Yanling Ge

Composition and temperature dependence of the crystal structure of Ni–Mn–Ga alloys

The crystal structure of ferromagnetic near-stoichiometric Ni2MnGa alloys with different compositions has been studied at ambient temperature. The studied alloys, with five-layered (5M) and seven-layered (7M) martensitic phases, exhibit the martensitic transformation temperature (TM) up to 353 K. Alloys with these crystal structures are the best candidates for magnetic-field-induced strain applications. The range of the average number of valence electrons per atom (e/a) was determined for phases 5M, 7M, and nonmodulated martensite. Furthermore, a correlation between the martensitic crystal structure, TM and e/a has been established. The lattice parameters ratio (c/a) as a function of e/a or TM has been obtained at ambient temperature for all martensitic phases. That the paramagnetic-ferromagnetic transition influences the structural phase transformation in the Ni–Mn–Ga system has been confirmed.

Various magnetic domain structures in a Ni–Mn–Ga martensite exhibiting magnetic shape memory effect

Magnetic domain structures of the Ni–Mn–Ga martensite were observed by means of type I and type II magnetic contrast in scanning electron microscope. The different configuration of magnetic domain patterns coupled together with the twin structures were studied in multivariant, two-variant, and single-variant martensite. The martensitic band contains broad stripelike magnetic domains following the easy axis of magnetization, i.e., the crystallographic c axis. These stripe domains are connected by 90° domain walls creating a staircaselike structure in the adjoining bands. It is found that the internal twins, substructures of the martensite twin domains, are distorted into a zig–zag shape in order to accommodate the main band magnetization. Furthermore, the dagger-shaped stripe domains occur only when the internal twins are present. When the sample exhibits the single-variant state, the internal twins disappear totally and the stripe magnetic domains spread over the whole specimen. The configuration observed...

Giant Magnetostrictive Materials

The magnetostrictive materials exhibit a strain caused by the orientation of the magnetic moment when exposed to a magnetic field. A particular class of magnetostrictive materials is called magnetic shape memory (MSM) alloys or ferromagnetic shape memory alloy (FSMA) materials, which can change their shape remarkably when subjected to magnetic field. In the following, the MSM materials are introduced and their extraordinary structure, properties, and performance are described.

Crystal structure and twinning in martensite of Ni1.96Mn1.18Ga0.86 magnetic shape memory alloy

Abstract The martensite structure in the Ni 1.96 Mn 1.18 Ga 0.86 magnetic shape memory single crystal is determined as 5R monoclinic at room temperature. Twin structure of martensite is found to be a result of the three twin types coexistence. Crystallography of macro- and internal twins in studied 5R martensite was determined using X-ray diffraction methods.

Direct optical observation of magnetic domains in Ni-Mn-Ga martensite

This letter reports the direct optical observation, i.e., without polarization, of the magnetic domain structure explained by a large surface relief in Ni–Mn–Ga martensite. The authors suggest that the relief is due to the different straining of the surface and the bulk caused by the internal stresses associated with the magnetic shape memory effect. As a result of the relief the projection of the (011) twin traces upon the (010) plane creates the observed zigzag pattern. The surface tilt angle calculated from the zigzag pattern is ∼3°.

DISLOCATION MECHANISM OF TWINNING IN Ni–Mn–Ga

Tensile tests were performed in situ in a transmission electron microscope to investigate the twinning mechanism in non-modulated Ni–Mn–Ga martensite. The reorientation of the twin variants occurs via twinning dislocations. Their generation and movement were followed; the glide plane and Burgers vector were verified. Individual twinning dislocations were visualized.

Process study on the formation of nanocrystalline α-alumina with novel morphology at 1000 °C

We have reported in a short communication that nanocrystalline α-alumina with novel morphology was synthesized at a relatively low temperature of 1000 °C without any seed material. The α-alumina nanocrystallites at a size of 5 nm were observed for the first time, and more interestingly the nanocrystallites organized themselves into nanorods having widths of about 15 nm and lengths of about 50–250 nm. In this follow up full paper, detailed processing parameters of such nanocrystalline α-alumina are reported and more characterizations are carried out in order to study the mechanism of the synthesis. The formation scheme of the novel morphology of such nanocrystalline α-alumina is illustrated and the phase transformation from transition aluminas to α-alumina is discussed.

