Giant magnetocaloric effect driven by structural transitions

Magnetic cooling could be a radically different energy solution substituting conventional vapour compression refrigeration in the future. For the largest cooling effects of most potential refrigerants we need to fully exploit the different degrees of freedom such as magnetism and crystal structure. We report now for Heusler-type Ni–Mn–In–(Co) magnetic shape-memory alloys, the adiabatic temperature change ΔT(ad) = ?3.6 to ?6.2?K under a moderate field of 2?T. Here it is the structural transition that plays the dominant role towards the net cooling effect. A phenomenological model is established that reveals the parameters essential for such a large ΔT(ad). We also demonstrate that obstacles to the application of Heusler alloys, namely the usually large hysteresis and limited operating temperature window, can be overcome by using the multi-response to different external stimuli and/or fine-tuning the lattice parameters, and by stacking a series of alloys with tailored magnetostructural transitions.     

Design and Qualification of Pr–Fe–Cu–B Alloys for the Additive Manufacturing of Permanent Magnets

The direct use of an advanced binder-free additive manufacturing technique, namely laser powder bed fusion (L-PBF), does not easily allow obtaining variously shaped, fully dense Nd–Fe–B magnets with high coercivity. The process inherently leads to the re-melting of the powder and appearance/disappearance of undesired/desired microstructural features responsible for low and large coercivity. In this work, the development of a useful microstructure responsible for high coercivity in Pr₂₁Fe_73.5Cu₂B_3.5 and Nd₂₁Fe_73.5Cu₂B_3.5 alloys and a possible way to produce fully dense permanent magnets via additive manufacturing processes is demonstrated using: (i) suction casting technique, which provides a high cooling rate and thus similar microstructures as in L-PBF but requires only very small amounts of powder; (ii) conventional L-PBF processing using kg of powder, and (iii) a subsequent annealing treatment that is similar to a conventional sintering treatment. The subsequent heat treatment is necessary to develop high coercivity by forming a novel microstructure: hard magnetic (Nd,Pr)₂Fe₁₄B grains embedded in a matrix of intermetallic (Nd,Pr)₆Fe₁₃Cu phase. Furthermore, it is demonstrated that Pr₂₁Fe_73.5Cu₂B_3.5 exhibits a higher coercivity than Nd₂₁Fe_73.5Cu₂B_3.5 because of a finer and more homogeneous grain size distribution of the Pr₂Fe₁₄B phase. The final L-PBF printed Pr₂₁Fe_73.5Cu₂B_3.5 samples provide a coercivity of 0.75 T.     

Improving the detectability of cracks during flow inspection by 3D tomography

The advantages of X-ray computer tomography over industrial radiography are described as applied to the problem of detecting arbitrarily oriented cracks. The approximate Feldkamp projection algorithm and the approximate reverse-projection algorithm with double differentiation filtration (RPDDF) are considered and methods for improving their accuracy are proposed. The possibility of increasing the reconstruction rate by using the pipelined algorithm and the spiral trajectory of a source is shown.     

Effect of shock-wave loading on the internal microstructure and mechanical properties of fine-grained copper

It is shown that preloading of fine-grained copper with a the grain size of 0.5 m by a shock wave of intensity ≈25–50 GPa does not lead to changes in its internal microstructure and mechanical properties, and the dislocation density increases only slightly from 1.8 · 10¹¹ cm^s-2 in the initial state to (3.1–3.6) · 1011 cm^s-2 after shockwave loading. An increase in shock wave intensity to pressures > 55 GPa leads to a decrease in the dislocation density to 2.5 · 10⁹ cm^s-2, an increase in the grain size to ≈19 fum, the occurrence of microtwins inside the grains, and a reduction in the mechanical properties of fine-grained copper to the level of coarse-crystalline copper.     

Magnetic anisotropy and magnetic properties of RTSi (R=Gd, Y; T=Mn, Fe) compounds

Magnetic properties of Gd_xY_1−xMnSi and GdMn_xFe_1−xSi compounds with CeFeSi-type structure have been studied by magnetization measurements in the temperature range 77–600 K in static magnetic fields up to 12 kOe. The measurements for some compounds have been carried out on single crystal samples. The easy magnetization direction of all single crystal samples was found to be the c axis. The value of the anisotropy constant K₁ for GdMnSi was estimated to be 2.0·10⁶ erg cm⁻³ at 77 K. The substitution in both rare earth and 3d sublattices leads to a sharp increase in magnetic anisotropy of these compounds. The concentration dependencies of magnetic ordering temperatures, effective magnetic moments and paramagnetic Curie temperatures have been determined. The obtained results can be explained by the modification of the band structure due to the change of interatomic distances and the filling of 3d band.     

Comparative analysis of techniques for revealing cracks in noisy X-ray tomography and introscopy images

A review is provided of techniques for digital processing of noisy X-ray computed tomography and introscopy data that are represented by halftone images. The most hazardous crack flaws placed against a background with a wide range of brightness have been studied. Difference median and slit filters are proposed for processing such images.     

Thermal destruction of superconductivity of YBa2Cu307−x thin films under various conditions of heat removal

The volt-ampere characteristics of thin films of YBa₂Cu₃0_7–x with different heat removals from a specimen are experimentally determined. The influence of boiling of liquid nitrogen on the destruction dynamics of the superconducting state in thin HTSC-films loaded with the carrier current is investigated.Institute of Thermal Physics, Ural Branch of the Russian Academy of Sciences, Ekaterinburg. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 64, No. 5, pp. 588–593, May, 1993.