Optimal orientation field to manufacture magnetostrictive composites with high magnetostrictive performance

Magnetostrictive properties have relationship with the applied orientation field during the preparation of giant magnetostrictive composites. To understand the dependence of the optimal orientation field on particle volume fraction, composites with 20%, 30% and 50% particles by volume were fabricated by distributing Terfenol-D particles in an unsaturated polyester resin under various orientation fields. Their magnetostrictive properties were tested without pre-stress at room temperature. The results indicate that as the particle volume fraction increases, the optimal orientation field increases. The main reason for this phenomenon is the packing density for the composites with higher particle volume fraction is larger than that for those with lower particle content.     

EPR spectroscopy of protein microcrystals oriented in a liquid crystalline polymer medium

Correlation of the g-tensor of a paramagnetic active center of a protein with its structure provides a unique experimental information on the electronic structure of the metal site. To address this problem, we made solid films containing metalloprotein (Desulfovibrio gigas cytochrome c(3)) microcrystals. The microcrystals in a liquid crystalline polymer medium (water/hydroxypropylcellulose) were partially aligned by a shear flow. A strong orientation effect of the metalloprotein was observed by EPR spectroscopy and polarizing optical microscopy. The EPR spectra of partially oriented samples were simulated, allowing for molecular orientation distribution function determination. The observed effect results in enhanced sensitivity and resolution of the EPR spectra and provides a new approach towards the correlation of spectroscopic data, obtained by EPR or some other technique, with the three-dimensional structure of a protein or a model compound.     

Magnetic alignment in nominally non-magnetic hexagonal metal hydrides: NMR

Powders of three hexagonal metal-hydrides or -deuterides are found to align in 4.4–8.3 T magnetic fields used for NMR. The field-alignment is unexpected, since all three systems have very small susceptibilities, as demonstrated by sharp NMR lines. The extent of alignment runs from nearly complete to barely detectable in ZrBe₂(H,D)_x, LuD₃, and YD₃, respectively. The preferred alignment direction in ZrBe₂(H,D)_x is with the crystallites’ c-axis perpendicular to B, while the c-axis and B tend to be parallel in LuD₃ and YD₃. The susceptibilities χ_|| and χ are determined from bulk magnetization measurements in aligned ZrBe₂H_1.4 powder. The alignment must be considered for proper analysis of NMR spectra in these and related materials.     

Characterisation of precipitate phases in magnesium alloys using electron microdiffraction

Recent results of the characterisation of the structure, morphology and orientation of fine-scale, strengthening precipitate phases in selected magnesium alloys using transmission electron microscopy and microdiffraction are reviewed. The strengthening precipitate phases in Mg–Y–Nd alloys, aged to maximum hardness at 250°C, have been found to include two metastable precipitate phases β′ and β₁, and the equilibrium precipitate β. The β′ phase has a globular form, a base-centred orthorhombic structure (potential point group of mmm), and an orientation relationship such that (100)_β′//(1 10)_α, [001]_β′//[0001]_α. The β₁ phase has an f.c.c. structure (space group and an orientation relationship that may be described by (100)_β1//[0001]_α, and forms as plates parallel to The β phase has an f.c.c. structure (space group ) and also forms as plates on with an orientation relationship with the matrix phase that is identical to that observed for β₁ phase. Precipitates in Mg–Al alloys, aged isothermally at 200°C, invariably have the b.c.c. structure of the equilibrium precipitate phase β (Mg₁₇Al₁₂). Three orientation relationships have been observed between β and the matrix phase. Most precipitates have an irrational orientation relationship that approximates to the Burger's relationship, (001)_β//(0001)_α, and a faceted lath morphology with habit plane parallel to (0001)_α. A minor fraction of precipitates posses an orientation relationship that is of the form

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and have a prismatic rod morphology. The long axes of these rods are parallel to [0001]_α, and their faceted surfaces are parallel to A few precipitates are observed to have an orientation relationship such that

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and a rod shape, with their long axes apparently inclined with respect to [0001]_α.     

