David Vokoun

Recovery stresses generated by NiTi shape memory wires under different constraint conditions

Recovery stresses generated by neat shape memory wires under different constraint conditions have been studied in detail. The thermomechanical conditions applied in the experiments were very close to the biasing conditions in polymer composites with embedded shape memory wires. The parameters of the present experiments were the predeformation of the shape memory wires and two other parameters related to the effect of thermal expansion and the stiffness of matrix materials. The influence of these parameters on the course of the generated recovery stresses has been examined. The results are helpful for understanding the functional behavior of composites with embedded shape memory wires and/or some other shape memory applications with the type of constraint analyzed in this paper.

A Simple Route to Fabricate Percolated Perpendicular Magnetic Recording Media

A simple approach is proposed for fabrication of percolated perpendicular media (PPM). Nano pore array with pore diameter of 12-15 nm and different pore densities has been prepared by anodizing aluminum on silicon wafers. The pore density increases from 3.2times1010 cm-2 to 11.0times1010 cm-2 with the decrease in anodizing voltage from 30 to 10 V. Then, Pt (7 nm)/{Co (0.5 nm)/Pt (2) nm}5 multilayers are deposited onto this porous anodized alumina (AAO) by sputtering. The pore size is reduced to 10-12 nm after deposition of magnetic layers. The Co/Pt multilayers on AAO exhibit perpendicular magnetic anisotropy, squareness ratios of unity, and negative nucleation fields. The perpendicular coercivity increases linearly with the increase in pore density due to the pinning effect imposed by the pores, which is consistent with theoretical calculation for PPM

3D flexible NiTi-braided elastomer composites for smart structure applications

While outstanding functional properties of thin NiTi wires are nowadays well recognized and beneficially utilized in medical NiTi devices, development of 2D/3D wire structures made out of these NiTi wires remains challenging and mostly unexplored. The research is driven by the idea of creating novel 2D/3D smart structures which inherit the functional properties of NiTi wires and actively utilize geometrical deformations within the structure to create new/improved functional properties. Generally, textile technology provides attractive processing methods for manufacturing 2D/3D smart structures made out of NiTi wires. Such structures may be beneficially combined with soft elastomers to create smart deformable composites. Following this route, we carried out experimental work focused on development of 3D flexible NiTi-braided elastomer composites involving their design, laboratory manufacture and thermomechanical testing. We describe the manufacturing technology and structural properties of these composites; and perform thermomechanical tests on the composites, focusing particularly on quasistatic tensile properties, energy absorption, damping and actuation under tensile loading. Functional thermomechanical properties of the composites are discussed with regard to the mechanical properties of the components and architecture of the composites. It is found that the composites indeed inherit all important features of the thermomechanical behavior of NiTi wires but, due to their internal architecture, outperform single NiTi wires in some features such as the magnitude of recoverable strain, superelastic damping capacity and thermally induced actuation strain.

Magnetostrictive and shape memory properties of Fe-Pd alloys with Co and Pt additions

Fe?~30?at.%?Pd is well known as a ferromagnetic shape memory alloy. The influence of Co and Pt additions on the shape memory and magnetostrictive behavior of Fe?Pd has been examined in the present investigation.

Magnetic properties of annealed Fe–29.9 at% Pd ribbons

Abstract The decomposition process of a disordered Fe–29.9xa0at% Pd ferromagnetic shape memory alloy into the αFe and the equiatomic FePd phases has been studied. The process has been found to correlate with a significant increase of coercive force and a decrease of magnetostriction.

The two-spin model with dipolar interactions for the exchange coupled composite media

We study exchange coupled composite (ECC) media where both the hard and soft coupled layers possess perpendicular anisotropy or one of the layers is superparamagnetic. Our model is used to demonstrate the effect of the dipolar interactions on the coercive field. A series of ECC samples with various thicknesses of Ru spacer is manufactured and analyzed. The structure of the samples prepared is CoPtCr-SiO2∕Ru∕CoPtCr-SiO2, with the composition varied around that of (CoPt17Cr10)90-(SiO2)10. The acquired experimental data are used to illustrate the significance of magnetostatic interactions between the two coupled magnetic layers.

Effect of Magnetostatic Interactions on Twin Boundary Motion in NiMnGa Magnetic Shape Memory Alloy

We investigated the effect of magnetostatic interactions on the field-induced reorientation of martensite variants in Ni50.0Mn27.5Ga22.5. The reorientation, achieved by sweeping a single Type-II twin boundary along the sample, was triggered by a twinning stress of about 0.1 MPa. However, depending on the initial position of the twin boundary, the magnetic field providing the critical stress varied in the range 832 kA/m. By taking into account the variants sizes and their mutual interactions, we explained the observed dependence of the switching field on the location of the boundary. The resulting match between model predictions and measurements illustrates the fundamental role played by demagnetization effects and magnetostatic interactions in magnetic shape memory effect.

Effect of Deposition Pressure on Switching Field Distribution of CoPtCr–SiO

We studied the effects of the deposition Ar pressure on the switching field distribution (SFD) of Tb/Pt/Ru/CoPtCr-SiO2 perpendicular media. In this study, we describe a simple method for obtaining parameters of the SFD such as the standard deviation (SD) while taking the effect of demagnetizing field into account. The SFD changed its character greatly with the change of Ar pressure. The differences in the SD of the SFD of various samples may originate from the variation of intergranular exchange and the demagnetizing field due to different Ar pressure

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Fe–29.9 at.% Pd shape memory ribbons have been manufactured by means of the free jet melt-spinning method. The microstructure, the shape memory effect and the texture of the manufactured melt-spun ribbons have been studied. Some of the results are compared with those obtained in similar studies on Fe–Pd shape memory melt-spun ribbons.

Perpendicular Magnetic Recording Thin Film Media

A recently developed Velcro-like fastener utilizes superelastic deformation of two interlocked NiTi hooks when pulled apart. This work focuses on experimental analysis (evaluation of normal detachment force at different temperatures) and modeling (simulation by a finite element implemented SMA model) of the unhooking process. It is claimed that nonlinear superelastic deformation of NiTi leads to unique properties of the NiTi hook fasteners such as high strength (~15 000 kg m − 2), a significant increase of strength with increasing temperature, absorption of impact loads, damping of mechanical vibrations, forceless contact or silent release and better functioning in dirty environments compared to conventional Velcro fasteners.