D.W. Lu

Studies of strong magnetic field produced by permanent magnet array for magnetic refrigeration

The success of a room temperature magnetic refrigerator (RTMR) depends critically on two essential parts: a high magnetic field and a magnetic refrigerant material with large magnetocaloric effect. A carefully designed hollow cylindrical permanent magnet array (HCPMA) can be used to provide strong magnetic field in the cavity, the magnitude of the resulting static field can be even greater than the remanence magnetization of the magnets comprising a HCPMA. A thorough understanding of the magnetic field distribution will provide an invaluable insight into the design and optimization of HCPMA in the reciprocating and rotary RTMR systems. Here, we show a construction of a 16 piece HCPMA with realistic dimensions and we illustrate the mechanism of generating a high magnetic field in such device. We present an effective way to calculate the field distribution of a permanent magnet array with finite size and an unsymmetrical geometry. Furthermore, detailed numerical results of the magnetic field distribution an...

Self-modification of the remanence distribution of a hollow cylindrical permanent magnet array

A hollow cylindrical permanent magnet array (HCPMA) used for refrigeration generates a strong magnetic field in the access hole and also inside the permanent magnets. The generated field modifies the remanence distribution of the magnets. Here, we describe a self-consistent calculation method for the remanence vectors and magnetic inductions using the Biot-Savart law. The method takes into account the anisotropic magnetization loops of commercial permanent magnets. Results obtained for the modified remanence distribution of a long HCPMA show demagnetization and deflection, including the reversal of original remanence vectors, when the generated field of the HCPMA exceeds the coercivity of the permanent magnets. The reversed remanence vectors lead to the formation of remanence vortices. In comparison with the field obtained from the ideal model assuming constant remanence vectors, remanence demagnetization and deflection result in obvious decreases in both the field magnitude and field homogeneity in the a...

Simulations of remanence and field distributions of permanent magnet array with rectangular cavity

The magnetic phenomena of a Halbach-type permanent magnet array under a strong self-field such as demagnetization, saturation, deflection, and rotation of remanence vectors occur deep in the interior of the magnets, and thus lead to an inhomogeneous field even for a hollow cylindrical permanent magnet array with circular cavity. Considering the anisotropic magnetization loops of commercially available permanent magnets, the self-consistent remanence and field distributions of a 16 piece hollow cylindrical permanent magnet array with rectangular cavity (HCPMARC) are presented here to simulate the magnetic phenomena inside the magnets. The field distribution of a HCPMARC can be modulated by array structure while keeping both the cross sectional areas of the cavity and the magnets constant. At the expense of field uniformity, a HCPMARC acquires greater center field uniformity than a HCPMACC does. Furthermore, we simulated the magnetization and field distributions for a yoked HCPMARC obtained by replacing the...

A Field Distribution of Dynamic-Link Magnetic Circuit for Room-Temperature Magnetic Refrigeration

A prevalent room-temperature magnetic refrigerator of permanent magnets usually utilize the structure of stationary magnetic circuit. A new notional method of dynamic-link magnetic circuit (DLMC) is proposed in this paper. The DLMC is made up of a changeable magnetic circuit, and it may generate a time-varying magnetic field in a certain space when some parts of the DLMC move. A typical DLMC is introduced in the paper, and a numerical value calculation and an experiment measuring for the DLMC are presented then. The research result shows that such a DLMC may produce a large variation of magnetic field when it is used in a rotary magnetic refrigerator. Although the minimum field is not equal to zero strictly, but it is much smaller than the maximum field, and the time-varying magnetic fields may be used to magnetize and demagnetize the magnetic refrigerants

Thermally assisted flux flow character of MgB2

Abstract As a new superconductor, MgB 2 has many different characteristic features in contrast to the traditional HTSC. The investigation of these features is important both for theoretical study and application of this material. Several groups reported that from the vortex phase diagram of MgB 2 , 3D vortex glass phase and vortex liquid phase are found. But the flux dynamic mechanism held by this new superconductor could not be considered as a merely a copy of those of the HTSC. While the former is a s-wave superconductor, the latter is d-wave superconductor. In the present article, we present an extensive investigation on the flux mechanism in MgB 2 by transport measurement from 5 K to T c of the sample at magnetic fields up to 7 T, and deal with the experimental data with the linear simulation method within framework of the thermally assisted flux flow theory, which was first proposed by Kes et al. Finally, we obtained an exact expression of the pinning barrier U 0 in the liquid phase of vortex matter of MgB 2 .

