by thermal nucleation of quantized vortices. Between 300 mK and 2 K linearly decreasing critical flow velocity has been observed earlier. As the temperature approaches T the size of the vortex core increases and becomes comparable to that of the orifice. We report here measurements of the critical mass current in this temperature range. An array of 24 3×0.17 m holes in parallel with a macroscopic parallel path and flexible-diaphragm Helmholtz resonator have been used. The temperature range explored was from 80 mK to 20 K below T. Preliminary analysis of the data shows that for a reduced temperature t=(T–T)/T1·10 ^–4 the critical current scales approximately as t ^1.25 . Closer to T the critical phase difference across the array becomes comparable to 2 and the results have to be analyzed in terms of Josephson effect. The superfluid density has been measured at the same time as the critical current.     

μ+SR Study on Bi2Sr2Ca1−x Y x (Cu1−y Zn y )2O8+δ around the Hole Concentration of \frac{1} {8} per Cu

In order to study the effect in different high-T _c oxides from the La-system, muon spin relaxation measurements were applied to the Zn-substituted Bi-2212 system, Bi ₂ Sr ₂ Ca _1–x Y _x (Cu _1–y Zn _y ) ₂ O ₈₊ , around the hole concentration p= per Cu. It has been revealed that the magnetic correlation between Cu spins is anomalously enhanced in the Zn-substituted samples at per Cu, proving the existence of the effect" in the Bi-2212 system as well.     

Interfacial and mechanical properties of environment-friendly “green” composites made from pineapple fibers and poly(hydroxybutyrate-co-valerate) resin

Physical and tensile properties of pineapple fibers were characterized. Tensile properties of pineapple fibers, like most natural fibers, showed a large variation. The average interfacial shear strength between the pineapple fiber and poly(hydroxybutyrate-co-valerate) (PHBV) was 8.23 MPa as measured by the microbond technique. Scanning electron microscopy (SEM) photomicrographs of the microbond specimens revealed an adhesive failure of the interface. Fully degradable and environment-friendly green composites were prepared by combining pineapple fibers and PHBV with 20 and 30% weight content of fibers placed in a 0°/90°/0° fiber arrangement. Tensile and flexural properties of these green composites were compared with different types of wood specimens. Even though tensile and flexural strength and moduli of these green composites were lower than those of some wood specimens tested in grain direction, they were significantly higher than those of wood specimens tested in perpendicular to grain direction. Compared to PHBV virgin resin, both tensile and flexural strength and moduli of these green composites were significantly higher. SEM photomicrographs of the fracture surface of the green composites, in tensile mode, showed partial fiber pull-out indicating weak bonding between the fiber and the matrix.