Porous polycrystals built up by uniformly and axisymmetrically oriented needles: homogenization of elastic propertiesPolycristaux poreux constitués d'aiguilles orientées de façon uniforme ou axisymétrique : homogénéisation des propriétés élastiques

Porous polycrystal-type microstructures built up of needle-like platelets or sheets are characteristic for a number of biological and man-made materials. Herein, we consider (i) uniform, (ii) axisymmetrical orientation distribution of linear elastic, isotropic as well as anisotropic needles. Axisymmetrical needle orientation requires derivation of the Hill tensor for arbitrarily oriented ellipsoidal inclusions with one axis tending towards infinity, embedded in a transversely isotropic matrix; therefore, Laws' integral expression of the Hill tensor is evaluated employing the theory of rational functions. For a porosity lower 0.4, the elastic properties of the polycrystal with uniformly oriented needles are quasi-identical to those of a polycrystal with solid spheres. However, as opposed to the sphere-based model, the needle-based model does not predict a percolation threshold. As regards axisymmetrical orientation distribution of needles, two effects are remarkable: Firstly, the sharper the cone of orientations the higher the anisotropy of the polycrystal. Secondly, for a given cone, the anisotropy increases with the porosity. Estimates for the polycrystal stiffness are hardly influenced by the anisotropy of the bone mineral needles. Our results also confirm the very high degree of orientation randomness of crystals building up mineral foams in bone tissues. To cite this article: A. Fritsch et al., C. R. Mecanique 334 (2006).     

Characterizing texture in polycrystalline materials by ultrasonic waves

A method for characterizing texture from measurements of ultrasonic wave velocities is proposed. In polycrystalline aggregates, ultrasonic wave velocities are strongly affected by orientation distribution coefficients (ODCs), which are usually used to describe the degree of preferred grain orientation in textured materials. In this work, velocities of longitudinal and transverse waves propagating into aluminum alloy 6061 were measured under pure shear, simple shear and uniaxial tension. From the measured ultrasonic wave velocities, the ODCs W₄₀₀ and W₄₂₀ were calculated to infer the deformation-induced texture. The predicted pole figures, obtained using ultrasonic velocities, were in good qualitative agreement with the finite element polycrystal model analyzed pole figures.     

聚对苯二甲酰对苯二胺纤维的热老化行为与机理研究

在250～400℃空气中对自由状态下对位芳纶进行等温热老化处理,采用万能材料试验机、红外光谱法、广角X射线衍射法、声速法和特性黏数法表征了老化过程中力学性能和结构的变化.结果表明,在老化初期,由于分子链的解取向,强度随时间快速下降;随后的热分解使强度随老化时间继续降低,符合二级反应动力学模型,其热老化表观活化能为32.4kJ/mol.老化样品的强度随温度升高显著下降,但高于350℃时热交联反应变得明显,同时结晶度增大和结晶结构完善,使强度的损失速率减小.老化样品的模量随老化温度的升高而增大,低于350℃时,非晶态分子链的解取向占优,模量较未热老化样品低;升高至350℃时,结晶结构完善占优,表现在第二类晶格畸变参数降低、表观微晶尺寸增大,特别是微晶横向融合使a,b轴方向尺寸显著增大,模量明显高于未老化样品.     

Multiscale modeling of the effect of carbon nanotube orientation on the shear deformation properties of reinforced polymer-based composites

A combination of molecular dynamics (MD), continuum elasticity and FEM is used to predict the effect of CNT orientation on the shear modulus of SWCNT-polymer nanocomposites. We first develop a transverse-isotropic elastic model of SWCNTs based on the continuum elasticity and MD to compute the transverse-isotropic elastic constants of SWCNTs. These constants are then used in an FEM-based simulation to investigate the effect of SWCNT alignment on the shear modulus of nanocomposites. Furthermore, shear stress distributions along the nanotube axis and over its cross-sectional area are investigated to study the effect of CNT orientation on the shear load transfer.     

On the geometric autocorrelation function of polycrystalline materials

Herein we derive an expression for direct determination of the geometric autocorrelation function W of a polycrystalline material from images of its grain boundary network (e.g., those delivered by orientation imaging microscopy). We also obtain an identity that relates the mean linear intercept function to a directional derivative of the geometric autocorrelation function. These formulae were applied to examine whether a widely-used formula for W, particularly in theoretical studies of attenuation of elastic waves in polycrystalline media, would be valid for the grain boundary structure of a commercial aluminum alloy. The conclusion was negative.