G.Q. Yuan

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...

Effect of Pb substitution in bulk superconducting MgB2

Abstract The samples of Mg 1− x Pb x B 2 ( x =0, 0.01, 0.03, 0.05, and 0.06) were successfully synthesized. The temperature dependence of resistance was measured at variant external magnetic field ranged from 0 to 6.15 T. The effects of Pb-doping on the critical temperatures in zero magnetic field, the upper critical fields and the coherence lengths of the samples are discussed. The experiment results show that the critical temperature of Mg 1− x Pb x B 2 decreases with increasing the content of Pb. Contrarily, the absolute value of derivative of H c2 , namely |d H c2 /d T |, increases as the content of Pb is increasing. The results imply that the impurities and defects due to Pb-doping play an important role in the system of Mg 1− x Pb x B 2 . Finally, according to the relationship between H c2 and T , the value of coherence lengths at zero temperature of the samples was estimated.

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...

Study of synthesis technology and superconductivity of MgB2

Abstract For fabricating a dense and superconducting bulk MgB 2 samples, a synthesis technique was employed. Our experimental results show that there is a suitable sintering temperature range, say from 800 to 850 °C. In this temperature range, the critical temperature of the samples is 38 K above, and the superconducting transition is sharper than that of those sintered out of the range. The temperature dependence of resistance of MgB 2 in high temperature region was also measured. The temperature dependence of the resistance, at temperature lower than 700 °C, obeys a form of a + bT n , where n ∼2.9. However, the resistance of the sample exhibits a negative temperature coefficient, as T is higher than 700 °C. This indicates that MgB 2 has semiconducting behavior.

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.

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.