The magnetic characteristics of iron oxide nanopowder (Fe
3
O
4
base phase) produced by electroerosion dispersion and consolidated at high pressures (2 GPa) and high temperatures (900, 1000, 1100, ...1200, and 1300°C) for 0.07 h in contact with hexagonal boron nitride were studied. The nanopowder was produced by dispersing iron granules or shavings in plasma induced at contact points between the granules when electric pulses of high current and voltage were passed through them. The metal granules being dispersed are in a constantly circulating liquid (water) atmosphere, creating a pseudo-boiling layer from the granules. The liquid (water in this case) cools the granules to prevent them from being welded and oxidizes the metal vapors that emerge in plasma, forming nanosized iron oxide grains carried by the liquid flow into sedimentation tanks (powders with different grain sizes sediment in different tanks). Room-temperature studies of the magnetic characteristics of samples consolidated from iron oxide powders showed that the materials sintered at 1200 and 1300°C were soft magnetics with virtually zero hysteresis. Their specific magnetic moments at 5000 Oe were 128.4 and 126.4 emu/g and the coercive force was negligibly small: 5.1 and 4.5 Oe. The materials sintered at 1100°C were characterized by a specific magnetic moment of 90.4 emu/g and a relatively low coercive force of 9.1 Oe. The specific magnetic moments of the samples sintered at 900 and 1000°C were significantly lower and the coercive force higher: 40.2 and 42.1 emu/g and 37.9 and 32.4 Oe, respectively. X-ray diffraction with Rietveld refinement revealed that the materials consolidated at 900 and 1000°C contained 75–80 wt.% FeO and 25–20 wt.% Fe, while the materials sintered at 1100°C contained, along with 32 wt.% FeO and 2 wt.% Fe, a significant amount of Fe
3
N (66 wt.%). The materials consolidated at 1200–1300°C contained 100% Fe
3
N phase. Hence, under high pressures and increasing sintering temperatures, iron oxides are reduced and then iron is nitrided with nitrogen released from boron nitride, which improves the soft magnetic characteristics of the sintered materials.
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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 development of hydrogen energy and, in particular, high-performance submersible liquid hydrogen (LH) pumps requires superconducting bearings which can trap magnetic fields up to 1 T at 20 K. MgB ...2 is hence a promising candidate for this application. The superconducting properties and microstructure of differently prepared MgB 2 were compared and the ability of different composite materials to trap magnetic fields was studied. Hollow cylinders of the same geometry were manufactured from hot pressed (under 30 MPa) blocks prepared from Mg:2B with Ti, TiC and Ti-O additives as well as from melt-textured YBCO ceramics. The high critical current densities and critical magnetic fields should ensure high trapped fields in all these materials. Indeed all materials demonstrated the required performance; however, flux jumps are a serious issue in MgB 2 even in crack free cylinders and impeded higher trapped fields.
The oxygen concentration and distribution in the microstructure of MgB 2 - and YBa 2 Cu 3 O 7-δ -based materials affect the formation of nanostructural defects and thus influence the critical current ...density, upper critical magnetic field and irreversibility field. For MgB 2 oxygen containing additions (Dy-O, Ti-O) in the form of nanograins occurred not to be very effective for an increase of critical current density, j c . Sn-O additions to MgB 2 can provoke even a significant decrease of j c due to a chemical interaction of Sn with MgB 2 . The processing pressure and temperature influence the defect density of the relevant pinning centers and the regularity of their distribution in doped and undoped MgB 2 as well as in MT-YBCO.
The correlations between structures and mechanical characteristics of α-AlB12-, AlB12C2-, B4C-based lightweight ceramics and composites synthesized or sintered by hot pressing (at 30 MPa) and ...SiC-based ceramics obtained by reactive sintering are compared. The effect of C, TiC and SiC additions on the properties of the resultant composites and the particularities of the ceramics destruction under shock loading are discussed. Ballistic tests performed on 10 mm thick plates fabricated from the developed ceramics (2.63 g/cm3 density; 78 wt% B4C and 22 wt% SiC composition according to X-ray diffraction analysis; B3.64CSi0.01 and SiC1.07 stoichiometry according to SEM microprobe X-ray analysis), showed that the material could withstand a 10 m distance shot using a bullet with initial kinetic energy of 3.7 kJ.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
In the paper, results of variations of structure, oxidation resistance, and electrical conductivity of novel MAX-phase composite coating are presented. The characteristics of highly dense Ti-Al-C ...composite bulks and vacuum-arc deposited 6 μm thick coatings before and after heating at 600 °C in air for 1000 h were compared. High electrical conductivity (σ = 1.3·106 S/m) of the highly resistant toward oxidation (Δm/S = 0.07 mg/cm2) Ti-Al-C coating was preserved after long-term heating in air. It was found that the specimen surface layers of MAX-phases Ti3AlC2 and Ti2AlC based bulks and chromium-containing Crofer 22APU steel became semiconductors because of high-temperature long-term oxidation (at 600 °C). The vacuum-arc deposited Ti-Al-C composite coating revealed high oxidation resistance and electrical conductivity along with good mechanical characteristics, namely nanohardness H(10mN) = 9.5 ± 1.5 GPa, and Young’s modulus E = 190 ± 10 GPa, which make it very promising for interconnects of solid oxide fuel cells (SOFCs).