Pinning effects and current density broadening of resistance on MgB2

Abstract We prepared the indium doped MgB 2 bulk sample by conventional solid state reaction. Systematic measurements of resistance versus temperature were performed on the sample with different transport current and different applied magnetic field. The curves of resistance in opposition to temperature are broadened with different transport current in transmitting temperature range. We present a method for the first time to analyze the experimental data and explore the pinning mechanisms of MgB 2 for such kind of experiments. By analyzing on the experimental data, we found that the effective potential U eff ( T , H , J ) is of logarithmic form in our experimental temperature and applied magnetic field regime. The curve U 0 ( T , J ) is linear dependent on temperature but the slope coefficient is changing lower near onset critical temperature.

Investigation on the anisotropy of the current density characteristics and flux pinning in Ag-sheathed Bi-2223 tape

Abstract In the J – T curves of Ag-sheathed Bi-2223 tape, when the applied magnetic field is perpendicular to the tape surface, the shape of the curves changes from the convex to concave. On the other hand, no such change is found in the curves for B ‖ ab plane. Furthermore, a linear temperature dependence is observed in both applied field directions under zero field. In the J – B curves for different given temperatures, for B ‖ c axis, J decreases monotonously with increasing field, while for B ‖ ab plane, a power-law relation of J ∝ B n −1 at temperatures from 23 to 60 K is observed with different exponent n in low field region, which could be attributed to the presence of surface pinning. Mchenry and Sutton [Prog. Mater. Sci. 38 (1994) 159] have suggested an empirical expression for the effective pinning energy, which we have employed together with Andersons thermal activation theory of the flux bundle to estimate the value of the effective pinning energy. It is shown that the effective energy of the B ‖ ab is about two times larger than that of B ‖ c axis due to the strong intrinsic pinning of the tape.

A new method of zooming in the cool power of the room temperature magnetic refrigeration

Publisher Summary This chapter proposes a new method of using the high heat conductivity materials to improve the heat conductivity of the magnetic refrigerants. The cycle speed of a room temperature magnetic refrigerator determines the cooling power. All the materials of giant magnetocaloric effect have much lower heat conductivity compared with copper, so the refrigerator with such a material has a less cooling power. The new method to be used in magnetic metal Gd with Ag may zoom in about 10 times of the cooling power. The possibility of increasing the heat transfer and keeping the high ME simultaneously by recomposing magnetic refrigeration materials with the high thermal conductivity materials has been explored. It is obvious that, as to different kinds of high heat conductivity materials, the perfect match of d1 and d2 is different (for the three compound materials, the optimal match are about 6:7(GaAI), 3:4(GdCu), and 8:9(GdAg) respectively), and ηmax is also different correspondingly. The enhancement using GdAg compound materials is highest, about ten times compared with using the pure Gd. In addition, the method may be also used to improve the performance of the cryogenic magnetic regenerative materials.

Chapter 40 – Room temperature magnetic refrigerator using both metal Gd or/and Gd-Si-Ge alloys and a permanent magnetic field source

Publisher Summary This chapter introduces a room temperature magnetic refrigerator using both metal Gd or/and Gd-Si-Ge alloys and permanent magnetic field source. The permanent magnet is assembled in a shape of cylinder in which there are 1.4 Tesla uniform magnetic fields. A temperature span of 25 K is reached for the alloys when environment temperature is about 290 K. In real circulation, the system reaches a temperature span more than 26 K when Gd spheres are used, and a temperature span of 25 K when Gd5Si1.985Ge1.985Ga0.03 spheres are used. When these two kinds of working materials are substituted with Gd5Si2Ge2 alloy, the temperature span is less than 20 K. But if the alloys are short of Ga, the less temperature range could be reached at the same temperature. In addition, the magnetic refrigerator is also filled with metal Gd as the refrigerant; a temperature span of 26 K is reached. Although it cannot be concluded that Gd-Si-Ge alloys have a better cooling effect than metal Gd, but the alloys are indeed valuable because of its optimal chemical stableness.

Pinning effects and magnetization hysteresis loops in MgB2

Abstract We propose a method to find the effective pinning potential from magnetization hysteresis loops at different temperature directly. As an example, the pinning characteristics of MgB 2 are studied. The form of the effective pinning potential U eff dependence on temperature, applied magnetic field and current density is also obtained.