**6. References**


The mixed oxides modified route (solid-state reaction) is a direct alternative to obtain highly densified BSTx ceramics. In this route, the high-energy ball milling and the applied heat treatment allowed the preparation of nanometric powders (less than 200nm) with Perovskite-type structure ABO3. The Curie temperature of the BSTx ceramics was unambiguosly determined as a function of temperature by several techniques: Raman spectroscopy, differential scaning calorimetry, and measurements of dielectric constant. This temperature was succesfully tuned from 87 K to 400 K by varying the Sr/Ba ratio, as expected. However, a shifting in the orthorhombic to tetragonal phase transition was observed in the sintered ceramics. For instance, the OTPT for the BST0-BST2 samples was shifted down 17-27 K with respect to the literature. The origin of this shifting is probably residual stresses associated to the fine-grained microstructure of the sintered samples.

The group of BST0, BST1, BST2 and BST3 ceramics present P(E) curves with ferroelectric behavior at room temperature. The other cases present paraelectric behavior. Moreover, the BST0, BST1and BST2 samples present rather low maximum and remanent polarization and coercive fields. BSTx ceramics (x = 0, 1, 2, 3) displayed piezoelectric response in the contact resonance piezoresponse force microscopy mode (CR-PFM). The polycrystalline BSTx ceramics showed ferroelectric domains with sizes several times smaller than the grains.

This work was partially supported by CONACYT, Mexico. The authors would like to thank Michael Boldrick Ph. D. and Rodrigo Mayen Mondragón Ph. D. for their help regarding the english translation. Besides, we thank Pedro García J., J. Eleazar Urbina A., M. Adelaido Hernández L., Francisco Rodríguez M., Agustín Galindo S., Rivelino Flores F., Alfredo

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**Part 6** 

**Superconducting Ceramics** 


**Part 6** 

**Superconducting Ceramics** 

466 Sintering of Ceramics – New Emerging Techniques

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**21** 

**Sintering Process and Its** 

*Tianjin Key Lab of Composite and Functional Materials,* 

Zongqing Ma and Yongchang Liu

*Tianjin University, Tianjin,* 

*P R China* 

*School of Materials Science & Engineering,* 

**Mechanism of MgB2 Superconductors** 

**1. Introduction** 

**1.1 The phase formation mechanism of MgB2 during sintering** 

investigate the sintering mechanism of MgB2 superconductors.

dependent magnetization (M–T) measurements [28].

The superconductivity at 39 K discovered in MgB2 among simple binary chemical composition attracted much interest in its fabrication techniques and practical applications [1]. MgB2 superconductor exhibits many impressive properties such as highest critical temperature amongst intermetallic superconductors which means low cooling costs, impressive grain boundary transparency to the flow of current which leads to greater critical current density [2- 4], comparatively large coherence length which allows a better Josephson junction fabrication, low material cost which will lead to a cheaper superconductor technology, simple crystal structure, etc. Hence, MgB2 superconductors, especially the MgB2 wires and coils, have the outstanding potential to be integrated into diverse commercial applications, such as, magnetic resonance imaging (MRI) [5, 6] , fault current limiters (FCL), Josephson junctions and SQUID [7, 8, 9], transformers, motors, generators, adiabatic demagnetization refrigerators, magnetic separators, magnetic levitation applications, energy storage, and high energy physical applications. But the MgB2 itself is mechanically hard and brittle and therefore not amenable to drawing into the desired wire and tape geometry. Thus, the powder-in-tube (PIT) technique that was used to make the Y-Ba-Cu-O oxide superconductor has been employed in the fabrication of MgB2 wires and tapes these years [10-14]. So far, in-situ sintering, including the in-situ PIT, from the mixture of magnesium and boron is the major method to fabricate MgB2 superconductors (bulks, wires and tapes). The corresponding sintering parameters have a significant influence on the superconducting properties of MgB2. Thus it is necessary to

The reaction process and MgB2 phase formation mechanism during the sintering have been studied by different methods, such as differential thermal analysis (DTA) [15-21], *in-situ* XRD measurement [22-25], *in-situ* resistance measurement [26, 27] and temperature

It can be seen from the DTA data of the Mg + 2Bamorphous precursor composition shown in Fig. 1 that the first exothermic peak occurs in the temperature range below 650 oC (the

**1.2 Sintering of Mg-B precursor powders over a wide temperature range** 
