Dense MgB2 Ceramics by Ultrahigh Pressure Field-Assisted Sintering

Abstract

Magnesium diboride (MgB2) ceramics, due to their impressive transition temperature of 39 K for superconductivity, have been widely investigated. The possibility to obtain highly dense MgB2 ceramics with fine microstructure and grain boundaries acting as pinning sites by novel high-pressure-assisted spark plasma sintering (HP-SPS) is reported in this article. HP-SPS was employed to reach 100% density in MgB2 ceramics, and high pressure was utilized in the consolidation of MgB2. An increase in pressure helped in stabilizing the MgB2 phase above thermal decomposition, thus avoiding the formation of non-superconducting phases such as MgO and MgB4. Pressure allowed strengthening of the covalent bond (condensation effect) to increase the thermal stability of MgB2. HP-SPS yielded high mechanical hardness in MgB2 (1488 HV). For better electrical connectivity, which leads to large magnetic moments in high density samples were obtained with the beneficial effect of high applied pressure (1.7–5 GPa) at high temperature (>1000 °C). The combination of the SPS process and high pressure ensured retention of the homogeneous fine microstructure required to obtain high current density and high hardness.

Recent Developments of High-Pressure Spark Plasma Sintering: An Overview of Current Applications, Challenges and Future Directions

“…In the same vein, increasing applied pressure becomes essential to allow sintering at temperatures below the decomposition temperature of the compounds (for example MgB 2 [ 31 ], BP [ 29 ], etc.) or to increase the thermal stability of precursors, for example by confining OH, H 2 O or other volatile elements [ 29 ].…”

High Pressure (HP) in Spark Plasma Sintering (SPS) Processes: Application to the Polycrystalline Diamond

“…In both cases, sintering occurred at lower temperature than in classic HP-HT setups. Another example showed the sintering of MgB 2 [ 16 ]. Here, the increase in pressure (in the range 2 to 5 GPa ) stabilized the phase above its decomposition temperature and high temperature promoted the sintering up to relative density of 100% with a fine-grain microstructure [ 16 ].…”

“…In particularly, pressure allows the synthesis of materials, even with different thermal stabilities precursors [ 9 ]: to orientate the chemical reaction in the direction of synthesis leading to the densest phase by the Le Chatelier principle (ex: synthesis of diamond to the detriment of graphite), e.g., [ 10 , 11 ]; to initiate a new finer microstructure by driving the phase transformation in polymorphic materials (ex: Al 2 O 3 : γ → α), e.g., [ 12 , 13 ]; to improve the chemical reactivity for refractory materials sintering (borides, nitrides, carbides) to better densification compared to lower pressure processes, e.g., [ 14 , 15 ]; to allow the sintering beyond the thermal decomposition temperature by the condensation effect, i.e., pressure stabilizing structure (ex: MgB 2 ), e.g., [ 16 ]; to sinter the high-pressure stable phase in the high-pressure stability domain (ex: c-C, c-BN), e.g., [ 17 ]; to adjust the porosity, close to 0% (ex: transparent ceramics) or high porosity ( p > 50% ) (ex: bone structure mimetic), e.g., [ 18 ]; to increase the thermal stability of precursors by condensation effect by avoiding the departure of OH − , H 2 O, others volatile elements) [ 19 ]; to decrease the sintering/consolidation/densification temperature by its driving force in order to avoid grain growth (which is always activated by high temperature), e.g., [ 20 ]; to favor the structural phase existing only at lower temperature (ex for amorphous calcium phosphate), e.g., [ 21 ]; to allow the consolidation of thermally unstable materials such as organic materials (ex: polymer) [ 22 ] and to allow the consolidation of composite constituted by materials of different thermal stability (ex: polymer composites) [ 23 ]. …”

“…to allow the sintering beyond the thermal decomposition temperature by the condensation effect, i.e., pressure stabilizing structure (ex: MgB 2 ), e.g., [ 16 ];…”

“…− to favor the structural phase existing only at lower temperature (ex for amorphous calcium phosphate), e.g., [21]; In particularly, pressure allows the synthesis of materials, even with different thermal stabilities precursors [9]: − to orientate the chemical reaction in the direction of synthesis leading to the densest phase by the Le Chatelier principle (ex: synthesis of diamond to the detriment of graphite), e.g., [10,11]; − to initiate a new finer microstructure by driving the phase transformation in polymorphic materials (ex: Al 2 O 3 : γ → α), e.g., [12,13]; − to improve the chemical reactivity for refractory materials sintering (borides, nitrides, carbides) to better densification compared to lower pressure processes, e.g., [14,15]; − to allow the sintering beyond the thermal decomposition temperature by the condensation effect, i.e., pressure stabilizing structure (ex: MgB 2 ), e.g., [16]; − to sinter the high-pressure stable phase in the high-pressure stability domain (ex: c-C, c-BN), e.g., [17]; − to adjust the porosity, close to 0% (ex: transparent ceramics) or high porosity (p > 50%)…”