Investigation of magnetic domains in Ni–Mn–Ga alloys with a scanning electron microscope

The magnetic domains of martensite have been investigated with a scanning electron microscope in three Ni–Mn–Ga alloys with five-layered, seven-layered and non-layered (T) martensite structure. Type I magnetic contrast provides an overview of the domain pattern. This contrast arises from the stray field of the specimen and it is observed in a secondary-electron image. The type II magnetic contrast of a backscattered electron image gives the detailed magnetic microstructure together with the crystal morphology. A stripe domain pattern is formed in all the alloys when there is one dominant martensite variant in the sample. The second minor variant might be distorted due to interaction with the magnetic domain structure of the major variant. The mechanism of the deformation is not entirely clear and a tentative explanation for this deformation is suggested.

ESOMAT 2009 - 8th European Symposium on Martensitic Transformations

The high temperature transformations (e.g. liquidus, ordering temperature) of the alloys Ni47.7Mn31.2Ga21.1, Ni49.7Mn28.7Ga21.6, and Ni49.6Mn24.0Ga26.6, Ni47.3Mn30.3Ga20.3Fe2.1, Ni49.9Mn28.3Ga20.1Fe1.7, Ni51.3Mn14.4Ga26.3Fe8.0, Ni47.3Mn25.5Ga24.5Cu2.7, Ni48.3Mn29.7Ga21.1Cu0.9, and Ni49.4Mn23.3Ga25.6Cu1.7 were studied for the practical melting and annealing purposes. At first the chemical compositions (SEM-EDS) and the martensitic and magnetic transition temperatures (DSC, ac magnetic susceptibility) of the alloys were determined. High temperature DSC measurements were made in argon with 10 K/min. Two first measurements were carried out in the solid state (301 1273 K) and in the third measurement the material was melted (max meas. temp. 1573 K). The ordering temperature was obtained from the measurements in the solid state. As the e/a ratio was above 7.53 the ordering temperature was in the range of 1019-1039 K, otherwise a clear change was observed. The variation in heating and cooling was less than 5 K with small quaternary additions, but alloying of 8% Fe increased this difference to 18 K. Alloys with close Ni/Mn/Ga-ratio showed only minor differences in solidus and liquidus temperatures, but if there was a clear change in the Ni/Mn-ratio even those alloys having close e/a ratios showed a clear difference in melting behavior. When Ni/Mn is 1.5with higher values not clear region could be determined.

Nanosilver-Silica Composite

Infected superficial wounds were traditionally controlled by topical antibiotics until the emergence of antibiotic-resistant bacteria. Silver (Ag) is a kernel for alternative antibacterial agents to fight this resistance quandary. The present study demonstrates a method for immobilizing small-sized (~5 nm) silver nanoparticles on silica matrix to form a nanosilver–silica (Ag–SiO2) composite and shows the prolonged antibacterial effects of the composite in vitro. The composite exhibited a rapid initial Ag release after 24 h and a slower leaching after 48 and 72 h and was effective against both methicillin-resistant Staphylococcus aureus (MRSA) and Escherichia coli (E. coli). Ultraviolet (UV)-irradiation was superior to filter-sterilization in retaining the antibacterial effects of the composite, through the higher remaining Ag concentration. A gauze, impregnated with the Ag–SiO2 composite, showed higher antibacterial effects against MRSA and E. coli than a commercial Ag-containing dressing, indicating a potential for the management and infection control of superficial wounds. Transmission and scanning transmission electron microscope analyses of the composite-treated MRSA revealed an interaction of the released silver ions with the bacterial cytoplasmic constituents, causing ultimately the loss of bacterial membranes. The present results indicate that the Ag–SiO2 composite, with prolonged antibacterial effects, is a promising candidate for wound dressing applications.