The structural and magnetic properties of functional Ni-Mn-Z (Z = Ga, In, Sn) Heusler alloys are studied by first-principles and Monte Carlo methods. The ab initio calculations give a basic understanding of the underlying physics which is associated with the strong competition of ferroand antiferromagnetic interactions with increasing chemical disorder. The resulting d-electron orbital dependent magnetic ordering is the driving mechanism of magnetostructural instability which is accompanied by a drop of magnetization governing the size of the magnetocaloric effect. The thermodynamic properties are calculated by using the ab initio magnetic exchange coupling constants in finite-temperature Monte Carlo simulations, which are used to accurately reproduce the experimental entropy and adiabatic temperature changes across the magnetostructural transition. PACS numbers: 75.50.-y, 75.10.-b, 75.30.SgFollowing the concepts of Hume-Rothery the influence of composition on martensitic and magnetic transformation temperatures is commonly condensed as a dependency of electrons per atom (e/a-ratio) [1]. Experiment and first-principles calculations, however, reveal that the Z element in Ni-Mn-Z Heusler alloys (Z = Ga, In, Sn) also affects the transformation temperatures substantially [2]. Moreover, recent experiments on samples with identical composition but different heat treatment indicate that chemical disorder also plays an important role [3][4][5]. Here, we use first-principles calculations to identify the influence of chemical disorder on the magnetic exchange parameters and derive guidelines for a further systematic improvement of magnetocaloric materials [6].Besides the magnetocaloric effect (MCE) in Gd and other alloys at room temperature [7,8], the metamagnetic Ni-Mn based Heusler materials [9,10], have attracted much interest recently [11,12]. In these alloys the metamagnetic features are responsible for magnetic glass behavior and frustration due to chemical disorder [13][14][15] as well as unusual magnetization behavior under an external magnetic field such as a large jump of the magnetization ∆M (T m ) at the martensitic/magnetostructural transformation temperature T m [16]. This gives rise to the large inverse MCE of the materials [9,10,17,18]. The MCE can be influenced when Ni is substituted in part by Co: It is strongly enhanced in the case of In-based intermetallics [19,20] (with adiabatic temperature change ∆T ad = −6 K in 2 T field [20]) while in the case of Ga the MCE is turned from direct to inverse by decoupling T m and Curie temperature T C [21] (with ∆T ad = −1.6 K in 1.9 T field [22,23]).Chemical disorder in the Mn-rich Heusler alloys is responsible for competing magnetic interactions (ferromagnetic versus antiferromagnetic) because the extra Mn atoms occupy lattice sites of the Z-sublattice which interact antiferromagnetically with the Mn atoms on the Y-sublattice due to RKKY-type interactions. This competition of magnetic interactions leads to the characteristic drop of magnetization curves at T m , wh...

No abstract

The interplay of structural and magnetic properties of magnetic shape memory alloys is closely related to their composition. In this study the influence of the valence electron concentration on the tetragonal transformation in Ni2Mn1 + xZ1 − x (Z = Ga, In, Sn, Sb) and Co2Ni1 + xGa1 − x is investigated by means of ab initio calculations. While the type of magnetic interaction is different for the two series, the trends of the total energy changes under a tetragonal transformation are very similar. We find that tetragonal structures become energetically preferred with respect to the cubic one as the valence electron concentration e/a is increased regardless of the system under consideration. In particular, the energy difference between the austenite and martensite structures increases linearly with e/a, which is in part responsible for the linear increase of the matensite transformation temperature. The substitution of nickel by platinum increases even further the transformation temperature.

No abstract

Broken drill bits constitute the largest proportion of broken orthopedic instruments. We report a new technique which allows atraumatic removal of cannulated drill bits. The technique is simple and does not require any special instrumentation.

No abstract

In this paper, a comprehensive approach to numerical and experimental analysis of microchamber filling in centrifugal microfluidics is presented. In the development of micro total analysis systems, it is often necessary to achieve complete, uniform filling of relatively large microchambers, such as those needed for nucleic acid amplification or detection. With centrifugal devices, these large microchambers must often be orientated perpendicularly to the direction of centrifugal force and are usually bounded by materials with varying surface properties. The resulting fluidic flow in such systems can be complex and is not well characterized. To gain further insight into complex fluidic behavior on centrifugal microfluidic platforms, numerical modeling using the Volume of Fluids method is performed to simulate microchamber filling in a centrifugal microfluidic device with integrated sample preparation, amplification, and detection capabilities. Parametric analyses are performed using numerical models to predict microchamber filling behavior for a range of pressure conditions. High-speed flow visualization techniques are used to track the liquid meniscus during filling of the microchambers, and comparison to the numerical predictions for experimental validation is achieved by analyzing the liquid volume fraction as a function of the non-dimensional temporal profile during filling. When channel filling profiles are compared, the numerical model predictions utilizing static conditions are in strong agreement with the experimental data. When dynamic modeling conditions are used, the numerical predictions are extremely accurate as compared to the experimental data.

In this work, the effects of small ternary additions to B2 NiTi structures was investigated through DFT calculations. The analysis considered deviations from stoichiometry arising from either simple substitution of host atoms in a given sublattice or from the formation of anti-sites. The calculations enabled the determination of the site preference of X ternary additions. Moreover, the results suggest that ternary additions located in the central region of the transition metal group across all periods tend to occupy Ni sites due to favorable X-Ti nearest neighbor (NN) interactions. This occupancy is achieved through substitution or through the generation of anti-site defects. On the other hand, ternary additions at both ends of a given transition metal row tend to occupy Ti sites due to favorable X-Ni NN interactions. Once site preferences are determined, the effect of alloying on the thermodynamic and mechanical properties of B2 NiTi-X structures are presented and trends are discussed.