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
—For structurally homotypical specimens different in the ZrO
2
content from the (94WC–6Co) + ZrO
2
matrix material used in diamond-containing С
diamond
–(WC–Co) composites formed by spark plasma ...sintering, the dependences of the relative density ρ
rel
, the ultimate strength under compression
R
cm
and bending
R
bm
, the microhardness
Н
V
, and the fracture toughness
K
Iс
on the zirconia content have been established. The addition of 6 wt % zirconia to the WC–6Co composite leads to an increase in the relative density from 0.948 to 0.990, the ultimate compression strength
R
cm
from 4950 ± 110 to 5600 ± 120 MPa, the ultimate bending strength
R
bm
from 1935 ± 80 to 2660 ± 115 MPa, and the fracture toughness
K
Iс
from 13.8 ± 0.71 to 16.9 ± 0.76 MPa m
0.5
at a slight decrease in hardness (from 15.9 ± 0.72 to 15.1 ± 0.33 GPa). Such values are caused by the decrease of main WC phase grains in size with tetragonal
t
-ZrO
2
phase transformation and, correspondingly, by the growing role of transformation strengthening mechanism and the active action of inner mechanical compressive microstresses. When the ZrO
2
additive to the WC–6Co composite is increased to 10%, the parameters ρ
rel
,
R
cm
,
R
bm
, and
K
Iс
are gradually decreased. At the same time, the material at the indentor imprint edge begins to destruct, and crack propagate in a chaotic way. It has been revealed that the properties ρ
rel
,
R
cm
,
R
bm
, and
K
Ic
are worsened at a zirconia nanopowder content above 6 wt % in the WC–Co composite due to the formation of agglomerates during the mixing of components, their separation under sintering, and the formation of micropores and microcracks.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
We prepared samples of composites based on tungsten carbide and cobalt with different concentrations (0–10 wt %) of chromium diboride, 10 mm in diameter and 8 mm in thickness, by cold pressing ...followed by vacuum hot pressing. A comprehensive study of composites using conventional methods of testing mechanical properties, combined with digital optical microscopy, transmission microscopy, and scanning atomic force microscopy, revealed stable correlations between the concentration of CrB
2
additive with the average WC grain size and microstructure parameters, hardness, and fracture toughness of the composites. A coarse-grained structure is observed in WC–6Co (wt %) composites, with direct contact of WC grains and large regions of a cobalt binder. The introduction of CrB
2
into the composite, on the contrary, ensures the formation of thin (~100 nm) and extended layers of a cobalt binder, even between fine WC grains. The CrB
2
additive yields a finer-grained structure, the parameters of which can be purposefully controlled by changing the additive concentration. The addition of 4 wt % of CrB
2
into the composite leads to a more than twofold increase in fracture toughness, that is, from 4.4 to 9.8 MPa m
1/2
, with a slight decrease in hardness from 15.1 to 13.0 GPa. With a further increase in the CrB
2
concentration from 4 to 10 wt %, fracture toughness and hardness gradually decrease.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
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.
The effect of the concentration of vanadium nitride additive (in the range from 0 to 10 wt %) on the phase formation, hardness, and fracture toughness of composite diamond-containing materials based ...on the 51Fe–32Cu–9Ni–8Sn matrix molded by cold pressing and subsequent vacuum hot pressing is investigated. It is found that the addition of 10 wt % of vanadium nitride to the 51Fe–32Cu–9Ni–8Sn composite is accompanied by an increase in the hardness from 3.86 to 8.58 GPa with a slight decrease in the fracture toughness from 5.55 to 4.76 MPa m
1/2
. Moreover, the
H
(
C
VN
) dependence has two characteristic segments that differ in the slope. The hardness increases insignificantly (from 3.86 to 5.26 GPa) in the range of 0 <
C
VN
< 4 wt %, while the second region (
C
VN
> 4 wt %) is characterized by a more substantial increase in the hardness and a more significant decrease in the grain size. It is shown that these parameters are achieved owing to the dispersion mechanism of strengthening and modification of the structure (a decrease in the mean particle size of the matrix phase, the formation of new (Fe
3
Ni)
0.5
and Cu
3
Fe
17
phases, and the precipitation of primary and secondary phases of vanadium nitride) and phase composition of the composites.
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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 structural characteristics and the results of a comprehensive study of the mechanical properties of the B
4
C and AlB
12
ceramic specimens and composites prepared from the B
4
C + 5–25 wt % SiC ...and AlB
12
+ 10–20 wt % TiC mixtures by hot pressing under a pressure of 30 MPa in the temperature range of 1950–2200°C by ultrasound, static, and dynamic methods are presented. The level of achieved mechanical characteristics of the developed composite materials makes them promising for manufacturing armor protection components and other products subjected to substantial dynamic loads. It is shown that one can use disk specimens made of brittle materials with a diameter of 5–15 mm to determine the strength under dynamic loads. The methods developed in the Bakul Institute for Superhard Materials of the National Academy of Sciences of Ukraine are used for studying the synthesized ceramic materials.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
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