Regularly distributed structural inhomogeneities in the MgB2 matrix, such as nano-areas with a high concentration of boron (MgBx) and impurity oxygen (Mg-B-O nano-layers or inclusions), are observed ...in all materials independently of the preparation method, pressure (0.1 MPa-2 GPa) and temperature (600-1100 °C), and in materials with different connectivity (18-98%) and density (55-99%). Such inhomogeneities can act as pinning centers in MgB2 because the variation of their size and distribution are well correlated with variations of the critical current density, jc. The decrease in size of MgBx inclusions, the transformation of 15-20 nm thick Mg-B-O nano-layers into separated inclusions, and the localization of impurity oxygen are accompanied by an increase in critical current density in low and medium magnetic fields. The efficiency of these defects is evidenced by a shift from grain-boundary pinning to point pinning.
We study the influence of technological media of solid-oxide fuel cells on the mechanical and physical properties of Crofer JDA alloy and the materials based on the Ti
3
AlC
2
-type MAX-phase. It is ...shown that Ti
3
AlC
2
and Ti
3
AlC
2
–Nb materials with a porosity of 1% are comparable with Crofer JDA alloy by electric conductivity but possess higher strength and heat resistance than this alloy for lower density. These results enable us to recommend the investigated materials for manufacturing the interconnects of solid-oxide fuel cells.
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DOBA, EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, IZUM, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UILJ, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
The volume pinning force,
F
p(max)
, increases with increasing synthesis or sintering pressure (0.1 MPa–2 GPa) in materials prepared at high temperature (1050 °C) while it stays practically unchanged ...in those prepared at low temperature (800 °C). The position of
F
p(max)
can be shifted to higher magnetic fields by: (1) increasing the manufacturing pressure or decreasing the temperature (2) additions (Ti, SiC, or C, for example), and (3) in-situ preparation. Grain boundary pinning (GBP) dominates in materials prepared at low temperatures (600–800 °C), while high-temperature preparation induces strong point pinning (PP) or mixed pinning (MP) leading to outstanding properties. In materials produced by spark plasma sintering (SPS), the position of
F
p(max)
is higher than expected for both grain boundary and point pinning. The distribution of boron and oxygen in MgB
2
based material, which can changed by additions or the preparation conditions, significantly affects the type and strength of pining.
Materials prepared under a pressure of 2 GPa with a nominal composition of Mg:7B or Mg:12B consist of 88.5 wt % MgB
12
, 2.5 wt % MgB
2
, 9 wt % MgO or 53 wt % MgB
12
, 31 wt % MgB
20
16 wt % MgO, respectively. Their magnetic shielding fractions at low temperatures are 10 % and 1.5 %, with a transition temperature,
T
c
of 37.4–37.6 K. Although their magnetic critical current density at zero field and 20 K was 2–5×10
2
A/cm
2
, they were found to be insulating on the macroscopic level.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
The distribution of nanostructural inhomogeneities, such as areas with high concentration of B and impurity O, plays one of the key roles on the variation of the critical current density,
j
c
, of ...MgB
2
materials. The effect of O and B redistribution can be enhanced by Ti addition and thus leads to the
j
c
increase. The effect of Ti addition and the mechanism of this effect are discussed based on experiments of MgB
2
synthesis under 2 GPa pressure at 800 and 1,050 °C.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
A structural Auger spectroscopy study of MgB2 thin (∼140 nm) oxygen-containing polycrystalline films produced by magnetron sputtering and 99% dense MgB2 bulks synthesized at 2 GPa allows us to ...conclude that jc of MgB2 depends to a high extent on the amount and distribution of oxygen in the material matrix. jc reached 7.8-2.7 MA cm2 below 1T at 20 K in the films and 0.3-0.9 MA cm2 (depending on the boron used) in the bulks. The higher jc in MgB2 thin films can be associated with finer oxygen-enriched Mg-B-O inclusions and their higher density in the film structure compared to the bulk. Calculations of the total electron density of states (DOS) in MgB2, MgB1.75O0.25, MgB1.5O0.5 and MgBO showed that all the compounds are conductors with metal-like behaviour. The DOS is even higher in MgB1.5O0 5 than in MgB2 and the binding energy is similar. So, the experimentally found presence of some dissolved oxygen in MgB2 does not contradict its high SC performance. The introduction of a high amount of oxygen into the MgB2 structure does not dramatically reduce the material's Tc and allows obtaining highjc as observed in our MgB2 films and bulks.
Bulk MgB 2 - and YBaCuO-based materials are competitive candidates for applications. The properties of both compounds can be significantly improved by high temperature-high pressure preparation ...methods. The transformation of grain boundary pinning to point pinning in MgB 2 -based materials with increasing manufacturing temperature from 800 to 1050 ° C under pressures from 0.1 MPa to 2 GPa correlates well with an increase in critical current density in low and intermediate magnetic fields and with the redistribution of boron and oxygen in the material structure. As the manufacturing temperature increases (to 2 GPa), the discontinuous oxygen-enriched layers transform into distinct Mg-B-O inclusions, and the size and amount of inclusions of higher borides MgB X (X>;2) are reduced. The effect of oxygen and boron redistribution can be enhanced by Ti or SiC addition. The oxygenation of melt-textured YBa 2 Cu 3 O 7 - δ (MT-YBaCuO) under oxygen pressure (16 MPa) allows one to increase the oxygenation temperature from 440°C to 700-800°C, which leads to an increase of the twin density in the Y123 matrix and to a decrease of dislocations, stacking faults, and the density of microcracks, and as a result, to an increase of the critical current density, J c , and the trapped magnetic field. In MT-YBaCuO, practically free form dislocations and stacking faults and with a twin density of 22-35 μm -1 , J c of 100 kA/cm 2 (at 77 K, 0 T) has been achieved, and the importance of twins in Y123 for pinning was demonstrated experimentally.
Regularly distributed structural inhomogeneities in the MgB sub(2) matrix, such as nano-areas with a high concentration of boron (MgB sub(x)) and impurity oxygen (Mg-B-O nano-layers or inclusions), ...are observed in all materials independently of the preparation method, pressure (0.1 MPa-2 GPa) and temperature (600-1100 degreesC), and in materials with different connectivity (18-98%) and density (55-99%). Such inhomogeneities can act as pinning centers in MgB sub(2) because the variation of their size and distribution are well correlated with variations of the critical current density, j sub(c). The decrease in size of MgB sub(x) inclusions, the transformation of 15-20 mn thick Mg-B-O nano-layers into separated inclusions, and the localization of impurity oxygen are accompanied by an increase in critical current density in low and medium magnetic fields. The efficiency of these defects is evidenced by a shift from grain-boundary pinning to point pinning.
Composite materials based on AlN-SiC have been produced by the pressureless sintering. It has been established that in the formation of the AlN-SiC-Y
3
Al
5
O
12
structure the yttrium-aluminum garnet ...locates along the boundaries of SiC and AlN grains, thus preventing the mutual dissolution of AlN-SiC. It has been found that the increase of the SiC amount from 20 to 50 wt % results in changes of the
a
and
c
parameters of the crystal lattice from 0.49821 to 0.49837 nm and from 0.5046 to 0.498 nm, respectively, and in increase of the composite absorbing ability from 8.8 to 31.4 dB/cm (at frequencies of 9.5–10.5 GHz).
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
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